Hydroboration of carbon dioxide enabled by molecular zinc dihydrides

Article abstract of DOI:10.1039/d0dt01090a

Neutral molecular zinc(ii) dihydrides supported by N-heterocyclic carbene ligands bearing a pendant phosphine group were synthesized and then reacted with carbon dioxide to afford zinc diformates. The zinc dihydrides were found to be active catalysts for hydroboration of carbon dioxide under mild conditions, selectively giving boryl formate, bis(boryl)acetal, or methoxy borane compounds by changing the nature of the borane reductant.

Full text of DOI:10.1039/d0dt01090a

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shall the Royal Society of Chemistry be held responsible for any errors
N-Heterocyclic carbene stabilized parent sulfenyl, selenenyl,
and tellurenyl cations (XH+, X = S, Se, Te)
or omissions in this Accepted Manuscript or any consequences arising
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Hydroboration of carbon dioxide enabled by molecular zinc
dihydrides
Received 00th January 20xx,
Accepted 00th January 20xx
Xiaoming Wang, Kejian Chang, Xin Xu*
DOI: 10.1039/x0xx00000x
Neutral molecular zinc(II) dihydrides supported by N-heterocyclic used for copolymerization of CO2 with epoxides10 and
carbene ligands bearing a pendant phosphine group were homogeneous hydrosilylation of However,
CO2.11
synthesized and then reacted with carbon dioxide to afford zinc hydroboration of CO2 by using a zinc catalyst remains scarce.12
diformates. The zinc dihydrides were found to be active catalysts Herein, we design and synthesize zinc(II) dihydrides supported
for hydroboration of carbon dioxide under mild conditions, by an N-heterocyclic carbene (NHC) bearing a pendant
selectively giving boryl formate, bis(boryl)acetal, or methoxy phosphine group, which are found active for the catalytic
borane compounds by changing the nature of the borane hydroboration of CO2. Furthermore, the reactions can
reductant.
selectively give different levels of reduction products, for
example, boryl formate, bis(boryl)acetal, and methoxy-borane
compounds, by simply varying the borane reductant.
The use of carbon dioxide (CO2) as a cheap, nontoxic, and
renewable C1 feedstock has attracted considerable attentions
from both academia and industry.1,2 Great effort has been
expended exploring selective and efficient methodologies for
the reduction of CO2 to value-added C1 chemicals such as
formic acid, formaldehyde, methanol, and their derivatives.3
Direct hydrogenation of CO2 with molecular hydrogen (H2)
represents the most atom-economical pathway to
corresponding reduction products, but usually requires harsh
conditions.4 To this end, replacement of H2 by silanes5 or
boranes6 as reductants has been explored and realized under
mild conditions because of the formation of strong Si-O or B-O
bonds. In the last decade, hydroboration of CO2 using a variety
of boranes has flourished and excellent performance in terms
of activity and selectivity has been achieved.7 In addition, the
resulting reduction products can be easily transformed to
other valuable organic compounds. To date, a number of
transition-metal complexes (including Ni,7a,7h Pd,7d,7j Cu,7c Fe,7f
Ru,7b,7e,7g and Co7i), alkali metal hydridoborates,8 and main-
group frustrated Lewis pairs9 have been successfully used as
catalysts for this transformation. Considering the abundance,
biocompatibility and environmental friendliness of metal ions,
the use of zinc-based complexes for CO2 transformation is
highly attractive. For example, zinc-based complexes can be
Okuda and coworkers showed that molecular zinc(II)
dihydrides or monohydride cations can be stabilized by using
well-known
NHC
ligands
such
as
1,3-bis(2,6-
diisopropylphenyl)imidazol-2-ylidene) (IPr) and 1,3-bis(2,4,6-
trimethylphenyl)imidazol-2-ylidene) (IMes).13 These complexes
were found to be reactive toward CO2, resulting in the
formation of formate species. However, only one precedent
for the catalytic transformation of CO2 has been reported: the
hydrosilylation of CO2 using a trinuclear cationic zinc hydride
cluster under harsh conditions (40 bar CO2, 60 ℃, 64 h).13b By
introducing a pendant phosphine arm on to NHC ligands, we
prepared corresponding zinc dihydrides using
a well-
established method, as shown in Scheme 1. Reactions of NHC
ligands with an equimolar amount of ZnEt2 gave zinc dialkyl
complexes 1, which were easily converted into zinc
diaryloxides
2
by adding two equivalents of 2,6-
diisopropylphenol. Finally, metathesis reactions of 2 with
PhSiH3 afforded the desired dihydride complexes 3 in high
yields (82% for 3a and 76% for 3b).
