An oxidovanadium(IV) complex with 4,4′-di-tert-butyl-2,2′-bipyridine ligand: Synthesis, structure and catalyzed cyclooctene epoxidation

Article abstract of DOI:10.1016/j.poly.2019.114305

The interaction of vanadium(III) chloride (VCl₃) with 4,4′-di-tert-butyl-2,2′-bipyridine (dbbpy) in air resulted in the monomeric oxidovanadium(IV) complex [VOCl₂(dbbpy)(H₂O)] (1) in high yield. The complex was characterized by IR and EPR spectroscopies, by elemental analysis, and by single crystal X-ray diffraction analysis. The vanadium atom has a distorted octahedral coordination environment. The EPR spectrum of 1 in CH₂Cl₂ demonstrates an eight-line signal typical of vanadium(IV) with a d¹ electronic configuration. Complex 1 exhibits catalytic activity in the cyclooctene oxidation with tert-butyl hydroperoxide (TBHP) in CHCl₃. Detailed EPR, NMR and GC–MS studies of the reaction revealed a few mechanistic details and the nature of by-products that are generated by involvement of the chloroform solvent.

Full text of DOI:10.1016/j.poly.2019.114305

Journal Pre-proofs
An oxidovanadium(IV) complex with 4,4’-di-tert-butyl-2,2’-bipyridine ligand:
synthesis, structure and catalyzed cyclooctene epoxidation
Iakov S. Fomenko, Sandrine Vincendeau, Eric Manoury, Rinaldo Poli, Pavel A.
Abramov, Vladimir A. Nadolinny, Maxim N. Sokolov, Artem L. Gushchin
PII:
S0277-5387(19)30750-8
DOI:
https://doi.org/10.1016/j.poly.2019.114305
POLY 114305
Reference:
Polyhedron
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Revised Date:
Accepted Date:
30 September 2019
30 October 2019
1 December 2019
Please cite this article as: I.S. Fomenko, S. Vincendeau, E. Manoury, R. Poli, P.A. Abramov, V.A. Nadolinny, M.N.
Sokolov, A.L. Gushchin, An oxidovanadium(IV) complex with 4,4’-di-tert-butyl-2,2’-bipyridine ligand: synthesis,
structure and catalyzed cyclooctene epoxidation, Polyhedron (2019), doi: https://doi.org/10.1016/j.poly.
2019.114305
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An oxidovanadium(IV) complex with 4,4’-di-tert-butyl-2,2’-
bipyridine ligand: synthesis, structure and catalyzed cyclooctene
epoxidation
Iakov S. Fomenko,b Sandrine Vincendeau,a Eric Manoury,a Rinaldo Poli,a,c, Pavel A.
Abramov,b Vladimir A. Nadolinny,b Maxim N. Sokolov,b,d Artem L. Gushchin,b,d*
a CNRS, LCC (Laboratoire de Chimie de Coordination), Université de Toulouse, UPS, INPT,
205 Route de Narbonne, BP 44099, 31077 Toulouse, France.
b Nikolaev Institute of Inorganic Chemistry, Siberian Branch of Russian Academy of Sciences,
3 Acad. Lavrentiev Avenue, Novosibirsk 630090, Russia. E-mail: gushchin@niic.nsc.ru
c Institut Universitaire de France, 1 Rue Descartes, 75231 Paris Cedex 05, France.
d Novosibirsk State University, 2 Pirogov Street, 630090 Novosibirsk, Russia
* Corresponding author. E-mail: gushchin@niic.nsc.ru (Artem L. Gushchin)
Abstract
The interaction of vanadium(III) chloride (VCl3) with 4,4’-di-tert-butyl-2,2’-bipyridine
(dbbpy) in air resulted in the monomeric oxidovanadium(IV) complex [VOCl2(dbbpy)(H2O)]
(1) in high yield. The complex was characterized by IR and EPR spectroscopies, by elemental
analysis, and by single crystal X-ray diffraction analysis. The vanadium atom has a distorted
octahedral coordination environment. The EPR spectrum of 1 in CH2Cl2 demonstrates an
eight-line signal typical of vanadium(IV) with a d1 electronic configuration. Complex 1
exhibits catalytic activity in the cyclooctene oxidation with tert-butyl hydroperoxide (TBHP)
in CHCl3. Detailed EPR, NMR and GC-MS studies of the reaction revealed a few mechanistic
details and the nature of by-products that are generated by involvement of the chloroform
solvent.
Keywords: oxidovanadium(IV) complexes; 2,2’-bipyridine ligands; crystal structure; EPR
spectroscopy; cyclooctene epoxidation.
1. Introduction
Oxidovanadium complexes containing the V=O moiety are nearly omnipresent in the
chemistry of vanadium [1]. They exhibit high catalytic activity in various organic
1

