Journal of Porphyrins and Phthalocyanines

Charge-transfer interactions of octaethylporphycenatozinc(II), (OEPc)Zn with 2,6-dichloro-3,5-dicyano-1,4-benzoquinone, DDQ, in non-aqueous solvents are reported. Both optical absorption and cyclic voltammetry studies reveal the formation of stable charge-transfer complexes between (OEPc)Zn and DDQ. New redox couples corresponding to reduction of the charge-transfer complex have been electrochemically detected. The formation of charge-transfer complexes between (OEPc)Zn and doubly reduced DDQ is examined and the present electrochemical studies reveal the possible existence of such complexes in solution. Based on semiempirical AM1 and PM3 calculations, interaction through the zinc(II) metal center of (OEPc)Zn and cyanide nitrogen of the quinone is proposed.

The synthesis and characterization of three monomeric aluminum porphycenes with anionic or σ-bonded axial ligands is reported. The investigated compounds are represented as (EtioPc)Al(CH³) and (EtioPc)AlX where EtioPc represents the dianion of etioporphycene and X = Cl⁻ or OH⁻. Each synthesized complex was characterized by mass spectrometry. ¹H NMR, IR and UV-visible spectroscopies as well as by electrochemistry. Comparisons are made between the properties of complexes in the aluminum etioporphycene series and related chloro- or methyl σ-bonded Al(III) porphyrins containing octaethylporphyrin (OEP) or tetraphenylporphyrin (TPP) macrocycles. Comparisons are also made between the currently investigated compounds and a previously reported Al(III) μ-oxo dimer, [(EtioPc)Al]₂O. In addition, the crystal and molecular structure of (EtioPc)Al(CH₃) was determined by X-ray diffraction. The molecular structure of this methyl-σ-bonded aluminum etioporphycene provides the first structural data for an aluminum porphycene compound. The aluminum(III) atom in (EtioPc)Al(CH₃) is pentacoordinated and is located 0.54 Å from the plane of the four N-nitrogens.

The solid state structure of dilithium tetraphenylporphyrin bis(etherate) has been determined by X-ray crystallography. The structure shows the lithium atoms are displaced out of the plane of the porphyrin, each coordinated in a square pyramidal fashion to the four nitrogen atoms in addition to a molecule of diethyl ether. The lithium atoms are identical, as required by the crystallographic inversion centre. X-ray data are as follows: (# 2) with a = 10.1908(4) Å, b = 11.6553(4) Å, c = 12.5199(5) Å, α = 111.644(1)°, β = 98.029(1)°, γ = 111.883(1)°, V = 1214.61(9)ų, d_calc = 1.171 g cm^-3 and Z = 1.

The electron-transfer oxidation-reduction chemistry for the alkyl derivatives of iron and cobalt porphyrins [(por)M^III−R] has been characterized on the basis of cyclic voltammetric and controlled-potential-electrolysis measurements. The electrogenerated anions of iron and cobalt porphyrins [(por)M⁻ and (por^−·)M⁻] are strong nucleophiles that react with alkyl halides (RX) via a nucleophilic displacement process to form metal-carbon bonds [(por)M-R and (por⁻·)M-R]. The difference in the reduction potentials for RX and (por)M^II provides an approximate measure of the (por)M-R bond-formation free energy (−ΔG_BF). The −ΔG_BF values for iron porphyrins (14–35 kcal mol⁻¹) and for cobalt porphyrins (20-38 kcal mol⁻¹) depend on the electron density of the porphyrin ring (OEP > TPP > Cl₈TPP > F₂₀TPP) and the structure of the alkyl group (1° > 2° > 3°). Thus, the apparent metal-carbon bond energy (−ΔG_BF) for (OEP)Fe^III-Bu-n is 28 ± 2 kcal mol⁻¹, and for [(MeO)₄TPP]Co^III-Bu-n is 36 ± 2 kcalmol⁻¹. The (por⁻·)M⁻ dianions react with carbon dioxide in an electrocatalysed reduction cycle to give CO and CO₃²⁻ via the apparent transient formation of a metal-carbon bond [(por⁻·)M-C(O)O⁻; −Δ G_BF ≥ 12 kcalmol⁻¹ for iron porphyrins].

