of 30
S
1
Supporting information
for
A Synthetic Single
-
Site Fe Nitrogenase: High
Turnover, Freeze
-
Quench
57
Fe Mössbauer Data,
and
a Hydride Resting State
T
revor
J. Del Castillo
,
Niklas B. Thompson
,
and J
onas
C. Peters
T.J.D.C. and N.B.T. contributed equally to
this work.
Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena,
California 91125, United States
Table of Contents:
S
1
3
Experimental and synthetic details
S3
6
Ammonia production and quantification studies
S
7
Ammonia generation reaction with periodic substrate reloading
S
8
1
2
Time
-
resolved NH
3
q
uantification via
l
ow
-
temperature
q
uenching
S1
3
Time
-
resolved H
2
q
uantification studies
S1
4
Solution IR calibration of [Na(12
-
crown
-
4)
2
][
P
3
B
Fe
-
N
2
] (
1
)
S15
17
Stoichiometric reaction of
(
P
3
B
)
-
H)Fe(H)(N
2
) (
4
-
N
2
) with HBAr
F
4
and KC
8
S18
2
8
Mössbauer Spectra
S29
Controlled Potential Electrolysis of [P
3
B
Fe][BAr
F
4
] and 10 equiv HBAr
F
4
S2
9
References
1
.
Experimental details
1.1
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 alumina column in a solvent purification syste
m by SG Water, USA LLC.
Nonhalogenated 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 Cambridge Isotope
Laboratories, Inc., degassed,
and dried over activated 3
-
Å molecular sieves prior to use.
KC
8
,
1
[Na(12
-
crown
-
4)
2
][
P
3
B
Fe
-
N
2
]
(
1
)
,
2
[K(OEt
2
)
0.5
][
P
3
C
Fe
-
N
2
]
(
2
),
3
[Na(12
-
crown
-
4)
2
][
P
3
Si
Fe
-
N
2
] (
3
)
,
4
(
P
3
B
)
-
H)Fe(H)(N
2
) (
4
-
N
2
),
5
(P
3
B
)
-
H)Fe(H)(N
2
) (
4
-
H
2
),
5
[
P
3
B
Fe
-
NH
3
][BAr
F
4
],
6
[
P
3
B
Fe
-
S
2
N
2
H
4
][BAr
F
4
],
6
P
3
B
Fe
-
NH
2
,
6
[P
3
B
Fe][BAr
F
4
],
6
P
3
B
Fe
-
NAd
,
7
and [P
3
B
Fe
-
NAd][BAr
F
4
]
7
were
prepared
according to literature procedures
.
NaBAr
F
4
and [H(OEt
2
)
2
][BAr
F
4
]
(HBAr
F
4
)
were prepared and purified
according to a procedure modified from the literature as described below
. All other reagents were purchased
from commercial vendors and used wit
hout further purification unless otherwise stated.
Diethyl ether
(Et
2
O) and THF
used
NH
3
generation
experiments were
stirred over Na/K (≥ 2 hours) and filtered before
use.
1.2
Physical Methods:
1
H chemical shifts are reported in ppm relative to
tetramethylsilane, using
1
H resonances from
residual solvent as internal standards. IR measurements were obtained as
solutions or
thin films formed by
evaporation of solutions using a Bruker Alpha Platinum ATR spectrometer with OPUS software
(solution
IR c
ollected in a cell with KBr windows and a 1 mm pathlength)
. Optical spectrosc
opy measurements were
collected with a Cary 50 UV
-
vis spectrophotometer usin
g a 1
-
cm two
-
window quartz cell
.
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.
Cyclic voltammetry
measurements were carried out in a
glovebox under an N
2
atmosphere in a
one
-
compartment
cell using a CH Instruments 600B electro
chemical
analyzer. A glassy carbon electrode was used as the working electrode and platinum wire was used as the
auxiliary electrode. The reference electrode was Ag/AgOTf in Et
2
O
isolated by a CoralPor
frit (obtained
from BASi)
. The ferrocene couple (Fc/F
c
+
) was used as an external reference. Et
2
O solutions of electrolyte
(0.1 M NaBAr
F
4
) and analyte were also prepared under an inert atmosphere.
