Zinc Complexes with Cyanoxime: Structural, Spectroscopic, and Catalysis Studies in the Pivaloylcyanoxime-Zn System

Article abstract of DOI:10.1021/acs.inorgchem.7b01891

Reaction of 2-hydroxyimino-4,4-dimethyl-3-oxo-pentanenitrile (common abbreviation HPiCO, pivaloyl-cyanoxime) with zinc sulfate in an aqueous solution results in the formation of the two new complexes: [Zn(PiCO){H(PiCO)₂}(H₂O)] (I) and tetranuclear Zn complex [Zn₄(μ³-OH)₂(PiCO)₆ (H₂O)₄] (II). Both complexes were characterized by elemental analysis, IR- and UV-visible spectra, DSC/TGA studies, and X-ray analysis. In complex II, the PiCO^- cyanoxime anion adopts three bidentate binding modes: O-monodentate, chelating (κ²), and bridging (²) coordinations. Also, the ligand represents the mixture of two diasteromers (cis-anti and cis-syn) that form five- and six-membered chelate rings with Zn atoms and cocrystallize in one unit cell at population of 0.57-0.43. There are two crystallographically different Zn-centers in the ASU, and two μ³-bridging hydroxo-groups arrange via inversion center the formation of an elegant tetranuclear complex. Each Zn atom has a molecule of coordinated water and is in the distorted octahedral environment. Because of the structural flexibility and multidentate propensity of the pivaloyl-cyanoxime, complex II may act as a structural model of naturally occurring Zn-containing enzymes. Indeed, compound I exhibits an efficient catalytic performance for transesterification reaction of various esters in ethanol under mild reaction conditions. Therefore, obtained results allow assignment of observed activity as green catalysis.

Full text of DOI:10.1021/acs.inorgchem.7b01891

Article
Cite This: Inorg. Chem. XXXX, XXX, XXX-XXX
pubs.acs.org/IC
Zinc Complexes with Cyanoxime: Structural, Spectroscopic, and
Catalysis Studies in the Pivaloylcyanoxime−Zn System
†
‡
‡
‡
́
Adedamola A. Opalade, Anirban Karmakar, G. M. D. M. Rubio, Armando J. L. Pombeiro,
,†
and Nikolay Gerasimchuk*
†
Department of Chemistry, Temple Hall 431, Missouri State University, Springfield, Missouri 65897, United States
‡
Centro de Química Estrutural (CQE), Instituto SuperiorTec
́
nico (IST) University of Lisbon, Avenida Rovisco Pais, 1049-001
Lisboa, Portugal
*
S Supporting Information
ABSTRACT: Reaction of 2-hydroxyimino-4,4-dimethyl-3-oxo-pentanenitrile (com-
mon abbreviation HPiCO, pivaloyl-cyanoxime) with zinc sulfate in an aqueous
solution results in the formation of the two new complexes: [Zn(PiCO){H-
3
(
(
PiCO) }(H O)] (I) and tetranuclear Zn complex [Zn (μ −OH) (PiCO)
2
2
4
2
6
H O) ] (II). Both complexes were characterized by elemental analysis, IR− and
2
4
UV−visible spectra, DSC/TGA studies, and X-ray analysis. In complex II, the
−
PiCO cyanoxime anion adopts three bidentate binding modes: O-monodentate,
2
2
chelating (κ ), and bridging (η ) coordinations. Also, the ligand represents the
mixture of two diasteromers (cis−anti and cis−syn) that form five- and six-
membered chelate rings with Zn atoms and cocrystallize in one unit cell at
population of 0.57−0.43. There are two crystallographically different Zn-centers in
3
the ASU, and two μ -bridging hydroxo-groups arrange via inversion center the
formation of an elegant tetranuclear complex. Each Zn atom has a molecule of
coordinated water and is in the distorted octahedral environment. Because of the structural flexibility and multidentate propensity
of the pivaloyl-cyanoxime, complex II may act as a structural model of naturally occurring Zn-containing enzymes. Indeed,
compound I exhibits an efficient catalytic performance for transesterification reaction of various esters in ethanol under mild
reaction conditions. Therefore, obtained results allow assignment of observed activity as green catalysis.
INTRODUCTION
Al(OR) , Mg−Al−O−t-Bu hydrotalcite, amberlyst-15, BF3·
3
■
23
OEt , and so on. Several metal complexes and metal organic
2
The chemistry, preparation, crystal structure, and properties of
multinuclear 3d transition metal complexes have received
considerable attention due to their probable application and
role in multi-metal-centered catalysis both in biological and
frameworks (MOFs) can also catalyze such reactions, but
usually, they require high reaction temperature and acidic
24,25
reaction conditions.
Therefore, the development of new
1
−7
types of catalysts based on cheap, nontoxic, environmentally
tolerable (green catalysts), and less moisture sensitive
compounds is highly desirable for practical utility. Zinc is
especially fit for this purpose considering its crucial catalytic
industrial reactions.
Their magnetic and optical properties
8
−11
also make these types of compounds very important.
study of multinuclear 3d metal complexes with primarily O- or
N-based ligation is today an area of modern science with
The
26
12
role in a variety of enzymes in biological systems.
interfaces in different facets of science. The chemistry of
Zn(II) multimetallic systems has not been fully explored. These
are significant, nevertheless, because of their importance as a
precursor to fabrication of ZnO-based materials, modeling of
active sites in enzymes, polymerization catalyst, molecular
material with interesting properties, and reagents in organic
Due to cited potential applications of these polymetallic
systems, there is a need for controlled synthesis methods for
preparation of these compounds, as well as a need for different
polydentate ligands that are able to act as bridges in such
systems. So far, polymetallic Zn complexes have been obtained
2
7
1
3−17
using ligands such as salicylaldo(keto)oximes, pyridyl
synthesis.
Thus, the development of catalyst-promoted
28 29
oximes, and aliphatic dioximes. Chemical structures of the
oximes and their derivatives used for the preparation of multi-
Zn complexes are presented in Scheme 1 with corresponding
literature citations highlighted in blue. Thus, a 36-membered
atom-economical and environmentally tolerable processes is a
topic of great interest in modern organic chemistry, e.g., in ester
1
8
formation. An attractive method of ester synthesis concerns
1
9
transesterification endorsed by a metal catalyst. Trans-
esterifications are important reactions in organic synthesis
and industrial and academic laboratories and have important
macrocyclic hexaoxime ester derivative with Zn La core/shell
3
20
tetranuclear cluster structure was obtained by condensation of
21
applications in polyester synthesis and biodiesel production.
