VARIABLE REDUCTION SEQUENCES FOR AXIAL (L) AND CHELATE LIGANDS (N-BOOLEAN-AND-N) IN RHENIUM(I) COMPLEXES [(N-BOOLEAN-AND-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'-bipyridazin e (bpdz), or 1,4,7,10-tetraazaphenanthrene (tap) and MQ(+) = N-methyl- 4,4'-bipyridinium) undergo four one-electron reduction steps which cou ld be analyzed using cyclic voltammetry, EPR, IR, and UV/vis spectroel ectrochemistry. Due to the rather low-lying pi orbital of tap, the co rresponding compound shows electron uptake by N Lambda N each time bef ore MQ(+) is reduced. The opposite is observed for the complexes of th e other chelate ligands N Lambda N, however, and the pi (N Lambda N) orbital approaches the pi (MQ(+)) level in the order bpy < bpym < bpd z. 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 spe ctroelectrochemical analysis. On reduction of the related compound [(b py)Re(CO)(3)(mpz)](PF6)(2) (mpz(+) = N-methylpyrazinium), the first tw o electrons are added to the axial ligand which has a lower-lying pi orbital than MQ(+) and cannot undergo intramolecular twisting.