The Crystal Structures of Two Hydro-closo-Borates with Divalent Tin in Comparison: Sn(H2O)3[B10H10] · 3 H2O and Sn(H2O)3[B12H12] · 4 H2O

Single crystals of Sn(H2O)3[B10H10] · 3 H2O and Sn(H2O)3[B12H12] · 4 H2O are easily accessible by reactions of aqueous solutions of the acids (H3O)2[B10H10] and (H3O)2[B12H12] with an excess of tin metal powder after isothermal evaporation of the clear brines. Both compounds crystallize with similar structures in the triclinic system with space group P1¯\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\bar{1 }$$\end{document} and Z = 2. The crystallographic main features are electroneutral ∞1{\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${}_{\infty }^{1} \{$$\end{document}Sn(H2O)3/1[B10H10]3/3} and ∞1{\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${}_{\infty }^{1} \{$$\end{document} Sn(H2O)3/1[B12H12]3/3} double chains running along the a-axes. Each Sn2+ cation is coordinated by three water molecules of hydration (d(Sn–O) = 221–225 pm for the B10 and d(Sn–O) = 222–227 pm for the B12 compound) and additionally by hydridic hydrogen atoms of the three nearest boron clusters (d(Sn–H) = 281–322 pm for the B10 and d(Sn–H) = 278–291 pm for the B12 compound), which complete the coordination sphere. Between these tin(II)-bonded water and the three or four interstitial crystal water molecules, classical bridging hydrogen bonds are found, connecting the double chains to each other. Furthermore, there is also non-classical hydrogen bonding between the anionic [BnHn]2− (n = 10 and 12) clusters and the crystal water molecules pursuant to B–Hδ−⋯\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\cdots$$\end{document}δ+H–O interactions often called dihydrogen bonds.


Introduction
Interactions between soft metal cations, e.g. Cu ? , Ag ? and Hg 2? , and hydro-closo-borate anions [B n H n ] 2-(n = 10 and 12) were firstly discussed in the 1960s [1]. The salt-like copper(I) compound Cu 2 [B 10 H 10 ] shows Cu Á Á Á B distances in the range from 214 to 233 pm, indicating a covalent interaction between the Cu ? cation and the hydro-closoborate anion which was suggested as a three-centered twoelectron Cu-H-B bond [2]. This assumption was encouraged by the infrared spectra, which unveils two distinct absorption bands in the B-H stretching area, one for the non-coordinating BH groups and another for the BH groups entailed in the Cu-H-B interactions [3]. Similar can be found in the infrared spectra of the compounds [Cu 2 (bpa) 2 B 10 H 10 ] and [Cu 2 (bpa) 2 (OH) 2 ] 2 [Cu 2 (B 10 H 10 ) 3 ] Á n CH 3 CN, both revealing stretching vibrations of BH groups in three-centered Cu-H-B bonds. Additionally the X-ray crystal structure of [Cu 2 (bpa) 2 (OH) 2 B 10 H 10 ] shows Cu-H contacts in the range of 263-273 pm [4]. For the higher homologous salt-like silver(I) compound Ag 2 [B 10 H 10 ] [1,5], up to now, no crystal structure could be determined, but the presence of three-centered Ag-H-B is supported by infrared spectroscopic data [5]. In the further case of Cu ? and Ag ? cations the review article by Avdeeva et al. [6] should be mentioned, as it provides a good overview of this field.
Up to now, these kind of connections can be found in several other salt-like hydro-closo-borates, especially with cations of the 6th period possessing lone-pair electrons. All of these compounds unveil interesting properties. The thallium(I) salt Tl 2 [B 12 H 12 ] [7] exhibits a yellow metalcentered luminescence at room temperature, based on an apparent covalent interaction between Tl ? and the hydridic hydrogen atoms of the [B 12 H 12 ] 2anions [8]. Besides Tl 2 [B 12 H 12 ], there can be also found two lead(II) dodecahydro-closo-dodecaborates, Pb[B 12 [13] all Tl ? cations are only coordinated by hydridic hydrogen atoms of the boron cluster anions. This compound crystallizes isotypically with the salt-like decahydro-closo-decaborates of some alkali metals (A = Na, K and Rb) [14], but with marked differences in the Tl Á Á Á H coordination spheres, based on stereochemically active lone-pair electrons. All these compounds can be seen as potential precursors for metalated boron clusters such as [Et 3 [18,19].
The crystallographically unique Sn 2? cation is coordinated by three oxygen atoms (O1-O3) from the corresponding water molecules of hydration forming the first coordination sphere with d(Sn-O) = 221-225 pm (Fig. 1 [7,11]. Furthermore, a second coordination sphere is built up by three decahydro-closodecaborate anions in such a way, that two of these three [B 10 H 10 ] 2clusters interact via their triangular faces and a third one via a single corner with each tin(II) cation. Together with the three water molecules of hydration they erect a distorted trigonal antiprism with their barycenters providing for CN = 10 (3 9 O ? 7 9 H) as overall coordination number (Fig. 1, right). This asymmetric coordination of the Sn 2? cation by the three boron clusters allows space for the stereochemically active 5sp lone pair of electrons. The corresponding Sn-H distances range from 281 to 322 pm and are in good accordance with the already known decahydro-closo-decaborates containing lone-pair cations with significant hydrogen interactions, e.g. The interatomic O Á Á Á H distances for these hydrogen bonds range from 171 to 189 pm and can be classified as decently strong, whereas the hydrogen bond interactions among the crystal water molecules O4w and O6w with H42w Á Á Á O6w distances of 217 pm are rather weak (Fig. 3, left) [21]. Beneath the classical hydrogen bonds, also unconventional hydrogen-bonds B-H d-Á Á Á d? H-O [22] can be observed (Fig. 3, right). These non-classical socalled dihydrogen bonds occur between the negatively polarized hydrogen atoms of the [B 10 H 10 ] 2anions and the protonic hydrogen atoms of the H 2 O molecules with d(H d-Á Á Á d? H) = 186-225 pm. Each [B 10 H 10 ] 2anion builds up non-classical hydrogen bonds to the nearest water molecules, represented by O3, O4w, O5w (2 9) and O6w (2 9), from which O3 is coordinated directly to a tin(II) cation as water molecule of hydration and the other oxygen atoms stand for free interstitial crystal water molecules (Fig. 4).
The higher homologous dodecaborate with the composition Sn(H 2 O) 3  providing for a total coordination number of six for the first coordination sphere of the divalent tin (Fig. 5, left). The irregular arrangement of the three oxygen and three hydrogen atoms leaves enough space for a 5sp lone pair of electrons with stereochemical activity. Further tin-hydrogen distances can be found in the range of 315-333 pm, so each of the three boron cages finally coordinates via one triangular face to every Sn 2? cation with a total coordination number of 12 for the first and second coordination sphere (Fig. 5, right).

