Adventures in Transition Metal and Actinide Chemistry: Multidentate N-Heterocyclic Ligands for the Study of d- and f-Element Bonding and Reactivity

Chapter 1:/nA series of actinide-transition metal heterobimetallics have been prepared, featuring thorium, uranium and cobalt. Complexes incorporating the binucleating ligand N[o-(NHCH2PiPr2)C6H4]3 with either Th(IV) (1.4) or U(IV) (1.5) and a carbonyl bridged [Co(CO)4]- unit were synthesized from the corresponding actinide chlorides (Th: 1.2; U: 1.3) and Na[Co(CO)4]. Irradiation of the resulting isocarbonyls with ultraviolet light resulted in the formation of new species containing actinide-metal bonds in good yields (Th: 1.6; U: 1.7); this photolysis method provides a new approach to a relatively unusual class of complexes. Characterization by single-crystal X-ray diffraction revealed that elimination of the bridging carbonyl and formation of the metal-metal bond is accompanied by coordination of a phosphine arm from the N4P3 ligand to the cobalt center. Additionally, actinide-cobalt bonds of 3.0771(5) Å and 3.0319(7) Å for the thorium and uranium complexes, respectively, were observed. The solution-state behavior of the thorium complexes was evaluated using 1H, 1H-1H COSY, 31P and variable-temperature NMR spectroscopy. IR, UV-vis/NIR, and variable-temperature magnetic susceptibility measurements are also reported./nChapter 2:/nThe first examples of actinide complexes incorporating corrole ligands are presented. Thorium(IV) and uranium(IV) macrocycles of Mes2(p-OMePh)corrole were synthesized via salt metathesis with the corresponding lithium corrole in remarkably high yields (93% and 83% respectively). Characterization by single-crystal X-ray diffraction revealed both complexes to be dimeric, having two metal centers bridged via bis(μ-chlorido) linkages. In each case, the corrole ring showed a large distortion from planarity, with the Th(IV) and U(IV) ions residing unusually far (1.403 Å and 1.330 Å respectively) from the N4 plane of the ligand. 1H NMR spectroscopy of both the Th and U dimers revealed dynamic solution behavior. In the case of the diamagnetic Th corrole, variable-temperature, DOSY and EXSY 1H NMR spectroscopy was employed, and supported that this behavior was due to an intrinsic pseudorotational mode of the corrole ring about the M-M axis. Additionally, the electronic structure of the actinide corroles was assessed using UV-visible spectroscopy, cyclic voltammetry and variable-temperature magnetic susceptibility. This novel class of macrocyclic complexes provides a rich platform in an underdeveloped area for the study of non-aqueous actinide bonding and reactivity./nChapter 3:/nA series of divalent first row triflate complexes supported by the ligand tris(2-pyridyl(methyl))amine (TPA) have been investigated as oxygen reduction catalysts for fuel cell applications. [(TPA)M2+]n+ (M= Mn, Fe, Co, Ni and Cu) derivatives were synthesized and characterized by X-ray crystallography, cyclic voltammetry, NMR spectroscopy, magnetic susceptibility, IR spectroscopy and conductance measurements. The stoichiometric and electrochemical O2 reactivity of the series were examined./nChapter 4:/nComplexes of the ligand tris(2-pyridyl(methyl))amine (TPA) {[(TPA)M2+]n+ (M= Mn, Fe, Co and Cu)} presented in chapter 3 were evaluated as electrocatalysts for oxygen reduction. Rotating-ring disk electrode (RRDE) voltammetry was used to examine the catalytic activity of the series of complexes on a carbon support in acidic media, emulating fuel cell performance. The iron complex displayed a selectivity of 89% for four-electron conversion and demonstrated the fastest reaction kinetics, as determined by a kinetic current of 7.6 mA. Additionally the Mn, Co and Cu complexes all showed selective four-electron oxygen reduction (< 28% H2O2) at onset potentials (~0.44 V vs. RHE) comparable to state of the art molecular catalysts, while being straightforward to access synthetically and derived from non-precious metals.

Advisor: Arnold, John