Tunable intramolecular multicenter H-bonding interactions in first-row metal complexes bearing bidentate redox-active ligands

Abstract

In this research article, we report the synthesis and structural characterization of a family of first-row metal complexes bearing redox-active ligands with tunable H-bonding donors. We observed that these coordination complexes can adopt three different geometries and that they are stabilized by intramolecular multicenter H-bonding interactions, which are systematically modified by changing the metal ion (Co, Ni, Cu, Zn), the ligand scaffold (variations in the diamine and ureanyl substituents used) and the solvent of crystallization.     

Intermolecular Hydrogen Bonding and Dimeric Structures in Bilirubin Esters from 1H-NMR Spectroscopy

Analysis of the N-H ¹H-NMR chemical shifts of various bilirubin dimethyl esters indicates intermolecularly H-bonded dimers in CDCl₃ solution with pairwise pyrromethenone associations. The presence of an exo (C-2 or C-18) vinyl or ethyl group on the lactam ring leads to relatively weaker H-bonding as compared with an endo (C-3 or C-17) vinyl or ethyl group. Consequently, in a non-symmetric bilirubin, e.g. IXα, the endo-vinylpyrromethenone to endo-vinylpyrromethenone intermolecular H-bonding becomes much stronger (with more deshielded N-H proton chemical shifts) than in corresponding symmetric isomers, e.g. XIIIα or IIIα.     

巴比妥酸楔形棒状液晶分子的合成、性质及与三嗪衍生物的氢键识别组装体

合成了一端含巴比妥酸, 另一端含带三条烷基链苯甲酸酯楔形单元, 中间为苯乙烯共轭单元的棒状分子5-{4-[3,4,5-三(十二烷氧基)苯甲酸酯]苯乙烯}-(1H,3H)-2,4,6嘧啶三酮(BA³/12); 采用偏光显微镜(POM)、 示差扫描量热法(DSC)、 X射线衍射(XRD)、 扫描电子显微镜(SEM)、 红外光谱(IR)、 核磁共振波谱(NMR)和荧光光谱等对其进行了表征. 结果表明, BA³/12能自组装形成晶格结构为c2mm的长方柱相液晶; 红外光谱及核磁共振氢谱表征数据初步证实了BA³/12与三嗪化合物6-(5-{4-[3,4-双(十二烷氧基)]苄氧基}苯基)噻吩-2,4-二氨基-1,3,5三嗪(1T²/12)形成的等摩尔氢键复合物BA/T的结构; BA/T不仅具有液晶性质且能形成具有三维网络状结构的超分子凝胶.     

A self assembled 3-D network propagated by coordination polymerization and H-bonding: synthesis and X-ray crystal structure of [{Co(L)2(H2O)2}(ClO4)2(CH3COCH3)2(H2O)2] n,where L = N,N-diisopropylisonicotinamide

Reaction of Co(ClO₄)₂?·?6H₂O with N,N-diisopropylisonicotinamide (L) has yielded a 1-D coordination polymer [{Co(L)₂(H₂O)₂}(ClO₄)₂(CH₃COCH₃)₂(H₂O)₂] _n (1). Complex 1 has been characterized by infrared (IR) and UV-Vis spectroscopies, thermal analysis, and single crystal X-ray diffraction techniques. The structure has alternate arrangement of parallel 1-D cationic metal-ligand chains and H-bonded anionic chains containing perchlorate, acetone and water in the lattice. Further hydrogen bonding among both chains leads to formation of 2-D networks along almost perpendicular planes. Interpenetrations of such perpendicular 2-D sheets create a 3-D supramolecular structure.     

Studies on the Halogen Substituted β-Amino Acids and Their Cu(Ⅱ) Coordination Complexes in Crystallography

Halogen substituted β-amino acids, D,L-3-amino-3-(4-fluoro)phenylpropionic acid(D,L-HL1, 1) and D,L-3-amino-3-(4-bromo)phenylpropionic acid(D,L-HL2, 2), as well as their Cu(Ⅱ) coordination complexes[Cu(L1)₂(CH₃OH)₂]·2CH₃OH(3) and[Cu(L2)₂(CH₃OH)₂]·2CH₃OH(4) were investigated and their single crystal structures were discussed in details. Supramolecular helical chains were found in β-amino acids 1 and 2 while there was no helix in their coordination complexes 3 and 4. The formation of supramolecular helixes could be due to the hydrogen bonds between terminal-NH³⁺ and adjacent-COO^- in β-amino acids 1 and 2. While, this kind of hydrogen bonds could not be observed in their Cu(Ⅱ) coordination complexes 3 and 4, in which central-symmetrical dimers could be formed via coplanar coordinated bonds(N-Cu-O) between-NH₂ and-COO^-.     

