Formation and reactivity of the [Cp′2Ta(H)2]+ cation (Cp′ = η5-tBuC5H4)

Article abstract of DOI:10.1021/om9809973

The complex Cp′₂Ta(H)(OCOCF₃)₂ (2) is synthesized by the acidolysis of Cp′₂TaH₃ (1) (Cp′ = η⁵-^tBuC₅H₅) in the presence of an excess of trifluoroacetic acid. 1 undergoes a loss of H^- upon the reaction of electrophile (H⁺ or CPh₃⁺). The resulting cation [Cp′₂Ta(H)₂]⁺ ([3]⁺) can be isolated as a solvento adduct [Cp′₂Ta(H)₂(SMe₂)IBF₄ (4·BF₄) by carring out the reaction in dimethyl sulfide. Oxidation of 1 by means of ferrocenium hexafluorophosphate gives 4· PF₆. Coordination of various neutral two-electron-donor L ligands affords the corresponding cationic species [Cp′₂Ta(H)₂(L)]⁺ as BF₄ or PF₆ salts with L = tetrahydrothiophene (5), CN-^tBu (6), PMe₂Ph (8), and CO (9). Microanalytical, mass spectrometric, and infrared and NMR (¹H, ¹⁹F, ³¹P) spectroscopic data for the complexes are reported, along with the X-ray crystal structure of [Cp′₂Ta(H)₂(SMe₂)]BF₄ (4·BF₄). The structural and spectroscopic properties of the dihydride complexes (4-8, 12, and 13) are consistent with a central position of the ligand at the tantalum center. The oxidation of 1 in the presence of acetonitrile results in the formation of the two isomeric azaalkenylidene complexes [Cp′₂Ta(H)(NCHMe)]PF₆ (9/9′· PF₆), but only one isomer, 10·PF₆, is obtained with pivalonitrile. By heating in acetonitrile, complex 4·BF₄ loses a molecule of dimethyl sulfide and is transformed to the mixture 9/9′· PF₆. The cationic heterobimetallic μ-hydrido complex [Cp′₂Ta(H)₂(μ-H)Nb(CO)Cp₂]PF ₆ (11· PF₆) is synthesized by reaction of [3]⁺ with Cp₂Nb(CO)H. The neutral dihydride complexes Cp′₂Ta(H)₂(X), with X = SEt (12), CN (13), are formed by treatment of [3]⁺ with the corresponding anionic species. The central position of the ligand X is proposed from ¹H NMR spectroscopic data. When treated with potassium hydroxide, [3]⁺ leads to the formation of the known oxo complex Cp′₂Ta(=O)H.

Full text of DOI:10.1021/om9809973

Organometallics 1999, 18, 2133-2138
2133
F or m a tion a n d Rea ctivity of th e [Cp ′2Ta (H)2]+ Ca tion
(Cp ′ ) η5-tBu C5H4)
Philippe Sauvageot,† Andre´ Sadorge,† Bernd Nuber,‡ Marek M. Kubicki,†
J ean-Claude Leblanc,† and Claude Mo¨ıse*,†
Laboratoire de Synthe`se et d’Electosynthe`se Organome´talliques, Universite´ de Bourgogne,
F-21000 Dijon, France, and Institut fu¨r Anorganische Chemie der Universita¨t Heidelberg,
D-69120 Heidelberg, Germany
Received December 7, 1998
The complex Cp′2Ta(H)(OCOCF3)2 (2) is synthesized by the acidolysis of Cp′2TaH3 (1) (Cp′
) η5-tBuC5H4) in the presence of an excess of trifluoroacetic acid. 1 undergoes a loss of H-
upon the reaction of electrophile (H+ or CPh3+). The resulting cation [Cp′2Ta(H)2]+ ([3]+)
can be isolated as a solvento adduct [Cp′2Ta(H)2(SMe2)]BF4 (4‚BF4) by carring out the reaction
in dimethyl sulfide. Oxidation of 1 by means of ferrocenium hexafluorophosphate gives 4‚
PF6. Coordination of various neutral two-electron-donor L ligands affords the corresponding
cationic species [Cp′2Ta(H)2(L)]+ as BF4 or PF6 salts with L ) tetrahydrothiophene (5), CN-
tBu (6), PMe2Ph (8), and CO (9). Microanalytical, mass spectrometric, and infrared and NMR
(1H, 19F, 31P) spectroscopic data for the complexes are reported, along with the X-ray crystal
structure of [Cp′2Ta(H)2(SMe2)]BF4 (4‚BF4). The structural and spectroscopic properties of
the dihydride complexes (4-8, 12, and 13) are consistent with a central position of the ligand
at the tantalum center. The oxidation of 1 in the presence of acetonitrile results in the
formation of the two isomeric azaalkenylidene complexes [Cp′2Ta(H)(NCHMe)]PF6 (9/9′‚
PF6), but only one isomer, 10‚PF6, is obtained with pivalonitrile. By heating in acetonitrile,
complex 4‚BF4 loses a molecule of dimethyl sulfide and is transformed to the mixture 9/9′‚
PF6. The cationic heterobimetallic µ-hydrido complex [Cp′2Ta(H)2(µ-H)Nb(CO)Cp2]PF6 (11‚
PF6) is synthesized by reaction of [3]+ with Cp2Nb(CO)H. The neutral dihydride complexes
Cp′2Ta(H)2(X), with X ) SEt (12), CN (13), are formed by treatment of [3]+ with the
1
corresponding anionic species. The central position of the ligand X is proposed from H NMR
spectroscopic data. When treated with potassium hydroxide, [3]+ leads to the formation of
the known oxo complex Cp′2Ta(dO)H.
