Synthesis and X-ray structural investigation of the hafnium(IV) complex with pivalyltrifluoroacetone

Synthesis and X-ray structural investigation of hafnium(IV) pivalyltrifluoroacetonate were carried out. Crystallographic data for C₃₂H₄₀F₁₂HfO₈ are: a = 19.913(3) Å, b = 11.8764(18) Å, c = 16.530(3) Å; β = 95.538(2)°; space group Cc, Z = 4, d _calc = 1.637 g/cm³, R = 0.035. The structure is molecular; it is composed of discrete mononuclear Hf(ptac)₄ molecules connected with each other by van der Waals interactions. The hafnium atom coordinates eight oxygen atoms belonging to four β-diketonate ligands; Hf-O distances are within the range 2.133–2.199 Å.     

Olefin polymerization by cyclopentadienyltris(dimethylamido)titanium(IV) complexes

Several titanium(IV) complexes of the type Cp′Ti(NMe₂)₃ [Cp′ = cyclopentadienyl ( 1 ), (dimethylaminoethyl)cyclopentadienyl ( 2 ), indenyl ( 3 ), and pentamethylcyclopentadienyl ( 4 )] were prepared, and their catalytic properties in the polymerization of α‐olefins were examined. Complexes 1 and 2 catalyzed the polymerization of ethylene in the presence of methylaluminoxane with a much higher activity than 3 or 4 . Complexes 3 and 4 polymerized ethylene with an activity similar to that of CpTiCl₃ ( 6 ). The preactivation of 2 , 3 , or 4 with trimethylaluminum (TMA) resulted in an increase in ethylene polymerization activities. Also, 1 and 2 were successfully used as ethylene/1‐hexene copolymerization catalysts, producing polymers with various amounts of 1‐hexene incorporation, depending on the amount of 1‐hexene in the feed mixture. Complex 1 likewise effectively polymerized styrene with a higher activity and higher syndiospecificity than the other three catalysts. Complexes 3 and 4 polymerized styrene with low syndiospecificity, whereas 2 produced only atactic polystyrene. The preactivation of 3 or 4 with TMA resulted in an increase in styrene polymerization activities and increased the syndiotacticity percentage of the polymers produced. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 39: 313–319, 2001     

Synthesis and X-ray structural study of octa-coordinated hexafluoroacetylacetone hafnium(IV) complex

Hexafluoroacetylacetonate hafnium(IV) complex has been synthesized and studied by X-ray structural analysis. Crystal data for C₃₀H₈F₃₆Hf₂O₁₄ are: a = 12.957(3) Å, b = 16.687(3) Å, c = 12.398(3) Å, β = 108.97°, space group P2₁/c, Z = 2, d _calc = 2.137 g/cm³, R = 0.047. The molecular structure is built from discrete binuclear molecules of Hf₂(OH)₂(hfac)₆ composition; the Hf...Hf distance in dimer is 3.533 Å. The structural units are connected by van der Waals interactions in the crystal.     

Synthesis and structural investigation of hafnium(IV) complexes with acetylacetone and trifluoroacetylacetone

Two volatile hafnium(IV) complexes with acetylacetone and trifluoroacetylacetone (HL) have been prepared and their structures have been studied at ?30°C. Crystal data for C₂₀H₂₈HfO₈: a = 21.5493(4) Å, b = 8.36720(10) Å, c = 13.9905(3) Å; β = 116.5550(10)°, space group C2/c, Z = 4, d _calc = 1.692 g/cm³, R = 0.015. Crystal data for C₂₀H₁₆F₁₂HfO₈: a = 8.1039(12) Å, b = 11.4499(14) Å, c = 15.790(2) Å; α = 99.341(4)°, β = 103.175(4)°, γ = 108.185(4)°, space group P?1, Z = 2, d _calc = 2.003 g/cm³, R = 0.074. Both structures are molecular and comprise isolated complex molecules HfL₄. The hafnium atom is coordinated with eight oxygen atoms of four β-diketonate ligands, Hf-O distances varying from 2.153 Å to 2.191 Å. The molecules make van der Waals contacts in the structures.     

Vinyl polymerization of acrylonitrile by the Ce(IV)-glucose redox system

Aqueous polymerization of acrylonitrile (M) initiated by the Ce(IV)-glucose (R) redox system has been studied under nitrogen in the temperature range of 30–40 °C. The rate of polymerization (Rp) is proportional to [M]², [R] and inversely proportional to [Ce(IV)]. The rate of ceric ion disappearance is proportional to [R] and [Ce(IV)]. The end group in the polymer is characterised by IR spectra. A suitable kinetic scheme has been proposed and explained in the light of these experimental findings.     

