Photochemical energy transition metals

Photochemical and transition-metal-mediated rearrangements are also known. In the former case the intermediates are high-energy diradicals, often leading to complex mixtures of... 

According to theoretical calculations, there is a low-energy pathway for the direct interaction of diazomethane with carbon monoxide [4j. However, experimental results so far show only evidences for two-step pathways. These involve first the dediazotation of the diazoalkanes in thermal, photochemical, or transition metal mediated reactions [5] resulting in free carbenes (Equation 8.2) or transition metal coordinated carbenes (Equation 8.3), which under proper reaction conditions couple in the second step with carbon monoxide (Equation 8.4 or 8.5) to form ketene products. 

The information available is discussed in light of the effects of excitation energy and the environment on the photofragmentation process of several transition metal cluster complexes. The photochemical information provides a data base directly relevant to electronic structure theories currently used to understand and predict properties of transition metal complexes (1,18,19). 

The development of comprehensive models for transition metal carbonyl photochemistry requires that three types of data be obtained. First, information on the dynamics of the photochemical event is needed. Which reactant electronic states are involved What is the role of radiationless transitions Second, what are the primary photoproducts Are they stable with respect to unimolecular decay Can the unsaturated species produced by photolysis be spectroscopically characterized in the absence of solvent Finally, we require thermochemical and kinetic data i.e. metal-ligand bond dissociation energies and association rate constants. We describe below how such data is being obtained in our laboratory. 

Conclusions. Time-resolved CO laser absorption spectroscopy can provide information useful in characterizing the primary photochemical channels in gas-phase transition metal carbonyls. We have found that product vibrational energy distributions indicate that W(CO)g and Cr(CO>6 dissociate via different... 

SN P spontaneously releases N O both thermally and photochemically [61-65], but is quite stable in the dark and in aqueous in vitro physiological media [66]. This implies that absorption of heat and light energy induces electron transfer from the Fe2+ center to the N 0+ ligand, resulting in weakening of the Fe-N O bond and subsequent release of NO [65]. SNP also decomposes in an aqueous environment in the presence of biological reductants [65, 66] and some transition metal ions to produce nitric oxide. 

Much of the work on the photoreduction of carbon dioxide centres on the use of transition metal catalysts to produce formic acid and carbon monoxide. A large number of these catalysts are metalloporphyrins and phthalocyanines. These include cobalt porphyrins and iron porphyrins, in which the metal in the porphyrin is first of all photochemically reduced from M(ii) to M(o), the latter reacting rapidly with CO to produce formic acid and CO. ° Because the M(o) is oxidised in the process to M(ii) the process is catalytic with high percentage conversion rates. However, there is a problem with light energy conversion and the major issue of porphyrin stability. 

When interaction between the metal-based components is weak, polynuclear transition metal complexes belong to the field of supramolecular chemistry. At the roots of supramolecular chemistry is the concept that supramolecular species have the potential to achieve much more elaborated tasks than simple molecular components while a molecular component can be involved in simple acts, supramolecular species can performIn other words, supramolecular species have the potentiality to behave as molecular devices. Particularly interesting molecular devices are those which use light to achieve their functions. Molecular devices which perform light-induced functions are called photochemical molecular devices (PMD). Luminescent and redox-active polynuclear complexes as those described in this chapter can play a role as PMDs operating by photoinduced energy and electron transfer processes. ... 

Because of the fast nonradiative deactivation of low lying energy states of transition metal complexes, the activation energy for the reactions that may occur from these states must be zero to enable them to compete effectively. For transition metal complexes both 4T2S and aEs states can be photochemically active but may follow different chemical pathways. 

Low-valence transition metal complexes of a-diimine ligands are highly colored because of the presence of low-energy metal to a-diimine charge transfer (MLCT) transitions. For a series of d6-M(CO)., (a-diimine) (M=Cr,Mo,W) and d8- M (CO)3 (a-diimine) (M =Fe, Ru) complexes, we have studied the spectroscopic and photochemical properties (1-10). The a-diimine ligands used are 1,4-diaza-1,3-butadiene (R-DAB), pyridine-2-car-baldehyde-imine (PyCa), 2,2 -bipyridine (bipy) or 1,10-phenanthroline (phen) molecules. A close relationship was deduced between the photochemical behavior of these complexes and their resonance Raman (rR) spectra, obtained by excitation into the low-energy MLCT band. 

Based on their easily tunable photophysical and redox properties, transition metal complexes are versatile components to be used in the construction of photochemical molecular devices. The studies presented in this article show that the combination of the Ru(bpy)22+ photosensitizer and cyanide bridging units allows the synthesis of a variety of polynuclear systems that exhibit interesting photochemical properties. Depending on the nature of the attached metal-containing units, supramolecular systems can be obtained that undergo efficient photoinduced intramolecular energy or electron transfer processes. 

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