Cleavage of carbon monoxide promoted by a dinuclear tantalum tetrahydride complex

Article abstract of DOI:10.1021/om300759p

The reaction of one equivalent of carbon monoxide with the dinuclear tetrahydride complex ([NPN]Ta)₂(μ-H)₄ (where NPN = PhP(CH₂SiMe₂NPh)₂) results in the cleavage of the CO triple bond and formation of ([NPN]Ta)(μ-O)(μ-H)(Ta[NPN′]) (where NPN′ = PhP(CH₂SiMe₂NPh)(CH ₂SiMe₂NC₆H₄)) via extrusion of CH₄. The identity of the ditantalum complex was confirmed by single-crystal X-ray analysis, isotopic labeling studies, and GC-MS analysis of the methane released. DFT calculations were performed to provide information on the initial adduct formed and likely transition states for the process.

Full text of DOI:10.1021/om300759p

Article
pubs.acs.org/Organometallics
Cleavage of Carbon Monoxide Promoted by a Dinuclear Tantalum
Tetrahydride Complex
Joachim Ballmann,† Fraser Pick,† Ludovic Castro,‡ Michael D. Fryzuk,*,† and Laurent Maron*,‡
†Department of Chemistry, University of British Columbia, 2036 Main Mall, Vancouver, BC Canada, V6T 1Z1
‡Universite
́
de Toulouse, INSA, UPS, LPCNO, 135 Avenue de Rangueil, F-31077 Toulouse, France, and CNRS, LPCNO UMR 5215,
F-31077 Toulouse, France
S
* Supporting Information
ABSTRACT: The reaction of one equivalent of carbon monoxide with the dinuclear tetrahydride complex ([NPN]Ta)2(μ-H)4
(where NPN = PhP(CH2SiMe2NPh)2) results in the cleavage of the CO triple bond and formation of ([NPN]Ta)(μ-O)(μ-
H)(Ta[NPN′]) (where NPN′ = PhP(CH2SiMe2NPh)(CH2SiMe2NC6H4)) via extrusion of CH4. The identity of the ditantalum
complex was confirmed by single-crystal X-ray analysis, isotopic labeling studies, and GC-MS analysis of the methane released.
DFT calculations were performed to provide information on the initial adduct formed and likely transition states for the process.
INTRODUCTION
RESULTS AND DISCUSSION
■
■
While the reaction of 1 with N2 proceeds smoothly with excess
N2,6,7 the corresponding reaction of 1 with CO requires
stoichiometric addition of CO, as excess carbon monoxide
results in a complicated mixture of products. Addition of exactly
1 equiv of CO to 1 results in the formation of a dark brown
solution, from which red crystals could be obtained in
reasonable yield (54%). The spectroscopic characteristics of
the isolated material show that it is an unsymmetrical species
with inequivalent phosphorus-31 nuclei (two sharp singlets at δ
20.2 and 12.1 in the 31P{1H} NMR spectrum) and a very
The analogy between carbon monoxide (CO) and dinitrogen
(N2) is often made to rationalize their relative abilities to be
activated by transition metal complexes.1 Even though these
molecules are isoelectronic, the more polar CO is a much better
ligand than N2 largely because of the smaller energy gap
between the HOMO and LUMO for CO as compared to N2,
which allows better overlap of these orbitals with appropriate
transition metal d orbitals. In addition, unlike dinitrogen,
carbon monoxide undergoes migratory insertion processes,
which are key to a number of industrial catalytic processes such
as hydroformylation, Fischer−Tropsch, and the acetic acid
synthesis;2 by comparison, the only industrial process that
utilizes N2 as a feedstock is the Haber−Bosch ammonia
synthesis,3 which does not involve any migratory insertion steps
with intact dinitrogen.4
1
complicated H NMR spectrum with eight distinct silyl methyl
groups, consistent with C1 symmetry. These patterns alone
made it clear that this product was not 3 (Scheme 1), the
anticipated analogue of the side-on end-on N2 complex 2, as
both species would have Cs symmetry. In particular, no peaks
1
were observed in the H NMR spectrum of the product in the
We recently described the facile activation of dinitrogen (N2)
by a dinuclear tantalum tetrahydride, ([NPN]Ta)2(μ-H)4 (1)
(where NPN = PhP(CH2SiMe2NPh)2), to generate the side-on
end-on ditantalum dinitrogen complex ([NPN]Ta)2(μ-η2:η1-
N2)(μ-H)2 (2).5,6 As this N2 complex displays a rich reactivity
that results in E−N bond formation (E = B, C, Al, Si) and even
N−N bond cleavage,5 we wondered if a similar kind of
activation process could be realized for carbon monoxide (CO)
by reaction with tetrahydride 1 (i.e., formation of 3 in Scheme
1). What we discovered was that CO can be activated by an
apparent series of migratory insertion processes involving 1 in a
manner quite different from its isoelectronic analogue, N2.