In tr od u ction
exhibited by these complexes. Even when acidic6 and
nucleophilic7 properties have been reported, studies
showing a somewhat hydridic character are rather
scarce.8
Organotransition metal polyhydride complexes have
received considerable attention because of their great
reactivity1 and their involvement in a number of stoi-
chiometric and catalytic hydrogenations.2 An important
step in such processes occurs via an intra- or intermo-
lecular tranfer of hydrogen from transition metal hy-
dride complexes as a hydride, a hydrogen atom, or a
proton.3
The reactivity of group 5 transition metal polyhydride
complexes, especially those with formal d0 configuration
as such Cp2MH3 is an area of continuous interest. The
loss of molecular hydrogen on heating4 and the insertion
reactions into the M-H bond5 are outstanding features
Following our recent studies on tantalocene hydride
complexes,9 we show now that the tantalocene Cp′2TaH3
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A.; Lorberth, J . Polyhedron 1996, 15, 1565. (d) Nikonov, G. I.;
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K. J . Organomet. Chem. 1997, 547, 183. (h) Nikonov, G. I.; Lorberth,
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T. J . Organomet. Chem. 1988, 342, 31. (d) Sauvageot, P. Ph.D. Thesis,
University of Burgundy, Dijon (France), 1996.
† Laboratoire de Synthe`se et d’Electosynthe`se Organome´talliques,
Universite´ de Bourgogne, F-21000 Dijon, France.
‡ Institut fu¨r Anorganische Chemie der Universita¨t Heidelberg,
D-69120 Heidelberg, Germany.
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10.1021/om9809973 CCC: $18.00 © 1999 American Chemical Society
Publication on Web 05/06/1999

2134 Organometallics, Vol. 18, No. 11, 1999
Sauvageot et al.
(1) (Cp′ ) η5-tBuC5H4) undergoes a loss of hydrogen
atom by oxidation with ferrocenium ion but behaves as
a hydride donor toward Lewis acid reagents (proton and
trityl cation). The electrophilic nature of the cationic
dihydride complex is confirmed by reactions with vari-
ous nucleophiles, leading to several new complexes of
tantalocene.
Sch em e 1
Resu lts a n d Discu ssion
Rea ction of Cp ′2Ta H3 w ith Ca r boxylic Acid s.
Treatment of 1 with trifluoroacetic acid (3 equiv) in
toluene at ambient temperature rapidly leads to the
formation of dicarboxylate tantalum(V) complex 2 as a
white solid with evolution of dihydrogen (eq 1):
yellow pale solution. However, the cationic intermediate
[Cp′2Ta(H)2]+ ([3]+) could be isolated as a solvento
adduct 4‚BF4 (72% yield) by carrying out the reaction
in dimethyl sulfide (route i; eq 2).
The loss of two hydride ligands is accompanied by the
monodentate coordination of two carboxylate ions at the
tantalum center. The excess of trifluoroacetic acid leads
neither to the trisubstituted derivative nor to a cyclo-
pentadienyl ligand substitution as observed in the
zirconocene series.11 From a mechanistic point of view,
we believe that the first step of the reaction path may
consist in the protonation of 1. Because 1 is a d0
coordinatively saturated complex, the protonation can-
not occur at the metal but must operate at the M-H
bond, affording a two-electron three-center cationic
intermediate [Ta- - -H- - -H]+. Such a pathway has
already been proposed for protonation of polyhydride
complexes of sixth-row transition metals.10 Complex 2
has been characterized by analytical and spectral
methods. Four signals observed for the cyclopentadienyl
protons in its 1H NMR spectrum indicate an unsym-
metrical distribution of the three σ-bonds. The low-field
resonance of residual hydride (δ ) 12.17 ppm) on Ta
agrees with strong electron-withdrawing properties of
the carboxylate ligands. Such a reaction fails with weak
carboxylic acids such as CH3COOH but works with
other strong monoacids (e.g., HCl): acidolysis takes
place upon the treatment of 1 with a toluene solution
of hydrochloric acid, giving the known Cp′2TaCl2H
dichloro complex.12
The analytical and spectroscopic data are consistent
with the formula shown in eq 2. The H NMR spectrum
of 4‚BF4 in the cyclopentadienyl region displays only two
signals at δ 5.84 and 4.95; the singlet at δ 1.70 is
assigned to the two equivalent (symmetrically distrib-
uted) metal hydrides.
1
Two other synthetical approaches of [Cp′2Ta(H)2-
(SMe2)]+ have been checked starting from a mixture of
1 with SMe2. The first one consists once more in the
hydride abstraction reaction that occurs when a di-
methyl sulfide solution of 1 is treated with an organic
Lewis acid species such as trityl cation (ii; Scheme 1).
4‚BF4 is formed in good yield (90%), and triphenyl-
methane is the only organic product detected. A hydrid
transfer mechanism through a cationic inner-sphere
intermediate [Ta- - -H- - -CPh3]+ can be suggested to
account for the formation of complex [4]+. However, the
alternative electron transfer/H atom transfer mecha-
nism cannot be definitively ruled out but seems less
favorable in view of the redox potentials of 1 (+0.5 V vs
SCE)14 and Ph3C+ (-0.32 V vs FC).13a
The second one (iii; Scheme 1) arises from a chemical
oxidation with ferrocenium ion. Room-temperature treat-
ment of a dimethyl sulfide solution of 1 with 1 equiv of
Cp2Fe‚PF6 immediately leads to the formation of 4‚PF6
(ca. 80-85%) and ferrocene, while a vigorous gas
evolution is observed. Ferrocenium ion operates the
clean one-electron outer-sphere oxidation and converts
Syn th esis of [Cp ′2Ta (H)2(SMe2)]BF 4 (4‚BF 4). We
thought that some intermediates in acidolysis could be
characterized by the use of other strong acids with
weakly coordinating anions. Thus, the addition of an
excess (1.5 equiv) of HBF4‚Et2O to a toluene solution of
1 results, as expected, in evolution of dihydrogen, but
no well-defined product could be extracted from the
(9) (a) Sabo-Etienne, S.; Chaudret, B.; Abou el Makarim, H.;
Barthelat, J .-C.; Daudet, J .-P.; Mo¨ıse, C.; Leblanc, J .-C. J . Am. Chem.
