EPR study of (η5-cyclopentadienyl)nickel(I) complexes with P-and N-donor ligands and 1,5-cyclooctadiene

Article abstract of DOI:10.1134/S1070328406060029

Mononuclear (η⁵-cyclopentadienyl)nickel(I) complexes with triphenylphosphine, triethylphosphite, 2,2'-bipyridyl, and 1,5-cyclooctadiene formed in the course of reduction of nickelocene were studied by EPR method. Monocyclopentadienyl Ni(I) complexes of the composition CpNiL₂ were shown to form during nickelocene reduction irrespective of the method applied (the use of organometallic compound, alkali metal, thermal conditions) in the presence of stabilizing ligands L (L = PPh₃, P(OEt)₃, Bipy/2, COD/2) and in the course of contradisproportionation reaction between nickelocene and the corresponding NiL₄ complex. It was found that in the structure of these CpNiL₂ complexes (L = PPh₃, P(OEt)₃, Bipy/2), the main molecular axis is perpendicular to the L-Ni-L plane and these complexes should be considered as derivatives of trigonal structures of D _3h symmetry distorted by Cp ligand. In CpNi(COD) complex, the main axis passes through the Cpring center and this complex should be treated as a derivative of pentagonal structure of C _5v symmetry distorted by COD ligand. Nonequivalence of ³¹P nuclei results from vibronic interaction effect in tricoordinate structures in pseudodegenerate electron state (Jahn-Teller effect). Pleiades publishing, Inc. 2006.

Full text of DOI:10.1134/S1070328406060029

ISSN 1070-3284, Russian Journal of Coordination Chemistry, 2006, Vol. 32, No. 6, pp. 397–401. © Pleiades Publishing, Inc., 2006.
Original Russian Text © V.V. Saraev, P.B. Kraikivskii, S.N. Zelinskii, A.I. Vil’ms, D.A. Matveev, A.Yu. Yunda, A.A. Fedonina, K. Lammertsma, 2006, published in Koordinatsion-
naya Khimiya, 2006, Vol. 32, No. 6, pp. 413–418.
EPR Study of (h5-Cyclopentadienyl)nickel(I) Complexes
with P- and N-Donor Ligands and 1,5-Cyclooctadiene
V. V. Saraeva, P. B. Kraikivskiia, S. N. Zelinskiia, A. I. Vil’msa, D. A. Matveeva, A. Yu. Yundaa,
A. A. Fedoninaa, K. Lammertsmab
a
Irkutsk State University, ul. Lermontova 126, Irkutsk, 664033 Russia
b
Vrije University, Amsterdam, the Netherlands
Received March 9, 2005
Abstract—Mononuclear (η5-cyclopentadienyl)nickel(I) complexes with triphenylphosphine, triethylphos-
phite, 2,2'-bipyridyl, and 1,5-cyclooctadiene formed in the course of reduction of nickelocene were studied by
EPR method. Monocyclopentadienyl Ni(I) complexes of the composition CpNiL2 were shown to form during
nickelocene reduction irrespective of the method applied (the use of organometallic compound, alkali metal,
thermal conditions) in the presence of stabilizing ligands L (L = PPh3, P(OEt)3, Bipy/2, COD/2) and in the
course of contradisproportionation reaction between nickelocene and the corresponding NiL4 complex. It was
found that in the structure of these CpNiL2 complexes (L = PPh3, P(OEt)3, Bipy/2), the main molecular axis is
perpendicular to the L–Ni–L plane and these complexes should be considered as derivatives of trigonal struc-
tures of D3h symmetry distorted by Cp ligand. In CpNi(COD) complex, the main axis passes through the Cp-
ring center and this complex should be treated as a derivative of pentagonal structure of C5vsymmetry distorted
by COD ligand. Nonequivalence of 31P nuclei results from vibronic interaction effect in tricoordinate structures
in pseudodegenerate electron state (Jahn–Teller effect).
