Transition-Metal Oxos as the Lewis Basic Component of Frustrated Lewis Pairs

Article abstract of DOI:10.1021/jacs.6b00705

The reaction of oxorhenium complexes that incorporate diamidopyridine (DAP) ligands with B(C₆F₅)₃ results in the formation of classical Lewis acid-base adducts. The adducts effectively catalyze the hydrogenation of a variety of unactivated olefins at 100 °C. Control reactions with these complexes or B(C₆F₅)₃ alone did not yield any hydrogenated products under these conditions. Mechanistic studies suggest a frustrated Lewis pair is generated between the oxorhenium DAP complexes and B(C₆F₅)₃, which is effective at olefin hydrogenation. Thus, we demonstrate for the first time that the incorporation of a transition-metal oxo in a frustrated Lewis pair can have a synergistic effect and results in enhanced catalytic activity.

Full text of DOI:10.1021/jacs.6b00705

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Transition Metal
Oxos as t
J
h
ou
e Lethewis
rnal
of
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Basic Component
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of Frustrated
LewisJo
P
urn
a
al
i
of the
r
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Nikola S. Lambic, Roger D.
Sommer, and Elon A. Ison
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δ+
PJ ao gu er n 1a lo of f1 t4 he American Chemical Society
B(C F )
6
5 3
Mes
δ-
Mes
N O
Re
X
N
O
X
N
1
2
3
4
B(C F )
Re
N
6
5 3
N
N
Mes
Mes
X
5
= Me, H, Ph, C(O)Ph
Frustrated Lewis Pair
6
7
8
9
1
1
H (50 psi)
2
2
5 mol % Re
R
2
R
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H
R
1
0 mol % B(C F )
6
5 3
R
H
R
R1
1
0
1
Toluene, 100 °C
R3
R
3

Journal of the American Chemical Society
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Transition Metal Oxos as the Lewis Basic Component of
Frustrated Lewis Pairs
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Nikola S. Lambic, Roger, D. Sommer, and Elon A. Ison*
Department of Chemistry, North Carolina State University, 2620 Yarbrough Drive, Raleigh, North Carolina 27695-
8
204, United States
Supporting Information Placeholder
that the incorporation of a transition metal oxo in an FLP
results in enhanced catalytic activity, in contrast to when the
ABSTRACT: The reaction of oxorhenium complexes that
incorporate diamidopyridine (DAP) ligands with B(C
6
F
)
5 3
transition metal oxo, or B(C
catalysts.
6 5 3
F ) , by themselves were used as
results in the formation of classical Lewis acid-base adducts.
The adducts effectively catalyze the hydrogenation of a variety
of unactivated olefins at 100 °C. Control reactions with these
Mes B(C
N
F )
6 5 3
Mes
complexes or B(C
6
F
5
)
3
alone did not yield any hydrogenated
O
X
N
O
X
products under these conditions. Mechanistic studies
suggest a frustrated Lewis pair is generated between the
B(C F )
Re
N
6
5 3
Re
N
N
N
Mes
oxorhenium DAP complexes and B(C
6
F
5
)
3
, which is effective
Mes
at olefin hydrogenation. Thus we demonstrate for the first
time that the incorporation of a transition metal oxo in an FLP
can have a synergistic effect and result in enhanced catalytic
activity.
X = H, Me, Ph
INTRODUCTION
In recent years the chemistry of frustrated Lewis pairs,
X"="H"
ΔG°"="'19.4"kcal/mol"
X"="Me"
ΔG°"="'19.6"kcal/mol"
X"="Ph"
ΔG°"="'14.8"kcal/mol"
(
FLPs), has emerged as a powerful new strategy for the
development of catalysts for a variety of different reactions.1
While most of the research effort has focused on the
development of metal-free catalysts, there have been a few
studies aimed at investigating the development of transition
metals as the Lewis base or Lewis acid component of FLPs.2
Increasing"Sterics"
4
Figure 1. B3PW91 calculated structures for ((C
O)Re(DAP)(X)) (X = H, Me, Ph). Structures were optimized
in the gas phase with the 6-31G* basis set on C, H, N, O, F
and B and the SDD basis set and effective core potential on
Re. Energies also included Grimme’s D3 dispersion
corrections as implemented in Gaussian09 and solvent
corrections by utilizing the PCM solvation model with
dichloromethane as the solvent.
6 5 3
F ) )B
(
7
We have recently shown that oxorhenium complexes that
incorporate diamidoamine (DAAm) and diamidopyridine
8
9
10
(
DAP) ligands exhibit ambiphilic reactivity, in that, the metal
11
3
oxo reacts with both Lewis acids and bases. The reaction of
these oxorhenium complexes with B(C results in the
formation of classical Lewis acid-base adducts. As shown in
Figure 1, DFT(B3PW91)4 calculated structures for ((C
6 5 3
F )
Scheme 1. Synthesis of 2
6
F
5
)
3
B(O)Re(DAP)(X)) (DAP = (2,6- bis((mesitylamino)methyl)-
pyridine) X = H, Me, Ph), reveal that as the size of the X-type
ligand increases, steric interactions between the substituent
Mes
Mes
B(C F )
6
5 3
N
O
N
O
Ph
N
1.2 equiv B(C6F5)3
C H , rt, 5 min
Ph
on the diamidopyridine nitrogen, (mesityl) and the C
groups of B(C increases. Further, the formation of
(C B(O)Re(DAP)(Ph)) is approximately 5 kcal/mol less
endergonic than ((C B(O)Re(DAP)(H)) and ((C
O)Re(DAP)(Me)). Thus the weak association between the
6 5
F
Re
Re
6 5 3
F )
N
N
6
6
N
(
6
F
5
)
3
Mes
Mes
6
F
5
)
3
6 5 3
F ) B
1
2
(
Lewis acid and the oxo ligands could result in complexes that
exhibit the high latent reactivity that is common for
traditional main group FLPs.1f, 5
RESULTS AND DISCUSSION
1. Synthesis of FLPs from Transition Metal Oxos.
A. Synthesis of ((C )B(O)Re(DAP)(Ph)). The
stoichiometric reaction of (O)Re(DAP)(Ph), 1, with B(C
leads to the formation of the classical Lewis acid/base adduct,
In this manuscript, we show that FLPs generated from
(O)Re(DAP)(X) are efficient catalysts for the hydrogenation of
6
5 3
F )
6 5 3
F )
6
unactivated olefins. Thus we demonstrate for the first time
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Journal of the American Chemical Society
(
(C
6
F
5
)
3
)B(O)Re(DAP)(Ph)), 2, (Scheme 1). Similar reactivity
was increased, these signals broadened and coalesced at ca.
51 °C. Cooling the solution to room temperature resulted in
the original spectrum (see Figure S13). Similar dynamic
behavior was observed by 1H NMR spectroscopy. The data
provide strong evidence for a dynamic equilibrium between 2
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
was observed for other oxorhenium complexes published by
our group. This moisture sensitive complex was isolated in
63% yield and exhibits the characteristic splitting pattern of
the diastereotopic methylene protons syn (5.7 ppm) and anti
3
1
(
4.9 ppm) to the rhenium oxo, by H NMR spectroscopy.
and B(C
6 5 3
F ) as shown in Scheme 2. From the chemical shifts
and the coalescence temperature the free energy of activation,
X-Ray quality crystals of 2 were obtained by cooling a
benzene/pentane solution of the complex to -40 °C (Figure 2).
Rhenium occupies a distorted square pyramidal geometry in
‡
12
(
ΔG = 14.1 kcal/mol) can be estimated for this process.
B(C F )
6
5
3
this molecule with the oxo ligand as well as B(C
6
F
5
)
3
in the
B(C6F5)3
Mes
B(C6F5)3
Mes
Mes
H/D
apical position. As shown in Figure 2, the solid-state structure
of 2 is sterically crowded, as the phenyl group is no longer
parallel with the mesityl groups as seen in the structure for 1.
