Ansa-cyclopentadienyl-arene tantalum complexes: Structure and reactivity of neutral, cationic, and dicationic derivatives

Article abstract of DOI:10.1021/om300421m

The cationic tantalum complex {[η⁶-Ar-CMe ₂-η⁵-C₅H₄]TaPr}[B(C ₆F₅)₄] (1; Ar = 3,5-Me₂C ₆H₃) serves as a starting material for a series of neutral, monocationic, and dicationic derivatives. The cationic hydride {[η⁶-Ar-CMe₂-η⁵-C₅H ₄]TaH}[B(C₆F₅)₄] (2) that results from hydrogenolysis of 1 inserts the di- and trisubstituted olefins cyclopentene and 2-methyl-2-pentene; it reacts with styrene to give the 2,1-insertion product, for which the Ta-CH(Me)Ph group is bound in a σ³- allylic fashion. A neutral complex is obtained from 1 by reaction with Br ^-, and a dicationic derivative is available by hydride abstraction from 2 using the Lewis acidic trityl cation. All compounds described here retain the unusual ansa-(η⁵-cyclopentadienyl, η⁶-arene) coordination mode of the ligand that stabilizes the formally Ta(III) center. X-ray structures and DFT calculations show that the metal-arene interaction contains a significant π back-donation component (arene←Ta(III)) that differs only slightly in the series, despite the variation of charge at the metal center.

Full text of DOI:10.1021/om300421m

Article
pubs.acs.org/Organometallics
ansa-Cyclopentadienyl-Arene Tantalum Complexes: Structure and
Reactivity of Neutral, Cationic, and Dicationic Derivatives
Edwin Otten,* Auke Meetsma, and Bart Hessen
Stratingh Institute for Chemistry, University of Groningen, Nijenborgh 4, 9747 AG, Groningen, The Netherlands
S
* Supporting Information
ABSTRACT: The cationic tantalum complex {[η6-Ar-CMe2-
η5-C5H4]TaPr}[B(C6F5)4] (1; Ar = 3,5-Me2C6H3) serves as a
starting material for a series of neutral, monocationic, and
dicationic derivatives. The cationic hydride {[η6-Ar-CMe2-η5-
C5H4]TaH}[B(C6F5)4] (2) that results from hydrogenolysis of
1 inserts the di- and trisubstituted olefins cyclopentene and 2-
methyl-2-pentene; it reacts with styrene to give the 2,1-insertion product, for which the Ta−CH(Me)Ph group is bound in a σ3-
allylic fashion. A neutral complex is obtained from 1 by reaction with Br−, and a dicationic derivative is available by hydride
abstraction from 2 using the Lewis acidic trityl cation. All compounds described here retain the unusual ansa-(η5-
cyclopentadienyl,η6-arene) coordination mode of the ligand that stabilizes the formally Ta(III) center. X-ray structures and DFT
calculations show that the metal−arene interaction contains a significant π back-donation component (arene←Ta(III)) that
differs only slightly in the series, despite the variation of charge at the metal center.
formal oxidation states are stabilized by delocalization of the
metal d electrons into the π system of the coordinated arene
ring.8 X-ray crystallography shows puckering in the arene that
indicates resonance contribution of a reduced “dienediyl” ring
bound to an oxidized metal center (Scheme 1, structure 1B).
Thus, it appears that the ansa-cyclopentadienyl-arene ligand
framework provides opportunities to access low-valent
oxidation states in cationic early-transition-metal chemistry,
without the need for (strongly bound) classical π acids such as
CO and isocyanides. The nature of the metal−arene interaction
and the stability that it provides in these low-valent compounds
is expected to depend on the other ligands present and on the
electrophilicity of the metal center. Kochi and co-workers
reviewed the effects of charge-transfer bonding in metal arene
complexes from the viewpoint of structural changes in the
arene fragment and showed that several effects (including π
bond localization and loss of planarity) may occur, depending
on the nature of the charge transfer interaction and arene
hapticity.9
Herein we explore in more detail the reactivity of cationic
tantalum(III) complexes bearing the ansa-cyclopentadienyl-
arene ligand. Insertion of olefins into the Ta−H bond results in
new cationic alkyl and benzyl derivatives, and conversion to
neutral and dicationic derivatives is described. All compounds
retain the ansa-cyclopentadienyl-arene coordination motif,
which allows us to explore the influence of charge on the
nature of the metal−arene interaction by crystallographic and
computational methods. Substantial arene folding is observed,
regardless of the charge at the metal center, indicating arene←
INTRODUCTION
■
A wide range of industrially important transition-metal-
catalyzed transformations involve oxidative addition and
reductive elimination as key steps in the catalytic cycle (e.g.,
cross-coupling1 and cycloaddition2 reactions). A prerequisite
for productive catalysis is the presence of two metal oxidation
states (n and n + 2) that are relatively similar in energy so that
these oxidation states can be reversibly addressed. An example
of such a redox couple is Pd(0)/Pd(II), which is widely applied
to catalytic C−C bond forming reactions. While the
prominence of (catalytic) oxidative addition−reductive elimi-
nation sequences holds true for a variety of late transition
metals, the high thermodynamic stability of the d0 high-valent
state in the case of the early transition metals has hampered
development of processes involving these elementary reaction
steps using these metals. Complexes that are able to stabilize
early-transition-metal centers in their low-valent state by
(partially) delocalizing metal d electron density into the ligand
framework have gained increased attention in this context.
Indeed, metal complexes with such “redox-active” ligands3 can
facilitate (catalytic) redox reactions that are otherwise not
accessible, as shown elegantly for example in the work of
Heyduk and co-workers.4
Previously, we have shown that cationic titanium complexes
with cyclopentadienyl-arene (Cp-arene) ligands are highly
active catalysts for the selective trimerization of ethylene to
1-hexene.5 This unusual reaction most likely occurs by a
Ti(IV)/Ti(II) redox mechanism with the involvement of
metallacyclic intermediates.6 The unique contribution of the
aromatic substituent from the ansa-Cp-arene ligand in
stabilizing various intermediates in the catalytic cycle has
been validated for a series of model compounds.7 Also, Cp-
arene ligated complexes of the group 5 metal tantalum with low
Received: May 16, 2012
Published: August 16, 2012
© 2012 American Chemical Society
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Scheme 1. Synthesis of Compounds 3−5 by Olefin Insertion into the Ta−H Bond of 2
metal back-donation from Ta(III) is important in stabilizing
these species across the series.
