Chromium Derivatives Research Articles

1,2-Diphenyl-o-carborane and Its Chromium Derivatives: Synthesis, Characterization, X-ray Structural Studies, and Biological Evaluations.

The objective of this study is to design and synthesize substituted η6-chromium(0) tricarbonyl metal complexes carrying o-carborane units as potential boron neutron capture therapy (BNCT) agents. In this study, 1,2-diphenyl-o-carborane (1) units were used as starting materials to generate biologically active species. We investigated how the structural changes of 1 substituted with chromium(0) tricarbonyl affect the biological properties, and 1-(Phenyl-η6-chromium(0) tricarbonyl)-2-phenyl-o-carborane (2) and 1,2-bis(phenyl-η6-chromium(0) tricarbonyl)-o-carborane (3) species were produced in moderate yields. The molecular structures of compounds 1-3 were identified and established by infrared (IR); 1H, 11B, and 13C nuclear magnetic resonance (NMR) and X-ray crystallography analyses. Crystal structures of 1,2-diphenyl-o-carborane and the corresponding chromium complexes 1, 2, and 3 were obtained. In an in vitro study using B16 and CT26 cancer cells containing the triphenyl-o-carboranyl chromium(0) complexes Ph3C2BCr2 and Ph3C2BCr3, which we reported previously, compounds 2 and 3 accumulated at higher levels than compounds Ph3C2BCr2 and Ph3C2BCr3. However, the phenylated o-carboranyl chromium complexes have been found to be more cytotoxic than p-boronophenylalanine (BPA).

1,3-Diphosphacyclobutadiene sandwich compounds as bidentate ligands in metal carbonyl chemistry: Binuclear chromium derivatives

The structures and thermochemistry of the binuclear 1,3-diphosphacyclobutadiene chromium carbonyl complexes (Me2C2P2)2Cr2(CO)n (n = 6, 5, 4, 3) have been examined by density functional theory. The only low-energy structure for the hexacarbonyl (Me2C2P2)2Cr2(CO)6 consists of a bidentate chelating (η4-Me2C2P2)2Cr(CO)2 sandwich diphosphine ligand bonded to a Cr(CO)4 unit with a long ∼4.0 Å Cr⋯Cr distance indicating lack of a direct metal-metal bond. The lowest energy structure for the pentacarbonyl (Me2C2P2)2Cr2(CO)5 is a triplet structure derived from this singlet hexacarbonyl structure by loss of a CO group from the sandwiched chromium atom. The bridging η1,4-Me2C2P2 ligands found in these two structures, using the four π-electrons of the P2C2 ring to bond to one chromium atom and a phosphorus lone pair to bond to the other chromium atom but not sandwiching a chromium atom, are features of the next lowest energy triplet and singlet (Me2C2P2)2Cr2(CO)5 structures. In contrast to the hexa- and pentacarbonyls, the lowest energy structures of the more highly unsaturated tetra- and tricarbonyls (Me2C2P2)2Cr2(CO)n (n = 4, 3) have terminal tetrahapto η4-Me2C2P2 rings and chromium-chromium multiple bonds.

Catalytic Asymmetric C-H Arylation of (η6-Arene)Chromium Complexes: Facile Access to Planar-Chiral Phosphines.

A catalytic asymmetric direct C–H arylation of (η6-arene)chromium complexes to obtain planar-chiral compounds is reported. The use of the hemilabile ligand H8-BINAP(O) is key to providing high enantioselectivity in this transformation. We show that this methodology opens the door to the synthesis of a variety of planar-chiral chromium derivatives which can be easily transformed into planar chiral mono- or diphosphines. Mechanistic studies, including synthesis and characterization of Pd and Ag complexes and their detection in the reaction mixture, suggest a Pd-catalyzed/Ag-promoted catalytic system where Ag carries out the C–H activation step.

Characterization and Hypoglycemic Activity of a Rhamnan-Type Sulfated Polysaccharide Derivative.

