Formation of hypervalent complexes of PhCCSiF3 with pyridine through intermolecular silicon...nitrogen interaction

We report here the synthesis of trifluoro-phenylethynyl-silane (1) that forms with pyridine (Py), through intermolecular Si...N interaction, the pentacoordinate 1.Py complex and at low temperatures also the hexacoordinate 1.Py2 complex. 1H, 19F, 29Si, and 15N NMR spectra, as well as the first report for an intermolecular 29Si...15N one-bond spin-spin coupling, are presented for the two complexes. Quantum mechanical ab initio calculations (MP2/6-31G*) suggest a distorted trigonal bipyramid structure for the 1.Py complex and a nearly ideal octahedral structure for the 1.Py2 complex. The hypervalent complexes of 1 with Py described here imply a possible application of such Si...N intermolecular interactions in supramolecular chemistry.     

Formation of hexacoordinate complexes of PhCCSiF3 with 2,2'-bipyridine and with 1,10-phenanthroline through intermolecular silicon...nitrogen interactions

Bidentate intermolecular Si...N interactions were utilized to form new hypervalent complexes of trifluoro-phenylethynyl-silane with 2,2'-bipyridine and with 1,10-phenanthroline. X-ray structures obtained for these complexes display a somewhat distorted octahedral geometry about the silicon atom. Binding constants ranging from 170 to 1600 M(-1) at 25 degrees C in CDCl3 were measured for the formation of these complexes, suggesting that such hypervalent complexes of silicon could be used as new motifs in supramolecular chemistry.     

Effect of solvents on dielectric,spectroscopic and thermodynamic manifestations of intermolecular interactions. Iodine complexes of pyridine and picoline

Dipole moments of pyridine and γ-picoline complexes with I₂ in a number of nonpolar and weakly polar solvents were determined and the contributions of the dative structure F_N in the ground state were estimated. The results are consistent with those calculated from the force constants of stretching vibrations v(N-l) and v (I-I). A good correlation of F_N was found with respect to the solvent shift of the transition energy {ie355-1} in the I₂ molecule bounded to amines and also to empirical parameters of solvent activities like rate constants of the Menshutkin reaction or the Dimroth-Reichardt parameter. No correlation exists between the solvent induced enhancement of the dipole moment and dielectric permittivity in terms of the Onsager reaction field theory.     

Steric effect on construction of extended architectures of Ni(II) complexes directed by intermolecular C-H...F and C-H...O interactions

Five new Ni(II) complexes with pyridine carboxamide ligands have been synthesized and the crystal structures of three of the complexes were determined. Strong distortion effects of 6-methyl substitution were observed in the complexes with 6-methyl-substituted pyridyl bpb ligands. The C-H...F and C-H...O hydrogen bond interactions build extended architectures in the crystals studied. This result suggests that the steric effect of 6-methyl substitution plays an important role in the distortion of the structure, and the 6-methyl substitution can facilitate hydrogen bond interactions between methyl hydrogen atoms and O(carbonyl) or F atoms. Twelve Ni(II) complexes, including seven complexes reported previously, show reversible redox behavior, implying that the reduced Ni(I) state of each complex is stable in the time scale of CV measurement. The steric effect of R₁ substituent and the electronic effects of X₁ and X₂ groups were found to be the main factors contributing to the shift of the redox potential of the Ni(II) complexes     

Secondary hypervalent I(III)...O interactions: synthesis and structure of hypervalent complexes of diphenyl-lambda3-iodanes with 18-crown-6

Reported here for the first time are the synthesis and characterization of supramolecular complexes between diaryl-lambda(3)-iodanes and 18-crown-6 (18C6). Slow evaporation of solvents afforded 1:1 and 2:1 complexes between Ph(2)IBF(4) and 18C6 as stable crystals, depending on the conditions. X-ray crystal structures of these complexes indicated that each iodine atom contacts with the three adjacent oxygen atoms of 18C6 through two hypervalent secondary bonding and a weak interaction. (1)H NMR analyses and CSI-MS spectra showed that, in dichloromethane solution, Ph(2)IBF(4) exclusively forms the 1:1 complex with 18C6 (binding constant K(a), 1.02 x 10(3) M(-)(1)). The binding constants decrease with the increased solvent donor ability (Gutmann's DN). Changing the heteroatom ligand from BF(4) to the less nucleophilic PF(6) and AsF(6) increased the binding constant by about six times. Substitution of an electron-withdrawing group onto the para position of Ph(2)IBF(4) tends to increase in the complex stability. A linear Hammett relationship (rho = 0.59) between log K(a) and sigma(p)(+) values of substituents indicates that the diaryl-lambda(3)-iodanes with electropositive iodine(III) interact more efficiently with 18C6. Decreased binding magnitude was measured with 15C5, dibenzo-18C6, dibenzo-21C7, and dibenzo-30C10.     

