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

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journal contribution
posted on 2009-08-10, 00:00 authored by Merle Arrowsmith, Alex Heath, Michael S. Hill, Peter B. Hitchcock, Gabriele Kociok-Köhn
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 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(ImtBu)3}M{N(SiMe3)2}(N-ImtBu)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(ImtBu)3}CaI(THF)] and [{HB(ImtBu)3}CaI(N-ImtBu)] following deprotonation of the boronium salt ligand precursor with [KN(SiMe3)2] in the presence of CaI2 or to dimeric [{HB(ImtBu)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(ImtBu)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.