Cationic rhenium(iii) complexes: synthesis, characterization, and reactivity for hydrosilylation of aldehydes

Article abstract of DOI:10.1039/c7dt00271h

A series of novel cationic Re(iii) complexes [(DAAm)Re(CO)(NCCH₃)₂][X] [DAAm = N,N-bis(2-arylaminoethyl)methylamine; aryl = C₆F₅ (a), Mes (b)] [X = OTf (2), BAr^F₄ [BAr^F₄ = tetrakis[3,5-(trifluoromethyl)phenyl]borate] (3), BF₄ (4), PF₆ (5)], and their analogue [(DAmA)Re(CO)(Cl)₂] [DAmA = N,N-bis(2-arylamineethyl)methylamino; aryl = C₆F₅] (6) were synthesized. The catalytic efficiency for the hydrosilylation reaction of aldehydes using 4a (0.03 mol%) has been demonstrated to be significantly more active than rhenium catalysts previously reported in the literature. The data suggest that electron-withdrawing substituents at the diamido amine ligand increase the catalytic efficiency of the complexes. Excellent yields were achieved at ambient temperature under neat conditions using dimethylphenylsilane. The reaction affords TONs of up to 9200 and a TOF of up to 126 h^-1. Kinetic and mechanistic studies were performed, and the data suggest that the reaction is via a non-hydride ionic hydrosilylation mechanism.

Full text of DOI:10.1039/c7dt00271h

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Efficient extraction of sulfate from water using
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Cationic Rhenium(III) Complexes: Synthesis, Characterization, and
Reactivity for Hydrosilylation of Aldehydes
Received 00th January 20xx,
Accepted 00th January 20xx
a
b
a
c
a
Damaris E. Pérez, Jessica L. Smeltz, Roger D. Sommer, Paul D. Boyle, and Elon A. Ison *
DOI: 10.1039/x0xx00000x
Abstract:
arylaminoethyl)methylamine; aryl
trifluoromethyl)phenyl]borate)] (3), BF
arylamineethyl)methylamino; aryl = C ] (6) were synthesized. The catalytic efficiency for the hydrosilylation reaction of
A
series of novel cationic Re(III) complexes [(DAAm)Re(CO)(NCCH
(a), Mes (b)] [X OTf (2), BAr
2
(4), PF (5)], and the analogue [(DAmA)Re(CO)(Cl) ] [DAmA = N,N-bis(2-
3
)
2
][X] [DAAm
=
N,N-bis(2-
F
4
F
4
=
C
6
F
5
=
[BAr
=
tetrakis[3,5-
www.rsc.org/
(
4
6
6 5
F
aldehydes using 4a (0.03 mol%) has been demonstrated to be significantly more active than previous rhenium catalysts
reported in the literature. The data suggest that electron-withdrawing substituents at the diamido amine ligand increases
the catalytic efficiency of the complexes. Excellent yields were achieved at ambient temperature in neat conditions using
-1
dimethylphenylsilane. The reaction affords TONs of up to 9,200 and a TOF of up to 126 h . Kinetic and mechanistic studies
were performed, and the data suggest that the reaction is via a non-hydride ionic hydrosilylation mechanism.
26, 27
reaction of carbonyl compounds.
INTRODUCTION
Hydrosilylation reactions of organic carbonyls are useful
because a protected alcohol can be generated in a single step
1
under mild conditions. These reactions are more attractive
RESULTS AND DISCUSSION
Synthesis and Characterization of Cationic DAAm
Rhenium(III) cationic
(DAAm)Re(CO)(NCCH N,N-bis(2-
6 5
arylaminoethyl)methylamine; aryl = C F (a), Mes (b)] [X = OTf
Complexes.
A
series
of
and convenient than the conventional two-step methodology
of reduction by a metal hydride followed by silyl protection. In
recent years several examples of mono-oxorhenium,
dioxorhenium, and mono-oxomolybdenum complexes were
shown to be highly active for the hydrosilylation of aldehydes
[
3
)
2
][X]
[DAAm =
F
F
(
(
3
), BAr
4
[BAr
), PF (6
6
4
= tetrakis[3,5-(trifluoromethyl)phenyl]borate)]
4
), BF4
(
5
)] complexes were synthetized from the
2
-24
DAAm rhenium(III) acetate complex [(DAAm)Re(CO)(OAc)] (1a
and ketones.
In general, cationic rhenium(V) mono-oxo
28
) by treatment with 1 equiv of a non-coordinating acid
or 1b
complexes exhibit the highest activities for catalytic
hydrosilylation reactions compared to their neutral rhenium(V)
in acetonitrile (Scheme 1).
1
0, 15
complexes counterparts.
However, recently our group
R
CO
R
X
reported a rhenium(III) complex that is significantly more
CO
Re
1
equiv HX
3
CN, rt, 5 min
25
R
N
N
R
NCCH
NCCH
3
active that its rhenium(V) metal mono-oxo precursors, and
we hypothesized that when oxorhenium complexes are
employed as catalysts, the active species are not high-valent
rhenium oxos, but species that are generated upon
deoxygenation with the organosilane. Here, we report the
synthesis of cationic rhenium(III) complexes that are efficient
catalysts for the hydrosilylation of aldehydes. A few examples
of low-valent rhenium complexes not bearing an oxo ligand
have been shown to be efficient catalysts for hydrosilylation
Re OAc
N
N
N
+ HOAc
CH
3
N
(
1a) R = C
6
F
5
6 5
(2a) R = C F , X = OTf; (2b) R = Mes, X = OTf
(3a) R = C F , X = BArF4; (3b) R = Mes, X = BArF4
(1b) R = Mes
6
5
(4a) R = C
6
F
5
, X = BF
, X = PF
4
; (4b) R = Mes, X = BF
; (5b) R = Mes, X = PF
6
4
(
5a) R = C
6
F
5
6
Scheme 1. Synthesis of Cationic DAAm Rhenium(III) Complexes
X-ray quality crystals were obtained by the slow diffusion of
ether into a concentrated acetonitrile solution of 3a (Figure 1).
