Facial coordination of cyclooctatetraene to a Co2Ni triangle in a heterometallic trinuclear cluster complex

Article abstract of DOI:10.1016/S0022-328X(01)01302-X

Reaction of [(η-C₅H₅)NiCo₃(CO)₉] (5) with 1,3,5,7-cyclooctatetraene or 1,4-(SiMe₃)₂C₈H₆, respectively, yields the complexes [Co₂Ni(CO)₆(μ₃

Full text of DOI:10.1016/S0022-328X(01)01302-X

Journal of Organometallic Chemistry 641 (2002) 142–147
www.elsevier.com/locate/jorganchem
Facial coordination of cyclooctatetraene to a Co2Ni triangle in a
heterometallic trinuclear cluster complex
Hubert Wadepohl *, Stefan Gebert, Ru¨diger Merkel, Hans Pritzkow
Anorganisch-Chemisches Institut der Ruprecht-Karls-Uni6ersita¨t, Im Neuenheimer Feld 270, D-69120 Heidelberg, Germany
Received 4 July 2001; accepted 12 August 2001
Dedicated to Professor Rolf Gleiter on the occasion of his 65th birthday
Abstract
Reaction of [(h-C5H5)NiCo3(CO)9] (5) with 1,3,5,7-cyclooctatetraene or 1,4-(SiMe3)2C8H6, respectively, yields the complexes
[Co2Ni(CO)6(m3-h8-C8H6R2)] (R=H, SiMe3) (7a, b). Dramatic modifications of the tetrametallic cluster core and the ligand
sphere of 5 to give the trinuclear complex 7 are driven by the preference of the cyclopolyenes for facial (m3-h8) coordination. The
title complexes are the first examples of facial cyclooctatetraene coordination to a heterometallic (Co2Ni) triangle. © 2002
Published by Elsevier Science B.V.
Keywords: Cluster complexes; Cyclopolyenes; Facial coordination; Cobalt; Nickel
1. Introduction
The cyclic p-perimeters CnHn have been shown to
facilitate the formation and enhance the stability of
oligonuclear molecular transition metal aggregates.
This can be attributed to the capability of such ligands
to cap a triangular face of a metal cluster. For example,
benzene (n=6) may be used as a template to aid the
self-assembly of organometal fragments. Three (h-
C5H5)Rh fragments can be assembled sequentially on a
face of benzene, to ultimatively form the trinuclear
cluster complex [{(h-C5H5)Rh}3(m3-C6H6)] (1), with
benzene in the facial (m3-) coordination mode [1]. Gen-
eration of the complexes [{(h-C5H5)Co}3(m3-arene)] (2)
proceeds in a similar and often highly efficient fashion,
but not without the additional help of an exocyclic
alkenyl group, which guides the metals to the arene
nucleus [2].
Co, Rh) by cyclooctatetraene proceeds readily and
leads to the formation of cluster complexes like, e.g.
[M4(CO)8(m3-C8H8)] (3, M=Co), [4] (4, M=Rh) [5] in
very high yield. Recently, evidence was also found for a
pronounced template effect due to the cyclopolyolefin
when 3 is generated from cyclooctatetraene and dinu-
clear cobalt carbonyl [Co2(CO)8] [6]. These results,
along with a now substantial amount of theoretical [7]
and experimental [3–5,8] evidence, consistently indicate
a high tendency of formation and stability of the
M3(m3-C8H8) moiety.
Facial coordination is a prominent feature of the
metal cluster chemistry of cycloheptatrienyl (n=7) and
cyclooctatetraene (n=8) [3]. For example, capping of a
trimetal face in the binary carbonyls [M4(CO)12] (M=
In the present paper we report the remarkable forma-
tion of trinuclear cyclooctatetraene-capped mixed metal
(Co2Ni) clusters from a tetranuclear precursor, where
* Corresponding author. Tel.: +49-6221-544827; fax: +49-6221-
544197.
E-mail address: bu9@ix.urz.uni-heidelberg.de (H. Wadepohl).
