Pyridyl-imine Units Research Articles

A supramolecular 3D structure constructed from a new metal chelate self-assembled from Sn(NCS)2 and phenyl(pyridin-2-yl)methylenepicolinohydrazide

Synthesis and detailed structural characterization of a new coordination compound [Sn(HL)2(NCS)2]·0.5MeOH (1·0.5MeOH), synthesised from a mixture of SnSO4 and KNCS with a bulky organic ligand phenyl(pyridin-2-yl)methylenepicolinohydrazide (HL) in MeOH, is reported. The metal is N6O2 octacoordinated by two tridentate ligands HL through the pyridyl-imine units and the carbonyl oxygen atoms, and its coordination sphere is completed by two N-bound isothiocyanate anions to form the mononuclear heteroleptic complex. The crystal packing of 1·0.5MeOH is strongly dictated by aromatic pyridine and benzene substituents in the structure of coordinated ligands HL. The pyridine rings are involved in intermolecular π(Py)···π(Py) stacking interactions, yielding a 2D supramolecular layer. These 2D layers are further interlinked in the orthogonal direction through intermolecular π(Ph)···π(Ph) stacking interactions, yielding a 3D supramolecular structure. The structure of 1·0.5MeOH is further stabilized by weak non-covalent interactions of the type C–H···S and C–H···π(Py). Latter interactions have been studied using density functional theory (DFT) and molecular electrostatic potential (MEP) surface calculations and characterized using the quantum theory of atoms-in-molecules (QTAIM).

Novel Pb(II) Complexes: X-Ray Structures, Hirshfeld Surface Analysis and DFT Calculations

Herein we report the synthesis and detailed structural characterization of two new centrosymmetric dinuclear coordination compounds of Pb(II) [Pb2L2(NCS)4] (1) and [Pb2L2(NO3)4]∙2MeOH (2), using the organic ligand 1,2-diphenyl-1,2-bis((methyl(pyridin-2-yl)methylene)hydrazono)ethane (L). In both complexes, each subunit [PbLX2 (X = NO3 or NCS)] adopts a quasi-aromatic Möbius metal chelate structure. Each ligand L is coordinated in a tetradentate coordination mode to Pb(II), yielding the 12π electron chelate ring via two pyridyl-imine units. In compound (1), the coordination sphere is completed by one disordered N,S-coordinated thiocyanate anion and two μ1,1-bridging N-coordinated thiocyanate anions. In compound (2), the coordination sphere of Pb(II) is completed by two monodentate and two bidentate nitrato ligands (two of them acting as bridging ligands). Crystal packing of both compounds is stabilized by intermolecular hydrogen bonds, intra- and intermolecular C–H∙∙∙π interactions. The Hirshfeld molecular surfaces of (1) and (2) demonstrate that their packing is dominated by C–H∙∙∙O/N/S interactions as well as by far less favored H∙∙∙H contacts.

New metal chelate constructed from Ni(NCS)2 and 1,2-diphenyl-1,2-bis((phenyl(pyridin-2-yl)methylene)hydrazono)ethane

Synthesis and detailed structural characterization of a new coordination compound [Ni(NCS)2L]∙3MeOH, synthesised from a mixture of Ni(NO3)2 and NaNCS with a bulky helical organic ligand 1,2-diphenyl-1,2-bis((phenyl(pyridin-2-yl)methylene)hydrazono)ethane (L) in MeOH, is reported. The metal is tetracoordinated by ligand L with its 12 π electron chelate ring via the pyridyl-imine units and completes its coordination sphere with two N-bound thiocyanate anions to form the mononuclear [Ni(NCS)2L] complex that adopts a quasi-aromatic Möbius metal chelate structure. Crystal packing of [Ni(NCS)2L]·3MeOH is stabilized by intermolecular O–H∙∙∙S and O–H∙∙∙O hydrogen bonds, intra- and intermolecular C–H∙∙∙π interactions as well as by intermolecular C–S∙∙∙π interactions. The Hirshfeld molecular surface of [Ni(NCS)2L] is dominated by highly favoured H∙∙∙X (X = C, N, O and S) contacts as well as by far less favoured H∙∙∙H contacts. The Kubelka-Munk based absorption spectrum of [Ni(NCS)2L]·3MeOH exhibits bands in the UV region, corresponding to intraligand π → π* and/or n → π* transitions, bands in the visible region, arising from metal-to-ligand charge transfers from both the organic ligand L and thiocyanate anions, as well as from the spin allowed d–d transitions 3A2g → 3T1g (3P), 3A2g → 3T1g (3F), 3A2g → 3T2g and 3A2g → 1Eg transitions. Finally, theoretical DFT calculations have been performed to analyze an unconventional H3C⋯π(C=S) interaction between the co-crystallized MeOH molecule and the thiocyanate ligand that can be classified as a σ-hole carbon bonding interaction with an electron transfer from the π-system of NCS to the antibonding C–O sigma orbital, π(NCS) → σ*(C–O).

