of 30
S
1
Supporting information for
Fe
-
Mediated Nitrogen Fixation with a Metallocene
Mediator: Exploring p
K
a
Effects and Demonstrating
Electrocatalysis
Matthew J. Chalkley,
Trevor J. Del Castillo,
Benjamin D. Matson,
and Jonas C.
Peters*
M.J.C., T.J.D.C.
,
and B.D.M. contributed equally to this work.
Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena,
California 91125, United State
s
Table of Contents:
S2
-
3
S
1. Experimental details
S
3
-
4
S
2. Synthetic Details
S
4
-
5
S
3. Ammonia generation
details
S
5
-
7
S
4.
H
2
monitoring details
S
8
-
9
S
5
.
Mössbauer Spectr
oscopy
S
9
-
11
S
6
. EPR Spectra
S
12
-
12
S
7
. Acid Quench of P
3
B
FeN
2
S
12
-
13
S
8.
Solubility Measurement
S13
-
1
8
S
9. Controlled Potential Electrolysis and Cyclic Voltammetry
S1
8
-
S2
3
S
10. Computational Details
S2
4
-
2
8
S
11. XPS Spectroscopy
S2
8
-
2
9
S
12. p
K
a
Determination Strategy
S2
9
-
30
S
13
. References
S
2
S
1. Experimental
Details
S1.1. General Considerations
All manipulations were carried out using standard Schlenk or glovebox techniques under
an N
2
atmosphere. Solvents were deoxygenated and dried by thoroughly sparging with N
2
followed by passage through an activated alum
ina column in a solvent purification system by SG
Water, USA LLC. Non
-
halogenated solvents were tested with sodium benzophenone ketyl in
tetrahydrofuran (THF) in order to confirm the absence of oxygen and water. Deuterated solvents
were purchased from Camb
ridge Isotope Laboratories, Inc., degassed, and dried over activated 3
-
Å molecular sieves prior to use.
Cp*
2
Co,
1
[P
3
B
Fe][BAr
F
4
],
2
[P
3
B
FeN
2
][Na(Et
2
O)
3
],
3
[P
3
B
FeN
2
][Na(12
-
crown
-
4)
2
],
3
[H(OEt2)][BAr
F
4
] (HBAr
F
4
; BAr
F
4
= tetrakis
-
(3,5
-
bis(trifluoromethyl
)phenyl)borate)
4
,
sodium
BAr
F
4
(
NaBAr
F
4
)
4
,
and
15
N
-
diphenylammonium triflate (
[Ph
2
15
NH
2
][OTf]
)
5,6
were prepared
according to literature procedures. All other reagents were purchased from commercial vendors
and used without further purification unless
otherwise stated. Diethyl ether (Et
2
O) used in the
experiments herein was stirred over Na/K (≥ 2 hours) and filtered through celite before use.
S1.2. Gas Chromatography
H
2
was quantified on an Agilent 7890A gas chromatograph (HP
-
PLOT U, 30 m, 0.32 mm
ID; 30 °C isothermal; nitrogen carrier gas) using a thermal conductivity detector. A 10 mL
manual injection was used and integration area was converted to percent H
2
compositio
n by use
of a calibration obtained from injection of H
2
solutions in N
2
of known concentration.
S
1.3. Mössbauer Spectroscopy
Mössbauer
spectra were recorded on a spectrometer from SEE Co. (Edina, MN)
operating in the constant acceleration mode in a transmission geometry. The sample was kept in
an SVT
-
400 cryostat form Janis (Wilmington, MA). The quoted isomer shifts are relative to the
c
entroid of the spectrum of a metallic foil of α
-
Fe at room temperature (RT). Solution samples
were transferred to a sample cup and
freeze
-
quenched with liquid nitrogen inside of the glovebox
and
then
immersed in liquid N
2
until mounted in the cryostat. Dat
a analysis was performed using
version 4 of the program WMOSS (www.wmoss.org) and quadrupole doublets were fit to
Lorentzian lineshapes. See discussion below for detailed notes on the fitting procedure.
