Catalysis of copper(II) chelate-amine complexes in the oxidative coupling of 2,6-dialkylphenols

The oxidative coupling reaction of 2,6-dimethylphenol and 2,6-di-tert-butylphenol with molecular oxygen was performed by using a series of copper(II) chelate complexes as a catalyst, derived from copper(II), β-diketone, and some Shiff bases. Under the applied reaction conditions, the reaction products of 2,6-dimethylphenol were poly(2,6-dimethyl-1,4-phenylene oxide) (C? O coupling product) and 3,3′,5-5′-tetramethyl-4,4′-diphenoquinone (C? C coupling product), and that of 2,6-di-tert-butylphenol oxidation was only 3,3′,5-5′-tetra-tert-butyl-4,4′-diphenoquinone (C? C coupling product). The catalytic activity has been shown to be dependent on the properties of the copper(II) chelates used as catalysts and the mole ratios of amine ligand to copper(II) chelate (ligand ratio). The basicity and the steric bulkiness of the amine used as a ligand for copper(II) β-diketonato catalysts were found to be two of the main factors that govern the oxidative coupling mode (C? O and/or C? C coupling) of 2,6-dimethylphenol. The oxidative coupling activity of 2,6-dialkylphenol is discussed in terms of both the stabilities of the copper(II) chelates and of the copper(II) chelate-amine adducts. The rate of oxygen absorption for 2,6-dimethylphenol catalyzed by the copper(II) acetylacetonato-piperidine system is first order in oxygen partial pressure and zero order in 2,6-dimethylphenol concentration, respectively. A Cu(II)-oxygen, as an intermediate is suggested on the basis of the results obtained.     

A copper(I) oxygenation precursor in the entatic state: two isomers of a copper(I) compound of a rigid tetradentate ligand

Oxygenation of [CuI(L1)(NC-CH3)]+ (L1 = dimethyl 2,4-bis(2-pyridinyl)-3,7-diazabicyclo-[3.3.1]-nonane-9-on-1,5-dicarboxylate) leads to a relatively stable mu-peroxo-dicopper(II) product. The stability of this type of oxygenation product has been shown before to be the result of the square pyramidal geometry of L1; preorganization by a dinucleating ligand has been shown to increase the stability of the mu-peroxo-dicopper(II) compound. The structural data presented here indicate that destabilization of the copper(I) precursor is another important factor. There are two isomers of [CuI(L1)(NCCH3)]+; one is yellow, and the other is red. X-ray crystallography indicates that one pyridinyl donor is not coordinated in the yellow compound and that the red compound is 5-coordinate. In the light of the X-ray structure of the metal-free ligand and that of the corresponding copper(II) compound, it emerges that the ligand cavity is well suited for copper(II), whereas the copper(I) compounds are highly strained. This is supported by 1H NMR spectra of the copper(I) species where a fast dynamic process leads to line broadening and by electrochemical data, which indicate that the copper(II) products are exceptionally stable. Also presented are structural (copper(II)), electrochemical, and spectroscopic data (1H NMR, copper(I)) of the derivative [Cu(L2)(X)]n+ with a methyl substituent at the alpha-carbon atom of the two coordinated pyridinyl groups (L2 = dimethyl 2,4-bis(2-pyridinyl-6-methyl)-3,7-diazabicyclo-[3.3.1]-nonane-9-on-1,5-dicarboxylate). There are two structural forms of [CuII(L2)(X)]n+ (X = NCCH3, Cl), which depend on the steric demand of the fifth donor X. For both, van der Waals repulsion leads to a destabilization of the copper(II) products, and this is also evident from an increase in the reduction potential (-110 mV vs. -477 mV, Ag/AgNO3).     

