Metal phthalocyanines

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. 

The phthalocyanines can be laid down as vividly coloured thin films by sublimation at high temperatures, e.g. at 400-600 °C under vacuum. The thin blue films of the bis-phthalocyanines are polyelecfiochromic, undergoing electrochemical oxidation to give first green and then yellow-red or red colours (anodic products) and blue-purple colours on reduction (cathodic products). The various structures of these coloured products are shown in Table 1.12. 

Metal oxidation state Red (yellow/red) green blue purple 

---

Depending upon the metallic compound used, different metallic phthalocyanine derivatives are obtained, e.g. when copper chloride is used copper phthalocyanine (Monastral Fast Blue B) is obtained. 

Phthabc anhydride reacts with urea and metal diacetates to form metal phthalocyanines (qv). 

Phthalocyanine compounds have been synthesized with various metals (1,2,4). The most important metal phthalocyanines are derived from phthalodinitnle, phthaUc anhydride. Pc derivatives, or alkaU metal Pc salts. 

Metal phthalocyanines may also be prepared using alkaU metal salts or from metal-free phthalocyanine by boiling the latter in quinoline with metal... 

Very unstable modifications, like the reddish, chlorine-free a-copper phthalocyanine, can be stabilized with amides or salts of copper phthalocyanine sulfonic acids (59—63). Mixture with other metal phthalocyanines, eg, tin, vanadium, aluminum, or magnesium, also inhibits crystallization change and poor performance in binders and prints (flocculation) due to the hydrophobic character of unsubstituted phthalocyanines. 

Phthalocyanine sulfonic acids, which can be used as direct cotton dyes (1), are obtained by heating the metal phthalocyanines in oleum. One to four sulfo groups can be introduced in the 4-position by varying concentration, temperature, and reaction time (103). Sulfonyl chlorides, which are important intermediates, can be prepared from chlorosulfonic acid and phthalocyanines (104). The positions of the sulfonyl chloride groups are the same as those of the sulfonic acids (103). Other derivatives, eg, chlormethylphthalocyanines (105—107), / /f-butyl (108—111), amino (112), ethers (109,110,113—116), thioethers (117,118), carboxyl acids (119—122), esters (123), cyanides (112,124—127), and nitrocompounds (126), can be synthesized. 

Dyes and Pigments. Several thousand metric tons of metallated or metal coordinated phthalocyanine dyes (10) are sold annually in the United States. The partially oxidized metallated phthalocyanine dyes are good conductors and are called molecular metals (see Semiconductors Phthalocyanine compounds Colorants forplastics). Azo dyes (qv) are also often metallated. The basic unit for a 2,2 -azobisphenol dye is shown as stmcture (11). Sulfonic acid groups are used to provide solubiHty, and a wide variety of other substituents influence color and stabiHty. Such complexes have also found appHcations as analytical indicators, pigments (qv), and paint additives. 

CNTs were also synthesized at lower temperatures starting from some metal phthalocyanines [18]. Nickel-, cobalt- and iron-phthalocyanines were deposited in vacuum and CNTs were grown perpendicularly on a quartz substrate at 700 and 800°C at relatively high yield. At the base of the nanotubes, a cluster of metal... 

PcH2 was obtained for the first time in 1907 as a byproduct during the preparation of 2-cyanobenzamide.50 However, this discovery initiated no interest at that time. In 1927, PcCu was prepared in 23% yield by heating 1,2-dibromobenzene with copper(I) cyanide in pyridine.51 The term phthalocyanine was first used by Linstead in 1933.52 From 1929 to 1939, Linstead et al. elucidated the structure of phthalocyanines and developed improved synthetic methods for several metal phthalocyanines.52 - 61... 

However, they can also be prepared by metal exchange from alkali-metal phthalocyanines. If proton donors like hydrochloric acid, water or methanol are added to the reaction mixture of a freshly prepared alkali-metal phthalocyanine, metal-free phthalocyanines (PcH2) are formed (see Section 2.1.4.1,). If, on the other hand, the appropriate metal salt is added to a solution of an alkali-metal phthalocyanine, the product is the metalated compound (PcM) (see Section 2.1.6.). 

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. 

Usually metal-free phthalocyanine (PcH2) can be prepared from phthalonitrile with or without a solvent. Hydrogen-donor solvents such as pentan-l-ol and 2-(dimethylamino)ethanol are most often used for the preparation.113,127 128 To increase the yield of the product, some basic catalyst can be added (e.g., DBU, anhyd NH3). When lithium or sodium alkoxides are used as a base the reaction leads to the respective alkali-metal phthalocyanine, which can easily be converted into the free base by treatment with acid and water.129 The solvent-free preparation is carried out in a melt of the phthalonitrile and the reductive agent hydroquinone at ca. 200 C.130 Besides these and various other conventional chemical synthetic methods, PcH2 can also be prepared electrochemically.79... 

