The Interaction of Water with Free Mn4O4+ Clusters: Deprotonation and Adsorption‐Induced Structural Transformations

As the biological activation and oxidation of water takes place at an inorganic cluster of the stoichiometry CaMn₄O₅, manganese oxide is one of the materials of choice in the quest for versatile, earth‐abundant water splitting catalysts. To probe basic concepts and aid the design of artificial water‐splitting molecular catalysts, a hierarchical modeling strategy was employed that explores clusters of increasing complexity, starting from the tetramanganese oxide cluster Mn₄O₄⁺ as a molecular model system for catalyzed water activation. First‐principles calculations in conjunction with IR spectroscopy provide fundamental insight into the interaction of water with Mn₄O₄⁺, one water molecule at a time. All of the investigated complexes Mn₄O₄(H₂O)_n⁺ (n=1–7) contain deprotonated water with a maximum of four dissociatively bound water molecules, and they exhibit structural fluxionality upon water adsorption, inducing dimensional and structural transformations of the cluster core.     

Toward the assignment of the manganese oxidation pattern in the water-oxidizing complex of photosystem II: a time-dependent DFT study of XANES energies

Density functional theory (DFT) and time‐dependent DFT (TDDFT) calculations are reported on three sets of isomeric models of the {Mn₄/Ca} water‐oxidizing complex of photosystem II (PS II), which share the general formula [CaMn₄O₄(N₂C₃H₄)(RCOO)₆]^q?(H₂O)_n (R=H, CH₃; q=?1, 0, +1, +2; n=3, 4, 5, 6, 7). Comparison with the full range of available data on Mn K‐edge X‐ray absorption energy values determined for the photosystem allows us to validate the structures that correspond to the particular S states and to determine their Mn oxidation patterns. By using a new TDDFT procedure, it is shown that variations in the absolute K‐edge energy values for a particular S state, reported by different research groups, can be quantitatively explained by different geometries adopted by the Mn cluster, which demonstrates flexibility in the position of the fourth ‘dangling’ Mn atom in relation to a cubane structure created by the Ca atom and the three other Mn atoms. Computational results show that each step of the S cycle occurs by removal of one electron directly from the Mn cluster. This Mn‐centered oxidation still agrees with the small difference observed experimentally between the K‐edge energy values of the S₂ and S₃ states of the photosystem, thus resolving a controversy as to whether this represents ligand‐centered or metal‐centered oxidation. The overall oxidation state of Mn atoms in the tetramanganese cluster during functional turnover changes from 2.75 for S₀, 3.00 for S₁, and 3.25 for S₂ up to 3.50 for the S₃ state, which is systematically 0.50 lower than the previously proposed oxidation states of the cluster. The calculations give insight into why these earlier, purely empirical, assignments of the Mn oxidation levels in PS II could be in error.     

Hydrogen bonding in two ammonium salts of 5‐sulfosalicylic acid: ammonium 3‐carboxy‐4‐hydroxybenzenesulfonate monohydrate and triammonium 3‐carboxy‐4‐hydroxybenzenesulfonate 3‐carboxylato‐4‐hydroxybenzenesulfonate

The structures of two ammonium salts of 3‐carboxy‐4‐hydroxybenzenesulfonic acid (5‐sulfosalicylic acid, 5‐SSA) have been determined at 200 K. In the 1:1 hydrated salt, ammonium 3‐carboxy‐4‐hydroxybenzenesulfonate monohydrate, NH₄⁺·C₇H₅O₆S⁻·H₂O, (I), the 5‐SSA⁻ monoanions give two types of head‐to‐tail laterally linked cyclic hydrogen‐bonding associations, both with graph‐set R₄⁴(20). The first involves both carboxylic acid O—H...O_water and water O—H...O_sulfonate hydrogen bonds at one end, and ammonium N—H...O_sulfonate and N—H...O_carboxy hydrogen bonds at the other. The second association is centrosymmetric, with end linkages through water O—H...O_sulfonate hydrogen bonds. These conjoined units form stacks down c and are extended into a three‐dimensional framework structure through N—H...O and water O—H...O hydrogen bonds to sulfonate O‐atom acceptors. Anhydrous triammonium 3‐carboxy‐4‐hydroxybenzenesulfonate 3‐carboxylato‐4‐hydroxybenzenesulfonate, 3NH₄⁺·C₇H₄O₆S²⁻·C₇H₅O₆S⁻, (II), is unusual, having both dianionic 5‐SSA²⁻ and monoanionic 5‐SSA⁻ species. These are linked by a carboxylic acid O—H...O hydrogen bond and, together with the three ammonium cations (two on general sites and the third comprising two independent half‐cations lying on crystallographic twofold rotation axes), give a pseudo‐centrosymmetric asymmetric unit. Cation–anion hydrogen bonding within this layered unit involves a cyclic R₃³(8) association which, together with extensive peripheral N—H...O hydrogen bonding involving both sulfonate and carboxy/carboxylate acceptors, gives a three‐dimensional framework structure. This work further demonstrates the utility of the 5‐SSA⁻ monoanion for the generation of stable hydrogen‐bonded crystalline materials, and provides the structure of a dianionic 5‐SSA²⁻ species of which there are only a few examples in the crystallographic literature.     

