Anant Kumar Srivastava

Synthesis and magnetothermal properties of a ferromagnetically coupled NiII–GdIII–NiII cluster

A linear trimeric cluster of molecular formula [Ni2Gd(L(-))6](NO3) (1) (L(-) = (C14H12NO2) has been isolated with its structure determined via single crystal X-ray diffraction. Magnetic susceptibility measurements of 1 show that the nickel and gadolinium ions are coupled ferromagnetically, with a ground total spin state (S) of 11/2. Best fit spin Hamiltonian parameters obtained for 1 are J(1(Ni-Gd)) = +0.54 cm(-1), g = 2.01. EPR measurements confirm a low magnetic anisotropy (D = -0.135 cm(-1)) for 1. Heat capacity determination of the magnetocaloric effect (MCE) parameters for 1 shows that the change in magnetic entropy (-ΔS(m)) achieves a maximum of 13.74 J kg(-1) K(-1) at 4.0 K, with the ferromagnetic coupling giving a rapid change in low applied fields, confirming the potential of Gd molecular derivatives as coolants at liquid helium temperature.

Hydroxo‐Bridged Dimers of Oxo‐Centered Ruthenium(III) Triangle: Synthesis and Spectroscopic and Theoretical Investigations

The homometallic hexameric ruthenium cluster of the formula [Ru(III)6(μ3-O)2(μ-OH)2((CH3)3CCO2)12(py)2] (1) (py = pyridine) is solved by single-crystal X-ray diffraction. Magnetic susceptibility measurements performed on 1 suggest that the antiferromagnetic interaction between the Ru(III) centers is dominant, and this is supported by theoretical studies. Theoretical calculations based on density functional methods yield eight different exchange interaction values for 1: J1 = -737.6, J2 = +63.4, J3 = -187.6, J4 = +124.4, J5 = -376.4, J6 = -601.2, J7 = -657.0, and J8 = -800.6 cm(-1). Among all the computed J values, six are found to be antiferromagnetic. Four exchange values (J1, J6, J7 and J8) are computed to be extremely strong, with J8, mediated through one μ-hydroxo and a carboxylate bridge, being by far the largest exchange obtained for any transition-metal cluster. The origin of these strong interactions is the orientation of the magnetic orbitals in the Ru(III) centers, and the computed J values are rationalized by using molecular orbital and natural bond order analysis. Detailed NMR studies ((1)H, (13)C, HSQC, NOESY, and TOCSY) of 1 (in CDCl3) confirm the existence of the solid-state structure in solution. The observation of sharp NMR peaks and spin-lattice time relaxation (T1 relaxation) experiments support the existence of strong intramolecular antiferromagnetic exchange interactions between the metal centers. A broad absorption peak around 600-1000 nm in the visible to near-IR region is a characteristic signature of an intracluster charge-transfer transition. Cyclic voltammetry experiments show that there are three reversible one-electron redox couples at -0.865, +0.186, and +1.159 V with respect to the Ag/AgCl reference electrode, which corresponds to two metal-based one-electron oxidations and one reduction process.

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.

Functional metal-organic molecules and materials derived from rigid and flexible P-N scaffolds

GRAPHICAL ABSTRACT Abstract Employing phosphoramide ligands of formula, (RNH)3PO in reaction with Pd(OAc)2, a facile route to access the highly basic trianions, [(RN)3PO]3− ((X)3−) analogous to PO43− ion were developed. The (X)3− ligand, containing a central binding group, acts as a rigid cis-blocking ligand and stabilizes trimeric or prismatic Pd(II) clusters of formula {Pd3X(OAc)3}1 or 2. Further, the trinuclear (Pd3X)3+ motifs were utilized as supramolecular synthons for obtaining neutral cluster cages for Pd(II) ions in tetrahedral and cubic topologies via a bridging ligand substitution strategy. In contrast, phosphoramides containing peripheral metal binding groups provide a flexible ligand platform and yield multi-metallic assemblies in various dynamic architectures. By using pyridyl functionalized flexible dipodal phosphoramide ligand (L) we were able to generate interesting examples of centrosymmetric and noncentrosymmetric {CuIIL2}n frameworks. Investigation of the electric field-dependent polarization (P-E loop) of the noncentrosymmetric {CuIIL2}n frameworks leads to the observation of a potentially ferroelectric behavior in them that are governed by the counter anions and topology of the framework.

Anion-induced ferroelectric polarization in a luminescent metal–organic cage compound

Metal–organic crystalline solids with ferroelectric properties have attracted significant attention recently as materials for high-tech applications. Here, we describe two crystalline assemblies that contain cationic metal–organic cages {[Zn6(H2O)12][TPTA]8}(NO3)12·26H2O (1) and {[Zn6(H2O)12][TPTA]8}(ClO4)12·18.75H2O (2) featuring the tripodal ligand [PS(NH3Py)3] (TPTA). Ferroelectric measurement on a single crystal of 1 gave a remnant (Pr) polarization of 1.2 μC cm−2 at room temperature. The ferroelectric response originates from the toggling of nitrate anions and solvate molecules found in pockets between the cages. The temperature-dependent permittivity of 1 shows an anomalous dielectric peak at 20 °C. This is attributed to the desolvation-assisted dielectric relaxation behaviour and signifies the role of solvate molecules in the ferroelectric behaviour of 1. In addition, this material is highly luminescent exhibiting a bright blue emission under UV light. This multifunctional behaviour is unique among metal–organic cage frameworks.

A tetrakis(amido)phosphonium cation containing 2-pyridyl (2Py) substituents,[P(NH2Py)4]+ and its reactivity studies with Ag(I) salts

AbstractPoly-imido analogues of various phosphorus oxo anions have gained recent attention in inorganic chemistry. Current methods to obtain these anions require strong organometallic deprotonating agents in reaction with phosphonium salt like [(NHPh)4P]Cl or phosphoramides such as [(RNH)3P=E] (E = NSiMe3, O, S or Se) in non-polar solvents. Recently, employing salts of soft and reactive transition metal ions, we have developed methods to obtain these anions in polar and protic solvents. Herein, we have described a facile anion exchange route that stabilizes the highly labile tetrakis(2-pyridylamino)phosphonium cation as its nitrate salt, [P(NH2Py)4]NO3. This molecule exhibits a double chain structure mediated by H-bonding interactions of the pyridylamino segments (N-H...N). The phosphonium salt upon reaction with excess silver triflate results in a pentanuclear Ag(I) complex, {Ag5[P(N 2Py)2(NH 2Py)2]} ⋅(F3CSO3)3, stabilized by two imido-phosphinate [P(N2Py)2(NH 2Py)2]− ligands. Formation of a similar penta-nuclear cluster has been observed before when AgClO4 was used as a base. Our previous results with the related phosphate precursor, [PO(NH2Py)3], in reaction with various Ag(I) salts have shown to yield complexes of the corresponding neutral, mono- and dianionic ligands. However, the stability of the Ag5-cluster within the mono-anionic casing of the [P(N2Py)2(NH2Py)2]− ligand have seemingly overwhelmed the subtle reactivity changes offered by various Ag(I) salts. Graphical AbstractEmploying an anion exchange route, synthesis of a stable tetrakis(2-pyridylamino)phosphonium cation as its nitrate salt, [P(NH2Py)4]NO3 has been described. The precursor phosphonium salt upon reaction with excess silver triflate results in a pentanuclear Ag(I) complex, {Ag5[P(N2Py)2(NH2Py)2]}•(F3CSO3)3, stabilized by two imido-phosphinate [P(N2Py)2(NH2Py)2]− ligands.