S
1
Supporting information for:
Generating potent C-H PCET donors: Ligand-induced Fe-to-ring proton
migration from a Cp*Fe
III
-H complex demonstrates a promising strategy
Dirk J. Schild
†
, Marcus W. Drover
†
, Paul H. Oyala, and Jonas C. Peters*
Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena,
California 91125, United States
†
These authors contributed equally
*E-mail:
jpeters@caltech.edu
1. General Considerations
S2
2. Synthethic Procedures
S6
3. Spectroscopic Data
S8
4. Crystallographic details
S51
5. DFT Calculations
S59
6. Square Scheme
S62
7. References
S63
S
2
1. General Considerations
All experiments were carried out employing standard Schlenk techniques under an atmosphere of dry
nitrogen or argon employing degassed, dried solvents in a solvent purification system supplied by SG
Water, LLC. Non-halogenated solvents were tested with a standard purple solution of sodium
benzophenone ketyl in tetrahydrofuran to confirm effective moisture removal. Fe
II
(
5
-Cp*)(dppe)X (X = Cl,
H, CH
3,
OTf)
1
were prepared according to a literature procedure. All other reagents were purchased from
commercial vendors and used without further purification unless otherwise stated.
1.1. Nuclear Magnetic Resonance Spectroscopy
1
H and
13
C chemical shifts are reported in ppm relative to tetramethylsilane, using residual solvent
resonances as internal standards.
31
P chemical shifts are reported in ppm and referenced externally to 85%
aqueous H
3
PO
4
at 0 ppm.
1.2.
57
Fe Mössbauer Spectroscopy
Mössbauer spectra were recorded on a spectrometer from SEE Co. (Edina, MN) operating in the constant
acceleration mode in transmission geometry. The sample was kept in an SVT-400 cryostat form Janis
(Wilmington, MA), using 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 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 quickly removed from the glovebox and immersed in liquid N
2
until mounted
in the cryostat. Data analysis was performed using WMOSS version 4 (www.wmoss.org) and quadrupole
doublets were fit to Lorentzian lineshapes.
2
1.3. Infrared Spectroscopy
Solid and thin film IR measurements were obtained on a Bruker Alpha spectrometer equipped with a
diamond ATR probe.
1.4. UV-VIS Spectroscopy
UV-Visible spectroscopy measurements were collected with a Cary 50 UV-Vis spectrophotometer using a
1 cm two-window quartz cell.
S
3
1.5. EPR Spectroscopy
Continuous wave X-band EPR spectra were obtained on a Bruker EMX spectrometer on 2-9 mM solutions
prepared as frozen glasses in 2-MeTHF. Pulse EPR spectroscopy: All pulse X-band (9.4-9.7 GHz) EPR,
electron nuclear double resonance (ENDOR), and hyperfine sublevel correlation spectroscopy (HYSCORE)
experiments were acquired using a Bruker ELEXSYS E580 pulse EPR spectrometer. X-band ENDOR
experiments were performed using a Bruker MD-4 X-band ENDOR resonator, and X-band HYSCORE
experiments were performed using a Bruker MS-5 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 pulse X-band (ν ≈ 9.4-9.7 GHz) EPR and electron nuclear double resonance (ENDOR) experiments were
aquired using a Bruker (Billerica, MA) ELEXSYS E580 pulse EPR spectrometer equipped with a Bruker
MD-4 resonator. Temperature control was achieved using an ER 4118HV-CF5-L Flexline Cryogen-Free VT
cryostat manufactured by ColdEdge (Allentown, PA) equipped with an Oxford Instruments Mercury ITC.
Pulse X-band ENDOR was acquired using the Davies pulse sequence (
π−T
ୖ
− π
ୖ
−T
ୖ
− π/2
–
τ
–
π
–
echo), where
T
ୖ
is the delay between mw pulses and RF pulses,
π
ୖ
is the length of the RF pulse and the
RF frequency is randomly sampled during each pulse sequence.
