Chemistry of molybdaboranes: Synthesis, structures, and characterization of a new class of open-cage dimolybdaheteroborane clusters

Article abstract of DOI:10.1021/ic100520k

Reaction of [CpMoCl₄], 1 (Cp* = η⁵-C ₅Me₅), with [LiBH₄ thf] in toluene at -70 °C, followed by pyrolysis with excess dichalcogenides RE-ER (R = Ph, CH ₂Ph, 2,6-(^tBu)₂-C₆H₂OH, (CH₃)₃C = ^tBu); E = S, Se) yielded a new class of hybrid clusters, 3-8: (3, [(CpMo)₂(μ-η¹-SPh) ₂(μ₃-S)(H₂BSPh)]; 4, [(CpMo) ₂B₅H₈(SPh)]; 5, [(CpMo)₂B ₅H₈(SePh)]; 6, [(CpMo)₂B₂S ₂H₂(μ-η¹-S)]; 7, [(CpMo) ₂B₂H₅(BSR)₂(μ-η¹- SR)], (R = 2,6-(^tBu)₂-C₆H₂OH); and 8, [(CpMo)₂B₂H₅(BSePh)₂(μ- η¹-SePh)]. Compounds 3-8 have been isolated in modest yields as green or brown crystalline solids. In parallel with 3-8, [(CpMo) ₂B₅H₉] was isolated as a major product in all cases. The isolation and structural characterization of compounds 3 and 6-8 provided the first direct evidence of the existence of [(CpMo)₂B ₄H₈], 2, an intermediate in the formation of [(CpMo) ₂B₅H₉]. These new compounds have been characterized in solution by mass spectrometry, ¹H, ¹¹B, ¹³C NMR, and IR spectroscopy, and elemental analysis. The structural types were unequivocally established by X-ray crystallographic analysis of compounds 3-8.

Full text of DOI:10.1021/ic100520k

Inorg. Chem. 2010, 49, 7741–7747 7741
DOI: 10.1021/ic100520k
Chemistry of Molybdaboranes: Synthesis, Structures, and Characterization
of a New Class of Open-Cage Dimolybdaheteroborane Clusters
Rajendra Singh Dhayal,† Kiran Kumar Varma Chakrahari,† Babu Varghese,‡ Shaikh M. Mobin,§ and Sundargopal Ghosh*,†
†Department of Chemistry, and ‡Sophisticated Analytical Instruments Facility, Indian Institute of Technology
§
Madras, Chennai 600036, India, and National Single Crystal X-ray Diffraction Facility, Indian Institute of
Technology Bombay, Mumbai 400 076, India
Received March 18, 2010
Reaction of [Cp*MoCl4], 1 (Cp*=η5-C5Me5), with [LiBH4 thf] in toluene at -70 °C, followed by pyrolysis with excess
3
dichalcogenides RE-ER (R=Ph, CH2Ph, 2,6-(tBu)2-C6H2OH, (CH3)3C=tBu); E=S, Se) yielded a new class of hybrid
clusters, 3-8: (3, [(Cp*Mo)2(μ-η1-SPh)2(μ3-S)(H2BSPh)]; 4, [(Cp*Mo)2B5H8(SPh)]; 5, [(Cp*Mo)2B5H8(SePh)]; 6,
[(Cp*Mo)2B2S2H2(μ-η1-S)]; 7, [(Cp*Mo)2B2H5(BSR)2(μ-η1-SR)], (R = 2,6-(tBu)2-C6H2OH); and 8, [(Cp*Mo)2B2-
H5(BSePh)2(μ-η1-SePh)]. Compounds 3-8 have been isolated in modest yields as green or brown crystalline solids.
In parallel with 3-8, [(Cp*Mo)2B5H9] was isolated as a major product in all cases. The isolation and structural
characterization of compounds 3 and 6-8 provided the first direct evidence of the existence of [(Cp*Mo)2B4H8], 2, an
intermediate in the formation of [(Cp*Mo)2B5H9]. These new compounds have been characterized in solution by mass
spectrometry, 1H, 11B, 13C NMR, and IR spectroscopy, and elemental analysis. The structural types were
unequivocally established by X-ray crystallographic analysis of compounds 3-8.
Introduction
accessible to any combinations of main-group elements that
have the same numbers of valence electrons, this chemistry is
dominated typically by carboranes and metallacarboranes.1-3
Metallaheteroborane compounds containing group 16 ele-
ments as cluster constituents4,5 have mostly been generated
from the reaction of metal centers with preformed polyhed-
ral heteroborane substrates.6,7 However, after it had been
An area of continuing importance in polyhedral metalla-
borane chemistry is the development of new, efficient methods
in which atom-insertion reactions leading to expanded-cage
clusters may be accomplished. Although the structural vari-
ety of polyhedral metallaborane chemistry is in theory also
*To whom correspondence should be addressed. E-mail: sghosh@iitm.ac.in.
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r
2010 American Chemical Society
Published on Web 08/06/2010
pubs.acs.org/IC

7742 Inorganic Chemistry, Vol. 49, No. 17, 2010
Dhayal et al.
Scheme 1
discovered that the reaction of [Cp*MoCl4] with [BH3 thf ]
and [PhSe-SePh], respectively. Descriptions of the characte-
rizations of 3-8 from mass spectrometric, NMR, and X-ray
diffraction studies follow.
