Syntheses,structures, and properties of imidazolate-bridged Cu(II)-Cu(II) and Cu(II)-Zn(II) dinuclear complexes of a single macrocyclic ligand with two hydroxyethyl pendants

The imidazolate-bridged homodinuclear Cu(II)-Cu(II) complex, [(CuimCu)L]ClO(4).0.5H(2)O (1), and heterodinuclear Cu(II)-Zn(II) complex, [(CuimZnL(-)(2H))(CuimZnL(-)(H))](ClO(4))(3) (2), of a single macrocyclic ligand with two hydroxyethyl pendants, L (L = 3,6,9,16,19,22-hexaaza-6,19-bis(2-hydroxyethyl)tricyclo[22,2,2,2(11,14)]triaconta-1,11,13,24,27,29-hexaene), have been synthesized as possible models for copper-zinc superoxide dismutase (Cu(2),Zn(2)-SOD). Their crystal structures analyzed by X-ray diffraction methods have shown that the structures of the two complexes are markedly different. Complex 1 crystallizes in the orthorhombic system, containing an imidazolate-bridged dicopper(II) [Cu-im-Cu](3+) core, in which the two copper(II) ions are pentacoordinated by virtue of an N4O environment with a Cu.Cu distance of 5.999(2) A, adopting the geometry of distorted trigonal bipyramid and tetragonal pyramid, respectively. Complex 2 crystallizes in the triclinic system, containing two similar Cu-im-Zn cores in the asymmetric unit, in which both the Cu(II) and Zn(II) ions are pentacoordinated in a distorted trigonal bipyramid geometry, with the Cu.Zn distance of 5.950(1)/5.939(1) A, respectively. Interestingly, the macrocyclic ligand with two arms possesses a chairlike (anti) conformation in complex 1, but a boatlike (syn) conformation in complex 2. Magnetic measurements and ESR spectroscopy of complex 1 have revealed the presence of an antiferromagnetic exchange interaction between the two Cu(II) ions. The ESR spectrum of the Cu(II)-Zn(II) heterodinuclear complex 2 displayed a typical signal for mononuclear trigonal bipyramidal Cu(II) complexes. From pH-dependent ESR and electronic spectroscopic studies, the imidazolate bridges in the two complexes have been found to be stable over broad pH ranges. The cyclic voltammograms of the two complexes have been investigated. Both of the two complexes can catalyze the dismutation of superoxide and show rather high activity.     

E.S.R., magnetic, electronic and superoxide dismutase studies of imidazolate-bridged Cu(II)-Cu(II) complexes with ethylenediamine as capping ligand

X-band E.S.R., magnetic and electronic spectra of some imidazolate-bridged homometallic complexes [(en)2Cu-R-Im-Cu(en)2](ClO4)3 where en, ethylenediamine; R-ImH, R = H imidazole (ImH); if R = CH3, 2-methylimidazole (M-ImH) and if R = C2H5, 2-ethylimidazole (E-ImH), and mononuclear complexes [(en)Cu-dien](ClO4)2 and [(en)Cu-PMDT](ClO4)2 where dien, diethylenetriamine; PMDT, pentamethyldiethylenetriamine have been described. Superoxide dismutase (SOD) activity has also been measured and compared with earlier reported complexes. In frozen solution at 77 K, the spectra show axial symmetry with a d(x2-y2) ground state. Difference in lambda(max) between mononuclear and binuclear complexes was found to be approximately 65-75 nm. Magnetic susceptibility and E.S.R. spectral measurements for all these binuclear complexes revealed that the copper(II) ions are involved in antiferromagnetic exchange interactions propagated by the imidazolate bridge.     

Synthesis and spectral studies of copper(II) complexes with amide group ligands

Complexes of copper(II) with 2-(acetylamino)benzoic acid, 2-(benzoylamino) benzoic acid, 2-(aminocarbonyl)benzoic acid, 2-[(phenylamino)carbonyl]benzoic acid, 2-[(1-naphthalenylamino)carbonyl]benzoic acid, 2-[(2-aminophenylamino)carbonyl]benzoic acid, 2-aminobenzanilide, 2(aminobenzoyl)benzoic acid, maleanilic acid and malea-1-naphthalanilic acid have been prepared and characterized by chemical analyses, molar conductivity, magnetic susceptibility measurements, thermal data, IR, electronic and ESR spectra. The visible and ESR spectral studies of these complexes (except those of maleanilic acid and malea-1-naphthalanilic acid) indicate that they are monomeric having either square planar or distorted octahedral geometry around Cu(II). The Cu(II) complexes of maleanilic acid and malea-1-naphthalanilic acid have been tentatively assigned dimeric structures. From the ESR spectra of Cu(II) complexes various parameters have been calculated.     

