Length dependence of coherent electron transportation in metal–alkanedithiol–metal and metal–alkanemonothiol–metal junctions

We have applied the elastic-scattering Green’s function theory to study the coherent electron transportation processes in both metal–alkanedithiol–metal (gold–[S(CH₂)_nS]–gold, n = 8–14) and metal–alkanemonothiol–metal (gold–[H(CH₂)_nS]–gold, n = 8–14) at the hybrid density functional theory level. It is shown that the current decreases exponentially with the molecular length. At the low temperature limit the electron decay rate, β, for alkanedithiol junction is found to be around 0.30/CH₂ at 1.0 V bias, much smaller than the calculated value of 0.60/CH₂ for alkanemonothiol junction. The decay rate for alkanedithiol junction at the room temperature is neither sensitive to the activation of the Au–S stretching vibrational mode nor to the external bias. The calculated current–voltage characteristics and decay rates for both junctions are in excellent agreement with the corresponding experimental results.     

Structure and redox properties of CexPr1−xO2−δ mixed oxides and their catalytic activities for CO, CH3OH and CH4 combustion

A series of Ce_xPr_1−xO_2−δ mixed oxides were synthesized by a sol–gel method and characterized by Raman, XRD and TPR techniques. The oxidation activity for CO, CH₃OH and CH₄ on these mixed oxides was investigated. When the value x was changed from 1.0 to 0.8, only a cubic phase CeO₂ was observed. The samples were greatly crystallized in the range of the value x from 0.99 to 0.80, which is due to the formation of solid solutions caused by the complete insertion of Pr into the CeO₂ crystal lattices. Raman bands at 465 and 1150 cm⁻¹ in Ce_xPr_1−xO_2−δ samples are attributed to the Raman active F_2g mode of CeO₂. The broad band at around 570 cm⁻¹ in the region of 0.3 ≤ x ≤ 0.99 can be linked to oxygen vacancies. The new band at 195 cm⁻¹ may be ascribed to the asymmetric vibration caused by the formation of oxygen vacancies. The TPR profile of Pr₆O₁₁ shows two reduction peaks and the reduction process is followed: . The reduction temperature of Ce_xPr_1−xO_2−δ mixed oxides is lower than those of Pr₆O₁₁ or CeO₂. TPR results indicate that Ce_xPr_1−xO_2−δ mixed oxides have higher redox properties because of the formation of Ce_xPr_1−xO_2−δ solid solutions. The presence of the oxygen vacancies favors CO and CH₃OH oxidation, while the activity of CH₄ oxidation is mostly related to reduction temperatures and redox properties.     

Titanium imido complexes containing 1,3,5-triazacyclohexane ligands

The syntheses of the 1,3,5-trimethyl- and tri-tert-butyl-1,3,5-triazacyclohexane-supported imido complexes [M(NR)(R′₃tach)Cl₂] (M = Ti or Zr (NMR only); R = Bu^t or 2,6-C₆H₃Prⁱ₂; R′ = Me or Bu^t) are reported, along with that of the thermally robust dibenzyl derivative [Ti(NBu^t)(Me₃tach)(CH₂Ph)₂]. The tert-butylimido ligand in [Ti(NBu^t)(Me₃tach)Cl₂] undergoes exchange with ArNH₂ (Ar = 4-C₆H₄Me or 2,6-C₆H₄Me or 2,6-C₆H₃Prⁱ₂) to form the corresponding arylimides [Ti(NAr)(Me₃tach)Cl₂]. The Me₃tach ring in [Ti(NR)(Me₃tach)Cl₂] undergoes slow exchange with Bu^t₃tach or Me₃tacn (1,4,7-trimethyl-1,4,7-triazacyclononane) to give the ring-exchanged products [Ti(NR)(Bu^t₃tach)Cl₂] and [Ti(NR)(Me₃tacn)Cl₂], respectively. The complexes [Ti(NR)(Me₃tach)X₂] (R = Bu^t or 2,6-C₆H₃Prⁱ₂; X = Cl or CH₂Ph) exhibit room-temperature dynamic NMR behaviour via an unusual trigonal twist of the facially coordinated Me₃tach ligand, and the activation parameters for these processes have been measured and are discussed. The X-ray structures of [Ti(NR)(Bu^t₃tach)Cl₂] (R = Bu^t or 2,6-C₆H₃Prⁱ₂) and [Ti(NBu^t)(Me₃tach)(X)₂] [X= Cl or CH₂Ph) are reported. Me₃tach and Bu^t₃tach = 1,3,5-trimethyl- and tri-tert-butyl-1,3,5-triazacyclohexane, respectively.     

