The Jahn-Teller effect of the Cr(2+) ion in aqueous solution: Ab initio QM/MM molecular dynamics simulations

The hydration structure of Cr(2+) has been studied using molecular dynamics (MD) simulations including three-body corrections and combined ab initio quantum mechanical/molecular mechanical (QM/MM) MD simulations at the Hartree-Fock level. The structural properties are determined in terms of radial distribution functions, coordination numbers, and several angle distributions. The mean residence time was evaluated for describing ligand exchange processes in the second hydration shell. The Jahn-Teller distorted octahedral [Cr(H(2)O)(6)](2+) complex was pronounced in the QM/MM MD simulation. The first-shell distances of Cr(2+) are in the range of 1.9-2.8 A, which are slightly larger than those observed in the cases of Cu(2+) and Ti(3+). No first-shell water exchange occurred during the simulation time of 35 ps. Several water-exchange processes were observed in the second hydration shell with a mean residence time of 7.3 ps.     

Quantum mechanical charge field molecular dynamics simulation of the TiO2+ ion in aqueous solution

Structural and dynamical properties of the TiO(2+) ion in aqueous solution have been investigated by using the new ab initio quantum mechanical charge field (QMCF) molecular dynamics (MD) formalism, which does not require any other potential functions except those for solvent-solvent interactions. Both first and second hydration shell have been treated at Hartree-Fock (HF) quantum mechanical level. A Ti-O bond distance of 1.5 A was observed for the [Ti=O](2+) ion. The first hydration shell of the ion shows a varying coordination number ranging from 5 to 7, five being the dominant one and representing one axial and four equatorial water molecules directly coordinated to Ti, which are located at 2.3 A and 2.1 A, respectively. The flexibility in the coordination number reflects the fast exchange processes, which occur only at the oxo atom, where water ligands are weakly bound through hydrogen bonds. Considering the first shell hydration, the composition of the TiO(2+) hydrate can be characterized as [(H(2)O)(0.7)(H(2)O)(4) (eq)(H(2)O)(ax)](2+). The second shell consists in average of 12 water molecules located at a mean distance of 4.4 A. Several other structural parameters such as radial and angular distribution functions and coordination number distributions were analyzed to fully characterize the hydration structure of the TiO(2+) ion in aqueous solution. For the dynamics of the TiO(2+) ion, different sets of dynamical parameters such as Ti=O, Ti-O(eq), and Ti-O(ax) stretching frequencies and ligands' mean residence times were evaluated. During the simulation time of 15 ps, 3 water exchange processes in the first shell were observed at the oxo atom, corresponding to a mean residence time of 3.6 ps. The ligands' mean residence time for the second shell was determined as 3.5 ps.     

"yl"-Oxygen exchange in uranyl(VI) ion: a mechanism involving (UO2)2(μ-OH)2(2+) via U-O(yl)-U bridge formation

Szabó and Grenthe (Inorg. Chem. 2007, 46, 9372-9378) suggested from NMR spectroscopy that the "yl"-oxygen exchange in dioxo uranium(VI) ion in acidic solution occurs via an OH-bridged binuclear complex (UO(2))(2)(μ-OH)(2)(2+). Here, an "yl"-oxygen exchange pathway involving the (UO(2))(2)(μ-OH)(2)(2+) is studied by B3LYP density functional theory calculations. The oxygen exchange takes place via an intramolecular proton shuttle between the oxygen atoms in (UO(2))(2)(μ-OH)(2)(H(2)O)(6)(2+). The direct proton transfer from the hydroxo bridge or from the coordinating water to the "yl"-oxygen in (UO(2))(2)(μ-OH)(2)(H(2)O)(6)(2+) appears to be negligible because of an exceedingly high activation barrier (～170 kJ mol(-1)). The exchange mechanism in (UO(2))(2)(μ-OH)(2)(H(2)O)(6)(2+) can be described by a multistep pathway that leads to the formation of an oxo bridge between two uranyl(VI) centers (U-O(yl)-U bridge). The activation enthalpy Δ(?)H of the reaction obtained at the B3LYP level is 94.7 kJ mol(-1) and is somewhat larger than the experimental value of 80 ± 14 kJ mol(-1). However, the discrepancy between theory and experiment is at the acceptable level. The formation of an oxo bridge between the two uranyl(VI) centers was found to be the key step in proton shuttling, indicating that uranyl(VI) complexes with a stable oxo bridge (such as trinuclear (UO(2))(3)(μ(3)-O)(OH)(3)(+)) may have even faster "yl"-oxygen exchange rates than (UO(2))(2)(μ-OH)(2)(2+).     

