Bis(imidazolin-2-ylidene-1-yl)borate Complexes of the Heavier Alkaline Earths: Synthesis and Studies of Catalytic Hydroamination

Heteroleptic complexes of the heavier alkaline earth elements, calcium and strontium, containing bis(imidazolin-2-ylidene-1-yl)borate ligands may be synthesized by deprotonation of boronium salt ligand precursors with [KN(SiMe3)2] in the presence of CaI2 or SrI2. The silylamide complexes [{H2B(ImtBu)2}M{N(SiMe3)2}(THF)n] (M = Ca, n = 1; M = Sr, n = 2), containing N-tert-butyl-substituted imidazolin-2-ylidene fragments, are stable to intermolecular solution redistribution despite some evidence of solution fluxionality in the case of M = Ca. Attempts to synthesize heteroleptic iodide-containing species led to the isolation of the homoleptic compounds [{H2B(ImtBu)2}2MTHF)n] (M = Ca, n = 1; M = Sr, n = 2) most likely because of the reduced steric demands of the halide coligand. Although extension of the “one pot” synthetic method to precursors containing N-mesityl-substituted imidazoles resulted in extensive ligand degradation, use of [Ca{N(SiMe3)2}2(THF)2] as both base and group 2 element source provided the target heteroleptic silylamide complex. X-ray diffraction analyses demonstrated that the borate ligands adopt a facial η3-binding mode in which coordination is provided through the two N-heterocyclic carbene (NHC) donors and an additional agostic-type interacton from one hydrogen of the BH2 residue. Analysis of this ligand binding by DFT methods indicates that, although the bonds from the carbon centers are similar in character to those provided by neutral NHC ligands, the bonding is supplemented by an electrostatic component which is manifested as a coordination of a hydridic BH bond. The ionic nature of the overall interaction is sufficient to maintain the metal−ligand binding in the presence of amine donors and has allowed the evaluation of the Ca and Sr complexes [{H2B(ImtBu)2}M{N(SiMe3)2}(THF)n] as precatalysts for the intramolecular hydroamination of aminoalkenes. While the calcium catalyst provides activities commensurate with our previously reported β-diketiminato complex, [CH{C(Me)N(2,6-iPr2C6H3)}Ca{N(SiMe3)2}(THF)], the strontium derivative is superior in all cases.