Fluorinated triazapentadienyl ligand supported ethyl zinc(ii) complexes: Reaction with dioxygen and catalytic applications in the Tishchenko reaction

Article abstract of DOI:10.1039/c6dt00257a

Ethyl zinc complexes [N{(C₃F₇)C(Dipp)N}₂]ZnEt, [N{(C₃F₇)C(Cy)N}₂]ZnEt, [N{(CF₃)C(2,4,6-Br₃C₆H₂)N}₂]ZnEt and [N{(C₃F₇)C(2,6-Cl₂C₆H₃)N}₂]ZnEt have been synthesized from the corresponding 1,3,5-triazapentadiene and diethyl zinc. X-ray data show that [N{(C₃F₇)C(Dipp)N}₂]ZnEt has a distorted trigonal planar geometry at the zinc center. The triazapentadienyl ligand binds to zinc in a κ²-mode. The zinc-ethyl bonds of [N{(C₃F₇)C(Dipp)N}₂]ZnEt, [N{(C₃F₇)C(Cy)N}₂]ZnEt, [N{(CF₃)C(2,4,6-Br₃C₆H₂)N}₂]ZnEt and [N{(C₃F₇)C(2,6-Cl₂C₆H₃)N}₂]ZnEt readily undergo oxygen insertion upon exposure to dry air to produce the corresponding zinc-ethoxy or zinc-ethylperoxy compounds. The ethoxy zinc adducts {[N{(CF₃)C(2,4,6-Br₃C₆H₂)N}₂]ZnOEt}₂ and {[N{(C₃F₇)C(2,6-Cl₂C₆H₃)N}₂]ZnOEt}₂ as well as the ethylperoxy zinc adduct {[N{(C₃F₇)C(Cy)N}₂]ZnOOEt}₂ have been isolated and fully characterized by several methods including X-ray crystallography. They feature dinuclear structures with four-coordinate zinc sites and bridging-ethoxy or -ethylperoxy groups. The ethyl zinc complexes catalyze the Tishchenko reaction of benzaldehyde under solventless conditions affording benzyl benzoate. The reaction of ethyl zinc complexes with dioxygen and their catalytic behaviour in the Tishchenko reaction are affected by the electronic and steric factors of the triazapentadienyl ligand. {[N{(C₃F₇)C(Cy)N}₂]ZnOOEt}₂ is an excellent reagent for the epoxidation of trans-chalcone.

Full text of DOI:10.1039/c6dt00257a

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Page 1 of 32
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DOI: 10.1039/C6DT00257A
Fluorinated triazapentadienyl ligand supported ethyl zinc(II)
complexes: reaction with dioxygen and catalytic applications in
the Tishchenko reaction
Naveen V. Kulkarni, Animesh Das, Shawn G. Ridlen, Erin Maxfield, Venkata A. K. Adiraju,
Muhammed Yousufuddin, H. V. Rasika Dias*
Department of Chemistry and Biochemistry, The University of Texas at Arlington, Arlington,
Texas 76019, USA
TOC Entry
Ethyl, ethoxide and ethylperoxide zinc(II) complexes featuring polyhalogenated 1,3,5-
triazapentadienyl ligands and their chemistry are reported.
1

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DOI: 10.1039/C6DT00257A
Abstract
Ethyl
zinc
complexes
[N{(C3F7)C(Dipp)N}2]ZnEt,
[N{(C3F7)C(Cy)N}2]ZnEt,
[N{(CF3)C(2,4,6-Br3C6H2)N}2]ZnEt and [N{(C3F7)C(2,6-Cl2C6H3)N}2]ZnEt have been
synthesized from the corresponding 1,3,5-triazapentadiene and diethyl zinc. X-ray data show
that [N{(C3F7)C(Dipp)N}2]ZnEt has a distorted trigonal planar geometry at the zinc center.
The triazapentadienyl ligand binds to zinc in a κ2-mode. The zinc-ethyl bonds of
[N{(C3F7)C(Dipp)N}2]ZnEt,
[N{(C3F7)C(Cy)N}2]ZnEt,
[N{(CF3)C(2,4,6-
Br3C6H2)N}2]ZnEt and [N{(C3F7)C(2,6-Cl2C6H3)N}2]ZnEt readily undergo oxygen insertion
upon exposure to dry air to produce the corresponding zinc-ethoxy or zinc-ethylperoxy
compounds.
The ethoxy zinc adducts {[N{(CF3)C(2,4,6-Br3C6H2)N}2]ZnOEt}2 and
{[N{(C3F7)C(2,6-Cl2C6H3)N}2]ZnOEt}2 as well as the ethylperoxy zinc adduct
{[N{(C3F7)C(Cy)N}2]ZnOOEt}2 have been isolated and fully characterized by several
methods including X-ray crystallography. They feature dinuclear structures with four-
coordinate zinc sites and bridging -ethoxy or –ethylperoxy groups. The ethyl zinc complexes
catalyze the Tishchenko reaction of benzaldehyde under solventless conditions affording
benzyl benzoate. The reaction of ethyl zinc complexes with dioxygen and their catalytic
behaviour in the Tishchenko reaction are affected by the electronic and steric factors of the
triazapentadienyl ligand. {[N{(C3F7)C(Cy)N}2]ZnOOEt}2 is an excellent reagent for the
epoxidation of trans-chalcone.
Keywords: Triazapentadiene, zinc, alkoxy, Tishchenko reaction, catalysis.
2

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DOI: 10.1039/C6DT00257A
Introduction
1,3,5-Triazapentadienyl systems represent an important class of monoanionic, nitrogen-based
ligands.1 Despite the similarity to the widely used 1,5-diazapentadienyl (-diketiminate)
ligands (Figure 1),2, 3 they have not received the same attention as metal ion chelators.1, 4-27
1,3,5-Triazapentadienyl ligands offer a great deal of tunability in their steric and electronic
properties. One major area of research focus in our laboratory has been the chemistry of
highly fluorinated metal ion chelators28-30 including polyfluorinated triazapentadienyl ligands.
Such ligands are attractive because some of their metal adducts display interesting reactivity
and notable improvement in the thermal stability, oxidative resistance, improved volatility
and fluorocarbon solubility.9, 31-34
Figure 1. 1,3,5-Triazapentadiene and 1,5-diazapentadiene skeletons (R, R’ = various
substituents like alkyl or aryl groups) and sketch of [N{(C3F7)C(Dipp)N}2]H and
[N{(C3F7)C(2,6-Cl2C6H3)N}2]H.