Ultraviolet photofragmentation spectroscopy of alkaline earth dication complexes with pyridine and 4-picoline (4-methyl pyridine)

A detailed experimental and theoretical study has been undertaken of the UV photofragmentation spectroscopy of the alkaline earth metal dications Mg(2+), Ca(2+), and Sr(2+) complexed with pyridine and 4-methyl pyridine (4-picoline). The ion complexes have been prepared using the pick-up technique and held in an ion trap where their internal temperature has been reduced to <150 K. Exposure of the trapped ions to tunable UV laser radiation leads to the appearance of photofragments with intensities that show significant variation as a function of wavelength. For all three metal dications, the resultant spectra show evidence of resolved features. Time-dependent density functional theory (TDDFT) has been used to identify possible electronic transitions that might be present in the [M(pyridine)(4)](2+) complexes (M = Mg, Ca, and Sr) within the wavelength range studied. These calculations show that the spectra are dominated by strong π* ← π and weaker π* ← n transitions localized on the pyridine ligands. The calculations correctly identify those regions of the experimental spectra where UV transitions begin to occur in the complexes and also the wavelengths at which absorption maxima are reached; however, more subtle features of the spectra are difficult to assign with confidence.     

Ferromagnetic and antiferromagnetic intermolecular interactions in a new family of Mn4 complexes with an energy barrier to magnetization reversal

A new family of tetranuclear Mn complexes [Mn4X4L4] (H2L = salicylidene-2-ethanolamine; X = Cl (1) or Br (2)) and [Mn4Cl4(L')4] (H2L' = 4-tert-butyl-salicylidene-2-ethanolamine, (3)) has been synthesized and studied. Complexes 1-3 possess a square-shaped core with ferromagnetic exchange interactions between the four Mn(III) centers resulting in an S = 8 spin ground state. Magnetochemical studies and high-frequency EPR spectroscopy reveal an axial magnetoanisotropy with D values in the range -0.10 to -0.20 cm(-1) for complexes 2 and 3 and for differently solvated forms of 1. As a result, these species possess an anisotropy-induced energy barrier to magnetization reversal and display slow relaxation of the magnetization, which is observed as hysteresis for 1 and 3 and frequency-dependent peaks in out-of-phase AC susceptibility measurements for 3. The effective energy barrier was determined to be 7.7 and 7.9 K for 1 and 3, respectively, and evidence for quantum tunneling of the magnetization was observed. Detailed magnetochemical studies, including measurements at ultralow temperatures, have revealed that complexes 1 and 2 possess solvation-dependent antiferromagnetic intermolecular interactions. Complex 3 displays ferromagnetic intermolecular interactions and approaches a ferromagnetic phase transition with a critical temperature of approximately 1 K, which is coincident with the onset of slow relaxation of the magnetization due to the molecular anisotropy barrier to magnetization reversal. It was found that the intermolecular interactions have a significant effect on the manifestation of slow relaxation of the magnetization, and thereby, these complexes represent a new family of "exchange-biased single-molecule magnets", where the exchange bias is controlled by chemical and structural modifications.     

