of 68
S
1
Supporting information for
Nitrogen Fixation via a Terminal Fe(IV) Nitride
Niklas B. Thompson,
Michael T. Green
,
Jonas C. Peters
Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California
91125, USA
Department of Chemistry, University of California
Irvine, Irvine, California 92697, USA
Table of Contents:
S
2
9
Experimental and
S
ynthetic details
S9
12
Supplementary Discussion
S
13
26
NMR Spectra
S
27
28
UV
-
vis Spectra
S
29
37
Mössbauer Spectra
S38
42
XAS Spectra
S
43
44
Cyclic Voltammograms
S45
Ab initio
EXAFS Scattering Paths for (P
3
B
)Fe(NNMe
2
)
S46
Calibration of DFT Method for Prediction of Mossbauer Spectra
S47
Fitting of VT NMR Data for (P
3
B
)Fe(NNMe
2
)
S48
Comparison of EXAFS Simulations of
(P
3
B
)Fe(NNMe
2
) and
(P
3
B
)Fe(NNH
2
) Excluding Fe
N
N Multiple Scattering
S49
Comparison of First and Second Scan XAS Data for [(P
3
B
)Fe≡N]
+
.
S50
Tabulated Crystallographic Details
S51
53
Tabulated Mössbauer Simulation Parameters
S55
Tabulated XANES
Simulation Parameters
S56
59
Tabulated EXAFS Simulation Parameters
S60
Tabulated NH
3
/N
2
H
4
Quantification Results
S61
Fit Parameters of VT NMR data of
(P
3
B
)Fe(NNMe
2
)
S62
Comparison of DFT
-
optimized S
tructures
(P
3
B
)Fe(NNMe
2
)
S63
Comparison of DFT
-
predicted Spin State Energetics of
(P
3
B
)Fe(NNMe
2
)
S64
Tabulated Data used to Calibrate the
DFT Method for Prediction of
Mossbauer Spectra
S65
Calculated Spin State Energetics
for
(P
3
B
)Fe(NNH
2
)
and [(P
3
B
)Fe≡N]
+
S66
Comparison of DFT
-
optim
ized
S
tructures of
[(P
3
B
)Fe≡N]
+
(
S
= 0) and
(P
3
B
)Fe≡N (
S
= 1/2).
S67
Supplementary Acknowledgments and
References
S
2
Experimental and Synthetic Details
Unless noted otherwise, 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 system by SG Water, USA LLC. Deoxygenated,
anhydrous 2
-
methyltetrahydrofuran (2
-
Me
THF) was purified by stirring over sodium
-
potassium alloy and filtering through a short column of activated alumina prior to use.
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.
The compounds tris(
o
-
diisopropylphosphinophenyl)borane
(P
3
B
),
1
(P
3
B
)Fe
(
Br
),
2
(P
3
B
)Fe(Me)
,
3
[Na(Et
2
O)
2
]
[(P
3
B
)Fe(N
2
)]
,
2
[Na(12
-
crown
-
4)
2
]
[(P
3
B
)Fe(N
2
)]
,
2
[H(OEt
2
)
2
][BAr
F
4
] (HBAr
F
4
)
,
4
and
57
FeCl
2
,
5
were prepared according to literature
procedures.
57
Fe
-
labelled
[Na(12
-
crown
-
4)
2
]
[(P
3
B
)Fe(N
2
)]
was prepared as usual, but
using (P
3
B
)
57
Fe(Cl) as the
precursor. (P
3
B
)
57
Fe(Cl) was prepared by a synthesis analogous
to that of the bromide analog, but replacing FeBr
2
with
57
FeCl
2
as t
h
e Fe precursor
. All
other reagents were purchased from commercial vendors and used without further
purification unless
otherwise stated.
NMR
Spectroscopy
Chemical shifts for
1
H and
13
C are reported in ppm relative to tetramethylsilane,
using resonances from residual solvent as internal standards;
31
P and
11
B resonances are
reported in ppm, referenced to the signal of the
deuterated solvent used to lock the
instrument.
IR
Spectroscopy
IR measurements were obtained as powders or thin films formed by evaporation of
solutions using a Bruker Alpha Platinum ATR spectrometer with OPUS software.
UV
-
Visible
Spectroscopy
Optical spectroscopy measurements were collected with a Cary 50 UV
-
vis
spectrophotometer using a 1
-
cm two
-
window quartz cell.
Electrochemistry
Cyclic voltammetry measurements were carried out in a glovebox under an N
2
atmosphere in a one
-
compartment cell
using a CH Instruments 600B electrochemical
analyzer. A glassy carbon electrode was used as the working electrode and a carbon rod
was used as the auxiliary electrode. The reference electrode was AgOTf/Ag in THF
isolated by a CoralPor™ frit (obtained from
BASi). The ferrocenium/ferrocene couple
(Fc
+
/Fc) was used as an external reference. THF solutions of electrolyte (0.1 M
[NBu
4
][PF
6
]) and analyte were also prepared under an inert atmosphere.
