Complexes derived from hydrolytically ‘unstable’ hydrazone ligands – Some unexpected products

Distinctive coordination behavior of a pyrazole imine-oxime compound towards Co(II) and Ni(II)

The polytopic Schiff base 5-methyl-1H-pyrazole-3-carboxylic acid 2-(hydroxyimino-1-methyl-propylidene)-hydrazide (H₂L)was synthesized by the condensation of 5-methyl pyrazole-3-carbohydrazide and 3-(hydroxyimino)butan-2-one and its coordination ability was tested against cobalt (II) and nickel (II) nitrates. The ligand exhibited two different binding modes to form a unique binuclear triply bridged Co(III) cationic complex [Co₂(1κN²:2κN²-L) (1κN³:2κO¹-HL)₂](NO₃)₂ (1). With the Ni(II) precursor, H₂L was hydrolyzed to N′,N˝-butane-2,3-diylidenebis (5-methyl-1H-pyrazole-3-carbohydrazide) (H₂L¹) which bound the metal cation in a tetradentate N₃O₁ fashion leading to the neutral square planar complex [Ni(κN³O¹-L¹)]·MeOH (2·MeOH). Complexes 1 and 2 were characterized by IR, NMR, UV-Vis and single crystal X-ray crystallography. The probable mechanism for the Ni(II) mediated transformation of H₂L into H₂L¹ has been investigated by ESI-MS.

Bis(N,N'-dimethyl-ethylenediammonium) tris-(oxalato-κO,O)cobaltate(II) dihydrate: an ion-pair complex

The Co^II ion in the title complex, (C₄H₁₄N₂)₂[Co(C₂O₄)₃]·2H₂O, is coordinated by three oxalate ions, resulting in a distorted octa­hedral geometry. Two uncoordinated water mol­ecules are present in asymmetric unit. Inter­molecular N—H⋯O and O—H⋯O hydrogen bonds between the different entities stabilize the crystal structure.

2,2'-{1,1'-[2,2'-Oxalylbis(hydrazin-2-yl-1-yl-idene)]diethyl-idyne}dipyridinium bis-(perchlorate) dihydrate

The title salt, C₁₆H₁₈N₆O₂²⁺·2ClO₄⁻·2H₂O, was obtained unintentionally as a major product in the reaction of Zn(ClO₄)₂·6H₂O with the N′,N′²-bis­[(1E)-1-(2-pyrid­yl)ethyl­idene]ethanedihydrazide (H₂L) ligand. The (H₄L)²⁺ cation lies across a centre of inversion. The pyridiniumimine fragments of (H₄L)²⁺ adopt syn orientations. Intra­molecular N—H⋯N and N—H⋯O hydrogen bonds lead to the formation of S(5) motifs. In the crystal, neighbouring cations are connected by π–π inter­actions between pyridinium units with a centroid–centroid distance of 3.600 (1) Å. Moreover, the crystal components are assembled into two-dimensional layers via N—H⋯O and O—H⋯O hydrogen bonds, with no direct hydrogen-bonding inter­actions between cations.

{μ-N,N′-Bis[1-(2-pyridyl)ethylidene]benzene-1,2-diamine}di-μ-chlorido-bis[diaquanickel(II)] dichloride ethanol disolvate

In the title compound, [Ni₂Cl₂(C₂₀H₁₈N₄)(H₂O)₄]Cl₂·2C₂H₆O, the coordination environment of each Ni²⁺ ion is distorted octa­hedral formed by two N atoms from the Schiff base ligand, two O atoms from water mol­ecules and two chloride anions acting as μ₂ bridges between the metal ions. The coordinated water mol­ecules are linked to the uncoordinated ethanol mol­ecules and chloride anions by O—H⋯O and O—H⋯Cl hydrogen bonds, although the assignment of some of these is tentative. A weak inter­molecular O—H⋯N inter­action within the ligand is also observed.

The cocrystal μ-oxalato-κO,O:O,O-bis-(aqua-(nitrato-κO){[1-(2-pyridyl-κN)eth-ylidene]hydrazine-κN}copper(II)) μ-oxalato-κO,O:O,O-bis-((methanol-κO)(nitrato-κO){[1-(2-pyridyl-κN)eth-ylidene]hydrazine-κN}copper(II)) (1/1)

The title cocrystal, [Cu₂(C₂O₄)(NO₃)₂(C₇H₉N₃)₂(H₂O)₂][Cu₂(C₂O₄)(NO₃)₂(C₇H₉N₃)₂(CH₄O)₂], is a 1:1 cocrystal of two centrosymmetric Cu^II complexes with oxalate dianions and Schiff base ligands. In each mol­ecule, the Cu^II centre is in a distorted octa­hedral cis-CuN₂O₄ environment, the donor atoms of the N,N′-bidentate Schiff base ligand and the bridging O,O′-bidentate oxalate group lying in the equatorial plane. In one mol­ecule, a monodentate nitrate anion and a water mol­ecule occupy the axial sites, and in the other, a monodentate nitrate anion and a methanol mol­ecule occupy these sites. In the crystal structure, inter­molecular N—H⋯O, O—H⋯O and N—H⋯N hydrogen bonds link the mol­ecules into a network. Weak intra­molecular N—H⋯O inter­actions are also observed.

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.