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

The preparation and reactivity with H2of two Ru complexes of the novel ZnPhos ligand (ZnPhos = Zn(o-C6H4PPh2)2) are described. Ru(ZnPhos)(CO)3(2) and Ru(ZnPhos)(IMe4)2(4; IMe4= 1,3,4,5-tetramethylimidazol-2-ylidene) are formed directly from the reaction of Ru(PPh3)(C6H4PPh2)2(ZnMe)2(1) or Ru(PPh3)3HCl/LiCH2TMS/ZnMe2with CO and IMe4, respectively. Structural and electronic structure analyses characterize both 2and 4as Ru(0) species in which Ru donates to the Z-type Zn center of the ZnPhos ligand; in 2, Ru adopts an octahedral coordination, while 4displays square-pyramidal coordination with Zn in the axial position. Under photolytic conditions, 2loses CO to give Ru(ZnPhos)(CO)2that then adds H2over the Ru–Zn bond to form Ru(ZnPhos)(CO)2(μ-H)2(3). In contrast, 4reacts directly with H2to set up an equilibrium with Ru(ZnPhos)(IMe4)2H2(5), 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)L2intermediates in which Zn sits in a basal site: for L = CO, this is readily accessed and allows H2to add across the Ru–Zn bond, but for L = IMe4, 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 IMe4. Steric effects associated with the larger IMe4ligands are not significant. Species 4can be considered as a Ru(0)L4species that is stabilized by the Ru→Zn interaction. As such, it is a rare example of a stable Ru(0)L4species devoid of strong π-acceptor ligands.