A zwitterionic triosmium cluster bearing a metallated azulene ligand coordinated perpendicularly as an alkylidene bridge

Article abstract of DOI:10.1016/j.jorganchem.2004.03.028

The cluster [Os₃(CO)₁₀(MeCN)₂] reacts readily with azulene in refluxing cyclohexane to give the oxidative addition product [Os₃(μ-H) (μ₂-η¹- C₁₀H₇) (CO)₁₀] 1 which was shown by its X-ray crystal structure to contain the C₅ ring of the azulenyl ligand bonded through a single carbon atom at the 1-position. We propose that the compound is zwitterionic, with the 7-membered ring a tropylium cation and the 5-membered ring coordinated as a μ-alkylidene to the metal cluster, which carries a formal negative charge. Thermal loss of one CO ligand leads by further oxidative addition to the known cluster [Os₃(μ-H)₂ (μ₃-η¹: η¹:η¹- C₁₀H₆) (CO)₉] 2.

Full text of DOI:10.1016/j.jorganchem.2004.03.028

Journal of Organometallic Chemistry 689 (2004) 2025–2028
www.elsevier.com/locate/jorganchem
A zwitterionic triosmium cluster bearing a metallated azulene
ligand coordinated perpendicularly as an alkylidene bridge
a,
a
a
a
*
Ysaura De Sanctis , Alejandro J. Arce , Farrah Ca n~ avera , Ruben Machado ,
b
Antony J. Deeming , Teresa Gonz ꢀa lez , Esperanza Galarza
a
c
a
Centro de Qu ꢀı mica, Instituto Venezolano de Investigaciones Cient ꢀı ficas (IVIC), Apartado 21827, Caracas 1020-A, Venezuela
b
Department of Chemistry, University College London, 20 Gordon Street, London WC1H 0AJ, UK
Universidad del Valle, Departamento de Qu ꢀı mica, Ciudad Universitaria Mel ꢀe ndez AA 25360, Cali, Colombia
c
Received 28 January 2004; accepted 22 March 2004
Abstract
The cluster [Os3(CO)10(MeCN)2] reacts readily with azulene in refluxing cyclohexane to give the oxidative addition product
1
[
bonded through a single carbon atom at the 1-position. We propose that the compound is zwitterionic, with the 7-membered ring a
Os3(l-H)(l2-g -C10H7)(CO)10] 1 which was shown by its X-ray crystal structure to contain the C5 ring of the azulenyl ligand
tropylium cation and the 5-membered ring coordinated as a l-alkylidene to the metal cluster, which carries a formal negative charge.
1
1
1
Thermal loss of one CO ligand leads by further oxidative addition to the known cluster [Os3(l-H)2(l3-g :g :g -C10H6)(CO)9] 2.
Ó 2004 Elsevier B.V. All rights reserved.
Keywords: Osmium; Azulene; Zwitterion; Alkylidene; Double oxidative addition
1
. Introduction
terminal alkylidene in the 7-membered ring with the
planar azulene system perpendicular to the Os plane.
3
The co-ordination of aromatic rings to transition-
Overall there has been a double oxidative addition with
cleavage of C–H bonds at the 1- and 8-positions. The
reactivity of azulene with triosmium is in contrast with
that of iron and ruthenium where the azulene is parallel
metal clusters has been extensively studied by several
research groups [1]. Apart from a wish to form novel
ligand systems, there has been a drive to produce mo-
lecular models for the chemisorption of aromatics on
metal surfaces and for the catalytic transformations of
these compounds. Commonly, arene complexation re-
duces the electron density of the p-electrons, thereby
enhancing unusual nucleophilic addition and substitu-
tion reactions. In this communication we report an
addition to our earlier report on azulene in which the
5
5
3
to the metal plane as in [Ru3(l3-g :g :g -C10H8)(l-
CO)(CO)6] [3] or parallel to the metal–metal axis as in
5
3
the dinuclear compound [Ru2(l-g :g -C10H8)(CO)5]
[4].
In light of the double oxidative addition of azulene in
refluxing octane (125 °C), we wished to establish whe-
ther the single oxidative addition product could be ob-
tained at lower temperatures and, if so, whether the
initial C–H activation was at the 5- or 7-membered ring.
In free azulene there is a significant dipole moment with
positive charge at the 7-membered ring and negative
charge at the 5-membered ring, and therefore which ring
is activated might depend upon the electrophilic or nu-
cleophilic nature of the oxidative addition. This paper
will show that it is the 5-membered ring which is acti-
vated first.
1
1
double oxidative addition product [Os3(l-H)2(l3-g :g :
1
g -C10H6)(CO)9] 2 was obtained from [Os3(CO)11
(MeCN)] [2]. The Os3 triangle was coordinated to a
bridging alkylidene in the 5-membered ring and to a
*
Corresponding author. Tel.: +582125041386; fax: +582125041350.
E-mail address: ysantis@ivic.ve (Y. De Sanctis).
0
022-328X/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2004.03.028

