Pyridine-triazole ligands for copper-catalyzed aerobic alcohol oxidation

Article abstract of DOI:10.1039/c5ra06933e

A series of Cu(NN′)₂(OTf)₂ complexes containing pyridine-triazole ligands [OTf = OSO₂CF₃; NN′ = NN′_Ph (1), NN′_hex (2), NN′_py (3)] with different substituents at the triazole N4 position or 2,2′-bipyridine (bpy; 4) have been synthesized. Crystal structures of 1 and 3 reveal a trans-isomer with strong preference for regular-type triazole coordination (for 3) whereas the Cu-bipyridine complex 4 is more stable in a cis-form. Cyclic voltammetry of 1-4 suggest that the electron-donating strength follows the trend: bpy > NN′_py > NN′_hex ～ NN′_Ph. The catalyst systems consisting of 5 mol% Cu(OTf)₂/NN′/TEMPO (TEMPO = (2,2,6,6-tetramethylpiperidin-1-yl)oxy) in the presence of 2 × 2.0 cm² Cu⁰ sheets as a reducing agent and 10 mol% N-methylimidazole (NMI) exhibit good activities for aerobic oxidation of benzyl alcohol to benzaldehyde. Catalytic studies have shown that the activities were higher with more electron-rich N-based ligands. Furthermore, oxidation of aliphatic alcohols such as 1-hexanol and 2-methyl-1-pentanol using the Cu catalyst system with the NN′_py ligand at room temperature afforded the corresponding aldehydes in >99% and 46% yields, respectively after 24 h.

Full text of DOI:10.1039/c5ra06933e

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ARTICLE
Pyridine–Triazole Ligands for Copper–Catalyzed Aerobic Alcohol
Oxidation
a,b
a,b
b
b
Pech Thongkam, Sudarat Jindabot, Samran Prabpai, Palangpon Kongsaeree, Taveechai
b
b
a,b
Wittisuwannakul, Panida Surawatanawong and Preeyanuch Sangtrirutnugul
2 2 2 3
A series of [Cu(NN')] (OTf) complexes containing pyridine-triazole ligands [OTf = OSO CF ; NN' = NN'Ph (1), NN'hex (2), NN'py
(3)] with different substituents at the triazole N4 position or 2,2'-bipyridine (bpy; 4) have been synthesized. Crystal
structures of 1 and 3 reveal a trans-isomer with strong preference for regular-type triazole coordination (for 3) whereas
the Cu-bipyridine complex 4 is more stable in a cis-form. Cyclic voltammetry of 1–4 suggest that the electron-donating
strength follows the trend: bpy > NN'py > NN'hex ~ NN'Ph. The catalyst systems consisting of 5 mol% CuOTf
2
/NN'/TEMPO
2
0
(
TEMPO = (2,2,6,6-tetramethylpiperidin-1-yl)oxy) in the presence of 2x2.0 cm Cu sheets as a reducing agent and 10 mol%
Received 00th January 20xx,
Accepted 00th January 20xx
N-methylimidazole (NMI) exhibit good activities for aerobic oxidation of benzyl alcohol to benzaldehyde. Catalytic studies
have shown that the activities were higher with more electron-rich N-based ligands. Furthermore, oxidation of aliphatic
alcohols such as 1-hexanol and 2-methyl-1-pentanol using the Cu catalyst system with the NN'py ligand at room
temperature afforded the corresponding aldehydes in >99% and 46% yields, respectively after 24 h.
DOI: 10.1039/x0xx00000x
www.rsc.org/rscadv
o
o
2
efficient at the oxidation of 1 and 2 alcohols under 1 atm of O at
o
12, 13
o
1
. Introduction
90 C.
Sheldon also reported oxidation of 1 alcohols catalyzed
t
by CuBr
2
(bpy) (bpy = 2,2'-bipyridine), TEMPO, and KO Bu in a 2:1
Oxidation of alcohols to aldehydes is one of the most important
1
4
1
,
2
3 2
mixture of CH CN:H O under air at room temperature. By
classes of organic transformation.
Classical alcohol oxidation
t
replacing KO Bu with organic base, Koskinen has shown that the
reactions to aldehydes usually employ stoichiometric amounts of
3
catalyst system CuX (bpy)/TEMPO/base [X = Br or OTf; base = N-
2
strong oxidants such as chromium(VI) oxide-pyridine,
4
5
methylimidazole (NMI) or 1,8-diazabicycloundec-7-ene (DBU)] could
oxalylchloride/DMSO, Dess-Martin periodinane (DMP), and
6
efficiently catalyze benzylic and allylic alcohol oxidation under mild
NaOCl/TEMPO (TEMPO = (2,2,6,6-tetramethylpiperidin-1-yl)oxy).