X-ray diffraction structural investigation of two hafnium(IV) complexes with dipivaloylmethane

Synthesis and single crystal X-ray diffraction study of hafnium(IV) dipivaloylmethanate Hf(dpm)₄ and chloro-tris-(dipivaloylmethanato)hafnium(IV) Hf(dpm)₃Cl have been carried out. Crystal data: a = 22.6606(5) Å, b = 11.3990(4) Å, c = 19.8513(7) Å, β = 106.458(1)°, Pc, Z = 4, d _calc = 1.231 g/cm³, R = 0.075 for C₄₄H₇₆HfO₈; a = 10.6376(13) Å, b = 10.6701(10) Å, c = 19.4400(22) Å, α = 74.970(3)°, β = 75.672(3)°, γ = 61.725(2)°, P-1, Z = 2, d _calc = 1.366 g/cm³, R = 0.031 for C₃₃H₅₇ClHfO₆. The structures are molecular and are built from discrete mononuclear complexes joined by van der Waals interactions. Disordering of carbon atoms preserving at low temperature is observed for the compound Hf(dpm)₄. It has been found out that the structure contains two crystallographically unique complexes of hafnium(IV) with central atoms coordinated with eight oxygen atoms of four dipivaloylmethane ligands, bond lengths Hf-O fall within 2.084–2.222 Å, the distances Hf...Hf between the molecules are 10.07–13.87 Å. In Hf(dpm)₃Cl the hafnium atom is seven-coordinated with six oxygen atoms of three β-diketonate ligands and one chlorine atom, the distances Hf-O fall within 2.087–2.179 Å, the lengths of the bond Hf-Cl for two crystallographically independent molecules Hf(1) and Hf(2) are 2.466 Å and 2.442 Å, respectively.     

Kinetic studies on the reduction of a nickel (III) oxime-imine complex by sulphur(IV) and selenium(IV)

The kinetics of the reduction of [Ni^III(L¹)]²⁺ (where HL¹ = 15-amino-3-methyl-4,7,10,13-tetraazapentadec-3-en-2-one oxime) by sulphur(IV) and selenium (IV) over the regions pH 2.50–8.02 and 2.01–4.00 respectively have been investigated at 30°C. Attempts were made to evaluate the reactivity of all the reacting species, of sulphur(IV) and selenium(IV) by considering suitable pH ranges. The oxidation of SC₂·H₂O and HSO ₃ ⁻ is proposed to proceed through the formation of a hydrogen-bonded adduct. The reaction with SO ₃ ²⁻ seems to follow a direct outer-sphere route which is well supported by Marcus crossrelation calculation. The oxidation of HSeO _l3 ⁻ is ≈ 10³ times slower than that of H₂SeO₃. The kinetic data indicate that the oxidation of sulphur(IV) by [Ni^IIIL¹)]²⁺ is much more favourable as compared to the corresponding oxidation of selenium(IV).     

(Salen)Ti(IV) complex catalyzed asymmetric ring-opening of epoxides using dithiophosphorus acid as the nucleophile

A series of chiral bis-Schiff bases were synthesized starting from (1R,2R)-(+)-diaminocyclohexane, (+)-cis-1,2,2-trimethyl-1,3-diaminocyclopentane, (R)-2,2^′-diamino-1,1^′-binaphthalene, and (1S,2S)-diphenyl-1,2-ethanediamine. The enantioselective ring-opening of meso epoxides with dithiophosphorus acids catalyzed by a (salen)Ti(IV) complex formed in situ upon the treatment of Ti(OPr-i)₄ and the aforementioned chiral Schiff base was realized. The resulting products were obtained with low to good enantioselectivities (up to 73% ee). The (salen)Ti(IV) complex containing the backbone of 1,2-diaminocyclohexane exhibited the best enantioselectivity. The substituents in dithiophosphorus acids and those on the salen aromatic ring have a significant influence on the reaction. Moderate enantioselectivity were obtained for the (salen)Ti(IV) complex catalyzed ring-opening of racemic monosubstituted epoxides. High regioselectivity was observed for the alkyl substituted epoxides, whereas poor regioselectivity was obtained for the aryl substituted ones.     

