Suppor
ting
Information
Light
Alters
the
NH
3
vs N
2
H
4
Product
Profile
in Iron-catalyzed
Nitrogen
Reduction
via Dual
Reactivity
from
an Iron
Hydrazido
(Fe
=
NNH
2
) Intermediate
P. Garrido-Barros,
M. J. Chalkley,
J. C. Peters*
SUPPORTING INFORMATION
2
Table of Contents:
2
-
4
S1.
Methods
5
-
7
S2. Catalytic set
-
up and N
-
fixed products quantification
8
-
1
0
S3. Catalytic runs under light/dark conditions
1
1
-
1
3
S4. Control
experiments
1
4
-
15
S5. Variable
-
temperature catalytic runs
16
-
17
S6. Freeze
-
quench Mössbauer
18
-
20
S7. Rotating ring disk electrode experiment
21
-
24
S8. UV
-
vis spectroscopy
25
-
2
9
S9. Electrochemistry
30
-
3
6
S10. DFT calculations
3
7
S11. References
SUPPORTING INFORMATION
S
3
S1. Methods:
S1.1. General methods
All manipulations were carried out using standard Schlenk or glovebox techniques under
an N
2
. Solvents were deoxygenated and dried by thoroughly sparging with N
2
followed by passage
through an activated alumina column in a solvent purification system by SG Water, USA LLC.
Subsequently, the solvents were further dried and stored under N
2
atmosphere inside a glove box
with molecular sieves obtained from Sigma Aldric
h that were activated at 200ºC overnight under
vacuum. Non
-
halogenated solvents were tested with sodium benzophenone ketyl in
tetrahydrofuran (THF) in order to confirm the absence of oxygen and water. The diethyl ether
(Et
2
O)
employed for catalytic and mec
hanistic investigations was further dried using Na/K
overnight and filter over celite prior use. Deuterated solvents were purchased from Cambridge
Isotope Laboratories, Inc., and use as received.
N
2
gas in a MB
-
Unilab Pro SP (2500/780) Glovebox System (MB
raun Company) was
purified by passing through two filter beds: molecular sieve and copper catalyst. The purity of N
2
gas was assessed via colorimetric, gas chromatography and NMR methods, with regard to NH
3
,
N
2
O, NO
2
−
and NO
3
−
impurities.
Cp*
2
Co,
1
[P
3
B
Fe][BAr
F
4
],
2
[H(OEt
2
)][BAr
F
4
] (HBAr
F
4
; BAr
F
4
= tetrakis
-
(3,5
-
bis(trifluoromethyl)phenyl)borate),
3
and sodium BAr
F
4
(NaBAr
F
4
),
3
were prepared according to
literature procedures. All other reagents were purchased from commercial vendors and used
without further purification unless otherwise stated.
Whenever water was specified as solvent,
deionized water OmniSolv (Supelco, Sigma Al
dich) was used to prepare the solutions.
S1.3. UV
-
vis spectroscopy
UV
-
vis measurements were taken on a Cary 50 UV
-
visible spectrophotometer using a 1
cm quartz cell sealed with a Teflon stopcock. The temperature was controlled by using a Unisoke
probe equ
ipped with magnetic stir plate.
S1.4. Gas chromatography
Gas chromatography coupled to a thermal conductivity detector (GC
-
TCD) was
performed in the Environmental Analysis Center (Caltech) using a HP 5890 Series II instrument
with N
2
as the carrier gas fo
r H
2
detection. A calibration curve was determined by direct injection
of known volumes of H
2
.
S1.5. Electrochemistry
A CHI 600B potentiostat was used for all electrochemical data collection.
Cyclic voltammetry (CV) experiments were carried out in a on
e
-
compartment three
-
electrode cell using a glassy carbon (GC) disk as the working electrode (3 mm diameter), a Pt
disk as the counter electrode, and a Ag/AgOTf (5 mM) reference electrode. Details for the CVs
are noted as they appear. For all measurements I
R compensation was applied accounting for
85% of the total resistance. All the reported potentials are referenced to the ferrocenium/ferrocene
couple (Fc
+/0
) used as an external standard.
