Reduction of carbon dioxide promoted by a dinuclear tantalum tetrahydride complex

Article abstract of DOI:10.1021/ic302438f

The reaction of 1 equiv of carbon dioxide with the dinuclear tetrahydride complex ([NPN]Ta)₂(μ-H)₄ [where NPN = PhP(CH ₂SiMe₂NPh)₂] results in the formation of ([NPN]Ta)₂(μ-OCH₂O)(μ-H)₂ via a combination of migratory insertion and reductive elimination. The identity of the ditantalum complex containing a methylene diolate fragment was confirmed by single-crystal X-ray analysis, NMR analysis, and isotopic labeling studies. Density functional theory calculations were performed to provide information on the structure of the initial adduct formed and likely transition states and intermediates for the process.

Full text of DOI:10.1021/ic302438f

Communication
pubs.acs.org/IC
Reduction of Carbon Dioxide 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, British Columbia, Canada V6T
‡Universite
́
de Toulouse, INSA, UPS, LPCNO, 135 avenue de Rangueil, F-31077 Toulouse, France
§CNRS, LPCNO UMR 5215, F-31077 Toulouse, France
S
* Supporting Information
What emerges from this work is a rare example of a dinuclear
metal hydride system that functionalizes CO2 and retains its
dinuclearity.
ABSTRACT: The reaction of 1 equiv of carbon dioxide
with the dinuclear tetrahydride complex ([NPN]Ta)2(μ-
H)4 [where NPN = PhP(CH2SiMe2NPh)2] results in the
formation of ([NPN]Ta)2(μ-OCH2O)(μ-H)2 via a
combination of migratory insertion and reductive elimi-
nation. The identity of the ditantalum complex containing
a methylene diolate fragment was confirmed by single-
crystal X-ray analysis, NMR analysis, and isotopic labeling
studies. Density functional theory calculations were
performed to provide information on the structure of the
initial adduct formed and likely transition states and
intermediates for the process.
Our initial inspiration to examine the activation of CO2 was
based on the report that certain zirconium and hafnium
dinitrogen complexes react productively with CO2 to generate
new N−C bonds and regioisomeric hydrazides.13 However, the
reaction of CO2 and the tantalum side-on end-on N2 complex 2
led to the formation of a multitude of products even when the
stoichiometry of added CO2 was controlled. Undaunted, we
turned to the reaction of the ditantalum tetrahydride 1 with CO2
and discovered that a single product could be obtained provided
that strict control of the stoichiometry was followed. For
example, if excess CO2 is used, very complicated spectra are
obtained, indicative of a mixture of products, perhaps a
consequence of migratory insertion of CO2 in the tantalum−
amido linkages of the NPN ligand. However, if exactly 1 equiv of
CO2 is employed, a clean reaction ensues with the formation of
only one very symmetrical product (60% recrystallized yield) on
the basis of a singlet at δ −13.1 observed in the 31P NMR
spectrum. The corresponding 1H NMR spectrum shows a triplet
resonance downfield at δ 6.81, which simplifies to a singlet upon
31P decoupling and integrates to two H atoms. Analysis by
heteronuclear single quantum coherence indicates that these H
atoms are not C-bound, consistent with the presence of bridging
tantalum hydrides, and are likely a Ta2(μ-H)2 moiety. Also
arbon dioxide (CO2) is potentially a plentiful C1 source
Cthat continues to occupy discussions related to climate
change.1 While conversion of CO2 to higher-value carbon-based
materials is a worthy goal, it is clear that these kinds of
approaches are not realistic as a way to sequester this greenhouse
gas,2 particularly if dihydrogen (H2) derived via steam reforming
is involved. Nevertheless, from a fundamental point of view,
discovering systems that can transform CO2 with3−5 or without
6
H2 is of considerable interest7−9 and may provide hints on ways
to better utilize this ever-more-abundant resource.10
We have described the facile activation of dinitrogen (N2) by
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;11,12 Scheme 1). Given that N2 is a very stable, inert molecule,
the question arose as to what the outcome would be in the
reaction of tetrahydride 1 with the very stable CO2 molecule.
Herein we report our efforts to examine the reactivity of CO2
with the strongly reducing ditantalum tetrahydride complex 1.
1
diagnostic in the H NMR spectrum is a singlet at δ 6.11 that
again integrates for two H atoms. That these two sets of proton
resonances are derived from the bridging hydrides of 1 was
confirmed by the use of 1-d12, in which all four bridging hydrides
and all eight of the o-NPh protons are deuterated (see the
Supporting Information); in this reaction, the peaks at δ 6.81 and
6.11 both disappear in the 1H NMR spectrum, as does a peak at δ
6.89 due to the o-H atoms of the NPh moiety of the NPN ligand.