1
5
conditions. More recently, Stahl and co-workers reported that the
Thus, an improvement toward greener and more economical
synthetic alternatives including the use of molecular oxygen as a
primary oxidant and recyclable metal catalysts is needed. Up to
now, various types of transition metal catalysts such as vanadium,
cobalt, and molybdenum have been employed in aerobic alcohol
I
3
catalyst system Cu OTf/bpy/NMI in CH CN exhibited better
II
oxidation performance compared to its Cu analogue and, based on
I
kinetic and spectroscopic studies, the mechanism of Cu /TEMPO-
1
6-18
catalyzed alcohol oxidation has been proposed (Scheme 1).
In
7
addition, the effect of bpy ligand on redox potentials at copper
oxidation. In particular, copper complexes have attracted
center and consequently catalytic activities toward alcohol
considerable attentions since copper species are found in active
sites of several oxidase enzymes, which are known to catalyze
1
7
oxidations has been studied.
8
-10
aerobic oxidation reactions under mild conditions.
In 1984,
Over the past decade, 1,2,3-triazole (“click”) compounds have
Semmelhack and co-workers reported effective alcohol oxidation
1
9
I
11
been widely used in applications including polymer, material
catalyzed by Cu Cl/TEMPO in the presence of O
later demonstrated that CuCl(Phen–DEADH
phenanthroline; DEADH2 diethylhydrazinodicarboxylate) was
2
gas. Markó et. al.
2
0
research, and catalysis mainly due to ease of preparation and
structural modification. For catalytic application, reported organic
reactions mediated by transition metal complexes featuring 1,2,3-
2
)
(Phen 1,10-
=
2
1
triazole ligands are, for example, alkyne-azide cycloaddition,
1
9, 22, 23
24-26
27
polymerization,
C–C coupling,
epoxidation,
and
2
8
propargyl alcohol dehydration. In comparison to pyridine, the
electronically similar 1,2,3-triazole compounds are aromatic with
better
π
accepting ability, based on photophysical and
29
electrochemical studies of the Ru(II) complexes.
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(
(
NN'py)] were prepared following those reported in the literature
24
2
Figure 1). Reactions of Cu(OTf) and 2.1 equiv of the pyridine-
triazole ligands in either a mixture of CH CN/CH Cl or CH CN at
3
2
2
3
room temperature afforded the corresponding, distorted
octahedral Cu(II) complexes Cu(NN') (OTf) [NN' = NN'Ph (1), NN'hex
2), NN'py (3)] in good yields (81–83% yields). For comparison, the
analogous Cu(bpy) (OTf) was also prepared under similar reaction
2
2
(
2
2
conditions, giving blue crystalline solids as the product 4 in 87%
yield. The resulting Cu(II) complexes 1–4 were isolated via
3
crystallization from the CH CN solution and characterized by UV-vis
spectroscopy (Figure S4–7, ESI), single-crystal X-ray diffraction (XRD)
studies (for 1, 3, and 4), powder XRD (for 3 and 4, Figure S8, ESI),
and elemental analysis.
I
Scheme 1. Catalytic mechanism for Cu /TEMPO-catalyzed aerobic
alcohol oxidation.
Another interesting feature of 1,2,3-triazole compounds is that the
substituents at the 1- and 4-positions can be conveniently adjusted
and this facilitates the study of ligand substituent effect. In this
work, we investigated a series of pyridine-triazole ligands, related
to the more well known bidentate N-based ligand 2,2'-bipyridine,
for copper-catalyzed alcohol oxidation. The use of oxidatively more
Figure 1. Structures of the ligands NN'Ph, NN'hex, NN'py, and bpy
0
stable Cu(II) complexes in the presence of a reducing agent Cu to
2
.2 Description of crystal structures
generate the catalytically active Cu(I) species in situ was also
explored. In addition, the effect of substituents at the 1,2,3-triazole
ring and triazole configurations on redox potentials at the copper
center and catalytic oxidation activities were studied.
The solid state structures of 1 and 3 reveal distorted octahedral
geometry with two bidentate NN' ligands and inner-sphere triflate
ions bound to the Cu(II) center in a trans stereoisomer (Figure 2).