There are many catalysts available for transesterification,
Received: August 14, 2017
22
namely, Ti(OiPr)4, and diverse catalysts such as BuSn(OH)3,
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.7b01891
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Scheme 1. Oximes Used in Earlier Preparation of Zn-Based
Complexes
photoluminescence with maxima ∼375 nm upon maximum
12
excitation at 314 nm.
In terms of applications of the multimetallic Zn(II)
complexes, there have been various Zn(II) complexes with
different ligands that have been reported for different
applications. Recently, Badetti et al. reported how the
utilization of trimetallic Zn(II) complexes of two modified
tris(2-pyridylmethyl)amine ligands and amino acids for the
54
determination of amino acids ee. Also, multimetallic zinc
complexes have been shown to be very promising catalysts for
55
ring opening polymerization (ROP) of esters. Recently, alkyl
Zn(II) complexes with dpg, derived from the Ph C(X)−dpgH
2
(
X = OH, NH ) were reported to give tetranuclear ring
2
complexes [RZn(dpg)] (R = Me, Et, 2-CF C H , and 2,4,6-
4
3
6
4
F C H ). The complexes were reported to be active for ROP of
3
6
2
56
ε-Cl, rac-LA, and δ-VL in the absence of benzyl alcohol.
Heterometallic complexes of Zn have also been reported for
their catalytic activities toward transesterification reaction. Two
novel one-dimensional heterometallic complexes represented as
[
Zn Co(O CPh) ] (bpa) and [Zn Cd(O CPh) ] (bpa) , con-
2 2 6 n n 2 2 6 n n
taining two bridging ligands: 1,2-Bis(4-pyridyl)ethane (bpa)
and benzoate were used as catalysts for transesterification of
esters by methanol using different substrates at room
dialdehyde with diamine using La3+ (core metal) and Zn
(
2+
30
57
shell metal) as a novel core/shell template. The first example
of a 12-metallacrown-4 with an “anti-metallacrown” structure
type inv-(OH) [12-MCZn N(pko) ] (a tetranuclear species
Zn (OH) (O CMe) (pko) ]) made by using an oxime based
ligand, bis(2-pyridyl)ketone oxime (later Hpko), was reported
to have a Zn O core structure similar to Fe O2 core
temperature under mild conditions. Also, an octanuclear
Zn(II) complex of meta-bis(imidazolylmethyl)-benzene ligand
was reported as a stable but very active catalyst for the
transesterification reaction of a broad range of substrates and
2
+
−4
2
[
4 2 2 2 4
25
l
alcohols; displaying enhanced efficiency and very high yields .
The enhanced catalytic activity of the multimetallic Zn(II)
complexes over monometallic analogues to aid chemical
transformation could be based on the cooperative and
synergistic effect of the metal centers, due to their close
proximity thus favoring multisite interactions with the substrate.
This could lead to the substrate activation. Metal centers could
also alter the electronic properties of its neighbors by playing
31
4
2
4
3
2
structure. A synthesis route to cationic pentanuclear ZnII/pko
clusters and to a new type of 12-MCZnIIN(pko)-4 compound
−
−
−
−
featuring SCN , PhCO , N , and acac (−1) ion and NCO
2
3
coligands was reported, and this pathway provided access to six
new polymetallic Zn complexes of which four were tetranuclear
3
3
and two were pentanuclear. Three tetranuclear ZnII
3
complexes of the general formula [Zn (μ -OH) (LR1−
58
4
2
the role of a ligand toward them.
3
N,N,O) (LR1−3N,N) ] were also reported from the reaction
4
2
There are only two reports containing preparation and some
of ZnCl with respective 2-pyridyl ketone oximes, HLR1−3
38,39
2
properties of Zn-cyanoximates,
but no crystallographic
(
R1−3 = H, Ph, and Py) where the complexes with R = Ph and
characterization was carried out. Cyanoximes represent an
Py have hydroxyl group encapsulated in the cavity of the
40
34
important subclass of oximes that has received intense studies
tetranuclear ZnII cluster. The synthesis of octanuclear 12-
membered metallacrown, [Zn ]{[Zn -(pko) ][12-MCZn(II)N-
shi)-4](CH OH) }, was also performed using ZnCl dissolved
in methanol, salicylhydroxamic acid, and 2-pyridyl-ketonoxime,
deprotonated by NaOH in methanol. Solvothermal synthesis
of Zn complex of pyridine-2-amidoxime, (py)C(NH )NOH
resulted in the formation of [Zn (OH) {(py)C(NH )-
NO} Cl ]·3MeCN where each ZnII ion is coordinated by a
μ -bridging hydroxide ion. The reaction of syn-2-pyridineal-
doxime (HL) with ZnII metal salt in a mixed C H OH/
CH CN solution resulted in a new complex with the
tetranuclear unit [Zn (L) Cl (μ -OH) ·4CH CN]. When
pyridine-2-carbaldehyde oxime (later as paoH) reacted with
Zinc(II) benzoate in the absence and in the presence of azide
ions, the clusters [Zn (OH) (O CPh) (pao) ] and
Z n ( O H ) ( p a o ) ( N ) ] w e r e o b t a i n e d .
Zn (OH) (pao) (N ) ] has an inverse 12-metallacrown-4
in the last decades because of their greater acidity, diversity of
donor atoms for metal ions binding, and, subsequently,
2
2
4
(
3
2
2
41
excellent properties as polydentate ligands. Complexes of
35
cyanoximes exhibited a variety of practically interesting
42
properties. The pivaloylcyanoxime, 2-hydroxyimino-4,4-di-
2
methyl-3-oxo-pentanenitrile (HPiCO), was first described in
4
2
2
43
1
993 by Fedorenko and represents a rare example of sterically
4
2
3
36
hindered chelating oximes. However, this cyanoxime is
insufficiently studied with only a handful of publications.
Thus, the Tl+ and Ag+ PiCO-based complexes have been
structurally characterized before. The crystal structure of the
Ag-PiCO compound is a layered 2D coordination polymer
2
5
3
3
37
4
4
2
2
3
−
where the PiCO anion displayed bridging mode between four
different silver(I) atoms, while the crystal structure of Tl−PiCO
1
2
4
2
16
4
complexes have been reported as coordination polymer with
[
4 2 4 3 2
−
42f,44,59
three different binding modes of the PiCO anion.
4
2
4
3 2
3
configuration with two μ -OH accommodated in the metal-
lacrown ring while the [Zn (OH) (O CPh) (pao) ] can be
described as consisting of a central {Zn (μ -(pao) )} cubane
subunit linked to four {Zn (μ−OH)} subunits via a
hydroxide group of the latter, which becomes the μ -type.
