ISSN 09655441, Petroleum Chemistry, 2015, Vol. 55, No. 3, pp. 217–223. © Pleiades Publishing, Ltd., 2015. Original Russian Text © I.S. Maksakova, I.G. Pervova, G.P. Belov, I.I. Khasbiullin, N.A. Ivanova, E.N. Frolova, I.N. Lipunov, 2015, published in Neftekhimiya, 2015, Vol. 55, No. 3, pp. 228–235.

Synthesis, Structure, and Catalytic Properties of Iron Complexes with Mono and Bisformazans in Sulfide Oxidation and Ethylene Oligomerization Reactions I. S. Maksakovaa, I. G. Pervovaa, G. P. Belovb, I. I. Khasbiullinb, c, N. A. Ivanovad, E. N. Frolovad, and I. N. Lipunova a

Ural State Forest Engineering University, Sibirskii trakt 37, Yekaterinburg, Sverdlovsk oblast, 620100 Russia email: [email protected] b Institute of Problems of Chemical Physics, Russian Academy of Sciences, pr. Akademika Semenova 1, Chernogolovka, Moscow oblast, 142432 Russia email: [email protected] c Kazan National Research Technological University, ul. Karla Marksa 68, Kazan, Tatarstan, 420015 Russia email: [email protected] d Zavoiskii Physicotechnical Institute, Kazan Scientific Center, Russian Academy of Sciences, Sibirskii trakt 10/7, Kazan, Tatarstan, 420029 Russia Received October 16, 2014

Abstract—Metal complexes with different numbers of iron atoms in a predominantly nitrogencontaining coordination environment have been synthesized on basis of benzothiazole mono and bisformazans. The composition and structures of the resulting iron coordination compounds have been characterized by elec tronic absorption spectroscopy, elemental analysis, mass spectrometry, electron spin resonance, and magne tochemistry. The catalytic behavior of these metal chelates in the liquidphase oxidation of sulfides and the oligomerization of ethylene has been studied. Keywords: bisformazans, iron formazanates, liquidphase sulfide oxidation, ethylene oligomerization DOI: 10.1134/S0965544115030068

Ironcontaining catalytic systems are characterized by high activity and selectivity for higher linear ethylene oligomers under mild reaction conditions [1–5] and, as such, hold much promise for industry to provide a sus tainable use of feedstock. Furthermore, both the nature of the ligand itself and the type of substituents that bears, which may affect the course of oligomerization, play an important role in the catalytic process. For example, the presence of chlorine atoms in the coordi nation sphere of a complex compound influences the selectivity of oligomerization and promotes catalytic ability of the complex [1–5]. A wide range of metal for mazanates as chemical objects were well studied in [6]. The catalytic properties of metal complexes with for mazan ligands in ethylene oligomerization were previ ously tested only for nickel formazanates [7]. The cata lytic behavior of some Fe(II) formazanates in ethylene oligomerization in the presence of different organoalu minum compounds was shown in [8, 9]. Nevertheless, particular attention is given to the iron complexes formed by formazans that bear oxy gencontaining coordinating substituents, in which the metal ion can change its valency as a result of com plex formation. It was found [10] that in the presence

of oxygencontaining coordinating substituents in the structure of the initial formazan ligand, the reactions with Fe(III) salts give rise to trivalent iron complexes, whereas the unsubstituted formazans undergo partial oxidation of the ligand to the tetrazolium salt, thereby facilitating the reduction of Fe3+ to Fe2+ and the for mation of divalent iron formazanates. In the present study, we prepared a set of iron com plexes 1Fe–10Fe using ligands L1, L2, and L3 described previously [7]. The methoxy group R = OMe introduced into the structure of both monomeric and bisformazans possesses potential coordinating ability toward the metal atom [11]. Thus, differences in the composition, structure, and arrangement of the metal coordination sphere will determine to a considerable extent the catalytic activity of iron chelates in redox reactions. EXPERIMENTAL Methods and Equipment The purity of the resulting iron complexes was monitored using thin layer chromatography (TLC) on Silufol UV254 silica gel precoated sheets.