Complexes 1 - 3 were fully characterized by elemental
analysis and multinuclear NMR spectroscopy. The structure of
complex 2b was further confirmed by single-crystal X-ray
diffraction (XRD); the molecular structure is depicted in the
Supporting Information. In the solid state, the phosphine arm
of 2b was bound to the zinc center with a Zn-P bond distance
of 2.636(5) Å, which hampered the formation of dimeric
structure obtained in the case of previously reported
Key Laboratory of Organic Synthesis of Jiangsu Province, College of Chemistry,
Chemical Engineering and Materials Science, Soochow University, Suzhou 215123,
P. R. China. E-mail: xinxu@suda.edu.cn
†
Electronic Supplementary Information (ESI) available. CCDC 1978325 (5), [(IMes)Zn(μ-OBn)]2 complex.14 A solution 13C{1H} NMR
1978326 (2b).
spectrum of 2b showed a carbene carbon resonance at δ 174.1
For ESI and crystallographic data in CIF or other electronic format see
DOI:10.1039/x0xx00000x
2
ppm as a doublet with a JPC coupling constant of 48.7 Hz,
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downfield resonance of a formate unit at δ 8.31 ppm (13C: δ
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N
N
N
Ar
N
Ar
Et
ZnEt2
167.6 ppm) with an integration ratio wDitOhI:t1h0e.10N39H/CD0lDigTa0n10d90oAf
2:1. The 31P NMR spectrum of 4 featured a singlet at δ -20.1
ppm, which was almost identical to that of starting dihydride
3a (-20.0 ppm). When the mesityl-substituted NHC zinc
complex was used, the situation changed markedly. Reaction
of 3b with CO2 rapidly gave trinuclear complex 5 with
concomitant formation of the NHC·CO2 adduct 6. Complex 5
was isolated and characterized by single-crystal XRD and its
molecular structure contained three zinc centers, six formate
moieties, and two NHC ancillary ligands, as shown in Figure 1.
The structure of complex 5 is quite similar to that of the
reported addition product of [IPrZnH2]2 with CO2.13a The
central zinc adopted an octahedral geometry and was
coordinated by six η1-formate ligands, whereas the other two
zinc centers were tetrahedrally coordinated by one NHC ligand
and three formate ligands. Adduct 6 was also isolated and
further confirmed by independent reaction of NHCb with CO2
(see the Supporting Information for details).
Zn
PPh2
P
Ph
Et
Ph
NHCa: Ar = DIPP
NHCb: Ar = Mes
1
HO-DIPP
PPh2
Ar
N
N
H
N
Ar
DIPP
H
N
PhSiH3
Zn
H
H
Zn
N
Zn
P
O
Ph
O
N
Ph
DIPP
Ar
Ph2P
2
3
DIPP = 2,6-iPr2-C6H3
Mes = 2,4,6-Me3-C6H2
Scheme 1. Synthesis of zinc dihydrides 3.
possibly indicating the retention of the Zn-P interaction in the
solution. Attempts to obtain single crystals of zinc dihydrides 3
suitable for XRD measurements failed. Nevertheless, the
solution behavior of complexes 3 was investigated to elucidate
their structures. In 1H NMR spectra, two distinct hydride
signals were observed at δ 4.15 and 3.50 ppm for 3a and δ
º
4.32 and 3.46 ppm for 3b in d8-toluene at -30 C. However,
these signals were not detected when the temperature was
raised to 0 ºC (see the Supporting Information for details). This
observation is consistent with the behavior of Okuda's
complex [IMesZnH2]2.13a Complexes 3 featured 31P NMR
resonances at δ -20.0 ppm (3a) and -19.5 ppm (3b), which
were shifted negligibly compared with those of the parent
NHC ligands (-22.1 ppm for NHCa and -22.7 ppm for NHCb),
indicating the absence of Zn-P interactions in 3. Therefore, we
proposed that complexes 3 contain a binuclear hydride
bridged core structure in solution, like that in Okuda's
complexes. Moreover, the diffusion coefficient of complex 3b
(4.8×10-10 m2 s-1) determined by DOSY NMR was smaller than
that for monomeric 2b (5.9×10-10 m2 s-1), which is consistent
with this hypothesis.15 Zinc dihydrides 3 were not thermally
º
stable and decomposed when heated at 50 C in solution for 2
h.