The three‐dimensional hydrogen‐bonded structures in the ammonium and sodium salt hydrates of 4‐aminophenylarsonic acid

The structures of two hydrated salts of 4‐aminophenylarsonic acid (p‐arsanilic acid), namely ammonium 4‐aminophenylarsonate monohydrate, NH₄⁺·C₆H₇AsNO₃⁻·H₂O, (I), and the one‐dimensional coordination polymer catena‐poly[[(4‐aminophenylarsonato‐κO)diaquasodium]‐μ‐aqua], [Na(C₆H₇AsNO₃)(H₂O)₃]_n, (II), have been determined. In the structure of the ammonium salt, (I), the ammonium cations, arsonate anions and water molecules interact through inter‐species N—H...O and arsonate and water O—H...O hydrogen bonds, giving the common two‐dimensional layers lying parallel to (010). These layers are extended into three dimensions through bridging hydrogen‐bonding interactions involving the para‐amine group acting both as a donor and an acceptor. In the structure of the sodium salt, (II), the Na⁺ cation is coordinated by five O‐atom donors, one from a single monodentate arsonate ligand, two from monodentate water molecules and two from bridging water molecules, giving a very distorted square‐pyramidal coordination environment. The water bridges generate one‐dimensional chains extending along c and extensive interchain O—H...O and N—H...O hydrogen‐bonding interactions link these chains, giving an overall three‐dimensional structure. The two structures reported here are the first reported examples of salts of p‐arsanilic acid.     

Stabilization of Substituted Triazenide Oxides: Synthesis and X‐ray Structural Features of Polymeric Potassium 3‐(4‐carboxylatophenyl)‐1‐methyltriazene N‐oxide‐hydrate, {K[O2C‐C6H4‐N(H)NN(CH3)O]·4H2O}n

3‐(4‐carboxyphenyl)‐1‐methyltriazene N‐oxide reacts with KOH in methanol/pyridine to give {K[O₂C‐C₆H₄‐N(H)NN(CH₃)O]·4H₂O}_n, Potassium‐3‐(4‐carboxylatophenyl)‐1‐methyltriazene N‐oxide). The terminal carboxylato group of the anion does not interact with the cation. In the crystal lattice of {K(C₈H₈N₃O₃)·4H₂O}_n each three of the four water molecules interact with two potassium cations, every K⁺ ion being the centre of six bridging K···O interactions. Potassium cations interact further with the terminal N‐oxigen atom of single [C₈H₈N₃O₃]^? anions achieving two parallel {C₈H₈N₃O₃^?K⁺}_n chains, which are linked through water molecules. The resulting polymeric, one‐dimensional chain, is operated by a screw axis 2₁ parallel to the crystallographic direction [010], along and equidistant to the K⁺ centres. The coordination of the K⁺ centres involves a distortion of the boat conformation of elementary sulfur (S₈) with the ideal C_2v symmetry.     