The geometry around rhenium is best described as a distorted
octahedron with the carbonyl ligand and amine nitrogen
occupying the apical position. The acetate ligand is replaced by
two acetonitrile ligands in the equatorial plane and the Re1-N4
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1
4 2
Figure 2. H NMR spectrum for 4a formation after addition of HBF ·OEt to 1a in
3 2
(
2.168 Å) and Re1-N5 (2.120 Å) bond lengths are comparable CD CN at room temperature. Peaks at 3.6 and 1.9 ppm are from Et O.
2
9
to cationic Re-acetonitrile bond lengths.
1
The H NMR spectrum of 4a is shown in Figure 2 and is
representative of the series of cationic complexes. The Synthesis and Characterization of DAmA Rhenium(III)
methylene protons from the ligand backbone resonate as Complex. The complex [(DAmA)Re(CO)(Cl) ] [DAmA = N,N-
2
three distinct multiplets at δ 4.05 (2H), δ 3.72 (2H) and δ 3.01 bis(2-arylamineethyl)methylamino; aryl
(4H) ppm. A singlet at δ 2.85 ppm is assigned to the methyl synthetized by treating 1a with excess hydrochloric acid (HCl)
= C F ] (6) was
6 5
group on the amine from the ligand backbone. The singlet in methylene chloride at room temperature (Scheme 2).
corresponding to the methyl groups from the acetonitrile
C
6
F
5
C6F5
ligands resonate at δ 2.14 ppm (6H). The IR stretch of the
CO
H
CO
-1
C
6
F
5
C6F
Cl
Cl
carbonyl ligand in 4a is observed at 1882 cm . This stretch is at
2
higher frequency than the average CO stretch in 1a (1872).
N
N
5
N
N
excess HCl
CH Cl , 1 d
Re OAc
Re
8
2
2
N
N
1
a
6
Scheme 2. Synthesis of DAmA rhenium(III) dichloride complex 6.
X-ray quality crystals were obtained by the slow diffusion of
pentanes into a concentrated reaction mixture of in
methylene chloride (Figure 3). The geometry around rhenium
in is best described as distorted octahedral. The acetate
6
6
ligand is replaced by two chloride ligands in the equatorial
plane. The X-ray crystal structural data suggest that one of the
chelating ligand nitrogen atoms has been protonated.
Evidence for this protonation and the change in hybridization
2
3
from sp to sp is provided by the analysis of the angles around
N1 and N3 in . A comparison between 1a and reveals a
decrease in the Re-N1-C8 angle from 118° to 108°. Also in
6
6
6,
the Re-N1 bond length (2.2 Å) is longer than Re-N3 bond
length (1.92 Å).
Figure 1. X-ray structure of 3a. Thermal ellipsoids are at 50% probability level.
Hydrogen atoms and counter ion were omitted and the pentafluorophenyl
groups on the ligand are shown in wireframe format for clarity. Selected bond
lengths (Å) and angles (deg.): Re1-C1, 1.867(4); Re1-N1, 1.932(3); Re1-N2,
2
.220(3); Re1-N3, 1.928(1); Re1-N4, 2.167(3); Re1-N5, 2.120(2), N1-Re1-N3,
0.04(15); N1-Re1-N2, 80.79(13); N1-Re1-C1, 94.26(16); N2-Re1-C1, 172.91(16);
N1-Re1-N4, 85.09(13); N1-Re1-N5, 160.96(15); Re1-N1-C8, 119.2(2); Re1-N3-C12,
16.9(3)
1
1
The synthesis of 2-5 provided a diverse set of cationic
rhenium(III) complexes bearing either electron-withdrawing (R
6 5
= C F ) or electron-donating (R = Mes) analogues of the DAAm
ligand framework. These complexes are easily synthesized, are
air and moisture stable, and a direct comparison of the
reactivity could be made based on the electronics and sterics
of the ligand framework.
Figure 3. X-ray structure of
and counter ion were omitted and the pentafluorophenyl groups on the ligand
are shown in wireframe format for clarity. Selected bond lengths (Å) and angles
deg.): Re1-C1, 1.862(3); Re1-N1, 2.200(2); Re1-N2, 2.262(2); Re1-N3, 1.920(2);
Re1-Cl1, 2.4361(6); Re1-Cl2, 2.3685(6), N1-Re1-N3, 93.97(8); N1-Re1-N2,
8.18(7); N1-Re1-C1, 98.15(9); N2-Re1-C1, 174.44(9); N1-Re1-Cl1, 80.01(6); N1-
6. Thermal ellipsoids are at 50%. Hydrogen atoms
(
7
Re1-Cl2, 163.79(5); Re1-N1-C8, 107.95(14); Re1-N3-C12, 120.44(16).
1
In the H NMR spectrum of
6 (S37) a singlet at 3.36 ppm is
observed for the methyl group on the amine nitrogen. The
methylene protons from the ligand backbone are observed as
three distinct multiplets at δ 4.16 (2H), δ 3.45 (2H) and δ 3.31
1
4H) ppm. The amine proton was not observed by H NMR
(
spectroscopy.
