Thermoanalytical investigations of niobium(V) complexes of 4-isopropylphenol

Thermal behaviour of newly synthesized niobium(V) aryloxides of composition [NbCl_5−n (OC₆H₄CH(CH₃)₂-4) _n ] (where n = 1 → 5) synthesized by the reactions of niobium pentachloride with 4-isopropylphenol in predetermined molar ratios in carbon tetrachloride has been studied by thermogravimetric (TG) and differential thermal analysis (DTA) techniques. The results showed that thermal decomposition of complex of composition [NbCl₄(OC₆H₄CH(CH₃)₂-4)] resulted in the formation of NbOCl₃ as the ultimate decompositional product while all other complexes yielded Nb₂O₅ as the final product of thermal decomposition. From the mathematical analysis of TG data, the kinetic and thermodynamic parameters viz. energy of activation, frequency factor, entropy of activation, etc. have been evaluated using Coats–Redfern equation.     

Solid-state 13C NMR and quantum chemical investigation of metal diene complexes

This paper presents novel measurements and calculations of the olefinic (13)C chemical shift tensor principal values in several metal diene complexes. The experimental values and the calculations show shifts as large as 70 ppm with respect to the values in the parent olefinic compounds. These shifts are highly anisotropic, with the largest ones observed in the less shielded principal components and the smallest ones in the most shielded principal components of the tensor. The orientations of the principal components of the tensors remain, within 10 degrees , at their directions in ethylene and other olefinic compounds. The calculations, performed using the GIAO method and the LanDZ pseudopotential basis set, show good agreement with the experiments, and were used to establish definite evidence for the existence of a Cl-bridge structure in the bicyclo[2.2.1]hepta-2,5-diene (BCHD)dichlororuthenium(II) polymer.     

Synthesis and structure of half-sandwich SmII and YIII cyclopentadienyl halide complexes with the penta(benzyl)cyclopentadienyl ligand

Russian Chemical Bulletin - The exchange reactions of equimolar amounts of potassium penta(benzyl)cyclopentadienide CpBn5K (CpBn5 = C5(CH2Ph)5) and SmI2(THF)2 or YCl3 afforded the corresponding...     

Thermoanalytical investigations of niobium(V) chloride aryloxides

Niobium(V) chloride aryloxides [NbCl₃(OAr)₂] and [NbCl₂(OAr)₃] (Oar = —OC₆H₄Bu ^t -4 and —OC₆H₄OMe-4) have been prepared by reacting NbCl₅ with two and three equivalents of the respective phenol in CCl₄. The complexes have been characterized by elemental analysis, molecular weight determination, i.r., ¹H-n.m.r., u.v.–vis. and MS techniques. Thermal behaviour (t.g.–d.t.) of the complexes has also been studied and decomposition schemes proposed. The kinetic and thermodynamic parameters namely, the activation energy 'E ^*', the frequency factor 'A', entropy of activation 'S' and specific rate constant 'kr' etc. have been calculated employing the Coats–Redfern equation. The non-isothermal t.g. data has also been utilized to determine the most probable mechanism and corresponding activation energy for the decomposition of niobium(V) complexes by testing seven different theoretically possible decomposition mechanisms.     

Solid-state (13)C NMR chemical shift anisotropy tensors of polypeptides.

Carbon-13 chemical shift anisotropy (CSA) tensors for various carbon sites of polypeptides, and for carbon sites in alpha-helical and beta-sheet conformations of poly-L-alanine, and polyglycine, are presented. The carbonyl (13)C CSA tensors were determined from one-dimensional CPMAS spectra obtained at a slow spinning speed, whereas the CSA tensors of C(alpha) and other carbons in side chains of peptides were determined using 2D PASS experiments on powder samples. The results suggest that the spans of (13)Carbonyl CSA tensors of alanine and glycine residues in various peptides are similar, even though the magnitude of individual components of the CSA tensor and the isotropic chemical shift are different. In addition, the delta(22) element is the only component of the (13)Carbonyl CSA tensor that significantly depends on the CO.HN hydrogen-bond length. Solid-state NMR experimental results also suggest that (13)Carbonyl and (13)C(alpha) CSA tensors are similar for alpha-helical and beta-sheet conformations of poly-L-alanine, which is in agreement with the reported quantum chemical calculation studies and previous solid-state NMR experimental studies on other systems. On the other hand, the (13)C(alpha) CSA tensor of the first alanine residue is entirely different from that of the second or later alanine residues of the peptide. While no clear trends in terms of the span and the anisotropic parameter were predicted for (13)C(beta) CSA tensors of alanine, they mainly depend on the conformation and dynamics of the side chain as well as on the packing interactions in the solid state of peptides.     