Soc. 1994, 116, 9335. (b) Sadorge, A.; Sauvageot, P.; Leblanc, J .-C.;
Mo¨ıse, C. C. R. Acad. Sci., Ser. IIb 1997, 325, 593. (c) Bach, H.-J .;
Brunner, H.; Wachter, J .; M. M. Kubicki, M. M.; Leblanc, J .-C.; Mo¨ıse,
C.; Volpato, F.; Nuber, B.; M. L. Ziegler, M. L. Organometallics 1992,
11, 1403. (d) Leboeuf, J .-F.; Lavastre, O.; Leblanc, J .-C.; Mo¨ıse, C. J .
Organomet. Chem. 1991, 418, 359.
(10) Crabtree, R. H.; Hlatky, G. G.; Parnell, C. P.; Segmu¨ller, B. E.;
Uriarte, R. J . Inorg. Chem. 1984, 23, 354.
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(12) Brunner, H.; Kubicki, M. M.; Leblanc, J .-C.; Moı¨se, C.; Volpato,
F.; Wachter, J . J . Chem. Soc., Chem. Commun. 1993, 851.
(13) (a) Ryan, O. B.; Tilset, M.; Parker, V. D. J . Am. Chem. Soc.
1990, 112, 2618. (b) Ryan, O. B.; Tilset, M. J . Am. Chem. Soc. 1991,
113, 9554. (c) Smith, K.-T.; Tilset, M. J . Organomet. Chem. 1992, 431,
55. (d) Skagestad, V.; Tilset, M. J . Am. Chem. Soc. 1993, 115, 5077.
(e) Pedersen, A.; Tilset, M. Organometallics 1994, 13, 4887. (f) Roullier,
L.; Lucas, D.; Mugnier, Y.; Antin˜olo, A.; Pajardo, M.; Otero, A. J .
Organomet. Chem. 1991, 412, 353.
(14) Mugnier, Y.; Lucas, D. Private communication.

[Cp′2Ta(H)2]+ Cation (Cp′ ) η5-tBuC5H4)
Organometallics, Vol. 18, No. 11, 1999 2135
1 into the corresponding [3]+ cation by dihydrogen
elimination. Formation of dihydrogen apparently con-
trasts with oxidations of metal hydrides studied by M.
Tilset13a-e and Y. Mugnier,13f but could arise from the
protonation of Cp′2TaH3 by the strong Brønsted-acidic
cation radical [Cp′2TaH3]•+ (route i, vide supra). The
electrochemical oxidation of a thf solution (0.2 M
Bu4NPF6) of 1 in the presence of SMe2 is a one-electron
process and leads to formation of complex 4‚PF6 in
quantitative yield.
Str u ctu r al Ch ar acter ization of [Cp′2Ta(H)2(SMe2)]-
BF 4. Coordination of the solvent molecule in 4‚BF4 at
tantalum in a central position has been confirmed by
an X-ray structure analysis. There are three independ-
ent cations and anions in the noncentrosymmetric
triclinic cell. One of them is depicted in Figure 1. The
cations 1 and 2 have rougly the same geometries with
a slightly staggerred conformation of the rings. The
cation 3 may be considered as an optical isomer of 1
and 2 because it is a mirror plane image of 1 and 2.
The Ta-S bond length (mean 2.558(6) Å) falls in the
upper range of values (2.417-2.567 Å) observed for
tantalocenes with a single Ta-S bond.12,15 It is, however,
much shorter than those reported by Cotton et al.
(2.698-2.730 Å)16 for Ta(III) inorganic complexes with
terminal dimethyl sulfide.
F igu r e 1. ORTEP drawing of one (2) of the three cations
in the structure of 4‚BF4. Selected bond distances (Å) and
angles (deg) for 2: Ta(2)-S(2) 2.55(1), S(2)-C(39) 1.69(2),
S(2)-C(40) 1.76(2), Ta(2)-S(2)-C(39) 115.3(9), Ta(2)-
S(2)-C(40) 113.3(8), C(39)-S(2)-C(40) 103(1).
Sch em e 2
Rea ction of [Cp ′2Ta (H)2]+ w ith Oth er Neu tr a l
Tw o-Electr on -Don or Liga n d s. The electrophilic na-
ture of [Cp′2Ta(H)2]+ [3]+ is confirmed by the adduct
formation with other ligands (alkyl sulfide, isocyanide,
phosphines, and carbon monoxide) as reported in eq 3.
symmetrical isomer obtained by protonation of the
Cp2Ta(H)(PMe2Ph) metallophosphine:9d the 1H NMR
spectrum of 7‚PF6 displays one doublet (2J H,P ) 71.2 Hz)
at -0.52 ppm for the two equivalent hydride ligands,
and the 31P{1H} NMR exhibits a signal at -16.5 ppm
for the metallophosphonium nucleus.
In addition to sulfide, isocyanide, and phosphine,
carbon monoxide also adds to [3]+. When a sample of
[Cp′2Ta(H)2][PF6] is exposed in toluene to a CO atmo-
sphere, selective formation of the trans [Cp′2TaH2(CO)]-
[PF6] (8) is observed and identified by its spectroscopic
properties similar to the complex previously character-
ized.17
Stirring a solution of Cp′2TaH3 in a toluene/tetrahy-
drothiophene (10/1) mixture with CPh3BF4 results in
the formation of [Cp′2Ta(H)2(tht)][BF4] (5‚BF4) in 85%
yield. Spectroscopic data of 5‚BF4 are similar to those
of complex 4‚BF4.
