Cr Centers Research Articles

Synthesis and Catalytic Hydrogenation Reactivity of a Chromium Catecholate Porous Organic Polymer

A single-site chromium catecholate POP (catPOP A2B1) was synthesized and characterized via AT-IR, XAS, and EPR spectroscopy. The well-defined, four-coordinate, 11-electron Cr(III) centers bound to catecholate POP were demonstrated to be active hydrogenation catalysts for nonpolar unsaturated organic substrates under mild conditions (5 mol % of Cr, 200 psi of H2, 60 °C). This material constitutes the first example of a well-defined, supported organometallic chromium hydrogenation precatalyst.

The neutral complex [CrIII4(μ3-O)2(μ2-CH3CO2)7(tbpy0)(tbpy•)]0 — a tetranuclear Cr(III) species containing a neutral (bpy0) and a π-radical anion (bpy•)1–

The tetranuclear neutral species [CrIII4(μ3-O)2(μ2-CH3CO2)7(tbpy0)(tbpy•)]0·3THF has been prepared from a mixture of [CrII2(μ2-CH3CO2)4]·2H2O and 4,4′-di-tert-butyl-2,2′-bipyridine (1:2) in tetrahydrofuran (THF) under anaerobic conditions. Black crystals of [CrIII4(μ-O)2(μ-CH3CO2)7(tbpy)2]0 were grown and investigated by X-ray crystallography: a [Cr4O2]8+ butterfly core structure has been identified. In addition, the presence of seven μ2-acetate bridges, a neutral (tbpy0) ligand, and a single π-radical anion (tbpy•)1– have been clearly identified. The magnetic properties have been investigated by solid-state SQUID (superconducting quantum interference device) measurements (2–300 K) and a coupling model is presented. Its X-band electron paramagnetic resonance spectrum has been recorded at 10 and 300 K in toluene solution and successfully simulated. The spin of the (tbpy•)1– radical couples strongly antiferromagnetically to one Cr(III) center (J4 = –129 cm−1). These properties are compared with those reported previously for the monocationic species [CrIII4(μ3-O)2(μ2-CH3CO2)7(bpy0)2](PF6) in Bino et al. (Inorg. Chem. 1991, 30, 856).

Anomalous discrete disorder response of high-symmetry impurity centers spectra in garnet solid solutions.

The zero-phonon electronic transitions in Cr(3+) and Ce(3+) impurity ions in a series of Lu3xY3-3xAl5O12 (0 < x < 1) garnet solid solution crystals were studied experimentally. It was observed that in contrast to the case of Ce(3+), the modification of zero-phonon R-line ((2)E-(4)A2) fluorescence spectra of Cr(3+) ions with changing x occurs in a discrete fashion and is not accompanied by strong inhomogeneous broadening, as usually happens in solid solutions. The effect is ascribed to the high C3i symmetry of the Cr(3+)(Al(3+)) sites that allows only a limited number of non-equivalent Cr(3+) centers in a mixed environment. The energies and radiative lifetimes of the (2)E states of locally identical Cr(3+) centers inside different mixed garnet matrices were studied and the observed dependences on the Lu content x are discussed in terms of lattice compression and dilation.

Synthesis and porous properties of chromium azolate porous coordination polymers.

We developed a new route for synthesis of Cr-based porous coordination polymers (PCPs) with azole ligands and characterized the unique open structures by single-crystal X-ray studies and other spectroscopy techniques. Chromium-based PCPs have been prepared from azolate ligands 3,5-dimethyl-1H-pyrazole-4-carboxylic acid (H2dmcpz) and 1,4-di(1H-tetrazole-5yl)benzene (H2BDT) by solvothermal reactions under an Ar atmosphere. [Cr3O(Hdmcpz)6(DMF)3]⊃DMF (1⊃DMF) is a coordination compound that forms a hydrogen-bonded porous network. [Cr3O(HBDT)2(BDT)Cl3)]⊃DMF (2⊃DMF) possesses a new type of trinuclear chromium μ3-O unit cluster and the novel topology of a Cr-based PCP with 700 m(2) g(-1) of Brunauer-Emmett-Teller surface area. [Cr(BDT)(DEF)]⊃DEF (3⊃DEF) is structurally flexible and reactive to O2 molecules because of the unsaturated Cr(2+) centers. This is the first report of a Cr-based PCP/metal-organic framework with noncarboxylate ligands and characterization by single-crystal X-ray diffraction.

