Synthesis and Coordination Chemistry of a Chelating P–Sn Ligand Based on Stanna-closo-dodecaborate

Graphical Abstract

The heteroborate cluster [SnB₁₁H₁₁]^2– was substituted at the upper belt of the icosahedron by one of the triarylphosphine moieties of xantphos under the catalysis of palladium acetate. Transition-metal electrophiles, like the platinum halide PtCl₂ or the rhodium carbonyl [Rh(CO)₂Cl]₂, react with the bidentate ligand, with the formation of cis-coordination products.

Abstract

Substitution at a B–H unit of the heteroborate cluster stanna-closo-dodecaborate [SnB₁₁H₁₁]^2– at the upper belt of the icosahedron by one of the triarylphosphine moieties of xantphos was carried out under the catalysis of palladium acetate. The resulting coupling product is a monoanion offering two coordination sites: a low-valent tin-cluster vertex and a triaryl phosphine moiety. The basicity and nucleophilicity of the anion were investigated in reactions with a strong acid or an alkylating agent. Transition-metal electrophiles like platinum halide PtCl₂ or rhodium carbonyl [Rh(CO)₂Cl]₂ react with the bidentate ligand, with the formation of cis-coordination products.

Introduction

Selective transition-metal-catalyzed substitution reactions at the B–H units of a boron cluster compound are currently of interest and have been studied in the case of o-carborane and the carba-closo-dodecaborate anion.1 Substitution chemistry at the sphere of borane and heteroborane clusters was studied in general for many decades, and electrophilic substitution is an especially well-known reaction in borane-cluster chemistry.[1], 2

We have been studying heteroborane-cluster chemistry for more than fifteen years, with a focus on heavy Group 14 element heteroborates.3 These clusters are characterized by a lone pair at the heteroatom vertex.4 In particular, the interaction of the electron pair at the tin or germanium atom with transition-metal fragments was studied in many experiments. It turned out that the dianionic closo-borates [GeB₁₁H₁₁]^2–, [SnB₁₁H₁₁]^2–, and [Sn₂B₁₀H₁₀]^2– are versatile ligands in coordination chemistry, offering strong trans influence and labile coordination properties.5 Furthermore, the electron-donor ability of the dianionic ligands was verified in coinage-metal coordination chemistry: clusters based on bond formation between coinage metals, due to dispersive interactions, are the main structural motif in this chemistry.6

A couple of years ago, we studied the coordination chemistry of the germa- and stanna-closo-dodecaborate nucleophiles at a palladium xantphos electrophile.7 In these reactions, we found a difference in reactivity between the germanium and tin ligands. The germanium-based nucleophile exhibits well-established η¹-coordination through the germanium atom at the palladium electrophile. The tin nucleophile only shows the same coordination behavior below –30 °C. At room temperature, we found further reactivity in the tin case, and coupling reactions between xantphos [4,5-bis(diphenylphosphanyl)-9,9-dimethylxanthene] and the heteroborate occurred. In one case, activation of a B–H bond and formation of a P–B bond was found to give a chelating ligand, such as in A (Figure 1). In another reaction pathway, we also characterized the product of a P–C bond activation in B (Figure 1). However, in both cases, a bidentate chelating P–Sn ligand was formed in the coordination sphere of palladium. On the basis of these results, we became interested in systematically investigating B–H activation at the cluster sphere.

Figure 1

Characterized coupling products between xantphos and [SnB₁₁H₁₁]^2–.7

Consequently, we aimed to develop a rational synthesis for the chelating P–Sn ligand found in the palladium coordination chemistry in A (Figure 1). In this manuscript, we present a synthetic procedure for the synthesis of a stanna-closo-dodecaborate-based bidentate P–Sn ligand, along with the first reactions of this tin and phosphorus donor with electrophiles.

Results and Discussion

Inspired by our palladium-based B–H activation and P–B bond-formation reaction, we started to investigate palladium-based coupling procedures between xantphos and the tributylammonium salt of stanna-closo-dodecaborate 1 (Scheme 1).7 Although the stoichiometric coupling reaction proceeds at room temperature with formation of the palladium complexes A and B, 22 mol-% of [Pd(OAc)₂] and higher temperatures are necessary for the coupling of xantphos and heteroborate 1 in a catalytic reaction. The reaction of xantphos with [SnB₁₁H₁₁]^2– (Scheme 1) was monitored by means of ³¹P{¹H} NMR spectroscopy. After heating an equimolar mixture of xantphos and closo-borate [SnB₁₁H₁₁]^2– in the presence of 22 mol-% of [Pd(OAc)₂] for 40 h at 120 °C in acetonitrile, we found quantitative formation of a P–B bond, corresponding to the P–Sn ligand in complex A (Figure 1). The reaction product 2 (Scheme 1) was purified by column chromatography and characterized by NMR spectroscopy, together with elemental analysis and single-crystal X-ray diffraction structure determination. Unambiguous identification is possible on the basis of the NMR spectroscopic data: the signal in the ¹¹⁹Sn{¹H} NMR spectrum occurs at δ = –524 ppm, which is a good indicator for an unsubstituted tin vertex. In comparison, [SnB₁₁H₁₁]^2– shows a resonance at δ = –546 ppm.8 Seven overlapping ¹¹B{¹H} NMR spectroscopic signals appear in the respective spectrum, due to C_s symmetry, and the assignment by a two-dimensional ¹¹B{¹H}–¹¹B{¹H} COSY NMR spectroscopic experiment allows the identification of the position of phosphorus substitution at the cluster sphere. In the ³¹P{¹H} NMR spectrum, a broad signal at 4.7 ppm, due to coupling with a boron nucleus, is detected, and a sharp singlet at δ = –23.6 ppm lies in the range typical for triaryl phosphines. The molecular structure of the anion of salt 2, determined by single-crystal X-ray diffraction structure analysis, is shown in Figure 2; details of the structure solution are placed in the Supporting Information. Single crystals suitable for X-ray diffraction analysis of 2 were obtained by slow diffusion of diethyl ether into an acetonitrile solution of the salt. The salt 2 crystallizes in the monoclinic space group P2₁/c. The B–P bond length is 1.942(2) Å, which lies in the range of values known from the literature for this kind of compound [1.928(2) Å in B₁₂H₁₁(PPh₃)]^–.[1] The interatomic B–B distances vary from 1.775(2) to 1.845(2) Å, and they do not show any significant deviation from the unsubstituted closo-borate [SnB₁₁H₁₁]^2–.9 The Sn–B distances lie in the range of 2.382(2) to 2.414(2) Å, with an average of 2.402 Å, which is larger than the average of the alkylated cluster [Me-SnB₁₁H₁₁]^– (2.296 Å) or the unsubstituted dianion (2.385 Å).8 The bond lengths found in the xantphos substituent are typical for this moiety.

