Rabindra Nath Mukherjee

Coordination chemistry with pyrazole-based chelating ligands: molecular structural aspects

Pyrazole-based chelating ligands form a variety of coordination complexes with a number of metal ions, providing varying coordination geometry and nuclearity. Recent years have seen considerable interest in the designing of various pyrazole-based ligands and to study their structural properties to serve specific stereochemical requirement of a particular metal-binding site. Using over 120 pyrazole-based chelating ligands, the stereochemical properties of over 110 discrete coordination complexes, studied by single-crystal X-ray crystallography were analysed. Various bonding modes for a given chelating ligand are involved, and are reviewed with reference to ligand structure and the resulting coordination complexes. It is shown that the metal coordination number of the resulting complexes varies from two to eight. The ligands are introduced systematically as a function of their denticity, making easy access to information on specific type of ligands and coordination complexes thereof. X-ray crystallographically-determined bond lengths of various donor atoms/groups are collected in a table, thus providing an accessible source for reference purposes. Source material for the review amounts to about 130 references.

Catecholase activity of dinuclear copper(II) complexes with variable endogenous and exogenous bridge

A group of a diverse family of dinuclear copper(II) complexes [Cu2(L1O)(μ-OH)][ClO4]2 (1), [Cu2(L2O)(μ-OH)][ClO4]2 (2), [Cu2(L3O)(μ-C3H3N2)(OClO3)(H2O)][ClO4]·H2O (3), [Cu2(L4)(μ-OH)2][ClO4]2·H2O (4), [Cu2(L5O)2(OClO3)2] (5) and [Cu2(bpy)2(μ-OH)(μ-OH2)(μ-O2CMe)][ClO4]2 (6) has been chosen to investigate their potential as catalysts for the aerial oxidation of 3,5-di-tert-butylcatechol (3,5-DTBC) to 3,5-di-tert-butylquinone (3,5-DTBQ) (catecholase activity) [L1OH, 2,6-[N-methyl-N-(2-pyridylethyl)aminomethyl]phenol; L2OH, 2,6-bis[N-(2-pyridylethyl)iminomethyl]phenol; L3OH, 1,3-bis[N-methyl-N-(2-pyridylethyl)amino]propan2-ol; L4, α,α′-bis[N-(6-methyl-2-pyridylmethyl)-N-(pyrazol-1-ylmethyl)amino]m-xylene; L5OH, 4-methyl-2,6-bis(pyrazol-1-ylmethy)phenol; bpy, 2,2′-bipyridine; C3H3N2, pyrazolate ion; H, dissociable hydrogen atom]. The synthesis of a new m-xylyl-based dinucleating ligand L4 is described. Using this ligand a new dihydroxy-bridged copper(II) complex 4 has been synthesised and characterised by elemental analysis and physicochemical measurements. The types of bridges present are endogenous phenoxo- and exogenous hydroxo- (1 and 2), endogenous alkoxo- and exogenous pyrazolate- (3), exogenous hydroxo- (4), endogenous diphenoxo-bridge (5) and exogenous hydroxo-, aqua- and acetate- (6). Barring complex 6, which is ferromagnetically coupled, all other complexes are antiferromagnetically coupled and all reported complexes are structurally characterised. The reaction between the dicopper(II) complexes and tetrachlorocatechol was investigated spectrophotometrically to investigate whether or not, during catalysis, binding of 3,5-DTBC is occurring to the copper centre(s). Kinetic experiments have been performed to determine initial rate of reactions and the activity order follows: 1>2≫3>4≫6 (for 5 initial rate could not be determined). For a fixed 3,5-DTBC concentration, a linear relationship for the initial rates and the complex concentration is obtained, suggestive of a first-order dependence on the catalyst concentration. Analysis of Michaelis–Menten kinetic treatment has been done for complexes 1, 2, 3 and 4. Complex 1 is the most active catalyst to show catecholase activity.

One-dimensional Co(II) and Cu(II) coordination polymers and a discrete Cu(II)4 complex of carboxylate-appended (2-pyridyl)alkylamine ligands: spin-canting and anti-/ferromagnetic coupling.

