Spectrophotometric study of dehalogenation of benzyl halides with nickel in aprotic dipolar solvents in the presence of oxygen

Article abstract of DOI:10.1134/S1070363216020079

Oxidative dissolution of nickel in the benzyl halogenide - dipolar aprotic solvent system in the presence of air oxygen proceeds with the formation of benzaldehyde, benzyl alcohol, 1,2-diphenylethane, and trace amounts of 4,4'-ditolyl. Spectrophotometry studies have revealed that the oxidation products are formed in the solution rather than at the nickel surface. It has been shown that the oxygen adsorbed at the nickel surface practically does not pass into the solution.

Full text of DOI:10.1134/S1070363216020079

ISSN 1070-3632, Russian Journal of General Chemistry, 2016, Vol. 86, No. 2, pp. 251–255. © Pleiades Publishing, Ltd., 2016.
Original Russian Text © А.М. Egorov, А.N. Novikova, Е.V. Stepanova, 2016, published in Zhurnal Obshchei Khimii, 2016, Vol. 86, No. 2, pp. 222–226.
Spectrophotometric Study of Dehalogenation of Benzyl Halides
with Nickel in Aprotic Dipolar Solvents in the Presence of Oxygen
А. М. Egorova, А. N. Novikovab, and Е. V. Stepanovaa
a Tula State University, pr. Lenina 92, Tula, 300600 Russia
e-mail: stepanova_ev@yahoo.com
b FSBI “Tula Interregional Veterinary Laboratory,” Tula, Russia
Received August 6, 2015
Abstract—Oxidative dissolution of nickel in the benzyl halogenide – dipolar aprotic solvent system in the
presence of air oxygen proceeds with the formation of benzaldehyde, benzyl alcohol, 1,2-diphenylethane, and
trace amounts of 4,4'-ditolyl. Spectrophotometry studies have revealed that the oxidation products are formed
in the solution rather than at the nickel surface. It has been shown that the oxygen adsorbed at the nickel surface
practically does not pass into the solution.
Keywords: nickel, benzyl halide, oxygen, dipolar aprotic solvent, spectrophotometry
DOI: 10.1134/S1070363216020079
Nickel complexes with organic ligands have been
used for modification of polymers to prepare the
materials with desired properties. They have been
often used as efficient light stabilizers of different
polymers [1]. Halogenated nickel complex compounds
are generally prepared via direct dissolution of the
metal in the organic halide – dipolar aprotic solvent
system under inert atmosphere. Carbon tetrachloride
and benzyl halides have been used [2] as the halide
component. Performing these processes in the presence
of oxygen has been scarcely studied, their mechanism
has not been established, and the kinetic features have
not been determined; this has prevented scaling up of
these reactions to be used in industry.
presence of oxygen was studied aiming to develop the
“green” technology for preparation of the nickel
complex compounds.
The analysis of the products by chromato–mass
spectrometry revealed that the organic products of the
reaction of fine-disperse nickel powder of the “PNE–1.
Lux” grade with benzyl halides were 1,2-
diphenylethane, benzaldehyde, and benzyl alcohol, on
top of trace amounts of 4,4'-ditolyl, benzoic acid, and
benzyl benzoate. Oxidative dissolution of nickel in the
benzyl halide–dipolar aprotic solvent–oxygen system
proceeded according to the Scheme 1.
Treatment of the reaction mixtures containing benzyl
bromide with diethyl ether gave the nickel(II) complex
compounds: [Ni(DMF)6]Br2 and [Ni(DMSO)6]Br2.
In this work, mechanism of the benzyl halides
reaction with nickel in dipolar aprotic solvents in the
Scheme 1.
O
H
O2
+
C
Ni +
CH2Hlg + L
NiHlg2 nL +
CH2 CH2
O
O
O
+
H3C
+
C
+
CH2 OH
CH3 +
C
OH
CH2
Hlg = Br, Cl; L = DMF, DMSO, n = 3, 6.
251

252
EGOROV et al.
radical reaction mechanism with the formation of
benzyl radical as the intermediate, its recombination
and isomerization occurring in the radical pair [3]. On
top of that, benzaldehyde and benzyl alcohol were
isolated, apparently, the products of benzyl radical
oxidation with oxygen [4].
