Exploring the coordination chemistry of 2-picolinic acid to zinc and application of the complexes in catalytic oxidation chemistry

Article abstract of DOI:10.1016/j.inoche.2014.06.020

Oxidation reactions are one of the fundamental processes in organic chemistry and in recent years the application of low-toxic and abundant catalysts became a significant research target. In this regard, the application of zinc catalysis has been established. For instance the in situ mixture of zinc(II) bromide and pyridines substituted with carboxylic acid functions has been demonstrated to oxidize e.g. alcohols to aldehydes with hydrogen peroxide as terminal oxidant. However, no information on the structural composition of the in situ systems has been accounted so far. In this study we tried to shed light on these in situ systems and report on the synthesis, isolation and characterization of zinc(II) complexes modified with 2-picolinic acid (2). In more detail, [Zn(2-H)(2)Br] and [Zn(2-H)₂(H₂O) ₂] were obtained and subsequently tested in catalytic oxidation reactions. Both complexes showed catalytic activity in the oxidation of benzyl alcohol to benzaldehyde. Importantly, a lower activity was observed compared to the in situ system. Several experiments were performed to understand the different reactivity and it was found that a significant effect on oxidation processes revealed by the HBr, which is produced as side product in the in situ complex formation. 2014 Elsevier B.V.

Full text of DOI:10.1016/j.inoche.2014.06.020

Inorganic Chemistry Communications 46 (2014) 320–323
Contents lists available at ScienceDirect
Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Exploring the coordination chemistry of 2-picolinic acid to zinc and
application of the complexes in catalytic oxidation chemistry
b,
a
c
a
Stephan Enthaler a, , Xiao-Feng Wu , Maik Weidauer , Elisabeth Irran , Peter Döhlert
⁎
⁎
a
Technische Universität Berlin, Department of Chemistry, Cluster of Excellence “Unifying Concepts in Catalysis”, Straße des 17. Juni 135/C2, D-10623 Berlin, Germany
Leibniz-Institut für Katalyse e.V. an der Universität Rostock, Albert-Einstein-Strasse 29a, 18059 Rostock, Germany
Technische Universität Berlin, Institute of Chemistry: Metalorganics and Inorganic Materials, Straße des 17. Juni 135/C2, D-10623 Berlin, Germany
b
c
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 12 May 2014
Received in revised form 12 June 2014
Accepted 22 June 2014
Available online 24 June 2014
Oxidation reactions are one of the fundamental processes in organic chemistry and in recent years the application
of low-toxic and abundant catalysts became a significant research target. In this regard, the application of zinc ca-
talysis has been established. For instance the in situ mixture of zinc(II) bromide and pyridines substituted with
carboxylic acid functions has been demonstrated to oxidize e.g. alcohols to aldehydes with hydrogen peroxide
as terminal oxidant. However, no information on the structural composition of the in situ systems has been
accounted so far. In this study we tried to shed light on these in situ systems and report on the synthesis, isolation
and characterization of zinc(II) complexes modified with 2-picolinic acid (2). In more detail, [Zn(2-H)(2)Br] and
[Zn(2-H)2(H2O)2] were obtained and subsequently tested in catalytic oxidation reactions. Both complexes
showed catalytic activity in the oxidation of benzyl alcohol to benzaldehyde. Importantly, a lower activity was
observed compared to the in situ system. Several experiments were performed to understand the different reac-
tivity and it was found that a significant effect on oxidation processes revealed by the HBr, which is produced as
side product in the in situ complex formation.
Keywords:
Zinc complexes
Bidentate ligand
Catalysis
Oxidation chemistry
© 2014 Elsevier B.V. All rights reserved.
Oxidation reactions are one of the fundamental transformations in
modern organic chemistry [1]. In this regard, countless important pro-
cedures have been developed and used during the past decades. A cur-
rent trend is to establish a “greener” chemistry and “sustainable”
synthetic approaches [2]. Especially with the aid of catalysis oxidations
can be performed at milder conditions, with reduced amounts of
waste, and obtains products in high yields and selectivities [3]. An inten-
sive studied area is the application of “bio”metal, e.g. iron, zinc as cata-
lyst core, which is advantageous because of low-toxicity, low price
and abundance, in combination with “greener” oxidants, such as hydro-
gen peroxide [4,5]. For instance simple zinc salts unmodified or
modified by straightforward ligands were demonstrated as useful
precatalysts in several oxidation reactions e.g. oxidation of alcohols, ox-
idative esterification, oxidative amidation, etc. [6]. Recently, some of us
reported on the oxidation as well as oxidative esterification of alcohols
to form the corresponding aldehydes or esters applying hydrogen per-
oxide as terminal oxidant (Scheme 1) [7]. Interestingly, with catalytic
amounts of zinc(II) bromide and pyridines substituted with carboxylic
acid functions an active catalyst is formed in situ, which allows the syn-
thesis of a variety of products under mild reaction conditions. Moreover,
the addition of trifluoroacetic acid enhanced the activity of the system.
In contrast to that, the composition of the precatalysts/catalysts is cur-
rently unknown. However, for a better understanding and future im-
provements of this type of zinc catalysis such information can be
helpful. In this regard, we report herein on the isolation, characteriza-
tion and application of potential zinc precatalysts in oxidation
chemistry.
Initially, an aqueous solution of zinc(II) bromide was mixed with
an aqueous solution of 2-picolinic acid (2) at room temperature
(Scheme 2). After several hours colorless crystals suitable for X-ray
measurements were obtained at room temperature in 64% yield. The
solid-state structure of complex 4 [Zn(2-H)(2)Br] has been character-
ized by single-crystal X-ray diffraction analysis. Thermal ellipsoid
plots, selected bond lengths and angles are shown in Fig. 1. The zinc
complex 4 showed a square-pyramidal geometry. In more detail, the
pentacoordinated zinc is coordinated by one 2-picolinate ligand
and one 2-picolinic acid ligand binding in a bidentate fashion [8]. The
N- and O-coordination created two five-membered rings with the zinc
⁎
Corresponding authors.
E-mail addresses: stephan.enthaler@tu-berlin.de (S. Enthaler),
xiao-feng.wu@catalysis.de (X.-F. Wu).
Scheme 1. Oxidation of benzyl alcohol with in situ formation of the catalyst.
http://dx.doi.org/10.1016/j.inoche.2014.06.020
1387-7003/© 2014 Elsevier B.V. All rights reserved.