X-band electron paramagnetic resonance, optical spectra and some biological (SOD and antimicrobial activity) studies of the copper(II) complexes: a plausible model for superoxide dismutase

The synthesis and characterization of homobinuclear complex by 2-ethylimidazole is reported along with two mononuclear complexes. Magnetic measurements and electron parameter resonance (e.p.r.) spectroscopy of the homobinuclear complex have shown an antiferromagnetic exchange interaction. Superoxide dismutase and antimicrobial activities of these complexes have also been measured.     

Magnetic,epr and SOD studies of some Cu(II)-Cu(II), Cu(II)-Ni(II) and Cu(II)-Zn(II) imidazolate bridged complexes

Magnetic, spectroscopic and superoxide dismutase activity of imidazolate bridged [(Salala)Cu-Im-Cu(Salala)]Na, [(Salala)Cu-Im-Zn(Salala)]Na and [(Salala)Cu-Im-Ni(Salala)]Na (Salala=Salicyledenealiniate, Im=Imdiazolate) are described. The epr and electronic spectra of related mononuclear complexes, viz., [(Salala)Cu-OH(2)] and [(Salala)Cu-ImH] also described. Appearance of a half-field signal in polycrystalline and decrease in mu(eff) per copper(II) ion indicate super exchange coupling between copper(II) ion in [(Salala)Cu-Im-Cu(Salala)]Na binuclear complex. A pH-dependent epr and UV-vis study of 50% aqueous DMSO solution of binuclear complexes suggest that the complexes are stable in narrow pH range.     

Synthesis, structures and photophysical properties of luminescent copper(I) and platinum(II) complexes with a flexible naphthyridine-phosphine ligand

Condensation of Ph(2)PH and paraformaldehyde with 2-amino-7-methyl-1,8-naphthyridine gave the new flexible tridentate ligand 2-[N-(diphenylphosphino)methyl]amino-7-methyl-1,8-naphthyridine (L). Reaction of L with [Cu(CH(3)CN)(4)]BF(4) and/or different ancillary ligands in dichloromethane afforded N,P chelating or bridging luminescent complexes [(L)(2)Cu(2)](BF(4))(2), [(micro-L)(2)Cu(2)(PPh(3))(2)](BF(4))(2) and [(L)Cu(CNN)]BF(4) (CNN = 6-phenyl-2,2'-bipyridine), respectively. Complexes [(L)(2)Pt]Cl(2), [(L)(2)Pt](ClO(4))(2) and [(L)Pt(CNC)]Cl (CNC = 2,6-biphenylpyridine) were obtained from the reactions of Pt(SMe(2))(2)Cl(2) or (CNC)Pt(DMSO)Cl with L. The crystal structures and photophysical properties of the complexes are presented.     

Synthesis,characterization and biological activities of homo-binuclear Cu(II) and Zn(II) complexes derived from 2-hydroxy-5-methyl-1,3-benzenedicarboxaldehyde derivatives

Few novel binuclear Schiff base metal complexes [M₂LCl₃], where M = Cu(II) and Zn(II); L= 2,6-bis-({2-[(3-hydroxy-4-nitrobenzylidene)amino]ethylimino}methyl)-4-methylphenol (BHEM), 2,6-bis-({2-[(3,4-dimethoxybenzylidene)amino]ethylimino} methyl)-4-methylphenol (BDEM) and 2,6-bis-({2-[(2,3,5-trichlorobenzylidene)amino]ethylimino}methyl)-4-methylphenol (BTEM), have been synthesized and characterized by analytical and spectral data. The data suggest that BHEM/BDEM/BTEM ligands afford square-pyramidal/distorted square-pyramidal geometry on metalation with Zn(II)/Cu(II). The binding behaviour of these complexes with DNA has been investigated using electronic absorption spectroscopy as well as viscosity and voltammetric measurements; the results show that they interact with DNA through intercalating way. From the DNA cleavage study of these complexes, investigated by gel electrophoresis, we found that they efficiently cleave supercoiled pUC19 DNA in the presence of a reducing agent (3-mercaptopropionic acid) and on irradiation with UV light of 360 nm wavelength. The mechanism reveals that singlet oxygen (¹O₂) plays a significant role in the photo cleavage. The superoxide dismutase (SOD) mimetic activity of the synthesized complexes demonstrates that most of the complexes have promising SOD-mimetic activity. The antimicrobial study indicates that the complexes inhibit the growth of bacteria and fungi more than the free ligands.     

