Iron complexes, photochemistry

There have been two books devoted to the chemistry of iron, " and many reviews devoted to various aspects of its coordination chemistry, including structures and photochemistry (iron(III)). Iron complexes appear in a multi-author volume on the history of coordination chemistry, but there is disappointingly little about iron—just a brief mention of hexacyanoferrates in connection with pigments—in an otherwise excellent overview of the history of chemistry. ... 

There is a brief but eomprehensively referenced section on iron in a review of homoleptie 2,2-bipyridyl eomplexes " iron-terpy (3 pages) and iron-quaterpy (1/2 page) complexes have also been briefly reviewed." Absorption spectra and photochemistry of apprcmriate iron diimine complexes are ineluded in a text on polypyridyl and porphyrin complexes." A review of the applieation of ehiral 2,2 -bipyridines, 1,10-phenanthrolines, and 2,2 6, 2"-terpyridines in homogeneous eatalysis eontains a little material on iron complexes of such ligands." ... 

The photochemistry of the iron complexes in this family has been extensively investigated and four different processes have been proposed by different authors for the primary photochemistry step. These are CO dissociation [Eq. (86)], shift of the Cp ring [Eq. (87)], CO insertion into the metal-alkyl bond [Eq. (88)], and metal-alkyl bond homolysis [Eq. (89)]. 

Fig. 8. Photochemical and chemical reduction of the semiquinone-iron complex in PS-II membranes (a) sample frozen in the dark, (b) sample illuminated for 10 m at 5 K, (c) sample illuminated for 10 m at 77 K, (d) sample illuminated for 4 m at 200 K. and (e) sample frozen in the dark in the presence of dithionite (no mediators present). EPR spectra were measured at 4.8 K. Figure source Rutherford and Mathis (1983) A relationship between the midpoint potential of the primary acceptor and low temperature photochemistry in photosystem II. FEES Lett 154 332.

Figure 9 illustrates the complicated interacticMis of iron-oxalato complex photochemistry with radical chemistry and the chemistry of organic substances. The main impacts of iron complex photochemistry are ultimately (1) breaking of C-C bonds and thus degradation of the hgand (oxalate) and (2) formation of radicals... 

A prerequisite to simulate the impact of iron complex photochemistry in atmospheric aqueous systems is the characterization of its efficiency. Figure 10 presents an overview of quantum yield measurements in the ferrioxalate system as a function of wavelength. 

Iron complex photolysis is mie of the processes that produce reduced iron (Fe(n)) in a highly oxidizing enviromnent like the atmospheric aqueous phase. There are numerous other processes such as reactions with HO species or Cu(I)/ Cu(n) which can reduce or oxidize iron in the troposphere. These reactions can take place simultaneously and cause iron to undergo a so-caUed redox-cycling [167]. Because of the large number of complex interactions in the atmospheric chemistry of the transition metal iron, it is useful to utilize models to assess the impact of the complex iron photochemistry. 

The photochemistry of cyclopentadienyl iron complexes has been extensively studied. When Cp2Fe(CO)2 is photolysed at low temperatures CO loss leads to CpFe(CO)(n -Cp)Two isomers of this complex were identified. An examination of the photochemistry of Cp2Fe2(CO)4 has shown that Cp2Fe2(CO)3 is the intermediate in photochemical substitution of CO by MeCN. ... 

Other studies devoted to mononuclear iron complexes include evidence for a photoinduced haptotropic shift of the coordinated pyrrolyl ligand in (C5H5)(C4H4N)Fe(CO)2. The photochemistry was studied in low-temperature matrices and in room-temperature solution. The preparation and properties of carbonyliron complexes of l-aza-4-oxa-1,3-butadiene have also been described. ... 

The photochemistry of several of the iron porphyrin halocarbene complexes Fe(TPP)(=CXY) (CXY = CCF, CBr2, CCIF. CCl(CN) and the vinylidene complex Fe(TPP)=C=CAr2 (Ar = p-C6Fl4Cl) has been studied in degassed benzene... 

Barbeau K (2006) Photochemistry of Organic Iron (HI) Complexing Ligands in Oceanic Systems. Photochem Photobiol 82 1505... 

Reviews of kinetics and mechanisms deal with long-range electron transfer and with the uptake and release of dioxygen species, especially uptake from organic peroxides. " " The photochemistry of iron porphyrin complexes has been documented." ... 

Among transition metal complexes the iron compounds are those for which LMCT photochemistry is of crucial environmental relevance [51, 61, 62], Their photoreductions yield the Fe11 species and ligand radicals (L ), eg ... 

Szacilowski K, Macyk W, Stochel G, Stasicka Z, Sostero S, Traverso O. Ligand and medium controlled photochemistry of iron and ruthenium mixed-ligand complexes prospecting for versatile systems. Coord Chem Rev 2000 208 277-97. 

Photochemical methods offer a convenient tool to study intra- and interprotein ET because of their time resolution and selectivity. Various mechanistic and design approaches based on photochemistry of metal complexes have been undertaken. Most of the studies on protein electron transfer processes have been done for hae-moproteins using among others ruthenium complex as a photosensitizer, modified haemoproteins in which haem iron is substituted by another metal (mainly Zn), and CO-bonded haem proteins [6,7],... 

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