ISSN 10231935, Russian Journal of Electrochemistry, 2015, Vol. 51, No. 11, pp. 1069–1078. © Pleiades Publishing, Ltd., 2015. Original Russian Text © O.S. Fomina, Yu.A. Kislitsyn, V.M. Babaev, I.Kh. Rizvanov, O.G. Sinyashin, J.W. Heinicke, D.G. Yakhvarov, 2015, published in Elektrokhimiya, 2015, Vol. 51, No. 11, pp. 1206–1215.

Electrochemical Properties and Catalytic Activity in the Ethylene Polymerization Processes of Nickel Complexes with 2,2'Bipyridine in the Presence of orthoPhosphinophenol Derivatives1 O. S. Fominaa, Yu. A. Kislitsynb, V. M. Babaeva, I. Kh. Rizvanova, O. G. Sinyashina, J. W. Heinickec, and D. G. Yakhvarova, b, z aArbuzov

Institute of Organic and Physical Chemistry, Kazan Scientific Center of the Russian Academy of Sciences, ul. Arbuzova 8, 420088 Kazan, Russia b Kazan Federal University, ul. Kremlevskaya 18, 420008 Kazan, Russia cInstitute of Biochemistry, University of Greifswald, FelixHausdorffStraße 4, D17487 Greifswald, Germany Received February 09, 2015

Abstract—Electrochemical properties of the [NiBr2(bpy)2] complex, where bpy = 2,2'bipyridyl, have been studied in the presence of derivatives of orthophosphine phenol: 2diphenyl phosphanyl4methyl phenol CH3C6H3(PPh2)OH (1), 2diphenyl phosphanyl4methylphenyldiphenyl phosphinate CH3C6H3(PPh2)OP(O)Ph2 (2), and 2diphenylphosphoryl4methyl phenol CH3C6H3(P(O)Ph2)OH (3). It is found that interaction of products of electrochemical reduction of complex [NiBr2(bpy)2] with 1 and 2 results in formation of active catalysts of the process of homogeneous oligomerization/polymerization of ethylene. Keywords: 2,2'bipyridyl, nickel complexes, orthophosphine phenols, polymerization, electrochemistry, ethylene, cyclic voltammetry, preparative electrolysis DOI: 10.1134/S102319351511004X

INTRODUCTION It is known that complexes of transition metals are effective catalysts for ethylene oligomerization and polymerization processes [1–4]. One of the most widespread technological procedures of producing linear αolefins by means of homogeneous oligomer ization of ethylene is SHOP (Shell Higher Olefin Pro cess) based on using organophosphorus compounds containing PCCO chelate centers [5–7]. In this pro cess, the catalytically active forms of the catalysts are organonickel complexes formed by derivatives of αphosphonecarboxylic acids and βphosphorylated alcohols that can enter reactions of oxidative addition with nickel(0) complexes with formation of catalyti cally active hydride complexes containing PCCO che late centers. Some of such compounds are derivatives of tertiary orthophosphine phenols; their nickel com plexes manifest high catalytic activity in the course of homogeneous oligomerization/polymerization of eth ylene [8]. Synthesis of the active form of organonickel catalyst in these processes is carried out via the reac tion of oxidative addition of complex [Ni0(COD)2], 1 Presented

at the 18th AllRussian Conference on Organic Elec trochemistry, Tambov, September 15–19, 2014. z Corresponding author: [email protected] (D.G. Yakhvarov).

where COD = cyclooctadiene1,5, via the O–H bond in orthophosphone phenols or O–R bond in their chemically active ethers. However, stringent thermal conditions requiring high energy consumption (the process temperature of 100°C) are required for imple mentation of such a process [8]. Moreover, catalyti cally inactive [Ni0(COD)2] is a very unstable com pound, which imposes a number of significant limita tions for industrial implementation of the developed technologies. Therefore, search for new alternative ways of generation of active organonickel catalysts of ethylene oligomerization and polymerization using stable precursors is of great interest for modern cata lytic chemistry. Our studies are concerned with development of new methods of generation of active ethylene oligo merization and polymerization catalysts using electro chemical methods that have been successfully used up to date to obtain catalytically active organonickel sigmacomplexes [9] and also nanosized catalysts and nanomaterials [10, 11]. However, at present, there is no information in the current scientific literature that describes the possibility of electrochemical generation of active organonickel catalysts of processes of homo geneous ethylene oligomerization/polymerization based on PCCO chelate ligands.

1069

1070

FOMINA et al.

