2880
Ind. Eng. Chem. Res. 1999, 38, 2880-2887
APPLIED CHEMISTRY Electrosynthesis of Phthalocyanines: Influence of Solvent B. I. Kharisov,* L. M. Blanco, L. M. Torres-Martinez, and A. Garcı´a-Luna Facultad de Ciencias Quı´micas, Universidad Auto´ noma de Nuevo Leo´ n, San Nicola´ s de los Garza, Nuevo Leo´ n, A.P.18-F, C.P.66450, Me´ xico
Several methods for the synthesis of metal-free phthalocyanine (Pc or PcH2) and copper phthalocyanine (PcCu) from urea and phthalic anhydride, phthalimide, 1,3-diiminoisoindoline (1,3-D), and phthalonitrile have been studied. The results show that the nature of the solvent is the most important factor in the conventional chemical and direct electrochemical synthesis of the final products. Different routes for phthalocyanine preparation are discussed. Introduction The well-known metal-free phthalocyanine (Pc or PcH2) and its numerous metal complexes have been intensively studied since the early 1930s1-4 and are widely used in the pigment industry. Pc can be obtained by the classic template reactions starting from diverse precursors, such as phthalonitrile (PN), o-cyanobenzamide, phthalimide (PM), 1,3-diiminoisoindoline (1,3-D), phthalic acid, etc., generally in high-boiling nonaqueous solvents at elevated temperatures,5-7 or electrochemically from phthalonitrile.8-10 Phthalocyanine forms metal complexes with “strong” (for example, Fe, Cu, Ni) or “weak” (Mg, Sb) metals (according to their resistance to being eliminated out of the product) which can be synthesized chemically from metal salts2-4 or electrochemically from the bulk metals or their salts.9,10 The first type of reaction employs elemental metals2-6,11-14 or their salts,5-6 the above precursors and nonaqueous solvent. High-boiling substances, such as nitrobenzene, o-dichloro- and trichlorobenzene, ethyleneglycol, R-methylnaphthaline, quinoline, etc., are usually used as solvents,11,15,16, although some alcohols6,17 or benzene18 have been successfully applied using PN as a precursor of Pc. The yields of these reactions are on the order of 90-100%. A series of articles are devoted to the preparation of Pc from metal alloys (ref 19 and references therein). The most important advantage of the alloys is an easier reaction between phthalonitrile and the alloy’s component(s), due to a concentration gradient of metal particles on the alloy surface. As a consequence of such an interaction, it is possible to obtain polynuclear phthalocyanines and separate the alloy.19 This relatively new and intriguing field in Pc research undoubtedly should be taken into account. Metal phthalocyanines can also be obtained by electrochemical process. The feasibility of the electrosynthesis of PcM was reported by C. H.Yang et al., who obtained “PcM” of Cu, Ni, Co, Mg, and Pb using metal * To whom all correspondence should be addressed. Phone: (52-8)312-1228. Fax: (52-8)375-3846. E-mail: bkhariss@ ccr.dsi.uanl.mx.
salts as a source for the central atom.9 Furthermore, Petit’s research group10,20 studied the electrosynthesis of PcCu by electroreduction of Pc with a copper sheet or an electrodeposited layer of copper on platinum as an anode. EtOH or its mixtures with DMA or H2O were used as solvents; reactions were carried out in an electrochemical cell with separated anode and cathode zones. It is shown that an increase of DMA or H2O proportion leads to a decrease of the final product’s yield.10 In all cases, the electrosyntheses of PcCu were carried out employing PN as a starting material. Among all Pc precursors, PN1-6,8,18,21,22 and 1,3-D (it is also an intermediate product in the condensation of phthalonitrile or urea and phthalic anhydride)23,24 are the most studied, since the preparation of metal-free Pc starting from these substances is the most simple and reproducible route. However, these precursors can be mainly used for academic purposes, since their relatively high cost makes them economically unprofitable (especially 1,3-D) for industrial production of Pc or its metal complexes (mainly PcCu). Industrial research in this area is devoted mainly to the synthesis of Pc or “PcM” (M ) Cu, Ni, Fe, Al, etc.) starting from urea and phthalic anhydride (or its derivatives) as the cheapest precursors. A survey of the literature shows that most of the articles and patents (among them refs 25-33) in the “phthalocyanine” area during the last 15 years are devoted to the search for the optimal conditions for Pc or “PcM” (M ) Cu, Fe, Al, etc.) preparation, as well as the study and applications of different phthalocyanine modifications,34-40 synthesis of various Pc substituted derivatives,41-50 study of reaction mechanisms of Pc formation,9,10,18,21-23 and much more.51 To carry out the interaction between urea and phthalic anhydride, molybdenum compounds, such as MoO3, Na2MoO4, (NH4)2MoO4, etc., are usually employed as catalysts,25,27-32 as well as other metal salts such as TiCl426 or tungsten compounds.25 Tetramethylurea, 1-methyl-2-pyrolidinone, and other organic compounds are added to the reaction system as promoters.25 The reactions are carried out in high-boiling solvent, such as nitrobenzene, a mixture of trichlorobenzene isomers, high alkanes, etc., at 150-250 °C.6,11,25 Central atom sources for PcM are usually the following transi-