transformations including alkane oxidation [2–10], olefin epoxidation [11–18], aromatization
of α,β-unsaturated cyclohexanone derivatives [19,20], alcohol oxidation [21–25], C-C bond
cleavage of glycols [26–28], naphthol coupling [29] and α-oxidation of hydroxyl esters and
amides [30] etc. The use of oxovanadium complexes in the oxidation reactions of various
substrates, in particular saturated and unsaturated hydrocarbons is due to the following
factors: (a) the ability of vanadium to adopt different coordination numbers (5 and 6), (b) the
stability of vanadium in oxidation states +4 and +5 under aerobic conditions, (c) the Lewis
acidic nature of the vanadium centers and (d) the high oxophilicity of vanadium.
Our recent studies have shown that oxovanadium(IV) complexes with redox-active
diimine ligands exhibit high catalytic activity towards alkanes and alcohols oxygenation with
H2O2 or tert-butyl hydroperoxide in MeCN. In particular, the square-pyramidal
oxidovanadium(IV)
complex
[VOCl2(dpp-bian)]
(dpp-bian
=
bis[N-(2,6-
diisopropylphenyl)imino]acenaphthene) [10] and polymeric complexes of general formula
[VO(L)X2]n (L = bpy, phen and X = Cl, Br) [31] were synthesized and tested as oxidation
catalysts. In this framework, we report herein further results on the preparation of a novel
octahedral oxidovanadium(IV) complex [VOCl2(dbbpy)(H2O)] (1), as well as its catalytic
properties in the cyclooctene oxidation with tert-butyl hydroperoxide in CHCl3, which is
another commonly used solvent for catalyzed oxidations. Careful analysis of the reaction
products revealed the presence of side reactions that depend on the solvent nature and purity.
2. Experimental Section
2.1. General procedures
All synthetic manipulations were carried out in air. Catalytic experiments were carried
out in argon. Reagent grade chemicals were obtained from Aldrich and used without further
purification. All organic solvents were distilled by standard methods before use.
2.2. Physical Measurements
The CHN elemental analysis was performed on a Euro EA 3000 analyzer. The FT-IR
spectrum in the range of 4000-400 cm-1 was recorded on a Scimitar FTS 2000 spectrometer.
EPR spectra were recorded at the X- and Q- bands at 77 K and 300 K on an E-109 Varian
spectrometer equipped with an analog-to-digital signal converter and were analyzed and
simulated with the EasySpin (Matlab) software package [32].
2.3. X-ray crystallography
Single crystals of 1·1.5CH2Cl2 suitable for the X-ray diffraction analysis were obtained
by recrystallization from a CH2Cl2/Et2O mixture. The diffraction data for 1·1.5CH2Cl2 were
2