S1.6. 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
centroid 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 N2 until mounted in the cryostat. Data 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.
SUPPORTING INFORMATION
S
4
S1.7. Computational details
All single point energy,
frequency and localized orbital analysis were performed with the
ORCA 4.0 package
4
,
5
using DFT
-
D3 (Grimmes D3 dispersion correction)
6
with an BP86
functional,
7
with the zeroth order regular approximation (ZORA) to account for relativistic effects,
8
in
combination with the scalar relativistically recontracted versions of the def2
-
TZVP basis set on
transition metals and a def2
-
SVP basis set on all other atoms.
9
,
10
This methodology is based on
previous reported results showing that the ZORA
-
BP86 method
produces the most accurate
geometry, as well as a singlet
-
triplet ΔH that agrees well with the experimental value.
11
The
localized orbitals have been calculated based on the Pipek
-
Mezey (PM) localization method as
implemented in ORCA.
12
NBO
single
-
point
ca
lculations
13
to attain
natural population analys
e
s
were performed in Gaussian09
.
14
The BP86 functional and
def2
-
TZVP basis set on transition
metals and a def2
-
SVP basis set on all other atoms
.
SUPPORTING INFORMATION
S
5
S2. Catalytic set
-
up and N
-
fixed products
quantification
S2.1. Standard procedure for the catalytic runs
All manipulations were done in a nitrogen
-
filled glovebox unless otherwise mentioned. Before use,
Et
2
O solvent was stirred over Na/K for more than 1 hour and filtered through alumina plug pr
ior
to use. The [(P
3
B
)Fe][BAr
F
4
] precatalyst (ca. 2.3 μmol) was weighed into a vial, dissolved in THF
and transferred quantitatively into an oven
-
dried Schlenk tube as a solution. The solvent was then
evaporated resulting in a solid layer of
precatalyst at the bottom of the Schlenk tube. The acid
and reductant were then added as solids and the tube was equipped with a stir bar previously
oven
-
dried. The tube was cooled down to 77 K in the Coldwell using liquid nitrogen and 2.0 mL
of Et
2
O were
subsequently added through the walls of the tube to allow for fast freezing before
contact with the reagents. The temperature of the system was allowed to equilibrate for 5 minutes
and then the tube was sealed with a Teflon screw
-
valve. This tube was broug
h out of the glove
-
box into a liquid nitrogen bath and transported to a fume hood. The tube was then transferred to
a dry ice/acetone bath (
–
78 °C) where it thawed and was allowed to stir at
–
78 °C. For runs under
light irradiation, LED was turned on as th
e tube was in the ice/acetone bath and constant flow of
nitrogen was employed to prevent water condensation on the tube interfering with the light
irradiation. For runs utilizing HBAr
F
4
, reactions were stirred at
–
78 °C for 1 hour, followed by
stirring at
room temperature for 45 minutes. For all other runs, reactions were allowed to stir and
gradually warm to room temperature overnight. To ensure reproducibility, all experiments were
conducted in 200 mL Schlenk tubes (51 mm OD) using 25 mm stir bars, and st
irring was
conducted at ~900 rpm.
S2.2. Quantification of H
2
After the reaction time, the headspace of the reaction vessel was sampled using a 10 ml Hamilton
syringe and subsequently injected in a Agilent 7890A gas chromatograph (HP
-
PLOT U, 30 m,
0.32 mm
ID; 30 °C isothermal; nitrogen carrier gas) using a thermal conductivity detector.
Integration area was converted to percent H
2
composition by use of a calibration obtained from
injection of H
2
solutions in N
2
of known concentration and correcting for the
vapor pressure of
Et
2
O and the removed H
2
from previous samplings as previously reported.
15
S2.3. Quantification of NH
3
and N
2
H
4
After the reaction time, the catalytic reaction mixture was cooled to 77 K and allowed to freeze.
