Paramita Kar

Antiferromagnetic Porous Metal–Organic Framework Containing Mixed-Valence [MnII4MnIII2(μ4-O)2]10+ Units with Catecholase Activity and Selective Gas Adsorption

A multifunctional porous metal organic framework based on mixed-valence hexa-nuclear [Mn(III)(2)Mn(II)(4)O(2)(pyz)(2)(C(6)H(5)CH(2)COO)(10)] (pyz = pyrazine) units has been synthesized. The complex has been characterized by elemental analysis, IR spectroscopy, single-crystal X-ray diffraction analysis, and variable-temperature magnetic measurements. The structural analysis reveals that the bidentate pyz molecules connect each [Mn(6)] unit to its four [Mn(6)] neighbors through the peripheral Mn(II) centers, giving rise to a three-dimensional (3D) distorted diamond-like porous framework. Variable-temperature (2-300 K) magnetic susceptibility measurements show the presence of dominant antiferromagnetic interactions within the discrete [Mn(6)] cluster that have been fitted with a model containing three exchange constants developed for the complex (J(1) = -8.6 cm(-1), J(2) = -3.9 cm(-1), and J(3) = -100.0 cm(-1)). Using 3,5-di-tert-butyl catechol (3,5-DTBC) as the substrate, catecholase activity of the complex has been studied; the turn over number is determined to be of 2547 h(-1) in acetonitrile. This porous compound shows remarkable selectivity for adsorption of CO(2) over N(2) that may be correlated with the effect of window flexibility of the pore to the corresponding adsorbate molecules.

A Rare Phenoxido/Acetato/Azido Bridged Trinuclear and an Unprecedented Phenoxido/Azido Bridged One-Dimensional Polynuclear Nickel(II) Complexes: Synthesis, Crystal Structure, and Magnetic Properties with Theoretical Investigations on the Exchange Mechanism

The reaction of a tridentate Schiff base ligand HL (2-[(3-dimethylaminopropylimino)-methyl]-phenol) with Ni(II) acetate or perchlorate salts in the presence of azide as coligand has led to two new Ni(II) complexes of formulas [Ni(3)L(2)(OAc)(2)(μ(1,1)-N(3))(2)(H(2)O)(2)]·2H(2)O (1) and [Ni(2)L(2)(μ(1,1)-N(3))(μ(1,3)-N(3))](n)(2). Single crystal X-ray structures show that complex 1 is a linear trinuclear Ni(II) compound containing a μ(2)-phenoxido, an end-on (EO) azido and a syn-syn acetato bridge between the terminal and the central Ni(II) ions. Complex 2 can be viewed as a one-dimensional (1D) chain in which the triply bridged (di-μ(2)-phenoxido and EO azido) dimeric Ni(2) units are linked to each other in a zigzag pattern by a single end-to-end (EE) azido bridge. Variable-temperature magnetic susceptibility studies indicate the presence of moderate ferromagnetic exchange coupling in complex 1 with J value of 16.51(6) cm(-1). The magnetic behavior of 2 can be fitted in an alternating ferro- and antiferromagnetic model [J(FM) = +34.2(2.8) cm(-1) and J(AF) = -21.6(1.1) cm(-1)] corresponding to the triple bridged dinuclear core and EE azido bridge respectively. Density functional theory (DFT) calculations were performed to corroborate the magnetic results of 1 and 2. The contributions of the different bridges toward magnetic interactions in both compounds have also been calculated.

The importance of an additional water bridge in making the exchange coupling of bis(μ-phenoxo) dinickel(II)complexes ferromagnetic

Two new nickel(II) complexes [Ni(2)L(2)(PhCOO)(2)(H(2)O)] (1), [Ni(2)L(2)(PhCH(2)COO)(2)(H(2)O)] (2) have been synthesized using a tridentate Schiff base ligand, HL (2-[(3-dimethylamino-propylimino)-methyl]-phenol) and the carboxylate monoanions, benzoate and phenylacetate, respectively. The complexes have been characterized by spectral analysis, variable temperature magnetic susceptibility measurement and crystal structure analysis. The structural analyses reveal that both complexes are dinuclear in which the distorted octahedral Ni(2+) ions share a face, bridged by one water molecule and two μ(2)-phenoxo oxygen atoms. A monodentate benzoate or phenylacetate anion and two nitrogen atoms of the chelating deprotonated Schiff base (L) complete the hexa-coordination around the metal ion. Variable-temperature magnetic susceptibility studies indicate the presence of dominant ferromagnetic exchange coupling in complexes 1 and 2 with J values of 11.1(2) and 10.9(2) cm(-1) respectively. An attempt has been made to rationalize the observed magneto-structural behavior considering the importance of the additional water bridge in the present two complexes and also in other similar species.

A new family of trinuclear nickel(II) complexes as single-molecule magnets.