Furthermore, for both complexes 1 and 3, the pyridyl N and the
triazole N donors are mutually trans to each other. This structural
arrangement has previously been observed in related octahedral
2
. Results and discussion
3
0
31
32
bis(pyridine-triazolyl) complexes of Cu(II), Ni(II), and Zn(II) as
2
.1 Synthesis and characterization
33
well as square planar Pd(II) complexes.
A series of bidentate pyridine-triazole ligands of the type 2-(4-R-
1
6 5 6 5 4
,2,3-triazol-1-yl)pyridine [R = C H (NN'Ph), n-C H13 (NN'hex), NC H
2
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Figure 2. ORTEP representations of complexes 1 (left), 3 (right), and 4 (middle) showing 30% probability displacement ellipsoids and
partial atom-labeling scheme. Hydrogen atoms and water molecules (for 1) were omitted for clarity.
Table 1. Selected bond lengths (Å).
o
Table 2. Selected bond angles ( ).
a
Bite angle
For complex 3, due to the more electron rich nature, the the d-d band shifts are associated with ligand field strength, the
proximal triazole N donor (N2) or the 2-(1H-1,2,3-triazol-4- blue-shifted d-d transitions indicate stronger ligand fields, giving the
yl)pyridine chelate pocket (i.e., regular-type triazole ligand) order of ligand field factor as follows: NN'py > NN'hex > NN'Ph > bpy.
exclusively binds to Cu(II) instead of the medial triazole N (N3), as
shown in Figure 2. The selective formation of the regular triazole
complexes has been previously observed with the related Pd(II)
The electrochemical properties of complexes 1–4 were
investigated by cyclic voltammetry (CV) using dry CH CN solvent
3
under Ar (Figure 3) and the CV data were summarized in Table 3.
Complexes 1 and 2 featuring NN'Ph and NN'hex ligands exhibit similar
quasi-reversible redox waves with cathodic reductions
2
4
compound (NN' )PdCl
and by ligand exchange experiments
py
2
3
4
reported by Crowley and co-workers. In general, the Cu–N bond
lengths of 1 are marginally shorter than those of 3 while the bond
distances of Cu–N(py) and Cu–N(trz) are within the expected ranges
corresponding to the Cu(II)/Cu(I) couple (Ep ) at –0.050 and –0.070
c
V and slightly weaker anodic responses (Ep ) at 0.373 and 0.323 V,
a
II
compared to similar Cu complexes supported by pyridine-triazole
respectively (Table 3). For complex 3 with regular-type triazole
ligands [i.e., 2.003–2.062 Å for Cu–N(py) and 1.985–2.037 Å for Cu–
binding to Cu(II), there is a significant negative shift of the reduction
30
N(trz)], with slightly shorter Cu–N(trz) bonds compared to Cu–
potential (Ep = –0.473 V) and unexpectedly weak Cu(I)→Cu(II)
c
N(py) (Table 1). In contrast to 1 and 3, the solid state structure of
oxidation wave, suggesting the Cu(I) species supported by NN' is
py
-1
Cu(bpy) (OTf) (4; Figure 2) shows a cis isomer, where both triflate
2
2
relatively unstable. However, at scan rate 100 mV s , no anodic
1
0,
ions are located trans to pyridyl N donors. For all complexes, ligand
stripping of copper, a result of the Cu(I)/Cu(0) couple, is observed.
o
o
3
5, 36
bite angles are in the range of ca. 80.0 –80.8 (Table 2). The purity
of the representative crystalline complexes 3 and 4 in bulk phase
was confirmed by powder X-ray diffraction (XRD) data compared to
the simulative patterns (Figure S8, ESI).
In addition, of all complexes, 4 possesses the most negative
reduction potential with Ep = –0.666 V. It is worth mentioning that
c
Stahl and co-workers previously reported the Cu(II)/Cu(I) reduction
potential of –0.18
V for an CH CN solution mixture of
3
I
16
2
.3 Electronic spectra and electrochemical properties
The profiles of UV-visible spectra of 1–4 in CH CN appear similar
Supporting Information Figures S4–S7). Broad low-intensity bands
[Cu (CH CN) ]OTf and 4 equiv of bpy. Since the more negative
3 4
reduction potentials assignable to Cu(II)→Cu(I) are attributed to
higher electron density at the copper center and consequently the
electron-donating ability of ligands, the electron-donating property
decreases following the trend: bpy > NN'py > NN'hex ~ NN'Ph.