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The electron absorption spectra in the visible and near UV regions were recorded on a Shimadzu UV1800 spectrophotometer in the wavelength range of 250–1000 nm. To perform titration, solutions of formazans L1–L3 with a concentration of Сf = 3 × 10–5 mol L–1 were prepared by dissolving analytical samples of formazans L1–L3 in 25 mL of alcohol. The resulting solution was titrated with an aqueous iron(III) perchlorate solution (СFe = 10–3 mol L–1). The absorption spectra were recorded after each addition of the salt solution [8]. The elemental analysis was carried out with a CHN PE2400SII automated analyzer (Perkin Elmer Instru ments). The analysis for metals was carried out by rapid gravimetry on Khimlabpribor equipment in the Laboratory of Elemental Analysis [8]. The mass spectrophotometric investigations of the formazans and the complexes were performed on a LCMS2010 Shimadzu liquid chromatograph–mass spectrometer in the chemical ionization mode at ambient pressure and electrospray ionization with detection of positive and negative ions. The samples were introduced into the mass spectrometer through the chromatograph with an SPDM10Avp diode array detector by direct injection into the ion source.

R N S

№ МC:

NH N N N

1Fe: R = Н, n = 1 4Fe: R = OCH3, n = 1 7Fe: R = Н, n = 2

O n

L1

R N S

NH N N N

2Fe: R = H, n = 1 5Fe: R = OCH3, n = 1 8Fe: R = H, n = 2

S n

L2

R N S

NH N N N N

3Fe: R = H, n = 1 6Fe: R = OCH3, n = 1 9Fe: R = H, n = 2 10Fe: R = OCH3, n = 2 n

L3

The magnetic characteristics of the compounds were measured by the Faraday method at 293 K. The electron spin resonance measurements were conducted in the Xband and the temperature interval of 20–300 K on a Bruker EMXplus spectrometer. The parameters of the spectra and their relative contribu tion into the total spectrum were determined on the basis of model calculations of the line shape using the program Easyspin. Synthesis of Iron Metal Complexes The initial formazan ligands were synthesized according to the procedures described elsewhere [7, 9]. Synthesis of iron(III) bis1(2methoxyphenyl)3 thienyl5(benzothiazol2yl)formazanate (5Fe). A hot aqueous Fe(ClO4)3 · H2O (0.019 g/0.10 mmol) solution was added with heating and stirring to a solution of 0.36 g (0.20 mmol) of formazan L2 dissolved in acetone (30 mL). Depending on the formazan structure, mixing of the solutions led to a drastic change of color from red or violet to dark brown, vinous, and green. The resulting complex was stirred for 30 min and then held in a por celain dish until solvent removal. Then, the suspension was filtered off on a vacuum filter, washed with warm water (~30 ± 2°C), and dried at room temperature. Iron(III) formazanates 1Fe–4Fe, 6Fe–10Fe were synthesized in the same manner. The characteristics of resulting iron formazanates 1Fe–10Fe are presented in Tables 1 and 2. Catalytic oxidation of sodium sulfide solution. The catalytic activity of iron(III) formazanates was studied in a conventional setup consisting of a microcompres sor and a sparger. An analytical sample of 0.0008– 0.0009 g (for complexes 1Fe–6Fe) or 0.0015–0.0017 g (for bismetal chelates 7Fe–10Fe) was dissolved in eth anol (5 mL). After complete dissolution, this mixture was poured into a frittedglass sparger, then, a solution of Nа2S · 9Н2О (25 mL) with a concentration of 0.3 g/L was added. Air was blown through the sparger with the reaction mixture using the microcompressor for 30 min. After air blowing, an aliquot (10 mL) was taken from the sparger and added to a beaker with the copper chloride solution (15 mL) (the concentration of CuCl2 was five times higher than that of S2– ions). Mixing of the solutions resulted in the reaction and precipitation of dark brown copper sulfide (CuS). The precipitate was filtered off on a paper filter, the clari fied solution was analyzed for residual copper(II) ions, and the concentration of sulfide ions was calculated on this basis [12]. Ethylene oligomerization reaction. Toluene was puri fied according to the standard procedure [13] and dis tilled over molecular sieves 4–5 Å. The catalytic exper iments were carried out using ethylene of 99.9% purity. The cocatalyst, Al(C2H5)2Cl in heptane, was used without further purification. Toluene was used as a PETROLEUM CHEMISTRY

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Table 1. Characteristics of iron complexes based on benzothiazolylformazans MC

