Ir-catalyzed tandem hydroformylation-transfer hydrogenation of olefins with (trans-/cis-)formic acid as hydrogen source in presence of 1,10-phenanthroline

Article abstract of DOI:10.1016/j.jcat.2020.03.008

The one-pot tandem hydroformylation-reduction to synthesize alcohols from olefins is in great demand but suffering from low yields, poor selectivity and harsh condition. Herein, 1,10-phenanthroline (L1) modified Ir-catalyst proved to exhibit multiple cata

Full text of DOI:10.1016/j.jcat.2020.03.008

Journal of Catalysis 385 (2020) 183–193
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Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Ir-catalyzed tandem hydroformylation-transfer hydrogenation of
olefins with (trans-/cis-)formic acid as hydrogen source in presence
of 1,10-phenanthroline
a,b,
Lei Liu a, Han Gao a,b, Shu-Qing Yang a, Xiao-Chao Chen a, Yong Lu a, Ye Liu a, , Fei Xia
⇑
⇑
a Shanghai Key Laboratory of Green Chemistry and Chemical Processes, School of Chemistry and Molecular Engineering, East China Normal University, 3663 North Zhongshan
Road, Shanghai 200062, China
b NYU-ECNU Center for Computational Chemistry at NYU Shanghai, 3663 Zhongshan Road North, Shanghai 200062, China
a r t i c l e i n f o
a b s t r a c t
Article history:
The one-pot tandem hydroformylation-reduction to synthesize alcohols from olefins is in great demand
but suffering from low yields, poor selectivity and harsh condition. Herein, 1,10-phenanthroline (L1)
modified Ir-catalyst proved to exhibit multiple catalysis in terms of FA dehydrogenation, hydroformyla-
tion of olefins, and transfer hydrogenation of aldehydes to accomplish the tandem hydroformylation-
transfer hydrogenation of olefins with formic acid (FA) as hydrogen source, as the result of 52–70% yields
for the target alcohols and the corresponding formate esters (obtained upon esterification of the alcohol
with FA). In this sequence, only the use of FA with trans- and cis-conformers could fulfill the reduction of
aldehydes to the alcohols through transfer hydrogenation as well as greatly depress hydrogenation of the
olefin, which universally occurred in the high pressured gaseous H2. Both the in situ FT-IR spectroscopic
analysis and the DFT-calculations verified that, over L1-[Ir(COD)Cl]2 catalyst, cis-FA serving as the
hydride-ligand was responsible for the efficient dehydrogenation of FA to release H2 along with CO2,
and trans-FA serving as the carbonyl O-containing ligand corresponded to the transfer hydrogenation
of the aldehydes to the alcohols under the same catalytic conditions.
Received 14 November 2019
Revised 9 March 2020
Accepted 11 March 2020
Keywords:
Co-catalysis
One-pot tandem reaction
Hydroformylation
Transfer hydrogenation
Iridium catalyst
Formic acid
In situ FT-IR spectral analysis
Ó 2020 Elsevier Inc. All rights reserved.
1. Introduction
ligands, the incorporated phosphino-fragment was responsible
for the hydroformylation of olefins upon coordinating to Rh-
Hydroformylation allows for the production of aldehydes from
syngas and olefins, which is one of large-scale processes of homo-
geneous catalysis [1–5]. The resultant products of aldehydes are
typically reduced in a separate step to the corresponding alcohols
which have wide applications as solvents as well as feedstock for
plasticizers and detergents [6,7]. The one-pot tandem
hydroformylation-reduction to synthesize alcohols from olefins is
an ideal protocol with advantages of lowering reaction time, labor
and cost [8–16].
To date, many catalytic systems have been reported in an
attempt to advance this sequence. Notably, the co-catalysis over
Rh-complex with the presence of the bi-functional ligands contain-
ing phosphino-fragment and NH-amino group enables this tandem
reaction with high selectivity to the target alcohols, as reported by
Breit [9,10]. It was illustrated that, in the developed bi-functional
center, and that the NH-amino group (such as guanidinyl) was in
charge of effective hydrogenation of the aldehydes through devel-
oping hydrogen-bond with carbonyl group [RNHꢀꢀꢀ(O = C)HR’]. In
addition, the co-catalysis over the bi-metal (like Rh-Ru) catalytic
systems with involvement of the elaborately designed phosphines
ligands also proved successful for this one-pot tandem sequence
[11–13], wherein the Ru-based Shvo’s catalyst was accountable
for the subsequent hydrogenation of aldehydes. Encouragingly,
the sole and low-cost Ru-catalytic systems with the involvement
of imidazolyl-tailed phosphines reported by M. Beller’s group also
proved to be the promising alternative to the reported bi-
functional catalytic ones. Although there are many phosphine-
modified Rh-catalytic systems for hydroformylation, the one-pot
tandem hydroformylation-reduction to produce alcohols rarely
happened over the sole Rh-catalysts because of its very low activity
towards hydrogenation [14]. Anyway, the uses of the nitrogen
ligands were found to enhance the activity of Rh-catalysts for this
tandem reaction [16–18].
⇑
Corresponding authors.
E-mail addresses: yliu@chem.ecnu.edu.cn (Y. Liu), fxia@chem.ecnu.edu.cn
(F. Xia).
https://doi.org/10.1016/j.jcat.2020.03.008
0021-9517/Ó 2020 Elsevier Inc. All rights reserved.

