NHC-Coordinated Diphosphene-Stabilized Gold(I) Hydride and Its Reversible Conversion to Gold(I) Formate with CO2

Article abstract of DOI:10.1002/anie.201909798

An NHC-coordinated diphosphene is employed as ligand for the synthesis of a hydrocarbon-soluble monomeric Au^I hydride, which readily adds CO₂ at room temperature yielding the corresponding Au^I formate. The reversible reaction can be expedited by the addition of NHC, which induces β-hydride shift and the removal of CO₂ from equilibrium through the formation of an NHC-CO₂ adduct. The Au^I formate is alternatively formed by dehydrogenative coupling of the Au^I hydride with formic acid (HCO₂H), thus in total establishing a reaction sequence for the Au^I hydride mediated dehydrogenation of HCO₂H as chemical hydrogen storage material.

Full text of DOI:10.1002/anie.201909798

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Accepted Article
Title: NHC-Coordinated Diphosphene Stabilized Gold(I) Hydride and its
Reversible Conversion to Gold(I) Formate with CO2
Authors: Anukul Jana, Debabrata Dhara, Shubhajit Das, Swapan Pati,
David Scheschkewitz, and Vadapalli Chandrasekhar
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201909798
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NHC-Coordinated Diphosphene Stabilized Gold(I) Hydride and its
Reversible Conversion to Gold(I) Formate with CO2
Debabrata Dhara,[a] Shubhajit Das,[b] Swapan K. Pati,*[b] David Scheschkewitz,*[c] Vadapalli
Chandrasekhar,*[a],[d] and Anukul Jana*[a]
Dedicated to Professor C. N. R. Rao on occasion of his 85th birthday.
Abstract: An NHC-coordinated diphosphene is employed as ligand
for the synthesis of a hydrocarbon-soluble monomeric Au(I) hydride,
which readily adds CO2 at room temperature yielding the
corresponding Au(I) formate. The reversible reaction can be
expedited by the addition of NHC, which induces β-hydride shift and
the removal of CO2 from equilibrium through the formation of an
NHC-CO2 adduct. The Au(I) formate is alternatively formed by
dehydrogenative coupling of the Au(I) hydride with formic acid
(HCO2H), thus in total establishing a reaction sequence for the Au(I)
hydride mediated dehydrogenation of HCO2H as chemical hydrogen
storage material.
almost exclusively consists of a p-orbital at the formally
negatively charged dicoordinate phosphorus center. Theoretical
calculations further suggest that the NHC-coordinated
diphosphene should be a stronger donor than Ph3P.[11] Moreover,
the calculated binding energy of IV to AuCl is −61.1 kcal/mol
which is higher in comparison to I (−50.1 kcal/mol), II (−38.5
kcal/mol), and III (−55.8 kcal/mol).[11] Armed with this knowledge,
we sought to utilize IV as a ligand towards gold(I) hydride. The
monomeric parent Au(I) hydride (AuH) is kinetically unstable and
has only been observed in cold matrices[12] and considered as
an intermediate in numerous gold-catalyzed organic
transformations.[13] With an NHC as a stabilizing ligand, it was
isolated as a room temperature stable compound.[14]
Tertiary phosphines, I (Scheme 1) are ubiquitous ligands and
thus play an important role in the organometallic chemistry of
transition metals[1] and thereby also in the area of homogenous
catalysis.[2] Celebrated examples of the use of phosphines
include, Wilkinson’s catalyst,[3] Noyori’s catalyst,[4] and the first
generation Grubbs catalyst.[5] Other phosphorus-based species
such as diphosphenes II,[6] and base-stabilized phosphinidenes
III[7] (Scheme 1) have also found applications as ligands in
transition metal complexes. In contrast, Bertrand's base-free
phosphino phosphinidene is electrophilic in nature.[8] None of
these P-centered donors (I-III), however, can compete with
carbenes in terms of the stabilization of reactive intermediates.[9]
Recently, we have reported the reversible coordination of
an NHC to a diphosphene to yield IV (Scheme 1), which
possesses two nonbonding electron pairs at the dicoordinate P-
center.[10] Frontier orbital analysis revealed that the HOMO
Herein, we thus disclose the use of IV as a new P-
centered neutral ligand that ultimately allowed for the isolation of
a monomeric Au(I) hydride complex, which in turn is shown to
undergo the first hydroauration reaction of CO2 to form the
corresponding Au(I) formate. Surprisingly, the Au(I) formate
spontaneously releases CO2 even at room temperature, a
process that is facilitated by the presence of NHC. We further
show that the Au(I) formate is also accessible by the
dehydrogenation of HCO2H (a chemical hydrogen storage
material)[15] by the Au(I) hydride, thus stoichiometrically
demonstrating its principal potential as dehydrogenation catalyst.
