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

We have investigated both the chemical reduction of (BDI)NbV imido complexes (BDI = HC[C(Me)NAr]2; Ar = 2,6-iPr2-C6H3) to the formal Nb(III) oxidation state and the ability of these
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(NtBu)(NAr)(L′)
(L′ = py, thf) were formed by either KC8 reduction
of (BDI)Nb(NtBu)Cl2(py) in
the absence of strong π-acids or by H2 reduction
of the Nb(V) dimethyl complex (BDI)Nb(NtBu)Me2 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(NtBu)Cl2(py) was arrested at the Nb(IV) oxidation state to give (BDI)Nb(NtBu)Cl(dmpe), which was characterized by solution-state
EPR spectroscopy as a Nb-centered paramagnet with strong coupling
to the two equivalent phosphorus nuclei (Aiso{93Nb} = 120.5 × 10−4 cm−1, Aiso{31P} = 31.0 ×
10−4 cm−1, giso = 1.9815). When strong π-acids were used to intercept
the thermally unstable Nb(III) complex (BDI)Nb(NtBu)(py) prior to reductive cleavage of the ligand C−N
bond, the thermally stable Nb(III) species (BDI)Nb(NtBu)(CX)2(L′′) (X = O, L′′=
py; X = NXyl, L′′ = CNXyl; Xyl =2,6-Me2-C6H3) were obtained in good yields. The Nb(III) complexes
(BDI)Nb(NtBu)py, (BDI)Nb(NtBu)(CO)2(py), and (BDI)Nb(NtBu)(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 π-systems. The reduction of mesityl azide by (BDI)Nb(NtBu)(py) and diphenyl sulfoxide by (BDI)Nb(NtBu)(CO)2 led to the monomeric
bis(imido) and dimeric oxo complexes (BDI)Nb(NtBu)(NMes)(py) and [(BDI)Nb(NtBu)]2(μ2-O)2, respectively. MeLi addition
to (BDI)Nb(NtBu)(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−Cacylate bond distance (2.059(4) Å), consistent
with multiple Nb−C bond character resulting from Nb(III) back-bonding
into the acylate carbon. The interaction of (BDI)Nb(NtBu)(CO)2 with 2 equiv of 4,4′-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(NtBu)(CO)2 formed
a thermally unstable 1:1 Nb/alkyne complex, as characterized 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.