When carried out in acetonitrile, the oxidation reac-
tion of Cp′2TaH3 with ferrocenium ion leads to a mixture
of isomeric azaalkenylidene complexes 9‚PF6 and 9′‚PF6.
The formulation of these two inseparable isomers is
based on their analytic and spectroscopic data. The
infrared spectrum shows a weak absorption at 1892
cm-1, which is characteristic of the Ta-H vibration and
Addition of trityl hexafluorophosphate to a solution
of Cp′2TaH3 in the presence of a slight excess of tert-
butyl isocyanide results in an immediate coordination
of isocyanide at the tantalum leading to complex 6‚PF6.
The infrared spectra exhibits a strong ν(NC) stretching
absorption at 2210 cm-1
.
1
the ν(CN) stretching absorption at 1714 cm-1. The H
The metallophosphonium complex 7‚PF6 is obtained
by addition of 1.2 equiv of PMe2Ph to a thf solution of
[3]+ prepared in situ (Scheme 1, iii). The spectroscopic
data of this complex are very similar to those of the
NMR spectrum of the mixture indicates a 2/1 ratio for
the two isomers and gives rise for each isomer to the
four signals expected for protons of the two equivalent
cyclopentadienyl ligands.
(15) (a) Daran, J .-C.; Meunier, B.; Prout, K. Acta Crystallogr., Sect.
B 1979, B35, 1709. (b) Hunter; J . A.; Lindsell, W. E.; McCullough; K.
J .; Parr; R. A.; Sholes, M. L. J . Chem. Soc., Dalton Trans. 1990, 2145.
(c) Nelson, J . E.; Bercaw, J . E.; Marsh R. E.; Henling M. L. Acta
Crystallogr., Sect. C 1992, C48, 1023. (d) Proulx G.; Bergman R. G.
Organometallics 1996, 15, 133.
A probable mechanism for their formation (Scheme
2) may involve a rapid rearrangement of the initially
formed nitrile cationic adduct consisting of a hydride
migration (in the bisecting plane of the Cp′-Ta-Cp′
(16) (a) Campbell J . C.; Canish J . A. M.; Cotton F. A.; Duraj S. A.;
Haw J . F. Inorg. Chem. 1986, 25, 287. (b) Canish J . A. M.; Cotton F.
A. Inorg. Chem. 1987, 26, 3473.
(17) Reynoud, J .-F.; Leboeuf, J .-F.; Leblanc, J .-C.; Mo¨ıse, C. Or-
ganometallics 1986, 5, 1863.

2136 Organometallics, Vol. 18, No. 11, 1999
Sauvageot et al.
Sch em e 3
angle) from tantalum to the electrophilic carbon atom
of the nitrile ligand.
Recently Grimes et al. reported insertion reactions of
nitriles with tantalum(V) carborane alkyl complexes;
however these reactions lead to a unique azaalken-
ylidene isomer.18
The existence of the two isomers 9‚PF6 and 9′‚PF6
could result from a probably linear heteroallene TaNC
arrangement (Scheme 3) where the azaalkenylidene
group acts as a three-electron-donor ligand to form an
18-electron Ta(V) complex, as observed for the titano-
cene, zirconocene, scandocene, and tungsten com-
plexes.19 The stereostability of these isomers at room
temperature indicates a restricted rotation about the
Ta-N bond.
In a fashion analogous to that for isomers 9‚PF6 and
9′‚PF6, complex [Cp′2Ta(H)(NdCH-tBu)]‚PF6 is isolated
by using the hindered pivalonitrile and exists as one
stereoisomer, 10‚PF6. The “tBu-outside” assigment for
complex 10‚PF6 is based on an NOE experiment. Selec-
tive irradiation of the hydride signal (at 6.01 ppm) leads
to an expressed NOE of 4.5% on the CH-tBu proton at
8.20 ppm.
two metal centers are connected in a central position
via a hydrido bridge.21 The IR ν(CO) stretching band in
the cation 11‚PF6 exhibits a blue shift of 25 cm-1 with
respect to the parent Cp2Nb(CO)H complex.
The positive charge can be delocalized onto both
metallic nuclei since complex 11‚PF6 can also be con-
sidered as the adduct of Cp′2TaH3 to the Lewis acid
[Cp2Nb(CO)]+ niobium moiety. It is worth noting that
[Cp′2TaH2]+ remains unreactive in the presence of the
chloro Cp2Nb(CO)Cl complex.
Rea ction of [Cp ′2Ta (H)2]+ w ith An ion ic Sp ecies.
Neutral tantalocene complexes 12 and 13 (eq 5) are
formed by reaction of [Cp′2Ta(H)2]+ with sodium ethane-
thiolate (in thf) and with sodium cyanide (in thf/water),
respectively. These compounds are soluble in benzene,
toluene, and chloroform but insoluble in aliphatic
solvents and can be isolated as white solids after
chromatographic workup, to principally eliminate a
small amount of the oxo complex 14 (cf. experimental).
Both azaalkenylidene isomers 9‚PF6 and 9′‚PF6 are
also formed by heating the dimethyl sulfide adduct 4‚
PF6 in acetonitrile (Scheme 2), suggesting the lability
of the coordinated solvent molecule (SMe2). Note, how-
ever, that the displacement of the SMe2 ligand by NCMe
does not operate with the stoichiometric ratio in thf at
room temperature.
The similar reactivity observed for Cp*2ZrH2
19d
and
[Cp′2Ta(H)2]+ toward nitriles does not recur in the
reaction with the carbonyl-niobium Cp2Nb(CO)H com-
plex. Whereas Cp*2ZrH2 leads to the formation of the
zirconoxy carbene complex Cp2(H)NbdCHOZr(H)Cp*2,20
[3]+ reacts with Cp2Nb(CO)H to form in good yield (75%)
the heterodinuclear cationic complex 11‚PF6 (eq 4).
