Cage Opening of a Carborane Ligand by Metal Cluster Complexes

Article abstract of DOI:10.1002/chem.201601075

The reaction of Os₃(CO)₁₀(NCMe)₂ with closo-o-C₂B₁₀H₁₀ has yielded two interconvertible isomers Os₃(CO)₉(μ₃-4,5,9-C₂B₁₀H₈)(μ-H)₂ (1 a) and Os₃(CO)₉(μ₃-3,4,8-C₂B₁₀H₈)(μ-H)₂ (1 b) formed by the loss of the two NCMe ligands and one CO ligand from the Os₃ cluster. Two BH bonds of the o-C₂B₁₀H₁₀ were activated in its addition to the osmium cluster. A second triosmium cluster was added to the 1 a/1 b mixture to yield the complex Os₃(CO)₉(μ-H)₂(μ₃-4,5,9-μ₃-7,11,12-C₂B₁₀H₇)Os₃(CO)₉(μ-H)₃ (2) that contains two triosmium triangles attached to the same carborane cage. When heated, 2 was transformed to the complex Os₃(CO)₉(μ-H)(μ₃-3,4,8-μ₃-7,11,12-C₂B₁₀H₈)Os₃(CO)₉(μ-H) (3) by a novel opening of the carborane cage with loss of H₂.

Full text of DOI:10.1002/chem.201601075

DOI: 10.1002/chem.201601075
Communication
&
Coordination Chemistry
Cage Opening of a Carborane Ligand by Metal Cluster Complexes
[
a]
Richard D. Adams,* Joseph Kiprotich, Dmitry V. Peryshkov,* and Yuen Onn Wong
Abstract: The reaction of Os (CO) (NCMe) with closo-o-
3
10
2
C B H
has yielded two interconvertible isomers
2
10 10
Os (CO) (m -4,5,9-C B H )(m-H) (1a) and Os (CO) (m -3,4,8-
3
9
3
2
10
8
2
3
9
3
C B H )(m-H) (1b) formed by the loss of the two NCMe li-
2
10
8
2
gands and one CO ligand from the Os cluster. Two BH
3
bonds of the o-C B H were activated in its addition to
2
10 10
the osmium cluster. A second triosmium cluster was
added to the 1a/1b mixture to yield the complex
Os (CO) (m-H) (m -4,5,9-m -7,11,12-C B H )Os (CO) (m-H) (2)
3
9
2
3
3
2
10
7
3
9
3
that contains two triosmium triangles attached to the
same carborane cage. When heated, 2 was transformed to
the complex Os (CO) (m-H)(m -3,4,8-m -7,11,12-C B H )Os -
3
9
3
3
2
10
8
3
(
CO) (m-H) (3) by a novel opening of the carborane cage
9
with loss of H2.
Cage stability is one of the hallmarks of the closo polyhedral
[1–2]
boranes and carboranes.
The stability is created by electron
Figure 1. An ORTEP diagram of the molecular structure of Os
3 9 3
(CO) (m -4,5,9-
delocalization manifested in their highly delocalized bonding
molecular orbitals. Carboranes are structures in which some of
C
2
B
10
H
10)(m-H) (1a) showing 30% thermal ellipsoid probability. Selected in-
2
teratomic bond distances [Š] are as follows: Os(1)ÀB(5)=2.168(10), Os(3)À
B(4)=2.181(10), Os(2)ÀB(9)=2.624(10), Os(2)ÀH(9a)=1.64(16), B(9)À
H(9a)=1.41(16), and C(1)ÀC(2)=1.594(13).
[
2]
the BH groups of the boranes are replaced by CH groups.
These substitutions lead to inequivalences and differences in
[3]
reactivity of the remaining BH groups. Studies have shown
the strong bases can attack the most electron deficient BH
groups leading to cage rupture including, in many cases, com-
[
4]
plete removal (deboronation) of a BH group from the cage.
