Photoisomerization of coordinated

Qualitative photochemical studies concentrate on diene complexes because of their greater thermal inertness and ease of characterization. Formation of olefin complexes is induced in situ either by photochemical or thermal means and their presence determined by spectroscopy. The photocatalyzed hydrogenation and hydrosilation of 1,3-di-enes the photocatalyzed valence isomerization of norbomadiene to quadricyc-lane and the cis trans photoisomerization of coordinated olefins are potentially usefulHowever, these transformations are not photosubstitution reactions and are not discussed here the reader should consult ref. 1 and references cited therein. Photolysis of olefin complexes leads to olefin loss with high quantum efficiency unless the olefin is a chelating di- or polyene where, as with most chelating ligands, other reactions occur. 

In other instances, irradiation of the d-d transition leads to no observable reaction. Examples of this behavior are found for complexes having a variety of d electron configurations and coordinative geometries square planar Ni(II) (3d)3 in Ni(CN)42 124 and mww-Ni(gIy)2 124 square planar Pd(II) in Pd(CN)42-,124 and tra -Pd(gly)2 square planar Pt(II) in Pt(CN)42" (5d)3 124 octahedral Co(III) (3d)6 in a variety of complexes (cf. Sect. III-C and III-D). A striking example of this type of behavior is afforded by the nonreversible photoisomerization of cis-Pt(gly)2 (5d)8 to trans-Pt(g y)2 [reaction (2)].124 It has been proposed that irradiation of either of these square planar complexes leads to the same tetrahedral intermediate which decays exclusively to mwj-Pt(gly)2. This behavior may be contrasted with the reversible photoisomerization shown in reaction (3).3... 

J. Troe Professor Marcus, you were mentioning the 2D Sumi-Marcus model with two coordinates, an intra- and an intermolecu-lar coordinate, which can provide saddle-point avoidance. I would like to mention that we have proposed multidimensional intramolecular Kramers-Smoluchowski approaches that operate with highly nonparabolic saddles of potential-energy surface [Ch. Gehrke, J. Schroeder, D. Schwarzer, J. Troe, and F. Voss, J. Chem. Phys. 92, 4805 (1990)] these models also produce saddle-point avoidances, but of an intramolecular nature the consequence of this behavior is strongly non-Arrhenius temperature dependences of isomerization rates such as we have observed in the photoisomerization of diphenyl butadiene. 

The trans-to-cis photoisomerization of stilbene-like organic species can be carried out upon their coordination to the Re1 centre to form the fac-[Re(CO)3(NN)( rans-L)]+ complexes [26]. Another spectacular example of photoisomerization is generation of several different isomers by photochemical rearrangement of norbornadiene in the presence of different metal catalysts, such as Cu1 complexes or carbonyls of Fe, Ni, and Cr [27, 28]. 

Geometrical Photoisomerizations. in this section geometrical photoisomerization reactions of OTM compounds are considered. Recent accounts of photoinduced rearrangements of coordinated ligands and OTM complex photoassisted isomerization of olefins, dienes, and so on are available elsewhere (3,4,91). 

Photoisomerization of five-coordinate compounds has also been observed in low-temperature media. Photolysis of Mo(CO)5 (PCX3), (Cx = cyclohexyl) in a hydrocarbon glass at 77°K produces two isomers of Mo(CO) (PCx3) (145) ... 

The photoisomerization of the five-coordinate species probably occurs via decay of 03, (trigonal bipyramld) excited states (57,130). 

Ohmine, I., Morokuma, K., Photoisomerization of Polyenes Reaction Coordinate and Trajectory in Triplet Mechanism, J. Chem. Phys. 1981, 74, 564 569. 

Scheme 7 Coordination synchronized trans-cis photoisomerization of dmbipy-azo driven by Cu(II)/Cu(I) redox change...

Due to interligand 7i-stacking effect, the coordination structure of [Cu(trans2-o-AB)2] was found to be much more stable than that of [Cu(c/i2-o-AB)2]. The destabilization of the cis isomer was used to drive a ligand exchange reaction between c 2-< -AB and 2,2 -bipy, and concomitantly to shift the Cu VCu redox potential by ca. 0.6 V. The combination of the trans/cis photoisomerization of azobenzene with the ligand exchange reaction was nicely apphed to construct an... 

Harkins SB, Peters JC (2006) Unexpected photoisomerization of a pincer-type amido ligand leads to facial coordination at Pt(IV). Inoig Chem 45 4316-4318... 

At photoisomerization of (trans-2d)Mg at Co= 10 - lO M, as in the case with (trans-2c)Mg " an intramolecular cis-isomer complex with short-wave absorption spectrum is formed. The constant of complexation of cis-2d with Mg ", also estimated by the competitive reaction metht, is eight times greater for cis-2c isomer. It is probably attributed to the fact that the larger the cycle the easier it satisfies the attempts of cis-2d to form an intramolecular coordination bond between 803 group and the metal cation. 

Basing on these data we assume that in the course of (trans-2c)Mg photoisomerization in the diluted solution an "open" form of (cis-2c)Mg is produced in the form of dimers. Such a situation is possible solely when the (trans-2c)Mg complex at Cq >5x10 M is at least partially dimerized, and in the course of photoisomerization of dimers the intermolecular coordination bond is retained. 

Kawashima and co workers have shown that it is possible to switch the Lewis acidity of catecholboronate (170) by photoisomerization of the 2-(phenylazo)phenyl group [101]. In the trans form, (170)-E presents an N-B dative bond whereas in the cis form (170).Z, it lacks the coordination capability, thus providing a more Lewis acidic species (Figure 5.12). 

I. Ohraine and K. Morokuma, Photoisomerization of polyenes reaction coordinate and trajectory in triplet mechanism, J. Chem. Phys. 74 564 (1981). 

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