Chemical oxidation of the ligand in nickel(II) cyclam: formation of a novel dinuclear complex and of a related cation containing a ligand radical ion

Diastereomeric dinickel(II) complexes with non-innocent bis(octaazamacrocyclic) ligands: isomerization, spectroelectrochemistry, DFT calculations and use in catalytic oxidation of cyclohexane

Diastereomeric dinickel(II) complexes with bis-octaazamacrocyclic 15-membered ligands [Ni(L1-3-L1-3)Ni] (4-6) have been prepared by oxidative dehydrogenation of nickel(II) complexes NiL1-3 (1-3) derived from 1,2- and 1,3-diketones and S-methylisothiocarbohydrazide. The compounds were characterized by elemental analysis, ESI mass spectrometry, and IR, UV-vis, 1H NMR, and 13C NMR spectroscopy. Single crystal X-ray diffraction (SC-XRD) confirmed the isolation of the anti and syn isomers of bis-octaazamacrocyclic dinickel(II) complexes 4a and 4s, the syn-configuration of 5s and the anti-configuration of the dinickel(II) complex 6a. Dimerization of prochiral nickel(II) complexes 1-3 generates two chiral centers at the bridging carbon atoms. The anti-complexes were isolated as meso-isomers (4a and 6a) and the syn-compounds as racemic mixtures of R,R/S,S-enantiomers (4s and 5s). The syn-anti isomerization (epimerization) of the isolated complexes in chloroform was disclosed. The isomerization kinetics of 5a was monitored at five different temperatures ranging from 20 °C to 50 °C by 1H NMR spectroscopy indicating the clean conversion of 5a into 5s. The activation barrier determined from the temperature dependence of the rate constants via the Eyring equation was found to be ΔH‡ = 114 ± 1 kJ mol-1 with activation entropy ΔS‡ = 13 ± 3 J K-1 mol-1. The complexes contain two low-spin nickel(II) ions in a square-planar coordination environment. The electrochemical behavior of 4a, 4s, 5s and 6a and the electronic structure of the oxidized species were studied by UV-vis-NIR-spectroelectrochemistry (SEC) and DFT calculations indicating the redox non-innocent behavior of the complexes. The dinickel(II) complexes 4a, 4s, 5s and 6a/6s were investigated as catalysts for microwave-assisted solvent-free oxidation of cyclohexane by tert-butyl hydroperoxide to produce a mixture of cyclohexanone and cyclohexanol (KA oil). The best value for KA oil yield (16%) was obtained with a mixture of 6a/6s after 2 h of microwave irradiation at 100 °C.

Electron exchange columns through entrapment of a nickel cyclam in a sol-gel matrix

An electron exchange column (analogous to ion exchange columns) was developed using the unique redox properties of the nickel-tetraazamacrocyclic complexes (nickel cyclam [Ni(II)L(1)](2+)) and nickel-trans-III-meso-5,7,7,12,14,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane, ([Ni(II)L(2)](2+)), and the physical and chemical stability of the ceramic materials using the sol-gel process to entrap the complexes. The entrapment by the biphasic sol-gel method is based on non-covalent bonds between the matrix and the complex; therefore the main problem was leaching. Parameters controlling the leaching were investigated. Redox cycles with the reducing agent ascorbic acid, and persulfate as the oxidizing agent were performed.

Tunable Charge Delocalization in Dinickel Quinonoid Complexes

When a 2,5-diamino-1,4-benzoquinonediimine C6H2(=NR)2(NHR)2 (2) is used as a bridging ligand, new dinickel(II) complexes [(acac)Ni[mu-C6H2(=NPh)4]Ni(acac)] (3a: R=Ph) and [(acac)Ni[mu-C6H2(=NCH2tBu)4]Ni(acac)] (3b: R=CH2tBu) are obtained; upon one-electron oxidation of these complexes delocalized mixed-valence compounds are formed. An X-ray diffraction study on 3b reveals equalization of the bond lengths within each of the ligand 6 systems and a lack of conjugation between them. The oxidized state in 3b+ involves both the bridging quinonoid ligand and the metal centers, with a major contribution coming from the bridging ligand. Electrochemical and spectroscopic methods were used to study the influence of the N-substituents of the tetranitrogen donor ligands 2. In this combined experimental and theoretical (DFT) study, it is also shown that the electronic structure within the dinickel system can be altered by addition of a coordinating ligand such as pyridine. The latter favors the high-spin configuration with semi-occupied metal-centered orbitals, leading to a metal-metal interaction in the mixed-valence Ni(II)-Ni(III) 3b+ system.