Scheme 1

Formation of the P–Sn ligand salt 2.

Figure 2

ORTEP of the structure of the anion of [Bu₃HN][2-(xantphos)-SnB₁₁H₁₀] (2) in the solid state. Ellipsoids are drawn at 50 % probability. The countercation and the phenyl groups of the xantphos moiety, except for their ipso carbon atoms, are omitted for the sake of clarity. Selected interatomic distances [Å] and angle [°]: Sn–B2 2.414(2), Sn–B3 2.414(2), Sn–B4 2.382(2), P1–B2 1.942(2), Sn–B2–P1 111.8(1).

Despite the possibility of forming two different P–Sn ligands, represented in the complexes A and B (Figure 1), we found high selectivity for the formation of ligand salt 2: the monoanionic coupling product between xantphos and the closo-cluster already characterized in complex A is the major reaction product. Two side products, which were isolated in very small amounts as crystals and only characterized by low-quality single-crystal X-ray diffraction, show B–P bond formation at both phosphine moieties of xantphos, and in one case, there is also the activation of a P–C bond.

Protonation of the Sn–P Nucleophile 2

To locate the position of highest basicity in the tin–phosphine bidentate donor, the anion 2 was protonated with the strong acid HCl (1 m HCl in 1,4-dioxane). The zwitterionic molecule 3 (Scheme 2) is formed as the product of protonation of the triarylphosphine moiety. After filtration and evaporation of the solvent, the product was isolated in a yield of 72 %. Purity of compound 3 was proven by elemental analysis. Due to its zwitterionic character, molecule 3 is barely soluble in common organic solvents.

Scheme 2

Protonation of the Sn–P anion.

Single crystals suitable for X-ray diffraction analysis of 3 were obtained by diffusion of 1 m HCl dioxane solution into an acetonitrile solution of salt 2. Zwitterion 3 crystallizes in the monoclinic space group P2₁/n, together with one acetonitrile molecule in the asymmetric unit. In Figure 3, the molecular structure of 3 is presented and selected bond lengths and angles are listed. In comparison with the starting material 2, the interatomic distances in 3 exhibit almost no change.

Figure 3

ORTEP of the zwitterion 3 in the solid state. Ellipsoids are drawn at 50 % probability. Phenyl groups of the xantphos moiety, except for their ipso carbon atoms, are omitted for the sake of clarity. Selected interatomic distances [Å] and angle [°]: Sn–B4 2.389(2), Sn–B2 2.423(2), P1–B2 1.949(2), B2–P1–C4 111.22(9).

The NMR spectra obtained in CD₂Cl₂ are in accordance with the structure of 3 in the solid state. The most interesting feature in the proton NMR spectrum is a doublet at δ = 9.04 ppm, with a ¹J(³¹P–¹H) coupling constant of 548 Hz for the phosphorus-bound proton. The ¹¹⁹Sn{¹H} NMR spectrum exhibits a broad signal at δ = –522 ppm, which is a chemical shift typical of an unsubstituted tin atom at the vertex of the cluster skeleton of stanna-closo-dodecaborate.8, 9 The ³¹P{¹H} NMR spectrum exhibits two signals: one broad multiplet for the B–P unit at δ = 11.9 ppm, broadened due to coupling with the ^11/10B nucleus, and one singlet at δ = –12.4 ppm for the P–H group, which splits into a doublet of multiplets in the proton-coupled experiment. In the ¹¹B{¹H} NMR spectrum, only one broad singlet, integrating to one boron atom, is resolved at δ = –4.3 ppm, while another broad multiplet between –5 and –14 ppm belongs to the other ten boron atoms of the cluster.

Alkylation of the Anionic Sn–P Nucleophile of Salt 2

Alkylation of the unsubstituted closo-borate [SnB₁₁H₁₁]^2– is a straightforward reaction, leading to the formation of a tin–carbon bond. To check the position of highest nucleophilicity in the anion of compound 2, the reaction of salt 2 with methyl iodide in CH₂Cl₂ was carried out. On the basis of NMR spectroscopy, we can conclude that the tin vertex of the cluster was alkylated by methyl iodide (Scheme 3). Unfortunately, single crystals of 4 (Scheme 3) were not obtained under various conditions.

Scheme 3

Methylation of the Sn–P anion.

Clear evidence for the alkylation of the tin vertex is found in the ¹¹⁹Sn{¹H} NMR spectrum: the signal at δ = –197 ppm lies in the typical range known for the alkylated tin vertex of stanna-closo-dodecaborate.8 The ¹¹B{¹H} NMR spectrum shows resonances between –11 and –18 ppm, which are shifted to the higher field, in comparison with the starting material. This is a common sign for substitution at the tin vertex.[4], 8 Furthermore, the signal for the tin-bound methyl group was found in the ¹H NMR spectrum at δ = 1.17 ppm, and it shows tin satellites, with coupling constants of ²J(¹¹⁹Sn–¹H) = 101.2 Hz and ²J(¹¹⁷Sn–¹H) = 96.8 Hz.

