Dihydroperimidine-Derived PNP Pincer Complexes as Intermediates en Route to N-Heterocyclic Carbene Pincer Complexes

Abstract

The reaction of N,N′-bis(dicyclohexylphosphinomethyl)dihydroperimidine (H2C(NCH2PCy2)2C10H6-1,8, 1a) with [RuCl2(PPh3)3] in THF affords the perimidinylidene-based N-heterocyclic carbene (per-NHC) pincer complex [RuCl2(OC4H8){═C(NCH2PCy2)2C10H6}] (2) via chelate-assisted double C–H activation. In contrast, the reactions of the tetraphenyl analogue H2C(NCH2PPh2)2C10H6 (1b) with [RuCl2(PPh3)3] and of 1a with [RuCl(R)(CO)(PPh3)2] (R = Ph, CH═CHPh) do not result in C–H activation but rather give the asymmetric, PNP-coordinated complexes [RuCl2(PPh3){κ3P,N,P′-CH2(NCH2PPh2)2C10H6}] (3) and [RuCl(R)(CO){κ3P,N,P′-CH2(NCH2PCy2)2C10H6}] (R = Ph (4), CH═CHPh (5)), respectively, in which the ruthenium migrates rapidly between nitrogen donors. This provides insight into the mechanistic pathway by which the proligands 1 undergo per-NHC formation, as demonstrated by the thermal conversion of 4 to [RuHCl(CO){═C(NCH2PCy2)2C10H6}] (6) and benzene.