0022-328X/02/$ - see front matter © 2002 Published by Elsevier Science B.V.
PII: S0022-328X(01)01302-X

H. Wadepohl et al. / Journal of Organometallic Chemistry 641 (2002) 142–147
143
partial destruction of the cluster core and
rearrangement of the ligand shell is governed by the
preferences of the facial ligands. A preliminary account
of part of this work has been given [9].
2. Results and discussion
From a structural point of view, in the complexes 3
and 4 the facial C8H8 ligand replaces three axial car-
bonyl ligands around a trimetal face of the parent
carbonyl [M4(CO)12]. However, in order to accommo-
date the eight electrons donated by a m3-h8-C8H8 ligand
and keep the total valence electron (v.e.) count at the
‘magic number’ of 60, [10] a total of four CO molecules
have to be displaced, resulting in the formation of an
apical [11] M(CO)2 from the original M(CO)3 group. In
the complex [(h-C5H5)NiCo3(CO)9] (5) only three (ax-
ial) carbonyls can be displaced with ease. Introduction
of a facial C8H8 ligand in 5 was expected to either result
in an only m3-h6 coordination of this ligand, or in the
formation of a (potentially paramagnetic) 62 v.e. spe-
cies with m3-h8-C8H8. There is literature precedence for
both possibilities. When heated with cyclooctatetraene,
the alkylidyne-bridged tricobalt complex [Co3(CO)9(m3-
CPh)] was reported to give the species [Co3(CO)6(m3-
CPh)(m3-C8H8)] (6) [12]. Coordination of only six of the
eight ring carbon atoms to the cluster conserves the 60
v.e. count [12]. Electrochemical reduction of
[Co4(CO)6(L)(m3-C8H8)] (L=h4-C8H8, h4-C6H8, h4-6,6-
Ph2C6H4) proceeds in two reversible steps; most likely
the h8 coordination of the facial C8H8 ligand is retained
in the 61 v.e. monoanion and the 62 v.e. dianion [7,8].
Fig. 1. Molecular structure of [Co2Ni(CO)6(m3-C8H8)] (7a).
erotrinuclear cluster complexes [NiCo2(CO)6(m3-h8-
C8H6R2)] (7a, R=H; 7b, R=SiMe3). The metal com-
position of 7 was unambiguously derived from the mass
spectra, where the shapes of the peaks due to the
molecular ions were in perfect agreement with the
calculated intensities based on the presence of two
cobalt atoms and one nickel atom. In addition, XPS
analysis of a sample of 7a also gave an atomic ratio
Co:Ni=2:1.
The presence of [(h-C5H5)Co(CO)2] (8) was spectro-
scopically detected in the reaction mixtures. It is likely
that the reaction involves transfer of a cyclopentadienyl
ligand from the nickel to a cobalt atom, which then
leaves the cluster in the form of 8. Related effects have
been observed in the course of metal exchange reactions
in tri- and tetranuclear cluster complexes [13].
Disregarding the trimethylsilyl groups the two com-
plexes 7a and b have very similar structures. Views of
the molecules are depicted in Figs. 1 and 2, respectively;
a superposition of the two molecules is shown in Fig. 3.
Important bond lengths and angles are summarized in
Table 1.
The molecules consist of trimetal triangles, which are
capped by the cyclooctatetraene [or 1,4-bis(trimethylsi-
lyl)cyclooctatetraene] in the facial coordination mode.
Three terminal and three edge-bridging carbonyls com-
plete the coordination sphere of the clusters.
Reaction of 5 with excess C8H8 or 1,4-C8H6(SiMe3)2,
respectively, in boiling n-heptane gave dark red crys-
talline products 7a and b in 25 and 38% yield, respec-
tively. The infrared spectra of 7 indicated terminal and
bridging carbonyl ligands; the large number of bands (3
terminal and 3 bridging wCO) points to a rather low
molecular symmetry for both derivatives. The NMR
spectra were consistent with the presence of facial
C8H6R2 ligands. Resonances that could have been at-
tributed to a cyclopentadienyl ligand were absent.
