of 61
S
I
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1
Improving Molecular Iron Ammonia Oxidation Electrocatalysts via Substituent Effects
T
hat Modulate Standard Potential and Stability
Michael D. Zott and Jonas C. Peters*
Corresponding Author Email: jpeters@caltech.edu
Division of
Chemistry and Chemical Engineering, California Institute of Technology, Pasadena,
California 91125, United States
Contents:
S1. General
p
rocedures
p. S
I
-
2
S2. Synthetic procedures
p. S
I
-
4
S3.
Additional voltammetry data related to
E
°
p. S
I
-
1
7
S4.
Additional voltammetry data for R = CF
3
p. S
I
-
1
8
S5.
E
1
v
oltammetry
p. S
I
-
20
S
6
. Catalytic controlled potential coulometry experiments
p. S
I
-
2
1
S
7
. CV of [(BPM
OMe
)Fe(N')
2
]
2+
under catalytic conditions
p. S
I
-
2
4
S
8
. Evans method
data
p. S
I
-
2
5
S
9
. UV
-
vis titration data for stability
a
gainst
demetallation
p. S
I
-
3
9
S1
0
. Mass spectrometry
p. S
I
-
4
5
S11. N
MR Spectra
p. S
I
-
4
9
S12. References
p. S
I
-
6
0
S
I
-
2
S.1 General p
rocedures
General Considerations:
All manipulations
of inorganic complexes
were carried out using
standard Schlenk or glovebox
techniques under an N
2
or
Ar atmosphere.
Unless otherwise noted,
solvents were deoxygenated and dried by
thoroughly sparging with N
2
gas followed by passage
through an activated alumina column in
a
solvent purification system
(
SG Water, USA LLC
)
. For
electrochemical measurements under an Ar atmosphere, solvents were further degassed and then
stored
under Ar.
All solvents were stored over activated
3 or
4 Å molecular sieves prior to use.
A
nhydrous ammonia gas was dried by passage through a calcium oxide drying tube. All reagents
were purchased from commercial vendors and used without further purification unles
s otherwise
stated. Tris(2
-
pyridylmethyl)amine (TPA)
,
1
tris(2
-
pyridylmet
hylamine) iron(II) triflate bis
-
acetonitrile
([(TPA)Fe(MeCN)
2
]OTf
2
)
,
2
6
-
(1,1
-
di(pyridin
-
2
-
yl)ethyl)
-
2,2'
-
bipyridine
(
BPM
),
3
6
-
(1,1
-
di(pyridin
-
2
-
yl)ethyl)
-
2,2'
-
bipyridine
iron(II)
triflate
bis
-
acetonitrile
([(
BPM
)Fe(MeCN)
2
]OTf
2
),
4
bis(4
-
(dimethylamino)
pyridin
-
2
-
yl)methanone
,
5
tris(4
-
(dimethylamino)
-
2
-
picolyl)amine
(TPA
NMe2
)
,
6
tris(4
-
methoxy
-
2
-
picolyl)amine
(TPA
OMe
),
6
4
-
(trifluoromethyl)pyridine
-
2
-
carboxaldehyde,
7
4
-
(trifluoromethyl)
-
2
-
ethylpyridine,
8
bis(4
-
(dimethylamino)pyridin
-
2
-
yl)methanone
,
9
and
m
ethylenetriphenylphosphorane
10
were
synthesized according to literature procedures.
Electrochemistry:
Voltammetry
experiments were carried out
with
a Biologic
VSP
-
300
or
CH
Instruments 600B
p
otentiostat using a one
-
compartment
three
-
electrode cell, and
coulometry
experiments were carried out
with
a Biologic VSP
-
300
p
otentiostat using a one
-
compartment
three
-
electrode cell
with a septum capped 14/20 joint for headspace analysis
.
For
voltammetry
,
a
boron
-
d
oped
d
iamond (BDD
)
working
electrode (3 mm
diameter),
a
Pt wire counter electrode
,
and
a
n
Ag/AgOTf
reference electrode
(
5 mM AgOTf and 0.1 M TBAPF
6
in MeCN) were employed.
For
controlled potential coulometry (CPC)
, the same reference electrode was used, but a BDD
plate (
geometric area
:
4
cm
2
) and a Pt mesh were used as working and count
er electrode
s,
respectively
. All redox potentials in the present work are reported versus
the
Fc/Fc
+
couple,
measured before e
ach
experiment
to be
approximately
+
0.1
2
V
versus
the
Ag/AgOTf reference
electrode
.
Cyclic voltammograms (CVs)
were collected at 10
0 mV·s
1
unless
otherwise
specified
. E
1/2
values
for the reversible waves were obtained from the half potential between th
e oxidative and reductive
peaks
.
