Aloke Kumar Ghosh

Self-Assembled Tetra- and Pentanuclear Nickel(II) Aggregates From Phenoxido-Based Ligand -Bound {Ni2} Fragments: Carboxylate Bridge Controlled Structures

Three different carboxylato bridges (R = C2H5, CF3, and PhCH2 in RCO2(-)) have been used to obtain the supramolecular aggregates [Ni5(μ-H2bpmp)2(μ3-OH)2(μ1,3-O2CC2H5)6]·2H2O·4DMF (1·2H2O·4DMF), [Ni4(μ3-H2bpmp)2(μ3-OH)2(μ1,3-O2CCF3)2](CF3CO2)2·H2O (2·H2O), and [Ni4(μ3-H2bpmp)2(μ3-OH)2(μ1,3-O2CCH2Ph)2](PhCH2CO2)2·4H2O (3·4H2O) (H3bpmp =2,6-bis-[(3-hydroxy-propylimino)-methyl]-4-methyl-phenol) from the hydroxido-bridged dinuclear motif [Ni2(μ-H2bpmp)(OH)](2+). These complexes have been characterized by X-ray crystallography and magnetic measurements. A change from propanoate group to trifluoroacetate and phenylaceate groups provided different course of cluster assembly based on Ni2(μ-H2bpmp)2 fragments. The {Ni5(μ3-OH)2(μ1,3-O2CC2H5)6}(2+) core in 1 contains five Ni(II) ions in an hourglass (pentanuclear vertex-shared double cubane) arrangement. These compounds are new examples of [Ni5] and [Ni4] complexes where aggregation of the building motifs are guided by the nature of the carboxylate anions, which allows an effective tuning of the self-aggregate process within same ligand environment. The study of the magnetic properties reveals that 1 exhibits an S = 3 ground state. Nevertheless, the magnetization increases above the expected saturation value of 6 μB at higher fields, because of the suppression of antiferromagnetic exchange between the central and peripheral Ni(II) ions. Complexes 2 and 3 exhibit ferromagnetic exchange interactions that result in the S = 4 ground state. Examination of AC magnetic susceptibility showed that complex 2 in finely ground form behaves as spin glass with the spin-freezing temperature of ∼5.5 K. This behavior was attributed to the collapse of the structure upon the loss of interstitial solvent. Such property was not observed for complex 3, in which the bulkier carboxylate ligands provide for a more robust crystal packing and larger separation between the [Ni4O4] clusters.

A dodecanuclear copper(II) cage self-assembled from six dicopper building units

Reaction of the dinucleating phenol-based ligand, H3bpmp (2,6-bis-[(3-hydroxy-propylimino)-methyl]-4-methyl-phenol), with Cu(2+) ions in the presence of a hybrid base (NEt3 and NaN3) necessary for the in situ generation of required numbers of hydroxido ions, results in the formation of a novel NO3(-) capped and HO(-) supported {Cu12} coordination complex {Cu6(μ3-OH)3(μ3-Hbpmp)3(μ3-NO3)}2(NO3)2(OH)2·2H2O·2MeOH (1). When the components are combined in right proportions (metal : ligand : NEt3 : NaN3 = 2 : 1 : 3 : 2) in MeOH, twelve Cu(2+) ions assemble in a cuboctahedral geometry, containing six square and eight triangular faces around a considerable void space. Six of the eight [Cu3] triangular faces are bound by the six Hbpmp(2-) ligands with six free pendant propanol arms around the central hexagonal plane. X-ray structure determination indicates new geometrical features for the core formation and reveals the face-capping potential of the H3bpmp ligand for the growth of a cuboctahedral coordination cage with the support of anions like HO(-) and NO3(-). The experimentally observed (J/kB = -173 K) strong antiferromagnetic coupling within the Cu12 complex has been justified by the DFT calculations.

Direct C–N Coupling in an in Situ Ligand Transformation and the Self-Assembly of a Tetrametallic [NiII4] Staircase

A [Ni(II)4] staircase complex was serendipitously prepared from the reaction of the binucleating Schiff base proligand 2,6-bis[[(3-hydroxypropyl)imino]methyl]-4-methylphenol (H3L2) and 3,5-dimethylpyrazole (Me2pzH) with nickel(II) nitrate in a reaction at room temperature, initially aimed to yield a dinuclear complex. From a room temperature metal ion/ligand reaction, the proligand H3L2 in situ transformed to modified forms HL3(2-) and HL4(2-), allowing the [Ni4] formation. Variable-temperature magnetic behavior of a [Ni4] complex reveals antiferromagnetic interactions with stabilization of a diamagnetic ground state (ST = 0).

Self-Assembled PdII6 Molecular Spheroids and Their Proton Conduction Property

A series of molecular spheroids (SP1-SP4) was synthesized using pseudolinear bisimidazole and bisbenzimidazole donors in combination with Pd(NO3)2 acceptor via coordination-driven self-assembly. They were characterized by NMR and mass spectrometry, and the solid-state structures of SP1 and SP3 were confirmed by X-ray diffraction. Crystal packing revealed the presence of molecular channels with water molecules in the channels as proton source. All the systems showed proton conductivity across a wide range of temperature and relative humidity. Furthermore, the mode of proton conduction in these molecular spheroids was explored by performing a control experiment using 2,4-dinitrophenol molecule, which indicates that the proton conductivity in the present case increases with increasing surface area of these molecular spheroids.