collected on a Bruker X8 Apex diffractometer with MoKα radiation (λ = 0.71073) by doing φ
and ω scans of narrow (0.5°) frames at 150 K. The absorption was empirically corrected using
SADABS [33]. Crystallographic data and refinement details for 1·1.5CH2Cl2 are given in
Table 1. The structure was solved with SHELXT [34] and refined by full-matrix least-squares
cycles against |F|2 in anisotropic approximation with SHELXL 2017/1 [35] in ShelXle
program [36]. Hydrogen atoms were refined in geometrically calculated positions. The
number of highly disordered CH2Cl2 molecules was found with the SQEEZE procedure [37],
which gave 63.5 e in a void with a 154.05 Å3 volume. It corresponds to 1.5 CH2Cl2, which has
42 e and a molecular volume of 106 Å. The crystallographic data have been deposited in the
Cambridge Crystallographic Data Centre under the deposition code CCDC 1945435.
Table 1. Main crystallographic characteristics and X-ray diffraction experimental details for
1·1.5CH2Cl2.
1·1.5CH2Cl2
Chemical formula
Mr
C19.5H29Cl5N2O2V
551.65
Crystal system, space group
Temperature (K)
a, b, c (Å)
Orthorhombic, Cmca
100
17.8155 (8), 11.4397 (6), 24.1718 (13)
V (Å3)
4926.3 (4)
8
Z
Radiation type
µ (mm-1)
Mo Kα
0.63
Crystal size (mm)
Diffractometer
Absorption correction
0.22 × 0.20 × 0.15
Bruker X8Apex
Multi-scan
SADABS (Bruker-AXS, 2004)
Tmin, Tmax
0.596, 0.746
No. of measured, independent
and
11345, 2933, 2630
observed [I > 2σ(I)] reflections
Rint
0.032
θ values (°)
θmax = 27.6, θmin = 3.9
0.652
(sin θ/λ)max (Å-1)
Range of h, k, l
R[F2 > 2σ(F2)], wR(F2), S
-23 ≤ h ≤ 17, -10 ≤ k ≤ 14, -26 ≤ l ≤ 31
0.056, 0.152, 1.21
2933, 121, 0
No. of reflections, parameters,
restraints
3

H-atom treatment
Weighting scheme
Δρmax, Δρmin (e Å-3)
H atoms treated by a mixture of independent and constrained
refinement
2
w = 1/[σ2(Fo ) + (0.0518P)2 + 26.942P]
where P = (Fo2 + 2Fc2)/3
0.55, -0.54
2.4. Synthesis of [VIVOCl2(dbbpy)(H2O)] (1)
To a suspension of VCl3 (115 mg, 0.7 mmol) in acetonitrile (14 ml) was added 4,4’-di-tert-
butyl-2,2’-bipyridine (196 mg, 0.7 mmol) and the mixture was refluxed for 4 hours in air. The
resulting solution was evaporated to dryness and the residue was dissolved in CH2Cl2 (7 ml).
Slow diffusion of an Et2O layer into the dichloromethane solution produced green crystals of
1·1.5CH2Cl2. These crystals readily lost part of the co-crystallized solvent when separated
from the mother liquor. Anal. Calc. for C18H26Cl2N2O2V.0.8CH2Cl2: С, 45.9; H, 5.7; N, 5.7
%. Found: C, 45.7; H, 5.6; N, 5.8 %. Extensive drying 1·1.5CH2Cl2 under vacuum gave 1.
Anal. Calc. for C18H26Cl2N2O2V: С, 50.9; H, 6.2; N, 6.6 %. Found: C, 50.9; H, 6.4; N, 6.6 %.
Yield: 327 mg (95%). IR (KBr) ν/cm−1: 3440(br.s), 3066(w), 2961(s), 2910(w), 2871(w),
1642(m), 1616(s), 1548(m), 1480(m), 1410(s), 1362(w), 1306(w), 1256(m), 1201(w),
1119(w), 1026(m), 989(s), 901(m), 865(m), 852(m), 746(w), 721(w), 609(m), 560(w).
2.5. Procedure for the catalyzed epoxidation reaction
With GC monitoring. Cyclooctene (135 mg, 1.2 mmol), complex 1 (10 mg, 0.024 mmol,
2%), trioxane (72 mg, 0.8 mmol) and 4 mL of chloroform were mixed in a Schlenk tube. The
mixture was stirred and a first aliquot (0.1 mL) was withdrawn, before adding the TBHP
solution (0.43 mL, 5.5 M in decane, 2.4 mmol). A second aliquot (0.1 mL) was then rapidly
taken. After that the reaction mixture was warmed to 55°C with stirring for 5 h. Aliquots were
analyzed by GC after quenching the residual TBHP by addition of excess PPh3. A control
31
experiment with GC and P NMR showed that the reaction between TBHP and PPh3 is
immediate and quantitative to produce tBuOH and Ph3P=O.
With 1H NMR monitoring. In a Schlenk flask, 1 (10 mg, 0.024 mmol) was dissolved in 4 ml
of chloroform. Subsequently, cycloooctene (160 l, 1.2 mmol) and trioxane (620 l, 0.8
mmol, internal standard) were added. The mixture was stirred and a first aliquot (0.1 mL) was
withdrawn. Then, 0.43 ml of a decane solution of TBHP (5.5 M, 2.4 mmol) were added to the
resulting solution. The mixture was stirred at 55°C and 0.1 mL aliquots were periodically
withdrawn for the 1H NMR monitoring. With the exception of the first aliquot which does not
contain TBHP, all other aliquots were introduced into an NMR tube containing ca. 63 mg
(0.24 mmol) of triphenylphosphine solubilized in CDCl3 in order to quench the residual
4