Synthesis and electrochemical investigation of chromium(0) ferrocenyl-substituted carbene complexes

The series [(CO)5CrC(R)Fc], (1, R=OEt; 2, R=NHnBu) as well as [(OC)5CrC(R)Fc′(R)CCr(CO)5], (3, R=OEt; 4, R=NHnPr) of mono- and biscarbene chromium(0) complexes with Fc=FeII(C5H5)(C5H4) and Fc′=FeII(C5H4)2, were synthesized and characterized spectroscopically, electrochemically and computationally. Electrochemical studies on the new aminocarbene complexes revealed that 2 and 4 are oxidized (and reduced) at lower potential than the EtO derivatives 1 and 3 while the biscarbene complexes 3 and 4 provided evidence for interaction between different redox sites, including the Cr-centers. The Cr0/I couples are electrochemically reversible with the difference in formal potentials ΔE°′ for the consecutive oxidations of the chromium carbene entities being 151 and 105mV in 3 and 4, respectively. Computational and electrochemical results were mutually consistent in showing unambiguously that the Cr(0) center is oxidized before the ferrocenyl group in the carbene complexes 2, 3 and 4. Electrochemical experiments on 1 were also consistent with this redox sequence of events, but differently, our calculations suggest that for 1, the ferrocenyl group may be oxidized before Cr(0) oxidation. The ethoxycarbenes 1 and 3 also showed a carbene double bond reduction to an anion radical, −CrC, while the new aminocarbene 4 was the only derivative to show irreversible Cr(I) oxidation to Cr(II) within the potential window of the solvent.

Proton transfers are key elementary steps in ethylene polymerization on isolated chromium(III) silicates.

Mononuclear Cr(III) surface sites were synthesized from grafting [Cr(OSi(O(t)Bu)3)3(tetrahydrofurano)2] on silica partially dehydroxylated at 700 °C, followed by a thermal treatment under vacuum, and characterized by infrared, ultraviolet-visible, electron paramagnetic resonance (EPR), and X-ray absorption spectroscopy (XAS). These sites are highly active in ethylene polymerization to yield polyethylene with a broad molecular weight distribution, similar to that typically obtained from the Phillips catalyst. CO binding, EPR spectroscopy, and poisoning studies indicate that two different types of Cr(III) sites are present on the surface, one of which is active in polymerization. Density functional theory (DFT) calculations using cluster models show that active sites are tricoordinated Cr(III) centers and that the presence of an additional siloxane bridge coordinated to Cr leads to inactive species. From IR spectroscopy and DFT calculations, these tricoordinated Cr(III) sites initiate and regulate the polymer chain length via unique proton transfer steps in polymerization catalysis.

Synthesis, characterization and catalytic activity of new Cr(III) complex in oxidation of primary alcohols to aldehydes

New mononuclear complex of chromium(III), [Cr(bp)(N3)(CH3OH)]·2CH3OH has been synthesized where H2bp is N,N-bis(2-hydroxy-3,5-di-tert-butylbenzyl)-N’,N’-dimethylethylene-1,2-diamine. The complex was characterized by elemental analyses and spectroscopic methods and X-ray diffraction analysis. Single crystal X-ray diffraction structure analysis of the complex revealed that the chromium ion is octahedrally coordinated where the bis(phenolate) ligand (bp2−) is N2O2-bonded to the Cr(III) center in a tetradentate dinegative fashion. The catalytic potential of this complex has been tested for the oxidation of primary alcohols using H2O2 as oxidant. The effects of various parameters including molar ratio of oxidant to substrate, temperature, and solvent have been studied. Excellent conversions have been obtained for the oxidation of benzyl alcohols containing electron-donating groups on para-position.

