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 = O 3 SCF 3 and PNP = [κ 3 - P, N, P -(4-Me-2-( i Pr 2 P)-C 6 H 3 ) 2 N]), has been prepared by oxidation of the rhodium(I) precursor (PNP)Rh(H 2 C═CHBu t ) ( 1 ) with AgOTf. A series of related rhodium(II) complexes of the general formula (PNP)Rh(X) (where X = OAc ( 3 ), OSiPh 3 ( 4 ), OC 6 H 4 F ( 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.