Reaction of 2 with Iodine

The addition of iodine to stanna-closo-dodecaborate was already studied, resulting in the product of an oxidative addition at the tin vertex [I₂SnB₁₁H₁₁]^2–. The tin is oxidized to the oxidation-four state and this reaction product is only stable below –30 °C.10 The anionic phosphino-substituted cluster of salt 2 was also reacted with iodine (1 equiv.) and the reaction product shows a comparable temperature sensitivity (Scheme 4). At room temperature, the originally red reaction product 5 (Scheme 4) changes its color to yellow. Crystallization of the reaction mixture, directly after mixing the reaction components at –25 °C, resulted in red crystals of the reaction product 5, with a yield of 89 %. The zwitterionic compound is only soluble in DMF, but the NMR spectroscopic data of the DMF solution are not in accordance with the solid-state structure. We speculate that the Sn–P bond is cleaved and a DMF adduct is formed. Therefore, product 5 was only characterized by elemental analysis and structure determination. In Figure 4, the molecular structure in the solid state is shown, and selected angles and interatomic distances are presented. The Sn–B distances range from 2.321(4) to 2.402(3) Å, with an average of 2.347 Å. In comparison with the starting material 2, the cluster in 5 shows a slight contraction upon oxidation with iodine. This is a common effect in this type of chemistry, and it was also found for the diiodide [I₂SnB₁₁H₁₁]^2–, showing a Sn–B distance average of 2.314 Å.10 The Sn–I distance of 2.8031(3) Å can be compared with the Sn–I distances found in [I₂SnB₁₁H₁₁]^2– of 2.797(1) and 2.862(1) Å. In the case of the Sn–P distance of 2.6708(9) Å, we compare this bond length with values found in Sn^IV phosphine adducts: SnCl₄(PEt₃)₂ 2.615(5) Å, SnCl₄(PMe₃)₂ 2.5654(7) Å.11

Scheme 4

Addition of iodine to the Sn–P anion.

Figure 4

ORTEP of the molecular structure of the zwitterion 5 in the solid state. Ellipsoids are drawn at 50 % probability. Phenyl groups of the xantphos moiety, except for their ipso carbon atoms, are omitted for the sake of clarity. Selected interatomic distances [Å] and angle [°]: I–Sn 2.8031(3), Sn–P2 2.6708(9), Sn–B2 2.402(4), P1–B2 1.938(4), I–Sn–P2 87.95(2).

Complexation of the Anionic P–Sn Ligand of Salt 2

The main objective of this chemistry was the synthesis of the chelating P–Sn ligand, based on stanna-closo-dodecaborate, and to compare its coordination chemistry with the coordination properties of [SnB₁₁H₁₁]^2–. Therefore, we reacted the anionic ligand of salt 2 with platinum and rhodium coordination compounds. In the platinum case, we reacted PtCl₂, as well as K₂[PtCl₄], with compound 2 in acetonitrile (Scheme 5). Due to the low solubility of the platinum reagents in acetonitrile, we had to increase the temperature to 120 °C and close the reaction vessel for 30 min. From the yellow reaction solution, the product was isolated by diffusion of diethyl ether, as light-brown crystals, in a yield of 44 %. Characterization was carried out by means of elemental analysis, NMR spectroscopy, and single-crystal X-ray diffraction structure analysis. The molecular structure of the anion of 6 (Scheme 5) is presented in Figure 5, together with selected interatomic distances and angles. Details of the structure solution were placed in the Supporting Information.

Scheme 5

Pt coordination of the bidentate P–Sn ligand.

Figure 5

ORTEP of the anionic platinum complex of salt 6 in the solid state. Ellipsoids are drawn at 50 % probability. The countercation, as well as the phenyl groups of the xantphos moiety, except for their ipso carbon atoms, are omitted for the sake of clarity. Selected interatomic distances [Å] and angles [°]: Pt–Sn 2.5066(3), Pt–P2 2.2121(10), Pt–Cl1 2.3600(10), Pt–Cl2 2.3783(9), Sn–B2 2.328(4), P1–B2 1.936(4), Sn–Pt–P2 95.64(3), Sn–Pt–Cl1 84.33(3), Sn–Pt–Cl2 172.76(3), P2–Pt1–Cl1 170.88(3), P2–Pt–Cl2 90.78(3), Cl1–Pt–Cl2 89.85(3).

The geometry of the anionic coordination compound of salt 6 can be compared with the homologous palladium complex, which we isolated from the reaction of [xantphosPdCl₂] with [SnB₁₁H₁₁]^2–.7 In both coordination compounds, the ligand is cis-coordinated at the transition metal, exhibiting an Sn–M–P angle close to 90° (Pd 92.93°, Pt 95.64°). The Pt–Sn distance of 2.5066(3) Å is slightly shorter than the Pt–Sn bonds characterized in the SnB₁₁H₁₁–platinum chemistry.[5], [5], 12 In the case of the Pt–Cl bonds of 2.3600(10) and 2.3783(9) Å, the longer bond is trans to the tin site. This might be an indicator for the stronger trans influence of the tin ligand, in comparison with the phosphine ligand. The interatomic distances inside the P–Sn ligand do not show any significant variation, compared with the uncoordinated ligand. The average Sn–B distance might point to the amount of electron density transferred to the Pt substituent. Heteroborates with naked Sn vertices show the largest Sn–B average distance: 2 2.4021 Å, [SnB₁₁H₁₁]^2– 2.385 Å. In contrast, alkylated stanna-closo-dodecaborate clusters like [Me–SnB₁₁H₁₁]^– exhibit the shortest average Sn–B distance of 2.296 Å. Shorter Sn–B distances point towards a higher oxidation state of the tin. This was also manifested by various ¹¹⁹Sn Mössbauer measurements. In the case of complex 6, we found a Sn–B average of 2.3274 Å and this can be interpreted as a middle position of Sn-donor activity in the series of stanna-closo-dodecaborate derivatives.

