Mohamedally Kurmoo

Rigid Pillars and Double Walls in a Porous Metal-Organic Framework: Single-Crystal to Single-Crystal, Controlled Uptake and Release of Iodine and Electrical Conductivity

A highly stable pillared and double-walled zinc(II) metal-organic framework with regular nanochannels displays single-crystal to single-crystal transformation upon desolvation and a large quantity of iodine uptake, controlled release, and electrical conductivity elevation due to synergy between the iodine guests and the host framework.

Hard magnets based on transition metal complexes with the dicyanamide anion, {N(CN)2}-,

We present the crystal structures and magnetic properties of a series of magnetic compounds, MII{N(CN)2}2, where M=Cu (1), Ni (2), Co (3) and Fe (4), and [Mn{N(CN)2}2(C2H5OH)2]Z·(CH3)2CO (5). In the isostructural compounds 1–4, the dicyanamide anion is triply coordinating through its three nitrogen atoms. It bridges the metal ions to form infinite 3D metal-organic frameworks with a rutile-type structure. The framework contains doubly bridged M(–Nâ–·C–N–Câ–·N–)2 ribbons that link approximately orthogonally through the amide nitrogen atoms. The Jahn–Teller distortion in 1 has a strong influence on the packing arrangement (M–N bond lengths: 1.98 and 2.47 A for 1 and 2.10 and 2.15 A for 3). On lowering the temperature the bond distances in 1 remain unchanged except for a decrease of the M–Namide length to 2.45 A. Magnetic data for 1 obey the Curie–Weiss law (Θ=-2.1 K). 2 and 3 are ferromagnets with Curie temperatures (TC) of 9 and 21 K and are characterized by hysteresis loops of 710 and 7975 Oe at 2 K, remnant magnetization, magnetization approaching the expected saturation (gS) of 2 and 3 µB in high field, absorptive component (χ″) in the AC magnetization and λ peak in the heat capacity data. 4 is similarly characterized and shows behaviour that is characteristic of a canted antiferromagnet: the Weiss constant is temperature dependent (+3 K in the range 200–300 K), there is a sharper peak than for 1 or 2 in the AC magnetization and the isothermal magnetization at 3 K increases monotonically to ≈1.3 µB (expected to be 4 µB for ferromagnetic alignment of the spins) in a field of 8 T. Its coercive field (17800 Oe) is the largest observed for any metal-organic compound and exceeds those of alloys of SmCo5 and Nd2Fe14B. The maximum energy product (B · H) is the highest for 3 and is comparable to alloys of Sm–Co. We attribute the large coercive field to a combination of single ion and particle shape anisotropies. 5 is paramagnetic at high temperature with Θ=-3 K. Below 16 K it behaves as a canted antiferromagnet with a very weak resultant spontaneous magnetization.

Mn3(HCOO)6: a 3D porous magnet of diamond framework with nodes of Mn-centered MnMn4 tetrahedron and guest-modulated ordering temperature

Mn3(HCOO)6, a 3D highly stable and flexible porous diamondoid framework based on Mn-centered MnMn4 tetrahedral nodes, exhibits a wide spectrum of guest inclusion behaviour and long-range magnetic ordering with guest-modulated critical temperature.

Metal–Organic Perovskites: Synthesis, Structures, and Magnetic Properties of [C(NH2)3][MII(HCOO)3] (M=Mn, Fe, Co, Ni, Cu, and Zn; C(NH2)3= Guanidinium)

We report the synthesis, crystal structures, and spectral, thermal, and magnetic properties of a family of metal-organic perovskite ABX(3), [C(NH(2))(3)][M(II)(HCOO)(3)], in which A = C(NH(2))(3) is guanidinium, B = M is a divalent metal ion (Mn, Fe, Co, Ni, Cu, or Zn), and X is the formate HCOO(-). The compounds could be synthesized by either diffusion or hydrothermal methods from water or water-rich solutions depending on the metal. The five members (Mn, Fe, Co, Ni, and Zn) are isostructural and crystallize in the orthorhombic space group Pnna, while the Cu member in Pna2(1). In the perovskite structures, the octahedrally coordinated metal ions are connected by the anti-anti formate bridges, thus forming the anionic NaCl-type [M(HCOO)(3)](-) frameworks, with the guanidinium in the nearly cubic cavities of the frameworks. The Jahn-Teller effect of Cu(2+) results in a distorted anionic Cu-formate framework that can be regarded as Cu-formate chains through short basal Cu-O bonds linked by the long axial Cu-O bonds. These materials show higher thermal stability than other metal-organic perovskite series of [AmineH][M(HCOO)(3)] templated by the organic monoammonium cations (AmineH(+)) as a result of the stronger hydrogen bonding between guanidinium and the formate of the framework. A magnetic study revealed that the five magnetic members (except Zn) display spin-canted antiferromagnetism, with a Néel temperature of 8.8 (Mn), 10.0 (Fe), 14.2 (Co), 34.2 (Ni), and 4.6 K (Cu). In addition to the general spin-canted antiferromagnetism, the Fe compound shows two isothermal transformations (a spin-flop and a spin-flip to the paramagnetic phase) within 50 kOe. The Co member possesses quite a large canting angle. The Cu member is a magnetic system with low dimensional character and shows slow magnetic relaxation that probably results from the domain dynamics.

