Effect of magnetic exchange,double exchange,vibronic coupling,and asymmetry on magnetic properties in d2–d3 mixed‐valence dimers

The effect of magnetic exchange, double exchange, vibronic coupling, and asymmetry on magnetic properties of d²–d³ systems is discussed. The temperature‐dependent magnetic moment was calculated with the semiclassical adiabatic approach. The results show that the vibronic coupling from the out‐of‐phase breathing vibration on the metal sites (Piepho, Krausz, and Schatz [PKS] model) and the vibronic coupling from the stretching vibration between the metal sites (P model) favor the localization and delocalization of the “extra” electron in mixed‐valence dimers, respectively. The magnetic properties are determined by the interplay among magnetic exchange, double exchange, and vibronic coupling. The results obtained by analyzing d²–d³ systems can be generalized to other full delocalized dinuclear mixed valence systems with a unique transferable electron. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2005     

Self‐Complementary Dimers of Oxalamide‐Functionalized Resorcinarene Tetrabenzoxazines

Self‐complementarity is a useful concept in supramolecular chemistry, molecular biology and polymeric systems. Two resorcinarene tetrabenzoxazines decorated with four oxalamide groups were synthesized and characterized. The oxalamide groups possessed self‐complementary hydrogen bonding sites between the carbonyls and amide groups. The self‐complementary nature of the oxalamide groups resulted in self‐included dimeric assemblies. The hydrogen bonding interactions within the tetrabenzoxazines gave rise to the formation of dimers, which were confirmed by single‐crystal X‐ray diffractions analysis and supported by NMR spectroscopy and mass spectrometry. The self‐included dimers were connected by numerous and strong intermolecular N?H???O and C?H???O hydrogen bonds supplemented with C?H???π interactions, forming one‐dimensional polymers, which were then further linked into three‐dimensional networks.     

Light‐Induced Self‐Assembly and Decay of J Aggregates of Thiamonomethinecyanine Dyes

Formation of J aggregates, that is, one‐dimensional supramolecular self‐organizations in which the transition moments of individual molecules are aligned parallel to the line joining their centers through a “head‐to‐tail” arrangement, normally proceed via electrostatic interactions between oppositely charged molecular groups; this is facilitated by an electrolyte medium. Here, we show that J aggregates of thiamonomethinecyanine dyes in a solution can be assembled from dye dimers by illuminating the solution with light of the appropriate wavelength to induce excitation of the dye dimers. The reverse process is also demonstrated by application of light of the correct wavelength to induce excitation of the J aggregates. Our results indicate that spontaneous J aggregation in the dark and formation of J aggregates through illumination proceed through different mechanisms; the former resulting in an increase in the number of the aggregates and the latter in an increase in the size of the aggregates.     

Molecular Mechanics Study of the Complexes of β-Cyclodextrin with 4-(dimethylamino)benzonitrile and Benzonitrile

Molecular Mechanics calculations with the Tripos Force Field were employed to study the complexation of 4-(dimethylamino)benzonitrile (DMABN) and/or benzonitrile (BN) with -cyclodextrin (CD). The systems studied have 1 : 1 (DMABN : CD and BN : CD), 2 : 2 (DMABN : CD) and 1 : 1 : 2 (DMABN : BN : CD) stoichiometries. Evidence for the formation of such complexes, binding constants and other thermodynamic parameters were extracted from the analysis of the steady state fluorescence measurements performed in a previous work. The Molecular Mechanics study, based on the energy changes upon guest-host approaching, was performed in vacuo and in the presence of water as a solvent. Results show that the driving forces for 1 : 1 complexation are mainly dominated by non-bonded van der Waals host : guest interactions. However, the driving forces for association between 1 : 1 complexes to give 2 : 2 homo- or 1 : 1 : 2 heterodimers are dominated by non-bonded electrostatic interactions. Head-to-head electrostatic interactions between CDs, which are presumably due to the hydrogen bonding formation between secondary hydroxyl groups of CDs, are responsible for most of the stability of the dimers.     

新型桥连双卟啉化合物的合成及结构表征

通过将4,4′-二羧基-2,2′-联吡啶、2,6-二溴甲基吡啶、2,6-二羟甲基吡啶和1,8-二氨基萘分别与5-(4-羟基苯基)-10,15,20-三苯基卟啉(1a)、5-(4-甲酰苯基)-10,15,20-三苯基卟啉(1b)和5-[4-(4′-溴代丁氧基)苯基]-10,15,20-三苯基卟啉(1c)反应,合成了3类新型的双卟啉化合物2a-2e,经IR,¹HNMR,MS,UV-Vis光谱及元素分析对中间体和目标化合物的结构进行了表征.     