Synthesis and structures of binuclear half-sandwich cobalt (III) ortho-carboranedithiolato complexes with silyl-bridged bis(cyclopentadienyl) ligands

Five binuclear half-sandwich cobalt complexes, [(η⁵-C₅H₄)Co(CO)I₂]₂SiMe₂ (3), [(η⁵-C₅H₄)Co(S₂C₂B₁₀H₁₀)]₂SiMe₂ (4), [(η⁵-C₅H₄)]₂Co₂(μ₂-S₂C₂B₁₀H₁₀)SiMe₂ (5), [(η⁵-C₅H₃)CoI₂](μ-I)[(η⁵-C₅H₃)Co(CO)I](SiMe₂)₂ (8), [(η⁵-C₅H₃)Co(S₂C₂B₁₀H₁₀)]₂(SiMe₂)₂ (9), were successfully synthesized in moderate yield by the reactions of corresponding ligands, (C₅H₅)₂SiMe₂ (1) and (C₅H₄)₂(SiMe₂)₂ (6), respectively. The molecular structures of 3, 5, 6, 8 and 9 was determined by X-ray crystallographic analysis, which distinctly depict various molecular structures containing the Cp rings and the metal centers with halide or 1,2-dicarba-closo-dodecaborane-1,2-dithiolato ligands. For the (η⁵-C₅H₄)₂SiMe₂ complexes, coordination of the fragments CpCo favors a exo conformation. With the rigid structure of the di-bridged ligand (C₅H₄)₂(SiMe₂)₂, only cis isomers of the corresponding (η⁵-C₅H₃)₂(Si₂Me₂)₂ complexes are formed. All the complexes have been well characterized by elemental analysis, NMR and IR spectra.     

Binuclear half-sandwich cobalt(III) and rhodium(III) ortho-carboranedichalocogenolato complexes with ether chain-bridged bis(cyclopentadienyl) ligand

1, 1'-(3-Oxapentamethylene)dicyclopentadiene [O(CH(2)CH(2)C(5)H(5))(2)], containing a flexible chain-bridged group, was synthesized by the reaction of sodium cyclopentadienide with bis(2-chloroethyl) ether through a slightly modified literature procedure. Furthermore, the binuclear cobalt(III) complex O[CH(2)CH(2)(eta(5)-C(5)H(4))Co(CO)I(2)](2) and insoluble polynuclear rhodium(III) complex {O[CH(2)CH(2)(eta(5)-C(5)H(4))RhI(2)](2)}(n) were obtained from reactions of with the corresponding metal fragments and they react easily with PPh(3) to give binuclear metal complexes, O[CH(2)CH(2)(eta(5)-C(5)H(4))Co(PPh(3))I(2)](2) and O[CH(2)CH(2)(eta(5)-C(5)H(4))Rh(PPh(3))I(2)](2), respectively. Complexes react with bidentate dilithium dichalcogenolato ortho-carborane to give eight binuclear half-sandwich ortho-carboranedichalcogenolato cobalt(III) and rhodium(III) complexes O[CH(2)CH(2)(eta(5)-C(5)H(4))Co(PPh(3))(E(2)C(2)B(10)H(10))](2) (E = S and Se), O[CH(2)CH(2)(eta(5)-C(5)H(4))](2)Co(2)(E(2)C(2)B(10)H(10)) (E = S and Se), O[CH(2)CH(2)(eta(5)-C(5)H(4))Co(E(2)C(2)B(10)H(10))](2) (E = S and Se and O[CH(2)CH(2)(eta(5)-C(5)H(4))Rh(PPh(3))(E(2)C(2)B(10)H(10))](2) (E = S and Se). All complexes have been characterized by elemental analyses, NMR spectra ((1)H, (13)C, (31)P and (11)B NMR) and IR spectroscopy. The molecular structures were determined by X-ray diffractometry.     

