Tailoring Enhanced Elasticity of Crystalline Coordination Polymers.

The approach for enhancing the elasticity of crystals with suboptimal elastic performances through a rational design was presented. A hydrogen-bonding link was identified as a critical feature in the structure of the parent material, the Cd(II) coordination polymer [CdI2(I-pz)2] n (I-pz = iodopyrazine), to determine the mechanical output and was modified via cocrystallization. Small organic coformers resembling the initial organic ligand but with readily available hydrogens were selected to improve the identified link, and the extent of strengthening the critical link was in an excellent correlation with the delivered enhancement of elastic flexibility materials.

Exploring the Co-Crystallization Landscape of One-Dimensional Coordination Polymers Using a Molecular Electrostatic Potential-Driven Approach.

The ability of coordination polymers (CPs) to form multicomponent heteromeric materials, where the key structural features of the parent CP are retained, has been explored via molecular electrostatic potential-driven co-crystallization technologies. Thirteen co-formers presenting hydrogen-bond donors activated through a variety of electron-withdrawing functionalities were employed, and the extent of activation was evaluated using molecular electrostatic potential values. Attempted co-crystallizations of the seven most promising co-formers with a family of nine CPs ([CdX'2(X-pz)2]n; X' = I, Br, and Cl; X = I, Br, and Cl) resulted in six successful outcomes; all four of the structurally characterized compounds displayed the intended hydrogen bond. The rationalization of the main structural features revealed that strict structural and electrostatic requirements were imposed on effective co-formers; only co-formers with highly activated hydrogen-bond donors, and with a 1,4-orientation of electron-withdrawing moieties bearing effective acceptor sites, were successfully implemented into the three-dimensional architectures composed of one-dimensional building units of CPs.

Correction to Two-Dimensional Anisotropic Flexibility of Mechanically Responsive Crystalline Cadmium(II) Coordination Polymers.

[This corrects the article DOI: 10.1021/acs.chemmater.2c00062.].

Two-Dimensional Anisotropic Flexibility of Mechanically Responsive Crystalline Cadmium(II) Coordination Polymers.

Crystals of a family of six one-dimensional (1D) coordination polymers of cadmium(II) with cyanopyridines [[CdX2L2] n , where X = Cl, Br, or I and L = 3-cyanopyridine (3-CNpy) or 4-cyanopyridine (4-CNpy)] presented a variety of morphologies and mechanical responses with dominant two-dimensional (2D) anisotropic flexibility, which has not been previously reported. All mechanically adaptable crystals were 2D flexible and displayed a variety of direction-dependent responses; in addition to 2D isotropic flexibility observed for solely elastic materials, 2D anisotropic flexibility was noticed for both elastic and elastic → plastic crystals. The consequences of fine and controlled structural variations on mechanical behavior were additionally explored via microfocus single-crystal X-ray diffraction and complementary theoretical studies, revealing that the relative strength and direction of the hydrogen bonding interactions were the key parameters in delivering a specific mechanical response.

Synthesis and crystal structure of a 6-chloro-nicotinate salt of a one-dimensional cationic nickel(II) coordination polymer with 4,4'-bi-pyridine.

A 6-chloro-nicotinate (6-Clnic) salt of a one-dimensional cationic nickel(II) coordination polymer with 4,4'-bi-pyridine (4,4'-bpy), namely, catena-poly[[[tetra-aqua-nickel(II)]-µ-4,4'-bi-pyridine-κ2 N:N'] bis-(6-chloro-nicotinate) tetra-hydrate], {[Ni(C10H8N2)(H2O)4](C6H3ClNO2)2·4H2O} n or {[Ni(4,4'-bpy)(H2O)4](6-Clnic)2·4H2O} n , (1), was prepared by the reaction of nickel(II) sulfate hepta-hydrate, 6-chloro-nicotinic acid and 4,4'-bi-pyridine in a mixture of water and ethanol. The mol-ecular structure of 1 comprises a one-dimensional polymeric {[Ni(4,4'-bpy)(H2O)4]2+} n cation, two 6-chloro-nicotinate anions and four water mol-ecules of crystallization per repeating polymeric unit. The nickel(II) ion in the polymeric cation is octa-hedrally coordinated by four water mol-ecule O atoms and by two 4,4'-bi-pyridine N atoms in the trans position. The 4,4'-bi-pyridine ligands act as bridges and, thus, connect the symmetry-related nickel(II) ions into an infinite one-dimensional polymeric chain extending along the b-axis direction. In the extended structure of 1, the polymeric chains of {[Ni(4,4'-bpy)(H2O)4]2+} n , the 6-chloro-nicotinate anions and the water mol-ecules of crystallization are assembled into an infinite three-dimensional hydrogen-bonded network via strong O-H⋯O and O-H⋯N hydrogen bonds, leading to the formation of the representative hydrogen-bonded ring motifs: tetra-meric R 2 4(8) and R 4 4(10) loops, a dimeric R 2 2(8) loop and a penta-meric R 4 5(16) loop.

