Showing papers on "Tetrahedral molecular geometry published in 2006"

On the Composition and Crystal Structure of the New Quaternary Hydride Phase Li4BN3H10

Abstract: X-ray data on single crystals of the quaternary metal hydride near the composition LiB0.33N0.67H2.67, previously identified as “Li3BN2H8”, reveal that its true composition is Li4BN3H10. The structure has body-centered-cubic symmetry [space group I213, cell parameter a = 10.679(1)−10.672(1) A] and contains an ordered arrangement of BH4- and NH2- anions in the molar ratio 1:3. The borohydride anion has an almost ideal tetrahedral geometry (∠H−B−H ∼ 108−114°), while the amide anion has a nearly tetrahedral bond angle (∠H−N−H ∼ 106°). Three symmetry-independent Li atom sites are surrounded by BH4- and NH2- anions in various distorted tetrahedral configurations, one by two B and two N atoms, another by four N atoms, and the third by one B and three N atoms. The Li configuration around B is nearly tetrahedral, while that around N resembles a distorted saddlelike configuration, similar to those in LiBH4 and LiNH2, respectively.

Phenoxyl radicals: H-bonded and coordinated to Cu(II) and Zn(II)

Abstract: Two pro-ligands (RLH) comprised of an o,p-di-tert-butyl-substituted phenol covalently bonded to a benzimidazole (BzLH) or a 4,5-di-p-methoxyphenyl substituted imidazole (PhOMeLH), have been structurally characterised. Each possesses an intramolecular O–H⋯N hydrogen bond between the phenolic O–H group and an imidazole nitrogen atom and 1H NMR studies show that this bond is retained in solution. Each RLH undergoes an electrochemically reversible, one-electron, oxidation to form the [RLH]˙+ radical cation that is considered to be stabilised by an intramolecular O⋯H–N hydrogen bond. The RLH pro-ligands react with M(BF4)2·H2O (M = Cu or Zn) in the presence of Et3N to form the corresponding [M(RL)2] compound. [Cu(BzL)2] (1), [Cu(PhOMeL)2] (2), [Zn(BzL)2] (3) and [Zn(PhOMeL)2] (4) have been isolated and the structures of 1·4MeCN, 2·2MeOH, 3·2MeCN and 4·2MeCN determined by X-ray crystallography. In each compound the metal possesses an N2O2-coordination sphere: in 1·4MeCN and 2·2MeOH the {CuN2O2} centre has a distorted square planar geometry; in 3·2MeCN and 4·2MeCN the {ZnN2O2} centre has a distorted tetrahedral geometry. The X-band EPR spectra of both 1 and 2, in CH2Cl2–DMF (9 : 1) solution at 77 K, are consistent with the presence of a Cu(II) complex having the structure identified by X-ray crystallography. Electrochemical studies have shown that 1, 2, 3 and 4 each undergo two, one-electron, oxidations; the potentials of these processes and the UV/vis and EPR properties of the products indicate that each oxidation is ligand-based. The first oxidation produces [M(II)(RL)(RL˙)]+, comprising a M(II) centre bound to a phenoxide (RL) and a phenoxyl radical (RL˙) ligand; these cations have been generated electrochemically and, for R = PhOMe, chemically by oxidation with Ag[BF4]. The second oxidation produces [M(II)(RL˙)2]2+. The information obtained from these investigations shows that a suitable pro-ligand design allows a relatively inert phenoxyl radical to be generated, stabilised by either a hydrogen bond, as in [RLH]˙+ (R = Bz or PhOMe), or by coordination to a metal, as in [M(II)(RL)(RL˙)]+ (M = Cu or Zn; R = Bz or PhOMe). Coordination to a metal is more effective than hydrogen bonding in stabilising a phenoxyl radical and Cu(II) is slightly more effective than Zn(II) in this respect.

