Anions of α-Amino Acids as (O,N)-Donor Ligands in Si-, Ge- and Sn-Coordination Chemistry

Seidel, Anne; Wagler, Jörg

doi:10.3390/molecules30040834

Open AccessReview

Anions of α-Amino Acids as (O,N)-Donor Ligands in Si-, Ge- and Sn-Coordination Chemistry

by

Anne Seidel

and

Jörg Wagler

^*

Institut für Anorganische Chemie, TU Bergakademie Freiberg, D-09596 Freiberg, Germany

^*

Author to whom correspondence should be addressed.

Molecules 2025, 30(4), 834; https://doi.org/10.3390/molecules30040834

Submission received: 30 December 2024 / Revised: 2 February 2025 / Accepted: 3 February 2025 / Published: 11 February 2025

(This article belongs to the Special Issue Exclusive Feature Papers in Inorganic Chemistry, 2nd Edition)

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Abstract

:

This review gives an overview of Si-, Ge- and Sn-compounds with (O,N)-bi- and -oligodentate ligands, which have the α-amino carboxylic acid motif N–C(R,R′)–C(=O)O in common (R,R′ = H or hydrocarbyl). While the amino acids themselves are encountered as mono- and di-anionic ligands, modifications at the N-terminus (e.g., extension of the ligand backbone by, e.g., additional alkane carboxylic acid groups) give rise to a wealth of ligands, which bear the α-amino carboxylic acid motif. With particular interest in the coordination features of these ligands, crystallographically characterized complexes are the focus of this review.

Keywords:

amine acetic acid; chelate; chiral pool; complexation; hypercoordination; peptide; tetrel

Graphical Abstract

1. Introduction

Coordination chemistry of the heavier tetrels Si, Ge and Sn has attracted researchers’ interest for decades. Simple complexes of tetravalent tetrels with a higher coordination number were reported more than one century ago, e.g., hexafluorosilicate ([SiF₆]^2–) [1], a related ammonia adduct ([SiF₄(NH₃)₂]) [2], hexachlorostannate ([SnCl₆]^2–) [3] and the Ge-analog ([GeCl₆]^2–) [4]. (The somewhat later dates of reports of various Ge compounds can be attributed to the discovery of the element germanium late, in 1886 [5].) In further studies, chelating ligands have given rise to a wealth of compounds in which these tetrels exhibit coordination numbers five, six or even higher. Thus, coordination chemistry of these tetrels has been thoroughly reviewed over the past decades; for examples, see [6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25]. As these tetrels (when tetravalent) are oxidophilic, which is manifest in the natural abundance of minerals such as cassiterite (SnO₂) and quartz (SiO₂), chelators with anionic O-anchor groups have proven successful in binding additional lone-pair donor sites (e.g., amine or imine N atoms) to the tetrel atoms, thus enhancing their coordination numbers. Figure 1 shows a selection of tetrel complexes I–XI [26,27,28,29,30,31,32,33,34,35,36] with two simple bidentate (O,N)-donor chelating ligands (mono-anions of 8-oxyquinoline and of N,N-dimethylaminoethanol). They reveal that chelators with the N-C-C-O sequence can bind to the tetrels with the formation of five-membered ring motifs and that they can enhance the tetrel coordination number to, e.g., five, six or seven. Moreover, as the pair of complexes IX and X shows, formation of ionic tetrel complexes must be taken into consideration as well, and pair X, XI demonstrates that isomeric coordination modes of the bidentate ligands may be encountered with otherwise related complexes.

In principle, a great variety of (O,N)-chelators is available from the natural chiral pool (i.e., α-amino acids), and further chiral and non-chiral derivatives thereof (of natural or artificial origin, such as aminoacetic acid (glycine) and α-amino isobutyric acid, respectively) are available as well. Interestingly, tetrel coordination chemistry with this class of ligands has been explored only scarcely so far (vide infra), but more complex (oligodentate) derivatives of these ligands also contribute to the portfolio of tetrel complexes investigated in the past decades. In combination with our group’s ongoing investigation of (O,N)-chelate complexes of silicon [37,38,39,40,41,42,43,44], this served as our motivation to review the current state of insights into the Si-, Ge- and Sn-coordination chemistry with α-amino acid-derived chelating ligands. Figure 2a–e give a selected impression as to which types of ligands may feature an α-amino acid motif. (For the sake of simplicity, these molecules are drawn in their carboxylic acid form. We are aware that α-amino acids form zwitterionic species.) The two examples shown in Figure 2f indicate that, starting from α-amino acids, modifications at the N and/or C-terminus may give access to various other (O,N)-chelating ligands. In this regard, this review mentions compounds which feature a genuine α-amino carboxylic acid motif in the ligand backbone and compounds which lack the amine or amide motif at the N-terminus (e.g., Schiff bases) and/or the carboxylate at the C-terminus (e.g., alcoholates, phosphonates). Thus, for the sake of clarity, the former are labeled with Arabic numerals and the latter with Roman numerals. Moreover, labels of generic drawings, which resemble groups of compounds, are italicized to emphasize their variable character. The crystal structures of compounds, which are referred to in this review, can be accessed from the Cambridge Structural Database (CSD). For the readers’ convenience, the access codes (CSD reference codes) are summarized in Appendix A (Table A1, Table A2 and Table A3, for the compounds labeled with Roman numerals, with Arabic numerals and for additional compounds apart from these labeling schemes, respectively).

2. Si-, Ge- and Sn-Complexes of α-Amino Carboxylic Acid-Derived (O,N)-Chelating Ligands

2.1. Si-, Ge- and Sn-Complexes Derived from α-Amino Monocarboxylic Acids

2.1.1. Si-Complexes Derived from α-Amino Monocarboxylic Acids

Literature reports from the past decades suggest that α-amino carboxylic acids can be utilized as mono-anionic (O,N)-chelators at Si, e.g., in complexes generated by reaction of Me₃SiCl or Ph₃SiCl and the mono-sodium salts of amino acids such as glycine, alanine, methionine, leucine, valine or phenylalanine [45]. This series of proposed pentacoordinate silicon chelates is complemented by a series of proposed hexacoordinate silicon bis-chelates obtained in a similar manner from Me₂SiCl₂ and two equivalents of the amino acids’ mono-sodium salts [46]. The protic solvent used for the syntheses (i.e., methanol), however, is likely to react with the chlorosilane starting materials (i.e., formation of methoxysilanes [47]), and none of the proposed chelates has been supported by crystallographic evidence. So far, only six examples of Si-complexes with (O,N)-chelating mono-anions of α-amino acids have been characterized crystallographically (Figure 3) [48]. The authors obtained these complexes from reactions of bis(trimethylsilylated) α-amino acids and isocyanatosilanes (Si(NCO)₄, MeSi(NCO)₃, PhSi(NCO)₃). The hexacoordinate Si-complex 1 derived from l-alanine features cis-arranged monodentate NCO groups trans to the chelates’ carboxylate O atoms. The two formally dative N–Si bonds are trans to one another. The pentacoordinate silicon complexes 2a–2e feature both a mono- and a di-anionic (O,N)-chelator derived from the same amino acid. In complexes 2a–2d (l-alanine and l-phenylalanine used as ligands), the NH₂ group of the mono-anionic chelator occupies an axial position in the almost trigonal–bipyramidal Si-coordination sphere. In the l-tert-leucine derivative 2e, both axial positions are occupied by the carboxylate groups. Selected geometric features of the Si-coordination spheres of these compounds are listed in Table 1. Poor solubility and decomposition in [D6]DMSO inhibited NMR spectroscopic investigations of the stereochemistry of the pentacoordinate Si-compounds 2a–2e in solution [48].

In previous studies, Tacke et al. reported on zwitterionic Si-bis-chelates, in which two di-anions of α-amino acids serve as the (O,N)-chelators (Figure 4) [49,50]. The syntheses of this class of compounds involved a dehydrogenative coupling of the amino acids with the silane H₃Si–CH₂–(NC₅H₆-2,2,6,6-Me₄). In these compounds 3a–3f, the di-anions of glycine, l-alanine, l-phenylalanine, l-valine, l-tert-leucine and l-proline, respectively, are employed as chelators. A common feature of these complexes is the trigonal–bipyramidal Si-coordination sphere with axial orientation of both Si–O bonds. In such a trigonal–bipyramidal arrangement, chiral amino acids (as in 3b–3f) may give rise to the formation of two different diastereomers (3 and 3′). The solid-state structures reveal that the one or the other may crystallize (e.g., 3b corresponds to type 3, 3c corresponds to type 3′). In solution, as monitored NMR spectroscopically, pairs of these diastereomers may coexist in epimerization equilibria (as in the case of 3b), whereas others (e.g., 3c) do not undergo epimerization in solution at room temperature.

So far, there is no crystallographic evidence for the successful isolation of silacycles of α-amino acid di-anions with a tetracoordinate Si atom (neither cyclic 4 nor spirocyclic types 4′ in Figure 5). Nonetheless, the formation of compounds of type 4 as reaction intermediates has been reported in the literature [51]. In principle, silylation of the anionic C- and N-termini of the amino acid is possible, and two-fold silylated derivatives can be isolated, e.g., the bis(triisopropylsilyl) derivative of tryptophan 5 [52]. The five-membered heterocycles, however, exhibit so-called ring-strain release Lewis acidity [53], and therefore they may require a fifth donor atom in the Si-coordination sphere for further stabilization. The successful isolation and structural characterization of ammonia adduct 6a [39] and of N-methylimidazole adducts 6b and 6c [37] are in support of the Si atom’s affinity to a fifth donor atom when incorporated in a five-membered cycle with an amino acid’s di-anion. These compounds were prepared from the respective amino acid and a silicon precursor with Si–N bonds, which either releases the required Lewis base (i.e., hexamethyldisilazane with the release of ammonia in the case of 6a) or triggers the addition of an alternative Lewis base (i.e., starting from Me₂Si(NHtBu)₂ and N-methylimidazole in the cases of 6b and 6c). In adducts 6a–6c, the Si-coordination spheres are close to trigonal–bipyramidal, in all cases with the carboxylate O and the additional Lewis base in the axial position while the anionic N-terminus of the amino acid occupies an equatorial site. In chloroform solution, these adducts dissociate (with the formation of a variety of products in the case of 6a [39] and in coexistence with the silacycle+NMI system in a dynamic equilibrium for 6b, 6c and the related l-valine derivative [37]).

