Stereochemistry of lead(II) complexes containing sulfur and selenium donor atom ligands

The stereochemistry of lead(II) complexes with S- and Se-donor atom ligands, including mixed ligand complexes is reviewed with respect to the geometry of the first coordination sphere of the Pb(II) atom in these compounds and rationalized in terms of the valence shell electron-pair repulsion (VSEPR) model. The most comprehensively structurally characterized classes of lead(II) thio and seleno complexes are discussed, including monothio-, dithio(seleno)-, trithio- and tetrathio-complexes, as well as Pb(II) dialkyldithio(seleno)carbamates, alkylxanthates and dialkyl(aryl) phosphorodithio(seleno)lates. Data about the polyhedral shape of the primary coordination sphere, coordination number (CN), bond lengths (primary and secondary) and bond angles of the Pb(II) atom in the compounds under investigation are systematized in comprehensive tables. The particularities of the stereochemistry of Pb(II) complexes with S(Se)-donor atom ligands are comparatively discussed with the stereochemistry of lead(II) complexes with oxygen donor ligands.     

An investigation of methods for the application of molecular mechanics to lanthanide complexes

Methods are described for molecular mechanics calculations on lanthanide complexes. The irregularity of the coordination spheres of these metals necessitate special treatment in a molecular mechanics force field. Three different methods for treating the metal coordination sphere in the complexes are evaluated. In the first method, we include bond stretch terms between metal and donor atoms and 1,3 interactions between donor atoms. The second method utilizes a nonbonded potential between metal atoms and donor atoms to determine the geometry of the coordination sphere, and the third method involves coulombic interactions as well as a nonbonded potential to describe the van der Waals interactions. Evaluations of the three methods have been carried out by calculating the r.m.s. deviations between experimental structures and minimized structures. Results indicate that it is possible to achieve good agreement by all three methods, but that the second method provides the most consistent results, as well as being relatively straightforward to paramaterize.     

Liquid adsorption chromatography of chelates : The effect of structure on the thin-layer chromatographic behaviour of chelates

The thin-layer chromatographic behaviour of chelates belonging to different classes is discussed. The major adsorption interactions of chelates are hydrogen bonding between the ligand donor atoms and the surface hydroxyl groups and reactions between the metal atom and the electron-donor active centres of the sorbent. Predominance of either of these general mechanisms depends on the chelate structure and particularly on the coordination saturation of the chelate. Coordination-saturated chelates are retained because of hydrogen bonding, while the metal atom does not participate directly but can influence sorption by affecting the electron density distribution in the chelating ring. Atomic electronegativity is used as a measure of the electron-acceptor ability of the metal. Electronegative atoms located outside the functional group of the chelate can participate in the adsorption either directly or by affecting the proton-acceptor ability of the donor atoms as a result of induction and steric effects. The relationship between chelate retention factors and the parameters characterizing the electron and spatial structure of ligands can be described quantitatively by an equation of the type log [(/R_f)?1] = A + Σσ. In the case of coordination-unsaturated chelates, adsorption interactions with participation of the metal atom predominate, either by ion exchange (with ligand replacement) or by a donor-acceptor mechanism (with introduction of the adsorption centre into the coordination sphere without decomposition). In general, the adsorbability of chelates is directly related to the proton-acceptor ability of donor atoms and the acceptor ability of the metal atom. Classification of chelates by their adsorption interactions is proposed. Recommendations are given for selecting the optimal chelating reagent for the separation of metals by liquid-adsorption chromatography.     

The n.m.r. of paramagnetic complexes of vanadyl acetylacetonate with organic phosphites and hydroperoxides

The vanadium IV ion in vanadyl acetylacetonate (V^IV) forms labile paramagnetic complexes with organic phosphites in the first coordination sphere. The enthalpy of complex formation between V^IV and triphenyl phosphite was 2.6 kcal mol^?1. Complex formation enthalpies ΔH and the activation energies E of ligand (hydroperoxide) escape from the metal ion sphere were determined from the temperature dependence of paramagnetic broadening of the n.m.r. lines of hydroperoxides in the presence of vanadyl acetylacetonate. At low temperatures the phosphite sharply weakens the bond between the metal ion and hydroperoxide in the second coordination sphere (ΔH decreases fivefold). Taken in excess, phosphite displaces the hydroperoxide molecules from the coordination sphere of the V^IV ion and thus blocks it. The observed n.m.r. characteristics of the paramagnetic complexes explain, on the model level, the kinetic regularities of the reaction of hydroperoxides with phosphite catalysed by transient metal ions.     