Polysaccharide chromium (III) derivatives are gaining increasing attention in improving type 2 diabetes. In this study, the sulfated polysaccharide from Enteromorpha prolifera (SPE) with 4.8 kDa was prepared by specific enzymatic hydrolysis. The obtained SPE was used to prepare a rhamnan-type sulfated polysaccharide derivative (SPED). Results indicated that O-H, C=O, and S=O were effectively involved in the chelation of SPED (chromium content 20.26%). Acute (half lethal dose > 2.38 g/kg) and sub-acute toxicity showed that SPED had no damaging effects on mice. Anti-diabetic experiment demonstrated that SPED improved glucose metabolism. Moreover, SPED promoted the PI3K/PKB/GSK-3β signaling pathway by regulating mRNA expression of insulin receptors (IR), insulin receptor substrate 2 (IRS-2), phosphatidylinositol 3 kinase (PI3K), protein kinase B (PKB), and glycogen synthase kinase 3β (GSK-3β). In conclusion, the SPED might represent a novel marine-derived candidate against hyperglycemia, which may undergo further pharmaceutical development as a hypoglycemic agent.

B-phenylated o-carboranes and its chromium derivatives: Synthesis, electrochemical properties, and X-ray structural studies

The electron accepting capability of B-phenylated o-carborane is greatly improved through favorable electronic interactions between the two adjoining phenyl-π* and cage carbon-σ* orbitals. Phenyl substitution at the boron atom in the 3-position directs the two phenyl groups of the cage carbon atoms to a face-to-face position and locks them into a rigid conformation, which gives rise to extensive electronic delocalization and maximum energy stabilization from the lowest unoccupied molecular orbital (LUMO). A reversible reduction peak from the cyclovoltammogram (CV) at −1.72 V and a broad and extended UV absorption (λmax = 277 nm) substantiate such LUMO stabilization, thereby augmenting the electron accepting capability of o-carborane. Due to electron depletion from the adjacent o-carborane, the adjoined phenyl groups act as potential π-acid; indeed, when B-phenylated 1,2-diphenyl-o-carborane was reacted with Cr(CO)6, bi- and trimetallic chromium complexes were selectively formed in an η6-bonding fashion between the chromium atoms and phenyl groups.

Further Theoretical Studies of the Aquation of Chromium(III) Chloride Nutritional Supplement: Effect of pH and Solvation

We have explored speciation of halochromium(III) complexes under physiological conditions involving both conjugate-base formation and halide-water exchange using DFT. Conjugate base formation is spontaneous under neutral and alkaline conditions. Several different pathways were explored for the water exchange reaction of cis- or trans-Cr(III) complexes in the series trans-[Cr(H2O)3(OH)X2]0 (X=Cl− or Br−) and cis- or trans-[Cr(H2O)4(OH)X]+, (X= F−, Cl−, Br−, or I−) including, associative interchange (Ia), dissociative interchange (Id) and dissociative (D) mechanisms. We have also investigated nucelophilic attack by OH− on [Cr(H2O)5X]+ species, (X=Cl− or Br−). The lowest overall activation enthalpies (ΔH≠) obtained for the F and Cl systems of [Cr(H2O)4(OH)X]+, are from the Ia pathway, whereas for the Br and I systems the Id pathway was found, consistent with experiment. The importance of outer sphere solvation was investigated with the addition of up to five explicit water molecules in the second coordination sphere. The ΔH≠ values for the aquation of cis-[Cr(H2O)4(OH)X]+…3H2O, are lower than the corresponding values for [Cr(H2O)5X]2+. This indicates that there will be greater speciation of chromium(III) halides under physiological conditions. These values also imply that chromium(III) derivatives that may be active in the treatment of physiological disorders such as Type(II) diabetes will be formed at different rates depending on the identity of the starting halide.