3
led to the isolation of two eight-vertex oxamolybdaborane
clusters, [(Cp*Mo)2B5(μ3-OEt)H6R] (R = H and n-BuO),8
an investigation of metallaheteroboranes containing group
16 elements, other than oxygen, became of interest. As di-
molybdaheteroboranes are rare, and structurally characte-
rized examples even more so, we have begun to investigate
the use of different chalcogen sources which might result in
the generation of new types of cluster systems. In this paper,
we present a set of open-cage dimolybdaheteroborane com-
pounds obtained from the pyrolysis of dichalcogenides and
an in situ generated intermediate, produced from the reaction
[(Cp*Mo)2(μ-η1-SPh)2(μ3-S)(H2BSPh)], 3. Mass spectral
data of 3suggest a molecular formula of C38H47BS4Mo2, and
the 11B NMR spectrum indicates the presence of a single
resonances at δ=-17.7 ppm. The 11B chemical shift is con-
sistent with hydrogen-bridged rather than direct Mo-B link-
ages. The 1H NMR spectrum reveals a single Cp* resonance
(relative intensity 30) and one Mo-H-B resonance at δ =
-11.50 ppm (intensity 2). The 13C NMR spectrum also re-
veals the presence of one set of Cp* resonances. The IR spec-
trum of 3 shows a band at 459 cm-1 in a region characteristic
of the Mo-S stretching frequency. Single crystal X-ray dif-
fraction structure of 3, shown in Figure 1, confirms the struc-
tural inferences made on the basis of spectroscopic results.
Compound 3 may be described as an edge-fused molyb-
dathiaborane cluster, giving each Mo center a formal oxi-
dation state of (IV). The molecular structure shows two
Cp*Mo units linked by a pair of bridging SPh ligands and
in a side-on fashion by a SPhH2BS unit. The interesting
feature of the structure is the nearly square planar arran-
gement of one boron and three sulfur atoms, perpendi-
cular to the Mo1-Mo2 bond and parallel to the two Cp*
planes. Two types of sulfur ligands are found: (i) μ2-S, (S2
and S3) and (ii) μ3-S (S4). The μ3-S ligand is connected
to the boron atom and the observed B(1)-S(4) bond
between 1 and [LiBH4 thf].
3
Results and Discussion
Although compounds 3-8 are produced in a mixture
along with [(Cp*Mo)2B5H9]9 (Scheme 1), these compounds
can be separated by preparative thin layer chromatography
(TLC) which allowed spectroscopic and structural characte-
rization of pure materials. These reactions utilized dichalco-
genide reagents as a chalcogen source, and it is noteworthy
that in half of the cases the dimetallaheteroborane com-
pounds retain the same number of boron framework atoms.
Further, the difference in reactivity between the dichalco-
genide reagents is also reflected in the isolation of edge-fused
cluster 3 in the PhS-SPh case, in contrast to the clean
synthesis of 7 and 8 by the use of [(2,6-(tBu)2-C6H2OH)2S2]
˚
length of 1.852(5) A is comparable to that of other sulfur-
containing metallathiaborane compounds.6 On the other
hand, the μ2-S ligands, bridged between two molybdenum
centers, are symmetric with an average metal-sulfur
(8) Sahoo, S.; Dhayal, R. S.; Varghese, B.; Ghosh, S. Organometallic
2009, 28, 1586.
(9) Kim, D. Y.; Girolami, G. S. J. Am. Chem. Soc. 2006, 128, 10969.

Article
Inorganic Chemistry, Vol. 49, No. 17, 2010 7743
Figure 1. Molecular structure of [(Cp*Mo)2(μ3-S)(μ-η1-SPh)2(H2BSPh)],
3 (Cp* ligands are not shown for clarity). Selected interatomic distances
˚
(A) and angles (deg): Mo(1)-Mo(2) 2.6962(5), Mo(2)-B(1) 2.443(5),
Mo(1)-B(1) 2.428(4), Mo(2)-S(4) 2.4539(11), Mo(1)-S(4) 2.4424(10),
Mo(1)-S(3) 2.4496(10), Mo(2)-S(3) 2.4635(10), Mo(2)-S(2) 2.4492(11),
Mo(1)-S(2) 2.4727(12), S(6)-B(1) 1.874(5), S(4)-B(1) 1.852(5); Mo(1)-
S(4)-B(1) 67.23(14), Mo(1)-S(3)-Mo(2) 66.57(3), S(3)-Mo(2)-S(4)
112.02(4).
˚
bond length of 2.458 A and an Mo-S-Mo angle of 66.5°,
which is comparable to the symmetric coordination of the
disulfide bridges in [(Cp* Mo)2(μ-S2)(μ-S)2]. The Mo-S
bond lengths of S(2), S(3), and S(4) are slightly longer
than those in related Mo (IV) complexes.10 Bridging of
the dimetal unit by B(1) and S(4) is analogous to the inter-
action of the B2H62- ligand with a dinuclear group 5 unit
in [(C5Me5M)2(B2H6)2] (M=V, Nb, or Ta), [(p-tol)NCH-
N(p-tol)4Ta2B2H6], and [(C5Me5TaBr)2B2H6].11-16 Simi-
Figure 2. Molecular structure of compounds 4and 5. (Cp* ligands are not
˚
shown for clarity). Selected interatomic distances (A) and angles (deg): 4:
˚
larities in the Mo-Mo bond length (2.696(5) A) and
Mo(1)-Mo(2) 2.8106(10), Mo(1)-B(1) 2.324(4), Mo(1)-B(2) 2.225(4),
Mo(1)-B(3) 2.196(4), Mo(1)-B(4) 2.218(4), Mo(1)-B(5), 2.318(4),
Mo(2)-B(1) 2.322(4), Mo(2)-B(2) 2.217(5), Mo(2)-B(3) 2.182(4), Mo(2)-
B(4) 2.213(4), Mo(2)-B(5) 2.324(4), B(1)-B(2) 1.747(5), B(2)-B(3)
1.715(6), B(3)-B(4) 1.711(8), B(5)-S(1) 1.887(4); B(2)-B(1)-Mo(1)
64.45(17). 5: Mo(1)-Mo(2) 2.8165(3), Mo(1)-B(1) 2.326(3), Mo(1)-B(2)
2.220(3), Mo(1)-B(3) 2.197(3), Mo(1)-B(4) 2.218(3), Mo(1)-B(5),
2.314(3), Mo(2)-B(1) 2.330(3), Mo(2)-B(2) 2.219(3), Mo(2)-B(3)
2.196(3), Mo(2)-B(5) 2.316(3), B(1)-B(2) 1.753(4), B(4)-B(5) 1.739(5),
B(5)-Se(1) 2.025(3); B(4)-B(3)-B(2) 121.3(2).