184
L. Liu et al. / Journal of Catalysis 385 (2020) 183–193
On the other hand, compared to Rh (typically applied in hydro-
multi-catalysis of the selected Ir-catalyst should be compatible
for each step under the same reaction conditions.
formylation) or Ru (typically applied in hydrogenation/transfer
hydrogenation) with task specific activities, many Ir-complexes
were reported to be active homogeneous catalysts not only for
hydroformylation [19–24], but also for hydrogenation/transfer
hydrogenation. For example, the nitrogen-ligand-modified Ir-
complexes were highly catalytic active for reduction of aldehy-
des/ketones via transfer hydrogenation with formic acid as hydro-
gen source (hydride donor) [25–31]. Then it was highlighted that
Ir-catalysts if with the involvement of the suitable ligands might
be more ideal systems for the tandem hydroformylation-
reduction than Rh-/Ru-systems.
2. Results and discussion
The one-pot tandem hydroformylation-transfer hydrogenation
for the synthesis of the alcohols (3-phenylpropanol and 2-
phenylpropanol) from styrene was investigated as a model reac-
tion. The important role of the nitrogen ligands in the hydrogen
transfer [25–31] and decomposition of FA [35,44] inspired us to
explore this sequence by using the nitrogen ligands.
Besides the significance of the applied catalytic systems, the
hydrogen source also plays an important role for the success of this
tandem hydroformylation-reduction. Reasonably, over the Ir-/Ru-
based catalysts with high activity for hydrogenation, when the typ-
ical hydrogen source like the high pressured H2 in the mixture of
syngas (CO/H2) was used, the side-reaction of olefin hydrogenation
universally occurred [8,15]. In order to overcome such shortcom-
ing, in the literature, H2O [20,32,33] or formic acid (FA, HCOOH)
[34–41] was widely applied as the H2-reservior, from which H2
was released in situ in the course of reaction. Specifically, FA with
a high hydrogen content of 53 gꢀL-1 or 4.4 wt% is the ideal hydrogen
source in term of H2-carrier or hydride-donor [41,42]. It has been
well known that the decomposition of FA can proceed via two
pathways: dehydrogenation (or decarboxylation) towards CO2
and H2, and decarbonylation to CO and H2O [41]. Fortunately, the
orientation of each reaction pathway can be controlled selectively
by using suitable catalyst and reaction conditions [40,43].
Although the aforementioned facts indicated that Ir-complexes
with presence of elaborately selected ligands are efficient catalysts
for hydroformylation as well as transfer hydrogenation, no exam-
ple has been reported up to date to explore the related tandem
hydroformylation-reduction (via transfer hydrogenation) using
FA as hydrogen source. Herein, for the first time we studied this
tandem reaction over nitrogen ligand modified Ir-catalyst with
FA as the hydrogen source. In this synthesis protocol, the three-
step tandem reaction is included: (1) dehydrogenation of FA (to
release H2 along with CO2), (2) hydroformylation of olefins, (3)
reduction of aldehydes towards alcohols via transfer hydrogena-
tion with FA (Scheme 1). The use of FA as the hydrogen source
was aimed to (1) depress the side-reaction of hydrogenation of
the olefin while high pressured gaseous H2 was applied, (2) facili-
tate the transfer hydrogenation of aldehydes to the target alcohols
instead of the conventional H2-hydrogneation. It means that FA not
only serves as the H2-carrier to provide H2 in situ (for hydroformy-
lation of olefins), but also as the hydride-donor (as a nucleophile to
attack carbonyl group in the aldehyde) for transfer hydrogenation.
In principle, in order to fulfil this three-step sequence, the follow-
ing requirements were concerned: (1) Ir-complexes should be
selected as be the ideal catalyst candidates, which possess the
multi-catalysis for this sequence inducing dehydrogenation of for-
mic acid, hydroformylation, and transfer hydrogenation; (2) The
Firstly, 1,10-phenanthroline (L1) was applied as the bidental
nitrogen ligand in combination of [Ir(COD)Cl]2 precursor (Table 1).
It was found that L1-based Ir-catalyst exhibited good activity not
only for hydroformylation but also for reduction of 3-/2-
phenylpropanals, along with 97% conversion of styrene and 64%
selectivity to the target alcohols (Entry 1). Under the selected con-
ditions (styrene 5 mmol, N/Ir molar ratio of 2, 16 h, 120 °C), the
aldehydes completely transformed into 3-/2-phenylpropanols.
Due to the presence of excess FA (25 mmol), the obtained 3-/2-
phenylpropanols mostly converted to the corresponding formate
esters with alcohol/formate ester ratio of 13/87. Meanwhile, the
side-reactions were observed unavoidably including hydrogena-
tion of styrene (Sel.ethylbenzene, 29%) and addition of styrene with
FA to produce 1-phenylethyl formate (Sel.1-phenylethyl formate, 7%) fol-
lowing Markownikoff’s rule (Entry 1). In comparison, under the
same conditions, the use of H2 (1.0 MPa, equiv. to 15 mmol) just
led to the hydroformylation along with the more serious hydro-
genation of styrene (Sel.ethylbenzene, 49%). The expected reduction
of 3-/2-phenylpropanals didn’t happen at all (Entry 2). Evidently,
the use of FA instead of H2 not only greatly depressed the hydro-
genation of styrene but also gave rise to the subsequent reduction
of the obtained aldehydes, which implied that the reduction of the
aldehydes was resulted from the transfer hydrogenation by FA but
not the hydrogenation by H2 (released from FA). Since the acid (FA)
with strong acidity could promote esterification and addition reac-
tion of styrene with FA as observed in Entry 1, the base of HCOONa
[29,30,35] or Et3N was added respectively in order to form buffer
salt solution to mitigate the acidity of FA, resulting in the appar-
ently depressed addition reaction and esterification (Entries 4 vs
1). It was found that the use of more amount of HCOONa or Et3N
(FA: additive = 50:1 or 100:1 or molar ratio) gave the similar
results, probably due to the buffering effect of the formed buffer
salt solution. As shown in Entries 3 and 4 respectively, the forma-
tion of 1-phenylethyl formate was completely inhibited and the
ratio of alcohol/formate increase to 1/1.7 from 1/7.0 in comparison
to the reaction free of HCOONa or Et3N (Entries 3 and 4 vs 1). The
experiments of solvent screening indicated that DCE was the best
choice in comparison to the solvent like DMF, MeOH, toluene or
anisole, corresponding to the most efficient transformation of styr-
ene to the target alcohols (Entries 4 vs 5–8). It was noted that the
use of MeOH as the solvent resulted in the much higher selectivity
Scheme 1. Tandem hydroformylation-transfer hydrogenation for the synthesis of alcohols (and the corresponding formates) with formic acid as hydrogen source.

