Vadapalli Chandrasekhar

Organotin assemblies containing SnO bonds

Abstract This review deals with the recent progress in the area of organotin assemblies that contain SnO bonds. Various kinds of tri-, di- and monoorganotin compounds are described in terms of their preparation by methods such as hydrolysis of organotin halides, reactions of suitable oragnotin compounds with various kinds of substrates such as carboxylic acids, sulfonic acids, oxide transfer reagents etc. The structural characterization of these compounds by the use of 119 Sn-NMR, 119 Sn Mossbauer and X-ray crystallography is presented in considerable detail. The amazing structural diversity present in this family of compounds is discussed.

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.

Synthesis, structure, and magnetism of heterobimetallic trinuclear complexes {[L2Co2Ln][X]} [Ln = Eu, X = Cl; Ln = Tb, Dy, Ho, X = NO3; LH3 = (S)P[N(Me)N=CH-C6H3-2-OH-3-OMe]3]: A 3d-4f family of single-molecule magnets.

Sequential reaction of LH3 (LH3 = (S)P[N(Me)N=CH-C6H3-2-OH-3-OMe]3) with Co(OAc)2 x 4 H2O followed by reaction with lanthanide salts afforded trinuclear heterobimetalllic compounds {[L2Co2Ln][X]} [Ln = Eu (1), X = Cl; Ln = Tb (2), Dy (3), Ho (4), X = NO3] in excellent yields. These compounds retain their integrity in solution as determined by electrospray ionization mass spectrometry studies. The molecular structures of 1-4 were confirmed by a single-crystal X-ray structural study and reveal that these are isostructural. In all of the compounds, the three metal ions are arranged in a perfectly linear manner and are held together by two trianionic ligands, L3-. The two terminal Co(II) ions contain a facial coordination environment (3N, 3O) comprising three imino nitrogen atoms and three phenolate oxygen atoms. The coordination geometry about the cobalt atom is severely distorted. An all-oxygen coordination environment (12O) is present around the central lanthanide ion, which is present in a distorted icosahedral geometry. The coordination sphere around the lanthanide ion is achieved by utilizing three phenolate oxygen atoms and three methoxy oxygen atoms of each ligand. In all of these trinuclear complexes (1-4), the Co-Ln distances are around 3.3 A, while the Co-Co distances range from 6.54 to 6.60 A. The screw-type coordination mode imposed by the ligand induces chirality in the molecular structure, although all of the complexes crystallize as racemates. Magnetic properties of 1-4 have been studied in detail using dc and ac susceptibility measurements. Dynamic measurements reveal that 2-4 display a single-molecule magnet behavior, while the Co2Eu (1) analogue does not show any out-of-phase ac susceptibility.

Phosphazenes as scaffolds for the construction of multi-site coordination ligands

Chlorocyclophosphazenes containing a reactive periphery and a robust framework offer an ideal platform for the construction of a large variety of multi-site coordination ligands. Various types of neutral or (multi)anionic multi-site coordinating ligands can be assembled containing different numbers, type and stereochemical orientations of coordination sites reflecting the ease of modulation of these ligand types. Also it is possible to construct new types of macrocyclic ligands using cyclophosphazenes as scaffolds. The interaction of these ligands with transition and main group metals leads to a number of different complex types showing the rich diversity that is present in this family of compounds. A study of these ligands in principle also allows the assembly of the corresponding polymeric ligands.

Advances in the chemistry of chlorocyclophosphazenes

Publisher Summary This chapter deals with chlorocyclophosphazenes and their reactions. Cyclophosphazenes are inorganic heterocyclic ring compounds containing an [N=PX 2 ] repeat unit in a valence-unsaturated skeleton that are isoelectronic with the corresponding siloxanes that contain the [OSiX 2 ] repeat unit. Within these compounds phosphorus is pentavalent and tetracoordinate while nitrogen is trivalent and dicoordinate. The cyclophosphazene ring systems constitute a regular and homologous series and the ring size varies considerably. The chemistry of cyclophosphazenes comes from different directions. Firstly, the replacement of halogens on halogenocyclophosphazenes by a variety of nucleophiles has been a fertile area of research fraught with many interesting problems connected with the question of regio- and stereo-selectivity of the substitution reaction, the kinetics of the reactions involved, and the structural elucidation of the products formed. Secondly, substituted cyclophosphazenes can serve as excellent ligands for a number of transition metal ions. The traditional and most extensively used method of synthesis of chlorocyclophosphazenes [NPCls] n involves the reaction of finely ground ammonium chloride with phosphorus pentachloride in a high boiling solvent such as sym -tetrachloroethane.

