Impact of the Novel Z-Acceptor Ligand Bis{(-diphenylphosphino)phenyl}zinc (ZnPhos) on the Formation and Reactivity of Low-Coordinate Ru(0) Centers.

The preparation and reactivity with H of two Ru complexes of the novel ZnPhos ligand (ZnPhos = Zn(-CHPPh)) are described. Ru(ZnPhos)(CO) () and Ru(ZnPhos)(IMe) (; IMe = 1,3,4,5-tetramethylimidazol-2-ylidene) are formed directly from the reaction of Ru(PPh)(CHPPh)(ZnMe) () or Ru(PPh)HCl/LiCHTMS/ZnMe with CO and IMe, respectively. Structural and electronic structure analyses characterize both and as Ru(0) species in which Ru donates to the Z-type Zn center of the ZnPhos ligand; in , Ru adopts an octahedral coordination, while displays square-pyramidal coordination with Zn in the axial position. Under photolytic conditions, loses CO to give Ru(ZnPhos)(CO) that then adds H over the Ru-Zn bond to form Ru(ZnPhos)(CO)(μ-H) (). In contrast, reacts directly with H to set up an equilibrium with Ru(ZnPhos)(IMe)H (), the product of oxidative addition at the Ru center. DFT calculations rationalize these different outcomes in terms of the energies of the square-pyramidal Ru(ZnPhos)L intermediates in which Zn sits in a basal site: for L = CO, this is readily accessed and allows H to add across the Ru-Zn bond, but for L = IMe, this species is kinetically inaccessible and reaction can only occur at the Ru center. This difference is related to the strong π-acceptor ability of CO compared to IMe. Steric effects associated with the larger IMe ligands are not significant. Species can be considered as a Ru(0)L species that is stabilized by the Ru→Zn interaction. As such, it is a rare example of a stable Ru(0)L species devoid of strong π-acceptor ligands.