X
-
ray
Crystallography
X
-
ray diffraction studies were carried o
ut at the Caltech Division of Chemistry and
Chemical Engineering X
-
ray Crystallography Facility
using a dual source Bruker D8
S
3
Venture, four
-
circle diffractometer with a PHOTON CMOS detector.
Data was collected
at 100K using Mo Kα radiation (λ = 0.71073 Å).
The crystals were mounted on a glass
fiber under Paratone N oil.
Structures were solved
using SHELXT
6
and refined against
F
2
on all data by full
-
m
atrix least squares with SHELXL.
7
All non
-
hydrogen atoms were
refined anisotropically. All hydrogen atoms wer
e placed at geometrically calculated
positions and refined using a riding model. The isotropic displacement parameters of all
hydrogen atoms were fixed at 1.2 (1.5 for methyl groups) times the
U
eq
of the atoms to
which they are bonded.
57
Fe
Mössbauer
Spec
troscopy
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), using liquid He as a cryogen
for temperatures below 80 K, and liquid N
2
as a cryogen for 80 K measurements. The
quoted isomer shifts are relative to 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 De
lrin 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 quickly removed from the glovebox and immersed in liquid N
2
until
mounted in the cryostat. Data an
alysis was performed using version 4 of the program
WMOSS (www.wmoss.org) and quadrupole doublets were fit to Lorentzian lineshapes.
8
X
-
ray
Absorption
Spectroscopy
(XAS)
Samples for XAS measurements were prepared in modified Mössbauer sample
cups in whic
h the bottom of the Delrin cup was removed and sealed with Kapton tape.
All samples thus prepared were analyzed by Mössbauer spectroscopy at 80 K prior to
collection of XAS data. Samples were maintained at temperatures of 80 K and below at
all times.
XAS d
ata collection was conducted at the Stanford Synchrotron Radiation
Laboratory (SSRL) with the SPEAR 3 storage ring containing 500 mA at 3.0 GeV. Fe K
-
edge data were collected on the beamline 9
-
3 operating with a wiggler field of 2 T and
employing a Si(220)
double
-
crystal monochromator. Beamline 9
-
3 is equipped with a
rhodium
-
coated vertical collimating mirror upstream of the monochromator and a bent
-
cylindrical focusing mirror (also rhodium
-
coated) downstream of the monochromator.
Harmonic rejection was acc
omplished by setting the energy cutoff angle of the mirrors to
10 keV. The incident and transmitted X
-
ray intensities were monitored using nitrogen
-
filled ionization chambers, and for dilute samples X
-
ray absorption was monitored by
measuring the Fe Kα flu
orescence intensity using an array of 100 Canberra germanium
detectors. For concentrated samples (≥ 10 mM), fluorescence was measured using a
single
-
channel PIPS detector. During data collection, samples were maintained at a
temperature of approximately 10
K using an Oxford instruments liquid helium flow
cryostat. The energy was calibrated by reference to the absorption of a standard iron
metal foil measured simultaneously with each scan, assuming a lowest energy inflection
point of the iron foil to be
7111
.3 eV
. Samples were monitored for photodamage by
comparing the pre
-
edge region between consecutive scans. In cases where photodamage
was detected, the sample was moved to a previously un
-
exposed region and single scans
S
4
were collected; in this fashion, six
first scans could be collected and integrated for each
sample.
The raw XAS data were analyzed using the EXAFSPAK suite of programs.
9
Data
were calibrated to the first inflection of the iron foil reference and averaged over all first
scans for each sample.
The edge region was background corrected by fitting a Gaussian
function through the pre
-
edge region and subtracting this from the entire spectrum. A
four
-
segment fourth
-
order spline was fit to the EXAFS region, and the spectrum was
normalized to the edge
jump. In most cases, a monochromator glitch at
k
≈12 Å
1
was
removed by fitting a cubic polynomial to the raw data. For dilute samples, a step at the
Co K
-
edge (7709 eV) due to a small Co impurity detected in the incident X
-
rays was
observed; note that sin
ce the step was present in the incident channel, the impurity was
due to Co on the slits which focus the X
-
rays on the sample, not in the sample itself. This
impurity was corrected by fitting a fourth
-
order polynomial through the step in the raw
data to de
termine a constant offset, which was subsequently subtracted from the data after
the step. Interatomic distances obtained from simulation of the raw, uncorrected EXAFS
data were found to be identical to those obtained from simulation of the data deglitched
in this manner (
vide infra
). No smoothing, filtering, or related operations were performed
on the data. The pre
-
edge region was fit between 7108 and 7119 eV using the EDG_FIT
utility. Resonances were fit with pseudo
-
Voigt lineshapes, where the weight of t
he
Lorentzian and Gaussian components were allowed to refine freely. The EXAFS
oscillations
χ
(
k
)
were quantitatively analyzed by non
-
linear least square curve
-
fitting. The
k
3
-
weighted data were fit from
k
= 3
15 Å
1
.