Figure 1. Molecular structure of complex 5. Hydrogen atoms (except formates)
are omitted for clarity; displacement ellipsoids are drawn at the 30% probability
level. Symmetry code: 2-x, 1-y and 1-z.
We subsequently monitored the reactions of zinc dihydrides 3
with CO2 (1 bar) at room temperature (Scheme 2). Complex 3a
resulted in the formation of diformate 4 in 69% isolated yield.
Because zinc dihydrides 3 are highly active toward CO2, we
decided to investigate their catalytic performance for
hydroboration of CO2. All the reactions were conducted in C6D6
at room temperature and the yields of reduction products
were determined by 1H NMR spectroscopy using
hexamethylbenzene as an internal standard (Table 1). Initially,
3a (5 mol% based on Zn) was exposed to 1 bar of CO2 with
pinacol borane (HBpin) as a reductant.16 The reaction
1
The H NMR spectrum of 4 showed a characteristic
PPh2
Ar
N
Ph2P
N
N
DIPP
O
H
H
N
CO2 (1 bar)
Zn
Zn
Zn
N
H
O
O
toluene
Ar = DIPP
H
N
O
Ar
Ph2P
H
H
3
4
1
predominantly produced boryl formate HCOOBpin [A, H: δ
CO2 (1 bar)
toluene
DIPP = 2,6-iPr2-C6H3
Mes = 2,4,6-Me3-C6H2
8.14 ppm (HCOO-)] in 68% yield after 48 h (entry 1), along with
Ar = Mes
the concomitant formation of a small amount of diborate
1
ether (Bpin)2O (13%, H: δ 1.01 ppm) and trace amounts of
H
PPh2
O
1
H
unidentified species. No bis(boryl)acetal [B, H: δ 5.49 ppm (-
H
Mes
N
1
OCH2O)]7b or methoxy borane [C, H: δ 3.51 ppm (CH3O-)]7b
O
O
O
O
O
O
O
N
O
H
Zn
Zn
Ph2P
Zn
N
N
O
Mes
was detected during the reaction. Under analogous conditions
(1 bar of CO2, room temperature, 48 h), complex 3b resulted in
a much lower yield of HCOOBpin (37%, entry 2) which was
probably caused by the dissociation of the NHC ligand during
the reaction. To promote the reaction, the CO2 pressure was
O
H
N
+
N
O
Mes
O
O
PPh2
H
6
5
Scheme 2. Reactions of zinc dihydrides 3 with CO2.
2 | J. Name., 2012, 00, 1-3
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increased to 10 bar when 3a was used as a catalyst. As (1 bar of CO2, room temperature, C6D6 as the solvent). The
View Article Online
DOI: 10.1039/D0DT01090A
expected, this reaction selectively gave HCOOBpin in 85% yield reaction using catechol borane (HBcat) as a reductant
within a short reaction time (12 h), along with a trace amount selectively produced six-electron reduction product methoxy
of (Bpin)2O (ca. 2%) (entry 3). In addition, the resulting boryl borane C [1H: δ 3.32 ppm (CH3O-)] in 85% yield within 12 h
formate A can be directly used as a formylation reagent for N- (entry 9), along with diborate ether (Bcat)2O and trace amount
methylaniline (see the Supporting Information for details). The of undefined boron-containing species.19 Subsequent
isolated CO2 addition product 4 was also active toward CO2 treatment of the reaction mixture with excess water rapidly
borylation and did not noticeably change the reaction results gave the hydrolysis product methanol (see the Supporting
in terms of selectivity and activity compared with the case for Information). Remarkably, the challenging four-electron
parent zinc dihydride 3a (entries 4 vs 3). This result suggests reduction product bis(boryl)acetal B [1H: δ 5.34 ppm (-
that the Zn-H insertion product serves as an intermediate for OCH2O-)] was obtained as the major product (75%) when 9-
the catalytic hydroboration. It has been reported that NHC borabicyclo[3.3.1]nonane (9-BBN) was used as
a
alone can be used as an efficient organocatalyst for reductive hydroboration reagent, along with the formation of a small
hydrosilylation of CO2.17 Therefore, compound NHCa was amount of methoxy borane (6%) within 4 h (entry 10). The
investigated under identical conditions, but it showed much bis(boryl)acetal form generated in the hydroboration of CO2 is
lower activity than that of 3a in the hydroboration reaction relatively rare compared with the formation of formic acid or
(entry 5). To make further comparisons, three previously methanol.7f,7h Finally, borane reagent BH3·SMe2 was
