Semester

Summer

Date of Graduation

2004

Document Type

Thesis

Degree Type

MS

College

Eberly College of Arts and Sciences

Department

Chemistry

Committee Chair

Alan M. Stolzenberg.

Abstract

Equilibrium constants and thermodynamic data are reported for coordination of various axial ligands to the zinc(II) complexes of tetra( p-tolyl)porphyrin, tetra(3,5-bis(biphenylmethoxy)phenyl)porphyrin, tetra(2,6-(perfluorobenzoxy)phenyl)porphyrin, and tetra(2,6-di(methoxy)phenyl)porphyrin in toluene at 25.0°C. With respect to coordination of a planar ligand to a zinc(II) porphyrin, the equilibrium constant varied with the identity of the meso-substituent in the order 3,5-bis-(biphenylmethoxy)phenyl > p-tolyl > 2,6-(perfluorobenzoxy)phenyl >> 2,6-di-(methoxy)phenyl. With respect to coordination of a non-planar ligand to a zinc(II) porphyrin, the order was 3,5-bis(biphenylmethoxy)phenyl > p-tolyl >> 2,6-(perfluorobenzoxy)phenyl ≈ 2,6-di(methoxy)phenyl. Thermodynamic data for the coordination of 3- and 4-phenylpyridine to zinc tetra(2,6-di(methoxy)phenyl) porphyrin show a greater loss of entropy than for the coordination of 3- and 4-phenylpyridine to zinc tetra (2,6-(perfluorobenzoxy)phenyl)porphyrin. Space-filling models show that the 2,6-dimethoxy groups of zinc tetra(2,6-di(methoxy)phenyl) porphyrin's four-coordinate form have rotational degrees of freedom as opposed to the rotationally restricted 2,6-perfluorobenzoxy groups of zinc tetra(2,6-(perfluoro-benzoxy)phenyl)porphyrin's four-coordinate form. The resulting loss of entropy due to coordination of an axial ligand (which restricts rotation of the meso-substituents) causes the 2,6-di(methoxy)phenyl groups to, in effect, be more sterically demanding than 2,6-(perfluorobenzoxy)phenyl groups in terms of lower axial ligation equilibrium constants. Also, experimental data suggest that 3,5-bis(biphenylmethoxy)phenyl groups at the meso-positions of a metalloporphyrin are no more sterically demanding than p-tolyl groups at the meso-positions.

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