New heterometallic copper zinc alkoxides: synthesis, structure properties and pyrolysis to Cu/ZnO composites

The copper compound [(THF)KCu(O^tBu)₃]_∞ 1 was obtained by interaction of a 1:1 mixture of ZnCl₂/CuCl₂ with KO^tBu. Bi- and trifunctional aminoalcohols were used to synthesize the intramolecularly donor stabilized Cu(II) alkoxides Cu(OCH(R)CH₂NMe₂)₂ (3: R=Me, 4: =CH₂NMe₂) where 4 was structurally characterized. Lewis acid–base adduct formation with (Me₃Si)₃CZnCl gave the heterodinuclear compounds (Me₃Si)₃CZnCl · Cu(OCH(R)CH₂NMe₂)₂ (5: R=Me, 6: R=CH₂NMe₂), which were characterized by X-ray single-crystal structure analysis. The two metal centers Cu and Zn of 5 and 6 are bridged by two oxygen atoms to form a Cu–O–Zn core. Pyrolysis of compounds 5 and 6 in dry argon or a H₂/N₂ mixture at atmospheric pressure forms metallic copper and zinc oxide, whereas pyrolysis under O₂/Ar forms additionally oxidized copper species. Elemental analysis of the pyrolysis products showed carbon and nitrogen contamination. Scanning electron microscopy and energy dispersive X-ray analysis were performed to get information on the morphology and the chemical composition of the pyrolysis products.     

Comparative characteristics of texture and properties of hybrid organic–inorganic adsorbents functionalized by amine and thiol groups

The structure and texture characteristics of the hybrid organic–inorganic adsorbents, which were obtained by using of two-component systems of “structure-forming agent/trifunctional silane”, are compared as follows: the first component is Si(OC₂H₅)₄ or (C₂H₅O)₃Si–A–Si(OC₂H₅)₃, where A = –(CH₂)₂– or –C₆H₄–; the second one is alkoxysilane with amine (–NH₂, NH, –NH(CH₂)₂NH₂) and thiol (–SH) groups. The adsorbents, derived from TEOS, have more accessible functional groups (2.6–4.2 mmol/g) than xerogels, which are based on bis(triethoxysilanes) (1.0–2.6 mmol/g). On another hand xerogels derived from bis(triethoxysilanes) have a more extended porous structure (S_sp =516–968 m²/g, V_s = 0.418–1.490 cm³/g, d = 2.5–15.0 nm) than those that are based on TEOS (S_sp = 4–631 m²/g, V_s = 0.005–1.382 cm³/g, d = 2.3–17.7 nm). The geometric dimensions of functional groups have a more essential effect on the parameters of porous structure in the case of TEOS-derived xerogels. Using solid-state NMR spectroscopy, it has been shown that in synthesis of xerogels with the use of TEOS, the molecular frame of globules is formed by structural units Qⁿ (n = 2,3,4), and the functional groups exist as structural units of Tⁿ (n = 2,3). The xerogels obtained with using bis(triethoxysilanes) consist only of structural units of Tⁿ-type (n = 1,2,3).     

Crystalline polysilicate magadiite with intercalated n-alkylmonoamine and some correlations involving thermochemical data

