二维网状Pb(Ⅱ)配合物的合成、晶体结构及荧光性质

利用4-吡啶-3-苯甲酸(4,3-pybz)和醋酸铅在水热条件下合成了一种新型金属配合物[Pb(4,3-pybz)2]_n(1),并对其进行了元素分析、红外光谱、热重表征、X 射线单晶衍射测定。该配合物为正交晶系,Pccn空间群,a=1.070 74(7) nm,b=2.138 03(13) nm, c=0.865 65(5) nm,V=1.981 7(2) nm³,Z=4,D_c=2.023 Mg·m^-3,F(000)=1 152,GOF=1.050,μ=8.549 mm^-1,残差因子R₁=0.013 9,wR₂=0.032 4。该配合物展现了一个具有(4,4)拓扑二维波浪状网络结构,进而通过弱的π-π相互作用拓展为三维超分子体系。此外,室温下配合物1展现了弱的荧光性质。     

Metal-metal bond length variability in Co(3)(dipyridylamide)(4)Cl(2): bond-stretch isomerism, crystal field effects, or spin transition process? A DFT study

The unprecedented structural behavior of Co(3)(dipyridylamide)(4)Cl(2), characterized in two crystalline forms in which the tricobalt framework is either symmetric or highly nonsymmetric at room temperature is investigated by means of gradient-corrected DFT calculations. The isolated molecule is assigned a single energy minimum associated with a low-spin (doublet) electronic configuration. The optimal geometry closely reproduces the X-ray structure observed for the isomer displaying equivalent metal-metal distances. However, the ground-state potential energy surface is extremely shallow with respect to a distortion of the Co(3) framework. A "weak" distortion, similar to that observed for the unsymmetrical complex at low temperature (Deltad(Co-Co) = 0.08 A at 110 K) induces a destabilization of 1.1 kcal.mol(-1) only. The distortion observed at room temperature (Deltad(Co-Co) = 0.17 A) destabilizes the isolated complex by 4.2 kcal.mol(-1). These results are rationalized in terms of the "three-electron three-center" concept applied to the sigma-bonding electrons of the cobalt framework. A phenomenological model based upon the Heisenberg Hamiltonian successfully reproduces the calculated potential energy curve and assigns the relative stability of the symmetric structure to local forces (Pauli repulsion, ligand bite, etc.) distinct from delocalized sigma bonding. In view of these results, the two structures characterized from X-rays cannot be termed "bond-stretch isomers" according to the strict definition given by Parkin. To investigate the origin of the distorted form, an electric field was applied to the isolated molecule, but it did not shift the equilibrium position toward asymmetry, despite a strong polarization of the electron density. Finally, the quartet state of lowest energy ((4)A state) has an optimal structure that is distorted and that reproduces most of the distinctive features observed in the nonsymmetric structure. Despite the high relative energy calculated for this quartet state, we assign the occurrence of the nonsymmetric form and its extreme variability with temperature to a progressive population of this excited state as temperature increases.     

Reactivity for boryl(phosphino)carbenyl carbene analogues with group 14 elements (C, Si, Ge, Sb, and Pb) as a heteroatom: a theoretical study

The potential energy surfaces for the chemical reactions of group 14 carbenes have been studied using density functional theory (B3LYP/LANL2DZ). Five boryl(phosphino)-based carbene (B-?-P) species, where ? = C, Si, Ge, Sn, and Pb, have been chosen as model reactants in this work. Also, four kinds of chemical reactions; intramolecular 1,2-migration, water insertion, alkene cycloaddition, and intermolecular dimerization, have been used to study the chemical reactivities of these group 14 carbenes. The present theoretical investigations suggest that the relative carbenic reactivity decreases in the order C > Si > Ge > Sn > Pb. That is, the heavier the group 14 atom (E), the more stable is the boryl(phosphino)-based B-?-P species towards chemical reactions. Our theoretical findings thus demonstrate that all boryl(phosphino)-based carbenes are isolable at room temperature because they are quite inert to chemical reactions, except that they are also moisture-sensitive molecules. Furthermore, the singlet-triplet energy splitting of the B-?-P, as described in the configuration mixing model attributed to the work of Pross and Shaik, can serve as a diagnostic tool for a better understanding and predicting of their chemical reactivities, kinetically and thermodynamically. The results obtained allow a number of predictions to be made.     