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Catalytic Hydrosilylation of Aldehydes. The catalytic Complex
competency of complexes 4a-b and for the hydrosilylation heating because of the poor solubility of the complex. The
reaction of benzaldehyde were examined according to difference in reactivity may also be attributable to the fact that
Equation 1. Reactions were conducted at different does not contain open coordination sites or L-type ligands
6 is not as active as 4a (Table 1, entry 1) and requires
6
6
temperatures, catalyst loadings, and with different silane that can easily dissociate.
reagents in acetonitrile or under neat conditions. The results Reactivity was not observed when 4b was utilized as the
are shown in Table 1.
catalyst (Table 1, entries 6 and 7). These results suggest that
the electronics of the diamido amine ligand are crucial for the
reactivity of the complex. Alternatively, the decreased
reactivity may be a result of the increased steric demands of
the mesityl substituents.
Table 1. Catalytic hydrosilylation of benzaldehyde according to equation 1.
O
OR
3
x mol% [Re]
3
.2 equiv HSiR , CH CN, T
H
(1)
H
H
1
3
The effect of various silane substrates was investigated. The
data suggest that 4a performed efficiently in the
a
b
hydrosilylation of benzaldehyde with HSiMe
Ph and HSiOEt
2 3
Entry
[Re] mol %
R
3
T (°C) Time (h) Conversion (%)
(
Table 1, entry 17). Conversion was poor when HSiPh and
3
c
1
6
1
Me
Me
Me
Me
Me
Me
Me
2
Ph
80
40
60
80
rt
95
118
48
48
8
79
41
46
49
95
NR
NR
NR
51
12
HSiEt were utilized as substrates (Table 1, entries 8–10). The
3
d
2
4a
4a
4a
4a
4b
4b
4a
4a
4a
0.10
0.10
0.10
0.50
0.10
0.10
0.10
0.50
0.50
2
2
2
2
2
2
Ph
Ph
Ph
Ph
Ph
Ph
reaction with HSiMePh (Table 1, entry 18) was slower than
2
d
3
the reactions using HSiMe Ph (24 h) or HSiOEt (18 h).
2
3
d
4
Presumably the poor catalytic reactivity of 4a with HSiPh and
3
e
5
HSiMePh are due to the steric bulk of these silanes. Similar
2
d
6
rt
24
34
114
48
46
results using these reagents were observed previously by our
25, 28
group with the precursor 1a.
d
7
40
rt
d
The optimized conditions for the hydrosilylation of
benzaldehyde using 4a as the (Table 2). A catalyst loading of
0.03 mol% was chosen with HSiMe Ph or HSiOEt at room
8
Ph
3
e
9
Et
3
rt
e
2
3
1
0
Ph
3
rt
temperature under neat conditions as. Catalysis with a variety
of aldehydes was efficient, and most reactions were complete
in 24 h (Table 2).
f,k
1
1
4a
4a
4a
4a
4a
4a
4a
4a
0.01
0.03
0.05
0.10
0.50
1
Me
Me
Me
Me
Me
Me
2
Ph
Ph
Ph
Ph
Ph
Ph
rt
rt
rt
rt
rt
rt
rt
rt
72
24
16
9
92
g,k
h,k
d,k
e,k
i,k
12
13
14
15
2
2
2
2
2
100
100
100
100
100
100
71
Table 2. Hydrosilylation of Various Aldehydes Catalyzed by 4a.
2.5
1
O
2
OSiMe Ph
0.03 mol% 4a
1
6
g,k
H
H
(2)
R
H
1.2 equiv HSiMe Ph, neat, rt, time R
2
1
1
7
0.03
0.03
OEt
3
18
240
g,k
8
MePh
2
a
General Reaction Conditions: 2 mg of the catalyst, 1.2 equiv silane, 0.5 equiv 2-
bromomesitylene (internal standard), 0.35 mL CH CN, and under an
3
N
2
b
c
d
atmosphere. NMR yield. Re (0.003 mmol), and aldehyde (0.3 mmol). Re (0.003
e
f
mmol), and aldehyde (3 mmol). Re (0.003 mmol), and aldehyde (0.6 mmol). Re
g
(
0.003 mmol), and aldehyde (28 mmol). Re (0.003 mmol), and aldehyde (9
h
i
mmol). Re (0.003 mmol), and aldehyde (6 mmol). Re (0.007 mmol), and
k
aldehyde (0.7 mmol). Neat Conditions. rt = room temperature. NR = no reaction.
Complex 4a is the most efficient catalyst in the series: 100%
conversion was attained at room temperature with 0.03 mol%
catalyst loading under neat conditions achieving a TON 3,000
-1
and a TOF 138 h (Table 1, entry 12); with 0.01 mol% catalyst
loading, 92% conversion was observed and TONs 8,586 and a
-1
TOF 119 h was achieved (Table 1, entry 11). These data
suggests that cationic catalysts are significantly more active
than the neutral rhenium acetate precursors. For example, the
data at room temperature with 4a at 0.01 and 0.03 mol%
catalyst loadings, suggest that this catalyst is more efficient at
catalytic hydrosilylation than the precursor 1a, which was
-1
reported to attain a yield of 93.5% (TON = 935; TOF = 62.3 h )
2
5, 28
under similar reaction conditions.
not improve the catalytic competency of 4a (Table 1, entries 2-
) presumably due to catalyst decomposition.