3
(
at
λmax = 678, 664, 657, and 733 nm (for 1–4, respectively) are
assigned to the low-energy d-d transitions of the Cu(II) center. Since
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Although, based on the reduction potentials, bpy appears to be the evidenced by previously reported crystal structures of Cu(I)
most electron-donating ligand for the Cu(II) complexes of the type complexes with bidentate N-donor ligands including
Cu(NN') (OTf) , we cannot ignore the difference in stereoisomers, [Cu(bpy) ]ClO and [Cu(phen) ]ClO . Our calculated structure of
which may also influence the observed redox potentials.
39
40
2
2
2
4
2
4
37
+
[Cu(bpy)2] is in good agreement with the available crystal
structures (Supporting Information Table S2). Here, the reduced
species of 1 and 3 are calculated as tetrahedral complexes,
+
+
[
Cu(NN'Ph
)
2
]
2
and [Cu(NN'py) ] (Figure 4), consistent with the
previously reported tetrahedral structure of the related Cu(I)
complex [Cu(L) ]ꢀBF (L = 2-(1-benzyl-1,2,3-triazol-4-yl)pyridine).
2
4
Similar to their corresponding Cu(II) complexes, these Cu(I)
complexes display slightly shorter Cu–N(trz) bonds compared to the
Cu–N(py) bonds.
Figure 3. Overlaid cyclic voltammograms of 1–4 in CH
Scan rate 100 mV s .
3
CN solution.
-1
a
Table 3. Cyclic Voltammetry Analysis of Cu(NN') OTf complexes
2
2
b
Complex!
Eox![V]!
Ered![V]!
ꢀE ![mV]! E1/2 ![V]!
p
1
2
3
&
&
&
0.373!
–0.050!
423!
393!
0.162!
0.126!
0.323!
!
–0.070!
–0.473!
4
&
–0.537!
–0.666!
129!
–0.601!
a
Conditions: 1 mM Cu(NN')
2
OTf
2
, in CH
3
4 6
CN, 0.1 M [NEt ][PF ],
glassy carbon working electrode, platinum counter electrode,
+
Ag/Ag reference electrode, scan rate 100 mV/s, potentials
+
b
+
+
reported in relative to Fc/Fc .
E1/2 = (Ep,a + Ep,c)/2.
Ph 2 py 2
Figure 4. Optimized geometries of [Cu(NN' ) ] and [Cu(NN' ) ]
(
the reduced forms of 1 and 3, respectively) with APT charges
shown on selected atoms. Calculated bond distances are given in Å.
All H atoms are omitted for clarity. Cu atoms are shown in white, C
atoms in grey, and N atoms in blue.
Interestingly, the complex 2 with a saturated and electron-
donating hexyl substituted NN'hex ligand resulted in only slightly
more cathodic shift by –20 mV compared to that of 1, containing an
aromatic phenyl substituent. This small potential difference
suggests that the type of substituents at the triazole N4 position
+
Molecular orbitals (MOs) and MO energies of [Cu(NN'Ph
)
2
] and
+
2
9, 38
[
(
Cu(NN'py
2
) ] show that all five metal d-based orbitals are occupied
have little effect on the electron density at the copper center.
On the other hand, triazole configurations involving chelation
1
0
Figure 5) representing d configuration. Although both complexes
through regular-type or inverse-type triazole N donor exert more have HOMO with a σ-antibonding interaction between the Cu dxz
+
impact on the electron density at copper. In particular, the regular- and the NN' ligands, the HOMO energy of [Cu(NN'py
)
] is relatively
2
+
type triazole ligand using proximal triazole N donor affords more higher than that of [Cu(NN'Ph
electron-rich copper center than the inverse-type triazole ligand.
