COSTA-TYPE-B-12 MODELS - SYNTHESIS, STRUCTURAL CHARACTERIZATION, AND ELECTROCHEMISTRY OF CHLORO DERIVATIVES

Complexes of the type [LCo((DO)(DOH)pn)Cl]PF6, where (DO)(DOH)pn = N2,N2'-propanediylbis(2,3-butanedione 2-imine 3-oxime) and L = pyridine (py) (1), 4-CNpy (2), 3-Br-py (3), 4-CH3Opy (4), and 4-(Me2N)py (5), and [(py)2Co((DO)(DOH)pn)](PF6)2 (6) have been synthesized. Compound 1 crystallizes in the monoclinic space group P2(1)/n with the following crystallographic parameters: a = 6.913 (4) angstrom, b = 15.248 (5) angstrom, c = 21.59 (1) angstrom, beta = 96.28 (5)-degrees, V = 2262 (2), Z = 4, R = 0.055, and R(w) = 0.060 for 2875 independent reflections. The axial N-Co-Cl fragment is characterized by Co-N and Co-Cl distances of 1.993 (6) and 2.216 (2) angstrom. A comparison with the structure of the analogous methyl complex [pyCo((DO)(DOH)pn)CH3]PF6 (Co-N bond = 2.106 (3) angstrom) shows that the Co-N(py) distance reflects the sigma-donor power of the trans axial ligand. The cyclic voltammogram (CV) of 1 in CH3CN has a first cathodic peak at -0.44 V vs Ag/AgNO3 (0.01 M) and two subsequent cathodic-anodic peak systems at E1/2 = -0.61 V and E1/2 = ca. -1.0 V, due to quasi-reversible and reversible charge-transfer processes, respectively. The CVs of 2-4 had profiles similar to that of 1. As L becomes more basic, the first cathodic peak is shifted cathodically. For 5, with the very basic 4-(Me2N)py ligand, the cathodic peak occurred at -0.65 V and overlapped the cathodic/anodic peak system at -0.61 V. However, the cathodic/anodic peak system at -0.61 V was clearly observed for 1-4 and did not shift; it occurred at ca. the same potential as the quasi-reversible Co(III)/Co(II) couple in Co(III)((DO)(DOH)pn)Cl2 (7). The CV of 7 had a reversible Co(II)/Co(I) couple at -1.02 V. The CVs of 1-4 (and probably 5) provide strong evidence for the presence of 7, a finding which can be explained by its formation either by a chemical reaction or during the electrochemical process. The H-1 NMR spectra of 1-5 in CD3CN indicate at most trace amounts of 7. Therefore, 7 was formed during the reduction process. Probably, the initially electrogenerated Co(II)((DO)(DOH)pn)Cl attacks the Co(III) starting compound to form 7 and a non-chloro Co(II) derivative. Then, 7 is reduced in the second cathodic wave at -0.61 V. The one-electron reduction of all Co(II) species formed is responsible for the reversible wave at ca. -1.0 V. The Co(II)/Co(I) E1/2 values for all the complexes studied are similar to the value reported for base-off aquocobalamin (-1.03 V vs Ag/AgNO3 (0.01 M)).