of 32
1
Supporting Information for
:
Characterization of a
Proposed
Terminal Iron(III) Nitride
Intermediate of Nitrogen
Fixation
Stabilized by a Trisphosphine
-
Borane Ligand
Dirk J. Schild, Lucie Nurdin, Marc
-
Etienne Moret, Paul H. Oyala
*
, and Jonas C. Peters*
Division of Chemistry and Chemical
Engineering, California Institute of Technology (Caltech),
Pasadena, California 91125, United States
Table of Contents
1.
Experimental Section
................................
................................
................................
.........................
2
2. NMR Spectra
................................
................................
................................
................................
....
10
3. IR Spectra
................................
................................
................................
................................
.........
12
4. EPR Spectra
................................
................................
................................
................................
.....
13
5. Mӧssbauer Spectra
................................
................................
................................
..........................
24
6. UV
-
Vis Spectra
................................
................................
................................
................................
25
7. Crystallographic Details and Tables
................................
................................
..............................
27
8. DFT Calcula
tions
................................
................................
................................
.............................
30
9. References:
................................
................................
................................
................................
.......
32
2
1.
E
xperimental Section
General considerations.
All manipulations were carried out using standard Schlenk or glovebox
techniques under an N
2
atmosphere
.
Unless otherwise noted, solvents were deoxygenated and
dried by thoroughly sparging with
N
2
gas followed by passage through an activated alumina
column
in the
solvent
purification system by SG Water, USA LLC. 2
-
MeTHF was degassed by
three
freeze
-
pump
-
thaw
cycles, followed by drying over NaK to remove traces of water. All
reagents were purchased from commercial vendors and used without further purification unless
stated
otherwise.
P
3
B
,
1
[(
P
3
B
)Fe(N
2
)],
2
[(
P
3
B
)Fe][
BAr
F
],
3
and were synthesized following literature
procedures. Deuterated solvents were purchased from Cambridge Isotope Laboratories, Inc.,
degassed, and dried over activated 3
-
Å molecul
ar sieves
prior to
use.
1
H and
13
C chemical shifts are reported in ppm
relative to
tetramethylsilane, using residual solvent
proton and
13
C resonances as internal standards.
3
1
P
chemical shifts are reported
relative to
85
%
aqueous H
3
PO
4
.
Solution
phase magnetic measurement
s
were performed by the
Evans
’ method
.
4
IR Spectroscopy
.
IR measurements
were performed
on a Bruker Alpha Platinum ATR
spectrometer.
CW
-
EPR Spectroscopy.
All
X
-
band
CW
-
EPR spectra were obtained on a Bruker
(Billerica, MA,
USA)
EMX spectrometer
. Spectra at 77 K were measured using Bruker SHQE resonator and a
vac
uum insulated quartz finger dewar filled with liquid nitrogen. For all 5 K CW
-
EPR spectra, a
Bruker dual mode resonator
tuned to perpendicular mode
was used
with an Oxford Instruments
ESR900 helium flow cryostat and ITC503 temperature controller
.
Pulse EP
R
S
pectroscopy
.
All pulse Q
-
band (34 GHz) EPR, electron nuclear double resonance
(ENDOR) and hyperfine sublevel correlation spectroscopy (HYSCORE) experiments were
3
acquired using a Bruker ELEXSYS E580 pulse EPR spectrometer equipped
with
a Bruker D
-
2
Q
-
band ENDOR resonator. Temperature control was achieved using an ER 4118HV
-
CF5
-
L Flexline
Cryogen
-
Free VT cryostat manufactured by ColdEdge equipped with an Oxford Instruments
Mercury ITC temperature controller.
All
Q
-
band
electron spin
-
echo detected EPR
(ESE
-
EPR) field
-
swept spectra were acquired using
the 2
-
pulse “Hahn
-
echo” sequence (
!
/
2
$
!
$
echo)
where
$
is a fixed delay
.
Q
-
band Davies ENDOR spectra were acquired using the pulse sequence (
!
&
!"
!
!"
(
!"
!
/
2
$
!
$
echo), where
&
!"
is the delay between mw pulses and RF pulses,
!
!"
is the
length of the RF pulse and the RF frequency is randomly sampled during each pulse sequence. For
all ENDOR scans the same
(
!"
of 1 μs was used, all other acquisition paramete
rs are detailed in
the caption for each ENDOR figure.