When 13C-labeled CO2 was utilized, the resonance at δ 6.11
1
Scheme 1
becomes a doublet with JCH = 110 Hz. Given our earlier
publication14 of the complete disassembly of CS2 to generate the
ditantalum species with a bridging methylene, an analogous
structure was considered. However, there are no bridging
hydrides in the CS2 disassembly product, and the chemical shift
of the resonance due to the bridging methylene of this material
occurs at δ 4.5 in the 1H NMR spectrum, which is considerably
Received: November 12, 2012
Published: January 29, 2013
© 2013 American Chemical Society
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Communication
upfield of the methylene resonance observed for the CO2
product 3. In fact, a recent report of the reaction of CO2 with
a mononuclear tantalum hydride to generate a ditantalum species
with a methylene diolate fragment proved to be a better
analogy.15
The solid-state structure of this complex is shown in Figure 1,
along with the transformation in eq 1; the most notable feature of
Figure 2. Gibbs free-energy profile of the reaction between the
tetrahydride complex 1 and CO2. NPN ligands have been simplified for
clarity. Atoms 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.
The first step of the reaction is coordination of CO2 to
complex 1, involving the two Ta centers. In Ta2H4·CO2, two
bridging hydride groups have become terminal in order to
liberate one coordination site on each Ta center and thus allow
the μ-η2:η2-coordination mode of CO2, all of which happens in a
concerted fashion. The computed O−C−O angle is 132.8° and
both C−O bonds are equal to 1.26 Å, suggesting that CO2 has
been reduced by complex 1. The Ta−Ta distance has increased
from 2.59 to 3.01 Å, so that the Ta−Ta bond has been broken.
This is confirmed by natural bond order (NBO) analysis, which
gives an oxidation state of +5 for each Ta, suggestive of the
presence of CO22−, at least formally. NPA charges show that the
C atom of CO2 is strongly positively charged (+0.70). The
formation of this adduct is slightly exergonic by −3.4 kcal/mol,
but it readily transforms to give the dinuclear μ-η2:η2-formato
Ta2H3·HCO2. Indeed, the transition state corresponding to the
hydrogen transfer from one Ta center to CO2 lies at only +10.7
kcal/mol with respect to Ta2H4·CO2, while the formation of
Ta2H3·HCO2 is exergonic by −11.4 kcal/mol with respect to the
adduct. The geometry of the first transition state is standard
because one O atom of CO2 has just moved away from the Ta−
Ta−CO2 plane of Ta2H4·CO2 in order to allow the terminal
hydride to bridge from the Ta center to the C atom of CO2. All
other bond lengths or angles are mostly unchanged.
The intermediate Ta2H3·HCO2 presents a nearly symmetrical
μ-η2:η2-HCO2 moiety. The Ta−Ta distance is 2.95 Å, which is
not consistent with a formate unit between two TaIV centers
unless the two d electrons remain unpaired. NBO analysis shows
that there are no unpaired electrons in the d orbitals of the Ta
centers or the presence of a Ta−Ta bond. The Lewis
configuration extracted from the NBO shows a covalent bond
between Ta1 and the C atom and two dative bonds from both
negatively charged O atoms to each Ta center. The system can
thus be described with two electrons delocalized between both
Ta−C bonds and formally TaV centers. The formation of the
experimental product involves the transition state TS2-CO2,
which corresponds to the transfer of the second terminal hydride
to the C atom of the HCO2 unit. The activation barrier is
calculated to be +20.5 kcal/mol, which is high but still kinetically
accessible. The geometry of this transition state looks like that of
TS1-CO2, discussed above. The formation of 3 is calculated to be
exergonic by −17.4 kcal/mol with respect to the dinuclear
formate complex Ta2H3·HCO2. The computed structure of 3 is
similar to the experimental one, for example, with C−O, Ta−O,
Figure 1. Selected bond lengths (Å) and bond angles (deg) for 3: Ta1−
N1 2.091(7), Ta1−N2 2.062(6), Ta1−P1 2.628(2), Ta1−O1 1.980(5),
Ta1−Ta1 2.7688(7), N1−Si1 1.742(7), N2−Si2 1.739(7), C1−O1
1.384(7); O1−C1−O1′ 117.0(9), Ta1−O1−C1 126.4(5), O1−Ta1−
O1′ 117.0(9), O1−Ta1−N1 91.6(2), O1−Ta1−N2 91.6(2), N1−
Ta1−N2 118.2(3), O1−Ta1−P1 160.99(15), N1−Ta1−P1 77.82(19),
N2−Ta1−P1 79.08(19), O1−Ta1−Ta1′ 83.45(14).
