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

Orientational preferences in alkene\nand alkyne complexes arise\nfrom differences in the π backbonding capabilities of the relevant\ndπ orbitals, which typically are engendered by an unsymmetrical\narrangement of ancillary ligands. The metal <i>trans</i>-bis(iminoxolene) fragment is <i>C</i><sub>2</sub>-symmetric\nbut discriminates between perpendicular dπ orbitals because\nonly one of them has a strong π interaction with the iminoxolenes.\nTo assess this effect, square pyramidal bis(iminoxolene) alkene and\nalkyne complexes (Diso)<sub>2</sub>Ru(L) (Diso = <i>N</i>-(2,6-diisopropylphenyl)-4,6-di-<i>tert</i>-butyl-<i>o</i>-iminobenzoquinone) are prepared via the bis-acetonitrile\ncomplex <i>cis</i>-(Diso)<sub>2</sub>Ru(NCCH<sub>3</sub>)<sub>2</sub>. The alkenes and alkynes align roughly along the O–Ru–O\naxis but are turned slightly toward the cleft between the iminoxolene\nligands, which orients the ligand π* orbital with the <i>d</i>π orbital that is not engaged in bonding with the\niminoxolenes. In the alkyne complexes, π donation from the alkyne\ncompetes effectively with the ruthenium-iminoxolene π bonding,\nforming a favorable four-electron, three-orbital system. The barrier\nto rotation in the 1-hexyne complex is 19.0 kcal mol<sup>–1</sup>, while allylbenzene dissociates more readily than it undergoes rotation,\nwith a barrier of 17.4 kcal mol<sup>–1</sup>. The strong orientational\npreference leads to high facial selectivity of alkene binding, with\nonly one diastereomer of the 1-alkene adducts observed by NMR (>30:1\nselectivity).