Synthesis and electrochemical investigation of covalently linked porphyrin dimers containing a...

Synthesis and electrochemical investigation of covalently linked porphyrin dimers containing a β-brominated subunit. Crystal structure of H2[tripp-tpp(Br8)]H2

Zhongping Oua, Pietro Tagliatesta*b, Mathias O. Sengec, Jianguo Shaoa and Karl M. Kadish*a

a Department of Chemistry, University of Houston, Houston, TX 77204-5003, USA b Dipartimento di Scienze e Tecnologie Chimiche, Università di Roma “Tor Vergata”, 00133 Roma, Italy c Institut für Chemie,Universität Potsdam, Karl-Liebknecht-Str. 24-25, D-14476 Golm, Germany

Received 10 July 2003Accepted 1 September 2003

ABSTRACT: Ten meso-tetraphenylporphyrin-type heterodimers containing a partly or completely β-brominated subunit were synthesized and characterized by UV-visible spectroscopy, cyclic voltammetry and spectroelectrochemistry, showing the presence of low electronic interactions between the two subunits. The investigated compounds are represented as M[(tripp-tpp(Br4)]M and M[tripp-tpp(Br8)]M (M = 2H, Zn, Ni, Co and Cu) where tripp-tpp(Br4) is the tetraanion of 1-[5-(10,15,20-triphenylporphyrinyl)]-4-[10-(2,3,12,13-tetrabromoporphyrinyl)]-benzene and tripp- tpp(Br8) is the tetraanion of 1-[5-(10,15,20-triphenylporphyrinyl)]-4-[10-(2,3,7,8,12,13,17,18-octabromoporphyrinyl)]-benzene. One of the synthesized dimers, H2[tripp-tpp(Br8)]H2, was characterized by a single-crystal X-ray investigation. Copyright © 2003 Society of Porphyrins & Phthalocyanines.

KEYWORDS: porphyrin dimers, halogenated porphyrins, metalloporphyrins.

INTRODUCTIONSynthetic porphyrin dimers have been extensively

studied as models for photosynthetic processes of bacteria and for their use as potentially useful materials for photonics [1-10]. This class of compounds has been also proposed as photosensitizers in the photodynamic therapy of cancer [11] and as models for catalytic oxygenation reactions [12].

Our own attention has in part been directed towards the synthesis and study of linked tetrapyrrolic heterodimers with two different types of macrocycles, one an alkyl substituted porphyrin and the other a meso-tetraphenylporphyrin [13-14] or one a corrole and the other a meso-tetraphenylporphyrin

[15]. Several other groups have also been involved in this field [16-22] and have developed different strategies for obtaining synthetic porphyrin dimers which are able to undergo electron and/or energy transfer between the two subunits. The goal of all these efforts was generally to study simple models in order to better understand which parameters govern the efficiency of the natural so-called special pair for tunneling the energy to the final acceptor, the ubiquinone.

The geometry between the donor and acceptor moieties and their redox potentials have been proposed as two key factors which govern the dynamics of the photosynthetic processes [23]. In this paper we report the synthesis, electrochemical and spectroelectrochemical characterization of a series of porphyrin phenyl-linked heterodimers bearing bromine atoms on the β-positions of one but not both of the two meso-tetraphenylporphyrin subunits. The

*Correspondence to: Pietro Tagliatesta, email: pietro. tagliatesta@uniroma2.it, fax: +39 06-72594754 and Karl M. Kadish, email: kkadish@uh.edu, fax: +1 713-743-2740

Journal of Porphyrins and Phthalocyanines Published at http://www.u-bourgogne.fr/jpp/

J. Porphyrins Phthalocyanines 2003; 7: 595-609

Z. OU ET AL.596

investigated compounds are represented as M[tripp-tpp(Br4)]M and M[tripp-tpp(Br8)]M (M = 2H, Zn, Ni, Co and Cu) where tripp-tpp(Br4) is the tetra-anion of 1-[5-(10,15,20-triphenylporphyrinyl)]-4-[10-(2,3,12,13-tetrabromoporphyrinyl)]-benzene and tripp-tpp(Br8) is the tetraanion of 1-[5-(10,15,20-triphenylporphyrinyl)]-4-[10-(2,3,7,8,12,13,17,18-octabromoporphyrinyl)]-benzene. One of the com-pounds, H2[tripp-tpp(Br8)]H2, was also structurally characterized and, to our knowledge, this is the first phenyl-linked dimer whose structure has been obtained.

Structures of the investigated free-base porphyrin dimers are shown in Fig. 1 which also includes a representation of the porphyrin monomers for comparison.

EXPERIMENTAL

Chemicals

Silica gel 60 (70-230 mesh, Merck) and neutral alumina (70-230 mesh, grade I, Merck) were used for column chromatography. N-bromosuccinimide was purified by a literature method [24]. Elemental analyses were carried out at the Microanalytical Laboratory of the University of Padova-Italy.

Benzonitrile (PhCN) was purchased from Aldrich Chemical Co. and distilled over P2O5 under vacuum prior to use. Tetra-n-butylammonium perchlorate (TBAP) from Fluka Chemical Co., was recrystallized from ethyl alcohol and dried under vacuum at 40 °C

for at least one week prior to use. All other reagents and solvents were of analytical grade and were used as received.

Instrumentation

Cyclic voltammetry was carried out with an EG&G Model 173 Potentiostat or an IBM Model EC 225 Voltammetric Analyzer. Current-voltage curves were recorded on an EG&G Princeton Applied Research Model RE-0151 X-Y recorder. A three electrode system was used and consisted of a glassy carbon or platinum button working electrode, a platinum wire counter electrode and a saturated calomel reference electrode (SCE). The reference electrode was separated from the bulk of the solution by a fritted-glass bridge filled with the solvent / supporting electrolyte mixture. All potentials are referenced to the SCE. UV-visible spectroelectrochemical experiments were performed with an optically transparent platinum thin-layer electrode of the type described in the literature [25].

Potentials for oxidation or reduction of each compound were applied with an EG&G Model 173 potentiostat. Time-resolved spectra were recorded with a Hewlett Packard Model 8453 diode array spectrophotometer. Mass spectra were obtained on a VG-4 mass spectrometer using m-nitrobenzyl alcohol or 2-amino glycerol as the matrix. 1H NMR spectra were recorded in CDCl3 on a Bruker AM-400 spectrophotometer using TMS as internal standard. UV-vis spectra were obtained with a HP-8452A spectrophotometer.

