Copper(II) complexes with carboxylic- or sulfonic-functionalized arylhydrazones of acetoacetanilide and their application in cyanosilylation of aldehydes

Article abstract of DOI:10.1016/j.jorganchem.2017.02.006

The new copper(II) complexes [Cu(HL¹)(H₂O)(CH₃OH)] (1, H₃L¹?= (Z)-2-(2-(1,3-dioxo-1-(phenylamino)butan-2-ylidene)hydrazinyl)benzoic acid) and [Cu(HL²)(H₂O)(CH₃OH)] (2,

Full text of DOI:10.1016/j.jorganchem.2017.02.006

Accepted Manuscript
Copper(II) complexes with carboxylic- or sulfonic-functionalized arylhydrazones of
acetoacetanilide and their application in cyanosilylation of aldehydes
Atash V. Gurbanov, Kamran T. Mahmudov, Manas Sutradhar, Fátima C. Guedes da
Silva, Talib A. Mahmudov, Firudin I. Guseinov, Fedor I. Zubkov, Abel M. Maharramov,
Armando J.L. Pombeiro
PII:
S0022-328X(17)30067-0
10.1016/j.jorganchem.2017.02.006
JOM 19795
DOI:
Reference:
To appear in: Journal of Organometallic Chemistry
Received Date: 12 December 2016
Revised Date: 4 February 2017
Accepted Date: 6 February 2017
Please cite this article as: A.V. Gurbanov, K.T. Mahmudov, M. Sutradhar, F.C. Guedes da Silva,
T.A. Mahmudov, F.I. Guseinov, F.I. Zubkov, A.M. Maharramov, A.J.L. Pombeiro, Copper(II)
complexes with carboxylic- or sulfonic-functionalized arylhydrazones of acetoacetanilide and their
application in cyanosilylation of aldehydes, Journal of Organometallic Chemistry (2017), doi: 10.1016/
j.jorganchem.2017.02.006.
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ACCEPTED MANUSCRIPT
Graphical abstract

ACCEPTED MANUSCRIPT
Copper(II) complexes with carboxylic- or sulfonic-functionalized
arylhydrazones of acetoacetanilide and their application in
cyanosilylation of aldehydes
a
a
Atash V. Gurbanov,a,b,c Kamran T. Mahmudov,a,b, Manas Sutradhar, Fátima C. Guedes da Silva,
*
Talib A. Mahmudov,b Firudin I. Guseinov,d,e Fedor I. Zubkov,c Abel M. Maharramov,b
Armando J. L. Pombeiro a,*
a Centro de Química Estrutural, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1049–
001 Lisbon, Portugal
b Department of Chemistry, Baku State University, Z. Xalilov Str. 23, Az 1148 Baku, Azerbaijan
c Organic Chemistry Department, RUDN University, 6 Miklukho-Maklaya str., Moscow 117198, Russian
Federation
d Institute for Chemistry and Biology Immanuel Kant Baltic Federal University, Universitetskaya str.,
236000 Kaliningrad, Russian Federation
e Department of Chemistry, M. V. Lomonosov Moscow State University, 1 Leninskie Gory, 119992
Moscow, Russian Federation
Corresponding authors
Phone: +351 218419237
E-mail addresses:
kamran_chem@mail.ru, kamran_chem@yahoo.com (K.T. Mahmudov),
pombeiro@tecnico.ulisboa.pt (Armando J. L. Pombeiro)
Abstract
The new copper(II) complexes [Cu(HL1)(H2O)(CH3OH)] (1, H3L1 = (Z)-2-(2-(1,3-dioxo-1-
(phenylamino)butan-2-ylidene)hydrazinyl)benzoic acid) and [Cu(HL2)(H2O)(CH3OH)] (2, NaH2L2
= sodium (Z)-2-(2-(1,3-dioxo-1-(phenylamino)butan-2-ylidene)hydrazinyl)benzenesulfonate) were
synthesized and characterized by IR and ESI-MS spectroscopies, elemental and X-ray crystal
structural analyses. The coordination environment of the central copper(II) has a square pyramidal
geometry, three sites being occupied by (HL1,2)2−, which chelates in the O,N,O fashion, while the
two other sites are filled with the water and methanol ligands. Multiple intra- and intermolecular
non-covalent interactions between the (HL1,2)2−, water and methanol ligands lead to supramolecular
network. Both compounds 1 and 2 act as homogenous catalysts for the cyanosilylation reaction of a
variety of both aromatic and aliphatic aldehydes with trimethylsilyl cyanide affording the
corresponding cyanohydrin trimethylsilyl ethers in high yields (up to 90 %) and at room
temperature.
Keywords: Arylhydrazones of acetoacetanilide; CuII complexes; cyanosilylation of aldehydes.