In this work, we report the first detailed investigations of the
−
Zn−PiCO system and present (1) syntheses and character-
1
2
4
2
16
4
3
4+
ization of the first Zn-cyanoxime complexes, (2) crystallo-
4
4
3
+
graphic data for a new Zn(II) complex that turned out to be an
2
3
3
interesting tetranuclear compound of [Zn (μ -OH) (PiCO)
4
2
6
The [Zn (OH) (O CPh) (pao) ] was found to exhibit
(H O) ] composition, and (3) results of significant catalytical
1
2
4
2
16
4
2 4
B
DOI: 10.1021/acs.inorgchem.7b01891
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Scheme 2. Preparation of the Cyanoxime and Its Zn Complexes
activity study for another complex [Zn(PiCO){H(PiCO)2}
later on referred to as tetra-Zn complex II (Scheme 2). Elemental
analysis for C H N O Zn calculated (found), %: C, 39.21 (39.25),
(
H O)] obtained in this system.
2
42 64 12 18
4
H, 5.01 (4.98), N, 13.06 (13.22). When bulk precipitate was triturated
with ether (not dry, technical grade, commonly contains up to 4% of
moisture) for several hours at room temperature, tetra-Zn complex II
EXPERIMENTAL PART
■
3
Synthesis. The preparation of the HPiCO was carried out
according to previously published procedure,
[Zn (μ -OH) (PiCO) (H O) ] was obtained again as well as a small
4
2
6
2
4
43,45
while the reactions
amount of the protonated pyvaloylcyanoxime HPiCO. Similarly, when
in the cyanoxime−Zn system are presented in Scheme 2 and in a set of
equations below. For the preparation of Zn(II) complex, 0.300 g (1.95
mM) of white powdery HPiCO was placed in a beaker containing 5
mL of deionized water, to which 1.95 mL of 1 M KOH solution was
added at once. The color quickly changes to yellow following a
complete dissolution of the cyanoxime (eq 1). Then, 0.2797 g (0.97
mM) of ZnSO ·7H O was dissolved in 5 mL of deionized water, and
bulk precipitate was dissolved in CH CN and reprecipitated by an
3
addition of ether (via vapor deposition method in closed vial), tetra-Zn
complex II was obtained once more as well as a free HPiCO
cyanoxime. More specifically, 200 mg of the bulk precipitate was
dissolved in 2 mL of acetonitrile and filtered over glass wool. The clear
filtrate was collected in a glass tube that then was inserted into a large-
mouth screw-cap tube containing ether and sealed up for several
weeks. The inner test tube then contained the above mixture of tetra-
Zn complex II and HPiCO, which was washed from the complex by
CHCl . An identity of the oxime was confirmed by TLC (R = 0.67;
4
2
the solution was added dropwise under continuous stirring to the
−
yellow PiCO cyanoximate solution prepared above (eq 3). The
mixture was allowed to stir continuously for ∼5 h until precipitate
appeared in the system. The mixture was gravity-filtered through the
paper filter leaving behind some bright-yellow precipitate (later cited
as the compound I). The bulk precipitate was weighed, and the yield
was 26% (0.2049 g). The precipitate was left in the filter to dry at
room temperature for a week, while clear bright-yellow filtrate was
collected in a plastic centrifuge tube and kept in a vacuum desiccator
charged with concentrated sulfuric acid to expedite the crystal growth.
Elemental analysis for C H N O Zn calculated (found), %: C, 46.37
3
f
CH OH/benzene = 1:9) and thermal measurements: The compound
3
melts at ∼150 °C followed by its rapid decomposition.
Also, an examination of crystalline samples of the HPiCO side-by-
side under the microscope has confirmed the same crystal habits. Also,
a suitable single crystal specimen of HPiCO after ether trituration was
inspected using a single crystal diffractometer (Figure S1), and
determined unit cell parameters matched the recently published data
21
30
6
7
4
5
for the structure of HPiCO. Hence, both solvent treatments of the
complex I lead to the same outcome (see Results and Discussion
below). A series of reactions that describes observed chemistry in the
(
47.07), H, 5.56 (5.65), N, 15.45 (15.40). This composition
corresponds to the formula of I as [Zn(PiCO){H(PiCO) } (H O)].
2
2
Small plate-like bright yellow crystals were formed from the aqueous
yellow solution in the plastic centrifuge tube after ∼2 months, and the
crystals were harvested for structural studies. This compound will be
−
PiCO −Zn system is presented below.
In water/CH CN:
3
C
DOI: 10.1021/acs.inorgchem.7b01891
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
HPiCO (colorless) + KOH → K PiCO (bright‐yellow) + H O
Article
+
−
source. The first full-sphere data set obtained from four runs in ω
scanning mode allowed only determination of the atoms’ connectivity.
We used 60 s/frame exposure time with 0.5° step at 120 K and still
were able to obtain the high R(int) value of ∼0.26 from a small and
poorly diffracting crystal. Therefore, the second data set was collected
using the high-brilliance photon source at Lawrence Berkeley National
Laboratory (LBNL). The crystal was placed in the MiTeGen cryoloop,
and intensity data were collected at 150 K on a D8 goniostat equipped
with a Bruker PHOTON100 CMOS detector. The Si[111] channel
cut crystal was used as monochromator. The Beamline 11.3.1 at the
Advanced Light Source (LBNL) with synchrotron radiation tuned to λ
= 0.7749 Å was used for data collection. Thus, frames were measured
for a duration of 2 s for data at 0.5° intervals of ω with a maximum 2θ
value of ∼60° using ω−φ scans. The data frames were collected using
the program suite on APEX 2 and processed using the program
SAINT routine within the package. The data were corrected for
absorption and beam corrections based on the multiscan technique as
implemented in SADABS. A series of DISP cards had to be used for
the final refinement accounting for the elements scattering factors at
0.7749 Å crystal irradiation wavelength (Supporting Information page
2
(1)
Hydrolysis of the anion:
−
−
−
H O + 2PiCO ↔ OH + H(PiCO)
(2)
(3)
2
2
Complexation reaction:
2
+
−
−
[
Zn(H O) ] + PiCO + H(PiCO)2
2
6
→
[Zn(PiCO){H(PiCO) }(H O)] + 6H O
2
2
2
off‐yellow bulk precipitate,compound I
Trituration with ether:
[Zn(PiCO){H(PiCO) }(H O)]
4
2
2
3
→
[Zn (μ ‐OH) (PiCO) (H O) ] + 6HiPCO
4
2
6
2
4
bright‐yellow plates,tetra‐Zn‐compound II
+
4CH CN + 2H O
(4)
3
2
3
). Hydrogen atoms on methyl groups were placed in their idealized
Overall reaction:
positions and refined isotropically, while H-atoms on two water
molecules and bridging hydroxyl group were found on the difference
map. The structure was successfully solved using the synchrotron data
set by direct methods and will be presented and discussed herein.
