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1,4,8,11-Tetraazacyclotetradecane-5,7-dione (dioxocyclam) copper(II) and aqueous copper(III) complexes
No full textDioxocyclam (H2L = 1,4,8,11-tetraazacyclotetradecane-5,7-dione), prepd. from di-Et malonate and 1,9-diamino-3,7-diazanonane, was reacted with Cu(OAc)2.H2O in aq. EtOH to give CuL. CuL was electrochem. or chem. (by K2S2O8) oxidized to [CuL]+
Solution chemistry in noncoordinating solvents (methylene chloride, chloroform) of nickel(II) and nickel(III) complexes of the lipophilic macrocycle 1-hexadecyl-1,4,8,11-tetraazacyclotetradecane. The emphasized role of the anion coordination
No full textNiLX2 (X = ClO4, BF4, I, Cl, Br, SCN, NO3; L = the lipophilic tetramine macrocycle 1-hexadecyl-1,4,8,11-tetraazacyclotetradecane) were prepd. and dissolve in noncoordinating solvents such as CH2Cl2 and CHCl3 as intact NiLX2, whose spin state and color depend on the coordinating tendencies of the apically bound X- anions: Cl-, and SCN-, blue, high-spin complexes; ClO4-, BF4-, and L-, yellow, low-spin complexes. NiLBr2 exists in soln. as an equil. mixt. of the blue and yellow species and the blue-to-yellow conversion is exothermic; moreover, the equil. is displaced to the right on further addn. of the background electrolyte, R4NBr (R = alkyl) or Et4PBr. A general model is proposed to explain the thermodn. aspects of the high-spin/low-spin interconversion in both coordinating (e.g. water) and noncoordinating media. Finally, NiLX2 in CH2Cl2 soln. (0.1 mol dm-3 Bu4NX) undergo reversible 1-electron oxidn. processes to give [NiLX2]X. The NiIII-NiII redox potential decreases dramatically with increasing coordinating tendency of the X- ions. A spectrochem. series was obtained for electrochem. inert inorg. anions. On this basis, it appears that ClO4- is a more strongly coordinating ligand than BF4-
Metal complexes that transport electrons across liquid membranes
No full textLipophilic metal complexes (displaying 1-electron redox activity) are used as carriers for the transport of electrons across bulk liq. membranes from an aq. reducing phase to an aq. oxidizing phase. The [FeIII,II(bpy)2]3=/2+ redox system (bpy = 4,4'-di-tert-butyl-2,2'-dipyridyl) transports electrons from both cationic and anionic aq. reducing agents to aq. Ce(IV) and counter-transports perchlorate ions in the opposite direction. Electron/chloride ion cross transport between aq. persulfate and aq. metal centered reducing agents, mediated by the membrane dissolved [NiII,IIILCl2]2+/3+ redox system (L = (N-cetylcyclam) takes place at a much higher rate. The overall transport rate in the 2 types of expts. controlled by that of the redox process occurring at the membrane/aq. reducing phase interface and is to be related to the type of mechanism (inner or outer sphere) by which the 2-phase electron transfer process takes place
σ and π Effects on the copper(II)/copper(I) redox couple potential in tetraazamacrocyclic complexes
No full textThe electrochem. redn. was studied of Cu(II) complexes with 14 membered tetraaza macrocycles with different structural features and different degrees of unsatn. Each complex undergoes a reversible 1-electron redn. at the Hg electrode in MeOH contg. 0.1 M Et4NBF4. The Cu(II)/Cu(I) process in macrocycles systems can be promoted through modification of the ligands that (a) reduce its σ-donor ability by destabilizing Cu(II) and (b) increase its π-acceptor properties by stabilizing Cu(I). Geometrical effects favorable to formation of Cu(I) can be induced in a fixed coplanar array of 4 N atoms by rearranging the system of fused chelate rings
Metal complexes that transport electrons across liquid membranes: the iron [FeII,IIIL3]2+,3+ redox system (L = 4,4'-di-tert-butyl-2,2'-bipyridine)
