Phthalocyanines metallophthalocyanines

Phthalocyanines(metallophthalocyanines) Oxygen, iodine, bromine (Laurs and Hedand 1987) nitrogen dioxide (Hu et al. 2000 Bouvet 2006) ozone (Bouvet et aL 2(X)la, b Bouvet 2006) alcohols, ketones, thiols, nitriles, esters, ring compounds (Crone et al. 2001)... 

Keywords Spectroelectrochemistry Spectroelectrochemistry of phthalocyanines Phthalocyanines Metal-free phthalocyanines Metallophthalocyanines (MPcs-RlAM) bearing redox Inactive metal centers Metallophthalocyanines (MPcs-RAM) bearing redox active metal centers Electrochemistry and spectroelectrochemistry of CoPcs Electrochemistry and spectroelectrochemistry of X-Mn(lll)Pcs Electrochemistry and spectroelectrochemistry of EePcs Electrochemistry and spectroelectrochemistry of TiOPcs Sandwich metallophthalocyanines (MPc2)... 

Electrical polymorphism is widespread in phthalocyanines. Metallophthalocyanines generally crystallize in an inclined stacked arrangement called the a- or jff-... 

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... 

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... 

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). 

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... 

Metal phthalocyanines are extremely important materials in a variety of fields ont-side their traditional nse as dyes and pigments (Chapter 2, sections 2.3.1.6 and 2.4.1.4), inclnding electrochromism. The properties of metallophthalocyanines make them attractive as potential electrochromic materials, e.g. high light stability, high molar absorption coefficients, stable and sublimable at high temperatures, and the possibility of multi-colours within one structure. 

The metallophthalocyanines which have found application as elecfiochromes are mainly the rare earth derivatives, especially lutetium, and second row fiansition metals such as zirconium and molybdenum. Synthesis of these molecules follows the fiaditional routes, e.g. condensation of 1,2-dicyanobenzene with a metal acetate in a high boiling solvent (see Chapter 2). These compounds have structures in which the rare earth element is sandwiched between two phthalocyanine rings, e.g. zirconium bisphthalocyanine (1.92 M = Zr) and lutetium bisphthalocyanine (192 M = Lu), the latter protonated on one of the meso N atoms to balance the charge. 

Cyclotetramerization of phthalonitrile, or its analogues, is the traditional and facile method of phthalocyanine preparation.196-199 In the presence of an appropriate metal template, a variety of metallophthalocyanines have been synthesized (Scheme 58). 

Of the metallophthalocyanines that crystallize in forms other than the a-, j3-, and y-polymorphs, the most notable are Pb(pc)2031 and Ga(pc)F1301. The Pb(pc) molecules are stacked metal-over-metal, as shown in Fig. 28, and the Pb-Pb spacing is 3.73 A. The phthalocyanine ring deviates markedly from planarity, although the separate isoindole moieties retain their planarity. In Ga(pc)F the Ga atoms are symmetrically bridged by F, with a Ga-F distance of 3.92 A the pc rings are eclipsed, rather than staggered1301. 

Phthalocyanine itself is best prepared3 by self-condensation of phthalimidine, which is available from the reaction of phthalonitrile with ammonia. However, in many cases, direct metalation of the macrocycle cannot be achieved. Instead, metalation by means of dilithium phthalocyanine or a template reaction, whereby the macrocycle is formed around the metal using phthalonitrile (or one of its derivatives), must be employed for the synthesis of metallophthalocyanins. 

Dilithium phthalocyanine is obtained as dark-blue crystals. The compound has high thermal stability, as is typical of many phthalocyanines. It is soluble in acetone, giving a deep-blue solution that deposits phthalocyanine when in contact with even trace amounts of water. The material is also soluble in ethanol and tetrahydrofuran, but it is insoluble in diethyl ether, hexane, or chloroform. Solutions of the dilithium complex in ethanol react rapidly and quantitatively with a variety of metal salts to give the metallophthalocyanines, which precipitate, in very pure form, from solution. The electronic spectrum contains the bands (acetone solution) 370 (e = 24,800), 596 (e = 17,300), 630 (e = 16,100), 655 nm (e = 11,100). 

To aid in locating the vanadyl ionization, the spectra of the related phthalocyanine complexes were also examined. Gas-phase spectra of Mg(pc) and other metallophthalocyanines have been reported. The ionization of Mg(pc) that corresponds to band 2 of Mg(oep) is stabilized because of the extra nitrogen atoms of phthalocyanine, which opens a window of ionization energy to allow observation of other ionizations. The spectrum of VO(pc) is similar to that ofMg(pc), with band 1 located at 6.49 eV. Most significantly, the VO(pc) spectrum contains a broad ionization at 7.59 eV... 

Phthalocyanines could be prepared in high yields by reacting phthalo-nitriles with alcohols in the presence of DBU (80CL1277 83JAP(K)23854 83JAP(K)105962). If a metal salt was also present in the reaction mixture, metallophthalocyanines were obtained (83CL313 83JAP(K) 127763). 

Radiative and Nonradiative Decay Processes - Due to the potential application of these compounds as photosensitizers for photodynamic therapy" the photophysical properties of porphyrins and phthalocyanines, and their corresponding metal complexes, have been investigated extensively over the past decade. The photophysical properties of water-soluble metalloporphyrins, and especially the tetraphenylsulfonates," have been re-examined but nothing new has been found. The disulfonated metallophthalocyanines (MPcS2, where M = Al ", Ga" , or Zn") form complexes with fluoride ions for which the fluorescence yields and lifetimes are decreased with respect to the parent dyes while there are... 

Figure 2. UV-vis spectra of metallophthalocyanines (dotted line) and metal-free phthalocyanines (contin-...

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