Metal Complexes with Macrocyclic Ligands. PartXLV Axial coordination tendency in reinforced tetraazamacrocyclic complexes

The Cu²⁺ and Ni²⁺ complexes of three reinforced tetraazamacrocycles, containing a piperazine subunit and one or two alkyl substituents at the other two N-atoms have been prepared and their structural properties studied. In solution, the Ni²⁺ complexes are square-planar and show no tendency to axially coordinate a solvent molecule or an N ion. In contrast, the Cu²⁺ complexes change their geometry depending upon the donor properties of the solvent, being square-planar in MeNO₂ and pentacoordinate in DMF. They also easily react in aqueous solution with N to give ternary species with pentacoordinate geometry, the stabilities of which have been determined. In the solid state, the X-ray crystal structures of three Cu²⁺ complexes also show both geometrical arrangements, two having a square-planar, the other one a distorted square pyramidal geometry. The difference behavior of Ni²⁺ and Cu²⁺ stems from the fact that the structural change from square-planar to square-pyramidal can easily be accomplished for Cu²⁺, whereas, for Ni²⁺, it is accompanied by an electronic rearrangement from the low-spin to the high-spin configuration. The relatively rigid ligands cannot Adapt to the somewhat larger high-spin Ni²⁺ion.     

Cuprous complexes and dioxygen. Part 11. Concomitant one- and two-electron reduction of O2 by the Cu ion

The autoxidation of Cu^I in aqueous MeCN has been studied using a Clark oxygen electrode in the presence and absence of Cu¹¹. The reaction is inhibited by Cu¹¹ in the pH range of 0.5 to 5.0, reaching a lower limiting value at the highest concentrations. The reaction order changes from 1 to 2 with respect to Cu^I under the influence of Cu²⁺ ion. Detailed kinetics analysis of a total of 275 measurements has shown that an unstable primary adduct CuO⁺₂ decomoses to give ^.O or HO, depnding on pH, and also reacts directly with a second Cu⁺ ion, avoiding one-electrton reduction of O₂ by this path. Reaction of HO is faster with Cu^I than with Cu¹¹ by a factor of 20, and single-electron transfer within CuO⁺₂ to Cu²⁺ and ^.O predominates over reaction with a second copper ion for [Cu^I_tot] < 2. 10^?3M in the absence of Cu²⁺. The most likely value for the reaction of ^.O with Cu^I is 5.3 · 10⁸ M ^?1S^?1, but even this high rate constant is at the limit of significance. All secondary reactions followinfg the initial formation of CuO⁺₂ are shown to be very fast, a fact that should be properly considered in the discussion of mechanisms of copper-catalyzed oxidations and oxygenations.     

Zur Stabilität der Metallkomplexe von Methantriessigsäure und cis,cis-1,3,5-Cyclohexantricarbonsäure

The stabilities of the Mn²⁺-, Co²⁺-, Ni²⁺-, Cu²⁺- and Zn²⁺-complexes with 2-(carboxymethyl)glutaric acid ( 2 ) and cis,cis-1,3,5-cyclohexanetricarboxylic acid ( 3 ) were measured potentiometrically at 25° and I = 0.5 (KNO₃). Beside the complexes ML^? protonated species MLH and MLH are also formed. Their stability constants are given in Table 1. A comparison between the stabilities of 2 or 3 and those of acetate, as a model for a monocarboxylate, or succinate and glutarate, as examples for dicarboxylates, indicates that in all species only one carboxylate is strongly bound whereas the second and third ones are probably not. The observation that Δlog K₁ = log K ? log K as well as Δlog K₂ = log K ? log K are practically constants with values of 0.34 ± 0.05 and 0.49 ± 0.07, respectively, for both ligands and the five metal ions studied is also in line with the proposed monodentate structures of the complexes ML^?, MLH and MLH.     