Table 1 contains selected parameters of the Si-coordination spheres of the compounds mentioned in Section 2.1.1. As compound 1 is the only hexacoordinate Si-complex in this series, we cannot deduce any trends in dependence on substitution patterns. Comparison of the other 14 compounds, all of which feature pentacoordinate Si, reveals some mutual features. The distorted trigonal–bipyramidal Si-coordination spheres are founded on axial positioning of O atoms or a combination of one O atom and one non-ionic N-donor moiety. None of the complexes have any amide N atoms in the axial position. The complexes of the series 2a–2e reveal the most pronounced deviation of this axis from linearity. However, there is no clear trend toward forming a square-base pyramidal coordination sphere by pronounced widening of one of the equatorial angles. Each of the compounds 2a–2e features two widened equatorial angles in the range of 120.1–125.9°. The (all-equatorial) Si–N(amide) bonds of the 14 complexes were found in a rather narrow range of 1.69–1.74 Å. The (all-axial) Si–O bonds of the di-anionic chelators span a wider range (1.79–1.88 Å), which exceeds the span of the bond length range of the axial Si–N bonds to the non-ionic donor sites (1.97–2.04 Å). As to the latter, the positional change of this formally dative bond into an equatorial position (encountered with compound 2e, 1.88 Å) has greater influence on this bond length than the variation in substitution patterns in the other 13 compounds. This shortening of the formally dative Si–N bond is accompanied by lengthening of the Si–O bond to the same chelating ligand. The resultant Si–X (X = O,N) bond lengths of the mono-anionic chelator in 2e are similar to those encountered with the mono-anionic chelators in the hexacoordinate Si-complex 1.

2.1.2. Ge-Complexes Derived from α-Amino Monocarboxylic Acids

While in the field of Si-coordination chemistry, a series of different classes of complexes with mono- and/or di-anionic α-amino acid-derived (O,N)-chelate complexes have been reported in the past two decades, there is no crystallographic evidence for the isolation of related Ge complexes. Moreover, no monodentate binding modes of an HN–C–C(=O)O motif to Ge are yet represented in the Cambridge Structural Database (CSD). Nonetheless, some amino acid-derived germacycles were reported by Lavayssiere et al., e.g., the Et₂Ge derivatives of the di-anions of valine and of α-amino isoburyic acid [54,55]. The authors report the use of Et₂GeCl₂ and a supporting base (Et₃N) as well as the route via aminogermane Et₂Ge(NEt₂)₂ for syntheses of that kind of germacycle. In a very recent report, Ignatyev et al. mentioned the formation of the compound (glycinate)Ge(OH)₃, prepared from GeO₂ and glycine, in which the Ge atom is chelated by a mono-anion of glycine [56].

2.1.3. Sn-Complexes Derived from α-Amino Monocarboxylic Acids

As found for Ge, the CSD currently lacks entries for (O,N)-chelate complexes of Sn with mono- and/or di-anionic α-amino acid-derived ligands. However, Ho et al. reported the structure of trimethyltin glycinate 7a [57] (Figure 6), obtained from Me₃SnOH and glycine upon azeotropic removal of the water formed in their reaction. In this polymeric compound, the Sn atom is pentacoordinate because of the two-fold monodentate (and thus bridging) coordination mode of the amino acid’s mono-anion. In a related manner, the (O,S)-di-anion of cysteine operates as a bridging ligand in compound 7b [58]. 7b and related derivatives of l-cysteine (with SnEt₂, SnBu₂, SnPh₂ and SnBn₂ moiety) were prepared from the respective diorganotin dichloride, l-cysteine and NaOEt. In contrast to 7a, compound 7b features a five-membered chelate about the Sn atom, but this (S,N)-bidentate binding mode is special to the anion of this particular amino acid. Some related chelates of Sn(IV) (8a [59], 8b [60]) and Sn(II) (8c [61]) were reported for mono-anions of cysteine esters. Even though they are somewhat out of scope (lacking the (O,N)-chelating motif), they allow for a comparison of the Sn-coordination of the amino acid’s NH₂ group. Comparison of 7a and 7b shows that the chelate and the presence of S (instead of the third C atom) in the trigonal–bipyramidal Sn-coordination sphere strengthen the H₂N–Sn-coordination to some extent, whereas the trans-disposed Sn–O bonds are similar in both compounds. Compound 8a, albeit featuring an axial Sn–Cl bond, resembles a cutout of the structure of 7b with similar bond lengths associated with the (N,S)-chelate at Sn. Comparison of 8a and 8b indicates that enhanced Lewis acidity of the Sn center (Sn-bound Me groups replaced by Cl and another (N,S)-chelate) strengthens the coordinative H₂N–Sn bonds. Longer Sn–N and Sn–S bonds are observed with the related Sn(II) compound 8c.

Even though there is no crystallographic analysis in support of the (O,N)-chelating properties of α-amino acid mono-anions at tin, some stannacycles of that kind were reported in the literature. Djurdjevic and Djokic reported complexation of Sn(II) by glycinate [62], and Nath and Yadaf reported the (O,N)-chelate of the mono-anion of methionine at SnMe₃ [63]. In the context of the known polymeric structure of (glycinate)SnMe₃ (7a) [57], a detailed elucidation of the coordination features of these two compounds is desirable for comparison. Moreover, related chelates were reported for other stannane and amino acid combinations by Cashion et al. [64] and Nath et al. [65].

2.1.4. Concluding Remarks Regarding Si-, Ge- and Sn-Complexes Derived from α-Amino Monocarboxylic Acids

In the course of the past two decades, structural studies of Si-complexes of amino acids revealed that this portfolio of compounds is capable of binding to Si in a chelating manner, both as a mono- and as a di-anionic (O,N)-chelator. In contrast, crystallographic characterization of tin compounds of simple amino acids dates back more than four decades, and yet the structural data available to date are still very scarce. Besides the mono-anion of glycine, which was reported as a bridging ligand between SnMe₃ groups, only cysteine and derivatives thereof were found to chelate the Sn atom (both Sn(IV) and Sn(II)) in a (S,N)-bidentate fashion. In all cases, the amino group of the ligand operates as a neutral donor moiety. Structural evidence of simple α-amino acid anions in (O,N)-chelates at Sn is yet to be delivered. This and the current lack of corresponding structural data for related Ge compounds clearly mark out some fields for further exploration. In principle, bidentate (O,N)-chelating motifs with anions of simple α-amino acids should be feasible at the heavier tetrels as well. Even though Sn is larger and more thiophilic than Si, some studies of the even heavier and more thiophilic Pb(II) (i.e., compounds Pb(leucinate)(NO₃) [66], Pb(phenylalaninate)₂ [67], Pb(prolinate)₂ [68] and Pb(aspartate) [69], cf. entries A3a, A3b, A3c and A3d, respectively, in Table A3) have clearly shown that mono-anions of various α-amino acids can be bound to Pb as (O,N)-chelators, thus being capable of binding at the smallest, i.e., Si(IV) (e.g., in compounds 1 and 2a–2e), and the largest, i.e., Pb(II), of the heavier tetrels. The particularly high stability of Pb(II) and the therefore possible syntheses in aqueous solution with no risk of oxidation of the tetrel and the availability of Pb(II) salts with weakly coordinating anions such as nitrate or perchlorate may have contributed to the success of the exploration of this field of coordination chemistry from this end of the tetrel series, because amino acids (as zwitterionic compounds with good solubility in water) may represent good reactants in an aqueous medium, in particular. The (O,N)-chelation by di-anionic amino acid-derived ligands was reported for Si and Ge (even though representatives of the latter still require detailed structural elucidation). In Sn-coordination chemistry, the (O,N)-di-anions of amino acids still need to be explored as ligands.

Apart from structural elucidation and gaining additional information about the tetrel coordination behavior of mono- and di-anions of simple α-amino acids, Cota et al. pointed out that complexes such as 2a and 2c (which comprise a SiMe motif and amino acid ligands as hydrolysable groups) may be interesting precursors for methylsilanetriol (MeSi(OH)₃) as a food supplement [48]. Moreover, the combination of amino acid-derived chelators and amines at the same Si atom (as in 6a) requires further exploration as starting materials for syntheses of the respective α-amino carboxylic amides [39].

2.2. Si-, Ge- and Sn-Complexes Derived from Dipeptides

2.2.1. Si-Complexes Derived from Dipeptides

In 2021, the first structure of an organosilicon complex with a di-anionic homodipeptide ligand was published [39]. This compound 9a (Figure 7) formed in a triethylamine supported reaction of Me₂SiCl₂ and l-valine and represents a Si-templated synthesis of a dipeptide from an unprotected amino acid. In 2022, a crystallographic analysis of a related compound (9b) was published by Hattori and Yamamoto. The heterodipeptide ligand of compound 9b was deliberately synthesized in the Si-coordination sphere by using Ph₂Si(imidazolyl)₂, α-amino isobutyric acid and the tBu-ester of glycine in a Ta(OEt)₅-catalyzed manner [51]. The authors reported the suitability of this route for the deliberate formation of a great variety of Si-dipeptide complexes. Cota et al. already mentioned the formation of peptides as a side-reaction when utilizing silylated amino acids as starting materials (in the context of their preparation of compounds 1 and 2a–2e) [48].

In compounds 9a and 9b, the Si atoms are situated in distorted trigonal–bipyramidal coordination spheres with the Si–O bond and the formally dative Si–NH₂ bond in axial positions. In this regard, they are related to compounds 2a–2d and 6a–6c. The axial angles (165.2 and 165.4° in 9a, 166.4–167.6° in 9b) exhibit slightly greater deviation from linearity than in compounds 2a–2d, which can be attributed to the tridentate (O,N,N)-chelate with two five-membered rings about Si. The lengths of the axial Si–N bonds correspond very well to the related axial bond lengths of compounds 2a–2d and 6a–6c. The di-anionic motif in compounds 9a and 9b, however, exhibits shorter axial Si–O bonds than in 6a–6c and longer equatorial Si–N bonds (with respect to 2a–2d and 6a–6c).

2.2.2. Ge-Complexes Derived from Dipeptides

Crystallographic studies of related germanium complexes with di-anionic dipeptide ligands were reported for compounds 10a [70] and 10b [71], and further compounds (such as (gly-ala)GeMe₂ [71]) were prepared in a similar manner, i.e., from Me₂GeBr₂ and the free dipeptide in toluene with Et₃N as a sacrificial base. Selected bond lengths of their Ge coordination spheres are shown in Figure 7. The coordination spheres of Ge in 10a and 10b are essentially related to those of Si in 9a and 9b. The larger covalent radius of Ge causes a more pronounced deviation of the axial angle from linearity (O-Ge-NH₂ 161.8(1)° in 10a, 164.4(2)° in 10b). Interestingly, the different bonds of the dipeptide ligand respond differently to the change to the heavier tetrel. Whereas both the axial and the equatorial Si–N bonds are ca. 0.1 Å longer in the Ge compounds, the Ge–O bonds are ca. 0.2 Å longer.

Another class of Ge-complexes with dipeptide ligands (GeCl moiety with a mono-anionic and a di-anionic glycylglycine-derived ligand) has been reported by Giuffrida et al. [72]. The authors studied the molecular conformation of this compound with the aid of computational methods.

