A cationic imido complex of permethyltantalocene: H2 and carbon-hydrogen bond activation, [2 + 2] cycloaddition reactions, and an unusual reaction with carbon dioxide That affords coordinated isocyanate

Article abstract of DOI:10.1021/om970815p

Treatment of the imido hydride complex Cp*₂Ta(=NCMe₃)H (Cp* = (η⁵-C₅Me₅)) with [Ph₃C]-[B(C₆F₅)₄] in tetrahydrofuran solution yields the cationic imido complex [Cp*₂Ta(=NCMe₃)-(THF)][B(C₆F ₅)₄] (1). Cation 1 reacts cleanly with H₂ to yield [Cp*₂Ta(NHCMe₃)H][B(C₆F₅)4]. Carbon-hydrogen bond-activation reactions are observed with propyne or phenylacetylene to afford [Cp*₂Ta(NHCMe₃)(C=CR)][B(C₆F ₅)₄] (R = CH₃, C₆H₅). In the reaction with propyne, an initial mixture of the [2 + 2] cycloaddition product and C-H activation product is thermally driven to the C-H activation product. The heterolytic cleavage reactions for 1 may be rationalized in terms of the presence of both electrophilic and nucleophilic sites of reactivity in the same molecule. Intramolecular activation of a carbon-hydrogen bond of a Cp* methyl group to ultimately yield [Cp*Ta(η⁵-C₅Me₄CH ₂NCMe₃)H][B(C₆F₅)₄] precludes C-H activation of the carbon-hydrogen bonds of methane. An unusual reaction occurs with carbon dioxide: dealkylation of the imido group is observed, liberating isobutylene and yielding the isocyanate complex [Cp*₂Ta(OH)(NCO)][B(C₆F₅)₄]. Complex 1 reacts with HCl to afford [Cp*₂Ta(NHCMe₃)Cl][B(C₆F ₅)₄] and decomposes cleanly in methylene chloride solution to give [Cp*₂Ta(NHCMe₃)Cl][B(C₆F ₅)₄] by formal HCl abstraction. X-ray crystal structure determinations for [Cp*Ta(η⁵-C₅Me₄CH ₂NCMe₃)H][B(C₆F₅)₄], [Cp*₂Ta-(NHCMe₃)Cl][B(C₆F ₅)₄], [Cp*₂Ta(NHCMe₃)H][B(C₆F₅) ₄], [Cp*₂Ta(NHCMe₃XC=CC₆H ₅)][B(C₆F₅)₄] and [Cp*₂Ta(OH)(NCO)][B(C₆F₅)₄] are reported.

Full text of DOI:10.1021/om970815p

718
Organometallics 1998, 17, 718-725
A Ca tion ic Im id o Com p lex of P er m eth ylta n ta locen e: H2
a n d Ca r bon -Hyd r ogen Bon d Activa tion , [2 + 2]
Cycloa d d ition Rea ction s, a n d a n Un u su a l Rea ction w ith
Ca r bon Dioxid e Th a t Affor d s Coor d in a ted Isocya n a te
Robert E. Blake, J r.,† David M. Antonelli,† Larry M. Henling,†
William P. Schaefer,† Kenneth I. Hardcastle,‡ and J ohn E. Bercaw*,†
Arnold and Mabel Beckman Laboratories of Chemical Synthesis,
California Institute of Technology, Pasadena, California 91125, and
California State University, Northridge, Northridge, California 91330
Received September 15, 1997
Treatment of the imido hydride complex Cp*2Ta(dNCMe3)H (Cp* ) (η5-C5Me5)) with [Ph3C]-
[B(C6F5)4] in tetrahydrofuran solution yields the cationic imido complex [Cp*2Ta(dNCMe3)-
(THF)][B(C6F5)4] (1). Cation 1 reacts cleanly with H2 to yield [Cp*2Ta(NHCMe3)H][B(C6F5)4].
Carbon-hydrogen bond-activation reactions are observed with propyne or phenylacetylene
to afford [Cp*2Ta(NHCMe3)(C≡CR)][B(C6F5)4] (R ) CH3, C6H5). In the reaction with propyne,
an initial mixture of the [2 + 2] cycloaddition product and C-H activation product is
thermally driven to the C-H activation product. The heterolytic cleavage reactions for 1
may be rationalized in terms of the presence of both electrophilic and nucleophilic sites of
reactivity in the same molecule. Intramolecular activation of a carbon-hydrogen bond of a
Cp* methyl group to ultimately yield [Cp*Ta(η5-C5Me4CH2NCMe3)H][B(C6F5)4] precludes
C-H activation of the carbon-hydrogen bonds of methane. An unusual reaction occurs
with carbon dioxide: dealkylation of the imido group is observed, liberating isobutylene and
yielding the isocyanate complex [Cp*2Ta(OH)(NCO)][B(C6F5)4]. Complex 1 reacts with HCl
to afford [Cp*2Ta(NHCMe3)Cl][B(C6F5)4] and decomposes cleanly in methylene chloride
solution to give [Cp*2Ta(NHCMe3)Cl][B(C6F5)4] by formal HCl abstraction. X-ray crystal
structure determinations for [Cp*Ta(η5-C5Me4CH2NCMe3)H][B(C6F5)4], [Cp*2Ta-
(NHCMe3)Cl][B(C6F5)4], [Cp*2Ta(NHCMe3)H][B(C6F5)4], [Cp*2Ta(NHCMe3)(C≡CC6H5)][B(C6F5)4]
and [Cp*2Ta(OH)(NCO)][B(C6F5)4] are reported.