Metal Complexes with Macrocyclic Ligands. XI. Ring size effect on the complexation rates with transition metal ions

The 12-16 membered tetraazamacrocycles 1 - 6 were synthesized, their protonation constants and complexation kinetics measured at 25° and I = 0.50. The results of Table 1 Show that pK is strongly influenced by the ring size whereas pK and pK are relatively insensitive to it. This can be understood in terms of electrostatic interactions of the positive charges when located on adjacent amino groups. The kinetics of complex formation between the macrocyclic ligands and several transition metal ions have been studied by pH-stat and stopped-flow techniques and the results have been analyzed as bimolecular reactions between the metal ion and the different protonated species of the ligands. The rate constants, given in Table 2, show that the macrocycles react less rapidly than analogous open chain amines. However, for a given protonated species of the ligand the rate of complexation follows the order Cu²⁺ > Zn²⁺ > Co2⁺ > Ni²⁺ which parallels the sequence of their water exchange rates. For the diprotonated tetraamines LH reacting with Cu²⁺ the slower rates seem to be mainly a consequence of electrostatic interactions, since a correlation between logk and pK exists. For LH⁺, however, the complexation rates of a metal ion with the different macrocycles are all in one order of magnitude and do not depend in a regular way on the ring size or the basicity of the ligand. It is therefore suggested that in this case other factors such as unfavourable preequilibria must be considered as important.     

Metal complexes with macrocyclic ligands. Part XX. Influence of coordinated ligands onto the complexation rate of Ni2+ with 1,4,8,11-tetraazacyclotetradecane

The kinetics of the reaction between 1,4,8,11-tetraazacyclotetradecane (Cy) and Ni²⁺ in the presence of series of ligands L = fluoride, acetate, glycolate, oxalate, malonate, succinate, methanetriacetate, 1,3,5-cyclohexanetriacetate, tricarballylate, picolinate, glycinate, iminodiacetate, nitrilotriacetate. N,N′ -ethylenediiminodiacetate, ammonia, pyridine, ethylenediamine, 1,3-propanediamine and diethylenetriamine were studied by pH-static and spectrophotometric methods at 25° and I = 0.5. By analysis of the log k/log [L] and/or log k/pH profiles the resolved bimolecular rate constants K (Table 3) were determined using a non-linear least-square fitting procedure. Practically for all systems the rate constant K, describing the reaction between the 1:1 Ni²⁺ complex and the monoprotonated form of the macrocycle, was obtained. In some cases, however, also K and K were found. Since the experimental conditions were choosen so that NiL was mainly formed, the reactivity of NiL₂ was generally not measurable. The effect of the number of coordinated donor groups in NiL and of the charge of NiL on K is discussed. Both effects seem to indicate that for the reaction between NiL and CyH⁺ first bond formation is not the rate-determining step.     

Cuprous complexes and dioxygen. VIII. Steric factors controlling one- or two-electron reduction of O2 by cuprous complexes with substituted imidazoles

In aqueous acetonitrile (AN), Cu (I) forms the complexes Cu(AN)L⁺ and CuL with a series of substituted imidazoles (L). Stability constants logK of Cu(AN)⁺ + L ? Cu(AN)L⁺ and logβ₂ were near 5 and 12, resp., log units for all ligands. The rate of autoxidation is described by ?d[O₂]/dt=[CuL]²[O₂](k_a/(1+k_b[CuL]) + (k_c[L]+k_d)/([CuL] + k_e[Cu])), implying competition between one- or two-electron reduction of O₂. The value of k_c decreases from 5500M ^?2S ^?1 for unsubstituted imidazole to about 40M ^?2S ^?1 for 2-methylimidazole or 1,2-dimethyl-imidazole and essentially zero for the corresponding 2-ethyl-derivatives. On the other hand, k_a and k_b are much less influenced by the nature of the ligands, all values being near 5 · 10⁴M ^?2S ^?1 and 10³M ^?1, respectively, for the complexes with the last four bases. Thus rather subtle sterical changes may strongly influence the relative importance of different pathways in the reduction of dioxygen by cuprous complexes.     

Stability,Formation and Dissociation Kinetics of the Pentacoordinate Ni2+ and CO2+ Complexes with 1,5-Diazacyclooctane-N,N′-diacetic Acid

The stability constants of the Ni²⁺ and Co²⁺ complexes with 1,5-diazacyclooctane-N,N′-diacetic acid (H₂DACODA) have been determined potentiometrically in 0.5M KNO₃ at 25°. Only M(DACODA) and M(DACODA)OH^? were observed. In addition the formation and dissociation kinetics of the pentacoordinate complexes M(DACODA) has been studied in aqueous solution using a stopped-flow technique. Formation follows the rate law v_f = k_f [M²⁺] [HDACODA^?]/[H⁺], which can be interpreted as a bimolecular process either between M²⁺ and DACODA^2? (k) or between MOH⁺ and HDACODA^? (k). The second order rate constants k are much higher than those expected from water exchange and can only be explained by a strong internal conjugate base effect. In the limiting case, however, this is equivalent to the second possible explanation, which assumes MOH⁺ and HDACODA^? as reactive species. The dissociation rate is given by v_d = (k^ML + k [H⁺]) · [M(DACODA)].     

