Diverse Reactivity of Amidinate-Supported Boron Centers with the Hypersilyl Anion and Access to a Monomeric Secondary Boron Hydride

Diverse reactivity of the bulky tris(trimethylsilyl)silyl substituent [Si(SiMe3)3], also known as the hypersilyl group, was observed for amidinate-supported dichloro- and phenylchloroborane complexes. Treatment of the dichloroborane with potassium tris(trimethylsilyl)silyl led to the activation of the backbone β-carbon center and formation of saturated four-membered heterocyclic chloroboranes R′{Si(SiMe3)3}C(NR)2BCl [R′ = Ph, R = Cy (3); R′ = Ph, R = iPr (6); R′ = tBu, R = Cy (8)], whereas the four-membered amidinate hypersilyl-substituted phenyl borane 4 {PhC(NCy)2B(Ph)[Si(SiMe3)3]} was observed for the case of an amidinate-supported phenylchloroborane. The highly deshielded 11B NMR spectroscopic resonance and the distinct difference in the 29Si NMR spectrum confirmed the presence of a σ-donating hypersilyl effect on compounds 3, 6, and 8. Reaction of 3 with the Lewis acid AlCl3 led to the formation of complex 11 in which an unusual cleavage of one of the C–N bonds of the amidinate backbone is observed. Nucleophilic substitution at the boron center of saturated chloroborane 3 with phenyllithium generated the phenylborane derivative 12, whereas the secondary monomeric boron hydride 13 was observed after treatment with alane (AlH3). All compounds (2–13) have been fully characterized by NMR spectroscopy and single-crystal X-ray structure determination studies.


■ INTRODUCTION
The early 21 st century witnessed enormous interest in the development of bidentate monoanionic nitrogen−donor amidinate ligands of the general formula [R′C(NR) 2 ] − , as a popular replacement for the cyclopentadienyl ligand, as well as a close association with guanidinates as the ligand architecture. 1Among a plethora of ligands, amidinates have been designed in such a way that the steric and electronic effects can be readily modified by tailoring the C-and Ncentered substituents to stabilize and tune the reactivity of various transition metals, lanthanides, and more recently, main group compounds.Following the first use with rare-earth metals in 1992 by Edelmann et al., 2 the application of amidinates rapidly expanded and fulfilled the quest for the preparation and stabilization of low-valent main group compounds 3−10 together with applications in homogeneous catalysis 11−15 and materials chemistry. 12Within the domain of p-block chemistry, group 13 and group 14 metal halide and metal−alkyl fragments have been well established using the amidinate ligand scaffold.For example, amidinate-supported silylenes have been reported for various small molecule activation and hydroboration reactions, 11,16−19 and amidinate-supported alkyl aluminum cations have been shown as active catalysts for olefin polymerization. 20,21In addition, amidinate-based tetra-coordinated boron compounds have also been proven to act as photoluminescent materials 22 and have been shown to activate gaseous CO and CO 2 , as well as carbonyl and nitrile functional groups. 18Surprisingly, a survey of known amidinate boron compounds reveals that the majority of amidinate boranes contain either halogen, 22,24,25 Ph, 27,28 or Me 29 groups at the boron center (Scheme 1A).We hypothesize that alternative reactivity or catalytic activity of amidinate-supported boranes could be achieved by adding strong σ-donor ligands and effectively reducing the HOMO− LUMO gap. 26However, this would be more synthetically challenging.In 1981, Brook discovered the first well-defined organometallic compound containing a Si�C double bond by the photolysis of an acylsilane, using the hypersilyl group, [tris(trimethylsilyl)silyl], with main group compounds. 30−44 The rapid growth of the use of the hypersilyl group in the synthesis and stabilization of several organometallic and main group compounds is a result of its pronounced steric effect, along with the strong σ-donor strength of the silyl ligand.The commercial readiness of the precursors and the easy access of further functionalization of the SiMe 3 moieties makes the hypersilyl group advantageous with a delicate balance between strong σ-donation and kinetic protection.−47 Herein, we investigate the different reactivities of the hypersilyl group on amidinate-supported dichloroboranes and phenyl-chloroborane by the treatment of the four-membered CN 2 B heterocycle with KSi(SiMe 3 ) 3 (Scheme 1C).The reactivity of the Si(SiMe 3 ) 3 -substituted products is also explored.