Recent Advances in Hypervalent Iodine Chemistry

In recent years, an explosive growth of reactivities of hypervalent iodine reagents has witnessed and these reagents, featuring facile availability and easy handleness, offer multiple advantages over establised methods as an efficient multipurpose oxidants, whose reactivities are similar to derivatives of mercury, chromium, lead, thallium and other heavy metals, but without the toxicity and environmental problems of these heavy metal agents. Thus, hypervalent iodine reagents have received much more attention from the synthetic chemists. This account mainly summarizes our recent research progress in the area of hypervalent iodine chemistry, especially focusing on the excellent performances and unique applications brought about by the new hypervalent iodine reagents and new combinations of them both developed in our group: (1) first utilization of a recyclable iodine(III) reagent iodosodilactone for the direct esterification, amidation and peptide synthesis and high efficient liquid-phase oligo-peptide synthesis mediated by a more powerful iodosodilactone-type reagent, 6-(3,5-bis-(trifluoromethyl)phenyl)-1H,4H-2aλ-ioda-2,3-dioxacyclopenta[hi]indene-1,4-dione (FPID); (2) design, synthesis and reactivity exploration of a new water-soluble hypervalent iodine(V) reagent, 5-trimethylammonio-1,3-dioxo-1,3-dihydro-1λ-benzo[d][1,2]iodoxol-1-ol anion (AIBX); (3) systematic studies of reactivities of the first hypervalent iodine(III) reagent iodobenzene dichloride.

Effects of occupational exposure to chromium and its compounds: a systematic review

To review the scientific literature on the health effects of exposure to chromium and chromium compounds on workers. A systematic review of the scientific literature contained in the MEDLINE (via PubMed), EMBASE, Web of Knowledge, Cochrane Library Plus and LILACS databases through January 2013. The terms searched for, both as subject headings and in free text, were: "chromium", "chromium compounds", "occupational exposure" and "occupational diseases". The search was completed through additional review of the bibliographic references included in the selected papers. We recovered 227 references, from which we selected 22 articles after applying inclusion and exclusion criteria. Of these, genetic alterations were examined in eight studies; in one study, alterations in male reproduction; in fourstudies, respiratory alterations; in seven studies, cancer; and in two studies, dermatitis. Hexavalent chromium was the agent most frequently associated with the described health effects. Occupational exposure to chromium and chromium derivatives is associated with human health effects of varying severity. This is why adequate control measures should be implemented in the workplace to minimize exposure and diminish the number of associated diseases. More restrictive exposure limits for this agent, based on health criteria, should be established in the future.

Bacterial mechanisms for Cr(VI) resistance and reduction: an overview and recent advances

Chromium pollution is increasing incessantly due to continuing industrialization. Of various oxidation states, Cr(6+) is very toxic due to its carcinogenic and mutagenic nature. It also has deleterious effects on different microorganisms as well as on plants. Many species of bacteria thriving in the Cr(6+)-contaminated environments have evolved novel strategies to cope with Cr(6+) toxicity. Generally, decreased uptake or exclusion of Cr(6+) compounds through the membranes, biosorption, and the upregulation of genes associated with oxidative stress response are some of the resistance mechanisms in bacterial cells to overcome the Cr(6+) stress. In addition, bacterial Cr(6+) reduction into Cr(3+) is also a mechanism of specific significance as it transforms toxic and mobile chromium derivatives into reduced species which are innocuous and immobile. Ecologically, the bacterial trait of reductive immobilization of Cr(6+) derivatives is of great advantage in bioremediation. The present review is an effort to underline the bacterial resistance and reducing mechanisms to Cr(6+) compounds with recent development in order to garner a broad perspective.