1
31
TBHP and then directly used for the H and P analyses. A control experiment showed that
the reaction between TBHP and PPh3 is immediate and quantitative to produce tBuOH and
Ph3P=O (identified by GC and 31P NMR). Another control experiment was run in the absence
of cyclooctene, in order to make sure that trioxane is not altered under the oxidizing
conditions, using the solvent peak as internal standard: integration of the chloroform signal at
7.3 ppm and of the trioxane signal at 5.15 ppm showed a constant intensity ratio over a 21 h
period at 55°C.
Large-scale epoxidation and isolation of the trans-1,2-dichlorocylooctane product.
Cyclooctene (2.07 g, 18.8 mmol), complex 1 (80 mg, 0.188 mmol, 1 mol%), and 30 mL of
chloroform were mixed in a Schlenk tube under argon. The mixture was stirred and a TBHP
solution (6.8 mL, 5.5 M in decane, 37.6 mmol) was added by syringe. The reaction mixture
was then warmed to 55°C with stirring for 42 h, with GC monitoring, and evaporated to
dryness to give an oily residue, which was redissolved in diethyl ether and filtered on a small
patch of silica to remove the catalyst. The resulting solution was evaporated again to dryness
yielding a yellow oil, which was dissolved in 4 mL of ethanol. 8 mL of 2 M HCl (16 mmol)
were added to this solution and the reaction mixture was stirred for 24 h at room temperature.
The organic phase was then extracted by dichloromethane, placed over Na2CO3, filtered and
evaporated. The oily residue was purified by chromatographic separation on a silica column
eluting with pentane. A colorless oily pure compound was obtained (yield: 30 mg, 1%) and
characterized as trans-1,2-dichlorocyclooctane by NMR analysis. A few other fractions
containing trans-1,2-dichlorocyclooctane together with other byproducts were also obtained
and discarded.
3. Results and discussion
3.1. Synthesis and characterization
The title complex was synthesized from vanadium(III) chloride (VCl3) and 4,4’-di-tert-
butyl-2,2’-bipyridine (dbbpy), with concomitant oxidation in air to give the {V=O}2+ moiety,
see Scheme 1. The same synthetic procedure was previously used for the preparation of other
oxidovanadium(IV) complexes with 2,2’-bipyridine, 1,10-phenathroline [31], bis(N-(2,6-
diisopropylphenyl)-imino)acenaphthene (dpp-bian) [10], dehydrophenanthroline and
diazofluorene ligands [42]. The reaction of VCl3 with 4,4’-di-tert-butyl-2,2’-bipyridine
(dbbpy) (1:1 molar ratio) in acetonitrile in air results in [VOCl2(dbbpy)(H2O)] (1) in high
yield (95%). The complex was isolated as a solvate with СH2Cl2 molecules (1·1.5CH2Cl2) as
confirmed by the X-ray structure data. Thе isolated crystals easily lose СH2Cl2 molecules to
5

