Polynuclear Chromium Research Articles

The tetracyanoborate anion as a building block for the heterocubane cluster cage [Cr4{B(CN)4}4]4.

The tetracyanoborate anion [B(CN)4]- (TCB) was utilized as a building block for the synthesis of polynuclear chromium carbonyl compounds upon photolytic reaction with [Cr(CO)6]. Up to four κN-coordinated cyano groups of TCB can be involved in binding to chromium, giving mixtures of [B(CN)4-x{CN-Cr(CO)5}x]- (x = 1-4; 1-4) and [{Cr(CO)4(B(CN)4)}2]2- (5). The reaction of [B(CN)4]- with fac-[Cr(CO)3(MeCN)3] led to isolation of salts of the tetraanionic heterocubane cage [{Cr(CO)3(B(CN)4)}4]4-.

Adsorption characteristics of Cr (III) onto starch‐graft‐poly(acrylic acid)/organo‐modifed zeolite 4A composite: A novel path to the adsorption mechanisms

Superabsorbent polymer are extensively synthesized in our daily life, and have become important scavengers of metal ions dissolved in aquatic environment. Herein, Herein, an in‐depth study was performed to examine the morphology and adsorption properties of the starch‐graft‐poly(acrylic acid)/organo‐modifed zeolite 4A composite by coupling SEM, XRF, and ICP‐MS, and to completely characterize the Cr (III) coordination sphere within the complex by combining DFT calculations and XAS. The results showed that the adsorption capacity of the synthesized composite could reach to a very large value of 651.42 mg g−1 with initial Cr3+ concentration ascending to 50 mmol L−1, while the removal rate decreased continuously. By virtue of DFT calculations and XAS, it was found that the adsorbed chromium was coordinated by four carboxyl O in quasi‐equator plane and two water O in axial direction to form a quasi‐octahedral geometry. The Cr–O/C distance of our compound were very close to those reported values of Cr‐adsorbed organic compounds and polynuclear chromium carboxylate compounds. Finally, the combination of DFT calculations and XAS was demonstrated to be a powerful method to explore the mechanisms of metal ion adsorption onto synthesized composite. POLYM. COMPOS., 39:1223–1233, 2018. © 2016 Society of Plastics Engineers

Mixed Metal Multinuclear Cr(III) Cage Compounds and Coordination Polymers Based on Unsubstituted Phenolate: Design, Synthesis, Mechanism, and Properties

Oxidative substitution reactions of chromium(II) chloride with lithium phenoxide in the presence of water give access to new polynuclear chromium(III) aryloxide complexes and coordination polymers. While a one-dimensional coordination polymer based on chromium(III), [Cr2(μ3-OPh)2(μ-OPh)6(μ-OH)(μ3-Li)4(μ3-Cl)2Li(THF)6]n, is first obtained by serendipity, the controlled addition of water to the initial Cr(II) complex as well as variation of the stoichiometry lead to three new discrete chromium(III) aggregates, namely, [Cr(OPh)4(μ-OH)Li2(THF)3]2, [Cr3Li3(OPh)9(μ3-OH)(μ3-O)(THF)6], and [Cr4(μ-OPh)8(μ3-OH)6(μ-OH)2Cl2Li6(THF)6]·2THF. The use of deuterated species allowed confirmation of the oxidation based on the addition of water by detection of H2, HD, and D2. During these investigations, we have also identified a THF-adduct of chromium(II) chloride, [CrCl2(THF)2], used in the literature as a precursor in numerous syntheses, but with a hitherto unknown structure.

