Iron-Catalyzed Homogeneous Hydrogenation of Alkenes under Mild Conditions by a Stepwise, Bifunctional Mechanism

Article abstract of DOI:10.1021/acscatal.5b02674

Hydrogenation of alkenes containing polarized C=C double bonds has been achieved with iron-based homogeneous catalysts bearing a bis(phosphino)amine pincer ligand. Under standard catalytic conditions (5 mol % of (PNHP^iPr)Fe(H)₂(CO) (PNHP^iPr = NH(CH₂CH₂PiPr₂)₂), 23 °C, 1 atm of H₂), styrene derivatives containing electron-withdrawing para substituents reacted much more quickly than both the parent styrene and substituted styrenes with an electron-donating group. Selective hydrogenation of C=C double bonds occurs in the presence of other reducible functionalities such as -CO₂Me, -CN, and N-heterocycles. For the α,β-unsaturated ketone benzalacetone, both C=C and C=O bonds have been reduced in the final product, but NMR analysis at the initial stage of catalysis demonstrates that the C=O bond is reduced much more rapidly than the C=C bond. Although Hanson and co-workers have proposed a nonbifunctional alkene hydrogenation mechanism for related nickel and cobalt catalysts, the iron system described here operates via a stepwise metal-ligand cooperative pathway of Fe-H hydride transfer, resulting in an ionic intermediate, followed by N-H proton transfer from the pincer ligand to form the hydrogenated product. Experimental and computational studies indicate that the polarization of the C=C bond is imperative for hydrogenation with this iron catalyst.

Full text of DOI:10.1021/acscatal.5b02674

Subscriber access provided by MAHIDOL UNIVERSITY (UniNet)
Article
Iron-Catalyzed Homogeneous Hydrogenation of Alkenes
under Mild Conditions by a Stepwise, Bifunctional Mechanism
Ruibo Xu, Sumit Chakraborty, Sarina M. Bellows, Hongmei Yuan, Thomas R. Cundari, and William D. Jones
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.5b02674 • Publication Date (Web): 11 Feb 2016
Downloaded from http://pubs.acs.org on February 20, 2016
Just Accepted
“Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted
online prior to technical editing, formatting for publication and author proofing. The American Chemical
Society provides “Just Accepted” as a free service to the research community to expedite the
dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts
appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been
fully peer reviewed, but should not be considered the official version of record. They are accessible to all
readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered
to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published
in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just
Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor
changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers
and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors
or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
ACS Catalysis is published by the American Chemical Society. 1155 Sixteenth Street
N.W., Washington, DC 20036
Published by American Chemical Society. Copyright © American Chemical Society.
However, no copyright claim is made to original U.S. Government works, or works
produced by employees of any Commonwealth realm Crown government in the course
of their duties.

Page 1 of 27
ACS Catalysis
1
2
3
4
5
6
7
8
9
IronꢀCatalyzed Homogeneous Hydrogenation of
Alkenes under Mild Conditions by a Stepwise,
Bifunctional Mechanism
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Ruibo Xu,a,b Sumit Chakraborty,a,d Sarina M. Bellows,a,d Hongmei Yuan,a,d Thomas R.
Cundari,c,d and William D. Jones*,a,d
aDepartment of Chemistry, University of Rochester, Rochester, New York 14627, USA.
bSchool of Pharmacy, Huaihai Institute of Technology, Lianyungang, Jiangsu, 222005, P. R.
China.
cDepartment of Chemistry and CASCaM, University of North Texas, Denton, TX 76203, USA.
dCenter for Enabling New Technologies through Catalysis (CENTC)
KEYWORDS. olefin/alkene hydrogenation, iron catalysis, bifunctional mechanism, metalꢀligand
cooperativity, DFT calculations
ACS Paragon Plus Environment
1

ACS Catalysis
Page 2 of 27
1
2
3
4
5
6
7
8
ABSTRACT. Hydrogenation of alkenes containing polarized C=C double bonds has been
achieved with ironꢀbased homogeneous catalysts bearing a bis(phosphino)amine pincer ligand.
9
Under standard catalytic conditions (5 mol% (PNHPiPr)Fe(H)2(CO), PNHPiPr
=
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
NH(CH2CH2PiPr2)2), 23 °C , 1 atm H2), styrene derivatives containing electronꢀwithdrawing
paraꢀsubstituents reacted much faster than both the parent styrene and substituted styrenes with
an electronꢀdonating group. Selective hydrogenation of C=C double bonds occurs in the presence
of other reducible functionalities such as –CO2Me, –CN, and Nꢀheterocycles. For the α,βꢀ
unsaturated ketone benzalacetone, both C=C and C=O bonds have been reduced in the final
product, but NMR analysis at the initial stage of catalysis demonstrates that the C=O bond is
reduced much more rapidly than the C=C bond. Although Hanson and coworkers have proposed
a nonꢀbifunctional alkene hydrogenation mechanism for related nickel and cobalt catalysts, the
iron system described here operates via a stepwise metalꢀligand cooperative pathway of Fe–H
hydride transfer, resulting in an ionic intermediate, followed by N–H proton transfer from the
pincer ligand to form the hydrogenated product. Experimental and computational studies indicate
that the polarization of the C=C bond is imperative for hydrogenation with this iron catalyst.
1. INTRODUCTION
The chemistry community has been aiming towards the use of Earthꢀabundant catalysis in
recent years.1 Although alkene hydrogenation is wellꢀknown with Rh,2 Ir,3 and Ru4 catalysts,5,6
only a handful of catalysts with Earthꢀabundant, first row metals have been discovered. Notably,
the use of Fe,7 Co,789ꢀ10 and Ni9,11 catalysts with pincer supporting ligands have been highly
successful in the past few years.6 For example, Peters, et al. discovered that the (PBPtBu)Co(N)2
complex (PBPtBu = bis(phosphino)boryl with tꢀbutyl substituents on the phosphines) can catalyze
ACS Paragon Plus Environment
2

