A hydridotantalum(V)-carborane analogue of Schwartz's reagent: Synthesis and reactivity

Article abstract of DOI:10.1016/S0022-328X(97)00555-X

The dichlorotantalum metallacarborane CpCl₂Ta(Et₂C₂B₄H₄) (1) on treatment with LiA1H₄ in THF gave a hydridotantalum dimer [CpTa(H)(Et₂C₂B₄H₄)] ₂(μ,-Cl)₂ (2), which was found to undergo alkyne insertion with p-tolyl acetylene to generate exclusively trans-CpCl(p-MeC₆H₄-CH=CH)Ta(Et₂C ₂B₄H₄) (3); the reaction of the latter complex with anhydrous HCl afforded p-tolylstyrene and 1. In contrast, reaction of 2 with diphenyl acetylene or methyl phenyl acetylene gave hydridotantalum-alkyne complexes, which are apparently the first examples of alkyne π-coordination to a formal d0 metal. Treatment of 2 with styrene produced the alkyl-tantalum species CpTaCl(CH₂CH₂Ph)(Et₂C₂B ₄H₄) (5), but no reactivity was observed between 2 and the sterically hindered olefins cis-stilbene, trans-stilbene, and cyclohexene. Addition of anhydrous HCl to 5 generates 1 and styrene via an apparent β-elimination mechanism.

Full text of DOI:10.1016/S0022-328X(97)00555-X

Ž
.
Journal of Organometallic Chemistry 550 1998 469–472
Short communication
ž /
A hydridotantalum V -carborane analogue of Schwartz’s reagent:
1,2
synthesis and reactivity
)
3
Michael A. Curtis, M.G. Finn , Russell N. Grimes
Department of Chemistry, UniÕersity of Virginia, CharlottesÕille, VA 22901, USA
Received 9 April 1997
Abstract
Ž
. Ž .
The dichlorotantalum metallacarborane CpCl2Ta Et2C2B4H4 1 on treatment with LiAlH4 in THF gave a hydridotantalum dimer
w
Ž .Ž .x Ž Ž .
.
CpTa H Et2C2B4H4 2 , which was found to undergo alkyne insertion with p-tolyl acetylene to generate exclusively
m-Cl
2
2
Ž
.
Ž
. Ž .
trans-CpCl p-MeC6H4-CH5CH Ta Et2C2B4H4 3 ; the reaction of the latter complex with anhydrous HCl afforded p-tolylstyrene and
1. In contrast, reaction of 2 with diphenyl acetylene or methyl phenyl acetylene gave hydridotantalum–alkyne complexes, which are
apparently the first examples of alkyne p-coordination to a formal d0 metal. Treatment of 2 with styrene produced the alkyl–tantalum
Ž
.Ž
. Ž .
species CpTaCl CH2CH2Ph Et2C2B4H4 5 , but no reactivity was observed between 2 and the sterically hindered olefins cis-stilbene,
trans-stilbene, and cyclohexene. Addition of anhydrous HCl to 5 generates 1 and styrene via an apparent b-elimination mechanism.
q 1998 Elsevier Science S.A.
Keywords: Carboranes; Metallacarboranes; Tantalacarboranes; Tantalum; Hydridometallacarboranes
1. Introduction
alkenyl fragment to be used in a variety of ways for
4
Ž .
organic synthesis
.
Ž
.
The Group 5 metallacarborane CpCl2Ta Et2C2 B4 H4
1 , was recently prepared in our laboratories 10,11 ,
The insertion of alkenes and alkynes into the Zr–H
Ž .
w
x
Ž .
Ž
.
bond of Cp2 Zr H Cl Schwartz’s reagent to form alkyl-
and we recognized that substitution of one of the Cl
Ž
.
Ž
.
or alkenylzirconium IV complexes Cp2 ZrClR is well
Ž .
ligands with H in this complex would create a Ta V
4
w
x
documented 2–5 . Treatment of these complexes with
analogue of Schwartz’s reagent in which one C5Hy5
ligand is replaced by Et2C2 B4 H24y. Such a complex
might well exhibit potentially useful differences in reac-
tivity from Schwartz’s reagent, and moreover could be
readily tailored via regiospecific introduction of sub-
Ž
.
electrophiles such as HCl or n-bromosuccinimide NBS
cleaves the Zr–C bonds and liberates the organic prod-
w
x
uct in high yield 2,3 ; in the alkenylzirconium species,
retention of the C5C stereochemistry is observed 2 .
