Counterdisproportionation between cationic Ni(II) and phosphine Ni(0) complexes

Article abstract of DOI:10.1134/S1070328408060080

The interaction of Ni(II) bis-tetrafluoroborate complexes [Ni(Dppe) ₂](BF₄)₂ and [Ni(CH₃CN) ₆](BF₄)₂ (where Dppe = 1,2- bis(diphenylphosphino)ethane)) with Ni(0) phosphine complexes Ni(Dppe) ₂ and Ni(PPh₃)₄ in 1 : 1 mixture of toluene-acetonitrile was studied by the EPR method. The counter- disproportionation was shown to occur in a solution between the cationic Ni(II) complexes and the Ni(0) complexes to give Ni(I) complexes almost in quantitative amounts. The structures of the cationic Ni(I) complexes obtained were found to depend on both the solvent nature and the presence of a free phosphine in a solution.

Full text of DOI:10.1134/S1070328408060080

ISSN 1070-3284, Russian Journal of Coordination Chemistry, 2008, Vol. 34, No. 6, pp. 438–442. © Pleiades Publishing, Ltd., 2008.
Original Russian Text © V.V. Saraev, P.B. Kraikivskii, D.A. Matveev, A.S. Kuzakov, A.I. Vil’ms, A.A. Fedonina, 2008, published in Koordinatsionnaya Khimiya, 2008, Vol. 34,
No. 6, pp. 446–450.
Counterdisproportionation between Cationic Ni(II)
and Phosphine Ni(0) Complexes
V. V. Saraev, P. B. Kraikivskii, D. A. Matveev, A. S. Kuzakov, A. I. Vil’ms, and A. A. Fedonina
Irkutsk State University, ul. Lermontova 126, Irkutsk, 664033 Russia
E-mail: saraev@admin.isu.ru
Received May 18, 2007
Abstract—The interaction of Ni(II) bis-tetrafluoroborate complexes [Ni(Dppe)2](BF4)2 and
[Ni(CH3CN)6](BF4)2 (where Dppe = 1,2-bis(diphenylphosphino)ethane)) with Ni(0) phosphine complexes
Ni(Dppe)2 and Ni(PPh3)4 in 1 : 1 mixture of toluene–acetonitrile was studied by the EPR method. The counter-
disproportionation was shown to occur in a solution between the cationic Ni(II) complexes and the Ni(0) com-
plexes to give Ni(I) complexes almost in quantitative amounts. The structures of the cationic Ni(I) complexes
obtained were found to depend on both the solvent nature and the presence of a free phosphine in a solution.
DOI: 10.1134/S1070328408060080
The catalytic systems based on Ni compounds and tional hydride or metallocyclic mechanism, respec-
known as catalysts of a low-molecular oligomerization tively. At the same time, the available data on the stabil-
of unsaturated hydrocarbons [1, 2] cause an increased ity of electroneutral Ni(II) phosphine complexes in
interest recently due to the high activity of some Ni aprotic solvents indicate the opposite situation, i.e., the
complexes in polymerization of the lower olefins [3] predominant
and in polymerization and cyclotrimerization of Ni(II) + Ni(0)
counterdisproportionation
process
2Ni(I). For example, in the reactions
alkynes [4].
Despite a great number of publications devoted to
occurring in aprotic solvents
(Ph3P)2NiX2 + Ni(PPh3)4
2(Ph3P)3NiX,
(1)
(2)
the nickel complex catalysts, the question of the struc-
ture of the active complexes remains open. The major-
ity of the researchers traditionally assume that the oli-
gomerization of olefins is directly associated with
Ni(II) hydride complexes. These assumptions are based
on the classic works of Tolman and his adherents con-
cerned with the NMR identification of the Ni(II)
hydride complexes and, first of all, in the model sys-
tems [5–7].
(Ph3P) NiX2 + (η2-C2H4)Ni(PPh3)
2
2
2(Ph3P)2NiX + C2H4,
where X = Cl, Br, the equilibrium is fully shifted to the
right [12], which is widely used in the synthesis of the
Ni(I) halide complexes with various phosphine ligands
[13, 14].
