ABSTRACT
Built from L-histidine amino acid ligand and cadmium ions, two new 3D chiral metal-organic frameworks, [α-Cd(HIS)] (1) and [ß-Cd(HIS)] (2), which contain metal-histidine bonds mimicking the structure features of carbonic anhydrase, demonstrate interesting properties of catalyzing the hydrolysis of p-nitrophenylacetate (p-NPA) to para-nitrophenol (p-NP).
ABSTRACT
New hybrid layered vanadates, M(bpy)V4O10 (I, M = Cu+; II, M = Ag+; bpy = 4,4'-bipyridine), were prepared from hydrothermal reactions at 220-230 degrees C, and their structures were characterized by single-crystal X-ray diffraction [I, P21/c (No. 14), Z = 4, a = 3.6154(3) A, b = 21.217(1) A, c = 20.267(1) A, and beta = 90.028(3) degrees ; II, P (No. 2), Z = 2, a = 3.5731(4) A, b = 10.429(1) A, c = 21.196(2) A, alpha = 89.031(5) degrees , beta = 89.322(5) degrees , and gamma = 85.546(5) degrees ]. The structures of I and II are closely related, though not isostructurally, with both containing partially reduced V4O10- layers that are constructed from zigzag chains of edge-sharing VO5 tetragonal pyramids. Neighboring zigzag chains within a layer condense via shared vertices and alternate between versions containing V4.5+ and V5+ ions, such that two out of four symmetry-unique V atoms are reduced by a half-electron on average. The interlayer spaces contain unusual M(bpy)+ chains formed from the coordination of two bridging bpy ligands to Ag+/Cu+ in a nearly linear fashion and each with a third bond to a single apical O atom of the reduced (V4.5+) VO5 tetragonal pyramids. Both I and II are stable until approximately 350-400 degrees C in O2, at which point the ligands are liberated to yield the purely inorganic MxV4O10 (M = Ag, Cu) solids. The electrical conductivities of both compounds show a temperature dependence that is consistent with Mott's variable-range-hopping model for randomly localized electrons. Magnetic susceptibilities of both I and II can be fitted to a Curie-Weiss expression (theta = -25 and -31 K, respectively; C approximately 0.40 emu.mol-1.K for both) at higher temperatures and one unpaired spin per formula. However, at below approximately 12-18 K, both show evidence for an antiferromagnetic transition that can be fitted well to the Heisenberg linear antiferromagnetic chain model. These results are analyzed with respect to related reduced vanadates and help to provide new structure-property insights for strongly correlated electron systems.
ABSTRACT
Reduced layered M(C4H4N2)V4O10 ((I, M = Co; II, M = Ni; III, M = Zn); C4H4N2 = pyrazine, pyz) hybrid solids were synthesized via hydrothermal reactions at 200-230 degrees C, and their structures determined by single-crystal X-ray diffraction (Cmcm, No. 63, Z = 4; a = 14.311(2), 14.2372(4), 14.425(1) A; b = 6.997(1), 6.9008(2), 6.9702(6) A; and c = 11.4990(8), 11.5102(3), 11.479(1) A; for I, II, and III, respectively). All three solids are isostructural and contain V4O102- layers condensed from edge- and corner-shared VO5 square pyramids. A single symmetry-unique V atom is reduced by 1/2 electron (on average) and bonds via its apical oxygen atom to interlayer Mpyz2+ chains. Magnetic susceptibility measurements show a strong temperature dependence and a Curie constant that is consistent with two randomly localized spins per V4O10(2-) formula for III. Further, the unusual discovery of a remarkably well-defined transition to a singlet ground state, as well as formation of a spin gap, is found for III at 22(1) K. The temperature-dependent electrical conductivities show apparent activation energies of 0.36 (I), 0.46 (II), and 0.59 eV (III). During heating cycles in flowing N2, the samples exhibit weight losses corresponding to the removal of predominantly pyrazine, pyrazine fragments, and CO2 via reaction of pyrazine with the vanadate layer. The complete removal of pyrazine without loss of crystallinity is found for well-ground samples of I and III. The SEM images of I and II after heating at 400-500 degrees C show relatively intact crystals, but at 600 degrees C further structural collapse results in the formation of macropores at the surfaces.