Vibrational Spectroscopy
Despite the two different boron clusters the infrared (IR) and Raman spectra of both compounds are very similar to each other (Fig. 9) and all observed peaks could be assigned successfully. The splitting of the B-H stretching mode in a low-frequency and high-frequency region is caused by B-H groups participating in hydrogen bonding (low-frequency shift) and groups that do not participate in hydrogen bonding (high-frequency shift). This clearly signalizing the presence of non-classical hydrogen bonds [24].
Possible occurring B-HÁ Á ÁSn stretching vibrations are expected in the low field at 2100 to 2000 cm -1 [25,26] [7,11]. But in comparison, the Sn-H distances are substantially strong acidic ion-exchange column (Merck, Amberlite IR-120). The reaction solution was heated up to 50°C for 2 days, filtered and then evaporated at room temperature to yield colorless single crystals within a few days.
An empirical absorption correction was performed by using the program Scalepack [34]. The crystal structure solutions and refinements were carried out with the program package SHELX-97 [35]. Both structure refinements include the positions of all hydrogen atoms without any constraints. The coefficients of the equivalent isotropic displacement parameters are defined as U eq = 1 / 3 [U 11 (aa*) 2 ? U 22 (bb*) 2 ?U 33 (cc*) 2 ? 2U 12 aba*b*cosc ? 2U 13 aca*c*cosb ? 2U 23 bcb*c*cosa] in pm 2 [36]. Further details of the crystal structure investigations may be  The infrared spectroscopic data were measured as potassium-bromide powder pellets on a Bruker ALPHA FT/IR spectrometer (Bruker Optik GmbH, Ettlingen, Germany). Raman spectra of the single crystals were recorded by using a XploRA Raman microscope (HORIBA Jobin Yvon GmbH, Bensheim, Germany).