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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
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.