Synthesis, Dynamics, and DFT Studies of Rhenium Dicarbonyl PNN Pincer Complexes in Three Different Oxidation States

The synthesis and characterization of ReI and ReII dicarbonyl halides 1 and 2 and the ReIII dicarbonyl dihalide 3 supported by a PNN pincer ligand are described. Complex 1 (1a, X = Br; 1b, X = Cl) was synthesized by refluxing the PNN ligand with Re(CO)5X (X = Br, Cl) in toluene. One-electron oxidation of 1 by [(4-BrC6H4)3N][SbCl6] gave 2, which when oxidized by PhIO afforded 3. X-ray and IR analysis of 1–3 revealed a systematic increase of Re–C(≡O) and decrease of C≡O bond lengths and increase in the corresponding C≡O stretching frequency. All of these results are consistent with weakening in the Re–C(≡O) bond from 1 to 3 upon increasing the oxidation state from +1 in 1 to +3 in 3, with 2 being in between. The heptacoordinate complex 3 exhibited temperature-dependent fluxional behavior, caused by the pseudorotation of the rhenium center. This phenomenon was observed by NMR and supported computationally by density functional theory (DFT) calculations.