Ab initio
theoretical phase and amplitu
de
functions were calculated using the program FEFF version 7.
10
Synthetic
Details
Synthesis of [(P
3
B
)FeN
2
]
2
:
From
[Na(Et
2
O)
2
]
[(P
3
B
)Fe(N
2
)]
.
A solution of
[Na(Et
2
O)
2
]
[(P
3
B
)Fe(N
2
)]
in THF (7 mM) was passed iteratively 3 times through a short
column of KC
8
(~ 0.7 × 0.7 mm) packed on top of a glass microfiber filter. An aliquot of
this solution was dried to a thin film on the sample plate of an ATR
-
IR spectrometer. The
resultant IR spectrum displays an intense vibration at 1803 cm
1
attributable to the N
N
vibration of
[(P
3
B
)FeN
2
]
2−
(Fig.
S12
).
The difference in N
2
stretching frequencies
between
[(P
3
B
)FeN
2
]
2−
produced from reduction of
[Na(Et
2
O)
2
]
[(P
3
B
)Fe(N
2
)]
and
[(P
3
B
)FeN
2
]
2−
produced from reduction of
[Na(12
-
crown
-
4)
2
]
[(P
3
B
)Fe(N
2
)
]
(Δν
NN
= 31
cm
-
1
,
vide infra
) is nearly identical to th
at observed for
[Na(12
-
crown
-
4)
2
]
[(P
3
B
)Fe(N
2
)]
versus
[Na(Et
2
O)
2
]
[(P
3
B
)Fe(N
2
)]
(Δν
NN
= 29 cm
-
1
), suggesting coordination of Na
+
to the
N
2
ligand of
[(P
3
B
)FeN
2
]
2−
produced
in situ
from
[Na(Et
2
O)
2
]
[(P
3
B
)Fe(N
2
)]
.
From
[Na(12
-
crown
-
4)
2
]
[(P
3
B
)Fe(N
2
)]
.
A solution of
[Na(12
-
crown
-
4)
2
]
[(P
3
B
)Fe(N
2
)]
in DME (1 to 10 mM) was similarly reduced via iterative passage
through a column of KC
8
. The filtered supernatant was layered with an equal volume of
Et
2
O and placed in a freezer at −35 °C. After ca. 24 hrs, the mother liquor was decanted
off of black crystalline solids, which were liberally washed with Et
2
O before drying
in
vacuo
. The solid state IR spectrum shows an intense resonance at 1836 cm
1
attribu
table
to the N
N vibration of [Na(12
-
crown
-
4)][K(DME)
x
]
[
(P
3
B
)Fe(N
2
)
]
. For NMR analysis,
deuterated THF was employed as the solvent, and the filtered black solution was sealed
in an NMR tube fitted with a J
-
Young adapter containing a spatula tip of KC
8
. Based on
the NMR data,
[(P
3
B
)FeN
2
]
2−
maintains
C
3
symmetry in solution, with a single set of
S
5
aromatic resonances in the
1
H spectrum, and one singlet in the
31
P spectrum.
As
[(P
3
B
)Fe(N
2
)]
2−
(−3.2 V vs Fc
+
/Fc) is more reducing than its alkali metal counter
ions (e.g.
−3.04 V vs Fc
+
/Fc for Na
+/0
) in ethereal solvents
, it is subject to disproportionation in
solution, and such preparations invariably contain [K(DME)
x
][(P
3
B
)Fe(N
2
)] as a
contaminant
NN
= 1893 cm
1
)
.
Thus
e
lemental analysis was not collected. Fo
r
Mössbauer studies,
57
Fe
-
enriched [Na(12
-
crown
-
4)][K(Solv)
x
]
[(P
3
B
)FeN
2
]
was prepared
in situ from 100%
57
Fe
-
enriched [Na(12
-
crown
-
4)]
[(P
3
B
)FeN
2
]
using the same method
with THF or 2
-
MeTHF as the solvent, and the filtered solution was immediately frozen
into a Delrin sample holder.
