Copolymerization of butadiene and styrene with a gadolinium tricarboxylate catalyst

Homo- and copolymerizations of butadiene (BD) and styrene (St) were carried out by gadolinium catalysts having various tricarboxylate ligands [Gd(OCOR)₃: R = CH₃, CH₂Cl, CHCl₂, CCl₃, and CF₃], to investigate the effects of ligands and discuss the cis polymerization mechanism. Polymerization of BD with Gd(OCOR)₃—(i—Bu)₃Al—Et₂AlCl catalysts was carried out in hexane at 50°C. By each catalyst, poly(BD) having a high cis content (cis = 97–99%) in 22–85% yields for 2–24 h were obtained. The ligands with low pKa values increased the polymerization activity as follows: R of Gd(OCOR)₃: CF₃ > CCl₃ > CHCl₂ > CH₂Cl ～ CH₃. On the other hand, in the polymerization of St or copolymerization of BD and St under similar conditions, the highest activity was attained by a Gd(OCOCCI₃)₃- based catalyst. The difference in the optimum ligand among the homo- and copolymerization of BD and St was discussed on the basis of energy levels of the catalysts. In the copolymers of BD and St, the cis-1,4 content of the BD unit decreased with increasing St content. Furthermore, according to the diad analysis of copolymers (St content ～ 14.5 mol %) by ¹³C NMR spectroscopy, the low cis value of the BD unit was observed in the St-BD diad (cis/trans/vinyl = 24/53/23), while the high cis value of the BD unit remained in the BD-BD diad (cis/trans/vinyl = 89/10/1). These results suggest that the terminal BD unit is controlled by the cis configuration by the coordination between the penultimate cis vinylene unit and the gadolinium metal catalyst, whereas the presence of the penultimate St unit interferes with cis polymerization of the terminal BD unit. The difference in the coordination mechanism in the course of polymerization between rare earth metal and transition metal catalysts such as the Ni(acac)₂ and Co(acac)₃-based catalyst was also discussed. © 1995 John Wiley & Sons, Inc.     

Synthesis and structural characterization of ferrocene-based bis(pyrazol-1-yl)borate ligands: FcB(Me)pz2K, Fc2Bpz2K, and 1,1′-fc[B(Me)pz2]2K2 (Fc: ferrocenyl, fc: ferrocenylene, pz: pyrazolyl)

Reaction of the ferrocenyl(dimethylamino)boranes FcB(Me)NMe₂, Fc₂BNMe₂, and 1,1′-fc[B(Me)NMe₂]₂ with 1:1 mixtures of pyrazole and potassium pyrazolide in refluxing THF gave the potassium salts of the ferrocene-based bis(pyrazol-1-yl)borate ligands FcB(Me)pz₂K, Fc₂Bpz₂K, and 1,1′-fc[B(Me)pz₂]₂K₂ in good yield (Fc: ferrocenyl, fc: ferrocenylene, pz: pyrazolyl). In the solid state, FcB(Me)pz₂K and Fc₂Bpz₂K form centrosymmetric dimers with short K?Cp contacts suggesting an η⁵ coordination mode of the potassium ion. The crystal lattice of the ditopic ligand 1,1′-fc[B(Me)pz₂]₂K₂ consists of coordination polymer strands featuring essentially the same structural motif that has been observed for the monotopic derivatives. All three scorpionate ligands are thus promising building blocks for the preparation of ferrocene-containing multiple-decker sandwich complexes.     

Rigid aromatic fractal polyamides

The preparation of rigid aromatic, highly branched polyamides is described. Owing to the method of preparation and the chosen ratio of difunctional to trifunctional monomers, these entities are highly porous and not dendrimeric in nature. They better conform with the fractal model and are therefore called fractal polyamides (FPs). The effects of variations in the polymerization procedure, in total monomer concentration, in the ratio of amine to carboxyl groups and in the duration of the polycondensation reaction are investigated. Some characterization was performed and the results are presented and briefly discussed.     

