Structural and Hirshfeld Surface Analysis of Thallium(I) and Indium(III) Complexes of a Soft Scorpionate Ligand

Abstract

Two complexes containing a soft sulfur-substituted tris(pyrazolyl)hydroborate ligand, namely [Tl^I(Tm^tBu)]₂∙2H₂O and [In^III(Tm^tBu)₂](InCl₄), where Tm^tBu is the tris(3-tert-butyl-2-sulfanylidene-1H-imidazol-1-yl)hydroborate anion, have been characterized. The {TlS}₂ core of the former has the shape of a diamond. Each S atom of the Tm^tBu anion coordinates differently: one S is connected to one Tl atom, one bridges both Tl atoms, while the third S atom connects solely to the second Tl atom. The S₄ donor set defines a seesaw geometry. The independent H₂O molecule forms O–H···S and localized O–H···π(pyrazolyl) contacts. Flattened octahedral geometries defined by S₆ donor sets are noted for the two independent cations in [In^III(Tm^tBu)₂](InCl₄). In the crystal of [Tl^I(Tm^tBu)]₂∙2H₂O, pyrazolyl-C–H···O(water) interactions connect the dimeric units into a linear supramolecular chain, chains pack without directional interactions between them. In the crystal of [In^III(Tm^tBu)₂](InCl₄), alternating rows of independent cations are interspersed by anions. The primary points of contact within a three-dimensional architecture are of the type In–Cl···π(pyrazolyl) and C–H···Cl. The assessment of the molecular packing was complemented by considering the calculated Hirshfeld surfaces and two-dimensional fingerprint plots (overall and delineated into individual contacts).

1. Introduction

Tripodal nitrogen-containing ligands, such as tris(pyrazolyl)hydroborate, have been utilized in the fields of inorganic and coordination chemistry [1,2]. One reason why the chemistry of this type of ligand has been studied so extensively relates to the fact that it is relatively facile to introduce substituents in the pyrazolyl rings with varying steric and electronic profiles. Recently, we developed transition metal complexes ligated by tris(pyrazolyl)hydroborate anion and/or their neutral analogues, i.e., tris(pyrazolyl)methanes, to determine how to control small molecule activation and their magnetism [3,4,5,6]. The history of the development and use of the tris(pyrazolyl)hydroborate ligand, often referred to as ‘scorpionate’, has been outlined by the founder of this chemistry, the late Prof. Swiatoslaw Trofimenko [7]. In Trofimenko’s historical account, it was noted that new ligand architectures could also be obtained by the introduction of other heteroatoms, such as oxygen, sulfur, and phosphorus [7].

Thallium and indium are toxic metal p-block elements [8,9]. Recently, indium(III) oxide has been used as a transparent conductive coating on glass substrates in electroluminescent panels, i.e., ITO [9]. Thallium(I) and indium(III) are stable formal oxidation states and have electron configurations of [Xe]4f¹⁴5d¹⁰6s²6p⁰ and [Kr]4d¹⁰5s⁰5p⁰, respectively. With respect to tripodal ligands, the introduction of sulfur gives rise to S₃-tripod type ligands, e.g., tris(3-tert-butyl-2-sulfanylidene-1H-imidazol-1-yl)hydroborate (denoted Tm^tBu) (Figure 1, left; R = tBu), being a soft tris(pyrazolyl)hydroborate derivative that readily complexes heavy metal p-block elements, such as bismuth(III) [10]; for relevant reviews of the coordination chemistry of p-block elements with soft S₃-type ligands, see [11,12,13].

In continuation of previous work, the crystal and molecular structures, as well as a detailed analysis of the calculated Hirshfeld surfaces, are described for thallium(I), [Tl^I(Tm^tbu)]₂, characterized as a dihydrate, and indium(III), [In^III(Tm^tBu)₂]⁺, complexes ligated by the same soft tripod sulfur-containing type ligand employed in an earlier study [10], namely Tm^tBu. This work compliments the literature precedents of thallium(I), thallium(III), indium(I), and indium(III) complexes ligated by tris(3-R-2-sulfanylidene-1H-imidazol-1-yl)hydroborate (Tm^R), tris(3-R-2-sulfanylidene-1H-benzimidazol-1-yl)hydroborate (Tm^RBenz), and tris(2-sulfanylidene-1H-benzothiazol-1-yl)hydroborate (Tbz) ligands (Figure 1)—thallium(I): [Tl(Tm^MeBenz)] [14], [Tl(Tm^tBuiBenz)]·C₆H₆ [14], [Tl(Tm^tBu)]₂ [15], [Tl(Tm^Ph)]₂ [16], and [Tl(Tbz)]_∞·CH₂Cl₂ [17]; thallium(III): [Tl(Tm)₂](TlI₄) [18], [Tl(Tm)₂](I) [14], and [Tl(Tm^Ph)₂](ClO₄) [16]; indium(I): [In(Tm^tBu)] [15], [In(Tm^tBu){B(C₆F₅)₃}] [15], [In(Tm^tBu)(κ²-S₄)] [15]; and [In(Tm^tBu)₂](I) [15]; indium(III): [In(Tm^Ad)₂](InI₄) [19], [In(Tm^Ad)(κ²-mim^Ad)](Cl) [19], [In(Tm^Ad){B(C₆F₅)₃}](Cl) [19], [In(Tm^tBu)] [15], and [In(Tm^Me)₂](I) [20].

