Ligand dissociation, photo

I propose to develop and apply such methods, based on ultrafast X-ray absorption spectroscopy, to study the ultrafast molecular motions of organometallics in solutions. In particular, initial studies will focus on photo-induced ligand dissociation and substitution reactions of transition metal carbonyls and related compounds in various solvent systems. 

Besides ruthenium porphyrins (vide supra), several other ruthenium complexes were used as catalysts for asymmetric epoxidation and showed unique features 114,115 though enantioselectivity is moderate, some reactions are stereospecific and treats-olefins are better substrates for the epoxidation than are m-olcfins (Scheme 20).115 Epoxidation of conjugated olefins with the Ru (salen) (37) as catalyst was also found to proceed stereospecifically, with high enantioselectivity under photo-irradiation, irrespective of the olefmic substitution pattern (Scheme 21).116-118 Complex (37) itself is coordinatively saturated and catalytically inactive, but photo-irradiation promotes the dissociation of the apical nitrosyl ligand and makes the complex catalytically active. The wide scope of this epoxidation has been attributed to the unique structure of (37). Its salen ligand adopts a deeply folded and distorted conformation that allows the approach of an olefin of any substitution pattern to the intermediary oxo-Ru species.118 2,6-Dichloropyridine IV-oxide (DCPO) and tetramethylpyrazine /V. V -dioxide68 (TMPO) are oxidants of choice for this epoxidation. 

The life-time of the photo chemically generated excited state should be shorter than the dissociation of the ligand-receptor complex, but long enough to spend sufficient time in a close proximity to a target site for covalent linkage. 

Dissociation of the carbonyl ligand was initially postulated as the role of the photon (Route A, Scheme 3) [4], Mechanistic investigations by Goldman et al. indicated, however, that C-H bond oxidative addition to the photo-excited sixteen-elec-... 

The photo substitution pathway probably involves intramolecular energy transfer from an f-f excited state to an intraligand state leading to the dissociation of ligand thd from the complex. Excitation of ligand absorption bands of Tb(thd)3 also leads to the same substitution reaction [111],... 

CO, NO and O2. It was shown that the most probable coordinate of photo-dissociation is the e coordinate of the ligand bending mode, which is a superposition of two symmetrized distortions corresponding to the rotation of the ligand and to its displacement parallel to the porphyrin ring. Different APES s are obtained for ligands in linear or bent coordination the different photolysis quantum yields (0l 1 for linear and 10 10 for... 

Transition metal catalysts can also be used in photochemical organic synthesis. For example, the photo-Bergman reaction (cycloaromatization see also Scheme 6.62) of (Z)-hex-3-en-l,5-diyne [(Z)-589], which is obtained by photostationary E Z isomerization (Section 6.1.1) from ( )-589, occurs via transition metal-catalysed cycloaromatization reaction followed by photochemical dissociation of the arene ligand from the complex 590 (Scheme 6.289).1530 The product, 1,2-di( -propyl)benzene (591), is obtained in 91% chemical yield. 

The novel complexes of N-heterocyclic carbene ligands (NHC), /ac-[Re(CO)3(N C)X] (N C = l-phenyl-3-(2-pyridyl)imidazole or 1-quinoli-nyl-3-(2-pyridyl)imidazole X = Cl or Br), were found to undergo a solvent-dependent photochemical CO dissociation under excitation to their lowest excited state. The key step of the mechanism is the formation of a photo-chemically active tricarbonyl solvato-complex, following the dissociation of the CO in trans to the strongly cr-donatingNHC moiety. The photoinduced CO dissociation of c-[Re(CO)3(NHC)L] complexes, upon excitation at 370 nm, has been proven by the change in both IR and NMR spectra, as well as by the red shift in the emission profile after photolysis. Photochemical studies suggest that Re-C bond cleaves from a MLCT-type excited state. ... 

---