Spectroscopic and biochemical studies of some manganese(II), oxovanadium(V) and dioxomolybdenum(VI) complexes S/O and N donor agents synthesized under microwave conditions

Abstract: The synthesis of some new manganese(II), oxovanadium(V) and dioxomolybdenum(VI) complexes with 5-chloro-1,3-dihydro-3-[2-(phenyl)-ethylidene]-2H-indol-2-one-hydrazinecarbothioamide (L1H) and 5-chloro-1,3-dihydro-3-[2-(phenyl)-ethylidene]-2H-indol-2-one-hydrazinecarboxamide (L2H) were carried out in unimolar and bimolar ratios. All the new derivatives have been characterized by elemental analyses, molecular weight determinations, molar conductance, magnetic measurements, i.r., 1H-n.m.r. and e.s.r. studies. The i.r. and n.m.r. spectral data suggest the involvement of sulphur/oxygen and azomethine nitrogen in coordination to the central metal ion. The magnetic moment values of the manganese(II) complexes are in the 5.85–6.13 B.M. range, suggesting a high spin state in these complexes. Based on these spectral studies tetrahedral geometry for the manganese(II) complexes and octahedral geometry for the molybdenum complexes has been proposed. In the case of the oxovanadium complexes vanadium is in the penta and hexa coordinated environments. Compounds have been synthesized in an open vessel under microwave irradiation (MWI) using a domestic microwave oven. The reaction time decreases from hours to minutes with improved yield as compared to conventional heating. The free ligands and their metal complexes have been tested in vitro against a number of microorganisms in order to assess their antimicrobial properties. The results indicate that the ligands and their respective metal derivatives possess antimicrobial properties.

Synthesis, X-ray crystallography, and computational analysis of 1-azafenestranes.

Abstract: The tandem [4+2]/[3+2] cycloaddition of nitroalkenes has been employed in the synthesis of 1-azafenestranes, molecules of theoretical interest because of planarizing distortion of their central carbon atoms. The synthesis of c,c,c,c-[5.5.5.5]-1-azafenestrane was completed in good yield from a substituted nitrocyclopentene, and its borane adduct was analyzed through X-ray crystallography, which showed a moderate distortion from ideal tetrahedral geometry. The syntheses of two members of the [4.5.5.5] family of 1-azafenestranes are also reported, including one with a trans fusion at a bicyclic ring junction which brings about considerable planarization of one of the central angles (16.8° deviation from tetrahedral geometry). While investigating the [4.5.5.5]-1-azafenestranes, a novel dyotropic rearrangement that converts nitroso acetals into tetracyclic aminals was discovered. Through conformational analysis, a means to prevent this molecular reorganization was formulated and realized experimentally with th...

Determination of the geometric structure of the metal site in a blue copper protein by paramagnetic NMR

Abstract: The biological function of metalloproteins is closely tied to the geometric and electronic structures of the metal sites. Here, we show that the geometric structure of the metal site of a metalloprotein in solution can be determined from experimentally measured electron-nuclear spin–spin interactions obtained by NMR. Thus, the geometric metal site structure of plastocyanin from Anabaena variabilis was determined by including the paramagnetic relaxation enhancement of protons close to the copper site as restraints in a conventional NMR structure determination, together with the distribution of the unpaired electron onto the ligand atoms. Also, the interproton distances (nuclear Overhauser enhancements) and dihedral angles (scalar nuclear spin–spin couplings) normally used in NMR structure determinations were included as restraints. The structure calculations were carried out with the program x-plor and a module that takes into account the specific characteristics of the paramagnetic restraints. A well defined metal site structure was obtained with the structural characteristics of the blue copper site, including a distorted tetrahedral geometry, a short Cu–Cys Sγ bond, and a long Cu–Met Sδ bond. Overall, the agreement of the obtained metal site structure of Anabaena variabilis plastocyanin with those of other plastocyanins obtained by x-ray crystallography confirms the reliability of the approach.

