Chromium Centers Research Articles

Development of novel chromium oxide/metallocene hybrid catalysts for bimodal polyethylene

The investigation of a multicomponent catalyst in polyolefin field came up as an alternative for synthesizing bimodal polymers in only one step process under constant reaction conditions. In the present work, new bifunctional catalysts were prepared by combining chromium and metallocene species on the same solid and tested in ethylene polymerization in order to evaluate the possibility of producing bimodal polyethylene. The catalytic system methylaluminoxane (MAO)/(nBuCp) 2ZrCl 2 was immobilized on activated chromium catalysts supported onto several inorganic carriers (silica, silica–alumina, aluminophosphate and mesostructured SBA-15-type materials). The reaction results showed a clear influence of the physicochemical properties of the support on the relative contribution of metallocene and chromium centers as well as on polymers molecular weight distribution. A bimodal polyethylene was obtained by supporting the MAO/metallocene system on a mesostructured chromium catalyst prepared by direct synthesis.

Spectral separation of Cr3+ optical centers in stoichiometric magnesium-doped lithium niobate crystals

The broadband luminescence of chromium optical centers with strongly overlapping spectral lines and similar emission probabilities from excited 4T2 states of red and green Cr3+ centers in stoichiometric magnesium-doped lithium niobate crystals has been separated for the first time. The spectral-luminescence characteristics and parameters of intracenter interaction between red and green optical Cr3+ centers in stoichiometric lithium niobate have been calculated. The luminescence quantum efficiencies of red and green chromium centers are determined.

A high field and frequency electron paramagnetic resonance study of the S = 2 chromium(II) complex trans-[Cr(NCCH 3) 4(FBF 3) 2] and investigation of its reaction with dioxygen: Crystal structure of [(CH 3CN) 5CrOCr(NCCH 3) 5](BF 4) 4·2CH 3CN

From high field and frequency electron paramagnetic resonance (EPR) powder spectra of the high spin chromium(II) complex trans-[Cr(NCCH 3) 4(FBF 3) 2] ( 1) recorded at T = 5–40 K the EPR parameters were found to be D = −2.19 cm −1, E ≈ 0 cm −1, and g ‖ = g ⊥ = 1.9833. The reaction between a MeCN solution of 1 and dioxygen gave dark green crystals of [(CH 3CN) 5CrOCr(NCCH 3) 5](BF 4) 4·2CH 3CN ( 2). From the X-ray crystal structure of 2 at T = 122 K a Cr–O–Cr angle of 180° and a Cr–O distance of 1.7556(5) Å was found. This is the shortest among the known Cr–O distances in dinuclear complexes containing a CrOCr 4+ core, and this can partly explain the strong antiferromagnetic coupling between the two chromium(III) centers in 2.

Synthesis and Crystal Structures of Two N-Heterocyclic Carbene Adducts of CrCl2

The reactions of CrCl2 with two N-heterocyclic carbenes (NHC) have afforded the complexes, trans-[CrCl2(NHC)2] (NHC = :C{N(C6H3Pr 2 i -2,6)C(H)}2 1 or :C{N(Pri)C(Me)}2 2). The cell parameters of the compounds are 1: P212121, a = 12.9445(4), b = 20.7364(5), c = 21.2841(6); 2: Pca21, a = 21.7583(14), b = 9.9465(8), c = 11.4414(9). The crystal structures of the complexes are the first for NHC adducts of CrCl2, and both exhibit square planar chromium centers. The synthesis and crystal structures of two N-heterocyclic carbene adducts of CrCl2, trans-[CrCl2(NHC)2] (NHC = :C{N(C6H3Pr 2 i -2,6)C(H)}2 or :C{N(Pri)C(Me)}2) are reported.