Mo(μ-SPh)Mo angles (66.57, 66.43°) of the Mo(μ-SPh)2-
i
Mo unit with those reported for [(C5H4 Pr)Mo(μ-Cl)2]2
(2.607(1) A and 63.17, 63.34°, respectively)17 are consis-
˚
tent with a single bond connecting two Mo(IV) centers.
[(Cp*Mo)2B5H8(EPh)], (4: E = S; 5: E = Se). Com-
pounds 4 and 5 have been isolated following preparative
TLC in yields of 9 and 7%, respectively, and characte-
rized by comparison of their spectroscopic data with other
similar B-H substituted compounds.18 The mass spectro-
metry measurements of 4 and 5 gave molecular ions
corresponding to C26H43B5SMo2 and C26H43B5SeMo2,
respectively. The 11B NMR spectrum of 5 indicates the pre-
sence of five boron environments in a ratio of 1:1:1:1:1.
The resonances at δ=34.2 and 30.3 ppm for 4 and 5, res-
pectively, showed no coupling, thus confirming the B-SPh
and B-SePh environments. Furthermore, the 1H{11B} NMR
spectra of 4 and 5 showed one type of Cp* protons, four
types of B-H protons, and two types of Mo-H-B protons.
Single crystals suitable for X-ray diffraction analysis of
4 and 5 were obtained from a hexane solution at -4 °C,
thus allowing structural characterization and confirma-
tion of the structural inferences made on the basis of
spectroscopic results. The solid state structure of 4 and 5,
shown in Figure 2, is similar to that of [(Cp*M)2B5H9],
(M=Cr, Mo, W)9,19 and [(Cp*ReH)2B5Cl5],20 and is best
described as a bicapped closo trigonal bipyramid. The obser-
˚
ved Se-B bond length of 2.025(4) A in 5 is significantly
longer than those observed in [NEt4]2[Se3B11H9], a disub-
stituted derivative of [closo-B11H11]2- 21
.
(10) Brunner, H.; Meier, W.; Wachter, J.; Guggolz, E.; Zahn, T.; Ziegler,
M. L. Organometallics 1982, 1, 1107.
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Nuber, B.; Ziegler, M. L. J. Organomet. Chem. 1992, 436, 313.
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2586. (b) Aldridge, S.; Fehlner, T. P.; Shang, M. J. Am. Chem. Soc. 1997, 119, 2339.
(16) Huheey, J. E. Inorganic Chemistry; Principles of structure and
Rectivity; Harper and Row: New York, 1972.
Existence of compounds 4 and 5 permits a structural
comparison with [(Cp*Mo)2B5H9] and its derivatives,9,18,22
and the important geometrical parameters and 11B chemical
(19) (a) Bullick, H. J.; Grebenik, P. D.; Green, M. L. H.; Hughes,
A. K.; Leach, J. B.; McGowan, P. C. J. Chem. Soc., Dalton Trans. 1995,
67. (b) Aldridge, S.; Hashimoto, H.; Kawamura, K.; Shang, M.; Fehlner, T. P.
Inorg. Chem. 1998, 37, 928. (c) Weller, A. S.; Shang, M.; Fehlner, T. P.
Organometallics 1999, 18, 53.
(20) Guennic, B. Le.; Jiao, H.; Kahlal, S.; Saillard, J.-Y.; Halet, J.-F.;
Ghosh, S.; Shang, M.; Beatty, A. M.; Rheingold, A. L.; Fehlner, T. P. J. Am.
Chem. Soc. 2004, 126, 3203.
(21) Freisen, G. D.; Little, J. L.; Huffman, J. C.; Todd, L. J. Inorg. Chem.
1979, 18, 755.
(22) Dhayal, R. S.; Chakrahari, K. K. V.; Ramkumar, V.; Ghosh, S.
J. Cluster Sci. 2009, 20, 565.
(17) Grebenik, P. D.; Green, M. L. H.; Izquierdo, A.; Mtetwa, V. S. B.;
Prout, K. J. Chem. Soc., Chem. Commun. 1987, 9.
(18) (a) Dhayal, R. S.; Sahoo, S.; Ramkumar, V.; Ghosh, S. J. Organo-
met. Chem. 2009, 694, 237. (b) Sahoo, S.; Reddy, K. H. K.; Dhayal, R. S.; Mobin,
S. M.; Ramkumar, V.; Jemmis, E. D.; Ghosh, S. Inorg. Chem. 2009, 48, 6509.

7744 Inorganic Chemistry, Vol. 49, No. 17, 2010
Dhayal et al.
Table 1. Selected Structural Parameters and 11B Chemical Shifts of 4, 5, and Related Species
a
˚
˚
compounds
d[M-M] [A]
avg.[Mo-B] [A]
dihedral angle [deg]b
11B NMR (ppm)
62.9, 25.8
66.4, 57.2, 55.8, 41.5, 22.6
65.7, 57.8, 37.9, 22.6
50.6, 45.1, 44.1, 42.6, 24.3
62.1, 61.5, 34.2, 24.2
64.1, 62.4, 59.8, 30.7, 24.7
9
(Cp*Mo)2B5H9
(Cp*Mo)2B5H8(n-Bu)22
2.80
2.79
2.81
2.82
2.81
2.81
2.31
2.33
2.30
2.29
2.32
2.31
121.5
122.9
122.3
122.7
121.8
121.9
(Cp*Mo)2B5H8Cl18a
(Cp*W)2B5H8Cl18b
4
5
a The average Mo-B distance found in cage. b Dihedral angles of the hydrogen bridged butterfly face.
[(CpCo)2B2S2H2] and other analogous systems23 (Table 2),
the metal-metal distances in 3 and 6-8 are much shor-
ter in comparison with those of their nonchalcogenide
analogues.24-26
[(Cp*Mo)2B2H5(BER)2(μ-η1-ER), (7: E = S, R = 2,6-
(tBu)2-C6H2OH), 8: E = Se, R = Ph). In contrast to the
results obtained using diphenyldisulfide, pyrolysis with
(2,6-(tBu)2-C6H2OH)2S2 and PhSe-SePh generated 7 and 8,
respectively. The molecular structures of 7 and 8, shown
in Figure 4, clearly show that both the compounds retain
the same number of boron atoms as the intermediate
during the course of reaction with dichalcogenides. These
clusters are closely related to 6, in that the μ2-S and μ3-S
ligands are replaced by -E(R) and -B(HER) respectively,
resulting in the same number of cluster framework elec-
trons. The occurrence of two more bridged sulfur atoms
would be expected to contract the Mo-Mo bond in 6.
