• Volume 123, Issue 6

November 2011,   pages  771-998

• Foreword

• Metal selective co-ordinative self-assembly of 𝜋-donors

Metal selective co-ordinative nanostructures were constructed by the supramolecular co-assembly of pyridine appended TTF (TTF-Py) and pyrene (PYR-Py) derivatives in appropriate solvent composition mixtures with metal ions.Microscopic analyses show that TTF-Py shows distinctive morphology with either of these metal ions, forming I-D tapes with 1:1 Pb2+ complex and 2-D sheets with 1:2 Zn2+ complex. A rationale has been provided from molecular level packing for such hierarchical changes. In case of Cu2+, we have observed an anomalous binding of metal ion to the core sulphur groups causing redox changes in the TTF core. PYR-Py on the other hand is shown to be a fluorescent sensor for Pb2+, Zn2+, Hg2+ and Ag+. With different fluorescent response for metal complexes, we essentially obtained similar 1-D assemblies suggesting similar binding modes for all of them. Supramolecular approach through which morphology of an electron donor moiety can be engineered by metal ions can be a new tool in nanoelectronics.

• Gold(I)-selenolate complexes: Synthesis, characterization and ligand exchange reactions

In this paper, the synthesis and characterization of some imidazole-based gold-selenolates are described. This study indicates that the nature of selenolate plays an important role in ligand exchange reactions in gold(I) selenolates. Furthermore, the reactivity of imidazole-based gold(I) selenolates toward nucleophiles such as selenols and phosphines is strikingly different from that of the $N,N$-dimethylaminobenzylamine-based gold(I) complexes. The presence of Se$\cdots$N non-bonded interactions in $N,N$-dimethylaminobenzylamine-based gold(I) complexes modulates the reactivity of Au(I) centre towards incoming nucleophiles.

• Ruthenium complexes of chelating amido-functionalized N-heterocyclic carbene ligands: Synthesis, structure and DFT studies

Synthesis, structure and density functional theory (DFT) studies of a series of new ruthenium complexes, [1-(R)-3-𝑁-(benzylacetamido)imidazol-2-ylidene]RuCl(𝑝-cymene) [R = Me (1c), 𝑖-Pr (2c), CH2Ph (3c); 𝑝-cymene = 4-𝑖-propyltoluene] supported over $N/O$-functionalized N-heterocyclic carbene (NHC) ligands are reported. In particular, the ruthenium (1-3)c complexes were synthesized from the respective silver complexes, [1-(R)-3-𝑁-(benzylacetamido)imidazol-2-ylidene]2Ag+Cl− [R = Me (1b), 𝑖-Pr (2b), CH2Ph (3b)] by the treatment with [Ru(𝑝-cymene)Cl2]2 in 65-76% yields. The molecular structures of (1-3)c revealed the chelation of the N-heterocylic carbene ligand through the carbene center and an amido sidearm of the ligand in all of the three complexes. The density functional theory studies on the ruthenium (1-3)c complexes indicated strong binding of the NHC ligand to the metal center as was observed from the deeply buried NHC-Ru 𝜎-bonding molecular orbitals.

• A RhIII-N-heterocyclic carbene complex from metal-metal singly bonded [RhII−RhII] precursor

Metal-metal singly bonded [Rh2(CO)4(acac)2][OTf]2 (1) has been synthesized and characterized by spectroscopic and analytical techniques. A density functional theory (DFT) optimized structure has been computed for the unbridged centro-symmetric structure. Reaction of 1 with PIN.HBr results in the [Rh(PIN)2(H2O)Br][OTf]2 (2) in high yield. The reaction involves metal-oxidation from RhII to RhIII accompanied by the metal-metal bond cleavage. The X-ray structure of 2 has been determined which reveals the incorporation of two N-heterocyclic carbene (NHC) ligands to each rhodium. This work demonstrates the general utility of the metal-metal bonded compounds for the easy synthesis of metal-NHC compounds.