2
to form its reduced complex [Cu(NN'py) ] is expected to be less
) ] . Thus, one-electron reduction of 3
2
+
+
favored than the same process of 1 to form [Cu(NN' ) ] . Using the
Ph 2
2
.4 DFT calculation
same reasoning, O2 activation by Cu(I) catalysts to afford the
Since the triazole chelation through inverse-type (in 1) vs. regular- corresponding Cu(II) superoxo species, proposed as turnover-
1
7, 18
type (in 3) configurations can affect the electron donating ability of limiting step for activated benzyl alcohol substrates,
should
+
the ligands and activity for alcohol oxidation of the catalysts, we occur more readily with [Cu(NN' ) ] . This result is also in
py 2
performed density functional calculations to gain insights into the agreement with the observed reduction potentials, which are more
electronic structures related to the redox properties of 1 and 3. negative for complex 3 relative to 1. Moreover, the APT charges on
Generally, Cu(I) complex often favors a tetrahedral geometry, as N2 and N4 of the triazole are rather negative while the charge on
4
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+
+
N3 is almost neutral for both [Cu(NN'Ph
)
2
] and [Cu(NN'py
)
2
] (Figure Complete conversions to benzaldehyde were observed with NN'py
4
). Therefore, the regular-type binding mode of the triazole should and bpy ligands while the catalyst systems containing NN'Ph and
+
lead to a stronger ligand σ-donation in [Cu(NN'py
)
2
] and potentially NN'hex ligands afforded the product in 77% and 80% yields,
the more active alcohol oxidation catalyst.
respectively. Based on its high activity, the molecular complex
0
Cu(NN'py
)
2
(OTf)
2
(3) and Cu was investigated as a catalyst under the
same reaction conditions and found that the catalytic oxidation of
2
.5 Catalytic test for aerobic alcohol oxidation
0
3
/Cu was only slightly faster than that of the system
To optimize reaction conditions, benzyl alcohol was chosen as a
model substrate for copper-catalyzed alcohol oxidation. A mixture
0
Cu(OTf)
2
/NN'py/Cu , giving 86% and 73% product yields,
respectively, after 2 h (entries 5 and 3). Since we expected
0
2
of Cu(OTf)
0 mol%), and NN' ligand (5 mol%) was used as the catalyst system
in the presence of TEMPO (5 mol%) radical. The reactions were
carried out at room temperature in CH CN under aerobic
2
(5 mol%), Cu sheet (4.0 cm ), N-methylimidazole (NMI,
II
0
I
reduction of Cu species by Cu to generate the active Cu catalyst, a
1
0
controlled reactions were carried out in the absence of Cu (entry 6)
and Cu(OTf)
22% and 66% after 24 h, respectively). In addition, even though
reduced catalyst loadings of Cu(OTf) /NN'py gave slower conversions
Table 4, entries 8–10), the high percentage yield of 87% was still
2
(entry 7), both of which gave a lower product yields
3
(
conditions. In a typical experiment, after a given time, the product
in the reaction mixture was analyzed using GC-MS. In general, the
reaction gave good to excellent yields of benzaldehyde after 3 h,
with no over-oxidized benzoic acid observed (Table 4, entries 1–4).
2
(
2
obtained with 0.05 mol% of Cu(OTf) /NN'py after 24 h.
+
+
2 2
Figure 5. Calculated molecular orbitals (MOs) and MO energies (in eV) of [Cu(NN'Ph) ] and [Cu(NN'py) ] .
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Other substituted benzyl alcohols were also used as substrates activity follows the order: bpy > NN'py > NN'hex ~ NN'Ph. This
and the oxidation results showed that all of them gave excellent observed activity trend is consistent with the ligand electron-
and selective conversions to the corresponding aldehydes (entries donating ability, based on the CV results (vide supra). In other
11–14). The non-activated aliphatic alcohol 1-hexanol could also be words, more electron-rich ligands resulted in more active copper
quantitatively converted to hexanal after 24 h although the more catalyst systems for oxidation. Along the same line, Stahl and co-
sterically hindered 2-methyl-1-pentanol resulted in a lower yield workers have recently observed that reaction rates of copper-
under the same conditions (entries 15 and 16). Noted that for catalyzed aerobic alcohol oxidation were higher with 4,4'-
4
1
secondary alcohols, no oxidation of cyclohexanol to cyclohexanone dimethoxy-2,2'-bipyridine ligand compared to 2,2'-bipyridine. In
was observed whereas the more reactive 2-cyclohexen-1-ol was addition, previous catalyst screenings reported by Repo and co-
oxidized at room temperature to afford the corresponding ketone workers have shown that rigid and strongly coordinating ligands
product in 55% yield after 24 h, based on GC-MS (entries 17 and generally afforded more active copper catalysts for oxidation of
o
18).
benzyl alcohol in alkaline-water solutions at 80 C under 10 bar of
4
2
0
To evaluate the effect of N-based ligands NN' and bpy on the
O
2
.
