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Article

Trigonal Bipyramidal Rhodium(I) Methyl and Phenyl Complexes: Precursors of Oxidative Methyl and Phenyl Radical Generation

by
Urs Fischbach
1,
Matthias Vogt
2,
Peter Coburger
3,
Monica Trincado
3,* and
Hansjörg Grützmacher
3,*
1
Cilag AG, 8200 Schaffhausen, Switzerland
2
Institut für Chemie, Naturwissenschaftliche Fakultät II, Martin-Luther-Universität Halle-Wittenberg, Kurt-Mothes-Str. 2, 06120 Halle, Germany
3
Department of Chemistry and Applied Biosciences, ETH Zürich, Vladimir-Prelog-Weg 1-5/10, 8093 Zürich, Switzerland
*
Authors to whom correspondence should be addressed.
Inorganics 2022, 10(3), 28; https://doi.org/10.3390/inorganics10030028
Submission received: 7 February 2022 / Revised: 15 February 2022 / Accepted: 18 February 2022 / Published: 24 February 2022
(This article belongs to the Special Issue Phosphorus Ligands for the Stabilization of Unusual and Elusive Structures in Transition Metal Complexes)
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Abstract

:
The new complexes [Rh(Me)(trop3P)] (2) and [Rh(Ph)(trop3P)] (3) (trop = 5H-dibenzo[a,d]cyclohepten-5-yl) were synthesised by addition of organolithium reagents (MeLi and PhLi) to the parent pentacoordinated chloride complex [RhCl(trop3P)]. The compounds have a trigonal bipyramidal structure with olefin-only ligands in the equatorial position and the methyl or phenyl substituent in the axial position. Oxidation of complexes 2 and 3 leads to the liberation of methyl and phenyl radicals, which were indirectly detected by reaction with common spin trapping reagents.

