Ethylene complexes, photochemical

Another interesting example of aurophilic interactions influencing electronic excited state properties is the unusual photochemical cis to trans conversion observed in dinudear goldhalide bis(diphenylphosphino)ethylene complexes, Au2X2(dppee)... 

Ethylene complexes of transition metals near the beginning of the transition series are now known. A neutral complex was prepared in 1960 by the photochemical displacement of carbon monoxide by ethylene from cyclopentadieny 1-manganese tricarbonyl 156) ... 

As an example, photochemical excitation of donor-acceptor complexes may be considered. Irradiaiion into the CT band of the anthracene-tetracyano-ethylene complex leads directly to the radical ion pair, the components of which are identifiable from their UV-visible spectra. The transient absorptions decay in 60 ps after excitation, as the radical ion pairs undergo rapid back electron transfer to afford the original donor-acceptor complex (Hilinski et al., 1984). With tetranitromethane as acceptor, however, an addition product is obtained in both high quantum and chemical yield. This is due to the fact that the tetranitromethane radical anion undergoes spontaneous fragmentation lo a NO, radical and a trinitromethyl anion, which is not able to reduce the anthracene radical cation (Masnovi et al., 1985) ... 

Both reactions, with participation of tetramethyltin, to a certain extent model the interaction of the Pt(lV) -1- Pt(ll) system with saturated hydrocarbons. The cleavage of the Sn-CHa bond by the PtCU complex is accelerated by irradiation [31]. The formation of the complex cr-CiHs-PtCI has been observed by h NMR in the photochemical reaction in CD3COOD. Light apparently accelerates the conversion of the ethyl a-complex into the rt-ethylene complex... 

Nickel dimethyl complexes NiMe2(PRs)2 decompose readily in solution, either thermally or photochemically, providing an efficient method for the generation of Ni(0) complexes. The complex NiMe2(dtbpe) undergoes reductive elimination in benzene at 50-60 °C, giving rise to a binuclear Ni(0) 7r-arene complex. Careful thermolysis in THF-/ 8 over several days at 20 °G affords a 1 2 1 mixture of the ethylene complex 159, the hydride 160, and ethane (Scheme 47). If the reaction is carried out in the presence of ethylene, equimolecular amounts of 159 and ethane are formed. These results can be explained by assuming that the decomposition process involves the formation of a complex. In contrast with this behavior, the bis(carbene) dimethyl complex 45 decomposes... 

Kaczmarek, H. and Szalla, A. (2006) Photochemical transformation in poly (acrylic acid)/poly (ethylene oxide) complexes./. Photochem. Photobiol. B Chem., 180, 46-53. 

Various methods have been used to convert precatalysts into the active species [7]. Ethylene can be easily displaced from the central atom of the corresponding complexes in solution, even at room temperature. CO-ligands in carbonyl complexes can conveniently be removed photochemically [8], Increasing the temperature is a further common method used to labilize precatalysts with respect to stabilizing ligands [9],... 

In attempts to cany out cyclisation from butadiene 1,3 in the gaseous phase and under photochemical conditions R Srinivasan (J.Amer.Chem.Soc. 88, 3765 (1966) showed that irradiation yields a complex mixture of ethylene, acetylene, but-1 ene, hydrogen and polymers. The quantum yields of acetylene and ethylene formation increase with pressure. 

Ring closure of the diene in [Ru(phen)2(217)] using Grubb s catalyst generates a macrocyclic complex, the photochemical behavior of which has been studied in MeCN solution. Irradiation at >300 nm leads to the expulsion of [Ru(phen)2(MeCN)2], and quantitative regeneration of the macrocyclic complex can be achieved by heating in ethylene glycol. " ... 

Cyano-substituted ethylenes react in a different way with aliphatic ketones. The orientation of photochemical cycloaddition (4.661 is the opposite of that found for electron-rich alkenes, and the reaction is highly stereoselective (4.69) in the early stages. These processes involve the formation and subsequent decay of an excited complex (exciplex) from the (n,n ) singlet state of the ketone and the alkene. Aryl ketones undergo intersystem crossing so efficiently that such a singlet-state reaction is rarely observed, but the reaction of a benzoate ester with an electron-rich alkene 14.70 rnay well be of this type, with the ester acting as electron-acceptor rather than electron-donor. 

The study of species in which ethylene is coordinated to transition metal centres holds great interest in areas of catalytic and polymerization chemistry (7). The bonding of the ethylene ligand to the metal centre in such species has been compared to that of the dihydrogen complexes described above (14,15,22). Photolysis of chromium hexacarbonyl, Cr(CO)6, in conventional solvents in the presence of dissolved ethylene gas is known to lead initially to a highly labile species in which one CO ligand is replaced by ethylene. Further photolysis leads to a more stable compound which contains two ethylene ligands trans to each other across the metal centre (25), equation 3. The conventional synthesis is experimentally difficult the two photochemical... 

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