J. Chem. Sci. (2018) 130:116 © Indian Academy of Scienceshttps://doi.org/10.1007/s12039-018-1515-3

REGULAR ARTICLE

Geometric isomerism effect on catalytic activities ofbis(oxalato)diaquochromates(III) for 2-chloroallyl alcohololigomerization

JOANNA DRZEZDZON∗ , LECH CHMURZYNSKI and DAGMARA JACEWICZFaculty of Chemistry, University of Gdansk, Wita Stwosza 63, 80-308 Gdansk, PolandE-mail: joanna.drzezdzon@ug.edu.pl

MS received 17 May 2018; revised 29 June 2018; accepted 30 June 2018; published online 10 August 2018

Abstract. The cis- and trans-potassium bis(oxalato)diaquochromates(III) have been studied towards theircatalytic activity for the 2-chloroallyl alcohol oligomerization. The geometric isomerism effect on theoligomerization products under mild reaction conditions has been investigated. The molecular masses andtactic structures of obtained poly(2-chloroallyl alcohols) have been compared and analyzed. The mechanismsof oligomerization processes have been proposed. It has been proven that two studied complexes – cis-K[Cr(C2O4)2(OH2)2] and trans-K[Cr(C2O4)2(OH2)2]·3H2O are highly active catalysts for the oligomerizationof the beta-olefin derivative.

Keywords. Chromium(III) complexes; catalytic activities; polymerization; oxalate ion.

1. Introduction

The products of olefin polymerization are used inindustry for the production of various plastics, e.g.containers, household articles, orthopedic prostheses,laboratory and medical devices.1–3 The chromium(III)complexes are commonly used as catalysts to the olefinand its derivatives polymerization.4,5 The catalytic activ-ity of the non-metallocene chromium(III) complexes forthe olefin polymerization was discovered in the first halfof the 1990s.4 This kind of chromium(III) complexesis the focus for the olefin polymerization researchersbecause these catalysts are very active and stable in theindustrial production of oligomers.6,7

Poly(vinyl alcohol) (PVA) and its derivatives haveorthopedic applications.8 PVA is used as a synthetic car-tilage.8 Moreover, PVA and its derivatives are used in theworld production of adhesives, hydrogels and stabilizersfor emulsions.9,10 Besides PVA, polypropylene (PP) isthe material commonly used in the polymer industries,11

such as, HVDC cable insulation.12 The derivative ofvinyl alcohol and propylene, i.e. 2-chloroallyl alcoholis a monomer used for the production of polymerichydroxy compounds. The copolymers of 2-chloroallylalcohol is used for the production of artificial glassesand elastomers.13–15 In the literature, there is little

*For correspondence

information regarding the chromium(III) complexesused in the polymerization of 2-chloroallyl alcohol (2-chloro-2-propen-1-ol). Recently, the catalytic prop-erties of the dipicolinate chromium(III) complexeswith 2,2′-bipyridine and 4,4′-dimetoxy-2,2′-bipyridinefor the preparation of poly(2-chloroallyl alcohol) atthe atmospheric pressure and room temperature weredescribed.7 These dicarboxylate chromium(III) com-plexes exhibit very high catalytic activities, i.e. 2609.86and 2254.57 g·mmol−1 ·h−1 for [Cr(dipic)2][Cr(bipy)(dipic)H2O]·2H2O and [Cr(dipic)2]Hdmbipy·2.5H2O,respectively.7

The aim of the our studies described in this report isfirst of all to investigate the catalytic activities of dicar-boxylate chromium(III) complexes cis-K[Cr(C2O4)2

(OH2)2] and trans-K[Cr(C2O4)2(OH2)2]·3H2O for the2-chloro-2-propen-1-ol oligomerization. Secondly, thegeometric isomerism effect in the bis(oxalato)diaquoch-romates(III) has been analyzed for the values of thecatalytic activity of the synthesized complexes and theidentity of the oligomerization products.

2. Experimental

2.1 Materials

All the reagents used in this work, potassium dichromate (99%purity), oxalic acid (99% purity), 2-chloro-2-propen-1-ol

1

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(technical grade, 90% purity), modified methylaluminoxane(MMAO-12, 7 wt% aluminum in toluene), toluene (99%purity), were purchased from Sigma-Aldrich.