1. Introduction

A large number of methods for the generation of C-centred radicals have been discovered in the past few years. The most relevant include thermolysis or photolysis of organic peroxides, azo, and carbonyl compounds [1,2,3,4]. Methyl radicals were observed for the first time by Kolbe in 1849 in the electrolysis of acetic acid [5]. Nowadays, the most important starting materials for the production of methyl radicals are acetyl peroxide and azomethane [6]. Common thermal sources of methyl radicals include (tBuO)2, HgMe2, CdMe2, ZnMe2, AsMe3, SbMe3, BiMe3, AgMe, SiMe4, SnMe4, PbMe4, SnCl2Me2, and CH3CHO [1]. The photolytic generation is often reported to be favourable with respect to the thermal generation because of the more controllable and less drastic conditions. Another important method for the generation of methyl radicals in aqueous media is the reaction of sulphoxides with hydroxyl radicals, produced either by redox chemistry (Fenton), hydroperoxide photolysis, or pulse radiolysis [7,8,9]. Less common methods embrace photolysis of acetone, azoethane, HgMe2, CH3CHO, ketene, biacetyl, and MeI [10]. Methyl radicals decay extremely rapidly. In the gas or liquid phase, either by reaction with a second methyl radical forming ethane or by reaction with a molecule in the surrounding, e.g., a solvent molecule from which often hydrogen is abstracted. The stability of C-centred radicals can conveniently be expressed using the isodesmic H-transfer reaction: •CH3 + R-H → CH4 + •R. The reaction enthalpy of this process is commonly referred to as the radical stabilisation energy (RSE) of the newly formed radical relative to the unsubstituted methyl radical. For the unsubstituted phenyl radical, for example, an RSE value of +32.9 kJ mol−1 has been determined (Ph-H BDE = 472.2 kJ mol−1, H3C-H BDE = 439.3 kJ mol−1) [11], indicating that phenyl radicals are significantly less stable than alkyl radicals. This is simply due to the fact that the C–H bonds in benzene are significantly stronger than the primary C–H bonds in alkanes. The phenyl radical (•C6H5) has been shown to be a very important transient intermediate in the formation of polycyclic aromatic hydrocarbons (PAHs) and soot in hydrocarbon combustion [12,13,14]. Therefore, it has been thoroughly characterised by UV, [15] IR, [16] and Raman spectroscopy [17]. The kinetics of its reactivity with various reagents (e.g., propene, [18] ketene, [19] methylacetylene, [20] or n-Bu3SnH [21]) have been extensively studied. Various precursors can be chosen in order to obtain phenyl radicals such as benzoyl peroxides, [22,23,24,25] benzoic anhydride, [26] halogenobenzenes, [27] and nitrosobenzene [28,29]. Photodissociation of aryl halides in solution proved to be useful in chemical synthesis. Classical methods such as laser flash photolysis [30,31] or pulse radiolysis are the tools of choice for investigations of phenyl radicals [32]. The photodissociation of halobenzenes has been studied in molecular beams with femtosecond time resolution by pump–probe spectroscopy [33,34]. Another general approach for the generation of hydrocarbyl radicals is the irradiation of organometallic compounds. Soon after the structural elucidation of vitamin B12 [35,36,37]. it was found that cobalt–carbon bonds, in general, are prone to undergo homolysis under irradiation. This was confirmed for simple compounds such as [Co(dmgH)2(CH3)(H2O)] (dmg = dimethylglyoxime) and [Co(CH3)([14]aneN4)(H2O)](ClO4)2 ([14]aneN4 = 1,4,8,1l-tetraazacyclotetradecane), which can be used as a source for methyl radicals in kinetic investigations [38]. Homolytic fission of the Rh–CH3 bond was observed in the electrochemical one-electron reduction of [RhIII(η5-C9H7)(CH3)(dppp*)]BPh4 (dppp* = Ph2PCH(CH3)CH2PPh2) in acetonitrile, which produced [Rh(η5-C9H7)(dppp*)] [39]. In a different report, the reaction of the paramagnetic RhII compound [Rh(2,4,6-iPr3C6H2)2(tht)2] (tht = tetrahydrothiophene) with tBuNC in d8-toluene yielded [Rh{tBuN=C-(2,4,6-iPr3C6H2)}(tBuNC)3] and monodeuterated triisopropylbenzene as a result of an deuterium abstraction reaction by the formed aryl radical from the solvent [40]. Electrolysis of Vaska type organometallic complexes as [Ir(CH3)(CO)(PPh3)2] in 1,2-dichloroethane in presence of excess PPh3 yielded [Ir(CO)(PPh3)3]+, [IrCl(CO)(PPh3)2] and methane [41]. This outcome suggests that the formed d7 species undergo radical dissociation, followed by a fast reaction of the formed •CH3 with the solvent. For dialkyl PdII and PtII complexes, the oxidative cleavage of hydrocarbyl radicals upon one-electron oxidation is commonly observed [42]. Nevertheless, none of the aforementioned methods has ever been used for the quantitative and controlled release of methyl or phenyl radicals and for their application in sequential chemical transformations.
Recently, we established the synthesis of the new bulky ligand trop3P (tris(5H-dibenzo[a,d]cyclohepten-5-yl)phosphane) and transition metal complexes thereof [43]. The Rh–P bond in complexes such as [RhCl(trop3P)] (1) proved to be especially strong and, therefore, exhibited a large trans-influence on the opposing axial ligands. In preliminary investigations, we found that the diphenylphosphinyl compound [Rh(PPh2)(trop3P)] can easily be oxidised by FcOTf to give the pentacoordinated complex [Rh(OTf)(trop3P)] and a •PPh2 radical [44]. The in situ formation of diphenylphosphinyl radicals was evidenced by spin trapping reactions. Herein, we report the synthesis of the organometallic methyl and phenyl analogous compounds [Rh(Me)(trop3P)] (2) and [Rh(Ph)(trop3P)] (3). The oxidation of these complexes gives rise to the controlled release of methyl or phenyl radicals, respectively.