DOI: 10.1134/S1070328406060029
Nickel(I) complexes stabilized with phosphine
ligands are indispensable components of nickel com-
plex catalysts of oligomerization of unsaturated hydro-
carbons [1, 2] irrespective of the catalyst formation
method: reaction of Ni(0) oxidation in the starting com-
pound with the Lewis [3–5] or Brönsted [6] acid or
reaction of Ni(II) reduction in the starting compound
with organometallic compounds of nontransition ele-
ments [7, 8]. It was shown for catalytic system
Ni(PPh3)4–BF3 · OEt2 as an example that the phosphine
cationic complexes of Ni(I) participate directly in the
catalytic transformation of unsaturated hydrocarbons
[9]. In connection with this, Ni(I) complexes stabilized
with cyclopentadienyl ligands (Cp) are of great interest
as potentially active forms that can be used in processes
such as oligomerization of ethylene [10–16], polymer-
ization of alkenes [17] and alkynes [18].
2{CpNiR}
{CpNiR}
{(NiCp)2} + (R–R),
.
{(NiCp)2} + R.
This paper reports the results of the EPR study of
mononuclear (η5-cyclopentadienyl)nickel(I) com-
plexes with triphenylphosphine, triethylphosphite,
2,2'-bipyridyl, or 1,5-cyclooctadiene formed in the
reaction of nickelocene reduction.
EXPERIMENTAL
All manipulations were carried out in the atmo-
sphere of purified argon following Schlenk procedure.
The precipitates were filtered on glass Schlenk filters.
All reagents were stored in sealed tubes in the argon
atmosphere.
As was reported in [19, 20], the first stage of inter-
action of nickelocene NiCp2 with organolithium and
organomagnesium compounds MR (M = Li or MgX) is
the exchange of one cyclopentadienyl group for the
alkyl group with the formation of unstable 16-electron
complex {CpNiR}. As a result of the coupling reaction
of the alkyl groups [21, 22] or homolysis of these
groups [23], this complex transforms to univalent
nickel complex occurring as a dimer in a solution in the
absence of stabilizing ligands:
Toluene, benzene, hexane, and cyclohexane (special
purity grade) were distilled before use over metallic
sodium in the presence of benzophenone.
Commercial reagents (Merck), i.e., 1,5-cyclooctadi-
ene (COD), 2,2'-bipyridyl (Bipy), triphenylphosphine
(PPh3), triethylphosphite (P(OEt)3) were used as
received. Commercial Et2AlOEt was distilled in vac-
uum.
The NiCp2, Ni(P(OEt)3)4, Ni(COD)2, and Ni(PPh3)4
complexes were synthesized by the known procedures
[24–27], respectively.
NiCp2 + MR
{CpNiR} + MCp,
397

398
SARAEV et al.
3a
3b
1a
1b
3a*
2a
2b
4a
4b
290
325
360 290
Magnetic field, mT
325
360
Fig. 1. (a) Experimental and (b) model EPR spectra of CpNiL complexes in toluene solution at T = 77 K: L = (1) PPh , (2) P(OEt) ,
2
3
3
(3) Bipy/2, (4) COD/2; (*) is the second derivative of the EPR spectrum
Synthesis of CpNi(PPh3)2 (I). Sublimated crystal-
For C15H13N2Ni
anal. calcd. (%): C 64.35; H 4.64; N 10.01; Ni 20.98
Found (%): C 64.19; H 4.60; N 9.65; Ni 20.96.
line NiCp2 (1 mmol) was added to 10 ml of toluene
solution of Ni(PPh3)4 (1 mmol); the reaction mixture
was stirred at room temperature for 12 h. The solution
obtained was cooled to −20°ë and toluene was distilled
off at this temperature in vacuum until a pasty mass was
formed. The mass was washed with 20 ml of hexane
cooled to –20°ë with stirring. Then, a major part of the
liquid was removed by siphon decantation. The solid
was dried in vacuum (0.01 mmHg) at room tempera-
ture. The complex was formed as fine amorphous parti-
cles, which at a temperature above +15°C, fuse into
tarry red-brown mass. The complex readily dissolves in
toluene. The yield was 40%.
The elemental analysis of the complexes was per-
formed on a FLACH 1112 analyzer (EA series). The
EPR spectra were recorded on a CMS-8400 spectro-
meter (working frequency 9.6 GHz) at the boiling point
of liquid nitrogen. The sweep range was calibrated
using the DDPG free radical and Mn2+ in MgO. The
EPR spectra were simulated with our own program
[28], where the hyperfine interaction (HFI) is limited to
the second-order term and the main axes of the g-tensor
and HFI tensors coincide. The NMR spectra were
recorded at 25°C on a VXR-500S Varian spectrometer.
