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(SiMe3)(2) in the presence of Cal(2), Srl(2),, 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'Bu)(3)}M{N(SiMe3)(2)(N-Im'Bu)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'BU)(3)}Cal(THF)] and [{HB(Im'Bu)(3)}Cal(N-Im'Bu)] following deprotonation of the boronium salt ligand precursor with [KN(SiMe3)(2)] in the presence of Cal(2) or to dimeric [{HB(Im'Bu)(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'Bu)(3)}(2)Sr] probably due to the 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 C-3-symmetric facial K-3-binding mode via the three N-heterocyclic carbene (NHC) sigma-donors.