Spectroscopic and analytic characterizations confirm
the formulas of these materials. In particular, the two
compounds exhibit similar 1H NMR spectra with a
single resonance for the two hydride ligands and two
resonances for the cyclopentadienyl protons. On the
basis of these 1H NMR data, we provide the C2v
symmetric strutures for these complexes shown in eq
5.
Attempt to prepare the hydroxido derivative Cp′2Ta-
(H)2(OH) by reaction of complex [3]+ prepared in thf
with a large excess of a 10 M solution of KOH in water
was unsuccessful; however, this reaction leads to the
known oxo complex 14 in nearly quantative yield (eq 6)
and provides a convenient route to the preparation of
this complex.
While the mechanistic details of the formation of 14
are unknown, this reaction clearly involves nucleophilic
attack which would lead to the unstable hydroxide
complex; the analogous sulfur Cp′2Ta(H)2(SH) complex,
which is accessible by a different route,22 is less sensitive
but decomposes slowly when heated in solution and
reverts to the starting sulfido complex Cp′2Ta(dS)H.
The pronounced Lewis acid character of [Cp′2Ta(H)2]+
is emphasized by the formation of complex 11‚PF6 due
to coordination of the donor hydrido-niobocene complex.
11‚PF6 is a stable brown-red solid soluble in thf and
acetone, but slow decomposition is observed in solution.
The high-field proton resonance at -19.08 ppm and the
presence of two equivalent Cp′ ligands confirm that the
(18) Boring, E.; Sabat, M.; Finn, M. G.; Grimes, R. N. Organo-
metallics 1997, 16, 3993.
(19) (a) Bochmann, M.; Wilson, L. M.; Hursthouse, M. B.; Motevalli,
M. Organometallics 1988, 7, 1148. (b) Fro¨mberg, W.; Erker, G. J .
Organomet. Chem. 1985, 280, 343. (c) Alelyunas, Y. W.; J ordan, R. F.;
Echols, S. C.; Borkowsky, S.; Bradley, P. K. Organometallics 1991, 10,
1406. (d) Bercaw, J . E.; Davies, D. L.; Wolczanski, P. T. Organo-
metallics 1986, 5, 443. (e) Feng, S. G.; White, P. S.; Templeton, J . L.
J . Am. Chem. Soc. 1994, 116, 8613. (f) Brent Gunnoe, T.; White, P. S.;
Templeton, J . L. J . Am. Chem. Soc. 1996, 118, 6916.
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M. L. Chem. Ber. 1984, 117, 79.
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(20) Threlkel, R. S.; Bercaw, J . E. J . Am. Chem. Soc. 1981, 103, 2650.

[Cp′2Ta(H)2]+ Cation (Cp′ ) η5-tBuC5H4)
Organometallics, Vol. 18, No. 11, 1999 2137
cm-1): ν(CdO) 1716 br. Anal. Calcd for C22H27F6O4Ta: C,
40.63; H, 4.18. Found: C, 40.43; H, 4.26.
P r ep a r a tion of [Cp ′2Ta (H)2(SMe2)]BF 4 (4‚BF 4). Meth od
i. A solution of 1 (0.100 g, 0.23 mmol) in dimethylsulfide (10
mL) was treated at room temperature with 0.1 mL of HBF4‚
Et2O (54 wt % in Et2O); the solution was stirred for an
additional 0.5 h. After evaporating, the crude solid was washed
with ether and the complex was recrystallized from thf.
Yield: 120 mg (90%).
Meth od ii. A solution of 1 (0.100 g, 0.23 mmol) in dimeth-
ylsufide (10 mL) was treated at room temperature with [Ph3C]-
BF4 (0.077 g, 0.23 mmol); the solution was stirred for an
additional 0.5 h. After evaporating, the crude solid was washed
with pentane and the complex was recrystallized from thf.
Yield: 120 mg (90%).
Con clu sion
Meth od iii. A solution of 1 (0.100 g, 0.23 mmol) in
dimethylsufide (10 mL) was treated at room temperature with
[Cp2Fe][BF4] (0.064 g, 0.23 mmol); the solution was stirred for
an additional 0.5 h. After evaporating, the crude solid was
washed with pentane and the complex was recrystallized from
thf. Yield: 120 mg (90%).
4‚BF4: 1H NMR (200 MHz, CDCl3, 20 °C, TMS) δ ) 5.84
(m, 4H, C5H4), 4.95 (m, 4H, C5H4), 2.54 (s, 6H, SMe2), 1.70 (s,
2H, TaH), 1.31 (s, 18, C(CH3)3); MS (FD) m/z 487.2 [M+]
([C20H34STa+] requires 487.18). Anal. Calcd for C20H34BF4-
STa: C, 41.83; H, 5.97. Found: C, 41.76; H, 5.87.
P r ep a r a tion of [Cp ′2Ta (H)2(th t)]P F 6 (5‚P F 6). To a solu-
tion of 1 (0.100 g, 0.23 mmol) in 11 mL of 10/1 toluene tht was
added 1 equiv of [Ph3C]PF6 (0.77 g, 0.23 mmol). After stirring
45 min, the solvent was eliminated under reduced pressure
and the crude material was washed with pentane to give a
white solid (ca. 85% yield). Recrystallization from thf/pentane
gave white crystals.
In conclusion, the tantalocene trihydrides are ampho-
teric compounds exhibiting both the protic (acid) and
hydride (base) reactivity. They can be deprotonated with
strong bases leading to the [Cp2Ta(H)2]- anions6 or may
undergo acidolysis with strong acids, as shown here. The
cationic intermediate [Cp′2Ta(H)2]+ has been character-
ized as a solvento complex. It behaves as a versatile
Lewis acid toward various inorganic, organic, and
organometallic nucleophiles and can be used as a
reducing agent of nitriles. Generation of this coordina-
tively and electronically unsaturated species is of syn-
thetic interest for building extended molecular systems,
and forthcoming studies will address its utilization as
a reactive precursor.