Möbius-like metal chelates constructed from CdHal2 (Hal = Cl, Br, I) and benzilbis(pyridin-2-yl)methylidenehydrazone

We report design and structural characterization of three new coordination compounds fabricated from Cd(NO3)2·4H2O and a helical organic ligand benzilbis((pyridin-2-yl)methylidenehydrazone (L) in the presence of two equivalents of NaHal (Hal = Cl, Br, I) in a mixture of MeOH and EtOH, namely [CdCl2L]·1.5MeOH (1), [CdBr2L]·EtOH (2) and [CdI2L] (3). The synthesis and design strategy involved using a branched tube, where temperature differential led to slow crystallization of the compounds. The ligand L is bound via two pyridyl-imine units with a tetradentate coordination mode yielding the 12π electron chelate ring. All the structures are stabilized by intermolecular hydrogen bonding and C–H⋯π interactions. Hirshfeld surface analysis showed that the structures of all the complexes are highly dominated by H⋯X (X = H, C and Hal) contacts. The optical properties of 1–3 have been investigated by diffuse reflectance spectroscopy. 1–3 each exhibit bands almost exclusively in the UV region up to about 450 nm, and the band gap energies were estimated to be 2.92, 3.03 and 3.11 eV, respectively.

Metal chelates constructed from CdHal2 (Hal = Cl, Br, I) and 1,2-diphenyl-1,2-bis((phenyl(pyridin-2-yl)methylene)hydrazono)ethane

Synthesis and structural characterization of three new isomorphous coordination compounds, fabricated from CdHal2 (Hal = Cl, Br, I) and a bulky helical organic ligand 1,2-diphenyl-1,2-bis((phenyl(pyridin-2-yl)methylene)hydrazono)ethane (L), namely [CdHal2L] (Hal = Cl (1), Br (2), I (3)), are reported. The ligand L is coordinated in a tetradentate coordination mode via two pyridyl-imine units, yielding the 12 π electron chelate ring. Herein, we indicated that using the pyridine based bulky ligand along with CdHal2 salts led to formation of desirable helical metallo-organic molecules with octahedral geometry. It was found that all the structures exhibit intermolecular π⋯π stacking and CH⋯π interactions. The Hirshfeld analysis indicated that the H⋯X (X = H, C and Hal) contacts are highly favoured in 1–3. The optical properties of 1–3 have been investigated by diffuse reflectance spectroscopy.

Quasi-aromatic Möbius Metal Chelates.

We report the design as well as structural and spectroscopic characterizations of two new coordination compounds obtained from Cd(NO3)2·4H2O and polydentate ligands, benzilbis(pyridin-2-yl)methylidenehydrazone (LI) and benzilbis(acetylpyridin-2-yl)methylidenehydrazone (LII), in a mixture with two equivalents of NH4NCS in MeOH, namely [Cd(SCN)(NCS)(LI)(MeOH)] (1) and [Cd(NCS)2(LII)(MeOH)] (2). Both LI and LII are bound via two pyridyl-imine units yielding a tetradentate coordination mode giving rise to the 12 π electron chelate ring. It has been determined for the first time (qualitatively and quantitatively), using the EDDB electron population-based method, the HOMA index, and the ETS-NOCV charge and energy decomposition scheme, that the chelate ring containing CdII can be classified as a quasi-aromatic Möbius motif. Notably, using the methyl-containing ligand LII controls the exclusive presence of the NCS- connected with the CdII atom (structure 2), while applying LI allows us to simultaneously coordinate NCS- and SCN- ligands (structure 1). Both systems are stabilized mostly by hydrogen bonding, C-H···π interactions, aromatic π···π stacking, and dihydrogen C-H···H-C bonds. The optical properties have been investigated by diffused reflectance spectroscopy as well as molecular and periodic DFT/TD-DFT calculations. The DFT-based ETS-NOCV analysis as well as periodic calculations led us to conclude that the monomers which constitute the obtained chelates are extremely strongly bonded to each other, and the calculated interaction energies are found to be in the regime of strong covalent connections. Intramolecular van der Waals dispersion forces, due to the large size of LI and LII, appeared to significantly stabilize these systems as well as amplify the aromaticity phenomenon.