2. Results and Discussion

The addition of two equivalents of MeLi to the complex [RhCl(trop3P)] ((1) resulted in a fast colour change from the suspended yellow starting material to an intensely deep green solution. This solution was quenched with a small amount of water (Et2O containing traces of water) to give the methyl complex [Rh(CH3)(trop3P)] (2) (Scheme 1). The use of exactly two equivalents of MeLi was crucial to obtain a quantitative yield of the final product. Compound 2 is remarkably stable to air and moisture at room temperature; however, heating the product in the presence of water or alcohol solvents resulted in decomposition and elimination of methane.
Complex 2 was fully characterised by spectroscopic methods (see ESI for details). The 1H NMR spectrum shows one resonance at δ = −1.01 ppm, which is attributed to the methyl group coordinated to rhodium and displays coupling to both nuclei, 31P (3JPH = 5.6 Hz) and 103Rh (2JRhH = 1.1 Hz). The large 13C 31P coupling, 2JPC = 114 Hz, in the 13C NMR spectrum indicated a trans-position of both nuclei within the trigonal pyramidal coordination sphere. All benzylic and olefinic protons are in magnetically equivalent positions, which is in accord with the proposed C3v symmetric structure. Single crystals of the compound were grown by layering a toluene solution of the complex with n-hexane and investigated using X-ray diffraction methods. Unfortunately, we failed to obtain a dataset which allows discussing the molecular structure of complex 2 in detail, but the overall connectivity and form could be determined (Figure 1a; see also Table S1 in Supplementary Materials). The methyl group resides in an axial position oriented trans to the P atom of the trop3P ligand. This feature is remarkable in the realm of RhI–CH3 complexes. Very few crystal structures of RhI–CH3 species have been reported. The most similar structure is [Rh(CH3)(CO){κ3-H3CC(CH2PPh2)3}], [45] where the methyl group is also coordinated trans to a phosphane ligand and the complex exhibits trigonal bipyramidal geometry. Other structures with a Rh–Me unit show the more common square planar coordination sphere [46] or dimeric structures [47,48,49]. Following a similar synthetic route, the synthesis of complex [Rh(Ph)(trop3P)] (3) was performed using an excess of PhLi (2.0 equiv) in the reaction with complex 1 (Scheme 1). The initial orange suspension turned rapidly into a deep-green solution. The reaction mixture was subsequently quenched with ethanol, forming compound 3. The NMR spectra of the product indicate high symmetry and confirm the formation of a pentacoordinated complex bearing the phenyl group in the axial position and the tris(olefinic) phosphine ligand in the three equatorial (olefins) and remaining axial position (phosphorus). The low-frequency 13C NMR and 1H NMR chemical shifts (5.85 and 75.8 ppm, respectively) of the coordinated olefin units, compared with the free trop3P ligand, indicate substantial metal-to-olefin electron back-bonding which strengthen the Rh–olefin bonds. The NMR signal to the ipso-carbon nucleus of the phenyl ligand bound to Rh was observed at 166.8 ppm (in the range of previously reported Ph–RhI complexes) [46,50], exhibiting the expected splitting due to coupling to both 103Rh (1JRhC = 20 Hz) and to 31P (2JPC = 114 Hz) in trans-position.
Extraction with toluene and slow evaporation yielded yellow crystals of the complex, suitable for an X-ray diffraction analysis. The molecular structure of 3 is presented in Figure 1b (crystal data and structural refinement details can be found in Table S2 in Supplementary Materials).
Likewise, complex [Rh(Ph)(trop3P)]×(1.5 C7H8) (3) has an uncommon structure. As compound 2, complex 3 is a pentacoordinated complex with the double bonds of the trop3P ligand arranged in the equatorial plane in an almost perfect coplanar fashion. According to Addison’s τ5 criterion [51], the coordination environment around the central atom can be described practically as an ideal trigonal bipyramid for both complexes (τ5 = 1.01 (2) and 0.96 (3)), with the phosphorous atom and alkyl ligands located in axial positions. The only comparable compounds reported previously are the square planar rhodium(I) complexes [Rh(Ph)(PMe3)3] [52] and [Rh(Ph)(PMe3)2(CO)] [46], where the phenyl group is located trans to a triphenylphosphane or a carbonyl ligand, respectively. Another loosely related example is the dimeric complex [Rh2(Ph)2(μ2-CO)(κ2-dppm)2] [50], having an ‘A-frame’ type structure and including a Rh–Rh bond. Table 1 lists some selected crystallographic data of these compounds. Noteworthy is the long bond between the Rh centre to the ipso-C atom of the phenyl substituent (2.149(3) Å), when compared with previously reported RhI phenyl complexes (Table 1), which indicates a weakened Rh–C bond.
