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.