Kausikisankar Pramanik

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.

Chemistry of [Ru(tpy)(pap)(L′)n+ (tpy = 2,2′,6′,2″-terpyridine; pap = 2-(phenylazo)pyridine; L′ = Cl−, H2O, CH3CN, 4-picoline, N3−; n = 1,2). X-ray crystal structure of [Ru(tpy)(pap)(CH3CN)](ClO4)2 and catalytic oxidation of water to dioxygen by [Ru(tpy)(pap)(H2O)]2+

Abstract The reaction of [Ru(tpy)Cl3] with pap has afforded [Ru(tpy)(pap)Cl]+ which has been isolated and characterized as the perchlorate salt. Treatment of [Ru(tpy)(pap)Cl]+ with Ag+ in aqueous solution gives [Ru(tpy)(pap)(H2O)]2+. This aquo-complex has been reacted with three monodentate ligands (L′ = CH3CN, 4-picoline and N3− to afford complexes of type [Ru(tpy)(pap)(L′)]n+. Structure determination of [Ru(tpy)(pap) (CH3CN)](ClO4)2 by X-ray crystallography shows that tpy is coordinated to ruthenium in the usual meridional fashion and the pap ligand is bound to ruthenium with the azo-nitrogen trans to CH3CN. All these complexes except [Ru(tpy)(pap)H2O)]2+ show a Ru(II)–Ru(III) oxidation in the range 1.11–1.50 V vs SCE and three ligand(pap)/(tpy)-based reductions on the negative side of SCE. The aquo-complex shows a RuIIOH2/RuIVO couple in aqueous solution (pH = 1–4), the E° of this oxidation is estimated to be 0.82 V vs SCE. Attempt to chemically oxidize the aquo-complex by Ce4+ in aqueous solution (1 M HClO4) results in the catalytic oxidation of water to dioxygen.

Synthesis of a tetra- and a trisaccharide related to an anti-tumor saponin “Julibroside J28” from Albizia julibrissin

Simple and convergent synthesis of a tetra- and a trisaccharide portions of an antitumor compound Julibroside J28, isolated from Albizia julibrissin, that showed significant in vitro antitumor activity against HeLa, Bel-7402 and PC-3M-1E8 cancer cell lines is reported. The tetrasaccharide has been synthesized as its p-methoxyphenyl glycoside starting from commercially available d-glucose, l-rhamnose and l-arabinose. The trisaccharide part has been synthesized from commercially available N-acetyl d-glucosamine, d-fucose and d-xylose using simple protecting group manipulations. Sulfuric acid immobilized on silica has been used successfully as a Brönsted acid catalyst for the crucial glycosylation steps.

Insight into luminescent bisazoaromatic CNN pincer palladacycle: synthesis, structure, electrochemistry and some catalytic applications in C–C coupling

Chelating behavior of a potential flexidentate bisazoaromatic molecule 2-(phenylazo)azobenzene (PAAB) toward palladium have been examined. Activation of one instead of the two C(Ph)–H bonds of PAAB (HL) in the course of the metallation furnishes an exclusive unsymmetrical mode of ligation. Consequently, another member of the important class of CNN pincer palladacycles is achieved in almost quantitative yields. Crystallographic analysis authenticates the monoanionic terdentate ligation of the bisazoaromatic molecule and neutral [PdII(L)X] (X = acetate and halides) complexes with planar palladacycle and out-of-plane pendant phenyl ring. Unlike the N-donor pincers incorporating imine moieties, the introduction of two azo chromophores not only makes the PAAB ligand strongly π-acidic but also significantly modulates the FMOs in the palladacycles. An appreciable lowering of ligand-centered (LC) π* orbitals is apparent upon complexation which is in accordance with the electrochemical responses where large anodic shifts of LC LUMO and LUMO + 1 by 0.8–0.9 V have been noted. The significant modification of FMOs leads to interesting optoelectronic properties in the palladacycles. The cyclopalladated compounds are luminescent in solution at room temperature. This property is plausibly a result of the LC emissive states (azo π*). The optoelectronic features are interpreted by the Time-Dependant (TD) density functional theory (DFT) and natural transition orbital (NTO) analyses. In addition, scrutiny of Suzuki–Miyaura and Heck-type reactions indicates that the pincer palladacycles may be employed in the development of useful catalysts for C–C coupling reactions.

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.

Iridium(III) Mediated Reductive Transformation of Closed-Shell Azo-Oxime to Open-Shell Azo-Imine Radical Anion: Molecular and Electronic Structure, Electron Transfer, and Optoelectronic Properties.