CuII‐Catalyzed Oxidative Formation of 5‐Alkynyltriazoles

In an alcoholic solvent under the catalysis of Cu(OAc)₂?H₂O, organic azide and terminal alkyne could oxidatively couple to afford 5‐alkynyl‐1,2,3‐triazole (alkynyltriazole) at room temperature under an atmosphere of O₂ in a few hours. The involvement of 1,5‐diazabicyclo[4.3.0]non‐5‐ene (DBN) is essential, without which the redox neutral coupling instead proceeds to produce 5‐H‐1,2,3‐triazole (protiotriazole) as the major product. Therefore, DBN switches the redox neutral coupling between terminal alkyne and organic azide, the copper‐catalyzed “click” reaction to afford protiotriazole, to an oxidation reaction that results in alkynyltriazole. The organic base DBN is effective in accelerating the copper(II)‐catalyzed oxidation of terminal alkyne or copper(I) acetylide, which is intercepted by an organic azide to produce alkynyltriazole. The proposed mechanistic model suggests that the selectivity between alkynyl‐ and protiotriazole, and other acetylide or triazolide oxidation products is determined by the competition between copper(I)‐catalyzed redox neutral cycloaddition and copper(II)/O₂‐mediated acetylide oxidation after the formation of copper(I) acetylide.     

Dioxygen reactivity of copper and heme-copper complexes possessing an imidazole-phenol cross-link

Recent spectroscopic, kinetics, and structural studies on cytochrome c oxidases (CcOs) suggest that the histidine-tyrosine cross-link at the heme a3-CuB binuclear active site plays a key role in the reductive O2-cleavage process. In this report, we describe dioxygen reactivity of copper and heme/Cu assemblies in which the imidazole-phenol moieties are employed as a part of copper ligand LN4OH (2-{4-[2-(bis-pyridin-2-ylmethyl-amino)-ethyl]-imidazol-1-yl}-4,6-di -tert-butyl-phenol). Stopped-flow kinetic studies reveal that low-temperature oxygenation of [CuI(LN4OH)]+ (1) leads to rapid formation of a copper-superoxo species [CuII(LN4OH)(O2-)]+ (1a), which further reacts with 1 to form the 2:1 Cu:O2 adduct, peroxo complex [{CuII(LN4OH)}2(O2(2-))]2+ (1b). Complex 1b is also short-lived, and a dimer Cu(II)-phenolate complex [CuII(LN4O-)]2(2+) (1c) eventually forms as a final product in the later stage of the oxygenation reaction. Dioxygen reactivities of 1 and its anisole analogue [CuI(LN4OMe)]+ (2) in the presence of a heme complex (F8)FeII (3) (F8 = tetrakis(2,6,-difluorotetraphenyl)-porphyrinate) are also described. Spectroscopic investigations including UV-vis, 1H and 2H NMR, EPR, and resonance Raman spectroscopies along with spectrophotometric titration reveal that low-temperature oxygenation of 1/3 leads to formation of a heme-peroxo-copper species [(F8)FeIII-(O2(2-))-CuII(LN4OH)]+ (4), nu(O-O) = 813 cm(-1). Complex 4 is an S = 2 spin system with strong antiferromagnetic coupling between high-spin iron(III) and copper(II) through a bridging peroxide ligand. A very similar complex [(F8)FeIII-(O2(2-))-CuII(LN4OMe)]+ (5) (nu(O-O) = 815 cm(-1)) can be generated by utilizing the anisole compound 2, which indicates that the cross-linked phenol moiety in 4 does not interact with the bridging peroxo group between heme and copper. This investigation thus reveals that a stable heme-peroxo-copper species can be generated even in the presence of an imidazole-phenol group (i.e., possible electron/proton donor source) in close proximity. Future studies are needed to probe key factors that can trigger the reductive O-O cleavage in CcO model compounds.     

Mechanistic study of asymmetric oxidative biaryl coupling: evidence for self-processing of the copper catalyst to achieve control of oxidase vs oxygenase activity

Copper(I) and copper(II) 1,5-diaza-cis-decalin complexes [(N2)Cu] are effective precatalysts for aerobic oxidative coupling of naphthol substrates. Mechanistic studies, however, reveal that these complexes are not the reactive form of the catalyst under steady-state conditions. Rather, the active catalyst forms in a presteady-state self-processing step that involves oxygenation of the naphthol substrate. The oxygenated substrate, NapHOX, serves as a cofactor that combines with the (N2)Cu complexes to achieve highly selective, steady-state oxidase reactivity (aerobic oxidative biaryl coupling).     

