A R T I C L E S
Tsai et al.
Celite to remove the [PPN][NO2]. Further addition of hexane (15
mL) to the filtrate afforded the insoluble dark-red solid
[PPN][(ONO)2Fe(NO)2] (3) [IR νNO (cm-1): 1704(vs), 1774(s)
(THF)] (0.051 g, 68% yield)11a and the upper mixture solution.
The upper mixture solution was dried under vacuum to obtain light-
yellow solid OPPh3, which was characterized by 31P NMR
spectroscopy [δ ) 30.4 ppm (CDCl3)].
Reaction of Complex 3 and [C4H7N2][BF4]. To a THF solution
(2 mL) of complex 3 (0.15 g, 0.2 mmol) was added a THF/CH3CN
solution of [C4H7N2][BF4] prepared from the reaction of 1-MeIm
(16 µL, 0.2 mmol) and HBF4 (29 µL, 0.2 mmol) at 0 °C. The
reaction mixture was stirred for 1 h at 0 °C, and diethyl ether (3
mL) was then added to precipitate a white solid (presumably
[PPN][BF4]). The resulting mixture was filtered through Celite to
remove the insoluble solid. Addition of a large amount of hexane
(40 mL) to the filtrate led to the precipitation of the known
compound [(1-MeIm)(ONO)Fe(NO)2] (4) (0.044 g, 95% yield). IR
νNO (cm-1): 1729(vs), 1800(s) (MeOH).9
Temperature-Dependent Dynamic Equilibrium between Com-
plexes 4 and 5 in CH2Cl2 Monitored by UV-Vis Spectroscopy.
Complex [(1-MeIm)2(η2-ONO)Fe(NO)2] (5) (0.033 g, 0.1 mmol)
and 1-MeIm (0.8 mL, 10 mmol) were dissolved in CH2Cl2 (20 mL)
at 0 °C. The diluted solution (6.25 × 10-4 M) was cooled to 183
K. The absorption at 417 nm was monitored by UV-vis spectros-
copy from 193 to 283 K. The enthalpy and entropy were calculated
as -11.0 kJ mol-1 and -14.2 J K-1 mol-1, respectively. Complex
5: Absorption spectrum (CH2Cl2) λmax/nm (ε/M-1 cm-1) (183 K):
417 (1467), 568 (719), 910 (402). Complex 4: Absorption spectrum
(CH2Cl2) λmax/nm (ε/M-1 cm-1) (303 K): 520 (263), 618 (140),
750 (72).
(νCO) (THF). Absorption spectrum (THF) λmax/nm (ε/M-1 cm-1):
507 (528), 668 (191). Anal. Calcd for C40H36N3O6P2Fe: C, 62.13;
H, 4.66; N, 5.44. Found: C, 61.85; H, 4.62; N, 5.29. In the isotopic
experiment, to a stirred THF suspension of complex 7 (0.193 g,
0.2 mmol) in an ice bath was added a THF solution of CH3COOD
(24 µL, 0.4 mmol). The THF solvent containing DHO (produced
by D/H exchange between D2O and THF) was trapped by liquid
nitrogen under vacuum. The trapped DHO in THF was identified
2
by H NMR spectroscopy [2H NMR (THF): δ 2.84 (DHO) vs δ
5.32 (CD2Cl2)]. In order to further corroborate the generation of
H2O from the reaction of complex 7 and glacial acetic acid
described above, the reaction of complex 7 (0.193 g, 0.2 mmol)
and 2 equiv of benzoic acid (0.05 g, 0.4 mmol) in THF-d8 (3 mL)
1
was conducted. The formation of H2O was identified by H NMR
spectroscopy. The 1H NMR chemical shift [δ ) 2.54 ppm (DHO)]
was detected as a result of H/D exchange between the produced
H2O and C4D8O.
Reaction of Complex 8, [PPN][NO2], and [C4H7N2][BF4]. To
a THF suspension (2 mL) of complex 8 (0.15 g, 0.2 mmol) and
[PPN][NO2] (0.25 g, 0.4 mmol) was added a THF/CH3CN solution
of [C4H7N2][BF4] prepared from the reaction of 1-MeIm (33 µL,
0.4 mmol) and HBF4 (58 µL, 0.4 mmol) at 0 °C. The reaction
mixture was stirred for 1 h at 0 °C, and the resulting mixture was
filtered through Celite to remove the insoluble solid (presumably
[PPN][BF4]). Addition of a large amount of hexane (40 mL) to the
filtrate led to the precipitation of [PPN][(ONO)2Fe(NO)2] (3) (0.12
g, 85% yield). IR νNO (cm-1): 1704(vs), 1774(s) (THF).
EPR Spectroscopy. X-band EPR measurements were performed
using a Bruker EMX spectrometer equipped with a Bruker TE102
cavity. The microwave frequency was measured with a Hewlett-
Packard 5246 L electronic counter. X-band EPR spectra of
complexes 2 and [PPN][Cl3Fe(NO)] in CH2Cl2 were recorded under
microwave powers of 19.971 and 17.773 mW at frequencies of
9.485 and 9.480 GHz and a modulation amplitude of 1.60 G at
100 kHz. X-band EPR spectra of complex 8 were recorded under
a microwave power of 20.020 mW at a frequency of 9.480 GHz
and a modulation amplitude of 0.8 G at 100 kHz.