downfield region between δ 8 and 18, which is diagnostic for a
bridging hydride of the type Ta2(μ-H)x. Moreover, the aromatic
protons displayed a complicated upfield-shifted pattern of four
coupled resonances in the range δ 4.8−6.4. The use of carbon-
13-labeled carbon monoxide (13CO) was confusing, as the
isolated product did not display any isotope-enhanced peaks in
the 13C NMR spectrum. However, we did observe an intense
peak at δ −4.5 when the solution and gas phase were carefully
analyzed after separation of the product from the crude reaction
Received: August 7, 2012
Published: December 11, 2012
© 2012 American Chemical Society
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Scheme 1
mixture; this peak corresponds to 13CH4. The reaction of the
polydeuteride (1-d12, vide infra) with 1 equiv of CO resulted in
the formation an isotopologue of the final product, whose
upfield 1H NMR resonance at δ 4.98 is absent. While a number
of possible structures were considered, particularly in light of
previous studies wherein upfield shifted aromatic proton
resonances of a cyclometalated N-Ph unit were observed,8
the isolation of suitable crystals for X-ray analysis provided an
unequivocal answer.
As shown in Figure 1, the product 4 has a dinuclear structure
and there is both a bridging hydride unit and a bridging oxo.
Also interesting is the π-bound cyclometalated N-C6H4 group
of one of the amido donors; this feature in 4 rationalizes the
upfield shifted proton resonances observed, as they are due to
this unique cyclometalated ring. As a resonance for the bridging
hydride was not detected in the range from δ −20 to +50 in the
1H NMR spectrum, we suggest that it is buried under the
aromatic resonances. Attempts to confirm this using the various
deuterated forms of 1 (cf., 1-d12 or 1-dx) were inconclusive.
A mechanism for the formation of 4 is proposed in Scheme 2
that takes advantage of previous studies on the reaction of
carbon monoxide with tantalum hydrides.9−11 The structure of
the initial adduct of CO with the tetrahydride, Ta2H4·CO,
could not be detected, and therefore its formulation is
speculatively shown as a simple end-on CO bound to one of
the Ta centers of the dinuclear tetrahydride. Subsequent
migratory insertion generates the bridging η2-formyl
Ta2H3·CHO, which is then converted to the methylene-oxy
species, Ta2H2·CH2O, via migratory insertion. The next step is
a reductive ring-opening of a ditantalamethylene oxy four-
membered ring to generate a coordinated methylidene and
tantalum-oxo species. Another migratory insertion of the
methylidene and one of the hydrides generates the methyl-μ-
oxo hydride Ta2H·CH3O, which upon reductive elimination of
CH4 would generate the highly coordinatively unsaturated
species Ta2O; it is this species that we propose undergoes
activation of the N-Ph moiety to generate the observed
cyclometalated derivative 4.
Figure 1. ORTEP diagram of 4 (silylmethyls omitted and only ipso
carbons of the N- and P-phenyls are shown, except for the
cyclometalated phenyl). Selected bond lengths (Å) and bond angles
(deg): Ta1−Ta2 2.7240(6), Ta1−C18 2.095(4), Ta2−C18 2.399(4),
Ta−C17 2.501(4), Ta1−O1 1.925(3), Ta2−O1 2.009(3), Ta1−H1
1.96(5), Ta2−H1 1.92(5), Ta1−N1 2.127(3), Ta1−N2 2.081(4),
Ta1−P1 2.6706(12), Ta2−N3 2.043(4), Ta2−N4 2.068(3), Ta2−P2
2.6154(12), Ta1−N1−C13 94.8(2), N1−C13−C18 104.4(3), Ta1−
C18−C13 95.0(3), C18−Ta1−N1 64.81(14), C17−C18−Ta1
143.2(3), Ta1−C18−Ta2 74.29(13), Ta1−O1−Ta2 87.60(11), O1−
Ta1−N1 107.72(12), O1−Ta1−N2 126.07(13), O1−Ta1−C18
100.74(14), N2−Ta1−C18 116.15(15), N2−Ta1−N1 122.77(13),
N3−Ta2−N4 113.35(13), N3−Ta2−C18 102.07(14), N4−Ta2−C18
144.54(14), O1−Ta2−N3 91.87(12), O1−Ta2−N4 88.40(12), O1−
Ta2−C18 88.85(13).
Carbon monoxide can be cleaved by both mononuclear and
multinuclear early transition metal12 and lanthanide com-
plexes.13 In certain cases, the systems utilized are low oxidation
14
early transition metal complexes, such as Ta(silox)3 or
15
W2(silox)4Cl2 (where silox = OSiBut3), and the cleavage
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Scheme 2
process operates via reduction of the CO.16 Other examples
include multistep processes involving migratory insertion,9−11
sometimes through the intermediacy of a coordinated formyl
unit. As proposed above in Scheme 2, the dinuclear ditantalum
system cleaves CO by a combination of migratory insertion
steps and a reductive cleavage step. In an effort to better
understand the mechanism of the reaction of tetrahydride 1
with CO, we examined possible intermediates and transition
states computationally.
