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A planar per-borylated digermene

Xiongfei Zheng , Agamemnon E. Crumpton , Mathias A. Ellwanger and Simon Aldridge *
Inorganic Chemistry Laboratory, Department of Chemistry, University of Oxford, South Parks Road, Oxford, OX1 3QR, UK. E-mail: simon.aldridge@chem.ox.ac.uk

Received 15th October 2023 , Accepted 31st October 2023

First published on 1st November 2023


Abstract

A tetraboryl digermene synthesized by the reaction between a dianionic digermanide nucleophile and a boron halide electrophile is dimeric both in the solid state and in hydrocarbon solution. It features both a planar ‘alkene-like’ geometry for the Ge2B4 core, and an exceptionally short Ge[double bond, length as m-dash]Ge double bond. These structural features are consistent with the known electronic properties of the boryl group, and with lowest energy (in silico) fragmentation into two triplet bis(boryl)germylene fragments.


Carbenes and their heavier group 14 analogues (tetrylenes) have received considerable recent attention, not least because of their ability to activate small molecules such as H2, CO and alkenes in ‘transition-metal like’ fashion.1,2 The ambiphilic nature of two-coordinate systems of the type EX2 (E = C–Pb), and the availability of frontier orbitals of both σ and π symmetry allow the possibility for synergistic interactions with small molecules which have both donor and acceptor properties.3 As with d-block systems, the extent of such inter-actions is influenced significantly by the energies of the frontier orbitals, which in turn reflect factors such as the identity of the group 14 element E, the X–E–X angle, and the electronic properties of the X-substituents. X groups featuring highly electronegative donor atoms and/or strong π-donor capabilities tend to lead to a stabilised σ symmetry orbital (typically the HOMO), a destabilised π orbital (often the LUMO, or an orbital close to it), and consequently a wide HOMO–LUMO energy gap.3 On the other hand, boryl substituents (i.e. X = BR2) offer the opposite scenario – namely a narrow HOMO–LUMO gap, brought about by the electropositive and non π-donor nature of the donor atom.4 As such, boryl-substituted tetrylenes have shown unusually high levels of reactivity in the activation of molecules, being implicated in the first examples of both silylenes and stannylenes to oxidatively add H2 (I-Si and II-Sn; Fig. 1).5,6
image file: d3dt03416j-f1.tif
Fig. 1 Boryl-substituted tetrylene and related systems of relevance to the current study.

Steric factors are also important in tetrylene systems – not only in influencing the magnitude of the X–E–X angle, but also their tendency towards dimerization to give ditetrelenes, X2EEX2.7 These heavier alkene analogues typically adopt trans-bent C2h skeletal geometries, which can formally be regarded as being constructed by the dimerization of two singlet EX2 fragments (which become increasing stabilized with respect to the corresponding triplet state on descending group 14).8

Given the heightened reactivity shown by bis(boryl)-stannylene II, and the accessibility of digermavinylidene III,9 we were interested here to explore strategies for the synthesis of the corresponding bis(boryl)germylene, II-Ge (Fig. 1). In the event, the combination of an energetically accessible triplet state for the Ge(boryl)2 fragment, allied to steric limitations in the boryl groups available synthetically, led to the isolation of the corresponding (planar) tetraboryl-digermene.

While the germanium (and silicon) analogues of the heteroleptic (amido)(boryl)stannylene Sn{N(SiMe3)Dipp}(boryl) (I-Sn), are readily accessible,5 attempts to synthesize Ge(boryl)2 (II-Ge) via reactions of GeCl2·L (L = dioxane, NHC) or Ge{N(SiMe3)2}2 with two or more equivalents of (boryl)Li(thf)2[thin space (1/6-em)]10 proved unsuccessful in our hands (where boryl = –B(NDippCH)2). In addition, the reaction of the hexa-germanium system Ge6(boryl)4 (IV)11 with reducing agents such as KC8, targeting Ge(boryl)2 by liberation of the known tetrahedral cluster nido-[Ge4]4−,12 also did not lead to the isolation of the desired germylene product. With this in mind, we turned our attention to an umpolung synthetic strategy, via the use of nucleophilic sources of the [(boryl)Ge] fragment, with the idea of assembling a second Ge–B bond by reaction of M2Ge2(boryl)2 (M = Group 1 metal)9 with a boron-centred electrophile.