2. Materials and Methods

2.1. Chemicals and Instrumentation

The preparation and handling of the two complexes were performed under an argon atmosphere using standard Schlenk tube techniques. Dichloromethane was carefully purified by refluxing and distilling under an argon atmosphere over phosphorous pentoxide. Heptane, toluene, and tetrahydrofuran were carefully purified by refluxing and distilling under an argon atmosphere over sodium benzophenone ketyl [21]. Dry ethanol was purchased from Wako Pure Chemical Ind. Ltd. and deoxygenated by purging with argon gas. Deuteriochloroform was obtained from Cambridge Isotope Laboratories, Inc. (Tewksbury, MA, USA). Other reagents were commercially available and used without further purification. The potassium salt of tris(3-tert-butyl-2-sulfanylidene-1H-imidazol-1-yl)hydroborate (KTm^tBu) was prepared by published methods [22,23,24,25].

2.2. Instrumentation

IR spectra (4000–400 cm⁻¹) were recorded as KBr pellets using a JASCO FT/IR-6300 spectrophotometer (JASCO, Tokyo, Japan). Raman spectra (4000–200 cm⁻¹) were measured as powders on a JASCO RFT600 spectrophotometer with a YAG laser 650 mW (JASCO, Tokyo, Japan). Abbreviations used in the description of vibrational data are as follows: s, strong; m, medium; w, weak. ¹H-NMR (500 MHz) and ¹³C-NMR (125 MHz) spectra were obtained on a Bruker AVANCE III-500 NMR spectrometer at room temperature (298 K) in CDCl₃ (Bruker Japan, Yokohama, Japan). ¹H and ¹³C chemical shifts were reported as δ values relative to residual solvent peaks (7.26 and 77.16 ppm, respectively). UV–Vis spectra (solution CH₂Cl₂, 1050–250 nm) were recorded on an Agilent 8453 UV–visible spectroscopy system (Agilent, Tokyo, Japan). The elemental analyses (C, H, and N) were performed by the Chemical Analysis Center of Ibaraki University.

2.3. Preparation of Complexes

2.3.1. [Tl(Tm^tBu)]₂∙2H₂O

A solution of K(Tm^tBu) (0.5114 g, 0.991 mmol) in dichloromethane (15 mL) was added to a solution of thallium(I) acetate (0.2649 g, 1.006 mmol) in degassed ethanol (10 mL). After allowing the reaction to proceed overnight, the solvent was removed under reduced pressure, and the resulting solid was extracted by dichloromethane (15 mL). Colorless crystals were obtained by slow evaporation from a saturated dichloromethane/heptane (1:1 v/v) solution and were characterized crystallographically as [Tl(Tm^tBu)]₂∙2H₂O (0.2820 g, 0.410 mmol, yield: 41%). Elemental analysis (bulk material): Anal. Calcd. for [Tl(Tm^tBu)]∙1/3H₂O: C 36.66, H 5.08, N 12.22%; Found: C 36.43, H 4.90 N 11.93%. IR (KBr, cm⁻¹): 3147 w ν(C–H), 2975 m ν(C–H), 2922 w ν(C–H), 2455 w ν(B–H), 1626 m, 1560 w, 1407 m, 1396 m, 1357 s, 1273 m, 1199 s, 1165 m, 1099 m, 981 w, 737 w, 715 m, 682 m. Raman (cm⁻¹): 3188 w (C–H), 3145 w ν(C–H), 2979m ν(C–H), 2959 m ν(C–H), 2922m ν(C–H), 2445 w ν(B–H), 1564 s, 1458 m, 1358 s, 1258 w, 1226 w, 1153 w, 1099 w, 1058 w, 1026 w, 931 w, 821 w, 716 w, 611 w, 564 w, 407 w, 314 w. ¹H NMR (CDCl₃, 298 K): 1.79 (s, 27H, CH₃), 6.12 (3H, imidazole H), 6.86 (3H, imidazole H). ¹³C NMR (CDCl₃, 298 K): 28.7 (CCH₃), 58.5 (CCH₃), 115.9 (imidazole C₄ or C₅), 123.1 (imidazole C₄ or C₅), 161.2 (imidazole C=S). UV–vis (CH₂Cl₂, 298 K; λ_max, nm (ε, M⁻¹cm⁻¹)): 270 (20,700).