Poly[hexaaquabis(μ4‐4‐carboxybenzenesulfonato)bis(μ3‐4‐carboxybenzenesulfonato)calcium(II)dipotassium(I)]

This study presents the coordination modes and crystal organization of a calcium–potassium coordination polymer, poly[hexaaquabis(μ₄‐4‐carboxybenzenesulfonato‐κ⁴O¹:O^1′:O^1′′:O⁴)bis(μ₃‐4‐carboxybenzenesulfonato‐κ²O¹:O^1′)calcium(II)dipotassium(I)], [CaK₂(C₇H₅O₅S)₄(H₂O)₆]_n, displaying a novel two‐dimensional framework. The potassium ion is seven‐coordinated by four sulfonate and one carboxyl O atom located on five different acid ligands, two of which are unique, and by two symmetry‐independent water O atoms. A pair of close potassium ions share two inversion‐related sulfonate O‐atom sites to form a dimeric K₂O₁₂ unit, which is extended into a one‐dimensional array along the a‐axis direction. The six‐coordinate Ca²⁺ ion occupies a special position () at (0, , ) and is surrounded by four sulfonate O atoms from two inversion‐related pairs of unique acid monoanions and by two O atoms from aqua ligands. The compound displays a layered structure, with K₂O₁₂ and CaO₆ polyhedra in the layers and aromatic linkers between the layers. The three‐dimensional scaffold is open, with nano‐sized channels along the c axis.     

Visible‐Light‐Driven Water Oxidation by a Molecular Manganese Vanadium Oxide Cluster

Photosynthetic water oxidation in plants occurs at an inorganic calcium manganese oxo cluster, which is known as the oxygen evolving complex (OEC), in photosystem II. Herein, we report a synthetic OEC model based on a molecular manganese vanadium oxide cluster, [Mn₄V₄O₁₇(OAc)₃]^3?. The compound is based on a [Mn₄O₄]⁶⁺ cubane core, which catalyzes the homogeneous, visible‐light‐driven oxidation of water to molecular oxygen and is stabilized by a tripodal [V₄O₁₃]^6? polyoxovanadate and three acetate ligands. When combined with the photosensitizer [Ru(bpy)₃]²⁺ and the oxidant persulfate, visible‐light‐driven water oxidation with turnover numbers of approximately 1150 and turnover frequencies of about 1.75 s^?1 is observed. Electrochemical, mass‐spectrometric, and spectroscopic studies provide insight into the cluster stability and reactivity. This compound could serve as a model for the molecular structure and reactivity of the OEC and for heterogeneous metal oxide water‐oxidation catalysts.     

Ion‐exchanged binuclear Ca2OX clusters,X = 1–4, as active sites of selective oxidation over MOR and FAU zeolites

A new series of calcium oxide clusters Ca₂O_X (X = 1–4) at cationic positions of mordenite (MOR) and faujasite (FAU) is studied via the isolated cluster approach. Active oxide framework fragments are represented via 8‐membered window (8R) in MOR, and two 6R and 4R windows (6R+4R) possessing one common Si? O? Si moiety in FAU. Structural similarities between the Ca₂O_X(8R) and Ca₂O_X(6R+4R) moieties are considered up to X = 4. High oxidation possibilities of the Ca₂O₂(nR) and Ca₂O₃(nR) systems are demonstrated relative to CO, whose oxidation over the Ca‐exchanged zeolite forms is well studied experimentally. Relevance of the oxide cluster models with respect to trapping and desorption of singlet dioxygen is discussed. © 2009 Wiley Periodicals, Inc. J Comput Chem 2010     

Hydrogen‐bonded two‐ and three‐dimensional polymeric structures in the ammonium salts of 3,5‐dinitrobenzoic acid, 4‐nitrobenzoic acid and 2,4‐dichlorobenzoic acid

The structures of ammonium 3,5‐dinitrobenzoate, NH₄⁺·C₇H₃N₂O₆⁻, (I), ammonium 4‐nitrobenzoate dihydrate, NH₄⁺·C₇H₄NO₄⁻·2H₂O, (II), and ammonium 2,4‐dichlorobenzoate hemihydrate, NH₄⁺·C₇H₃Cl₂O₂⁻·0.5H₂O, (III), have been determined and their hydrogen‐bonded structures are described. All three salts form hydrogen‐bonded polymeric structures, viz. three‐dimensional in (I) and two‐dimensional in (II) and (III). With (I), a primary cation–anion cyclic association is formed [graph set R₄³(10)] through N—H...O hydrogen bonds, involving a carboxylate group with both O atoms contributing to the hydrogen bonds (denoted O,O′‐carboxylate) on one side and a carboxylate group with one O atom involved in two hydrogen bonds (denoted O‐carboxylate) on the other. Structure extension involves N—H...O hydrogen bonds to both carboxylate and nitro O‐atom acceptors. With structure (II), the primary inter‐species interactions and structure extension into layers lying parallel to (001) are through conjoined cyclic hydrogen‐bonding motifs, viz.R₄³(10) (one cation, an O,O′‐carboxylate group and two water molecules) and centrosymmetric R₄²(8) (two cations and two water molecules). The structure of (III) also has conjoined R₄³(10) and centrosymmetric R₄²(8) motifs in the layered structure but these differ in that the first motif involves one cation, an O,O′‐carboxylate group, an O‐carboxylate group and one water molecule, and the second motif involves two cations and two O‐carboxylate groups. The layers lie parallel to (100). The structures of salt hydrates (II) and (III), displaying two‐dimensional layered arrays through conjoined hydrogen‐bonded nets, provide further illustration of a previously indicated trend among ammonium salts of carboxylic acids, but the anhydrous three‐dimensional structure of (I) is inconsistent with that trend.     