Figure 3. Molecular structure of [(Cp*Mo)2B2S2H2(μ-η1-S)], 6 (Cp*
˚
ligands are not shown for clarity). Selected interatomic distances (A)
and angles (deg): Mo(1)-Mo(2) 2.6359(16), Mo(1)-B(1) 2.372(12),
Mo(2)-B(1) 2.341(13), Mo(2)-S(2) 2.389(3), B(1)-B(2) 1.69(4), B(1)-
S(2) 1.829(19); Mo(1)-S(1)-Mo(2) 68.11(13), Mo(1)-S(2)-Mo(2)
67.07(9), Mo(1)-S(2)-B(1) 67.1(4), Mo(2)-S(2)-B(1) 68.7(5).
shifts are listed in Table 1. Although the Mo1-Mo2 bond
lengths in 4 and 5 are longer than those observed in
[(Cp*Mo)2B5H9], the dihedral angles of the hydrogen-
bridged butterfly face are similar. The observed discre-
pancies in structural parameters and 11B chemical shifts
may be due to the perturbation of the electronic environ-
ment of the boron atoms by ligands.
˚
Indeed, the Mo-Mo bond length of 2.6375(12) A in 6, is
˚
about 0.07 A shorter than the Mo-Mo bond in 7 or 8. In
6, the Mo-Mo bond is bridged by three sulfido ligands,
whereas there is only one such bridge in 7 and 8. Com-
pounds 6, 7, and 8 can be described as (Cp*Mo)2(BH)2
dimetallatetrahedranes capped on each Mo2B face with a
μ3-S group in 6 and -B(HER) group in 7 and 8.
[(Cp*Mo)2B2S2H2(μ-η1-S)], 6. Compound 6 was iso-
lated as a green, relatively air-stable solid. The mass spectro-
metric data suggest a molecular formula of C20H32Mo2B2S3
and the 11B NMR data (single resonance at δ=18.8 ppm)
suggests a structure, if static, of higher symmetry. Con-
sistent with this observation, 6 indeed shows one kind of
Cp* signal. Molecular structure of 6, shown in Figure 3,
reveals that there are two independent molecules cocrys-
tallized in a 1:1 ratio, one is the known [(Cp*Mo)2(μ-S2)-
(μ-S)2],10 and the other is 6. It is a notable example of
organomolybdenum chalcogenide clusters in which the
[Mo2B2S2] atoms define a bicapped tetrahedral framework,
with one μ-η1-S ligand bridged between two molybdenum
centers.
Consistent with the X-ray results, the 11B NMR spectra
of both 7 and 8 rationalize the presence of four boron reso-
nances in a ratio of 1:1:2. The resonances appearing in the
high field regions showed no coupling in the 11B NMR
spectrum confirming the B-SePh or B-S(2,6-(tBu)2-
1
C6H2OH) environments. The H{11B} NMR spectrum
of compound 8 reveals two types of B-H protons and
two kinds of Mo-H-B protons each at δ = -6.58 and
-8.58 ppm with intensity ratio 2:1. Furthermore, 1H and
13C NMR spectra of compound 7 and 8 confirmed the
presence of two kinds of Cp* ligands in a 1:1 ratio. The
Cp* ligands in both 7 and 8 are asymmetrically bonded to
the molybdenum centers and are tilted from the mean
plane of the four boron and one sulfur atom such that the
dihedral angles between them are 7.9°, 8.7° for 7 and 9.1°,
12.8° for 8. The 1H{11B}-11B{1H} HETCOR and variable
temperature 1H{11B} NMR also support the structures of
7 and 8 as shown in Scheme 1. At room temperature the
1H{11B} NMR of compound 8 shows two resonances at
δ = -6.58 and δ = -8.58 ppm. As the temperature is
A similar structural interpretation has also been sug-
gested by Sneddon for [(CpCo)2B2S2H2] (Cp=C5H5).6a,b
The cage has C2v symmetry with the Mo atoms located in
the apical positions and the boron and sulfur atoms in
equatorial positions of the parent pentagonal bipyrami-
dal polyhedron. In the molecular structure of 6 the two
Mo atoms and the two sulfur atoms are situated on the
open face of the cluster and allow two sulfur atoms to
adopt lower coordinate positions in the polyhedron. Con-
comitantly a contraction of the metal-metal bond to
˚
(23) (a) Volkov, O.; Rath, N. P.; Barton, L. Organometallics 2002, 21,
5505. (b) Kang, O. S.; Carroll, P. J.; Sneddon, L. G. Inorg. Chem. 1989, 28, 961.
(c) Zimmerman, G. J.; Sneddon, L. G. J. Am. Chem. Soc. 1981, 103, 1102.
(24) Ho, J.; Deck, K. J.; Nishihara, Y.; Shang, M.; Fehlner, T. P. J. Am.
Chem. Soc. 1995, 117, 10292.
(25) Ghosh, S.; Shang, M.; Fehlner, T. P. J. Organomet. Chem. 2000,
614-615, 92.
(26) Bose, S. K.; Geetharani, K.; Varghese, B.; Mobin, S. M.; Ghosh, S.
Chem.;Eur. J. 2008, 14, 9058.
2.6359(16) A was observed. This short distance is within
the range typical for ModMo bonds, as in binuclear alkoxy-
bridge Mo(IV) complexes,10 and comparable to that of oxa-
molybdaboranes.8 The metal-metal interaction seems to
be affected by the formal oxidation state of metal, as well
as by the nature of the sulfur bridges. Although the avg.
