One of the most crucial challenges for society in the 21st century is finding renewable sources of energy. Apart from abandoning fossil fuels, this would help diminishing pollution by lowering the levels of greenhouse gases like carbon dioxide (CO2) or methane (CH4). If we were able to transform methane (around 84 times more potent than CO2 as a greenhouse gas) in a controlled and selective manner, we would be able to use a pollutant as a source of carbon and energy. In fact, this is what some organisms called methanotrophic bacteria do. However, this is still not possible for us to achieve nowadays. The way these organisms transform CH4 into different hydrocarbons relies on some proteins called methane monooxygenase (MMO) enzymes, which possess two metal atoms in the place where methane transformation takes place (dinuclear active sites). These two metals are in close proximity and surrounded by organic fragments from amino acid residues, hence they can all cooperate to carry out the chemical transformation of interest. There are different types of these MMO proteins, depending on the metals: dicopper or diiron methane-transforming enzymes can be found in nature. However, nature has developed this mechanism due to thousands of years of evolution.
In our case, we need to speed up this process by making molecular models of these proteins in the laboratory. Through chemical synthesis, we can mimic the active sites of these enzymes by making analogues of the amino acid chains that hold the metals close together in the active site. These analogues are called ‘ligands’, and they are usually organic fragments that can be made in the lab in a short number of synthetic steps from commercially available reagents. The combination of rational ligand design and chemical synthetic methods is a powerful strategy to achieve this goal, since chemical modification of the ligands has a dramatic impact on the structure/activity relationship of the resulting bimetallic complexes.
Related to CH4 transformation, catalytic C-H activation is one of the most active areas of chemistry nowadays, given that selective functionalization of C-H bonds gives rise to a wide range of products that we use in our everyday lives. Particularly, C-H borylation (transformation of a C-H bond into a C-B bond) is an instrumental process in fields like carbohydrate analysis, novel materials or therapeutic agents, to name a few. In the latter case, the ability to selectively fuse organoboron species with other organic fragments via cross-coupling reactions is one of the main strategies employed in the pharmacological industry for the synthesis of new drugs. The utilization of copper-based catalysts in borylation reactions is an attractive methodology due to the low cost of the metal and the generally mild conditions required for these reactions to take place. In these processes, copper(I) boryl species have been invoked as reactive intermediates, but they are extremely challenging to isolate and study (they tend to decompose in solution releasing elemental copper). If we could synthesize, stabilize and isolate these compounds, we would be able to understand and modulate their properties so that we can design catalysts with better properties in terms of activity and/or selectivity.
In this project, the utilization of dinucleating 1,8-naphthyridine-based ligands has allowed us to synthesize a series of dicopper complexes, among which we can find dicopper(I) mu-boryl complexes which exhibit remarkable thermal stability (see next section). Additionally, structural models of some of the previously mentioned enzymes have been succesfully achieved with the synthesis of iron-containing homo- and heterobimetallic complexes stabilized by naphthyridine-based scaffolds.
