Synthesis of novel molybdaboranes from (η5-C5R5)MoCl(n) precursors (R = H, Me; N = 1,2,4)

Article abstract of DOI:10.1021/ja973720n

Reaction of Cp*MoCl₄(1), or (Cp*MoCl₂)₂ (2), Cp* = η⁵-C₅Me₅, with BH₃·THF ultimately generates the Mo(II) cluster (Cp*Mo)₂B₅H₉ (7), together with the Mo(III) species (Cp*MoCl)₂B₄H₁₀, 4. Prereduction of 2 before reaction with BH₃·THF yields only 7. The structure of 4 consists of two Cp*Mo units bridged by two chlorides and a [B₂H₅(B₂H₅)]^2- ligand in which the two diboron moieties are connected by a B-B-B three center bond. Closer inspection of the reaction by ¹¹B and ¹H NMR reveals the existence of three intermediate species (Cp*MoCl)₂B₂H₆ (3), (Cp*MoCl)₂B₃H₇ (5), and (Cp*Mo)₂(B₂H₆)₂ (6). Each of these species has been characterized spectroscopically, and crystal structures have been obtained for 3 and 5. Compound 3 features molybdenum centers bridged by two chlorides and an ethane-like [B₂H₆]^2- ligand such that the B-B bond is perpendicular to the Mo-Mo bond. Replacing one terminal H by [B₂H₅] generates 4. The structure of 5 is based on a trigonal bipyramidal Mo₂B₃ core, and the molecule is electronically unsaturated although the Mo-Mo distance (3.096 A?) precludes the existence of multiple bonding between the metal centers. 5 exists as a relatively stable molecule despite having too few electrons and too few atoms to adopt a capped structure based on a polyhedron with fewer vertexes. Comparison of MO descriptions of the electronic structure of 5 with that of the later transition metal species (Cp*Co)₂B₃H₇ (8) shows that this stabilization is derived from the appropriate energy match between Cp*Mo and borane based orbitals which elevates the energy of the Mo-B antibonding LUMO, a cluster orbital which would normally be filled, into the region of unoccupied orbitals. The concentration vs time behavior for the final products 4 and 7, for the intermediates 3, 5, and 6, for the monoboron species BH3·THF and BH₂Cl, and selected non-boron containing species is used to define a pathway for the molybdaborane cluster condensation. With 1, use of LiBH₄ as the monoboron source yields 6 as the primary product via 3 as an intermediate, whereas prereduction of 2 with [Et₃BH]^- results in the formation of 7. The varied cluster building abilities of BH₃·THF vs LiBH₄ originate in the differing reduction and coordination properties of the two monoboranes. Investigation of the analogous Cp = η⁵-C₅H₅ system reveals similar chemistry albeit simpler and on a shorter time scale.

Full text of DOI:10.1021/ja973720n

2586
J. Am. Chem. Soc. 1998, 120, 2586-2598
5
Synthesis of Novel Molybdaboranes from (η -C R )MoCl Precursors
5
5
n
(R ) H, Me; n ) 1, 2, 4)
Simon Aldridge,* Maoyu Shang, and Thomas P. Fehlner*
Contribution from the Department of Chemistry and Biochemistry, UniVersity of Notre Dame,
Notre Dame, Indiana 46556
ReceiVed October 27, 1997
5
Abstract: Reaction of Cp*MoCl4 (1), or (Cp*MoCl2)2 (2), Cp* ) η -C5Me5, with BH3‚THF ultimately generates
the Mo(II) cluster (Cp*Mo)2B5H9 (7), together with the Mo(III) species (Cp*MoCl)2B4H10, 4. Prereduction
of 2 before reaction with BH3‚THF yields only 7. The structure of 4 consists of two Cp*Mo units bridged by
two chlorides and a [B2H5(B2H5)]2 ligand in which the two diboron moieties are connected by a B-B-B
-
1
1
1
three center bond. Closer inspection of the reaction by B and H NMR reveals the existence of three
intermediate species (Cp*MoCl)2B2H6 (3), (Cp*MoCl)2B3H7 (5), and (Cp*Mo)2(B2H6)2 (6). Each of these
species has been characterized spectroscopically, and crystal structures have been obtained for 3 and 5.
2
-
Compound 3 features molybdenum centers bridged by two chlorides and an ethane-like [B2H6] ligand such
that the B-B bond is perpendicular to the Mo-Mo bond. Replacing one terminal H by [B2H5] generates 4.
The structure of 5 is based on a trigonal bipyramidal Mo2B3 core, and the molecule is electronically unsaturated
although the Mo-Mo distance (3.096 Å) precludes the existence of multiple bonding between the metal centers.
5
exists as a relatively stable molecule despite having too few electrons and too few atoms to adopt a capped
structure based on a polyhedron with fewer vertexes. Comparison of MO descriptions of the electronic structure
of 5 with that of the later transition metal species (Cp*Co)2B3H7 (8) shows that this stabilization is derived
from the appropriate energy match between Cp*Mo and borane based orbitals which elevates the energy of
the Mo-B antibonding LUMO, a cluster orbital which would normally be filled, into the region of unoccupied
orbitals. The concentration vs time behavior for the final products 4 and 7, for the intermediates 3, 5, and 6,
for the monoboron species BH3‚THF and BH2Cl, and selected non-boron containing species is used to define
a pathway for the molybdaborane cluster condensation. With 1, use of LiBH4 as the monoboron source yields
-
6
as the primary product via 3 as an intermediate, whereas prereduction of 2 with [Et3BH] results in the
formation of 7. The varied cluster building abilities of BH3‚THF vs LiBH4 originate in the differing reduction
5
and coordination properties of the two monoboranes. Investigation of the analogous Cp ) η -C5H5 system
reveals similar chemistry albeit simpler and on a shorter time scale.
5
12
Introduction
and to the molybdaboranes (Cp′Mo)2B5H9 (Cp′ ) η -C5H4Me),
1
3
5
1
14
3
2
Cp2Mo(H)B2H5, CpMo(η :η -C5H4)B4H7 and CpMo(η :η -
5
14
Metallaboranes that incorporate metals to the left of iron are
a relatively sparsely explored area of cluster chemistry.
Previous to this work, with the exception of coordination
C H )C B H (Cp ) η -C H ).
3
3
2
9
9
5
5
1
-4
The challenge of the early transition metals is to find synthetic
routes. That is, traditional synthetic routes to metallaboranes
have focused on two broad but complementary approaches,
-
5
- 6
7
1-4
complexes containing the BH4 , B3H8 , or B2H4(PR3)2
ligands, structurally authenticated compounds of the group 6
metals are confined to the electronically unsaturated chrom-
aborane (Cp*Cr)2B4H8 (Cp* ) η -C5Me5) and its derivatives
namely (i) insertion or fragmentation reactions of “ready-
assembled” borane frameworks and (ii) condensation reactions
of monoboron precursors. Typically, products are isolated by
chromatography from reaction mixtures in low yields owing to
the accessibility of several minima on the reaction pathway
5
8
9-11
(
(
(
7.
(
1) Kennedy, J. D. Prog. Inorg. Chem. 1984, 32, 519.
2) Kennedy, J. D. Prog. Inorg. Chem. 1986, 34, 211.
3) Housecroft, C. E.; Fehlner, T. P. AdV. Organomet. Chem. 1982, 21,
4) Grimes, R. N. In Metal Interactions with Boron Clusters; Grimes,
15
under the conditions employed. Only the most stable/least
reactive compounds are characterized greatly restricting the
scope of the chemistry particularly when early transition metals
are of interest.
Recently we have illustrated the use of dimeric metal-
containing precursors as a method of controlling the number of
5
R. N., Ed.; Plenum: New York, 1982; p 269.
(
(
(
5) Marks, T. J.; Kolb, J. R. Chem. ReV. 1977, 77, 263.
6) Guggenberger, L. J. Inorg. Chem. 1970, 9, 367.
7) Shimoi, M.; Katoh, K.; Ogino, H. J. Chem. Soc., Chem. Commun.
1
990, 811.
8) Deck, K. J.; Nishihara, Y.; Shang, M.; Fehlner, T. P. J. Am. Chem.
Soc. 1995, 117, 10292.
9) Hashimoto, H.; Shang, M.; Fehlner, T. P. J. Am. Chem. Soc. 1996,
18, 8164.
(12) 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.
(13) Grebenik, P. D.; Green, M. L. H.; Kelland, M. A.; Leach, J. B.;
Mountford, P.; Stringer, G.; Walker, N. M.; Wong, L. L. J. Chem. Soc.,
Chem. Commun. 1988, 799.
(14) Grebenik, P. D.; Leach, J. B.; Kelland, M. A.; Green, M. L. H.;
Mountford, P. J. Chem. Soc., Chem. Commun. 1989, 1397.
(15) Meng, X.; Bandyopadhyay, A. K.; Fehlner, T. P.; Grevels, F.-W.
J. Organomet. Chem. 1990, 394, 15.
(
(
1
1
(10) Hashimoto, H.; Shang, M.; Fehlner, T. P. Organometallics 1996,
5, 1963.
(11) Aldridge, S.; Hashimoto, H.; Kawamura, K.; Shang, M.; Fehlner,
T. P. Inorg. Chem. In press.
S0002-7863(97)03720-7 CCC: $15.00 © 1998 American Chemical Society
Published on Web 03/05/1998

