Synthesis and structural characterization of diruthenium cluster containing germylene ligand

Article abstract of DOI:10.1016/j.jorganchem.2013.02.004

Mild thermolysis of arachno-[(Cp*RuCO)₂B₂H ₆], 1 in presence of GeCl₂·dioxane yielded di-μ-germylene complex [(Cp*Ru)₂(μ-GeCl₂) ₂(CO)₂], 2 (Cp* = η⁵-C ₅Me₅). The molecular structure of 2 exhibits a diruthenium unit bridged by a pair of μ-GeCl₂ moieties. The Cp* and the terminal CO ligands are oriented in anti fashion with respect to Ru-Ru bond. In addition, the reaction of 2 with arachno-[CpFe(CO)B₃H₈] (Cp = η⁵-C₅H₅) transformed to germenocene, [Cp₂Ge:], 3 via an unstable intermediate [CpFe(CO)B₂H ₅GeCl₂], I. The ¹¹B NMR spectroscopy was used as a tool to establish a probable reaction pathway for the formation of germenocene 3. Cluster 2 has been characterized by IR and NMR spectroscopy, and the geometric structure was unequivocally established by crystallographic analysis.

Full text of DOI:10.1016/j.jorganchem.2013.02.004

Journal of Organometallic Chemistry 731 (2013) 18e22
Contents lists available at SciVerse ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Note
Synthesis and structural characterization of diruthenium cluster containing
germylene ligand
*
R.S. Anju, K. Geetharani, Dipak Kumar Roy, Sundargopal Ghosh
Department of Chemistry, Indian Institute of Technology Madras, Chennai 600 036, Tamil Nadu, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Mild thermolysis of arachno-[(Cp*RuCO)2B2H6], 1 in presence of GeCl2$dioxane yielded di-m-germylene
complex [(Cp*Ru)2(
uthenium unit bridged by a pair of
anti fashion with respect to RueRu bond. In addition, the reaction of 2 with arachno-[CpFe(CO)B3H8]
m
-GeCl2)2(CO)2], 2 (Cp* ¼
h
5-C5Me5). The molecular structure of 2 exhibits a dir-
Received 27 November 2012
Received in revised form
7 February 2013
m
-GeCl2 moieties. The Cp* and the terminal CO ligands are oriented in
Accepted 8 February 2013
(Cp ¼
h
5-C5H5) transformed to germenocene, [Cp2Ge:], 3 via an unstable intermediate [CpFe(CO)
B2H5GeCl2], I. The 11B NMR spectroscopy was used as a tool to establish a probable reaction pathway for
the formation of germenocene 3. Cluster 2 has been characterized by IR and NMR spectroscopy, and the
geometric structure was unequivocally established by crystallographic analysis.
Ó 2013 Elsevier B.V. All rights reserved.
Keywords:
Germanium
Germenocene
Germylene
Metallaborane
Ruthenium
1. Introduction
heteroboranes. For instance, clusters containing group 14 elements,
[E9]xꢀ (E ¼ Si, Ge, Sn, Pb; x ¼ 3, 4) were found to be structurally and
Since after the discovery of germylene by Lappert and co-
workers in 1976 [1], germanium analogue of carbene received
significant interest because of their essential importance as well as
their rich chemistry. For example, in the precursor of bimetallic
nanoparticles, catalytic activity and application in organic chem-
istry for its stereo chemical consequences [2e6]. A number of
germylene complexes for various transition metals have been
prepared, mainly by the insertion of GeCl2 into a metal chloride
bond [7e12]. It is well established that germylene ligand gets
inserted into bonds between two main group elements providing
easy access to tetravalent germanium compounds [13]. In contrast,
the insertion of germylenes into the metalemetal bonds has been
less explored [14e17]. The first example of mononuclear organo-
metallic complex containing germylene ligand was isolated and
synthesized by Power using sterically encumbered aryl sub-
stituents [18]. On the other hand the incorporation of group 14
elements other than carbon into polyhedral borane species has
proved unusually elusive, although a few carbaborane derivatives
of silicon, germanium, and tin are known [19e21]. Furthermore,
several naked-metal clusters have structures reminiscent of
electronically analogous to boranes with delocalized bonding that
follow Wade’s electron-counting principles [22e26], and also the
4ꢀ
carborane analogue of Ge11
ligand C2B9H11 in transition metal complexes [27,28].