Two products, Os (CO) (m -4,5,9-C B H )(m-H) (1a) and
3
9
3
2
10
8
2
Os (CO) (m -3,4,8-C B H )(m-H) (1b) were obtained from the re-
3
9
3
2
10
8
2
action of Os (CO) (NCMe) with closo-o-C B H in toluene sol-
3
10
2
2 10 10
vent at 110 8C for 1.5 h. ORTEP diagrams of the molecular
structures 1a and 1b are shown in Figures 1 and 2. Com-
pounds 1a and 1b are isomers formed by the loss of the two
NCMe ligands and one CO ligand from the Os (CO) (NCMe) ,
3
10
2
and the addition of one equivalent of closo-C B H to the tri-
2
10 10
osmium cluster. The carborane is a triply bridging ligand con-
taining two direct Os ÀB bonds, and one agostically-coordinat-
ed BH group to the third metal atom in each isomer. The dis-
tances of the direct OsÀB bonds, Os(1)ÀB(5)=2.168(10), Os(3)À
B(4)=2.181(10) Š for 1a, and Os(1)ÀB(3)=2.189(11), Os(3)-
[
a] Prof. R. D. Adams, J. Kiprotich, Prof. D. V. Peryshkov, Dr. Y. O. Wong
Department of Chemistry and Biochemistry
University of South Carolina
JM Palms Ctr for GSR, Columbia, SC 29208 (USA)
E-mail: ADAMSRD@mailbox.sc.edu
Figure 2. An ORTEP diagram of the molecular structure of Os
3 9 3
(CO) (m -3,4,8-
C
2
B
10
H
10)(m-H) (1b) showing 30% thermal ellipsoid probability. Selected in-
2
Supporting information for this article and ORCID for two of the authors
are available on the WWW under
http://dx.doi.org/10.1002/chem.201601075.
teratomic bond distances [Š] are as follows: Os(1)ÀB(3)=2.189(11), Os(3)À
B(4)=2.176(12), Os(2)ÀB(8)=2.629(13), Os(2)ÀH(8a)=1.70(18), B(8)À
H(8a)=1.03(18), and C(1)ÀC(2)=1.645(15).
Chem. Eur. J. 2016, 22, 6501 – 6504
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Communication
B(4)=2.176(12) Š for 1b, are considerably shorter than the dis-
tance to the agostically coordinated BH groups, B(9)ÀH(9a)
bond, B(9)ÀH(9a)=1.41(16), Os(2)ÀB(9)=2.624(10), and Os(2)À
H(9a)=1.64(16) Š, in 1a, and B(8)ÀH(8a)=1.03(18), Os(2)À
B(8)=2.629(13), and Os(2)ÀH(8a)=1.70(18) Š for 1b. Agostical-
ly coordinated BÀH bonds exhibit characteristically longer M–B
[
7]
distances. Two BÀH bonds were cleaved from the cage in the
ligation process and the hydrogen atoms were shifted to the
metal atoms to become bridging hydrido ligands as shown in
1
Figures 1 and 2: H NMR: d=À17.90 (s, 1H), À20.25 ppm (s,
1
H) for 1a and d=À17.93 (s, 1H), À20.22 ppm (s, 1H) for 1b.
The boron atoms from the cleaved BÀH bonds are the ones
coordinated directly to the metal atoms. The hydrogen atoms
of the agostically coordinated BH groups are also characteristi-
1
cally shifted upfield and appear as a quartet due to J coupling
to the neighboring boron atom: for 1a: d=À9.97 ppm (q, br,
1
BÀH!Os), J =76 Hz; for 1b: d=À9.91 ppm (q, br, BÀH!
BÀH
1
[8]
Os), J =78 Hz. Isomers 1a and 1b differ principally by the
BÀH
B3 triangle through which the carborane is coordinated to the
Os triangle. For 1a it is coordinated to the 4,5,9 triangle, but
3
for 1b it is coordinated to the 3,4,8 triangle. The locations of
the carbon atoms in each carborane ligand were identified by
their characteristically short C–C bond distance.
Figure 3. An ORTEP diagram of the molecular structure of Os
3 9 2 3
(CO) (m-H) (m -
4,5,9-m -7,11,12-C B H )Os (CO) (m-H) (2) showing 25% thermal ellipsoid
3
2
10
7
3
9
3
probability. Selected interatomic bond distances [Š] are as follows: Os(1)À
B(4)=2.169(11), Os(2)ÀB(9)=2.593(11), Os(3)ÀB(5)=2.188(11), Os(4)À
B(11)=2.189(11), Os(5)ÀB(7)=2.185(11), Os(6)ÀB(11)=2.211(11), Os(2)À
H(9A)=1.86(10), B(9)ÀH(9A)=1.30(9), and C(1)ÀC(2)=1.603(14).