K2Ge2(boryl)2 (1) is available via the exhaustive reduction of (boryl)Ge(IPrMe)Cl using KC8,9 and we sought to expand the range of nucleophilic group 1 metal digermanide derivatives by synthesizing a related lithium compound. However, rather than a reductive protocol starting from (boryl)Ge(IPrMe)Cl, the dilithium species {(IPrMe)Li}2Ge2(boryl)2 (2) is most conveniently prepared from 1 by a salt metathesis reaction with lithium iodide, in the presence of the carbene IPrMe (Scheme 1 and Fig. 2).2 can be obtained via this route in ca. 30% yield after recrystallization from toluene/hexane. Its solid-state structure, determined by X-ray crystallography, is based around a Ge2 unit featuring two boryl groups arranged in 1,2-fashion (and trans to one another), with each germanium centre being additionally bound to a single lithium atom. Each lithium centre is also coordinated by a single NHC donor (d(Li–C) = 2.133(5) Å), and engages in weaker contacts with two of the carbon atoms of a Dipp group associated with the boryl ligand attached to the other germanium centre (d(Li⋯C) = 2.702(7), 2.743(7) Å). In contrast to the solid-state structure of 1 (and related terphenyl-ligated systems) in which the alkali metal cation sits above the centre of the Ge2 unit and is further encapsulated by π-interactions with the flanking aryl rings,9,13 the positioning of the lithium atoms in 2 suggests greater directionality in the Ge–Li interactions. The associated Ge–Li distances (2.541(7) Å) are slightly greater than the sum of the respective covalent radii (1.20 + 1.28 Å),14 and the geometry at each germanium centre defined by the proximal boron, germanium and lithium centres is trigonal planar (Σ(angles at Ge) = 358.8(3)°).


image file: d3dt03416j-s1.tif
Scheme 1 Synthesis of lithiated and borylated derivatives of K2Ge2(boryl)2 (1) via metathesis processes with LiI/IPrMe and CatBBr, respectively. (IPrMe = C{N(iPr)CMe}2).

image file: d3dt03416j-f2.tif
Fig. 2 Molecular structures of (upper) {(IPrMe)Li}2Ge2(boryl)2 (as the pentane solvate, 2·C5H12) and (lower) of one of the (four) components of the asymmetric unit of Ge2(boryl)2(Bcat)2 (3) in the solid state as determined by X-ray crystallography. Hydrogen atoms and solvate molecules omitted, and certain carbon atoms depicted in wireframe format for clarity; thermal ellipsoids set at the 25% probability level. Key bond lengths (Å) and angles (°): (for 2) Ge–Ge 2.3453(5), Ge–B 2.091(3), Ge–Li 2.541(7), Li–CNHC 2.133(5), Li–Caryl 2.702(7), 2.743(7), B–Ge–Ge–B 166.7(1); (for 3) Ge–Ge 2.234(1), 2.235(1), 2.235(1), 2.238(1); Ge–BBcat 2.022(3), 2.025(3), 2.026(3), 2.026(3), 2.028(3), 2.028(3), 2.031(3), 2.033(3), Ge–Bboryl 2.023(2), 2.025(2), 2.025(3), 2.027(2), 2.027(2), 2.028(3), 2.028(3), 2.028(3); B–Ge–Ge–B 180.0(1), 177.3(1), 179.6(1), 180.0(1).

The straightforward substitution of the K+ cations in 1 by Li+ in the reaction with LiI/IPrMe led us to investigate its reactivity towards boron-based electrophiles, with the aim of generating a diborylgermylene. However, neither 1 or 2 reacts with (boryl)Br to give a species of empirical composition Ge(boryl)2 under any conditions examined – presumably due to the excessive steric demands of the boryl Dipp substituents and relatively low electrophilicity of (boryl)Br enforced by the presence of two α-amido π donors. With this in mind we turned to the less sterically demanding boron electrophile, catBBr (cat = 1,2-O2C6H4) as has previously been employed by Cui in related silicon chemistry.15 Reaction with 1 leads to clean conversion in ca. 65% yield (after recrystallization from pentane), to yield the unsymmetrical trans-1,2-tetraboryl digermene, Ge2(boryl)2(Bcat)2 (3; Scheme 1).3 is characterized by two 11B signals at δB = 28.9 and 43.6 ppm, and its dimeric structure in the solid state, together with the relative arrangement of the two different sets of boryl ligands has been confirmed by X-ray crystallography (Fig. 2).