(C5H9)}[B(C6F5)4] (3; Scheme 1). Similar to the case for the
8
1
n-propyl derivative 1, the H NMR spectrum of 3 at −30 °C
indicates a C1-symmetric complex with diagnostic resonances at
δ −0.69 and −7.04 ppm for the diastereotopic cyclopentyl-β-
CH2 group. A small amount of crystals suitable for X-ray
diffraction was isolated after slow diffusion of cyclohexane into
the C6D5Br solution. Although the diffraction data are of poor
quality and only serve to establish connectivity (Figure S1,
Supporting Information), the presence of a tantalum-cyclo-
pentyl group (instead of cyclopentene-hydride) is evidenced by
a normal Ta−C(alkyl) distance of 2.204(15) Å for Ta−Cα, with
the other Ta−C bonds being significantly longer (>2.47 Å).
In a similar fashion, the trisubstituted olefin 2-methyl-2-
pentene (ca. 10 equiv) was reacted with 2 in C6D5Br solution
to result in formation of the new tantalum alkyl species 4
RESULTS AND DISCUSSION
■
Monocationic Derivatives via Insertion into the Ta−H
Bond. Convenient access to new tantalum complexes with the
ansa-cyclopentadienyl-arene ligand is provided by the isolable
base-free monocationic derivative {[η6-Ar-CMe2-η5-C5H4]-
TaPr}[B(C6F5)4] (1; Ar = 3,5-Me2C6H3). As shown in a
preliminary communication, compound 1 can be cleanly
converted to the monocationic monohydride {[η6-Ar-CMe2-
η5-C5H4]TaH}[B(C6F5)4] (2).8 Although compound 2 is
relatively stable in solution, it decomposes upon attempted
isolation but may be isolated in crystalline form as its THF
adduct. Early-transition-metal and rare-earth-metal hydrides are
known to be highly reactive species, and compound 2 is no
exception; it readily reacts with aliphatic α-olefins (propene, 1-
hexene) to give the corresponding Ta-alkyl cations.8 While
most neutral hydrides of the early transition metals tend to
aggregate into higher nuclearity polyhydride compounds,10 the
solubility of cationic 2 in bromobenzene suggests it to be
monomeric with a terminal hydride ligand. This is not
unexpected: cationic early-transition-metal and rare-earth-
metal hydride derivatives, although still quite rare,11 tend to
have lower aggregation states due to unfavorable electrostatic
interactions. For example, Piers and co-workers were able to
prepare the monomeric cationic titanium hydride [Cp*(tBu3
N)TiH][B(C6F5)4] and structurally characterize its THF
adduct.12 The previously observed facile insertion of terminal
olefins (propene, 1-hexene) into the Ta−H bond in 2,8 which
stands in contrast to the isoelectronic neutral group 5
metallocene hydrides that have olefin-hydride ground states,13
and its relevance to olefin polymerization by cationic early-
transition-metal catalysts prompted us to explore the reactivity
of 2 toward the di- and trisubstituted olefins cyclohexene and 2-
methyl-2-pentene.
1
(Scheme 1). In the H NMR spectrum at −30 °C, three
different diastereotopic CH2 groups are observed for the Ta-
alkyl moiety, one of which has broadened resonances at δ −1.3
and −6.6 ppm that indicate a β-agostic interaction. These data
are consistent with 4 being the 4-methylpentyl compound {[η6-
Ar-CMe2-η5-C5H4]Ta(CH2CH2CH2CHMe2)}[B(C6F5)4].
Complex 4 likely results from isomerization of the initial
insertion product by a series of β-hydrogen elimination−
reinsertion (“chain walking”)14 events; the observed linear alkyl
group is the thermodynamic product for steric reasons.
The formation of tantalum alkyl complexes is in contrast with
the observations made by Bercaw and co-workers for the
isoelectronic neutral metallocene complexes of niobium and
tantalum, which were found to have olefin-hydride ground
states.15 Competitive binding studies for niobocene complexes
allowed the determination of relative ground-state energies,
which revealed a significantly larger ground-state stabilization
for styrene-hydride complexes relative to derivatives with
aliphatic olefins.15b The electronic difference between aliphatic
and aromatic olefins that is responsible for this stabilization
prompted us to investigate whether formation of an olefin-
hydride species with our system was possible using styrene.
Treatment of 2 with styrene resulted in clean 2,1-insertion of
styrene into the Ta−H bond. Precipitation with pentane
afforded brownish {[η6-Ar-CMe2-η5-C5H4]TaCH(Me)Ph}[B-
Treatment of 2 with cyclopentene (1.1 equiv) on an NMR
tube scale in C6D5Br solution resulted in full conversion of the
starting materials in the course of ca. 5 h and clean formation of
the tantalum cyclopentyl species {[η6-Ar-CMe2-η5-C5H4]Ta-
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(C6F5)4] (5) as an analytically pure material in 67% yield
(Scheme 1). The electronic preference for the observed 2,1-
insertion of styrene has been reported for many early- and late-
transition-metal compounds,16 although sometimes 1,2-inser-
tion is competitive and mixtures of terminal and internal
insertion products are observed.17 In this case, the product
arising from 2,1-insertion is the only observable species, as is
clear from the characteristic mutually coupled 1H NMR signals
at δ 1.51 (doublet, 3H) and −0.22 ppm (quartet, 1H) for the
CH3 and CH groups of the Ta-CH(CH3)Ph moiety,
respectively. One of the o-H resonances of the benzyl group
is shifted upfield to δ 2.39 ppm, whereas the other is located at
δ 6.62 ppm, which suggests the benzyl ligand is η3-bound.16c
For the d2 tantalum center in Cp*2Ta(η3-C3H5), π back-
donation into the allyl ligand leads to metallabicyclobutane
character (σ3-allyl), evidenced by high-field resonances and low
CH coupling constants (147−149 Hz) for the C3H5 moiety.18
A similar bonding description may apply for 5, with the “allylic”
aromatic region indicates retention of the η6-arene coordination
and partial disruption of aromaticity (Ta(V)/dienediyl
resonance structure), similar to the case for the monocationic
Ta-alkyl derivatives described above.