Ready Access to Anhydrous Anionic Lanthanide Acetates by Using Imidazolium Acetate Ionic Liquids as the Reaction Medium

Access to lanthanide acetate coordination compounds is challenged by the tendency of lanthanides to coordinate water and the plethora of acetate coordination modes. A straightforward, reproducible synthetic procedure by treating lanthanide chloride hydrates with defined ratios of the ionic liquid (IL) 1-ethyl-3-methylimidazolium acetate ([C₂mim][OAc]) has been developed. This reaction pathway leads to two isostructural crystalline anhydrous coordination complexes, the polymeric [C₂mim]_n[{Ln₂(OAc)₇}_n] and the dimeric [C₂mim]₂[Ln₂(OAc)₈], based on the ion size and the ratio of IL used. A reaction with an IL : Ln-salt ratio of 5 : 1, where Ln=Nd, Sm, and Gd, led exclusively to the polymer, whilst for the heaviest lanthanides (Dy−Lu) the dimer was observed. Reaction with Eu and Tb resulted in a mixture of both polymeric and dimeric forms. When the amount of IL and/or the size of the cation was increased, the reaction led to only the dimeric compound for all the lanthanide series. Crystallographic analyses of the resulting salts revealed three different types of metal-acetate coordination modes where η²μκ² is the most represented in both structure types.     

Electropolymerization of [Ru(bpy)(CO)2Cl2] (bpy=2,2′-bipyridine: a revisited trans versus cis(Cl) isomeric influence study

The mono-bipyridine bis carbonyl complex [Ru(bpy)(CO)₂Cl₂] exists in two stereoisomeric forms having a trans(Cl)/cis(CO) (1) and cis(Cl)/cis(CO) (2) configuration. In previous work we reported that only the trans(Cl)/cis(CO) isomer 1 leads by a two-electron reduction to the formation of [Ru(bpy)(CO)₂]_n polymeric film on an electrode surface. This initial statement was overstated, as both isomers allowed the build up of polymers. A detailed comparison of the electropolymerization of both isomers is reported here, as well as the reduction into dimers of parent stereoisomer [Ru(bpy)(CO)₂(C(O)OMe)Cl] complexes 3 and 4 obtained as side products during the synthesis of 1 and 2.     

Syntheses,structures, and magnetic properties of dinuclear/1-D copper(II) acetato and formato derivatives with methylpyrazine and dimethylpyrazine

In this study, {[Cu₂(µ-HCO₂)₄](µ-Mepyrz)} _n (1), [Cu₂(µ-HCO₂)₄(Mepyrz)₂] (2), {[Cu₂(µ-AcO)₄](µ-Mepyrz)} _n (3), [Cu₂(µ-AcO)₄(Mepyrz)₂] (4), [Cu₂(µ-AcO)₄(2,3-Me₂pyrz)₂] (5), [Cu₂(µ-AcO)₄(2,6-Me₂pyrz)₂] (6), and {[Cu₂(µ-AcO)₄](µ-2,5-Me₂pyrz)} _n (7) have been synthesized and characterized by chemical analysis and electronic spectroscopy. Compounds 2, 4, 5, and 6, characterized by single-crystal X-ray diffraction, are composed of molecular dimers based on a paddle-wheel motif with two coppers, four syn–syn carboxylates, and two ligands coordinated to copper in the axial positions. In 7, chains of [Cu₂(µ-AcO)₄] dimers with 2,5-Me₂pyrz as bridging ligands are formed. Magnetic properties and electron paramagnetic resonance results of the compounds are also described.     

N-Hydroxyurea dimers: A matrix isolation and theoretical study

The dimerization of N-hydroxyurea (NH₂CONHOH) has been investigated by FTIR matrix isolation spectroscopy and DFT(B3LYP)/6-311++G(2d,2p) calculations. The analysis of NH₂CONHOH/Ar matrix spectra and comparison with theoretical ones reveal the formation of two types of dimers with a strong OH⋯O hydrogen bond. There is an additional weak interaction between the oxygen atom of the OH group of the proton donor molecule and the NH or NH₂ group of the proton acceptor in both dimers, respectively. The identified structures correspond to local minima on the PES. The formation of the less stable structures not the most stable ones indicates that the creation of N-hydroxyurea dimers is related to the dipole-dipole interaction at the initial stage of the dimerization process, which favours generation of polar dimers.