Dual facet of gold(III) in the reactions of gold(III) and porphyrins

We have firstly demonstrated the dual facet of gold(III) in the reaction between gold(III) and porphyrins, which could be tuned through changing the counter ions, ligands and the electronic effect of the substituents of the porphyrins.     

Spectral properties and solution behavior of gold(III) coordination compounds with water-soluble porphyrins

Gold(III) coordination compounds with three water-soluble porphyrins―5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrin (H₂TSPP^4–), 5,10,15,20-tetrakis(4-N-methylpyridyl)porphyrin (H₂TMPyP⁴⁺), and 5,10,15,20-tetrakis(4-N,N,N-trimethylaminophenyl)porphyrin (H₂TTMAPP⁴⁺)―have been studied. Complex [Au(TTMAPP)]⁵⁺ has been prepared for the first time. The analysis of coordination-induced shifts of proton signals in NMR spectra and intensities of Q bands in absorption spectra indicates the high degree of bond covalence in the studied metal porphyrins and a partial transfer of electron density from porphyrin to gold ion. The cationic complexes [Au(TMPyP)]⁵⁺ and [Au(TTMAPP)]⁵⁺ in aqueous solutions has been found to exist in monomeric form, while anionic complex [Au(TSPP)]^3– undergoes dimerization upon growth of concentration and solution ionic strength. Equilibrium constant for dimerization has been calculated, the constant has been found to decrease when temperature rises. Thermodynamic parameters of dimerization process have been determined: ΔH° =–31.8 kJ/mol and ΔS° =–13.8 J/mol K.     

Triplet state spectroscopic studies on some 5,10,15,20-tetrakis(methoxyphenyl)porphyrins

Extensive triplet state spectroscopic investigations were carried out with a series of 5,10,15,20-tetrakis(methoxyphenyl)porphyrins. Triplet absorption spectra, triplet lifetime, triplet quantum yield and quantum yield for singlet oxygen production were determined with different absorption and emission techniques, using the frequency-doubled beam of a Nd:YAG laser. It has been found that these synthetic porphyrins are effective photosensitizers which can be used as model compounds to investigate the theoretical and instrumental aspects of PDT.     

Evidence that gold(III) porphyrins are not electrochemically inert: facile generation of gold(II) 5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)porphyrin

Gold(III) porphyrin 1 is shown to undergo reduction at the central metal ion to give the first known gold(II) porphyrin overturning the long held assumption that reduction of such complexes only occurs at the macrocycle.     

Photochemistry and photophysics of Cr(III) macrocyclic complexes

The phosphorescence and photochemical behavior of the macrocyclic complexes (1,4,7,10,13,16-hexaazacyclooctadecane)chromium(III) (Cr([18]-aneN(6))(3+); 1) and (4,4',4'-ethylidynetris(3-azabutan-1-amine)) chromium(III) (Cr(sen)(3+); 2) have been compared to each other and to the complex Cr(en)(3)(3+). For both macrocyclic complexes, phosphorescence from room temperature aqueous solutions is too weak to be observed, contrasting with Cr(en)(3)(3+), though both had somewhat longer 77 K lifetimes than Cr(en)(3)(3+). Phosphorescence lifetimes for these macrocyclics decreased with increasing temperature much faster than for Cr(en)(3)(3+) and a conventional extrapolation based on a fit of reciprocal lifetimes (corrected for the low-temperature contribution) to the Arrhenius equation gave estimated room temperature phosphorescence lifetimes of a few nanoseconds, consistent with the failure to observe room temperature emission. Fitting of the nonlinearity of the data seen in these plots suggested that two high-temperature processes were occurring with estimated activation parameters (E in kJ mol(-1) and A in s(-1)) for Cr([18]-aneN(6))(3+): E(1) = 40, A(1) = 1 x 10(16); E(2) = 24, A(2) = 1 x 10(14): Cr(sen)(3+); E(1) = 45, A(1) = 2 x 10(15); E(2) = 29, A(2) = 7 x 10(11). Cr([18]-aneN(6))(3+) was photochemically inert on irradiation. On irradiation into the lowest quartet ligand field absorption band, Cr(sen)(3+) photolyzes with a quantum yield of 0.098 +/- 0.001 at room temperature. Laser flash photolysis with conductivity detection showed that this photoreaction occurred faster than protonation of the liberated amine ligand at all practical proton concentrations. The quantum yield for irradiation directly into the doublet absorption band of Cr(sen)(3+) was 0.077 +/- 0.003. Photoaquation of Delta-Cr(sen)(3+) led to loss of optical activity and product analysis by capillary electrophoresis showed that both racemic and Delta-Cr(sen)(3+) photoaquate to a single main product, trans-Cr(sen-NH)(H(2)O)(4+). The product stereochemistry is shown to be consistent with predictions based on the angular overlap model for Cr(III) photochemistry, recognizing the additional constraints imposed by the ligand. The abnormally short room temperature solution lifetime of the doublet state is a result of a radiation-less process that competes with other processes depleting the doublet state. However, this doublet-state deactivation process does not lead to photoaquation but competes with BISC and photoaquation via the quartet state, resulting in an unprecedented reduction in photoaquation quantum yield on direct irradiation into the doublet state.     