ABSTRACT
The layered molybdate [M2(pzc)2(H2O)x][Mo5O16] (I: M = Ni, x = 5.0; II: M = Co, x = 4.0; pzc = pyrazinecarboxylate) hybrid solids were synthesized via hydrothermal reactions at 160-165 degrees C. The structures were determined by single-crystal X-ray diffraction data for I (Cc, Z = 4; a = 33.217(4) A, b = 5.6416(8) A, c = 13.982(2) A, beta = 99.407(8) degrees , and V = 2585.0(6) A3) and powder X-ray diffraction data for II (C2/c, Z = 4; a = 35.42(6) A, b = 5.697(9) A, c = 14.28(2) A, beta = 114.95(4) degrees , and V = 2614(12) A3). The polar structure of I contains new [Ni2(pzc)2(H2O)5]2+ double layers that form an asymmetric pattern of hydrogen bonds and covalent bonds to stair-stepped [Mo5O16]2- sheets, inducing a net dipole moment in the latter. In II, however, the [Co2(pzc)2(H2O)4]2+ double layers have one less coordinated water and subsequently exhibit a symmetric pattern of covalent and hydrogen bonding to the [Mo5O16]2- sheets, leading to a centrosymmetric structure. Thermogravimetric analyses and powder X-ray diffraction data reveal that I can be dehydrated and rehydrated with from 0 to 6.5 water molecules per formula unit, which is coupled with a corresponding contraction/expansion of the interlayer distances. Also, the dehydrated form of I can be intercalated by approximately 4.3 H2S molecules per formula unit, but the intercalation by pyridine or methanol is limited to less than one molecule per formula unit.
ABSTRACT
A new helically pillared and chiral solid, Cu(pzc)2AgReO4 (I, pzc = pyrazinecarboxylate), was synthesized from hydrothermal reactions at 95-125 degrees C. The structural origin of its chirality, relative to the achiral M(pzc)2(H2O)2AgReO4 (II, M = Co; III, M = Ni) analogues, arises from significantly tilted pillars and hydrogen bonds to the AgReO4 layers. The new pillared structure exhibits second harmonic generation activity, CO2 absorption, thermal stability to approximately 250 degrees C, and Curie-Weiss magnetism expected for isolated Cu2+.
ABSTRACT
A polymeric hybrid cluster-based compound with a double-layered honeycomb framework built of octahedral niobium cyanochloride clusters and [Zn(en)]2+ metal complexes was designed and synthesized at room temperature, and structurally characterized by single crystal X-ray diffraction.
Subject(s)
Niobium/chemistry , Organometallic Compounds/chemistry , Zinc/chemistry , Models, Molecular , Organometallic Compounds/chemical synthesis , X-Ray DiffractionABSTRACT
The preparation, structure, and magnetic properties of the first three-dimensional framework containing octahedral niobium cyanochloride clusters as building units are reported. Reactions of aqueous solutions of (Me(4)N)(2)K(2)[Nb(6)Cl(12)(CN)(6)] (2) with aqueous solutions of MnCl(2) result in the precipitation of the compound (Me(4)N)(2)[MnNb(6)Cl(12)(CN)(6)] (3). The structure of 3 was determined from single-crystal X-ray diffraction study (crystal data: cubic, Fm3macrom(No. 225), a = 15.513(4) A, V = 3733.2(12) A(3), Z = 4). Its 3D framework is based on edge-bridged [Nb(6)Cl(12)](2+) clusters and Mn(2+) ions bridged by cyanide ligands to form a cfc lattice [MnNb(6)Cl(12)(CN)(6)](2)(-) in which all tetrahedral sites are occupied by the cations (Me(4)N)(+) which act as charge compensating template. The structure of 3 can be considered as an expansion of the Prussian blue framework in which [Fe(CN)(6)](4)(-) is replaced by the cluster [Nb(6)Cl(12)(CN)(6)](4)(-). Magnetic susceptibility measurements indicate that Mn(2+) is present in a high spin d(5) configuration. No magnetic ordering is observed.