Mass spectroscopic analysis and X-ray crystal struc-
ture determinations identified the products as the het-
With complex 7a, the X-ray data do not allow a
distinction to be made between nickel and cobalt. We

144
H. Wadepohl et al. / Journal of Organometallic Chemistry 641 (2002) 142–147
Fig. 2. Molecular structure of [Co2Ni(CO)6{m3-C8H6(SiMe3)2}] (7b).
,
,
1.396(8)–1.434(8) A in 7a, 1.406(3)–1.451(3) A in 7b),
and a difficult-to-define hapticity, in between m3-
h2:h3:h3 and m3-h3:h3:h3 (Figs. 1 and 2). The bulky
trimethylsilyl groups do not have much influence on the
geometry of the bridging eight-membered ring and on
the overall geometry of the molecule (Fig. 3). This was
also noted for the tetrarhodium complex [Rh4(CO)6{h4-
C8H6(SiMe3)2}{m3-h8-C8H6(SiMe3)2}] [5].
can neither completely rule out static disorder of the
metal triangles, nor detect definitive signs of it in the
shapes and orientations of the anisotropic displacement
ellipsoids. In the only structurally characterized com-
parable Co2Ni cluster complex, viz. [NiCo2(m2-
CO)3(PMe3)6] (9) the metals are disordered, and an
average threefold symmetry is imposed upon the
molecule by the lattice symmetry [14]. In contrast, the
metals could be assigned in the structure of 7b (Fig. 2).
Superposition of the molecular structures of 7a and 7b
was particularly good in one of the three obvious
orientations, which can be generated from permutation
of the metals (Fig. 3). This would place the nickel atom
on M1 in 7a.
The NMR spectra of 7 show a high molecular sym-
metry on the NMR time scale. Only one resonance is
observed for the cyclooctatetraene ligand in 7a both in
the 1H- (l=3.42) and 13C-NMR spectra (l=71.8).
Likewise, the proton (one silyl resonance along with a
singlet overlaid by an (AB)2 pattern around l=4.3)
and carbon spectra of 7b correspond to the highest
possible symmetry of the 1,4-(SiMe3)2C8H6 ligand. The
The metal–metal distances in 7a and b span the
,
range 2.500(1)–2.540(1) A, comparable to those in the
isoelectronic and isostructural anion [Co3(CO)6(m3-
C8H8)]− (10) (2.507(2), 2.530(2) A) [8]. Compared to
,
the Co3 frame in complex 10, a contraction of the
Co2Ni triangle would have been expected, due to the
smaller covalent radius of nickel (compare the metal–
,
metal separation of 2.438 A in 9). The bond distances
between the metals and the terminal CO ligands show
,
little variation (range 1.774(5)–1.783(5) A in 7a;
,
d(NiꢀC)=1.792(2), d(CoꢀC)=1.771(2), 1.773(2) A in
7b). The carbonyl bridges are somewhat asymmetric in
both structures. As expected, metal–C(O) distances are
a little longer than those in the tricobalt anion 10 [8].
The cyclooctatetraene ligands exhibit the geometrical
features that are typical for the facial coordination
mode: [3] a rather flat chair-like ring conformation,
with the mean plane through the ring carbons parallel
to the trimetal plane, a non-systematic variation of the
lengths of the endocyclic carbon–carbon bonds (range:
Fig. 3. Superposition of the molecules of 7a (dashed lines) and 7b
(solid lines).