reported zinc hydrides were also used for the hydroboration of investigated in the present hydroboration reaction; no
CO2 with HBpin. Okuda's zinc dihydride [IMesZnH2]2 afforded a reduction product was detected after 48 h (entry 11). Based on
31% yield of boryl formate A after 48 h (entry 6). Its analogue the above preliminary observations, we tentatively propose
[IPrZnH2]2 only gave (Bpin)2O in 27% yield and no signals of A, that the mechanistic framework of hydroboration of CO2 using
B, or C was observed (entry 7). A zinc monohydride complex our zinc dihydride catalyst also involves three sequential
[(DIPP-nacnac)ZnH]18 supported by
a
conventional β- catalytic cycles that are responsible for the formation of
diketiminato ligand was also prepared and tested, but proved products with different reduction levels (Scheme S10 in the
to be inactive for the hydroboration of CO2 (entry 8). Overall, Supporting Information), similar to CO2 hydroboration using
zinc dihydride 3a exhibited much better performance than other transition-metal hydrides.7 HBcat promotes CO2
other zinc hydrides under the explored conditions. We reduction to methanol level probably because it is less steric
tentatively propose that this is because the NHC ligand in 3a is hindrance and electron rich compared with HBpin.
less sterically hindered than IMes, IPr, and DIPP-nacnac
ligands.
In summary, molecular Zn(II) dihydrides based on neutral
NHC ligands bearing a tethered phosphine arm were
Controlling the selectivity of the reduction of CO2 is of synthesized that underwent stoichiometric reactions with CO2
considerable interest and can be achieved by some late to give zinc diformates. Remarkably, Zn(II) dihydride 3a was
transition-metal catalysts by reagent control.7i,7j Thus, zinc found to be an active catalyst for the hydroboration of CO2
dihydride 3a-mediated hydroboration reactions with different under mild conditions. The selectivity of the hydroboration
borane reagents were carried out under our typical conditions
reactions can be easily controlled by varying the borane
reagent used and the resulting reduction products were able
to be converted into other valuable organic compounds using
facile approaches. Although the details of this hydroboration
reaction currently remain unclear, we propose that a tandem
reaction containing three catalytic cycles is involved, as in the
case of late transition-metal hydrides-catalyzed hydroboration.
Our study represents the first examples of zinc hydrides as
catalysts in the hydroboration of CO2. Further investigation of
the reactivity of these zinc dihydrides and mechanistic studies
of the hydroboration reaction are underway in our laboratory.
This work was supported by the National Natural Science
Foundation of China (21871204), 1000-Youth Talents Plan, and
PAPD.
Table 1. Zinc-catalyzed reduction of CO2 with hydroboranes.a
O
[catalyst]
+ H3CO BR2
OBR2
HBR2 + CO2
+
R2BO
H
OBR2
C6D6, r. t.
B
A
C
Yield (%)b
A/B/C
Time
(h)
48
48
12
12
48
48
48
48
12
4
Entry
Borane
Catalyst
1
2
3c
4c
5
6
7
8
9
HBpin
HBpin
HBpin
HBpin
HBpin
HBpin
HBpin
HBpin
HBcat
3a
3b
3a
68/0/0
37/0/0
85/0/0
83/0/0
34/0/0
31/0/0
0/0/0
4
NHCa
[(IMes)ZnH2]2
[(IPr)ZnH2]2
(DIPP-nacnac)ZnH
0/0/0
Conflicts of interest
There are no conflicts to declare.
3a
3a
3a
0/0/85
0/75/6
0/0/0
10
11
9-BBN
BH3·SMe2
48
aReaction conditions: unless noted otherwise, [catalyst] = 0.0075 mmol (based on
Zn), [Borane] = 0.15 mmol, 1 bar of CO2, C6D6, room temperature. bDetermined
by 1H NMR integration using hexamethylbenzene as an internal standard. c10 bar
of CO2 was used.
Notes and references
1
Carbon Dioxide as Chemical Feedstock, ed. M. Aresta, Wiley-
VCH, Weinheim, 2010.
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 3
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as a reductant under identical conditions was performed and
did not give desired products A, B, or C. For more details, see
ESI.
4 | J. Name., 2012, 00, 1-3
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