CV
measurement
s
were
performed
apply
ing IR compensation
,
compensating 85% of the
resistance measured at one high frequency value (100 kHz).
CVs were col
lected using either 100 mM TBAPF
6
or 50 mM NH
4
OTf supporting electrolyte.
Under our conditions,
NH
4
OTf
was more conductive than TBAPF
6
;
thus
, a lower concentration
could be used w
hile maintaining high conductivity
.
For
CPC experiments,
NH
4
OTf
was
the sole
electrolyte
used
since it
enabled
higher turnover numbers to be achieved
.
Gas Chromatography:
Gas chromatography was performed in the
Caltech
Environmental
Analysis Center using HP 5890 Series II instruments.
Gas quantification was performed
with
a
molecular sieve column attached to a thermal conductivity detector
using
a
rgon as the carrier gas.
S
I
-
3
Standard curves were generated by direct injection
of hydrogen or nitrogen gas. Quantification of
background nitrogen was determined using the background oxygen signal. Isotopic measurements
were performed with a separate HP 5890 Series II
instrument
equipped with a GasPro column
using helium as the carri
er gas.
NMR:
NMR spectroscopy was performed using Varian
and Bruker
400 MHz NMR spectrometer
s
equipped with
broadband auto
-
tune probe
s
.
1
H NMR chemical shifts are reported in ppm relative
to tetramethylsilane, using residual solvent resonances as internal
standards.
UV
-
Vis:
Spectra
were collected
at 25 °C
using a
Cary 6
0 instrument
equipped with a thermostat
and
Cary WinUV software.
X
-
r
ay Crystallography:
XRD studies were carried out at the
Caltech
Beckman Institute
Crystallography Facility on a Bru
ker
D8 Venture diffractometer (Cu
Kα radiation).
The crystals
were mounted on a glass fiber under Paratone N oil.
Structures were solved using
direct methods
with
SHELXS or SHELXT and refined against F
2
on all data by full
-
matrix least squares with
SHELXL.
11
A
ll of the solutions were performed in the Olex2 program.
12
S
I
-
4
S
2
.
Synthetic procedures
TPA derivatives
TPA
CF3
(
tris(4
-
(
trifluoromethyl
)
-
2
-
picolyl)amine)
Ammonium chloride (15 mg, 1 eq), 4
-
(
trifluoromethyl
)
-
pyridine
-
2
-
carbaldehyde (250 mg, 5 eq),
and triethylamine (44 μL, 1.1 eq) were combined in dichloromethane (5 mL). Sodium
triacetoxyborohydride (270 mg, 4.5 eq) was added as a solid, and the mixture was stirred at
room
temperature for 48 h
. A saturated aqueous solution of sodium carbonate was added, and the organic
phase was separated. The aqueous phase was extracted twice with dichloromethane, dried over
sodium sulfate, filtered, and
concentrated
under reduced pressure. The remaining oil w
as dissolved
in ethyl acetate and added to a silica plug. The silica plug was eluted with ethyl acetate until the
eluent was colorless, then the product was eluted with methanol, dried over sodium sulfate, and
concentrated
under reduced pressure to yield a
yellow oil (48 mg, 34%
yield
).
1
H NMR (CDCl
3
, 400 MHz): δ (ppm) = 8.
71
(d,
J
= 5.
1
Hz, 3H), 7.
72
(
d
,
J
= 1.8 Hz,
3H), 7.3
6
(
d
d,
J
=
5
.
1, 1.7
Hz, 3H), 4.0
7
(s, 6H).
13
C
{
1
H}
NMR (CDCl
3
, 101 MHz
): δ (ppm) =
160.
55
(s)
, 150.
19
(s)
, 138.
8
5 (q,
J
= 34.1 Hz),
122.
74
(q
,
J
= 273.
3
Hz), 118.
82
(q,
J
= 3.6 Hz), 117.
87
(q,
J
= 3.
8
Hz), 60.
55
(s)
.
19
F{
1
H} NMR (CDCl
3
, 376 MHz): δ (ppm) =
65.1
(s)
.
MS (ESI,
UHP
LC
-
MS (CH
3
CO
2
H),
m/z
): calculated
for
C
21
H
1
5
F
9
N
4
+
H,
[
M+
H]
+
: 495.1
,
found:
495.1.
S
I
-
5
General note about
the
synthesis of iron compounds:
For the preparation of the
following
iron
complexes, only the crystalline yield for the first crop of crystals is reported. Higher yields can
be obtained if the supernatant is concentrated and recrystallized.
[(TPA
NMe2
)Fe(MeCN)
2
]OTf
2
White solids TPA
NMe2
(
tris(4
-
(
dimethylamino
)
-
2
-
picolyl)amine) (30 mg
, 1 eq
) and FeOTf
2
·
2
MeCN (31 mg
, 1 eq
) were each dissolved in 0.4 mL acetonitrile.
