M. Angeles Máñez

Hydrogen and copper ion induced molecular reorganizations in two new scorpiand-like ligands appended with pyridine rings.

The synthesis of two new ligands constituted of a tris(2-aminoethyl)amine moiety linked to the 2,6 positions of a pyridine spacer through methylene groups in which the hanging arm is further functionalized with a 2-pycolyl (L1) or 3-pycolyl (L2) group is presented. The protonation of L1 and L2 and formation of Cu(2+) complexes have been studied using potentiometric, NMR, X-ray, and kinetic experiments. The results provide new information about the relevance of molecular movements in the chemistry of this kind of so-called scorpiand ligand. The comparison between these two ligands that only differ in the position of the substituent at the arm reveals important differences in both thermodynamic and kinetic properties. The Cu(2+) complex with L1 is several orders of magnitude more stable than that with L2, surely because in the latter case the pyridine nitrogen at the pendant arm is unable to coordinate to the metal ion with the ligand acting as hexadentate, a possibility that occurs in the case of [CuL1](2+), as demonstrated by its crystal structure. Significant differences are also found between both ligands in the kinetic studies of complex formation and decomposition. For L1, those processes occur in a single kinetic step, whereas for L2 they occur with the formation of a detectable reaction intermediate whose structure corresponds to that resulting from the movement typical of scorpiands. Another interesting conclusion derived from kinetic studies on complex formation is that the reactive form of the ligand is H(3)L(3+) for L1 and H(2)L(2+) for L2. DFT calculations are also reported, and they allow a rationalization of the kinetic results relative to the reactive forms of the ligands in the process of complex formation. In addition, they provide a full picture of the mechanistic pathway leading to the formation of the first Cu-N bond, including outer-sphere complexation, water dissociation, and reorganization of the outer-sphere complex.

Kinetics and mechanism of formation and decomposition of copper(II) complexes with a binucleating hexaazamacrocycle

Abstract -The kinetics of formation and decomposition of mono- and binuclear copper(II) complexes of the macrocycle 3,6,9,17,20,23-hexaazatricyclo[23.3.1.111,15]triaconta-1(29), 11(30),12,14,25(26),27-hexaene (L) has been studied at 25°C and 1.0 M ionic strength under a variety of conditions. All reactions occur in the stopped-flow time-scale and results indicate that upon addition of a large excess of H+ binuclear complexes convert rapidly into mononuclear species in which some nitrogens of the ligand are uncoordinated. The kinetics of decomposition of the resulting mononuclear species is intermediate between that of complexes with linear polyamines and those with mononucleating macrocycles. On the other hand, the formation of CuII complexes at high concentrations of OH- occurs essentially through reaction of Cu(OH)3- with the unprotonated form of the ligand, at a rate similar to that observed for reactions with simpler ligands. Coordination of the second CuII is very rapid under these conditions.

Stability and kinetics of decomposition of binuclear Cu(II) complexes with a symmetrical hexaaza macrocycle: the effect of SCN− as ancillary ligand

Abstract The effect of added KBr and KSCN on the stability constants of the mono and binuclear Cu(II) complexes with a symmetrical hexaazamacrocycle L has been examined in 0.1 M KNO 3 . The presence of these salts does not cause any change in the ligand protonation constants, which indicates that, in the presence of 0.1 M KNO 3 , there is not preferential interaction of the Br − and SCN − anions with the highly protonated forms of the ligand. No ternary CuLBr complexes are detected in the potentiometric study of the equilibrium, but several mono and binuclear CuLSCN complexes are formed at significant amounts and their stabilities are reported. The kinetics of decomposition of the binuclear CuL and CuLSCN complexes upon addition of an excess of acid has been also measured. The results obtained for the CuL complexes agree well with those previously reported in 1.0 M KNO 3 , and they indicate that the release of both Cu(II) ions is statistically controlled. The existence of some differences between the kinetic data corresponding to decomposition of solutions at different starting pH is interpreted in terms of parallel decomposition of the binuclear Cu 2 L 4+ , Cu 2 L(OH) 3+ and Cu 2 L(OH) 2 2+ complexes, the kinetic parameters for the three complexes being slightly different. This interpretation is also supported by the kinetics of decomposition of the CuLSCN − complexes that also reveals differences between the several complexes in solution. If the present data are interpreted in terms of the classical mechanism for decomposition of Cu(II)-polyamine complexes, they suggest that the nature of the ancillary ligands does not cause large changes in the lability of the CuN bonds but it largely affects to the relative rates of attack by H + and water.