Compounds 1a and 1b can be interconverted thermally.
The equilibrium ratio 1a/1b=0.63 in toluene solvent was ach-
ieved by heating 1a at 1088C for 48 h. In the transformation
of 1a to 1b, the carborane cage must shift by two B triangles
3
and must involve the addition of a hydrogen atom to B5 and
a cleavage of the hydrogen atom from B3. It is likely that the
hydrido ligands on the metal cluster participate in these hydro-
gen exchanges, but details of the mechanism are not available
at this time.
Except for the presence of one of the bridging hydrido ligands,
H(2), on the Os(1)-Os(2)-Os(3) triangle, compound 2 contains
overall an approximate reflection symmetry with the symmetry
plane passing through the cage atoms C(1), C(2), B(9), and
B(12).
Interestingly, 1a and 1b react with a second equivalent of
Os (CO) (NCMe) at 978C to yield two new hexaosmium com-
An ORTEP diagram of the molecular structure of 3 is shown
in Figure 4. Compound 3 also contains two triangular triosmi-
um clusters that are coordinated to a significantly ruptured
C B H cage. Each triosmium cluster contains two directly-co-
3
10
2
pounds: Os (CO) (m-H) (m -4,5,9-m -7,11,12-C B H )Os (CO) (m-
3
9
2
3
3
2
10
7
3
9
H)3 (2) (38% yield) and Os (CO) (m-H)(m -3,4,8-m -7,11,12-
3
9
3
3
2
10
8
C B H )Os (CO) (m-H) (3) (5% yield). Compound 2 can be con-
ordinated boron atoms, one triply-bridging agostic BH group
and one bridging hydrido ligand. The Os–B distances to the
triply-bridging agostic BH groups, B(3) and B(7), are significant-
ly shorter, Os(1)ÀB(3)=2.132(10), Os(2)ÀB(3)=2.200(10), Os(3)À
B(3)=2.199(10), Os(1)ÀH(3A)=1.85(15), Os(5)ÀB(7)=2.150(11),
Os(4)ÀB(7)=2.235(10), Os(6)ÀB(7)=2.191(11) Š, than those to
the agostically coordinated BH groups that are coordinated to
only one metal atom as found in compounds 1a, 1b, and 2.
The resonances of the triply bridging agostic BH groups are
shifted to slightly higher field values, d=À11.83 (br, 1H), and
À13.56 ppm (br, 1H), than those of 1a, 1b, and 2. The
osmium triangle Os(4)-Os(5)-Os(6) is bonded to the same
group of boron atoms, 7,11,12, as found in 2, but the Os(1)-
2
10
8
3
9
verted to 3 in 15% yield by heating a solution of 2 in nonane
solvent to reflux for 1.25 h. Compounds 2 and 3 have both
been characterized by a combination of IR, NMR and single-
crystal X-ray diffraction analyses. An ORTEP diagram of the mo-
lecular structure of 2 is shown in Figure 3.
Compound 2 contains a C B H cage sandwiched between
2
10
7
two triangular triosmium carbonyl clusters. The metal clusters
bridge the 4,5,9 and 7,11,12 B triangles of the closo-C B H
7
3
2
10
cage. Two boron atoms, Os(1)ÀB(4)=2.169(11), Os(3)ÀB(5)=
.188(11) Š, and one agostically coordinated BH group,
2
1
Os(2)ÀB(9)=2.593(11), Os(2)ÀH(9 A)=1.86(10) Š, H NMR d=
1
À9.70 ppm (q, 1H), J = ~82 Hz, are bonded to the Os(1)-
BÀH
Os(2)-Os(3) triangle. The Os(1)-Os(2)-Os(3) triangle also contains
two bridging hydrido ligands that were presumably transferred
to the cluster from the two boron atoms that are directly
Os(2)-Os(3) group is bonded instead to the B group 3,4,8 and
3
thus was shifted in the course of the conversion of 2 to 3.
