title: Mechanistic Spectro-electrochemistry of Group-6 and -7 Metal Carbonyl Electrocatalysts of CO2 Reduction
Creators: name: Taylor, James
Publisher: name: University of Reading
Year: 2022

description: Bio-inspired transition metal complexes have recently revealed their potential to catalytically convert waste CO2 from industry into a sustainable source of carbonaceous fuel and chemical feedstock. Ultimately working to close the carbon cycle and helping to alleviate some of the most concerning impacts of anthropogenic climate warming. Significant research efforts have been expended thus far in the race to understand the underlying mechanisms, the trends in reactivity, and the factors which can aid rational design of improved catalysts. There are now hundreds of reported transition metal catalysts for the most facile 2e– reduction of CO2 to CO or formate, many of which have high activity and tunable selectivity. The majority of catalysts feature a diverse ligand framework, which works cooperatively with the metal centre(s) to reduce CO2, and much of the work that goes into optimizing the electrocatalytic behaviour actually involves systematic alteration of the non-innocent coordination sphere. A lot of attention has been paid in the recent decade to the Earth abundant materials such as Mn, Fe, Co, Ni and Cu. Much less attention however has been paid to the Group-6 triad (Cr, Mo, W), leading to the primary aim of this thesis, which is to explore the rich reduction-induced chemistry of these metals and their coordinated ligands. In Chapter 1, a concise overview of the major milestones concerning the most promising catalysts, as well as an outlook on the field as a whole is presented, while in Chapter 2, an introduction to the major experimental techniques utilized in the thesis is given. The novel research presented in this thesis begins initially with Chapter 3, which reports an in depth study using mainly cyclic voltammetry and spectroelectrochemistry on the [Mo(x,x0 - dmbipy)(CO)4 ] (x = 4-6, dmbipy = dimethyl-2,20 -bipyridine) series of complexes. A low energy pathway to the active catalyst is probed by systematic changes to the ligand, electrode and solvent. The most promising results are obtained with [Mo(6,60 -dmbipy)(CO)4 ] on an Au cathode in conjunction with the solvent, NMP (N-methyl-pyrrolidone), revealing an important synergy between ligand, electrode and solvent effects. This initial research chapter serves to highlight that with careful, rational control, the Group-6 metals are far more promising than previously imagined. In Chapter 4, a new cooperative approach between photo and electrochemistry toward reducing the catalytic overpotential is trialled with [Mn(bipy)(CO)3Br] (bipy = 2,20 - bipyridine) and [Mo(6,60 -dmbipy)(CO)4 ]. In the 1e– reduced state, both catalyst precursors have photochemistry which may be exploited to produce the active catalyst already at the 1e– reduced state, without shifting the electrochemical potential more negatively. In both cases, this represents a significant saving of energy, as there is an appreciable separation between the initial reduction and the follow-up reduction which produces the active catalyst. In the future, the novel approach may aid the design of functional devices for CO2 reduction. Chapter 5 covers the characterization of a new series of complexes with the structure [Mo(η 3 -allyl)(x,x0 -dmbipy)(CO)2 (NCS)] (x = 4-6) and an in-depth study of their varied cathodic paths under Ar and CO2 using a combination of cyclic voltammetry, IR/UV-Vis spectroelectrochemistry and supporting DFT calculations. It is revealed that the cathodic path of [Mo(η 3 -allyl)(x,x0 -dmbipy)(CO)2 (NCS)] bears striking similarity to the well-studied and highly promising catalyst, [Mn(bipy)(CO)3Br]. Seemingly small, but systematic changes in the ligand sphere induces significant change in cathodic path, particularly in the stability of the 1e– reduced state and the proclivity for the formation of an inactive dimer (x = 4), vs formation of the active catalyst (x = 6). Under CO2, catalytic conversion to CO and formate is observed by IR spectroelectrochemistry. Chapter 6 follows directly from the work presented in Chapter 5, beginning with the characterization of several new complexes, with the structures [Mo(η 3 -allyl)(6,60 - dmbipy)(CO)2Cl] (dmbipy = dimethyl-2,20 -bipyridine, allyl = allyl or 2-methyl-allyl) and [Mo(η 3 -2-methyl-allyl)(pTol-bian)(CO)2Cl] (pTol-bian = bis(p-tolylimino)acenaphthene). These complexes were specifically designed in order to systematically probe the innocence of the allyl and halide/pseudo-halide ligands. The results reveal that both types of ligands also have a strong impact on the cathodic path, revealing previously unknown complexity and questions about the non-innocence of the allyl ligand. Finally, Chapter 7 describes the unusual cathodic path of the Group-7 complexes, [Re(3,30 -dhbipy)(CO)3X]n (X = Cl, n = 0; X = PrCN, n = +1; dhbipy = dihydroxy-2,20 - bipyridine) which bear local proton sources, namely two hydroxyl groups in close proximity. Upon reaching the initial reduction, reductive deprotonation occurs, allowing a strong hydrogen bond to form, which radically alters the subsequent cathodic path. The complexes display markedly different behaviour even when compared to their close relative, [Re(4,40 -dhbipy)(CO)3Cl] and the progenitor [Re(bipy)(CO)3Cl] complex. Following a second concerted reduction involving multiple electrons, [Re(3,30 -dhbipy-2H+)(CO)3 ] 3– forms, which under CO2 is revealed, by cyclic voltammetry and bulk electrolysis, to be catalytically active toward the reductive disproportionation of CO2 producing CO and CO2– 3 . With careful potentiostatic control, evidence of an electrode catalysed reaction is also revealed. This reaction ultimately allows the radical anion of the parent complex to form in a limited potential range. That is to say, following the initial reductive deprotonation but prior to its close-lying reduction to the active CO2 reduction catalyst.