Inorganic Chemistry
In line with the significant growth of the use of the electropositive tris(trimethylsilyl)silyl group for the synthesis of numerous organometallic compounds, 49 we treated compounds 1 and 2 with equimolar amounts of KSi(SiMe 3 ) 3 in toluene at room temperature.While 2 smoothly afforded the hypersilyl-substituted phenylborane derivative (4) in high yields (94%) from substitution at the boron atom via traditional salt metathesis; under similar reaction conditions, 1 led to the formation of compound 3 instead of the analogous hypersilyl-substituted chloroborane 3′ (Scheme 3).Here, substitution at the backbone β-carbon of the amidinate moiety occurred giving the product 3 in 90% yield.Concentrated toluene solutions of both 3 and 4 at low temperature (−20 °C) produced high quality colorless crystals within 3−4 days.Single-crystal X-ray diffraction studies confirmed the structures of both compounds.The single crystal of compound 3 undergoes a solid-state phase transition at 200 K from the high temperature form (290 K) to the low temperature form (120 K), and the transformation occurs in a single-crystal-to-singlecrystal manner (discussed in Supporting Information).The molecular structure of 3 contains a B−Cl bond length of 1.772(3) Å (Figure 2), which is shorter than that of the previously reported dichloroborane precursor 1 [1.832(3)Å].A slightly longer Si−C bond than their covalent radii was observed for 3 at 1.977(2) Å, and an elongated B−Si bond of 2.102(2) Å was observed for 4 (Figure 2). 3 and 4 were further characterized by multinuclear NMR spectroscopy.Although the 1 H NMR spectroscopic resonance for the three SiMe 3 groups for 3 and 4 were observed in the same range, at δ = 0.40 and 0.53 ppm, respectively, the distinguishable 11 B resonances at δ = 21.8 and 9.1 ppm validated the different reactivity and coordination around the boron center of 3 and 4, respectively.The 29 Si NMR spectroscopic signals were detected at δ = −70.28[Si(SiMe 3 )] and −13.58 [Si(SiMe 3 )] ppm with a large resonance gap between two Si centers for 3, whereas for 4, these were observed at δ = −10.77[Si(SiMe 3 )] and −11.30[Si(SiMe 3 )] ppm.The molecular ion peaks were detected with the highest relative intensity at m/z 577.3232 and 619.3937 for 3 and 4, respectively.Interestingly, when we tried similar reactions with difluoro and dibromo boranes, we had no success in observing either the backbone-substituted or the hypersilyl-substituted borane products by either NMR spectroscopic studies or single-crystal X-ray diffraction.
To understand the preferred formation of 3 instead of 3′ and to investigate the effect of the substituents on the nitrogen center of the amidinate ligand, we prepared the previously reported dichloroborane 5, 23 containing an isopropyl group attached to the nitrogen center of the amidinate ligand, and performed the reaction with KSi(SiMe 3 ) 3 under similar reaction conditions (Scheme 4).As for dichloroborane 3, an attack at the backbone of the amidinate occurred, and the formation of 6 was observed in 94% yield.The downfield shift in the 11 B and 29 Si NMR spectrum at δ = 22.1 ppm and δ = −69.73[Si(SiMe 3 )] and −13.52 [Si(SiMe 3 )] ppm, respectively, further confirmed the formation of 6, along with 1 H    In an attempt to prevent attack at the backbone β-carbon center of the amidinate ligand and to encourage substitution at the boron center, we introduced further steric hindrance into the ligand by changing the phenyl group attached to the carbon atom of the amidinate ligand with a more bulky tertbutyl group.Compound 7 was prepared following the same literature procedure 23 for 5 using tert-butyl lithium instead of phenyllithium in the first step.7 was then reacted with KSi(SiMe 3 ) 3 under similar reaction conditions to those described for the synthesis of 6.Again, an attack occurred at carbon, yielding the analogous saturated chloroborane compound 8 in high yields (94%) (Scheme 4).Colorless single crystals of 7 and 8 suitable for X-ray diffraction studies were grown from saturated toluene solutions at −20 °C.The notable downfield 11  To introduce further steric hindrance at the backbone carbon, we synthesized the Dipp (Dipp = 2,6-diisopropylphenyl) amidinate-substituted dichloroborane [DippC-(NCy) 2 BCl 2 ] (9, 86% yield) and treated it with KSi(SiMe 3 ) 3 following the same reaction procedure.Despite a distinct color change in the reaction from pale yellow to red, neither the backbone-substituted nor the hypersilyl-substituted boron product was observed (Scheme 5).A saturated toluene solution of 9 formed suitable single crystals for X-ray diffraction studies and proved the desired steric protection with proximal alignment of the hydrogens from the isopropyl groups of the Dipp substituents and cyclohexyl groups of the amidinate ligand framework in compound 9 (molecular structure with H atoms shown in Figure S48 in the Supporting Information) to prevent the backbone attack by the hypersilyl group (Figure 6).