Targeted guanine oxidation by a dinuclear copper(II) complex at single stranded/double stranded DNA junctions

A dinuclear copper(II) complex [Cu(II)2(PD'O-)(H2O)2](ClO4)3 (5) with terminal Cu(II)-H(2)O moieties and a Cu...Cu distance of 4.13 A (X-ray structure) has been synthesized and characterized by EPR spectroscopy (ferromagnetic coupling observed) and cyclic voltammetry. Dizinc(II) and mononuclear copper(II) analogues [Zn(II)2(PD'O-)(H2O)2]3+ (7) and [Cu(II)(mPD'OH)(H2O)]2+ (6), respectively, have also been synthesized and structurally characterized. Reacting 5/MPA/O(2) (MPA = 3-mercaptopropionic acid) with DNA leads to a highly specific oxidation of guanine (G) at a junction between single- and double-stranded DNA. Mass spectrometric analysis of the major products indicates a gain of +18 and +34 amu relative to initial DNA strands. The most efficient reaction requires G at the first and second unpaired positions of each strand extending from the junction. Less reaction is observed for analogous targets in which the G cluster is farther from the junction or contains less than four Gs. Consistent with our previous systems, the multinuclear copper center is required for selective reaction; mononuclear complex 6 is not effective. Hydrogen peroxide as a substitute for MPA/O2 also does not lead to activity. Structural analysis of a [Cu(II)2(PD'O-)(G)]3+ complex (8) and dizinc analogue [Zn(II)(2)(PD'O-)(G)](ClO4)3 (9) (G = guanosine) reveals coordination of the G O6 and N7 atoms with the two copper (or zinc) centers and suggests that copper-G coordination likely plays a role in recognition of the DNA target. The Cu2-O2 intermediate responsible for guanine oxidation appears to be different from that responsible for direct-strand scission induced by other multinuclear copper complexes; the likely course of reaction is discussed.     

Complex of copper(II) with phenoxo and hexamethylphosphoramide ligands and its decomposition

The 2,4,6-trichlorophenoxo–hexamethylphosphoramide(HMPA)–copper(II) complex was isolated by the reaction of copper(II) chloride with 2,4,6-trichlorophenol, sodium methoxide, and HMPA in methanol solvent under an atmosphere of nitrogen. This complex has the composition of Cu·HMPA·2(C₆H₂Cl₃O). The molecular weight determination was consistent with the copper(II) complex of binuclear structure. The infrared, electronic, and ESR spectra and magnetic susceptibility of the copper(II) complex are discussed in relation to its structure. Decomposition of the copper(II) complex in refluxing benzene yielded poly(dichlorophenylene oxide), coupling product of the 2,4,6-trichlorophenoxo ligand of the copper(II) complex. Electron spin resonance (ESR) measurements on the copper(II) complex in the solid state in a degassed sealed tube at 120 ± 5°C indicated that the phenoxy radical was generated during the period of decomposition and the intensity of the ESR spectra based on copper(II) ion decreased with the measurement time. From these ESR spectra, a possible initial step involving one electron transfer of the decomposition of the 2,4,6-trichlorophenoxo–HNPA–copper(II) complex is discussed.     

Synthesis of the reported structure of crassiflorone, a naturally occurring quinone isolated from the African ebony Diospyros crassiflora, and regioisomeric pentacyclic furocoumarin naphthoquinones

A synthesis of the structure reported for the natural product crassiflorone, a furocoumarin naphthoquinone, is described. The key steps are a Diels-Alder reaction to form 2-bromo-8-hydroxy-6-methylnaphthoquinone, followed by O-protection and copper(II) mediated coupling to 4-hydroxy-5-methylcoumarin to establish the pentacyclic framework whose structure was unambiguously confirmed by X-ray crystallography. Since the spectroscopic data of the synthetic material did not match those reported for the natural product, three further regioisomeric furocoumarin naphthoquinones were prepared by copper(II) mediated coupling of 4-hydroxy-5- or 8-methyl coumarins with 5-benzyloxy-2-bromo-7-methyl- or 8-benzyloxy-2-bromo-6-methyl-1,4-naphthoquinone. Again the spectroscopic data did not match those of the natural material and therefore the true structure of crassiflorone remains unknown.     

