Synthesis, Structure, and Reactivity of Dinuclear Nickel Amino-Thiophenolate Complexes Bearing Bridging VO₂(OH)₂^– and VO₂(OR)₂^– Coligands

A series of novel mixed ligand dinickel complexes of the type [Ni^II₂L­(μ-L′)]⁺, where L′ is a tetrahedral oxo-alkoxo vanadate (L′ = [O₂V^V(OR)₂]⁻, R = H or alkyl) and L a macrocyclic N₆S₂ supporting ligand, have been prepared, and their esterification reactivity has been studied. The orthovanadate complex [Ni₂L­(μ-O₂V­(OH)₂)]⁺ (<b>2</b>), prepared by reaction between [Ni₂L­(μ-Cl)]­ClO₄ with Na₃VO₄ and a phase transfer reagent in CH₃CN, reacts smoothly with MeOH and EtOH forming the vanadate diesters [Ni₂L­(μ-O₂V­(OMe)₂)]⁺ (<b>3</b>) and [Ni₂L­(μ-O₂V­(OEt)₂)]⁺ (<b>4</b>). The dialkyl orthovanadate esters in <b>3</b> and <b>4</b> are readily transesterified with mono- and difunctional alcohols. Complex <b>3</b> can also be generated from <b>4</b> by transesterification with MeOH. Complexes <b>3</b> and <b>4</b> react with diols (ethylene glycol, propylene glycol and diethylene glycol) as well to afford the complexes [Ni₂L­(μ-O₂V­(OH)­(OCH₂CH₂OH))]⁺ (<b>5</b>), [Ni₂L­(μ-O₂V­(OCH₂)₂CH₂)]⁺ (<b>6</b>), and [Ni₂L­(μ-O₂V­(OCH₂CH₂)₂O)] (<b>7</b>). The crystal structures of the tetraphenylborate salts of complexes <b>3</b>–<b>7</b> reveal in each case four-coordinate O₂V^V(OR)₂^– groups bonded in a μ_1,3-bridging mode to generate trinuclear complexes with a central N₃Ni­(μ-S)₂(μ_1,3-O₂V­(OR)₂)­NiN₃ core. The stabilization of the four-coordinate V^VO₂(OR)₂^– moieties is a consequence of both the two-point coordinative fixation to and the steric protection of the bowl-shape binding pocket of the [Ni₂L]²⁺ fragment. Cyclic voltammetry experiments reveal that the encapsulated vanadate esters are not reduced in a potential window of −2.0 to +2.5 V vs SCE. The spins of the nickel­(II) (<i>S</i>_i = 1 ions) in <b>3</b> are weakly ferromagnetically coupled (<i>J</i> = +23 cm^–1, (<b>H</b> = −2<i>J</i><b>S</b>₁<b>S</b>₂)) to produce an <i>S</i> = 2 ground state

Synthesis, Structure, and Reactivity of Dinuclear Nickel Amino-Thiophenolate Complexes Bearing Bridging VO₂(OH)₂^– and VO₂(OR)₂^– Coligands