Ab initio Molecular Dynamical Investigation of the Finite Temperature Behavior of the Tetrahedral Au$_{19}$ and Au$_{20}$ Clusters

Abstract: Density functional molecular dynamics simulations have been carried out to understand the finite temperature behavior of Au$_{19}$ and Au$_{20}$ clusters. Au$_{20}$ has been reported to be a unique molecule having tetrahedral geometry, a large HOMO-LUMO energy gap and an atomic packing similar to that of the bulk gold (J. Li et al., Science, {\bf 299} 864, 2003). Our results show that the geometry of Au$_{19}$ is exactly identical to that of Au$_{20}$ with one missing corner atom (called as vacancy). Surprisingly, our calculated heat capacities for this nearly identical pair of gold cluster exhibit dramatic differences. Au$_{20}$ undergoes a clear and distinct solid like to liquid like transition with a sharp peak in the heat capacity curve around 770 K. On the other hand, Au$_{19}$ has a broad and flat heat capacity curve with continuous melting transition. This continuous melting transition turns out to be a consequence of a process involving series of atomic rearrangements along the surface to fill in the missing corner atom. This results in a restricted diffusive motion of atoms along the surface of Au$_{19}$ between 650 K to 900 K during which the shape of the ground state geometry is retained. In contrast, the tetrahedral structure of Au$_{20}$ is destroyed around 800 K, and the cluster is clearly in a liquid like state above 1000 K. Thus, this work clearly demonstrates that (i) the gold clusters exhibit size sensitive variations in the heat capacity curves and (ii) the broad and continuous melting transition in a cluster, a feature which has so far been attributed to the disorder or absence of symmetry in the system, can also be a consequence of a defect (absence of a cap atom) in the structure.

Synthesis and antibacterial activity of cephradine metal complexes

Abstract: Cephradine (Hcephra) interacts with transition metal ions to give [Fe(cephra)Cl2] and [M(cephra)Cl] complexes (M = Mn(II), Co(II), Ni(II), Cu(II) and Zn(II)) which were characterized by physicochemical and spectroscopic methods; a tetrahedral geometry is suggested for their structures where cephradine behaves as monoanionic tridentate ligand. The complexes have been screened for antibacterial activity against several bacteria, and the results showed that all metal complexes tested had lower antibiotic activity than the free ligand.

Synthesis, Structures, and Luminescent Properties of d10 Group 12 Metal Complexes with Substituted 2,2′-Bipyridine Ligands

Abstract: Four d10 group 12 metal complexes, 6-(2-methoxyphenyl)-2,2′-bipyridinezinc dichloride (2a), -mercury dichloride (2b), 6-[2-(dimethylamino)phenyl]-2,2′-bipyridinezinc dichloride (2c), and -mercury dichloride (2d), were synthesized and the structures determined by single-crystal X-ray crystallography. Complexes 2a and 2b are four-coordinate and adopt a distorted tetrahedral geometry, while complexes 2c and 2d are five-coordinate with a distorted trigonal bipyramidal geometry for the metal center. Luminescent properties of complexes 2a–2d in both solution and the solid state were studied. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)

Coordination chemistry of Group 12 metals with phosphorodiselenoates: syntheses, structures and VT 31P NMR study of monomer-dimer exchange equilibrium.

Abstract: The reactions of diselenophosphates, [dsep, (RO)2PSe2-; R = Et, (n)Pr and (i)Pr] with cadmium(II) and mercury(II) perchlorates in a 2 : 1 molar ratio formed compounds of stoichiometry M[Se2P(OR)2]2{M = Cd, R = Et (1), (n)Pr (2), (i)Pr (3); Hg, Et(4), (n)Pr (5), (i)Pr (6)}, and with zinc(II) perchlorates, chalcogen centered tetranuclear clusters, [Zn4(micro4-E){Se2P(OR)2}6]{E = Se, R = Et (7), (n)Pr (8), (i)Pr (9); E = O, R = Et (10), (n)Pr (11), (i)Pr (12)} were formed. All these complexes have been characterized with the help of analytical data, X-ray crystallography (1, 3, 6, 10, 11 and 12), and FAB-mass spectrometry (7-12). Compound 1 is a linear double-chain polymer, in which each pair of Cd atoms is bridged by two dsep ligands; the mercury 6 polymer has a helical chain structure, in which two Hg atoms are bridged by one dsep ligand, and the other ligand chelates the Hg atom. The chelating dsep ligands lie on either side of the helical chain. Compound 3 exists as a dimer in which two cadmium atoms are connected by two bridging dsep ligands, and each cadmium atom is further chelated by a dsep ligand. The metal atoms in 1, 3 and 6 are each coordinated by four selenium atoms in a distorted tetrahedral geometry. Clusters 10-12 have tetrahedral array of zinc atoms with an oxygen atom in the center with edge-bridging dsep ligands. Positive FAB-mass spectra support the formation of selenium-centered clusters,7-9, of which the cluster 8 was structurally confirmed earlier. The solution state behavior of compounds 1-12 has been studied by using multinuclear NMR spectroscopy. Dimer 3 in CD2Cl2 showed monomer-dimer exchange equilibrium in the temperature range 20 to -90 degrees C and the free energy of activation is calculated from the coalescence temperature as DeltaG++(223 K)= 38.5 kJ mol(-1). Polymer undergoes depolymerization in CDCl3 and exhibits monomer-dimer exchange equilibrium in the temperature range 20 to -60 degrees C.

