Fused bis(bipyridine) ligand systems: coordination and electrochemistry with copper, rhenium and manganese

Abstract

Bimetallic coordination complexes are of interest due to the unique properties which may arise from the positioning of two metal centres in proximity. In this thesis, research targeting redox-active bimetallic coordination complexes using ligands with fused bis(bipyridine) coordination motifs is presented in a series of studies with copper, rhenium and manganese. These studies comprise (i) self-assembly of bi- and multi-metallic coordination complexes with copper, and (ii) the use of bi- and mono-metallic rhenium and manganese complexes as catalysts for electrochemical CO2 reduction. Chapter 1 provides the background to the major concepts that underlie the content of this thesis: these being metal-metal cooperativity, and reduction of CO2 in electrochemical systems, and provides discussion of the methods that will be used to study electrochemical reactivity in this project. The design and synthesis of appropriate binucleating ligands is prerequisite for the study of redox-active bimetallic coordination complexes. In Chapter 2, the ligand designs enabling this project are discussed, with reference to (i) the redox activity of 2,2'-bipyridine; (ii) ligand designs containing multiple 2,2'-bipyridyl or other diimine donor motifs; and (iii) ligands containing fused oligopyridine cores such as the subjects of study: bis(6-R-2-pyridyl)-1,8-naphthyridine (R = H, L1; R = Me, L2), and bis(6-R-2-pyridyl)-1,8-diazaanthracene (R = H, L3; R = Me, L4). Bi- and multi-metallic copper complexes are of interest for study into mixed valence and (photo)redox systems. In Chapter 3 the synthesis of copper complexes of ligands L2-L4 is described. The complexes self-assemble into homoleptic helicate ([Cu2(L2)2](PF6)2, [3-1][PF6]2) or 2x2 grid ([Cu4(L3/L4)4][PF6]4, [3-2][PF6]4/[3-3][PF6]4) structures. These are studied using crystallographic, spectroscopic, and electrochemical methods, with attention given to metal- and ligand-centred redox activity. Rhenium bipyridine complexes of the form [Re(bpy)(CO)3X] (bpy = 2,2'-bipyridine, X = Cl, 4-A) are known catalysts for the electroreduction of CO2. In Chapter 4, an experimental description of rhenium complexes using L3 is given. Complexes [Re(L3)(CO)3Cl], (4-1), and the dirhenium complex [Re2(L3)(CO)6(Cl)2], (4-2), are studied via spectroscopic, crystallographic and electrochemical techniques. Evidence for catalytic rate enhancement was observed for 4-2 and data from our systems is compared to known monometallic and bimetallic rhenium systems to attempt to understand this cooperative behaviour. Extending from the previous chapter, Chapter 5 then discusses the effects of catalyst modifications on the structure and function of rhenium complexes within diazaanthracene ligand systems via the synthesis of rhenium complexes of L4. The complexes [Re(L4)(CO)3Cl] (5-1) and [Re2(L4)(CO)6(Cl)2] (5-2) are introduced and investigated, including comparisons to complexes 4-1 and 4-2 discussed in Chapter 4, and a description of isomerism at rhenium for 5-2. As was seen for 4-2, 5-2 was found to have high activity towards CO2 reduction compared to monometallic analogues. For the final study presented in this work, Chapter 6 considers the use of manganese as an earth-abundant alternative to rhenium for CO2 reduction catalysis. Here, complexes [Mn(L1)(CO)3Br] (6-1), [Mn(L2)(CO)3Br] (6-2), [Mn(L3)(CO)3Br] (6-3), [Mn2(L4)(CO)3Br] (6-4), [Mn(L3)(CO)6(Br)2] (6-5), [Mn(L4)(CO)6(Br)2] (6-6), [Mn(L3)(CO)6(NCMe)2][BF4]2 (6-7) and [Mn(L3)(CO)6(NCMe)2][BF4]2 (6-7)[Mn(L3)(CO)6(NCMe)2][BF4]2 (6-8) are described by various methods. It was found that the high activity of bimetallic rhenium systems described earlier did not translate directly to the manganese systems, and factors relating to this such as the stability of complexes under electrochemical conditions are discussed. Chapter 7 then concludes the thesis by providing a summary of the work as a whole, and discusses the prospectus for future work in this area. Show more