2D-QSPR/DFT studies of aryl-substituted PNP-Cr-based catalyst systems for highly selective ethylene oligomerization

1-Hexene and 1-octene are important comonomers for the synthesis of high performance polyolefins. Recently, various N-substituted Cr-bis(diphenylphosphino)amine (PNP-Cr) catalysts show the potential as excellent candidates for highly selective ethylene trimerization/tetramerization. In this work, a series of aryl-substituted PNP-Cr catalysts were studied by two-dimensional quantitative structure-property relationship (QSPR) method based on density functional theory (DFT) calculations. The heuristic method (HM) and best multi-linear regression (BMLR) were used to establish the best linear regression models to describe the relationship between selectivities and catalyst structures. Both Cr(I) and Cr(II) active site models for ethylene trimerization/tetramerization were considered. It was found that 1) the relativity and stability of the models were increased by using self-defined descriptors based on DFT calculations; 2) Cr(I)/Cr(III) centers were the most plausible active sites for ethylene trimerization, while Cr(II)/Cr(IV) active sites were most possibly responsible for ethylene tetramerization; and 3) the skeleton structures of the PNP-Cr system with good complanation and symmetry were crucial for achieving excellent catalytic selectivity of 1-octene, while the PNP-Cr backbone with a large steric effect on N atom would benefit ethylene trimerization. Six new PNP ligands with high selectivity toward ethylene trimerization/tetramerization were predicted based on descriptor analysis and the best linear regression models providing a good basis for further development of novel catalyst systems with better performance.

Carbon dioxide adsorption on MIL-100(M) (M = Cr, V, Sc) metal–organic frameworks: IR spectroscopic and thermodynamic studies

Interaction between carbon dioxide and the coordinatively unsaturated Cr(III), V(III) and Sc(III) cationic centers in MIL-100(Cr), MIL-100(V) and MIL-100(Sc), respectively, was studied by means of variable-temperature infrared (VTIR) spectroscopy, a technique that affords determination of standard adsorption enthalpy (ΔH0) and entropy (ΔS0) from analysis of IR spectra recorded over a temperature range while simultaneously measuring equilibrium pressure inside a closed IR cell. ΔH0 was found to be −63, −54 and −48kJmol−1 for MIL-100(Cr), MIL-100(V) and MIL-100(Sc), respectively, which are among the highest values so far reported for CO2 adsorption on metal–organic frameworks containing open metal sites. Corresponding values for ΔS0 resulted to be −210, −198, and −178Jmol−1K−1, thus showing a positive correlation between ΔH0 and ΔS0. The observed values of standard adsorption enthalpy are discussed in the broader context of corresponding data reported in the literature for the adsorption of carbon dioxide on other MOFs, as well as on zeolites.

Alkylamine‐Tethered Stable Metal–Organic Framework for CO2 Capture from Flue Gas

Different alkylamine molecules were post-synthetically tethered to the unsaturated Cr(III) centers in the metal-organic framework MIL-101. The resultant metal-organic frameworks show almost no N2 adsorption with significantly enhanced CO2 capture under ambient conditions as a result of the interaction between amine groups and CO2 molecules. Given the extraordinary stability, high CO2 uptake, ultrahigh CO2 /N2 selectivity, and mild regeneration energy, MIL-101-diethylenetriamine holds exceptional promise for post-combustion CO2 capture and CO2 /N2 separation.

Chromium coordination compounds with bis(3,5-dimethylpyrazol-1-yl)acetic acid or its anion

Two chromium(III) coordination compounds synthesized from a solution of the tridentate (NON) ligand bis(3,5-dimethylpyrazol-1-yl)acetic acid (HL=Hbdmpza), namely [Cr(L)2](H(NO3)2) (1) and [Cr(HL)Cl3] (2), are reported. The centrosymmetric central coordination sphere in compound 1 is fulfilled by two deprotonated ligands L− giving sterically closed coordination shell with pink-orange colored CrO2N4 chromophore. The coordination cation is accompanied by the dinitratohydrogenate(1-) counter anion. Contrary to compound 1, the compound 2 shows only one Hbdmpza ligand, which is protonated HL, though with the same three coordinate N,O,N atoms. Both title compounds 1 and 2 were isolated from the acetonitrile solution with the starting counter-anion remaining in the final product as a new counter anion H(NO3)2− in 1, or the coordinated chloride anions in 2 with green colored CrON2Cl3 chromophore. We were able to isolate the single crystals of 1, while only polycrystalline material for 2. Nevertheless, the crystal structure of 2 containing two octahedral coordination molecules in the asymmetric unit was determined via the combination of DFT calculations and the powder XRD analysis. Though both compounds reveal a mononuclear octahedral coordination motif, the catalytic accessibility of the compound 2, due to only one tridentate ligand L (HL) per one Cr(III) centre clearly promises more to this aspect.