Reactions of Metal-Metal Quintuple Bonds with Alkynes: [2+2+2] and [2+2] Cycloadditions

Benzene, C6H6, is one of the most important molecules in organic chemistry. The term aromaticity was developed from the study of benzene and has become an important and fundamental concept in contemporary chemistry since Kekul proposed a ringlike structure for benzene in 1865. Today, aromatic compounds are not only limited to carbocycles, but also extended to heterocycles containing maingroup elements. For example, one or two CH groups of benzene can be replaced by group 14 and 15 elements, such as silicon, nitrogen, and phosphorus, to give heterocyclic aromatics. Furthermore, an aromatic compound can be a transition-metal-containing organometallic species. Hoffmann et al. predicted that a metallabenzene could be possible by replacing one CH group with a transition-metal fragment. On the experimental side, since the recognition of the first metallabenzene, osmabenzene, in 1982, many metallabenzenes have been subsequently synthesized and structurally characterized. There are many ways to synthesize aromatic compounds. Among the methods, the [2+2+2] cycloaddition reaction of alkynes is a powerful tool to construct highly substituted homoand heterocyclic aromatic molecules. Such a reaction is thermally allowed, but it does not proceed at ambient temperature without transition-metal catalysts. It is however worth noting that Sekiguchi et al. have demonstrated a catalyst-free [2+2+2] cycloaddition reaction for the synthesis of 1,2-disilabenzene by reacting a kinetically stable disilyne RSi SiR (R= SiiPr[CH(SiMe3)2]2) and phenylacetylene at room temperature. The chemistry of transition-metal multiple bonding was recently reawakened by the recognition the first isolable quintuply bonded dimeric chromium complex Cr2Ar’2 (Ar’= C6H3-2,6-(C6H3-2,6-iPr2)2 by Power and co-workers. [10] Since then, several quintuply bonded dinuclear group VI metal complexes stabilized by nitrogen donors have been reported. Besides interesting bonding, the chromium derivatives have recently been shown to display novel reactions with AlMe3, [11f] N2O, RN3, [14] and P4. [15] Reactions of the chromium dimers with alkynes through a [2+2] cycloaddition process to give 1:1 adducts were recently explored as well. In light of the bonding analogy between the C C p components of alkynes and metal–metal d components in the quintuply bonded species (Figure 1), we became inter-

Triple-Decker Sandwiches and Related Compounds of the First-Row Transition Metals Containing Cyclopentadienyl and Benzene Rings

The triple-decker sandwich compound trans-Cp(2)V(2)(eta(6):eta(6)-mu-C(6)H(6)) has been synthesized, as well as "slipped" sandwich compounds of the type trans-Cp(2)Co(2)(eta(4):eta(4)-mu-arene) and the cis-Cp(2)Fe(2)(eta(4):eta(4)-mu-C(6)R(6)) derivatives with an Fe-Fe bond (Cp = eta(5)-cyclopentadienyl). Theoretical studies show that the symmetrical triple-decker sandwich structures trans-Cp(2)M(2)(eta(6):eta(6)-mu-C(6)H(6)) are the global minima for M = Ti, V, and Mn but lie approximately 10 kcal/mol above the global minimum for M = Cr. The nonbonding M...M distances and spin states in these triple decker sandwich compounds can be related to the occupancies of the frontier bonding molecular orbitals. The global minimum for the chromium derivative is a singlet spin state cis-Cp(2)Cr(2)(eta(4):eta(4)-mu-C(6)H(6)) structure with a very short CrCr distance of 2.06 A, suggesting a formal quadruple bond. A triplet state cis-Cp(2)Cr(2)(eta(4):eta(4)-mu-C(6)H(6)) structure with a predicted Cr[triple bond]Cr distance of 2.26 A lies only approximately 3 kcal/mol above this global minimum. For the later transition metals the global minima are predicted to be cis-Cp(2)M(2)(eta(6):eta(6)-mu-C(6)H(6)) structures with a metal-metal bond, rather than triple decker sandwiches. These include singlet cis-Cp(2)Fe(2)(eta(4):eta(4)-mu-C(6)H(6)) with a predicted Fe=Fe double bond distance of 2.43 A, singlet cis-Cp(2)Co(2)(eta(3):eta(3)-mu-C(6)H(6)) with a predicted Co-Co single bond distance of 2.59 A, and triplet cis-Cp(2)Ni(2)(eta(3):eta(3)-mu-C(6)H(6)) with a predicted Ni-Ni distance of 2.71 A.

