Showing papers on "Tetrahedral molecular geometry published in 2019"

Large Thermal Conductivity Drops in the Diamondoid Lattice of CuFeS2 by Discordant Atom Doping

Abstract: Doping in a lattice refers to the introduction of very small quantities of foreign atoms and has a generally small effect on decreasing the lattice thermal conductivity, unlike alloying which involves large fractions of other elements and strongly enhances point defect phonon scattering. Here, we report that, by alloying only 3% of In on the Cu sites of the diamond-like lattice of CuFeS2 chalcopyrite compound (Cu1-xInxFeS2, x = 0.03) has a disproportionally large effect in reducing the lattice thermal conductivity of the compound from 2.32 to 1.36 Wm-1K-1 at 630 K. We find that In is not fully ionized to +3 when on the Cu sublattice and exists mainly in the +1 oxidation state. The 5s2 lone pair of electrons of In+ makes this atom incompatible (referred to as discordant) with the tetrahedral geometry of the crystallographic site. This causes strong local bond distortions thereby softening the In-S and Cu-S chemical bonds and introducing localized low frequency vibrations. The latter couple with the base phonon frequencies of the CuFeS2 matrix enhancing the anharmonicity and decreasing the phonon velocity, and consequently the lattice thermal conductivity. The control material in which the In doping is on the Fe3+ site of the structure at the same doping level (and found in the site-compatible In3+ state), has a far smaller effect on the phonon scattering.

Symmetry breaking charge transfer as a means to study electron transfer with no driving force

Abstract: Symmetry breaking charge transfer (SBCT) is a process where a symmetrically disposed pair of identical chromophores forms a charge transfer excited state with the hole and electron on different chromophores, i.e. chr-chr + hν → chr+-chr-. Herein we explore this process in two dipyrrin-based bichromophoric systems. One of these bisdipyrrins involved a pair of BODIPY chromophores linked by a single bond at their meso-positions (compound 1) and the other involved two dipyrrin ligands coordinated in a tetrahedral geometry at the Zn2+ ion (compound 2). Both compounds show rapid SBCT in polar solvents and only dipyrrin based emission in nonpolar solvents, the latter arising from a dipyrrin localized excited sate (LE). By "tuning" the solvent polarity the equilibrium between the LE and SBCT states can be shifted to favor either state. Ultrafast transient absorption spectroscopy (TA) was used to probe the kinetics of the charge transfer for 2 in solvents where the electron transfer is endergonic, exergonic and has a ΔG close to zero. Our TA derived rates were used to predict fluorescence efficiencies in each of the different solvent systems and showed a good correspondence to measured values. Detailed density functional theory (DFT) and time dependent DFT were used to model the ground states as well as the LE and SBCT states of 1 and 2, in both polar and nonpolar media. The ground and LE excited states show small dipole moments, while the SBCT states show dipole moments of 16.4 and 20.3 D for 1 and 2, respectively.

Influence of a Counteranion on the Zero-Field Splitting of Tetrahedral Cobalt(II) Thiourea Complexes.

Abstract: Four mononuclear cobalt(II) complexes with pseudo tetrahedral geometry were isolated with different counteranions; their structure solution reveals the molecular formula as [Co(L1)4]X2 [where L1 = thiourea (NH2CSNH2) and X = NO3 (1), Br (2), and I (3)] and [Co(L1)4](SiF6) (4). The detailed analysis of direct-current (dc) magnetic data reveals a zero-field splitting (ZFS; D) with mS = ±3/2 as the ground levels (D < 0) for the four complexes. The magnitude of the ZFS parameter is larger, in absolute value, for 1 (D = -61.7 cm-1) than the other three complexes (-5.4, -5.1, and -12.2 cm-1 for 2-4, respectively). The sign of D for 1, 2, and 4 was unambiguously determined by X-band electron paramagnetic resonance (EPR) spectroscopy of the diluted samples (10%) at 5 K. For 3, the sign of D was naturally endorsed from the frequency-dependent out-of-phase signal (χM″) observed in the absence of an external dc magnetic field and confirmed by high-frequency EPR (70-600 GHz) experiments performed on a representative pure polycrystalline 3, which gave a quantitative D value of -5.10(7) cm-1. Further, the drastic changes in the spin Hamiltonian parameters and their related relaxation dynamics phenomena (of 2-4 compared to 1) were rationalized using ab initio complete-active-space self-consistent field/n-electron valence perturbation theory calculations. Calculations disclose that the anion-induced structural distortion observed in 2-4 leads to a nonfavorable overlap between the π orbital of cobalt(II) and the π* orbital of the sulfur atom that reduces the overall |D| value in these complexes compared to 1. The present study demonstrates that not only the first but also the second coordination sphere significantly influences the magnitude of the ZFS parameters. Particularly, a reduction of D of up to ∼90% occurs (in 2-4 compared to 1) upon a simple variation of the counteranions and offers a viable approach to modulate ZFS in transition-metal-containing single-molecule magnets.