We have reported the synthesis of several fluorinated 1,3,5-triazapentadienyl ligands
such as [N{(C3F7)C(Dipp)N}2], [N{(C3F7)C(Mes)N}2], [N{(C3F7)C(2,6-Cl2C6H3)N}2]
and [N{(C3F7)C(C6F5)N}2] (Dipp = 2,6-(i-Pr)2C6H3; Mes = 2,4,6-(Me)3C6H2) and their
utility in the isolation of metal adducts with interesting bonding and structural features.7-9, 17, 31,
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33, 35-40 For example, silver(I) isocyanide adduct t-BuNCAg[N{(C3F7)C(Dipp)N}2]DfOeIa: 1t0u.1r0e3Vs9ie/wC6A1Dr-tiTcl0e0O2n5li7nAe
bonded triazapentadienyl ligand linked to silver via the central nitrogen atom whereas the
triphenyl phosphine complex [N{(C3F7)C(C6F5)N}2]Ag(PPh3)2 has
a
κ1-bonded
triazapentadienyl ligand attached to the metal fragment via one of the terminal nitrogen
atoms.8, 39 The gold(I) alkene and alkyne adducts [N{(C3F7)C(2,6-Cl2C6H3)N}2]Au(C2H4)33
and [N{(C3F7)C(Dipp)N}2]Au(EtC≡CEt)31 feature 2-bonded ligand with a U-shaped
triazapentadienyl backbones. Tl[N{(C3F7)C(Dipp)N}2] is a mono-coordinate thallium
adduct with a W-shaped triazapentadienyl backbone.17 Some metal complexes of these
fluorinated triazapentadienyl ligands exhibit useful catalytic33, 38 and biological activities.37
In this article, we describe the synthesis of first mono(1,3,5-triazapentadienyl) zinc(II)
complexes of these fluorinated ligands and some of their chemistry. In particular, we report
the isolation of ethyl zinc adducts supported by fluorinated triazapentadienyl ligands, their
reactivity towards oxygen leading to ethoxide and ethylperoxide products, and the catalytic
applications in Tishchenko reaction, which involves the dimerization of aldehydes to give
esters. We also show the oxygen atom transfer chemistry of triazapentadienyl ligand
supported zinc ethylperoxide leading to epoxides from olefins.
Results and discussion
The fluoroalkyl substituted triazapentadienes were obtained in excellent yield from the
reaction of the perfluoro-aza-alkene with the appropriate amine in ether as depicted in Figure
2.5, 7-9 The syntheses of [N{(C3F7)C(Dipp)N}2]H8, 9 and [N{(C3F7)C(2,6-Cl2C6H3)N}2]H33
have been reported while [N{(C3F7)C(Cy)N}2]H and [N{(CF3)C(2,4,6-Br3C6H2)N}2]H were
obtained from the reaction of CyNH2 (cyclohexyl amine) with perfluoro-5-aza-4-nonene5, 6, 41
or 2,4,6-Br3C6H2NH2 with perfluoro-3-aza-2-pentene,42 respectively. Triazapentadienes
[N{(C3F7)C(Cy)N}2]H and [N{(CF3)C(2,4,6-Br3C6H2)N}2]H were characterized by the
4

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solution NMR spectroscopic studies and elemental analyses. These compounds exhibit
DOI: 10.1039/C6DT00257A
1
somewhat complicated H, 19F and 13C NMR spectra due to the presence of different
conformational isomers and/or tautomers in solution that are close in energy as noted
previously for the related systems.5, 6, 8
Figure 2. Synthetic route to triazapentadienes and ethylzinc adducts of triazapentadienyl
ligands.
The ethyl zinc adducts 1-4 (Figure 2) were prepared by heating (~ 80 oC) a mixture of
diethyl zinc and the corresponding 1,3,5-triazapentadiene [N{(C3F7)C(Dipp)N}2]H,
[N{(C3F7)C(Cy)N}2]H, [N{(C3F7)C(2,6-Cl2C6H3)N}2]H or [N{(CF3)C(2,4,6-Br3C6H2)N}2]H
in toluene.
They were obtained in 82-85% yield. The zinc(II) complexes
[N{(C3F7)C(Ph)N}2]2Zn and [N{(C3F7)C(H)N}2]2Zn featuring fluorinated triazapentadienyl
5
ligands have been reported but they are bis(1,3,5-triazapentadienyl)zinc adducts.4,
1
Furthermore, no structural data are available for these adducts. The H NMR spectrum of
[N{(C3F7)C(Dipp)N}2]ZnEt (1) in C6D6 displays two doublets for CH(CH3)2 and a septet for
CH(CH3)2 of the isopropyl moieties and a triplet and quartet for the ethyl group. The two
doublets for methyl groups of isopropyl moieties are a result of restricted rotation of the aryl
and isopropyl groups creating two environments for methyl groups with one set directed at
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the zinc atom side while second set pointing towards the ligand backbone (see Figure 3). In
DOI: 10.1039/C6DT00257A
the 19F NMR spectrum, only three sets of signals were observed for the two C3F7- groups
indicating the C2-symmetric nature of the triazapentadienyl fragment.
X-ray quality crystals of [N{(C3F7)C(Dipp)N}2]ZnEt (1) were obtained from a hexane
o
solution at -10 C and the structure is illustrated in Figure 3 (Crystal data are given in Table
1). It reveals a three-coordinate, distorted trigonal planar geometry at the zinc center (with C-
Zn-N angles of 133.69(6)o, 133.94(6)o and a 92.37(5)o N-Zn-N angle). The Y-shaped bonding
at zinc is similar to those previously reported for copper and silver complexes like
[N{(C3F7)C(Dipp)N}2]CuCO and [N{(C3F7)C(Dipp)N}2]AgPPh3 involving the same
ligand.8, 9 The U-shaped ligand backbone is essentially planar and the aromatic rings of
“Dipp” moieties are arranged nearly perpendicular to the triazapentadienyl plane (the
aromatic rings are twisted by about 90o from the NCNCN plane). The triazapentadienyl
ligand binds to the zinc center in a κ2-mode and the two Zn-N bond lengths are nearly the
same (Zn-N = 1.9931(13), 1.9855(13) Å). These bond distances are comparable to the
reported Zn-N bond lengths of four-coordinate, triazapentadiene zinc(II) complexes such as
[HN{(Ph)C(Ph)N}2]ZnCl243 but are slightly longer than the Zn-N bonds of a related three-
coordinate diazapentadienyl ethyl zinc complex [HC{(CH3)C(Dipp)N}2]ZnEt (Zn-N =
1.979(3) and 1.943(3) Å).44 Note that [HC{(CH3)C(Dipp)N}2]ZnEt has a relatively electron
rich and better donating supporting ligand than the fluorinated [N{(C3F7)C(Dipp)N}2]. The
Zn-C bond distance of [HC{(CH3)C(Dipp)N}2]ZnEt (1.963(5) Å) is however similar to that
of [N{(C3F7)C(Dipp)N}2]ZnEt (1.9515(16) Å).