Lowering melting points in asymmetrically substituted Salen-copper(II) complexes exhibiting mesomorphic behavior. Structure of the mesogen Cu(5-hexyloxySalen)

In comparison with their symmetrical analogues, unsymmetrically substituted Cu-Salen complexes show mesophases with lowered melting points. For terminally substituted complexes, symmetrical ones (R(1) = R(2)) have only an S(A) phase, while for unsymmetrical alkoxy substitution a monotropic S(E) phase occurs and the melting temperature decreases with no loss in mesophase stability. Lateral substitution, when it is symmetrical (R(3) = R(4)), lowers mesophase stability but not melting temperature, and when it is unsymmetrical, it greatly lowers both mesophase stability and melting temperature compared with the parent compound. Substitution at the imine carbon (R(5), R(6)) also lowers chemical stability (decomposition) of the compounds. The structure of the 5-hexyloxy complex (R(1) = R(2) = OC(6)H(13), R(3) = R(4) = R(5) = H) shows the pre-mesophasic arrangement likely adopted after melting.     

The structure of xenon hexafluoride in the solid state

According to single crystal X-ray diffraction, neutron powder diffraction, solid state MAS NMR data, and differential scanning calorimetry, XeF₆ exists in at least six different modifications. Three of them are formed at temperatures above room temperature, one exists at room temperature, while two have been found at low temperatures. In the high temperature modifications XeF₆ forms a non-symmetric tetramer, better described as a cyclic trimer with a weakly associated monomer. The normal temperature modification is the previously described cubic phase IV, having disordered tetrameric and hexameric units. The low temperature modifications are regular tetramers. Only in presence of HF symmetric dimers are formed.     

A study of the hydrogen abstraction reactions of C2H radical with CH3CN, C2H5CN, and C3H7CN by dual-level generalized transition state theory

The hydrogen abstraction reactions C2H + CH3CN --> products (R1), C2H + CH3CH2CN --> products (R2), and C2H + CH3CH2CH2CN --> products (R3) have been investigated by dual-level generalized transition state theory. Optimized geometries and frequencies of all the stationary points and extra points along the minimum-energy path (MEP) are performed at the BH&H-LYP and MP2 methods with the 6-311G(d, p) basis set, and the energy profiles are further refined at the MC-QCISD level of theory. The rate constants are evaluated using canonical variational transition state theory (CVT) with a small-curvature tunneling correction (SCT) over a wide temperature range 104-2000 K. The calculated CVT/SCT rate constants are in good agreement with the available experimental values. Our calculations show that for reaction R2, the alpha-hydrogen abstraction channel and beta-hydrogen abstraction channel are competitive over the whole temperature range. For reaction R3, the gamma-hydrogen abstraction channel is preferred at lower temperatures, while the contribution of beta-hydrogen abstraction will become more significant with a temperature increase. The branching ratio to the alpha-hydrogen abstraction channel is found negligible over the whole temperature range.     

Pb1-xTbxTi1-xMnxO3(O≤x≤0.10)固溶体的合成、结构与表征

用固相反应合成了Pb1-xTbxTi1-xMnxO3(0≤x≤0.10)固溶体,并用X射线粉末衍射进行了表征,室温下其空间群为P4mm.热分析仪测试结果显示,随着Tb和Mn掺杂量的增加,该固溶体的相变温度Tc降低.介电常数在Tc附近出现峰值,表明对应的相变是铁电相变.磁性测量显示,当x=0.08和x=0.10时,Pb1-xTbxTi1-xMnxO3分别在25和29 K附近有顺磁性向反铁磁性的转变.     