Correlating magnetic anisotropy with the subtle coordination geometry variation of a series of cobalt(II)-sulfonamide complexes

Abstract: Systematic substitution on the N-(pyridine-2-ylmethyl)-sulfonamide ligand leads to the subtle variation of the CoN4 coordination geometry in a series of cobalt(II) complexes sharing the common formula of Co[R1(C6N2H5)R2]2, where R1 = H, R2 = 4-tert-butylphenylsulfonyl (tBuphs) 1, R2 = 5-(dimethylamino)naphthalen-1-ylsulfonyl (DNps) 2, R2 = mesitylsulfonyl (Ms) 3, R2 = tosyl (Tos) 4, and R2 = naphthalen-1-ylsulfonyl (Nps) 5; R1 = Me, R2 = tBuphs 6. Magnetic studies show that the axial zero-field splitting parameter (D) is subtlely correlated with the coordination geometric variation subjected to the peripheral substituted groups. Specifically, the distortion from the ideal tetrahedral geometry (Td symmetry) to the seesaw geometry (D2d symmetry) increases uniaxial magnetic anisotropy. The degree of distortion measured by the continuous symmetry measure (CSM) shows that a narrow interval of CSM (6–7), which corresponds to 14–15 degree deviation from the standard tetrahedron, is ideal for maximising the D value in this coordination geometry, while the direction of the D tensor is less sensitive to such a structural variation.

A Stable N-Hetero-Rh-Metallacyclic Silylene.

Abstract: A cyclic (amino)metal-substituted dicoordinated silylene derivative has been synthesized and fully characterized. Of particular interest is that the N-hetero-RhI -metallacyclic silylene exhibits a distorted tetrahedral geometry around the rhodium atom and a considerably shortened Si-Rh bond (2.138 A) compared to classical Si-Rh single bonds (ca. 2.30-2.35 A). A theoretical investigation reveals that the geometrical deviation around the rhodium center from the classical square-planar to a tetrahedral geometry increases the π-donating and σ-accepting character of the rhodium atom, thereby efficiently stabilizing the silylene moiety.

From mononuclear to two-dimensional cobalt(II) complexes based on a mixed benzimidazole–dicarboxylate strategy: syntheses, structures, and magnetic properties

Abstract: Based on a benzimidazole derivative and three different dicarboxylates, three new cobalt(II) complexes, namely [Co(1,3-BDC)2(MBIm)2] (1), [Co(SCA)2(MBIm)2]n (2), and {[Co(1,4-BDC)3(MBIm)2]·2DMF}n (3), (1,3-BDC = benzene-1,3-dicarboxylic acid, SCA = succinic acid, 1,4-BDC = benzene-1,4-dicarboxylic acid, MBIm = 5,6-dimethylbenzimidazole), have been hydrothermally synthesized and characterized structurally and magnetically. Single crystal X-ray diffraction revealed that 1 is a mononuclear neutral complex possessing a distorted tetrahedral geometry, 2 is a one-dimensional (1D) chain coordination polymer with the Co2+ centers arranged in Td symmetry, and 3 is a geometrically frustrated two-dimensional (2D) framework constructed from linear trimeric Co3 units with a mixed Td–Oh–Td geometry. Magnetic investigation revealed the easy-axis magnetic anisotropy of the Co2+ ions in 1 and 2, and field-induced slow magnetic relaxation behaviors were observed at very low temperatures with an effective energy barrier of Ueff = 51.8 K and 11.3 K for 1 and 2, respectively. Notably, complex 2 is the first example of a 1D SIM featuring four-coordinate metal centers. For 3, no long-range magnetic ordering was established even at 1.8 K, which is mainly attributed to spin frustration. These studies illustrate that a mixed benzimidazole–dicarboxylate strategy is an effective strategy for the design and construction of functional molecular magnetic materials with distinct magnetic properties and structural diversity.

