Monomeric Rhodium(II) Complexes Supported by a Diarylamido/Bis(phosphine) PNP Pincer Ligand and Their Reactivity Toward Dihydrogen

A rhodium(II) complex of a diarylamido/bis(phosphine) PNP pincer ligand, (PNP)Rh(OTf) (2, where OTf = O3SCF3 and PNP = [kappa(3)-P,N,P-(4-Me-2-((Pr2P)-Pr-i)-C6H3)(2)N]), has been prepared by oxidation of the rhodium(I) precursor (PNP)Rh(H2C=CHBut) (1) with AgOTf. A series of related rhodium(II) complexes of the general formula (PNP)Rh(X) (where X = OAc (3), OSiPh3 (4), OC6H4F (5), Cl (6)) was synthesized via simple anion metathesis reactions starting from 2. In addition, complexes 3 and 6 could be prepared by hydrogen atom abstraction from (PNP)Rh(H)(OAc) (7) or (PNP)Rh(H)(Cl) (8) with TEMPO ((2,2,6,6-tetramethylpiperidin-1-yl)oxyl). Solid-state X-ray structures of compounds 2-6 revealed an approximately square-planar environment about Rh. Analysis of the structural features of 2-6, EPR spectroscopic data, and DFT computational studies are most consistent with a +2 oxidation state for rhodium Reactions of 2, 3, 5, and 6 with H-2 were explored. The reaction of 2 with H-2 gave the new complex (PN(H)P)Rh(H)(2)(OTf) (9), and the reaction of 3 with H-2 produced (PNP)Rh(H)(OAc) (7), whereas the reaction of 5 with H-2 gave the known (PNP)Rh(H-2) (10), all with complete consumption of the starting rhodium(II) complexes. In contrast, the reaction of 6 with H-2 produced a mixture of (PNP)Rh(H)(Cl) (8) and (PN(H)P)Rh(H)(2)(Cl) (11) in apparent equilibrium with 6 and H-2. (PNP)Rh(H-2) (10) was identified as an elongated dihydrogen complex.