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

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Abstract
A model of the iron hydrogenase active site with the structure [(μ-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)2]2+ 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)2]2+ 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)2]2+. 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)2]2+ 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 [(μ-ADT)Fe2(CO)6] portion of the supermolecule. Following this procedure, the first azadithiolate-bridged dinuclear iron complex coordinated by a phosphine ligand [(μ-ADT)Fe2(CO)5PPh3] (4, R = 4-bromophenyl) was synthesized.