2
G. Argouarch et al. / Journal of Organometallic Chemistry xxx (2017) 1e6
solution (2 M, 30 mL) and brine (20 mL), followed by extraction of
the reaction mixture with diethyl ether, washing of the extract with
water and drying over anhydrous magnesium sulfate, filtration and
evaporation to dryness, gave 1-bromo-4-(ethoxymethyl)benzene
as a pure yellow oil (380 mg, 1.78 mmol; 88%).
2.4. X-ray diffraction study
Data collection of 2[PF6] was carried out on a Bruker Apex-II CCD
diffractometer at 150 K. The structure was solved by direct methods
using the SIR97 program [18], and then refined with full-matrix
least-squares methods based on F2 (SHELXL-97) [19] with the aid
of the WINGX [20] program. All non-hydrogen atoms were refined
with anisotropic atomic displacement parameters. H atoms were
included in the final cycle of refinement in their calculated posi-
tions. Details of the data collection, cell dimensions, and structure
refinement are given in Table S1 (Supporting Information). CIF files
for 2[PF6] have been deposited at the Cambridge Crystallographic
Data Center and were allocated the deposition number CCDC
863975.
Scheme 1. Organoiron catalysts for the reductive etherification of aldehydes.
chemicals were obtained commercially and used without further
purification. High-field NMR spectra were obtained on a multinu-
clear Bruker 300 MHz instrument. Chemical shifts are given in parts
per million (ppm) relative to tetramethylsilane (TMS) for 1H NMR
spectra. Transmittance-FTIR spectra were recorded using a Bruker
IFS28 spectrometer (400-4000 cmꢀ1). Cyclic voltammograms were
recorded using a PAR 263 instrument in methylene chloride at
20 ꢁC (0.1 M [n-Bu4N][PF6]) with 100 mV/s scan rate at a platinum
disk (1 mm diameter) using a SCE reference electrode and ferrocene
as internal calibrant (0.46 V) [17]. Photolyses were performed with
a Heraeus UV lamp (TQ150, 150 W, medium pressure) equipped
with a quartz jacket. ESR X-Band spectra were recorded on a Bruker
3. Results and discussion
3.1. Synthesis and characterization of 2[PF6]
€
EMX-8/2.7 (X-band) spectrometer. Mossbauer spectra were recor-
A cyclic voltammetric study of 2 was conducted first, to obtain
information about the oxidation of its electron-rich ferrocenyl (Fc)
fragment (Supporting Information). Some unexpected features
were immediately seen. The first oxidation occurs at a much lower
potential (ꢀ0.36 V vs. FcH) than that of ferrocene, although the Fp*
group was not expected to be very strongly electron-releasing given
the electron-withdrawing nature of its two carbonyl ligands (e.g.
oxidation potentials much above that of ferrocene are found for
complexes 1a-c [0.48e0.61 V vs. FcH]). The second oxidation, pre-
sumed to be localized on the Fp* moiety, occurs at a significantly
higher potential (þ0.92 V vs. FcH) than in 1a-c and exhibits partial
chemical reversibility on the timescale of the measurement,
whereas 1a-c are irreversible under similar conditions. This in-
dicates a kinetic increase in the stability of the dication 22þ
compared to the cations 1a-cþ. Furthermore, considering the po-
ded with a 2.5 ꢂ 10ꢀ2 C (9.25 ꢂ 108 Bq) 57Co source using a sym-
metric triangular sweep mode in the LCC (Toulouse). The
spectrometer calibration was effected using natural iron foil at
20 ꢁC and all isotropic shifts (IS) are reported with respect to the
centroid of this spectrum. Mass spectrometry (MS) analyses were
performed at the “Center Regional de Mesures Physiques de
l'Ouest” (CRMPO, University of Rennes) on a high-resolution MS/
MS ZABSpec TOF Micromass Spectrometer.
