Electronic Control of Conformation in Mixed-Phosphine Complexes of the Ruthenium η⁵-Indenyl Fragment

The solid-state conformations of a series of new, mixed secondary and tertiary phosphine complexes of the general formula [Ru(η5-indenyl)Cl(PPh3)(HPR2)] have been shown to persist in solution, largely because of the electronic requirements of the ancillary η5-indenyl ligand in these complexes. The crystallographically observed conformations, with the indenyl benzo ring lying anti to the secondary phosphine across the Ru−η5-indenyl bond, were diagnosed in solution from chemical shift differences for PPh3 signals in the low-temperature-limiting 1H and 13C NMR spectra. These conformations were tuned by varying the relative trans influences of the different piano-stool legs, in particular through preparation of the analogous hydrido complexes [Ru(η5-indenyl)H(PPh3)(HPR2)], which resulted in structures with the indenyl benzo ring anti to the hydride ligand. These studies provide evidence for the reasonably strong trans influence of coordinated secondary phosphines relative to PPh3. Implications for the ancillary ligand behavior of secondary phosphines are discussed.