Benzyl radical was detected in the reaction mixture
using a chemical radical trap – dicyclohexyldeutero-
phosphine (DCPD) [2, 4]. When performing the
reaction in the presence of DCPD (molar ratio Ni :
DCPD = 1 : 5), α-deuterotoluene and trace amounts of
1,2-diphenylethane were detected in the reaction
mixture. Benzaldehyde, benzyl alcohol, benzoic acid,
benzyl benzoate, and 4,4'-ditolyl were absent,
indicating the reaction mechanism via the formation of
benzyl radical [2, 4].
Oxidation of benzyl radical might occur both in
solution and at the nickel surface. In order to identify
the reactive site, we investigated the nickel–benzyl
halogenide–dipolar aprotic solvent–oxygen system by
means of spectrophotometry.
The setup for investigation of oxidative dissolution of
nickel in the benzyl halide–solvent–oxygen system by
spectrophotometric method. (1) quartz cuvette (l = 2 cm),
(2) dropping funne, (3) Schlenk tube, (4) input for selection
of the reaction mixture, (5) a cover of polyamide-6 with a
rubber sealing gasket, and (6) micromixer.
It is known that certain dipolar aprotic solvents
form charge transfer complexes (CTC) with molecular
oxygen [5]. It was found that for CTC between DMSO
(DMF) and oxygen could be identified by the
appearance of the absorption band at 256 (264) nm
[6, 7].
After oxidative dissolution of nickel in the benzyl
chloride–DMSO–oxygen system, [Ni(DMSO)6][NiCl4]
and [Ni(DMSO)6]Сl2 were isolated from the reaction
mixture in the ratio depending on the concentration of
DMSO and the temperature.
The interactions in the nickel–benzyl halide–
dipolar aprotic solvent–oxygen system were invest-
tigated using the setup depicted in the figure.
Similar results were obtained using nickel nano-
powder of Hongwu International Group Ltd (China)
production or of the oxygen-stabilized nanoparticles
(oxygen content <0.15 %) from “Advanced Powder
Technologies” (Russia).
The setup was made on the basis of standard quartz
cell placed in a cell holder of CF-26 spectrophoto-
meter, allowing for unambiguous detection of
appearance and disappearance of the absorption bands
of CTCs between oxygen and dipolar aprotic solvent,
as well as for performing the experiments in the semi-
automated mode.
Dissolution of nickel nanopowders stabilized with
carbon (carbon content 0.08%) or oxygen (oxygen
content > 1 %), in the benzyl halide – dipolar aprotic
solvent – oxygen systems proceeded with substantial
inductive period, owing to the presence of dense oxide
or carbide film at the nanoparticle surface. However,
the ratio of the products was always the same. Except
for fine-disperse nickel powder “PNE–1. Lux,” all the
used nanopowders formed floating suspensions in the
dipolar aprotic solvents, making the dosing procedure
easy. Therefore, we used the nickel powder produced
in argon atmosphere or stabilized with oxygen (oxygen
content < 0.15 %) for the spectrophotometry studies.
In the first series of the experiments, oxygen was
passed through the solvent till the appearance of the
absorption band characteristic of the CTC between
DMSO (DMF) and oxygen [5–7]. Then, nickel
nanopowder prepared in argon atmosphere or the
oxygen-stabilized nanopowder (oxygen content
<0.15 %) was added. The absorption bands instantly
disappeared, likely due to fast adsorption of oxygen at
the nickel surface. After addition of benzyl bromide to
the reaction mixture and carrying out the reaction
during 2 h, the reaction products were analyzed.
The presence of 1,2-diphenylethane and trace
amounts of 4,4'-ditolyl in the products suggested the
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SPECTROPHOTOMETRIC STUDY OF DEHALOGENATION OF BENZYL HALIDES
253
The chromato–mass spectrometry showed that in
the both cases the products of the reaction were 1,2-
diphenylethane (99.9 %) and trace amounts of 4,4'-
ditolyl (<0.1 %). Similar results were obtained for
benzyl chloride. Noteworthily, the same products were
formed in the course of oxidative dissolution of nickel
in the benzyl halide – dipolar aprotic solvent systems
in the absence of oxygen [8]. However, the metal
dissolution was much faster in the presence of small
amounts of oxygen. The absence of the products of
oxidation of benzaldehyde, benzyl alcohol as well as
even trace amounts of benzoic acid and benzyl
benzoate in the first series of the experiments, along
with disappearance of the absorption band, suggested
that oxygen was adsorbed at the nickel surface and
practically did not pass to the solution; no oxidation
with oxygen occurred at the metal surface.
and analysis of the reaction mixtures in the first and
second experiments were performed under the same
conditions.