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DOI: 10.1039/C6DT00257A
Figure 3. ORTEP diagram of [N{(C3F7)C(Dipp)N}2]ZnEt (1). Selected bond lengths (Å) and
angles (o): Zn-N(1) =1.9931(13), Zn-N(3) = 1.9855(13), Zn-C(33) = 1.9515(16), N(1)-C(1) =
1.301(2), N(2)-C(1) = 1.340(2), N(2)-C(2) = 1.334(2), N(3)-C(2) = 1.306(2), C(1)-C(3) =
1.541(2), C(2)-C(6) = 1.551(2) Å; C(34)-C(33)-Zn = 115.93(12), N(1)-C(1)-N(2) =
128.59(14), C(33)-Zn-(N1) = 133.69(6), C(33)-Zn-(N3) = 133.94(6), N(1)-Zn-N(3) =
92.37(5), C(1)-N(2)-C(2) = 124.99(13)o.
Ethyl zinc complexes 2-4 were characterized by NMR spectroscopy and elemental
analyses. The data suggest the formation [N{(C3F7)C(Cy)N}2]ZnEt (2), [N{(CF3)C(2,4,6-
Br3C6H2)N}2]ZnEt (3) and [N{(C3F7)C(2,6-Cl2C6H3)N}2]ZnEt (4) featuring rather symmetric
2-bound triazapentadienyl ligands as observed in [N{(C3F7)C(Dipp)N}2]ZnEt (1). For
example, the solution NMR data of [N{(C3F7)C(Cy)N}2]ZnEt (2) in C6D6 at room
temperature demonstrate the presence triazapentadienyl and ethyl groups at 1:1 ratio. The
19
presence of only three set of signals for the two C3F7- groups in the F NMR spectrum of 2
1
indicates a C2-symmetric bonding mode of the ligand.8, 31 The H NMR chemical shift value
of methylene protons of the Zn-CH2CH3 moiety in complex 2, which bears N-cyclohexyl
substituents on the triazapentadienyl ligand is slightly higher (δ 0.55 ppm) compared to the
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corresponding resonance in complexes 1, 3 and 4 (δ 0.13, 0.14 and 0.17 ppm, rDeOspI: 1e0c.1t0i3v9e/Cly6D)T00257A
which have aromatic substituents on the nitrogen. The relative upfield shift of the latter
group of signals is probably a result of ring-current effects of flanking arenes. For
comparison, the corresponding signal in [HC{(CH3)C(Dipp)N}2]ZnEt appears at δ 0.24
ppm.44
Figure 4. Synthesis of zinc(II) ethoxide and ethylperoxide adducts from the corresponding
ethyl zinc adducts.
These triazapentadienyl ethyl zinc complexes are sensitive to air and moisture even in
the solid form and gradually decompose upon exposure for longer time. Note that reactivity
of alkyl zinc compounds with oxygen is not that straight forward and there are even air stable
zinc ethyl adducts like [HB(3,5-(CF3)2Pz)3]ZnEt and [HB(3-(t-Bu)Pz)3]ZnEt in the
literature.45, 46 The reaction of triazapentadienyl ethyl zinc complexes 2-4 with dry air under
carefully controlled conditions leads to isolable zinc-ethoxide or ethylperoxide derivatives.
For example, when toluene solutions of [N{(CF3)C(2,4,6-Br3C6H2)N}2]ZnEt (3) and
[N{(C3F7)C(2,6-Cl2C6H3)N}2]ZnEt (4) were exposed to an excess of dry air at room
o
o
temperature (20 C) for 5 min, and then stored at -20 C overnight, colourless crystalline
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precipitates of the corresponding alkoxide {[N{(CF3)C(2,4,6-Br3C6H2)N}2]Zn(μ-OEt)}2 (5)
DOI: 10.1039/C6DT00257A
and {[N{(C3F7)C(2,6-Cl2C6H3)N}2]Zn(μ-OEt)}2 (6) were obtained in about 70% yield
(Figure 4).
Interestingly, the [N{(C3F7)C(Cy)N}2]ZnEt (2) adduct, in contrast, produced a rare,
isolable, zinc-peroxide complex {[N{(C3F7)C(Cy)N}2]Zn(μ-OOEt)}2 (7) under the same
conditions (Figure 4). On the other hand, when
a
toluene solution of
[N{(C3F7)C(Dipp)N}2]ZnEt (1) was exposed to air for a short period of time and then stored
o
1
at -20 C, a mixture of compounds were obtained as indicated by the H NMR data of the
reaction mixture. We were not been able to isolate analytically pure -ethoxy or –ethylperoxy
adducts cleanly thus far from this reaction. One of the reaction mixtures, however produced
a crystalline product in low yield, which is a trinuclear species {[N{(C3F7)C(Dipp)N}2]Zn(μ-
OEt)(μ-OOEt)}2Zn containing both -ethoxy and –ethylperoxy moieties (see supporting
information for X-ray structure, Figure S1). In all the cases, the air exposure time (should not
exceed 5 min) and the storage temperature (-20 ºC) were found to be critical to obtain zinc-
ethoxide or ethylperoxide adducts in good yield. Longer period of air exposure and room
temperature storage leads to the decomposition of the complexes.47
The formation of zinc-ethoxy vs zinc-ethylperoxy compound upon oxygen insertion is
perhaps governed by the ligand effects. The [N{(C3F7)C(Cy)N}2] which bears electron
donating groups on the nitrogen atoms is a better donor and might be capable of stabilizing
the
zinc-ethylperoxy
complex
7
formed.
Whereas
[N{(CF3)C(2,4,6-
Br3C6H2)N}2]and[N{(C3F7)C(2,6-Cl2C6H3)N}2] are relatively weaker donors which drive
the reaction towards zinc-ethoxy complexes 5 and 6. Similar observations are made by
Lewinski et al48 where they have systematically studied the reactions of zinc dialkyls with
dioxygen in the presence of different donor ligands. The strong N-donor ligand 4-
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methylpyridine is found to stabilize the alkylperoxide moiety, while THF, a weaker donor,
DOI: 10.1039/C6DT00257A
leads to an alkoxide compound.
The 1H NMR spectroscopy is very useful in determining the oxygen insertion into the
Zn-Et bond. The signal due to zinc-bound methylene protons (Zn-CH2CH3) in complexes 3-4
which appears in the range of δ 0.14-0.17 ppm (in C6D6) disappears upon exposure to oxygen
containing air. A new signal was observed around δ 3.8 ppm (in DMSO-d6; these ethoxide
compounds are not very soluble in C6D6) which is attributable to the -OCH2-CH3 moiety,
indicating complete oxygenation of the Zn-C bond.