give 1, which is stable in air, both in solution and in the solid state, and is well-soluble in
CH3CN, CH2Cl2, CHCl3 and other organic solvents. The same complex was recently used by
us as a building block to obtain a binuclear complex [VO(dbbpy)Cl(CA)Cl(dbbpy)VO] with
bridging chronilate CA2- [43]. Other previously reported oxidovanadium complexes
-
-
containing dbbpy ligands are cis-[VIVO(OH)(dbbpy)]Y (Y = BF4 , SbF6 ), cis-
−
−
[VIVO(F)(dbbpy)2]A (A = BF4 , ClO4 ), [V2O2(μ-MeO)2(μ-MO4)2(dbbpy)2] (M = Mo, W),
[(VO)2(μ2-SO4)2(dbbpy)2(CH3OH)2] and [(VO)2(μ2-O)2(μ2-SO4)(dbbpy)2] [38–41].
R
R
R
R
O
N
N
N
N
Cl
Cl
O2
R =
V
V
Cl3
+
H3C
CH3
CH3CN/H2O
OH2
CH3
Scheme 1. Reaction scheme for the synthesis of 1
The IR spectrum of 1 shows a strong band at 989 cm-1, assigned to a V=O stretching
mode in agreement with those reported for other oxidovanadium(IV) complexes [10,44–46].
The broad vibration band at 3440 cm-1 for the OH group in H2O and vibration bands for the
CH group (2871-2961 cm-1) and for the C=C and C=N groups (1642-1548 cm-1) of the dbbpy
ligand (Fig. S1, see ESI) were also observed. A band at 901 cm-1 is assigned to the V-O
stretching vibration of the V-H2O moiety.
3.2. Crystal structure
Crystals of 1·1.5CH2Cl2 were grown by recrystallization of 1 from a CH2Cl2/Et2O and
were studied by single crystal X-ray diffraction analysis. The molecular structure of 1 is
represented in Fig. 1. Selected bond distances and angles are given in Table 2. Vanadium(IV)
has a distorted octahedral environment. The equatorial plane of the octahedron is defined by
two chlorides and two nitrogen atoms of the dbbpy ligand while the oxido and H2O ligands
occupy the axial positions.
6

Fig. 1 Molecular structure of [VOCl2(dbbpy)(H2O)] in 1·1.5CH2Cl2.
Table 2. Selected geometric parameters (Å, º) for 1·1.5CH2Cl2
1·1.5CH2Cl2
1·1.5CH2Cl2
2.3662 (9)
2.3663 (9)
2.128 (3)
V-O(1)
V-O(2)
1.589 (3)
2.259 (3)
V-Cl1
V-Cl2
V-N
Cis-angles
N(1)-V-N(2)
Cl(1)-V-Cl(2)
N(1)-V-Cl(1)
N(2)-V-Cl(2)
Trans-angles
O(1)-V-O(2)
N(1)-V-Cl(2)
N(2)-V-Cl(1)
O(1)-V-Cl
O(2)-V-Cl
O(1)-V-N
O(2)-V-N
98.16 (9)
84.39 (6)
97.74 (12)
79.30 (10)
76.14 (13)
94.58 (4)
92.42 (7)
92.42 (7)
176.21 (15)
161.52 (8)
161.52 (8)
The V=O distance of 1.5905(1) Å is within the 1.56-1.63 Å range reported for other
vanadium(IV) complexes [10,31,47]. The V-Cl (2.3655(1) Å) and V-N (2.1289(1) Å) V-O
(2.2589(1) Å) distances are similar to those found elsewhere [10,31,48–50].
3.3. EPR spectra
The EPR spectrum of 1 in CH2Cl2 at room temperature is shown in Fig. 2. The spectrum
shows an eight-line pattern due to hyperfine interaction with the 51V isotope (I = 7/2, 100%),
which is typical for oxidovanadium(IV) complexes [10,51]. The V4+ ion has a 4d1 electronic
state with S = 1/2 and g = 1.967. The line splitting varies from 8.98 mT to 11.16 mT, and the
7

