Ligand Binding by Metalloporphyrins. III. Thermodynamic Functions for the Addition of Substituted Pyridines to Nickel(II) and Zinc(II) Porphyrins

Abstract

Free energy, enthalpy, and entropy changes for the addition of substituted pyridines to various nickel and zinc porphyrins have been determined in chloroform and benzene solutions. The stoichiometry of the reactions indicates formation of mooo pyridinates with some nickel porphyrins and bis pyridinates with others, the latter requiring very high ligand concentrations. In contratt to the situation in iron porphyrin pyridinates, the ir-acceptor ability of the ligands has little effect on the enthalpy of reaction of the nickel and zinc complexes. The dependence of the enthalpy of reaction on ligand basicity is strong for nickel porphyrin pyridinates in chloroform and for the corresponding zinc complexes in benzene but is less noticeable when these solvents are reversed. Specific solute-solvent interactions are suggested to explain this behavior. It is suggested that in nickel porphyrins hydrogen bonding between chloroform and the solvent-accessible side of the five-coordinate nickel porphyrins is very sensitive to changes in the metal-ligand bond strength on the opposite side of the porphyrin plane. In agreement with the expectation that zinc compounds will moee readily accommodate the forced square-pyramidal structuee than nickel, the changes attributed to hydrogen-bond effects in chloroform weee found to be smaller in the zinc than the nickel compounds studied. In this respect the zinc porphyrins moee closely resemble the iron(II) porphyrins which have been studied previously than do the nickel porphyrins and interactions between the aromatic porphyrin plane and benzene are thought to be moee important. to illustrate this complexity. We would agree with this referee's comment but would point out that the supposition that hydrogen bonding changes are reflected by the differences in heats of mixing seems to u.s a reasonable interpretation for the purposes of this discussion.