Hafnium chloride and hafnium methyl complexes bearing substituted pyrrolyl ligands: synthesis, characterization, and ethylene polymerization

Reactions of HfCl₄ with 2 and 3 equiv. of Li[C₄H₃N(CH₂NMe₂)-2] in toluene afford HfCl₂[C₄H₃N(CH₂NMe₂)-2]₂ (1) and HfCl[C₄H₃N(CH₂NMe₂)-2]₃ (2), respectively. Transmetallation reaction of 1 with 2 equiv. of MeLi results in a hafnium dimethyl compound HfMe₂[C₄H₃N(CH₂NMe₂)-2]₂ (3). A variable-temperature ¹H NMR spectroscopic study shows that the activation energy for the dissociation/association of the NMe₂ units of compound 2 in solution is ca. 13.6 kcal/mol. Compounds 1-3 are characterized by NMR spectroscopy and single crystal X-ray diffraction. A polymerization study shows that compounds 1 and 3 exhibit moderate activity toward ethylene in the presence of TIBA and MAO.     

Asymmetric binuclear titanocenes linked by alkyl benzyl ethers: Synthesis, characterization and use as catalysts for ethylene polymerization

A series of novel asymmetric binuclear titanocenes linked with alkyl benzyl ethers p-[(C₅H₅TiCl₂)C₅H₄CH₂]C₆H₄O(CH₂)_n[C₅H₄(TiCl₂C₅H₅)] (n = 2-5) (13-16) have been synthesized by treating p-(LiC₅H₄CH₂)C₆H₄O(CH₂)_n(C₅H₄Li) (n = 2-5) (9-12) with C₅H₅TiCl₃. The new complexes have been characterized by elemental analysis and NMR spectra. Their catalytic activity for ethylene polymerization was investigated in the presence of aluminoxane (MAO). The results show that 13-16 are efficient catalysts for producing polyethylene (PE) with a broad molecular weight distribution (MWD). Their catalytic activity is highly dependent on the length of the alkyl chain and the polymerization conditions. A longer alkyl chain increases the catalytic activity, whereas the molecular weight of the produced polyethylene decreases.     

New bis(azolylcarbonyl)pyridine chromium(III) complexes as initiators for ethylene polymerization

Reaction of 2,6-pyridinedicarbonyl dichloride with 3,5-dimethylpyrazole and 1H-indazole, respectively, yield the tridentate ligands 2,6-bis(3,5-dimethylpyrazol-1-ylcarbonyl)pyridine (1) and 2,6-bis(indazol-1-ylcarbonyl)pyridine (2). The molecular structure of the new compound (2) was determined by single-crystal X-ray diffraction. These ligands react with CrCl₃(THF)₃ in THF to form neutral complexes of general formula [CrCl₃{2,6-bis(azolylcarbonyl)pyridine-N,N,N}] (3, 4) which were isolated in high yield as air stable green solids and characterized by elemental analysis, magnetic moment, IR, and mass spectroscopies. Theoretical calculations predict that the thermodynamically preferred structure of the complexes is the fac configuration. After reaction with methylaluminoxane (MAO) the chromium(III) complexes are active for the polymerization of ethylene.     

Nickel(II) complexes bearing pyrazolylimine ligands: Synthesis, characterization, and catalytic properties for ethylene oligomerization

Four phenyl-substituted pyrazolylimine ligands 2-(C₃HN₂Me₂-3,5)(C(Ph)N(4-R₂C₆H₂(R₁)₂-2,6)) (L1: R₁ = ⁱPr, R₂ = H; L2: R₁ = H, R₂ = NO₂; L3: R₁ = R₂ = H; L4: R₁ = H, R₂ = OCH₃) were synthesized. The influences of steric bulk and electronic effect of pyrazolylimine ligands on the structures of their corresponding nickel complexes were investigated. Ligands with more bulky and electron withdrawing substituents on N-phenyl ring produced four-coordinate nickel complexes (2-(C₃HN₂Me₂-3,5))(C(Ph)(4-R₂C₆H₂(R₁)₂-2,6)NiBr₂ (1, R₁ = ⁱPr, R₂ = H; 2, R₁ = H, R₂ = NO₂)), whereas the ligands with less bulky and electron donating substituents on N-phenyl ring formed bis-pyrazolylimine dinickel tetrahalides (bis-2-(C₃HN₂Me₂-3,5))(C(Ph)N(4-R₂C₆H₂ (R₁)₂-2,6)Ni₂Br₄ (3, R₁ = R₂ = H; 4, R₁ = H, R₂ = OCH₃)) and six-coordinate nickel dihalides (bis-2-(C₃HN₂Me₂-3,5))(C(Ph)N(4-R₂C₆H₂(R₁)₂-2,6) NiBr₂ (5, R₁ = R₂ = H;6, R₁ = H, R₂ = OCH₃)). The solid-state structures of complexes 1, 4 and 5 have been confirmed by X-ray single-crystal analyses. Activated by methylaluminoxane (MAO), complexes 1, 2, 5 and 6 showed moderate to high activity for ethylene oligomerization, and complex 5 revealed the highest activity up to 8.96 × 10⁵ g oligomer/(mol Ni · h). The proportions of resultant oligomers were mainly C4-C8 and a little C10-C14 determined by gas chromatography/mass spectrometry.     

