Charge Transfer Interaction in (Riboflavin)tricarbonylrhenium(I) Chloride

Riboflavin (vitamin B2) is one of the most im­ portant organic redox coenzymes [1 -4], It partic­ ipates in biological electron transfer with redoxactive metalloenzymes. Accordingly, the nature of flavin transition metal complexes is of consider­ able interest [5-16]. If electron transfer between metal and riboflavin takes place in the coordinated state (inner sphere electron transfer) this process can be related to optical charge transfer transi­ tions. Since riboflavin is an oxidant, its complexes with reducing metals should display metal-to-ligand charge transfer (MLCT) absorptions in their elec­ tronic spectra. Indeed, such MLCT transitions have been assigned to long-wavelength absorptions of various riboflavin complexes with donor metals [11,12, 15,16] including [ReI(riboflavin)(CO)3Cl] [16]. Unfortunately, the original preparation of this compound yielded only a mixture of the prod­ uct with the starting material [16]. Moreover, only very little is known on the electronic spectra of this complex. In order to gain more insight in the CT interaction we decided to prepare and examine [ReI(riboflavin)(CO)3Cl] in a pure state.