Homoleptic zinc(II) complexes with first and second coordination shells of 5-tert-butylpyrazole.

Abstract: Reaction of hydrated Zn[NO3]2 or Zn[BF4]2 with four or more equivalents of 3{5}-tert-butylpyrazole (LtBu) yields [Zn(LtBu)4]X2 (X- = NO3- or BF4-). The nitrate complex contains C2-symmetric four-coordinate zinc(II) centers with a slightly flattened tetrahedral geometry, and each nitrate anion hydrogen bonds to two pyrazole N−H groups. Similar reactions with Zn[ClO4]2 or ZnCl2 in the presence of 2 equiv of AgPF6 or AgSbF6 yield instead [{Zn(LtBu)4}(LtBu)4][ClO4]2 and [{Zn(LtBu)4}(LtBu)2]Y2 (Y- = PF6- or SbF6-). Crystals of [{Zn(LtBu)4}(LtBu)4][ClO4]2 are composed of discrete [{Zn(LtBu)4}(LtBu)4]2+ supramolecules that are formed from N−H···N hydrogen bonding between zinc-bound and uncoordinated pyrazole rings. The [{Zn(LtBu)4}(LtBu)4]2+ moieties are linked into planar 44 nets by hydrogen bonding to bridging ClO4- anions. The ClO4- ions are almost perfectly encapsulated in near-spherical cavities of approximate dimensions 5.0 × 5.0 × 4.5 A that are formed by two interlocked supramolecular dications. Similarl...

Synthesis, characterization and nonlinear optical effects of M4(µ4-O) core complexes with large two-photon absorption cross-section

Abstract: Four tetranuclear 2-mercaptobenzothiazole cluster compounds [M4O(MBT)6] (M = Fe, Co, Ni, Cu; MBT = 2-mercaptobenzhothiazole) were synthesized and characterized by elemental analyses, IR and UV–vis spectra, and single crystal X-ray crystallography. The crystal structures of [M4O(MBT)6] (M = Fe, Co, Ni) confirm that the four metal(II) atoms locate four capsheaves of a tetrahedral skeletal structure and a tetracoordinate O2− as an interstitial atom occupies the centre position of this tetrahedron. The metal atoms all possess slightly distorted tetrahedral geometry. The thermal gravimetry data indicate that these clusters all have good thermal stability. The nonlinear absorption of four cluster solutions (in DMF) were measured by open-aperture Z-scan technique at a 532 nm wavelength. The results of Z-scan experiments show that these clusters all have remarkable and very strong nonlinear optical absorption effects. The largest two-photon absorption cross-section is 172880 GM for cluster [Fe4O(MBT)6].

Coordination diversity of new mononucleating hydrazone in 3d metal complexes : Synthesis, characterization and structural studies

Abstract: The mononucleating hydrazone ligand LH 3 , a condensation product of salicyloylhydrazine and (2-formylphenoxy)acetic acid, was synthesized and its coordination behavior with first row transition metal(II) ions was investigated by isolating and elucidating the structure of the complexes using elemental analysis, conductivity and magnetic susceptibility measurements, as well as IR, 1 H-NMR, electronic and EPR spectral techniques. The ligand forms mononuclear metal(II) complexes of the type [COLH(H 2 O) 2 ], [NiLH(H 2 O) 2 ], [CuLH] and [ZnLH]. The ligand field parameters, Dq, B and β values, in the case of the cobalt and nickel complexes support not only the octahedral geometry around the metal ion, but also imply the covalent nature of the bonding in the complexes. The EPR study revealed the presence of a spin exchange interaction in the solid copper complex and the covalent nature of the bonding. The 'H-NMR study of the zinc(II) complex indicated the non-involvement of the COOH group in the coordination. The physico-chemical study supports for the presence of octahedral geometry around cobalt(II), nickel(II) and tetrahedral geometry around copper(II) and zinc(Il) ions.