The analogous processes are observed also in the
case of the neutral nickel cyclopentadienyl complexes
in toluene [15]:
On the other hand, according to the EPR data, the
process of formation of the nickel complex catalysts
includes the formation and stabilization of the Ni(I)
complexes, independent of the valence state of a transi-
tion metal in the initial compound [8, 9]. It has been
found recently using a number of the most active cata-
lytic systems (Ni(PPh3)4/BF3 · OEt2/ROH) that their
activity in oligomerization of unsaturated hydrocarbons
(ethylene, styrene) depends linearly on the concentra-
tion of the cationic Ni(I) complexes in these systems
[10, 11], which allows one consider the Ni(I) cationic
complexes as catalytically active species of the coordi-
nation-ionic transformation of unsaturated hydrocar-
bons.
NiCp2 + Ni(PPh3)4
2CpNi(PPh3)2.
(3)
To establish the participation in the counterdispro-
portionation reaction of the cationic Ni(II) complexes
in addition to the neutral complexes, we studied in this
work the interaction of Ni(II) bis-tetrafluoroborate
complexes
([Ni(Dppe)2](BF4)2
and
[Ni(CH3CN)6](BF4)2, where Dppe = 1,2-bis(diphe-
nylphophino)ethane) with Ni(0) phosphine complexes
in the 1 : 1 mixture of toluene–acetonitrile.
However, the discovered regularities do not exclude
the possible disproportionation of the cationic Ni(I)
complexes to the Ni(II) and Ni(0) complexes at the ini-
EXPERIMENTAL
All the procedures were performed in the atmo-
tial moment and further reaction on the Ni complexes sphere of dry argon using the Schlenk technique. The
in the common oxidation states according to a tradi- precipitates were filtered on glass Schlenk filters. The
438

COUNTERDISPROPORTIONATION BETWEEN CATIONIC Ni(II)
439
Parameters of EPR spectra of Ni(I) complexes
Complex*
[Ni(Dppe)2]X
g||
g⊥
A||, mT
A⊥, mT
X = BF4
2.160
2.160
2.036
2.031
5.46(4P)
5.3(4P)
6.76(4P)
6.7(4P)
X = AlCl3Et [23]
[(PPh3)3NiLNi(PPh3)3](BF4)2
Tricoordinate Ni(I)
Tetracoordinate Ni(I)
[Ni(PPh3)3]BF4 [24]
2.454**
1.980***
2.385z
2.109**
2.425***
2.12y
6.1(1P)z
6.4(1P)y
8.1(1P)x
2.07x
[LNi(PPh3)3]BF4
1.973z
2.420y
2.476x
* L = CH CN.
3
** Signal 1'b (Fig. 2).
*** Signal 1"b (Fig. 2).
prepared and synthesized reagents were kept in sealed to 77 K, the EPR spectra were recorded. The EPR spec-
tubes under argon.
tra were characteristic of the Ni(I) compounds, and the
integrated intensity corresponded within the experi-
mental error to the total concentration of nickel in a sys-
tem. Hence, in a toluene-acetonitrile mixture, the pro-
cess of couterdisproportionation occurs between the
cationic Ni(II) complexes and the Ni(0) complexes
with the formation of the Ni(I) complexes almost in the
quantitative amount. Thus, not only neutral Ni(II) com-
plexes [12−15], but also the cationic Ni(II) complexes
in combination with phosphine Ni(0) complexes coun-
terdisproportionate in a solution to the Ni(I) complexes.
Toluene, hexane (Merck) were distilled before use
over metallic sodium in the atmosphere of argon. Ace-
tonitrile (Merck) was additionally dried over molecular
sieves (Aldrich 3A). All the solvents before use were
degassed in vacuum and saturated with argon.
Commercial reagents triphenylphosphine, trieth-
ylphosphite, diphenylphosphinoethane (Merck) were
used as received.