S. Enthaler et al. / Inorganic Chemistry Communications 46 (2014) 320–323
321
and with the N-atoms as well as the O-atoms in trans-position. More-
over, the vertex of the square pyramid is occupied by a bromide ligand.
A similar geometry was reported for the isomorphic chloro complex
[Zn(2-H)(2)Cl] [9]. Moreover, the proton signals of the ligand backbone
were shifted as noticed by 1H NMR measurements in comparison to the
uncoordinated 2-picolinic acid (2). Noteworthy, the proton of the acid
function of the 2-picolinic acid ligand was not observed in the 1H
NMR, probably because of exchange reaction with the solvent D2O.
However, in accordance to the previously reported [Zn(2-H)(2)Cl] in
IR investigations the presence of COO− and COOH functions were
detected.
After isolation and characterization of the complex 4 its reactivity
was studied with regard to oxidation chemistry (Scheme 2). In a first
experiment an aqueous solution of complex 4 was treated with an ex-
cess of hydrogen peroxide (5.0 equiv., 30 wt.% in water) at room tem-
perature. The clear solution was stored at 6 °C and after several days
colorless crystals were formed, which were suitable for single-crystal
X-ray diffraction analysis. Noteworthy, gas evolution at the crystal sur-
face was observed, which probably revealed the decomposition of hy-
drogen peroxide. Thermal ellipsoid plots, selected bond lengths and
angles are shown in Fig. 2. In contrast to complex 4 both 2-picolinic
acid (2) ligands are coordinated in its 2-picolinate form [10,11]. In the
Fig. 1. Molecular structure of 4. Thermal ellipsoids are drawn at the 50% probability level.
Selected bond lengths [Å] and angles [°] of 4: Br\Zn: 2.3603(4), Zn\N1: 2.0590(18),
Zn\N2: 2.0575(18), Zn\O1: 2.1333(18), Zn\O2: 2.1418(18), N2\Zn\N1: 126.55(7),
N2\Zn\O1: 89.61(7), N1\Zn\O1: 78.51(6), N2\Zn\O2: 78.48(6), N1\Zn\O2:
89.53(7), O1\Zn\O2: 153.39(7), N2\Zn\Br: 116.72(5), N1\Zn\Br: 116.72(5),
O1\Zn\Br: 103.30(5), O2\Zn\Br: 103.31(5).
Scheme 2. Synthesis of zinc complexes and reactivity study.

