Multiple-Timescale Photoreactivity of a Model Compound Related to the Active Site of [FeFe]-Hydrogenase Anna R. Ridley, † A. Ian Stewart, ‡ Katrin Adamczyk, § Hirendra N. Ghosh, § Boutheı ¨na Kerkeni, | Z. Xiao Guo, | Erik T. J. Nibbering, § Christopher J. Pickett,* ,† and Neil T. Hunt* ,‡ Department of Physics, UniVersity of Strathclyde, SUPA, 107 Rottenrow East, Glasgow G4 0NG, U.K., School of Chemical Sciences and Pharmacy, UniVersity of East Anglia, Norwich NR4 7TJ, U.K., Max Born Institut fuer Nichtlineare Optik and Kurzzeitspektroskopie, Max Born Strasse 2A, D-12489 Berlin, Germany, and Department of Chemistry, UniVersity College London, 20 Gordon Street, London WC1H OAJ, U.K. Received March 31, 2008 Ultraviolet (UV) photolysis of (µ-S(CH 2 ) 3 S)Fe 2 (CO) 6 (1), a model compound of the Fe-hydrogenase enzyme system, has been carried out. When ultrafast UV-pump infrared (IR)-probe spectros- copy, steady-state Fourier transform IR spectroscopic methods, and density functional theory simulations are employed, it has been determined that irradiation of 1 in an alkane solution at 350 nm leads to the formation of two isomers of the 16-electron complex (µ-S(CH 2 ) 3 S)Fe 2 (CO) 5 within 50 ps with evidence of a weakly associated solvent adduct complex. 1 is subsequently recovered on timescales covering several minutes. These studies constitute the first attempt to study the photochemistry and reactivity of these enzyme active site models in solution following carbonyl ligand photolysis. The Fe-only hydrogenases reversibly catalyze the reduction of protons to molecular hydrogen. Understanding the chemi- cal basis of this process is of significance to the design of new technological systems for hydrogen production and utilization, and considerable advances have been made in studies of the biochemistry of these enzymes and of synthetic active site models; 1 these have been extensively reviewed. 2,3 X-ray crystallographic studies of the active site of an Fe- only hydrogenase show a {4Fe4S} cubane structure linked via a cysteinyl bridge to a diiron subsite, a structure previously unprecedented in nature. 4,5 One important facet of the chemistry of the enzyme is the inhibition of catalysis by CO bound to the active site. Studies using irradiation of the CO-inhibited state at cryogenic temperatures have shown evidence for a structure identical with that of the oxidized form of the enzyme, as well as the loss of the infrared (IR) band associated with the bridging carbonyl ligand. 6 Pho- tolysis of diiron carbonyl analogues is thus important in the context of the photolability of CO in the biological hydro- genase system and provides a vital basis for future studies of the enzyme; however, the ultrafast dynamics of these species remains unexplored. Ultraviolet (UV)-Fourier trans- form IR (FTIR) studies of iron carbonyl sulfide, Fe 2 (CO) 6 (µ- S 2 ), in a Nujol matrix show that the two lowest-energy transitions correspond to Fe-Fe bond activation at 450 nm, resulting in a change of the geometry, while higher-energy absorptions (285 nm < λ < 420 nm) correspond to metal- to-ligand charge-transfer transitions and result in the loss of a carbonyl ligand. 7 The role of CO loss in the enzyme mechanism makes the latter transition the most pertinent to this study. To establish the general photochemical behavior of diiron carbonyl dithiolate systems related to the subsite of Fe-only hydrogenase in solution, results of the photolysis of 1 under ambient conditions using ultrafast UV-pump IR-probe tech- niques and continuous photolysis are presented. UV-pump IR-probe spectroscopy is a well-established technique. Briefly, UV-pump pulses (60 fs duration, λ ) 330 nm) were incident upon the sample, where they were overlapped spatially and temporally with mid-IR pulses (100 fs) with a wavelength resonant with the carbonyl ligand stretching vibrations of 1 (ca. 2000 cm -1 ). The time delay between pump and probe pulses was variable up to 1 ns using an * To whom correspondence should be addressed. E-mail: c.pickett@uea.ac.uk (C.J.P.), nhunt@phys.strath.ac.uk (N.T.H.). † University of East Anglia. ‡ University of Strathclyde. § Max Born Institut fuer Nichtlineare Optik and Kurzzeitspektroskopie. | University College London. (1) Evans, D. J.; Pickett, C. J. Chem. Soc. ReV. 2003, 32, 268. (2) Liu, X. M. I. S.; Tard, C.; Pickett, C. J. Coord. Chem. ReV. 2005, 249, 1641. (3) Capon, J. F.; Gloaguen, F.; Schollhammer, P.; Talarmin, J. Coord. Chem. ReV. 2005, 240, 1664. (4) Peters, J. W.; Lanzilotta, W. N.; Lemon, B. J.; Seefeldt, L. C. Science 1998, 282, 1853. (5) Nicolet, Y.; Piras, C.; Legrand, P.; Hatchikian, C. E.; Fontecilla-Camps, J. C. Struct. Fold. Des. 1999, 7, 13. (6) Chen, Z.; Lemon, B. J.; Huang, S.; Swartz, D. J.; Peters, J. W.; Bagley, K. A. Biochemistry 2002, 41, 2036. (7) Silaghi-Dumitrescu, I.; Bitterwolf, T. E.; King, R. B. J. Am. Chem. Soc. 2006, 128, 5342. Inorg. Chem. XXXX, xx,0 10.1021/ic800568k CCC: $40.75  XXXX American Chemical Society Inorganic Chemistry, Vol. xx, No. x, XXXX A Published on Web 07/30/2008