Synthesized hydrated lamellar acidic crystalline magadiite (H₂Si₁₄O₂₉·2H₂O) nanocompound was used as host for intercalation of polar n-alkylmonoamine molecules of the general formula H₃C(CH₂)_nNH₂ (n = 1–6) in aqueous solution. The original interlayer distance (d) of 1500 pm, determined by X-ray powder diffraction patterns, increases after intercalation. The values correlated with the number of aliphatic amine carbon (n_c) atoms: d = [(1312 ± 11) + (21 ± 2)]n_c. The amount of intercalated amines (N_s), decreased as n_c increased: N_s = [(5.82 ± 0.04) − (0.45 ± 0.01)]n_c. The acidic layered nanocompound was calorimetrically titrated with the amines and the thermodynamic data gave exothermic values for all guest molecules, as shown by the correlation: Δ_intH = −[(24.45 ± 0.49) − (1.91 ± 0.10)]n_c and d = [(1576 ± 16) − (10.8 ± 1.0)]Δ_intH. The negative values of the Gibbs energies and the positive entropies also presented the correlations: Δ_intG = −[(22.8 ± 0.2) − (0.2 ± 0.1)]n_c and Δ_intS = [(6 ± 1) + (5 ± 1)]n_c, respectively.     

Novel cyclopentadienyl silanes and disilanes: synthesis, structure and gas-phase pyrolysis

The new chloro(cyclopentadienyl)silanes Cp′SiH_yCl_3−y (Cp′=Me₄EtC₅, y=1: 1; Cp′=Me₄C₅H, y=1: 2; y=0: 3; Cp′=Me₃C₅H₂, y=1: 4 and pentachloro(cyclopentadienyl)disilanes Cp′Si₂Cl₅ (Cp′=Me₅C₅ 5, Me₄EtC₅ 6, Me₄C₅H 7, Me₃C₅H₂ 8, Me₃SiC₅H₄ 9) are synthesized in good yields via metathesis reactions. Treatment of 1–9 with LiAlH₄ leads under Cl–H exchange to the hydridosilyl compounds Cp′SiH₃ (Cp′=Me₄EtC₅ 10, Me₄C₅H 11, Me₃C₅H₂ 12) and to the hydridodisilanyl compounds Cp′Si₂H₅ (Cp′=Me₅C₅ 13, Me₄EtC₅ 14, Me₄C₅H 15, Me₃C₅H₂ 16, Me₃SiC₅H₄ 17). Complexes 1–17 are characterized by ¹H, ¹³C, and ²⁹Si-NMR spectroscopy, IR spectroscopy, mass spectrometry and CH-analysis. The structures of 6, 7 and 9 are determined by single-crystal X-ray diffraction analysis. Pyrolysis studies of the cyclopentadienylsilanes 10–12 and disilanes 13–17 show their suitability as precursors in the MOCVD process.     

Ab initio study on the kinetics and mechanisms of the formation of Agn (n = 2–6) clusters

The CCSD(T)/11e-RECP//MP2/11e-RECP method was used to explore the potential energy surfaces (PESs) of the formation of Ag_n (n = 2–6) clusters. Two kinds of reaction mechanisms were revealed in the formation of Ag_n clusters, the association mechanism for the formation of Ag₂, Ag₅, and Ag₆ clusters and the association–isomerization mechanism for the formation of Ag₃ and Ag₄ clusters. Based on the canonical transition state theory, the calculated rate constants of the formation of Ag_n clusters displayed an odd–even effect: the rate constants of formation of Ag_n clusters with odd number were larger than those with even number. The rate constant of formation of Ag₄ was the lowest, whereas that of Ag₅ was the highest among Ag_n (n = 2–6) clusters. The formation of Ag₄ was the most difficult step in the aggregation process of the silver clusters. The formation of Ag₄ may be related with the critical point in the silver aggregation process.     

Square planar diphosphinoazine rhodium(I) amido carbonyl complexes with an unsymmetrical PNP′ pincer-type coordination

A series of novel diphosphinoazine rhodium amido carbonyl complexes [{R₂PCHC(Bu^t)–NNC(Bu^t)CH₂PR₂}Rh(CO)] (R = Ph, Prⁱ, c-C₆H₁₁, Bu^t) was prepared by deprotonation of cationic diphosphinoazine rhodium amino carbonyl complexes. The complexes were characterized by NMR as were also their precursors. The crystal structures of two cationic and one neutral deprotonated complex were determined by X-ray diffraction showing the complexes to be essentially planar with mutual trans arrangement of phosphine groups and nitrogens trans to carbonyl ligands. Measurement of valence vibration frequencies of carbonyl groups in the complexes allowed to estimate the electron density on the rhodium centre. The ene-hydrazone ligand backbone (nitrogen covalently bonded) is more electron donating than the azine backbone (nitrogen bonded by electron pair donation) as expected. In the neutral series of complexes electron donation increases with phosphine substitution in the order Ph < Prⁱ = c-C₆H₁₁ < Bu^t with the corresponding decrease of carbonyl valence vibration frequency. The tert-butyl cationic complex undergoes in a low yield an unusual diphosphinoazine bond cleavage with simultaneous oxidation of the metal resulting in a binuclear bis(iminophosphine)dirhodium complex [{(Bu^t)₂PCH₂C(Bu^t)NH}Rh(Cl)₂(μ-Cl)]₂ the structure of which was also determined by X-ray diffraction.     

