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 CuL2+ species with both ligands, the L1 complex being 3–4 log units more stable than the L2 complex. The crystal structure of [Cu(L1)](ClO4)2·H2O shows that the coordination geometry around the Cu2+ 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 CuL22+ 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

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 Cu2+ complexes. The values of the stability constants and paramagnetic 1H 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

Equilibrium, Kinetic, and Computational Studies on the Formation of Cu2+ and Zn2+ Complexes with an Indazole-Containing Azamacrocyclic Scorpiand: Evidence for Metal-Induced Tautomerism

Cu2+ and Zn2+ coordination chemistry of a new member of the family of scorpiand-like macrocyclic ligands derived from tris(2-aminoethyl)amine (tren) is reported. The new ligand (L1) contains in its pendant arm not only the amine group derived from tren but also a 6-indazole ring. Potentiometric studies allow the determination of four protonation constants. UV−vis and fluorescence data support that the last protonation step occurs on the indazole group. Equilibrium measurements in the presence of Cu2+ and Zn2+ reveal the formation of stable [ML1]2+, [MHL1]3+, and [ML1(OH)]+ complexes. Kinetic studies on the acid-promoted decomposition of the metal complexes were carried out using both absorbance and fluorescence detection. For Zn2+, both types of detection led to the same results. The experiments suggest that [ZnL1]2+ protonates upon addition of an acid excess to form [ZnHL1]3+ within the mixing time of the stopped-flow instrument, which then decomposes with a first-order dependence on the acid concentration. The kinetic behavior is more complex in the case of Cu2+. Both [CuL1]2+ and [CuHL1]3+ show similar absorption spectra and convert within the mixing time to a new intermediate species with a band at 750 nm, the process being reverted by addition of base. The intermediate then decomposes with a secondorder dependence on the acid concentration. However, kinetic experiments with fluorescence detection showed the existence of an additional faster step. With the help of DFT calculations, an interpretation is proposed in which protonation of [CuL1]2+ to form [CuHL1]3+ would involve dissociation of the tren-based NH group in the pendant arm and coordination of a 2H-indazole group. Further protonation would lead to dissociation of coordinated indazole, which then will convert to the more stable 1H tautomer in a process signaled by fluorescence changes that would not be affecting to the d−d spectrum of the complex

Combined kinetic and DFT studies on the stabilization of the pyramidal form of H3PO2 at the heterometal site of [Mo3M’S4(H2O)10]4+ clusters (M’= Pd, Ni)

Kinetic and DFT studies have been carried out on the reaction of the [Mo3M’S4(H2O)10]4+ clusters (M’= Pd, Ni) with H3PO2 to form the [Mo3M’(pyr-H3PO2)S4(H2O)9]4+ complexes, in which the rare pyramidal form of H3PO2 is stabilized by coordination to the M’ site of the clusters. The reaction proceeds with biphasic kinetics, both steps showing a first order dependence with respect to H3PO2. These results are interpreted in terms of a mechanism that involves an initial substitution step in which one tetrahedral H3PO2 molecule coordinates to M’ through the oxygen atom of the P=O bond, followed by a second step that consists in tautomerization of coordinated H3PO2 assisted by a second H3PO2 molecule. DFT studies have been carried out to obtain information on the details of both kinetic steps, the major finding being that the role of the additional H3PO2 molecule in the second step consists in catalysing a hydrogen shift from phosphorus to oxygen in O-coordinated H3PO2, which is made possible by its capability of accepting a proton from P-H to form H4PO2 + and then transfer it to the oxygen. DFT studies have been also carried out on the reaction at the Mo centres to understand the reasons that make these metal centres ineffective for promoting tautomerizatio

Equilibrium and kinetics studies on bibrachial lariat aza-crown/Cu(II) systems reveal different behavior associated with small changes in the structure