Figure 5. ORTEP diagram of {[N{(CF3)C(2,4,6-Br3C6H2)N}2]Zn(μ-OEt)}2 (5). Selected
bond lengths (Å) and angles (o): Zn-N(1) = 2.041(6), Zn-N(3) = 2.030(6), Zn-O = 1.929(5),
Zn-O1 = 1.969(5), N(3)-C(1) = 1.305(9), N(1)-C(2) = 1.297(10), N(2)-C(2) = 1.339(10),
N(2)-C(1) = 1.337(10), O-C(18) = 1.430(11), Zn•••Zn1 = 2.9222(16) Å; O-Zn-O1 = 82.9(2),
Zn-O-Zn1 = 97.1(2), N(1)-Zn-O = 118.1(3), N(1)-Zn-O1 = 120.3(2), N(3)-Zn-O = 125.9(2),
N(3)-Zn-O1 = 118.5(2), N(1)-Zn-N(3) = 94.1(2), N(1)-C(2)-N(2) = 129.4(7), N(2)-C(1)-N(3)
= 130.2(7)º.
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Figure 6. ORTEP diagram of {[N{(C3F7)C(2,6-Cl2C6H3)N}2]Zn(μ-OEt)}2 (6). Selected bond
lengths (Å) and angles (o): Zn-N(1) = 2.021(3), Zn-N(3) = 2.030(3), Zn-O = 1.929(3), Zn-O1
= 1.956(3), N(3)-C(2) = 1.307(5), N(1)-C(1) = 1.301(5), N(2)-C(2) = 1.334(5), N(2)-C(1) =
1.340(5), O-C(21) = 1.431(5), Zn•••Zn1 = 2.9267(8) Å; O-Zn-O1 = 82.25(12), Zn-O-Zn1 =
97.75(12), N(1)-Zn-O = 111.32(13), N(3)-Zn-O = 135.22(13), N(1)-Zn-O1 = 122.29(13),
N(3)-Zn-O1 = 116.93(12), N(1)-Zn-N(3) = 92.58(12), N(1)-C(1)-N(2) = 127.6(4), N(2)-C(2)-
N(3) = 129.3(3)o.
The molecular structures of the zinc-ethoxy complexes {[N{(CF3)C(2,4,6-
Br3C6H2)N}2]Zn(μ-OEt)}2 and {[N{(C3F7)C(2,6-Cl2C6H3)N}2]Zn(μ-OEt)}2 are illustrated in
Figures 5 and 6, respectively. They show that {[N{(CF3)C(2,4,6-Br3C6H2)N}2]Zn(μ-OEt)}2
(5) and {[N{(C3F7)C(2,6-Cl2C6H3)N}2]Zn(μ-OEt)}2 (6) exist as dinuclear aggregates of
triazapentadienyl zinc-ethoxy units linked by the μ2-type bridging of ethoxide groups. These
molecules sit on crystallographically imposed centers of inversion. The central [Zn2N4O2]
core in both complexes is similar to each other and those observed for the related structurally
49-59
authenticated diazapentadienyl zinc alkoxides44,
such as {[HC{(CH3)C(2,6-
Et2C6H3)N}2]Zn(μ-OMe)}2 and {[HC{(CH3)C(Dipp)N}2]Zn(μ-OiPr)}2.44, 49 Zinc atoms
adopt distorted tetrahedral geometry. The U-shaped N2C3-ligand backbone is relatively flat
and the aromatic rings on nitrogen atoms are arranged nearly perpendicular to the
triazapentadienyl plane. The two zinc atoms are slightly displaced from the triazapentadienyl
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ligand planes but are co-planar with each-other and with the two bridging oxygen atoms.ViTewhAerticle Online
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Zn•••Zn separation is similar in 5 and 6 (2.9222(16) and 2.9267(8) Å, respectively). The two
Zn-O bond lengths are slightly different both in 5 and 6 (e.g., 1.929(5) and 1.969(5) Å in 5
and 1.929(3) and 1.956(3) Å in 6) but fall in the typical range observed in other systems (e.g.,
Zn-O bond lengths of {[HC{(CH3)C(2,6-Et2C6H3)N}2]Zn(μ-OMe)}2 are 1.931(2) and
1.984(2) Å).44, 51
Figure 7. ORTEP diagram of {[N{(C3F7)C(Cy)N}2]Zn(μ-OOEt)}2 (right) and a view
showing the [ZnOOEt]2 core (left). Representative bond lengths (Å) and angles (o): Zn-N(1)
= 2.004(2), Zn-N(3) = 1.999(2), Zn-O(1)1 = 2.0487(18), Zn-O(1) = 1.9570(17), N(3)-C(2) =
1.316(3), N(1)-C(1) = 1.307(3), N(2)-C(2) = 1.351(3), N(2)-C(1) = 1.357(3), O(1)-O(2) =
1.483(2), O(2)-C(21) = 1.421(3), Zn•••Zn1 = 3.0474(7)Å; O(1)-Zn-O(1)1 = 80.96(7), Zn-
O(1)-Zn1 = 99.04(7), N(1)-Zn-O(1) = 123.80(8), N(3)-Zn-O(1) = 136.02(8), N(1)-Zn-O(1)1 =
109.82(8), N(3)-Zn-O(1)1 = 115.14(8), N(1)-Zn-N(3) = 90.49(8), N(1)-C(1)-N(2) = 128.3(2),
N(2)-C(2)-N(3) = 128.8(2)º.
The IR spectrum (ATR-IR, solid powder) of ethylperoxy derivative
{[N{(C3F7)C(Cy)N}2]Zn(μ-OOEt)}2 (7) displays a weak absorption band at 853 cm-1 which
is attributable to the characteristic peroxidic ῡ(O-O) stretching vibration (for example, the
corresponding band for {[HC{(CH3)C(Dipp)N}2]Zn(μ-OOEt)}2 was observed at 854 cm-1).60
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The X-ray crystal structure of 7 is illustrated in Figure 7. Such isolable and well aDuOthI:e10n.1t0ic39a/tCe6dDT00257A
alkyl peroxides of zinc are relatively uncommon.47, 48, 60-68
The compound {[N{(C3F7)C(Cy)N}2]Zn(μ-OOEt)}2 (7) is a dinuclear species linked
by the μ2-ethylperoxy groups and sits on a crystallographically imposed center of inversion.
The central [Zn2O2] core is quite similar to the zinc-ethoxide complexes described above and
to the closely related alkylperoxide complex, {[HC{(CH3)C(Dipp)N}2]Zn(μ-OOEt)}2.60 The
Zn•••Zn separation (3.0474(7)Å) and the Zn-O bond lengths (2.0487(18), 1.9570(17) Å)
however, are relatively longer compared to the related distances of ethoxide compounds 5
and 6 but are comparable to the reported 1,5-diazapentadienyl zinc-ethylperoxy complex
{[HC{(CH3)C(Dipp)N}2]Zn(μ-OOEt)}2. The U-shaped, NCNCN ligand backbone remains
relatively planar, while the tetrahedral zinc atom sits above or below the ligand plane forming
an envelope-type shape for the ZnNCNCN metallacycle.