The Ni complexes [Ni(PPh3)3]BF4, Ni(PPh3)4,
Ni(Dppe)2, [Ni(Dppe)2](BF4)2, [Ni(CH3CN)6](BF4)2
were synthesized by the known procedures [10, 16–19],
respectively, while the Co complexes N2Co(PPh3)3 and
[Co(CH3CN)6](BF4)2 were prepared as described in
[19, 20].
The EPR spectra were recorded on a CMS-8499
spectrometer (working frequency 9.6 GHz) at a boiling
point of liquid nitrogen. The sweep range was cali-
brated using the diphenylpicrylhydrazyl (DPPH) free
radical and Mn2+ in MgO. The EPR spectra were simu-
lated with our own program [21], where the hyperfine
interaction (HFI) is limited to the second-order term
and the main axes of the g-tensor and HFI tensors coin-
cide.
Now let us consider the structures of the obtained
cationic Ni(I) complexes using the EPR data (see in the
table).
The system [Ni(Dppe)2](BF4)2–Ni(Dppe)2. The
EPR spectrum of a complex formed in this system
(Fig. 1, 1) contains hyperfine structure (HFS) from four
31
practically equivalent P nuclei and corresponds to a
tetragonal symmetry of the complex and quantitatively
coincides with the spectrum of the [Ni(Dppe)2]AlCl3Et
complex identified previously in the Ni(Dppe)2–
AlCl2Et system formed in toluene [23]. Therefore, the
cationic Ni(I) complex [Ni(Dppe)2]+ is also formed in
the system [Ni(Dppe)2](BF4)2–Ni(Dppe)2 as a result of
the counterdisproportionation reaction
[Ni(Dppe)2](BF4)2 + Ni(Dppe)2
RESULTS AND DISCUSSION
(4)
2[Ni(Dppe)2]BF4.
The starting Ni(II) and Ni(0) complexes
([Ni(Dppe)2](BF4)2, [Ni(CH3CN)6](BF4)2, Ni(Dppe)2,
The system [Ni(CH3CN)6](BF4)2–Ni(Dppe)2. The
and Ni(PPh3)4) in 1 : 1 toluene–acetonitrile mixture EPR spectrum of a given system (Fig. 1, 2) appears as
(v/v) at 77 K do not produce EPR signals. The use of superposition of signals, one of them being due to the
vitrifying toluene–acetonitrile mixture [22] provided [Ni(Dppe)2]BF4 complex. If the counterdisproportion-
satisfactory solubility of the starting cationic Ni(II) ation reaction is performed in the presence of a free
complexes. After the solutions of the Ni(II) and Ni(0) ligand Dppe (in equimolar amount with respect to the
complexes were mixed, stirred for 30 s at room temper- total content of nickel or in a higher amount), then only
ature (the molar ratio Ni(II) : Ni(0) = 1 : 1), and cooled the spectrum of the [Ni(Dppe)2]BF4 complex is
RUSSIAN JOURNAL OF COORDINATION CHEMISTRY Vol. 34 No. 6 2008

440
SARAEV et al.
1‡
1
2
3
*
*
1'b
1b
*
*
*
1''b
4
2‡
3‡
*
250
290
330
370
Magnetic field, mT
250
290
330
370
Magnetic field, mT
Fig. 1. The EPR spectra of the systems (1)
[Ni(DÂ) ](BF ) –Ni(DÂ) , (2) [Ni(CH CN) ](BF ) –
Fig. 2. (‡) The experimental and (b) simulated EPR spectra of
cationic Ni(I) complexes in vitrifying solutions at í = 77 ä:
2
4 2
2
3
6
4 2
Ni(DÂ) , (4) [Ni(CH CN) ](BF ) –Ni(PPh ) in a mix-
2
3
6
4 2
3 4
(1) [(PPh ) NiLNi(PPh ) ](BF ) ; (2) [Ni(PPh ) ]BF ; (3)
3 3
3 3
4 2
3 3
4
ture toluene-acetonitrile at í = 77 K. Spectrum 3 is pro-
duced by subtraction of signal 1 from signal 2; (*) is a signal
from DPPH.