• Two new 1D chains of Ni2Na2 heterometallic double half-cubane building units: Synthesis, structures and variable temperature magnetic study

An equimolar mixture of Ni(NO3)2·6H2O and pyridine-2-aldehyde with two equivalents of NaN3 in methanol in the presence of NaOMe resulted in the formation of light green precipitate which upon crystallization from dimethylformamide (DMF) yielded light green single crystals [{Ni2Na2(pic)4(N3)2(H2O)2(MeOH)}· MeOH·3H2O]$_n$ (1) and [{Ni2Na2(pic)4(N3)2(H2O)4}$\cdot$2DMF$\cdot$H2O]$_n$ (2) (pic = pyridine-2-carboxylate) at room temperature and high temperature (100°C), respectively. Variable temperature magnetic studies revealed the existence of overall ferromagnetic behaviour with $J \approx +10$ cm-1 and $D \approx −2$ to −7 cm-1 for 1 and 2, respectively. Negative 𝐷 values as well as variation of 𝐷 upon slight distortion of structure by varying reaction temperature were observed. The X-band Electron Paramagnetic Resonance (EPR) spectra of both 2 and 3 were recorded below 50 K. The structural distortion was also implicated from the EPR spectra. Density Functional Theory (DFT) calculations on both complexes were performed in two different ways to corroborate the magnetic results. Considering only Ni$^{\text{II}}_2$ dimeric unit, results were $J = +20.65$ cm-1 and $D = −3.16$ cm-1 for 1, and $J =+24.56$ cm-1 and $D =−4.67$ cm-1 for 2. However, considering Ni$^{\text{II}}_2$Na$^I_2$ cubane as magnetic core the results were $J =+16.35$ cm-1 (1), +19.54 cm-1 (2); $D =−3.05$ cm-1 (1), −4.25 cm-1 (2).

• Synthesis and characterization of some heteroleptic copper(II) complexes based on meso-substituted dipyrrins

The syntheses and characterizations of meso-substituted dipyrrins, 5-(4-imidazol-1-yl-phenyl)-dipyrromethene (4-impdpm), 5-(4-nitro-imidazol-1-yl-phenyl)-dipyrromethene, (4-nimpdpm), 5-(4-benzimidazol-1-yl-phenyl)-dipyrromethene (4-bimp-dpm) and heteroleptic complexes [Cu3(4-impdpm)2(hfacac)4] 1, [Cu(4-nimpdpm)(acac)] 2, [Cu(4-nimpdpm)(hfacac)] 3, [{Cu(4-bimpdpm)(acac)}$_n$] 4 and [{Cu(4-bimpdpm)-(hfacac)}$_n$] 5, imparting acetylacetonato (acac) and hexafluoroacetylacetonato (hfacac) groups as co-ligand have been described. The dipyrrins and complexes 1-5 have been characterized by elemental analyses and spectral (IR ESI-MS, NMR, electronic absorption and emission) studies. Crystal structures of 1, 3 and 4 have been authenticated by X-ray single crystal analyses. The reaction between 4-impdpm and Cu(hfacac)2 gave a trimetallic complex, under analogous conditions 4-nimpdpm and 4-bimpdpm reacted with Cu(acac)2 and Cu(hfacac)2·2H2O to afford mononuclear (2, 3) and 1D polymeric (4, 5) complexes.

• Axial phenoxide coordination on di-iron(III) bisporphyrin: Insights from experimental and DFT studies

Synthesis, structure and properties of new five-coordinate phenolate complexes of di-iron(III) bisporphyrin are reported here, in which phenol binds in $\eta^1$-fashion as an axial ligand. The solid and solution EPR at 120K and 1H NMR spectral pattern in solution provide unequivocal evidence for the high spin (S = 5/2) nature of the complex. Mulliken spin density calculation using DFT demonstrates the positive spin densities at the meso carbons and negative spin densities at the methylene carbons and, as a result, the meso and methylene protons are shifted in the upfield and down field regions, respectively in the 1H NMR spectra of the molecule. Also, the ortho- and para-protons of the phenolate ligands are observed to be shifted in the upfield region while meta-protons are shifted downfield. The alternating shift pattern, which is the opposite sign of the chemical shifts for meta-versus ortho- and para-protons, was also explained due to negative and positive spin densities, respectively on the carbons and indicative of 𝜋 spin delocalization on the phenolate ligand. Thus, the calculated spin density maps accounted for the essential 1H NMR spectroscopic features that are observed here for the phenolate complexes of di-iron(III) bisporphyrin. The temperature dependence of the signals follows the Curie law which is indicative of single spin state throughout the temperature range of −40 to +40°C. The single crystal X-ray structure of the corresponding chloro derivative, trans 1,2-bis(chloroiron(III) octaethyl porphyrinyl)ethene, has also been reported here which authenticates the high-spin nature of the complex.