2.2.3. Sn-Complexes Derived from Dipeptides

In contrast to the few examples of structurally characterized dipeptide complexes of Ge, a great variety of related diorganotin complexes has been synthesized (and investigated crystallographically). The syntheses started from the free dipeptide, and the synthesis routes can be divided into three general approaches: (A) reaction of a dipeptide and a diorganotin oxide with formation of water, (B) reaction of a dipeptide and a diorganotin dialkoxide with liberation of the respective alcohol, (C) reaction of a dipeptide with sodium alkoxide (for in situ preparation of the dipeptide di-anion with liberation of the respective alcohol) and a diorganotin dichloride with formation of NaCl. As a representative example, compound 11a [73] is listed in Figure 7. In this particular compound, the Sn atom is also coordinated in a distorted trigonal–bipyramidal manner, and with a more pronounced position of the heavier tetrel out of the tridentate (O,N,N)-chelate clamp (the axial angle is 153.0(2)°). With respect to the related Ge compounds, the formal move out of the chelate is accompanied by rather proportional lengthening of the three bonds to the chelating ligand. The variety of dipeptide-Sn-complexes, which were structurally characterized in the course of these studies, revealed a portfolio of tin coordination patterns in their solid-state structures, which depend on the absence vs. presence of additional intramolecular coordination interactions (Figure 8). They range from tin pentacoordination in a distorted trigonal–bipyramidal fashion (Figure 8a, as in 11a), via [5 + 2]- or [5 + 1]-coordination of two O atoms or one O atom of an adjacent carboxylate group (Figure 8b,c, respectively) to [5 + 1]-coordination by a nitrogen atom of an adjacent histidine moiety (Figure 8d).

Table 2 contains selected bond lengths and angles of compounds 11a–11p [73,74,75,76,77,78,79,80,81,82,83,84,85]. The features of the Sn-coordination spheres of the pentacoordinate Sn-compounds of this series are related to those of 11a. Additional remote [5 + 1]- or [5 + 2]-coordination, however, caused pronounced widening of the C-Sn-C angle in most cases (less pronounced in 11k). Interestingly, with respect to bonds to the tridentate dipeptide ligand, the increase in Sn-coordination number mainly causes lengthening of the formally covalent bonds (of Sn–O, in particular), whereas the formally dative bond to the NH₂ group is less affected. We attribute this lengthening of the Sn–N bond to the fact that the increase in coordination number occurs in this equatorial plane where Sn–N is located. Pronounced lengthening of Sn–O rather Sn–NH₂ on the O-Sn-NH₂ axis indicates a pronounced affinity of Sn atom of these compounds to the coordination of the amino group. This is also supported by the fact that the Sn–NH₂ bonds in compounds 11a–11p are markedly shorter (by ca. 0.2 Å) than the corresponding bonds in the pentacoordinate Sn(IV) compounds 7a, 7b and 8a. The remote [5 + 1]- or [5 + 2]-coordination in this class of compounds can be interpreted as an effect in the solid state. Solution-state ¹¹⁹Sn NMR data of compounds such as 11j (δ ¹¹⁹Sn–198.1 ppm) and 11k (δ ¹¹⁹Sn –200.7 ppm) indicate pentacoordination of the Sn atoms (in this case, in [D6]DMSO solution) [81]. In deuterated methanol, Sn pentacoordination is retained as well, as shown for 11b (δ ¹¹⁹Sn–160.6 ppm) [74] as well as 11m (δ ¹¹⁹Sn–175.9 ppm) and 11n (δ ¹¹⁹Sn–183.4 ppm) [83].

This set of structurally characterized diorganotin compounds with (O,N,N)-chelating dipeptide ligands is complemented by further organotin compounds of amino acids and dipeptides, e.g., [86,87,88]. The particular interest in this class of compounds arose from the physiological activity of many organotin compounds, which made these combinations with bio-related molecules (i.e., dipeptides) attractive candidates for the exploration of their, for example, anti-tumor [76,83] anti-bacterial [81] and anti-inflammatory activity [80].

2.2.4. Concluding Remarks Regarding Si-, Ge- and Sn-Complexes Derived from Dipeptides

Regarding the structural motifs encountered with di-anionic ligands of peptides at the heavier tetrels, only compounds with a single (O,N,N)-ligand chelating the tetrel have been reported so far. The ligands’ di-anionic nature in combination with the stability of E = Si(IV), Ge(IV), Sn(IV) and these tetrels’ capability of forming stable hexacoordinate complexes, however, should allow for access to dipeptide complexes of the type E(O,N,N)₂. Even though structural evidence is scarce, some representatives of E(O,N,N)₂ with other tridentate di-anionic chelators capable of chelating via the formation of five-membered rings only (e.g., the compounds XII [89] and XIII [90] shown in Figure 9) serve as motivation for the exploration of such complexes with dipeptide-derived ligands as well.

Moreover, to our knowledge, structural characterization of Si-, Ge- or Sn-complexes of anions of longer oligopeptides has not been reported so far. However, Hattori and Yamamoto [51] reported the utilization of silicon dipeptide complexes as starting materials for selective peptide chain elongation. This provides a perspective on a way to access and characterize silicon complexes of longer oligopeptides. Also, the utilization of silicon compounds as templates for the deliberate syntheses of peptides from unprotected [39,91] or protected amino acids [51] is a very young and promising field of amino acid coordination chemistry at heavier main group elements. In addition to organosilicon compounds, organoaluminium compounds appear to be useful tools in peptide syntheses as well [92].

From the viewpoint of coordination chemistry, the special amino acid motif of proline, which should exhibit different coordination features when incorporated into the dipeptides’ N-terminus positions because of its cyclic amine moiety, still warrants exploration.

2.3. Si-, Ge- and Sn-Complexes Derived from Amineoligoacetic Acids and Related Hydroxyalkylamineacetic Acids

2.3.1. Si-Complexes Derived from Amineoligoacetic Acids and Related Hydroxyalkylamineacetic Acids

Silatranes (general motif 12 in Figure 10) represent an old and well-reviewed class of silicon complexes derived from triethanolamine. Substitution of one, two or three alcoholate groups with carboxylate groups leads to the classes of silatranones, silatranediones and silatranetriones, respectively. The molecular structures of silatranones 12a–12f [93,94,95,96,97] and silatranedione 12g [98] were analyzed by X-ray crystallography, and the molecules were found to resemble the molecular architecture of related silatranes. For example, in methylsilatrane [99] and phenylsilatrane [100], the Si–N bond lengths are 2.16 Å, the Si–O bond lengths are in the range of 1.65–1.68 Å and the sum of O-Si-O angles amounts to 356°. The ranges of Si–N bond lengths (spanning 2.09 Å in 12b and 2.15 Å in 12a) are very similar, and the sums of O-Si-O angles (spanning 355° in 12c and 357° in 12b) are very similar, too. Even though the Si–O bonds are located in similarly planarized equatorial environments around the Si atoms, their bond lengths are noticeably different for the alkoxy- vs. carboxy-O atoms. The former span a range of 1.64–1.66 Å, whereas the latter are longer (in the range of 1.71–1.72 Å in compounds 12a–12f and 1.70 Å in silatranedione 12g). The equatorially situated Si–O(carboxylate) bonds, however, are markedly shorter than the axial Si–O(carboxylate) bonds in the compounds listed in Table 1 and compounds 9a and 9b. Silatranetriones have also been reported [101], but their molecular structures have been analyzed by spectroscopic means only. Their ²⁹Si NMR shifts (e.g., δ –135.8 ppm for methylsilatranetrione N(CH₂COO)₃SiMe in [D6]DMSO, whereas the corresponding methylsilatrane N(CH₂CH₂O)₃SiMe, methylsilatranone (12f) and methylsilatranedione (12g) have ²⁹Si NMR shifts of δ –69.7, –73.2 and –77.2 ppm, respectively, in the same solvent [101]) indicate a higher coordination number of their Si atoms, and the authors attributed this to the additional coordination of a solvent molecule. The presence of a single set of ¹H and ¹³C NMR signals for the three CH₂COO moieties indicates rapid site exchange of the coordinated solvent [101]. A detailed elucidation of their molecular structures is yet to be delivered to allow for discussion of the effects of solvent coordination on the bond lengths and angles of the atrane cage.

Structural analyses of quasisilatranones and quasisilatranediones (i.e., silatranone-related Si-compounds with tridentate (O,N,O)-chelators with a central NH moiety) have not been reported so far. Moreover, we are not aware of reports of silicon complexes of chelating anions derived from ethylenediamine tetraacetic acid, diethylenetriamine pentaacetic acid or related extended versions of nitrilotriacetic acid.

2.3.2. Ge-Complexes Derived from Amineoligoacetic Acids and Related Hydroxyalkylamineacetic Acids

In sharp contrast to the structural information available in the field of silatranone-type silicon complexes, the current state of crystallographic characterization of related Ge compounds lies beyond the Si congeners, with no overlap that would allow for a direct comparison of molecular structures. Even though germatranones, germatranediones and germatranetriones have been reported in the literature [101], only one crystal structure has been reported out of this group of compounds, i.e., the hydrate of hydroxygermatranetrione 13 [102] (Figure 11). In this complex, the Ge atom is hexacoordinated, and a water molecule serves as a sixth ligand moiety, located in the otherwise equatorial GeO₃ plane of the atrane motif. In this compound, the length of the Ge–OOC bond trans to the Ge-bound water molecule (1.88 Å) is only marginally shorter than the mutually trans Ge–OOC bonds (1.89 and 1.90 Å). They are closer to the length of the dative Ge–OH₂ bond (1.92 Å) than to the length of the formally covalent Ge–OH bond (1.76 Å). The structure of the corresponding anionic complex 13′ (K⁺ salt, two OH groups bound to Ge) has been reported as well [103]. The conformation of this anion resembles the molecular shape of 13. The stronger coordination of the additional OH group trans to the carboxylate (Ge–OH bond length 1.82 Å vs. Ge–OH₂ bond length 1.92 Å in 13) causes particular lengthening of the trans-disposed Ge–OOC bond (1.97 Å), while the mutually trans Ge–OOC bonds (1.91 Å) remain shorter. The related compound 14 features a phosphonic acid moiety in one of its equatorial atrane positions (with respect to the N-Ge-OH axis) [104]. In this case, the anionic phosphonate serves as a ligand, which enhances the Ge coordination number to six by dimerization of this anionic complex via Ge-O-P-O-Ge bridges. The two Ge–OP bonds (1.87 and 1.88 Å) are shorter than the two Ge–OOC bonds (1.90 and 1.93 Å). The Ge–OH bond (1.77 Å) is similar to the corresponding bond in 13. The Ge–N bonds are very similar in compounds 13, 13′ and 14 (2.08, 2.08 and 2.09 Å, respectively).

