Reactions of Aluminium Tribromide with Tetrakis(triphenylphosphine) nickel(0) and Tetrakis(triethylphosphite)nickel(0) Complexes

Article abstract of DOI:10.1023/A:1024042429252

Reactions of aluminium tribromide with the Ni(0) phosphine and phosphite complexes are studied by EPR method. AlBr₃ was found to cause the oxidation of the transition metal in the (PPh₃)₄Ni complex to the univalent state with the formation of the tetracoordinated (PPh₃)₃NiBr complex. With an excess of AlBr₃, the phosphine ligands are eliminated from the coordination sphere of Ni(I), and the coordinatively unsaturated complexes are destroyed to give the colloidal nickel. In the reaction of (P(OEt)₃)₄Ni with AlBr ₃, Ni(0) is also oxidized to Ni(I), but the acido ligand is not eliminated even with a 15-fold excess of the Lewis acid. The activity of catalytic systems on the basis of the Ni(0) phosphine complexes and the Lewis acids in the low-molecular oligomerization reactions of olefines is determined by the cationic coordinatively unsaturated Ni(I) complexes formed in these systems.

Full text of DOI:10.1023/A:1024042429252

Russian Journal of Coordination Chemistry, Vol. 29, No. 6, 2003, pp. 431–434. Translated from Koordinatsionnaya Khimiya, Vol. 29, No. 6, 2003, pp. 461–464.
Original Russian Text Copyright © 2003 by Kraikivskii, Saraev, Matveev, Zelinskii, Tkach.
Reactions of Aluminium Tribromide
with Tetrakis(triphenylphosphine)nickel(0)
and Tetrakis(triethylphosphite)nickel(0) Complexes
P. B. Kraikivskii, V. V. Saraev, D. A. Matveev, S. N. Zelinskii, and V. S. Tkach
Irkutsk State University, ul. Lermontova 126, Irkutsk, 664033 Russia
Received May 29, 2001; in final form, April 26, 2002
Abstract—Reactions of aluminium tribromide with the Ni(0) phosphine and phosphite complexes are studied
by EPR method. AlBr3 was found to cause the oxidation of the transition metal in the (PPh3)4Ni complex to the
univalent state with the formation of the tetracoordinated (PPh3)3NiBr complex. With an excess of AlBr3, the
phosphine ligands are eliminated from the coordination sphere of Ni(I), and the coordinatively unsaturated
complexes are destroyed to give the colloidal nickel. In the reaction of (P(OEt)3)4Ni with AlBr3, Ni(0) is also
oxidized to Ni(I), but the acido ligand is not eliminated even with a 15-fold excess of the Lewis acid. The activ-
ity of catalytic systems on the basis of the Ni(0) phosphine complexes and the Lewis acids in the low-molecular
oligomerization reactions of olefines is determined by the cationic coordinatively unsaturated Ni(I) complexes
formed in these systems.
The catalytic systems produced from the Ni(0) recondensation in vacuum. The organophosphorus
phosphine complexes and Lewis acids exhibit the high compounds were prepared by standard procedures [8].
activity in the transformation reactions of the unsatur- All reactions were performed in glass reactors under
ated hydrocarbons [1].
the atmosphere of argon free from oxygen and moisture
traces. The solvents (toluene and hexane) were dehy-
drated, degassed in vacuum, and kept over the sodium
surface under argon for a day. The EPR spectra were
recorded on a SE/X-2542 spectrometer (9.6 GHz) at
T = 77 K. The sweep range was calibrated using the
DPPG free radical and Mn2+ in MgO. The EPR spectra
were simulated with our own program [3], where the
hyperfine interaction (HFI) is limited to the second-
order term and the main axes of the g-tensor and HFI
tensors coincide.