In tr od u ction
ancillary ligands.5 Recent work with electrophilic imido
complexes has revealed that they are capable of activat-
ing the carbon-hydrogen bonds of hydrocarbons (eq 1).6
There is still great interest in the factors which
Complexes containing metal-ligand multiple bonds
constitute extremely interesting subjects of research in
organometallic chemistry. Many of these complexes
have been shown to be effective reagents or catalysts
for oxidations, metathesis reactions, and ammoxida-
tions.1 Transition-metal-imido complexes have been
studied as molecular precursors to thin films,2 models
of hydrodenitrogenation,3 and probes of electronic struc-
ture via their luminescent properties.4 For related
alkylidene systems, which are effective metathesis and
polymerization catalysts, imido ligands serve as tunable
R′
H
(1)
L′nM NR
L′nM NR
L′nM NR + R′
H
control the extreme reactivity or (nearly) total inertness
of metal-ligand multiple bonds.7
We have described an imido complex of permethyl-
tantalocene Cp*2Ta(dNCMe3)H (Cp* ) (η5-C5Me5))
obtained by the reaction of in-situ-prepared tetrahydro-
furan solutions of Cp*2TaCl(THF) with Li(NHCMe3).8
† California Institute of Technology.
‡ California State University, Northridge.
(1) (a) Wigley, D. E. Prog. Inorg. Chem. 1994, 42, 239-482. (b)
Nugent, W. A.; Mayer, J . M. Metal-Ligand Multiple Bonds; Wiley: New
York, 1988.
(2) (a) McKarns, P. J .; Yap, G. P. A.; Rheingold, A. L.; Winter, C.
H. Inorg. Chem. 1996, 35, 5968. (b) J ayaratne, K. C.; Yap, G. P. A.;
Haggerty, B. S.; Rheingold, A. L.; Winter, C. H. Inorg. Chem. 1996,
35, 4910.
(3) (a) Fox, P. A.; Gray, S. D.; Bruck, M. A.; Wigley, D. E. Inorg.
Chem. 1996, 35, 6027. (b) Gray, S. D.; Weller, K. J .; Bruck, M. A.;
Briggs, P. M.; Wigley, D. E. J . Am. Chem. Soc. 1995, 117, 10678.
(4) (a) Williams, D. S.; Thompson, D. W.; Korolev, A. V. J . Am. Chem.
Soc. 1996, 118, 6526. (b) Heinselman, K. S.; Hopkins, M. D. J . Am.
Chem. Soc. 1995, 117, 12340.
(5) (a) Schrock, R. R.; Luo, S. F.; Lee, J . C.; Zanetti, N. C.; Davis,
W. M. J . Am. Chem. Soc. 1996, 118, 3883. (b) Fujimura, O.; de la Mata,
F. J .; Grubbs, R. H. Organometallics 1996, 15, 1865. (c) Schaverian,
C. J .; Dewan, J . C.; Schrock, R. R. J . Am. Chem. Soc. 1986, 108, 2771.
(6) (a) Walsh, P. J .; Hollander, F. J .; Bergman, R. G. J . Am. Chem.
Soc. 1988, 110, 8729. (b) Cummins, C. C.; Baxter, S. M.; Wolczanski,
P. T. J . Am. Chem. Soc. 1988, 110, 8731. (c) de With, J .; Horton, A. D.
Angew. Chem., Int. Ed. Engl. 1993, 32 (6) 903. (d) Schaller, C. P.;
Cummins, C. C.; Wolczanski, P. T. J . Am. Chem. Soc. 1996, 118, 591.
(7) (a) Huber, S. R.; Baldwin, T. C.; Wigley, D. E. Organometallics
1993, 12, 91. (b) Cundari, T. R. J . Am. Chem. Soc. 1992, 114, 7879.
(8) Antonelli, D. M.; Schaefer, W. P.; Parkin, G.; Bercaw, J . E. J .
Organomet. Chem. 1993, 462, 213.
S0276-7333(97)00815-7 CCC: $15.00 © 1998 American Chemical Society
Publication on Web 02/16/1998

A Cationic Imido Complex of Permethyltantalocene
Organometallics, Vol. 17, No. 4, 1998 719
This reaction likely proceeds by an initial displacement
of chloride by tert-butylamide, followed by an R-hydro-
gen elimination. Cp*2Ta(dNCMe3)H serves as a con-
venient synthetic precursor to a cationic imido complex
with the formula [Cp*2Ta(dNCMe3)]X, where X is a
noncoordinating (or weakly coordinating) anion. The
frontier orbitals of the bent metallocene are oriented
such that a vacant orbital remains in the equatorial
plane of the tantalocene moiety, giving rise to an
interesting combination: an electrophilic tantalum
center in close proximity to a nucleophilic nitrogen of
the imido ligand.9
ether. If the reaction is warmed too rapidly or Cp*2-
Ta(dNCMe3)H is of insufficient purity, large amounts
of polytetrahydrofuran are formed, precluding the isola-
tion of the desired product.
If the reaction is carried out in benzene, a product
precipitates immediately. Dissolution of this solid in
methylene chloride and subsequent recrystallization
affords the unusual cyclometalated product that arises
from the intramolecular activation of a C-H bond of a
Cp* ligand. An X-ray structure determination (Table
1) for [Cp*Ta(η5-C5Me4CH2NCMe3)H][B(C6F5)4] (2) re-
veals the connectivity shown in Figure 1.
It is noteworthy that the methylene group ends up
attached to nitrogen rather than tantalum, opposite to
that expected on the basis of the previously observed
regiochemistry given in eq 1. Presumably, the stability
of the less strained pseudo-five-membered ring tautomer
vis-a´-vis the pseudo-four-membered ring alternative is
the rationale for the product observed. Since no inter-
mediates are observed during this reaction, it is not
known whether the initial activation occurs in the
traditional fashion, followed by a ring-expansion, or
whether the isolated product is directly formed during
the activation step (Scheme 1).