Synthesis of NoVel Molybdaboranes
J. Am. Chem. Soc., Vol. 120, No. 11, 1998 2587
Table 1. 11B NMR of the Metallaboranes 3-7a,b
3c
4c
5c
6
7
B-M
d
d
105.5 {155}
B(1)
34.2
d
62.9 {130} (3B, accid.
overlap of two signals)
25.8 (2B)
B-H-M -44.1 [290, 200] B(1), B(2) -56.7 [760, 630]
B(2)
B(1)
-58.6
[220, 190] (all 4B)
-
-
-
27.2 [910, 550]
11.9 [1320, 720]
8.0 [1440, 1020]
B(2), B(2′)
B(3), B(4)
}
a
b
11
1
Toluene solutions at 21 °C for 3-6; hexane solution at 21 °C for 7. Coupling constant J( B- H) in Hz given where appropriate in the form
1
c
{
150}; fwhm in Hz given in the form [290, 200] refers to the coupled and H decoupled spectra, respectively. Numbering scheme for each
d
molecule taken from the appropriate figure. Not applicable.
Table 2. 1H NMR of the Metallaboranes 3-7a,b
3
4
5
6
7
B-H-M
B-H
-13.4 (4)
-12.6 (2), -12.1 (2)
-4.8 (4, 70)
-12.4 (8)
-7.0 (4)
1.1 (2, 120)
0.4 (1) ≈2.5 (4), ≈3.6 (1)
3.4 (2), 8.5 (1)
0.6 (4, 130)
3.9 (1), 5.37 (2), 6.22 (2)
a
11
1
b
2
Relative intensities and coupling constants J( B- H) in Hz where appropriate given in parentheses. Solutions in [ H
6
]benzene at 21 °C.
prepared according to literature methods.2
2,24-26
metal atoms in the product during condensation reactions of
3
The BH ‚THF and
BH3‚THF.8
,16
LiBH4 solutions (Aldrich) were used as received without further
purification.
NMR spectra were measured on a Varian-300 or Varian-500 FT-
NMR spectrometer. Residual protons of solvent were used for reference
The extension of this approach to molybdenum
species of the type (Cp*MoCln)2 offers a promising route to
clusters of the stoichiometry Mo2Bx. Using this method we have
17
18
been successful in isolating Mo2B2, Mo2B3, Mo2B4, and
Mo2B518 metallaboranes. These high yield reactions permit
mechanistic studies of cluster condensation from halogenated
precursors including the role of Cp* vs Cp ligand on the reaction
chemistry. To this end qualitative experiments have been carried
out resulting in the identification of at least three intermediate
species (all of which have been subsequently isolated and two
crystallographically characterized). This allows the delineation
of the pathway of the stepwise build-up of metallaborane clusters
thereby defining the similarities and differences of borane and
borohydride as cluster building units.
1
for H NMR (δ, ppm: benzene, 7.15; toluene, 2.09), while a sealed
tube containing [(NEt
4
)(B
reference for B NMR. Infrared spectra were measured on a Nicolet
05 FT-IR spectrometer. Mass spectra were measured on a JEOL JMS-
3 8
H
)] (δ
B
-29.7 ppm) was used as the external
11
2
AX 505HA mass spectrometer using the EI ionization mode. Perfluo-
rokerosene was used as the standard for high-resolution EI mass spectra.
Synthesis of Final Molybdaborane Products. B and H NMR
data for the molybdaboranes 3-7 are listed in Tables 1 and 2,
respectively.
10 (4). Reaction of (Cp*MoCl
‚THF in toluene at 45 °C for 120 h yields a mixture
of 4 and 7, as determined by B NMR spectroscopy. 7 is easily
removed by washing with hexane, and crystals of 4 can then be grown
by layering a toluene solution with hexane (yield ca. 45% based on
the amount of 2 taken). An identical mixture of products 4 and 7 is
11
1
(
i) (Cp*MoCl)
2
B
4
H
2 2
) (2) with a
5
-fold excess of BH
3
11
18
A key intermediate in the formation of one final product
(
(
Cp*Mo)2B5H9 is the electronically unsaturated species
Cp*MoCl)2B3H7. This cluster is of interest not only because
17
of its role as a reaction intermediate but also as a rare example
of a metallaborane which has insufficient skeletal electrons based
obtained on treating Cp*MoCl (1) with 7 equiv of BH ‚THF under
4
3
11 1
similar reaction conditions. 4 has been characterized by B and H
on the Wade-Mingos rules to justify its observed molecular
NMR, solid-state IR, mass spectrometry, and single-crystal X-ray
structure.1
9,20
It is the first such unsaturated metallaborane for
18
+
diffraction. Additional spectroscopic data for 4: MS (EI), P ) 587,
two Mo, two Cl, four B atoms, calcd for weighted average of 81
isotopomers falling within the resolution limits of the spectrometer at
which a later transition metal analogue exists containing the
16
same borane fragment [(Cp*Co)2B3H7 ]; this allows identifica-
tion of the electronic factors stabilizing the unsaturated cluster.
-1
the measured ion peak, 587.1078, obsd 587.1107. IR (KBr, cm ) 2979
m, 2960 m, 2912 s, 2857 w sh, ν(C-H); 2493 s, 2462 s, 2624 vs,
ν(B-H
ii) (Cp*Mo)
in 0.75-1.5% yield has previously been reported by the reaction of
MeMoCl with LiBH (7) is most
in diethyl ether,12 (Cp*Mo)
conveniently prepared by the reaction of either Cp*MoCl (1),
Cp*MoCl (2), or “[Cp*MoCl]” with BH
‚THF.18 Reaction of 1
with a 7-fold excess of BH ‚THF in toluene at 45 °C over a period of
20 h yields a mixture of 7 and the Mo(III) metallaborane
Cp*MoCl)
10 (4),18 from which 7 is easily separated by extraction
t
); 1994 m br, ν(B-H
b
); 1376 s, 1028 m, δ(CH
3
).
Experimental Section
(
2
B
5
H
9
(7). Although the synthesis of (C
5
H
4
MeMo)
2 5 9
B H
General Methods. All manipulations were carried out under a
nitrogen or argon atmosphere using standard Schlenk line or drybox
techniques. Solvents were predried over 4-Å molecular sieves (tet-
rahydrofuran) or KOH (hexanes, toluene) and purged with nitrogen
prior to distillation. Tetrahydrofuran (THF), toluene, and hexanes were
all distilled from sodium benzophenone ketal. Starting materials for
C H
5 4
4
4
2 5 9
B H
4
(
2
)
2
3
3
1
(
2 4
B H
the molybdaboranes, Cp*MoCl
according to the methods of King, Schrock, and Green. The
analogous Cp compounds CpMoCl (1′) and (CpMoCl (2′) were also
4 2 2
(1) or (Cp*MoCl ) (2), were prepared
2
1
22
23
with hexane. Concentration of a hexane solution followed by cooling
to -40 °C allows orange crystals of 7 to be isolated in yields of 30-
4
2 n
)
4
0% based on the amount of 1 taken. Reaction of (Cp*MoCl
with BH
the products 4 and 7. On the other hand, reduction of 2 (140 mg, 0.23
mmol) by 2 equiv of LiEt BH in toluene at room temperature [to yield
the Mo(II) halide “Cp*MoCl”] followed by removal of volatiles in
vacuo and subsequent reaction with a 3-fold excess of BH ‚THF (also
in toluene) produced an orange solution from which tiny orange crystals
of (Cp*Mo) could be isolated as the only metallaborane product
2
)
2
(2)
(
16) Nishihara, Y.; Deck, K. D.; Shang, M.; Fehlner, T. P.; Haggerty,
B. S.; Rheingold, A. L. Organometallics, 1994, 13, 4510.
17) Aldridge, S.; Shang, M.; Fehlner, T. P. J. Am. Chem. Soc. 1997,
19, 11120.
18) Aldridge, S.; Fehlner, T. P.; Shang, M. J. Am. Chem. Soc. 1997,
19, 2339.
3
‚THF under similar conditions gives an identical mixture of
(
3
1
1
(
3
(
(
(
(
19) Wade, K. New Scientist 1974, 62, 615.
20) Mingos, D. M. P. Acc. Chem. Res. 1984, 17, 311.
21) King, R. B.; Bisnette, M. B. J. Organomet. Chem 1967, 8, 287.
22) Murray, R. C.; Blum, L.; Liu, A. H.; Schrock, R. R. Organometallics
2 5 9
B H
(24) Cousins, M.; Green, M. L. H. J. Chem. Soc. 1964, 1567.
(25) Krueger, S. T.; Owens, B. E.; Poli, R. Inorg. Chem. 1990, 29, 2001.
(26) Linck, R. G.; Owens, B. E.; Poli, R.; Rheingold, A. L. Gazz. Chim.
Ital. 1991, 121, 163.
1
985, 4, 953.
23) Green, M. L. H.; Hubert, J. D.; Mountford, P. J. Chem. Soc., Dalton
Trans. 1990, 3793.
(