Theoretical study of the complexes of horminone with Mg2+ and Ca2+ ions and their relation with the bacteriostatic activity

The coordination of the horminone molecule with hydrated magnesium and calcium divalent ions was studied by means of the density functional theory. All-electron calculations were performed with the B3LYP/6-31G method. The first layer of the water molecules surrounding the metallic cations was included. It was found that the octahedral [horminone(O(a)-O(d))-Mg-(H(2)O)(4)](2+) complex is more stable than [Mg(H(2)O)(6)](2+). That is, horminone is able to displace two water units from the hexahydrated complex. This behavior does not occur for Ca(2+). Consistently, [horminone(O(a)-O(d))-Mg-(H(2)O)(4)](2+) has a greater metal-ligand binding energy than [horminone(O(a)-O(d))-Ca-(H(2)O)(4)](2+). The preference of horminone by Mg(2+) is enlightened by these results. Moreover, its electronic structure, as shown by huge changes in the atomic populations, is strongly perturbed by Mg(2+). Indeed, horminone, bonded to [Mg(H(2)O)(4)](2+), is able to cross the bacterial membrane cell. Once inside, [horminone(O(a)-O(d))-Mg-(H(2)O)(4)](2+) binds to rRNA phosphate groups yielding [horminone(O(a)-O(d))-Mg-(H(2)O)(PO(4)H(2))(PO(4)H(3))(2)](+). These results give insights into how horminone may inhibit the initial steps of protein synthesis. The stability of the studied systems is accounted for in terms of the calculated structural and electronic properties: Mg-O and Ca-O bond lengths, charge transfers, and binding energies.     

Theoretical Modeling of Water Exchange on [Pd(H(2)O)(4)](2+), [Pt(H(2)O)(4)](2+), and trans-[PtCl(2)(H(2)O)(2)

Density functional theory is applied to modeling the exchange in aqueous solution of H(2)O on [Pd(H(2)O)(4)](2+), [Pt(H(2)O)(4)](2+), and trans-[PtCl(2)(H(2)O)(2)]. Optimized structures for the starting molecules are reported together with trigonal bipyramidal (tbp) systems relevant to an associative mechanism. While a rigorous tbp geometry cannot by symmetry be the actual transition state, it appears that the energy differences between model tbp structures and the actual transition states are small. Ground state geometries calculated via the local density approximation (LDA) for [Pd(H(2)O)(4)](2+) and relativistically corrected LDA for the Pt complexes are in good agreement with available experimental data. Nonlocal gradient corrections to the LDA lead to relatively inferior structures. The computed structures for analogous Pd and Pt species are very similar. The equatorial M-OH(2) bonds of all the LDA-optimized tbp structures are predicted to expand by 0.25-0.30 ?, while the axial bonds change little relative to the planar precursors. This bond stretching in the transition state counteracts the decrease in partial molar volume caused by coordination of the entering water molecule and can explain qualitatively the small and closely similar volumes of activation observed. The relatively higher activation enthalpies of the Pt species can be traced to the relativistic correction of the total energies while the absolute DeltaH() values for exchange on [Pd(H(2)O)(4)](2+) and [Pt(H(2)O)(4)](2+) are reproduced using relativistically corrected LDA energies and a simple Born model for hydration. The validity of the latter is confirmed via some simple atomistic molecular mechanics estimates of the relative hydration enthalpies of [Pd(H(2)O)(4)](2+) and [Pd(H(2)O)(5)](2+). The computed DeltaH() values are 57, 92, and 103 kJ/mol compared to experimental values of 50(2), 90(2), and 100(2) kJ/mol for [Pd(H(2)O)(4)](2+), [Pt(H(2)O)(4)](2+), and trans-[PtCl(2)(H(2)O)(2)], respectively. The calculated activation enthalpy for a hypothetical dissociative water exchange at [Pd(H(2)O)(4)](2+) is 199 kJ/mol. A qualitative analysis of the modeling procedure, the relative hydration enthalpies, and the zero-point and finite temperature corrections yields an estimated uncertainty for the theoretical activation enthalpies of about 15 kJ/mol.     

