Coordination motifs in modern supramolecular chemistry

Engineered π⋯π interactions favour supramolecular dimers X@[FeL3]2 (X = Cl, Br, I): solid state and solution structure

Ditopic bis-pyrazolylpyridine ligands usually react with divalent metal ions (M2+) to produce dinuclear triple-stranded helicates [M2L3]4+ or, via π⋯π interactions, dimers of monoatomic complexes ([ML3]2)4+. The introduction of an additional benzene ring at each end of ligand L increases the number of aromatic contacts within the supramolecular aggregate by 40%, driving the self-recognition process in an irreversible manner. Consequently, the mixing of new bis-pyrazolylquinoline L2 with FeX2 salts leads to crystallization of the tripartite high-spin assemblies (X@[Fe(L2)3]2)3+ (X = Cl, Br or I). The aggregates exhibit exceptional stability, as confirmed by a combination of paramagnetic 1H NMR techniques, demonstrating their persistence in solution. Our investigations further reveal that the guests Br- and I- are retained inside the associate in solution but Cl- is immediately released, resulting in the formation of the empty supramolecular dimer ([Fe(L2)3]2)4+.

Ancillary-Ligand-Assisted Variation in Nuclearities Leading to the Formation of Di-, Tri-, and Tetranuclear Copper(II) Complexes with Multifaceted Carboxylate Coordination Chemistry

The self-assembly of a carboxylate-based dinucleating ligand, N,N'-bis[2-carboxybenzomethyl]-N,N'-bis[2-pyridylmethyl]-1,3-diaminopropan-2-ol (H₃cpdp), and copper(II) ions in the presence of various exogenous ancillary ligands results in the formation of the new dinuclear complex [Cu₂(cpdp)(μ-Hisophth)]₄·2H₂isophth·21H₂O (1), trinuclear complex [Cu₃(Hcpdp)(Cl)₄] (2), and tetranuclear complex [Cu₄(cpdp)(μ-Hphth)(μ₄-phth)(piconol)(Cl)₂]·3H₂O (3) (H₂phth = phthalic acid; H₂isophth = isophthalic acid; piconol = 2-pyridinemethanol; Cl^- = chloride). In methanol-water, the reaction of H₃cpdp with CuCl₂·2H₂O at room temperature leads to the formation of 2. On the other hand, 1 and 3 have been obtained by carrying out the reaction of H₃cpdp with CuCl₂·2H₂O/m-C₆H₄(CO₂Na)₂ and CuCl₂·2H₂O/o-C₆H₄(CO₂Na)₂/piconol, respectively, in methanol-water in the presence of NaOH at ambient temperature. All three complexes have been characterized by elemental analysis, molar electrical conductivity and magnetic moment measurements, FTIR, UV-vis spectroscopy, and PXRD, including single-crystal X-ray structural analyses. The molecular structure of 1 is based on a μ-alkoxide and μ-isophthalate-bridged dimeric [Cu₂] core; the structure of 2 represents a trimeric [Cu₃] core in which a μ-alcohol-bridged dinuclear [Cu₂] unit is exclusively coupled with a [CuCl₂] species by two μ:η¹:η¹-syn-anti carboxylate groups forming a triangular motif; the structure of 3 embodies a tetrameric [Cu₄] core, with two copper(II) ions in a distorted-octahedral coordination environment, one copper(II) ion in a distorted-trigonal-bipyramidal coordination environment, and the other copper(II) ion in a square-planar coordination environment. In fact, 2 and 3 represent rare examples of copper(II)-based multinuclear complexes showing outstanding features of rich coordination chemistry: (i) using a symmetrical dinucleating ligand, trinuclear complex 2 is generated with four- and five-coordination environments around copper(II) ions; (ii) the unsymmetrical tetranuclear complex 3 is obtained by using the same ligand with four-, five- and six-coordination environments around copper(II) ions; (iii) tetracopper(II) complex 3 shows four different bridging modes of carboxylate groups simultaneously such as μ:η², μ:η¹:η¹, μ₃:η²:η¹:η¹, and μ₄:η¹:η¹:η¹:η¹, the μ₄:η¹:η¹:η¹:η¹ mode of phthalate being unprecedented. The formation of these [Cu₂], [Cu₃], and [Cu₄] complexes can be controlled by changing the exogenous ancillary ligands and pH of the reaction solutions, thus allowing an effective tuning of the self-assembly. The magnetic susceptibility measurements suggest that the copper centers in all three complexes are antiferromagnetically coupled. The thermal properties of 1-3 have been investigated by thermogravimetric and differential thermal analytical (TGA and DTA) techniques, indicating that the decomposition of all three complexes proceeds via multistep processes.