Reactions of superoxo and oxo metal complexes with aldehydes. Radical-specific pathways for cross-disproportionation of superoxometal ions and acylperoxyl radicals

The aquachromyl(IV) ion, Cr(aq)O(2+), reacts with acetaldehyde and pivaldehyde by hydrogen atom abstraction and, in the presence of O(2), produces acylperoxyl radicals, RC(O)OO(*). In the next step, the radicals react with Cr(aq)OO(2+), a species accompanying Cr(aq)O(2+) in our preparations. The rate constant for the Cr(aq)OO(2+)/CH(3)C(O)OO(*) cross reaction, k(Cr) = 1.5 x 10(8) M(-1) s(-1), was determined by laser flash photolysis. The evidence points to radical coupling at the remote oxygen of Cr(aq)OO(2+), followed by elimination of O(2) and formation of CH(3)COOH and Cr(V)(aq)O(3+). The latter disproportionates and ultimately yields Cr(aq)(3+) and HCrO(4)(-). No CO(2) was detected. The Cr(aq)OO(2+)/C(CH(3))(3)C(O)OO(*) reaction yielded isobutene, CO(2), and Cr(aq)(3+), in addition to chromate. In the suggested mechanism, the transient Cr(aq)OOOO(O)CC(CH(3))(3)(2+) branches into two sets of products. The path leading to chromate resembles the CH(3)C(O)OO(*) reaction. The other products arise from an unprecedented intramolecular hydrogen transfer from the tert-butyl group to the CrO entity and elimination of CO(2) and O(2). A portion of C(CH(3))(3)C(O)OO(*) was captured by (CH(3))(3)COO(*), which was in turn generated by decarbonylation of acyl radicals and oxygenation of tert-butyl radicals so formed.

Generation and reactivity of rhodium(IV) complexes in aqueous solutions

At pH = 1 and 25 degrees C, the Fenton-like reactions of Fe(aq)(2+) with hydroperoxorhodium complexes LRh(III)OOH(2+) (L = (H(2)O)(NH(3))(4), k = 30 M(-1) s(-1), and L = L(2) = (H(2)O)(meso-Me(6)-[14]aneN(4)), k = 31 M(-1) s(-1)) generate short-lived, reactive intermediates, believed to be the rhodium(IV) species LRh(IV)O(2+). In the rapid follow-up steps, these transients oxidize Fe(aq)(2+), and the overall reaction has the standard 2:1 [Fe(aq)(2+)]/[LRhOOH(2+)] stoichiometry. Added substrates, such as alcohols, aldehydes, and (NH(3))(4)(H(2)O)RhH(2+), compete with Fe(aq)(2+) for LRh(IV)O(2+), causing the stoichiometry to change to <2:1. Such competition data were used to determine relative reactivities of (NH(3))(4)RhO(2+) toward CH(3)OH (1), CD(3)OH (0.2), C(2)H(5)OH (2.7), 2-C(3)H(7)OH (3.4), 2-C(3)D(7)OH (1.0), CH(2)O (12.5), C(2)H(5)CHO (45), and (NH(3))(4)RhH(2+) (125). The kinetics and products suggest hydrogen atom abstraction for (NH(3))(4)RhO(2+)/alcohol reactions. A short chain reaction observed with C(2)H(5)CHO is consistent with both hydrogen atom and hydride transfer. The rate constant for the reaction between Tl(aq)(III) and L(2)Rh(2+) is 2.25 x 10(5) M(-1) s(-1).

Shape-selective transport through rectangle-based molecular materials: thin-film scanning electrochemical microscopy studies

Microporous thin films (approximately equal to 50 to 400 nm) composed of discrete, cavity-containing molecular rectangles have been prepared. The films, which contain both amorphous and microcrystalline domains, display shape-selective transport behavior. They are permeable to small molecules and to molecules that are short or narrow in at least one dimension--for example, elongated planar molecules--but are impermeable to molecules lacking a narrow dimension. However, the shape selectivity is based on transport through intramolecular rather than intermolecular cavities. By using redox-active probe molecules, rates of transport through the rectangle-based material have been extracted from electrochemical measurements. Spatially resolved measurements obtained via scanning electrochemical microscopy have permitted transport through individual microcrystals to be evaluated semiquantitatively. The measurements reveal that transport is roughly two orders of magnitude slower than observed with thin microcrystalline films of molecular squares featuring similar-sized cavities. The differences likely reflect the fact that cavities within the square-based materials, but not the rectangle-based material, align to form simple one-dimensional channels.