Water versus acetonitrile coordination to uranyl. Density functional study of cooperative polarization effects in solution

Optimizations at the BLYP and B3LYP levels are reported for mixed uranyl-water/acetonitrile complexes [UO(2)(H(2)O)(5-n)(MeCN)(n)](2+) (n = 0-5), in both the gas phase and a polarizable continuum modeling acetonitrile. Car-Parrinello molecular dynamics (CPMD) simulations have been performed for these complexes in the gas phase, and for selected species (n = 0, 1, 3, 5) in a periodic box of liquid acetonitrile. According to structural and energetic data, uranyl has a higher affinity for acetonitrile than for water in the gas phase, in keeping with the higher dipole moment and polarizability of acetonitrile. In acetonitrile solution, however, water is the better ligand because of specific solvation effects. Analysis of the dipole moment of the coordinated water molecule in [UO(2)(H(2)O)(MeCN)(4)](2+) reveals that the interaction with the second-shell solvent molecules (through fairly strong and persistent O-H···N hydrogen bonds) causes a significant increase of this dipole moment (by more than 1 D). This cooperative polarization of water reinforces the uranyl-water bond as well as the water solvation via strengthened (UO(2))OH(2)···NCMe hydrogen bonds. Such cooperativity is essentially absent in the acetonitrile ligands that make much weaker (UO(2))NCMe···NCMe hydrogen bonds. Beyond the uranyl case, this study points to the importance of cooperative polarization effects to enhance the M(n+) ion affinity for water in condensed phases involving M(n+)-OH(2)···A fragments, where A is a H-bond proton acceptor and M(n+) is a hard cation.     

Photoinduced water oxidation by a tetraruthenium polyoxometalate catalyst: ion-pairing and primary processes with Ru(bpy)3(2+) photosensitizer

The tetraruthenium polyoxometalate [Ru(4)(μ-O)(4)(μ-OH)(2)(H(2)O)(4)(γ-SiW(10)O(36))(2)](10-) (1) behaves as a very efficient water oxidation catalyst in photocatalytic cycles using Ru(bpy)(3)(2+) as sensitizer and persulfate as sacrificial oxidant. Two interrelated issues relevant to this behavior have been examined in detail: (i) the effects of ion pairing between the polyanionic catalyst and the cationic Ru(bpy)(3)(2+) sensitizer, and (ii) the kinetics of hole transfer from the oxidized sensitizer to the catalyst. Complementary charge interactions in aqueous solution leads to an efficient static quenching of the Ru(bpy)(3)(2+) excited state. The quenching takes place in ion-paired species with an average 1:Ru(bpy)(3)(2+) stoichiometry of 1:4. It occurs by very fast (ca. 2 ps) electron transfer from the excited photosensitizer to the catalyst followed by fast (15-150 ps) charge recombination (reversible oxidative quenching mechanism). This process competes appreciably with the primary photoreaction of the excited sensitizer with the sacrificial oxidant, even in high ionic strength media. The Ru(bpy)(3)(3+) generated by photoreaction of the excited sensitizer with the sacrificial oxidant undergoes primary bimolecular hole scavenging by 1 at a remarkably high rate (3.6 ± 0.1 × 10(9) M(-1) s(-1)), emphasizing the kinetic advantages of this molecular species over, e.g., colloidal oxide particles as water oxidation catalysts. The kinetics of the subsequent steps and final oxygen evolution process involved in the full photocatalytic cycle are not known in detail. An indirect indication that all these processes are relatively fast, however, is provided by the flash photolysis experiments, where a single molecule of 1 is shown to undergo, in 40 ms, ca. 45 turnovers in Ru(bpy)(3)(3+) reduction. With the assumption that one molecule of oxygen released after four hole-scavenging events, this translates into a very high average turnover frequency (280 s(-1)) for oxygen production.     