Nickel and palladium complexes bearing ortho-phenoxy modified anilido-imine ligands: Drastic steric effect on coordinated geometry, and catalytic property toward olefin polymerization

A series of new nickel complexes and palladium complexes bearing ortho-phenoxy modified anilido-imine ligands have been synthesized and characterized. X-ray diffraction analyses of the single crystal structures reveal that there are no direct metal-O interactions in all of the complexes. The steric hindrance of complexes has an importance influence on their coordinated geometries. The bulky complexes with 2,6-diisopropylphenyl substituent exist as a dimers with bromine-bridged structure while those with 2,6-dimethylphenyl or phenyl substituents adopt a distorted tetrahedral geometry with four nitrogen atoms of two anilido-imine ligands. The nickel complexes exhibited high activity up to 7.33 × 10⁶ g/(mol of Ni · h) and palladium complexes showed very high activity up to 2.63 × 10⁸ g/(mol of Pd · h) for norbornene polymerization with methylaluminoxane as cocatalyst. The nickel catalysts were attempted to polymerize ethylene at atmosphere pressure, however, only oligomers were produced.     

Methylene bridged binuclear bis(imino)pyridyl iron(II) complexes and their use as catalysts together with Al(i-Bu)3 for ethylene polymerization

Three novel methylene bridged binuclear iron(II) complexes: (R,R′ = i-C₃H₇ (6); R = i-C₃H₇, R′ = CH₃ (7); R,R′ = CH₃ (8))} have been synthesized. Activated by Al(i-Bu)₃, complex 6 shows very poor activity for the polymerization of ethylene at one bar ethylene pressure, whereas, 7 and 8 exhibit much higher activity than mononuclear iron catalysts {[ArNC(Me)C₅H₃N(Me)CNAr′]FeCl₂ (Ar,Ar′ = 2,6-C₆H₃-i-Pr (9); Ar = 2,6-C₆H₃-i-Pr₂, Ar′ = 2,6-C₆H₃–Me₂ (10); Ar,Ar′ = 2,6-C₆H₃–Me₂ (11))}. The molecular weight (M_w) of PE produced by 7 and 8 are in the range 13.2–46.0 × 10⁴ and much higher than those produced by mononuclear iron catalysts 9 and 10. GPC results demonstrate that 7 and 8 yield PE with a broad/bimodal molecular weight distribution (MWD). In contrast, 9 and 10 yield PE with relatively narrow and unimodal MWD (4.26 and 3.55). Elevating the temperature and Al/Fe molar ratio will narrow the MWD of PE.     

Bacterial metabolism of ethylene glycol

Metabolism of ethylene glycol as the sole source of carbon by a species of Flavobacterium was affected by the dissolved oxygen tension of the growth medium. Under strongly aerobic conditions the diol was exclusively metabolised to glycollate by an initial oxidase, subsequently metabolised to acetyl-CoA with no net change in ATP, and then oxidised to CO₂, by the tricarboxylic acid cycle yielding large amounts of reduced nicotinamide nucleotides which were used to generate a net gain in ATP by oxidative phospsorylation. Under miccroaerophilic conditions, some ethylene glycol after initial metabolism to acetyl-CoA by the oxidase-initiated pathway, was subsequently catabolised to acetyl phosphate and then acetate, yielding a net gain in ATP by substrate-level phosphorylation: additionally some diol was catabolised by an inducible diol dehydratase to acetaldehyde and subsequently reduced to ethanol as a terminal metabolite.     