B-B and B-S distances in 6 are comparable to those of

Article
Inorganic Chemistry, Vol. 49, No. 17, 2010 7745
Table 2. Selected Structural Parameters and Dihedral Angles of 3, 6-8, and Other Related Compounds
b
sepa
d[M-M] [A]
avg. d[M-B] [A]
d[B-B] [A]
dihedral angle [deg]c
11B NMR [ppm]
˚
˚
˚
compound
[(Cp*Cr)2B4H8]24
5
6
5
8
6
6
6
6
2.87
2.81
2.89
3.06
2.69
2.63
2.69
2.70
2.06
2.17
2.38
2.16
2.43
2.48
2.29
2.27
3.02
3.20
3.68
3.11d
2.79e
3.93g
3.72
3.69
147.6
163.4
167.7
34.3, 126.5
1.30, 68.7
0.3, 16.6
17.6
-17.7
18.8
[(Cp*ReH2)2B4H4]25
26
[(Cp*Ta)2B4H10
]
[(CpCo)2 B2S2H2]
3
6
7
8
94.3f
161.9
164.2
168.0
84.4, 49.8, 30.45
78.8, 54.0, 19.2
a sep = skeletal electron pair. b Distance between two capped boron atoms (B1 and B4) in the open face of bicapped tetrahedral geometry. c Dihedral
angle of M2B2 tetrahedral geometry. d Distance between two capped sulfur atoms (S2 and S3) in the open face. e Distance between S2 and S3. f Dihedral
angle between Mo1S2Mo2 and Mo1S3Mo2 planes. g Distance between two capped sulfur atoms in the open face of bicapped tetrahedron.
1
1
low temperature H{11B} NMR and H{11B}-11B{1H}
HETCOR spectra (Figure 5).
As shown in Table 2, the structural parameters and
chemical shift values of 6-8 and their Cr, Re and Ta
analogues exhibit several contrasting features. The M-M
distances in 6-8 are shorter, but the average M-B
distances are markedly longer. The distance between the
two capping boron atoms differ to some extent with res-
pect to the size of the open face of the cluster, which is
more open in the Mo system. The 11B resonances, on going
from the lighter to the heavier metal atom, appeared at
low field. The 77Se NMR of 8 shows two resonances, one
at δ=461 ppm for the bridged μ2-SePh and another more
upfield one at δ = 327 ppm for the cage B-SePh group.
The average B-Se bond length in 8 is longer comparable
to that observed in [(Cp*Mo)2B4H4Se2].27 However, they
are significantly shorter when compared to those obser-
ved in other selenaborane clusters.4b,e,5d,5e This may be
due to the tendency of the boron and selenium atoms to
form polarized bonds that have a localized two-center
character resulting in the observed distances.28
Conclusion
These results and others6,29 demonstrate that many tran-
sient metallaborane intermediates can be stabilized by brid-
ged chalogenide ligands. Thus, the isolation and structural
characterization of compounds 6-8 provided the first direct
evidence of the existence of intermediate 2, a proposed
unstable intermediate in the formation of [(Cp*Mo)2B5H9].
Although analogous insertions into transition metal-boron
bonds have also been claimed, to our knowledge compounds
3 and 6-8 are the first structurally characterized products of
such an insertion. It should also be noted that some of the
byproducts generated by these reactions employing dichalco-
genide as reactant were [(Cp*MoS)2(μ-η1-S)2], [(Cp*MoO)2-
(μ-S2)], [(Cp*Mo)2(μ-S2)(μ-η1-SPh)2], or [(Cp*MoSe)2(μ-
η1-Se)2].30 This observation suggests that metallaborane
reactions may be of use in the future not only for the synthesis
Figure 4. Molecular structure of 7 and 8 (Mo-Mo bond and aryl groups
are not shown for clarity). Selected interatomic distances (A) and angles
˚
(deg): 7: Mo(1)-Mo(2) 2.6941(6), Mo(1)-B(1) 2.291(6), Mo(1)-B(2)
2.302(6), Mo(1)-B(3) 2.285(6), Mo(1)-B(4) 2.381(6), Mo(2)-B(1)
2.232(6), Mo(2)-B(2) 2.246(6), Mo(2)-B(3) 2.299(6), Mo(2)-B(4)
2.345(6), Mo(1)-S(3) 2.5234(14), Mo(2)-S(3) 2.4676(13), B(1)-B(2)
1.752(9), B(2)-B(3) 1.626(9), B(1)-S(1) 1.835(6), B(4)-S(2) 1.888(6);
B(3)-Mo(1)-B(1) 82.0(2), Mo(2)-S(3)-Mo(1) 65.33(4). 8: Mo(1)-
Mo(2) 2.7081(3), Mo(1)-B(1) 2.339(3), Mo(1)-B(2) 2.244(3), Mo(1)-
B(3) 2.228(4), Mo(1)-B(4) 2.195(3), Mo(2)-B(1) 2.366(3), Mo(2)-B(2)
2.259(3), Mo(2)-B(4) 2.300(3), Mo(1)-Se(1) 2.6065(4), Mo(2)-Se(1)
2.6677(4), B(2)-B(3) 1.647(5), B(4)-Se(3) 1.988(3), B(1)-Se(2)
2.005(3); B(4)-Mo(1)-B(3) 46.80(12), Mo(1)-Se(1)-Mo(2) 61.77(10).
lowered, the resonance at δ = -6.58 ppm (two Mo-
H-B) splits into two distinct peaks at δ = -6.20, and
-6.75 ppm in a 1:1 ratio (Figure 5). Thus, there are three
different types of bridging protons which are connected to
distinct boron atoms. Two of the Mo-H-B protons
bridge the open face boron atoms, while the other one is
bonded to the tetrahedral core (Mo2B2). Not all of the
bridging hydrogen atoms of molecules 7 and 8 have
been positioned by X-ray diffraction studies; however,
their connectivity with regard to the metal and boron
atoms (Scheme 1), has been confidently determined by
(27) Sahoo., S.; Mobin, S. M.; Ghosh, S. J. Organomet. Chem. 2010, 695,
945.
(28) (a) Bicerano, J.; Lipscomb, W. N. Inorg. Chem. 1980, 19, 1825.