A homobimetallic complex of chromium(0) with a σ-borane component

The synthesis, characterization, and reactivity of a chromium(0) complex bearing an amine-borane moiety (η(6)-C(6)H(5)CH(2)NMe(2)·BH(3))Cr(CO)(3) (2) is reported. Photolysis of complex 2 results in the elimination of a CO ligand followed by the formation of an intramolecular σ-borane complex (η(1)-(η(6)-C(6)H(5)CH(2)NMe(2)·BH(2)-H))Cr(CO)(2) (3). This species was characterized in solution by NMR spectroscopy. Reaction of complex 2 with photochemically generated (OC)(5)Cr(THF) affords a novel homobimetallic σ-borane complex (OC)(3)Cr(η(6)-C(6)H(5)CH(2)NMe(2)·BH(2)-H-Cr(CO)(5)) (4), wherein one of the BH moieties is bound to the chromium center in an η(1)-fashion. The σ-borane complex 4 was isolated in moderate to good yield (72%). The BH(3) fragment in the complexes 3 and 4 are highly dynamic involving exchange of the BH hydrogen bound to the metal with the terminal BH hydrogen atoms. The dynamics has been studied using variable-temperature NMR spectroscopy. Complexes 2 and 4 have been characterized by X-ray crystallography.

Highly symmetrical chromium(II) bisdiketiminate complexes

Reaction of the lithium salts of N, N′-dialkyl-2-amino-4-imino-pent-2-enes, nacnac RLi(THF) (R = CH 2Ph, Cy, nPr, iBu or S–CH(Me)Ph), with half an equivalent of CrCl 2(THF) x yielded the homoleptic complexes ( nacnac R) 2Cr. All complexes were characterized by X-ray diffraction studies and displayed a highly symmetric, square-planar coordination around the chromium center with strong boat-like distortions of the diketiminate ligands. Reaction of nacnac RLi(THF) (R = CH 2Ph, Cy) with one equivalent of CrCl 2(THF) x afforded the dimeric complexes { nacnac RCr(μ-Cl)} 2.

Alkali metal cation complexation and solvent interactions by robust chromium(III) fluoride complexes

Interaction of robust chromium(III) fluoride complexes with sodium or lithium cations in solution lead to hypsochromic spectral shifts of increasing magnitude along the series: trans-[CrF 2(py) 4] +, mer-[CrF 3(terpy)], and fac-[CrF 3(Me 3tacn)]. Crystalline products isolated from solution exhibit μ 2-bridging by the fluoride ligands in a linear fashion between Na +-ions and chromium centres in catena-[Na(H 2O) 4(μ-F)- trans-{CrF(py) 4}](HCO 3) 2 and in the dimers [Li(H 2O) n (μ-F)- trans-{CrF(py) 4}] 2+ ( n = 3, 4). The uncharged chromium complexes fac-[CrF 3(Me 3tacn)] and mer-[CrF 3(terpy)] have been synthesized from mer-[CrF 3(py) 3] and shown to precipitate sodium salts from solution, of which 3[CrF 3(Me 3tacn)]·2Na(Bph 4)·solv and 6[CrF 3(terpy)]·4Na(Bph 4)·solv have been crystallographically characterized. In these clusters, the neutral fluoride complexes bring the Na + cation separation down to 3.610 Å and 3.369 Å, respectively, which is much closer than the inter-cation distance in NaCl and comparable to that of NaF. DFT calculations support the notion of a strong interaction between Na + ions and neutral chromium(III) fluoride complexes. The calculations reproduce the magnitude and the counter-intuitive sign of the spectral shifts induced by second sphere complexation in solution, which originates in a breakdown of the assumption of parameter transferability in ligand-field descriptions.

CrIII−CrIIIInteractions in Two Alkoxo-Bridged Heterometallic Zn2Cr2Complexes Self-Assembled from Zinc Oxide, Reinecke’s Salt, and Diethanolamine