is well-known as the dicarbollide
2ꢀ
As part of our interest in synthesizing metallaboranes containing
a range of early and late transition metals [29e41], several triply-
bridged heterotrinuclear borylene clusters [(m3-BH)(Cp*RuCO)2
(m-CO){Fe(CO)3}], [{(m3-BH)(Cp*Ru)Fe(CO)3}2(m-CO)] and capped-
cubane cluster [(Cp*Ru)2(m3-CO)2B2H( -H){Fe(CO)2}2Fe(CO)3] have
m
been isolated using arachno-1,2-[(Cp*RuCO)2B2H6], 1 as the starting
reagent [42]. Further, as shown in Scheme 1, (vide infra) compound 1
catalyzes the cyclotrimerization of a variety of internal- and terminal
alkynes to yield mixtures of 1,3,5- and 1,2,4-substituted benzenes
[43]. Subsequently, upon availability of these triply bridged borylene
compounds the chemistry was expanded by the use of alkynes under
photolytic conditions. As a result, the bis-bridged borylene complex
on reaction with alkynes yielded ruthenacarborane [44], while the
mono-bridged borylene complex generated novel vinyl borylene
compounds [45]. As an extension to these results, we proposed to
add GeCl2$dioxane to metallaborane 1 in order to prepare germa-
borane derivatives. However, reaction of 1 with GeCl2$dioxane led to
the formation of di-m-germylene complex 2. In addition, the chem-
istry of 2 has been elaborated with [CpFe(CO)B3H8] [46], which led to
the isolation of germenocene, [Cp2Ge:].
*Corresponding author. Tel.: þ91 44 2257 4230; fax: þ91 44 2257 4202.
E-mail address: sghosh@iitm.ac.in (S. Ghosh).
0022-328X/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jorganchem.2013.02.004

R.S. Anju et al. / Journal of Organometallic Chemistry 731 (2013) 18e22
19
Scheme 1. Summary of the reactions of the arachno-[(Cp*RuCO)2B2H6], 1 and synthesis of di-m-germylene complex 2 reported here in.
ꢀ
2. Results and discussion
[Ru3(
m
-GeMe2)3(CO)9], 2.482(11)e2.500(12) A [47]. The RueRu dis-
ꢀ
tance of 3.1015 (4) A in 2 is longer than that observed in 1 and other
related species [48,49]. The RueCring bond lengths in 2 ranges be-
tween 2.1980(2) and 2.2980(3) A. The RueCO bond length of 2 has
2.1. Isolation and structural characterization of 2
As shown in Scheme 1, reaction of 1 in toluene with excess of
ꢀ
ꢀ
a value of 1.8680(3) A, which is shorter than those of the
GeCl2$dioxane resulted in the formation of 2. This new di-m-ger-
mylene complex has been isolated in 85% yield. Compound 2 has
been separated by preparative thin-layer chromatography (TLC),
allowing the characterization of pure materials. Compound 2 was
characterized by IR, NMR and a single-crystal X-ray diffraction
analysis. The IR spectrum displayed intense band at 1944 cmꢀ1
,
characteristic of terminal carbonyl groups. The 1H NMR spectrum
shows Cp* resonance at
d
¼ 2.06 ppm. Consistent with this obser-
vation, the 13C NMR spectrum of 2 shows resonances correspond to
both the Cp* and terminal CO groups.