We have analyzed the transformation of 2 into 3 in terms
bonded to the osmium atoms. Three boron atoms, Os(4)À a two-step mechanism as shown in Scheme 1. The process
B(11)=2.189(11), Os(5)ÀB(7)=2.185(11), and Os(6)ÀB(11)=
.211(11) Š are directly coordinated to the Os(4)-Os(5)-Os(6) tri-
begins with a shift of the Os(1)-Os(2)-Os(3) cluster from the
4,5,9 B -triangle to the 3,4,8 B -triangle. In the process, a hydro-
2
3
3
angle. Accordingly, this Os triangle contains three hydrido li-
gen atom is shifted to B(5) and one is cleaved from B(3). This
step would lead to the formation of an intermediate represent-
3
gands with one bridging each of the three OsÀOs bonds.
Chem. Eur. J. 2016, 22, 6501 – 6504
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Communication
Scheme 1. A proposed mechanism for the transformation of 2 to 3. The red lines in the Intermediate 2a indicate the cage bonds that must be cleaved in the
cage-opening process.
and B(7)ÀB(8), to complete the cage opening process. Two hy-
drogen atoms were eliminated, presumably as H , in the trans-
2
formation of 2 to 3.
The metalated boron cages reported herein are the rare ex-
amples of the unsupported metal–boryl complexes of unsub-
stituted icosahedral carboranes. The formation of transition-
metal B-carboranyl bonds by nondirected BÀH activation has
been reported only for the reaction of o-C B H and
2
10 12
[
9]
[
Ir(PPh ) Cl] or [Ir(AsPh ) Cl]. Furthermore, compound 2 is the
3 3 3 3
first example of the complex containing a total of five metal–
boryl bonds on a single icosahedral carborane cage.
It is anticipated that similar cluster-induced cage-opening re-
actions can be obtained with other carboranes and by using
clusters of different transition metals. Indeed, Du et al. have
shown that reaction of the carborane, closo-4-CB H , with
8
9
[
Ru (CO) ] proceeds by formation of an Ru raft from which
3 12 6
three of the ruthenium atoms have been inserted into the car-
[
10]
borane cage to form a 12-vertex triruthenacarborane. It will
be interesting to see if carboranes will also open when they
are deposited on clean metal surfaces and, if so, could carbor-
Figure 4. An ORTEP diagram of the molecular structure of Os
3 9 3
(CO) (m-H)(m -
3
,4,8-m -7,11,12-C )Os (CO) (m-H) (3) showing 20% thermal ellipsoid
3
2
B
10
H
8
3
9
[
11]
probability. Selected interatomic bond distances [Š] are as follows Os(1)À
B(3)=2.132(10), Os(2)ÀB(3)=2.200(10), Os(3)ÀB(3)=2.199(10), Os(1)À
H(3A)=1.85(15), Os(2)ÀB(4)=2.291(11), Os(3)ÀB(8)=2.304(10), Os(5)À
B(7)=2.150(11), Os(4)ÀB(7)=2.235(10), Os(6)ÀB(7)=2.191(11), Os(4)À
B(11)=2.307(10), Os(6)ÀB(12)=2.335(10), and C(1)ÀC(2)=1.446(14).
anes serve as precursors to “carborene” hybrids of graphenes
[
12]
and borophenes on metal surfaces?
Experimental Section
Synthesis of 1a and 1b
ed as 2a in Scheme 1. This step of the rearrangement is for-
mally equivalent to the isomerization of 1a to 1b described
above. The opening of the C2B10 cage requires the cleavage of
five cage bonds, B(3)ÀB(7), B(3)ÀC(1), B(3)ÀC(2), B(7)ÀC(2), and
B(7)ÀB(8). The sequence of these cleavages is not known at
present, but we are willing to speculate that the process
begins with cleavage of the bonds between B(3) and the two
carbon atoms, C(1) and C(2), because previous studies have
shown that these B(3)ÀC cage bonds are the first to cleave
C B H (7.4 mg, 0.0513 mmol) was added to Os (CO) (NCMe)
2
2
10 12
3
10
(48.0 mg, 0.0515 mmol) dissolved in toluene (30 mL) and then
heated to reflux for 1.5 h. After removal of the solvent in vacuo,
TLC analysis was performed on alumina which provided 1a and
1
b in yields of 26 and 9%, respectively.