The hydrogen bonded bis azo-oximato [IrCl2(L(NOH))(L(NO))] 2 and its deprotonated form (Et3NH)[IrCl2(L(NO))2] (Et3NH)(+)3(-) have been isolated in the crystalline state by a facile synthetic method. The azo-oxime frameworks in 3(-) have been conveniently transformed to the azo-imine by reduction with NaBH4 or ascorbic acid. Notably, the coordinated azo-imines accept an extra electron thereby furnishing the azo-imine radical anion complex 4. The underlying reductive transformation can be best described by proton-coupled electron transfer (PCET) process. Both the coordinated ligands (azo-oxime) in 3(-) are typically closed-shell monoanion (L(NO-)), but their reduced form (azo-imine) can behave as open-shell monoanion (L(NH•-)) owing to the presence of highly stabilized virtual orbitals. Remarkable enhancement of the π-acidity in azo-imine relative to the precursor azo-oxime has also been reflected from the electrochemical study. The irido complexes display rich optoelectronic properties, and the origin of the transitions has been scrutinized by the TD-DFT method. The molecular geometries of the complexes 2 and 3(-) reveal that the syn orientation of the azo-oximes frameworks is favored because of strong noncovalent H-bonding and π-π stacking interactions. In the course of the reduction of 3(-), the sterically encumbered disposition of the azo-oximes is converted to the relaxed anti form in the transformed azo-imines. Diffraction study reveals the electronic structure of 4 as [Ir(III)Cl2{(L(NH))2(•-)}]. The superior stabilization of the unpaired spin on the ligand array rather than metal has also been substantiated from EPR and DFT studies. Theoretical analysis reveals that the odd electron delocalizes primarily over both the azo-imine moieties ([IrCl2(L(NH•-))(L(NH))] ↔ ([IrCl2(L(NH))(L(NH•-))]) with no apparent contribution from metal, and this type of ligand-centered mixed valency (LCMV) can be best expressed as Robin-Day class III (fully delocalized) in nature.

Valence specific chelation of ruthenium to Schiff mono-bases of 2,6-diformyl-4methylphenol : synthesis and structure of trivalent salicylaldiminato species of coordination type RuN2O2PCl

Abstract The reaction of Schiff mono-bases, HRL (R = Me, Et), of 2,6-diformyl-4-methylphenol with K2 RuCl,(H2O) has afforded RuIII(RL)2(PPh3)Cl. The X-ray structure of the R = Et complex has revealed metal chelation at the salicylaldimine segment of RL−, the two phenolic oxygen and azomethine nitrogen atoms lying in mutually trans and cis positions, respectively. The trivalent state of the metal is stabilized in Ru(RL)2(P-Ph3)Cl, the ruthenium (III)/ruthenium(II) E 1 2 being ∼0.40 V vs SCE. These distorted low-spin (t25) complexes display rhombic EPR spectra and are characterized by a pair of ligand field transitions (in the near-IR region) within the split t2 shell. The complexes provide a striking contrast with the ruthenium(II) organometallics arising from the reaction of HRL with RuII(PPh3)3Cl2.

Synthesis and characterisation of a pair of azo anion radicals bonded to ruthenium(II)

The reactions of 1-methyl-2-(p-chlorophenylazo)imidazole (L1) and 2-(phenylazo)pyridine (L2) with [Ru(H)(X)- (CO)(PPh3)3] (X = Cl, Br) have afforded the green paramagnetic (S = ½) and EPR-active (g ≈ 2.00) title anion radical complexes [Ru(L1·–)(Cl)(CO)(PPh3)2] 1 and [Ru(L2·–)(Br)(CO)(PPh3)2] 2 in which the N–N bond lengths lie near 1.35 A.

Luminescent closed shell nickel(ii) pyridyl-azo-oximates and the open shell anion radical congener: molecular and electronic structure, ligand redox behaviour and biological activity

Luminescent nickel(II) complexes of type [NiII(L−I)2], 2, have been synthesized using photosensitizer redox-active oxime ligands HL, 1, incorporating the π-acidic azo and pyridyl functions. The redox non-innocent behaviour of the ligands has been exploited to isolate the open shell Ni(II)-bound azo-oxime anion radical complexes of type Et4N[NiII{(L−I)2}•−], 3, via reduction with NaBH4. The superior stabilization of the unpaired spin over the ligand framework has been established by EPR and DFT studies. The anti-bacterial activity of 2 has been scrutinized and it has been found to exhibit potential radical scavenging activity.

Ambient-Stable Bis-Azoaromatic-Centered Diradical [(L•)M(L•)] Complexes of Rh(III): Synthesis, Structure, Redox, and Spin–Spin Interaction

Bis-azoaromatic electron traps, viz. 2-(2-pyridylazo)azoarene 1, have been synthesized by colligating electron-deficient pyridine and azoarene moieties, and they act as apposite proradical templates for the formation of stable open-shell diradical complexes [(1•-)RhIII(1•-)]+ ([2]+), starting from the low-valent electron reservoir [RhI]. The less stable monoradical [RhIII(1•-)Cl2(PPh3)3] (3) has also been isolated as a minor product. These π-radical complexes are multiredox systems, and the electron transfer processes occur exclusively within the pincer-type NNN ligand backbone 1. Molecular and electronic structures of the diradicals and monoradicals have been ascertained with the aid of X-ray diffraction, electrochemical, spectroelectrochemical, and spectral (electronic, IR, NMR, and EPR) studies. In the diradicals [2]+, the orthogonal disposition of two ligand π orbitals linked via a closed-shell metal center (t26) impedes significant coupling between the radicals. Indeed, the observed magnetic moment of [2a]+ lies near ∼2.3 μB over the temperature range 50-300 K. A very weak antiferromagnetic (AF) intramolecular spin-spin interaction between two ligand π arrays in [(1•-)RhIII(1•-)]+ have been found experimentally (J ≈ -5 cm-1), and this is further substantiated by density functional theory (DFT) calculations at the (U)B3LYP/6-31G(d,p) level.