2
588 J. Am. Chem. Soc., Vol. 120, No. 11, 1998
Table 3. Amounts of Reagents Used for 11B NMR-Monitored Reactions
amount of reagents used
toluene (cm3)
overall concn of BH
Aldridge et al.
Cp*MoCl
4
BH
3
‚THFa
3
‚THF (M)
4 3
ratio Cp*MoCl :BH ‚THF
3
1
2
3
20 mg, 0.054 mmol
20 mg, 0.054 mmol
20 mg, 0.054 mmol
1.08 cm , 0.216 mmol
1.08
1.35
1.89
0.1
0.1
0.1
1:4
1:5
1:7
3
1.35 cm , 0.270 mmol
3
1.89 cm , 0.378 mmol
a
0
.2 M solution in THF.
[
i.e., none of the Mo(III) cluster 4 was formed]. By contrast, the
reaction of 1 with 5 equiv of LiBH , analogous to that reported by
Green et al. in their synthesis of (C MeMo) gives very low
yields of the Mo cluster 7, the predominant product being the bis-
) adduct 6 (vide infra). Characterization of 7 was achieved by
comparison of the measured NMR spectra with those reported for (C
of 6 is by the reaction of 1 with 10 equiv of LiBH
4
in toluene. The
4
solution of LiBH in thf was added to a solution/suspension of 1 in
4
5
H
4
2
B
5
H
9
toluene at -80 °C, and the reaction mixture was allowed to warm
slowly to room temperature. After 72 h 6 was shown to be the
predominant metallaborane product by 11B NMR (there being less than
2
B
5
2 6
(B H
5 4
H -
2 5
5% of the Mo B species 7), and volatiles were removed in vacuo to
1
2
MeMo)
2
B
5
H
9
.
yield a brown solid. Extraction with hexane, concentration of the
resulting brown solution, and cooling to -10 °C gave a brown
microcrystalline precipitate of 6. All attempts to obtain a crystalline
sample of 6 suitable for study by X-ray diffraction instead gave
microcrystalline material. 6 has been characterized by 11B and H NMR
Synthesis and Isolation of Intermediate Species. NMR experi-
ments revealed the existence of three intermediates in the formation of
(
Cp*MoCl
shown to be (Cp*MoCl)
Cp*Mo)
2
B
4
5
H
9
(7) and (Cp*MoCl)
(1) with 7 equiv of BH ‚THF. These were subsequently
(3), (Cp*MoCl) (5), and (Cp*Mo)
(6). If only 4 equiv of BH ‚THF are used, then the
2 4
B H10 (4) by the reaction of
1
3
B H
2 2 6
B H
2 3 7
2
-
and mass spectrometry. Additional spectroscopic data for 6: MS(EI),
+
(
B H )
2 6 2
3
P
) 518, two Mo, four B, atoms; isotopic distribution in excellent
Mo
NMR-Monitored Reactions. To investigate possible mechanistic
pathways linking the fully halogenated starting material Cp*MoCl (1)
and the metallaborane end-products (Cp*MoCl) 10 (4) and
(Cp*Mo) (7) a series of experiments was carried out in which
concentrations of these intermediates do not fall to zero, and the
concentrations of the final products 4 and 7 remain low even after long
reaction times (ca. 6 days). It seems probable that there is insufficient
agreement with formulation C20
H
42
B
2
2
.
4
BH
ate species formed to the final products 4 and 7.
i) (Cp*MoCl) (3). By optimizing the reaction conditions (45
C, 6 days reaction time, 4:1 ratio of BH ‚THF:1) it proved possible
to obtain a reaction mixture in which by far the largest component (as
3
‚THF present under these conditions to convert all of the intermedi-
2 4
B H
2 5 9
B H
(
B H
2 2 6
the reaction mixture was monitored periodically by NMR. Reaction
mixtures were made up in 10 mm NMR tubes as listed in Table 3sthe
total amount of 1 in each was kept constant as was the concentration
°
3
11
of BH ‚THF, but the ratio of 1:BH ‚THF was varied by using a different
determined by B NMR) was the intermediate 3. Removal of volatiles
in vacuo at this point and extraction into hexane gave a green-brown
solution and an insoluble brown solid (which was subsequently shown
by H NMR to be unreacted 2). Repeated concentration and cooling
of the hexane solution gave dark brown crystals of 3 in yields of 15-
1
3
3
volume of borane solution in each tube. The BH ‚THF solution was
3
added to the solution/suspension of 1 in toluene at -40 °C, and the
reaction mixture was allowed to warm to room temperature. After 90
min at room temperature the reaction temperature was raised to 55 °C
1
11
for the remainder of the experiment, and the composition of the reaction
8% based on the amount of 1 taken. 3 has been characterized by B
1
mixture monitored periodically by 11B NMR.
and H NMR, mass spectrometry, and single-crystal X-ray diffraction.
An added complication was the presence of ≈14% of 2 which
A further experiment was carried out in which the reaction was
1
1
cocrystallizes with 3, the existence of which is shown by H NMR
monitored by H NMR in order to reveal the concentration/time
spectra and which complicates the interpretation of the crystal structure
data. The initial single crystals examined contained ≈85% of 2, and
repeated recrystallization was required to reduce the impurity level to
dependence for 11B silent species such as (Cp*MoCl
2
)
2
and possible
molybdenum hydride intermediates. To this end 10 mg (0.03 mmol)
3
2
of 1 was loaded into a 5 mm NMR tube and 1.0 cm of [ H
was added; to this solution/suspension was added BH ‚THF (0.11
mmol). Both the ratio of 1:BH ‚THF (1:4) and the concentrations of
the reagents used were the same as those used in tube 1 of the
6
]benzene
+
acceptable limits. Additional spectroscopic data for 3: MS(EI), P )
3
5
60, two Mo, two B, two Cl atoms; isotopic distribution in excellent
3
1
1
agreement with formulation C20
H
36
B
2
Cl
2
2
Mo .
B
(
ii) (Cp*MoCl) (5). Of the three intermediate species 5 is
2
B
3
H
7
NMR-monitored reaction, although in order to allow for the smaller
volume of the 5 mm tube the absolute quantities of all reagents were
reduced from those used for the 10 mm tube reaction. As before, the
first 90 min of reaction time were at room temperature with the tube
subsequently being heated to 55 °C for the remainder of the experiment;
the composition of the reaction mixture was monitored periodically
present in the lowest concentration at all reaction times, presumably
because its electronic unsaturation renders it extremely reactive to
further BH
mixtures (containing lower concentrations of BH
possible to isolate this intermediate and then only in low yield.
Accordingly, reaction of Cp*MoCl (1) with 4 equiv of BH ‚THF in a
dilute toluene solution (ca. 50 mM in BH ‚THF) at 55 °C over a period
of 72 h produces the cluster 5 in ca. 5% yield. Removal of volatiles
in vacuo, extraction with hexane, and filtration yields a solution
containing a mixture of 3-5 from which the dark green microcrystalline
product 5 can be isolated by cooling to -10 °C (5 is less soluble in
hexane than either 3 or 4). Dissolution in toluene and layering of the
solution with hexane allows the growth of rectangular platelike crystals
of 5 by slow diffusion over a period of 2-3 days at room temperature.
3
‚THF (vide infra). Only by the use of more dilute reaction
3
‚THF) did it prove
1
by H NMR.
4
3
3
Cp Analogues. Reactions of the corresponding cyclopentadienyl
molybdenum species CpMoCl
excess of BH
4 2 n
(1′) and (CpMoCl ) (2′) with a large
3
‚THF (7 or 5 equiv, respectively) were also investigated
by 11B NMR spectroscopy. In each case only one intermediate and
one final product was observed. The final product is characterized by
1
1
11
three B NMR signals (2:1:2) at δ
B
61.0, 59.4 [both doublets, J( B-
H) ≈ 140 Hz], and 27.1 (broad multiplet); the observed spectrum is
assigned to 7′ as it is essentially identical to that reported for (C
1
5
4
H -
1
1
1
12
5
has been characterized by B and H NMR, IR spectroscopy, and
single-crystal X-ray diffraction. Additional spectroscopic data for 5:
MeMo)
B H
2 5 9
and (Cp*Mo)
B H
2 5 9
(7). The only intermediate detected
-45.4 (3′) [broad doublet,
11
gives rise to a single B NMR signal at δ
B
-
1
11
1
IR (KBr, cm ) 2982 w, 2962 w, 2916 s, 2855 w sh, ν(C-H); 2498 s,
455 s, ν(B-H ); 1475 m, 1448 w sh, 1415 w, 1377 s, 1071 w, δ-
CH ); 752 s, 675 w sh, 664 m, F(CH ) [δ ) deformation mode, F )
J( B- H) ≈ 140 Hz]; the reaction is considerably faster than in the
Cp* casesat room temperature the concentration of the intermediate
species is maximized after a 24 h reaction time and is completely
consumed after 72 h.
2
(
t
3
3
rocking mode]. Attempts to obtain the mass spectrum of 5 were
frustrated by its extreme air and moisture sensitivity.
Use of LiBH as the Boron Source. The reaction of C H MeMoCl
4
5
4
4
(
iii) (Cp*Mo)
identified as an intermediate in the formation of 7 by the reaction of
Cp*MoCl (1) with 7 equiv of BH ‚THF, the most convenient synthesis
2
(B
2
H
6
)
2
(6). Although the molybdaborane 6 has been
with 3.5 equiv of LiBH
investigated by Green et al. who isolated (C
4
in diethyl ether has been previously
MeMo) in 0.75-
1.5% yield.12 We have repeated the reaction using toluene as the solvent
H
5 4
2 5 9
B H
4
3

