Jian Xua,
Shenglai Yaoa,
Verònica Postils
b,
Eduard Matito
cd,
Christian Lorent
e and
Matthias Driess
*a
aMetalorganic and Inorganic Materials, Department of Chemistry, Technische Universität Berlin, 10623 Berlin, Germany. E-mail: matthias.driess@tu-berlin.de
bTheoretical Chemistry Group, Molecular Chemistry, Materials and Catalysis Division (MOST), Institute of Condensed Matter and Nanosciences, Université Catholique de Louvain, Place Louis Pasteur 1, B-1348 Louvain-la-Neuve, Belgium
cDonostia International Physics Center (DIPC), 20018 Donostia, Euskadi, Spain
dIkerbasque Foundation for Science, Plaza Euskadi 5, 48009 Bilbao, Euskadi, Spain
ePhysical and Biophysical Chemistry, Department of Chemistry, Technische Universität Berlin, 10623 Berlin, Germany
First published on 19th May 2025
Utilizing the chelating bis(silylenyl)carborane [SiII(closo-CB)SiII] (A, SiII = PhC(NtBu)2Si, CB = o-C2B10H10) ligand, a series of unprecedented bis(silylene)-stabilized monovalent bismuth complexes {[SiII(closo-CB)SiII]Bi}X (X = I, 1a; X = OTf, 1b), {[SiII(nido-CB)SiII]Bi} (2) and ({[SiII(nido-CB)SiII]Bi}K(thf)2)2 ([3K(thf)2]2) were synthesized, isolated and characterized. The electronic structures of the bismuth complexes are significantly influenced by the redox-active nature of the CB scaffold. Remarkably, a one-electron injection to 1b with KC8 does not furnish a Bi0 complex but reduction of the CB backbone giving rise to the neutral BiI radical complex 2. Notably, compound 1b can also undergo a two-electron reduction with two molar equiv. of potassium naphthalenide, resulting in the formation of the diamagnetic BiI anion complex 3 as a dimer bridged via two K(thf)2 cations. Density functional theory calculations reveal that upon reduction from 1a to 2, and 2 to 3, the added electron predominantly localizes within the carborane cage, with a marked preference for the carbon atoms, ruling out that these species exhibit characteristics of a molecular bismuth(0) electride.
Monoatomic zero-valent complexes of Group 14 elements, known as tetrylones, adopt the general formula L: → E0 ←: L (E = C, Si, Ge, Sn, Pb; L = Lewis donor ligands).17–21 Mono-valent cationic bismuth species can be regarded as the heaviest isoelectronic analogs of tetrylones, that is plumbylones, possessing two lone pairs on the central atom.22,23 The first cationic BiI compound (I, Fig. 1)24 was synthesized by reducing BiCl3 in the presence of cyclic alkyl(amino)carbenes (cAACs). Subsequently, the bis(silylene)-supported BiI cation complex [{SiII(TBD)SiII}BiI][BArF4] II (ref. 25) (SiII = PhC(NtBu)2Si, TBD = 1,8,10,9-triazaboradecalin, ArF = 3,5-(CF3)2-C6H3) was prepared through a one-pot reaction between the bis(silylene) {SiII(TBD)SiII} ligand and [IPr → BiBr3] (IPr = 1,3-bis(2,6-diisopropylphenyl)-imidazole-2-ylidene), followed by treatment with Na[BArF4] in THF at −30 °C and reduction with two molar equiv. of potassium graphite (KC8). Recently, our group reported the bis(germylene)-supported cationic BiI complex III.26 Due to the redox non-innocent nature of the germylene moieties, the positive charge of the BiI cation migrates to one of the Ge atoms within the bis(germylene) ligand, resulting in the chelating germylium–germylene BiI complex III.
