J. Appl. Cryst.の発刊に際して

Endohedral metallofullerenes (EMFs), a new class of hybrid molecules formed by encapsulation of metallic species inside fullerene cages, exhibit unique properties that differ distinctly from those of empty fullerenes because of the presence of metals and their hybridization effects via electron transfer. This critical review provides a balanced but not an exhaustive summary regarding almost all aspects of EMFs, including the history, the classification, current progress in the synthesis, extraction, isolation, and characterization of EMFs, as well as their physiochemical properties and applications in fields such as electronics, photovoltaics, biomedicine, and materials science. Emphasis is assigned to experimentally obtained results, especially the X-ray crystallographic characterizations of EMFs and their derivatives, rather than theoretical calculations, although the latter has indeed enhanced our knowledge of metal–cage interactions. Finally, perspectives related to future developments and challenges in the research of EMFs are proposed. (381 references)

Current status and future developments of endohedral metallofullerenes

We present a facile synthesis of bioinspired manganese oxides for chemical and photocatalytic water oxidation, starting from a reliable and versatile manganese(II) oxalate single-source precursor (SSP) accessible through an inverse micellar molecular approach. Strikingly, thermal decomposition of the latter precursor in various environments (air, nitrogen, and vacuum) led to the three different mineral phases of bixbyite (Mn2 O3 ), hausmannite (Mn3 O4 ), and manganosite (MnO). Initial chemical water oxidation experiments using ceric ammonium nitrate (CAN) gave the maximum catalytic activity for Mn2 O3 and MnO whereas Mn3 O4 had a limited activity. The substantial increase in the catalytic activity of MnO in chemical water oxidation was demonstrated by the fact that a phase transformation occurs at the surface from nanocrystalline MnO into an amorphous MnOx (1<x<2) upon treatment with CAN, which acted as an oxidizing agent. Photocatalytic water oxidation in the presence of [Ru(bpy)3 ](2+) (bpy=2,2'-bipyridine) as a sensitizer and peroxodisulfate as an electron acceptor was carried out for all three manganese oxides including the newly formed amorphous MnOx . Both Mn2 O3 and the amorphous MnOx exhibit tremendous enhancement in oxygen evolution during photocatalysis and are much higher in comparison to so far known bioinspired manganese oxides and calcium-manganese oxides. Also, for the first time, a new approach for the representation of activities of water oxidation catalysts has been proposed by determining the amount of accessible manganese centers.

Nanostructured manganese oxides as highly active water oxidation catalysts: a boost from manganese precursor chemistry.

In continuation of our previous brief review of structural concepts of novel supramolecular proton transfer compounds and their metal complexes by Aghabozorg et al. [1], we briefly surveyed the current research in the field of proton transfer compounds supramolecular synthons and their self-assembled metallic complexes from the points of view of Crystal Engineering and Density Functional Theory (DFT) since 2008. Our research groups have recently focused on the proton delivery from acids, which are considered to be suitable proton donors, to amines as proton acceptors. The results were the production of several proton transfer ion pairs possessing some remaining donor sites applied for coordination to metal centers in the preparation of metal-organic compounds. Some of the complexes showed contributions of both cationic and anionic fragments of the starting ion pair, while some others contained only one of these species as ligand. Our review and investigation of compounds revealed that they mainly focused on the concept of supramolecular systems, co-crystallization, stereochemically active lone pairs, coordination polyhedron, mainly on the various interactions involved, including van der Waals, ion pairing, hydrogen bondings, face to face ν-β stackings and edge to face C-Hπ, C-Oπ, N-H3π and SS. These interactions were the most commonly used strategies in the extension of supramolecular structures.

A brief review of structural concepts of novel supramolecular proton transfer compounds and their metal complexes (part II)