The aim of this research was to study electrochem ical properties of nickel(II) complexes with 2,2'bipy ridyl in the presence of derivatives of orthophosphine phenol using the methods of cyclic voltammetry, pre parative electrolysis, and mass spectrometry, and also to study catalytic activity of compounds formed as a result of electrochemical transformations in the course of homogenous oligomerization/polymeriza tion of ethylene. EXPERIMENTAL All experiments related to preparation of the initial reagents and conducting electrochemical studies were carried out in an inert atmosphere (nitrogen) using a standard Schlenk technique. The method of mass spectrometry with electrospray ionization (ESI) was used to establish the composition and structure of products of electrochemical processes. ESI mass spectra were obtained using a AmazonX mass spectrometer (Bruker Daltonik GmbH, Bremen, Germany) in the positive ion detection mode in the range of m/z of 100 to 2800. The voltage on the capil lary was –4500 V. The drier gas was nitrogen with the temperature of 300°C and consumption of 8 L min–1. The sample was introduced at the rate of 10 µL/min using a syringe pump. The data of ESI experiments were processed using the DataAnalysis 4.0 software (Bruker Daltonik GmbH, Bremen, Germany). A stationary disk electrode of glassy carbon (GC) with the working surface area of 3.14 mm2 is used as a working electrode in the studies using the method of cyclic voltammetry (CV). Voltammograms (CVcurves) were recorded in a three–electrode cell. Registration of CV curves was carried out in dimethyl formamide (DMFA) in the background solution of 0.1 M (NBu4)BF4 using a PI501 potentiostat at the linear potential sweep rate of 50 mV/s. The potential sweep program was set using a computer and a E14440 ana log–to–digital converter. The reference electrode was the system of Ag/0.01 M AgNO3 in acetonitrile (Е0(Fc/Fc+) = +0.20 V; +0.17 V vs. the saturated calomel electrode). All potentials in the papers are presented vs. this reference electrode. The auxiliary electrode was a Pt wire with the diameter of 1 mm. Measurements were carried out in a thermostated (20°C) cell in a nitrogen atmosphere. In the experi ments, the substrate concentration was 5 × 10–3 M. The values of peak potentials registered in CV curves of nickel complexes studied in the work both in the absence and in the presence of organic substrates used in the work are presented in Table 1. Preparative elec trolysis was carried out in a 30 mL electrochemical cell with separate compartments equipped with a DURAN porous ceramic diaphragm (pore size 3), a glassy carbon cathode (with the working surface area of 10 cm2) and a nickel anode (with the working sur face area of 5 cm2).

DMFA was purified by triple vacuum distillation with intermediate drying over calcium hydride (10 g/L) and was stored in a nitrogen atmosphere. The com pounds used in the work, i.e., 2diphenyl phosphanyl 4methyl phenol (1) [12], 2diphenyl phosphanyl4 methylphenyldiphenyl phosphinate (2), and 2diphenyl phosphoryl4methyl phenol (3) [8], were synthesized in Laboratory of Inorganic Chem istry of Institute of Biochemistry of University of Greifswald (Germany). Triphenylphosphine (PPh3) and phenol (PhOH) are commercially available reagents (Alfa Aesar) and were used without additional purification. Preparation of Solution for Catalysis (Method A) The solution for electrolysis was prepared by disso lution of 53.1 mg (0.1 mM) of complex [NiBr2(bpy)2] and 0.1 mM organophosphorus ligand (29.2 mg of 2diphenyl phosphanyl4methyl phenol (1) or 49.2 mg of 2diphenyl phosphanyl4methylphenyl diphenyl phosphate (2), or 30.8 mg of 2diphenyl phosphoryl4methyl phenol (3)) on 20 mL of DMFA at the room temperature. Then the obtained mixture was transferred into an electrochemical cell and con stant current of 11 mA was passed through electrolyte (the current density was 1.1 mA cm–2) for 30 min under continuous solution stirring (2e/Ni atom) and at a controlled potential of the working electrode (cathode) in the range of –(1.45–1.65) V (the cell voltage was 6.5–6.8 V). The amount of electricity passed through the solution was 5.5 mA h. When the electrochemical process was over, the solvent (DMFA) was evaporated in vacuum and products of electrolysis were extracted by toluene (20 mL). The obtained tol uene extract was filtered using a fine filter (Whatman) and then was used in catalytic experiments. Preparation of Solution for Catalysis (Method B) The solution for electrolysis for prepared by disso lution of 53.1 mg (0.1 mM) of complex [NiBr2(bpy)2] in 20 mL of DMFA at the room temperature. Then the obtained solution was transferred into an electro chemical cell and constant current of 11 mA was passed through electrolyte (the current density was 1.1 mA cm–2) for 30 min under continuous solution stirring (2e/Ni atom) and at a controlled potential of the working electrode (cathode) in the range of ⎯(1.45–1.65) V (the cell voltage was 6.5–6.8 V). The amount of electricity passed through the solution was 5.5 mA h. When electrolysis was over, a dark violet homogeneous solution was obtained. After this, the solvent (DMFA) was evaporated in vacuum and prod ucts of electrolysis were extracted by toluene (15 mL) and filtered on a fine filter (Whatman) and 5 mL of tol uene solution containing 0.1 mM of organophosphorus lligand (29.2 mg (1), or 49.2 mg (2), or 30.8 mg (3)) were added to the obtained filtrate. The solution

RUSSIAN JOURNAL OF ELECTROCHEMISTRY

Vol. 51

No. 11

2015

ELECTROCHEMICAL PROPERTIES AND CATALYTIC ACTIVITY

1071

Table 1. Potentials of peaks (±0.01 V)* in CV curves of the studied compounds (the substrate concentration was 5 × 10–3 M, the cathode is GC, 0.1 M nBu4NBF4, v = 50 mV/s) Cathodic peaks

Epred, V

Anodic peaks

Epox, V

1 (1) (CH3C6H3(PPh2)OH)

C1

–2.25

A1A2

+0.40 + 0.92

2 (2) (CH3C6H3(PPh2)OP(O)Ph2)

C1

–2.25

A1

+0.67

3 (3) (CH3C6H3(P(O)Ph2)OH)

C1

–2.25

A1

+0.94

4 PPh3

–

–

A1

+0.83

5 PhOH

–

–

A1

+1.05

6 (4) ([NiBr2(bpy)2])

C1

–1.54

A1

–1.35

C2

–2.25

A2

–2.16

A5

+0.37

A6

+0.88

No.

Compound

7 (4) + (1)

8 (4) + (2)

9 (4) + (3)

10 (4) + PPh3

11 (4) + PhOH

C1

–1.53

A1

–

C2

–

A2

–

C3

–1.94

A3

–1.93

A5

+0.35

A6

–

A7

+0.65

C1

–1.53

A1

–1.32

C2

–2.25

A2

–

A5 + A7

+0.45

A6

+0.95

C1

–1.57

A1

–1.40

C2

–2.25

A2

–

A5

+0.37

A6 + A8

+1.05

C1

–1.54

A1

–1.33

C2

–2.35

A2

–

A5

+0.41

A6

+0.85

A7

+0.25

C1

–1.58

A1

–1.35

C2

–2.35

A2

–

A5

+0.40

A6 + A8

+1.04

* CV curves were registered without IR compensation. RUSSIAN JOURNAL OF ELECTROCHEMISTRY

Vol. 51

No. 11

2015

1072

FOMINA et al.

Table 2. Conditions and results of processes of catalytic oligomerization/polymerization of ethylene under the action of nickel complexes and derivatives of orthophosphone phenol (1) and (2) (Pinit = 50 atm, 100°C, t = 16 h) No. 1 2 3 4 5 6 7 8 9 10 11 12 13 14

Catalyst (1)/Ni*a (1)/Ni*a (1)/Ni* (1)/Ni** (1)/Ni** (1)/Ni** (1)/Ni*** [8] (1)/Ni*** [8] (2)/Ni* (2)/Ni* (2)/Ni** (2)/Ni** (2)/Ni*** [8] (2)/Ni*** [8]

C2H4, g (mM)

Ethylene conversion, g (%); TON4*; mol/mol

14.9 (532) 8.9 (317) 9.6 (342) 13.3 (475) 15.5 (553) 9.0 (321) 14.6 (521) 13.2 (471) 14.8 (528) 10.2 (364) 13.6 (485) 11.0 (392) 14.2 (507) 13.3 (475)

0.06 (0.4), 21 0.08 (0.9), 28 0.5 (5.2), 178 13.0 (97), 4642 10.6 (68), 3785 8.3 (92), 2964 10.9 (74), 3875 13.1 (99), 4670 0.07 (0.5), 25 0.1 (0.9), 35 9.2 (68), 3285 8.3 (75), 2964 11.1 (77), 3900 13.1 (99), 4700

Polyethylene, g tmelt, °C; r, g/cm3 0.06, 119–122, – 0.08, 118–120, – 0.5, 118–125, 0.952 13.0, 119–123, 0.960 10.6, 118–124, 0.958 8.3, 118–123, 0.959 10.9, 118–121, 0.958 13.0, 124–125, 0.950 0.07, 115–122, – 0.1, 114–122, – 9.2, 116–121, 0.956 8.3, 114–120, 0.954 11.1, 116–120, 0.955 13.1, 95–125, 0.950