L. Liu et al. / Journal of Catalysis 385 (2020) 183–193
185
Table 1
Tandem hydroformylation-reduction of styrene with FA as hydrogen source over Ir(I)-catalyst under different conditionsa.
Entry
P
(MPa) FA (mmol)
25
Solvent
DCE
Additive Conv. (%)b Sel. (%)b
Aldehydes Alcohols (Alcohols/Formate esters)c Ethylbenzene 1-Phenylethyl formate
CO
1
2
3
4
5
6
7
8
3
3
3
3
3
3
3
3
2
4
3
3
–
–
97
98
–
51
1
64 (13/87)
–
29
49
28
25
30
25
31
29
33
20
26
26
7
–
ꢁ (H2 1.0 MPa) DCE
25
25
25
25
25
25
25
25
15
25
DCE
DCE
DMF
MeOH
Toluene Et3N
Anisole
DCE
DCE
DCE
DCE
HCOONa 97
69 (25/75)
72 (38/62)
59 (83/17)
62 (92/8)
67 (37/63)
69 (54/46)
65 (39/61)
78 (27/73)
66 (59/41)
70 (26/74)
trance
trace
4
1
2
trace
1
trace
Et3N
Et3N
Et3N
95
48
55
92
93
94
97
76
98
2
7
12
–
2
1
2
6
–
Et3N
Et3N
Et3N
Et3N
–
9
10
11
12d
2
4
a
b
c
Styrene 5 mmol, [Ir(COD)Cl]2 0.025 mmol (1 mol %), L1 0.05 mmol (N/Ir = 2/1), solvent 2 mL, FA: Additive = 200:1 (molar ratio), temperature 120 °C, time 16 h;
Determined by GC and GC-Mass analyses;
Sel. to alcohols is the sum of Sel. to 3-/2-phenylpropanols and Sel. to 3-/2-phenylpropyl formates, which is defined to evaluate the efficiency of tandem hydroformylation-
reduction;
d
L1 0.175 mmol (N/Ir = 7/1).
to the pure alcohols due to the involved transesterification of the
formate esters. Nevertheless the lower conversion of styrene of
55% was obtained disappointingly (Entry 6). As for toluene and ani-
sole as the solvents instead, the relatively worse results were
obtained in comparison to those with DCE (Entries 7 and 8). Rea-
sonably, the increase of CO pressure up to 4.0 MPa could further
depress styrene hydrogenation to ethylbenznene (Entries 10 vs 4
and 9). The amount of FA obviously influenced the reaction rate
and the product distributions. The decreased concentration of FA
resulted in the lower transformation along with the less trend to
esterification (Entry 11: Conv. 76%; alcohols/formate esters ratio
of 59/41). Notably, L1 in the increased amount of (N/Ir molar ratio
of 7) also could play the role of a buffering reagent instead of Et3N
with the negligible effect on the conversion of styrene and selectiv-
ity to the alcohols (Entry 12 vs 4).
with the overwhelming side-reactionsover hydroformylation
(Entry 7: Sel.ethylbenzene 40%, Sel.1-phenylethyl 30%; Entry 8:
formate
Sel.ethylbenzene 54%, Sel.1-phenylethyl
22%). Especially over L8-
formate
based Ir-catalyst, the hydroformylation and the subsequent reduc-
tion of the aldehydes were completed inhibited, but supplanted by
hydrogenation of styrene, and addition reaction of styrene with FA
as well as the styrene dimerization (Entry 8: Sel.styrene dimers 24%).
Accordingly, not only the bi-dental chelating effect of the involved
nitrogen ligands but also the electronic effect did play significant
role in tailoring the activity of Ir-catalyst for this
hydroformylation-reduction sequence. Evidently, the rigid bi-
dental nitrogen ligands, possessing stronger
p-accepting ability
(i.e. less -donating ability) like L1 and L3–L6 due to the delocal-
r
ization of lone-pair electrons into aryl ring, could more effectively
promote the performance of Ir-catalysts than those (L7 and L8)
Next, the effect of nitrogen-ligands on the performance of Ir-
catalysts was investigated in Table 2. A series of nitrogen-ligands
(L1–L8) were selected for this sequence. When L2 with the steric
structure similarity to L1 but featured with mono-nitrogen coordi-
nation was applied under the same conditions, the poor conversion
of styrene and the low selectivity to the alcohols were obtained
along with the serious side-reactions (Entry 2), which suggested
that the lack of bi-dental N-chelation to Ir-center did go against
the activity of the corresponding Ir-catalyst. It was also found that,
the rigid bi-dental nitrogen-ligands possessing N-sp2-hybrid char-
acter like L1 and L3–L6 corresponded to much better catalytic per-
formance, leading to more efficient formation of the target alcohols
with yields of 64–72% (Entries 1 and 3–6). While L7 and L8 only
possessing N-sp3-hybrid character were used respectively in place
of L1, the much lower conversions of styrene were observed along
with stronger r-donating ability. When the phosphines such as
PPh3, Dppp and Xantphos were used respectively instead of L1,
the conversions of styrene were dramatically decreased along with
the poor selectivities to the aldehydes/alcohols (Entries 9–11). In
contrast, over L1-based [Rh(COD)Cl]2 system, no matter using FA
(Entry 12) or H2 (Entry 13) as the hydrogen source, the expected
hydroformylation didn’t occur at all, indicating that the corre-
sponding Rh-catalyst was neither active towards the decomposi-
tion of FA nor towards the hydroformylation of styrene with the
presence of the nitrogen ligand. When the other Ir-precursors such
as Ir(acac)(CO)2, IrCl3ꢀ3H2O, and iridium(III) acetate were applied
instead of [Ir(COD)Cl]2, it was found that Ir(acac)(CO)2 and
IrCl3ꢀ3H2O corresponded to the much lower selectivity of ca. 40%
to the target alcohols (Entries 14 and 15). Although iridium(III)
acetate exhibited the competitive activity for this sequence in

186
L. Liu et al. / Journal of Catalysis 385 (2020) 183–193
Table 2
The effects of ligands and the catalyst precursors on tandem hydroformylation-reduction of styrene with FA as hydrogen sourcea.
