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The photochemistry of (μ2-CRCR′)Co2(CO)6 complexes (R = pyrenyl, R′ = H; R = pyrenyl, R′ = ferrocenyl; R = ferrocenyl, R = H) was investigated by ps-time-resolved infrared spectroscopy at room temperature in dichloromethane solution. The main focus of these studies was to determine the primary photoprocess relevant to the light assisted Pauson–Khand reaction. These studies were supported by spectro-electrochemical investigations and density functional calculations which suggest that the primary process to initiate the Pauson–Khand reaction involves a homolytic cleavage of the Co–Co bond forming a high-spin diradical species and not CO-loss as previously thought. [ABSTRACT FROM AUTHOR]

The photochemistry and photophysics of three model "half-sandwich" complexes (η6-benzophenone)-Cr(CO)3, (η6-styrene)Cr(CO)3, and (η6-allylbenzene)Cr(CO)3 were investigated using pico-second timeresolved infrared spectroscopy and time-dependent density functional theory methods. The (η6-benzophenone) Cr(CO)3 complex was studied using two excitation wavelengths (470 and 320 nm) while the remaining complexes were irradiated using 400 nm light. Two independent excited states were detected spectroscopically for each complex, one an unreactive excited state of metal-to-arene charge-transfer character and the other with metal-to-carbonyl charge transfer character. This second excited state leads to an arrested release of CO on the pico-second time-scale. Low-energy excitation (470 nm) of (η6-benzophenone)Cr(CO)3 populated only the unreactive excited state which simply relaxes to the parent complex. Higher energy irradiation (320 nm) induced CO-loss. Irradiation of (η6-styrene)Cr(CO)3, or (η6-allylbenzene)Cr(CO)3 at 400 nm provided evidence for the simultaneous population of both the reactive and unreactive excited states. The efficiency at which the unreactive excited state is populated depends on the degree of conjugation of the substituent with the arene p-system and this affects the efficiency of the CO-loss process. The quantum yield of CO-loss is 0.50 for (η6-allylbenzene)Cr(CO)3 and 0.43 for (η6-styrene)Cr(CO)3. These studies provide evidence for the existence of two photophysical routes to CO loss, a minor ultrafast route and an arrested mechanism involving the intermediate population of a reactive excited state. This reactive excited state either relaxes to reform the parent species or eject CO. Thus the quantum yield of the CO-loss is strongly dependent on the excitation wavelength. Time-dependent density functional theory calculations confirm that the state responsible for ultrafast CO-loss has significant metal-centred character while the reactive state responsible for the arrested CO-loss has significant metal-to-carbonyl charge-transfer character. The CO-loss product (η6-allylbenzene)-Cr(CO)2 formed following irradiation of (η6-allylbenzene)Cr(CO)3 reacts further with the pendent alkenyl group to form the chelate product (η6,η²-allylbenzene)Cr(CO)2. [ABSTRACT FROM AUTHOR]

Two porphyrin-cobaloxime complexes; [{Co(dmgH)2Cl}{MPyTPP}] (1a) and [{Co(dmgH)2Cl}{ZnMPyTPP}] (2a) (dmgH = dimethylglyoxime, MPyTPP = 5-(4-pyridyl)-10,15,20-triphenylporphyrin) have been synthesised as model systems for the generation of hydrogen from water. Although initially envisaged as photocatalytic systems neither complex catalysed the reduction of water to hydrogen following irradiation. However, both complexes are molecular precursors for hydrogen evolution under electrochemical conditions. Turnover numbers for hydrogen production of 1.8 x 103 and 5.1 x 103 were obtained for 1a and 2a respectively following potentiostatic electrolysis at -1.2 V vs. Ag/AgCl while cobaloxime alone produced a turnover-number of 8.0 x 103. The photophysical properties of 1a and 2a were examined to provide an explanation for the lack of photochemical activity. These results, coupled with quantum chemical calculations, confirm that porphyrins fail to act as light-harvesting units for these systems and that the lowest energy excited states are in fact cobaloxime-based rather than porphyrin based. [ABSTRACT FROM AUTHOR]