Chemical Studies on Oxomolybdenum(VI,IV) Complexes as Bioinorganic Models for the Molybdenum Oxidases

Author

James Thomas Lyon IIIFollow

DOI

https://doi.org/10.25772/T1S3-9P73

Defense Date

1985

Document Type

Dissertation

Degree Name

Doctor of Philosophy

Department

Chemistry

First Advisor

Joseph Topich

Abstract

The synthesis, characterization, and chemical properties of Mo(VI)o₂(5-X-SSP) and Mo(VI)o₂(5-X-SSE), (5-X- SSP²- = 2-((5-X-salicylidene)amino)benzenethiolate; 5-X- SSE^2- = 2-((5-X-salicylidene)amino)ethanethiolate; X = Br, Cl, H, CH₃O), which contain tridentate (ONS) Schiff base ligands is described. The chemical properties of these molybdenum complexes are compared with those which possess tridentate (ONO) Schiff base ligands. Cyclic voltammetry was used to obtain cathodic reduction potentials (E_PC) for the quasi-reversible reduction of the cis-dioxomolybdenum (VI) complexes. Although the reductions are quasi-reversible, trends are observed in E_PC both within series and when different series are compared. Cathodic reduction potentials for the four series of complexes examined span the range -l.53 to -l.05 V versus NHE.

The oxygen atom transfer reactions for Mo(VI)o₂(5-X-SSP) and Mo(VI)o₂(5-X-SSE) with PEtPh₂ were studied in detail between 30 and 60°C. The applicable rate law is +d[Mo(IV)OL]/dt = k₁[Mo- (VI)o₂L][PEtPh₂]. The specific rate constants span the range from 8.4 x 10 (X = CH3O) to 19.6 x 10^-4 M^-1S^-1 (x = Br) for Mo(VI)o₂(5-X-SSP) at 30°C and from 21.4 x 10^-4 M^-1S^-1 (x = CH₃O) to 34.8 x 10^-4 M^-1S^-1 (X = Br) for Mo(VI)o₂(5-X-SSE) at 60°C. Only oxomolybdenum (IV) complexes are observed as products of these reactions. This is a significant result. A linear dependence is observed between log(k_1X/k_1H) and the Hammett o_P parameter for the ligand x substituents for the two series Mo(VI)o₂(5-X-SSP) (p = +0.75) and Mo(VI)o₂(5-X-SSE) (p = +0.42). Activation parameter data were obtained for Mo(VI)0₂(5-H-SSP) (E_a = 67.9 kJ/mol, ΔH+ = 65.2 kJ/mol, ΔS+ = -86.5 J/(mol-K)) and 70.3 kJ/mol, ΔS+ Mo(VI)o₂(5-H-SSE) (E_a 72.0 kJ/mol, ΔH+ = -82.6 J/(mol-K)). There are three ligand features whose effect systematically alters both the cis-dioxomolybdenum- (VI) cathodic reduction potentials and specific rate constants. These include (1) the X-substituent on the salicylaldehyde portion of each ligand; (2) the degree of ligand delocalization; and (3) the substitution of a sulfur donor atom for an oxygen donor atom. Each of these effects is considered separately with regard to both the cis-dioxo- molybdenum(VI) cathodic reduction potentials and specific rate constants, then their cumulative effect is discussed. There exists a correlation between the specific rate constants and E_pc for Mo(VI)0₂(5-X-SSP) and Mo(VI)0₂(5-X- SSE). Initial results are included on the reaction of Mo(IV)0₂(5-H-SSP) with NO₃^-. Spectral results suggest conversion of Mo(IV)0₂(5-H-SSP) to Mo(VI)0₂(5-H-SSP) by way of an oxygen atom transfer reaction.

Comments

Scanned, with permission from the author, from the original print version, which resides in University Archives.

Rights

© The Author

Is Part Of

VCU University Archives

Is Part Of

VCU Theses and Dissertations

Date of Submission

2-5-2018