Computational Details. Tantalum atoms were treated
with the small-core Stuttgart−Dresden relativistic effective core
potential in combination with its adapted basis set.17,18 Carbon,
oxygen, nitrogen, and hydrogen atoms have been described
with a 6-31G(d,p) double-ζ basis set.19 Silicon and phosphorus
atoms were treated with the Stuttgart−Dresden ECP in
combination with its adapted basis set and additional d
polarization functions.20,21 Calculations were carried out at the
DFT level of theory using the hybrid functional B3PW91.22,23
Geometry optimizations were performed without any symme-
try restrictions, and the nature of the extremes (minima and
transition states) was verified with analytical frequency
calculations. Gibbs free energies were obtained at T = 298.15
K within the harmonic approximation. IRC calculations were
performed to confirm the connections of the optimized
transition states. DFT calculations were carried out with the
Gaussian09 suite of programs.24 The electronic density (at the
DFT level) has been analyzed using the natural bond orbital
(NBO) technique.25 Calculations have been realized in the gas
phase, and the real NPN ligands have been computed.
Figure 2. Computed structure of complex 1. NPN ligands have been
simplified for clarity. Selected bond distances (Å), bond angles (deg),
and torsion angles (deg); comparisons to the incomplete structure of 1
(see Supporting Information) are given where applicable in brackets:
Ta1−Ta2 2.589 [2.57], Ta−P 2.605, Ta1−N1 2.104 [2.08], Ta1−N2
2.124 [2.09], Ta2−N3 2.124 [2.09], Ta2−N4 2.104 [2.08], Ta1−H1
1.965, Ta1−H2 1.918, Ta1−H3 1.978, Ta1−H4 1.909, Ta2−H1
1.918, Ta2−H2 1.965, Ta2−H3 1.909, Ta2−H4 1.978, P1−Ta1−P2
154.57, Ta2−Ta1−P1 128.97, Ta2−Ta1−N1 122.69, Ta1−H1−Ta2
83.61, P1−Ta1−Ta2−P2 179.99 [180], N1−Ta1−Ta2−N4 179.98.
and angles. It confirms the presence of four bridging hydrides,
and the global structure matches the structure observed
experimentally26 for the complex ([P2N2]Ta)2(μ-H)4. The
NBO analysis is consistent with the presence of the Ta(IV)
oxidation state and the presence of a Ta−Ta bond. This is
further confirmed by analyzing the molecular orbitals. Indeed,
the HOMO of this complex corresponds to the σ-interaction of
To begin, we computed the structure of the ditantalum
tetrahydride 1 and compared it to an incomplete X-ray crystal
structure (see Supporting Information). The computed
structure is presented in Figure 2 with selected bond lengths
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two d orbitals of tantalum (see Figure 3), suggesting that the
electrons stored in the Ta−Ta bond can be used to reduce
substrates.
the CO to be unsymmetrically bridging in a μ-η1-η1 mode with
quite different Ta1−C and Ta2−C distances of 2.08 and 2.43
Å, respectively. The C−O bond length is 1.19 Å, which is
elongated with respect to free CO (1.13 Å), which supports
some π-back-donation from the two tantalum centers. The
HOMO of this system corresponds to the interaction between
the Ta centers and CO and is depicted in Figure 5. The
Figure 3. Schematic depiction of the HOMO for complex 1,
corresponding to the metal−metal overlap.
The reaction between 1 and CO is complex. The first part of
the Gibbs free energy profile of the reaction between 1 and CO
is presented in Figure 4. Each minimum of the profile has been
optimized in its singlet and triplet spin-states, and the singlet
spin-states are also found to be the most stable for all minima,
by 8−30 kcal/mol. Thus, the reactivity takes place on the
singlet potential energy surface.
Figure 5. Schematic depiction of the HOMO for Ta2H4·CO,
corresponding to the metal−CO overlaps; the bridging hydrides are
not shown, and most of the ligands except for the two amido donors
on each tantalum are omitted.
The first step of the reaction is the coordination of CO to 1
involving both Ta centers. Interestingly, two of the bridging
hydride groups have become terminal in order to allow the
coordination; this was not considered in the original proposed
mechanism shown in Scheme 1. The coordination mode shows
coordination of CO is exergonic by −7.8 kcal/mol.
Interestingly, this resembles the aforementioned product of
CO activation shown in Scheme 1, by analogy to the N2
activation; however, in this calculation of the side-on, end-on
Figure 4. Gibbs free energy profile of the reaction between the tetrahydride complex 1 and CO. NPN ligands have been simplified for clarity. Atom
colors: Ta: green, P: orange, N: blue, O: red, C: black, H: white. For atom numbering, Ta1 is on the left, while Ta2 is on the right.
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Figure 6. Continuation from Figure 4 starting with the putative tetradeuteride ([NPN]Ta)2(μ-D)4; three proposed pathways leading to the
experimental product with concomitant release of deuterated methane from Ta2H2·(CH2)O. In pathways B and C, CD3H is produced, whereas in
path A, CD4 would result.
CO species, loss of H2 does not occur as in the case of
dinitrogen activation. Rather, the adduct Ta2H4·CO easily
undergoes migratory insertion of CO into a terminal Ta−H
bond. Indeed, the transition state of insertion lies at only +1.9
kcal/mol above the adduct, and the formation of the
corresponding formyl, Ta2H3·HCO, is exergonic by −8.2
kcal/mol with respect to Ta2H4·CO. The structure of the first
transition state, TS1-CO, has the hydride poised for migratory
insertion to generate the formyl group in a μ-η1-η2 coordination
mode.