Characterization and biological activities of two copper(II) complexes with diethylenetriamine and 2,2'-bipyridine or 1,10-phenanthroline as ligands

Two new mixed ligand copper(II) complexes with diethylenetriamine, 2,2'-bipyridine and 1,10-phenanthroline have been synthesized. The crystal and molecular structures of [Cu(dien)(phen)](ClO(4))(2) and [Cu(dien)(bipy)](BF(4))(2) (dien=diethylenetriamine, phen=1,10-phenanthroline, bipy=2,2'-bipyridine) were determined by X-ray crystallography from single crystal data. These two complexes have similar structures. The EPR spectral data also suggest that these complexes have distorted square pyramidal geometry about copper(II). Anti-microbial and superoxide dismutase activities of these complexes have also been measured. They show the higher SOD activity than the corresponding simple Cu(II)-dien/Cu(II)-PMDT (PMDT=N,N,N',N',N'-pentamethyldiethylenetriamine) complexes because of a strong axial bond of one of the nitrogen atoms of the alpha-diimine. Both the complexes have been found to cleave plasmid DNA in the presence of co-reductants such as ascorbic acid and glutathione.     

Spectral and magnetic studies on manganese(II), cobalt(II) and nickel(II) complexes with Schiff bases

Mn(II), Co(II) and Ni(II) complexes of 2-methylcyclohexanone thiosemicarbazone(MCHTSC L(1)) and 2-methylcyclohexanone-(4)N-methyl-3-thiosemicarbazone (MCHMTSC L(2)), general composition [M(L)(2)X(2)] (where M = Mn(II), Co(II), Ni(II), L = L(1) or L(2) and X = Cl(-), NO(3)(-), and [(1/2)SO(4)(2-)) have been synthesized and characterized by elemental analysis, magnetic susceptibility measurements, UV-vis, IR, EPR, and mass spectral studies. Various physico-chemical techniques suggest an octahedral geometry for all the complexes.     

2-(Phenyltelluromethyl)tetrahydrofuran (L) and 2-(2-{4-methoxyphenyl}telluroethyl)-1,3- dioxolane (L) and their palladium(II) and platinum(II) complexes

The nucleophile [ArTe⁻] generated in situ borohydride solution of Ar₂Te₂, reacts with 2-(chloromethyl) tetrahydrofuran and 2-(2-bromoethyl)-1,3-dioxolane resulting in L¹ and L², respectively. The complexes of palladium(II) and platinum(II) with L¹/L² having stoichiometries [MCl₂·L₂], [ML₂](ClO₄)₂, [(DPPE)ML₂](ClO)₄)₂, [(PPh₃)₂ML₂](ClO₄)₂ and [(phen)ML₂](ClO₄)₂ (where L = L¹/L² DPPE = Ph₂PC H₂CH₂PPh₂, PHEN = 1,10-phenanthroline and M = Pd/Pt) have been synthesized. IR, ¹H, ¹²⁵Te{¹H} and ³¹P{¹H} NMR and UV-vis spectral data of these species in conjunction with their molar conductance and molecular weight data have been used to authenticate the new species. In all complexes (1–20) the ligands L¹ and L² are coordinated through tellurium and in the complexes of formula [ML₂](ClO₄)₂ (M = Pd, Pt) the ligand is bidentate with the oxygen atom used in complexation. In solution, complexes PtCl₂L₂ exist as a mixture of cis and trans isomers whereas only the trans isomer was observed for the palladium analogues. The [(phen)PdL₂](ClO₄)₂(Q) quenches ¹O₂ readily. The plot of log [Q] vs time is linear. Mechanism compatible with the experimental observations is proposed.     