Crystal and refinement data are presented in Table 1, while selected
bond lengths and angles are summarized in Table 2 with geometries
around metal centers presented in Figure 3. The molecular structure of
tetra-Zn complex II is shown in Figure 1. The complex is registered as
CCDC 1562109, and its PLATON check CIF report is shown in the
Supporting Information.
+
−
3
8
K PiCO + 4ZnSO ·7H O → [Zn (μ ‐OH) (PiCO) (H O) ]
4
2
4
2
6
2
4
+
2HPiCO + 4K SO + 22H O
(5)
2
4
2
Physical and Analytical Methods. Elemental analyses for all
−
obtained PiCO −Zn system compounds were analyzed for elemental
C−H−N content at Atlantic Microlab at Norcross, GA, USA.
Differential Scanning Calorimetry−Thermogravimetric
Analysis (DSC−TGA). The TG/DSC measurements of I, II, and
HPiCO were carried out under N (UHP grade) flow using a TA Q−
2
Powder Diffraction Data. The XRD patterns were recorded at
room temperature from 8 to 15 mg amounts of powdery samples
adhered by vacuum grease to the plastic MiTeGen loop. The Agilent
single crystal diffractometer equipped with Cu tube (λ = 1.54056 Å)
operated in the powder Θ−2Θ mode was used to record patterns to
6
00 instrument. Results are presented in Figure 1.
6
0° in 2Θ. Results are presented in Figure S3. Caution: Although we
have not encountered any problems during many years of laboratory work
and handling cyanoxime compounds, it is an absolute necessity to wear
protective gloves at all times when working with them due to their
pronounced biological activity.
RESULTS AND DISCUSSION
■
Preparation. The cyanoximes are much more acidic than
their other related family members aldoximes and ketoximes
due to the strong electron-withdrawing effect of the nitrile
40
group. Being easily ionizable in aqueous and nonaqueous
solutions, cyanoximes form a large array of metal com-
4
1,42,46
plexes.
Recently, we reported the pK value for the
a
45
HPiCO as equal to 4.91 ± 0.01. For comparison, the acidity
of the above aldoximes and ketoximes (including conventional
Figure 1. Molecular structure of the tetranuclear zinc complex II; an
aliphatic and aromatic α-dioximes) is in the pK range of 9−
a
47
ORTEP drawing at 50% probability level. Well-modeled geometry
12. In this context, it was unexpected to find that to this date
the HPiCO cyanoxime despite presence of the t-butyl group
attached to the core is one of the top four most acidic
−
with two chelating diastereomers of PiCO ; labeled L1−L4 are
differently coordinated to metal centers anions.
4
6a
cyanoximes after benzoylcyanoxime HBCO (pK = 4.27 ±
a
41a
IR and UV−Visible Spectroscopy. The IR spectra of compound
I, tetra-Zn complex II, and the initial HPiCO ligand were recorded at
ambient temperature using Bruker Vertex 70 Spectrophotometer in 13
mm KBr disks pressed at 9 tons of pressure delivered by a Carver
hydraulic press. Electronic spectra of the ligand and its zinc complexes
were recorded in solid state with the help of Cary 100 Bio
spectrophotometer equipped with the solid-state accessory by
Labsphere (Figure S2).
X-ray Analysis. Single Crystal Data. Two low- temperature data
sets were collected for a small bright-yellow plate of the Zn complex II:
one using an APEX 2 diffractometer with monochromated Mo
radiation (λ = 0.71073 Å) and the second using the synchrotron
0.01), and two isomeric 3-pyridyl and 4-pyridyl cyanoximes
(pK = 3.25 and 4.00 respectively). Deprotonation of HPiCO
a
leads to the formation of a yellow anion that exhibits strong
45
solvatochromism. In general, the reaction of room temper-
ature hydrolysis of cyanoximes was found to generate rather
unique “acid-salt monoanions” of HL2− composition, in
which H atom can be either symmetrically (equidistantly), or
asymmetrically bound to both anions. This was also the
40
−
situation in the case of the PiCO anion (eq 2) which led to the
formation of bulk precipitate I of [Zn(PiCO){H(PiCO) }]
2
composition in eq 3, with this product characterized by well-
D
DOI: 10.1021/acs.inorgchem.7b01891
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Table 1. Crystal and Refinement Data for the Tetranuclear Zn−Hydroxo-Cyanoximate II
chemical formula
formula weight, g/mol
temperature
C H N O Zn
42 64 12 18 4
1286.59
150(2) K
wavelength
0.77490 Å (synchrotron data; Si [111] filter)
0.005 · 0.030 · 0.040
clear bright yellow plate
triclinic
crystal size, mm
crystal habit, color
crystal system
space group
̅
P1, no. 2
unit cell dimensions
a = 11.1704(8) Å
b = 11.2712(8) Å
c = 11.5404(9) Å
1424.82(18)
α = 82.712(3)°
β = 88.895(3)°
γ = 81.342(3)°
volume, Å3
Z
1
3
density (calculated; g/cm )
absorption coefficient, mm−
F(000)
1.499
1
2.194
664
Θ range for data collection
index ranges
2.61 to 31.15°
−14 ≤ h ≤ 14, −15 ≤ k ≤ 15, −15 ≤ l ≤ 15
30299
reflections collected
independent reflections
max. and min transmission
absorption method
7069 [R(int) = 0.0573]
0.9870 and 0.9610
multiscan, from equivalents
direct methods
structure solution technique
structure solution program
refinement method
refinement program
function minimized
data/restraints/parameters
goodness-of-fit on F2
Δ/σmax
Bruker SHELXTL v6.14
Full-matrix least-squares on F2
Bruker SHELXTL v6.14
2
2 2
Σ w(Fo − F )
c
7069/134/469
1.026
0.001
final R indices
5481 data; I > 2σ(I) all data
R = 0.0316, wR = 0.0601
1
2
R = 0.0508, wR = 0.0649
1
2
2
2
2
weighting scheme
w = 1/[σ (F ) + (0.0191P) + 0.1749P]
o
where P = (F 2 + 2F )/3
2
0
c
largest diff. peak and hole, e·Å−
3
0.351 and −0.455
0.066
RMS deviation from mean, e·Å−
3
matching elemental analysis and IR spectra. Redissolving of
and 1.64% weight loss (Figure 1A). Then, molecules of
coordinated water leave the system at 134 °C (calculated/
found total % of water loss: 5.01/4.78), after which two
molecules of the HPiCO cyanoxime sublime from 165 to 192
°C (calculated/found % of weight loss: 29.63/30.18). Pure
uncomplexed cyanoxime undergoes sublimation within 176−
complex I in CH CN with subsequent reprecipitation with
3
ether has led to further nucleation of the complex into
tetrameric, tetra-Zn complex II. It was established in the past
−
that in the presence of water the HL2 acid-salts of cyanoximes
monoanions undergo dissociation to their initial components or
exist under equilibrium conditions:
2
16 °C with the maximum rate of the process at 193 °C
(
Figure S5). The tentative decomposition path for the complex
HL2 ↔ HL + L−
−
(6)
I is outlined in Figure S5. Tetra-Zn complex II experiences
continuous loss of four water molecules ending at ∼154 °C:
calculated/(found) 5.59% (5.41%) respectively (Figure 1B).
