Zinc(II) and Cadmium(II) Monohydroxide Bridged, Dinuclear Metallacycles: A Unique Case of Concerted Double Berry Pseudorotation

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• 作者：Daniel L. Reger ; Andrea E. Pascui ; Perry J. Pellechia ; Mark D. Smith
• 刊名：Inorganic Chemistry
• 出版年：2013
• 出版时间：October 7, 2013
• 年：2013
• 卷：52
• 期：19
• 页码：11638-11649
• 全文大小：660K
• 年卷期：v.52,no.19(October 7, 2013)
• ISSN：1520-510X

文摘

The reactions of M(ClO₄)₂路6H₂O [M = Zn(II), Cd(II)] and the ligands m-bis[bis(1-pyrazolyl)methyl]benzene, L_m, or m-bis[bis(3,5-dimethyl-1-pyrazolyl)methyl]benzene, L_m*, in the presence of a base yield the hydroxide bridged dinuclear metallacycles [M₂(渭-OH)(渭-L)₂](ClO₄)₃, L = L_m, M = Zn(II) (1); L = L_m*, M = Zn(II) (2), Cd(II) (3). In the solid state, the coordination environment of the metals is distorted trigonal bipyramidal with the bridging hydroxide in an equatorial position and M-O-M angles greater than 161掳. The observation of two equal intensity resonances for each type of pyrazolyl-ring hydrogen in the ¹H NMR for all three complexes coupled with the determination of the hydrodynamic radius based on the diffusion coefficient of 1 that matches that observed in the crystal structure, demonstrate this structure is retained in solution. Additional proof of the dinuclear structures in solution is given by the ¹¹³Cd NMR spectrum of [Cd₂(渭-OH)(渭-L_m*)₂](ClO₄)₃ showing ^111/113Cd satellites (J¹¹¹_Cd-¹¹³_Cd = 173 Hz). Complex 1 is dynamic in solution, with the resonances for each type of pyrazolyl-ring hydrogen broadening and averaging at higher temperatures. Detailed variable temperature studies show that 螖G_pz = 15.2(卤0.2) kcal/mol, 螖H_pz = 6.6(卤0.1) kcal/mol, and 螖S_pz = 鈭?8.8(卤0.4) cal/mol路K at 25 掳C for this process. The same 螖G value for the dynamic process was also determined by saturation transfer experiments. The most plausible mechanism for this dynamic process, which exchanges the axial and equatorial positions of the pyrazolyl rings in the trigonal bipyramidal arrangement, involves Berry pseudorotation at both metal sites using the bridging oxygen atom as the pivot ligand, coupled with the ring flip of the ligand鈥檚 phenylene spacer by 180掳, a rearrangement process we termed the 鈥淐olumbia Twist and Flip鈥? This process was shown to be influenced by trace amounts of water in the solvent, with a linear relationship between the water concentration and 螖G_pz; increasing the water concentration lowers 螖G_pz. Spin saturation transfer experiments demonstrated the exchange of the hydrogens between the water in the solvent and the bridging hydroxide group, with 螖G_OH = 16.8(卤0.2) kcal/mol at 25 掳C, a value larger than the barrier of 螖G_pz = 15.2(卤0.2) kcal/mol for the 鈥淐olumbia Twist and Flip鈥? Compounds 2 and 3 do not show dynamic behavior involving the pyrazolyl-rings in solution because of steric crowding caused by the methyl group substitution, but do show the exchange between the water in the solvent and the bridging hydroxide group.