Solubilization of silica: Synthesis, characterization and study of penta-coordinated pyridine N-oxide silicon complexes

In an effort to design agents that could solubilize silica in water, under ambient conditions and pH, as takes place in nature, novel zwitterionic, penta-oxo-coordinated silicon compounds with siliconate cores have been prepared from 4-substituted pyridine N-oxides (H, OMe, morpholino, NO₂) as donor ligands, their structures established by¹H,¹³C and MS, and the coordination number of silicon, by²⁹Si NMR. The formation of complexes from pyridine N-oxides is noteworthy since they arise from interaction with a weakly nucleophilic oxygen centre. The ability of the pyridine N-oxides to enhance the solubilization of silica in water has been experimentally demonstrated. Possible rationalization of this observation on the basis of O → Si coordination via the oxygen atom of pyridine N-oxide is suggested Dedicated to Professor S Swaminathan on the occasion of his 80th birthday     

Coordination complexes exhibiting anion...pi interactions: synthesis, structure, and theoretical studies

The polydentate ligand 2,4,6-tris(dipyridin-2-ylamino)-1,3,5-triazine (dpyatriz) in combination with the Cu(ClO 4) 2/CuX 2 salt mixtures (X (-) = Cl (-), Br (-), or N 3 (-)) leads to the formation of molecular coordination aggregates with formulas [Cu 3Cl 3(dpyatriz) 2](ClO 4) 3 ( 2), [Cu 3Br 3(dpyatriz) 2](ClO 4) 3 ( 3), and [Cu 4(N 3) 4(dpyatriz) 2(DMF) 4(ClO 4) 2](ClO 4) 2 ( 4). These complexes consist of two dpyatriz ligands bridged via coordination to Cu (II) and disposed either face-to-face in an eclipsed manner ( 2 and 3) or parallel and mutually shifted in one direction. The copper ions complete their coordination positions with Cl (-) ( 2), Br (-) ( 3), or N 3 (-), ClO 4 (-), and N, N-dimethylformamide (DMF) ( 4) ligands. All complexes crystallize together with noncoordinate ClO 4 (-) groups that display anion...pi interactions with the triazine rings. These interactions have been studied by means of high level ab initio calculations and the MIPp partition scheme. These calculations have proven the ClO 4 (-)...[C 3N 3] interactions to be favorable and have revealed a synergistic effect from the combined occurrence of pi-pi stacking of triazine rings and the interaction of these moieties with perchlorate ions, as observed in the experimental systems.     

Solvent-induced aggregation through metal...metal/pi...pi interactions: large solvatochromism of luminescent organoplatinum(II) terpyridyl complexes

A dramatic color change and tremendous emission enhancement have been "switched on" upon increasing diethyl ether ratio in acetonitrile or acetone solution of [Pt(tpy)(CC-CCH)]OTf, attributed to the formation of Pt...Pt and pi-pi interactions. Two crystal forms (dark-green and red) of [Pt(tpy)(CC-CCH)]OTf, together with [Pt(tBu3-tpy)(CC-CCH)]OTf, show different crystal-packing modes as revealed by X-ray crystallography.     

Structure and intermolecular interactions in complexes with hydrogen bond of a series of bifunctional nitrogen-containing compounds

By means of IR absorption spectra, low temperature ¹H and ¹⁵N NMR spectra, and quantum chemical calculations we study the structure of complexes with hydrogen bond of molecular and ionic character formed by nitrogen-containing compounds able to be both proton donors and acceptors simultaneously and the interaction specificity in them. Spectroscopic, steric, and thermodynamic characteristics of diphenyltriazene, 3,5-dimethylpyrazole, and diphenylformamidine homoassociates and heterocomplexes of these compounds with carboxylic acids and various proton donating molecules are obtained. Quantum chemical calculations of the structure of complexes and vibrational frequencies in IR spectra are made in the harmonic approximation and with regard to anharmonicity corrections. Calculations taking into account anharmonicity of vibrations are shown to produce the results closest to the experimental data.     

The nature of intermolecular CuI...CuI interactions: a combined theoretical and structural database analysis

The nature of intermolecular interactions between dicoordinate Cu(I) ions is analyzed by means of combined theoretical and structural database studies. Energetically stable Cu(I).Cu(I) interactions are only found when the two monomers involved in the interaction are neutral or carry opposite charges, thus allowing us to speak of bonding between the components of the bimolecular aggregate. A perturbative evaluation of the components of the intermolecular interaction energies, by means the IMPT scheme of Stone, indicates that both the Coulombic and dispersion forces are important in determining the Cu(I).Cu(I) bonding interactions, because only a small part of that energy is attributable to Cu.Cu interactions, while a large component results from Cu.ligand interactions. The electrostatic component is the dominant one by far in the interaction between charged monomers, while in the interaction between neutral complexes, the electrostatic component is found to be of the same order of magnitude as the dispersion term. Bimolecular aggregates that have like charges are repulsive by themselves, and their presence in the solid state results from anion.cation interactions with ions external to this aggregate. In these cases, the short-contact Cu.Cu interactions here should be more properly called counterion-mediated Cu.Cu bonds.     