1
H NMR (400 MHz,
d
8
-
THF, 293 K, ppm): δ 7.14 (d,
J
= 5 Hz, Ar
-
C
H
, 3H) , 6.84
(d,
J
= 5 Hz, Ar
-
C
H
, 3H), 6.45 (t,
J
= 6.5
Hz, Ar
-
C
H
, 3H), 6.29 (t,
J
= 6.5 Hz, Ar
-
C
H
,
3H), 3.61 (br, 12
-
crown
-
4), 3.59 (br, THF), 2.35 (br,
-
C
H
(CH
3
)
2
, 6H), 1.78 (br,
-
CH(C
H
3
)), 1.73 (br,
THF
); N.B., a
ccurate integrations for the isopropyl methyl protons of
the ligand and the methylene protons of
the [(12
-
crown
-
4)
2
Na]
+
ion could not be obtained
due to overlap with the residual THF resonances appearing at 1.73 and 3.59 ppm.
31
P{
1
H} (162 MHz, C
6
D
6
, 293 K, ppm): δ 79.65.
Synthesis of (P
3
B
)Fe(N
NMe
2
)
:
To a suspension of (P
3
B
)FeBr (200 mg, 0.275 mmol)
in 5.5 mL Et
2
O was added MeOTf (65 μL, 0.578 mmol), and the mixture was
subsequently cooled to −78 °C with stirring in the cold well of an N
2
-
filled glove box. A
scintillation vial containing KC
8
(123 mg, 0.909 mmol) suspended in 2.5 mL Et
2
O was
similarly
chilled, and then transferred to the stirring (P
3
B
)FeBr/MeOTf mixture via
pipette; this vial was additionally washed with 1 mL of pre
-
chilled Et
2
O, which was
subsequently added to the reaction mixture. The mixture was allowed to stir at −78 °C for
1 hr, an
d then allowed to warm to room temperature and stirred an additional 3 hrs. The
solvent was removed
in vacuo
, and the remaining solids extracted with pentane and
filtered over a pad of celite until the filtrate, containing crude product, is colorless (ca.
50
mL). The filtrate was concentrated to dryness, and THF (ca. 5 mL) and 0.7 wt% Na(Hg)
(1.375 mmol Na
0
) were added. This mixture is stirred rapidly overnight (ca. 12 hrs), at
which point the dark solution is decanted from the excess Na(Hg), the solvent re
moved
in
vacuo
, and the remaining solids extracted with pentane and filtered through a celite pad
until the filtrate runs colorless. The pentane extract is concentrated to ca. 5 mL and then
cooled to −35 °C. After 2 days, the mother liquor is decanted, the
remaining solids
washed with cold pentane (5 × 1 mL), and dried
in vacuo
to yield
(P
3
B
)Fe(NNMe
2
)
as
dark brown crystals (24 mg, 13%). Crystals suitable for XRD were obtained by slow
evaporation of a pentane solution of
(P
3
B
)Fe(NNMe
2
)
at room temperature.
1
H NMR (400 MHz, C
6
D
6
, 293 K, ppm): δ 9.73 (d,
J
= 7 Hz, Ar
-
C
H
, 3H) , 8.19 (t,
J
= 7 Hz, Ar
-
C
H
, 3H), 6.97 (d,
J
= 7 Hz, Ar
-
C
H
, 3H), 5.40 (t,
J
= 7 Hz, Ar
-
C
H
, 3H), 5.29
(br,
-
C
H
(CH
3
)
2
, 3H), 4.06 (br,
-
C
H
(CH
3
)
2
, 3H), 1.11 (d,
J
= 6 Hz,
-
CH(C
H
3
), 9H), 0.95
(
d,
J
= 6 Hz,
-
CH(C
H
3
), 9H), 0.64 (d,
J
= 6 Hz,
-
CH(C
H
3
), 9H), 0.06 (d,
J
= 6 Hz,
-
CH(C
H
3
), 9H), −18.66 (br,
-
NN(C
H
3
)
2
, 6H).
13
C{
1
H} NMR (101 MHz, C
6
D
6
, 293 K,
ppm): δ 241.88 (Ar
-
C), 137.78 (Ar
-
C
H), 120.34 (Ar
-
C
H), 119.28 (Ar
-
C
H), 75.39 (Ar
-
C
),
62.24 (Ar
-
C
H), 45.38 (
-
C
H(CH
3
)
2
), 33.56 (
-
CH(
C
H
3
)), 22.62 (
-
CH(
C
H
3
)), 17.67 (
-
CH(
C
H
3
)), 16.92 (
-
CH(
C
H
3
)), −10.74 (
-
C
H(CH
3
)
2
); N.B., a resonance for the
N
-
methyl
carbon atom could not be located in the chemical shift range from 1000 to −500 ppm,
even by
1
H
-
detected HS
QC/HMQC. The resonance is likely too broad at 293 K by
S
6
exchange with the paramagnetic excited state to be observed (
vide infra
).
31
P{
1
H} (162
MHz, C
6
D
6
, 293 K, ppm): δ 806.61 (br, FWHM = 2741 Hz).