Rea ctivity of [Cp *2Ta (dNCMe3)(THF )][B(C6F 5)4].
The reaction of 1 with methylene chloride proceeds
cleanly at 60 °C to give a single product. 1H NMR
spectra reveal a resonance at δ 7.18, the position and
broadness of the peak suggesting an N-H. An X-ray
structure determination (Figure 2, Table 1) identified
the compound as [Cp*2Ta(NHCMe3)Cl][B(C6F5)4] (3) (eq
3).
Since the electrophilic nature of early transition metal
centers is undoubtedly important to the high reactivity
for their metal-imido groups, we reasoned that a
cationic group 5 complex would exhibit greater reactiv-
ity than its isoelectronic neutral zirconium analogs,
which have already been shown to activate the C-H
bonds of benzene. In this article, we report the suc-
cessful synthesis of [Cp*2Ta(dNCMe3)(THF)][B(C6F5)4]
and our investigations of the reactivity of this complex
toward methylene chloride, H2, the C-H bonds of
acetylenes, and unsaturated C-C and C-O bonds.
Resu lts a n d Discu ssion
P r ep a r a tion a n d Ch a r a cter iza tion of [Cp *2Ta -
(dNCMe3)(THF )][B(C6F 5)4]. The reaction between
[Ph3C][B(C6F5)4] and Cp*2Ta(dNCMe3)H at low tem-
perature in THF gives an orange solution of [Cp*2Ta-
(dNCMe3)(THF)][B(C6F5)4] (1), triphenylmethane, and
a small amount of polytetrahydrofuran (eq 2).
The tantalum-nitrogen distance of 1.94 Å is consis-
tent with a single bond10 and a Ta-N-C1 angle of
155.4°. A relatively weak absorption at 3355 cm-1 in
the infrared spectrum is attributable to ν(N-H). In-
terestingly, when this reaction is carried out in meth-
ylene chloride-d2, the N-H resonance of the final
product is prominent, suggesting either that the solvent
(10) See, for example: Allen, F. H.; Davies, J . E. Galloy, J . J .;
J ohnson, O.; Kennard, O.; Macrae, C. F.; Mitchell, E. M.; Mitchell, G.
F.; Smith, J . M.; Watson, D. G. J . Chem. Inf. Comput. Sci. 1991, 31,
187. For 16 examples of TadNsR, d(Ta-N) from 1.707 to 1.830 Å
mean 1.778 Å, angle range from 165.35° to 180°, mean of 173.23°. For
41 examples of Ta-NRR′, d(Ta-N) from 1.929 to 2.275 Å, mean of
2.004 Å, angle range from 104.21° to 144.62°, mean of 124.3°.
Large orange crystals of 1 could be isolated by
layering methylene chloride solutions with petroleum
(9) Parkin, G.; van Asselt, A.; Leahy, D. J .; Whinnery, L. L., J r.;
Hua, N. G.; Quan, R. W.; Henling, L. M.; Schaefer, W. P.; Santarsiero,
B. D.; Bercaw, J . E. Inorg. Chem. 1992, 31, 82.

720 Organometallics, Vol. 17, No. 4, 1998
Blake et al.
Ta ble 1. Cr ysta l Da ta a
compd no.
formula
mol wt
color
2
3
4
7
8
C
48H39BF20NTa
C48H40BClF20NTa
1238.02
C48H41BF20NTa
1203.58
yellow
C56H45BF20NTa
1303.71
yellow
C45H31BF20NO2Ta
1189.47
orange
1201.57
yellow
orange
habit
irregular
0.15 × 0.37 × 0.78
triclinic
P1h (No. 2)
15.692(3)
17.045(4)
19.208(4)
94.23(2)
113.87(2)
96.90(2)
4621(2)
4
prismatic
0.04 × 0.15 × 0.37
monoclinic
P21/c (No. 14)
15.200(3)
16.540(3)
18.702(4)
90
prismatic
0.37 × 0.44 × 0.55
triclinic
P1h (No. 2)
10.719(2)
15.076(3)
15.275(3)
80.21(3)
83.08(3)
85.50(3)
2410.5(8)
2
blade
rod
cryst size, mm
cryst syst
space group
a, Å
b, Å
c, Å
R, deg
â, deg
γ, deg
V, Å3
0.15 × 0.30 × 0.40
triclinic
P1h (No. 2)
12.470(3)
14.748(4)
15.880(4)
100.32(2)
109.39(2)
97.98(2)
2648(1)
2
0.30 × 0.36 × 0.45
triclinic
P1h (No. 2)
12.626(3)
12.763(3)
15.352(3)
70.84(3)
69.45(3)
77.43(3)
2173.4(8)
2
92.30(3)
90
4698(2)
Z
d
4
calc, g cm-3
1.727
24.63
0.69, 1.13
1.9
1.750
25.13
0.09, 1.05
1.658
23.92
0.94, 1.06
3.3
1.635
21.56
0.83, 1.09
3.2
1.818
26.56
0.76, 1.10
1.0
µ, cm-1
trans coeff
decay, %
2θ range, deg
h min, max
k min, max
l min, max
no. of reflns
no. of indep reflns
GOF merge (mult)
R(F)
2, 40
2, 40
2, 40
3, 40
3, 45
-15, 15
-16, 16
-18, 18
17 795
-3, 14
-15, 15
-17, 17
9479
-10, 10
-14, 14
-14, 14
9274
-12, 12
-14, 14
-15, 15
9449
-12, 13
-12, 13
-14, 16
11 863
8619
4366
4489
4915
5675
1.07 (8618)
1.02 (4171)
1.16 (3545)
1.16 (4134)
1.05 (5674)
0.080b
0.089c
0.076c
0.049b
0.058c
Rw(F2)d
0.084
1.55
0.080
1.28
0.127
2.84
0.020
1.41
0.069
1.47
GOFe
data, parameters
8617, 969
1.46, -1.04
4366, 640
0.68, -1.26
4489, 641
1.75, - 0.64
4915, 712
0.98, -1.00
5675, 631
1.01, - 0.86
∆F max, min, e Å-3
a
b
Nonius CAD-4 diffractometer, Mo KR radiation, λ ) 0.7107 Å, graphite monochromator, ω scans, 293 K. R(F) ) ∑|Fc - |Fo||/∑Fo, Fo
> 0. c R(F) ) ∑||Fc| - Fo|/∑|Fo|. Rw(F2) ) [∑w(Fo - Fc2)2/∑w(Fo2)]1/2
.