Two new tetranuclear complexes, [Zn(2)Cr(2)(NCS)(4)(Dea)(2)(HDea)(2)].4DMSO (1; DMSO = dimethyl sulfoxide) and [Zn(2)Cr(2)(NCS)(4)(Dea)(2)(HDea)(2)].2CH(3)CN (2), were prepared from zinc oxide, Reinecke's salt, NH(4)[Cr(NCS)(4)(NH(3))(2)].H(2)O, ammonium thiocyanate, and a nonaqueous solution of diethanolamine (H(2)Dea) in a reaction carried out under open air. Both compounds have similar centrosymmetric crystal structures based on a tetranuclear {Zn(2)Cr(2)(mu(3)-O)(2)(mu-O)(4)} core. Variable-temperature magnetic susceptibility measurements of 1 and 2 show weak antiferromagnetic coupling between chromium centers. The magnetic data and high-field, high-frequency electron paramagnetic resonance spectra were analyzed in terms of the spin Hamiltonian H = JS(1).S(2) - j(S(1).S(2) + mu(B)B{g(1)}S(1) + D(Cr){S(z1)(2) - S(1)(S(1) + 1)/3} + E(Cr)(S(x1)(2) - S(y1)(2)) + mu(B)B{g(2)}S(2) + D(Cr){S(z2)(2) - S(2)(S(2) + 1)/3} + E(Cr)(S(x2)(2) - S(y2)(2)) + D(12){S(z1)S(z2) - S(1).S(2)/3} + E(12)(S(x1)S(x2) - S(y1)S(y2)) with J = 13.7 cm(-1), j = 1.1 cm(-1), D(Cr) = 0.3864 cm(-1), E(Cr) = -0.1104 cm(-1), D(12) = -0.1873 cm(-1), and E(12) = -0.0155 cm(-1) for 1 and J = 9.4 cm(-1), j = 0.8 cm(-1), D(Cr) = 0.3564 cm(-1), E(Cr) = -0.0647 cm(-1), D(12) = -0.1850 cm(-1), and E(12) = -0.0112 cm(-1) for 2. Density functional theory (DFT) calculations were employed to calculate the zero-field splitting on Cr(3+) ions. Calculations of the exchange integrals J were attempted by using the "broken-symmetry" DFT method.

EPR study of chromium centres formed in hexagonal RbMgF3 and RbZnF3 single crystals

AbstractEPR measurements have been made on single crystals of RbMgF3 doped only with chromium or codoped with chromium and lithium. Three kinds of trigonally symmetric Cr3+ spectra were observed. Strong spectrum observed from codoped RbMgF3 is ascribed to Cr3+–Li+ pair centre. Other two centres are identified by considering the signs of the fine structure parameters $b_2^0$. Weak spectrum observed from RbMgF3 doped only with chromium is ascribed to a simply‐substituting Cr3+ ion at the MgII site in the face‐sharing Mg2F9 unit. The spectrum observed after X‐ray irradiation for the above crystal is ascribed to a simply‐substituting Cr3+ ion at the MgI site in the single octahedron unit. Similar Cr3+ centres are identified for hexagonal‐RbZnF3 codoped with chromium and lithium. An empirical rule for the sign and magnitude of the $b_2^0$ parameter among the Cr3+ centres in the hexagonal‐BaTiO3 type fluorides is presented. Site preference substitution by trivalent chromium ion in RbMgF3 was found.

An extremely rare example of asymmetric tetranuclear core {M4(μ3-X)2(μ-X)4} in a novel alkoxo-bridged heterometallic Cr/Cd complex

A novel heterometallic tetranuclear complex [Cr(3)Cd(NCS)(4)(H(2)tea)(Htea)(3)].3CH(3)OH () has been prepared using cadmium oxide, Reinecke's salt, ammonium thiocyanate and a methanol solution of triethanolamine (H(3)tea) in air. Crystallographic investigations reveal that the molecular structure of the complex is based on an uncommon asymmetric {Cr(3)Cd(mu(3)-O)(2)(mu-O)(4)} core with firstly observed terminal coordination of the NCS-groups in such molecular structure type. A network of O-HO hydrogen bonds as well as SS interactions link the molecules into a two-dimensional supramolecular network. Variable-temperature (1.8-300 K) magnetic susceptibility measurements show a change of the mu(B) value at low temperature, indicative of antiferromagnetic interactions (J = -7.45 cm(-1)) between chromium(iii) centers.

