Relative Reactivity of the Metal−Amido versus Metal−Imido Bond in Linked Cp-Amido and Half-Sandwich Complexes of Vanadium

Treatment of (η5-C5H4C2H4NR)V(N-t-Bu)Me (R = Me, i-Pr) and CpV(N-p-Tol)(N-i-Pr2)Me (Cp = η5-C5H5) with B(C6F5)3 or [Ph3C][B(C6F5)4] results in formation of the corresponding cations, [(η5-C5H4C2H4NR)V(N-t-Bu)]+ and [CpV(N-p-Tol)(N-i-Pr2)]+. The latter could also be generated as its N,N-dimethylaniline adduct by treatment of the methyl complex with [PhNMe2H][BAr4] (Ar = Ph, C6F5). Instead, the analogous reaction with the linked Cp-amido precursor results in protonation of the imido-nitrogen atom. Sequential cyclometalation of the amide substituents gave cationic imine complexes [(η5-C5H4C2H4NCR′2)V(NH-t-Bu)]+ (R′ = H, Me) and methane. Reaction of cationic [(η5-C5H4C2H4NR)V(N-t-Bu)]+ with olefins affords the corresponding olefin adducts, whereas treatment with 1 or 2 equiv of 2-butyne results in insertion of the alkyne into the vanadium−nitrogen single bond, affording the mono- and bis-insertion products [(η5-C5H4C2H4N(i-Pr)C2Me2)V(N-t-Bu)]+ and [(η5-C5H4C2H4N(i-Pr)C4Me4)V(N-t-Bu)]+. The same reaction with the half-sandwich compound [CpV(N-p-Tol)(N-i-Pr2)]+ results in a paramagnetic compound that, upon alcoholysis, affords sec-butylidene-p-tolylamine, suggesting an initial [2+2] cycloaddition reaction. The difference in reactivity between the V−N bond versus the VN bond was further studied using computational methods. Results were compared to the isoelectronic titanium system CpTi(NH)(NH2). These studies indicate that the kinetic product in each system is derived from a [2+2] cycloaddition reaction. For titanium, this was found as the thermodynamic product as well, whereas the insertion reaction was found to be thermodynamically more favorable in the case of vanadium.

将(η⁵-C₅H₄C₂H₄NR)V(N-叔丁基(tert-butyl, t-Bu))甲基配合物[(η⁵-C₅H₄C₂H₄NR)V(N-t-Bu)Me，其中R为甲基(methyl, Me)、异丙基(isopropyl, i-Pr)]以及环戊二烯基(Cp, η⁵-C₅H₅)钒(V)-N-对甲苯基(p-tolyl, p-Tol)-N-二异丙基酰胺甲基配合物CpV(N-p-Tol)(N-i-Pr₂)Me，分别与三(五氟苯基)硼(B(C₆F₅)₃)或三苯基四(五氟苯基)硼酸盐([Ph₃C][B(C₆F₅)₄])反应，可得到对应的阳离子配合物[(η⁵-C₅H₄C₂H₄NR)V(N-t-Bu)]⁺与[CpV(N-p-Tol)(N-i-Pr₂)]⁺。后者还可通过将该甲基配合物与N,N-二甲基苯胺鎓四芳基硼酸盐([PhNMe₂H][BAr₄]，Ar=苯基(Ph)、五氟苯基(C₆F₅))反应，制备得到其N,N-二甲基苯胺加合物。与之相反，与桥联Cp-酰胺前驱体的类似反应则会发生亚氨基氮原子的质子化过程。酰胺取代基的连续环金属化反应，可生成阳离子亚胺配合物[(η⁵-C₅H₄C₂H₄NCR′₂)V(NH-t-Bu)]⁺（R′=氢(H)、甲基(Me)）与甲烷。阳离子配合物[(η⁵-C₅H₄C₂H₄NR)V(N-t-Bu)]⁺与烯烃反应可得到相应的烯烃加合物；而与1当量或2当量的2-丁炔反应时，炔烃会插入钒-氮单键中，生成单插入和双插入产物[(η⁵-C₅H₄C₂H₄N(i-Pr)C₂Me₂)V(N-t-Bu)]⁺与[(η⁵-C₅H₄C₂H₄N(i-Pr)C₄Me₄)V(N-t-Bu)]⁺。针对半夹心型配合物[CpV(N-p-Tol)(N-i-Pr₂)]⁺的同类反应则生成顺磁性产物，该产物经醇解后可得到仲丁叉基对甲苯胺，这表明反应初始步骤为[2+2]环加成反应。研究团队通过计算化学方法进一步探究了钒-氮单键(V-N)与钒-氮双键(V=N)反应活性的差异，并将实验结果与等电子钛体系CpTi(NH)(NH₂)进行了对比。上述研究表明，两个体系中的动力学产物均源自[2+2]环加成反应。对于钛体系而言，该产物同时也是热力学稳定产物；而对于钒体系来说，插入反应路径在热力学上更为有利。