Novel class of heterometallic cubane and boride clusters containing heavier group 16 elements

Article abstract of DOI:10.1021/ic3008602

Thermolysis of an in situ generated intermediate, produced from the reaction of [CpMoCl₄] (Cp* = η⁵-C ₅Me₅) and [LiBH₄.THF], with excess Te powder yielded isomeric [(CpMo)₂B₄TeH₅Cl] (2 and 3), [(CpMo)₂B₄(μ₃-OEt)TeH₃Cl] (4), and [(CpMo)₄B₄H₄(μ₄-BH) ₃] (5). Cluster 4 is a notable example of a dimolybdaoxatelluraborane cluster where both oxygen and tellurium are contiguously bound to molybdenum and boron. Cluster 5 represents an unprecedented metal-rich metallaborane cluster with a cubane core. The dimolybdaheteroborane 2 was found to be very reactive toward metal carbonyl compounds, and as a result, mild pyrolysis of 2 with [Fe₂(CO)₉] yielded distorted cubane cluster [(CpMo)₂(BH)₄(μ₃-Te){Fe(CO)₃}] (6) and with [Co₂(CO)₈] produced the bicapped pentagonal bipyramid [(CpMoCo)₂B₃H₂(μ₃-Te) (μ-CO){Co₃(CO)₆}] (7) and pentacapped trigonal prism [(CpMoCo)₂B₃H₂(μ₃-Te)(μ-CO) ₄{Co₆(CO)₈}] (8). The geometry of 8 is an example of a heterometallic boride cluster in which five Co and one Mo atom define a trigonal prismatic framework. The resultant trigonal prism core is in turn capped by two boron, one Te, and one Co atom. In the pentacapped trigonal prism unit of 8, one of the boron atoms is completely encapsulated and bonded to one molybdenum, one boron, and five cobalt atoms. All the new compounds have been characterized in solution by IR, ¹H, ¹¹B, and ¹³C NMR spectroscopy, and the structural types were unambiguously established by crystallographic analysis of 2 and 4-8

Full text of DOI:10.1021/ic3008602

Article
pubs.acs.org/IC
Novel Class of Heterometallic Cubane and Boride Clusters
Containing Heavier Group 16 Elements
Arunabha Thakur, Soumik Sao, V. Ramkumar, and Sundargopal Ghosh*
Department of Chemistry, Indian Institute of Technology Madras, Chennai 600 036, India
S
* Supporting Information
ABSTRACT: Thermolysis of an in situ generated intermedi-
ate, produced from the reaction of [Cp*MoCl4] (Cp* = η5-
C5Me5) and [LiBH4.THF], with excess Te powder yielded
isomeric [(Cp*Mo)2B4TeH5Cl] (2 and 3), [(Cp*Mo)2B4(μ3-
OEt)TeH3Cl] (4), and [(Cp*Mo)4B4H4(μ4-BH)3] (5).
Cluster 4 is a notable example of a dimolybdaoxatelluraborane
cluster where both oxygen and tellurium are contiguously
bound to molybdenum and boron. Cluster 5 represents an
unprecedented metal-rich metallaborane cluster with a cubane
core. The dimolybdaheteroborane 2 was found to be very
reactive toward metal carbonyl compounds, and as a result,
mild pyrolysis of 2 with [Fe2(CO)9] yielded distorted cubane
cluster [(Cp*Mo)2(BH)4(μ3-Te){Fe(CO)3}] (6) and with
[Co2(CO)8] produced the bicapped pentagonal bipyramid
[(Cp*MoCo)2B3H2(μ3-Te)(μ-CO){Co3(CO)6}] (7) and pentacapped trigonal prism [(Cp*MoCo)2B3H2(μ3-Te)(μ-
CO)4{Co6(CO)8}] (8). The geometry of 8 is an example of a heterometallic boride cluster in which five Co and one Mo
atom define a trigonal prismatic framework. The resultant trigonal prism core is in turn capped by two boron, one Te, and one
Co atom. In the pentacapped trigonal prism unit of 8, one of the boron atoms is completely encapsulated and bonded to one
molybdenum, one boron, and five cobalt atoms. All the new compounds have been characterized in solution by IR, 1H, 11B, and
13C NMR spectroscopy, and the structural types were unambiguously established by crystallographic analysis of 2 and 4−8
CO)2B2H(μ-H){Fe(CO)2}2Fe(CO)3], III, similar to the
geometry of I and II.5 Although homometallic and hetero-
metallic cubane clusters containing chalcogen elements are well
documented in the literature,6−8 up until now only a few
cubane clusters containing boron as a main group element are
known9,10 (Chart 1). In addition, structurally characterized
metallaheteroboranes having a cubane core containing a Te
atom as one of the vertices are very rare. Therefore, our efforts
to incorporate main group elements, in particular heavier
chalcogen elements, into transition metal cubane and boride
clusters have recently been focused on the Te atom.
Compound 2, isostructural with [(Cp*Mo)2B5H9], was found
to be a good precursor for the generation of this type of
heterometallic cubane-type and boride clusters. We report
herein the synthesis and structural characterization of several
unusual molybdaheteroborane clusters with a novel cluster
core.