The kinetics and mechanisms of reactions involving the dihydrogen complex trans-[FeH(H2)(DPPE)2]+ and related compounds

Abstract The kinetic and mechanistic aspects of reactions involving the dihydrogen complex trans -[FeH(H 2 )(DPPE) 2 ] + and related Fe(II) and Ru(II) complexes are reviewed. Despite the observation that substitution of coordinated H 2 usually goes through a limiting dissociative mechanism, the reactions of the title complex involve associative activation and are proposed to occur through the initial opening of a DPPE chelate ring followed by rate-determining attack by the entering ligand. The kinetics of reactions between cis -[MH 2 (diphosphine) 2 ] compounds and acids to form dihydrogen complexes is also reviewed. The rate of protonation is strongly dependent on the nature of the acid and shows an inverse kinetic isotope effect; the mechanism proposed consists of attack by the acid to yield a transition state involving a dihydrogen-bonded adduct. For these complexes, the kinetics of protonation can be summarised in two parameters, R and S , that measure the intrinsic reactivity and selectivity of the complexes towards acids. The lack of reaction of [CpRuH(diphosphine)] complexes with some acids poses some questions about the validity of an aqueous p K a scale to measure the acidity of dihydrogen complexes.

Kinetics of reaction of the FeII-cyclam complex with H2O2 in acetonitrile and the mechanism of catalyzed epoxidation of cyclohexene

The FeII complex with macrocycle cyclam (1,4,8,11-tetraazacyclotetradecane) reacts with H2O2 in acetonitrile to give a mixture of products which results from oxidation both at the metal center and the ligand. At 25°C in the presence of 0.05 M Bu4NBF4 the reaction occurs with three kinetically distinguishable steps. The first two steps are first order with respect to both H2O2 and the metal complex. The values of the second order rate constants are k1 = 3.7 M−1 s−1 and k2 = 0.83 M−1 s−1. EPR spectra suggest that intermediates formed in these two steps are low spin FeIII complexes. The kinetics of the third step is more complicated, with a dependence on the concentration of H2O2 of the form k2[H2O2] + k4, with k3 = 2.72 × 10−3 M−3 s−1 and k4 = 2.18 × 10−4 s−1. This rate law is interpreted in terms of two parallel pathways leading to FeIII and extensively dehydrogenated cyclam. NMR experiments suggest that the active catalyst in the iron-cyclam catalyzed epoxidation of cyclohexene is the intermediate formed in the first step. However, this intermediate is not able to transfer an oxygen atom directly to the substrate and requires the participation of additional H2O2, in a mechanism very different to that proposed for porphyrin complexes.

Structurally Different Dinuclear Copper(II) Complexes with the Same Triazolopyrimidine Bridging Ligand

Four binuclear copper(II) compounds with the anionic form of the ligand 4,5-dihydro-5-oxo[1,2,4]triazolo[1,5-a]pyrimidine (5tpO - ) have been isolated, their formulae being [Cu2(5tpO)4(H2O)2]·2H2 O( 1), [Cu2(phen)2(5tpO)2(H2O)2](NO3)2·4H2 O( 2), [Cu2(biim)2(5tpO)2(H2O)](ClO4)2·5.5H2O (3), and [Cu2(CH3CO2)2(5tpO)2(H2O)2 ]( 4) (phen = 1,10phenanthroline, biim = bisimidazole). A related mononuclear complex, [Cu(phen)2(5HtpO)2](NO3)2 (5), has also been prepared. The crystal structure of compounds 1-3 has been determined by X-ray diffraction, showing their binuclear nature with four (1) or two (2, 3) bridging 5tpO- moieties. The triazolopyrimidine ligand binds the copper atoms through N3 and N4 in compounds 1 and 3, whereas a novel binding mode through N3 and the exocyclic oxygen atom has been found

Copper(II) complexes of quinoline polyazamacrocyclic scorpiand-type ligands: X-ray, equilibrium and kinetic studies

