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

**Kiyoshi Fujisawa 1,\*, Ayaka Kuboniwa 1, Sang Loon Tan <sup>2</sup> and Edward R. T. Tiekink 2,\***


**Abstract:** Two complexes containing a soft sulfur-substituted tris(pyrazolyl)hydroborate ligand, namely [TlI (TmtBu)]2·2H2O and [InIII(TmtBu)2](InCl4), where TmtBu is the tris(3-*tert*-butyl-2-sulfanylidene-1*H*-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 TmtBu 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 [InIII(TmtBu)2](InCl4). In the crystal of [TlI (TmtBu)]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 [InIII(TmtBu)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).

**Keywords:** thallium; indium; main group chemistry; tripodal ligand; thiol ligand; crystal structure; hydrogen bonding; Hirshfeld surface analysis

## **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 [3–6]. 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]4*f* 145*d*106*s*26*p*<sup>0</sup> and [Kr]4*d*105*s*05*p*0, 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-1*H*-imidazol-1-yl)hydroborate (denoted TmtBu) (Figure 1, left; R = tBu), being a soft tris(pyrazolyl)hydroborate derivative that readily

**Citation:** Fujisawa, K.; Kuboniwa, A.; Tan, S.L.; Tiekink, E.R.T. Structural and Hirshfeld Surface Analysis of Thallium(I) and Indium(III) Complexes of a Soft Scorpionate Ligand. *Crystals* **2023**, *13*, 745. https://doi.org/10.3390/ cryst13050745

Academic Editor: Waldemar Maniukiewicz

Received: 10 April 2023 Revised: 26 April 2023 Accepted: 26 April 2023 Published: 29 April 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

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 [11–13].

**Figure 1.** Schematic drawing of the ligands: tris(3-R-2-sulfanylidene-1*H*-imidazol-1-yl)hydroborate (TmR), tris(3-R-2-sulfanylidene-1*H*-benzimidazol-1-yl)hydroborate (TmRBenz), and tris(2-sulfanylidene-1*H*-benzothiazol-1-yl)hydroborate (Tbz).

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), [TlI (Tmtbu)]2, characterized as a dihydrate, and indium(III), [InIII(TmtBu)2] +, complexes ligated by the same soft tripod sulfur-containing type ligand employed in an earlier study [10], namely TmtBu. This work compliments the literature precedents of thallium(I), thallium(III), indium(I), and indium(III) complexes ligated by tris(3-R-2-sulfanylidene-1*H*-imidazol-1-yl)hydroborate (TmR), tris(3-R-2-sulfanylidene-1*H*-benzimidazol-1-yl)hydroborate (TmRBenz), and tris(2-sulfanylidene-1*H*-benzothiazol-1-yl)hydroborate (Tbz) ligands (Figure 1)—thallium(I): [Tl(TmMeBenz)] [14], [Tl(TmtBuiBenz)]·C6H6 [14], [Tl(TmtBu)]2 [15], [Tl(TmPh)]2 [16], and [Tl(Tbz)]∞·CH2Cl2 [17]; thallium(III): [Tl(Tm)2](TlI4) [18], [Tl(Tm)2](I) [14], and [Tl(TmPh)2](ClO4) [16]; indium(I): [In(TmtBu)] [15], [In(TmtBu){B(C6F5)3}] [15], [In(TmtBu)(κ2-S4)] [15]; and [In(TmtBu)2](I) [15]; indium(III):[In(TmAd)*2*](InI4) [19], [In(TmAd)(κ2-mimAd)](Cl) [19], [In(TmAd){B(C6F5)3}](Cl) [19], [In(TmtBu)] [15], and [In(TmMe)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-1*H*-imidazol-1-yl)hydroborate (KTmtBu) was prepared by published methods [22–25].

#### *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<sup>−</sup>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*