(b) Dolansky, J.; Hermanek, S.; Zahradnik, R. Collect. Czech. Chem.
Commun. 1981, 46, 2479.
(29) Bose, S. K.; Geetharani, K.; Varghese, B.; Ghosh, S. Inorg. Chem.
2010, 49, 2881.
(30) (a) Holm, R. H. Chem. Rev. 1987, 87, 1401. (b) Endrich, K.; Guggolz,
E.; Serhadle, O.; Ziegler, M. L.; Korswagen, R. P. J. Organomet. Chem. 1988,
349, 323. (c) Kawaguchi, H.; Yamada, K.; Lang, J.-P.; Tatsumi, K. J. Am. Chem.
Soc. 1997, 119, 10346.

7746 Inorganic Chemistry, Vol. 49, No. 17, 2010
Dhayal et al.
Figure 5. (a) 1H{11B}-11B{1H} HETCOR and (b) Variable-temperature 1H{11B} NMR of 8 in [D8]toluene.
of new types of metallaheteroborane clusters but also for the
preparation of other transition-metal main-group hetero-
nuclear clusters.31-36 The factors that lead to such insertion
rather than a direct π-coordination to dichalcogen bonds are
not yet understood. Further, systematic reactivity studies
combined with theoretical treatments are required to fully
understand these systems. Such work is in progress.
on 3 cm of silica gel in a 2.5 cm diameter column. TLC was
carried on 250 mm diameter aluminum-supported silica gel TLC
plates (MERCK TLC Plates). NMR spectra were recorded on
400 and 500 MHz Bruker FT-NMR spectrometers. Residual
solvent protons were used as reference (δ, ppm, 22 °C, CDCl3,
7.26), while a sealed tube containing [Bu4N(B3H8)] in C6D6 (δB,
ppm, -30.07) was used as an external reference for 11B NMR
spectra. Microanalyses for C, H, and N were performed on
Perkin Elimer Instruments series II model 2400. Infrared spectra
were obtained on a Nicolet 6700 FT-IR spectrometer. Mass
spectra were obtained on a Jeol SX 102/Da-600 mass spectrometer/
Experimental Section
General Procedures and Instrumentation. All the operations
were conducted under an Ar/N2 atmosphere using standard
Schlenk techniques. Solvents were distilled prior to use under
Argon. MeI was freshly distilled prior to use. All other reagents
Cp*H, Mo(CO)6, BuLi, LiBH4 in tetrahydrofuran (THF, Aldrich)
were used as received. [Cp*MoCl4]37 and the external reference,
[Bu4N(B3H8)],38 for the 11B NMR were synthesized by the litera-
ture method. The dichalcogenide ligands Ph2S2,39 (PhCH2)2S2,39
Ph2Se2,40 and [(2,6-(tBu)2-C6H2OH)2S2]41 were prepared as
described in the literature. Chromatography was carried out
˚
Data System using Argon/Xenon (6 kV, 10 mA) as the FAB gas.
Synthesis of 3-8. In a Flame-dried Schlenk tube, [Cp*MoCl4],
(0.5 g, 1.34 mmol) was suspended in toluene (20 mL) and cooled to
-70 °C. LiBH4 thf (4.02 mL, 8.05 mmol) was added via syringe,
3
and the reaction mixture was warmed slowly over 20 min to room
temperature and left stirring for an additional hour. The solvent
was evaporated under vacuum, and the residue extracted into
hexane. The yellowish green hexane extract was thermolyzed at
90 °C with excess of diphenyldisulfide PhS-SPh (1.17 g, 5.36 mmol)
for 20 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 work up using silica
gel TLC plates. Elution with hexane/CH2Cl2 (95:05 v/v) yielded
brown [(Cp*Mo)2(μ-η1-SPh)2(μ3-S)(H2BSPh)], 3 (0.06 g, 5%) and
[(Cp*Mo)2B5H8(SPh)], 4 (0.08 g, 9%). Under the same reaction
conditions, PhSe-SePh (1.67 g, 5.36 mmol) yielded [(Cp*Mo)2-
B5H8(SePh)], 5 (0.07 g, 7%) and [(Cp*Mo)2B4H5(SePh)2(μ-η1-
SePh)], 8 (0.18 g, 14%). Further, in a similar fashion, other di-
chalogenide ligands, for example, PhCH2S-SCH2Ph (1.32 g, 5.36
mmol) and (2,6-(tBu)2-C6H2OH)2S2 (2.54 g, 5.36 mmol) provided
[(Cp*Mo)2B2S2H2(μ-η1-S)], 6 (0.11 g, 14%) and [(Cp*Mo)2B2H5-
(BSR)2(μ-η1-SR), 7 (0.18 g, 11%, R = 2,6-(tBu)2-C6H2OH),
respectively.
(31) Roberts, D. A.; Geoffroy, G. L. In Comprehensive Organometallic
Chemistry; Wilkinson, G., Stone, F. G. A., Abel, E. W., Eds.; Pergamon: Oxford,
England, 1982; Vol. 6, p 763.
(32) Adams, R. D. In Comprehensive Organometallic Chemistry II; Abel,
E. W., Stone, F. G. A., Wilkinson, G., Eds.; Pergamon: Oxford, England, 1995;
Vol. 10, p 1.
(33) (a) Vahrenkamp, H. Comments Inorg. Chem. 1985, 4, 253. (b) Jensen,
S. D.; Robinson, B. H.; Simpson, J. Organometallics 1986, 5, 1690. (c) Adams,
R. D.; Kwon, O.-S.; Smith, M. D. Organometallics 2002, 21, 1960. (d) Adams,
R. D.; Kwon, O.-S.; Smith, M. D. Inorg. Chem. 2002, 41, 1658.
(34) (a) Cowie, M.; Dekock, R. L.; Wagenmaker, T. R.; Seyferth, D.;
Henderson, R. S.; Gallagher, M. K. Organometallics 1989, 8, 119. (b) Song,
L.-C.; Shen, J.-Y.; Hu, Q.-M.; Wang, R.-J.; Wang, H.-G. Organometallics 1993,
12, 408. (c) Song, L.-C.; Shen, J.-Y.; Hu, Q,-M.; Huang, X.-Y. Organometallics
1995, 14, 98.