Moreover, detailed structural analyses are available for Ge-complexes derived from oligoamine–oligoacetic acids. This class of compounds is accessible in acidic aqueous medium from GeCl₄ [105]. Most of them feature the ethylenediamine tetraacetic acid motif (compounds 15 [106,107,108,109,110], Figure 12, Table 3), and one of them has a propane-1,3-diamine backbone (compound 16 [111]). This difference, i.e., extension of the ligand backbone by one additional carbon atom, allows the diamine tetraacetate to bind to the Ge atom in a hexadentate manner, as shown in Figure 12 for compound 16, whereas in compounds of type 15, the diamine oligoacetate ligand always binds in a pentadentate manner, leaving a dangling ligand arm R, while an OH group occupies the sixth ligand site at Ge. As shown in Table 3, the seven representatives of compound class 15 (15a–15f [106,107,108,109,110], where the crystal structure of 15e features two independent molecules in the asymmetric unit) exhibit very similar bond length patterns on their Ge coordination spheres. There is a trend that the Ge–N bond a of the atrane-like N atom (which features three Ge-binding arms) is similar to those in compounds 13, 13′ and 14, whereas the Ge–N bond b of the N atom with a dangling substituent is slightly longer. As bond a is trans to a more tightly bound anionic O-donor than bond b (i.e., bond f is shorter than bond d), we attribute the shortness of bond a (and the similarities of this bond length to those in compounds 13, 13′ and 14) to the bond-enforcing action of the tripodal ligand motif in atranes and related compounds. These interpretations are consistent with the bond lengths observed in compound 16, which features shorter Ge–N bonds. In 16, both N atoms are part of tripodal ligand motifs, and the Ge-coordination sphere is devoid of such a tightly bound O-donor site (i.e., OH) as in compounds of type 15, which would compete with the trans-disposed Ge–N bond. If in compounds of type 15 the dangling arm R is a chelator such as CH₂CH₂N(CH₂COO⁻)₂ (i.e., 15 being a derivative of diethylenetriamine pentaacetic acid), additional cations can be added to this remote ligand site with the formation of related heteronuclear complexes, such as [“Ge”-CH₂CH₂N(CH₂COO)(CH₂COOH)]₂Cu, with “Ge” being Ge-complex motif 15 with the R′ = CH₂CH₂ backbone [112] (cf. entry A3e in Table A3). A different kind of heteronuclear germatranone originates from 2-propanol-1,3-diamine tetraacetic acid (i.e., from the 2-OH functionalized derivative of the ligand used in 16). This ligand gives rise to the formation of germatranedione cages with the alcoholate ligand arm bridging the Ge site and the adjacent heterometallic site in complexes of types 17 and 17′. In motif 17 (encountered with lanthanide complexes 17a–17e [103,113,114,115,116]), the alcoholate donor site at Ge is trans to a Ge–OH bond, and the two Ge–OOC bonds are mutually trans. In type 17′, the two Ge–OOC bonds are mutually cis, and another equatorial O-donor ligand (“O”) is bound in one of the cis positions next to the alcoholate. In compound 17f, “O” is a water molecule. In ML_n = Cu(bipy) [117] and Zn(H₂O)₂ complexes [118] (cf. Table A3 entries A3f and A3g, respectively) of a more complicated multinuclear architecture, a carbonyl O atom of an adjacent complex moiety occupies this position. The Ge–N, Ge–OOC and Ge–OH bond lengths in the complexes 17a–17f (Table 4) are longer than corresponding bonds in germatranetrione 13.

2.3.3. Sn-Complexes Derived from Amineoligoacetic Acids and Related Hydroxyalkylamineacetic Acids

In contrast to the Si- and Ge-chemistry shown in Section 2.3.1 and Section 2.3.2, respectively, structural characterization of tin complexes of amineoligoacetic acid derivatives has been successful with a greater variety of ligands. Particularly interesting is the set of compounds derived from diacetic acid derivatives (compounds 18, Figure 13). The N-bound substituent R may be H or hydrocarbyl (a non-coordinating residue), and for a set of diorganotin compounds derived therefrom (18a–18g [119,120,121,122,123,124,125]), two principle coordination motifs are encountered. As shown for 18a, the di-anionic (O,N,O)-ligand was found to bind to Sn in an axial–equatorial–axial manner, and the equatorial Sn–C bonds span an angle closer to the expected 120° for equatorial positions in a trigonal bipyramid. With the aid of additional ligands L (such as water) and dimerization via additional remote coordination of a carboxylic O atom of an adjacent complex molecule, the Sn-coordination sphere transitions toward pentagonal–bipyramidal with axial Sn–C bonds and equatorial positioning of N and O atoms (as shown in Figure 13 for 18c). As shown in Table 5, this adoption of pentagonal–bipyramidal coordination is similar for compounds 18c–18g, all resembling a [6 + 1]-coordination sphere ([4 + 1] in the equatorial plane) with the carboxylate O of an adjacent molecule being the remote lone-pair donor. The C-Sn-C angle widening (toward axial positions) is more pronounced than in the dipeptide complexes 11e, 11g and 11i, which also feature additional remote coordination in the solid state (cf. Table 2). Moreover, the related complex 19 (Figure 13, [126]) adopts an intermediate role in this regard. Its Sn atom is hexacoordinated, and its C-Sn-C angle is 141.7°. The different electronic situation in 19, however, causes further differences, and therefore rather withdraws this complex from comparison within a series. That is, the different bridging atom (alcoholate instead of carboxylate) forms short bonds to both Sn atoms (2.07 and 2.34 Å of the intra- and intermolecular bond length, respectively), whereas the Sn–O(carboxylate) bond is rather long (2.49 Å). Other changes in the ligand (such as a coordinating substituent R, a 2-hydroxyethyl group) may give rise to different coordination patterns, i.e., the stannatranedione motif with hexacoordinate Sn in 20 [127]. In spite of the additional ligand arm, the Sn–N bond length in 20 (2.32 Å) is similar to those in compounds 18a–18g. Whereas in compounds 18 and in 20 the (O,N,O)-ligand binds to Sn in a mer-like fashion (positioning of both O,N,O and Sn close to an idealized plane), the anionic complex 21 [128] represents a structurally characterized example of this ligand binding to octahedrally coordinated Sn in a fac configuration. In addition to the different configuration of the ligand and the set of electron-withdrawing substituents at Sn, the Sn–N bond in 21 (2.22 Å) is markedly shorter than those in compounds 18 and 20. Extension of the ligand backbone, toward ethylendiamine diacetate (OOCCH₂NHCH₂CH₂NHCH₂COO)^2–, also allows for the formation of octahedral Sn-complexes, as shown for the SnMe₂ derivative, which has a trans-N-Sn-C and trans-O-Sn-O arrangement [122] (cf. Table A3 entry A3h).

Using nitrilotriacetate, Sn-complexes with a penta- (22) [129], hexa- (23) [130], hepta- (24) [131] and octacoordinated Sn atom (25) [132,133] were prepared, the molecular structures of which were characterized (Figure 14). Compound 22 was prepared from the corresponding triiodostannyl complex and nitrilotriacetic acid in dichloromethane in a base-supported substitution reaction with retention of the Sn–Os bond. Compound 23 formed in a Sn–C(Ph) bond cleavage reaction of Me₂N(CH₂)₃SnPh₃ and nitrilotriacetic acid in DMF. Hence, in addition to the liberation of benzene as the expected leaving group, the NMe₂ moiety was converted into the corresponding amine N-oxide, which furnishes the Sn-coordination sphere of 23. Salts of the anions 24 and 25 were accessible in aqueous solution.

The Sn-coordination sphere in 22 is trigonal–bipyramidal, with the Sn–Os bond in the axial position and a trans-disposed N–Sn bond. In 23, the tripodal ligand adopts a configuration as shown in Figure 14, which is related to the structures of compounds 20 and 21, combining their Sn-OOC positions in one molecule. This arrangement in an octahedral Sn-coordination sphere may play an important role in the Sn–N bond length of this molecule, because this bond length is in the lower range for this set of three octahedral Sn-complexes (2.23 Å in 23, 2.22 Å in 21, 2.32 Å in 20), whereas the penta-, hepta- and octacoordinated Sn-compounds in Figure 14 feature markedly longer Sn–N bonds (2.36 Å in 22, 2.32 and 2.35 Å for the tetra- and the tridentate ligand site, respectively, in 24, 2.37 Å in the K-salt of 25, 2.38 and 2.40 Å in the methylammonium salt of 25).

The molecular conformation of anion 24 can be related to the atrane-type cage (as in 22) for the tetradentate ligand, and the three bonds to the donor atoms of the tridentate ligand are staggered about the O-Sn-O angles of the former. In contrast, in di-anionic octacoordinated Sn-complex 25, the two tetradentate ligand moieties adopt conformations which are comparable to those in 23, and a Sn–O bond of one ligand opposes the vacant face of the other ligand (and vice versa), as indicated by the bold-style O and N atoms in Figure 14 (vacant face at bold N of one ligand exposed to the bold O of the other ligand and vice versa).

This trend toward coordination numbers 7 or even 8 of the Sn atom, which contrasts the Sn-coordination chemistry from related Ge- and Si-chemistry, is also evident in Sn-complexes of oligoamine oligoacetic acid-derived ligands. Various derivatives of Sn(EDTA) (EDTA being the tetra-anion ethylenediamine tetraacetate) of the general molecular building pattern 26 [134,135,136,137,138,139] (Figure 15) were characterized crystallographically, and their mutual feature is a heptacoordinate Sn in a coordination sphere, which can be described as a distorted pentagonal bipyramid. As shown in Figure 15, the axial positions are occupied by carboxylate O atoms (forming the shortest Sn–O bonds in this coordination sphere), and the sets of atoms in the idealized pentagonal plane (two N atoms, two further carboxylate O atoms, one additional lone-pair donor ligand L) are distorted against one another in terms of a twist of the N₂Sn plane against the SnO₂L plane. This building pattern (although more distorted off the bipyramidal shape of compounds of type 26) is in general retained in Sn(IV) complexes derived from diethylenetriaminepentaacetic acid (motif 27, both the protio form [140,141] and the ammonium salt [142] were characterized crystallographically). Their building pattern is related to that of compounds 26 by replacing an equatorial O atom with the third amine N, accompanied by the replacement of ligand L with O atoms of the additional two carboxylate moieties. Thus, in compounds of type 27, the Sn atom is octacoordinated and completely encapsulated in the octadentate ligand. Also related to the molecular structures of 26 and 27 is tin(II) compound (EDTA)Sn₂ (28) [143]. The solid-state structure of 28 features two different Sn(II) coordination spheres; one Sn atom is located in a SnO₄-coordination sphere, surrounded by carboxylate O atoms (it serves as the bridging Sn site, indicated by “(Sn)” in Figure 15), whereas the other Sn atom is located in the EDTA chelate in a SnN₂O₄-coordination sphere (in a [4 + 2]-coordination with two markedly longer Sn-O distances trans to the Sn⋯N bonds), which relates to those of 26 by replacing the seventh ligand L with the Sn-located lone pair, and it relates to 27 in terms of the pronounced distortion off the axial geometry. In sharp contrast to 28, tin(II) compound (EDTA)H₂Sn (29) [144] adopts a tetrel coordination sphere related to those of Ge compounds of type 15 (the site of the Ge-bound OH group in 15 is replaced with the Sn-located lone pair in 29).