S
1.4. Ammonia Quantification
Reaction mixtures are co
oled to 77 K and allowed to freeze. The reaction vessel is then
opened to atmosphere and to the frozen solution is slowly added a twofold excess (with respect
to acid) solution of a NaO
t
Bu solution in MeOH (0.25 mM) over 1
-
2 minutes. This solution is
allow
ed to freeze and a Schlenk tube adapter is added and the headspace of the tube is evacuated.
After sealing the tube is then allowed to warm to RT and stirred at RT for at least 10 minutes. An
additional Schlenk tube is charged with HCl (3 mL of a 2.0 M sol
ution in Et
2
O, 6 mmol) to serve
as a collection flask. The volatiles of the reaction mixture are vacuum transferred at RT into this
collection flask. After completion of the vacuum transfer, the collection flask is sealed and
warmed to RT and stirred
vigor
ously
for 10 minutes. Solvent is removed in vacuo, and the
remaining residue is dissolved in DMSO
-
d
6
containing 20 mM 1,3,5
-
trimethoxybenzene
as an
S
3
internal standard
. The ammonium chloride is quantified by integration relative to the 1,3,5
-
trimethoxybenzen
e internal standard.
S1.5. Computational Methods
All stationary point geometries were calculated using DFT
-
D
3
(Grimmes D
3
dispersion
correction
7
) with an TPSS functional,
8
a def2
-
TZVP
9
basis set on transition metals and a def2
-
SVP
7
basis set on all other atoms. Calculations were performed, in part, using Xtreme Science
and Engineering Discovery Environment (XSEDE) resources.
10
Calculations were performed on
the full P
3
B
Fe scaffold. Geometries were optimized using the NWChem 6.5 pack
age.
11
All single
point energy, frequency and solvation energy calculations were performed with the ORCA
package.
1
2
Frequency calculations were used to confirm true minima and to determine gas phase
free energy values (G
gas
). Single point solvation calcula
tions were done using an SMD solvation
model
1
3
, 1
4
with diethyl ether solvent and were used to determine solvated internal energy (E
soln
).
Free energies of solvation were approximated using the difference in gas phase internal energy
(E
gas
) and solvated in
ternal energy (∆G
solv
≈ E
soln
E
gas
) and the free energy of a species in
solution was then calculated using the gas phase free energy (G
gas
) and the free energy of
solvation (G
soln
= G
gas
+ ∆G
solv
).
1
5
,1
6
All reduction potentials were calculated referenced
to Fc
+/0
and
using the standard Nernst relation
G =
nFE
0
.
S2. Synthetic Details:
S2.1. General Procedure for the Synthesis of the Anilinium Triflates
17,18
Prior to use the amine was purified (aniline and 2,6
-
dimethylaniline by distillation and
the
remaining substituted anilines by sublimation). To a 100 mL round bottom flask in the
glovebox was added the desired aniline which was subsequently dissolved in 50 mL of Et
2
O (no
additional drying with NaK). To this was added dropwise (1 equiv) of HOTf wi
th stirring over
five minutes. Immediate precipitation of white solid was observed and the reaction mixture was
allowed to stir for thirty minutes. The reaction mixture was then filtered and the resulting white
powder was washed with Et
2
O (50 mL) and penta
ne (50 mL). The resulting white
microcrystalline material was then dried under vacuum. Yields of greater than 90% of
microcrystalline material was obtained in this manner in all cases.
4
-
methoxyanilinium triflate
([
4
-
OMe
PhNH
3
][OTf])
:
1
H NMR (DMSO
-
d
6, 400
MHz): 7.52 (m, 2
H), 7.28 (m, 2H), 4.30 (br, 3H).
anilinium triflate
([PhNH
3
][OTf])
:
1
H NMR (DMSO
-
d
6, 400 MHz): 7.50 (m, 2 H), 7.41 (m, 1H),
7.34 (m, 2H).
2,6
-
dimethylanilinium triflate
([
2,6
-
Me
PhNH
3
][OTf])
:
1
H NMR (DMSO
-
d
6, 400 MHz): 7.14 (m,
3H), 2.32 (
br, 6H).
2
-
chloroanilinium triflate
([
2
-
Cl
PhNH
3
][OTf])
:
1
H NMR (DMSO
-
d
6, 400 MHz): 7.32 (m, 1H),
7.15 (m, 1H), 7.02 (m, 1H), 6.82 (m, 1H).