The angle of distortion is approximately 22.4°. The symmetry
of the boron atom is also quite different than in other
examples of acid/base adducts,3 with one –C
completely in the plane with the pyridine ligand, and the other
two perpendicular to the plane.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
H/D
Ph
N
O
N
O
N
O
Ph
Ph
H2/D2
Re
N
Re
Re
N
N
N
N
N
Mes
Mes
Mes
6
F
5
unit
H /D ꢀ(1:1)ꢀ
2
2
Bꢀ
O1ꢀ
Re1ꢀ
1
Figure 3. H NMR spectra showing A) the initial spectrum
N2ꢀ
N1ꢀ
N3ꢀ
for a 1:1 mixture of H
equiv of B(C and B) complete isotope scrambling of the
/D mixture by 1:1 2/B(C after 90 min in toluene-d
2 2
and D in the presence of 2 and one
C26ꢀ
6 5 3
F )
H
2
2
6
F
5
)
3
8
.
8
C. H/D Exchange. A toluene-d solution of 2 was exposed to
6
0 psi (total pressure) of a 1:1 mixture of H /D in a J-Young
2
2
1
tube at 100 °C. H NMR analysis of the mixture after 90 min
showed signals consistent with the catalytic isotopic
scrambling of hydrogen and deuterium (Figure 3).
Figure 2. Thermal ellipsoid plot for 2
5
.
Ellipsoids are at the
0% probability level. Hydrogen atoms were omitted for
Equilibration was not observed when solutions of B(C
alone were employed.
6 5 3
F )
clarity and the mesityl substituents are depicted in
wireframe. Selected bond lengths (Å) and angles (deg).
Re1-O1, 1.7674(17); Re1-N3, 1.963(2); Re1-N1, 1.954(2);
Re1-N2, 2.050(2); O1-B, 1.535(3); Re1-C26, 2.084(3); Re1-
O1-N1, 113.95(9); O1-Re1-N3, 111.25(8); N1-Re1-N3,
To summarize, complex 1 forms the classic Lewis acid-base
adduct 2 with B(C . Upon thermal activation, a frustrated
Lewis pair is generated that is capable of activating H . This
6 5 3
F )
2
1
34.14(9); O1-Re1-N2, 118.28(8); N1-Re1-N2, 75.92(9);
N3-Re1-N2, 76.47(9); O1-Re1-C26, 104.27(9); B-O1-Re1,
72.64(17).
reactivity is consistent with previously reported FLP
reactivity.1m, 1o, 1q, 1r
1
2
. Catalytic Hydrogenation with Transition Metal
Oxos.
6
Scheme 2. Equilibrium between 2 and B(C F5)3
6 5 3
A. Substrate Scope. The combination of 1 and B(C F )
B(C F )
6
5 3
effectively catalyzes the hydrogenation of a variety of
terminal, internal, cyclic and conjugated olefins at 100 °C
under the optimized conditions (Chart 1). Neither
Mes
Mes
B(C6F5)3
Mes
N
O
N
O
N
O
Ph
B(C F )
Ph
Ph
6
5 3
Re
Re
N
Re
6 5 3
complex 1 nor B(C F ) catalyze the hydrogenation of
N
N
N
olefins under the optimized reaction conditions.
Hydrogenation occurred smoothly for many of the small-
linear-sterically-unhindered alkenes, such as ethylene,
and monosubstituted olefins such as propylene, 1-octene,
N
N
Mes
Mes
Mes
1
2
Classic LA/LB Adduct
FLP
3
,3-dimethylbut-1-ene, allylbenzene, and 4-phenyl-1-
butene. In all cases, high conversion was achieved in 16 h.
B. Dynamic equilibrium with B(C
6
F
5
)
4
.
A variable
solution
, provided evidence for a
dynamic equilibrium in solution. For example, distinct signals
for B(C
and 2 were observed in the 19F NMR spectrum at
room temperature (21 °C) at -132 (ortho-C , 2) and -129
ortho-C , borane) ppm respectively. As the temperature
temperature H and 19F NMR analysis of a toluene-d
1
8
6 5 3
of a 1:1 mixture of 2/B(C F )
6 5 3
F )
6 5
F
(
6 5
F
2ꢀ
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Journal of the American Chemical Society
Chart 1. Substrate Scope for Hydrogenation with
Page 4 of 14
tetramethylethylene was utilized, however low yields (13%)
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
a
were observed. The hydrogenation of the cyclic olefin
cyclooctene was achieved with 99% conversion. However, low
conversions (12%) were observed for cyclohexene and
cyclooctadiene was not hydrogenated. Dienes, 1-4 hexadiene
(99%)13 and 1-9 decadiene (76%) were also successfully
hydrogenated.
1
/B(C
6
F
5 3
)
H2 (50 psi)
5 mol % 1
R2
R
2
1
0 mol % B(C F )
H
R
6
5 3
R
R
H
R1
1
Toluene, 100 °C
R3
R
3
1
6 h
B. Kinetic and Mechanistic Studies. Kinetic studies
6 5 3
were performed using 1/B(C F ) under optimized
H
H
H
H
H
H
H
H
conditions (Scheme 3). Rate data were collected in order
to determine the order with respect to substrate,
catalyst/cocatalyst and hydrogen. The substrate chosen
for the kinetic analysis was 3,3-dimethyl-1 butene
(neohexene) because of the relative ease of 1H NMR
assignments. In addition, this substrate cannot undergo
double bond isomerization, thus simplifying the kinetic
analysis.
H
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
H
ethene
prop-1-ene
99%
oct-1-ene
99%
8
2%
H
H
H
H
H
H
H
H
H
3
,3-dimethylbut-1-ene
allylbenzene
99%
but-3-en-1-ylbenzene
99%
Scheme 3. Conditions for Hydrogenation of Neohexene
9
9%
H
H2
(5 mol %)
H
Ph
H
H
1
B(C F ) (2:1 vs Re mol %)
H
6
5 3
Ph
H
2-methylprop-1-ene ethene-1,1-diyldibenzene (E)-hex-3-ene
Toluene, 100 °C
9
9%
84%
99%
Full Conversion
Ph
H
H
H
H
The catalytic reaction exhibits first order dependencies on
/B(C and neohexene and a zeroth order dependence on
(Figure 4). Thus the rate law for the overall catalytic
H
H
Ph
Ph
Ph
1
H
6 5 3
F )
(
Z)-hex-2-ene
(E)-1,2-diphenylethene (Z)-1,2-diphenylethene
2
99%
ND
ND
reaction is given by equation 1:
H
H
d[neohexane]
= k[neohexene][1/ B(C F ) ]
(1)
H
6 5 3
dt
H
H
,3-dimethylbut-2-ene (E)-hexa-1,4-dieneb (Z)-cyclooctene
In addition to these kinetic studies, the kinetic isotope effect
was determined when using D instead of H under previously
optimized conditions. Time profiles for parallel reactions were
2
1
3%
99%
2
2
9
9%
H
D
monitored over time, and
extrapolated. From these experiments a KIE of 1.11(1) was
kobs and kobs values were
obtained which suggests that H
the turnover-limiting step.
2
activation does not occur in
deca-1,9-diene
6%
(1Z,5Z)-cycloocta-1,5-diene
ND
7
Scheme 4. Hydrogenation of Neohexene with
Generated in Situ
2
H
H
H (50 psi)
2
cyclohexene
2%
3-methyl-6-(prop-1-en-2-yl)cyclohex-1-ene
99% (external)
1
(5 mol%)
1
B(C F ) (5 or 10 mol%)
6
5 3
a) %Conversion determined by integrating alkenic protons
with respect to the corresponding methylene or methyl
protons of the product. Conditions: 1 (0.0061 mmol),
Toluene, 100 °C, 16 h
6 5 3
B(C F ) (0.0124 mmol), Olefin (0.1224 mmol), toluene (0.2
ml) in a 3 ml storage tube. b) Reaction resulted in a mixture
of hexane (33%) and 3-hexene and 2-hexene (66%).
C. Catalytic Competency of 2. Complex
2
was
generated in situ and examined as a catalyst for the
hydrogenation of neohexene (Scheme 4). When 1 equiv
of B(C F ) was utilized in catalysis, the activity was
The use of geminal substituted olefins such as isobutylene
6
5 3
(
99%) and 1,1-diphenylethylene (84%) resulted in good
conversion as well. The use of the vicinal substituted olefins,
E)-3-hexene (99%) and (Z)-2-hexene (99%) also proceeded
approximately 10 times slower than the comparable
reaction with an additional equiv of B(C F ) (Figure S8).