322
S. Enthaler et al. / Inorganic Chemistry Communications 46 (2014) 320–323
Table 1
Comparative study of in situ catalyst formation and isolated precatalysts 4 and 5.
Entry
Complex [mol%]
Additive
Conv. [%]
Yield [%]
1
2
3
4
5
6
7
8
ZnBr2 [10]
ZnBr2 [10]
–
2
38
b1
b1
b1
50
50
52
50
b1
20
N99
b1
21
N99
80
88
N99
90
55
68
70
69
b1
30
b1
b1
b1
47
45
49
48
b1
17
88
b1
16
90
66
65
69
75
47
31
56
59
75
32
35
TFA [0.1 mL]
TFA [0.1 mL]
–
2 [10]
2 [10]
ZnBr2 [10]/2 [10]
ZnBr2 [10]/2 [10]
–
TFA [0.1 mL]
–
TFA [0.1 mL]
–
ZnBr2 [10]/2 [20]
9
ZnBr2 [10]/2 [20]
4 [10]
4 [10]
4 [10]
5 [10]
5 [10]
5 [10]
5 [5]
5 [2.5]
5 [10]
5 [10]
–
TFA [0.1 mL]
–
Fig. 2. Molecular structure of 5. Thermal ellipsoids are drawn at the 50% probability
level. Solvent molecules are omitted for clarity. Selected bond lengths [Å] and angles
[°] of 5: Zn(1)\O(2): 2.0781(13), Zn(1)\N(1): 2.1195(17), Zn(1)\O(3): 2.1575(14),
O(2)#1\Zn(1)\O(2): 180.00(11), O(2)#1\Zn(1)\N(1): 100.43(6), O(2)\Zn(1)\
N(1): 79.57(6), O(2)\Zn(1)\O(3)#1: 90.98(6), O(2)\Zn(1)\O(3): 89.02(6), N(1)\
Zn(1)\O(3): 95.66(6).
10
11
12
13
14
15
16
17
18
20
21
22
23
24
25a
26b
27b
TFA [0.1 mL]
TFA [0.1 mL]/HBr [0.1 mL]
–
TFA [0.1 mL]
TFA [0.1 mL]/HBr [0.1 mL]
TFA [0.1 mL]/HBr [0.1 mL]
TFA [0.1 mL]/HBr [0.1 mL]
TFA [0.5 mL]/HBr [0.1 mL]
HBr [0.1 mL]
octahedral zinc complex the 2-picolinate ligands are in a plane and the
N- and O-coordination created two five-membered rings with the zinc
and with the N-atoms as well as the O-atoms in trans-position. The
axial positions are occupied by two water molecules. Interestingly, the
same complex 5 was formed and isolated in 77% yield or 31% yield, re-
spectively when complex 4 was dissolved in water and benzyl alcohol
(5.0 equiv.) or benzyl alcohol [12] (5.0 equiv.) and hydrogen peroxide
(5.0 equiv., 30 wt.% in water) [13]. In the initial work on the in situ for-
mation of the catalyst the addition of trifluoroacetic acid was beneficial
to improve the yield of the products. To study the influence of
trifluoroacetic acid on complex 4, an excess of trifluoroacetic acid (5.0
equiv.) was added at room temperature to an aqueous solution of 4.
Again complex 5 was isolated from the reaction mixture as colorless
crystals in 62% yield. In accordance to Vargová et al. the complex 5 is
also accessible by the reaction zinc(II) trifluoroacetate with two equiv-
alents of 2-picolinic acid (2) in water (Scheme 2). Moreover, the reac-
tion of complex 4 with silver trifluoroacetate revealed the formation
of the octahedral complex 5 (Scheme 2). With complex 5 in hand the re-
activity against hydrogen peroxide, benzyl alcohol and hydrogen perox-
ide/benzyl alcohol was studied. However, in all cases complex 5 was
isolated and confirmed by single-crystal X-ray diffraction analysis
(Scheme 2).
After isolation, characterization and reactivity study of the zinc
complexes 4 and 5 the catalytic abilities were examined in the oxidation
of benzyl alcohol 1 with hydrogen peroxide (Table 1). Initially,
unmodified zinc(II) bromide was tested in catalytic amounts (Table 1,
entry 1). However, no product formation was observed. Addition of
trifluoroacetic acid (1.14 equiv.) to the reaction mixture revealed the
formation of benzaldehyde in 30% yield (Table 1, entry 2). In contrast
to that, the zinc(II) bromide was necessary, while in its absence no
product was formed (Table 1, entry 3). On the other hand the need for
zinc(II) bromide was proven by the zinc-free system ligand 2 and ligand
2/TFA (Table 1, entries 4 and 5). An increase of activity was observed
for the in situ combination of zinc(II) bromide and ligand 2 (Table 1,
entry 6). Interestingly, addition of TFA showed no positive effect on
the yield of the product (Table 1, entry 7). Moreover, the increase of
the ratio zinc:2 from 1:1 to 1:2, which is the ratio found in the isolated
complexes 4 and 5, revealed no increase of the yield (Table 1, entries 8
and 9). Next, the abilities of the isolated complex 4 were studied. With-
out additives no product was observed, while in the presence of TFA a
yield of 17% was observed, which is significant to the in situ system
(Table 1, entry 11). In case of the in situ system 20 mol% HBr is formed
as side product from the reaction of zinc(II) bromide with ligand 2.
Hence TFA and HBr were added to the isolated complex 4. Interestingly,
full conversion was attained and benzaldehyde was formed in 88% yield
(Table 1, entry 12). Moreover, the isolated complex 5 was tested as
HBr [0.1 mL]
–
HBr [0.1 mL]/TFA [0.1 mL]
HBr [20 mol%]
HBr [20 mol%]/TFA [0.1 mL]
TFA [0.1 mL]/HBr [0.1 mL]
TFA [0.1 mL]
–
–
5 [10]
4 [10]
5 [10]
89
51
48
TFA [0.1 mL]
Reaction conditions: Benzyl alcohol (1 mmol), THF (2 mL), H2O2 (4 mmol, 30 wt.%
in H2O), r.t., air, 6 h. Conversions and yields were determined by GC using hexadecane
as internal standard.
a
2 mmol H2O2.
24 h.
b
precatalyst. Similar to complex 4 no reactivity was monitored in the ab-
sence of any additive, while the addition of TFA resulted in a similar
yield as found for complex 4 (Table 1, entries 13 and 14). Again the ad-
dition of HBr improved the yield significantly to 90% (Table 1 entry 15).
Reducing the loading of the zinc complex 5 to 2.5 mol% reduced the
yield to 65% (Table 1, entries 16 and 17). In addition the yield was re-
duced to 75% in the absence of TFA (Table 1, entry 20). Importantly, it
was observed that the reaction also works in the absence of any zinc
complex with catalytic amounts of HBr (Table 1, entry 23). Comparing
the results with and without catalytic amounts of zinc complexes indi-
cates that the main product formation is caused by the HBr and only a
minor part is produced by the zinc complexes. Based on the stoichio-
metric experiments figured out in Scheme 2, as key complex for the
zinc-catalyzed oxidation complex 5 can be envisaged. The dissociation
of water molecules in the axial position and coordination of substrate
molecules can be assumed. In addition, hydrogen peroxide can be acti-
vated by trifluoroacetic acid to form peroxytrifluoroacetic acid [14],
which reacts with the activated substrate to form benzaldehyde,
trifluoroacetic acid and water. Moreover, a zinc complex is formed
with an open coordination site, at which the next substrate molecule
can be activated.
In summary, we shed light on an in situ generated catalyst composed
of zinc(II) bromide and pyridines substituted with carboxylic acid func-
tions, which has been recently applied in the oxidation of alcohols to al-
dehydes with hydrogen peroxide as terminal oxidant. Two zinc
complexes [Zn(2-H)(2)Br] and [Zn(2-H)2(H2O)2] were isolated and
characterized. Both complexes showed catalytic activity in the oxidation
of benzyl alcohol to benzaldehyde. Importantly, a lower activity was ob-
served compared to the in situ system. Several experiments were per-
formed to understand the different reactivity and it was found that a
significant effect on oxidation processes is revealed by the HBr, which
is produced as side product in the in situ complex formation.

