E. Lagerspets, et al.
Molecular Catalysis 468 (2019) 75–79
2. Experimental
2.4. Crystal structure determinations
Commercially available compounds were purchased and used
The single-crystal X-ray diffraction studies were carried out on a
Bruker D8 Venture diffractometer with Photon100 detector at 123(2) K
using Mo-Kα radiation (α = 0.71073 Å). Heavy Atom Methods
(SHELXS-97) [28] were used for structure solution and refinement was
1
13
without further purification unless otherwise stated. The H and
C
NMR spectra were recorded on a Varian Mercury 500 MHz spectro-
meter GC–MS analyses were performed on Agilent Technologies 7890B
GC equipped with 5977B MSD, using an Agilent DB-WAX (30 m,
2
carried out using SHELXL-2014 (full-matrix least-squares on F ) [29].
0
.25 mm, 0.25 μm) column. The oxidation products were identified
Hydrogen atoms were localized by difference electron density de-
termination and refined using a riding model (H(O) free). Semi-em-
pirical absorption corrections and extinction corrections were applied.
In C2-Br there is a 1/4 water per asymmetric unit (calculated using
SQUEEZE [30,31], and from free refinement of highest electron dif-
ference density as water). The hydrogen atoms could not be localized
and are not included in the refinement (see cif-.file for details). C8 is a
redetermination of ALAFOK (CCDC-1403822) at 123 K, for more de-
tailed information see ESI [32].
with GC–MS by comparison with commercial samples. The yield de-
terminations were conducted with the GC–MS using calibration curves
with acetophenone or 1,2-dichlorobenze as an internal standard. The
UV–vis absorption measurements were performed on a Hewlett Packard
8453 instrument using 1.0 cm plastic cuvettes at room temperature. For
™
the in situ IR measurements a Mettler Toledo ReactIR 15 was used with
a 6.3 mm AgX DiComp as the probe. The single-crystal X-ray diffraction
study were carried out on a Bruker D8 Venture diffractometer with
Photon100 detector at 123(2)
α = 0.71073 Å). ATR-IR spectra were measured with Bruker Alpha-p,
with a diamond ATR-unit.
K
using Mo-Kα radiation
C2-Br: orange crystals, C22
crystal size 0.30 × 0.12 × 0.04 mm, monoclinic, space group P2
(No. 14), a = 12.5965(5) Å, b = 13.2221(6) Å, c = 12.6993(5) Å,
H
16BrCuF
2
N
2
O
2
∙ ¼ H
2
O, M
r
= 526.32,
/n
(
1
3
−3
β = 101.498(1)°, V = 2072.65(15) Å , Z = 4, ρ = 1.687 Mg/m
,
-
1
α
μ(Mo-K ) = 3.022 mm , F(000) = 1050, 2θmax = 55.0°, 36,381 re-
2.1. Ligand synthesis
flections, of which 4765 were independent (Rint = 0.053), 276 para-
meters, R = 0.028 (for 3944 I > 2σ(I)), wR = 0.060 (all data),
S = 1.04, largest diff. peak/hole = 0.348/-0.578 e Å
1
2
The ligands (L1-L7) were prepared according to literature protocols
27]. Due to air sensitivity of the copper(I) complexes we carried out
−
3
.
[
CCDC 1865444 (C2-Br) and 1865445 (C8) contain the supplemen-
tary crystallographic data for this paper. These data can be obtained
the reactions under argon. Furfural (1 eq.) and substituted anilines (1.1
eq.) were mixed in a 50 ml round bottom flask with dry MeOH (5 ml)
and 3 Å molecular sieves (MS). The reaction was stirred in RT for 1 h.
1
13
The MS were filtered out and H, C NMR spectra were recorded from
1
3 3
the crude oily product in CDCl . L2: H NMR (CDCl , 500 MHz): δ 8.24
3
. Results and discussion
(
s, 1H, N = CH), δ 7.67 (1 H, ar-H), δ 7.19 (2H, ar-H), δ 7.05 (2H, ar-H),
δ 6.93 (1H, ar-H), δ 6.53 (1H, ar-H). All the ligands were synthesized
accordingly.
