SYNTHESIS, SPECTROSCOPIC CHARACTERIZATION, AND ELECTROCHEMICAL PROPERTIES OF [(5-CL-SALEN)MNIV(MU-O]2 (5-CL-SALEN = 1,2-BIS(5-CHLOROSALICYLIDENEAMINO) ETHANE)

Reaction of the complex (5-Cl-SALEN)MnII (5-Cl-SALEN = 1,2-bis(5-chlorosalicylideneamino)ethane), formed by electrochemical reduction of [(5-Cl-SALEN)MnIII]+ or initially synthesized in the Mn(II) form, with dioxygen in CH3CN produces the bis(mu-oxo) dimer [(5-Cl-SALEN)MnIV(mu-O)]2. Cyclic voltammetry experiments reveal that the dimer forms in less than 75 ms and that oxygen binding by (5-Cl-SALEN)MnII is most likely the rate-determining step in the dimerization process. The dimer has a moderately intense vibrational band in its FTIR spectrum at 652 cm-1, the region characteristic of the bis(mu-oxo)manganese core, and a moderately intense absorption band in the UV-vis spectrum at lambda(max) = 482 nm (epsilon = 9300 M-1 cm-1 in DMF) arising from an oxo ligand to metal dpi* charge-transfer transition. A cyclic voltammogram of the dimer in 0.1 M Bu4NPF6/DMF shows what appears to be a reversible redox couple at E-degrees = -270 mV vs SSCE, but a bulk electrochemical reduction of a solution of the dimer and a spectrophotometric titration with CP2Co as an outer-sphere reductant reveal that the dimer is unstable. The ESR spectrum of this decomposition product taken as a frozen DMF solution at 6 K displays a 16-line pattern typical of an antiferromagnetically coupled S = 1/2 mixed-valent manganese species, consistent with the presence of a bridging oxo ligand.