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 H2 of 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/ZnMe2 with CO and IMe4, respectively. Structural and electronic structure analyses characterize both 2 and 4 as 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 4 displays square-pyramidal coordination with Zn in the axial position. Under photolytic conditions, 2 loses CO to give Ru­(ZnPhos)­(CO)2 that then adds H2 over the Ru–Zn bond to form Ru­(ZnPhos)­(CO)2(μ-H)2 (3). In contrast, 4 reacts directly with H2 to 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)­L2 intermediates in which Zn sits in a basal site: for L = CO, this is readily accessed and allows H2 to 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 IMe4 ligands are not significant. Species 4 can be considered as a Ru(0)­L4 species that is stabilized by the Ru→Zn interaction. As such, it is a rare example of a stable Ru(0)­L4 species devoid of strong π-acceptor ligands.