Dark-brown cubic crystals of 6 are obtained by crystallization
from toluene. The X-ray crystal structure determination (Figure
1; pertinent interatomic distances and bond angles are given in
Table 1) confirms the ansa-cyclopentadienyl-arene coordina-
tion in the neutral compound 6. A closer inspection of the
metric parameters of the coordinated C6 ring reveals a similar
extent of π localization, whereas the deviation from planarity in
6 is slightly larger than that in 1 (Table 2). The fold angle
(14.7(4)°), however, is not as large as those found for related
neutral early-transition-metal arene complexes in which the
arene is not linked to the ligand system. For example, arene
folding of up to 34.4° has been observed for the Ta(III)
complex (C6Me6)Ta(OAr)2Cl,21 whereas corresponding Ti(II)
arene complexes have fold angles between 20 and 30°.22 This
suggests that the strained ansa-cyclopentadienyl-arene coordi-
nation that is enforced by the short C1 bridge between the
cyclopentadienyl and arene moieties prevents optimal overlap
between the metal d and arene π* orbitals. A comparison with
structurally characterized high-valent, d0 metal-arene complexes
shows that in these systems the arene ring retains its delocalized
π system and planarity.23,24 The loss of planarity in the Ta(III)
compounds described here underscores the importance of
arene←metal back-bonding interactions to approach a Ta(V)/
dienediyl resonance structure.
ortho carbon at δ 73.7 ppm in the 13C NMR spectrum (JCH
=
145 Hz). In the absence of π back-donation, η3-benzyl groups
generally show resonances for the coordinated CH moiety at δ
>105 ppm.16c,19
Dynamic exchange was observed in the 2D EXSY spectrum
of 5, with positive intensity cross peaks correlating the two
benzyl o-H signals. Analysis of EXSY spectra at five temper-
atures in the range of 20−65 °C gave the activation parameters
of the process as ΔH⧧ = 71(3) kJ mol−1 and ΔS⧧ = −28(5) J
mol−1 K−1. No exchange correlations are observed for the Cp-
arene ligand, indicating that inversion at the metal center does
not occur at an appreciable rate and that the arene moiety is
strongly bound. Exchange processes in early-transition-metal d0
metal allyl compounds have been investigated, and η3 → η1
interconversions were shown to occur with lower enthalpies
and more negative entropies of activation (e.g., ΔH⧧ = 41(5) kJ
mol−1 and ΔS⧧ = −63(17) J mol−1 K−1 for Cp*2Sc(η3-C3H5) in
Et2O-d10).20 The negative entropy of activation found for 5
(and related allyl species)20 is most consistent with solvent-
assisted loss of the η3 coordination in the transition state,
followed by rotation around the C−C bond, as depicted in
Scheme 2.
In agreement with its 18-valence-electron count, compound
6 is relatively robust and fails to react with H2, CO, or PhSSPh
at room temperature. Monitoring solutions of 6 at 80 °C in the
presence of CO or PhSSPh shows gradual disappearance of the
tantalum starting material, but the products of these reactions
1
are paramagnetic (on the basis of H NMR integration) and
have not been identified.
Dicationic Tantalum Compound with ansa-Cp−Arene
Ligation. Treatment of the hydride cation 2 with the strong
Lewis acid [Ph3C][B(C6F5)4] results in hydride abstraction to
give the dicationic tantalum compound {[η6-Ar-CMe2-η5-
C5H4]Ta(solvent)}2+[B(C6F5)4]− (7) and concomitant for-
2
mation of Ph3CH (Scheme 3). Although the low solubility of 7
in C6D5Br hampers full NMR spectroscopic characterization, its
1H NMR spectrum is indicative of a Cs-symmetric complex.
Singlet resonances are observed at δ 5.26 (1H) and 4.86 (2H)
ppm, which can be attributed to the para and ortho H atoms of
the coordinated arene ring.
Scheme 2. Representation of the Process Leading to
Exchange of the o-Ph Environments of the Benzyl Ligand in
5
Addition of THF-d8 to an NMR sample of 7 in C6D5Br
results in a clear solution of the Cs-symmetric THF adduct {[η6-
Ar-CMe2-η5-C5H4]Ta(THF-d8)2}2+[B(C6F5)4]− (7-THF) in
2
near-quantitative yield. The coordinated arene moiety of 7-
THF gives rise to resonances at δ 5.32 (1H, p-H) and 4.92
1
(2H, o-H) ppm in the H NMR spectrum, consistent with
Ta(V)/dienediyl resonance contributions. These signals are
shifted downfield in comparison to those in the monocationic
and neutral derivatives 1 and 6, respectively, suggesting that
reduction of the arene ring is somewhat less pronounced in 7-
THF. Compound 7-THF could be isolated as dark blue crystals
from THF/pentane, and a crystal structure determination
confirms the proposed formulation (Figure 2; pertinent
interatomic distances and bond angles are given in Table 1).
The X-ray data are indicative of some disorder in the
orientation of one of the coordinated THF molecules, but
the salient structural features are not affected by this disorder.