Effects of 1-MCP and exogenous ethylene on fruit ripening and antioxidants in stored mango

Mango (Mangifera indica L. cv. Tainong) fruits were harvested at the green-mature stage in Hainan and air-freighted to the laboratory at Peking. The fruits were treated with either 1 μl l⁻¹ 1-MCP or 5 μl l⁻¹ ethylene for 24 h and stored at 20°C for up to 16 days. 1-MCP maintained fruit firmness, whereas exogenous ethylene decreased fruit firmness. Exogenous ethylene accelerated the increase in ethylene and 1-aminocyclopropane-1-carboxylate (ACC) oxidase, whereas 1-MCP reduced both. Exogenous ethylene stimulated and 1-MCP inhibited the production of H₂O₂ of mango fruit during storage. Ascorbic acid was maintained at a high concentration in 1-MCP-treated fruit but was low in ethylene-treated fruit. 1-MCP inhibited activities of antioxidant enzymes including catalase, superoxide dismutase and ascorbate peroxidase. These results suggest that 1-MCP could play a positive role in regulating the activated oxygen metabolism balance. Baogang Wang and Jianhui Wang contributed equally to this work.     

The stimulation of cell extension by ethylene and auxin in aquatic plants

Elongation of the shoots of three aquatic plants (Hydrocharis morsus-ranae, Regnellidium diphyllum and Ranunculus sceleratus) is stimulated by treatment with ethylene or IAA. The effects of the two hormones are additive, and experiments with an ethylene biosynthesis inhibitor and silver ions indicate that the mechanisms by which ethylene and IAA stimulate growth may be different. Hydrocharis and Ranunculus leaf discs synthesize [¹⁴C]ethylene from [¹⁴C]methionine, but no [¹⁴C]ethylene is formed by Regnellidium, suggesting the existence of an alternative pathway of ethylene biosynthesis in the fern.Abbreviations IAA Indole-3-acetic acid - RBA 1,2-amino-4-(2-aminoethoxy)-trans-3-butenoic acid     

Thiocarbonyl, thiocarbyne and thiocarbene ligands in di- and polynuclear complexes

Thiocarbonyl (CS), homologous to the ubiquitous carbonyl ligand, has interesting and unique properties as ligand. Nevertheless it did not reach the widespread use of CO in the formation of transition metal complexes. This short account, dedicated to professor R.J. Angelici, is focused on the multisite coordination of thiocarbonyl ligand in di- and poly-nuclear transition metal complexes, and to its transformation into thiocarbyne and thiocarbene ligands. These latter, in turn, can be transformed, providing access to a variety of new ligands and functionalities, which are here briefly reviewed.     

Synthesis, structure, and catalytic ethylene oligomerization of nickel(II) and cobalt(II) complexes with symmetrical and unsymmetrical 2,9-diaryl-1,10-phenanthroline ligands

A series of nickel(II) and cobalt(II) complexes, NiX₂L (X = Cl, Br; 1-6) and CoCl₂L (7-9), with 2,9-diaryl-1,10-phenanthroline ligands (L¹-L³) have been synthesized and characterized by elemental analysis, UV-Vis, IR spectroscopy, and X-ray crystal structural study (for 1, 4-7, 9). The solid-state structures of 1, 5-7 and 9 show four-coordinate, slightly flattened tetrahedral geometry at the Ni(II) or Co(II) center, while 4 is five-coordinated (square-pyramidal), containing a THF molecule as an auxiliary ligand. The title complexes (1-9) display good catalytic activities in ethylene oligomerization when activated with methylaluminoxane (MAO). While the Co(II) precatalysts produce primarily C₄ isomers, the Ni(II) complexes give ethylene dimers and trimers at normal pressure. The activities and yields of linear α-olefins increase with increasing ethylene pressure for the Ni(II) complexes, leading to more high-molar-mass products (C₈-C₁₈). Complex 6 displays the best catalytic activity among the complexes studied (up to 1518 kg/mol[Ni] h at 10 atm).     

