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Cooperative Two-Electron Reagents of Lower Transition Metals of Group 10

Chatterjee, Sayandev
http://rave.ohiolink.edu/etdc/view?acc_num=ucin1250266435

Abstract Details

2009, PhD, University of Cincinnati, Arts and Sciences : Chemistry.
Increase in worldwide fossil fuel consumption and environmental concerns aremotivating efforts to develop efficient multi-electron catalysts for energetically or kinetically unfavorable reactions. Examples include splitting water to make oxygen and hydrogen fuel and fixing carbon dioxide produced in combustion processes. Squareplanar d8- and octahedral d6- electron complexes of 2nd and 3rd row of Group 10 are attractive candidates for catalysis, as they undergo cooperative two electron changes in oxidation states. The cooperativity arises from the stability of the d6- and d8-electron configurations with respect to the intermediate d7, causing an inversion of the one-electron reduction potentials: E1°′(d6/d7)<E2°′(d7/d8). Fine control over these redox potentials and rates represents a powerful strategy for improving and designing new catalysts. However, attempts to implement this strategy are hampered by the fact that the redox reactions involve drastic changes in molecular geometry, making them irreversible. To address this problem, this work is focused on designing metal complexes with ligand architectures capable stabilizing four-coordinate geometry favored by the d8- electron configuration and six-coordinate geometry favored by the d6-electron configuration. In certain instances, these ligand scaffolds allow for facile two-electron transfer and measurement of the two-electron redox couple. This dissertation reports the synthesis of two new classes of palladium and platinum complexes with ligand architectures capable of supporting outer-sphere two electron transfer. The first class includes complexes with a combination of monodentate, bidentate and pincer ligands. In these systems, the pincer ligand is capable of interconverting between a monodentate coordination mode and a meridional coordination mode, thus allowing for interconversion. Electrochemical studies demonstrate that these complexes are capable of undergoing two-electron transfer. The reactions show a wide variation in the redox potentials, chemical reversibility, and electron-transfer kinetics that can be rationalized in terms of the electronic, steric and conformational properties of the ligands, as also the electronic properties of the metal. One critical discovery is that the metal can act as both a Brønsted base and Lewis acid. Accumulated data suggest that the Lewis acid behavior causes preorganization of the d8-electron complex and is therefore a crucial element in the cooperative two-electron reactivity. The second category of complexes is composed of a bidentate ligand and a potentially facially coordinating tripodal ligand. In the presence of nucleophiles, these systems undergo remarkably reversible outer-sphere two-electron transfer. Mechanistic studies indicate interconversion between four- and six-coordinate geometries, in which the nucleophile bonds at an open axial site. Varying the ligand, metal and exogenous nucleophile allows for tuning the redox potential over an astoundingly wide range, exceeding 1 V. The major findings from this work are significant because they establish fundamental guidelines for designing two-electron reagents and controlling their redox potentials. Furthermore, the compounds in the new class of two-electron reagents have open coordination sites that can potentially interact with substrate. Thus, these new compounds afford the opportunity to couple two-electron reactivity with bond-making and bond-breaking steps necessary for substrate activation. Knowledge derived from this dissertation work is expected to lead to the development of new multielectron catalysts, photocatalysts, and redox mediators for solar cells.
William Connick, PhD (Committee Chair)
Michael Baldwin, PhD (Committee Member)
William Heineman, PhD (Committee Member)
407 p.
Chemistry
redox; electron transfer; platinum; palladium; complexes

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Citations

  • Chatterjee, S. (2009). Cooperative Two-Electron Reagents of Lower Transition Metals of Group 10 [Doctoral dissertation, University of Cincinnati]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=ucin1250266435

    APA Style (7th edition)

  • Chatterjee, Sayandev. Cooperative Two-Electron Reagents of Lower Transition Metals of Group 10. 2009. University of Cincinnati, Doctoral dissertation. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=ucin1250266435.

    MLA Style (8th edition)

  • Chatterjee, Sayandev. "Cooperative Two-Electron Reagents of Lower Transition Metals of Group 10." Doctoral dissertation, University of Cincinnati, 2009. http://rave.ohiolink.edu/etdc/view?acc_num=ucin1250266435

    Chicago Manual of Style (17th edition)

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© 2009, all rights reserved.
This open access ETD is published by University of Cincinnati and OhioLINK.