Synthesis of NoVel Molybdaboranes
J. Am. Chem. Soc., Vol. 120, No. 11, 1998 2589
Table 4. Crystal Data and Structure Refinement Details for Compounds 3-5
3
4
5
formula weight
crystal system
space group
a (Å)
560.92
586.54
monoclinic
P2 /c
572.71
tetragonal
P4
8.5231(13)
8.5231(13)
32.956(7)
monoclinic
P2
1
1
3 1
2 2
8.1278(7)
29.447(6)
10.601(4)
108.61(3)
2404.6(10)
4
11.5162(11)
15.261(2)
14.815(2)
103.183(10)
2535.1(5)
4
b (Å)
c (Å)
â (deg)
3
V (Å )
2394.0(7)
4
1.589
1160
0.71073
1.275
Z
F
calcd (g cm-3)
1.565
1.537
F(000)
1146
1192
wavelength (Å)
abs. coeff (mm-1)
crystal size (mm)
θ range (deg)
index ranges
0.71073
0.71073
1.298
1.205
0.48 × 0.18 × 0.02
2.03-28.39
-9 e h e 10
0.25 × 0.16 × 0.10
2.25-22.99
0 e h e 12
0 e k e 16
-16 e l e 15
3717
0.40 × 0.33 × 0.02
2.47-24.96
0 e h e 10
0 e k e 10
0 e l e 39
2422
-
30 e k e 37
13 e l e 10
-
no. of reflns collected
no. of independent reflns
9999
6360
3523
2099
[
R(int) ) 0.0606]
[R(int) ) 0.0587]
[R(int) ) 0.0497]
no. of observed reflnsa
refinement method
4952
2910
1826
full-matrix on F2
6342/95/526
1.094
full-matrix on F2
3522/19/284
1.216
full-matrix on F
2
data/restraints/parameters
2099/0/137
1.143
2
goodness of fit on F
final R indices [I > 2σ(I)]
R
1
) 0.0683
wR ) 0.1466
) 0.0982
wR
R
1
) 0.0708
wR ) 0.2148
) 0.0830
wR ) 0.2290
1.270 and -1.848
R
1
) 0.0420
wR ) 0.0944
) 0.0542
wR ) 0.1053
0.934 and -0.472
2
2
2
R indices (all data)b
R
R
R
1
1
1
2
) 0.1745
2
2
largest diff. peak and hole (e Å-3)
0.985 and -0.935
a
I > 2σ(I)]. b wR2 ) {[∑w(F
2
2)2]/[∑w(F 2)2]}1/2.
[
c
- F
o
o
and monitoring the progress of the reaction by 11B NMR. Using 3.5
equiv of LiBH , added to a solution/suspension of Cp*MoCl at -78
C and warming to room temperature, the B NMR spectrum measured
after 30 min indicates the presence of both 3 and 6, together with a
small amount of unreacted LiBH and a much larger quantity of BH ‚-
THF, presumably produced during the initial reduction of Cp*MoCl
to (Cp*MoCl by LiBH (vide infra). At longer reaction times the
Table 5. Selected Interatomic Distances (Å) and Angles (deg) for
Cp*MoCl)
4
4
(
2 4
B H10 (4)
11
°
Distance
2.228(12)
Mo(1)-C(1)
Mo(1)-C(2)
Mo(1)-C(3)
Mo(1)-C(4)
Mo(1)-C(5)
Mo(1)-B(1)
Mo(1)-B(2)
Mo(1)-Cl(1)
Mo(1)-Cl(2)
Mo(1)-Mo(2)
Mo(1)-H(2)
Mo(1)-H(4)
Mo(2)-C(11)
Mo(2)-C(12)
Mo(2)-C(13)
Mo(2)-C(14)
Mo(2)-C(15)
Mo(2)-B(1)
Mo(2)-B(2)
Mo(2)-Cl(1)
Mo(2)-Cl(2)
Mo(2)-H(3)
Mo(2)-H(5)
B(1)-B(2)
B(1)-H(1)
B(1)-H(2)
B(1)-H(3)
B(2)-B(3)
B(2)-B(4)
B(2)-H(4)
B(2)-H(5)
B(3)-B(4)
2.328(11)
2.44(2)
2.422(13)
2.463(3)
2.478(3)
1.80(2)
1.80(2)
1.70(2)
1.06(2)
1.08(2)
1.05(2)
1.77(2)
1.87(2)
1.05(2)
1.05(2)
1.79(3)
4
3
2.273(11)
2.324(12)
2.342(12)
2.254(13)
2.42(2)
4
2
)
2
4
concentration of 3 falls (such that after 72 h there is very little of this
species remaining), whereas the concentration of 6 continues to grow.
1
1
2.42(2)
After 72 h the B spectrum reveals that 6 is the predominant
2.468(3)
2.486(3)
2.7113(13)
1.82(2)
metallaborane species in solution, there being additional weak signals
due to the Mo
5 °C, such that 6 remains the major component of the mixture even
after several weeks and the addition of further aliquots of LiBH . If
the reaction is carried out with 10 equiv of LiBH (rather than 3.5),
2 5
B species 7. Conversion of 6 to 7 is very slow even at
5
4
1.80(2)
4
2.270(12)
2.228(11)
2.262(10)
2.348(11)
then the maximum concentration achieved by the intermediate 3 is much
lower, and the amount of 7 ultimately produced is also lower.
X-ray Structure Determinations. Details of the data collection
and structure refinement procedures for compounds 3-5 are sum-
marized in Table 4. Data collection for compounds 4 and 5 was carried
out using an Enraf-Nonius CAD4 diffractometer, whereas that for 3
was carried out on an Enraf-Nonius FAST area detector diffractometer;
all data were collected at room temperature.
Angle
C(1)-Mo(1)-B(1)
97.9(5)
C(12)-Mo(2)-B(1) 153.7(5)
C(2)-Mo(1)-B(1) 125.7(5)
C(3)-Mo(1)-B(1) 158.4(5)
C(4)-Mo(1)-B(1) 136.3(6)
C(5)-Mo(1)-B(1) 102.8(5)
C(13)-Mo(2)-B(1) 116.3(5)
C(14)-Mo(2)-B(1)
97.0(5)
C(15)-Mo(2)-B(1) 110.3(5)
C(11)-Mo(2)-B(2) 117.1(5)
C(13)-Mo(2)-B(2) 142.7(5)
C(14)-Mo(2)-B(2) 108.1(5)
2 4
(Cp*MoCl) B H10 (4). A dark green-brown acicular crystal of 7
C(1)-Mo(1)-B(2)
98.0(5)
was grown by slow diffusion of hexane into a concentrated toluene
solution at -40 °C over a period of 6 days and mounted on a glass
fiber prior to data collection. Most of the non-hydrogen atoms were
located by direct methods and the remainder found in successive
difference Fourier syntheses. All non-hydrogen atoms were refined
anisotropically and all hydrogen atoms were then located in a difference
Fourier map. Both borane and methyl hydrogens were included in the
final refinement, the former being treated isotropically with bond length
C(2)-Mo(1)-B(2) 103.2(4)
C(3)-Mo(1)-B(2) 135.7(5)
C(4)-Mo(1)-B(2) 158.9(5)
C(5)-Mo(1)-B(2) 126.1(5)
C(15)-Mo(2)-B(2)
Cl(1)-Mo(2)-Cl(2)
96.5(5)
76.46(12)
Mo(2)-Cl(1)-Mo(1) 66.71(8)
Cl(1)-Mo(1)-Cl(2) 76.21(12) Mo(2)-Cl(2)-Mo(1) 66.21(8)
B(1)-Mo(1)-Mo(2) 56.4(4)
B(2)-Mo(1)-Mo(2) 56.0(3)
B(1)-B(2)-B(3)
B(1)-B(2)-B(4)
B(3)-B(2)-B(4)
B(3)-B(4)-B(2)
82.9(11)
141.3(12)
58.6(10)
58.0(9)
B(1)-Mo(1)-B(2)
B(1)-Mo(2)-B(2)
41.2(5)
41.1(5)
restraints, the latter as idealized riding atoms [r(C-H) ) 0.96 Å, Uiso
-
2
(
H) ) 1.5 Ueq(C)]. The structure was refined on F by full-matrix
C(11)-Mo(2)-B(1) 145.1(5)
27
least-squares methods using SHELXTL V5. Selected bond distances
are listed in Table 5; other data have been deposited in conjunction
1
8
4 days and mounted on a glass fiber for data collection. Most of the
non-hydrogen atoms were shown on an E-map; ensuing least-squares
refinement and successive difference Fourier syntheses revealed the
with the original communication.
Cp*MoCl) (3). A brown platelike crystal of 3 was grown
by cooling a concentrated hexane solution to -10 °C over a period of
(
2 2 6
B H
remaining non-hydrogen atoms and the disorder model of a B group
H
2 6
(27) SHELXTL.-V5; Siemens Industrial Automation Inc.: 1994.
with two bridging chlorine atoms in each of the independent Mo cluster
2

2590 J. Am. Chem. Soc., Vol. 120, No. 11, 1998
Aldridge et al.
Table 6. Selected Interatomic Distances (Å) and Angles (deg) for
Table 7. Selected Interatomic Distances (Å) and Angles (deg) for
a
(
Cp*MoCl) (3)
2
B
2
H
6
(Cp*MoCl)
2 3 7
B H (3)
Distance
Mo(2)-B(2)
Distance
Mo-H(3)
Mo(1)-C(1)
Mo(1)-C(2)
Mo(1)-C(3)
Mo(1)-C(4)
Mo(1)-C(5)
Mo(1)-B(1)
Mo(1)-B(2)
Mo(1)-Cl(2)
Mo(1)-Cl(1)
Mo(1)-Mo(2)
Mo(1)-H(12)
Mo(1)-H(22)
Mo(2)-B(1)
2.27(2)
2.24(2)
2.18(2)
2.29(2)
2.36(2)
2.34(2)
2.42(3)
2.474(5)
2.490(5)
2.696(2)
1.6(2)
2.40(2)
2.493(5)
2.502(5)
1.8(2)
Mo-B(1)
2.144(8)
1.74(6)
Mo(2)-Cl(2)
Mo(2)-Cl(1)
Mo(2)-H(13)
Mo(2)-H(23)
B(1)-B(2)
Mo-B(2)#1
Mo-B(2)
Mo-C(1)
Mo-C(5)
Mo-C(2)
Mo-C(3)
Mo-C(4)
Mo-Cl
2.257(8)
2.286(8)
2.302(7)
2.312(7)
2.353(8)
2.355(6)
2.358(7)
2.423(2)
3.0958(12)
B(1)-B(2)#1
B(1)-B(2)
B(1)-Mo#1
B(1)-H(1)
B(2)-Mo#1
B(2)-H(2)
B(2)-H(3)
B(2)-H(4)
1.838(11)
1.838(11)
2.144(8)
1.06(14)
2.257(8)
1.27(5)
1.6(2)
1.63(3)
1.37(5)
1.36(5)
1.36(5)
1.36(5)
1.37(5)
1.37(5)
B(1)-H(11)
B(1)-H(12)
B(1)-H(13)
B(2)-H(21)
B(2)-H(22)
B(2)-H(23)
1.26(6)
1.06(7)
Mo-Mo#1
Angle
1.8(2)
B(1)-Mo-B(2)#1
B(1)-Mo-B(2)
B(2)#1-Mo-B(2)
B(1)-Mo-C(1)
B(1)-Mo-C(2)
B(1)-Mo-C(3)
B(1)-Mo-C(4)
B(1)-Mo-C(5)
B(2)-Mo-C(1)
B(2)-Mo-C(2)
B(2)-Mo-C(3)
B(2)-Mo-C(4)
B(2)-Mo-C(5)
49.3(3) B(2)#1-Mo-C(5)
149.1(3)
135.3(2)
98.3(3)
112.1(3)
43.8(2)
47.4(2)
46.6(2)
117.8(9)
69.6(4)
68.5(4)
68.5(4)
69.6(4)
92.4(4)
62.2(3)
61.5(3)
85.9(3)
2.37(2)
48.9(3) B(1)-Mo-Cl
Angle
C(1)-Mo(1)-B(1) 103.1(9) B(2)-Mo(1)-Cl(1)
C(2)-Mo(1)-B(1) 98.9(9) B(2)-Mo(1)-Cl(2)
87.7(3) B(2)-Mo-Cl
85.6(7)
109.6(6)
76.3(2)
55.7(6)
55.6(6)
72.7(14)
71.0(12)
69.8(6)
69.2(11)
67.4(13)
68.0(7)
89.6(3) B(2)#1-Mo-Cl
82.1(3) B(1)-Mo-Mo#1
109.8(3) B(2)#1-Mo-Mo#1
140.3(3) B(2)-Mo-Mo#1
124.5(3) B(2)-B(1)-B(2)#1
92.9(3) B(2)#1-B(1)-Mo#1
112.4(3) B(2)-B(1)-Mo#1
147.1(4) B(2)#1-B(1)-Mo
144.3(3) B(2)-B(1)-Mo
109.0(3) Mo#1-B(1)-Mo
C(3)-Mo(1)-B(1) 126.6(9) Cl(2)-Mo(1)-Cl(1)
C(4)-Mo(1)-B(1) 156.8(8) B(1)-Mo(1)-Mo(2)
C(5)-Mo(1)-B(1) 135.8(9) B(2)-Mo(1)-Mo(2)
C(1)-Mo(1)-B(2) 125.2(9) B(2)-B(1)-Mo(1)
C(2)-Mo(1)-B(2)
98.8(9) B(2)-B(1)-Mo(2)
C(3)-Mo(1)-B(2) 103.8(8) Mo(1)-B(1)-Mo(2)
C(4)-Mo(1)-B(2) 136.2(8) B(1)-B(2)-Mo(2)
C(5)-Mo(1)-B(2) 156.5(8) B(1)-B(2)-Mo(1)
B(1)-Mo(1)-B(2)
B(1)-Mo(1)-Cl(1) 110.0(7) Mo(1)-Cl(1)-Mo(2)
B(1)-Mo(1)-Cl(2) 84.8(6) Mo(1)-Cl(2)-Mo(2)
39.9(8) Mo(2)-B(2)-Mo(1)
B(2)#1-Mo-C(1) 120.7(3) B(1)-B(2)-Mo#1
65.37(13)
65.74(12)
B(2)#1-Mo-C(2)
B(2)#1-Mo-C(3)
90.9(3) B(1)-B(2)-Mo
93.2(3) Mo#1-B(2)-Mo
B(2)#1-Mo-C(4) 124.5(3)
units. Note: The ratio of cocrystallized 3 and 2 is ≈6 as independently
a
1
Symmetry transformations used to generate equivalent atoms: #1
determined from H NMR of the single crystals used. In the refinement
y, x, -z+2.
free variables were used to restrain divergence of the B-B and Cl‚‚‚-
Cl distances. After all non-hydrogen atoms were refined anisotropically,
all hydrogen atoms were then located in a difference Fourier map.
Both borane and methyl hydrogens were included in the final
refinement, the former being treated isotropically with bond length
restraints, the latter as idealized riding atoms [r(C-H) ) 0.96 Å, Uiso
-
2
(
H) ) 1.5 Ueq(C)]. The structure was refined on F by full-matrix
27
least-squares methods using SHELXTL V5. Coordinates and selected
bond distances are listed in Tables S2 (Supporting Information) and 6,
respectively.
2 3 7
(Cp*MoCl) B H (5). A dark green platelike crystal of 5 was grown
by slow diffusion of hexane into a concentrated toluene solution at
room temperature over a period of 2-3 days and mounted in a capillary
under argon prior to data collection. Most of the non-hydrogen atoms
were located by direct methods, and the remainder found in successive
difference Fourier syntheses. All non-hydrogen atoms were refined
anisotropically, and all hydrogen atoms were then located in a difference
Fourier map. Both borane and methyl hydrogens were included in the
final refinement, the former being treated isotropically, the latter as
idealized riding atoms [r(C-H) ) 0.96 Å, Uiso(H) ) 1.5 Ueq(C)]. The
2 4
Figure 1. ORTEP drawing of (Cp*MoCl) B H10 (4).
2
structure was refined on F by full-matrix least-squares methods using
2
7
molybdenum halides 1 and 2 with monoboron reagents are
summarized in Schemes 1-3. The structures shown are based
upon analyses of the spectroscopic and crystallographic data
which follow. The reaction pathway is based upon the results
of NMR-monitored experiments which are described subse-
quently.
SHELXTL V5. Selected bond distances are listed in Table 7; other
data have been deposited in conjunction with the original communica-
1
7
tion.
MO Calculations. Fenske-Hall calculations28,29 were carried out
for the molecules (CpMoCl) (3′) and (CpCo) (8′); in both
cases the geometry used was that derived from the solid-state structure
2
B
H
3 7
2 3 7
B H
5
determination of the corresponding Cp* (η -C
5 5
Me ) derivative (includ-
(Cp*MoCl)2B4H10 (4). Reaction of 1 (or 2) with a 7-fold
ing hydrogen atom positions) with the simplification that the per-
5
15,24
(5-fold) excess of BH3‚THF generates two products which are
methylated Cp* ligand was replaced by Cp (η -C
H
5 5
).
For both
18
easily separated by solubility. The less hexane-soluble mate-
molecules 3′ and 8′ the minimal atomic orbital (AO) basis set
calculations were transformed into fragment basis sets for two CpM
11
rial has a molecular formula (Cp*MoCl)2B4H10, but the B and
1
(
M ) MoCl or Co) and B
3
H
7
fragments.
H NMR spectra imply a low symmetry structure featuring a
single plane of symmetry. For this reason the structure was
only defined by a single-crystal X-ray diffraction study, and it
is given in Figure 1; relevant interatomic distances and angles
are listed in Table 5. The structure is consistent with the
observed spectroscopic data and shows two Cp*Mo units linked
by a pair of bridging chloride ligands and in a side-on fashion
Results and Discussion
Synthesis and Characterization of New Molybdaboranes.
The results of the reactions of the pentamethylcyclopentadienyl
(
28) Fenske, R. F. Pure Appl. Chem. 1988, 27, 61.
(29) Hall, M. B.; Fenske, R. F. Inorg. Chem. 1972, 11, 768.

