Journal of the American Chemical Society
Article
Electronic Paramagnetic Resonance Spectroscopy. X-band
EPR spectra were obtained on a Bruker EMX spectrometer on 5 mM
solutions as frozen glasses in toluene. Samples were collected at
powers ranging from 1 to 10 mW and temperatures ranging from 5 to
CONCLUSIONS
■
In summary, a nonheme Fe−O complex has been prepared
2
via exposure of O to a phosphinimide-iron(II) compound. Its
2
structural and spectroscopic features corroborate an anti-
ferromagnetically coupled, high-spin Fe(III)-superoxide site
analogous to that predicted for many nonheme iron oxygenase
enzymes. This amphoteric oxidant engages in both electro-
philic O-atom transfer reactivity and nucleophilic aldehyde
deformylation. These combined results demonstrate the utility
in using phosphinimide ligands to simultaneously stabilize and
harness reactive inorganic species. Further investigations aimed
at probing the nature of the Fe-containing intermediates of
these transformations are currently underway.
6
5 K with modulation amplitudes of 8 G. Spectra were simulated
59
using the EasySpin suite of programs in Matlab 2021.
Optical Spectroscopy. Measurements were taken on a Hewlett-
Packard 8453 UV−vis Spectrophotometer using a 1 cm quartz cell
sealed with a Teflon stopcock. Variable Temperature measurements
were performed using a UNISOKU Unispec Cryostat.
Electrochemistry. Electrochemical measurements were carried
n
out in 0.2 M THF solutions of electrolyte ([ Bu N][PF ]). Data
4
6
collection were performed on a BioLogic SP-50 Potentiostat using a
freshly polished glassy carbon electron as the working electrode and a
platinum wire as the auxiliary electrode. All reported potentials are
+
referenced to the ferrocene/ferrocenium couple (Fc/Fc ).
EXPERIMENTAL SECTION
General Considerations. Unless otherwise noted, all manipu-
lations were carried out using standard Schlenk or glovebox
57
■
Fe Mossbauer Spectroscopy. 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 from Janis (Wilmington, MA). The quoted
isomer shifts are relative to the centroid of the spectrum of a metallic
foil of a-Fe at room temperature. Samples were prepared by
suspending polycrystalline material in an eicosane matrix and
quadrupole doublets were fit to either Lorentzian or Voigt line shapes.
DFT Calculations. All calculations were carried out using
techniques under an N atmosphere. Acetonitrile (MeCN), benzene,
2
diethyl ether (Et O), pentane, tetrahydrofuran (THF), and toluene
2
were deoxygenated by thorough sparging with N gas; they were then
2
passed through an activated alumina column in a solvent purification
system from Pure Process Technology and further dried over 4 Å
molecular sieves for 48 h prior to use. Solvents were routinely tested
with a THF solution of sodium benzophenone ketyl. Deuterated
solvents were purchased from Cambridge Isotope Laboratories, Inc.,
60
and were distilled under N , degassed via freeze−pump−thaw cycles,
2
Gaussian 09 rev. D.01. Coordinates for all heavy (non-H) atoms
were taken from the structures determined via X-ray crystallography.
To improve the efficiency of the calculations, the adamantyl
substituents were truncated to tert-butyl substituents. Gas-phase
geometry optimizations and single-point and frequency calculations
employed the unrestricted TPSSH functional. The 6-31g(d) basis set
was employed for C and H atoms, and Def2-TZVPP was used for Fe,
N, P, and O atoms. Successful optimization to a minimum was
confirmed by the absence of imaginary frequencies in a subsequent
and stored over 4 Å molecular sieves prior to use. Oxygen was
purchased in Ultra High Purity from Praxair and was further dried by
passing it through two traps immersed in a dry ice/isopropanol bath.
All reagents were purchased from commercial vendors and used
without further purification unless otherwise stated. 1,3,5-tris(2-
49
50
bromophenyl)benzene, di-(1-adamantyl)phosphine, O-(2,4-dini-
5
1
52
trophenyl)-N-hydroxyphthalimide, Fe(HMDS)2, and d -diphenyl-
2
5
3
hydrazine were prepared according to literature procedures.