NMR studies of the tautomerism of cyclo-tris(4-R-2,6-pyridylformamidine) in solution and in the solid state

Two novel cyclo-tris(4-R-2,6-pyridylformamidine)s (R = H, CH(3)) have been studied by solution- and solid-state NMR. Both compounds show fast exchange of NH protons in dimethyl sulfoxide at room temperature. The E-syn configuration of the formamidine group with the NH protons in the outer ring position could be proved by low-temperature (1)H NMR experiments. The influence of deuteration of the NH group on the exchange rate has been demonstrated qualitatively. Proton exchange at the formamidine groups results in both a symmetric (S) and an asymmetric (A) isomer which could be spectroscopically identified and characterized at 193 K. Whereas two degenerate forms exist for S, six degenerate A forms can be distinguished. Prototropic tautomerism at one formamidine group results in exchange from S into A, whereas A is transformed to a degenerate A form or to S. It could be shown that some transitions between substructures are impossible by a single -NH-CH=N-/-N=CH-NH- exchange. The S isomer with three equivalent formamidine groups is the preferred isomer in solution as indicated by the S/A ratio at 193 K. From this result we conclude that in polyformamidines, ordered sequences of formamidine tautomers are also formed at low temperatures. Prototropic exchange was not observed in the solid state, neither by (13)C nor by (15)N solid-state NMR. For one trimer (R = CH(3)), three molecules dimethyl sulfoxide per trimer molecule are within the lattice as could be proved by (13)C CPMAS NMR.     

Structural study of 2,6-bis[(dimethylaminomethyl)phenyl]butyl stannanes: nonconventional behaviour of triorganotin(IV) halides

The four organotin (IV) compounds ([2,6-bis(dimethylaminomethyl)phenyl](n-butyl)R(1)R(2)stannane, with R(1)=R(2)=nBu (1), R(1)=nBu, R(2)=Cl (2), R(1)=nBu, R(2)=Br (3) and R(1)=R(2)=Br (4)), have been prepared and their structures have been investigated in various solvents and at various temperatures (NMR). The structures of these compounds in solution are solvent- and temperature-dependent. The solid state structures of 2 and 3 were studied using CP/MAS NMR spectroscopy and Xray diffraction techniques. The tetraorganotin compound 1 exhibits tetrahedral geometry with very weak Sn-N coordination. The dynamic process of Sn-N bond(s) association/dissociation was observed using low-temperature NMR measurements. The tin central atom in 2 and 3 is [4+2]-coordinated in toluene solutions and the NMR low-temperature measurements reveal the same dynamic behavior as for 1 in this solution, with retention of the covalent halogen-tin bond. However, this bond is dissociated in methanol solutions, yielding ionic species, where the tin atom is only [3+2]-coordinated, and the halogen atom lies outside of the primary coordination sphere of the tin atom. In addition, while the same ionic structure as in methanol was found in the whole measured temperature range in the chloroform solution of 3, the structure of 2 varies in this solvent. In this compound, the covalent Sn-Cl bond (similar structure as in toluene solution), which is retained at room temperature in chloroform solution, is continuously dissociated with a decrease in temperature, leading to ionic bonding (a similar structure as in methanol solution). All the above-mentioned processes are reversible in all the solvents and at all temperatures. In the solid state, the covalent Sn-Cl bond is observed for 2, while an ionic bond was found in 3.     

Structural Versatility in Polyoxometalates and in Some Linear Trimetallic Complexes: An Electronic Interpretation

Bond stretch isomerism, for which the molecular conformations differ only in the length of one or more bonds, is often difficult to evidence on purely experimental grounds. Quantum chemical modelling allows to make a clear distinction between the effects of an instability, either of steric, electronic, or magnetic origin, inherent to the molecular system and those that should be assigned to the molecular environment. DFT calculations carried out on compounds of the type M^II ₃(dpa)₄Cl₂ (M=Co, Cr; dpa=dipyridylamide) show that the unprecedented structural variability observed in these complexes should be related to the electronic structure of the molecule itself. In the case of Co₃(dpa)₄Cl₂, a single minimum corresponding to the symmetric metal framework observed in the orthorhombic crystal was characterized on the doublet ground state potential energy surface. The presence of a unique energy minimum for the ground state rules out the possibility for bond stretch isomerism. However, a spin transition from doublet to quartet shifts the equilibrium geometry towards the unsymmetrical structure characterized at room temperature in the tetragonal crystal. The quintet ground state of Cr₃(dpa)₄Cl₂ also corresponds to a symmetric equilibrium geometry, but a second quintet state corresponding to a slightly higher energy and a different spin coupling of the localized metal electrons breaks the symmetry of the metallic framework. If the relative energy of this second quintet state could be lowered by a change in the axial coordination, an instability favorable to bond stretch isomerism would be generated on the ground state potential surface. Finally, calculations carried out on some functionalized polyoxometalates suggest that the tuning of intramolecular redox processes involving polyoxoanions as rigid electron reservoirs could provide a route toward the design of chemical architectures displaying bond stretch isomerism.     

