A [2 × 2] nickel(ii) grid and a copper(ii) square result from differing binding modes of a pyrazine-based diamide ligand

Recent developments in supramolecular complexes of azabenzenes containing one to four N atoms: synthetic strategies, structures, and magnetic properties

For the last couple of decades, azabenzene-based ligands have drawn much attention from inorganic chemists due to their ability to coordinate with different metal ions to form supramolecular clusters. These azabenzenes are weak σ donors and strong π acceptors and electron-deficient. Metallogrid complexes and non-grid oligomers are well-defined supramolecular clusters, formed by appropriate chelating ligands, and can show interesting optical, magnetic, and electronic properties. Self-assembly of [n × n] metallogrid complexes is dominated by the entropic factor while the formation of oligonuclear metal ion complexes is dominated by other effects like CFSE, electrostatic factors, ligand conformational characters, etc. Herein, the present article gives an overview of six-membered heterocyclic azine-based ligands and their potential for different metal ions to form polynuclear complexes. Moreover, their temperature-dependent magnetic properties and SCO phenomena are well described and tabulated.

A Cuprous [4 × 4] Grid: Single-Crystal to Single-Crystal Transformation and Fading of Luminescence by Solvent Inclusion

The subtle balance of the metrics of the highly supple 5,5'-pyridyl-3,3'-bis-1 H-pyrazolate and the variable coordination numbers of Cu(I) favor the stabilization of a [4 × 4] grid complex with all Cu(I). It has numerous cuprophilic bonds (Cu···Cu = 2.56-3.02 Å), and consequently, exhibits strong orange luminescence that fades upon insertion of solvents without loss of crystallinity.

Crystal structure of bis((pyrazin-2-ylmethyl)(pyrazine-2-carbonyl)amido-κ3 N,N′,N′′)copper(II), C20H16CuN10O2

C20H16CuN10O2, tetragonal, I41/a (no. 88), a = 8.8634(6) Å, c = 27.731(4) Å, V = 2178.5(4) Å3, Z = 4, R gt(F) = 0.0541, wR ref(F 2) = 0.1521, T = 296 K.

Self-Assembly of Cyclohelicate [M3 L3 ] Triangles Over [M4 L4 ] Squares, Despite Near-Linear Bis-terdentate L and Octahedral M

Self-assembly of 1:1:2 M^II (BF₄ )₂ (M=Zn or Fe), pyrazine-2,5-dicarbaldehyde (1) and 2-(2-aminoethyl)pyridine gave trimetallic triangle architectures rather than the anticipated tetrametallic [2×2] squares. Options for the nontrivial synthesis of 1 are considered, and synthetic details provided for both preferred routes. Rare cyclohelicate triangle architectures are observed for the pair of structurally characterized yellow-brown [Zn₃ L₃ ](BF₄ )₆ and dark green [Fe₃ L₃ ](BF₄ )₆ complexes of the neutral bis-terdentate Schiff base L. In order to form these pyrazine-edged triangles, the octahedral metal ions-with all 6 N-donors provided by the terdentate binding pockets of two L-are located 0.4-0.5 Å out of the plane of the bridging pyrazines, towards the center of the triangle. Density functional theory calculations confirm that simple particle counting entropic arguments, which predict triangles over squares, are correct here, with the triangles shown to be energetically favored over the corresponding squares. However, importantly, DFT analysis of these and related triangle versus square systems also show that vibrational contributions to entropy dominate and may significantly influence the preferred architecture, such that simple particle counting cannot in general be reliably employed to predict the observed architecture.

Crystal structure of a pyrazine-2,3-dicarboxamide ligand and of its silver(I) nitrate complex, a three-dimensional coordination polymer

The title ligand, C18H16N6O2·2H2O (L1) [N2,N3-bis-(pyridin-4-ylmeth-yl)pyrazine-2,3-dicarboxamide], crystallized as a dihydrate. The mol-ecule is U-shaped with the carboxamide groups being cis to one another, making a dihedral angle of 81.6 (5)°. The terminal pyridine rings are inclined to one another by 58.5 (4)°. There is an intra-molecular N-H⋯Npyrazine hydrogen bond present, forming an S(5) ring motif. In the crystal, adjacent mol-ecules are linked by N-H⋯Ocarboxamide hydrogen bonds, forming a chain along [001]. A chain of hydrogen-bonded water mol-ecules is linked to the chain of (L1) mol-ecules by O-H⋯N hydrogen bonds, forming columns propagating along the c axis. The columns are linked by C-H⋯O and C-H⋯N hydrogen bonds, forming a three-dimensional supra-molecular structure. The reaction of ligand (L1) with silver(I) nitrate led to the formation of a new three-dimensional coordination polymer, {[Ag(C18H16N6O2)]NO3} n , poly[[[μ4-N2,N3-bis-(pyridin-4-ylmeth-yl)pyrazine-2,3-dicarboxamide]-silver(I)] nitrate] (I). The asymmetric unit is composed of half of one silver ion, located on a twofold rotation axis, half a ligand mol-ecule and half a positionally disordered nitrate anion located about a twofold rotation axis. The full mol-ecule of the ligand is generated by twofold rotational symmetry, with this twofold axis bis-ecting the Car-Car bonds of the pyrazine ring and the Ag-Ag bond. The carboxamide groups are now trans to one another, making a dihedral angle of 65.8 (4)°. The two terminal pyridine rings are inclined to one another by 6.6 (3)°. Two ligands wrap around an Ag-Ag bond of 3.1638 (11) Å, forming a figure-of-eight-shaped complex mol-ecule. Each silver ion is coordinated by two pyridine N atoms and by two carboxamide O atoms of neighbouring mol-ecules, hence forming a three-dimensional framework. The nitrate anion is linked to the framework by N-H⋯O and C-H⋯O hydrogen bonds.