10.1021/ie9806545 CCC: $18.00 © 1999 American Chemical Society Published on Web 06/19/1999
Ind. Eng. Chem. Res., Vol. 38, No. 8, 1999 2881
tion metal salts: NiCl2, CuCl, CuSO4, CoCl2, FeSO4‚ 4H2O, Fe2(SO4)3‚9H2O, FeCl2, FeCl3, etc.6,27-32 The products obtained have purity on the order of 50-100% and can be used as pigments after the corresponding purification. Some solvents promote the formation of side products (for example, biphenyls52) or competitive reactions preventing the normal reaction course; therefore, sometimes the syntheses are carried out without any solvent,25 which could influence the morphology and crystalline structure of the formed products.53 Despite the abundance of reported data on PcH2 or “PcM” preparation, there is no information in the available literature on the use of direct electrochemical synthesis to obtain the above products starting from urea and phthalic anhydride (or phthalimide) as the cheapest Pc precursors. A solution to the problem of obtaining Pc in one step could be very useful for the pigment industry. So, the present work is devoted to study and compare the conventional chemical and direct electrochemical synthesis of metal-free and metal phthalocyanines starting from standard precursors in different solvents. Experimental Part Materials. Phthalonitrile, 1,3-diiminoisoindoline, standard copper phthalocyanine, and PcH2 (all from Aldrich) and urea, phthalic anhydride, and phthalimide (all from “Productos Quı´micos Monterrey”, technical quality) were used as supplied. Solvents (all of Productos Quı´micos Monterrey, technical quality) were distilled by standard methods before use. n-Tetrabutylammonium chloride was dried in a vacuum oven until use. Electrochemical Synthesis. The electrochemical synthesis was carried out according to the method described by Tuck et al.54,55 The electrochemical cell was a 100 mL beaker with reflux. The anode was a copper sheet (1 g); the cathode was a platinum foil. n-Bu4NBr (∼0.05 g) was used as the supporting electrolyte (in the case of solvents having low and medium dielectric constant). A current of dry nitrogen was passed through the reaction mixture at all times during the processes. All other details (temperature, time of electrolysis, etc.) are presented in the tables below. Purification and Identification of the Products. Phthalocyanines formed were purified by washing with hot ethanol in a Soxhlet equipment and dried in air. The products were characterized by metal content (in case of PcCu and PcFe, by atomic absorption spectroscopy), organic microanalysis carried out by standard methods for some phthalocyanines obtained, infrared and UV/visible (in pyridine) spectra recorded on PerkinElmer and Lamboda 12 instruments, respectively, and X-ray powder data (SIEMENS D-5000). According to elemental analysis data, a composition of the obtained products corresponds to typical phthalocyanines {metal-free PcH2 or MPc, M ) Cu(II), Fe(II)}. Some variations of composition (0.05-0.30%) were observed in different experiments. The IR spectra (KBr pellet) of copper phthalocyanine contain the following main bands (cm-1), among others: 35003380(vs,w), 2815m, 2504m {n(C-H)}; 2300-2280(s), 1730(vs), 1607(s), 1524(m) {n(C-C) of benzene rings}; 1448(m) {n(C-C) of pyrrol rings}; 1385(vs), 1365(s) (pyrrol nuclei-mesoatoms of N); 1320(m), 1150(s) {g(C-H)}. Some absorption bands (2865 and 2705 cm-1) are absent in the PcH2 spectra, which has additional bands at 622, 675, 690, 720, 1309, and 1500 cm-1. IR
spectra of PcH2 or PcCu, obtained in different syntheses, are almost identical (the difference is in the peak intensity). UV/vis data (nm) of PcH2 are the following: 692-693, 659-660 (Q-band), 640, 602-604. Results and Discussion I. Metal-Free Phthalocyanine. The first part of this work is related to the study of the parameters that influence on the formation and yields of metal-free phthalocyanine, such as starting materials, reaction temperature, solvent, additives, and use of the conventional chemical or direct electrochemical routes. Different Pc precursors such as phthalonitrile, 1,3-diiminoisoindoline, phthalimide, urea, and phthalic anhydride were used. I.1. Use of Phthalonitrile as Precursor. The first approach was using phthalonitrile