Thorium(IV) molecular clusters with a hexanuclear Th core

Three polynuclear thorium(IV) molecular complexes have been synthesized under ambient conditions from reactions of an amorphous Th precipitate, obtained via hydrolysis, with carboxylate functionalized ligands. The structures of Th(6)(OH)(4)O(4)(H(2)O)(6)(HCO(2))(12)·nH(2)O (1), Th(6)(OH)(4)O(4)(H(2)O)(6)(CH(3)CO(2))(12)·nH(2)O (2), Th(6)(OH)(4)O(4)(H(2)O)(6)(ClCH(2)CO(2))(12)·4H(2)O (3) each consist of a hexanuclear Th core wherein six 9-coordinate Th(IV) cations are bridged by four μ(3)-hydroxo and four μ(3)-oxo groups. Each Th(IV) center is additionally coordinated to one bound "apical" water molecule and four oxygen atoms from bridging carboxylate functionalized organic acid units. "Decoration" of the cationic [Th(6)(μ(3)-O)(4)(μ(3)-OH)(4)](12+) cores by anionic shells of R-COO(-) ligands (R = H, CH(3), or CH(2)Cl) terminates the oligomers and results in the formation of discrete, neutral molecular clusters. Electronic structure calculations at the density functional theory level predicted that the most energetically favorable positions for the protons on the hexanuclear core result in the cluster with the highest symmetry with the protons separated as much as possible. The synthesis, structure, and characterization of the materials are reported.     

Probing defect sites on TiO2 with [Re3(CO)12H3]: spectroscopic characterization of the surface species

Samples of the anatase phase of titania were treated under vacuum to create Ti(3+) surface-defect sites and surface O(-) and O(2) (-) species (indicated by electron paramagnetic resonance (EPR) spectra), accompanied by the disappearance of bridging surface OH groups and the formation of terminal Ti(3+)-OH groups (indicated by IR spectra). EPR spectra showed that the probe molecule [Re(3)(CO)(12)H(3)] reacted preferentially with the Ti(3+) sites, forming Ti(4+) sites with OH groups as the [Re(3)(CO)(12)H(3)] was adsorbed. Extended X-ray absorption fine structure (EXAFS) spectra showed that these clusters were deprotonated upon adsorption, with the triangular metal frame remaining intact; EPR spectra demonstrated the simultaneous removal of surface O(-) and O(2) (-) species. The data determined by the three complementary techniques form the basis of a schematic representation of the surface chemistry. According to this picture, during evacuation at 773 K, defect sites are formed on hydroxylated titania as a bridging OH group is removed, forming two neighboring Ti(3+) sites, or, when a Ti(4+)-O bond is cleaved, forming a Ti(3+) site and an O(-) species, with the Ti(4+)-OH group being converted into a Ti(3+)-OH group. When the probe molecule [Re(3)(CO)(12)H(3)] is adsorbed on a titania surface with Ti(3+) defect sites, it reacts preferentially with these sites, becoming deprotonated, removing most of the oxygen radicals, and healing the defect sites.     

Equatorial and apical solvent shells of the UO2 2+ ion

First principles molecular dynamics simulations of the hydration shells surrounding UO(2)(2+) ions are reported for temperatures near 300 K. Most of the simulations were done with 64 solvating water molecules (22 ps). Simulations with 122 water molecules (9 ps) were also carried out. The hydration structure predicted from the simulations was found to agree with very well-known results from x-ray data. The average U=O bond length was found to be 1.77 A. The first hydration shell contained five trigonally coordinated water molecules that were equatorially oriented about the O-U-O axis with the hydrogen atoms oriented away from the uranium atom. The five waters in the first shell were located at an average distance of 2.44 A (2.46 A, 122 water simulation). The second hydration shell was composed of distinct equatorial and apical regions resulting in a peak in the U-O radial distribution function at 4.59 A. The equatorial second shell contained ten water molecules hydrogen bonded to the five first shell molecules. Above and below the UO(2)(2+) ion, the water molecules were found to be significantly less structured. In these apical regions, water molecules were found to sporadically hydrogen bond to the oxygen atoms of the UO(2)(2+), oriented in such a way as to have their protons pointed toward the cation. While the number of apical waters varied greatly, an average of five to six waters was found in this region. Many water transfers into and out of the equatorial and apical second solvation shells were observed to occur on a picosecond time scale via dissociative mechanisms. Beyond these shells, the bonding pattern substantially returned to the tetrahedral structure of bulk water.     