* Catalytically active forms of complexes were obtained in situ by electrolysis of solutions containing orthophosphine phenol and complex [NiBr2(bpy)2] (method A). ** Catalytically active forms of complexes were obtained in situ by mixing solutions of orthophosphine phenol and with the solution of the electrochemical obtained complex [NiBr2(bpy)2] (method B). *** Catalytically active forms of complexes were obtained in situ by mixing solutions of orthophosphine phenol and with the solution of complex [Ni(COD)2] (method C) [8]. **** TON (turnover number) is the amount of ethylene (mol) converted by 1 mol of the catalyst.

obtained in 10 min of stirring was used in catalytic experiments. Preparation of Solution for Catalysis (Method C [8]) The solution containing 0.1 mM of the organo phosphorus ligand (29.2 mg of 2diphenyl phospha nyl4methyl phenol (1) or 49.2 mg of 2diphenyl phosphanyl4methylphenyldiphenyl phosphinate (2), or 30.8 mg of 2diphenyl phosphoryl4methyl phe nol (3)) in 10 mL of toluene was added to the solution containing 27.5 mg (0.1 mM) of complex [Ni(COD)2] in 10 mL of toluene. When the organophosphorus ligand was added, the color of the nickel complex solution changed from paleyellow to palebrown. The obtained solution was used for carrying our cata lytic experiments. Catalytic Oligomerization/Polymerization of Ethylene The solution obtained according to method A, B, or C and containing an organonickel catalyst was placed into a 75 mL steel autoclave. After the auto clave with the solution was weighed, ethylene was loaded under pressure (30–50 atm) into the autoclave and its mass was determined on the basis of variation of the autoclave mass before and after loading the monomer. After this, the autoclave was placed into a

silicon bath heated to 70–100°C and the reaction mixture was mixed for 16 h. After the reaction was over, the autoclave was cooled to the room tempera ture, the remains of unreacted ethylene were vented through a low–temperature trap (–40°C) allowing to capture dimerization products (butenes). The formed macromolecular polymerization products were con secutively washed first by a mixture of methanol/2 N hydrochloric acid (1 : 1 by volume), then by pure methanol, and dried in vacuum at the room tempera ture. The density of the obtained products was deter mined by immersing a tablet (IR press, 1.0 × 104 atm) into a mixture of water and ethanol (on the basis of the density of the aqueous ethanol solution). The results of catalytic tests and properties of the obtained products are presented in Table 2. RESULTS AND DISCUSSION Electrochemical properties and reactivity of com plex [NiBr2(bpy)2] (4), where bpy = 2,2'bipyridyl, with respect to derivatives of orthophosphine phenol: 2diphenyl phosphanyl4methyl phenol (1), 2diphe nyl phosphanyl4methylphenyldiphenyl phospho nate (2), and 2diphenylphosphoryl4methyl phe nol (3) selected as model compounds (Fig. 1) were studied to investigate the possibility of using electro chemical methods for generating active catalysts for

RUSSIAN JOURNAL OF ELECTROCHEMISTRY

Vol. 51

No. 11

2015

ELECTROCHEMICAL PROPERTIES AND CATALYTIC ACTIVITY

O PPh2

PPh2

PPh2

O PPh2 O 2

OH 1

OH 3

Fig. 1. Structure of derivatives of orthophosphine phe nol (1)–(3).

processes of homogeneous oligomerization/polymer ization of ethylene. It is known that 2,2'bipyridyl (bpy) can well sta bilize reduced forms of nickel in metal complexes [13–15]. The choice of complex [NiBr2(bpy)2] for performing these studies is related to the higher reac tivity of the reduced form of complex [Ni0(bpy)2] as compared to trisbipyridyl complex [Ni0(bpy)3] satu rated by 2,2'bipyridyl and its higher stability as com pared to that of complex [Ni0(bpy)] that is coordina tion–unsaturated by 2,2'bipyridyl and is character ized by disproportionation reactions on [Ni0(bpy)2] and metallic nickel [13–15]. Such differences in reac tivity affect the electrochemical behavior of complexes of nickel(II) at different molar ratios of Ni/bpy (Fig. 2). Electrochemical properties of organophosphorus compounds (1)–(3) used in the work were studied ear lier [16] to investigate the possibility of electrochemical generation of active organonickel catalysts of ethylene oligomerization and polymerization on the basis of organophosphorus ligands containing PCCO chelate centers. Data for triphenylphosphine (PPh3) and phe

6 I, µA

The CV curve of complex [NiBr2(bpy)2], similar to complexes [NiBr2(bpy)] and [Ni(bpy)3](BF4)2 contain two quasireversible reduction peaks С1 and C2 with anodic reoxidation components А1 and А2, and also characteristic oxidation peaks of free bromide anions А5 and А6 (Fig. 2, Table 1). The process of qua sireversible electrochemical reduction of complex [NiBr2(bpy)2] occurs at the potential of peak C1 with formation of a new complex of nickel(0), [Ni0(bpy)2], which is reoxidized at the potentials of anodic peak А1 (Scheme 1). [NiBr2(bpy)2] + 2e

C1 A1

[Ni0(bpy)2] + 2Br–

Scheme 1.