The rhodium methyl compound 2 rapidly reacted with one equivalent of FcOTf. The 31P{1H} NMR spectrum shows the known rhodium complex [Rh(OTf)(trop3P)] (4) as the only phosphorus-containing oxidation product [44]. This observation indicates that the other product of the irreversible oxidation might be a methyl radical acting as a reactive intermediate. This hypothesis was corroborated by carrying out the oxidation reaction in the presence of TEMPO (2,2,6,6-tetramethyl-1-piperidinyloxyl radical), which is a known radical trap and should lead to the formation of TEMPO–Me [53]. Indeed, TEMPO–Me was detected by both 1H NMR spectroscopy and GC–MS. In a further experiment, the methyl radicals released upon oxidation of 2 were trapped with nitrosophenol which yielded the paramagnetic compound HO-C6H4-N(O•)CH3. This compound could easily be detected by EPR spectroscopy. Additionally, indeed, an EPR spectrum was obtained which is characteristic for a radical with one large nitrogen coupling and three large proton couplings, which stem from the methyl group. The obtained radical HO-C6H4-N(O•)CH3 has not been described in the literature, to the best of our knowledge, but the closely related anisole derivative H2N-C6H4-N(O•)CH3 has been reported, and the coupling constants are comparable (Figure 2) [54].
The reaction of 3 with FcOTf was much slower than with the methyl complex 2, and complete conversion was achieved only after ca. 30 min. In analogy to the oxidation process observed for the methyl analogue, the organometallic product of this oxidation was the same complex 4, observed spectroscopically. In order to show that oxidation of the phenyl complex leads to the production of phenyl radicals, we used in situ the spin trap nitrosobenzene, Ph–N=O. When the oxidation with FcOTf was carried out, an intense and nicely resolved EPR spectrum of the spin trap adduct Ph2N-O∙ was observed by EPR spectroscopy (Figure 3), which is in agreement with the literature data [55].
DFT methods were used in order to calculate the energy required to dissociate the Rh–C bond (methyl or phenyl ligand) from either the neutral d8-valence electron configured RhI complexes 2 or 3 (Equations (1) and (2)) or the d7-valence electron configured RhII complexes 2+ or 3+, respectively, which are the products of the one-electron oxidation (Equations (3) and (4)). As expected, the Rh–C bond in the RhI phenyl complex 3 was about 16% stronger than in the methyl complex 2 and, in both dissociations, was endothermic by more than 60 kcal mol−1, making these compounds stable under the experimental conditions. This dramatically changed upon oxidation, whereby the bond energy in 2 diminished by about 75% in 2+ (13.8 kcal mol−1) and 67% in 3+ (23.5 kcal mol−1). Nevertheless, the significantly higher calculated dissociation energy in 3+ is consistent with the experimentally observed much slower decay of this complex in comparison with 2+.
ΔE (kcal∙mol−1)
[Rh(I)(CH3)(trop3P)] (2) → [Rh(I)(trop3P]• + CH3•60.6(1)
[Rh(I)(C6H5)(trop3P)] (3) → [Rh(I)(trop3P]• + C6H5•70.3(2)
[Rh(II)(CH3)(trop3P)]+ (2+) → [Rh(I)(trop3P]+ + CH3•13.8(3)
[Rh(II)(C6H5)(trop3P)]+ (3+) → [Rh(I)(trop3P]+ + C6H5•23.5(4)
In summary, facile synthesis of the new RhI methyl and phenyl compounds [Rh(R)(trop3P)] was accomplished. The trigonal pyramidal arrangement of the ligands with the organic substituent in axial position is unprecedented for d8 RhI compounds. The complexes [Rh(R)(trop3P)] (2: R = Me; 3: R = Ph) reacted with FcOTf to give the [Rh(trop3P)]+ cation and methyl or phenyl radicals, indicating that the one-electron oxidation product [RhIIR(trop3P)]+ (R = Me, Ph) was unstable and decomposed under homolysis of the Rh–C bond. The formation of the radicals R∙ was evidenced by reactions with either a persistent radical to give a diamagnetic radical combination product or with spin trap reagents which gave persistent radicals as products that could be easily detected by EPR spectroscopy. To the best of our knowledge, this is the first time that methyl or phenyl radicals could be generated oxidatively from the corresponding alkyl and aryl metal complexes. This reactivity of 2 and 3 is similar to the one previously reported for the phosphinyl complex [Rh(PPh2)(trop3P)]. We attribute this reactivity to the formation of the very stable and chemically inert complex [Rh(trop3P)(OTf)] and the special property of the phosphane ligand trop3P whose phosphorus centre seemingly exercises a strong trans-influence and trans-effect on the trans bound ligand, especially in the RhII complexes which are the likely reactive intermediates. Based on the calculated Rh–C bond dissociation enthalpies, the formation of alkyl and aryl radicals from cationic RhII intermediates are rather favourable. The homolytic bond cleavage is easier for methyl than for the phenyl derivative, according to the DFT predictions and experimental observations. This eventually opens the possibility to use metal complexes of the general formula [MX(trop3P)] as sources for X∙ radicals which we currently explore in our laboratory.