H. Wadepohl et al. / Journal of Organometallic Chemistry 641 (2002) 142–147
145
Table 1
,
Selected bond lengths (A) and bond angles (°) for the complex [Co2Ni(CO)6(m3-C8H6R2)] (7a, R=H; 7b, R=SiMe3)
a
7a
7b
7a
7b a
Bond lengths
M1ꢀM2
M2ꢀM3
M1ꢀC2
M2ꢀC3
M2ꢀC5
M3ꢀC7
C1ꢀC2
2.512(1)
2.526(1)
2.140(5)
2.333(5)
2.236(5)
2.088(5)
1.396(8)
1.417(8)
1.411(7)
1.412(7)
2.5183(4)
2.5402(4)
2.198(2)
2.238(2)
2.278(2)
2.109(2)
1.412(3)
1.451(3)
1.409(3)
1.419(3)
M1ꢀM3
M1ꢀC1
M1···C8
M2ꢀC4
M3ꢀC6
M3ꢀC8
C1ꢀC8
C3ꢀC4
C5ꢀC6
C7ꢀC8
2.503(1)
2.118(5)
2.473(5)
2.087(5)
2.190(5)
2.427(5)
1.434(8)
1.418(7)
1.432(8)
1.404(7)
2.5003(4)
2.088(2)
2.474(2)
2.070(2)
2.151(2)
2.482(2)
1.426(3)
1.406(3)
1.421(3)
1.424(3)
C2ꢀC3
C4ꢀC5
C6ꢀC7
Bond angles
C2ꢀC1ꢀC8
C2ꢀC3ꢀC4
C4ꢀC5ꢀC6
C6ꢀC7ꢀC8
133.2(5)
136.5(5)
135.1(5)
132.8(5)
135.1(2)
136.5(2)
136.1(29
128.4(2)
C1ꢀC2ꢀC3
C3ꢀC4ꢀC5
C5ꢀC6ꢀC7
C7ꢀC8ꢀC1
134.4(5)
131.6(5)
134.8(5)
136.5(5)
129.9(2)
132.2(2)
135.7(2)
138.8(2)
a M1=Ni1, M2=Co2, M3=Co3.
3. Conclusions
spectra are consistent with a rapid rotation of the facial
ligands on top of the trimetal triangles. The observed
1H and 13C chemical shifts compare well with those of
10 and other (tetranuclear) C8H6R2-capped cluster
complexes [3–5,8].
Formation of the title complexes 7 from the Co3Ni
cluster complex 5 involves dramatic modifications of
both the metal cluster core and the ligand sphere of the
starting material. These processes are obviously driven
by the needs of the facial h8-cyclooctatetraene ligand,
which cannot be accommodated in a Co3Ni complex.
The high tendency of formation of 7 is also illustrated
by our recent observation that complex 7a can also be
generated from [Co4(CO)6(h4-C8H8)(m3-C8H8)] and
nickelocene [16]. In the terminology of Vahrenkamp
these are truly ‘basic cluster reactions’ [17], where the
metal core is changed in a very specific way. Het-
erometallic cluster complexes are often more reactive
than their homometallic analogues. This offers a lot of
potential for cluster-mediated and cluster-catalyzed re-
actions [18], but also causes problems due to an en-
hanced tendency of the cluster to fragment. It has been
demonstrated by the present work that facial cy-
clopolyene ligands are capable of playing an important
role in the generation and stabilization of such systems.
Owing to the effort of several research groups in the
last 15 years, facial coordination has now become
firmly established in the metal complex chemistry of
cyclopolyenes [3,19]. Interestingly, apart from the title
complexes only one other example of facial coordina-
tion involving a heterometallic site has so far been
reported, with a benzene derivative capping a Ru2Pt
face of a nonanuclear Ru6Pt3 cluster [20]. Such com-
plexes may help to understand the sometimes enormous
difference between the catalytic properties of metal
In the chemical shift region typical of terminal car-
bonyl ligands both 7a and 7b show a sharp resonance
at l(13C)=185. No further low field resonances were
found in the spectra of 7a. Complex 7b shows an
additional rather broad resonance at l(13C)=247.