A novel two‐dimensional layered framework built of strontium(II) with 4‐nitrobenzene‐1,3‐dicarboxylic acid

In poly[[aquastrontium(II)]‐μ₆‐4‐nitrobenzene‐1,3‐dicarboxylato‐κ⁷O¹:O²:O²:O³:O³,O⁴:O⁴], [Sr(C₈H₃NO₆)(H₂O)]_n, the Sr^II ion displays a distorted bicapped triangular prismatic configuration, defined by seven carboxyl O atoms from six symmetry‐related ligands and one water molecule. The ligand molecules connect the Sr^II ions into a two‐dimensional layered framework in the ac plane, with close O...O contacts between the nitro groups and with each nitro group providing one acceptor O atom for a weak intermolecular C—H...O hydrogen bond.     

Neutral 4‐phenyl‐1‐[phenyl(pyridin‐2‐yl)methylidene]semicarbazide and its salt forms with inorganic anions

Semicarbazones can exist in two tautomeric forms. In the solid state, they are found in the keto form. This work presents the synthesis, structures and spectroscopic characterization (IR and NMR spectroscopy) of four such compounds, namely the neutral molecule 4‐phenyl‐1‐[phenyl(pyridin‐2‐yl)methylidene]semicarbazide, C₁₉H₁₆N₄O, (I), abbreviated as HBzPyS, and three different hydrated salts, namely the chloride dihydrate, C₁₉H₁₇N₄O⁺·Cl^?·2H₂O, (II), the nitrate dihydrate, C₁₉H₁₇N₄O⁺·NO₃^?·2H₂O, (III), and the thiocyanate 2.5‐hydrate, C₁₉H₁₇N₄O⁺·SCN^?·2.5H₂O, (IV), of 2‐[phenyl({[(phenylcarbamoyl)amino]imino})methyl]pyridinium, abbreviated as [H₂BzPyS]⁺·X^?·nH₂O, with X = Cl^? and n = 2 for (II), X = NO₃^? and n = 2 for (III), and X = SCN^? and n = 2.5 for (IV), showing the influence of the anionic form in the intermolecular interactions. Water molecules and counter‐ions (chloride or nitrate) are involved in the formation of a two‐dimensional arrangement by the establishment of hydrogen bonds with the N—H groups of the cation, stabilizing the E isomers in the solid state. The neutral HBzPyS molecule crystallized as the E isomer due to the existence of weak π–π interactions between pairs of molecules. The calculated IR spectrum of the hydrated [H₂BzPyS]⁺ cation is in good agreement with the experimental results.     

CdII and CoII coordination polymers constructed from benzene‐1,4‐dicarboxylic acid and 2‐(pyridin‐3‐yl)‐1H‐benzimidazole ligands

In poly[aqua(μ₃‐benzene‐1,4‐dicarboxylato‐κ⁵O¹,O^1′:O¹:O⁴,O^4′)[2‐(pyridin‐3‐yl‐κN)‐1H‐benzimidazole]cadmium(II)], [Cd(C₈H₄O₄)(C₁₂H₉N₃)(H₂O)]_n, (I), each Cd^II ion is seven‐coordinated by the pyridine N atom from a 2‐(pyridin‐3‐yl)benzimidazole (3‐PyBIm) ligand, five O atoms from three benzene‐1,4‐dicarboxylate (1,4‐bdc) ligands and one O atom from a coordinated water molecule. The complex forms an extended two‐dimensional carboxylate layer structure, which is further extended into a three‐dimensional network by hydrogen‐bonding interactions. In catena‐poly[[diaquabis[2‐(pyridin‐3‐yl‐κN)‐1H‐benzimidazole]cobalt(II)]‐μ₂‐benzene‐1,4‐dicarboxylato‐κ²O¹:O⁴], [Co(C₈H₄O₄)(C₁₂H₉N₃)₂(H₂O)₂]_n, (II), each Co^II ion is six‐coordinated by two pyridine N atoms from two 3‐PyBIm ligands, two O atoms from two 1,4‐bdc ligands and two O atoms from two coordinated water molecules. The complex forms a one‐dimensional chain‐like coordination polymer and is further assembled by hydrogen‐bonding interactions to form a three‐dimensional network.     

