Two nickelocenes and ferrocene in a rigid cis/trans chain

Article abstract of DOI:10.1016/S0022-328X(02)01674-1

With the aim of studying the conformation of bridged paramagnetic metallocenes and the conformation-dependent electron spin delocalization therein, a trimetallic model compound was synthesized. First, a nickelocene bonded to cyclopentadienyl (Cp) anion by

Full text of DOI:10.1016/S0022-328X(02)01674-1

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Two nickelocenes and ferrocene in a rigid cis/trans chain
Martin Herker a, Frank H. Kohler a,*, Markus Schwaiger b, Bernd Weber a
¨
a Anorganisch-Chemisches Institut, Technischen Universitat Munchen, Lichtenbergstrasse 4, D-85747 Garching, Germany
¨
¨
b Institut fur Organische Chemie und Biochemie der, Technischen Universitat Munchen, Lichtenbergstrasse 4, D-85747 Garching, Germany
¨
¨
¨
Received 4 March 2002; received in revised form 2 May 2002; accepted 2 May 2002
Abstract
With the aim of studying the conformation of bridged paramagnetic metallocenes and the conformation-dependent electron spin
delocalization therein, a trimetallic model compound was synthesized. First, a nickelocene bonded to cyclopentadienyl (Cp) anion
by two SiMe2 groups was made in three steps. Further reaction with solvated iron dichloride gave a trimetallic metallocene (6) that
had the metal sequence NiFeNi. X-ray crystal analysis revealed that in one-half of 6 the arrangement of the metal atoms Ni and Fe
with respect to the ligand bridge was trans; in the other half it was cis. The Cp ligands were neither staggered nor eclipsed, the
1
metallocene axes were slightly bent, and bending was also observed about three vectors in the bridging ligand. The H-, 13C-, and
29Si-NMR spectra were in accord with two unpaired electrons per nickelocene, with both cis and trans arrangement of the metal
atoms, and with bending of the ligand bridges. Large NMR signal shifts indicated transfer of spin density from the nickelocenes into
the SiMe2 bridges and the central ferrocene. The efficiency of the spin transfer was found to depend on the bending angle of the
ligand bridge. MO calculations illustrated that spin delocalization in the cis half of 6 was less pronounced than in the trans half.
# 2002 Elsevier Science B.V. All rights reserved.
Keywords: Nickelocene; Ferrocene; Bridged cyclopentadienyls; NMR spectroscopy; Electron spin delocalization
1. Introduction
For motif C additional isomers are possible if
different fragments (M and M?) and/or more than two
fragments are bridged. For instance, trinuclear metallo-
cenes derived from the bridging ligand 1 should be able
to adopt conformations F, G, and H (Fig. 2), regardless
whether M and M? are equal or not. The arrangement of
M and M? in conformation H is particularly interesting,
because it is the only one that would lead to small rings
if no ligand other than the bridging one were present.
This has actually been found in a ring, which contains
seven ferrocenes (Fig. 3) [7]. On the other hand, only
conformer F could be isolated when a vanadocene was
flanked by two ferrocenes, i.e. when the metal sequence
Linking metallocenes (most often ferrocenes) by a
single bridge per cyclopentadienyl (Cp) ligand can lead
to organometallic polymers [1]. These compounds
would be conformationally very flexible as has been
found for oligomeric ferrocenes by, for instance, one
Me2Si group per Cp [2]. When the bridging is effected by
two adjacent groups per Cp the resulting molecules are
more rigid, and various more stable geometric motifs
become available. A widely studied example is the
dianion 1 [3] which forms two pentahapto bonds to
metals thereby yielding the structurally established
motifs A [4], B [3,5], C [6], D [5], and E [6c] in Fig. 1.
In these motifs the bar is a Cp in side view, and M
represents a fragment consisting of a metal ion and
auxiliary ligands.
was FeVFe [6d]. This suggests that the different Mꢀ
/Cp
distances of the metallocenes [8] have some bearing on
the formation of conformers F, G, and H. Actually, the
steric hindrance between the axial methyl groups at the
silicon bridges and adjacent Cp protons (see frames in
Fig. 4) must be considered: if the Mꢀ/Cp distances of the
central and terminal metallocenes are long and short
(Fig. 4, F?), respectively, steric hindrance is small. This
scenario has been realized with the metal sequence
FeVFe [6d]. Fig. 4, Fƒ shows the reverse case where
* Corresponding author. Tel.:
28913762
E-mail address: f.h.koehler@lrz.tu-muenchen.de (F.H. Kohler).
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0022-328X/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved.
PII: S 0 0 2 2 - 3 2 8 X ( 0 2 ) 0 1 6 7 4 - 1

