The case of [(bpm)Re(CO)3Br] and anti-[Br(CO)3Re(μ-bpm)Re(CO)3Br] (bpm: 2,2′-bipyrimidine) towards multi-metallic ReI species and their properties

Abstract The reaction of [Re(CO)3(THF)Br]2 and 2,2′-bipyrimidine (bpm) in dichloromethane at room temperature employing a stoichiometric 2:1 ratio leads to a mixture of [(bpm)Re(CO)3Br] (21%) and anti-[Br(CO)3Re(μ-bpm)Re(CO)3Br] (79%). When a 1:1 ratio is used, only anti-[Br(CO)3Re(μ-bpm)Re(CO)3Br] is obtained in a 89% yield. The reaction of [Re(CO)5Br] and (bpm) leads to [(bpm)Re(CO)3Br] as a single product. [(bpm)Re(CO)3Br] and anti-[Br(CO)3Re(μ-bpm)Re(CO)3Br] display one and two rhenium(I) centers of distorted octahedral coordination geometry respectively, each with a facial ligand correlation. The intramolecular metal to metal distance is 5.8508(16) A in anti-[Br(CO)3Re(μ-bpm)Re(CO)3Br]. The HOMO-LUMO gap for [(bpm)Re(CO)3Br] results 0.74 eV larger (3.03 vs 2.29 eV) than the computed for anti-[Br(CO)3Re(μ-bpm)Re(CO)3Br]. In the same sense, the anti-isomer is computed to be 0.14 eV (13.4 kJ mol−1) more stable compared to the hypothetical syn-[Br(CO)3Re(μ-bpm)Re(CO)3Br]. Cyclic voltammetry shows that the difference between the ReI/Re0 reduction and its ReI/ReII oxidation in terms of potential is about 3.46 V for [(bpm)Re(CO)3Br], while this value diminishes to 2.62 V for anti-[Br(CO)3Re(μ-bpm)Re(CO)3Br]. Following this tendency, the MLCT band for [(bpm)Re(CO)3Br] is centered at 385 nm (DMF), while it is red-shifted almost 100 nm for anti-[Br(CO)3Re(μ-bpm)Re(CO)3Br]. While [(bpm)Re(CO)3Br] emits at 680 nm (excited at 380 nm), anti-[Br(CO)3Re(μ-bpm)Re(CO)3Br] remains non-emissive, which is attributed to a favored vibrational deactivation path.