Synthesis of 2 and 3
Complex 1 (27.0 mg, 0.0279 mmol) was added to Os (CO) (NCMe)
2
[4]
3
10
upon the addition of bases to 1,2-C B H . Hydrogen atoms,
2
10 12
(
22.5 mg, 0.0241 mmol) dissolved in heptane (30 mL) and then
presumably from the metal atoms, are added to the boron
atoms B(3) and B(7). These additions would facilitate cleavage
of the bonds to B(7), particularly B(3)ÀB(7), and also B(7)ÀC(2)
heated to reflux for 1.25 h. After removal of solvent in vacuo, TLC
analysis was performed on silica which provided 2 and 3 in yields
of 38 and 5%, respectively.
Chem. Eur. J. 2016, 22, 6501 – 6504
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Communication
Interconversion of 1a/1b
Acknowledgements
Conversion of 1a to 1b and vice versa was performed by dissolv-
This research was supported by grant CHE-1464596 from the
U. S. National Science Foundation to R.D.A. Acknowledgement
is made to the donors of the American Chemical Society Petro-
leum Research Fund (award: 54504-DNI3 to D.V.P.) for the par-
tial support of this research.
ing either 1a or 1b in [D ]toluene and heating at 1088C for 43 h.
8
The ratio of the two isomers was then measured by NMR spectro-
scopic integration. Conversion of 2 to 3 was performed by simply
heating 2 in a nonane solution at reflux for 1.25 h.
Single-crystal structure determination
The intensity data were collected by using a Bruker SMART APEX
CCD-based diffractometer with MoKa radiation (l=0.71073 Š) at
Keywords: cage-opening
structure · osmium
· carborane · clusters · crystal
2
94 K. All nonhydrogen atoms were refined with anisotropic ther-
mal parameters. Hydrogen atoms were placed in geometrically ide-
alized positions and included as standard riding atoms during the
least-square refinements.
Crystal data for 1a: Os O C B H monoclinic; P2 /n; M =
3
9
11 10 12
1
r
À1
9
9
2
0
66.91 gmol ; a=8.1347(12), b=20.133(3), c=13.984(2) Š; a=
3
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=
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.812 gcm ; m=16.690 mm ; 2qmax =56.708; F(000)=1712; Rint
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o
À3
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1
9
3
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=
=
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.822 gcm ; m=16.750 mm ; 2qmax =52.188; F(000)=1712; Rint
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À3
6
18 20 10 12
1
r
À1
789.79 gmol ; a=9.6636(2), b=15.4422(4), c=25.6755(6) Š; a=
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=
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.0740; no.collected/unique/I >2sI data=45720/6784/5836; R1/
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o
o
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18 20.5 10
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808.31 gmol ; a=19.2832(13), b=18.0798(12), c=23.6626(16) Š;
3
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L.-S. Wang, Nat. Chem. 2014, 6, 727–731; c) W.-L. Li, Y.-F. Zhao, H.-S. Hu,
J. Li, L.-S. Wang, Angew. Chem. 2014, 53, 5540–5545.
a=90.00, b=108.5790(10), g=90.008; V=7819.7(9) Š ; Z=8;
1calcd =3.072 gcm
6
6
0
À3
À1
;
m=19.523 mm
;
2qmax =56.788; F(000)=
332; Rint =0.0551; no.collected/unique/I >2sI data=41237/6912/
o
o
178; R1/wR2 (alldata)=0.0372/0.0849; R1/wR2(I >2sI )=0.0319/
o
o
À3
.0823; max/min. electron density=1.456/À1.516 e-Š .
CCDC 1448072 (1a), 1448073 (1b), 1448074 (2) and 1448075 (3)
contain the supplementary crystallographic data for this paper.
These data are provided free of charge by The Cambridge Crystal-
lographic Data Centre.
Received: March 7, 2016
Published online on March 23, 2016
Chem. Eur. J. 2016, 22, 6501 – 6504
www.chemeurj.org
6504
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim