Anisotropic π Bonding in Bis(iminoxolene)ruthenium: Consequences for Alkene and Alkyne Binding

Orientational preferences in alkene and alkyne complexes arise from differences in the π backbonding capabilities of the relevant dπ orbitals, which typically are engendered by an unsymmetrical arrangement of ancillary ligands. The metal trans-bis(iminoxolene) fragment is C2-symmetric but discriminates between perpendicular dπ orbitals because only one of them has a strong π interaction with the iminoxolenes. To assess this effect, square pyramidal bis(iminoxolene) alkene and alkyne complexes (Diso)2Ru(L) (Diso = N-(2,6-diisopropylphenyl)-4,6-di-tert-butyl-o-iminobenzoquinone) are prepared via the bis-acetonitrile complex cis-(Diso)2Ru(NCCH3)2. The alkenes and alkynes align roughly along the O–Ru–O axis but are turned slightly toward the cleft between the iminoxolene ligands, which orients the ligand π* orbital with the dπ orbital that is not engaged in bonding with the iminoxolenes. In the alkyne complexes, π donation from the alkyne competes effectively with the ruthenium-iminoxolene π bonding, forming a favorable four-electron, three-orbital system. The barrier to rotation in the 1-hexyne complex is 19.0 kcal mol–1, while allylbenzene dissociates more readily than it undergoes rotation, with a barrier of 17.4 kcal mol–1. The strong orientational preference leads to high facial selectivity of alkene binding, with only one diastereomer of the 1-alkene adducts observed by NMR (>30:1 selectivity).