The Tishchenko reaction, which involves the dimerization of two aldehydes to form
the corresponding carboxylic ester, has been known for nearly a century,69 and of significant
interest to both the industrial and academic scientists due to its atom efficiency and utility.69,
70
During all these years, in addition to the traditional aluminium based catalysts, chemists
have investigated new catalytic systems such as those involving alkali71, 72 and alkaline earth
metals,73-75 lanthanides,76, 77 actinides78, 79 and d-block elements80-83 to mediate Tishchenko
chemistry. More recently, the zinc based catalysts have also gained attention for this
purpose.52, 73, 84 We found that the ethyl zinc complex [N{(C3F7)C(Dipp)N}2]ZnEt (1) is a
very effective catalyst in the dimerization of benzaldehyde, providing the corresponding ester
benzyl benzoate, in good yield (Table 2). The catalytic reaction was performed as a
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solventless process in neat aldehyde using 1:100 catalyst:aldehyde molar ratiDoOI(: 2105.10μ39m/Co6DlT00257A
catalyst: 2.3 mmol aldehyde) at 80 oC for 3 h. The progress of the reaction was monitored by
1H NMR spectroscopy. Gradual disappearance of the aldehyde proton signal (C(O)H; δ =
10.02 ppm) and the appearance of the CH2 proton resonance of the corresponding ester
product (PhC(O)OCH2Ph (δ = 5.38 ppm) was noted (in CDCl3). The NMR yield was
calculated by integrations of the above mentioned signals.79, 83 Benzyl benzoate was found to
be the exclusive product from the reaction mixture, as demonstrated by the NMR and GC/MS
analyses, while rest of the aldehyde remained unreacted. The temperature is an important
factor in the present catalytic reaction. When the same reaction (catalyst:aldehyde ratio =
o
1:100, reaction time = 3 h) was conducted at room temperature (20 C), no dimerization was
observed and the benzaldehyde remained essentially unreacted, even after extended periods
o
of reaction time (24 h). Subsequently, when the reaction temperature was increased to 50 C,
only about 4% of the product was obtained in 3 h. In contrast, when the reaction temperature
o
was increased to 80, 110 and 140 C, a significant increase in the aldehyde dimerization was
observed, providing products in 78, 98 and 100% yield, respectively (Table 2). This hike in
the catalytic activity observed at the higher temperatures (and inactivity or lower activity at
the lower temperatures) can be attributed to the high activation energy of the reaction. Longer
reaction time at 80 ºC (18-20 rather than 3 h) also leads to essentially complete conversion.
The catalytic properties of other ethyl zinc compounds (2-4) were also investigated at
80 oC under the similar conditions. The results (Table 2) show a discernible influence of the
steric and electronic features of the ligand skeleton on the catalytic ability of these
complexes. Noticeably, the other common zinc compounds such as ZnEt2, ZnCl2, Zn(OAc)2
o
and Zn(OTf)2 did not show significant activity at 80 C or higher temperatures (i.e., to about
140 ºC) under similar conditions (catalyst:benzaldehyde = 1:100; reaction time = 3 h).
Compared to [N{(C3F7)C(Dipp)N}2]ZnEt (1), [N{(C3F7)C(Cy)N}2]ZnEt (2), in which the
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bulky “Dipp” groups on the nitrogens are replaced by a flexible and less sterically demanding
DOI: 10.1039/C6DT00257A
but better donating cyclohexyl groups, exhibits a lower activity giving only 32% of the
dimerized product. [N{(CF3)C(2,4,6-Br3C6H2)N}2]ZnEt (3), in contrast, produced the
aldehyde dimerization product in 87% yield under similar conditions. Although it is tempting
to attribute the lower and higher reactivity of the latter two systems compared to that of 1 to
the changes in Lewis-acidity of the zinc center (which would affect the aldehyde
coordination), the good activity of [HC{(CH3)C(Dipp)N}2]ZnEt (which contains a non-
fluorinated diazapentadienyl ligand) in Tishchenko chemistry points to more complex ligand
effects.52 Lower activity of less sterically protected triazapentadienyl zinc adducts could also
be a result of the coordination of more than one aldehyde (or Lewis base) to the zinc site
affecting the acidity.85 In fact, studies involving aluminium alkoxides suggest two different
rate-determining steps for bulky alkoxide and less-bulky alkoxide containing catalysts (i.e.,
the rate determining step is the coordination of aldehyde for the former while the hydride-
shift from alkoxide for the latter).86 Furthermore, t-BuOCuL catalyzed processes show that
electron donating ligands (L) provide higher activity in the benzaldehyde to benzyl benzoate
conversion (t-BuOCu alone is inactive).87
These triazapentadienyl-zinc ethyl complexes can also efficiently catalyse the
dimerization other aldehydes apart from benzaldehyde.
For example,
[N{(C3F7)C(Dipp)N}2]ZnEt (1) can catalyse the dimerization of 2,3,4,5,6-
pentafluorobenzaldehyde and 1-naphthaldehyde to give the corresponding esters,
perfluorobenzyl 2,3,4,5,6-pentafluorobenzoate (36%) and naphthalen-2-ylmethyl 2-
naphthoate (60%) in moderate yields under similar reaction conditions. In the case of
acetaldehyde, a mixture of compounds including the expected dimer (ethyl acetate) and other
oligomers was obtained.70, 88
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Although we have not explored the mechanism in detail, it is possible to speculate various
DOI: 10.1039/C6DT00257A
options based on the results of this study and literature data. A widely used mechanism
proposed for the Tishchenko reaction involves metal-alkoxides as key intermediates in the
catalytic reaction (see supporting information, Figure S2).69, 76 Furthermore, some studies
have also reported an equal catalytic activity for the metal-alkyl and the corresponding metal-
alkoxy complexes.79 Accordingly, zinc-ethoxide complexes {[N{(CF3)C(2,4,6-
Br3C6H2)N}2]Zn(μ-OEt)}2 (5) and {[N{(C3F7)C(2,6-Cl2C6H3)N}2]Zn(μ-OEt)}2 (6) we have
obtained from the oxygen insertion reaction may also facilitate the dimerization of
benzaldehyde. However, to our surprise, 5 and 6 and the ethylperoxy complex and the
ethylperoxy complex {[N{(C3F7)C(Cy)N}2]Zn(μ-OOEt)}2 (7) did not produce benzyl
o
benzoate from benzaldehyde even at higher temperatures 100–140 C and longer reaction
time (24 h). This inactivity is may be due to the dimeric nature (Figures 5-7) of the zinc-
alkoxide/peroxide complexes. The steric constraints caused by the crowded environment at
zinc of these dimeric species might prevent the effective coordination of the incoming
aldehyde to the zinc center, hence impeding the catalytic process. Yet another mechanism
used commonly for the Tishchenko reaction is the one first proposed by Lin et al for
aluminium alkoxides, which involve a direct hydride shift between aldehydes facilitated by a
Lewis acidic metal site.69, 89 This route does not alter the ligands originally on the catalyst
(see supporting information, Figure S3).