Slow Magnetic Relaxation in Co(III)–Co(II) Mixed-Valence Dinuclear Complexes with a CoIIO5X (X = Cl, Br, NO3) Distorted-Octahedral Coordination Sphere

The reaction of the multisite coordination ligand (LH4) with CoX2·nH2O in the presence of tetrabutylammonium hydroxide affords a series of homometallic dinuclear mixed-valence complexes, [Co(III)Co(II)(LH2)2(X)(H2O)](H2O)m (1, X = Cl and m = 4; 2, X = Br and m = 4; 3, X = NO3 and m = 3). All of the complexes have been structurally characterized by X-ray crystallography. Both cobalt ions in these dinuclear complexes are present in a distorted-octahedral geometry. Detailed magnetic studies on 1-3 have been carried out. M vs H data at different temperatures can be fitted with S = 3/2, the best fit leading to D(3/2) = -7.4 cm(-1), |E/D| < 1 × 10(-3), and g = 2.32 for 1 and D(3/2) = -9.7 cm(-1), |E/D| <1 × 10(-4), and g = 2.52 for 2. In contrast to 1 and 2, M vs H data at different temperatures suggest that compound 3 has comparatively little magnetic anisotropy. In accordance with the large negative D values observed for compounds 1 and 2, they are single-molecule magnets (SMMs) and exhibit slow relaxation of magnetization at low temperatures under an applied magnetic field of 1000 Oe with the following energy barriers: 7.9 cm(-1) (τo = 6.1 × 10(-6) s) for 1 and 14.5 cm(-1) (τo = 1.0 × 10(-6) s) for 2. Complex 3 does not show any SMM behavior, as expected from its small magnetic anisotropy. The τo values observed for 1 and 2 are much larger than expected for a SMM, strongly suggesting that the quantum pathway of relaxation at very low temperatures is not fully suppressed by the effects of the applied field.

Novel chemosensor for the visual detection of copper(II) in aqueous solution at the ppm level

A new water-soluble, multisite-coordinating ligand LH(7) was prepared by the condensation of tris(hydroxymethyl)aminomethane with 2,6-diformyl-p-cresol. LH(7) is a selective chemosensor for Cu(2+), under physiological conditions, with visual detection limits of 20 ppm (ambient light conditions) and 4 ppm (UV light conditions). LH(7) can also be used in biological cell lines for the detection of Cu(2+).

Syntheses, structures, and magnetic properties of a family of heterometallic heptanuclear [Cu5Ln2] (Ln = Y(III), Lu(III), Dy(III), Ho(III), Er(III), and Yb(III)) complexes: observation of SMM behavior for the Dy(III) and Ho(III) analogues.

Sequential reaction of the multisite coordination ligand (LH3) with Cu(OAc)2·H2O, followed by the addition of a rare-earth(III) nitrate salt in the presence of triethylamine, afforded a series of heterometallic heptanuclear complexes containing a [Cu5Ln2] core {Ln = Y(1), Lu(2), Dy(3), Ho(4), Er(5), and Yb(6)}. Single-crystal X-ray crystallography reveals that all the complexes are dicationic species that crystallize with two nitrate anions to compensate the charge. The heptanuclear aggregates in 1-6 are centrosymmetrical complexes, with a hexagonal-like arrangement of six peripheral metal ions (two rare-earth and four copper) around a central Cu(II) situated on a crystallographic inversion center. An all-oxygen environment is found to be present around the rare-earth metal ions, which adopt a distorted square-antiprismatic geometry. Three different Cu(II) sites are present in the heptanuclear complexes: two possess a distorted octahedral coordination sphere while the remaining one displays a distorted square-pyramidal geometry. Detailed static and dynamic magnetic properties of all the complexes have been studied and revealed the single-molecule magnet behavior of the Dy(III) and Ho(III) derivatives.

Silver-guided excimer emission in an adenine-pyrene conjugate: fluorescence lifetime and crystal studies

This Communication describes a novel adenine-pyrene conjugate (1) and its solid-state structure with silver and copper ions. Single-crystal studies of metal complexes of 1 offer insight into molecular interactions and provide a basis to rationalize possible interactions in the solution state, leading to excimer formation. The robust nature of this interaction was further confirmed by deposition of the silver complex on a graphite surface, which exhibited a remarkable resemblance to its solid-state structure. The structural basis of selective excimer formation in the presence of Ag(+) ions presents a viable approach for ratiometric detection of these ions.

A distorted cubic tetranuclear copper(II) phosphonate cage with a double-four-ring-type core

The reaction of Cu2(O2CMe)(4).2H2O with tert-butylphosphonic acid and 3,5-di-tert-butylpyrazole in the presence of triethylamine leads to a high-yield synthesis of the tetranuclear compound [Cu2(3,5-t-Bu2PzH)2(t-BuPO3)2]2 (1). The latter has a distorted cubic cage structure and its core resembles the D4R (double-four-ring) motif found in zeolites. The phosphonate, [t-BuPO3]2-, functions as a dianionic tridentate ligand, while the pyrazole ligands are neutral and are monodentate. The coordination geometry at each copper atom is distorted square planar with a 3O,1N coordination environment. Magnetic measurements on 1 reveal that the chiT product continuously decreases to reach a value very close to zero at 1.8 K, indicating dominant antiferromagnetic interactions between Cu(II) ions that leads to an S=0 ground state. The tetranuclear cage 1 functions as a very effective artificial nuclease in the presence of an external oxidant, magnesium monoperoxyphthalate.