Energetics and bonding in beryllium metallized carbon clusters

Small carbon clusters C, C₂ and C₃ metallized with beryllium were studied by first principles within the hybrid density functional approach with generalized gradient correction. Cluster isomer structures for the ground state and several excited states where systematically calculated for C_xBe_y with x = 1–3 and y = 1–4 including the vibrational analysis of all states. Ionization potentials and electron affinities are calculated for the ground state cluster isomers. The thermal stability of the ground state isomers was verified within the harmonic approximation of the Helmholtz free energy up to temperatures of about 1000 K. Within the family of clusters studied, C₂Be₃ undergoes a solid-like structural transition at about 260 K changing from a planar structure at low temperatures to a linear isomer at high temperatures.     

Oxovanadium(IV) based coordination polymers and their catalytic potentials for the oxidation of styrene, cyclohexene and trans-stilbene

Reaction between 5,5′-methylenebis(salicylaldehyde) or 5,5′-dithiobis(salicylaldehyde) and 1,2-diaminocyclohexane in equimolar ratio leads to the formation of new polymeric chelating ligands [–CH₂(H₂sal-dach)–]_n (I) and [–S₂(H₂sal-dach)₂–]_n (II). These ligands react with [VO(acac)₂] in DMF to give coordination polymers [–CH₂{VO(sal-dach)·DMF}–]_n (1) and [–S₂{VO(sal-dach)·DMF}–]_n (2). Both complexes are insoluble in common solvents and exhibit a magnetic moment value of 1.74 and 1.78μ_B, respectively. IR spectral studies confirm the coordination of ligands through the azomethine nitrogen and the phenolic oxygen atoms to the vanadium. These complexes exhibit good catalytic activity towards the oxidation of styrene, cyclohexene and trans-stilbene using tert-butylhydroperoxide as an oxidant. Concentration of the oxidant and reaction temperature has been optimised for the maximum oxidation of these substrates. Under the optimised conditions, oxidation of styrene gave a maximum of 76% (with 1) or 85% (with 2) conversion having following products in order of selectivity: benzaldehyde > styreneoxide > 1-phenylethane-1,2-diol > benzoic acid. A maximum of 98% conversion of cyclohexene was obtained with both the catalysts where selectivity of cyclohexeneoxide varied in the order: 2 (62%) > 1 (45%). With the conversion of 33% (with 1) and 47% (with 2), oxidation of trans-stilbene gives benzaldehyde, benzil and trans-stilbeneoxide as major products.     

Preparation, properties, and reactions of metal-containing heterocycles: Part CV. Synthesis and structure of polyoxadiphosphaplatinaferrocenophanes

A series of novel heterobimetallic crown ether-like polyoxadiphosphaplatinaferrocenophanes cis-[1,1′-Fc(CH₂O(CH₂CH₂O)_nCH₂CH₂PPh₂)₂]PtCl₂ (n=1–3) (4a–c) was synthesized in good yield by cyclization of the bis(phosphine) ligands 1,1′-Fc(CH₂O(CH₂CH₂O)_nCH₂CH₂PPh₂)₂ (n=1–3) (3a–c) and (PhCN)₂PtCl₂ under high dilution conditions in CH₂Cl₂. The bisphosphines 3a–c are obtained by reaction of the corresponding diols 1,1′-Fc(CH₂O(CH₂CH₂O)_nCH₂CH₂OH)₂ (n=1–3) (1a–c) with: (i) CH₃SO₂Cl in CH₂Cl₂ and (ii) LiPPh₂ in THF. Although the X-ray crystal structure of 4a shows that the cavity is large enough for the encapsulation of small metal cations, inclusion experiments of 4a–c with Group 1 cations, and Mg²⁺, or NH₄⁺ in solution applying NMR titration and cyclovoltammetric methods reveal no evidence for the formation of host–guest complexes for 4a,b. In the case of 4c only the addition of Na⁺ or K⁺ leads to an insignificant effect.     