A two‐dimensional cadmium(II) coordination polymer constructed from 4‐carboxy‐1‐(4‐carboxylatobenzyl)‐2‐propyl‐1H‐imidazole‐5‐carboxylate and 1‐(4‐carboxylatobenzyl)‐2‐propyl‐1H‐imidazole‐4‐carboxylate ligands

Crystals of poly[[aqua[μ₃‐4‐carboxy‐1‐(4‐carboxylatobenzyl)‐2‐propyl‐1H‐imidazole‐5‐carboxylato‐κ⁵O¹O^1′:N³,O⁴:O⁵][μ₄‐1‐(4‐carboxylatobenzyl)‐2‐propyl‐1H‐imidazole‐4‐carboxylato‐κ⁷N³,O⁴:O⁴,O^4′:O¹,O^1′:O¹]cadmium(II)] monohydrate], {[Cd₂(C₁₅H₁₄N₂O₄)(C₁₆H₁₄N₂O₆)(H₂O)]·H₂O}_n or {[Cd₂(Hcpimda)(cpima)(H₂O)]·H₂O}_n, (I), were obtained from 1‐(4‐carboxybenzyl)‐2‐propyl‐1H‐imidazole‐4,5‐dicarboxylic acid (H₃cpimda) and cadmium(II) chloride under hydrothermal conditions. The structure indicates that in‐situ decarboxylation of H₃cpimda occurred during the synthesis process. The asymmetric unit consists of two Cd²⁺ centres, one 4‐carboxy‐1‐(4‐carboxylatobenzyl)‐2‐propyl‐1H‐imidazole‐5‐carboxylate (Hcpimda²⁻) anion, one 1‐(4‐carboxylatobenzyl)‐2‐propyl‐1H‐imidazole‐4‐carboxylate (cpima²⁻) anion, one coordinated water molecule and one lattice water molecule. One Cd²⁺ centre, i.e. Cd1, is hexacoordinated and displays a slightly distorted octahedral CdN₂O₄ geometry. The other Cd centre, i.e. Cd2, is coordinated by seven O atoms originating from one Hcpimda²⁻ ligand and three cpima²⁻ ligands. This Cd²⁺ centre can be described as having a distorted capped octahedral coordination geometry. Two carboxylate groups of the benzoate moieties of two cpima²⁻ ligands bridge between Cd2 centres to generate [Cd₂O₂] units, which are further linked by two cpima²⁻ ligands to produce one‐dimensional (1D) infinite chains based around large 26‐membered rings. Meanwhile, adjacent Cd1 centres are linked by Hcpimda²⁻ ligands to generate 1D zigzag chains. The two types of chains are linked through a μ₂‐η² bidentate bridging mode from an O atom of an imidazole carboxylate unit of cpima²⁻ to give a two‐dimensional (2D) coordination polymer. The simplified 2D net structure can be described as a 3,6‐coordinated net which has a (4³)₂(4⁶.6⁶.8³) topology. Furthermore, the FT–IR spectroscopic properties, photoluminescence properties, powder X‐ray diffraction (PXRD) pattern and thermogravimetric behaviour of the polymer have been investigated.     