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1. Introduction
On account of the variability of the active methylene fragment and attachment of substituents to the
aromatic moiety, arylhydrazones of active methylene compounds (AHAMC) are versatile ligands in
coordination chemistry [1]. Thus, multifunctional AHAMC can be prepared from a variety of
aromatic amines and active methylene compounds by the easy and well-studied two-step
Japp−Klingemann procedure (diazotization and azocoupling) [2]. The attachment of –COOH, –
SO3H, etc. on the ortho-position of the aromatic moiety of AHAMC can provide a stable six-
membered H-bonded cycle, stabilized by intramolecular hydrogen bonding, in particular resonance-
assisted hydrogen bonding (RAHB) (Scheme 1) [3]. In many cases, these H-bonded cycles
determine the reactivity of AHAMC ligands towards metal ions; for example, copper can easily
enter into this pocket, but for zinc coordination it has to be destroyed by a base [1]. Furthermore,
the variation of the active methylene fragment of AHMAC ligands can also influence the
coordination ability of these ligands; for example, replacement of acetylacetone by barbituric acid
provides more donor sites [4,5]. Hence, replacement of acetylacetone, barbituric acid, etc. by
another β-diketone, i.e., acetoacetanilide, and attachment of the –COOH or –SO3H group to the
ortho-position of the aromatic moiety of the arylhydrazone fragment of the AHAMC ligands give
carboxylic- or sulfonic-functionalized arylhydrazones of acetoacetanilide (Scheme 1), which can be
used as new promising ligands in the synthesis of new coordination compounds and multi-
component supramolecular architectures.
Scheme 1. Carboxylic- or sulfonic-functionalized arylhydrazones of acetoacetanilide.
On the other hand, the cyanation reaction of aldehydes (or ketones) with cyanide sources
(HCN and its salts, acyl cyanides, alkyl cyanoformates and trialkylsilyl cyanides) is an important
procedure for the formation of new C–C bonds (Scheme 2). Cyanohydrins or cyanohydrin
trimethylsilyl ethers are key intermediates for the synthesis of many valuable molecules such as α-
hydroxy aldehydes, α-hydroxy acids and β-amino alcohols [6]. Trimethylsilyl cyanide (TMSCN) is
one of the most used cyanating sources for the nucleophilic addition to carbonyl compounds, since
it is easy to handle, provides a high atom economy without side reactions, and possesses low
dissociation energy of the Si–C bond [7−12]. Many organocatalysts or transition metal complexes
have been applied as catalysts in the nucleophilic addition of cyanide to carbonyl compounds [6].
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However, there is only a limited number of metal complex approaches to this transformation in
homogenous catalytic systems [13−17]. Thus, the search for efficient and environmentally friendly
homogenous protocol for cyanosilylation of carbonyl compounds with TMSCN is still highly
desirable [6].
Scheme 2. Cyanosilylation of aldehydes.
Hence, on the basis of the above considerations, we focused this work on the following aims: i) to
synthesize new copper(II) complexes, [Cu(HL1)(H2O)(CH3OH)] (1, H3L1 = (Z)-2-(2-(1,3-dioxo-1-
(phenylamino)butan-2-ylidene)hydrazinyl)benzoic acid) and [Cu(HL2)(H2O)(CH3OH)] (2, NaH2L2
= sodium (Z)-2-(2-(1,3-dioxo-1-(phenylamino)butan-2-ylidene)hydrazinyl)benzene-sulfonate); ii) to
evaluate the catalytic activity of the prepared new complexes in the cyanosilylation of aldehydes
with TMSCN.
2. Experimental
2.1. Materials and instrumentation
All the chemicals were obtained from commercial sources (Aldrich) and used as received. The
NaH2L2 was synthesized according to the reported procedure [18]. Infrared spectra (4000–400
cm−1) were recorded on a Vertex 70 (Bruker) instrument in KBr pellets. Carbon, hydrogen, and
nitrogen elemental analyses were done using a "2400 CHN Elemental Analyzer" (Perkin Elmer).
1
The H and 13C NMR spectra were recorded at room temperature on a Bruker Avance II + 300
(UltraShieldTM Magnet) spectrometer operating at 300.130 and 75.468 MHz for proton and carbon-
13, respectively. The chemical shifts are reported in ppm using tetramethylsilane as the internal
reference. Electrospray mass spectra (ESI-MS) were run with an ion-trap instrument (Varian 500-
MS LC Ion Trap Mass Spectrometer) equipped with an electrospray ion source. For electrospray
ionization, the drying gas and flow rate were optimized according to the particular sample with 35
p.s.i. nebulizer pressure. Scanning was performed from m/z 0 to 1100 in methanol solution. The
compounds were observed in the positive mode (capillary voltage = 80–105 V).
2.2. Synthesis of H3L1
H3L1 was synthesized according to a well-established procedure applied to the preparation of
NaH2L2 [18] (Scheme 1), by azocoupling of 2-carboxybenzenediazonium chloride with
acetoacetanilide in the presence of sodium hydroxide.
Diazotization: A 10 mmol portion of 2-aminobenzoic acid was dissolved in 50 mL of water upon
addition of 0.8 g of crystalline NaOH. The solution was cooled in an ice bath to 0 °C and 10 mmol
3

