Alkali metal and magnesium enamides from metallation of the alkyl ligands [(2-Pyr)(SiMe3)CH2] and [6-Me-(2-Pyr)(SiMe3)CH2]: a solid state and ab initio study†
作者:Philip C. Andrews、David R. Armstrong、Colin L. Raston、Brett A. Roberts、Brian W. Skelton、Allan H. White
DOI:10.1039/b009596f
日期:——
The alkali metal complexes [rNa·(pmdta)]2, 1, [rK·(pmdta)]2, 2, [rNa·(tmeda)]2, 3, [r′Li·(pmdta)], 4, and the magnesium complex, [(r)2Mg·(hmpa)2], 5, formed on metallation of the monosilylated ligands [(2-Pyr)(SiMe3)CH2] (= rH) and [6-Me-(2-Pyr)(SiMe3)CH2] (= r′H), rather than being metal alkyls, are all metal enamides in the solid state, as evidenced by single crystal X-ray diffraction studies. Even in the presence of the bi- and tri-dentate donors tmeda (= N,N,N′,N′-tetramethylethylenediamine) and pmdta (= N,N,N′,N′,N″-pentamethyldiethylenetriamine) the heavier alkali metal complexes are found to be dimeric. The Li complex is monomeric and adopts the enamide configuration despite the presence of additional steric bulk on introduction of a Me group on the 6-position of the pyridyl ring. This preference for an enamide configuration, rather than that of carbanion or aza-allyl, has been studied by ab initio MO calculations. These confirmed that as the coordination environment of the metal increases the enamide form rather then the aza-allylic form becomes the most energetically favoured configuration.
碱金属配合物[rNa-(pmdta)]2, 1、[rK-(pmdta)]2, 2、[rNa-(tmeda)]2, 3、[r′Li-(pmdta)]4 和镁配合物[(r)2Mg-(hmpa)2]5、单硅烷化配体[(2-Pyr)(SiMe3)CH2](=rH)和[6-Me-(2-Pyr)(SiMe3)CH2](=r′H)金属化后形成的镁配合物[(r)2Mg-(hmpa)2],5,在固态下都是金属酰胺,这一点可以从单晶 X 射线衍射研究中得到证明。即使在存在二齿和三齿供体 tmeda(=N,N,N′,N′-四甲基乙二胺)和 pmdta(=N,N,N′,N′,N″-五甲基二乙烯三胺)的情况下,较重的碱金属配合物也是二聚体。尽管在吡啶基环的 6 位上引入一个 Me 基团会产生额外的立体体积,但 Li 复合物是单体,并采用了烯酰胺构型。ab initio MO 计算研究了这种偏好烯酰胺构型而不是碳离子或偶氮烯丙基构型的情况。这些计算证实,随着金属配位环境的增加,烯酰胺构型而不是偶氮烯丙基构型成为能量上最有利的构型。