An antiferromagnetic {Mn8} ring supported by planar multidentate ligands

Complexation of the planar multidentate ligand 3,5-bis-(2-hydroxyphenyl)pyrazole (H 3 L) with manganese chloride leads to the formation of the polynuclear complex [Mn Ⅲ 8 L 4 O 4 (MeO) 4 (MeOH) 8 ] (1). 1 has an octanuclear macrocyclic core in which the MnⅢ ions are bridged by four L molecules to form a ring type structure. Antiferromagnetic interactions were shown to be operative between metal centers.     

Host-guest chemistry of the chromium-wheel complex [Cr8F8(tBuCO2)16]: prediction of inclusion capabilities by using an electrostatic potential distribution determined by modeling synchrotron X-ray structure factors at 16 K

The structure and detailed electron density distribution (EDD) of the large octanuclear chromium-wheel host complex [Cr8F8(tBuCO2)16] (1) has been determined from synchrotron X-ray structure factors collected at 16(5) K. The complex has a central cavity with a minimum entry distance between carbon atoms of the pivalate methyl groups (pivalic acid = tBuCO2H) of 4.027(4) A on one side of the molecule and 7.273(4) A on the other. The screened side of the molecule can be "opened" by rotation of methyl groups to create a strained host structure, which is compensated for by improved host-guest and host-solvent interaction. The EDD of the 272-atom complex (1144 e-) was determined by multipole modeling based on the experimental structure factors. 3d orbital populations on the Cr atoms and topological analysis of the EDD show that the covalent part of the metal-ligand interactions consists mainly of sigma donation from the ligands, but that overall the interactions are predominantly electrostatic. The electrostatic potential (EP) has been calculated from the experimental EDD. Knowledge of the geometry of the naked complex 1 as well as the EP in the central cavity of this molecule allows us to deduce which characteristic properties guest molecules must have to be accepted into the void. To probe these predictions, a series of complexes of 1 with different guest inclusions were synthesized (2 = 1 + N,N'-dimethylformamide (DMF), 3 = 1 + N,N'-dimethylacetamide (DMA), 4 = 1 + DMA + DMF, 5 = 1 + 2CH3CN), and their structures were examined by using X-ray diffraction data measured at 120(1) K. Results of these studies indicate that in the crystalline state, the optimal guest molecule should be linear and possess a permanent dipole. Attempts to crystallize the host complex with cations incorporated into the cavity were fruitless, although electrospray ionization mass spectrometry showed that a [1 + potassium]+ entity pre-exists in solution and can be transferred intact into the gas phase.     

Magnetic circular dichroism spectroscopy of antiferromagnetically coupled hetero-metallic rings [H2NR2][Cr7MF8(O2CCMe3)16

The optical and magnetic properties of the multi-metal rings [NH(2)R(2)][Cr(7)MF(8)(O(2)CCMe(3))(16)], where M = Cd(II), Mn(II) or Ni(II), have been studied using variable-field and variable-temperature magnetic circular dichroism (MCD) in the UV-visible spectra. Spectra of samples were recorded in a frozen organic matrix or cast in a polymethacrylate (PMMA) polymer film between 1.7 and 75 K. The spectra are characteristic of the Cr(III) ion (d(3)) in a rhombic field when M = Cd(II). In the case that M = Ni(II) additional optical transitions arise from the d(8) ion whereas for M = Mn(II) no additional transitions are observed. The influence of magnetic exchange is apparent from a change in the sign of the MCD signal between complexes in which the hetero-atom has a local spin moment greater, or less, than that of Cr(III), S = 3/2, namely, Mn(II), S = 5/2, and Ni(II), S = 1. The exchange coupling generates a manifold of thermally accessible electronic states that give rise to variations in MCD intensity as well as additional spectral features as the temperature is raised. Equations have been derived to relate the splittings observed in the optical spectrum to the single-ion ground state zero-field splittings of chromium(III). There is reasonable agreement between the sign and magnitude of the contribution to the cluster anisotropy from that of the single ion with values estimated from other techniques.     

A novel synthesis of the benzo[F]quinazoline ring system

An approach to the synthesis of the benzo[f]quinazoline ring system using a preformed pyrimidine is described. A facile cyclization of a ketone function into the 5-position of uracil is the key step in this sequence.     