Transmetalation reactions allow the generated alkyl or
w x
w
x
stituents at boron positions 12–14 . We present here
the synthesis and initial reactivity studies of the first
metallacarborane of this type.
)
Corresponding author.
1 Organotransition-Metal Metallacarboranes. 49. For part 48, see
w x
Ref. 1 .
2 Dedicated to Professor Kenneth Wade on the occasion of his
2. Results and discussion
65th birthday.
3 Also corresponding author.
4 For synthetic applications of the Schwartz reagent, see Refs.
As shown in Scheme 1, reaction of 1 with LiAlH in
w
Ž .Ž
.
2 x4
Ž
w
x
THF generated the dimer CpTa H Et2C2 B4 H4
m-
6–9 .
0022-328Xr98r$19.00 q 1998 Elsevier Science S.A. All rights reserved.

(
)
470
M.A. Curtis et al.rJournal of Organometallic Chemistry 550 1998 469–472
Scheme 1.
5
.
Cl
Ž .
2 , which in contrast to Schwartz’s reagent is
Ž
ligand on 2 is well defined by its IR band yTa – H 1839
2
1
cmy1 and H NMR shift dTa – H 14.51 ppm .
Treatment of with 2.0 equivalent of 4-
6
.
Ž
.
soluble in THF and benzene at room temperature.
The product 2 is always accompanied by a smaller
quantity of 1 in an approximate 7:3 ratio, and we have
not yet found a satisfactory method for its removal.
However, the presence of small amounts of 1 furnishes
an internal standard for NMR observations, and control
experiments on 1 have established that it is a nonpartici-
pant in the reactions of 2 to be described. The hydride
2
7
ethynyltoluene gave only the trans vinylic complex 3,
indicating that Ta–H addition occurs in a cis fashion
Scheme 1 ; the vinyl proton shifts appear at doublets
with JH – H s18.6 Hz at d 7.39 and 6.68 ppm. In order
to explore the steric effects of Ta–H addition to alkynes,
2 was reacted with 2.0 equivalent of diphenylacetylene.
The expected vinylic species was not observed; instead,
Ž
.
5
Ž
Under a N2 atmosphere the orange-air-stable complex 1 500 mg,
1.12 mmol was treated with 0.33 equivalent of LiAlH4 1 M in
6 In support of this assignment, the absorption at 1839 cmy1 is
absent in the corresponding deuteride. The Ta–D band was not
identified and is assumed to lie within the complex pattern of
.
Ž
.
.
THF in THF at y788C . The solution turned golden yellow as it
warmed to room temp over 3 h. The solvent was removed in vacuo
and the residue extracted with toluene. Removal of the toluene from
absorptions in the range 400 to 1500 cmy1
A 150 mg sample of a 30:70 mixture of 1 and 2 122 mg, 0.148
.
mmol of 2 was dissolved in 1.5 ml of C6 D6, and 4-ethynyltoluene
1
.
7
Ž
.
Ž
the filtrate afforded 380 mg of a 30:70 mixture of 1 72 mg and 2
1
Ž
.
Ž
.
Ž
.
308 mg . H NMR 300 MHz, C6 D6, d : 14.51 TaH, s, 2H , 5.36
Cp, s, 10H , 2.92 ethyl CH2 , m, 4H , 2.62 ethyl CH2 , m, 2H ,
Ž
.
Ž
.
Ž
.
.
Ž
.
Ž
34 mg, 0.296 mmol was added at room temperature. H NMR 300
. Ž . Ž
MHz, C6 D6, d : 7.39 vinyl H, d, 1H, JH – H s18.6 Hz , 7.11 Ph, d,
Ž
Ž
.
Ž
2.44 ethyl CH2 , m, 2H , 1.44 ethyl CH3, t, 6H, JH – H s7.5 Hz ,
1.19 ethyl CH3, t, 6H, JH – H s7.5 Hz . 13C NMR 75.4 MHz,
.
Ž
. Ž . Ž
2H, JH – H s7.8 Hz , 6.89 Ph, d, 2H, JH – H s8.8 Hz , 6.68 vinyl
.