Different transition metal coordination compounds based on py-2,5-dcH2 have been synthesised, i.e. (H2-a-4-mpym)2[Zn(py-2,5-dc)2(H2O)2] (1), (H9-aacr)2[Zn(py-2,5-dc)2(H2O)2]·4H2O (2), (H9-aacr)2[Ni(py-2,5-dc)2(H2O)2]·2H2O (3), (H9-aacr)2[Cu(py-2,5-dc)2(H2O)]·H2O·DMF (4), and (H9-aacr)3[Cr(CN)4(py-2,5-dc)]·5H2O (5), starting from two proton-transfer compounds, (H2-a-4-mpym)2(py-2,5-dc) and (H9-aacr)2(py-2,5-dc) [2-a-4-mpym = 2-amino-4-methylpyrimidine, py-2,5-dcH2 = pyridine-2,5-dicarboxyxlic acid, and 9-aacr = 9-aminoacridine]. They were characterized by elemental analyses, IR and 1H-, 13C-NMR spectroscopy, TGA, and single crystal X-ray diffraction analysis. Results revealed that following various experimental conditions such as temperature, solvent, and pH, the py-2,5-dcH2 ligand undertakes different coordination modes to the metal centres determined by their stereoelectronic requirements and affording mononuclear complex anions with the coordination environments (either pseudo-octahedral or square pyramidal geometries) being completed by water molecules (1–4) or CN− ligands (5). However, in all cases py-2,5-dcH2 acts as a bidentate ligand through the oxygen atom of a carboxylate group and the N atom of the pyridine ring. Complex anions and (H2-a-4-mpym)+/(H9-aacr)+ units together with co-crystallised solvent molecules interact with each other via hydrogen bonds, C–H⋯π, and/or π⋯π interactions to afford various interaction synthons that play an important role in building the overall supermolecular crystal lattices. In order to better understand the crystalline network of compounds 1–5, their respective non-covalent interactions have been studied by means of high level DFT-D calculations. These calculations have shown that complexes containing different spatial-directed py-2,5-dcH2 ligands following the stereoelectronic requirements of the central metal ion could have different predictable effects on the structure of pertinent monomers and networks. The method meets the need of contemporary crystal engineering concepts and is useful for practical synthesis and/or design.

Insight into the connecting roles of interaction synthons and water clusters within different transition metal coordination compounds of pyridine-2,5-dicarboxylic acid: experimental and theoretical studies

The title polymer, [Mn(C2O4)(H2O)2]n, was synthesized by the reaction of manganese(II) nitrate tetra­hydrate with a proton-transfer compound, (pipzH2)(ox) (where oxH2 is oxalic acid and pipz is piperazine), in aqueous solution. The structure features a six-coordinate Mn2+ ion located on a twofold rotation axis, with a distorted octa­hedral geometry [Mn—O = 2.1728 (8)–2.1914 (9) A]. The bond angles show that the two coordinated water mol­ecules are arranged trans. One-dimensional linear chains are formed through bis-bidentate oxalate ligands bridging the MnII ions. These structural units are held together by O—H⋯O hydrogen bonds. A neutron diffraction study of [Mn(μ-ox)(D2O)2]n, using powdered samples, has been published recently in the same space group [Sledzinska, Murasik & Fischer (1987). J. Phys. Solid State Phys. 20, 2247–2259]. However, it should be noted that an ortho­rhom­bic polymorph for the title polymer is also known [Huizing, van Hal, Kwestroo, Langereis & van Loosdregt (1977). Mater. Res. Bull. 12, 605–611; Lethbridge, Congreve, Esslemont, Slawin & Lightfoot (2003). J. Solid State Chem. 172, 212–218]. The present single-crystal X-ray study deals with the monoclinic polymorph. This phase is isostructural with the CoII analogue.

The monoclinic polymorph ofcatena-poly[[diaquamanganese(II)]-μ-oxalato-κ4O1,O2:O1′,O2′]

The reaction of nickel(II) nitrate hexa­hydrate, guanidine (G) and 4-hydroxy­pyridine-2,6-dicarboxylic acid (hypydcH2) in a 1:2:2 molar ratio in aqueous solution resulted in the formation of the title compound, (CH6N3)2[Ni(C7H3NO5)2]·2H2O or (GH)2[Ni(hypydc)2]·2H2O. The six donor atoms of the two 4-hydroxy­pyridine-2,6-dicarboxyl­ate or (hypydc)2− ligands form a distorted octa­hedral arrangement around the NiII centre. Considerable C—O⋯π stacking inter­actions between the CO groups of carboxyl­ate fragments and the pyridine rings of (hypydc)2− with a distance of 3.3212 (8) A are observed. In the crystal structure, a wide range of noncovalent inter­actions consisting of hydrogen bonding (of the types O—H⋯O and N—H⋯O), ion pairing, and π–π [centroid–centroid distance 3.8037 (5) A], N—H⋯π and C—O⋯π stacking inter­actions connect the various components into a supra­molecular structure.

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Bis(guanidinium) bis-(4-hydroxy-pyridine-2,6-dicarboxyl-ato-κO,N,O)nickel-ate(II) dihydrate.

