Swapan K. Chandra

Synthesis, structure and magnetism of a new dicubane-like ferromagnetic tetranuclear nickel cluster containing versatile azido-only bridges and a bis(bidentate) Schiff base blocker

A new tetranuclear double-open dicubane complex [Ni4(mu 2-N3)4(mu 3-N3)2(N3)2(enbzpy)2].2H2O (enbzpy = [N,N-bis(pyridin-2-yl)benzylidene]ethane-1,2-diamine) has been characterised structurally and magnetostructurally.

Synthesis, crystal structure and magnetic properties of a new ferromagnetic nickel(II) dimer derived from a hexadentate Schiff base ligand

Abstract A new end-on bis(μ-azido) nickel(II) complex of the formula [Ni2(L1)(N3)4] (1) (L1=hexadentate Schiff base ligand made from the reaction of 2 moles of 2-benzoyl pyridine and 1 mole of triethylenetetramine) has been prepared and characterized by single crystal X-ray diffraction studies. Each nickel center is coordinated in a distorted octahedral (NiN6) fashion consisting of three nitrogen atoms from the hexadentate ligand moiety, two nitrogen atoms from two end-on azido ligands and the last one from another terminal azido ion. The variable temperature magnetic susceptibility of compound 1 shows that the nickel centers are ferromagnetically coupled. The NiNNi bridge angles are very similar [98.36(8)° and 97.66(9)°] and the Ni⋯Ni separation is 3.155(1) A. The magnetic exchange coupling constant J=21.8 cm−1 is consistent with the structure and the value has been compared with some similar reported compounds.

A third isolated oxidation state for the family of [Mn12O12(O2CR)16(H2O)4] compounds. Synthesis, characterization and single-molecule magnetism properties of (PPh4)2[Mn12O12(O2CR)16(H2O)x] (x=3 or 4)

Abstract The synthesis and characterization are reported of the two-electron reduced version of the [Mn12O12(O2CR)16(H2O)4] family of single-molecule magnets (SMMs). Reduction of the neutral complexes with 2 equiv. of PPh4I gives the (PPh4)2[Mn12O12(O2CR)16(H2O)x] (x=3 or 4). The crystal structure shows the overall Mn12 structure is maintained, with a 2Mn(II), 6Mn(III), 4Mn(IV) trapped oxidation state description. Magnetic characterization indicates a S=10 ground state and retention of SMM behavior.

Synthesis, structure, EPR and electrochemical studies of a μ2-phenoxo bridged manganese(II) dimer afforded by a binucleating macrocylic ligand

Abstract The reaction of 2 moles of tris(2-aminoethyl)amine and 3 moles of 4-methyl-2,6 diformyl phenol in acetonitrile solution affords a light yellow crystalline solid having composition of H 3 L, which on reacting with manganese(II) acetate tetrahydrate in alkaline methanol produced the binuclear complex [Mn II 2 L] + . This has been isolated as a perchlorate salt. The X-ray structure of [Mn II 2 L]ClO 4 ·H 2 O is reported. The lattice consists of two crystallographically independent but metrically very similar binuclear units. Manganese(II) ions are tris μ 2 -phenolato bridged and the distorted octahedral metal coordination spheres are of type MnN 3 O 3 . The frozen (77 K) EPR spectrum of the complex shows group of 11 hyperfine lines with coupling constants of 43.7 G showing that the unpaired electrons interact with both 55 Mn centres. The complex displays two successive oxidation couples with E 1 2 values 0.54 and 1.07 V vs saturated calomel electrode.

Synthesis, crystal structure and magnetic behavior of two new binuclear complexes bridged by a pentadentate ligand: [Ni2L2(NCS)2](ClO4)2 and [Ni2L2(NCO)2](ClO4)2 [L=pentadentate ligand]

Abstract Two new binuclear nickel(II) complexes [Ni2L2(NCS)2](ClO4)2 (1) and [Ni2L2(NCO)2](ClO4)2 (2), where L is a pentadentate Schiff base ligand derived from the reaction of 2 mol of 2-benzoyl pyridine and 1 mol of diethylene triamine, have been prepared and characterized. The structures of complexes 1 and 2 have been studied by single crystal X-ray diffraction analysis. The structure analysis reveals that each nickel(II) center is coordinated in a distorted octahedral (NiN6) fashion consisting of three nitrogen atoms from one and two nitrogen atoms from other pentadentate ligand and the last one from terminal thiocyanato/cyanato ion. The variable temperature magnetic susceptibility of both compounds 1 and 2 have been studied and found a very weak magnetic interaction between two nickel(II) centers consistent with the structures (Ni⋯Ni distance of 5.809(1) A for 1 and 5.695(1) A for 2 with no suitable bridge or linkage to propagate magnetic interaction).