Reversible hydrogenation of CO2 is known by the bacterial
enzyme carbondioxide reductase.[16] In an analogous manner we
report a synthetic system that shows reversible hydroauration
behavior of a Au(I) hydride.
[a]
[b]
Dr. D. Dhara, Prof. V. Chandrasekhar, Dr. A. Jana
Tata Institute of Fundamental Research Hyderabad
Gopanpally, Hyderabad-500107
Telangana, India
E-mail: ajana@tifrh.res.in
Scheme 1. Chemical structures of I-IV (R = monoanionic ligand, NHC = N-
heterocyclic carbene).
Dr. S. Das,† Prof. S. K. Pati
Theoretical Sciences Unit, Jawaharlal Nehru Centre for Advanced
Scientific Research, Bangalore-560064, India
E-mail: pati@jncasr.ac.in
[17]
The reaction of AuClSMe2 with a 1:1 solution of NHCMe
†Present Address: Laboratory for Computational Molecular Design
Institute of Chemical Sciences and Engineering, Ecole
Polytechnique Federale de Lausanne (EPFL), 1015 Lausanne,
Switzerland
4
and diphosphene 1[18] in THF at −78 °C yields the
NHCMe4/diphosphene-coordinated Au(I) chloride complex
2
(Scheme 2).[11] Formation of 2 reveals the ability of the
diphosphene motif to act simultaneously both as a Lewis acid
and a Lewis base in analogy to compounds with heavier Group
14 multiple bonds.[19] The 31P NMR spectrum of 2 exhibits two
[c]
[d]
Prof. Dr. D. Scheschkewitz
Krupp-Chair of General and Inorganic Chemistry, Saarland
University, 66123 Saarbrücken, Germany
E-mail: scheschkewitz@mx.uni-saarland.de
Prof. V. Chandrasekhar
Department of Chemistry, Indian Institute of Technology Kanpur,
Kanpur-208016, India
1
doublets at  = 1.34 and −31.46 ppm with JPP = 462 Hz, which
is in between the values of 1·NHCMe (1JPP = 423 Hz)[10] and the
4
E-mail: vc@iitk.ac.in
monoaurated adduct, Mes*(AuCl)P=PMes* (1JPP = 539 Hz).[20]
Supporting information for this article is given via a link at the end of
the document.
1
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10.1002/anie.201909798
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Hz) and at  = −14.5 ppm as a doublet of doublets (1JPP = 470
Hz, 2JPH = 138 Hz).
Scheme 3. Synthesis of NHC-coordinated diphosphene stabilized Au(I)-
hydride, 3 and it's rearrangement to 4.
Scheme 2. Synthesis of NHCMe4-coordinated diphosphene stabilized Au(I)-Cl
complex, 2 (Ar = 2,6-Mes2C6H3, Mes = 2,4,6-Me3C6H2).
In the 1H NMR spectrum, the doublet at  = 4.60 ppm (2JPH
= 138 Hz) can be unambiguously assigned to the Au‒H
resonance, which is upfield shifted in comparison to that of
NHCDip-stabilized Au(I)-hydride (5.11 ppm).[12a] The IR spectrum
of 3 shows a strong sharp band at 1893 cm-1 for the Au-H motif,
in good agreement with the calculated value (1880.2 cm-1).[11]
The molecular structure of 3 (Figure 2) reveals a P‒P bond
distance of 2.197(1) Å, which is slightly shorter than the P‒P
bond distance in 2 due to less pronounced -back-donation.
Indeed, the P‒Au bond distance in 3 of 2.3297(9) Å is larger
than the 2.2540(8) Å in 2 suggesting a stronger trans influence
of the hydride compared to the chloride ligand. The Au(I) hydride
3 is stable in presence of degassed water in toluene overnight; a
solid-crystalline sample even persists in open air at least for two
days.
Interestingly, in solution, the coordinated NHCMe does not
4
dissociate unlike 1·NHCMe that exists in equilibrium with 1 and
4
[10]
NHCMe
.
The stability of 2 is probably due to the coordination
4
of the diphosphene moiety to AuCl, which enhances the
electrophilicity of the second P center resulting in stronger
binding to NHCMe4. This is also supported by DFT calculations
on the highly endergonic dissociation of NHCMe4 from 2 (21.8 vs.
