Hydroxide-Bridged Cubane Complexes of Nickel(II) and Cadmium(II): Magnetic, EPR, and Unusual Dynamic Properties

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• 作者：Daniel L. Reger ; Andrea E. Pascui ; Perry J. Pellechia ; Mark D. Smith ; Julia Jezierska ; Andrew Ozarowski
• 刊名：Inorganic Chemistry
• 出版年：2014
• 出版时间：May 5, 2014
• 年：2014
• 卷：53
• 期：9
• 页码：4325-4339
• 全文大小：718K
• 年卷期：v.53,no.9(May 5, 2014)
• ISSN：1520-510X

文摘

The reactions of M(ClO₄)₂路xH₂O (M = Ni(II) or Cd(II)) and m-bis[bis(1-pyrazolyl)methyl]benzene (L_m) in the presence of triethylamine lead to the formation of hydroxide-bridged cubane compounds of the formula [M₄(渭₃-OH)₄(渭-L_m)₂(solvent)₄](ClO₄)₄, where solvent = dimethylformamide, water, acetone. In the solid state the metal centers are in an octahedral coordination environment, two sites are occupied by pyrazolyl nitrogens from L_m, three sites are occupied by bridging hydroxides, and one site contains a weakly coordinated solvent molecule. A series of multinuclear, two-dimensional and variable-temperature NMR experiments showed that the cadmium(II) compound in acetonitrile-d₃ has C₂ symmetry and undergoes an unusual dynamic process at higher temperatures (螖G_Lm^{鈥?/sup> = 15.8 卤 0.8 kcal/mol at 25 掳C) that equilibrates the pyrazolyl rings, the hydroxide hydrogens, and cadmium(II) centers. The proposed mechanism for this process combines two motions in the semirigid L_m ligand termed the 鈥淐olumbia Twist and Flip:鈥?twisting of the pyrazolyl rings along the C_pz鈥揅_methine bond and 180掳 ring flip of the phenylene spacer along the C_Ph鈥揅_methine bond. This dynamic process was also followed using the spin saturation method, as was the exchange of the hydroxide hydrogens with the trace water present in acetonitrile-d₃. The nickel(II) analogue, as shown by magnetic susceptibility and electron paramagnetic resonance measurements, has an S = 4 ground state, and the nickel(II) centers are ferromagnetically coupled with strongly nonaxial zero-field splitting parameters. Depending on the Ni鈥揙鈥揘i angles two types of interactions are observed: J₁ = 9.1 cm鈥? (97.9 to 99.5掳) and J₂ = 2.1 cm鈥? (from 100.3 to 101.5掳). 鈥淏roken symmetry鈥?density functional theory calculations performed on a model of the nickel(II) compound support these observations.}