Three‐dimensional networks in 5‐methylimidazolium 3‐carboxy‐4‐hydroxybenzenesulfonate and bis(5‐methylimidazolium) 3‐carboxylato‐4‐hydroxybenzenesulfonate

The two title proton‐transfer compounds, 5‐methylimidazolium 3‐carboxy‐4‐hydroxybenzenesulfonate, C₄H₇N₂⁺·C₇H₅O₆S⁻, (I), and bis(5‐methylimidazolium) 3‐carboxylato‐4‐hydroxybenzenesulfonate, 2C₄H₇N₂⁺·C₇H₅O₆S²⁻, (II), are each organized into a three‐dimensional network by a combination of X—H...O (X = O, N or C) hydrogen bonds, and π–π and C—H...π interactions.     

syn and anti conformations in 2‐hydroxy‐5‐[(E)‐(4‐nitrophenyl)diazenyl]benzoic acid and two related salts

The crystal structures of 2‐hydroxy‐5‐[(E)‐(4‐nitrophenyl)diazenyl]benzoic acid, C₁₃H₉N₃O₅, (I), ammonium 2‐hydroxy‐5‐[(E)‐phenyldiazenyl]benzoate, NH₄⁺·C₁₃H₉N₂O₃⁻, (II), and sodium 2‐hydroxy‐5‐[(E)‐(4‐nitrophenyl)diazenyl]benzoate trihydrate, Na⁺·C₁₃H₈N₃O₅⁻·3H₂O, (III), have been determined using single‐crystal X‐ray diffraction. In (I) and (III), the phenyldiazenyl and carboxylic acid/carboxylate groups are in an anti orientation with respect to each other, which is in accord with the results of density functional theory (DFT) calculations, whereas in (II), the anion adopts a syn conformation. In (I), molecules form slanted stacks along the [100] direction. In (II), anions form bilayers parallel to (010), the inner part of the bilayers being formed by the benzene rings, with the –OH and –COO⁻ substituents on the bilayer surface. The NH₄⁺ cations in (II) are located between the bilayers and are engaged in numerous N—H...O hydrogen bonds. In (III), anions form layers parallel to (001). Both Na⁺ cations have a distorted octahedral environment, with four octahedra edge‐shared by bridging water O atoms, forming [Na₄(H₂O)₁₂]⁴⁺ units.     

A solid‐state oxidation of 1,1,3,3‐tetramethylguanidinium 4‐methylbenzenesulfinate to 1,1,3,3‐tetramethylguanidinium 4‐methylbenzenesulfonate

The organic acid–base complex 1,1,3,3‐tetramethylguanidinium 4‐methylbenzenesulfonate, C₅H₁₄N₃⁺·C₇H₇O₃S⁻, was obtained from the corresponding 1,1,3,3‐tetramethylguanidinium 4‐methylbenzenesulfinate complex, C₅H₁₄N₃⁺·C₇H₇O₂S⁻, by solid‐state oxidation in air. Comparison of the two crystal structures reveals similar packing arrangements in the monoclinic space group P2₁/c, with centrosymmetric 2:2 tetramers being connected by four strong N—H...O=S hydrogen bonds between the imine N atoms of two 1,1,3,3‐tetramethylguanidinium bases and the O atoms of two acid molecules.     

Hydration preferences for Mn4Ca cluster models of photosystem II: location of potential substrate-water binding sites

Density functional theory calculations are reported on a set of three model structures of the Mn₄Ca cluster in the water‐oxidizing complex of Photosystem II (PSII), which share the structural formula [CaMn₄C₉H₁₀N₂O₁₆]^q+ ? (H₂O)_n (q=?1, 0, 1, 2, 3; n=0–7). In these calculations we have explored the preferred hydration sites of the Mn₄Ca cluster across five overall oxidation states (S₀ to S₄) and all feasible magnetic‐coupling arrangements to identify the most likely substrate–water binding sites. We have also explored charge‐compensated structures in which the overall charge on the cluster is maintained at q=0 or +1, which is consistent with the experimental data on sequential proton loss in the real system. The three model structures have skeletal arrangements that are strongly reminiscent, in their relative metal‐atom positions, of the 2.9‐, 3.7‐, and 3.5 Å‐resolution crystal structures, respectively, whereas the charge states encompassed in our study correspond to an assignment of (Mn^III)₃Mn^II for S₀ and up to (Mn^IV)₃Mn^III for S₄. The three models differ principally in terms of the spatial relationship between one Mn (Mn(4)) and a generally robust Mn₃Ca tetrahedron that contains Mn(1), Mn(2), and Mn(3). Oxidation‐state distributions across the four manganese atoms, in most of the explored charge states, are dependent on details of the cluster geometry, on the extent of assumed hydration of the clusters, and in some instances on the imposed magnetic‐coupling between adjacent Mn atoms. The strongest water‐binding sites are generally those on Mn(4) and Ca. However, one structure type displays a high‐affinity binding site between Ca and Mn(3), the S‐state‐dependent binding‐energy pattern of which is most consistent with the substrate water‐exchange kinetics observed in functional PSII. This structure type also permits another water molecule to access the cluster in a manner consistent with the substrate–water interaction with the Mn cluster, seen in electron spin‐echo envelope modulation (ESEEM) studies of the functional enzyme in the S₀ and S₂ states. It also rationalizes the significant differences in hydrogen‐bonding interactions of the substrate water observed in the FTIR measurements of the S₁ and S₂ states. We suggest that these two water‐binding sites, which are molecularly close, model the actual substrate‐binding sites in the enzyme.     