as a starting material for the preparation of Pc and studying the effect of protic or aprotic solvents of low or high boiling points, in the chemical or electrochemical process. I.1.1. Solvent Effect in Pc Formation. Numerous solvents are reported in the literature for the preparation of Pc;8,11,18,24,56-58 for this study, six protic and seven aprotic solvents were selected (Table 1). According to the recommendations to use alkali metal alcoholates,8,18,21,24 1,8-diazabicyclo[5.4.0]undece-7-ene (DBU) or 1,5-diazabicyclo[4.3.0]non-5-ene (DBN), a 25% solution of sodium methoxide in methanol was employed to provide a nucleophilic attack in the carbon atom (CN group) of PN. In case of aprotic solvent use (DMSO, Py, dioxane, THF, DMF, acetone, nitrobenzene), no PcH2 was obtained from phthalonitrile in the electrosynthesis conditions, as well as without electrolysis. In these experiments, different contents of SM (0-1 mL) have been used. It was almost impossible or difficult to carry out the electrolysis in dioxane, DMF, acetone, and nitrobenzene due to unstable voltage. In the case of SM use, a visible interaction between the solvent and sodium methylate takes place, including formation of insoluble sediment (reactions in DMSO, DMF, THF, acetone). The absence of sodium methylate does not influence the reaction course. The absence of phthalocyanine being formed could be explained by the impossibility of nucleophilic attack of SM on the CN group of the phthalonitrile due to the interaction between SM and the aprotic solvent. As a suggestion, instead of SM, other agents compatible with a solvent are needed to carry out a nucleophilic attack in further investigations with use of aprotic solvents. The most reproducible results have been obtained in protic solvents such as i-BuOH and dimethyletanolamine with almost quantitative yields. A successful synthesis in protic media is in agreement with the opinion of the authors of ref 10 that “a protic solvent is required for the electrosynthesis of PcH2”. Sodium methylate as a source of alkoxide anions performs a nucleophilic attack at the cyano group of PN to form 1-alkoxy-3-iminoisoindoline as an intermediate which is further reduced and cyclizated.21 This mechanism is similar to the one described by Petit10 and Tomoda,21 where metal alkoxides were used for the electrochemical preparation of Pc. Higher yields of Pc in alcohol media with application of electrochemical procedure can be explained by formation of RO- ions from ROH during the process. These ions, together with the action of CH3ONa, perform a
2882 Ind. Eng. Chem. Res., Vol. 38, No. 8, 1999 Table 1. Protic Solvent Effect in the Electrosynthesis of PcH2 Starting from Phthalonitrile expt 1 2 3 4 5 6 7 8 9
initial systema
temp, °C
yield, %
n-BuOH TBA (0.1 g) n-BuOH
50
25
1.3
90
78
0
0
1.3
90
43
i-BuOH TBA (0.1 g) i-BuOH n-C5H11OH TBA (0.2 g) n-C5H11OH ethylene glycol glycerine
25
150
1
90
98
0 25
0 150
1 1.3
90 95
0 93
0 50 80
0 10 37
1.3 1 2
95 100 100
65 62
40
115
2
90-100
97
0
0
2
90-100
66
N,N-dimethylethanolamine TBA (0.1 g) N,N-dimethylethanolamine
10
current, init voltage, time, mA V h
observations After addition of SM an increase of temperature is observed. After addition of SM an increase of temperature is observed. Simultaneous use of ultrasonic treatment stabilizes the voltage. No product observed without electrolysis. Low conductivity of the solution, so it is necessary to use more TBA. No reaction observed without SM. No reaction observed without SM. High conductivity of the solution. Formed phthalocyanine is inseparable from the solvent, so it is impossible to evaluate the yield. The conductivity increases gradually during the process.
a
The reaction yield is calculated on the charge passed through the solution in the case of the electrosynthesis or on the phthalocyanine formed in the conventional chemical synthesis. Amounts of 1 g of SM solution and 5 g of PN have been used in all experiments. Tetran-butylammonium bromide (TBA) was selected according to the dielectric constant of the corresponding solvent. An amount of 100 mL of each solvent was used in all experiments. Platinum sheets were used as an anode and a cathode. Table 2. Temperature Effect in the Electrosynthesis of Metal-Free Phthalocyanine Using Phthalonitrile as a Precursor expt
initial systema
1
i-BuOH (100 mL) TBA (0.2 g) “ “ “ “ “ dimethyl ethanolamine (100 mL) TBA (0.1 g) “ “
2 3 4 5 6 7 8 9 a
current, init time, temp, yield, mA voltage, V h °C % 25
115
2
55
3
“ “ “ “ “ 40
104 97 88 79 70 118
2 2 2 2 2 2
70 75 80 85 90 90
9 22 37 66 97 97
“ “
127 136
2 2
80 70
86 56
observations Pc is formed starting from 52-55 °C
Pc formation starts from 55 °C. The conductivity is increased gradually during 2 h of the process.
