Template reactions/synthesis phthalocyanines

One of the first examples of metal ions facilitating macrocycle formation was the synthesis of metal phthalocyanines (95) from 1,2-dicyanobenzene or 2-cyanobenzamide (Scheme 53).229-231 This example of macrocycle synthesis has been applied to a very wide range of metal salts of various valencies, and is of great commercial interest. As this area of chemistry has been reviewed frequently and is dealt with in Chapter 21.1, only a brief consideration of the most important template reactions will be presented here. 

The formation of the macrocyclic complex (98) is a template reaction reminiscent of phthalocyanine synthesis, and aza linkages are formed (Scheme 56).242 The resulting complex (98) can be transformed into a complex with a fully conjugated macrocyclic ligand. The dibromocobaIt(III) complex related to (98) can also be prepared and can function to some extent as a model for vitamin... 

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. 

Template reactions of 1,2-dicyanobenzenes with uranyl salts lead to the synthesis of super-phthalocyanines (Scheme 54).237,23s jjjg uranyl ion can be displaced from the macrocycle by a range of metal salts, but a ring contraction occurs to yield normal phthalocyanine complexes (Scheme... 

It should be noted that the basic reactions used to prepare phthalocyanine derivatives today are fundamentally those developed by Linstead and coworkers in the 1930s [52-54]. Due to the large number of substituted phthalocyanines described in the literature, space limitations mean that a detailed review of synthetic aspects cannot be provided here. The following discussion is concerned with the synthesis of lanthanide phthalocyanines via (i) template tetramerization of phthalonitrile with lanthanide salts, (ii) direct metalation of the metal-free ligands by the salts or (iii) metal exchange of a labile metal ion or ions for a lanthanide. 

The template methods have also been used for the synthesis of a number of substituted Ln di(naphthalocyanine) complexes, LnNc2 [82-88]. Apart from thermal fusion by conventional heating processes, complexation has been initiated by microwave radiation, although only a few publications are devoted to the template synthesis of lanthanide bis(phthalocyanine) complexes by this method [89, 90]. The use of microwave radiation (MW) reduces the reaction time from several hours to several minutes. Unsubstituted complexes LnPc2 (Ln = Tb, Dy, Lu) were prepared [90] by irradiation (650-700 W) of a mixture of phthalonitrile with an appropriate lanthanide salt for 6-10 min (yields >70%). 

Although direct reaction of lanthanide mono-porphyrins with free phthalo-cyanine or its lithium derivatives is generally more efficient than the template synthesis, and gives rise to mixed-ligand complexes, the template strategy can also be applied for synthesis of phthalocyanine-porphyrin complexes, as in the case of unsymmetric bisphthalocyanine complexes (Scheme 8.2, B(b)) [106, 136, 145, 146]. Thus, metallation of free porphyrins with lanthanide salts in TCB or n-octanol leads to single-decker complexes, which then react with phthalonitriles under the action of DBU in alcoholic media to give the desired compounds. 

In this context it is interesting to note that benzonitrile, Ph—C=N, trimerizes to a triazine on a Raney nickel surface. It was assumed that Jt-bonded nitriles were involved in the reaction mechanism.10 This reaction resembles the well-known template synthesis of phthalocyanine complexes from phthalodinitrile. Formation of linear polymers [—C(R)—N—] occurs on heating aryl or alkyl cyanides with metal halides.11... 

Several syntheses of tetrabenzoporphyrin complexes make use of similar template strategies to those used in phthalocyanine synthesis, so they will be discussed here. An isoindole has been found to undergo a very complex series of reactions promoted by a range of metal ions in refluxing 1,2,4-trichlorobenzene to produce tetrabenzoporphyrin complexes (Scheme 55).240 In contrast, an extremely simple synthesis of tetrabenzoporphyrin can be achieved from 2-acetylbenzoic acid, using zinc as templating metal (equation 44).241... 

Both porphyrins and phthalocyanines are prepared by template Schiff base type condensation rections. For example, the use of a large template is evident in the synthesis of the superphthalocyanine 3.83, in which five repeat units are organised about the pentagonal bipyramidal U022+ core, instead of four as in more traditional phthalocyanine complexes such as 3.82. Smaller templates result in the formation of the trimeric subphthalocyanine 3.84. The reversible nature of the condensation reaction means that both 3.83 and 3.84 can be converted into normal tetrameric phthalocyanine, 3.85, Scheme 3.23. 

The template synthesis represents an elegant method that uses metal ions to direct reactions of ligands and provides a useful route to macrocyclic structures. Several books159-161 describe the template processes that involve reactions on matrices used to synthesize polyazamacrocyles, crown ethers, cryptands, rotaxanes, knots,159 clathrochelates,160 phthalocyanines,161 etc. which are applied, e.g., as molecular switches, in ion exchange, electron transfer or catalysis. An example of clathrochelate synthesis is given in Chapter 1.33... 

Baeyer first reported a 4 4 cyclization product for the reaction of pyrrole and dry acetone catalyzed by the addition of one drop of hydrochloric acid (Baeyer, 1886). The macrocycle was later isolated in an 88% yield in a less violent reaction (Rothemund and Gage, 1955). Metal derivatives of the phthalocyanines were obtained when o-diaminobenzene, o-cyanobenzamide, or related compounds were pyrolized with metals or their salts (Braun and Tcherniac, 1907). This was possibly the first observed template synthesis of a macrocyclic ligand. 

Unsymmetrically substituted crown ether phthalocyanines H2L642 have been assembled from o-dicyanobenzo-15-crown-5 and L641 in the presence of Zn" and a strong organic base, or from the corresponding diirainoisoindoline derivatives on Pt as template in hexan-l-ol or pentan-l-ol and isolated as [M(L642)] (Eqs 2.250 and 2.251) [508]. Hexyl or pentyl ester derivatives are formed by the transesterification reaction that accompanies the process of template synthesis. The products have been purified by preparative TLC and identified by FAB mass spectrometry. Hydrolysis of metal(II) monoester phthalocyaninate leads to the acid derivative, which can be converted into its methyl ester by reaction with methanol. 

Toledo et al. [66] described an in situ synthesis of cobalt phthalocyanine in the pores of silica gel. The synthetic route can be visualized in the reaction scheme shown in Fig. 16a. In the first step Co(II) is adsorbed on the silica surface by an ion-exchange reaction. In the second step, the adsorbed metal served as the template for the formation of the metallated macrocyclic complex by reaction with phthalonitrile. Extraction and characterization by UV-Vis spectroscopy of the... 

The various strategies for preparation of zeolite encapsulated phthalocyanine complexes have largely involved the condensation of dicyanobenzene (DCB) around an intrazeolite metal ion to form the MPc complex. The efficiency of this template synthesis depends on the nature and location of the intrazeolite metal ion to be complexed. For example, metals have been introduced to the zeolite by ion exchange (7-13), metal carbonyls (14-19) and metallocene complexes (2-5,19-21) prior to reaction with DCB. Some of the advantages and disadvantages of these methods have been detailed by Jacobs (2). However, there are several problems that are inherent to the template synthesis in general. Often there is incomplete... 

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