Isocyanide and Phosphine Oxide Coordination in Binuclear Chromium Pacman Complexes.

The new binuclear chromium Pacman complex [Cr2(L)] of the Schiff base pyrrole macrocycle H4L has been synthesized and structurally characterized. Addition of isocyanide, C≡NR (R = xylyl, tBu), or triphenylphosphine oxide donors to [Cr2(L)] gives contrasting chemistry with the formation of the new coordination compounds [Cr2(μ-CNR)(L)], in which the isocyanides bridge the two Cr(II) centers, and [Cr2(OPPh3)2(L)], a Cr(II) phosphine oxide adduct with the ligands exogenous to the cleft.

Substituent Effects on the Electrochemical, Spectroscopic, and Structural Properties of Fischer Mono- and Biscarbene Complexes of Chromium(0)

A series of ten ferrocenyl, furyl, and thienyl mono- and biscarbene chromium(0) complexes were synthesized and characterized spectroscopically and electrochemically. The single crystal structure of the biscarbene complex [(CO)5Cr═C(OEt)-Fu'-(OEt)C═Cr(CO)5] (4a) was determined: C20H12Cr2O13; triclinic; P1; a = 6.2838(5), b = 12.6526(9), c = 29.1888(19) Å, α = 89.575(2), β = 88.030(2), γ = 87.423(2)°; Z = 4. Results from an electrochemical study in CH2Cl2 were mutually consistent with a computational study in showing that the carbene double bond of 1-6 is reduced to an anion radical, (-)Cr-C• at formal reduction potentials < -1.7 V vs FcH/FcH(+). The Cr centers are oxidized in two successive one electron transfer steps to Cr(II) via the Cr(I) intermediate. Only Cr(I) oxidation is electrochemically irreversible. Dicationic Cr(II) species formed upon two consecutive one-electron oxidation processes are characterized by a peculiar bonding situation as they are stabilized by genuine CH···Cr agostic interactions. With respect to aryl substituents, carbene redox processes occurred at the lowest potentials for ferrocene derivatives followed by furan complexes. Redox process in the thiophene derivatives occurred at the highest potentials. This result is mutually consistent with a (13)C NMR study that showed the Cr═C functionality of furyl complexes were more shielded than thienyl complexes. The NHBu carbene substituent resulted in carbene complexes showing redox processes at substantially lower redox potentials than carbenes having OEt substituents.

Syntheses, crystal structures, and magnetic properties of a series of cyanide-bridged trinuclear chromate(III)−nickel(II)−chromate(III) complexes based on dicyanidechromate(III) building blocks

Four cyanide-bridged trinuclear CrIII−NiII−CrIII complexes [Ni(cyclam)][Cr(bpb)(CN)2]2·2H2O (1) (cyclam=1,4,8,11-tetraazacyclotetradecane, bpb2–=1,2-bis(pyridine-2-carboxamido)-benzenate), [Ni(cyclam)][Cr(bpClb)(CN)2]2·4H2O (2) (bpClb2–=1,2-bis(pyridine-2-carboxamido)-4-chloro-benzenate), [Ni(cyclam)][Cr(bpmb)(CN)2]2·4H2O (3) (bpmb2–=1,2-bis(pyridine-2-carboxamido)-4-methyl-benzenate) and [Ni(cyclam)][Cr(bpdmb)(CN)2]2 (4) (bpdmb2–=1,2-bis(pyridine-2-carboxamido)-4,5-dimethyl-benzenate) have been synthesized by the reaction of [Ni(cyclam)](ClO4)2 with a series of dicyanidechromate(III) building blocks. Single crystal X-ray diffraction analyses show that the four complexes have similar trinuclear structures with CrIII–CN–NiII–NC–CrIII linkages. Magnetic investigations indicate the ferromagnetic coupling between Cr(III) and Ni(II) centers through the cyanide bridge, with JCrNi=4.64(3)cm−1 for 1, 4.33(4)cm−1 for 2, 3.57(3)cm−1 for 3 and 5.3(1)cm−1 for 4. The study on magneto-structural correlation for cyanide-bridged CrIII−NiII systems reveals that the cyanide-bridging bond angle is related to the strength of magnetic exchange coupling: the larger the Ni–NC bond angle, the stronger the Ni⋯Cr magnetic interaction.