INTRODUCTION
■
Boron, being the only nonmetal element in group 13, forms a
number of different clusters with both main group and
transition metal elements. Over the past few decades, extensive
efforts have been made to synthesize mixed-metal chalogenide
clusters of high nuclearity, and exploitation of new cluster
components continues to be an imperative stage for
accomplishing large-core structures.1 Transition metal−chalc-
ogen chemistry that deals with compounds containing a
cubane-type core and in which the metal and chalcogen
atoms occupy adjacent vertices is a unique area of research.
These compounds are of interest not only because of their
contribution to the growth of modern organometallic chemistry
but also for their potential use as models for various industrial
and biological catalytic processes.2−4
A continuous research interest in our laboratory has been the
synthesis and structural characterization of cubane or boride
clusters containing transition metal and chalcogen atoms. As
shown in Chart 1, a number of such species have recently been
synthesized from the thermolysis of [(Cp*Mo)2B4E2] (E = S,
Se) in the presence of [Fe2(CO)9], which generates cubane-
type clusters [(Cp*Mo)2(μ3-E)2B2H(μ-H){Fe(CO)2}2Fe-
(CO)3], I and II (I: E = S; II: E = Se).5 In a similar fashion,
reaction of [(Cp*CO)2-arachno-Ru2B2H6] with excess
[Fe2(CO)9] resulted in the formation of [(Cp*Ru)2(μ3-
RESULTS AND DISCUSSION
■
Synthesis of 2−5. As shown in Scheme 1, thermolysis of an
in situ generated intermediate, produced from the reaction of
[Cp*MoCl4] (Cp* = η5-C5Me5) and [LiBH4·THF] with an
Received: April 27, 2012
Published: July 19, 2012
© 2012 American Chemical Society
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Chart 1. Metal-Rich Metallaboranes with Cubane-Type Geometry
Scheme 1. Synthesis of Compounds 2−5
excess of Te powder yielded [(Cp*Mo)2B4TeH5Cl] (2 and 3),
[(Cp*Mo)2B4(μ3-OEt)TeH3Cl] (4), and [(Cp*Mo)4B4H4(μ4-
BH)3] (5) in moderate to good yields. In parallel with the
formation of 2−5 compounds [(Cp*Mo)2B4H4Te2]11 and
[(Cp*Mo)2B5H9]12 have also been isolated in good yields.
The 11B{1H} NMR spectrum of 2 indicates the presence of
four boron resonances at δ = 95.5, 73.2, 40.7, and 26.7 ppm in
an intensity ratio of 1:1:1:1. Interestingly, compound 3 also
shows four boron resonances at δ = 98.8, 71.8, 51.6, and 24.7
ppm, which are similar to those of [(Cp*Mo)2B4SH6] and
[(Cp*Mo)2B4SeH5(Ph)].13 The peak at δ = 40.7 ppm for 2
and δ = 71.8 ppm for 3 appears as a singlet in the 11B NMR,
indicating the absence of terminal hydrogen in boron.
Therefore, compounds 2 and 3 could be illustrated as
geometrical isomers based on their spectroscopic data. The
resonances at δ = 26.7 and 24.7 ppm have been assigned to the
unique boron, attached to the tellurium atom. The chemical
shift of this boron appeared significantly upfield shifted relative
to the S analogue (δ = 10.1 ppm). A similar trend has also been
observed in [(Cp*Mo)2B4H4E2]11 systems (E = S, Se, Te). The
upfield chemical shift of these compounds may be due to the
electronegativity and size differences between S, Se, and Te.
The 1H NMR spectra of 2 and 3 show the presence of four B−
H resonances, one Cp* resonance in the range δ = 2.01−2.11
ppm and Mo−H−B resonances in the range δ = −6.27 to
−6.60 ppm. The 13C NMR spectra also reveal the presence of
one type of Cp* resonance. The IR spectra of both 2 and 3
show bands at 2407−2443 cm−1 in a region characteristic of the
B−H stretching vibration. Both clusters 2 and 3, having six
skeleton electron pairs (sep), appear to follow the regular
electron-counting rules appropriate for a bicapped trigonal
bipyramid.14
The solid-state structure of one of the isomers (2), shown in
Figure 1, is fully consistent with the spectroscopic data. All the
B−B and Mo−B bond distances are in the normal range.
However, the Mo−Mo bond length in 2 is slightly longer (0.03
Å) as compared with the corresponding selenaborane clusters
[(Cp*Mo)2B4SeH5(Ph)]. This may be due to the reduced
tendency of boron and tellurium atoms to form polarized
bonds (compared to boron−oxygen and boron−selenium
atoms) that have a localized two-center character, resulting in
the observed distances.15,16
[(Cp*Mo)2B4(μ3-OEt)TeH3Cl], 4. Compound 4 has been
isolated as a brown solid in 4% yield. The 11B NMR spectrum
exhibits four boron resonances at δ = 87.2, 77.5, 44.1, and
1
−12.0 ppm with an intensity ratio of 1:1:1:1. The H NMR
spectrum shows the presence of three B−H terminal protons
1
and a single Cp* resonance at δ = 2.18 ppm. The H{11B}
NMR spectrum displays three singlets in the range δ = 7.54−
2.43 ppm (1:1:1), implying that one of the boron atoms has no
terminal hydrogen. In addition to the signal attributable to the
η5-C5Me5 protons, the presence of an ethoxy group was also
confirmed by the 1H NMR spectrum. The presence of Cp* and
ethoxy moieties was further confirmed by a 13C NMR
spectrum. The formation of oxamolybdaborane species is
possibly due to the ring-opening insertion of THF into the
molybdaborane framework.17
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is comparable with other oxamolybdaborane clusters,21 but is
shorter compared to those observed in other oxametallaborane
clusters, for example, 1.523 Å in [9,9-(PMe2Ph)2-arachno-9,6-
PtOB8H10],22 1.510 Å in [7-(η5-C5Me5)-nido-7,12-RhOB10H9-
8-Cl-11-(PMe2Ph)],23 and 1.481 Å in 2-[(η6-C6(CH3)3H3)]Fe-
O-6-B8H10].24 Cluster 4 is unprecedented and a remarkable
example of an oxametallaborane containing a heavier group 16
element (Te) as a cluster constituent.