The next step of the reaction is the facile migration of the
other terminal hydride (transition state TS2-CO at +11.4 kcal/
mol), which generates Ta2H2·H2CO, which contains a
coordinated formaldehyde unit, whose formation is slightly
endergonic by +5.0 kcal/mol with respect to the formyl
Ta2H3·HCO. The formaldehyde group is unsymmetrically
bridging the two Ta centers, with the Ta1−C, Ta2−C, Ta1−O,
and Ta2−O distances being 2.16, 2.41, 2.24, and 2.16 Å,
respectively. At this point, the Ta−Ta distance is equal to 2.66
Å, consistent with the presence of a Ta−Ta bond and the
Ta(IV) formal oxidation state. This intermediate can further
react by breaking the C−O bond of the formaldehyde moiety
with concomitant oxidation of both Ta(IV) centers to Ta(V).
Overcoming this third transition state, TS3-CO, is the most
difficult part of the reaction since the activation barrier is equal
to +19.2 kcal/mol with respect to Ta2H2·H2CO; interestingly,
the overall barrier can be defined as the conversion of
Ta2H3·HCO to TS3-CO and requires +24.2 kcal/mol.
However, the next intermediate is Ta2H2·(CH2)O, which is
exergonic by −23.5 kcal/mol with respect to Ta2H2·H2CO.
Ta2H2·(CH2)O displays a bridging μ-oxo group as well as a
bridging hydride, a terminal methylene group on Ta1, and a
terminal hydride on Ta2. From this point on, different pathways
have been examined computationally in order to generate the
experimentally observed product 4, but only three were found
to be competitive, and these are summarized schematically in
Figure 6.
The three pathways A, B, and C, in Figure 6, differ in the
order of the migratory insertion of the methylene unit and the
activation of the N-phenyl moiety, which results in differences
in where the ortho proton of the activated N-phenyl group
ends up. In path A, the ortho-C-H ends up as the bridging
hydride, whereas in paths B and C, one of the original bridging
hydrides of 1 remains in the bridging position. We anticipated
that testing this would be straightforward based on our earlier
report6 on the preparation of ([NPN]Ta)2(μ-D)4, 1-d4, which
should generate CD4 via path A, whereas both paths B and C
generate CD3H. However, we have since discovered that the
preparation of 1-d4 by addition of excess D2 to the precursor
trimethyl [NPN]TaMe3 is more complicated and actually
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results in the formation of 1-d12, wherein all eight of the ortho-
C-H units on the N-Ph moieties have also been deuterated, in
addition to formation of the four bridging deuterides (Scheme
3). None of ortho-C-Hs of the phenyl on the phosphine moiety
The Gibbs free energy profile of pathway A is presented in
Figure 7. It begins by the migration of the bridging hydride
group of Ta2H2·(CH2)O to the carbene group in order to form
Ta2H·(CH3)O. The activation barrier is equal to +16.1 kcal/
mol with respect to Ta2H2·(CH2)O, and the energetic gain is
only 2.3 kcal/mol. The Ta−Ta distance has decreased from
3.15 Å in Ta2H2·(CH2)O to 2.83 Å in Ta2H·(CH3)O, which is
consistent with the passage from Ta(V) to Ta(IV). The
terminal hydride becomes a bridging hydride in order to
slightly stabilize the complex. Then, this new bridging hydride
can also migrate toward the methyl group in order to release a
methane molecule and form the Ta(III) μ-oxo complex Ta2O.
This latter complex presents a bent Ta−O−Ta angle of 89.1°,
allowing the formation of a double Ta−Ta bond. This is
confirmed by analyzing the HOMO and HOMO−1 of the
complex, which correspond respectively to one π and one σ
orbital arising from the overlap of the d orbitals of the metal
centers (Figure 7).
Scheme 3
The activation barrier corresponding to its formation is equal
to +27.9 kcal/mol, and the reaction is exergonic by −14.2 kcal/
mol, both with respect to Ta2H·(CH3)O. This thermodynamic
gain is mainly due to the formation of the stable methane
molecule along with the entropic gain due to the release of a
small molecule (TΔS estimated to 10−15 kcal/mol at room
temperature).27 Thus, the formation of the coordinately
unsaturated Ta2O complex itself is not a favorable process;
otherwise the energetic gain would be much larger than the
14.2 kcal/mol calculated. In order to stabilize the complex, the
last step of the reaction is the C−H activation of the ortho C−
H bond of an N-phenyl group of the NPN ligand, leading to
the observed product 4. The transition state lies only at +2.1
kcal/mol above the oxo complex, and the reaction is exergonic
by −22.9 kcal/mol with respect to Ta2O, which is consistent
with the low stability of Ta2O. The product exhibits a bridging
oxo group (Ta1−O bond length of 2.02 Å and Ta2−O bond
length of 1.95 Å, versus 2.009(3) and 1.925(3) Å exptl), a
bridging hydride (Ta1−H bond length of 2.02 Å and Ta2−H
bond length of 1.92 Å, versus 1.92(5) and 1.96(5) Å exptl), and
a phenyl bridging ligand, coordinated by the ortho-carbon
(Ta1−C bond length of 2.42 Å and Ta2−C bond length of 2.11
Å versus 2.399(4) and 2.095(4) Å exptl). It is thus a Ta(IV)
complex, which is confirmed by the short Ta−Ta bond distance
(2.75 Å versus 2.7240(6) Å exptl). It is noteworthy that the
quintet spin-state of the Ta(III) μ-oxo complex is more stable
than the singlet spin-state by 3.6 kcal/mol, but in the first case,
the Ta−O−Ta angle becomes linear, which would make the
last reaction step forbidden. We can thus assume without risk
that there is equilibrium between the singlet and the quintet
spin-states and that the singlet spin-state complex Ta2O reacts
directly to generate 4.