In all the experiments, 5 g of PN and 1 mL of SM were used.
nucleophilic attack at the CN group of PN resulting higher rate of Pc formation. In contrast to N,N-dimethylethanolamine, the interaction in i-BuOH without electrolysis of the reaction mixture does not produce Pc. This fact could be successfully used to electrosynthesize various metal phthalocyanines, synchronizing the Pc formation on the cathode and metal anode dissolution. This way could prevent a formation of mixtures of metal-free and metal phthalocyanines.59 As will be shown below, N,N-dimethyletanolamine could be also successfully used as a “model solvent”, in which the formation of the metalfree Pc takes place even at room temperature in the conditions of UV irradiation (see Table 4). I.1.2. Temperature Effect in Pc Formation. The abovementioned solvents (i-BuOH and N,N-dimethyletanolamine) were selected to study the effect of the temperature on the reaction yields. Syntheses have been carried out at 55-90 °C. Results are presented in Table 2. As can be seen in Table 2, the Pc yields increase as temperature increases in both cases as expected. In contrast to any solvent used, an increase of the conductivity of the reaction mixture is observed in N,Ndimethyletanolamine during the electrolysis (at constant temperature). This solvent can be recommended for the conventional Pc synthesis (Table 1). Further increase of temperature (till boiling) does not improve yields. I.2. 1,3-Diiminoisoindoline as a Precursor. 1,3-Diiminoisoindoline (1,3-D) is the intermediate product in
Scheme 1
successive Pc formation, and it has been studied in detail,23,24 as well as the mechanism of formation of the complexes derived from 1,3-D-containing ligands60,61 or intermediates to metal Pc.62 As mentioned above, its main disadvantage for industrial use, as well as that of phthalonitrile, is its relatively high cost, which makes both substances very inattractive as precursors for industrial production of Pc. The 1,3-D‚HNO3 whose structure can be represented by the tautomeric formulas23 of Scheme 1 can be obtained starting from urea, phthalic anhydride, and NH4NO3 in PhNO2 in the presence of catalysts such as (NH4)2MoO4 or MoO323 with further treatment by cold NaOH to produce 1,3D.63 More pure 1,3-D can be obtained starting from phthalonitrile and NH364,65 (NH3 is also used for direct preparation of PcCu from urea and phthalic acid66). To synthesize Pc starting from 1,3-D, inert organic solvents such as trichlorobenzene, o-dichlorobenzene, etc. are generally used.1-6,24 I.2.1. Solvent Effect and Ligand Concentration in Pc Formation. In this part of the work, we have used 1,3-D as a precursor for Pc in different protic and aprotic systems, without catalysts or promoters. An amount of
Ind. Eng. Chem. Res., Vol. 38, No. 8, 1999 2883 Table 3. Electrosynthesis of Phthalocyanine Starting from 1,3-Diiminoisoindoline expt
initial system
1
EtOH TBA (0.1 g) 1,3-D (1 g) i-BuOH TBA (0.15 g) 1,3-D (1 g) N,N-dimethylethanolamine 1,3-D (1 g) N,N-dimethylethanolamine TBA (0.1 g) 1,3-D (1 g) N,N-dimethylethanolamine TBA (0.1 g) 1,3-D (3 g) nitrobenzene 1,3-D (1 g) nitrobenzene TBA (0.1 g) 1,3-D (1 g) DMSO 1,3-D (1 g) DMSO 1,3-D (1 g) DMSO 1,3-D (1 g) o-dichlorobenzene 1,3-D (1 g) DMF 1,3-D (1 g) DMF 1,3-D (1 g) DMF 1,3-D (3 g) trichlorobenzene 1,3-D (1 g) trichlorobenzene 1,3-D (3 g)
2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
current, mA
init voltage V
time, h
temp, °C
yield, %
observations
30
150
1
75
15
150
1
100
0
0
1
133
0 (8% in case of use of 3 g of 1,3-D) 0 (5% in case of use of 3 g of 1,3-D) 52
40
75
1
133
76
40
72
1
133
92
0
0
1
190
11
The crystalline product is formed.
1
190
44
Unstable voltage.
