Center for Computational Quantum Chemistry, South China Normal University, Guangzhou 510631, PR China.
J Phys Chem A. 2011 Aug 18;115(32):9022-32. doi: 10.1021/jp203956k. Epub 2011 Jul 26.
Compounds of the type M(2)Bz(3) (Bz = benzene, C(6)H(6)) have been of interest since the related triple-decker mesitylenechromium sandwich (1,3,5-Me(3)C(6)H(3))(3)Cr(2) has been synthesized and characterized structurally by X-ray crystallography. Theoretical studies predict the lowest-energy M(2)Bz(3) structures of the early transition metals Ti, V, and Cr to be the triple-decker sandwiches trans-Bz(2)M(2)(η(6),η(6)-μ-C(6)H(6)) having quintet, triplet, and singlet spin states, respectively. In these structures, the central benzene ring functions as a hexahapto ligand to each metal atom. The singlet rice-ball cis-Bz(2)M(2)(μ-C(6)H(6)) structures with a 2.64-Å Mn═Mn double bond or a 2.81-Å Fe-Fe single bond are preferred for the central transition metals Mn and Fe. Singlet triple-decker-sandwich structures trans-Bz(2)M(2)(μ-C(6)H(6)) return as the lowest-energy structures for the late transition metals Co and Ni but with the central benzene ring only partially bonded to each metal atom. Thus, the lowest-energy cobalt derivative has a trans-Bz(2)Co(2)(η(3),η(3)-μ-C(6)H(6)) structure in which the central benzene ring acts as a trihapto ligand to each metal atom. Similarly, the lowest-energy nickel derivative has a trans-Bz(2)Ni(2)(η(2),η(2)-μ-C(6)H(6)) structure in which the central benzene ring acts as a dihapto ligand to each metal atom, leaving an uncomplexed C═C double bond. The metal-metal bond orders in the singlet "rice-ball" structures cis-Bz(2)M(2)(μ-C(6)H(6)) (M = Mn, Fe) and the hapticities of the central benzene rings in the singlet late-transition-metal triple-decker-sandwich structures trans-Bz(2)M(2)(μ-C(6)H(6)) (M = Co, Ni) are governed by the desirability for the metal atoms to attain the favored 18-electron configuration.
一直以来，M(2)Bz(3)（Bz = 苯，C(6)H(6)）类化合物都很受关注，因为相关的三联苯甲叉铬夹心物（1,3,5-Me(3)C(6)H(3))(3)Cr(2)已通过 X 射线晶体学结构进行了合成和表征。理论研究预测，最低能量的 M(2)Bz(3)结构的早期过渡金属 Ti、V 和 Cr 是三重苯甲叉夹心物 trans-Bz(2)M(2)(η(6),η(6)-μ-C(6)H(6))，分别具有五重态、三重态和单重态自旋态。在这些结构中，中心苯环作为每个金属原子的六配位配体。具有 2.64-Å Mn═Mn 双键或 2.81-Å Fe-Fe 单键的单重“饭团”顺式-Bz(2)M(2)(μ-C(6)H(6))结构是中间过渡金属 Mn 和 Fe 的首选。单重三重夹心结构 trans-Bz(2)M(2)(μ-C(6)H(6))返回作为晚期过渡金属 Co 和 Ni 的最低能量结构，但中心苯环仅与每个金属原子部分键合。因此，最低能量的钴衍生物具有 trans-Bz(2)Co(2)(η(3),η(3)-μ-C(6)H(6))结构，其中中心苯环作为每个金属原子的三配位配体。同样，最低能量的镍衍生物具有 trans-Bz(2)Ni(2)(η(2),η(2)-μ-C(6)H(6))结构，其中中心苯环作为每个金属原子的二配位配体，留下未配位的 C═C 双键。单重“饭团”结构 cis-Bz(2)M(2)(μ-C(6)H(6))（M = Mn，Fe）中的金属-金属键序和单重晚期过渡金属三重夹心结构 trans-Bz(2)M(2)(μ-C(6)H(6))（M = Co，Ni）中中心苯环的配位体都受金属原子达到理想的 18 电子构型的影响。