Neutral Tantalum Compound with ansa-Cp−arene
Ligation. The monocationic Ta-propyl compound 1 reacts in
bromobenzene with Bu4NBr to give the salt [Bu4N][B(C6F5)4]
and the neutral tantalum bromide complex [η6-Ar-CMe2-η5-
C5H4]Ta(nPr)Br (6; Scheme 3), which could be isolated in
83% yield as dark brown blocks. The number of resonances in
1
the H NMR spectrum of 6 is consistent with a C1-symmetric
compound, and no evidence is found for agostic interactions for
the Ta-propyl moiety. The 1H NMR signals for the C6 ring are
observed at δ 4.33 and 3.36/3.27 ppm for the para and ortho
hydrogens, respectively. The significant upfield shift from the
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Scheme 3. Synthesis of Neutral [η6-Ar-CMe2-η5-C5H4]Ta(Pr)Br (6) and Dicationic {[η6-Ar-CMe2-η5-
C5H4]Ta(THF)2}2+[B(C6F5)4]− (7-THF)
2
While dicationic rare-earth-metal alkyl species have been
implicated as active species in olefin polymerization,25 well-
characterized examples of dicationic early transition metals are
rare.26 Green and Saßmannshausen have reported a dicationic
zirconocene complex in which the cyclopentadienyl ligands
have a pendant arene moiety which is believed to coordinate to
the metal center, but no structural confirmation of this binding
mode is available.27 In contrast to the case for 7-THF, the
metal−arene interaction in this zirconocene(IV) system is
mainly electrostatic in nature, as there are no d electrons
available for arene←metal back-donation.
The dicationic tantalum complex 7 is isostructural and
isoelectronic with the proposed active species for ethylene
trimerization catalyzed by cationic cyclopentadienyl-arene
titanium complexes.5b,6,28 It was therefore of interest to
investigate the reactivity of 7 toward ethylene. Treatment of
7 (generated in situ in C6D5Br) with 1 bar of ethylene and
monitoring by NMR spectroscopy, however, shows no
indication of ethylene consumption. Also, reaction of 7-THF
Figure 1. Molecular structure of 6 showing 50% probability ellipsoids.
The hydrogen atoms are omitted for clarity.
1
with ethylene gives no observable changes in the H NMR
Table 1. Selected Bond Distances (Å) and Angles (deg) for
1, 6 and 7-THF
spectrum after 5 h at room temperature. Heating this mixture
to 80 °C for several hours results in gradual disappearance of
resonances due to 7-THF, but no conversion of ethylene is
observed. The poor solubility of 7 in nonpolar solvents limits
further reactivity studies. For the 18-electron complex 7-THF it
is likely that any reactivity requires dissociation of either the
coordinated arene ring or a THF molecule. The observation
that 7-THF is unreactive even toward CO suggests that
dissociative displacement of ligands in 7-THF is unfavorable
under the conditions applied.
Computational Studies. Density functional theory (DFT)
calculations were performed to evaluate the nature of the
metal−arene interaction in these neutral, monocationic, and
dicationic ansa-cyclopentadienyl-arene tantalum compounds.
The optimized geometries of 1, 6, and 7-THF are in good
agreement with the crystallographically determined structures.
The largest difference between calculated and experimental
structures is observed for 7-THF, for which the calculated Ta−
O(THF) bonds are somewhat elongated (by ca. 0.05 Å) in
comparison to the X-ray structure. Importantly, the metrical
parameters for the ansa-cyclopentadienyl-arene ligand are
similar to the crystallographic values, with arene fold angles
that are somewhat larger (Table 2). Nevertheless, the
1 (x = 1)
6 (x = none) 7-THF (x = 1)
Ta(x)−Cg(Cp)
Ta(x)−Cg(Ar)
Ta(x)−C(x17)
Ta(x)−C(x9)
Ta(x)−C(x10)
Ta(x)−C(x11)
Ta(x)−C(x12)
Ta(x)−C(x13)
Ta(x)−C(x14)
2.016(3)
1.917(3)
2.234(9)
2.300(6)
2.361(6)
2.474(6)
2.352(6)
2.425(7)
2.391(6)
80.7(5)
2.044(2)
1.961(2)
2.294(5)
2.275(5)
2.400(5)
2.473(5)
2.366(5)
2.526(6)
2.460(5)
121.6(3)
93.7(3)
2.033(2)
1.9628(17)
2.283(5)
2.428(5)
2.500(4)
2.372(4)
2.527(4)
2.407(5)
Ta(x)−C(x17)−C(x18)
C(x5)−C(x6)−C(x9)
Cg(Cp)−Ta−Cg(Ar)
95.0(5)
92.0(3)
128.79(13)
57.7(4)
127.79(9)
59.0(3)
126.81(8)
61.3(3)
a
α
a
α is the interplanar angle between the least-squares planes of the
cyclopentadienyl and arene rings.
The fold angle of the coordinated C6 ring and the bond length
alternation (Table 2) shows that back-donation from the metal
d orbitals into the arene π system occurs despite the high
electrophilicity of the dicationic tantalum center in 7-THF.
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Table 2. C−C Bond Lengths (Å) within the Coordinated C6 Fragment and Fold Angle along the C(x9)−C(x12) Vector in 1, 6,
and 7-THF
1 (x = 1)
1calcd
6 (x = none)
6calcd
7-THF (x = 1)
7-THFcalcd
C(x9)−C(x10)
C(x10)−C(x11)
C(x11)−C(x12)
C(x12)−C(x13)
C(x13)−C(x14)
C(x14)−C(x9)
1.450(10)
1.390(10)
1.426(9)
1.447(11)
1.378(10)
1.431(9)
8.6(5)
1.447
1.395
1.429
1.438
1.392
1.438
12.5
1.441(6)
1.392(6)
1.427(6)
1.441(7)
1.374(6)
1.440(6)
14.7(4)
1.453
1.389
1.430
1.451
1.376
1.451
21.1
1.440(7)
1.391(6)
1.428(6)
1.431(7)
1.389(8)
1.447(6)
13.8(3)
1.437
1.392
1.436
1.434
1.388
1.446
16.8
a
fold angle
a
Folding along the C(x9)−C(x12) vector.