70
150-250
0
0
1
189
5
40
37
1
189
26
40
42
1
189
98
0
0
3
180
7
0
0
1
145
5
40
35
1
145
25
40
33
1
145
94
0
0
1
180
33
0
0
1
180
95
100 mL of solvent and different amounts of the 1,3-D (1 and 3 g) and TBA have been used in the experiments. The results are presented in Table 3. As can be observed, in this case it is possible to carry out the chemical and electrochemical synthesis of Pc in aprotic solvents, such as DMF or DMSO, in contrast to the results with PN. It is surprising that the yields of Pc in ROH are comparatively small. The N,N-dimethyletanolamine is characterized by the best yields, as in the case when PN was used as a precursor. Increasing the concentration of 1,3-D in the reaction mixture leads to higher yields which are almost quantitative. With low-conducting solvents such as trichlorobenzene or o-dichlorobenzene we carried out the synthesis of Pc without electrolytic conditions due to an absence of conductivity in their solutions or very low conductivity of their mixtures with DMF or DMSO. In our opinion, successful chemical and electrochemical synthesis of Pc from 1,3-D in aprotic solvents in comparison with those with PN use shows that the highest influence of a solvent’s nature on a reaction course takes place in the first stage of the process (1,3-D formation). For further reactions (cyclization and reduction of 1,3-D), a solvent’s nature is not very important, as results presented in Table 3 show. The formation of Pc from 1,3-D takes place in all the solvents used; higher yields can be achieved by optimization of the processes (variation of concentration of 1,3-D, use of electrosynthesis, and/or selection of the best solvent applied). I.2.2. UV Irradiation Effect on Pc Formation. Tomoda and co-workers reported58 that the Pc could be produced from PN even at room temperature by UV irradiation
No product observed without electrolysis No product observed without electrolysis
High conductivity of the solution.
Formation of Pc is observed at 120 °C on the cathode surface.
of the reaction mixture in different alcoholic solvents. The authors concluded that the UV treatment is effective only at the initial stage of the reaction because of an absence of the product after heating the reaction mixture in the dark and further exposure to UV light.58 In this part of the work, the behavior of 1,3-D in some nonaqueous solvents under UV irradiation was studied. Solutions of 2 g of 1,3-D in 100 mL of different solvents were placed into quart ampules and irradiated by a mercury lamp (1200 W) for 4 h. No electrolysis was used in these experiments. The results are presented in Table 4. As well as in our previous experiments, N,N-dimethyletanolamine has demonstrated its especial properties in relation to the Pc formation. Pc appears slowly even at 7 °C and can be isolated (with different yields, depending on the temperature) at any temperature of the reaction mixture. When PN is used as a precursor,58 free radicals of RO• appear due to UV irradiation of the alcohol solutions of PN, contributing to the nucleophilic attack on the carbon atom of the CN group, together with the action of CH3ONa. Probably, this is a cause of Pc formation at room temperature. In our experiments using 1,3-D as a starting material, the formation of free radicals (CH3)2NCH2CH2O• (RO•) from N,N-dimethylethanolamine probably contributes to the conglomeration of the intermediate products, which appear as a result of the transformation of 1,3-D.23,61 Another version which can explain “especial” properties of this solvent in all the experiments above is a possible participation of the
2884 Ind. Eng. Chem. Res., Vol. 38, No. 8, 1999 Table 4. Influence of UV Irradiation on the Synthesis of Phthalocyanine Starting from 1,3-D in Different Solvents expt
solvent
1 2
DMF DMSO
3 4
trichlorobenzene nitrobenzene
5
N,N-dimethylethanolamine
temp, °C
observations
7, 20, 45, 65, 75, 85, 100 7, 20, 45, 65, 75 85, 100 7, 20, 45, 65, 75, 85, 100 7, 20, 45, 65, 75 80, 100 7-100
No visible changes in the system. No visible changes in the system. Dark color of the solution. No phthalocyanine formed. No visible changes in the system. No visible changes in the system. Dark color of the solution. Small amount of PC (∼0.2 g) is formed. Pc is formed at any temperature. Intensity of the color of the solution increases with temperature. In absence of UV irradiation, the Pc is formed slowly starting from 45-50 °C.
Table 5. Urea and Phthalic Anhydride Effects on the Formation of Pc Starting from 1,3-D expt
initial system
1
Presence of urea: DMF (100 mL) 1,3-D (1 g) urea (1; 2; 3 g) TBA (0.1 g) Presence of urea: DMF (100 mL) 1,3-D (1 g) urea (1; 2; 3 g)
2
current, mA
init voltage, V
time, h
temp, °C
yield, %
observations
40
35-40
1
145
23; 18; 22
Product is formed without any difference in comparison with the experiments where urea is absent.