Cuprous complexes and dioxygen. IX. Formation of a unique trimeric species Cu3L and Ligand Oxidation (L  cis,cis-1, 3, 5-cyclohexanetriamine)

The complexation of Cu⁺ by the potentially tripod like ligand cis, cis-1, 3, 5 cyclohexanetriamine (chta) has been studied potentiometrically in aqueous acetonitrile (an). The expected tetracoordinated species Cu (chta) ? (an)⁺ was formed only at rather high pH with log K (Cu (an)⁺ + chta ? Cu (chta) · (an)⁺) = 6.94. Quite unexpectedly the most stable complex in neutral solution was the trimetric species Cu³ (chta) with log K (3 Cu⁺ + 2 chta ? Cu³ (chta)) = 31.75. In addition, the ternary complexes Cu (LH²) · (an)³⁺ and Cu (LH) · (an)²⁺ (L = chta) are formed at low pH. From model considerations, Cu³ (chta) must contain two ligand molecules with all amino groups in equatorial position, linked by three linearly coordinated Cu⁺-ions. Cu₃ (chta)³⁺₂ shows no measurable reactivity towards dioxygen. At pH values above 9, very rapid O₂-uptake due to Cu (chta) · (an)⁺ is observed. In this reaction, Cu⁺-autoxidation is stoichiometrically coupled to ligand oxidation, followed by a much slower Cu-catalyzed secondary reaction of the primary oxidation product of chta. Hydrogen peroxide and likely also superoxide, are involved in the coupled Cu⁺/ligand oxidation.     

Variable-Temperature and Variable-Pressure Kinetic and Equilibrium studies of formation and dissociation of [V(H2O)5NCS]2+

The kinetics of formation and dissociation of [V(H₂O)₅NCS]²⁺ have been studied, as a function of excess metal-ion concentration, temperature, and pressure, by the stopped-flow technique. The thermodynamic stability of the complex was also determined spectrophotometrically. The kinetic and equilibrium data were submitted to a combined analysis. The rate constants and activation parameters for the formation (f) and dissociation (r) of the complex are: k/M ^?1 · S^?1 = 126.4, k/s^?1 = 0.82; ΔH /kJ · mol^?1 = 49.1, ΔH/kJ · mol^?1 = 60.6; ΔS/ J·K^?1·mol^?1= ?39.8, ΔSJ·K^?1·mol^?1 = ?43.4; ΔV/cm³·mol^?1 = ?9.4, and ΔV/cm³ · mol^?1 =?17.9. The equilibrium constant for the formation of the monoisothiocynato complex is K²⁹⁸/M ^?1 = 152.9, and the enthalpy and entropy of reaction are ΔH⁰/kJ · mol^?1 = ? 11.4 and ΔS⁰/J. K^?1mol^?1 = +3.6. The reaction volume is ΔV⁰/cm³· mol^?1 = +8.5. The activation parameters for the complex-formation step are similar to those for the water exchange on [V(H₂O)₆]³⁺ obtained previously by NMR techniques. The activation volumes for the two processes are consistent with an associative interchange, I_a, mechanism.     

Variable-Temperature and Variable-Pressure 17O-NMR Study of Water Exchange of Hexaaquaaluminium(III)

The transverse relaxation rate of H₂O in Al(H₂O) has been measured as a function of temperature (255 to 417 K) and pressure (up to 220 MPa) using the ¹⁷O-NMR line-broadening technique, in the presence of Mn(II) as a relaxation agent. At high temperatures the relaxation rate is governed by chemical exchange with bulk H₂O, whereas at low temperatures quadrupolar relaxation is prevailing. Low-temperature fast-injection ¹⁷O-NMR was used to extend the accessible kinetic domain. The samples studied contained Al³⁺ (0.5 m), Mn²⁺ (0.2–0.5 m), H ⁺ (0.2–3.1 m) and ¹⁷O-enriched (20–40%) H₂O. Non-coordinating perchlorate was used as counter ion. The following H₂O exchange parameters were obtained: k = (1.29 ± 0.04) s⁻¹, ΔH* = (84.7 ± 0.3) kJ mol⁻¹, ΔS* = +(41.6 ± 0.9) J K ⁻¹ mol⁻¹, and ΔV = +(5.7 ± 0.2) cm³ mol⁻¹, indicating a dissociative interchange, I_d, mechanism. These results of H₂O exchange on Al(H₂O) are discussed together with the available complex-formation rate data and permit also the assignment of I_d mechanisms to these latter reactions.     