The reaction vessel was
then opened to atmosphere and to the frozen solution was added
dropwise
3 mL of 2M HCl in Et
2
O over 1
-
2 mins through the wall of the tube. The tube was allowed
to equilibrate for 5 mins and then the headspace of the tube was evacuated to a constant pressur
e
and subsequently sealed. The tube was allowed to warm up to room temperature and stir for 10
mins. The resulting solution was then evaporated to dryness under vacuum. The tube with the
remaining solids was cooled sown in a dry ice/acetone bath for 10 min
s. A 1 M aqueous HCl
solution (4 ml) was delivered through the wall of the tube, allowed to freeze and the tube was
subsequently sealed and allowed to warm up to room temperature. After 10 mins of stirring, the
resulting suspension was filtered through cel
ite into a 10 ml volumetric flask. The tube was further
extracted twice with 2 mL of 1 M aqueous HCl solution, followed by filtration through celite into
the same 10 mL volumetric flask. More HCl solution was added up to 10 mL total volume and 5
mL of this
solution were transferred to a 5 mL volumetric flask, where it was extracted 5x with 1
mL of n
-
butanol to remove remaining metal containing impurities. After extraction, the 5 mL flask
was refilled to the mark and 100 μL aliquots were used for the colorim
etric detection methods to
analyze for the presence of NH
3
(present as [NH
4
][Cl]) by the indophenol method, or N
2
H
4
(present
as [N
2
H
5
][Cl]) by a standard colorimetric method. Quantification was performed with UV
-
visible
spectroscopy by analyzing the
absorbance at 635 nm (NH
3
) and 458 nm (N
2
H
4
) in a quartz cuvette.
Calibration curves for ammonia
16
and hydrazine (see below) were constructed.
SUPPORTING INFORMATION
S
6
Figure S
1
.
UV
-
vis absorption spectra for standard solutions containing different concentration of
N
2
H
4
after
color development using the previously described method. The absorption at 458 nm
was measured and plotted versus the concentration of hydrazine in the analyte to build a
calibration curve for hydrazine quantification.
Figure S
2
.
Calibration curve f
or the detection of hydrazine using the colorimetric method
previously described. The plot was built using different N
2
H
4
solutions in aqueous HCl (1M)
obtained by dilution of a standard
N
2
H
4
solution. The absorption at 435 nm was measured and
plotted versus the concentration of hydrazine in the analyte.
SUPPORTING INFORMATION
S
7
Figure S
3
.
Calibration curve for the quantification of H
2
in the reaction headspace using GC
-
TCD
using 10 ml sample manual injection.
SUPPORTING INFORMATION
S
8
S3
. Catalytic runs under light/dark conditions
For catalytic runs performed under illumination,
a
440
nm LED lamp
(Kessil PR160
-
440 nm lamp,
45 W)
or a Hg lamp
(HBO 100 Mercury Arc lamp,100 W)
were
employed as the light source and
place close to the wall of
the reaction tube above the dry ice/acetone bath, with a separation no
longer than 10 cm. Continuous flow of nitrogen over the tube external wall was employed during
the experiment to prevent condensation of water affecting the illumination.
Table S1
.
Catalytic
runs under light irradiation
Entry
Light source
Acid
Reductant
NH
4
Cl
equiv.
(%
)
N
2
H
5
Cl
equiv
.