Kinetics and Mechanism of the Reaction between Chromium(III) and 2,3‐Dihydroxybenzoic Acid in Weak Acidic Aqueous Solutions

The reaction between chromium(III) and 2,3-dihydroxybenzoic acid (2,3-DHBA) takes place in at least three stages, involving various intermediates. The ligand (2,3-DHBA)-to-chromium(III) ratio in the final product of the reaction is 1 : 1. The first stage is suggested to be the reaction of [Cr(H2O)5(OH)]2+ with the ligand in weak acidic aqueous solutions that follows an I d mechanism. The second and third stages do not depend on the concentrations of chromium(III), and their activation parameters are ΔH ≠ 2(obs) = 61.2 ± 3.1 kJmol−1, ΔS ≠ 2(obs) = −91.1 ± 11.0 JK−1mol−1, ΔH ≠ 3(obs) = 124.5 ± 8.7 kJmol−1, and ΔS ≠ 3(obs) = 95.1 ± 29.0 JK−1mol−1. These two stages are proposed to proceed via associative mechanisms. The positive value of ΔS ≠ 3(obs) can be explained by the opening of a four-membered ring (positive entropy change) and the breaking of a hydrogen bond (positive entropy change) at the associative step of the replacement of the carboxyl group by the hydroxyl group at the chromium(III) center (negative entropy change in associative mechanisms). The reactions are accompanied by proton release, as shown by the pH decrease.

Bis[N,N′-bis(2,6-diisopropylphenyl)ethane-1,2-diimine]-1κ2N,N′;2κ2N,N′-tri-μ-trichlorido-1:2κ6Cl:Cl-chlorido-1κCl-tetrahydrofuran-2κO-dichromium(II) dichloromethane 4.5-solvate

In the mol­ecular structure of the title compound, [Cr2Cl4(C26H36N2)2(C4H8O)]·4.5CH2Cl2, the two CrII centers are bridged by three Cl atoms, forming a dinuclear complex. Each CrII center is coordinated by one chelating bis­(2,6-diisopropyl­phen­yl)ethane-1,2-diimine ligand via both N atoms. An additional chloride ion binds to one chromium center, whereas an additional tetra­hydro­furan mol­ecule coordinates to the second CrII center. The coordination geometry at each CrII center can be best described as distorted octa­hedral.

Photoredox reactions of Cr(III) mixed-ligand complexes

Irradiation of chromium(III) complexes with oxalate and pyridinedicarboxylate ligands (pda = 2,3-, 2,4-, or 2,5-dicarboxylate) leads to diverse behaviors, dictated by light energy, presence of oxygen and the ligand nature. Irradiation within the MC bands is unaffected by O 2 and results in ligand substitution. The LMCT excitation is effective only when oxalate is coordinated to Cr(III); then electron transfer from oxalate to central ion generates an intermediate, consisted of a Cr(II)species and the C 2O 4 − radicals. The species undergo fast redox reactions dependent on the presence of O 2 and the pda ligand. (1) In anoxic medium the fast outersphere electron transfer from Cr(II) to solvent, generates hydrated electrons and re-oxidizes the chromium centre to Cr III. Then geminate recombination regenerates substrate, whereas competitive release of the C 2O 4 − radical leads to substitution of one oxalate ligand by two water molecules (aquation induced by the LMCT excitation). In the presence of the pda ligand the outersphere electron transfer is accompanied by the innersphere CT, generating Cr(III) coordinated to two radical ligands: C 2O 4 − and pda 3−; the intermediate releases also e aq −, but this reaction is slower than that of the homoleptic oxalate complex. Hydrated electrons are scavenged also by the released radicals. All these processes are completed within microseconds and in consequence, the Cr(III) complexes irradiated in deoxygenated solutions are insensitive to subsequent oxygenation. (2) When UV-irradiation is carried out in oxygenated medium reaction of Cr(II) species with molecular oxygen competes with the outer- or inner electron transfer observed in anoxic medium. Both these pathways result in generation of chromate(VI). Quantum yield of the Cr(VI) production is sensitive to the presence and structure of pda ligand, decreasing within the series 2,3-pda > 2,4-pda > 2,5-pda.

