Reactivity of Niobium and Tantalum Imido and Bis(imido) Complexes

Chapter 1. Previous collaborative work between the Arnold and Bergman groups on niobium and tantalum systems supported by N,N’-diaryl-β-diketiminate (BDI, aryl = 2,6-iPr2-C6H3) and tert-butylimido ligands is summarized. Routes to prepare high-valent niobium and tantalum halide and alkyl starting materials are described, and the reactions of these complexes with small molecule substrates are discussed. Reactions to access low-valent niobium systems are described, and several examples are provided in which low-valent niobium systems promote unusual stoichiometric and catalytic transformations. In addition, the chemistry of high-valent BDI niobium bis(imido) systems is discussed.Chapter 2. The nitrene transfer reactivity of a series of BDI niobium bis(imido) complexes is reported. A novel metathesis process results in nitrene fragment exchange between alkylimido groups in Nb(V) bis(imido) complexes and arylisocyanide substrates. Calculations and experimental evidence indicate that these reactions likely proceed through cycloaddition and cycloreversion steps involving Nb(V) η2-carbodiimide intermediates, and occurs at a single metal center without elimination of carbodiimide. In contrast, reaction with unhindered alkylisocyanides results in extrusion of dialkylcarbodiimide, a process that was rendered catalytic in the presence of excess azide. This represents a rare example of oxidative nitrene transfer promoted by an early transition metal complex, in which reactivity across an imido group and two-electron metal-based redox chemistry are used in tandem to effect catalytic turnover.Chapter 3. BDI niobium bis(imido) complexes are shown to react with a variety of substrates across their Nb-N π-bonds by cycloaddition and 1,2-addition. Niobium bis(imido) intermediates generated by treatment of a Nb(III) precursor with azide substrates react further with azide substrates via [3+2] cycloaddition to give niobium tetrazene complexes. These compounds exist in equilibrium with the corresponding bis(imido) complexes, and are competent catalyst precursors for oxidative nitrene transfer to generate carbodiimides. Isolated bis(imido) complexes are also shown to engage in stoichiometric 1,2-addition reactions with dihydrogen, silanes, boranes, thiols, and terminal alkynes. These bis(imido) compounds also exhibit [2+2] cycloaddition and cycloreversion reactivity with carbon dioxide and carbon disulfide. Through these studies, the first example of an isolated early transition metal (group 3 to 5) complex bearing both a terminal imido and a terminal oxo ligand is accessed. Likely as a consequence of π-loading effects, this compound is reactive toward 1,2-addition of silane substrates across its oxo group.Chapter 4. The cyclometallated tantalum(V) hydride complex {ArNC(Me)CHC(Me)N[2-(CHMeCH2)-6-iPr-C6H3]}Ta(NtBu)H was prepared from hydrogenolysis of (BDI)TaNtBuMe2 (BDI = N,N′-diaryl-β-diketiminate, aryl = 2,6-iPr2-C6H3). Based on mechanistic studies, formation of the hydride complex likely proceeds through a dihydride intermediate generated from successive σ-bond metathesis steps. Low-valent Ta(III) dicarbonyl derivatives are accessed by either introducing CO atmosphere to the DMAP adduct at room temperature, or by directly adding CO at low temperature. Instead of promoting reductive elimination, 2,6-dimethylphenylisocyanide reacts with the tantalum hydride complex via migratory insertion to give a product containing a unique metallaimidazole ring. This complex can be reversibly oxidized by one electron either electrochemically or chemically to give a tantalum radical cation species. DFT calculations, as well as X-ray crystallographic and EPR spectroscopic data are consistent with the electron being removed from an orbital primarily composed of Ta-C π-bonding character, but also delocalized over the tantalaimidazole ring.Chapter 5. The synthesis and reactivity of niobium and tantalum complexes bearing tert-butylimido and N,N′-di-2,4,6-trimethylphenyl-β-diketiminate (BDIAr) or N,N′-di2,6-dichlorophenyl-β-diketiminate (BDICl) ligands is reported. Reaction of dimethyl complex (BDIAr)Nb(NtBu)Me2 with dihydrogen leads to generation of bis-μ-hydrido Nb(IV)-Nb(IV) complex {(BDIAr)Nb(NtBu)(μ-H)}2. One-electron oxidation of this complex results in release of dihydrogen to generate the imido-bridged mixed-valent Nb(III)-Nb(IV) dimer [{(BDIAr)Nb(μ-NtBu)}2][B(C6F5)4]. EPR spectroscopic measurements and DFT calculations are consistent with a symmetric structure containing a Nb-Nb single bond in which the unpaired electron is delocalized over both niobium centers