The addition of up to 3.5 equivalents of does not result in
any further increase in the rate of hydrogenation (Figure
S9) for complete dependence on B(C F ) ).
6
5 3
(
efficiently. However, no alkane was detected when cis and
trans stilbene were utilized. The tetrasubstituted olefin,
6
5 3
3ꢀ
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42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Figure 4. a) Time profiles for hydrogenation of neohexene with 1/B(C
.042 mM, (3.4 mol%); [1] = 0.031 mM, [B(C ] = 0.062 mM, (4.7 mol%); [1] = 0.041 mM, [B(C
); [olefin] = 0.62 mM. b) Dependence of kobs on [1]. c) Dependence of kobs on pH2. Conditions: [1] = 0.020 mM; [B(C
0.040 mM; [olefin] = 0.30 mM.
6
F
5
)
3
. Conditions: 2:1 B:Re: [1] = 0.021 mM, [B(C
] = 0.082 mM, (6.7 mol
] =
6 5 3
F ) ] =
0
%
6
F
5
)
3
6 5 3
F )
6 5 3
F )
When 1 and one equiv B(C
identified at the end of the reaction was (DAP)Re(O)C
contrast, when 2 equiv of B(C were used in the reaction,
6
F
5
)
3
were utilized, the species
Scheme 6. Olefin Hydrogenation with Oxorhenium
Complexes
6 5
F
, 3. In
6 5 3
F )
the species identified throughout the reaction was 2. This
establishes 2, or the FLP generated from 2, as the resting state
of the catalytic system. The reduced rate observed with one
H2 (50 psi)
Re (5 mol %)
B(C F ) (10 mol%)
6
5 3
6 5 3 6 5
equiv of B(C F ) results from competitive C F transfer to 1
to produce 3. As shown below catalysis with 3 is significantly
slower than with 1. The addition of excess equivalents of
Toluene, 100 °C, 16 h
Mes
B(C
6
F
5
)
3
is necessary to prevent C
6
F
5
/Ph group exchange
Mes
Mes
(
Scheme 5).
N
O
Me
N
N
O
Ph
N
O
N
C6F5
Scheme 5. Competitive Formation of an FLP and 3 from
1
Re
Re
Re
and B(C F )
N
6
5 3
N
N
N
Mes
Mes
Mes
Mes
B(C
O
F )
6 5 3
Mes
Mes
4
1
3
N
O
Mes
Ph
N
Mes
N
O
B(C F )
6 5 3
Ph
C6F5 B(C F )
6
5 3
Re
Re
Re
N
O
C(O)Ph
O
N
N
H
N
O
N
N
N
Mes
N
Re
Me
Mes
Mes
Re
Re
N
3
1
FLP
N
N
O
O
N
Mes
slow catalysis
fast catalysis
Mes
5
6
7
D. Catalysis with Other Oxorhenium Catalysts. Other
oxorhenium catalysts (Scheme 6, Figure 5) were
explored in order to probe the steric and electronic
demands of the reaction. Reduced rates were observed
with catalysts 3, and 4, bearing pentaflourophenyl and
methyl substituents respectively, however, catalyst 5
which contains the benzoyl substituent, exhibited similar
reactivity to 1. An induction period of ~ 250 min was
observed when the hydride complex 6 was used as a
catalyst. Similarly an induction period (~ 200 min) was
observed when methyltrioxorhenium (MTO), 7 was
utilized.14 Low conversions were obtained when the
rhenium dioxo complex 8, was used as a catalyst. While
complexes 9 and 10, which contain a phenoxyoxazoline
ligand, did not show any activity.15
B(C F )
5 4
6
O
O
PPh3
Cl
N
O
N
O
N
O
Re
I
Re
O
Re
O
PPh3
O
O
O
N
O
NCCH3
O
8
10
9
E. The Nature of the Induction Period for 6. The
most prominent induction period (~ 250 min) was
observed with 6. Notably, catalyst 4, which contains a
methyl substituent, proceeded at approximately the same
rate compared to 6 during the induction period (Figure
5
). We therefore sought to investigate the nature of the
induction period and identify the possible catalytically
active species along the reaction pathway.
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Page 6 of 14
coordination environment in 11 with the oxo ligand in the
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
1
apical position. The expected elongation of the Re-O bond is
observed, due to the reduced bond order upon binding of
Lewis acid. Other bond lengths are consistent with previously
1
00
80
7
6
5
reported structures bearing the same ligand set. The hydride
3
3
4
ligand could not be located in the difference map, but other
physical methods (described below) were able to confirm its
presence.
6
4
2
0
0
0
0
Complex 11 was characterized by 1H, 13C, 19F and NMR
spectroscopy, X-ray crystallography, IR spectroscopy and
elemental analysis. By 1H NMR spectroscopy, the hydride
resonance shifts from 6.02 ppm in 6, to 11.83 ppm in 11. This
signal is not observed in the deuterated analog 11-d which
was prepared in a similar fashion. The downfield shift for the
hydride ligand in 11 is consistent with the increased acidity of
the Re-H bond, because of the electron withdrawing B(C F ) .
6 5 3
A shift of fluorine resonances to new frequencies is also
observed in the 19F NMR spectrum (Figure S2).
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
8
0
200
400
600
800
1000 1200
Time (min)
Figure 5. Competency of other oxorhenium catalysts.
Conditions: [Re] = 0.0306 mM ; [B(C6F5)3] = 0.0613 mM
1
[
Olefin] = 0.613 mM. Conversions determined by H NMR
spectroscopy by integrating the ratios of tert-butyl singlets of
the product with respect to the reactant.
B1ꢀ
Traditional organometallic mechanism. A Re-H could
be potentially generated by a σ-bond metathesis of H
Given that olefin insertion into Re-H bonds is a well-known
reaction, we were prompted to explore the viability of an
insertion / elimination pathway. One potential mechanism
could involve the activation of the Re-H by coordination of
2
with 1.
O1ꢀ
Re1ꢀ
N3ꢀ
N1ꢀ
N2ꢀ
B(C
olefin insertion into the Re-H bond and heterolytic cleavage of
the Re-alkyl bond with H (Scheme 7).
6 5 3
F ) to the oxo ligand in 6. This would be followed by
2
Scheme 7. Traditional Organometallic Mechanism
Mes
N
O
.
Figure 6. Thermal ellipsoid plot for 11 Ellipsoids are at the
H
Re
50% probability level. Hydrogen atoms were omitted for
clarity. Selected bond lengths (Å) and angles (deg). Re1-
O1, 1.757(3); Re1-N3, 1.939(4); Re1-N1, 2.042(4); Re1-N2,
N
N
Mes
6
1
.947(4); O1-B1, 1.529(7) O1-Re1-N3, 113.51(18); O1-Re1-
N2, 113.39(18); N3-Re1-N2, 133.09(19); O1-Re1-N1,
24.15(16); N3-Re1-N1, 76.63(16); N2-Re1-N1, 76.66(17);
B(C F )
6
5
3
1
Mes B(C F )
6
5
3
B1-O1-Re1, 175.3(4).