2.2 Synthesis of cis-K[Cr(C2O4)2(OH2)2] andtrans-K[Cr(C2O4)2(OH2)2]·3H2O

The cis-K[Cr(C2O4)2(OH2)2] and trans-K[Cr(C2O4)2(OH2)2]·3H2O complexes were synthesized according to themethod of Werner and his coworkers.16,17 The compositionand purity of the synthesized bis(oxalato)diaquochromates(III) complexes were confirmed by elemental analysis of car-bon and hydrogen using the CARLO ERBA type CHNS–O1108 analyzer. Anal. Calcd. for trans-K[Cr(C2O4)2(OH2)2]·3H2O: C, 13.66; H, 2.85%; and for cis-K[Cr(C2O4)2(OH2)2]C, 15.91; H, 1.95%. Found for trans-K[Cr(C2O4)2(OH2)2]·3H2O: C, 13.41; H, 2.82%, and for cis-K[Cr(C2O4)2(OH2)2]:C, 15.19.; H, 1.80%.

The crystallographic data for cis-K[Cr(C2O4)2(OH2)2]and trans-K[Cr(C2O4)2(OH2)2]·3H2O have been describedin the literature.16,18 The physicochemical properties ofcis-K[Cr(C2O4)2(OH2)2] and trans-K[Cr(C2O4)2(OH2)2]·3H2O have also been characterized earlier in the litera-ture.19 The TG–MS analysis confirmed the thermodynamicstability of the synthesized isomers and it shows that thetrans-K[Cr(C2O4)2(OH2)2]·3H2O is more stable in the solidstate than cis-K[Cr(C2O4)2(OH2)2]. Moreover, both isomersundergo dehydration and decarboxylation.19

2.3 MS spectra

The positive-ion mode MALDI-TOF mass spectra of obtainedpoly(2-chloroallyl alcohols) with the 2,5-dihydroybenzoicacid (DHB) matrix were recorded on the Bruker Biflex IIIspectrometer.

2.4 NMR spectra

The 1H and 13C NMR spectra of the following mixtureswere obtained using the Bruker Avance III 500 (500.13/

125.76 MHz) instrument (300 K): (1) poly(2-chloroallylalcohols), cis-K[Cr(C2O4)2(OH2)2] with MMAO-12; (2)poly(2-chloroallyl alcohols), trans-K[Cr(C2O4)2(OH2)2]·3H2O with MMAO-12. 1,1,2,2-Tetrachloroethane-D2(C2D2Cl4) was used as the solvent.

2.5 Synthesis of poly(2-chloroallyl alcohols)

The oligomerization processes were carried out underconstant conditions: at the atmospheric pressure, at 21 ◦C,under the nitrogen atmosphere, continuous mixing using amagnetic stirrer. Cis-K[Cr(C2O4)2(OH2)2] (3 μmol,0.84 mg)was dissolved in toluene (2 mL). This chromium(III)complex was used as a catalyst. The obtained solution wasplaced in the glass cell with a sealed stopper. Subsequently,3 mL of the MMAO-12 solution were dropped in to the com-plex solution in the glass cell. In the next step, 3 mL of2-chloro-2-propen-1-ol were added to the MMAO-12 and cis-K[Cr(C2O4)2(OH2)2] mixture in the glass cell. After 20 min,the oligomerization process was finished. The product of theoligomerization (2.17 g) has been studied by MALDI-TOFmass spectrometry and NMR spectroscopy.

The same oligomerization procedure was carried outusing trans-K[Cr(C2O4)2(OH2)2]·3H2O (3 μmol, 1.00 mg)

instead of cis-K[Cr(C2O4)2(OH2)2]. The product of theoligomerization weighed 3.49 g.

3. Results and Discussion

Two geometric isomers cis-K[Cr(C2O4)2(OH2)2] andtrans-K[Cr(C2O4)2(OH2)2]·3H2O (Figure 1) after reac-tion with MMAO-12 were used as the catalyst sys-tems for the preparation of poly(2-chloroallyl alcohols).Both the synthesized complexes contain two oxalate[O−, O−] ligands, however, with different positions inspace. In our studies, two processes of 2-chloro-2-propen-1-ol oligomerization have been conducted: inthe first sample, the cis isomer was used as a catalyst,

Figure 1. (A) trans-K[Cr(C2O4)2(OH2)2]·3H2O; (B) cis-K[Cr(C2O4)2(OH2)2].

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Figure 2. The 1H NMR spectra for (1) the prod-uct of 2-chloro-2-propen-1-ol oligomerization catalyzedby cis-K[Cr(C2O4)2(OH2)2]/MMAO-12; (2) the productof 2-chloro-2-propen-1-ol oligomerization catalyzed bytrans-K[Cr(C2O4)2(OH2)2]·3H2O/MMAO-12.

