Birinchi K. Das

Structural, thermal and spectroscopic properties of supramolecular coordination solids

Molecules in metal isonicotinate tetrahydrates, M(NC5H4-p-CO2)2.4H2O with M = Mn, Fe, Co, Ni, Cu, Zn, consisting of the hexacoordinate complexestrans-[M(NC5H4-p-CO2)(OH2)4], participate in exhaustive hydrogen-bond formation among themselves to lead to a robust 3D supramolecular network in the solid state. Solid-state diffuse reflectance UV-Vis-NIR spectra of the complexes have been assigned to ligand field and charge transfer transitions. Sharp weight loss due to dehydration, as shown by TGA of Cu(NC5H4-p-CO2)2.4H2O, suggests the suitability of this complex for the gravimetric estimation of copper.

Catalytic Properties of Cobalt(III)−Oxo Cubanes in the TBHP Oxidation of Benzylic Alcohols

Two series of oxo-bridged Co(III) complexes of the type Co(4)O(4)(O(2)CC(6)H(4)-X)(4)(py)(4) (1) and Co(4)O(4)(O(2)CC(6)H(4)-X)(4)(4-Mepy)(4) (2), where X = H (a), Me (b), MeO (c), Cl (d), NO(2) (e), have been synthesized and characterized in detail. The molecular structures of the complexes consist of a cubelike Co(4)(mu(3)-O)(4) core having Co and O atoms at alternate vertices with carboxylato ligands bridging the Co(3+) ions along four face diagonals of the approximate cube. Nitrogen atoms from pyridyl ligands complete the distorted-octahedral coordination around each Co(III). These neutral Co(III) complexes undergo a nearly reversible one-electron oxidation involving the redox couple [Co(4)(III)(mu(3)-O)(4)](4+) <--> [Co(3)(III)Co(IV)(mu(3)-O)(4)](5+) at potentials (approximately 0.7-1.0 V) that linearly depend on the electronic influence of X. The cobalt(III) clusters of types 1 and 2 have been found to effectively promote the TBHP oxidation of benzylic alcohols under homogeneous conditions to produce the corresponding carbonyl compounds.

Investigation of the fate of poplar lignin during autohydrolysis pretreatment to understand the biomass recalcitrance

Lignocellulosic biomass is the most abundant renewable resource for the potential replacement of fossil fuels, though to fully realize this vision, an improved understanding of the chemical structures of its components within the biomass and after bioprocessing is critical. In this study, we investigated the fate of isolated poplar lignin during autohydrolysis pretreatment at different temperatures and subsequently analyzed the structural changes by gel permeation chromatography, 13C–1H HSQC and phosphorylation/31P NMR. Our results suggested that an increase in temperature and time of autohydrolysis of lignin resulted in an increase in phenolic hydroxyl groups coupled with a decrease in aliphatic hydroxyl groups. This may be attributed to the cleavage of β-O-4 linkages via acidolysis. Molecular weight determination revealed that lignin depolymerization predominates over condensation. Our results also highlight that the cleavage of lignin side-chain units is relatively fast in lignin autohydrolysis compared to the autohydrolysis of biomass. This study provides an enhanced understanding of the fundamental autohydrolysis pretreatment lignin chemistry and will facilitate improved methodology to reduce biomass recalcitrance.

Enhanced selectivity in green catalytic epoxidation using a supported cobalt complex

A supported reagent made by immobilising a tetrameric cobalt(III) cubane cluster on mesoporous silica catalyses the aerobic oxidation of α-pinene with enhanced epoxidation selectivity compared with other systems.

Solvothermal synthesis, molecular structures and spectroscopic characterization of the cluster compounds (Ph4P) 2[Fe4Te2(CO)14] and Cs[HFe3Te(CO)9]