Hexacoordinated complexes of bivalent zinc incorporating thioether binding

Abstract The reaction of zinc(II) acetate dihydrate with dithiaalkyl-substituted triazene 1-oxides, [RN(O)NNHC6H4S]2(CH2)2 (H2RL; RMe, Ph) has afforded light-yellow Zn(MeL) and light-green Zn ( PhL )· 1 2 CH 2 Cl 2 , the first hexacoordinated of zinc bonded by acyclic ligands incorporating thioether sites. Spectral features ff the complexes are briefly noted. Both the complexes have been structurally characterized revealing distorted octahedral binding of the metal ion in ZnO2N2S2 coordination spheres; the ZnO, ZnN and ZnS distances lie in the ranges 2.056(7)−2.105(8), 2.005(8)−2.029(8) and 2.649(4)−2.720(3) A, respectively. Crystal data: Zn(MeL): monoclinic, space group P21/c, a = 26.529(9), b = 7.974(3), c = 19.396(8) A , β = 110.57(3)°, V = 3842(3) A 3 , Z = 8, R = 5.40%, Rw = 5.73%; Zn ( PhL )· 1 2 CH 2 Cl 2 ; orthorhombic, space group Pbcb, a = 9.540(3), b = 19.449(6), c = 29.534(8) A, V = 5480(3) A 3 , Z = 8, R = 5.65%, Rw = 5.42%.

Stepwise formation of a pentanuclear Ni4Cu heterometallic complex exhibiting a vertex-sharing defective double-cubane core and diphenoxo- and phenoxo/azide bridging groups: a magnetostructural and DFT theoretical study.

Sequential reaction of a N5O3 octadentate tripodal ligand with Ni(2+) and subsequently with Cu(2+) and azide ligand afforded the first example of a heterobridged (phenoxo/μ(1,1)-azido) pentanuclear heterometallic (Ni4Cu) compound, which exhibits a centrosymmetric vertex-sharing defective double-cubane structure. The study of the magnetic properties reveals that the compound shows ferromagnetic interaction interactions, leading to an S = 9/2 spin ground state. Density functional theory calculations on the X-ray structure and model compounds predict ferromagnetic interactions through the magnetic exchange pathways involving each couple of metal ions.

A third isolated oxidation state for the Mn12 family of single-molecule magnets

Dianionic members of the Mn12 family of single-molecule magnets have been prepared by two-electron reduction of the neutral cluster; the [PPh4]2[Mn12O12(O2C CHCl2)16- (H2O)4] representative has been shown to possess an S = 10 ground state and to exhibit out-of-phase ac susceptibility signals diagnostic of single-molecule magnetism behavior.

Synthesis, structure and metal redox of a family of copper complexes derived from hexadentate ligands incorporating thioether and triazene 1-oxide functions

Abstract Hexadentate ligands of type RN(O)NN(H)C6H4S(CH2)xSC6H4(H)NN(O)NR (H2L1: R = Me, x = 2; H2L2: R = Ph, x = 2 ; H2L3: R = Me, x = 3 ; general abbreviation H2L) have afforded copper(II) complexes of type [CuL]. The X-ray structure of [CuL2] · 1 2 CH 2 Cl 2 reveals the presence of a distorted CUS2N2O2 coordination sphere which is strongly elongated along an OCuS axis. The axial and equatorial bond lengths are: CuS, 2.636(5) and 2.473(6) A Cu0, 2.222(10) and 2.039(11) A respectively. Both the CuN bonds, 1.939(13) and 1.904(12) A lie on the equatorial plane. The structure is compared with that of [ZnL2] · 1 2 CH 2 Cl 2 , which does not display the above-noted axial elongation. The findings are rationalized in terms of simple angular overlap considerations. The EPR spectrum of [CuL2] · 1 2 CH 2 Cl 2 has g|; > g⊥, showing that the hole lies in the dx2- y2 orbital. The [CuL] complexes are electroactive in dichloromethane solutions and successive quasireversible responses due to the [CuIIL]/[CuIL]− and [CuIIIL]+/[CuIIL] couples are observed, the E 1 2 values being ∼ −0.9 V and ∼ 0.8 V vs SCE, respectively. The E 1 2 of the copper(IIIcopper(II) copple lies close to that of the corresponding nickel (III)nickel(II) couple of [NiL]. A thermodynamic rationale is provided.

Electron Transfer. 127 Intermediate Oxidation States in the Reduction of Chromium(Vi) With Formate

When HCrO4− is reduced by formate in solutions buffered by 2-ethyl-2-hydroxybutanoic acid and its anion, chelated complexes of both Cr(IV) and Cr(V), both of them stabilized in the medium used, are formed. It appears that Cr(V) is not generated directly from the Cr(VI)-formate reaction but arises instead from oxidation of Cr(IV) by Cr(VI). When the Cr(VI)-formate reaction is allowed to go to completion in the presence of [Cl(NH3)5Co]2+, a scavenger for Cr(II), 84–86% of the Cr(VI) taken is found to be converted to Cr(II), indicating that nearly all of the reacting system proceeds through Cr(IV) and bypasses the more usual state Cr(III). Initial rates for formation of Cr(IV) lead to a rate law pointing to a transition state containing the two redox partners, two ligating carboxyl groups, and two units of H+. Substitution of DCO2− for HCO2− retards formation of Cr(IV) by a factor of 3.3, whereas the solvent isotope affect, rateD2O/rateH2O, favors the deuterated system by a factor of 1.4. Our observations are in accord with a sequence initiated by the ligation of HCrO4− to a chelate derived from the buffering carboxylate anion. Conversions of Cr(VI) to Cr(IV), and Cr(IV) to Cr(II) appear to entail hydride shifts from formate to the Cr(=O) function.