Metal macrocyclics metalloporphyrins

Vibrational spectra of metalloporphyrins have been studied extensively because of their biological importance as prosthetic groups of a variety of heme proteins. Thus, many review articles have been published on this subject, and most of them discuss the vibrational spectra of metalloporphyrins together with those of heme proteins. Nakamoto has reviewed several articles concerning these macrocycles and discussed the state of the art of the normal coordinate analysis, the assignment of the structure-sensitive vibrations, the metal-macrocycle ring coordination, the metal-metal bonded porphyrins and the n-n complex formation and dimerization. ... 

The first-principles DFT calculations of ORR on various M-N4 macrocyclic complexes have been carried out by several research groups [93, 168-173]. Through comparative study of O2 dissociation on different metalloporphyrins (MnP, FeP, CoP, NiP), the trends of the activation barriers for the O2 dissociation with respect to LUMO-HOMO characters of these metalloporphyrins have been discussed by Tsuda et al. [172]. FeP is demonstrated to be the best one due to the large d electrons contribution to the LUMO-HOMO level of the FeP and the stable Fe-O bond [172]. Shi and Zhang performed the DFT calculation on the O2 adsorption on various iron and cobalt porphyrins and phthalocyanines [171]. The catalytic activities of the transition metal macrocyclic complexes were positively related with... 

The simple porphyrin category includes macrocycles that are accessible synthetically in one or few steps and are often available commercially. In such metallopor-phyrins, one or both axial coordinahon sites of the metal are occupied by ligands whose identity is often unknown and cannot be controlled, which complicates mechanistic interpretation of the electrocatalytic results. Metal complexes of simple porphyrins and porphyrinoids (phthalocyanines, corroles, etc.) have been studied extensively as electrocatalysts for the ORR since the inihal report by Jasinsky on catalysis of O2 reduction in 25% KOH by Co phthalocyanine [Jasinsky, 1964]. Complexes of all hrst-row transition metals and many from the second and third rows have been examined for ORR catalysis. Of aU simple metalloporphyrins, Ir(OEP) (OEP = octaethylporphyrin Fig. 18.9) appears to be the best catalyst, but it has been little studied and its catalytic behavior appears to be quite distinct from that other metaUoporphyrins [CoUman et al., 1994]. Among the first-row transition metals, Fe and Co porphyrins appear to be most active, followed by Mn [Deronzier and Moutet, 2003] and Cr. Because of the importance of hemes in aerobic metabolism, the mechanism of ORR catalysis by Fe porphyrins is probably understood best among all metalloporphyrin catalysts. 

Pharmaceutical discoveries are principally made by thoughtful structural variation on a lead compound which has been found, by chance or design, to have a certain amount of the desired activity. It is clear that with metalloporphyrins, there are additional structural variations to be had. In the first place, it is possible to vary the metal. It was realized at an early stage that inserting and varying the metal would modify PDT activity.61 The possibility also exists of structural changes in axial ligands in those metalloporphyrins which possess them. This structural variation occurs in the space immediately above and below the macrocycle, which is a space not readily accessible to controlled variation in metal-free compounds of this series. 

The synthesis of metalloporphyrins which contain a metal-carbon a-bond can be accomplished by a number of different methods(l,2). One common synthetic method involves reaction of a Grignardreagent or alkyl(aryl) lithium with (P)MX or (PMX)2 where P is the dianion of a porphyrin macrocycle and X is a halide or pseudohalide. Another common synthetic technique involves reaction of a chemically or electrochemically generated low valent metalloporphyrin with an alkyl or aryl halide. This latter technique is similar to methods described in this paper for electrosynthesis of cobalt and rhodium a-bonded complexes. However, the prevailing mechanisms and the chemical reactions... 

As is known, the triplet and singlet tetrapyrrolic pigment states are caused by excitation of delocalized ir electrons of the heteroaromatic macrocycle (1 ). The most favourable structure of dimeric complexes for metal 1oporphyrin TTA seems to be a sandwich dimer as in the case when molecular orientation of plane to plane electronic shells reaches the maximum overlap. Quantum chemistry calculations of metalloporphyrin dimers indicates that for the dimeric emission process the 2 state is split into states of higher energy (SJ) and lower energy ( 2). In the case of a sandwich dimer of 04 symmetry, the following states are seen ... 

Coordinated transition metal redox-active macrocycles, 39 108-124 ammonium cation, 39 128-133 crown ether and bis crown ether ligands containing bipyridyl transition metal recognition sites, 39 111 crown ether dithiocarbamate and dithiolene complexes, 39 123-124 metalloporphyrin crown ether compounds, 39 108-109... 

Dendritic derivatives of these macrocycles can be placed in the wider context of studies on metalloporphyrins with sterically hindered faces which have been designed in attempts to mimic the properties of heme proteins and chlorophylls, and there are suggestions that steric isolation of the metalloporphyrin nucleus is important in certain biological functions, The redox properties of metalloporphyrins are well-documented they are dominated by two, reversible one-electron transfers involving both the metal and the ligand. The first dendritic porphyrins of general structure 47 and their Zn complexes were reported by Inoue et al. who... 

The ligand field of the macrocycle is also modified by ring distortion.154 The metal to alkylated nitrogen distance is 2.33 A (Fe) to 2.53 A (Zn) which is considerably longer than the normal value of 2.0-2.1 A for the first row transition metalloporphyrins in a high spin state, while the extension is small for the other three M—Npor bonds. The reduced and distorted ligand field of the macrocycle makes it possible for even the Co complexes as well as other metals to take a high spin state. 

The geometry of metalloporphyrins and other tetrapyrroles have been studied in detail by molecular mechanics. The effect of (i) the size of the metal ion, (ii) axial ligation by planar ligands, such as imidazoles, (iii) the phenyl group orientation in tet-raphenyl porphinato complexes, and (iv) the flexibility of the porphyrin macrocycle,... 

As demonstrated in this chapter, the binding of metal ions to maclocyclic ligands (e.g., porphyrins) results in the change in both the thermodynamic and dynamic properties of ET reactions of metalloporphyrins. Excellent models of the photosynthetic reaction center were developed by the appropriate choice of combination of metal ions and macrocyclic ligands. The lifetimes of the CS states in models of photosynthetic reaction center composed of electron donors and acceptors also were controlled by binding of metal ions to radical anions of electron acceptor moieties in the electron donor-acceptor hnked molecules. The control of ET processes by coordination of metal ions to the dyads led us to develop a unique fluorescence sensor for the ion. The binding of metal ions to radical anions of electron acceptors results in acceleration of thermal ET reactions, which would otherwise be impossible to occur. Such effects of metal ions to enhance the ET... 

---