Heptahapticity in Binuclear (Cycloheptatrienyl)molybdenum Carbonyl Derivatives: The Interplay between Ring Hapticity/Planarity and Metal–Metal Multiple Bonding

Theoretical studies on the binuclear (cycloheptatrienyl)molybdenum carbonyl complexes [(C(7)H(7))(2)Mo(2)(CO)(n)] (n=6, 5, 4, 3, 2, 1, 0) indicate structures with fully bonded heptahapto eta(7)-C(7)H(7) rings and four or fewer carbonyl groups to be energetically competitive. This is in striking contrast to the corresponding chromium derivatives. The lowest energy of such a structure for [(eta(7)-C(7)H(7))(2)Mo(2)(CO)(4)] is a singlet unbridged structure with a formal Mo--Mo single bond with a length of approximately 3.2 A. A higher-energy pentahapto structure [(eta(5)-C(7)H(7))(2)Mo(2)(CO)(4)] is also predicted with a formal Mo [triple bond]Mo triple bond with a length of approximately 2.56 A. Low-energy structures are predicted for [(C(7)H(7))(2)Mo(2)(CO)(3)] with two heptahapto eta(7)-C(7)H(7) rings, either one or two bridging carbonyl groups, and formal Mo=Mo double bonds with a length of approximately 2.8 A. However, the global minimum for [(C(7)H(7))(2)Mo(2)(CO)(3)] is a [(eta(7)-C(7)H(7))(eta(5)-C(7)H(7))Mo(2)(CO)(3)] structure with a formal Mo[triple bond]Mo triple bond with a length of approximately 2.53 A. The lowest-energy structures for [(C(7)H(7))(2)Mo(2)(CO)(2)] and [(C(7)H(7))(2)Mo(2)(CO)] have heptahapto eta(7)-C(7)H(7) rings and predicted metal-metal bond lengths of approximately 2.54 and 2.31 A, respectively, consistent with the formal triple and quadruple bonds, respectively, needed to give both metal atoms the favored 18-electron configuration. The lowest-energy structures for the carbonyl-richer systems [(C(7)H(7))(2)Mo(2)(CO)(n)] (n=6, 5) contain one trihapto eta(3)-C(7)H(7) ring and one pentahapto eta(5)-C(7)H(7) ring.

Highly Selective and Reactive (salan)CrCl Catalyst for the Copolymerization and Block Copolymerization of Epoxides with Carbon Dioxide

Chromium(III) derivatives of the tetradentate phenoxyamine ligand H2salan, where salan = the N,N′-dimethylated bis(aminophenoxide) ligand or the saturated version of the corresponding salen ligand, in the presence of 1 equiv of [PPN]N3 (PPN=Ph3P═N+═PPh3) are shown to effectively catalyze the copolymerization of cyclohexene oxide and carbon dioxide. X-ray crystallographic analysis reveals the structure of the complex to be different from that of its salen analogue, with an all cis arrangement of the nitrogen and oxygen atoms. Although these catalysts are selective for copolymerizing propylene oxide and CO2 at ambient temperature with a high degree of regioselectivity, the copolymerization of cyclohexene oxide and CO2 requires higher temperatures (e.g., 60 °C). Nevertheless, the random polymerization of cyclohexene oxide and propylene oxide with CO2 at ambient temperature provides a terpolymer with a nearly statistical distribution of monomer units. In addition, these catalysts have been shown to be efficie...

An Exploration of the Coupling Reactions of Epoxides and Carbon Dioxide Catalyzed by Tetramethyltetraazaannulene Chromium(III) Derivatives: Formation of Copolymers versus Cyclic Carbonates