Oxyfluorotitanophosphate cluster [Ti10P4O16F44]16-: synthesis and characterization of K16[Ti10P4O16F44

One oxyfluorotitanophosphate cluster compound, K16[Ti10P4O16F44], has been synthesized and structurally characterized. As far as we know, it is the first cluster compound for titanophosphate.     

8 and 16-Membered ring phosphorus compounds. Ring size dependence of the NMR parameters

The ¹H NMR spectral analysis of four 8-membered rings, 2-thioxo- (or 2-oxo-) 2-R-1,3,6,2- trithiaphocane, is reported. The stereochemistry of the 8-membered ring is discussed. The ³¹P NMR spectral parameters [δ³¹P, ¹J(PC)] obtained on several cyclic 1,3,2-dithiaphospha compounds of variable size (5, 6, 8, 12 and 16-membered rings) are discussed as a function of the ring size and of the geometry of the molecule.     

[Cr8(PhCO2)16O4]·4CH3CN·2H2O: structural origin of magnetic anisotropy in a molecular spin cluster

The Cr₄O₄ hetero‐cubane‐centered octachromium(III) cluster [Cr₈(PhCO₂)₁₆O₄] crystallizes from fluorobenzene–acetonitrile as dodeca‐μ₂‐benzoato‐tetrabenzoatotetra‐μ₄‐oxido‐octachromium(III) acetonitrile tetrasolvate dihydrate, [Cr₈(C₇H₅O₂)₁₆O₄]·4C₂H₃N·2H₂O, (I). Crystals produced by this method are significantly more stable than the originally published dichloromethane pentasolvate, [Cr₈(PhCO₂)₁₆O₄]·5CH₂Cl₂ [Atkinson et al. (1999). Chem. Commun. pp. 285–286], leading to a significantly higher quality structure and allowing the production of large quantities of high‐quality nondeuterated and deuterated material suitable for inelastic neutron scattering (INS) measurements. Compound (I) reveals a higher symmetry structure in which the cluster sits on a twofold rotation axis, and is based on an asymmetric unit containing four crystallographically independent Cr positions, two oxide ligands, eight benzoate ligands, two acetonitrile solvent molecules and one disordered water molecule. All the Cr atoms are six‐coordinate, with an octahedral geometry for the inner cubane and a more highly distorted coordination environment in the outer positions. Despite the higher symmetry, the coordination geometries observed in (I) are largely similar to the dichloromethane pentasolvate structure, indicating that crystal‐packing effects have little influence on the molecular structure of [Cr₈(PhCO₂)₁₆O₄]. Close structural analysis reveals that the high magnetic anisotropy observed in the INS measurements is a consequence of the distorted coordination geometry of the four outer Cr atoms.     

Theoretical study of the magnetic behavior of [Fe8] and [Fe16] wheels

The reaction of the trimetallic species [Fe(3)O(PhCOO)(6)(H(2)O)(3)]NO(3) with 1,1,1-tris(hydroxymethyl)ethane (H(3)thme) affords either the octametallic species [Fe(8)(PhCOO)(12)(thme)(4)] 1 or the hexadecametallic species [Fe(16)(EtO)(4)(PhCOO)(16)(Hthme)(12)](NO(3))(4) 2, depending on the nature of the solvent used for crystallization. The structure of 1 can be described as a nonplanar wheel of eight Fe(III) ions bridged by a combination of PhCOO(-) and thme(3)(-) ligands, and 2 as a nonplanar wheel of sixteen Fe(III) ions bridged by PhCOO(-), Hthme(2)(-), and EtO(-) ligands. Both compounds can be broken down into simple units of two metal ions and the bridging ligands that connect them. The best fits of the chi vs T curves in the 300-10 K temperature range were obtained with the parameters g = 2.0, J(1) = -24.0 cm(-1), and J(2) = -8.59 cm(-1) for [Fe(8)] and g = 2.0, J(1) = -25.0 cm(-1), J(2) = -11.73 cm(-1), and J(3) = -69.3 cm(-1) for [Fe(16)]. Density functional theory (DFT) calculations show that the antiferromagnetic interactions between the metals in the dinuclear units decrease when two types of bridging ligands are present, as expected for an orbital counter-complementarity effect.     