Fig. 1. Molecular structure of monomers and dimers

Dimer 5 Dimer 11

H2tpp(Br4)H2tpp H2tpp(Br8)

Dimer 5 Dimer 11

H2tpp(Br4)H2tpp H2tpp(Br8)

PORPHYRIN DIMERS CONTAINING A β-BROMINATED SUBUNIT 597

Synthesis

2,3,12,13-tetrabromo-5-(4ʼ-acetyloxymethyl-phenyl)-10,15,20-triphenylporphyrin (2). 200 mg (0.29 mmol) of 5-(4ʼ-acetyloxymethylphenyl)-10,15,20-triphenylporphyrin, 1 [15] were dissolved in 50 ml of dry CHCl3 in a 100 ml flask. N-bromosuccinimide, 310 mg (1.88 mmol) was added and the resulting solution was refluxed for two hours in air, protected from the moisture by a CaCl2 valve. After cooling, a few drops of pyridine were added to the mixture which was washed with water and dried on anhydrous Na2SO4. After evaporation of the solvent, the residue was chromatographed on a silica gel column, eluting with CHCl3. The fraction containing the desired product was evaporated under vacuum and the residue recrystallized from CHCl3 / hexane, 1:3, affording 219 mg (0.22 mmol) of product. Yield 75%. UV-vis (CHCl3): λmax, nm 438, 536, 684. 1H NMR (400 MHz, CDCl3): δ, ppm 8.70 (s, 4H), 8.12-8.23 (m, 8H), 7.61-7.82 (m, 11H), 5.45 (s, 2H), 2.30 (s, 3H), -2.8 (s, 2H). FAB-MS: m/z 1000 [M-2H]+. Anal. calcd. for C47H30N4O2Br4: C, 56.32; H, 3.02; N, 5.59; Found: C, 56.55; H, 3.22; N, 5.66.

2,3,12,13-tetrabromo-5-(4ʼ-hydroxymethyl-phenyl)-10,15,20-triphenylporphyrin (3). 210 mg (0.21 mmol) of 2,3,12,13-tetrabromo-5-(4ʼ-acetyloxymethylphenyl)-10,15,20-triphenyl-porphyrin, 2 were dissolved, under nitrogen, in 50 ml of THF in a 100 ml flask. A solution of 201 mg (5.04 mmol) of NaOH, dissolved in a minimum amount of CH3OH, was added and the resulting mixture was stirred for two hours. Water, 200 ml, was added and the resulting solution was extracted with three 50 ml portions of CHCl3. The organic solution was washed with water and dried on anhydrous Na2SO4. After evaporation of the solvent, the residue was chromatographed on a silica gel column, eluting with CHCl3. The fraction containing the desired product was evaporated under vacuum and the residue recrystallized from CHCl3 / methanol, 1:3, affording 137 mg (0.14 mmol) of product. Yield 68%. UV-vis (CHCl3): λmax, nm 439, 535, 685. 1H NMR (400 MHz, CDCl3): δ, ppm 8.68 (s, 4H), 8.15-8.23 (m, 8H), 7.70-7.82 (m, 11H), 5.10 (s, 2H), -2.8 (s, 2H). FAB-MS: m/z 958 [M-2H]+. Anal. calcd. for C45H28N4OBr4: C, 56.28; H, 2.94; N, 5.83; Found: C, 56.48; H, 3.07; N, 5.77.

2,3,12,13-tetrabromo-5-(4ʼ-formylphenyl)-10,15,20-triphenylporphyrin (4). 1.200 g (1.25 mmol) of 2,3,12,13-tetrabromo-5-(4ʼ-hydroxy-methylphenyl)-10,15,20-triphenylporphyrin], 3 were dissolved in 50 ml of dry CH2Cl2 in a 100 ml flask. 160 mg (1.36 mmol) of 4-methylmorpholine N-oxide were added to the solution and, after its complete dissolution, tetrapropylammonium perruthenate was added in small portions. The reaction was continously

monitored by TLC plates and more catalyst added if necessary. After evaporation of the solvent, the residue was chromatographed on a silica gel column, eluting with CHCl3 / hexane, 1:1. The fraction containing the desired product was evaporated under vacuum and the residue recrystallized from CHCl3 / hexane, 1:3, affording 219 mg (0.22 mmol) of product. Yield 75%. UV-vis (CHCl3): λmax, nm 440, 536, 657. 1H NMR (400 MHz, CDCl3): δ, ppm 10.40 (s, 1H), 8.80 (s, 4H), 8.52 (dd, J = 8 Hz, 4H), 8.16-8.28 (m, 6H), 7.72-7.83 (m, 9H), -2.8 (s, 2H). FAB-MS: m/z 957 [M-H]+. Anal. calcd. for C42H26N4OBr4: C, 56.40; H, 2.73; N, 5.85; Found: C, 56.66; H, 2.88; N, 5.99.

H2[tripp-tpp(Br4)]H2 (5). Porphyrin aldehyde, 4 (0.4 g, 0.417 mmol) was dissolved in anhydrous dichloromethane (1000 ml). Pyrrole (0.35 g, 5.0 mmol) and benzaldehyde (0.47 g, 4.63 mmol) were added and the resulting solution was purged by bubbling argon for 15 minutes. Boron trifluoride etherate (0.3 ml) was added and the solution was kept 2 hours under continous bubbling. Chloranile (0.4 g 1.61 mmol) was added and the solution was refluxed 1 hour in air. Evaporation of the solvent, afforded a residue which was purified by silica gel column chromatography, eluting with CHCl3 / hexane, 1:1. The fraction containing the dimer was evaporated under vacuum and the residue recrystallized from CHCl3 / hexane affording the desired pure product (0.159 g, 26%). UV-vis (CHCl3): λmax, nm 419, 441, 518, 587, 686. 1H NMR (400 MHz, CDCl3): δ, ppm 9.35 (d, br, 2H), 9.27 (d, 1H), 9.01 (d, 2H), 8.90 (m, 4H), 8.74 (s, 4H), 8.62 (dd, 4H), 8.2-8.3 (m, 12H), 7.7-7.9 (m, 18H), -2.64 (s, 2H), -2.69 (s, 2H). FAB-MS: m/z 1467 [M]+. Anal. calcd. for C82H50N8Br4: C, 67.14; H, 3.44; N, 7.64; Found: C, 67.34; H, 3.23; N, 7.27.

[5-(4ʼ-acetyloxymethylphenyl)-10,15,20-triphenylporphyrin]copper(II) (6). 500 mg (0.73 mmol) of 5-[4ʼ-acetyloxymethylphenyl)-10,15,20-triphenylporphyrin], 1 [15] were dissolved in CHCl3 under nitrogen and 2 ml of saturated methanol solution of Cu(CH3COO)2 were added. The mixture was refluxed for three hours under nitrogen and, after cooling, washed with water and evaporated to dryness under vacuum. The residue was recrystallized from CHCl3 / methanol giving 490 mg of the product. Yield 90%. UV-vis (CHCl3): λmax, nm 430, 543. FAB-MS: m/z 748, [M]+. Anal. calcd. for C47H32N4O2Cu: C, 75.44; H, 4.31; N, 7.49; Found: C, 75.12; H, 4.46; N, 7.62.