The intermediate products formed between 400−1000 °C are
unknown but may include remains of the decarboxylation
process, loss of cyanogen (CN) , and so on. The final
decomposition product for both complexes at ∼600 °C
appeared to be ZnO (Figure S6).
Vibrational and Electronic Spectra. Bands in the IR
spectrum of the pure HPiCO cyanoxime and its anion in the
bulk precipitate I and tetra-Zn complex II showed a
considerable decrease in the ν(CO) vibrational frequency
in the latter, which evidence coordination of the anion to metal
center through the oxygen atom of the carbonyl group (Figures
−
In this case, only deprotonated anionic cyanoxime L is able to
coordinate to the metal center because of its much higher
nucleophilicity, while the protonated cyanoxime HL is left
behind and can be easily separated from the mixture via
extraction or trituration with the solvent (eq 4). Therefore, an
overall reaction that describes preparation of the tetranuclear
Zn−cyanoximates complex in the system with 1:2 stoichio-
2
−
metric ratio between metal ion and PiCO can be presented as
in eq 5.
Thermal Analysis. The thermal decomposition patterns of
I and tetra-Zn complex II are completely different for both
complexes (Figure 1) and also different from that for the pure
cyanoxime (Figure S5). The decomposition pattern of I is such
that at first there is a loss of one molecule of water (presumably
as,s
S7 and S8). The increase in the frequency of the ν(N−O)
vibrations indicates involvement of the oxime fragment into
−
from the H{PiCO}2 anion) at 103 °C with 1.65% calculated
E
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Table 2. Selected Bonds and Angles in the Structure of Anionic Cyanoxime Ligands in the Tetranuclear Zn−Hydroxo Complex
II
ligand
bonds, Å (Figure 3)
geometry (in Scheme 3)
binding mode (in Scheme 4)
O1−N1 = 1.314
N1−C1 = 1.312
C1−C2 = 1.434
L1
L2
trans−anti, oxime form
O-monodentate, 2
C1−C3 = 1.474
C3−O2 = 1.230
C2−N2 = 1.148
O3−N3 = 1.291
N3−C8 = 1.317
C8−C9 = 1.439
cis−anti, nitroso form
cis−anti, nitroso form
cis−syn, nitroso form
N,O five-membered chelate + bridging, 8
N,O five-membered chelate + bridging, 8
O,O six-membered chelate + bridging, 10
C8−C10 = 1.489
C10−O4 = 1.230
C9−N4 = 1.143
O5a−N5a = 1.296
N5a−C15a = 1.342
C15a−C17a = 1.446
C15a−C16a = 1.413
C17a−O6a = 1.236
C16a−N6a = 1.433
O5−N5 = 1.286
N5−C15 = 1.325
C15−C17 = 1.446
C15−C16 = 1.434
C17−O6 = 1.241
C16−N6 = 1.144
diastereomeric mixture: L3
L4
Metal Centers
Zn1−O1 = 1.9987(12)
Zn1−O7B = 2.0235(13)
Zn1−O4 = 2.1332(13)
Zn2−O7B = 2.0652(13)
Zn2−O3 = 2.0760(13)
Zn1−N3 = 2.1201(15)
Zn1−O8W = 2.1746(15)
Zn2−O7B = 2.0710(14)
Zn2−O9W = 2.1071(14)
Zn−L Distances in Diastereomers
Zn1−O5 = 2.164(4)
N5A−Zn2 = 2.133(4)
O6A−Zn2 = 2.325(6)
Zn1−O5A = 2.219(5)
O5−Zn2 = 2.170(3)
O6−Zn2 = 2.088(4)
41a,e,42g,48
coordination either via O- or N-binding
(Table S1).
Scheme 3. Geometrical Isomerism Observed in Cyanoximes
Both were confirmed by the X-ray analysis results and are
presented below. The IR spectrum of I to some extent
resembles that of the pure ligand HPiCO (Figure S9) with only
few new bands related to anionic cyanoxime in its nitroso
4
5
form, while two isolated complexes I and tetra-Zn complex II
possess yellow color with a broad band with a shoulder at ∼420
45,46e,48a
nm which belongs to the n → π* ligands transition
−
concept of the bidentate ligands such as classical NCX and
(
Figure S10). Both these Zn complexes represent non-
−
XCN anions (X = O, S, Se, and Te) when a new series of the
cyano-methanides, cyano-amides, and cyanoximes and their
fluorescent compounds.
X-ray Powder Diffraction. The two XRD patterns for I
and II have some similarities with some peaks present in both
profiles but do not match exactly (Figure S3). Therefore, two
60
numerous complexes were discovered and studied.
−
Specifically, for the PiCO anion the most straightforward
ways of coordination are presented in Scheme 4. As an
illustration for this concept, we should mention that the variety
of binding modes of other cyanoximes has been well-
Thus, coordination type 1 was
observed for the benzoylcyanoxime anion BCO in its
−
Zn(II) complexes obtained with PiCO are not isostructural.
Structural Studies. We were successful in growing crystals
only for tetra-Zn complex II suitable for X-ray analysis, despite
numerous attempts to obtain diffracting crystalline material for
precipitating in bulk complex I. The crystal structure of the
41e,49
documented in the past.
−
50
macrocyclic complex, while types 3 and 4 were found in
45
41a−c,51
uncomplexed cyanoxime was recently reported and evidence
the adoption of trans−anti geometry by the ligand (Scheme 3).
Displayed here are four principal geometrical isomers which
have been observed for other cyanoximes and their anions in
abundance in a variety of Tl(I) complexes.