line half-width varies from 1.28 mT to 2.4 mT, due to the vanadium complex mobility in
solution and to an incomplete averaging of the g tensor anisotropy.
280
300
320
340
360
380
400
Magnetic field, mT
Fig. 2. X-Band CW EPR spectrum of 1 in CH2Cl2 at 298K
The EPR spectrum of the frozen solution of 1 at 77 K is shown in Fig. 3. Simulation
shows that the experimental spectrum is described by the spin Hamiltonian:
H = gxxβHxSx +gyyβHySy + gzzβHzSz + A(V)xxSxIx + A(V)yySyIy + A(V)zzSzIz
with the following parameters: S = 1/2, I = 7/2, gxx = gyy = 1.978, gzz = 1.945, Axx = Ayy = 6.5
mT, Azz = 17.86 mT. Similar values were previously reported for related oxidovanadium(IV)
complexes [10,42]. The EPR spectrum of solid 1 reveals a single-line signal with a half-width
width (∆H1/2) of 8.6 mT and g = 1.967 due to the exchange interaction between paramagnetic
vanadium ions (Fig. S2, see ESI). The analysis of the hyperfine splitting parameters allowed
us to estimate the isotropic and anisotropic contributions to the hyperfine interaction with
vanadium: Aiso = 10.29 mT and Aaniso = 3.79 mT. By using these parameters and assuming a
100% localization on the s or d orbitals [52], we could estimate the spin density of the
unpaired electron on the vanadium atom: (3.79/6.246 + 10.29/148.62)*100% = 67.60%.
8

DPPH
a
b
250
275
300
325
350
375
400
425
Magnetic field, mT
Fig. 3. EPR spectra of 1 in CH2Cl2 at 77 K. DPPH indicates the single EPR resonance of 2,2-
diphenyl-1-picrylhydrazyl, which was used as an internal standard, with g = 2.0036. a –
experimental, b – simulated.
3.4. Cyclooctene epoxidation
Cyclooctene was chosen as a representative substrate for catalytic studies. It was oxidized
by tert-butyl hydroperoxide (TBHP) under mild conditions (55o C) in the presence of catalytic
amounts of complex 1 in chloroform. The reaction gave, as expected, cyclooctene epoxide as
the main product and traces of the cis-cyclooctan-1,2-diol, whereas no significant amount of
the trans-cyclooctan-1,2-diol, which is the expected product of acid-catalyzed epoxide ring
opening in the presence of water, was detected. However, a non-negligible amount of a
secondary product 2 was also noted. The formation of this product did not depend on the
method of quenching for the reaction monitoring (vide infra). All products, except for the
1
latter secondary product, were unambiguously identified by the H NMR resonance of the
methyne (CH) proton and by the comparison of the GC retention time with genuine standards.
The CH proton of residual cyclooctene yields a multiplet at δ 5.6, whereas that of cyclooctene
epoxide resonates at δ 2.9 and that of the cis-diol at δ 3.9. The corresponding proton of the
trans-diol is expected at δ 3.6 ppm [53], but no significant intensity was observed in this
region. The secondary product 2 is characterized by a multiplet at δ 4.3, the shape of which is
1
quite similar to that of the epoxide product. A representative H-NMR spectrum, recorded
9