[2,3,7,8,12,13,17,18-octabromo-5-(4ʼ-acetyloxy-methylphenyl)-10,15,20-triphenylporphyrin]-copper(II) (7). 250 mg (0.33 mmol) of Cu[5-(4ʼ-acetyloxymethylphenyl)-10,15,20-triphenyl-porphyrin], 6 were dissolved in 500 ml of a 50% mixture of CHCl3 in CCl4. Bromine (1.1 g, 6.87 mmol), dissolved in 50 ml of the same solvent mixture

Z. OU ET AL.598

reported above, was added after 2 hours. The mixture was left to react for three hours after which 0.3 g of pyridine (4.16 mmol) in 25 ml of CHCl3 was added in 1 hour and the resulting solution stirred overnight. 200 ml of a 20% Na2S2O4 aqueous solution was added to destroy the excess of bromine and after 1 hour, the resulting mixture was separated. The organic layer was washed with water, brine and dried on anhydrous Na2SO4. The solvent was evaporated and the residue chromatographed on neutral alumina eluting with chloroform. The fraction containing the desired product was evaporated and the residue recrystallized from CHCl3 / hexane to give 0.273 g (0.198 mmol). Yield 60%. UV-vis (CHCl3): λmax, nm 450 (sh), 466, 580. FAB-MS: m/z 1379, [M]+. Anal. calcd. for C47H24N4O2Br8Cu: C, 40.92; H, 1.75; N, 4.06; Found: C, 41.09; H, 1.88; N, 4.19.

[2,3,7,8,12,13,17,18-octabromo-5-(4ʼ-hydroxy-methylphenyl)-10,15,20-triphenylporphyrin]-copper(II) (8). 200 mg (0.145 mmol) of Cu[2,3,7,8,12,13,17,18-octabromo-5-(4ʼ-acetyl-oxymethylphenyl)-10,15,20-triphenylporphyrin], 7 were dissolved under nitrogen in 50 ml of THF in a 100 ml flask. A solution of 100 mg (2.32 mmol) of NaOH, dissolved in a minimum amount of CH3OH, was added and the resulting mixture was stirred for two hours. Water, 200 ml, was added and the resulting solution was extracted with three 50 ml portions of CHCl3. The organic solution was washed with water and dried on anhydrous Na2SO4. After evaporation of the solvent, the residue was chromatographed on a silica gel column, eluting with CHCl3. The fraction containing the desired product was evaporated under vacuum and the residue recrystallized from CHCl3 / methanol, 1:3, affording 174 mg (0.13 mmol) of product. Yield 90%. UV-vis (CHCl3): λmax, nm 448(sh), 468, 583. FAB-MS: m/z 1337 [M]+. Anal. calcd. for C45H22N4OBr8Cu: C, 40.41; H, 1.66; N, 4.19; Found: C, 40.66; H, 1.75; N, 4.23.

[2,3,7,8,12,13,17,18-octabromo-5-(4ʼ-formyl-phenyl)-10,15,20-triphenylporphyrin]-copper(II) (9). 200 mg (0.149 mmol) of Cu[2,3,7,8,12,13,17,18-octabromo-5-(4ʼ-hydroxy-methylphenyl)-10,15,20-triphenylporphyrin], 8 were dissolved in 50 ml of dry CH2Cl2 in a 100 ml flask. 21 mg (0.178 mmol) of 4-methylmorpholine N-oxide were added to the solution and, after its complete dissolution, tetrapropylammonium perruthenate was added in small portions. The reaction was continously checked by TLC plates and more catalyst added if necessary. After evaporation of the solvent, the residue was chromatographed on a silica gel column, eluting with CHCl3 / hexane, 1:1. The fraction containing the desired product was evaporated under vacuum and the residue recrystallized from CHCl3 / hexane, 1:3, affording 159 mg (0.12 mmol) of product. Yield 80%. UV-vis (CHCl3): λmax, nm 449(sh), 468, 580. FAB-

MS: m/z 1335 [M]+. Anal. calcd. for C45H20N4OBr8Cu: C, 40.47; H, 1.51; N, 4.20; Found: C, 40.40; H, 1.44; N, 4.32.

H2[tripp-tpp(Br8)]Cu (10). Porphyrin aldehyde 9, (0.2 g, 0.156 mmol) was dissolved in anhydrous dichloromethane (250 ml). Pyrrole (0.1 g, 0.160 mmol) and benzaldehyde (0.158 g, 1.58 mmol) were added and the resulting solution was purged by bubbling argon for 15 minutes. Boron trifluoride etherate (0.1 ml) was added and the solution was kept 2 hours under continous bubbling. Chloranile (0.155 g, 0.624 mmol) was added and the solution was refluxed 1 hour in air. Evaporation of the solvent, afforded a residue which was purified by silica gel column chromatography, eluting with CHCl3 / hexane, 1:1. The fraction containing the metalated dimer was evaporated under vacuum and the residue recrystallized from CHCl3 / methanol, affording 40 mg of the desired pure product. Yield 24%. UV-vis (CHCl3): λmax, nm 419, 451(sh), 469, 514, 586, 646. FAB-MS: m/z 1844 [M]+. Anal. calcd. for C82H44N4Br8Cu: C, 53.41; H, 2.41; N, 6.08; Found: C, 53.65; H, 2.30; N, 6.22.

H2[tripp-tpp(Br8)]H2 (11). H2[tripp-tpp(Br8)]Cu 10, was demetalated in a 1:2 conc. H2SO4/CF3COOH solution, under nitrogen for 2 hours. The resulting solution was diluted in ice-water and carefully neutralized with solid K2CO3. The solution was extracted with chloroform which was then dried on anhydrous Na2SO4 to give a residue which was recrystallized from CHCl3 / methanol, affording quantitatively the desired pure 11. UV-vis (CHCl3): λmax, nm 419, 471, 515, 556, 627, 646. 1H NMR (400 MHz, CDCl3): δ, ppm 9.35 (s, 1H), 8.95 (s, 1H), 8.70 (s, 4H), 8.75 (m, 6H), 8.22-8.33 (m, 12H), 7.76-7.90 (m, 18H), -2.52 (s, 2H), -2.72 (s, 2H). FAB-MS: m/z 1784 [M+H]+. Anal. calcd. for C82H46N8Br8: C, 55.25; H, 2.60; N, 6.29; Found: C, 55.59; H, 2.70; N, 6.06.