One‐dimensional CuII coordination polymers containing C2h‐symmetric 1,1′:4′,1′′‐terphenyl‐3,3′‐dicarboxylate linkers

Two new one‐dimensional Cu^II coordination polymers (CPs) containing the C_2h‐symmetric terphenyl‐based dicarboxylate linker 1,1′:4′,1′′‐terphenyl‐3,3′‐dicarboxylate (3,3′‐TPDC), namely catena‐poly[[bis(dimethylamine‐κN)copper(II)]‐μ‐1,1′:4′,1′′‐terphenyl‐3,3′‐dicarboxylato‐κ⁴O,O′:O′′:O′′′] monohydrate], {[Cu(C₂₀H₁₂O₄)(C₂H₇N)₂]·H₂O}_n, (I), and catena‐poly[[aquabis(dimethylamine‐κN)copper(II)]‐μ‐1,1′:4′,1′′‐terphenyl‐3,3′‐dicarboxylato‐κ²O³:O^3′] monohydrate], {[Cu(C₂₀H₁₂O₄)(C₂H₇N)₂(H₂O)]·H₂O}_n, (II), were both obtained from two different methods of preparation: one reaction was performed in the presence of 1,4‐diazabicyclo[2.2.2]octane (DABCO) as a potential pillar ligand and the other was carried out in the absence of the DABCO pillar. Both reactions afforded crystals of different colours, i.e. violet plates for (I) and blue needles for (II), both of which were analysed by X‐ray crystallography. The 3,3′‐TPDC bridging ligands coordinate the Cu^II ions in asymmetric chelating modes in (I) and in monodenate binding modes in (II), forming one‐dimensional chains in each case. Both coordination polymers contain two coordinated dimethylamine ligands in mutually trans positions, and there is an additional aqua ligand in (II). The solvent water molecules are involved in hydrogen bonds between the one‐dimensional coordination polymer chains, forming a two‐dimensional network in (I) and a three‐dimensional network in (II).     

Three novel topologically different metal–organic frameworks built from 3‐nitro‐4‐(pyridin‐4‐yl)benzoic acid