Synthesis of the pivalamidate-bridged pentanuclear platinum(II,III) linear complexes with Pt...Pt interactions

Pentanuclear linear chain Pt(II,III) complexes [[Pt2(NH3)2X2((CH3)3CCONH)2(CH2COCH3)]2[PtX'4]].nCH3COCH3 (X = X' = Cl, n = 2 (1a), X = Cl, X' = Br, n = 1 (1b), X = Br, X' = Cl, n = 2 (1c), X = X' = Br, n = 1 (1d)) composed of a monomeric Pt(II) complex sandwiched by two amidate-bridged Pt dimers were synthesized from the reaction of the acetonyl dinuclear Pt(III) complexes having equatorial halide ligands [Pt2(NH3)2X2((CH3)3CCONH)2(CH2COCH3)]X' ' (X = Cl (2a), Br (2b), X' ' = NO3-, CH3C6H4SO3-, BF4-, PF6-, ClO4-), with K2[PtX'4] (X' = Cl, Br). The X-ray structures of 1a-1d show that the complexes have metal-metal bonded linear Pt5 structures, and the oxidation state of the metals is approximately Pt(III)-Pt(III)...Pt(II)...Pt(III)-Pt(III). The Pt...Pt interactions between the dimer units and the monomer are due to the induced Pt(II)-Pt(IV) polarization of the Pt(III) dimeric unit caused by the electron withdrawal of the equatorial halide ligands. The density functional theory calculation clearly shows that the Pt...Pt interactions between the dimers and the monomer are made by the electron transfer from the monomer to the dimers. The pentanuclear complexes have flexible Pt backbones with the Pt chain adopting either arch or sigmoid structures depending on the crystal packing.     

Polymeric and monomeric dipicolinate complexes with 4-hydroxymethyl pyridine: spectral, structural, thermal and electrochemical characterization

Polymeric copper(II), [Cu(μ-dpc)(μ-4-hymp)] _n (1), and monomeric nickel(II), [Ni(dpc)(4-hymp)(H₂O)₂]·H₂O (2), (dpc: dipicolinate, 4-hymp: 4-hydroxymethyl pyridine), dipicolinate complexes have been prepared and characterized by spectroscopic (IR, UV–Vis, EPR), thermal (TG/DTA), X-ray diffraction technique and electrochemical methods. In both the dipicolinate complexes, the dpc dianion acts as a tridentate ligand. In polymeric copper(II) complex, the 4-hymp and dpc ligands adopt a bridging position between the Cu(II) centers, forming the elongated octahedral geometry. The polymeric chains are linked to one another via O–H···O hydrogen bond interactions, forming the 3-D polymeric structure. The Ni(II) ion is bonded to dpc ligand through pyridine N atom together with one O atom of each carboxylate group, two aqua ligands and N pyridine atom of 4-hymp, forming the distorted octahedral geometry. The Ni(II) complexes are connected to one another via O–H···O hydrogen bonds, forming R ₄²(18) motifs in 2-D pattern. The powder EPR spectra of copper(II) complex have indicated that the paramagnetic center is in rhombic symmetry with the Cu²⁺ ion having distorted octahedral geometry. IR and UV–Vis spectroscopes all agree with the observed crystal structure.     

Crystal structures and intermolecular interactions of two copper(II) complexes with an asymmetric diazine ligand

Reactions of asymmetric ligand N-phenylacetyl picoloylhydrazide (HL) and copper(II) acetate/chloride give two complexes CuL₂ (1) and Cu₂Cl₂L₂ (2). The coordination geometries of Cu(II) in 1 and 2 are a severely distorted octahedron and a distorted square pyramid, respectively. The binuclear copper complex 2 contains a centrosymmetric Cu₂(μ-Cl)₂ core. Individual molecules of 1 and 2 further self-assemble through non-covalent intermolecular bonds in the solid state to form extended 2-D polymers. The magnetic properties, IR, EA, and solid-state photoluminescence properties of the title complexes are presented.     