X-band HYSCORE spectra were acquired using the 4-pulse sequence (
π/2−τ− π/2−t
ଵ
− π
–
t
ଶ
–
π/2
–
echo), where
τ
is a fixed delay, while
t
ଵ
and
t
ଶ
are independently incremented by Δ
t
ଵ
and Δ
t
ଶ
, 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. For
2
H-
1
H difference spectra, the time
domain of the HYSCORE spectrum of the
1
H sample was subtracted from that of the
2
H sample, and the
same data processing procedure detailed above was used to generate the frequency spectrum.
In general, the ENDOR spectrum for a given nucleus with spin
I
= ½ (
1
H,
31
P) coupled to the S = ½ electron
spin exhibits a doublet at frequencies
ν
±
=
ฬ
A
2
±
ν
ฬ
(1)
Where
ν
is the nuclear Larmor frequency and
A
is the hyperfine coupling. For nuclei with
I ≥1
(
2
H), an
additonal splitting of the
ν
±
manifolds is produced by the nuclear quadrupole interaction (P)
ν
±
,
୫
=
ฬ
ν
±
3P
(
2
m
୍
−
1
)
2
ฬ
(2)
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 submanifold to be differentiated, as well as separating features
from hyperfine couplings in the weak-coupling regime (
|
A
|
<2
|
ν
୍
|
) in the (+,+) quadrant from those in the
strong coupling regime (
|
A
|
>2
|
ν
୍
|
) in the (−,+) quadrant. The (−,−) and (+,−) quadrants of these frequency
spectra are symmetric to the (+,+) and (−,+) quadrants, thus typically only two of the quadrants are
displayed in literature.
S
4
For systems with appreciable hyperfine anisotropy in frozen solutions or solids, HYSCORE spectra
typically do not exhibit sharp cross peaks, but show ridges that represent the sum of cross peaks from
selected orientations within the excitation bandwidth of the MW pulses at the magnetic field position at
which the spectrum is collected. The length and curvature of these correlation ridges can allow for the
separation and estimation of the magnitude of the isotropic and dipolar components of the hyperfine
tensor, as shown in Fig. S1.
Figure S1.
a) HYSCORE powder patterns for an
S
= 1/2,
I
= 1/2 spin system with an isotropic hyperfine
tensor A. b) HYSCORE powder patterns for an
S
= 1/2,
I
= 1/2 spin system with an axial hyperfine tensor
which contains isotropic (
a
୧ୱ୭
) and dipolar (
T
) contributions. Blue correlation ridges represent the strong
coupling case; red correlation ridges represent the weak coupling case.
EPR Simulations. Simulations of all CW and pulse EPR data were achieved using the EasySpin
3
simulation
toolbox (release 5.2.21) with Matlab 2018b using the following Hamiltonian:
H
=
μ
B
ሬ
ሬ
⃑
g
S
+
μ
g
B
ሬ
ሬ
⃑
I
መ
+
h
S
∙
퐀
∙
I
መ
+
h
I
መ
∙
퐏
∙
I
መ
(3)
In this expression, the first term corresponds to the electron Zeeman interaction term where
μ
is the Bohr
magneton, g is the electron spin g-value matrix with principle components g = [g
xx
g
yy
g
zz
], and
S
is the
electron spin operator; the second term corresponds to the nuclear Zeeman interaction term where
μ
is
the nuclear magneton,
g
is the characteristic nuclear g-value for each nucleus (e.g.
1
H,
2
H
,
31
P) and
I
መ
is the
nuclear spin operator; the third term corresponds to the electron-nuclear hyperfine term, where
퐀
is the
S
5
hyperfine coupling tensor with principle components
퐀
= [A
xx
, A
yy
, A
zz
]; and for nuclei with
I ≥1
, the final
term corresponds to the nuclear quadrupole (NQI) term which arises from the interaction of the nuclear
quadrupole moment with the local electric field gradient (efg) at the nucleus, where
퐏
is
the
quadrupole
coupling tensor. In the principle axis system (PAS),
퐏
is traceless and parameterized by the quadrupole
coupling constant
e
ଶ
Qq/h
and the asymmetry parameter
η
such that:
퐏
=
ቌ
P
୶୶
0
0
0
P
୷୷
0
0
0
P
ቍ
=
e
ଶ
Qq
/
h
4I
(
2I
−
1
)
൭
−
(
1
−
η
)
0
0
0
−
(
1
+
η
)
0
0
0
2
൱
(4)
where
ୣ
మ
୕୯
୦
=2I
(
2I−1
)
P
and
η=
౮౮
ି
౯౯
. 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 principle axis systems and the g-matrix reference
frame are defined by the Euler angles (α, β, γ).
