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New Donor-Acceptor Charge-Transfer Complexes of Transition Metals Incorporating Redox-Active Ligands

  • Author(s): Seraya, Elaine
  • Advisor(s): Heyduk, Alan F
  • et al.
Abstract

The work described in this dissertation is subdivided into two main parts. Part 1 addresses how new donor-acceptor charge-transfer complexes, of mixed redox-active ligands, can be realized with coordinatively saturated transition-metal centers. Part 2 details synthetic strategies for installing phosphonic acid anchoring groups on frequently used redox-active ligands.

Chapter 1 provides a general background on redox-active ligands and their donor-acceptor charge-transfer complexes. The molecular structure of classical donor-acceptor complexes is introduced along with their benefits and inherent deficiencies. The need for improved molecular designs is highlighted through several non-classical donor-acceptor complexes of coordinatively saturated metal ions.

Chapter 2 discusses the coordination and reactivity patterns of the new β-ketoaminato ligand, (acNacPh)–, on square-planar RhI and octahedral RhIII centers. Oxidation of the RhI structure, (acNacPh)Rh(dmbpy) (dmbpy = 4,4-dimethyl-2,2-bipyridine), by two electrons produces two different forms of the RhIII complex: the electron donor ( acNacPh)– and the electron-acceptor dmbpy are found either in a coplanar arrangement or non-coplanar arrangement. The propensity of (acNacPh)– to participate in ligand-to-ligand charge-transfer (LL’CT) on RhIII centers is scrutinized by optical spectroscopic methods, revealing that the ligand is primarily redox innocent.

Chapter 3 reports on the synthesis and characterization of octahedral RuII centers coordinated by the redox-active catecholate donor and bipyridine acceptor ligands. Solid-state analysis of two independent products indicates that both contain RuII in a six-coordinate environment with non-coplanar donor-acceptor pairs. However, characterization of the putative products by solution spectroscopic techniques provides evidence that alternative ruthenium configurations must also be prevalent.

Chapter 4 introduces the concept of chemical anchoring groups for covalent modification of metallic oxide surfaces. The intrinsic advantages, suitable syntheses, and the real-life applications of phosphonic acid anchors are discussed. The current and future role of the phosphonate group is highlighted in the context of redox-active ligands.

Chapter 5 compares the redox properties of two new catecholate ligands containing protected phosphonate diester groups in the 4-position of the catecholate ring. The electronic properties of the phosphonate anchors are assessed in solution through a series of (catecholate)Pd(diimine) charge-transfer complexes. Direct attachment of the phosphonate group to the catecholate ligand significantly perturbs the electron donor ability of the ligand whereas the incorporation of a single methylene linker between the phosphonate group and the catecholate ring effectively preserves the donating properties of the ligand. The charge-transfer complexes of the catecholate ligands with deprotected phosphonic acids are shown to bind strongly on the surface of titanium dioxide.

Chapter 6 outlines a modular synthetic approach for the redox-active β-diketiiminyl ligand, (NacNacPhos)–, functionalized with a phosphonate diester substituent at the central carbon of the ligand backbone. The new ligand is coordinated on RhI and RhIII centers, (NacNacPhos)Rh(phdi) and (NacNacPhos)RhCl2(phdi) (phdi = phenanthrene-9,10-diimine), respectively. For the RhIII structure, significant involvement of the (NacNacPhos)– donor ligand in ligand-to-ligand charge-transfer is verified through systematic comparisons with reported (NacNacR)RhCl2(phdi) complexes (R = CH3 or CF3 at the 1,3-positions of the ligand backbone). The LL’CT band energy is directly influenced by the donor-ligand substitution according to the trend: (NacNacCF3)– > (NacNacPhos)– > (NacNacCH3)–.

Chapter 7 details a general route for the preparation of redox-active amido-bis(phenolate) ligands, [ONO], with doubly substituted phosphonate diester substituents. Based on this route, two independent ONO-ligand derivatives are accessed with different solubility characteristics, which is attributed to the presence of bulky tert-butyl substituents at the 3-position of the aromatic rings or lack thereof.

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