e GOF ) [∑w(Fo - Fc2)2/(n - p)]1/2
.
d
2
2
F igu r e 1. Drawing of the cation of [Cp*Ta(η5-C5Me4CH2-
NCMe3)H][B(C6F5)4] (2) (50% ellipsoids). Ta1-Cp*(1) )
2.081 Å, Ta1-Cp*(2) ) 2.131 Å, Ta1-N1 ) 1.976(10) Å,
N1-C1 ) 1.555(17) Å, N1-Me1 ) 1.441(16) Å, Cp1-Me1
) 1.479(19) Å; Cp*1-Ta1-Cp*2 ) 137.8°, Ta1-N1-C1 )
146.1(8)°, Ta1-N1-C1 ) 108.4(9)°.
F igu r e 2. Drawing of the cation of [Cp*2Ta(NHCMe3)Cl]-
[B(C6F5)4] (3) (50% ellipsoids). Ta-Cp*(1) ) 2.158 Å, Ta-
Cp*(2) ) 2.173 Å, Ta-N ) 1.940(11) Å, Ta-Cl ) 2.372(3)
Å, N-C(1) ) 1.511(14) Å, N-H(1) ) 1.180(11) Å, Ta-H(1)
) 2.31 Å; Cp*1-Ta-Cp*2 ) 132.5°, N-Ta-Cl ) 94.1(3)°,
Ta-N-C1 ) 155.4(9)°.
is not the source of the hydrogen or that [N-H] is
readily exchanged with residual protons of the glass.
No experiments have been conducted to attempt to
resolve this issue, however.
principally 3, but also another Cp*-containing product
and a white precipitate are formed. The minor product
does not contain a tertiary butyl group and is tentatively
postulated to be [Cp*2TaCl2][B(C6F5)4]. It is likely that
to a small extent HCl reacts further with the initial
amido-chloro product 3 to cleave the tert-butylamido
group from the metal, which precipitates as Me3CNH3-
Cl.
Since the reaction between [Cp*2Ta(dNCMe3)(THF)]-
[B(C6F5)4] and methylene chloride is formally abstrac-
tion of HCl from the solvent, the direct reaction of 1
with 1 equiv of HCl was carried out on an NMR-tube
scale in methylene chloride-d2. The reaction affords

A Cationic Imido Complex of Permethyltantalocene
Organometallics, Vol. 17, No. 4, 1998 721
Sch em e 1
Sch em e 2
Reaction of methane with 1 was attempted in meth-
ylene chloride solution, but even under a several atmo-
spheres of methane pressure, the only product observed
was complex 3. In the less-reactive solvent chloroben-
zene, the attempted reaction with methane yielded only
the “tuck-in” product 2. Thus, the intramolecular C-H
bond-activation process for 1 appears to preclude reac-
tions with methane.
of Cp*2Ta(dNCMe3)Cl rather than [Cp*2Ta(NHCMe3)-
(CH3)][B(C6F5)4] and LiCl.
[Cp*2Ta(dNCMe3)(THF)][B(C6F5)4] (1) does, however,
react cleanly with dihydrogen (eq 4).
The availability of [Cp*2Ta(NHCMe3)Cl][B(C6F5)4] (3)
prompted the attempted synthesis of [Cp*2Ta(NHCMe3)-
(CH3)][B(C6F5)4], the expected product from the activa-
tion of a carbon-hydrogen bond of methane by 1.
Should this compound exhibit sufficient thermal stabil-
ity, one could conclude that the lack of reactivity of 1
with methane is likely kinetic not thermodynamic in
nature. Unfortunately, we have been unable to cleanly
convert 3 to [Cp*2Ta(NHCMe3)(CH3)][B(C6F5)4]. 1H
NMR experiments indicated that the alkylating agent
dimethylmercury (as well as diethylzinc) undergoes no
reaction with 3. Whereas addition of methyllithium to
3 does lead to rapid evolution of methane, simple
deprotonation occurs in preference to metathesis of the
Ta-Cl bond with LiCH3, as evidenced by the formation
The NMR spectrum of the recrystallized product (4)
shows two broad downfield peaks at δ 7.74 and 8.52,

722 Organometallics, Vol. 17, No. 4, 1998
Blake et al.
F igu r e 4. Drawing of the cation of [Cp*2Ta(NHCMe3)-
(C≡CC6H5)][B(C6F5)4] (7) (50% ellipsoids). Ta-Cp(A) )
2.157 Å, Ta-Cp(B) ) 2.172 Å, Ta-N ) 1.928(7) Å, Ta-C1
) 2.097(9) Å, N-C9 ) 1.570(12) Å, C1-C2 ) 1.202(13) Å,
C2-C3 ) 1.459(14) Å; Cp(A)-Ta-Cp(B) ) 134.7°, N-Ta-
Cl ) 92.5(3)°, Ta-N-C9 ) 151.8(6)°.