Electron transfer of quinone self-assembled monolayers on a gold electrode

Dialkyl disulfide-linked naphthoquinone, (NQ-C_n-S)₂, and anthraquinone, (AQ-C_n-S)₂, derivatives with different spacer alkyl chains (C_n: n = 2, 6, 12) were synthesized and these quinone derivatives were self-assembled on a gold electrode. The formation of self-assembled monolayers (SAMs) of these derivatives on a gold electrode was confirmed by infrared reflection-absorption spectroscopy (IR-RAS). Electron transfer between the derivatives and the gold electrode was studied by cyclic voltammetry. On the cyclic voltammogram a reversible redox reaction between quinone (Q) and hydroquinone (QH₂) was clearly observed under an aqueous condition. The formal potentials for NQ and AQ derivatives were −0.48 and −0.58 V, respectively, that did not depend on the spacer length. The oxidation and reduction peak currents were strongly dependent on the spacer alkyl chain length. The redox behavior of quinone derivatives depended on the pH condition of the buffer solution. The pH dependence was in agreement with a theoretical value of E_1/2 (mV) = E′ − 59pH for 2H⁺/2e⁻ process in the pH range 3–11. In the range higher than pH 11, the value was estimated with E_1/2 (mV) = E′ − 30pH , which may correspond to H⁺/2e⁻ process. The tunneling barrier coefficients (β) for NQ and AQ SAMs were determined to be 0.12 and 0.73 per methylene group (CH₂), respectively. Comparison of the structures and the alkyl chain length of quinones derivatives on these electron transfers on the electrode is made.     

Synthesis and structural behavior of the P-functional organotin chlorides [Ph2P(CH2)3]2SnCl2, Ph2P(CH2)3SnCl2Me, and Ph2P(CH2)nSnCl3 (n=2, 3)

The P-functional organotin dichloride [Ph₂P(CH₂)₃]₂SnCl₂ (3) is synthesized by reaction of Ph₂P(CH₂)₃MgCl with SnCl₄ independently of the molar ratio of the starting compounds. The corresponding organotin trichlorides Ph₂P(CH₂)_nSnCl₂R (4: n=2, R=Cl; 5: n=3, R=Cl; 6: n=3, R=Me) are formed in a cleavage reaction of Ph₂P(CH₂)_nSnCy₃ (n=2, 3) with SnCl₄ or MeSnCl₃, respectively. The main features of the crystal structures of 3–6 are both intra- and intermolecular PSn coordinations and the existence of intermolecular Sn---ClSn bridges. For further characterization of the title compounds, the adducts 4(Ph₃PO)₂ (7) and 5(Ph₃PO) (8), as well as the P-oxides and P-sulfides of 3–6 (9–15), are synthesized. The results of crystal structure analyses of 7, 11, 12, and 14 are reported. The structures of 9–15 are characterized by intramolecular P=XSn interactions (X=O, S). A first insight into the structural behavior of the compounds 3–15 in solution is discussed on the basis of multinuclear NMR data.     

Molecular motion and phase transition in perovskite-type (CH3)nNH4−nSnCl3 (n=1–4) studied by proton NMR spin–lattice relaxation

Powder X-ray diffraction, ¹¹⁹Sn NMR spectra, and ¹H NMR spin–lattice relaxation times, T₁, were measured for (CH₃)_nNH_4−nSnCl₃ (n=1–4). From the Rietveld analysis, it is shown that all four compounds crystallize into deformed perovskite-type structures at room temperature. The temperature dependence of ¹H T₁ was analyzed in terms of the CH₃ reorientation and other motions of the whole cation. Except for the phase transition in CH₃NH₃SnCl₃, which is from monoclinic to rhombohedral at 331 K, ¹H T₁ was continuously changed at other phase transitions in this compound as well as in the n=2–4 compounds, suggesting that the transitions are not caused by the change of the motional state of the cation but by an instability of the [SnCl₃]_nⁿ⁻ perovskite lattice.     

