THE CIS-INFLUENCE OF HYDROPORPHYRIN MACROCYCLES ON THE AXIAL LIGATION EQUILIBRIA OF COBALT(II) AND ZINC(II) PORPHYRIN COMPLEXES

Stability constants and thermodynamic data are reported for coordination of piperidine, pyridine, and substituted pyridines to the cobalt(II) and zinc(II) complexes of octaethylporphyrin (OEP), t-octaethylchlorin (OEC) and the tct- and ttt-isomers of octaethylisobacteriochlorin (OEiBC) in toluene, cyclohexane, and chloroform solution at 25.0-degrees-C. Under the conditions of the study, only 1:1 complexes are formed. With the exception of the case of 2-substituted pyridines, the stability constants, log K, correlate roughly with the base strength of the nitrogenous ligand but correlate closely with the log K for coordination of the base to Zn(OEP). A cis-influence of the macrocycle saturation level on the stability constants is observed. Stability constants for coordination of a given ligand to OEiBC complexes are typically 4 times greater than those for coordination to OEP complexes and 1.8 times greater than those for coordination to OEC complexes. The stability constants of both Co- and Zn(OEiBC) complexes were unaffected by the stereochemistry (tct vs ttt) of the ethyl substituents, unlike the case for nickel. DELTAH and DELTAS vary between -8 and -12 kcal/mol and -12 and -24 cal K-1 mol-1, respectively, and correlate linearly with each other. They do not correlate directly with either log K or the saturation level of the macrocycle. For most bases, log K is greater for the zinc complexes than for the cobalt complexes. However, for 3,5-dichloropyridine, log K is greater for the cobalt complexes. The acid dissociation constants for the free-base compounds H2(OEP), H2(OEC), and H2(OEiBC) were measured in THF/n-butanol solution. All three compounds ionize to dianions by simultaneous loss of two protons. OEP and OEiBC have pK(a) = 15.9. OEC is a weaker acid with pK(a) = 16.6. The increase in log K with macrocycle saturation level does not correlate with the acidity of the respective free bases, but the latter is not necessarily representative of the sigma-donor strength of the macrocycle dianion. Solvation and pi-effects are not responsible for either the dependence of log K on macrocycle saturation level or the reversal for weak bases of the relative Lewis acid strengths of the cobalt and zinc complexes. The latter is attributed to a relief of strain due to core expansion that occurs upon ligand coordination to cobalt complexes. Cobalt complexes are more sensitive than zinc complexes to steric interactions with the ortho-substituents of a pyridine ligand owing to the much smaller out-of-plane displacement of the cobalt atom compared to the zinc atom in five-coordinate complexes.