F. Wasgestian

Photocatalytic reduction of nitrate ions on TiO2 by oxalic acid

Abstract Using oxalic acid as a hole scavenger, the photocatalytic reduction of nitrate in aqueous dispersions of TiO 2 has been investigated. Oxalic acid accelerates the reaction considerably compared to the reaction in the absence of oxalic acid. In the presence of nitrate, the oxidation of oxalate is also accelerated. The reduction product of the nitrate is mainly ammonia. The effects of concentrations of nitrate, of oxalate, and of pH were studied. The adsorption of the reactants on TiO 2 is an important factor for accelerating the reaction.

Charge transfer photochemistry of Reinecke's salt and of some of its analogues containing organic amines

Abstract N-substituted Reineckates of the compositions [CrA2(NCS)4]− (A=NH3, aniline, n-propylamine, morpholine) and [Cr(A-A)(NCS)4]− (A-A=en, tn) have been prepared. The ligand field absorption maxima were slightly shifted to longer wavelengths in the order NH3

Photochemical chloride anation and aquation of hexammine chromium(III) in acid solution

Abstract Photolysis of aqueous solutions of [Cr(NH 3 ) 6 ] 3+ (pH = 2.0) containing chloride ions produces [Cr(NH 3 ) 5 H 2 O] 3+ and [Cr(NH 3 ) 5 Cl] 2+ . The competition between photoanation and photoaquation was studied as a function of Cl − concentration (0–8 M LiCl). The quantum yield for ammonia release remained constant over the whole concentration range (ф = 0.41). Comparable efficiency of anation and aquation was found between 2–3 M Cl − . A mechanism assuming a trigonal prismatic transition state is proposed for photosubstitution reactions of Cr(III) complexes.

HIGHER STABILITY CONSTANTS FOR COMPLEXATION OF LOW VALENT, THAN OF HIGH VALENT, TRANSITION METAL IONS BY SATURATED TERTIARY AMINE MACROCYCLIC TETRAAZA LIGANDS

Recent results indicate that N-methylation of tetraaza macrocyclic ligands stabilizes low valent transition metal complexes [l-7]. It has been recently suggested [8] that at least for nickel*+‘l+ couples the redoxpotentials are in the order Ni(5,7,7,12,14,14hexamethyl-1,4,8,11-tetraaza-cyclotetradecane)2~‘1+ < Ni(H20),*+” + <Ni(1,4,5,7,7,8,11,12,14,14-decamethyl-1,4,8,11-tetraaza-cyclotetradecane)*+”+. In this series the redox potential of Ni(H20)62+“+ could only be obtained as a rough estimate. This conclusion, if corroborated, suggests that tertiary amine ligands bind preferentially to the low valent nickel ion, and probably to other low valent transition metal ions. It was therefore decided to measure the redox potentials of trans-CrL(Hz02 ‘+a+ ions with L= L’ = 1,4,8,11-tetraza-cyclotetradecane and L2’ 1,4,8,1l-tetramethyl-1,4,8,1l-tetraza-cyclotetradecane. The experimental results indeed corroborate the above suggested hypothesis. The free ligands cyclam and tmc were synthesized according to the literature [9, lo]. trans-CrL’(H20)23+ was prepared via the steps cis-CrL1C12+ [ll] and trans-CrL’(CN)2+ [12] which was treated with tetrafluoroboric acid. The synthesis of the novel complex truns-CrL2(H20)23+ was accomplished by a modification of a described procedure [13]. The spectral characteristics, including luminescence of the known rr~ns-CrL’(H~O),~+ are identical to those CH2 vibration in accord with expectations for a truns isomer, whereas the cis isomer should have a doublet at least. The UV-Vis spectrum showed the expected shift to lower ligand field and the characteristics of a D4,, configuration (h,,(nm) (c(mol-’ dm3 cm-‘)) 582 (28); 393sh; 334 (92)). The redox potentials of the complexes were determined by cyclic voltammetry on a hanging mercury drop electrode (HMDE). Voltammograms of ~ransCrL’(H,O);+ and trans-CrL2(H20)23+ are shown in Fig. 1. Besides the more anodic potential the L* complex exhibits a better reversibility of the redox process E,, -Ered = 0.08 f 0.01 and 0.12 f 0.01 V, respectively. The redox potentials determined depend on the pH as expected. For truns-CrL’ the following potentials are measured: -0.90, -0.96 and 1.05 V versus SCE at pH 2.9, 4.2 and 7.2, respectively (accuracy&O.010 V). At these pHs the major forms of the complex present in the solution are trunsCrL’(H,O)?+, tram-CrL’(H,O)(OH)*+ and frunsCrL’(OH)*+, respectively. For truns-CrL* the following potentials are measured: 0.250, 0.115 and 0.005 Vversus SCE at pH 2.9,5.2 and 7.9, respectively. (The measurement at pH 2.9 could not be carried out on the mercury electrode due to its oxidation at these potentials, therefore a solid gold electrode was used. The background current on this electrode at the required potentials was not negligible and the

Homogeneous catalysis by tetra-t-butylphthalocyanine iron: intermediates in the oxidative dimerization of 2,6-di-t-butylphenol

The oxygen-bridged dinuclear complex derived from tetra-t-butylphthalocyaninato-iron efficiently catalyses the oxidative dimerization of o,o′-disubstituted phenols such as 2,6-di-t-butylphenol to yield 3,3′,5,5′-tetra-t-butyl-p-bisphenoquinone. UV and NMR spectroscopy, as well as an independent synthesis of the product, clearly indicate that the intermediate 3,3′,5,5′-tetra-t-butylbiphenyl-4,4′-diol (3) is accumulated at high substrate to catalyst ratios, i.e. about 1550, whereas at ratios of about 100 the bisphenoquinone 4 is exclusively formed.