Synthesis of NoVel Molybdaboranes
J. Am. Chem. Soc., Vol. 120, No. 11, 1998 2591
by a B4H10 unit. Bridging of the dimetal unit by B(1) and B(2)
Scheme 1
2-
is reminiscent of the interaction of the B2H6 ligand with a
30
dinuclear group 5 unit in (C5Me4EtNb)2(B2H6)2, (p-tolNCHNp-
3
1
32
tol)4Ta2(B2H6),
Cp*TaBr)2B2H6.32 Comparison of the bonding parameters for
with analogous M2B2 units reveals B-B [1.70(2) Å] and
B-Hb distances [1.05(2)-1.08(2) Å] which are within the range
(Cp*Ta)2(B2H6)2,
and one form of
(
4
3
0,31
of values expected for this mode of coordination.
The
similar M-B distances for the molybdenum [2.42(2)-2.44(2)
Å], tantalum [2.48(1)-2.51(1) Å for (p-tolNCHNp-tol)4Ta2-
3
1
(
(
B2H6) ], and niobium [2.401(4)-2.408(4) Å for (C5Me4EtNb)2-
3
0
B2H6)2 ] compounds reflect similar radii for the metals
involved.
3
3
The structure of 4 is viewed as a dimolybdenum unit bridged
by two chlorides and by a [B2H6]2- ligand in which one single
terminal hydrogen has been replaced by a µ-B2H5 fragment. In
common with most other structurally characterized B2H5 units,
the H2B‚‚‚BH2 unit is planar and approximately perpendicular
to the plane defined by the BBHB unit. The B‚‚‚B distance
within the B2H5 unit [1.79(3) Å] is consistent with those
3
4-36
obtained for other related compounds.
Furthermore, the
chemical shift is consistent with hydrogen-bridged rather than
direct Mo-B linkages. The H NMR spectrum reveals a single
Cp* resonance (relative intensity 30), a single Mo-H-B signal
B(2)-B(3) and B(2)-B(4) distances [1.77(2) and 1.87(2) Å,
respectively] are similar to the analogous values in [CpCo(Me3-
Si)2C2B4H3][B2H5] [1.789(2) and 1.799(2) Å ] and also to those
in B3H7CO [1.820(6) and 1.803(6) Å ]. In common with these
compounds the B2H5 unit in 4 is viewed as being linked to the
third boron atom via a three-center boron-boron-boron bond.
1
3
5
(
intensity 4), and one BH resonance (intensity 2). The spectral
36
2-
data are consistent with an ethane-like B2H6 unit sym-
metrically bridging a dimolybdenum framework as shown in
Scheme 2. Similar spectra reported for the minor isomer of
2
-
32
30
As a terminal hydrogen from an ethane-like [B2H6] unit
has been replaced by B2H5, the bridging ligand is formally
B4H102 , and the molybdenum centers are formally in the +III
2 2 6 5 4 2 2 6
(Cp*TaBr) B H and for (C Me EtNbCl) B H , together with
the absence of any resonance attributable to a BHB hydrogen
-
11
corroborate this assignment. In addition, the B NMR shift
3
2
1
oxidation state. The ditantalum compound (Cp*TaBr)2B2H6
and the H NMR shift for the Mo-H-B protons are similar to
is said to have a TadTa double bond suggesting a single bond
between the two molybdenum centers in 4. Similarities in the
Mo-Mo bond length [2.711(1) Å] and Mo(µ-Cl)Mo angles
66.71(8), 66.21(8)°] of the Mo(µ-Cl)2Mo unit with those
reported for [(C5H4 Pr)Mo(µ-Cl)2]2 [2.607(1) Å and 63.17(3),
3.34(3)°, respectively ] are consistent with a single bond
linking two Mo(III) centers.
Cp*Mo)2B5H9 (7). Of the two final products produced by
the reaction of 1 (or 2) with BH3‚THF, 7 is much more soluble
in hexane. The NMR data are virtually identical to those
reported for the structurally characterized molybdaborane (C5H4-
those found in 4 which contains a related ethane-like B H R
2
5
unit.
The single-crystal X-ray diffraction structure of 3 is repro-
duced in Figure 2; relevant interatomic distances and angles
are listed in Table 6. The asymmetric unit contains two
independent (Cp*MoCl) B H molecules which differ little in
[
i
37
6
2
2
6
terms of geometric parameters (Table 6 lists parameters for only
one of the two molecules). The structure is consistent with the
observed spectroscopic data and shows two Cp*Mo units linked
by a pair of bridging chloride ligands and in a side-on fashion
by a B2H6 unit. The B-B distance [1.63(3) Å] is in the range
(
12
2-
MeMo)2B5H9, and the pattern of resonances is also very similar
expected for the directly bonded ethane-like (H3B-BH3)
1
8
to that found for the chromaborane (Cp*Cr)2B5H9.
C5H4MeMo)2B5H9 and (Cp*Cr)2B5H9 are best viewed as 46
cve (six sep) clusters based around a M2B3H3 trigonal bipyra-
midal core (M ) Cr or Mo) capped by two BH3 fragments,
and an analogous structure for 7 is therefore proposed on the
Both
ligand predicted spectroscopically, being very slightly shorter
(
than the corresponding distance in (p-tolNCHNp-tol)4Ta2(B2H6)
3
1
[1.68(2) Å ] and markedly shorter than that observed for the
1
1,12
2-
hydrogen-bridged (H2BHBH3) ligand found for (Cp*TaBr)2-
3
2
B2H6 in the solid state [1.88(3) Å ].
basis of this spectroscopic evidence.
Viewed as a metallaborane cluster, 3 has the correct electron
count for an M2B2 tetrahedron (six sep’s or 40 cve); such a
formulation implies a Mo-Mo bond, and the observed inter-
atomic distance [2.696(2) Å] is consistent with this. 3 can
therefore be seen as a tetrahedral Mo2B2 cluster with a doubly
chloride-bridged Mo-Mo bond and four hydrogen-bridged
Mo-B linkages. Alternatively, viewing 3 as a complex
(Cp*MoCl)2B2H6 (3). Mass spectral data for 3 are consistent
11
with the formulation (Cp*MoCl)2B2H6; the B NMR spectrum
indicates the presence of a single boron environment, and the
(
30) Brunner, H.; Gehart, G.; Wachter, J.; Wrackmeyer, B.; Nuber, B.;
Ziegler, M. L. J. Organomet. Chem. 1992, 436, 313.
31) Cotton, F. A.; Daniels, L. M.; Murillo, C. A.; Wang, X. J. Am.
Chem. Soc. 1996, 118, 4830.
(
2-
featuring two Mo(III) centers and a bridging B2H6 ligand
(isoelectronic with C2H6), the molybdenum centers each attain
an 18-electron configuration by the formation of an Mo-Mo
(
32) Ting, C.; Messerle, L. J. Am. Chem. Soc. 1989, 111, 3449.
(33) Huheey, J. E. Inorganic Chemistry; Principles of Structure and
ReactiVity; Harper and Row: New York, 1972.
2-
single bond. This requires the B2H6 ligand to act as an eight-
electron ligand (four to each metal center) which it accomplishes
by utilizing four BH bonds each as a two-electron donor. This
view of 3 is given further weight by the description of the
(
34) Borelli, A. J.; Plotkin, J. S.; Sneddon, L. G. Inorg. Chem. 1982,
2
1, 1328-1331.
(
(
35) Briguglio, J. J.; Sneddon, L. G. Organometallics 1985 4, 721.
36) Glore, J. D.; Rathke, J. W.; Schaeffer, R. Inorg. Chem. 1973, 12,
2
175.
37) Grebenik, P. D.; Green, M. L. H.; Izquierdo, A.; Mtetwa, V. S. B.;
Prout, K. J. Chem. Soc., Chem. Commun. 1987, 9.
3
2
analogous group 5 compounds (Cp*TaBr)2B2H6 and (C5Me4-
(
3
0
EtNbCl)2B2H6 as MdM doubly bonded dimers.