NMR spectroscopic data obtained from measurements in solution confirm the solid-state structure of the anion of 6. In the ¹⁹⁵Pt{¹H} NMR spectrum, a resonance at δ = –4360 ppm, exhibiting coupling to one phosphorus atom, with a ¹J coupling constant of 3720 Hz, is detected. The chemical shift lies in the range known for Pt^II coordination compounds and the ¹J(³¹P–¹⁹⁵Pt) coupling is slightly larger than reported coupling constants.[12], [12], 13 The ¹¹⁹Sn{¹H} NMR spectroscopic signal found at δ = –493 ppm is very broad and exhibits ¹⁹⁵Pt satellites, with a coupling constant of 22232 Hz. The chemical shift of the ¹¹⁹Sn signal is typical for a transition-metal-substituted stanna-closo-dodecaborate cluster and the size of ¹J_Pt–Sn coupling constants is also known from other examples of this chemistry.[12] In the ³¹P{¹H} NMR spectrum, the connectivity of the coordination compound can be assigned. The signal for the platinum-coordinated diphenylphosphine moiety shows ¹⁹⁵Pt, ¹¹⁹Sn, and ¹¹⁷Sn satellites. Furthermore, the ³¹P{¹H} NMR spectroscopic signal of the P–B unit exhibits a quartet, due to coupling to the ¹¹B nucleus, with a ¹J_P–B = 125 Hz.

Apart from the coordination chemistry at platinum, we have also studied transition-metal complexes of the anion of 2 with rhodium fragments. On the basis of NMR spectroscopy, a rhodium coordination compound was quantitatively formed at room temperature in a reaction between the ligand salt 2 and the dinuclear rhodium carbonyl complex [RhCl(CO)₂]₂ (Scheme 6). However, the rhodium complex 7 is sensitive at room temperature in solution, and we found that destruction and formation of a new compound (vide infra) starts after a few minutes. Therefore, the neutral coordination compound 7 has to be stored at temperatures around –25 °C. After several weeks, analytically pure yellow crystals of rhodium complex 7 (Scheme 6) were obtained by slow diffusion of diethyl ether into a dichloromethane solution of the reaction product.

Scheme 6

Rh coordination of the bidentate P–Sn ligand.

Characterization was carried out by means of elemental analysis, NMR spectroscopy, and single-crystal X-ray diffraction structure analysis. The molecular structure of complex 7 is shown in Figure 6, and selected interatomic distances and angles are listed. Details of the structure solution are placed in the Supporting Information. As in the platinum complex, the P–Sn ligand is coordinated in a cis fashion at the transition metal, showing an Sn–Rh–P2 angle of 96.00(1)°. The Rh–Sn and Rh–P distances can be compared with chelating P–Sn ligands published in the literature.14 The average Sn–B distance of 2.335 Å lies in the middle of the stanna-closo-doecaborate derivatives characterized so far and is indicative of electron donation by the heteroborate moiety (vide supra). The carbonyl absorptions were detected in the IR spectrum at 2027 and 2082 cm^–1. These vibrations point toward less back-bonding character of the carbonyl ligands, and therefore, weak donation of the P–Sn ligand.[14], 15 A characteristic feature of the rhodium complex 7 in the ¹¹⁹Sn{¹H} NMR spectrum is a broad ¹¹⁹Sn NMR spectroscopic signal at δ = –359 ppm, indicating coordination of the tin ligand. Two signals in the ³¹P{¹H} NMR spectrum at δ = 19.8 ppm, which exhibits coupling with the ¹⁰³Rh nucleus of 119 Hz and the ^119/117Sn nuclei of 420 Hz, and a broad resonance at δ = 11.2 ppm for the B–P unit, are detected.

Figure 6

ORTEP of the molecular structure of complex 7 in the solid state. Ellipsoids are drawn at 50 % probability. Phenyl groups of the xantphos moiety, except for their ipso carbon atoms, are omitted for the sake of clarity. Selected interatomic distances [Å] and angles [°]: Rh–Sn 2.5927(2), Rh–P2 2.3519(5), Rh–C2 1.901(2), Rh–C1 1.892(2), C2–O2 1.124(3), C1–O1 1.129(3), Sn–B2 2.370(2), P1–B2 1.938(2), Sn–Rh–P2 96.000(14), Sn–Rh–C2 81.46(6), Sn–Rh–C1 171.68(6), P2–Rh–C2 169.81(7), P2–Rh–C1 92.29(6), C2–Rh–C1 90.25(9).

As already stated, solutions of compound 7 show decomposition of the complex at room temperature. Only a very small amount of the crystals of one decomposition product could be isolated, and they were only characterized by single-crystal X-ray diffraction structure analysis. Obviously, complex 7 loses a carbon monoxide ligand and dimer 8 (Scheme 7) is formed. In this dimer (Figure 7), one rhodium atom is coordinated by two heteroborates through the tin-donor site, with distances close to literature values for P–Sn ligands and complex 7.[14], 15, 16 The other rhodium atom is coordinated by two B–H units of a borate. This type of coordination is well known in boron-cluster coordination chemistry, and here, two B–H units act as a bidentate ligand.17 Both rhodium atoms are further coordinated in a cis arrangement by one carbonyl ligand and a phosphine moiety of the P–Sn ligand. The distances for these ligands are comparable to those in complex 7.

