Probing the role of coordinate bonds versus covalent bonds in mediating electron transfer in porphyrin-spacer-quinone complexes: Synthesis, spectroscopy, and electron transfer kinetics.

摘要

The role of other-than-covalent interactions in electronic coupling between D and A in long range electron transfer (ET) in proteins has not been sufficiently assessed with model compounds. This thesis focuses primarily on the design, synthesis, and study of ET complexes to delineate the role coordinate bonds play in mediating ET in biological systems as compared to that of covalent bonds. The synthesis of the compounds, {lcub}5-[4'-(2 '',5''-benzoquinonyl-1 ''-yl)-phenyl-1'-yl]-10,15,20-triphenylporphinato{rcub}zinc(II), {lcub}5-[4''-(2''',5 '''-benzoquinonyl-1''' -yl)-[1',1'']-biphenyl-4 '-yl]-10,15,20-triphenylporphinato{rcub}zinc(II) , {lcub}5-[4''' -(2'''',5'''' -benzoquinonyl-1'''-yl)-[1 ',1'',4'',1 ''']-terphenyl-4'-yl]-10,15,20-triphenylporphina to{rcub}zinc(II), and the (5,10,15,20-tetraphenylporphinato)zinc(II) complexes of 2-pyridin-4-yl-[1,4]benzoquinone, 2-[4'- (pyridin-4 ''-yl)-phenyl]-[1,4]benzoquinone, and 2-(4'pyridin-4-yl-biphenyl-4-yl)-[1,4]benzoquinone followed a convergent strategy utilizing palladium catalyzed cross coupling methodologies. The compounds allow for direct comparison of covalent attachment of the acceptor to the porphyrin periphery versus axial ligation through a pyridine; by varying the number of spacer phenyl units the effect of increasing D-A distance is also evaluated. Photoinduced ET rates were measured to demonstrate a difference in electronic coupling between covalent bonds and coordinate bonds distinguishable form the also observed distance dependence of electron transfer efficiency. In direct answer to a controversy in the role of coordinated ligands in biological Et systems, we further conclude that in these model systems, the axial ligand should be considered part of the spacer and not part of the donor.; Novel synthetic strategies developed to access ET model compounds by simplified molecular methods is also an important subject in this thesis. As part of this synthetic strategy, the novel orgnaometallic porphyrins, [5-(4 ',4',5',5' -tetramethyl-[1',3',2 ']dioxaborolan-2'-yl)-10,20-diphenylporphinato]zinc(II) and [5,15-bis(4',4,'5,' 5'- tetramethyl-[1',3 ',2']dioxaborolan-2'-yl)-10,20-diphenylporphinato]zinc(II) in which boronic esters are appended to a (porphinato)zinc(II) framework were fabricated. These are the first examples of meso -metalated porphyrin species. The synthesis, structural characterization, and spectroscopic properties of these systems are presented. The use of boronate porphyrins as versatile synthons for the simplified preparation of functionalized porphyrins and porphyrin arrays by Suzuki-type cross-coupling reactions is demonstrated. In addition an unusual (5,15-dioxo-10,20-diphenylporphinato)zinc(II) compound was discovered and characterized and the solid-state structure was also determined. The design and synthesis of porphyrin-spacer-quinone systems incorporating rigid bicyclo[2.2.2]octyl units is also discussed.