Ferromagnetic Cu8 pinwheel clusters as building blocks for larger magnetic networks: Long-range structural and magnetic ordering

Diversity of Coordination Modes in a Flexible Ditopic Ligand Containing 2-Pyridyl, Carbonyl and Hydrazone Functionalities: Mononuclear and Dinuclear Cobalt(III) Complexes, and Tetranuclear Copper(II) and Nickel(II) Clusters

Syntheses, crystal structures and characterization are reported for four new complexes [Cu4Br2(L)4]Br2 (1), [Ni4(NO3)2(L)4(H2O)](NO3)2 (2), [Co2(L)3](ClO4)3 (3) and [Co(L)2](ClO4) (4), where L− is the monoanion of the ditopic ligand N′-(1-(pyridin-2-yl)ethylidene)pyridine-2-carbohydrazide (LH) built on a picolinoyl hydrazone core fragment, and possessing a bidentate and a tridentate coordination pocket. The tetranuclear cation of 1·0.8H2O·MeOH is a strictly planar, rectangular [2 × 2] grid. Two 2.21011 L− ligands bridge adjacent CuII atoms on the short sides of the rectangle through their alkoxide oxygen atoms, and two 2.11111 ligands bridge adjacent CuII atoms on the long sides of the rectangle through their diazine groups; two metal ions are 5-coordinate and two are 6-coordinate. The tetranuclear cation of 2·0.2H2O·3EtOH is a square [2 × 2] grid. The two 6-coordinate NiII atoms of each side of the square are bridged by the alkoxide O atom of one 2.21011 L− ligand. The dinuclear cation of 3·0.8H2O·1.3MeOH contains two low-spin octahedral CoIII ions bridged by three 2.01111 L− ligands forming a pseudo triple helicate. In the mononuclear cation [Co(L)2]+ of complex 4, the low-spin octahedral CoIII center is coordinated by two tridentate chelating, meridional 1.10011 ligands. The crystal structures of the complexes are stabilized by a variety of π–π stacking and/or H-bonding interactions. Compounds 2, 3 and 4 are the first structurally characterized nickel and cobalt complexes of any form (neutral or anionic) of LH. The 2.01111 and 1.10011 coordination modes of L−, observed in the structures of complexes 3 and 4, have been crystallographically established for the first time in coordination complexes containing this anionic ligand. Variable-temperature, solid-state dc magnetic susceptibility and variable-field magnetization studies at 1.8 K were carried out on complexes 1 and 2. Antiferromagnetic metal ion···metal ion exchange interactions are present in both complexes. The study reveals that the cation of 1 can be considered as a practically isolated pair of strongly antiferromagnetically coupled (through the diazine group of L−) dinulear units. The susceptibility data for 2 were fit to a single-J model for an S = 1 cyclic tetramer. The values of the J parameters have been rationalized in terms of known magnetostructural correlations. Spectral data (infrared (IR), ultraviolet/visible (UV/VIS), 1H nuclear magnetic resonance (NMR) for the diamagnetic complexes) are also discussed in the light of the structural features of 1–4 and the coordination modes of the organic and inorganic ligands that are present in the complexes. The combined work demonstrates the ligating flexibility of L−, and its usefulness in the synthesis of complexes with interesting structures and properties.

A new nitrogen rich open chain diazine ligand system: synthesis coordination chemistry and magneto-structural studies