Scheme 7

Formation of a dinuclear rhodium complex.

Figure 7

ORTEP of the dimer of 8 in the solid state. Ellipsoids are drawn at 50 % probability. Phenyl groups of the xantphos moiety, except for their ipso carbon atoms, are omitted for the sake of clarity. Selected interatomic distances [Å] and angles [°]: Rh1–Sn1 2.6314(2), Rh1–Sn2 2.5625(2), Rh1–P1 2.2878(6), Rh1–C1 1.843(2), C1–O1 1.145(3), Rh2–P3 2.2373(6), Rh2–C2 1.835(2), C2–O2 1.139(3), Rh2–B23 2.399(1), Rh2–B27 2.348(1), P2–B2 1.936(3), P4–B22 1.953(2), Sn1–Rh1–Sn2 88.431(7), Sn1–Rh1–P1 96.691(15), Sn1–Rh1–C1 172.47(7), Sn2–Rh1–P1 171.291(17), Sn2–Rh1–C1 84.15(7), P1–Rh1–C1 90.83(7), P3–Rh2–C2 90.72(7).

Conclusion

Palladium acetate facilitates phosphino substitution of a B–H unit at the upper belt of stanna-closo-dodecaborate, in the proximity of the tin heteroatom. The reaction of stanna-closo-dodecaborate with xantphos results in a monoanionic substitution product. This substitution product features two donor sites, a triaryl phosphine moiety, and a stannylene-type tin atom inside an icosahedral closo-cluster. The phosphine moiety reacts with strong acids and is the more basic site, whereas the tin vertex forms a tin–carbon bond in reaction with methyliodide and is therefore the position of higher nucleophilicity. In reaction with transition-metal complexes, the P–Sn anion acts as a bidentate ligand, and in such complexes, it forms up to eleven-membered rings. The coordination compounds are still reactive and show labile coordination of the tin site at room temperature, as already known from experiments of the coordination chemistry of stanna-closo-dodecaborate.

Experimental Section

All manipulations were carried out under the exclusion of air and moisture in an argon atmosphere using standard Schlenk techniques. Solvents were purified by either a Solvent Purification System MBraun-SPS or a Solvent Purification System MBraun-SPS-800 equipped with alumina dry columns. [Bu₃HN]₂[SnB₁₁H₁₁] was synthesized by a modified protocol of Todd et al., while [Rh(µ-Cl)(CO)₂]₂ was synthesized following a literature procedure.8, 18 All other chemicals were purchased commercially from common chemical suppliers and were used as received.

NMR Spectroscopy: NMR spectra were recorded with a Bruker DRX-250 NMR spectrometer equipped with a 5 mm BBO ATM probe head and operating at 250.13 (¹H), 80.25 (¹¹B), 62.90 (¹³C), 101.23 (³¹P), and 93.25 MHz (¹¹⁹Sn), as well as a Bruker Avance AVII+ 500 NMR spectrometer equipped with a 5 mm PABBO ATM probe head and operating at 500.13 (¹H), 160.50 (¹¹B), 125.76 (¹³C), 202.46 (³¹P), 186.50 MHz (¹¹⁹Sn), and 107.50 MHz (¹⁹⁵Pt). Chemical shifts are reported in δ values in parts per million (ppm) relative to external SiMe₄ (¹H, Ξ = 100 %; ¹³C, Ξ = 25.145020 %), BF₃·Et₂O (¹¹B, Ξ = 32.083974 %), H₃PO₄ (³¹P, Ξ = 40.480747 %), SnMe₄ (¹¹⁹Sn, Ξ = 37.290632 %), and Na₂PtCl₆ (¹⁹⁵Pt, Ξ = 21.496784 %) using the chemical shift of the solvent ²H resonance frequency. The given temperatures are uncorrected.

IR Spectroscopy: The infrared spectra were recorded with a Bruker Vertex 70 spectrometer equipped with an attenuated total reflexion (ATR) unit.

X-ray Diffraction Crystallography: X-ray diffraction data for all compounds were collected with a Bruker Smart APEXII diffractometer with graphite monochromated Mo-K_α radiation. The software programs used, therefore, were Bruker′s APEX2 v2011.8–0, including SADABS for multiscan absorption correction and SAINT for structure solution, as well as the WinGX suite of programs v1.70.01, including SHELXL for structure refinement. Furthermore, the programs ORTEP-3 and PLATON for Windows Taskbar 1.17 were used for the analysis and validation of structures.19

CCDC 1546355 (for 6), 1546356 (for 3·CH₃CN), 1546357 (for 8·Et₂O·2DMF), 1546358 (for 2), 1546359 (for 5), and 1546360 (for 7·CH₂Cl₂) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre.

Elemental Analysis: Elemental analyses were performed at the Institute of Inorganic Chemistry of the Eberhard Karls Universität Tübingen with a Vario EL analyzer or a Vario MICRO EL from Elementar Co. in the CHNS mode.