For C41H35NiP2
anal. calcd. (%): C 75.96; H 5.40; Ni 9.06;
RESULTS AND DISCUSSION
Found (%):
C 75.08; H 5.01; Ni 8.96.
Formation of Ni(I) complexes. Nickelocene is a
high-spin compound (S = 1), but is EPR-undetectable
even at T = 4.2 K due to strong splitting of the electron
levels in a zero field [29]. After an equivalent amount of
Et2AlOEt is added to a toluene solution of nickelocene
at room temperature with a twofold molar excess of
PPh3, the intense EPR signal appears with well resolved
Synthesis of CpNi(Bipy) (II). To a solution of
NiCp2 (1mmol) in 20 ml cyclohexane, a sublimated
2,2'-bipyridyl (1 mmol) was added and the mixture was
stirred for 2 h at 75°ë. The reaction was terminated in
20 min after the maximum intensity of EPR signal from
CpNi(Bipy) complex was attained. The reaction mix-
ture was cooled to –20°ë and kept at the same temper-
ature for 3 h. The precipitated violet crystals were fil-
tered off at –20°ë and dried in vacuum (0.01 mm Hg)
at room temperature. At a temperature of +140°ë, the
31
hyperfine structure (HFS) from two nonequivalent P
nuclei (Fig. 1). The same EPR spectrum is observed in
the case of NiCp2 interaction with a fourfold excess of
PPh3 in the absence of organoaluminium compound, if
product decomposed and slightly sublimated. The com- the reaction is performed at boiling point of toluene
plex yield was 83% of the theoretical value. (110°C) or under the action of dispersed metallic
RUSSIAN JOURNAL OF COORDINATION CHEMISTRY Vol. 32 No. 6 2006

EPR STUDY OF (η5-CYCLOPENTADIENYL)NICKEL(I) COMPLEXES
The EPR parameters of the CpNiL2 complexes
399
Complex
gx
gy
gz
Ax, mT
Ay, mT
Az, mT
CpNi(PPh3)2 (I)
2.030
2.047
2.119
12.4 (P1)
11.6 (P2)
1.2 (N)
12.0 (P1)
11.4 (P2)
1.1 (N)
10.9 (P1)
9.35 (P2)
0.8 (N)
CpNi(Bipy) (II)
2.032
2.079
2.183
2.033 [30]
2.032
2.080 [30]
2.032
2.184 [30]
2.093
1.1 (N) [30]
20.1 (P1)
22.0 (P2)
0.9 (N) [30]
20.1 (P1)
22.0 (P2)
0.8 (N) [30]
18.9 (P1)
20.5 (P2)
CpNi(P(OEt)3)2 (III)
CpNi(COD) (IV)
2.015
2.021
2.180
2.018 [31]
2.018 [31]
2.168 [31]
sodium in a toluene–THF (3 : 1) mixture at room tem- ment) in the presence of stabilizing ligands L (L = PPh3,
perature. Under these conditions, the intensity of the P(OEt)3, Bipy/2, COD/2), as well as in the reaction of
spectrum reaches a maximum point and then decreases, contradisproportionation between nickelocene and the
which points to a subsequent change in the Ni oxidation corresponding NiL4 complex.
state: Ni(II)
Ni(I)
Ni(0). Moreover, the analo-
Structure of (h5-Cp)NiL2 complexes. The EPR
spectral parameters of 19-electron CpNiL2 complexes
(table) calculated from characteristic points of the
experimental spectra of solid toluene solutions and
refined by computer simulation of these spectra indi-
cate three-axis anisotropy of the EPR parameters. An
exception is the CpNi(P(OEt)3)2 complex, which has
two-axis anisotropic EPR parameters. It is necessary to
establish the nearest and the highest symmetry to which
the first coordination sphere of Ni(I) belongs, in order
to assign the g-factor components to the corresponding
molecular axes. Let us begin our consideration with
complexes with P- and N-donor ligands.
gous EPR spectrum was produced by a complex
formed in contradisproportionation reaction between
NiCp2 and Ni(PPh3)4, which quickly proceeds in tolu-
ene at room temperature (the maximum intensity of the
spectrum is attained in less than 2 h after the compo-
nents were mixed together and then, the spectrum
remains unchanged for indefinitely long time). There-
fore, the recorded spectrum can be assigned to mono-
nuclear complex (I). The EPR spectrum of this com-
plex in liquid solution does not exhibit nonequivalence
of the phosphine ligands [30].