Structural characterization of the newly synthesized complexes [M(I)I(L(1)OO)(xH2O)][ClO4] x 2 H2O [M = Co, x = 1 (1); M = Cu, x = 0 (2); L(1)OO- = 3-[(2-(pyridin-2-yl)ethyl){2-(pyridin-2-yl)methyl}amino]propionate] reveals that 1 and 2 are 1D chainlike coordination polymers. A tridentate variety of this ligand afforded a discrete tetranuclear complex {[Cu(I)I(L(2)OO)(OClO3)]}4 x MeCN (3) [L(2)OO- = 3-[N-methyl-{2-(pyridine-2-yl)ethyl}amino]propionate]. Analysis of the crystal packing diagrams reveals extensive pi-pi stacking in 1 and C-H...O hydrogen bonding interactions in 3, leading to the formation of network structures. For these complexes, absorption spectral properties have been investigated. All three complexes exhibit exchange interaction between the M(II) ions through a syn-anti bridging carboxylate pathway. Magnetic studies on 1 show spontaneous magnetization below 5 K, which corresponds to the presence of spin-canted antiferromagnetism. At T = 2 K, the values of coercive field (H(c)) and remnant magnetization (M(r)) are 200 G and 0.019 muB, respectively. Analysis of the magnetic data through spin Hamiltonians in the form HH = sigma(i<j)(n) -J(ij)S(i)S(j) (J is positive for a ferromagnetic interaction and negative for an antiferromagnetic interaction) leads to the following set of best-fit parameters: J = -2.65(2), -0.66(1), and +12.2(2) cm(-1) for 1, 2, and 3, respectively. An attempt has been made to rationalize the observed magnetic behavior.

Homochiral 1D-helical metal–organic frameworks from achiral components. Formation of chiral channel via C–H⋯Cl interaction

Using an achiral ligand α,α′-bis(pyrazolyl)-m-xylene (L6) a coordination polymer [(L6)ZnIICl2]∞1 (1a and 1b are two enantiomorphs) has been prepared and authenticated from their crystal structure determination. Structural analysis revealed that each Zn(II) centre is coordinated by two chloride ions and two pyrazole nitrogens from two different L6 ligands, giving pseudo-tetrahedral coordination environment around a metal ion. For these structures 1-D homochiral helical polymeric chain running along a crystallographic 21 axis in the b-direction is observed. In continuation of our recent activity on inorganic crystal engineering from the standpoint of metal–ligand coordination chemistry and C–H⋯Cl2MII hydrogen bonding, in this work we have investigated the potential of coordination modules present in 1a and 1b in bringing about generality of homochiral helix formation, strengthened by C–H⋯Cl interactions. Notably, in the case of 1 both right-(P) (1a) and left-(M) (1b) handed helices from two different crystals from same batch were identified. This work demonstrates a case of conglomeration (racemic mixture of chiral crystals). Helical channels are formed via noncovalent C–H⋯Cl hydrogen-bonding supported by π–π interactions.

Modeling Tyrosinase Monooxygenase Activity. Spectroscopic and Magnetic Investigations of Products Due to Reactions between Copper(I) Complexes of Xylyl-Based Dinucleating Ligands and Dioxygen: Aromatic Ring Hydroxylation and Irreversible Oxidation Products.

A full account of a chemical system possessing features that mimic the reactivity aspects of tyrosinase is presented. Using dinucleating ligands with a m-xylyl spacer three new dicopper(I) complexes have been synthesized and their reactivity with dioxygen investigated. The six-membered chelate ring forming ligands provide only two nitrogen coordinations to each copper. The complexes [CuI2L(CH3CN)2]X2 (X = ClO4- (1a), SbF6- (1b)) and [CuI2(L-NO2)(CH3CN)2][SbF6]2 (1c) [L = α,α‘-bis[N-methyl-N-(2-pyridylethyl)amino]-m-xylene; L-NO2 = para-nitro derivative of L] have been characterized by IR and 1H NMR spectroscopy. The reaction of O2 with 1a−c in CH2Cl2 or THF is instantaneous and causes stoichiometric xylyl hydroxylation reactions producing phenol products. Thus 1a produces phenoxo-/hydroxo-bridged product [CuII2(L-O)(OH)][ClO4]2 (2a). The existence of putative peroxo−dicopper(II) species could not be detected even at −80 °C. A trend is observed for the extent of aromatic ring hydroxylation (298 K):  CH3CN ...