[(Cp*Mo)4B4H4(μ4-BH)3], 5. Compound 5 has been
isolated in 5% yield as a green solid. The spectroscopic and
structural characterization of cluster 5 has been reported in our
preliminary communication.25 As shown in Figure 3, the
Figure 1. Molecular structure of [(Cp*Mo)2B4H5(μ3-Te)Cl], 2.
Selected bond lengths (Å) and angles (deg): Mo1−Mo2 2.845(2),
B1−B2 1.697(3), B4−Te 2.219(4), Mo2−Te 2.753(2), Mo2−B1
2.155 (6), B1−Cl 1.929(19), Mo2−B4 2.191(9); Mo1−Te−Mo2
81.220(8), B2−B3−B4 114.0(10).
The solid-state structure of 4, shown in Figure 2, is fully
consistent with the spectroscopic data. Although the average
Figure 3. Molecular structure of [(Cp*Mo)4B4H4(μ4-BH)3], 5. Cp*
are omitted for clarity. Selected bond lengths (Å) and angles (deg):
Mo(1)−Mo(3) 2.659 (10), Mo(2)−Mo(4) 2.651 (9), Mo(2)−Mo(3)
2.847(10), Mo(1)−B(1) 2.220(9), Mo(1)−B(2) 2.136(11), Mo(1)−
B(3) 2.309(10), Mo(2)−B(2) 2.158(9), Mo(3)−B(6)−Mo(1)
69.71(11).
geometry of 5 can be viewed as cubane, made of four Mo and
four B atoms, and out of the six square faces of the cube three
of them are capped by BH units to build a [Mo4B7] core.
Alternatively, the observed geometry of 5 can be viewed as a
tetracapped tetrahedron in which four molybdenum atoms
define a pseudotetrahedron. Compound 5 has five skeleton
electron pairs, two electrons fewer for the structure based on a
tetrahedron. An alternative electron-counting method is also
possible for this cluster. The 11-vertex polyhedron is a
deltahedron with six degree 4 vertices for the four molybdenum
atoms. The elemental polyhedral topology demands that
among the remaining seven vertices two must be of degree 5
and the remaining five must be of degree 4. This is indeed
observed in cluster 5. Alternatively, this structure can be
interpreted as a cluster having 24 skeletal electrons required by
Wade−Mingos rules for an 11-vertex deltahedron. In counting
the skeletal electrons, the three external Mo−Mo bonds
indirectly contribute six skeletal electrons by using what
would otherwise be lone pair electrons to make the three
bonds. Thus, the skeletal electron count for 5 is as follows: 7
BH vertices = 14 skeletal electrons; 4 Cp*Mo vertices using
four internal orbitals = 4 skeletal electrons; 3 external Mo−Mo
bonds = 6 skeletal electrons; total skeletal electrons = 24 = 2n +
2 for n = 11.
Figure 2. Molecular structure of [(Cp*Mo)2B4(μ3-OEt)TeH3Cl], 4.
Selected bond lengths (Å) and angles (deg): Mo1−Mo2 2.660(7),
Mo1−B2 2.195(7), B1−B2 1.654(10), B4−Te 2.169(8), Mo2−Te
2.759(6), Mo2−B1 2.330(6), B4−O 1.438 (9), Mo2−O 2.140(5);
Mo1−Te−Mo2 81.220(8), Mo1−Te−Mo2 76.85(14).
Mo−B (2.281(3) Å) and Mo−Mo bond length (2.6603(7) Å)
are comparable to those observed in oxamolybdaborane
clusters,18 they are significantly shorter in comparison with
other molybdaborane clusters.19,20 This might be due to the
presence of an oxygen atom that withdraws electron density
from the cluster. The boron−tellurium bond length in 4 was
found to be 2.169(8) Å, which is significantly shorter compared
to the boron−tellurium bond distance in 2 (2.219(4) Å).
Further, the boron−oxygen bond length B4−O1 (1.438(9) Å)
Utilization of [(Cp*Mo)2B4H5(μ3-Te)Cl], 2, As a Pre-
cursor to a Cubane Cluster. Transition metal carbonyl
compounds have received significant attention in polyhedron
boranes,26 carboranes,27 metallacarboranes,28 and metallabor-
ane chemistry29 in connection with their potential as versatile
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a
Scheme 2. Synthesis of Heterometallic Cubane 6 and Boride Clusters 7 and 8
a
The interstitial boron atoms in 8 are not shown for clarity.
reagents in cluster growth reactions. Therefore, upon the
availability of 2, the chemistry was elaborated by means of a
cluster expansion reaction with [Co2(CO)8] and [Fe2(CO)9].