The solution of FeOTf
2
was added
to the solution of TPA
NMe2
, instantly producing a pale orange
-
brown
sol
ution.
The
solution was
filtered through Celite, and d
iethyl ether (2 mL) was layered on top of
the filtrate.
This
mixture
was placed in a freezer
(
30
°
C)
until a purple
-
gray (this compound changes color with varying
temperature) precipitate appeared
. The precipitat
e was isolated by decanting the supernatant and
drying under vacuum
(50.7 mg, 83%
yield
)
.
1
H NMR (
25
°
C
, C
D
3
CN
, 400
.15
MHz): δ
(ppm)
=
94.92
(s),
93.33
(s),
43.73
(
s
)
,
35.32
(s),
15.88
(
s
)
.
19
F
{
1
H} NMR (
25
°
C
, C
D
3
CN
,
376
MHz
): δ
(ppm)
=
77
.1
.
UV
-
vis (MeCN):
nm [cm
-
1
M
-
1
]):
225 [20000]
,
283 [40000], 342 [3900]
.
MS (ESI, direct injection in MeCN,
m/z
):
calculated
for C
25
H
33
N
7
O
3
F
3
SFe
,
[M]
+
: 624.2, found:
624.3
.
E
lectrochemistry
:
E
1/2
= 0.21 V vs Fc/Fc
+
(MeCN, 0.1 M TBAPF
6
,
BDD disk electrode)
.
[(TPA
O
Me
)Fe(MeCN)
2
]OTf
2
White solids TPA
OMe
(
tris(4
-
methoxy
-
2
-
picolyl)amine) (25 mg
, 1 eq
) and FeOTf
2
·
2 MeCN (29
mg
, 1 eq
) were each dissolved in 0.25 mL acetonitrile. The solution of FeOTf
2
was added to the
solution of TPA
OMe
, instantly producing a purple
-
red solution. The solution was filtered through
Celite, and diethyl ether (2 mL) was layered on top of the filtrate. This mixture was placed in a
freezer (
30
°
C) until dark purple needle
-
s
haped crystals appeared
.
The precipitate was isolated
by decanting the supernatant and drying under vacuum
(34.7 mg, 64%
yield
)
.
S
I
-
6
1
H NMR (25 °C, CD
3
CN, 400.15 MHz): δ (ppm) =
38.51
(s),
30.31
(s),
22.62
(s),
20.02
(s),
4.63
(s).
19
F{
1
H} NMR (25
°C, CD
3
CN,
376
MHz): δ (ppm) =
7
8.1
.
UV
-
vis (MeCN): nm [cm
-
1
M
-
1
]): 235 [24
000],
339
[
5100
],
375
[
4700
].
MS (ESI, direct injection in MeCN,
m/z
): calculated
for C
22
H
24
N
4
O
6
F
3
SFe, [M]
+
: 585.1, found:
585.2.
Electrochemistry:
E
1/2
= 0.55 V vs Fc/Fc
+
(MeCN, 0.1 M TBAPF
6
,
BDD disk electrode)
.
[(TPA
CF3
)Fe(MeCN)
2
]OTf
2
Yellow oil
TPA
CF3
(
tris(4
-
(
trifluoromethyl
)
-
2
-
picolyl)amine) (
72
mg
, 1 eq
) and
white solid
FeOTf
2
·
2 MeCN (
63
mg
, 1 eq
) were each
dissolved in 0.
5
mL acetonitrile. The solution of FeOTf
2
was
added to the solution of TPA
CF3
, instantly producing a purple
-
red solution. The solution was
filtered through Celite, and diethyl ether (2 mL) was layered on top of the filtrate. This mixture
was
placed in a freezer (
30
°
C) until dark
red
-
brown
crystals appeared
. The precipitate was
isolated by decanting the supernatant and drying under vacuum
(
47.5
mg,
35
%
yield
)
.
1
H NMR (25 °C, CD
3
CN, 400.15 MHz): δ (ppm) =
11.07
(s),
8.43
(s),
5.86
(s),
3.66
(s).
19
F{
1
H} NMR (25
°C, CD
3
CN, 376 MHz): δ (ppm) =
65
.9 (s),
78.
9 (s)
.
UV
-
vis (MeCN): nm [cm
-
1
M
-
1
]):
225 [9900], 262
[
13000
],
388
[
6600
],
42
8
[
8900
].
MS (ESI, direct injection in MeCN,
m/z
): calculated
for C
22
H
15
N
4
O
3
F
12
SFe, [M]
+
:
699.0
, found:
699.0
.
Electrochemistry:
E
p
/2
=
1.09
V (irreversible) vs Fc/Fc
+
(MeCN, 0.1 M TBAPF
6
,
BDD disk
electrode)
.