5‚PF6: 1H NMR (200 MHz, CDCl3, 20 °C, TMS) δ ) 5.83
(m, 4H, C5H4), 4.86 (m, 4H, C5H4), 2.96 (m, 4H, tht), 1.97 (m,
4H, tht), 1.90 (s, 2H, Ta-H), 1.32 (s, 18H, C(CH3)3); MS (FD)
m/z 513.3 [M+] ([C22H36STa+] requires 513.2). Anal. Calcd for
Exp er im en ta l Section
Gen er a l P r oced u r es. All reactions were carried out under
an argon atmosphere by using Schlenk techniques. Solvents
were dried and purified by known procedures and distilled
under argon prior to use. Dimethyl sulfide (99%, Sigma-
Aldrich) was degassed and stored under argon. Except for
PMe2Ph (Strem) all chemicals were purchased either from
J anssen Chimica or from Sigma-Aldrich. Ethanethiol (98%,
Sigma-Aldrich) was distilled before use and stored over 4 Å
molecular sieves under argon. Column chromatography was
performed under argon with silica gel (70-230 mesh, Merck).
The following compounds were prepared as described else-
where: Cp′2TaH3 (1),23a Cp2Nb(CO)H,23b and ferrocenium
salts.23c
1H, 19F, and 31P spectra were recorded at ambient temper-
ature on a Bruker AC200 spectrometer. Infrared spectra were
obtained on a Bruker IFS 66v spectrophotometer. The molec-
ular weights were determined by field desorption (FD) mass
spectra on a Finnigan MAT 311A instrument. Elemental
analyses were carried out using an EA 1108 Fisons instru-
ment.
P r ep a r a tion of Cp ′2Ta (H)(OCOCF 3)2 (2). A toluene (20
mL) solution of 1 (0.200 g, 0.47 mmol) was treated at room
temperature with an excess of trifluoroacetic acid (0.132 g, 1.5
mmol). The colorless solution turned pale yellow, and gas
evolution was observed. The solution was stirred for an
additional 30 min, and the volatiles were removed in vacuo.
Recrystallization from diethyl ether gave white crystals.
Yield: 250 mg (82%).
1H NMR (200 MHz, C6D6, 20 °C, TMS): δ ) 12.17 (s, 1H,
TaH), 6.02 (m, 2H, C5H4), 5.90 (m, 2H, C5H4), 4.83 (m, 2H,
C5H4), 3.92 (m, 2H, C5H4), 1.02 (s, 18H, C(CH3)3). 19F NMR
(188.33 MHz, C6D6, 20 °C, CFCl3): δ ) -75.2, -76.0. IR (KBr,
C
22H36PF6STa: C, 40.13; H, 5.51. Found: C, 40.06; H, 5.49.
P r ep a r a tion of [Cp ′2Ta (H)2(CN-tBu )]P F 6 (6‚P F 6). To a
solution of 1 (0.100 g, 0.23 mmol) in thf (10 mL) were added
[Ph3C]PF6 (0.091 g, 0.23 mmol) and a slight excess of CN-tBu
(0.029 g, 0.35 mmol). After stirring 30 min, the volatiles were
removed in vacuo and the crude material was washed with
pentane to give 6‚PF6 in good yield (90%). Recrystallization
from chloroform/ether gave white crystals.
6‚PF6: 1H NMR (200 MHz, CDCl3, 20 °C, TMS) δ ) 5.69
(m, 4H, C5H4), 4.89 (m, 4H, C5H4), 1.46 (s, 9H, CN-tBu), 1.26
(s, 18H, C(CH3)3), -0.86 (s, 2H, TaH); IR (KBr, cm-1) ν(Ta-
H) 1910 w, ν(CdN) 2210 s; MS (FD) m/z 508.3 [M+] ([C23H37
-
NTa+] requires 508.2). Anal. Calcd for C23H37PF6NTa: C,
42.28; H, 5.71. Found: C, 41.97; H, 5.78.
P r ep a r a tion of [Cp ′2Ta (H)2(P Me2P h )]P F 6 (7‚P F 6). To a
solution of 1 (0.100 g, 0.23 mmol) in thf (10 mL) were added
[Cp2Fe][PF6] (0.093 g, 0.23 mmol) and a slight excess of PMe2-
Ph (0.038 g, 0.28 mmol). After stirring 10 min, the volatiles
were removed in vacuo and the crude material was washed
with pentane to give 7‚PF6 with a good yield (90%). Recrys-
tallization from thf/ether gave white crystals.
7 PF6: 1H NMR (200 MHz, CDCl3, 20 °C, TMS) δ ) 7.50-
7.30 (m, 5H, C6H5), 5.53 (m, 4H, C5H4), 4.64 (m, 4H, C5H4),
1.88 (d, 2J H,P ) 8.8 Hz, 6H, PMe2), 1.18 (s, 18H, C(CH3)3), -0.52
(d, 2J H,P ) 71.2 Hz, 2H, Ta-H); 31P {1H} NMR (81 MHz, CDCl3,
1
20 °C, H3PO4) δ ) -16.5 (s), -146.1 (hep, J F,P ) 713 Hz).
Anal. Calcd for C26H39P2F6Ta: C, 44.08; H, 5.55. Found: C,
44.49; H, 5.81.
P r ep a r a tion of [Cp ′2Ta (H)2(CO)]P F 6 (8‚P F 6). To a solu-
tion of 1 (0.100 g, 0.23 mmol) in thf (10 mL) was added [Cp2Fe]-
[PF6] (0.091 g, 0.23 mmol) under CO atmosphere. After stirring
10 min, the volatiles were removed in vacuo and the crude
material was washed with pentane to give a white powder.