Sc(III) porphyrins. The molecular structure of two Sc(III) porphyrins and a re-evaluation of the parameters for the molecular mechanics modelling of Sc(III) porphyrins

The crystal structures of two new Sc(III) porphyrins, [Sc(TPP)Cl]·2.5(1-chloronaphthalene), (5,10,15,20-tetraphenylporphyrin)-chloro-scandium(III)·2.5(1-chloronaphthalene) solvate, (Mo Kα, 0.71073 Å, triclinic system  = 9.9530(2) Å, b = 15.4040(3) Å, c = 17.7770(3) Å, α = 86.5190(10)°, β = 89.7680(10)°, γ = 86.9720(10)°, 13101 independent reflections, R₁ = 0.0712) and the dimeric [μ₂-(OH)₂(Sc(TPP))₂], bis-(μ-hydroxo)-(5,10,15,20-tetraphenylporphyrin) scandium(III) (Mo Kα, 0.71073 Å, monoclinic system C2, a = 24.2555(16) Å, b = 11.1598(7) Å, c = 25.6468(17) Å, β = 91.980(2)°, 13084 independent reflections, R₁ = 0.0485) are reported. In [Sc(TPP)Cl] the metal is five-coordinate and the porphyrin is domed with the metal displaced by 0.63 Å from the mean porphyrin towards the axial Cl⁻ ligand. The average Sc-N bond length is 2.143(3) Å, which is shorter than the average bond length of previously reported structures. Two of the phenyl rings are nearly orthogonal to the porphyrin core and the other two are significantly tilted because of contacts with 1-chloronaphthalene solvent molecules, and the phenyl rings of neighbouring porphyrins. In [μ₂-(OH)₂(Sc(TPP))₂] both porphyrins are domed, with the metal displaced from the mean porphyrin plane towards the bridging hydroxo ligands. The average Sc-N bond length is 2.197(12) Å, which is in the upper range of Sc-N bond lengths in known Sc(III) porphyrins but not dissimilar to the average Sc-N bond lengths in another other bis-μ₂-hydroxo Sc(III) porphyrin, [μ₂-(OH)₂(Sc(OEP))₂]. One porphyrin is rotated relative to the upper porphyrin by 25° due to steric contacts between the phenyl substituents. We have used these new structures to re-evaluated our previously reported molecular mechanics force field parameters for modelling Sc(III) porphyrins using the MM2 force field; the training set was augmented from two to seven structures by using all available Sc(III) porphyrin structures and the two new structures. The modelling reproduces the porphyrin core very accurately; bond lengths are reproduced to within 0.01 Å, bond angles to within 0.5° and torsional angles to within 2°. The optimum parameters for modelling the Sc(III)-N bond lengths, determined by finding the minimum difference between the crystallographic and modelling mean bond lengths with the aid of artificial neural network architectures, were found to be 0.90 ± 0.03 mdyn Å⁻¹ for the bond force constant and2.005 ± 0.005 Å for the strain-free bond length. Modelling the seven Sc(III) porphyrins with the new parameters gives an average Sc-N bond length of 2.182 ± 0.018 Å, indistinguishable from the crystallographic mean of 2.181 ± 0.024 Å.     