2592 J. Am. Chem. Soc., Vol. 120, No. 11, 1998
Aldridge et al.
Scheme 2
(
Cp*MoCl)2B3H7 (5). Of the three intermediate species
The structure of 5 is best viewed as a trigonal bipyramid; the
required number of skeletal electron pairs is six (42 cve), and
since the electron count for (Cp*MoCl)2B3H7 is only five pairs
(40 cve) this implies that the molecule is electronically
unsaturated.
identified, 5 is present in the lowest concentration and is by far
17
the most sensitive to air and moisture. Mass spectral data
and 11B and H NMR data are consistent with the molecular
structure shown in Scheme 2. This is confirmed by a single-
crystal X-ray diffraction structure reproduced in Figure 3;
relevant interatomic distances and angles are listed in Table 7.
1
Comparison of the structural metrics of 5 with those of the
12
saturated species {(C5H4Me)Mo}2B5H9 reveals a longer Mo-

Synthesis of NoVel Molybdaboranes
J. Am. Chem. Soc., Vol. 120, No. 11, 1998 2593
B-B distances are considerably longer for the 5 than for {(C5H4-
Me)Mo}2B5H9. The principal geometric differences between
saturated {(C5H4Me)Mo}2B5H9 and unsaturated 5 are mirrored
by similar changes between saturated and unsaturated derivatives
8
,44
of the Cr2B4 system.
Comparison with the seven skeletal pair dicobalt species
16
(
Cp*Co)2B3H7 (8) is instructive. The differences between the
two species are consistent with the differing electron counts.
Corresponding to its predicted nido structure, the Co-Co
distance in (Cp*Co)2B3H7 is long and nonbonding, and the
B-B-B angle is 100.6(7)° which approximates that expected
for three boron atoms on adjacent vertexes of an octahedron.
In 5 the Mo-Mo distance is shorter and, as indicated above,
Mo-Mo bonding. In addition, the B3 ligand is more “opened
out” in 5 than in (Cp*Co)2B3H7 with longer B-B distances
and a wider B-B-B angle [117.8(9)°]. Both differences are
consistent with the two Mo atoms and three boron atoms of 5
occupying two equatorial and two apical and one equatorial
positions, respectively, of a trigonal bipyramid.
Interestingly, the two chloride ligands in 5 adopt terminal
positions, rather than the µ2-bridging coordination mode seen
for both chlorides in 3 and 4. In the latter compounds the four
extra electrons supplied by the chlorides adopting µ2-bridging
rather than terminal positions allows the skeletal electron count
to reach the six pairs required for a tetrahedron-based structure.
In 5 the presence of an extra BH group in the cluster (B3H7 vs
B2H6, in effect) means that the electron pair count would be
five with two terminal chlorides or seven if both were µ2-
bridging. The steric requirement of the B3H7 ligand is clearly
greater than that of B2H6 and this may force the chlorides to
adopt terminal positions with the consequence that the Mo-
Mo distance is longer [3.096(1) Å for 5 vs. 2.696(2) for 3 and
Figure 2. ORTEP drawing of the two independent molecules making
up the asymmetric unit of (Cp*MoCl) B H (3).
2 2 6
2
.710(1), 2.690(2) Å for 4] and the angle between the two Cp*
rings is larger [49.7(2)° for 5 vs 1.5(8), 4.9(9) for 3 and 9.85°
for 4].
(
Cp*Mo)2(B2H6)2 (6). In addition to being one of three
intermediates in the formation of 4 and 7 from Cp*MoCl4, 1,
Cp*Mo)2(B2H6)2 (6) is the predominant product of the reaction
(
of 1 with LiBH4, being formed in ca. 70% yield when 10 equiv
of the borohydride are used. 6 is converted slowly to the Mo2B5
species 7 by reaction with further LiBH4 at 55 °C over a period
of several weeks. Mass spectral data for 6 are consistent with
the formulation (Cp*Mo)2(B2H6)2, and the single signal observed
2 3 7
Figure 3. ORTEP drawing of (Cp*MoCl) B H (5).
Mo distance [3.0958(12) vs 2.812(1) Å] and shorter M-B
distances. The former precludes multiple bonding between the
molybdenum centers which is one possible response to electronic
unsaturation. That is, species containing triple or quadruple
bonds between molybdenum centers typically have internuclear
distances of less than 2.5 Å [e.g., (Me3SiCH2)3MotMo(CH2-
SiMe3), 2.167 Å;38 Me(Me2N)2MotMo(NMe2)2Me, 2.201 Å;
1
1
in the B NMR spectrum indicates a highly symmetrical
1
structure in solution. Three H NMR signals are observed with
the intensity ratio 15:4:2 attributed to the Cp*, MoHB, and BH
protons, respectively. These spectral data are consistent with
a dimeric (Cp*Mo)2 unit symmetrically bridged by a pair of
equivalent ethane-like B2H6 units as shown in Scheme 2.
Electronic Structure of an Unsaturated Molybdaborane.
Fenske-Hall molecular orbital (MO) calculations were carried
out in order to determine (i) why the unsaturated species 5 is
stable despite having two fewer electrons than required by the
Wade-Mingos rules; (ii) why the metallaborane clusters 5 and
39
40
Me2(Me3P)2MotMo(PMe3)2Me2, 2.153 Å ]. On the other
hand, the observed Mo-Mo distance of 5 is shorter than that
41
2-
observed in both [CpMo(CO)3]2 (3.22 Å ) and Mo2(CO)10
4
2
(
3.123 Å ) with metal-metal single bonds and much shorter
than the 3.575 Å found in the cation [{(MeO)3P}2(OC)2Mo(µ-
Cl)3Mo(CO)2{P(OMe)3}2] which has no Mo-Mo bond. The
8
are both stable despite skeletal electron counts differing by
n+
43
two pairs; and (iii) why the two examples of unsaturated
metallaboranes synthesized to date (5 and (Cp*Cr)2B4H8, 9)
display similar geometric changes with respect to saturated
analogues (i.e., lengthening of the M-M bond and shortening
of M-B linkages).
(
38) Huq, F.; Mowat, W.; Shortland, A.; Skapski, A. C.; Wilkinson, G.
J. Chem. Soc., Chem. Commun. 1971, 1079.
39) Girolami, G. S.; Mainz, V. V.; Andersen, R. A.; Vollmer, S. H.;
Day, V. W. J. Am. Chem. Soc. 1981, 103, 3953.
40) Chisholm, M. H.; Cotton, F. A.; Extine, M. W.; Murillo, C. A. Inorg.
Chem. 1978, 17, 2338.
41) Gould, R. O.; Barker, J.; Kilner, M. Acta Crystallogr. 1988, C44,
61.
(
(
Relevant sections of the eigenvalue spectra calculated for
(
(
CpMoCl)2B3H7 (5′) and (CpCo)2B3H7 (8′) by Fenske-Hall
4
7
(
42) Handy, L. B.; Ruff, J. K.; Dahl, L. F. J. Am. Chem. Soc. 1970, 92,
molecular orbital methods are reproduced in Figure 4. Also
312.
(43) Drew, M. G. B. J. Chem. Soc., Dalton Trans. 1975, 1984.
(44) Fehlner, T. P. J. Organomet. Chem. In press.

2594 J. Am. Chem. Soc., Vol. 120, No. 11, 1998
Aldridge et al.
2 3 7 2 3 7 2 4 8
Figure 4. Molecular orbital scheme for (CpMoCl) B H (5′), (CpCo) B H (8′), and (CpCr) B H (9′).
included is part of the MO scheme calculated previously for
the unsaturated chromaborane (CpCr)2B4H8 (9′).8,44 Consistent
with the stability of both 8′ and 5′, a comparison of the MO
pictures reveals a significant HOMO/LUMO gap for the
saturated 8′ as expected as well as for the unsaturated 5′.
In 5′ MOs 45 and 49 constitute a Mo-Mo σ bonding/
antibonding pair of orbitals; MO 45 is low-lying and filled,
whereas MO 49 is high-lying and empty. The corresponding
pair of orbitals in 8′ constitute part of the filled, nonbonding
set lying at low energy. In essence, the Mo-Mo bonding
interaction implied by the MO diagram is consistent with the
Mo-Mo distance as well as the view of 5 as based on a trigonal
bipyramidal geometry, i.e., one of the two extra pairs of
electrons present in the nido cobalt system is unneeded in the
closo molybdenum system. The other pair of pertinent orbitals
is occupied MO 38 for 8′, which lies at -12.94 eV, and
unoccupied MO for 5′ (48) at -4.27 eV. Both MOs are M-B
antibonding and the occupation numbers agree with the differ-
ences in M-B distances observed in the structures. The large
difference in energy between the two MOs is traced to the
difference in energies of the 3d functions of the two metals and
the differences in cluster geometry. In the case of Co the 3d
functions are more core-like and low-lying. For Cr they are
found at higher energy and are a better match in energy and
size for the borane fragment orbitals thereby enhancing the
M-B antibonding character of MO 48. The net result is that
9′ has one fewer occupied MO.44 Further, the MO description
of the metal-metal bonding in 5′ and 9′ is similar. The pleasing
correspondence between the geometric and MO characteristics
of 5 and 9 corroborates the description of these compounds as
unsaturated metallaboranes.
Pathways for Reaction of [Cp*MoCln], n ) 1, 2, 4, with
Monoboranes. The reaction of the monocyclopentadienyl
molybdenum halides 1, 2, and [CpMoCl] (produced by the
-
18
stoichiometric reduction of 2 with [Et3BH] ) with BH3‚THF
have been examined by multinuclear NMR. Further the reaction
of LiBH4 with 1 and [CpMoCl] have been examined in like
manner. The results are summarized in Schemes 1-3 (BH3‚-
-
THF is represented by BH3 and LiBH4 by BH4 ) and discussed
below. The compounds in brackets are postulated inter-
mediatessall other compounds have been isolated and charac-
terized as described above.
-
Borane. The reduction of 2 with 2 mol of [Et3BH] followed
by the removal of the Et3B and treatment with BH3‚THF results
in the formation of 7 as the single metallaborane product along
1
8
with BH2Cl as a coproduct. We suggest (Cp*MoBH3)2 as an
intermediate as (Cp*CoBH3)2 has been identified in the reaction
of (Cp*CoCl)2 with monoboranes leading ultimately to
1
6
(Cp*Co)2B3H7 and other products. Further, we have shown
that (Cp*Cr)2B4H8, 9, adds borane to yield the chromium
analogue of 7 thereby lending credance to a stepwise build-up
1
8
of the borane fragment in these metallaboranes.
5
′ has two fewer bonding MOs and two fewer skeletal pairs
than 8′.
Similar orbital interactions account for the unsaturation in
CpCr)2B4H8 (9′). Relative to the saturated chromaboranes
CpCr)2B4H8Fe(CO)3 and (CpCr)2B4H6(CO)2, there is a length-
As already indicated in the earlier sections, the reaction of
(Cp*MoCl2)2 (2) with borane gives a more complicated set of
products showing that more than a simple reduction of 2 to
[CpMoCl] takes place. The discussion is simplified by estab-
lishing that reaction of Cp*MoCl4 (1) with BH3‚THF yields 2
as the sole product. In a reaction of 1 with borane at room
(
(
9,44
ening of the Cr-Cr bond and shortening of the Cr-B bonds.
11
Since the LUMO for 9′ is Cr-Cr bonding and Cr-B antibond-
ing, it is found at high energy and unoccupied. As the
corresponding orbital is occupied in its saturated counterparts,
temperature, integration of the B NMR spectra measured after
90 min shows that the initial step involves the consumption of
2 equiv of BH3‚THF and formation of 2 equiv of BH2Cl. At