0
0
1
145
7; 9; 8.5
Product is formed without any difference in comparison with the experiments where urea is absent.
a In the presence of phthalic anhydride (1-3 g) or its mixture with urea (3 and 4 g, respectively), no Pc was observed in electrosynthesis conditions starting from 1,3-D (1 g), as well as without electrolysis.
electron pair of the nitrogen atom of N,N-dimethylethanolamine in nucleophilic attack. I.2.3. Urea and PA Effects on Pc Formation. The presence of urea or PA can also influence the reaction course of Pc starting from 1,3-D. We have introduced these Pc precursors into the reaction system in some experiments in order to establish which component’s abundance could affect the Pc formation. DMF was chosen as a solvent in this study. The resulting data are presented in Table 5. As can be observed, in DMF medium the presence of urea does not affect Pc formation. This fact is in agreement with earlier reports6 where urea was used as a solvent. Urea and, as it will be shown below, tetramethylurea are “inert” solvents with respect to 1,3D. On the contrary, the presence of PA decreases reaction yields to zero. Its presence provokes competitive reactions (probably, due to the attack of oxygen atoms of PA on N atoms of 1,3-D) preventing 1,3-D cyclization in the reaction system that leads to the absence of the final product. I.3. Use of Urea and Phthalic Anhydride as Precursors. As was mentioned above, urea and PA are the cheapest phthalocyanine precursors and are produced in an industrial scale, so it is not surprising that numerous articles and patents have been dedicated to the study of their interactions.6,23,25-32 However, only copper and some other “strong” metals (in relation to the “PcM” formation and stability) form their phthalocyanines using these precursors. There are no reports in the available literature about intents to electrosynthesize PcH2 or “PcM” starting from urea and phthalic anhydride. So, we have studied the interaction between these two precursors as well as phthalimide in various nonaqueous solutions by conventional chemical and electrochemical methods. Three series of experiments were carried out: (1) using N,N-dimethylethanolamine, nitrobenzene, trichlorobenzene, DMSO, or the mixtures “DMSO-trichlorobenzene” and “nitrobenzene-trichlorobenzene” as solvents (100 mL) and urea (4 g), PA (3 g), TBA (0.1-0.2 g), and ammonium molybdate (AM, 0.04 g); (2) the same
solvents (100 mL) and urea (0; 3; 6 g), phthalimide (3 g), TBA (0.1-0.2 g), and ammonium molybdate (0.04 g); (3) mixtures of DMSO (or nitrobenzene) (50 mL) and trichlorobenzene (50 mL), TMU or 1-methyl-2-pyrolidinone as promoters (0.12 g), urea (4 g), PA (3 g), TBA (0.05-0.2 g), and MoO3 (0.015 g). As a result of all these experiments, it is impossible to obtain metal-free phthalocyanine from urea and phthalic anhydride as well as phthalimide in a one-step interaction either by conventional chemical or electrochemical methods. The presence of catalysts and promoters for CuPc (TMU or 1-methyl-2-pyrolidinone) manufacture does not influence the formation of the Pc. Similarly, the presence of small amounts of Pc introduced into the reaction mixture does not provoke further PcH2 formation starting from the above precursors in the solvents used. It does not mean that metal-free Pc could not be theoretically obtained from these precursors (see Conclusions); additional detailed studies (combination of different techniques, such as UV irradiation, microwave treatment, use of inert solvents, electrolysis in the systems producing free radicals, etc.) are required for successful resolution of this problem. However, in the same conditions it is possible to obtain some transition metal complexes of the Pc due to the template effects (see below). II. Transition Metal Phthalocyanines. Phthalocyanines of copper and other transition and p-metals have been intensively studied.1-6,18,25-32,67-79 Their syntheses have been carried out using all possible precursors for Pc (see Introduction), in different polar or nonpolar solvents or without any solvent,1-6,18,25-32,59,67-79 in the presence of NH3,66 N2, or H2,6 using microamounts of PcH2,79 or applying microwave conditions.67 In addition to “standard” metal phthalocyanines such as PcCu, polynuclear complexes of “double sandwich”type6,47,59 or water-soluble complexes80 have been obtained. However, the electrochemical method has been used only for the synthesis of PcCu, using a copper anode9,10 or CuSO49 and some other metal (Ni,Co,Mg,Pb) phtha-
Ind. Eng. Chem. Res., Vol. 38, No. 8, 1999 2885 Table 6. Use of Tetramethylurea as a Solvent and Promoter Simultaneously expt 1 2 3b 4
initial systema PA (3 g) Cu (1 g)
current, mA
init voltage, V
time, h
(a) Interaction between Urea and PA in the Presence of Copper 20 45 3 0
0
3
(b) Interaction between Phthalimide and Urea in the Presence of copper phthalimide (3 g) 30 50 3 Sb or Mg (1 g) 0 0 3
temp, °C
yield, %
170
35
170
47
170
17
170
33
a
TMU (100 mL), urea (4 g), TBA (0.2 g), and MoO3 (0.015 g) have been used in all the experiments. b Copper transfer from the anode to the cathode is observed. In the absence of urea, no phthalocyanine formation is observed.
