Tetramethylsilane Coordination to Neutral, Heavier Alkaline Earth Metal Complexes

The bulky xanthene-bridged diamido-barium complex, [(EtNONTCHP)Ba(η6-toluene)] 1-Ba (EtNONTCHP = 4,5-bis{(TCHP)N}-2,7-diethyl-9,9-dimethyl-xanthene, TCHP = 2,4,6-tricyclohexylphenyl), has been prepared and exists as a weakly associated polymer in the solid-state. While this is unreactive toward tetramethylsilane (TMS), its lighter analogues [(EtNONTCHP)M(η6-toluene)] 1-M (M = Ca and Sr) coordinate to TMS giving the 1:1 complexes [(EtNONTCHP)M(η6-toluene)(κ2-H,H-TMS)] 2-M (M = Ca or Sr). In the solid-state, the TMS ligands in these species appear to ligate the metal centers in a κ2-H,H-mode. NMR spectroscopic studies imply that the TMS ligands of 2-M dissociate from the metal center, even in noncoordinating solvents. A theoretical analysis of the strength and nature of the metal-TMS bonding in 2-M has been carried out. First, precursor complexes 1-M were computed to be strong Lewis acids, which coordinate the poorly nucleophilic TMS ligand, to give 2-M. DFT analyses revealed the strength of bonding between TMS and the 1-M fragments to be weak (ΔG298 K° = -6.6 and -6.9 kcal mol-1 for 2-Ca and 2-Sr, respectively). ETS-NOCV and NBO analyses of the bonding in 2-Ca indicated that, while electrostatic interactions between the TMS and 1-Ca units dominate (ca. 68%), both dispersion (ca. 20%) and orbital (ca. 12%) interactions make significant contributions to the total attractive energy.