Designing neutral coordination networks using inorganic supramolecular synthons: Combination of coordination chemistry and C–H⋯Cl hydrogen bonding

Crystal structure determination of two compounds, [(L3)MnIICl2(EtOH)] (1) and [(L6)FeIIICl3] (2) [L3 = 2-[3-(2′-pyridyl)pyrazol-1-ylmethyl]pyridine; L6 = methyl[2-(2-pyridyl)ethyl](2-pyridylmethyl)amine], the former one reported for the first time, demonstrates that these coordination units can be used effectively as supramolecular synthons in crystal engineering to construct neutral networks. In continuation of our recent contribution on inorganic crystal engineering from the standpoint of metal–ligand coordination chemistry and C–H⋯Cl2MII hydrogen-bonding, in this work we have investigated the potential of coordination modules present in 1 and 2 in bringing about generality and diversity of C–H⋯Cl hydrogen bonding interactions. Special attention has been directed to the influence of additional (i) coordinated solvent molecule, which can act as hydrogen bond donor/acceptor and (ii) chloride ion in the assembly of supramolecular architectures via noncovalent interactions. Inorganic supramolecular synthons 1 and 2 give rise to molecular staircase and zig-zag network, respectively. Re-examination of previously reported compound [(L6)ZnCl2] with extended limit of hydrogen bonding interaction reveals additional helical network.

Synthesis and Properties of Diphenoxo-Bridged CoII, NiII, CuII, and ZnII Complexes of a New Tripodal Ligand: Generation and Properties of MII-Coordinated Phenoxyl Radical Species

Four dinuclear complexes of composition [MII2(L)2].xS [M=Co, x=0.5, S=1,4-dioxane (1.0.5 1,4-dioxane); Ni, x=0 (2) [single crystals have x=2, S=diethyl ether (2.2 diethyl ether)]; Cu, x=0 (3); Zn, x=0.5, S=1,4-dioxane (4.0.5 1,4-dioxane)] have been synthesized using a new tripodal ligand [2,4-di tert-butyl-6-{[(2-pyridyl)ethyl](2-hydroxybenzyl)-aminomethyl}-phenol (H2L)], in its deprotonated form, providing a N 2O 2 donor set. Crystallographic analyses reveal that the complexes have a similar diphenoxo-bridged structure. Each metal ion is terminally coordinated by 2,4-di tert-butyl-phenolate oxygen, a tertiary amine, and a pyridyl nitrogen. From each ligand, unsubstituted phenolate oxygen provides bridging coordination. Thus, each metal center assumes M (II)N 2O 3 coordination. Whereas the geometry around the metal ion in 1.0.5 1,4-dioxane, 2.2 diethyl ether and, 4.0.5 1,4-dioxane is distorted trigonal-bipyramidal, in 3 each copper(II) center is in a square-pyramidal environment. Temperature-dependent magnetic behavior has been investigated to reveal intramolecular antiferromagnetic exchange coupling for these compounds (-J=6.1, 28.6, and 359 cm(-1) for 1.0.5 1,4-dioxane, 2, and 3, respectively). Spectroscopic properties of the complexes have also been investigated. When examined by cyclic voltammetry (CV), all four complexes undergo in CH2Cl2 two reversible ligand-based (2,4-di tert-butylphenolate unit) one-electron oxidations [E1/2(1)=0.50-0.58 and E1/2(2)=0.63-0.75 V vs SCE (saturated calomel electrode)]. The chemically/coulometrically generated two-electron oxidized form of 3 rearranges to a monomeric species with instantaneous abstraction of the hydrogen atom, and for 4.0.5 1,4-dioxane the dimeric unit remains intact, exhibiting an EPR spectrum characteristic of the presence of ZnII-coordinated phenoxyl radical (UV-vis and EPR spectroscopy). To suggest the site of oxidation (metal or ligand-centered), in each case DFT calculations have been performed at the B3LYP level of theory.

Syntheses, X-ray Structures, and Physicochemical Properties of Phenoxo-Bridged Dinuclear Nickel(II) Complexes: Kinetics of Transesterification of 2-Hydroxypropyl-p-nitrophenylphosphate