F igu r e 3. Drawing of the cation of [Cp*2Ta(NHCMe3)H]-
[B(C6F5)4] (4) (50% ellipsoids). Ta-Cp*(1) ) 2.14 Å, Ta-
Cp*(2) ) 2.12 Å, Ta-N ) 1.93(2) Å, N-C1 ) 1.59(3) Å;
Cp*1-Ta-Cp*2 ) 137°, Ta-N-C1 ) 159(2)°.
supports this hypothesis. The other product has been
identified as the [2 + 2] addition product, azametalla-
cyclobutene, tentatively assigned to have the following
isomeric structure: [Cp*2TaN(CMe3)C(CH3)dCH]-
[B(C6F5)4] (6). Heating the initially formed ca. 60:40
mixture of 5 and 6 to 60 °C for several hours results in
conversion entirely to the C-H activation product 5
(Scheme 2).
On the basis of the hypothesis that the C-H activa-
tion is essentially a heterolysis reaction, a more acidic
substrate was chosen in the hopes that the C-H
activation product would predominate. Indeed, phenyl-
acetylene reacts rapidly and cleanly with [Cp*2Ta-
(dNCMe3)(THF)][B(C6F5)4] at room temperature to give
the expected C-H addition product [Cp*2Ta(NHCMe3)-
(CtCC6H5)][B(C6F5)4] (7) (eq 6). In addition to the
consistent with the expected amido-hydride product
[Cp*2Ta(NHCMe3)H][B(C6F5)4]. An infrared resonance
is observed at 3336 cm-1 attributable to ν(N-H), and
another is observed at 1862 cm-1 attributable to ν(Ta-
H). An X-ray structural determination (Table 1) con-
firmed the structure of 4 (Figure 3). When solutions of
4 in methylene chloride are allowed to stand for
prolonged periods at room temperature, 3 is slowly
formed.
Given the relative ease with which 1 reacts with H2,
we then examined acetylenes in the hopes that their
C-H bonds, with a relatively large amount of s char-
acter, might prove more reactive than the sp3-hybridized
C-H bonds of methane. Thus, reaction of 1 and
propyne was examined. After a few hours at room
1
temperature, the H NMR signals for 1 are replaced by
two sets of new Cp* and tert-butyl resonances. The
presence of a broad peak at δ 7.02 suggested that the
expected C-H activation had taken place to form [Cp*2-
Ta(NHCMe3)(CtCCH3)][B(C6F5)4] (5) (eq 5).
corroborating spectroscopic evidence, a crystal structure
determination (Table 1) unambiguously identified the
structure of this product (Figure 4).
An interesting and unexpected product is obtained
from the reaction of [Cp*2Ta(dNCMe3)(THF)][B(C6F5)4]
with carbon dioxide. Previous work in our group and
The observation of a resonance in the infrared spec-
trum at 3338 cm-1 attributable to ν(N-H) further

A Cationic Imido Complex of Permethyltantalocene
Organometallics, Vol. 17, No. 4, 1998 723
carbon dioxide, which results in dealkylation of the
imido ligand.
The importance of unreactive ancillary ligands once
again is underscored by the observed reactivity of [Cp*2-
Ta(dNCMe3)(THF)][B(C6F5)4]. In this regard, the par-
ent tantalocene analog [(η5-C5H5)2Ta(dNR)(L)]+ might
prove reactive toward a wider range of C-H bonds and
unsaturated substrates. The original proposal that
more electrophilic metal imido complexes would be more
reactive toward hydrocarbon C-H bonds is supported
by these studies. In reactions with acetylenes, the C-H
activation products proved to be more stable than the
[2 + 2] addition products. While C-H activation
undoubtedly is encouraged by the polar TadN bond, the
key feature is likely the highly electrophilic, cationic
tantalum center.
Exp er im en ta l Section
F igu r e 5. Drawing of the cation of [Cp*2Ta(OH)(NCO)]-
[B(C6F5)4] (7) (50% ellipsoids). Ta-Cp*(1) ) 2.131 Å, Ta-
Cp*(2) ) 2.142 Å, Ta-O1 ) 1.926(5) Å, Ta-N1 ) 1.944(7)
Å, N1-C1 ) 1.076(11) Å, C1-O2 ) 1.320(12) Å, O1-H1
) 0.77 Å; Cp*(1)-Ta-Cp*(2) ) 138.9°, O1-Ta-N1 ) 99.4-
(3)°, Ta-N-C1 ) 163.8(10)°, Ta-O1-H1 ) 114.5(2)°.
Gen er a l Con sid er a tion s. All air- and/or moisture-sensi-
tive compounds were manipulated using standard Schlenk
techniques or in a drybox under a nitrogen atmosphere. 1H
and 13C NMR spectra were recorded on a General Electric QE
300 (300.152 MHz for 1H) spectrometer and referenced to
residual proton impurities or carbon signals in the NMR
solvent. Fluorine spectra were recorded on a Bruker AM500
(500.13 MHz for 1H) spectrometer. Infrared spectra were
recorded on a Perkin-Elmer 1600 series spectrophotometer as
Nujol mulls. Elemental analyses were performed by Fenton
Harvey at Caltech. Preliminary NMR reactions were all
performed using Teflon-valved NMR tubes. 1H NMR and 13C
NMR spectra are summarized in Tables 2 and 3, respectively.
Syn th esis of [Cp *2Ta (dNCMe3)(THF )][B(C6F 5)4] (1).
Cp*2Ta(NHCMe3)H (2.13 g, 4.07 × 10-4 mol) was treated with
Ph3C[B(C6F5)4] (3.75 g, 4.07 × 10-4 mol) in 75 mL of THF at
room temperature. The color changed from deep purple to
brown to orange over a period of 1 h. After a total of 2 h of
stirring, THF was removed to a volume of 10 mL in vacuo and
50 mL of petroleum ether was condensed onto the residue.