In order to confirm the spectroscopic assignments and determine
the molecular and crystal structure of 2, an X-ray analysis was un-
dertaken. The crystal structure of 2, shown in Fig. 1, corresponds to
discrete molecules of [(Cp*Ru)2(m-GeCl2)2(CO)2] separated by normal
van der Waals distances. The crystal structure revealed that 2 is a
diruthenium complex having two Cp* ligands, two bridging germa-
nium dichloride and two carbonyl ligands. Cluster 2 is a transition
metal bridged germylene complex, where each ruthenium atom is
having a Cp* ligand and a terminally bound carbonyl group. The
carbonyl ligands and the Cp* units are in anti orientation with
respect to RueRu bond. The RueGe bond distances to the bridging
Fig. 1. Molecular structure and labelling diagram for 2. Elꢁlipsoids are drawn at the 50%
ꢀ
probability level. Selected bond lengths (A) and angles ( ): Ru1eRu2 3.1015(4), Ge1e
ꢀ
GeCl2 groups lie in the range 2.4000(3)e2.4086(3) A and are shorter
than those found for the bridging GeMe2 groups in the compound
Ru1 2.4000(3), Ru2eGe1 2.4086(3), Ge1eCl2; Ru1eGe1eRu2 80.329(10), Cl1eGe1e
Ru1 116.99(2), Cl2eGe1eRu1 125.88(2), Cl2eGe1eCl1 97.20(3), O1eC11eRu1 175.2(2).

20
R.S. Anju et al. / Journal of Organometallic Chemistry 731 (2013) 18e22
dimetalladichlorogermanes complexes [CpMo(CO)3]2GeCl2 (average
was found to be highly unstable and air and moisture sensitive. The
oxidative addition of the divalent germanium centre to the cluster
was not observed; instead it led to the formation of germanocene, 3
resulting probably from the cleavage of the cyclopentadienyl
ligand. Germanocene 3 was characterized based on a comparison of
its 1H and 13C NMR chemical shift values (1H: 5.38 ppm, 13C:
99 ppm) [64]. Further, to support the identity of 3, a solid state
X-ray study was performed that confirms the germanocene core.
Although the mechanism for the formation of 3 is not fully clear;
a possible pathway is shown in Scheme 2. To confirm the formation
of any intermediates, mild pyrolysis of [CpFe(CO)B3H8] with 2
was performed and monitored by 11B spectroscopy. After 6 h
of pyrolysis, the reaction generated a new boron containing product
ꢀ
MoeCO 1.9930(5) A) and [CpW(CO)3]2GeCl2 (average WeCO
ꢀ
1.9900(10) A) [50]. The tetrahedral environment of the germanium
atom is strongly distorted as shown by the CleGeeCl bond angle
ꢀ
(97.20 A), which is smaller than that of a regular tetrahedron. Simi-
ꢀ
larly, the RueGeeRu bond angle (80.33 A), is much smaller than that
of a regular tetrahedron. The Ru(m-Ge)2Ru core of 2 is planar, and the
dihedral angle between the plane of the central core relative to the
Cp* ligand is 110.54ꢁ. The terminal CO ligands are almost linear, with
a RueCeO angle of 175.2ꢁ.
Whilst the transition metal bridge methylene compounds have
been investigated extensively, the transition metal linked cluster of
heavier counterpart received little interest. Table
1 offers a
comparative study of the RueRu distances in RueEeRu (where
E ¼ C, Si, Ge, Sn, Pb) bridged complexes of different group 14 an-
alogues. Moving on from the bridged methylene analogue to the
plumbylenes, a gradual increase in the RueRu distance can be
observed in most of the entries. An exceptionally higher RueRu
bond distance, even higher than that of the lead analogue in
compound 2 is unique. The average bridging RueE (E ¼ C, Si, Ge, Sn,
Pb) distance is increasing on going from the lighter to the heavier
group 14 analogues. Difference in the bond distances of 2 with
other related compounds may be due to the perturbation of the
electronic environment of the group 14 atoms by the ligands.
Compound 1 has 42 electrons with a butterfly structure and
isoelectronic with 2. Considering 2 as Ru2Ge2 cluster, it possesses a
total valence count (tvc) of 42 electrons {[2(CO) þ 5(Cp*) þ
8(Ru) þ 6(GeCl2)] ꢂ 2} [51,52]. Alternatively, the structure of 2 can be
explained in simple organometallic terms, giving rise to an electron
count of 17 at the ruthenium centre (8 þ 5 þ 2 þ 1 þ 1 ¼ 17; each 1
corresponding to one of the two electrons of the bridging germy-
lenes). Although the RueRu interatomic distance in 2 is longer than
usual [53], the structure can also be seen as a 34 electron bimetallic
I, having two boron resonances each appeared at
d
¼
5.9
and ꢀ37.2 ppm. The resonances weakened with the progress of the
reaction and finally faded away after the reaction was stirred at
90 ꢁC for 12 h. The reaction could not be controlled to yield the
intermediate complex [CpFe(CO)B2H5GeCl2], expected to form
during the course of the reaction. Therefore, to ensure the forma-
tion of germanocene by the release of BH3, we performed the re-
action in presence of Lewis base, PMe2Ph. The reaction was
monitored by 11B NMR spectroscopy, which revealed the formation
of a phosphineeborane adduct, BH3$PMe2Ph, as indicated by the
presence of a single resonance in the 11B{1H} NMR spectrum at
d
¼ ꢀ36.2 ppm. However, we have been unable to determine, up to
date, the fate of the dissociated metal fragments.
