• Volume 127, Issue 2

February 2015,   pages  176a-352

• Editorial Board

• Notes on the preparation of Papers

• Foreword

• Structural variations in aromatic 2𝜋-electron three-membered rings of the main group elements

Structural variations of different 2𝜋-aromatic three-membered ring systems of main group elements, especially group 14 and 13 elements as compared to the classical description of cyclopropenyl cation has been reviewed in this article. The structures of heavier analogues as well as group 13 analogues of cyclopropenyl cation showed an emergence of dramatic structural patterns which do not conform to the generalnorms of carbon chemistry. Isolobal analogies between the main group fragments have been efficiently used to explain the peculiarities observed in these three-membered ring systems.

• Cyclodiphosphazanes as synthetic probes: P-C/P-N bond formation from the reaction with functionalized propargyl alcohols and 𝑁-hydroxy substrates

Phosphano-indoles were synthesized in a fairly straightforward route from the reaction of simple cyclodiphosphazanes [XP(𝜇-N-t-Bu)2PY] [X=Y=NH-$t$-Bu (1a); X=Y=NH-i-Pr (1b)] with o-aminophenyl functionalized propargyl alcohols. The reaction occurs via an allene intermediate formed by PIII-O-C$\to$PV(O)-C rearrangement, followed by cyclization utilizing the central allenic carbon and the –NH2 functionality. In a similar way, cyclodiphosphazanes [XP(𝜇-N-t-Bu)2PY] [X=Y=Cl (1c); X=Cl, Y=NH-t-Bu(1d)] have been treated with N-hydroxy substrates to obtain novel PIII-O-N$\to$PV(O)-N rearranged products.X-ray structures of the four products, 2-(1-phenyl-ethyl)-3-[(t-Bu)NH)P(𝜇-N-t-Bu)2P(O)]-indole [14], cis-{[-C(=O)-C6H4-C(=O)-]-N-P(=O)-N-t-Bu}2[cis-18], trans-{[-C(=O)-C6H4-C(=O)-]-N-P(=O)-N-t-Bu}2 [trans-18] and cis-[(t-BuNH)P(𝜇-N-t−Bu)2P(=O)-N{-C(=O)-CH2-CH2-C(=O)-}] [cis-19] are also reported

• Metal — metal multiple bonded intermediates in catalysis

Metal–metal bonded Rh2 and Ru2 complexes having a paddlewheel-type structure are exceptional catalysts for a broad range of organic transformations. I review here the recent efforts towards the observation and characterization of intermediates in these reactions that have previously eluded detection. Specifically, mechanistic investigations of carbenoid and nitrenoid reactions of Rh2(II,II)-tetracarboxylate compounds have led to the observation of a metastable Rh2(II,II) carbene complex as well as a mixed-valent Rh2(II,III)-amido intermediate. Related Ru2 nitrido compounds have been studied and found to undergo intramolecular C–H amination reactions as well as intermolecular reaction with triphenylphosphine

• Studies on cluster, salt and molecular complex of zinc-quinolinate

Reactions of zinc halides with 8-hydroxyquinoline (hydroxQ) in equimolar ratio were carried out in different solvents. Respective solvates of tetranuclear clusters, namely [Zn4(oxyQ)6X2].(solvent)2, (when X=Cl, Solvent=dimethylformamide (1), dimethylacetamide (2) and dimethysulphoxide (3); X = Br, solvent = dimethylformamide (4), oxyQ=quinolinate anion) were obtained. Bond parameters of these isostructural clusteres 1–4 are compared from their single crystal structures. Anhydrous form of the cluster have porous packing and is thermally stable below 250° C. Surface area of the clusters 1 and 4 are 8.933 and 6.172 m2/g, respectively. Complexes 1 and 4 can be reversibly hydrated, which is reflected in colour changes. The reaction of zinc chloride with 8-hydroxyquinoline in equimolar ratio followed by crystallization from water gave salt (HhydroxQ)2 [ZnCl4] (5) and a similar reaction followed by crystallization from 3-methylpyridine (3mepy) resulted in the molecular complex [Zn(oxyQ)2(3mepy)]. [Zn(oxyQ)2(3mepy)2].3H2O (6). Complex 5 is formed from a hydrolytic equilibrium of water with zinc chloride yielding tetrachloro zinc anion and zinc hydroxide. Taking advantage of this reaction, a composite material of ZnO@complex 5 exhibiting dual fluorescence at 450 and 575 nm on excitation at 390 nm was prepared. Fluorescence emission properties of all the complexes in solid state are compared with fluorescence emission of the ligand