Influence of the coordination geometry on the physicochemical properties of a copper(II) complex with a tailor-made calixarene-based ligand bearing dipyridyl pendants. An ESR, UV-Vis and CV study

Dipyridyl pendants, suitably attached onto a calix[4]arene fixed in its 1,3-alternate conformation, form stable complexes with copper(ii) in acetonitrile. Both the mononuclear and the homobinuclear species ([Cu(1)](2+) and [Cu(2)(1)](4+)), previously detected through a UV-Vis investigation, have been studied by ESR, UV-Vis and cyclic voltammetry. ESR and UV-Vis data clearly indicate that in the [Cu(1)](2+) species the nitrogen atoms of dipyridyl pendants are tetrahedrally arranged around copper(II). Computer models, optimised through molecular mechanics methods, further support these spectroscopic findings. The tetrahedral arrangement, achieved thanks to the anchoring of the dipyridyl moieties onto the calixarene platform, accounts for the easy reversible reduction of this species. The aspects associated with the reduction-oxidation of the homobinuclear species are also discussed.     

Novel copper(II)-dien-imidazole/imidazolate-bridged copper(II) complexes. Crystal structure of [Cu(dien)(Him)](ClO4)2 and of [(dien)Cu(mu-im)Cu(dien)](ClO4)3, a homobinuclear model for the copper(II) site of the CuZn-superoxide dismutase

The imidazolate-bridged binuclear copper(II)-copper(II) complex [(dien)Cu(mu-im)Cu(dien)](ClO(4))(3) and related mononuclear complexes [Cu(dien)(H(2)O)](ClO(4))(2), [Cu(dien)(Him)](ClO(4))(2) were synthesized with diethylenetriamine (dien) as capping ligand. The crystal structure of mononuclear [Cu(dien)(Him)](ClO(4))(2) and binuclear complex [(dien)Cu(mu-im)Cu(dien)](ClO(4))(3) have been determined by single crystal X-ray diffraction methods. The mononuclear complex [Cu(dien)(Him)](ClO(4))(2) crystallizes in the orthorhombic, Pca2(1) with a = 9.3420(9) A, b = 12.3750(9) A, c = 14.0830(9) A, beta = 90.000(7)(o) and Z = 4 and binuclear complex [(dien)Cu(mu-im)Cu(dien)](ClO(4))(3) crystallizes in the monoclinic space group P2(1)/a, with a = 15.017(7) A, b = 11.938(6) A, c = 15.386(6) A, beta = 110.30(4)(o) and Z = 4. The molecular structures show that copper(II) ions in an asymmetrically elongated octahedral coordination (type 4 + 1 + 1) and in binuclear complex Cu(1) atom has a asymmetrically elongated octahedral coordination (type type 4 + 1 + 1) and Cu(2) atom exhibits a square base pyramidal coordination (type 4 + 1). The bridging ligand (imidazolate ion, im) lies nearly on a straight line between two Cu(2+), which are separated by 5.812 A, slightly shorter than the value in copper-copper superoxide dismutase (Cu(2)-Cu(2)SOD). Magnetic measurements and electron spin resonance (ESR) spectroscopy of the binuclear complex have shown an antiferromagnetic exchange interaction. From pH-dependent cyclic voltametry (CV) and electronic spectroscopic studies the complex has been found to be stable over a wide pH range (7.75-12.50).     

Antimicrobial,antioxidant and SOD activities of copper(II) complexes derived from 2-aminobenzothiazole derivatives

A series of Cu(II) complexes have been synthesized from bidentate Schiff base ligands (by condensation of Knoevenagel condensate of acetoacetanilide (obtained from substituted benzaldehydes and acetoacetanilide) and 2-aminobenzothiazole). They were characterized by elemental analysis, IR, ¹H NMR, ¹³C NMR, UV–vis., molar conductance, magnetic moment, ESR spectra and electrochemical studies. Based on the magnetic moment, ESR, and electronic spectral data, a distorted square planar geometry has been suggested for the complexes. Antibacterial and antifungal screening of the ligands and their complexes reveal that all the complexes show higher activities than the ligands. The antioxidant activities of the ligands and complexes were determined by superoxide and hydroxyl radical scavenging methods in vitro, indicating that the complexes exhibit more effective antioxidant activity than the ligands alone. The results show that the Cu(II) complexes also have similar superoxide dismutase activity to that of native Cu, Zn-SOD. All complexes exhibit suitable Cu(II)/Cu(I) redox potential (E_1/2) to act as synthetic antioxidant enzyme mimics.     