Abstract The iron carbonyl tellurido cluster (Ph 4 P) 2 [Fe 4 Te 2 (CO) 14 ] ( 1 ) has been synthesized at 80°C in moderate yield by heating a mixture of Fe 3 (CO) 12 , Na 2 Te 2 , and Ph 4 PBr in a 2:1:6 molar ratio in a sealed Pyrex tube containing 0.4 ml of methanol. Another cluster, Cs[HFe 3 Te(CO) 9 ] ( 2 ) has been prepared similarly by allowing Fe 3 (CO) 12 to react with Cs 2 Te 3 (4:1 molar ratio) in 0.2 ml of H 2 O at 130°C. Compound 1 crystallizes in the monoclinic space group C 2/ c (No. 15) with a = 23.324(3) A, b = 11.793(2) A, c = 23.499 (5) A, β = 102.02(1)°, V = 6322(2) A 3 , Z = 4. A central ‘Fe 2 ( μ 3 -Te) 2 (CO) 6 ’ butterfly unit remains connected to two Fe(CO) 4 fragments through the tellurium atoms in the anion of 1 . Compound 2 crystallizes in the orthorhombic space group Pbca (No. 61) with a = 9.672(2)A, b = 15.816(4) A, c = 21.430(5) A, V = 3278(2) A 3 , Z = 8. In the anion of this compound, a Te 2− ligand caps a triiron triangle to form an approximate tetrahedron, with a hydride ligand bridging one of the FeFe bonds. Both compounds have been characterized by solid state and solution infrared spectroscopy and 125 Te NMR spectroscopy.

A novel immobilised cobalt(III) oxidation catalyst

A complex form of cobalt(III) has been successfully immobilised on a chemically modified silica and proven to be an active catalyst for the selective oxidation of alkylaromatics using air as the source of oxygen and in the absence of solvent.

Methanothermal synthesis of polynuclear ruthenium telluride carbonyl clusters

Abstract The tellurido ruthenium carbonyl cluster compounds (Ph 4 P) 2 [Ru 4 Te 2 (CO) 10 ] ( 1 ) and (Ph 2 P) 2 [Ru 5 Te(CO) 14 ] ( 2 ) have been prepared by methanothermal reactions at 80°C using a reaction mixture of Ru 3 (CO) 12 , Na 2 Te 2 and Ph 4 PBr in appropriate molar ratios. Both compounds have pseudo-octahedral cores formed by ruthenium and tellurium atoms.

Superheated solvent media for organometallic (poly)chalcogenide cluster synthesis

Abstract Solvothermal reactions in superheated aqueous and nonaqueous solvent media have led to many new metal-carbonyl chalcogenido clusters. This efficient synthetic technique, usually applied to synthesize solid state materials, is also surprisingly useful in preparing molecular clusters in crystalline form. Thus, many of the often laborious purification and separation procedures which are associated with conventional solution methods can be avoided. This method holds substantial promise for the exploratory synthesis of inorganic/organometallic compounds. In this short account we intend to focus on the contributions of the solventothermal technique to the organometallic polytelluride cluster chemistry.

Novel C–H⋯C contacts involving 3,5-dimethylpyrazole ligands in a tetracoordinate Co(II) complex

A violet-blue cobalt(II) complex [Co(4-nbz)(2)(DMP)(2)] (1), where 4-nbz = 4-nitrobenzoate and DMP = 3,5-dimethylpyrazole, has been prepared at room temperature. Crystallographic studies on 1·0.5H(2)O reveal that the molecules of 1 are linked by a variety of non-covalent bonds including a novel type of C-H···C contact forming, with assistance from N-H···O, C-H···O and C-H···π interactions, an intricate 3-D supramolecular network. Theoretical calculations suggest that the observed C-H···C interactions are energetically quite significant.

Novel supramolecular aggregation in [Ni(H2O)4(4-CNpy)2](BPh4)2·2(4-CNpy)·4H2O

The crystal structure of the compound [Ni(H2O)4(4-CNpy)2](BPh4)2·2(4-CNpy)·4H2O (1) containing a substituted pyridine ligand, 4-CNpy (4-cyanopyridine), has been studied. It has been prepared by a self-assembly reaction between nickel(II) acetate with 4-cyanopyridine carried out in the presence of tetraphenylborate anions. All four water molecules and the pyridyl-N bound 4-CNpy ligands of the dicationic trans-[Ni(H2O)4(4-CNpy)2]2+ complexes participate in hydrogen-bonding interactions in tandem with the 4-CNpy and H2O molecules present in the crystal lattice to form a supramolecular 3-D network with large pores partially occupied by the bulky BPh4− ions. The analogous Co(II) and Mn(II) species [Co(H2O)4(4-CNpy)2](BPh4)2·2(4-CNpy)·4H2O (2) and [Mn(H2O)4(4-CNpy)2](BPh4)2·2(4-CNpy)·4H2O (3) have also been made.