Coordination chemistry of bis(2-pyridylimine) ligands with Ag(I): formation of two structurally different coordination polymers and one metallocycle controlled by linker and the solvent system

Reaction of equimolar amounts of AgClO4 and bis[4-(2-pyridylmethyleneamino)phenyl] methane (L1) or bis[4-(2-pyridylmethyleneamino)phenyl] ether (L2) in a 1:1 solvent mixture of CH3CN and CH2Cl2 leads to the formation of two infinite coordination polymers of the composition {[Ag(L1)]ClO4·CH3CN}n (1) and {[Ag(L2)]ClO4·CH2Cl2}n (2). Whereas 1 represents a homochiral single-stranded helicate the related complex 2 shows a typical zigzag chain arrangement. Both structures are characterized by a distorted tetrahedral coordination environment of the Ag(I) centres each based on a N4 coordination pattern of two ligand molecules. The resulting strands are connected by a hydrogen bonding network including ClO4− anions and solvent molecules forming 2-D layers. Additional π–π and CH–π interactions between the aromatic parts of the ligand molecules give a 3-D arrangement of the packing. In contrast, a discrete dinuclear metallocycle, [Ag2(L2)2](ClO4)2·CH3OH (3), has been formed by reaction of AgClO4 with L2 when CH2Cl2 in the solvent mixture was replaced by CH3OH. Again each Ag(I) has a distorted tetrahedral geometry and is coordinated to two pyridylimine units of two ligand molecules. Additional weak hydrogen bonds involving perchlorate and solvent molecules as well as edge-to-face and face-to-face π–π interactions allow a 3-D packing arrangement.

Spacially Confined M2Centers (M = Fe, Co, Ni, Zn) on a Sterically Bulky Binucleating Support: Synthesis, Structures and Ethylene Oligomerization Studies

Two new bulky aryl-bridged pyridyl-imine compartmental (pro)ligands, 2,6-{(2,6-i-Pr(2)C6H3)N=C(Me)C5H3N}2C6H3Y (Y = H L1, OH L2-H), have been prepared in moderate to good overall yields via a Stille-type cross-coupling approach. The molecular structure of L2-H reveals a transoid configuration within the pyridyl-imine units with a hydrogen-bonding interaction maintaining the phenol coplanar with one of the adjacent pyridine rings. The interaction of 2 equiv of MX2 with L1 in n-BuOH at 110 degrees C gives the binuclear complexes, [(L1)M2X4] (M = Fe, X = Cl (1a); M = Co, X = Cl (1b); M = Ni, X = Br (1c); M = Zn, X = Cl (1d)), in which the metal centers adopt distorted tetrahedral geometries and occupy the two pyridyl-imine cavities in L1. In contrast, deprotonation of L2-H occurs upon reaction with 2 equiv of MX2 to afford the phenolate-bridged species [(L2)M2(mu-X)X2] (M = Fe, X = Cl (2a); M = Co, X = Cl (2b); M = Ni, X = Br (2c); M = Zn, X = Cl (2d)). 1H NMR studies of diamagnetic 1d and 2d reveal that the limited rotation of the N-aryl groups in 1d is further impeded in 2d by steric interactions imparted by the two closely located N-aryl groups. Partial displacement of the bridging bromide in 2c results upon its treatment with acetonitrile to afford [(L2)Ni2Br3(NCMe)] [2c(MeCN)]; no such reaction occurs for 2a, 2b, or 2d. Upon activation with excess methylalumoxane (MAO), 1b, 1c, 2b, and 2c show some activity for alkene oligomerization forming low molecular-weight materials with methyl-branched products predominating for the nickel systems. Single-crystal X-ray diffraction studies have been performed on L2-H, 1c, 2b, 2c, 2c(NCMe), and 2d.

Binding sites on the outside of metallo-supramolecular architectures; engineering coordination polymers from discrete architectures.