For
57
Fe ENDOR spectra, a
n LP
-
2500 low
-
pass
RF
filter
(Vectronics, Starkville, MS) with a cutoff frequency of 35 MHz was attached in
line with the RF
amplifier and ENDOR coils to eliminate contribution
s from
1
H harmonics in the RF region of
interest.
Q
-
band HYSCORE spectra were acquired using the 4
-
pulse sequence (
!
/
2
$
!
/
2
(
#
!
(
$
!
/
2
$
echo), where
$
is a fixed delay, while
(
#
and
(
$
are independently incremented by
Δ
(
#
and Δ
(
$
, respectively. The time domain data was baseline
-
corrected (third
-
order polynomial)
to eliminate the exponential decay in the echo intensity, apodized with a Hamming window
function, zero
-
filled to eight
-
fold
points, and fast Fourier
-
transformed to yield the 2
-
dimensional
frequency domain.
In general, the ENDOR spectrum for a given nucleus with spin
)
= ½ (
1
H
,
57
Fe
) coupled to the S
= ½ electron spin exhibits a doublet at frequencies
4
*
±
=
,
-
2
±
*
&
,
(E1)
Where
*
&
is the nuclear Larmor frequency and
-
is the hyperfine coupling. For nuclei with
)
1
(
14
N,
2
H
,
11
B
), an additional splitting of the
*
±
manifolds is produced by the nuclear quadrupole
interaction (P)
*
±
,
(
!
=
,
*
&
±
3
2
(
2
4
)
1
)
2
,
(E2)
In
HYSCORE spectra, these signals manifest as cross
-
peaks or ridges in the 2
-
D frequency
spectrum which are generally symmetric about the diagonal of a given quadrant. This technique
allows hyperfine levels corresponding to the same electron
-
nuclear submanifo
ld to be
differentiated, as well as separating features from hyperfine couplings in the weak
-
coupling regime
(
|
-
|
<
2
|
*
)
|
) in the (+,
+
) quadrant from those in the strong coupling regime (
|
-
|
>
2
|
*
)
|
) in the
(
-
,
+
) quadrant. The (
-
,
-
) and (+,
-
)
quadrants of these frequency spectra are symmetric to the (+,+)
and (
-
,+) quadrants, thus typically only two of the quadrants are typically displayed in literature.
For systems with appreciable hyperfine anisotropy in frozen solutions or solids, HYSCORE
sp
ectra typically do not exhibit sharp cross peaks, but show ridges that represent the sum of cross
peaks from selected orientations at the magnetic field position at which the spectrum is collected.
The length and curvature of these correlation ridges allow
for the separation and estimation of the
magnitude of the isotropic and dipolar components of the hyperfine tensor, as shown in Figure S1.
5
Figure S
1
.
HYSCORE powder patterns for an
S
= 1/2,
I
= 1/2 spin system with an axial hyperfine
tensor which conta
ins isotropic (
9
*+,
) and dipolar (
&
) contributions. Blue correlation ridges
represent the strong coupling case; red correlation ridges represent the weak coupling case.
For weakly coupled nuclei (
-
<
2
*
)
),
*
-
and
*
.
are both positive, appearing i
n the (+,+)
quadrant, while for strongly coupled nuclei they will show up in the (−,+) quadrant. In the
intermediate coupling regime where
-
2
*
)
, peaks will often appear in both the (+,+) and (−,+)
quadrants of the HYSCORE spectrum.
All EPR spectra (CW,
ENDOR, HYSCORE) were simulated using the
EasySpin
5
simulation
toolbox (version 5.2.
33
) with Matlab
20
2
0
b
using the following Hamiltonian:
;
<
=
=
/
>
?
0
A
B
C
+
=
&
A
&
>
?
0
)
C
+
B
C
G
)
C
+
)
C
H
)
C
(E3)
In this expression, the first term corresponds to the electron Zeeman interaction term where
=
/
is the Bohr magneton,
g
is the electron spin
g
-
value matrix with principal components g = [g
xx
,
g
yy
, g
zz
], and
B
C
is the electron spin operator; the second term corresponds to the nuclear Zeeman
interaction term where
=
&
is the nuclear magneton,
A
&
is the characteristic nuclear g
-
value for
each nucleus (e.g.