N
O
H
Scheme 8. Synthesis of 12
Re
H
H
H
N
H
H
N
H
Mes
B(C F )
6 5 3
Mes
H
H
11
H
H
N
O
ethylene
H
Olefin insertion
Heterolytic cleavage
Re
Benzene, 80 °C, 1 h
N
B(C F )
Mes
6
5 3
N
Mes
N
O
H2
Mes B(C
6
F
5
)
3
11
Re
N
O
N
Mes
N
Re
Mes
N
N
Mes
N
O
B(C
F )
6 5 3
12
1
2
Re
Benzene, rt, 1 h
N
N
Similar to the formation of 2 (Scheme 1), treatment of
complex 6 with B(C in aromatic solvents yields the
classical Lewis acid/base adduct 11. X-ray quality crystals
were obtained by vapor diffusion of pentane into the
concentrated toluene solution of 11 (Figure 6). Similar to the
structure for 2, rhenium occupies a square pyramidal
Mes
6 5 3
F )
13
While the hydride precursor 6 does not show any reactivity
with olefins, even at elevated temperatures, complex 11
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smoothly reacts with a variety of olefins to yield the
corresponding Re-alkyl species. For example, the reaction of
11 with ethylene at 80 °C results in the formation of (C
(O)Re(DAP)(Et), 12 (Scheme 8), which was characterized by
1H, 13C and 19F NMR spectroscopy (see Supporting
Information). Complex 12 can also be synthesized from the
6 5 3
Reactions of 6 with B(C F ) under catalytic conditions are
summarized in Scheme 10. Olefin insertion to produce 12,
was shown to be facile, however, cleavage of the Re-carbon
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
6 5 3
F ) B
2
bond with H does not occur which suggests that this pathway
is not a viable catalytic pathway. Complexes 3 and 14 are
catalytically active (Figure S10), however, the major
component of catalytic reactions is 12. These data suggest
that during catalysis with 6, more active catalytic species (3,
6 5 3
reaction of (O)Re(DAP)(Et), 13, with B(C F ) .
By 1H NMR spectroscopy, 12 shows characteristic alkyl
1
2, and 14) are generated. This accounts for the sigmoidal
resonances corresponding to the ethyl ligand, as well as a shift
of diastereotopic methylene protons to different frequencies.
Two additional signals belonging to the ethyl ligand are also
behavior observed during catalysis. Olefin insertion into the
Re-H bond also occurs but since this pathway does not lead to
catalysis it is mainly a deactivation pathway.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
observed by 13C NMR spectroscopy.
pressurized with H , no ethane was observed under optimized
conditions (50 psi H , 100 °C). Complex 11 was also not
When 12 was
2
Scheme 10. Nature of Induction Period for 6.
2
B(C6F5)3
observed. Instead, 12 displayed excellent stability even at
higher temperatures. These results suggest that a traditional
organometallic olefin insertion pathway as described in
Scheme 7 is not a catalytically viable pathway for this
reaction.
Mes
Mes
B(C6F5)3
O
Mes
N
O
N
N
O
H
5 3
B(C F )
H
B(C6F5)3
H
N
6
Re
Re
Re
N
N
N
N
N
Mes
Mes
Mes
Identification of catalytically active species. Two
species were identified in catalytic reactions with 6. Complex
FLP
Slow Catalysis
1
1
6
Classic LA/LB Adduct
3
6 5
, which contains the C F group, was crystallized at the end
of the catalytic reaction and characterized. This reactivity is
not uncommon as pentafluorophenyl transfer can occur from
borane Lewis acids to the metal center.16 As shown above,
(
Figure 5), 3 exhibits moderate activity for the hydrogenation
B(C F )
B(C F )
6
5
3
6
5
3
6 5 3
B(C F )
Mes
Mes
Mes
of t-butyl-ethylene (76% after 16 h).
H
N
O
N
O
O
C6F5
H
N
Scheme 9. Synthesis of 14
ethyl
Re
Re
Re
N
N
N
B(C F )
N
N
Mes
6
5 3
N
Mes
Mes
Mes
N
O
N
O
2 equiv B(C6F5)3
3
14
12
H
Re
Re
N
Fast Catalysis
N
Toluene, 100 °C, 16 h
N
N
Mes
3
. Computational Studies
6
14
In order to gain additional insights into the reaction
mechanism, DFT (B3PW91) calculations were performed. For
these calculations catalytic reactions employing catalyst 4
with ethylene was modeled because: 1) as shown in Figure 5,
4 shows significant albeit modest reactivity, 2) the size of the
Additionally, the cyclometalated product, 14, was observed
during the stoichiometric reaction of 6 with B(C (Scheme
). The cyclometalated product formed by C-H activation of
6 5 3
F )
9
1
mesityl substituent was observed by H NMR spectroscopy,
and its structure was elucidated on the basis of chemical shifts
and signal integrations (See Supporting Information). The H
FLP generated from this complex and B(C
accurate calculations at a reasonable computational time, and
as shown in Chart 1, ethylene was successfully
6 5 3
F ) allows for
1
3
)
NMR spectrum for 14 features 5 methyl signals for the
mesityl ligands, as a result of the broken symmetry of the
molecule, and C-H activation of the methyl fragment.
Furthermore, 6 different diastereotopic methylene protons
were observed in the expected region, indicating again,
different chemical environments due to the asymmetric
nature of the compound. A similar cyclometalation reaction
was observed by Schrock and co-workers for the C-H
hydrogenated under catalytic conditions. This last point is
important because traditional olefin hydrogenation
mechanisms proceed by transient hydrogen activation by an
FLP, followed by proton transfer to an olefin to form a
carbocation, which subsequently leads to irreversible reaction
with the hydridoborate to liberate the hydrocarbon and
regenerate the FLP (Scheme 11).1s, 6b, 18 Ethylene, as a
substrate allowed us to test the viability of this mechanism
because protonation will result in the generation an unstable
primary carbocation.
activation of a mesityl subsituent of a Zr-Np complex bearing a
diamidoamine ligand.17 Complex 14 was found to be
catalytically active in the hydrogenation of ethylene.
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Scheme 11. Typical Mechanism for Olefin Hydrogenation with FLPs.
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
H
R'
B(C F )
H
H
R'
R'
H
H
R'
R'
6
5 3
H2
H
R'
+
HB(C F ) ]
[HPR3]
H
[HB(C F ) ]
6 5 3
[
H
H
6
5 3
PR3
PR3
B(C6F5)3
Structure optimizations included Grimme’s D3 dispersion
corrections9 as implemented in Gaussian09.10 For these
calculations Gibbs free energies at 373 K are reported and
include solvation corrections by utilizing the PCM solvation
model11 with benzene as the solvent. The viability of a
traditional FLP mechanism for a system that involves
oxorhenium complexes as the Lewis base component of the
frustrated Lewis pair was considered first.
FLP Generation: The structure and bonding in
frustrated Lewis pairs. The optimized structures of
frustrated Lewis acid pairs generated from 1, 4, and 6 are
depicted in Figure 7. Interaction of B(C F ) with the rhenium
6 5 3
oxo bond generates the frustrated Lewis acid pairs, 18, 15,
and 19 respectively. For 15 the formation of this FLP from the
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
6 5 3
infinitely separated B(C F ) and (O)Re(DAP)(Me) fragment
is slightly endergonic (ΔG373 = 1.09 kcal/mol) and is
generated from the classical Lewis/acid 14, (ΔG373 = -16.8
A. Viability of Typical FLP Mechanism.
‡
kcal/mol) which proceeds through the TS(14-15) (ΔG =
As noted above, for the oxorhenium complexes examined,
upon thermal activation, a frustrated Lewis pair is generated
6
.30 kcal/mol).
that is capable of activating H
generated from 1 and B(C
mixture for H and D
2
(Scheme 2). Thus the FLP
F ) was shown to scramble a
The most important characteristic of FLPs 18, 15 and 19 is
the long B-O bond lengths (3.67 Å, 18; 3.74 Å, 15; 3.65 Å, 19).
Further, the plane formed by B and the three C
parallel to the Re=O bond, suggesting that there is an absence
of a dative bond between the oxo and B(C . This is
6 5 3
.
6 5
F rings is now
2
2
Scheme 12. Proposed FLP Mechanism with
Oxorhenium Catalysts
6 5 3
F )
illustrated further in the (lowest unoccupied molecular
orbital), LUMO, for 15, where it is shown that the electron
Mes
density is exclusively localized on the B(C
Figure 10).
6 5 3
F ) fragment (see
N
O
CH3
N
Re
N
The structural characteristics and the nature of the B-O bond
length are quite similar to the traditional organic frustrated
Lewis pairs. For example Zeonjuk and coworkers have
calculated the optimal distances necessary for FLP reactivity
between the boron and phosphorus atoms. This range was
Mes
4
B(C6F5)3
Mes
5 3
B(C F )
estimated to be between 3-5
Å between boron and
6
H
H
H
H
N
O
phosphorus metal center. For example, the calculated
equilibrium structure of the well understood Mes
FLP has a calculated B-P bond distance of 3.86 Å
correlates well with the calculated distance for the highest
active catalyst generated from 1.
