LONG-LIVED OSMIUM(II) CHROMOPHORES CONTAINING 2,3,5,6-TETRAKIS(2-PYRIDYL) PYRAZINE

A series of complexes of the form [Os(tpypz)(LLL)](2+), [Os(tpypz)(PPh(3))(2)X](n+), and [Os(tpypz)(PPh(3))(2)X](n+) (tpypz = 2,3,5,6-tetrakis (2-pyridyl)pyrazine; tpy = 2,2',2''-terpyridine; LLL = tpy, tpypz; dppe = bis (diphenylphosphino) ethane; X = Cl-, CH3CN) have been prepared and characterized. These complexes represent the first polyazine bridging ligand complexes in which the polyazine ligand is capable of bridging and the light-absorbing metal is also coordinated to a phosphine ligand. The incorporation of the phosphine ligands into the osmium tpypz framework has lead to interesting reactivity. Even in the presence of a diphosphine ligand, trans substitution is observed to yield two dangling diphosphine ligands although the cis chelated product is also formed. In the [Os(tpypz)(PPh(3))(2)X](n+) series, cis-trans isomerization is observed upon production of the solvato complex. The incorporation of the tpypz ligand into the osmium framework has resulted in the production of a series of complexes with relatively long-lived metal-to-ligand charge-transfer excited states, hundreds of nanoseconds in fluid solution at room temperature. The synthesis and characterization of this new class of osmium phosphine complexes are outlined herein. An ESR study of the singly reduced form of one of these complexes, [Os(tpypz)(PPh(3))(2)Cl], indicates that the electrochemically occupied orbital is based on the pyrazine portion of the tpypz ligand. It is this orbital that represents the LUMO in all of the complexes reported herein. These systems are of interest due to their long excited state lifetime coupled to the presence of the tpypz ligand which makes possible their incorporation into stereochemically defined supramolecular complexes. In addition, the solvato species are designed to facilitate synthetic variation and coupling to additional fragments in these supramolecular complexes.