Cr(II) complex: water reductant and starting compound for new Cr(III) compounds

The use of alkali aryloxide reagents in organo-metallic synthesis often depends on their solubility, a property derived from their structure. The regain of interest of alkali aryloxides also originates from the discovery of high-temperature superconducting compounds, which has generated a great interest in the formation of oxide materials and other ceramics. Thus, many alkoxides of yttrium and copper are common precursors for oxide materials. Moreover, the synthesis of heterobimetallic alkoxides has provided a facile route for obtaining soluble, volatile, and generally monomeric species. These heterobimetallic complexes can thus serve as valuable precursors for making metal oxides but it is not the only possible application for this type of compounds. These complexes can be used as starting compounds for syntheses of more complex structures. In this work, the salt elimination and ligand exchange reaction of chromium(II) chloride with lithium phenoxide yields a mixed metal lithium-chromium(II) phenoxide. Using this latter as intermediately formed starting material and combining the substitution reaction with an oxidation process, we have gained access to new polynuclear chromium(III) aryloxide complexes. While a 1D coordination polymer based on chromium(III) is obtained in a first reaction by serendipity, the controlled addition of water to the Cr(II) complex leads to three new discrete chromium(III) cluster compounds. The use of deuterated species allowed to confirm the oxidation based on the addition of water by detection of H2, HD and D2. During these investigations, we have also identified a THF-adduct of chromium(II) chloride, used in the literature as precursor in numerous syntheses, but with a hitherto unknown structure. Figure 1: Oxidation of a Cr(II) complex by water and formation of a new Cr(III) complex.

Prediction and correlations between the stability constants of mono- and polynuclear chromium(III) and iron(III) complexes

Correlations between the experimentally determined stability constants of mono- and polynuclear chromium(III) and iron(III) complexes are discussed. An equation to evaluate the stability constants of mono- and polynuclear chromium(III) complexes is obtained: \(\log \beta [Cr_p^{3 + } (L_i )_{q_i } ] = 0.84\log \beta [Fe_p^{3 + } (L_i )_{q_i } ]\).

Chromium(III) Complexation to Natural Organic Matter: Mechanisms and Modeling

Chromium is a common soil contaminant, and it often exists as chromium(III). However, limited information exists on the coordination chemistry and stability of chromium(III) complexes with natural organic matter (NOM). Here, the complexation of chromium(III) to mor layer material and to Suwannee River Fulvic Acid (SRFA) was investigated using EXAFS spectroscopy and batch experiments. The EXAFS results showed a predominance of monomeric chromium(III)-NOM complexes at low pH (<5), in which only Cr···C and Cr-O-C interactions were observed in the second coordination shell. At pH > 5 there were polynuclear chromium(III)-NOM complexes with Cr···Cr interactions at 2.98 Å and for SRFA also at 3.57 Å, indicating the presence of dimers (soil) and tetramers (SRFA). The complexation of chromium(III) to NOM was intermediate between that of iron(III) and aluminum(III). Chromium(III) complexation was slow at pH < 4: three months or longer were required to reach equilibrium. The results were used to constrain chromium-NOM complexation in the Stockholm Humic Model (SHM): a monomeric complex dominated at pH < 5, whereas a dimeric complex dominated at higher pH. The optimized constant for the monomeric chromium(III) complex was in between those of the iron(III) and aluminum(III) NOM complexes. Our study suggests that chromium(III)-NOM complexes are important for chromium speciation in many environments.

Electrode processes and their kinetic parameters in the reduction and oxidation of Cr(III) and Cr(II) in molten CsCl

The reduction of chromium ions in the CsCl-CrCl3 melt in wide concentration and temperature ranges is reported. It is assumed that the melt contains polynuclear chromium complexes. The reduction of Cr(III) to Cr(II) is a reversible one-electron process. The electroreduction of divalent chromium to chromium metal is an irreversible process involving dinuclear and mononuclear chromium complex ions. The electron transfer rate constant has been calculated for various temperatures and chromium chloride concentrations in the salt melt. The activation energy of electron transfer and its dependence on the CrCl3 concentration in the electrolyte have been determined for the Cr(III) ↔ Cr(II) process.