1
after 3.5 h of reaction, is shown in Fig. S3 (ESI). The results of the H NMR monitoring are
summarized in Fig. 4 (full data in Table S1). The slight erosion of the mass balance, which is
accentuated at longer reaction times (see table S1) indicates that other unknown products are
also obtained. The catalyst is presumably deactivated during the reaction, since the residual
cyclooctene amount stagnates after 5 h.
100
Cyclooctene
Epoxide
Trans-dichloride
Cis-diol
Mass balance
80
60
40
20
0
0
1
2
3
4
5
Time (h)
1
Fig. 4. H NMR monitoring of the cyclooctene epoxidation by TBHP in CHCl3 at 55°C
catalyzed by compound 1.
Identification of the significant secondary product 2 was not easy. The GC-MS analysis
revealed a molecular ion corresponding to the C8H14Cl+ formula that did not match with
available library spectra. Unambiguous identification required a large scale catalysis
experiment followed by product separation and NMR analysis of the pure product. In this
experiment, decane was used as internal standard for a monitoring by GC. The separation was
accomplished by quenching the TBHP excess with PPh3, followed by exhaustive hydrolysis
of the epoxide product 1 to yield the trans-diol, since the epoxide and compound 2 could not
be separated in our hands, and by chromatographic separation on a silica column, eluting with
pentane, as eluent. Monitoring of the separation step was complicated by the lack of
chromophores in both 2 and the decane internal standard, but 30 mg of pure isolated
compound could eventually be obtained. The hydrocarbons (decane, residual cyclooctene)
were eluted first, followed by the desired compound, while the diols remained trapped on the
1
silicagel. Full H and 13C analyses of the recovered product allowed its unambiguous
identification as trans-1,2-dichlorocycooctane. The 1H and 13C NMR data (see SI, Fig. S4 and
10

S5) were identical to those reported in the literature for this сompound [54]. It is worth
pointing out that trans-1,2-dichlorocycooctane is the only product of dichlorination. The other
isomer, cis-1,2-dichlorocycooctane, with an expected signal at δ 4.6-4.1ppm [55], was not
observed. This perfect steroselectivity rules out radical dichlorination, which would provide a
mixture of diastereoisomers [56]. Because of the trans stereoselectivity, we propose that an
electrophilic dichlorination is taking place.
Understanding the origin of chlorine for this reaction is of interest. Formation of 2 may
be rationalized as the result of 1,2-dichlorination by HCl (a common contaminant of
chloroform) under oxidizing conditions. This transformation occurs without need of metal
catalysis and is trans-selective, presumably via the formation of a chloronium intermediate
[57]. In order to check this hypothesis, the catalytic oxidation was repeated in a carefully
purified chloroform, and also by adding HCl (as a solution in diethyl ether) but the yield of
trans-1,2-dichlorocycooctane were not significantly decreased or improved under these
conditions. From these results, we conclude that the source of chlorine is not HCl. The only
other possible sources of Cl atoms are compound 1 and chloroform itself. The amount of
compound 1 (2%) used in the catalytic reactions is not enough to provide the necessary Cl
atoms to produce the observed by-product 2 (23.4% after 72 h, see table S1). We can
therefore safely conclude that chloroform (41 equivalents relative to cyclooctene) is the
primary source of chlorine atoms. Further studies are necessary to obtain additional insights
into the chloroform activation mechanism, but we can tentatively speculate that chloroform
may react with TBHP, with or without the intervention of vanadium catalysts, to yield
methanol and tBuOCl or an ether and hypochloric acid, which are possible sources of
electrophilic chloronium [57]. To the best of our knowledge, the formation of this by-product
has not been previously mentioned, although chloroform is a rather frequently used as solvent
for the catalyzed alkene epoxidations with H2O2 or TBHP [14,16,17].
1
The above mentioned reaction monitoring by H NMR and GC were conducted using
PPh3 to quench the excess TBHP, which rapidly transforms the oxidant to tBuOH and
31
generates Ph3P=O, as verified by control experiments and P NMR analysis. Preliminary
catalytic experiments were also conducted with MnO2 as reagent to quench the excess TBHP.
This procedure led to the same product distribution as with the PPh3 quench, but substantial
amounts of an additional heavy molecular weight product were also observed in the GC trace.
This product could only be detected in very small amounts during the experiments run with a
PPh3 quench (see Fig. S6). The GC-MS analysis indicated that this additional product
corresponds to 3-trichloromethylcyclooctene. Its formation results from the formal
11