Compound 7 was identified by its 1H NMR data.17
(23) (a) Leblanc, J .-C.; Reynoud, J .-F.; Mo¨ıse, C. C. R. Acad. Sci.,
Ser. II 1982, 295, 755. (b) Labinger, J . A. Adv. Chem. Ser. 1978, 167,
149. (c) Connelly, N. G.; Geiger, W. E. Chem. Rev. 1996, 96, 877.

2138 Organometallics, Vol. 18, No. 11, 1999
Sauvageot et al.
8 PF6: 1H NMR (200 MHz, thf-d8, 20 °C, TMS) δ ) 6.13 (t,
4H, C5H4), 5.56 (m, 4H, C5H4), 1.38 (s, 18H, C(CH3)3), -1.62
(s, 2H, Ta-H).
12: 1H NMR (200 MHz, C6D6, 20 °C, TMS) δ ) 5.05 (m, 4H,
3
C5H4), 4.33 (m, 4H, C5H4), 2.80 (q, J H,H ) 7.3 Hz, 2H, CH2-
3
Me), 2.73 (s, 2H, Ta-H), 1.45 (t, 3H, J H,H ) 7.3 Hz, CH2-Me),
1.34 (s, 18H, C(CH3)3); MS (FD) m/z 486.3 [M+] ([C20H33STa+]
requires 486.2); IR (KBr, cm-1) ν(Ta-H) 1801 w. Anal. Calcd
for C20H33STa: C, 49.38; H, 6.84. Found: C, 49.58; H, 6.79.
P r ep a r a tion of [Cp ′2Ta (H)(NdCHMe)]P F 6 (9‚P F 6/9′‚
P F 6). A solution of [Cp2Fe][PF6] (0.078 g, 0.33 mmol) in
acetonitrile (5 mL) was added to a solution of 1 (0.100 g, 0.33
mmol) in acetonitrile (10 mL). The mixture was stirred at room
temperature for 30 min. The solvent was removed under
vacuum and the residue washed with pentane (overall yield
P r ep a r a tion of Cp ′2Ta (H)2(CN) (13). To a solution of 1
(0.100 g, 0.23 mmol) in thf (10 mL) was added 1 equiv of [Ph3C]-
PF6 (0.091 g, 0.23 mmol). After stirring 5 min, a large excess
of potassium cyanide (0.130 g, 2 mmol) in solution in water
was added. The solvent was eliminated under reduced pressure
to give a mixture of 13 and 14. The crude material was purified
by column chromatography (h ) 9 cm, Ø ) 2.5 cm). Elution
with toluene eliminated triphenylmethane, and elution with
9/1 toluene/thf furnished compound 13. Recrystallization from
chloroform/ether gave pale green crystals.
The presence of OH- in the aqueous solution of potassium
cyanide leads to the formation of 14 and gave a large range of
yield (40-90%).
13: 1H NMR (200 MHz, C6D6, 20 °C, TMS) δ ) 4.76 (m, 4H,
C5H4), 4.31 (m, 4H, C5H4), 1.23 (s, 18H, C(CH3)3), -0.14 (s,
2H, TaH); IR (KBr, cm-1) ν(Ta-H) 1802 w, ν(CdN) 2109 w.
Anal. Calcd for C19H28NTa: C, 50.56; H, 6.25. Found: C, 50.41;
H, 6.16.
1
about 80%). The H NMR spectrum of the mixture indicates a
2/1 ratio for the isomers 9‚PF6/9′‚PF6, respectively. Beige
microcrystals were obtained by crystallization from chloroform/
pentane (unchanged ratio). MS (FD): m/z 466.3 [M+] ([C20H31
-
NTa] requires 466.19. IR (KBr, cm-1): ν(Ta-H) 1892 w,
ν(CdN) 1714 s. Anal. Calcd for C20H31NPF6Ta: C, 39.29; H,
5.11. Found: C, 38.73; H, 5.00.
9‚PF6: 1H NMR (200 MHz, CDCl3, 20 °C, TMS) δ ) 8.36
4
3
(dq, J H,H ) 3.3 Hz, J H,H ) 5,5 Hz, 1H,NdCHMe), 6.15 (m,
2H, C5H4), 6.11 (m, 3H, C5H4 and TaH), 5.61 (m, 2H, C5H4),
3
5.37 (m, 2H, C5H4), 1.90 (d, J H,H ) 5.5 Hz, 3H, NdCHMe),
1.27 (s, 18H, C(CH3)3).
9′‚PF6: 1H NMR (200 MHz, CDCl3, 20 °C, TMS) δ ) 8.19
4
3
(dq, J H,H ) 3.0 Hz, J H,H ) 5,5 Hz, 1H, NdCHMe), 6. 28 (m,
2H, C5H4), 6.11 (d, 1H, TaH), 6.04 (m, 2H, C5H4), 5.56 (m, 2H,
3
C5H4), 5.48 (m, 2H, C5H4), 2.10 (d, J H,H ) 5.5 Hz, 3H,
NdCHMe), 1.27 (s, 18H, C(CH3)3).
By heating at 60 °C, a solution of 4‚PF6 in acetonitrile leads
to the formation of 9‚PF6 and 9′‚PF6 (2/1 ratio).