Synthesis

[Bu₃HN][2-(xantphos)-SnB₁₁H₁₀] (2): Under an argon atmosphere, an autoclave with a teflon inlet was charged with [Bu₃HN]₂[SnB₁₁H₁₁] (4.0 g, 6.4 mmol), xantphos (3.7 g, 6.4 mmol), and Pd(OAc)₂ (0.3 g, 1.4 mmol). MeCN (320 mL) was added. The reaction was stirred at 120 °C internal temperature and 2–5 bar internal pressure for 40 h. Thereafter, it was cooled to room temperature. The resulting dark suspension was filtered and the solvent was removed in vacuo. The residue was redissolved in CHCl₃ and purified by column chromatography on silica gel using CHCl₃ and CHCl₃/MeCN (4:1) or CHCl₃/EtOAc (4:1) as the eluant. By diffusion of Et₂O into a MeCN solution, 2.3 g (2.3 mmol, 35 %) of the pure product were obtained as off-white crystals. The [Et₃HN] salt was prepared accordingly. ¹H NMR (500 MHz, CD₃CN, 26 °C): δ = 9.2–9.1 [m, 1 H, H(3)], 7.9 [m, 1 H, H(1)], 7.8–7.7 [m, 4 H, H(14), H(18), H(20), H(24)], 7.5 [m, 1 H, H(8)], 7.4 [m, 1 H, H(2)], 7.3 [m, 2 H, H(28), H(34)], 7.2 [m, 4 H, H(27), H(29), H(33), H(35)], 7.1 [m, 2 H, H(16), H(22)], 7.1–7.0 [m, 4 H, H(15), H(17), H(21), H(23)], 6.9 [m, 1 H, H(7)], 6.7 [m, 4 H, H(26), H(30), H(32), H(36)], 6.5 [m, 1 H, H(6)], 3.1–3.0 (m, 6 H, N-CH₂), 1.7–1.6 (m, 6 H, N-CH₂-CH₂), 1.57 [s, 6 H, H(11a-11c), H(12a-12c)], 1.4–1.3 (m, 6 H, CH₂-CH₃), 0.95 [t, ³J(¹H–¹H) = 7.4 Hz, 9 H, CH₂-CH₃] ppm. ¹H{¹¹B} NMR (500 MHz, CD₃CN, 26 °C): δ = 9.2–9.1 [m, 1 H, H(3)], 7.9 [m, 1 H, H(1)], 7.8–7.7 [m, 4 H, H(14), H(18), H(20), H(24)], 7.5 [m, 1 H, H(8)], 7.4 [m, 1 H, H(2)], 7.3 [m, 2 H, H(28), H(34)], 7.2 [m, 4 H, H(27), H(29), H(33), H(35)], 7.1 [m, 2 H, H(16), H(22)], 7.1–7.0 [m, 4 H, H(15), H(17), H(21), H(23)], 6.9 [m, 1 H, H(7)], 6.7 [m, 4 H, H(26), H(30), H(32), H(36)], 6.5 [m, 1 H, H(6)], 3.2 [s, vbr, 1 H, B(12)-H] 3.1–3. 0 (m, 6 H, N-CH₂), 2.1 [s, vbr, 1 H, B(10)-H], 1.9 [2, vbr, 4 H, B(7)-H, B(8)-H, B(9)-H, B(11)-H] 1.7–1.6 (m, 6 H, N-CH₂-CH₂), 1.57 [s, 6 H, H(11a-11c), H(12a-12c)], 1.4–1.3 (m, 6 H, CH₂-CH₃), 1.3 [s, vbr, 2 H, B(4)-H, B(5)-H], 1.0 [s, vbr, 2 H, B(3)-H, B(6)-H] 0.95 (t, 9 H, CH₂-CH₃) ppm. ¹¹B{¹H} NMR (161 MHz, CD₃CN, 26 °C): δ = –4.3 [br. s, 1 B, B(12)], –6.5 [br. s, 1 B, B(10)], –8.0 [br. s, 2 B, B(4), B(5)], –8.9 [br. s, 2 B, B(3), B(6)], –10.5 [br. s, 3 B, B(2), B(9), B(11)], –11.7 [br. s, 2 B, B(7), B(8)] ppm. ¹³C{¹H} NMR (126 MHz, CD₃CN, 26 °C): δ = 153.7 [d, ²J(³¹P–¹³C) = 22 Hz, C(10a)], 153.4 [br. m, C(4a)], 139.5 [d, ¹J(³¹P–¹³C) = 19 Hz, C(13), C(19)], 139.0 [br. m, C(3)], 135.7 [m, C(6)], 134.0 [br. m, C(26), C(30), C(32), C(36)], 133.9 [m, C(14), C(18), C(20), C(24)], 132.3 [br. m, C(1), C(9a)], 131.5 [m, C(28), C(34)], 131.4 [m, C(8a)], 129.8 [d, ¹J(³¹P–¹³C) = 69 Hz, C(5)], 129.2 [d, ³J(³¹P–¹³C) = 6 Hz, C(15), C(17), C(21), C(23)], 128.9 [br. s, C(16), C(22)], 128.6 [br. m, C(8), C(27), C(29), C(33), C(35)], 125.3 [d, ¹J(³¹P–¹³C) = 23 Hz, C(25), C(31)], 125.1 [s, C(7)], 123.9 [br. d, ³J(³¹P–¹³C) = 13 Hz, C(2)], 115.0 [d, br, ¹J(³¹P–¹³C) = 64 Hz, C(4)], 54.1 (s, N-CH₂), 35.4 [s, C(9)], 32.2 [s, C(11), C(12)], 26.3 (s, N-CH₂-CH₂), 20.5 (s, CH₂-CH₃), 13.9 (s, CH₃) ppm. ³¹P{¹H} NMR (203 MHz, CD₃CN, 26 °C): δ = 4.7 (m, vbr, 1 P, XantPh₂P-B), –23.6 (s, 1 P, XantPh₂P) ppm. ¹¹⁹Sn{¹H} NMR (187 MHz, CD₃CN, 26 °C): δ = –524 (s, vbr) ppm. C₅₁H₇₀B₁₁NOP₂Sn (1012.67 g/mol): calcd. C 60.49, H 6.97, N 1.38; found C 60.40, H 7.05, N 1.61.