6.7 kcal/mol from 1·NHCMe4) in THF.[11]
The molecular structure of 2 was confirmed by single
crystal X-ray diffraction (Figure 1). The P‒Au bond distance
(2.2540(8) Å) in 2 is longer than those of Ph3P·AuCl (Au‒P
2.235 Å)[21] and the corresponding diaurated adduct of Mes*-
substituted diphosphene, Mes*(AuCl)P=P(AuCl)Mes* (2.201
Å).[20] The P1P2 bond distance is 2.219(1) Å and thus
considerably longer than in free diphosphene 1 (2.029 Å)[22] or
1·NHCMe4 (2.134 Å).[10]
Figure 2. Molecular structure of 3 with thermal ellipsoids at 50% probability
level. All H atoms except Au-H and one co-crystallized molecule of benzene
are omitted for clarity.
Figure 1. Molecular structure of 2 with thermal ellipsoids at 50% probability
level. All hydrogen atoms and one molecule of co-crystallized toluene are
omitted for clarity.
In solution, however, 3 slowly undergoes a 1,3-hydrogen
shift from the Au center to the -phosphorus atom resulting in
the Au(I) phosphinophosphide 4 as shown by the appearance of
1
1
a H NMR doublet of doublets at  = 4.19 ppm (dd, JPH = 214.
2
The 1:1 reaction of 2 with N-selectride at −78 °C affords
the NHC/diphosphene-stabilized Au(I) hydride, 3 in 90 % yield
as light yellow crystals (Scheme 3). The 31P NMR spectrum of 3
exhibits two resonances at  = 1.6 ppm as doublet (1JPP = 470
Hz, JPH = 9 Hz). The concomitant migration of the NHC ligand
from the phosphorus to the gold center is evident from the
significantly smaller coupling of the 13C{1H} signal at  = 193.6
ppm of the carbenic carbon atom to the nearest 31P nucleus
2
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1
(2JCP = 53 Hz for 4 vs. JCP = 99 Hz for 2). Conversion is
crystallography (Figure 4). The Au‒O bond distance in the Au(I)
formate 5 of 2.140(4) Å is slightly longer than that of a reported
Au(III) formate (2.102 Å).[25] To the best of our knowledge, the
formation of 5 represents the first example of any gold formate
obtained by direct hydroauration of CO2.
completed by heating to 65 °C for one hour. According to our
DFT results, the rearrangement of 3 to 4 is exergonic by 26.1
kcal/mol.[11] The structure of the NHC-stabilized Au(I)
phosphinophosphide 4 was finally confirmed by X-ray diffraction
on single crystals (Figure 3). The P‒P bond distance of 4 is
2.218(1) Å and thus slightly longer than in the one of the
reported boryl substituted lithium phosphinophosphide (2.1775
Å).[23]
Figure 4. Molecular structure of 5 with thermal ellipsoids at 50% probability
level. All hydrogen atoms and one molecule of toluene are omitted for clarity.
Figure 3. Molecular structure of 4 with thermal ellipsoids at 50% probability
level. All hydrogen atoms and one molecule of co-crystallized hexane solvent
molecule are omitted for clarity.
We had noted that the 31P NMR spectrum of the residue
after removal of the solvent shows the presence of about 5 % of
the starting Au(I) hydride, 3. This observation prompted us to
further investigate a possible spontaneous release of CO2 from 5.
The release of CO2/HCO2− from transition-metal formates is well-
known[26] and the reductive elimination of CO2 from a binuclear
Au(II)/CO2 complex has been reported.[27] Indeed, the
application of 0.12 mbar vacuum for 15 h results in the original
Au(I)-hydride 3 in about 50%. Decarboxylation of 5 above 65 °C
proceeds to complete conversion, but also affords 4, the thermal
isomerization product of 3 as side product.
In order to address the hydridic character of the Au-H
moiety of 3, we considered the hydroauration reaction with CO2.
The hydrometallation of carbonyl compounds, in particular of
CO2, is a key step of catalytic conversions to access C1-
feedstock materials.[24] In fact, the formation of 5 from 3 and CO2
is computed to be exergonic by 11.6 kcal/mol.[11] Upon passing
CO2 gas into a toluene solution of 3 at room temperature, the
quantitative formation of Au(I) formate 5 was observed based on
31P NMR of the reaction mixture (Scheme 4).