Page 3 of 27
ACS Catalysis
1
2
hydrogenation of terminal alkenes quantitatively with TOF’s of 1000 h–1; dimeric
[(PBPCy)CoH]2 catalyzes hydrogenation of terminal and internal olefins, albeit at much slower
rates with TOF’s from 2.5 to 8 h–1 (1 atm H2, C6D6, 2 mol% catalyst, 23 °C).9 The Ni analogue
of the Peters catalyst forms a monomer which also hydrogenates terminal (TOF = 25 h–1) and
internal (TOF = 0.6 h–1) olefins catalytically (1 atm H2, C6D6, 2 mol% catalyst, 23 °C).9 The Niꢀ
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
boryl catalyst is more active than Hanson’s (PNHPCy)Ni(H)+ catalyst (PNHPCy
=
HN(CH2CH2PCy2)2) based on the latter having only 0.2 – 0.4 h–1 TOF for hydrogenation of
terminal alkenes (hydrogenation of internal alkenes was not reported).11 Both Ni catalysts are
proposed to hydrogenate alkenes via 1,2ꢀinsertion of the alkene into the Ni–H bond. Chirik’s
cobaltꢀdiphosphine catalysts hydrogenate allyl alcohols and other unhindered, activated olefins at
25 °C under 4 atm H2.10 Milstein et al. have reported an acridineꢀbased (PNP)Fe catalyst that can
partially hydrogenate alkynes to Eꢀalkenes.12
Figure 1. Examples of bifunctional catalysts with a metalꢀhydride and ligandꢀproton.13,14
The (PNPiPr)Fe(H)(CO) (PNPiPr = N(CH2CH2PiPr2)2) catalyst (1) can hydrogenate (using 2 in
Figure 1) and dehydrogenate CN and CO bonds.15,16 The latter underwent experimental and
computational mechanistic studies to conclude that a bifunctional mechanism with a concerted
sixꢀmembered cyclic transition state of hydride (Fe–H) and proton (N–H) transfer to the C and O
atoms of C=O substrates, respectively, was operating.16 This metalꢀligand cooperativity, yielding
a bifunctional hydrogenation mechanism, has been proposed for a number of catalysts. Figure 1
displays a few selected examples, including species 2 formed from the catalyst used in this paper
ACS Paragon Plus Environment
3

ACS Catalysis
Page 4 of 27
1
2
3
4
5
6
7
8
9
(1); note that these hydride complexes possess an acidic proton on the supporting ligand. Metalꢀ
ligand cooperativity can occur via a simultaneous (concerted) hydride/proton transfer step or via
consecutive steps. Interestingly, DFT calculations of Milstein’s transꢀ[Ru(H)2(PNN)(CO)]
catalyst (PNN = 2ꢀCH2NEt2ꢀ6ꢀCH2PtBu2ꢀpyridine) for dimethylcarbonate hydrogenation to
methanol propose the metalꢀligand cooperative pathway to be higher in energy than hydride
transfer followed by C–OMe cleavage.17 In the present work, hydrogenation of polar C=C bonds
is investigated under mild conditions with catalyst 1 and the in situ formed 2. Nonꢀpolarized
C=C bonds cannot be hydrogenated by this catalyst. DFT calculations suggest a stepwise,
bifunctional mechanism of hydride transfer followed by proton transfer that only occurs for
polarized C=C bonds. This stepwise mechanism is in contrast to the concerted hydride/proton
transfer to C=O substrates. Experiments and calculations suggest that polarization of the C=C
bond is necessary for hydrogenation with this catalyst system.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
2. RESULTS AND DISCUSSION
Hydrogenation of Olefins. The present investigation was initiated by employing iron
complexes 1, 3, and 4 as homogeneous catalysts (or catalyst precursors) for the hydrogenation of
styrene. At 23 °C under 1 atm of H2 pressure, styrene was quantitatively hydrogenated to form
ethylbenzene with 5 mol% of either catalyst 1 or 3 within a day (Table 1). In the case of the iron
complex 3, a slight excess of KOtBu (3 equiv. with respect to the iron catalyst) was required to
initiate catalysis (entry 2), but it gives results identical to those seen with 1 as catalyst. No
catalytic activity was observed in the absence of a base. On the other hand, catalyst 1 did not
require any base to carry out the hydrogenation reaction. The iron hydridoꢀborohydride complex
(4) was reported to be the best catalyst for ester18 and nitrile hydrogenation19 at higher
temperatures (100 – 135 °C). When this catalyst was used for the hydrogenation of styrene at
ACS Paragon Plus Environment
4

Page 5 of 27
ACS Catalysis
1
2
Table 1. Ironꢀcatalyzed hydrogenation of styrene.a
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
entry
catalyst (loading)
pH2
(atm)
time (h)
temp. (°C)
NMR
conv. (%)
1
1 (5 mol %)
3 (5 mol %)
+ KOtBu (15 mol %)
3 (5 mol %)
4 (5 mol %)
4 (5 mol %)
1 (2 mol %)
ꢀ
1
1
24
24
23
23
100
100
2b
3c
4
1
1
1
1
1
24
24
24
48
24
23
23
0
37
86
23
0
5
100
23
6
7
23
asolvent: ~0.7 mL of C6D6, monitored over time by 1H NMR spectroscopy;
bsolvent: 1 mL of THF; cno base added.
23 °C (1 atm H2), only 37% conversion was achieved after one day. From the current
mechanistic understanding of the hydrogenation reactions involving these iron catalysts,20 it is
proposed that complex 1 is one of the active intermediates in all of these reactions. But
generating this species from the iron hydridoꢀborohydride preꢀcatalyst requires either heating or
a BH3ꢀtrapping reagent like trimethylamine.21 Therefore, the low catalytic activity of complex 4
is attributed to the lack of BH3 dissociation at room temperature, so the active iron dihydride
catalyst is not formed.
ACS Paragon Plus Environment
5

ACS Catalysis
Page 6 of 27
1
2
o
3
To probe this hypothesis, hydrogenation of styrene was carried out at 100 C using catalyst 4;
in this case a much higher conversion was achieved after one day (86%, entry 5). From these
catalytic studies, it was evident that complex 1 was the best catalyst for the hydrogenation of
styrene under these conditions. However, when the catalyst loading of 1 was reduced from 5
mol% to 2 mol%, only 23% of styrene was converted to ethylbenzene after two days, suggesting
that the reaction has a strong dependence on the concentration of the iron catalyst. A control
experiment in the absence of an iron catalyst did not produce any ethylbenzene indicating that
this reaction is indeed catalytic in iron. The solution stayed homogeneous during the course of
the reaction with no visible particles being formed. A mercury poisoning test did not affect the
hydrogenation activity.22,23 However, mercury does not always inhibit iron nanoparticle
catalysts.24 We also observed reproducible kinetic behavior with no induction period for this
catalytic process which is supportive of the homogeneous nature of the catalyst.22b Furthermore,
a successive addition experiment was also performed to reinforce the homogeneity of the
catalytic system.25
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Next, the alkene substrate scope was expanded to investigate the electronic and steric factors
involved in this reaction (Table 2). Under standard catalytic conditions (5 mol% 1, 23 °C, 1 atm
H2), styrene derivatives containing electronꢀwithdrawing substituents reacted much faster than
both the parent styrene and styrenes with electronꢀdonating groups. For example, 4ꢀ(CF3)ꢀ
styrene was quantitatively hydrogenated in 1.5 hours, but styrene and 4ꢀ(CH3)ꢀstyrene were
completed in 24 h and 48 h, respectively. For an even more electron rich substrate, 4ꢀ(OCH3)ꢀ
styrene, no hydrogenation product was formed at 23 °C and a higher temperature (60 °C) and a
much longer reaction time of 168 h were required to achieve full conversion (entry 3). These
results suggest that the hydrogenation reactions are strongly dependent on the electronic
ACS Paragon Plus Environment
6