As shown in Scheme 2, reaction of 2 with [Fe2(CO)9] led to
the isolation of a heterometallic cubane-type cluster (6)
containing Te as one of the vertices.
Table 1. Selected Structural and Spectroscopic Data of 5 and
6 and Other Related Compounds
d[M−
a
compound
sep
10
M] [Å]
E1/2
λmax
ref
b
[(Cp)4Ni4B4H4]
2.35
-
543, 423,
335, 284
9
9
b
The solid-state structure of 6, shown in Figure 4, shows that
the cubane core, {Mo2B4FeTe}, is highly distorted. As shown in
[(Cp)4Co4B4H4]
8
2.47
-
548, 365,
302, 257
b
II
6
6
6
2.92
-
-
-
-
-
-
5
5
b
III
2.72
c
[(Cp*Ru)3(μ3-CO)
Co(CO)2B3H3]
2.93
10
b
5
6
5
7
2.71
0.42,
0.63
440, 367,
315, 239
this
work
b
2.95
0.16,
0.49
c
490, 403,
312, 235
this
work
a
b
sep = skeletal electron pair. M = Mo. M = Ru.
distorted (92.2(3) Å). Note that the top and bottom four-
membered rings of Mo2B2 are not planar (dihedral angle
7.74°).
Figure 4. Molecular structure of [(Cp*Mo)2(BH)4(μ3-Te){Fe-
(CO)3}], 6. Cp* are not shown for clarity. (a) Cubane view; (b)
bicapped octahedron view. Selected bond lengths (Å) and angles
(deg): Mo(1)−Mo(2) 2.9518(11), Mo(2)−Te(1) 2.6641(10),
Fe(1)−Mo(1) 2.7985(16), Fe(1)−Te(1) 2.6270(15), Mo(1)−B(3)
2.323(11), Fe(1)−B(1) 2.226(13), B(1)−B(2) 1.749(16), B(1)−B(3)
1.741(16); Fe(1)−Te(1)−Mo(2) 64.25(4).
Consistent with the X-ray results, the 11B NMR spectrum of
6 rationalizes the presence of four boron atoms in the ratio
2:1:1 at δ = 85.6, 45.6, and −4.09 ppm, respectively. The
11B{1H} NMR spectrum of 6 features one downfield resonance
at δ = 85.6 ppm, accounting for two four-coordinated boron
atoms (B1 and B4). The chemical shifts at δ = 45.6 and −4.09
ppm have been assigned to the five- and three-coordinated
1
Table 1, the average Mo−Mo bond distance (2.951(9) Å) in
cluster 6 is significantly longer than the Mo−Mo bond
distances in 5 (2.714 Å) as well as in other single- and
double-cubane clusters.30 Alternatively, the observed geometry
of 6 can be viewed as a bicapped octahedral structure in which
Mo1, Mo2, B1, B3, B4, and Fe, occupying the vertices of the
core octahedron, and the two triangular faces Mo1−Mo2−Fe
and Mo1−Mo2−B3 are capped by Te and B2, respectively. All
the Mo−B and B−B distances are in the normal range for other
molybdaborane clusters.19,20 The Mo−Fe distances (2.798(16)
and 2.813(16) Å) are consistent with the existence of a single
bond. The localization of Mo−Mo bonds renders the top four-
membered ring (Mo2B1FeTe) and the bottom ring
(Mo1B2B3B4) twisted with respect to each other by ca.
1.21°. In this way the B2−Mo2−Te bond angle of the cube is
boron atoms (B3 and B2), respectively. The H{11B} NMR
spectrum reveals three types of B−H protons in the ratio 2:1:1.
1
The H NMR spectral data of 6 suggest a plane of symmetry
for a static molecule as well as the presence of a single Cp*
resonance at 1.93 ppm. The 13C NMR spectrum contains
signals attributable to the one type of Cp* ligand and one
Fe(CO)3 fragment. The IR spectrum of 6 shows a band at 2471
cm−1 in a region characteristic for the B−H stretching
frequency and three strong bands at 2020, 1971, and 1943
cm−1, which are assigned to the CO stretching modes of the
Fe(CO)3 unit.
The electronic spectrum of compound 6 in the visible region
(measured in CH2Cl2 solution) shows that there is a single
absorption band at 235 nm (ε = 2700), with indications of
several weaker bands at lower energy. The pattern of this
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Figure 5. Cyclic voltammetry of compound 6 in CH2Cl2.
spectrum is very similar to those of [(CpNi)4B4H4] and 5. As
shown in Table 1 (vide infra), the higher intensity band at 235
nm has been blue-shifted by 41 nm on going from
[(CpNi)4B4H4] to compound 6.31 Further, in order to evaluate
the redox properties, cyclic voltammetry experiments of cluster
6 were carried out in CH2Cl2 using glassy carbon as a working
electrode, Pt wire as counter electrode, Ag/Ag+ as reference
electrode, tetrabutylammonium perchlorate as supporting
electrolyte, and Fc/Fc+ as internal standard. The cyclic
voltammogram, shown in Figure 5, exhibits one reversible
redox potential at E1/2 = 0.42 V and one quasi-reversible peak at
to the substantial decomposition of compound 2. Clusters 7
and 8 are air-stable brown solids, and they were isolated by
thin-layer chromatography (TLC) in 8% and 35% yield,
respectively.