1.
3
M
ö
ssbauer Spectroscopy:
M
ö
ssbauer 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 t
o the centroid of the spectrum of a metallic
foil of
α
-
Fe at room temperature. Solid samples were prepared by grinding solid material into a fine
powder
and then mounted in to a Delrin cup fitted with a screw
-
cap as a boron nitride pellet. Solution samples
were
transferred to a sample cup and chilled to 77 K inside of the glovebox, and unless noted otherwise, quickly
removed from the glovebox and immersed in liquid N
2
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.
1.4 Ammonia Quantification:
Reaction mixtures are cooled to 77 K and allowed to freeze. The reaction vessel
is then opened to
atmosphere and to the frozen solution is slowly added a fourfold excess (with respect to acid) solution of a
NaO
t
Bu solution in MeOH (0.25 mM) over 1
2 minutes. This solution is allowed to freeze, then the
headspace of the tube is evacuat
ed and the tube is sealed. The tube is then allowed to warm to RT and
stirred at room temperature for 10 minutes. An additional Schlenk tube is charged with HCl (3 mL of a 2.0
M solution in Et
2
O, 6 mmol) to serve as a collection flask. The volatiles of the
reaction mixture are vacuum
transferred into this collection flask. After completion of the vacuum transfer, the collection flask is sealed
and warmed to room temperature. Solvent is removed in vacuo, and the remaining residue is dissolved in
H
2
O (1 mL). An aliquot of this solution (20
100 μL) is then analyzed for the presence of NH
3
(present as
NH
4
Cl) by the indophenol method.
8
Quantification is performed with UV−vis spectroscopy by analyzing
absorbance at 635 nm.
2
.
Synthetic
Details:
2.1
S
ynthesis and Purification of NaBAr
F
4
and
[H(OEt
2
)
2
][BAr
F
4
]
:
Crude NaBAr
F
4
was prepared according to a literature procedure.
9
The crude material, possessing
a yellow
-
tan hue, was purified by a modification to the procedure published by Bergman,
10
as follows. The
S
3
crude
NaBAr
F
4
was
ground into a fine powder and partially hydrated by exposure to air for at least 24 hours
(
NaBAr
F
4
is a hygroscopic solid and crystallizes as a hydrate containing between 0.5
3.0 equivalents of
H
2
O when isolated under air
).
This material
was
first wash
ed
with
dichloromethane (~3
mL/g, in three
portions
), washing liberally with pentane between each portion of dichloromethane. The remaining solids
were
washed with boiling fluorobenzene
(~1 mL/g, in three
portions
), to yield
a bright white powder.
Anhydrous NaBAr
F
4
was
obtained by drying this material under vacuum at 100
°
C over P
2
O
5
for at least 18
hours. Note that additional NaBAr
F
4
may be recrystallized from the fluorobenzene washings via slow
diffusion of pentane vapors a
t room temperature, and further purified if necessary.
Crude
[H(OEt
2
)
2
][BAr
F
4
]
was prepared according to a literature procedure
, using NaBAr
F
4
purified
as described above
.
11
The crude material was purified by iterative recrystallization from 4 mL/g Et
2
O
layered with an equivalent volume of pentane at
-
30
°
C. The purity of the recrystallized
[H(OEt
2
)
2
][BAr
F
4
]
was assayed by collecting a UV
-
vis spectrum of a 10 mM solution in Et
2
O
, where the presence of yellow
-
brown impurities appears as a broad
absorbance centered at ~
330
nm
(see Fig. S
2.
1
)
. Typically 2
3
recrystallizations were required to obtain material of suitable purity for catalytic reactions.
Figur
e S
2.
1
:
UV
-
vis traces of 10 mM solutions of [H(OEt
2
)
2
][BAr
F
4
] in
Et
2
O at various stages of purity.