Anionic NaCl-type frameworks of [MnII(HCOO)3−], templated by alkylammonium, exhibit weak ferromagnetism

We present the synthesis, characterization by IR, TGA, single crystal X-ray structure and magnetic properties of a novel series of NaCl-type frameworks of [AmineH(+)][Mn(HCOO)(3)(-)], templated by alkylammonium. The anionic NaCl-framework of [Mn(HCOO)(3)(-)] is counter-balanced by the alkylammonium cations located in the cavities of the framework to which they are hydrogen-bonded. The divalent manganese ions have octahedral geometry and are bridged by the formate in an anti-anti mode of coordination. All the compounds exhibit long-range antiferromagnetism below 9 K with a slight non-collinear arrangement of the moments. The canting, likely due to second-order spin-orbit coupling, is via a Dzyaloshinski-Moriya antisymmetric exchange mechanism. A spin-flop is observed in each case at fairly low fields. An orthorhombic to monoclinic transformation was observed for the protonated cyclotrimethyleneamine that is accompanied by localization of the cations into two positions below 240 K from the rapid dynamic flipping of the ring observed at room temperature.

Tracking the Formation of a Polynuclear Co16 Complex and Its Elimination and Substitution Reactions by Mass Spectroscopy and Crystallography

We present the syntheses and structures of the biggest chiral cobalt coordination cluster, [Co16(L)4(H3L)8(N3)6](NO3)2·16H2O·2CH3OH (1, where H4L = S,S-1,2-bis(1H-benzimidazol-2-yl)-1,2-ethanediol). 1 consists of two Co4O4 cubes (Co4(L)2(H3L)2) alternating with Co2(EO-N3)2Co2 (Co4(L)2(H3L)2(N3)2), bridged by the benzimidazole and azide nitrogen atoms to form a twisted ring. The ligand adopts both cis and trans forms, and all the rings have the same chiralilty. ESI-MS of 1 from a methanol solution of crystals reveals the fragment [Co16(L)4(H3L)8(N3)6+2H](4+), suggesting the polynuclear core is stable in solution. ESI-MS measurements from the reaction solution found smaller fragments, [Co4(H3L)4-H](3+), [Co4(H3L)4-2H](2+), [Co4(H3L)4(N3)2](2+), and [Co2(H3L)2](2+), and ESI-MS from a methanol solution of the solid deposit found in addition the Co16 core. These results and the dependence on the synthesis time allow us to propose the process for the formation of 1, which opens up a new way for the direct observation of the ligand-controlled assembly of clusters. In addition, the isolation of [Co4(H3L)4](NO3)4·4H2O (2) consisting of separate Co4O4 cubes with the ligands being only cis in crystalline form supports the proposal. Interestingly, N3(-) is replaced by either CH3O(-) or OH(-), and this is the first time that high-resolution ESI-MS is successfully utilized to examine both the step-by-step elimination and substitution of inner bridging ligands in such a high nuclear complex. Increasing the voltage results in stepwise elimination of azide from the parent cluster. The preliminary magnetic susceptibility of 1 indicates ferromagnetic cubes antiferromagnetically coupled to the squares within the cluster, though in a field of 2.5 kOe, weak and slow relaxation is observed below 4 K.

Tandem Postsynthetic Modification of a Metal–Organic Framework by Thermal Elimination and Subsequent Bromination: Effects on Absorption Properties and Photoluminescence

Inside and outside: Two consecutive postsynthetic modifications, first an elimination reaction in the channels and then bromination at the surface, were realized in a new hybrid metal-organic framework. The dramatic effects of the different groups in the channels and at the surface were studied using gas sorption and the loading/release of solvent and iodine.

