Synthesis of Unsymmetrical Diboron(5) Compounds and Their Conversion to Diboron(5) Cations

Abstract

Reaction of bis-catecholatodiboron-NHC adducts, B2Cat2(NHC), (NHC = IMe (tetramethylimidazol-2-ylidene), IMes (1,3-dimesitylimidazol-2-ylidene) or IDIPP (1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene)) with BCl3 results in the replacement of the catecholato group bound to the four coordinate boron with two chlorides to yield diboron(5) Lewis acid-base adducts of formula CatB-BCl2(NHC). These compounds are precursors to diboron(5) monocations, accessed by adding AlCl3 or K[B(C6F5)4] as halide abstraction agents in the presence of a Lewis base. The substitution of the chlorides of CatB-BCl2(NHC) for hydrides is achieved using Bu3SnH and a halide abstracting agent to form 1,1-dihydrodiboron(5) compounds, CatB-BH2(NHC). Attempts to generate diboron(4) monocations of formula [CatB-B(Y)(NHC)] + (Y = Cl or H) led to the rapid formation of CatBY.

Chemistry of Dicationic Diboranes

“…[8] More recently, Prokofjefs proposed the formation of a sp 2 -sp 3 diborane(5) monocation [L(H) 2 B-BHL] + as a possible key intermediate in the intramolecular activation of aliphatic C-H bonds. [9] Another notable work on the synthesis of unsymmetrical NHC-coordinated diborane compounds and their conversion to the corresponding cations was published by Braunschweig and Ingleson et al [10] Treating of tetramethylimidazol-2ylidene (IMe)substituted diboranes(5) with a Lewis base and AlCl 3 or K[B(C 6 F 5 ) 4 ] led in this case to the formation of stabilized monocations.…”

A Facile Synthesis of Robinson's NHC‐Stabilised Diborane(4)

“…An 1 H{ 11 B} NMR spectrum recorded for [(IMes)BH 2 B(cat)] allowed an additional singlet at δ = 0.68 ppm to be detected. [ 6,13 ] The room temperature 1 H NMR spectrum of 1 shows no obvious BH resonances, although the presence of an extremely broad resonance at around δ = 0.5 ppm can be deduced. We have studied compound 1 in deuterated toluene by 1 H NMR spectroscopy between 295 K and 193 K. Over this range, the resonances for the isopropyl hydrogen environments appear as two doublets and one septet, are remarkably unchanged and do not significantly broaden, even though the very low temperature spectra show the 3 J coupling poorly resolved.…”

“…Compound 5· 0.5C 6 H 6 was studied by single‐crystal X‐ray diffraction and crystallised with a full molecule in the asymmetric unit (see Figure 2) showing a B1–B2 bond length of 1.685(5) Å {cf. 1.686(4) Å in [(IMes)BH 2 B(cat)]}, IMes = [(HCNMes) 2 C:, Mes = 2,4,6‐Me 3 C 6 H 2 , cat = catecholato], [ 13 ] and a B1–C1 bond lengths of 1.592(5) Å {cf. 1.605(3) Å in [(IMes)BH 2 B(cat)]}.…”

“…[(IMes)BH 2 B(cat)] and [(IDip)BH 2 B(cat)] were previously prepared by the reaction of the respective NHC adduct of B 2 cat 2 with BCl 3 to afford [(IMes)BCl 2 B(cat)] or [(IDip)BCl 2 B(cat)] followed by hydrogenation using two equivalents of tri‐ n ‐butyltinhydride and 10 mol‐% sodium tetrakis(3,5‐dichlorophenyl)borate. [ 13 ] Also, hydride abstraction of related [(IMe)BH(Mes)B(pin)] {IMe = (MeCNMe) 2 C:} leads to a cationic diborane species that activates dihydrogen at low temperatures. [ 14 ]…”

Spontaneous N2-diboranylation of [W(N2)2(dppe)2] with B2Br4(SMe2)2

“…The B-B bonds in 1-L (≈1.69 Å) are typical of singly Lewis-base stabilised diboranes and borylboronium complexes. 49,[54][55][56] Although structurally reminiscent of platinum borylborates obtained by reacting [Pt(PCy 3 ) 2 ] with B 2 X 4 (X = Cl, Br; B-B ≈ 1.77 Å), 57 3-DMAP shows a much shorter B-B bond (1.701(15) Å) due to the additional attraction of the vicinal positive and negative charges at the boron atoms. The B-B bond in complex 2 (1.681(5) Å) is similar to that in B 2 Br 2 (NMe 2 ) 2 (1.682(16) Å).…”