Porphyrins, literature review

This volume of Progress in Heterocyclic Chemistry is the tenth annual review of the literature, eovering the work published on most of the important heterocyclic ring systems during 1997. In addition there are two specialised reviews. The synthesis of chlorins and related reduced porphyrins is reviewed by Franz-Peter Montforts and Martina Glasenapp-Breiling in Chapter 1. Steven Collier and Richard Storr have presented an overview of the generation and reactions of heterocyclic o-quinodimethanes in Chapter 2. These intermediates are transient and reactive dienes that are useful for the synthesis of a wide variety of fused heterocycles. 

These results agree, in the literature review performed, with the studies published by numerous authors and refered in the Doctoral Thesis of Gonzalez-Lopez (1992) and others (Sakai, 2000 Gurer-Orhan et al., 2004). All of them showed the influence of Pb on the heme biosynthesis pathway and the effects produced in some of the enzymes which take part in the biosynthesis of porphyrins, phenomena demonstrated in clinical research, in vitro studies and in experiments on animals. 

This review essentially comprises a survey of the developments in the organometallic chemistry of porphyrin complexes over the last decade, continuing on from the three reviews published during 1986-1988. Literature since the mid-1980s has been surveyed, and work reported prior to this will be touched on primarily to put the more recent developments into context and will not be described in depth. A new multivolume set encompassing the entire range of porphyrin chemistry has been recently published, and this contains a chapter on organometallic porphyrin chemistry. ... 

Over the past 20 years, the principal biomedical application for (free or metal-substituted) porphyrins has been Photodynamic Therapy (PDT), with extensive literature in the area including a number of comprehensive reviews (114-116). Porphyrins offer scope for optical imaging as they are potent fluorophores in the red region of the electromagnetic spectrum. Although, to date, the emphasis has been on their therapeutic effects, due to their... 

Table 2. Specification of the porphyrin part (P) of the general metalloporphyrin formula M(P)LX [4] and other abbreviations used (abbreviations in accord with the recent porphyrin review literature (21, 22) R, M, L, X specified in the text)...

In short, this volume contains a variety of chapters concerned with the fate of porphyrin molecules that have absorbed a photon. Most of the chapters examine changes that occur after this process. In some cases the change is ligand loss, in other cases nuclear motion, and yet other cases electron transfer. These processes also occur in biological systems and have important practical application. Because this book collects works from various groups, we hope that it will find a useful place in many research libraries and laboratories and that it will fill a gap in the current review literature. 

When molecules absorb a photon and produce an electronic excited state, the energy can be dissipated in several ways luminescence, radiationless decay to the ground state, and photochemistry. Luminescence dominated the older literature because it was easy to observe. A good review of luminescence is in Volume 3 of David Dolphin s seven-volume series The Porphyrins. Picosecond laser spectroscopy allowed for exploration of the radiationless decay pathways, particularly the initial steps that compete with luminescence and lead to photochemistry. Two principal forms of radiationless decay lead to long-term metastables ligand ejection and electron transfer. 

Spin equilibria are an important feature of the chemistry of porphyrins and heme proteins. The extensive literature on this subject cannot be reviewed here. Only some relevant implications from the studies of spin equilibria in metal complexes will be described. 

As already mentioned, the coordination type of the Ru, Os, Rh and Ir porphyrins is C (Fig. 2), i.e. distorted octahedral about the metal which is in most cases a d6 system occasionally, d5, d4, or d2 systems are encountered. Table 2 gives a compilation of compounds the molecular structures of which have been determined by X-ray crystallography. The numbers of entries given for the individual noble metals approximately reflects the intensity of research done for the respective metal. For details and special structural aspects, the reader is referred to the original literature and a previous review by Scheidt and Lee [143]. Here, just a few general notes will be made. Phthalocyanine systems are not incorporated. 

A thorough electrochemical characterization of new metalloporphyrins is nowadays state of the art for the synthetic inorganic chemist. In many of the papers cited in Sects. 3 and 4, a characterization of the new complexes by cyclic voltametry and electrolysis at controlled potential has been done. Thin-layer spectroelectrochemistry is very fruitful [346]. Fortunately, apart from classical articles of Davis et al. [347], Felton et al. [292], Fuhrhop et al. [293], Buchler et al. [190], more recent reviews of Kadish et al. are available which systematically cover the field of general metalloporphyrins [294] or organometallic porphyrin complexes [306]. Therefore, a short, update of these articles will be given in the form of Table 7. For details, the reader is referred to the original literature. 

In view of the extensive literature on the development of synthetic methodologies in porphyrin chemistry as well as applications in a variety of areas, a number of comprehensive review articles have been published (10ACR300, 10CCR77, 01CR2751) and their discussion is beyond our scope. However, important aspects of structural modifications, reorganization, and expansion of a porphyrin scaffold (Figure 1), as outlined below (a-e), are discussed to highlight developments. 

The vast majority of the dyad models for photosynthetic electron transfer have consisted of synthetic porphyrins covalently linked to quinones. The first such models were reported in the late 1970 s. Kong and Loach prepared the ester-linked dyad 2 in 1978 [38], and the amide 3 was reported by Tabushi and coworkers in 1979 [39]. A large number of these P-Q systems have now appeared in the literature. The reader is referred to several reviews [13, 34, 40], including the recent review by Connolly and Bolton [41] for a complete compilation of these results. 

This article is not intended to present an exhaustive review of the extensive literature on haemoprotein spectra, which dates back almost 90 years. Our aim is to discuss the origin of the absorption bands of ferric haems and haemoproteins in the light of the most recent physico-chemical and theoretical studies. We consider that the spectra of ferrous haems and haemoproteins are reasonably well understood in terms of the theory of porphyrin spectra those of the ferric compounds are much more complex, and the number of distinct assignments and interpretations which have been offered in the literature in the last twelve years is approaching double figures. A critical summary of the relevant experimental facts and theoretical interpretations seems timely. 

Much of the older work (including some useful practical procedures) concerned with natural iron porphyrins is contained in Falk s classic book Porphyrins and Metalloporphyrins , which was published in 1964.1013 Subsequently an enlarged multiauthor edition appeared1014 that covered the literature to the end of 1974. Since then a number of reviews concerned with iron porphyrins have been published.1015"1022 The most important recent books are the seven volumes entitled The Porphyrins edited by Dolphin,1013 and Iron Porphyrins edited by Lever and Gray,1016 while Berezin s book1017 is a useful guide to some of the less familiar Russian literature. 

As noted in Section 3.1.2, the introduction of the crown ethers in the late 1960s gave legitimacy to the concept of stable coordination complexes of the alkali metals. Their presence, and that of many other macrocyclic counterparts (e.g., porphyrins) and three-dimensional chelators (e.g., cryptands, calixar-enes) is now pervasive in both alkali and alkaline-earth coordination chemistry, and the literature on these complexes is vast. Early work in this area was summarized in CCC (1987), and examined in a more focused manner in Comprehensive Supramolecular Chemistry. It is not the intent of this section to repeat such material, but rather to highlight new developments since the mid-1990s. In some cases, specialist reviews are available on these subjects they will be noted where relevant. 

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