H
H
CH3
N
Re
3
6 5 3
P-B(C F )
FLP Generation
TS (14-15)
N
19
.
This
Mes
Product Generation
14
B(C6F5)3
H
H
H
B(C6F5)3
H
Mes
Mes
H
H
N
O
N
O
CH3
CH3
Re
Re
N
N
N
N
Mes
Mes
1
7
15
B(C6F5)3
H
Mes
Ethylene Activation
H
H2
N
O
CH3
H2 Splitting
B-O: 3.67 Å
B-O: 3.74 Å
B-O: 3.65 Å
H
H
H
H
Re
N
19
1
5
18
N
Mes
16
Figure 7. B3PW91+D3 calculated structure for FLPs 19
Re-Ph), 15 (Re-Me) and 18 (Re-H).
(
2 6 5 3
H splitting with 4/B(C F ) was therefore investigated. The
proposed FLP mechanism for the catalytic hydrogenation of
ethylene is shown in Scheme 12. It begins with the generation
of an FLP from the classical Lewis acid/ base pair, 14. This
A more in depth analysis of the bonding in the calculated FLP
structures reveals several interesting features (Figure S28).
Two of the aryl rings are oriented perpendicular to the Re-oxo
bond in 15. The distances of the oxo ligand to the center of
these rings are 2.68 and 3.39 Å at angles of 125° and 126°
respectively. These values are reminiscent of the studies of
2
FLP activates H to generate the onium hydridoborate, 16,
which activates ethylene by sequential protonation to form a
cation, followed by hydride transfer from the hydridoborate
portion of 17. Each of the steps is analyzed in detail below.
) by Rhee and coworkers.20
3 6 5 3
main group FLPs (t-Bu P:B(C F )
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Further, within this structure there is one H-F bond between
the Mes methyl groups on the diamidopyridine ligand and the
groups on B(C at a distance of 2.49 Å and an angle of
Given that the idealized bond lengths and angles for H-F
hydrogen bonding are < 2.5 Å and 180°, these data suggest
that hydrogen bonding may not be a significant factor in
stabilizing FLPs and the stability may be driven by similar
Hydride Transfer. The viability of hydride transfer from
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
the hydridoborate portion of 16 was also investigated. As
shown in Figure 8, the formation of the alkoxy complex, 24,
from 16 is exergonic at 373 K (ΔG373 = -2.66 kcal/mol).
However, hydride transfer occurs with an activation barrier
C
112°
6
F
5
6 5 3
F )
.
‡
(ΔG ) of 45.4 kcal/mol. Thus the data suggest that hydride
transfer is also inaccessible at the reaction temperature
employed.
lone-pair (Re=O) to π orbital (from the C
6 5
F group of
B(C
6 5 3
F )
) interactions as observed by Rhee.20 However, unlike
To summarize, FLPs generated from the oxorhenium
main group FLPs, a direct P-B interaction as in classical Lewis
pairs does not appear to exist. This is evident in Figure 7 as
the plane encompassing the boron atom and its aryl
substituent is oriented orthogonal to the lone pair on the
Re=O.
complexes effectively split
H
2
.
However, the onium
hydridoborate generated is not kinetically competent to
activate ethylene via protonation or transfer a hydride to an
oxo alkyl from the hydridoborate. Thus these results suggest
that a traditional FLP mechanism is not viable for this catalytic
system.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
H
(
2
Splitting. At 373 K splitting of H
ΔG373 = 8.47 kcal/mol) (Scheme 13). However, it was
observed experimentally that FLPs generated from
2
by 15 is endergonic
H
TS24’-25ꢀ
45.4ꢀ
H
H
H
6 5 3
B(C F )
H
oxorhenium complexes resulted in the scrambling of H
2
/D
2
Mes
H
B(C6F5)3
N
O
CH
3
mixtures (Scheme 2). This suggests that the barrier for H
2
H
Re
Mes
N
B(C6F5)3
splitting by these complexes is low. The data are also
consistent with previously reported reactivity of main group
H
N
H
H
H
N
O
CH3
Mes
H
H
H
Mes
Re
21ꢀ
N
N
O
Mes
N
N
Mes
CH
3
FLPs that have been shown to lead to facile H
2
splitting at mild
Re
N
temperatures.1m, 1q Thus the activation of ethylene with 16
O
16ꢀ
N
Me
N
19.1ꢀ
Mes
+
ꢀꢀ
Re
N
was explored with this catalytic system.
ethylene
ꢀ
24’ꢀ
Mes
H
B(C6F5)3
9
.56ꢀ
H
Scheme 13. Splitting of H by 15.
4ꢀ
B(C6F5)3
TSꢀ14-15ꢀ
6.3ꢀ
H
2
H
+
H
Mes
H
6
.90ꢀ 7.04ꢀ
+ꢀH2ꢀ
N
O
CH3
N
0.0ꢀ
Re
B(C
6
F
5
)
3
B(C6F5)
B(C F )
1.09ꢀ
3
N
6
5 3
H
B(C F )
Mes
6
5 3
Mes
H
δ-
Mes
N
H
H
Mes
Mes
Mes
H
H
25ꢀ
B(C6F5)3
O
H
3
H
Oδ+
Me
N
N
O
CH
N
N
Me
-8.75ꢀ
N
O
Re
N
Re
N
O
Me
Re
Me
H2
N
N
N
Mes
Mes
Re
-16.8ꢀ
Re
Mes
24ꢀ
14ꢀ
15ꢀ
N
N
N
N
Mes
Mes
Figure 8. Free energy (373 K) for hydride transfer from the
hydridoborate portion of 16.
1
6
1
5
1
.09 kcal/mol
9.56 kcal/mol
δ+
H H
Mes
N
B(C
H
6
F
5
)
3
B(C
6
F
5
)
3
H
B(C
6
F
5
)
3
Mes
H
O
H
δ-
O
H H
H
H
H
Me
Free energies (373 K) are reported relative to the infinitely
separated species 4, H , and ethylene.
Mes
N
Re
N
N
Me
TSꢀ26-27ꢀ
30.5ꢀ
Re
N
O
H
N
2
Me
N
Mes
N
Re
N
Mes
28ꢀ
30ꢀ
Ethylene Activation. Traditionally, olefin activation by
main group FLPs is believed to proceed by initial protonation
to generate a carbocation, followed by irreversible hydride
transfer from a hydridoborate.1s, 6b Electron rich olefins are
typically employed to stabilize the carbocation intermediate
that results from protonation. In the case of ethylene a
primary carbocation would be generated, as a result, catalytic
hydrogenation with main group FLPs are generally not
effective with unactivated olefins. In order to test the viability
of the traditional FLP mechanism in the catalytic system
described here, we examined ethylene activation with 16.
TS29’-30ꢀ
23.0ꢀ
Mes
23.6ꢀ
+ꢀH2ꢀ
26ꢀ
18.6ꢀ
11.3ꢀ
Mes
N
B(C
6 5 3
F )
TSꢀ14-15ꢀ
6.3ꢀ
H
O
4ꢀꢀ
H
H
Me
N
8.48ꢀ
Re
7.45ꢀ
+
ꢀB(C
6
F
5
)
3ꢀ
H
N
0.0ꢀ
Mes
1
.09ꢀ
27ꢀ
H
Mes
N
1
5ꢀ
B(C6F5)3
B(C
6
F
5
)
3
OH
Me
Mes
H
Re
N
N
OH
N
Mes
Me
N
-12.3ꢀ
Re
2
9’ꢀ
N
The activation of ethylene by 16 proceeds with unreasonably
-16.8ꢀ
4ꢀ
Mes
1
29ꢀ
high barriers to generate a new classical borane, (C
6 5 2
F ) ((6-Et-
1
6 5
-H-C F
)B·(O)Re(DAP)(Me), 22 (Figure S30), (ΔG‡373 = 45.3
Figure 9. Free energy (373 K) for the concerted attack of
the frustrated Lewis acid-base pair, 15, on the olefinic
double bond of ethylene.
kcal/mol) or to oxidize ethylene to ethanol with the
subsequent reduction of the Re(V) component, to Re(III)
(
Figure S31), (ΔG‡373 = 73.7 kcal/mol). Importantly, neither
of the reactions depicted in Figures 9 and 10 result in the
formation of ethane. Thus the data suggest that the activation
B. Concerted Attack of Frustrated Lewis Acid–Base
Pairs on Olefinic Double Bonds. Having shown that a
mechanism that involves initial H splitting followed by
2
sequential protonation and hydride transfer is not viable in
this catalytic system, alternative mechanisms were explored.