1.6. Electrochemistry
Electrochemical measurements were carried out using a CD instruments 600B electrochemical analyzer. A
freshly polished glassy carbon electrode was used as the working electrode and a graphite rod was used
as the auxiliary electrode. Solutions (THF) of electrolyte (0.4 M tetra-n-butylammonium
hexafluorophosphate) contained ferrocene (0.1 mM), to serve as an internal reference, and analyte (0.2 mM).
All reported potentials are referenced to the ferrocene/ferrocenium couple, [Cp
2
Fe]
+
/Cp
2
Fe.
S
6
2. Synthetic Procedures
[Fe
III
(
5
-Cp*)(dppe)H]BAr
F
4
([
1
]BAr
F
4
: C
68
H
52
BF
24
FeP
2
; 1453.3 gmol
-1
): To a solution of
Fe
II
(
5
-Cp*)(dppe)H
(40 mg, 0.07 mmol, 1 equiv.) in Et
2
O (2 mL) at -78
°
C was added an Et
2
O (2 mL) solution of [Fc]BAr
F
4
(60
mg, 0.06 mmol, 0.85 equiv.). Following addition, the resulting dark orange mixture was stirred at 25
°
C over
10 min giving a dark red-orange solution. Removal of volatiles
in-vacuo
and washing with pentane gave
[
1
]BAr
F
4
as a red solid (72 mg, 74%).
N.B.
The PF
6
-
salt has been prepared previously, though
1
H NMR data
was not provided.
4
1
H NMR (THF-d
8
, 400 MHz, 298 K)
:
= 54.12 (br), 26.46 (br), 9.88 (br), 8.71 (br), 7.80
(BAr
F
4
), 7.58 (BAr
F
4
), 7.40 (br), 5.61 (br), -0.15 (br), -2.51 (br), -9.00 (br).
Fe(
endo
4
-Cp*H)(dppe)(CO)
(
endo
-
3
): Prepared as previously reported.
5
The solid-state molecular
structure was not reported and is shown in the crystallography section.
UV-VIS (THF, nm {cm
-1
M
-1
)
: 450
{1130}.
IR (ATR, C
6
D
6
film)
: 1864 cm
-1
(
ν
CO).
[Fe(
endo
4
-Cp*H)(dppe)(CO)]BAr
F
4
(
endo
-[
3
]BAr
F
4
): To a solution of [
1
]BAr
F
4
(10 mg, 0.007 mmol) in 2-
MeTHF (2 mL) at -78
°
C was added CO (~1 atm) in a J. Young EPR tube, giving a green solution. CW X-
band EPR spectroscopy evidenced complete consumption of the Fe
III
-H starting material.
UV-VIS (THF,
nm {cm
-1
M
-1
)
: 891 {252}, 712 {425}, 459 {870}, 383 {1530}.
EPR Parameters (30 K, 2-MeTHF, 9.717 GHz):
g =
[2.085, 2.039, 2.004]; A(
31
P
1
) = [72, 59, 58] MHz; A(
31
P
2
) = [49, 42, 51] MHz;
A
(
1
H) = ± [24, 20, 34.5] MHz.
Fe(
exo
4
-Cp*H)(dppe)(CO)
(
exo
-
3
): To a solution of [Fe(
5
-Cp*)(dppe)CO]OTf (229.9 mg, 0.3 mmol, 1
equiv.) in Et
2
O (20 mL) at -78
°
C was added drop-wise a solution of 1 M LiBEt
3
H in Et
2
O (0.3 mL, 0.3 mmol,
1 equiv. Following addition, the resulting mixture was stirred at 25
°
C over 10 min giving a clear yellow
solution. Volatiles were removed
in-vacuo
and the sample was extracted with 200 mL pentane, and filtered
over celite. Removal of pentane
in-vacuo
yields
exo
-
3
as a yellow solid (158 mg, 85%). X-ray quality crystals
are formed by cooling down a concentrated pentane solution of
exo
-
3
to -35 °C.