Highly enantioselective hetero-Diels-Alder reaction between trans-1-methoxy-2-methyl-3-trimethylsiloxybuta-1,3-diene and aldehydes catalyzed by (R)-BINOL-Ti(IV) complex

An efficient enantioselective approach to 2,5-disubstituted dihydropyrones was developed. Some easily accessible inexpensive diol ligand metal complexes were employed, and [(R)-BINOL]₂-Ti(OiPr)₄ complex was found to be the most effective catalyst (up to 99% yield and 99% ee in the presence of 5 mol% catalyst) for the hetero-Diels-Alder reaction between trans-1-methoxy-2-methyl-3-trimethylsiloxybuta-1,3-diene (1) and aldehydes. The potential and generality of this catalyst were evaluated by a variety of aldehydes including aromatic, heteroaromatic, α,β-unsaturated and aliphatic aldehydes. Based on the isolated intermediate from the reaction of benzaldehyde being confirmed by ¹H, ¹³C NMR and HRMS data, the mechanism was proposed as a Mukaiyama aldol pathway.     

Melt polymerization of hexachlorocyclotriphosphazene in the presence of organotin (IV) compounds

The melt polymerization of hexachlorocyclotriphosphazene to poly(dichlorophosphazene) has been studied in the presence of organotin (IV) compounds. Unlike other typical Lewis acids, diethyltin (IV) chloride, Et₂SnCl₂, has shown to be an inhibitor for the thermal polymerization of the trimer, raising the activation energy to 70 kcal/mol. Diethyltin(IV) chloride remarkably delayed the rate of polymerization and efficiently inhibited the crosslinking reaction, thus leading to an improvement in the yield of the linear polymer without sacrifice to its molecular weight. © 1993 John Wiley & Sons, Inc.     

Olefin polymerization catalyzed by amide vanadium(IV) complexes: The stereo‐ and regiochemistry of propylene insertion

The reaction of VCl₃(THF)₃ with 1 equiv of the lithium salt of ligand ArNH(Me₂SiCH₂CH₂SiMe₂)NHAr or ArNH(SiMe₃) (Ar = 2,6‐Me₂C₆H₃) afforded the corresponding V(IV) amide complexes, [1,2‐CH₂CH₂(Me₂SiNAr)₂]VCl₂ ( 3 ) and (Me₃SiNAr)₂VCl₂ ( 4 ). The activation of 3 and 4 with the alkyl aluminum compound Al₂Et₃Cl₃ or AlEt₂Cl produced active ethylene polymerization catalysts exhibiting productivity values among the highest reported for vanadium amide based catalysts. Moreover, syndiotactic specific propylene polymerization was successfully conducted at ?40 °C in the presence of 3 /Al₂Et₃Cl₃ and 4 /Al₂Et₃Cl₃. Syndiotactic polypropylenes with moderate stereoregularity ([rr] = 0.66) and a concentration of regioirregular propylene of 6.9 mol % were obtained. Monomodal molecular weight distributions and polydispersity indices lower than 2 were observed in the polymerization runs carried out in heptane solutions. Thus, ethylene–propylene copolymers with propylene concentrations up to 45 mol % were synthesized and characterized by ¹³C NMR and thermal analysis. Good alternation and random distribution of the two monomers were actually obtained. Samples with elevated concentrations of propylene were completely amorphous, with a glass‐transition temperature of ?50 °C. The properties and structure of the copolymers produced with amide vanadium catalysts 3 and 4 were similar to those reported for ethylene–propylenes produced with industrial vanadium‐based catalysts, suggesting the presence of the same active catalyst species. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 3279–3289, 2006     

Effect of alkylaluminum on ethylene polymerization catalyzed by 2,6‐bis(imino)pyridyl complexes of Fe(II)

In the presence of tetraethylaluminoxane (TEAO), iron complexes were used to catalyze ethylene polymerizations with extremely high activities and generally produced polyethylene with a bimodal molecular weight distribution (MWD). This bimodal MWD of polyethylene was mainly derived from residual triethylaluminum in TEAO and was produced through a mechanism of chain transfer to aluminum. Ethylaluminoxane and tetraisobutylaluminoxane also were used to polymerize ethylene with high activities in the presence of iron complexes, and only polyethylene with a unimodal MWD was produced. The ratio of the rate constant of chain transfer to aluminum (k_trA) to the rate constant of chain propagation (k_p) was determined to be 0.12 for {[ArN?C(Me)]₂C₅H₃N}FeCl₂ when Ar was 2,6‐diisopropylphenyl ( 1 ) and 2.48 for {[ArN?C(Me)]₂C₅H₃N}FeCl₂ when Ar was 2,6‐dimethylphenyl ( 2 ); these values are far larger than those for metallocene‐based catalysts. This explains why an iron complex usually produces polyethylene with a broader MWD than metallocene‐based catalysts. Additionally, it can be concluded from the great difference between 1 and 2 with respect to k_trA/k_p that an iron complex with less congested aryl substituents is subjected to chain transfer to aluminum. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 1599–1606, 2005     