Ž
.
Ž
.
.
Ž
.
. Ž . Ž
H, d, 1H, JH – H s18.6 Hz , 5.50 Cp, s, 5H , 3.00 ethyl CH2 , m,
C6 D6, d : 104.8 Cp , 24.5 ethyl CH2 , 23.7 ethyl CH2 , 14.9
y1
Ž
.
Ž
Ž
.
. Ž . Ž . Ž
2H , 2.65 ethyl CH2 , m, 1H , 2.34 ethyl CH2 , m, 1H , 2.00 tolyl
ethyl CH3 , 14.3 ethyl CH3 . IR neat film, cm : yB – H 2560,
. Ž . Ž
CH3, s, 3H , 1.20 ethyl CH3, t, 3H, JH – H s7.8 Hz , 1.06 ethyl
yTa – H 1839. Mass spectrum: mrz 823 corresponding to the parent
.
CH3, t, 3H, JH – H s7.8 Hz .
ion envelope.

(
)
M.A. Curtis et al.rJournal of Organometallic Chemistry 550 1998 469–472
471
Scheme 2.
the hydridotantalum –alkyne product CpTaCl-
H PhC[CPh Et2C2 B4 H4 4a was obtained. Co-
ordination of alkynes to Group 5 metals has precedent
The stereochemistry of tantalacarborane–alkyne
complexes was further explored via reaction of 2 with
3-phenylpropyne in benzene. As anticipated, two iso-
meric products were observed, in a ratio of 1:6 based on
8
Ž .Ž
.Ž
. Ž
.
Ž
.
Ž
.
Ž
.
in bis cyclopentadienyl tantalum III and –niobium III
1
w
x
dialkylacetylene complexes 15,16 , but is highly un-
integration of Ta–H singlets in the H NMR spectrum.
usual for d0 early transition metal species. Characteriza-
If it is assumed that the favored configuration places the
bulky phenyl group close to the Cl, while the smaller
methyl substituent resides in the congested area near the
Cp and carborane ligands, the predominant isomer can
be tentatively assigned this geometry.
1
13
Ž
.
tion of 4a included a H– C connectivity HETCOR
study, which revealed no carbon atom association with
1
the H NMR resonance at d 9.23. The IR spectrum of
4a exhibits bands at 1726 and 1807 cmy1 that are
tentatively assigned to Ta–H and C[C stretches, re-
spectively. The possibility of a Ta–H . . . B interaction
cannot be discounted, since bridging between metal
Addition of styrene to a benzene solution of 2 imme-
diately generated the alkyltantalum complex
9
Ž
.Ž
. Ž .
CpTaCl CH2CH2 Ph Et2C2 B4 H4 5 , as shown in
Scheme 2. However, no reactivity was observed with
the sterically hindered olefins cis-stilbene, trans-stil-
bene, and cyclohexene. The sharp contrast with the
facile formation of alkyne complexes 4a and 4b implies
that complexation with tantalum is significantly depen-
Ž
.
hydride ligands e.g., Co–H, Fe–H, and Ru–H and
boron atoms in small metallacarboranes is well known
w
x
17 ; this is conceivable in 2 but has not been demon-
strated.
8
Ž
A 150 mg sample of a 30:70 mixture of 1 and 2 122 mg, 0.148
.
mmol of 2 was dissolved in 1.5 ml of C6 D6, and diphenyl acetylene
52 mg, 0.296 mmol was added at room temperature. The reaction
was monitored by H NMR and was complete in 13 h. H NMR 300
Ž
.
1
1
9
Ž
Ž
A 150 mg sample of a 30:70 mixture of 1 and 2 122 mg, 0.148
.
Ž
.
Ž
.
Ž
MHz, C6 D6, d : 9.35 Ta–H, s, 1H , 7.67 Ph, d, 2H, JH – H s6.8
mmol of 2 was dissolved in 1.5 ml of C6 D6, and styrene 30 mg,
0.296 mmol was added at room temperature. The formation of 5
.
Ž
.
Ž
.
.
Hz , 7.26 Ph, d, 2H, JH – H s7.8 Hz , 7.06y6.91 Ph, m, 6H , 5.29
1
Ž
Ž
.
Ž
.
Ž
.