2,
5
-
chloroanilinium triflate
([
2,5
-
Cl
PhNH
3
][OTf])
:
1
H NMR (DMSO
-
d
6, 400 MHz): 7.19
(
apparent
d, 1H,
3
J(H
-
H) = 8.5 Hz)
, 6.83 (
apparent
d
d
, 1H
,
3
J(H
-
H) = 2.5 Hz, 0.9 Hz
),
6.56
(m,
1H).
S
4
2,6
-
chloroanilinium triflate
([
2,6
-
Cl
PhNH
3
][OTf])
:
1
H NMR (DMSO
-
d
6, 400 MHz): 7.22 (d, 2H,
3
J(H
-
H) = 8.0 Hz), 6.57 (t, 1H,
3
J(H
-
H) = 8.0 Hz).
2,4,6
-
chloroanilinium triflate
([
2,4,6
-
Cl
PhNH
3
][
OTf])
:
1
H NMR (DMSO
-
d
6, 400 MHz): 7.37 (s,
2H).
S2.2.
Preparation
of
decamethylcobaltocenium
tetrakis(3,5
-
bis(trifluoromethyl)phenyl)borate)
,
[Cp*
2
Co][BAr
F
4
]
A
RT
solution of HBAr
F
4
(96.1 mg, 0.095 mmol) in Et
2
O (6 mL) is added dropwise to a
stirred
,
RT
solution of Cp*
2
Co (32.9 mg, 0.1 mmol) in Et
2
O (6 mL). This mixture is allowed to
stir 30 min and then reduced to dryness in vacuo. The resulting solid residue is washed with
pentane (3 x 2 mL) to yield [Cp*
2
Co][BAr
F
4
] as a bright yellow solid
(104 mg iso
lated, 92%
yield)
.
1
H NMR (THF
-
d
8, 300 MHz): δ
7.79 (8H, s, BAr
F
4
), δ 7.
58
(
4
H, s, BAr
F
4
)
, δ 1.75 (30H, s,
Cp*
2
Co).
S3. Ammonia Generation Details
S3.1. Standard NH
3
Generation Reaction Procedure
All solvents are stirred with Na/K for ≥
2 hours and filtered prior to use. In a nitrogen
-
filled glovebox, the precatalyst (2.3 μmol) was weighed into a vial. The precatalyst was then
transferred quantitatively into a
long
tube
with a female 24
-
40 joint at the top
using THF. The
THF was then evap
orated to provide a thin film of precatalyst at the bottom of
tube
. The tube
is
then charged with a stir bar,
the acid (108 eq
uiv
)
,
and
Cp*
2
Co
(41.2 mg, 54 eq
uiv
) as solids. The
tube is then sealed at
RT
with a septum that is secured with copper wire
(this
ensures a known
volume of N
2
in the reaction vessel, which is important for H
2
detection)
. The tube is then chilled
to 77 K and allowed to equilibrate for 10 minutes. To the chilled tube is added 1 mL of Et
2
O.
The temperature of the system is allowed to e
quilibrate for 5 minutes. This tube is passed out of
the box into a liquid N
2
bath and transported to a fume hood. The tube is then transferred to a dry
ice/acetone bath where it thaws and is allowed to stir at
78 °C for four hours. At this point the
headspace of the tu
be is sampled with a 10 mL sealable gas syringe
which is used to analyze for
H
2
by GC
. The tube is then allowed to warm to RT with stirring and then stirred at
RT
for a
further ten minutes. At this point the previously described procedur
e for quantifying ammonia
was employed. To ensure reproducibility, all experiments were conducted in 395 mL tubes (
51
mm OD) using 25 mm stir bars
and stirring was conducted at ~
650
rpm.