Tandem intramolecular silylformylation and silicon-assisted cross-coupling reactions. synthesis of geometrically defined alpha,beta-unsaturated aldehydes

The palladium- and copper-catalyzed cross-coupling reactions of cyclic silyl ethers with aryl iodides are reported. Silyl ethers 3 were readily prepared by intramolecular silylformylation of homopropargyl silyl ethers 2 under a carbon monoxide atmosphere. The reaction of cyclic silyl ethers 3with various aryl iodides 7 in the presence of [(allyl)PdCl](2), CuI, a hydrosilane, and KF.2H(2)O in DMF at room temperature provided the alpha,beta-unsaturated aldehyde coupling products 8 in high yields. The need for copper in this process suggested that transmetalation from silicon to copper is an important step in the mechanism. Although siloxane 3 and the product 8 are not stable under basic conditions, KF.2H(2)O provided the appropriate balance of reactivity toward silicon and reduced basicity. The addition of a hydrosilane to [(allyl)PdCl](2) was needed to reduce the palladium(II) to the active palladium(0) form.     

Fine-tuning of copper(I)-dioxygen reactivity by 2-(2-pyridyl)ethylamine bidentate ligands

Copper(I)-dioxygen reactivity has been examined using a series of 2-(2-pyridyl)ethylamine bidentate ligands (R1)Py1(R2,R3). The bidentate ligand with the methyl substituent on the pyridine nucleus (Me)Py1(Et,Bz) (N-benzyl-N-ethyl-2-(6-methylpyridin-2-yl)ethylamine) predominantly provided a (mu-eta(2):eta(2)-peroxo)dicopper(II) complex, while the bidentate ligand without the 6-methyl group (H)Py1(Et,Bz) (N-benzyl-N-ethyl-2-(2-pyridyl)ethylamine) afforded a bis(mu-oxo)dicopper(III) complex under the same experimental conditions. Both Cu(2)O(2) complexes gradually decompose, leading to oxidative N-dealkylation reaction of the benzyl group. Detailed kinetic analysis has revealed that the bis(mu-oxo)dicopper(III) complex is the common reactive intermediate in both cases and that O[bond]O bond homolysis of the peroxo complex is the rate-determining step in the former case with (Me)Py1(Et,Bz). On the other hand, the copper(I) complex supported by the bidentate ligand with the smallest N-alkyl group ((H)Py1(Me,Me), N,N-dimethyl-2-(2-pyridyl)ethylamine) reacts with molecular oxygen in a 3:1 ratio in acetone at a low temperature to give a mixed-valence trinuclear copper(II, II, III) complex with two mu(3)-oxo bridges, the UV-vis spectrum of which is very close to that of an active oxygen intermediate of lacase. Detailed spectroscopic analysis on the oxygenation reaction at different concentrations has indicated that a bis(mu-oxo)dicopper(III) complex is the precursor for the formation of trinuclear copper complex. In the reaction with 2,4-di-tert-butylphenol (DBP), the trinuclear copper(II, II, III) complex acts as a two-electron oxidant to produce an equimolar amount of the C[bond]C coupling dimer of DBP (3,5,3',5'-tetra-tert-butyl-biphenyl-2,2'-diol) and a bis(mu-hydroxo)dicopper(II) complex. Kinetic analysis has shown that the reaction consists of two distinct steps, where the first step involves a binding of DBP to the trinuclear complex to give a certain intermediate that further reacts with the second molecule of DBP to give another intermediate, from which the final products are released. Steric and/or electronic effects of the 6-methyl group and the N-alkyl substituents of the bidentate ligands on the copper(I)-dioxygen reactivity have been discussed.     

The influence of metal(II) salts and solid supports on the nature of the photochemical reaction of 1,1-diphenyl-2-haloethene

Photolysis of 1,1-diphenyl-2-haloethene in methanol solution in the presence of copper(II) acetate, copper(II) triflate or cadmium(II) acetate results in the formation of ionic and radical products. The ratio of ionic to ionic and radical products depends on the halogen atom and the salt used. The effect of a solid support (silica gel, silica gel impregnated with copper(II) or cadmium(II) salts) was studied and it was found that the product distribution and the ratio of ionic to ionic and radical products are different from the results obtained during irradiation in solution.     