Joining of Trinuclear Heterometallic CuII2-MII (M = Mn, Cd) Nodes by Nicotinate to Form 1D Chains: Magnetic Properties and Catalytic Activities

In the present work, four new heterometallic coordination complexes, {[(CuL)₂Mn(nic)(H₂O)₂](ClO₄)(0.5H₂O)}_n (1), {[(CuL)₂Cd(nic)(H₂O)₂](ClO₄)(H₂O)}_n (2), [(CuL)₂Mn(nic)₂]·2CH₃OH (3), and [(CuL)₂Cd(nic)₂]·2CH₃OH (4) (where H₂L = N,N'-bis(α-methylsalicylidene)-1,3-propanediamine and nic = nicotinate ion), have been synthesized and characterized by single-crystal X-ray crystallography. In complexes 1 and 2, the nicotinate ion acts as a bifunctional linker (N,O donor) and joins the linear trinuclear nodes to form 1D polymeric chains. However, in complexes 3 and 4, the nicotinate ion uses only the oxygen atoms of the carboxylic acid (O donor) to bind to the metal centers, forming discrete linear trinuclear units, while the pyridyl nitrogen (N donor atom) remains free. The dc magnetic susceptibility measurements show that the Cu^II and Mn^II ions are antiferromagnetically coupled in both 1 and 3, with exchange coupling constants (J_Mn-Cu) of -20.57 ± 0.08 and -9.38 ± 0.08 cm^-1, respectively. Among the four complexes, 1 and 3 show catechol oxidase and phenoxazinone synthase like catalytic activities. The turnover numbers (k_cat) of complexes 1 and 3 for catecholase activity are 1121 and 720 h^-1, respectively, at an optimum pH of 8.0 and for phenoxazinone synthase activity are 429 and 398 h^-1, respectively, at an optimum pH of 9.7. The higher k_cat values of 1 for both reactions are attributable to a water molecule coordinated to the central Mn^II atom that facilitates the substrate-catalyst binding. An ESI-mass spectral analysis indicates that trinuclear heterometallic species, e.g., [(CuL)₂Mn(nic)(H₂O)]⁺ for 1 and [(CuL)₂Mn(nic)]⁺ for 3, are the active species that bind to the substrate, and on that basis, probable mechanisms through the formation of radical intermediates have been proposed.

Allan White and Solvento/Aqua Complexes: ScIII Solvation

Addition of various oxygen-donor ligands (L, all dipolar, aprotic solvents), to a solution of hydrated scandium picrate in weakly coordinating solvents (S), has yielded several crystalline adducts of the form Sc(pic)3(L)m(·nS) in which all ligands L are coordinated, subsequently characterised by a series of single crystal X-ray studies. For L=dmso, m=3 and the picrate anions are all bound as phenoxide-O donor units, while for nmp, dma, tmp, and hmpa, m=2 and one of the bound picrates becomes bidentate through phenoxide- and nitro-O donation. For L=ompa, m=2 and two picrate ligands are bound through phenoxide-O while one is displaced from the primary coordination sphere. All complexes contain six-coordinate ScIII, confirming that this coordination number is a consequence of ligand bulk.

Cross-π-conjugated enediyne with multitopic metal binding sites

The synthesis of an enediyne molecule functionlized with different metal coordination sites in a cross-π-conjugated fashion is reported. Using Pd-mediated cross-coupling reactions, 2,2′-bipyridine units were attached at the periphery of diazafluorenemethylidene to obtain a multitopic ligand. UV-vis spectrosopic investigations along with electrochemical analyses reveal electronic communication along the conjugated path reflected in red-shifted absorption spectra and shifts of reduction potentials. The properties of the ligand could be manipulated by coordinating [Ru(bpy)₂]²⁺ fragments at all three coordination spheres of the molecule while the different complexing imine moieties serve as possible coordination sites for various metal centres.

Synthesis and structural characterization of a geminal enediyne molecule with three imine metal binding sites based on diazafluorenemethylidene in a cross-π-conjugated fashion is reported.

Self-assembly of cucurbiturils and cyclodextrins to supramolecular millstones with naphthalene derivatives capable of translocations in the host cavities

Naphthylpyridine derivatives and cavitands form inclusive complexes of different structure and stoichiometry; the guests are capable of translocations within cucurbit[7,8]uril cavities.