are exchanged. To distinguish paths A−C in Figure 6, the use of
1-d12 would not be useful, as CD4 would result in all cases.
However, we have determined that the deuteration of the
trimethyl complex in hexanes results in less deuterium
incorporation; this arises from precipitation of the partially
deuterated complex, which presumably intercepts the isotopic
1
incorporation process. By H NMR spectroscopy, there is no
incorporation of deuterium into the ortho-C-H positions of the
N-Ph groups and partial deuteration of the bridging tantalum
hydrides (Scheme 3). In fact, the formation of a mixture of
isotopologues ([NPN]Ta)2(μ-D)4−x(μ-H)x (x = 0−4) can
easily be determined by the isotopically perturbed signals in the
1H NMR spectrum in the tantalum hydride region.6,10 While
the mechanism of this process is currently under investigation,
by correlating the number of deuterium atoms of the partially
deuterated dinuclear hydride 1-dx to the amount of deuterium
contained in the methane product, we are able to provide some
support for one of the pathways. By GC-MS analysis, we
observe the formation of approximately equal amounts of CD4
and CD3H from the reaction of 1-dx with CO; CD2H2 is also
CONCLUSIONS
■
The reaction of CO with the highly reducing ditantalum
tetrahydride complex 1 proceeds by a series of migratory
insertion reactions. Of particular interest for this process is how
CO interacts with the starting hydride. By using a computa-
tional approach, an adduct structure is proposed that involves
CO interacting with tetrahydride 1 wherein two of the bridging
hydrides isomerize to terminal hydrides, presumably to open up
a coordination site. In fact, one of the interesting insights that is
suggested from these calculations is the importance of the
terminal hydride unit in the migratory insertion processes
documented in this work. While the starting tetrahydride 1
+
likely produced, but its parent ion overlaps with CD3 ions
from CD4. The deuteration of the trimethyl complex in Et2O is
homogeneous throughout, which likely promotes exchange of
both the bridging hydrides and the ortho-C−H bonds (Scheme
3). Reaction of CO with the fully deuterated material 1-d12
results in the formation of CD4 exclusively as expected.
While the other pathways are found to be energetically
accessible, only the Gibbs free energy profile for path A is
shown in detail; the profiles for paths B and C are presented in
the Supporting Information.
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Figure 7. Gibbs free energy profile of pathway A. NPN ligands have been simplified for clarity. Atom colors: Ta: green, P: orange, N: blue, O: red, C:
black, H: white. For atom numbering, Ta1 is on the left, while Ta2 is on the right.
contains four bridging hydrides in its ground state, each
insertion process involves a terminal hydride interacting with a
small organic moiety in a bridging position. While dinuclear
complexes have offered unique activation modes for small
molecules, mainly by invoking simultaneous interactions with
two metal centers,16,28,29 in this work, we show that a strongly
reducing dinuclear center in concert with available hydrides for
migratory insertion can convert an important C1 molecule such
as CO into CH4 via complete utilization of the four bridging
hydrides. We are presently examining the effect of new ligand
designs30−32 on the overall reducing power of a series of
ditantalum tetrahydrides.
EXPERIMENTAL SECTION
■
All manipulations were performed under an atmosphere of dry and
oxygen-free dinitrogen by means of standard Schlenk or glovebox
techniques. Anhydrous Et2O and hexanes were purchased from
Aldrich, sparged with dinitrogen, and dried further by passage through
towers containing activated alumina and molecular sieves. Benzene
and (Me3Si)2O were purchased from Aldrich dried over sodium
benzophenone ketyl and distilled under positive pressure. Toluene-d8
and benzene-d6 were refluxed over sodium, vacuum transferred, and
1
freeze−pump−thaw degassed. H, 31P, and 13C NMR spectra were
recorded on a Bruker AV-400 MHz spectrometer; unless noted
1
otherwise, all spectra were recorded at room temperature. H NMR
spectra were referenced to residual proton signals in C6D6 (7.16 ppm)
or C7D8 (2.09); 31P NMR spectra were referenced to external
P(OMe)3 (141.0 ppm with respect to 85% H3PO4 at 0.0 ppm); 13C
NMR spectra were referenced to the solvent resonances of C6D6
(128.0 ppm) or C7H8 (20.4 ppm). Microanalyses (C, H, N) and mass
spectrometry (low-resolution EI) were performed at the Department
of Chemistry at the University of British Columbia. CO (99.9%),
13CO (99% 13C enriched, break-seal flask), H2 (99.99%), and D2 (HD
Figure 8. Calculated HOMO (top) and HOMO−1 (bottom) for the
TaTa of the putative intermediate Ta2O.