Role of ethylene action in ethylene production and poststorage leaf senescence and survival of pelargonium cuttings

This study investigated the role of ethylene action in ethylene production and in poststorage performance of pelargonium cuttings. Cuttings of zonal pelargonium (Pelargonium x hortorum L.H. Bailey) of the cultivars Isabell and Mitzou were treated with ethylene and with the ethylene action inhibitors 1-methylcyclopropene (MCP), silver-thiosulfate (STS) and silver nitrate (SN) and were stored in the dark at different temperatures (5, 12, and 20 °C) for 48 h. Ethylene concentrations in the storage boxes were monitored and poststorage leaf senescence, survival and root formation of cuttings were determined. Applications of MCP resulted in a significant increase of ethylene evolution by cuttings of both cultivars which was more pronounced with increasing storage temperature. After 48 h of storage at 20 °C, ethylene concentrations were more than 20-fold higher for the MCP-treated cuttings as compared to those of the untreated controls. Also preharvest applications of STS and SN increased ethylene evolution by cuttings, even though these effects were less pronounced. However, application of these inhibitors caused severe poststorage leaf injury. Application of ethylene during storage had no effect on subsequent leaf damage. Leaf senescence during rooting and decay of cuttings, which raised with increasing storage temperature, could only partially been reduced by MCP. The results strongly support the conclusion, that in zonal pelargonium cuttings, ethylene production is controlled by autoinhibition, and clearly indicate, that temperature dependent processes other than ethylene action are substantially involved in storage-induced leaf senescence and decay.     

Role of polyamines and ethylene as modulators of plant senescence

Under optimal conditions of growth, senescence, a terminal phase of development, sets in after a certain physiological age. It is a dynamic and closely regulated developmental process which involves an array of changes at both physiological and biochemical levels including gene expression. A large number of biotic and abiotic factors accelerate the process. Convincing evidence suggests the involvement of polyamines (PAs) and ethylene in this process. Although the biosynthetic pathways of both PAs and ethylene are interrelated, S-adenosylmethionine (SAM) being a common precursor, their physiological functions are distinct and at times antagonistic, particularly during leaf and flower senescence and also during fruit ripening. This provides an effective means for regulation of their biosynthesis and also to understand the mechanism by which the balance between the two can be established for manipulating the senescence process. The present article deals with current advances in the knowledge of the interrelationship between ethylene and PAs during senescence which may open up new vistas of investigation for the future.     

Effects of silver on ethylene synthesis and action in cut carnations

Silver, applied as silverthiosulphate, completely blocked the ethylene surge preceding the wilting of the petals. As a consequence, vase life was extended by nearly 100%. In addition, a pretreatment with silverthiosulphate caused the flowers to become insensitive to an ethylene treatment.     

Living enantiosymmetric and enantioasymmetric polymerization of methylthiirane in homogeneous phase

The polymerization of racemic methylthiirane in homogeneous phase, initiated by bis(isopropyl-S-cysteinato) cadmium is a living process. The resulting polymers are isotactic and optically active at partial conversion. The optical purity of the residual monomer may reach 27% at half conversion. The propagation occurs mainly on one valency of Cd, however oligomers grow slowly on the second valency. The stereoregularity of the polymer chain appears only when the length of the oligomer becomes high enough, making possible a bicoordination of the Cd counterions. The stereoregularity of the polymer is characterized by the molar fraction σ of isotactic diads which varies from 0.5 for atactic chains—formed at the beginning—to about one for isotactic segments formed for longer chains. The stereospecifictity also depends on temperature of propagation and on initiator concentration. The kinetics observed (zero order in monomer and one-half in Cd) are explained by monomer coordination before insertion and dimeric association of the thiolate end groups. The enantioasymmetric process observed results from an unbalance in the number of the two different types of active sites and possibly from a difference in their reactivities. Enantioasymmetry has been found to decrease significantly when the dielectric constant ε of the medium increases.     