interaction in these compounds is best characterized as a
donor−acceptor interaction in terms of the Chatt−Dewar−
Duncanson model.29b As expected for early-transition-metal
centers with a formal d2 electron count, there is a substantial
arene←metal back-donation component (b) in all complexes
studied (Figure 3 and Table 3). This back-bonding interaction
is primarily located in the HOMO of the complex that arises
from interaction of the arene LUMO with the HOMO of the
metal-containing fragment. Surprisingly, there is little variation
in the amount of back-donation in the series of compounds;
even the highly electrophilic dicationic Ta center in 7-THFcalcd
is able to accommodate a substantial back-bonding interaction
with the coordinated arene ring. For comparison, CDA analysis
was performed for the corresponding “naked” dication in two
different geometries: 7calcd corresponds to simple removal of
the two THF molecules from 7-THFcalcd without further
geometry optimization, and 7calcd,opt is the geometry after
relaxation to its minimum energy conformation. To complete
the bonding picture, we computed the atomic charges from
natural population analysis (NPA) for the central Ta atom and
the coordinated arene ring (summation over the 6 C atoms,
Table 3). The coordinated THF molecules in 7-THFcalcd result
in a decrease in charge at the Ta center (+0.88) in comparison
to the “naked” dication 7calcd (+1.27), which gives rise to a
lower total binding energy (Eb = 54.1 and 93.6 kcal mol−1 for 7-
THFcalcd and 7calcd, respectively). For the optimized structure
7calcd,opt the metal−C(arene) distances are somewhat con-
tracted, concomitant with a decrease in the donation term (d)
and a larger amount of back-donation (b). In all the complexes
studied, the arene←metal back-bonding interaction in the
HOMO of the complex contributes at least 10% to the total
Figure 2. Molecular structure of 7-THF showing 50% probability
ellipsoids. The B(C6F5)4 anions, THF solvate molecule, and hydrogen
atoms are omitted for clarity.
experimentally observed trends are correctly reproduced in the
calculations. Charge decomposition analysis (CDA)29 was used
to estimate the relative importance of donation (arene→metal)
and back-donation (arene←metal) contributions to the overall
metal−arene interaction (see the Supporting Information for
details). This fragment-based analysis gives values for the
amount of electron transfer (number of electrons) for each
molecular orbital upon complex formation. The positive total
values (summation over all MOs) for donation (d) and
backdonation (b) and negligible residual term (Δ) obtained by
CDA analysis (Table 3) indicate that the metal−arene
a
Table 3. Charge Decomposition Analysis of the Ta−Arene Interaction and NPA Charges
Eb
NPA charge
∑(Ar)
d
b
Δ
d/b
total
HOMO
Ta
1calcd
0.716
0.801
0.767
0.713
0.613
0.236
0.232
0.248
0.234
0.287
0.026
0.063
3.0
3.5
3.1
3.0
2.1
63.0
25.3
54.1
93.6
99.5
6.9
3.5
0.50
0.47
0.88
1.27
1.26
−0.66
−0.83
−0.66
−0.68
−0.71
6calcd
7-THFcalcd
7calcd
−0.020
−0.022
−0.027
6.8
12.4
15.5
7calcd,opt
a
Donation (d), back-donation (b), residual term (Δ) values (in numbers of electrons) and binding energies (Eb, in kcal/mol) upon complex
formation, as obtained from CDA analysis.
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Figure 3. Fragment molecular orbitals that give rise to arene←metal back-donation upon complexation (top) and HOMOs of 1calcd, 6calcd, and 7-
THFcalcd (bottom left to right) showing the metal−arene back-bonding interaction. The values given are the total amount of back-donation, with the
contribution from the HOMO being given in parentheses.
99.9%) was passed over a column of LiAlH4 prior to use; cyclopentene
(Aldrich) and styrene (Aldrich) were vacuum-transferred from LiAlH4.
[Ph3C][B(C6F5)4] (Strem) and Bu4NBr (Fluka) were used as
received. The compounds {[η6-Ar-CMe2-η5-C5H4]Ta(Pr)}[B(C6F5)4]
(1; Ar = 3,5-Me2C6H3) and {[η6-Ar-CMe2-η5-C5H4]TaH}[B(C6F5)4]
(2) were synthesized according to published procedures.8 NMR
spectra were recorded on Varian VXR 300 or Varian Inova 500
spectrometers. The 1H and 13C NMR spectra were referenced
internally using the residual solvent resonances and reported in ppm
relative to TMS (0 ppm); J values are reported in Hz. Assignment of
NMR resonances was aided by gradient-selected COSY, NOESY,
HSQC, and/or HMBC experiments using standard pulse sequences.
Elemental analyses were performed at the Microanalytical Department
of the University of Groningen or Kolbe Microanalytical Laboratory
binding energy. Taken together, these data indicate that while
electrostatic contributions are important, delocalization of Ta d
electron density into the arene π* orbitals (back-donation) is a
key component in stabilizing these complexes.
SUMMARY AND CONCLUSIONS
■
The insertion chemistry of olefins into the Ta−H bond of the
cation [η6-Ar-CMe2-η5-C5H4]TaH+ has been examined, and its
high reactivity is exemplified by the facile insertion of the
trisubsituted olefin 2-methyl-2-pentene. Insertion of styrene
exclusively proceeds in a 2,1-fashion to give a Ta-CH(Me)Ph
group that shows σ3-benzylic character. Neutral and dicationic
tantalum complexes with an ansa-cyclopentadienyl-arene ligand
are readily available from monocationic precursors by addition
of Br− and removal of H−, respectively, and their X-ray crystal
structures are described. All of the complexes show back-
donation from the d2 Ta(III) center into the arene π* system,
as judged from the puckering of the C6 ring to result in
dienediyl character. The structural and computational data
show that enhanced electrostatic interactions on going to more
positively charged metal centers increase the binding energy.
The charge at the metal center (electrophilicity) does not alter
the fundamental nature of the metal−arene interaction, which
in all cases may be described by a donor−acceptor type
interaction in accordance with the Chatt−Dewar−Duncanson
model that includes significant arene←metal back-donation.
This suggests that ansa-cyclopentadienyl-arene ligation in early-
transition-metal compounds is a more generally applicable
motif for stabilization of complexes in low formal oxidation
states.
(Mulheim an der Ruhr, Germany).
̈
Ionic compounds contain the anion [B(C6F5)4]−, the NMR data for
which are identical for all compounds and are as follows. 13C NMR
(125.7 MHz, C6D5Br): δ 148.7 (d, JCF = 242, o-CF), 138.6 (d, JCF
=
238, p-CF), 136.6 (d, JCF = 241, m-CF), 124.7 (br, ipso-C). 19F NMR
(470.3 MHz, C6D5Br): δ −131.9 (br d, o-F), −161.7 (t, J = 21, p-F),
−165.8 (br d, m-F).