After initially oiling out, the residue became a solid in about
5 min. The yellow solid was collected by filtration and washed
with petroleum ether twice. After pumping on the solid
overnight, the yellow product (4.21 g, 82%) was collected.
Recrystallization by layering a methylene chloride solution
with petroleum ether afforded 2.4 g of large orange crystals.
Anal. Calcd for C52H37BF20NO: C, 49.43; H, 2.95; N, 1.11.
Found: C, 48.48, 48.75; H, 3.62, 3.68; N, 0.99, 0.80. IR: 1213
others gave precedent for a formal [2 + 2] addition to
form an η2-carbamate complex. Instead, a rapid reac-
tion gave a product that is lacking the tert-butyl group,
according to elemental analysis and 1H NMR. An X-ray
structure determination (Figure 5, Table 1) was re-
quired to finally establish the structure as [Cp*2Ta(OH)-
(NCO)][B(C6F5)4] (8) (Scheme 3).
Although rather speculative, we propose that the
initially formed [2 + 2] product likely undergoes a
productive retro-[2 + 2] to generate an oxo intermediate
that subsequently abstracts a proton from tert-butyl
with the release of isobutylene. (Scheme 4).
Re-examination of the data from the original sealed-
tube NMR experiment confirmed the presence of isobu-
tylene in the product mixture. No other 1H NMR
signals, in particular those for isobutane, were observed,
•
arguing against the intermediacy of CMe3 radicals
during the formation of 8. Reaction of 1 with 13CO2
allowed unambiguous assignment of ν(C-O) at 2252
cm-1 for the isocyanato complex, which shifts by 62 cm-1
upon isotopic substitution. This unusual transforma-
tion represents an interesting new carbon-nitrogen
bond-cleavage reaction promoted by carbon dioxide,
albeit with simultaneous formation of a (CO2-derived)
carbon-nitrogen bond.
(m), 847 (m) cm-1
.
Syn th esis of [Cp *2Ta (NHCMe3)Cl][B(C6F 5)4] (3). [Cp*2-
Ta(dNCMe3)(THF)][B(C6F5)4] (0.5 g, 3.95 × 10-4 mol) was
placed in a small Teflon-valved flask with 5 mL of methylene
chloride in the glovebox. This flask was placed in an oil bath
and heated to 55 °C for 12 h. The yellow-brown solution from
the reaction was pumped down to a brown oily solid in vacuo.
The product was purified by recrystallization from a methylene
chloride and diethyl ether solution, which was layered with
petroleum ether. The orange crystals were isolated by decant-
ing as much solvent as possible and evaporation of the residual
in vacuo (0.322 g isolated, 69% yield). A suitable crystal for
structural analysis by X-ray diffraction was cleaved from a
larger orange needle. Anal. Calcd for C48H40BClF20N: C,
Con clu sion s
The hoped-for reactivity of [Cp*2Ta(dNCMe3)(THF)]-
[B(C6F5)4] with methane was not observed due to a facile
intramolecular C-H activation reaction. The noninno-
cence of the ancillary Cp* ligand has proven to be
problematic for this extremely reactive compound. Even
so, many reactions were found to be competitive with
the intramolecular reaction, permitting the synthesis
and characterization of a number of new cationic
complexes of permethyltantalocene. The anticipated
C-H activation is observed for the fairly reactive and
unhindered sp C-H bonds. Particularly interesting was
the reaction of [Cp*2Ta(dNCMe3)(THF)][B(C6F5)4] with
46.57; H, 3.26; N, 1.13. Found: C, 46.45; H, 3.39; N, 0.98. 19
NMR (methylene chloride-d2): δ -135.2 (d), -165.9 (t), -169.7
(t). IR: 3355 (w), 1181 (m) cm-1
F
.
Alter n a te Syn th esis of [Cp *2Ta (NHCMe3)Cl][B(C6F 5)4]
(NMR-tu be sca le). [Cp*2Ta(dNCMe3)(THF)][B(C6F5)4] (0.1
g, 8 × 10-5 mol) was placed in a small Teflon-valved NMR
tube with 0.7 mL of methylene chloride in the glovebox.
A
6.9 mL gas bulb was attached to the NMR tube, evacuated,

724 Organometallics, Vol. 17, No. 4, 1998
Blake et al.
Sch em e 3
Ta ble 2. 1H NMR Da ta a
Sch em e 4
δ
(coupling,
Hz)
compd
assignment
[Cp*2Ta(dNCMe3)(THF)][B(C6F5)4] (1)
C5(CH3)5
(CH3)3C
OCH2
2.14
1.32
4.19
OCH2CH2 2.21
[Cp*Ta(η5-C5Me4CH2NCMe3)H][B(C6F5)4]
(2)
C5(CH3)5
(CH3)3C
TaH
2.19
1.18
8.58
4.59 (d of d)
1.95
NCH2
C5(CH3)4
2.06
2.27
2.64
[Cp*2Ta(NHCMe3)Cl][B(C6F5)4] (3)
[Cp*2Ta(NHCMe3)H][B(C6F5)4] (4)
C5(CH3)5
(CH3)3C
NH
C5(CH3)5
(CH3)3C
NH
2.22
1.26
7.18
2.22
1.30
7.74
TaH
8.52
2.17
1.24
and filled with 220 Torr of HCl at 25 °C. The contents of the
tube were frozen at 77 K, and the HCl was opened to the tube.
