Sumit Khanra

Synthesis, Structural and Magnetochemical Studies of Iron Phosphonate Cages Based on {Fe3O}7+ Core

We report the synthesis, structures, and magnetic properties of twelve iron(III) phosphonate cages: [Fe(4)(mu(3)-O)Cl(PhCO(2))(3)(PhPO(3))(3)(py)(5)] 1, [Fe(4)(mu(3)-O)((t)BuCO(2))(4)(C(10)H(17)PO(3))(3)(py)(4)] 2 (C(10)H(17)PO(3)H(2) = camphylphosphonic acid), [Fe(7)(mu(3)-O)(2)(PhPO(3))(4)(MeCO(2))(9)(py)(6)] 3, [Fe(7)(mu(3)-O)(2)(PhPO(3))(4)(PhCO(2))(9)(py)(6)] 4, [Fe(7)(mu(3)-O)(2)((t)BuPO(3))(4)((t)BuCO(2))(8)(py)(8)](NO(3)) 5, [Fe(7)(mu(3)-O)(2)(PhPO(3))(4)(MeCO(2))(8)(py)(8)] 6, [Fe(9)(mu(3)-O)(2)(mu(2)-OH)(PhPO(3))(6)((t)BuCO(2))(10)(MeCN)(H(2)O)(5)] 7, [Fe(9)(mu(3)-O)(2)(mu(2)-OH)(C(10)H(17)PO(3))(6)(PhCO(2))(10)(H(2)O)(6)] 8, [Fe(6)(mu(3)-O)(2)(O(2))((t)BuCO(2))(8)(PhPO(3))(2)(H(2)O)(2)] 9, [Fe(6)(mu(3)-O)(2)(O(2))((t)BuCO(2))(8)(C(10)H(17)PO(3))(2)(H(2)O)(2)] 10, [Fe(6)(mu(3)-O)(2)(O(2))((t)BuCO(2))(8)((t)BuPO(3))(2)(py)(2)] 11, and [Fe(14)(mu(3)-O)(4)(O(2))(2)(PhPO(3))(8)((t)BuCO(2))(12)(H(2)O)(12)](NO(3))(2) 12. The results have allowed us to compare the magnetic exchange found with magneto-structural correlations found previously for iron-oxo cages.

Star-shaped molecule of MnII4O6 core with an St=10 high-spin state. A theoretical and experimental study with XPS, XMCD, and other magnetic methods.

We report a comprehensive study of the electronic and magnetic properties of a star-shaped molecule comprising a MnII4O6 core. One feature of this compound is weak magnetic coupling constants compared to other similar polyoxo compounds. This leads to complicated low-lying magnetic states in which the ground state is not well separated from the upper-lying states, yielding a high-spin molecule with a giant magnetic moment of up to 20 microB/formula unit. We apply X-ray diffraction and magnetometry as well as other X-ray spectroscopic techniques, namely, X-ray photoelectron spectroscopy, X-ray magnetic circular dichroism, and X-ray emission spectroscopy. We compare our experimental results with ab initio electronic band structure calculations as well as the localized electronic structure around the Mn2+ ions with charge-transfer multiplet calculations.

A Star-Shaped Heteronuclear CrIIIMnII3 Species and Its Precise Electronic and Magnetic Structure: Spin Frustration Studied by X-Ray Spectroscopic, Magnetic, and Theoretical Methods

Molecular magnets incorporate transition-metal ions with organic groups providing a bridge to mediate magnetic exchange interactions between the ions. Among them are star-shaped molecules in which antiferromagnetic couplings between the central and peripheral atoms are predominantly present. Those configurations lead to an appreciable spin moment in the nonfrustrated ground state. In spite of its topologically simple magnetic structure, the [Cr(III)Mn(II)(3) (PyA)(6)Cl(3)] (CrMn(3)) molecule, in which PyA represents the monoanion of syn-pyridine-2-aldoxime, exhibits nontrivial magnetic properties, which emerge from the combined action of single-ion anisotropy and frustration. In the present work, we elucidate the underlying electronic and magnetic properties of the heteronuclear, spin-frustrated CrMn(3) molecule by applying X-ray magnetic circular dichroism (XMCD), as well as magnetization measurements in high magnetic fields, density functional theory, and ligand-field multiplet calculations. Quantum-model calculations based on a Heisenberg Hamiltonian augmented with local anisotropic terms enable us not only to improve the accuracy of the exchange interactions but also to determine the dominant local anisotropies. A discussion of the various spin Hamiltonian parameters not only leads to a validation of our element selective transition metal L edge XMCD spin moments at a magnetic field of 5 T and a temperature of 5 K but also allows us to monitor an interesting effect of anisotropy and frustration of the manganese and chromium ions.