(
Red dash
-
dotted trace
)
[H(OEt
2
)
2
][BAr
F
4
] prepared from crude NaBAr
F
4
without additional purification;
(
Purple dotted trace
)
[H(OEt
2
)
2
][BAr
F
4
] prepared from NaBAr
F
4
purified a
ccording to the above procedure,
and recrystallize
d once;
(
Blue solid trace
)
[H(OEt
2
)
2
][BAr
F
4
] prepared from NaBAr
F
4
purified according
to the above procedure, and recrystallized twice.
2.2 Preparation of 10 wt% Na(Hg) shot:
In a three
-
neck round bottom flask equipped with a mechanical stirrer, reflux condenser, and a
dropping funnel was added Na (0.5 g) and a sufficient volume of toluene to completely submerse the Na.
The dropping funnel was charged with 5 grams of Hg. The to
luene was brought to reflux and the molten
Na was finely dispersed by rapid agitation with the mechanical stirrer, at which point the Hg was added in
one shot.
Caution: upon contact with Na, the Hg vapors boil and there is a brief but intense exotherm.
The
pelleted 10 wt% Na(Hg) immediately
forms, at which point the toluene is decanted, and the shot is washed
with Et
2
O and pentane before being dried in vacuo. After breaking up coagulated pieces, this procedure
yields somewhat uniform shot ranging 1
3 mm in
diameter.
3. Ammonia production and quantification studies
3.1
Standard NH
3
Generation Reaction Procedure
with
[Na(12
-
crown
-
4)
2
]
[
P
3
B
Fe
-
N
2
] (
1
)
:
S
4
All solvents are stirred with Na/K for ≥2 hours and filtered prior to use.
In a nitrogen
-
filled
glovebox, a
stock solution of
1
in THF
(
9
.
5
mM
)
is prepared
.
Note that a fresh stock solution is prepared
for each experiment and used
immediately
. An aliquot
of this stock solution (50
200 μL, 0.47
1.9 μmol)
is added to a Schlenk tube
and
evaporated to dryness under
vacuum, depositing a film of
1
. The tube is then
charged with a stir bar and cooled to 77 K in
a cold well. To the cold tube is added a solution of
H
BAr
F
4
in
Et
2
O
. Th
is solution is allowed to cool and freeze for 5 minutes. Then a suspension of KC
8
in Et
2
O
(1.2
equiv relative to
HBAr
F
4
)
is added to the cold tube.
The temperature of the system is allowed to equilibrate
for 5 minutes and then the tube is sealed
with a Teflon screw
-
valve
. 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 the desired length of time. At this point the tube is allowed to
warm to
room temperature
with stirring, and stirred at
room
temperature
for 5 minutes.
To ensure
reproducibili
ty, all experiments were conducted in 200 mL Schlenk tubes (51 mm OD) using 25 mm stir
bars, and stirring was conducted at ~900 rpm.
Table S
3.
1
: UV
-
vis quantification results for standard NH
3
generation e
xperiments with
1
Entry
Total
volume of
Et
2
O (mL)
1
μ
mol (mM)
[H
(OEt
2
)
2
][BAr
F
4
]
equiv (mM)
NH
4
Cl
(
μmol
)
Eq
uiv
NH
3
/
Fe
% Yield
Based on
H
+
A
1.5
1.9 (1.3)
48 (63)
13.2
7.0
43.2
B
1.5
1.9 (1.3)
48 (63)
14.5
7.6
47.2
Avg
.
7.3
± 0.
5
45
±
3
C
3.0
1.9 (
0.6
4
)
97 (63)
22.1
11.6
35.9
D
3.0
1.9 (0.6
4
)
97 (63)
25.1
13.2
40.8
Avg.
12 ± 1
38 ± 3
E
1.1
0.48 (
0.43
)
1
50
(63)
8.34
17.5
36.1
F
1.1
0.48 (0.43)
150
(63)
8.19
17.2
35.5
Avg.
17.4 ± 0.2
35.8 ± 0.4
G
5.5
0.48 (
0.087
)
7
30
(63)
23.3
48.9
20.2
H
5.5
0.48 (0.087)
730 (63)
20.3
42.5
17.6
I
5.5
0.48 (0.087)
730 (63)
19.1
40.2
16.6
J
1.4
0.12 (
0.087
)
7
30
(63)
4.82
40.5
16.7
Avg.
