Synthesis and reactivity of reduced niobium imido complexes
In this dissertation is presented the synthesis and reactivity of Nb(III) imido complexes supported by the BDI ligand (BDI = &beta-diketimate).
Chapter I. The discovery of a Nb(III)-mediated catalytic hydrogenation of internal alkynes to Z-alkenes is reported and found to proceed through an unprecedented mechanism. The mechanistic proposal involves initial reduction of the alkyne by the Nb(III) complex (BDI)Nb(NtBu)(CO)2 to provide a Nb(V) metallacyclopropene, itself capable of &sigma-bond metathesis reactivity with H2. The resulting alkenyl hydride species then undergoes reductive elimination to provide the Z-alkene product and regenerate a metal complex in the Nb(III) oxidation state. Support for the proposed mechanism is derived from i) the dependence of product selectivity on the relative concentrations of CO and H2, ii) the isolation of complexes closely related to those proposed to lie on the catalytic cycle, iii) H/D crossover experiments, and iv) DFT studies on multiple possible reaction pathways.
Chapter II. Monometallic niobium arene complexes [Nb(BDI)(NtBu)(&mu-RC6H5)] (R = H and Me) were synthesized and were found to slowly converted into the diniobium inverted arene sandwich complexes [[(BDI)Nb(NtBu)]2(&mu-RC6H5)] (R = H and Me) in solution. The kinetics of this reaction were followed by 1H NMR spectroscopy, and is in agreement with a dissociative mechanism. These compounds showed a lack of reactivity towards small molecules - even at elevated temperatures - which is unusual in the chemistry of inverted sandwich complexes. However, protonation of the BDI ligands occurred readily on treatment with [H(OEt2)][B(C6F5)4], resulting in the mono-protonated cationic inverted sandwich complex [[(BDI#)Nb(NtBu)][(BDI)Nb(NtBu)](&mu-C6H5)][B(C6F5)4] and the dicationic complex [[(BDI#)Nb(NtBu)]2(&mu-RC6H5)][B(C6F5)4]2 (BDI# = (ArNC(Me))2CH2). NMR and UV-vis spectroscopies were used to characterize this unique series of diamagnetic molecules as a means of determining how best to describe the Nb-arene interactions. The X-ray crystal structures, UV-visible spectra, arene 1H NMR chemical shifts and large JCH coupling constants provide evidence for donation of electron density from the Nb d-orbitals into the antibonding π system of the arene ligands. However, the lack of sp3 hybridization of the arene carbon indicate that the Nb-arene donation is not accompanied by an increase in formal oxidation state, and suggest that 4d2 electronic configurations are appropriate to describe the Nb atoms in all four complexes.
Chapter III. Inverted sandwich complexes have seen interesting recent developments both in the nature of their bonding and in their use as chemical reactants. Although discussions concerning the electronic delocalization lend credit to their use as potential electronic and spintronic devices, mixed valent inverted sandwich complexes are rarely reported. We show in this work that the selective single electron oxidation of a neutral benzene inverted sandwich complex of niobium leads to an isolable cationic mixed valent benzene inverted sandwich complex. The latter complex shows unique structural features elucidated through studies with an arsenal of physical methods, including cyclic voltammetry, 1H NMR, UV-Vis, magnetism, EPR spectroscopies, in addition to DFT calculations. These analyses indicate that although delocalization is allowed over the benzene ring from both niobium atoms, the single electron is unequally shared between the two metal centres. Under certain conditions, this complex reforms the neutral benzene complex along with a highly reactive Nb(IV) species, which is of great interest for potential chemical reactivity.
Chapter IV. All three C-F bonds in CF3-substituted arenes are activated by a niobium imido complex, driven by the formation of strong Nb-F bonds. The mechanism of this transformation was studied by NMR spectroscopy which revealed the involvement of Nb(III). Attempts to extend this chemistry to non-aromatic CF3 groups led to intramolecular reactivity.The mechanism of activation of C-F bonds in fluoroarenes using a well-defined niobium (III) imido complex has been investigated.
Chapter V. The Nb(III) arene species [BDI]Nb(NtBu)(C6H6), reacts stoichiometrically with fluoroarenes to yield niobium (V) aryl fluorides. Spectroscopic analysis supported by DFT calculations revealed the critical involvement of a Nb(III) fluoroarene-bound species. In contrast to previous reports of related reactivity, we found that perfluorinated arenes (i.e. those normally assumed to bear more `activated' C-F bonds) are, in the present system, much less reactive towards C-F bond cleavage than mono- or difluoro-substituted arenes. In addition to demonstrating stoichiometric hydrodefluorination reactions, we also describe an efficient and mild hydrodefluorination of mono- and di-fluoroarenes that is catalytic in niobium.