Double‐stranded copper(II) string complexes of varying nuclearity, from di‐ to tetranuclear species, have been prepared by the CuII‐mediated self‐assembly of a novel family of linear homo‐ and heteropolytopic ligands that contain two outer oxamato and either zero ( 1 b ), one ( 2 b ), or two ( 3 b ) inner oxamidato donor groups separated by rigid 2‐methyl‐1,3‐phenylene spacers. The X‐ray crystal structures of these CuIIn complexes (n=2 ( 1 d ), 3 ( 2 d ), and 4 ( 3 d )) show a linear array of metal atoms with an overall twisted coordination geometry for both the outer CuN2O2 and inner CuN4 chromophores. Two such nonplanar all‐syn bridging ligands 1 b – 3 b in an anti arrangement clamp around the metal centers with alternating M and P helical chiralities to afford an overall double meso‐helicate‐type architecture for 1 d – 3 d . Variable‐temperature (2.0–300 K) magnetic susceptibility and variable‐field (0–5.0 T) magnetization measurements for 1 d – 3 d show the occurrence of S=nSCu (n=2–4) high‐spin ground states that arise from the moderate ferromagnetic coupling between the unpaired electrons of the linearly disposed CuII ions (SCu=1/2) through the two anti m‐phenylenediamidate‐type bridges (J values in the range of +15.0 to 16.8 cm?1). Density functional theory (DFT) calculations for 1 d – 3 d evidence a sign alternation of the spin density in the meta‐substituted phenylene spacers in agreement with a spin polarization exchange mechanism along the linear metal array with overall intermetallic distances between terminal metal centers in the range of 0.7–2.2 nm. Cyclic voltammetry (CV) and rotating‐disk electrode (RDE) electrochemical measurements for 1 d – 3 d show several reversible or quasireversible one‐ or two‐electron steps that involve the consecutive metal‐centered oxidation of the inner and outer CuII ions (SCu=1/2) to diamagnetic CuIII ones (SCu=0) at relatively low formal potentials (E values in the range of +0.14 to 0.25 V and of +0.43 to 0.67 V vs. SCE, respectively). Further developments may be envisaged for this family of oligo‐m‐phenyleneoxalamide copper(II) double mesocates as electroswitchable ferromagnetic ‘metal–organic wires’ (MOWs) on the basis of their unique ferromagnetic and multicenter redox behaviors. 相似文献
We present a method to produce anti‐fouling reverse osmosis (RO) membranes that maintains the process and scalability of current RO membrane manufacturing. Utilizing perfluorophenyl azide (PFPA) photochemistry, commercial reverse osmosis membranes were dipped into an aqueous solution containing PFPA‐terminated poly(ethyleneglycol) species and then exposed to ultraviolet light under ambient conditions, a process that can easily be adapted to a roll‐to‐roll process. Successful covalent modification of commercial reverse osmosis membranes was confirmed with attenuated total reflectance infrared spectroscopy and contact angle measurements. By employing X‐ray photoelectron spectroscopy, it was determined that PFPAs undergo UV‐generated nitrene addition and bind to the membrane through an aziridine linkage. After modification with the PFPA‐PEG derivatives, the reverse osmosis membranes exhibit high fouling‐resistance.