No full textLipophilic [FeL3][ClO4]2 (L = 4,4'-di-tert-butyl-2,2'-bipyridine) was prepd. to be used as a carrier for the transport of electrons from an aq. oxidizing phase to an aq. reducing phase, sepd. by a CH2Cl2 bulk liq. membrane. Two-phase redox expts. indicated that CH2Cl2-dissolved [FeL3][ClO4]2 is oxidized by aq. CeIv to give [FeL3][ClO4]3. [FeL3][ClO4]3 was reduced, under 2-phase conditions, by a series of aq. reducing agents according to the rate sequence: NO2- > [Fe(CN)64- > FeII > SO32-. Three-phase expts. were carried out in which electrons are transported by [FeL3][ClO4]2 from the aq. reducing phase to the aq. phase contg. CeIV, across the bulk liq. membrane, and ClO4- ions are transported by [FeL3][ClO4]3 in the opposite direction. The rate of the electron transport is controlled by the rate of the slowest of the 2 redox processes at the 2 sides of the membrane. By varying the concns. of the aq. reactants, it is possible to det. the 2-phase rate-detg. step of the overall 3-phase process and to perform selective oxidn. by CeIV of the investigated reducing agents
Redox active liquid membranes: transport of electrons across a dichloromethane layer mediated by the [CoIIIL1X2]+-[CoIIL1X2] cobalt redox system (L1 = 1-hexadecyl-1,4,8,11-tetraazacyclotetradecane, X = Cl or ClO4)
No full textLipophilic [CoIIILCl2]Cl (L = 1-hexadecyl-1,4,8,11-tetraazacyclotetradecane) can transport electrons, through the CoIII-CoII redox change, from an aq. phase contg. CrII to an aq. phase contg. the mild oxidizing agents FeIII or [NiIIIL1]3+ (L1 = 1,4,8,11-tetraazacyclotetradecane), in aq. HCl, across a bulk CH2Cl2 membrane. The transport of electrons, to which a counter transport of Cl- is coupled, implies that the potential assocd. with the [CoIIILCl2]Cl + e- → CoIILCl2 + Cl- half-reaction (-0.60 V vs. ferrocenium-ferrocene, measured in a CH2Cl2 soln., 0.1 mol dm-3 in NBu4Cl) should lie between that assocd. with the aq. CrIII-CrII couple and that assocd with the aq. Q-Q- couple (Q = FeIII or [NiIIIL1]3+). Replacement of Cl- by ClO4- stops the electron transport from CrII to FeIII or [NiIIIL1]3+. This is ascribed to the esp. high potential assocd. with the [CoIIIL(ClO4)2]ClO4 + e- → CoIIL(ClO4)2 + ClO4- half-reaction. CoIIL(ClO4)2 in a CH2Cl2 soln., covered by a layer of aq. HClO4, is not oxidized even by bubbling O2. This complex can transport electrons, across the bulk CH2Cl2 membrane, from aq. CrII only to the strong oxidizing agent CeIV in aq. HClO4
Entropy contribution to the nickel(III)/nickel(II) redox change in six-coordinating systems. Investigation of the role of the negative charge of the ligand in the stabilization of high oxidation states of the metal center
No full textEntropy changes ΔS°rc assocd. with the 2 half-reactions (i) [NiIIIL2]3+ + e- = [NiIIL2]2+ and (ii) [NiIIIY]0 + e- = [NiIIY] (L = 1,4,7-triazacyclononane H3Y = 1,4,7-triazacyclononane-N,N',N''-triacetic acid) were detd. through the investigation of the temp. dependence of the E1/2(NiIII/NiII) parameter over the 0.1-3.5M concn. range of the background electrolyte (NaCl). The ΔS°rc for half-reaction (i) is pos., and its value decreases with an increasing concn. of the supporting electrolyte, whereas ΔS°rc for half-reaction ii is neg. and its value increases (becoming less neg.) with an increasing concn. of NaCl. The above results were interpreted in terms of variation in the size of the hydration sphere of the electroactive species, which are related to changes of the elec. charge on the complexes. In particular, it was shown that such polyneg. charged ligands as Y3- favor access to unusually high oxidn. states of the metal center due a very favorable entropy term: this reflects a substantial increase of translational entropy, assocd. with the release of H2O mols. from the hydration sphere of the complex during the [NiIIY]/[NiIIIY]0 oxidn. proces
Ring-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
N-(AMINOETHYL)CYCLAM - A TETRAAZA MACROCYCLE WITH A COORDINATING TAIL (SCORPIAND) - ACIDITY CONTROLLED COORDINATION OF THE SIDE-CHAIN TO NICKEL(II) AND NICKEL(III) CATIONS
No full textStepwise 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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