Approaching the gas-phase structures of [AgS8]+ and [AgS16]+ in the solid state

Upon treating elemental sulfur with [AgSbF(6)], [AgAl(hfip)(4)], [AgAl(pftb)(4)] (hfip=OCH(CF(3))(2), pftb =OC(CF(3))(3)) the compounds [Ag(S(8))(2)][SbF(6)] (1), [AgS(8)][Al(hfip)(4)] (2), and [Ag(S(8))(2)](+)[[Al(pftb)(4)](-) (3) formed in SO(2) (1), CS(2) (2), or CH(2)Cl(2) (3). Compounds 1-3 were characterized by single-crystal X-ray structure determinations: 1 by Raman spectroscopy, 2 and 3 by solution NMR spectroscopy and elemental analyses. Single crystals of [Ag(S(8))(2)](+)[Sb(OTeF(5))(6)](-) 4 were obtained from a disproportionation reaction and only characterized by X-ray crystal structure analysis. The Ag(+) ion in 1 coordinates two monodentate SbF(6) (-) anions and two bidentate S(8) rings in the 1,3-position. Compound 2 contains an almost C(4v)-symmetric [AgS(8)](+) moiety; this is the first example of an eta(4)-coordinated S(8) ring (d(Agbond;S)=2.84-3.00 A). Compounds 3 and 4, with the least basic anions, contain undistorted, approximately centrosymmetric Ag(eta(4)-S(8))(2) (+) cations with less symmetric eta(4)-coordinated S(8) rings (d(Agbond;S)=2.68-3.35 A). The thermochemical radius and volume of the undistorted Ag(S(8))(2) (+) cation was deduced as r(therm)(Ag(S(8))(2) (+))=3.378+ 0.076/-0.120 A and V(therm)(Ag(S(8))(2) (+))=417+4/-6 A(3). AgS(8) (+) and several isomers of the Ag(S(8))(2) (+) cation were optimized at the BP86, B3LYP, and MP2 levels by using the SVP and TZVPP basis sets. An analysis of the calculated geometries showed the MP2/TZVPP level to give geometries closest to the experimental data. Neither BP86 nor B3LYP reproduced the longer weak dispersive Agbond;S interactions in Ag(eta(4)-S(8))(2) (+) but led to Ag(eta(3)-S(8))(2) (+) geometries. With the most accurate MP2/TZVPP level, the enthalpies of formation of the gaseous [AgS(8)](+) and [Ag(S(8))(2)](+) cations were established as Delta(f)H(298)([Ag(S(8))(2)](+), g)=856 kJ mol(-1) and Delta(f)H(298)([AgS(8)](+), g)=902 kJ mol(-1). It is shown that the [AgS(8)](+) moiety in 2 and the [AgS(8)](2) (+) cations in 3 and 4 are the best approximation of these ions, which were earlier observed by MS methods. Both cations reside in shallow potential-energy wells where larger structural changes only lead to small increases in the overall energy. It is shown that the covalent Agbond;S bonding contributions in both cations may be described by two components: i) the interaction of the spherical empty Ag 5s(0) acceptor orbital with the filled S 3p(2) lone-pair donor orbitals and ii) the interaction of the empty Ag 5p(0) acceptor orbitals with the filled S 3p(2) lone-pair donor orbitals. This latter contribution is responsible for the observed low symmetry of the centrosymmetric Ag(eta(4)-S(8))(2) (+) cation. The positive charge transferred from the Ag(+) ion in 1-4 to the coordinated sulfur atoms is delocalized over all the atoms in the S(8) ring by multiple 3p(2)-->3sigma* interactions that result in a small long-short-long-short Sbond;S bond-length alternation starting from S1 with the shortest Agbond;S length. The driving force for all these weak bonding interactions is positive charge delocalization from the formally fully localized charge of the Ag(+) ion.     

Magnetic anisotropy of cyclopropane and epoxide rings

MO LCAO results are presented for the interatomic contribution to the anisotropy of the diamagnetic susceptibility; values of _d=–23.5 · 10^–6 and _d=–13.8 · 10^–6 cm³/mole are obtained for the cyclopropane and epoxide ring systems, respectively. The results are used to interpret PMR spectra in terms of the configuration and conformation of the and forms of caran-4-ol and two stereoisomers of caranone.     