2
026
Y. De Sanctis et al. / Journal of Organometallic Chemistry 689 (2004) 2025–2028
SHELXTL
-
PLUS [7]. Crystal data: C H O Os , formula
20 8 10 3
2
. Experimental
weight ¼ 977.86, monoclinic, space group P21=c, a ¼
1
ꢁ
ꢁ
ꢁ
9
1
0
:740ð7Þ A, b ¼ 8:936ð3Þ A, c ¼ 25:963ð12Þ A, b ¼
2
3
2
7
.1. Synthesis of cluster [Os (l-H)(l -g -C10H )
CO)10] 1
ꢁ
3
00:39ð4Þ°, V ¼ 2223ð2Þ A , Z ¼ 4, green prisms, 0.60 ꢁ
.30 ꢁ 0.20 mm. 4176 measured reflections (h ¼ 0 to 11,
(
k ¼ 0 to 10, l ¼ ꢀ30 to 30), 3927 independent reflec-
A solution of [Os (CO) (MeCN) ] (0.050 g, 0.054
3
10
2
2
tions, R ¼ 0:0754, R ¼ 0:0482, wRðF Þ ¼ 0:1259 for
int
mmol) and azulene (0.013 g, 0.101 mmol) in cyclohexane
3
3
067 reflections with I P 2rðI Þ, 298 parameters.
(
50 cm ) was refluxed under nitrogen. By following the
0
0
colour change from yellow to green, the changes in
the IR spectrum, and using analytical TLC, we stopped
the reaction after 40 min when it appeared that the
starting cluster had been consumed. TLC work-up (sil-
ica, eluent petroleum spirit, b.p. 40–60 °C) gave unre-
acted azulene, the known deep yellow-orange cluster 2
3
. Results and discussion
Azulene was added to
Os3(CO)10(MeCN)2] in cyclohexane and the mixture
a
suspension of
[
refluxed during which time the color of the solution
(
1
10% yield) and a green band, which yielded compound
(15%). Cluster 2 was characterized by comparison of
its spectroscopic data with those published [2] and
changed from yellow to green. The green azulene de-
1
rivative [Os (l-H)(l -g -C H )(CO) ] 1 was isolated
3
2
10
7
10
1
ꢀ
1
in 15% yield. The H NMR spectrum confirmed the
presence of a hydride (d )14.40) and there were seven
multiplets for the remaining seven hydrogen atoms on
the ligand. Two signals coupled by 4.1 Hz for the 5-
membered ring and analysis of the remainder of the
spectrum, showed that there were still five hydrogens on
the 7-membered ring, confirming that the metallation
had occurred at the 1-position on the 5-membered ring.
spectral data for 1 are: m(CO)/cm (cyclohexane): 2095
m, 2074 s, 2061 s, 2054 ms, 2037 m, 2012 ms, 2000 s,
1
1
2
974 w; H NMR (CDCl3, 20 °C, J in Hz): d 9.34 (d, H ,
4
J2;3 ¼ 4:1), 8.54 (ddd, H , J4;5 ¼ 9:7, J4;6 ¼ 0:9,
8
J
¼ 0:8), 8.25 (dd, H , J ¼ 9:2, J ¼ 0:9), 8.11
3
;4
7;8
6;8
7
(
ddd, H , J ¼ 9:7, J ¼ 9:2, J ¼ 0:9), 7.99 (ddd,
5 6
4
6
;7
7;8
5;7
H , J ¼ 9:7, J ¼ 9:7, J ¼ 0:9), 7.88 (dddd, H ,
;5
5;6
5;7
3
J6;7 ¼ 9:7, J5;6 ¼ 9:7, J4;6 ¼ 0:9, J6;8 ¼ 0:9), 7.46 (dd, H ,
1
3
1
13
An analysis of the C NMR spectrum, including H–
13C COSY spectra confirmed these findings.
J2;3 ¼ 4:1, J3;4 ¼ 0:8), )14.40 (s, OsHOs); C NMR
1
9
3
(
(
(
CDCl3, 20 °C): d 29.7 (C ), 72.0 (C ), 122.6 (C ), 128.9
7 5 4 6 8
The molecular structure of 1 (Fig. 1) confirmed the
site of metallation but also that the 1-azulenyl ligand is
attached to the cluster through the 1-position alone. The
organic ligand is planar and orientated perpendicularly
to the metal triangle (dihedral angle between the azulene
C ), 132.4 (C ), 138.4 (C ), 141.1 (C ), 151.1 (C ), 154.0
2
1
0
C ), 178.5 (C ). Crystals of 1 suitable for X-ray
structure determination were obtained by slow evapo-
ration of a hexane solution at room temperature.
1
and the Os planes ¼ 85.9°, See Table 1 for selected bond
2
.2. Thermolysis of cluster [Os
CO)10] 1
3
(l-H)(l -g -C10
H
7
)
3
2
lengths and angles). The metal triangle has three Os–Os
ꢁ
(
bonds with the shortest [Os(1)–Os(2) ¼ 2.7638(11) A]
associated with the bridging C(1) atom of the azulenyl.
The other two Os–Os bonds are similar but longer
A solution of cluster 1 (0.030 g) in cyclohexane (50
3
cm ) was refluxed under nitrogen for 4 h by which time
the green solution had become yellow-orange. TLC
work-up gave [Os (CO) ] (3%), unreacted cluster 1
3
12
(10%) and cluster 2 (80%), which was characterized by
comparison of its spectra with those reported [2].
2
.3. Crystal structure determination for cluster 1
The structure was determined using a Rigaku AFC7S
diffractometer (graphite-monochromated Mo Ka radi-
ꢁ
ation, 0.71073 A) at 298(2) K. Corrections for L effects
p
and semi-empirical absorption corrections [5] were ap-
plied. Structure solution was by direct methods [6] and
expanded using Fourier techniques [7]. All non-H atoms
were refined anisotropically, using a riding model for
H-atoms with thermal parameters 1.2 ꢁ Ueq of the cor-
responding C atoms. The final cycle of full-matrix least-
2
squares was based on F . teXsan crystallographic
software package for data reduction and graphical
representations was used [8], with refinement using
Fig. 1. ORTEP view (35% probability ellipsoids, H atoms omitted for
clarity) of the molecular structure of 1 in the crystal.