Spectral shifts and structures of phenol...Ar(n) clusters

A laser spectroscopic investigation of phenol...Ar(n) (n = 1-6) clusters in the first electronically excited state (S(1)) and the cationic ground state (D(0)) is reported. Resonance enhanced two-photon ionisation (R2PI) spectra have been recorded for the investigation of the S(1) state. The origins of S(1)← S(0) (S(1)0(0)) transition of phenol...Ar(n) (n = 1, 2,4-6) are all red shifted compared to the S(1)0(0) state of the monomer by 33 cm(-1), 67 cm(-1), 10 cm(-1), 20 cm(-1), 44 cm(-1), respectively. However, the origin of the phenolAr(3) cluster is blue shifted by 25 cm(-1). For the investigation of the ionic ground state photoionization efficiency (PIE) and mass-analyzed-threshold ionization (MATI) spectroscopy have been applied. The spectra of phenol...Ar(3) and phenol...Ar(4) yield values for the ionization energy (IE) of 68,077 ± 15 cm(-1) and 67,948 ± 15 cm(-1). With the combination of theoretical methods and R2PI, PIE and MATI spectroscopy, the major species present have been positively identified.     

Theoretical designs for germaacetylene (RC≡GeR'): a new target for synthesis

The effect of substitution on the potential energy surfaces of RC≡GeR (R = F, H, OH, CH(3), SiH(3), Tbt, Ar*, SiMe(SitBu(3))(2) and SiiPrDis(2)) was explored using density functional theories (B3LYP/LANL2DZdp and B3PW91/6-31G(d)). Our theoretical studies indicate that all the triply bonded RC≡GeR species prefer to adopt trans-bent geometry, which is in agreement with the theoretical model (mode (B)). Additionally, we show that the stabilities of the RC≡GeR species bearing smaller substituents (R = F, H, OH, CH(3) and SiH(3)) decrease in the order R(2)C=Ge: > RC≡GeR > :C=GeR(2). On the other hand, the triply bonded RC≡GeR molecules with bulkier substituents (R = Tbt, Ar*, SiMe(SitBu(3))(2), SiiPrDis(2)) were found to possess the global minimum on the singlet potential energy surface and are both kinetically and thermodynamically stable. That is to say, both electronic and steric effects of bulky substituents play a crucial role in making triply bonded germaacetylenes (RC≡GeR) an interesting synthetic target.     

Behaviour of some dialkyl- and trialkyl-lead compounds in the hydride-generation procedure using a non-dispersive atomic-fluorescence detector

The method of generating covalent hydrides by reduction with sodium tetrahydroborate was applied to aqueous solutions containing traces of R(3)Pb(+) and R(2)Pb(2+) compounds (R = methyl, ethyl). For each compound the effects of sample acidity, sodium tetrahydroborate and hydrogen peroxide concentrations, reaction-vessel and transfer-line materials, were measured and the experimental conditions defined for obtaining maximum sensitivity. Experimental evidence indicating the formation of organolead hydrides during the reduction step at room temperature was adduced. The reaction efficiency was found to be 90% for trimethyl-, triethyl- and diethyl-lead and 59% for dimethyl-lead. In the case of inorganic lead the efficiency was only 27%. Without an intermediate preconcentration step, the detection limits obtained for trimethyl-, triethyl-, dimethyl- and diethyl-lead (3 times the standard deviation) were 3-5 ng of lead per litre. A procedure for discriminating between R(3)Pb(+), R(2)Pb(2+) and Pb(2+) compounds is proposed.     