Perturbing the structure of the 2-norbornyl cation through C-H...N and C-H...pi interactions

Quantum chemical studies on complexes of the 2-norbornyl cation with ammonia and benzene are described. These calculations demonstrate that the delocalized, bridged, nonclassical geometry that is usually favored for this carbocation can be perturbed significantly toward the normally less favorable classical geometry through appropriately oriented noncovalent interactions. Since such cations have been proposed as intermediates in enzyme and antibody catalyzed reactions, these results have implications for the nature of the cationic species that may be generated in the presence of electron-rich amino acid side chains of the sort that may be present in the active sites of biocatalysts.     

Vibrational dynamics, intermolecular interactions, and compound formation in GeH4-H2 under pressure

Optical microscopy, spectroscopic and x-ray diffraction studies at high-pressure are used to investigate intermolecular interactions in binary mixtures of germane (GeH(4)) + hydrogen (H(2)). The measurements reveal the formation of a new molecular compound, with the approximate stoichiometry GeH(4)(H(2))(2), when the constituents are compressed above 7.5 GPa. Raman and infrared spectroscopic measurements show multiple H(2) vibrons substantially softened from bulk solid hydrogen. With increasing pressure, the frequencies of several Raman and infrared H(2) vibrons decrease, indicating anomalous attractive interaction for closed-shell, nonpolar molecules. Synchrotron powder x-ray diffraction measurements show that the compound has a structure based on face-centered cubic (fcc) with GeH(4) molecules occupying fcc sites and H(2) molecules likely distributed between O(h) and T(d) sites. Above ca. 17 GPa, GeH(4) molecules in the compound become unstable with respect to decomposition products (Ge + H(2)), however, the compound can be preserved metastably to ca. 27 GPa for time-scales of the order of several hours.     

The suppressed reactivity of pyridine towards electrophiles as a result of an interplay between intra- and intermolecular interactions

A new accounting is suggested for the suppressed relative reactivity of all carbon atoms of pyridine as compared to benzene in the bimolecular electrophilic substitution (S_E2) processes in terms of an interplay between intra- and intermolecular interactions. Introduction of a nitrogen atom is shown to be accompanied by emergence of intramolecular interactions between the usual molecular orbitals of benzene that, in turn, give birth to definite mixed terms containing products of intra- and intermolecular interactions in the perturbative expansions for populations transferred between orbitals of the aromatic ring and those of electrophile. For ortho and para directions of the electrophilic attack, all the principal intramolecular interactions are shown to yield negative contributions to the intermolecular charge transfer in accordance with the dramatically reduced reactivities of these positions in pyridine. By contrast, contributions of different signs are shown to emerge in the relevant expressions referring to the meta attack. In this connection, conditions are revealed and analyzed that ensure the reduced intermolecular charge transfer for the meta position as well. Electrophiles of sufficiently high electron-accepting ability are shown to meet these conditions.     

Chemical interactions of pyridine and cyanide with silver

From a quantitative determination of pyridine and cyanide adsorbed on a silver electrode, by a radiochemical technique, we have shown that the two adsorbate - silver systems are different. After a dissolution - redeposition electrochemical cycle the quantity of pyridine adsorbed depends on the charge transfer. For low charge transfer (<50 mC cm^?2) the quantity increases from three to nine monolayers and depends on the nature of the supporting electrolyte, which suggests the formation of new bonds between pyridine, Ag and the anion of the supporting electrolyte. For high charge transfers the quantity of pyridine increases, the rate of increase depending on the supporting electrolyte (KI>KCl>KClO₄); in our opinion this is due to a trapping of pyridine in the salt formed between the support electrolyte anion and silver. The quantity of pyridine adsorbed at the silver electrode which can be as large as 100 equivalent monolayers can explain part of the enhancement of the Raman signal observed for this system.After a dissolution - redeposition electrochemical cycle the quantity of cyanide adsorbed remains constant, the cyanide - silver system is reversible and the Raman enhancement observed at the rest potential, is due only to Ag-CN interactions.