Y. De Sanctis et al. / Journal of Organometallic Chemistry 689 (2004) 2025–2028
2027
Table 1
[
Os(1)–Os(3) ¼ 2.895(2) A, Os(2)-Os(3) ¼ 2.8626(13) A],
ꢁ
ꢁ
ꢁ
1
Selected bond lengths (A) and angles (°) for [Os3(l-H)(l2-g -
C10H7)(CO)10] 1
which is consistent with an approximate mirror plane
through the azulenyl ligand bisecting Os(1) and Os(2).
We interpret the bonding as involving an alkylidene
bridge between Os(1) and Os(2), and a charge distribu-
tion tending towards a cationic tropylium arrangement
for the 7-membered ring and a negative charge at the
metal atoms. The m(CO) values lend support to some
build-up of negative charge at the metal atoms. The C–C
distances in the ligand are similar but their uncertainties
do not allow us to discuss any differences in bond order
in the carbon–carbon bonds around the two rings.
The green cluster 1 converts in good yield by loss of a
CO ligand to the previously reported yellow-orange
cluster 2 on heating a cyclohexane solution for 4 h
Os(1)–Os(2)
Os(1)–Os(3)
Os(2)–Os(3)
Os(1)–C(1)
Os(2)–C(1)
C(1)–C(2)
C(1)–C(9)
C(2)–C(3)
C(3)–C(10)
C(4)–C(5)
C(4)–C(10)
2.7638(11)
2.895(2)
2.8626(13)
2.235(14)
2.301(13)
1.459(19)
1.38(2)
C(5)–C(6)
C(6)–C(7)
1.35(3)
1.41(2)
1.40(2)
1.38(2)
1.45(2)
90.9(30)
90.4(4)
75.1(4)
121.2(10)
120.3(11)
112.9(9)
120.1(9)
C(7)–C(8)
C(8)–C(9)
C(9)–C(10)
Os(3)–Os(1)–C(1)
Os(3)–Os(2)–C(1)
Os(1)–C(1)–Os(2)
Os(1)–C(1)–C(2)
Os(1)–C(1)–C(9)
Os(2)–C(1)–C(2)
Os(2)–C(1)–C(9)
1.36(2)
1.38(2)
1.40(3)
1.34(3)
(Scheme 1). This conversion requires a rotation of the
alkylidene bridge so that the 7-membered ring can ap-
proach the third Os atom at which it is metallated, ra-
ther than away from it as in 1. Scheme 2 shows two
possible mechanisms. The alkylidene could rotate by
180° to give A followed by loss of CO and C–H acti-
vation. Alternatively the ligand could convert from
bridging to terminal, rotate by 90° to form B so that the
+
[
Os (CO)10(MeCN)2]
(CO) Os
Os(CO)3
3
4
-
cyclohexane reflux
Os
CO)
4
0 min
H
3
(
1
cyclohexane reflux
4
h
+
+
PPh3
Ph P
3
Rotation
(
CO) Os
Os(CO)
4
3
Os (CO)
H
(
CO) Os
3
(
CO) Os
Os(CO)3
H
-
4
3
-
Os
(CO)
Os
CO)
H
Os
H
3
2
(
(CO)3
3
Scheme 1.
Scheme 3.
+
+
rotation
-CO
(
CO) Os
Os (CO)3
H
4
(
CO) Os
Os (CO)3
(CO)
Os
(CO)3
4
3
Os
H
-
-Os
Os
CO)3
Os
(CO)3
(
H
A
H
2
1
(CO)3
-CO
H
(
CO) Os
Os (CO)3
(CO)3
C
4
(CO) Os
Os
4
Os
Os
(CO)3
H
CO)3
B
H
H
(
Scheme 2.