The desire to form a stable boron compound with unusual structure, bonding, and reactivity has led to the synthesis of several low-valent and low coordinated boron compounds 50  and has challenged the predominance of transition metals in catalysis, 51 small molecule and bond activation, 52,53 and has furthered the development of functional materials. 54Lowcoordinate cationic boron centers are attractive and reactive Lewis acidic entities.Noẗh and co-workers introduced the nomenclature of boronium, borenium, and borinium cations for the four-, three-, and two-coordinate boron-centered complexes, respectively. 55These highly Lewis acidic boron complexes are important in the field of bond activation and catalysis. 56Subsequently, we attempted to prepare the corresponding boron cations from complexes 2 and 3 by abstracting the chloride ligand.−59 However, the addition of 1 equiv of AlCl 3 to a benzene solution of 3 in the absence of a Lewis base led to the cleavage of the C−N bond of the amidinate backbone and binding of N amidinate to AlCl 3 , leading to zwitterionic 11 in 73% yield (Scheme 6).The reaction is expected to be driven by the formation of a tetracoordinated anionic aluminum fragment through N → Al bond formation.A possible reaction mechanism is proposed in the Supporting Information (Scheme S1) for the formation of 11.Colorless single crystals of 11 suitable for X-ray diffraction studies were grown from the saturated benzene solution at −4 °C in a freezer; the molecular structure of 11 validates our spectroscopic evidence.Compound 11 crystallizes in the triclinic space group, P1̅ .The B−Cl bond length of 11 is 1.785(2) Å, slightly longer than the saturated chloroborane 3 [1.772(3)Å], and the new N amidinate −Al bond is observed at a distance of 1.868(1) Å (Figure 7).The aluminum center adopts a tetrahedral geometry with bond angles of 109.04(6),108.99 (5), 107.3(1), and 114.76(6)°.The only partial solubility of adduct 11 in most organic solvents did not allow us to record 13 C, 29 Si, or 27 Al NMR spectra even after several attempts, as with time some decomposed solid compounds precipitated out.Gradual decomposition of 11 was also observed in 1 H NMR spectroscopy with the appearance of undesired peaks (Figure S26 in the Supporting Information), whereas the 11 B NMR spectrum showed a Scheme 5. Synthesis of Dipp-Amidinate-Substituted Dichloroborane 9  Cleavage of the stable B−Cl bond of saturated chloroborane 3 was unsuccessful under harsh reaction conditions, such as using Li-naphthalide or KC 8 as a reducing agent, except in the presence of phenyllithium.The toluene solution of 3 produced nucleophilic-substituted product 12 when treated with phenyllithium, exhibiting a B−C Ph bond, with removal of LiCl over a prolonged reaction time more than 36 h (Scheme 6).Compound 12 crystallizes in triclinic symmetry with a P1̅ space group.The boron center adopts a trigonal-planar geometry (Figure 8) with a B−C Ph bond length of 1.570(3) Å, which is slightly shorter than the phenyl-substituted tetracoordinated B−C Ph bond [1.612(3)Å] of compound 4. The 1 H NMR spectrum displays a singlet at δ = 0.46 ppm for the three Si(SiMe 3 ) groups, and the 29 Si NMR spectrum exhibits resonances at δ = −73.29 [Si(SiMe 3 )] and −13.42 [Si(SiMe 3 )] ppm for both the Si centers, while the 11 B NMR spectroscopic signal is observed at δ = 32.8ppm.
