Subphthalocyanines

Although these methods were applied for the synthesis of a number of various phthalocyanines with different central atoms (e.g., H2, Cu, Zn, Ni, Pt, Pd, Lu, etc.) not all metal phthalocyanines can be prepared by one of these methods. For example, the synthesis of silicon phthalocyanine, rhenium phthalocyanine and boron subphthalocyanine need more drastic conditions. In the following, an overview of the synthesis of phthalocyanines containing all central metals which have hitherto been inserted into the ring is given. 

In contrast to phthalocyanines (tetra- or octasubstituted) in which the isoindoline units carry all the same substituents, reports of phthalocyanines with lower symmetry, which have been prepared by using two different phthalonitriles, have rarely appeared. This is due to the problems which are associated with their preparation and separation. For the preparation of unsymmetrical phthalocyanines with two different isoindoline units four methods are known the polymer support route,300 " 303 via enlargement of subphthalocyanines,304 " 308 via reaction ofl,3,3-trichloroisoindoline and isoindolinediimine309,310 and the statistical condensation followed by a separation of the products.111,311 319 Using the first two methods, only one product, formed by three identical and one other isoindoline unit, should be produced. The third method can be used to prepare a linear product with D2h symmetry formed by two identical isoindoline units. For the synthesis of the other type of unsymmetrical phthalocyanine the method of statistical condensation must be chosen. In such a condensation of two phthalonitriles the formation of six different phthalocyanines320 is possible. 

Ring enlargement from subphthalocyanines has been used to synthesize unsymmetrically substituted metal-free phthalocyanines of the AAAB type (see p 738). 

Subphthalocyanines are the products of the cyclotrimerization of molecules like phthalonitriles or isoindolinediimines. The formation of structural isomers occurs if these precursors are un-symmetrically substituted at the benzene nucleus (see also p736). 

Besides unsubstituted phthalonitrile, 4-mono- and 4,5-disubstituted phthalonitriles can also be employed as precursors of subphthalocyanines 1. 

A 5,6-disubstituted isoindolinediimine 1 has been used in subphthalocyanine synthesis.68... 

The chloro group on the central boron atom of a chloroboron subphthalocyanine may be substituted by a phenyl or hydroxy group. The latter product can be further transformed into an ether.68... 

Hydroxyboron subphthalocyanine was obtained in very low yield (2%) from chloroboron subphthalocyanine in the presence of sodium hydroxide and a crown ether in refluxing xylene, the major product (8%) being, u-oxo-bis(boron subphthalocyanine).68 Silyl ethers, however, are formed in reasonable yields.68... 

Figure 13 Ring expansion of a haloboron(III) subphthalocyanine (49) to give an unsymmetrically substituted phthalocyanine, here the zinc(II) benzonaphthoporphyrazine (51).252,253 As usual, the product is...

The synthetic methods of symmetrically substituted Pcs are similar to those of the parent unsubstituted Pcs. However, unsymmetrically substituted Pcs are usually prepared by statistical condensation methods, subphthalocyanine expulsion methods or cross-condensation methods. 

Fig. 3. Molecular structure of phenylboron subphthalocyanine B(SubPc)(C6H5). ...

Spesia, M. B. and Durantini, E. N. (2008). Synthesis and antibacterial photosensitizing properties of a novel tricationic subphthalocyanine derivative. Dyes Pigm. 77, 229-237. 

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. 

In corroles and other systems described above, the macrocyclic ring is contracted by leaving out one of the four porphyrinic meso bridges. A more radical modification, namely an omission of one the cyclic subunits, results in even smaller macro-cyclic rings, collectively termed triphyrins. Boron subphthalocyanines (65, Fig. 25), known for 35 years [191], are an archetype of triphyrin structure. However, their... 

Some representative examples of fullerene-porphyrin dyads are shown in Scheme 9. In other examples, porphyrin analogs such as phthalocyanines and subphthalocyanines have been used for the construction of efficient dyads. Again, the most straightforward approach for their synthesis involved 1,3-dipolar cycloaddition of the appropriate azomethine ylides to C60 [203-205]. Also, with the aid of the Bingel reaction, other phthalocyanine-fullerene systems have been prepared [206,207] with the most prominent example being the one that contains a flexible linker possessing an azacrown subunit [208]. The novelty of this dyad can be found in the nature of the linker that could, in principle, induce conformational changes in the multicomponent system when certain ions (e.g., alkaline ions) are present. As a direct consequence this would potentially allow an external control over the electronic interactions between the phthalocyanine and fullerene units. 