S. Enthaler et al. / Inorganic Chemistry Communications 46 (2014) 320–323
(g) E. Nakamura, N. Yoshikai, J. Org. Chem. 75 (2010) 6061–6067;
323
Acknowledgments
(h) C.-L. Sun, B.-J. Li, Z.-J. Shi, Chem. Rev. 111 (2011) 1293–1314.
[6] (a) X.-F. Wu, Tetrahedron Lett. 53 (2012) 4328–4331;
(b) X.-F. Wu, Tetrahedron Lett. 53 (2012) 3397–3399;
(c) X.-F. Wu, Tetrahedron Lett. 53 (2012) 4328–4331;
Financial support from the Cluster of Excellence “Unifying Concepts
in Catalysis” (funded by the Deutsche Forschungsgemeinschaft EXC
314/1 and administered by the Technische Universität Berlin) is grate-
fully acknowledged. Furthermore, S. Conway and Prof. Dr. A. L. Spek
are thanked for helpful discussions.
(d) Z.-Z. Song, J.-L. Gong, M. Zhang, X.-F. Wu, Asian J. Org. Chem. 1 (2012) 214–217;
(e) X.-F. Wu, Tetrahedron Lett. 53 (2012) 6123–6126;
(f) X.-F. Wu, C.B. Bheeter, H. Neumann, P.H. Dixneuf, M. Beller, Chem. Commun. 48
(2012) 12237–12239;
(g) M. Zhang, X.-F. Wu, Tetrahedron Lett. 54 (2013) 1059–1062;
(h) X.-F. Wu, M. Sharif, A. Pews-Davtyan, P. Langer, K. Ayub, M. Beller, Eur. J. Org.
Chem. (2013) 2783–2787;
Appendix A. Supplementary material
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[7] X.-F. Wu, Chem. Eur. J. 18 (2012) 8912–8915.
Supplementary data to this article can be found online at http://dx.
doi.org/10.1016/j.inoche.2014.06.020. These data include MOL files
and InChiKeys of the most important compounds described in this
article.
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