Photochemical ligand substitution

Neutral (cyclobutadiene)Fe(CO)3 complexes undergo thermal and photochemical ligand substitution with phosphines, with alkenes such as dimethyl fumarate and dimethyl maleate and with the nitrosonium cation to generate the corresponding (cyclobutadiene)Fe(CO)2L complexes15. These types of complexes are presumably intermediates in the reaction of (cyclobutadiene)Fe(CO)3 complexes with perfluorinated alkenes and alkynes to generate the insertion products 266 or 267 respectively (Scheme 70)15,238. 

The parent TMM complex (190 R = H) undergoes photochemical ligand substitution with trifluorophosphine or trimethylamine Al-oxide assisted substitution with tertiary phosphines or t-butyl isocyanide (Scheme 5A) Trimethylamine A-oxide assisted substitution using isoprene as the incoming ligand results in C-C bond formation to afford the bis-TT-allyl complex (197). An intramolecular version of this reaction is also known.The parent complex (190 R = H) reacts with electrophiles. Addition of HCl or Br2 gives the methallyl complexes (192) and (198), respectively. Tetrafluoroethylene adds across the Fe bond to afford (199) under photochemical conditions. Complex (190) undergoes Friedel-Crafts-type acylation with... 

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)... 

The next section about photochemical reactions includes ligand substitution, homolysis, and reactions of the ligand on the rhenium complexes. This section also includes synthesis of emissive multinuclear rhenium(I) complexes using the photochemical ligand substitution. 

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).

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. 

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). 

Monstad, L. Monstad, G. Mechanism of Thermal and Photochemical Ligand Substitution Reactions of Chromium(III) and other Octahedral Metal Complexes, Coord. Chem. Revs. 1989,94,109-150. 

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