By a mixed condensation of 1,3-bis(3″,4″-dicyanophenoxy)-2-ethyl-2-methylpropane with a large amount of 4,5-bis(3,3-dimethyl-1-butynyl)phthalonitrile, 1,3-bis-2″-(9″,10″,16″,17″,23″,24″-hexakis(3″,3″-dimethyl-1″-butynyl)phthalocyaninoxyl)-2-ethyl-2-methylpropane, was synthesized as a binuclear phthalocyanine having a single isomer. NMR studies at various temperatures and concentrations suggested that the interactions between the phthalocyanine macrocycles were dominated by intermolecular aggregation at lower temperatures or high concentrations whereas intramolecular aggregation was predominant at higher temperatures or lower concentrations.

μ-Carbido diporphyrinates and diphthalocyaninates of general formula [{Mp²⁻}₂(μ-C)] (p²⁻ = tpp (M = Fe), oep (Fe), pc (Fe, Ru); H₂tpp: 21H,23H-5,10,15,20-tetraphenylporphine; H₂oep: 21H,23H-2,3,4,8,12,13,17,18-octaethylporphine; H₂pc: 29H,31H-phthalocyanine) of formally Fe^IV and Ru^IV are prepared by a new and improved ‘one-pot’ synthesis. The corresponding chloro complexes of the tervalent metal ions react successively with potassium hydroxide in boiling 2-propanol and then with trichloromethane. Potassium hydroxide is proven to be a very versatile and powerful reductant in tetrapyrrolic chemistry. As evidenced from electron spin resonance and UV vis spectral measurements, the precursor is reduced primarily to an ate-complex of type [M^Ip²⁻]⁻ of a formally monovalent metal ion. This active species is assumed to react with trichloromethane via a dichlorocarbene complex of type [M^II(CCl₂)p²⁻] to yield the actual -carbido complex. [{Feoep²⁻}₂(μ-C)] is crystallographically characterized. It is monoclinic, space group C12/c1 (15), with a = 18.279(3) Å, b = 15.005(2) Å, c = 23.392(7) Å, β = 107.12(2)°, Z = 4, R₁ = 0.0773. The iron atom is displaced by 0.192(3) Å out of the centre (Ct) of the (N_p)₄ plane toward the (μ-C) atom. Average d(Fe-N_p) is 1.986(5) Å; d(Fe-(μ-C)) is 1.6638(9) Å. The Fe-C-Fe skeleton is linear (179.5(3)°). The two slightly waving porphyrinato cores are in a staggered conformation, the (N_p-Fe-Fe″-N_p″) torsion angle being 21.0(3)°. Solutions of each μ-carbido complex in pyridine/dichloromethane show four distinct quasi-reversible redox processes in their differential-pulse voltammograms and these are assigned to the successive one-electron reduction and oxidation of the macrocyclic ligands. ¹³C CP MAS NMR spectra indicate effective four-fold symmetry within the series of the μ-carbido complexes with isotropic shifts occurring at similar fields to those of the corresponding macrocyclic complex of a closed-shell metal ion. Resonances of the bridging carbon atom are not detected. A characteristic increase of line broadening within the series tpp²⁻ > oep²⁻ > pc²⁻ may be due to Fermi contact interactions with the strongly coupled low-spin M^IV centres. The magnetic susceptibility studies show that the complexes all display non-zero μ values at 295 K increasing from pc²⁻ to tpp²⁻. Mössbauer spectra confirm the low-spin Fe^IV oxidation state for the iron centres. Isomer shift, δ, and quadrupole splitting, ΔE_Q, for [{Fepc²⁻}₂(μ-C)] and [{Fetpp²⁻}₂(μ-C)] are identical to those previously reported. Data for [{Feoep²⁻}₂(μ-C)] are essentially the same as for the pc and tpp complex. Thus the order of δ is tpp ≈ oep > pc whilst that of ΔE_Q is pc >> oep > tpp. Small impurity lines are observed which help explain the magnetic data. UV vis/NIR spectra of the μ-carbido complexes show the characteristic π-π* transitions. These are shifted with respect to the corresponding mononuclear complexes to higher energy because of excitonic interactions. Vibrational spectra are discussed in detail ν_as(M-C-M) (in cm⁻¹) is at 937 (M = Fe; tpp) < 976 (Fe/oep) < 997 (Fe/pc) < 1050 (Ru/pc), ν_s(M-C-M) (in cm⁻¹ at 433 (Fe/tpp) < 460 (Fe/oep) < 477 (Fe/pc). Hence, valence force constants increase significantly in the order tpp < oep < pc. ν_s(Fe-C-Fe) of [{Fepc²⁻}₂(μ-C)] is selectively resonance Raman enhanced. As evidenced from the excitation profile a C→Fe charge transfer, not detected in the vis spectrum, is assumed to be present at 22 000 > ν > 25 000 cm⁻¹.