Calculations of the contribution of ring currents to the chemical shielding anisotropy

Ring currents can exert a large effect upon the chemical shielding of NMR resonances. The analytical expression developed by Waugh and Fessenden (Waugh, J. S.; Fessenden, R. W. J. Am. Chem. Soc. 1957, 79, 846) and Johnson and Bovey (Johnson, C. E.; Bovey, F. A. J. Chem. Phys. 1957, 29, 1012) only quantifies the contribution of ring currents to the isotropic component of the shielding tensor. In the work described here an additional analytical expression is developed so that the contribution of ring currents to the full shielding tensor can be calculated, allowing an estimate of the influence of ring currents upon the chemical shielding anisotropy (CSA, Deltasigma). To test that this pair of analytical expressions can provide a reasonable estimate of the contribution of ring currents to the full shielding tensor a series of density functional calculations (DFT) were carried out. A shielding tensor in a model compound was calculated in two distinct ways. For the first series, DFT shielding calculations of the model compound were carried out in the presence of a benzene ring. For the second series a ring current contribution to the shielding tensor was calculated via the new expressions, and this was added to the result of a DFT shielding calculation which used in place of benzene the nonaromatic analogue 1,3 cyclohexadiene. The two series of results proved to be in excellent agreement. The pair of analytical expressions are used to calculate ring current contributions to the CSA (Deltasigma) of 1H(N) backbone amide resonances in a structure of the second type 2 module from the protein fibronectin. Significant CSA variations are predicted in particular for the 1H(N) of G42 which is most likely involved in a N-H...tpi aromatic hydrogen bond.     

Structural, magnetic and DFT studies of a hydroxide-bridged [Cr8] wheel

Structural, magnetic and theoretical studies of an octanuclear chromium(III) wheel are reported, containing hydroxide and pivalate bridges.     

Microstructure and diffusion kinetics at the bondedFe-16Al/Cr18-Ni8interface

Summary Fe-16Al alloy and Cr18-Ni8 austenitic stainless steel were diffusion bonded in a vacuum of 10^-4Pa. Microstructure characteristics at the bonded Fe-16Al/Cr18-Ni8 interface were analyzed by scanning electron microscope (SEM) and X-ray diffraction (XRD) technologies. The results indicated that FeAl, Fe₃Al, Ni₃Al and α-Fe (Al) solid solutions were produced at the bonded Fe-16Al/Cr18-Ni8 interface after diffusion bonding. These phases may accelerate the diffusion and even the formation of the bonded Fe-16Al/Cr18-Ni8 interface. The growth of the interface obeyed the parabolic law and its thickness (X) was expressed by X²=7.0×10^-4exp(-82900/RT)(t-t₀) withtemperature (T) and time (t).     

Magnetic circular dichroism and absorption spectra of the mononegative and dinegative ions of [16] annulene

The magnetic circular dichroism (MCD) and absorption spectra of the mononegative and dinegative ions of [16] annulene have been measured in the 16000–30000 cm^?1 region. The results are interpreted using a PPP molecular orbital calculation. It is found that the D_4h symmetry, using Slater type atomic orbitals excellent agreement between experiment and calculations is obtained. The excited ²E_g and ¹E_u states of the mono- and dinegative ions have experimental averaged angular monomenta of 1.0 h and 2.3 h respectively and we conclude to planar ring structures for the ions. Variations in the parameters for the PPP calculations, i.e. B, do not influence the MCD values to any great extent.In addition, from temperature dependent measurements between 20°C and ?150°C, the thermodynamic parameters ΔH and ΔS for the equilibrium observed between molecule and ions in solution have been determined.     

A dinuclear cobalt(II) complex of calix[8]arenes exibiting strong magnetic anisotropy

The solvothermal reaction of cobalt(II) acetate with p-tert-butylcalix[8]arene (calix) and triethylamine affords the compound (Et3NH)2 [CoII2(calix)2] (.2Et3NH) that shows a hydrogen bond bridged dinuclear complex [CoII2(calix)2]2- () with cobalt(II) ions in a tetrahedral geometry. The compound crystallises in the monoclinic, space group P2(1)/n with cell parameters a=14.89(1) A, b=20.90(2) A, c=30.87(4) A, beta=102.57(7) degrees, V=9376(16) A3, Z=2. The magnetic studies together with ab initio calculations are evidence of an important role of the geometry of the second coordination sphere of carbon and hydrogen atoms around the CoO4 core in quantifying the zero field splitting on cobalt sites. This results in strong magnetic anisotropies with a negative axial component on the cobalt fragments.