A General Synthetic Route to NHC-Phosphinidenes: NHC-mediated Dehydrogenation of Primary Phosphines

Abstract

The dehydrocoupling of primary phosphines with N-heterocyclic carbenes (NHCs) to yield NHC-phosphinidenes is reported. The reaction of two equivalents of the NHCs Me2Im (1,3-dimethylimidazolin-2-ylidene), Me4Im (1,3,4,5-tetramethylimidazolin-2-ylidene), iPr2Im (1,3-di-iso-propylimidazolin-2-ylidene) and Mes2Im (2,4,6-trimethylphenylimidazolin-2-ylidene) with PhPH2 and MesPH2 led to the NHC stabilized phosphinidenes (NHC)PAr: (iPr2Im)PPh (1), (Mes2Im)PPh (2), (Me4Im)PPh (3), (Mes2Im)PMes (4), (Me2Im)PMes (5), (Me4Im)PMes (6) and (iPr2Im)PMes (7). The reaction of tBuPH2 with two equivalents of the NHCs afforded the corresponding NHC stabilized parent phosphinidenes (NHC)PH: (iPr2Im)PH (8), (Mes2Im)PH (9) and (Me4Im)PH (10). Reaction of 1 with oxygen and sulfur led to isolation of iPr2Im-P(O)2Ph (11) and iPr2Im-P(S)2Ph (12), whereas the reaction with elemental selenium and tellurium gave (NHC)PPh cleavage with formation of (iPr2Im)Se (13), iPr2ImTe (14) and different cyclo-oligophosphines. Furthermore, the complexes [{(iPr2Im)PPh}W(CO)5] (15), [Co(CO)2(NO){(iPr2Im)PPh}] (16) and [(η5-C5Me5)Co(η2-C2H4){(iPr2Im)PPh}] (17) have been prepared starting from 1 and a suitable transition metal complex precursor. The complexes 16 and 17 decompose in solution upon heating to ca. 80 °C to yield the NHC complexes [Co(iPr2Im)(CO)2(NO)] and [(η5-C5Me5)Co(iPr2Im)(η2-C2H4)] with formation of cyclo-oligophosphines. The reaction of 1 with [Ni(COD)2] afforded the diphosphene complex [Ni(iPr2Im)2(trans-PhP=PPh)] 18.