High-frequency tunable EPR spectroscopy of non-Kramers ions in AgGaSe2: Cr, AgGaS2: Cr, and CdGa2S4: Cr crystals

Crystals of the chalcopyrite family, AgGaSe2, AgGaS2, and CdGa2S4, doped with chromium ions have been investigated using high-frequency broad-band EPR spectroscopy in the range 65–530 GHz at T = 4.2 K. It has been revealed that, in the AgGaSe2 and AgGaS2 crystals, the Cr2+ ions occupy positions with orthorhombic and tetragonal symmetry, whereas the previously investigated CdGaS4 crystals contain only tetragonal centers. The observed spectra have been described in the framework of the spin-Hamiltonian formalism. Apart from the divalent chromium centers, the EPR lines attributed to non-Kramers ions are observed in the frequency range 300–450 GHz for all the crystals under investigation. The nature of these lines has been discussed.

Diacetonitrile[N,N′-bis(2,6-diisopropylphenyl)ethane-1,2-diimine]dichloridochromium(II) acetonitrile solvate

The title compound, [CrCl2(CH3CN)2(C26H36N2)]·CH3CN, was synthesized by the reaction of CrCl2(THF)2 with N,N′-bis­(2,6-diisopropyl­phen­yl)ethane-1,2-diimine in dichloro­methane/acetonitrile. The chromium center is coordinated by two N atoms of the chelating diimine ligand, two chloride ions in a trans configuration with respect to each other, and by two N atoms of two acetonitrile mol­ecules in a distorted octa­hedral geometry.

Study of divalent and trivalent chromium in forsterite by high-frequency EPR spectroscopy

Divalent and trivalent chromium ions Cr2+ and Cr3+ replacing magnesium ions at octahedral positions in Mg2SiO4: Cr and Mg2SiO4: Cr: Li crystals are investigated by submillimeter EPR spectroscopy in the frequency range 65–230 GHz. The crystals are grown from the melt by the Czochralski method. The content of mixed-valence chromium species in forsterite is analyzed. It is demonstrated that, in crystals grown in argon (the oxygen partial pressure is \( P_{O_2 } \) = 0.01 kPa), approximately half of the chromium ions are in the divalent form. The Cr2+ ions are distributed over the M1 and M2 positions in a ratio of approximately 2: 1. A change in the oxygen partial pressure \( P_{O_2 } \) and the chromium concentration, as well as an additional doping with lithium, does not lead to substantial changes in the distribution of divalent chromium ions over the positions. It is shown that an increase in the oxygen partial pressure \( P_{O_2 } \) from 0.01 to 2.00 kPa results in a decrease in the coefficient of divalent chromium distribution between the crystal and the melt. Doping with lithium also decreases the concentration of Cr2+ centers. In crystals grown without lithium, approximately half of the trivalent chromium ions are associated with magnesium vacancies. The addition of lithium leads to the destruction of these associates, an increase in the concentration of individual Cr3+ centers, and the formation of lithium associates with trivalent chromium ions. The conditions for the formation of associates of trivalent chromium ions with lithium ions are optimum when the crystal contains approximately identical amounts of Cr3+ and Li+ ions. Doping with lithium increases the concentration of Cr3+ ions and, thus, decreases the fraction of Cr2+ and Cr4+ ions in the total content of chromium centers.

Poly[1-ethyl-3-methylimidazolium [tri-μ-chlorido-chromate(II)

The title compound, {(C6H11N2)[CrCl3]}n, was generated via mixing of the ionic liquid 1-ethyl-3-methyl­imidazolium chloride with CrCl2 in ethanol. Crystals were obtained by a diffusion method. In the crystal structure, the anion forms one-dimensional chains of chloride-bridged Jahn–Teller distorted chromium(II) centers extending along the [100] direction. The imidazolium cations are positioned between these chains.

