Building Block Approach to the Construction of Long-Lived Osmium(II) and Ruthenium(II) Multimetallic Complexes Incorporating the Tridentate Bridging Ligand 2,3,5,6-Tetrakis(2-pyridyl)pyrazine

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Abstract
Two classes of synthetically useful bimetallic complexes of the form [(tpy)M(tpp)RuCl3](PF6) and [(tpy)M(tpp)Ru(tpp)](PF6)4 have been prepared and their spectroscopic and electrochemical properties investigated (tpy = 2,2‘:6‘,2‘‘-terpyridine, tpp = 2,3,5,6-tetrakis(2-pyridyl)pyrazine, and M = RuII or OsII). Synthetic methods have been developed for the stepwise construction of tpp-bridged systems using a building block approach. In all four complexes, the tpp that serves as the bridging ligand is the site of localization of the lowest unoccupied molecular orbital (LUMO). The nature of the HOMO (highest occupied molecular orbital) varies depending upon the components present. In the systems of the type [(tpy)M(tpp)RuCl3](PF6), 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)](PF6)4 systems, the HOMO is based on the metal, M, that is varied, either Ru or Os. This gives rise to systems which possess a lowest lying excited state that is always a metal-to-ligand charge transfer state involving tpp but can be tuned to involve Os or Ru metal centers in a variety of coordination environments. The synthetic variation of the components within this framework has allowed for understanding the spectroscopic and electrochemical properties. Bimetallic systems incorporating this tpp ligand have long-lived excited states at room temperature (lifetimes of ca. 100 ns). The bimetallic system [(tpy)Ru(tpp)Ru(tpp)](PF6)4 has a longer excited state lifetime than the monometallic system from which it was constructed, [(tpy)Ru(tpp)](PF6)2. Details of the spectroscopic and electrochemical studies are reported herein.