Yield, %

Tmp, °C

λmax (L), nm

λmax(MC), nm

M+, m/z (I, %)*

1Fe 2Fe 3Fe 4Fe 5Fe 6Fe 7Fe 8Fe 9Fe 10Fe

72 63 67 72 58 71 42 53 39 41

252 >250 218 243 197 214 246 191 222 226

470 485 415 490 495 500 500 500 450 570

700 710 680 660 640 740 655 717 705 770

748 (64) 780 (100) 889 (98) 808 (100) 840 (100) 830 (100) 442.1 (100) 438 (90), 642.6 (100) 743 (100) 830 (100), 696 (67)

Mr, g/mol 748.84 780.94 888.40 808.88 841.00 830.94 1443.66 1603.92 1698.75 1802.86

* Molecular peaks of fragment ions with an abundance of more than 20% are presented.

Table 2. Characteristics of iron complexes based on benzothiazolylformazans Found, %

Empirical formula

Calculated, %

MC C

H

N

Fe

H

C

H

N

Fe

1Fe 2Fe

57.66 55.31

3.23 3.11

18.68 17.89

7.46 7.19

(C18H12N5SO)2 · Fe (C18H12N5S2)2 · Fe

57.74 55.36

3.24 3.10

18.71 17.94

7.48 7.17

3Fe

51.17

3.19

18.88

6.28

(C19H13N6S)2 · Fe · Cl O 4 · H2O

51.37

3.18

18.92

6.30

4Fe 5Fe 6Fe 7Fe 8Fe

56.24 54.30 60.07 60.12 55.53

3.50 3.35 3.81 3.59 3.16

17.25 16.59 19.40 19.35 17.46

6.95 6.64 6.48 3.88 6.98

(C19H14N5SO2)2 · Fe (C19H14N5S2O)2 ⋅ Fe (C20H15N6SO)2 ⋅ Fe (C36H25N10S2O2)2 ⋅ Fe (C36H22N10S4)2 ⋅ Fe2 ⋅ C2H5OH

56.42 54.27 57.81 59.90 55.41

3.50 3.36 3.65 3.50 3.15

17.32 16.66 20.23 19.41 17.47

6.92 6.66 6.73 3.88 6.98

9Fe

53.02

2.97

19.54

3.24

(C38H24N12S2)2 ⋅ Fe · 2Cl O 4 · H2O

53.73

2.85

19.79

3.29

10Fe

55.21

3.15

19.16

2.95

(C40H29N12S2O2)2 ⋅ Fe ⋅ 2Cl O 4

55.29

3.25

18.65

3.12

2–

2–

2–

medium to prepare the desired concentration of orga noaluminum compounds. Ethylene oligomerization was carried out in toluene in a stainless steel reactor (0.2 L) equipped with a mag netic stirrer and a pressure gauge, which was used to control ethylene pressure. The temperature in the reactor was controlled by supplying thermostated water into the reactor jacket. The clean reactor purged with an inert gas was evacuated for 1–2 h at 353 K. Then, the reactor heated to the required temperature was charged with calculated amounts of the solvent; ethyl ene; and sequentially, using metallic syringes according to the Schlenk technology, solutions of iron formazan ate and the cocatalyst, the components of the catalytic. Ethylene oligomerization was carried out for 1 h. The products of ethylene oligomerization were determined by GLC on a CHROM 5 chromatograph with a flame ionization detector. The mixture was sep arated on a Thermo Fisher Scientific TR5MS capil PETROLEUM CHEMISTRY

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lary column with the bonded phase composed of 95% of phenylenesiloxane and 5% of phenylpolysilane. The column had a length of 30 m, an outer diameter of 0.25 mm, and film thickness of the phase of 0.25 µm; helium used as the carrier gas had a flow rate of 15 mL min–1; the flow rates of air and hydrogen were 100 and 25 mL min–1, respectively. The column tem perature was programmed from 40 to 80°C a heating rate of 6°C/min and from 80 to 160°C at a rate of 10°C/min. The evaporator and detector temperatures were maintained at 200°C. The liquid sample of a 0.2 µL size was introduced with a microsyringe [13]. The recording and processing of chromatograms was carried out using a Multikhrom hardwareand software complex. The product composition of oligo merization was determined by the normalization of peak areas to 100%.