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0.4%, lecture bottle) were purchased from Praxair or from Cambridge
Isotopes Ltd. and passed through activated molecular sieves prior to
use. [PhP(CH2SiMe2NPh)2]TaMe3 ([NPN]TaMe3) was prepared
according to the literature method.6
(app t, 1H, J = 5.8 Hz), 4.98 (d, 1H, J = 6.5 Hz), 1.57 (m, 5H), 1.27
(m, 1H), 1.06 (m, 1H), 0.81 (m, 1H), 0.45 (s, 3H), 0.41 (s, 3H), 0.35
(s, 3H), 0.08 (s, 3H), −0.07 (s, 3H), −0.10 (s, 3H), −0.20 (s, 3H),
−0.29 (s, 3H); 13C{1H} NMR (δ in ppm, C6D6 293 K, 101 MHz)
133.8 (d, 3JCP = 13.63 Hz), 131.8 (d, 3JCP = 11.50 Hz), 130.4 (s), 129.1
(s), 129.0 (s), 129.0 (s), 128.7 (s), 128.6 (s), 127.6 (s), 127.3 (s),
127.2 (s), 127.1 (s), 126.0 (s), 125.3 (s), 122.6 (s), 122.2 (s), 121.5
(s), 17.7 (s), 14.8 (s), 13.9 (s), 5.21 (s), 3.56 (d, 3JCP = 4.42 Hz), 3.03
PhP(CH2SiMe2NPh)2Ta(μ-O)(μ-H)Ta(κ-o-C-C6H4-NSiMe2CH2)-
PhP(CH2SiMe2NPh) ([NPN]Ta)(μ-O)(μ-H)(Ta[NPN′]) (4). A pale
yellow solution of [NPN]TaMe3 (600 mg, 0.908 mmol) in Et2O (50
mL) was transferred into a thick-wall glass vessel equipped with a
Teflon valve and thoroughly degassed via three freeze−pump−thaw
cycles. The vessel was immersed in liquid dinitrogen, filled with H2
gas, and sealed under atmospheric pressure. The reaction mixture was
allowed to warm to room temperature, while the pressure inside the
vessel slowly rose to approximately four atmospheres. After stirring at
room temperature overnight, a deep purple solution of ([NPN]-
Ta)2(μ-H)4 (1) was obtained. The vessel was cooled in liquid
nitrogen, the headspace evaporated, and the solution thoroughly
degassed at −78 °C. The vessel was immersed in liquid dinitrogen and
carbon monoxide (10 mL, 0.409 mmol, 0.9 equiv) condensed into the
vessel from a calibrated glass bulb. Note that exposure to dinitrogen
has to be avoided during this procedure, due to the N2-sensitivity of 1.
The vessel was then sealed under static vacuum, and the reaction
mixture slowly warmed to room temperature. Within 30 min the
purple solution turned brown-orange. After stirring for another 15
min, all volatiles were removed under vacuum. Hexamethyldisiloxane
(approximately 10 mL) was added to the tacky residue, resulting in the
precipitation of the product as a brown powder, which was filtered off,
rinsed with hexamethyldisiloxane (2 × 2 mL), and dried under
vacuum. Yield: 310 mg, 0.249 mmol, 61%. 31P{1H} NMR (δ in ppm,
C6D6 293 K, 162 MHz) 20.2 (s), 12.1 (s); 1H NMR (δ in ppm, C6D6
293 K, 400 MHz) 7.93 (m, 2H), 7.74 (m, 2H), 7.31 (m, 4H), 7.05 (m,
8H), 6.85 (m, 4H), 6.72 (m, 3H), 6.59 (t, 1H, J = 7.2 Hz), 6.47 (d,
2H, 7.3 Hz), 6.34 (dd, 1H, J = 5.3 Hz, 7.8 Hz), 6.04 (t, 1H, J = 7.3
Hz), 5.48 (app t, 1H, J = 5.8 Hz), 4.98 (d, 1H, J = 6.5 Hz), 1.57 (m,
5H), 1.27 (m, 1H), 1.06 (m, 1H), 0.81 (m, 1H), 0.45 (s, 3H), 0.41 (s,
3H), 0.35 (s, 3H), 0.08 (s, 3H), −0.07 (s, 3H), −0.10 (s, 3H), −0.20
(s, 3H), −0.29 (s, 3H); 13C{1H} NMR (δ in ppm, C6D6 293 K, 101
3
3
(s), 2.49 (d, JCP = 7.07 Hz), 2.38 (s), 2.09 (s),), 1.12 (d, JCP = 10.6
3
Hz) 0.53 (d, JCP = 5.69 Hz), −4.26 (s).