Page 7 of 27
ACS Catalysis
1
2
Table 2. Ironꢀcatalyzed hydrogenation of alkenes.a
3
4
5
6
7
entry
substrate
product
time (h)
temperature
NMR conv.
(%)
8
9
1
2
3
24
48
23 °C
23 °C
100
96
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
168
60
°C
100
MeO
MeO
4
5
6
7
0.3
0.7
53
1
23 °C
23 °C
23 °C
23 °C
100
100
30
F3C
MeO2C
NC
F3C
MeO2C
NC
N
N
100
8
9
240
0.8
60
°C
100
100
Ph
Ph
Ph
Ph
O
O
23 °C
OMe
OMe
10
11
144
2.5b
80
°C
44
O
OH
23 °C
100
a
Conditions: 0.7 mL of C6D6; [1] = 0.014 M; [alkene] = 0.285 M; catalytic reactions were
performed in a J. Young NMR tube with a large head–space (~9 mL).
b Time to reduce both alkene and ketone.
properties of the substrate, i.e., the polarity of the C=C double bond. DFT calculations, reported
below, will investigate the electronic effect of the substrates on the rate of hydrogenation. These
ACS Paragon Plus Environment
7

ACS Catalysis
Page 8 of 27
1
2
hydrogenation reactions were found to follow good first order kinetics (R2 > 0.997), with rate
constants of 8.31(5) x 10–5, 2.18(7) x 10–5, and 8.8(9) x 10–4 s–1 for styrene, 4ꢀmethylstyrene, and
4ꢀtrifluoromethylstyrene, respectively (Figure 2).
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
100
90
80
70
60
50
40
30
20
10
0
0
50000
100000
time, s
150000
200000
Figure 2. Kinetic traces for the hydrogenation of styrenes at 23 °C by 1. ꢀ= 4ꢀCF3ꢀstyrene; ꢁ
= styrene; ꢂ= 4ꢀMeꢀstyrene. Conditions: [1] = 0.014 M, [styrene] = 0.285 M, 1 atm of H2, 0.7
mL C6D6, 23 °C. Lines are first order fits.
The rate of hydrogenation was increased for 4ꢀ(CO2Me)ꢀstyrene, consistent with rate effects of
other electronꢀwithdrawing paraꢀsubstituents (entry 5, Table 2). Notably, under 1 atm of H2
pressure and at room temperature, hydrogenation of the C=C double bond occurred in a
chemoselective fashion as the ester functionality remained intact after the reaction.18a Similarly,
methyl methacrylate, a nonꢀbenzylic alkene containing a polar ester group adjacent to the C=C
double bond, readily yielded the desired hydrogenation product methyl isobutyrate, keeping the
ester moiety intact (entry 9). Although the –CN group is generally considered to be an electronꢀ
withdrawing group, pꢀCNꢀstyrene reacted much more sluggishly and only reached ~30%
conversion after 53 hours. As mentioned earlier, Beller and coworkers have reported nitrile
hydrogenation with the same iron catalyst under more forcing conditions;19 however nitrile
hydrogenation product was not observed under these mild reaction conditions with 1. The slow
ACS Paragon Plus Environment
8

Page 9 of 27
ACS Catalysis
1
2
3
4
5
6
7
8
9
rate of hydrogenation observed in the present case could be a result of catalyst inhibition by
substrate coordination to the fiveꢀcoordinate iron center in 1. In fact, when a stoichiometric
amount of pꢀ(CN)ꢀstyrene was added to a C6D6 solution of 1, there was significant broadening of
the resonances of the vinyl group of the styrene, suggesting an interaction between the iron
complex and the substrate double bond (eq 1). An IR spectrum of a similar reaction in THF
showed a CN stretch that was identical to that of free 4ꢀcyanostyrene in THF (2228 cm–1),
arguing against coordination through the nitrile. Based on these results, we conclude that under
catalytic conditions of excess pꢀ(CN)ꢀstyrene substrate, the substrate can coordinate to
(PNPiPr)Fe(CO)(H) through the double bond and thereby inhibit catalysis. This effect is not seen
with excess styrene, which is likely due to the olefin of pꢀ(CN)ꢀstyrene being a better πꢀacceptor
ligand than styrene.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
The pyridine analogue of styrene, 2ꢀvinylpyridine, was also hydrogenated at a much faster rate
than styrene, affording 2ꢀethylpyridine quantitatively within 1 h (entry 7, Table 2). It is thought
that the presence of the electronegative nitrogen atom adjacent to the C=C double bond in 2ꢀ
vinylpyridine increases the polarity of the alkene C=C bond and therefore the reaction proceeds
with a faster rate than styrene. Notably, no product resulting from the hydrogenation of the
pyridine ring was detected under these conditions even though 2 has been shown to hydrogenate
2,6ꢀdimethylpyridine (3 mol% 3, 10 mol% KOtBu, 80 °C, 10 atm H2).15 Remarkably, catalytic
ACS Paragon Plus Environment
9

ACS Catalysis
Page 10 of 27
1
2
3
4
5
6
hydrogenation of 2ꢀvinylpyridine could also be performed under neat conditions affording 100%
conversion in four hours.
7
8
9
If only electronic effects dictate the outcome of the hydrogenation activity, then installing a
second phenyl ring at the αꢀposition of styrene should in theory increase the rate of reaction.
However, for such a substrate, no reactivity was observed at room temperature and in fact it
reacted much more slowly even at a higher temperature (entry 8). We rationalize this behavior on
the basis of the steric crowding imposed by the second phenyl ring at the alkene moiety which
likely increases the related kinetic barrier. An internal alkene, βꢀmethylstyrene, also reacted
slowly possibly for both steric and electronic reasons.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Although several research groups have independently performed hydrogenation of carbonyl
groups utilizing these iron catalysts,16,18,19 it was of interest to examine a substrate, benzalacetone
(entry 11, Table 2), which contains both C=O and C=C bonds to determine if the hydrogenation
reaction would proceed in a chemoselective fashion at room temperature (23 °C) and low
hydrogen pressure (1 atm) in benzene. Interestingly, when benzalacetone was subjected to
catalysis, a mixture containing the starting material (a), two partially reduced species (b and c),
and the fully hydrogenated species (d) resulted in a relative ratio of 1:1:10:7 after 15 min with an
overall ~95% conversion of the reactant (Figure 3). With a longer reaction time, b and c both
converted to the fully reduced species d. Although both C=O and C=C bonds are ultimately
hydrogenated by this iron catalyst, the product distribution at the early stage of the reaction
suggests that the rate of hydrogenation of the C=C bond is much slower than that of the C=O
bond.
ACS Paragon Plus Environment
10