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of NaNO2 were added with subsequent addition of 2 mL 33 % HCl in portions of 0.2 mL for 1 h,
under vigorous stirring. During the reaction, the temperature of the mixture must not exceed +5 °C.
The obtained diazonium salt was used for the next stage (see below).
Azocoupling: 10 mmol of NaOH were added to a solution of 10 mmol of acetoacetanilide in 50 mL
water-ethanol (1/5, v/v). The solution was cooled in an ice bath, and a suspension of 2-
carboxybenzenediazonium chloride (prepared according to the procedure described above) was
added in two equal portions under rigorous stirring for 1 h. Then the system was acidified by
addition of concentrated HCl (until pH 5). The formed yellow precipitate of (Z)-2-(2-(1,3-dioxo-1-
(phenylamino)butan-2-ylidene)hydrazinyl)benzoic acid (H3L1) was filtered off, recrystallized from
methanol and dried in air.
H3L1: Yield 84 % (based on acetoacetanilide), yellow powder, soluble in water, DMSO, ethanol and
dimethylformamide and insoluble in non-polar solvents. Anal. Calcd. for C17H15N3O4 (M =
325.32): C, 62.76; H, 4.65; N, 12.92. Found: C, 62.67; H, 4.74; N, 12.98 %. IR (KBr): 3436 (br.)
ν(H2O), 3240 ν(NH), 1696 and 1663 ν(C=O), 1599 ν(C=N) cm−1. MS (ESI) (positive ion mode):
m/z: 326.11 [Mr+H]+. 1H NMR (DMSO-d6): δ 2.54 (s, 3H, CH3), 7.15−8.00 (9H, Ar−H), 11.08 (s,
1H, NH), 14.84 (s, 1H, NH). 13C{1H} (DMSO-d6): δ 26.56 (CH3), 115.67 (Ar−H), 116.47(Ar−H),
120.66 (2Ar−H), 124.06 (Ar−H), 124.96 (Ar−H), 129.54 (2Ar−H), 131.06 (Ar−H), 131.86
(Ar−COOH), 135.00 (C=N), 137.85 (Ar−CNH), 143.96 (Ar−NH−N), 161.07 (−COOH), 168.41
and 199.26 (C=O).
2.3. Synthesis of [Cu(HL1)(H2O)(CH3OH)] (1)
0.1 mmol (32 mg) of H3L1 were dissolved in 20 mL of methanol, then 2 drops of Et3N and 0.1
mmol (23 mg) of Cu(NO3)2ꢀ2.5H2O were added. The mixture was stirred for 5 min and left for slow
evaporation. Greenish crystals of the product started to form after ca. 2 d at room temperature; they
were filtered off and dried in air.
1: Yield, 56 % (based on Cu). Calcd. for C18H19CuN3O6 (Mr = 436.06): C 49.48, H 4.38, N 9.62;
found C 49.31, H 4.26, N 9.52. MS (ESI, positive ion mode), m/z: 387.0 [Mr−CH3OH−H2O+H]+.
IR (KBr): 3461 (s, br) ν(OH), 3007 ν(NH), 1674 (s) ν(C=O), 1591 (s) ν(C=N) cm–1.
2.4. Synthesis of [Cu(HL2)(H2O)(CH3OH)] (2)
0.1 mmol (37 mg) of NaH2L2 were dissolved in 25 mL of methanol, then 0.1 mmol (23 mg) of
Cu(NO3)2ꢀ2.5H2O was added and the system was stirred for 10 min. After ca. 2 d at room
temperature, greenish crystals precipitated, which were then filtered off and dried in air.
2: Yield, 53 % (based on Cu). Calcd. for C17H19CuN3O7S (Mr = 472.96): C 43.17, H 4.05, N 8.88;
found C 43.24, H 3.98, N 8.77. MS (ESI, positive ion mode), m/z: 423.9 [Mr−CH3OH−H2O+H]+.
IR (KBr): 3415 (s, br) ν(OH), 3280 ν(NH), 1627 (s) ν(C=O), 1599 ν(C=N) cm–1.
4