A5

A6

A4

A2 0

nol (PhOH) (Table 1) were obtained for comparison of electrochemical properties of the studied organophos phorus compounds with unphosphorylated counterparts with a close structure. The performed studies showed that derivatives of orthophosphine phenol (1)–(3) are stable towards reduction in a wide range of cathodic potentials. Such electrochemical stability of ortho phosphine phenols in the cathodic range makes them convenient reagents for the process of electrochemical in situ generation of active organonickel catalysts according to the reaction of oxidative addition of elec trochemically generated complexes of nickel(0) via the O–H (or O–R) bond [13–15]. However, one must point out that when the working electrode potential is swept into the anodic region, the CV curves of all the studied organophosphorus compounds contain oxida tion peaks corresponding to oxidation of moieties – PPh2 and –OH that are electrochemically active in the anodic region (Table 1) [16].

[NiBr2(bpy)2] [Ni(bpy)3](BF4)2 [NiBr2(bpy)] A1

12

1073

A3

–6 C2

C1

C3 –2.8

–1.4

0

1.4

E, V Fig. 2. CV curves of complexes [NiBr2(bpy)2] (solid line), [Ni(bpy)3](BF4)2 (dashed line) and [NiBr2(bpy)] (dotted line) in the DMFA medium in the presence of nBu4NBF4 (0.1 M) (the concentration of substrate is 5 × 10–3 M, the cathode is GC, v = 50 mV/s). RUSSIAN JOURNAL OF ELECTROCHEMISTRY

Vol. 51

No. 11

2015

1074

FOMINA et al. 12

6 I, µA

A7 A 6

[NiBr2(bpy)2] [NiBr2(bpy)2] + 1 (1 eq.) [NiBr2(bpy)2] + 1 (3 eq.)

A5

A1 A2

A4

A3

0

–6 C2

C3

C1 –1.4

–2.8

1.4

0 E, V

Fig. 3. CV curves of complex (4) in the DMFA medium in the absence (solid line) and in the presence of a single (dashed line) and three (dotted line) equivalents of orthophosphine phenol (1) (the concentration of the complex is 5 × 10–3 M, the cathode is GC, v = 50 mV/s).

At higher cathodic potentials (С2), reversible trans fer of yet another electron to complex [Ni0(bpy)2] occurs with formation of anion–radical complex – [Ni0(bpy)2] • , in which an unpaired electron is local ized at the ligand (Scheme 2) [13–15]. [Ni0(bpy)2] + e

C2 A2

peak results from formation of new complex (5) of nickel (II) in the solution (Scheme 3). PPh2 [NiBr2(bpy)2] +

[Ni0(bpy)2]–•

–bpy

Scheme 2.

We assumed that addition of increasing amounts of orthophosphine phenol (1) to the solution of complex [NiBr2(bpy)2] must result in a change in morphology of the CV curve due to formation of organonickel complexes according to the reaction of oxidative addition of electrochemically generated complexes [Ni0(bpy)2] with molecules of orthophos phine phenol. It is found experimentally that a slight splitting of the first reduction peak С1 and appear ance of a new peak С3 at higher cathodic potentials in the CV curve of complex (4) occurs in the presence of (1) (Fig. 3). Appearance of prepeak С1 is rather com mon for organophosphorus ligands, which is related to the presence of the effect of reverse πdoping of delectrons of the metal to the coordinated atom of phosphorus, which summarily results in a certain decrease in the electron density on the metal atom [17–19]. Thus, one can conclude that orthophos phone phenol (1) can act as a ligand with respect to both the oxidized (Ni(II)) and reduced (Ni(0)) forms of complex (4) and splitting of the first reduction

OH 4 1 [NiBr2(bpy)(CH3C6H3(PPh2)OH)] 5 Scheme 3.

One must point out that the presence of a hydroxyl fragment in structure (1) also admits formation of salt structures with the Ni–O bond as a result of elimina tion of HBr. Therefore, compound (2) containing no hydroxyl group was used to establish the coordination bonding type in the case of formation of coordination compounds of nickel (II) in the solution. It was shown using the method of mass spectrometry that addition of (2) to the solution of complex [NiBr2(bpy)2] indeed resulted in formation of a new cationic complex (6) according to the ligand exchange reaction (Scheme 4). PPh2 [NiBr2(bpy)2] +

–bpy

OP(O)Ph2 4 2 [NiBr(bpy)CH3C6H3(PPh2)OP(O)Ph2]+Br– 6 Scheme 4.