Page 11 of 27
ACS Catalysis
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Figure 3. Ironꢀcatalyzed hydrogenation of benzalacetone at different times. Conditions: [1] =
0.014 M, [benzalacetone] = 0.285 M, 1 atm of H2, 0.7 mL C6D6, 23 °C.
Under 1 atm of H2 pressure, hydrogenation of nonꢀpolar aliphatic alkenes such as 1ꢀhexene
and tertꢀbutylethylene did not occur in the presence of catalyst 1 (Figure 4). Attempts to
hydrogenate these substrates even under a higher H2 pressure (5 atm) and higher temperatures
(100 ꢀ 150 °C) were not successful. Nonetheless, these results are consistent with our hypothesis
that the C=C double bond needs to be sufficiently polarized in order to interact with the active
iron dihydride complex. DFT calculations support the hypothesis that the activation barriers of
nonꢀpolarized (ethylene) C=C bonds are higher (vide infra). In the case of 1ꢀhexene, products
resulting from the isomerization of the terminal C=C double bond were not observed. Aliphatic
alkenes containing functional groups such as –OH and –CO2H were also not viable substrates as
these groups reacted with the fiveꢀcoordinate iron center present in complex 1 to generate the
catalytically inactive iron alkoxide or carboxylate complexes, respectively (Figure 4).26 Although
Hazari, Schneider, and Bernskoetter have reported increased rates for ironꢀcatalyzed
dehydrogenation and hydrogenation reactions in the presence of a Lewis acidic coꢀcatalyst such
ACS Paragon Plus Environment
11

ACS Catalysis
Page 12 of 27
1
2
as LiBF4 or LiOTf,27 such effects were not observed in the alkene hydrogenation reaction with
3
4
5
6
iron catalyst 1.
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Figure 4. Incompatible hydrogenation substrates.
Hanson and coworkers have reported nickel11 and cobalt28 catalysts, supported by a similar
PNHPCy ligand (Cy = cyclohexyl), for alkene hydrogenation. Interestingly, for both the nickel
and cobaltꢀbased systems, hydrogenation of alkenes proceeded smoothly without the
involvement of the protic N–H moiety in the pincer ligand indicating that the PNHPCy ligand is
not participating in these hydrogenation reactions. For the nickel system, migratory insertion of
1ꢀoctene into the Ni–H bond generated the corresponding linear nickelꢀoctyl species, which was
isolated and characterized.11 The insertion proved to be reversible as the parent nickel hydride
complex reformed upon heating the nickelꢀoctyl complex to 80 °C. The nickel hydride species,
in the absence of H2, also catalyzed isomerization of 1ꢀoctene to form internal octene isomers at
80 °C. In the case of the cobalt system, a similar alkene isomerization activity was observed in
the absence of dihydrogen; however, formation of the corresponding cobalt alkyl species via the
insertion of a C=C double bond into a Co–H bond was not experimentally observed. As the N–H
moiety on the pincer ligand is not involved in the hydrogenation of alkenes, a direct
hydrogenolysis of nickel or cobalt alkyl intermediates was proposed to release the product during
catalysis.
ACS Paragon Plus Environment
12

Page 13 of 27
ACS Catalysis
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Scheme 1. Experiments to support a metalꢀligand cooperative pathway.
In contrast to the aforementioned cobalt and nickel complexes, when complex 1 or inꢀsitu
generated iron dihydride complex (2) was treated with 1ꢀhexene (1 : 5 ratio), neither an iron
alkyl intermediate nor the internal hexene isomers were formed (eq 3, Scheme 1). When a
stoichiometric reaction between 1 and styrene was performed, no insertion product was
generated at room temperature (eq 2). Chemical shifts associated with 1 and styrene did not
change, indicating there is minimal interaction between these two species. On the other hand,
the reaction between 2 and styrene quantitatively produced ethyl benzene and 1 at room
temperature (eq 4). No iron alkyl species was detected during this reaction (Scheme 1). These
results seem to indicate that the mechanism of the C=C double bond hydrogenation using the
iron system differs from the nickel and cobalt systems discussed above. To test if the iron
system operates via a metalꢀligand cooperative pathway, we synthesized the related PNMeP
derivative of one of the active iron catalysts (4-Me)18b,29 and investigated its catalytic activity. 4-
ACS Paragon Plus Environment
13

ACS Catalysis
Page 14 of 27
1
2
3
4
5
6
7
8
9
Me turned out to be catalytically inactive in the hydrogenation of styrene under 1 atm of H2
pressure at 100 oC (eq 5, Scheme 1).30 No ethyl benzene was formed after 24 h. This result is in
contrast to what has been previously observed for the related nickel and cobalt systems and
implies that these iron catalysts act in a bifunctional pathway even for a much lessꢀpolar C=C
double bond as compared to the more highly polarized C=O and C=N bonds. Further discussion
of this point is presented below in the context of DFT simulations.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
DFT Calculations of the Mechanism. The mechanism of styrene hydrogenation with the
truncated complex (PNPMe–H)Fe(H)2(CO) ((PMe2)2 -with methyl substituents on the phosphines)
was modeled using a DFT level of theory similar to that used in simulating the mechanism of
alcohol dehydrogenation by Chakraborty and coꢀworkers.16 Truncation of the ligand is not
expected to have a substantial influence on the activation barriers due to minimal steric
hindrance between the flanking phosphine substituents and the βꢀCH2 of the styrene substrates.
Previous mechanistic studies of the hydrogenation and dehydrogenation of substrates with
complex (PMe2)2 were proposed to occur via a concerted, bifunctional H–/H+ transfer mechanism
through a 6ꢀmembered transition state, i.e., a Noyoriꢀstyle H2 transition state.4 Interestingly, a
transition state for the concerted transfer of the NHꢀproton and FeHꢀhydride for styrene could not
be found in the present case. Instead, a transition state for Fe–H transfer (“hydride transfer”) to
the βꢀcarbon of the substrate was found (TS-A) with an imaginary vibrational mode at 545i cm–1
corresponding to the Fe–H bond breaking and the C–H bond forming. The hydride transfer
activation barrier in benzene is ꢁG‡ = 14.1 kcal/mol and ꢁH‡ = 0.9 kcal/mol. An intrinsic
reaction coordinate (IRC) calculation of this transition state revealed formation of an ionic
intermediate complex (int-C) with a sigmaꢀcomplexed C–H bond (C–H = 1.22 Å). The
intermediate is endergonic relative to the starting materials by only 13.3 kcal/mol in free energy.
ACS Paragon Plus Environment
14