Bis(triazacyclohexane) sandwich complexes of (Cu(I))(2), Cu(II) and Zn(II): complexes with cuprophilic attraction between two cationic copper(I) leading to unusual reactivity with dioxygen

As the first 1st-row transition metal complexes having six tertiary amine donor groups, bis(triazacyclohexane) sandwich complexes [L2M](BF4)2 (L = benzyl- or p-fluorobenzyl-triazacyclohexane, M = Cu or Zn) have been obtained by the protonolysis of Et2Zn in the presence of L or by reaction of [Cu(MeCN)4](BF4) with L in CH2Cl2 and subsequent air oxidation via an unprecedented Cu(I)(2) sandwich complex containing a short Cu-Cu contact.     

E.s.r., magnetic, optical and biological (SOD and antimicrobial) studies of imidazolate bridged Cu(II)-Zn(II) and Cu(II)-Ni(II) complexes with tris(2-amino ethyl)amine as capping ligand: a plausible model for superoxide dismutase

X-band e.s.r. and optical absorption spectra of the imidazolate bridged heterobimetallic complexes [(tren)Cu-E-Im-Zn-(tren)](ClO(4))(3) and [(tren)Cu-E-Im-Ni-(tren)](ClO(4))(3), where trentris(2-aminoethyl)amine, E-Im=2-ethylimidazolate ion and the related mononuclear complexes [Cu(tren)](ClO(4))(2) and [(tren)Cu-E-ImH)](ClO(4))(2) have been described. Biological activities (superoxide dismutase and antimicrobial) have also been measured and compared with reported complexes.     

Cobalt(II) and copper(II) complexes of (2-thiophene)-(5,6-diphenyl-[1,2,4]-triazin-3-yl)hydrazone

New complexes having the formulae [L₂CoX₂] and [LCuCl₂], [LCuCl] and [LCu](ClO₄)₂ where L?=?(2-thiophene)-(5,6-diphenyl-[1,2,4]-triazin-3-yl)hydrazone TDPTH; X?=?Cl, OAc or ClO₄ have been synthesized and characterized on the basis of elemental analyses, conductance, magnetic moments and infrared, electronic and ESR spectral data. The IR spectra indicate that TDPTH is a neutral bidentate ligand, coordinating via a triazine-N and azomethine-N in [L₂CoX₂] and [LCuCl₂] with the thiophene-S not coordinated but is tridentate in [LCuCl] and [LCu](ClO₄)₂ through the same two nitrogen atoms and thiophene-S. The magnetic moment and electronic spectral data suggest a distorted octahedral structure for Co(II) complexes, a dimeric square pyramidal geometry for [LCuCl₂] through chloride bridges and a dimeric diamagnetic, four-coordinate copper in [LCu](ClO₄)₂ through thiophene-S bridges. The X-band ESR spectra of Co(II) complexes, in the solid state, are rhombic with three g values consistent with a high-spin distorted octahedral structure. The X-band ESR spectrum of the powdered sample of both [LCuCl₂]·2H₂O and [LCu](ClO₄)₂ at room temperature and at 77?K showed only one broad signal due to?ΔM _s?=?±?1 transition and a weak signal due to the forbidden?ΔM _s?=?±?2 transition, indicating an antiferromagnetic interaction between copper(II) centers whereas [LCuCl] is ESR silent, indicating a monovalent copper ion in this complex.     

Solution chemical properties and catecholase-like activity of the copper(II)-Ac-His-His-Gly-His-OH system, a relevant functional model for copper containing oxidases

The solution chemical properties, superoxide dismutase and catecholase activity of the copper(ii)-Ac-His-His-Gly-His-OH (hhgh) complexes were studied to identify functional and structural models of copper-containing oxidases. The solution speciation was determined in the pH range 3-11 by two independent methods (potentiometry and pH-dependent EPR measurements). The results obtained by the two methods agree very well with each other and show the formation of differently protonated CuH(x)L complexes (where x= 2 ,1, 0, -1, -2, -3) in aqueous solution. The spectroscopic (UV-Vis, CD, EPR) data indicate that the coordination of the imidazole rings is a determinant factor in all these complexes. Amide coordinated complexes are dominant only above pH 8. This offers excellent possibilities for structural/functional modelling of copper(ii) containing metalloenzymes. Indeed, the {3N(im)} coordinated CuL species (pH = 6-7) has efficient superoxide dismutase-like activity. The {3N(im),OH(-)} coordinated CuH(-1)L possesses outstanding activity to catalyze the oxidation of 3,5-di-tert-butylcatechol (H(2)dtbc) by dioxygen in 86 wt% methanol-water, providing the first example that copper(ii)-peptide complexes are able to mimic copper containing oxidases.     