Fig. 2 (a) Kobayashi ting opening reaction of subphthalocyanines. (b) Schematic structure of a metallophthalocyanine with ligands in axial positions... 

Pcs are one class of such compounds with their delocalized two-dimensional 18 7t-electron systems and exceptional stabilities, which make them suitable candidates for NLO applications. From this point of view, Pcs, subphthalocyanines, and related compounds have been investigated extensively in recent years [14]. Besides the excellent 18 Tt-electron systems, the substituents at the benzene rings, coordinative metal, and axial coordination ligands of the Pcs affect the degree of the NLO properties significantly. However, correlation between these factors has not been fully established yet. 

The phthalocyanines (e.g., 2.277) are well known for their ready availability and their tendency to form stable complexes with a wide variety of metals. They are typically prepared via the metal-templated macrocyclization of a phthalonitrile derivative (e.g., 2.276) as shown in Scheme 2.3.1. The ease of preparation as well as the remarkable coordinative ability of the phthalocyanines led Meller and Ossko in 1972 to attempt to prepare boron-containing phthalocyanine derivatives. This they tried via the reaction of phthalonitrile with haloboranes. What they in fact obtained, however, was not the desired tetrameric phthalocyanine derivative, but rather a contracted trimeric phthalocyanine 2.284, a species that has since come to be known as subphthalocyanine (Scheme 2.3.2). A similar fluorine-containing subphthalocyanine 2.285 was also prepared using this procedure. 

Since the initial disclosure of subphthalocyanines 2.284 and 2.285, several other examples of subphthalocyanines have been reported. The first of these, the bromo-derivative of the /-butyl-substituted macrocycle 2.286 was prepared in 1990 by Kobayashi, et al The synthesis of unsubstituted and t-butyl-substituted subphthalocyanines 2.287 and 2.288 (Scheme 2.3.2), in which the axial substituent at the boron center is a phenyl group, has also been reported. (In the case of the /-butyl-substituted compound, the two possible structural isomers 2.288a and 2.288b (Figure 2.3.1) were separated by high-performance liquid chromatography (HPLC). )... 

In addition to the above subphthalocyanines, the hexakis(alkylthio)-substi-tuted system 2.291 was prepared by Bekaroglu and coworkers from the bis(alk-ylthio)phthalonitrile derivative 2.290 (Scheme 2.3.3). Also, in what may be considered an intellectual extension on the synthesis of subphthalocyanines. 

Subphthalocyanines contain a delocalized 14 -electron conjugated pathway, and are brightly colored compounds, both in the solid state and in organic solution. As such, they exhibit fairly strong absorption bands in their visible electronic spectrum. For instance, subphthalocyanine 2.284 exhibits a Soret-like absorbance band at ca. 305 nm and a more intense Q-like transition at 565 nm (e = 50 100 M cm and 89 100 M cm , respectively, in CHCI3). These bands are blue-shifted relative to those of the metallophthalocyanines (by c. 20-30 nm and 120-130 nm in the Soret and Q-band regions, respectively), and exhibit absorption coefficients that typically are smaller than those of the metallophthalocyanines. ... 

A single crystal X-ray diffraction analysis has been carried out on both the chloro- and the phenyl-substituted subphthalocyanines 2.284 and 2.287 (Figures 2.3.2 and 2.3.3). These analyses served to show that subphthalocyanines lie in a bowl-shaped conformation. This is, of course, very different from the near-planar conformation of the parent phthalocyanines. Presumably, this bowl-shaped structure accounts, in part, for the decreased molar absorptivities of the subphthalocyanines relative to their phthalocyanine parents . Nevertheless, despite the non-planar nature of these macrocycles, the subphthalocyanines are capable of supporting an induced diamagnetic ring current (as judged by NMR spectroscopy). Thus, they may appropriately be considered as being aromatic. 

While the subphthalocyanines are interesting in their own right, much of the recent impetus for preparing these macrocycles derives from their use as precursors in the synthesis of unsymmetrically substituted phthalocyanines, which are otherwise... 

Figure 2.3.2 Single Crystal X-Ray Diffraction Structure of Subphthalocyanine(Cl) 2.284.

The first documented example of a ring-expansion reaction involving a sub-phthalocyanine came in 1990. Here, Kobayashi and Osa and coworkers reported that by treating the t-butyl-substituted subphthalocyanine 2.286 with succinimide (2.296), one could obtain the unsymmetrically substituted phthalocyanine 2.297 in 13% yield (Scheme 2.3.5). Similarly, treating subphthalocyanine 2.286 with the dii-minoisoindoline analogs 2.298-2.300, afforded the unsymmetrically substituted... 

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