Page 15 of 27
ACS Catalysis
1
2
3
4
5
6
7
8
9
The separation of these charged species is highly endergonic in benzene at ꢁG = 44.7 kcal/mol
(ꢁH = 58.4 kcal/mol). Note that in continuum THF solvent, the separation of charged species is
also endergonic, but much less so than calculated in benzene, ꢁG = 15.4 kcal/mol (ꢁH = 29.1
kcal/mol) for THF. After int-C, the transition state for the N–H proton transfer (TS-B) was
located with a single imaginary vibrational mode at 792i cm–1 corresponding to N–H bond
breaking and C–H bond forming. This barrier is ꢁG‡ = 14.0 kcal/mol (ꢁH‡ = 0.5 kcal/mol) from
the starting materials, and thus 1.0 kcal/mol lower in energy than int-C. An IRC calculation
indicated that this transition state connects int-C to (PMe2)1 + ethylbenzene. The stability of the
wavefunction of each complex was checked to ensure it is a true minimum energy solution. The
present result indicates that the potential energy surface for proton transfer is very flat and that
although the bifunctional hydride/proton transfer occurs through a calculated stepwise
mechanism, the second step is facile, essentially making this an asynchronous ionic
hydrogenation (Figure 5).31 With an electron withdrawing paraꢀNO2 substituent on styrene, the
PES (Figure S1b) becomes less shallow with a small barrier to proton transfer of ꢁG‡ = 3.6
kcal/mol (ꢁH‡ = −3.2 kcal/mol). With an electron donating paraꢀNMe2 substituent on styrene,
the barrier to proton transfer (Figure S1a) becomes even more endergonic from the ionic
intermediate, ꢁG‡ = −1.3 kcal/mol (ꢁH‡ = −1.9 kcal/mol). Comparing the relative energies of the
ionic intermediates to the hydride and proton transfer transition states suggests that the ionic
intermediate becomes less stable (increases in free energy) with electron donating substituents on
the para position of the styrene phenyl moiety.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
ACS Paragon Plus Environment
15

ACS Catalysis
Page 16 of 27
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Figure 5. Calculated potential energy diagram for the hydrogenation of styrene in benzene
(black) and THF (red) by (PMe2)2. Free energies and (enthalpies) are given in kcal/mol at STP.
Table 3 presents the calculated barriers for hydride transfer from (PMe2)2 to paraꢀXꢀstyrene
substrates where X = NMe2, OMe, Me, H, CF3, CN, and NO2. The transition state structure of
hydride transfer significantly changes with the electronic character of the styrene substrate. As
the paraꢀsubstituent becomes more electron withdrawing, the FeHꢂꢂꢂCβ and NHꢂꢂꢂCα distances
increase, thereby yielding even earlier transition states (Figure 6). This indicates that the more
polarized the C=C bond, the earlier the TS, and the lower the activation barrier for
hydrogenation. The Hammett plot in Figure 7 shows a strong correlation between the electronic
influence of the Xꢀsubstituent and the rate where the calculated ρ = 3.00 in the gasꢀphase, ρ =
3.13 in benzene, and ρ = 3.65 in THF. Overall, hydrogenation activation barriers decreased
markedly as the substituent became more electron withdrawing, which is consistent with
experimental results. The increase in polarity of the solvent increases the influence of the paraꢀ
substituent because of the significant electronic charge buildup present in the transition state. For
example, the activation barriers in benzene (ε = 2.27) are less than those in THF (ε = 7.43) by ca.
ACS Paragon Plus Environment
16

Page 17 of 27
ACS Catalysis
1
2
3
4
5
6
7
8
9
2 kcal/mol. The increase in activation barrier in a more polar solvent may be due to the early
transition state of hydride transfer. The βꢀcarbon of styrene must be polarized to have significant
partial positive charge for the hydride to transfer. This partial positive site may be stabilized by
the polar, THF solvent, thus decreasing the carbon’s electrophilicity and increasing the activation
barrier for hydride transfer. This conclusion was also seen experimentally. Sideꢀbyꢀside reaction
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
1
of 1 with styrene and H2 (1 atm) in C6D6 vs. THFꢀd8 were monitored by H NMR spectroscopy
at 23 °C. The first order rate constants were 9.6 (3) x 10–5 s–1 and 4.4 (1) x 10–5 s–1, respectively
(see Supporting Information).
Table 3. Calculated activation parameters for hydrogenation of pꢀXꢀstyrene substrates in
kcal/mol.
ꢁG‡ (ꢁH‡), ꢁG‡ (ꢁH‡), ꢁG‡ (ꢁH‡),
32
σp
X
gas
THF
benzene
17.5 (3.8)
15.8 (2.6)
14.6 (1.6)
–0.83
–0.27
–0.17
0
NMe2
OMe
Me
14.0 (0.3)
19.5 (5.8)
12.6 (−0.6) 17.6 (4.4)
11.4 (−1.5) 16.4 (3.4)
H
11.2 (−2.1) 15.9 (−2.3) 14.1 (0.9)
9.4 (−3.9) 14.0 (0.7) 12.4 (−1.0)
+0.54
+0.66
+0.78
CF3
CN
8.1 (−4.8) 12.3 (−0.6) 12.1 (−0.8)
7.2 (−5.7) 11.1 (−1.8) 9.7 (−3.1)
NO2
ACS Paragon Plus Environment
17