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MAKSAKOVA et al. a b c

d

1 2

0

100

200

300

400

500

600 H, mT

Fig. 1. ESR spectra of formazanates (1) 9Fe and (2) 10Fe. ν = 9.47 GHz; T = 160 K.

RESULTS AND DISCUSSION Synthesis and Characterization of Iron(II,III) Formazanates It is well known [14] that formazans readily form complexes with transition metal ions. The ease of complexation is shown by spectrophotometric titra tion of ethanol solutions of the free ligands with an aqueous solution of a metal salt; however, we did not manage to study in detail the complexation of both mono and bisformazans with iron(III) ions because of the absence of a distinct spectral pattern of titration. Nonetheless, the direct synthesis by heating in accord ing with the procedure described above afforded iron formazanates 1Fe–10Fe in the solid state, which were characterized by elemental analysis, electron spec troscopy, mass spectrometry, magnetochemistry, and electron spin resonance (ESR). Unfortunately, the structure of isolated solid iron(II,III) formazanates could not be determined by Xray diffraction analysis because of their amorphous state. According to the elemental analysis and mass spec tral data, all 1Fe–6Fe iron chelates based on monofor mazans have the composition L2M. The results of mag netochemical (μeff = 0 mB) and ESR (using the 3Fe, 6Fe compounds as an example) measurement show the formation of diamagnetic coordination compounds 1Fe–6Fe that contain the iron(II) atoms in the low spin state (S = 0). According to published data [15, 16], it is obvious that the Fe(II) complexes formed have a

pseudooctahedral structure with the FeN6 coordina tion sphere and the methoxy group introduced in the orthoposition of the aryl moiety being not involved in the coordination to the iron atom. The complication of the ligand molecules by dou bling the formazan unit led to the production of coor dination compounds with more diverse compositions depending on the substituent nature in the mesoposi tion of the formazan molecule. Formazanates 9Fe and 10Fe, obtained on the basis on bisligands L3 bearing the pyridinyl substituent in the mesoposition of the formazan chain, are para magnetic (μeff = 1.2 mB for compound 9Fe) and have composition L2M. The ESR spectra of the test samples of 9Fe and 10Fe represent a superposition of several types of signals (Fig. 1). Signals (a) and (b) belong to the lowspin com plexes of Fe(III) ions with a spin of S = 1/2. Signal (c) can be associated with the existence of a certain amount of free radicals in the bulk of the sample. Sig nal (d) with geff ≅ 4.2 belongs to the highspin com plexes of Fe(III) ions with a spin of S = 5/2, which are characterized by strong axial (D  hν) and rhombic (E/D ~ 1/3) distortions. The results of calculations of the parameters of spectra (a)–(c) and their relative contributions to the total spectrum are presented in Table 3 and Fig. 2. From the above consideration, it follows that the iron ions in compounds 9Fe and 10Fe are mainly in the trivalent state and form lowspin centers of type (a). The gfactors of spectra (a) of the lowspin Fe(III) complexes were analyzed in the oneelectron approximation within the lower orbital triplet as had been done in [16]. The results of the analysis are pre sented in Table 4. The identity of the ESR parameters of these centers in both compounds indicates the similarity of the inner coordination spheres formed by the iron ions. The structural differences between compounds 9Fe and 10Fe are in the rather remote iron ion coordination spheres and do not affect the crystal field parameters (within the experiment accuracy). The lowspin state (with the invariable parameters of spectra) is retained in the entire range of temperatures (20–300) K, thereby suggesting the absence of a spin transition. The high value of coefficient B of the wave function for the fun damental Kramer’s doublet shows that the main state is the |dxy>| orbital.

Table 3. Parameters of ESR spectra of samples 9Fe and 10Fe Spectrum (a)

Spectrum (b)

Spectrum (c)

MC 9Fe 10Fe

g_|

g||

na

g1

g2

g3

nb

g

nc

2.175 2.175

1.937 1.937

0.62 0.64

2.110 2.122

2.045 2.122

1.937 1.985

0.34 0.35

2.00 2.00

0.04 0.01

ni (i = a, b, c) are the weight coefficients of (a)–(c) type spectra in the total spectrum. PETROLEUM CHEMISTRY

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9Fe

221

10Fe

a a c

b

240

260

280

300

320

b

340

360

380 240 H, mT

260

280

300

320

c

340

360

380 H, mT

Fig. 2. Fragments of ESR spectra of samples 9Fe and 10Fe (black and red lines refer to the experimental spectrum and the calcu lated total spectrum, respectively).