Zinc, cobalt, nickel and copper dimers

The metal dimers M[(tripp-tpp(Br4)]M and M[tripp-tpp(Br8)]M, where M = Zn, Ni, Co or Cu, were obtained by standard procedure [26] in 60-80% yield. Their analytical data are reported below.

Zn[tripp-tpp(Br4)]Zn (12). UV-vis (CHCl3): λmax, nm 422, 439, 566. 1H NMR (400 MHz, CDCl3): δ, ppm 9.52 (d, 1H), 9.22 (d, 1H), 9.12 (m, 2H), 9.01 (m, 6H), 8.80 (s, 2H) 8.1-8.4 (m, 12H), 7.7-8.0 (m, 18H). MS (FAB/NBA): m/z 1594 [M]+. Anal. calcd. for C82H46N8Br4Zn2: C, 61.80; H, 2.91; N, 7.03; Found: C, 61.68; H, 3.02; N, 7.22.

Zn[tripp-tpp(Br8)]Zn (13). UV-vis (CHCl3): λmax, nm 419, 468, 549, 591. 1H NMR (CDCl3): δ, ppm 9.44 (d, 2H), 9.12 (d, 2H), 8.99 (q, 4H), 8.63 (d, 2H), 8.53 (d, 2H), 8.26 (m, 12H), 7.81 (m, 18H); MS (FAB/NBA): m/z 1909 [M]+. Anal. calcd. for

C82H42N8Br8Zn2: C, 51.59; H, 2.22; N, 5.87; Found: C, 51.70; H, 2.42; N, 5.68.

Co[tripp-tpp(Br4)]Co (14). UV-vis (CHCl3): λmax, nm 410, 430, 532. MS (FAB/NBA): m/z 1578 [M-2H]+. Anal. calcd. for C82H46N8Br4Co2: C, 62.30; H, 2.93; N, 7.09; Found: C, 62.33; H, 2.87; N, 7.15.

Co[tripp-tpp(Br8)]Co (15). UV-vis (CHCl3): λmax, nm 412, 449, 529, 558. MS (FAB/NBA): m/z 1908 [M-H]+. Anal. calcd. for C82H42N8Br8Co2: C, 51.94; H, 2.23; N, 5.91; Found: C, 52.08; H, 2.16, N, 6.12.

Ni[tripp-tpp(Br4)]Ni (16). UV-vis (CHCl3): λmax, nm 417, 434, 533; 585. MS (FAB/NBA): m/z 1578 [M-2H]+. 1H NMR (CDCl3): δ, ppm 9.11 (d, 2H), 8.97 (d, 1H), 8.89 (d, 2H), 8.81 (s, 4H), 8.71 (d, 1H), 8.59 (s, 2H), 8.35 (d, 2H), 8.8.21 (d, 2H), 8.08 (m, 6H), 7.92 (m, 6H), 7.74 (m, 18H). Anal. calcd. for C82H46N8Br4Ni2: C, 62.32; H, 2.93; N, 7.09; Found: C; 62.40, H, 2.90, N, 7.22.

Ni[tripp-tpp(Br8)]Ni (17). UV-vis (CHCl3): λmax, nm 415, 452, 520, 557. MS (FAB/NBA): m/z 1896 [M+H]+. 1H NMR (CDCl3): δ, ppm 9.06 (d, 2H), 8.86 (d 2H), 8.76 (s, 4H), 8.34 (d, 2H), 8.24 (d, 2H), 8.03 (m, 6H), 7.94 (m, 6H), 7.68 (m, 18H). Anal. calcd. for C82H42N8Br8Ni2: C, 51.95; H, 2.23; N, 5.91; Found: C, 51.72; H, 2.12; N, 5.86.

Cu[tripp-tpp(Br4)]Cu (18). UV-vis (CHCl3): λmax, nm 414, 430, 541. MS (FAB/NBA): m/z 1588, [M-2H]+. Anal. calcd. for C82H46N8Br4Cu2: C, 61.94; H, 2.92; N, 7.05; Found: C, 62.01, H, 3.02, N, 7. 08.

Cu[tripp-tpp(Br8)]Cu (19). UV-vis (CHCl3): λmax, nm 414, 470, 539, 580. MS (FAB/NBA): m/z 1906 [M]+. Anal. calcd. for C82H42N8Br8Cu2: C, 51.68; H, 2.22; N, 5.88; Found: C, 51.78; H, 2.19; N, 5.94.

X-ray data collection, structure solution and refinement

Black crystals of H2[tripp-tpp(Br8)]H2 suitable for X-ray analysis were obtained by slow diffusion of hexane (containing 2-methyl-pentane) into a concentrated solution of the free-base dimer in methylene chloride. After selection and mounting of the crystals as described by Hope [27], diffraction data were collected at 92 K on a Bruker SMART 1000 system. Mo-Kα (0.71073 Å) radiation was used and the data were corrected for absorption, extinction [28]. The structure was solved with direct methods using the SHELXS-97 [29] program and refined against |F2| using the program SHELXL-97 [30]. Except for a 2-methyl-pentane molecule of solvation, all nonhydrogen atoms were refined with anisotropic thermal parameters. Hydrogen atoms were placed into geometrically calculated positions and refined using a standard riding model. Selected crystallographic data are summarized in Table 1 while

selected conformational and structural parameters are compiled in Table 2. Final details for the structure determination of H2[tripp-tpp(Br8)]H2 have been deposited with the Cambridge Crystallographic Data Center, deposition number CCDC 211116.

RESULTS AND DISCUSSION

Synthesis

The goal of the synthetic work was to link two electronically different monomers through a spacer group having the conformational rigidity necessary for our purposes. The starting porphyrin 1 [15] was easily converted to the tetrabrominated compound 2 in 75% yield using NBS (6 equivalents) in boiling chloroform. After silica gel column purification, the ester group was hydrolized in alkaline media giving quantitatively the benzylic derivative 3 which was subjected to 4-methylmorpholine N-oxide / perruthenate oxidation to give the final aldehydic synthon 4 in 75% yield as shown in Scheme 1. Compound 4 was then converted to the tetrabrominated dimeric compound, 5, in 26%

Table 1. Selected crystallographic data for H2[tripp-tpp(Br8)]H2

chemical formula C82H46N8Br8·1.5C6H14·C6H12

formula weight 2018.01

cryst color, habit black, parallelepiped

T, K 92(2)

cryst. system triclinic

space group P-1

a, Å 13.9373(9)

b, Å 18.5359(11)

c, Å 18.5716(12)

α, deg 112.795(1)

β, deg 93.500(1)

γ, deg 107.835(1)

V, Å3 4124.2(4)

λ, Å 0.71073

dcalcd, g.cm-1 1.625

μ, mm-1 3.945

R1a 0.0496

Rw2b 0.1302

a R1 = Σ||Fo| - |Fc||/Σ|Fo|. b RW2 = [Σ[w(Fo2 - Fc

2)2] / Σw(Fo

2)2]]1/2.