S
I
-
7
BPM
dipyridyl substitution
2,2'
-
(eth
e
ne
-
1,1
-
diyl)bis(
4
-
(dimethylamino)pyridine
)
Note:
This compound
was prepared
with modifications to the literature procedure for
2,2'
-
(ethene
-
1,1
-
diyl)
dipyridine.
13
Notably,
poor
performance
was
observed
when
m
ethylenetriphenylphosphorane
was generated
in
situ
,
presumably due to residual potassium salts
(i.e.
,
KBr).
B
is(4
-
dimethylamino
pyridin
-
2
-
yl)methanone
(406 mg, 1.5 mmol, 1 eq) was suspended in
anhydrous tetrahydrofuran (15 mL, 0.1 M) in a glovebox and cooled to −30
°C
, after which isolated
m
ethylenetriphenyl
phosphorane
(435 mg, 1.58 mmol, 1.05 eq) was added in one portion.
This
mixture was warmed to room temperature and allowed to react for at least 4
h
(reaction times up
to overnight have no deleterious impact).
Outside of the glovebox, t
he reaction was quen
ched with
minimal water and concentrated to yield a viscous orange
-
red oil.
The oil was
purified by silica
gel column chromatography by
load
ing
with dichloromethane
,
elut
ing
the triphenylphosphine
oxide with 3:1 ethyl acetate:acetone
plus
1% triethylamine
,
t
hen elut
ing
the product with acetone
plus
1% triethylamine
to afford
a white crystalline solid (346 mg, 86% yield).
R
f
= 0.3
(
acetone
plus
1% triethylamine
;
TLC plates treated with triethylamine
)
.
1
H NMR (CDCl
3
, 400 MHz): δ (ppm) =
8.29 (d,
J
= 5.9 Hz, 2H), 6.67 (d,
J
= 2.
7
Hz, 2H), 6.46
(dd,
J
= 5.9, 2.6 Hz, 2H), 5.94 (s, 2H), 3.00 (s, 12H).
13
C
{
1
H}
NMR (CDCl
3
, 101 MHz
): δ (ppm) =
158.66
(s)
, 154.77
(s)
, 150.57
(s)
, 149.32
(s)
, 118.66
(s)
, 106.38
(s)
, 105.47
(s
)
, 39.18
(s)
.
HR
MS (ESI,
TOF
(CH
3
CO
2
H),
m/z
): calculated
for C
16
H
2
0
N
4
+
H
, [M+H]
+
:
269.1761
, found:
269.1763
.
2,2'
-
(ethane
-
1,1
-
diyl)bis(
4
-
(
dimethylamino
)
pyridine
)
2,2'
-
(ethene
-
1,1
-
diyl)bis(4
-
(dimethylamino)pyridine)
(268 mg, 1 mmol, 1 eq), Pd/C (43 mg, 5%
by mass Pd, 0.02 eq), and activated carbon (134 mg, 0.5 mass eq) were suspended in methanol (10
mL, 0.1 M). The headspace of this mixture was purged with nitrogen
and then hydrogen. A
S
I
-
8
hydrogen atmosphere was maintained using a balloon, and the mixture was allowed to react for 12
h. The suspension was allowed to settle, and the solution was filtered through
C
elite. The
remaining solids were washed with methanol
, and
the supernatant was filtered through
C
elite. The
combined solution was concentrated
, dissolved in dichloromethane, dried over sodium sulfate, and
concentrated again
to afford an off
-
white crystalline solid after gentle heating under vacuum
.
(247
mg, 91%
yield
)
. This material was used without further purification.
1
H NMR (CDCl
3
, 400 MHz): δ (ppm) =
8.
19
(d,
J
= 6.0 Hz, 2H), 6.
58
(d,
J
= 2.6 Hz, 2H), 6.3
4
(dd,
J
= 6.0, 2.6 Hz, 2H), 4.2
1
(q,
J
= 7.2 Hz, 1H), 2.9
5
(
s
, 12H), 1.7
1
(d,
J
= 7.2 Hz, 3H).
13
C
{
1
H
}
NMR (CDCl
3
, 101 MHz
): δ (ppm) =
164.30
(s)
, 154.84
(s)
, 149.03
(s)
, 105.02
(s)
, 104.81
(s)
, 50.39
(s)
, 39.13
(s)
, 19.94
(s)
.
MS (ESI,
UHP
LC
-
MS (CH
3
CO
2
H),
m/z
):
calculated
for C
16
H
22
N
4
+ H, [M+H]
+
:
271.2
, found:
271.2
.
HR
MS (ESI,
TOF
(CH
3
CO
2
H),
m/z
):
calculated
for C
16
H
22
N
4
+
H
, [M+H]
+
:
271.1917
, found:
271.1919
.