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2.5. Crystal structure determination
X-ray diffraction patterns of 1 and 2 were collected using a Bruker SMART APEX-II CCD area
detector equipped with graphite-monochromated Mo-Kα radiation (λ = 0.71073 Å) at room
temperature. Absorption correction was applied by SADABS [19,20]. The structure was solved by
direct methods and refined on F2 by full-matrix least-squares using Bruker's SHELXTL-97 [21]. All
non-hydrogen atoms were refined anisotropically. The details of the crystallographic data, selected
bond distances and angles for 1 and 2 are summarized in Tables 1 and 2. Crystallographic data for
the structural analysis have been deposited to the Cambridge Crystallographic Data Center (CCDC
1509418 for 1 and 1509417 for 2). Copy of this information can be obtained free of charge from
The Director, CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (Fax: (+44) 1223-336033; E-mail:
deposit@ccdc.cam.ac.uk or www.ccdc.cam.ac.uk/data_request/cif).
Table 1. Crystallographic data and structure refinement details for 1 and 2.
1
2
Empirical formula
fw
Temperature (K)
Cryst. Syst.
Space group
a (Å)
b (Å)
c (Å)
α, °
C18H19CuN3O6 C17H19CuN3O7S
436.90
298
472.95
298
triclinic
P-1
triclinic
P-1
7.5448(10)
9.9017(13)
12.6326(16)
88.454(6)
81.918(6)
88.950(5)
933.9(2)
2
7.7830(5)
10.8272(7)
12.5543(8)
72.749(2)
74.012(3)
79.382(2)
965.17(11)
2
β
, °
γ, °
V (Å3)
Z
ρcalc (g cm-3)
ꢀ(Mo Kα) (mm-1)
F (000)
1.554
1.627
1.211
1.286
450
486
R1a (I
≥
2
≥
σ
2σ)
)
0.0318
0.0812
1.060
0.0280
0.0772
wR2b (I
GOOF
1.030
a R1 = Σ||Fo| – |Fc||/Σ|Fo|. b wR2 = [Σ[w(Fo – Fc2)2]/ Σ[w(Fo ) ]]1/2.
2
2 2
Table 2. Selected bond distances (Å) and angles (°) for 1 and 2.
1
2
1.9298(17)
1.9381(15)
1.9380(15)
1.9843(19)
2.548(2)
93.33(7)
90.69(7)
170.86(8)
90.27(7)
153.33(7)
1.9534(16)
1.9654(14)
1.9270(14)
1.9861(15)
2.2872(16)
94.60(6)
91.97(6)
91.04(6)
176.57(6)
159.88(7)
Cu1
Cu1
Cu1
Cu1
Cu1
−N1
−O1
−O4
−O5
−O6
Cu1
Cu1
Cu1
Cu1
Cu1
−
−
−
−
−
Cu1
Cu1
Cu1
Cu1
Cu1
N1
O1
O4
O6
O5
N1
N1
N1
N1
O1
−
Cu1
Cu1
Cu1
Cu1
Cu1
−O1
−O4
−O5
−O6
−O4
N1
N1
N1
N1
O1
−
−
−
−
−
−
O1
O4
O5
O6
O4
−
−
−
−
−
−
−
−
5