RUSSIAN JOURNAL OF ELECTROCHEMISTRY

Vol. 51

No. 11

2015

ELECTROCHEMICAL PROPERTIES AND CATALYTIC ACTIVITY (a)

(b) Experimental isotopic distribution

787.1 785.1

1075

Theoretical isotopic distribution

787.1 785.1

786.1

784

788.1 789.1 790.1 791.1 788

788.1 789.1 786.1 790.1 791.1792.1 793.1

792.1

792

m/z

796 784

788

(c) 1123.1

(d) Experimental isotopic distribution

1123.1

1121.1

1121.1

1124.1 1125.1 1122.1

1122.1

1124

792

796

Theoretical isotopic distribution

1124.1 1125.1 1126.1 1127.1 1128.1 1129.1

1126.1 1127.1 1128.1 1120

m/z

1128

1120

1124

m/z

1128

1132

m/z

Fig. 4. (a, c) Experimental and (b, d) theoretical isotopic distributions of compounds (a, b) (6) and (c, d) (7).

Moreover, at an excess amount of 2 and solution conditioning for several hours, molecules of organo phosphorus compound can fully replace bpy mole cules in the initial complex with formation of bische late complexes 7 (Scheme 5). 6+2

–bpy

[NiBr{CH3C6H3(PPh2)OP(O)Ph2}2]+Br– 7 Scheme 5.

Formation of complexes 6 and 7 in the solution was confirmed using the method of mass spectrometry (Fig. 4). Thus, the peak of m/z 785.1 corresponding to complex cation (6) is present in the mass spectrum of the solution of complex [NiBr2(bpy)2] after addition of the equivalent amount of 2diphenyl phosphanyl4 methylphenyldiphenyl phosphinate (2), while the peak with m/z 1123.1 corresponding to complex (7) is

With account for the earlier obtained results, one can conclude on the basis of the developed methods of electrochemical generation of organonickel sigma complexes [20] that the new reduction peak С3 appearing in the CV curve of complex 4 in the pres ence of (1) corresponds to reduction of the product of oxidative addition of complex [Ni0(bpy)2] generated electrochemically at potentials C1 to the added ortho phosphine phenol (1) via the O–H bond and repre sents a hydride complex of nickel (8) in accordance with the earlier described processes with participation of complex [Ni0(COD)2] [8] (Scheme 6). Ph2 P H Ni (bpy) O

PPh2

[Ni0(bpy)2] +

present in the mass spectrum of the solution at an excess (>2 eq.) in the added organophosphorus ligand. Experimental isotopic distributions agree exactly with the theoretically calculated ones (Fig. 4).

–bpy

OH 1

Ph2 P H Ni + bpy O 䊐 8

䊐 vacant coordination site Scheme 6.

RUSSIAN JOURNAL OF ELECTROCHEMISTRY

Vol. 51

No. 11

2015

1076

FOMINA et al. A7

[NiBr2(bpy)2] [NiBr2(bpy)2] + 2 (3 eq.) [NiBr2(bpy)2] + 3 (2 eq.)

24

A8

18

I, µA

12

A5

A6

A1

6 A2 0 –6

C1

C2 –1.9

–2.6

–1.2

–0.5

0.2

0.9

E, V Fig. 5. CV curves of complex (4) in the DMFA medium in the absence (solid line) and in the presence of (2) (dashed line) and (3) (dotted line) (the concentration of the complex is 5 × 10–3 M, the cathode is GC, v = 50 mV/s).

Absence of a hydroxyl fragment in the structure of (2) and chelating ability of compound (3) can cause the low reactivity of these compounds in the reaction of oxidative addition. Thus, the CV curve of complex (4) in the presence of 2diphenyl phosphanyl4methylphe nyldiphenyl phosphinate (2) or 2diphenyl phospho ryl4methyl phenol (3) manifests only an increase in the anodic current peaks due to oxidation of the phos phorus center in molecule (2) (peak А7, Fig. 5) and hydroxyl fragment in (3) (peak А8, Fig. 5). It is notable that such behavior was characteristic for systems con taining complex (4) in the presence of PPh3 and PhOH used as model compounds containing either no hydroxyl or no phosphanyl fragments, accordingly (Fig. 6, Table 1). It should be pointed out that complex (8) is a coun terpart of catalytically active particles of the process of ethylene oligomerization under the action of nickel