Functional Supramolecular Architectures of Dipyrrin Complexes

Dynamic formation of self-assemblies from molecular components is a useful and efficient way to produce molecular and supramolecular architectures with sophisticated functions. The labile coordination bond and dynamic covalent bond as a reversible bond have often been used to create a well-organized supramolecular self-assembly. In order to realize sophisticated novel functions of the supramolecular self-assemblies, dipyrrin complexes have recently been employed as a functional unit and incorporated into the supramolecular architectures because of their outstanding properties and functions such as a high photostability and strong light absorption/emission. This review article summarizes recent development in functional supramolecular architectures of the dipyrrin complexes produced by coordination to a metal ion and dynamic covalent bond formation. We first describe the synthesis and unique functions of a series of discrete supramolecular architectures: helicates, macrocycles, and cages. The polymeric supramolecular self-assemblies with 1D, 2D, and 3D structures are then introduced as a functional infinite supramolecular architecture.

Creating capsules with cubanes

A sextuply linked capsule incorporating two Ni(ii) cubane units is formed when Ni(ii) salts are reacted with N-aminoalkyl salicylamidato ligands in a 4 : 3 ratio in methanol. With a short (propyl) alkyl substituent, additional chloride counter ions are included in the cavity by occupation of one apex of each of the linked cubanes, while with a longer (pentyl) substituent, perchlorate and tetrafluoroborate anions are included, probably by H-bonding involving hydroxyl groups on the inner apices of the cubanes, indicating that the cavity can be regarded as suited to binding of hydrophilic units despite the lipophilic character of its links. The dicubane capsules show ferromagnetic behaviour typical of Ni(ii) cubanes in general, with coupling constants dependent upon the nature of the salt-derived counter anions.

Joining of trinuclear (CuL)2M (M = MnII and CdII) nodes by 1,3- and 1,4-benzenedicarboxylate linkers: positional isomeric effect on co-crystallization

Among the four one-dimensional coordination polymers obtained by joining (CuL)₂M (M = Mn^II and Cd^II) nodes with 1,3- and 1,4-benzenedicarboxylate linkers, the chains derived from 1,3-benzenedicarboxylate are co-crystallized with dimeric Cu^II units.

Tailoring the local environment around metal ions: a solution chemical and structural study of some multidentate tripodal ligands

Manganese(ii), copper(ii) and zinc(ii) complexes of four polydentate tripodal ligands (tachpyr (N,N',N''-tris(2-pyridylmethyl)-cis,cis-1,3,5-triaminocyclohexane), trenpyr (tris[2-(2-pyridylmethyl)aminoethyl]amine, tach3pyr (N,N',N''-tris(3-pyridylmethyl)-cis,cis-1,3,5-triaminocyclohexane) and tren3pyr (tris[2-(2-pyridylmethyl)aminoethyl]amine)) were characterized in both solution and solid states. A combined evaluation of potentiometric, UV-VIS, NMR and EPR data allowed the conclusion of both thermodynamic and structural information about the complexes formed in solution. The four tailored polydentate tripodal ligands studied here exhibit a high thermodynamic stability, and a variety of coordination environments/geometries for the studied transition metal ions. Our data indicate that tachpyr is a more efficient zinc(ii) chelator and a similar copper(ii) chelator compared to trenpyr. Considering the higher number of N-donors and conformational flexibility of trenpyr, as well as the energy demanding switch to the triaxial conformation required for metal ion binding of tachpyr, the above observation is surprising and is very likely due to the encapsulating effect of the more rigid tachpyr skeleton. This relative binding preference of tachpyr for zinc(ii) may be related to the observation that zinc(ii) is one of the principal metals targeted by tachpyr in cells. In contrast, trenpyr is a considerably more efficient manganese(ii) chelator, since it acts as a heptadentate ligand in the aqueous Mn(trenpyr) complex. The crystal structures of copper(ii) and zinc(ii) complexes of tachpyr indicated important differences in the ligand conformation, induced by the position of counter ions, as compared to earlier reports. The closely related new ligands, tach3pyr and tren3pyr, have been designed to form oligonuclear complexes. Indeed, we obtained a three dimensional polymer with a copper(ii)/tren3pyr ratio of 11/6. Within this metal-organic framework, three distinctly different copper geometries can be identified: square pyramidal, trigonal bipyramidal and tetrahedral. Two square pyramidal and four trigonal bipyramidal copper centres create a hexanuclear subunit with a large inside cavity. These moieties are linked by tetrahedral copper(ii) centres, constructing the three-dimensional polymer structure. The formation of such polynuclear complexes was not detected in solution. Both tach3pyr and tren3pyr form only mononuclear complexes with square pyramidal and trigonal bipyramidal geometries, respectively.