Metal Complexes with Macrocyclic Ligands. Part XXXVIII. Steric effects in the copper(II) and nickel(II) complexes with tetra-N-alkylated 1,4,8,11-tetraazacyclotetradecanes

A series of tetra-N-alkylated 1,4,8,11-tetraazacyclotetradecanes have been synthesized and their complexation potential towards Ni²⁺ and Cu²⁺ studied. In the case of sterically demanding alkyl substituents, such as i-Pr, PhCH₂, or 2-MeC₆H₄CH₂, no metal complexes are formed, whereas for substituents such as Me, Et, and Pr, the metal ion is incorporated into the macrocycle. The spectroscopic properties of the Ni²⁺ and Cu²⁺ complexes in aqueous solution indicate that, depending on the sterical hindrance of the N-substituents, the complexes are either square planar or pentacoordinated. All these Ni²⁺ and Cu²⁺ complexes react with N to give ternary species, the stability of which have been determined by spectrophotometric titrations. The tendency to bind N decreases with increasing steric hindrance of the alkyl substituents. The X-ray studies of the Ni²⁺ complex with the macrocycle 11 , substituted by two Me and two Pr groups, and that of the Cu²⁺ complex with the tetraethyl derivative 8 show that in the solid state, the metal ions exhibit square planar coordination with a small distortion towards tetrahedral geometry.     

Complex formation equilibria of L-Amino-Acid amides with copper(II) in aqueous solution

Protonation and Cu(II) complexation equilibria of L -phenyhilaninamide, N²-methyl-L-phenylalaninamide, N², N²-dimethyl-L-phenylalaninamide, L -valinamide, and L -prolinamide have been studied by potentiometry in aqueous solution. The formation constants of the species observed, CuL²⁺, CuL, CuLH, CuL₂H and CuL₂H_?2, are discussed in relation to the structures of the ligands. Possible structures of bisamidato complexes are proposed on the ground of VIS and CD spectra. Since Cu(II) complexes of the present ligands (pH range 6–8) perform chiral resolution of dansyl- and unmodified amino acids in HPLC (reversed phase), it is relevant for the investigation of the resolution mechanism to know which are the species potentially involved in the recognition process.     

Komplexe mit makrocyclischen Liganden,II. Zweischrittmechanismus bei der Komplexbildung von Cu2+ mit meso-5,7,7,12,14,14-Hexamethyl-1,4,8,11-tetraazacyclotetradecan

The complex formation between Cu^II and the title compound (tet a) is studied by spectrophotometry and pH-stat techniques. Between pH 4 and 5,5 the reaction proceeds in two steps, the first giving a blue intermediate, Cu(teta)²⁺ (blue), exhibiting a broad absorption band at 620 nm. Titration with NaOH and the absorption spectrum suggest that, in the intermediate, Cu^II is coordinated to only two amino groups of the ligand. Both steps are slow compared to other complex formation reactions of Cu^II. The rate of the first step, in which Cu(tet a)²⁺ (blue) is formed, is given by v₁ = k₁ · [Cu²⁺] [(tet a) H]/[H⁺] with k₁ = 2,7 · 10^?6 s^?1 at 40° and I = 0,1. In the second step the last two nitrogens of the quadridentate ligand are bound to Cu^II, giving the mauve end product. The rate of this step is given by v₂ = k₂ · [Cu(tet a)OH⁺ (blue)] [OH^?] with k₂ = 1,2 · 10³ M^?1 s^?1 at 50° and I = 0,5.     

