Hemilability and nonrigidity in a series of mixed P,P=S donor ligands has been studied in the complexes [Pd(P,P=S)Cl₂], [Pd(η³-C₃H₅)(P,P=S)][SbF₆], and [Rh(cod)(P,P=S)][SbF₆] (P,P=S = Ph₂P-Q-P(S)Ph₂).  The effects of bite angle, the rigidity of the ligand backbone, and the role of the ancillary ligands have been investigated.  Hemilability and allyl exchange mechanisms of palladium and rhodium complexes of P,P=S ligands were examined by variable temperature NMR.  Larger bite angle ligands were found to have decreased barriers to hemilability.  A manuscript summarizing the work to date has been submitted to J. Organometal. Chem.

            [Pd(η³-C₃H₅)(P,P=S)][SbF₆] complexes catalyze the allylic alkylation of carbonates with sodium dimethylmalonate with unusually high selectivity for addition to the substituted allylic terminus.  We have been investigating the origin of this unusual selectivity.  Larger bite angle P,P=S ligands give increased branched selectivity and it appears that ligand backbone flexibility correlates with increases in the rate of the reaction.  There is also a significant memory effect observed with some of the P,P=S ligands. Low temperatures increase the regioselectivity and the memory effect for the reaction.  While the effect of bite angle is most likely the result of better embracing of the substrate, variations in the rate of hemilability influences the rate of allyl equilibration, and can alter contributions from the memory effect by slowing cis-trans exchange.  A difference in linear and branched substrate reactivity was observed.  The ability to tune ligands for high branched selectivity is a first step in preparing catalysts with high selectivity for enantiopure branched product for palladium catalyzed allylic alkylations and aminations.

Binding of heterobidentate P-S ligands introduces metal-centered chirality to the planar chiral parent complex Ru(η⁶: η¹-NMe₂C₆H₄C₆H₄PCy₂)Cl₂.  Observed diastereomeric ratios for the kinetic product vary dramatically depending upon ring size of the chelate formed with the P-S ligand.  The complexes epimerize very slowly to thermodynamic product ratios that are substantially different from the kinetic product ratios.  We have attributed this interconversion to a process involving a hemilabile intermediate.  A paper describing this work has appeared in the J. Organometal. Chem.

            Graduate students who have participated in this project were Philip Fontaine, who is currently carrying out a postdoctoral project at the University of Maryland, and Suzanna Milheiro who will start teaching at the University of Hartford in the Fall of 2007.  Happy Kanyili, a minority undergraduate, worked on this project for her undergraduate research.