locyanines using metal salts, dissolved in a reaction system containing PN.9 In our work, we have chosen four metals for the interaction with the Pc precursors in nitrobenzene and trichlorobenzene, according to their capacity to form stable (Cu,Fe) and unstable (Mg,Sb) compounds with Pc.1-4 Each experiment is carried out using urea (4 g), PA (3 g), TBA (0-0.2 g), TMU (0.12 mL), and MoO3 (0.015 g). A mixture of nitrobenzene (50 mL) and trichlorobenzene (50 mL) is used as a solvent. The processes are carried out during 3 h at 170 °C with or without electrolysis. As a result, use of Cu and Fe leads to their phthalocyanines formation; the yields are considerably higher in pure chemical experiments (69-77%). Applying the electrosynthesis, only a small amount of CuPc (7%) is observed. Mg and Sb do not produce phthalocyanines in above conditions. As a conclusion, it is impossible to obtain phthalocyanines electrochemically from the mentioned metals in the conditions above, except PcCu with low yields, although without electrolysis the copper and iron complexes are formed with significantly higher yields. According to Linstead,2 among other metal phthalocyanines, PcCu is the most stable due to the highest affinity of copper(II) to the phthalocyanine macrocycle. Copper cannot be “extracted” from PcCu without destruction of organic matter. PcCu is formed generally in inert nonaqueous solvents or without any solvent;6 use of polar solvents (in order to create favorable conditions for electrolysis) leads to zero yields of the final product. It was confirmed in the series of experiments using urea (4 g), PA (3 g), (0.12 g), Cu (1 g), MoO3 (0.015 g), 1-methyl-2-pyrolidinone (0.5 mL), and 100 mL of solvent (DMEA, ethyleneglycol, glycerine, DMSO, DMF, mesytilene, butylcellosolve, hysol, and xylene). Only in hysol, the yield of PcCu was 85% (without electrolysis); in all other mentioned solvents no phthalocyanines were observed (except PcCu traces in mesytilene). Use of N,Ndimethyletanolamine with simultaneous UV irradiation does not produce PcCu (copper transfer from the anode to the cathode was observed in this experiment). Therefore, in our opinion, the solvent used for successful electrosynthesis of PcCu should be “inert” in relation to PA and, of course, should have electroconductivity. The compounds used as promoters25 could theoretically serve as such solvents. We have chosen tetramethylurea (TMU) and 1-methyl-2-pyrolidinone among other promoters used in the work.25 The first one has a nature close to that of the principal precursor (urea) and, thus, should not negatively influence the reaction course. The TMU has a sufficient conductivity, permitting electrolysis to be carried out in this medium, and a reasonable viscosity. The boiling point 174-178
°C is ideal for the present research, since usual syntheses of Pc from urea and PA are carried out at similar temperatures. The results of TMU use as a solvent are presented in Table 6. The results with its use seem promising, and this solvent is recommended to study the Pc formation in its medium in further research works. In the case of 1-methyl-2-pyrolidinone use, no phthalocyanine formation was observed. No phthalocyanine was observed also in the following systems: (1) urea, PA, TBA, TMU (without copper); (2) urea, PA, TBA, TMU, Sb or Mg (anodes); (3) TMU, urea (or without urea), phthalimide, TBA (in all cases with or without electrolysis). As was mentioned earlier (Table 5), the abundance of urea does not affect the reaction course of 1,3-D cyclization. Tetramethylurea can participate in similar intermediate reactions as urea due to the proximity of their nature. Theoretically, it is possible to obtain the metal-free phthalocyanine by adding other catalysts and/or promoters into the reaction mixture on the basis of TMU or other derivatives of urea. To conclude this work, it is necessary to mention that it is still difficult to evaluate real reaction mechanisms in each synthetic procedure applied. It is clear that the use of such polar protic solvents as alcohols contributes to higher yields of Pc from PN in the electrosynthesis conditions due to easiness of nucleophilic attack of the generated additional RO-. In the further steps of Pc formation from PN or 1,3-D, a solvent’s nature has no significant importance. These data concerning the importance of, first of all, the initial stage correspond to those reported on UV irradiation58 of PN solutions where such a treatment is effective only at the beginning of the process (see I.2.2). However, in the case of the use of urea and PA, a solvent must be completely inert (or be close to urea’s nature) to carry