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.

Reversible Multistimuli Switching of a Spiropyran-Functionalized Organic Cage in Solid and Solution.

A spiropyran-decorated covalent organic cage (PC2) has been designed, employing dynamic imine chemistry followed by imine bond reduction. The molecule is capable of altering its color upon exposure to external stimuli such as heat and light. Construction of a 3D organic cage introduces a new piece to the system by swapping the closed form with the open form in the solid state with diverse color change. Moreover, this material has high chemical stability and is capable of reversible stimuli-responsive color change without any degradation for an extended period.

Versatility of Two Diimidazole Building Blocks in Coordination-Driven Self-Assembly.

A series of discrete molecular architectures was synthesized via self-assembly of two "linear" diimidazole donors [L1 = 1,4-di(1H-imidazol-1-yl)benzene and L2 = 1,1'-(2,5-dimethyl-1,4-phenylene)bis(1H-imidazole)] independently with cis-[(tmeda)Pd(NO3)2] [tmeda = N,N,N',N'-tetramethylethane-1,2-diamine], cis-[(tmeda)Pt(NO3)2], a diplatinum(II) acceptor (bisPt), and Pd(NO3)2. The donors L1 and L2 are isostructural with four hydrogens in the benzene ring of L1, whereas in L2 two of such hydrogens are substituted by methyl groups. The assemblies were characterized by NMR spectroscopy and electrospray ionization mass spectrometry analyses along with single-crystal structure determination of three of them. While the self-assembly of L1 with cis-[(tmeda)Pd(NO3)2] solely formed a [3 + 3] self-assembled molecular triangle (1), L2 with the same acceptor predominantly resulted in the formation of a [4 + 4] molecular square (3). Such a dramatic change in the final outcome in the coordination-driven self-assembly by simple alkyl substitution of isostructural donors is remarkable. Interestingly, self-assembly of L1 and L2 with analogous Pt(II) acceptor cis-[(tmeda)Pt(NO3)2] yielded mixtures of [3 + 3] triangle and [4 + 4] square, where the molecular triangles (4 and 6) were the predominant products in both the cases. The same donors in combination with a 0° acceptor bisPt independently formed the expected [2 + 2] metallo-macrocycles (8 and 9). Surprisingly, the macrocycle (9) involving L2 is found to exist in more than one conformation at room temperature. Moreover, the diimidazole donors formed unprecedented Pd6L12 molecular spheres (10 and 11) when they were separately treated with Pd(NO3)2. The imidazole moieties in the ligands are found to appear in versatile orientations in the synthesized molecules due to their rotational flexibility to produce required bite angles for the particular architecture.

A Pd8 Tetrafacial Molecular Barrel as Carrier for Water Insoluble Fluorophore.

A new carbazole-based tetraimidazole ligand 1,3,6,8-tetra(1H-imidazol-1-yl)-9-methyl-9H-carbazole (L) has been synthesized. The unsymmetrical nature of L as well as the rotational freedom of imidazole donor moieties around C-N bond make it a special building unit, which upon treatment with cis-(tmeda)Pd(NO3)2 produced an unprecedented single linkage-isomeric Pd8 tetrafacial molecular nanobarrel (PSMBR-1) [tmeda = N,N,N',N'-tetramethylethane-1,2-diamine]. Unlike closed architectures, open barrel architecture of water-soluble PSMBR-1 makes it an ideal host for some water insoluble polyaromatic hydrocarbons in aqueous medium; one such inclusion complex coronene⊂PSMBR-1 was characterized by X-ray diffraction study. Moreover, the potential application of PSMBR-1 as carrier in aqueous medium for the transportation of water insoluble fluorophore (perylene) for live cell imaging is explored.

Direct C-N Coupling in an in Situ Ligand Transformation and the Self-Assembly of a Tetrametallic [Ni(II)4] 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).

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.

Self-assembly of a [Ni8] carbonate cube incorporating four µ4-carbonato linkers through fixation of atmospheric CO2 by ligated [Ni2] complexes.

The communication reports the synthesis, characterization, and magnetic behavior of a novel µ4-carbonato supported and imidazole capped ligated nickel cage [Ni8(µ-H2bpmp)4(µ4-CO3)4(ImH)8](NO3)4·2H2O (1) through self-assembly of ligand bound ferromagnetic Ni2 building blocks. Structural analysis indicates newer geometrical features for the coordination cage formation and dominant interdimer antiferromagnetic coupling resulting in a diamagnetic ground state.

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.

Coordination induced fluorescence enhancement and construction of a Zn3 constellation through hydrolysis of ligand imine arms.

The phenoxido and alkoxido bridged neutral Zn(3) complex [Zn(3)(µ-H(2)bemp)(2)(µ(3)-emp)(2)] (1), with an angular Zn(3)(µ-OPh)(2)(µ-OEt)(2) core and capping nitrogen donors, was synthesized via simultaneous chelation-cum-bridging of the parent and hydrolysed ligands. Zinc(II) coordination triggered the solution phase imine (C=N) bond hydrolysis of H(3)bemp (2,6-bis-[(2-hydroxyethylimino)methyl]-4-methylphenol) and yielded the unexpected angular trinuclear Zn(II) complex 1, having structural similarity with the Zn(3) active site of P1 nuclease. H(3)bemp also displays a zinc(II) selective chelation-enhanced fluorescence response from strong metal ion coordination. Complexation of zinc(II) with H(3)bpmp (2,6-bis-[(3-hydroxypropylimino)methyl]-4-methylphenol), a close analogue of H(3)bemp, instead provides only mononuclear [Zn(H(2)bpmpH(N))(2)](ClO(4))(2)·2H(2)O (2·2H(2)O) (H(N) is the proton attached to an imine nitrogen atom) of two zwitterionic ligands, generated through a kind of coordination driven acid-base reaction, without showing any aggregation reaction. As the sole metal-organic precursor, both the complexes under pyrolytic conditions give ZnO nano structures of two morphologies.