NMR-Scale Synthesis of {[η6-Ar-CMe2-η5-C5H4]Ta(C5H9)}[B-
(C6F5)4] (3). A solution of 2 (ca. 8.7 μmol) in C6D5Br was degassed
on a vacuum line by three freeze−pump−thaw cycles. Subsequently,
ca. 1.1 equiv of cyclopentene was condensed into the tube using a
calibrated gas bulb (9.4 mL, ∼20 mmHg) and the reaction was
monitored by 1H NMR spectroscopy. Full conversion was achieved in
5 h, and NMR spectroscopy was consistent with the formation of 3.
Layering the C6D5Br solution with cyclohexane afforded a small
amount of dark green crystals, and X-ray analysis confirmed the
proposed formulation. 1H NMR (500 MHz, C6D5Br, −30 °C): δ 4.52
(s, 1H), 4.44 (s, 1H), 4.25 (s, 1H), 4.21 (s, 1H), 4.18 (s, 1H), 4.12 (s,
1H), 2.03 (s, 1H), 1.81 (s, 3H, ArMe), 1.65 (s, 3H, ArMe), 0.52 (s,
3H, CMe2), 0.40 (s, 3H, CMe2), −0.69 (br, 1H, β-CHH′), −7.04 (br,
1H, β-CHH′), broad overlapped signals between 1.9 and 1.0 ppm due
to C5H9.
EXPERIMENTAL SECTION
■
NMR-Scale Synthesis of {[η6-Ar-CMe2-η5-C5H4]Ta-
(CH2CH2CH2CHMe2)}[B(C6F5)4] (4). A solution of 2 (ca. 3.6 μmol)
in C6D5Br was degassed on a vacuum line by three freeze−pump−
thaw cycles. Subsequently, ca. 10 equiv of 2-methyl-2-pentene was
condensed into the tube using a calibrated gas bulb (9.4 mL, ∼70
General Considerations. All manipulations were carried out
under a nitrogen atmosphere using standard glovebox, Schlenk, and
vacuum-line techniques. Toluene and pentane (Aldrich, anhydrous,
99.8%) were passed over columns of Al2O3 (Fluka), BASF R3-11-
supported Cu oxygen scavenger, and molecular sieves (Aldrich, 4 Å).
THF (Aldrich, anhydrous, 99.8%) was dried by percolation over
columns of Al2O3 (Fluka). Cyclohexane (Labscan) was distilled from
Na/K alloy and bromobenzene (Merck) from CaH2. All solvents were
degassed prior to use and stored under nitrogen. Deuterated solvents
were vacuum-transferred from Na/K alloy (THF-d8, C6D6, Aldrich) or
from CaH2 (C6D5Br, Aldrich) and stored under nitrogen. H2 (AGA,
1
mmHg) and the reaction was monitored by H NMR spectroscopy.
1
NMR spectroscopy was consistent with the formation of 4. H NMR
(500 MHz, C6D5Br, −30 °C): δ 4.52 (s, 2H, Cp + Ar o-H), 4.51 (s,
1H, Cp), 4.32 (s, 1H, Cp), 4.24 (s, 1H, Ar o-H), 4.18 (s, 1H, Cp), 2.14
(s, 1H, Ar pH), 1.83 (s, 3H, ArMe), 1.75 (s, 3H, ArMe), 1.37 (m, 1H,
TaCH2CH2CH2CHMe2), 1.25 (m, 1H, TaCH2CH2CHH′CHMe2),
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0.78 (d, J = 6.7, 3H, TaCH2CH2CH2CHMe2), 0.77 (d, J = 6.7, 3H,
TaCH2CH2CH2CHMe2), 0.62 (m, 1H, TaCHH′CH2CH2CHMe2),
0.56 (s, 3H, CMe2), 0.52 (m, 1H, TaCH2CH2CHH′CHMe2), 0.41 (s,
3H, CMe2), −0.46 (m, 1H, TaCHH′CH2CH2CHMe2), −1.3 (br, 1H,
TaCH2CHH′CH2CHMe2), −6.6 (br, 1H, TaCH2CHH′CH2CHMe2).
13C NMR38 (125.7 MHz, C6D5Br, −30 °C): δ 127.7 (s, Ar m-C),
128.2 (s, Ar m-C), 103.5 (d, J ≈ 185, Cp CH), 100.6 (d, J ≈ 180, Cp
CH), 96.9 (d, J ≈ 176, Ar o-CH), 93.9 (d, J ≈ 178, Ar p-CH), 93.6 (d,
J ≈ 174, Ar o-CH), 72.8 (d, J ≈ 184, Cp CH), 72.3 (s, Cp ipso-C), 71.4
(d, J ≈ 188, Cp CH), 60.4 (s, Ar ipso-C), 40.2 (t, J ≈ 129,
TaCH2CH2CH2CHMe2), 36.2 (s, CMe2), 29.6 (d, J ≈ 123,
freeze−pump−thaw cycles. The tube was frozen in liquid N2 and back-
filled with 1 bar of H2, after which the tube was closed. After it was
warmed to room temperature, the resulting red solution was analyzed
by 1H NMR, showing full conversion to 2.8 Excess H2 was pumped off,
and [Ph3C][B(C6F5)4] (2.2 mg, 2.4 μmol) was added in the glovebox.
The reaction was complete after ca. 1 h, and NMR spectroscopy
showed clean formation of Ph3CH and 7. 1H NMR (300 MHz,
C6D5Br, 20 °C): δ 7.15−6.95 (Ph3CH), 5.64 (s, 2H, Cp), 5.40
(Ph3CH), 5.26 (s, 1H, Ar p-H), 4.86 (s, 2H, Ar o-H), 4.66 (s, 2H, Cp),
2.02 (s, 6H, ArMe), 0.50 (s, 6H, CMe2).
