Ramamoorthy Boomishankar

Trinuclear Heterobimetallic Ni2Ln complexes [L2Ni2Ln][ClO4] (Ln = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, and Er; LH3 = (S)P[N(Me)N═CH−C6H3-2-OH-3-OMe]3): From Simple Paramagnetic Complexes to Single-Molecule Magnet Behavior

The reaction of LH3 with Ni(ClO4)(2).6H 2O and lanthanide salts in a 2:2:1 ratio in the presence of triethylamine leads to the formation of the trinuclear complexes [L2Ni2Ln][ClO4] (Ln=La (2), Ce (3), Pr (4), Nd (5), Sm (6), Eu (7), Gd (8), Tb (9), Dy (10), Ho (11) and Er (12) and L: (S)P[N(Me)NCH-C6H3-2-O-3-OMe]3). The cationic portion of these complexes consists of three metal ions that are arranged in a linear manner. The two terminal nickel(II) ions are coordinated by imino and phenolate oxygen atoms (3N, 3O), whereas the central lanthanide ion is bound to the phenolate and methoxy oxygen atoms (12O). The Ni-Ni separations in these complexes range from 6.84 to 6.48 A. The Ni-Ni, Ni-Ln and Ln-O phenolate bond distances in 2-12 show a gradual reduction proceeding from 2 to 12 in accordance with lanthanide contraction. Whereas all of the compounds (2-12) are paramagnetic systems, 8 displays a remarkable ST=(11)/2 ground state induced by an intramolecular Ni. . .Gd ferromagnetic interaction, and 10 is a new mixed metal 3d/4f single-molecule magnet generated by the high-spin ground state of the complex and the magnetic anisotropy brought by the dysprosium(III) metal ion.

Thermochromic and Mechanochromic Luminescence Umpolung in Isostructural Metal–Organic Frameworks Based on Cu6I6 Clusters

Two isostructural metal-organic framework (MOF) materials, namely, {[MeSi((3)Py)3]6(Cu6I6)}n (1) and {[ MeSi((3)Qy)3]6(Cu6I6)}n (2), featuring Cu6I6 clusters were synthesized from tridentate arylsilane ligands of the type MeSi((3)Py)3 ((3)Py = 3-pyridyl) and MeSi((3)Qy)3 ((3)Qy = 3-quinolyl), respectively. While the MOF 1 displays the usual thermochromism associated with traditional Cu4I4Py4 clusters, the MOF 2 shows (3)XLCT/(3)MLCT emission due to the Cu6I6 cluster core at both 298 and 77 K, albeit with some marginal variations in its emission wavelengths. Interestingly, an unusual reversal in the mechanochromic luminescent behavior was observed for these isostructural MOFs at 298 K wherein a pronounced blue-shifted high energy emission for 1 (from orange to yellowish-orange) and a red-shifted low-energy emission for 2 (from green to orange) were obtained upon grinding these samples. This is primarily due to the variations in their cuprophilic interactions as 1 displays shorter Cu···Cu distances (2.745(1) Å) in comparison with those present in 2 (3.148(0) Å). As a result, the ground sample of 2 exhibits a prominent red shift in luminescence owing to the reduction of its Cu···Cu distances to an unknown value closer to the sum of van der Waals radii between two Cu(I) atoms (2.80 Å). However, the blue-shifted emission in 1 is presumably attributed to the rise in its lowest unoccupied molecular orbital energy levels caused by changes in the secondary packing forces. Furthermore, the absorption and emission characteristics of 1 and 2 were substantiated by time-dependent density functional theory calculations on their discrete-model compounds. In addition, the syntheses, reactivity studies, and photophysical properties of two one-dimensional MOFs, namely, {[MeSi((3)Qy)3]2(Cu2I2)}n (3) and {[MeSi((3)Qy)3](CuI)}n (4), having dimeric Cu2I2 and monomeric CuI moieties, respectively, were examined.

Noncovalent synthesis of hierarchical zinc phosphates from a single zn(4)o(12)p(4) double-four-ring building block: dimensionality control through the choice of auxiliary ligands