Synthesis of NoVel Molybdaboranes
J. Am. Chem. Soc., Vol. 120, No. 11, 1998 2595
Figure 5. Concentration vs time graphs for reaction mixture 3 (see
Table 3) obtained by periodic monitoring of 11B NMR signal intensi-
ties: (a) monoboron species; (b) intermediates 3 and 6; and (c) final
products 4 and 7.
Figure 6. Concentration vs time graphs for reaction mixture 1 (see
Table 3) obtained by periodic monitoring of B NMR signal intensi-
ties: (a) monoboron species; (b) intermediates 3, 5, and 6; and (c) final
products 4 and 7.
11
the same time all of the purple Cp*MoCl4 suspended in the
toluene disappears, gas is evolved, and the resulting solution is
brown in color. These results taken together with the finding
that 1 and 2 give near identical mixtures of products under
similar conditions show that the initial step in the reaction
sequence is reduction of 1 to 2 via reaction 1
Cp*MoCl (1) + 2BH ‚THF f
4
3
1
/2
(Cp*MoCl ) (2) + 2BH Cl + H (1)
2 2
2
2
1
This is corroborated by H NMR monitored experiments that
reveal the growth of signals due to 2 in the same time span.
For convenience, then, 1 was used in the detailed NMR
experiments described next.
Figure 7. Relative concentration vs time graphs for reaction mixture
Three 11B NMR-monitored reactions were carried out in
which the ratio of reactants 1:BH3‚THF was varied between
1
1 (see Table 3) obtained by periodic monitoring of H NMR signal
intensities: 11B NMR silent species (Cp*MoCl
hydride intermediate.
2 2
) (2) and a molybdenum
1:4 and 1:7 (see Table 3). Relevant concentration vs time graphs
for the various components of the three different reaction
mixtures are reproduced in Figures 5 and 6 (see also Figure 8,
Supporting Information). Concentration vs time graphs for the
BH3‚THF and BH2Cl. As expected BH3‚THF is consumed, and
its concentration diminishes sharply with a corresponding growth
in the concentration of BH2Cl. The total concentration of
monoboron species decreases with time as the amount of boron
incorporated within the various metallaboranes increases. Figure
5, parts b and c shows how the concentrations of the intermedi-
ates 3 and 6 and the two final products (Cp*MoCl)2B4H10 (4)
11
1
B silent components of the H-monitored reaction mixture are
reproduced in Figure 7. Several features are apparent from
analysis of the data. The 11B NMR spectra obtained from
reaction mixture 3 are used to generate Figure 5. Figure 5a
shows the variation in concentration of the monoboron species

2596 J. Am. Chem. Soc., Vol. 120, No. 11, 1998
Aldridge et al.
and (Cp*Mo)2B5H9 (7) vary with time. The concentrations of
and 7 show the expected concentration/time behavior, and it
Scheme 3
4
is noteworthy that 4 appears to be the first formed of the two.
For t < 1000 min the concentration of 4 is greater than that of
7
by at least a factor of 3, and only at longer reaction times
does the ratio appear to diminish noticeably. Of the intermediate
species, 3 is the first to be detected; its concentration reaches
nearly 10 mM after 480 min but dies away to zero after 4000
min. 6 is formed much later in the course of the reaction,
reaching its maximum of concentration at ca. 2500 min and is
not fully consumed even after 8000 min. No other borane
containing species are observed during the course of the reaction.
Analogous concentration/time graphs for reaction mixture 1
(containing less BH3‚THF) are shown in Figure 6 and reveal
different time-dependent behavior. First, the concentrations of
the species 3 and 6 do not die away completely due to the
insufficient amounts of BH3‚THF present, which prevents
complete reaction to 4 and 7. In addition, the intermediate
(Cp*MoCl)2B3H7 (5) is clearly detected in this reaction mixture,
albeit at low concentration peaking at around t ) 600 min. 5
was barely detectable in reaction mixtures containing higher
concentrations of BH3‚THF. If the cluster expansion proceeds
from 5 to 7 by addition of two BH fragments and loss of Cl (as
BH2Cl), then the rate of loss of 5 will depend strongly on the
concentration of BH3‚THF. Only when the rate of the cluster
expansion step is slowed significantly by reducing the amount
of BH3‚THF available is the highly reactive unsaturated species
5
detected.
The other major difference between reaction mixtures 1 and
is that the concentrations of two species which appeared to
3
be ultimate products in reaction 3 [BH2Cl and (Cp*MoCl)2B4H10
4)] reach maxima and then begin to diminish. In reaction
(
mixture 1 the concentration of 4 decays to zero at t ) 4400
min and is accompanied by a similar decrease in the concentra-
tion of BH2Cl; however no resonances attributable to new
metallaboranes were observed to grow in. Possibly 4 reacts
with BH2Cl to yield a molybdenum chloride species (rather than
a metallaborane); cluster degradation yielding metal halide
products has previously been observed to occur in the reaction
4
7
Mo(µ-Cp)(µ-H)Mo(H)Cp and -11.45 for CpMo(CO)2(µ-H)(µ-
4
8
I)Mo(CO)2Cp ], and the signals for both molybdenum-bound
protons in Cp2Mo(µ-Cp)(µ-H)Mo(H)Cp are described by Green
4
7
et al. as “broad.”
It is certainly not out of the question that
a bridging hydride could feature in the early stages of the
reaction pathway from Cp*MoCl4 to (Cp*Mo)2B5H9 although
it is difficult to state this with confidence on the basis of these
observations.
45
of (Cp*Cr)2B4H8 with species containing a B-Cl bond. This
assertion was confirmed by an independent experiment; reaction
of 4 with BH2Cl in toluene under reaction conditions similar to
those used above resulted in consumption of 4 over a 2 week
period and the detection of no new metallaborane products by
The summaries of the borane reactions in Schemes 1 and 3
require no further comment; however, that in Scheme 2 does.
In the reaction of BH3‚THF with 1 three competing reactions
which consume the key intermediate 3 are proposed to account
for the observations. First, reaction to give 4 is irreversible,
and since 4 is unreactive toward further BH3‚THF (but reactive
1
1
B NMR.
1
The results of the H NMR-monitored experiment yield
similar concentration/time behavior for the metallaboranes 3-7
to those obtained by 11B NMR spectroscopy under the same
conditions. Figure 7 depicts concentration vs time behavior for
-
to BH4 ssee below), a product is isolated in which not all of
the chloride ligands have been replaced by borane units.
Formation of 4 from 3 by replacement of a terminal BH unit
by B(B2H5) is an unusual reaction, but one which finds precedent
in the formation of [B5H8][B2H5] and [1,6-C2B4H5][B2H5] from
B5H9 and 1,6-C2B4H6, respectively. Second, insertion of a
BH unit into the B2H6 fragment of 3 can generate the reactive
intermediate 5 (and ultimately the Mo B species 7). Third,
replacement of the remaining two bridging chlorides in 3 by
B H to give 6 also takes place, and appreciable concentrations
of 6 are observed in the reaction mixtures; reaction of 6 with
1
1
B silent speciess(Cp*MoCl2)2 (2) and a second unknown
species which gives rise to a resonance at δH -11.5 with no
1
1
1
apparent coupling to B in the H NMR and which has no
corresponding 11B signal. This species is observed during the
early stages of the reaction and subsequently dies away, showing
behavior characteristic of a reaction intermediate. It is possible
that this signal is due to some form of molybdenum
hydridesalthough the resonance is broader than might be
expected for an Mo-H species.46 The signal is certainly in the
correct region of the spectrum for a hydrogen-bridged Mo-
H-Mo linkage [cf. -12.23 for the bridging hydrogen in Cp2-
49
2
5
2
6
(
47) Bashkin, J.; Green, M. L. H.; Poveda, M. L.; Prout, C. K. J. Chem.
Soc., Dalton Trans 1982, 2485.
48) Curtis, M. D.; Fotinos, N. A.; Han, K. R.; Butler, W. M. J. Am.
(
(
(
45) Aldridge, S.; Shang, M.; Fehlner, T. P. Acta Crystallogr. In press.
46) See, for example: Jesson, J. P. In Transition Metal Hydrides;
Chem. Soc. 1983, 105, 2686.
(49) Corcoran, E. W.; Sneddon, L. G. J. Am. Chem. Soc., 1985, 107,
Muetterties, E. L., Ed.; Marcel Dekker Inc.: New York, 1971; p 87.
7446.