Novel poly(methimazolyl)borate complexes of niobium(V) and tantalum(V)

The reactions of [MCl4(eta-C5H5)](M = Nb, Ta) with Ph3Sn{HB(mt)3} (mt = methimazolyl) provide structurally characterised complexes of the new chlorobis(methimazolyl)borate ligand, [MCl3(eta-C5H5){kappa2-S,S'-HClB(mt)2}].     

Solid-state (63)Cu and (65)Cu NMR spectroscopy of inorganic and organometallic copper(I) complexes

Solid-state 63Cu and 65Cu NMR experiments have been conducted on a series of inorganic and organometallic copper(I) complexes possessing a variety of spherically asymmetric two-, three-, and four-coordinate Cu coordination environments. Variations in structure and symmetry, and corresponding changes in the electric field gradient (EFG) tensors, yield 63/65Cu quadrupolar coupling constants (CQ) ranging from 22.0 to 71.0 MHz for spherically asymmetric Cu sites. These large quadrupolar interactions result in spectra featuring quadrupolar-dominated central transition patterns with breadths ranging from 760 kHz to 6.7 MHz. Accordingly, Hahn-echo and/or QCPMG pulse sequences were applied in a frequency-stepped manner to rapidly acquire high S/N powder patterns. Significant copper chemical shielding anisotropies (CSAs) are also observed in some cases, ranging from 1000 to 1500 ppm. 31P CP/MAS NMR spectra for complexes featuring 63/65Cu-31P spin pairs exhibit residual dipolar coupling and are simulated to determine both the sign of CQ and the EFG tensor orientations relative to the Cu-P bond axes. X-ray crystallographic data and theoretical (Hartree-Fock and density functional theory) calculations of 63/65Cu EFG and CS tensors are utilized to examine the relationships between NMR interaction tensor parameters, the magnitudes and orientations of the principal components, and molecular structure and symmetry.     

Solid-state 13C NMR investigations of cyclophanes: [2.2]paracyclophane and 1,8-dioxa[8](2,7)pyrenophane

Solid-state NMR (ssNMR) and ab initio quantum mechanical calculations are used in order to understand and to better characterize the molecular conformation and properties of [2.2]paracyclophane and 1,8-dioxa[8](2,7)pyrenophane. Both molecules are cyclophanes, consisting of an aromatic ring assembly and a cyclic aliphatic chain connected to both ends of the aromatic portion. The aliphatic chain causes curvature in the six-membered aromatic ring structures. This led us to examine how the ring strain due to curvature affects the chemical shifts. Using X-ray structures of both [2.2]paracyclophane and 1,8-dioxa[8](2,7)pyrenophane as our starting model, we calculate the chemical shielding tensors and compare these data with those collected from the (13)C ssNMR FIREMAT experiment. We define curvature of [2.2]paracyclophane and 1,8-dioxa[8](2,7)pyrenophane using the π-orbital axis vector (POAV) pyramidalization angle (θ(p)).     

47, 49Ti NMR spectra of half-sandwich titanium(IV) complexes

The 47, 49Ti chemical shifts, resonance line half-widths (Deltanu1/2) and energies of the first electronic charge-transfer transitions (lambdamax1.CT) of Cp'TiX3, where Cp' = eta5-C5H5 (Cp), eta5-C5H4Me (MeCp), eta5-C5HMe4 (Me4Cp), eta5-C5Me5 (Me5Cp), eta5-C5H4SiMe3 (SiCp), eta5-C5H4SnMe3 (SnCp) and eta5-C5H4SiMe2Cl (Si'Cp) and X = Cl, Br, I and OBut, half-sandwich complexes are reported. For the compounds studied, a direct linear relationship between delta(49Ti) and lambdamax1.CT was found.     