43 ± 4
18 ± 2
K
11.0
0.48 (
0.043
)
1
500
(63)
28.4
59.5
12.3
L
11.0
0.48 (0.043)
1500
(63)
27.1
56.8
11.7
M
11.0
0.48 (0.043)
1500 (63)
22.9
48.1
9.9
N
11.0
0.48 (0.043)
1500 (63)
25.4
53.4
11.0
O
11.0
0.48 (0.043)
1500 (63)
30.2
63.5
13.1
P
2.8
0.12 (
0.043
)
1500
(63)
7.67
64.4
13.3
Q
2.8
0.12 (0.043)
1500 (63)
7.53
63.3
13.0
R
2.8
0.12 (0.043)
1500 (63)
7.67
64.4
13.3
S
2.8
0.12 (0.043)
1500 (63)
6.85
57.5
11.9
Avg.
59 ± 6
12
±
1
Hydrazine was not detected in the catalytic runs using a standard UV
-
Vis quantification method
.
12
3.2
Standard NH
3
Generation Reaction Procedure with
[
K(OEt
2
)
0.5
][
P
3
C
Fe
-
N
2
]
(
2
):
The procedure was identical to that of the standard NH
3
generation reaction protocol with the
changes noted. The precursor used was
2
.
S
5
Table S
3.
2
: UV
-
vis quantification results for standard
NH
3
generation experiments with
2
Entry
Total
volume of
Et
2
O (mL)
2
μ
mol (mM)
[H(OEt
2
)
2
][BAr
F
4
]
equiv (mM)
NH
4
Cl
(μmol)
Equiv
NH
3
/Fe
% Yield
Based on
H
+
A
*
2.5
2.5 (
1.0
)
37 (37)
4.6
± 0.8
36
± 6
B
1.1
0.63 (
0.56
)
1
10
(60)
6.66
10.7
29.7
C
1.1
0.63
(0.56)
110 (60)
7.41
11.9
33.0
Avg.
11
.
3
± 0.
9
31
±
2
D
2.3
0.63
(0.
28
)
2
20
(60)
7.41
11.9
16.4
E
2.3
0.63 (0.28)
220 (60)
9.89
15.8
21.9
Avg.
14
±
3
19
±
4
F
2.0
0.16
(
0.080
)
7
50
(60)
2.49
15.6
6.2
G
2.0
0.16 (0.080)
750 (60)
3.50
21.9
8.8
Avg.
19
±
4
7
±
2
H
4.0
0.16 (
0.040
)
1
500
(60)
7.46
46.8
9.3
I
4.0
0.16 (0.040)
1500 (60)
4.63
29.0
5.8
J
4.0
0.16 (0.040)
1500 (60)
5.82
36.5
7.3
K
4.0
0.16 (0.040)
1500 (60)
5.56
34.8
6.9
L
4.0
0.16 (0.040)
1500 (60)
5.13
32.1
6.4
Avg.
36
±
7
7
±
1
Hydrazine was not detected in the catalytic runs using a standard UV
-
Vis quantification method.
12
*
Data for entry A
is an
average
of experiments described in reference
3
.
3.3
Standard NH
3
Generation Reaction Procedure with [
Na(12
-
crown
-
4)
2
][
P
3
Si
Fe
-
N
2
] (
3
):
The procedure was identical to that of the standard NH
3
generation reaction protocol with the
changes noted. The precursor used
was
3
.
Table S
3.
3
: UV
-
vis quantification results for standard NH
3
generation experiments with
3
Entry
Total
volume of
Et
2
O (mL)
3
μmol (mM)
[H
(OEt
2
)
2
][BAr
F
4
]
equiv (mM)
NH
4
Cl
(μmol)
Equiv
NH
3
/Fe
% Yield
Based on
H
+
A*
3.25
1.9 (
0.58)
49
(
28
)
0
.
8
± 0.
5
5
±
3
B
3.0
0.12 (
0.039)
1
500
(60)
0.516
4.4
0.9
C
3.0
0.12 (0.039)
1500
(60)
0.380
3.2
0.6
Avg.
