Number Of Nitrogen Donors Research Articles

Factors affecting the reactivity and selectivity in the oxidation of sulfides with tetra- n-butylammonium peroxomonosulfate catalyzed by Mn(III) porphyrins: Significant nitrogen donor effects

The oxidation of aryl sulfides by tetra- n-butylammonium peroxomonosulfate ( n-Bu 4NHSO 5) was carried out in the presence of six different manganese (III) tetraarylporphyrins [Mn(Por)s] as biomimetic catalysts and a number of nitrogen donors as co-catalysts. There is no noticeable difference between the reactivity of sulfides, in the presence of electron-rich Mn(por)s, whereas, for electron-deficient catalysts, conversion rates are different. Nevertheless, the over-oxidation of sulfoxide is more sensitive to both the nature of substituents attached to the sulfur atom in substrates as well as porphyrin complex structure. The degree of catalytic activity of Mn(Por)s for the formation of sulfone product increases as the following order: Mn(TPFPP)OAc < Mn[T(4-NO 2P)P]OAc < Mn(TDCPP)OAc < Mn(TPP)OAc < Mn(TMP)OAc < Mn[T(4-OMeP)P]OAc. Our results show that in the presence of electron-rich Mn(Por)s, the strong π-donor N–H imidazoles possess co-catalytic activity greater than that of strong σ-donor amines and weak π-donor pyridines. When electron-deficient Mn(Por)s were employed as catalyst, pyridines demonstrated a higher co-catalytic activity than that of N–H imidazoles. The pronounced effect of protic solvents on the rate and selectivity of oxidation reactions, particularly in the presence of electron-deficient Mn(Por)s has been observed. The outcome of our investigations accompanied by UV–Vis and Raman spectral data confirms the involvement of different active oxidant such as a high valent Mn-oxo species as well as a six-coordinate [(L)(Por)Mn–OHSO 4] complex.

Excited-State Properties of Octahedral Hexarhenium(III) Complexes with Redox-active N-heteroaromatic Ligands

New sulfide-capped octahedral hexarhenium(III) complexes containing 4-phenylpyridine (ppy) or 1,2-bis(4-pyridyl)ethane (bpe) ((n-C(4)H(9))(4)N)[mer-{Re(6)S(8)Cl(3)(ppy)(3)}] ((Bu(4)N)[1]), ((n-C(4)H(9))(4)N)(2)[trans-{Re(6)S(8)Cl(4)(ppy)(2)}] ((Bu(4)N)(2)[2a]), ((n-C(4)H(9))(4)N)(2)[cis-{Re(6)S(8)Cl(4)(ppy)(2)}] ((Bu(4)N)(2)[2b]), ((n-C(4)H(9))(4)N)(2)[trans-{Re(6)S(8)Cl(4)(bpe)(2)}] ((Bu(4)N)(2)[3a]), and ((n-C(4)H(9))(4)N)(2)[cis-{Re(6)S(8)Cl(4)(bpe)(2)}] ((Bu(4)N)(2)[3b]) were prepared, and X-ray single-crystal structure determination was carried out for (Bu(4)N)(2)[2a] and (Bu(4)N)(2)[3a]. The photophysical properties of these complexes were studied both in acetonitrile at 298 K and in the solid state at 298 and 80 K, along with those of the known 4,4'-bipyridine (bpy) analogues ((n-C(4)H(9))(4)N)[mer-{Re(6)S(8)Cl(3)(bpy)(3)}] ((Bu(4)N)[4]), ((n-C(4)H(9))(4)N)(2)[trans-{Re(6)S(8)Cl(4)(bpy)(2)}] ((Bu(4)N)(2)[5a]), and ((n-C(4)H(9))(4)N)(2)[cis-{Re(6)S(8)Cl(4)(bpy)(2)}] ((Bu(4)N)(2)[5b]). The photophysical data of [5a](2-) and [5b](2-) in solution and in the solid state were significantly different from those of other complexes. On the basis of experimental observations of [2a](2-) and [5a](2-) and density-functional theory (DFT) calculations, it was concluded that [5a](2-) and [5b](2-) exhibited metal (Re(6)S(8) core)-to-ligand (bpy) charge transfer (MLCT) type emission. This is the first unambiguous demonstration of MLCT type emissions for the hexarhenium complexes. The MLCT components, where present, are only minor in the case of the emissions of [1](-), [2a](2-), [2b](2-), and [4](-); these can be explained primarily as the contributions of the intracore electronic transitions. The emissions of [3a](2-) and [3b](2-) can be assigned almost completely to the electronic transitions within the Re(6)S(8) core. The different emission characteristics of the bis(bpy) complexes ([5a](2-) and [5b](2-)) from the tris(bpy) complex ([4](-)) are a result of the increase in the number of nitrogen donors on the Re(6)S(8) core, which stabilizes the Re(6)S(8) core energy to a lower level than the energy of the bpy ligand pi* orbital. On the other hand, it has been shown that the emissions of the bis(ppy) ([2a](2-) and [2b](2-)) and bis(bpe) complexes ([3a](2-) and [3b](2-)) are best characterized by the higher pi* energy level of each N-heteroaromatic ligand, which lead to a stronger metal character in the emissive excited state of the complex.