[LNi(PPh ) ]BF ; L = CH CN. Curves 1' and 1" are the com-
3 3
4
3
ponents of signal 1, (*) is a signal from DPPH.
recorded. Obviously, in the presence of a free Dppe, the system of a number of Ni(I) complexes with the
number of coordinated phosphine ligands less than 3.
several Ni(I) complexes are formed in the system with
different number of bidentate phosphine ligands. Signal 3
in the EPR spectrum (Fig. 1, 3) presents the result of the
computer subtraction of individual signal 1 from
signal 2. This signal (g|| > g⊥) contains HFS from the 31P
nuclei, but it is not individual signal, which indicates
that the system contains different in composition and
(or) geometry Ni(I) complexes with one bidentate
phosphine ligand. With account for the stoichiometry
aspects, the system should also contain the Ni+ ions sta-
bilized only by the acetonitrile molecules. The rela-
tively high stability of the Ni(I) acetonitrile complexes
is evidenced by the fact no colloidal ferromagnetic
Ni(0) particles as the most probable products of decom-
position of the Ni(I) acetonitrile complexes were dis-
covered.
When the reaction of couterdisproportionation is
carried out in the presence of a free thiphenylphosphine
in the equimolar amount with respect to the total
amount of nickel, a novel EPR spectrum is recorded
(Fig. 2, 1a), which can be considered as a superposition
of two signals from axially-symmetric structures with
the inverse ratio of g|| and g⊥. Given in the table are the
parameters of the EPR modeled signals (1'b and 1"b),
whose linear combination in equal ratio (Fig. 2, 1b)
reproduces most adequately the experimental spectrum
(Fig. 2, 1a). The variation of a free triphenylphosphine
quantity in a wide range (1–10) and of the total nickel
concentration (10–2–10–3 mol/l) did not almost influ-
ence the shape of the EPR signals. On the one hand, this
indicates the coordination saturation of the Ni+ ion with
the phosphine ligands in the complex, and, on the other
hand, suggests steady relationship between the axially-
symmetric structures.
The system [Ni(CH3CN)6](BF4)2–Ni(PPh3)4. The
EPR spectrum of Ni(I) complex in this system
(Fig. 1, 4) corresponds to the ratio g|| > g⊥ and contains,
like spectrum 3, the HFS from the 31P nuclei, but is not
To establish the composition of a new complex, we
individual. This indicates the possibility of formation in studied the effect of the solvent nature on the EPR spec-
RUSSIAN JOURNAL OF COORDINATION CHEMISTRY Vol. 34 No. 6 2008

COUNTERDISPROPORTIONATION BETWEEN CATIONIC Ni(II)
441
trum of individual complex [Ni(PPh3)3]BF4 synthe-
The equatorial and azimuthal delocalization of the
sized by the previously developed procedure [10]. The lone electrons in two bonded Ni(I) complexes is, prob-
[Ni(PPh3)3]BF4 complex in the ether–toluene mixture ably, the main reason for their weak magnetic interac-
(1 : 1) given at í = 77 K the EPR spectrum characteris- tion in a binuclear structure.
tic for tricoordinate structures [21, 24] with g|| > g⊥
The formation of a binuclear Ni(I) complex can be
represented as follows:
(Fig. 2, 2‡), which does not change after the introduc-
tion of a free triphenylphosphine to the system. In a
mixture toluene–acetonitrile, the spectrum of this com-
plex fully coincides with that of the system
[Ni(CH3CN)6](BF4)2–Ni(PPh3)4, which is formed in
the presence of a free triphenylphosphine. Hence, in the
counterdisproportionation reaction in the presence of a
free triphenylphosphine, the cationic Ni(I) complex is
formed that contains three phosphine ligands and ace-
tonitrile. This complex was isolated from a solution by
vacuum evaporation at room temperature as a green-
brown powder. Attempts to obtain its crystals suitable
for X-ray diffraction analysis were failure. The com-
plex is likely to occur in the amorphous form only.