•
replacement of an allylic hydrogen atom by CCl3, suggesting the involvement of CCl3
radicals. This is corroborated by the parallel observation of C2Cl6 in the GC spectrum. The
•
production of a large amount of CCl3 radicals during the TBHP quench with MnO2 can be
attributed to the formation of HO• and/or tBuO• radicals, followed by H abstraction from the
solvent.
Additional investigations were carried out in order to learn more about the catalyst action.
When TBHP was added to a solution of complex 1, which is pale green, the color turned
immediately to violet. The EPR spectrum recorded 10 minutes after the addition of TBHP at
20°C shows the overlap of a new 8-line pattern, similar to but different from that of 1 (g =
1.990, A ~ 94 G) and a strong single line at g = 2.014, see Figure 5. The 8-line pattern slowly
decreased and completely disappeared, while the second peak still remained after 40 min, and
completely disappeared only two days later. In a second similar experiment, carried out at
55°C, the initial spectrum before the addition of TBHP was identical to that recorded at 20°C,
showing that the precatalyst is stable at this temperature. Addition of TBHP accelerated the
degradation. These experiments suggest an immediate reaction between complex 1 and TBHP
at 20°C, with the formation of an intermediate VIV complex (8-line pattern) and an organic
radical (single peak), which then decompose at different rates. The VIV intermediate is likely
the product of H2O substitution by TBHP, by analogy with the intermediate proposed in Mo-
catalyzed epoxidation [58]. Its decomposition, in the absence of substrate, presumably yields
a VV product by homolytic O-O cleavage, releasing an OH• radical (see scheme 2). The latter
•
abstracts the H atom from the solvent and yields the long-lived CCl3 radical, which
presumably accounts for the second observed product in the EPR spectrum. Indeed, as stated
above, the formation of 3-trichloromethylcylooctene, though in rather small amounts, was
also observed in the experiments carried out without a MnO2 quench. It should be underlined
that the reactions leading to disappearance of the VIV EPR signals are fast at 55°C but, when
the substrate is present, the catalytic oxidation at this temperature continues at moderate
activity for several hours. Thus, the VV species produced by oxidation of the VIV precatalyst is
certainly catalytically active [15]. However, the initially higher catalytic activity may result
from either a VV or, more likely, a VIV species.
12

Fig. 5. EPR spectra of the solution of complex 1 in CHCl3 before and after TBHP addition at
20°C. The spectra recorded 10 and 40 min after the TBHP addition have a vertical scale
expanded by a factor of 10.
TBHP
N
N
N
N
O
V
V
Cl
OH2
tBu
Cl
Cl
O
Cl
O
O
H
1
CHCl3
CCl3 + H2O
OH
N
N
V
tBu
Cl
O
Cl
O
Scheme 2. Proposed action of TBHP on compound 1
4. Conclusion
The octahedral oxidovanadium(IV) complex, [VOCl2(dbbpy)(H2O)] (1), containing the
4,4’-di-tert-butyl-2,2’-bipyridine (dbbpy) ligand was synthesized and characterized using X-
ray diffraction, IR and EPR spectroscopies. This complex was tested as a catalyst for
13

epoxidation of cyclooctene with TBHP in chloroform. A conclusion to be drawn from this
study is that one-electron processes occur in the presence of the vanadium(IV) precatalyst, the
oxidant and chloroform, generating radical species. In addition, using chloroform in
combination with the TBHP oxidant leads to the formation of trans-1,2-dichlorocyclooctane
as an unwanted side product of 1,2-dichlorination. It therefore seems wise to avoid the use of
chloroform as a solvent in catalyzed oxidations with TBHP (and likely also with H2O2), at
least with a vanadium(IV) precatalyst.
Acknowledgements
We acknowledge the Russian Foundation for Basic Research (grant 18-03-00155-а) and
the Centre National de la Recherche Scientifique (CNRS, France) for funding. I. S. F. also
thanks the French Embassy in Moscow for a Mechnikov fellowship.
14

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this paper.
Graphical abstract
21

A new oxidovanadium(IV) complex has been synthesized and fully characterized. It catalyzes
the oxidation of cyclooctene with tert-butyl hydroperoxide in chloroform.
22