The solid-state structure of 3 contains four molecules within the asymmetric unit, and reveals an exceptionally short Ge–Ge separation (2.234(1)–2.238(1) Å). This distance can be compared to 2.4 Å for the sum of the (single bond) covalent radii,14 2.347(2) Å for the digermene Ge2{CH(SiMe3)2}4,16 and 2.285(1) Å for the digermyne Ge2ArDipp2 (where ArDipp = 2,6-C6H3Dipp2).17 Moreover, in contrast to the trans-bent structures observed for digermenes such as Ge2{CH(SiMe3)2}4, 3 adopts a planar geometry at each germanium centre (Σ(angles at germanium) = 358.2–360.0°) with an inter-planar angle (between the least-squares planes defined by the two GeB2 units) in the range 0–4.8°.

The Ge–Ge distance measured for 3 (2.236 Å (mean)) is successively shorter than those determined for dilithium compound 2 (2.3453(5) Å) and dipotassium species 1 (2.392(1) Å).9 Notwithstanding the predominantly non-bonding nature of the (ag, n+) HOMO in doubly reduced digermyne species such as 1,13 this sequential shortening presumably reflects a reduction in the partial negative charge at each germanium centre on assimilation of the Li(NHC) and (particularly) the more covalently bound Bcat fragment. In a broader context, the ‘alkene-like’ form of 3 is reminiscent of the geometries determined for tetrasilyldigermenes, which also evidence near-planar structures with very small trans-bending and inter-planar angles.18 The Ge[double bond, length as m-dash]Ge distances measured for 3, are however, even shorter than those measured for silyl-substituted systems, and are among the shortest measured for any digermene.19

Viewing 3 as having a geometric structure closer to a conventional alkene than the trans bent geometry typical of heavier group 14 E2R4 species, implies that it can be formally constructed by combining two triplet (rather than singlet) germylene fragments. Dimerization via ‘conventional’ σ and π bond formation rather than a two-way donor–acceptor interaction offers a rationale for the structural features observed for 3 and related silyl-substituted systems.7,8 This in turn is consistent with the influence of the strongly electro-positive boryl (and silyl) substituents at germanium, which are known to lead to a small HOMO–LUMO energy gaps (and low-lying triplet states) for tetrylene systems of the type E(boryl)2. By means of comparison, singlet–triplet energy separations of −0.3, 17.9 and 39.2 kJ mol−1 have previously been calculated for the tetrylene systems E(boryl)2 (where E = Si, Ge and Sn, respectively).5

With this in mind, we examined computationally the fragmentation of a range of digermenes, Ge2X4, into both singlet and triplet germylenes (GeX2) by ETS-NOCV methods (Table 1).§ In the cases of 3 and Ge2(SiMe2tBu)4,18b cleavage into triplet germylene fragments clearly represents the lowest energy fragmentation pathway (e.g. ΔEorb = +155.4 vs. +299.2 kJ mol−1 for 3). On the other hand, the cleavage of Lappert's tetraalkyl digermene is most easily accomplished to give singlet Ge{CH(SiMe3)2}2 fragments (ΔEorb = +157.0 vs. +142.6 kJ mol−1), consistent with the observed trans bent structure of the digermene.16 Only in the case of the trans bent, but twisted system Ge2Trip3(SiPh3) (among the digermene systems examined),20 were the two fragmentation pathways found to be of comparable energy (ΔEorb = +159.1 and +160.5 kJ mol−1, respectively).

Table 1 Structural and electronic properties of 3, Ge2(SiMe2tBu)4, Ge2{CH(SiMe3)2}4 and Ge2Trip3(SiPh3)16,18b,20
  d(Ge–Ge) (Å) ∑(angles at Ge) trans-Bending angle, θ (°) Inter-planar anglea (°) ΔEorb (triplet) kJ mol−1 ΔEorb (singlet) kJ mol−1
a Defined as the angle between the two least squares GeXX’ planes. b The crystal structure of 3 contains four independent molecules in the asymmetric unit. c Metrical data taken from current study and from CIFs JORGIJ, LIGTAY and CENWAT (CCDC).16,18b,20
Ge2(boryl)2(Bcat)2 (3)b 2.234(1) 359.2(3), 359.2(3) 4.0, 4.0 0 155.4 299.2
2.235(1) 360.0(3), 360.0(3) 8.5, 8.5 0
2.235(1) 360.0(3), 359.9(3) 2.8, 5.3 3.1
2.238(1) 359.2(3), 358.2(3) 14.6, 9.3 4.8
Ge2(SiMe2tBu)4[thin space (1/6-em)]c 2.270(1) 360.0(1) 1.3 7.5 139.4 227.9
Ge2{CH(SiMe3)2}4[thin space (1/6-em)]c 2.347(2) 348.5 32.8 0 157.0 142.6
Ge2Trip3(SiPh3)c 2.328(1) 345.3(2), 346.5(2) 34.6, 35.1 13.6 159.1 160.5