2.3.4. Concluding Remarks Regarding Si-, Ge- and Sn-Complexes Derived from Amineoligoacetic Acids and Related Hydroxyalkylamineacetic Acids

The exploration of Ge- and Sn-coordination chemistry benefits from the availability of compounds such as EDTA complexes from an aqueous medium. This allows for access to a variety of complexes which feature, for example, an atranetrione or -dione ligand backbone about Ge or Sn. Because of the size difference between Ge and Sn, different coordination modes of EDTA and related anions are encountered (vide supra). Interestingly, EDTA complexes of hs-Fe³⁺, which has an ionic radius between those of the formal ionic radii of Ge⁴⁺ and Sn⁴⁺ [145], include the motifs encountered with Ge (in [Fe(HEDTA)(H₂O)] [146]) and with Sn (in K[Fe(EDTA)(H₂O)] [147]) (cf. Table A3, entries A3i and A3j, respectively). The class of complexes presented in Section 2.3.3. comprises some structurally characterized examples of Sn(II) compounds. In this regard, it serves as a motivator for further exploration of related Sn(II) complexes with ligands mentioned in Section 2.1 and Section 2.2. Moreover, the wealth of coordination chemistry encountered with the Ge- and Sn-complexes in Section 2.3. raises questions as to the facets of Si-coordination chemistry of anions of amineoligoacetic acids beyond silatranediones, with the Si-hexacoordinate silatranetriones as an entrance into this world of coordination chemistry of interesting compounds yet to be characterized in detail.

3. Perspectives Outside the Box

3.1. α-Amino Acid-Derived Schiff Bases as Ligands

In principle, the primary amine motif of an α-amino acid motivates researchers to create related ligands by Schiff base condensation of this group with suitable carbonyl compounds (such as o-hydroxyarylcarbaldehydes, -ketones or acetylacetone). In 1992, Smith et al. reported the first crystal structure of an (O,N,O)-chelated tin compound with a Schiff base derived from an α-C substituted α-amino acid, a compound of type XIV (Figure 16, compound XIVa [148]). It features a di-anionic (O,N,O)-tridentate Schiff base ligand derived from salicylaldehyde and valine, and further compounds of that kind were reported in the course of this investigation. The Schiff base ligands were accessible by condensation reaction of the aldehyde and the free amino acid, and the tin complexes were prepared from the Schiff base ligands and diorganotin oxides [148]. Further compounds of that kind were reported thereafter, e.g., compounds XIVb [149] and XIVc [150] with ligands derived from leucine and alanine, respectively, and it was reported that racemization of the chiral Schiff base compounds may occur [150]. Moreover, modifications at the aryl part of the ligand backbone enhanced the portfolio of these compounds (e.g., XIVd and XIVe [151]); they were pursued as this kind of tin compound was of interest for the exploration of their cytotoxic activity. In the course of the past decade, acetylacetone-derived Schiff bases (e.g., XVa [152], XVb [153], XVc [154]) have enhanced the portfolio of this class of tin compounds even further (also with the aim of exploring their biological activity, e.g., for use against moscito larvae [154]). As found with other diorganotin(IV) compounds with di-anionic tridentate (O,N,O)-ligands (cf. Section 2.2.3 and Section 2.3.3), Schiff base complexes such as compounds of type XIV may establish additional intermolecular interactions, which enhance the Sn-coordination number to 6 (e.g., compound XIVf [155]), [6 + 1] (compound XIVg [156]) or 7 (in XIVh [157]). Within the portfolio of amino acid-derived Schiff base complexes of tin, compounds XVI [158] and XVII [159] are particularly noteworthy as they enhance our understanding of the coordination properties of this class of tridentate ligands. The former features a mono-anionic (O,N,O)-ligand, which also binds to tin in a tridentate manner. The latter represents (to date) the only crystallographically characterized tin complex which features two of these amino acid-derived Schiff base ligands. In both cases, the Sn atom is hexacoordinate, and the (O,N,O)-ligands occupy meridional positions in the octahedral Sn-coordination spheres.

In the past two decades, these studies were complemented by the investigation of some related Ge- and Si-compounds. In 2002, Nath and Goyal reported the syntheses of some organosilicon compounds with amino acid-derived Schiff base ligands and the investigation of their antimicrobial activity (even though the authors also mentioned that these compounds are sensitive to hydrolysis) [160]. Detailed insights into the molecular structures of such complexes were delivered for aldehyde-derived compounds such as XVIII [159], XIXa [161] and XIXb [159], acetylacetone-derived compounds such as XXa [162] and XXb [163] and even for compounds of sterically more demanding imine backbones (XXIa [164] and XXIb [165]) (see Figure 17). It was also pointed out that racemization can present an issue in case of the preparation of silicon compounds of amino acid-derived Schiff base ligands [161].

3.2. α-Amino Acid-Derived Alcohols as Ligands

In addition to the preparation of complexes with mono- or di-anionic α-amino acid ligand motifs, amino acids (especially the chiral representatives thereof) offer great potential for transferring their chiral information to other classes of ligands. The chiral amino alcohol H₂N-C*HiPr-CPh₂-OH, which can be obtained from valine, can be utilized for the preparation of chiral Schiff base ligands and Si-complexes thereof as well, compound XXII (Figure 18) being a representative thereof [166]. Compound XXIII represents a Si-complex with a related aminoalcohol motif in a tridentate (O,N,O)-ligand that features an amine moiety as the formal dative bond donor [167]. To name a second class of oligodentate ligands, chiral triethanolamine derivatives (and complexes thereof) may also be obtained from chiral amino acids, e.g., compounds of type XXIV, which are based on alanine and valine as the chiral starting materials [168]. While other routes may also be employed (e.g., the -CHPhCH₂O- and -CH₂CHMeO- moieties in the ligands of compounds XXV [169] and XXVI [170], respectively, originate from ring-opening of styreneoxide and propyleneoxide, respectively), the amino acids from the natural chiral pool offer a variety of building blocks, which can be converted into different kinds of other chiral ligands.

3.3. Ligands Derived from Non-Carboxylic α-Amino Acids

Even though silicon analogs of the type R₂N-CH₂-Si(=O)OH are not known, the α-amino carboxylic acid motif is related to other heavier main group element (E)-based acids of the type R₂N-CH₂-E(=O)R′OH (R, R′ being various kinds of residues, e.g., alkyl, aryl, H and, for R′, extending to OH, alkyoxy and others) such as aminoalkylphosphonic or -sulfonic acid derivatives. For some simple representatives thereof, coordination chemistry has been explored to some extent, e.g., syntheses and characterization of the copper complexes of H₂N-CH₂-PR(=O)O^– (R = Me, Ph) [171] and H₂N-CH₂-S(=O)₂O^– [172] (cf. Table A3, entries A3k, A3l and A3m, respectively), in which these mono-anions operate as (O,N)-chelators. A search for crystallographic evidence of these special kinds of α-amino acid motifs acting as chelators in Si/Ge/Sn-coordination chemistry merely yielded some hits for phosphonic and phosphinic acid derivatives (XXVII [173], 14 [104], XXVIII [174], Figure 19). Interestingly, only the Ge representative, which has N clamped to Ge by additional carboxylate buttresses, exhibits a rather short bond between the amine N atom and the heavier tetrel. The coordinative N–E bonds for the E = Sn, Si representatives are markedly longer. For the series of Si-compounds XXVIIIa-e shown in Figure 19, this was rather unexpected; the anion of the trifunctional aminomethylphosphinic acid (nitrilotris(phenylphosphinic acid) [175]) allowed for rather long N–Si distances in spite of the additional buttresses. Of note, the Sn representative XXVII features N–Sn-coordination at the Sn(II) site of this Sn(II)₂Sn(IV) trinuclear complex. Sn-complexes of [4-HOOC-C₆H₄-CH₂N(CH₂PO₃)₂]^4– (Sn–N 2.92 Å) [176], [–CH(CH₂CH₂)₂NCH₂PO₃^2–]₂ (Sn–N 2.94, 2.96, 3.23 Å; 2.81, 3.13, 3.19 Å) [177] and CyN(CH₂PO₃^2–)₂ (Sn–N 2.96 Å) [178] (cf. Table A3, entries A3n, A3o and A3p, respectively) contribute to the portfolio for the heavier tetrel. All of them feature amine-Sn-coordination at Sn(II) with rather long Sn–N distances (listed in parentheses). As for Sn(IV) representatives, structures of other Sn-compounds of α-amino phosphonic and phosphinic acid derivatives were reported in which the respective molecule or anion binds to Sn via O only, leaving behind a vacant N lone-pair donor site. Figure 19 shows some selected examples (XXIX [179], XXX [180], XXXI [181]), and the SnCl₂Me₂-complex of [(EtO)₂(O=)PCH(Ph)-NH-CH₂]₂ [182] (cf. Table A3 entry A3q) represents a further example. Of note, coordination of amine N to phosphonate-functionalized Sn(IV) at a rather short distance (2.45 Å) is in principle feasible, as shown by Dakternieks et al. for compound MeN(CH₂CH₂CH₂)₂SnMe(O(OH)O=PtBu) [183] (cf. Table A3 entry A3r).

As for sulfur analogs, compound XXXII shown in Figure 20 [184] represents the only crystallographically characterized compound with a five-membered (N-C-S-O)Sn-chelate motif, and representatives of Ge and Si are yet to be explored. In principle, compounds such as the SiMe₃-derivative XXXIII shown in Figure 20 [185] may turn out to be suitable starting materials for syntheses of (N-C-S-O)E (E = Si, Ge, Sn)-chelates. The compound itself would be a neutral (N-C-S-O)-chelator, but the SiMe₃ groups as leaving groups offer its transformation into an anionic chelator. In its Li salt (XXXIV), the anion forms a five-membered (N-C-S-O)Li-chelate [185].

