Transition metals, photoreactive complexes

Photoreactions that involve transition metal ions, complexes or compounds can usually be classified as (photo)redox (simultaneous oxidation and reduction) processes. A representative non-photoassisted catalytic system is Fenton s reagent that produces HO radicals on reaction of ferrous ions (Fe2 +) and hydrogen peroxide (Scheme 6.287a). Its photochemical counterpart is the photo-Fenton process,1527 in which ferric (Fe3 + ) complexes in aqueous solutions (absorbing over 300 nm) are reduced to ferrous ions (Scheme 6.287b). 

Consideration of the nature of the LMCT transitions, redox energetics, and photoredox behaviour of transition-metal ammine complexes has allowed Endicott and co-workers18 to propose new models for the potential energy surfaces describing their photoredox reactions. These models have been used to discuss the differences in photoreactivity of [Co(NH3)5Br]2+ and [Co(NH3)5-N03]2+.21 These differences are ascribed to (i) more Co-radical bonding in the... 

Among outputs, luminescence emission is considered to be mie of the most attractive, owing to the ease of detection and the cheap fabrication of devices in which it is detected. Many examples of fluorescent photochromic molecules have been published, by combining a DTE unit with a fluorophore [4]. Incorporation of the DTE fragment into the ligands of transition-metal polypyridine complexes allows the photoreaction to proceed via a triplet state leading to a photoregulation of phosphorescence. 

Transition-metal-ion complexes have been introduced as templates to direct [2-1-2] photodimerizations in the solid state. The photoreaction has been achieved in both discrete complexes and metal-organic frameworks (MOFs). MOFs are intriguing platforms to control the photodimerization owing to changes to bulk physical properties (e.g., porosity) that can occur in the porous frameworks. 

Daniel C (2004) Electronic Spectroscopy and Photoreactivity of Transition Metal Complexes Quantum Chemistry and Wave Packet Dynamics. 241 119-165... 

In C-NMR spectra, the signals for the carbene carbon are usually shifted upheld by about 20-30 ppm upon complexation of the free NHC to a transition metal. Cr-NMR data of [LCr(CO)s] complexes underline that NHC are a special case of carbene ligands because of their lack of tt-acceptor ability. Photoreactions of metal complexes containing NHCs by laser flash and continuous photolysis show that NHCs are quite inert ligands in photolysis reactions. He I and He II photoelectron spectra of platinum(O)- and palladium(O) bis(imidazolin-2-ylidene)... 

Mo(r75-C5H5)2H2] and [MoH dppe ]. Our studies of the di- and trihydride complexes of ruthenium and iridium, described above and published previously (27,35), and those of other workers (discussed at the beginning of this chapter), indicate that photoinduced elimination of molecular hydrogen is a common reaction pathway for di- and polyhydride complexes. To demonstrate the photoreaction s generality and its utility for generating otherwise unattainable, extremely reactive metal complexes, we have begun to study the photochemistry of polyhydride complexes of the early transition metals. We focused initially... 

Although only rarely luminescent in ambient fluid solutions, square-planar transition metal bis(dithiolene) complexes do display significant and varied photochemical reactivity. Much of the photoreactivity described above for dianionic bis(dithiolene) complexes involves excited-state oxidation and often leads to radical formation. In addition, the excited states of these complexes are receiving attention for their potential as materials for optical (15), nonlinear optical (10-13), and electrooptical (16) devices. The relevance of this work to those applications is addressed in other parts of chapter 8 in this volume (87b). 

Three steps are Involved In predicting the photoreactions of a transition metal complex with this model. First, the molecular axes that are labillzed In the excited state are determined. 

In contrast to the typical behavior of organic compounds discussed above, many photoreactions of transition metal complexes have wavelength-dependent quantum yields (7). Generally, these wavelength effects have been interpreted in terms of more than one reactive excited state of the photolyzed species. The photoreactivity of V(CO) L (L = amine), for example, has been interpreted in this manner with the previously mentioned model of substitutional photoreactivity proposed by Wrighton et al. (42, 49,73). Assuming ligand dissociation to be the only primary photochemical process (Section III-B-1), photolysis of W(C0)5L could produce three primary products ... 

An excited state of the substrate S reacts with the transition metal complex, whereas S in the ground state would not react. Such a situation was labeled catalyzed photoreaction by Wubbels [1], Salomon [9], Mirbach [29], and Hennig [8]. 

Complexation of halocarbons with natural substances can enhance the rates of photoreactions that provide sinks. Ionizable halocarbons, such as hal-ogenated organic carboxylic acids, potentially could form complexes with pho-toreactive transition metals, such as iron. In addition, dissolved NOM and sediments are known to sorb or bind ionic and nonionic halocarbons, and sorbed halocarbons may photoreact more efficiently (eq 7). 

Ionic halocarbons, including halogenated carboxylic acids, may form pho-toreactive complexes with transition metals in the aquatic environment. Indeed, complexes of carboxylates with dissolved Fe(III) and iron oxides are very photoreactive under solar radiation (28, 29). Photoreactions of such complexes may help to explain the enhanced photoreactivity of chlorinated acetates in natural water samples. 

The enhanced photoreactivity of sorbed nonionic halocarbons may involve photoreactive complexes with amines and other electron-donating substances. The enhanced photoreactivity of ionic halocarbons (e.g., chloroacetates) may involve complexes with DOM and transition metals. Additional studies are needed to examine the role of complexation in the aquatic photochemistry of halocarbons. 

---