Cross-coupling methodology in the synthesis of luminescent metal complexes and multimetallic assemblies
The synthesis of a series of complexes of the type [M(C^N)2(N^N)]+ is reported, where M is either iridium or rhodium, C^N is a cyclometalating ligand such as 2-phenylpyridine (ppy) and N^N is 2,2'-bipyridme (bpy) or a substituted bpy ligand. Several complexes are produced via an in situ Suzuki cross-coupling reaction between a bromo-substituted metal complex and an organic boronic acid. The introduction of fluoro-substituents in the C^N ligand is found to significantly perturb the excited state properties of such complexes, whilst species containing bpy ligands appended with dimethylamino, pyridyl or hydroxy functionalities display pH responsive photophysical properties. The Suzuki cross-coupling reaction is further extended to the controlled synthesis of multimetallic arrays containing from two to eight metal centres. This is achieved via the coupling of bromo-substituted metal complexes with previously unreported ruthenium, iridium or rhodium complexes bearing bpy ligands with boronic acid substituents. The regioselective brominatìon of ppy ligands already bound to metal centres allows for the elaboration of dimeric systems to tetrameric systems, and tetrameric systems to octameric systems by exploiting the same coupling reaction. Access to well-defined mixed metal species is achieved, including a tetrameric species which incorporates rhodium, iridium and ruthenium moieties. Similarly, imsymmetrical bridging units are simply created and can contain either bpy, ppy or 2,2'：6',2"-terpyridine (tpy) coordinating units. Photophysical investigations reveal that the energy transfer properties of these multimetallic assemblies, which contain simple phenyl bridging units between the bpy, ppy or tpy ligands, can be reliably predicted from the properties of the constituent building blocks. That is to say that the bridging ligand does not alter the relative excited state energies of the complexes when they are incorporated as building blocks in extended systems. This allows for the design of energy-channelling multimetallic species, potentially applicable to solar energy conversion devices.