Synthesis of NoVel Molybdaboranes
J. Am. Chem. Soc., Vol. 120, No. 11, 1998 2597
BH3‚THF is slow, such that appreciable amounts of 6 persist
in solution even after 6 days at 55 °C [see Figure 5b].
The proposal of 3 as an intermediate common to formation
of both 5 and 6 is consistent with the concentration/time
behavior for all three species seen in Figure 5. Furthermore,
the idea that 5 is formed from 3 is consistent with the
observation [Figure 6b] that the concentration of 5 builds up to
appreciable levels only when the concentration of 3 is high (so
that the rate of formation of 5 is high) and the concentration of
BH3‚THF is low (so that the rate of consumption of 5 is low).
Insertion of BH into a BnHn+4 fragment to give a Bn+1Hn+5 unit
finds precedent in the cluster expansion reaction of (Cp*Cr)2B4H8
to (Cp*Cr)2B5H9 with BHCl2‚SMe2 or BH3‚THF.11,18 For the
same reason, (Cp*Mo)2B4H8 is postulated as an intermediate
species leading from 5 to 7.
Although one of the final products (Cp*MoCl)2B4H10 (4) is
unreactive toward BH3‚THF and can be isolated from the
-
Cp*MoCl /BH ‚THF reaction mixture, it does react with BH .
4
3
4
The bis(B H ) adduct 6 is formed together with a small amount
2
6
of 7. As indicated in reactions 3 and 4 replacement of bridging
toluene/THF
(
Cp*MoCl) B H + 2LiBH
8
2
4
10
4
(Cp*MoH)
B
H
10 + 2BH
‚THF + 2LiCl (3)
(B (4)
2
4
3
(Cp*MoH)
B
H
10 f (Cp*Mo)
H )
2 6 2
2
4
2
chlorides with hydrides by BH4- to give (Cp*MoH)2B4H10 is
followed by rearrangement to give (Cp*Mo)2(B2H6)2 (6). An
attractive alternative route by which the pendant B2H5 is clipped
Borohydride. A summary of the results for this monoborane
-
off to produce 3 and [B3H8] followed by reaction of 3 with
is also included in Schemes 1-3. The reaction of LiBH4 with
-
-
additional [BH4] to give 6 was ruled out as no [B3H8] was
-
[
Cp*MoCl] prepared in situ from 2 and [Et3BH] yields only
observed as a product.
7
. This was unexpected, and it is not clear how the presumed
Role of the Ancillary Ligand: Cp vs Cp*. To investigate
the possible role of the Cp* ligand (electronic and steric) the
analogous reactions of the Cp starting materials 1′ and 2′ with
BH3‚THF were carried out. These give only a single metall-
aborane end product identified by the spectroscopic data as
(Cp*MoBH3)2 intermediate is converted into 7 by borohydride.
Other products must be involved and the best route to a product
mixture containing 7 alone remains the reaction of borane with
[Cp*MoCl]. Note that in the case of the reaction of (Cp*CrCl)2
with borohydride, (Cp*CrBH4)2 is the sole product at room
(CpMo)2B5H9 (7′). In addition, only one intermediate species
temperature, and only on heating is 9 produced in a mixture of
is detected; given the extremely similar spectra measured for
this species and the Cp* intermediate 3 it is identified as
8
products some of which are paramagnetic.
As with borane, reduction of 1 by BH4- leads exclusively to
(CpMoCl)2B2H6 (3′). The reaction is considerably more facile
2
and is accompanied by the release of BH3‚THF, according to
in the Cp case, complete conversion to the final product 7′ being
accomplished in 3 days at room temperature (in contrast to the
55 °C temperature and 6 day reaction times required in the Cp*
case). No Cp compounds analogous to the Mo(III) metallabo-
ranes 4 and 5 or to the bis(B2H6) species 6 were detected. In
the Cp* case three reaction pathways are observed from the
intermediate 3. The differing chemistry of the Cp and Cp*
derivatives suggests that in the Cp case addition of extra BH
fragments to the intermediate 3′ (to give 5′ and hence 7′) is
much more facile than for Cp*. Consequently this route is the
predominant one, and CpMo clusters analogous to 4 and 6 are
not observed. If addition of BH fragments to the borane
framework is more facile for Cp than Cp*, then one might
reasonably expect the Cp analogue of 5 to be even more reactive
than its Cp* counterpart, and it never attains a concentration
reaction 2
toluene/THF
Cp*MoCl (1) + 2LiBH
8
4
4
1
/2
(Cp*MoCl ) (2) + 2BH ‚THF + 2LiCl + H (2)
2 2 3 2
The NMR experiments show that in the presence of a large
-
excess of LiBH4, 2 then reacts with 2 equiv of BH4 to generate
3
(with release of H2) which then reacts with 2 further equiv to
generate 6. This chemistry is completely analogous to that
observed for the corresponding niobium and tantalum systems,
in which Cp′M(µ-Cl)4MCp′ is converted to Cp′M(µ-B2H6)2-
MCp′ via the intermediate Cp′M(µ-Cl)2(µ-B2H6)MCp′ [M )
Nb, Cp′ ) C5Me4Et; M ) Ta, Cp′ ) C5Me5], i.e., by stepwise
replacement of pairs of chloride ligands by B2H62
.
- 30,32
1
1
Further reaction of 6 to 7 is very slow, as demonstrated by
the minimal conversion at 55 °C over a period of several weeks.
high enough to be observed by B NMR.
On electronic grounds one would expect the more electron
rich Cp*Mo species to be more reactive toward electrophilic
species such as BH3; it seems likely therefore that the higher
reactivity of the Cp species is based on steric rather than
electronic grounds. Build-up of the borane cage by addition
of successive BH fragments to 3 or 3′ would be expected to be
easier in the sterically less crowded CpMo dimers.
-
On the other hand, if 3.5 equiv of BH4 are used, the 2 equiv
consumed in the initial reduction of 1 to 2 results in a low
-
concentration of BH4 in solution after the next step (i.e.,
formation of 3 from 2). Consequently the rate of conversion
of 3 to 6 (which would be expected to be dependent on the
-
concentration of BH4 ) will be lower, and the concentration of
3
remaining will be higher. This is observed to be the case.
-
BH3‚THF vs BH4 . Much of the known chemistry of early
Moreover, with 2 equiv of BH3‚THF being generated in the
initial reduction step, reaction of 3 with BH3‚THF will compete
transition metal monocyclopentadienyl halides involves reaction
5
0
with nucleophiles or Lewis bases. The observed reactivity
-
with the reaction with BH4 . 3 could thus react with BH3‚-
-
of [BH4] is consistent with these observations. It now appears,
THF and be converted via 5 into 7. This would then account
for the larger amount of 7 produced when 3.5 equiv of LiBH4
are used rather than 10. Note, however, that 7 is synthesized
in far greater yield from 1 when BH3‚THF is used.
however, that high oxidation state early transition metal species
5
1
(
such as Cp*TaCl4 and Cp*MoCl4) also exhibit significant
reactivity toward electrophilic reagents such as BH3‚THF. The
initial step in the reaction may involve interaction of the BH3
unit with one of the chloride ligands to give a coordinated BH3-
Cl fragment. In fact, selected chloride salts are found to react
The completely different products produced by the reaction
of 2 vs [Cp*MpCl] with borohydride shows that reduction of 2
-
to [Cp*MoCl] by BH4 is not competitive with the reactions
shown in Scheme 2. Thus, although both monoborane reagents
take 1 to 2 they are not sufficiently reducing to produce
(
50) See, for example, Poli, R. Chem. ReV. 1991, 91, 509.
(51) Aldridge, S.; Hashimoto, H.; Shang, M.; Fehlner, T. P. J. Chem.
[Cp*MpCl] on the time scale of the cluster building reactions.
Soc., Chem. Commun. 1998, 207.

2598 J. Am. Chem. Soc., Vol. 120, No. 11, 1998
Aldridge et al.
with borane to give chloroborohydride ion.52 Elimination of
BH2Cl could then occur (as is observed experimentally) leaving
a metal hydride intermediate. A hydride intermediate (Cp*Ta-
Based on this work as well as earlier studies, borane seems to
be the more versatile reagent for cleanly generating metallabo-
ranes, but both reagents have distinct value, e.g., the best route
1
-
Cl2)2H2(BH3) is observed in the tantalum system, and a H signal
to 6 is via [BH ] , whereas the best route to 7 is via BH ‚THF.
4
3
at δH -11.5 observed in the molybdenum system displays a
concentration/time behavior consistent with that of a hydride
intermediate early in the reaction scheme. Elimination of H2
could then complete the necessary reduction of the metal center,
whereas addition of another equivalent of borane could lead to
metallaborane products.
For Cp*MXn species in lower oxidation states borohydride
generates metal borohydrides rather than hydrides, and these
eliminate hydrogen to give metallaboranes such as 3 and 6. On
the other hand, the Lewis acidity of the borane fosters cluster
condensation which competes with chloride abstraction/reduc-
tion so that “mixed” species containing both borane fragments
and chloride ligands are obtained (e.g., molybdaboranes 3-5).
In the case of both chromium and molybdenum the final product
is a metallaborane [(Cp*Cr)2B4H8 or (Cp*Mo)2B5H9] in which
all chloride ligands have been replaced; the difference in
products produced is attributed to larger kinetic barriers to cluster
expansion in the chromium case.
As illustrated in Schemes 1-3, one set of products is
accessible via either borane or borohydride. The coordination
ability of borane, however, leads to facile cluster expansion and
a greater variety of metallaborane products. Although the broad
outline of reactivity illustrated in the schemes is clear, the
mechanistic details are not, e.g., in the conversion of 5 to 7 the
stage at which Cl is lost will only be learned from more detailed
mechanistic studies.
The intermediate (Cp*MoCl)2B3H7 (5) represents the second
reported example of an unsaturated metallaborane cluster, and
the first for which direct comparison is possible with a saturated
later transition metal species containing the same borane
fragment. 5 contains no multiple bonding between the molyb-
denum centers and has too few skeletal atoms to adopt a
structure based on a polyhedron with fewer vertexes, these being
typical responses to electron deficiency in organometallic and
cluster chemistry, respectively. The unsaturation in 5 must be
delocalized over the cluster framework.
Comparison with (Cp*Co)2B3H7 shows that the existence of
a metal-metal bond together with the elevation of M-B
antibonding orbitals resulting from a better metal-borane energy
match are vital in stabilizing a structure with two fewer electron
pairs than the cobalt counterpart. Similar behavior observed
for (Cp*Cr)2B4H8 identify this as a more general phenomenon
in which a good energy match between metal and borane
fragment orbitals can elevate the energy of a M-B skeletal
orbital such that the cluster can support one fewer skeletal
bonding pair than predicted.
Acknowledgment. The support of the National Science
Foundation is gratefully acknowledged. S.A. also thanks the J.
William Fulbright Scholarship Board for the award of a research
scholarship.
Conclusions
Supporting Information Available: Tables of crystal data,
positional and equivalent isotropic thermal parameters, bond
distances and angles, and general displacement parameter
expressions and Figure 8, the NMR data for reaction mixture
Both monoborane reagents act in a dual role in which their
characteristic reducing properties and coordination abilities lead
to both similarities and differences in the products produced
from reaction with a given monocyclopentadienylmetal chloride.
2
, Table 3 (27 pages). See any current masthead page for
ordering and Web access instructions.
(52) Lawrence, S. H.; Shore, S. G.; Koetzle, T. F.; Huffman, J. C.; Wei,
C.-Y.; Bau, R. Inorg. Chem. 1985, 24, 3171.
JA973720N