ACS Catalysis
Page 18 of 27
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Figure 6. Plot of CꢂꢂꢂH distances in hydride transfer transition states between the alpha and beta
carbon of pꢀXꢀstyrene substrates and the N–H proton and Fe–H hydride of (PMe2)2, respectively.
Figure 7. Calculated Hammett plot in gasꢀphase (black, y = 3.00x + 0.17), benzene (blue, y =
3.13x + 0.06), and THF (red, y =3.65x + 0.10).
The present calculations indicate that the polarizability of the C=C bond significantly
influences the activation barrier. Therefore, it is of interest to ask, why doesn’t 2 hydrogenate
less polarized C=C bonds, e.g., that of ethylene or 1ꢀhexene? The hydrogenation of ethylene was
calculated to have an activation barrier of ꢁG‡ = 19.1 kcal/mol (ꢁH‡ = 7.4 kcal/mol) in
continuum benzene solvent with an imaginary frequency of 456i cm–1. It is interesting that this
barrier is only 5 kcal/mol higher in free energy than the barrier to hydrogenation of styrene, yet
aliphatic monoꢀolefins cannot be hydrogenated. Even more interesting, the IRC calculation of the
ethylene hydrogenation transition state revealed a concerted transfer of the hydride and proton,
ACS Paragon Plus Environment
18

Page 19 of 27
ACS Catalysis
1
2
3
4
5
6
7
8
9
suggesting a different mechanism for ethylene hydrogenation than for the styrene
hydrogenations. Based on these differences between ethylene and styrene hydrogenation, we
propose that C=C hydrogenation only occurs when there is sufficient polarization of the double
bond to permit the stepwise hydrogenation by hydride transfer followed by proton transfer.
Calculations of the hydrogenation of O=C bonds show a hydrogen bonded adduct between
O=CH2 and (PNPMe–H)Fe(H)2(CO) only 1.1 kcal/mol lower in free energy than the concerted
hydrogenation TS.16 For Noyori’s catalyst, hydrogen bonded adducts between X=C (X = N, O)
and the model complex (C6H6)Ru(H)(H2N(CH2)2O) are more stable than the starting materials.13
This initial outerꢀsphere interaction between the transꢀdihydride complex and the X=C substrate
may be the first necessary step for hydrogenation to proceed.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
3. CONCLUSIONS
In summary, we have shown that the fiveꢀcoordinate iron complex (1) bearing a
bis(phosphino)amine ligand can effectively catalyze the baseꢀfree hydrogenation of a variety of
alkenes under mild conditions. Polarity of the C=C double bond seems to be critical and
therefore a large electronic influence on the rate of the hydrogenation of different styrene
derivatives was observed. This catalytic system is tolerant of other reducible functional groups
such as –CO2Me, –CN, Nꢀheterocycle and the catalytic reaction can be performed under solventꢀ
free conditions. Unlike the similar nickel and cobalt systems developed by Hanson and
coworkers,11,28 the iron catalyst described in this report operates via a metalꢀligand cooperative
pathway via a stepwise hydride transfer then proton transfer mechanism.
4. EXPERIMENTAL DETAILS
General Experimental Information. Unless otherwise noted, all of the organometallic
compounds were prepared and handled under a nitrogen atmosphere using standard Schlenk and
ACS Paragon Plus Environment
19

ACS Catalysis
Page 20 of 27
1
2
3
4
5
6
7
8
9
glovebox techniques. Dry and oxygenꢀfree solvents (pentane) were collected from an Innovative
Technology PSꢀMDꢀ6 solvent purification system and used throughout the experiments. THF,
toluene, C6D6, and THFꢀd8 were vacuum distilled from purple solutions of Na and benzophenone
under a nitrogen atmosphere. All other commercially available chemicals were used as received.
1H NMR and 31P NMR spectra were recorded on a Bruker Avanceꢀ400 MHZ or Avanceꢀ500
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
1
MHz spectrometers. Chemical shift values in H NMR spectra were referenced internally to the
residual solvent resonances. 31P NMR spectra were referenced externally to 85% H3PO4 (0 ppm).
[PNPiPr]Fe(H)(CO) (1), [PNHPiPr]Fe(H)(CO)Br (3), [PNHPiPr]Fe(H)(CO)(HBH3) (4), and
[PNMePiPr]Fe(H)(CO)(HBH3) (4-Me) were prepared according to previously published
procedures.18a,18b,33
General Procedure for the Hydrogenation of Alkenes. In a typical catalytic experiment,
complex 1 (4 mg, 10 ꢁmol), a substrate (0.2 mmol), and 0.7 mL of C6D6 were placed in a J.
Young NMR tube adapted with a 9 mL cylindrical bulb just below the valve. Before passing H2
1
into this reaction mixture, a H NMR spectrum was recorded. Then the NMR tube was
thoroughly degassed by performing three freezeꢀpumpꢀthaw cycles and 1 atm of H2 was
introduced. The resulting solution was mixed either at 23 °C or at a higher temperature and the
1
progress of the reaction was monitored periodically by H NMR spectroscopy. The percentage
1
conversion values were obtained from the relative H NMR integrations of the reactants and
products.
Stoichiometric Reaction between Complex 1 and Styrene. In a glovebox, a J. Young NMR
tube was charged with complex 1 (8 mg, 20 ꢁmol), styrene (2.3 ꢁL, 20 ꢃmol) and 0.7 mL of
C6D6. The resonances associated with the iron complex and styrene did not change at all
indicating that there is no molecular interaction between these two species at room temperature.
ACS Paragon Plus Environment
20

Page 21 of 27
ACS Catalysis
1
2
3
4
5
6
7
8
9
Stoichiometric Reaction between Complex 1 and 4-Cyanostyrene. 10 mg (26 ꢁmol) of
complex 1 was introduced into a JꢀYoung tube. 3.3 mg (0.11 mmol) 4ꢀcyanostyrene was diluted
in 0.5ml C6D6 in a small vial, then this solution was added to the JꢀYoung tube. About 0.5 mL
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
1
C6D6 was added which was used to wash the vial contents into the Jꢀyoung tube. A H NMR
spectrum showed that the olefinic resonances of the substrate broadened substantially (See
Supporting Information).
Side-by-side Reaction of 1 with Cyanostyrene in C6D6 and THF-d8. 4 mg (12.3 ꢀmol) of 1
was measured in a vial. C6D6 was used to dissolve the catalyst and wash the vial contents into a
JꢀYoung NMR tube. The total volume of C6D6 was 0.7 mL. 23.5 ꢀL (0.205 mmol) styrene was
added and the sample placed under 1 atm H2 following freezeꢀpumpꢀthaw degassing. The
1
progress of the reaction was monitored by H NMR spectroscopy. A similar sample was
prepared using 0.75 mL THFꢀd8, 4.3 mg (11.0 ꢀmol) 1, and 25.3 ꢀL (0.221 mmol) styrene and
monitored by 1H NMR spectroscopy. The samples were slowly rotated to invert the sample and
1
mix the liquid with the H2 gas at 23 °C. The rate of reaction was monitored by H NMR
spectroscopy.
Stoichiometric Reaction between the Iron Dihydride Species and Styrene. H2 (1 atm) was
passed to a J. Young NMR tube containing a red C6D6 solution of complex 1 (8 mg, 20 ꢁmol).
The formation of the transꢀFe(H)2 and cisꢀFe(H)2 (62 : 38) was supported by H and 31P{1H}
1
NMR spectroscopy and the color of the solution turned yellow. To this solution, styrene (2.3 ꢁL,
20 ꢁmol) was quickly added via a syringe and the resulting mixture was mixed at 23 °C. Over a
period of 14 h, the color of the yellow solution changed to orange and H and 31P{1H} NMR
1
spectra recorded at this point showed a mixture containing ethyl benzene, 1, and iron dihydride
species. Styrene was completely consumed at this point.
ACS Paragon Plus Environment
21