1
H NMR (400 MHz, C
6
D
6
)
:
= 7.91 (t,
3
J
H,H
= 8.2 Hz, 4H), 7.33 (h,
3
J
H,H
= 3.8, 2.9 Hz, 4H), 7.27 – 7.18 (m, 4H), 7.14 – 7.03 (m, 8H), 3.04 (q,
3
J
H,H
= 6.8 Hz, 1H), 2.23 – 2.01 (m, 2H), 1.83 (q,
3
J
H,H
= 2.2 Hz, 9H), 1.65 (td,
3
J
H,H
= 15.7, 12.7, 6.8 Hz, 2H), 0.97
(s, 6H),
13
C NMR (101 MHz, THF-d
8
)
:
= 139.7 – 138.8 (m), 138.3 (d,
J
= 16.8 Hz), 133.8 (t,
J
= 6.0 Hz), 133.1
(d,
J
= 5.3 Hz), 128.6 (d,
J
= 27.6 Hz), 127.3 (dt,
J
= 11.6, 4.1 Hz), 92.3, 60.8, 58.1, 31.0 – 29.9 (m), 13.1, 11.3.
31
P
NMR (162 MHz, C
6
D
6
)
:
= 85.3.
UV-VIS (THF, nm {cm
-1
M
-1
)
: 441 {1850}.
IR (ATR, C
6
D
6
film)
: 2711 cm
-1
(
ν
C–H), 2612 cm
-1
(
ν
C–H), 1854 cm
-1
(
ν
CO).
[Fe(
exo
-
4
-Cp*H)(dppe)(CO)]BAr
F
4
(
exo
-[
3
]BAr
F
4
): In an 4 mm EPR tube, a frozen solution of
exo
-
3
(1.3 mg,
0.002 mmol) in 2-MeTHF (0.25 mL) was layered with a frozen solution of [Fc]BAr
F
4
(2.2 mg, 0.002 mmol, 1
equiv) in 0.25 mL 2-MeTHF. The two frozen solutions where slow thawed and stirred with a needle by
taking the tube out of a cold well cooled with liquid nitrogen. Upon mixing, a color change to green can be
observed. CW X-band EPR spectroscopy evidenced complete consumption of the Fe
III
-H starting material.
UV-VIS (THF, nm {cm
-1
M
-1
)
: 923 {130}, 767 {180}, 441 {1645}.
EPR Parameters (30 K, 2-MeTHF, 9.717 GHz):
g = [2.116, 2.073, 1.997]; A(
31
P
1
) = [96, 88, 47] MHz; A(
31
P
2
) = [78, 75, 63] MHz;
A
(
1
H) = ± [85, 84, 83] MHz,
HStrain = [70, 22, 22] MHz for conformer
A
(0.6 weight) and g = [2.093, 2.045, 2.013]; A(
31
P
1
) = [46, 44, 15]
MHz; A(
31
P
2
) = [70, 64, 64] MHz;
A
(
1
H) = ± [76, 74, 70] MHz, HStrain = [70, 22, 22] MHz for conformer
B
(0.4
weight).
S
7
[Fe(
endo
-
4
-Cp*H)(dppe)(CNXyl)]BAr
F
4
(
endo
-[
4
]BAr
F
4
): To a solution of [
1
]BAr
F
4
(10 mg, 0.007 mmol) in
2-MeTHF (2 mL) at -78
°
C was added CNXyl (~1 mg, 0.008 mmol, ~1.1 equiv.), giving a green solution. CW
X-band EPR spectroscopy evidenced complete consumption of the Fe
III
-H starting material.
UV-VIS (THF,
nm):
828.