BPM
NMe2
(6
-
(1,1
-
bis(4
-
(
dimethylamino
)pyridin
-
2
-
yl)ethyl)
-
2,2'
-
bipyridine)
This compound was prepared by analogy to the parent ligand.
3
In a Schlenk tube under nitrogen,
2,2'
-
(ethane
-
1,1
-
diyl)bis(4
-
(dimethylamino
)pyridine (
135
m
g,
0.5 mmol,
1 eq) was dissolved in
2
mL dry tetrahydr
ofuran and cooled in a dry ice/acetone bath.
A
1.6
M
n
-
BuLi solution
in
hexanes
(0.
31
mL,
0.5 mmol,
1 eq) was added dropwise
via syringe
, and the solution turned
yellow
-
orange
.
The mixture was stirred for
4
5 additional minutes,
then
6
-
bromo
-
2,2'
-
bipyridine (
118 mg, 0.5
mmol,
1 eq)
was added as a
solution in 0.5 mL tetrahydrofuran
. The solution was warmed to room
temperature
and stirred for 16 h
, after which
time
the reaction was quenched with water. The
solution was concentrated,
tran
sferred to a separatory funnel
,
and extracted with dichloromethane.
The combined organic fractions were
concentrated and purified via column chromatography
on
neutral alumina
using
methanol:dichloromethane
(1:4
9
) to afford a
waxy colorless solid.
(
163
m
g,
77
%
yield
).
R
f
= 0.2
(
methanol
; TLC plates
treated
with
methanol
)
.
1
H NMR (CDCl
3
, 400 MHz): δ (ppm) =
8.56 (ddd,
J
= 4.8, 1.9, 0.9 Hz, 1H), 8.31 (dt,
J
= 8.1, 1.1
Hz, 1H), 8.19 (dd,
J
= 5.8, 0.6 Hz, 2H), 8.15 (dd,
J
= 7.8, 0.9 Hz, 1H), 7.65 (td,
J
= 7.7, 1.8 Hz,
S
I
-
9
1H), 7.60 (
t
,
J
= 7.8 Hz, 1H), 7.17 (ddd,
J
= 7.5, 4.8, 1.3 Hz, 1H), 7.11 (dd,
J
= 7.9, 0.9 Hz, 1H),
6.31
6.25 (m, 4H), 2.79 (s, 1
2
H), 2.33 (s, 3H).
13
C
{
1
H}
NMR (CDCl
3
, 101 MHz
): δ (ppm) =
166.41
(s)
, 165.4
4
(s)
, 156.89
(s)
, 154.50
(s)
, 154.27
(s)
, 148.86
(s)
, 148.79
(s)
, 136.66
(s)
, 136.48
(s)
, 124.42
(s)
, 123.30
(s)
, 121.47
(s)
, 117.99
(s)
,
106.72
(s)
, 104.38
(s)
, 60.37
(s)
, 39.04
(s)
, 27.16
(s)
.
MS (ESI,
UHP
LC
-
MS (CH
3
CO
2
H),
m/z
):
calculated
for C
2
6
H
2
8
N
6
+ H, [M+H]
+
:
425.3
, found
:
425.3.
HR
MS (ESI,
TOF
(CH
3
CO
2
H),
m/z
): calculated
for C
2
6
H
2
8
N
6
+
H
, [M+H]
+
:
425.2448
, found:
425.2448
.
bis(4
-
methoxypyridin
-
2
-
yl)methanone
This compound was prepared by analogy to the known
analogue
,
bis(4
-
(dimethylamino)
pyridin
-
2
-
yl)methanone
.
5
In a 100 mL flask under nitrogen, 2
-
bromo
-
4
-
methoxypyridine (0.7 g, 3.7 mmol,
1 eq) was diss
olved in 30 mL
dry
diethyl ether and cooled in a dry ice/acetone bath.
A
1.7 M
t
-
BuLi solution in pentane (4.4 mL, 7.4 mmol, 2 eq) was added dropwise
via syringe
, and the
solution slowly turned red
-
orange. The mixture was stirred for 5 additional minutes b
efore neat
ethyl chloroformate (0.18 mL, 1.8 mmol, 0.5 eq) was added all at once via syringe, after which the
solution darkened. The solution was stirred 30 additional minutes in the dry ice/acetone bath,
warmed to approximately 0 °C,
and then quenched wit
h water. Th
e
mixture was transferred to a
separatory funnel and extracted with additional diethyl ether. The combined organic fractions were
dried over sodium sulfate
and concentrated
under reduced pressure. Th
e
crude material was eluted
through a plug of silica using
acetone and then purified by
silica gel
column chromatography (3:1
ethyl acetate:acetone
plus
1% triethylamine; R
f
= 0.3) to yield an off
-
white solid (0.267 g, 59%
yield
).