[NiL6]X2 + Ni(PPh3)4 + 2PPh3
[(PPh3)3NiLNi(PPh3)3]X2 + 5L,
(5)
where L = CH3CN, X = BF4.
The existence of the cationic binuclear Ni(I) com-
plex indirectly suggests that the binuclear structure is
formed prior to the stage of the electron transfer
between the Ni atoms. In the THF solution, the binu-
clear complex dissociates to monomers.
Note in conclusion that a single-electron transfer in a
mixture toluene-acetonitrile occurs not only between Ni(II)
and Ni(0), but also between Ni(II) and Co(0) and Co(II) and
Ni(0), which results in the formation of the Ni(I) complexes.
For instance, an intense EPR signals almost coinciding with
a signal in Fig 2 (1‡) is observed in the systems
After the powder was dissolved in the toluene–ace-
tonitrile mixture, the signal of the former intensity
(Fig. 2, 1‡) was recorded again. If the above powder
was dissolved in THF, the EPR spectrum became more
simple and only the signal due to a complex with g|| < g⊥
was observed (Fig. 2, 3‡), which is characteristic of the
trigonally distorted tetrahedral structures [25]. The
[Ni(CH3CN)6](BF4)2–N2Co(PPh3)3
and
[Co(CH3CN)6](BF4)2–Ni(PPh3)4, which are formed in the
presence of a free triphenylphosphine. Obviously, the com-
plexes with the composition [(PPh3)3NiLNi(PPh3)3]X2 and
[(PPh3)3NiLCo(PPh3)3]X2 are formed in the above systems
in addition to the [(PPh3)3CoLCo(PPh3)3]X2. In the former
complexes, the Co+ ions in both tetra- and tricoordinate sur-
rounding are likely to occur in a high-spin state and are not
registered in the EPR spectrum at í = 77 K. The magnetic
interaction between the Ni+ and Co+ ions in a heteronuclear
complex is, obviously, responsible for the additional broad-
ening of the EPT lines of the Ni+ ions, whose major width
(25−30 mT) is determined by the orbital degeneracy of the
Ni+ ground state in the tetra- and tricoordinate surrounding.
31
absence of the HFS from the P nuclei indicates that
the trigonal axis, along which the lone electron is delo-
calized, does not pass along the Ni-P bond. On the basis
of the above data, the following structure was suggested
for the paramagnetic complex
+
L
BF4–
Ni
,
Ph3P
PPh3
Ph3P
where L = CH3CN.
ACKNOWLEDGMENTS
Two signals observed in a mixture toluene–acetoni-
trile are likely to be due to both tri- and tetracoordinate
Ni(I) complexes, which are bonded, for example, by
acetonitrile bridges:
This work was supported by the Russian Foundation
for Basic Research—CRDF (grant no. 07-03-91123)
and DAAD (grant no. A/05/56802).
2+
Ph3P
Ph3P
PPh3
REFERENCES
Ni
1. Keim, W., Behr, A., Poper, M., Comprehensive Organo-
metallic Chemistry, Wilkinson, G., Stone, F.G.A., and
Abel, E.W., Eds., Oxford: Pergamon, 1982, vol. 8,
p. 371.
2BF4– .
L
Ni
Ph3P
PPh3
2. Shmidt, F.K., Kataliz kompleksami perekhodnykh met-
allov pervogo perekhodnogo ryada reaktsii gidriro-
vaniya i dimerizatsii (Catalysis of Hydrogenation and
Dimerization by First Transition Row Metal Com-
plexes), Irkutsk: IGU, 1986.
Ph3P
The elemental analysis data agree with the empirical
formula of the complex
For C110H93B2F8NNi2P6
3. Zhang, L., Brookhart, M.A., and White, P.S., Organome-
tallics, 2006, vol. 25, no. 8, p. 1868.
anal. calcd (%):
Found (%):
C, 69.33; H, 4.92; N, 0.73.
C, 70.24; H, 5.1; N, 0.82.
