Dinitrosyl rhenium complexes for ring-opening metathesis polymerization (ROMP)

The treatment of benzene solutions of the cations [Re(NO)2(PR3)2][BArF4] (R = Cy and R = iPr; [BArF4] = tetrakis{3,5-bis(trifluoromethyl)phenyl}borate) with phenyldiazomethane afforded the moderately stable cationic rhenium(I) benzylidene dinitrosyl bis(trialkyl) phosphine complexes as [BArF4]- salts in good yields. The cationic rhenium dinitrosyl bisphosphine complexes catalyze the ring-opening metathesis polymerization (ROMP) of highly strained nonfunctionalized cyclic olefins to give polymers with relatively high polydispersity indices, high molecular weights, and Z configurations of the double bonds in the polymer chain backbones of over 80 %. The benzylidene derivatives are almost inactive in ROMP catalysis with norbornene and in olefin metathesis. NMR experiments gave first hints for the initial formation of carbene complexes when [Re(NO)2(PR3)2][BArF4] was treated with norbornene. The carbene formation is initiated by an unique reaction sequence where the cleavage of the strained olefinic bond starts with phosphine migration forming a cyclic ylid carbene complex. The [2+2] addition of a norbornene molecule to the Re=C bond leads to the rhenacyclobutane complex, which is expected to be converted into an iminate complex by attack of the ylid function onto one of the NNO atoms followed by Wittig-type phosphine oxide elimination. The formation of phosphine oxide was confirmed by NMR spectroscopy. This species is thought to drive the ROMP metathesis with alternating rhenacyclobutane formations and cycloreversions. The proposed mechanism is supported by density functional theory (DFT) calculations.