Syntheses and structures of two new lithium-heptamolybdates

The synthesis, crystal structures, IR, UV–vis, ⁷Li NMR spectra, electrochemical investigations, and conductivity studies of two new lithium-heptamolybdates, (NH₄)₄[Li₂(H₂O)₇][Mo₇O₂₄]·H₂O (1) and (NH₄)₃[Li₃(H₂O)₄(μ₆-Mo₇O₂₄)]·2H₂O (2), are reported. In 1 the (NH₄)⁺ and [Li₂(H₂O)₇]²⁺, cations are charge balanced by the heptamolybdate anion. In 2, the [Mo₇O₂₄]^6? anion is coordinated to three unique Li⁺ ions via a μ₆-hexadentate-binding mode resulting in the formation of a two-dimensional (2-D) [Li₃(H₂O)₄(μ₆-Mo₇O₂₄)]^3? anionic complex, charge neutralized by three (NH₄)⁺ ions. The cations, anions, and the lattice water molecules in 1 and 2 are linked by weak H-bonding interactions.     

Taming Chiral Quaternary Stereocenters via Remote H-Bonding Stereoinduction in Palladium-Catalyzed (3+2) Cycloadditions

Ring-opening transformations of donor-acceptor (D-A) cyclopropanes enable the rapid assembly of complex molecules. However, the enantioselective formation of chiral quaternary stereocenters using substrates bearing two different acceptors remains a challenge. Herein, we describe the first palladium-catalyzed highly diastereo- and enantioselective (3+2) cycloaddition of vinyl cyclopropanes bearing two different electron-withdrawing groups, a subset of D-A cyclopropanes. The key to the success of this reaction is the remote stereoinduction through hydrogen bond from chiral ligands, which thereby addressed the aforementioned challenge. A variety of chiral five-membered heterocycles were produced in good yields and with high stereoselectivity (up to 99 % yields, 99 : 1 er and >19 : 1 dr). In-depth mechanistic investigations, including control experiments and theoretical calculations, revealed the origin of the stereoselectivity and the importance of H-bonding in stereocontrol.     

Theoretical investigation (DFT and MP2) of the intermolecular proton transfer in the supersystems uracil-(H2O) n and uracil-(CH3OH) n (n = 1, 2)

Twelve binary and eight ternary H-bonded systems between uracil and water/methanol were investigated at the B3LYP and MP2 theoretical levels using 6 − 31 + G(d) basis functions. The binary and ternary systems that contain the hydroxo-uracil tautomer H-bonded with water and methanol were found to be the most stable complexes. The calculated energy barriers of the intermolecular proton exchange showed that the methanol molecule provokes larger reduction of the energy barrier of the intermolecular proton exchange reactions than the water molecule. Correspondence: Vassil B. Delchev, Department of Physical Chemistry, University of Plovdiv, Plovdiv, Bulgaria     

Intramolecular Hydrogen Bonding in Bilirubin Amides as Detected by Circular Dichroism Spectroscopy

Analysis of the circular dichroism spectra of bilirubin-IXα amides of S'-(-)-1 -phenylpropylamine reveals a preference in organic solvents for folded, intramolecularly H-bonded conformations involving the amide N-H hydrogen.     

Intramolecular Hydrogen Bonding in Bilirubin Monomethyl Esters Determined by 1H-NMR Spectroscopy

Analysis of the N-H ¹H-NMR chemical shifts of the monomethyl esters of bilirubin-IIIα, IXα and XIIIα reveals a preference in CDCl₃ for folded conformations in which the unesterified propionic acid carboxyl group is intramolecularly H-bonded to the opposing pyrromethenone unit. The remaining pyrromethenone unit may be weakly intramolecularly H-bonded to the propionic ester carbomethoxy group.