4. Pasynkiewicz, S., Oledzka, E., and Pietrzykowski, A., J.
Mol. Catal. A, 2004, vol. 224, nos. 1–2, p. 117.
RUSSIAN JOURNAL OF COORDINATION CHEMISTRY Vol. 34 No. 6 2008

442
SARAEV et al.
5. Tolman, C.A., J. Am. Chem. Soc., 1970, vol. 92, no. 14, 16. Venanzi, L.M., J. Chem. Soc., 1958, vol. 63, no. 2,
p. 4217.
p. 719.
6. Shmidt, F.K., Mironova, L.V., Kalabina, A.V., et al.,
17. Hartung, H., Baumeister, U., Walter, B., and
Maschmeier, M., Z. Anorg. Allg. Chem., 1989, vol. 578,
no. 1, p. 177.
Neftekhimiya, 1976, vol. 16, no. 4, p. 547.
7. Keim, W., Angew. Chem., 1990, vol. 102, no. 3, p. 251.
8. Saraev, V.V. and Shmidt, F.K., Koord. Khim., 1997,
vol. 23, no. 1, p. 45 [Russ. J. Coord. Chem. (Engl.
Transl.), vol. 23, no. 1, p. 40].
18. Miedaner, A., Curtis Haltiwanger, R., and DuBois, D.L.,
Inorg. Chem., 1991, vol. 30, no. 3, p. 417.
19. De Souza, R.F., Monteriro, A.L., Seferin, M., et al., J.
Coord. Chem., 1996, vol. 40, no. 4, p. 311.
9. Saraev, V.V. and Shmidt, F.K., J. Mol. Catal. A, 2000,
vol. 158, no. 1, p. 149.
20. Speier, G. and Marko, L., Inorg. Chim. Acta, 1969,
10. Saraev, V.V., Kraikivskii, P.B., Zelinskiy, S.N., et al., J.
Mol. Catal. A, 2005, vol. 236, nos. 1–2, p. 125.
vol. 3, p. 126.
21. Saraev, V.V., Kraikivskii, P.B., Lazarev, P.G., et al.,
Koord. Khim., 1996, vol. 22, no. 9, p. 648 [Russ. J.
Coord. Chem. (Engl. Transl.), vol. 22, no. 9, p. 608].
11. Saraev, V.V., Kraikivskii, P.B., Annenkov, V.V., et al.,
ARKIVOC, 2005, no. XV, p. 44.
12. Heimbach, P., Angew. Chem., 1964, vol. 76, no. 13,
22. Meyer, B., Low Temperature Spectroscopy, New York:
p. 586.
American Elsevier, 1971.
13. Saraev, V.V., Shmidt, F.K., Mironova, L.V., and
Gruznykh, V.A., Koord. Khim., 1976, vol. 2, no. 9,
p. 1249.
23. Saraev, V.V., Shmidt, F.K., Gruznykh, V.A., et al., Koord.
Khim., 1979, vol. 5, no. 6, p. 897.
24. Kraikivskii, P.B., Saraev, V.V., Matveev, D.A., et al.,
Koord. Khim., 2003, vol. 29, no. 6, p. 461 [Russ. J.
Coord. Chem. (Engl. Transl.), vol. 29, no. 6, p. 431].
14. Nilges, M.J., Barefield, E.K., Belford, R.L., and
Davis, P.H., J. Am. Chem. Soc., 1977, vol. 99, no. 3,
p. 755.
15. Saraev, V.V., Kraikivskii, P.B., Zelinskii, S.N., et al., 25. Saraev, V.V., Kraikivskii, P.B., Lazarev, P.G., et al.,
Koord. Khim., 2006, vol. 32, no. 6, p. 413 [Russ. J.
Coord. Chem. (Engl. Transl.), vol. 32, no. 6, p. 397].
Koord. Khim., 1996, vol. 22, no. 9, p. 655 [Russ. J.
Coord. Chem. (Engl. Transl.), vol. 22, no. 9, p. 615].
RUSSIAN JOURNAL OF COORDINATION CHEMISTRY Vol. 34 No. 6 2008