1,721,011 research outputs found
Mono- and di-protonated 1,4-dihydrazinophthalazine (C8H10N6). Structures of C8H11N6.Cl and C8H12N6.2Cl.2H2O
No full textTermodynamic Aspects of the Ni(III)/Ni(II) Redox Change in Poly-Aza-Macrocyclic Complexes.
No full textThe reaction entropies of the Ni(III)/Ni(II) redox couple for some polyaza macrocyclic complexes in aqueous solution have been
determined by measuring E1/2(Ni(III)/Ni(II) values through voltammetry experiments at varying temperatures using a nonisothermal
cell. reaction entropies values are in any case positive, and their magnitudes decrease with the increasing number of nitrogen atoms of the
coordinating system (from four to six): this is ascribed to the release of water molecules from the coordination sphere and/or
the hydration sphere during the Ni"'/Ni" redox change. The presence of negative charges on the coordinating system (deprotonated
amido or carboxylate groups) makes the reaction entropies ,(Ni"'/Ni1') values substantially less positive. Redox equilibria of the type M"'L, + M"L2 in equilibrium with M"L1 + M"'L2 have been considered and the corresponding values of the enthalpy term evaluated for chosen
pairs of ligating systems L1, and L2. Inspection of the above values indicates that the enthalpy contribution (which includes bond
energy terms) favors the attainment of trivalent nickel along the sequence N(amine) > N(deprotonated amido groups) = -COOand
that a square arrangement of four nitrogen atoms (cyclam) is the very most favorable coordinative situation (from the enthalpic
point of view)
Nickel(III) and copper(III) complexes with 13- and 14-membered tetra-aza macrocycles. Ring-size and medium effects on the MIII/MII redox couple potentials
No full textThe one-electron oxidn. of the NiII and CuII complexes with 13- and 14-membered macrocycles I (n = 2, 3) that produce authentic NiIII and CuIII species was electrochem. investigated and the E1/2 values assocd. with the reversible MIII/MII redox couple were detd. by a differential pulse voltammetry technique. The 13-membered ring favors the attainment of the CuIII state, whereas the 14-membered ring favors the formation of the NiIII complex. This opposite behavior is satisfactorily interpreted in terms of size of the metal ions and aperture of the macrocyclic cave. Moreover the effect of the concn. of the inert electrolyte (NaClO4, 0.1-7.0M) on the E1/2(MIII/MII) values was investigated: the increase of the NaClO4 concn. favors the formation of the CuIII complexes and does not favor the formation of the NiIII species. This behavior is ascribed to the destabilization of NiIII and CuII complexes induced by perchlorate ion through the formation of H bonds with water mols. axially coordinated to the metal ions
Copper(II) complexes of 2-carbamoylethyl derivatives of linear diamines. Crystallographic evidence for amido oxygen binding prior to deprotonation of amide nitrogens
No full textDicopper(II) and dicopper(III) complexes with a double-ring octaaza macrocycle
No full textThe bicyclic amine I was prepd. in 30% yield by condensation of tetra-Et 1,1,2,2-ethanetetracarboxylate with 1,4,8,11-tetraazaundecane in refluxing EtOH for 2 days and formed a 1:2 complex with Cu2+ in aq. soln. The 2Cu2+-I complex showed weak metal-metal interaction and was easily oxidized to the trivalent state by 2 consecutive one-electron steps sepd. by 110 mV
A ditopic tetradentate pyridyl amine ligand containing an anthracene fragment: fluorescence intensity and ‘closed’ vs. ‘open’ species formation in the presence of Cu2+, as a function of pH
No full textThe compounds 9,10-bis(2-pyridylmethylaminoethyl)anthracene L1 and 9-(2-pyridylmethylaminomethyl)anthracene L2, based on nitrogen donors of the amine and pyridine type, were prepared and their properties examined in aqueous solution (water–1,4-dioxane 1∶4 v/v). The protonation constants of both have been determined by means of potentiometric titrations and the obtained species vs. pH distribution diagrams superimposed on the fluorescence intensity vs. pH profiles. This revealed an ‘off-on-offhair space’ window behaviour: the fluorescence intensity is high only in the pH range in which both free pyridine and protonated amines exist. Moreover, the formation constants of the complex species which form in the presence of 1 or 0.5 (in the case of L1 and L2, respectively) equivalent of CuII have been determined by means of potentiometric titrations. In particular, in the case of the tetradentate ditopic ligand L1 the ‘closed’ two ligand–two metal species [Cu2L12]4+ and [Cu2L12(OH)]3+ form in basic solution, in addition to two ligand–one metal partially protonated ‘open’ complex species. Moreover, a strong variation of the fluorescence intensity of the anthracene fragment signals the formation of the ‘closed’ species with respect to the ‘open’ ones, while a more subtle variation of the fluorescence intensity also allows one to distinguish between [Cu2L12]4+ and [Cu2L12(OH)]3+
Scandium trifluoromethanesulfonate as catalyst in the hydrogen/deuterium exchange of 1,3,5-trimethoxybenzene: evidence for a direct interaction of scandium with the aromatic ring
No full textRing-size effects on the formation of dicopper(II) and dicopper(III) complexes with bimacrocycles, 13- and 14-membered tetraaza subunits
No full textThe double-ring macrocycle I, having 13-membered subunits, incorporates 2 CuII ions in aq. soln. in 3 pH-controlled steps, which involve progressive deprotonation of the coordinated amido groups of the ligand. The fully deprotonated neutral dicopper(II) complex undergoes a reversible oxidn. process to the dicopper(III) species according to 2 1-electron redox changes sepd. by 100 mV. This behavior is compared with that of the corresponding bimacrocyclic ligand having 14-membered subunits as well as with single-ring ref. systems. The 14-membered subunit forms the more stable complexes with CuII ion, whereas the 13-membered cavity favors the access to the trivalent state, with both single- and double-ring systems
Stepwise incorporation of copper(II) into a double-ring octaaza macrocycle and consecutive oxidation to the trivalent state
No full textThe double-ring octaaza macrocycle bisdioxocyclam (I) was prepd. by the single-step aminolytic condensation of the appropriate tetraester and linear tetraamine in 1:2 molar ratio. The complexation of Cu(II) by I in aq. soln. was studied by potentiometry. Monometallic and dimetallic complex species form according to the ligand:metal ratio. In general, incorporation of Cu(II) ion into each tetraaza subunit promotes a simultaneous deprotonation of the 2 amido groups. The log k values of complexation equil. are compared with those for the ref. single-ring tetraaza macrocycle dioxocyclam. The [CuII2(bisdioxocyclamato(4-))] complex undergoes a reversible oxidn. to the dicopper(III) complex through 2 consecutive 1-electron steps, whose electrode potentials are sepd. by 110 mV
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