Magnetic excitations in molecular magnets with complex bridges: The tetrahedral molecule Ni$_4$Mo$_{12}$

Abstract: We investigate the spectroscopic magnetic excitations in molecular magnets with complex intermediate structure among the magnetic ions. Our approach consists in introducing a modified spin Hamiltonian that allows for discrete coupling parameters accounting for all energetically favorable spatial distributions of the valence electrons along the exchange bridges connecting the constituent magnetic ions. We discuss the physical relevance of the constructed Hamiltonian and derive its eigenvalues. The model is applied to explore the magnetic excitations of the tetrameric molecular magnet Ni$_4$Mo$_{12}$. Our results are in a very good agreement with the available experimental data. We show that the experimental magnetic excitations in the named tetramer can be traced back to the specific geometry and complex chemical structure of the exchange bridges leading to the splitting and broadness of the peaks centered about 0.5 meV and 1.7 meV.

Magnetic excitations in molecular magnets with complex bridges: the tetrahedral molecule Ni 4 Mo 12

Abstract: We investigate the spectroscopic magnetic excitations in molecular magnets with complex intermediate structure among the magnetic ions. Our approach consists in introducing a modified spin Hamiltonian that allows for discrete coupling parameters accounting for all energetically favorable spatial distributions of the valence electrons along the exchange bridges connecting the constituent magnetic ions. We discuss the physical relevance of the constructed Hamiltonian and derive its eigenvalues. The model is applied to explore the magnetic excitations of the tetrameric molecular magnet Ni4Mo12. Our results are in a very good agreement with the available experimental data. We show that the experimental magnetic excitations in the named tetramer can be traced back to the specific geometry and complex chemical structure of the exchange bridges leading to the splitting and broadness of the peaks centered about 0.5 meV and 1.7 meV.

Synthesis and crystal structure of bis(O-methyl hydrogenato carbonodithioate)-Pb(II): structural, optical and photocatalytic studies of PbS nanoparticles from the complex

Abstract: Lead(II) methyl xanthate [Pb(S2COMe)2] was synthesized and characterized by single crystal X-ray crystallography. The molecular structure showed a distorted tetrahedral geometry around Pb(I...

A new tri-nuclear Cu-carbonate cluster utilizing CO2 as a C1-building block - reactive intermediates, a probable mechanism, and EPR and magnetic studies.

Abstract: This work demonstrates that simple copper-bipyridine compounds and atmospheric CO2 react to produce useful/complex materials under appropriate conditions. Starting from a distorted square planar copper(II) complex, [(tbubpy)CuCl2](tbubpy = 4-tert-butyl-2-(4-tert-butylpyridin-2-yl)pyridine), an air-sensitive, copper(I) complex, [(tbubpy)2CuI][BF4], which exhibits a distorted tetrahedral geometry, was synthesized and characterized. Reactions of [(tbubpy)2CuI][BF4] with CO2 inside a sealed tube and with air were carried out over a week and three weeks, respectively. A new tricopper(II)-carbonato cluster, [{(tbubpy)2Cu}3(μ-CO3)][PF6]4, was isolated with three distorted octahedral copper(II) centres bound by a carbonate-bridge formed from atmospheric CO2. NMR and UV-Vis spectroscopic analyses coupled with previous reports point to a multi-step process in the formation of a trinuclear CuII-carbonato cluster that includes the probable involvement of μ-hydroxo-bridged dicopper(II) type intermediates.

Corrigendum: Entrapment of a Pseudo-Tetrahedral CoII Center by Thioether Sulfur Bound {Co2 (μ-L)} Fragments: Synthesis, Field-Induced Single-Ion Magnetism and Catechol Oxidase Mimicking Activity.