Chromium(III) complexes bearing 2-benzazole-1,10-phenanthrolines: synthesis, molecular structures and ethylene oligomerization and polymerization

The 2-benzazole-1,10-phenanthrolines and their chromium(III) complexes were synthesized and characterized by elemental and spectroscopic analyses as well as by single-crystal X-ray crystallography. X-Ray crystallographic analyses of C6 and C8 reveal an octahedral geometry around both chromium centers. The electronic spectra of the chromium complexes exhibit d-d transitions typically of a pseudo-octahedral coordinated d(3) ion, falling into the region nu(1)(4)A(2g)-->(4)T(2g) 600-700 nm,nu(2)(4)A(2g)-->(4)T(1g)(F) 430-470 nm, and the 10Dq values between 15674 and 16529 cm(-1). The variation of the heteroatoms (X = N, O, S) in benzazole group shows regular influence on the 10Dq values as 10Dq (O) > 10Dq (N) > 10Dq (S), which reflects the sigma-donor/pi-acceptor properties of these benzazole ligands. Upon activation with modified methylaluminoxane (MMAO), these complexes exhibited high activities for ethylene oligomerization (up to 7.36 x 10(6) g mol(-1) (Cr) h(-1)) and ethylene polymerization (up to 1.28 x 10(6) g mol(-1) (Cr) h(-1)). The effects of non-coordinated N, O or S heteroatoms in the ligands have been examined, and interestingly catalytic activities also strictly adhered to the trend O > N > S under various ethylene pressures. Reaction conditions significantly influenced the catalytic properties of the complexes.

One- and Two-Electron Reduced 1,2-Diketone Ligands in [CrIII(L•)3] (S= 0) and Na2(Et2O)2[VIV(LRed)3] (S=1/2)

The electronic structures of chromium and vanadium centers coordinated by three reduced 1,2-diketones have been elucidated by using density functional theory (DFT) calculations and a host of physical methods: X-ray crystallography; cyclic voltammetry; ultraviolet-visible (UV-vis), nuclear magnetic resonance (NMR), and electron paramagnetic resonance (EPR) spectroscopy; and magnetic susceptibility measurements. The metal center in octahedral [CrIII(L*)3]0 (1), a CrIII (d3) ion is coupled antiferromagnetically to three monoanionic ligand pi-radicals affording an S ) 0 ground state. In contrast, Na2(Et2O)2[VIV(LRed)3] (2) (S ) 1/2), possesses a central VIV (d1) ion O,OE-coordinated to three closed-shell, doubly reduced ligands which in turn are coordinated by two Na cations enforcing a trigonal prismatic geometry at the vanadium center. 2 can be oxidized electrochemically by one and two electrons generating a monoanion, [V(L)3]1-, and a neutral species, [V(L)3]0, respectively. DFT calculations atthe B3LYP level show that the one-electron oxidized product contains an octahedral VIV ion coupled antiferromagnetically to one monoanionic ligand pi-radical [VIV(L*)(LRed)2]1- (S ) 0). In contrast, the two-electron oxidized product contains a VIII ion coupled antiferromagnetically to three ligand pi-radicals in an octahedral field[VIII(L*)3]0 (S ) 1/2).

Synthesis and Structural Comparison for a Series of Cr(II) (iodo) NacNac Complexes

AbstractA series of complexes of the form [bis(2,6-diisopropylphenyl)pentane-2,4-ketiminato](iodo)(solvent) chromium were synthesized and structurally characterized (solvent = tetrahydrofuran, acetonitrile, α-picoline). The tetrahydrofuran adduct crystallized in the P2 1 /n space group with the crystal cell parameters a = 17.628(6) A, b = 10.218(3) A, c = 20.646(7) A, β = 109.209(4)°, V = 3512(2) A 3 and Z = 4. The acetonitrile adduct crystallized in the P2 1 /c space group with the crystal cell parameters a = 16.104(7) A, b = 11.965(5) A, c = 18.201(8) A, β = 116.091(6)°, V = 3150(2) A 3 and Z = 4. The α-picoline adduct crystallized in the C2/c space group with the crystal cell parameters a = 39.982(9) A, b = 11.637(2) A, c = 16.560(4) A, β = 108.085(4)°, V = 7325(3) A 3 and Z = 8. For each compound, the coordination around each chromium center is square planar with varying degrees of distortion.Graphical AbstractA series of Cr(II) complexes of the form LCrI(S) (where L = nacnac = β-diketiminate; S = solvent) were synthesized and structurally characterized.[IMAGE]