Tris(imidazolin-2-ylidene-1-yl)borate complexes of the heavier alkaline earths: Synthesis and structural studies

Heteroleptic tris(imidazolin-2-ylidene-1-yl)borate complexes of the heavier alkaline earth elements calcium, strontium, and barium have been synthesized by deprotonation of boronium salt ligand precursors with [KN(SiMe ₃)₂] in the presence of CaI₂, SrI₂, or BaI₂. Complex formation invariably involved partial B-N bond cleavage of the ligand precursors, leading to the formation of the silylamide complexes [{HB(Im^tBu)₃}M{N(SiMe₃) ₂}(N-Im^tBu)_n] (M = Ca, n = 0; Sr, n = 1; Ba, n = 1.5). All three silylamide complexes are stable toward Schlenk-type ligand redistribution in solution and show catalytic activity in the intramolecular hydroamination of aminoalkenes. In the case of M = Ca attempts to synthesize heteroleptic halide-containing species led to a 1:9 mixture of monomeric [{HB-(Im^tBu)₃)CaI(THF)] and [(HB(Im^tBu) ₃)CaI(N-Im^tBu)] following deprotonation of the boronium salt ligand precursor with [KN(SiMe₃)₂] in the presence of CaI₂ or to dimeric [{HB(Im^tBu)₃}CaBr] ₂ when using [Ca{N(SiMe₃)₂} (THF)₂] as both base and calcium source. However, similar reactions with M = Sr resulted in the formation of only homoleptic [{HB(Im^tBu)₃} ₂Sr] probably due to the larger ionic radius of the strontium center combined with a less sterically demanding halide co-ligand. X-ray diffraction analyses of all compounds demonstrated in each case that the monoanionic borate ligand coordinates to the alkaline earth metal center in a C₃- symmetric facial κ³-binding mode via the three N-heterocyclic carbene (NHC) σ-donors.