Aggregation of metallo-supramolecular architectures through additional coordination is explored by introducing metal-binding units onto the outside of the supramolecular architectures. This is achieved within the framework of our imine-based approach to supramolecular architecture, by replacing the pyridylimine units with pyrazylketimine units. An advantage of the design is that it retains the ease-of-synthesis which characterises our imine-based approach. Silver(I) complexes of three pyrazylketimine ligand systems are described. The complexes demonstrate that introducing pyrazine donor units does indeed allow higher-order assembly of the distinct supramolecular architectures into engineered coordination polymers. Two distinct types of aggregation are observed. In the first, the donors on the outside of one architecture bind to the metals of another to link the units into a polymeric array. In the second type, the donors on the outside of the architectures bind to separate metal centres which are themselves not part of the architectures, and these separate metal centres link the units to form the macromolecular array. The weaker donor nature of the pyrazine nitrogens (compared to pyridine) also introduces an additional element into the design; higher coordination numbers are favoured and this can lead to arrays with higher connectivity than those observed in the discrete pyridylimine architectures.

Helical (isotactic) and syndiotactic silver(I) metallo-supramolecular coordination polymers assembled from a readily prepared bis-pyridylimine ligand containing a 1,5-naphthalene spacer.

A ligand in which two pyridylimine binding units are linked by a 1,5-naphthalene spacer is prepared and its silver(I) coordination chemistry investigated. In the solid state, a pair of C-H triplebond N interactions between pyridylimine units link the free ligands into chain structures, with further C-H triplebond N and some -stacking interactions linking these chains into a three-dimensional structure. The spacer constrains the ligand to dinucleate, and with silver(I) the metal coordinates to two pyridylimine units from two separate ligands and this leads to the formation of coordination polymers with a range of different anions. Different twisting motifs within the ligand control the tacticity of these coordination polymers and both isotactic, helical polymers and syndiotactic (achiral) polymers result. The core of the isotactic polymer strands contains two metallo-vectors and results in long-range ordering of the metal centres into a 2 x n grid arrangement. The solution behaviour indicates that exchange between the diastereomeric forms occurs. Since this must involve inversion at the metal centres, atactic species may also form a component of the solution library.

Design and crystal structures of triple helicates with crystallographic idealized D3 symmetry: the role of side chain effect on crystal packing.

Novel crystallographic D3-symmetric binuclear triple molecular helices [Co2L(1)3][BF4]4 (1), [Zn2L(1)3][BF4]4 (2), [Mn2L(1)3][BF4]4 (3), [Co2L(2)3][BF4]4 (4), [Zn2L(2)3][BF4]4 (5), and [Mn2L(2)3][BF4]4 (6) have been achieved to establish the side chain effect on molecular packing, where L1 is [(C5H4N)C(CH3)=N-(C6H4)-]2CH2 and L2 is [(C5H4N)C(CH3)=N-(C6H4)-]2O, respectively. Crystal structure analyses show that each helix crystallizes in a hexagonal crystal system with space group Pc1 and the general axis of the helix occupies the crystallographic 3-fold axial position with the other three crystallographic 2-fold symmetries perpendicular to it. Each metal center is bound to three pyridylimine units to attain C3 pseudooctahedral coordination geometry with respective equivalent metal-N (CH=N) and metal-N (pyridyl) bonds. It is speculated that the existence of the methyl group might minimize the potential intermolecular interactions, which would be the essential factor controlling the helices formed in idealized crystallographic D3 symmetry. Moreover, crystallographic idealized C3-symmetric helicates [Co2L(3)3][BF4]4 (7), [Zn2L(3)3][BF4]4 (8), [Ni2L(3)3][BF4]4 (9), and [Cu2L(3)3][BF4]4 (10) were also structurally characterized for comparison, where L3 is [(C5H4N)C(CH3)=N-]2. All the results indicate that the existence of the methyl group in the side chain of aromatic ligands could effectively reduce the potential - intermolecular interactions and the side chain effect of the methyl group in crystal packing is robust enough to be exchanged from one network structure to another, which ensures the generality and predictability of the crystallographic idealized symmetry formation to a certain extent.