Organophophorous Ester Degradation by Chromium(III) Terephthalate Metal–Organic Framework (MIL-101) Chelated to N,N-Dimethylaminopyridine and Related Aminopyridines

Porous materials based on chromium(III) terephthalate metal organic frameworks (MOF) MIL-101(Cr) and their complexes with dialkylaminopyridines (DAAP) were synthesized via a DAAP-MOF complexation, and tested for hydrolytic degradation of organophosphorous esters such as diethyl 4-nitrophenyl phosphate (paraoxon). Elemental analysis, TGA, XRD, FT-IR, TEM, SEM, and nitrogen adsorption measurements indicated that the DAAP units were incorporated into MIL-101 pores by complexation, keeping the parent framework intact. The DAAP-MOF enabled facile paraoxon hydrolysis in water/acetonitrile mixtures under ambient conditions (100% conversion after 24 h at pH 10). The MOF-DAAP complexes showed synergistic effects, being 7-fold and 47-fold more active than the parent MIL-101 or DAAP materials, respectively. The high hydrolysis reaction turnover was realized by simultaneous action of the Lewis acid Cr(III) center of the MOF as well as the electron-rich nucleophile, DAAP. This study demonstrates a simple and efficient method of generating catalytically active MOF materials for environmental detoxification as well as defensive applications.

New diethoxo-bridged dinuclear Cr(III) complexes with derivatives of the quinoxaline-2,3-dione ligand and 2,2′-bipyridine as a co-ligand: Syntheses, spectral characterizations, magnetic properties, antimicrobial inhibitory activities and interpretation of the electronic absorption spectra using the ZINDO/S-CI semi-empirical method

Two new diethoxo-bridged dinuclear Cr(III) complexes, [Cr(QX)(bpy)EtO]2 [QX=6,7-dichloroquinoxaline-2,3-dione (1); 6,7-dimethylquinoxaline-2,3-dione (2)], have been synthesized and characterized. The complexes were initially characterized on the basis of their elemental and mass analyses. The infrared studies were useful in assigning the coordination mode of the quinoxaline-2,3-dione ligand to the chromium metal. In addition, the presence of μ-ethoxo bridges was inferred from the characteristic vibrational bands in the IR spectra of both complexes. The structural and vibrational behaviors of both complexes have been elucidated using a parameterized PM3 semi-empirical method. The magnetic susceptibility, measured at 298K, indicated exchange interactions between the two Cr(III) centers. The observed effective magnetic moments have been correlated to the calculated Cr⋯Cr distances and Cr–O–Cr angles of the Cr(OEt)2Cr cores in both complexes. The ESR spectra have been recorded on powder samples at 293K. The dominant quintet state has been computer-simulated with the parameters J=23cm−1, g=2.11, D=0.074cm−1 and E=0.008cm−1 for 1. On the other hand, the spectrum of complex 2, which showed two slightly different Cr centers, has been simulated with J=17cm−1, g1=2.17, D1=0.063cm−1, E1=0.012cm−1 for site 1 and g2=2.055, D2=0.065cm−1 and E2=0.0087cm−1 for site 2. The electronic spectra of the studied complexes were dominated by charge-transfer, [Cr(dπ)→bpy(π∗) and QX(O-pπ)→Cr(dπ∗)], and spin-allowed d–d transitions. In addition, low-energy maxima characteristics of the dinuclear transition metal complexes were observed in the 550–1050nm region. Theoretical studies of the electronic spectra by the ZINDO/S-CI method were useful in interpreting the observed electronic transitions. The antimicrobial activity studies have indicated a significant inhibitory activity of complex 2 against the studied bacteria and complex 1 showed the highest inhibitory activity against the studied fungi.

