Molybdenum
17- and 18-Electron Bis- and Tris(Butadiene) Complexes: Electronic
Structures, Spectroscopic Properties, and Oxidative Ligand Substitution
Reactions
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Abstract
New results on the
electronic structures, spectroscopic properties, and reactivities
of the molybdenum tris(butadiene) and tris(2,3-dimethylbutadiene)
complexes [Mo(bd)<sub>3</sub>] (<b>1</b><sup><b>bd</b></sup>) and [Mo(dmbd)<sub>3</sub>] (<b>1</b><sup><b>dmbd</b></sup>), respectively, are reported. Importantly, the metal ligand
bonding interaction can be weakened by oxidizing the metal center
with ferrocenium salts. The addition of the bidentate phosphine ligand
1,2-bis(diphenylphosphino)ethane then leads to a new type of stable
17-electron complex, [Mo(dmbd)<sub>2</sub>(dppe)](X) (<b>2</b>; X = BF<sub>4</sub><sup>–</sup>, PF<sub>6</sub><sup>–</sup>, BPh<sub>4</sub><sup>–</sup>), where one of the butadiene
ligands is exchanged by a chelating phosphine. Reduction of the cationic
complexes <b>2</b> generates the corresponding 18-electron complex
[Mo(dmbd)<sub>2</sub>(dppe)] (<b>3</b>), thus establishing a
new strategy for ligand substitution reactions in [Mo(bd)<sub>3</sub>] complexes via one-electron oxidized intermediates. The new heteroleptic
molybdenum complexes are characterized by X-ray structure analysis;
vibrational, NMR, and EPR spectroscopy; and electrochemistry. DFT
calculations are performed to explain the structural and specroscopic
trends observed experimentally. For compound <b>1</b><sup><b>bd</b></sup>, a normal coordinate analysis is presented, providing
additional information on the bonding situation in this type of complex