3. Materials and Methods

General methods and Supplementary Materials are available online, including spectroscopic data for complexes 2 and 3 and crystallographic data for 3.
Data collection for the X-ray structure determinations was performed on a Bruker SMART 1K platform with graphite-monochromated Mo–Kα radiation (λ = 0.71073 Å). The reflex intensities were measured by CCD area detectors. The collected frames were processed with the proprietary software SAINT, and an absorption correction was applied (SADABS). Solution and refinement of the structures were performed with SHELXS-97 and SHELXL-97, respectively. All non-hydrogen atoms were refined with anisotropic displacement parameters. Hydrogen atoms were placed in their idealised positions and were allowed to ride on the respective carbon atoms. CCDC 631208 contains the supplementary crystallographic data for 3. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif (accessed on 31 January 2022)
DFT calculations were carried out with the ORCA program package [56]. Unless stated otherwise, all calculations were carried out on isolated molecules. Density fitting techniques, also called resolution-of-identity approximation (RI) [57], were used for GGA and meta-GGA calculations, and the RIJCOSX [58] approximation was used for hybrid-GGA and CASSCF calculations. Atom-pairwise dispersion corrections with the Becke–Johnson damping (D3BJ) [59,60] were used for all DFT calculations (D3(0) in case of M06). All geometries were obtained using the B97-3c method developed by the Grimme group [61]. Final energy calculations were performed using the M06 [62] functional, the def2-TZVPP [63] basis set, and the CPCM [64] model for THF.
Complex 2. Complex RhCl(trop3P)] 1 (260 mg, 0.35 mmol, 1.0 equiv) was suspended in dry THF (25 mL) and cooled in an ice bath. A solution of MeLi in Et2O (1.6 M, 0.45 mL 0.72 mmol, 2.0 equiv) was added dropwise until all the solids had dissolved. The colour of the resulting solution changed from light yellow to a strong dark green. Conversion of the reaction was checked by 31P NMR spectroscopy. Et2O (1.5 mL 1% v/v water) was added carefully to the reaction mixture provoking an immediate colour change to light yellow. The resulting solution was concentrated to dryness and extracted with CH2Cl2 (2 × 25 mL) to remove residual salts. The solution was concentrated, and the residue was extracted with hexane (3 × 50 mL). The combined extractions were concentrated to dryness and recrystallised from Et2O. The precipitated product was collected by filtration, washed with Et2O (2 × 3 mL), hexane (2 × 3 mL), and pentane (2 × 3 mL), and then dried in a high vacuum. The obtained product was a microcrystalline light yellow powder. Yield: 95 mg (38%).
Complex 3. Complex [RhCl(trop3P)] (1 (214 mg, 0.29 mmol, 1.0 equiv) was suspended in dry THF (15 mL) and a solution of PhLi in dibutyl ether (2.0 M, 0.29 mL 0.58 mmol, 2.0 equiv) was added dropwise. Upon addition, the suspended starting material dissolved, and the colour of the solution changed from light yellow to a strong dark green. Ethanol (50 μL) was added to the dark green solution, provoking an immediate colour change to orange. The solution was concentrated to dryness, and the residue was extracted with dry toluene (2 × 15 mL). The toluene solution was concentrated to ca. 5 mL and cooled to −18 °C. After 16 h, the precipitated product was collected by filtration and washed with diethyl ether (2 × 5 mL). After drying under high vacuum, complex 3 was isolated as a yellow crystalline powder. Yield: 127 mg (56%).