Complexation of dimethylmagnesium with alpha-diimines; structural and EPR characterisation of single electron and alkyl transfer products

Treatment of dimethylmagnesium with the alpha-diimine ligands Ar'N=C(R)C(R)=NAr' [R = naphth-1,8-diyl (1), H (2), CH3 (3); Ar' = 2,6-diisopropylphenyl] in diethyl ether provides the neutral methyl-bridged dimeric complexes [(alpha-diimine-.)Mg+(mu-CH3)]2 via single electron transfer (SET) to the coordinated diimine and elimination of a methyl radical. These biradical species have been characterised by EPR spectroscopy and, for the ligand , X-ray crystallography. In the presence of THF the reaction of ligand proceeds to the diamagnetic [(ene-1,2-diamide)Mg(THF)3] complex in which the diimine ligand has been doubly reduced to an ene-diamide by two successive SET processes. Comparison of the structural data for the free ligand with that obtained for the alpha-diimine radical anion and ene-diamide complexes shows the expected increases in C-N, and decreases in C-C, bond lengths within the N-C-C-N unit consistent with the progressive reduction of the ligand. In the case of ligand , reaction at low temperature provides the complex [Mg(mu2-Me){Ar'NC(Me)2C(Me)NAr'}]2 in which methyl transfer to a ligand imine carbon atom has occurred. This species has also been structurally characterised. This contrasts with the formation of the radical species at room temperature, and indicates the involvement of an intermediate in which the radical products of the SET process are held in close proximity by the solvent cage. Two competing processes of methyl radical escape and methyl transfer to the ligand account for the formation of the observed products at different temperatures.     

Theoretical investigations of the reactivities of saturated five-membered ring N-heterocyclic carbenes with heavier group 14 elements

The potential energy surfaces for the chemical reactions of group 14 carbenes have been studied using density functional theory (B3LYP/LANL2DZ). Five saturated five-membered-ring N-heterocyclic carbene Dipp[upper bond 1 start]N(CH(2))(2)N(Dipp)E[upper bond 1 end]: (five-ring-E:) species, where E = C, Si, Ge, Sn and Pb, have been chosen as model reactants in this work. Also, four kinds of chemical reactions; addition of water, methane insertion, alkene cycloaddition and dimerization, have been used to study the chemical reactivities of these group 14 carbenes. The present theoretical investigations suggest that the relative carbenic reactivity decreases in the order: C > Si > Ge > Sn > Pb. That is, the heavier the group 14 atom (E), the more stable is the carbene towards chemical reactions. This may be the reason that there have been many instances reported of the synthesis and characterization of stable group 14 five-membered-ring N-heterocyclic carbene species with various alkyl protecting substituents at room temperature. Furthermore, the singlet-triplet energy splitting of the five-ring-E:, as described in the configuration mixing model attributed to the work of Pross and Shaik, can be used as a diagnostic tool to predict their reactivities. The results obtained allow a number of predictions to be made.     

NO3-+Cl2→ClONO2+Cl-反应势能面和势能阱

采用密度泛函理论B3LYP方法和6-311+G(d)基组, 计算构建离子-分子气相反应NO3-+Cl2→ClONO2+Cl-的三维势能面. 三维反应势能面证明该反应没有过渡态和势能垒, 但是存在一个深达-55.0 kJ/mol的势能阱(以氯气分子和硝酸根离子相隔无穷远为参量). 在势能阱底部, 有个化合物(O2NOClCl)- 称为势阱化合物, 依赖于势能阱而稳定存在. 理论红外光谱预测低温红外光谱能检测该势阱化合物. 低温条件下, 该反应由热力学控制, 反应产物是势阱化合物(O2NOClCl)-. 当温度升高, 该反应由动力学控制, 势阱化合物(O2NOClCl)-不稳定, 发生分解反应, 重新生成NO3-和Cl2. 研究结果可用来解释低温时ClONO2与Cl-气相反应不能产生Cl2的原因.     