2
Reusability studies of the catalyst system Cu(OTf) /NN'py/Cu
catalyst activities, the reaction mixtures from entries 1–4 (Table 4) were also evaluated. It was found that the copper catalysts with
were sampled and percent formation of the benzaldehyde product added benzyl alcohol substrate and 5 mol% TEMPO remained
was monitored over time using GC-MS (Figure 6). The results effective giving >98% conversion for at least five reaction cycles
showed the fastest initial conversion of benzyl alcohol oxidation for without a loss in catalytic activities (Figure 7).
0
the catalyst system Cu(OTf)
2
/bpy/Cu . In general, the catalytic
a
Table 4. Aerobic Oxidation of selected alcohols
b
Entry
Cu source
Cu(OTf) /Cu
Cu(OTf) /Cu
Ligand
NN'Ph
NN'hex
NN'py
bpy
Substrate
Product
Time (h)
Yield (%)
77
0
0
0
0
1
2
3
4
5
2
Benzyl alcohol
Benzyl alcohol
Benzyl alcohol
Benzyl alcohol
Benzyl alcohol
Benzyl alcohol
Benzaldehyde
Benzaldehyde
Benzaldehyde
Benzaldehyde
Benzaldehyde
Benzaldehyde
3
3
80
2
c
Cu(OTf)
Cu(OTf)
2
/Cu
/Cu
3
>99 (73)
>99
2
3
0
c
3/Cu
-
3
98 (86)
d
6
3
-
24
22
e
0
7
8
Cu
NN'py
NN'py
Benzyl alcohol
Benzyl alcohol
Benzaldehyde
Benzaldehyde
24
24
66
>99
f
0
0
0
0
0
0
0
0
Cu(OTf)
Cu(OTf)
Cu(OTf)
Cu(OTf)
Cu(OTf)
Cu(OTf)
2
2
2
2
2
2
/Cu
/Cu
/Cu
/Cu
/Cu
/Cu
g
9
NN'py
NN'py
NN'py
NN'py
NN'py
NN'py
NN'py
Benzyl alcohol
Benzaldehyde
24
24
2
92
h
10
11
12
13
14
15
Benzyl alcohol
Benzaldehyde
87
i
4-MeO-benzyl alcohol
4-MeO-benzaldehyde
95 (>99)
4-NO
2
-benzyl alcohol
4-NO
2
-benzaldehyde
2
>99
j
j
2-MeO-benzyl alcohol
2-MeO-benzaldehyde
2
82 (>99)
67 (>99)
99
Cu(OTf) /Cu
2-NO -benzyl alcohol
2-NO -benzaldehyde
2
2
2
2
Cu(OTf)
2
/Cu
1-Hexanol
Hexanal
24
0
1
1
6
7
Cu(OTf)
Cu(OTf)
2
2
/Cu
/Cu
NN'py
NN'py
2-Methyl-1-pentanol
Cyclohexanol
2-Methylpentanal
Cyclohexanone
24
24
46
0
0
0
1
8
Cu(OTf)
2
/Cu
NN'py
2-Cyclohexen-1-ol
2-Cyclohexen-1-one
24
55
a
2
Typical Conditions: 2.0 mmol of the substrate, 0.10 mmol (5 mol%) of Cu(OTf) /Ligand, 0.10 mmol (5 mol%) of TEMPO, 0.20 mmol (10
0
2
b
c
3
mol%) of NMI, 8 Cu sheets (0.4 cm /mL) in CH CN (10 mL), 1 atm of air. % yield was obtained based on GC analyses. yields obtained
d
0
e
f
g
h
2 2 2
after 2 h. In the absence of Cu sheet. In the absence of Cu(OTf) . 1 mol% of CuOTf /NN'py. 0.1 mol% of CuOTf /NN'py. 0.01 mol% of
i
j
2
CuOTf /NN'py. yields obtained after 3 h. yields obtained after 4 h.
6
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ARTICLE
1
H (500.1 MHz) NMR spectra were acquired on Bruker AV-500
spectrometer equipped with a 5 mm proton/BBI probe at room
temperature and referenced to protic impurities in the deuterated
solvent. Elemental analyses were conducted at Chemistry
department, Mahidol University. Product yields and percent
substrate conversions obtained from catalytic experiments were
analyzed by GLC on
a 6890N Agilent Technologies gas
chromatograph equipped with a 5973N Agilent Technologies
quadrupole mass detector. UV-vis spectra of the CH CN solutions
3
(
0.03 mM and 1 mM) were collected using a Jasco Model V-530 UV-
vis spectrophotometer. Powder X-ray diffraction (XRD)
measurements were collected using Bruker D8 ADVANCE
diffractometer (Cu K
α radiation with Ge-crystal (Johansson type)
monochromator ( = 0.15406 nm)).