Resonance Raman and semiempirical electronic structure studies of an odd-electron dinickel tetraiminoethylenedimacrocycle complex

Resonance Raman studies of Ni2TIED3+ (TIED = tetraiminoethylenedimacrocycle) reveal that many modes couple to the intense electronic transition centered at 725 nm, a feature that is nominally similar to the intense delocalized intervalence absorption bands observed in the same region for Fe2(TIED)L4(5+) and Ru2(TIED)L4(5+) (L is any of several axial ligands). Time-dependent spectral modeling of the Raman and absorption spectra for the nickel compound was undertaken to understand the electronic transition. We were unable to model the Raman and absorption spectra successfully with a single electronic transition, suggesting that the absorption band is made up of two overlapping transitions. Semiempirical electronic structure calculations corroborate the suggestion. Additionally, these calculations indicate that the transitions are in fact ligand-localized transitions, with little metal involvement and no charge-transfer character. Furthermore, the ground-state electronic structure is best described as an identical pair of NiII centers bridged by a radical anion rather than a three-site mixed-valence assembly. Previous EPR studies (McAuley and Xu, Inorg. Chem. 1992, 31, 5549) had indicated primarily ligand character for the radical. The assignments are consistent with the resonance Raman results where the dominant modes coupled to the transitions are assigned as totally symmetric bridge vibrations.

Stepwise Complexation of Ni(II) and Cu(II) Ions by 6,6'-C-spirobi(cyclam) (cyclam = 1,4,8,11-Tetraazacyclotetradecane), L(1). Syntheses and Redox Chemistry of [M(H(2)L(1))]X(4) (M = Cu(2+), Ni(2+)), [Cu(2)(L(1))]X(4), and [CuNi(L(1))]X(4) (X = ClO(4)(-)) and the X-ray Crystal Structure of [Cu(2)(L(1))](ClO(4))(4)

In aqueous HClO(4), the cation [H(4)(L(1))](4+) where L(1) is 6,6'-C-spirobi(cyclam) (cyclam = 1,4,8,11-tetraazacyclotetradecane), complexes Cu(2+) and Ni(2+) ions in a stepwise fashion to form [M(H(2)L(1))](ClO(4))(4) (M = Cu(2+) and Ni(2+)) from which [CuNi(L(1))](ClO(4))(4) has been prepared selectively. The preparation and the structure of [Cu(2)(L(1))](ClO(4))(4) (empirical formula, C(19)H(44)N(8)Cu(2)Cl(4)O(16); space group, triclinic; P&onemacr;; a = 8.1815(6) Å, b = 12.6098(9) Å, c = 16.6565(12) Å, alpha = 80.3890(10) degrees, beta = 76.5840(10) degrees, gamma = 87.1750(10) degrees, V = 1647.9(2) Å(3), and Z = 2; of the 9531 total reflections collected, 6779 reflections with I > 2sigma(I) on least-squares refinement provided final R(1) = 0.0657 and wR(2) = 0.1424) are also reported. The cyclic voltammograms (1.0 M NaCl, 0.1 M H(+); Pt electrodes; all E(1/2) vs NHE) of [M(H(2)L(1))](4+) (M = Cu(2+) and Ni(2+)) ions show single waves for the Cu(II)/Cu(III) couple (E(1/2) = 0.79 V, irreversible) and the Ni(II)/Ni(III) couple (E(1/2) = 0.56 V, reversible), respectively. In CH(3)CN (0.1 M Et(4)NClO(4)), the [Cu(2)(L(1))](4+) ion shows a reversible wave for the Cu(II)-Cu(II)/Cu(II)-Cu(III) couple ((1)E(1/2) = 1.120 V) and an irreversible wave for the Cu(II)-Cu(III)/Cu(III)-Cu(III) couple ((2)E(1/2) = 1.430 V). Similarly, a reversible wave for the Cu(II)-Ni(II)/Cu(II) -Ni(III) couple ((1)E(1/2) = 0.750 V) and an irreversible wave for the Ni(III)-Cu(II)/Ni(III)-Cu(III) couple ((2)E(1/2) = 1.20 V) are observed in the case of the [Cu(II)Ni(II)(L(1))](4+) ion. The [Cu(2)(L(1))](4+) ion (g( perpendicular) = 2.120, g( parallel) = 2.256 and 2.196, A( parallel) = 150 G, and D( parallel) = 75 G) and the mixed valent species [Cu(II)Ni(III)(L(1))](5+) (for Cu(2+), g( perpendicular) = 2.060, g( parallel) = 2.219, and A( parallel) = 100 G; for Ni(3+) in sulfate media, g( perpendicular) = 2.204 and g( parallel) = 1.967) and [Cu(II)Cu(III)(L(1))](5+) (g(xx)() = 1.982, g(yy)() = 2.155, g(zz)() = 2.386, A(yy)() = 80 G, and A(zz)() = 120 G) show dipolar-dipolar interaction. In the mixed-valent ions, due to strong electrostatic repulsion from either the Ni(III) or Cu(III) ions, significantly smaller A( parallel) (or A(zz)()) values are observed for the Cu(2+) ion compared to 200 G in the mononuclear ions. Also, the [Cu(II)Ni(II)(L(1))](4+) in aqueous HClO(4) and the [Ni(2)(L(1))](4+) ion in CH(3)NO(2) show a tendency to reduce perchlorate very slowly.