The high catalytic activity of a tetramethyltetraazaannulene (tmtaa) chromium complex toward the copolymerization of cyclohexene oxide and carbon dioxide to discriminatively provide poly(cyclohexylene carbonate) has directed further studies into the capabilities of the catalyst system. Various [PPN]X (PPN(+) = bis(triphenylphosphoranylidene)ammonium) cocatalysts, where X = Cl, N(3), Br, CN, and OBzF(5), in the presence of (tmtaa)CrCl were examined for catalytic reactivity and selectivity for polycarbonate formation, achieving turnover frequencies of 1500 h(-1) at 80 degrees C in the case of PPNCl. The catalyst system was examined under varied pressures and found to be active even at 1 bar of CO(2) pressure. In addition to cyclohexene oxide, the (tmtaa)CrCl complex was investigated for catalytic activity toward the coupling of carbon dioxide with propylene oxide, isobutylene oxide, 1,2-epoxyhexane, styrene oxide, and 4-vinyl cyclohexene oxide. Activation energies were found for the copolymerization reaction between cyclohexene oxide and carbon dioxide utilizing the tetramethyltetraazaannulene catalyst system to be 67.1 +/- 4.2 kJ.mol(-1) and 65.2 +/- 2.5 kJ.mol(-1) in neat epoxide and with methylene chloride cosolvent, respectively, upon monitoring these processes by in situ infrared spectroscopy. Supplementary to the studies involving (tmtaa)CrCl, electronic effects at the metal center on catalytic activity were examined through derivatization of the tmtaa ligand, resulting in increased activity as electron-donating substituents were added.

2,2,6,6-Tetramethylpiperidinogallium as a Terminal and Bridging Ligand in Homo- and Heteroleptic Chromium, Nickel, and Cobalt Complexes

By reaction of the gallium(I) derivative Ga(4)tmp(4) (tmp = 2,2,6,6-tetramethylpiperidino) with Cr(CO)(5)(cyclo-octene), Co(2)(CO)(8), and Ni(cyclooctadiene)(2), respectively, the Gatmp complexes [Cr(CO)(5)Gatmp], (CO)(3)Cr(mu(2)-Gatmp)(3)Cr(CO)(3), (CO)(3)Co(mu(2)-Gatmp)(2)Co(CO)(3), and (tmpGa)(2)Ni(mu(2)-Gatmp)(3)Ni(Gatmp)(2) were obtained. The latter are described as derivatives of the binuclear metal carbonyls Cr(2)(CO)(9), Co(2)(CO)(8), and Ni(2)(CO)(7), where some or all carbonyls are replaced by the amino gallylene group. All compounds are characterized by spectroscopy and crystal structure analysis. The change of the bonding situation from localized two-center gallium metal bonds in the chromium derivative to three-center bonds in the cobalt complex is discussed by means of density functional theory calculations.

Stoichiometric and Homogeneous-Catalytic Diboration of the NN Double Bond of Azobenzene

The diboration of the N=N double bond of azobenzene was achieved by reaction of the [3]diboraplatinametalloarenophanes derived from ferrocene, [Fe(eta5-C5H4)B(NMe2)Pt(PEt3)2B(NMe2)(eta5-C5H4)], and bis(benzene)chromium, [Cr(eta6-C6H5)B(NMe2)Pt(PEt3)2B(NMe2)(eta6-C6H5)], with an excess of azobenzene in toluene at elevated temperature. The formation of the anticipated ansa-bis(boryl)hydrazines was substantiated by the determination of the molecular structure of the chromium derivative by X-ray diffraction. The synthesis of the ansa-bis(boryl)hydrazine derivative of ferrocene could also be accomplished under homogeneous catalysis conditions. Hence, reaction of the well-known [2]boraferrocenophane, [Fe(eta5-C5H4)2B2(NMe2)2], and azobenzene in the presence of 5 mol % [Pt(PEt3)3] afforded the bis-borylated hydrazine derivative in good yields.