Note that compound [(Cp*Mo)2B5H9] was isolated as a
major product in all cases and characterized by comparison of
its spectroscopic data with those reported earlier.9
(35) (a) Rauchfuss, T. B.; Rodgers, D. P. S.; Wilson, S. R. J. Am. Chem.
Soc. 1986, 108, 3114. (b) Schwarz, D. E.; Rauchfuss, T. B.; Wilson, S. R. Inorg.
Chem. 2003, 42, 2410.
(36) (a) Braunstein, P.; Rose’, J. In Metal Clusters in Chemistry; Braunstein, P.,
Oro, L. A., Raithby, P. R., Eds.; Wiley-VCH: Weinheim, Germany, 1999; Vol. 2, p 616.
(b) Braunstein, P.; Rose', J. In Catalysis by Di- and Polynuclear Metal Cluster
Complexes; Adams, R. D., Cotton, F. A., Eds.; Wiley-VCH: New York, 1998; p 443.
(37) Green, M. L. H.; Hubert, J. D.; Mountford, P. J. Chem. Soc., Dalton
Trans. 1990, 3793.
(38) Ryschkewitsch, G. E.; Nainan, K. C. Inorg. Synth. 1975, 15, 113.
(39) Yiannios, C. N.; Karabinos, J. V. J. Org. Chem. 1963, 28, 3246.
(40) Bhasin, K. K.; Gupta, V.; Sharma, R. P. Indian J. Chem. 1919, 30A,
632.
(41) Suzuki, R.; Matsumoto, K.; Kurata, H.; Oda, M. Chem. Commun.
2000, 1357.
3: MS (ESIþ) m/z: 834 (isotopic pattern for 2Mo, 1B, 4S
atoms); 11B NMR: δ=-17.7 (t, JB-H=139 Hz, 1B); 1H NMR:
δ = 7.78 - 6.86 (m, 15H, 3Ph), 1.74 (s, 30H, 2Cp*), -11.50
(partially collapsed quartet (pcq), 2Mo-H-B); 13C NMR: δ =
134.04, 133.54, 128.56, 128.02, 125.91 (s, C6H5), 103.29 (s, η5-
C5Me5), 12.29 (s, η5-C5Me5); IR (hexane, cm-1): 459 m
(Mo-S-Mo); elemental analysis calcd (%) for C38H47BMo2S4:
C 54.68, H 5.68; found: C 55.33, H 5.81.
4: MS (FAB) Pþ(max): m/z (%) 634 (isotopic pattern for 2Mo,
5B, 1S atoms); 11BNMR:δ=62.1 (br, 1B), 61.5 (d, JB-H=133 Hz,

Article
Inorganic Chemistry, Vol. 49, No. 17, 2010 7747
Table 3. Crystallographic Data for Compounds 3-8
3
4
5
6
7
8
a
formula
C38H47BS4Mo2.
0.5CH2Cl2
877.15
C26H43B5SMo2
C26H43B5SeMo2
C40 H62B2Mo4S7
C69H90Mo2B4S3O3 C38H51B4Se3Mo2
formula weight
space group
633.59
P1
680.49
P1
1170.68b
P21/m
1298.71
P21/c
979.79
P21/c
P1
˚
a (A)
10.6733(3)
11.8836(4)
17.4329(6)
71.821(2)
82.061(2)
65.939(2)
1918.09(11)
2
1.519
898
0.968
1.23-28.34
1.021
10.198(2)
11.376.(2)
13.934(3)
111.15(3)
104.01(3)
92.30(3)
1448.2(5)
2
1.453
648
0.952
1.63-28.37
1.056
10.3687(3)
11.3702(3)
13.8112(4)
110.2440(10)
104.4370(10)
92.3050(10)
1465.03(7)
2
1.543
684
2.112
1.64-28.29
0.901
12.744(5)
14.785(5)
14.632(5)
90.000(5)
95.380(5)
90.000(5)
2744.8(17)
2
1.416
1180
1.183
1.40-25.00
1.486
10.6528(2)
27.0706(6)
25.9190(6)
90.00
20.1362(14)
11.4533(4)
17.2837(13)
90.00
˚
b (A)
˚
c (A)
R (deg)
β (deg)
γ (deg)
100.631(2)
90.00
106.944(9)
90.00
3
˚
V (A )
Z
7346.2 (3)
4
1.174
3818.0(4)
4
1.707
Dcalc (Mg/m3)
F (000)
2720
0.467
1940
3.548
μ (mm-1
)
θ rRange (deg)
goodness-of-fit
3.29-25.00
0.893
3.27-25.00
0.891
R indices [I > 2σ(I)]
R1 = 0.0460
wR2 = 0.1015
R1 = 0.0753
wR2 = 0.1153
R1 = 0.0341
wR2 = 0.0743
R1 = 0.0461
wR2 = 0.0814
R1 = 0.0269
wR2 = 0.0676
R1 = 0.0375
wR2 = 0.0745
R1 = 0.0932
wR2 = 0.3123
R1 = 0.1123
wR2 = 0.3482
R1 = 0.0512
wR2 = 0.1251
R1 = 0.0979
wR2 = 0.1338
R1 = 0.0208
wR2 = 0.0409
R2 = 0.0319
wR2 = 0.0419
0.481 and -0.384
R indices (all data)
largest difference in
peak and hole (e/ A )
1.530 and -1.543 0.707 and -1.141 1.137 and -0.972 3.447 and -1.503 1.456 and -0.653
3
˚
a Sum of the two chemical formula of (C20H32S3B2Mo2) and (C20H30S4Mo2). b Sum of the two formula masses of 6 and C20H30S4Mo2.