PPh2 + 2e OH

[NiBr2(bpy)2] +

complexes with PCCO chelate ligands (SHOP). Therefore, it was of interest to study catalytic activity of the forming organonickel intermediates in the pro cess of catalytic oligomerization of ethylene. Solutions obtained both by electrochemical coreduction of complex (4) in the presence of derivatives of ortho phosphine phenol (1) and (2) (method A) and also solutions obtained by the mixing of electrochemically synthesized complex [Ni0(bpy)2] [21, 22] with the used organophosphorus ligands (method B) were used to study catalytic activity of nickel complexes formed in the course of the electrochemical process. The general scheme of the process of electrochem ical formation of catalytically active complexes of nickel (nickel hydride complexes) (8) according to the reaction of reduced forms of the [NiBr2(bpy)2] com plex with orthophosphine phenol (1) can be presented in the following form (Scheme 7):

Ph2 P H Ni (bpy) O

EpC1 –bpy –2Br–

1

Ph2 P H Ni + bpy O 䊐 8

䊐 vacant coordination site Scheme 7.

Preparative electrolysis was carried out in the cell with separated anodic and cathodic compartments in DMFA in the absence of specially added background electrolyte. Conductivity of the solution was provided by [Ni(bpy)2]2+ ions and bromide anions forming in the

course of dissociation of the initial complex (4). To carry out the process of oligomerization/polymerization of ethylene, the working solution was evaporated after elec trolysis and the products of electrolysis were extracted using toluene. The obtained toluene extract was used to

RUSSIAN JOURNAL OF ELECTROCHEMISTRY

Vol. 51

No. 11

2015

ELECTROCHEMICAL PROPERTIES AND CATALYTIC ACTIVITY A8

[NiBr2(bpy)2] [NiBr2(bpy)2] + PPh3 (1 eq.) [NiBr2(bpy)2] + PhOH (1 eq.)

24 18

1077

A5 A7

A6

I, µA

12 A1

6 A2 0 –6 C1

C2 –2.6

–1.9

–1.2

–0.5

0.2

0.9

E, V Fig. 6. CV curves of complex (4) in the DMFA medium in the absence (solid line) and in the presence of PPh3 (dashed line) and PhOH (dotted line) (the concentration of the complex is 5 × 10–3 M, the cathode is GC, v = 50 mV/s).

perform catalytic tests. The results and conditions of the performed catalytic experiments are presented in Table 2. As can be seen from the obtained data (experi ments 4–6 and 11–12, Table 2), electrochemically generated complexes [Ni0(bpy)2] can react with organo phosphorus derivatives (1) and (2) forming active cat alysts of oligomerization/polymerization of ethylene allowing to convert more than 90% of loaded ethylene into macromolecular products. This is very interesting and promising, as electrochemical methods have never been used earlier to obtain active catalysts of processes of ethylene oligomerization and polymerization using organophosphorus PCCO of chelate ligands. More over, catalytic activity of electrochemically obtained complexes is as high as that of the known counterparts based on using low–stability complex [Ni0(COD)2] as a precatalyst (experiments 7–8 and 13–14, Table 2). One must also point out here that the process of elec trochemical generation of the active catalyst form consumes much less energy, as complex [Ni0(COD)2] requires thermal activation and large energy con sumption for generation of the very complex of nickel(0) [8]. Moreover, the nature and properties of the obtained polymer products (Table 2) argue for the similar structure of catalytically active forms of the organonickel catalyst in the both cases. Unfortunately, the catalytic activity of solutions obtained in situ in the case of electrochemical core duction of complex [NiBr2(bpy)2] in the presence of (1) or (2) (method A) proved to be low as compared to the method of preparation of the solution of orga nonickel catalyst by mixing solutions of electrochem ically generated complex [Ni0(bpy)2] and the organo phosphorus ligand (method B). This can be related to formation in the solution in the course of electro RUSSIAN JOURNAL OF ELECTROCHEMISTRY

