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Spectroscopic and Computational Studies on [(PhTttBu)2Ni2(μ-O)2]:  Nature of the Bis-μ-oxo (Ni3+)2 “Diamond” Core

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journal contribution
posted on 29.10.2002, 00:00 by Ralph Schenker, Beaven S. Mandimutsira, Charles G. Riordan, Thomas C. Brunold
Spectroscopic and density functional theory (DFT) electronic structure computational studies on a binuclear bis-μ-oxo bridged (Ni3+)2 complex, [(PhTttBu)2Ni2(μ-O)2] (1), where (PhTttBu) represents phenyl-tris((tert-butylthio)methyl)borate, are presented and discussed. These studies afford a detailed description of the Ni2O2 core electronic structure in bis-μ-oxo (Ni3+)2 dimers and provide insight into the possible role of the (PhTttBu) thioether ligand in the formation of 1 from a Ni1+ precursor by O2 activation. From a normal coordinate analysis of resonance Raman data, a value of kNi-O = 2.64 mdyn/Å is obtained for the Ni−O stretch force constant for 1. This value is smaller than kCu-O = 2.82−2.90 mdyn/Å obtained for bis-μ-oxo (Cu3+)2 dimers possessing nitrogen donor ligands, indicating a reduced metal−oxo bond strength in 1. Electronic absorption and magnetic circular dichroism spectroscopic techniques permit identification of several O→Ni and S→Ni charge transfer (CT) transitions that are assigned on the basis of DFT calculations. The dominant O→Ni CT transition of 1 occurs at 17 700 cm-1, red-shifted by ∼7000 cm-1 relative to the corresponding transition in bis-μ-oxo (Ni3+)2 dimers with nitrogen donor ligands. This red-shift along with the relatively low value of kNi-O are due primarily to the presence of the thioether ligands in 1 that greatly affect the compositions of the Ni2O2 core MOs. This unique property of the thioether ligand likely contributes to the reactivity of the Ni center in the precursor [(PhTttBu)Ni1+CO] toward O2. DFT computations reveal that conversion of a hypothetical side-on peroxo (Ni2+)2 dimer, [(PhTttBu)2Ni2(μ-η22-O2)], to the bis-μ-oxo (Ni3+)2 dimer 1 is energetically favorable by 32 kcal/mol and occurs without a significant activation energy barrier (ΔH = 2 kcal/mol).