Co-catalytic effects of nitrogen donors on the epoxidation of cyclooctene with tetra- n-butylammonium hydrogen monopersulfate in the presence of manganese(III)tetraarylporphyrins: A comparative study

The epoxidation of cyclooctene by tetra- n-butylammonium hydrogen monopersulfate was performed in the presence of four different manganese(III)tetraarylporphyrins as catalysts and a number of nitrogen donors as co-catalysts under a fixed co-catalyst/catalyst ratio (100:1). It was observed that the σ-bonding abilities of the nitrogen donors, as related to their p K a (BH +) values, were not a reliable factor for determining their co-catalytic activities, unless there were no π-interactions and/or no significant involvement of steric hindrance. On the other hand, the π-donating ability of the nitrogen donors was found to be of importance in defining their role as co-catalysts. The strong π-donor N–H imidazoles displayed co-catalytic activities greater than strong σ-donor amines and weak π-donor pyridines in the presence of manganese(III)tetraphenylporphyrin acetate (MnTPP(OAc)). In the case of hindered manganese(III)tetramesitylporphyrin acetate and manganese(III)tetrakis(2,6-dichlorophenyl)porphyrin acetate steric hindrance of the nitrogen donors played a dominant role, and the least hindered imidazole (ImH) was the best co-catalyst. When manganese(III)tetrakis(pentafluorophenyl)porphyrin acetate(MnTPFPP(OAc)) was employed as catalyst then pyridines, in general, demonstrated a higher co-catalytic activities than imidazoles. The same trend was also observed under various co-catalyst/catalyst ratios (1–200), and pyridines were always more effective co-catalysts than imidazoles with MnTPFPP(OAc), whereas for MnTPP(OAc) the order for co-catalytic activities of these nitrogen donors was inverted. Also, in the presence of electron deficient manganese(III)tetrakis(4-nitrophenyl)porphyrin acetate imidazoles acted generally as more effective co-catalysts than pyridines, at co-catalyst/catalyst ratios < 100. It is proposed that the varied attractive C–H⋯F–C interactions between the ortho-C–F σ* orbitals of the pentafluorophenyl groups in MnTPFPP(OAc) complex and C–H σ bonds adjacent to the nitrogen donor sites of pyridines or imidazoles could be responsible for the observed differences. The UV–vis spectra of the catalytic systems containing pyridine and ImH in association with MnTPP(OAc) and MnTPFPP(OAc) were examined for the active oxygen intermediates.

Cd(II) complexation in aqueous solution with dipyridine- and phenanthroline-containing polyamine macrocycles

The coordination features toward Cd(II) of four phenanthroline-containing and three dipyridine-containing macrocyclic polyamines have been studied by means of potentiometric and spectrophotometric UV–Vis measurements in aqueous solutions. All ligands form stable 1:1 metal complexes. The stability constants of the Cd(II) are generally lower than those reported for Cd(II) complexes with aliphatic macrocyclic polyamines containing the same number of nitrogen donors. This effect is mainly related to the stiffening of the ligands, due to the insertion of a large and rigid heteroaromatic moiety within the aliphatic polyamine framework. This structural feature does not allow the nitrogen donor to achieve an optimal arrangement around the metal ion. The decreased stability is often accompanied by facile protonation of the complexes, suggesting that some amine groups are not involved in metal binding.

Interaction of long polyazaalkanes with zinc(II) and cadmium(II) ions. A thermodynamic and13C nuclear magnetic resonance study

The interaction of Zn2+ and Cd2+ ions with a series of terminally methylated open-chain polyazaalkanes CH3NHCH2CH2NH[CH2CH2NH]nCH2CH2NHCH2CH2NHCH3(n= 1–5, L1–L5) has been studied by potentiometry and direct microcalorimetry at 298 K in 0.15 mol dm–3 NaClO4, as well as by 1H and 13C NMR techniques. All these ligands form mononuclear complexes with Zn2+ and Cd2+. The stability constants and the enthalpy of formation of the species [ML]2+ for both metal ions are roughly constant along the series. The 13C NMR spectra of the complexes [ZnL1]2+ and [CdL1]2+ indicate the presence of a C2 symmetry axis. All the experimental evidence suggests five-co-ordination for the zinc complexes and six-co-ordination for the cadmium complexes. The great upfield shifts experienced by the methyl carbons at both ends of the chain suggest the involvement of the terminal nitrogens in the co-ordination to the metals. The three largest ligands form binuclear zinc and cadmium complexes, the stability of which increases with the number of nitrogen donors. Comparison with analogous polyazacycloalkanes shows that a macrocyclic effect is observed only for the zinc mononuclear complexes of the shortest ligand L1 and for the cadmium complexes of L1 and L2. For all the other complexes, mono- or binuclear, a higher stability of the complexes of the open-chain ligands is observed. On the other hand, 13C NMR spectra of the ligands L3–L5 allow a general protonation pattern to be established based on minimization of repulsive interactions between same-sign charges.