Coordination chemistry of a new imine-ether ligand, N, N′-bis(2-pyridylmethylene)-2,2′-(ethylenedioxy)bis(ethylamine): mono-helical and single-helical-strand coordination modes identified by X-ray crystallography

A new Schiff base ligand, N, N′-bis(2-pyridylmethylene)-2,2′-(ethylenedioxy)bis(ethylamine), L, and the complexes, [AgL](BF 4) ( 1), [Zn 2LCl 4] ( 2), [Hg 2LCl 4] ( 3), [Cd 2LI 4] ( 4), [Ni 2L 3](BF 4) 4 ( 5), and [M 2L 3](ClO 4) 4 with M=Co(II) ( 6) Fe(II) ( 7) and Zn(II) ( 8), have been synthesised and characterised crystallographically and spectroscopically. The silver(I) complex 1 consists of a mono-helical structure where one ligand molecule, coordinating in a tetradentate manner, wraps itself around the Ag(I) ion, giving rise to a distorted tetrahedral arrangement. Single-helical-strand binuclear complexes were obtained with zinc(II), mercury(II) and cadmium(II), 2, 3 and 4, respectively. Complexes 2 and 3 are isomorphous and both contain one ligand molecule coordinating to two metal atoms via the two pyridylimine N-atoms. The two remaining coordination sites on the metals are occupied by chlorine atoms. In the fivefold coordinate cadmium(II) complex, 4, the ligand coordinates in a bis-tridentate manner via the two pyridylimine units and the two oxygen atoms. The two remaining coordination sites on the metals are occupied by iodine atoms. It was possible to combine both coordination modes of the ligand, mono-helical and single-helical-strand, in the isomorphous binuclear octahedrally coordinated nickel(II), cobalt(II), iron(II) and zinc(II) complexes, 5– 8, respectively. One ligand molecule is wrapped around each metal ion, which in turn are linked by a third ligand molecule, so forming mono-bridged species.

The synthesis and structural characterisation of helical silver(i) complexes with short bis(pyridylimine) ligands

Three new disilver(I) double helicate complexes [Ag2L2][ClO4]2 (1), [Ag2L2][BF4]2 (2) [Ag2L2][NO3]2 (3) and a polymeric monohelical species [Ag2L′2][BF4]2 (4) have been generated via a self-assembly procedure to establish the effect of a side-chain on the subtle metal–ligand coding. The ligands, L = (C5H4N)C(CH3)N–NC(CH3)(C5H4N) and L′ = (C5H4N)CHN–NCH(C5H4N), were both readily prepared imine-based systems, in which the two bis-bidentate chelating units are separated by a single bond. Complexes 1, 2 and 3 adopt a double helical architecture, in which each silver centre is bound to two pyridylimine units in a distorted tetrahedral coordination geometry. Complex 4 is formed as an infinite monohelical polymer in which each silver centre is coordinated in a distorted tetrahedral geometry by two bidentate binding units from different ligands and the ligands are in turn coordinated to two silver(I) centres. While the two pyridylimine units of the ligand L are able to twist both in its free form and in the double helicates 1, 2 and 3, ligand L′ presents a planar conformation in both the free ligand and its monohelical complex 4. It is speculated that the arrangement of the two pyridylimine units around the N–N bond is essential to encode for a double helicate. Intra- and inter-molecular non-covalent interactions are found to stabilize the polymer 4 and provide opportunities for complexes 1, 2 and 3 to crystallize in different space groups.

Self-assembled dinuclear molecular box [Ag2L2]2+ and triple helicates [Co2L3]4+, [Ni2L3]4+ {L = bis[4-(2-pyridylmethyleneamino)phenyl] ether}

The self-assembly and structural characterization of the new silver(I) molecular box [Ag2L2]2+ and cobalt(II), and nickel(II) molecular helices [Co2L3]4+ and [Ni2L3]4+ have been achieved. These complexes are derived from an inexpensive and easy-to-prepare bis-bidentate Schiff base ligand bis[4-(2-pyridylmethyleneamino)phenyl] ether (L). The cage-like cation [Ag2L2]2+ is located at an inversion center with two tetrahedrally co-ordinated silver(I) atoms and two ether oxygen atoms in the corner of a box. The edge length Ag⋯O is ca. 7.1 A. Each silver(I) center is bound to two pyridylimine units, one from each ligand. One ligand passes above the silver(I)–silver(I) axis, while the other passes beneath, giving a non-helical metallo-cyclophane with Ag⋯Ag separation ca. 12.3 A. Both triple helical [Co2L3]4+ and [Ni2L3]4+ cations contain two metal atoms and three ligands. Each metal center is bound to three pyridylimine units to attain a pseudo-octahedral co-ordination geometry. The ligand wraps in a helical arrangement around the two metal ions. Edge-to-face and face-to-face π–π interactions play important roles in the metal-assisted self-assembling process.