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Reactions of Terphenyl-Substituted Digallene AriPr4GaGaAriPr4 (AriPr4 = C6H3‑2,6-(C6H3‑2,6‑iPr2)2) with Transition Metal Carbonyls and Theoretical Investigation of the Mechanism of Addition

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journal contribution
posted on 29.01.2016, 00:00 by Madison L. McCrea-Hendrick, Christine A. Caputo, Christopher J. Roberts, James C. Fettinger, Heikki M. Tuononen, Philip P. Power
The neutral digallene AriPr4GaGaAriPr4 (AriPr4 = C6H3-2,6-(C6H3-2,6-iPr2)2) was shown to react at ca. 25 °C in pentane solution with group 6 transition metal carbonyl complexes M­(CO)6 (M = Cr, Mo, W) under UV irradiation to afford compounds of the general formula trans-[M­(GaAriPr4)2(CO)4] in modest yields. The bis­(gallanediyl) complexes were characterized spectroscopically and by X-ray crystallography, which demonstrated that they were isostructural. In each complex, the gallium atom is two-coordinate with essentially linear geometry, which is relatively rare for gallanediyl-substituted transition metal species. The experimental data show that the gallanediyl ligand :GaAriPr4 behaves as a good σ-donor but a poor π-acceptor, in agreement with prior theoretical analyses on related systems. In addition, the monogallanediyl complex Mo­(GaAriPr4)­(CO)5 was synthesized by reacting AriPr4GaGaAriPr4 with two equivalents of Mo­(CO)5NMe3 in THF solution. The mechanism of the reaction between AriPr4GaGaAriPr4 and Cr­(CO)6 was probed computationally using density functional theory. The results suggest that the reaction proceeds via an intermediate monogallanediyl complex, Cr­(GaAriPr4)­(CO)5, that can be generated via two pathways, one of which involves the dimeric AriPr4GaGaAriPr4, that are possibly competing. AriPr4GaGaAriPr4 was also shown to react readily under ambient conditions with Co2(CO)8 to give the monosubstituted dicobalt complex Co2(μ-GaAriPr4)­(μ-CO)­(CO)6 by X-ray crystallography. The :GaAriPr4 unit bridges the Co–Co bond unsymmetrically in the solid state. No evidence was found for incorporation of more than one :GaAriPr4 unit into the dicobalt complex.