Trianionic Pincer Complexes of Niobium and Tantalum as Precatalysts for ROMP of Norbornene

This report details the synthesis and characterization of a series of Nb complexes and one tantalum complex supported by the [CF3–ONO]3– trianionic pincer-type ligand. Access to the trianionic Nb dichloride complex, [CF3–ONO]­NbCl2(OEt2) (3-Et2O), allows for the synthesis of Nb-dialkyl complexes, [CF3–ONO]­NbR2 (where R = benzyl (5), neopentylsilyl (6), neophyl (7)). The sterically encumbered Nb-neophyl complex (7) is thermally stable and fails to convert to the alkylidene even in the presence of donor ligands. Complex 7, however, promotes catalytic ring-opening metathesis polymerization (ROMP) of norbornene, suggesting the plausible intermediacy of a Nb-alkylidene. The corresponding Ta analogue, [CF3–ONO]­Ta­(CH2C­(CH3)2(C6H5))2 (9), requires dramatically higher temperatures to initiate ROMP and provides poor yields of polymer. Complexes 5 and 6 also promote ROMP of norbornene. Characterization of all new complexes includes multinuclear NMR spectroscopy and combustion analysis. For complex 7, characterization also includes solid-state structure elucidation via a single crystal X-ray diffraction experiment.