Supplementary Materials

The supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/inorganics10030028/s1, including spectroscopic and crystal data of complexes 2 and 3.

Author Contributions

Conceptualization, U.F., M.T. and H.G.; methodology, U.F.; validation, M.V, P.C. and M.T.; formal analysis, M.V. and P.C. and M.T.; investigation, U.F. and M.T.; writing—original draft preparation, U.F; writing—review and editing, M.T. and H.G.; supervision, M.T. and H.G.; project administration, H.G.; funding acquisition, H.G.All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by ETHZ (Eidgenössische Technische Hochschule Zürich; grant 0-20214-16) and the SNF (Schweizerischer Nationalfonds; grant 2-77199-18).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Spectroscopic and crystal data are given in the supporting information. Crystallographic data can be obtained free of charge from the Cambridge Crystallographic Data Centre.

Acknowledgments

We thank W. Koppenol as well as R. Kissner for the supply and support of EPR instrumentation.

Conflicts of Interest

The authors declare no conflict of interest.

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Inorganics 10 00028 sch001 550
Scheme 1. Synthesis of methyl and phenyl rhodium(I) complexes 2 and 3 and their reactivity upon oxidation. (FcOTf = ferrocenyl triflate).
Scheme 1. Synthesis of methyl and phenyl rhodium(I) complexes 2 and 3 and their reactivity upon oxidation. (FcOTf = ferrocenyl triflate).
Inorganics 10 00028 sch001
Inorganics 10 00028 g001 550
Figure 1. (a) Molecular structure of [Rh(CH3)(trop3P)]×(0.5 C7H8) (2). Hydrogen atoms and solvent molecules (toluene) are omitted for clarity; (b) ORTEP plot of [Rh(Ph)(trop3P)]x(1.5 C7H8) (3). Hydrogen atoms, phenyl substituent splitting, and toluene molecules were omitted for clarity. Thermal ellipsoids at 50% probability; selected bond lengths (Å) and angles (°): Rh1–P1: 2.2099(7); Rh1–C46: 2.149(3); P1–Rh1–C46: 178.37(8); ∑°P1 318.2.
Figure 1. (a) Molecular structure of [Rh(CH3)(trop3P)]×(0.5 C7H8) (2). Hydrogen atoms and solvent molecules (toluene) are omitted for clarity; (b) ORTEP plot of [Rh(Ph)(trop3P)]x(1.5 C7H8) (3). Hydrogen atoms, phenyl substituent splitting, and toluene molecules were omitted for clarity. Thermal ellipsoids at 50% probability; selected bond lengths (Å) and angles (°): Rh1–P1: 2.2099(7); Rh1–C46: 2.149(3); P1–Rh1–C46: 178.37(8); ∑°P1 318.2.
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Figure 2. EPR spectrum of the spin trap adduct HO-C6H4-N(O•)CH3, generated in the reaction of complex 2 with p-HO-C6H4-N=O in the presence of FcOTf in THF. Top: simulated spectrum, bottom: experimental spectrum. Frequency: 9.727 GHz; g-value (vs. DPPH): 2.0044; hyperfine coupling constants: aN: 30.9 MHz, aH,methyl: 27.9 MHz, aH,ortho: 7.9 MHz, aH,meta: 2.6 MHz.