11
B (128 MHz, C
6
D
6
, 293
K, ppm) δ −396.23 (br, FWHM = 909 H
z). UV
-
Visible (toluene, 293 K, nm {ε, cm
-
1
M
-
1
}): 551 {636}, 774 {139}. Anal. Calc. for C
3
8
H
60
BFeN
2
P
3
: C, 64.79; H, 8.58; N, 3.98.
Found: C, 65.06; H, 8.56; N, 3.70.
Synthesis of (P
3
B
)Fe
(OTf)
:
A suspension of (P
3
B
)Fe(Me) (25 mg, 0.038 mmol) in 1
mL of
Et
2
O was chilled to −78 °C in the cold well of an N
2
filled glove box. A similarly
chilled solution of TfOH (3.5 mL, 0.040 mmol) in 1 mL of Et
2
O was added dropwise to
the suspension of (P
3
B
)Fe(Me), and the resultant mixture was removed from the cold well
a
nd allowed to warm to room temperature with stirring over the course of 1 hour. The
solvent was removed
in vacuo
, and the remaining brown
-
green solids were extracted with
pentane (3 × 15 mL) and filtered through a pad of celite. The filtrate was dried
in v
acuo
to yield analytically pure
(P
3
B
)Fe(OTf)
as a yellow
-
green powder (25 mg, 83%). Crystals
suitable for XRD were grown by slow evaporation of a pentane solution of
(P
3
B
)Fe(OTf)
at room temperature.
1
H NMR (400 MHz, C
6
D
6
, 293 K, ppm): δ 57.27, 35.29, 26.5
2, 24.70, 5.32, 4.38,
2.93, 0.86,
-
3.15,
-
26.04.
μ
eff
(
C
6
D
6
, Evans
method
,
293 K
):
4.1
μ
B
.
UV
-
Visible (2
-
MeTHF, 293 K, nm {ε, cm
-
1
M
-
1
}): 562 {126}, 749 {124}. Anal. Calc. for
C
37
H
54
BF
3
FeO
3
P
3
S
: C, 55.87; H, 6.84. Found: C, 55.78; H, 6.76.
Protonation
studies
of
[(P
3
B
)FeN
2
]
2−
Using TfOH:
As described above, a solution of
[(P
3
B
)FeN
2
]
2−
(0.0028 mmol) in
500
L 2
-
MeTHF was prepared
in situ
from
57
Fe
-
labelled
[Na(12
-
crown
-
4)
2
]
[(P
3
B
)Fe(N
2
)]
, and immediately transferred to a Mössbauer or XAS
sample holder and
glassed (91 K) in the cold well of an N
2
filled glove box chilled to 77 K. A solution of
TfOH in 2
-
MeTHF (200
L 80 mM, 0.016 mmol, 6 equiv) was layered on top and
allowed to form a glass (final [Fe] = 4 mM, final [TfOH] = 24 mM). Using p
re
-
chilled
stainless steel forceps, the sample cup was lifted off of the bottom of the cold well, and
the mixture was allowed to de
-
glass. A pre
-
chilled stainless steel stir rod was used to mix
the viscous, supercooled 2
-
MeTHF briefly before replacing the
cup on the bottom of the
well and allowing the solvent to re
-
glass. This procedure was repeated until the desired
mixing time was reached, at which point the sample was allowed to re
-
glass on the
bottom of the cold well before it was transferred quickly ou
t of the glovebox and stored at
77 K prior to analysis.
N.B., at early reaction times (< 15 min. of mixing) it is critical that temperature be
maintained low enough that the 2
-
MeTHF appears as a very viscous gel. However, with
enough reaction time, the mix
ture occasionally flash
-
freezes (typically after about 20
min. of mixing), at which point the frozen mixture must be carefully thawed to 137 K
before re
-
glassing and repeating the above procedure.
Using HBAr
F
4
:
The procedure is identical to that using TfOH
, but HBAr
F
4
was
used as the proton source (58 mg, 0.057 mmol, 20 equiv).
Protonation
of
[Na(12
-
crown
-
4)
2
]
[(P
3
B
)Fe(N
2
)]
The procedure use was identical to that described above for protonation of
[(P
3
B
)FeN
2
]
2−
, with the following changes:
[(P
3
B
)FeN
2
]
2−
wa
s replaced with
57
Fe
-
labelled
S
7
[Na(12
-
crown
-
4)
2
]
[(P
3
B
)Fe(N
2
)]
(2.0 mg, 0.0019 mmol); 15 equiv of TfOH was used (2.5
L, 0.028 mmol); and the total reaction volume was 500
l (final [Fe] = 4 mM). This
mixture was stirred for 15 min. before re
-
glassing and
collecting Mössbauer spectra.
S
tudies
with
NH
3
/N
2
H
4
quantification:
Protonation of
[(P
3
B
)FeN
2
]
2−
or
[Na(12
-
crown
-
4)
2
]
[(P
3
B
)Fe(N
2
)]
with TfOH was
carried out as described above, but in a 20 mL scintillation vial, on larger scale (0.0095
mmol Fe, [Fe] = 4
mM), and with a higher concentration of TfOH (80 mM, 20 equiv).
