Complexation photoinduced dissociation

Photochemical Reactions of Metal Complexes. The major photoinduced reactions of metal complexes are dissociation, ligand exchange and reduc-tion/oxidation processes. The quantum yields of these reactions often depend on the wavelength of the irradiating light, since different excited states are populated. This is seldom the case with organic molecules in which reactions take place almost exclusively from the lowest states of each multiplicity Sj and Tj. 

Unlike (TPP)Mn(NO), the five-coordinate (TPP)Mn(ONO) complex undergoes linkage isomerization to (TPP)Mn(N02) following laser flash photolysis of a toluene solution of (TPP)Mn(ONO) (i66). The proposed mechanism involves the photoinduced dissociation of the axial ligand to (TPP)Mn and NO2. (TPP)Mn andN02 then recombine to form (TPP)Mn(N02). The latter complex readily isomeiizes back to the structurally characterized ground state (TPP)Mn(ONO) compound (Figure 32) (48). 

In a few cases alkyl complexes can be synthesized by the photoaddition of dichloromethane or chloroform to a low-valent transition metal complex, but the usual products from these halogen abstraction reactions are the dichloro complexes. Photoinduced oxidative addition reactions are not always the result of the generation of an excited state that is more reactive than the ground state. An alternative photochemical pathway can involve the initial dissociation of a ligand to generate a coordinately unsaturated intermediate, which is then activated toward oxidative addition of the substrate. An example of such a reaction is... 

Discussion of the mechanism of photosubstitution in Cr(III) complexes has largely concerned the possibility of a seven-coordinate intermediate. Such an associative type mechanism is not proven by any means, but there is some circumstantial data to support it and some data that tends to rule out a purely dissociative mechanism involving the photoinduced formation of a discrete five-coordinate intermediate capable of being competitively scavenged by nucleophiles in the medium. 

The complexation/decomplexation of a linear guest by a macrocyclic host can be exploited [ref. 7] (Fig. 10.2) to switch reversibly between two states. When the two components are dissociated, the system is in its State 0. When they are associated in the form of a [2]pseudorotaxane, the system is in its State 1. Switching between these two states can be achieved [ref. 7] by protonation/ deprotonation, by oxidation/reduction, or by photoinduced isomerization of the guest. Alternatively, the addition/removal of a sequestering agent able to... 

As part of the work on model heme FeNO complexes, mechanistic studies on the reversible binding of nitric oxide to metmyoglobin and water soluble Fe, Co and Fe porphyrin complexes in aqueous solution, ligand-promoted rapid NO or NO2 dissociation from Fe porphyrins, reductive nitrosylation of water-soluble iron porphyrins, activation of nitrite ions to carry out O-atom transfer by Fe porphyrins, demonstration of the role of scission of the proximal histidine-iron bond in the activation of soluble guanylyl cyclase through metalloporphyrin substitution studies, reactions of peroxynitrite with iron porphyrins, and the first observation of photoinduced nitrosyl linkage isomers of FeNO heme complexes have been reported. 

Neumann (777) showed that irradiation of the carbene complexes (CO)sW CPh2, (CO)5 V C(OCH3)Ph, and (CO)5Cr CO(CH2)2CH2 under CO atmosphere led to CO incorporation into the complexes, a result which implies photoinduced CO dissociation. They also irradiated these complexes under H2 atmosphere and observed slow, if any, hydrogenation of the carbene ligands in the tungsten complexes but more rapid hydrogenation for the Cr complex [Eq. (137)] (777). 

Casey and co-workers 178, 179) also studied the photochemistry of olefin-substituted carbene complexes and observed CO dissociation. For example, irradiation of a 1.38 1 mixture of the Z and E isomers of 145 led to CO substitution by the allyl-carbene group to give the isolable carbene-olefin complex 146 [Eq. (138)], a reaction which demonstrates CO dissociation. (Z)-145 has the proper stereochemistry to yield 146 upon photoinduced CO loss. However, the E isomer does not. Yet, the authors observed simultaneous disappearance of the E isomer upon irradiation of... 

By analogy with thermal processes (92), photoinduced isomerization s may occur either by intramolecular or intermolecular mechanisms. Intramolecular isomerizations are further classified either as "twisting" mechanisms, which involve no metal-ligand bond breaking, or as bond-rupture mechanisms. The latter term applies to reactions of chelate compounds in which one of the ligands is for a short time partially dissociated. Intermolecular mechanisms involve species other than those that make up the reactant complex. 

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