Copper represents a highly versatile redox-active metal whose facile Cu(I/II) interconversion is accompanied by distinct coordination geometries. Controlling the resulting structural reorganization through ligand architecture is the basis for increasing the efficiency of natural copper proteins. The steric restriction and rigid coordination sphere that results from this steric control is also known an entatic state. The guanidine quinolinyl ligands provide a suitable scaffold for such studies, as guanidine substituents allow systematic variation of steric bulk and donor strength while the quinolinyl backbone ensures structural rigidity. In the first part of this work, coordination-variant models using tripodal tetradentate guanidine quinolinyl ligands were studied. Their tetracoordinate Cu(I) complexes were found to adopt a rare umbrella distorted trigonal-pyramidal geometry that increases with guanidine substitution. The corresponding Cu(II) species were found to expand their coordination sphere, yielding penta-coordinate structures that were further investigated by DFT calculations. Cyclic voltammetry and stopped-flow measurements revealed quasi-reversible redox processes involving conformational changes and small self-exchange rate constants. The complexes follow an addition-oxidation pathway rather than the predominantly observed oxidation-addition sequence, indicating that the observed umbrella distortion in the Cu(I) state can be utilized to manipulate electron transfer mechanisms. The second part of the thesis focuses on coordination-invariant Cu(GUAqu)2+/2+ systems with sp2-hybridized substituents. X-ray diffraction and computational analysis revealed several conformers of the phenyl-substituted Cu(TMG2Phqu)2+ complex, yet cyclic voltammetry showed similar redox potentials for both oxidation states, implying a common redox-active conformer. Correlation of charge-transfer energies with redox potentials demonstrated that structural rather than electronic factors dominate the redox behavior. Kinetic investigations confirmed that minimized internal reorganization in the phenyl-substituted complex affords the fastest self-exchange rate among comparable systems. The third part of the thesis extends the entatic concept to photoexcited states. DFT and transient XAFS studies established that increased steric bulk suppresses flattening distortions and prolongs triplet lifetimes. Electrochemical and spectroscopic data revealed highly negative redox potentials and short-lived but reactive excited states. All complexes exhibited measurable activity in atom transfer radical addition reactions, with yields scaling with expected lifetimes.
Tobias Seitz (Wed,) studied this question.