Ring Size Effects on the Structures of Sandwich Compounds with a Stoichiometry of C12H12M (M = Ti–Ni)

Ring size effects on geometries and electronic structures were investigated for the (CnHn)M(CmHm) (n = 4, 5, or 6; m = 8, 7, or 6; m + n = 12; M = Ti–Ni) systems using density functional theory. The lowest-energy C12H12M structures for the early transition metals titanium, vanadium, and chromium are the experimentally known singlet (η5-C5H5)Ti(η7-C7H7), doublet (η5-C5H5)V(η7-C7H7), and singlet (η6-C6H6)2Cr, respectively. The likewise experimentally known singlet (η6-C6H6)2Ti, doublet (η6-C6H6)2V, and singlet (η5-C5H5)Cr(η7-C7H7) are the second-lowest-energy structures with only a small energy difference between the two vanadium structures. For the later transition metals, dibenzenemetal complexes are the lowest-energy C12H12M species with two fully bonded hexahapto benzene rings in the lowest-energy manganese and iron derivatives and one hexahapto and one dihapto benzene ring in the lowest-energy cobalt and nickel derivatives. The lowest-energy (C5H5)M(C7H7) structures for the later transition metals iron, cobalt, and nickel have partially bonded nonplanar C7H7 rings with one or two uncomplexed CC bonds. The (C4H4)M(C8H8) (M = Ti–Ni) structures with the metal sandwiched between four- and eight-membered rings were found to be much higher in energy than their (C5H5)M(C7H7) and (C6H6)2M isomers.本研究采用密度泛函理论（density functional theory），针对(CₙHₙ)M(CₘHₘ)（其中n=4、5或6；m=8、7或6；m+n=12；M=Ti~Ni）体系，系统探究了环尺寸对其几何结构与电子结构的影响。对于早期过渡金属钛、钒和铬，能量最低的C₁₂H₁₂M结构分别为实验中已表征的单重态(η⁵-C₅H₅)Ti(η⁷-C₇H₇)、二重态(η⁵-C₅H₅)V(η⁷-C₇H₇)以及单重态二(η⁶-苯)铬((η⁶-C₆H₆)₂Cr)。同样经实验表征的单重态二(η⁶-苯)钛((η⁶-C₆H₆)₂Ti)、二重态二(η⁶-苯)钒((η⁶-C₆H₆)₂V)以及单重态(η⁵-C₅H₅)Cr(η⁷-C₇H₇)，则为第二低能量的结构；且两种钒基结构之间的能量差极小。对于后期过渡金属，能量最低的C₁₂H₁₂M物种为二苯金属配合物：其中锰和铁的最低能量衍生物具有两个完全配位的六哈普托苯环，而钴和镍的最低能量衍生物则分别带有一个六哈普托苯环与一个二哈普托苯环。针对后期过渡金属铁、钴和镍，能量最低的(C₅H₅)M(C₇H₇)结构具有部分配位的非平面C₇H₇环，其带有1个或2个未配位的碳碳双键。金属被夹在四元环与八元环之间的(C₄H₄)M(C₈H₈)（M=Ti~Ni）结构，其能量远高于对应的(C₅H₅)M(C₇H₇)与二(η⁶-苯)金属((C₆H₆)₂M)异构体。