Magnetic behaviour of spin-mixed iron(III) porphyrins

Detailed average magnetic susceptibility (295-4.2 K) and average magnetisation (20-2 K, 50-10 kOe) are reported for two novel spin-mixed iron(III) porphyrins, namely Fe(TPP)ClO4 and Fe(OEP)ClO4. The results confirm an S = 3/2 ground state substantially spin-mixed with a low-lying S = 5/2 state, but do not agree in detail with the crystal-field model of Maltempo.     

Six-coordinate nitrato complexes of iron(III) porphyrins

The interaction of tetrahydrofuran (THF) with thin films of the nitrato complexes Fe(III)(Por)(eta(2)-O(2)NO) [Por = meso-tetraphenylporphyrinato (TPP) and meso-tetratolylporphyrinato (TTP) dianion] at low temperature leads to the formation of the six-coordinate nitrato complex Fe(Por)(THF)(NO(3)), which was characterized by IR and UV-visible spectroscopies. Formation of the THF adduct was accompanied by nitrate linkage isomerization from bidentate to monodentate coordination. The iron(III) center remains in a high spin state in contrast with the previously observed low-spin nitratonitrosyl complex Fe(TPP)(NO)(eta(10-ONO(2)). Upon warming, THF dissociates to restore the initial five-coordinate bidentate nitrato complex.     

Monooxygenase activity of immobilized iron(III) porphyrins

New silica-immobilized iron(III) porphyrin systems possessing monooxygenase activity in aniline oxidation to p-aminophenol, have been found. The catalytic activity depends on both the porphyrin structure and coordination by an axial ligand. The maximum activity in this reaction is observed for imidazol-coordinated hemin. The hydroxylation in the above system is shown to follow a non-radical mechanism.

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This term refers to catalysis with the formation of monooxygenated products using dioxygen oxidants     

Substituent effects on the site of electron transfer during the first reduction for gold(III) porphyrins

Gold(III) porphyrins of the type (P-R)AuPF(6), where P = 5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)porphyrin and R is equal to H (1), NO(2) (2), or NH(2) (3) which is substituted at one of the eight beta-pyrrolic positions of the macrocycle, were investigated as to their electrochemistry and spectroelectrochemistry in nonaqueous media. Each compound undergoes three reductions, the first of which involves the central metal ion to give a Au(II) porphyrin or a Au(III) porphyrin pi-anion radical depending upon the nature of the porphyrin ring substituent. A similar metal-centered reduction also occurs for compounds 1, 3, and Au(III) quinoxalinoporphyrin, (PQ)AuPF(6) (4), where PQ = 5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)quinoxalino[2,3-b]porphyrin, and these results on the three Au(III) porphyrins overturn the long held assumption that reductions of such complexes only occur at the macrocycle. In contrast, when a NO(2) group is introduced on the porphyrin ring to give (P-NO(2))AuPF(6) (2), the site of electron transfer is changed from the gold metal to the macrocycle to give a porphyrin pi-anion radical in the first reduction step. This change in the site of electron transfer was examined by electrochemistry combined with thin-layer UV-vis spectroelectrochemistry and ESR spectroscopy of the singly reduced compound produced by chemical reduction. The reorganization energy (lambda) of the metal-centered electron transfer reduction for (P-H)AuPF(6) (1) in benzonitrile was determined as lambda = 1.23 eV by analyzing the rates of photoinduced electron transfer from the triplet excited states of an organic electron donor to 1 in light of the Marcus theory of electron transfer. The lambda value of the metal-centered electron transfer of gold porphyrin (1) is significantly larger than lambda values of ligand-centered electron transfer reactions of metalloporphyrins.     