A series of novel mixed ligand dinickel complexes of the type [Ni^II₂L­(μ-L′)]⁺, where L′ is a tetrahedral oxo-alkoxo vanadate (L′ = [O₂V^V(OR)₂]⁻, R = H or alkyl) and L a macrocyclic N₆S₂ supporting ligand, have been prepared, and their esterification reactivity has been studied. The orthovanadate complex [Ni₂L­(μ-O₂V­(OH)₂)]⁺ (<b>2</b>), prepared by reaction between [Ni₂L­(μ-Cl)]­ClO₄ with Na₃VO₄ and a phase transfer reagent in CH₃CN, reacts smoothly with MeOH and EtOH forming the vanadate diesters [Ni₂L­(μ-O₂V­(OMe)₂)]⁺ (<b>3</b>) and [Ni₂L­(μ-O₂V­(OEt)₂)]⁺ (<b>4</b>). The dialkyl orthovanadate esters in <b>3</b> and <b>4</b> are readily transesterified with mono- and difunctional alcohols. Complex <b>3</b> can also be generated from <b>4</b> by transesterification with MeOH. Complexes <b>3</b> and <b>4</b> react with diols (ethylene glycol, propylene glycol and diethylene glycol) as well to afford the complexes [Ni₂L­(μ-O₂V­(OH)­(OCH₂CH₂OH))]⁺ (<b>5</b>), [Ni₂L­(μ-O₂V­(OCH₂)₂CH₂)]⁺ (<b>6</b>), and [Ni₂L­(μ-O₂V­(OCH₂CH₂)₂O)] (<b>7</b>). The crystal structures of the tetraphenylborate salts of complexes <b>3</b>–<b>7</b> reveal in each case four-coordinate O₂V^V(OR)₂^– groups bonded in a μ_1,3-bridging mode to generate trinuclear complexes with a central N₃Ni­(μ-S)₂(μ_1,3-O₂V­(OR)₂)­NiN₃ core. The stabilization of the four-coordinate V^VO₂(OR)₂^– moieties is a consequence of both the two-point coordinative fixation to and the steric protection of the bowl-shape binding pocket of the [Ni₂L]²⁺ fragment. Cyclic voltammetry experiments reveal that the encapsulated vanadate esters are not reduced in a potential window of −2.0 to +2.5 V vs SCE. The spins of the nickel­(II) (<i>S</i>_i = 1 ions) in <b>3</b> are weakly ferromagnetically coupled (<i>J</i> = +23 cm^–1, (<b>H</b> = −2<i>J</i><b>S</b>₁<b>S</b>₂)) to produce an <i>S</i> = 2 ground state

Preservation of hemicellulose remnants in sedimentary organic matter

Cellulose has been identified in fossil material, but the preservation potential of hemicelluloses, which are less resistant to decomposition than cellulose, is generally considered very low. Here we present the discovery of hemicellulose in Miocene xylites (fossil wood) and Cretaceous xylo-detritic coals from Poland. The main building blocks of hemicelluloses in softwood are mannose, glucose, xylose with lesser amounts of galactose and arabinose. These saccharides were detected in the coals and xylites using independent geochemical methods. Based on chemical analysis, the lignites contained significant holocellulose (22–37%), α-cellulose (8–29%) and hemicellulose (7–13%). In the smoke from a xylite burn test, levoglucosan and mannosan were dominant, the latter a specific hemicellulose alteration product. Glucose and mannose products dominated after methanolysis, with minor galactose and xylose. The main hemicellulosic polysaccharides in lignite appear to be glucomannan and/or galactoglucomannan but with a lower mannose content, possibly connected to wood degradation by fungi. The preservation of hemicelluloses in fossil material may be due to structural interconnection between lignin, cellulose and hemicellulose (i.e. lignocellulose), common in extant wood. This is the first documentation of hemicelluloses in fossil material. Our results show that not only cellulose, but also hemicelluloses can persist for millions of years under favorable conditions with only minor structural changes due to slow microbial and/or diagenetic decay. In fossil wood, types of hemicellulose can help assess whether the ancient plants were related to gymnosperms or angiosperms.This work was supported by the National Science Centre, Poland (grant 2018/31/B/ST10/00284 to LM

Stabilization of Hypophosphite in the Binding Pocket of a Dinuclear Macrocyclic Complex: Synthesis, Structure, and Properties of [Ni₂L(μ‑O₂PH₂)]BPh₄ (L = N₆S₂ Donor Ligand)