In fact,
a
reaction between
[N{(C3F7)C(Dipp)N}2]ZnEt (1, 0.2 mmol) and benzaldehyde (0.5 mmol) (i.e.,
o
catalyst:aldehyde molar ratio of 1:2.5) in hexane at 60 C for 3 h afforded benzyl benzoate
quantitatively while the zinc ethyl complex 1 remained essentially unchanged (no zinc-
alkoxide moiety was detected in the 1H NMR spectrum). Similar observation was also noted
in a closely related study involving diazapentadienyl zinc-ethyl complexes [HC{(CH3)C(2,6-
Et2C6H3)N}2]ZnEt and [HC{(CH3)C(Dipp)N}2]ZnEt.52 Thus a later mechanism involving
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zinc-alkyl catalysts is a valid option for these catalysts although we cannot rule out alkoxide
DOI: 10.1039/C6DT00257A
pathway mediated by small quantities of zinc alkoxide species (that could generate under
reaction conditions) without data from additional studies.
The epoxidation of electron-deficient olefins is an important process in organic
chemistry, and the zinc alkylperoxides are believed to be promising reagents for this
reaction.60, 64 The well defined {[N{(C3F7)C(Cy)N}2]Zn(μ-OOEt)}2 (7) we have isolated may
be a good reagent for this process. Indeed, when it was treated with an equimolar amount of
o
trans-chalcone at 20 C in toluene, the epoxidation product, phenyl(3-phenyloxiran-2-
yl)methanone was obtained in quantitative yield in 15 min. This indicates an outstanding
oxidizing ability of 7.
Summary and conclusions
We have successfully prepared a series of zinc ethyl complexes supported by fluoroalkyl
substituted triazapentadienyl ligands. The zinc-ethyl bonds of these adducts are sensitive
towards oxygen and undergo oxygen insertion leading to the corresponding zinc-ethoxy or
zinc-ethylperoxy compounds within few mins. Prolonged exposure to air at room
temperature causes decomposition. These zinc-ethyl complexes serve as good catalysts in the
Tishchenko reaction of benzaldehyde under solventless conditions, whereas the
corresponding oxygenated, dinuclear complexes remain inactive. A rare zinc-ethylperoxo
compound has also been isolated and fully characterized and successfully employed in
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epoxidation of trans-chalcone. Additional studies on metal mediated Tishchenko reaction as
DOI: 10.1039/C6DT00257A
well as the oxidation chemistry of zinc alkylperoxy adducts are in progress.
Experimental
All manipulations were carried out under an atmosphere of purified dry nitrogen using
standard Schlenk techniques or in a MBRAUN LABMaster glove-box equipped with a -10
°C refrigerator. Solvents were purchased from commercial sources, purified by distilling over
conventional drying agents or by using Innovative Technology SPS-400 PureSolv solvent
drying system and degassed by the freeze-pump-thaw method prior to use. Glassware was
oven-dried at 150 °C overnight. NMR spectra were recorded on a JEOL Eclipse 500. Proton
and carbon chemical shifts are reported in ppm, and referenced using the residual proton or
carbon signals of the deuterated solvent. 19F NMR chemical shifts were referenced relative to
CFCl3 (external). Elemental analyses were performed at Intertek Pharmaceutical Services,
Whitehouse, NJ. The precursors for the ligands, perfluoro-5-aza-4-nonene5, 6 and perfluoro-
3-aza-4-pentene,42 and the ligands [N{(C3F7)C(Dipp)2C6H3)N}2]H8, 9 and [N{(C3F7)C(2,6-
Cl2C6H3)N}2]H33, 37 were prepared by using the published procedure. Rest of the materials
used in this work were obtained from commercial vendors (mainly Sigma Aldrich) and used
without further purification unless otherwise stated. The products of the catalytic processes
were identified by NMR studies by comparison with previously reported data or commercial
samples.79, 83
Synthesis of [N{(C3F7)C(Cy)N}2]H
Perfluoro-5-aza-4-nonene (5.41 g, 12.5 mmol) was added drop-wise to a solution of
o
cyclohexylamine (6.23 g, 63 mmol) in diethylether (30 mL) at 0 C. The solution was
allowed to stir overnight at room temperature. The resulting mixture was filtered, the filtrate
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was collected and washed first with 10% HCl (30mL) and then twice with distilled water
DOI: 10.1039/C6DT00257A
(30mL×2). The ether layer was separated and dried over CaCl2. The solvent was removed
o
under reduced pressure and the resulting residue was recrystallized from hexane at 0 C to
obtain the colourless crystals of [N{(C3F7)C(Cy)N}2]H. Yield 70%. M.P. 58-60 oC. 1H NMR
(CDCl3, 500.16 MHz, 298 K):  1.10 to 1.95 (several overlapping peaks, br and sharp, 20H,
19
CH2), 3.13 and 3.53 (m, 2H, ipso Cy). 5.27 (br, 1H, NH). F NMR (CDCl3, 470.62 MHz,
298 K): -80.39 (t, J = 9.1 Hz, CF3), -80.53 (t, J = 9.1 Hz, CF3), -115.35 and -117.37 (AB
multiplet, JAB = 264 Hz, α-CF2), -115.86 (br, α-CF2), -125.45 (s, -CF2), -125.57 (s, -CF2), -
126.60 (s, -CF2). 13C NMR (CDCl3, 125.77 MHz, 298 K):  24.5, 25.0, 25.6, 31.2, 32.9,
33.5, 51.0, 58.6, 105.0 – 125.0 (m, CF2CF2CF3), 141.0 (t, J = 26.3 Hz, NCN), 146.4 (t, J =
26.3 Hz, NCN). Anal.Calcd. for C20H23F14N3: C, 42.04; H, 4.06; N, 7.35 %. Found: C, 42.19;
H, 3.88; N, 7.19%.
[N{(CF3)C(2,4,6-Br3C6H2)N}2]H
Perfluoro-3-aza-2-pentene (2.91 g, 12.5 mmol) was added drop-wise to a solution of
trimethylamine (3.78 g, 37.5 mmol) and 2,4,6-tribromoaniline (8.22 g, 25 mmol) in ether at 0
oC (30 mL). The solution was allowed to stir overnight at room temperature. The resulting
mixture was filtered, the filtrate was collected and washed first with 10% HCl (30 mL) and
then twice with distilled water (30 mL × 2). The ether layer was separated and dried over
CaCl2. The solvent was removed under reduced pressure and the resulting residue was
o
recrystallized from hexane at 0 C to obtain the desired ligand as a light yellow solid. Yield
75%. M.P. 155-157 oC. 1H NMR (CDCl3, 500.16 MHz, 298 K):  6.62 (s, 1H, NH), 7.59 and
19
13
7.79 (s, 4H, m-Ar). F NMR (CDCl3, 470.62 MHz, 298 K): -80.03 and -79.50 (s, CF3). C
NMR (CDCl3 125.77 MHz, 298 K):  109.5 (q, J = 273.5 Hz, CF3) 117.2 (q, J = 273.5 Hz,
CF3), 117.3, 124.0, 125.0, 132.0, 134.4, 135.0 (Ar), 141.1 (q, J = 29.9 Hz, NCN), 147.5 (q, J
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= 29.9 Hz, NCN). Anal.Calcd. for C16H5Br6F6N3: C, 23.08; H, 0.61; N, 5.05DO%I: .10.F10o3u9/nCd6D:T00257A
23.44, 0.38, 4.04%.