Three new trinuclear nickel (II) complexes with the general composition [Ni3 L3 (OH)(X)](ClO4 ) have been prepared in which X=Cl(-) (1), OCN(-) (2), or N3(-) (3) and HL is the tridentate N,N,O donor Schiff base ligand 2-[(3-dimethylaminopropylimino)methyl]phenol. Single-crystal structural analyses revealed that all three complexes have a similar Ni3 core motif with three different types of bridging, namely phenoxido (μ2 and μ3 ), hydroxido (μ3 ), and μ2 -Cl (1), μ1,1 -NCO (2), or μ1,1 -N3 (3). The nickel(II) ions adopt a compressed octahedron geometry. Single-crystal magnetization measurements on complex 1 revealed that the pseudo-three-fold axis of Ni3 corresponds to a magnetic easy axis, being consistent with the magnetic anisotropy expected from the coordination structure of each nickel ion. Temperature-dependent magnetic measurements indicated ferromagnetic coupling leading to an S=3 ground state with 2J/k=17, 17, and 28 K for 1, 2, and 3, respectively, with the nickel atoms in an approximate equilateral triangle. The high-frequency EPR spectra in combination with spin Hamiltonian simulations that include zero-field splitting parameters DNi /k=-5, -4, and -4 K for 1, 2, and 3, respectively, reproduced the EPR spectra well after a anisotropic exchange term was introduced. Anisotropic exchange was identified as Di,j /k=-0.9, -0.8, and -0.8 K for 1, 2, and 3, respectively, whereas no evidence of single-ion rhombic anisotropy was observed spectroscopically. Slow relaxation of the magnetization at low temperatures is evident from the frequency-dependence of the out-of-phase ac susceptibilities. Pulsed-field magnetization recorded at 0.5 K shows clear steps in the hysteresis loop at 0.5-1 T, which has been assigned to quantum tunneling, and is characteristic of single-molecule magnets.

Formation of two drastically different MOFs based on Mn(II)–benzoate and pyrazine with a change in seasonal temperature: structural analysis and magnetic study

Two new stoichiometrically and structurally very different Mn(II) coordination compounds {[Mn(pyz)(C6H5COO)2](C2H5OH)}n (1) and [Mn5(pyz)(C6H5COO)10(H2O)2]n (2) (where pyz = pyrazine) have been synthesized by the reaction of Mn(C6H5COO)2·4H2O and pyz in 1 : 1 molar ratio in different seasons of the year. The structural analyses reveal that complex 1, which is synthesized in summer when the temperature remains around 28–40 °C, features a 1D molecular ladder formed by joining of syn–syn benzoate-bridged dimeric Mn(II) units by pyz. Complex 2, which is obtained in winter (temperature around 10–20 °C), is an unprecedented 2D rectangular sheet motif based on a 1D column constructed from syn–syn benzoate bridged repetitive Mn5 units which are further interconnected by the pyz spacer. The compounds are interconvertible in solution: when an ethanol solution of 1 is allowed to evaporate slowly in winter or in a refrigerator (5–6 °C) it transformed into 2 whereas crystallization of 2 from ethanol medium in summer (28–40 °C) yields 1. Variable-temperature (1.8–300 K) magnetic susceptibility measurements showed presence of weak antiferromagnetic coupling in 1 that has been fitted with the van Vleck equation. The interaction in benzoate-bridged Mn2 unit was characterized to be J = −0.586(1) cm−1, where the spin-Hamiltonian is defined as H = −2JS1S2. The interaction in 2 was also characterized with the modified van Vleck equation to be weakly antiferromagnetic.

The first alternating MnII–MnIII 1D chain: structure, magnetic properties and catalytic oxidase activities

An unprecedented one-dimensional mixed-valence chain, [{MnIII(L)2}{MnII(bpy)2}(ClO4)(H2O)0.5]∞ (H2L = N-salicylidene-L-alanine and bpy = 2,2′-bipyridine), has been synthesized. The complex has been characterized by elemental analysis, IR spectroscopy, single-crystal X-ray diffraction analysis, and variable-temperature magnetic measurements. The structural analysis reveals that the [Mn(bpy)2]2+ unit is linked to the pendent carboxylate arm of the [MnIII(L)2]− unit to produce a unique syn–anti carboxylate bridged 1D coordination polymer. Variable-temperature (2–300 K) magnetic susceptibility measurements show the presence of antiferromagnetic interaction between the MnIII and MnII centres with J = −0.98(2) cm−1. Using 3,5-di-tert-butylcatechol (3,5-DTBC) and o-aminophenol (OAPH) as the substrates, the catecholase-like activity and phenoxazinone synthase-like activity of the complex have been studied; the turnover numbers (kcat) for these oxidase reactions have been calculated to be 126.9 and 738.0 h−1, respectively.