Crystal structure of N,N′–bis(pyridin-4-ylmethyl)pyrazine-2,3-dicarboxamide dihydrate, C18H20N6O4

C18H20N6O4, monoclinic, Pc, a = 4.4412(8) Å, b = 14.100(3) Å, c = 15.206(3) Å, β = 95.920(4)°, V = 947.1(3) Å3, Z = 2, R gt(F) = 0.0352, wR ref(F 2) = 0.0770, T = 296.15 K.

Crystal structures of N2,N3,N5,N6-tetra-kis-(pyridin-2-ylmeth-yl)pyrazine-2,3,5,6-tetra-carboxamide and N2,N3,N5,N6-tetra-kis-(pyridin-4-ylmeth-yl)pyrazine-2,3,5,6-tetra-carboxamide

The title compounds, C32H28N10O4· unknown solvent, (I), and C32H28N10O4, (II), are pyrazine-2,3,5,6-tetra-carboxamide derivatives. In (I), the substituents are (pyridin-2-ylmeth-yl)carboxamide, while in (II), the substituents are (pyridin-4-ylmeth-yl)carboxamide. Both compounds crystallize in the monoclinic space group P21/n, with Z' = 1 for (I), and Z' = 0.5 for (II). The whole mol-ecule of (II) is generated by inversion symmetry, the pyrazine ring being situated about a center of inversion. In (I), the four pyridine rings are inclined to the pyrazine ring by 83.9 (2), 82.16 (18), 82.73 (19) and 17.65 (19)°. This last dihedral angle involves a pyridine ring that is linked to the adjacent carboxamide O atom by an intra-molecular C-H⋯O hydrogen bond. In compound (II), the unique pyridine rings are inclined to the pyrazine ring by 33.3 (3) and 81.71 (10)°. There are two symmetrical intra-molecular C-H⋯O hydrogen bonds present in (II). In the crystal of (I), mol-ecules are linked by N-H⋯O and N-H⋯N hydrogen bonds, forming layers parallel to (10-1). The layers are linked by C-H⋯O and C-H⋯N hydrogen bonds, forming a three-dimensional framework. In the crystal of (II), mol-ecules are linked by N-H⋯N hydrogen bonds, forming chains propagating along the [010] direction. The chains are linked by a weaker N-H⋯N hydrogen bond, forming layers parallel to the (101) plane, which are in turn linked by C-H⋯O hydrogen bonds, forming a three-dimensional structure. In the crystal of compound (I), a region of disordered electron density was treated with the SQUEEZE routine in PLATON [Spek (2015 ▸). Acta Cryst. C71, 9-18]. Their contribution was not taken into account during refinement. In compound (II), one of the pyridine rings is positionally disordered, and the refined occupancy ratio for the disordered Car-Car-Npy atoms is 0.58 (3):0.42 (3).

Valence directed binding mode of [2 × 2] iron grids of an unsymmetrical picolinic hydrazone based ligand

Valence directed binding mode of the unsymmetrical ditopic ligand was observed in tetranuclear trivalent (FeIII4) and mixed valent (FeII2FeIII2) [2 × 2] iron grid complexes.

Ferromagnetic coupling in copper(II) [2 × 2] grid-like complexes

Two copper(II) [2 × 2] grid-like complexes were synthesized and structurally characterized. Investigation of the magnetic properties showed for both the occurrence of intramolecular ferromagnetic interactions.

Recent Advances in Molecular Magnetic Materials

This review describes advances made in three areas of molecular magnetic materials of the types A: extended frameworks (coordination polymers) showing long-range magnetic order, B: spin-coupled clusters with emphasis on single molecule magnets and (n × n) grid species, C: polynuclear spin-switching (spin crossover) compounds of FeII with emphasis on dinuclear compounds and one-dimensional (1D) and three-dimensional (3D) (framework) materials, including porous ‘hybrid’ systems. The work of the author and his group is largely used to provide examples, together with results from other groups and collaborators that are included for comparison and completeness. Supramolecular aspects such as cluster–cluster and chain–chain interactions are discussed where relevant. A brief discussion is also given of the recent studies, carried out elsewhere, dealing with aspects of spintronics and the possible future relevance to molecular computers (type B materials) and with memory and other device possibilities (type C materials)