Composition and Structure of Polynuclear Chromium(III) Hydroxo Complexes

The structures of the polynuclear hydrolysis complexes of chromium(III) formed in acidic and alkaline aqueous solution have been determined by means of extended X-ray absorption fine structure and large angle X-ray scattering. Chromium(III) is present as mononuclear hydrated ions below pH 3-4, with the upper limit depending on concentration, ionic strength, and temperature. Chromium(III) hydrolyzes in aqueous solution above this pH in concentrated aged solutions to mainly cationic tetrameric complexes containing both single and double hydroxo bridges between the chromium(III) ions. Above pH 5, the solubility of chromium(III) decreases sharply to form solid hydrated chromium(III) hydroxide. The solubility of chromium(III) increases sharply at elevated pH, and fairly concentrated solutions can be prepared in concentrated aqueous solutions of sodium hydroxide. According to the literature, mononuclear tetrahydroxochromate(III) complexes, Cr(OH)(4)(-), are formed in alkaline aqueous solution. However, this study shows polymeric chains of six-coordinated chromium(III) ions connected through double hydroxo bridges and with two terminal hydroxo groups giving an overall composition of (Cr(OH)(4))(n)(n-). This polymer is stable for a long time in solution at pH = 15. However, at pH < 15, this polymer precipitates slowly with time, over months, to an amorphous phase with a structure around chromium similar to that in alpha-chromium(III) oxide hydroxide.

Electrode processes in the course of Cr(III) and Cr(II) reduction/oxidation in KCl melt

A hypothesis about formation of a polynuclear chromium complex is made on the basis of a voltammetric study of the processes of trivalent and divalent chromium reduction within complex chloride ions in a potassium chloride melt. The effect of the reaction of Cr2Cl73− formation on deviation of theoretical dependences from the experimentally obtained dependences under the terms of linear voltammetry is shown. Models are suggested for the complex chromium ions, quantum-chemical calculations of the bond lengths in them are performed and steric energies of ions are determined. Diffusion coefficients of Cr(III) and Cr(II) are calculated, diffusion activation energies are determined.

Supramolecular approach to crystallization of polynuclear chromium(III) aqua hydroxo complexes: synthesis and crystal structures of complexes [Cr2(OH)2(H2O)8]4+ and [Cr4(OH)6(H2O)12]6+ with cucurbit[n]uril (n = 7, 8)

Supramolecular compounds of the compositions {[Cr2(OH)2(H2O)8](C42H42N28O14)2}-(NO3)4·18.75H2O (1) and {[Cr4(OH)6(H2O)12](C48H48N32O16)3(NO3)6·55H2O (2) were synthesized from aqueous solutions of chromium(III) nitrate and the macrocyclic cavitand cucurbit[n]uril (C6n H6n N4n O2n , where n = 7 or 8, respectively). According to the X-ray diffraction study, the polynuclear chromium aqua complexes are disposed in cavities formed by the cucurbit[n]uril molecules and are linked to these molecules through hydrogen bonds between the hydroxo and aqua ligands of the polycations and the portal oxygen atoms of the macrocycles. Compound 1 is the first example of supramolecular compounds of cucurbit[7]uril with metal aqua complexes. The isolation of the supramolecular adduct with cucurbit[8]uril 2 in the single-crystalline state allows the determination of the structure of the tetranuclear chromium aqua complex having an adamantane-like structure, [Cr4(μ2-OH)6(H2O)12]6+, which has been previously unknown in the solid state.

Structural and magnetic studies on mono- and polynuclear chromium ascorbate complexes

Polynuclear chromium ascorbate complexes were isolated and physicochemically analyzed in a comparative manner with their mononuclear analog (1). Characterization by elemental analysis, electronic, vibrational, 13C-n.m.r and mass spectroscopies, and variable temperature magnetic susceptibility studies, allowed structural proposals for the binuclear, [Cr2(μ-OH)2(H2O)(C6H7O6)3(OH)] · 4H2O (2), and trinuclear, [Cr3(μ-O)3(H2O)6(C6H7O6)3] · 4 H2O (3) complexes. The pseudo-octahedral Cr III centers were suggested to be connected through hydroxo bridges in (2) and in (3) by oxo bridges forming a hexocyclic ring.