Luminescence Properties of Platinum(II) Dithiooxamide Compounds

Tight contact ion pairs of general formula {Pt(H(2)-R(2)-dto)(2)(2+),(X(-))(2)} have been prepared, and their absorption spectra and luminescence properties (at room temperature in dichloromethane fluid solution and at 77 K in butyronitrile rigid matrix) have been studied (dto = dithiooxamide; R = methyl, X = Cl (1); R = butyl, X = Cl (2); R = benzyl, X = Cl (3); R = cyclohexyl, X = Cl (4); R = cyclohexyl, X = Br (5); R = cyclohexyl, X = I (6)). The absorption spectra of all the compounds are dominated by moderately strong Pt(dpi)/S(p) to dithiooxamide (pi) charge transfer (Pt/S --> dto CT) bands in the visible region (epsilon in the 10(4)-10(5) M(-)(1) cm(-)(1) range). Absorption features are also present at higher energies, due to pi-pi transitions centered in the dto ligands (ligand centered, LC). All the compounds exhibit a unstructured luminescence band in fluid solution at room temperature, with the maximum centered in the 700-730 nm range. The luminescence bands are blue-shifted about 4000 cm(-)(1) on passing to the rigid matrix at 77 K. Luminescence lifetimes are on the 10(-)(8)-10(-)(7) s time scale at room temperature and 1 order of magnitude longer at 77 K. Luminescence is assigned to triplet Pt/S --> dto CT excited states in all cases. Compounds 3-6 also exhibit a second higher-energy luminescence band at room temperature, centered at about 610 nm, attributed to a LC excited state. Charge transfer interactions between halides and dto ligands destabilize dto-centered orbitals, affecting the energy of Pt/S --> dto CT transitions and states. The X counterions and X --> dto CT levels are proposed to play a role in promoting excited state conversion between LC and Pt/S --> dto CT levels. The R substituents on the nitrogen atoms of the dto ligands influence the absorption and photophysical properties of the compounds, by affecting proximity of the ion pairs. The possibility to functionalize the R substituents may open the way to interface these luminescent compounds with desired substrates and to construct supramolecular assemblies.     

Amino-imino tautomerization reaction of the 4-aminopyrimidine/acetic acid system

The amino-imino tautomerization of the 4-aminopyrimidine (4APM)/acetic acid (AcOH) system through dual hydrogen bonding in n-hexane at room temperature was investigated using UV absorption and fluorescence spectroscopies, fluorescence time-profile measurements, and molecular orbital calculations, with those of the imino-model compound of 3-methyl-4(1H)-pyrimidinimine (3M4PMI). From the experimental results, it was confirmed that the imino-tautomer was formed in the first excited singlet state (S1) state through the double-proton transfer of the dual hydrogen-bonded complex of 4APM with AcOH. The fluorescences of the free 4APM monomer (band maximum at 353nm), imino-tautomer (at 414nm), and 3M4PMI monomer (at 437nm) exhibit the single-exponential decays of 98, 73, and 19ps time constants, respectively. The shorter decay time of the imino-tautomer fluorescence compared with the free monomer one is suggestive of the low activation energy process in the S1 state. From the result of the shortest decay time of the 3M4PMI fluorescence, it can be deduced that 3M4PMI has a non-planar structure in the S1 state. The theoretical calculations on the S0 and S1 state double-proton transfer for the 4APM/AcOH dual hydrogen-bonded system were performed with the use of formic acid (FoOH) in place of AcOH for the sake of simplicity. Only one peak of transition state was resolved in the S0 and S1 states. The energy barrier for the S1 state double-proton transfer of the 4APM/FoOH complex-->3H-4(1H)-pyrimidinimine/FoOH tautomer was estimated to be approximately 2kJmol(-1) using the CIS/6-31G(d) methods. On the other hand, the energy barrier for the S0 state reverse proton transfer of the 3H-4(1H)-pyrimidinimine/FoOH tautomer-->4APM/FoOH complex was estimated to be almost zero kJmol(-1) at B3LYP/6-31G(d) level.     

Quantum Monte Carlo simulations of selected ammonia clusters (n = 2-5): isotope effects on the ground state of typical hydrogen bonded systems

Variational Monte Carlo, diffusion Monte Carlo, and stereographic projection path integral simulations are performed on eight selected species from the (NH(3))(n), (ND(3))(n), (NH(2)D)(n), and (NH(3))(n-1)(ND(3)) clusters. Each monomer is treated as a rigid body with the rotation spaces mapped by the stereographic projection coordinates. We compare the energy obtained from path integral simulations at several low temperatures with those obtained by diffusion Monte Carlo, for two dimers, and we find that at 4 K, the fully deuterated dimer energy is in excellent agreement with the ground state energy of the same. The ground state wavefunction for the (NH(3))(2-5) clusters is predominantly localized in the global minimum of the potential energy. In all simulations of mixed isotopic substitutions, we find that the heavier isotope is almost exclusively the participant in the hydrogen bond.