Vol. 51

chemical process of catalytically inactive bischelate complexes (7), which is typical for polar solvents, such as DMFA that was used in the process of preparative reduction [6]. Carrying out such catalytic tests of solu tions containing nickel complexes obtained on the basis of organophosphorus compound (3) resulted in no pos itive result, which was expected, as the absence of a phosphorus atom capable of coordination in the mole cule of 2diphenyl phosphoryl4methyl phenol (3) causes a significant decrease in stability of nickel– hydride complexes due to the absence of the chelating ability of the organophosphorus ligand. Thus, this study shows for the first time that elec trochemical methods can be efficiently used to obtain active forms of organonickel catalysts for processes of homogeneous oligomerization/polymerization of eth ylene on the basis of derivatives of ternary orthophos phine phenols and complexes of nickel(II) stabilized by 2,2'bipyridyl. ACKNOWLEDGMENTS The work was supported by the Russian Founda tion for Basic Research and Academy of Sciences of the Republic of Tatarstan (project no. 154302667 r_povolzhye_a). REFERENCES 1. Kuhn, P., Sémeril, D., Matt, D., Chetcuti, M.J., and Lutz, P., Dalton Trans., 2007, p. 515. 2. Ittel, S.D., Johnson, L.K., and Brookhart, M., Chem. Rev., 2000, vol. 100, p. 1169. 3. Gibson, V.C. and Spitzmesser, S.K., Chem. Rev., 2003, vol. 103, p. 283. No. 11

2015

1078

FOMINA et al.

4. Late Transition Metal Polymerisation Catalysis, Rieger, B., Saunders, L., Kacker, S., and Striegler, S., Eds., Wein heim: WileyVCH Verlag GmbH & Co. KGaA, 2003. 5. Keim, W., in Industrial Applications of Homogeneous Catalysis, Mortreux, A. and Petit, F, Eds., Dordrecht: D. Riedel Publishing Company, 1988, pp. 335–347. 6. Keim, W., Angew. Chem., 1990, vol. 102, p. 251. 7. Keim, W., Vysokomol. Soed., Ser. A, 1994, vol. 36, p. 1644. 8. Yakhvarov, D.G., Basvani, K.R., Kindermann, M.K., Dobrynin, A.B., Litvinov, I.A., Sinyashin, O.G., Jones, P.G., and Heinicke, J., Eur. J. Inorg. Chem., 2009, vol. 9, p. 1234. 9. Yakhvarov, D.G., Tazeev, D.I., Sinyashin, O.G., Giam bastiani, G., Bianchini, C., Segarra, A.M., Lonnecke, P., and HeyHawkins, E., Polyhedron, 2006, vol. 25, p. 1607. 10. Zhou, Z.Y., Tian, N., Huang, Z.Z., Chen, D.J., and Sun, S.G., Faraday Discuss., 2008, vol. 140, p. 81. 11. Yang, H., Ouyang, J., Tang, A., Xiao, Y., Li, X., Dong, X., and Yu, Y., Mater. Res. Bull., 2006, vol. 41, p. 1310. 12. Heinicke, J., Kadyrov, R., Kindermann, M.K., Koesling, M., and Jones, P.G., Chem. Ber., 1996, vol. 129, p. 1547. 13. Budnikova, Yu.G., Yakhvarov, D.G., Morozov, V.I., Kargin, Yu.M., Il’yasov, A.V., Vyakhireva, Yu.N., and

14. 15. 16. 17. 18. 19. 20. 21.

22.

Sinyashin, O.G., Russ. J. Gen. Chem., 2002, vol. 72, p. 168. Yakhvarov, D.G., Budnikova, Yu.G., and Sinyashin, O.G., Russ. J. Electrochem., 2003, vol. 39, p. 1261. Yakhvarov, D.G., Samieva, E.G., Tazeev, D.I., and Budnikova, Yu.G., Russ. Chem. Bull., 2002, vol. 51, p. 796. Fomina, O.S., Sinyashin, O.G., Heinicke, J., and Yakhvarov, D.G., Butlerov Commun., 2012, vol. 32, no. 10, p. 65. Budnikova, Y.H. and Kargin, Y.M., J. Organomet. Chem., 1997, vol. 265, p. 536. Budnikova, Y., Kargin, Y., Nédélec, J.Y., and Peri chon, J., J. Organomet. Chem., 1999, vol. 575, p. 63. Budnikova, Yu.H., Perichon, J., Yakhvarov, D.G., Kargin, Yu.M., and Sinyashin, O.G., J. Organomet. Chem., 2001, vol. 630, p. P. 185. Yakhvarov, D.G., Trofimova, E.A., Rizvanov, I.Kh., Fomina, O.S., and Sinyashin, O.G., Russ. J. Electro chem., 2011, vol. 47, p. 1100. Yakhvarov, D.G., Petr, A., Kataev, V., Büchner, B., GómezRuiz, S., HeyHawkins, E., Kvashennikova, S.V., Ganushevich, Yu.S., Morozov, V.I., and Sinyashin, O.G., J. Organomet. Chem., 2014, vol. 750, p. 59. Yakhvarov, D.G., Khusnuriyalova, A.F., and Sinya shin, O.G., Organometallics, 2014, vol. 33, p. 4574.

RUSSIAN JOURNAL OF ELECTROCHEMISTRY

Translated by M. Ehrenburg

Vol. 51

No. 11

2015