2.3.2. [In(Tm^tBu)₂](InCl₄)

A solution of [Tl(Tm^tBu)]₂∙2H₂O (0.0706 g, 0.050 mmol) in dichloromethane (5 mL) was added to a solution of InCl₃∙4H₂O (0.0294 g, 0.100 mmol) in tetrahydrofuran (5 mL). After allowing the reaction to proceed overnight, the solvent was removed under reduced pressure, and the resulting solid was extracted with dichloromethane (10 mL). Colorless crystals were obtained by slow evaporation from a saturated dichloromethane/toluene (1:1 v/v) solution as [In(Tm^tBu)₂](InCl₄) (0.0586 g, 0.044 mmol, yield: 88%). Elemental analysis: Anal. Calcd. for [In(Tm^tBu)₂](InCl₄): C 38.03, H 5.17, N 12.67%; Found: C 38.04, H 5.09 N 12.33%. IR (KBr, cm⁻¹): 3181w ν(C–H), 3147 w ν(C–H), 2979 m ν(C–H), 2928 m ν(C–H), 2423 w ν(B–H), 1567 w, 1480 m,1420 s, 1398 m, 1357 s, 1308 m, 1260 w, 1228 m, 1098 s, 1177 s, 1072 w, 822 w, 768 w, 732 m, 687 m, 590 w, 552 w, 496 w, 457 w. Raman (cm⁻¹): 3184 w ν(C–H), 3152 w ν(C–H), 3111 w ν(C–H), 2986 m ν(C–H), 2928 m ν(C–H), 2425 w ν(B–H), 1568 m, 1450 w, 1431 w, 1359 s, 1307 w, 1255 w, 1248 w, 1073 w, 1041 w, 986 w, 932 w, 825 w, 738 w, 637 w, 590 w, 401 w, 320 w. ¹H NMR (CDCl₃, 298 K): 1.74 (s, 9H, CH₃), 1.79 (s, 18H, CH₃), 6.82 (d, 1H, J = 2.0 Hz, imidazole H), 6.89 (d, 2H, J = 2.0 Hz, imidazole H), 7.08 (d, 1H, J = 2.0 Hz, imidazole H), 7.12 (d, 2H, J = 2.0 Hz, imidazole H). ¹³C NMR (CDCl₃, 298 K): 29.9 (CCH₃), 30.1 (CCH₃), 60.1 (CCH₃), 60.8 (CCH₃), 118.3 (imidazole C₄ or C₅), 123.9 (imidazole C₄ or C₅), 153.2 (imidazole C=S). UV–vis (CH₂Cl₂, 298 K; λ_max, nm (ε, M⁻¹cm⁻¹)): 268 (17,900), 307 (8000).

2.4. X-ray Crystallography

Colorless crystals of [Tl(Tm^tBu)]₂∙2H₂O and [In(Tm^tBu)₂](InCl₄) were coated with Paratone-N oil (Hampton Research, Aliso Viejo, CA, USA) and mounted on a Dual-Thickness MicroLoop LD (200 μM) (MiTeGen, New York, NY, USA). X-ray intensity data were measured at T = 178 K on a Rigaku/Oxford Diffraction Rigaku XtaLAB P200 diffractometer (Rigaku Oxford Diffraction, Oxfordshire, UK) fitted with MoKα radiation (λ = 0.71073 Å) so that 100% data completeness was achieved at θ_max = 25.2°. Data reduction, including empirical absorption correction, was accomplished with CrysAlisPro (Rigaku Oxford Diffraction, Oxfordshire, UK) [26]. The structures were solved by direct methods [27] and refined (anisotropic displacement parameters and C-bound H atoms in the riding model approximation) on F² [28]. For 2, the positions of the water-bound H atoms were idealized based on chemically reasonable positions with the O–H and H···H distances initially refined with restraints 0.840 ± 0.001 and 1.30 ± 0.001 Å, respectively, and fixed at these values in the final cycles of refinement. A weighting scheme of the form w = 1/[σ²(F_o²) + (aP)² + bP], where P = (F_o² + 2F_c²)/3), was applied toward the latter stages of each refinement. At the conclusion of each refinement, relatively large residual electron density peaks were noted; details are given in the respective CIFs. The molecular structure diagrams were generated with ORTEP for Windows [29] with 50% displacement ellipsoids, and the packing diagrams were drawn with DIAMOND [30]. Additional data analysis was made with PLATON [31]. Crystal data and refinement details are given in

Table 1. Crystallographic data and refinement details for [Tl(Tm^tBu)]₂∙2H₂O and [In(Tm^tBu)₂](InCl₄).