Boranes containing at least one B−H bond have appeared as powerful synthetic tools for hydroboration reactions.Among the many hydroborating borane catalysts, secondary boranes are very effective for the transformation, moreover, Piers' borane [HB(C 6 F 5 ) 2 ] has emerged as a highly effective catalyst for hydroboration and acts as an efficient reagent in various other organic transformations due to its high Lewis acidity. 60lectrophilic secondary boranes are typically preferred and stable as monomer−dimer equilibria in solution, 61,62 and isolable monomeric secondary boranes are extremely rare without sufficient steric provision. 63Recently, the research group of Martin reported a significantly high Lewis acidic monomeric secondary borane [tris(ortho-carboranyl)borane] containing two electron-withdrawing ortho carborane substituents and further utilized it as a potent hydroborating reagent. 64Inspired by Martin's work, the addition of alane (AlH 3 •NMe 2 Et) to a toluene solution of 3 smoothly afforded the stable boron hydride 13 in 90% yield (Scheme 6).Surprisingly, compound 13 is monomeric in both solid and solution state with the absence of any steric protection, which was confirmed by NMR spectroscopy, mass spectrometry, and infrared spectroscopy.Variable temperature 1 H NMR spectroscopic studies of a toluene-d 8 solution of 13 displayed a resonance of the B−H at δ = 4.62 ppm (Figures S33 and S34 in the Supporting Information) along with a singlet for the Si(SiMe 3 ) 3 group, even at −60 °C.The 11 B NMR spectroscopic resonance was observed at δ = 26.61ppm for the tricoordinated boron center.The distinguishable IR stretching frequency with one sharp band at 2557 cm −1 confirmed the monomeric nature of B−H bond (Figure S38 in the Supporting Information), which is in good agreement with the monomeric secondary boron hydride reported by Martin and co-workers. 64Thermal characterization by differential scanning calorimetry (DSC) shows an endothermic melting event at 150.4 °C (Figure S40 in the Supporting Information), corroborating the analogous melting point measurement (∼162−164 °C).In addition, 13 shows thermal stability up to 250 °C, and no exothermic crystallization point could be observed in the cooling curve, suggesting that the crystallization of the sample takes place at temperatures below 30 °C (Figure S39 in the Supporting Information).Colorless crystals of 13 were grown from cooling a concentrated toluene solution.13 crystallizes in the triclinic space group P1̅ adopting a distorted trigonal planar geometry (Figure 9).The hydride was detected in the difference Fourier map and refined isotropically.Due to the stability of the hydride compound, the traditional hydroboration reaction using 13 as an effective catalyst did not provide any significant results compared to the reported Piers' borane [HB(C 6 F 5 ) 2 ] and/or tris(orthocarboranyl)borane.Thus, we are currently exploring other organic transformations using monomeric boron hydride as well as catalytic studies with compounds 4 and 13, which will be published in the future.

■ CONCLUSIONS
The chemistry of the amidinate ligand in main-group chemistry is in its infancy compared to other ligand scaffolds.In this work, we have shown a unique β-carbon activation of  the amidinate ligand with a hypersilyl group for dichlorosubstituted amidinate−borane complexes with various electronic and steric C-and N-substituents (3, 6, and 8).The treatment with amidinate-supported phenylborane finally produced the expected hypersilyl-substituted phenylborane derivative (4) in high yields.Further attempts to cleave the exceptionally stable B−Cl bond of 3 led to the formation of ionic compound 11 upon cleavage of the ligand backbone and nucleophilic-substituted phenylborane 12 in the presence of AlCl 3 and PhLi, respectively.Moreover, the formation of a rare and thermally stable monomeric secondary borohydride 13 was achieved from compound 3 from the reaction with alane, without significant steric or electronic support to compound 13.

Data Availability Statement
Information about the data that underpins the results presented in this article can be found in the Cardiff University data catalogue at 10.17035/d.2024.0315834338.
General experimental information, structural description of 2−13, details of synthesis, representative NMR spectra, thermal characterization data for compound 13, tentative mechanism for the formation of compound 11, crystallographic data for the structural analysis of compounds 2−13, crystal structure of compounds 2−13 with thermal ellipsoids, solid-state phase transformation details of compound 3, and computational details (PDF)

Figure 3 .
Figure 3. Energy profile for the formation of 3-SiMe 3 versus 3′-SiMe 3 .Relative energies are given in kcal/mol.B is shown in pink, C in gray, N in blue, Cl in green, K in purple, and Si in beige.Hydrogen atoms are omitted for clarity.