ACS Catalysis
Page 22 of 27
1
2
3
4
5
6
7
8
9
Iron-Catalyzed Hydrogenation of 2-vinylpyridine under Neat Conditions. In a glovebox,
complex 1 (4 mg, 10 ꢁmol) and 2ꢀvinylpyridine (21.6 ꢁL, 0.2 mmol) were placed in a J. Young
NMR tube. H2 (1 atm) was introduced to this reaction mixture after performing freezeꢀpumpꢀ
thaw cycles. The resulting solution was mixed at 23 °C for 4 h, ~0.6 mL of C6D6 was added, and
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
1
a H NMR spectrum was recorded to calculate the percent conversion. Quantitative conversion
of 2ꢀvinylpyridine to 2ꢀethylpyridine was observed under these conditions.
Isomerization of 1-Hexene. In a glovebox, complex 1 (4 mg, 10 ꢁmol) or inꢀsitu formed 2
(10 ꢁmol), 1ꢀhexene (0.2 mmol, 24.8 ꢁL), and 0.7 mL of C6D6 were placed in a J. Young NMR
tube equipped with a Teflon stopcock. The reaction mixture was heated to 60 °C for 1 day and
1
monitored by H NMR spectroscopy. No isomerization reaction leading to the formation of
internal isomers was observed during this time period.
Computational Details. All calculations were performed using Gaussian0934 using a DFT
level of theory similar to that used in calculating the mechanism of alcohol dehydrogenation.16
Geometry optimizations were performed in the gasꢀphase with the B3LYP functional and def2ꢀ
SVP basis set and enthalpy and free energy corrections were obtained at 298 K using unscaled
vibrational frequencies. Single point calculations were performed on the gasꢀphase optimized
structure with the B3PW91/def2ꢀQZVPP level of theory including the GD3 parameters for
empirical dispersion. Solvent corrections were performed at optimized stationary points
(B3PW91/def2ꢀQZVPP) with the SMD solvent model35 for benzene (ε = 2.3) and
tetrahydrofuran (ε = 7.4). Free energies are reported in kcal/mol at STP unless otherwise noted.
See Supporting Information for a sample input file.
ACS Paragon Plus Environment
22

Page 23 of 27
ACS Catalysis
1
2
3
4
ASSOCIATED CONTENT
5
6
7
8
Supporting Information. Spectroscopic data for reactions are included. Kinetic data and fits,
and xyz coordinates of calculated structures with single point energies (in Hartrees) and free
energy and enthalpy corrections. This material is available free of charge via the internet at
http://pubs.acs.org.
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
AUTHOR INFORMATION
Corresponding Author
*Eꢀmail: jones@chem.rochester.edu
Author Contributions
‡These authors (R. X., S. C., and S. M. B.) contributed equally. The manuscript was written
through contributions of all authors. All authors have given approval to the final version of the
manuscript.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENT
This work was funded by the NSF under the CCI Center for Enabling New Technologies through
Catalysis (CENTC), CHEꢀ1205189. S.M.B. thanks the University of North Texas Center for
Advanced Scientific Computing and Modeling (CASCaM) for access to the CASCaM
supercomputer, the purchase of which was supported by grant CHEꢀ0741936.
ACS Paragon Plus Environment
23

ACS Catalysis
Page 24 of 27
1
2
3
4
References and Notes
5
6
7
8
9
(1) Catalysis without Precious Metals; R. M. Bullock, Ed.; WileyꢀVCH: Hoboken, NJ, 2010.
(2) Osborn, J. A.; Jardine, F. H.; Young, J. F.; Wilkinson, G. J. Chem. Soc. A 1966, 1711ꢀ1732.
(3) Crabtree, R. Acc. Chem. Res. 1979, 12, 331ꢀ337.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
(4) Noyori, R.; Yamakawa, M.; Hashiguchi, S. J. Org. Chem., 2001, 66, 7931ꢀ7944.
(5) Wang, D.; Astruc, D. Chem. Rev. 2015, 115, 6621ꢀ6686.
(6) Werkmeister, S.; Neumann, J.; Junge, K.; Beller, M. Chem. Eur. J. 2015, 21, 12226ꢀ12250.
(7) Chirik, P. J. Acc. Chem. Res. 2015, 48, 1687ꢀ1695.
(8) Friedfeld, M. R.; Shevlin, M.; Hoyt, J. M.; Krska, S. W.; Tudge, M. T.; Chirik, P. J.
Science 2013, 342, 1076ꢀ1080.
(9) Lin, T.ꢀP.; Peters, J. C. J. Am. Chem. Soc. 2014, 136, 13672ꢀ13683.
(10) Friedfeld, M. R.; Margulieux, G. W.; Schaefer, B. A.; Chirik, P. J. J. Am. Chem. Soc.
2014, 136, 13178ꢀ13181.
(11) Vasudevan, K. V.; Scott, B. L.; Hanson, S. K. Eur. J. Inorg. Chem. 2012, 4898ꢀ4906.
(12) Srimani, D.; DiskinꢀPosner, Y.; BenꢀDavid, Y.; Milstein, D. Angew. Chem. Int. Ed. 2013,
52, 14131ꢀ14134; Angew. Chem. 2013, 125, 14381ꢀ14384.
(13) Yamakawa, M.; Ito, H.; Noyori, R., J. Am. Chem. Soc. 2000, 122, 1466ꢀ1478.
(14) (a) Mikhailine, A. A.; Maishan, M. I.; Morris, R. H. Org. Lett., 2012, 14, 4638–4641. (b)
Mikhailine, A. A.; Maishan, M. I.; Lough, A. J.; Morris, R. H. J. Am. Chem. Soc., 2012, 134,
12266–12280. (c) Lagaditis, P. O.; Lough, A. J.; Morris, R. H. J. Am. Chem. Soc., 2011, 133,
ACS Paragon Plus Environment
24