The reaction was mixed for 30 min. at T ≤ 137 K. (Mössbauer experiments under
identical conditions shows that the yield of [(P
3
B)Fe≡N]
+
is typically ~ 50%; see
Fig.
S24
.) At this point, a stir
bar was added to the mixture, which was allowed to warm to
room temperature with stirring over the course of 15 min. The warmed solution was then
transferred to Schlenk tube and refrozen in the liq. N
2
chilled cold well before a solution
of NaO
t
Bu
(37 mg, 0.38 mmol) in 1 mL THF was added and frozen on top of the reaction
mixture. The Schlenk tube was sealed and thawed to room temperature with stirring over
the course of 15 min. At this point, the Schlenk tube was removed from the N
2
-
filled
glove bo
x, the volatiles vacuum
-
transferred onto an excess of 2.0 M HCl in Et
2
O and
analyzed for [NH
4
][Cl] and [N
2
H
6
][Cl]
2
as described previously.
4
For experiments in which reductant was added after initial protonation of
[(P
3
B
)FeN
2
]
2−
, an identical procedure w
as used, with the following modification. After
mixing
[(P
3
B
)FeN
2
]
2−
and TfOH in supercooled 2
-
MeTHF for 30 min., the solution was
re
-
glassed, and a solution of CoCp*
2
in 2
-
MeTHF (31 mg, 0.095 mmol, 10 equiv) was
layered on top and allowed to glass. The mi
xture was mechanically stirred at T ≤ 137 K
for an additional 30 min. At this point, a stir bar was added to the mixture, which was
allowed to warm to room temperature with stirring over the course of 15 min. The
reaction was subsequently worked up as desc
ribed above.
The following procedure was employed for a catalytic reaction:
In a nitrogen
-
filled glovebox, a stock solution of
[(P
3
B
)Fe][BAr
F
4
]
in
Et
2
O
was
prepared. An aliquot of
this stock solution (
2.3
μmol)
was
added to a Schlenk tube and evaporated to
dryness
under vacuum, depositing a film of
[(P
3
B
)Fe][BAr
F
4
]
. The tube
was
then charged with a
stir bar and cooled to 77 K in a cold well. To the cold tube
was added solid Cp*
2
Co
(0.124 mmol, 54 equiv) and a solution of TfOH in Et
2
O (0.247 mmol, 107 equiv)
. The
final volume of solvent was 1 mL
. This solution
was
allowed to cool and freeze for 5
minutes. 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
passed out of the box into a
liquid N
2
bath and transported to a fume hood. The tube
was
then transferred to a dry
ice/acetone bath where
was thawed and
allowed to stir at
195 K for 3 hrs
. At this point
the tube
was
warm
ed
to room temperature with stirrin
g, and stirred at room temperature
for 5 minutes.
The reaction was subsequently worked up and quantified for the presence
of NH
3
/N
2
H
4
as described above.
The results of these experiments are tabulated in Table
S11
. (N.B., during vacuum
transfer the temper
ature of the system was maintained at 298 K and below to prevent
decomposition of N
2
H
4
.)
Computational
Methods
S
8
All calculations were carried out using version 3.0.3 of the ORCA package
.
11
Given the ex
perimentally measured structure
and ground state
/excite
d state energy
splitting
for
(P
3
B
)Fe(NNMe
2
)
(
vide infra
)
,
this was used
as
a model
for testing
the pure
exchange
-
correlation functionals
BP86,
12
M06
-
L,
13
and TPSS.
14
For the purposes of
testing, gas
-
phase geometry optimizations were carried out using the d
ef2
-
SVP(C,H)/def2
-
TZVP basis set (with atomic coordinates from XRD as inputs)
,
15
followed by a frequency calculation at the same level of theory to ens
ure a true minimum.
Calculations employed a fine integration grid (ORCA Grid5) during geometry
optimization, as well as during the final single
-
point calculation (Grid6). The importance
of relativistic effects were tested by inclusion of the zeroth order regular approximation
(ZORA) with the BP86 functional,
16
using the scalar relativistically
-
recon
tracted
def2
-
ZORA
-
SVP
(C,H)/def2
-
ZORA
-
TZ
VP
basis sets and
def2
-
SVP(C,H)/def2
-
TZVP
auxiliary basis sets.
17
The results of these calculations are collected in Table
S
13
and
S
14
,
from which it can be seen that
the ZORA
-
BP86 method produces the most accurate
ge
ometry, as well as a singlet
-
triplet ΔH that agrees well with the experimental value. All
subsequent geometry optimizations and single
-
point energy calculations employed this
method.