Synthesis and characterization of new paramagnetic tetraaryl derivatives of chromium and molybdenum

The low-temperature reaction of [CrCl 3(thf) 3] with LiC 6H 3Cl 2-2,6 yields the organochromium(III) compound [Li(thf) 4][Cr III(C 6H 3Cl 2-2,6) 4] ( 1) in 48% yield. The homoleptic, anionic species [Cr III(C 6H 3Cl 2-2,6) 4] − is electrochemically related to the neutral one [Cr IV(C 6H 3Cl 2-2,6) 4] ( 2) through a reversible one-electron exchange process ( E 1/2 = 0.16 V, Δ E p = 0.09 V, i pa/ i pc = 1.18). Compound 2 was isolated in 74% yield by chemical oxidation of 1 with [N(C 6H 4Br-4) 3][SbCl 6]. Attempts to prepare the salt [NBu 4][Cr III(C 6Cl 5) 4] ( 4) by direct arylation of [CrCl 3(thf) 3] with LiC 6Cl 5 in the presence of [NBu 4]Br gave the organochromium(II) salt [NBu 4] 2[Cr II(C 6Cl 5) 4] ( 3) instead, as the result of a reduction process. The salt [NBu 4][Cr III(C 6Cl 5) 4] ( 4) was cleanly prepared by comproportionation of 3 and [Cr IV(C 6Cl 5) 4]. The reaction of [MoCl 4(dme)] with LiC 6Cl 5 in Et 2O solution proceeded with oxidation of the metal center to give the paramagnetic ( S = 1/2), five-coordinate salt [Li(thf) 4][Mo VO(C 6Cl 5) 4] ( 5). The crystal and molecular structures of 1 and 2 have been established by X-ray diffraction methods. The magnetic properties of 1 and 4 ( S = 3/2) as well as those of 2 ( S = 1) have been established by EPR spectroscopy as well as by ac and dc magnetization measurements.

Metal Salen Derivatives as Catalysts for the Alternating Copolymerization of Oxetanes and Carbon Dioxide To Afford Polycarbonates

Metal salen derivatives of chromium and aluminum, along with n-Bu4NX (X = Cl or N3) salts, have been shown to be effective catalysts for the selective coupling of CO2 and oxetane (trimethylene oxide) to provide the corresponding polycarbonate with only trace quantities of ether linkages. The formation of copolymer is suggested, based on circumstantial evidence, not to proceed via the intermediacy of trimethylene carbonate, which was observed as a minor product of the coupling reaction. For a reaction catalyzed by (salen)CrCl in the presence of n-Bu4NN3 as the cocatalyst, both matrix-assisted laser desorption ionization time-of-flight mass spectrometry and infrared spectroscopy revealed an azide end group in the copolymer.

Switchable Chromium(II) Ethylene Oligomerization/Polymerization Catalyst

The tri- and divalent chromium derivatives of [N-Me,tripyrr] ([N-Me,tripyrr] = 2,5-bis(diphenyl(pyrrol-2-yl)methyl)-1-methylpyrrole) [N-Me,tripyrr]CrCl (1), and [N-Me,tripyrr]Cr(L)2·(THF)0.5 (2) [L = THF (2a), pyridine (2b)] were obtained in good yield from the reaction of the dipotassium salt of the ligand with CrCl3(THF)3 and CrCl2(THF)2, respectively. Reaction of the dilithium salt of the ligand with CrCl2(THF)2 led to the divalent analogue [N-Me,tripyrr]Cr(μ-Cl)Li(THF)3 (3), wherein a Li−Cl unit was retained by the metal center. The compounds were tested for ethylene oligomerization activity upon activation with two different alumoxanes. In the case of MAO, the Cr(II) catalyst precursors 2b and 3 showed a marked increase in activity with respect to the trivalent 1. The product distributions from compounds 1 and 2b were nearly identical, indicating that the Cr(II) oxidation state provides better catalyst precursors. Use of the i-BuAlO activator instead of MAO afforded exclusively polymerization with high activity and produced linear, low molecular weight polyethylene.

The First Structurally Characterized Homoleptic Aryl-Manganese(III) Compound and the Corresponding Isoleptic and Isoelectronic Chromium(II) Derivative

The homoleptic organochromium(II) compound [Mg(THF)6][CrII(C6F5)4] (1) was obtained as an extremely air-sensitive solid by reaction of [CrCl2(THF)] with C6F5MgBr. The arylation of MnBr2 under simil...