3
.
8
± 0.
8
0.8
±
0.
2
Hydrazine was not detected in the catalytic runs
using a standard UV
-
Vis quantification method.
12
* Data for entry A is an average of experiments described in reference
13
.
3.4
Standard NH
3
Generation Reaction Procedure with
(
P
3
B
)
-
H)Fe(H)(N
2
) (4
-
N
2
):
The procedure was identical to that
of the standard NH
3
generation reaction protocol with the
changes noted. The precursor used was
4
-
N2
.
Note that
4
-
N
2
is not indefinitely stable in the solid state,
even at
-
30
°
C; accordingly
4
-
N
2
was used within 24 hours after isolation as a solid.
The a
ddition of
toluene was necessary to load the precatalyst volumetrically. The final solvent composition for entries A
and B was 3% toluene in Et
2
O. The final solvent composition for entries C and D was 25% toluene in
Et
2
O.
Table
S
3.
4
: UV
-
vis quantification
results for standard NH
3
generation experiments with
4
-
N
2
S
6
Entry
Total
volume of
(mL)
4
-
N
2
μmol (mM)
[H(OEt
2
)
2
][BAr
F
4
]
equiv (mM)
NH
4
Cl
(μmol)
Equiv
NH
3
/Fe
% Yield
Based on
H
+
A
1.1
0.48 (*)
150 (63
)
0.582
1.19
2.56
B
1.1
0.48 (*)
150 (63)
0.490
1.00
2.15
Avg.
1.1
± 0.1
2.4
± 0.3
C
1.7
0.74 (0.
44
)
150 (63)
3.66
4.95
10.3
D
1.7
0.74 (0.
44
)
150 (63)
4.63
6.26
13.0
Avg.
5.6 ± 0.9
12 ± 2
Hydrazine was not detected in the catalytic runs using a standard UV
-
Vis quantification method.
12
*
4
-
N
2
not fully soluble under these conditions
.
3.5 NH
3
Generation Reaction Procedure with (1) with the inclusion of NH
3
:
A standard
catalytic reaction was prepared according to the procedure detailed in section 3.1. After
the frozen Schlenk tube was removed from the glovebox, it was brought to a Schlenk line and attached to
the line via a 0.31 mL calibrated volume
(corresponding to 12.
7 μmol gas when filled at 21 °C and 1 atm)
.
The gas manifold of the line was first filled with N
2
by three pump
-
refill cycles, and subsequently sparged
(through a mineral oil bubbler) with NH
3(g)
for 30 minutes, passing the NH
3(g)
through a
-
30
°
C trap to
remove adventitious water. At this point the calibrated volume was filled with NH
3(g)
via 5 pump
-
refill
cycles, and then sealed from the gas manifold. The
frozen
Schlenk tube was opened
and allowed to
equilibrate with the calibrated volume for 1 hour before it was resealed, and the reaction carried out in the
usual manner. As a control, several trials were conducted
with only an 2.0 M ethereal solution of HCl frozen
in the tube
, and it as
sumed that the average amount of NH
3
recovered in those trials was added to the
catalytic reactions.
Table S3.
5
: UV
-
vis quantification results for NH
3
generation experiments with
1
with the inclusion of
NH
3
Entry
Total
volume of
Et
2
O (mL)
1
μmol (mM)
[H(OEt
2
)
2
][BAr
F
4
]
equiv (mM)
NH
4
Cl
(μmol)
NH
4
Cl
due to Fe
(μmol)
Equiv
NH
3
/Fe
A
3.0
0
0
12.5
N/A
N/A
B
3.0
0
0
11.8
N/A
N/A
C
3.0
0
0
12.1
N/A
N/A
D
3.0
0
0
12.0
N/A
N/A
E
3.0
0
0
12.2
N/A
N/A
Avg.
12.1 (95 % of expected value)
C
1.
1
0.48 (0.43)
150 (63)
15.1
3.0
6.3
D
1.
1
0.48 (0.43)
150 (63)
15.2
3.1
6.5
Avg.
6.4 ± 0.1
Hydrazine was not detected in the catalytic runs using a standard UV
-
Vis quantification method.