Higher temperatures did
4
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a
b
time profile for this reaction under pseudo-first order
1
conditions as monitored by H NMR spectroscopy is depicted
Entry
Substrate
Time (h)
24
Conversion (%)
in Figure 4. The results suggest a first-order overall rate law
1
2
100
(Figure 4 (left)). A first order dependence on [4a] was also
obtained by plotting the observed rate constants (kobs) versus
the concentration of rhenium (Figure 4 (right)).
24
100
3
4
18
20
100
91
5
6
24
24
92
Figure 4. Left: Time profile for the catalytic hydrosilylation of benzaldehyde by
2
4a. Reaction Conditions: [Benzaldeyde] = 2.77 M; [HSiMe Ph] = 3.324 M; [2-
bromomesitylene] = 1.385 M; [4a] = 0.0279 mmol (0.01 mol%), 0.829 mM (0.03
mol%), 1.385 mM (0.5 mol%), 2.77 (1 mol%). Reactions were performed in a vial
2
under neat conditions at room temperature in a N atmosphere for a fixed time.
100
Each point represents a representative aliquot of the reaction mixture at a
1
different time point. The percent conversion was determined by H NMR
spectroscopy by integrating the product peak against the internal standard.
-
6
-1
-5 -1
Observed rate constants (kobs) 6.2 x 10 s , 2.3 x 10 s , 4.31 x 10
-5 -1
s and 1.0 x
0-4 s were obtained. Right: Plot kobs (s ) vs Concentration (mM) of 4a for
-
1
-1
1
determination of the order with respect to 4a
7
24
65
The order with respect to [benzaldehyde] was determined by
2
performing the kinetic experiments in neat HSiMe Ph. The
decay of benzaldehyde was monitored by GC with 2-
bromomesitylene as the internal standard. As shown in A
linear decay over time was observed with suggest a pseudo-
zeroth order dependence with respect to [benzaldehyde]
c
8
24
24
48
24
100
90
(Figure 6).
1
c
9
-
5
-1
Rate = -3.3(3) x 10 Ms
1
1
0
1
73
2
R = 0.9681
0
.6
65
1
2
29
20
100
100
0
.2
c
1
3
4
4
4
0
1 x 10
2 x 10
Time / s
3 x 10
a
General Reaction Conditions: Re (0.00277 mmol), aldehyde (9.21 mmol), 1.2
equiv HSiMe Ph, 0.5 equiv 2-bromomesitylene, neat, and under an
2
b
N
2
c
atmosphere. NMR yield. Silane: HSiOEt
a minimum amount of CH CN.
3
. Note: Solid aldehydes were dissolved in Figure 5. Kinetic plot of [Benzaldehyde] vs time (s). Reaction Conditions: [4a] =
.921 mM; [Benzaldehyde] = 0.921 M; [HSiMe Ph] = 5.48 M. Benzaldehyde
0
2
3
concentration was determined by GC by integrating the benzaldehyde peak
versus the internal standard, 2-bromomesitylene (0.438 M). The reaction was
performed under neat conditions at room temperature in 10 different 4 mL
screw cap vials for a fixed period of time.
Kinetic Studies. Kinetic experiments were performed to
determine the dependences with respect to each reagent. The
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Given that the reaction is first order overall, a zeroth order
dependence on [benzaldehyde] suggests that the reaction is
first order with respect to silane. Based on the kinetic
experiments the rate equation for the hydrosilylation of
benzaldehyde catalyzed by 4a is given by:
d[PhCH OSiMe Ph]
2
2
=
k[4a][HSiMe Ph] (1)
2
dt
Hammett Correlation. Competition experiments between
benzaldehyde and the corresponding para-substituted
3 3 2
benzaldehyde (para-OCH , -Cl, -CF , -NO ) were monitored by
1
H NMR spectroscopy after 1 d at room temperature. As
shown in Figure 6, the Hammett plot obtained showed a linear
correlation with a positive slope (ρ) of 0.36. The slope suggests
that for the product-forming step, electron-withdrawing
substituents accelerate the rate of the reaction.
0
.4
.2
0
Scheme 3. Stoichiometric reaction of 4a and HSiMe
for the Hydrosilylation Reaction Catalyzed by 4a
2
Ph and Proposed Mechanism
p-NO2
This result suggests that HSiMe
of benzaldehyde. Complex
2
Ph is activated in the absence
has been reported and
0
7
characterized previously by our group in the stoichiometric
2
5
p-CF3
reaction of 1a with HSiMe
exhibit minimal activity in hydrosilylation reactions described
< 5% conversion in 5 h). A similar pathway for its formation is
proposed here and is depicted in Scheme 3. In this mechanism,
HSiMe Ph is activated to produce the rhenium hydride
complex , in the absence of benzaldehyde two molecules
react to produce H and . Consistent with the kinetic data, no
reactivity was observed in the stoichiometric reaction of 4a
with benzaldehyde in the absence of HSiMe Ph.
2
Ph at 80 °C, and was shown to
p-Cl
(
p-H
2
R = 0.9252
2
8
p-OCH3
2
7
-0.4
0
0.4
0.8
2
sigma
Proposed Mechanism. The following mechanism has been
proposed for the catalytic hydrosilylation of benzaldehyde by
Figure 6. Hammett Plot for the hydrosilylation of various para substituted
benzaldehydes catalyzed by 4a. Reaction Conditions: [4a] = 0.00277 mmol; 4a (Scheme 4) based on kinetic data, the Hammett correlation,
[
Benzaldehyde] = 0.554 mmol; [para-substituted benzaldehyde] = 0.554 mmol;
1
[
NMR spectroscopy at room temperature.