An alternative approach to probe the nature of the Ge[double bond, length as m-dash]Ge bonds in planar and trans-bent cases (exemplified by 3 and Ge2{CH(SiMe3)2}4), involves the use of localised molecular orbitals (LMOs) to visualise the primary interactions between the two metal centres.§ In the case of 3, the LMOs which account for significant electron density at the bond critical point (BCP) and 1 Å above the BCP (Fig. 3) resemble classical σ- and π-type bonding orbitals. By contrast, in the case of Ge2{CH(SiMe3)2}4, two essentially equivalent LMOs each account for 48% of the electron density at the BCP (Fig. 4), and are of a form consistent with the ‘slipped’ π-bond description, distorted in each case towards one of the two germanium centres.7,18


image file: d3dt03416j-f3.tif
Fig. 3 Localized molecular orbitals (LMOs) contributing significantly to the electron density (left) at the bond critical point (BCP) and (right) 1 Å above the BCP (and above the Ge2B4 plane) for Ge2(boryl)2(Bcat)2, 3. The σ-type LMO accounts for 91% of the electron density at the BCP and 48% at 1 Å above the BCP; the π-type LMO accounts for 44% of the electron density at 1 Å above the BCP.

image file: d3dt03416j-f4.tif
Fig. 4 Localized molecular orbitals (LMOs) contributing significantly to the electron density at the BCP for Ge2{CH(SiMe3)2}4 (48% of the electron density, in each case).

The speciation of 2 and 3 in hydrocarbon solution was also investigated. Most informatively, the UV-Vis spectrum of 2 in methylcyclohexane and 3 in pentane feature strong bands at 423 and 431 nm, respectively, i.e. similar to those measured for species containing Ge[double bond, length as m-dash]Ge double bonds (e.g. 455 nm for [Ge(IDipp)]2,21 and 434 nm for (boryl)Ge[double bond, length as m-dash]Ge(IPrMe)(boryl)).9 In the case of 3, the assignment of the band at 431 nm is aided by the results of TD-DFT calculations. The S1 absorption (2.932 eV, 423 nm) is predominantly due to a Ge[double bond, length as m-dash]Ge π to π* transition, being composed of 93% NTOd_S1 → NTOa_S1 (Fig. S12(a) and (b)) with a minor contribution from the digermene σ to σ* orbitals (4% NTOd_S1s → NTOa_S1s; Fig. S12(c) and (d)). The higher energy S3 transition (3.911 eV, 317 nm) also accesses the Ge[double bond, length as m-dash]Ge π* orbital, in this case from the boryl ligand π-system (99% NTOd_S3 → NTOa_S3; Fig. S12(e) and (f)).

Spectroscopic and quantum chemical evidence is therefore consistent with retention of the dimeric structure of 3 in hydrocarbon solution at room temperature. We also investigated whether the speciation of 3 might be temperature dependent. However, while heating a solution in benzene-d6 over a period of 5 days, did lead to the formation of a single major new species, this could be shown by X-ray crystallographic studies to be the unsymmetrical diboron(4) species catBB(NDippCH)2 (4; Fig. S8),22 formed by reductive B–B bond formation. Despite repeated attempts, we were unable to ascertain the nature of the germanium-containing species formed under these conditions – which according to in situ NMR measurements (Fig. S7) is only very sparingly soluble in benzene-d6 solution.

Conclusions

In conclusion, we have explored a range of synthetic routes to systems of the type [Ge(boryl)2]n, with the reaction between a dianionic digermanide nucleophile and a boron halide electrophile proving to be the most viable. The tetraboryl digermene so generated is dimeric both in the solid state and in hydrocarbon solution and features both a planar ‘alkene-like’ geometry for the Ge2B4 core, and an exceptionally short Ge[double bond, length as m-dash]Ge double bond. These structural features are consistent with the known electronic properties of the boryl group, and with conceptual fragmentation into two triplet bis(boryl)-germylene fragments.

Conflicts of interest

There are no conflicts to declare.

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Footnotes

Electronic supplementary information (ESI) available: Full synthetic/characterizing data, X-ray CIFs and details of calculations. CCDC 2300941–2300943. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d3dt03416j
See ESI for synthetic and characterizing data for 2 and 3.
§ See ESI for details of quantum chemical calculations.

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