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Fig. 1 Known examples of cationic BiI complexes I–III and the BiI species 1–3 of this work bearing a redox non-innocent carborane cage. |
In recent years, our group has successfully designed and synthesized several chelating bis(NHSi) ligands (NHSi = N-heterocyclic silylene) featuring electronically and geometrically diverse spacers.27 These bis(NHSi) ligands have facilitated the isolation of various low-valent main-group compounds, including zero-valent group 14 and mono-valent group 15 complexes, which exhibit fascinating electronic structures and unique chemical reactivities.28–35 The bis(silylenyl)-o-carborane ligand [SiII(closo-CB)SiII] (A, SiII = PhC(NtBu)2Si, CB = o-C2B10H10)36 (Scheme 1), featuring a relatively short SiII⋯SiII distance of approximately 3.3 Å, was first reported by our group in 2016. The latter ligand acts as a strong chelating Lewis donor due to the silylene moieties and exhibits interesting redox non-innocence attributed to the carborane spacer. These properties have proven effective in stabilizing monoatomic Si0 and Ge0 complexes33,34 as well as containing the isoelectronic NI cation.35 Herein, we report the synthesis and characterization of the BiI cation complexes {[SiII(closo-CB)SiII]Bi}X (X = I, 1a; X = OTf, 1b) supported by the bis(silylenyl)-o-carborane A. Strikingly, the one-electron and two-electron reductions of 1b using KC8 and KC10H8, respectively, yield no Bi0 species but the neutral and anionic BiI complexes {[SiII(nido-CB)SiII]Bi} 2 and ({[SiII(nido-CB)SiII]Bi}K(thf)2)2 ([3K(thf)2]2), both featuring a nido-C2B10 core, yet in different reduced states. The electronic structures of this series of BiI complexes are further elucidated through Density Functional Theory (DFT) calculations.
The molecular structures of 1a and 1b were determined by single-crystal X-ray diffraction (scXRD) analysis. Both exhibit a discrete ionic structure with a similar five-membered C2Si2Bi ring in the cation, where the central BiI site is coordinated to two silicon atoms. The Si–Bi bond lengths range from 2.5774(6) to 2.5958(9) Å (Fig. 2), similar to those in the [{SiII(TBD)SiII}BiI][BArF4] complex II (Fig. 1, 2.557(1) and 2.561(8) Å).25 The Si–C bond lengths span from 1.923(2) to 1.937(4) Å, while the C1–C2 distances in 1a and 1b [1.691(5) and 1.692(3) Å] are very close to that in A (1.71 Å).36 Notably, the Si1–Bi1–Si2 angles of 79.28(3)° and 79.486(18)° in 1a and 1b are slightly more acute than in complex II [82.10(3)°].25
Notably, the cyclic voltammogram (CV) of 1b exhibits two quasi-reversible reduction processes at E1/2 = −1.36 V and −1.68 V vs. Fc/Fc+ (see ESI Fig. S11†). This prompted us to explore its chemical reduction. Upon mixing 1b with one equivalent of KC8 in THF at −30 °C, a deep-red solution formed, from which complex 2 was isolated in 43% yield (Scheme 2).
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Scheme 2 Reversible redox reactions between 1b, 2 (with its resonance structure 2′) and [3K(thf)2]2. |
An scXRD analysis revealed that compound 2 crystallizes as a neutral BiI radical complex in the orthorhombic space group Cmcm. Its molecular structure features an open-cage nido-carborane backbone with a C⋯C distance of 2.268 Å (Fig. 3). The Si1–Bi1 bond length of 2.576(2) Å is comparable to those in complexes 1a and 1b (2.5774(6)–2.5958(9) Å). However, the Si1–C1 bond in complex 2 (1.851(8) Å) is significantly shorter than in 1a and 1b (1.923(2)–1.937(4) Å). Notably, the Si1–Bi1–Si1a bond angle in complex 2 (86.25(8)°) is substantially larger than in 1a and 1b (∼79°), likely due to the open-cage nature of the carborane backbone.
Compound 2 is paramagnetic and shows broad resonance peaks in solution 1H NMR spectra at room temperature (see ESI, Fig. S13†). Accordingly, the electron paramagnetic resonance (EPR) spectrum of 2 recorded at room temperature in THF (Fig. 4) exhibits an isotropic signal at g = 2.0229 (line width = 29.1 G). Though the band shape is very similar to the one of the known NI anionic carborane radical {[SiII(nido-CB)SiII]NI},35 the g-value is slightly higher than those typical for such organic radicals. Magnetic interactions with the nearby very heavy Bi nucleus could be the origin. An unpaired spin located at the Bi itself can be excluded based on the EPR spectrum at 10 K (see ESI, Fig. S14†), which lacks the characteristic broad and multiline features of a Bi-centred radical (Bi0).26 Overall, the observations indicate that the unpaired electron is localized in the carborane cage (see electronic structure discussion below).