A one-pot synthesis of a paramagnetic high-nuclearity nickel(II) cluster: an octadecanuclear NiII16NaI2 metal aggregate

An unusual NiII16NaI2 cluster which features formate as a bridging clamp between two octanuclear nickel cages is reported; preliminary magnetic studies exhibit paramagnetic low-lying states resulting from dominating antiferromagnetic interactions between the nickel(II) centers.

An oximate-based hexanuclear mixed-valence MnIII4MnII2 edge-sharing bitetrahedral core with an St = 5 spin ground state

The synthesis, structures and magnetic properties of two hexanuclear Mn6 clusters are reported: Mn6(mu4-O)2(dapdo)2(dapdoH)4(mu2-OH)2](ClO4)(2).6MeCN (1.6MeCN) and [Mn6(mu4-O)2(dapdo)2(dapdoH)4(mu2-OCH3)2](ClO4)(2).2Et2O (2.2Et2O) [dapdo2- is the dianion of 2,6-diacetylpyridine dioxime and dapdoH- is the monoanion of the aforesaid dioxime ligand]. Both complexes are mixed-valent with two Mn(II) and four Mn(III) atoms disposed in an edge-sharing bitetrahedral core. Both complexes 1 and 2 display the same [Mn(III)4Mn(II)2(mu4-O)2(mu2-OR)2]10+ core in which R = H for 1 and R = Me for 2. The [Mn(III)4Mn(II)2] core is rather uncommon compared to the reported [Mn(III)2Mn(II)4] core in the literature. DC magnetic susceptibility measurements on 1 and 2 reveal the presence of competing exchange interactions resulting in an St = 5 ground spin state. The magnetic behavior of the compounds indicates antiferromagnetic coupling between the manganese(III) centers, whereas the coupling between the manganese(III) and manganese(II) is weakly antiferromagnetic or ferromagnetic depending on the bridging environments. Finally the interaction between the manganese(II) centers from the two fused tetrahedra is weakly ferromagnetic in nature stabilizing St = 5 ground spin state in compounds 1 and 2.

A spin-frustrated star-shaped heterotetranuclear CrIIIMnII3 species and its magnetic and HF-EPR measurements

A tetranuclear complex [Cr(III)Mn(II)(3)(PyA)(6)Cl(3)] 1 containing pyridine-2-aldoximato monoanion, PyA, has been structurally and magnetochemically characterized. The compound is a rare example of a tetranuclear star-shaped metal topology containing pyridine-2-aldoximato ligands. Static magnetic studies have demonstrated very weak antiferromagnetic exchange interactions between the paramagnetic centers (S(Cr) = 3/2 and S(Mn) = 5/2) resulting in closely spaced low-lying levels, which undergo splitting and crossing. Preliminary high-field EPR measurements (20 < nu < 388 GHz) indicate the presence of zero-field splitting D of the order of 0.7 cm(-1).

A magnetostructural study of linear NiIIMnIIINiII, NiIICrIIINiII and triangular NiII3 species containing (pyridine-2-aldoximato)nickel(II) unit as a building block

Three trinuclear complexes, NiII MnIII NiII, NiII CrIII NiII and Ni(II)3 based on (pyridine-2-aldoximato)nickel(II) units are described. Two of them, and , contain metal-centers in linear arrangement, as is revealed by X-ray diffraction. Complex is a homonuclear complex in which the three nickel(II) centers are disposed in a triangular fashion. The compounds were characterized by various physical methods including cyclic voltammetric and variable-temperature (2-290 K) susceptibility measurements. Complexes and display antiferromagnetic exchange coupling of the neighbouring metal centers, while weak ferromagnetic spin exchange between the adjacent Ni II and Cr III ions in is observed. The experimental magnetic data were simulated by using appropriate models.

Families of Molecular Hexa- and Trideca-Metallic Vanadium(III) Phosphonates

The synthesis and structural characterization of two families of low-valent vanadium(III) {V6P4} and vanadium(III/IV) {V13P8} phosphonate complexes are reported. Magnetic characterization is reported for representative examples.

Synthesis, Structure, and Valency Verification of a Mn6IIIO2-Cluster

Abstract A new oxime-based hexanuclear MnIII complex containing the structural core [Mn6III(μ3-O)2] was synthesized through a general route. We describe the preparation and characterization of the manganese(III) complex [Mn6III(μ3-O)2(Salox)6(CH3CH2OH)4{(CH3)3CCOO}2] by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS). Therefore we offer the first core level XPS, and XAS reference spectra of polynuclear complexes comprising only Mn(III) ions in octahedral surrounding.