{[η6-Ar-CMe2-η5-C5H4]Ta(THF-d8)2}[B(C6F5)4]2 (7-THF). An
NMR tube containing a solution of 1 (23.0 mg, 0.0181 mmol) in
0.4 mL of C6D5Br was degassed on a vacuum line by three freeze−
pump−thaw cycles. The tube was frozen in liquid N2 and back-filled
with 1 bar of H2, after which the tube was closed. After it was warmed
TaCH2 CH2 CH2 CHMe2 ), 22. 2, 22. 0 (q,
J ≈ 124,
TaCH2CH2CH2CHMe2), 21.1, 20.7 (q, J ≈ 129, ArMe), 20.3, 19.9
(q, J ≈ 127, CMe2), 9.7 (t, J ≈ 145, TaCH2CH2CH2CHMe2), 1.6
(TaCH2CH2CH2CHMe2).
1
{[η6-Ar-CMe2-η5-C5H4]TaCH(Me)Ph}[B(C6F5)4] (5). A solution of
1 (38.1 mg, 30.0 μmol) in 1 mL of bromobenzene was degassed on a
vacuum line by three freeze−pump−thaw cycles. The reaction vessel
was pressurized with 1 bar of H2, and the mixture was stirred at room
temperature for 15 min. The excess H2 was pumped off. To the red
solution was added 3.8 μL of styrene (33 μmol) by microsyringe, and
the mixture was allowed to react at −30 °C overnight. The product
was precipitated by addition of pentane and the supernatant decanted.
The yellow-brown product was washed with toluene and then pentane
and subsequently dried in vacuo to give 23.8 mg of brownish
microcrystalline 5 (20.2 μmol, 67%). 1H NMR (500 MHz, C6D5Br, 20
°C): δ 7.20 (t, 1H, J = 7.6, Ph), 7.15 (t, 1H, J = 7.6, Ph), 6.67 (t, 1H, J
= 7.5, Ph), 6.62 (d, 1H, J = 8.3, Ph o-H), 4.55 (s, 1H, Ar o-H), 4.25 (s,
1H, Ar o-H), 4.12 (s, 1H, Cp), 3.82 (s, 1H, Cp), 3.50 (s, 1H, Cp), 2.87
(s, 1H, Ar p-H), 2.39 (br, 1H, Ph o-H), 2.04 (s, 3H, ArMe), 1.72 (s,
3H, ArMe), 1.63 (s, 1H, Cp), 1.51 (d, 3H, J = 6.1, TaCH(Me)Ph, 0.58
(s, 3H, CMe2), 0.44 (s, 3H, CMe2), −0.22 (q, 1H, J = 6.1,
TaCH(Me)Ph. 13C NMR (125.7 MHz, C6D5Br, 20 °C): δ 143.8 (d, J
= 163, Ph CH), 137.2 (d, J = 161, Ph CH), 133.17 (s, Ar m-C), 133.15
(s, Ph ipso-C), ∼126 (Ar m-C), 122.0 (d, overlapped, Ph CH), 121.9
(d, overlapped, Ph CH), 111.5 (d, J = 180, Cp CH), 107.3 (d, J = 179,
Cp CH), 98.1 (d, J = 172, Ar o-CH), 96.8 (d, J = 168, Ar p-CH), 88.2
(d, J = 174, Ar o-CH), 75.3 (d, J = 183, Cp CH), 73.7 (d, J = 145, Ph
o-CH), 72.2 (s, Cp ipso-C), 70.3 (d, J = 184, Cp CH), 61.4 (s, Ar ipso-
C), 41.1 (d, J = 135, TaCH(Me)Ph), 34.5 (s, CMe2), 20.9 (q, J = 128,
ArMe), 20.8 (q, J = 128, ArMe), 20.6 (q, J = 128, CMe2), 20.1 (q, J =
128, CMe2). Anal. Calcd for C48H28BF20Ta: C, 49.00; H, 2.40. Found:
C, 48.78; H, 2.34.
to room temperature, the resulting red solution was analyzed by H
NMR, showing full conversion to 2. Excess H2 was pumped off, and
[Ph3C][B(C6F5)4] (16.7 mg, 0.0181 mmol) was added in the
glovebox. A viscous dark oil precipitated over the course of 1 h,
which was subsequently dissolved by addition of ca. 0.2 mL of THF-d8.
NMR spectroscopy was consistent with the formation of Ph3CH and
7-THF. 1H NMR (500 MHz, C6D5Br/THF-d8, 20 °C): δ 7.10 (t, 6H,
J = 7.5, Ph3CH m-H), 7.04 (t, 3H, J = 7.3, Ph3CH p-H), 6.99 (d, 6H, J
= 7.4, Ph3CH o-H), 6.24 (2H, Cp), 5.40 (s, 1H, Ph3CH), 5.32 (s, 1H,
Ar p-H), 5.17 (s, 2H, Cp), 4.92 (s, 2H, Ar o-H), 1.99 (s, 6H, ArMe),
0.65 (s, 6H, CMe2). 13C NMR (125.7 MHz, C6D5Br/THF-d8, 20 °C):
δ 144.1 (s, Ph3CH ipso-C), 140.9 (s, Ar m-C), 129.6 (d, J = 160,
Ph3CH CH), 128.4 (d, J = 159, Ph3CH CH), 126.4 (d, J = 159,
Ph3CH CH), 118.8 (d, J = 180, Cp CH), 102.9 (d, J = 173, Ar p-CH),
98.4 (d, J = 179, Ar o-CH), 94.5 (s, Cp ipso-C), 88.9 (s, Ar ipso-C),
78.9 (d, J = 185, Cp CH), 57.1 (d, J = 127, Ph3CH), 33.2 (s, CMe2),
22.1 (q, J = 130, ArMe), 20.2 (q, J = 128, CMe2).
Diffusion of cyclohexane into the C6D5Br/THF-d8 solution
precipitated a dark oil, from which the supernatant was decanted.
Repeated trituration of the oil with pentane gave 7-THF as a sticky
solid material (34.2 mg, 0.0181 mmol, 99%). Anal. Calcd for
C72H35B2F40O2Ta: C, 45.65; H, 1.86. Found: C, 46.33; H, 2.25. A
small amount of the solid material was dissolved in THF, and pentane
was layered on top of the solution. A dark oil precipitated, which
slowly solidified in the course of several days to give dark blue needles
of 7-THF.