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Fig. 4 EPR spectrum of compound 2 in THF (top) recorded at 293 K and the corresponding simulation (bottom). The g-value of the radical species is 2.023 and the line width 29.1 G. |
To investigate the reversibility of the latter one-electron reduction, compound 2 was allowed to react with equimolar amount of AgOTf in THF at room temperature. Indeed, compound 1b was quantitatively regenerated after stirring for 10 minutes (Scheme 2). Given the carborane moiety of bis(NHSi) can store one or two electrons, we further explored the two-electron reduction of 1b. The reaction of 1b with two molar equiv. of potassium naphthalenide (KC10H8) in THF at −30 °C yielded complex ({[SiII(nido-CB)SiII]Bi}K(thf)2)2 ([3K(thf)2]2) as a brown powder in 51% yield (Scheme 2). Its 1H NMR spectrum in THF-d8 displays one singlet at δ = 1.29 ppm for the tert-butyl groups, while the 29Si{1H} NMR spectrum exhibits a singlet at δ = 51.8 ppm, significantly upfield-shifted compared to 1a (δ = 68.7 ppm) and 1b (δ = 66.9 ppm).
An scXRD analysis reveals that ([3K(thf)2]2) adopts a dimeric structure in the solid state, with two [K(thf)2]+ moieties acting as linkers via B–H⋯K⋯H–B interactions (Fig. 5). The Si–Bi bond lengths in ([3K(thf)2]2) (2.6266(13) and 2.6138(13) Å) are slightly longer than those in 1a and 1b (2.5774(6)–2.5958(9) Å) and in 2 (2.576(2) Å). Additionally, the Si–Bi1–Si bond angle in ([3K(thf)2]2) (90.27(4)°) is larger than in 1a and 1b (∼79°) and 2 (86.25(8)°). In line with that, the carborane backbone in ([3K(thf)2]2) features a C⋯C distance of 2.576 Å, notably longer than that in 2 (2.268 Å), indicating a further reduced carborane cage. Furthermore, the Si–C distances in ([3K(thf)2]2) (1.786(5) and 1.789(5) Å) are significantly shorter than those in 1a and 1b (1.923(2)–1.937(4) Å) and in 2 (1.851(8) Å). These structural features confirm that the open cage in ([3K(thf)2]2) corresponds to a nido-carborane anion. It should be mentioned that complex 2 can also be synthesized via a metathesis reaction between 1b and ([3K(thf)2]2) in THF at room temperature. In addition, complex ([3K(thf)2]2) can be obtained through the one-electron reduction of complex 2 with KC8 in THF (Scheme 2). This indirectly confirms the redox reversibility between the family members of this BiI series.
The reactivity of 1b towards methyl trifluoromethanesulfonate (MeOTf) was also investigated to evaluate the nucleophilic character of BiI. Upon addition of MeOTf at 40 °C in DCM, the yellow solution of 1b gradually decolorized over 4 hours, yielding colorless {[SiII(closo-CB)SiII]BiMe} [OTf]2 (4) in 61% yield after workup (Scheme 3). The 1H NMR spectrum of 4 in DCM-d2 displays a singlet at δ = 2.47 ppm for the methyl group,37 downfield-shifted compared to free BiMe3 (δ = 1.11 ppm).37a The 29Si{1H} NMR spectrum exhibits a singlet at δ = 62.0 ppm, significantly upfield-shifted compared to 1a (δ = 68.7 ppm) and 1b (δ = 66.9 ppm) but downfield-shifted relative to ([3K(thf)2]2) (δ = 51.8 ppm). The 19F NMR spectrum shows a singlet at δ = −78.7 ppm. Notably, treatment of 4 with PMe3 in DCM at room temperature quantitatively regenerated 1b immediately along with the formation of PMe4OTf.