Figure 3. The 13C NMR spectra for (1) the prod-uct of 2-chloro-2-propen-1-ol oligomerization catalyzedby cis-K[Cr(C2O4)2(OH2)2]/MMAO-12; (2) the productof 2-chloro-2-propen-1-ol oligomerization catalyzed bytrans-K[Cr(C2O4)2(OH2)2]·3H2O/MMAO-12.

and the trans isomer was used in the second one. Theoligomerization products have been characterized by1H, 13C NMR spectroscopy and MALDI-TOF massspectrometry.

The 1H and 13C NMR spectra for the products of 2-chloro-2-propen-1-ol oligomerization catalyzed by cis-K[Cr(C2O4)2(OH2)2] and trans-K[Cr(C2O4)2(OH2)2]·3H2O are presented in Figures 2 and 3, respectively.A low number of signals in the range of 20–80 ppmin the 13C NMR spectra confirms the isotactic structureof the prepared oligomers.7,20 The specific carbon and

Figure 4. The MALDI-TOF mass spectrum for theproduct of 2-chloro-2-propen-1-ol oligomerization wherecis-K[Cr(C2O4)2(OH2)2]/MMAO-12 was used as a catalystsystem.

Figure 5. The MALDI-TOF mass spectrum for theproduct of 2-chloro-2-propen-1-ol oligomerization wheretrans-K[Cr(C2O4)2(OH2)2] ·3H2O/MMAO-12 was used asa catalyst system.

hydrogen atoms present in the obtained oligomers havebeen associated with the characteristic peaks in the 13CNMR and 1H and spectra (see Figures 2 and 3).21 In the1H NMR spectrum shown in Figure 2 at the panel (2),the satellite peaks are observed. These peaks confirm theprotons interactions with carbon atoms in the oligomer.

The peaks corresponding to the signals from carbonatoms in the 13C NMR spectra are shifted towards lowervalues of the chemical shift [ppm] as a result of the for-mation of the oligomer, in comparison with the signalsof carbon atoms present in the 2-chloro-2-propen-1-olmolecule (monomer).22,23

Moreover, the molecular masses of the obtainedoligomers have been determined by MALDI-TOF massspectrometry. The molecular peaks of the products

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Figure 6. The proposed mechanism for the oligomerization reaction of 2-chloro-2-propen-1-ol usingcis-K[Cr(C2O4)2(OH2)2] and MMAO-12.

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Figure 7. The proposed mechanism for the oligomerization reaction of 2-chloro-2-propen-1-ol usingtrans-K[Cr(C2O4)2(OH2)2]·3H2O and MMAO-12.

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of the 2-chloro-2-propen-1-ol oligomerization occur at665.4 m/z and 666.3 m/z for the process catalyzedby cis-K[Cr(C2O4)2(OH2)2]/MMAO-12 and trans-K[Cr(C2O4)2(OH2)2] ·3H2O/MMAO-12, respectively(Figures 4 and 5). It can, therefore, be concluded thatthe obtained products consist of seven monomers of 2-chloro-2-propen-1-ol. The MS spectra analysis showsthat the same 2-chloroallyl alcohol oligomerizationproducts have been obtained using cis-K[Cr(C2O4)2

(OH2)2] and trans-K[Cr(C2O4)2(OH2)2]·3H2O as cat-alysts. In the MS spectra of both oligomers (Fig-ures 4 and 5), besides the molecular peak, the peaksat 469.4 m/z and 469.3 m/z corresponding to themolecules of five monomers of 2-chloro-2-propen-1-ol are observed for cis-K[Cr(C2O4)2(OH2)2]/MMAO-12 and trans-K[Cr(C2O4)2(OH2)2] ·3H2O/MMAO-12,respectively. In the case of peaks occurring at the higherm/z values than the m/z values of the molecular peaks,it can be observed that they are the results of attach-ing two monomers to the obtained dominant oligomer.The difference in the values of the successive m/z peaksis within the range 178.0–196.1 m/z. This relationshipresponds to the molecular mass of two molecules of 2-chloro-2-propen-1-ol (185 g·mol−1).

