Metallophthalocyanine

Oxidation of pollutants catalyzed by metallophthalocyanins 97ACR470. Recent developments in the use of A-phthaloyl-amino acid derivatives in synthesis 98SL457. 

The prevalence of the heme in O2 metabolism and the discovery in the 1960s that metallophthalocyanines adsorbed on graphite catalyze four-electron reduction of O2 have prompted intense interest in metaUoporphyrins as molecular electrocatalysts for the ORR. The technological motivation behind this work is the desire for a Pt-ffee cathodic catalyst for low temperature fuel cells. To date, three types of metaUoporphyrins have attracted most attention (i) simple porphyrins that are accessible within one or two steps and are typically available commercially (ii) cofacial porphyrins in which two porphyrin macrocycles are confined in an approximately stacked (face-to-face) geometry and (iii) biomimetic catalysts, which are highly elaborate porphyrins designed to reproduce the stereoelectronic properties of the 02-reducing site of cytochrome oxidase. 

Kobayashi N, Lam H, Nevin WA, Janda P, Leznoff CC, Lever ABP. 1990. Electrochemistry and spectroelectrochemistry of 1,8-naphthalene- and 1,8-anthracene-hnked cofacial binuclear metallophthalocyanines. New mixed-valence metaUophthalocyanines. Inorg Chem 29 3415. 

The first catalysts reported for the electroreduction of C02 were metallophthalocyanines (M-Pc).126 In aqueous solutions of tetraalkylammonium salts, current-potential curves at a cobalt phthalocyanine (Co-Pc)-coated graphite electrode showed a reduction current peak whose height was proportional to the C02 concentration and to the square root of the potential sweep rate at a given C02 concentration. On electrolysis, oxalic acid and glycolic acid were detected, but formic acid was not. Mn and Pd phthalocyanines were inactive, while Cu and Fe phthalocyanines were slightly active. At the potentials used for C02 reduction, M-Pc catalysts would be in their dinegative state, and the occupied dz2 orbital of the metal ion in the metallophthalocyanine was suggested to play an important role in the catalytic activity. 

Several metallophthalocyanines have been reported to be active toward the electroreduction of C02 in aqueous electrolyte especially when immobilized on an electrode surface.125-127 CoPc and, to a lesser extent, NiPc appear to be the most active phthalocyanine complexes in this respect. Several techniques have been used for their immobilization.128,129 In a typical experiment, controlled potential electrolysis conducted with such modified electrodes at —1.0 vs. SCE (pH 5) leads to CO as the major reduction product (rj = 60%) besides H2, although another study indicates that HCOO is mainly obtained.129 It has been more recently shown that the reduction selectivity is improved when the CoPc is incorporated in a polyvinyl pyridine membrane (ratio of CO to H2 around 6 at pH 5). This was ascribed to the nature of the membrane which is coordinative and weakly basic. The microenvironment around CoPc provided by partially protonated pyridine species was suggested to be important.130,131 The mechanism of C02 reduction on CoPc is thought to involve the initial formation of a hydride derivative followed by its reduction associated with the insertion of C02.128... 

Interesting results have been obtained using metallophthalocyanines supported on porous carbon gas diffusion electrodes.132-136 In the case of CoPc and NiPc, CO is formed with a current efficiency of almost 100%.135 With Sn, Pb, and In phthalocyanines, mainly HC02H is formed, while Cu and Ti phthalocyanines promote the formation of CH4. The reason why some metal Pc complexes give CO or CH4, while others yield HC02H, has been interpreted in terms of the electron configuration in the metal.137 A rather different type of reaction is the very recent demonstration of the simultaneous reduction of C02 and N02 to give urea (NH2)2CO, which can be achieved with an efficiency up to 40% at similar gas-diffusion electrode devices with a NiPc supported catalyst.138... 

The electrochromism of the phthalocyanine ring-based redox processes of vacuum-sublimed thin films of [Lu(Pc)2] was first reported in 1970,32 and since that time this complex has received most attention, although many other (mainly lanthanide) metallophthalocyanines have been investigated for their electrochromic properties.1 Lu(Pc)2 has been studied extensively by Collins and Schiffrin33,34 and by... 

On account of their particularly extensive delocalized 7r-systems, MPs have received comparatively more attention for their cubic, as opposed to quadratic, NLO properties. From the viewpoint of practical applications, such complexes (and also metallophthalocyanines and other closely related compounds) are of major interest for OL due to their tendency to exhibit RSA behavior. These materials are particularly well suited in this regard because they often exhibit strongly absorbing, long-lived triplet excited states as well as reasonably wide transparency windows over the visible region of interest between the intense B- and Q-bands. 

The NLO properties of metallophthalocyanines (MPcs) and metallo-2,3-naphthalocyanines (MNPcs) have been extensively studied, primarily due to their OL potential. This area has been reviewed,197-200 and although citations are near-comprehensive, discussion here will be limited to selected highlights and more recent developments. 

At the outset it is to be observed that these metal complexes, although they need to be circumspectly handled, are user-friendly substances, and are usually readily obtained. The preparation of metalloporphyrins and metallophthalocyanines is accomplished in three ways, as described below. 

This reaction has been observed for porphyrins, and used to establish relative stabilities.51,53 It is of practical use in the preparation of metal complexes, e.g., metallophthalocyanines by metal exchange with dilithium(I) phthalocyanine.54... 

The typical metalloporphyrins and metallophthalocyanines have in general very high thermodynamic stabilities. However, because it is difficult to establish conditions under which the... 

Figure 6 An outline of synthetic routes to metallophthalocyanines. The syntheses are often carried out in a melt, or in the presence of a high boiling solvent, such as 1,3,4-trichlorobenzene, or under other conditions eg LiOR, Me2NCH2CH2OH see later examples (Figures 8, 10, 12, 13).

In general, for a particular metal state, metallophthalocyanines are more difficult to demetallate than are metalloporphyrins (the central hole is marginally smaller in the phthalocyanine ligand). Thus, copper(II) phthalocyanine, dissolved in concentrated sulfuric acid, is precipitated unchanged when the solution is poured into ice/water. However, metallochlorins are demetallated more readily than the corresponding metalloporphyrins (basicity chlorin < porphyrin). 

Table 6 Rates of intersystem crossing, kisc, for some metallophthalocyanines in 1-chloronaphthalene at 298 K.

The most studied of the water-soluble metallophthalocyanines (Zn, Al, Ga) are the sulfonic acids. These are prepared by cyclotetramerization of 4-sulfophthalic acid or its derivatives (Figure 6),246,247 or by the direct sulfonation (oleum) of the metallophthalocyanine.248 The latter method gives complex mixtures, since direct sulfonation occurs at both exo and endo positions. These mixtures may be separated by chromatographic methods (usually reverse-phase HPLC). Thus... 

The preparation of metallophthalocyanines under solvent-free conditions from 1,2-phthalonitrile or phthalic anhydride and urea in the presence of metal templates... 

Since the electrochemical behaviour of metallophthalocyanines has been reviewed,103 we will simply demonstrate the ligand based nature of their redox processes. As a typical example, Figure 57 shows the redox aptitude of zinc-phthalocyanine [Zn(Pc)] CH2C12 in solution,96 together with its molecular structure.104... 

See also Metallophthalocyanines absorption spectra during electrochemical oxidation, 321-325 absorption spectra during photolysis, 321-325 Iron porphine... 

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