Complex	[Tl(TmtBu)]2∙2H2O	[In(TmtBu)2](InCl4)
Formula	C42H68B2N12S6Tl2, 2(H2O)	C42H68B2InN12S6, InCl4
Molecular weight	1399.86	1326.50
Crystal size/mm3	0.03 × 0.10 × 0.13	0.13 × 0.22 × 0.26
Colour	colorless	colorless
Crystal system	triclinic	triclinic
Space group	P$\overline{1}$	P$\overline{1}$
a/Å	9.5421(2)	11.1900(1)
b/Å	11.8656(2)	11.4449(2)
c/Å	14.5839(3)	23.2153(3)
a/°	67.963(2)	94.421(1)
β/°	71.203(2)	92.606(1)
γ/°	73.428(2)	90.262(1)
V/Å3	1423.33(6)	2961.14(7)
Z	1	2
Dc/g cm−3	1.633	1.488
μ/mm−1	5.918	1.212
Measured data	48,875	101474
θ range/°	2.5–29.9	2.6–29.8
Unique data	7633	15775
Observed data (I ≥ 2.0σ(I))	6786	14551
No. of parameters	298	616
R, obs. data; all data	0.032; 0.038	0.028; 0.030
a; b in weighting scheme	0.055; 1.294	0.041; 2.674
Rw, obs. data; all data	0.085; 0.087	0.075; 0.076
Range of residual electron density peaks/eÅ−3	−1.38–2.58	−0.78–1.76

.

3. Results and Discussion

3.1. Synthesis and Characterization

The reactions of the ligand K(Tm^tBu) [22,23,24,25] with one equivalent of thallium(I) acetate (TlOAc) were carried out at room temperature, and single crystals of the thallium(I) complex, formulated as [Tl(Tm^tBu)]₂∙2H₂O, were obtained by slow evaporation of a dichloromethane/ethanol solution at room temperature (Figure 2). The indium(III) complex, [In(Tm^tBu)₂](InCl₄), was obtained by the reaction of [Tl(Tm^tBu)₂] with indium(III) chloride InCl₃∙4H₂O. The colorless crystals were obtained from the mixed solution of the saturated dichloromethane/toluene solution (Figure 2).

The expected signals in IR and Raman spectra were obtained for each [Tl(Tm^tBu)]₂ and [In(Tm^tBu)₂](InCl₄). Noteworthy were the B–H stretching bands at 2455 cm⁻¹ for [Tl(Tm^tBu)]₂ and 2423 cm⁻¹ for [In(Tm^tBu)₂](InCl₄), which were clearly evident and redshifted compared to 2480 cm⁻¹ for K(Tm^tBu) [25] (Figures S1 and S2 of the Supplementary Materials). The ¹H-and ¹³C NMR spectra of [Tl(Tm^tBu)]₂ in CDCl₃ occurred at chemical shifts identical to those of KTm^tBu (Figures S3–S5 of the Supplementary Materials). For [In(Tm^tBu)₂](InCl₄), all chemical shifts in the ¹H-NMR, and all those except for C=S carbon shifts in the ¹³C-NMR, were clearly split due to the different structural arrangements, i.e., occupying equatorial and axial coordination sites. The UV–Vis absorption spectra of [Tl(Tm^tBu)]₂ and [In(Tm^tBu)₂](InCl₄) were also measured (Figure S6 of the Supplementary Materials). Two characteristic absorption bands at 268 and 307 nm were observed for [In(Tm^tBu)₂](InCl₄), but for [Tl(Tm^tBu)]₂, only a band at 261 nm was noted. From this observation, the high-energy bands at 260 and 268 nm band were assigned to a ligand Tm^tBu-based absorption, and the low-energy band for [In(Tm^tBu)₂](InCl₄) is metal centered. For more detailed assignments, computational chemistry calculations are required, which are beyond the scope of this study. The ¹H-NMR and UV–Vis spectroscopy results indicate that the structures remained intact in the solution state.