Role of inner- and outer-sphere bonding in the sensitization of EuIII-luminescence deciphered by combined analysis of experimental electron density distribution function and photophysical data

A series of lanthanide adducts with different amounts of 1,10-phenanthroline, chloride ions, and water molecules in the inner and outer coordination spheres are investigated with the aim of relating the chemical bonding patternin the crystals to the luminescence properties of the Eu ion: [LnCl1Phen2(H2O)3]Cl2(H2O) (Ln ) Eu, 1Eu; Gd, 1Gd;Tb, 1Tb), [EuCl2Phen2(H2O)2]Cl1(H2O) (2), and [EuCl2Phen1(H2O)4]Cl1(H2O) (3). The influence of inner- versus outersphere ligands on the Ln-X bond lengths and angles in the structure is examined. A detailed topological analysis of the electron density function derived from the X-ray diffraction data for 1Gd is performed within the frame of the"atoms in molecule" theory for the first time for a lanthanide complex. The chemical bonding pattern is interpreted in terms of net atomic charges, bond energies, and electron transfers from the ligands to the metal ion. A noteworthy finding is that the energy of extended noncovalent interactions occurring in the second coordination sphere (H-bonding and pi-stacking interactions) is comparable to that of Ln-ligand bonds. The luminescence properties of the three Eu adducts are interpreted with the results of electron density distribution function topology. An intraligand charge transfer state is identified, and its contribution in the ligand-to-europium energy transfer process is analyzed.The outcome of this study is that specific interionic interactions which are usually not considered in theoretical calculations or in the interpretation of luminescence properties play an important role in the sensitization of the Eu luminescence.     

The Maximum Hardness Principle and the Composition of Zn(II) and Cd(II) Tetrafluoroborate Complexes with Nitrogen-Containing Organic Bases

The hardness/softness parameters of the central complexing ion, the donor nitrogen atom of the ligand, and the coordination bond were calculated for zinc and cadmium tetrafluoroborate complexes with nitrogen-containing organic bases of various composition (both experimentally determined and hypothetical). On the basis of the quantitative estimate of the hard/soft characteristics of the whole series of the coordination compounds considered, it was found that the hardest parameters are inherent in the complexes isolated in a pure state by a preparative method.     

Synthesis, spectroscopic characterization, and 3D molecular modeling of lead(II) complexes of unsymmetrical tetradentate Schiff-base ligands

Solid complexes of Pb(II) with unsymmetrical Schiff-base ligands (H₂L) derived from 2-aminobenzophenone, thiosemicarbazide, semicarbazide, salicylaldehyde, 2-hydroxynaphthaldehyde, and o-hydroxyacetophenone have been synthesized and characterized by elemental analysis, conductance measurements, molecular weight measurement, and UV–Vis, FTIR, ¹H NMR, and ¹³C NMR spectroscopy. The spectral studies suggest the ligands behave as dibasic tetradentate ligands with ONNO/ONNS donor atom sequences toward the central metal ion. From the microanalytical data, the stoichiometry of the complexes was found to be 1:1 (metal:ligand). The physicochemical data suggest a tetracoordinated environment around the metal ion. Three-dimensional molecular modeling and analysis of bond lengths and bond angles have also been conducted for a representative compound, [PbL¹], to substantiate the proposed structures.     

Mössbauer study of the donor character of oxygen in Si−O bonds

The donor strength of oxygen atoms in the Si?O?C and Si?O?Si groups has been studied by Mössbauer spectroscopy, using SnI₄ as acceptor. At 80 K trimethylalkoxysilanes and the disiloxanes studied show characteristic isomer shifts on the basis of which the existence of pentacovalent SnI₄·D and hexacovalent SnI₄·D₂ may be assumed. The changes in isomer shift due to the first and second oxygen atoms are ?0.60 and ?0.55 mm/sec, respectively. The coordination number is determined by the steric requirements of the donor relative to the coordination sphere of the acceptor, provided that the donor strength is sufficiently high. No donor-acceptor interactions have been observed with compounds of the-(OSiR₂)-O-type. In the case of hexamethyldisiloxane, iodine is displaced from the coordination sphere. No quadrupole splitting has been observed in any of the cases studied. Thus the ratio of point charges assignable to the iodine and the donor oxygen atom is 6:1 for the trigonal bipyramidal arrangement; for a tetragonal pyramidal structure the angle between the xy plane and the Sn?I bond is ～35°.     