EPR Parameters (20 K, 2-MeTHF, 9.716 GHz):
g = [2.132, 2.042, 2.004]; A(
31
P
1
) = [75, 35, 54] MHz;
A(
31
P
2
) = [76, 64, 64] MHz;
A
(
1
H) = ± [17.0, 22.0, 32.5] MHz;
A
(
14
N) = [7.4, 7.4, 9] MHz.
[Fe(
5
-Cp*)(dppe)CO]BAr
F
4
([
5
]BAr
F
4
): This molecule and H
2
cleanly result (>99%) by annealing of
solutions of
exo
- or
endo
-[
3
]
+
to room temperature. Characterization data is consistent with that reported in
ref. 6.
[Fe(
5
-Cp*)(dppe)CNXyl]BAr
F
4
([
6
]BAr
F
4
): To a solution of
Fe
II
(
5
-Cp*)(dppe)H
(9.5 mg, 0.016 mmol, 1
equiv.) and CNXyl (2.1 mg, 0.016 mmol, 1 equiv.) in Et
2
O (2 mL) at -78
°
C was added drop-wise a chilled (-
78
°
C) solution of [H(OEt)
2
]BAr
F
4
(16.3 mg, 0.016 mmol, 1 equiv.) in Et
2
O (1 mL). Following addition, the
resulting mixture was stirred at 25
°
C over 10 min giving a clear yellow solution. Removal of volatiles
in-
vacuo
and washing with pentane gave [
6
]BAr
F
4
as a yellow solid (20 mg, 80%).
1
H NMR (THF-d
8
, 400 MHz,
298 K)
:
= 7.79 (s, 8H; BAr
F
4
), 7.57 (s, 4H; BAr
F
4
), 7.55 (m, 10H; Ph), 7.39 (m, 10H; Ph), 7.08 (t,
3
J
H,H
= 7.2 Hz,
1H; CNXyl), 7.00 (d,
3
J
H,H
= 7.2 Hz, 2H; CNXyl), 2.67 (m, 2H; CH
2
), 2.45 (m, 2H; CH
2
), 1.63 (s, 6H; CNXyl),
1.55 (s, 15H; Cp*).
31
P{
1
H} NMR (THF-d
8
, 162 MHz, 298 K)
:
= 94.0.
13
C NMR (THF-d
8
, 100 MHz, 298 K)
:
= 162.5 (q,
1
J
C,B
= 37 Hz, BAr
F
4
, ipso quaternary C), 135.6 (BAr
F
4
, ortho C), 135.0, 134.6, 133.7, 131.9, 129.9,
129.9 (q,
2
J
C,F
= 31 Hz, BAr
F
4
, meta quaternary C), 129.6, 129.4, 129.2, 129.1, 125.4 (q,
1
J
C,F
= 273 Hz, BAr
F
4
, CF
3
),
117.9 (m, BAr
F
4
, para C), 93.6 (Cp*), 30.5 (CH
2
; dppe), 18.8, 10.1 (Cp*).
IR (THF film)
: 2050 cm
-1
(
C≡N
).
57
Fe
Mössbauer (80 K, Et
2
O solution, mm/s)
:
= 0.16,
E
Q
= 1.75.
[Fe(
5
-Cp*)(dppe)N
2
]BAr
F
4
([
7
]BAr
F
4
): To a solution of Fe(
5
-Cp*)(dppe)CH
3
1
(21.8 mg, 0.036 mmol, 1
equiv.) in Et
2
O (2 mL) at -78
°
C was added drop-wise a chilled (-78
°
C) solution of [H(OEt)
2
]BAr
F
4
(36.5 mg,
0.036 mmol, 1 equiv.) in Et
2
O (1 mL). Following addition, the resulting mixture was stirred at 25
°
C over 10
min giving a clear yellow solution. Removal of volatiles
in-vacuo
and washing with pentane gave [
7
]BAr
F
4
as a yellow solid (53 mg, 92%). [
7
]BAr
F
4
is also the product of H
2
release by [
1
]BAr
F
4
in THF (< 5% yield after
1 week, + 80
o
C, THF-d
8
).
