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Cortlandt G. PIERPONT
Office Phone: 303 492 8420
E-mail: Cortlandt.Pierpont@colorado.edu
FAX: 303 492 0951
Lab: Ekeley M394
Lab Phone: 303 492 8025
Group Website:
Ph.D.: Brown University, 1971
Physical and Chemical Properties of Transition Metal Complexes Containing Quinone Ligands
Professor Pierpont is studying the structural, electrochemical, magnetic, and photochemical properties of transition metal complexes that contain organic ligands bearing quinone functionalities. o-Benzoquinones have vacant electronic levels that are similar in energy to the valence d-orbitals of transition metal ions. Electron transfer reactions have been investigated, and the results of these studies have shown that the distribution of charge in a metal-quinone coordination complex is dependent upon parameters that influence the relative energies of metal-d and quinone electronic-pi levels. These include the bonding effects of coligands, the overall charge of the complex, quinone-ligand substituent effects, and periodic variations in metal d-orbital energy. In cases where metal and quinone electronic levels are particularly close in energy, electron transfer may be induced at low energy using light in the infrared or by thermal changes. Reactions of this type have been investigated both in solution and for samples in the solid state. Shifts in charge distribution may be accompanied by large changes in metal ion radius, and this is particularly true of the low-spin Co(III)/high-spin Co(II) redox couple. Thermal and light-induced changes in charge distribution for complexes prepared with 3,6-di-tert-butyl-1,2- benzoquinone (DBBQ) have been used to study low-energy physical effects. Photomechanical properties have been observed as the consequence of cooperative interactions between monomeric subunits in supported polymers that exhibit valence tautomeric equilibria between CoIII(N-base-N)(DBSQ)(DBCat) and CoII(N-base-N)(DBSQ)2 redox isomers, where the N-base-N coligand serves to bridge Co subunits in a linear polymer. A view of the polymer formed with bridging pyrazine coligands is shown before. Radial changes at the metal may result in a shift in coligand structure. The dipyridyl ether (Py2O) coligand is planar for CoII(Py2O) (DBSQ)2 and folded for CoIII(Py2O)-(DBSQ)(DBCat). This structural change is responsible for hysteresis in the Co(III) --> Co(II) and Co(II) --> Co(III) electron transfer steps, and these changes have formed the basis for investigations on molecular switching. Views of the redox isomers and hysteresis in magnetism associated with electron transfer are shown.
Current projects under investigation include studies on one-dimensional conductive polymers, a molecular reactor for light-induced water-splitting, and investigations on molecular ensembles formed with quinone-functionalized macrocyclic ligands as systems that may exhibit multiple electron/proton transfers to a metal-bound substrate molecule.
Selected Publications
A.S. Attia, S. Bhattacharya, and C.G. Pierpont. "Studies on Complexes of the Fe III (N-N)(DBSQ)(DBCat) Series with 2,2'-Bipyridine and N,N,N',N'-Tetramethyl-ethylenediamine Coligands. The Potential for Valence Tautomerism in Quinone Complexes of Iron(III)," Inorg. Chem., 34, 4427-4433 (1995).
C.G. Pierpont and O.-S. Jung. "Thermodynamic Parameters for Cobalt-Quinone Electron Transfer and Spin Transition Steps of the CoIII(bpy)(3,5-DBSQ)(3,5-DBCat)/CoII(bpy>)(3,5-DBSQ)2 Valence Tautomeric Equilibrium," Inorg. Chem., 34, 4281-4283 (1995).
A.S. Attia, B.J. Conklin, C.W. Lange, and C.G. Pierpont. "Iron-Semiquinone, Semi-quinone-Semiquinone, and Iron-Iron Magnetic Exchange in Monomeric and Dimeric Ferric Complexes Containing the Radical 3,6-Di-tert-butyl-1,2-semiquinonate Ligand," Inorg. Chem., 35, 1033-1038 (1996).
G. Speier, J. Csihony, A.M. Whalen, and C.G. Pierpont. "Studies on Aerobic Reactions of Ammonia/3,5-Di-tert-butylcatechol Schiff-Base Condensation Products with Cu, Cu(I), and Cu(II). Strong Cu(II)-Radical Ferromagnetic Exchange and Observations on a Unique N-N Coupling Reaction," Inorg. Chem., 35, 3519-3524 (1996).
O.-S. Jung, D.H. Jo, Y.-A. Lee, Y.S. Sohn, and C.G. Pierpont. "Ancillary Ligand Dependent Shifts in Charge Distribution for Cobalt-Quinone Complexes," Angew. Chem., 108, 1796-1797 (1996).
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