b
Entry
Precursor
Ligand
Conv. (%)
Sel. (%)b
Aldehydes
Alcohols (Alcohols/Formate esters)c
Ethylbenzene
1-Phenylethyl formate
1
[Ir(COD)Cl]2
[Ir(COD)Cl]2
[Ir(COD)Cl]2
[Ir(COD)Cl]2
[Ir(COD)Cl]2
[Ir(COD)Cl]2
[Ir(COD)Cl]2
[Ir(COD)Cl]2
[Ir(COD)Cl]2
[Ir(COD)Cl]2
[Ir(COD)Cl]2
[Rh(COD)Cl]2
[Rh(COD)Cl]2
Ir(acac)(CO)2
IrCl3ꢀ3H2O
L1
L2
L3
L4
L5
L6
L7
L8
Xantphos
Dppp
PPh3
L1
L1
L1
95
23
84
97
99
98
25
49
36
44
48
15
<5
93
97
97
2
12
4
2
–
2
10
–
72 (38/62)
19 (21/79)
64 (80/20)
72 (10/90)
70 (9/91)
72 (8/92)
20 (50/50)
–
25
40
30
25
28
25
40
54
40
42
41
–
trace
21
2
1
2
2d
3
4
5
6
1
7
30
22
35
20
23
65
–
4
7
–
8e
9f
–
–
10g
11h
12
13i
14j
15k
16l
15
11
–
–
32
–
15 (80/20)
9 (67/23)
–
–
–
43 (12/88)
41 (10/90)
81 (52/48)
21
52
19
L1
L1
Iridium(III) acatate
5
a
Styrene 5 mmol, [M(COD)Cl]2 0.025 mmol (M = Ir or Rh, [M] = 1 mol %), ligand 0.05 mmol, PCO 3.0 MPa, FA 25 mmol, Et3N 0.125 mmol (FA:Et3N = 200:1, molar ratio),
120 °C, 16 h, DCE 2 mL.
b
Determined by GC and GC-Mass analyses.
Sel. to alcohols is the sum of Sel. to 3-/2-phenylpropanols and Sel. to 3-/2-phenylpropyl formats.
8% styrene dimmers detected.
24% styrene dimmers detected.
Xantphos (0.05 mmol). 25% styrene dimmers detected.
Dppb (0.05 mmol). 8% styrene dimmers detected.
PPh3 (0.05 mmol). 16% styrene dimmers detected.
H2 (2.0 MPa) was applied in place of FA (25 mmol).
c
d
e
f
g
h
i
j
Ir(acac)(CO)2 0.05 mmol.
k
IrCl3ꢀ3H2O 0.017 mmo.
l
Iridium(III) acetate ([Ir3(
l3-O)(OAc)6][OAc]ꢀ3H2O)0.017 mmol.
comparison to [Ir(COD)Cl]2 (Entry 16), its much higher price lim-
ited the practical use. As for [Ir(COD)Cl]2, the weak coordinating
ability of COD and the bridged -Cl ligands might give rise to their
timely dissociation from Ir-center to accommodate L1 and FA for
the effective formation of the active Ir-complex intermediates as
proposed in Scheme 2.
The result in Table 1 (Entry 2) indicated that in syngas
(CO/H2 = 3/1) without the presence of FA, the formed aldehydes
from the first-step hydroformylation couldn’t be reduced by H2
effectiely over L1-based [Ir(COD)Cl]2 system. Only the presence
of FA instead of H2 corresponded to successful reduction of the
aldehydes into the target alcohols. These results coupled with the
data in Entry 2 of Table 1 implied that the role of FA was not just to
in situ release H2, which participated hydroformylation as well as
hydrogenation supposedly.
3-phenylpropanol (Entry 3). Subsequently,
a
portion of
3-phenylpropanol was esterificated with FA to afford
3-phenylpropyl formate with the yield of 19% (Entry 3). In compar-
ison, over L1-based [Rh(COD)Cl]2 system, the reduction of
3-phenylpropanal by FA was quite sluggish only with 38%
conversion (Entry 4). These results implied that the reduction of
3-phenylpropanal by H2 and FA underwent the different reaction
pathways, which further confirmed that the reduction of
3-phenylpropanal to afford the alcohols was derived from the
transfer hydrogenation by FA, but not due to the hydrogenation
by H2 (obtained upon dehydrogenation of FA in situ).