[(Cp*MoCO)2B3H2(μ3-Te)(μ-CO){Co3(CO)6}], 7. The mo-
lecular structure of 7, shown in Figure 6, reveals that the cage
E1/2 = 0.63 V, which indicate the delocalization nature of metal
electrons in the species. Further, differential pulse voltammetry
analysis was carried out to obtain the better resolution of redox
peaks, and it shows a similar result to that obtained from the
CV analysis. For a scan from 0 to 1 V at 50 mV s−1, these redox
potentials were probably caused by the dimolydenum-based
reduction processes [Mo(II)Mo(I)/Mo(I)Mo(0)]. On the
other hand, compound 5 underwent one quasi-reversible
redox along with one reversible redox process at 0.16 V (for
50/5+) and 0.49 V (5+/52+), respectively.
Utilization of [(Cp*Mo)2B4H5(μ3-Te)Cl], 2, As a Pre-
cursor to Boride Clusters. The study of transition metal
interstitial borides has produced a richly developed area of
cluster chemistry.32 Metallaboranes are predominantly exem-
plified by boron-rich clusters rather than metal-rich clus-
ters.33−39 The characteristic feature that separates the boride
clusters40 from the metallaboranes is the greater number of
boron-to-metal bonding contacts at the expense of boron−
hydrogen bonds. Although a number of transition metal
clusters containing an encapsulated main group element have
been synthesized, only few structurally characterized clusters
containing an internal boron atom are known.34−39
The cluster expansion reactions of main group and transition
metal polyhedral metallaboranes using a metal carbonyl
fragment are well documented.41,42 However, examples
proceeding in high yield with clean and well-defined
stochiometry are limited. As shown in Scheme 2 (vide infra),
compound 2 undergoes a cluster buildup reaction with
[Fe2(CO)9] to generate single cubane cluster 6 and on mild
thermolysis with an excess of [Co2(CO)8] yielded two
heterometallic boride clusters, 7 and 8. Thermolysis of 2 with
[Mn2(CO)10] and [Re2(CO)10] at elevated temperature leads
Figure 6. Molecular structure of [(Cp*MoCO)2B3H2(μ3-Te)(μ-
CO){Co3(CO)6}], 7. Cp* and CO are omitted for clarity. Selected
bond lengths (Å) and angles (deg): Co1−Co2 2.543(9), Mo1−Co3
2.731(7), Co3−B1 2.055(5), Mo2−B1 2.274(5), Mo1−B2 2.315(5);
Co3−B1−B3 118.2(3).
geometry is based on a pentagonal bipyramid with a semi-
interstitial boron atom. The three cobalt atoms (Co1, Co2, and
Co3) and two boron atoms (B1 and B3) constitute the
pentagonal plane, while the one molybdenum atom (Mo2) and
B2 occupy the apical positions of the pentagon. In addition, the
triangular faces, Mo1−Co1−Co2 and B1−B2−Co3, are capped
by a Te and Mo1 atom, respectively. Clusters containing
bicapped pentagonal bipyramidal geometry are very rare, and
only a few examples are known, for example, [B6H6Cp*Ni]2−,43
[B5H7 (Cp*Co)2],44 closo-[B5H4PPh3{Fe(CO)3{Ir-
(CO)2PPh3}],45 and [Cp*Ir(B3H3-C2Me2){Mo(CO)3}(μ-
CO)].46 Cluster 7 is the first example of this kind of geometry,
having Te and a Cp*Mo fragment occupying the capping
positions and boron occupying the semi-interstitial position.
The apical Mo−B2 separation in 7 (2.428(5) Å) is longer than
a common Mo−B bond (2.21 Å)14 but comparable with a
Mo−Bequatorial distance. The Mo2capping−Co3 distance is
considerably longer (2.987(7) Å) than the average Mo−Co
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Figure 7. Molecular structure of [(Cp*MoCO)2B3H2(μ3-Te)(μ-CO)4{Co6(CO)8}], 8. Cp* and CO are omitted for clarity. (a) Without interstitial
boron atom and (b) with encapsulated boron atom. Selected bond lengths (Å) and angles (deg): Co1−Co2 2.647 (7), Co3−Co5 2.791(6), Mo2−
B2 2.280(4), Co5−Co6 2.562(7), Co6−B2 2.109(4), Mo1−Co6 2.731(5), Co5−Te 2.453(5), B1−B2 1.750(5), B1−Co1 2.140(4); Co6−Mo1−
Co1 55.604(13).
distance (2.809 Å). The Co1−Co2 distance in the pentagon is
2.543(9) Å, while the internal angles within this pentagon are in
the range 97.48(10)−121.74(3)° (107.66° av), comparable
with the expected value (108°) for a planar structure.47 Indeed,
the equatorial Co1−Co2−Co3−B1−B3 ring in 7 is in the
plane.