For both complex compounds, cis-K[Cr(C2O4)2

(OH2)2] and trans-K[Cr(C2O4)2(OH2)2] ·3H2O, cataly-tic activities have been calculated. The values of the cata-lytic activities equal to 2163.25 and 3479.15 g·mmol−1·h−1 for cis-K[Cr(C2O4)2(OH2)2] and trans-K[Cr(C2

O4)2(OH2)2]·3H2O, respectively. The value of the catal-ytic activity of the compound was calculated as follows:

Catalytic activity = mp

nCr · t

where, mp = the weight of obtained polymer [g], nCr =the quantity of Cr3+ moles (from the weight of the com-plex compound used in the experiment) [mmol], and t =the time of the polymerization [h]. The unit of catalyticactivity was selected in this way so that the obtainedvalue could be comparable with other values of the cat-alytic activity of the chromium(III) complex compoundsdescribed in the literature. The results of the calcu-lations shows that trans-K[Cr(C2O4)2(OH2)2] ·3H2Ocauses that a larger amount of the oligomer is formedthan in the case of the oligomerization catalyzed by cis-K[Cr(C2O4)2(OH2)2]. The catalyst system containingtrans-K[Cr(C2O4)2(OH2)2]·3H2O exhibits about 1.5-fold higher catalytic activity for the oligomeriztionof poly(2-chloroallyl alcohol) in comparison with thecatalytic activity of [Cr(dipic)2][Cr(bipy)(dipic)H2O] ·2H2O and [Cr(dipic)2]Hdmbipy·2.5H2O where dipicdenotes dipicolinate, bipy means 2,2′-bipyridine and

Hdmbipy denotes the 4,4′-dimethoxy-2,2′-bipyridineorganic cation.7

In addition, the mechanisms of the oligomeriza-tion have been proposed (Figures 6 and 7) on thebasis of the knowledge of the behavior of Ziegler-Natta catalysts.24–26 The catalyst complex is formedbetween the chromium(III) complex compound andMMAO-12. The important stage is the connection ofthe monomer molecule to the oligomerization cata-lyst complex. Then the π bond is formed as a resultof the interactions between the Cr(III) cation and thedouble bond in the monomer molecule. In the nextstep, the oligomer molecule is formed. The attachmentof further monomer molecules is possible by creatingpartial positive and negative charges on the monomermolecule and the catalyzing complex. The proposedmechanisms may explain the different catalytic activityof the chromium(III) complexes described in this work.Trans-K[Cr(C2O4)2(OH2)2]·3H2O complex causes thata larger amount of oligomer is obtained than in the caseof use of cis-K[Cr(C2O4)2(OH2)2] complex. It may beexplained by the fact that in the oligomerization mech-anism, in the case of the catalytic complex containingtrans-K[Cr(C2O4)2(OH2)2] ·3H2O, the bonds betweenAl3+ from MMAO-12 and donor atoms from Cr(III)complex are formed with one kind of ligand (H2O) intrans-K[Cr(C2O4)2(OH2)2] ·3H2O, and oxalate anionsdo not play a direct role in the formation of the catalyticcomplex. In the mechanism of the oligomer forma-tion using trans-K[Cr(C2O4)2(OH2)2] · 3H2O, there isno steric hindrance such as oxalate anion.

4. Conclusions

In present study, the geometric isomerism effect on theoligomerization products of 2-chloro-2-propen-1-ol hasbeen investigated using two complex compounds, cis-K[Cr(C2O4)2(OH2)2] and trans-K[Cr(C2O4)2(OH2)2]·3H2O. The mechanisms of the oligomerization reactionhave been proposed taking into account the geometricisomerism effect.

The investigated chromium(III) complexes exhibitvery high catalytic activity. The advantage of the cat-alysts described in this report is the high-rate oligomer-ization in undemanding reaction conditions at theatmospheric pressure and at 21 ◦C, and that tacticaloligomers are formed. Trans-K[Cr(C2O4)2(OH2)2]·3H2O has higher catalytic activity when compared to thechromium(III) complexes reported so far in the literatureto obtain poly(2-chloroallyl alcohol). It has been foundthat the geometric isomerism of the chromium(III) com-plex compounds has no effect on the chain length of

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the obtained oligomer. The cis-K[Cr(C2O4)2(OH2)2]and trans-K[Cr(C2O4)2(OH2)2]·3H2O complexes understudy, after the induction by MMAO-12, can be classi-fied as the kind of Ziegler-Natta catalysts.

Acknowledgements

This work was supported by National Science Centre, Polandunder Grant Number 2015/19/N/ST5/00276.

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