No abstract

Chemical actinometry is an indispensable analytical tool in preparative photochemistry that allows for a precise measurement of radiant fluxes inside photoreactors. An actinometer thus enables an absolute determination of the quantum yield of a photochemical reaction of interest. The "gold standard" of chemical actinometry in liquid systems is the Hatchard-Parker actinometer, i.e. an aqueous solution of potassium trisoxalatoferrate(iii), which is based on the light-induced net transformation of ferric into ferrous oxalate complexes. Although the absolute photochemical quantum yield for this fundamental standard system has been accurately known for many years, the underlying molecular-level mechanisms and time scales associated with a photoreduction of the ferrioxalate actinometer remained so far largely obscured. Here, we use femtosecond mid-infrared spectroscopy combined with ultrafast laser photolysis to obtain unique structural-dynamical information associated with the primary light-triggered processes thereby finally providing the missing quantitative molecular-level foundations that ultimately justify a utilization of aqueous ferrioxalate as a true photochemical standard. Following photon absorption by the octahedral parent complex, an ultrafast decarboxylation occurs within 500 fs, which generates a penta-coordinated ferrous dioxalate that carries a bent carbon dioxide radical anion ligand in an "end-on" O-coordinated fashion. This unique intermediate structurally isomerizes on a tens of picoseconds time scale and subsequently loses a CO2˙--ligand to form a square-planar bisoxalatoferrate(ii) on a hundreds of picoseconds time scale.

The influence of the spin on the mode of binding between carbon dioxide (CO 2) and a transition-metal (TM) center is an entirely open question. Herein, we use an iron(III) oxalato complex with nearly vanishing doublet-sextet gap, and its ultrafast photolysis, to generate TM-CO 2 bonding patterns and determine their structure in situ by femtosecond midinfrared spectroscopy. The formation of the nascent TM-CO 2 species according to [L 4 Fe III (C 2 O 4)] + + hn ! [L 4 Fe(CO 2)] + + CO 2 , with L 4 = cyclam, is evidenced by the coincident appearance of the characteristic asymmetric stretching absorption of the CO 2-ligand between 1600 cm À1 and 1800 cm À1 and that of the free CO 2-co-fragment near 2337 cm À1. On the high-spin surface (S = 5/2), the product complex features a bent carbon dioxide radical anion ligand that is O-"end-on"-bound to the metal. In contrast, on the intermediate-spin and low-spin surfaces, the product exhibits a "side-on"-bound, bent carbon dioxide ligand that has either a partial open-shell (for S = 3/2) or fully closed-shell character (for S = 1/2).

The activation of carbon dioxide by transition metals is widely recognized as a key step for utilizing this greenhouse gas as a renewable feedstock for the sustainable production of fine chemicals. However, the dynamics of CO binding and unbinding to and from the ligand sphere of a metal have never been observed in the time domain. The ferrioxalate anion is used in aqueous solution as a unique model system for these dynamics and femtosecond UV-pump mid-infrared-probe spectroscopy is applied to explore its photoinduced primary processes in a time-resolved fashion. Following optical excitation, a neutral CO molecule is expelled from the complex within about 500 fs to generate a highly intriguing pentacoordinate ferrous dioxalate that carries a bent carbon dioxide radical anion ligand, that is, a reductively activated form of CO , which is end-on-coordinated to the metal center by one of its two oxygen atoms.