Cyclohexadienyl niobium complexes and arene hydrogenation catalysis

Hydrogenolysis of (R)[P2N2]NbCH2SiMe3 (where (R)[P2N2] = RP(CH2SiMe2NSiMe2CH2)(2)PR; R = cyclohexyl, Cy, or phenyl, Ph) in benzene or toluene causes hydride addition to the aromatic solvent resulting in the formation of the pi-bonded complexes (R)[P2N2]Nb(eta(5)-C6H7) (R = Cy, 1; R = Ph, 2) and (R)[P2N2]Nb(eta(5)-C7H9) (R = Cy, 3; R = Ph, 4) in benzene and toluene, respectively. Performing the hydrogenation at a higher pressure of 29 atm at room temperature causes the catalytic hydrogenation of benzene to cyclohexane as determined by NMR spectroscopy and GC-MS analysis. The hydrogenation of toluene to methylcyclohexane can also be performed, but the turnover frequency is considerably lower. Examination of the solid residues from the high-pressure hydrogenations indicates the formation of the pi-bonded complexes 1 and 2. The additionof 29 atm of H-2 to these cyclohexadienyl derivatives in benzene or toluene, however, shows no hydrogenation products, indicating these species are not catalytically active.