The high-yield synthesis of a new bibrachial lariat azacrown constituted by two tris(2-aminoethyl)amine (tren) units functionalized in one of its arms with a 4-methylquinoline group linked by dimethylene pyridine spacers (L2) is reported for the first time. The speciation studies show formation of mono- and binuclear Cu2+ complexes of similar stability. Comparisons are established with the complexes formed by the precursor tren-quinoline derivative (L4) and with the previously reported ligands containing naphthalene instead of quinoline as the fluorophore (L1, L3). The kinetics of formation and decomposition of Cu2+ complexes with L1 and L2 has been studied. For L1, the acid-promoted decomposition of mono and dinuclear complexes occurs in all cases with a rapid step within the stopped-flow mixing time that leads to the formation of an intermediate that decomposes in two additional steps. In the dinuclear complexes, both metal ions dissociate from the ligand with statistically-controlled kinetics. Complex formation with L1 occurs through the same intermediate observed during the decomposition. For L2, only the formation and decomposition of binuclear complexes could be studied, and the kinetic data show that the metal ion can coordinate both in square pyramidal sp and trigonal bipyramidal (tbp) geometries, coordination being faster in the sp environment and dissociation being faster from tbp. DFT and TD-DFT have been also carried out to determine the geometries with both coordination environments as well as their electronic spectra. The results of calculations indicate that the appearance or not of a mixture of coordination geometries does not necessarily require the participation of the quinoline ring

Benchmarking of DFTmethods using experimental free energies and volumes of activation for the cycloaddition of alkynes to cuboidalMo(3)S(4)clusters

Here, the kinetics of the concerted [3 + 2] cycloaddition reaction between the [Mo3(μ3‐S)(μ‐S)3Cl3(dmen)3]+ (dmen = N,N′‐dimethyl‐ethylenediamine) ([1]+) cluster and various alkynes to form dithiolene derivatives is thoroughly studied, with measurements at different temperatures and pressures allowing the determination of the free energies and volumes of activation. These parameters, together with the available single‐crystal X‐ray diffraction structures, are used to test a number of commonly used density functional theory (DFT) methods from Jacob's ladder, as well as the effects associated with the size of the basis sets, the way in which solvent effects are taken into account, or the inclusion of dispersion effects. Overall, a protocol that leads to average deviations between experimental and computed ΔV‡ and ΔG‡ values similar to the uncertainty of the experimental measurements is obtained

Base-Free Catalytic Hydrogen Production from Formic Acid Mediated by a Cubane-Type Mo3S4 Cluster Hydride

Formic acid (FA) dehydrogenation is an attractive process in the implementation of a hydrogen economy. To make this process greener and less costly, the interest nowadays is moving toward non-noble metal catalysts and additive-free protocols. Efficient protocols using earth abundant first row transition metals, mostly iron, have been developed, but other metals, such as molybdenum, remain practically unexplored. Herein, we present the transformation of FA to form H2 and CO2 through a cluster catalysis mechanism mediated by a cuboidal [Mo3S4H3(dmpe)3]+ hydride cluster in the absence of base or any other additive. Our catalyst has proved to be more active and selective than the other molybdenum compounds reported to date for this purpose. Kinetic studies, reaction monitoring, and isolation of the [Mo3S4(OCHO)3(dmpe)3]+ formate reaction intermediate, in combination with DFT calculations, have allowed us to formulate an unambiguous mechanism of FA dehydrogenation. Kinetic studies indicate that the reaction at temperatures up to 60 °C ends at the triformate complex and occurs in a single kinetic step, which can be interpreted in terms of statistical kinetics at the three metal centers. The process starts with the formation of a dihydrogen-bonded species with Mo–H···HOOCH bonds, detected by NMR techniques, followed by hydrogen release and formate coordination. Whereas this process is favored at temperatures up to 60 °C, the subsequent β-hydride elimination that allows for the CO2 release and closes the catalytic cycle is only completed at higher temperatures. The cycle also operates starting from the [Mo3S4(OCHO)3(dmpe)3]+ formate intermediate, again with preservation of the cluster integrity, which adds our proposal to the list of the infrequent cluster catalysis reaction mechanisms.Funding for open access charge: CRUE-Universitat Jaume