• Interesting cationic (Li+/Fe3+/Te6+) variations in new rocksalt ordered structures

A new series of layered oxides, Li3(Li1.5xFe3−(x+1.5x)Tex)O6, (0.1 ≤ 𝑥 ≤ 1.0) possessing rock-salt superstructures crystallizing in monoclinic (S.G. C2/m) symmetry is reported here. Investigations based on single crystal and powder X-ray diffraction studies for the 𝑥 = 1 member, Li3(Li1.5Fe0.5Te)O6, (a=5.1834(1); b=8.8858(2); c=5.16840(8) Å; 𝛽=110.660(1)°) confirmed the stabilization of (Li1.5Fe0.5Te1.0O6)3− honeycomb arrays with a very high amount of lithium ions. The structure for the x = 0.5 member (Li3.75Fe1.75Te0.5O6) has also been confirmed by the powder X-ray diffraction Rietveld refinements. Li3(Li1.5Fe0.5Te)O6 and Li3(Li0.75Fe1.75Te0.5)O6 oxides exhibited Curie–Weiss behaviour in the temperature range of 50–300 K with negative 𝜃 values. Their respective ionic conductivities were found to be 6.76 × 10−5 S cm−1 and 2.21 × 10−6 S cm−1 at 573 K. The UV-visible diffuse reflectance measurements for the differentmembers of the series Li3(Li1.5xFe3−(x+1.5x)Tex)O6, (0.1 ≤ 𝑥 ≤ 1.0) show the expected shifts in their absorptionedges based on the increasing amount of Fe3+ions starting from 𝑥 = 1.0 member to 𝑥 = 0.1 member

• Effect of 𝛽−𝛽′ fusion on metal ion complexation of porphycene

Complexation of 𝛽−𝛽′ fused 𝜋-extended porphycene, namely dinaphthoporphycene was carried out successfully with copper(II) and its solid state structure shows a square-type planar N4-coordination core. The photophysical and electrochemical properties of this complex, along with the nickel(II) complex were also investigated. Further, electronic paramagnetic resonance (EPR) analysis of this complex is also reported.

• Frustrated Lewis pairs: Design and reactivity

The interaction of a Lewis acid with a Lewis base results in the formation of a Lewis acid–base adduct. Understanding Lewis acids and bases is central to conceptualizing chemical interactions and constitutes a major portion of metal–ligand chemistry. Sterically encumbered/constrained Lewis pairs cannot form acid–base adducts, but such ‘Frustrated Lewis Pairs’ (FLPs), with their unquenched electronic demands can be elegantly used to simultaneously react with a third species, resulting in unusual reactivity of small molecules. Such unusual reactions, explored only in the last few years, have found several applications, e.g., heterolytic splitting of H2, activation of small molecules (CO2, N2O, etc.). FLPs have opened new opportunities in synthetic chemistry, covering organic, main group as well as transition metal chemistry. The design strategies adopted for FLP systems and their unique reactivity are discussed here.