The design and synthesis of metal–organic frameworks (MOFs) have attracted much interest due to the intriguing diversity of their architectures and topologies. However, building MOFs with different topological structures from the same ligand is still a challenge. Using 3‐nitro‐4‐(pyridin‐4‐yl)benzoic acid (HL) as a new ligand, three novel MOFs, namely poly[[(N,N‐dimethylformamide‐κO)bis[μ₂‐3‐nitro‐4‐(pyridin‐4‐yl)benzoato‐κ³O,O′:N]cadmium(II)] N,N‐dimethylformamide monosolvate methanol monosolvate], {[Cd(C₁₂H₇N₂O₄)₂(C₃H₇NO)]·C₃H₇NO·CH₃OH}_n, ( 1 ), poly[[(μ₂‐acetato‐κ²O:O′)[μ₃‐3‐nitro‐4‐(pyridin‐4‐yl)benzoato‐κ³O:O′:N]bis[μ₃‐3‐nitro‐4‐(pyridin‐4‐yl)benzoato‐κ⁴O,O′:O′:N]dicadmium(II)] N,N‐dimethylacetamide disolvate monohydrate], {[Cd₂(C₁₂H₇N₂O₄)₃(CH₃CO₂)]·2C₄H₉NO·H₂O}_n, ( 2 ), and catena‐poly[[[diaquanickel(II)]‐bis[μ₂‐3‐nitro‐4‐(pyridin‐4‐yl)benzoato‐κ²O:N]] N,N‐dimethylacetamide disolvate], {[Ni(C₁₂H₇N₂O₄)₂(H₂O)₂]·2C₄H₉NO}_n, ( 3 ), have been prepared. Single‐crystal structure analysis shows that the Cd^II atom in MOF ( 1 ) has a distorted pentagonal bipyramidal [CdN₂O₅] coordination geometry. The [CdN₂O₅] units as 4‐connected nodes are interconnected by L^? ligands to form a fourfold interpenetrating three‐dimensional (3D) framework with a dia topology. In MOF ( 2 ), there are two crystallographically different Cd^II ions showing a distorted pentagonal bipyramidal [CdNO₆] and a distorted octahedral [CdN₂O₄] coordination geometry, respectively. Two Cd^II ions are connected by three carboxylate groups to form a binuclear [Cd₂(COO)₃] cluster. Each binuclear cluster as a 6‐connected node is further linked by acetate groups and L^? ligands to produce a non‐interpenetrating 3D framework with a pcu topology. MOF ( 3 ) contains two crystallographically distinct Ni^II ions on special positions. Each Ni^II ion adopts an elongated octahedral [NiN₂O₄] geometry. Each Ni^II ion as a 4‐connected node is linked by L^? ligands to generate a two‐dimensional network with an sql topology, which is further stabilized by two types of intermolecular OW—HW…O hydrogen bonds to form a 3D supramolecular framework. MOFs ( 1 )–( 3 ) were also characterized by powder X‐ray diffraction, IR spectroscopy and thermogravimetic analysis. Furthermore, the solid‐state photoluminescence of HL and MOFs ( 1 ) and ( 2 ) have been investigated. The photoluminescence of MOFs ( 1 ) and ( 2 ) are enhanced and red‐shifted with respect to free HL. The gas adsorption investigation of MOF ( 2 ) indicates a good separation selectivity (71) of CO₂/N₂ at 273 K (i.e. the amount of CO₂ adsorption is 71 times higher than N₂ at the same pressure).     

Proton‐transfer compounds of isonipecotamide with the aromatic dicarboxylic acids 4‐nitrophthalic, 4,5‐dichlorophthalic, 5‐nitroisophthalic and terephthalic acid

The structures of the 1:1 proton‐transfer compounds of isonipecotamide (piperidine‐4‐carboxamide) with 4‐nitrophthalic acid [4‐carbamoylpiperidinium 2‐carboxy‐4‐nitrobenzoate, C₆H₁₃N₂O₈⁺·C₈H₄O₆⁻, (I)], 4,5‐dichlorophthalic acid [4‐carbamoylpiperidinium 2‐carboxy‐4,5‐dichlorobenzoate, C₆H₁₃N₂O₈⁺·C₈H₃Cl₂O₄⁻, (II)] and 5‐nitroisophthalic acid [4‐carbamoylpiperidinium 3‐carboxy‐5‐nitrobenzoate, C₆H₁₃N₂O₈⁺·C₈H₄O₆⁻, (III)], as well as the 2:1 compound with terephthalic acid [bis(4‐carbamoylpiperidinium) benzene‐1,2‐dicarboxylate dihydrate, 2C₆H₁₃N₂O₈⁺·C₈H₄O₄²⁻·2H₂O, (IV)], have been determined at 200 K. All salts form hydrogen‐bonded structures, viz. one‐dimensional in (II) and three‐dimensional in (I), (III) and (IV). In (I) and (III), the centrosymmetric R₂²(8) cyclic amide–amide association is found, while in (IV) several different types of water‐bridged cyclic associations are present [graph sets R₄²(8), R₄³(10), R₄⁴(12), R₃³(18) and R₆⁴(22)]. The one‐dimensional structure of (I) features the common `planar' hydrogen 4,5‐dichlorophthalate anion, together with enlarged cyclic R₃³(13) and R₄³(17) associations. In the structures of (I) and (III), the presence of head‐to‐tail hydrogen phthalate chain substructures is found. In (IV), head‐to‐tail primary cation–anion associations are extended longitudinally into chains through the water‐bridged cation associations, and laterally by piperidinium–carboxylate N—H...O and water–carboxylate O—H...O hydrogen bonds. The structures reported here further demonstrate the utility of the isonipecotamide cation as a synthon for the generation of stable hydrogen‐bonded structures. An additional example of cation–anion association with this cation is also shown in the asymmetric three‐centre piperidinium–carboxylate N—H...O,O′ interaction in the first‐reported structure of a 2:1 isonipecotamide–carboxylate salt.