• Iron and cobalt complexes of 4,4,9,9-tetramethyl-5,8-diazadodecane-2, 11-dione dioxime ligand: Synthesis, characterization and reactivity studies

Two oximate bridged dinuclear complexes [Co$^{\text{II}}_2$ (HL)2](ClO4)2 (1) and [Fe^{text{II}}_2$(HL)2](ClO4)2 (2), and a biomimetic iron(III)-catecholate complex [FeIII(HL)(DBC)] (3) of a dioxime ligand (H2L = 4,4,9,9-tetramethyl-5,8-diazadodecane-2,11-dione dioxime and DBCH2 = 3,5-di-tert-butylcatechol) were synthesized and characterized. X-ray single-crystal structures of both the dinuclear complexes exhibit an out-of-plane oximate bridge where the six-membered M2(NO)2 ring adopt a boat conformation with the metal ions in a fivecoordinate distorted trigonal bipyramidal geometry. Complexes 1 and 2 react with dioxygen at ambient condition to form the corresponding hydroxo- or oxo-bridged dinuclear cobalt(III) or iron(III) complexes. On the other hand, the iron(III)-catecholate complex (3) activate dioxygen to undergo oxidative C-C bond cleavage of catechol. The selective formation of extradiol catechol cleavage products in the reaction of 3 with dioxygen mimics the functional aspect of extradiol-cleaving catechol dioxygenases. The flexibility of ligand backbone is proposed to control the dioxygen reactivity of metal complexes. • Synthesis and reactivity of dimolybdathiaborane cluster [(CpMo)2B4SH6] (Cp =$\eta^5^-C5Me5)

Chemistry and reactivity of dimolybdathiaborane, [(CpMo)2B4SH6], 1, obtained from the reaction of 2-mercaptobenzothiazole, [CpMoCl4] and [LiBH4.thf], has been explored with dinuclear metal carbonyl [Fe2(CO)9]. As a result, reaction of 1 with [Fe2(CO)9] yielded heterometallathiaborane, [(CpMo)2B4H6SFe(CO)3], 2 in good yield. Both the new compounds have been characterized in solution by 1H, 11B and 13C NMR spectroscopy and the structural types were unequivocally established by X-ray crystallographic analysis of compound 2. Cluster 2 has a bicapped octahedral geometry with the {Fe(CO)3} fragment occupying one of the high-connectivity cluster vertexes rather than a capping position. Interestingly, cluster 1 undergoes geometric changes (bicapped trigonal bipyramid → bicapped octahedron) on the addition of two-electron {Fe(CO)3} fragment to 1.

• Identification and characterization of intermediates in the formation of the cyclobutadiene linked dimeric cobalt sandwich compound [($\eta^5$-RCp)Co($\eta^4$-C4Ph3)]2 [R = H, CH3C(O)O]