Page 25 of 27
ACS Catalysis
1
2
3
4
5
6
7
8
9662–9665. (d) Lagaditis, P. O.; Lough, A. J.; Morris, R. H. Inorg. Chem., 2010, 49, 10057–
10066. (e) Mikhailine, A. A.; Lough, A. J.; Morris, R. H. J. Am. Chem. Soc., 2009, 131, 1394–
1395.
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
(15) Chakraborty, S.; Brennessel, W. W.; Jones, W. D. J. Am. Chem. Soc. 2014, 136, 8564ꢀ
8567.
(16) Chakraborty, S.; Lagaditis, P. O.; Förster, M.; Bielinski, E. A.; Hazari, N.; Holthausen, M.
C.; Jones, W. D.; Schneider, S. ACS Catal. 2014, 4, 3994ꢀ4003.
(17) Hasanayn, F.; Baroudi, A.; Bengali, A. A.; Goldman, A. S. Organometallics 2013, 32,
6969ꢀ6985.
(18) (a) Chakraborty, S.; Dai, H.; Bhattacharya, P.; Fairweather, N. T.; Gibson, M. S.; Krause,
J. A.; Guan, H. J. Am. Chem. Soc. 2014, 136, 7869ꢀ7872. (b) Werkmeister, S.; Junge, K.; Wendt,
B.; Alberico, E.; Jiao, H.; Baumann, W.; Junge, H.; Gallou, F.; Beller, M. Angew. Chem., Int. Ed.
2014, 53, 8722ꢀ8726. (c) Chakraborty, S.; Dai, H.; Bhattacharya, P.; Fairweather, N. T.; Gibson,
M. S.; Krause, J. A.; Guan, H. Homogeneous Hydrogenation of Esters Employing a Complex of
Iron as Catalyst, U.S. patent application with serial number 61/972927, 2014. (d) Fairweather,
N. T.; Gibson, M. S.; Guan, H. Organometallics 2015, 34, 335ꢀ339.
(19) Bornschein, C.; Werkmeister, S.; Wendt, B.; Jiao, H.; Alberico, E.; Baumann, W.; Junge,
H.; Junge, K.; Beller, M. Nat. Commun. 2014, 5, 4111ꢀ4122.
(20) Chakraborty, S.; Bhattacharya, P.; Dai, H.; Guan, H. Acc. Chem. Res. 2015, 48, 1995ꢀ
2003.
ACS Paragon Plus Environment
25

ACS Catalysis
Page 26 of 27
1
2
3
4
5
6
(21) Qu, S.; Dai, H.; Dang, Y.; Song, C.; Wang, Z.ꢀX.; Guan, H. ACS Catal. 2014, 4, 4377ꢀ
7
8
4388.
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
(22) (a) Widegren, J. A.; Finke, R. G. J. Mol. Catal. A: Chem. 2003, 198, 317ꢀ341. (b)
Crabtree, R. H. Chem. Rev. 2012, 112, 1536ꢀ1554.
(23) A PMe3 poisoning experiment was not conducted, as complex 4 itself decomposed in the
presence of excess of PMe3, and therefore, the result from this experiment would be
inconclusive.
(24) (a) Sonnenberg, J. F.; Coombs, N.; Dube, P. A.; Morris, R. H. J. Am. Chem. Soc. 2012,
134, 5893ꢀ5899. (b) Sonnenberg, J. F.; Morris, R. H. Catal. Sci. Technol. 2014, 4, 3426ꢀ3438.
(25) After the first catalytic run, additional styrene (0.2 mmol) was introduced and the
hydrogenation reaction continued. Catalytic activity of 1 was found to be fully retained and
hydrogenation of styrene followed the same kinetic profile in the second run.
(26) Bielinski, E. A.; Förster, M.; Zhang, Y.; Bernskoetter, W. H.; Hazari, N.; Holthausen, M.
C. ACS Catal. 2015, 5, 2404ꢀ2415.
(27) (a) Bielinski, E. A.; Lagaditis, P. O.; Zhang, Y.; Mercado, B. Q.; Würtele, C.;
Bernskoetter, W. H.; Hazari, N.; Schneider, S. J. Am. Chem. Soc. 2014, 136, 10234ꢀ10237. (b)
Zhang, Y.; Maclntosh, A. D.; Wong, J. L.; Bielinski, E. A.; Williard, P. G.; Mercado, B. Q.;
Hazari, N.; Bernskoetter, W. H. Chem. Sci. 2015, 6, 4291ꢀ4299.
(28) (a) Zhang, G.; Scott, B. L.; Hanson, S. K. Angew. Chem., Int. Ed. 2012, 51, 12102ꢀ12106.
(b) Zhang, G.; Vasudevan, K. V.; Scott, B. L.; Hanson, S. K. J. Am. Chem. Soc. 2013, 135, 8668ꢀ
8681.
ACS Paragon Plus Environment
26

Page 27 of 27
ACS Catalysis
1
2
3
4
5
6
7
8
(29) Sharninghausen, L. S.; Mercado, B. Q.; Crabtree, R. H.; Hazari, N. Chem. Commun. 2015,
51, 16201ꢀ16204.
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
(30) Note that 4 is an active catalyst in the hydrogenation reaction at 100 °C (see entry 5, Table 1).
(31) Bullock, R. M. Chem. Eur. J. 2004, 10, 2366ꢀ2374.
(32) Hansch, C.; Leo, A.; Taft, R. W. Chem. Rev. 1991, 91, 165ꢀ195.
(33) Koehne, I.; Schmeier, T. J.; Bielinski, E. A.; Pan, C. J.; Lagaditis, P. O.; Bernskoetter, W.
H.; Takase, M. K.; Würtele, C.; Hazari, N.; Schneider, S. Inorg. Chem. 2014, 53, 2133ꢀ2143.
(34) Gaussian 09, Revision D.01, Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G.
E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.;
Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H. P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.;
Sonnenberg, J. L.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.;
Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, J. A., Jr.; 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.;
Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, Ö.; Foresman, J. B.; Ortiz, J. V.;
Cioslowski, J.; Fox, D. J. Gaussian, Inc., Wallingford CT, 2009.
(35) Marenich, A. V.; Cramer, C. J.; Truhlar, D. G. J. Phys. Chem. B 2009, 113, 6378ꢀ6396.
ACS Paragon Plus Environment
27