The first coordination compound of 6-fluoro-nicotinate: the crystal structure of a one-dimensional nickel(II) coordination polymer containing the mixed ligands 6-fluoro-nicotinate and 4,4'-bi-pyridine.

A one-dimensional nickel(II) coordination polymer with the mixed ligands 6-fluoro-nicotinate (6-Fnic) and 4,4'-bi-pyridine (4,4'-bpy), namely, catena-poly[[di-aqua-bis-(6-fluoro-pyridine-3-carboxyl-ato-κO)nickel(II)]-µ-4,4'-bi-pyri-dine-κ2 N:N'] trihydrate], {[Ni(6-Fnic)2(4,4'-bpy)(H2O)2]·3H2O} n , (1), was prepared by the reaction of nickel(II) sulfate hepta-hydrate, 6-fluoro-nicotinic acid (C6H4FNO2) and 4,4'-bi-pyridine (C10H8N2) in a mixture of water and ethanol. The nickel(II) ion in 1 is octa-hedrally coordinated by the O atoms of two water mol-ecules, two O atoms from O-monodentate 6-fluoro-nicotinate ligands and two N atoms from bridging 4,4'-bi-pyridine ligands, forming a trans isomer. The bridging 4,4'-bi-pyridine ligands connect symmetry-related nickel(II) ions into infinite one-dimensional polymeric chains running in the [10] direction. In the extended structure of 1, the polymeric chains and lattice water mol-ecules are connected into a three-dimensional hydrogen-bonded network via strong O-H⋯O and O-H⋯N hydrogen bonds, leading to the formation of distinct hydrogen-bond ring motifs: octa-meric R 8 8(24) and hexa-meric R 8 6(16) loops.

The first coordination compound of deprotonated 2-bromo-nicotinic acid: crystal structure of a dinuclear paddle-wheel copper(II) complex.

A copper(II) dimer with the deprotonated anion of 2-bromo-nicotinic acid (2-BrnicH), namely, tetrakis(µ-2-bromonicotinato-κ2 O:O')bis[aquacopper(-II)](Cu-Cu), [Cu2(H2O)2(C6H3BrNO2)4] or [Cu2(H2O)2(2-Brnic)4], (1), was prepared by the reaction of copper(II) chloride dihydrate and 2-bromo-nicotinic acid in water. The copper(II) ion in 1 has a distorted square-pyramidal coordination environment, achieved by four carboxyl-ate O atoms in the basal plane and the water mol-ecule in the apical position. The pair of symmetry-related copper(II) ions are connected into a centrosymmetric paddle-wheel dinuclear cluster [Cu⋯Cu = 2.6470 (11) Å] via four O,O'-bridging 2-bromo-nicotinate ligands in the syn-syn coordination mode. In the extended structure of 1, the cluster mol-ecules are assembled into an infinite two-dimensional hydrogen-bonded network lying parallel to the (001) plane via strong O-H⋯O and O-H⋯N hydrogen bonds, leading to the formation of various hydrogen-bond ring motifs: dimeric R 2 2(8) and R 2 2(16) loops and a tetra-meric R 4 4(16) loop. The Hirshfeld surface analysis was also performed in order to better illustrate the nature and abundance of the inter-molecular contacts in the structure of 1.

Mechanically Responsive Crystalline Coordination Polymers with Controllable Elasticity.

Crystalline coordination polymers tend to be brittle and inelastic, however, we now describe a family of such compounds that are capable of displaying mechanical elasticity in response to external pressure. The design approach successfully targets structural features that are critical for producing the desired mechanical output. The elastic crystals all comprise 1D cadmium(II) halide polymeric chains with adjacent metal centres bridged by two halide ions resulting in the required stacking interactions and short "4 Å" crystallographic axes. These polymeric chains (structural "spines") are further organized via hydrogen bonds and halogen bonds perpendicular to the direction of the chains. By carefully altering the strength and the geometry of these non-covalent interactions, we have demonstrated that it is possible to control the extent of elastic bending in crystalline coordination compounds.