An scXRD analysis revealed that 4 crystallizes in the monoclinic space group P21/n. The dication in 4 features a five-membered C2Si2Bi ring, with a methyl group and two triflate anions coordinated to the bismuth atom (Fig. 6). The bismuth center thus adopts a distorted tetragonal pyramidal geometry (τ5 = 0.25),37b with the methyl group occupying the apical position, suggesting the presence of a lone electron pair opposite the tetragonal plane. The Si–Bi bond lengths in 4 (2.7003(17) and 2.7160(19) Å) are significantly longer than those observed in complexes 1–3 (1: 2.5774(6)–2.5958(9) Å; 2: 2.576(2) Å; 3: 2.6266(13) and 2.6138(13) Å). The Bi1–C3 bond length of 2.278(9) Å in 4 is comparable to that in the [BiMe]2+ complexes derived from II (2.300 Å)25 and III (2.247(7) Å).26 Notably, the shortest Bi⋯O interaction between the bismuth center and the two triflate anions in 4 is 3.025 Å, indicating weak coordination between the bismuth center and the triflate anions.
The skeleton of 1a, 2, and 3 is divided into four parts: the Bi center, the two amidinato silylene units, and the carborane cage. The amidinato silylene units barely change upon reduction from 1a to 2, and 2 to 3, and their data is only included in the ESI† for the sake of completion. Fig. 7 and Table 1 contain data concerning the electron distribution of the relevant parts of structures 1a, 2, and 3.
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Fig. 7 Evolution of selected QTAIM atomic charges (in black) and delocalization indices (shaded and blue) of 1a, 2 and 3 structures upon reduction. |
We focus on the Bi center, the two carbon atoms in the carborane cage, and the rest of the cage (BH cage, hereafter). 1a exhibits a regular 2c–2e bond (DI = 1.01) between Bi and Si (which is essentially maintained after reduction to 2 and 3), while the bonds between Si and the C atoms in the carborane cage are partially covalent (DI = 0.49) with a high ionic component (Si holds a +2.06 charge, whereas C has a large negative charge of −1.75e). 1a also exhibits a C–C covalent bond (DI = 0.97) within the carborane cage.
Upon reduction of 1a to 2, the most significant change is the reduction of the C–C bonding interaction (DI = 0.38), which further decreases upon reduction to 3 (DI = 0.22). This C–C weakening is caused by the opening of the cage at the top, as reflected in the increase of the C–C bond distance from 1.69 Å in 1a to 2.27 Å in 2, and subsequently to 2.58 Å in 3. The cage opening is accompanied by a slight increase of the covalent bond orders between C and the neighboring B atoms (see ESI, Fig. S31†), and a slight increase of the Si–C bond strength, which is reflected by the higher covalent character (DI = 0.56 in 2, DI = 0.63 in 3). This electron reorganization is also reflected in the picture of the Laplacian of the electron density given in Fig. S32 and S33,† and the number of electrons localized in the C atoms (see ESI, Fig. S31†).
Hence, upon reduction from 1a to 2, the extra electron mostly localizes in the carborane cage, especially in the carbon atoms, as the BH cage actually loses some electron density upon reduction. The Bi atomic charge also increases by 0.4e. Based on the calculated partial charge distribution, resonance structure 2′ (Scheme 2) is proposed for compound 2, in which the negative charges are delocalized over the bismuth atom and the carborane cage, while the two positive charges are distributed over the two silicon atoms. Upon reduction from 2 to 3, 0.26 additional electrons localize in each C atom, whereas the other half electron is split between the Bi, which has now −0.45 electrons, and the BH cage, which restores the +2 charge it had in 1a.
The analysis of the molecular orbitals aligns well with the conclusions drawn thus far (see Fig. 8). The SOMO of the BiI compound 2 shows a large fraction of the electron density of the unpaired electron on the C atoms of the carborane (Fig. 8). This stabilized σ* orbital is reminiscent of the NI radical {[SiII(closo-CB)SiII]NI} supported by the same bis(silylenyl) carborane ligand35 and the diphenyl-o-carborane system reported by Adillon et al.44
Footnote |
† Electronic supplementary information (ESI) available. CCDC 2428091–2428095. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d5sc02644j |
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