X-ray Crystallography. Suitable crystals of 6 and 7-THF were
mounted on top of a glass fiber in a drybox and transferred, using
inert-atmosphere handling techniques, into the cold nitrogen stream of
a Bruker SMART APEX CCD diffractometer. The final unit cell was
obtained from the xyz centroids of 7262 (6) and 7391 (7-THF)
reflections after integration. Intensity data were corrected for Lorentz
and polarization effects, scale variation, and decay and absorption: a
multiscan absorption correction was applied, on the basis of the
intensities of symmetry-related reflections measured at different
angular settings (SADABS).30 The structures were solved by Patterson
methods, and extension of the model was accomplished by direct
methods applied to difference structure factors using the program
DIRDIF.31 The hydrogen atoms were generated by geometrical
considerations and constrained to idealized geometries and allowed to
ride on their carrier atoms with an isotropic displacement parameter
related to the equivalent displacement parameter of their carrier atoms.
For 6, the polarity of the structure of the molecule actually chosen was
determined by Flack’s refinement parameter32 (x = 0.016(14)). For 7-
THF, refinement was frustrated by disorder in the position of C(124).
A two-site occupancy model was applied to C(124), for which the
displacement parameters were separately refined. The sof of the major
fraction refined to a value of 0.55(4). All refinement and geometry
calculations were performed with the program packages SHELXL33
and PLATON.34 Crystal data and details on data collection and
refinement are presented in the Supporting Information.
[η6-Ar-CMe2-η5-C5H4]Ta(Pr)Br (6). Solid 1 (123.4 mg, 0.0971
mmol) and Bu4NBr (32.9 mg, 0.101 mmol) were mixed in the
glovebox, and 2 mL of cold (−30 °C) bromobenzene was added.
While it was warmed to room temperature, the mixture was shaken to
dissolve all starting materials. The resulting brownish yellow solution
was allowed to react overnight at room temperature. Upon addition of
5 mL of pentane, a white precipitate was formed, from which the
supernatant was decanted. The residue was washed with 10 mL of a
toluene/pentane (1/1) mixture. The combined filtrate was evaporated
to dryness, yielding 6 as a brown-green microcrystalline material (41.3
1
mg, 0.0802 mmol, 83%). H NMR (500 MHz, C6D6, 25 °C): δ 5.69
(s, 1H, Cp), 5.05 (s, 1H, Cp), 4.36 (s, 1H, Cp), 4.33 (s, 1H, Ar p-H),
3.85 (s, 1H, Cp), 3.36 (s, 1H, Ar o-H), 3.27 (s, 1H, Ar o-H), 2.02 (s,
3H, ArMe), 1.52 (s, 3H, ArMe), 1.41 (m, 1H, TaCH2CHH′Me), 1.40
(ps t, overlapped, 1H, TaCHH′CH2Me), 1.21 (t, J = 7.1, 3H,
TaCH2CH2Me), 0.96 (m, 1H, TaCH2CHH′Me), 0.53 (s, 3H, CMe2),
0.43 (ps t, J = 13.6, 1H, TaCHH′CH2Me), 0.20 (s, 3H, CMe2). 13C
NMR (data taken from HSQC/HMBC experiments, C6D6, 25 °C): δ
134.9, 129.9, 115.3 (Cp CH), 106.7 (Cp CH), 100.8 (Ar o-CH), 95.4
(Ar p-CH), 85.6 (Ar o-CH), 78.2 (Cp ipso-C), 72.8 (Cp CH), 71.4
(Cp CH), 58.4 (Ar ipso-C), 33.4 (CMe2), 30.4 (TaCH2CH2Me), 23.8
(TaCH2CH2Me), 22.3 (CMe2), 21.4 (ArMe), 21.3 (CMe2), 20.1
(ArMe), 15.1 (TaCH2CH2Me). Anal. Calcd for C19H26BrTa: C, 44.29;
H, 5.09. Found: C, 44.52; H, 5.12.
Computational Studies. Calculations were performed with the
Gaussian03 program using density functional theory (DFT).35
Geometries were fully optimized starting from the X-ray structures
using the B3LYP exchange-correlation functional with the LANL2DZ
NMR-Scale Synthesis of {[η6-Ar-CMe2-η5-C5H4]Ta(C6D5Br)x}[B-
(C6F5)4]2 (7). An NMR tube containing a solution of 1 (3.0 mg, 2.4
μmol) in 0.4 mL of C6D5Br was degassed on a vacuum line by three
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basis set on the metal center and 6-311G(d) basis sets on all other
atoms. The anions were omitted in the calculations. Optimizations
were performed without (symmetry) constraints, and the resulting
structures were confirmed to be minima on the potential energy
surface by frequency calculations (number of imaginary frequencies 0).
For 7-THFcalcd, another stationary point (number of imaginary
frequencies 0) was identified for which the orientation of the
coordinated THF molecules was somewhat different: the structural
features of the [Cp-arene]Ta fragment were virtually identical and the
energies of these two minima were within 1 kJ/mol.
Charge decomposition analysis was performed using the CDA
program by Dapprich and Frenking.36 The input for the CDA program
was generated from single-point calculations on the appropriate
optimized geometry (without bridging atom, as described previously7)
with double-ζ basis sets used throughout (LANL2DZ on the metal, 6-
31G(d) on all nonmetal atoms). For visualization of the computed
structures and molecular orbitals the program MOLEKEL 4.3 was
used.37
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ASSOCIATED CONTENT
* Supporting Information
■
S
Tables giving Cartesian coordinates for the optimized geo-
metries of 1calcd, 6calcd, 7-THFcalcd, and 7calcd,opt and tables, a
figure, and CIF files giving crystallographic data for compounds
3, 6, and 7-THF. This material is available free of charge via the
Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: edwin.otten@rug.nl.
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Inorg. Chem. Commun. 2003, 6, 1287.
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Soc. 1987, 109, 6525. (b) Arney, D. J.; Wexler, P. A.; Wigley, D. E.
Organometallics 1990, 9, 1282.
■
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
Financial support was provided by NRSC-Catalysis and The
Netherlands Organisation for Scientific Research (NWO-Veni
grant to E.O.).
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