Lanthanide-tungstobismuthate clusters based on [BiW9O33]9- building units: synthesis, crystal structures, luminescent and magnetic properties

The reaction of Na(12)[Bi(2)W(22)O(74)(OH)(2)]·44H(2)O, Na(9)[BiW(9)O(33)]·16H(2)O, lanthanide chloride and Na(2)CO(3) in aqueous solution at a pH value of about 7.0 resulted in the three unprecedented giant lanthanide-tungstobismuthate clusters Na(x)H(22-x)[(BiW(9)O(33))(4)(WO(3)){Bi(6)(μ(3)-O)(4)(μ(2)-OH)(3)}(Ln(3)(H(2)O)(6)CO(3))]·nH(2)O {Ln = Pr(3+) (1), Nd(3+) (2), La(3+) (3), x = 22 (1), 22 (2), 20 (3), n = 95 (1), 91 (2), 73 (3)}. These three complexes represent the first examples of lanthanide ions encapsulated in polyoxotungstobismuthates and the largest polytungstobismuthates so far. Furthermore, a [{Bi(6)(μ(3)-O)(4)(μ(2)-OH)(3)}](7+) polyoxo cation was incorporated into the structure of these compounds. All complexes are characterized by single-crystal X-ray diffraction, IR spectra, electronic spectroscopy, thermogravimetric and elemental analysis. Magnetic investigation revealed that the progressive depopulation of excited Stark sublevels of the lanthanide ions at low temperature and the weak antiferromagnetic interaction between the neighboring metal centres are responsible for the magnetic properties of 1 and 2. The original synthesis strategy in this work may open a gateway to assembly of large lanthanide-tungstobismuthates clusters and novel multifunctional solid materials in aqueous solution under mild conditions.     

Structure and hydration of the C4H4●+ ion formed by electron impact ionization of acetylene clusters

Here we report ion mobility experiments and theoretical studies aimed at elucidating the identity of the acetylene dimer cation and its hydrated structures. The mobility measurement indicates the presence of more than one isomer for the C(4)H(4)(●+) ion in the cluster beam. The measured average collision cross section of the C(4)H(4)(●+) isomers in helium (38.9 ± 1 A?(2)) is consistent with the calculated cross sections of the four most stable covalent structures calculated for the C(4)H(4)(●+) ion [methylenecyclopropene (39.9 A?(2)), 1,2,3-butatriene (41.1 A?(2)), cyclobutadiene (38.6 A?(2)), and vinyl acetylene (41.1 A?(2))]. However, none of the single isomers is able to reproduce the experimental arrival time distribution of the C(4)H(4)(●+) ion. Combinations of cyclobutadiene and vinyl acetylene isomers show excellent agreement with the experimental mobility profile and the measured collision cross section. The fragment ions obtained by the dissociation of the C(4)H(4)(●+) ion are consistent with the cyclobutadiene structure in agreement with the vibrational predissociation spectrum of the acetylene dimer cation (C(2)H(2))(2)(●+) [R. A. Relph, J. C. Bopp, J. R. Roscioli, and M. A. Johnson, J. Chem. Phys. 131, 114305 (2009)]. The stepwise hydration experiments show that dissociative proton transfer reactions occur within the C(4)H(4)(●+)(H(2)O)(n) clusters with n ≥ 3 resulting in the formation of protonated water clusters. The measured binding energy of the C(4)H(4)(●+)H(2)O cluster, 38.7 ± 4 kJ/mol, is in excellent agreement with the G3(MP2) calculated binding energy of cyclobutadiene(●+)·H(2)O cluster (41 kJ/mol). The binding energies of the C(4)H(4)(●+)(H(2)O)(n) clusters change little from n = 1 to 5 (39-48 kJ/mol) suggesting the presence of multiple binding sites with comparable energies for the water-C(4)H(4)(●+) and water-water interactions. A significant entropy loss is measured for the addition of the fifth water molecule suggesting a structure with restrained water molecules, probably a cyclic water pentamer within the C(4)H(4)(●+)(H(2)O)(5) cluster. Consequently, a drop in the binding energy of the sixth water molecule is observed suggesting a structure in which the sixth water molecule interacts weakly with the C(4)H(4)(●+)(H(2)O)(5) cluster presumably consisting of a cyclobutadiene(●+) cation hydrogen bonded to a cyclic water pentamer. The combination of ion mobility, dissociation, and hydration experiments in conjunction with the theoretical calculations provides strong evidence that the (C(2)H(2))(2)(●+) ions are predominantly present as the cyclobutadiene cation with some contribution from the vinyl acetylene cation.     

