Synthesis, Photophysics and Photochemistry of Uncommon Transition Metal Complexes

2,2'-bipyridine and related alpha-diimine ligands form luminescent complexes with a variety of d6 and d8 metals, many of which are very well investigated. The phosphorous analog of 2,2'-bipyridine is called 2,2'-biphosphinine and has been known since 1991. Several transition metal complexes with 2,2'-biphosphinine have been synthesized but the photophysical and photochemical properties of these complexes have remained essentially unexplored. For well-selected complexes with bi- and tridentate phosphinine ligands there is good reason to expect similar (emissive) metal-to-ligand charge transfer (MLCT) excited states as for many alpha-diimine complexes of d6 and d8 metals. However, 2,2'-biphosphinine tends to stabilize metals in lower oxidation states than 2,2'-bipyridine, and some complexes of 2,2'-biphosphinine adopt a trigonal-prismatic geometry rather the octahedral coordination usually encountered in homoleptic 2,2'-bipyridine d6 metal complexes. In addition, 2,2'-biphosphinine is more redox non-innocent than 2,2'-bipyridine. One can therefore expect certain analogies but also some pronounced differences between the photophysics of 2,2'-biphosphinine and 2,2'-bipyridine complexes. The goal of the first part of this proposal (sub-project 1) is to synthesize new luminescent metal complexes with bi- or tridentante phosphinine ligands and to obtain a detailed fundamental understanding of their photophysical and photochemical properties. The work will not be limited to 2,2'-biphosphinine and its derivatives but will encompass bi- and tridentate chelating ligands including for example pyridylphosphinines and phenylphosphinines. This research has the potential to lead to new luminescent materials, photo- or electrocatalysts for CO2 reduction, photosensitizers for dye-sensitized solar cells, potent photooxidants for electron transfer studies in proteins, DNA intercalators, or chemical sensors for volatile organic compounds. The second part of the proposed research (sub-project 2) concentrates on novel bipyridine and terpyridine ligands which are expected to show catechol-like redox properties (Figure 2). The ultimate goal is to develop new transition metal complexes which can undergo multiple oxidations in a reversible fashion at relatively modest electrochemical potentials. Such complexes are of interest as catalysts for a variety of different multi-electron conversions including for example water oxidation or CO2 fixation. Common alpha-diimine complexes such as [Ru(2,2'-bipyridine)3]2+ exhibit simple one-electron redox chemistry, and they usually cannot undergo multi-electron reactions. The ligands in Figure 2 are expected to be highly redox-active, and when coordinated to metal complexes unusually rich multi-electron (photo)redox chemistry might result. Multiple oxidations of these complexes should be possible over a relatively small potential window due to the fact that the respective ligands can release protons upon oxidation, thereby avoiding the formation of highly charged species. Initial work will focus mostly on substitution-inert d6 metal complexes, but in the mid- to long-term more earth-abundant 3d metals will be investigated.