3. Conclusions
The di-m-germylene diruthenium complex 2 was synthesized by
thermal reaction of cluster 1. Further, reaction of 2 with [CpFe(CO)
B3H8] yielded germanocene by slow conversion of an in situ
generated intermediate proposed to be [CpFe(CO)B2H5GeCl2]. This
work has further demonstrated the ability of metallaborane clus-
ters to engage in multiple additions of germanium containing li-
gands to metal-carbonyl clusters. These and related cluster
complexes containing group 14 elements could be used as pre-
cursors to a range of new bimetallic nanoparticle catalysts.
species, analogous to cis-[Cp2Ru2(m-CH2)2(CO)2], with a RueRu single
bond in agreement with the diamagnetic property. Also, a classical
electron precise and 2ce2e bonded bridged borylene ligand, linked
to the homo or hetero transition metal centres can be compared
remarkably with the GeCl2 unit bridged between two Ru metal in
compound 2 [54,55].
4. Experimental section
2.2. Reactivity of [CpFe(CO)B3H8] with 2
4.1. General procedure and instrumentation
Ferrocene-like sandwich molecules of the heavier group 14 el-
ements (Si, Ge, Sn, and Pb) have been studied most extensively
because of their structural, bonding and reactivity aspects [61e63].
The most investigated area of these sandwich complexes is the
interpretation of stereochemically active lone pair at the central
atom of group 14 metallocenes. Expecting the borane fragment vs
germanium fragment displacement, we performed the reaction of 2
with [CpFe(CO)B3H8], wherein the in-situ generated intermediate
All the manipulations were conducted under an Ar/N2 atmo-
sphere using standard Schlenk techniques or glove box. All the
solvents were distilled prior to use under Argon. [Cp*RuCl2]2,
[Mo(CO)6], LiBH4 2.0 M in THF, GeCl2$dioxane (Aldrich) were used
as received. [(Cp*CORu)2B2H6] [42] and [CpFe(CO)B3H8] [46] were
prepared according to literature methods. The external reference
for the 11B NMR, [Bu4N(B3H8)], was synthesized with the literature
Table 1
Selected structural parameters of 2 and other related clusters (E ¼ C, Si, Ge, Sn, Pb).
Avg. d[RueE]a [A]
Avg. RueEeRu [ꢁ]
Avg. d[EeX] [A]
Ref.
ꢀ
ꢀ
ꢀ
Entry
Complex
d[RueRu] [A]
1
2
3
cis-[Cp2Ru2(
m
m
-CH2)2(CO)2]
-SiPh2)( -H)}2]
-SiMe2)[(Me2SiCH2) C5H3N{CH2Si(Me)CH2}
C5H3N (CH2SiMe3)](CO)4]
[Ru4(m4-GePh)2( -GePh2)2(
[(Cp*Ru)2( -GeCl2)2(CO)2], 2
[Ru5(CO)8( -SnPh2)4(C6H6)(m5-C)]
[Ru3( -PbR2)2( -CO)(CO)9]
2.6813(6)
2.665(0)
2.9313(3)
2.07
2.36
2.49
80.60
68.70
71.99
0.97
eb
[56]
[57]
[58]
[{(C5Me5)Ru(
[Ru2(
m
m
1.88
4
5
6
7
m
m
-CO)2(CO)8]
2.95c
2.48
2.40
2.62
2.77
72.70
80.33
66.78
64.33
1.96
2.19
2.14
2.29
[49]
This paper
[59]
m
m
3.1015(4)
2.91c
m
m
2.96c
[60]
a
E ¼ C, Si, Ge, Sn, Pb.