Z. OU ET AL.600

yield using the method of Lindsey and Wagner [31] as shown in Scheme 2. The 1H NMR pattern and the presence of a strong m/z peak at 1379 are in agreement with the formulation and confirm the proposed dimeric structure.

Furthermore, we decided to synthesize the copper derivative 6 of the porphyrin 1 as a starting material for obtaining the octabrominated heterodimer 11 because the final cyclization step failed using the corresponding aldehyde free base. Compound 6 was halogenated using excess liquid bromine in a 1:1 mixture of CHCl3/CCl4 at room temperature as reported by Bhyrappa and Krishnan [32]. After silica gel column chromatography, compound 7 was obtained in 60% yield. Compounds 8 and 9 were obtained in 90 and 80% yields, respectively, (Scheme 3) following procedures previously reported

for compounds 4 and 5. It should be emphasized the great importance of compounds 4, 5, 8 and 9 which, to our knowledge, have not so far been reported in the literature. In fact, such compounds can be used as building blocks for further derivatizations of the β-brominated porphyrins.

The heterodimer 10 was successfully obtained by the Lindsey method [31] in 24% yield after silica gel column chromatography, after which the compound was quantitatively demetalated in a TFA/H2SO4 1:1 mixture to give the free-base porphyrin dimer 11 as shown in Scheme 4. Again, the mass spectrum (m/z 1784; [M]+) and the 1H NMR spectrum are both in agreement with the proposed structure.

It is interesting to note that the Adler method [33], which consists in cyclization of pyrrole and an aldehyde to give a porphyrin in boiling propionic

Table 2. Selected averaged bond lengths and angles and conformational parameters for H2[tripp-tpp(Br8)]H2, H2[tppBr8] and H2(tpp)f

H2[tripp-tpp(Br8)]H2H2[tpp(Br8)] H2(tpp)

[-tpp(Br8 )]H2 half H2(tripp-) half

Bond lengths, Å

N-Ca 1.371(5) 1.372(5) 1.362 1.372

Ca-Cb 1.450(5) 1.446(5) 1.435 1.442

Ca-Cm 1.409(5) 1.404(5) 1.414 1.400

Cb-Cb 1.363(5) 1.360(5) 1.348 1.351

Bond angles, deg

N-Ca-Cb 107.3(3) 108.8(3) 107.5 108.8

N-Ca-Cm 124.1(3) 126.3(3) 123.1 126.2

Ca-N-Ca 109.8(3) 107.9(3) 109.4 107.7

Ca-Cm-Ca 123.0(3) 125.0(3) 120.9 125.6

Ca-Cb-Cb 107.7(3) 107.3(3) 107.7 107.5

Cm-Ca-Cb 128.4(3) 124.8(3) 129.4 124.9

Conformational parameters

core size, Åa 2.067 2.062 2.091 2.06

Δ24, Åb 0.524 0.09 0.616 0.05

δCm, Åc 0.14 0.08 0.32 0.14

δCa, Åc 0.385 0.09 0.41 0.12

δCb, Åc 1.068 0.19 1.263 0.23

ϕpyr, degd 28.65 4.95 39.1 7.5

ϕaryl, dege 44.4 65.9 29.6 60.0

a Average vector length from the geometric center of the four nitrogen atoms to the nitrogen atoms. b Average deviation of the 24 macrocycle atoms from their least-squares plane. c Average deviation of the relevant pyrrole positions from the 4N-plane. d Average tilt angle of the individual pyrrole rings against the 4N-plane. e Average tilt angle of the meso phenyl rings against the 4N-plane. f Triclinic modification.

acid, always afforded the debrominated dimers when applied to the synthesis of 5 and 10. This undesired dehalogenation reaction was also found in some cases to occur during electrochemical reduction of the β-halogenated porphyrins (as determined also by FAB-mass spectrum studies of the β-bromo meso-tetraphenylporphyrins) and is attributed to a relief from steric hindrance which occurs between the β-pyrrole substituents, e.g. the bromine atoms and the phenyls in the meso positions [34-35].

Electronic absorption spectra

The UV-vis spectra of 5 and 11 appear to be a superposition of absorption bands for the two

individual macrocycles. For example, compound 5 in a 99:1 CHCl3 / pyridine mixture, shows porphyrin Soret bands which are characterized by two maxima centered at 419 and 440 nm. These absorptions are slightly shifted from Soret bands of the monomers and this situation parallels what has previously been reported for similar compounds [12-13].

In benzonitrile, the difference between the peak maximum of the Soret bands is less pronounced for the metal derivatives and depends upon the specific metal ion, with a Δλmax ranging from 17 nm for zinc, copper and nickel to 20 nm in the case of the cobalt derivative (see Table 3). Compound 11 shows a similar situation with two well-separated Soret bands at 419 and 471 nm, both of which are slightly shifted

Scheme 1.

NBS/CHCl3

KOH/MeOH,THF

4-methylmorpholine N-oxide

[CH3(CH2)3]4NRuO4/CH2Cl2

CH2OCOCH3

Scheme 2.

N HNHN

PhCHO/Pyrrole/CH2Cl2

BF3. Et2O/Chloranile

Z. OU ET AL.602

Scheme 3.

Br2, Py/CHCl3/CCl4

MeOH/THF

4-methylmorpholine N-oxide

[CH3(CH2)3]4NRuO4/CH2Cl2

CH2OCOCH3

Scheme 4.

PhCHO/Pyrrole/CH2Cl2

BF3. Et2O/Chloranile

N HNHN

TFA/H2SO4

Br BrN

from bands obtained for the individual monomeric macrocycles under similar experimental conditions.