The presence of type (b) centers in the ESR spectra of the test samples also does not rule out the formation of iron complexes in which the axial positions are occupied by the perchlorate ions or solvent molecules. It was noted that the reaction of unsubstituted bis formazan L1, which bears the furyl substituent in the mesoposition, with Fe(III) ions under the aforemen tioned conditions leads to the formation of trivalent iron paramagnetic complex 7Fe (μeff = 2.0 mB) of L2M composition. Furthermore, the use of thienyl containing formazan L2 made it possible to prepare binuclear (L2M2) diamagnetic complex 8Fe, which was characterized by the data of Mössbauer spectros copy and magnetochemistry (μeff = 0 mB). Catalytic Activity The catalytic properties of the resulting metal che lates of di and trivalent iron were tested in the reac tions with different mechanisms, the model reaction of liquidphase oxidation of sulfides and the industri ally important ethylene oligomerization reaction. In the presence of metalcontaining catalysts 1Fe– 10Fe, the liquidphase oxidation of sodium sulfide at room temperature (20°C) under conditions of homoge neous catalysis proceeds via the following scheme [17]: Na2S + 2O2

Fe(formazan)2 20°С

Na2SO4.

The catalytic tests showed that the iron formazan ates 1Fe–10Fe exhibit activity in this reaction; the cal culated degrees of sulfur conversion (i.e., the ratio of sulfur transformed into sulfates as a result of reaction to the amount of sulfur in the initial sulfide solution) vary within 19–42%). Of the unsubstituted (R = H) monomeric ben zothiazolylformazanates, formazanate 3Fe showed the highest catalytic activity; the degree of sulfur con version on it was 42%. The presence of an additional group R = ОСН3 in the structures of formazanates 4Fe, 5Fe, and 6Fe compared to their unsubstituted analogs 1Fe (21%), 2Fe (21%), and 3Fe (32%) did not substantially affect the catalytic activity: 4Fe (24%), 5Fe (22%), and 6Fe (32%). The degrees of sulfur conversion in the liquid phase sulfide oxidation reaction catalyzed by bisfor mazanates are comparable with those achieved with their monomeric analogs. The highest value of 27% was found for unsubstituted biscomplex 7Fe. The ethylene reactions leading to the formation of ethylene dimers, oligomers, and polymers are of par ticular interest both from the scientific (novel types of ligands) and industrial points of view. These reactions can yield the linear and branched compounds, which find application in different fields of engineering and technology, for example, in production of detergents and polyethylene [18]. Oligomerization processes

Table 4. Results of analysis of gfactors for lowspin Fe(III) complexes of type (a) in the test compounds MC

g_|

g||

A

B

C

K

Δ/ξ

ΔE12/ξ

ΔE13/ξ

9Fe, 10Fe

2.175

1.937

0.10906

0.99403

0.0

0.6483

2.289

6.522

7.445

A, B, and C are the coefficients of wave functions for the fundamental Kramer’s doublet, K is the spinorbital coupling suppression coefficient, ξ is the oneelectron spinorbital coupling constant, Δ is the tetragonal component of the crystal field, and ΔE12 and ΔE13 are the values of the corresponding energy intervals between the orbital triplet levels. PETROLEUM CHEMISTRY

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MAKSAKOVA et al. 1

(а) 1.1

Ethylene consumption, L

Ethylene consumption, L

1.2 3

1.0 4

0.9 2

0.8 0.7

1.2

(b)

1

1.0

2

0.8 0.6 0.4 0.2

0.6 10

0

20

30 40 Time, min

50

0

60

10

20

30 40 Time, min

50

60

Fig. 3. Kinetic curves of ethylene consumption in oligomerization reaction in the presence of iron formazanates based on (a) monoligands: (1) 1Fe, (2) 2Fe, (3) 4Fe, (4) 5Fe and (b) bisligands: (1) 10Fe, (2) 9Fe. Reaction conditions: Т = 80°C, Р = 2.0 MPa, СFe = 4 × 10–5 mol/L, Vtoluene = 60 mL, Al : Fe = 20 : 1 (mol).