Data not available.
Average bond distance.
b
c

R.S. Anju et al. / Journal of Organometallic Chemistry 731 (2013) 18e22
21
Scheme 2. Possible reaction pathway for the formation of 3 via intermediate, I.
method [65]. Preparative thin-layer chromatography was per-
formed with Merck 105554 TLC Silica gel 60 F254, layer thickness
were solved by heavy atom methods using SHELXS-97 or SIR92 [66]
and refined using SHELXL-97 (Sheldrick, G.M., University of Göt-
tingen) [67,68].
Crystal data for 2: CCDC 891146, formula, C22H30O2Cl4Ge2Ru2;
formula weight, 815.71 g/mol; crystal system, space group: mono-
250
recorded on a 400 and 500 MHz Bruker FT-NMR spectrometer.
Residual solvent protons were used as reference ( , ppm, CDCl3,
mm on aluminium sheets (20 ꢂ 20 cm). NMR spectra were
d
7.26), while a sealed tube containing [Bu4N(B3H8)] in benzene-d6
(
NMR. Infrared spectra were recorded on a Nicolet iS10 spectrom-
eter. Microanalyses for C and H were performed on Perkin Elmer
Instruments series II model 2400.
clinic, P21/n. Unit cell dimensions, a ¼ 9.6298(4), b ¼ 14.2062(6),
dB, ppm, ꢀ30.07) was used as an external reference for the 11B
c ¼ 10.1923(5) A,
b
¼ 99.389(2)ꢁ; Z ¼ 4. Final R indices [I > 2sigma (I)]
ꢀ
R1 ¼ 0.0314, wR2 ¼ 0.0792.
q
range for data collection 2.48e31.20ꢁ.
ꢂ
Crystal size 0.45
ꢂ
0.35
0.20 mm3. Density (calculated)
1.969 Mg/m3. Reflections collected 10,258, independent reflections
3722 [R(int) ¼ 0.0314], goodness-of-fit on F2 1.031.
4.2. Synthesis of 2
Acknowledgements
In a 50 mL Schlenk tube, containing 1 (0.025 g, 0.045 mmol) in
toluene (12 mL) was stirred with GeCl2$dioxane (0.021 g,
0.09 mmol) for 12 h at 65 ꢁC. The solvent was removed in vacuo, the
residue was extracted in hexane and passed through Celite. The
filtrate was concentrated and the residue was chromatographed on
silica gel TLC plates. Elution with a hexane/CH2Cl2 (7:3) mixture
Generous support of the Indo-French Centre for the Promotion
of Advanced Research (IFCPAR-CEFIPRA), No. 4405-1, New Delhi is
gratefully acknowledged. We thank Prof. Jean-Francois Halet,
CNRS-Universite de Rennes-1 for helpful discussions on metal-
laborane chemistry. RSA thank the University Grants Commission,
India, for Junior Research Fellowship. KG and DKR thank Council of
Scientific and Industrial Research (CSIR), India for research
fellowship. We thank V. Ramkumar for X-ray crystallography
analysis.
yielded air stable yellow coloured 2 (0.03 g, 85%).
1
2: H NMR (400 MHz, CDCl3, 22 ꢁC):
d
¼ 2.06 (30H, s, Cp*). 13
C
NMR (100 MHz, CDCl3, 22 ꢁC):
d
¼ 193 (CO), 98.6 (s; C5Me5), 10.3 (s;
C5Me5). IR (hexane, cmꢀ1): 1994 (CO). Microanalysis found: C,
33.48; H, 4.12; C22H30Cl4Ge2O2Ru2 requires C, 32.40; H, 3.71%.
Appendix A. Supplementary material
4.3. Synthesis of I and 3
CCDC 891146 (2) contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge from
the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.
uk/data_request/cif.
The di-m-germylene complex 2 (0.076 g, 0.094 mmol) was added
to a solution of [CpFe(CO)B3H8] (0.010 g, 0.053 mmol) in toluene-d8
(1 mL). The solution when heated at 90 ꢁC for 6 h generated the new
boron containing product I, as indicated by the presence of reso-
nances at
d
¼ ꢀ37.2 and 5.9 ppm. These resonances slowly
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