General procedure for synthesis of ethyl zinc complexes (1-4)
Diethyl zinc (1 M solution in toluene/hexane, 0.41 mmol, 1.5 eqv.) was added to a solution of
1,3,5-triazapentadiene (0.27 mmol) in toluene (~5 mL) at room temperature. The resulting
clear solution was heated at 80 ºC for 48-72 h in a sealed tube. The reaction mixture was
cooled down to room temperature and the volatiles were removed under reduced pressure.
Ethyl zinc complexes were obtained as colorless to light yellow solids. These ethyl zinc
complexes (1-4) are soluble in common solvents like hexane, toluene, benzene, THF,
chloroform and dichloromethane.
[N{(C3F7)C(Dipp)N}2]ZnEt (1)
Pale-yellow solid. Yield 82%. The X-ray quality colorless crystals were obtained from a
1
hexane solution kept at -10 ºC overnight. H NMR (C6D6, 500.16 MHz, 298 K):  0.13 (q,
2H, J = 8.0 Hz, CH2CH3), 0.59 (t, 3H, J = 8.0 Hz, CH2CH3), 1.14 (d, J = 6.9 Hz, 12H,
CH(CH3)2), 1.18 (d, J = 6.9 Hz, 12H, CH(CH3)2), 3.04 (sept, J = 6.9 Hz, 4H, CH(CH3)2),
19
6.96-7.10 (m, 6H, m-and p-Ar). F NMR (C6D6, 470.62 MHz, 298 K):  -80.39 (t, J = 11.0
13
Hz, 6F, CF3), -106.26 (q, J = 11.0 Hz, 4F, α-CF2), -121.97 (br, 4F, -CF2). C NMR (C6D6,
125.77 MHz, 298 K):  -0.09 (s, CH2CH3), 10.7 (s, CH2CH3), 23.0, 24.3 (s, CH(CH3)2), 29.0
(s, CH(CH3)2, 105.0 – 125.0 (m, CF2CF2CF3), 127.8, 128.0, 139.3, 139.9 (Ar), 155.4 (t, J =
32.7 Hz, NCN). Anal.Calcd. for C34H39N3F14Zn: C, 49.74; H, 4.79; N, 5.12 %. Found: C,
49.05; H, 4.01; N, 4.86 %.
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DOI: 10.1039/C6DT00257A
[N{(C3F7)C(Cy)N}2]ZnEt (2)
Colorless solid. Yield 80%. 1H NMR (C6D6, 500.16 MHz, 298 K):  0.55 (q, 2H, J = 8.0 Hz,
CH2CH3), 1.39 (t, 3H, J = 8.0 Hz, CH2CH3), 0.99, 1.29, 1.51 (m, 20H, Cy), 3.71 (m, 2H,
ipsoCy).19F NMR (C6D6, 470.62 MHz, 298 K):  -80.01 (t, J = 11.0 Hz, 6F, CF3), -105.79 (q,
13
J = 11.0 Hz, 4F, α-CF2), -123.68 (br, 4F, -CF2). C NMR (C6D6, 125.77 MHz, 298 K): 
6.6 (s, CH2CH3), 12.2 (s, CH2CH3), 24.8, 24.9, 35.1, 59.4, 105.0 – 125.0 (m, CF2CF2CF3),
153.3 (t, J = 23.9 Hz, NCN). Anal. Calcd. for C22H27F14N3Zn: C, 39.74; H, 4.09; N, 6.32 %.
Found: C, 40.89; H, 3.73; N, 6.04 %.
[N{(CF3)C(2,4,6-Br3C6H2)N}2]ZnEt (3)
1
Pale-yellow solid. Yield 85%. H NMR (C6D6, 500.16 MHz, 298 K):  0.14 (q, 2H, J = 8.0
Hz, CH2CH3), 0.73 (t, 3H, J = 8.0 Hz, CH2CH3), 7.19 (s, 4H, m-Ar). 19F NMR (C6D6, 470.62
13
MHz, 298 K):  -67.27 (s, CF3). C NMR (C6D6 125.77 MHz, 298 K):  -2.2 (s, CH2CH3),
10.8 (s, CH2CH3), 117.2 (q, J = 273.5 Hz, CF3), 119.0, 120.3, 134.4, 141.5 (Ar), 157.2 (q, J =
31.5 Hz, NCN). Anal.Calcd. for C18H9Br6F6N3Zn: C, 23.34; H, 0.98; N, 4.54%. Found: C,
22.95; H, 0.93; N, 4.31 %.
[N{(C3F7)C(2,6-Cl2C6H3)N}2]ZnEt (4)
1
Pale-yellow solid. Yield 82%. H NMR (C6D6, 500.16 MHz, 298 K):  0.17 (q, 2H, J = 8.0
Hz, CH2CH3), 0.71 (t, 3H, J = 8.0 Hz, CH2CH3), 6.21 (t, 2H, J = 8.0 Hz, p-Ar), 6.72 (d, 4H, J
= 8.0 Hz, m-Ar). 19F NMR (C6D6, 470.62 MHz, 298 K): ):  -80.40 (apparent triplet, J = 11.0
and 7.3 Hz, 6F, CF3), -109.89 (apparent quartet, J = 10.9 Hz, 4F, α-CF2), -122.66 (br, 4F, -
CF2).13C NMR (C6D6, 125.77 MHz, 298 K):  -2.4 (s, CH2CH3), 10.5 (s, CH2CH3), 105.0 –
125.0 (m, CF2CF2CF3), 122.7, 127.6, 129.4, 139.6 (Ar), 156.5 (t, J = 24.5 Hz, NCN). Anal.
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Calcd. for C22H11Cl4F14N3Zn: C, 33.42; H, 1.40; N, 5.32 %. Found: C, 32.92; DHO,I: 110..150439;/CN6D,T00257A
4.85 %
General procedure for the synthesis of ethoxy- or ethylperoxy-zinc complexes
The ethyl zinc complex (2-4) (0.25 mmol) was placed in a Schlenk flask and dry
toluene/benzene (~10 mL) was added. Resulting solution was stirred under the positive
pressure of dry air for 5 min at 20 oC. Then the Schlenk flask was sealed and placed in freezer
o
at -20 C overnight to obtain the corresponding ethoxy- (5 or 6) or ethylperoxy (7) complex
as colorless to pale yellow crystalline materials. These adducts are less soluble in solvents
like hexane, benzene, chloroform and dichloromethane but show good solubility in DMSO.
