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 = [κ³-P,N,P-(4-Me-2-(ⁱPr₂P)-C₆H₃)₂N]), has been prepared by oxidation of the rhodium­(I) precursor (PNP)­Rh­(H₂CCHBu^t) (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 H₂ were explored. The reaction of 2 with H₂ gave the new complex (PN­(H)­P)­Rh­(H)₂(OTf) (9), and the reaction of 3 with H₂ produced (PNP)­Rh­(H)­(OAc) (7), whereas the reaction of 5 with H₂ gave the known (PNP)­Rh­(H₂) (10), all with complete consumption of the starting rhodium­(II) complexes. In contrast, the reaction of 6 with H₂ produced a mixture of (PNP)­Rh­(H)­(Cl) (8) and (PN­(H)­P)­Rh­(H)₂(Cl) (11) in apparent equilibrium with 6 and H₂. (PNP)­Rh­(H₂) (10) was identified as an elongated dihydrogen complex.