2
HSiMe Ph] = 0.277 mmol; [2-bromomesitylene] = 0.274 mmol. Monitored by H and mechanistic observations. Kinetic data suggest that the
turnover-limiting step is the activation of the silane as no
dependence was observed on [benzaldehyde] and a first order
dependence on [silane] was observed. Activation of the silane
Determination of the Activation Parameters. Kinetic
experiments were performed to investigate the activation
parameters in a temperature range of 25-80 °C. The enthalpy
1
2
could occur via ƞ or ƞ coordination. Treatment of 4a with
HSiMe Ph in the absence of benzaldehyde results in the
formation of The electronic dependence on the
‡
2
of activation, ΔH , was found to be 13.1(4) kcal/mol, and the
‡
entropy of activation, ΔS , was found to be -34(1) cal/mol·K.
‡
Thus, the overall free energy of activation, ΔG (298 K) was
7.
benzaldehyde substrate, suggest that for the product-forming
step, electron-withdrawing substituents in the para position of
the benzaldehyde substrate accelerate the formation of the
reduced carbonyl products.
23.5 kcal/mol.
Possible Intermediates. Attempts were made to isolate
possible intermediates in the reaction. The stoichiometric
In recent years, an ionic hydrosilylation mechanism has been
11-
reaction of 4a with HSiMe
2
Ph resulted in the formation of a
(Scheme 3).
1
8
proposed by Oestreich and computationally studied by Wei
DAAm dirhenium(II) complex
7
1
3, 22, 23, 30
for catalysis by oxorhenium and oxomolybdenum
complexes. In this mechanism aldehyde does not coordinate to
the metal center; instead, silane is activated by coordination to
N
rhenium, this is followed by nucleophilic S 2-Si attack of the
carbonyl substrate at the activated silicon leading to the
cleavage of the Si-H bond and formation of an ion pair.
Product formation results from transfer of the hydride from
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DOI: 10.1039/C7DT00271H
Journal Name
ARTICLE
rhenium to the carbonyl carbon of the resultant ion-pair. As mmol; 1b = 50 mg, 0.08 mmol) was added to a screw cap vial
shown in Scheme 4, the proposed mechanism is consistent and dissolved in 0.35 mL of acetonitrile. Then, the
F
with these findings.
corresponding acid HX (X = HOTf, HBAr
4
, HBF
4
·OEt
2
, HPF
6
·H
2
O)
CO
(
1 equiv) was added to the solution. The solvent was removed
NCCH
NCCH
3
3
(
DAAm) Re
under pressure. The resulting oil was dissolved in diethyl ether
and the corresponding bisacetonitrile product was precipitated
with excess of hexanes. The final powder was collected via
vacuum filtration.
NCCH
3
OSiR
3
CO
(DAAm) Re NCCH3
[
(DAAm-C
6 5 3 2
F )Re(CO)(NCCH ) ][OTf], 2a. Following the
H
H
HSiR3
general synthesis, complex 2a was synthetized in quantitative
1
yield. H NMR (300 MHz, CD
.01 (m, 4H), 2.85 (s, 3H), 1.96 (s, 6H). C NMR (101 MHz,
CD CN) δ: 188.2, 172.9, 143.5, 142.7, 142.1, 141.3, 140.3,
3
CN) δ: 4.05 (m, 2H), 3.71 (m, 2H),
NCCH
3
1
3
3
NCCH
3
3
1
9
1
39.0-138.2, 137.8, 137.5, 65.9, 59.1, 55.2. F NMR (376 MHz,
CO
DAAm) Re
CO
OSiR
H
3
CD CN) δ: -79.4 (s, 3F), -149.8 (dd, J = 22.1, 5.7 Hz, 2F), -150.8
3
CO
SiR
H
3
(
H SiR
3
or(DAAm)Re
(dd, J = 14.0, 8.0 Hz, 2F), -162.9 (t, J = 20.9 Hz, 2F), -165.8 (m,
(
DAAm) Re H +
-1
8
2F), -166.7 (m, 2F). IR (FTIR, cm ):
ͪ(CO) 1882. Anal. Calc. for
C H F N O ReS: C, 30.95; N, 7.85; H, 1.92. Found: C, 30.39;
O
2
3
17 13
5
4
H
N, 7.74; H, 1.90.
(DAAm-Mes)Re(CO)(NCCH ) ][OTf], 2b. Following the
[
3
2
Scheme 4. Proposed Mechanism for the Hydrosilylation Reaction Catalyzed by general synthesis, complex 2b was synthetized in quantitative
4a.
1
yield. H NMR (400 MHz, CD CN) δ: 6.87 (s, 2H), 6.76 (s, 2H),
3
4
.03 (m, 2H), 3.56 (m, 2H), 3.07 (m, 2H), 2.91 (m, 5H), 2.33 (s,
CONCLUSION
1
H), 2.25 (s, 6H), 1.96 (m, 6H), 1.84 (s, 6H). C NMR (126 MHz,
3
6
In conclusion, we have demonstrated that cationic DAAm
rhenium(III) complexes are highly efficient catalysts for the
hydrosilylation of aldehydes. Kinetic data revealed that the
CD CN) δ: 193.4, 159.2, 135.3, 132.1, 131.6, 130.4, 129.7, 66.5,
3
1
9
6
0.2, 49.4, 20.4, 20.3, 20.0. F NMR (376 MHz, CD CN) δ: -79.6
3
-1
st
(s, 3F). IR (FTIR, cm ): ͪ(CO) 1890. Anal. Calc. for
C H F N O ReS·H O: C, 43.71; N, 8.79; H, 4.93. Found: C,
reaction rate is 1 order in [silane] and [4a], and is
2
9
39
3
5
4
2
independent of aldehyde concentration.
