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

Abstract

The solid-state conformations of a series of new, mixed secondary and tertiary phosphine complexes of the general formula [Ru(η 5 -indenyl)Cl(PPh 3 )(HPR 2 )] have been shown to persist in solution, largely because of the electronic requirements of the ancillary η 5 -indenyl ligand in these complexes. The crystallographically observed conformations, with the indenyl benzo ring lying anti to the secondary phosphine across the Ru-η 5 -indenyl bond, were diagnosed in solution from chemical shift differences for PPh 3 signals in the low-temperature-limiting 1 H and 13 C 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(η 5 -indenyl)H(PPh 3 )(HPR 2 )], 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 PPh 3 . Implications for the ancillary ligand behavior of secondary phosphines are discussed.

Diastereoselective synthesis of a “chiral-at-Ru” secondary phosphine complex

“…2) shows an extreme shift to high field (d2.70 ppm in d 1 -chloroform, d 2.81 ppm in d 6 -benzene), consistent with its proximity to the center of a phenyl ring that lies perpendicular to the C-H1/3 bond axis. Similar to what we described previously for the complexes [Ru(g 5 -indenyl)Cl(PR 2 H)(PPh 3 )], where R = Cy or Ph [1], we observe broadening of many of the aromatic signals due to PPh 3 in the room temperature 1 H and ( 13 C{ 1 H}) NMR spectra of mixtures of 2a and 2b. This arises from the combination of slowed rotation around the Ru-PPh 3 bond (due to steric congestion at ruthenium) and disparate chemical environments experienced by each of these three phenyl groups.…”

“…5 Similar to what we reported in Ref. [1] for the symmetrically substituted analogues of 2, the crystallographic and IR data obtained for 2 provide no evidence for hydrogen bonding between the secondary phosphine P-H and the Ru-Cl.…”

“…Deuterated solvents were purchased from Cambridge Isotope Labs (CIL), freeze-pump-thaw degassed, and vacuum transferred from sodium/benzophenone (d 6 -benzene, d 8 -toluene) or calcium hydride (d-chloroform) before use. [RuCl(g 5 -indenyl)(PPh 3 ) 2 ] was prepared as described previously [1]. HPBu t Ph was purchased from Sigma-Aldrich Canada and used as received.…”

“…We have previously described the synthesis of half-sandwich ruthenium complexes of symmetrically substituted secondary phosphines of the formula [Ru(g 5 -indenyl)Cl(PR 2 H)(PPh 3 )] [1]. Their subsequent dehydrohalogenation reactions generate highly reactive terminal phosphido complexes containing a Ru-PR 2 p-bond [2].…”

“…This is a preliminary report of our efforts to extend this chemistry to the stereoselective P-C bond-forming reactions of unsymmetrically-substituted secondary phosphines. The chirality at ruthenium in the symmetric secondary phosphine complexes in Scheme 1 is evident from their 13 C and 1 H NMR spectra, which show distinct chemical shifts for peaks due to the diastereotopic substituents at the coordinated secondary phosphine [1]. An intriguing strategy for the production/resolution of chiral phosphines would capitalize on this chirality through the selective enhancement of the rate of P-C bond formation for one of two diastereomers resulting from the addition of a racemic mixture of a P-chiral secondary phosphine to [Ru(g 5 -indenyl)Cl(PPh 3 ) 2 ].…”