Synthesis and characterization of [FeFe]-hydrogenase mimics appended with a 2-phenylazopyridine ligand

Two new complexes in which 2-phenylazopyridine (pap) chelates iron in hydrogenase mimics, 1,2-(-benzenedithiolato)-2′- phenylazopyridinediirontetracarbonyl and 1,3-(-propanedithiolato)-2′- phenylazo- pyridinediirontetracarbonyl have been synthesized and fully characterized, including X-ray crystal structure determinations. The electronic structures of the two complexes are compared with the analogous 1,2-(-benzenedithiolato)diironhexacarbonyl and 1,3-(-propanedithiolato) diironhexacarbonyl complexes. Based on comparison of the crystal structures, the overall bonding in the 2Fe2S core of the molecules is little perturbed by replacing two carbonyl ligands with the pap ligand. Also, the coordinated pap ligand retains a similar structure and NN bond distance to that of the uncoordinated ligand. However, the charge asymmetry in the 2Fe2S core that results from chelating the pap ligand on one of the iron atoms induces substantial localization of the individual orbital characters in the 2Fe2S core. Most interesting, the pap-substituted complexes feature a novel strong long wavelength absorption in the visible region that imparts a deep blue color to the molecules. TDDFT calculations reveal the nature of this absorption as excitation to a low-lying empty orbital on the pap ligand mixed with filled primarily metal d orbitals of the 2Fe2S core.