Structural and magnetic studies on mono- and polynuclear chromium ascorbate complexes

Structural and magnetic studies on mono- and polynuclear chromium ascorbate complexes

Polynuclear chromium(III) carboxylates. 3. Cyclic and cubane type hexachromium acetates

Heating an aqueous solution of the trinuclear ‘basic’ chromium(III) acetate led to the formation of several products which were separated by ion-exchange chromatography. Crystals of a new cyclic, hexanuclear Cr(III) compound, [Cr 6(OH) 10(O 2CCH 3) 6(H 2O) 4]Cl 2·13H 2O ( 3·Cl 2·13H 2O) were obtained upon elution of the violet complex 3 with 0.5 M NaCl and slow evaporation of the eluent. The six chromium atoms in complex 3 form an almost planar, irregular hexagon with an overall symmetry close to C 2 h . By heating solid ‘basic’ chromium(III) acetate at 300 °C, followed by ion-exchange separation, a new hexanuclear complex, [Cr 6O 3(OH)(O 2CCH 3) 9(H 2O) 4] 2+ ( 4) has been obtained. Complex 4 has a {Cr 6O 4} core, which consists of a {Cr 4O 4} cubane type inner core with two external chromium centers attached to μ 4-oxo(cube) ligands. A similar procedure, using ‘basic’ chromium(III) propionate led to the isolation of the dodecanuclear complex [Cr 12O 8(O 2CCH 2CH 3) 16(H 2O) 8] 4+ ( 5) which has a {Cr 12O 8} core. The {Cr 6O 4} core in complex 4 can be regarded to be formed from a tetranuclear {Cr 4O 2} butterfly unit and a dinuclear {Cr 2O 2} unit. Similarly, the {Cr 12O 8} core in 5 can be considered to be constructed from two orthogonal {Cr 6O 4} units as in complex 4.

Polynuclear chromium(III) carboxlylates: Part 2. Chromium(III) acetate — what's in it?

Chromium(III) acetate has been widely used in industry for decades. The commercial material is an ill-defined substance, which represents a large number of compounds having different compositions, physical properties and appearances. Several samples of Cr(III) acetate, from various commercial sources were examined by ion-exchange chromatography. All the samples were found to contain several species such as [Cr 3O(O 2CCH 3) 6(H 2O) 3] + and other positively charged purple complexes. They also contain various amounts of the neutral violet complex [Cr 8(OH) 8(O 2CCH 3) 16] ( 1) which crystallizes upon slow evaporation of its aqueous solution. 1 is a cyclic octanuclear complex with hydroxo and acetate ligands bridging the adjacent Cr(III) ions. The structure of a well-defined Cr(III) acetate, namely, [Cr(H 2O) 6](O 2CCH 3) 3 ( 2) has been determined crystallographically and its decomposition products were examined by ion-exchange chromatography. Compound 2 decomposes under ambient conditions, releasing acetic acid and water producing neutral and charged polynuclear Cr(III) complexes.

Modeling chromium sulfate complexes in relation to chromium tannage in leather technology: a computational study

Structures that may lead to polynuclear chromium complexes to cross-link the collagen fibrils in chromium tanning processes are modeled. Pathways are suggested to the olation reaction. The role of sulfate as a facilitator has been demonstrated. Copyright © 2000 John Wiley & Sons, Ltd.

Polynuclear chromium(III) carboxylates. 1. Synthesis, structure, and magnetic properties of an octanuclear complex with a ring structure.