Electrochemical illumination of thienyl and ferrocenyl chromium(0) Fischer carbene complexes

A series of ferrocenyl and thienyl mono- and biscarbene chromium(0) complexes 1-6 were synthesised. The complexes were characterised both spectroscopically and electrochemically, and the single crystal X-ray structure of 3 was determined. Electrochemical measurements in CH2Cl2 revealed that the carbene double bond of 1-6 is reduced to an anion radical, (-)Cr-C˙ at formal reduction potentials <-1.7 V vs. FcH/FcH(+). A computational study on 1, 3 and 4 (B3LYP/def2-SVP level) is consistent with electrochemical results in showing that electrochemically generated chromium(I) species may be further electrochemically irreversibly oxidised to chromium(II) at Epa > 0.95 V. The reactivity towards follow-up chemical reactions of the anodically produced Cr(II) species is much higher than the reactivity of the cathodically produced radical anions as the latter was still observably reoxidised to the parent Cr=C species at fast scan rates. The ferrocenyl group is oxidised electrochemically reversibly to ferrocenium at larger potentials than the electrochemically reversible oxidation of the Cr(0) centre to Cr(I). That all redox centres in 1-6 are involved in one-electron transfer steps was confirmed by comparing the ferrocenyl voltammetric wave with those of the other redox centres in linear sweep voltammetric experiments. The ferrocenyl group was electrochemically shown to stabilise the Cr=C centre almost as much as the NHBu, and much more than the ethoxy and thienyl groups.

Observation of Magnetic Anisotropy in GaN:Cr Single Crystals

Over the last decade GaN doped with transition-metals (TM) has been extensively studied both experimentally and theoretically. Among the prominent candidates for future spintronics are GaN:Mn and GaN:Cr. However, there is still lack of su cient experimental data on diluted magnetic semiconductors (DMS) based on Cr, especially on paramagnetic phase of GaN:Cr [1, 2]. The electron con guration of free Cr atom is [Ar]3ds and neutral electronic con guration of Cr substituting Ga in GaN is Cr(d). There are reports that Cr substituting Ga ions in the GaN lattice forms Cr/ deep acceptor level about 2 eV above GaN valence band [3, 4]. Since, in n-type GaN the Fermi level is located near the conduction band, the expected electron structure of Cr ion is Cr(d). Such electronic con guration is non-spherical and should result in pronounced magnetic anisotropy. Similar situation was reported for GaN:Mn, co-doped with Mg [5], where manganese was found to form Mn(d) center and strong magnetic anisotropy was observed [5]. In this communication we report on magnetic anisotropy of Cr in GaN, which we assign to Cr(d) centers.

The nature of Cr center in GaN: Magnetic anisotropy of GaN:Cr single crystals

Magnetization measurements of the strain-free bulk GaN:Cr single crystals of wurtzite structure are reported. Strong magnetic anisotropy at low temperatures (2–10 K) was observed. The data were analyzed assuming Cr2+(d4) configuration. The crystal field model taking into account cubic field of tetrahedral symmetry, trigonal field along the c-axis simulating hexagonal structure, tetragonal static Jahn-Teller distortion, and the spin-orbit interaction provide a good description of the experimental magnetization data.

Synthesis, crystal structure, antimicrobial activity and electrochemistry study of chromium(III) and copper(II) complexes based on semicarbazone Schiff base and azide ligands

A tridentate NNO donor Schiff base ligand HL [HL: C5H4NCH=NNHCONH2] was obtained by condensation of pyridine 2-carbaldehyde with semicarbazide. The HL and azide ligands with Cr(III) and Cu(II) ions have been used to synthesize a dinuclear complex [Cr(L)(N3)(OCH3)]2(1) and a coordination polymer complex [Cu(L)(N3)]n(2). The ligand, 1 and 2 were clearly characterised by elemental analysis, FT-IR, UV–Vis spectral studies and the structures of the 1 and 2, have been studied by single crystal X-ray diffraction analysis. The results of X-ray diffraction analysis revealed Cr(III) and Cu(II) center are coordinated in a distorted octahedral and a square pyramidal geometries for 1 and 2 respectively. The ligand and the complexes were tested for their efficiency towards antimicrobial activity and the MIC data revealed that the HL, 1 and 2 have not strong activity in comparison to the standard drugs. The electrochemistry of the HL and its complexes were studied by cyclic voltammetry.