Four dinuclear nickel(II) complexes [Ni(II)(2)(L(1))(O(2)CMe)(2)(H(2)O)(2)][PF(6)].MeOH.3H(2)O (1), [Ni(II)(2)(L(1))(O(2)CMe)(2)(NCS)] (2), [Ni(II)(2)(L(2))(O(2)CMe)(2)(MeOH)(H(2)O)][ClO(4)] (3), and [Ni(II)(2)(L(2))(O(2)CMe)(2)(MeOH)(H(2)O)][BPh(4)].3MeOH.H(2)O (4) have been synthesized [HL(1): 2,6-bis[N-methyl-N-(2-pyridylethyl)amino]-4-methylphenol; HL(2): 2,6-bis[3-(pyridin-2-yl)pyrazol-1-ylmethyl]-4-methylphenol]. Complexes 1, 3, and 4 are new while complex 2 was reported previously by Fenton and co-workers (the structure of 2 was presented but no physicochemical properties of this complex were reported; in this work such studies have been completed). X-ray crystallographic analyses of 1 and 4 reveal that each nickel(II) center is six-coordinate, terminally coordinated by two nitrogen donors [(pyridin-2-yl)ethylamine unit in 1 and 3-(pyridin-2-yl)pyrazole moiety in 4], and bridged by an endogenous phenolate ion. Each of the acetate ions in 1 adopts a eta(2)-coordination mode (chelating) whereas in 4 each is coordinated in a mu-eta(1):eta(1) syn-syn bridging mode. In 1 each Ni(II) center has water coordination whereas in 4 one Ni(II) center has a methanol and the other has water coordination. The X-ray structure of 3 could not be determined. The physicochemical properties (electronic spectroscopy and cyclic voltammetry) of the cation of 3 are identical to that of 4. Magnetic susceptibility measurements have revealed the occurrence of ferromagnetic coupling of spins of the nickel(II) centers in 2 [J = +9.80 cm(-1)]. The nickel(II) centers in 1 and 3 are antiferromagnetically coupled, but to different extents [J = -48.4 cm(-1) (1); J = -1.24 cm(-1) (3)]. The magnetic properties are correlated with the nature of bridges between the nickel(II) ions. The two coordinated aqua ligands in 1 and the aqua and methanol ligands in 3 have enabled these dinuclear nickel(II) complexes to function as catalysts in the hydrolysis of 2-hydroxypropyl-p-nitrophenylphosphate (HPNP). Complex 1 is more effective in the conversion of substrate to product (p-nitrophenolate ion) than 3, under identical experimental conditions. Pseudo first-order kinetic treatment has been done for complexes 1 and 3. Temperature-dependent measurements were done to evaluate kinetic/thermodynamic parameters for the hydrolysis/transesterification reaction of HPNP and to propose a mechanistic pathway. The activation parameters are DeltaH(++) = 64 kJ mol(-1), DeltaS(++) = -104 J mol(-1) K(-1) for 1 and DeltaH(++) = 68 kJ mol(-1), DeltaS(++) = -109 J mol(-1) K(-1) for 3. A mechanism consistent with the kinetic data is presented.

Catalytic oxidation of hindered phenols by a copper(I) complex and dioxygen

The dioxygen reaction product of a binuclear copper(I) complex of a new m-xylyl-based ligand has proven to be a catalyst in the promotion of oxidative carboncarbon coupling of hindered phenols, which leads to bisphenol and diphenoquinones.

Magneto–structural studies of monohydroxo-bridged dicopper(II) complexes M[Cu2L2(OH)]·2H2O (M=Na+ (1) and K+ (2); H2L=2,6-bis[N-(phenyl)carbamoyl]pyridine). Effect of CuOHCu bridge angle on antiferromagnetic coupling

Abstract Using a tridentate bis-amide ligand 2,6-bis[N-(phenyl)carbamoyl]pyridine (H2L), in its deprotonated form, two new monohydroxo-bridged dicopper(II) complexes M[Cu2L2(OH)]·2H2O (M=Na+ (1) and K+ (2)) have been prepared and characterised by a number of methods, including X-ray crystallography. Each copper(II) ion is terminally coordinated by one pyridyl and two amide nitrogen donors. The two copper(II) centres are bridged by a hydroxo group, with each copper(II) centre assuming a distorted square planar geometry. The observation of short CuNpy and long CuNamide bonds is caused by the steric requirement of the ligand. Interestingly, each cation Na+/K+ is coordinated to four different [Cu2L2(OH)]− units through the amide O-donors, in an uncommon distorted tetrahedral coordination environment. Temperature-dependent magnetic susceptibility measurements revealed that the compounds have S=0 ground state with singlet–triplet energy separation, 2J=−334 and −296 cm−1 for 1 and 2, respectively. The larger CuOHCu bridge angle in 1 (131.1(6)°) causes better antiferromagnetic exchange coupling than that in 2 (125.7(6)°).