Chemistry of the PCNHCP ligand:: Silver and ruthenium complexes, facial/meridional coordination, and catalytic transfer hydrogenation

The silver complex of (PCP)-P-NHC was identified as [Ag-3(mu-Cl)((PCP)-P-NHC)(2)]Cl-2 (1). Anion exchange of 1 with AgNO3 affords a white compound, which crystallizes into two forms, [Ag-3(mu-Cl)-((PCP)-P-NHC)](2)(NO3)(2) (2a) and [Ag-3(Cl)(NO3)(0.5DMF)(pC(NHC)P)(2)]NO3 (2b), from different solvent combinations. Their structural determination reveals interesting structural features, including a short Ag center dot center dot center dot Ag interaction in both of them. Unlike the common preparation of ruthenium(II) NHC complexes in the literature, a novel silver carbene transfer reaction between 1 and RuCl2(PPh)(3) produces fac-[Ru-2(kt-Cl)(3)((PCP)-P-NHC)(2)]Cl (3) in good yield. The nonredox nature of the carbene transfer reaction is different from that of an earlier report and shows that the preparation of divalent ruthenium NHC complexes via silver carbene transfer is a feasible synthetic route. Significantly, complex 3 can also be prepared directly from (PCP)-P-NHC center dot HCl and RuCl2(PPh3)3 in the absence of base. Reactions of 3 with CO, phenylacetylene, and pyridine afford mer complexes mer,cis-RuCl2(CO)(pCNHCP) (4), mer,trans-RuCl2(=C=CHPh)((PCP)-P-NHC) (6), and mer,trans- [RuCl(py)(2)((PCP)-P-NHC)]Cl (7), respectively. The fac complex, fac,cis-[RuCl(bipy)((PCP)-P-NHC)]Cl (8), was produced in reaction of either 3 or 7 with 2,2'-bipyridine. Treatment of 4 with excess NaBH4 produced the hydride complex mer-RuHCl(CO)(pC(NHC)p) (5). Complex 7 is slowly oxidized to mer-[(RuCl3)-Cl-III(pC(NHC)P)] (9) in the air. The structures of 2a, 2b, 3, 4, 7, 8, and 9 have been confirmed by X-ray crystallography. The reactivity study shows that pC(NHC)p is capable of accommodating extra steric requirement by readily switching from facial to meridional coordination mode, and the preliminary catalytic study indicates that 3 is effective in transfer hydrogenation of ketones.