Table S1:
NMR quantification results for standard NH
3
generation e
xperiments with
[P
3
B
Fe][BAr
F
4
]
Entry
Acid
Integration Relative
to Internal Standard
% Yield NH
3
(error)
% Yield H
2
(error)
1
[
4
-
OMe
PhNH
3
][OTf]
0.01, 0.02
0.2 ± 0.1
89.1 ± 0.2
2
[PhNH
3
][OTf]
3.42, 3.33
40.4 ± 0.5
48.6 ± 0.7
S
5
3
[
2,6
-
Me
PhNH
3
][OTf]
4.30,
3.63
47.5 ± 4.0
37.8 ± 0.2
4
[
2
-
Cl
PhNH
3
][OTf]
4.98, 4.92
59.3 ± 0.4
26.1 ± 1.9
5
[
2,5
-
Cl
PhNH
3
][OTf]
6.78, 6.15
77.5 ± 3.8
10.5 ± 1.1
6
[
2,6
-
Cl
PhNH
3
][OTf]
6.81, 6.00
76.7 ± 4.9
12.6 ± 2.5
7
[
2,6
-
Cl
PhNH
3
][OTf]
*
6.60, 5.81
74.4 ± 4.7
14.2 ± 3.4
8
[
2,6
-
Cl
PhNH
3
][BAr
F
4
]
4.12, 3.0
42.7 ± 6.7
18.8 ± 0.8
9
[
2,4,6
-
Cl
PhNH
3
][OTf]
5.73, 6.10
70.9 ± 2.2
12.0 ± 0.8
10
pentachloroanilinium triflate
([
per
-
Cl
PhNH
3
][OTf])
1.62, 1.70
19.9 ± 0.5
63.5 ± 1.1
*Run performed with [P
3
B
FeN
2
][Na(Et
2
O)
3
] as the precatalyst.
S4.
H
2
Monitoring Details
S4.1. Standard Background Generation Reaction Procedure
All solvents are stirred with Na/K for ≥
2 hours and filtered prior to use. In a nitrogen
-
filled glovebox, a long tube with a female 24
-
40 j
oint is charged with a stir bar,
the acid (108
eq
uiv
) and
Cp*
2
Co
(41.2 mg, 54 eq
uiv
). The tube is then sealed at
RT
with a septum that is
secured with copper wire. The tube is then chilled to 77 K and allowed to equilibrate for 10
minutes. To the chilled tube is added 1 mL of Et
2
O. The te
mperature of the system is allowed to
equilibrate for 5 minutes. This tube is passed out of the box into a liquid N
2
bath and transported
to a fume hood. The tube is then transferred to a dry ice/acetone bath where it thaws and is
allowed to stir at
78 °C
for four hours. At this point the headspace of the tube is sampled with a
10 mL
sealable gas syringe
which is used to analyze for H
2
.
Table S2:
Data for Background H
2
Quantification Experiments
Acid
GC Integration for H
2
% Yield H
2
[
4
-
OMe
PhNH
3
][OTf]
49.8
31.5
[PhNH
3
][OTf]
24.0
15.2
[
2,6
-
Me
PhNH
3
][OTf]
8.2
5.2
[
2
-
Cl
PhNH
3
][OTf]
47.2
29.9
[
2,5
-
Cl
PhNH
3
][OTf]
37.1
23.5
[
2,6
-
Cl
PhNH
3
][OTf]
77.8
49.2
[
2,4,6
-
Cl
PhNH
3
][OTf]
34.8
22.0
S
6
[
per
-
Cl
PhNH
3
][OTf]
98.3
62.3
S4.2. H
2
Evolution Kinetics
All solvents
are stirred with Na/K for ≥
2 hours and filtered prior to use. For the catalyzed
run, the precatalyst was then transferred quantitatively into a Schlenk tube using THF. The THF
was then evaporated to provide a thin film of precatalyst at the bottom of the
long tube with a
female 24
-
40 joint. The tube is then charged with a stir bar and the
[
2
,6
-
Cl
PhNH
3
][OTf]
(77.9 mg,
108 eq
uiv
) and
Cp*
2
Co
(41.2 mg, 54 eq
uiv
) are added as solids. The tube is then sealed at
RT
with a septum that is secured with copper wire.
The tube is then chilled to 77 K and allowed to
equilibrate for 10 minutes. To the chilled tube is added 1 mL of Et
2
O. The temperature of the
system is allowed to equilibrate for 5 minutes. This tube is passed out of the box into a liquid N
2
bath and tran
sported to a fume hood. The tube is then transferred to a dry ice/acetone bath where
it thaws and is allowed to stir at
78 °C. As soon as the stir bar is freed from the frozen solution
and stirring begins the timing is started. At the time points noted
below the headspace was
sampled for H
2
with a 10 mL
sealable gas syringe
.