Scheme 4. Synthesis of NHC-coordinated diphosphene stabilized Au(I)-
formate, 5.
The 1H NMR spectrum of 5 exhibits a doublet centered at 
= 9.50 ppm (4JHP = 7 Hz), in line with the suggested formate as
is a prominent IR band at 1884 cm-1 for the C=O stretching
frequency. The molecular structure of 5 was confirmed by X-ray
Me
Scheme 5. NHCiPr 2-mediated release of CO2 from Au(I)-formate, 3.
2
3
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COMMUNICATION
Me [14]
To facilitate the release of CO2, we added NHCiPr
in
2
2
India and Research Group Linkage Programme (between TIFR
Hyderabad, India and Saarland University, Germany), AvH
Foundation, Germany. VC is thankful to the DST, New Delhi,
India, for a National J. C. Bose fellowship. SD thanks CSIR, New
Delhi, India and JNCASR for fellowship. SKP acknowledges
research support from DST, New Delhi, India. We are grateful to
the reviewers for their critical insights to improve the quality of
the manuscript.
the anticipation that it might induce the required 1,3-H shift (β-
hydride elimination)[28] by coordination to the carbonyl group and
removal of CO2 from equilibrium as NHCiPr 2-CO2 adduct 6.
Me
[29]
2
Me
Addition of one equivalent of NHCiPr
to a solution of 5 at
room temperature indeed resulted in the immediate formation of
3 (Scheme 5).
2
2
In order to verify the CO2 release at lower temperatures and
to check for intermediates, we carried out a VT-NMR study of a
1:1 toluene-d8 solution of NHCiPr
and 5. At −78 °C, the 31P
Me
2
2
Keywords: carbon dioxide • diphosphene • gold • ligand design
• phosphorus
NMR spectrum does not show any indication for the release of
CO2. At −10 °C, we observed one new set of peaks at  = −35.2
and 0.9 ppm (1JPP = 465 Hz). These resonances disappear while
approaching room temperature with the concomitant
appearance of the resonances of 3. The occurrence of an
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intermediate suggests that the reaction may indeed proceed
Me
through the initial coordination of NHCiPr
to the carbonyl
2
2
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Subsequent hydride migration (β-hydride elimination) would lead
Me
to the NHCiPr
adduct of CO2 6 and Au(I) hydride 3 (Scheme
2
2
5). The calculated Gibbs free energy values confirm that the
Me
[11]
reaction 5 + NHCiPr 2 → 3 + 6 is endergonic by 7.4 kcal/mol.
2
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with elimination of H2 (Scheme 6).
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Scheme 6. Au(I)-hydride, 3 mediated release of H2 and CO2 from HCO2H
(Inset: Reaction of HCO2H to CO2 and H2).
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In conclusion, we herewith disclosed
a water-stable
monomeric terminal Au(I)-hydride coordinated by an
NHC/diphosphene adduct. Like other heavier Group 14 multiple
bonds, the diphosphene can simultaneously act as a Lewis acid
and as a Lewis base. The Au(I) hydride exhibits pronounced
hydridic character and thus reacts with CO2 to the corresponding
Au(I)-formate, which spontaneously releases CO2 at room
temperature, a feature that typically requires much higher
temperatures.[31] The alternative formation of formate from Au(I)-
hydride and HCO2H with release of H2 suggests that a thermally
more stable Au(I)-hydride might indeed be a competent catalyst
for the release of H2 from HCO2H, a chemical hydrogen storage
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Acknowledgements
This work is supported by the Tata Institute of Fundamental Re-
search Hyderabad, Gopanpally, Hyderabad-500107, Telangana,
4
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10.1002/anie.201909798
Angewandte Chemie International Edition
COMMUNICATION
Entry for the Table of Contents
COMMUNICATION
Debabrata Dhara, Shubhajit Das,
Swapan Pati,* David Scheschkewitz,*
Vadapalli Chandrasekhar,* and Anukul
Jana*
Page No. – Page No.
NHC-Coordinated Diphosphene
Stabilized Gold(I) Hydride and its
Reversible Conversion to Gold(I)
Formate with CO2
NHC-coordinated diphosphene has been used as a ligand for the isolation of
monomeric gold(I) hydride which readily converted to the corresponding Au(I)
formate in either by hydroauration of CO2 or by dehydrogenative coupling with
formic acid. The Au(I) formate in turn loses CO2 at room temperature upon
application of vacuum and this process can be expedited at lower temperatures by
using NHC.
6
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