Abstract
The photochemistry and photophysics of three model "half-sandwich" complexes (η(6)-benzophenone)Cr(CO)3, (η(6)-styrene)Cr(CO)3, and (η(6)-allylbenzene)Cr(CO)3 were investigated using pico-second time-resolved infrared spectroscopy and time-dependent density functional theory methods. The (η(6)-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 (η(6)-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 (η(6)-styrene)Cr(CO)3, or (η(6)-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 π-system and this affects the efficiency of the CO-loss process. The quantum yield of CO-loss is 0.50 for (η(6)-allylbenzene)Cr(CO)3 and 0.43 for (η(6)-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 (η(6)-allylbenzene)Cr(CO)2 formed following irradiation of (η(6)-allylbenzene)Cr(CO)3 reacts further with the pendent alkenyl group to form the chelate product (η(6),η(2)-allylbenzene)Cr(CO)2.
Highlights
Our understanding of the photophysics of CO-loss from metal carbonyl complexes has traditionally been informed by the behaviour of homoleptic systems such as Cr(CO)6.1–6 The general PaperRecently the development of fast time-resolved spectroscopic methods has provided greater insights into the photophysical processes underpinning CO-loss from metal carbonyl systems.[9]
The chelate product was produced in low-temperature steady state photolysis at −20 °C as an air sensitive solid. These results provide a fuller picture of the photophysical processes proceeding the ejection of CO from (η6-arene)Cr(CO)[3] complexes
The most important consequence of this is an increase in the importance of metal-to-arene chargetransfer state (MACT) excited states in the photophysics of these complexes
Summary
The development of fast time-resolved spectroscopic methods has provided greater insights into the photophysical processes underpinning CO-loss from metal carbonyl systems.[9] It is clear from these studies that not all photoinduced CO-losses from metal carbonyl complexes are ultrafast.[10,11,12,13,14,15] In particular half sandwich complexes of the type (η6-arene)M(CO)[3] (M = Cr, Mo, or W) and some Fischer carbene complexes of Cr(CO)[5], have been shown to undergo so-called “arrested CO-loss” where CO is ejected on the ps rather than the fs timescale. Fast ( ps) time-resolved infrared spectroscopy ( ps-TRIR) techniques are available which can probe early-stage processes in the photoinduced CO-loss reaction by monitoring changes in the mid-infrared region of the spectrum where metal carbonyl stretching modes are observed.[9] The high extinctions of the metal carbonyl absorptions make this class of complex suited to such studies. In addition the availability of time-dependent density functional theory (TDDFT) provides a method of describing excited state dynamics at a reasonable computational cost which can assist in explaining the results obtained in ps-TRIR experiments.[16,17,18] For the systems in this investigation these calculations provide a plausible explanation for the arrested CO-loss observed in the ps-TRIR experiments and can help in the design of systems with specific photophysical and photochemical properties
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