• Exploration of the structural features and magnetic behaviour in a novel 3-dimensional interpenetrating Co(II)-based framework

A new Co(II)-based three-dimensional (3D) framework having the molecular formula [Co(C4O4)(4-bpmh))H2O)2]n·2nH2O·2nMeOH·(1)(4-bpmh = N, N-bis-pyridin-4-ylmethylene-hydrazine) has been synthesized using a mixed ligand system and characterized by elemental analysis, IR spectroscopy, thermogravimetric analysis, single crystal X-ray diffraction and variable temperature magnetic study. The framework is constructed by the bridging squarate (C4O2−4) and N, N-bis-pyridin-4-ylmethylene-hydrazine (4-bpmh) ligands and interpenetration of the 2D grid-like frameworks at definite angles gives rise to 2D$\to$3D inclined polycatenation with sql/Shubnikov tetragonal plane net topology. Extensive non-covalent interactions (H-bonding as well as 𝜋$\cdots$𝜋 interactions) are also observed which stabilises the 3D arrangement. Additionally, complex 1 contains 1D channels of large dimensions (10.91 × 11.78 Å2) that runs along the b-axis. Variable temperature DC magnetic susceptibility study reveals dominant spin–orbit coupling effect typical of the 4T1g ground state of octahedral high-spin Co(II) ion at a higher temperature range.

• Syntheses and structures of dimeric sodium and potassium complexes of 2,6-diisopropyl-anilidophosphine borane ligand

We report here the syntheses and structural studies of dimeric sodium and potassium complexes of composition [Na(THF)2{Ph2P(BH3)N(2,6-iPr2C6H6)}]2 (2) and [K(THF)2{Ph2P(BH3)N(2,6-iPr2C6H6)}]2(3). The sodium complex 2 was readily prepared by the reaction of sodium bis(trimethylsilyl)amide with 2,6-diisopropylanilidophosphine-borane ligand [2,6-iPr2C6H3NHP(BH3)Ph2] (1-H) at ambient temperature. The potassium complex 3 was prepared by two synthetic routes: in the first method, the ligand 1-H was made to react with potassium hydride at room temperature to afford the corresponding potassium complex. The potassium bis(trimethylsilyl)amides were made to react with protic ligand 1-H in the second method to eliminate the volatile bis(trimethyl)silyl amine. Solid-state structures of both the new complexes were established by single crystal X-ray diffraction analysis. In the molecular structures of complexes 2, the sodium metal is coordinated by the anilido nitrogen (𝜂1) and borane group (𝜂1) attached to the phosphorus atom of ligand 1. In contrast, for compound 2, ligand 1 displays 𝜂6𝜋-arene interaction from 2,6-diisopopylphenyl ring with potassium atom along with 𝜂3 interaction of BH3 group due to larger ionic radius of potassium ion.

• Role of peripheral phenanthroline groups in the self-assembly of self-assembled molecular triangles

Self-assembled molecular triangles [Pd3(phen)3(imidazolate)3](NO3)3, 1a and [Pd3(phen)3 (imidazolate)3](PF6)3, 1b are prepared by the combination of imidazole with Pd(phen)(NO3)2 and Pd(phen) (PF6)2, respectively. Imidazole was deprotonated during the complexation reactions and the imidazolate so formed acted as a bis-monodentate bridging ligand to form the bowl-shaped trinuclear architectures of 1a/b. Relative orientation of the imidazolate moieties can be best described as syn,anti,anti as observed in the crystal structure of 1b. However, in solution state, slow conformational changes are assumed on the basis of 1HNMR spectral data. The molecular triangles are crafted with three peripheral phen units capable of 𝜋−𝜋 stacking interactions. Well-fashioned intermolecular 𝜋−𝜋 interactions are observed in the solid-state, wherein further self-assembly of already self-assembled triangle is observed.