2-(H-xantphos)-SnB₁₁H₁₀ (3): [Bu₃HN][2-(xantphos)-SnB₁₁H₁₀] (15 mg, 0.02 mmol) was dissolved in DCM (0.5 mL). Thereafter HCl (0.1 mL, 0.1 mmol) in 1,4-dioxane (1 m) was added and the mixture was shaken for 10 seconds. Immediately, a white precipitate was formed, which was filtered, washed with 1 mL DCM, and dried in vacuo. The product was obtained as a colorless powder, with a yield of 9 mg (0.01 mmol, 72 %). Crystals were grown by diffusion of HCl (1m solution in dioxane) into a solution of [Bu₃HN][2-(xantphos)-SnB₁₁H₁₀] in MeCN. ¹H NMR (500 MHz, CD₂Cl₂, 26 °C): δ = 9.04 [d, ¹J(³¹P–¹H) = 547 Hz, 1 H, XantPh₂P-H], 8.0–7.0 (br. m, 26 H, H_ar), 1.87 [s, 6 H, C(CH₃)₂] ppm. ¹¹B{¹H} NMR (161 MHz, CD₂Cl₂, 26 °C): δ = –4.3 [br. s, 1 B, B(12)], –5 to –14 [br. m, 10 B, B(2)-B(11)] ppm. ³¹P{¹H} NMR (203 MHz, CD₂Cl₂, 26 °C): δ = 11.9 (br. m, 1 P, XantPh₂P-B), –12.4 (s, 1 P, XantPh₂P-H) ppm. ³¹P NMR (203 MHz, CD₂Cl₂, 26 °C): δ = 11.9 (br. m, 1 P, XantPh₂P-B), –12.4 [dm, br, ¹J(³¹P–¹H) = 548 Hz, 1 P, XantPh₂P-H] ppm. ¹¹⁹Sn{¹H} NMR (187 MHz, CD₂Cl₂, 26 °C): δ = –522 (s, vbr) ppm. C₃₉H₄₃B₁₁OP₂Sn (827.32 g/mol): calcd. C 56.62, H 5.24; found C 55.99, H 5.19.

1-(Me)-2-(xantphos)-SnB₁₁H₁₀ (4): [Et₃HN][2-(xantphos)-SnB₁₁H₁₀] (25 mg, 0.03 mmol) was dissolved in DCM (15 mL). Ten drops of MeI (excess) were added. The reaction mixture was stirred for 5 h at room temperature. Thereafter, water (10 mL) was added and the mixture was shaken for 1 min. The phases were separated and the organic phase was evaporated to dryness. Ultimately, 10 mg (0.01 mmol, 44 %) of the product were obtained as an off-white powder. ¹H NMR (250 MHz, CD₃CN, 26 °C): δ = 8.7–8.6 [m, 1 H, ortho-H(Xant)], 8.1 [m, 1 H, para-H(Xant)], 7.8–7.7 [m, 4 H, ortho-H(Ph)], 7.6–7.5 [m, 2 H, para-H(Xant), meta-H(Xant)], 7.3–7.1 [m, 12 H, para-H(Ph), meta-H(Ph)], 7.0 [m, 1 H, meta-H(Xant)], 6.7–6.6 [m, 4 H, ortho-H(Ph)], 6.5 [m, 1 H, ortho-H(Xant)], 1.66 [s, 6 H, C(CH₃)₂], 1.17 [s, ²J(¹¹⁹Sn–¹H) = 101.2, ²J(¹¹⁷Sn–¹H) = 96.8 Hz, 3 H, Sn-CH₃] ppm. ¹¹B{¹H} NMR (80 MHz, CD₂Cl₂, 26 °C): δ = –11.1 (br. s, 2 B), –12 to –18 (br. m, 9 B) ppm. ³¹P{¹H} NMR (101 MHz, CD₂Cl₂, 26 °C): δ = 1.4 [q, br, ¹J(³¹P–¹¹B) = 130 Hz, 1 P, XantPh₂P-B], –23.4 [s, ⁸J(^119/117Sn–³¹P) = 18 Hz, 1 P, XantPh₂P] ppm. ¹¹⁹Sn{¹H} NMR (93 MHz, CD₂Cl₂, 26 °C): δ = –197 (s, vbr) ppm.

1-I-1,2-(xantphos)-SnB₁₁H₁₀ (5): [Bu₃HN][2-(xantphos)-SnB₁₁H₁₀] (95 mg, 0.09 mmol) was dissolved in DCM (10 mL). A solution of I₂ (25 mg, 0.10 mmol) in DCM (5 mL) was added in one portion. The intensely red solution was stirred at room temperature for one minute. It was, thereafter, layered with 25 mL Et₂O and stored at –25 °C. After two months, 80 mg (0.08 mmol, 89 %) of red crystals were collected, washed, and dried under reduced pressure. C₃₉H₄₂B₁₁IOP₂Sn (953.21 g/mol): calcd. C 49.14, H 4.44; found C 49.14, H 4.06.