The evolving reaction processes of styrene with CO and FA in
DCE against the reaction time over L1-based [Ir(COD)Cl]2 systems
were presented in Fig. 1. It was indicated that the hydroformyla-
tion was the rate-determining step for this sequence. Once the
aldehydes were formed, the subsequent reduction via transfer
hydrogenation to the corresponding alcohols/formates proceeded
smoothly. In the initial 4 h, the hydrogenation of styrene inten-
sively competed with the hydroformylation, giving 74% conversion
of styrene along with 21% selectivity to ethylbenzene. Later on, the
hydrogenation was completely depressed as the olefin greatly con-
sumed, then giving way to the hydroformylation-transfer hydro-
genation sequence.
In order to further elucidate the role of FA in reducing the alde-
hydes, the separate reduction of 3-phenylpropanal over L1-based
[Ir(COD)Cl]2 system was investigated in Table 3. It was indicated
that, in H2 atmosphere (1.0 MPa, ca. 15 mmol) under the similar
reaction condition as in Entry
4 of Table 1, only 30%
3-phenylpropanal converted to 3-phenylpropanol (Entries 1). The
addition of FA as an additive (0.5 mmol) has negligible promoting
effect on the reaction rate of hydrogenation (Entry 2). In compar-
ison, when FA (15 mmol, equivalent to 15 mmol H2) was used
instead of H2, 3-phenylpropanal was reduced completely to
In order to deeply understand decomposition of FA into CO2
and H2 over L1-[Ir(COD)Cl]2 catalytic system, the in situ FT-IR

L. Liu et al. / Journal of Catalysis 385 (2020) 183–193
187
Scheme 2. The proposed catalytic mechanism over L1-[Ir(COD)Cl]2 for tandem hydroformylation- reduction of olefins using FA as hydrogen source.
spectroscopic analysis was carried out to monitor the decarboxyla-
tion processes of FA at different temperatures (Fig. 2). Knowingly,
FA can form several hydrogen-bonded dimeric structures at ambi-
ent conditions mainly in the forms of trans-trans and trans-cis
dimers. The structure of the trans-trans FA dimer, formed by the
hydrogen bonds both involving the hydroxy hydrogen atom and
carbonyl oxygen atom of the individual trans-conformers, should
possess the most thermodynamic stability with the lowest energy

188
L. Liu et al. / Journal of Catalysis 385 (2020) 183–193
Table 3
The separate reduction of 3-phenylpropanal towards 3-phenylpropanol over L1-based Ir-catalysta.
Entry
Hydrogen source
Catalyst precursor
Additive
Conv. (%)b
Sel. (%)b
3-Phenylpropanol
3-Phenylpropyl formate
1
2
3
4
H2 (1.0 MPa, 15 mmol)
H2 (1.0 MPa, 15 mmol)
FA (15 mmol)
[Ir(COD)Cl]2
[Ir(COD)Cl]2
[Ir(COD)Cl]2
[Rh(COD)Cl]2
–
30
34
100
38
100
100
81
–
–
19
66
FA (0.5 mmol)
–
–
FA (15 mmol)
34
a
3-Phenylpropanol 5 mmol, [Ir(COD)Cl]2 (or [Rh(COD)Cl]2) 0.025 mmol, L1 0.05 mmol, 120 °C, 16 h, DCE 2 mL.
Determined by GC and GC-Mass analyses.
b
the synchronous disappearance of vibration of 1370 cmꢁ1 due to
the rupture of the hydrogen bond to afford cis-/trans-FA mononer.
Accompanied by the appearance of the adsorption at 1791 cmꢁ1
,
the characteristic double vibrations of CO2 at 2367 and
2324 cmꢁ1 (see Fig. S1 in ESI with more distinct CO2 adsorptions)
as well as the peak at 2050 cmꢁ1 ascribed to hydride-Ir (H-Ir) inter-
mediate [20] emerged concurrently. And their adsorption reached
the strongest intensity at 120 °C. In comparison, without the pres-
ence of L1-[Ir(COD)Cl]2 system (Fig. 2B), although the two split
peaks at ca. 1794 and 1751 cmꢁ1 (indicating the co-existence of
cis-FA and trans-FA conformers) as well as the gradually disap-
peared adsorption of the hydrogen bond-related vibration at ca.
1371 cmꢁ1 were also observed while the temperature increasing
up to 80–120 °C, the concomitant formation of CO2 derived from
the decarboxylation of FA was not found at all. It was also noted
that an unidentified weak absorption at ca. 2018 cmꢁ1 appeared
concurrently with the absorption of cis-FA (m
1794 cmꢁ1) no matter
the catalyst was present or not. In parallel, over L1-[Rh(COD)Cl]2,
the decarboxylation (along with the release of H2) of FA didn’t
occur either (see Fig. S2 in ESI for the corresponding in situ FT-IR
analysis). The results in Fig. 2 confirmatively demonstrated that
Fig. 1. Evolution profiles of the tandem hydroformylation-reduction of styrene
with CO and FA in DCE vs reaction time catalyzed by L1-[Ir(COD)Cl]2 system
[conditions: [Ir(COD)Cl]2 0.025 mmol, L1 0.05 mmol, styrene 5 mmol, FA 25 mmol,
Et3N 0.125 mmol, PCO 3 MPa, DCE 2 mL, 120 °C.
the appearance of cis-FA (m
1794 cmꢁ1) and then its interaction
with L1-[Ir(COD)Cl]2 catalyst to form the H-Ir intermediate closely
co-related to the efficient decarboxylation of FA to afford CO2 as
well as H2. Herein, since CO was absent in the testing system, the
state, whereas the structure of the trans-cis FA dimer, formed by
two different types of the hydrogen bonds, should be preferred
according to the reported calculations [45,46]. Hence, the most sta-
bilized structures of trans-trans/trans-cis FA dimers as depicted in
Fig. 2 were applied as the representative modes of FA. As shown
in Fig. 2A, over L1-[Ir(COD)Cl]2 system, while the temperature
increased from 30 to 80 °C, the characteristic absorptions of FA typ-
ically in the forms of trans-trans FA dimer and trans-cis FA dimer
were observed at ca. 1720 (broad, C@O vibration) and 2940
(CAH vibration) cmꢁ1 without clear distinction. Concomitantly,
an unidentified strong adsorption coherently related to OꢀꢀꢀH
hydrogen bond formation in FA-dimer was clearly observed at ca.
