Ligand substitution, photo

In solution, at room temperature Ru(bpy) does not undergo a photo-induced ligand substitution over a wide pH range, whereas at 95°C complete lab Llization occurs in a few hours. In acidic solution Cr(bpy)3 is photo-inert. At room temperature at neutral or alkaline pH, a bidentate complex is formed rapidly... 

The photochemistry of lanthanide ions is limited to photoredox reactions. Photo substitution has not been studied because of the lability of metal ions, and since ligand substitution reactions are rapid even under thermal conditions. 

Mechanically interlocked molecnles containing ruthe-ninm(ll) complexes have also been considered as good candidates for the realization of photo-driven molecular machines since the rntheninm(ll) complexes can be easily involved in photo-indnced ligand substitution reactions to generate relative motion of mechanically interlocked molecules. ... 

Arakawa and coworkers [45] developed the on-line photoreaction cell depicted in Figure 5.3 and performed a series of studies on the detection of reaction intermediates in photosubstitution and photooxidation of Ru(II) complexes. The photosubstitution of Ru(bpy)2B [bpy = 2,2 -bipyridine B = 3,3 -dimethyl-2,2 -bipyridine (dmbpy) or 2-(aminomethyl)pyridine (ampy)] was studied in acetonitrile and pyridine. Irradiation of Ru(bpy)2B and related complexes yields a charge-transfer excited species with an oxidized Ru center and an electron localization on the bpy moiety. The excited-state complex underwent ligand substitution via a stepwise mechanism that includes ant] bidentate ligand (Scheme 5.6). Photoproducts such as Ru(bpy)2S2 (S = solvent molecule) and intermediates with a monodentate (mono-hapfo-coordination) B ligand, Ru(bpy)2BS, and Ru(bpy)2BSX+ (X=C10/, PF ) were detected. Other studies also identified photo-oxidized products of several mixed-valence Ru(II) complexes upon irradiation (7i> 420 nm) [31b, 46]. 

Early photochemical observations with coordination compounds typically amounted to the finding that the known thermal reaction, be it redox decomposition or ligand substitution, was accelerated by light. Such behavior was often referred to as "photocatalysis", as we did in Ref. 1. Later, it seemed better to use more specific terms such as "photochemistry", "photo-induced reaction", "photosubstitution", etc. [19]. There was a progression in the sophistication of photochemical studies. With exceptions, early reports simply noted that the compound changed on exposure to... 

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. 

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. 

Photo substitution reactions of Pr, Eu and Ho complexes with the ligand L = 2,2,6,6-tetramethyl-3,5-heptanedione (thd) of the formula Ln(thd)3 have been studied by irradiation into the f-f bands [110]. The corresponding thermal reactions are slow. The ligand thd is replaced by solvent thus... 

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],... 

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