λ
Figure 6. Percentage of benzaldehyde product vs. time
3
3
.2 Synthesis and characterization
.2.1 Synthesis of Cu(II) Complexes 1, 3, and 4. To a stirring 5 mL of
CH CN solution of Cu(OTf) (0.45 mmol) was added a 5 mL CH Cl
2
3
2
2
solution of NN' ligand (0.90 mmol) at room temperature. After 4 h,
the reaction mixture was filtered and the precipitates were washed
with 20 mL of CH Cl .
2
2
2 2
trans-Cu(NN'Ph) (OTf) (1). The product was obtained as a light
green solid in 81% yield (0.29 g, 0.36 mmol). Anal. Calcd. for
C H N CuF O S : C, 41.72; H, 2.50; N, 13.90. Found: C, 41.67; H,
2
8
20
8
6
6
2
2
.30; N, 14.21.
trans-Cu(NN'py
)
2
2
(OTf) (3). The product was obtained as a blue solid
in 83% yield (0.30 g, 0.37 mmol). Anal. Calcd. for C H N CuF O S :
26
18 10
6
6 2
C, 38.64; H, 2.25; N, 17.33. Found: C, 39.02; H, 2.39; N, 17.70.
cis-Cu(bpy) (OTf) (4). The product was obtained as a blue solid in
7% yield (0.26 g, 0.39 mmol). Anal. Calcd. forC22 CuF : C,
39.20; H, 2.39; N, 8.31. Found: C, 39.33; H, 2.41; N, 8.28.
3.2.2 Synthesis of trans-Cu(NN' ) (OTf) (2). To a stirring 30
2
2
8
H
16
N
4
6 6 2
O S
Figure 7. Reusability tests based on percent benzyl alcohol
conversions over five reaction cycles
hex 2
2
mLCH
3
CN solution of Cu(OTf)
2
(0.53 g, 1.46 mmol) was added a 20
mL CH
3
CN solution of NN'hex (0.67 g, 2.91 mmol) at room
temperature. After 5 h, the solvent was removed under vacuum.
3
. Experimental
The remaining solid was purified by crystallization in CH CN at 0 °C
3
to afford 2, as a blue crystalline solid in 81% yield (0.97 g,
3
.1 General procedure
1
1
36 8 6 6 2
.18mmol). Anal. Calcd. for C28H N CuF O S : C, 40.90; H, 4.41; N,
3.63. Found: C, 40.70; H, 4.39; N, 13.71.
Air-sensitive syntheses were carried out under Ar using standard
Schlenk techniques or in a Braun drybox. Toluene (Fisher Scientific)
was dried using PURE SOLV MD-5 solvent purification system from
Innovative Technology Inc. All other solvents were used as received.
3
.3. Cyclic voltammetry
Voltammograms were recorded at ambient temperatures with
Autolab Type III potentiostat and NOVA software. The Cu(II)
complexes Cu(NN') (OTf) (1.0 mM) was dissolved in dry CH CN
containing 0.1 M [Et N][PF ] electrolyte. Measurements were
ꢁ
(
CuOTf) •C H (Aldrich) was used as received and stored under Ar.
2
6 6
Cu(OTf)
2
, 2-bromopyridine, phenylacetylene, 2-ethynylpyridine, N-
2
2
3
methylimidazole (NMI), benzyl alcohol, other alcohol substrates,
and 2,2'-bipyridine were purchased from Aldrich and used without
further purification. Ethylenediaminetetraacetic acid (EDTA;
LobaChemie), 1-octyne (Merck), anisole (Merck), 1-hexanol (Fluka),
4
6
-1
performed under Ar at a scanning rate of 0.1 V s with a glassy
carbon working electrode, a platinum counter electrode, and a
+
Ag/Ag reference electrode. The sample was referenced to the
ferrocene internal standard and its potentials were reported versus
2
,2,6,6-tetramethylpiperidin-1-yl)oxy (TEMPO; Acros Organics) were
also used as received. 2-azidopyridine and 2-(4-R-1,2,3-triazol-1-
yl)pyridine [R = C (NN'Ph), n-C (NN'py)] were
prepared according to the literature.
+
those of Fc/Fc .