Stephan and coworkers have shown that ethylene and other
of ethylene via initial proton transfer to generate
a
carbocation is unlikely for this catalytic system. In fact, results
suggest that the carbocation generated is so reactive that it
reacts either with the electron rich aryl ring to generate 21, or
abstracts the oxo ligand to generate 23.
alkenes react readily with mixtures of P(t-Bu)
to yield products where the phosphine and borane add across
8ꢀ
3 6 5 3
and B(C F ) ,
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the olefinic double bond.21 In addition, Erker and coworkers
olefin to the boron and oxygen atoms in 26 with boron-
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
have developed the intramolecular version of this reaction
with P/B FLPs.22 Further, theoretical studies by Papai and
coworkers, have shown that the Lewis acid and base centers
act in concert in an FLP and lead to activation of the π-bond of
the olefin and regio and stereoselective B–C and P–C covalent
bond formation.23
carbon bond formation preceding oxygen-carbon bond
formation. This is evident by inspection of the B-C and C-O
Wiberg bond orders,25 (0.617 and 0.236 respectively) and
bond lengths (1.81 and 2.08 Å respectively). The cis olefin
addition observed here is in contrast to the trans olefin
addition observed by Li,26 Stephan,21 and Papai27. However,
the data are consistent with the cis olefin addition observed
Despite the apparent facile reactivity of FLPs with olefins, to
the best of our knowledge, a hydrogenation mechanism
involving initial concerted olefin addition has not been
considered. As a result we investigated a new mechanism that
by
Erker and coworkers.22c In fact the B-C bond length
observed in TS (26-27), (1.81 Å), is similar to the TS
reported by the Erker group, (1.83 Å), for the addition of an
intramolecular frustrated Lewis pair to norbornene.22c
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
begins with initial activation of the olefin, followed by fast H
2
activation. Importantly, such a mechanism is consistent with
the experimental kinetic data, where a first order dependence
Scheme 14. Pathway for the Addition of H to 27
2
on olefin and a zeroth order dependence on H
(see Figure 4).
2
were observed
B(C F )
6 5 3
Mes
N
The calculated mechanism for the catalytic hydrogenation of
ethylene with 4 is depicted in Figure 9. This catalytic cycle
begins with the formation of the frustrated Lewis pair, 15
from the classical Lewis acid/base adduct 14. This is followed
by olefin activation via the association complex, 26 to
generate zwitterion, 27. Hydrogen cleavage occurs by
O
Me
N
+
H2
Re
N
Mes
2
7
1
8.6 kcal/mol
addition across the polarized Re=OàCH
in 27 to generate the
((Et)B(C
2
CH
2
(B(C
6
F
5
)
3
) bond
pair,
ion
[
6
F
5
)
3
)][H(O)Re(DAP)(Me)], 29, which subsequently
A
B
C
rearranges to its isomer 29’. The acidic Re=OH group in 29
results in protonation of the ethyl fragment in the
Et(B(C F ) ) group to release of ethane and regenerate the
6 5 3
catalytic cycle. Importantly, the highest barrier (30.5
kcal/mol) for this mechanism is the olefin activation step,
which is consistent with the kinetic data presented above.
B(C F )
6 5 3
B(C6F5)3
B(C F )
6 5 3
Mes
OH
Mes
N
H
Mes
N
O
H
N
H
N
OH
Me
N
H
Re
Re
Re
Analysis of the key species for the critical steps of the
calculated mechanism, (olefin activation, hydrogen cleavage,
proton transfer) are described below.
Me
Me
N
N
N
N
Mes
Mes
Mes
Olefin Activation. The LUMO depicted in Figure 10 suggest
31
9.4 kcal/mol
32
2
9
that the B(C
activation of the olefin. Addition of the (O)Re(DAP)(Me) and
B(C fragments across the olefin is endergonic (ΔG373 =
6 5 3
F ) in 15 is perfectly aligned for electrophilic
3
40.1 kcal/mol
8.48 kcal/mol
6
F
5
)
3
Free energies (373 K) are reported relative to the infinitely
separated species 4, H , and ethylene.
7
2
.3 kcal/mol) and results in the formation of the zwitterion,
7 via TS(26-27) (ΔG‡ = 19.2 kcal/mol). The optimized
2
Hydrogen Cleavage. As shown in Scheme 14, three
pathways were explored for the addition of H to 27. In the
first pathway, addition of hydrogen across the rhenium oxo
bond (sigma bond metathesis) leads to
[HRe(DAP)(Me][HOCH CH ], 31. Pathway B involves
the activation of H at Re via the dihydrogen adduct, 32, and
structures for 26 and TS (26-27) are depicted in Figure 10.
2
1.81ꢀÅꢀ
1.66ꢀÅꢀ
a)ꢀ
b)ꢀ
c)ꢀ
2
2 6 5 3
B(C F )
2
lastly, pathway C involves cleavage of the oxygen-carbon bond
in 27 to produce 29. At 373 K, pathways A and B are
endergonic with free energies of 20.8 and 21.5 kcal/mol
respectively. In contrast, hydrogenolysis of the carbon-oxygen
bond in 27 is exergonic (ΔG373 = -10.1 kcal/mol). The
unstable nature of 31 and 32 suggest that hydrogen cleavage
does not proceed through these species. Further, given that
these intermediates are higher in energy than TS (26-27),
hydrogen cleavage through these intermediates is unlikely
since the kinetics for the catalytic reaction suggest that olefin
activation is the turnover-limiting step. Structure 27
maintains Re-oxo multiple bond character (1.77 Å). For
example, by comparison, the Re-OEt bond in trans-
2.08ꢀÅꢀ
1.47ꢀÅꢀ
3.12ꢀÅꢀ
2.96ꢀÅꢀ
15ꢀ
TSꢀ(26-27)ꢀ
27ꢀ
Figure 10. a) B3PW91+D3 calculated LUMO for FLP
generated from 4. Kohn-Sham orbitals are depicted using
an isocontour value of 0.045. b) Optimized structure for
TS(26-27) with selected bond lengths; c) Optimized
structure for 27 with selected bond lengths.
ReOCl
2
(OEt)(t-Bu
2
P(OEt)(t-Bu
2
P(H)O)28
is
significantly
Multiple bond character in the rhenium oxo bond20, 24 is
longer, (1.85(2) Å). Further, the carbon-oxygen bond length is
significantly longer in 27 (1.47 Å), than in trans-
maintained in both TS (26-27) and in 27 as both species
exhibit short bonds to the oxo ligand (1.72 and 1.77 Å,
respectively). The transition state for the addition of ethylene
involves an asynchronous concerted cis 1,2-addition of the
ReOCl
2 2 2
(OEt)(t-Bu P(OEt)(t-Bu P(H)O) (1.41 Å). These factors
result in the facile cleavage of the C-O bond in 27 by H
2
.
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Journal of the American Chemical Society
Product Generation. Product is generated from 29’ via
proton transfer from the acidic Re=OH group to the alkyl
fragment in EtB(C . This step proceeds via TS(29’-30),
studies indicate first order rate dependencies on
[rhenium/cocatalyst] and [substrate] and a zero order
dependence on molecular hydrogen. In addition a KIE of
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
6 5 3
F )
with an activation barrier of 23.0 kcal/mol. The acidity of
hydroxyl groups on rhenium has been noted by Neurock and
coworkers, as these groups have been proposed to be
responsible for the hydrogenolysis of polyols and cyclic
ethers.29 This acidity of the hydroxyl groups is derived from
the electron affinity of the high-valent oxo, which depletes
electron density from the OH bond and ultimately results in
the generation of a proton. In contrast, as noted above, (see
Figure 8) transfer of a hydride to a rhenium alkoxy ligand
proceeds with a significantly higher barrier (45.4 kcal/mol).