Crystal structure of H2[tripp-tpp(Br8)]H2

A view of the molecular structure of H2[tripp-tpp(Br8)]H2 is shown in Fig. 2. The two porphyrin macrocycles are tilted slightly against each other by 16.5°. The molecule exhibits structural features typical for isolated octabromo-tetraarylporphyrins and tetraarylporphyrins, respectively (Table 1). The tetraphenylporphyrin part of the compound shows structural and conformational parameters very similar to the triclinic modification of free-base 5,10,15,20-tetraphenylporphyrin [36]. The overall structural parameters observed in the octabromo-tetraphenylporphyrin part of the molecule are also close to those found for the related free-base 2,3,7,8,12,13,17,18-octabromo-5,10,15,20-tetraphenylporphyrin [37]. The conformation of this macrocycle is characterized by a major contribution of sad distortion (large Cb displacements) and a

minor ruf contribution (Cm displacements) [38]. The observed differences in conformational parameters, bond lengths and angles between the dodecasubstituted part and the tetra-meso-substituted part are in full agreement with other highly substituted porphyrins [39]. The overall degree of macrocycle distortion in the octabromo part of the molecule (Δ24 = 0.524 Å) is somewhat smaller than that found in 2,3,7,8,12,13,17,18-octabromo-5,10,15,20-tetraphenylporphyrin (Δ24 = 0.616 Å). In addition, the conformation of the octabromo-macrocycle is somewhat asymmetric (Fig. 3). While the average degree of the displacement for all β-pyrrole positions from the 4N-plane is 1.07 Å, this value is significantly larger for the two Cb atoms (1.35 Å) located in the pyrrole ring containing N23. The packing of the molecules is a typical layer structure with the octabromoporphyrin units lying on top of each other. The only short intermolecular contact observed is N23-Br1 with a separation of 3.264 Å. The Br1 atom lies on top of the cavity of a neighboring octabromoporphyrin unit and this might account for

Table 3. UV-visible data of neutral complexes in PhCN

Metal Macrocycleλmax, nm (ε × 10-4, M-1.cm-1)

Soret bands Visible bands

Zn(II) tpp(Br4) 439 (19.8) 569 (1.0) 613 (0.6)

tripp-tpp(Br4) (12) 432 (35.8) 442 (19.6) 567 (1.6) 605 (0.6)

tpp 427 (36.7) 556 (1.7) 597 (0.7)

tripp-tpp(Br8) (13) 430 (37.2) 479 (19.9) 562 (1.8) 605 (0.7) 678 ( )

tpp(Br8) 477 (19.7) 622 (1.0) 685 (1.6)

Co(II) tpp(Br4) 434 (13.0) 552 (1.4)

tripp-tpp(Br4) (14) 419 (18.8) 434 (17.4) 533 (2.3)

tpp 416 (18.8) 531 (1.2)

tripp-tpp(Br8) (15) 418 (14.5) 458 (14.6) 533 (2.0) 560 (1.8)

tpp(Br8) 457 (13.3) 566 (1.7)

Ni(II) tpp(Br4) 433 (10.3) 546 (1.4) 587 (1.1)

tripp-tpp(Br4) (16) 419 (14.0) 437 (16.6) 533 (1.6) 592 (0.5)

tpp 417 (19.8) 529 (1.5)

tripp-tpp(Br8) (17) 418 (13.1) 455 (18.8) 529 (1.9) 564 (1.8)

tpp(Br8) 454 (7.9) 563 (0.9)

Cu(II) tpp(Br4) 428 (27.2) 552 (1.3) 593 (0.3)

tripp-tpp(Br4) (18) 420 (30.2) 438 (18.6) 542 (3.0)

tpp 419 (39.4) 541 (2.1)

tripp-tpp(Br8) (19) 420 (26.1) 474 (14.9) 542 (2.8) 584 (2.4)

tpp(Br8) 472 (12.6) 581 (2.0)

Z. OU ET AL.604

the observed asymmetric macrocycle distortion. The compound crystallizes with two molecules of hexane and a molecule of methylcyclopentane of solvation which are located in the void between neighboring porphyrin layers.

Electrochemistry of M[tripp-tpp(Brx)]M, where M = Co, Ni, Cu or Zn and x = 4 or 8

The electrochemistry of M[tripp-tpp(Brx)]M was carried out in PhCN containing 0.1 M TBAP and half-wave potentials of the eight metalloporphyrin dimers are given in Table 4. As a comparison, the redox potentials for monomers of M[tpp(Brx)] (x = 0, 4 or 8) containing the same central metal ion are also included in the table.

Electrochemistry of Co[tripp-tpp(Br4)]Co and Co[tripp-tpp(Br8)]Co

Cyclic voltammograms of both halogenated cobalt dyads, Co[tripp-tpp(Br4)]Co and Co[tripp-tpp(Br8)]Co, as well as the corresponding monomers, are shown in Figs 4 and 5. Both cobalt dimers undergo five oxidations and two reductions between 1.8 and -1.5 V vs SCE. The reductions occur at E1/2 = -0.58 and -0.85 V for Co[tripp-tpp(Br4)]Co and at -0.33 and -0.84 V for Co[tripp-tpp(Br8)]Co. The first reduction of Co[tripp-tpp(Br4)]Co is close to the measured E1/2 value of -0.61 V for the first reduction of Co[tpp(Br4)] while Co[tripp-tpp(Br8)]Co is reduced at an E1/2 close to the E1/2 = -0.32 V for the

first reduction of Co[tpp(Br8)]. Also, the E1/2 values for the second reduction of Co[tripp-tpp(Br4)]Co and Co[tripp-tpp(Br8)]Co are similar to the first reduction of Co(tpp) (see Figs 4 and 5). These data indicate that the first reduction of Co[tripp-tpp(Br4)]Co and Co[tripp-tpp(Br8)]Co involves the [-tpp(Br4)]Co or [-tpp(Br8)]Co parts of the dimer while the second reduction involves the Co(tripp-) part. The oxidations of Co[tripp-tpp(Br4)]Co and Co[tripp-tpp(Br8)]Co also occur via stepwise electron transfer pro-

Fig. 2. Views of the molecular structure of the dimer H2[tripp-tpp(Br8)]H2 in the crystal

Fig. 3. Linear display of the skeletal deviations of the octabromo-macrocycle H2[tripp-tpp(Br8)]H2 from the 4N-plane

Fig. 4. Cyclic voltammograms of Co[tripp-tpp(Br4)]Co with corresponding monomers in PhCN, 0.1 M TBAP

Fig. 5. Cyclic voltammograms of Co[tripp-tpp(Br8)]Co with corresponding monomers in PhCN, 0.1 M TBAP

cesses involving alternatively the [-tpp(Br4)]Co or [-tpp(Br8)]Co and Co(tripp-) parts of the dimer.

The sites of the electron transfer in the two reductions and the first two oxidations of Co[tripp-tpp(Br4)]Co and Co[tripp-tpp(Br8)]Co were confirmed by thin-layer UV-visible spectroelectrochemistry. As an example, the spectral changes obtained during controlled-potential electrolysis of Co[tripp-tpp(Br4)]Co are shown in Fig. 6a (oxidation) and 6b (reduction).

The spectrum of neutral Co[tripp-tpp(Br4)]Co in PhCN containing 0.2 M TBAP is characterized by two Soret bands at 419 and 432 nm, one visible band at 533 nm and a shoulder at about 550 nm. The spectrum of neutral Co(tpp) has bands at 416 and 531 nm while Co[tpp(Br4)] has bands at 434 and 552 nm under the same solution conditions (see Table 3). The spectrum of Co[tripp-tpp(Br4)]Co thus closely

resembles the superimposed spectra of Co(tpp) and Co[tpp(Br4)] under the same solution conditions. The absorption bands of Co[tripp-tpp(Br4)]Co at 419 and 533 nm are similar to the those of Co(tpp) at 416 and 531 nm and can thus be assigned to the Co(tripp-) part of the dimer. At the same time, the bands of the dimer at 432 and 550 nm correspond to the spectral features of Co[tpp(Br4)] at 434 and 552 nm and can be assigned to the [-tpp(Br4)]Co part of the dimer.

The UV-visible spectral changes of Co[tripp-tpp(Br4)]Co obtained upon the first and second oxidations are shown in Fig. 6a. The bands at 419 and 533 nm, assigned to the Co(tripp-) part of the dimer disappear upon the first oxidation while the Soret band at 432 nm shifts to 441 nm and a new band appears at 585 nm. Similar spectral features are seen upon the formation of Co(III) complexes during oxidation of Co(II) porphyrins under the same

Table 4. Half-wave potentials (V vs SCE) of M[tripp-tpp(Brx)]M, where M = Zn, Co, Ni or Cu and x = 4 or 8, in PhCN containing 0.1 M TBAP

Metal MacrocycleOxidation Reduction

5th 4th 3rd 2nd 1st 1st 2nd 3rd 4th

Zn tpp(Br4) 1.19 0.95 -1.03 -1.32

tripp-tpp(Br4) (12) 1.21 0.95 0.81 -1.05 -1.31a -1.45a -1.90a

tpp 1.14 0.82 -1.32 -1.74

tripp-tpp(Br8) (13) 1.25 1.00 0.82 -0.83 -1.15 -1.43 -1.70

tpp(Br8) 1.15 0.96 -0.82 -1.15

Co tpp(Br4) 1.39 1.29 0.68a -0.61 -1.66a -1.80a

tripp-tpp(Br4) (14) 1.40 1.29 1.21 0.70a 0.56a -0.58 -0.85 -1.68a -1.82a

tpp 1.38 1.20 0.56a -0.84

tripp-tpp(Br8) (15) 1.43 1.29 1.16 0.74 0.55a -0.33 -0.84 -1.60a -1.76a

tpp(Br8) 1.41 1.29 0.74 -0.32 -1.60a -1.78a

Ni tpp(Br4) 1.17b -0.96 -1.27

tripp-tpp(Br4) (16) 1.15c -0.95 -1.22 -1.36 -1.94a

tpp 1.15 1.08 -1.28 -1.81

tripp-tpp(Br8) (17) 1.32 1.14d -0.74 -1.11 -1.29 -1.92a

tpp(Br8) 1.30 1.08 -0.96 -1.26

Cu tpp(Br4) 1.41 1.08 -0.96 -1.26

tripp-tpp(Br4) (18) 1.44 1.35 1.06 -0.94 -1.24 -1.32 -1.85a

tpp 1.31 1.02 -1.28 -1.73

tripp-tpp(Br8) (19) 1.56 1.36 1.06 -0.71 -1.06 -1.29 -1.85a

tpp(Br8) 1.52 1.01 -0.75 -1.10

a Peak potentials at a scan rate of 0.1 V/s. b Two electron transfer process. c Four electron transfer process. d Three electron transfer process.

Z. OU ET AL.606

experimental conditions [34]. The results therefore indicate that the abstraction of one-electron from the Co(tripp-) subunit leads to formation of a species formulated as [CoIII[tripp-tpp(Br4)]CoII]+. Upon the second oxidation, the band at 441 nm decreases in intensity and a new band at 454 nm appears. These results are consistent with the abstraction of one electron from the [-tpp(Br4)]Co part of the dimer with a subsequent formation of [CoIII[tripp-tpp(Br4)]CoIII]2+. The UV-visible spectral changes obtained upon the two reductions of Co[tripp-tpp(Br4)]Co] are shown in Fig. 6b. These results suggest that the addition of one electron to the [-tpp(Br4)]Co part of the dimer leads to the formation of a [CoI[tripp-tpp(Br4)]CoII]- species. Upon addition of a second electron to [CoI[tripp-tpp(Br4)]CoII]-, the band at 419 nm decreases in intensity and a new band appears at 376 nm. This is similar to what has been reported for Co[tpp(Brx)] (x = 0, 6-8) which undergoes a metal-centered reduction and forms a Co(I) complex [34]. Thus, the second reduction of the dyad which occurs on the Co(tripp-) part of the molecule leads to formation of a complex formulated as [CoI[tripp-tpp(Br4)]CoI]2-.

Pyridine binding reactions of Co[tripp-tpp(Brx)]Co (x = 4 or 8)

Co(II) porphyrins can axially bind pyridine molecules to form five- or six-coordinated complexes [40]. Pyridine binding reactions of the bis-cobalt porphyrins were investigated by electrochemical titration methods which involves recording cyclic voltammograms in PhCN solutions containing different pyridine concentrations and then plotting of E1/2 values for reduction versus the pyridine concentrations, log[py]. The plots of E1/2 for the first and the second reduction vs log[py] are given in Fig. 7 and are linear with straight line slopes of -53 mV. These values can be compared to a theoretical slope of -59 mV per log[py] for the case where each Co(II) center binds one py molecule but dissociates after the electroreduction to give the Co(I) porphyrin which does not axially bind py. Based on the data in Fig. 7, the py binding constants (logK) for each neutral Co(II) center of the neutral dimers could be calculated. The values of logK for both cobalt dimers are listed in Table 5. For comparison, the py binding constants of the corresponding cobalt monomers were also measured under the same solution conditions and are given in this Table. Only slight differences in logK can be

Fig. 6. UV-vis spectral changes of Co[tripp-tpp(Br4)]Co (a) obtained upon the first and second oxidation and (b) upon the first and second reduction in PhCN containing 0.2 M TBAP

seen between the individual units of the dimer and the related monomer.