Benchmarking of DFT methods using experimental free energies and volumes of activation for the cycloaddition of alkynes to cuboidal Mo3S4 clusters

Here, the kinetics of the concerted [3 + 2] cycloaddition reaction between the [Mo3(μ3‐S)(μ‐S)3Cl3(dmen)3]+ (dmen = N,N′‐dimethyl‐ethylenediamine) ([1]+) cluster and various alkynes to form dithiolene derivatives is thoroughly studied, with measurements at different temperatures and pressures allowing the determination of the free energies and volumes of activation. These parameters, together with the available single‐crystal X‐ray diffraction structures, are used to test a number of commonly used density functional theory (DFT) methods from Jacob's ladder, as well as the effects associated with the size of the basis sets, the way in which solvent effects are taken into account, or the inclusion of dispersion effects. Overall, a protocol that leads to average deviations between experimental and computed ΔV and ΔG values similar to the uncertainty of the experimental measurements is obtained

Efficient (Z)-selective semihydrogenation of alkynes catalyzed by air-stable imidazolyl amino molybdenum cluster sulfides

Imidazolyl amino cuboidal Mo3(μ3-S)(μ-S)3 clusters have been investigated as catalysts for the semihydrogenation of alkynes. For that purpose, three new air-stable cluster salts [Mo3S4Cl3(ImNH2)3]BF4 ([1]BF4), [Mo3S4Cl3(ImNH(CH3))3]BF4 ([2]BF4) and [Mo3S4Cl3(ImN(CH3)2)3]BF4 ([3]BF4) have been isolated in moderate to high yields and fully characterized. Crystal structures of complexes [1]PF6 and [2]Cl confirm the formation of a single isomer in which the nitrogen atoms of the three imidazolyl groups of the ligands are located trans to the capping sulfur atom which leaves the three bridging sulfur centers on one side of the trimetallic plane while the amino groups lie on the opposite side. Kinetic studies show that the cluster bridging sulfurs react with diphenylacetylene (dpa) in a reversible equilibrium to form the corresponding dithiolene adduct. Formation of this adduct is postulated as the first step in the catalytic semihydrogenation of alkynes mediated by molybdenum sulfides. These complexes catalyze the (Z)-selective semihydrogenation of diphenylacetylene (dpa) under hydrogen in the absence of any additives. The catalytic activity lowers sequentially upon replacement of the hydrogen atoms of the N–H2 moiety in 1+ without reaching inhibition. Mechanistic experiments support a sulfur centered mechanism without participation of the amino groups. Different diphenylacetylene derivatives are selectively hydrogenated using complex 1+ to their corresponding Z-alkenes in excellent yields. Extension of this protocol to 3,7,11,15-tetramethylhexadec-1-yn-3-ol, an essential intermediate for the production of vitamin E, affords the semihydrogenation product in very good yield

Characterization of a Ferryl Flip in Electronically Tuned Nonheme Complexes. Consequences in Hydrogen Atom Transfer Reactivity

Two oxoiron(IV) isomers (R2a and R2b) of general formula [FeIV(O)(RPyNMe3)(CH3CN)]2+ are obtained by reaction of their iron(II) precursor with NBu4IO4. The two isomers differ in the position of the oxo ligand, cis and trans to the pyridine donor. The mechanism of isomerization between R2a and R2b has been determined by kinetic and computational analyses uncovering an unprecedented path for interconversion of geometrical oxoiron(IV) isomers. The activity of the two oxoiron(IV) isomers in hydrogen atom transfer (HAT) reactions shows that R2a reacts one order of magnitude faster than R2b, which is explained by a repulsive noncovalent interaction between the ligand and the substrate in R2b. Interestingly, the electronic properties of the R substituent in the ligand pyridine ring do not have a significant effect on reaction rates. Overall, the intrinsic structural aspects of each isomer define their relative HAT reactivity, overcoming changes in electronic properties of the ligand.10 página