The formation of Cu(II) complexes with two isomeric quinoline-containing scorpiand-type ligands has been studied. The ligands have a tetraazapyridinophane core appended with an ethylamino tail including 2-quinoline (L1) or 4-quinoline (L2) functionalities. Potentiometric studies indicate the formation of stable CuL(2+) species with both ligands, the L1 complex being 3-4 log units more stable than the L2 complex. The crystal structure of [Cu(L1)](ClO(4))(2)·H(2)O shows that the coordination geometry around the Cu(2+) ions is distorted octahedral with significant axial elongation; the four Cu-N distances in the equatorial plane vary from 1.976 to 2.183 Å, while the axial distances are of 2.276 and 2.309 Å. The lower stability of the CuL2(2+) complex and its capability of forming protonated and hydroxo complexes suggest a penta-dentate coordination of the ligand, in agreement with the type of substitution at the quinoline ring. Kinetic studies on complex formation can be interpreted by considering that initial coordination of L1 and L2 takes place through the nitrogen atom in the quinoline ring. This is followed by coordination of the remaining nitrogen atoms, in a process that is faster in the L1 complex probably because substitution at the quinoline ring facilitates the reorganization. Kinetic studies on complex decomposition provide clear evidence on the occurrence of the molecular motion typical of scorpiands in the case of the L2 complex, for which decomposition starts with a very fast process (sub-millisecond timescale) that involves a shift in the absorption band from 643 to 690 nm.

Stability and kinetics of the acid-promoted decomposition of Cu(II) complexes with hexaazacyclophanes: kinetic studies as a probe to detect changes in the coordination mode of the macrocycles

The synthesis, protonation and Cu(II) coordination features of the novel azacyclophane type receptors 2,6,10,13,17,21-hexaza[22]-(2,6)-pyridinophane (L2), 2,6,9,12,15,19-hexaza[20]-(2,6)-pyridinophane (L5) and 2,6,9,12,15,19-hexaza[20]metacyclophane (L6) are presented. The protonation and Cu(II) constants are analysed and compared with the previously reported open-chain polyamines 4,8,11,15-tetrazaoctadecane-1,18-diamine (L1) and 4,7,10,13-tetraazahexadecane-1,16-diamine (L4) and of the cyclophane 2,6,10,13,17,21-hexaaza[22]paracyclophane (L3). All the systems form mono- and dinuclear complexes whose stability and pH range of existence depend on the type of hydrocarbon chains and molecular topology. The effects of the cyclic or open-chain nature and of the presence of the pyridine rings on the protonation and formation of mono- and dinuclear complexes are discussed. Stopped-flow kinetic measurements on the acid-promoted decomposition of the Cu(II) complexes have been carried out for the different systems. With respect to the decomposition of the dinuclear complexes, because the size of the macrocycles forces both metal ions to be close to each other, the release of the first ion occurs within the mixing time of the stopped-flow except for the dinuclear complexes of L2. However, the most interesting kinetic result is the observation of different kinetics of decomposition for the different mononuclear complexes formed by a given ligand. This effect is especially evident for L3 and L6 and indicates a change in the coordination mode of the ligand for the different mononuclear species. Therefore the Cu(II) ion performs a slippage motion through the macrocyclic cavity driven by pH changes. The stopped-flow experiments are an excellent tool to detect these slippage processes that may be present for the complexes with other macrocycles.

Equilibrium and kinetic studies on complex formation and decomposition and the movement of Cu2+metal ions within polytopic receptors

Potentiometric studies carried out on the interaction of two tritopic double-scorpiand receptors in which two equivalent 5-(2-aminoethyl)-2,5,8-triaza[9]-(2,6)-pyridinophane moieties are linked with 2,9-dimethylphenanthroline (L1) and 2,6-dimethylpyridine (L2) establish the formation of mono-, bi- and trinuclear Cu(2+) complexes. The values of the stability constants and paramagnetic (1)H NMR studies permit one to infer the most likely coordination modes of the various complexes formed. Kinetic studies on complex formation and decomposition have also been carried out. Complex formation occurs with polyphasic kinetics for both receptors, although a significant difference is found between both ligands with respect to the relative values of the rate constants for the metal coordination steps and the structural reorganizations following them. Complex decomposition occurs with two separate kinetic steps, the first one being so fast that it occurs within the stopped-flow mixing time, whereas the second one is slow enough to allow kinetic studies using a conventional spectrophotometer. As a whole, the kinetic experiments also provide information about the movement of the metal ion within the receptors. The differences observed between the different receptors can be interpreted in terms of changes in the network of hydrogen bonds formed in the different species.

Hydrogen-ion driven molecular motions in Cu2+-complexes of a ditopic phenanthrolinophane ligand.

One of the first kinetic evaluations of a metal ion interchange between the two coordination sites of a ditopic macrocycle is presented.