Iron complexes, photolysis

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 therapeutic effects of sodium nitroprusside depend on release of nitric oxide which relaxes vascular muscle. Sodium nitroprusside is best formulated as a nitrosonium (NO+) complex. Its in vivo activation is probably achieved by reduction to [Fe(CN)5NO]3, which then releases cyanide to give [Fe(CN)4NO]2, which in turn releases nitric oxide and additional CN to yield aquated Fe(II) species and [Fe(CN)6]4 (502). There are problems associated with its use, namely reduced activity due to photolysis (501) and its oxidative breakdown due to the action of an activated immune system (503), both of which release cyanide from the low-spin d6 iron complex. 

Photolysis of cationic alkoxycarbene iron complexes [193] or alkoxycarbene manganese complexes [194] has been used to replace carbonyl groups by other ligands. The alkylidene ligand can also be transferred from one complex to another by photolysis [195], Transfer of alkylidene ligands occurs particularly easily from diaminocarbene complexes, and has become a powerful synthetic method for the preparation of imidazoline-2-ylidene complexes [155,196]. 

Equation 17.26 is directly involved in DOM photomineralization, and Equation 17.25 yields Fe2+. Complexation of Fe(III) by organic ligands is in competition with the precipitation of ferric oxide colloids [79], and the formation of ferrous iron on photolysis of Fe(III)-carboxylate complexes is an important factor in defining the bioavailability of iron in aquatic systems. Iron bioavailabihty, minimal for the oxides and maximal for Fe2+, is considerably enhanced by the formation of Fe(III)-organic complexes and their subsequent photolysis. Iron bioavailabihty plays a key role in phytoplankton productivity in oceans [80-82], while that of freshwater is mainly controlled by nitrogen and phosphoms. 

Intramolecularly alkoxy base-stabilized bis(germylene)-, 54, or (germylene)(silylene)-, 55, iron complexes, of type IV, were obtained from the photolysis of digermyl or germylsilyl iron complexes (equation 43)14. These are the same type of complexes noted above for base-free tungsten analogs146 involving a-elimination subsequent to photochemical elimination of CO as noted below. 

Scheme 23. One-pot syntheses of the pheno l-tria I lyl iron complex and metal-free dendron by variation of the experimental conditions. The iron complex can be demetalated by visible-light photolysis the metal-free phenol-triallyl dendron can be obtained more rapidly direct from the p-ethoxytoluene-iron complex.

Neutral (cyclobutadiene)iron complexes undergo thermal and photochemical ligand substitution with phosphines, with alkenes such as dimethyl fumarate and dimethyl maleate, and with the nitrosonium cation. Cationic nitrosyl complexes (e.g. 210) undergo ligand substitution by treatment with phosphines. Photolysis of (tetraphenylcyclobutadiene)Fe(CO)3 in THF at -40 °C is reported to give the novel bimetallic complex (214), which reacts with carbon monoxide (140 atm, 80 °C) to regenerate the starting material.An X-ray diffraction analysis of (214 R = Ph, R = t-Bu) reveals a very short Fe-Fe distance of 2.117 A. 

Cyclic 1,3-diene iron tricarbonyl complexes eliminate hydrogen on electron impact to give predominant odd electron ions with iron bonded to an aromatic system 57) These same molecules eliminate hydrogen and iron on photolysis to give aromatic hydrocarbon products. 

Photolysis of bulky permethylated Fp complexes such as FpSi[Si(CH3)3]3 does not cause deoligomerization, possibly because stable intermediate iron-silyl(silylene) complexes are not formed (27). Other less bulky transi-tion-metal-oligosilane complexes are also unreactive under the photolysis conditions. For example, the ruthenium analogues of the iron complexes, [( ri -C5H5)Ru(CO)2-Si ], are essentially photostable (23). Whether this behavior is due to the strength of the Ru-CO bond or to the enhanced stability of the Si-Si bond is not clear, and this problem is currently under investigation. 

The alkylcarbene complex (14) has been produced by photolysis of (15). The reactive intermediate Pe(DMPE)2 (DMPE -Me2PCH2CH2PMe2) generated by photolysis of FeH2(DMPE)2 has been shown to be sufficiently reactive to add intermolecularly to the H bonds in pentane at -90 C to form la-(1-pentyl)PeH(DMPE)2 At -30 C the analogous reaction leads to E- and Z- ia-(pent-l-enyl)PeH(DMPE)2- The ring-expanded complexes (16) are formed on photolysis of the (aminocyclobutyl)iron complexes (17) (R Me, CHMe2). ... 

Hydroxyl radicals are produced in the aqueous environment by photolysis of nitrate (Figure 12.3a), nitrite, and aqueous iron complexes, or, in water treatment, from photolysis of HOCl and catalytic decay of aqueous O3. OH radicals formed in natural waters react principally with dissolved organic matter (see Figure 11.10c). 

Several possible mechanisms are available for UV-induced photoreactions of iron complexes. First, direct photoreactions involving ligand-to-metal charge transfer are likely to be one of the most important mechanisms for photoreaction [117,198,224]. Second, iron complexes can be reduced by photochemically-produced superoxide [207-209]. Superoxide ions are formed via the photoreduction of molecular oxygen by CDOM and it is one of the most concentrated radicals in seawater and is the precursor to hydrogen peroxide [Chapter 8]. Superoxide-induced reduction of Fe(iii) is an important mechanism in certain lakes [207]. However, the fact that Fe(ii) photoproduction can be more rapid in oxygen-free water than in air-saturated water in acidic estuaries [59] or model systems with well-defined organic acid complexes of Fe(iii) [117] indicates that direct photolysis of Fe(iii) is likely to be a dominant mechanism for Fe(ii) photoproduction in many aquatic systems. 

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