Binding
modes 5 and 7 were observed in transition metal complex-
46a,b,d,49
42b,d,f,g
es,
types 6 and 8 in Ag(I) cyanoximates,
and
52
coordination types 2 and 11 in organometallic antimony,
and tellurium compounds (Scheme 4). More
specifically, for the PiCO anion its ampolydentate behavior
4
1c,48a
46c
53
the recent past
dentate ligands.
as these compounds represent ampoly-
This term was coined to broaden the
tin,
4
0,41e,49
−
F
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Inorganic Chemistry
Article
−
Scheme 4. Possible Binding Modes of the PiCO Anion in
Complexes
t-butyl group: see Supporting Information page 3 for detail),
and EADP for one of the carbon atoms that showed rather a
“
flat” feature.
There are in total four different binding modes of the PiCO−
anion in this structure which are marked as L1−L4 in Figure 2
and subsequently explained in Table 2. Fragments of crystal
structure with unobstructed views of different parts (L1, L2 and
L3, L4) together with the numbering scheme can be found in
Figure 2. First, a mixture of two diastereomers in one chelating
cyanoxime is cocrystallized in one unit cell and statistically
−
distributed throughout the crystal. Thus, PiCO adopts cis−
anti geometry (Scheme 3) that is reflected in binding mode 8
(
3
Scheme 4; L3 in Figure 2B) and cis−syn geometry (Scheme
) shown in binding mode 10 (Scheme 4; L4 in Figure 2B) at
the ratio 0.57:0.43. It should be noted that it is not two
positional disorders but indeed a cocrystallized mixture of two
geometrical isomers that have similar lattice energies and crystal
packing. The presence of aforementioned four different binding
modes of the same cyanoxime anion in one complex is rather
−
remarkable in itself. The geometry of the PiCO anion in the
monodentate O-binding mode in L1 (Figure 2A) is very similar
45
to that in the structure of uncomplexed HPiCO. There are
two kinds of chelating + bridging cyanoximes L2, L3 and L4
(
Figure 2). The first two have formed five-membered
metallocycles with the values of “bite angles” O4−Zn1−N3 =
7
5.16° and O6a−Zn2−N5a = 71.31°, while cis−syn diaster-
eomer L4 forms six-membered chelate and has a “biting angle”
O6−Zn2−O5 equal to 80.31°.
There are two crystallographically independent Zn1 and Zn2
atoms in the ASU of the structure of the tetra-Zn complex II
and both possess distorted octahedral geometry with metal
centers environment depicted in (Figure 3). The average Zn- -
-
Zn distance is 3.273 Å and is large to consider any intermetallic
interactions. An inversion center generates tetra-zinc complex
3
(
Figure 1) in which the hydroxo-group O(7)−H acts as a μ -
bridge. The distorted trigonal-pyramidal geometry around
O(7) in this group is shown in Figure 4. It is quite similar to
the geometry of some active catalysts and Zn-based enzymes,
for which, presented in this work, tetra-Zn complex II may act
as a structural model. This complex, however, is stable at
was crystallographycally confirmed as coordination modes 2−4
44a
45
in its Tl complex, mode 7 in Pd, Pt complexes, and binding
42f
mode 12 in Ag complex (Scheme 4). The IR spectroscopy
evidence the adoption of modes 2 and 11 in organoantimony-
4
3
43
(
V) and organotin(IV) compounds, respectively. At the
ambient conditions to reaction with CO present in the air.
2
same time, uncomplexed protonated cyanoxime HPiCO has
trans−anti geometry shown in Scheme 3.
There are other important features in the structure of tetra-
Zn complex II: intramolecular and intermolecular H-bonding.
The geometry of a strong intramolecular H-bond is best seen in
an unobstructed view in Figure S12. There is a ver−y
pronounced “tilt” of the monodentate coordinated PiCO
anion (as L1, Figure 1) bound to Zn1 toward coordinated to
Zn2 water molecule O9w (Figure S12). The intermolecular H-
bonding has two parts. One is the bond between the CN-group
and water molecule O8w (Figure S13). The geometry of this
medium strength H-bond is best seen viewing the unit cell
content (Z = 2) in the Figure S13, namely, these H-bonds are
holding molecules of the tetra-Zn complex II in a chain
structure along c-direction as it is shown in Figure S14. The
second part of intermolecular H-bonding represents 2.164 Å
45
The crystal structure of tetra-Zn complex II is shown in
Figure 1, and there are several interesting features that have to
be discussed. First, the successful structure solution and
refinement demonstrates overall quite reasonable connectivity
of atoms, but five of them show rather large thermal
displacement parameters unexplainable in comparison with
those of other adjacent atoms in the same molecule (Figure
S11). It is interesting to mention that the Platon checkCIF
report for this nonmodeled structure did not contain A- or B-
type alerts. Nevertheless, the structure did not look right or
completely acceptable, and needed improvement. It was
possible to find on a difference map Q-peaks that correspond
to two alternative structures one of the cyanoxime anions: syn
and anti diastereomers. It was necessary to apply a series of
restrains to keep all atoms connected in proper order with
correct geometry of functional groups affected. More
specifically, these were DFIX commands for one of the water
molecules (O8W H8Wa, O9W H9 Wb with 0.85 Å bond
distance), RIGU (for two alternative positions of the cyanoxime
including hydrogen atoms attached to the carbon atoms of the
3
bond between the oxygen atom of the keto-group and μ -
bridging hydroxo group O7 (Figure S15).
The crystal packing diagram of the tetra-Zn complex II is
displayed in Figure S16 and shows intermolecular van der
Waals contacts between hydrophobic t-butyl groups directed at
each other. These contacts are responsible for the formation of
layers of the complex in the structure. On the basis of results of
the thermal analysis, IR-spectra, and data of elemental analysis,
G
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Figure 2. Fragments of the crystal structure of tetra-Zn complex II showing the atomic numbering sheme in monodentate coordinated L1 (trans−
anti), bridging-chelating L2 (cis−anti) (A), and in well-modeled geometry of two diastereomers L3 (cis−anti, solid bonds) and L4 (cis−syn, open
bonds) in the complex (B).
the suggested structure of complex I is shown in the Supporting
Information, below Table S1.
Catalytic Activity in the Transesterification Reaction.
We have tested the catalytic activity of our compound I toward
the transesterification reaction of methyl-4-nitrobenzoate. In a
typical reaction, a mixture of this ester and catalyst I in EtOH
transesterification reaction, and the obtained reaction yield was
only 15% (entry 14, Table 3).
When we used 3 mol % catalyst I, a conversion of 94% of
methyl-4-nitrobenzoate into ethy-l-4-nitrobenzoate was reached
after 24 h of reaction time (entry 5, Table 4). The increase of
the catalyst amount enhances the product yield from 65 to 94%
for the corresponding amounts of 1.0 and 3.0 mol % catalyst I
(
2 mL) was added into a capped glass vessel and the resulting
(
entries 6 and 5, respectively, Table 4), but a further increase in
the catalyst amount to 7 mol % improves the yield only by 1%
entries 6−7, Table 4). Increasing the reaction temperature
from room temperature to 75 °C (entries 10, 11, and 5 Table
) resulted in yield improvement from 10 to 94%, but a further
mixture was stirred at 75 °C for 24 h (Scheme 5). The solvent
was evaporated in vacuum, leading to a crude mixture of
(
1
products which was analyzed by H NMR spectroscopy (Figure
S17).
