Intramolecular "Aryl-metal chelate ring" pi,pi-interactions as structural evidence for metalloaromaticity in (aromatic alpha,alpha'-diimine)-copper(II) chelates: molecular and crystal structure of aqua(1,10-phenanthroline)(2-benzylmalonato)copper(II) three-hydrate

Schiff Base in Ketoamine Form and Rh(η4-cod)-Schiff Base Complex with Z' = 2 Structure from Pairwise C-H···Metallochelate-π Contacts

Condensation of 2-hydroxybenzaldehyde (salicylaldehyde) or 2-hydroxy-1-naphthaldehyde with 2-ethylaniline yields the Schiff base compound of (E)-2-(((2-ethylphenyl)imino)methyl)phenol (HL¹) or (E)-1-(((2-ethylphenyl)imino)methyl)naphthalen-2-ol (HL²), which in turn react with the dinuclear complex of [Rh(η⁴-cod)(µ-O₂CCH₃)]₂ (cod = cycloocta-1,5-diene) to afford the mononuclear (η⁴-cod){(E)-2-(((2-ethylphenyl)imino)methyl)phenolato-κ²N,O}rhodium(I), [Rh(η⁴-cod)(L¹)] (1) or (η⁴-cod){(E)-1-(((2-ethylphenyl)imino)methyl)naphthalen-2-olato-κ²N,O}rhodium(I), [Rh(η⁴-cod)(L²)] (2) (L¹ or L² = deprotonated Schiff base ligand). The X-ray structure determination revealed that the HL² exists in the solid state not as the usual (imine)N···H-O(phenol) form (enolamine form) but as the zwitterionic (imine)N-H⁺···^-O(phenol) form (ketoamine form). ¹H NMR spectra for HL² in different solvents demonstrated the existence of keto-enol tautomerism (i.e., keto ⇆ enol equilibrium) in solution. The structure for 1 and 2 showed that the deprotonated Schiff base ligand coordinates to the Rh(η⁴-cod)-fragment as a six-membered N^O-chelate around the rhodium atom with a close-to-square-planar geometry. Two symmetry-independent molecules (with Rh1 and Rh2) were found in the asymmetric unit in 1 in a structure with Z' = 2. The supramolecular packing in HL² was organized by π-π and C-H···π contacts, while only two recognized C-H···π contacts were revealed in 1 and 2. Remarkably, there were reciprocal or pairwise C-H···π contacts between a pair of each of the symmetry-independent molecules in 1. This pairwise C-H contact to the Rh-N^O chelate (metalloaromatic) ring may be a reason for the two symmetry-independent molecules in 1. Differential scanning calorimetry (DSC) analyses revealed an irreversible phase transformation from the crystalline-solid to the isotropic-liquid phase and subsequently confirmed the thermal stability of the compounds. Absorption spectra in solution were explained by excited state properties from DFT/TD-DFT calculations.

Schiff base ligands derived from 1,2-bis(2′-nitro-/amino-phenoxy)-3-R-benzene and 2-hydroxy-1-naphthaldehyde and their Cu/Zn(ii) complexes: synthesis, characterization, X-ray structures and computational studies

Racemic crystals of Cu/Zn(ii)-tetradentate Schiff base ligands with Λ/Δ-chirality induction at-metal center.

Enantio- and Site-Selective α-Fluorination of N-Acyl 3,5-Dimethylpyrazoles Catalyzed by Chiral π-CuII Complexes

Catalytic enantioselective α-fluorination reactions of carbonyl compounds are among the most powerful and efficient synthetic methods for constructing optically active α-fluorinated carbonyl compounds. Nevertheless, α-fluorination of α-nonbranched carboxylic acid derivatives is still a big challenge because of relatively high pK_a values of their α-hydrogen atoms and difficulty of subsequent synthetic transformation without epimerization. Herein we show that chiral copper(II) complexes of 3-(2-naphthyl)-l-alanine-derived amides are highly effective catalysts for the enantio- and site-selective α-fluorination of N-(α-arylacetyl) and N-(α-alkylacetyl) 3,5-dimethylpyrazoles. The substrate scope of the transformation is very broad (25 examples including a quaternary α-fluorinated α-amino acid derivative). α-Fluorinated products were converted into the corresponding esters, secondary amides, tertiary amides, ketones, and alcohols with almost no epimerization in high yield.

Recurrent π–π stacking motifs in three new 4,5-dihydropyrazolyl–thiazole–coumarin hybrids: X-ray characterization, Hirshfeld surface analysis and DFT calculations

Two different π–π stacking modes are described, studied and characterized in the crystal structures of 4,5-dihydropyrazolyl–thiazole–coumarin hybrids, including a partial aliphatic ring.