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Fig. 3. Sketch of seven bridged ferrocenes forming a ring.
Fig. 1. Metal binding motifs of ligand 1.
Fig. 4. Steric interactions between the framed parts of trimetallic
bridged metallocenes depending on the MꢀCp distance and the
metallocene sequence. The differences of the MꢀCp distances are
exaggerated.
/
/
Fig. 2. Conformers of three metallocenes bridged by four silyl groups
of two ligands 1.
whether conformation G can be realized with the metal
sequence NiFeNi and how the unpaired electrons would
be delocalized.
the Mꢀ
/
Cp distance of the central metallocene is shorter
than that of the terminal ones and where steric
hindrance is stronger. The latter type of compounds is
expected to prefer conformation G with less steric
repulsion. While the conformational variety is interest-
ing in itself, it is expected to influence the distribution of
the unpaired electrons in such molecules, and this in
turn has been shown to influence the magnetic interac-
tion in compounds with adjacent paramagnetic metal-
locenes [6d]. In the present work, we wanted to check
2. Results and discussion
2.1. Synthesis
The assembly of different metals in trinuclear bridged
metallocenes such as the target molecule 6 (Scheme 1) is
based on the stepwise deprotonation of the starting

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M. Herker et al. / Journal of Organometallic Chemistry 658 (2002) 266Á273
/
Scheme 1. Synthesis of compound 6. (a) n-BuLi; (b) CpNa,
NiBr2(THF)1.5; (c) lithium piperidid; (d) FeCl2(THF)1.5. Only one of
two isomers is shown for 2 and 4.
ligand precursor 2. Thus, removal of one proton gave
anion 3 [3], which was converted to the mixed-ligand
nickelocene 4 [9]. Compound 4 was formed together
with Cp2Ni and a nickelocene, which had two ligands 3.
The yield of the latter nickelocene was kept low by using
an excess of CpNa in step b; it formed insoluble
coordination polymers after steps c and d and was not
studied further. The other nickelocenes were separated
from ionic impurities by extraction with hexane before
mild deprotonation was achieved in step c. Anion 5 and
Cp2Ni were separated by extracting the latter with
hexane so that, after step d, the only product soluble
in hexane was expected to be 6. Actually, some Cp2Ni
and 4 had to be removed from 6 by medium pressure
chromatography. When the volume of a toluene solu-
tion of 6 was slowly reduced, crystals containing one
solvent molecule per formula unit were obtained.
Fig. 5. Molecular structure of compound 6 (top and bottom: ORTEP
drawings, middle: sketch showing interplane angles).
˚
1.65 A) is smaller than that of the nickelocene moieties
˚
(average 1.82 A). The steric constraints outlined in Fig.
4 would, therefore, favor motif G which has actually
been found. Compound 6 features various distortions
that are also illustrated in Fig. 5. Thus, there are
interplane angles of 1.8Á6.48 at the junctions between
/
the Cp rings and the adjacent halves of the six-
membered rings, and all metallocene moieties are
2.2. Crystal structure of 6
slightly bent (1.5Á5.98). Quite remarkably, the bending
/
about the Si, Si vector is smaller for the cis than for the
trans arrangement (complementary angles: 169.0 and
134.28, respectively). In consequence, there are axial and
equatorial methyl groups on the trans side of 6, while on
the cis side the methyl groups are neither axial nor
equatorial, but syn and anti with respect to the adjacent
nickelocene. It can be seen at the bottom of Fig. 5, that
the molecular symmetry is low (C1), that is, the two
ferrocene ligands are not staggered. However, with a
twist angle of 15.98 they are not eclipsed either.
Intermediate twist angles (19.0 and 18.08) also exist for
After prolonged standing of a saturated solution of 6
in benzene-d6, solvent-free crystals were obtained
wherein 6 has the molecular structure shown in Fig. 5.
It turns out that both the cis and trans arrangement of
the nickel and iron atoms relative to the bridging ligand
are realized, that is, motifs D and C of Fig. 1,
respectively. Since different neighboring metallocenes
are present, it is not sufficient to classify the trans side of
6 as C; there is still a choice between the trans versions
of motifs F and G in Fig. 2. As expected, in 6 the metalꢀ
/
Cp centroid distances of the ferrocene moiety (average