[ N ( P P h 3 ) 2 ] [ R u 6 H 2 - ( C O ) 1 8 B ] 4 8 a n d [ A s -
Ph4]2[HFe7(CO)20B].49 However, the boron-centered trigonal
prismatic structure of 8 containing Te and the Cp*Mo
fragment as a capping unit appears to be the first cluster of
its type, not only with respect to the environment of the boron
atom but also with respect to the metal framework. The boron
atom (B1) lies at the center of a trigonal prismatic cage, bonded
to six Co atoms, two boron atoms (B2 and B3), and one Mo
atom (Mo2). The average Co−Co bond distance (2.592 Å) in
the trigonal prismatic core is considerably longer than the sum
of the van der Waals radii for metallic cobalt (2.5 Å). The
Mo2−Co distances are in the range 2.847(5)−3.010(5) Å,
which are significantly longer than the average Mocapping−Co
distance (2.771(5) Å).
Consistent with the X-ray results, the 11B NMR spectrum of
7 rationalizes the presence of three boron resonances in the
ratio 1:1:1. The 11B NMR chemical shift at δ = 110 ppm is
tentatively assigned to the unique boron situated at the apical
1
position of the pentagon in a semi-interstitial manner. The H
and 13C NMR spectra of 7, measured at room temperature, are
1
consistent with its unsymmetrical structure. In particular, H
NMR spectral data reveal the presence of two kinds of Cp*
signals, and the 13C NMR spectrum confirms the presence of
CO. The IR spectrum in the carbonyl region shows both
terminal and bridging carbonyl frequencies at 2024, 1996, and
1786 cm−1, which have been assigned to Co(CO)2 and
Mo(CO)2 fragments, and bands at 2498 and 2368 cm−1 are due
to the terminal B−H stretches.
The X-ray structure of 8 rationalizes the presence of two
resonances in the 11B NMR spectrum at δ = 166.6 and 102.1
1
ppm (1:2). The H NMR spectrum shows one broad signal in
the lower field for the terminal BH proton and two sets of
distinct chemical shifts for two Cp* ligands. The 13C NMR
spectrum also confirms the presence of two Cp* ligands and
CO groups. The position of the boron chemical shift for B1 (δ
= 166.6 ppm) demands some comments. Interstitial boron
atoms are usually characterized by their downfield chemical
shifts and a sharp signal. Although B1, the interstitial boron
atom in 8, is directly bonded to seven metals, the 11B NMR
chemical shifts appear comparatively upfield.49 This may be due
to the change in connectivity associated with the interstitial
boron (B1), which is also directly linked to two boron atoms.
Resonances of relative intensity two at δ = 102.1 ppm can be
assigned to the five- and six-connected boron atoms (B2 and
B3). The IR spectrum shows the presence of characteristic
stretching frequencies for CO groups attached to Mo and Co
atoms.
[(Cp*MoCO)2B3H2(μ3-Te)(μ-CO)4{Co6(CO)8}], 8. Com-
pound 8 was isolated in 35% yield as a brown solid. Single
crystals suitable for X-ray diffraction analysis of 8 were obtained
from a mixture of hexane and CH2Cl2 solution at −10 °C. The
crystal structure showed CH2Cl2 of solvation. The identity of
the heterometallic boride cluster 8 was not easily determined
purely on the basis of spectroscopic data, but resolved properly
with the support of crystal structure determination. The
molecular structure of the boride cluster is shown in Figure
7. The central core of 8 can be regarded as a tetracapped
trigonal prism. As shown in Figure 7b, a group of five cobalt
and one molybdenum atom forms an idealized trigonal prism
with an encapsulated boron atom (B1). Three rectangular faces
of the trigonal prism are in turn capped by two boron atoms
(B2, B3) and one cobalt atom (Co4), and one of the triangular
faces (Mo2−Co5−Co6) is capped by one Te atom. The
resulting trigonal face (Co1−B3−Co6) is further capped by
one Mo atom; thus molecule 8 can be considered a
pentacapped trigonal prism containing 118 cluster valence
electrons. Like the M4 butterfly, the M5 square-pyramid, and
the M6 octahedron, the trigonal prism framework is not
unusual in boride chemistry and was found previously in
CONCLUSION
■
We report herein the synthesis and structural characterization
of two novel dimolybdatellurabone clusters (2 and 3) and
systematic study of the reactivity of one of the isomers (2) with
metal carbonyl compounds. Dimolybdaoxatelluraborane 4 has
its tellurium and oxygen atom in the open-face position of
cluster connectivity three, and this is unprecedented.