In contrast to the well-known reaction of phosphonic acids RP(O)(OH)(2) with divalent transition-metal ions that yields layered metal phosphonates [RPO(3)M(H(2)O)](n), the 2,6-diisopropylphenyl ester of phosphoric acid, dippH(2), reacts with zinc acetate in methanol under ambient conditions to afford tetrameric zinc phosphate [(ArO)PO(3)Zn(MeOH)](4) (1). The coordinated methanol in 1 can be readily exchanged by stronger Lewis basic ligands at room temperature. This strategy opens up a new avenue for building double-four-ring (D4R) cubane-based supramolecular assemblies through strong intercubane hydrogen-bonding interactions. Seventeen pyridinic ligands have been used to synthesize as many D4R cubanes [(ArO)PO(3)Zn(L)](4) (2-18) from 1. The ligands have been chosen in such a way that the majority of them contain an additional functional group that could be used for noncovalent synthesis of extended structures. When the ligand does not contain any other hydrogen-bonding donor-acceptor sites (e.g., 2,4,6-trimethylpyridine (collidine)), zero-dimensional D4R cubanes have been obtained. The use of pyridine, lutidine, 2-aminopyridine, and 2,6-diaminopyridine, however, results in the formation of linear or zigzag one-dimensional assemblies of D4R cubanes through strong intermolecular C-H...O or N-H...O interactions. Construction of two-dimensional assemblies of zinc phosphates has been achieved by employing 2-hydroxypyridine or 2-methylimidazole as the exo-cubane ligand on zinc centers. The introduction of an alcohol side chain on the pyridinic ligand in such a way that the -CH(2)OH group cannot participate in intracubane hydrogen bonding (e.g., pyridine-3-methanol, pyridine-4-methanol, and 3,5-dimethylpyrazole-N-ethanol) leads to the facile noncovalent synthesis of three-dimensional framework structures. Apart from being useful as building blocks for noncovalent synthesis of zeolite-like materials, compounds 1-18 can also be thermolyzed at approximately 500 degrees C to yield high-purity zinc pyrophosphate (Zn(2)P(2)O(7)) ceramic material.

Tri-, hepta- and octa-nuclear Ag(I) complexes derived from 2-pyridyl-functionalized tris(amido)phosphate ligand

Mild deprotonation of a 2-pyridyl (py)-functionalized phosphoric triamide [PO(NHpy)(3)] in the absence of an external base was studied in the presence of various silver(I) salts. Interesting examples of octa- and hepta-nuclear Ag(I) complexes coordinated to imido and pyridyl groups were obtained when more reactive Ag(I) salts, such as AgClO(4) and AgBF(4), were used, while the less reactive AgNO(3) reacts only with the peripheral pyridyl groups leading to a tri-nuclear cluster. Structural determination of these Ag(I) complexes show that sequential deprotonation of the ligand amino protons were achieved forming imido P(V) species analogous to the H(2)PO(4)(-) and HPO(4)(2-) ions.

A phosphorus-supported multisite coordination ligand containing three imidazolyl arms and its metalation behaviour. An unprecedented co-existence of mononuclear and macrocyclic dinuclear Zn(II) complexes in the same unit cell of a crystalline lattice

The reaction of (S)P[N(Me)NH(2)](3) with three equivalents of 4(5)-imidazolecarboxaldehyde afforded, in situ, (S)P[N(Me)N[double bond, length as m-dash]CHIm](3) (Im = imidazolyl). The reaction of the latter with metal salts afforded mononuclear complexes [{(S)P[N(Me)N[double bond, length as m-dash]CHIm](3).M}][X](2) [M = Co, X = NO(3); M = Ni, X = ClO(4); M = Cd, X = NO(3)]. In these compounds, in the cationic part, the metal ion is bound by three imino and three imidazolyl nitrogen atoms to generate a trigonal prismatic type of coordination environment. Structural analysis of the zinc complex showed the presence of a mononuclear [{(S)P[N(Me)N[double bond, length as m-dash]CHIm](3).Zn}][NO(3)](2) and a dinuclear 20-membered metallamacrocycle [{(S)P[N(Me)N[double bond, length as m-dash]CHIm](3).Zn}(2)][NO(3)](4) in the same unit cell. Solution studies of the zinc complex revealed a monomer-macrocycle equilibrium.

Stereochemically Distinct Cyclotetrasiloxanes Containing 3-Pyridyl Moieties and Their Functional Coordination Polymers

Synthesis of new cyclotetrasiloxane scaffolds containing peripherally functionalized 3-pyridyl moieties, [MeSiO(CH═CH(3)Py)]4 (L(1)) and [MeSiO(CH2CH2(3)Py)]4 (L(2)), and their reactivity studies with certain d(10) metal ions are reported. The ligand L(1) is obtained by the Heck-coupling reaction of tetramethyl tetravinyl tetrasiloxane (D4(vi)) and 3-bromopyridine in the presence of the Pd(0) catalysts. The as-synthesized ligand L(1) shows the presence of three stereoisomers, cis-trans-cis (L(1A)), cis-cis-trans (L(1B)), and all-trans (L(1C)), which are quantitatively separated by column chromatography. Subsequent reduction of L(1A), L(1B), and L(1C) with triethylsilane in the presence of catalytic amounts of Pd/C leads to the formation of the ligands L(2A), L(2B), and L(2C) with retention of stereochemistry due to the precursor moieties. Treatment of ZnI2 with L(1A) gives a one-dimensional coordination framework [(L(1A))4(ZnI2)2]∞, 1. These 1D-chains are further connected by π-π stacking interactions between the pyridyl groups of the adjacent chains leading to the formation of a three-dimensional network with the topology of a PtS net. The reaction of silver nitrate with ligand L(1B) gives a chain like one-dimensional cationic coordination polymer {[(L(1B))4Ag2]·2NO3·H2O·CH3OH }∞, 2, consisting of two different kinds of 32-membered macrocycles. Treatment of the all-trans ligand L(2C) with copper(I) iodide salt results in the formation of a cubane-type Cu4I4 cluster MOF [(L(2C))4Cu4I4]∞, 3, in a two-dimensional 4-connected uninodal sql/Shubnikov tetragonal plane net topology represented by the Schläfli symbol {4(4).6(2)}. This MOF displays a thermochromic luminescence behavior due to Cu4I4 clusters showing an orange emission at 298 K and a blue emission at 77 K.