Magnetic Measurements. The magnetic data were recorded on
a SQUID magnetometer (Quantum Design MPMS5) under a 1 T
external magnetic field over the temperature range 2-300 K. The
magnetic susceptibility data were corrected for temperature-
independent paramagnetism (TIP, 2 × 10-4 cm3 mol-1) and for
ligand diamagnetism using tabulated Pascal’s constants.16
Crystallography. Crystallographic data and structure refinement
parameters of complexes 1, 2, 7, and 8 are summarized in Tables
S1-S4 in the Supporting Information. Each crystal was mounted on
a glass fiber and quickly coated in epoxy resin. The crystals of
complexes 1, 2, 7, and 8 chosen for X-ray diffraction studies had
dimensions of 0.30 × 0.30 × 0.25, 0.35 × 0.28 × 0.25, 0.30 × 0.25
× 0.25, and 0.30 × 0.30 × 0.25 mm3, respectively. Unit-cell
parameters were obtained by least-squares refinement. Diffraction
measurements for complexes 1, 2, 7, and 8 were carried out on a Bruker
X8 APEX II CCD diffractometer with graphite-monochromatized Mo
KR radiation (λ ) 0.7107 Å) between 1.91 and 26.40° for 1, 1.91 and
28.28° for 2, 1.27 and 27.13° for 7, and 1.98 and 26.42° for 8. In
complex 1, the high residual electron density (Q1 ) 3.425) was
localized near O(4). This was due to the thermal disorder of the
chelating nitrito ligands. Examination using the PLATON program
supported the orthorhombic assignment. Also, the difference (0.3015
Å) between the a and c axes is larger than their deviation (3σ ) 0.0015
Å), supporting the orthorhombic assignment. Least-squares refinement
of the positional and anisotropic thermal parameters of all non-
Preparation of [PPN][(PPh3)(η1-NO2)Fe(NO)2] (7). To a stirred
THF solution of the complex [(PPh3)(1-MeIm)Fe(NO)2] (6) (0.09 g,
0.2 mmol) in an ice bath was added dropwise a CH3CN solution of
[PPN][NO2] (0.11 g, 0.2 mmol). The reaction mixture was stirred for
1 h in the ice bath. Diethyl ether/hexane (15 mL) was then added
slowly to layer above the green solution. The flask was tightly sealed
and kept in the refrigerator at -20 °C for 2 weeks to yield green
crystals of [PPN][(PPh3)(η1-NO2)Fe(NO)2] (7) (0.16 g, 80% yield)
suitable for X-ray diffraction analysis. IR (cm-1): 1642(vs), 1693(s)
(νNO) (CH2Cl2); 1637(vs), 1685(s) (νNO), 1306 (νO-N-O), 1263
(νO- N-O) (KBr). Absorption spectrum (CH2Cl2) λmax/nm (ε/M-1
15
cm-1): 601 (208). Anal. Calcd for C54H45N4O4P3Fe ·CH3CN ·(C2H5)2O:
C, 66.82; H, 5.38; N, 6.50. Found: C, 66.81; H, 4.97; N, 6.73.
Reaction of Complex 7 and 2 Equiv of Glacial Acetic
Acid. A THF suspension (5 mL) of complex 7 (0.193 g, 0.2 mmol)
was prepared under a N2 atmosphere in a vial. The vial containing
the THF solution of 7 was then placed in a larger vial containing
a THF/MeCN solution of [PPN]2[S5Fe(µ-S)2FeS5] (0.078 g, 0.05
mmol). The larger vial was then capped with a well-sealed septum.
A THF solution of glacial acetic acid (HOAc) (24 µL, 0.4 mmol)
was then added by syringe to the vial containing the THF solution
of complex 7. The solution containing complex 7 and HOAc was
stirred for 12 h at room temperature. Then the resulting green
solution in the larger vial was transferred to Schlenk tube and dried
under vacuum. The remaining dark-green crude solid was redis-
solved in THF and filtered through Celite to remove the insoluble
solid. Addition of hexane to the filtrate led to the precipitation of
the known dark-green solid [PPN][S5Fe(NO)2] (60%), which was
characterized by its IR spectrum.15e At the same time, the red
solution produced in the small vial [νNO (cm-1): 1693(vs), 1771(s)
(THF)] was transferred to a Schlenk tube and then filtered through
Celite to remove the insoluble solid. Hexane was added to the filtrate
to separate the insoluble red solid [PPN][(OAc)2Fe(NO)2] (8) (0.12
g, 65% yield) and the upper solution. The upper solution was dried
under vacuum to obtain the white solid PPh3, which was character-
ized by 31P NMR spectroscopy [δ ) -4.3 ppm (CDCl3)]. Red
crystals suitable for X-ray diffraction analysis were obtained from
a THF solution of complex 8 layered with diethyl ether/hexane at
-20 °C for 1 week. IR (cm-1): 1693(vs), 1771(s) (νNO), 1630(s)
(16) (a) Bain, G. A.; Berry, J. F. J. Chem. Educ. 2008, 85, 532–536. (b)
Kahn, O. Molecular Magnetism; VCH: New York, 1993.
(17) North, A. C. T.; Phillips, D. C.; Mathews, F. S. Acta Crystallogr.
1968, A24, 351–359.
(18) Sheldrick, G. M. SHELXTL: A Program for Crystal Structure
Determination; Siemens Analytical X-ray Instruments, Inc.: Madison,
WI, 1994.
9
5298 J. AM. CHEM. SOC. VOL. 132, NO. 14, 2010
Tsai, Fu-Te
