Preparing efficient and robust water oxidation catalyst (WOC) with inexpensive materials remains a crucial challenge in artificial photosynthesis and for renewable energy. Existing heterogeneous WOCs are mostly metal oxides/hydroxides immobilized on solid supports. Herein we report a newly synthesized and structurally characterized metal-organic hybrid compound [{Co3 (μ3 -OH)(BTB)2(dpe)2} {Co(H2O)4(DMF)2}0.5]n ⋅n H2O(Co-WOC-1) as an effective and stable water-oxidation electrocatalyst in an alkaline medium. In the crystal structure of Co-WOC-1, a mononuclear Co(II) complex {Co(H2O)4(DMF)2}(2+) is encapsulated in the void space of a 3D framework structure and this translationally rigid complex cation is responsible for a remarkable electrocatalytic WO activity, with a catalytic turnover frequency (TOF) of 0.05 s(-1) at an overpotential of 390 mV (vs. NHE) in 0.1 m KOH along with prolonged stability. This host-guest system can be described as a "ship-in-a-bottle", and is a new class of heterogeneous WOC.

Six cobalt(II) containing coordination polymers {Co(hfipbb)(L1)0.5} n (1), {Co(hfipbb)(L2)0.5} n (2), {Co(oba)(L1)0.5} n (3), {Co(oba)(L2)} n ·nH2O (4), {Co(1,2-pda)(L1)0.5} n (5), and [Co(1,2-pda)(L2)(H2O)] n ·nH2O (6), that are formed from two positional isomeric bis(pyridyl) ligands with a long flexible spacer 1,4-bis(2-pyridylaminomethyl)benzene (L1) and 1,4-bis(3-pyridylaminomethyl)benzene (L2) and three different bent carboxylic acids 4,4′-(hexa-fluoroisopropylidene)bis-(benzoicacid) (H2hfipbb), 4,4′-oxybenzoic acid (H2oba), and 1,2-phenylenediaceticacid (1,2-H2pda), have been synthesized under hydrothermal conditions. Compounds 1–6 are characterized by single crystal X-ray diffraction analysis, IR spectroscopy, and thermogravimetric (TG) and elemental analysis. In the crystal structures of compounds 1, 3, and 5, two-dimensional (2D) metal-carboxylic acid layers, composed of dicobalt tetracarboxylate paddle-wheel clusters, are formed whereby these layers are pillared by the secondary ligand L1 in a typical trans–trans–trans conformation to result in a three-dimensional (3D) layered-pillared structure. However, in the crystals of compounds 2, 4, and 6 with secondary ligand L2, it does not favor the formation of paddle-wheels resulting in three completely different coordination polymers. The geometry of the carboxylic acid influences the formation of 2D metal acid layers in the compounds 1, 3, and 5 to form interpenetrated helical double layers to single layers. In compound 2, the secondary ligand L2 diagonally connects the 3D metal acid framework in a regular trans–trans–trans conformation. In compound 4, the ligand L2 exists in cis–cis–trans conformation to form [Co2L22] loops (metallo-macrocycles) which are connected by the oba2– ligand to form polyrotaxane-like 2D polymers. In compound 6, ligand L2 exists in an unusual cis–trans–cis conformation to allow the pda2– in a rare cis conformation to form one-dimensional (1D) ladders. The conformations of the pyridyl ligands L1 and L2 have been explained based on the torsion angle measurement. The steric hindrance created by the isomeric flexible pyridyl ligands at the metal coordination sphere plays an important role in the modulation of the conformation of the secondary ligand that drives the self-assembly of the coordination polymers. Finally, temperature-dependent magnetic susceptibility studies for the compounds 1–5 have been described.

As a continuation of our recent investigations on coordination polymer/ metal−organic framework containing compounds, herein, four new coordination polymers, namely, {Co 2 (ADA) 2 (px2ampy)} n (1), {Cd(ADA)(px2ampy) 0.5 } n (2), {Co 2 (IPTA) 2 (px2ampy)} n (3), {Mn(IPTA)(px3ampy)} n (4) have been synthesized, based on the flexible dicarboxylate ligand H 2 ADA and rigid dicarboxylate ligand H 2 IPTA along with long flexible bis-pyridyl linkers as coligand (where H 2 ADA = 1,3adamantanediacetic acid; H 2 IPTA = isopthalic acid; px2ampy = 1,4-bis(2pyridylaminomethyl)benzene; px3ampy = 1,4-bis(3-pyridylaminomethyl)benzene). Compounds 1−4 have been characterized by routine elemental analysis, IR spectroscopy, and thermogravimetric (TG) analysis and unambiguously by single crystal X-ray diffraction analysis. The solid-state luminescent property of compound 2 has been additionally characterized. Compound 1 is a two-dimensional (2D) framework, constructed by the connectivity of Co 2 paddle-wheels as secondary building units (SBUs) and the organic linkers. The 2D sheet of compound 2 consists of Cd-dimers as SBUs and extends to a supramolecular three-dimensional (3D) framework through H-bonding interactions among the sheets. Compound 3 is a 3D coordination polymer, in which an unusual SBU is formed by two Co II centers: one in tetrahedral and the other one in octahedral coordination geometries. In the crystal structure of compound 4, the connectivity of the Mn 2 -dimers with the macrocyclic rings, formed from px3ampy coligands, results in the formation of 2D layer structures. The title compounds 1−4 illustrate an important structural relationship, which can be rationalized by the geometrical diversity of two different dicarboxylate ligands and the nature of different metal ions, employed in this study. The role of the metal ionic radii in assessing the formation of final architectures, thereby, influencing the dimensionality of the compounds has been discussed. The long flexible pyridyl spacers, present in all these compounds as an integral part, modulate themselves in different conformations depending on the requirement of the coordination environment.