12
3.6 Standard NH
3
Generation Reaction Procedure with 1 using Na(Hg) as the reductant:
The procedure was identical to that of the standard NH
3
generation reaction protocol with the
changes no
ted. The precursor used was
1
and 10 wt% Na(Hg) shot of approximately 1
3 mm diameter
was employed as the reductant (1900
Na atom
equiv relative to catalyst).
Table S3.6
: UV
-
vis quantification results for standard NH
3
generation experiments with
1
using N
a(Hg)
as the reductant
S
7
Entry
Total
volume of
Et
2
O (mL)
1
μmol (mM)
[H(OEt
2
)
2
][BAr
F
4
]
equiv (mM)
NH
4
Cl
(μmol)
Equiv
NH
3
/Fe
% Yield
Based on
H
+
A
1.
1
0.48 (0.43)
150 (63)
2.45
5.15
10.6
B
1.
1
0.48 (0.43)
150 (63)
2.30
4.84
9.97
Avg.
5.0
± 0.
2
10.3
± 0.
5
Hydrazine was not detected in the catalytic runs using a standard UV
-
Vis quantification method.
12
4.
NH
3
Generation Reaction with Periodic Substrate Reloading, Procedure with
1
:
All solvents are stirred with Na/K for ≥2 hours and filtered prior to use. In a nitrogen
-
filled
glovebox, a
stock solution of [Na(12
-
crown
-
4)
2
][
P
3
B
Fe
-
N
2
]
(
1
)
in THF (9.5 mM) is prepared. Note that a
fresh stock solution is prepared for each experiment and used
immediately
. An aliquot of this stock solution
(50
-
200 μL, 0.47
-
1.9 μmol) is added to a Schlenk tube. T
his aliquot is evaporated to dryness under vacuum,
depositing a film of
1
. The tube is then charged with a stir bar and cooled to 77 K in a cold well. To the
cold
tube is added a solution of
H
BAr
F
4
(
48 equiv with respect to
1
) in Et
2
O
. This solution is
allowed to
cool and freeze for 5 minutes. Then a suspension of KC
8
(1.3 equiv with respect to
HBAr
F
4
) in Et
2
O is
added to the cold tube. The temperature of the system is allowed to equilibrate for 5 minutes and then the
tube is sealed. The cold well coolin
g bath is switched from a N
2(
l
)
bath to a dry ice/acetone bath. In the cold
well the mixture in the sealed tube thaws with stirring and is allowed to stir at
-
78 °C for 40 minutes. Then,
without a
llowing the tube to warm above
-
78 °C
,
the cold well bath is
switched from dry ice/acetone to
N
2(
l
)
. After ten minutes the reaction mixture is observed to have frozen, at this time the tube is opened
.
To
the cold
tube is added a solution of
HBAr
F
4
(
48 equiv with respect to
1
) in Et
2
O. This solution is allowed
to
cool and freeze for 5 minutes. Then a suspension of KC
8
(1.3 equiv with respect to
HBAr
F
4
) in Et
2
O is
added to the cold tube. The temperature of the system is allowed to equilibrate for 5 minutes and then the
tube is sealed. The cold well cooling bath is s
witched from a N
2(
l
)
bath to a dry ice/acetone bath. In the cold
well the mixture in the sealed tube thaws with stirring and is allowed to stir at
-
78 °C for 40 minutes. These
last steps are repeated for the desired number of loadings. Then
the tube is all
owed to warm to RT with
stirring, and stirred at RT for 5 minutes.
Table S
4.1
: UV
-
vis quantification results for NH
3
generation experiments with 1, with reloading
Entry
Number of
Loadings
4
-
N
2
μmol
[H(OEt
2
)
2
][BAr
F
4
]
equiv
NH
4
Cl
(μmol)
Equiv
NH
3
/Fe
%
Yield
Based on
H
+
A
1
1.9
48
13.3
6.96
43.2
B
1
1.9
48
14.5
7.60
47.2
Avg.
7.3
± 0.
5
45
±
3
C
2
0.95
96
9.56
10.0
31.5
D
2
0.95
96
10.3
10.9
34.0
Avg.