The synthesis of novel cyclobutadiene linked dimers of [$\eta^5$-RCp]Co($\eta^4$-C4Ph4) was reported recently from a three-component reaction of 1,4-diphenylbutadiyne, diphenylacetylene and [$\eta^5$-RCp]Co(PPh3)2 [R = H or MeOC(O)]. The chiral bis oxazoline derivative of this dimer was found to form cobalt oxazolinyl palladacycles which were excellent chiral catalysts for the Overman-Claisen rearrangement of trichloroacetimidates, similar to the well-known COP catalysts. In this paper, we report reactions carried out for identifying the intermediates in the formation of the dimeric sandwich compound {[$\eta^5$-MeOC(O)Cp]Co($\eta^4$-C4Ph3)}2 (1). On varying the stoichiometric ratio of the reactants of the three component reaction, two new cobalt sandwich compounds [$\eta^5$-MeOC(O)Cp]Co[$\eta^4$-C4Ph3(C≡CPh)] (2) and [$\eta^5$-MeOC(O)Cp]Co[$\eta^4$-C4-Ph2(C≡CPh)2] (3) were found to form as viscous oily liquids. The identity of 2 was ascertained by oxidizing its alkyne moiety to yield crystalline [$\eta^5$-MeOC(O)Cp]Co{$\eta^4$-C4Ph3[C(O)-C(O)Ph]} (4) which was structurally characterized. Further, the role of phenylethynyl derived monomeric sandwich compound 2 as a precursor to the dimeric compound 1 was established by reacting 2 with diphenylacetylene and CpCo(COD) which yielded the first example of an unsymmetrical cyclobutadiene linked dimeric cobalt sandwich compound {$\eta^5$-[MeOC(O)Cp]Co($\eta^4$-C4Ph3)}[($\eta^5$-Cp)Co($\eta^4$-C4Ph3)] (5) which was also structurally characterized.

• Mono-, di- and tetranuclear rhodium(I) complexes of morpholine and 𝑁-methylpiperazine functionalized cyclodiphosph(III)azanes, cis-[(${}^t$BuN-𝜇)2(PNC4H8X)2] (X = O, NMe)

The reactions of cyclodiphosphazanes, cis-[(${}^t$BuN-𝜇)2(PNC4H8O)2] (1, R = NC4H8O; 2, R = NC4H8NMe ) (2) with [{RhCl(COD)}2] and [{RhCl(CO)2}2] are described. The reactions of 1 or 2 with [{RhCl(COD)}2] in 2:1 molar ratio yielded the mononuclear complexes [(COD)RhCl{(𝜇-N$^t$BuP)2(NC4H8X)2}] (3, X = O; 4, X = NMe) in good yield. Treatment of one equivalent of [{RhCl(CO)2}2] with four equivalents of 1 or 2 produced the trans-[(CO)RhCl{(𝜇-N$^t$BuP)2(NC4H8X)2}2] (5, X = O; 6, X = NMe) complexes as pale yellow solids. On changing the molar ratio of [{RhCl(CO)2}2] and 1 or 2 from 1:4 to 1:1 resulted in the formation of tetranuclear complexes [{(CO)Rh(𝜇-Cl)(𝜇-N$^t$BuP)(NC4H8X)$\}_4$] (7, X = O; 8, X = NMe) under similar reaction conditions. The tetranuclear rhodium(I) complex 8 upon stirring with wet acetonitrile or in the presence of four equivalents of water at room temperature afforded a novel hydroxo bridged zwitterionic dirhodium(I) complex [{(ClRh(CO))2(𝜇-OH)(MeNC4H8N)(𝜇-N$^t$BuP)2(NC4H8NHMe)}] (9). The crystal structures of 5, 6 and 9 were established by single crystal X-ray diffraction studies.

• Optical detection of sodium salts of fluoride, acetate and phosphate by a diacylhydrazine ligand by the formation of a colour alkali metal complex

A solution of N, N'-diacylhydrazine ligand in organic solvent is potential for colourimetric detection of F/AcO/PO$^{3−}_4$ via -NH deprotonation, tautomerization and its stabilization as a colour alkali metal complex.

• Synthesis of azamacrocycle stabilized palladium nanoparticles: Controlled size and one-dimensional growth

Palladium nanoparticles (PdNPs) of uniform size in the range of 3-5 nm are prepared in MeOH and MeCN:MeOH by solvent-assisted reduction of palladium acetate in the presence of a hexaazamacrocyclic ligand, L1. For the mixed solvent system different ratio of the solvents was tried i.e., 1:1, 1:3 and 3:1. In all cases the concentration and ratio of Pd(II) to L1 was maintained at 2mM:1mM. In another experiment NPs were prepared in MeOH by lowering the concentration of L1 to 0.5mM, where chain-like assemblies of PdNPs was observed. Importantly, the solutions are found to be stable at RT for months.