氯冉酸阴离子桥联双核铜(II)配合物的合成和磁性

本文合成了五种以氯冉酸二价阴离子为桥联配体的Cu(II)单核([CuLCA].H~2O)和双核([Cu~2L~2CA](ClO~4)~2配合物:[Ca(phen)CA].H~2O(1),[Cu~2(phen)~2CA](ClO~4)~2(2),[Cu(NO~2-phen)CA].H~2O(3),[Cu~2(NO~2-phen)~2CA](ClO~4)~2(4)和[Cu~2(bpy)~2CA](ClO~4)~2(5)。经元素分析、红外、固体紫外、顺磁共振、磁化率及变温磁化率的测定对上述各配合物进行了表征。配合物1,3可能是通过水分子中的氢键将两个[CuLCA]单元联接而缔合的假双聚体。配合物2,4,5则由阳[Cu~2L~2CA]^2^+阳离子和弱配位的ClO~4^-阴离子所组成。双核配合物中Cu(II)离子的几何构型可能为畸变的四方锥。所有五种配合物均难溶于水及常见有机溶剂。上述配合物的室温ESR谱呈现ΔM~s=2的从单重态到三重态的半场跃迁。配合物2,5的变温磁化率(4-300K)已测得,利用Heisenberg模型确定交换参数J值为-29.2和-25.7cm^-^1。表明在此类桥联配合物中,两核间存在着反铁磁性交换耦合作用。     

Synthesis and Properties of Mononuclear and Homobinuclear Chelates of Mn(II), Co(II), Ni(II), and Cu(II) with Phthalein Purple

Mono- and homobinuclear complexes of Mn(II), Co(II), Ni(II), and Cu(II) with phthalein purple are prepared and characterized by elemental analysis, thermal studies (TGA and DTA), spectral methods (IR, UV/Vis, and ESR), magnetic moment determination, and electrochemical reduction (DC polarography at DME and CV at HMDE). Thermal degradation of the complexes was studied by TGA and DTA where some thermodynamic parameters were determined. The mode of bonding and geometry of the complexes were determined from the spectral studies. Magnetic moment values showed some antiferromagnetism in the homobinuclear complexes. The reduction of the metal ions proceeds to the metallic state along an irreversible process.     

Copper(II/I) Complexes of a Hexakis(bipyridyl)cyclotriveratrylene Ligand: A Redox-Induced Conformational Switch

A series of copper(II) and copper(I) complexes have been synthesized with ligands combining 6-methyl-2,2'-bipyridines with cyclotriveratrylene (CTV) (1) and with catechol (2). The electrochemical, (1)H NMR, and mass spectrometry characterizations of these complexes are described and discussed. The six pendant bipyridines of ligand 1 allow for the formation of two trinuclear copper(I) complexes [(1)Cu(3)](BF(4))(3) differing only in the conformation "vic" or "int" adopted by the ligand to fit the tetrahedral cuprous ions. Similarly, 1 generates two trinuclear copper(II) complexes in which the conformation of the ligand fits the square planar geometry of cupric ions. In both the cuprous and cupric complexes, a conformational equilibrium exists. Ligand 2 bearing two methylbipyridines has proven to be a useful model of the coordinating sites of ligand 1. In this case, two homologous copper(I) complexes are obtained, [(2)Cu]BF(4) and [(2)(2)Cu(2)](BF(4))(2), modeling respectively two possible coordination conformations of ligand 1. With copper(II), ligand 2 yields only one complex [(2)Cu](CF(3)SO(3))(2), which allows for the unambiguous identification of the conformations observed for ligand 1 complexes. The different coordinating modes of ligand 1 in the complexes mentioned are in exchange but exhibit different physical properties, thus representing a new bistable system based on conformational isomerism which exhibits an electrochemical potential hysteresis. An equilibrium constant and thermodynamic data were obtained for this system by variable-temperature cyclic voltammetry. The influence of coordinating vs noncoordinating solvents was also studied.