C i -Symmetry, [2 × 2] grid, square copper complex with the N4,N5-bis(4-fluorophenyl)-1H-imidazole-4,5-dicarboxamide ligand: structure, catecholase activity, magnetic properties and DFT calculations

An antiferromagnetically coupled tetranuclear copper complex of a dinucleating bis amide ligand displays catecholase activity.

The Halogen Bond

The halogen bond occurs when there is evidence of a net attractive interaction between an electrophilic region associated with a halogen atom in a molecular entity and a nucleophilic region in another, or the same, molecular entity. In this fairly extensive review, after a brief history of the interaction, we will provide the reader with a snapshot of where the research on the halogen bond is now, and, perhaps, where it is going. The specific advantages brought up by a design based on the use of the halogen bond will be demonstrated in quite different fields spanning from material sciences to biomolecular recognition and drug design.

Spectrophotometric study on the proton transfer reaction between 2-amino-4-methylpyridine with 2,6-dichloro-4-nitrophenol in methanol, acetonitrile and the binary mixture 50% methanol+50% acetonitrile

Proton transfer reaction between 2-amino-4-methylpyridine (2AMP) as the proton acceptor with 2,6-dichloro-4-nitrophenol (DCNP) as the proton donor has been investigated spectrophotometrically in methanol (MeOH), acetonitrile (AN) and a binary mixture composed of 50% MeOH and 50% AN (AN-Me). The composition of the complex has been investigated utilizing Job(')s and photometric titration methods to be 1:1. Minimum-maximum absorbance equation has been applied to estimate the formation constant of the proton transfer reaction (K(PT)) where it reached high values in the investigated solvent confirming its high stability. The formation constant recorded higher value in AN compared with MeOH and mixture of AN-Me. Based on the formation of stable proton transfer complex, a sensitive spectrophotometric method was suggested for quantitative determination of 2AMP. The Lambert-Beer(')s law was obeyed in the concentration range 0.5-8 μg mL(-1) with small values of limits of detection and quantification. The solid complex between 2AMP with DCNP has been synthesized and characterized by elemental analysis to be 1:1 in concordant with the molecular stoichiometry in solution. Further analysis of the solid complex was carried out using infrared and (1)H NMR spectroscopy.

Isolation of proton transfer complexes containing 4-picolinium as cation and pyridine-2,6-dicarboxylate complex as anion: crystallographic and spectral investigations, antioxidant activities and molecular docking studies

Three novel proton transfer complexes have been synthesized and characterized by spectral and X-ray studies. Antioxidant activity and molecular docking is also examined for the complexes.

Combining coordination and supramolecular chemistry to explore uranyl assembly in the solid state

Supramolecular assembly of uranyl species via halogen–oxo and halogen–halogen interactions is explored in the solid state.

Dimeric packing of molecular clips induced by interactions between π-systems

We report the first observation synthesis and X-ray structures of seven glycoluril clips that feature extended aromatic sidewalls; compounds 1 and 2 are the first examples of the out–out dimeric motif.

Composite molecular assemblies: nanoscale structural control and spectroelectrochemical diversity

The controlled deposition of metal complexes from solution on inorganic surfaces offers access to functional materials that otherwise would be elusive. For such surface-confined interfaces to form, specific assembly sequences are often used. We show here that varying the assembly sequence of two well-defined and iso-structural osmium and ruthenium polypyridyl complexes results in interfaces with strikingly different spectroelectrochemical properties. Successive deposition of redox-active layers of osmium and ruthenium polypyridyl complexes, leads to self-propagating molecular assemblies (SPMAs) with distinct internal interfaces and individually addressable components. In contrast, the clear separation of these interfaces upon sequential deposition of these two complexes, results in charge trapping or electrochemical communication between the metal centers, as a function of layer thickness and applied assembly sequence. The SPMAs were characterized using a variety of techniques, including: UV–vis spectroscopy, spectroscopic ellipsometry, electrochemistry, synchrotron X-ray reflectivity, angle-resolved X-ray photoelectron spectroscopy, and spectroelectrochemistry. The combined data demonstrate that the sequence-dependent assembly is a decisive factor that influences and provides the material properties that are difficult to obtain otherwise.