METAL-ORGANIC ION-PAIRS WITH DITHIOOXALATE D(8) METAL DONORS AND VIOLOGENS - SPECTROSCOPIC, STRUCTURAL AND ELECTRONIC CHARACTERIZATION

Ion-pair charge-transfer complexes of the type {A(2+) [M(dto)(2)](2-)} where A(2+) represents bipyridinium and phenanthrolinium compounds of different redox potential, M = Ni, Pd, Pt, and dto = 1,2-dithiooxalat e, were synthesized and characterized with respect to their spectrosco pic, structural and electronic properties. A weak absorption band at t he red-tail of the low-energy two-band system of the complex donor com ponent can be attributed to the ion-pair charge-transfer (IPCT) transi tion type. As expected from electrochemical data, the IPCT band of the se systems is markedly hypsochromically shifted compared with ion pair s containing dithiolene donors. The different nature of the highest oc cupied MO may offer an explanation for the distinct electro-and photoc hemical properties. Applying the Hush model reorganization energies ar e estimated in the range 107-125 kJ mol(-1) for the thermal electron t ransfer from [Ni(dto)(2)](2-) to the acceptor site. The amount of char ge that is transferred from the donor to the acceptor (alpha(2)) is ca lculated as 10(-5) - 10(-6), lowered by 2-3 orders of magnitude compar ed with the metal dithiolenes. As shown by X-ray analysis of HEM[Ni(dt o)(2)] (HEM = 2-hydroxyethyl-N'-methyl-4,4'-bipyridiniumcation), the m olecular structure consists of alternating donor-acceptor stacks with a typical plane-to-plane distance of 345 pm, In addition to supramolec ular interactions both components are bonded by a hydrogen bridge betw een the OH group of the cation and the electron-rich oxygen of the dto ligand.