3.2. Crystal and Molecular Structures

3.2.1. Molecular Structures

The crystallographic asymmetric unit of [Tl^I(Tm^tBu)]₂∙2H₂O comprises one-half of a dimeric complex molecule, being located about a center of inversion and a water molecule of crystallization. The complex molecule is shown in Figure 3a and comprises a central, diamond-shaped core with almost equivalent Tl1–S1, S1ⁱ bond lengths (

Table 2. Selected geometric parameters (Å, °) for [Tl(Tm^tBu)]₂∙2H₂O.

Parameter	Value	Parameter	Value
Tl1–S1	3.0408(9)	S1–Tl1–S3i	82.04(3)
Tl1–S2	3.1696(9)	S2–Tl1–S1i	91.89(2)
Tl1–S1i a	2.9993(10)	S2–Tl1–S3i	156.32(3)
Tl1–S3i a	3.2764(11)	S1i–Tl1–S3i	104.23(3)
C5–S1	1.721(4)	Tl1–S1–Tl1i	86.10(2)
C12–S2	1.700(3)	Tl1–S1–C5	120.26(12)
C19–S3	1.694(4)	Tl1–S2–C12	83.37(12)
S1–Tl1–S2	114.53(2)	Tl1–S1i–C5i	86.11(12)
S1–Tl1–S1i a	93.90(2)	Tl1–S3i–C19i	127.75(14)
(N11,N12,C5-C7)/(N21,N22,C12-C14)	88.2(3)	(N21,N22,C12-C14)/(N31,N32,C19-C21)	78.7(3)
(N11,N12,C5-C7)/(N31,N32,C19-C21)	83.2(3)

^a symmetry operation: 2 − x, − y, − z.

). The four-coordinate geometry for thallium is completed by the thione-S2 and symmetry-related thione-S3ⁱ atoms. An indication of the coordination geometry defined by the S₄ donor set is τ₄, which is computed from [360 − (α + β)]/141, where α and β are the two widest angles subtended at the thallium(I) atom [32]. In this case, τ₄ = 0.63, which corresponds to a seesaw geometry (τ₄ = 0.64); the widest angle corresponds to S2–Tl1–S3ⁱ, i.e., 156.32(3)°. The Tm^tBu ligand is therefore tetra-coordinating, bridging two thallium(I) centers. A curious feature of the molecular structure is a close intramolecular B–H···ring centroid (Tl₂S₂) separation of 2.17 Å. This is a consequence of the coordination mode of the Tm^tBu ligand and has been observed in literature analogs, e.g., in the benzene mono-solvate of [Tl(Tm^tBu)]₂ [15] and in [Tl(Tm^Ph)]₂, as its chloroform mono-solvate [16], each of which features the same κ³ coordination mode as described above but with variations in the magnitudes of the Tl–S1 bond lengths.

As highlighted in Figure 3b, the water molecule of crystallization is closely associated with the complex molecule, forming a hydrogen bond to the thione-S2 atom and close contact with the N31-pyrazolyl ring; see

Table 3. Geometric parameters (Å, °) characterizing the specified intermolecular contacts operating in the crystal of [Tl(Tm^tBu)]₂∙2H₂O ^a.

A	H	B	H···B	A···B	A–H···B	Symmetry Operation
O1w	H1w	S2	2.47	3.2779(10)	160	x, y, z
O1w	H2w	Cg (1)	2.47	3.2816(18)	161	x, y, z
C13	H13	O1w	2.46	3.214(5)	136	−1 + x, y, z

^a Cg(1) is the ring centroid of the (N31,N32,C19-C21) ring.

for the geometric parameters defining these interactions. The H2w···ring centroid separation is 2.47 Å, with the closest contact to a specific atom within the ring being 2.45 Å, i.e., the C19 atom with the next closest interaction, i.e., 2.60 Å with N31, and the longest separation of 2.97 Å with C21. This pattern indicates that the interaction is best described as a localized H2w···π interaction [33].

The asymmetric-unit of [In^III(Tm^tBu)₂](InCl₄) comprises two independent complex cations, each disposed about a center of inversion, and a InCl₄⁻ anion; the molecular structures are shown in Figure 4a–c. Focusing on the In1-containing molecule, the In1 atom is coordinated by two tripodal Tm^tBu ligands to define a soft S₆ donor set, which defines an octahedral geometry. The geometry is slightly flattened, as the In1–S1 bond length of 2.5682(4) Å is systematically shorter than the In1–S2, S3 bond lengths of 2.6597 (4) and 2.6429(4) Å (

Table 4. Selected geometric parameters (Å, °) for [In(Tm^tBu)₂](InCl₄).

