Synthesis, structural studies, and redox chemistry of bimetallic [Mn(CO)] and [Re(CO)] complexes.

Manganese ([Mn(CO)]) and rhenium tricarbonyl ([Re(CO)]) complexes represent a workhorse family of compounds with applications in a variety of fields. Here, the coordination, structural, and electrochemical properties of a family of mono- and bimetallic [Mn(CO)] and [Re(CO)] complexes are explored. In particular, a novel heterobimetallic complex featuring both [Mn(CO)] and [Re(CO)] units supported by 2,2'-bipyrimidine (bpm) has been synthesized, structurally characterized, and compared to the analogous monomeric and homobimetallic complexes. To enable a comprehensive structural analysis for the series of complexes, we have carried out new single crystal X-ray diffraction studies of seven compounds: Re(CO)Cl(bpm), anti-[{Re(CO)Cl}(bpm)], Mn(CO)Br(bpz) (bpz = 2,2'-bipyrazine), Mn(CO)Br(bpm), syn- and anti-[{Mn(CO)Br}(bpm)], and syn-[Mn(CO)Br(bpm)Re(CO)Br]. Electrochemical studies reveal that the bimetallic complexes are reduced at much more positive potentials (ΔE≥ 380 mV) compared to their monometallic analogues. This redox behavior is consistent with introduction of the second tricarbonyl unit which inductively withdraws electron density from the bridging, redox-active bpm ligand, resulting in more positive reduction potentials. Re(CO)Cl was reduced with cobaltocene; the electron paramagnetic resonance spectrum of the product exhibits an isotropic signal (near g = 2) characteristic of a ligand-centered bpm radical. Our findings highlight the facile synthesis as well as the structural characteristics and unique electrochemical behavior of this family of complexes.