Ž
.
Ž
Cp, s, 5H , 2.77 ethyl CH2 , m, 1H , 2.57 ethyl CH2 , m, 1H , 2.30
occurred rapidly. H NMR 300 MHz, C6 D6, d : 7.23y7.17 Ph, m,
.
Ž
Ž
.
Ž
.
Ž
.
Ž
.
Ž
ethyl CH2 , m, 1H , 2.15 ethyl CH2 , m, 1H , 1.16 ethyl CH3, t,
5H , 5.62 Cp, s, 5H , 3.09 C H2 Ph, m, 1H , 2.88 ethyl CH2 , m,
3H, JH – H s7.8 Hz , 1.00 ethyl CH3, t, 3H, JH – H s7.8 Hz . 13C
.
.
.
Ž
.
Ž
.
1H , 2.77 ethyl CH2 , m, 1H , 2.72 ethyl CH2 , m, 1H , 2.69
Ž
.
Ž
.
Ž
Ž
.
Ž
.
Ž
.
NMR 75.4 MHz, C6 D6, d : 141.4 Ph, quaternary , 140.2 Ph,
Ta–CH2 , m, 1H , 2.46 CH2 Ph, m, 1H , 2.34 ethyl CH2 , m, 1H ,
.
Ž
.
Ž
.
Ž
.
.
Ž
.
Ž
Ž
.
Ž
.
quaternary , 131.2 Ph , 130.3 Ph , 127.8 Ph , 127.7 Ph , 127.0
Ph , 126.7 Ph , 102.7 Cp , 28.3 ethyl CH2 , 24.9 ethyl CH2 ,
15.1 ethyl CH3 , 14.3 ethyl CH3 . IR neat film, cm : yB – H
2574, yC ' C 1807, yTa – H 1726. Mass spectrum: mrz 589 corre-
sponding to the parent ion envelope.
1.61 Ta–CH2 , m, 1H , 1.35 ethyl CH3, t, 3H, JH – H s7.8 Hz ,
1.13 ethyl CH3, t, 3H, JH – H s7.8 Hz . 13C NMR 75.4 MHz,
Ž
.
Ž
.
Ž
Ž
.
Ž
.
Ž
.
.
Ž
y1
Ž
.
Ž
.
.
Ž
Ž
.
Ž
.
Ž
.
Ž
.
C6 D6, d : 148.4 Ph, quaternary , 128.8 Ph , 128.2 Ph , 125.9 Ph ,
Ž
.
.
Ž
Ž
.
110.5 Cp , 62.0 Ta–CH2 , 41.1 C H2 Ph, 23.3 ethyl CH2 , 22.8
ethyl CH2 , 14.8 ethyl CH3 , 14.5 ethyl CH3 .
Ž
.
Ž
.
Ž
.

(
)
472
M.A. Curtis et al.rJournal of Organometallic Chemistry 550 1998 469–472
dent on the greater electron density available in alkyne
vs. alkene moieties.
not common for neutral zirconocene derivatives. All of
these observations suggest that, despite its formal d0
electronic configuration, the tantalum center in 2 is
significantly more electron-rich than is the zirconium
Treatment of compounds 3 and 5 with 1 equivalent
of anhydrous HCl quantitatively released p-methyl
styrene and styrene, respectively, and regenerated 1 in
w
x
center of Cp2 ZrHCl n, which is expected given the
differences in electronegativity and orbital interactions
characteristic of carborane ligands as compared to cy-
10
both cases. The reaction of 5 was investigated further
via addition of 0.5 equivalent of anhydrous HCl to a
mixture of 2 and 5, which rapidly generated H2 and
converted all of the 2 to the dichloride 1. After 12 h, 1H
spectroscopy revealed the formation of styrene accom-
panied by a small amount of the hydridotantalum com-
plex 2. These observations suggest that 5 initially un-
dergoes b-elimination, generating 2 which reacts with
HCl to produce 1 and H2. Addition of excess HCl was
found to drive the reaction to completion, with quantita-
tive formation of styrene and 1.
w
x
clopentadienyl fragments 18 .
Acknowledgements
This work was supported by the National Science
Foundation, Grant No. CHE 9322490.