Parameter	Value	Parameter	Value
In1–S1	2.5682(4)	In2–S4	2.6110(4)
In1–S2	2.6597(4)	In2–S5	2.6709(5)
In1–S3	2.6429(4)	In2–S6	2.5905(5)
C5–S1	1.7236(17)	C26–S4	1.7223(19)
C12–S2	1.7279(17)	C33–S5	1.7249(19)
C19–S3	1.7253(17)	C40–S6	1.725(2)
S1–In1–S2	91.715(13)	S4–In2–S5	92.567(14)
S1–In1–S3	93.179(13)	S4–In2–S6	95.416(15)
S2–In1–S3	92.142(13)	S5–In2–S6	89.139(15)
In1–S1–C5	105.90(6)	In2–S4–C26	109.61(6)
In1–S2–C12	107.64(6)	In2–S5–C33	106.11(7)
In1–S3–C19	106.57(6)	In2–S6–C40	106.41(7)
(N11,N12,C5-C7)/(N21,N22,C12-C14)	83.99(11)	(N41,N42,C26-C28)/(N21,N22,C12-C14)	82.84(12)
(N11,N12,C5-C7)/(N31,N32,C19-C21)	89.18(11)	(N41,N42,C26-C28)/(N51,N52,C33-C35)	88.91(13)
(N21,N22,C12-C14)/(N31,N32,C19-C21)	87.98(11)	(N51,N52,C33-C35)/(N61,N62,C40-C42)	84.43(13)

). Despite this difference, the C5–S1 bond lengths are equal within experimental error. Otherwise, the deviations from the ideal octahedral geometry are small, as noted from the relevant angles included in Table 4. As highlighted by the dihedral angles between the pyrazolyl rings listed in Table 4 and the overlay diagram of Figure 4d, there is close agreement between the independent molecules. This observation notwithstanding, there are notable differences in the In–S bond lengths. Thus, in the first independent cation of [In^III(Tm^tBu)₂](InCl₄), one In–S bond length is 0.07–0.10 Å shorter than the others, whereas in the second independent cation, the differential between the In–S bond lengths is significantly smaller, i.e., the difference between the shortest and longer bond lengths is now 0.02–0.08 Å.

There are several literature precedents for [In(Tm^tBu)₂](InCl₄), including [In(Tm^tBu)₂]⁺ characterized as the Cl⁻ as an acetonitrile tri-solvate [15], and I⁻ as a benzene mono-solvate, salts [15], along with [In(Tm^Me)₂]⁺ characterized as the I⁻ salt as a diethyl ether mono- and dimethylformamide di-solvate [20]. While the indium(III) centers in all three literature complex cations exist within S₆ donor sets, the symmetries of these vary. Thus, in the chloride salt of [In(Tm^tBu)₂]⁺, each indium atom of the two independent molecules sits on an inversion center; in the iodide salt, the indium atom is located on a three-fold inversion center, implying that all In–S lengths are equivalent. The [In(Tm^Me)₂]⁺ cation is located on an inversion center.

3.2.2. Molecular Packing

The water molecule of solvation in [Tl(Tm^tBu)]₂∙2H₂O proves pivotal in assembling the dimeric molecules into a supramolecular chain along the a-axis, as shown in Figure 5a. Thus, the water molecule forms O–H···π(pyrazolyl) and O–H···S interactions, as shown in detail in Figure 3b, with the bay region of the Tm^tBu ligand. At the same time, the water-O atom accepts an interaction from a pyrazolyl-C–H atom; see Table 3 for geometric parameters. Chains pack in the crystal without directional interactions between them, as highlighted in the unit cell diagram of Figure 5b.

A view of the unit cell contents for [In(Tm^tBu)₂](InCl₄) is shown in Figure 6. Globally, when viewed down the b-axis, molecules of In1- and In2-containing cations and [InCl₄]⁻ anions stack in columns along this axis. The cations align in rows along the a-axis, and the rows alternate in an ···ABA··· fashion down the c-axis. Interspersed between rows are the [InCl₄]⁻ anions, and these are pivotal in providing links between the constituent cations. The primary mode of interaction is via C–H···Cl contacts of the type pyrazolyl-C–H···Cl and methyl-C–H···Cl; for geometric parameters characterizing these interactions, refer to

Table 5. Geometric parameters (Å, °) characterizing the specified intermolecular contacts operating in the crystal of [In(Tm^tBu)₂](InCl₄) ^a.

A	H	B	H···B	A···B	A–H···B	Symmetry Operation
C21	H21	Cl1	2.85	3.723 (2)	154	x, −1 + y, z
C41	H41	Cl3	2.83	3.773 (2)	174	1 − x, − y, 1 − z
C42	H42	Cl4	2.81	3.562 (2)	137	1 − x, − y, 1 − z
C1	H1b	Cl4	2.83	3.797 (2)	168	x, y, z
In3	Cl1	Cg(1)	3.7819 (11)	6.0631 (9)	162.17 (3)	x, −1 + y, z

^a Cg(1) is the ring centroid of the (N21,N22,C12-C14) ring.