2B), 34.2 (s, 1B), 24.2 (br, 1B); 1H NMR: δ=7.34-6.98 (m, 5H,
Ph), 5.72 (br, 1BHt), 5.24 (br, 1BHt), 4.67 (br, 1BHt), 3.21 (br,
1BHt), 1.97 (s, 30H, 2Cp*), -5.70 (pcq, 2Mo-H-B), -6.87 (br,
2Mo-H-B); 13C NMR: δ = 130.03, 128.32, 123.85 (s, C6H5),
107.97 (s, η5-C5Me5), 12.60 (s, η5-C5Me5); IR (hexane, cm-1):
2461w, 2490w (BHt); elemental analysis calcd (%) for C26H43-
B5Mo2S: C 49.28, H 6.84; found: C 48.53, H 6.68.
8: MS (FAB) Pþ(max): m/z (%): 979 (isotopic pattern for
2Mo, 4B, 3Se atoms); 11B NMR: δ=78.8 (br, 1B), 54.0 (br, 1B),
19.2 (s, 2B); 1H NMR: δ=7.48-6.78 (m, 15H, C6H5), 4.86 (br,
1B-Ht), 4.54 (br, 1B-Ht), 2.21 (s, 15H, Cp*), 1.80 (s, 15H,
Cp*), -6.58 (br, 2Mo-H-B), -8.58 (br, Mo-H-B); 13C NMR:
δ=134-128, (s, C6H5), 108.38, 107 (s, η5-C5Me5), 12.82, 11.79
(s, η5-C5Me5); 77Se NMR: δ = 327 (s, 2Se), 461 (s, Se); IR
(hexane, cm-1): 2476w (B-Ht); elemental analysis calcd (%) for
C38H50B4Mo2Se3: C 46.63, H 5.15; found: C 47.17, H 5.25.
X-ray Structure Determination. Suitable X-ray quality crys-
tals of 3-8 were grown by slow diffusion of a hexane/CH2Cl2
(9.5:0.5 v/v) solution and single crystal X-ray diffraction studies
were undertaken (Table 3). Crystal data for 3-6 were collected
and integrated using Bruker AXS (Kappa Apex II) and for 7 and
8 using Oxford Diffraction Xcalibur-S CCD system diffract-
ometer equipped with graphite monochromated Mo-KR (λ =
5: MS (FAB) Pþ(max): m/z (%) 680 (isotopic pattern for
2Mo, 5B, Se atoms); 11B NMR: δ=64.14 (br, 1B), 62.4 (br, 1B),
59.8 (d, JB-H=133 Hz, 1B), 30.7 (s, 1B), 24.7 (br, 1B); 1H NMR:
δ=7.48-7.01 (m, 5H, Ph), 5.69 (br, 1BHt), 5.29 (br, 1BHt), 4.21
(br, 1BHt), 3.21 (br, 1BHt), 2.33 (s, 30H, 2Cp*), -5.77 (pcq,
2Mo-H-B), -7.12 (br, 2Mo-H-B); 13C NMR: δ = 132.30,
128.46, 124.59, (s, C6H5), 108.84 (s, η5-C5Me5), 12.58 (s, η5-
C5Me5); IR (hexane, cm-1): 2464w, 2480w, (BHt); elemental
analysis calcd (%) for C26H43B5Mo2Se: C 45.89, H 6.37; found:
C 46.10, H 6.29.
˚
0.71073 A) radiation at 150 K. The structure was solved by
6: MS (ESIþ) m/z: 582 (isotopic pattern for 2Mo, 2B, 3S
atoms); 11B NMR: δ=18.8 (d, JB-H =133 Hz, 2B); 1H NMR:
δ=3.80 (br, 2B-Ht), 2.18 (s, 30H, 2Cp*). 13C NMR: δ=111.8,
(s, η5-C5Me5), 13.71, (s, η5-C5Me5); IR (hexane, cm-1): 2493s
(B-Ht), 474 m (Mo-S-Mo); elemental analysis calcd (%) for
C40H62B2Mo4S7: C 40.97, H 5.33; found: C 41.62, H 5.44.
7: MS (ESIþ) m/z: 1222 (isotopic pattern for 2Mo, 4B, 3S, 3O
atoms); 11B NMR: δ = 83.4 (d, JB-H = 134 Hz, 1B), 49.8 (br,
1B), 30.45 (s, 2B); 1H NMR: δ=7.51-7.03 (m, 6H, C6H2OH),
5.00 (s, 3H, C6H2OH), 4.39, (br, 1B-Ht), 3.58 (br, 1B-Ht), 1.82
(s, 15H, Cp*), 1.64 (s, 15H, Cp*), 1.35-1.47 (s, 54H, 6(CH3)3C),
-7.00 (br), Mo-H-B), -7.28 (br, Mo-H-B), -7.55 (br, Mo-H-
B); 13C NMR: δ=135.40, 131.75, 129.80 (s, C6H2OH), 107.65,
106.35 (s, η5-C5Me5), 34.69, 34.49, 34.41 (s, -C(CH3)3), 30.56,
30.30, 30.14 (s, C(CH3)3), 12.27, 11.95 (s, η5-C5Me5); IR
(hexane, cm-1): 2476w (B-Ht), 459 (s Mo-S(R)-Mo, R = 2,6-
(tBu)2-C6H2OH); elemental analysis calcd (%) for C62H98-
B4Mo2O3S3: C 60.90, H 8.08; found: C 59.97, H 8.19.
heavy atom methods using SHELXS-9742 and refined using
SHELXL-9743 (Sheldrick, G.M., University of Gottingen).
€
Acknowledgment. Generous support of the Department of
Science and Technology, DST, (Project No. SR/S1/IC-19/2006) is
gratefully acknowledged. R.S.D. and K.K.V. thank the University
Grants Commission (UGC), India, for a Junior Research Fellow-
ship. We would also like to thank SAIF, CDRI, Lucknow 226001
for FAB mass and SAIF, IIT Madras for HETCOR and VT NMR.
Supporting Information Available: The supplementary crys-
tallographic data and X-ray crystallographic files for 3-8. This
material is available free of charge via the Internet at http://
pubs.acs.org.
€
€
(42) Sheldrick, G. M. SHELXS-97; University of Gottingen: Gottingen,
Germany, 1997.
€
€
(43) Sheldrick, G. M. SHELXL-97; University of Gottingen: Gottingen,
Germany, 1997.