Polynuclear lanthanide hydroxo complexes: new chemical precursors for coordination polymers

The synthesis of hexanuclear lanthanide hydroxo complexes by controlled hydrolysis led to polymorphic compounds. The hexanuclear entities crystallize in four different ways that depend on the extent of their hydration. The four structures can be described as hexanuclear lanthanide entities with formula [Ln(6)(mu(6)-O)(mu(3)-OH)(8)(NO(3))(6)(H(2)O)(12)](2+). Two additional NO(3)(-) ions intercalate between the hexanuclear entities in order to ensure the electroneutrality of the crystal structure. Some crystallization water molecules fill the intermolecular space. The three first families of compounds (1-3) exhibit crystal structures that have previously been reported. The fourth family of compounds (4) is described here for the first time. Its chemical formula is [Ln(6)(mu(6)-O)(mu(3)-OH)(8)(NO(3))(6)(H(2)O)(12)](NO(3))(2).2H(2)O (Ln = Gd, Er, and Y). In this paper, the chemical and thermal stabilities of the hexanuclear lanthanide compounds are reported together with the magnetic properties of the Gd(III)-containing species. To use these entities as precursors for new materials, the substitution of the nitrato groups by chloride ions has been studied. Two byproduct compounds have so been obtained: The first (compound 5) is a nitrato/chloride hexanuclear compound of chemical formula [Er(6)(mu(6)-O)(mu(3)-OH)(8)(NO(3))(6)(H(2)O)(12)](NO(3))Cl.2H(2)O. The second one (compound 6) is a polymeric compound in which the hexanuclear entities are linked by an unexpected and original N(2)O(4) bridge. Its chemical formula is [Er(6)(mu(6)-O)(mu(3)-OH)(8)(NO(3))(4)(H(2)O)(11)(OH)(ONONO(2))]Cl(3).2H(2)O. Its crystal structure can be described as the juxtaposition of chainlike molecular motifs. To the best of our knowledge, this is the first example of a coordination polymer synthesized from an isolated polylanthanide hydroxo complex.     

Theoretical study of [Ni (H2O)n]2+(H2O)m (n ≤ 6, m ≤ 18)

The [Ni-(H(2)O)(n)](2+)(H(2)O)(m) (n ≤ 6, m ≤ 18) complexes were studied by means of first-principles all-electron calculations performed with the BPW91 gradient corrected functional and the 6-311+G(d,p) basis sets for the H, O, and Ni atoms. Triplet states were found as low-lying states for each (n, m) combination. The estimated Ni(2+)-(H(2)O)(n) binding energies (112.8-57.4 kcal/mol for the first layer and 52.0-23.0 kcal/mol for the second one) decreases and the Ni(2+)-OH(2) bond lengths lengthen as n + m increases. With six H(2)O moieties the Ni(2+) ion furnishes its first coordination sphere of octahedral geometry. Further water addition renders the formation of the second layer. The effect of Ni(2+) on the (H(2)O)(n)···(H(2)O)(m) hydrogen bond formation for several "n" and "m" combinations was studied, revealing an enhancement of this kind of bonding, which is of key importance for the stabilization and growth of the clusters. For some n + m isomers the second layer appears before the first octahedral layer is fully formed. For example, the square planar Ni(2+)-(H(2)O)(4) core originates two-dimensional 4 + 2 and 4 + 4 isomers, where each outer water molecule accepts two H-bonds, lying 2.0 kcal/mol above the 6 and 6 + 2 ground states. The clusters were also studied by IR spectra; the OH stretching vibrational frequencies allowed us to identify the outer solvation shells by the presence of red-shifted hydrogen bond regions.     

Can we understand the different coordinations and structures of closed‐shell metal cation‐water clusters?

We present a model potential for studying M(q+)(H(2)O)(n=1,9) clusters where M stands for either Na(+), Cs(+), Ca(2+), Ba(2+), or La(3+). The potential energy surfaces (PES) are explored by the Monte Carlo growth method. The results for the most significant equilibrium structures of the PES as well as for energetics are favorably compared to the best ab initio calculations found in the literature and to experimental results. Most of these complexes have a different coordination number in cluster compared to experimental results in solution or solid phase. An interpretation of the coordination number in clusters is given. In order to well describe the transition between the first hydration sphere and the second one we show that an autocoherent treatment of the electric field is necessary to correctly deal with polarization effects. We also explore the influence of the cation properties (charge, size, and polarizability) on both structures and coordination number in clusters, as well as the meaning of the second hydration sphere. Such an approach shows that the leading term in the interaction energy for a molecule in the second hydration sphere is an electrostatic attraction to the cation and not a hydrogen bond with the water molecules in the first hydration sphere.     