(%
)
% Yield
A
LED
440
nm
[H
2
NPh
2
][OTf]
Cp*
2
Co
7.4
(56)
4.4
(44)
73.8
B
LED
440
nm
[H
2
NPh
2
][OTf]
Cp*
2
Co
8.0
(69)
2.7
(31)
63.8
C
LED
440
nm
[H
2
NPh
2
][OTf]
Cp*
2
Co
6.6
(66)
2.5
(34)
54.9
D
LED
440
nm
[H
2
NPh
2
][OTf]
Cp*
2
Co
6.4
(62)
2.9
(38)
56.8
Average
LED
440
nm
[H
2
NPh
2
][OTf]
Cp*
2
Co
7.1
± 0.7
(
63 ± 6
)
3.1
± 0.8
(37
± 6
)
62
±
9
E
Hg Lamp
[H
2
NPh
2
][OTf]
Cp*
2
Co
8.6
(68)
3.1
(32)
70.7
F
Hg Lamp
[H
2
NPh
2
][OTf]
Cp*
2
Co
8.2
(68)
2.9
(32)
67.0
Average
Hg Lamp
[H
2
NPh
2
][OTf]
Cp*
2
Co
8.4
±
0.3
(68
±
0.3)
3.0
±
0.1
(32
±
0.3)
6
8.9
±
2.6
G
LED
440
nm
[
2,6
-
Me
PhNH
3
][OTf]
Cp*
2
Co
3.6
(46)
3.2
(54)
44.7
H
LED
440
nm
[
2,6
-
Me
PhNH
3
][OTf]
Cp*
2
Co
3.0
(49)
2.3
(51)
33.8
Average
LED
440
nm
[
2,6
-
Me
PhNH
3
][OTf]
Cp*
2
Co
3.3
±
0.4
(48
±
3)
2.7
±
0.6
(52
±
3)
3
9
±
8
I
LED
440
nm
[
2,4,6
-
Cl
PhNH
3
][OTf]
Cp*
2
Co
6.8
(69)
2.2
(30)
54.1
J
LED
440
nm
[
2,4,6
-
Cl
PhNH
3
][OTf]
Cp*
2
Co
6.2
(61)
2.9
(38)
55.92
Average
LED
440
nm
[
2,4,6
-
Cl
PhNH
3
][OTf]
Cp*
2
Co
6.5
±
0.4
(66
±
6)
2.5
±
0.5
(34
±
6)
55
±
1
K
Hg Lamp
HBAr
F
4
KC
8
4.5
(40)
5.0
(60)
22.3
L
Hg Lamp
HBAr
F
4
KC
8
4.0
(41)
4.3
(59)
19.5
Average
Hg Lamp
HBAr
F
4
KC
8
4.2
±
0.3
(41
±
1)
4.7
±
0.5
(59
±
1)
2
1
±
2
M
LED
440
nm
HBAr
F
4
KC
8
2.3
(24)
5.4
(75)
18.4
SUPPORTING INFORMATION
S
9
N
LED
440
nm
HBAr
F
4
KC
8
2.7
(29)
4.9
(71)
18.8
Average
LED
440
nm
HBAr
F
4
KC
8
2.5
±
0.3
(27
±
4)
5.1
±
0.4
(73
±
4)
18.6
±
0.3
Table S2
. Catalytic runs under dark conditions
Entry
Acid
Reductant
NH
4
Cl
equiv.
(%)
N
2
H
5
Cl
equiv.
(%)
% Yield
A
[H
2
NPh
2
][OTf]
Cp*
2
Co
11.7
(96)
0.35
(4)
67.6
D
[H
2
NPh
2
][OTf]
Cp*
2
Co
11.5
(95)
0.46
(5)
67.2
Average
[H
2
NPh
2
][OTf]
Cp*
2
Co
11.6
±
0.1
(96
±
1)
0.4
±
0.1
(4
±
1)
67.4
±
0.3
C
[
2,6
-
Me
PhNH
3
][OTf]
Cp*
2
Co
8.6
(98)
0.1
(2)
47.8
D
[
2,4,6
-
Cl
PhNH
3
][OTf]
Cp*
2
Co
12.3
(98)
0.2
(2)
68.7
Figure S
4
. Representative UV
-
vis spectra of the colorimetric methods for detection of NH
3
and
N
2
H
4
after a catalytic run under light irradiation using Cp*
2
Co and
[H
2
NPh
2
][OTf] as reactant
cocktail.
SUPPORTING INFORMATION
S
10
Figure S
5
. Representative UV
-
vis spectra of the colorimetric methods for detection of NH
3
and
N
2
H
4
after a catalytic run under light irradiation using KC
8
and
HBAr
F
4
as reactant cocktail, leading
to the highest selectivity for hydrazine.
Figure S
6
. Representative UV
-
vis spectra of the colorimetric methods for detection of NH
3
and
N
2
H
4
after a catalytic run under dark conditions using Cp*
2
Co and
[H
2
NPh
2
][OTf] as reactant
cocktail.