If 2,2'-bipyridyl, triethylphosphite, or 1,5-cyclooc-
tadiene are used as stabilizing ligands under similar
conditions described above (reduction or contradispro-
portionation), then complexes II, CpNi(P(OEt)3)2 (III),
and CpNi(COD) (IV) are formed. The EPR spectra of
these complexes are shown in Fig. 1. Unlike the spec-
trum of phosphine complex I, the spectrum of phos-
phite complex III shows more distinctly the nonequiv-
alence of the 31P nuclei due to an increase in HFS con-
stants and a decrease in the widths of individual lines.
Note that the EPR spectrum of analogous complex
CpNi(P(OMe)3)2 in liquid solution also shows non-
equivalence of the 31P nuclei [30].
The X-ray diffraction data [30] for the CpNi(Bipy)
complex show that the planes of cyclopentadienyl and
bipyridyl ligands in this complex are mutually ortho-
gonal, which makes it possible to select the following
system of coordinates (1, 2, 3) for complexes with P-
and N-donor ligands (Fig. 2a). Axis 2 lies in the
L−Ni−L plane, axis 3 is perpendicular to this plane. The
model suggested can be described as a structure with
the initial pentagonal C5v symmetry distorted by
1
1
In the EPR spectrum of bipyridyl complex II, the
HFS from the 14N nuclei is poorly resolved and can be
reliably resolved only by a doduble differentiation of
the EPR signal (Fig. 1, curve 3a*). Complexes I and II
were isolated as individual compounds and character-
ized by elemental analysis. The EPR parameters of
bipyridyl complex II and cyclooctadienyl complex IV
coincide within the limits of measurement error with
the literature data for these complexes obtained for the
solid solutions (THF solution at –196°C) [30, 31].
π 2
Ni
2
Ni
2
σ
L
L
3
3
π 1
(‡)
(b)
Thus, monocyclopentadienyl complexes of Ni(I) of
the composition CpNiL2 are formed during nickelocene
reduction irrespective of the method applied (the use of
organometallic compound, alkali metal, thermal treat-
Fig. 2. The Ni coordination sphere in the CpNi complexes
with (a) P- and N-donor ligands and (b) COD.
RUSSIAN JOURNAL OF COORDINATION CHEMISTRY Vol. 32 No. 6 2006

400
SARAEV et al.
ligands L or as a structure with the initial trigonal D3h At ψ = 0, formulas (2) acquire the form common for
symmetry distorted by ligand Cp. In both cases, the HFS constants expressed in terms of magnetic field
unpaired electron occupies pseudodegenerate orbital units, when the principal axes of the g-tensor and HFI
[29, 32], thus giving rise to the following relation tensor coincide.
between the g-factor components gz > gy, gx [28], which
At 0 < ψ < π/2, the apparent constants Ax and Az rep-
corresponds to the experimental data. However, in the
former case, the principal axis of the complex is axis 1,
while in the latter case, it is axis 3. The direction of the
principal axis of the complex can be determined from
the data on HFI with the nuclei of the ligands.
resent combinations of components of HFI tensors.
According to experimental data (table), the apparent
constant Az is the smallest among the three constants for
all Ni(I) complexes with P- and N-donor ligands. Since
|T||| > |T⊥|, then the relationship Az < Ay, Ax is likely to be
fulfilled only at sufficiently large ψ angles. Unfortu-
nately the system of equations (2) cannot be used
directly in order to quantitatively estimate the value of
ψ (as well as |T||| and |T⊥|) from the experimental data,
since in the Ni(I) complexes under study, unlike the
model, the g-tensor and the HFI tensor are triaxial-
anisotropic and therefore, three rotation angles (Euler
angles [34]) should be taken into consideration when
describing their relative orientation.
A great deal of experimental data on Ni(I) com-
plexes with ternary phosphines and phosphites [33]
indicate and the tensor of HFI with 31P nuclei is almost
axially symmetric; its principal axis is directed along
the Ni–P bond. Of the three main components of this
tensor (Tσ, Tπ1, Tπ2), the greatest is Tσ corresponding to
the direction of the axis along the Ni–P bond such that
Tπ1 ≈ Tπ2. Obviously, the principal axes of the g-tensor
(axis 1 or 3) and the HFI tensor (axis σ) do not coincide.
