Dinuclear organometallic dinitrogen complexes of niobium

The niobium(III) chloride precursors (R)[P2N2]NbCl stabilized by the bis-(amidophosphine) macrocycle (where (R)[P2N2] RP(CH2SiMe2NSiMe2CH2)(2)PR, R = phenyl, Ph, or cyclohexyl, Cy), react with MeMgCl under argon to form the paramagnetic (R)[P2N2]NbMe (R = Ph, 1; R = Cy, 2) complexes. The methyl complexes I and 2 can be stabilized by the donor solvent pyridine to form the paramagnetic adducts R [P2N2]NbMe(py) (R = Ph, 3; R = Cy, 4). Methyl complexes 1 and 2 readily react with N-2 to form the diamagnetic dinuclear dinitrogen compounds, (R[P2N2]NbMe)(2)(mu-N-2) (R = Ph, 5; R = Cy, 6), which have been characterized crystallographically. Dinitrogen complex 5 is fluxional and undergoes enantiomeric inversion of the six-coordinate niobium centres via axial rotation about the Nb-N-2-Nb axis. Complex 6 shows no enantiomeric inversion, likely due to the steric effects of the cyclohexyl substituents. The dinitrogen ligand itself is labile at elevated temperatures and undergoes reversible dissociation; the dinitrogen complexes are reformed upon cooling to room temperature. Reaction of 5 and 6 with CO displaces the N-2 fragment forming the bridging acyl complexes (R [P2N2]Nb)(2)(mu-COCH3)(2) (R = Ph, 7; R = Cy, 8) where the carbonyl fragments bond to the metal centres in a eta(2) fashion. Addition of H-2 to 5 and 6 results in the formation of the previously reported paramagnetic dinuclear dinitrogen complex, ((R)[P2N2]Nb)(2)(mu-N-2). (C) 2002 Elsevier Science B.V. All rights reserved.