The photo-oxidative stability of various annelated and substituted tetraazaporphyrin derivatives were investigated in N,N-dimethylformamide by irradiation in the presence of air. First-order rate constants were calculated. In addition, the positions of the HOMO and LUMO energy levels of the compounds were calculated using a commercially available program. A linear correlation between the experimental values of the rate constants and the theoretical values of the HOMO position exists. The method described allows one to predict the photo-oxidative stability by calculating their HOMO levels, which is very important for the use of macrocyclic metal complexes in photo-oxidation reactions in solution. From the calculated triplet energies it is considered that the macrocyclic metal complexes can convert by photoinduced energy transfer triplet oxygen to singlet oxygen. Experimentally, tetraazaporphyrin derivatives show high quantum yields of singlet oxygen formation under irradiation. No correlations of the quantum yields with the position of the HOMOs or with the rate constants of decomposition was observed.

Phthalocyanines and 2,3-naphthalocyanines are macrocyclic compounds which possess a variety of properties of potential importance in advanced technologies. The article discusses the influence of successive condensation of additional benzene rings to the phthalocyanine system. For this purpose three hexyloxy- and heptyl-substituted 2,3-dicyanonaphthalenes 3,7,11 and the respective 2,3-naphthalocyaninato nickel(II) complexes were prepared. In three statistical condensations of tetraphenylphthalodinitrile (TPPN) (12) and 2,3-dicyanonaphthalenes, 3,7,11, twelve unsymmetrical naphthobenzo-condensed porphyrazinato nickel(II) complexes 13b-15e were synthesized and separated by common column chromatography. All complexes were characterized by mass spectrometry, IR, UV and ¹H NMR spectroscopy.

The possible effects of the electron-donating properties of p-hydroxyphenyl groups on the catalytic activity of cobalt porphyrins toward the electro-reduction of O₂ were explored with three cobalt porphyrins having p-hydroxyphenyl substituents in the meso position of the porphyrin ring. The porphyrins were examined as electrocatalysts adsorbed on the surfaces of pyrolytic graphite electrodes in aqueous solution and as homogeneous catalysts dissolved in methanolic solutions. The adsorbed molecules exhibited a higher catalytic activity than those in solution and accomplished the reduction of a portion of the O₂ molecules reaching the electrode to H₂O. However, the four-electron reduction was indirect, with H₂O₂ being formed as an intermediate that was subsequently reduced. Although the introduction of the electron-donating p-hydroxyphenyl substituents on the porphyrin ring altered the catalytic activity of the cobalt porphyrins, the effect was not as great as can be obtained by coordinating ruthenium (II) complexes to π-accepting ligands in the meso position of the porphyrin ring.

A series of thiol-derivatized porphyrins and the corresponding cobalt and zinc complexes were synthesized from 5-(p-aminophenyl)-10,15,20-triphenylporphyrin (H₂TPP-NH₂), and 5α,15α-bis(o-aminophenyl)porphyrin (H₂DPE- (NH₂)₂). These derivatized porphyrins have a different number of thiol appendages attached via amide linkages at different locations. The thiol-derivatized porphyrins made from H₂TPP-NH₂, H₂TPP-NHC(O)(CH₂)_nSH (n = 2, 4, 5, 10), have one alkyl-thiol appendage attached to the porphyrin through an amide linkage on the p-aminophenyl position. The derivatives made from H₂DPE-(NH₂)₂ have two alkyl-thiol appendages attached to the porphyrin through amide linkages on the o-aminophenyl positions on the same face of the porphyrin ring. These thiol-derivatized porphyrins are important adsorbates for the preparation of thiol–porphyrin monolayers on gold.

Reaction of (2-nitro-5,10,15,20-tetraphenylporphyrinato)nickel(II) 3 or its copper(II) complex 4 with α-isocyanoacetic esters in the presence of 1,8-diazabicyclo[5.4.O]undec-7-ene accomplishes the first syntheses of β-fused pyrroloporphyrins (e.g. 5–10). The removal of the fused pyrrole ester protecting groups to give the di-α-free pyrroloporphyrins 2a and 13 is described, as well as some pyrrole-type chemistry. Unsymmetrical fused dipyrromethanoporphyrins (16–18) were synthesized by reacting 5-acetoxymethylpyrrole-2-carboxylates (14,15) with mono-α-free pyrroloporphyrins (5–8) in the presence of Montmorillonite clay. A symmetrical bis-porphyrinyl dipyrromethane 19 was also prepared by condensation of the β-fused methyl ester pyrroloporphyrin 7 with dimethoxymethane in ethanol and concentrated hydrochloric acid.