Concentration dependent ratiometric turn-on selective fluorescence detection of picric acid in aqueous and non-aqueous media

A phosphorictriamide scaffold, [(NHAQ)3PO] (TAQP), which has an electron-rich aminoquinoline (AQ) chromophore, is shown to exhibit a ratiometric turn-on response for picric acid (PA) at low (1 × 10−5 M) concentration via a proton-transfer pathway. However, due to the presence of strong H-bonding and π⋯π interactions, at higher concentrations (1 × 10−3 M), it exhibits only a luminescence quenching behaviour for PA. These effects can be observed in both protic and non-protic systems. The detection limit for this aqueous phase turn-on sensing was found to be 0.2 ppm.

A Neutral Cluster Cage with a Tetrahedral [Pd12IIL6] Framework: Crystal Structures and Host–Guest Studies

A charge-neutral tetrahedral [(Pd3X)4L6] cage assembly built from a trinuclear polyhedral building unit (PBU), [Pd3X](3+), cis-blocked with an imido P(V) ligand, [(N(i)Pr)3PO](3-) (X(3-)), and oxalate dianions (L(2-)) is reported. Use of benzoate or ferrocene dicarboxylate anions, which do not offer wide-angle chelation as that of oxalate dianions, leads to smaller prismatic clusters instead of polyhedral cage assemblies. The porosity of the tetrahedral cage assembly was determined by gas adsorption studies, which show a higher uptake capacity for CO2 over N2 and H2. The tetrahedral cage was shown to encapsulate a wide range of neutral guest solvents from polar to nonpolar such as dimethyl sulfoxide, benzene, dichloromethane, chloroform, carbon tetrachloride, and cyclopentane as observed by mass spectral and single-crystal X-ray diffraction studies. The (1)H two-dimensional diffusion ordered spectroscopy NMR analysis shows that the host and guest molecules exhibit similar diffusion coefficients in all the studied host-guest systems. Further, the tetrahedral cage shows selective binding of benzene, CCl4, and cyclopentane among other solvents from their categories as evidenced from mass spectral analysis. A preliminary density functional theory analysis gave a highest binding energy for benzene among the other solvents that were structurally shown to be encapsulated at the intrinsic cavity of the tetrahedral cage.

Altering polarization attributes in ferroelectric metallo-cavitands by varying hydrated alkali-metal guest cations

Supramolecular ferroelectrics have emerged as an exciting topic of research for both fundamental understanding and practical applications in the areas of energy and electronics. Here, we describe the synthesis of two metallo-cavitands [{M4L8(H2O)8}⊃9(H2O)]·(NO3)8·x(H2O) [MNi2+ (1) or Co2+ (2)] and demonstrate solid-state host–guest behavior and ferroelectricity in the presence of various hydrated alkali metal cations (Hy-A) in their intrinsic voids. Due to the confinement effects of the cavitands, the degree of hydration observed for these alkali metal ions is on the upper side; in particular, the K+, Rb+ and Cs+ ions show high hydration numbers of 8, 9 and 10, respectively. Ferroelectric studies on 1, [Hy-A]⊂1 and [Hy-A]⊂2 assemblies show remnant polarization (Pr) values ranging from 27 to 30 μC cm−2 with concomitant variations in the coercive field (Ec) values at a lower frequency of 0.1 Hz. The observed features for the P–E loop characteristics of all the assemblies are reminiscent of triglycine sulphate (TGS) and certain other supramolecular ferroelectrics. Interestingly, ferroelectric fatigue measurements on all these systems show sizable variations as the cavitands with hard Li+ ions exhibiting the maximum (fatigue) tolerance and the ones with higher polarizable Cs+ ions show a reduction in Pr values up to 35%, after 105 switching cycles. The results signify the effect of encapsulated guest molecules in altering the polarization attributes (Pr, Ec and fatigue tolerance) via host–guest interactions.

Light-driven Hydrogen Evolution from Water by a Tripodal Silane Based CoII6L18 Octahedral Cage

The octahedral cage assembly [CoII6L18Cl6(H2O)6]Cl6 has been synthesized in a single-step reaction by using a polypyridyl-functionalized tripodal silane ligand. The electrochemical behavior of the cage in water exhibits the pH dependence of potential as well as catalytic current indicating the possible involvement of proton-coupled electron transfer in H2 evolution. Electrocatalytic hydrogen evolution from an aqueous buffered solution gave a turnover frequency of 16 h-1. Further, this cage assembly has been explored as a photocatalyst (blue light irradiation λ 469 nm) for the evolution of H2 from water in the presence of Ru(bpy)32+ as a photosensitizer and ascorbic acid as a sacrificial electron donor. This catalytic reaction is found to be pseudo first order with a turnover frequency of 20.50 h-1.