Synthesis and Characterization of 2,4,6-tris(hydrazino)-s-triazine and its Metal Complexes

Abstract: A new multidentate ligand related to s-triazine herbicides 2,4,6-tris(hydrazino)-s-triazine (THSTZ) and its metal complexes was synthesized. The complexes were investigated by m.s., n.m.r., i.r., u.v.–vis and AA spectroscopic techniques. Furthermore, carbon, nitrogen, hydrogen, chloride, and metal analyses, conductivity, magnetic susceptibility measurements, and thermal analyses were carried out. The Co(II), Ni(II), Cu(I), and Zn(II) metal complexes were synthesized in methanolic media. Metal-to-ligand ratios were found to be 1:1 for Co(II) and Zn(II), 2:1 for Cu(I) and 3:2 for Ni(II) complexes. N.m.r. spectral and thermal analyses showed the presence of MeOH in all of the complexes. Conductivity measurements suggested that the complexes were 1:2 electrolytes. Ring nitrogens as well as the terminal nitrogens of hydrazine side chains in THSTZ were proposed as metal binding centres. Magnetic moments of Ni(II) (4.12μB) and Co(II) (4.2μB) indicate tetrahedral and octahedral geometry, respectively. Tetrahedral geometry for Cu(I) and Zn(II) complexes was suggested.

Carbon dioxide fixation by an unprecedented hydroxo lead-chromium carbonyl complex: synthesis, reactivity, and theoretical calculations.

Abstract: The novel hydroxo-bridged dimeric lead−chromium carbonyl complex [Et4N]2[{PbCr2(CO)10}2(μ-OH)2] ([Et4N]2[1]) was synthesized from the reaction of PbCl2 and Cr(CO)6 followed by metathesis with [Et4N]Br in a KOH/MeOH solution. The X-ray crystallographic structure shows that dianion 1 consists of two Pb{Cr(CO)5}2 units bridged by two hydroxo fragments in which the Pb atoms are further coordinated with two Cr(CO)5 groups, resulting in a distorted tetrahedral geometry. A CO2 molecule can insert itself into dianion 1 to form two new carbonate complexes, [Et4N]2[{PbCr2(CO)10}(CO3)] ([Et4N]2[2]) and [Et4N]2[{PbCr2(CO)10}2(CO3)] ([Et4N]2[3]), depending on the reaction conditions. In addition, complex 2 can be transformed into 3 in CH2Cl2 solution at an elevated temperature. While the carbonate group in dianion 2 is bonded to one Pb atom, which is coordinated with two Cr(CO)5 fragments, the carbonate group in 3 bridges the two Pb centers in a μ-1κ2OO‘:2κ2OO‘ fashion in which each Pb atom is further bonded to two Cr...

Two Novel Diamagnetic-metal Coordination Polymers of Nitronyl Nitroxide Radical Incorporating Isophthalate Bridges

Abstract: Two novel coordination polymers of formula [Zn(NIT4py)2(ip)]n (1) and [Cd(NIT4py)2(ip)(H2O)]n (2) (NIT4py=2-(4′-pyridinyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide and ip=isophthalate dianion), have been synthesized and determined by X-ray diffraction. The crystal structures of the complexes are both neutral 1-D chains. The Zn(II) ion adopts a distorted tetrahedral geometry and the Cd(II) ion adopts a distorted pentagonal bipyramidal geometry. Each isophthalate dianion binds two metal ions in bis-monodentate mode for complex (1) and in bis-bidentate mode for complex (2) leading to the 1-D chains. In complex (2) the 1-D chains develop into two-dimensional network via the hydrogen bond interactions. The magnetic studies show a weak antiferromagnetic coupling between two radicals through the diamagnetic Zn(II) in complex (1).