Complexation of (carbamoylmethyl)diphenylphosphine sulfide with silver nitrate. The structure of the polymeric complex [Ag2{Ph2P(S)CH2C(O)NH2}2(NO3)2] n

The reaction of (carbamoylmethyl)diphenylphosphine sulfide with AgNO₃ yields the polymeric complex [Ag₂{Ph₂P(S)CH₂C(O)NH₂}₂(NO₃)₂] _n . Its structure was established by X-ray diffraction analysis. The coordination environments about both Ag⁺ cations are formed by five donor atoms, two of which are bonded to the metal atom substantially more weakly than the remaining three atoms. The compositions of the coordination polyhedra are different: ({AgSO′(C)O(N)O₂(N′)} and {AgS′ SO(C)O₂(N)}). The coordinated ligands differ in their functions: one ligand chelates the metal cation and its sulfur atom is additionally bonded to the second cation, while the second ligand acts as a bridge between the two different cations. The structure of the complex and the character of the interaction between the ligand and AgNO₃ are substantially affected by the network of hydrogen bonds. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 838–845, April, 1997.     

EPR of complexes of divalent copper ions with organophosphorus derivatives of 2-aminopyridine

The complexation of copper(II) ion with organophosphorus derivatives of 2-aminopyridine was studied with EPR. It was found that, compared with 2-aminopyridine, the presence of a P-N bond in its derivatives substantially changes the character of the coordination of the ligand with respect to divalent copper, which includes an aminic nitrogen atom in the coordination sphere of the metal, and results in the formation of a four-membered chelate ring. The parameters of the EPR spectra and evaluations of the stability constants are given.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 12, pp. 2756–2763, December, 1990.     

A computational study of O-O bond formation catalyzed by mono- and bis-MnIV-corrole complexes

A detailed computational study of O-O bond formation, catalyzed by monomeric and dimeric Mn-corrole complexes, is reported. The model explicitly takes into account the solvent, with respect to the first and second coordination spheres, while the bulk solvent is described by the polarizable continuum model. Two reaction mechanisms are proposed and computationally characterized: the concerted and the two-step mechanisms. The concerted mechanism is based on a OH--MnIVO interaction via the outer-sphere pathway involving the bridging solvent molecules in the first coordinating sphere. The two-step mechanism is proposed to operate via the coordination of a hydroxide to the MnIV ion, forming a MnO(OH)--corrole complex with a strongly nonplanar corrole ligand. Comparison of the proposed mechanisms with available experimental data is performed.     

A modular approach toward regulating the secondary coordination sphere of metal ions: differential dioxygen activation assisted by intramolecular hydrogen bonds

