Photochemical ligand substitution isomerization

The cis-trans isomerization of PtCl2(Bu P)2 and similar Pd complexes, where the isomerization is immeasurably slow in the absence of an excess of phosphine, is very fast when free phosphine is present. The isomerization doubtless proceeds by pseudorotation of the 5-coordinate state. In this case an ionic mechanism is unlikely, since polar solvents actually slow the reaction. Similar palladium complexes establish cis/trans equilibrium mixtures rapidly. Halide ligand substitution reactions usually follow an associative mechanism with tbp intermediates. Photochemical isomerizations, on the other hand, appear to proceed through tetrahedral intermediates. 

Photochemical isomerization of la to the mer-isomer (7) in a CO-saturated THE solution proceeded by 313-nm irradiation instead of the ligand substitution reaction (Eq. 12) 52). 

The photochemical reactions " of Cr(III) include both ligand substitutions and isomerization in solution and solids. Intramolecular redox reactions also are noted when charge-transfer bands are excited as well as intermolecular ones in cases where a long-lived state is quenched by bimolecular electron transfer to another species in solution. ... 

Unfortunately, despite its poor luminescent properties, tpy is a desirable ligand from a structural point of view in the design of supramolecular photochemical devices. Metal bis(tpy) complexes are achiral, which means that they do not give rise to undesirable mixtures of diastereoisomers when more than one metal centre is present, and as a bridging ligand, substituted tpy compounds are easier to make into rigid linear connectors than bpy or phen derivatives. Furthermore, substituted tpy compounds do not exhibit facimer isomerism as do their bidentate counterparts (Figure 11.9). 

Photochemical reactions have been used for the preparation of various olefin, and acetylene complexes (7). Application to the coordination of dienes as ligands has not been used extensively, so far. In this article the preparative aspects of the photochemistry of carbonyls of the group 6 and group 7 elements and some key derivatives, with the exception of technetium, with conjugated and cumulated dienes will be described. Not only carbonyl substitution reactions by the dienes, but also C—C bond formation, C—H activation, C—H cleavage, and isomerizations due to H shifts, have been observed, thereby leading to various types of complexes. 

Open mthenocenes, Ru(jj -C5R7)2, with asymmetric substitution of the pentadienyl ligands (see Pentadienyl Ligand) allow for the synthesis of diastereomeric products. The treatment of (1) with 2-methyl-4-phenylpentadiene and zinc dust yielded bis(2-methyl-4-phenylpentadienyl)mthenium (94). The product consists of a pair of diastereomers, which were easily separated by fractional crystallization because of their substantially different solubilities. One isomer could exist as a pair of enantiomeric rotamers, each with C symmetry (94a) and (94b), while the other could be a pair of diastereomeric rotamers, both with C2 symmetry (94c) and (94d). Both of the former rotamers are observable, but (94c) is the only form of the latter, and interconversion to (94d) does not occur. The possibihty of isomerization with photochemical initiation still remains to be investigated. ... 

The c /.s-bis(phosphine)-substituted complexes 68 were found in our laboratory to isomerize to the trans derivatives under irradiation with visible light [Eq. (57)] (97). The isomerization reaction was postulated to involve a stereochemically nonrigid pentacoordinate intermediate which is generated by photochemical carbyne-carbonyl coupling. (The evidence for the ligand coupling step as well as other photoinduced coupling reactions are discussed in Section IV,F.)... 

The mechanism of this reaction is not as yet quite clear. Since cis/ trans isomerization of the replaced ligand is observed in the photochemical but not in the thermal substitution reaction, the mechanisms of these two reactions appear to be different 282>. The photochemical reaction must involve a species, which can rotate about the C=C bond. 

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