Table S3:
Time points for catalyzed H
2
evolution from 2,6
-
dichloroanlinium triflate and
Cp*
2
Co
Time
(min)
GC Integration for H
2
% Yield H
2
(error)
5
3.8, 6.4
3.3
± 0.9
15
11.6,
16.9
9.3
± 1.8
25
14.7, 26.2
13.4
±
3.8
35
22.5, 20.8
13.9
± 0.5
Table S4.
Time points for uncatalyzed H
2
evolution from 2,6
-
dichloroanlinium triflate and
Cp*
2
Co
Time
(min)
GC Integration for H
2
% Yield H
2
(error)
5
3.3, 2.9
2.0
±
0.1
15
7.0, 6.2
4.3
±
0.3
25
8.8, 11.1
6.3
± 0.8
65
20.7, 27.0
14.5
± 1.7
S
7
Figure S1:
Comparison of catalyzed and uncatalyzed H
2
evolution from 2,6
-
dichloroanlinium
triflate and
Cp*
2
Co
at early time poi
nts.
S5. Mössbauer Spectroscopy
S5.1. General Procedure for Freeze
-
Quench Mössbauer Spectroscopy
All solvents are stirred with Na/K for ≥
2 hours and filtered prior to use. In a nitrogen
-
filled glovebox, the desired
57
Fe species (0.0023 mmol) is quantitatively transferred using THF
to a
vial and then evaporated to yield a thin film. That vial is charged with a small stir bar and the
other reagents as solids. The vial is then chilled to 77 K in a liquid nitrogen bath and allowed to
equilibrate for five minutes. To the chilled tube is adde
d 1 mL of Et
2
O and this allowed to
equilibrate for another five minutes. The vial is then transferred to a cold well that has been pre
-
cooled for at least fifteen minutes to
78
°
C with a dry ice/acetone bath. When the stir bar is
freed from the frozen sol
vent and begins to stir the time is started. At the time noted the stirring
is stopped and using a prechilled pipette the reaction mixture is transferred in one portion to a
pre
-
chilled Mössbauer cup sitting in a vial. The vial is then placed in a liquid n
itrogen bath
causing the reaction mixture to freeze in approximately twenty seconds. The Mössbauer cup is
then submerged in the liquid nitrogen and then removed from the glovebox and standard
procedure is used to mount the sample on the Mössbauer spectrome
ter.
0
2
4
6
8
10
12
14
16
18
0
10
20
30
40
50
60
70
%H
2
Evolution
Time (min)
Catalyzed H2 Formation
Background H2 Evolution
S
8
Figure S2.
Mössbauer spectrum collected from a reaction freeze quenched after stirring for 5
minutes at
78 °C in 1 mL of Et
2
O between [P
3
B
(
57
Fe)N
2
][Na(Et
2
O)
3
]
and excess 2,6
-
[
2,6
-
Cl
PhNH
3
][OTf]
(50 equiv). Raw data shown as black points, simulation
as a solid red line, with
components in
green, purple, and yellow
(see Table S3 for parameters). The spectrum was
collected at 80 K with a parallel applied magnetic field of 50 mT in Et
2
O.
Fitting details for Figure S2:
Three quadrupole doublets were found
to be necessary to obtain an
adequate simulation. Although a variety of parameters could potentially simulate the relatively
broad absorptions observe
d here, previous reactivity of
P
3
B
FeN
2
with acid
19
suggested that
P
3
B
FeN
2
and P
3
B
Fe
+
were likely product
s. Satisfyingly
if the known isomer shift and quadrupole
splitting for one of those species was fixed during the fitting process and the other components
were allowed to refine freely the other major component was found to be the complementary
species
.
19
T
he third species was
always
unchanged in these simulations and represents an
unknown species
. Its presence in the fit
is demanded by the inflection point on the more negative
side of the right
-
hand absorbance. Modeling this feature also helps to capture the asymmetry of
the left
-
hand absorbance while using the symmetric line
-
shapes we expect for P
3
B
FeN
2
(green)
and
P
3
B
Fe
+
(purple)
.