2028
Y. De Sanctis et al. / Journal of Organometallic Chemistry 689 (2004) 2025–2028
8
-H can approach an osmium atom with an agostic in-
References
teraction, followed by C–H bond activation to give C,
all occurring before the loss of CO which occurs in the
final step. There is precedence for the rotation of a
bridging alkylidene as in the phosphonium zwitterionic
species shown in Scheme 3 [9]. There appears to be no
tendency for triosmium clusters to form p-interactions
with azulene and it is likely that the greater tendency for
osmium to undergo C–H bond activation than ruthe-
nium and iron and its ability to form stronger M–C
r-bonds accounts for the differences.
[
1] W.E. Silverthorne, Adv. Organomet. Chem. 13 (1975) 47;
E.L. Muetterties, J.R. Bleeke, E.J. Wucherer, T. Albright, Chem.
Rev. 82 (1982) 499;
H. Wadepohl, Angew. Chem., Int. Ed. Engl. 31 (1992) 247;
A.J. Deeming, Multiple interactions between arenes and metal
atoms, in: P. Braunstein, L.A. Oro, P.R. Raithby (Eds.), Metal
Clusters in Chemistry, vol. 1, Wiley-VCH, Weinheim, Germany,
1
999 (Chapter 1.14);
D. Braga, P.J. Dyson, F. Grepioni, B.F.G. Johnson, Chem. Rev. 94
1994) 1585.
(
[
[
2] S. Aime, A.J. Arce, D. Giusti, R. Gobetto, J.W. Steed, J. Chem.
Soc., Dalton Trans. (2000) 2215.
3] M.R. Churchill, J. Wormald, Inorg. Chem. 12 (1973) 191;
H. Nagashima, A. Suzuki, M. Nobata, K. Aoki, K. Itoh, Bull.
Chem. Soc. Jpn. 71 (1998) 2441.
4
. Supporting material
[
4] A.J. Arce, Y. De Sanctis, E. Galarza, M.T. Garland, R. Gobetto,
R. Machado, J. Manzur, A. Russo, E. Spodine, M.J. Stchedroff,
Organometallics 20 (2001) 359.
Complete crystallographic data for the structure of 1
have been deposited at the Cambridge Crystallographic
Data Centre, No. 229940.
[
[
5] A.C.T. North, D.C. Phillips, F.S. Mathews, Acta Cryst. A 24
(
1986) 351.
6] A. Altomare, M.C. Burla, M. Camalli, M. Cascarano, C. Giaco-
vazzo, A. Guagliardi, G.J. Polidori, Appl. Crystallogr. 27 (1994)
435, SIR-92.
Acknowledgements
[
[
[
7] G.M. Sheldrick, SHELXL NT 5.10: Program for Crystal Structure
Refinement, University of G o€ ttingen, Germany, 1993.
8] teXsan, Crystal Structure Analysis Package, Molecular Structure
Corporation, The Woodlands, TX, USA, 1995.
We thank the Royal Society of Chemistry for the
award of a Journals Grant for International Authors to
assist this collaboration and FONACIT, Venezuela, for
project LAB-97000665.
9] A.J. Deeming, N.I. Powell, C. Whittaker, J. Chem. Soc., Dalton
Trans. (1991) 1875.