Photochemical substitution reactions transition metal complexes

There are several recent reviews of the photochemistry of transition metal complexes 3,4,5,29,30,45,466,514,518,580)> the most comprehensive being 4>. However, these papers deal mainly with ionic coordination compounds containing inorganic ligands only one 466> is devoted to photochemical substitution reactions of metal carbonyls and their derivatives, while in another 4> this subject is discussed in a short paragraph. 

In this chapter, the focus will be on the application of high pressure techniques in the study of the photochemical behavior of transition metal complexes (coordination, organometallic and bio-inorganic) in solution. We will present a systematic treatment of pressure effects on the nature of excited states (ES) and on the photophysical and photochemical processes that lead to ligand substitution, electron or energy transfer and thermal reactions of reactive intermediates generated by ES reactions. Selected examples will be presented in detail to illustrate how pressure effects can provide valuable mechanistic insight when combined with other quantitative studies. 

A wide variety of compounds are known which contain at least one transition metal-tin bond. These derivatives undergo different types of reactions, such as substitution of ligands at the tin or the metal center, photochemical reactions and so on. Selected tin derivatives of such transition metal complexes are shown in Table 11. 

Many reviews of this area are available and no attempt will be made to be comprehensive here. Instead the focus will be on the relationship between photochemistry and photophysics with emphasis on the use of directly measured kinetic data (lifetimes and quantum yields) to draw mechanistic interpretations. The photochemical discussion will be limited to substitution, especially solvation, reactions. The effect of solvent motion will be explicitly treated. Reactions of a few hexacoordinated transition metal complexes will be used to illustrate the important ideas. 

Previously reported work demonstrated that substituents can be used to tune the energies of excited states responsible for the emission spectra of certain group VIII metal complexes (1) and to modify significantly the absorption spectra of complexes displaying metal-to-ligand charge transfer (MLCT) bands (2). In this presentation, we summarize some recent attempts to use ligand substituents in our studies of transition metal complex photochemical reaction mechanisms. The particular subjects of interest are the metal ammine complexes M(NH3)5L where M is Rh(III) or Ru(II) and L is a meta- or para-substituted pyridine. 

Whereas for thermal substitution the mechanistic picture is clearer for cobalt(m) complexes than for chromium(m) complexes, for photochemical reactions the reverse is the case. Currently there is much interest, and some success, in bringing the level of understanding of the photochemistry of cobalt(m) and other octahedral complexes up to that of chromium(in) complexes. A recent review of photochemical reactions of transition-metal complexes concentrates on chromium(iii) complexes, but also contains some information on cobalt(ni) and platinum(n) complexes. 

Our initial studies focused on the transition metal-catalyzed [4+4] cycloaddition reactions of bis-dienes. These reactions are thermally forbidden, but occur photochemically in some specific, constrained systems. While the transition metal-catalyzed intermole-cular [4+4] cycloaddition of simple dienes is industrially important [7], this process generally does not work well with more complex substituted dienes and had not been explored intramolecularly. In the first studies on the intramolecular metal-catalyzed [4+4] cycloaddition, the reaction was found to proceed with high regio-, stereo-, and facial selectivity. The synthesis of (+)-asteriscanoHde (12) (Scheme 13.4a) [8] is illustrative of the utihty and step economy of this reaction. Recognition of the broader utiHty of adding dienes across rc-systems (not just across other dienes) led to further studies on the use of transition metal catalysts to facilitate otherwise difficult Diels-Alder reactions [9]. For example, the attempted thermal cycloaddition of diene-yne 15 leads only... 

Ferrocene, Fe(Ti5-C5H5)2, and related cyclopentadienyl complexes of transition metals in fact are far more thermally stable, less reactive substances than ionic cyclopentadienides, and have an extensive derivative chemistry that is typically aromatic in that their C-H bonds can undergo such electrophilic substitution reactions as Friedel-Crafts alkylation or acylation, nitration, and so on. Moreover, as a substituent, the ferrocenyl group (ri -f sl l5)Fc(ri -( 5l I4) (=R) is even more effective than a phenyl substituent in stabilizing carbenium ions [RCH2]+. The redox and photochemical properties of many metaUocenyl residues make them versatile substituents with many chemical and materials applications. ... 

---