The reaction of magnesium(II) chloride tetra­hydrate with the proton-transfer compound piperazinediium pyridine-2-carboxyl­ate, (pipzH2)(pyc)2 (pipz is pirerazine and pycH is pyridine-2-carboxylic acid) in aqueous solution leads to the formation of the title compound, [Mg(C6H4NO2)2(H2O)2]·0.15H2O. The Mg atom is six-coordinated in a distorted octa­hedral environment by two bidentate pyridine-2-carboxyl­ate groups and two O atoms of coordinated water mol­ecules, which are located in cis positions. In the crystal structure, inter­molecular O—H⋯O and C—H⋯O hydrogen bonds, and π–π [π–π distance = 3.5616 (8) A for pyridine rings; symmetry code: 2-x, 1-y, 1-z] and C—H⋯π stacking, connect the various components into a supra­molecular structure.

Diaquabis(pyridine‐2‐carboxylato)magnesium(II) 0.15‐hydrate

The title compound, 4C12H9N2+·2C10H5O8−·C10H4O82−, contains singly and doubly deprotonated tetracarboxylic acids and the protonated form of 1,10-phenanthroline; the dianion is centrosymmetric. The crystal structure is stabilized by strong inter­molecular O—H⋯O and N—H⋯N hydrogen bonds, linking the ions to form a three-dimensional network.

A proton-transfer compound: 1,10-phenanthrolinium–2,4,5-tricarboxy­benzoate–2,5-dicarboxy­benzene-1,4-dicarboxyl­ate (4/2/1)

The title ligand, [1-(2-methoxyphenyl)-3-(4-chlorophenyl)]triazene, HL (1), was prepared. In a reaction with Hg(NO3)2 it forms the complex [Hg(C26H22Cl2N6O2)], [HgL2] (2). Both compounds were characterized by means of X-ray crystallography, CHN analysis, FT-IR, 1H NMR, and 13C NMR spectroscopy. In the structure of compound 1, two independent fragments are present in the unit cell. They exhibit trans arrangement about the –N=N– double bond. The dihedral angles between two benzene rings in both fragments are 4.36 and 18.79 A, respectively. Non-classic C–H···N hydrogen bonding and C–H···π interactions form a layer structure along the crystallographic ab plane [110]. In compound 2, the HgII atom is hexacoordinated by two tridentate [1-(2-methoxyphenyl)-3-(4-chlorophenyl)]triazenide ligands through a N2O2 set. In addition, in the structure of 2, monomeric complexes are connected to each other by C–H···π stacking interactions, resulting in a 2D architecture. These C–H···π edge-to-face interactions are present with H···π distances of 3.156 and 3.027 A. The results of studies of the stoichiometry and formation of complex 2 in methanol solution were found to support its solid state stoichiometry.

Jafar Attar Gharamaleki

Papers

The monoclinic polymorph ofcatena-poly[[diaquamanganese(II)]-μ-oxalato-κ4O1,O2:O1′,O2′]

Bis(guanidinium) bis-(4-hydroxy-pyridine-2,6-dicarboxyl-ato-κO,N,O)nickel-ate(II) dihydrate.

Diaquabis(pyridine‐2‐carboxylato)magnesium(II) 0.15‐hydrate

A proton-transfer compound: 1,10-phenanthrolinium–2,4,5-tricarboxybenzoate–2,5-dicarboxybenzene-1,4-dicarboxylate (4/2/1)

Synthesis, Structure, and Solution Study of a Mercury(II) Complex with the Ligand [1‐(2‐Methoxyphenyl)‐3‐(4‐chlorophenyl)]triazene

Jafar Attar Gharamaleki

Papers

The monoclinic polymorph ofcatena-poly[[diaquamanganese(II)]-μ-oxalato-κ4O1,O2:O1′,O2′]

Bis(guanidinium) bis-(4-hydroxy-pyridine-2,6-dicarboxyl-ato-κO,N,O)nickel-ate(II) dihydrate.

Diaquabis(pyridine‐2‐carboxylato)magnesium(II) 0.15‐hydrate

A proton-transfer compound: 1,10-phenanthrolinium–2,4,5-tricarboxy­benzoate–2,5-dicarboxy­benzene-1,4-dicarboxyl­ate (4/2/1)

Synthesis, Structure, and Solution Study of a Mercury(II) Complex with the Ligand [1‐(2‐Methoxyphenyl)‐3‐(4‐chlorophenyl)]triazene

A proton-transfer compound: 1,10-phenanthrolinium–2,4,5-tricarboxybenzoate–2,5-dicarboxybenzene-1,4-dicarboxylate (4/2/1)