1370 cmꢁ1[47]. Subsequently, from 80 to 120 °C, the broad peak
at ca. 1720 cmꢁ1 slightly blue-shifted and then gradually split into
the two sharp peaks at 1794 and 1751 cmꢁ1, which was ascribed to
the C@O vibrations in cis-FA monomer and trans-FA monomer
respectively according to the reported work [45,46], along with
formation of the observed Ir-H species (
m
2050 cmꢁ1) must be
attributed to the available cis-FA conformer as a hydride-source.
If CO were involved, the obtained syngas (H2 was released from
FA decomposition) also could have resulted in the formation of
Ir-H species upon complexation with [Ir(COD)Cl]2 and L1 (see
Scheme 2). Actually, the efficient decomposition of FA into CO2
and H2 via the coordination of cis-FA conformer to Co(I)-catalyst
had been proposed based on the DTF-calculations reported by M.
Beller [48]. In addition, the decomposition experiment of FA in
the stainless steel autoclave under the similar conditions applied
for this in situ FT-IR analysis also showed that over L1-[Ir(COD)
Cl]2 FA could rapidly decompose into CO2 and H2 which were
detected by TCD-GC successfully (see Fig. S3 in ESI).
On the basis of the obtained results (Tables 1, 3 and Fig. 2), the
catalytic mechanism over L1-[Ir(COD)Cl]2 system for the tandem
hydroformylation-transfer hydrogenation with FA as hydrogen

L. Liu et al. / Journal of Catalysis 385 (2020) 183–193
189
source was proposed in Scheme 2. Firstly, as observed in Fig. 2A
that, the cis-FA conformer is abundantly available upon heating
up to 120 °C. The co-existence of trans-cis FA and trans-trans FA
dimers rendered FA molecules the two alternative modes to coor-
dinate to L1-ligated Ir(I)-center, leading to the formation of cis-FA
(Path I, as a hydride-containing ligand) coordinated intermediate
A or trans-FA (Path-II, as an carbonyl O-containing ligand) coordi-
nated intermediate A0 upon ligand-exchange with COD and chlo-
ride. In Path I, the critical cis-FA/formate/L1 four-coordinated
intermediate A is formed firstly whose stability is guaranteed by
the available OꢀꢀꢀH hydrogen-bonding interaction between cis-FA
and formate (HCOOꢁ). In intermediate A, cis-FA serves as a
hydride-ligand, and the format serves as the anionic O-
containing ligand. Then the intermediate A undergoes the decar-
boxylation to release two molecules of CO2 (ascribed to
m 2367
and 2324 in Fig. 2A) along with the formation of di-hydride-Ir
intermediate (B), which corresponds to the observed adsorption
at ca. 2050 cmꢁ1 in Fig. 2A (obtained free of CO). Subsequently,
H2 is released from B along with the regeneration of intermediate
A upon the coordination of another cis-FA and formate. The forma-
tion of thermodynamically stable H2 is the driving force for Path I.
Herein, the released H2 reacts with CO and the olefin to carry out
the hydroformylation through the formation of the intermediates
of C, D, E, and F under the same catalytic conditions. Simultane-
ously, in Path II, the trans-FA/formate/L1 four-coordinated inter-
mediate A0 was formed alternatively, in which trans-FA serves as
a carbonyl O-containing ligand. The available OꢀꢀꢀH hydrogen-
bonding interaction for forming energy-satisfied six-member ring
structure also gives rise to the stability of intermediate A0, which
is the driving force for Path II. Upon the release of one molecule
of CO2, the mono-hydride-Ir intermediate B0 whose stability is
badly deteriorated due to the destroyed hydrogen bond and then
followed by the rapid ligand-exchange with the aldehyde to afford
intermediate G. The successful hydride-transfer in the intermedi-
ate G upon the nucleophilic attack to the carbonyl group as well
as the proton-transfer to O-atom leads to the formation of the tar-
get alcohols, along with the regeneration of intermediate A0 after
the re-coordination of trans-FA and formate. Evidently, the avail-
ability of cis-FA as the hydride-ligand is responsible for the dehy-
drogenation upon decarboxylation (Path I). The availability of
trans-FA as the carbonyl O-containing ligand corresponds to the
transfer hydrogenation to reduce the aldehyde to the correspond-
ing alcohol (Path II).
In order to further account for the feasible formation and stabil-
ity of the critical Ir-complex intermediates of A in Path I and A0 in
Path II, which should be co-existent in parallel to enable this tan-
dem hydroformylation-transfer hydrogenation sequence, their
geometric structures were optimized and Gibbs free energy (DG)
was calculated by using DFT methods [49]. The obtained possible
coordination patterns for A0 and A as well as X (as the reference
structure to A) is shown in Fig. 3. The intermediate complex A0 is
the most stable structure, whose Gibbs free energy (DG) is set as
the reference with the value of 0.0 kcal/mol. The optimized geo-
metric structure of A0 indicates that, besides of L1 ligand, one car-
bonyl O-atom (in trans-FA) and one negative-charged O-atom (in
HCOOꢁ) both coordinate to Ir(I)-center respectively. In addition,
the negative-charged O-atom can develop an O∙∙∙H hydrogen bond
with the hydroxyl H-atom of trans-FA with the bond distance of
1.6 Å. In comparison, as for the intermediate complex A, its Gibbs
free energy is just a little higher than that of A0 by 8.7 kcal/mol. In
A, the H-atom as hydride-donor in cis-FA coordinates to Ir(I)-
center, along with the same O∙∙∙H hydrogen bond with the bond
Fig. 2. The in situ FT-IR spectra recorded in N2 atmosphere from 30 to 120 °C for (A)
the mixture of FA, L1 and [Ir(COD)Cl]2 (The selected FT-IR spectra with clarity
demonstrating the evolving CO2-release at 30, 80, 100, and 120 °C respectively were
provided in ESI); (B) pure FA without the presence of L1 and [Ir(COD)Cl]2.