Synthesis and low temperature fluorination of the alkaline-earth palladates Ba2−xSrxPdO3 (x = 0–2)

Since the discovery of superconductivity in Sr₂CuO₂F_2+δ there has been an increased interest in ternary oxide-fluorides. Sr₂CuO₂F_2+δ is prepared via low temperature (T = 220 °C) reaction routes. Low temperature fluorination induces an interesting structural rearrangement in the parent compound Sr₂CuO₃, which is a one-dimensional material containing linear chains of vertex sharing CuO₄ squares along the crystallographic b axis. Upon fluorination, one oxide is substituted by two fluorides and Cu²⁺ becomes octahedrally coordinated by four oxides and two fluorides. The fluorinated compound Sr₂CuO₂F_2+δ displays the T-type structure (La₂CuO₄). Insertion of excess fluorine, δ, also takes place and this fluorine occupies interstitial sites in the T structure. Although the starting material Ca₂CuO₃ is isostructural to Sr₂CuO₃, Ca₂CuO₂F_2+δ displays the T′ (Nd₂CuO₄) structure due to the smaller radius of Ca²⁺ compared to that of Sr²⁺.

The alkaline-earth palladates with the general formula A₂PdO₃ (A = Ba, Sr) are isostructural with the A₂CuO₃(A = Ca, Sr) materials. We prepared the Ba_2−xSr_xPdO₃ (x = 0–2) series and performed low temperature fluorination, which led to the synthesis of the series Ba_2−xSr_xPdO₂F_2+δ (0 ≤ x ≤ 1.5). All the compounds in the Ba_2−xSr_xPdO₂F_2+δ series show T′ structure (Ca₂CuO₂F_2+δ). Similarities and differences with Sr₂CuO₂F_2+δ and Ca₂CuO₂F_2+δ will be discussed.     

The solution chemistry of organotin compounds V. Proton relaxation and dynamic NMR studies of solute-solvent interactions in solutions of dimethyltin dichloride plus 2,2′-bipyridine complex

¹H T₁ and dynamic NMR measurements (T₁ = relaxation time) were made with Me₂SnCl₂ and its bpy complex in mixed solvents of CH₂Cl₂ and weak bases to elucidate the role of the solvent in the dynamic behaviour of organotin compounds. In the line shape analysis of the exchange reaction between Me₂SnCl₂ and its bpy complex, the dissociation rate constant k_d of the Me₂SnCl₂ · bpy complex supported an exchange mechanism of the dissociation followed by a rapid recombination step. Weak bases in the mixed solvents decreased the Eyring activation parameters ΔH^‡ and ΔS^‡ for the dissociation reaction of the complex, indicating more extensive solvation in the activated state than in the ground state of the complex. In the ¹H T₁ measurements, the reorientational correlation time τ_c was shorter and its activation energy E_a was larger in Me₂SnCl₂ than in Me₂SnCl₂ · bpy, in conformity with their molecular sizes. E_a and τ_c increased on addition of weak bases, such as nitromethane, nitrobenzene and acetonitrile, even for the Me₂SnCl₂ · bpy complex, whose acidity is reduced considerably from that of the free tin compound. The major cause of this result is suggested to be the interaction of the weak bases with CH₂Cl₂, resulting in a change of the physical properties of the mixed solvent.     

Strong antiferromagnetism in the dinuclear 2-pyridone complex with N–C–O bridges: A paddle-wheel analogue of the dinuclear tetracarboxylates