There are several possible interpretations of these spec-
tra. In a completely rigid (non-exchanging) system, two
resonances each should be present for the terminal and
bridging carbonyl ligands. We expect the chemical shift
difference between the two groups of resonances (about
70–90 ppm corresponding to 3.5–4.5 kHz) to be much
larger than that (a few ppm) within a particular group
(i.e. terminal Co- and terminal Ni-bound, or CoCo-
and CoNi-bridging, respectively). It is reasonable to
assume CO exchange to take place via bridge opening-
bridge closing sequences, simultaneously involving all
types of terminal and bridging sites [15]. A situation
can be envisaged where this exchange process has al-
ready merged the individual terminal and individual
bridging resonances to give a total of two signals, which
are still far from coalescence to a single line. Alterna-
tively, there could be accidental degeneracy of some of
the carbonyl resonances. In addition, some resonances
may have escaped detection, due to extensive broaden-
ing. This is believed to be the case in the carbon spectra
of 7a.

146
H. Wadepohl et al. / Journal of Organometallic Chemistry 641 (2002) 142–147
2,4,6,8-C8H6 was prepared according to the literature
[22]. NMR spectra were obtained on Bruker AC 200
and AVANCE DRX200 instruments (200.1 MHz for
alloys and supported bimetallic clusters compared to
those of the pure metals [21].
1
1H, 50.3 MHz for 13C). H and 13C chemical shifts are
reported versus SiMe4 and were determined by refer-
ence to internal SiMe4 or residual solvent peaks. As-
signment of the carbon resonances was routinely
augmented by 13C–1H DEPT (135°) spectra. Infrared
spectra were recorded in CaF2 cells with a Bruker
IFS-28 Fourier transform spectrometer (optical resolu-
tion 0.5 cm−1). The XPS analysis of complex 7a was
carried out on a sample film, which was deposited from
solution on a gold foil.
4. Experimental
4.1. General procedures
All operations were carried out under an atmosphere
of purified nitrogen or argon (BASF R3-11 catalyst)
using Schlenk techniques. Solvents were dried by con-
ventional methods. Alumina used as a stationary phase
for column chromatography was heated to 180–200 °C
under vacuum for several days, deactivated with 5% of
degassed water and then stored under nitrogen. 1,4-
(SiMe3)2C8H6 (a mixture of the double bond shifted
isomers 1,4-(SiMe3)2-1,3,5,7-C8H6 and 1,4-(SiMe3)2-
4.2. [Co2Ni(CO)6(v3-C8H8)] (7a)
A mixture of 100 mg (0.2 mmol) of 5 and 0.3 ml (2.6
mmol) of 1,3,5,7-cyclooctatetraene in 10 ml of n-hep-
tane is heated to reflux for 1 h. After cooling of the
mixture to room temperature, the dark red precipitate
is collected on a fine frit, washed with several portions
of n-pentane and recrystallized from benzene. Yield: 20
mg (25%) of dark red microcrystalline [Co2Ni(CO)6(m3-
C8H8)] (7a).
Table 2
Crystal data and structure refinement parameters for the complex
[Co2Ni(CO)6(m3-C8H6R2)] (7a, R=H; 7b, R=SiMe3)
7a
7b
Empirical formula
Formula weight
Temperature (K)
X-ray, wavelength
C14H8Co2NiO6·C6H6 C20H24Co2NiO6Si2
526.91
203
Mo–Ka, graphite
monochromated,
0.71073
IR (wCO
,
in CH2Cl2, cm−1): 2059(m), 2020(s),
593.14
173
Mo–Ka, graphite
monochromated,
0.71073
1
2003(s), 1883(m), 1853(m), 1834(m). H-NMR (C6D6,
ppm): l 3.42 (C8H8). 13C{1H}-NMR (C6D6, ppm): l
71.8 (C8H8), 185.1 (CO).