3
.1. New catalysts
In the search of new homogenous catalyst systems, Schiff base li-
2
.2. Complex preparation
gands are particularly interesting, due to their straightforward mod-
ification. Furthermore, Schiff base ligands can give alternatives for bipy
and porphyrin ligands, needed in the copper(I) oxidation catalysts. In
this respect we examined our previous Schiff base ligands developed for
a copper(II)-system [16], which were active for the oxidation of pri-
mary benzylic alcohols. From those we chose the most promising ligand
The new copper(I) complexes (C2-C7) bearing the N-(X)-1-(furan-2-
yl)methanimine ligands (L2-L7) were synthesized by mixing the corre-
sponding ligand and with a 5 ml MeOH suspension of Cu(I)Br/Cu(I)OTf
(
0.33 eq.) at room temperature stirred for 2 h and the prepared com-
plexes were isolated by filtration. The precipitated complex was washed
with 10 ml of cold MeOH and dried under vacuum. The complex was
dried in vacuum and stored at -84 ⁰C under inert atmosphere. C2: IR
assignment: 1611 nm
1
(
N-(4-fluorophenyl)-1-(pyroll-2-yl)methanimine (L1, C11
H
9
2
FN ) [33,34]
for further studies with copper(I) catalysts.
To our surprise this copper(I) complex (C1 (C11H FN ) CuBr)) with
8 2 2
−
1
−1
(v= C]N), 1502 nm
(v= furan),
(v= furan),
L1 showed no catalytic activity for the oxidation of 1-octanol to octanal
under ambient conditions (Table 1, entry 1). We hypothesized that this
was due to the different coordination mode between copper(I) and the
bidentate ligand as in bipy and thus due to its inability to generate a
catalytically active copper(I) center. This led us to the idea of in-
troducing a five ring bearing oxygen as the second donor atom in the
−
1
−1
−1
223 nm
(v= ArF), 1019 nm
(v= Ar), 842 nm
−1
−1
7
53 nm (v= sub. Ar), 519 nm (v= furan). UV–vis: No Cu(II) signal
at around 600–700 nm was visible. A broad signal at around 300 nm
can be appointed to the ligand (aromatic groups). All other complexes
were synthesized accordingly with the appropriate ligand and char-
acterized with ATR-IR, UV–vis spectroscopy and Elemental Analysis
8 2
ligand. From the complex C2-Br ((C11H FNO) CuBr)crystals suitable
(
see ESI for more information).
for the single crystal X-ray measurement were obtained by slow eva-
poration of methanol. Since copper(I) is known to have commonly two-
coordinate linear, three-coordinate trigonal planar or four-coordinate
tetrahedral geometry [35], to find a stable Schiff base copper(I) com-
plex with a square pyramidal coordination sphere (Fig. 1) is interesting.
It is noteworthy that the bonds from furan oxygen donor to copper(I)
are significantly longer than the bonds between copper(I) and imine
nitrogen. We further expanded the series of complexes with furan based
Schiff-base ligands having different substituted patterns on aniline
(Tabel 1, C3-C7).
2
.3. Oxidations
A series of catalytic oxidation reactions was performed in 5 ml/3 ml
MeCN solutions at room temperature under open air. The reaction was
set up by adding 1 mmol of 1-octanol, 5 mol % of copper(I)catalyst,
5
was equipped with a magnetic stirrer bar. The reaction was stirred
1
1
reaction solution and an internal standard (acetophenon 40 μl or 1,2-
dichlorobenzene 40 μl, more information see ESI) were diluted with
EtOAc (100 ml). GC samples (1.5 ml) were prepared by filtrating the
solution through a layer of silica gel (1 cm thick). The yields were de-
termined using GC–MS with calibration curves.
mol % of TEMPO and 10 mol % of NMI into a 20 ml test tube, which
500 rpm for 24 h for 1-octanol, 3 h for cinnamyl alcohol and 3-phenyl-
-propanol and 1 h for benzyl alcohol. After the reaction, 0.7 ml of the
The impact these modified Schiff base ligands on the catalytic ac-
tivity by oxidizing 1-octanol under ambient air was studied (Table 1).
The highest yield for the oxidation of 1-octanol was obtained by using
L2 (C11
C2-Br ((C11
were recorded with C3, C5, C6 and C7 bearing aniline with different
H
8
FNO) as a ligand forming isolated or in situ made complexes
H
8
FNO) CuBr), Table 1, entry 2). Only modest reactivities
2
76