Elemental analyses were performed by the Microanalytical Laboratory
in the College of Chemistry at the University of California−Berkeley
using a PerkinElmer 2400 Series II combustion analyzer.
frequency calculation. Multireference character is expected for the
t‑Bu
STOT = 2 state of (L H)FeO , and a broken-symmetry solution is
2
found via quadratically convergent SCF procedures. The resultant
wave function was found to be the most stable using the “stable = opt”
command. Natural orbitals were constructed using the pop = no
Nuclear Magnetic Resonance Spectroscopy. Nuclear Mag-
netic Resonance (NMR) spectra were measured at Bruker AV-300,
AVQ-400, NEO-500, or AV-600 spectrometers. H and C chemical
shifts are reported in ppm relative to tetramethylsilane (TMS) at 0.00
ppm using residual solvent residues as internal standards. P chemical
shifts are reported in ppm relative to 85% aqueous H PO at 0 ppm.
1
13
61
command. Orbitals were visualized using VMD version 1.9.4a51.
Ad
3
1
Synthesis of (L H)Fe. In the glovebox, a 250 mL round-
bottomed flask was charged with a magnetic stir bar, L H (1.0 g, 0.8
Ad
3
3
4
mmol, 1 eq), and Et O (60 mL). Fe(HMDS) (296.0 mg, 0.8 mmol, 1
2
2
Solution phase magnetic measurements were performed using the
54
eq) was added dropwise to the flask as a solution in Et O (20 mL).
2
Evans’ method.
The mixture was stirred for 24 h resulting in a beige precipitate in a
brown solution. The beige solid was collected on a medium frit and
Infrared Spectroscopy. Solid IR measurements were obtained
on a Nicolet iS20 Spectrometer as KBr pellets.
X-ray Crystallography. XRD studies were performed at the Small
Molecule X-ray Crystallography Facility (CheXray) or at beamline
2.2.1 at the Advanced Light Source at Lawrence Berkeley National
Laboratory. For studies performed at ChexRay, crystals were mounted
on a Kapton loop under Paratone oil. Data were collected on a Rigaku
XtalLAB P200 (Mo Kα or Cu Kα radiation) equipped with a
MicroMax-007 HF microfocus rotating anode and a Pilatus 200 K
Ad
washed with Et O (3 × 15 mL) to give (L H)Fe as a beige powder
2
(
881.5 mg, 0.675 mmol, 84.5%). The filtrate was concentrated and
stored at −30 °C for 48 h to obtain a second crop of (L H)Fe (42.5
mg, 0.03 mmol, 4.1%) to give a combined yield of 88.6%. Single
crystals of (L H)Fe suitable for X-ray diffraction were grown from
layering pentane onto a concentrated Et O solution of (L H)Fe to
give faint yellow rods. H NMR (400 MHz, C D ) δ 47.76, 41.88,
Ad
1
Ad
Ad
2
1
6
6
39.83, 36.47, 27.96, 22.19, 19.33, 16.27, 15.76, 15.16, 14.33, 13.06,
hybrid pixel array detector at 100 K under a stream of N . Data
2
collection, integration, and scaling were carried out using the
9.52, 8.42, 7.94, 6.03, 5.10, 3.60, 2.99, 2.71, 1.73, 1.33, 0.96, 0.18,
Pro
55
CrysAlis software. For studies performed at the Advanced Light
Source, crystals were mounted on a MiTeGen loop under Paratone
oil. Data were collected on a Bruker D85 three-circle diffractometer
with a PHOTON II CCD area detector using silicon monochromated
synchrotron radiation (λ = 0.7288 Å). Bruker APEX2 software was
used for data collection. Bruker SAINT and SADABS software was
−2.20, −2.71, −3.25, −3.74, −9.40, −11.02, −29.16, −45.23. Anal:
calc. for C H N P Fe: C 77.22, H 8.18, N 3.22; found: C 75.74, H
x y 3 3
8.20, N 3.01. μeff (benzene-d , 298 K, 400 MHz): 4.94 μ . (KBr, 298
6 B
−1
K, cm ): 3344 ν(N−H).
Ad
Synthesis of (L H)FeO
. In the glovebox, a 1-dram vial was charged with a suspension of
2
. Method A: in-crystallo exposure to
2
O
56,57
Ad
utilized for data reduction and absorption correction, respectively.
Structures were solved using SHELXS and refined against F on all
data by full matrix least-squared with SHELXL using OLEX2
crystallographic software. All non-hydrogen atoms were refined
using anisotropic displacement parameters. Hydrogen atoms were
placed in idealized positions and refined using a riding model.
crystalline (L H)Fe in an ether/pentane mother liquor and placed
into a Schlenk tube. The tube was sealed with a glass stopper after
briefly applying a mild vacuum to the headspace. The tube was
subsequently transferred to a gas manifold and exposed to 1 atm of
2
5
8
dry oxygen. Following ∼90 min of O exposure, visibly red crystals
2
were mounted onto a diffractometer.
E
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX