FACILE INSERTION OF CLPPH2 INTO NB-H BONDS AS A SYNTHETIC ROUTE TO NEW PHOSPHINO AND PHOSPHIDO DERIVATIVES OF NIOBOCENE

Synthetic approaches to phosphorus derivatives of niobocene) including a metathesis of Cp2NbH2Li with ClPPh2 and insertion of ClPPh2 into the Nb-H bond, were studied. The smooth reaction of Cp'2NbH3 (a, Cp = C5H5; b, Cp' = C5H4Me) with ClPPh2 affords [Cp'2-NbH2PHPh2]Cl (3a,b) in high yield. Deprotonation of 3 leads exclusively to Cp'2NbH(PHPh2) (2), while the isomeric complex Cp'2NbH2PPh2 (1) was not observed; it seems to be the intermediate precursor to complex 2. Direct reaction of CP2NbH2Li with ClPPh2 also gives 2a but in poorer yield. 2 was found to selectively rearrange into Cp'2Nb(PHPhC6H4-) (5), the first ortho-metalated phosphine complex of the early transition metals. The formulation of 5 was supported by H-1, C-13, and P-31 NMR spectroscopy. However, X-ray diffraction study was not possible due to twinning problems. By repeated insertion of ClPPh2 into the Nb-H bond of 2, followed by deprotonation, we obtained [Cp'2Nb(PHPh2)2]Cl (7) and (C5H4-Me)2NbPPh2(PHPh2) (8b). Deprotonation of 7 and 8b affords a new niobocene anionic complex CP'2Nb(PHPh2)2M (M = Na, Li (9)). Thermal degradation of 2, 8, and 9, leading to 5, was studied and found to proceed via an intermediate Cp'2NbPPh2 (6) which was identified by NMR spectroscopy.