Tuning the ground and excited state properties of a series of polymetallic tridentate complexes incorporating Ru(II) or Os(II) as the chromophore

Abstract

The work reported in this thesis involves the synthesis of a series of monometallic and polymetallic complexes incorporating the tridentate polypyridyl bridging ligand tpp (where tpp = 2,3,5,6-tetrakis(2-pyridyl)pyrazine) and an investigation of their ground and excited state properties. A series of ruthenium monometallic chromophore complexes has been prepared varying the ligands coordinated on the metal center. The systematic alteration of the ligands has enhanced the understanding of the electrochemical and spectroscopic properties of ruthenium polypyridyl tridentate complexes. In contrast to [Ru(tpy)₂]²⁺ (where tpy = 2,2’:6’,2’’-terpyridine), these monometallic complexes and many of the polymetallic systems emit in solution at room temperature. Methylation of one of the remote pyridine nitrogens on tpp has been accomplished forming a covalently coupled viologen that can potentially function as an electron acceptor.

Two classes of synthetically useful bimetallic complexes of the form [(tpy)M(tpp)RuCl₃]⁺ and [(tpy)M(tpp)Ru(tpp)]⁴⁺ have been prepared (where M = Ru(II) or Os(II)). Synthetic methods have been developed for the stepwise construction of tpp bridged systems by a building block approach. In all bimetallic complexes, the tpp bridging ligand is the site of localization of the LUMO. In the [(tpy)M(tpp)RuCl₃]⁺ systems, the ruthenium metal coordinated to tpp and three chlorides is the easiest to oxidize and is the site of localization of the HOMO. In contrast, for the [(tpy)M(tpp)Ru(tpp)]⁴⁺ systems, the HOMO is localized on the metal being varied, either Ru or Os. This gives rise to complexes which possess a lowest lying excited state that is always a MLCT state involving tpp but can be tuned to involve Ru or Os metal centers. Bimetallic systems that incorporate this tpp ligand have long lived excited states in solution at room temperature (t >100 ns). The bimetallic complex [(tpy)Ru(tpp)IrCl₃]²⁺ has been studied and shown to be an electrocatalyst for the reduction of CO₂ to CO and formate.

The synthesis of the monometallic complexes has advanced the understanding of the ground and excited state properties of tridentate compounds. Incorporation of these complexes into bimetallic systems has aided in the understanding of the perturbations of these properties that occur upon formation of a polymetallic system.