Photochemical Ligand Substitution Reactions

The possibility of the practical application of the catalytic photode-composition of water based on the reactivity of the excited states of tris(2,2 -bipyridine) complexes of ruthenium(III) and ruthenium(II) has attracted considerable interest, but it is now clear that the efficiency of this process is limited not only by the lack of efficient catalysts, particularly for the dioxygen-evolving path, but also by both thermal and photochemical ligand oxidation 1,2) and ligand substitution reactions (3) of the 2,2 -bipyridine complexes. The stoichiometrically analogous tris(2,2 -bipyridine) and tris(l,10-phenanthroline) complexes of both... 

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. 

Re—Re, Re—M, and Re—C Bond Homolysis Reactions Photochemistry of Re(I) Diimine Tetracarbonyl Complexes Photochemical Ligand Substitution Reaction of/ac-[Re(Diimine)... 

D. Photochemical Ligand Substitution Reaction of fac- [Re(DiiMiNE)(CO)3PR3]... 

Most rhenimn diimine tricarbonyl complexes were considered to be relatively stable against photosubstitution, with some exceptions described above, until the photochemical ligand substitution reactions of complexes with phosphorous ligands, phosphorous complexes, and/ac-[Re(LL)(CO)3(PR3)] were reported. 

The energy of the MLCT excited state (F)oo( MLCT)) can be evaluated from the emission spectrum. Emission peak wavelength (le), emission quantum yields (0e), emission lifetimes (le), and reaction quantum yields of the photochemical ligand substitution reactions ( r) are summarized in Table III. The modification of the bipyridine ligand caused changes in. Boo( MLCT) as large as 2400 cm. ... 

The energy differences between the lowest excited MLCT states and the transition states of the photochemical ligand substitution reactions could be estimated from analysis of the temperature effects (Table IV). The evaluated AG values were found to vary with the substituents on the bpy ligand (3650-4820 cm ). 

Fig. 11. Temperature dependence of the emission yield the lifetime (te), and the quantum yield of the photochemical ligand substitution reaction (cPr) of 3a in a degassed CH3CN solution. Copyright 2002 American Chemical Society.

Interestingly, this effect was inversely proportional to the energy of the MLCT state (Eoo in Tables III and IV). Therefore, the energy gap between the ground state and the transition state of the photochemical ligand substitution reaction ( loo + AG ) is not affected by the modification of the bip5uidine ligand as much... 

Thermodynamic Data foe the Photochemical Ligand Substitution Reactions of [Re(X2bpy)(CO)3(PR3)]+ (3) in CH3CN. 

Scheme 5. Photochemical ligand substitution reactions for introducing replaceable solvent molecules (CH3CN).

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 ligand substitution reaction of la was investigated by ultrafast TR-IR spectroscopy (Fig. 16) 51). An acetonitrile solution of la was irradiated by a 266-nm laser pulse ( 150 fs pulse width). A broad IR absorption band which was attributed to the reaction products in higher vibrational excited states was produced within 1 ps after the laser flash. The broad band sharpened and a vqo peak at 1828 cm of the reaction product was observed in the 50- to 100-ps duration. This time scale is much shorter than the decay of the lowest MLCT excited state (right-hand side of Fig. 16). The TR-IR results indicate that this photochemical reaction proceeds from higher vibrational states or high-energy electronic excited states instead of the lower vibrational excited states of MLCT and thermal accessible states from MLCT such as the LF state. 

Although mononuclear metal carbonyls are purportedly less effective as catalysts for this process when compared with metal carbonyl clusters (14,15), investigations of these systems will provide for a better understanding of the fundamental steps in the homogeneous metal-catalyzed water gas shift reaction. Therefore, the primary objective of this work was to examine (i) the reversible nature of the reaction of hydroxide ion with Cr(CO)e, along with the concomitant formation of /A-H[Cr-(CO)5]2" and CO2 and (ii) the ligand substitution reactions of /x-H[Cr-(CO)5]2" with CO, both thermally and photochemically (Scheme 1). 

The photochemical stability of [Ru(bipy)3] " towards ligand substitution reactions has been the mainstay of its use as a photosensitizer. More recently however photosubstitution reactions have been reported. The EIP model has been modified by Houten and Watts to take into account high temperature substitution reactions by introducing a higher set of non-luminescent levels. Substitution photochemistry of [Ru(bipy)3] by Br is quenched by ferrocene three times as strongly as luminescence suggesting that the photoreactive LF state is not in thermal equilibrium with the luminescent excited state. Photolysis of [Ru(bipy)3] with X yields [RuXj-(bipy) ] (X-- = Cr, Br-, 1 , NCS, "NCO, NCSe, NOj , Nj , " mal-... 

The photochemical ligand substitution reactions of CpMn(CO)3 have been well studied and are synthetically very useful (IQ, JJ). Upon irradiation with li t of wavelength less than ca, 400 nm, CpMn(CO)3 readily dissociates one CO ligand (Scheme 1). 

The most common application of photochemistry in the syntheses of organometallic complexes is in ligand substitution reactions where an initial photochemical step is used to create a vacant coordination site (Equation (18)). Many of these reactions will also occur thermally, but the advantages of photochemistry are that milder conditions can often be used than in the corresponding thermal reactions, and the photochemical reactions are generally more selective than their thermal counterparts. 

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