Compound 5 is a novel example of a homometallic cuboidal
8327
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Inorganic Chemistry
Article
Table 2. Crystallographic Data and Structure Refinement Information for Compounds 2 and 4−8
2
4
5
6
7
8
empirical
formula
C20H35B4TeClMo2
C22H38B4ClOMo2Te C40H67B7Mo4
C25H34B4FeTeO5Mo2
C29H32B3TeO9Mo2Co3
C34H32B3TeO14Mo2Co6
fw
671.63
orthorhombic
Pbca
714.96
1007.37
triclinic
831.305
monoclinic
P2(1)/n
8.5599(5)
15.6973(10)
20.8469(11)
90.00
1050.28
triclinic
1368.024
monoclinic
P2(1)/n
11.0344(4)
17.3088(7)
23.3171(9)
90.00
cryst syst
space group
a (Å)
orthorhombic
Pbca
P1
̅
P1
̅
14.900(3)
21.774(4)
16.047(3)
90.00
15.2196(4)
16.0129(3)
22.6286(5)
90.00
11.2490(7)
11.9722(8)
18.7227(12)
77.172(3)
86.647(3)
87.738(3)
2455.1(3)
2
10.3465(7)
11.6799(7)
16.5804(10)
74.313(3)
76.110(3)
63.926(3)
1715.62(19)
2
b (Å)
c (Å)
α (deg)
β (deg)
γ (deg)
V (Å3)
Z
90.00
90.00
90.292
91.414(2)
90.00
90.00
90.00
90.00
5206.2(18)
8
5514.8(2)
8
2801.1(3)
4
4452.0(3)
4
Dcalc (g/cm3)
1.714
1.711
1.363
1.833
2.035
2.168
F(000)
μ (mm−1
2568
2808
1020
1496
1016
2804
)
1.619
2.056
1.022
2.436
3.014
3.547
cryst size (mm) 0.20 × 0.15 × 0.10
0.35 × 0.32 × 0.10
0.25 × 0.22 ×
0.10
0.15 × 0.10 × 0.08
0.20 × 0.18 × 0.10
0.42 × 0.35 × 0.20
θ range (deg)
2.6−24.50
2.8−26.5
2.05−28.05
2.35−28.30
2.40−30.12
2.51−34.73
no. of unique
reflns
collected
4580
4831
8424
6347
9662
13 016
goodness-of-fit
on F2
1.189
1.119
1.111
1.108
0.977
1.080
final R indices
[I > 2θ(I)]
R1 = 0.0448
R1 = 0.0405
R1 = 0.0624
R1 = 0.0688
R1 = 0.0456
R1 = 0.0366
wR2 = 0.1054
R1 = 0.0583
wR2 = 0.0983
R1 = 0.0636
wR2 = 0.1791
R1 = 0.0859
wR2 = 0.2021
R1 = 0.0837
wR2 = 0.1047
R1 = 0.0729
wR2 = 0.1295
R1 = 0.0453
R indices (all
data)
wR2 = 0.1100
wR2 = 0.1162
wR2 = 0.2077
wR2 = 0.2110
wR2 = 0.1179
wR2 = 0.1390
reduced pressure, and the residue was extracted into hexane. The
yellowish-green hexane extract was thermolyzed at 100 °C with an
excess of Te powder (0.683 g, 5.36 mmol) in toluene for 30 h. The
solvent was removed in vacuo, and the residue was extracted into
hexane. After removal of solvent from the filtrate, the residue was
subjected to chromatographic workup using silica gel TLC plates.
Elution with hexane/CH2Cl2 (95:05 v/v) yielded green 2 and 3 (2,
0.112 g, 25%; 3, 0.030 g 6%), brown 4 (0.023 g, 4%), and green 5
(0.038 g, 5%). Note that, along with the formation of 2−5, known
[(Cp*Mo)2B4Te2H4] (0.03 g, 6%) and [(Cp*Mo)2B5H9] (0.08 g,
24%) have also been isolated.
metallaborane cluster containing boron as a main group
element, whereas compound 6 is a heterometallic cuboidal
metallaborane cluster containing a Te atom as one of the
vertices. The clusters 7 and 8 are notable examples of
heterometallic boride clusters having one semi-interstitial
boron and one completely encapsulated boron atom,
respectively.
EXPERIMENTAL SECTION
■
General Procedures and Instrumentation. All manipulations
were conducted either under an atmosphere of dry argon inside a
glovebox or employing standard Schlenk techniques. Solvents were
distilled prior to use under argon. [Mo(CO)6], [PCl5], [Fe2(CO)9],
[Mn2(CO)10], [Co2(CO)8], and LiBH4 in THF (Aldrich) were used
as received. The [Cp*MoCl4] complex was prepared as described in
the literature.50 The external reference for the 11B NMR, [Bu4N-
(B3H8)], was synthesized with the literature method.51 Thin-layer
chromatography was carried on 250 mm diameter aluminum-
supported silica gel TLC plates (Merck). The NMR spectra were
recorded in a 400 MHz Bruker FT-NMR spectrometer. Residual
solvent protons were used as reference (δ, ppm, benzene, 7.16), while
a sealed tube containing [Bu4N(B3H8)] in benzene-d6 (δB, ppm,
−30.07) was used as an external reference for the 11B NMR. Infrared
spectra were recorded on a Nicolet iS10 FT spectrometer. The
absorption spectra were recorded with a JASCO V-650 UV−vis
spectrophotometer at 298 K. The CV measurements were performed
in a CH potentiostat model 668. Microanalyses for C, H, and N were
performed on a Perkin Elmer Instruments series II model 2400.