In order to determine the mutual orientation of these
tensors, let us use the results of theoretical studies of the
EPR of solid solutions of model species, where the
principal axes of the axially symmetric g-tensors and of
HFI tensor make an angle ψ [34]. According to the
results obtained, the coordinates of extreme points in
the spectral curve, in whose vicinity the resonance field
changes insignificantly with changing direction of
external magnetic field (characteristic points), are
determined in the first approximation by the following
expressions:
Let us estimate the lower limit of the ψ value for tri-
ethylphosphite complex III, whose axial-symmetric
tensors are the closest to the chosen model in terms of
symmetry and consider the inequality
A2z < A2y,
(3)
which gives
(g||/g⊥)2 – 1
2
-----------------------------
cos ψ <
.
(4)
(T||/T⊥)2 – 1
If the ratio of components of the HFI tensor with 31P
for the P(OEt)3 ligand is considered to change insignif-
icantly in the series of the Ni(I) complexes with thriethyl-
phosphite, then one can use the value |T|||/|T⊥| = 1.2 for
the complex [(P(OEt)3)Ni(PPh3)3]BF4, where the prin-
cipal axes of the axially-symmetric g-tensor and the
HFI tensor for the P(OEt)3 ligand coincide (triethyl-
phosphite is located at a trigonal pyramid vertex) [35].
By substituting the experimental values of the g-tensor
for complex III into formula (4), we obtain that ψ >
68°. Taking into account that the difference (Ay – Az) is
noticeably greater than the difference (Ax – Ay) for all
CpNiL2 complexes with P- and N-donor ligands, one
can conclude that the angle ψ between the principal
axis of the g-tensor and the HFI tensor is substantially
greater than 68° and is obviously close to π/2. This is
likely to occur when the principal axis of the g-tensor is
the molecular axis 3 (Fig. 2a). Hence, the principal
molecular in complexes I, II, and III is perpendicular
to the L−Ni−L plane and these complexes should be
considered as the derivatives of trigonal structures of
the D3h symmetry distorted by the Cp ligand.
Hy = (hν – T⊥ m)/g⊥βe,
Hx = (hν – [T|2| sin2 ψ + T2⊥ cos2 ψ]1/2m)/g⊥βe, (1)
Hz = (hν – [T|2| cos2 ψ + T2⊥ sin2 ψ]1/2m)/g||βe,
where hν is the energy of a quantum of electromagnetic
field, βe is the Bohr magneton, m is the magnetic quan-
tum number of a nucleus, g⊥, g|| are components of the
axially symmetric g-tensor, T⊥ and T|| are components
of axially symmetric HFI tensor. Indices x, y, z indicate
the direction of magnetic field along the corresponding
molecular axes.
The first of the three expressions (1) is an explicit
solution, whereas the other two equations are approxi-
mate solutions to the extremum problem that are
obtained on condition that HFI anisotropy is low [34].
In line with expressions (1), the apparent HFI con-
stants (the distances between neighboring characteris-
tic points) will be
Nonequivalence of the 31P nuclei seems to be due to
the vibronic interaction effect in tricoordinate struc-
tures in pseudodegenerate ground state (Jahn–Teller
pseudoeffect) [28].
The cyclooctadiene ligand in complex IV supplied 4
carbon atoms to the Ni coordination sphere (Fig. 2b). If
Ay = T⊥ /g⊥βe,
Ax = [T|2| sin2 ψ + T⊥2 cos2 ψ]1/2/g⊥βe,
Az = [T|2| cos2 ψ + T⊥2 sin2 ψ]1/2/g||βe.
(2)
RUSSIAN JOURNAL OF COORDINATION CHEMISTRY Vol. 32 No. 6 2006

EPR STUDY OF (η5-CYCLOPENTADIENYL)NICKEL(I) COMPLEXES
401
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and III, in complex IV, the principal axis correspond-
ing to the largest component of the g-factor passes
through the Cp ring center. Therefore, complex IV
should be treated as a derivative of pentagonal structure
with C5v symmetry distorted by the COD ligand.
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(long relaxation times) at a high temperature (77 K)
with triaxial anisotropy of the g-factor indicate the
orthorhombic distortion of the complex, probably, as a
result of Jahn–Teller effect, which has steady-state
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ACKNOWLEDGMENTS
This work was supported by the NWO Foundation
(the Netherlands) and the Russian Foundation for Basic
Research (International grant no.047.015.014).
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