Thallium phthalocyanine (PcT12) can be obtained by heating phthalonitrile with an inter-metallic alloy of indium and thallium in an evacuated glass ampule.145 The structure of this compound is unique among metal phthalocyanines as the two thallium cations occupy two opposite corners of an octahedron, which is formed by the thallium cations and the four iso-indolinc nitrogen atoms facing the center of the macrocycle.147 Another unusual type of phthalocyanine can be prepared by heating phthalonitrile with thallium metal.148 It was identified as a bicyclic thallium(III) phthalocyanine (Pc3/2T1).14S... 

Almost every metal atom can be inserted into the center of the phthalocyanine ring. Although the chemistry of the central metal atom is sometimes influenced in an extended way by the phthalocyanine macrocycle (for example the preferred oxidation state of ruthenium is changed from + III to + II going from metal-free to ruthenium phthalocyanine) it is obvious that the chemistry of the coordinated metal of metal phthalocyanines cannot be generalized. The reactions of the central metal atom depend very much on the properties of the metal. 

This compound has been investigated by several groups,379- 384 but due to its practical insolubility it has not been fully characterized. Another approach to obtain phthalocyanine network polymers is with the use of crown ethers385 or other groups like di oxy -para-p h enyle n e3 8 6 as bridging units between the macrocycles. Some attempts to obtain metal phthalocyanine substructures arranged as ladder polymers have also been proposed (see below).344... 

A mixture of4-(dodecylsulfanyl)phthalonitrile(9, R = SC,2H25 0.25 g, 0.76 mmol), anhyd NiCI2 (0.028 g, 0.22 mmol) and quinoline (0.4 mL) was heated and stirred at 200 C for 5h under argon. After cooling to rt, the mixture was treated with EtOH (5 mL), filtered, and washed successively with the same solvent. Isolation of the metal phthalocyanine was carried out using column chromatography (silica gel, hexane/ CH2C12 5 3) yield 0.067 g (26%). 

Solvents of basic character or the addition of basic compounds such as l,8-diazabicyclo[5.4.0]-undec-7-ene (DBU) to solvents like alcohols are often used in the preparation of soluble metal phthalocyanines 18. 

In presence of acids or other metal ions the five-unit macrocyclc of superphthalocyanines can be contracted to produce metal-free phlhalocyanine or metal phthalocyanines, respectively. This reaction might be more of scientific interest than of synthetic value. Nevertheless, one example is shown below. 

From Alkali-Metal Phthalocyanines By Metal/Metal Exchange... 

Alkali-metal phthalocyanines 1 are commonly prepared in situ by the reaction of the appropriate phthalocyanine with lithium in an alcohol like pentan-l-ol. If higher temperatures are required during the synthesis, octan-1-ol with its substantially higher boiling point is used. The reaction mixture is then refluxed with a compound containing the desired metal atom to yield the appropriate metal phthalocyanine 2. 

Metal-free phthalocyanines can also be transferred into the metal phthalocyanines (e.g., PcCu, PcZn, PcNi) by treatment with the respective metal acetates in penlan-l-ol.290... 

From Both Metal-Free and Metal Phthalocyanines... 

Metal atoms with a stable oxidation state of + IV are known to form unsubstituted bis(phthalo-cyanines) 9,10. They can be prepared by the reaction of a metal phthalocyanine, or an appropriate metal salt, and an alkali-metal phthalocyanine. 

If two differently substituted alkali-metal phthalocyanines are used (in the same molar ratio), unsymmetrically substituted bis(phthalocyanines) can be obtained. 

Tetrasubstituted phthalocyanines, e.g. 11, can be prepared from 3-substituted phthalonitriles via metal/metal exchange from tetrasubstituted alkali-metal phthalocyanines. 

In the same manner, zinc(II) phthalocyanine 12 can be prepared by metal/metal exchange from a 2,3,9,10,16,17,23,24-octasubstituted alkali-metal phthalocyanine. 

Radical phthalocyanine species, e.g. 6, can be prepared by the reaction of thionyl chloride or thionyl bromide with metal phthalocyanines. In these cases, the phthalocyanine ligand only carries the charge — l.349... 

The elucidation of the structure of the phthalocyanines followed some pioneering research into the chemistry of the system by Linstead of Imperial College, University of London. The structure that we now recognise was first proposed from the results of analysis of a number of metal phthalocyanines, which provided the molecular formulae, and from an investigation of the products from degradation studies. Finally, Robertson confirmed the structure as a result of one of the classical applications of single crystal X-ray crystallography. 

---