Syntheses and structures of isoindoline complexes of Zn(II) and Cu(II): an unexpected trinuclear Zn(II) complex

Deprotonation of the tridentate isoindoline ligand 1,3-bis[2-(4-methylpyridyl)imino]-isoindoline, 4'-MeLH, and reaction with hydrated zinc(II) perchlorate produces an unexpected trinuclear Zn(II) complex, [Zn(3)(4'-MeL)(4)](ClO(4))(2).5H(2)O (1), whereas reaction with hydrated copper(II) perchlorate in methanol produces the expected mononuclear product, [Cu(4'-MeL)(H(2)O)(2)]ClO(4) (2). X-ray diffraction shows that the trinuclear Zn(II) complex (1) contains a linear zinc backbone, and the arrangement of ligands about the outer chiral zinc(II) atoms is helical. The two terminal zinc ions exhibit approximate C(2) site symmetry, with tetrahedral coordination by two pyrrole and two pyridyl nitrogen atoms of the potentially tridentate isoindoline ligands. The central zinc ion exhibits approximate tetrahedral symmetry, with coordination by four pyridyl nitrogen atoms of four different isoindoline ligands. Pyridyl-pyrrole intramolecular pi-stacking interactions contribute to the stability of the trinuclear cation. The structure of the mononuclear copper(II) complex cation in 2 is best described as a distorted trigonal bipyramid. The isoindoline anion binds Cu(II) in both axial positions and one of the equatorial positions; water molecules occupy the other two equatorial positions.     

Helical-chain copper(II) complexes and a cyclic tetranuclear copper(II) complex with single syn-anti carboxylate bridges and ferromagnetic exchange interactions

Tridentate Schiff-base carboxylate-containing ligands, derived from the condensation of 2-imidazolecarboxaldehyde with the amino acids beta-alanine (H2L1) and 2-aminobenzoic acid (H2L5) and the condensation of 2-pyridinecarboxaldehyde with beta-alanine (HL2), D,L-3-aminobutyric acid (HL3), and 4-aminobutyric acid (HL4), react with copper(II) perchlorate to give rise to the helical-chain complexes [[Cu(mu-HL1)(H2O)](ClO4)]n (1), [[Cu(mu-L2)(H2O)](ClO4).2H2O]n (2), and [[Cu(mu-L3)(H2O)](ClO4).2H2O]n (3), the tetranuclear complex [[Cu(mu-L4)(H2O)](ClO4)]4 (4), and the mononuclear complex [Cu(HL5)(H2O)](ClO4).1/2H2O (5). The reaction of copper(II) chloride with H2L1 leads not to a syn-anti carboxylate-bridged compound but to the chloride-bridged dinuclear complex [Cu(HL1)(mu-Cl)]2 (6). The structures of these complexes have been solved by X-ray crystallography. In complexes 1-4, roughly square-pyramidal copper(II) ions are sequentially bridged by syn-anti carboxylate groups. Copper(II) ions exhibit CuN2O3 coordination environments with the three donor atoms of the ligand and one oxygen atom belonging to the carboxylate group of an adjacent molecule occupying the basal positions and an oxygen atom (from a water molecule in the case of compounds 1-3 and from a perchlorate anion in 4) coordinated in the apical position. Therefore, carboxylate groups are mutually cis oriented and each syn-anti carboxylate group bridges two copper(II) ions in basal-basal positions with Cu...Cu distances ranging from 4.541 A for 4 to 5.186 A for 2. In complex 5, the water molecule occupies an equatorial position in the distorted octahedral environment of the copper(II) ion and the Cu-O carboxylate distances in axial positions are very large (>2.78 A). Therefore, this complex can be considered as mononuclear. Complex 6 exhibits a dinuclear parallel planar structure with Ci symmetry. Copper(II) ions display a square-pyramidal coordination geometry (tau = 0.06) for the N2OCl2 donor set, where the basal coordination sites are occupied by one of the bridging chlorine atoms and the three donor atoms of the tridentate ligand and the apical site is occupied by the remaining bridging chlorine atom. Magnetic susceptibility measurements indicate that complexes 1-4 exhibit weak ferromagnetic interactions whereas a weak antiferromagnetic coupling has been established for 6. The magnetic behavior can be satisfactorily explained on the basis of the structural data for these and related complexes.     