The broad linewidths for P
3
B
Fe
+
have been observed previously and may be
explained by the existence of unbound and bound varieties of the species with the reaction
mixture providing potential ligands such as
OTf
,
2,6
-
Cl
PhNH
2
, and N
2
.
Table S5
:
Simulation parameters for Mossbauer spectrum in Figure S2.
Component
δ (mm s
1
)
ΔE
Q
(mm s
1
)
Linewidths,
Γ
L
/ Γ
R
(mm s
1
)
Relative area
A (green)
0.58 ± 0.02
3.28 ± 0.07
0.52/0.52
0.26
B (purple)
0.76 ± 0.02
2.57 ± 0.05
1.10/1.10
0.63
C (yellow)
0.13 ±
0.02
2.24 ± 0.04
0.50/0.50
0.11
S
9
S6. EPR Spectroscopy
S6.1 General Procedure for EPR Spectroscopy
All solvents are stirred with Na/K for ≥
2 hours and filtered prior to use. In a nitrogen
-
filled glovebox, the desired Fe species (0.0023 mmol
) is quantitatively transferred using THF to
a vial and then evaporated to yield a thin film. That vial is charged with a small stir bar and the
acid (0.116 mmol, 50 eq
uiv
) as solids ([
2,6
-
Cl
PhNH
3
][OTf] or [
2,6
-
Cl
PhNH
3
][BAr
F
4
]
). The vial is
then chilled to
77 K in a liquid nitrogen bath and allowed to equilibrate for five minutes. To the
chilled tube is added 1 mL of Et
2
O (for HOTf 50 eq
uiv
have been dissolved in this 1 mL of Et
2
O
at
RT
) and this allowed to equilibrate for another five minutes. The vial is then transferred to a
cold well that has been pre
-
cooled for at least fifteen minutes to
78
°
C with a dry ice/acetone
bath. When the stir bar is freed from the frozen solvent and begi
ns to stir the time is started. The
reaction mixture is stirred for five minutes and then stirring is stopped. Using a pre
-
chilled
pipette approximately 0.5 mL
of the
reaction mixture is rapidly transferred to a pre
-
chilled X
-
band EPR tube. The X
-
band EPR
tube is then placed in a liquid nitrogen bath causing the
reaction mixture to freeze in approximately twenty seconds. The EPR tube is then sealed and
removed from the glovebox in liquid nitrogen.
S6.2 Comment on Stoichiometric Reactivity
In our attempt to
model the catalytic reaction mixture we were interested in the reactivity
of P
3
B
FeN
2
(observed previously both from mixing
[P
3
B
Fe][BAr
F
4
]
with excess Cp*
2
Co and
under the catalytic reaction conditions) with acid. In order to achieve this we wanted to
prepare
independently known P
3
B
FeN
2
species to model the proposed catalytic intermediate
[P
3
B
FeN
2
][Cp*
2
Co]. We chose [P
3
B
FeN
2
][Na(Et
2
O)
3
] because we believed that its solubility in
Et
2
O likely modeled that of [P
3
B
FeN
2
][Cp*
2
Co].
S
10
Figure S3:
The continuous wave, X
-
band EPR at 77K in Et
2
O of reaction mixtures freeze
-
quenched after five minutes. In red is the reaction of [P
3
B
FeN
2
][Na(Et
2
O)
3
] with 50
eq
uiv
of
[
2,6
-
Cl
PhNH
3
]
[BAr
F
4
] clearly demonstrating the formation of [P
3
B
FeNNH
2
][BAr
F
4
]. In
green is
reaction of [P
3
B
FeN
2
][Na(Et
2
O)
3
] with 50 eq
uiv
of
[
2,6
-
Cl
PhNH
3
][OTf]
in which the small
residual species is neither the starting material ([P
3
B
FeN
2
][Na(Et
2
O)
3
]) or the desired product
([P
3
B
FeNNH
2
][OTf]). Although we do not know the chemical ident
ity of this species we note
that it is very similar to the EPR observed in the reaction of [P
3
B
FeN
2
][Na(12
-
crown
-
4)
2
] with 1
eq
uiv
of HBAr
F
4
.
1
9
We hypothesize therefore that it may represent a Fe
H side product.