P r ep a r a tion of [Cp ′2Ta (dO)(H)] (14). To a solution of 1
(0.260 g, 0.61 mmol) in thf (15 mL) was added 1 equiv of
[Cp2Fe][PF6] (0.202 g, 0.61 mmol). After stirring 5 min, a large
excess of KOH (0.5 g, 8.9 mmol) in water (1 mL) was added,
and the solution was evaporated in vaccuo. The pale pink
residue was extracted with toluene to give, after elimination
of the solvent, a white solid. Compound 13 was identified by
its 1H NMR data.12
P r ep a r a tion of [Cp ′2Ta (H)(NdCH-tBu )]P F 6 (10‚P F 6). To
a solution of 1 (0.120 g, 0.28 mmol) in thf (10 mL) were added
[Cp2Fe][PF6] (0.093 g, 0.28 mmol) and 2 equiv of tBu-CN (0.040
g, 0.056 mmol). After stirring 30 min, the volatiles were
removed in vacuo and the crude material was washed with
pentane to give 10‚PF6 with a good yield (ca. 90%). White
analytically pure microcrystals were obtained by crystalliza-
tion from chloroform/pentane.
14: 1H NMR (200 MHz, C6D6, 20 °C, TMS) δ ) 7.35 (s, 1H,
Ta-H), 5.82 (m, 2H, C5H4), 5.43 (m, 2H, C5H4), 5.33 (m, 2H,
C5H4), 5.03 (m, 2H, C5H4), 1.37 (s, 18H, C(CH3)3).
10‚PF6: 1H NMR (200 MHz, CDCl3, 20 °C, TMS) δ ) 8.20
4
(d, J H,H ) 3.3 Hz, 1H, NdCH(tBu)), 6.42 (m, 2H, C5H4), 6.01
4
(d, J H,H ) 3.3 Hz, Ta-H), 5.99 (m, 2H, C5H4), 5.51 (m, 2H,
C5H4), 5.19 (m, 2H, C5H4), 1.29 (s, 18H, C(CH3)3), 0.98 (s, 9H,
NdC(H)-tBu). MS (FD) m/z 508.3 [M+] ([C23H37NTa+] requires
508.2); IR (KBr, cm-1) ν(Ta-H) 1910 w, ν(CdN) 1703 s. Anal.
Calcd for C23H37NTaPF6: C, 42.28; H, 5.71. Found: C, 42.42;
H, 5.90.
Cr ysta l Da ta for [Cp ′2Ta (H)2(SMe2)]BF 4. C20H34BF4STa,
M ) 574.3, colorless prisms (0.20 × 0.45 × 0.65 mm), triclinic,
P1 (No. 1), a ) 10.305(2) Å, b ) 10.601(2) Å, c ) 17.272(2) Å,
R ) 77.28(2)°, â ) 75.77(2)°, γ ) 75.23(2)°, V ) 1743.4(7) Å3,
Z ) 3, dcalc ) 1.64 g/cm3, µ ) 4.85 mm-1, F(000) ) 852,
empirical absorption correction 8 reflections 7.0° < 2θ < 46.0°,
transmission factor (min/max) 0.55/1.00, Syntex R3 diffrac-
tometer, Mo KR radiation, T ) 296 K, 7042 unique reflections,
5930 with I > 2.5 σ(I); structure solution by Patterson-
difference Fourier (SHELXTL Plus), 728 parameters, hydride
ligands not found, other H atoms in calculated positions, R )
0.041, Rw ) 0.036, GOF ) 1.68. Residual electron density max/
min 1.49/-1.19 e/Å3. CCDC 101073.
P r ep a r a tion of [Cp ′2Ta (H)2(µ-H)Nb(CO)Cp 2]P F 6 (11‚
P F 6). To a solution of 1 (0.100 g, 0.23 mmol) in thf (10 mL)
were added [Ph3C]PF6 (0.77 g, 0.23 mmol) and a slight excess
of Cp2Nb(CO)H (0.071 g, 0.028 mmol) in thf (8 mL). After
stirring 15 min, the volatiles were removed in vacuo and the
crude material was washed with toluene to give 11‚PF6 in good
yield (ca. 75%). Brown-red analytically pure microcrystals were
obtained by a rapid crystallization from acetone/pentane.
11‚PF6: 1H NMR (200 MHz, CD3COCD3, 20 °C, TMS) δ )
5.79 (m, 4H, C5H4), 5.60 (s, 10H, C5H5), 5.10 (m, 4H, C5H4),
2
1.34 (s, 18H, C(CH3)3), 0.70 (d, 2H, J H,H ) 10.5 Hz,Ta(H)2),
Ack n ow legm en t is made to the MS laboratory of the
University of Regensburg for recording the field desorp-
tion mass spectra, and we are also grateful to Prof. Y.
Mugnier for helpful discussions. Prof. G. Huttner is
acknowledged for financial support of X-ray crystal-
lography.
2
-19.08 (broad t, 1H, J H,H ) 10.5 Hz, µ-H); IR (thf, cm-1
)
ν(CdO) 1930 s. Anal. Calcd for C29H39OTaNbPF6: C, 42.35;
H, 4.78. Found: C, 42.68; H, 4.83.
P r ep a r a tion of Cp ′2Ta (H)2(SEt) (12). To a solution of 1
(0.100 g, 0.23 mmol) in thf (15 mL) was added 1 equiv of
[Cp2Fe][PF6] (0.078 g, 0.23 mmol). After stirring 5 min, a large
excess (100 equiv) of a freshly prepared solution of sodium
thioethanolate in thf was added. The solvent was evaporated
in vacuo, and the crude material was purified by column
chromatography (h ) 12 cm, Ø ) 2.5 cm). Elution with toluene
eliminated ferrocene and elution with 9/1 toluene/thf furnished
compound 12 in good yield (80%). Recrystallization from thf/
pentane gave white needles.
Su p p or tin g In for m a tion Ava ila ble: Tables of crystal-
lographic data for 4‚BF4, including bond distances, bond
angles, atomic parameters, and isotropic and anisotropic
thermal parameters. This material is available free of charge
via the Internet at http://pubs.acs.org.
Traces of water in the solvent are responsible for the
formation of the oxo complex 14 that could be easely eliminated
by the chromatographic workup.
OM9809973