A
Hammett
4
2.50; N, 8.69; H, 5.00.
(DAAm-C F )Re(CO)(NCCH ) ][BAr ], 3a. Following the
correlation suggests that electron-withdrawing substituents on
the para position of benzaldehyde accelerate the reaction rate.
Based on this data, a non-hydride ionic outer-sphere
hydrosilylation mechanism has been proposed.
F
[
6
5
3 2
4
general synthesis, complex 3a was synthetized in quantitative
1
yield. H NMR (300 MHz, CD CN) δ: 7.69 (m, 12H), 4.03 (m,
3
1
3
2
H), 3.72 (m, 2H), 2.99 (m, 4H), 2.84 (s, 3H), 1.96 (s, 6H).
C
EXPERIMENTAL SECTION
2
8
-
NMR (175 MHz, CD CN) δ: 188.1, 163.0, 162.7, 162.4, 162.1,
3
General Considerations. 1a-b
31, 32
and [(3,5-(CF
3 2 6 3 4
) C H ) B]
+
F
143.7, 142.9, 142.2, 141.4, 140.4, 139.2-138.6, 137.9, 137.6,
[
2 2 4
H(OEt ) ] (HBAr )
were prepared according to previous
1
5
30.2, 130.0, 129.8, 129.6, 127.7, 126.2, 124.6, 123.1, 66.2,
procedures. All reactions were carried out in a nitrogen filled
glove box unless otherwise noted. All reagents were purchased
from commercial sources, placed in a nitrogen filled glove box
1
9
9.3, 47.5. F NMR (376 MHz, CD CN) δ: -63.25 (s, 24F), -149.8
3
(m, 2F), -150.8 (m, 2F), -162.9 (m, 2F), -165.8 (t, J = 20.7 Hz,
-1
1
13
2F), -166.2 (m, 2F). IR (FTIR, cm ): ͪ(CO) 1925. Anal. Calc. for
C H BF N ORe·H O: C, 39.92; N, 4.31; H, 1.92. Found: C,
and used as received without further purification. H, C and
1
F spectra were acquired on a Varian Mercury 700 MHz,
9
5
4
29
34
5
2
3
9.11; N, 4.80; H, 2.02.
(DAAm-Mes)Re(CO)(NCCH ) ][BAr ], 3b. Following the
Varian Mercury 400 MHz or Varian Mercury 300 MHz
spectrometer. NMR chemical shifts are listed in parts per
million (ppm) and are referenced to residual protons or
carbons of the deuterated solvents, respectively at room
temperature unless otherwise noted. The FTIR spectra were
obtained in KBr thin films on a JASCO FT/IR-4100 instrument.
Gas Chromatography was performed on an Agilent 7820A
GC/FIC using HP-5MS columns. Elemental analyses were
performed by Atlantic Micro Laboratories Inc. X-ray
crystallography was performed at the X-ray Structural Facility
at North Carolina State University by Dr. Roger D. Sommer and
Paul D. Boyle.
F
[
3
2
4
general synthesis, complex 3b was synthetized in quantitative
1
yield. H NMR (376 MHz, CD CN) δ: 7.69 (m, 12H), 6.87 (s, 2H),
3
6
.75 (s, 2H), 4.02 (m, 2H), 3.55 (m, 2H), 3.07 (m, 2H), 2.91 (m,
1
9
5
H), 2.33 (s, 6H), 2.25 (s, 6H), 1.96 (m, 6H), 1.84 (s, 6H).
F
-1
3
NMR (376 MHz, CD CN) δ: -63.4 (s, 24F). IR (FTIR, cm ): ͪ(CO)
1
1
890. The complex was not characterized by C NMR due to
3
its very poor stability. Elemental Analysis was not attempted
on this complex because of its instability.
[(DAAm-C
6 5 3 2 4
F )Re(CO)(NCCH ) ][BF ], 4a. Following the
general synthesis, complex 4a was synthetized in quantitative
1
yield. H NMR (300 MHz, CD
3.01 (m, 4H), 2.85 (s, 3H), 2.14 (s, 6H). C NMR (176 MHz,
CD CN) δ: 188.2, 143.5, 142.7, 142.1, 141.3, 140.2, 139.0-
38.4, 137.8, 137.6-137.3, 66.0, 59.1, 47.3. F NMR (376 MHz,
3
CN) δ: 4.05 (m, 2H), 3.72 (m, 2H),
General Synthesis [(DAAm-aryl)Re(CO)(NCCH
3
)
2
][X] (aryl =
); 2a-b, 3a-b, 4a-b, 5a-b.