out the onestep synthesis of metal phthalocyanines, to exclude any negative influence on the reaction course. The fact that the yields are almost always higher in the case of direct electrosynthesis could serve as an additional confirmation of the usefulness and necessity of this technique. Conclusions (1) We believe that it is very difficult (but not impossible) to carry out direct one-step electrosynthesis of metal-free Pc starting from urea and PA. The Pc could be synthesized from PN or 1,3-D in a one-step process, according to the reported9,10 results. Using urea and PA, the synthesis of Pc must include three steps: (a) interaction between PA, urea, and NH4NO3,23 forming 1,3-D‚HNO3; (b) elimination of NO3- by NaOH, forming 1,3-D;63 (c) (electro)synthesis of Pc from 1,3-D. (2) It is possible to improve existing technologies for the synthesis of PcCu and other metal phthalocyanines
2886 Ind. Eng. Chem. Res., Vol. 38, No. 8, 1999
from phthalimide or urea and PA, applying the electrosynthesis. These processes have many peculiarities. The solvent nature strongly affects the course of the majority of reactions of coordination compound formation.81,82 The solvent used for PcCu preparation must be inert in order not to influence the desirable reaction course and simultaneously must have electroconductivity to carry out electrolysis. It is recommended to use the derivatives of urea as such solvents and promoters at the same time. The use of the standard electrochemical procedure9-14,54,55,83 could be useful for the PcCu industry, since a typical industrial problem is the presence of initial Cu2+ salts in the final product. These impurities decrease the quality of the pigment and are hardly removed. (3) PcCu could be formed without use of any solvent.6 It is recommended to apply electrosynthesis in melt urea or its mixtures with high-boiling conducting solvents, avoiding such typical problems as, for example, sublimation of PA. (4) A probable success could be theoretically reached using “weak” metals (Sb,Mg) in the chemical or electrochemical interaction with the Pc precursors. Undoubtedly, this way should be developed, since it is a possible route to prepare metal-free Pc after removing of metal from the macrocycle. (5) Among the other solvents applied in the present research, N,N-dimethyletanolamine has especial properties, which make it a promising solvent for further investigations in the Pc area. (6) Electrochemical reactions with use of sacrificial cathode,54 which have not been carried out in the present research, could have theoretical success in the synthesis of “PcM”. The cathode dissolution takes place due to the action of free radicals formed in solution near the cathode surface;54 in the case of the synthesis of Pc (which is formed on the cathode surface), UV irradiation of the reaction mixture could serve as a source of free radicals. Literature Cited (1) Linstead, R. P. Phthalocyanines. Part I. A New Type of Synthetic Colouring Matters. J. Chem. Soc. 1934, 1016. (2) Byrne, G. T.; Linstead, R. P.; Lowe, A. R. Phthalocyanines. Part II. The Preparation of Phthalocyanine and Some Metallic Derivatives From o-Cyanobenzamide and Phthalimide. J. Chem. Soc. 1934, 1017. (3) Linstead, R. P.; Lowe, A. R. Phthalocyanines. Part III. Preliminary Experiments on the Preparation of Phthalocyanines From Phthalonitrile. J. Chem. Soc. 1934, 1022. (4) Dent, C. E.; Linstead, R. P. Phthalocyanines. Part IV. Copper Phthalocyanines. J. Chem. Soc. 1934, 1027. (5) Phthalocyanines. Properties and Applications. Leznoff, C. C., Lever, A. B. P., Eds.; VCH Publishers, Inc.: New York, Vol. 1, 1990; Vol. 2, 1992; Vol. 3, 1993; Vol. 4, 1996. (6) Thomas, A. L. Phthalocyanines. Research and Application; CRC Press: Boca Raton, 1990. (7) Thoma, P. F.; Habermann, W. M.; Kranz, J. L. Preparation of metal-free phthalocyanines. U.S. Patent 4,145,264, 1979. (8) Moser, F. H.; Thomas, A. L. Phthalocyanines. Properties; CRC Press: Boca Raton, 1983; Vol 1. (9) Yang, C. H.; Lin, S. F.; Chen, H. L.; Chang, C. T. Electrosynthesis of the Metal Phthalocyanine Complexes. Inorg. Chem. 1980, 19, 3541. (10) Petit, M. A.; Plichon, V.; Belkacemi, H. Electrosynthesis of Phthalocyanines. New J. Chem. 1989, 13, 459. (11) Garnovskii, A. D.; Ryabukhin, Yu. I.; Kuzharov, A. S. Direct Synthesis of Metal Complexes in Non-Aqueous Media. Koord. Khim. 1984, 10 (8), 1011. (12) Garnovskii, A. D.; Kharisov, B. I.; Go´jon-Zorrilla, G.; Garnovskii, D. A. Direct Synthesis of Coordination Compounds
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Received for review October 13, 1998 Revised manuscript received March 12, 1999 Accepted March 14, 1999 IE9806545