The dinickel­(II) complex [Ni₂L­(ClO₄)]­ClO₄ (<b>1</b>), where L^2– represents a 24-membered macrocyclic hexaamine-dithiophenolate ligand, reacts with [<i>n</i>Bu₄N]­H₂PO₂ to form the hypophosphito-bridged complex [Ni₂L­(μ-O₂PH₂)]⁺, which can be isolated as an air-stable perchlorate [Ni₂L­(μ-O₂PH₂)]­ClO₄ (<b>2</b>) or tetraphenylborate [Ni₂L­(μ-O₂PH₂)]­BPh₄ (<b>3</b>) salt. <b>3</b>·MeCN crystallizes in the triclinic space group <i>P</i>1̅. The bisoctahedral [Ni₂L­(μ-O₂PH₂)]⁺ cation has a N₃Ni­(μ_1,3-O₂PH₂)­(μ-S)₂NiN₃ core structure with the hypophosphito ligand attached to the two Ni^II ions in a μ_1,3-bridging mode. The hypophosphito ligand is readily replaced by carboxylates, in agreement with a higher affinity of the [Ni₂L]²⁺ dication for more basic oxoanions. Treatment of [Ni₂L­(μ-O₂PH₂)]­ClO₄ with H₂O₂ or MCPBA results in the oxidation of the bridging thiolato to sulfonato groups. The hypophosphito group is not oxidized under these conditions due to the steric protection offered by the supporting ligand. An analysis of the temperature-dependent magnetic susceptibility data for <b>3</b> reveals the presence of ferromagnetic exchange interactions between the Niⁱⁱ (<i>S</i> = 1) ions with a value for the magnetic exchange coupling constant <i>J</i> of +22 cm^–1 (<b>H</b> = −2<i>J</i><b>S</b>₁<b>S</b>₂). These results are additionally supported by DFT (density functional theory) calculations

Stabilization of Hypophosphite in the Binding Pocket of a Dinuclear Macrocyclic Complex: Synthesis, Structure, and Properties of [Ni₂L(μ‑O₂PH₂)]BPh₄ (L = N₆S₂ Donor Ligand)

The dinickel­(II) complex [Ni₂L­(ClO₄)]­ClO₄ (<b>1</b>), where L^2– represents a 24-membered macrocyclic hexaamine-dithiophenolate ligand, reacts with [<i>n</i>Bu₄N]­H₂PO₂ to form the hypophosphito-bridged complex [Ni₂L­(μ-O₂PH₂)]⁺, which can be isolated as an air-stable perchlorate [Ni₂L­(μ-O₂PH₂)]­ClO₄ (<b>2</b>) or tetraphenylborate [Ni₂L­(μ-O₂PH₂)]­BPh₄ (<b>3</b>) salt. <b>3</b>·MeCN crystallizes in the triclinic space group <i>P</i>1̅. The bisoctahedral [Ni₂L­(μ-O₂PH₂)]⁺ cation has a N₃Ni­(μ_1,3-O₂PH₂)­(μ-S)₂NiN₃ core structure with the hypophosphito ligand attached to the two Ni^II ions in a μ_1,3-bridging mode. The hypophosphito ligand is readily replaced by carboxylates, in agreement with a higher affinity of the [Ni₂L]²⁺ dication for more basic oxoanions. Treatment of [Ni₂L­(μ-O₂PH₂)]­ClO₄ with H₂O₂ or MCPBA results in the oxidation of the bridging thiolato to sulfonato groups. The hypophosphito group is not oxidized under these conditions due to the steric protection offered by the supporting ligand. An analysis of the temperature-dependent magnetic susceptibility data for <b>3</b> reveals the presence of ferromagnetic exchange interactions between the Niⁱⁱ (<i>S</i> = 1) ions with a value for the magnetic exchange coupling constant <i>J</i> of +22 cm^–1 (<b>H</b> = −2<i>J</i><b>S</b>₁<b>S</b>₂). These results are additionally supported by DFT (density functional theory) calculations

Complexation, computational, magnetic, and structural studies of the maillard reaction product isomaltol including investigation of an uncommon π interaction with copper(II)

The metal complexation properties of the naturally occurring Maillard reaction product isomaltol HL2 are investigated by measurement of its stability constants with copper(II), zinc(II), and iron(III) using potentiometric pH titrations in water, by structural and magnetic characterization of its crystalline complex, [Cu(L2) 2]·8H2O, and by density functional theory calculations. Strong complexation is observed to form the bis(isomaltolato) copper(II) complex incorporating copper in a typical (pseudo-)square-planar geometry. In the solid state, extensive intra- and intermolecular hydrogen bonding involving all three oxygen functions per ligand assembles the complexes into ribbons that interact to form two-dimensional arrays; further hydrogen bonds and π interactions between the furan moiety of the anionic ligands and adjacent copper(II) centers connect the complexes in the third dimension, leading to a compact polymeric three-dimensional (3D) arrangement. The latter interactions involving copper(II), which represent an underappreciated aspect of copper(II) chemistry, are compared to similar interactions present in other copper(II) 3D structures showing interactions with benzene molecules; the results indicate that dispersion forces dominate in the π system to chelated copper(II) ion interactions. © 2011 American Chemical Society