Synthesis of Neocaesalpin A,AA, and Nominal Neocaesalpin K**

The first total synthesis of heavily oxidized cassane-type diterpenoid neocaesalpin A ( 1 ) is disclosed. At the heart of the synthesis lies an intermolecular Diels–Alder reaction that rapidly assembles the target framework from commercial materials. A carefully orchestrated sequence of oxidations secured the desired oxygenation pattern. Late-stage release of the characteristic butenolide occurred through a novel mercury(II)-mediated furan oxidation. Successful extension of the route allowed preparation of neocaesalpin AA ( 2 ) as well as nominal neocaesalpin K ( 3 ) and suggested structural revision of neocaesalpin K, leading to the hypothesis that the two are likely the same natural product with correct assignment as 2 .     

Unique reactivity of a tetradentate N(2)S(2) complex of nickel: intermediates in the production of sulfur oxygenates

Complex 1 [(N,N'-dimethyl-N,N'-bis(2-sulfanylethyl)ethylenediamine)nickel(II)], previously shown to react with H(2)O(2) to produce the fully oxygenated disulfonate 5 [diaqua(N,N'-dimethyl-N,N'-bis(2-sulfonatoethyl)ethylenediamine)nickel(II)], has been explored in detail to explain the observed reactivity of this compound and to discern intermediates in the oxygenation reaction. Reaction of 1 with 1 equiv of methyl iodide results in the monomethylated square-planar nickel complex 2 [[(N,N'-dimethyl-N-(2-sulfanylethyl)-N'-(2-methylthioethyl)(ethylenediamine)nickel(II)] iodide], while a slight excess of methyl iodide results in the dimethylated complex 3 [diiodo(N,N'-dimethyl-N,N'-bis(2-methylthioethyl)ethylenediamine)nickel(II)], an X-ray structure of which has shown that the nickel ion is in an octahedral N(2)S(2)I(2) environment. Crystal data of 3: monoclinic, a = 8.865(3) A, b = 14.419(4) A, c = 14.389(6) A, beta = 100.19(3) degrees, V = 1810.2(12) A(3), space group P2(1)/n, Z = 4. The equatorial positions are occupied by the two cis-amine N-atoms and the coordinated iodides, while the axial positions are occupied by the thioether sulfur atoms. In organic solvents, the dithiolate complex 1 reacts with molecular oxygen or H(2)O(2) to produce the mixed sulfinato/thiolato complex 4 [(N,N'-dimethyl-N-(2-sulfanylethyl)-N'-(2-sulfinatoethyl)(ethylenediamine)nickel(II)], and the fully oxidized product 5. X-ray analysis of complex 4 reveals a square-planar geometry in which the nickel ion is coordinated by two cis-amine nitrogens, one thiolate sulfur donor, and one sulfinato sulfur donor. Crystal data of 4: orthorhombic, a = 11.659(2) A, b = 13.119(3) A, c = 16.869(3) A, V = 2580.2(9) A(3), space group Pbca, Z = 8. This complex is the only intermediate in the oxygenation reaction that could be isolated, and it is shown to be further reactive toward O(2) to yield the fully oxidized product 5. For a better understanding of the reactivity observed for 4, DFT calculations have been undertaken, which show a possible reaction path toward the fully oxidized product 5.     

Oxidative halogenation of substituted pyrroles with Cu(II). Part III. Bromination and chlorination of 2-benzoylpyrrole

The bromination of 2-benzoylpyrrole with copper(II) bromide in the homogeneous and the heterogeneous phase is described, giving 4- and 5-monobromo derivatives whose ratio decreases as the temperature is increased. The same reaction with copper(II) chloride in acetonitrile at 60° produces 5-chloro-2-benzoylpyrrole as the major product. 4,5-Dihalopyrroles in good yields are obtained with an excess of halogenating agent.     