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269
Table 1
1400 Hz) the non-equivalent protons are resolved except
for those of the terminal Cps which coincide acciden-
tally. The nickelocene carbon atoms, which are expected
to show extremely broad signals above 1000 ppm [11]
were not considered, because the signal-to-noise ratio
was too low. As outlined in Section 3, the NMR signal
˚
Selected bond distances (A) and angles (8) of compound 6 with eds’s in
units of the last significant figure in parentheses
Bond distances
Si1ꢀC6
1.855(6)
1.863(6)
1.861(8)
1.872(7)
1.854(6)
1.844(7)
1.842(8)
1.878(7)
1.826
Si3ꢀC16
Si3ꢀC22
Si3ꢀC35
Si3ꢀC36
Si4ꢀC17
Si4ꢀC21
Si4ꢀC37
Si4ꢀC38
1.837(7)
1.866(6)
1.863(7)
1.844(7)
1.867(7)
1.863(7)
1.858(7)
1.858(7)
1.657
Si1ꢀC12
Si1ꢀC31
Si1ꢀC32
Si2ꢀC7
Si2ꢀC11
Si2ꢀC33
Si2ꢀC34
Ni1ꢀD1
Ni1ꢀD2
Ni2ꢀD5
shifts in Table 2 are contact shifts, dcon
.
The detailed signal assignment was of interest,
because from the signal shifts it can be concluded how
the structure determines the transfer of the unpaired
electron spin density from one metallocene to the next.
The strongly shifted nickelocene protons were assigned
by comparison with cis- and trans-NiNi [10], and the
same applies for the methyl protons except for the
a
a
a
a
FeꢀD3
FeꢀD4
a
1.814
1.815
1.653
1.822
a
Ni2ꢀD6
Bond angles
C6ꢀSi1ꢀC12
C31ꢀSi1ꢀC32
C7ꢀSi2ꢀC11
C33ꢀSi2ꢀC34
107.0(3)
107.2(4)
107.0(3)
106.7(4)
C16ꢀSi3ꢀC22
C35ꢀSi3ꢀC36
C17ꢀSi4ꢀC21
C37ꢀSi4ꢀC38
104.1(3)
108.8(4)
104.2(3)
108.6(4)
a
D is the centroid of the respective Cp ring (see Fig. 5).
the Cps of the nickelocenes on the cis and trans side,
respectively. Further selected structural data are given in
Table 1.
2.3. 1H-, 13C-, and 29Si-NMR spectra
In solution Cs symmetry follows from all NMR
spectra. As compared to the signal sets known [6d] for
motif F in Fig. 2 (trans, trans conformer), there is a
doubling of the signal number owing to cis and trans
moieties. But there are no further signals that would
imply C1 symmetry. As can be seen from the data in
Table 2, the 1H- and 13C-NMR signals appear in ranges
that are known from simpler analogues containing two
nickelocenes (cis- and trans-NiNi) or one ferrocene and
one nickelocene (FeNi) [10]. For instance, the nickelo-
cene proton signals are shifted more than 200 ppm to
low frequency. Although the signals are broad (up to
Fig. 6. HMQC NMR spectrum of compound
benzene-d6, 298 K). S, solvent.
6 (500.12 MHz,
Table 2
a
1H and 13C contact shifts of compound 6
b
b
Position of nucleus
d
con(1H)
d
(
con 13CH)
Position of nucleus
d
con(1H)
d
(
con 13C)
c
c
c
c
c
c
c
c
Cp
ꢁ259
ꢁ239
ꢁ244
Cp?
2?
1?/3?
3a?/8a?
4a?/7a?
ꢁ259
ꢁ231
ꢁ233
2
1/3
3a/8a
4a/7a
144.6
16.6
98.8
2.1
9.6
d
d
5/7
6
ꢁ1.1
ꢁ11.7
6.5
5?/7?
2.1
5.8
6?
Mesyn
Meanti
33.6
121.2
115.8
Meax
Meeq
200.1
104.0
16.7
ꢁ2.6
1.7
a
In ppm at 298 K.
For numbering see Scheme 1.
b
c
Not observed.
Interchange of signal assignment for positions 5/7 and 5?/7? not excluded.
d