The synthesis and coordination chemistry of a new series of open chain diazine based ligands (L3aH₂, L3bH₂, and L3cH₂) is reported. The ligands comprise a central disubstituted bipyridine moiety with two bridging alkoxide oxygen donors, together with diazine and pyridine terminal groups strategically located to coordinate three metal centres. Reactions of L3aH₂ and L3bH₂ with CuX₂ (X^- = ClO₄, Cl, NO₃) yield a trinuclear complex (1) and 1-D copper chains (2, 3). In these complexes the ligands bind copper ions via N_bipyridne, trans N_diazine, O_hydrazone, and N_pyridne donors while vacant sites are occupied by counter ions or solvent molecules (methanol, water, acetonitrile). Reaction of L3cH₂ with MnCl₂ affords a linear trinuclear Mn complex (4), where the Mn(ii) ions are connected via μ₂-O_hydrazone linkers with no N-N bridging. Reaction of L3aH₂ with Fe(SO₃CF₃)₂ yields a tetranuclear mixed valence Fe complex (5), in which both trans N-N and O_hydrazone bridging is observed. Magnetic studies reveal the presence of moderate to strong antiferromagnetic interactions in complexes 1-4 while a mix of ferromagnetic and antiferromagnetic interactions is observed in complex 5.

The influence of different coordination environments on one-dimensional Cu(ii) coordination polymers for the photo-degradation of organic dyes

The role of coordination environment in 1D Cu(ii) coordination polymers for photo-catalytic activity has been investigated. Pyridine-2,6-dicarbohydrazide based polymeric complex 3 has the highest photo-catalytic rate constant (k = 0.72 h⁻¹) for rhodamine B degradation.

Mota AJ, Ephritikhine M. Structural, magnetic and theoretical calculations of a ferromagnetically coupled tetranuclear copper(ii) square complex

Ferromagnetic interactions in a tetranuclear copper(ii) square complex containing a Schiff base ligand.

Supramolecular Self-Assembled Polynuclear Complexes from Tritopic, Tetratopic, and Pentatopic Ligands: Structural, Magnetic and Surface Studies

Polymetallic, highly organized molecular architectures can be created by "bottom-up" self-assembly methods using ligands with appropriately programmed coordination information. Ligands based on 2,6-picolyldihydrazone (tritopic and pentatopic) and 3,6-pyridazinedihydrazone (tetratopic) cores, with tridentate coordination pockets, are highly specific and lead to the efficient self-assembly of square [3 x 3] Mn9, [4 x 4] Mn16, and [5 x 5] Mn25 nanoscale grids. Subtle changes in the tritopic ligand composition to include bulky end groups can lead to a rectangular 3 x [1 x 3] Mn9 grid, while changing the central pyridazine to a more sterically demanding pyrazole leads to simple dinuclear copper complexes, despite the potential for binding four metal ions. The creation of all bidentate sites in a tetratopic pyridazine ligand leads to a dramatically different spiral Mn4 strand. Single-crystal X-ray structural data show metallic connectivity through both mu-O and mu-NN bridges, which leads to dominant intramolecular antiferromagnetic spin exchange in all cases. Surface depositions of the Mn9, Mn16, and Mn25 square grid molecules on graphite (HOPG) have been examined using STM/CITS imagery (scanning tunneling microscopy/current imaging tunneling spectroscopy), where tunneling through the metal d-orbital-based HOMO levels reveals the metal ion positions. CITS imagery of the grids clearly shows the presence of 9, 16, and 25 manganese ions in the expected square grid arrangements, highlighting the importance and power of this technique in establishing the molecular nature of the surface adsorbed species. Nanoscale, electronically functional, polymetallic assemblies of this sort, created by such a bottom-up synthetic approach, constitute important components for advanced molecule-based materials.

Linear copper ‘chain’ complexes with bulky tritopic hydrazone ligands—structural and magnetic studies

Tritopic 2,6-picolyl-bis-hydrazone ligands with bulky terminal groups derived from phenyl-pyridyl ketone do not form the expected [3 x 3] grids on reaction with copper(II), but instead form Cu8 'pinwheels', and in the present case linear trinuclear, pentanuclear and chain structures also. Direct bridging between copper ions occurs through micro2-N-N diazine groups, and longer O-C-N hydrazone connections, leading to moderately strong antiferromagnetic exchange between adjacent metal centres. Structural and magnetic properties are discussed in the context of specific orthogonal and non-orthogonal bridges, which can be distinguished and quantified.