Surface diffusion in reversed-phase liquid chromatography using silica gels bonded with C1 and C18 ligands of different densities

Surface diffusion in reversed-phase liquid chromatography (RPLC) using silica gels bonded with C₁ and C₁₈ alkyl ligands of different densities was studied from the viewpoints of two extrathermodynamic relationships, i.e., enthalpy-entropy compensation (EEC) and linear free energy relationship (LFER). First, according to the four methods proposed by Krug et al., the values of surface diffusion coefficient (D_s) were analyzed to confirm that an actual EEC resulting from substantial physico-chemical effects takes place for surface diffusion. Then, it was also demonstrated that a LFER is observed between surface diffusion and the retention equilibrium. The establishment of EEC and LFER suggests a mechanistic similarity of molecular migration by surface diffusion, irrespective of the alkyl chain length and the densities of C₁ and C₁₈ ligands. Finally, a thermodynamic model for the LFER based on the real EEC was used to estimate D_s values under various RPLC conditions. The D_s values can be estimated with a mean square deviation of about 25–30%. The agreement between the D_s values estimated and those experimentally measured suggests that the total mass flux by surface diffusion consists of the two contributions due to C₁ and C₁₈ ligands and that the contribution of each ligand is proportional to the ligand density.     

Synthesis and Characterization of a [Mn12O12(O2CR)16(H2O)4] Complex Bearing Paramagnetic Carboxylate Ligands. Use of a Modified Acid Replacement Synthetic Approach

Summary.  A new modified approach for the synthesis of Mn₁₂ clusters, based on the use of complex [Mn₁₂O₁₂(O₂C ^t Bu)₁₆(H₂O)₄] (2) as starting material to promote the acidic ligand replacement, is presented here. This new synthetic approach allowed us to obtain complex [Mn₁₂O₁₂(O₂CC₆H₄N(O^•) ^t Bu)₁₆(H₂O)₄] (3), whose preparation remained elusive by direct replacement of the acetate groups of Mn₁₂Ac (1). Complex 3 bearing open-shell radical units, was prepared to increase the total spin number of its ground state, and consequently, to increase T _B , with the expectation that the radical ligands may couple ferromagnetically with the Mn₁₂ core. Unfortunately, magnetic measurements of complex 3 revealed that the sixteen radical carboxylate ligands interact antiferromagnetically with the Mn₁₂ core to yield a S = 2 magnetic ground state. Corresponding author. E-mail: vecianaj@icmab.es Received March 27, 2002; accepted May 2, 2002     

Scoring functions: A view from the bench

Computational approaches to drug design are presently hindered by the complexity of the physical chemistry which underlies weak, non- covalent interactions between protein targets and small molecule ligands. Although a number of programs are now available for the design of novel potential ligands, it remains a key problem to rank these rapidly and reliably by estimated binding affinity. Such a step is necessary to select only the most promising candidates for synthesis and experimental characterisation. To calculate ligand affinity quickly and reliably is an extremely difficult problem, but it may well prove possible to estimate sufficiently accurately given an appropriate set of parameters to score individual protein–ligand interactions. Improvements in the situation will require a wider set of thermodynamically characterised systems than is currently available.     

(μ3-S)FeCo2(CO)7(dppfe)的合成和晶体结构

通过简单取代反应,合成了一个新的混金属簇合物(μ3-S)FeCo2(CO)7(dppfe) (2) (dppfe=Ph2PC5H4FeC5H4PPh2).利用IR,1H NMR,MS 和 X-ray单晶衍射的方法对簇合物2进行了结构表征.簇合物2的晶体属于三斜晶系,空间群为Pī.晶胞参数: a=1.132 4(15) nm, b=1.3670(17) nm, c=1.5769(2) nm,α=114.646(2)°,β=100.340(2)°,γ=100.113(3)°, V=2.0953(5) nm3.     

Influence of axial coordination on the photophysical property of excited state of zinc porphyrin

The phenomenon that metalloporphyrins are coordinated to axial ligands is very common especially in green-plant and photosynthetic bacteria, where such coordination impacts a signifi-cant effect on both the primary charge separation in the reaction center and energy transfer in an-tenna systems[1]. In addition, it plays a key role in photodynamic therapy and medicine synthesis in a sense that some metalloporphyrins have been assembled by coordination bonds to anti-cancer drugs, enhancing the se…     

VO（Ⅱ）—组氨酸—π受体三元混配配合物的ESR波谱

钒的氨基酸配合物具有重要的生物和医学意义。VO(Ⅱ)-组氨酸配合物及VO(Ⅱ)-π受体二元配合物的ESR研究已有报道。但是VO(Ⅱ)-氨基酸-π受体三元配合物的结构及其ESR研究至今未见报道。本文考察了VOSO_4与组氨酸(His)和邻菲啰啉(Phen)及α,α′-联吡啶(bipy)三元体系在不同酸度(pH=1～14)的乙二醇/水(1:1)溶液中低温(173K)ESR波谱,获得了不同pH条件下配合情况的详细信息。