Tris(imidazolin-2-ylidene-1-yl)borate Complexes of the Heavier Alkaline Earths: Synthesis and Structural Studies

Abstract

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 CaI 2, SrI 2, or BaI 2 . Complex formation invariably involved partial B−N bond cleavage of the ligand precursors, leading to the formation of the silylamide complexes [{HB(Im t Bu) 3 }M{N(SiMe 3 ) 2 }(N-Im t 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 t Bu) 3 }CaI(THF)] and [{HB(Im t Bu) 3 }CaI(N-Im t Bu)] following deprotonation of the boronium salt ligand precursor with [KN(SiMe 3 ) 2 ] in the presence of CaI 2 or to dimeric [{HB(Im t Bu) 3 }CaBr] 2 when using [Ca{N(SiMe 3 ) 2 }(THF) 2 ] as both base and calcium source. However, similar reactions with M = Sr resulted in the formation of only homoleptic [{HB(Im t Bu) 3 } 2 Sr] 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 C 3 -symmetric facial κ 3 -binding mode via the three N-heterocyclic carbene (NHC) σ-donors.