Isolation of ([NPN]Ta)2(μ-H)4 (1). A pale yellow slurry of
[NPN]TaMe3 (2.00g, 1.51 mmol) in hexanes (50 mL) was transferred
into a thick-wall glass vessel equipped with a Teflon valve and
thoroughly degassed via three freeze−pump−thaw cycles. The vessel
was immersed in liquid dinitrogen, filled with H2 gas, and sealed under
atmospheric pressure. The reaction mixture was allowed to warm to
room temperature, while the pressure inside the vessel slowly rose to
approximately four atmospheres. Stirring at room temperature
overnight resulted in a purple precipitate and a light brown solution.
The hexanes supernatant was removed under argon via cannula, and
the resulting purple powder was dried in vacuo for 10 min. Note that
prolonged exposure to static or dynamic vacuum results in
decomposition, and once in solution the product is N2-sensitive;
however, the solid can be manipulated in a N2-filled glovebox. Yield:
1
1.37g, 1.11 mmol, 73%. H NMR (C6D6, 400 MHz) δ 0.12 (s, 12H,
3
SiCH3), 0.16 (s, 12H, SiCH3), 1.03 (m, 8H, SiCH2P), 6.91 (t, JH,H
=
7.0 Hz, 4H, p-NPh), 7.22 (br m, 6H, o,p-PPh), 7.27 (m, 8H, m-NPh),
7.33 (m, 8H, o-NPh), 7.39 (br m, 4 H, m-PPh), 10.63 (s, 4H, Ta2H4);
31P{1H} NMR (C6D6, 161 MHz) δ 21.81 (s).
([NPN]Ta)2(μ-D)4−x(μ-H)x (x = 0−4) (1-dx). This was prepared in
an identical manner to ([NPN]Ta)2(μ-H)4 (1), replacing H2 with D2.
Yield: 650 mg, 0.525 mmol, 69%. 1H NMR (C6D6, 293 K, 300 MHz) δ
0.12 (s, 12H, SiCH3), 0.16 (s, 12H, SiCH3), 1.03 (m, 8H, SiCH2P),
3
6.92 (t, JH,H = 7.3 Hz, p-NPh), 7.22 (m, 6H, m,p-PPh), 7.28 (m, 8H,
m-NPh), 7.35 (m, 8H, o-NPh), 7.42 (m, 2H, p-PPh), 10.55 (s, 0.36H,
Ta2D3H), 10.58 (s, 0.38H, Ta2D2H2), 10.61 (s, 0.15H, Ta2DH3), 10.63
(s, 0.04H, Ta2H4), which corresponds to the following ratio:
Ta2D4:Ta2D3H:Ta2D2H2:Ta2DH3:Ta2H4 = 0.38:0.34:0.20:0.07:0.01.
31P NMR (C6D6, 293 K, 121 MHz) δ 21.76 (s).
2
2
MHz) 163.5 (d, JCP = 12.7 Hz), 160.8 (d, JCP = 8.62 Hz), 156.4 (d,
3JCP = 7.27 Hz), 155.2 (d, JCP = 5.61 Hz), 154.8 (d, JCP = 6.31 Hz)
138.24 (d, 1JCP = 28.12 Hz), 136.5 (d, 1JCP = 27.58 Hz), 133.8 (d, 3JCP
= 13.63 Hz) 131.8 (d, 3JCP = 11.50 Hz) 130.4 (s), 129.1 (s), 129.0 (s),
129.0 (s), 128.7 (s), 128.6 (s), 127.6 (s), 127.3 (s), 127.2 (s), 127.1
(s), 126.0 (s), 125.3 (s), 122.6 (s), 122.2 (s), 121.5 (s), 107.2 (s), 98.1
(s), 19.5 (d, 2JCP = 3.64 Hz), 17.7 (s), 14.8 (s), 13.9 (s), 5.21 (s), 3.56
3
3
[PhP(CH2SiMe2NPh-o-d2)2]Ta)2(μ-D)4 (1-d12). A pale yellow
solution of [NPN]TaMe3 (600 mg, 0.908 mmol) in Et2O (50 mL)
was transferred into a thick-wall glass vessel equipped with a Teflon
valve and thoroughly degassed via three freeze−pump−thaw cycles.
The vessel was immersed in liquid dinitrogen, filled with D2 gas, and
sealed under atmospheric pressure. The reaction mixture was allowed
to warm to room temperature, while the pressure inside the vessel
slowly rose to approximately four atmospheres. After stirring at room
temperature overnight, a deep purple solution of [PhP-
(CH2SiMe2NPh-o-d2)2Ta]2(μ-D)4 (1-d12) was obtained. Note: This
3
3
(d, JCP = 4.42 Hz), 3.03 (s), 2.49 (d, JCP = 7.07 Hz), 2.38 (s), 2.09
3
3
(s), 1.12 (d, JCP = 10.6 Hz) 0.53 (d, JCP = 5.69 Hz); MS (EI) m/z
(%) 1246 (100%) [M+]. Anal. Calcd for C48H62N4O1P2Si4Ta2: C,
46.22; H, 5.01; N, 4.49. Found: C, 46.17; H, 5.24; N, 4.33.