• Role of ligands in controlling the regioselectivity in ruthenium-catalysed addition of carboxylic acids to terminal alkynes: A DFT study

Density functional studies are performed to understand the role of chelating bi-phosphine ligands [(Ph2P(CH2)mPPh2); m=1–4] in modulating the regio-selectivity of benzoic acid addition to 1-hexyne, in presence of ruthenium(II) catalyst [(Ph2P(CH2)mPPh2)Ru(methallyl)2]. The Markovnikov addition to 1-hexyne is observed when catalyst 1a [(Ph2P(CH2)PPh2)Ru(methallyl)2] is employed, whereas a reverse regio-selectivity is witnessed in presence of 1d [(Ph2P(CH2)4PPh2)Ru(methallyl)2]. Anti-Markovnikov addition occurs via the neutral vinylidene intermediates (5a/d) formed after 1,2-hydrogen shift in hexyne coordinated ruthenium(II) complexes 3a/d. The energy profile shows clear preference for Markovnikov addition by 15.0 kcal/mol (𝛥$G^S$L) in case of catalyst system 1a. In contrast, anti-Markovnikov pathway following neutral vinylidenes are more favourable by 9.1 kcal/mol (𝛥$G^S$L) for catalyst system 1d. The Z-enol ester formation is more predominantin the anti-Markovnikov pathway since the activation barrier for this step requires less energy (5.9 kcal/mol, 𝛥$G^S$L) than the one furnishing the E-product. The calculated results are in good agreement with the reported experimental findings.

• Modeling the active site of [FeFe]-hydrogenase: Electro-catalytic hydrogen evolution from acetic acid catalysed by [Fe2(𝜇-L)(CO)6] and [Fe2(𝜇-L)(CO)5(PPh3)] (L=pyrazine-2, 3-dithiolate, quinoxaline-2, 3-dithiolate and pyrido[2,3-b] pyrazine-2, 3-dithiolate)

Compounds [Fe2{𝜇-pydt}(CO)6] (pydt = pyrazine-2,3-dithiolate) (1), [Fe2{𝜇-qdt}(CO)6] (qdt = quinoxaline-2,3-dithiolate) (2), [Fe2{𝜇-ppdt}CO)6] (ppdt = pyrido[2,3-b]pyrazine-2,3-dithiolate) (3), [Fe2{𝜇-pydt}(CO)5PPh3] (4), [Fe2{𝜇-qdt}(CO)5PPh3] (5) and [Fe2{𝜇-ppdt}(CO)5PPh3] (6) have been synthesized in order to model the active sites of `[FeFe]-hydrogenase’. Compounds 1–6 have been characterized by routine spectral studies and unambiguously by single crystal X-ray crystallography. Supramolecular chemistry of compounds 1–6 have been described in terms of intermolecular interactions, observed in their respective crystal structures. Electro-catalytic hydrogen evaluation studies (from acetic acid) have been performed using compounds 1–6 as electro-catalysts. The mechanistic aspects of relevant electro–catalytic proton reductions have been discussed in detail.

• Functionalized silica materials for electrocatalysis

Electrocatalysis is an important phenomenon which is utilized in metal–air batteries, fuel cells, electrochemical sensors, etc. To increase the efficiency of the electrocatalytic process and to increase the electrochemical accessibility of the immobilized electrocatalysts, functionalized and non-functionalized mesoporous organo-silica (MCM41-type-materials) are used in this study. These materials possess several suitable properties to be durable catalysts and/or catalyst supports. Owing to the uniform dispersion of electrocatalysts (metal complex and/or metal nanoparticles (NPs)) on the functionalized and non-functionalized silica, an enormous increase in the redox current is observed. Long range channels of silica materials with pore diameter of 15–100 Å allowed metal NPs to accommodate in a specified manner in addition to other catalysts. The usefulness of MCM-41-type silica in increasing the efficiency of electrocatalysisis demonstrated by selecting oxygen, carbon dioxide and nitrite reduction reactions as examples

• Bulky iminophosphonamines for N–P–N coordination: Synthesis and structural characterization of lithium iminophosphonamides and homoleptic bis-chelates of Co(II), Ni(II) and Cu(II)