{[N{(CF3)C(2,4,6-Br3C6H2)N}2]Zn(μ-OEt)}2 (5)
Light yellow solid. Yield 71 %. 1H NMR (DMSO-d6, 500.16 MHz, 298 K):  1.07 (t, 6H, J =
6.9 Hz, OCH2CH3), 3.83 (q, 4H, J = 6.9 Hz, OCH2CH3), 7.33 (s, 8H, m-Ar). 19F NMR
(DMSO-d6, 470.62 MHz, 298 K):  -68.04 (s, CF3). 13C NMR (DMSO-d6, 125.77 MHz, 298
K):  13.7 (s, OCH2CH3), 71.4 (s, OCH2CH3), 119.1 (q, J = 285.1 Hz, CF3), 117.9, 128.8,
133.6, 148.7 (Ar), 147.58 (q, J = 32.5 Hz, NCN). Anal.Calcd. for C36H18Br12F12N6O2Zn2: C,
22.95; H, 0.96; N, 4.46. Found: C, 22.15; H, 0.83; N, 4.13 %.
{[N{(C3F7)C(2,6-Cl2C6H3)N}2]Zn(μ-OEt)}2 (6)
Pale-yellow solid. Yield 72 %. 1H NMR (DMSO-d6, 500.16 MHz, 298 K):  1.07 (t, 3H, J =
6.9 Hz, CH2CH3), 3.83 (q, 2H, J = 6.9 Hz, CH2CH3), 6.78 (t, 2H, J = 8.0 Hz, p-Ar), 7.14 (d,
4H, J = 8.0 Hz, m-Ar). 19F NMR (DMSO-d6, 470.62 MHz, 298 K):  -79.52 (apparent triplet,
13
J = 11.0 and 7.3 Hz, 12F, CF3), -111.35 (b, 8F, α-CF2), -123.99 (br, 8F, -CF2). C NMR
(DMSO-d6, 125.77 MHz, 298 K):  13.7 (s, OCH2CH3), 71.4 (s, OCH2CH3), 105.0-121.0
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(m, CF2CF3CF3), 122.7, 125.8, 128.7, 129.4 (Ar), 146.5 (t, J = 25.0 Hz, NCN). Anal. CVaielwcdAr.ticle Online
DOI: 10.1039/C6DT00257A
for C44H22Cl8F28N6O2Zn2, C, 32.76; H, 1.37; N, 5.21%. Found: 32.11, H 1.20, N 4.92%.
{[N{(C3F7)C(Cy)N}2]Zn(μ-OOEt)}2 (7)
Colorless solid. Yield 70%. 1H NMR (DMSO-d6, 5 00.16 MHz, 298 K):  1.20, 1.69 1.90 (m,
40H, Cy), 1.23 (t, 6H, J = 6.9 Hz, OOCH2CH3), 3.75 (m, 4H, ipsoCy), 3.97 (q, 4H, J = 6.9
Hz, OOCH2CH3). 19F NMR (DMSO-d6, 470.62 MHz, 298 K):  -79.64 (apparent triplet, J =
7.3 and 10.9 Hz, 12F, CF3), -105.80 (apparent quartet, J = 11.0 Hz, 8F, α-CF2), -123.88 (br,
13
8F, -CF2). C NMR (DMSO-d6, 125.77 MHz, 298 K):  14.4 (s, OOCH2CH3), 21.4, 25.3,
32.2, 34.2, 54.7, 62.5 (s, OOCH2CH3), 105.0 – 125.0 (m. CF2CF2CF3), 146.3 (t, J = 23.9 Hz,
NCN). Anal. Calcd. for C44H54F28N6O4Zn2: C, 37.92; H, 3.91; N, 6.03 %. Found: C, 37.60;
H, 3.79; N, 5.80 %.
Procedure for Tishchenko reaction
A 10 mL Schlenk tube was charged with the complex (25 μmol) in a drybox. The Schlenk
tube was connected to a vacuum line and the aldehyde (2.5 mmol) was added. The mixture
o
was placed in an oil-bath maintained at 80 C and stirred vigorously. After 3h, the tube was
opened to air and aliquots of the reaction mixture were used for the NMR83, 90 (in CDCl3) and
GC/MS studies. Further, the products were isolated by column chromatography on silica gel
(hexane/ether = 1:15 eluent).
Procedure for epoxidation of trans-chalcone
To a stirred solution of complex 7 (2 mmol) in toluene (5 mL), trans-chalcone was added (1.9
mmol) at 20ꢀ°C and the reaction mixture was stirred for 15 min and the reaction mixture was
analysed by 1H NMR (in CDCl3) and GCMS.60 Disappearance of the peaks corresponding to
23

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trans-chalcone and the appearance of the new peaks consistent with the epoxidatDioOnI: 1p0.r10o3d9u/Cc6tD,T00257A
phenyl(3-phenyloxiran-2-yl)methanone was observed.
X-ray crystallographic data:
A suitable crystal covered with a layer of hydrocarbon/Paratone-N oil was selected and
mounted on a Cryo-loop, and immediately placed in the low temperature nitrogen stream.
The X-ray intensity data for were measured at 100(2) K on a Bruker D8 Quest with a Photon
100 CMOS detector equipped with an Oxford Cryosystems 700 series cooler, a Triumph
monochromator, and a Mo Ka fine-focus sealed tube (λ = 0.71073 Å). Intensity data were
processed and the initial calculations for the structure determination were carried out using
the SHELXTL package (version 6.14).91 Final refinements were completed using SHELX
within Olex2.92 Hydrogen atoms were placed at calculated positions and refined riding on the
corresponding carbons. All the non-hydrogen atoms were refined anisotropically.
{[N{(CF3)C(2,4,6-Br3C6H2)N}2]Zn(μ-OEt)}2 (5) and {[N{(C3F7)C(2,6-Cl2C6H3)N}2]Zn(μ-
OEt)}2 (6) crystallize with molecules of benzene in the crystal lattice. Further details are
given in Tables 1. The 1446335-1446338 contain the supplementary crystallographic data.
These
data
can
be
obtained
free
of
charge
via
http://www.ccdc.cam.ac.uk/conts/retrieving.html or from the Cambridge Crystallographic
Data Centre (CCDC), 12 Union Road, Cambridge, CB2 1EZ, UK).
Acknowledgments. This work was supported by the National Science Foundation (CHE-
1265807 to H.V.R.D.) and the Robert A. Welch Foundation (Grant Y-1289 to H.V.R.D). We
are grateful to 3M Company and Dr. N. Somasiri for providing us with a sample of
fluorinated amine which is required for the ligand synthesis.
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DOI: 10.1039/C6DT00257A
ASSOCIATED CONTENT
Supporting Information
Additional information on molecular structures and possible reaction mechanism.
AUTHOR INFORMATION
Corresponding Author
TEL
:
(+1) 817 272 3813
E-mail
: dias@uta.edu
Homepage : http://www.uta.edu/chemistry/faculty/directory/Dias.php
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