The crystal structures and doublet emission properties of hexaam(m)ine Cr(III) complexes

Abstract Hexaam(m)ine Cr(III) complexes are reviewed on which X-ray data were published or of which the phosphorescence was measured in the crystalline state. In addition complexes of the type mer-[Cr(NH3)(diamine)(triamine)]3+ have been prepared. In all cases, the complex cations were crystallised as chloromercury(II) salts and single-crystal X-ray structures have been determined for six systems. Apart from mer-[Cr(NH3)(Me2tn)(2,3-tri)]HgCl5·1.5H2O (5) (monoclinic, C2/c) which has both endo and exo forms in the lattice, the other six complexes adopt the exo–mer-triamine configuration. The 1,2-diaminopropane (pn) ligand in mer-[Cr(NH3)(pn)(3,3-tri)]HgCl5·2H2O (2) (monoclinic, P21/c) is end-for-end disordered and the 1,2-cyclohexane diamine ligand (chxn) is the trans-(RR,SS)-isomer in both mer-[Cr(NH3)(chxn)(3,3-tri)]HgCl5 (3) (monoclinic, P21/n) and mer-[Cr(NH3)(chxn)(2,3-tri)]HgCl5·2H2O (6) (monoclinic, P21c). In 2, 3, 5 and 6 above, the chloromercury(II) formula unit is a distorted tetrahedral HgCl42− plus an isolated Cl− ion. The Hg2Cl73− formula unit in mer-[Cr(NH3)(tn)(2,3-tri)]Hg2Cl7·H2O (4) (triclinic, P1) adopts an HgCl3− plus HgCl42− arrangement. A novel [Hg4Cl17]9− stoichiometry is found in mer-[Cr(NH3)(en)(3,3-tri)]3[HgCl4]4·Cl·3H2O (1) (triclinic, P1). The 2Eg–4A2g (Oh) intercombination bands were measured at 20 K both in absorption and emission. The phosphorescence decay curves of selected single emission lines were determined at temperatures 18

Doublet versus quartet reactivity in the photoanation of hexammine chromium(III) ion

The photoanation of [Cr(NH3)6]3+ by Cl− and Br− was studied in acid solution. No difference in quantum yields was found for both anions. The photoanation by Cl− was further studied in alkaline solution and in acid solution containing Co(II). In those solutions the 2Eg state of Cr(III) is quenched. The limiting quantum yield for complete doublet quenching (unquenchable part of the photoreaction) is identical in acid and alkaline solution and has the same value as for the aquation (φnq = 0.14). The quenchable part of the photoreaction depends on the Co(II) concentration the same way as the phosporescence. The probable photochemical pathways are discussed.

Solvent dependence of the spin-allowed transitions in free base tetra-t-butylphthalocyanine

Abstract The electronic absorption spectrum of tetra-t-butylphthalocyanine (TTBPc) was studied in various solvents. All bands were slightly red-shifted on increasing polarity of the solvents, indicating π—π* transitions. The oscillator strength of the Q x band varied between 0.11 (t-butanol) and 0.27 ( n -octane) and that of the Q y band between 0.17 (t-butanol) and 0.28 (pyridine). The influence of the refractive index was masked by other solvent effects. Good agreement of the measured oscillator strength with modern theoretical values was obtained, when the random orientation of the solute molecules was taken into account.

Ligand field photochemistry of mixed-ligand cyanochromates. Comparative study of [Cr(CN)5(OH)]3− and [Cr(CN)4(N3)(OH)]3−

Abstract The novel mixed-ligand cyanochromate, [Cr(CN)4(N3)OH)]3−, has been synthesized and its LF-photochemistry has been studied and compared with that of the better known pentacyanohydroxochromate(III). In both the cases the reaction consists in the photosubstitution of one CN− ligand followed by fast thermal CN− release leading to the formation of one relatively stable product. The chemical and spectral analysis of the products showed their formulation as triacido-complexes, [Cr(CN)3(OH)n(H2O)3−n]n− and [Cr(CN)2(N3)-(OH)n(H2O)3−n]n−, respectively. The results contribute to the discussion on preferential ligand photosolvation.

The effect of N-methylation on the spectroscopical and electrochemical properties of 1,4,8,11-tetraazacyclotetradecane chromium(III) complexes

Abstract trans -[Cr(tmc)X 2 ] complexes (tmc=1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane, XF, Cl, Br, H 2 O, OH, NO 3 ) have been prepared and spectroscopically characterized. The redox potentials of the tmc complexes and of the analogous cyclam complexes (cyclam=1,4,8,11-tetraazacyclotetradecane) were determined by cyclic voltammetry. The potentials of the tmc complexes are 1.1±0.1 V more positive. Their long wavelength absorption is red-shifted by 50 to 100 nm with respect to that of the analogous cyclam complexes. The shift was consistent with a linear dependence of the polarographic half-wave potentials on the transition energies of the first spin allowed ligand field bands, observed on a series of chromium(III) amine complexes. The reaction entropies for the reduction of trans -[Cr(cyclam)(H 2 O) 2 ] 3+ , trans -[Cr(cyclam)(OH) 2 ] + , and trans -[Cr(tmc)(OH) 2 ] + , Δ S =88±2, 200±40 and 53±8 J mol −1 K −1 , respectively, were obtained from the temperature dependence of the redox potential. It was shown that trans -[Cr(cyclam)(H 2 O) 2 ] 3+ catalyzes the electroreduction of nitrate.