SUPPORTING INFORMATION
S
11
S4. Control experiments
S4.1. Compatibility of acids with the colorimetric methods for detection of NH
3
and N
2
H
4
To test for potential
interferences of the three acids employed in this work with the colorimetric
detection methods, a control experiment in the absence of catalyst was run following similar
procedure as for the catalytic runs in the dark. The acid was added to a Na/K died Et
2
O solution
in the presence or absence of a known amount of NH
3
or N
2
H
4
(12 μmol corresponding to 5 equiv
in a catalytic run) in a Schlenk tube. This mixture was brough out of the glove box and the solution
was subjected to quantification methods previously
described. Results confirmed that the acid
employed do not show any absorbance in these methods and that their presence did not affect
the quantification of the known amounts of added N
2
H
4
and NH
3
.
Figure S
7
. Representative UV
-
vis spectra for the hydr
azine test to evaluate the interference of
the acid (
[H
2
NPh
2
][OTf]) in the quantification.
The hydrazine detected corresponded to 0.88 of the
total hydrazine initially added, similar to the
detection in the absen
ce
of acid
.
SUPPORTING INFORMATION
S
12
Figure S
8
.
Representative UV
-
vis spectra for the ammonia test to evaluate the interference of
the acid (
[H
2
NPh
2
][OTf]) in the quantification.
The ammonia detected corresponded to 0.9 of the
total ammonia initially added
, similar to the detection in the absence of acid.
S4.2. Control experiments for the generation of NH
3
and N
2
H
4
under catalytically relevant
conditions
Table S
3
.
Summary of the control experiments.
Entry
Conditions
Acid
Reductant
N
2
H
5
Cl
(mM)
NH
4
Cl
(mM)
A
LED 450 nm
No catalyst, N
2
H
4
(0.023 mmol)
[H
2
NPh
2
][OTf]
Cp*
2
Co
0.02
0
B
LED 450 nm
P
3
B
Fe
+
(2.3 mM)
N
2
H
4
(0.023 mmol)
Ar atmosphere
[H
2
NPh
2
][OTf]
Cp*
2
Co
0.018
0
C
LED 450 nm
No catalyst
[H
2
NPh
2
][OTf]
Cp*
2
Co
0
0
SUPPORTING INFORMATION
S
13
Figure S
9
.
Representative UV
-
vis spectra of the colorimetric methods for detection of NH
3
and
N
2
H
4
after a control experiment using Cp*
2
Co and
[H
2
NPh
2
][OTf] as reactant cocktail in the
presence of the (P
3
B
)Fe
+
precatalyst and 10 equiv of N
2
H
4
under Ar atmosphere, showing that
N
2
H
4
is not further reduced to NH
3
.
SUPPORTING INFORMATION
S
14
S5. Variable
-
temperature catalytic runs
For catalytic runs performed at different temperatures, the reaction tubes were wrapped into
aluminum foil to maintain strict dark conditions and placed into either acetonitrile/dry ice or
aqueous NaCl solution/ice to provide the specific temperatures,
-
45
ºC or
-
10 ºC respectively.
Table S
4
. Catalytic runs under light irradiation
Entry
Temperature
Acid
Reductant
NH
4
Cl
N
2
H
5
Cl
% Yield
A
-
45 ºC
[H
2
NPh
2
][OTf]
Cp*
2
Co
8.5
1.3
57.1
B
-
45 ºC
[H
2
NPh
2
][OTf]
Cp*
2
Co
9.0
0.8
55.8
Average
-
45 ºC
[H
2
NPh
2
][OTf]
Cp*
2
Co
8.8
1
56.4
E
-
10 ºC
[H
2
NPh
2
][OTf]
Cp*
2
Co
3.4
0.8
34.8
F
-
10 ºC
[H
2
NPh
2
][OTf]
Cp*
2
Co
5.8
0.6
36.7
Average
-
10 ºC
[H
2
NPh
2
][OTf]
Cp*
2
Co
4.6
0.7
30.7
Figure S
10
. Representative UV
-
vis spectra of the N
2
H
4
colorimetric test after a catalytic run under
dark conditions, using Cp*
2
Co and
[H
2
NPh
2
][OTf] as reactant cocktail, at different temperatures.