• A two-fold interpenetrated flexible bi-pillared-layer framework of Fe(II) with interesting solvent adsorption property

A two-fold interpenetrated microporous bi-pillared-layer framework of Fe(II), {[Fe(2,6-napdc)(4,4'-bipy)](EtOH)(H2O)}$_n$ (1) (2,6-napdc =2,6-naphthalenedicarboxylate; 4,4'-bipy=4,4'-bipyridine) composed of mixed ligand system has been synthesized and structurally characterized. The 2,6-napdc linkers form a 2D corrugated sheet of {Fe(2,6-napdc)}$_n$ by linking the secondary building unit of Fe2(CO2)2 in the $ac$ plane, which are further connected by double 4,4'-bipy pillars resulting in a bi-pillared-layer type 3D framework. The 3D framework is two-fold interpenetrated and exhibits a 3D channel structure ($4.0 \times 3.5, 1.5 \times 0.5$ and $2.2 \times 2.1$ Å2) occupied by the guest water and ethanol molecules. Framework 1 shows high thermal stability, and the desolvated framework (1′) renders permanent porosity realized by N2 adsorption profile at 77K (BET surface area of $\tilde 52$ m2 g-1). Moreover, the framework 1′ also uptakes different solvent vapours (water, methanol and ethanol) and their type-I profile suggest strong interaction with pore surfaces and overall hydrophilic nature of the framework. Temperature dependent magnetic measurements suggest overall antiferromagnetic behaviour in compound 1.

• $\pi u$-Stacking interactions between G-quartets and circulenes: A computational study

Structures of planar and bowl-shaped circulenes as well as their stacks with G-quartet (G4) have been investigated through dispersion-corrected Density Functional Theory (DFT-D). The binding energies are substantial $\tilde ∼$10 kcal/mol with d ∼3.5 Å between the stacking rings. The calculations show that G4 binds much more effectively to planar circulenes as compared to bowl shaped molecules. The strength of binding between a G-quartet and a non-planar circulene molecule depends on the orientation of the circulene (concave or convex) with respect to G-quartet. An AIM analysis of the M05-2X wave-functions has also been performed to confirm the presence of weak intermolecular interactions between guanine quartets and circulenes. Apart from 𝜋-stacking interactions, the concave bowl-shaped circulenes also interact with G4 through C-H$\cdots \pi$ interactions. The charge transport properties between the two moieties have also been analysed through effective transport integral. The calculations provide an understanding for the basis of molecular recognition by G4 for non-planar systems.

• Silica nanodisks as platforms for fluorescence lifetime-based sensing of pH

Core-shell conjugates of silica nanodisks and fluorescent dyes have been prepared. Rhodamine B, the reference, has been attached to the core, by surface functionalization of the pristine SNDs. Then, a layer of silica has been deposited on the composite nanodisks. Finally, the surface has been functionalized with fluorescein in one case and protoporphyrin IX in the other. These dyes exhibit pH-dependent fluorescence properties. The nanoconjugates are found to sense the pH of the medium, through systematic variation of the fluorescence intensity ratios of the reporter dye at the surface and the reference dye at the core. Moreover, the fluorescence lifetimes and corresponding amplitudes of the reporter dyes have been found to be reliable parameters for assessing the pH of the medium, even though the variation in lifetimes of fluorescein is rather small. In case of protoporphyrin, however, this variation is significantly large. Besides, the change in amplitudes is prominent in acidic as well as alkaline solutions. The temporal parameters can thus be used to ascertain the pH of the medium, when used in conjunction with each other.

• Estimation of interfacial acidity of sodium dodecyl sulfate micelles

An enhancement in the excited state proton transfer (ESPT) processes of coumarin-102 (C-102) dye was observed upon addition of salicylic acid and hydrochloric acid in sodium dodecyl sulfate (SDS) micellar solution. The phenomenon was observed only in the micellar medium of anionic surfactant SDS and not in case of cationic (CTAB) or neutral (Trition X -100) surfactants. ESPT of C-102 was also observed in aqueous solutions but on addition of very high concentrations of hydrochloric acid. However, on comparing the ratio of the protonated species from the emission spectra in the presence and absence of SDS micelle, a conclusive estimation of the local proton concentration at the Stern layer of SDS micelles could be evaluated.