Upon warming to room temperature, a flocculate white
precipitate was present in the solution (multiple products by
NMR). The tube was warmed to 60 °C for 1 h, during which
time the precipitate dissolved. The primary product is [Cp*2-
Ta(NHCMe3)Cl][B(C6F5)4]
[Cp*2Ta(NHCMe3)(C≡CCH3)][B(C6F5)4] (5) C5(CH3)5
(CH3)3C
NH
7.02
CH3C≡C
1.95b
2.07
[Cp*2TaN(CMe3)CHdCCH3][B(C6F5)4] (6) C5(CH3)5
(CH3)3C
C≡CH
1.35
4.57 (q, J )
2 Hz)
2.84 (d, J )
2 Hz)
2.24
CH3C≡C
Syn th esis of [Cp *2Ta (NHCMe3)H][B(C6F 5)4] (4). [Cp*2-
Ta(dNCMe3)(THF)][B(C6F5)4] (0.5 g, 3.95 × 10-4 mol) was
placed in a small (15 mL) Teflon-valved flask with 5 mL of
methylene chloride in the glovebox. The flask was evacuated,
and 1 atm of hydrogen (approximately 6 × 10-4 mol) was added
at room temperature. The solution was stirred for 3 days, then
the resulting yellow solution was pumped down to a yellow
solid in vacuo. It was purified by recrystallization by layering
a methylene chloride solution of the compound with petroleum
ether (yield 0.380 g, 80%). A single crystal for structure
determination by X-ray diffraction was grown by layering a
methylene chloride solution with petroleum ether. Anal.
Calcd for C48H41BF20N: C, 47.90; H, 3.43; N, 1.16; Found: C,
47.54; H, 3.45; N, 0.96. 19F NMR (methylene chloride-d2): δ
-135.2 (d), -165.9 (t), -169.8 (t). IR: 3336 (w), 1862 (w, br),
[Cp*2Ta(NHCMe3)(C≡CC6H5)][B(C6F5)4] (7) C5(CH3)5
(CH3)3C
NH
1.29
7.33 (br)
7.35 (m)
7.40 (m)
2.21
C6H5
[Cp*2Ta(OH)(NCO)][B(C6F5)4] (8)
C5(CH3)5
OH
8.26 (br)
a
Spectra were taken in methylene chloride-d2 and referenced
to residual proton impurities at 5.32 ppm unless indicated
otherwise. Chemical shift was not unambiguously resolved.
b
reaction flask was allowed to stir overnight, during which time
the orange solution turned yellow-brown. The solution was
then pumped to dryness in vacuo. The brownish-yellow
product was recrystallized by layering a methylene chloride
solution with petroleum ether. Yield: 0.336 g (69%). Anal.
Calcd for C51H43BF20N: C, 49.66; H, 3.49; N, 1.13. Found: C,
49.59; H, 3.67; N, 1.04. 19F NMR (methylene chloride-d2): δ
-135.2 (d), -165.9 (t), -169.8 (t).
1182 (m) cm-1
.
In itia l Rea ction of [Cp *2Ta (dNCMe3)(THF )][B(C6F 5)4]
w ith P r op yn e. [Cp*2Ta(dNCMe3)(THF)][B(C6F5)4] (0.500 g,
3.95 × 10-4 mol) was placed in a Teflon-valved 100 mL flask,
and 10 mL of dry methylene chloride was vacuum transferred
into the flask. Once the solid was dissolved and the solution
warmed to room temperature, approximately 1 atm of propyne
(about 3.6 × 10-3 mol) was transferred into the flask. This
Syn t h esis of [Cp *2Ta (NH CMe3)(C≡CCH 3)][B(C6F 5)4]
(5). The solid from the initial reaction of [Cp*2Ta(dNCMe3)-

A Cationic Imido Complex of Permethyltantalocene
Organometallics, Vol. 17, No. 4, 1998 725
Ta ble 3. 13C NMR Da ta a
in a Teflon-valved 100 mL flask, and 10 mL of dry methylene
chloride was vacuum transferred into the flask. Approxi-
mately 1 atm of carbon dioxide (about 3.6 × 10-3 mol) was
transferred into the flask. This reaction vessel was allowed
to stir for 3 days, during which time the orange solution turned
greenish-yellow. The reaction mixture was then pumped to
dryness in vacuo. The yellow product was recrystallized by
layering a methylene chloride solution with petroleum ether.
Yield: 0.400 g (85%). Repeated recrystallizations were suc-
cessful for the isolation of a crystal suitable for an X-ray
structure determination. Anal. Calcd for C45H31BF20NO2: C,
δ
compd
assignment
(coupling, Hz)
[Cp*2Ta(dNCMe3)(THF)]-
[B(C6F5)4] (1)
C5(CH3)5
(CH3)3C
C5(CH3)5
(CH3)3C
OCH2
123.4
34.9
12.7
68.6
88 (br)
OCH2CH2 27.4
b
C6F5
136.3 (d, J CF )246)
138.3 (d, J CF ) 245)
148.1 (d, J CF ) 242)
124.5
31.9
[Cp*2Ta(NHCMe3)Cl][B(C6F5)4] (3) C5(CH3)5
45.44; H, 2.63; N, 1.18. Found: C, 45.52; H, 2.61; N, 1.18. 19
NMR (methylene chloride-d2): δ -135.2 (d), -165.9 (t), -169.7
(t). IR: 3490 (w, br), 3611 (w), 2252 (s), 2190 (s) cm-1
F
(CH3)3C
C5(CH3)5
11.9
(CH3)3C
65.5
.
[Cp*2Ta(NHCMe3)H][B(C6F5)4] (4) C5(CH3)5
118.4
34.0
12.1
(CH3)3C
C5(CH3)5
(CH3)3C
X-r a y Cr ysta l Str u ctu r e Deter m in a tion s. For each
compound, a suitable crystal was mounted with grease in a
capillary tube and optically centered on the diffractometer. The
unit cell was determined from setting angles of 25 reflections.