4
The effects of temperature, reaction time, amount of catalyst,
and the alcohol were investigated, and the obtained results are
presented in Table 3. A blank test was carried out with methyl-
rise in the temperature to 100 °C (entry 12, Table 4) led to a
decrease in the overall yield.
Extending the reaction time to 48 h did not lead to a
significant increase of the yield. The high conversion rate
suggests that I act as an efficient catalyst for the trans-
esterification reaction. The product yield increases with time
(Figure 5) until 24 h, but after 12 h, it already reached 85%
4-nitrobenzoate in the absence of any metal catalyst at 75 °C in
ethanol, and no considerable amount of ethy-l-4-nitrobenzoate
was detected after a reaction time of 24 h (entry 13, Table 4).
We have also checked the activity of ZnSO ·7H O in the
4
2
H
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Inorganic Chemistry
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Figure 3. Donor atoms environment and geometry of two
independent zinc(II) atoms in the ASU in the structure of tetranuclear
complex: A, bond length (Å); B, valence angles (°). Hydrogen atoms
on water molecules and bridging hydroxyl groups are shown while all
other atoms were removed for clarity. Symmetry code for #1: −x, −y,
−
z.
3
Figure 4. Geometry of μ -bridging hydroxo-group in the tetranuclear
o
complex: two orthogonal views. A, bond lengths (Å); B, angles ( ); C,
intermetallic distances. Symmetry transformation for #1 position: 1 −
(
entry 4). Thus, we have obtained the best reaction yield by
using 3 mol % of catalyst at 75 °C for 24 h. When 1-propanol
x, 1 − y, −z.
was used instead of ethanol the reaction yield decreased from
9
4 to 87% (entries 8 vs 5, Table 4). The reaction conversion
Scheme 5. Transesterification Reaction Used as Probe in
Catalysis Studies of Activity of Zn−Cyanoximates
further decreased to 17% with the use of 2-propanol, a
secondary alcohol (entry 9, Table 3). Hence, for further studies
we have used ethanol as the alcohol. Moreover, we have
performed control experiments using different ratios of water
and ethanol mixture and the results indicate that the reaction
yield significantly decreases upon increasing the water
concentration (entries 15−17, Table 3).
We also investigated the reaction of various substituted
methyl benzoates in ethanol. Those with the strongest electron-
withdrawing nitro substituent underwent the fastest trans-
esterification (entries 1−2, Table 4) whereas that with the
electron-donating methoxy group experienced the slowest
conversion (entry 5, Table 4). Ethyl-4-hydroxybenzoate also
converted to methyl-4-hydroxybenzoate upon reaction with
methanol, with a yield of 54% (entry 10, Table 4). We also
tested some aliphatic esters and obtained yields are in the range
of 21−63% (entries 8−9 and 11−12, Table 4). To check the
integrity of the complex catalyst after the catalytic reaction, we
have performed the transesterification reaction with a low
boiling ester, e.g., ethyl acetate, and isolated the catalyst by
taking the reaction solution to dryness. FT-IR of complex I
after the reaction was identical to the initial one indicating that
the structure of the solid was retained (Figure S18A). We
tested the recyclability of complex I by using ethyl acetate as
substrate. For this purpose, upon completion of a reaction
cycle, we separated the catalyst by evaporation of the solvent,
washing with dichloromethane, and drying at room temper-
ature before further use. We repeatedly recycled catalyst I, and
the catalytic system essentially maintained the full activity for at
least four consecutive cycles as shown in Figure S18B.
The transesterification of methyl-4-nitrobenzoate with
ethanol has not been broadly studied. Very few Zn(II)
compounds were reported to catalyze this reaction. For
example, the 3D zinc(II) MOF of 5-benzamidoisophthalate
I
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Table 3. Optimization of Parameters of the Transesterification Reaction of Methyl-4-Nitrobenzoate in Alcohols with Complex I
a
as Catalyst
b
c
entry
catalyst
time (h)
amount of catalyst (mol %)
T (°C)
alcohol
EtOH
yield (%)
TON
1
2
3
4
5
6
7
8
9
I
I
I
I
I
I
I
I
I
I
I
I
1
3
3.0
3.0
3.0
3.0
3.0
1.0
7.0
3.0
3.0
3.0
3.0
3.0
75
75
75
75
75
75
75
75
75
RT
50
100
75
75
75
75
75
51
63
67
85
94
65
95
87
17
10
63
77
17
21
22
28
31
65
13
29
6
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
1-PrOH
2-PrOH
EtOH
EtOH
EtOH
EtOH
EtOH
6
12
24
24
24
24
24
24
24
24
24
24
24
24
24
10
11
12
13
14
15
16
17
3
21
26
blank
ZnSO ·7H O
3.0
3.0
3.0
3.0
15
15
0.9
5
4
2
I
I
I
H O/EtOH (1:3)
2
5
H O/EtOH (1:1)
2
0.3
H O/EtOH (3:1)
2
no reaction
a
b
1
c
Reaction conditions: methyl ester (0.5 mmol), ethanol (2.0 mL) and catalyst I at 75 °C for 24 h. Calculated by H NMR method. Number of
moles of ethyl ester per mole of catalyst.
Table 4. Transesterification Reaction of Various Methyl/
Ethyl Esters in Ethanol/Methanol Catalysed with the Zn−
a
Cyanoximate Complex I
b
entries
substrate
alcohol
yield (%)
TON
1
2
3
4
5
6
7
8
9
methyl-3-nitrobenzoate
methyl-2-nitrobenzoate
methyl-4-aminobenzoate
methyl-4-iodobenzoate
methyl-4-methoxybenzoate
methyl-3-hydroxybenzoate
methyl-2,4-dihydroxybenzoate
methyl butyrate
91
89
72
47
10
22
18
33
21
54
63
37
30
29
24
16
3
ethanol
7
6
11
7
vinyl acetate
1
1
1
0
1
2
ethyl-4-hydroxybenzoate
ethyl acetate
18
21
12
methanol
ethyl butyrate
a
Reaction conditions: 3.0 mol % of catalyst, methyl/ethyl ester (0.5
b
mmol) and ethanol/methanol (2.0 mL) at 75 °C for 24 h. Calculated
by H NMR method.