Metal complexes with macrocyclic ligands. XVI. Spectrophotometric and thermodynamic studies of solvents and unidentate ligands interaction with the pentacoordinate Co2+-, Ni2+- and Cu2+-complexes of 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane

The interaction of solvents and of unidentate ligands such as N, SCN^?, OCN^? and OH^? with the Co²⁺-, Ni²⁺ and Cu²⁺-complexes of 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane (TMC) have been studied by Spectrophotometric and calorimetric techniques. The spectra in different solvents (Table 2) show that the Ni²⁺- and probably also the Cu²⁺-complex with TMC exist as square planar or pentacoordinate species or as a mixture of both, depending on the donor properties of the solvent. The [Co(TMC)]²⁺-complex is pentacoordinate in all the solvents studied. Ternary complexes [M(TMC)X]ⁿ⁺ are also formed by the unidentate ligands X = N, OCN^?, OH^?, F^? and NH₃ and their stability constants have been determined. Interesting is the high selectivity of [Ni(TMC)]²⁺ towards the addition of a further donor (Table 3). Only small ligands such as those listed above form stable adducts, whereas the larger ones such as imidazole or pyridine do not. This is a consequence of the special structure of the complex and of the trans-I-(RSRS)- conformation of the ligand in these complexes. Since the four methyl groups are all on the side of the macrocycle to which the additional unidentate ligand binds, steric interaction between the four methyl groups and the larger ligands prevents the formation of the adducts. The calorimetric measurements show that the stability of the complexes [M(TMC)X]ⁿ⁺ is due to both an enthalpic and entropic contribution which differ in their magnitude (Table 4). This indicates that several antagonistic factors are important in determining the overall stability.     

L'action de l'ammoniac sur l'oxyde cuivrique. et les hydroxo-complexes de cuivre (II)

Using a new mathematical treatment, the nature and stability constants of the simple and mixed complex-species of copper(II) with hydroxyde and ammonia as ligands have been determined. The solubility curves of CuO in heterogeneous equilibrium have been identified in function of pH only and in function of pH and pNH_3tot at 25° and unit ionic strength (NaClO₄). The predominent species in the relatively dilute system limited by the ionic strength are [Cu²⁺], [Cu(OH)₂], [Cu(OH)], [Cu(OH)], [Cu(NH₃)], [Cu(NH₃)], [Cu(NH₃)], [Cu(NH₃) (OH)⁺], [Cu(NH₃)₃(OH)⁺] and [Cu(NH₃)₂(OH)₂].     

Die Deprotonierung von Metall-Aquoionen I.: Be · aq2+ Solvatations-Isomerie

The reaction of Be · aq²⁺ with OH^? leeds not only to loss of protons by the metalaquo ion but also to structural changes in the solvation sphere. These can be studied by following the pH variations during the first decisecond after mixing the solutions of metal salt and alkali hydroxide. The equilibrium Be²⁺ ? BeOH⁺ is reached within 5 milliseconds if acid free Beryllium solutions are used. If the metal solution is strongly acidic, however, the establishment of the equilibrium needs more time because of the slowness of the process H⁺ + BeOH⁺ → Be²⁺ (k ～ 10⁵ M^?1, s^?1). The extraction of two protons produces in the first instance an unstable Be(OH) species which transforms into the stable isomer Be(OH)₂ (solvatation isomerism) in a first-order reaction of half-life of 7 ms. This isomerisation causes almost complete disappearance of BeOH⁺ from the equilibrium Be²⁺ ? BeOH⁺ ? Be(OH)₂. (KAKIHANA & SILLEN state that the relaxed solutions contain only Be²⁺, Be(OH)₂, Be₃(OH) and some Be₂OH³⁺.) The formation of the polynuclear species Be₃(OH) needs about 30 seconds to go to completion.     

Reduction Potentials of Tetraaminecopper(II/I) Couples

The difference in steric strain between the oxidized and the reduced forms of tetraaminecopper complexes is correlated with the corresponding reduction potentials. The experimentally determined data considered range from ?0.54 to ?0.04 V (vs. NHE) in aqueous solution and from ?0.35 to ?0.08 V (vs. NHE) in MeCN. The observed and/or computed geometries of the tetraaminecopper(II) complexes are distorted octahedral or square-pyramidal (4 + 2 or 4+1) with (distorted) square-planar CuN₄ chromophores (Cu^II? N = 1.99–2.06 Å; Cu? O ≈ 2.5 Å; Cu? O ≈ 2.3 Å), those of the tetraaminecopper(I) complexes are (distorted) tetrahedral (four-coordinate; Cu^I? N = 2.12–2.26 Å; tetrahedral twist angle ?? = 30–90°). The reduction potentials of Cu^II/I couples with primary-amine ligands and those with macrocyclic secondary-amine ligands were correlated separately with the corresponding strain energies, leading to slopes of 70 and 61 kJ mol^?1 V^?1, with correlation coefficients of 0.89 and 0.91, respectively. The approximations of the model (entropy, solvation, electronic factors) and the limits of applicability are discussed in detail and in relation to other approaches to compute reduction potentials of transition-metal compounds.     