Solid-state 109Ag CP/MAS NMR spectroscopy of some diammine silver(I) complexes

Solid-state cross-polarization magic-angle spinning (CP/MAS) NMR spectra were recorded for the compounds [Ag(NH3)2]2SO4, [Ag(NH3)2]2SeO4 and [Ag(NH3))]NO3, all of which contain the linear or nearly linear two-coordinate [Ag(NH3)2]+ ion. The 109Ag CP/MAS NMR spectra show centrebands and associated spinning sideband manifolds typical for systems with moderately large shielding anisotropy, and splittings due to indirect 1J(109Ag,14N) spin-spin coupling. Spinning sideband analysis was used to determine the 109Ag shielding anisotropy and asymmetry parameters Deltasigma and eta from these spectra, yielding anisotropies in the range 1500-1600 ppm and asymmetry parameters in the range 0-0.3. Spectra were also recorded for 15N and (for the selenate) 77Se. In all cases the number of resonances observed is as expected for the crystallographic asymmetric units. The crystal structure of the selenate is reported for the first time. One-bond (107, 109Ag,15N) coupling constants are found to have magnitudes in the range 60-65 Hz. Density functional calculations of the Ag shielding tensor for model systems yield results that are in good agreement with the experimentally determined shielding parameters, and suggest that in the solid compounds Deltasigma and eta are reduced and increased, respectively, from the values calculated for the free [Ag(NH3)2]+ ion (1920 ppm and 0, respectively), primarily as a result of cation-cation interactions, for which there is evidence from the presence of metal-over-metal stacks of [Ag(NH3)2]+ ions in the solid-state structures of these compounds.     

Some niobium(V) complexes and their relevance to the uptake of niobium by ascidians

Ascidians have been reported to absorb many unusual elements, including niobium. A study of some aqueous niobium(V) chemistry is used to identify potential ligating systems. It is proposed from the inorganic chemical evidence that these marine organisms may concentrate other early second and third row transition elements.     

Cyanide (13)C NMR hyperfine shifts in paramagnetic cyanide-bridged mixed-valence complexes

Paramagnetic (hyperfine) NMR shifts in the (13)C cyanide bridge and (31)P resonances in a set of mixed valence complexes [(eta(5)-C(5)R(5))Ru(PPh(3))L((13)CN)Ru(NH(3))(5)](n+) (R = H; L = PPh(3), CO, NO(+); R = Me; L = PPh(3)) are sensitive to the extent of intermetallic charge-transfer, and are strongly solvent dependent.     

Synthesis and reactivity of substituted cyclopentadienyl rhodium(I) and (III) complexes

New cyclopentadienyl derivatives of rhodium COD complexes [Cp*=C₅H₄COOCH₂CHCH₂ (1); C₅H₄CH₂CH₂CHCH₂ (2); C₅H(i-C₃H₇)₄ (3)] and carbonyl complex [Cp*=C₅H(i-C₃H₇)₄ (4)] were synthesized from [RhCl(COD)]₂ and [RhCl(CO)₂]₂. 1, 2 and 3 oxidized by iodine gave iodine bridged dimers 5, 6 and 7, respectively. Triphenyl phosphine, carbon monoxide and carbon disulfide molecules broke down the iodine bridged structure easily and produced monomer products Cp*RhI₂L [Cp*=C₅H₄COOCH₂CHCH₂, L=CS₂ (8); L=PPh₃ (9). Cp*=C₅H(i-C₃H₇)₄, L=CO (10)]. All of these new compounds were characterized by elemental analysis, ¹H NMR, IR, UV-Vis and mass spectroscopy. The crystal structure of 1 was solved in the triclinic space group with one molecule in the unit cell, the dimensions of which are a=7.082(9) Å, b=8.392(3) Å, c=13.889(5) Å, α=101.19(3)°, β=99.06(6)°, γ=105.11(5)°, and V=763(1) ų. The crystal structure of 3 was solved in the orthorhombic space group Pn21a with four molecules in the unit cell, the dimensions of which are a=9.748(3) Å, b=16.054(5) Å, and V=2319(1) ų. Least squares refinement leads to values for the conventional R₁ of 0.0251 for 1 and 0.0558 for 3, respectively. Compared to that in 1, a shorter metal-ligand bond length in 3 was observed and this is attributed to the rich electron density on Rh(I) metal center piled up by the C₅H(i-C₃H₇)₄ ligand.