• Utilizing dendritic scaffold for feasible formation of naphthalene excimer

Peripheral functionalization of PAMAM dendrimers with naphthalene units leads to significant ground state aggregation in the system above the critical aggregation concentrations (CAC). Upon photoexcitation of the ground state aggregates, static type excimer formation of naphthalene moiety is observed. Significant red-shifted emission from naphthalene excimers is achieved through generating the static type excimers in polar solvents such as methanol and acetonitrile-water mixtures. Control experiments suggest that the presence of dendritic scaffold in the system play a pivotal role in generating intense static excimer emission in naphthalene modified PAMAM dendrimers, in solution phase at room temperature.

• Nucleobase assemblies supported by uranyl cation coordination and other non-covalent interactions

We describe synthesis and solid state structural description of uranyl complexes of carboxylate functionalized adenine and uracil derivatives. The metal coordination through carboxylate pendant leads to the formation of dimeric assemblies, whereas the directional nature of hydrogen bonding interaction supported by nucleobases and aqua ligands, result in the generation of complex 3-D architectures containing embedded nucleobase ribbons.

• Di(1𝐻-benzo[𝑑][1,2,3]triazol-1-yl)methane: An efficient ligand for copper and amine-free palladium-catalysed Sonogashira coupling reaction

An efficient Pd-catalysed Sonogashira coupling reaction was achieved in the absence of copper and amine with inorganic base using phosphene-free, air stable di(1𝐻-benzo[𝑑][1,2,3]triazol-1-yl)methane as ligand. The cross coupling of electron-rich, electron-defficient and hindered aryl halides with terminal alkynes afforded the internal alkynes in good to excellent yields.

• Stereoselective synthesis of 2,3-disubstituted dihydrobenzofuran using alkyne Prins type cyclization to vinylogous carbonates

An intramolecular, alkyne Prins type cyclization of vinylogous carbonates derived from 𝑜-alkynyl phenols is developed for the stereoselective construction of trans-2,3-disubstituted dihydrobenzofuran derivatives. Strong Lewis acids like TMSOTf catalyse this reaction efficiently. The presence of mildly electron donating groups on aryl rings increases the efficiency of the reaction.

• SN2-type ring opening of substituted-𝑁-tosylaziridines with zinc (II) halides: Control of racemization by quaternary ammonium salt

Quaternary ammonium salt mediated highly regioselective ring opening of aziridines with zinc(II) halides to racemic and non-racemic 𝛽-halo amines in excellent yield and selectivity is described. The reaction proceeds via an SN2-type pathway and the partial racemization of the starting substrate and the product was effectively controlled by using quaternary ammonium salts to afford the enantioenriched products (er up to 95:5).

• Comparative self-assembly studies and self-sorting of two structurally isomeric naphthalene-diimide (NDI)-gelators

We have reported here a comparative self-assembly and gelation studies of two isomeric bis-amide functionalized NDI-derivatives. In one case (NDI-1) the two amide groups were placed symmetrically on either side of the chromophore while for the other system (NDI-2) they were located on same side. In non-polar solvent both isomers formed self-assembled structures by synergistic effect of 𝜋-stacking and hydrogen-bonding. The propensity for self-assembly of NDI-1 was greater due to symmetrical placement of two amide groups on either arms of this chromophore which allowed 𝜋-stacking in tandem with hydrogen-bonding, while NDI-2 formed thermally more stable self-assembled fibres possibly due to location of two amide groups in close proximity along single arm of this chromophore. The structural difference in these two isomers lead to distinctly different morphology of their respective self-assembled structures which was further reflected on their gelation properties. Morphology of the self-assembled array of NDI-1 showed organized and regular entangled bundles of nanorods which imparted better gelation ability to this chromophore while the self-assembled fibres of NDI-2 showed less ordered and irregular fibres. We also probed self- assembly of these two chromophores in their mixture which revealed orthogonal assembly of the individual chromophores and no molecular mixing was noticed.

• Subject index

• Author index

• # Journal of Chemical Sciences

Volume 132, 2020
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• # Editorial Note on Continuous Article Publication

Posted on July 25, 2019