1
H,
2
H,
31
P) and
)
C
is the nuclear spin operator; the t
hird term corresponds to the
electron
-
nuclear hyperfine term, where
G
is the hyperfine coupling tensor with principal
6
components
G
= [A
xx
A
yy
A
zz
]; and for nuclei with
)
1
, the final term corresponds to the nuclear
quadrupole (NQI) term which arises fr
om the interaction of the nuclear quadrupole
mo
ment with
the local electric field gradient (efg) at the nucleus, where
H
is
the
quadrupole coupling tensor. In
the principal axis system (PAS),
H
is traceless and parametrized by the quadrupole coupling
constant
I
$
JK
/
and the
electric field gradient
asymmetry parameter
L
such that
:
H
=
M
2
11
0
0
0
2
22
0
0
0
2
33
O
=
I
$
JK
/
4
)
(
2
)
1
)
Q
(
1
L
)
0
0
0
(
1
+
L
)
0
0
0
2
R
(E4)
Where
4
"
56
7
=
2
)
(
2
)
1
)
2
33
and
L
=
8
##
9
8
$$
8
%%
. The asymmetry parameter may have values
between 0 and 1, with 0 corresponding to an electric field gradient with axial symmetry and 1
corresponding to a fully
rhombic efg.
The orientations between the hyperfine and NQI tensor principal axis systems and the g
-
matrix reference frame are defined by the Euler angles (α, β, γ), with rotations performed within
the zyz convention where α rotates xyz counterclockwise ab
out z
-
axis to give x'y'z', β rotates x'y'z
counterclockwise about y'
-
axis to give x",y",z", γ rotates xyz counterclockwise about z"
-
axis to
give final frame orientation.
Mössbauer
Spectroscopy.
S
pectra
were recorded
on a spectrometer from SEE Co. operating in
the constant acceleration mode in a transmission geometry. Spectra were recorded with the
temperature of the sample maintained at 80 K. The sample was kept in an SVT
-
400 Dewar from
Janis. The quoted isomer shift
s are
relative to
the centroid of the spectrum of a metallic foil of α
-
Fe at room temperature. Data analysis was performed using the program
WMOSS
(www.wmoss.org) and quadrupole doublets were fit to Lorentzian lineshapes.
7
X
-
Ray Crystallography.
X
-
ray diffr
action studies were carried out at the Caltech Division of
Chemistry and Chemical Engineering X
-
ray Crystallography Facility on a Bruker three
-
circle
SMART diffractometer with a SMART 1K CCD detector, APEX CCD detector, or Bruker D8
VENTURE Kappa Duo PHOTON
100 CMOS detector. Data were collected at 100 K using Mo
Kα radiation (λ = 0.71073 Å) or Cu Kα radiation (λ = 1.54178 Å). Structures
were solved
by
direct
or
Patterson methods using SHELXS and refined against F2 on all data by
full
-
matrix
least squares
wit
h SHELXL
-
97.68 All non
-
hydrogen atoms were refined anisotropically. All hydrogen atoms
were 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 m
ethyl groups) times the
Ueq of the atoms to which they
are bonded
.
[(
P
3
B
)
)Fe(Cl)] (
1
).
A mixture of FeCl
2
(87 mg, 0.69 mmol),
P
3
B
(400 mg, 0.69
m
mol), iron powder
(415 mg, 7.4 mmol), and THF (20mL) was heated to 90 °C in a sealed bomb under
vigorous
stirring for 62 h, during which time the color of the liquid phase turned from pale yellow to brown.
The remaining iron powder was removed by filtration
, and the solvent
was removed
in vacuo
. The
brown residue was taken in toluene (10 mL) and drie
d
in vacuo
. The brown residue was extracted
with pentane (200 mL) to give a brown solution. Solvent evaporation
in vacuo
afforded the product
as a
greenish
-
brown
powder (422 mg, 90%). An analytically pure sample was obtained by slow
concentration of a saturated pentane solution. Crystals suitable for XRD
were obtained
upon
cooling a saturated solution of
(
P
3
B
)Fe
(
Cl
)
in pentane to −35 °C.
1
H NMR (C
6
D
6
, 300 MHz): δ
96.9 (1H), 35.0 (1H), 23.6 (1H), 9.8(1H), 5.8 (1H), 1.9 (3H),
-
0.3 (3H),
-
2.3 (3H), −22.4 (1H).
UV
-
vis (THF, nm {cm
1
M
1
}): 280 {2.0·10
4
}, 320 {sh}, 560 {sh}, 790 {150}, 960 {190}. μ
eff
(C
6
D
6
, Evans method, 20 °C): 4.0 μ
B
. Anal:
calcd
for C
36
H
54
BClFeP
3
: C 63.41, H 7.98; found: C
63.16, H 7.72.