4. Conclusions

This overview of heavier tetrel derivatives of α-amino acids, where anions of the latter operate as (O,N)-chelating ligands, demonstrates the variety of ligands with the α-amino acid motif in Si-, Ge- and Sn-coordination chemistry. Some classes of complexes (e.g., complexes of di-anions of dipeptides) were reported with structural characterization for the series of the three tetrels and thus allow for comparison of the coordination behaviors of the different tetrels. For some classes (e.g., complexes derived from amine diacetic acid), series of crystallographically characterized representatives were reported for one tetrel only (E = Sn), which at least allow for comparison of structural features within the set of complexes. In the context of the availability of a great variety of amino acids from the natural chiral pool, however, the portfolio of structurally characterized Si-, Ge- and Sn-complexes of these ligands is rather limited. Moreover, rather common amine oligoacetic acids (such as nitrilotriacetic acid, ethylenediamine tetraacetic acid) are also highly underrepresented in some fields of tetrel coordination chemistry as far as unequivocal characterization of the molecular structures of the resultant complexes is concerned. Some emerging fields of tetrel coordination chemistry with α-amino acid-derived ligands indicate that the overall topic of tetrel coordination chemistry with these kinds of (O,N)-chelators is a live and growing field. Just to name sections of the timeline and give some examples: The first crystallographic characterization of a tin complex of a chiral amino acid-derived Schiff base ligand dates back to 1992 [148], and examples with Si and Ge followed in 2012 [161] and 2018 [159], respectively. Whereas complexes of di-anions of dipeptides were structurally characterized as early as the 1970s for Sn [85] and 1980s for Ge [70,71], the first crystallographic report of a dipeptide-derived Si-complex was published in 2021 [39]. However, gaps in some series of classes of complexes predominate. Detailed structural characterization is yet to be delivered for, e.g., silatranetriones, which are interesting because of their tendency toward Si-hexacoordination (as indicated by ²⁹Si NMR spectroscopic data) [101]; for EDTA complexes of silicon, which would complement the examples of heptacoordinate Sn- and hexacoordinate Ge-complexes of this well-known chelator; for Ge-complexes of mono- and di-anions of simple α-amino acids, where there are examples of chelating mono- and di-anions of that kind at Si [37,39,48,49,50]; and structural evidence for the bridging coordination of the glycinate mono-anion in polymeric (gly)SnMe₃ [57], which raise questions as to the preferred chelating or bridging coordination of these ligands in Ge compounds. In addition to the apparent gaps in series of compounds, this overview sheds light on further fields of coordination chemistry yet to be explored. Tetrel coordination chemistry of other α-amino acids (such as α-amino phosphonic or phosphinic acids) is an almost blank space on the map of structurally characterized compounds. Moreover, in spite of the enhanced stability of di-valent tin (with respect to Ge(II) or Si(II)), the field of Sn(II)-compounds of α-amino acids is rather underrepresented. In this regard, chelates of the type (O,N)₂Sn(II) with amino acid-derived mono-anionic (O,N)-chelating ligands may represent interesting ligands themselves in transition metal coordination chemistry because of the lone-pair donor capability of Sn(II). For other chelates, some transition metal complexes have been characterized, e.g., (8-oxyquinolinate)₂GeCr(CO)₅ [186] and (8-oxyquinolinate)₂SnCr(CO)₅ [187] (cf. Table A3 entries A3s and A3t, respectively). For α-amino acid-derived complexes of Sn (or even Ge or Si), however, this kind of transition metal coordination chemistry with group 14 ylene ligands is an unexplored field. So far, stannatranetrione compound 22 [129], which features an Os–Sn bond, is the only crystallographically characterized TM–E compound (TM = any transition metal, E = Si,Ge,Sn), which features an E-bound α-amino acid-derived ligand backbone.

Author Contributions

A.S. and J.W. contributed to the writing and proofreading of this review. All authors have read and agreed to the published version of the manuscript.

Funding

Parts of this work were carried out with financial support from the European Union (European Social Fund, ESF) and the federal state of Saxony (Sächsische Aufbaubank, SAB, Dresden, Germany) via a Ph.D. scholarship (“Landesinnovationsstipendium”) to A. Seidel under Project 100670490-AP2.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Conflicts of Interest

The authors declare no conflicts of interest.

Appendix A

Table A1. List of literature references (Ref.) and CSD reference codes (CSD Refcode) of crystallographically characterized compounds mentioned in this paper with Roman numerals. The crystallographic data (in CIF format) can be accessed from the Cambridge Structural Database (CSD) using the respective CSD Refcode via https://www.ccdc.cam.ac.uk/structures/ (accessed on 1 February 2025 for availability check).

Compound	Ref.	CSD Refcode	Compound	Ref.	CSD Refcode	Compound	Ref.	CSD Refcode
I	[26]	IHAFAY	XIVe	[151]	KAXVIP	XXIII	[167]	FEYMON
II	[27]	EYASOO	XIVf	[155]	OGOCIZ	XXIVa	[168]	RABLUB
III	[28]	MOCCIT	XIVg	[156]	NEYDED	XXIVb	[168]	RABMAI
IV	[29]	DMSNQN	XIVh	[157]	JATHUI	XXV	[169]	IJAXEW
V	[30]	FEDYOC	XVa	[152]	ZEHQIP	XXVI	[170]	OGABAD
VI	[31]	FICNUA	XVb	[153]	KALYEE	XXVII	[173]	QIFBIT
VII	[32]	CEWJAP	XVc	[154]	ESOMAC	XXVIIIa	[174]	UGEKOK
VIII	[33]	WONQOG	XVI	[158]	JILXAH	XXVIIIb	[174]	UGELEB
IX	[34]	CEKWAR	XVII	[159]	ZIGWIY	XXVIIIc	[174]	UGELAX
X	[35]	WEQRIU	XVIII	[159]	ZIGTUH	XXVIIId	[174]	UGELIF
XI	[36]	VUKMAS	XIXa	[161]	ACIJED	XXVIIIe	[174]	UGEKUQ
XII	[89]	VUNKAR	XIXb	[159]	ZIGWAG ¹	XXIX	[179]	KISROT
XIII	[90]	IQATOK	XXa	[162]	MUFQAH	XXX	[180]	QIHWEJ
XIVa	[148]	JUNHAB	XXb	[163]	OFOYOZ	XXXI	[181]	JIPYEP ²
XIVb	[149]	TOZNEB	XXIa	[164]	JICTAU	XXXII	[184]	SUPHOC
XIVc	[150]	EHIZOK	XXIb	[165]	YILVOI	XXXIII	[185]	GIJJIT
XIVd	[151]	KAXVEL	XXII	[166]	WEKFAU	XXXIV	[185]	GIJJAL

¹ Another entry of a crystal structure of this compound can be found under CSD Refcode ZIGWEU. ² Another entry of a crystal structure of this compound can be found under CSD Refcode JIPYOZ.

Table A2. List of literature references (Ref.) and CSD reference codes (CSD Refcode) of crystallographically characterized compounds mentioned in this paper with Arabic numerals. The crystallographic data (in CIF format) can be accessed from the Cambridge Structural Database (CSD) using the respective CSD Refcode via https://www.ccdc.cam.ac.uk/structures/ (accessed on 1 February 2025 for availability check).

Compound	Ref.	CSD Refcode	Compound	Ref.	CSD Refcode	Compound	Ref.	CSD Refcode
1	[48]	WUYTIU	11g	[78]	KIHYAB	17e	[114]	GELJUE
2a	[48]	WUYTOA	11h	[79]	VUPRON	17f	[116]	RAGHIS
2b	[48]	WUYTOG	11i	[80]	IGUQIL	18a	[119]	OWEYAS
2c	[48]	WUYVAO	11j	[81]	MOCRAY	18b	[120]	IGEYOI
2d	[48]	WUYVES	11k	[81]	MOCREC	18c	[121]	KELLAO
2e	[48]	WUYVIW	11l	[82]	VUPDAL	18d	[122]	TOSRIC
3a	[49]	GAGJEE	11m	[83]	PATHEX	18e	[123]	GAMYUP
3b	[49]	GAGJII	11n	[83]	PATHIB	18f	[124]	SARREL
3c	[49]	GAGJOO	11o	[84]	SASZUH	18g	[125]	LUKDEB01
3d	[50]	JAZHAU	11p	[85]	GLDPSN	19	[126]	VAZHAG
3e	[50]	JAZHEY	12a	[93]	CASYOK	20	[127]	VIPDAZ
3f	[50]	JAZHOI	12b	[94]	CECXEN	21	[128]	QOVVEE
5	[52]	UYAYEZ	12c	[95]	FMESIA	22	[129]	QINWOZ
6a	[39]	JAPZOT	12d	[95]	FMESIB	23	[130]	HAWBOW
6b	[37]	HIVJOP	12e	[96]	FOFXED	24	[131]	LOFJEV
6c	[37]	HIVJUV	12f	[97]	GEKXID	25 ¹	[132]	FIBWAO
7a	[57]	GLYMSN10	12g	[98]	DIRSAY	25 ²	[132]	FIBWOC
7b	[58]	BIGSUG	13	[102]	VUHCIK	25 ³	[133]	IWEROR
8a	[59]	ECYSSN	13′	[103]	LAFHIL	25 ⁴	[133]	IWERIL
8b	[60]	BONYAE	14	[104]	TEBJOA	26 ⁵	[134]	HIVZUJ
8c	[61]	VUKLIW	15a	[106]	VEFGOC	26 ⁶	[135]	SNEDTA
9a	[39]	JAPZUZ	15b	[107]	VOCPIN	26 ⁷	[136]	NUQZEE
9b	[51]	QANSOR	15c	[108]	YUFYIH	26 ⁸	[137]	QELZUC
10a	[70]	VASWER	15d	[109]	XOHHEH	26 ⁹	[137]	QEMBAL
10b	[71]	VAWHEG	15e	[110]	VOCPOT	26 ¹⁰	[138]	TORTUP
11a	[73]	DOHBIL	15f	[108]	YUFYON	26 ¹¹	[139]	WEVNAP
11b	[74]	NOCQOM	16	[111]	XUZZEZ	27 ¹²	[140]	KIJWUV
11c	[75]	QOBXOU	17a	[103]	GEMPEV	27 ¹²	[141]	KIJWUV01
11d	[76]	VUKHAK	17b	[113]	ELOYIO	27 ¹³	[142]	LOHLEZ
11e	[76]	VUKHEO	17c	[114]	GELJOY	28	[143]	EDTASN
11f	[77]	ZUQBOC	17d	[115]	XAVRUL	29	[144]	HEDTAT

¹ Potassium salt. ² Rubidium salt. ³ Methylammonium salt. ⁴ Guanidinium salt. ⁵ L = HO(SnCl₃OH₂). ⁶ L = H₂O. ⁷ L = OH^– (counter-ions: Ba²⁺, Cl^–). ⁸ L = OH^– (counter-ion: Ba²⁺). ⁹ L = F^– (counter-ion: NH₄⁺). ¹⁰ L = OH^– (counter-ion: Na⁺). ¹¹ L = Cl^– (counter-ion: H₃O⁺). ¹² counter-ion: H₃O⁺. ¹³ counter-ion: NH₄⁺.

Table A3. List of literature references (Ref.) and CSD reference codes (CSD Refcode) of selected crystallographically characterized compounds mentioned in this paper. The crystallographic data (in CIF format) can be accessed from the Cambridge Structural Database (CSD) using the respective CSD Refcode via https://www.ccdc.cam.ac.uk/structures/ (accessed on 1 February 2025 for availability check).

Entry	Ref.	CSD Refcode	Entry	Ref.	CSD Refcode	Entry	Ref.	CSD Refcode
A3a	[66]	BOHNES	A3h	[122]	TOSRUO	A3o	[177]	PEGRUQ
A3b	[67]	ODEWEA	A3i	[146]	EDTAFE01	A3p	[178]	ZAYRIC
A3c	[68]	RIRRUG	A3j	[147]	CERFEK	A3q	[182]	TASSAH
A3d	[69]	NAGZAZ	A3k	[171]	TIRYAU	A3r	[183]	TUPMEW
A3e	[112]	DANRUG	A3l	[171]	TIRXUN	A3s	[186]	IGICEG
A3f	[117]	XAVROF	A3m	[172]	AMSOCU	A3t	[187]	RODLOL
A3g	[118]	TEKSEJ	A3n	[176]	JOVXAV

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Figure 1. Hypercoordinate Si-, Ge- and Sn-complexes I [26], II [27], III [28], IV [29], V [30], VI [31], VII [32], VIII [33], IX [34], X [35] and XI [36] with mono-anionic (O,N)-bidentate chelating ligands.