using predominantly homogeneous catalysts based on organometallic complexes bearing transition metals in their structures in combination with organoaluminum compounds are the most popular. nC2H4

Fe(formazan)2 + AlEt2Cl toluene, 80°С

the negative effect can be exerted both by oxygen atoms in the solvent molecules and those in the struc ture of the catalyst itself. It was found that the iron(III) catalyst systems based on monoformazans 4Fe–5Fe, which contain the R = ОСН3 group in their structure (Fig. 3a) exhib ited a lower activity than unsubstituted benzothiaz olylformazanates 1Fe–2Fe. At the same time, forma zanate 10Fe, which contains the pyridinyl moiety, showed the highest catalytic activity and the best on stream time among iron bisformazanates. The decrease in ethylene absorption with time can be due to two factors, a reduction in partial pressure of ethyl ene in the reactor because of the formation of butenes and hexenes during the reaction and a possible decrease in the amount of active sites of the catalyst. The analysis of reaction products by gas–liquid chromatography showed (Table 5) that ethylene oligo merization in the presence of diethylaluminum chlo

higher αolefins.

At the same time, the search for effective and selective catalytic systems is still an urgent task. For nickel formazanates tested in this reaction ear lier [7, 9], the optimal conditions of the process were as follow: Т = 80°C, Р = 2.0 MPa, СFe = 4 × 10–5 mol/L, cocatalyst: diethylaluminum chloride (Al(C2H5)2Cl). Figure 3 depicts rate curves for ethylene consump tion in the presence of catalytic systems based on some of the iron(II,III) formazanates obtained in this work. The performance of a catalytic system using orga noaluminum compounds as a cocatalyst is known [19] to depend on the presence in the reaction mixture of oxygen atoms that lead to their degradation. Note that

Table 5. Catalytic activity of iron formazanates in oligomerization of ethylene (T = 80°C, P = 2.0 MPa, CFe = 4 × 10–5 mol/L, Vtoluene = 60 mL, Al : Fe = 20 : 1 (mol)) MC

Activity, kg/g Fe h

Product composition, wt % 1C4H8

1C6H12

olefins C8+

1Fe

9.52

42

3

55

2Fe

9.52

63

25

12

4Fe

8.66

56

11

33

5Fe

8.66

29

7

64

9Fe

8.22

29

71

–

10Fe

10.39

58

42

–

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ride proceeds toward the formation of predominantly αolefins (butene1, hexene1) with a low amount of byproducts. In this study, we showed the feasibility of targetori ented synthesis of Fe(II,III) coordination compounds with diverse structures and compositions of the coor dination sphere (LM2, L2M2) by increasing the ligand denticity via duplication of the formazan unit. Heter ometallic complex 8Fe of L2M2 composition was pre pared using bisligand L2. These differences in the spa tial arrangement of the coordination sphere facilitated the elucidation of a correlation between the structures of the metal complexes obtained and their activity in catalytic oligomerization of ethylene. It was revealed that the iron(II,III) complexes under optimal condi tions exhibit catalytic activity and selectivity in ethyl ene oligomerization, resulting in a certain series of olefins (butenes, hexenes). It is noteworthy that the formazanate based on bisligand (10Fe) exhibited the highest catalytic activity of 10.39 kg/g Fe h. Investigating the catalytic properties of these iron(II,III) complexes, we left one problem unre solved—the catalyst onstream time. The activity of Fe(II,III) coordination complexes decrease with in time (Figs. 3a, 3b), a fact that can be due to the decomposition or rearrangement of the coordination sphere of the metal complexes until the complete deactivation of the catalytic process under given reac tion conditions. Therefore, it is the choice of the opti mal conditions to ensure both enhancement of ethyl ene consumption and increase in the catalyst time on stream that will be in focus of the further research. ACKNOWLEDGMENTS This work was supported by the Russian Founda tion for Basic Research, project nos. 110390724 mob_st and 110300181, and program no. 3 of the Presidium of the Russian Academy of Sciences “Energy Aspects of Deep Processing of Fossil and Renewable CarbonContaining Resources.” The authors are grateful to I. V. Ovchinnikov (Zavoiskii Physicotechnical Institute) for recording and discussion of the ESR spectra.

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Translated by K. Aleksanyan