6
H
5
6 5 4
H13 (NN'hex), NC H
3
.4. X-ray analysis
24
Crystals of 1, 3, and 4 were grown from slow evaporation of the
CH
3
CN solution of the corresponding Cu(II) complexes at room
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temperature. The single crystal X-ray analyses of Cu(II) complexes 1, alcohol and TEMPO followed by GC-MS measurements to
, and 4 were carried out at the Mahidol Crystallographic Facility. determine percent conversions were repeated for five cycles.
3
Diffraction measurements were recorded using a 4 K Bruker APEX
CCD area detector diffractometer with graphite-monochromated
Mo Kα radiation (λ = 0.71073 Å). The structures were solved by the
Conclusions
4
3
44
SHELX or OLEX2 crystallographic software packages, using direct We have reported the synthesis of Cu(II) complexes of the type
4
5
methods (SIR97) and refined by full-matrix least-squares method Cu(NN') (OTf) , [NN' = NN' (1), NN'hex (2), NN'py (3), and bpy (4)].
2
2
Ph
2
)
46
on (Fobs
using the SHELXL97 software package. Complexes For the pyridyl-substituted NN'py ligand, the crystal structure reveals
O and 3 crystallize in the orthorhombic system, space group an exclusive binding of Cu(II) ion to the pyridine N and the proximal
Pcba, and the triclinic system, space group P-1, respectively. In triazole N donor (i.e., regular-type triazole ligand). Results from CV
addition, both complexes 1•2H O and 3 show the Cu atom in a data and DFT calculations also indicate that the regular triazole
1•2H
2
2
centrosymmetric position whereas the complex4, in the C2/c space ligand (NN' ) is a more electron-rich and better σ-donating ligand,
py
group, has the Cu atom on a 2-fold axis. All non-hydrogen atoms compared to the inverse-type ligands (NN'Ph and NN'hex). On the
were refined anisotropically while the hydrogen atoms were other hand, based on reduction potentials, the substituents on the
included in calculated positions and refined as riding atoms with triazole ring do not exert much effect on the electron-donating
II
0
individual isotropic displacement parameters. Crystallographic data strength of the ligand. The systems Cu /NN'/Cu in the presence of
of 1•2H O, 3, and 4 are listed in Supporting Information Table S1.
the TEMPO radical and NMI have been shown as stable and
efficient catalysts for aerobic alcohol oxidation, both with aromatic
and unhindered aliphatic alcohol substrate. Catalytic studies reveal
that rates of activated benzyl alcohol oxidation depend on the
ligand electron-donating strength. More specifically, the
2
3.5. Computational detail
47
All calculations were performed with the Gaussian 09 program. All
4
8-50
structures were fully optimized using B3LYP
density functional
II
0
with default convergence criteria and frequencies were calculated
to ensure that there is no imaginary frequency for minima. The
Cu /NN'/Cu systems are more active oxidation catalysts in the
presence of the more electron-rich N-based ligands.
5
1
Stuttgart relativistic small core (RSC) 1997 ECP basis set was used
5
2-54
for Cu and 6-31++G(d,p)
for all other atoms. Molecular orbitals
5
5-57
were plotted by Jimp2 visualizing program.
Acknowledgements
The authors are grateful to Kasempong Srisawad for a technical
assistance in the recording of CV data. We acknowledge financial
supports from the Thailand Research Fund and Commission on
Higher Education for financial support under Grant No.
RSA5780058. This work is also supported by the Center of
Excellence for Innovation in Chemistry (PERCH-CIC) and Faculty of
Science, Mahidol University and Mahidol University under the
National Research Universities Initiative.
3
.6. General procedures for Cu-catalyzed aerobic alcohol
oxidation
3
2.0 mmol of alcohol was added into a 2 mL CH CN solution of
0
Cu(OTf)
surface area = 4.0 cm ). The reaction mixture was stirred for 10 min.
Then, a 5 mL CH CN solution of NN' (0.10 mmol) was added into the
reaction mixture, followed by addition of a 3 mL CH CN solution of
2
(0.10 mmol), followed by addition of Cu sheets (total
2
3
3
TEMPO (0.10 mmol) and NMI (0.20 mmol). The reaction solution
was allowed to stir at room temperature for a given time, after
which it was filtered through a silica column using ethyl acetate as
eluent. The product yields were determined using GC-MS methods
with anisole (0.20 mmol) as an internal standard. For kinetic
studies, the reaction solution was sampled for GC-MS analysis every
hour for 5 h. The kinetic experiments were repeated three times for
which the averaged data were used.
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