A Porous 4-Fold-Interpenetrated Chiral Framework Exhibiting Vapochromism, Single-Crystal-to-Single-Crystal Solvent Exchange, Gas Sorption, and a Poisoning Effect

The synthesis and characterization of a 4-fold-interpenetrated pseudodiamond metal-organic framework (MOF), Co(II)(pybz)2·2DMF [pybz = 4-(4-pyridyl)benzoate], are reported. N,N-Dimethylformamide (DMF) of the channels can be removed to give the porous framework, and it can also be exchanged for methanol, ethanol, benzene, and cyclohexane. It is a rare example of a stable MOF based on a single octahedral building unit. The single-crystal structures of Co(II)(pybz)2·2DMF, Co(II)(pybz)2, Co(II)(pybz)2·4MeOH, and Co(II)(pybz)2·2.5EtOH have been successfully determined. In all of them, the framework is marginally modified and contains a highly distorted and strained octahedral node of cobalt with two pyridine nitrogen atoms and two chelate carboxylate groups. In air, the crystals of Co(II)(pybz)2·2DMF readily change color from claret red to light pink. Thermogravimetric analysis and Raman spectroscopy indicate a change in coordination, where the carboxylate becomes monodentate and an additional two water molecules are coordinated to each cobalt atom. In a dry solvent, this transformation does not take place. Tests show that Co(II)(pybz)2 may be a more efficient drying agent than silica gel and anhydrous CuSO4. The desolvated Co(II)(pybz)2 can absorb several gases such as CO2, N2, H2, and CH4 and also vapors of methanol, ethanol, benzene, and cyclohexane. If Co(II)(pybz)2 is exposed to air and followed by reactivation, its sorption capacity is considerably reduced, which we associate with a poisoning effect. Because of the long distance between the cobalt atoms in the structure, the magnetic properties are those of a paramagnet.

Magnetotransport studies of the organic superconductor kappa -(BEDT-TTF)2Cu(NCS)2 under pressure: the relationship between carrier effective mass and critical temperature

Magnetotransport measurements have been carried out on the organic superconductor kappa -(BEDT-TTF)2Cu(NCS)2 at temperatures down to 500 mK and in hydrostatic pressures up to 16.3 kbar. The observation of Shubnikov-de Haas and magnetic breakdown oscillations has allowed the pressure dependences of the area of the closed pocket of the Fermi surface and the carrier effective masses to be deduced and compared with simultaneous measurements of the superconducting critical temperature Tc. The effective mass measured by the temperature dependence of the Shubnikov-de Haas oscillations is found to fall rapidly with increasing pressure up to a critical pressure Pc approximately=5 kbar. Above Pc a much weaker pressure dependence is observed; Tc also falls rapidly with pressure from 10.4 K at ambient pressure to zero at around Pc. This strongly suggests that the enhanced effective mass and the superconducting behaviour are directly connected in this organic superconductor. A simplified model of the band structure of kappa -(BEDT-TTF)2Cu(NCS)2 has been used to derive the bare band masses of the electrons from optical data. Comparisons of these parameters with cyclotron resonance data and the effective masses measured in the present experiments indicate that the greater part of the enhancement of the effective mass necessary for superconductivity in this material is due to quasiparticle interactions, with the electron-phonon interactions playing a secondary role. The dependence of Tc on the effective mass may be fitted satisfactorily to a suitably parametrized weak-coupling BCS expression, although this cannot be taken as a definitive proof of the nature of superconductivity in organic conductors.

Hierarchical Assembly of a {MnII15MnIII4} Brucite Disc: Step-by-Step Formation and Ferrimagnetism

In search of functional molecular materials and the study of their formation mechanism, we report the elucidation of a hierarchical step-by-step formation from monomer (Mn) to heptamer (Mn7) to nonadecamer (Mn19) satisfying the relation 1 + Σn6n, where n is the ring number of the Brucite structure using high-resolution electrospray ionization mass spectrometry (HRESI-MS). Three intermediate clusters, Mn10, Mn12, and Mn14, were identified. Furthermore, the Mn19 disc remains intact when dissolved in acetonitrile with a well-resolved general formula of [Mn19(L)x(OH)y(N3)36-x-y](2+) (x = 18, 17, 16; y = 8, 7, 6; HL = 1-(hydroxymethyl)-3,5-dimethylpyrazole) indicating progressive exchange of N3(-) for OH(-). The high symmetry (R-3) Mn19 crystal structure consists of a well-ordered discotic motif where the peripheral organic ligands form a double calix housing the anions and solvent molecules. From the formula and valence bond sums, the charge state is mixed-valent, [Mn(II)15Mn(III)4]. Its magnetic properties and electrochemistry have been studied. It behaves as a ferrimagnet below 40 K and has a coercive field of 2.7 kOe at 1.8 K, which can be possible by either weak exchange between clusters through the anions and solvents or through dipolar interaction through space as confirmed by the lack of ordering in frozen CH3CN. The moment of nearly 50 NμB suggests Mn(II)-Mn(II) and Mn(III)-Mn(III) are ferromagnetically coupled while Mn(II)-Mn(III) is antiferromagnetic which is likely if the Mn(III) are centrally placed in the cluster. This compound displays the rare occurrence of magnetic ordering from nonconnected high-spin molecules.