1
H NMR (THF-d
8
, 400 MHz, 298 K)
:
= 7.79 (s, 8H; BAr
F
4
), 7.57 (s, 4H; BAr
F
4
),
7.75-7.54 (m, 16H; Ph), 7.44 (m, 4H; Ph), 2.54 (m, 2H; CH
2
), 2.38 (m, 2H; CH
2
), 1.43 (s, 15H; Cp*).
31
P{
1
H}
NMR (THF-d
8
, 162 MHz, 298 K)
:
= 86.6.
13
C NMR (THF-d
8
, 100 MHz, 298 K)
:
= 162.5 (q,
1
J
C,B
= 37 Hz,
BAr
F
4
, ipso quaternary C), 135.6 (BAr
F
4
, ortho C), 135.4, 134.3, 133.2, 132.4, 132.2, 129.9 (q,
2
J
C,F
= 31 Hz, BAr
F
4
,
meta quaternary C), 125.4 (q,
1
J
C,F
= 273 Hz, BAr
F
4
, CF
3
), 118.0 (m, BAr
F
4
, para C), 117.0, 92.7 (Cp*), 28.8 (CH
2
;
dppe), 9.2 (Cp*).
IR (THF film)
: 2119 cm
-1
(
NN
).
S
8
3. Spectroscopic Data
Figure S2.
[
1
]BAr
F
4
,
1
H NMR,
THF-d
8
, 400 MHz, 298 K
Note:
The
31
P{
1
H} NMR spectrum (THF-d
8
, 162 MHz, 298 K) is featureless.
-15
-10
-5
0
5
10
15
20
25
30
35
40
45
50
55
60
f1
(ppm)
-8.90
-8.07
-1.91
0.87
0.89
0.90
1.10
1.11
1.13
1.25
1.28
1.29
1.31
1.32
1.34
1.73
1.77
1.79
2.31
2.51
3.36
3.38
3.39
3.41
3.58
3.61
3.63
4.11
5.64
5.69
7.13
7.19
7.20
7.27
7.30
7.39
7.57
7.79
8.58
9.68
24.87
27.44
-2
-1
0
1
2
3
4
5
6
7
8
9
10
11
12
13
f1
(ppm)
0.87
0.89
0.90
1.10
1.11
1.13
1.28
1.29
1.31
1.32
1.34
1.73
1.77
1.79
3.36
3.38
3.39
3.41
3.58
3.61
3.63
7.27
7.30
7.39
7.57
7.79
S
9
Figure S3.
[
1
]BAr
F
4
, FT-IR ATR, thin film, 298 K (
FeH
= 1874 cm
-1
)
S
10
Figure S4.
endo
-
3
,
1
H NMR,
C
6
D
6
, 400 MHz, 298 K. The data match that previously
reported.
5
(* = C
6
D
6
)
Figure S5.
endo
-
3-H
/
D
stacked plot,
1
H NMR,
C
6
D
6
, 400 MHz, 298 K showing
disappearance of a signal at
H
= 2.65.
*
S
11
Figure S6
endo
-
3
,
31
P{
1
H} NMR,
C
6
D
6
, 162 MHz, 298 K. The data match that previously
reported.
5
Figure S7.
endo
-
3-H/D
, FT-IR ATR, thin film, 298 K
S
12
Figure S8.
exo
-
3
,
1
H NMR,
C
6
D
6
, 400 MHz, 298 K (* = C
6
D
6
)
Figure
S9.
exo
-
3-H
/
D
stacked plot,
1
H NMR,
C
6
D
6
, 400
MHz,
298 K showing disappearance of a signal at
H
=
3.04.
Figure S9.
exo
-
3-H
/
D
stacked plot,
1
H NMR,
C
6
D
6
, 400 MHz, 298 K showing
disappearance of a signal at
H
= 2.65.
*
S
13
Figure S10.
exo
-
3
,
31
P{
1
H} NMR,
C
6
D
6
, 162 MHz, 298 K
Figure S11.
exo
-
3
,
13
C{
1
H} NMR,
THF-d
8
, 100 MHz, 298 K (* = THF-d
8
)
*
*