Two equivalent sets of data were collected. Table 1 sum-
marizes the crystallographic data. Lorentz and polarization
factors were applied, decay (based on 2 or 3 standards) and
absorption (based on ψ-scans of 6 reflections) corrections were
made, and the data were then merged in the appropriate point
group with CRYM11 programs. Weights w were calculated as
1/σ2(Fo2); variances (σ2(Fo2)) were derived from counting sta-
tistics plus an additional term (0.014I)2; variances of the
merged data were obtained by propagation of error plus
another additional term, (0.014hI)2.
Structures 3, 4, 7, and 8 were solved with SHELXS-8612 by
either direct methods or the Patterson technique; 2 was solved
using CRYM and a Patterson map. The remaining non-
hydrogen atoms were found by successive refinement-differ-
ence Fourier syntheses. The hydrogen atoms were included
in the refinement as riding atoms, except in 2 where the atoms
were fixed but repositioned toward the end of refinement. Most
hydrogen-atom coordinates were calculated from the expected
chemical positions. For 2, 14 of the 18 cyclopentadienyl methyl
groups (in the two independent cations) showed disordered
hydrogen atoms; these were modeled as two sets with 0.5
population each. The hydride on the tantalum was not
included. For 3, the hydrogen on the nitrogen was located in
a difference map and included as a riding atom. For 4, neither
the hydrogen bonded to the nitrogen nor the hydride on the
tantalum were included. For 7, the hydrogen on the nitrogen
was located in a difference map and included as a riding atom.
For 8, the hydroxyl hydrogen was located in a difference map
and included as a riding atom. All non-hydrogen atoms were
refined anisotropically. Refinement was full matrix least-
squares on F2 using either CRYM (2, 7) or SHELXL-9313 (3,
4, 8). For 4, extinction coefficient of 0.0011(3) was included.
64.3
[Cp*2Ta(NHCMe3)(C≡CCH3)]-
[B(C6F5)4] (5)
C5(CH3)5
(CH3)3C
C5(CH3)5
(CH3)3C
C≡CCH3
121.9
32.6
12.3
64.83
119.7
131.8
11.35c
121.9
33.4
11.9
61.1
122.0
32.6
12.4
C≡CCH3
C5(CH3)5
(CH3)3C]
C5(CH3)5
(CH3)3C
C5(CH3)5
(CH3)3C
C5(CH3)5
(CH3)3C
C≡CPh
[Cp*2TaN(CMe3)CHdCCH3]-
[B(C6F5)4] (6)
[Cp*2Ta(NHCMe3)(C≡CC6H5)]-
[B(C6F5)4] (7)
65.5
133.3
124.0
128.3
128.7
130.4
146.5
126.0
10.9
C6H5
[Cp*2Ta(OH)(NCO)][B(C6F5)4] (8) C5(CH3)5
C5(CH3)5
NCO
140.8
a
b
All spectra were recorded in methylene chloride-d2. The
chemical shifts of the anion for all compounds in this table were
within the experimental error of these values. c Peak assignment
is ambiguous due to impurities in the sample and/or low signal-
to-noise ratio.
(THF)][B(C6F5)4] with propyne was placed in a 25 mL Teflon-
valved flask, and 5 mL of dry methylene chloride was added
to the flask in the glovebox. This reaction vessel was placed
in a 60 °C oil bath for 12 h. After removal of the solvent in
vacuo, the crude product was recrystallized by layering a
methylene chloride solution with petroleum ether. The NMR
spectrum confirmed the product identity. IR: 3338 (w), 2115
(w), 1189 (m) cm-1
.
Syn th esis of [Cp *2Ta (NHCMe3)(C≡CP h )][B(C6F 5)4] (7).
[Cp*2Ta(dNCMe3)(THF)][B(C6F5)4] (0.500 g, 3.95 × 10-4 mol)
was placed in a Teflon-valved 15 mL flask, and 5 mL of dry
methylene chloride was pipetted into the flask in the glovebox.
Also in the glovebox, 45 µL of phenylacetylene was syringed
into the flask. The reaction mixture was stirred overnight.
The resulting red solution was reduced in volume to 2 mL in
vacuo, then 6 mL of petroleum ether was layered onto it.
Initially, a viscous oil settled out of the solution, but after
vigorous shaking, the product crystallized. X-ray quality
crystals were isolated from the crude product and a structure
Ack n ow led gm en t. This work was supported by the
National Science Foundation (Grant No. CHE-9506413),
through a National Sciences and Energy Research
Council of Canada (NSERC) postdoctoral fellowship to
D.M.A., and through a National Science Foundation
Predoctoral Fellowship to R.E.B.
OM970815P
determined. Yield: 0.453 g (88%). Anal. Calcd for C56H45
-
(11) The CRYM Crystallographic Computing System. Duchamp, D.
J . American Crystallography Association Meeting, Bozeman, Montana,
1964; Paper B14, p 29-30.
(12) SHELXS-86: Sheldrick, G. M. Acta Crystallogr. 1990, A46, 467.
(13) SHELXL-93: Sheldrick, G. M. Program for the Refinement of
Crystal Structures; University of Go¨ttingen: Go¨ttingen, Germany,
1993.
BF20N: C, 51.59; H, 3.48; N, 1.07. Found: C, 51.10; H, 3.56;
N, 1.10. 19F NMR (methylene chloride-d2) δ -135.2 (d), -165.9
(t), -169.8 (t). IR: 3342 (w), 2098, 1184 (m) cm-1
.
Syn t h esis of [Cp *2Ta (NCO)(OH)][B(C6F 5)4]. [Cp*2Ta-
(dNCMe3)(THF)][B(C6F5)4] (0.5 g, 3.95 × 10-4 mol) was placed