Coordination polymers based on aluminum(III) porphyrins

Aluminum(III) porphyrin carboxylate complexes have shown an affinity for a sixth nitrogenous ligand. The use of isonicotinic or nicotinic acid, which offers both a carboxylate and a nitrogen donor in the same molecule, resulted in the formation of one-dimensional (1-D) coordination polymers. The complexes and their linear oligomers have been characterized by (1)H NMR spectroscopy and nanoelectrospray ionization spectrometry. X-ray analyses confirmed the formation of the 1-D polymers in the solid state.     

Aluminium(III) porphyrins as supramolecular building blocks

Aluminium(III) porphyrin-carboxylate complexes, including a porphyrin pentamer, have been characterised by NMR spectroscopy, MALDI spectrometry and single crystal X-ray diffraction; these complexes can also be coordinated by a sixth, nitrogenous, ligand to the aluminium(III) centre.     

Low-temperature magnetisation study on manganese(III) porphyrins

Average magnetisation between 2 and 20 K and 10 and 50 kOe of two typical high-spin manganese(III) porphyrins, namely TPPMnCI and TPPMnCl(py), is reported for the first time. The results have been analysed in terms of spin hamiltonian formalism including both the crystal field and magnetic exchange.     

Some novel manganese(III) porphyrins with catalytic properties

Abstract

A series of manganese(III) porphyrins with 4-methylimidazole have been prepared. These are high-spin complexes having general formula [Mn^III(THMPP)X(4-MeIm)], where THMP?=?5,10,15,20-tetra(4-hydroxy-3-methoxyphenyl)porphine ligand, X?=?Cl^?, Br^?, NCS^?, or N₃^? and 4-MeIm?=?4-methylimidazole. All the complexes have been characterized by UV-visible, FT-IR, ESI-MS spectra, elemental analyses and magnetic susceptibility measurements. These manganese(III) porphyrins oxidize aromatic alcohols to aldehydes. The oxidation reactions have been carried out at room temperature in the presence of oxidants such as NaIO₄, H₂O₂, and NaOCl. The comparative studies proved that NaIO₄ behaves as the most efficient oxidant in these oxidative transformation reactions.     

Synthesis of cationic gold(III) complexes using iodine(III)

Abstract

We report the synthesis and characterization of cationic Au(III) complexes supported by nitrogen-based ligands. The syntheses are achieved by reacting Au(I) complexes [Au(N-Me-imidazole)₂]⁺ and [Au(pyridine)(NHC)]⁺ with iodine(III) reagents PhI(OTf)(OAc) and [PhI(pyridine)₂]²⁺ yielding a series of cationic gold(III) complexes. In contrast, reactions of phosphine ligated gold(I) complexes with iodine(III) reagents results in the oxidation of the phosphine ligand.     

Amine-amide equilibrium in gold(III) complexes and a gold(III)-gold(I) aurophilic bond

The ligands HN(CH2-2-C5H4N)2, BPMA, and PhCH2N(CH2-2-C5H4N)2, BBPMA, react with Na[AuCl4] to give the cationic complexes [AuCl(BPMA-H)]+ and [AuCl(BBPMA)]2+, respectively. The amido complex [AuCl(BPMA-H)]+ undergoes easy inversion at the amido nitrogen atom and can be reversibly protonated by triflic acid to give [AuCl(BPMA)]2+. The complex [AuCl(BBPMA)]2+ is easily decomposed in aqueous solution by cleavage of a carbon-nitrogen bond or, in dilute HCl solution, by protonation of the ligand to give [BBPMAH2]Cl[AuCl4] The complexes [BBPMAH2]Cl[AuCl4] and [BBPMAH2]Cl[AuCl2] can be formed by direct reaction of BBPMA with H[AuCl4]. Unusual forms of gold(III)...gold(III) and gold(III)...gold(I) aurophilic bonding are observed in the salts [AuCl(BPMA-H)][PF6] and [AuCl(BPMA-H)][AuCl2], respectively. The first comparison of the structures of gold(III) amine and amido complexes, in the cations [AuCl(BPMA-H)]+ and [AuCl(BPMA)]2+, indicates that there is little ppi-dpi bonding in the amido-gold bond and that the amide exerts a stronger trans influence than the amine group.