In a nitrogen filled glove box, the corresponding [(DAAm-
aryl)Re(CO)(OAc)] (aryl = C 1a), Mes(1b)) (1a = 50 mg, 0.07
1
3
F
6 5 4 4 6
C F , Mes) (X = OTf, BAr , BF , PF
3
1
9
1
6 5
F (
CD
3
CN) δ: -149.8 (dd, J = 22.1, 5.8 Hz, 2F), -150.8 (m, 2F), -
This journal is © The Royal Society of Chemistry 20xx
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DOI: 10.1039/C7DT00271H
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ARTICLE
1
51.7 (d, J = 20.2 Hz, 4F), -162.9(t, J = 20.9 Hz, 2F), -165.8 (m, dissolved in 0.35 mL of acetonitrile. HBF
4
·OEt
2
(1 equiv) was
-1
2F), -166.2 (m, 2F). IR (FTIR, cm ):
ͪ(CO) 1882. Anal. Calc. for added to the solution. The solvent was removed under
ORe: C, 31.82; N, 8.43; H, 2.06. Found: C, 31.40; N, reduced pressure to afford the corresponding oil. The
C
22
H17BF14
N
5
7.90; H, 2.12. aldehyde (27.7 mmol, 0.01 mol%; 9.23 mmol, 0.03 mol%; 5.54
[
3 2 4
(DAAm-Mes)Re(CO)(NCCH ) ][BF ], 4b. Following the mmol, 0.05 mol%; 2.77 mmol, 0.1 mol%; 0.554 mmol, 0.5
general synthesis, complex 4b was synthetized in quantitative mol%; 0.277 mmol, 1 mol%), silane (33.2, 11.1, 6.65, 3.32,
1
yield. H NMR (300 MHz, CD
3
CN) δ: 6.87 (s, 2H), 6.75 (s, 2H), 0.665 and 0.332 mmol respectively) and 2-bromomesitylene
4
6
1
4
1
.03 (m, 2H), 3.55 (m, 2H), 3.07 (m, 2H), 2.91 (m, 5H), 2.33 (s, (0.20 mL, 1.39 mmol) were sequentially added to the screw
1
H), 2.25 (s, 6H), 1.84 (d, 12H). C NMR (126 MHz, CD
3
3
CN) δ: cap vial. The reaction was stirred at room temperature for the
93.4, 159.3, 135.3, 132.1, 131.6, 130.4, 129.7, 66.5, 60.2, designated amount of time. An aliquot of the crude reaction
1
9
9.42, 20.43, 20.30, 20.02. F NMR (376 MHz, CD
3
CN) δ: - mixture was dissolved in CDCl
(CO) 1878. Anal. Calc. for conversion was determined by the H NMR ratio of the
O: C, 44.68; N, 9.30; H, 5.49. Found: C, product and starting material: ((integration of methylene
3.80; N, 8.99; H, 5.38. protons/2)/((integration of aldehyde peak) + (integration of
(DAAm-C )Re(CO)(NCCH ], 5a. Following the methylene protons/2))).
general synthesis, complex 5a was synthetized in quantitative General Procedure for Catalytic Hydrosilylation Reactions of
3 3
or CD CN. The percent
-
1
1
51.3 (s, 4F). IR (FTIR, cm ):
ORe·H
ͪ
C
28
H39BF
4
N
5
2
4
[
6
F
5
3 2 6
) ][PF
1
3
yield. H NMR (300 MHz, CD CN) δ: 4.07 (m, 2H), 3.74 (m, 2H), Aldehydes with 6.
1
.05 (m, 4H), 2.87 (s, 3H), 1.99 (s, 6H). C NMR (101 MHz, In a nitrogen-filled glove box complex
3
3
6
(2 mg, 0.003 mmol)
CD
3
CN) δ: 188.1, 144.1, 143.3, 141.6, 140.8, 139.3, 138.5, was added to a screw cap NMR tube and dissolved in 0.35 mL
1
9
37.8, 136.9, 66.0, 59.2, 47.3. F NMR (376 MHz, CD
1
3
CN) δ: - of acetonitrile. The aldehyde (28 µL, 0.274 mmol), silane (50
72.0 (s, 3F), -73.9 (s, 3F), -150.0 (dd, J = 22.1, 5.6 Hz, 2F), -150.9 µL, 0.326 mmol) and 2-bromomesitylene (21 µL, 0.136 mmol)
(
dt, J = 16.1, 4.7 Hz, 2F), -163.0 (t, J = 20.9 Hz, 2F), -165.8 (t, J = were sequentially added to the screw cap NMR tube. The
-1
2
0.0 Hz, 2F), -166.2 (m, 2F). IR (FTIR, cm ):
ͪ(CO) 1882. Anal. reaction was heated at 80 °C for the designated amount of
Calc. for C22
9.62; N, 7.19; H, 2.03.
(DAAm-Mes)Re(CO)(NCCH
17 16 5
H F N
OPRe: C, 29.74; N, 7.88; H, 1.93. Found: C, time. The percent conversion was determined by the proton
2
NMR ratio of the product and starting material ((integration of
[
3
)
2
][PF
6
], 5b. Following the methylene protons/2)/((integration of aldehyde peak) +
general synthesis, complex 5b was synthetized in quantitative (integration of methylene protons/2))).
1
yield. H NMR (300 MHz, CD
3
CN, δ): 6.88 (s, 2H), 6.75 (s, 2H), General Procedure to Obtain Order with Respect to
4
6
.03 (m, 2H), 3.55 (m, 2H), 3.07 (m, 2H), 2.97 (m, 5H), 2.33 (s, Benzaldehyde by Gas Chromatography.
1
3
H), 2.25 (s, 6H), 1.84 (m, 6H), 1.86 (s, 6H). C NMR (101 MHz, In a nitrogen filled glove box complex 1a (2 mg, 0.003 mmol),
CN, δ): 193.37, 159.31, 135.36, 132.15, 131.60, 130.40, and HBF ·OEt (1 equiv) were added to a screw cap vial and
29.70, 66.52, 60.21, 49.47, 20.48, 20.34, 20.07. F NMR (376 dissolved in 0.35 mL of acetonitrile. The solvent was removed
CD
3
4
2
1
9
1
-1
3
MHz, CD CN, δ): -71.96 (s, 3F), -73.84 (s, 3F). IR (FTIR, cm ): under reduced pressure to afford the corresponding oil. Then,
ͪ
(CO) 1878. Elemental Analysis was not attempted on this benzadehyde (2.8 mmol, 0.28 mL), dimethylphenylsilane (17
complex because of its instability.