Pd(OAc)(2)/3 is an efficient catalyst system for the base-free oxidative Heck reaction that outperforms the currently available catalysts for the more challenging substrates studied. The catalyst system is highly selective, and works at room temperature with dioxygen as the oxidant. 相似文献
Two new heterobimetallic manganese(II)-copper(II) coordination polymers of formulas [Mn(2)Cu(2)(Me(3)mpba)(2)(H(2)O)(6)]·8H(2)O (1) and [Mn(2)Cu(2)(Me(4)ppba)(2)(H(2)O)(6)]·8H(2)O (2) [Me(3)mpba = 2,4,6-trimethyl-N,N'-1,3-phenylenebis(oxamate) and Me(4)ppba = 2,3,5,6-tetramethyl-N,N'-1,4-phenylenebis(oxamate)] have been synthesized following a molecular-programmed self-assembly method from the corresponding dicopper(II) complexes acting as metalloligands toward Mn(II) ions. 1 and 2 consist of neutral Mn(II)(2)Cu(II)(2) layers with a brick-wall structure made up of oxamato-bridged Mn(II)Cu(II) chains connected through double meta- (1) and para-substituted (2) permethylated phenylene spacers. Overall magnetic (1) and nonmagnetic (2) layer ground states result from the ferro- and antiferromagnetic interchain interactions between the oxamato-bridged Mn(II)Cu(II) ferrimagnetic chains across m- and p-phenylene spacers, respectively. Interestingly, compound 1 exhibits a long-range ferromagnetic ordering with a rather high Curie temperature (T(C)) of 20.0 K. 相似文献
An irreversible Carnot cycle engine operating as a closed system is modeled using the Direct Method and the First Law of Thermodynamics for processes with Finite Speed. Several models considering the effect on the engine performance of external and internal irreversibilities expressed as a function of the piston speed are presented. External irreversibilities are due to heat transfer at temperature gradient between the cycle and heat reservoirs, while internal ones are represented by pressure losses due to the finite speed of the piston and friction. Moreover, a method for optimizing the temperature of the cycle fluid with respect to the temperature of source and sink and the piston speed is provided. The optimization results predict distinct maximums for the thermal efficiency and power output, as well as different behavior of the entropy generation per cycle and per time. The results obtained in this optimization, which is based on piston speed, and the Curzon–Ahlborn optimization, which is based on time duration, are compared and are found to differ significantly. Correction have been proposed in order to include internal irreversibility in the externally irreversible Carnot cycle from Curzon–Ahlborn optimization, which would be equivalent to a unification attempt of the two optimization analyses. 相似文献
Cyclopropyl metal carbenes formed in the 6-endo-dig cyclization may evolve to form seven-membered ring intermediates. This has been achieved for the first time by using highly electrophilic platinum(II) and gold(I) complexes. Gold(I) also triggers a remarkable rearrangement of certain enynes leading to complex cyclic systems. A new cationic platinacycle is described, which catalyzes skeletal rearrangements and other reactions of enynes at room temperature. 相似文献
Dispersive liquid–liquid microextraction is a novel miniaturized sample pre-treatment technique, which has been used successfully for the extraction of organic and inorganic compounds. This paper reviews the applications of this new procedure for sample preparation.
We herein present the preparation, crystal structure, magnetic properties, and theoretical study of new heterobimetallic chains of formula {[Fe(III)(bpym)(CN4)]2M(II)(H2O)2}.6H2O [bpym = 2,2'-bipyrimidine; M = Zn (2), Co (3), Cu (4), and Mn (5)] which are obtained by using the building block PPh4[Fe(bpym)(CN)4].H2O (1) (PPh4+= tetraphenylphosphonium) as a ligand toward the fully solvated MII ions. The structure of complex 1 contains mononuclear [Fe(bpym)(CN)4]- anions. Compounds 2-5 are isostructural 4,2-ribbonlike bimetallic chains where the [Fe(bpym)(CN)4]- unit acts as a bis-monodenate ligand through two of its four cyanide ligands toward the M atom. Water hexamer clusters (4) and regular alternating fused six- and four-membered water rings with two dangling water molecules (2, 3, and 5) are trapped between the cyanide-bridged 4,2-ribbonlike chains. 1 and 2 behave as magnetically isolated low-spin iron(III) centers. 3 behaves as a single-chain magnet (SCM) with intrachain ferromagnetic coupling, slow magnetic relaxation, hysteresis effects, and frequency-dependent ac signals at T < 7 K). As expected for a thermally activated process, the nucleation field (Hn) in 3 increases with decreasing T and increasing v. Below 1.0 K, Hn becomes temperature independent but remains strongly sweep rate dependent. In this temperature range, the reversal of the magnetization may be induced by a quantum nucleation of a domain wall that then propagates due to the applied field. 4 and 5 are ferro- and ferrimagnetic chains respectively, with metamagnetic-like behavior (4). DFT-type calculations and QMC methodology provided a good understanding of the magnetic properties of 3-5. 相似文献