This material can be crystallized from cold 1:3 acetone:diethyl ether.
1
H NMR (CDC
l
3
, 400 MHz): δ (ppm) =
8.56 (dd,
J
= 5.7, 0.5 Hz,
2
H), 7.61 (dd,
J
= 2.6, 0.5 Hz,
2
H), 6.99 (dd,
J
= 5.7, 2.6 Hz,
2
H)
, 3.93 (s, 6H)
.
13
C
{
1
H}
NMR (CDCl
3
, 101 MHz
): δ (ppm) =
19
3
.
11
(s)
, 166.
28
(s)
, 156.
11
(s)
, 150.
42
(s)
, 112.7
3
(s)
, 110.9
3
(s)
, 55.5
3
(s)
.
MS (ESI,
UHP
LC
-
MS (CH
3
CO
2
H),
m/z
):
calculated
for C
13
H
12
N
2
O
3
+ H, [M+H]
+
:
245.1,
found
:
245.1
.
HR
MS (ESI,
TOF
(CH
3
CO
2
H),
m/z
): calculated
for C
13
H
12
N
2
O
3
+
H
, [M+H]
+
:
245.
0921
,
found
:
245.
0927
.
S
I
-
10
2,2'
-
(eth
e
ne
-
1,1
-
diyl)bis(
4
-
(methoxypyridine
)
Note:
This compound
was prepared
with modifications to the literature procedure for
2,2'
-
(ethene
-
1,1
-
diyl)
dipyridine.
13
Notably,
poor
performance
was
observed
when
m
ethylenetriphenylphosphorane
was generated
in
-
situ
, presumably due to residual potassium salts
(i.e.
,
KBr).
B
is(4
-
methoxy
pyridin
-
2
-
yl)methanone
(3
66 mg, 1.5 mmol, 1 eq) was dissolved in anhydrous
tetrahydrofuran (15 mL, 0.1 M) in a glovebox and cooled to −30
°C
, after which isolated
m
ethylenetriphenylphosphorane
(435 mg, 1.58 mmol, 1.05 eq) was added in one portion. This
mixture was warmed to room t
emperature and allowed to react for at least 4
h
(reaction times up
to overnight have no deleterious impact).
Outside of the glovebox, t
he reaction was quenched with
minimal water and concentrated to yield a viscous orange
-
red oil.
The oil was purified by
silica
gel column chromatography by loading with dichloromethane, eluting the triphenylphosphine
oxide with 4:1 ethyl acetate:acetone plus 1% triethylamine, then eluting the product with 3:1 ethyl
acetate:acetone plus 1% triethylamine to afford an off
-
whit
e crystalline solid (269 mg, 74% yield).
R
f
= 0.3
(
4:1 ethyl acetate:acetone
plus
1% triethylamine
;
TLC plates treated with triethylamine
)
.
1
H NMR (CDCl
3
, 400 MHz): δ (ppm) =
8.49 (d,
J
= 5.7 Hz,
2
H), 6.95 (d,
J
= 2.4 Hz,
2
H), 6.79
(dd,
J
= 5.7, 2.5 Hz,
2
H), 6.04 (s,
2
H), 3.86 (s,
6
H).
13
C
{
1
H}
NMR (CDCl
3
, 101 MHz
): δ (ppm) =
166.05
(s)
, 159.50
(s)
, 150.57
(s)
, 148.72
(s)
, 120.38
(s)
, 109.55
(s)
, 108.54
(s)
, 55.14
(s)
.
HR
MS (ESI,
TOF
(CH
3
CO
2
H),
m/z
): calculated
for C
1
4
H
14
N
2
O
2
+
H
,
[
M+
H]
+
:
243.1128
, found:
243.1133
.
2,2'
-
(ethane
-
1,1
-
diyl)bis(
4
-
methoxypyridine
)
2,2'
-
(ethene
-
1,1
-
diyl)bis(4
-
methoxy
pyridine)
(242 mg, 1 mmol, 1 eq), Pd/C (43 mg, 5% by mass
Pd, 0.02 eq), and activated carbon (121 mg, 0.5 mass eq) were suspended in methanol (10 mL, 0.1
M). The headspace of this mixture was purged with nitrogen and then hydrogen. A hydrogen
atmosphere was maintai
ned using a balloon, and the mixture was allowed to react for 12 h. The
S
I
-
11
suspension was allowed to settle, and the solution was filtered through
C
elite. The remaining solids
were washed with methanol, and the supernatant was filtered through
C
elite. The com
bined
solution was concentrated and purified by column chromatography
to afford a
white
solid
(
127
mg, 52%
yield
).
C
hromatography:
S
ilica
was
slurry
-
packed
with 1:19 methanol:dichloromethane then equilibrate
d
with 1:49 methanol:dichloromethane.