Thus unlike hydride transfer to a rhenium alkoxy ligand which
was shown not to be kinetically competent, proton transfer
from an acidic hydroxyl group is facile under reaction
conditions.
1.11(1) was obtained from parallel reactions with H
These data suggest that H activation does not occur in the
turnover-limiting step. Mechanistic studies indicate that the
oxorhenium complexes interact with B(C to form a
frustrated Lewis pair. This results in electrophilic activation of
the olefin and ultimately heterolytic cleavage with H . The
2 2
and D
2
6 5 3
F )
2
mechanism presented fundamentally differs from traditional
transition metal catalyzed hydrogenation, in that; catalysis
does not involve coordination of the substrates to the metal
center.30 Other rhenium catalysts were shown to exhibit
catalytic activity. Preliminary evidence suggests that catalytic
activity correlates with the sterics of the X-type ligand bound
to rhenium.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
EXPERIMENTAL SECTION
To summarize, the mechanism most consistent with the
computational and experimental data is depicted in Scheme
General Considerations. Complexes 1, 4, 5, 6, 915 and
1015 were prepared as previously reported;31 all other
reagents were purchased from commercial resources and
1
5. The addition of B(C
an FLP, 15. Concerted attack of this FLP across the double
bond of ethylene is the turnover-limiting step. Fast H
6 5 3
F ) to 4 results in the generation of
used as received. B(C
6
F
5
)
3
was purchased from Strem
2
Chemicals and sublimed prior to use. H, 13C, 19F NMR spectra
1
cleavage then occurs to generate an acidic hydroxyl group that
protonates an alkyl borate to generate the product. This
mechanism is consistent with the empirical rate law and is
also consistent with the observed insignificant H/D kinetic
isotope effect.
were obtained on 300 or 400 MHz spectrometers at room
temperature. Chemical shifts are listed in parts per million
(
ppm) and referenced to their residual protons or carbons of
the deuterated solvents respectively. All reactions were run
under an inert atmosphere with dry solvents unless otherwise
noted. High-pressure reactions were performed in a stainless
steel Micro Bench Top Reactor. FTIR spectra were obtained in
KBr thin films. Elemental analyses were performed by
Atlantic Micro Labs, Inc.
Scheme 15. Summary of the Proposed Mechanism for
the Catalytic Hydrogenation of Olefins with FLPs
Generated from Transition Metal Oxos.
Mes
N
O
Computational Methods. Computations were performed
on clusters provided by NC State Office of Information
CH
N
3
Re
N
Technology High Performance Computing (HPC). Theoretical
Mes
calculations have been carried out using the Gaussian 0910
4
implementation of B3PW914 density functional theory. All
6 5 3
B(C F )
geometry optimizations were carried out in the gas phase
using tight convergence criteria (“opt = tight”) and pruned
ultrafine grids (“Int = ultrafine”). The basis set for rhenium
Mes
B(C6F5)3
O
H
H
H
H
N
H
H
CH
3
was the small-core (311111,22111,411)
→
[6s5p3d]
Re
N
FLP Generation
Stuttgart-Dresden basis set and relativistic effective core
N
Mes
potential (RECP) combination (SDD)8 with an additional f
Product Generation
14
polarization function.7b The 6-31G(d,p)7a basis set was used
for all other atoms. Cartesian d functions were used
throughout, i.e., there are six angular basis functions per d
function. Calculations also include dispersion effects by
utilizing Grimme’s D3 dispersion correction with Becke-
Johnson damping.9 All structures were fully optimized and
analytical frequency calculations were performed on all
structures to ensure either a zeroth-order saddle point (a
local minimum) or a first-order saddle point (transition state:
TS) was achieved. The minima associated with each transition
state was determined by animation of the imaginary
frequency and, if necessary, with intrinsic reaction coordinate
δ
+
H
B(C
6
F
5
)
3
Mes
Mes
B(C
Mes
6 5 3
F )
δ
-
OH
N
N
O
CH
N
3
Me
Re
Re
N
N
N
Mes
2
9'
15
FLP
6 5 3
B(C F )
Mes
δ
+
H
δ
-
N
O
H
CH
3
H
H
2
Re
(
IRC) calculations.
H
N
H
2
Cleavage (several steps)
N
fast, no dependance on H , KIE = 1.1
2
Mes
Ethylene Activation
Turnover limiting step
Energetics were calculated at 373
K
with the 6-
2
7
Rate = k[Re/B(C
6 5 3
F ) ][olefin]
Activation Barrier = 30.5 kcal/mol
311++G(d,p)32 basis set for C, H, N, O and F atoms and the
SDD basis set with an added f polarization function on Re.
Reported energies utilized analytical frequencies and the zero
point corrections from the gas phase optimized geometries
and included solvation corrections which were computed
using the PCM method, with benzene as the solvent as
implemented in Gaussian 09.
CONCLUSIONS
Oxorhenium/borane adducts have been shown to be
catalytically competent for the hydrogenation of unactivated
olefins. Good conversions of olefins to alkanes were obtained
for a variety of terminal, internal and cyclic olefins. Kinetic
10ꢀ
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Page 12 of 14
2
011, 40, 7475-7483; (m) Miller, A. J. M.; Labinger, J. A.; Bercaw, J.
ASSOCIATED CONTENT
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
E., J. Am. Chem. Soc. 2010, 132, 3301.
Supporting Information. Kinetic data. Experimental details
for the synthesis of 2, 3, 11 and 14. Computational details.
3.
Smeltz, J. L.; Lilly, C. P.; Boyle, P. D.; Ison, E. A., J. Am.
Chem. Soc. 2013, 135, 9433-9441.
4
.
Burke, K.; Perdew, J. P.; Wang, Y., Derivation of a
Full Gaussian reference. CIF files for 2,
3 and 11.
generalized gradient approximation: The PW91 density functional. In
Electronic Density Functional Theory, Springer: 1998; pp 81-111.
5.
K.; Leskela, M.; Repo, T., Dalton Trans. 2012, 41, 4310-4312; (b)
Rokob, T. A.; Hamza, A.; Stirling, A.; Papai, I., J. Am. Chem. Soc.
Computational coordinates and XYZ files. This material is
available free of charge via the Internet at http://pubs.acs.org
(a) Lindqvist, M.; Sarnela, N.; Sumerin, V.; Chernichenko,
AUTHOR INFORMATION
2
009, 131, 2029-2036.
(a) Nicasio, J. A.; Steinberg, S.; Ines, B.; Alcarazo, M.,
Corresponding Author
6
.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Email: eaison@ncsu.edu
Chem. Eur. J. 2013, 19, 11016-11020; (b) Paradies, J., Synlett
2013, 24, 777-780.
7
.
(a) Hariharan, P. C.; Pople, J. A., Theoret. Chim. Acta
973, 28, 213-222; (b) Hehre, W. J.; Ditchfield, R.; Pople, J. A., J.
Chem. Phys. 1972, 56, 2257-2261.
Andrae, D.; Haeussermann, U.; Dolg, M.; Stoll, H.; Preuss,
H., Theoret. Chim. Acta 1990, 77, 123-141.
9. (a) Grimme, S.; Antony, J.; Ehrlich, S.; Krieg, H., J. Chem.
Author Contributions
1
The manuscript was written through contributions of all
authors. All authors have given approval to the final version of
the manuscript.
8
.
Notes
Phys. 2010, 132, 154104; (b) Grimme, S.; Ehrlich, S.; Goerigk, L., J.
Comput. Chem. 2011, 32, 1456-1465.
The authors declare no competing financial interests.
1
0.
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.; et.al.
ACKNOWLEDGMENT
We acknowledge North Carolina State University and the
National Science Foundation via the CAREER Award (CHE-
11.
THEOCHEM 1999, 464, 211-226.
2. Sandstrom, J., Dynamic NMR Spectroscopy. Academic
Press Inc. Ltd.: London NW1 7DX, 1982.