New Cu(II) complexes based on the hydroxo-bridged dinuclear [Cu(OH)2Cu] units: step-like di- and trimerizations of [Cu(OH)2Cu] units

Four new Cu(II) complexes {[Cu(4)(bpy)(4)(OH)(4)(H(2)O)(2)]}(NO(3))(2)(C(7)H(5)O(2))(2)·6H(2)O 1, {[Cu(4)(bpy)(4)(OH)(4)(H(2)O)(2)]}(NO(3))(2)(C(5)H(6)O(4))·8H(2)O 2, {[Cu(4)(bpy)(4)(OH)(4)(H(2)O)(2)]}(C(5)H(6)O(4))(2)·16H(2)O 3 and {[Cu(6)(bpy)(6)(OH)(6)(H(2)O)(2)]}(C(8)H(7)O(2))(6)·12H(2)O 4 were synthesized (bpy = 2,2'-bipyridine, H(2)(C(5)H(6)O(4)) = glutaric acid, H(C(7)H(5)O(2)) = benzoic acid, H(C(8)H(7)O(2)) = phenyl acetic acid). The building units in 1-3 are the tetranuclear [Cu(4)(bpy)(4)(H(2)O)(2)(μ(2)-OH)(2)(μ(3)-OH)(2)](4+) complex cations, and in 4 the hexanuclear [Cu(6)(bpy)(6)(H(2)O)(2)(μ(2)-OH)(2)(μ(3)-OH)(4)](6+) complex cations, respectively. The tetra- and hexanuclear cluster cores [Cu(4)(μ(2)-OH)(2)(μ(3)-OH)(2)] and [Cu(6)(μ(2)-OH)(2)(μ(3)-OH)(4)] in the complex cations could be viewed as from step-like di- and trimerization of the well-known hydroxo-bridged dinuclear [Cu(2)(μ(2)-OH)(2)] entities via the out-of-plane Cu-O(H) bonds. The complex cations are supramolecularly assembled into (4,4) topological networks via intercationic ππ stacking interactions. The counteranions and lattice H(2)O molecules are sandwiched between the 2D cationic networks to form hydrogen-bonded networks in 1-3, while the phenyl acetate anions and the lattice H(2)O molecules generate 3D hydrogen-bonded anionic framework to interpenetrate with the (4,4) topological cationic networks with the hexanuclear complex cations in the channels. The ferromagnetic coupling between Cu(II) ions in the [Cu(4)(μ(2)-OH)(2)(μ(3)-OH)(2)] cores of 1-3 is significantly stronger via equatorial-equatorial OH(-) bridges than via equatorial-apical ones. The outer and the central [Cu(2)(OH)(2)] unit within the [Cu(6)(μ(2)-OH)(2)(μ(3)-OH)(4)] cluster cores in 4 exhibit weak ferromagnetic and antiferromagnetic interactions, respectively. Results about i.r. spectra, thermal and elemental analyses are presented.     

Synthesis and solution studies of the complexes of trivalent lanthanides with the tetraazamacrocycle TETA-(PO)(2)

A new potentially multidentate hexaprotic ligand H(6)[TETA-(PO)(2)] has been prepared by reaction of ethylenediamine-N,N'-diacetic acid (EDDA), paraformaldehyde, and phosphinic acid; its coordination properties with three lanthanide ions (La(3+), Gd(3+), and Lu(3+)) have been explored. The structures of the complexes were studied in aqueous solution by potentiometric pH titrations and by (31)P NMR spectroscopy. Four acidity constants were determined potentiometrically in the range 2.5 < pH < 14. The four measured pK(a) values can be divided into two groups, and within each group the initial deprotonation was found to have little effect on the second. Variable temperature (31)P and (31)P[(1)H] EXSY NMR spectra showed that, for [Lu(TETA-(PO)(2))](3-), the two phosphorus atoms exist in different chemical environments and undergo an exchange process which is very fast on the NMR time scale at room temperature. This result is consistent with one of the phosphinate residues coordinating the metal ion and exchanging with a free analogue. In the case of [La(TETA-(PO)(2))](3-), only one temperature invariant signal is observed in (31)P NMR spectra; it corresponds to both phosphinate residues remaining uncoordinated to La(3+). The stability of [Ln(TETA-(PO)(2))](3-) has an order of La(3+) > Gd(3+) > Lu(3+). The coordination of one phosphinate residue to Lu(3+) brings the metal ion closer to the plane of four nitrogens and farther from the four carboxylate arms, resulting in [Lu(TETA-(PO)(2))](3-) having a lower stability than the corresponding La(3+) and Gd(3+) complexes. A pM-pH distribution diagram showed that introducing two phosphinate groups into TETA renders [Gd(TETA-(PO)(2))](3-) more stable than [Gd(TETA)](-). The selectivity factor of the ligand for Gd(3+) vs Ca(2+), Zn(2+), and Cu(2+) has been calculated, and the hydration number for [Dy(TETA-(PO)(2))](3-) has been measured by (17)O NMR spectroscopy to be zero.     