2.2. Dicarbonyl(pentamethylcyclopentadienyl)(ferroceniumyl)iron
hexafluorophosphate (2[PF6])
A solution of complex 2 (0.160 g, 0.37 mmol) and ferrocenium
hexafluorophosphate (0.116 g, 0.35 mmol) in THF (15 mL) and
methylene chloride (10 mL) was stirred for 1 h. After evaporation to
dryness, the brown residue was purified by partial precipitation
from methylene chloride and diethyl ether, washed with diethyl
ether (10 mL) and n-pentane (10 mL), and then dried in vacuo to
afford complex 2[PF6] as a brown powder (0.180 g, 89%). Crystals
suitable for a single-crystal X-ray diffraction study were grown by
slow diffusion of n-pentane into a saturated methylene chloride
solution of complex 2[PF6]. Mp. 252 ꢁC (dec.). HRMS (ESI/CH2Cl2):
[M]þ (C22H24Fe2O2) requires 432.0469, found 432.0468. IR
(CH2Cl2): 2007 (s, nC≡O), 1959 (s, nC≡O), 849 (s, nPeF). 1H NMR
tential difference between the two redox events (
D
Eꢁ ¼ 1.28 V), the
monocation 2þ appears to be a very stable Fe(II)/Fe(III) mixed-
valent (MV) complex on thermodynamic grounds, with a stability
constant (Kc) z 4.95 ꢂ 1021 (eq. (1)) for the corresponding com-
proportionation reaction (eq. (2)).
Eꢁ=0:059Þ
Kc¼ 10ðD
(1)
2
þ þ 2þ⇔2 þ 22þ
(2)
(300 MHz, CD2Cl2):
d
¼ 6.1 (s, 15H), 24.9 (broad s, 5H), 38.8 (broad s,
We therefore decided to isolate and characterize this open-shell
2H), 49.4 (broad s, 2H). CV (CH2Cl2): E0 ¼ 0.10 V (
D
Ep ¼ 0.12 V, Iap/
Icp ¼ 1.0); E0 ¼ 1.38 V (
D
Ep ¼ 0.12 V, Iap1/Ipc ¼ 0.7). UVꢀVisꢀnear-IR
compound following chemical oxidation of 2 using ferrocenium
hexafluorophosphate as a chemical oxidant (Scheme 2). The salt 2
[PF6] was isolated as a brown powder and subsequently crystal-
lized, affording access to its structural data in the solid state (Fig. 1).
In this compound, the nCO stretching modes are shifted to higher
wavenumbers compared to those of 2, namely 1959 and 2007 cmꢀ1
(vs. 1935 and 1992 cmꢀ1 for 2), indicating a significant decrease in
the retrodonation of the iron(II) to the CO ligands after oxidation of
2
(CH2Cl2): lmax (ε/103 Mꢀ1 cmꢀ1) 250 (24.9), 278 (sh, 15.7), 340 (sh,
4.2), 420 (3.2), 568 (sh, 0.2), 890 (0.8). ESR (CH2Cl2/1,2-C2H4Cl2,
80 K): gII ¼ 3.47, g⊥ ¼ 1.83.
2.3. Reductive etherification of p-bromobenzaldehyde with 2[PF6]
the ferrocenyl unit (
D
ῡCO ¼ þ24 and þ15 cmꢀ1, respectively).1 Note
In a Schlenk tube under 1 atm of argon, a solution of p-bro-
mobenzaldehyde (2 mmol, 370 mg), 2[PF6] (0.04 mmol, 23 mg), and
diethoxymethylsilane (0.40 g, 3.0 mmol) in CH2Cl2 (15 mL) was
irradiated for 4 h. After removal of the solvent, methanol (5 mL) and
aqueous NaOH solution (2.5 M, 5 mL) were added and the resulting
suspension was stirred overnight. Neutralization with aqueous HCl
that these wavenumbers remain lower than those found for the
acetonitrile complex [Fe(h
5-C5Me5)(CO)2(NCMe)][PF6] (3[PF6];
Differences of only þ3 cmꢀ1 for each ῡCO mode are stated between 1a and 2 [1].
1
j.jorganchem.2017.04.038
Argouarch, Gilles