Metal ion function depends on the regulation of properties within the primary and second coordination spheres. An approach toward studying the structure-function relationships within the secondary coordination sphere is to construct a series of synthetic complexes having constant primary spheres but structurally tunable secondary spheres. This was accomplished through the development of hybrid urea-carboxamide ligands that provide varying intramolecular hydrogen bond (H-bond) networks proximal to a metal center. Convergent syntheses prepared ligands [(N'-tert-butylureayl)-N-ethyl]-bis(N' '-R-carbamoylmethyl)amine (H(4)1R) and bis[(N'-tert-butylureayl)-N-ethyl]-(N' '-R-carbamoylmethyl)amine (H(5)2R), where R=isopropyl, cyclopentyl, and (S)-(-)-alpha-methylbenzyl. The ligands with isopropyl groups H(4)1iPr and H(5)2iPr were combined with tris[(N'-tert-butylureayl)-N-ethyl]amine (H6buea) and bis(N-isopropylcarbamoylmethyl)amine (H(3)0iPr) to prepare a series of Co(II) complexes with varying H-bond donors. [CoIIH(2)2iPr]- (two H-bond donors), [CoIIH1iPr]- (one H-bond donor), and [CoII0iPr]- (no H-bond donors) have trigonal monopyramidal primary coordination spheres as determined by X-ray diffraction methods. In addition, these complexes have nearly identical optical and EPR properties that are consistent with S=3/2 ground states. Electrochemical studies show a linear spread of 0.23 V in anodic potentials (Epa) with [CoIIH(2)2iPr]- being the most negative at -0.385 V vs [Cp2Fe]+/[Cp2Fe]. The properties of [CoIIH3buea]- (H3buea, tris[(N'-tert-butylureaylato)-N-ethyl]aminato that has three H-bond donors) appears to be similar to that of the other complexes based on spectroscopic data. [CoIIH3buea]- and [CoIIH(2)2iPr]- react with 0.5 equiv of dioxygen to afford [CoIIIH3buea(OH)]- and [CoIIIH(2)2iPr(OH)]-. Isotopic labeling studies confirm that dioxygen is the source of the oxygen atom in the hydroxo ligands: [CoIIIH3buea(16OH)]- has a -(O-H) band at 3589 cm-1 that shifts to 3579 cm-1 in [CoIIIH3buea(18OH)]-; [CoIIIH(2)2iPr(OH)]- has -(16O-H)=3661 and -(18O-H)=3650 cm-1. [CoIIH1iPr]- does not react with 0.5 equiv of O2; however, treating [CoIIH1iPr]- with excess dioxygen initially produces a species with an X-band EPR signal at g=2.0 that is assigned to a Co-O2 adduct, which is not stable and converts to a species having properties similar to those of the CoIII-OH complexes. Isolation of this hydroxo complex in pure form was complicated by its instability in solution (kint=2.5x10-7 M min-1). Moreover, the stability of the CoIII-OH complexes is correlated with the number of H-bond donors within the secondary coordination sphere; [CoIIIH3buea(OH)]- is stable in solution for days, whereas [CoIIIH(2)2iPr(OH)]- decays with a kint=5.9x10-8 M min-1. The system without any intramolecular H-bond donors [CoII0iPr]- does not react with dioxygen, even when O2 is in excess. These findings indicate a correlation between dioxygen binding/activation and the number of H-bond donors within the secondary coordination sphere of the cobalt complexes. Moreover, the properties of the secondary coordination sphere affect the stability of the CoIII-OH complexes with [CoIIIH3buea(OH)]- being the most stable. We suggest that the greater number of intramolecular H-bonds involving the hydroxo ligand reduces the nucleophilicity of the CoIII-OH unit and reinforces the cavity structure, producing a more constrained microenvironment around the cobalt ion.     

Structures of Nickel(II) and Copper(II) Complexes of 14-Membered Macrocyclic Quadridentate Ligands

The structures of 41 Ni(II) and 17 Cu(II) complexes of macrocyclic quadridentate ligands have been analyzed, and are discussed about bond lengths, bond angles, conformations, and configurations, upon which many conclusions are formed. The inter- or intra-molecular hydrogen bonds exist among ligands and hydrates in many compounds and play an important role in the structures. There are exhibited two distinct peaks on the histogram of the average Ni-N distances, corresponding to four coordination and six coordination; these average Ni-N distances are 1.95(4) Å and 2.10(5) Å, respectively. The most probable structures of Ni(II) macrocyclic compounds have coordination number six for the metal ion, chair forms for six-membered rings, planar structure for the metal ion and the four donor atoms of the quadridentate ligand and an inversion center at the central metal ion.     

Tetragonal-pyramidal complex of copper nitrate with 2-Amino-5-ethyl-1,3,4-thiadiazole

A complex compound of Cu(II) nitrate with 2-amino-5-ethyl-1,3,4-thiadiazole was synthesized and its structure was studied by the methods of IR spectroscopy and X-ray crystal analysis. The complex has the composition Cu(NO₃)₂(2-amino-5-ethyl-1,3,4-thiadiazole)₄ with four molecules of the heterocyclic ligand (coordination through nitrogen atoms of thiadiazole rings) and one of nitrate ions (the other is replaced in the second sphere) entering into the coordination sphere of the complex polyhedron. The internal coordination sphere of the complex has the form of a tetragonal pyramid with 2-amino-5-ethyl-1,3,4-thiadiazole ligands in the sites of its base and the oxygen atom of the nitrate ion in a slightly distorted vertex of the pyramid.     