Figure 2. Selected (O,N)-chelators, which feature an α-amino acid motif (drawn with bold-style bonds). The fundamental features of N-alkylated amino acids, such as N,N-dimethylaminoacetic acid (a), and NH-bearing amino acids, such as glycine (b), may be contained in or give rise to more complex chelators such as chiral amino acids and peptides (c), amineoligoacetic acids (d) and alkanolamine acetic acids (e). This variety can be extended to amino acid-derived ligands, which are devoid of an amino and/or carboxylic acid group, e.g., in Schiff bases, which originate from α-amino acids (f). In (c) and (f), R represents a variety of substituents, and the asterisk (*) indicates a center of chirality. Related features may be contained in the other classes of ligands as well.

Figure 3. Crystallographically characterized hexa- (1) and pentacoordinate (2a–2e) silicon complexes, which feature a mono-anion of an α-amino acid among the (O,N)-chelating ligands. For clarity, the formally dative bond between Si and the amino group is represented by a dashed line.

Figure 4. Zwitterionic spirosilicates with two di-anions of α-amino acids as (O,N)-chelators (3a–3f) [49,50]. The substituents R,R′ at N and the α-C atom in the generic drawings of the diastereomers 3 and 3′ may also represent -CH₂CH₂CH₂- and thus account for the di-anion of l-proline as an (O,N)-chelator as well.

Figure 5. O,N-bis-silylated α-amino acids: Generic cyclic 4 and spirocyclic types 4′ (R may be various substituents, and the amino acid backbone may carry further substituents not shown in this generic sketch), bis-silylated l-tryptophan 5 [52] and structurally characterized Lewis-base adducts of silacycles of l-valine [39], α-amino isobutyric acid [37] and d-phenylglycine [37] (6a, 6b and 6c, respectively).

Figure 6. Sn(IV)-complexes of the amino acids glycine (7a) and l-cysteine (7b), as well as Sn(IV) (8a, 8b) and Sn(II) (8c) complexes of cysteine alkyl esters. The Sn–N as well as Sn–O and Sn–S (if applicable) bond lengths (Å, rounded to two decimal places, where their s.u.s for the 2nd decimal place are (3) for 7a and ≤(1) for the other compounds) are listed next to the respective bond. In the drawings of 7a and 7b, the atoms (Sn) and (O) in parentheses represent the respective atoms of adjacent molecules in the crystal structure.

Figure 7. Crystallographically characterized Si-complexes of di-anions of valinylvaline (9a) and α-aminoisobutyrylglycine (9b), Ge-complexes of di-anions of glycylmethionine (10a) and glycylglycine (10b) and, as a representative example, the related SnMe₂ complex of glycylmethionine (11a). Selected bond lengths (Å, rounded to two decimal places, where their s.u.s for the 2nd decimal place are <(1)) are listed next to the respective bond. For compound 9a, the values are listed for the two crystallographically independent molecules. For compound 9b, which contains six molecules in the asymmetric unit, the bond lengths vary in the ranges 1.80–1.82 Å for Si–O, 1.75–1.77 Å for the equatorial and 2.02–2.05 Å for the axial Si–N bond.

Figure 8. Generic representation of the Sn-coordination modes encountered in the crystal structures of dipeptide-derived diorganotin(IV) compounds (substituents at the dipeptide backbones and at the Sn-bound C atoms, if any, are omitted, and adjacent molecules are represented by the essential coordinating moiety only). (a) Pentacoordination of Sn (distorted trigonal–bipyramidal) with O(axial)-N(equatorial)-NH₂(axial) positioning of the di-anionic dipeptide-derived ligand and Sn–C bonds in equatorial positions. (b) [5 + 2]-coordination, where two remote O⋯Sn interactions with a carboxylate moiety of an adjacent molecule enhance the Sn-coordination number in the idealized Sn(O,N,N) plane. (c) [5 + 1]-coordination, where one remote O⋯Sn interaction with a carbonyl O atom of the carboxylate moiety of an adjacent molecule enhances the Sn-coordination number in the idealized Sn(O,N,N) plane. The other O atom of this carboxylate group may be involved in H-bonding with the NH₂ group (as indicated by an additional dashed line). (d) [5 + 1]-coordination, where one remote N⋯Sn interaction with an imidazole N atom of a histidine group of an adjacent molecule enhances the Sn-coordination number in the idealized Sn(O,N,N) plane.

Figure 9. Examples of hexacoordinate Si- and Sn-complexes (XII and XIII, respectively) with two tridentate di-anionic (O,N,N)-chelating ligands and chelation through exclusive formation of five-membered rings.

Figure 10. Generic structural motif of silatranes and various derivatives thereof (12), and listing of the crystallographically characterized silatranones (12a [93], 12b [94], 12c [95], 12d [95], 12e [96] and 12f [97]) and silatranedione 12g [98].

Figure 11. Crystallographically characterized germatranone 13 [102], its anionic relative 13′ (in the potassium salt) [103] and 14 [104]. The counter-ion in compound 14 is piperazinium.

Figure 12. Generic drawing of germanium complexes of type 15 derived from EDTA and related acids with C₂-backbone R′ [106,107,108,109,110] (for details of R and R′, see Table 3), complex 16 derived from a diamine tetraacetic acid with a C₃ backbone [111] as well as examples of Ge-containing heteronuclear complexes derived from 2-propanol-1,3-diamine tetraacetic acid (17 [103,113,114,115,116] and 17′ [117,118]) (for details of ML_n and “O”, see Table 4 and descriptions in the Discussion). Note: In each case, only one enantiomer is drawn as a representative example. In compound 16, the Ge–N, Ge–O(trans-N) and Ge–O(trans-O) bond lengths are 2.05, 1.84 and 1.88 Å, respectively.

Figure 13. Generic drawing of tin complexes 18 as well as examples 18a and 18c (for details of L, R and R′ in 18a–18g, see Table 5), related alcoholate bridged dimeric complex 19 [126] and complexes 20 [127] and 21 [128] with related ligands. The counter-ion in the crystal structure of the anionic complex 21 is piperazinium. Note: The Sn atoms of the bridging Me₂SnCl₂ moieties in 18a are located in distorted octahedral coordination spheres.

Figure 14. Molecular drawings of crystallographically characterized compounds with the stannatranetrione motif with a penta- (22) [129] hexa- (23) [130], hepta- (24) and octacoordinated Sn atom (25). The anionic complex 24 was characterized in its Cs salt [131], and the anion 25 was reported for its, for example, K [132], Rb [132], methylammonium [133] and guanidinium salt [133].

Figure 15. Molecular drawings of crystallographically characterized tin compounds of the anions of ethylenediaminetetraacetic acid and diethylenetriaminepentaacetic acid. In compounds of type 26, the ligand L may be formally charge-neutral (like OHSnCl₃(H₂O) [134] or H₂O [135]) or anionic (like hydroxide [136,137,138], fluoride [137] or chloride [139]), resulting in non-charged or anionic Sn-complexes, respectively. Compounds of type 27 were reported as the hydrated protio form [140,141] and as the ammonium salt [142]. In the drawing of 28, the atoms (Sn) in parentheses represent the additional Sn(II) sites in the crystal structure, which feature SnO₄-coordination spheres.

Figure 16. Molecular sketches of selected crystallographically characterized Sn-complexes with Schiff-base-type tridentate (O,N,O)-chelating ligands derived from α-amino acids. In compounds XIVf and XIVg, the (Sn) and (O) atoms in parentheses indicate additional coordination with the respective atomic sites of adjacent molecules in the crystal structure.

Figure 17. Molecular sketches of selected crystallographically characterized Ge- and Si-complexes with Schiff-base-type tridentate (O,N,O)-chelating ligands derived from α-amino acids.

Figure 18. Pentacoordinate Si- and Ge-compounds with chiral chelating ligands that originate from chiral α-amino acids (XXII [166], XXIII [167], XXIV [168]) and from epoxides (XXV [169] and XXVI [170]). The asterisk (*) denotes the asymmetric C atom.

Figure 19. Molecular sketches of Sn-, Ge- and Si-compounds with ligands of the α-aminophosphonic acid or α-aminophosphinic acid type. For compounds XXVII, 14 and the set of compounds XXVIII, the E⋯N distances (E = Sn, Ge, Si) in Å are listed. For compounds XXX and XXXI, dashed lines indicate intermolecular Sn⋯O coordination in the crystal structure.

Figure 20. Molecular sketches of compounds which feature SO moieties inside five-membered (O,N)-Sn-chelates (XXXII) and which may be considered precursors for SO-functionalized (O,N)-chelating ligands (XXXIII and XXXIV).

Table 1. Selected bond lengths (Å) and angles (deg.) of the Si-coordination spheres of compounds 1, 2a–2e, 3a–3f and 6a–6c. The values for each column are rounded to the same number of decimal places, and the s.u. on the last decimal place listed is ≤(1).

	Axis	Axial Angle	Si–N ^1,2	Si⋯N ^1,2	Si–O ^1,2	Si–O ^1,2
1	H₂N-Si-NH₂	172.1	n/a	1.89 ³	n/a	1.80 ³
	O-Si-(NCO)	176.8 ³
2a	O-Si-NH₂	167.6	1.69 ^e	1.99 ^a	1.79 ^a	1.71 ^e
2b	O-Si-NH₂	168.3	1.69 ^e	1.97 ^a	1.79 ^a	1.71 ^e
2c ⁴	O-Si-NH₂	169.0, 170.6	1.71, 1.71 ^e	1.97, 1.98 ^a	1.80, 1.80 ^a	1.72, 1.72 ^e
2d	O-Si-NH₂	169.4	1.70 ^e	1.97 ^a	1.80 ^a	1.70 ^e
2e	O-Si-O	164.9	1.70 ^e	1.88 ^e	1.79 ^a	1.82 ^a
3a	O-Si-O	176.3	1.71, 1.72 ^e	n/a	1.84, 1.82 ^a	n/a
3b	O-Si-O	178.5	1.71, 1.72 ^e	n/a	1.83, 1.81 ^a	n/a
3c	O-Si-O	175.8	1.73, 1.73 ^e	n/a	1.84, 1.82 ^a	n/a
3d ⁵	O-Si-O	173.1–174.5	1.71–1.72 ^e	n/a	1.81–1.83 ^a	n/a
3e	O-Si-O	176.8	1.71, 1.71 ^e	n/a	1.81, 1.83 ^a	n/a
3f ⁵	O-Si-O	177.6–179.3	1.73–1.74 ^e	n/a	1.81–1.85 ^a	n/a
6a	O-Si-NH₃	171.1	1.72 ^e	2.01	1.88 ^a	n/a
6b	O-Si-NMI	172.3	1.71 ^e	2.04	1.85 ^a	n/a
6c	O-Si-NMI	171.3	1.72 ^e	2.01	1.87 ^a	n/a

¹ The bonds refer to those of the amino acids’ anions in the order Si–N to the di-anionic ligand, Si⋯N to the mono-anionic ligand, Si–O to the di-anionic ligand, Si–O to the mono-anionic ligand. ² The superscript indices (^a) and (^e) indicate axial and equatorial positioning, respectively, of the respective donor atom (N or O) in the Si-coordination sphere ³ The two features of the respective entry are identical because of crystallographic symmetry. ⁴ The two lines contain entries for the two crystallographically independent molecules. ⁵ Ranges are given for the corresponding data of the multiple independent molecules of the crystal structures.