As we previously reported, the Lewis acids, such as
AlCl2Et [2] and BF3 · OEt2 [3], cause the quantitative
oxidation of the transition metal in the phosphine
(PPh3)4Ni complex to the univalent state and the forma-
tion of the Ni(I) halide complexes. In the excess of the
Lewis acid, both the phosphine and acido ligands are
eliminated from the Ni(I) coordination sphere, and the
Ni(I) complex is converted to the cationic form. In the
case of AlCl2Et, the Ni(I) complex occurs as the ionic
pair with the Lewis acid, while with BF3 · OEt2, it exists
as the isolated cations.
With the aim to elucidate the role of the Lewis acids
in the formation of the catalytically active Ni(I) com-
plexes, we studied the reactions of aluminium tribro-
mide with the tetrakis(triphenylphosphine)nickel(0)
and tetrakis(triethylphosphite)nickel(0) complexes in
toluene by the EPR method.
RESULTS AND DISCUSSION
After AlBr3 is added to the toluene solution of the
diamagnetic (PPh3)4Ni complex (with the molar ratio of
Al : Ni = 1), an intense signal (1‡ in figure) appears in
the EPR spectrum with unresolved hyperfine structure
(HFS). The signal 1‡ is typical of the tetrahedral Ni(I)
complexes that have the trigonal distortion [9]. The lack
of the resolved HFS lines suggests a short relaxation
time of the electron spin in a complex with the pseudo-
degenerate ground state. Further addition of AlBr3 to
the system results in a sharp reduction in the intensity
of the 1‡ signal and its disappearance when Al : Ni = 2.
EXPERIMENTAL
The (PPh3)4Ni and (P(OEt)3)4Ni complexes used in
the experiment were synthesized as described in [4] and
[5], respectively. The [(PPh3)3Ni]BF4 complex was pro-
duced by the oxidation of the (PPh3)4Ni complex with
boron trifluoride etherate [6]. The (PBu3)2NiBr2 com-
In order to interpret the EPR signal (1‡), we
plex was synthesized by the known procedure [7]. The obtained in situ the (PPh3)3NiBr complex in toluene
composite Ni(I) complex (PBu3)(PPh3)2NiBr was solution by oxidizing the (PPh3)4Ni complex with
obtained by the contradisproportionation reaction molecular bromine. In this case, we recorded the same
between the (PBu3)2NiBr2 and (PPh3)4Ni complexes in signal 1‡, which disappeared on the addition of AlBr3
toluene. The analytical grade AlBr3 was purified by to the system.
1070-3284/03/2906-0431$25.00 © 2003 åÄIä “Nauka /Interperiodica”

432
KRAIKIVSKII et al.
1a
4a
4b
2a
2b
5a
5b
3a
3b
250
300
350 250
mT
300
350
(a) Experimental and (b) model EPR spectra in toluene: (1) (PPh ) Ni–AlBr ; (2) (PPh ) Ni–(PBu ) NiBr ;
3 4
3
3 4
3 2
2
(3) (PBu )(PPh ) NiBr–AlBr ; (4) (P(OEt) ) Ni–AlBr ; and (5) (PPh ) Ni–BF · OEt , T = 77 K.
3
3 2
3
3 4
3
3 4
3
2
Thus, in the reaction of AlBr3 with (PPh3)4Ni, Ni(0) spectrum calculation does not provide the variation in
is oxidized to Ni(I) and the (PPh3)3NiBr neutral com- the angles between the axes of the g-tensor and the HFI
plex is formed that is decomposed with the excess tensors with the ligands, the found values of the HFI
AlBr3 in the system. One should note that in the EPR constants correspond not to the main values but to the
spectrum of the (PPh3)3NiCl with similar composition values reduced to the g-tensor axes. In accordance with
and geometry, the HFS is not resolved at 77 K ether [10]. the theoretical developments [9], the experimental data
(see table) satisfy the trigonal structure, where the main
According to the data in [2], the decomposition pro-
axis passes along the Ni–P bond with the tributylphos-
cess can be preceded by the formation of the coordina-
phine ligand.
tively unsaturated structures, which in the case of the
Ni(I) bromide complexes are likely to be unstable and
quickly disproportionate to form the Ni(II) complexes
and the colloidal nickel.