A novel dinuclear complex [Cu₂(μ-L)₄(HL)₂] (1) was isolated from starting 2-pyridone (HL) via a resonance and a tautomeric transformation. Each copper centre is in a square-pyramidal coordination sphere, defined by two oxygen atoms (Cu–O4 1.978(5), Cu–O11 1.964(4) Å) and two nitrogen atoms (Cu–N2 2.003(5), Cu–N3 2.007(5) Å) of four bridging deprotonated pyridin-2-olates and an oxygen atom on the top from a neutral 2-pyridone (Cu–O2 2.227(5) Å), analogous to tetracarboxylate paddle-wheel complexes. Compound 1 was compared with mixed pyridin-2-olato/methanoato analogues [Cu₂(μ-HCO₂)₂(μ-L)₂(HL)₂] · 2CH₃CN (2) and [Cu₂(μ-HCO₂)₂(μ-L)₂(HL)₂] (2a) (2a is an air stable form obtained from 2 outside mother-liquid). The EPR spectra of air stable 1 and 2a show three signals H_z1, H₂ and H_z2, typical for the binuclear systems with spin S = 1, both revealing strong antiferromagnetism 2J = −334 (1) and −324 cm⁻¹ (2a). Interestingly, only for 1 additional H₁ signal at 100 mT is noticed (D(1) = 0.293 cm⁻¹ < hν = 0.320 cm⁻¹ < D(2a) = 0.347 cm⁻¹). On the other hand, several broad signals in the 100–450 mT region, only in the high temperature spectrum for 2a are observed. These results are in agreement with the magnetic susceptibility analysis.     

Synthesis and chemistry of the bis(trimethylsilyl)amido bis-tetrahydrofuranates of the group 2 metals magnesium, calcium, strontium and barium. X-ray crystal structures of Mg[N(SiMe3)2]2·2THF and related Mn[N(SiMe3)2]2·2THF

Transamination reactions utilizing the compound mercuric bis(trimethylsilyl)amide, Hg{N(SiMe₃)₂}₂, in tetrahydrofuran (THF), and the metals Na, Mg, Ca, Sr, Ba and Al have been investigated. Thus the THF solvated compounds Na[N(SiMe₃)₂]·THF and M[N(SiMe₃)₂]₂·2THF, M = Mg, Ca, Sr and Ba (1–4), have been prepared. The X-ray crystal structures of 1 and the related manganese compound Mn[N(SiMe₃)₂]₂·2THF (5) are reported. Interaction of the silylamides, 2–4, with a range of crown ethers apparently proceeded with elimination of silylamine, (Me₃Si)₂NH, and novel ring opening of the crown ethers, generating species containing a donor alkoxide ligand with a vinyl ether function, presumably, ---O(CH₂CH₂O)_nCH=CH₂ (n = 3−5). The silylamides 2–4 were also cleanly converted to the corresponding alkoxides (from ¹H NMR data) in reactions with stoichiometric quantities of 3-ethyl-3-pentanol.     

N,N′-bis(substituted-phenyl)oxamides and their dinuclear pentacoordinate nickel(II) complexes

The preparation, spectroscopic characterization and magnetic study of N,N′-bis(substituted-phenyl)oxamidate-bridged nickel(II) dinuclear complexes of formula {[Ni(N₃-mc)]₂(μ-CONC₆H₄-X)}(PF₆)₂ (N₃-mc = 2,4,4-trimethyl-1,5,9-triazacyclo-dodec-1-ene (Me₃-N₃-mc) or 2,4,4,9-tetramethyl-1,5,9-triazacyclododec-1-ene (Me₄-N₃-mc), X = 2-Cl, 4-Cl, 2-OCH₃, 4-OCH₃) are reported. These paramagnetic nickel(II) complexes have been characterized by both one- and two-dimensional (COSY) ¹H NMR techniques. The COSY spectrum of 5 has allowed to achieve the assignment of the phenyl protons of the N,N′-diphenyloxamidate. The crystal structures of [Ni(Me₃-N₃-mc)(μ-CONC₆H₄-4-Cl)]₂(PF₆)₂ (6), [Ni(Me₃-N₃-mc)(μ-CONC₆H₄-4-OMe)]₂(PF₆)₂ (8) and [Ni(Me₄-N₃-mc)(μ-CONC₆H₄-2-Cl)]₂(PF₆)₂ (9) have been determined and their magnetic properties have been studied. The value of magnetic coupling between the two nickel(II) ions across the oxamidate bridge [J = − 37.6 (6), −39.9 (8) and −39.7 cm⁻¹ (9)] is sensitive to the distortion of the coordination sphere of the metal ions and the topology of the molecular bridge.