Two new sterically demanding iminophosphonamine ligands (2,4,6-Me3C6H2NH)P(Ph2)=N (C6H2-2,4,6-Me3) (1) and (2,6-iPr2C2H3NH)P(Ph2)=N(C6H2-2,4,6-Me3) (2) and their lithium derivatives as tmeda adducts (Li·tmeda)[(2,4,6-Me3C6H2)NP(Ph2)=N(C6H2-2,4,6-Me3)] (3) and (Li·tmeda)[(2,6-iPr2C2H3)NP(Ph2)=N(C6H2-2,4,6-Me3)] (4), respectively are reported here. Compounds 1–4 have been investigated by 1H, 13C and 13P{1H} NMR spectroscopy. The 7Li NMR for complexes 3 and 4 has also been reported. Utility of the ligands and their lithium derivatives have been shown in the synthesis of bis-homoleptic metal complexes M[Ph2P(NC6H2-2,4,6-Me3)2]2 (M = Co (5), Cu (6) and Ni (7). Metal-bis-silylamide generated in situ was reacted with the ligand (for 5 and 6) or the lithium derivative of the ligand was reacted with the metal chloride (for 7). Molecular structure of compounds 1–7 has been elucidated by single crystal X-ray diffraction analyses. The complexes are formed in good yields and are highly lipophilic in a wide range of solvents.

• Iron(III) and copper(II) complexes of trans-bis(ferrocenyl)porphyrin: Effect of metal ions on long-range electronic communication

A series of complex with a general formula of M(Fc2Ph2P) [Fc2Ph2P = 5, 10-bisferrocenyl-15,20-bisphenylporphyrin (2−); M = Fe(III)Cl Fe(III)(ClO4) and Cu(II)] have been synthesized and characterized. The single crystal X-ray structure of CuII(Fc2Ph2P) has been reported in which two ferrocene moieties are in anti form with respect to each other. The ferrocenyl groups of CuII(Fc2Ph2P) are more easily oxidized via a single two-electron quasi-reversible process compared to the free base ligand in which two 1e-oxidative response separated by 0.23 V are observed. Electrochemical study of FeIII(Fc2Ph2P)Cl revealed ferrocenebased two-electron quasi-reversible oxidation at 0.72 V indicating no observable coupling of the ferrocene moieties. The higher oxidation state of Fe(III) reduces the electron releasing tendency of the porphyrin ring and thus make the ferrocene oxidation difficult. The porphyrin, however, lack substituents at the 𝛽-pyrrolic positions, and the ferrocenyl moieties are therefore free to rotate. The observed electrochemical analyses thus demonstrate that the oxidation of the ferrocene subunit is strongly affected by porphyrin ring as well as the central metal through extended 𝜋-conjugation.

• Fluorescent naphthalene-based benzene tripod for selective recognition of fluoride in physiological condition

Aluminium complex of a naphthalene-based benzene tripod ligand system has been reported for the selective recognition of fluoride in aqueous medium in physiological condition. The ligand can selectively recognize Al3+ through enhancement in the fluorescence intensity and this in situ formed aluminium complex recognizes fluoride through quenching of fluorescence. The receptor system detects fluoride in nanomolar range. The sensing property was extended for practical utility to sense fluoride in tap water, pond water and river water.

• Structures, bonding and reactivity of iron and manganese high-valent metal-oxo complexes: A computational investigation

Iron and manganese ions with terminal oxo and hydroxo ligands are discovered as key intermediates in several synthetic and biochemical catalytic cycles. Since many of these species possess vigorous catalytic abilities, they are extremely transient in nature and experiments which probe the structure and bonding on such elusive species are still rare. We present here comprehensive computational studies on eight iron and manganese oxo and hydroxo (FeIII/IV/V-O, FeIII-OH and MnIII/IV/V-O, MnIII-OH) species using dispersion corrected (B3LYP-D2) density functional method. By computing all the possible spin states for these eight species, we set out to determine the ground state S value of these species; and later on employing MO analysis, we have analysed the bonding aspects which contribute to the high reactivity of these species. Direct structural comparison to iron and manganese-oxo species are made and the observed similarity and differences among them are attributed to the intricate metal–oxygen bonding. By thoroughly probing the bonding in all these species, their reactivity towards common chemical reactions such as C–H activation and oxygen atom transfer are discussed.

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