190
L. Liu et al. / Journal of Catalysis 385 (2020) 183–193
Fig. 3. The optimized structures and free energies (
D
G) of intermediates A0, A, and X (X is regarded as the reference structure to A0 but with the potential Nꢀꢀꢀhydrogen bond).
Table 4
Generality of L1-[Ir(COD)Cl]2 system for one-pot tandem hydroformylation-reduction of olefins with CO and FAa.
Entry
1
Olefin
Conv. (%)b
95
Sel. (%)b
Aldehydes
2
Alcohols (alcohols/formate esters)c
72 (38/62)
Alkane
25
Addition-compound of olefin with FA
trace
2
3
97
91
1
2
71 (45/55)
76 (46/54)
25
21
3
1
4
5
98
90
16
2
60 (17/83)
74 (39/61)
23
23
Trace
trace
6
7
90
91
2
2
64 (39/61)
73 (34/66)
33
24
trace
trace
8
94
86
86
3
56 (52/48)
22 (9/91)
76 (43/57)
29
18
17
ꢁ (12)d
ꢁ (25)d
ꢁ (10)d
9
35
3
10
a
b
c
Olefin 5 mmol, [Ir(COD)Cl]2 0.025 mmol (Ir 1 mmol %), L1 0.05 mmol, PCO 3.0 MPa, FA 25 mmol, Et3N 0.125 mmol (FA:Et3N = 200:1, molar ratio), 120 °C, 16 h, DCE 2 mL.
Determined by GC and GC-Mass analyses.
Sel. to alcohols is the sum of Sel. to the alcohols and Sel. to 3-/2-the corresponding formate esters.
Sel. of isomers of the olefin was indicated in the parenthesis.
d
distance of 1.6 Å. Undoubtedly, the developed O∙∙∙H hydrogen bond
interaction between trans-/cis-FA and formate to form a thermody-
namically favored six-member with Ir(I)-center definitely gives
rise to the stability of A and A0. In contrast, the geometric struc-
tures as well as Gibbs free energy of the complex X as the reference
analogue of A0 was calculated, wherein the same ligands of trans-
FA and HCOOꢁ as well as L1 were involved but the hydrogen bond
is developed between trans-FA and N-atom of L1. It was found that,
in the optimized structure of X, although a thermodynamically
favored six-member with Ir(I)-center is also formed upon the link-
age of HCOOꢀꢀꢀHAN hydrogen bond with the bond distance of 1.8 Å,
the Gibbs free energy of X is as much as 36.5 kcal/mol, which is
much higher than those of the complexes A and A0. It should be
noted that, due to the strong basicity, L1 is able to serve as an
acid-scavenger to result in HCOOꢀꢀꢀHAN hydrogen bond instead
of HCOO-HꢀꢀꢀN one. The DFT-calculation results further supports

L. Liu et al. / Journal of Catalysis 385 (2020) 183–193
191
that the complexes A0 and A are the most optimized structures
with much better stability, wherein the developed O∙∙∙H
(HCOOH∙∙∙OCHO) hydrogen bonds between cis-/trans-FA and for-
mate contribute more stabilization to the structures of A0 and A
than the O∙∙∙H hydrogen (HCOO∙∙∙HAN) between trans-FA and L1
to X. Based on the DFT-calculations coupled with in situ FT-IR anal-
ysis for decomposition of FA (Fig. 2), it is reasonable to believe that
the formation of the stabilized A and A0 via Path I and Path II is the
driving force to achieve the tandem hydroformylation-transfer
hydrogenation sequence as proposed in Scheme 2.
favored six-member with Ir(I)-center, were the driving force for
this successful hydroformylation-transfer hydrogenation
sequence. The formation of A in which cis-FA as the hydride-
ligand to coordinate to Ir(I)-center was responsible for the efficient
dehydrogenation of FA to release H2 along with CO2. The formation
of A0 in which trans-FA as the carbonyl O-containing ligand corre-
sponded to the transfer hydrogenation of the aldehydes to the
alcohols under the same catalytic conditions.
4. Experimental
Finally, the generality of L1-[Ir(COD)Cl]2 catalytic system for
tandem hydroformylation-reduction of the different olefins with
CO and FA was summarized in Table 4. It was found that as for styr-
ene derivatives, the steric and electronic effects of the substituents
had no obvious discrimination on the reaction rate of the first-step
hydroformylation. More than 90% of styrene derivatives were con-
verted respectively under the applied conditions, along with 66–
78% selectivities to the hydroformylated products including the
aldehydes and the corresponding alcohols (Entries 1–7). However,
as for 1-methyl-4-vinylbenzene, the second-step reduction of the
aldehydes to the alcohols was slowed down to some extend due
to the enhanced stability of 2-(p-tolyl)propanal while methyl
located at para-position of phenyl ring (Entry 4). And the most
intensive electronic effect coming from the para-positioned Cl-
atom led to the relatively lower efficiency for hydroformylation-
transfer hydrogenation sequence (Entry 6: Conv. 90%; Sel.aldehydes
2%; Sel.alcohols 64%), due to the relatively weakened coordination
of C = C double bond to Ir-center for formation of intermediate D
followed by CO-insertion to afford acylirdidium intermediate of F
(Scheme 2). When the linear aliphatic olefins such as 1-octene,
2-octnene, and 1-decene, were applied to repeat the reaction
respectively, the isomerization of the substrates occurred univer-
sally besides of the side-reaction of hydrogenation of the olefins
(Entries 8–10), resulting in the decreases selectivities of the tar-
get alcohols. Notably as for the internal olefin like 2-octene (Entry
9) or the longer carbon-chain olefin like 1-decene (Entry 10), the
reaction rate was decreased due to the increased steric hindrance.
