Synthesis, Characterization, and Reactions of Isolable (β-Diketiminato)niobium(III) Imido Complexes

We have investigated both the chemical reduction of (BDI)Nb-V imido complexes (BDI = [C(Me)NAr](2); Ar = 2,6-Pr-i(2)-C6H3) to the formal Nb(III) oxidation state and the ability of thcse Nb(III) complexes to behave as two-electron reductants. The reduction of the Nb(V) species was found to depend heavily on the nature of available supporting ligands, but the chemistry of the reduced compounds proceeded cleanly with a number of unsaturated organic reagents. Accordingly, novel Nb(V) bis(imido) complexes supported by the monoazabutadiene (mad) ligand (mad)Nb((NBu)-Bu-t)(NAr)(L') (L' = py, thf) were formed by either KC8 reduction of (BDI)Nb((NBu)-Bu-t)Cl-2(py) in the absence ofstrong pi-acids or by H-2 reduction of the Nb(V) dimethyl complex (BDI)Nb((NBu)-Bu-t)Me-2 in THF. These products are likely formed though an intramolecular 2e reductive C-N bond cleavage, as has been observed previously for related group 4 systems, suggesting that transient Nb(III) intermediates were present in both cases. In the presence of 1,2-bis(dimethylphosphino)ethane (dmpe), KC8 reduction of (BDI)Nb((NBu)-Bu-t)Cl-2(py) was arrested at the Nb(IV) oxidation state to give (BDI)Nb((NBu)-Bu-t)Cl(dmpe), which was characterized by solution-state EPR spectroscopy as a Nb-centered paramagnet with strong coupling to the two equivalent phosphorus nuclei (A(iso){Nb-93} = 120.5 x 10(-4) cm(-1), A(iso){P-31} = 31.0 x 10(-4) cm(-1), g(iso) = 1.9815). When strong pi-acids were used to intercept the thermally unstable Nb(III) complex (BDI)Nb((NBu)-Bu-t)(py) prior to reductive cleavage of the ligand C-N bond, the thermally stable Nb(III) species (BDI)Nb((NBu)-Bu-t)(CX)(2)(L '') (X = O, L '' = py; X = NXyl, L '' = CNXyl; Xyl =2,6-Me-2-C6H3) were obtained in good yields. The Nb(III) complexes (BDI)Nb((NBu)-Bu-t)py, (BDI)Nb((NBu)-Bu-t)(CO)(2)(py), and (BDI)Nb((NBu)-Bu-t)(CO)(2) were subsequently investigated for their ability to serve as two-electron reducing reagents for both metal ligand multiple bond formation and for the reduction of organic pi-systems. The reduction of mesityl azicie by (BDI)Nb((NBu)-Bu-t)(py) and diphenyl sulfoxide by (BDI)Nb(NtBu)(CO)(2) led to the monomeric bis(imido) and dimeric oxo complexes (BDI)Nb((NBu)-Bu-t)(NMes)(py) and [(BDI)Nb-((NBu)-Bu-t)](2)(mu(2)-O)(2), respectively. MeLi addition to (BDI)Nb((NBu)-Bu-t)(CO)(2)(py) resulted in the formation of a Nb acylate via methide addition to one of the carbonyl carbons. The acylate product was revealed to have a short Nb-C-acylate bond distance (2.059(4) angstrom), consistent with multiple Nb-C bond character resulting from Nb(III) back-bonding into the acylate carbon. The interaction of (BDI)Nb((NBu)-Bu-t)(CO)(2) with 2 equiv of 4,41-dichlorobenzophenone resulted in the clean, quantitative formation of the corresponding pinacol coupling product, but introduction of the ketone in 1:1 molar ratios resulted in mixtures of the pinacol product and the starting material, suggesting that ketone coordination to the Nb(III) complex may be reversible. In a related manner, addition of 1-phenyl-1-propyne to (BDI)Nb((NBu)-Bu-t)(CO)(2) formed a thermally unstable 1:1 Nb/alkyne complex, as caracterized by NMR and IR spectroscopy; reaction of this species with HCl/MeOH yielded a 2:1 mixture of 1-phenyl-1-propene and the free alkyne, suggesting a high degree of covalency in the Nb-C bonds.