After the addition of AlBr3 in the equimolar amount
to a solution of the mixed-ligand (PBu3)(PPh3)2NiBr
complex, the EPR spectrum exhibits a rapid transition
of the 2‡ signal to the 3‡ signal (see figure), where the
HFS from two equivalent 31P nuclei is resolved. The tri-
axial anisotropic signal 3‡ is typical of the tricoordi-
nated Ni(I) complexes (PR3)2NiBr [12]. It is notewor-
thy that the EPR parameters of a new complex are close
to the average values of similar parameters for two
complexes (PPh3)2NiBr and (PBu3)2NiBr (given in
table). The recorded spectrum is most likely produced
With the aim to identify the coordinatively unsatur-
ated Ni(I) bromide complexes, we studied the reaction
of AlBr3 with the tetracoordinated mixed-ligand
(PBu3)(PPh3)2NiBr complex in toluene solution. The
former complex contains the tributylphosphine ligand,
which stabilizes nickel in univalent state more effi-
ciently. In the EPR spectrum of the mixed-ligand com-
plex (see 2‡ in the figure), the HFS is resolved in the
parallel orientation from three 31P nuclei (IP = 1/2), two
of which are equivalent, and one 79,81Br nucleus (IBr =
3/2).
by
the
Ni(I)
complex
with
composition
(PBu3)(PPh3)NiBr, which is formed due to the elimina-
tion of the triphenylphosphine ligand from the
(PBu3)(PPh3)2NiBr complex. Assuming that the 31P
nuclei are magnetically equivalent in two different
phosphine ligands, it should be noted that in the mixed
ligand Ni(I) complexes [(PBu3)4 – n (PPh3)nNi]BF4, the
31P nuclei are also magnetically equivalent [9].
The EPR parameters corresponding to the best
visual coincidence of the experimental and model EPR
spectra are given in the table. Taking into account a
small (~10%) difference in the magnetic moments for
the 79Br and 81Br isotopes, whose natural content is
almost equal [11], the model spectra were calculated
Further introduction of AlBr3 into the model system
only for one isotope. Since the program for the EPR results in the intensity reduction of the 3‡ EPR signal,
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REACTIONS OF ALUMINIUM TRIBROMIDE
433
Parameters of EPR spectra of Ni(I) complexes
Complex
g||
g
A||, mT
A , mT
7.5(1P)
(PPh3)3NiBr
2.02
2.35
2.23
(PBu3)(PPh3)2NiBr
2.037
9.15(1P)
7.63(2P)
5.0(2P)
3.05(Br)
<5.0(Br)
(PBu3)(PPh3)NiBr
(PPh3)2NiBr
2.341z
2.108y
2.083x
2.122y [12]
2.085x [12]
4.8(2P)z
<2.0(Br)z
4.3(2P)z [12]
6.28(2P)y, x
3.14(Br)y, x
6.15(2P)y [12]
6.53(2P)x [12]
2.348z [12]
3.04(Br)y, x [12]
6.69(2P)y [12]
6.78(2P)x [12]
3.25(Br)y, x [12]
14.5(1P)
(PBu3)2NiBr
2.291z [12]
1.989
2.100y [12]
2.083x [12]
5.7(2P)z [12]
(P(OEt)3)3NiBr
2.207
17.0(1P)
5.7(1P)
6.5(1P)
4.5(1P)
5.5(1P)
3.4(Br)
<3.0(Br)
[(PPh3)3Ni]BF4
2.385z
2.385 [3]
2.12y
2.07x
2.09 [3]
6.1(1P)z
<3.2(2P)z
6.0(1P) [3]
6.4(1P)y
<3.2(2P)y
8.1(1P)x
<2.0(2P)x
7.5(1P) [3]
which fully disappears when the molar ratio ofAl : Ni =
The addition of AlBr3 to the toluene solution of the
3. An excess of AlBr3 is likely to cause the further elim- diamagnetic (P(OEt)3)4Ni complex (with the molar
ination of the phosphine ligands and decomposition of ratio of Al : Ni = 2) produces an intense EPR signal (4‡
the Ni(I) complexes.