Figure 2. EPR spectrum of the spin trap adduct HO-C6H4-N(O•)CH3, generated in the reaction of complex 2 with p-HO-C6H4-N=O in the presence of FcOTf in THF. Top: simulated spectrum, bottom: experimental spectrum. Frequency: 9.727 GHz; g-value (vs. DPPH): 2.0044; hyperfine coupling constants: aN: 30.9 MHz, aH,methyl: 27.9 MHz, aH,ortho: 7.9 MHz, aH,meta: 2.6 MHz.
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Figure 3. EPR spectrum of Ph2N-O•, generated in the reaction of complex 3 and Ph–N=O in the presence of FcOTf in THF. Top: simulated spectrum, bottom: experimental spectrum. Frequency: 9.746 GHz; g-value (vs. DPPH): 2.0047; hyperfine coupling constants: aN: 27.1 MHz, aH,ortho/para: 5.1 MHz, aH,meta: 2.5 MHz.
Figure 3. EPR spectrum of Ph2N-O•, generated in the reaction of complex 3 and Ph–N=O in the presence of FcOTf in THF. Top: simulated spectrum, bottom: experimental spectrum. Frequency: 9.746 GHz; g-value (vs. DPPH): 2.0047; hyperfine coupling constants: aN: 27.1 MHz, aH,ortho/para: 5.1 MHz, aH,meta: 2.5 MHz.
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Table
Table 1. Comparison of crystallographic data of RhI phenyl complexes.
Table 1. Comparison of crystallographic data of RhI phenyl complexes.
CompoundBond Length
Rh–Cipso		Bond Angle (trans)[Ref]
32.149(3) ÅP–Rh–C178.37(8)°this study
[Rh(Ph)(PMe3)3]2.079(4) ÅP–Rh–C170.0(1)°[52]
[Rh(Ph)(PMe3)2(CO)]2.096(4) ÅC–Rh–C180°[46]
[Rh2(Ph)2(μ2-CO)(κ2-dppm)2]2.066(5); 2.072(5) ÅRh–Rh–C166.4(1)°[50]
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Fischbach, U.; Vogt, M.; Coburger, P.; Trincado, M.; Grützmacher, H. Trigonal Bipyramidal Rhodium(I) Methyl and Phenyl Complexes: Precursors of Oxidative Methyl and Phenyl Radical Generation. Inorganics 2022, 10, 28. https://doi.org/10.3390/inorganics10030028

AMA Style

Fischbach U, Vogt M, Coburger P, Trincado M, Grützmacher H. Trigonal Bipyramidal Rhodium(I) Methyl and Phenyl Complexes: Precursors of Oxidative Methyl and Phenyl Radical Generation. Inorganics. 2022; 10(3):28. https://doi.org/10.3390/inorganics10030028

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Fischbach, Urs, Matthias Vogt, Peter Coburger, Monica Trincado, and Hansjörg Grützmacher. 2022. "Trigonal Bipyramidal Rhodium(I) Methyl and Phenyl Complexes: Precursors of Oxidative Methyl and Phenyl Radical Generation" Inorganics 10, no. 3: 28. https://doi.org/10.3390/inorganics10030028

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