Isopropylmaltol-Ti Synthesis and its Use as a Catalyst for Olefin Polymerization

Este artigo mostra a síntese e caracterização do ligante isopropilmaltol obtido a partir do furfural e também um novo complexo diclorobis-(3-hidroxi-2-isopropil-4-pirona)titânio(IV) com estrutura cis. Dados de ressonância magnética nuclear de H, análise elementar e UV-Vis confirmam a formação do complexo. Este complexo foi estudado em reações de polimerização de etileno utilizando metilaluminoxano (MAO) como co-catalisador. A atividade catalítica deste complexo é baixa, entretanto foram obtidos polietilenos com alto peso molecular que são interessantes para diferentes aplicações.


Introduction
Hydroxypyrones are versatile compounds that have been used in food, pharmaceutical, biological and medical applications.These compounds are six-membered heterocyclics with a hydroxyl group bonded in the ortho position, in relation to ketone group, providing two donor oxygens (Figure 1).Several studies have shown that these compounds can form stable complexes with various metal ions to be used in the body as a way to combat and prevent many diseases.Divalent ions such as Zn 2+ , Ru 2+ , [VO] 2+ , [MoO 2 ] 2+ and trivalent cations (Fe 3+ , Al 3+ , Ga 3+ ) may be used in the synthesis of thermodynamically stable complexes with hydroxypyrone ligands. 1 Some studies with the 2-methyl-3-hydroxypyrone complex (maltolato) are reported in the literature for biomedical purposes such as maltol-oxovanadium(III), 2 maltol-ruthenium, 3 maltolmolybdenum(IV), 4 among others.
In parallel to the studies focused on medicine, the coordination chemistry of organometallic complexes containing hydroxypyrone ligands with transition metals have also been investigated.These ligands can also form ML 2 X 2 complexes, where X = halogen and M = metal, with group IV transition metals, such as Ti and Zr. 5 In 2001, Sobota et al. 6 synthesized a new non-metallocene Ti-complex based on bidentate maltol ligand, dichlorobis-(3-hydroxy-2-methyl-4-pyrone)titanium(IV).This complex was tested in homogeneous olefin polymerization.The complex was obtained in octahedral coordination where the Ti metal is surrounded by four oxygen atoms from two ligand molecules and two chlorines in the cis position.This complex was active in ethylene polymerization reactions, suggesting that maltolato ligands could represent a good alternative to the cyclopentadienyl ring of metallocene catalysts. 6ubsequently, our research group synthesized the analogous zirconium complex, dichlorobis-(3-hydroxy-2methyl-4-pyrone)zirconium(IV), and also dichlorobis-(2hydroxy-1,4-naphthoquinone)zirconium(IV).The catalytic activity of these complexes was compared in homogeneous and heterogeneous ethylene polymerization.The complex from the methylpyrone ligand was more active than the one with naphthoquinone ligand, in both homogeneous and heterogeneous conditions, when supported on SiO 2 and SiO 2 /methylaluminoxane (MAO) modified silica. 7Later, we studied the catalytic activity of the dichlorobis-(3-hydroxy-2methyl-4-pyrone)titanium(IV) complex in different supports, such as SiO 2 , SiO 2 /MAO, MCM-41, Al 2 O 3 , ZrO 2 and MgO.Results of ethylene polymerization reactions showed that most of these heterogeneous systems were more active than the homogeneous complex, and the complexes supported in SiO 2 948, MCM-41 and alumina showed the highest catalytic activities. 8Basso noted that the catalytic activity of titanium and zirconium complexes from methylpyrone ligand depends on the metal and the Al/M ratio.In this study, the Zr complex was more active than the analogous Ti complex in the ethylene polymerization using Al/M = 2500, while the Ti complex was more active at lower Al/M ratios. 9ther analogous bidentate zirconium complex derived from 3-hydroxy-2-ethyl-4-pyrone (ethylmaltol) has been proposed for the ethylene polymerization.The polyethylene obtained showed, depending on the synthesis conditions, high molecular weight polyethylene.These studies have shown that the exchange of the methyl substituent by an ethyl in the pyrone ligand exerts a great influence on the catalytic activity, regardless of the metal used, resulting in higher catalytic activities. 10s part of our studies directed towards organometallics complex synthesis, this paper reports a new synthesis of a maltolato compound, isopropylmaltol, its complexation with TiCl 4 and the use as catalyst in the ethylene polymerization.

For synthesis of the complex
All experiments were performed under argon atmosphere using the schlenk technique.Titanium tetrachloride (Merck) was used without purification.Dichloromethane was dried by refluxing over phosphorus pentoxide.Ethyl ether, hexane and toluene (Nuclear) were dried by refluxing over metallic sodium, under nitrogen, and using benzophenone as indicator.MAO (Witco, 10% m/m Al in toluene solution) was employed as received.