Four novel Zn(ii)/Cu(ii) coordination polymers containing hydroxyl groups: synthesis, crystal structure, luminescence sensing and photocatalysis properties

CPs 1, 3 and 4 show a 2D network, while 2 possesses a 3D network. 1 is a highly selective and sensitive bifunctional luminescent sensor towards Cu²⁺ and Cr₂O₇²⁻ ions. 3 and 4 exhibit photocatalytic activities for degradation of dyes (MB/MO).

Towards understanding π-stacking interactions between non-aromatic rings

The first systematic study of π interactions between non-aromatic rings, based on the authors' own results from an experimental X-ray charge-density analysis assisted by quantum chemical calculations, is presented. The landmark (non-aromatic) examples include quinoid rings, planar radicals and metal-chelate rings. The results can be summarized as: (i) non-aromatic planar polyenic rings can be stacked, (ii) interactions are more pronounced between systems or rings with little or no π-electron delocalization (e.g. quinones) than those involving delocalized systems (e.g. aromatics), and (iii) the main component of the interaction is electrostatic/multipolar between closed-shell rings, whereas (iv) interactions between radicals involve a significant covalent contribution (multicentric bonding). Thus, stacking covers a wide range of interactions and energies, ranging from weak dispersion to unlocalized two-electron multicentric covalent bonding ('pancake bonding'), allowing a face-to-face stacking arrangement in some chemical species (quinone anions). The predominant interaction in a particular stacked system modulates the physical properties and defines a strategy for crystal engineering of functional materials.

Modulation of coordination in pincer-type isonicotinohydrazone Schiff base ligands by proton transfer

We report the synthesis and X-ray characterization and theoretical study of two Hg(ii) complexes with pincer-type isonicotinohydrazone Schiff base ligands to analyze chelate-ring π-stacked assemblies.

Recurrent supramolecular motifs in discrete complexes and coordination polymers based on mercury halides: prevalence of chelate ring stacking and substituent effects

We report the synthesis, X-ray characterization and DFT study of five Hg(ii) complexes with Schiff bases containing a nicotinohydrazide core to explore the formation of chelate-ring π-stacking interactions.

Chelate ring stacking interactions in the supramolecular assemblies of Zn(ii)and Cd(ii) coordination compounds: a combined experimental and theoretical study

We report seven Zn(ii)/Cd(ii) complexes with picolinoyl/isonicotinoyl hydrazone based ligands exhibiting relevant chelate–π interactions.

Synthesis, structural characterization, cytotoxic properties and DNA binding of a dinuclear copper(II) complex

In this study a novel dinuclear copper(II) complex with adenine and phenanthroline has been synthesized and its structure determined by single crystal X-ray diffraction. In the dinuclear complex [Cu₂(μ-adenine)₂(phen)₂(H2O)2](NO3)4·0.5H2O (phen=1,10-phenanthroline) (1) the two Cu(II) centres exhibit a distorted square pyramidal coordination geometry linked by two nitrogen donors from adenine bridges leading to a Cu-Cu distance of 3.242(3)Å. Intramolecular and intermolecular π⋯π interactions as well as an H-bonding network were observed. The antitumor capacity of the complex has been tested in vitro against human cancer cell lines, cervical carcinoma (HeLa) and colorectal adenocarcinoma (Caco-2), by metabolic tests, using 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl-tetrazolium bromide as reagent. The complex 1 has remarkable low IC50 values of 0.87±0.06μM (HeLa) and 0.44±0.06μM (Caco-2), when compared with values for cisplatin against the same cell lines. The interaction of complex 1 with calf thymus DNA (CT DNA) was further investigated by absorption and fluorescence spectroscopic methods. A binding constant of 5.09×10(5)M(-1) was obtained from UV-vis absorption studies.

Synthesis, X-ray characterization, DFT calculations and Hirshfeld surface analysis of thiosemicarbazone complexes of Mn+ ions (n = 2, 3; M = Ni, Cd, Mn, Co and Cu)

Herein we report the synthesis, X-ray characterization, DFT calculations and Hirshfeld surface analysis of seven complexes of transition metal ions with pyridine-based thiosemicarbazone ligands.

Aliphatic–aromatic stacking interactions in cyclohexane–benzene are stronger than aromatic–aromatic interaction in the benzene dimer

Stacking interactions between cyclohexane and benzene were studied in crystal structures from the Cambridge Structural Database and by ab initio calculations.

Mixed ligand copper(ii) dicarboxylate complexes: the role of co-ligand hydrophobicity in DNA binding, double-strand DNA cleavage, protein binding and cytotoxicity

Mixed ligand 5,6-dmp and 3,4,7,8-tmp complexes display more prominent cytotoxicity and higher apoptotic inducing ability.