Molecular Recognition in Anion Coordination Chemistry. Structure,Binding Constants and Receptor-Substrate Complementarity of a Series of Anion Cryptates of a Macrobicyclic Receptor Molecule

The crystal structures of four anion cryptates [X^? ? BT -6H⁺] formed by the protonated macrobicyclic receptor BT -6H⁺ with F^?, Cl^?, Br^? and N have been determined. They provide a homogeneous series of anion coordination patterns with the same ligand. The small F^?-ion is tetracoordinated, while Cl^? and Br^? are bound in an octahedron of H-bonds. The non-complementarity between these spherical anions and the ellipsoïdal cavity of BT -6H⁺ is reflected in ligand distortions. Structural complementarity is achieved for the linear triatomic substrate N, which is bound by two pyramidal arrays of three H-bonds, each interacting with a terminal N-atom of N. The formation constants of the complexes formed by BT -6H⁺ with a variety of anions (halides, N, NO, carboxylates, SO, HPO, AMP^2?, ADP^3?, ATP^4?, P₂O) have been determined. Very strong complexations are found, as well as marked electrostatic and structural effects on stability and selectivity; in particular the binding of F^?, Cl^?, Br^?, and N may be analyzed in terms of the crystal structure data. The cryptand BT -6H⁺ is a molecular receptor containing an ellipsoïdal recognition site for linear triatomic substrates of size compatible with the size of the molecular cacity. Further developments of various aspects of anion coordination chemistry are considered.     

Intramolecular Stacking in Ternary Complexes Containing Uridine 5′-Triphosphate, 2,2′-Bipyridyl,and a Divalent Metal Ion

The stability constants of the ternary complexes containing UTP, 2,2′-bipyridyl (bipy), and Co²⁺, Ni²⁺, Cu²⁺, or Zn²⁺ (M²⁺) have been determined by potentiometric titrations (Table 1). Changes in stability are quantified by Δlog K_M = log K–log K. For the Co²⁺, Ni²⁺, Cu²⁺, and Zn²⁺ systems Δlog K_M is 0.10, ?0.13, 0.36, and 0.15, respectively. All these ternary complexes are considerably more stable than would be expected on statistical grounds; indeed, for Co²⁺, Cu²⁺, and Zn²⁺, UTP^4? binds more tightly to M (bipy)²⁺ than to M²⁺. An UV. difference spectroscopic study suggests that stacked adducts between bipyridyl and the pyrimidine moiety of uridine are formed. ¹H-NMR. studies of the bipy/uridine, bipy/UTP, and bipy/UTP/Zn²⁺ systems (Figs. 1 and 2) confirm the presence of stacking in the binary adducts and in the ternary complex. There is also evidence for the existence of the stacked protonated complex, Zn(bipy) (HUTP)^?, with the proton at the γ-phosphate group. The acidity constant of this ternary complex has been measured (Fig. 3). The observed stability enhancement of stacked adducts by the formation of a metal ion bridge is discussed (Fig. 4) and biological implications are indicated.     

A Reduction Catalyst Powered by Its Own 10-Electron Battery: Synthesis and properties of a pentaviologen-linked corrinatocobalt complex

The synthesis and properties of the molecular reduction device 7b (Co^II–V) consisting of a reduction catalyst (a derivative of vitamin B₁₂, Co^II) and a covalently linked 10-electron reservoir (five viologen units, V⁺⁺) is described. The five viologen subunits were introduced at C(2) and C(3) of the side chains c and g and b, e and f, respectively, of an appropriate derivative of heptamethyl cob(III)yrinate by N-alkylation of 1-methyl-4,4′-bipyridinium iodide (see Scheme). The pentaviologen-linked corrinatocobalt(II) complex 7b behaves as a molecular electron trap with respect to the Co^III/Co^II redox couple. The phenomenon is related to the structural and thermodynamic relation of the corrin and viologen subunits in 7b , i.e. the relative redoc energies and the spherical inner-outer arrangement of the types of redox systems. When completely reduced to Co^I–V, 7b exhibits multiple reductive elimination of trans-1,2-dibromocyclohexane to cyclohexene under concomitant oxidation to Co^II–V. Rate measurements indicate that the reduction occurs via Co^I which is regenerated by intramolecular electron transfer from the periphery of the molecule, i.e. by V⁰ and V⁺⁺.