[R₃HN][Pt(SnBXP)Cl₂] (6): Route a) A suspension of K₂PtCl₄ (8 mg, 0.02 mmol) in MeCN (5 mL) was added to a solution of [Bu₃HN][2-(xantphos)-SnB₁₁H₁₀] (19 mg, 0.02 mmol) in DCM (5 mL). The mixture was heated to 120 °C in a closed vessel for one hour. The light-yellow solution was cooled to room temperature, and the solvents were removed in vacuo. The off-white residue was dissolved in MeCN (0.5 mL) and layered with Et₂O. After 4 d, a small amount of a light-brown precipitate was isolated and dried under reduced pressure. ¹H NMR (500 MHz, CD₃CN, 26 °C): δ = 8.0–6.5 (m, 26 H, H_ar), 3.1–3.0 (m, 6 H, N-CH₂), 1.74 [s, 3 H, (CH₃)C(CH₃)], 1.7 (m, 6 H, N-CH₂-CH₂), 1.4 [m, 9 H, CH₂-CH₃, (CH₃)C(CH₃)], 0.95 [t, ³J(¹H–¹H) = 7.4 Hz, 9 H, CH₂-CH₃] ppm. ¹H{¹¹B} NMR (500 MHz, CD₃CN, 26 °C): δ = 8.0–6.5 (m, 26 H, H_ar), 3.1–3.0 (m, 6 H, N-CH₂), 2.8 (s, vbr, 1 H, B-H), 1.8–1.7 (m, vbr, 2 H, B-H), 1.74 [s, 3 H, (CH₃)C(CH₃)], 1.7 (m, 6 H, N-CH₂-CH₂), 1.5 (s, vbr, 4 H, B-H), 1.4 [m, 9 H CH₂-CH₃, (CH₃)C(CH₃)], 1.3 (s, vbr, 1 H, B-H), 0.95 (t, 9 H, CH₃), 0.9 (s, vbr, 1 H, B-H), 0.6 (s, vbr, 1 H, B-H) ppm. ¹¹B{¹H} NMR (161 MHz, CD₃CN, 26 °C): δ = –10 to –18 [br. m, 11 B, B(2)-B(12)] ppm. ³¹P{¹H} NMR (203 MHz, CD₃CN, 26 °C): δ = 10.0 [q, br, ¹J(³¹P–¹¹B) = 125 Hz, 1 P, XantPh₂P-B], –4.7 [s, ¹J(¹⁹⁵Pt–³¹P) = 3710, ²J(¹¹⁹Sn–³¹P) = 263, ²J(¹¹⁷Sn–³¹P) = 254, ²J(¹¹⁵Sn–³¹P) = 223, ¹J(³¹P–¹³C) = 63 Hz, 1 P, XantPh₂P-Pt] ppm. ¹¹⁹Sn{¹H} NMR (187 MHz, CD₃CN, 26 °C): δ = –493 [s, vbr, ¹J(¹⁹⁵Pt–¹¹⁹Sn) = 22232 Hz] ppm. ¹⁹⁵Pt{¹H} NMR (108 MHz, CD₃CN, 26 °C): δ = –4360 [br. d, ¹J(¹⁹⁵Pt–³¹P) = 3720 Hz] ppm. Route b) [Et₃HN][2-(xantphos)-SnB₁₁H₁₀] (21 mg, 0.02 mmol) and PtCl₂ (6 mg, 0.02 mmol) were suspended in MeCN (10 mL). The reaction mixture was heated to 120 °C in a closed vessel for 30 min. The light-yellow solution was cooled to room temperature, and Et₂O was added by slow diffusion. After two weeks, 12 mg (0.005 mmol, 44 %) of light-brown crystals were isolated, washed with Et₂O, and dried under reduced pressure. ¹H NMR (250 MHz, CD₂Cl₂, 26 °C): δ = 8.0–6.5 (m, 26 H, H_ar), 3.2–3.1 [q, ³J(¹H–¹H) = 7.3 Hz, 6 H, N-CH₂], 1.74 [s, 3 H, (CH₃)C(CH₃)], 1.40 [s, 3 H, (CH₃)C(CH₃)], 1.28 [t, ³J(¹H–¹H) = 7.3 Hz, 9 H, N-CH₂-CH₃] ppm. ¹¹B{¹H} NMR (80 MHz, CD₂Cl₂, 26 °C): δ = –9 to –19 [br. m, 11 B, B(2)-B(12)] ppm. ³¹P{¹H} NMR (101 MHz, CD₂Cl₂, 26 °C): δ = 9.9 (m, 1 P, XantPh₂P-B), –4.6 (s, 1 P, XantPh₂P-Pt) ppm. C₄₅H₅₉B₁₁Cl₂NOP₂PtSn (1195.51 g/mol): calcd. C 45.21, H 4.97, N 1.17; found C 45.30, H 4.38, N 1.57.

[Rh(SnBXP)(CO)₂] (7): [Bu₃HN][2-(xantphos)-SnB₁₁H₁₀] (60 mg, 0.06 mmol) and [Rh(µ-Cl)(CO)₂]₂ (23 mg, 0.06 mmol) were dissolved in DCM (15 mL). The mixture was stirred for one minute to obtain a clear yellow solution. It was layered with Et₂O and stored at –25 °C. After two weeks, 19 mg (0.02 mmol, 32 %) of the product were obtained as yellow crystals. ¹H NMR (250 MHz, CD₂Cl₂, 26 °C): δ = 7.9–6.4 (m, 26 H, H_ar), 1.87 [s, 3 H, (CH₃)C(CH₃)], 1.59 [s, 3 H, (CH₃)C(CH₃)] ppm. ¹¹B{¹H} NMR (80 MHz, CD₂Cl₂, 26 °C): δ = –5 to –20 [br. m, 11 B, B(2)-B(12)] ppm. ³¹P{¹H} NMR (101 MHz, CD₂Cl₂, 26 °C): δ = 19.8 [d, ¹J(¹⁰³Rh–³¹P) = 119, ²J(cis–^119/117Sn–³¹P) = 420 Hz, 1 P, XantPh₂P-Rh], 11.2 (br. m, 1 P, XantPh₂P-B) ppm. ¹¹⁹Sn{¹H} NMR (93 MHz, CD₂Cl₂, 26 °C): δ = –359 (s, vbr) ppm. IR (KBr): ν̃ = 3098 (w, arom. C–H), 2954 (w, arom. C–H), 2849 (w, arom. C–H), 2517 (vst, B–H), 2082 (vst, CO), 2027 (vst, CO), 1486 (w), 1437 (st), 1402 (vst), 1219 (st), 1095 (w), 1013 (w), 743 (st), 694 (st), 542 (w), 514 (st), 478 (w) cm^–1. C₄₁H₄₂B₁₁O₃P₂RhSn (985.23 g/mol): calcd. C 49.98, H 4.30; found C 50.06, H 4.25.