In the second experiment, the continuous supply of
oxygen in the system under investigation led to the
formation of the oxidation products: benzaldehyde
(64.5±1.5) mol % and benzyl alcohol (31.5±1.5) mol %.
On top of that, 4 mol % of 1,2-diphenylethane and
trace amounts of 4,4'-ditolyl, benzoic acid, and benzyl
benzoate were found in the reaction mixture, the
concentration of 1,2-diphenylethane being much lower
than that in the first experiment. When excess oxygen
was supplied, the amount of 1,2-diphenylethane was
decreased to 0.1 mol %, and that of benzoic acid
increased to 1 mol %. Similar results were obtained for
benzyl chloride as the substrate. The determined ratio
of the reaction products suggested that the investigated
process proceeded mainly via the radical mechanism
(formation of benzyl radical that was rapidly oxidized
with oxygen in the solution), and no formation of CTC
was observed till the reaction was complete.
Using the methods of secondary ionization mass
spectrometry (SIMS), X-ray photoelectron spectro-
scopy (XPS), near-edge X-ray adsorption fine structure
(NEXAFS), temperature programmed desorption
(TPD), and low-energy electron diffraction (LEED) it
has been shown that the first stages of oxygen
interaction with different surfaces of pure nickel are
reversible chemisorption and physical adsorption [9–
11]. Reversible chemisorption of oxygen at nickel
surface proceeds with predominant formation of
Nin–O–O– and NinO2. It has been found that either
reversible chemisorption of oxygen or its partial
desorption from the metal surface follow the physical
adsorption of molecular О2 on Ni(100) at temperatures
above 80 K [10, 11].
Comparison of the results of those experiments led
to a conclusion that the formation of the reaction pro-
ducts (benzaldehyde, benzyl alcohol, 1,2-diphenyl-
ethane, and traces of 4,4'-ditolyl, benzoic acid, and
benzyl benzoate) proceeded mainly in the solution
rather than at the nickel surface. The oxygen adsorbed
on the nickel surface practically does not pass to the
solution.
EXPERIMENTAL
The reaction products were analyzed using an
Agilent (USA) chromato–mass spectrometer (mass
detector 5975C, gas chromatograph 7890A) equipped
with a capillary column (l = 30 m, d = 0.25 mm),
diphenyl (5%) on polydimethylsiloxane was used as
the stationary phase; column temperature was of 40–
250°С; heating rate was of 30 deg/min; helium (flow
rate 1 mL/min) was used as the carrier gas; injector
temperature was of 250°С; detector temperature was of
280°С.
The results obtained in this work and the data
reported in [9–11] coincided well with the data of [4],
the latter demonstrating that the intermediates of the
reaction of oxidative dissolution of copper in the
benzyl bromide – dimethylacetamide – oxygen system
are Cun+–O–O– species capable of electron transfer to
benzyl bromide.
The second series of the experiments was
performed similarly, but the oxygen supply was
continuous. In that case, addition of the fine-disperse
nickel powder led to a short-term slight weakening of
the corresponding absorption band. After recovery of
the band intensity, benzyl halide was added, and the
process was carried out during 2 h. Addition of benzyl
halide led to disappearance of the CTC absorption
band, being recovered only after complete dissolution
of nickel or after supply of excess oxygen. Sampling
Elemental analysis of deuterated organic com-
pounds was performed using a Carlo-Erba-1100 (Carlo
Erba Instruments, Italy) gas chromatograph analyzer
and a Tsvet-570 (Russia) gas chromatograph as described
in [12].
The absorption bands of CTC between the solvent
and oxygen were observed using a SF-26 spectro-
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254
EGOROV et al.
photometer by recording the spectra over the range of
186 to 1100 nm.
26.28; Ni 16.73. NiC6H18S3O3Cl2. Calculated, %: С
19.70; Н 4.95; С1 19.47; S 26.40; Ni 16.20. IR
spectrum, ν, cm–1: 425 (Ni–O), 940 (S=O···Ni) (KCl);
1000 (S=O···Ni), 1037 (S=O···Ni) (CH3NO2). Refe-
rence data: mp 122–124 °С [15], IR spectrum, ν, cm–1:
425 (Ni–O) [15]; 940 (S=O···Ni) (KCl); 1000
(S=O···Ni), 1037 (S=O···Ni) (CH3NO2) [16].