Table 2. Synthesis routes and selected bond lengths (Å) and angles (deg.) of the Sn-coordination spheres of crystallographically characterized tin compounds (11a–11p) of the type (a-b)SnR₂, which feature a tridentate di-anionic dipeptide-derived (O,N,N)-chelating ligand, with a and b being the C- and N-termini of the dipeptide-di-anion and R being the Sn-bound hydrocarbyl groups. The values for each column are rounded to the same number of decimal places, and the s.u. on the last decimal place listed is ≤(1).

	Ref.	Route	(a-b)R ²	O-Sn-N	C-Sn-C	Si–O	Si–N	Si⋯N	Motif
11a	[73]	B	(met-gly)Me	153.0	123.8	2.16	2.07	2.25	a
11b	[74]	C	(phe-tyr)Me	149.5	136.5	2.20	2.10	2.25	c ³
11c ¹	[75]	A	(his-met)Me	147.0	124.7	2.14	2.12	2.30	a
				152.3	126.1	2.18	2.07	2.24	a
11d	[76]	A	(met-met)Me	148.8	132.0	2.19	2.11	2.26	c ³
11e	[76]	A	(his-ala)Me	146.4	143.9	2.22	2.14	2.28	b ³
11f	[77]	A	(ala-trp)Me	151.5	123.8	2.18	2.06	2.27	a
11g ¹	[78]	A	(his-gly)Et	151.9	128.8	2.18	2.10	2.27	a
				150.0	148.5	2.24	2.13	2.27	d ³
11h	[79]	C	(tyr-gly)Et	152.2	131.4	2.19	2.08	2.29	a
11i	[80]	C	(gly-trp)nBu	147.9	151.9	2.25	2.12	2.29	b ³
11j	[81]	A	(leu-aib)nBu	148.7	127.8	2.20	2.11	2.28	a
11k	[81]	A	(ala-aib)nBu	147.6	129.8	2.24	2.12	2.24	c ³
11l	[82]	A,B,C	(val-gly)nBu	151.3	125.3	2.14	2.10	2.27	a
11m	[83]	A	(gly-gly)Cy	151.4	123.6	2.17	2.10	2.28	a
11n	[83]	A	(ala-gly)Cy	150.8	123.0	2.16	2.09	2.30	a
11o	[84]	C	(gly-gly)tBu	149.6	121.7	2.20	2.09	2.29	a
11p	[85]	A	(gly-gly)Ph	153.2	117.5	2.16	2.08	2.27	a

¹ Data listed for two independent molecules in the crystallographic asymmetric unit. ² The abbreviations correspond to ala = alanine, aib = α-amino isobutyric acid, gly = glycine, his = histidine, leu = leucine, met = methionine, phe = phenylalanine, trp = tryptophan, tyr = tyrosine, val = valine. ³ The additional atom distances, which contribute to the remote [5 + 1]- or [5 + 2]-coordination of the Sn atoms in these compounds, are: Sn⋯O 3.12 Å in 11b, Sn⋯O 3.14 Å in 11d, Sn⋯O 3.00 and 3.13 Å in 11e, Sn⋯N 2.79 Å in 11g, Sn⋯O 2.80 and 2.85 Å in 11i and Sn⋯O 2.82 Å in 11k.

Table 3. Bond lengths (Å) of the Ge-coordination spheres of crystallographically characterized Ge-complexes of type 15. The bond labels correspond to Figure 12. The values for each column are rounded to the same number of decimal places.

	Ref.	R′ ¹	Ge–N (a)	Ge–N (b)	Ge–O (c)	Ge–O (d)	Ge–O (e)	Ge–O (f)
15a	[106]	CH₂CH₂	2.08	2.11	1.88	1.88	1.89	1.77
15b	[107]	CH₂CH₂	2.09	2.10	1.89	1.88	1.90	1.76
15c	[108]	CH₂CHMe²	2.09	2.10	1.88	1.87	1.89	1.78
15d	[109]	1,2-C₆H₁₀³	2.07	2.13	1.88	1.86	1.89	1.77
15e ⁴	[110]	CH₂CH₂	2.09	2.09	1.88	1.90	1.91	1.76
			2.07	2.11	1.87	1.88	1.91	1.77
15f	[108]	CH₂CH₂	2.09	2.11	1.89	1.88	1.89	1.75

¹ The dangling ligand arms R are -CH₂COOH (in 15a, 15c, 15d), -CH₂COO as N,N′-diphenylguanidinium salt (in 15b), -CH₂CH₂OH (in 15e) and a zwitterionic form of CH₂CH₂N(CH₂COOH)₂ (in 15f). ² The Me group of the backbone is bound to the C atom at the tripod-center N-donor atom. ³ trans-1,2-cyclohexanediyl backbone. ⁴ Data are listed for two crystallographically independent molecules in the asymmetric unit.

Table 4. Bond lengths (Å) of the Ge-coordination spheres of crystallographically characterized heteronuclear complexes of type 17. The bond labels correspond to Figure 12. The values for each column are rounded to the same number of decimal places.

	Ref.	ML_n	Ge–N (a)	Ge–O (b)	Ge–O (b’)	Ge–O (c)	Ge–O (d)	Ge–O (e)
17a	[103]	La(H₂O)₄	2.10	1.93	1.93	1.86	1.81	1.80
17b	[113]	Nd(H₂O)₄	2.10	1.93	1.92	1.86	1.81	1.80
17c	[114]	Tb(H₂O)₃	2.10	1.93	1.92	1.87	1.81	1.81
17d	[115]	Tm(H₂O)₃	2.10	1.93	1.91	1.87	1.81	1.81
17e	[114]	Yb(H₂O)₃	2.10	1.93	1.92	1.86	1.81	1.81
17f	[116]	Cu(H₂O)	2.09	1.91	1.91	1.84	1.90 ¹	1.79

¹ In compound 17f, the ligand “O” is H₂O.

Table 5. Bond lengths (Å) and C-Sn-C angles (deg.) of the Sn-coordination spheres of complexes 18a–18g. The positions of L, R and R′ correspond to 18 in Figure 13. The values for each column are rounded to the same number of decimal places.

	Ref.	L,R,R′	C-Sn-C	Sn–N	Sn–O ¹	Sn–O ²	Sn–O(L)	Sn⋯O
18a	[119]	-,H,Me	134.3	2.27	2.17	2.17	n/a	n/a
18b	[120]	-,mTol,nBu	132.4	2.23	2.13	2.14	n/a	n/a
18c	[121]	H₂O,H,Me	161.1	2.29	2.20	2.37	2.38	2.79
18d	[122]	H₂O,Me,Me	162.5	2.36	2.19	2.35	2.36	2.74
18e	[123]	MeOH,H,Me	159.5	2.28 ³	2.16	2.35	2.37	2.95
18f	[124]	H₂O,H,nBu	162.7	2.32	2.21	2.35	2.36	2.77
18g	[125]	H₂O,H,nBu	163.4	2.30	2.20	2.33	2.31	2.76

¹ This bond is to the non-bridging carboxylate O of the (O,N,O)-ligand. ² This bond is to the bridging carboxylate O of the (O,N,O)-ligand, which is involved in remote O⋯Sn-coordination with an adjacent complex molecule. ³ Average value from two disorder positions of the N atom site.

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Seidel, A.; Wagler, J. Anions of α-Amino Acids as (O,N)-Donor Ligands in Si-, Ge- and Sn-Coordination Chemistry. Molecules 2025, 30, 834. https://doi.org/10.3390/molecules30040834

AMA Style

Seidel A, Wagler J. Anions of α-Amino Acids as (O,N)-Donor Ligands in Si-, Ge- and Sn-Coordination Chemistry. Molecules. 2025; 30(4):834. https://doi.org/10.3390/molecules30040834

Chicago/Turabian Style

Seidel, Anne, and Jörg Wagler. 2025. "Anions of α-Amino Acids as (O,N)-Donor Ligands in Si-, Ge- and Sn-Coordination Chemistry" Molecules 30, no. 4: 834. https://doi.org/10.3390/molecules30040834

APA Style

Seidel, A., & Wagler, J. (2025). Anions of α-Amino Acids as (O,N)-Donor Ligands in Si-, Ge- and Sn-Coordination Chemistry. Molecules, 30(4), 834. https://doi.org/10.3390/molecules30040834

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Anions of α-Amino Acids as (O,N)-Donor Ligands in Si-, Ge- and Sn-Coordination Chemistry

Abstract

1. Introduction

2. Si-, Ge- and Sn-Complexes of α-Amino Carboxylic Acid-Derived (O,N)-Chelating Ligands

2.1. Si-, Ge- and Sn-Complexes Derived from α-Amino Monocarboxylic Acids

2.1.1. Si-Complexes Derived from α-Amino Monocarboxylic Acids

2.1.2. Ge-Complexes Derived from α-Amino Monocarboxylic Acids

2.1.3. Sn-Complexes Derived from α-Amino Monocarboxylic Acids

2.1.4. Concluding Remarks Regarding Si-, Ge- and Sn-Complexes Derived from α-Amino Monocarboxylic Acids

2.2. Si-, Ge- and Sn-Complexes Derived from Dipeptides

2.2.1. Si-Complexes Derived from Dipeptides

2.2.2. Ge-Complexes Derived from Dipeptides

2.2.3. Sn-Complexes Derived from Dipeptides

2.2.4. Concluding Remarks Regarding Si-, Ge- and Sn-Complexes Derived from Dipeptides

2.3. Si-, Ge- and Sn-Complexes Derived from Amineoligoacetic Acids and Related Hydroxyalkylamineacetic Acids

2.3.1. Si-Complexes Derived from Amineoligoacetic Acids and Related Hydroxyalkylamineacetic Acids

2.3.2. Ge-Complexes Derived from Amineoligoacetic Acids and Related Hydroxyalkylamineacetic Acids

2.3.3. Sn-Complexes Derived from Amineoligoacetic Acids and Related Hydroxyalkylamineacetic Acids

2.3.4. Concluding Remarks Regarding Si-, Ge- and Sn-Complexes Derived from Amineoligoacetic Acids and Related Hydroxyalkylamineacetic Acids

3. Perspectives Outside the Box

3.1. α-Amino Acid-Derived Schiff Bases as Ligands

3.2. α-Amino Acid-Derived Alcohols as Ligands

3.3. Ligands Derived from Non-Carboxylic α-Amino Acids

4. Conclusions

Author Contributions

Funding

Institutional Review Board Statement

Informed Consent Statement

Data Availability Statement

Conflicts of Interest

Appendix A

References

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