title: Direct observation of key catalytic intermediates in a photoinduced proton reduction cycle with a diiron carbonyl complex.
abstract: The structure and reactivity of intermediates in the photocatalytic cycle of a proton reduction catalyst, [Fe2(bdt)(CO)6] (bdt = benzenedithiolate), were investigated by time-resolved spectroscopy. The singly reduced catalyst [Fe2(bdt)(CO)6](-), a key intermediate in photocatalytic H2 formation, was generated by reaction with one-electron reductants in laser flash-quench experiments and could be observed spectroscopically on the nanoseconds to microseconds time scale. From UV/vis and IR spectroscopy, [Fe2(bdt)(CO)6](-) is readily distinguished from the two-electron reduced catalyst [Fe2(bdt)(CO)6](2-) that is obtained inevitably in the electrochemical reduction of [Fe2(bdt)(CO)6]. For the disproportionation rate constant of [Fe2(bdt)(CO)6](-), an upper limit on the order of 10(7) M(-1) s(-1) was estimated, which precludes a major role of [Fe2(bdt)(CO)6](2-) in photoinduced proton reduction cycles. Structurally [Fe2(bdt)(CO)6](-) is characterized by a rather asymmetrically distorted geometry with one broken Fe-S bond and six terminal CO ligands. Acids with pK(a) ≤ 12.7 protonate [Fe2(bdt)(CO)6](-) with bimolecular rate constants of 4 × 10(6), 7 × 10(6), and 2 × 10(8) M(-1) s(-1) (trichloroacetic, trifluoroacetic, and toluenesulfonic acids, respectively). The resulting hydride complex [Fe2(bdt)(CO)6H] is therefore likely to be an intermediate in photocatalytic cycles. This intermediate resembles structurally and electronically the parent complex [Fe2(bdt)(CO)6], with very similar carbonyl stretching frequencies.
authors.affiliation: Ångström Laboratory, Department of Chemistry, Uppsala University , Box 523, 75120 Uppsala, Sweden.
authors.forename: Mohammad
authors.initials: M
authors.lastname: Mirmohades
authors.forename: Sonja
authors.initials: S
authors.lastname: Pullen
authors.forename: Matthias
authors.initials: M
authors.lastname: Stein
authors.forename: Somnath
authors.initials: S
authors.lastname: Maji
authors.forename: Sascha
authors.initials: S
authors.lastname: Ott
authors.forename: Leif
authors.initials: L
authors.lastname: Hammarström
authors.forename: Reiner
authors.initials: R
authors.lastname: Lomoth