Matrix photochemistry temperature matrices

C5Me5) was stable up to room temperature (108,109). A surprising feature of the matrix photochemistry (6,7) was that only the trans isomer of [CpFe(CO)2]2 was photolyzed, and it may be that this was the consequence of concerted CO loss from the trans isomer and isomerization to form the triply bridged product (6). 

Photochemistry and Matrix-isolated Species.—A potentially important development can be recognized in studies of the production of metal atoms and naked metal clusters e.g., Fcs from [Fe3(CO)i2] in the gas phase at ambient temperature by photodissociation or as a result of dissociation of metal carbonyls caused by metastable noble gases. In addition to ground-state species, significant numbers of excited-state atoms, many with long radiative lifetimes, can be produced by these methods. 

The matrix photochemistry of phenylnitrene (PhN) has many similarities to that of phenylcarbene, but also some significant differences. A comparison between the two, including flash-photolysis as well as low-temperature, matrix-isolation studies, can be found in a 1995 review. 

An elegant illustration of the low-temperature matrix technique is provided by the photochemistry of pleiadene precursor 1 (Scheme 5). Ojmpound 1 rearranges to pleiadene (2) only after population of an upper triplet state by biphotonic excitation in rigid glasses at 77K. It is important to note that solution irradiation both at room and low temperatures leads to no reaction, and that triplet sensitization in solution is similarly ineffective. The reaction was achieved only in matrices at 77K using both UV and visible radiation sources simultaneously in a biphotonic process, or with a single UV source in a monophotonic... 

The matrix experiments thus reveal some complex photochemistry of relevance to solution chemistry but the experiments do not provide information about kinetics. For this we need a fluid medium e.g. gas or liquid, and we consider such experiments in the next two sections. Flash photolysis suggests itself as the technique for detecting a species as reactive as Cr(C0)5 but before describing these experiments we show what can be achieved from low-temperature solutions. 

In this section, we outline a number of studies by various workers in which matrix isolation experiments have contributed to the understanding of some photochemical pathways and where the objective was to relate the experiments to room-temperature photochemistry. We then consider briefly some current problems involving dinuclear and polynuclear carbonyls. 

Jacox and Milligan (520a) favor the three-membered ring structure with an O—C—O angle of 65 from the analysis of infrared spectra of isotopic species of C03 in low temperature matrices. A new broad and weak absorption band at 4060 A with an absorption coefficient of 1.1 +0.3 atm -1 cm -1 is also found in 03—C02 matrix (551). However, no corresponding infrared absorption bands have been found in the gas phase photolysis [DeMorc and Dcde (277)]. The photochemistry of C02 in this region may be summarized as follows. 

If the photolysis takes place in an inert gas matrix, both the homolytic splitting of the metal-metal bond and the breaking of a metal-nitrogen bond will be followed by a fast backreaction to the parent compound. The radicals formed by homolysis of the metal-metal bond can not diffuse from the matrix site and will recombine to the parent compound. Moreover, the photoproduct obtained by breaking of a metal nitrogen bond, will not be stabilized by a coordinating solvent molecule and therefore react back to the parent compound. Because of this the photochemistry of some of these complexes has also been studied in a Cl-k-matrix at 10K and for comparison in a PVC film, which is a less rigid medium than the matrix especially at room temperature. 

The only claim of direct evidence for the intermediates proposed above come from ESR measurements subsequent to photolysis of Mn2(CO)j0 in tetrahydrofuran at room temperature.112) A long-lived, ESR detectable, radical was found and proposed to be Mn(CO)5 THF. The intermediate disappears upon addition of I2 and the formation of Mn(CO)sI is observed. These data seem to be wholly consistent with the photochemistry outlined above, but the interpretation of the ESR signal as that due to an Mn(CO)s moiety seems untenable because it is too long-lived. The Re(CO)s species proposed as an intermediate in the photolysis of Re2(CO)10 has recently been synthesized by atom/ligand co-condensation synthesis and infrared data in the matrix at low temperature support a square-pyramidal structure.113) An ESR signal was also observed from a species thought to be Mn(CO)s formed by subliming Mn2(CO)i0 on to a cold tip.114) The ESR detectable species is now believed to be OOMn(CO)s.115)... 

The wavelength (185-254 nm) dependent photochemistry of cyclo-octa- and cyclo-hepta-1,3-dienes has been studied. cis,cis -Cyclohepta-l,3-diene is converted to the trans -cIs -isomer (92) when irradiated in a matrix at low temperature. On warming to temperatures above -78°C this intermediate ring closes to afford bicyclo(3,2,0)hept-6-ene, the same product obtained by irradiation at room temperature. ... 

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