3. While most of the starting olefin was converted in this case,
Tomasi, J.; Mennucci, B.; Cances, E., J. Mol. Struct.
1
0
955636) for funding.
1
REFERENCES
the reaction produced a mixture of hexane (33%) and 2 isomers of
hexene 2-hexene and 3-hexene (66%). This was presumably
because there was insufficient H
substrate.
14.
at this time.
1.
(a) Stephan, D. W., J. Am. Chem. Soc. 2015, 137,
1
2
2
0018-10032; (b) Stephan, D. W.; Erker, G., Angew. Chem. Int. Edit.
015, 54, 6400-6441; (c) Paradies, J., Angew. Chem. Int. Edit.
014, 53, 3552-3557; (d) Houghton, A. Y.; Hurmalainen, J.;
2
to completely hydrogenate this
The induction period observed for MTO is not understood
Mansikkamaki, A.; Piers, W. E.; Tuononen, H. M., Nat. Chem. 2014,
, 983-988; (e) Stephan, D. W.; Erker, G., Chem. Sci. 2014, 5,
2625-2641; (f) Mahdi, T.; Stephan, D. W., Angew. Chem. Int. Edit.
013, 52, 12418-12421; (g) Berke, H.; Jiang, Y. F.; Yang, X. H.;
Jiang, C. F.; Chakraborty, S.; Landwehr, A., Top. Curr. Chem. 2013,
15.
(a) Ison, E. A.; Corbin, R. A.; Abu-Omar, M. M., J. Am.
6
Chem. Soc. 2005, 127, 11938-11939; (b) Ison, E. A.; Trivedi, E. R.;
Corbin, R. A.; Abu-Omar, M. M., J. Am. Chem. Soc. 2005, 127,
15374-15375.
2
1
6.
C., Organometallics 2000, 19, 3983-3990.
17. Schrodi, Y.; Schrock, R. R.; Bonitatebus, P. J.,
Organometallics 2001, 20, 3560-3573.
8. Hounjet, L. J.; Bannwarth, C.; Garon, C. N.; Caputo, C. B.;
Kalamarides, H. A.; Iyer, S.; Lipian, J.; Rhodes, L. F.; Day,
3
2
34, 27-57; (h) Lu, Z. P.; Ye, H. Y.; Wang, H. D., Top. Curr. Chem.
013, 334, 59-80; (i) Stephan, D. W.; Erker, G., Top. Curr. Chem.
2013, 332, 85-110; (j) Chernichenko, K.; Madarasz, A.; Papai, I.;
Nieger, M.; Leskela, M.; Repo, T., Nat. Chem. 2013, 5, 718-723; (k)
Stephan, D. W., Org. Biomol. Chem. 2012, 10, 5740-5746; (l)
Ghattas, G.; Chen, D. J.; Pan, F. F.; Klankermayer, J., Dalton Trans.
1
Grimme, S.; Stephan, D. W., Angew. Chem. Int, Edit. 2013, 52,
7492-7495.
19.
Röschenthaler, G.-V.; Eicher, J., Chem. Eur. J. 2013, 19, 17413-
17424.
2
2
012, 41, 9026-9028; (m) Stephan, D. W., Org. Biomol. Chem.
012, 10, 9747-9747; (n) Soos, T., Pure Appl. Chem 2011, 83,
Zeonjuk, L. L.; Vankova, N.; Mavrandonakis, A.; Heine, T.;
667-675; (o) Berkefeld, A.; Piers, W. E.; Parvez, M., J. Am. Chem.
Soc. 2010, 132, 10660-10661; (p) Eros, G.; Mehdi, H.; Papai, I.;
Rokob, T. A.; Kiraly, P.; Tarkanyi, G.; Soos, T., Angew. Chem. Int.
Edit. 2010, 49, 6559-6563; (q) Geier, S. J.; Stephan, D. W., J. Am.
Chem. Soc. 2009, 131, 3476; (r) Kim, H. W.; Rhee, Y. M., Chem.
Eur. J. 2009, 15, 13348-13355; (s) Chase, P. A.; Stephan, D. W.,
Angew. Chem. Int. Edit. 2008, 47, 7433-7437.
2
0.
Herrmann, W. A.; Roesky, P. W.; Wang, M.; Scherer, W.,
Organometallics 1994, 13, 4531-4535.
21.
Chem. Int. Edit. 2007, 46, 4968-4971.
22.
Organometallics 2011, 30, 584-594; (b) Sortais, J. B.; Voss, T.;
Kehr, G.; Frohlich, R.; Erker, G., Chem. Commun. 2009, 7417-7418;
c) Momming, C. M.; Fromel, S.; Kehr, G.; Frohlich, R.; Grimme, S.;
Erker, G., J. Am. Chem. Soc 2009, 131, 12280-12289.
23. Bako, I.; Stirling, A.; Balint, S.; Papai, I., Dalton Trans.
2012, 41, 9023-9025.
McCahill, J. S. J.; Welch, G. C.; Stephan, D. W., Angew.
(a) Voss, T.; Sortais, J.-B.; Fröhlich, R.; Kehr, G.; Erker, G.,
2.
Manners, I.; Wass, D. F., J. Am. Chem. Soc. 2016, 138, 1994-2003;
b) Flynn, S. R.; Metters, O. J.; Manners, I.; Wass, D. F.,
(a) Metters, O. J.; Forrest, S. J. K.; Sparkes, H. A.;
(
(
Organometallics 2016, ASAP (c) Cui, P.; Comanescu, C. C.; Iluc, V.
M., Chem. Commun. 2015, 51, 6206-6209; (d) Xu, X.; Kehr, G.;
Daniliuc, C. G.; Erker, G., J. Am. Chem. Soc. 2015, 137, 4550-
2
4.
25.
19, 610-627.
Mayer, J. M., Inorg. Chem. 1988, 27, 3899-3903.
Glendening, E. D.; Weinhold, F., J. Comput. Chem. 1998,
4
2
3
557; (e) Dobrovetsky, R.; Stephan, D. W., Isr. J. Chem. 2015, 55,
06-209; (f) Boone, M. P.; Stephan, D. W., Organometallics 2014,
3, 387-393; (g) Fromel, S.; Kehr, G.; Frohlich, R.; Daniliuc, C. G.;
Erker, G., Dalton Trans. 2013, 42, 14531-14536; (h) DeMott, J. C.;
Bhuvanesh, N.; Ozerov, O. V., Chem. Sci. 2013, 4, 642-649; (i) Xu,
X.; Kehr, G.; Daniliuc, C. G.; Erker, G., Organometallics 2013, 32,
26.
2505.
27.
Guo, Y.; Li, S., Eur. J. Inorg. Chem. 2008, 2008, 2501-
Stirling, A.; Hamza, A.; Rokob, T. A.; Papai, I., Chem.
7
306-7311; (j) Xu, X.; Kehr, G.; Daniliuc, C. G.; Erker, G., Angew.
Commun. 2008, 3148-3150.
8. Arif, A. M.; Bright, T. A.; Jones, R. A., J. Coord. Chem.
1987, 16, 45-50.
2
Chem. Int. Edit. 2013, 52, 13629-13632; (k) Flynn, S. R.; Wass, D.
F., ACS Catal. 2013, 3, 2574-2581; (l) Erker, G., Dalton Trans.
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Chia, M.; Pagán-Torres, Y. J.; Hibbitts, D.; Tan, Q.; Pham,
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2011, 40, 11815-11821; (b) Lilly, C. P.; Boyle, P. D.; Ison, E. A.,
Organometallics 2012, 31, 4295-4301.
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P. V. R., J. Comput. Chem. 1983, 4, 294-301.
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SYNOPSIS TOC
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δ+
B(C F )
6
5 3
Mes
δ-
Mes
N O
Re
X
N
O
X
N
Re
N
B(C F )
6
5 3
N
N
Mes
Mes
X = Me, H, Ph, C(O)Ph
Frustrated Lewis Pair
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H (50 psi)
5 mol % Re
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R2
R
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H
R
10 mol % B(C F )
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5 3
R
H
R
R1
1
Toluene, 100 °C
R3
R
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