3 is the bridging methylene diolate unit between the two Ta
centers. The Ta1−O1 bond length of 3 of 1.980(5) Å is slightly
longer that the Ta−O bonds of 1.929(5) and 1.917(2) Å found in
two dinuclear methylene diolate complexes formed via
intermolecular processes.15 The other parameters of this μ-
OCH2O unit compare unremarkably to other examples of this
rare kind of fragment with the exception that the O−C−O angle
in 3 of 117.0(9)° is larger than the aforementioned dinuclear
systems [cf. 111.4(7) and 109.7(3)°]15 and a tetrayttrium cluster
[cf. 107.6(3)°]16 that contain this unit.
The formation of 3 likely involves hydride addition reactions
most probably through a formate-type intermediate.3,17 In an
effort to shed light on this process, possible structures of
intermediates and transition states were examined computation-
ally. Density functional theory [B3PW91//SDDALl(Ta,P)/6-
31G** (other atoms)] calculations were carried out on the full
system.
The Gibbs free-energy profile of the reaction between 1 and
CO2 is presented in Figure 2. For each minimum, singlet and
triplet spin states have been considered, and the singlet spin
states are always the most stable, by 20−50 kcal/mol. Thus, the
reactivity takes place on the singlet potential energy surface.
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Communication
and Ta−Ta bond lengths of 1.40 Å [1.388(10) Å exp], 1.95 Å
[1.982(8) Å exp], and 2.80 Å [2.7693(9) Å exp], respectively.
The highest occupied molecular orbital (HOMO) still
corresponds to the σ interaction of the two d orbitals of Ta,
confirming the TaIV oxidation state. The importance of the metal
is clearly evident because calculations using copper surfaces detail
quite different intermediates.18
The reaction of CO2 with the highly reducing ditantalum
tetrahydride complex 1 proceeds by a migratory insertion
process followed by reductive elimination, as summarized in
Scheme 2.
AUTHOR INFORMATION
Corresponding Author
*E-mail: laurent.maron@irsamc.ups-tlse.fr (L.M.), fryzuk@
chem.ubc.ca (M.D.F.). Tel: +1 604 822-2471. Fax: +1 604
822-8710.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
M.D.F. thanks the 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.
Scheme 2
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Of particular interest is how CO2 interacts with the starting
hydride. By using a computational approach, a low-energy
structure emerged, wherein CO2 binds reductively with
tetrahydride 1 to generate Ta2H4·CO2, wherein two of the
bridging hydrides become terminal and the CO2 unit is formally a
2−
CO2 moiety. One of the interesting insights that result from
these calculations is the importance of the terminal hydride unit
in the transformation documented in this work. While the
starting tetrahydride 1 contains four bridging hydrides in its
ground state, each step in the process involves a terminal hydride
interacting with a small organic moiety in a bridging position. In
the first transfer, the μ-η2:η2-formato complex Ta2H3·HCO2 is
generated and finally the methylene diolate product 3; this latter
process is formally a dinuclear C−H reductive elimination,
wherein the two TaV centers of Ta2H3·HCO2 are converted to
TaIV moieties in 3.
(17) Langer, R.; Diskin-Posner, Y.; Leitus, G.; Shimon, L. J. W.; Ben-
David, Y.; Milstein, D. Angew. Chem., Int. Ed. 2011, 50, 9948.
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Nørskov, J. K. Energy Environ. Sci. 2010, 3, 1311.
While dinuclear complexes offer unique activation modes for
small molecules, mainly by invoking simultaneous interactions
with two metal centers, in this work, we show that the strongly
reducing ditantalum complex in concert with available hydrides
can convert the important C1 source CO2 to a reduced form, in
this case, a methylene diolate fragment. We are presently
examining the effect of new ligand designs on the overall
reducing power of a series of ditantalum tetrahydrides with CO2
and other hard-to-activate small molecules.
ASSOCIATED CONTENT
* Supporting Information
■
S
Experimental procedures, X-ray data collection and refinement
procedures for 3, including the corresponding CIF file,
computational details, Cartesian coordinates for optimized
structures, schematic of HOMO−5 of the adduct Ta2H4·CO2,
and representative NMR spectra. This material is available free of
charge via the Internet at http://pubs.acs.org.
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