,
(A)
Crystal system
Space group
Monoclinic
P21/n
Monoclinic
P21/n
4.3. [Co2Ni(CO)6{v3-C8H6(SiMe3)2] (7b)
Unit cell dimensions
,
a (A)
10.203(5)
16.674(8)
11.902(6)
101.19(2)
1986(2)
4
1.763
2.62
1056
4–56
14.9542(3)
8.9889(2)
18.2789(4)
97.256(1)
2437.41(9)
4
1.616
2.24
1208
3–57
A mixture of 1.0 g (1.8 mmol) of 5 and 0.5 g (2.0
mmol) of 1,4-(SiMe3)2C8H6 in 400 ml of n-heptane is
heated to reflux. The course of the reaction is moni-
tored by IR spectroscopy. The mixture is cooled to
room temperature when the absorptions of 5 are no
longer detectable (ca. 9 h). Solvent is removed under
reduced pressure and the dark residue is chro-
matographed on alumina (4.5×15 cm) using n-hexane
as the mobile phase. Solvent is removed from the deep
red eluate and the resulting dark red solid is recrystal-
lized from n-pentane. Yield: 420 mg (38%) of dark red
microcrystalline [Co2Ni(CO)6{m3-C8H6(SiMe3)2}] (7b).
FD-MS (m/z, relative intensity [%]): 592.0 (100, [M+]).
IR (wCO, in n-hexane, cm−1): 2059(m), 2023(s), 2008(s),
,
b (A)
,
c (A)
i (°)
V (A )
3
,
Z
Dcalc (Mg m−3
)
v(Mo–Ka) (mm−1
F(000)
)
2r range (°)
Index ranges
−135h513,
05k522, 05l515
3454
−195h519,
05k511, 05l524
6013
Unique reflections
Reflections observed 2261
[I]2|(I)]
Parameters refined
R (observed
reflections only)
4607
319
0.040
376
0.028
1
1882(m), 1857(m), 1843(m). H-NMR (CDCl3, ppm): l
wR2 (all reflections) 0.094
0.066
0.19 (s, 18H, SiMe3), 4.30 (s, 2H, H-2, -3), 4.32 ((AB)2-
pattern, 4H, H-5, -8). 13C{1H}-NMR (CDCl3, ppm): l
−1.0 (SiMe3), 74.0 (CH), 74.3 (CH), 76.0 (CH), 84.1
(CꢀSiMe3), 185.3 (CO), 247 (br, m-CO).
(w=1/[|2(F)
+(AP)2
+BP])
A, B
0.036, 0
0.037, 0
P
max((F2o, 0)+2F2c)/3
1.035
4.4. Crystal structure determinations
Goodness-of-fit
Largest difference
peak and hole
0.943
0.48 and −0.37
0.28 and −0.36
Single crystals of 7a·C6H6 were grown from a ben-
zene solution at 10 °C. These crystals were found to be
twinned (two interpenetrating individuals with different
−3
,
(e A
)

H. Wadepohl et al. / Journal of Organometallic Chemistry 641 (2002) 142–147
147
orientation). Intensity data were collected using an
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obtained from n-pentane at 8 °C. Crystal data are
compiled in Table 2.
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5. Supplementary material
Crystallographic data for the structural analysis have
been deposited with the Cambridge Crystallographic
Data Centre, CCDC nos. 182/1148 and 166312 for
compounds 7a and 7b, respectively. Copies of this
information may be obtained free of charge from The
Director, CCDC, 12 Union Road, Cambridge CB2
1EZ, UK (Fax: +44-1223-336033; e-mail: deposit@
ccdc.cam.ac.uk or www: http://www.ccdc.cam.ac.uk).
(c) H. Wadepohl, in: M. Gielen (Ed.), Physical Organometallic
Chemistry, vol. 3, Wiley, New York, 2001, in press.
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Acknowledgements
(
SHELXS-86);
This work was supported by the Sonderforschungs-
bereich 247 der Universita¨t Heidelberg and the Fonds
der Chemischen Industrie. Donation of valuable chemi-
cals by BASF AG Ludwigshafen is gratefully acknowl-
edged. We thank Professor Ch. Wo¨ll (Ruhr-Universita¨t,
Bochum) for the XPS analysis.
(b) G.M. Sheldrick, SHELXL-97, University of Go¨ttingen, Go¨ttin-
gen, Germany, 1997.
[24] (a) A.L. Spek, PLATON, Utrecht University, Utrecht, The Nether-
lands, 2001;
(b) A.L. Spek, Acta Crystallogr. Sect. A 46 (1990) C31.
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Edinburgh, Edinburgh, UK, 1988.