Stilbene complexes

The conversion of trans- to cis-stilbene by irradiation of either W(CO)6 or Mo(CO)6 is consistent with production of an excited metal carbonyl-stilbene complex which undergoes isomerization promptly.136, 137 The generation of the cis isomer represents movement away from the thermodynamic ratio of cis- and frans-stilbene which is 100% trans. Thus, an electronically excited state must be important in the reaction sequence. Two proposals have been made 137 (1) excitation of an M(CO) (stilbene)6 complex leads to a sigma bonded diradical intermediate as shown in reaction (49) or (2) electronic excitation may yield a perturbed IL excited state as for the styrylpyridine complexes discussed above. The sigma bonded olefin diradical... 

It is well known that alkynes may behave as variable electron donors to metal centers and efforts have been made to prepare (nonbridging) 4e-donor phosphaalkyne-containing complexes. Treatment of the trani -stilbene complex (5) with a stoichiometric amount of BuCP afforded the low-melting green solid (6). Further, treatment of this 4e-donor phosphaalkyne complex with another molar equivalent of BuCP afforded the 1,3-diphosphacyclobutadiene-containing complex (7) in good overall yield and showed the possibility... 

A different type of environmental effect is manifested in the photoisomerization of trans-stilbene complexed to cyclodextrin. In picosecond laser studies a single decay in a-cyclodextrin shows binding occurs at one site, whilst in the (3-forra... 

Silver also has been demonstrated to be reactive in solution systems. Thus, silver perchlorate has been shown to influence the photochemical reactivity of stilbene in acetonitrile and methanol. The fluorescence of the stilbene is quenched on addition of the perchlorate and this is good evidence for the enhancement of the So-Ti crossing induced by the heavy ion Ag+. It seems likely that an Ag+/stilbene complex is formed. The perturbation of the system is better in methanol than in acetonitrile. However, cis.trans isomerism of the stilbene is reduced within the excited Ag+/stilbene complex since it is difficult for the geometrical isomerism to occur. Enhanced isomerism is observed with the Ag+/azobenzene system. In this complex there are steiic problems encountered in the nitrogen rehybridization process that is operative in the isomerism . Enhanced So-T crossing is also seen in the Ag+/1 1 complex with tryptophan where the fluorescence is quenched and there is a threefold increase in phosphorescence . Complexes between Ag+ and polynucleotides and DNA cause quenching of the fluorescence. Enhancement of phosphorescence and a 20-fold increase in the dimerization of thymine moieties has also been observed when silver ions are added to the reaction system . ... 

Figure 2.13 Formation of chiral Pd(0) trans-stilbene complexes [104]. (Reproduced with permission.)...

Stilbene is an unusual arene as a ligand in that it offers the possibility of coordination to an arene ring or to the double bond. In spite of the abundance of rare earth naphthalene complexes in hterature (Bochkarev, 2002 Bochkarev et al., 1997 Fryzuk et al., 2000), ( )-stilbene complexes are rare. Evans et al. reported the synthesis of [(C5Me5)2Sm]2((E)-stilbene) from the direct reaction of (C5Me5)2Sm and ( )-stUbene (Chart 23 Evans et al., 1990). Due to the poor quality of the crystal, the structure of the molecule could not be unambiguously determined but the authors suggested an unsym-metrical coordination mode based on coimectivity. The analogous samarium styrene and butadiene complexes (styrene complex shown in Chart 23) showed two-electron reduction of the C=C bond and concomitant oxidation of Sm(ll) to Sm(lll) (Evans et al., 2001). Related yttrium and lutetium complexes of the readily available tetraphenylethylene dianion have been reported with similar stmctural features (Chart 23 Roitershtein et al., 1998, 2004). 

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