The following nickel powders were used: nickel
powder (fraction 0.050 mm) from “Ural electrochemical
plant” “PNE–1. Lux” (nickel content 99.8405%); nickel
nanopowder (average particle size 70 nm, surface area
6 m2/g) stabilized with oxygen (oxygen content <0.15 %
and >1 %) from “Advanced powder technologies
Ltd.;” nickel nanopowder (average particle size 40 nm)
stabilized with carbon (carbon content 0.08%) from
Hongwu International Group Ltd (China); nickel
nanopowder (average particle size 40 nm) prepared in
argon atmosphere (oxygen and carbon absent) from
Hongwu International Group Ltd (China).
Hexakis(dimethylsulfoxido)nickel(II)
chloride
[Ni(DMSO)6]Сl2. Yield 30%, mp 66–68°C. Found, %:
C 24.06, H 6.05, Cl 11.87, Ni 9.78, O 16.08, S 32.16.
C12H36NiS6O6Cl2. Calculated, %: C 24.09, H 6.06, Cl
11.85, Ni 9.81, O 16.04, S 32.15. Reference data: mp
66–68°C [15].
Hexakis(dimethylsulfoxido)nickel(II) bromide
[Ni(DMSO)6]Br2. Yield 90 %, mp 83–85°C. Found,
%: C 20.96, H 5.30, Br 23.23, Ni 8.57, O 13.98, S
27.96. C12H36Br2NiO6S6. Calculated, %: C 20.97, H
5.28, Br 23.25, Ni 8.54, O 13.97, S 27.99. IR spec-
trum, ν, cm–1: 444 (Ni–O), 380 (C–S–O), 347 (C–S–O),
315 (C-S-C), 306 (СН3) (Vaseline oil); 957 (S=O···Ni)
(KBr). Reference data: mp 83–85°C [15], IR spectrum,
ν, cm–1: 957 (S=O···Ni) (KBr) [16].
Benzyl chloride (“analytical pure” grade) was dried
with CaCl2 and distilled under reduced pressure, bp
65–66°C (11 mmHg) {bp 66°C (11 mmHg). [13]}.
The organic solvents were purified by conventional
procedures [14]. All the solvents and reagents were
degassed by repeated freeze-thawing under reduced
pressure and stored in sealed ampoules.
Dehalogenation of benzyl halides with nickel in
dipolar aprotic solvent in the presence of oxygen.
20 mL of dipolar aprotic solvent (DMF or DMSO) and
0.1 mol of benzyl halide were added to 0.02 mol of
nickel powder. The reaction was carried out in the
presence of dry air at 70°С. After dissolution of nickel
and cooling the vessel to 20°С, the reaction mixture
was treated with 20 mL of dry diethyl ether. The
precipitated nickel complex compound was separated,
and the ether solution was analyzed by means of
chromato-mass spectrometry.
Dehalogenation of benzyl halide with nickel in
dipolar aprotic solvent. a. Oxygen was passed for a
short time through 1 mL of dipolar aprotic solvent,
then 0.1 g of nickel powder (in the form of suspension
in 1 mL of the corresponding solvent) and 0.5 mL of
benzyl halogenide were successively added upon
stirring. Reaction time 2 h.
b. Oxygen was continuously passed through 1 mL
of dipolar aprotic solvent, and 0.1 g of nickel powder
(in the form of the suspension in 1 mL of the corres-
ponding solvent) and 0.5 mL of benzyl halogenide
were successively added upon stirring. Reaction time 2 h.
Hexakis(dimethylformamido)nickel(II) bromide
[Ni(DMF)6]Br2. Yield 78 %. Found, %: C 32.92, H
6.48, Br 24.35, N 12.77, Ni 8.90, O 14.57.
C18H42Br2N6NiO6. Calculated, %: C 32.90, H 6.44, Br
24.32, N 12.79, Ni 8.93, O 14.62. IR spectrum, ν, cm–1:
425 (Ni–O), 465 (Ni–O), 690 (OCN), 1030 (СН3),
1070 (СН3), 1120 (СН3), 1160 (СН3), 1255 (С–N),
1387 (СН3), 1425 (СН3), 1445 (СН3), 1635
(C=O···Ni), 1650 (C=O···Ni) (KBr). Reference data: ν,
cm–1: 425 (Ni–O), 465 (Ni–O), 690 (OCN), 1030
(СН3), 1070 (СН3), 1120 (СН3), 1160 (СН3), 1255 (С–
N), 1387 (СН3), 1425 (СН3), 1445 (СН3), 1635
(C=O···Ni), 1650 (C=O···Ni) (KBr) [8].
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