Abstract: Simultaneous incorporation of both CoII and CoIII ions within a new thioether S-bearing phenol-based ligand system, H3 L (2,6-bis-[{2-(2-hydroxyethylthio)ethylimino}methyl]-4-methylphenol) formed [Co5 ] aggregates [CoII CoIII 4 L2 (μ-OH)2 (μ1,3 -O2 CCH3 )2 ](ClO4 )4 ⋅H2 O (1) and [CoII CoIII 4 L2 (μ-OH)2 (μ1,3 -O2 CC2 H5 )2 ](ClO4 )4 ⋅H2 O (2). The magnetic studies revealed axial zero-field splitting (ZFS) parameter, D/hc=-23.6 and -24.3 cm-1 , and E/D=0.03 and 0.00, respectively for 1 and 2. Dynamic magnetic data confirmed the complexes as SIMs with Ueff /kB =30 K (1) and 33 K (2), and τ0 =9.1×10-8 s (1), and 4.3×10-8 s (2). The larger atomic radius of S compared to N gave rise to less variation in the distortion of tetrahedral geometry around central CoII centers, thus affecting the D and Ueff /kB values. Theoretical studies also support the experimental findings and reveal the origin of the anisotropy parameters. In solutions, both 1 and 2 which produce {CoIII 2 (μ-L)} units, display solvent-dependent catechol oxidation behavior toward 3,5-di-tert-butylcatechol in air. The presence of an adjacent CoIII ion tends to assist the electron transfer from the substrate to the metal ion center, enhancing the catalytic oxidation rate.

NMR Spectroscopy in an Advanced Inorganic Lab: Structural Analysis of Diamagnetic and Paramagnetic Ni(II) Schiff Base Complexes

Abstract: In a two-step synthesis starting with Ni(CH3COO)2·4H2O and salicylaldehyde, students prepare N-ethyl and N-isopropyl Ni(II) Schiff base (imine) complexes. After isolation and recrystallization, the products are characterized by room-temperature magnetic susceptibility measurements, FTIR spectroscopy, and 1H, 13C{1H}, and 13C DEPT-135 or 13C APT NMR spectroscopy. The results are analyzed by considering the steric requirements of the ethyl and isopropyl groups and the subsequent effect on the preferred geometries, either the diamagnetic square-planar geometry, as expected for a Ni(II) d8 complex, or the paramagnetic tetrahedral geometry. Students are challenged to explain why the spectra for these similar complexes are so different and what this difference reveals about steric requirements in determining geometry.

Synthesis, Characterization, Crystal Structure Determination and Luminescence Study of New Linear Zn(II) Coordination Polymers with 4,7-Phenanthroline Ligand

Abstract: Two linear coordination polymers containing Zn(II) and 4,7-phenanthroline (4,7-phen) ligand have been synthesized and structurally characterized by single-crystal X-ray diffraction method. Complexes {[Zn(η2-4,7-phen)X2] ° CH3CN}n (X = Br for 1 and X = I for 2) were synthesized from the reaction of one equivalent of ZnBr2(1) and ZnI2(2) in acetonitrile with one equivalent of 4,7-phen ligand in methanol. Crystals of title complexes suitable for X-ray diffraction experiments have been obtained by slow evaporation of the resulted solution at room temperature. Both complexes have linear polymeric structures which extend parallel to the b axis and contain Zn(II) centers coordinated to two bridging 4,7-phen ligands, and two coordinated halogen molecules, showing a distorted tetrahedral geometry around Zn atoms. In addition, π⋯π (in 1), C-H⋯N (in 1 and 2), C-H⋯Br (in 1) and C-H⋯I (in 2) hydrogen bonding interactions play important role on stabilization of the crystal packing. The luminescence properties of the free ligand as well as the complexes 1 and 2 were investigated in solution.

A Novel ZnII Complex Bearing Two Monodentate (4-Methoxyphenyl)[(1E, 2E)-3-phenylprop-2-en-1-ilidene] Schiff Bases: Crystal Structure and DFT Study

Abstract: A novel ZnII complex bearing two monodentate (4-methoxyphenyl)[(1E, 2E)-3-phenylprop-2-en-1-ilidene] Schiff bases was synthesized and investigated both in the solid state by single-crystal X-ray diffraction, elemental analysis, and FTIR and in solution by 1H NMR spectroscopy. The complex crystallizes in the P21/c monoclinic space group. The asymmetric unit contains one ZnII ion coordinated to two Schiff base ligands and two chloride ions, in a distorted tetrahedral geometry. The title complex was also investigated by DFT, using the hybrid functional B3LYP with basis set 6-31G++, which reproduced the geometry and structural features of the complex in the crystal, and was used to assign the main bands in the FTIR spectrum. In solution, the complex maintains its integrity against decomposition to the parent reagents.