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⋮N) by the terminal niobium
nitride anion [N⋮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 N2
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 N2 complexes, (Ar[R]N)3Mo-(μ-N2)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⋮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⋮Nb(N[Np]Ar)3 (1a-O). Utilization of 15N-labeled 15N2 gas in this chemistry affords a series of 15N-labeled
organic nitriles establishing the utility of anion [1a-N]- as a reagent for the 15N-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 provides the 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-N2]-) or [N2Mo(N[Np]Ar)3]- ([2b-N2]-),
respectively.

本文报道了通过等价的N取代O(Cl)的复分解交换反应，由末端铌氮化物阴离子[N⋮Nb(N[Np]Ar)₃]⁻（记为[1a-N]⁻，其中Np为新戊基（neopentyl），Ar为3,5-二甲基苯基（3,5-Me₂C₆H₃））将酰氯（RC(O)Cl）转化为有机腈（RC≡N）的过程。氮化阴离子[1a-N]⁻可通过以三酰胺钼体系Mo(N[R]Ar)₃（R为叔丁基（t-Bu）时记为2a，R为新戊基时记为2b）作为反应底物的异双核N₂裂解反应制备得到。使用钠汞齐对异双核桥联N₂配合物(Ar[R]N)₃Mo-(μ-N₂)Nb(N[Np]Ar)₃（R为叔丁基时记为3b，R为新戊基时记为3c）进行还原裂解，可分别得到等物质的量的盐Na[1a-N]与中性配合物N⋮Mo(N[R]Ar)₃（R为叔丁基时记为2a-N，R为新戊基时记为2b-N）。可便捷实现2-N与Na[1a-N]的分离。将盐Na[1a-N]与酰氯底物在四氢呋喃（tetrahydrofuran, THF）中反应，可得到对应的有机腈，同时生成氯化钠（NaCl）与氧代铌配合物O⋮Nb(N[Np]Ar)₃（记为1a-O）。在该反应体系中使用¹⁵N标记的¹⁵N₂气体，可得到一系列¹⁵N标记的有机腈，证实了阴离子[1a-N]⁻可作为有机分子¹⁵N标记的试剂。对模型铌体系的合成与计算研究表明，在生成有机腈的反应路径中，线性酰亚胺基与铌杂环丁烯物种均作为反应中间体存在。现已开发出将氧代配合物1a-O高效循环再生为适用于异双核N₂裂解反应的三氟甲磺酸盐铌配合物的方法。具体而言，向1a-O的乙醚（Et₂O）溶液中加入三氟甲磺酸酐（Tf₂O，其中Tf为-SO₂CF₃），可得到近定量产率的双三氟甲磺酸盐配合物Nb(OTf)₂(N[Np]Ar)₃（记为1a-(OTf)₂）。使用二茂钴（cobaltocene, Cp₂Co）或Mg(THF)₃(蒽)对1a-(OTf)₂进行单电子还原，可得到单三氟甲磺酸盐配合物Nb(OTf)(N[Np]Ar)₃（记为1a-(OTf)）；该配合物分别与二氮合钼阴离子[N₂Mo(N[t-Bu]Ar)₃]⁻（记为[2a-N₂]⁻）或[N₂Mo(N[Np]Ar)₃]⁻（记为[2b-N₂]⁻）反应时，可高效再生配合物3b与3c。