Apparent or real water exchange reactions on [Zn(H2O)4(L)](2+)·2H2O (L = sp-nitrogen donor ligands)? A quantum chemical investigation

The exchange of a second coordination sphere water molecule in [Zn(H(2)O)(4)(L)](2+)·2H(2)O (L = HN(3), HCN, FCN, ClCN, BrCN, CH(3)CN, (C(4)H(3))CN, PhCN, (CH(3))(3)CCN, CF(3)CN, CCl(3)CN, CHCl(2)CN, and CH(2)ClCN) against a coordinated water molecule was studied by quantum chemical calculations (RB3LYP/6-311+G**). The complete reaction consists of an associative binding of one H(2)O from the second coordination sphere leading to a six-coordinate intermediate [Zn(H(2)O)(5)(L)](2+)·H(2)O, followed by the dissociation of a water molecule to reach the product state [Zn(H(2)O)(4)(L)](2+)·2H(2)O. For a real water exchange reaction to occur two different transition states have to be included, otherwise only an apparent water exchange reaction takes place. For the water exchange reaction in [Zn(H(2)O)(4)(L)](2+)·2H(2)O, nearly iso-energetic cis- and trans-orientated transition states are crossed. The gas-phase proton affinity of L shows instructive correlations with structural parameters and energy gaps for the investigated reactions.     

Synthesis and characterization of two novel lanthanide coordination polymers with an open framework based on an unprecedented [Ln(7)(micro(3)-OH)(8)](13+) cluster

Two novel heptanuclear lanthanide clusters of the dicubane-like type [Ln(7)(micro(3)-OH)(8)](13+) (Ln = Ho (1), Yb (2)) were obtained via hydrothermal reaction, and as building blocks, they were formally assembled into porous three-dimensional networks through the linkage of 1,4-naphthalenedicarboxylate (1,4-NDA), forming first examples of porous lanthanide polymers with 1,4-NDA, [Ln(7)(micro(3)-OH)(8)(1,4-NDA)(6)(OH)(0.5)(Ac)(0.5)(H(2)O)(7)].4H(2)O (Ln = Ho, Yb). The coordinating water molecules and lattice water molecules are enclathrated in the cavities.     

Water oxidation intermediates applied to catalysis: benzyl alcohol oxidation

Four distinct intermediates, Ru(IV)═O(2+), Ru(IV)(OH)(3+), Ru(V)═O(3+), and Ru(V)(OO)(3+), formed by oxidation of the catalyst [Ru(Mebimpy)(4,4'-((HO)(2)OPCH(2))(2)bpy)(OH(2))](2+) [Mebimpy = 2,6-bis(1-methylbenzimidazol-2-yl) and 4,4'-((HO)(2)OPCH(2))(2)bpy = 4,4'-bismethylenephosphonato-2,2'-bipyridine] on nanoITO (1-PO(3)H(2)) have been identified and utilized for electrocatalytic benzyl alcohol oxidation. Significant catalytic rate enhancements are observed for Ru(V)(OO)(3+) (~3000) and Ru(IV)(OH)(3+) (~2000) compared to Ru(IV)═O(2+). The appearance of an intermediate for Ru(IV)═O(2+) as the oxidant supports an O-atom insertion mechanism, and H/D kinetic isotope effects support net hydride-transfer oxidations for Ru(IV)(OH)(3+) and Ru(V)(OO)(3+). These results illustrate the importance of multiple reactive intermediates under catalytic water oxidation conditions and possible control of electrocatalytic reactivity on modified electrode surfaces.