In continuation of our recent investigation on flexible ligands, three new metal-organic coordination framework containing compounds, formulated as {Co(2)(L2)2(px3ampy)(2)}(n)·npx3ampy (1), {Co(L2)(px3ampy)(0.5)}(n) (2) and {Co(2)(L3) (px3ampy)(2)(H(2)O)}(n)·3nH(2)O (3) have been synthesized using three structurally different meso-flexible polycarboxylate ligands, H(2)L1(4,4'-methylenebis-(oxy)dibenzoic acid), H(2)L2 (3,3'-methylenebis-(oxy)dibenzoic acid) and H(4)L3 (5,5'-methylene-bis(oxy)diisophthalic acid) bearing the flexible spacer in the middle of the skeleton, along with a long flexible pyridyl ligand px3ampy (1,4-bis(3-pyridylaminomethyl)benzene). Compounds 1-3 have been characterized by single crystal X-ray diffraction analysis, IR spectroscopy and thermogravimetric (TG) studies including elemental analysis. The crystal structure determinations reveal that compound 1 has a 1D ladder-like structure and compounds 2and 3 are characterized by 3D frameworks. Compound 2 possesses a tiling of a snz net and compound 3 has a 3D interpenetrated motif with a (4,4) connecting 2-nodal net. The variable temperature magnetic susceptibility measurements demonstrate the dominating antiferromagnetic nature of all three title compounds; interestingly, however, compounds 2 and 3 exhibit ferromagnetic interactions due to the uncompensated magnetic moment of the system at low temperatures. Compound 2 illustrates the occurrence of spin canted antiferromagnetic ordering at T(c) ≈ 25 K with a coercive field (H(c)) of 900 Oe at 10 K due to the inclusion of magnetic anisotropy caused by the twisting of the concerned ligand skeleton. Compound shows comparatively weak ferromagnetic ordering with T(c) ≈ 9.5 K, for which weak magnetic anisotropy is present because of two different coordination environments (octahedral and tetrahedral) between two cobalt centers. A structure-function relationship has been described based on the position of the coordinating groups with respect to the flexible center of the ligand skeleton as well as the coordination angle between the ligand and the metal ion.

Six new metal complexes with the formulae [Co­(pda)­(bix)] n (1), [Ni­(pda)­(bix)­(H2O)] n (2), [Cu­(pda)­(bix)2(H2O)2] n ·8nH2O (3), [Co2(μ-OH)­(pda)­(ptz)] n ·nH2O (4), [Co­(hfipbb)­(bix)0.5] n (5), and [Co­(2,6-pydc)­(bix)1.5] n ·4nH2O (6) have been synthesized by the reactions of Co­(II), Ni­(II), and Cu­(II) salts with two flexible ligands 1,4-phenylenediaceticacid (H2pda) and 1,4-bis (imidazole-1-ylmethyl)-benzene (bix) in the presence of coligands 5-(4-pyridyl) tetrazole (4-ptz), 4,4′(hexa-fluoroisopropylidene)­bis­(benzoicacid) (H2hfipbb), and 2,6-pyridine dicarboxylic acid (2,6-H2pydc) and characterized by single crystal X-ray diffraction analysis, IR spectroscopy, and thermogravimetric (TG) analysis. Because of the coordination geometry around the metal ions and the diverse coordination modes of the flexible ligands in combination with the rigid and flexible coligands, the obtained complexes show diverse structures from a one-dimensional (1D) chain to three-dimensional (3D) coordination polymers. 1, 4, 5, and 6 are Co­(II) complexes in which Co­(II) ions show tetrahedral coordination in 1, trigonal bipyrimidal coordination in 4, and octahedral coordination in 5 and 6. Complexes 2 and 3 are respectively Ni­(II) and Cu­(II) complexes in which the metal ions are present in octahedral coordination geometries. Factors affecting the conformational change of the flexible ligands in the self-assembly process of coordination polymers, such as, coordination geometry around the metal ions and geometry of the coligands have been systematically studied. The rotation of the bonds C­(sp3)–C­(sp2) and C­(sp3)–N­(sp3) in H2pda and bix ligands causes different conformations (cis, trans, gauche); these conformations have been studied by measuring the torsion angle. A comparative study between the torsion angle of the particular conformation of the ligands and coordination geometry of metal ion/geometry of the coligand has been undertaken. All the possible cis, trans, and gauche conformations of the flexible ligands have been obtained/observed in our complexes. Theoretical calculations were performed to determine the energies of the different conformations of the flexible ligands. The electronic properties of these complexes have also been investigated in the solid state at room temperature. Finally, the temperature-dependent magnetic studies for compounds 4 and 5 are described.