Reaction of 13CO with ([NPN]Ta)2(μ-H)4. A pale yellow solution
of [NPN]TaMe3 (53 mg, 0.0401 mmol) in C6D6 (5 mL) was
transferred into a thick-wall glass vessel equipped with a Teflon valve
and thoroughly degassed via three freeze−pump−thaw cycles. The
vessel was immersed in liquid dinitrogen, filled with H2 gas, and sealed
under atmospheric pressure. The reaction mixture was allowed to
warm to room temperature, while the pressure inside the vessel slowly
rose to approximately four atmospheres. After stirring at room
temperature overnight, a deep purple solution of [([NPN]Ta)2(μ-H)4
(1) was obtained. After removing the overpressure of hydrogen the
solution was cannula transferred to an NMR tube equipped with a
Teflon valve, and the tube was filled with C6D6 (∼3 mL) until 1 mL of
headspace remained. The NMR tube was immersed in liquid nitrogen,
and the volatiles were removed under high vacuum. 13CO (∼1 mL,
0.409 mmol) was condensed into the NMR tube from a break-seal
flask. The NMR tube was sealed and allowed to warm to room
temperature. Within 30 min the solution had turned brown-orange,
indicative that the reaction was complete. The reaction mixture was
characterized by NMR spectroscopy. 31P{1H} NMR (δ in ppm, C6D6
293 K, 162 MHz) 20.2 (s), 12.1 (s); 1H NMR (δ in ppm, C6D6 293 K,
400 MHz) 7.93 (m, 2H), 7.74 (m, 2H), 7.31 (m, 4H), 7.05 (m, 8H),
6.85 (m, 4H), 6.72 (m, 3H), 6.59 (t, 1H, J = 7.2 Hz), 6.47 (d, 2H, 7.3
Hz), 6.34 (dd, 1H, J = 5.3 Hz, 7.8 Hz), 6.04 (t, 1H, J = 7.3 Hz), 5.48
1
solution is vacuum and nitrogen sensitive. The H NMR spectrum is
identical to 1 except that the peaks at δ 10.63 and 7.35 are absent.
Methane Analysis for 1 and 1-dx. To an NMR tube equipped
with a Teflon valve were added ([NPN]Ta)2HxD4−x (1-dx) (50 mg,
0.0405 mmol) and diethyl ether (8 mL) under an argon atmosphere
such that ∼1 mL of headspace remained. The solution was degassed
via one freeze−pump−thaw cycle. The headspace was quickly
evacuated and backfilled with carbon monoxide (1 mL, 0.0409
mmol). The NMR tube was sealed, and after one hour the solution
had changed from purple to brown, indicative that the reaction was
complete. The headspace was analyzed on an Agilent GC-MS with an
electron impact ionizer. The temperature was held constant at 28 °C
for 2.00 min and then ramped up at a rate of 5.00 °C/min for 4.40
min. A sample chromatogram is shown in Figure S14 in the
Supporting Information.
ASSOCIATED CONTENT
■
S
* Supporting Information
X-ray data collection and refinement procedures for 1 and 4,
including incomplete X-ray data for 1 and corresponding CIF
8523
dx.doi.org/10.1021/om300759p | Organometallics 2012, 31, 8516−8524

Organometallics
Article
Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin,
K. N.; Staroverov, V. N.; Kobayashi, R.; Normand, J.; Raghavachari, K.;
Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega,
N.; Millam, J. M.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.;
Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.;
Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.;
Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador, P.;
files; additional figures that show the complete optimized
structure of 1, additional mechanistic pathways for generation
of 4; representative NMR spectra. This material is available free
of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: fryzuk@chem.ubc.ca. Fax: +1 604 822-8710. Tel: +1
604 822-2471.
■
̈
Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, O.;
Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. Gaussian 09,
Revision A.2; Gaussian, Inc.: Wallingford, CT, 2009.
(25) Reed, A. E.; Curtiss, L. A.; Weinhold, F. Chem. Rev. 1988, 88,
899.
(26) Fryzuk, M. D.; Johnson, S. A.; Rettig, S. J. Organometallics 2000,
19, 3931.
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(28) Cavigliasso, G.; Christian, G.; Stranger, R.; Yates, B. F. Dalton
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(29) Ariafard, A.; Brookes, N. J.; Stranger, R.; Yates, B. F. J. Am.
Chem. Soc. 2008, 130, 11928.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
M.D.F. thanks NSERC of Canada for a Discovery Grant. L.M.
is member of the Institut Universitaire de France. CINES and
CalMip are acknowledged for a generous grant of computing
time. J.B., M.D.F., and L.M. are also grateful to the Alexander
von Humboldt Foundation for fellowships.
(30) MacLachlan, E. A.; Hess, F. M.; Patrick, B. O.; Fryzuk, M. D. J.
Am. Chem. Soc. 2007, 129, 10895.
(31) Men
5253.
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ard, G.; Jong, H.; Fryzuk, M. D. Organometallics 2009, 28,
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