Irradiation, cobalt complexes

This may be of significance in connetion with the biosynthesis of acetate from carbon dioxide, because the next step, the fixation of carbon monoxide, was demonstrated by B. Krautler. He irradiated methyl cobalamin under Co at 30 atm and obtained the acyl cobalamin as the product. Interestingly, a radical mechanism was iproposed, involving the reaction of methyl radicals with CO to give acyl radicals, which then recombine with the cobalt complex /55/. 

Ethylenediaminetetraacetic acid, analogs, complexes of, 3 277 chelation by, 3 276-277 cobalt complex of, 3 281 complexes, 3 277-278 formation constant of, 3 273-274 -nickel, 3 17-18 stability of, 3 266-267 reaction with metal ions, 3 62 Ethylene dibromide, irradiation of, 5 196 4,5-Ethylenedithio-1,3-dithiole-2-thione based supramolecular complexes, 46 200-204 Ethylene glycol, 32 4... 

Several recent examples of this technique are outlined in Scheme 70.l99 20li20S 206 The starting cobalt complexes are highly colored, air stable compounds that require no special precautions in handling. The process is relatively cost effective most cobalt precursors are less expensive per mole than tributyltin hydride. The reactions are conducted by visible light irradiation in a variety of solvents and can often be followed by color changes characteristic of the different oxidation states of cobalt. A disadvantage is that... 

Substituents in the macrobicyclic cobalt complexes affect their photochemical properties. These effects are more pronounced for solutions of the complexes exhibiting high redox potentials and low electron-transfer rates (Tables 42 and 52). Clathrochelates with lower redox potentials and higher electron-transfer rates are less photochemically active. As mentioned above, the [Co(diNOsar-H)]2+ cation displays the highest photochemical activity among the other complexes studied spectral changes are observed on its irradiation... 

Although the mechanism of the reaction is not completely clear, it is certain that, organomercurials behave as alkylating agents of the cobalt complex. In improving this reaction, Seyferth found that carbonylation of mercurials can be carried out in the presence of catalytic amounts of Co2(CO)8 or Hg[Co(CO)4]2 under UV irradiation. 

Novel platinum(ii) Schiff base complexes were found to be efficient photosensitizers for H2 evolution from mixed solvents (e.g. DMF/H2O) under visible light irradiation, with a cobalt complex as catalyst and TEA as sacrificial electron donor. Catal5Aic activity is influenced by the presence of substituents on the ligand electron-withdrawing groups, in particular, have a positive effect. ... 

Yoshida and co-workers utilized a commercially available photochemical KeyChem-Lumino microreactor with a quartz window and a channel volume of 917 pL (width = 1000 pm, depth = 200 pm, length = 916 pm) to explore dissociation of a CO ligand from the di-cobalt complex 8 to initiate a Paulson-Khand reaction with 1-norbornene 9 (Scheme 2). For this study they employed a Peltier device to maintain the reactor temperature at 25 °C. They irradiated the reactor by placing it in front of a medium-pressure 80W Hg arc lamp. Reaction of 1-norbornene 9 with cobalt complex 8 afforded the cyclopentenone product 10 in excellent yield (90%) after only two minutes. This was superior to the corresponding batch reaction which only afforded a 32%... 

Hot atom chemistry of cobalt complexes was intensively studied (Tominaga and Saito 1979). Chemical varieties of recoil prodncts found in cobalt complexes of cyano, ammine, and ethylenediamine ligands are worth mentioning. For example, neutron irradiation of hexamminecobalt(lll) nitrate [Co(NH3)6](N03)3 prodnces [ Co(NH3)6], [ Co(NH3)5 NOzl ", cis- and fra s-[ Co(NH3)4(N02)2l", [ Co(NH3)3(N02)3l, [ Co(NH3)2(N02)4], and Co, which were detected by paper electrophoresis (Saito et aL 1962). 

Catalytic and photochemical processes using mixed metal carbonyls are also represented. Tin-cobalt carbonyl compounds have been used as catalysts for ring-opening of oxiranes by secondary and tertiary alcohols 121 and the photochemical reactions of Pt2M4(CO)jg (M=Os, Ru) with cycloocta-1,5-diene under UV irradiation 122,123,124 jjgve been reported. A new rhenium-cobalt complex has been characterised 126 and Pt(cod)2 has been used to prepare new complexes via its reaction with 0 3(CO) jQ(FlCMe)2 26. 

There are some examples of the use of so-called photobase generators in the preparation of polymers by anionic means. Photobase generators are compounds that produce bases upon irradiation with light, examples include cobalt complexes 0-acyloximines and benzyloxycarbonyl derivatives. PBGs are generally important in polymer science due to their application in photoinitiated transformation of polymeric materials and epoxy resins. Kutal et al. have reported the anionic polymerization of alkyl 2-cyanoacrylates initiated by anionic species upon the irradiation of group 8 metallocenes [YAM 98, YAM 99, YAM 00, SAN 02, BRI 02, DIN 03, SAN 05,... 

Palladium complexes also catalyze the carbonylation of halides. Aryl (see 13-13), vinylic, benzylic, and allylic halides (especially iodides) can be converted to carboxylic esters with CO, an alcohol or alkoxide, and a palladium complex. Similar reactivity was reported with vinyl triflates. Use of an amine instead of the alcohol or alkoxide leads to an amide. Reaction with an amine, AJBN, CO, and a tetraalkyltin catalyst also leads to an amide. Similar reaction with an alcohol, under Xe irradiation, leads to the ester. Benzylic and allylic halides were converted to carboxylic acids electrocatalytically, with CO and a cobalt imine complex. Vinylic halides were similarly converted with CO and nickel cyanide, under phase-transfer conditions. ... 

---