The interaction of 4a with either anhydrous or aque-
ous HCl initially afforded an as yet unidentified
References
Ž
organometallic product the same species forming in
.
both cases . In the aqueous system the Ta center facili-
w x
1
M.A. Curtis, T. Muller, V. Beez, H. Pritzkow, W. Siebert, R.N.
¨
Grimes, Inorg. Chem. 36 1997 360.
tated the generation of a Ph2 Et2C4 B4 H4carborane, the
formation and structure of which are currently under
investigation.
Ž
.
w x
2
D.W. Hart, T.F. Blackburn, J. Schwartz, J. Am. Chem. Soc. 97
Ž
.
1975 679.
w x
Ž
.
3
D.W. Hart, J. Schwartz, J. Am. Chem. Soc. 96 1974 8115.
w x
4
P.C. Wailes, H. Weigold, A.P. Bell, J. Organomet. Chem. 27
Ž
.
1971 373.
3. Conclusion
w x
5
J. Schwartz, J. Labinger, Angew. Chem., Int. Ed. Engl. 15
Ž
.
1976 333.
w x
6
Ž
.
T. Takahashi, N. Suzuki, in: L.A. Paquette Ed. , Encyclopedia
of Reagents for Organic Synthesis, Vol. 2, Wiley, Chichester,
1995, p. 1082.
The reactivity of the hydridotantalacarborane com-
plex 2 is thus shown to differ significantly from that of
the Schwartz reagent. The latter species performs facile
w x
Ž .
7
w x
8
E. Negishi, T. Takahashi, Aldrichimica Acta 18 1985 31.
Ž .
P. Wipf, H. Jahn, Tetrahedron 52 1996 12853.
Ž .
P. Wipf, W. Xu, Tetrahedron 50 1994 1935.
hydrometalation of a wide variety of alkynes and alkenes
3
w
x
Ž .
w x
2–5
, whereas 2 engages in the same reaction only
9
x
w
w
w
10 K.L. Houseknecht, K.E. Stockman, M. Sabat, M.G. Finn, R.N.
with terminal substrates. Complex 2 is unreactive with
several 1,2-disubstituted olefins and forms stable coor-
dination complexes with internal alkynes, which are
Ž
.
Grimes, J. Am. Chem. Soc. 117 1995 1163.
x
11 K.E. Stockman, K.L. Houseknecht, E.A. Boring, M. Sabat,
Ž
.
M.G. Finn, R.N. Grimes, Organometallics 14 1995 3014.
11
unknown for d0 metallocene complexes of group 4.
x
12 J.H. Davis Jr., M.D. Attwood, R.N. Grimes, Organometallics 9
Ž
.
Furthermore, the alkyl complex 5, derived from 2,
apparently undergoes b-hydride elimination, which is
1990 1171.
w
w
x
x
Ž
.
13 K.W. Piepgrass, R.N. Grimes, Organometallics 11 1992 2397.
14 K.W. Piepgrass, D.E. Stockman, M. Sabat, R.N. Grimes,
Ž
.
Organometallics 11 1992 2404.
w
x
15 J.A. Labinger, J. Schwartz, J.M. Townsend, J. Am. Chem. Soc.
10
Ž
.
Ž
96 1974 4009.
Compounds 3 and 5 were treated with 1 equivalent of HCl 1 M
w
w
x
x
Ž
.
.
16 J.A. Labinger, J. Schwartz, J. Am. Chem. Soc. 97 1975 1596.
17 X. Wang, M. Sabat, R.N. Grimes, J. Am. Chem. Soc. 117
in diethyl ether at 08C in C6 D6. The identities of the cleaved
products were confirmed by NMR spectroscopy following addition of
Ž
.
1995 12218, and references therein.
authentic samples.
Complexes of the type Cp2 M h -RCCR M5Ti, Zr, Hf may be
described in terms of alkyne coordination to an M II center, or as
11
2
w
x
Ž
.
Ž
. Ž
.
18 R.N. Grimes, in: E. Abel, F.G.A. Stone, G. Wilkinson Eds. ,
Comprehensive Organometallic chemistry II, Vol. 1, Chap. 9,
Pergamon Press, Oxford, England, 1995, pp. 373–430, and
references therein.
Ž .
Ž
.
M IV metallacyclopropene systems. In either case, these are funda-
mentally different from the alkyne complexes reported here.