. The anion also forms an In–Cl···π(pyrazolyl) interaction, which is discussed in more detail in Section 3.3.

3.3. Hirshfeld Surface Analysis

To further understand the nature of the contacts between molecules in their respective crystals, an analysis of the calculated Hirshfeld surfaces was conducted employing Crystal Explorer 17 [34] following established protocols [35]. The analysis shows that there are some close contacts present in each crystal, as evident from the red spots of various intensity observed on the respective d_norm maps indicating contact distances shorter than the sum of van der Waals radii [36].

For [Tl(Tm^tBu)]₂∙2H₂O, the d_norm map of the Tl(Tm^tBu) fragment shows several red spots of moderate to strong intensity (Figure 7), which can be attributed to O1w–H1w···S2, O1w–H2w···C19(π), and C13–H13···O1w close contacts, with the respective contact distances being 2.37, 2.35, and 2.34 Å compared to the respective sums of the van der Waals radii (ΣvdW) of 2.89, 2.79, and 2.61 Å for H···S, H···C, and H···O (

Table 6. d_norm contact distances (adjusted to neutron values) of intermolecular interactions identified in the crystals of [Tl(Tm^tBu)]₂∙2H₂O and [In(Tm^tBu)₂](InCl₄), as computed through the Hirshfeld surface analysis and in comparison to the corresponding sum of van der Waals radii (ΣvdW).

Contact	Distance (Å)	ΣvdW (Å)	Δ(ΣvdW—Distance)	Symmetry Operation
[Tl(TmtBu)]2∙2H2O
H1w···S2	2.37	2.89	0.52	x, y, z
H2w···C19	2.35	2.79	0.44	x, y, z
H13···O1w	2.34	2.61	0.27	− 1 + x, y, z
[In(TmtBu)2](InCl4)
H41···Cl3	2.40	2.84	0.44	1 − x, − y, 1 − z
H21···Cl1	2.49	2.84	0.35	x, − 1 + y, z
H37c···H30a	1.93	2.18	0.25	1 + x, y, z
H42···Cl4	2.72	2.84	0.12	1 − x, − y, 1 − z
H1b···Cl4	2.73	2.84	0.11	x, y, z
H6···Cl4	2.78	2.84	0.06	x, y, z
H35···C19	2.76	2.79	0.03	x, y, z

).

Hirshfeld surface analysis was also performed for the central thallium atom (Figure 8). The d_norm map displays several weak to intense red spots indicating additional intramolecular interactions between B1–H1···Tl (d_norm distance = 2.69 Å vs. ΣvdW = 3.05 Å) and C10–H10a···Tl (d_norm distance = 3.00 Å vs. ΣvdW = 3.05 Å) on top of S1···Tl, S2···Tl and S3···Tl contacts, as described above.

The d_norm analysis was also conducted for each of the In1- or In2-containing cations and In3-anion in [In(Tm^tBu)₂](InCl₄) (Figure 9). A number of red spots with moderate intensity are observed for close contacts comprising C41–H41···Cl3, C21–H21···Cl1, and C42–H42···Cl4, in accordance with those interactions identified through the geometric analysis conducted with PLATON [31]. However, additional contacts were noted through the d_norm map arising from H37c···H30a, with moderate intensity, as well as from C1–H1b···Cl4, C6–H6···Cl4, and C35–H35···C19 with weak intensity. Table 6 summarizes these contacts and compares the separation distances with the respective van der Waals radii.

In addition to the close contacts as identified through the direct observation of red spots on the Hirshfeld surface of [In(Tm^tBu)₂](InCl₄), two other important contacts, specifically In3–Cl1···π(pyrazolyl) and C1–H1b···In3, are detected based on the complementarity of shape as indicated by the hollows and bumps on the shape index mapped over the Hirshfeld surfaces between the independent In1 cation and In3 anion, despite the observation that the contact distances between Cl1 and the closest carbon atom of the five-membered imidazole ring, i.e., Cl1···C14, as well as H1b···In3, are longer than the corresponding ΣvdW, Figure 10.