A novel cyclic octanuclear chromium(III) complex with hydroxo and acetato bridging ligands was isolated and its structure determined by X-ray crystallography. The complex [Cr8(OH)12(OAc)12] (1) (OAc- = CH3CO2-), as found in crystals of 1.34H2O, is obtained by refluxing an aqueous solution of the trinuclear "basic" chromium acetate. 1.34H2O crystallizes in the tetragonal space group I42d with the following unit cell dimensions: a = 16.592(2) A, c = 31.557(4) A, V = 8687(1) A3, and Z = 2. A total of 2000 unique data with I > 3 sigma (I) were used to solve and refine the structure to R(Fo) = 0.066 and Rw(Fo) = 0.085. The structure consists of eight Cr(III) ions that form a ring structure and are bridged by hydroxo and acetato ligands. Each of the two neighboring metal atoms in 1 is bridged either by two OH- ligands and one OAc- ligand, with a Cr...Cr distance of 2.949(2) A, or by two OAc- ligands and one OH- ligand, with a Cr...Cr distance of 3.383(2) A in an alternating fashion. The complex resides on a crystallographic 4 center, and the overall symmetry of 1 is S4. The magnetic susceptibility of 1.34H2O was measured in the temperature range of 5-240 K. Our theoretical modeling of the susceptibility data indicates alternating antiferromagnetic exchange interactions between adjacent spin 3/2 Cr3+ ions around the ring, of magnitude J/kB = 13.7 and 8.9 K, respectively.

Modeling chromium sulfate complexes in relation to chromium tannage in leather technology: a computational study

Structures that may lead to polynuclear chromium complexes to cross-link the collagen fibrils in chromium tanning processes are modeled. Pathways are suggested to the olation reaction. The role of sulfate as a facilitator has been demonstrated.

Self-Assembled Superanions: Ionic Capsules Stabilized by Polynuclear Chromium(III) Aqua Cations

An inclusion complex of an inclusion complex: so can the superanion [{Na+⊂(18-crown-6)(OH2)2}⊂{(p-sulfonatocalix[4]arene4−)2}]7− (illustrated here) be described. These self-assembled ionnic capsules can facilitate the crystallization of polynuclear transition metal aqua ions that have different charge, size and shape, such as [Cr2(OH)2(H2O)8]4+, [Cr3(OH)4(H2O)10]5+ and [Cr4(OH)6(H2O)12]6+.

Unexpected copper(I) complexation behaviour observed in the synthesis of novel polynuclear chromium(0)–copper(I) complexes

A novel series of penta- and hexa-nuclear chromium(0)-copper(I) complexes, displaying either a two ligand to one copper (thermodynamic product) or a two ligand to two copper (kinetic product) ratio have been synthesized and their properties studied.

The nature of surface chromium species on CrO x/ZrO 2 catalysts

The nature and stability of chromium species on the surface of CrO x / ZrO 2 catalysts (Cr content, 0.05 – 5 wt.%) have been investigated by means of ESR, IR and XPS spectroscopies. On samples heated in oxygen at 773 K or at 923 K, chromium is present on the surface as Cr V (ESR) and Cr VI (IR and XPS). Experiments with the 53Cr isotope show that the species observed by ESR spectroscopy is a surface mononuclear chromyl complex in a square-pyramidal configuration. Surface chromates are demonstrated by IR spectroscopy. In more concentrated samples, polynuclear chromium species, and/or Cr VI and Cr V polychromates, are observed in addition to chromates and isolated Cr V Upon reduction with CO or H 2 at increasing temperatures, Cr V is reduced to Cr III (ESR and IR) even at ca. 383 K. On further reduction at higher temperatures (up to 623 K), Cr VI is also reduced to Cr II. Specifically, samples reduced at 623 K contain Cr III and Cr II in nearly equal amounts. The catalytic pattern of the CrO x /ZrO 2 system, examined in the terms of the chromium species, allows us to conclude that isolated Cr III species, arising from the reduction of Cr V, are the active sites for H 2-D 2 equilibration and propene hydrogenation. The role of Cr II, either isolated or as dimers, is ruled out in the case of CrO x /ZrO 2 catalysts. The zirconia matrix stabilizes the Cr III active species in surface sites of high coordinative unsaturation.