10.4
± 0.
6
33
± 2
C
2
0.95
150
13.4
14.1
32.7
D
2
0.95
150
14.9
15.6
29.4
Avg.
15
±
1
31
± 2
C
2
1.0
190
18.4
17.6
29.0
D
2
1.0
190
18.4
17.6
29
.0
Avg.
17.6
29
S
8
5.
General Procedure for T
ime
-
resolved NH
3
Q
uantification
via Low
-
temperature Q
uenching:
A typical catalytic reaction is prepared according to the
procedure described above. The timer is
set to zero as soon as the frozen reaction mixture is transferred to the dry ice/acetone bath;
note that the
average thaw time is 2.0 ± 0.3 minutes (measured for a 1.1 mL solution of Et
2
O over 8 trials).
A
t the desir
ed
reaction time, the Schlenk tube is rapidly transferred to a liquid N
2
bath and
the reaction mixture is allowed
to freeze
. Under N
2
counterflow, a solution of
t
BuLi (1.6 M in hexanes, 4 equiv with respect to
HBAr
F
4
is
added to the frozen reaction mixture
. The Schlenk tube is then sealed, thawed to
-
78
°
C, and stirred rapidly
for 10 minutes. The Schlenk tube is transferred to a liquid N
2
bath and the reaction mixture is re
-
frozen.
The reaction vessel is opened to atmosphere and to the frozen solution is sl
owly added a fivefold excess
(with respect to
HBAr
F
4
) solution of a NaO
t
Bu solution in MeOH (0.25 mM) over 1
2 minutes. This
solution is allowed to freeze, then the headspace of the tube is evacuated and the tube is sealed. The tube is
then allowed to warm
to RT and stirred at room temperature for 10 minutes. At this point the reaction is
quantified for the presence of NH
3
(
vide supra
)
.
As a control to determine that the action of
t
BuLi is sufficiently fast to enable rapid quenching of
catalytic reactions a
t low temperature, we added
t
BuLi to reaction mixtures prepared as described above
before allowing them to thaw to
-
78 °C for the first time (
effectively at time 0, Table S
5.1
, Entry
A) and
observed no detectable NH
3
formation.
Table S
5.1
:
Time profiles for NH
3
generation by
1
Entry
Total
volume of
Et
2
O (mL)
1
μmol (mM)
[H(OEt
2
)
2
][BAr
F
4
]
equiv (mM)
Quench
time (min)
[NH
3
]
(mM)
Equiv
NH
3
/Fe
A
3.0
1.9 (0.6
4
)
48 (31)
0
0
0
B
3.0
1.9 (0.6
4
)
48 (31)
5
1.21
1.91
C
3.0
1.9 (0.6
4
)
48 (31)
5
1.87
2.94
Avg.
5
1.5
± 0.
5
2.4
± 0.2
D
3.0
1.9 (0.6
4
)
48 (31)
10
4.36
6.86
E
3.0
1.9 (0.6
4
)
48 (31)
10
3.66
5.76
Avg.
10
4.0
± 0.5
6.3
± 0.
8
F
3.0
1.9 (0.6
4
)
48 (31)
1
5
4.87
7.68
G
3.0
1.9 (0.6
4
)
48 (31)
1
5
4.63
7.29
Avg.
15
4.8
± 0.
2
7.5
± 0.
3
H
3.0
1.9 (0.6
4
)
48 (31)
25
4.40
6.93
I
3.0
1.9 (0.6
4
)
48 (31)
25
4.79
7.54
Avg.
25
4.6 ± 0.3
7.2 ± 0.4
J
1.1
0.48 (0.43)
150 (63)
5
0.806
1.86
K
1.1
0.48 (0.43)
150 (63)
5
1.09
2.51
Avg.
5
0.9
±
0.2
2.2
± 0.
5
L
1.1
0.48 (0.43)
150 (63)
10
2.08
4.81
M
1.1
0.48 (0.43)
150 (63)
10
2.74
6.33
Avg.