For the calculation of Mössbauer parameters, the hybrid functional TPSSh
1
8
was
used with the def2
-
SVP(C,H)/def2
-
TZVP basis set on all non
-
Fe atoms and the “core
properties” CP(PPP) basis set for Fe.
19
The angular integration grid was set to Grid4
(NoFinalGrid), with increased radial accuracy for the Fe atom (IntAcc 7). To simul
ate
solid state effects, a continuum solvation model was included (COSMO) with a solvent of
intermediate dielectric (methanol). To calibrate the isomer shift scale and estimate the
error in the calculated quadrupole splitting using this method, the Mössbau
er parameters
of 8 (P
3
B
)Fe complexes were computed from crystallographically
-
determined structures;
in addition, the parameters of the previously
-
characterized nitrido complex
(PhBP
3
i
Pr
)Fe≡N were computed using coordinates from the ZORA
-
BP86 method (Fig.
S
34 and Table
S
15).
20
Given the accuracy of the predicted spectroscopic parameters, all
orbital analysis presented in the main text utilized the wavefunctions computed using this
method.
For the calculation of XAS spectra, the TPSSh functional was used in c
onjunction
with the def2
-
TZVP basis set on all non
-
Fe atoms and the CP(PPP) basis set for Fe. The
angular integration grid was set to Grid4 (NoFinalGrid), with increased radial accuracy
for the Fe atom (IntAcc 7). To simulate solid state effects, a continu
um solvation model
was included (COSMO) with an infinite dielectric. TD
-
DFT transitions were calculated
using the Tamm
-
Dancoff approximation with excitations restricted from the Fe 1
s
orbital.
The first 50 lowest
-
energy transitions were calculated, and the
total intensity was
computed including both dipole and quadrupole transition intensities. To calibrate the
energy scale of the computed spectra, the XAS spectrum of (PhBP
3
i
Pr
)Fe≡N was
calculated from BP86
-
ZORA optimized coordinates, and compared with the
experimentally
-
reported spectrum.
21
A constant shift of 154.25 eV was determined to
align the intense pre
-
edge transitions of the experimental and calculated spectra;
subsequently, this same shift was applied to all calculated spectra. A line broadening of
1.5 eV was applied to the calculated spectra to approximate the experimentally
-
observed
linewidth. Spectra were normalized by setting the area of the (PhBP
3
i
Pr
)Fe≡N spectrum to
0.92, which is the estimated area normalized to the edge
-
jump (based on the re
lated
S
9
(PhBP
3
CH
2
Cy
)Fe≡N variant).
21
To compare with the experimental pre
-
edge spectra, the
line
-
broadened TD
-
DFT spectrum was fit to 2 or 3 Gaussian functions, from which
predicted pre
-
edge areas were calculated.
Supplementary Discussion
Excited
state
paramagnetism
of
(P
3
B
)Fe(NNMe
2
)
The chemical shifts of the NMR resonances of complex
(P
3
B
)Fe(NNMe
2
)
were
f
ound to be strongly temperature
dependent, with significant deviations from linearity
when plotted versus T
−1
. This observation is consi
stent with the thermal occupation of a
paramagnetic excited state, as has been observed for the isoelectronic hydrazido complex
[(P
3
Si
)Fe
(
NNMe
2
)
]
+
.
22
This
temperature dependence can be modelled accurately as a low
-
spin/high
-
spin equilibrium by adopting a s
imple model assuming: (i) rapid
inte
rconversion of the spin states
on the NMR timescale; (ii)
temperature independence
of
the diamagnetic shift; (iii)
Curie
-
behavior for the
paramagnetic shift; and (iv) a
Boltzmann distribution of states.
Under these assum
ptions, the observed chemical shift
will be the
Boltzmann
-
weighted
sum of those of the diamagnetic state and the
paramagnetic state,
표푏푠
=
+
=
(
1
)
+
(
+
)
=
+
=
+
+
exp
(
1
(
Δ
Δ
)
)
(
1
)
Where
δ
d
is the diamagnetic shift,
C
is the Curie factor of the paramagnetic shift, and
g
p
is the electronic degeneracy of the excited state. Fitting this equation to both the
temperature dependence of the
N
-
CH
3
resonance from
1
H NMR (which has the larges
t
Curie factor) and the
31
P chemical shift produces ΔH = 3.7(1) kcal mol
−1
and ΔS = 2(3)
cal mol
−1
K
−1
for
g
p
= 3 and ΔH = 3.7(1) and ΔS = 0(2) for
g
p
= 5 (Fig.
S
35 and Table
S
12). The fits are of equivalent quality, therefore, while ΔH is well
-
determined
from the
variable temperature NMR data,
g
p
is not. However, on the basis of computational studies
(
vide
infra
, Table
S
14), we assign
g
p
= 3 (i.e. a triplet excited state), given that a quintet
state is predicted to be > 20 kcal mol
−1
higher in energy than the diamagnetic ground
state.
