Model of the Iron Hydrogenase Active Site Covalently Linked to a Ruthenium Photosensitizer:  Synthesis and Photophysical Properties

A model of the iron hydrogenase active site with the structure (mu-ADT)Fe2(CO)6 (ADT = azadithiolate (S-CH2-NR-CH2-S), (2: R = 4-bromophenyl, 3: R = 4-iodophenyl)) has been assembled and covalently linked to a Ru(terpy)22+ photosensitizer. This trinuclear complex 1 represents one synthetic step toward the realization of our concept of light-driven proton reduction. A rigid phenylacetylene tether has been incorporated as the linking unit in 1 in order to prolong the lifetime of the otherwise short-lived Ru(terpy)22+ excited state. The success of this strategy is demonstrated by comparison of the photophysical properties of 1 and of two related ruthenium complexes bearing acetylenic terpyridine ligands, with those of Ru(terpy)22+. IR and electrochemical studies reveal that the nitrogen heteroatom of the ADT bridge has a marked influence on the electronic properties of the Fe2(CO)6 core. Using the Rehm-Weller equation, the driving force for an electron transfer from the photoexcited *Ru(terpy)22+ to the diiron site in 1 was calculated to be uphill by 0.59 eV. During the construction of the trinuclear complex 1, n-propylamine has been identified as a decarbonylation agent on the (mu-ADT)Fe2(CO)6 portion of the supermolecule. Following this procedure, the first azadithiolate-bridged dinuclear iron complex coordinated by a phosphine ligand (mu-ADT)Fe2(CO)5PPh3 (4, R = 4-bromophenyl) was synthesized.