Hydrogen-bridged chelate ring-assisted π-stacking interactions

A salicylideneaniline (SA) derivative, (6Z)-6-({[2-(hydroxymethyl)phenyl]amino}methylidene)-3,5-dimethoxycyclohexa-2,4-dien-1-one monohydrate, has an increased aromaticity within its hydrogen-bridged chelate ring owing to its NH character. In the reported crystal structure, nonconventional π-stacking interactions, which are referred to as hybrid π-stacking interactions, are observed between a quasiaromatic chelate ring, formed as a result of the resonance-assisted intramolecular hydrogen bond and ordinary aromatic rings. Besides, π-stacking interactions are also seen between two hydrogen-bridged quasiaromatic chelate rings, which are referred to as pure π-stacking interactions. A CSD search has revealed that both kinds of interactions are frequently observed in molecular crystals of SA derivatives in fully or partially NH tautomeric form, and aromaticity levels of certain fragments of SA derivatives have dramatic effects on their stacking arrangements. These interactions are distinguished from the usual π...π interactions by their formation character, i.e. both σ- and π-deficient and σ-deficient character of pure interactions is more pronounced than that of the hybrid ones.

Complexation, computational, magnetic, and structural studies of the Maillard reaction product isomaltol including investigation of an uncommon π interaction with copper(II)

The metal complexation properties of the naturally occurring Maillard reaction product isomaltol HL(2) are investigated by measurement of its stability constants with copper(II), zinc(II), and iron(III) using potentiometric pH titrations in water, by structural and magnetic characterization of its crystalline complex, [Cu(L(2))(2)]·8H(2)O, and by density functional theory calculations. Strong complexation is observed to form the bis(isomaltolato)copper(II) complex incorporating copper in a typical (pseudo-)square-planar geometry. In the solid state, extensive intra- and intermolecular hydrogen bonding involving all three oxygen functions per ligand assembles the complexes into ribbons that interact to form two-dimensional arrays; further hydrogen bonds and π interactions between the furan moiety of the anionic ligands and adjacent copper(II) centers connect the complexes in the third dimension, leading to a compact polymeric three-dimensional (3D) arrangement. The latter interactions involving copper(II), which represent an underappreciated aspect of copper(II) chemistry, are compared to similar interactions present in other copper(II) 3D structures showing interactions with benzene molecules; the results indicate that dispersion forces dominate in the π system to chelated copper(II) ion interactions.

Concentration dependent tautomerism in green [Cu(HL¹)(L²)] and brown [Cu(L¹)(HL²)] with H₂L¹ = (E)-N'-(2-hydroxy-3-methoxybenzylidene)benzoylhydrazone and HL² = pyridine-4-carboxylic (isonicotinic) acid

The in situ formed hydrazone Schiff base ligand (E)-N'-(2-hydroxy-3-methoxybenzylidene)benzoylhydrazone (H₂L¹) reacts with copper(II) acetate in ethanol in the presence of pyridine-4-carboxylic acid (isonicotinic acid, HL²) to green-[Cu(HL¹)(L²)]·H₂O·C₂H₅OH (1) and brown-[Cu(L¹)(HL²)] (2) complexes which crystallize as concomitant tautomers where either the mono-anion (HL¹)⁻ or di-anion (L¹)²⁻ of the Schiff base and simultaneously the pyridine-carboxylate (L²)⁻ or the acid (HL²) (both through the pyridine nitrogen atom) function as ligands. The square-planar molecular copper(II) complexes differ in only a localized proton position either on the amide nitrogen of the hydrazone Schiff base in 1 or on the carboxyl group of the isonicotin ligand in 2. The proportion of the tautomeric forms in the crystalline solid-state can be controlled over a wide range from 1:2 = 95 : 5 to ∼2 : 98 by increasing the solution concentration. UV/Vis spectral studies show both tautomers to be kinetically stable (inert), that is, with no apparent tautomerization, in acetonitrile solution. The UB3LYP/6-31+G* level optimized structures of the two complexes are in close agreement with experimental findings. The solid-state structures feature 1D hydrogen-bonded chain from charge-assisted O((-))H-N and O-H((-))N hydrogen bonding in 1 and 2, respectively. In 1 pyridine-4-carboxylate also assumes a metal-bridging action by coordinating a weakly bound carboxylate group as a fifth ligand to a Cu axial site. Neighboring chains in 1 and 2 are connected by strong π-stacking interactions involving also the five- and six-membered, presumably metalloaromatic Cu-chelate rings.