6 5 2
Synthesis of complex [(DAAm-C F )Re(CO)(Cl) ], 6.
mmol, 2.5 mL), and 2-bromomesiltylene (1.4 mmol, 0.20 mL)
were added to the screw cap vial and mixed. The reaction
In a screw cap vial, complex 1a (50 mg, 0.07 mmol) was mixture was divided into 10 small screw cap vials equipped
dissolved in a minimal amount of methylene chloride. Excess with a stir bar. The reactions were stirred at room temperature
12 M HCl (5.0 µL, 60.0 µmol) was added to the screw cap vial. for the designated amount of time. At that time, the ratio
The solution was shaken and let to stand overnight. The benzaldehyde:2-bromomesitylene was determined by GC
product precipitates out of solution as green crystals (21.9 mg, using a calibration curve.
1
3.0% yield). H NMR (400 MHz, CD
4
3
CN) δ: 4.16 (m, 2H), 3.45 General Procedure to Competition Reaction for Hammett
(
m, 2H), 3.36 (s, 3H), 3.31 (m, 4H). The amine proton in the Plot.
ligand backbone was not observed either at room temperature In a nitrogen-filled glove box complex 1a (2 mg, 0.003 mmol),
or -60 °C indicating that the proton is too fast to be observed and HBF ·OEt (1 equiv), benzaldehyde (0.55 mmol, 56 µL), the
CN) δ: -150.1 indicated para-substituted benzaldehyde (0.55 mmol),
m, 2F), -150.5 (dd, J = 23.5, 6.6 Hz, 2F), -162.9 (m, 2F), -167.2 dimethylphenylsilane (0.28 mmol, 42 µL), and 2-
4
2
1
9
on the NMR time scale. F NMR (376 MHz, CD
3
(
(
1
3
m, 4F). The complex was not characterized by C NMR due to bromomesiltylene (0.27 mmol, 42 µL) were added to the screw
or methylene chloride- cap vial. The reaction mixture was dissolved in CD CN (0.35
(CO) 1873. Anal. Calc. for mL) and stirred for 24 h at room temperature. An aliquot of
its very poor solubility in acetonitrile-d
3
3
-1
d
2
.
IR (FTIR, cm ):
ORe: C, 29.48; N, 5.73: H, 1.65. Found: C, 29.79; the reaction mixture was placed in CD
integration against the internal standard (2-bromomesitylene)
General Procedure for Catalytic Hydrosilylation Reactions of of the methylene protons of the para-substituted silyl ether
Aldehydes with 4a and 4b. and the benzyloxydimethylphenylsilane were used to
In a nitrogen-filled glove box, the corresponding [(DAAm- determine the product ratio value (P /P ).
aryl)Re(CO)(OAc)] (aryl = C 1a), Mes (1b)) (1a = 2 mg, 0.003 General Procedure for Eyring Plot Data.
ͪ
1
C
18
H13Cl
2
F
10
N
3
3
CN. The H NMR
N, 6.01: H, 1.59.
H
X
6 5
F (
mmol; 1b = 0.003 mmol) was added to a screw cap vial and
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ARTICLE
In a nitrogen-filled glove box complex 1a (2 mg, 0.003 mmol) 20.
was added to a screw cap NMR tube and dissolved in 0.35 mL 236, 107-112.
of CD CN. Then HBF ·OEt (0.3 µL, 0.003 mmol) was added to 21. S. C. Sousa, I. Cabrita and A. C. Fernandes, Chem. Soc.
the solution and mixed. The benzaldehyde (0.554 mmol, 56 Rev., 2012, 41, 5641-5653.
B. Royo and C. C. Romão, J. Mol. Catal. A: Chem., 2005,
3
4
2
µL), dimethylphenylsilane (0.665 mmol, 0.1 mL) and 2- 22.
J. Wang, L. Huang, X. Yang and H. Wei,
bromomesitylene (0.277 mmol, 42 µL) were sequentially Organometallics, 2014, 34, 212-220.
added to the mixture. The reaction was heated at the 23.
J. Wang, W. Wang, L. Huang, X. Yang and H. Wei,
respective temperature (25–80 °C). The percent conversion ChemPhysChem, 2015, 16, 1052-1060.
was determined by the proton NMR ratio of the product and 24.
J. E. Ziegler, G. Du, P. E. Fanwick and M. M. Abu-Omar,
starting material.
Inorg. Chem., 2009, 48, 11290-11296.
2
2
2
5.
012, 31, 5994-5997.
6. A. Choualeb, E. Maccaroni, O. Blacque, H. Schmalle and
J. L. Smeltz, P. D. Boyle and E. A. Ison, Organometallics,
Acknowledgements
H. Berke, Organometallics, 2008, 27, 3474-3481.
We acknowledge North Carolina State University and the
2
1
2
7.
783-1788.
8. J. L. Smeltz, P. D. Boyle and E. A. Ison, J. Am. Chem.
H. Dong and H. Berke, Adv. Synth. Catal., 2009, 351,
National Science Foundation via the CAREER Award (CHE-
0955636) for funding.
Soc., 2011, 133, 13288-13291.
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DOI: 10.1039/C7DT00271H
Cationic Re(III) complexes are shown to be more active for the catalytic hydrosilylation of
benzaldehydes than their neutral acetate precursors.