Synthesis of 2−5. In a flame-dried Schlenk tube, [Cp*MoCl4]
(0.5 g, 1.34 mmol) was suspended in toluene (20 mL) and cooled to
−78 °C. LiBH4·THF (4.02 mL, 8.05 mmol) was added slowly, and the
reaction mixture was warmed slowly over 20 min to room temperature
and left stirring for an additional hour. The solvent was removed under
Compound 2. 11B NMR (128 MHz, [D6]benzene, 22 °C): δ 95.5
(s, br, 1B), 73.2 (s, br, 1B), 40.7 (s, br, 1B), 26.7 (s, br, 1B). 1H NMR
(400 MHz, [D6]benzene, 22 °C): δ 8.33 (partially collapsed quartet
(pcq), 1BHt), 4.80 (pcq, 1BHt), 3.43 (pcq, 1BHt), 2.01 (s, 30H,
C5Me5), −6.60 (s, 2H, Mo−H−B). 13C NMR (100 MHz, [D6]-
benzene, 22 °C): δ 107.34 (C5(CH3)5), 12.76 (C5(CH3)5). IR
(hexane, cm−1): 2443w, 2407w cm−1 (B−Ht). Anal. Calcd (%) for
C20H35B4TeClMo2: C 35.66, H 5.24. Found: C 36.23, H 4.99.
Compound 3. 11B NMR (128 MHz, [D6]benzene, 22 °C): δ 98.8
(s, br, 1B), 71.8 (s, br, 1B), 51.6 (s, br, 1B), 24.7 (s, br, 1B). 1H NMR
(400 MHz, [D6]benzene, 22 °C): δ 8.25 (partially collapsed quartet
(pcq), 1BHt), 4.81 (pcq, 1BHt), 3.43 (pcq, 1BHt), 2.11 (s, 30H,
C5Me5), −6.27 (s, 2H, Mo−H−B). 13C NMR (100 MHz, [D6]-
benzene, 22 °C): δ 108.69 (C5(CH3)5), 13.63 (C5(CH3)5). IR
(hexane, cm−1): 2431w, 2417w cm−1 (B−Ht). Anal. calcd (%) for
C20H35B4TeClMo2: C 35.66, H 5.24. Found: C 36.13, H 5.06.
Compound 4. 11B NMR (128 MHz, [D6]benzene, 22 °C): δ 87.2
1
(s, br, 1B), 77.5 (s, br, 1B), 44.1 (s, br, 2B), −12.0 (s, br, 1B). H
NMR (400 MHz, [D6]benzene, 22 °C): δ 7.54 (partially collapsed
quartet (pcq), 1BHt), 2.80 (pcq, 1BHt), 2.43 (pcq, 1BHt), 2.18 (s,
30H, C5Me5), −6.70 (s, 4H, Mo−H−B). 13C NMR (100 MHz,
[D6]benzene, 22 °C): δ 105.7 (C5(CH3)5), 12.7 (C5(CH3)5). IR
(hexane, cm−1): 2413w, 2397w cm−1 (B−Ht).
8328
dx.doi.org/10.1021/ic3008602 | Inorg. Chem. 2012, 51, 8322−8330

Inorganic Chemistry
Article
Compound 5. 11B NMR (128 MHz, [D6]benzene, 22 °C): δ 131.5
ACKNOWLEDGMENTS
■
1
(s, br, 2B), 94.5 (s, br, 2B), 58.8 (s, br, 3B). H NMR (400 MHz,
Generous support of the Indo-French Centre for the
Promotion of Advanced Research (IFCPAR-CEFIPRA), No.
4405-1, New Delhi, is gratefully acknowledged. A.T. thanks the
Council of Scientific and Industrial Research (CSIR), India, for
a Senior Research Fellowship. We would like to thank Dr. Babu
Varghese for X-ray crystallography analysis and Dr. Ramanujam
Kothandaraman of Indian Institute of Technology, Madras, for
helpful discussions on electrochemistry.
[D6]benzene, 22 °C): δ 9.92 (partially collapsed quartet (pcq), 2BHt),
8.93 (pcq, 2BHt), 6.12 (pcq, 3BHt), 2.14 (s, 15H, C5Me5), 1.58 (s,
15H, C5Me5). 13C NMR (100 MHz, [D6]benzene, 22 °C): δ 105.8
(C5(CH3)5), 103.4(C5(CH3)5), 14.0 (C5(CH3)5), 12.6 (C5(CH3)5).
IR (hexane, cm−1): 2490w, 2471w cm−1 (B−Ht). Anal. Calcd (%) for
C20H35B4TeClMo2: C 47.64, H 6.65. Found: C 47.13, H 5.94.
Synthesis of 6. A solution of 2 (0.15 g, 0.23 mmol) in hexane (15
mL) was stirred at 70 °C in the presence of 3 equivalents of
[Fe2(CO)9] (0.25 g, 0.69 mmol) for 3 h. The solvent was removed in
vacuo, and the residue was extracted into hexane and passed through
Celite. The filtrate was concentrated and kept at −40 °C to remove
Fe3(CO)12. The mother liquor was concentrated, and the residue was
chromatographed on silica gel TLC plates. Elution with a hexane/
CH2Cl2 (8:2 v/v) mixture yielded yellow 6 (0.028 g, 15%).
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ASSOCIATED CONTENT
■
S
* Supporting Information
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Supplementary crystallographic data and X-ray crystallographic
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AUTHOR INFORMATION
■
Corresponding Author
*Phone: (+91) 44 2257 4230. Fax: (+91) 44 2257 4202. E-
mail: sghosh@iitm.ac.in.
Notes
The authors declare no competing financial interest.
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