SUPPORTING INFORMATION
S
15
Figure S
11
. Representative UV
-
vis spectra of the NH
3
colorimetric test after a catalytic run under
dark conditions, using Cp*
2
Co and
[H
2
NPh
2
][OTf] as reactant cocktail, at different temperatures
SUPPORTING INFORMATION
S
16
S6. Freeze
-
quench Mössbauer
General procedure:
Et
2
O and THF were stirred with Na/K overnight and filtered ov
er alumina prior to use. In
a nitrogen filled glovebox, [(P
3
B
)
57
Fe][BAr
F
4
] (0.0023 mmol) was quantitatively transferred using
THF to a Schlenk tube and then evaporated to yield a thin film. The tube is charged with a oven
-
dried stir bar and the other reage
nts as solids. The tube is then cooled down to 77 K in a liquid
nitrogen bath inside the glove box and allowed to equilibrate for five minutes. To the tube is added
1 mL of Et
2
O over the tube wall and this allowed to equilibrate for another five minutes. T
he tube
is then transferred to a cold well that has been precooled for at least fifteen minutes to −78 °C
with a dry ice/acetone bath. After 30 mins from the beginning of the stirring, the reaction mixture
was sampled using a prechilled pipette and subsequ
ently transferred in one portion to a pre
-
chilled Mössbauer cup sitting in a vial. The vial is then placed in a liquid nitrogen bath causing the
reaction mixture to freeze in approximately twenty seconds. The Mössbauer cup is then
submerged in the liquid n
itrogen and then removed from the glovebox and standard procedure is
used to mount the sample on the Mössbauer spectrometer.
Figure S
12
.
Mössbauer spectrum collected from a reaction freeze quenched after stirring for 30
minutes at −78 °C in 1 mL of Et
2
O between [(P
3
B
)
57
Fe][BAr
F
4
] and excess Cp*
2
Co (54 equiv.) and
[H
2
NPh
2
][OTf] (108 equiv.) as reactant cocktail
. Raw data shown as black points, simulation as a
solid red line, with components in green, purple, blue, and yellow (see Table
S
5
for parameters).
The spectrum was collected at 80 K with a parallel applied magnetic field of 50 mT in Et
2
O.
Table S
5
. Fe speciation based on simulation parameters extracted from Mössbauer spectroscopy.
SUPPORTING INFORMATION
S
17
Species
Simulation color
δ
(mm sec
−1
)
|
Δ
eq
| (mm sec
−1
)
Weight (%)
P
3
B
Fe
–
N
2
green
0.56
3.24
29
P
3
B
Fe
+
purple
0.72
2.62
12
h.s. Fe
II
blue
1.29
2.88
39
unknown
orange
0.17
1.51
20
Comments on the Mössbauer spectrum:
Freeze
-
quench Mössbauer spectroscopy was performed after 30 minutes of turnover
during light irradiation to determine the speciation of
57
Fe labeled
-
catalyst (Figure S
12
). Four main
species with well
-
resolved sets of quadrupole doublets can be
identified by deconvolution of the
80 K spectrum; these correspond to the neutral (P
3
B
)Fe(N
2
) complex (δ = 0.56 mm/s, Δ
E
Q
= 3.24
mm/s, 29%),
17
the precatalyst [(P
3
B
)Fe]
+
(δ = 0.72 mm/s, Δ
E
Q
= 2.62 mm/s, 12%),
18
a high spin
Fe
II
species (δ = 1.29 mm/s, Δ
E
Q
= 2.88 mm/s, 39%) previously associated with off
-
path or
decomposition species, and an unidentified, likely P
3
B
-
metalated Fe species (δ = 0.17 mm/s, Δ
E
Q
= 1.51 mm/s, 20%). This speciation closely resembles that obtained in the absence of light
irradiation
using the same reductant and acid combination.
17