A
gradient
of 1:49 to 1:19 methanol:dichloromethane
was
used
to elute the product
,
as well as a dimeric product characterized below.
R
f
= 0.3
(
1:19 methanol:dichloromethane
)
.
1
H NMR (CDCl
3
, 400 MHz): δ (ppm) =
8.36 (d,
J
= 5.
8
Hz,
2
H), 6.81 (d,
J
= 2.5 Hz,
2
H), 6.64
(dd,
J
= 5.7, 2.5 Hz,
2
H), 4.34 (q,
J
= 7.2 Hz, 1H), 3.79 (s,
6
H), 1.71 (d,
J
= 7.2 Hz,
3
H).
13
C
{
1
H}
NMR (CDCl
3
, 101 MHz
): δ (ppm) =
166.09
(s)
, 165.52
(s)
, 150.34
(s)
, 108.26
(s)
, 107.90
(s)
, 55.03
(s)
, 49.99
(s)
, 19.67
(s)
.
MS (ESI,
UHP
LC
-
MS (CH
3
CO
2
H),
m/z
)
:
calculated
for C
1
4
H
16
N
2
O
2
+
H,
[
M+
H]
+
:
245.1
, found
:
245.1
.
HR
MS (ESI,
TOF
(CH
3
CO
2
H),
m/z
): calculated
for C
1
4
H
1
6
N
2
O
2
+
H
,
[
M+
H]
+
:
245.1285
, found:
245.1287
.
Byproduct from synthesis of
2,2'
-
(ethane
-
1,1
-
diyl)bis(
4
-
methoxypyridine
)
:
2,2',2'',2'''
-
2,2',2'',2'''
-
(butane
-
1,1,3,3
-
tetrayl)tetrakis(4
-
methoxypyridine)
Isolated from the above reaction
as a
pale oil
(75.6 mg
,
31
% yield
).
R
f
= 0.1
(
1:19
methanol:dichloromethane
)
.
1
H NMR (CDCl
3
, 400 MHz): δ (ppm) =
8.31 (dd,
J
= 5.6, 0.9 Hz,
2
H), 8.27 (dd,
J
= 5.7, 0.9 Hz,
2
H), 6.78 (d,
J
= 2.5 Hz,
2
H), 6.70 (d,
J
= 2.2 Hz,
2
H), 6.55 (dd,
J
= 2.5, 1.1 Hz,
2
H), 6.53 (dd,
J
= 2.4, 1.1 Hz,
2H
), 4.27 (t,
J
= 6
.0 Hz, 1H), 3.77 (s,
6
H), 3.74 (s,
6
H), 3.40 (d,
J
= 5.8 Hz,
2
H),
1.72 (s,
3
H).
13
C
{
1
H}
NMR (CDCl
3
, 101 MHz
): δ (ppm) =
168.19
(s)
, 165.72
(s)
, 165.58
(s)
, 165.11
(s)
, 149.76
(s)
, 149.64
(s)
, 108.52
(s)
, 108.12
(s)
, 107.20
(s)
, 54.96
(s)
, 54.87
(s)
,
52.99
(s)
, 51.92
(s)
, 44.21
(s)
, 25.63
(s)
.
S
I
-
12
HR
MS (ESI,
TOF
(CH
3
CO
2
H),
m/z
): calculated
for C
28
H
30
N
4
O
4
+
H
,
[
M+
H]
+
:
487.
2340
, found:
487.2361
.
BPM
OMe
(
6
-
(1,1
-
bis(4
-
methoxy
pyridin
-
2
-
yl)ethyl)
-
2,2'
-
bipyridine)
This compound was prepared by analogy to the parent ligand.
3
In a Schlenk tube under nitrogen,
2,2'
-
(ethane
-
1,1
-
diyl)bis(4
-
methoxy
pyridine (
122
m
g,
0.5 mmol,
1 eq) was dissolved in
2
mL dry
tetrahydrofuran and cooled in a dry ice/acetone bath.
A
1.6
M
n
-
BuLi solution
in
hexanes
(0.
31
mL,
0.5 mmol,
1 eq) was added dropwise
via syringe
, and the solution turned
yellow
-
orange
. The
mixture was stirred for
4
5 additional minutes,
then
6
-
bromo
-
2,2'
-
bipyridine (
118 mg, 0.5 mmol,
1
eq)
was added as a
solution in 0
.5 mL tetrahydrofuran
. The solution was warmed to room
temperature
and stirred for 24 h
, after which
time
the reaction was quenched with water. The
solution was concentrated,
transferred to a separatory funnel
,
and extracted with dichloromethane.
The combi
ned organic fractions were
concentrated and purified via column chromatography
to
afford a
white
solid
(166 mg, 83%
yield
).