10
2.4
± 0.5
6
±
1
N
1.1
0.48 (0.43)
150 (63)
15
3.79
8.75
O
1.1
0.48 (0.43)
150 (63)
15
4.06
9.39
Avg.
15
3.9
± 0.2
9.1
± 0.
4
P
1.1
0.48 (0.43)
150 (63)
25
5.73
13.2
Q
1.1
0.48 (0.43)
150 (63)
25
4.68
10.8
Avg.
25
5.2
± 0.
7
12
±
2
R
1.1
0.48 (0.43)
150 (63)
35
6.11
14.1
S
9
S
1.1
0.48 (0.43)
150 (63)
35
5.51
12.7
Avg.
35
5.8
± 0.
4
13
±
1
T
1.1
0.48 (0.43)
150 (63)
45
7.99
18.5
U
1.1
0.48 (0.43)
150 (63)
45
8.37
19.3
Avg.
45
8.2
± 0.
3
18.9
±
0.6
V
1.1
0.48 (0.43)
150 (63)
55
7.18
16.6
W
1.1
0.48 (0.43)
150 (63)
55
7.94
18.3
Avg.
55
7.6
± 0.5
17
±
1
Hydrazine was not detected in the catalytic runs using a standard
UV
-
Vis quantification method.
10
5.1
Kinetic S
tudy of NH
3
G
eneration by
1
via the M
ethod of
I
nitial
R
ates:
General procedure
:
Typical catalytic reactions were prepared at various concentrations of
1
and
HBAr
F
4
(1.1 mL Et
2
O total for each reaction).
For each given concentration of
1
and
HBAr
F
4
, the time
profile of NH
3
generation was measured over the first 15 minutes by quenching reactions at 5, 10 and 15
minutes, as described above.
The results of individual experiments are given in Table S
5.2
.
The i
nitial rate
of NH
3
formation,
0
=
[
N
H
3
]
t
(0)
was measured as the slope of a least
-
squares linear regression for these
data. For the cases where the timescale of the reaction was too fast to obtain pseudo
-
first
-
order behavior
over the first 15 minutes,
0
was approximated as the slope of the line be
tween the yield of NH
3
at 5 minutes
and a zero point at 2 minutes (the aver
age thaw time for the reaction
,
vide supra
); the results of this analysis
are given in Table S
5.3
and plotted in Figures S
5.1
and
S5.2
.
The reaction order in
1
and
HBAr
F
4
was
determined by applying a least
-
squares linear analysis to the initial rates determined for 5 different
concentrations in each reage
nt, ranging over a factor of 16 (Table
S5.4
).
Table
S
5.2
:
Time resolved NH
3
quantification data used in initial rates analysis for NH
3
generation by
1
Entry
Total
volume of
Et
2
O (mL)
1
μmol (mM)
[H(OEt
2
)
2
][BAr
F
4
]
equiv (mM)
Quench
time (min)
[NH
3
] (mM)
A
1.1
0.12
(0.11)
560
(
63
)
5
0.268
B
1.1
0.12
(0.
11
)
560
(
63
)
5
0.273
Avg.
5
0.270
± 0.
004
C
1.1
0.12 (0.11)
560 (63)
10
0.225
D
1.1
0.12 (0.11)
560 (63)
10
0.338
Avg.
10
0.28
± 0.
08
E
1.1
0.12 (0.11)
560 (63)
15
0.747
F
1.1
0.12 (0.11)
560 (63)
15
1.27
Avg.
15
1.0
± 0.
4
G
1.1
0.24 (0.22)
290 (63)
5
0.538
H
1.1
0.24 (0.22)
290 (63)
5
0.763
Avg.
5
0.7
± 0.
2
I
1.1
0.24 (0.22)
290 (63)
10
1.81
J
1.1
0.24 (0.22)
290 (63)
10
1.29
Avg.
10
1.5 ± 0.4
K
1.1
0.24 (0.22)
290 (63)
15
3.47
L
1.1
0.24 (0.22)
290 (63)
15
2.23
Avg.
15
2.9
± 0.
9
M
1.1
0.95 (0.87)
73 (63)
5
1.98
N
1.1
0.95 (0.87)
73 (63)
5
1.64