A nitridoniobium(V) reagent that effects acid chloride to organic nitrile conversion: Synthesis via heterodinuclear (Nb/Mo) dinitrogen cleavage, mechanistic insights, and recycling

The transformation of acid chlorides (RC(O)Cl) to organic nitriles (RC equivalent to N) by the terminal niobium nitride anion [N equivalent to Nb(N[Np]Ar)(3)](-) ([1a-N](-), where Np = neopentyl and Ar = 3,5-Me2C6H3) via isovalent N for O(Cl) metathetical exchange is presented. Nitrido anion [1a-N](-) is obtained in a heterodinuclear N-2 scission reaction employing the molybdenum trisamide system, Mo(N[R]Ar)(3) (R = t-Bu, 2a; R = Np, 2b), as a reaction partner. Reductive scission of the heterodinuclear bridging N-2 complexes, (Ar[R]N)(3)Mo-(mu-N-2)Nb(N[Np]Ar)(3) (R = t-Bu, 3b; R = Np, 3c) with sodium amalgam provides 1 equiv each of the salt Na[1a-N] and neutral N equivalent to Mo(N[R]Ar)(3) (R = t-Bu, 2a-N; R = Np, 2b-N). Separation of 2-N from Na[1a-N] is readily achieved. Treatment of salt Na[1a-N] with acid chloride substrates in tetrahydrofuran (THF) furnishes the corresponding organic nitriles concomitant with the formation of NaCl and the oxo niobium complex O equivalent to Nb(N[Np]Ar)(3) (1a-O). Utilization of N-15-labeled N-15(2) gas in this chemistry affords a series of N-15-labeled organic nitriles establishing the utility of anion [1a-N](-) as a reagent for the N-15-labeling of organic molecules. Synthetic and computational studies on model niobium systems provide evidence for the intermediacy of both a linear acylimido and niobacyclobutene species along the pathway to organic nitrile formation. High-yield recycling of oxo 1a-O to a niobium triflate complex appropriate for heterodinuclear N2 scission has been developed. Specifically, addition of triflic anhydride (Tf2O, where Tf = SO2CF3) to an Et2O solution of 1a-O providesthe bistriflate complex, Nb(OTf)(2)(N[Np]Ar)(3) (1a-(OTf)(2)), in near quantitative yield. One-electron reduction of 1a-(OTf)(2) with either cobaltocene (Cp2Co) or Mg(THF)(3)(anthracene) provided the monotriflato complex, Nb(OTf)(N[Np]Ar)(3) (1a-(OTf)), which efficiently regenerates complexes 3b and 3c when treated with the molybdenum dinitrogen anions [N2Mo(N[t-Bu]Ar)(3)](-) ([2a-N-2](-)) or [N2Mo(N[Np]Ar)(3)](-) ([2b-N-2](-)), respectively.