2-isopropyl-6-methoxy-3-pyrone (4)
To a solution of compound 3 (340.4mg, 2.0 mmol) in ethyl ether (4 mL) at 0 °C was added 5% aqueous Na 2 CO 3 (1 mL) and 15% H 2 O 2 (0.9 mL).The resulting mixture was stirred at 0 °C for 2.5 h.Next, 30% H 2 O 2 (0.45 mL) was additionally added and the reaction remained at the same temperature.After 2 h, 30% H 2 O 2 (0.45 mL) was further added and the reaction mixture was stirred at 0 °C for an additional 2 h.Then saturated NaCl (4 mL) was added and the aqueous layer was extracted with Et 2 O (4 × 5 mL).The combined organic layers were dried over anhydrous Na 2 SO 4 , filtered and evaporated under reduced pressure.The resulting oil was directly used for the next step without further purification.

Polymerization reactions
Ethylene polymerizations were performed in a PARR 4843 reactor with 100 mL capacity.Into the reactor was added 30 mL of toluene, methylaluminoxane as co-catalyst (co-catalyst/catalyst ratio: 1500 and 2500) and 1 µmol of catalyst.The polymerization reactions were performed at 2.8, 4 and 6 bar of ethylene at 40 °C during 30 min.The polymerization temperature of 40 °C was chosen because previous works 9 showed that similar Ti complexes have the highest catalytic activities at this temperature.Acidified ethanol with chloride acid was used to quench the process, Vol. 25, No. 12, 2014 and reaction products were separated by filtration, washed with ethanol and acetone, and finally dried.

Ligand and complex characterization
The 1 H NMR spectra of the organic compounds of ligand synthesis and the complex were recorded on a Varian Inova 300 Spectrometer, using CDCl 3 as solvent.The UV-Vis absorption spectra of the isopropylmaltol ligand and the complex were recorded on a Varian Cary 100 spectrophotometer with quartz cells of 1 cm path length at room temperature.The analyses were performed using toluene as solvent.Elemental analysis (C, H) of the complex was performed in a 240 PERKIN-ELMER.

Theoretical calculations
The geometries and energies of all possible isomeric species of the complex and their conformers, were obtained by full unconstrained optimizations performed at density functional theory (DFT) level using the B3LYP hybrid functional obtained by the three parameter fit of the exchange-correlation potential suggested by Becke, 11 and the gradient corrected correlation functional of Lee, Yang and Parr. 12The polarized Dunning-Huzinaga DZ basis setwas used for the hydrogen, carbon, oxygen and chlorine atoms. 13,14For the titanium atom, the inner shell electrons were represented by the Los Alamos effective core potential (LANL2) of Hay and Wadt, 15,16 and the valence electrons were explicitly included using the associated DZ basis set.All calculations were performed with the Gaussian 03 program using standard procedures and parameters. 17

Polymer characterization
The melting points (T m ) and crystallinities (X c ) of the polymers were determined using a differential scanning calorimeter (DSC) Q20 TA Instruments with heating of 20-160 °C and heating rate of 10 °C min −1 with 50 mL min −1 of N 2 flow rate.The heating cycle was performed twice, but only the results of the last scan were considered.The molar masses and molar mass distributions were measured by gel permeation chromatography (GPC) PL 220 Polymer with RI and VI detector equipped with Water columns (HT6, HT5, HT4, HT3).The mobile phase used was trichlorobenzene (TCB) with 0.1% of butyl hydroxytoluene (BHT) at 150 °C at a flow rate of 1 mL min −1 .