Electrochemistry of Ni[tripp-tpp(Br4)]Ni and Ni[tripp-tpp(Br8)]Ni

Ni[tripp-tpp(Br4)]Ni exhibits one four-electron oxidation and three one-electron reductions in PhCN containing 0.1 M TBAP. The potential of the first reduction is located at E1/2 = 0.95 V which is similar to the half-wave potential for reduction of the corresponding monomers Ni[tpp(Br4)] (see Fig. 8 and Table 4). The second and the third reductions of Ni[tripp-tpp(Br4)]Ni are located at -1.22 and -1.36 V and lead to formation of [Ni[tripp-tpp(Br4)]Ni]2- and [Ni[tripp-tpp(Br4)]Ni]3-, respectively. As seen in Fig. 8, the peak current, ipa for the first oxidation of Ni[tpp(Br4)] is twice as high as ipc for the first reduction which indicates that the first oxidation involves two electrons under the given experimental conditions.

The ratio of peak currents for the first oxidation and first reduction of Ni[tripp-tpp(Br4)]Ni, ipa/ipc, is close to 4 which suggests that the first oxidation at E1/2 = 1.15 V involves an overall four-electron transfer process in PhCN containing 0.1 M TBAP.

The spectrum of neutral Ni[tripp-tpp(Br4)]Ni has two Soret bands at 419 and 437 nm. Based on a comparison of the spectra for neutral Ni(tpp) and Ni[tpp(Br4)] the 419 nm band might be assigned to the tpp part of the molecule and the 437 nm band to the [-tpp(Br4)]Ni part of the nickel dimer (see Table 3). Thin-layer UV-visible spectral changes obtained during the first oxidation of Ni[tripp-tpp(Br4)]Ni are shown in Fig. 9. As seen in the figure, two sets of spectral changes are observed, each of which has an independent set of isobestic points upon the first oxidation. Therefore, the overall four-electron process can be divided into two oxidation steps. In the first step, the Soret band at 419 nm almost disappears while the 437 nm band decreases in intensity. Two isosbestic points can be seen at 402 and 459 nm. In the second oxidation step, the Soret band at 437 nm decreases further in intensity while a new set of isosbestic points at 397 and 469 nm is observed. The site of the individual electron transfers could be ascertained on the basis of the spectroelectrochemical or cyclic voltammetric data but the ratio of peak current for the first oxidation (ipa) over the first reduction (ipc) = 3.9 which is consistent with four one-electron transfer processes, all of which are overlapped in potentials. The oxidation occurs in two steps (see Fig. 9), each of which is proposed to involve two overlapping electron transfers and this would imply the formation

Fig. 7. Plots of E1/2 vs log[py] for Co[tripp-tpp(Br8)]Co

Table 5. Pyridine binding constants of Co[tpp(Brx)], where x = 0, 4 or 8, and bis-cobalt complexes obtained by electrochemical titration in PhCN containing 0.1 M TBAP

Compoundlog K

tripp unit tpp(Brx) unit

Co(tpp) 2.94 --

Co[tpp(Br4)] -- 3.02

Co[tpp(Br8)] -- 2.93

Co[tripp-tpp(Br4)]Co 2.72 3.04

Co[(tripp-tpp(Br8)]Co 2.75 2.77

Fig. 8. Cyclic voltammograms of Ni[tripp-tpp(Br4)]Ni and corresponding monomers in PhCN, 0.1 M TBAP

Z. OU ET AL.608

of two linked π-cation radicals followed by two linked dications, ie, [Ni[tripp-tpp(Br8)]Ni]2+ and [Ni[tripp-tpp(Br8)]Ni]4+, respectively.

Ni[tripp-tpp(Br8)]Ni undergoes four one-electron reductions which are located at E1/2 = -0.74, -1.11, -1.29 and -1.92 V. Two reversible oxidations are seen at E1/2 = 1.14 and 1.32 V. The first oxidation of this dimer indicates a three-electron transfer process due to an overlap of the first oxidation of [-tpp(Br8)]Ni part and the first and the second oxidation of Ni(tripp-) part of the dimer, thus leading to formation of the [Ni[tripp-tpp(Br8)]Ni]3+ complex. The potential for the second oxidation (1.32 V) of the dimer is similar to E1/2 for the second oxidation for the Ni[tpp(Br8)] monomer (1.30 V). Thus, the second oxidation of the dimer appears to be [-tpp(Br8)]Ni centered and leads to the formation of [Ni[tripp-tpp(Br8)]Ni]4+.

Electrochemistry of M[tripp-tpp(Brx)]M, where M = Cu or Zn and x = 4 or 8

Cu[tripp-tpp(Br4)]Cu and Cu[tripp-tpp(Br8)]Cu both undergo three oxidations and four reductions in PhCN containing 0.1 M TBAP (see Table 4). On the basis of the electrochemical behavior, the reactions are proposed to involve alternating oxidations of the tpp and tpp(Brx) subunits. The first oxidation of the dimers is a two-electron transfer process which arises from an overlap of two one-electron transfer oxidations involving the tpp or tpp(Brx) subunits. The

second oxidation is assigned as the first oxidation of the tripp unit while the third is assigned as the second oxidation of the [-tpp(Brx)]Cu part of the dimer. The reduction potentials of both dimers are much closer to those of their corresponding monomeric units (see Table 4). The first two electrode reactions of Cu[tripp-tpp(Brx)]Cu are thus assigned as the first and the second reductions of the [-tpp(Brx)]Cu unit. The last two electroreductions of the dimers occur at the Cu(tripp-) unit.

Four one-electron reductions and three oxidations can be observed for both Zn[tripp-tpp(Br4)]Zn and Zn[tripp-tpp(Br8)]Zn in PhCN containing 0.1 M TBAP. The redox potentials of both dyads are close to values of the corresponding monomers (see Table 4) and this indicates that no strong interaction exists between the two linked porphyrin macrocycles. The reductions and oxidations of both dyads must occur on the porphyrin ring due to the non-electroactive zinc ion and this leads to the formation of [Zn[tripp-tpp(Brx)]Zn]n- (n = 1-4) and [Zn[tripp-tpp(Brx)]Zn]n+ (n = 1-3), respectively.

Acknowledgements

The support of the Robert A. Welch Foundation (Grant E-680, K.M.K.) is gratefully acknowledged. The Italian CNR is also acknowledged for a travel grant. M.O.S. acknowledges financial support from the Deutsche Forschungsgemeinschaft (Se543/6-2) and the Fonds der Chemischen Industrie. We are indebted to the UC Davis crystallographic facility (Dr. M. M. Olmstead, director) for their cooperation and use of facilities. Finally we wish also to thank Dr. Ning Guo for help with the electrochemical measurements and Mrs. Alessandro Leoni and Giuseppe D ̓Arcangelo for their valuable technical assistance.

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Synthesis and electrochemical investigation of covalently linked porphyrin dimers containing a...

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