Hafnium (IV) bis(perfluorooctanesulfonyl)imide complex catalyzed synthesis of polyhydroquinoline derivatives via unsymmetrical Hantzsch reaction in fluorous medium

A facile and efficient synthesis of polyhydroquinoline derivatives was reported via four-component condensation reaction of aldehydes, dimedone, active methylene compounds, and ammonium acetate in the presence of Hf(NPf₂)₄ in C₁₀F₁₈ at 60 °C. The method offers several advantages including high yields, short reaction time, simple work-up procedure and catalyst reusability.     

Human serum albumin binding studies of a new platinum(IV) complex containing the drug pregabalin: experimental and computational methods

A new platinum(IV) complex, [Pt(en)(Cl)₂(Pregabalin)₂], containing the drug pregabalin was synthesized and characterized by elemental analysis, FT-IR, ¹H NMR, mass spectrum, thermogravimetric analysis (TGA), molecular docking and RHF/PM6 method. Also, the interaction of Pt(IV) complex with human serum albumin (HSA) was studied by using UV–vis, fluorometric, circular dichroism (CD) spectroscopies and molecular docking techniques. The results demonstrated that the binding of the complex to HSA caused strong fluorescence quenching of HSA through static quenching mechanism. Hydrogen bonds and van der Waals contacts are the major forces in the stability of protein-Pt(IV) complex and the process of the binding of complex with HSA was enthalpy driven (ΔH = –105.8?kJ·mol^?1). The results of CD and UV–vis spectroscopy indicated that the binding of the complex to HSA caused conformational changes in HSA. In addition, the study of molecular docking and RHF/PM6 method confirm the experimental results with respect to the mechanism of binding.     

Vinyl polymerization of norbornene catalyzed by 2‐Py‐amidine Ni(II) complex/methylaluminoxane systems

Two 2‐Py‐amidine ligands (2‐Py―NH―C(Ph)═N―Ar, Ar = 2,6‐Me₂C₆H₃ and 2,6‐ⁱPr₂C₆H₃) and the corresponding Ni(II) complexes ( 1 and 2 ) were synthesized and characterized using elemental analysis and FT‐IR, UV–visible, ¹H NMR and ¹³C NMR spectroscopies. X‐ray crystal structures indicate that the chelate ring conformation of the less bulky complex 1 is relatively planar compared with that of the bulky complex 2 . Paramagnetic ¹H NMR and ¹³C NMR studies show that, in solution, the time‐average structures of complexes 1 and 2 have mirror symmetry. Both complexes 1 and 2 were used as catalyst precursors for norbornene polymerization with methylaluminoxane as a co‐catalyst. The effects of Al/Ni ratio, temperature and structure of precursors on the catalytic performance were investigated. Copyright © 2014 John Wiley & Sons, Ltd.     

Electron‐pair‐donor reaction order in the cationic polymerization of isobutylene coinitiated by AlCl3

The cationic polymerizations of isobutylene (IB) initiated by an H₂O/AlCl₃ system were carried out in a mixture of n‐hexane/methylene dichloride of 60/40 v/v at ?50 °C in the presence of various external electron pair donors (EDs), including methyl benzoate (MB), ethyl benzoate (EB), and methyl acrylate (MA). The effects of the concentrations of EDs ([ED]) and polymerization time on monomer conversion and kinetics of IB polymerization were also investigated. The complexes between AlCl₃ and the esters were soluble in the polymerization system at ?50 °C. The polymers with high molecular weights and relatively narrow molecular weight distributions were obtained in the presence of the EDs. The rate of polymerization decreased with increasing [ED]. The kinetic orders of the EDs were remarkably dependent on their chemical structure, steric hindrance from moieties, and concentration in the polymerization system. The reaction order of MB was determined to be around ?1.3 when [MB] ≤ 0.40 mmol/L, whereas it was ?12.9 when [MB] ≥ 0.40 mmol/L. The reaction order of EB was determined to be ?1.36 when [EB] ≤ 0.41 mmol/L, whereas it was ?3.36 when [EB] ≥ 0.41 mmol/L. The reaction order of MA was determined to be ?1.85 when [MA] ≤ 0.48 mmol/L, whereas it was ?16.7 when [MA] ≥ 0.48 mmol/L. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 3053–3061, 2007     