High catalytic activities in the norbornene polymerization with neutral palladium complexes containing N4-type tetradentate chelating ligands

Norbornene polymerization catalyzed by new Pd(II) complexes bearing N4-type tetradentate ligands obtained from the reaction between a 6-methyl-2-picolinic acid or picolinic acid and appropriate diamines has been studied. A class of new palladium complexes, [Pd(X¹X²bpb)] and [Pd(X¹X²-6-Me₂bpb)] (X¹ = Me, X² = Me (1 and 4); X¹ = H, X² = H (2 and 5); X¹ = H, X² = NO₂ (3 and 6); bpb = N,N′-(o-phenylene)bis(pyridine-2-carboxamidate); 6-Me₂bpb = N,N′-(o-phenylene)bis(6-methylpyridine-2-carbox-amidate)) were synthesized and characterized. The molecular structure of Pd complex 5 was determined by X-ray crystallography, showing distorted square planar configurations. Using modified methylaluminoxanes (MMAO) as an activator, the palladium complexes exhibited high catalytic activities for the polymerization of norbornene. The catalytic activities up to 4.0 × 10⁶ g of PNBEs/mol_Pd·h and M_w up to 8.34 × 10⁵ g/mol with PDI < 2.53 were observed. Amorphous polynorbornenes (PNBEs) were obtained with good solubility in halogenated aromatic solvents. Interestingly, the structural modification with the methyl groups of pyridyl rings and the strong electron-withdrawing substituents induced improvement in solubility, thermal stability and catalytic activity. FT-IR, ¹H, and ¹³C NMR analyses of the polymers suggest that the catalytic polymerization occurs via vinyl addition mechanism.     

A novel ethylene receptor homolog gene isolated from ethylene-insensitive flowers of gladiolus (Gladiolus grandiflora hort.)

Gladiolus is an ethylene insensitive flower whose exogenous ethylene and ethylene inhibitors have no effect on the petal senescence process. To study which processes in gladiolus are associated with changes in ethylene perception, two types of gladiolus genes, named GgERS1a and GgERS1b, respectively, homologous to the Arabidopsis ethylene receptor gene ERS1 were isolated. GgERS1a is conserved in terms of exon numbers and intron positions, whereas GgERS1b is almost same with GgERS1a except lacking 636 nucleotide encoding first and second histidine kinase (HisKA) motifs. The sequence data on full length genomic DNA indicated that both GgERS1a and b were spliced from different genomic DNA. As the result of mRNA expression study, in spite of lacking the two significant motifs, the expression of GgERS1b dramatically changed with advance in petal senescence, whereas the level of GgERS1a expressed highly and constitutively. The result suggests that both the genes possess a significant role for the subfunctionalization process to provide ethylene insensitivity in gladiolus flowers.     

An increase in ethylene sensitivity following pollination is the initial event triggering an increase in ethylene production and enhanced senescence of Phalaenopsis orchid flowers

Cut Phalaenopsis (Phalaenopsis hybrid, cv. Herbert Hager) flowers usually last about 2 weeks. Following pollination however, there is a rapid acceleration of the wilting process, which is completed within 2 days. The first event detected following pollination was an increase in ethylene sensitivity. This increased sensitivity began about 4 h after pollination and peaked 6 h later. A subsequent increase in ethylene production could only be detected 12 to 14 h after pollination. Treatment of the flowers with silver thiosulfate or 1-methylcyclopropene, both inhibitors of ethylene action, completely inhibited the pollination-induced increase in ethylene production and the enhanced senescence of the flowers. This indicates that the pollination-induced increase in ethylene production is a response to the existing ethylene. Treatment of flowers with calcium and its ionophore A23187, which increased ethylene sensitivity and protein phosphorylation, also promoted ethylene production and senescence of unpollinated flowers, EGTA, a calcium chelator, decreased the sensitivity of pollinated flowers to ethylene and delayed and decreased the pollination-induced increase in ethylene production. We suggest that the pollination-induced increase in ethylene sensitivity is the initial pollination-induced event triggering the enhancement of ethylene production, which leads to enhanced senescence of Phalaenopsis flowers.     

The ethylene/metal(0) and ethylene/metal(I) redox system: model ab initio calculations

Ab initio calculations (coupled cluster with single and double excitations; CCSD) have been used to investigate the model redox systems ethylene:M(0) (M = Li, Na, K, Rb, Cs) and ethylene:M(I) (M = Be, Mg, Ca, Sr, Ba, Zn, Cd, Hg). Within C2v symmetry, the ground (2A1) states correspond to the charge distribution given in the title. The lowest (2B2) excited states correspond, somewhat counter intuitively, to the ethylene*-/=M(II)ion pair. These trends can be rationalized on the basis of simple electrostatic and configuration-mixing arguments that lead to two simple equations for predicting the electron-transfer energies for oxidation or reduction of the ethylene. The electron-transfer energies to the 2B2 ion pairs are dominated by the electrostatic ion-pairing energies.