Especially for the internal olefin of 2-octene, the second-step
reduction of the aldehydes was greatly depressed, along with only
22% selectivities of the alcohols as well as 35% selectivities of alde-
hydes (Entry 9).
4.1. Reagents and analysis
The chemical reagents were purchased from Shanghai Aladdin
Chemical Reagent Co. Ltd. and Alfa Aesar China, which were used
as received. The solvents were distilled and dried before use. Gas
chromatography (GC) was performed on
equipped with a DM-Wax capillary column (30 m ꢂ 0.25 mm ꢂ
0.25 m) and FID to analyze the conversions of olefin and the pro-
a -SHIMADZU-2014
l
duct distributions for tandem hydroformylation-transfer hydro-
genation. GC-mass spectrometry (GC–MS) was recorded on an
Agilent 6890 instrument equipped with an Agilent 5973 mass
selective detector. As for the experiment of FA decomposition over
L1-[Ir(COD)Cl]2 system in the stainless steel autoclave, the
obtained gases were analyzed by GC-TCD (Agilent 7890A)
equipped with a Pora PLOT Q (30 m ꢂ 530
l
m ꢂ 40
m) and HP-PLOT 5A
lm). The FT-IR spectra were recorded on
lm), Pora PLOT
Q
(2
m
ꢂ
530
m ꢂ 25
l
m
ꢂ
40
l
(60 m ꢂ 530
l
a Nicolet NEXUS 670 spectrometer.
4.2. General procedures for one-pot tandem hydroformylation-transfer
hydrogenation of olefins with CO and FA
In
a typical experiment, [Ir(COD)Cl]2 (0.025 mmol), L1
(0.05 mmol) (or the other ligand) were mixed with styrene
(5 mmol, or the other olefin), FA (25 mmol), Et3N (0.125 mol if
required) and DCE (2 mL, or the other solvent). The mixture was
added in a 50 mL sealed Teflon-lined stainless steel autoclave
which was purged with CO (0.5 MPa) for three times and then
pressured by CO to 3.0 MPa. Then the reaction mixture was stirred
vigorously at appointed temperature for some time. Upon comple-
tion, the autoclave was cooled down to room temperature and
slowly depressurized. The solution was analyzed by GC to deter-
mine the conversions (n-dodecane as internal standard) and the
selectivities (normalization method), and the products were fur-
ther identified by GC-mass spectrometry.
3. Conclusions
L1-based [Ir(COD)Cl]2 system proved to be highly efficient for
tandem hydroformylation-reduction for the one-pot synthesis of
alcohols from terminal olefins with FA as the hydrogen source.
The target alcohols including the corresponding formate esters
upon the corresponding esterification with FA were obtained in
the yields of 52–70%. L1 as a bidental N-containing ligand with
chelating effect rendered the Ir(I)-complex the multiple catalysis
in terms for FA dehydrogenation, hydroformylation and transfer
hydrogenation under the same reaction conditions. In this
sequence, FA doubled as a H2-carrier to provide H2 in situ (required
by hydroformylation with advantage of the depressed hydrogena-
tion of the olefin) and a hydrogen-transfer reagent to accomplish
the reduction of aldehydes (to the alcohols through transfer hydro-
genation). The in situ FT-IR spectroscopic analysis revealed that FA-
dimer completely dissociated into cis-FA monomer and trans-FA
monomer at 120 °C, then resulting in the efficient dehydrogenation
of FA along with the release of CO2 and H2 over L1-[Ir(COD)Cl]2 sys-
tem due to the availability of cis-FA conformer. The DFT calcula-
tions confirmed that, the formation of the Ir(I)-intermediate
complexes of A and A0, which were dramatically stabilized by the
developed O∙∙∙H hydrogen bond to form a thermodynamically
4.3. In situ FT-IR spectroscopic characterization
The in situ FT-IR spectra were recorded on a Nicolet NEXUS 670
spectrometer. The spectral resolution was about 4 cmꢁ1. A mixture
of 0.1 mL of FA and 0.0125 mmol of [Ir(COD)Cl]2 with 0.025 mmol
L1 (if [Ir(COD)Cl]2 and L1 required) was added in the specially
designed high-pressure IR cell, in which cylindric CaF2 was used
as the sealing sheets. Then, the IR cell was purged with N2
(0.5 MPa) for three times. The real time monitoring was performed
under the different reaction temperatures.
4.4. Computational methods
All the DFT calculations were carried out in the Gaussian09 soft-
ware package [49]. The geometrical structures of the three com-
plexes were optimized with using the
combined with the 6-31G* basis set for the nonmetal C, N, O and
H elements and the Lanl2dz [51,52] basis set for the metal atom
xB97XD [50] functional

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Acknowledgements
This work was supported by the National Natural Science Foun-
dation of China (No. 21673077, 21972045) and the Science and
Technology Commission of Shanghai Municipality (18JC1412100).
We acknowledge the support of the NYU-ECNU Center for Compu-
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