C
hromatography:
S
ilica
was slurry
-
packed
with 1:49 methanol:dichloromethane then
equilibrate
d
with 1:99 methanol:dichloromethane.
A gradient of
1:99 to 1:19 methanol:dichloromethane
was
used to elute the product
.
R
f
= 0.
3
(
1:9 methanol:dichloromethane
)
.
1
H NMR (CDCl
3
, 400 MHz): δ (ppm) =
8.56 (ddd,
J
= 4.9, 1.9, 1.0 Hz, 1H), 8.38 (dd,
J
= 5.6, 0.8
Hz, 2H), 8.20 (ddt,
J
= 8.8, 7.7, 1.1 Hz, 2H), 7.64 (td,
J
= 7.8, 0.9 Hz, 2H), 7.19
7.15 (m, 1H),
7.09 (dd,
J
= 7.8, 1.0 Hz, 1H), 6.60 (ddd,
J
= 5.6, 2.4, 0.9 Hz, 2H), 6.56 (dd,
J
= 2.4, 0.8 Hz, 2H),
3.66
(d,
J
= 0.9 Hz, 6H), 2.32 (d,
J
=
1.0
Hz, 3H).
13
C
{
1
H}
NMR (CDCl
3
, 101 MHz
): δ (ppm) =
167.66
(s)
, 165.72
(s)
, 164.37
(s)
, 156.55
(s)
, 154.55
(s)
, 150.12
(s)
, 148.88
(s)
, 136.87
(s)
, 136.71
(s)
, 123.94
(s)
, 123.46
(s)
, 121.30
(s)
, 118.37
(s)
,
110.37
(s)
, 106.96
(s)
, 60.23
(s)
, 54.96
(s)
, 27.10
(s)
.
MS (ESI,
UHP
LC
-
MS (CH
3
CO
2
H),
m/z
):
calculated
for C
24
H
22
N
4
O
2
+ H, [M+H]
+
:
399.2
, found
:
399.1
.
HR
MS (ESI,
TOF
(CH
3
CO
2
H),
m/z
):
calculated
for C
24
H
22
N
4
O
2
+
H
, [M+H]
+
:
399.1816
, found:
399.1821
.
S
I
-
13
2,2'
-
(ethane
-
1,1
-
diyl)bis(
4
-
(
trifluoromethyl
)
pyridine
)
This compound was prepared by analogy to the procedure reported for the parent
dipyridylethane.
14
In a Schlenk tube
under nitrogen
, 2
-
ethyl
-
4
-
(trifluoromethyl)pyridine (876 mg,
2
eq) was dissolved in THF (7 mL) and cooled to −78
°
C while stirring. A 2.0 M solution of lithium
diisopropylamide in THF/heptane/ethylbenzene (2.5 mL,
2
eq) was added dropwise
via syringe
,
and
the solution
t
urned
dark purple. Th
e
solution was warmed to room temperature
for 5 minutes
then cooled to −78
°
C prior to dropwise addition of a cooled solution of 2
-
fluoro
-
4
-
(trifluoromethyl)pyridine (413 mg,
1
eq) in THF (3 mL). The reaction was warmed to room
tempera
ture after which it was stirred for 30 minutes then quenched with water. The mixture was
concentrated, diluted in water, and extracted with dichloromethane. The organic extract was dried
over sodium sulfate, concentrated,
subjected to a silica plug with
1:
4
ethyl acetate:hexane
,
and
concentrated
to a yellow oil
that was
used without further purification
(0.15 g, 19%
yield
).
R
f
= 0.3
(
1:9 ethyl acetate:hexane
)
.
1
H NMR (CDCl
3
, 400 MHz): δ (ppm) =
8.73 (dt,
J
= 5.1, 0.8 Hz,
2
H), 7.5
7
7.5
4
(m,
2
H), 7.37
(ddd,
J
= 5.1, 1.7, 0.8 Hz,
2
H), 4.61 (q,
J
= 7.2 Hz,
1
H), 1.80 (d,
J
= 7.2 Hz,
3
H).
13
C
{
1
H}
NMR (CDCl
3
, 101 MHz
):
164.46
(s)
, 150.34
(s)
, 138.89 (q,
J
= 33.9 Hz), 122.80 (q,
J
=
273.3 Hz), 118.10 (q,
J
= 3.6 Hz), 117.49 (q,
J
= 3.5 Hz), 49.90
(s)
, 19.79
(s)
.
19
F
{
1
H} NMR (C
DCl
3
,
376
MHz): δ
(ppm)
=
64.8
(s)
.
MS (ESI,
UHP
LC
-
MS (CH
3
CO
2
H),
m/z
):
calculated for
C
14
H
10
N
2
F
6
+ H,
[
M+
H]
+
:
321.1
, found:
321.1
.