in figure) with the resolved complicated HFS from the
ligands. Analogous spectra were obtained in the oxida-
tion of the (P(OEt)3)4Ni complex in toluene with the
molecular bromine and by full replacement of the tri-
phenylphosphine ligands by the triethylphosphite
ligands in the (PPh3)3NiBr complex using the exchange
ligand technique [9]. Thus, aluminium tribromide also
causes the oxidation of Ni(0) in the (P(OEt)3)4Ni com-
plex to give the tetrahedral neutral complex
(P(OEt)3)3NiBr.
Thus, the detection in the model system of the coor-
dinatively unsaturated Ni(I) bromide complex with
tributylphosphine ligand indicates that the (PPh3)2NiBr
structures can form also in the starting system. These
structures are not detected since they occur in a narrow
interval of the Al : Ni ratios that can be controlled with
difficulty.
One should note that the EPR spectra did not exhibit
the signals from the bridging structures typical of the
systems based on AlCl2Et [2].
If an excess AlBr3 is introduced into the system after
the (P(OEt)3)3NiBr complex formation, the 4‡ EPR sig-
nal remains unchanged even at the molar ratio of Al :
Ni = 15. Hence, aluminium tribromide causes neither
the elimination of the triethylphosphite ligands nor the
conversion of the neutral bromide complex into the cat-
ionic form through the migration of the Br– anion to the
outer sphere. Therefore, the Ni(I) bromide phosphine
complexes can only occur in the neutral form indepen-
dent of the ligand environment.
The reaction of AlBr3 with (PPh3)4Ni can be written
as follows:
AlBr3
(PPh3)4Ni
(PPh3)3NiBr
AlBr3
AlBr3
(PPh3)2NiBr
Destruction of Ni(I).
In order to study the possibility of using AlBr3 for
eliminating the acido ligand from the Ni(I) coordina-
tion sphere, its reaction with the Ni(0) complex
(P(OEt)3)4Ni in toluene was investigated. This study
As we previously showed in [14], the activity of the
was based on the data on the formation of the cationic catalytic system (PPh3)4Ni–BF3 · OEt2 in the processes
Ni(I)
[(P(OEt)3)4Ni]AlCl3Et in the catalytic (P(OEt)3)4Ni– hydrocarbons is directly determined by the obtained
AlCl2Et system [13]. cationic Ni(I) complexes. One of the shortcomings of
complexes
of
the
composition of the low-molecular oligomerization of the olefine
RUSSIAN JOURNAL OF COORDINATION CHEMISTRY Vol. 29 No. 6 2003

434
KRAIKIVSKII et al.
this system is the need to use large amounts of the activity of the catalytic systems in the dimerization
boron trifluoride etherate (up to 80 molar fractions) in reactions of the olefine hydrocarbons produced from
order to ensure the required coordination unsaturation the Ni(0) phosphine complexes and the Lewis acids is
of the Ni(I) cationic complexes. Since AlBr3 actively determined by the formed Ni(I) cationic complexes.
eliminates the triphenylphosphine ligands, it was inter-
esting to estimate whether it can be used as the acceptor
of phosphines in the catalytic system. For this purpose,
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[(PPh3)3Ni]BF4 + AlBr3
(PPh3)3NiBr + AlBr2F · BF3.
Hence, the reaction of aluminium tribromide with
the cationic Ni(I) complex results, first of all, in the
ionic exchange of the acido ligands and in the conver-
sion of the potentially active Ni(I) cationic complex
into the inactive molecular form.
This conclusion was confirmed by the catalytic tests
for the activity in the dimerization reactions of propyl-
ene and ethylene. The (PPh3)4Ni–AlBr3 and
[Ni(PPh3)3]BF4–AlBr3 systems in toluene were found to
show zero activity in these processes. The results
obtained support the previous conclusion that the high
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