Partitioning effect of nitrogen catalyst into polymerizing particles on dispersion reversible chain transfer catalyzed polymerization (dispersion RTCP) of methyl methacrylate in supercritical carbon dioxide and organic solvents

Reversible chain transfer catalyzed polymerization (RTCP) in dispersion polymerization system (dispersion RTCP) of methyl methacrylate (MMA) was performed with N‐iodosuccimide (NIS) as a nitrogen catalyst in supercritical carbon dioxide (scCO₂). The solubility of NIS in scCO₂ can be controlled by tuning the pressure, and this led to promote NIS partitioning into polymerizing particles. As a result, the molecular weight distribution control was successfully improved by decreasing the NIS solubility in the medium by tuning the scCO₂ at a low pressure of 20 MPa. On the other hand, at the same NIS concentration, a solution RTCP of MMA in toluene as a homogeneous polymerization system did not proceed with a controlled/living manner. The importance of NIS partitioning into the polymerizing particles was also confirmed in hexane as well as scCO₂ medium. From these results, it was clarified that the NIS catalyst partitioning into the polymerizing particles as main polymerization loci is a key factor to control the molecular weight distribution in the dispersion RTCP of MMA in scCO₂. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2019 , 57, 613–620     

A simulation model for long-chain branching in vinyl acetate polymerization: 1. Batch polymerization

A new simulation model for the kinetics of long-chain branching formed via chain transfer to polymer and terminal double-bond polymerization is proposed. This model is based on the branching density distribution of the primary polymer molecules. The theory of branching density distribution is that each primary polymer molecule experiences a different history of branching and provides information on how each primary polymer molecule is connected with other chains that are formed at different conversions, therefore making possible a detailed analysis on the kinetics of the branched structure formation. This model is solved by applying the Monte Carlo method and a computer-generated simulated algorithm is proposed. The present model is applied to a batch polymerization of vinyl acetate, and various interesting structural changes occurring during polymerization (i.e., molecular weight distribution, distribution of branch points, and branching density of the largest polymer molecule) are calculated. The present method gives a direct solution for the Bethe lattice formed under nonequilibrium conditions; therefore, it can be used to examine earlier theories of the branched structure formation. It was found that the method of moments that has been applied successfully to predict various average properties would be considered a good approximation at least for the calculation of not greater than the second-order moment in a batch polymerization. © 1994 John Wiley & Sons, Inc.     

Effect of acrylamide concentration on the kinetics of oxidation of tartaric acid by cerium(IV) in sulfuric-perchloric acid media

The kinetics of oxidation of tartaric acid by Ce(IV) in the absence and presence of acrylamide has been investigated spectrophotometrically in aqueous H₂SO₄–HClO₄ media at a constant ionic strength 2.0M and 25°C. Oxidation of tartaric acid in both cases was first order with respect to Ce(IV). Kinetic data showed that the reaction involves the formation of an unstable complex and an intermediate free radical. The activation parameters were calculated to be E _a =91.3±0.4 kJ-mol^–1, S=20.2±1.0 J-mol^–1-K^–1, H=88.8±0.4 kJ-mol^–1. A polymerization mechanism is discussed.     

Titanium (IV) chloride complexes with salen ligands supported on magnesium carrier: Synthesis and use in ethylene polymerization

The magnesium support with the formula MgCl₂(THF)_0.32(Et₂AlCl)_0.36 was used for immobilization of salen complexes of titanium [Ti(salen)Cl₂, Ti(salen(OMe)₂)Cl₂]. The effects of the catalyst composition (i.e. type of titanium complex and type of activator), polymerization temperature, polymerization time, and the effect of comonomer (1‐octene) on the activity of the obtained supported catalysts, on the polymer characteristics (molecular weight, molecular weight distribution, melting point), and on the polymer morphology were studied. The findings were compared to those obtained for corresponding unsupported systems. Catalysts immobilization results in considerable changes in catalysts activity and in properties of resultant polymers. The studied supported catalysts are highly active in ethylene polymerization, their activity increases with increasing temperature and lasts at least 2 hours. Their copolymerizing ability towards 1‐octene is rather low. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 6693–6703, 2009