Quantification of the close contacts in each individual component of [Tl^I(Tm^tBu)]₂∙2H₂O and [In^III(Tm^tBu)₂](InCl₄) was performed through two-dimensional fingerprint plot analysis. The overall and delineated fingerprint plots for the individual components profiled in [Tl(Tm^tBu)]₂∙2H₂O, namely the Tm^tBu fragment, the water molecule, the thallium atom as well as the [Tl(Tm^tBu)]₂ dimer, are illustrated in Figure 11. The fingerprint profiles for both Tm^tBu and [Tl(Tm^tBu)]₂ resemble each other and have the shape of a flying fox, while those for H₂O and Tl display a cicada- and pincer-like profile, respectively. The decomposition of the corresponding overall profiles for Tm^tBu and [Tl(Tm^tBu)]₂ show that their close contacts are dominated by H···H (69.5 and 76.6%, respectively), H···S/S···H (12.8 and 9.8%), H···C/C···H (6.2 and 6.6%), H···Tl/Tl···H (3.7 and 3.3%), S···Tl (for Tm^tBu only, 2.9%), and H···O/O···H (2.3 and 2.6%) as well as by other minor contacts that constitute less than 1.0% of all surface contacts, including H···N/N···H, S···O, and others. Among those contacts, only S···H, C···H, H···Tl/Tl···H, and O···H feature a distinctive tip with d_i + d_e values corresponding to O1w–H1w···S2, O1w–H2w···C19, B1–H1···Tl/C10–H10a···Tl, and C13–H13···O1w, while the rest of the contacts present indistinct profiles.

As for the H₂O molecule and thallium atom, the major contacts for the former appear in the order H···H (43.8%) > O···H (30.4%) > H···S (10.0%) > H···C (7.2%) > H···N (6.2%) > O···S (2.5%), while the latter is dominated by H···Tl (63.1%) > S···Tl (26.2%) > C···Tl (6.9%) > N···Tl (3.9%). The d_i + d_e distances associated with the significant peaks in these profiles correspond to the reciprocal contacts as identified in the Tm^tBu and [Tl(Tm^tBu)]₂ profiles.

The overall and selected delineated two-dimensional fingerprint plots for the In1- and In2-cations as well as the In3-anion in [In(Tm^tBu)₂](InCl₄) are presented in Figure 12. As a general observation, the In1- and In2-cations exhibit a paw-like overall profile, which can be delineated mainly into H···H, H···Cl, and H···C/C···H contacts that constitute more than 93% of the Hirshfeld surfaces in each case. Specifically, the plots of the H···Cl contacts for the In1- and In2-cations display a distinctive spike in their decomposed fingerprint plots comprising 13–14% of all surface contacts with d_i + d_e tipped at about 2.70 Å, which can be, respectively, attributed to H1b···Cl4 and H42···Cl4. The (inner)-C···H-(outer) and (inner)-H···C-(outer) contacts, represented by the pair of pincer-like profiles in the decomposed H···C/C···H fingerprint plots for the In1- and In2-cations, are due to H35···C19, with both constituting about 5% of the Hirshfeld surfaces with a d_i + d_e distance of 2.76 Å, while the reciprocal (inner)-H···C-(outer) of the In1-cation and (inner)-C···H-(outer) of the In2-cation contribute 4.3 and 5.4%, respectively, to the contact surfaces but with a less significant d_i + d_e distance, being longer than ΣvdW. A distinct feature is observed in the decomposed H···H fingerprint plot for the In2-cation compared to that for the In1-cation, in that for the former, a relatively prominent tip at d_i + d_e = 1.93 Å is assignable to H37c···H30a.

On the other hand, the decomposition of the squid-like overall fingerprint plot for the In3-anion shows that about 94.4% of the contact surfaces are dominated by Cl···H with a d_i + d_e distance tipped at 2.66 Å ascribed to the close C41–H41···Cl3 contact, while other minor contacts include Cl···C and In···H, which contribute about 3.1 and 2.0%, respectively, to the total surface, both with d_i + d_e tips arising from the In3–Cl1···π(C14) and C1–H1b···In3 contacts as discussed above.

4. Conclusions

The synthesis, spectroscopic and X-ray crystallographic characterizations of two main group element complexes of a soft S₃-tripod-type ligand have been described. S₄-seesaw and S₆-flatenened octahedral geometries were found for the central atoms in Tl(Tm^tBu)]₂∙2H₂O and [In(Tm^tBu)₂](InCl₄), respectively. The analyses of the calculated Hirshfeld surfaces confirmed the geometric analysis of the molecular packing. These results suggest soft S₃-tripod-type ligands related to that discussed herein are a potentially useful class of ligands for coordination to p-block elements. The coordination of sulfur atoms gives rise to distinct electronic characteristics compared with the well-known hard N₃-type ligands. Therefore, the present chemistry paves the way for new coordination chemistry based on soft S₃-tripod-type ligands.