1
Figure 5. Plot of product yield vs time for the transesterification
reaction of methyl-4-nitrobenzoate and ethanol catalyzed by I.
led to an overall yield of 91% after 30 h of reaction time at 75
2
5
°C. Other 3D frameworks of zinc(II) constructed with Zn(II)
ligands, which takes 1.5 days for the complete conversion of
phenyl acetate to phenol and methyl acetate at same reaction
temperature. They have also reported polymer-supported Zn
and 2-propionamidoterephthalate ligand led to an overall yield
of 96% in the same reaction after 24 h at the higher
temperature of 100 °C. Nevertheless, our catalyst I leads to a
25
catalysts which are more efficient for such a reaction (required
25
25
yield of 94% after 24 h reaction time at 75 °C. The above two
examples show that our catalyst appears to be more efficient
than other reported Zn(II) compounds (Figure 5).
0.8 days at room temperature for complete conversion).
Moreover, a mononuclear Zn(II) complex of 2,2′-dipyridyl-
amine also effectively catalyzes the transesterification of phenyl
acetate at room temperature, with complete conversion into the
In addition, the transesterification reactions of phenyl or
substituted phenyl acetate in different alcohols catalyzed by zinc
compounds, to afford the corresponding substituted phenols
2
5a
corresponding products in 0.25 days.
A 1D Zn(II)
coordination polymer of meta-bis(imidazolylmethyl)benzene
ligand also catalyzes the reaction of phenyl acetate and benzyl
25
and acetates, have been explored well. For example, the
transesterification of phenyl acetate and methanol, in the
presence of a dinuclear Zn(II) complex constructed by
benzoate and quinoxaline ligands takes ca. 7 days for complete
25
alcohol in toluene with a 91% reaction yield at 120 °C in 5 h.
The mechanism of the metal-catalyzed transesterification
conceivably involves an electrophilic activation of the carbonyl
moiety of the substrate upon binding to the metal center of the
25
conversion to the corresponding products at 50 °C. The same
research group has reported the use of a 1D Zn(II)
coordination polymer of benzoate and 1,2-bis(4-pyridyl)ethane
19
catalyst. Accordingly, the Lewis acidity of the metal center is
relevant and a possible mechanism is shown in Figure 6. The
J
DOI: 10.1021/acs.inorgchem.7b01891
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Inorganic Chemistry
Article
Accession Codes
CCDC 1562109 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_
request@ccdc.cam.ac.uk, or by contacting The Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge CB2
1
EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Author
E-mail: NNGerasimchuk@MissouriState.edu.
■
*
ORCID
Figure 6. Proposed catalytic cycle for the transesterification reaction
catalyzed by I.
Nikolay Gerasimchuk: 0000-0003-4867-6475
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
ESI-MS spectrum (Figure S17) of the reaction mixture shows a
peak at m/z 765.16, which can be accounted for by the
formation of [Zn(PiCO){H(PiCO) }·4-NO PhCOOEt·EtOH·
H] . This suggests that 4-nitroethyl benzoate and ethanol add
to the Zn center as we are proposing in the reaction
mechanism.
Future Work. The observations and results obtained from
the studies of these compounds has fostered our interest for
further exploration of other analogues, for probable activity in
catalyzing useful reactions like epoxidation, transamidation, ring
opening polymerization, C−C bond forming reactions, and so
on.
■
We are very thankful to Dr. Jeannette Krause and Dr. Allen
Oliver for the data collection at the synchrotron facility through
the SCrALS (Service Crystallography at the Advanced Light
Source) program at Beamline 11.3.1 at the Advanced Light
Source (ALS) in the Lawrence Berkeley National Laboratory.
The Advanced Light Source is supported by the Director,
Office of Science, Office of Basic Energy Sciences, of the U.S.
Department of Energy under Contract No. DE−AC02−
05CH11231. We thank Bruker Analytical X-ray Instruments,
Inc., Madison, WI, for APEX2 v2014.11.0 and SAINT v8.34A
data collection and data processing programs, and Professor G.
2
2
+
̈
M. Sheldrick, University of Gottingen, Germany, for the use of
CONCLUSIONS
the SADABS v2014.5 semi-empirical absorption and beam
correction program. We also thank instructors Dr. Bruce Noll
(Bruker AXS, Inc.) and Dr. Roger Sommer (North Caroline
State University) at the American Crystallographic Association
Small Molecules summer school for assistance with the
structure refinement. This work has been supported by the
Foundation for Science and Technology (FCT) (project UID/
QUI/00100/2013), Portugal. A.K. expresses his gratitude to the
FCT for postdoctoral fellowship (ref. No. SFRH/BPD/76192/
■
First, zinc cyanoxime-based complexes were obtained in a Zn−
−
PiCO system and were characterized using spectroscopic
methods, thermal analysis, and X-ray crystallography. Second,
the crystal structure of the tetra-Zn complex II showed the
formation of the tetra-zinc complex with μ -bridging OH-
3
groups, with cyanoxime anions demonstrating three different
binding modes, as well coexistence of the oxime- and nitroso-
anions. Third, there is a rare phenomenon in the structure of II
of cocrystallization of the two dieastereomers of the cyanoxime
to same metal centers at 0.57 (cis−anti) to 0.43 (cis−syn) ratio.
Fourth, Zn(II) compound I acts as a good homogeneous
catalyst for the transesterification reactions of methyl
carboxylate esters. It effectively catalyzes the reaction of various
methyl benzoate esters with ethanol producing the correspond-
ing ethyl benzoate esters (and methanol) in high yields which
depend on the electrophilicity of the benzoate substituent,
under relatively mild conditions. The process has “green”
features that deserve to be further explored for other reactions.
2
011). N.G. is grateful to Dr. Sergey Lindeman (Marquette
University, Milwaukee, WI, USA) for recording XRD powder
diffraction patterns on a single crystal diffractometer in his
laboratory and to Ms. Janet Dyer for invaluable technical help
with the manuscript preparation.
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■
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ASSOCIATED CONTENT
■
(
*
S
Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.7b01891.
(
(
Crystallographic information, photograph of solid-state
accessory, applied restraints for final structure refine-
ment, XRD patterns for compounds I and II, DSC/TGA
profiles, IR spectra and vibrational frequencies, reflec-
tance spectrum, complex II structures, unit cell
4
(
(
Chem. 1994, 33, 490−497.
(9) Chen, L.; Thompson, L. K.; Bridson, J. N. Inorg. Chem. 1993, 32
(13), 2938−2943.
1
structures, elemental analyses, H NMR, FT-IR, and
ESI-MS spectra (PDF)
K
DOI: 10.1021/acs.inorgchem.7b01891
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