Results and Discussion
The synthesis of isopropylmaltol was previously reported by Thompson et al., that obtained 15% yield starting from furanaldehyde. 18For this synthesis, molecular bromine was used in the conversion of 1-(2-furyl)-2methyl-1-propanol in isopropylmaltol, this reaction is also known as Achmatowicz rearrangement which may alternatively employ different oxidants and reaction conditions. 19Several attempts made in our laboratory to obtain the isopropylmaltol using the procedure described by Thompson failed.Thus, we have developed a synthetic method for obtaining isopropylmaltol as shown in Scheme 1.The transformation of 1-(2-furyl)-2-methyl-1-propanol to pyranone was performed in presence of m-CPBA.Column chromatography provided the compound in 60% yield.Subsequent allylic alcohols were O-methylated with MeI and Ag 2 O followed by epoxidation with 30% H 2 O 2 .The resulting oil was directly used for the next step without further purification.
Next, isopropylmaltol was conveniently obtained by heating the intermediate in the presence of sulfuric acid and dioxane, as outlined by Torii protocol, to afford the product with 55% of yield over two steps. 20Figure 2 shows 1 H NMR and APT 13 C NMR of the isopropylmaltol ligand.
The complex isopropylmaltol-Ti was synthesized from the isopropylmaltol ligand and TiCl 4 (Figure 3).The complexation reaction in dichloromethane at room temperature is extremely rapid.When adding the metal salt to the ligand, an orange color typical of titanium complex, is observed immediately.The complex formation can be proved comparing the NMR spectra of the ligand and the complex (Figures 2 and 4).
The hydroxyl group resonance signal at 7.00 ppm in the 1 H NMR spectrum of the isopropylmaltol ligand disappears in the complex spectrum, indicating deprotonation of the ligand and insertion of the titanium metal.Furthermore, it was visualized the formation of two isomers of the isopropylmaltol-Ti complex, A and B, which, through the integrals of the respective protons, it was observed that A Scheme 1. Steps of the synthesis of isopropylmaltol ligand.
isomer is found in greatest proportion (90%) in relation to B isomer (10%).The coordination of the oxygen atoms ligand with the metal also deshielded proton H5 and H6 from the major isomer (A) showing a donation of electron density to the metal.The formation of isomers in the complex was studied by theoretical calculations using the DFT.Table 1 shows the relative energies and percentage population of each possible isomer.The calculations indicate the existence of five possible different stable geometric isomers resulting from different arrangements of chlorine atoms in the titanium complex.Two isomers can have trans configuration and three cis configuration.Each isomer can also have four conformers and, in the higher symmetry isomers, equivalent conformers can also occur.Table 1 shows the molecular and relative energies of the lowest energy configuration of the system, as well as a percentage population estimate for each isomeric species discriminated in its possible conformations considering 298 K temperature.shows calculated structure cis2 and cis3 isomer in its lowest energy conformation that it is responsible for the largest fraction population percentage.From all the isomeric structures calculated, only two isomers with cis configuration, cis2 and cis3, have energies related to lower energy configuration lower than 4 kcal mol −1 and population percentage over 0.01%.The isomer which is in greater proportion is cis3 with 96.72% of the total population, followed by cis2 with 3.28% percentage share.The other isomers have insignificant percentage and should not be observed in the NMR spectrum.In fact, Figure 5 shows the presence of only two isomers, A and B, in different proportions.Probably, the major isomer, A, is the cis3 structure and the lower abundance, B, is the cis2.The production of compounds with chlorides in the cis position is desirable because only the cis structures are active in the olefin polymerization. 9Therefore, the two isomers obtained in this complex synthesis can be considered active in ethylene polymerization reactions.
The isopropylmaltol-Ti complex was also characterized by UV-Vis spectroscopy.The ligand absorption due maltolate group appears as narrow peak at λ max = 378 nm.The spectra of the titanium complex presents an absorption at λ max = 355 nm, as a broad band attributable to metal-toligand charge transfer transitions between titanium and isopropylmaltol ligand suggesting the coordination of the maltolate ligand. 4

Ethylene polymerization
The results of the ethylene polymerization reactions with the synthesized isopropylmaltol-Ti complex are shown in Table 2.It can be observed that, despite the low catalytic activity presented by the complex, it was possible to polymerize ethylene in the presence of MAO as cocatalyst, at ratios of Al/Ti 1500 and 2500.Comparing these results with those presented by Basso, 9 with the analogue complex dichlorobis(3-hydroxy-2-methylpyrone)titanium(IV) under the same reaction conditions (Al/Ti = 2500, T = 40 °C) is observed that both complexes have very similar catalytic activities.
The polymers obtained were high density polyethylene as it can be seen by the melting and crystallization temperatures and the degree of crystallization.The materials have all very high molecular weight with distribution between 3.8-5.3.This high polydispersity could be attributed to the presence of more than one isomer active in the ethylene polymerization.However, a more detailed work must be done in the future in order to understand better the catalytic system.Those results show the potentiality of these products that can be interesting for production of high density polyethylene (HDPE) with good processability.

Conclusions
A new methodology was described for isopropylmaltol synthesis.The Ti-complex was synthesized using metal salt, TiCl 4 , and it was obtained as two isomers with the chlorine atoms in cis position, what it was proven by NMR and DFT theory.The isopropylmaltol-Ti complex was used as catalyst on ethylene polymerization.Different experimental conditions were tested such as ethylene pressures and cocatalyst (methylalumonixane)/catalyst ratios but the catalytic activities were low, however, similar to analogous complexes described in the literature.Moreover, it was obtained high density polyethylenes with very high molecular weights, which could be interesting for several uses (ex.fibers, etc.).

Table 1 .
Molecular energies and energies relating to more stable configuration of isopropylmaltol-Ti complex a at room temperature, 298 K.