XRD
refinement
details
for
(P
3
B
)Fe(NNMe
2
)
The crystal structure of
(P
3
B
)Fe(NNMe
2
)
suffered from two
-
part positional disorder
of the two
N
-
methyl carbons (CM1, CM2), coupled with two
-
part positional diso
rder of
the isopropyl substituents on P2. This disorder reflects rotation about the N1
N2 bond
by ca. 30°, which forces the isopropyl substituents on P2 to rotate away from CM2 to
avoid unfavorable steric clashing.
Each position
of the two
-
part disorder
wa
s located in
the difference map and refined anisotropically with hydrogen atoms calculated in the
usual
manner. To test the robustness of this model, the occupancies of both
S
10
conformations of the NNMe
2
ligand and both conformations of the P2 substituents we
re
refined separately. The major conformations refined to 64% and 65% occupancy,
respectively, confirming that the two conformational changes are coupled
.
Mössbauer
simulation
details
All spectra were fit assuming symmetric quadrupole doublets with Loren
tzian line
shapes. This is the correct model for frozen solution spectra in the limit of fast electronic
relaxation, which is typical at 80 K. However, the presence of small amounts of multiple
(possibly paramagnetic and not necessarily in the fast relaxat
ion limit) contaminants
prevents accurate integration of spectra collected from protonation experiments of
[(P
3
B
)FeN
2
]
2−
and
[Na(12
-
crown
-
4)
2
]
[(P
3
B
)Fe(N
2
)]
at long mixing times. However, given
the well
-
separated spectral features of nitrido [(P
3
B
)Fe≡N]
+
,
masking the spectra from δ =
−2.2 to 2.8 mm s
1
allowed for accurate integrations of [(P
3
B
)Fe≡N]
+
, when present. The
results of simulations including the minimal number of quadrupole doublets necessary for
a reasonable simulation (
2
near 1) are given in
Tables S2
S5,
where the integrations of
[(P
3
B
)Fe≡N]
+
from masking the interior region of the spectra are also given. The latter
integrations should be taken as more accurate.
EXAFS
simulation
details
The EXAFS spectrum of
(P
3
B
)Fe(NNMe
2
)
was initially ref
ined using phase and
amplitude functions from the McHale curved wave theory tables included in
EXAFSPAK, finding a prominent peak in the phase
-
uncorrected Fourier transform at
R +
Δ
~1.9 Å due to three P
-
atom scatterers and a smaller peak at
R + Δ
~ 1.5 Å
due to a
single N
-
atom scatterer. Using coordinates from XRD, a model was constructed
including the intact NNMe
2
ligand as well as a single
i
Pr
2
P moiety bonded to the B atom
through a phenylene linker (i.e. all “symmetry
-
inequivalent” atoms of the pseudo
-
C
3
symmetric molecule). This model was used as input for the calculation of
ab initio
phase
and amplitude functions using FEFF. In addition to single
-
scattering paths for the N
α
atom of the NNMe
2
ligand and the P atom, two single
-
scattering paths due to carbon
atoms were found to contribute significantly to the spectrum. One involves the C atom of
the phenylene linker bonded directly to B (Fe
C = 3.38 Å from XRD), while the other
involves a methyn
e C atom on the P
i
Pr substitutent (Fe
C = 3.42 Å from XRD). Finally,
a single multiple
-
scattering path was found involving the nearly linear Fe
N
N vector
(Fe
N
N = 2.97 Å and ˂ FeNN = 176° from XRD); inclusion of this multiple
-
scattering
path was necessa
ry to completely simulate the Fourier
-
transformed EXAFS in the region
from 0
3 Å. Inclusion of a path due to a B atom scatter improves the simulation
marginally; however, with
Z
= 5, the B atom only weakly scatters the Fe photoelectron,
which is reflected
in the relatively high uncertainty in the simulated parameters. If the
multiple
-
scattering path is omitted from the simulation, the data in the region
R + Δ
> 2 Å
is poorly fit (Fig. S36).
For simulation of the EXAFS spectra of
(P
3
B
)Fe(NN
H
2
)
and [(P
3
B
)Fe
≡N]
+
, the
phase and amplitude functions computed from the XRD coordinates of
(P
3
B
)Fe(NNMe
2
)
were employed. As expected, the EXAFS spectrum of
(P
3
B
)Fe(NN
H
2
)
was found be very
similar to that found for
(P
3
B
)Fe(NNMe
2
)
. A single N
-
atom scatterer and 3 P
-
atom
s
catterers account for the majority of the spectrum below
R + Δ
= 2.5 Å. Inclusion of two
C
-
atom scatterers and the Fe
N
N multiple scattering path found above for