Variable reduction sequences for axial (L) and chelate ligands (N∧N) in rhenium(I) complexes [(N∧N)Re(CO)3(L)]n

The rhenium(I) compounds [(N Lambda N)Re(CO)(3)(MQ)](PF6)(2) (N Lambda N = 2,2'-bipyridine (bpy), 2,2'-bipyrimidine (bpym), 3,3'-bipyridazine (bpdz), or 1,4,7,10-tetraazaphenanthrene (tap) and MQ(+) = N-methyl-4,4'-bipyridinium) undergo four one-electron reduction steps which could be analyzed using cyclic voltammetry, EPR, IR, and UV/vis spectroelectrochemistry. Due to the rather low-lying pi* orbital of tap, the corresponding compound shows electron uptake by N Lambda N each time before MQ(+) is reduced. The opposite is observed for the complexes of the other chelate ligands N Lambda N, however, and the pi* (N Lambda N) orbital approaches the pi* (MQ(+)) level in the order bpy < bpym < bpdz. Remarkably, the reduction processes of MQ(+) and bpdz in [(bpdz)Re(CO)(3)(MQ)](PF6)(2) are separated by only 74 mV as deduced from IR spectroelectrochemical analysis. On reduction of the related compound [(bpy)Re(CO)(3)(mpz)](PF6)(2) (mpz(+) = N-methylpyrazinium), the first two electrons are added to the axial ligand which has a lower-lying pi* orbital than MQ(+) and cannot undergo intramolecular twisting.