Synthesis,Coordination Chemistry and Bonding of Strong N‐Donor Ligands Incorporating the 1H‐Pyridin‐(2E)‐Ylidene (PYE) Motif

A range of N‐donor ligands based on the 1H‐pyridin‐(2E)‐ylidene (PYE) motif have been prepared, including achiral and chiral examples. The ligands incorporate one to three PYE groups that coordinate to a metal through the exocyclic nitrogen atom of each PYE moiety, and the resulting metal complexes have been characterised by methods including single‐crystal X‐ray diffraction and NMR spectroscopy to examine metal–ligand bonding and ligand dynamics. Upon coordination of a PYE ligand to a proton or metal‐complex fragment, the solid‐state structures, NMR spectroscopy and DFT studies indicate that charge redistribution occurs within the PYE heterocyclic ring to give a contribution from a pyridinium–amido‐type resonance structure. Additional IR spectroscopy and computational studies suggest that PYE ligands are strong donor ligands. NMR spectroscopy shows that for metal complexes there is restricted motion about the exocyclic C? N bond, which projects the heterocyclic N‐substituent in the vicinity of the metal atom causing restricted motion in chelating‐ligand derivatives. Solid‐state structures and DFT calculations also show significant steric congestion and secondary metal–ligand interactions between the metal and ligand C? H bonds.     

Different coordination modes and supramolecular aggregations in copper(II) pentane-2,4-dionato compounds with 2-pyridone and 3-hydroxypyridine

Abstract  

Copper(II) bis(pentane-2,4-dionato-κ² O,O′) compounds with 2-pyridone (1) and 3-hydroxypyridine (2) were prepared by the reaction of bis(pentane-2,4-dionato-κ² O,O′)copper(II) with selected ligands. The coordination of Cu(II) in both compounds is square pyramidal with the fifth coordination site occupied by the carbonyl oxygen atom of the 2-pyridone ligand in 1 and by the nitrogen atom of 3-hydroxypyridine in 2. The X-ray crystallographic studies revealed different crystal aggregation influenced by the ability of the 2-pyridone ligand to act as a hydrogen bond donor and acceptor, and 3-hydroxypyridine acting only as a hydrogen bond donor. Intermolecular N–H···O hydrogen bonding forms dimers in 1 and infinite chains in 2. Three-dimensional aggregation is achieved by π–π interactions and C–H···π (arene) hydrogen bonding.     

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.     

Mössbauer study of the influence of ligands and anions of the second coordination sphere in Fe(II) complexes with 1,2,4-triazole and 4-amino-1,2,4-triazole on the temperature of the 1A 1⇆5 T 2 spin transitions

For Fe(II) complexes with 1,2,4-triazole (Tr) and 4-amino-1,2,4-triazole (ATr) in low-spin phases, it has been revealed that the ligand and anion of the second coordination sphere affect the electronic state of the central atom and the symmetry of its local environment. For low-spin phases, a correlation between the Mössbauer spectrum parameters—the chemical shift and the bandwidth of the weakly resolved quadrupole doublet—and the spin transition temperature T_c has been established. The mechanism of the influence of complex composition on the spin transition temperature is accounted for by assuming that T_c is determined by the lattice energy of a complex compound.     

Peculiarities of metal—ligand bonding in europium trinitrate complexes: a viewpoint of comparative charge density analysis in crystals

The results of high-resolution X-ray diffraction studies of the charge density distribution in crystals of three europium trinitrate complexes, including those with N-donor "antenna"-ligands are summarized. It is shown that the charge transfer between lanthanide ion and "antenna" correlates with the energy of interaction between the metal and nitrate anion, and the total stabilization energy of the metal polyhedron depends weakly on the coordination number of the metal and the nature of the ligands. The statistical treatment of the crystal structural data for similar complexes and the energies of the corresponding metal—ligand interactions allowed us to suggest the stability of mer-arrangement of the coordinated nitrate anions and to propose a semiempirical relationship to estimate the energy of Eu—O interactions. The influence of vibration processes on the electron density distribution in the EuO₂NO fragment was additionally considered, and a feasibility to estimate the energy of interaction between the ligand and lanthanide ion at non-stationary points on the potential energy surface was validated.