Copper(II)-catalysed oxidative coupling reaction of 3-hydroxyacetanilide with 3-methyl-2-benzothiazolinone hydrazone for the spectrophotometric determination of traces of copper(II)

A spectrophotometric method is developed for the determination of traces of copper(II), based on the catalytic oxidative coupling reaction of 3-hydroxyacetanilide with 3-methyl-2-benzothiazolinone hydrazone in the presence of ammonia and hydrochloric acid. Beer's law is obeyed in the copper(II) concentration range of 0.008-0.16 microg mL(-1), and the molar absorptivity at 530 nm is 2.5x10(5) L mol(-1) cm(-1). The Sandell's sensitivity of the product is 0.000254 microg cm(-2). The optimum reaction conditions and other important analytical parameters have been investigated. The proposed method is applied to the analysis of water and soil samples and the results are compared with the literature method.     

Through Bridge Spin Coupling in Homo- and Heterobimetallic Porphyrin Dimers upon Stepwise Oxidations: A Spectroscopic and Theoretical Investigation

We have described copper(II)-iron(III) and copper(II)-manganese(III) heterobimetallic porphyrin dimers and compared them with the corresponding homobimetallic analogs. UV-visible spectra are very distinct in the heterometallic species while electrochemical studies demonstrate that these species, as compared to the homobimetallic analog, are much easier to oxidize. Combined Mössbauer, EPR, NMR, magnetic and UV-visible spectroscopic studies show that upon 2e-oxidation of the heterobimetallic complexes only ring-centered oxidation occurs. The energy differences between HOMO and LUMO are linearly dependent with the low-energy NIR band obtained for the 2e-oxidized complexes. Also, strong electronic communication between two porphyrin rings through the bridge facilitates coupling between various unpaired spins present while the coupling model depends on the nature of metal ions used. While unpaired spins of Fe(III) and the porphyrin π-cation radical are strongly antiferromagnetically coupled, such coupling is rather weak between Mn(III) and a porphyrin π-cation radical. Moreover, the coupling between two π-cation radicals are much stronger in the 2e-oxidized complexes of dimanganese(III) and copper(II)-manganese(III) porphyrin dimers as compared to their diiron(III) and copper(II)-iron(III) analogs. Furthermore, coupling between the unpaired spins of a π-cation radical and copper(II) is much stronger in the 2e-oxidized complex of copper(II)-iron(III) porphyrin dimer as compared to its copper(II)-manganese(III) analog. The Mulliken spin density distributions in 2e-oxidized homo- and heterobimetallic complexes show symmetric and asymmetric spread between the two macrocycles, respectively. In both the 2e-oxidized heterobimetallic complexes, the Cu(II) porphyrin center acts as a charge donor while Fe(III)/Mn(III) porphyrin center act as a charge acceptor. The experimental observations are also strongly supported by DFT calculations.     

Synthesis of binaphthols over mesoporous molecular sieves

Mesoporous aluminosilicates (Al-containing NaMCM-41) were applied as catalyst supports for oxidative coupling of β-naphthol and substituted β-naphthols due to their remarkable features such as surface area, ordered mesopores and high thermal stability. The NaMCM-41 supported copper catalysts prepared by impregnation method, and Cu-NaMCM-41 was prepared by incorporating copper during synthesis. Oxidative coupling of β-naphthol reaction was studied using molecular oxygen as oxidant. The copper supported NaMCM-41 catalysts were prepared with different Si/Al ratios and calcined from 120 to 420 °C were observed to show varied product selectivity. The NaMCM-41 supported copper catalysts and Cu-NaMCM-41 were more active than the corresponding Cu/Fe supported on NaY zeolite. The catalysts were characterized by X-ray diffraction (XRD), temperature programmed reduction (TPR), UV–DRS, ICPMS and BET surface area techniques and the reaction products were confirmed by ¹H-NMR, FTIR and HRMS. An attempt has been made to explain the product selectivity of the catalysts discussed with the above techniques. The high dispersion of Cu⁺² species observed in the catalysts having high Si/Al ratios in NaMCM-41 support and catalysts that are calcined at low temperatures, i.e. less than 420 °C, yielded an unexpected product perylene diol. A comparatively low dispersion of Cu⁺² species, noticed in catalysts having low Si/Al ratios and calcined at high temperatures, yielded binapthol as the coupled product. The effect of the variation of catalyst and the solvent are also studied.