STM Study of Au(111) Surface-Grafted Paramagnetic Macrocyclic Complexes [Ni₂L(Hmba)]⁺ via Ambidentate Coligands

Molecular anchoring and electronic properties of macrocyclic complexes fixed on gold surfaces have been investigated mainly by using scanning tunnelling microscopy (STM) and complemented with X-ray photoelectron spectroscopy (XPS). Exchange-coupled macrocyclic complexes [Ni₂L­(Hmba)]⁺ were deposited via 4-mercaptobenzoate ligands on the surface of a Au(111) single crystal from a mM solution of the perchlorate salt [Ni₂L­(Hmba)]­ClO₄ in dichloromethane. The combined results from STM and XPS show the formation of large monolayers anchored via Au–S bonds with a height of about 1.5 nm. Two apparent granular structures are visible: one related to the dinickel molecular complexes (cationic structures) and a second one related to the counterions ClO₄^– which stabilize the monolayer. No type of short and long-range order is observed. STM tip-interaction with the monolayer reveals higher degradation after 8 h of measurement. Spectroscopy measurements suggest a gap of about 2.5 eV between HOMO and LUMO of the cationic structures and smaller gap in the areas related to the anionic structures

Encapsulation of the 4‑Mercaptobenzoate Ligand by Macrocyclic Metal Complexes: Conversion of a Metallocavitand to a Metalloligand

Complexation of the ambidentate ligand 4-mercaptobenzoate (4-SH-C₆H₄CO₂H, H₂mba) by the macrocyclic complex [Ni₂L­(μ-Cl)]­ClO₄ (L^2– represents a 24-membered macrocyclic hexaazadithiophenolate ligand) has been examined. The monodeprotonated Hmba^– ligand reacts with the Ni₂ complex in a selective manner by substitution of the bridging chlorido ligand to produce μ_1,3-carboxylato-bridged complex [Ni₂L­(Hmba)]⁺ (<b>2⁺</b>), which can be isolated as an air-sensitive perchlorate (<b>2</b>ClO₄) or tetraphenylborate (<b>2</b>BPh₄) salt. The reactivity of the new mercaptobenzoate complex is reminiscent of that of a “free” thiophenolate ligand. In the presence of air, <b>2</b>ClO₄ dimerizes via a disulfide bond to generate tetranuclear complex [{Ni₂L}₂(O₂CC₆H₄S)₂]²⁺ (<b>3²⁺</b>). The auration of <b>2</b>ClO₄ with [AuCl­(PPh₃)], on the other hand, leads to monoaurated complex [Ni^II₂L­(mba)­Au^IPPh₃]⁺ (<b>4⁺</b>). The bridging thiolate functions of the N₆S₂ macrocycle are deeply buried and are unaffected/unreactive under these conditions. The complexes were fully characterized by electrospray ionization mass spectrometry, IR and UV/vis spectroscopy, density functional theory, cyclic voltammetry, and X-ray crystallography [for <b>3</b>(BPh₄)₂ and <b>4</b>BPh₄]. Temperature-dependent magnetization and susceptibility measurements reveal an <i>S</i> = 2 ground state that is attained by ferromagnetic coupling between the spins of the Ni^II ions in <b>2</b>ClO₄ (<i>J</i> = +22.3 cm^–1) and <b>4</b>BPh₄ (<i>J</i> = +20.8 cm^–1; <i>H</i> = −2<i>JS</i>₁<i>S</i>₂). Preliminary contact-angle and X-ray photoelectron spectroscopy measurements indicate that <b>2</b>ClO₄ interacts with gold surfaces