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91.73(8)
97.17(7)
88.18(7)
109.18(7)
81.55(8)
102.45(6)
82.24(6)
96.41(6)
90.59(7)
90.94(7)
O1
O1
O4
O4
O5
−
−
−
−
−
Cu1
Cu1
Cu1
Cu1
Cu1
−O5
−O6
−O5
−O6
−O6
O1
O1
O4
O4
O5
−Cu1
−Cu1
−Cu1
−Cu1
−Cu1
−
−
−
−
−
O5
O6
O5
O6
O6
2.6. Catalytic activity studies
In a typical cyanosilylation procedure, to a stirred solution of benzaldehyde (0.4 mmol), catalyst (1
or 2) (1–8 mol%) in dry solvent [THF, CH2Cl2 or MeOH; 2 mL] trimethylsilyl cyanide (TMSCN)
(0.6 mmol) was added dropwise. The resulting solution was stirred continuously and the progress of
the reaction was followed by TLC. After a certain time, the solvent was evaporated, and the residue
was diluted with 3 mL water and extracted with diethyl ether (3×10 mL). After removal of diethyl
1
ether under vacuum, the extract was taken for analysis by H-NMR spectroscopy in CDCl3, to
evaluate the yield of the cyanohydrin trimethylsilyl ethers [22−24]. The adequacy of the procedure
1
was confirmed by blank H NMR analyses with 1,2-dimethoxyethane as an internal reference. The
internal standard method proved that no side products were formed.
3. Results and discussion
3.1. Synthesis and characterization of H3L1, 1 and 2
The hydrazone ligand H3L1 was synthesized via C−N coupling reaction between 2-
carboxybenzenediazonium chloride and acetoacetanilide in a sodium hydroxide water-ethanol
solution, and characterized by IR, ESI-MS, NMR spectroscopies and elemental analysis. H3L1 is
stabilized in DMSO-d6 solution in the hydrazone form, as reported for analogues [1]. The hydrazone
=N−NH− signal is observed at δ 14.84 in the 1H-NMR spectrum, which confirms the RAHB system
[3] in H3L1.This is also supported by IR data: 3436 (br.) ν(H2O), 3240 ν(NH), 1696 and 1663
ν(C=O), 1599 ν(C=N) cm−1. Elemental analysis and ESI-MS in methanol (peak at m/z 326.11
[Mr+H]+) confirm the formulation of H3L1.
Reaction of Cu(NO3)2ꢀ2.5H2O with H3L1 or NaH2L2 in methanol in the presence or absence
of Et3N led to the CuII complexes [Cu(HL1)(H2O)(CH3OH)] (1) or [Cu(HL2)(H2O)(CH3OH)] (2),
respectively (Scheme 3). Both complexes are characterized by elemental analysis, ESI-MS, IR
spectroscopy and single crystal X-ray diffraction (see below). In the IR spectra of 1 and 2, the
ν(C=O) and ν(C=N) signals appear at 1674 and 1591, 1627 and 1599 cm–1, respectively, values that
are significantly shifted in relation to the corresponding signals of H3L1 (see above) or NaH2L2 [18].
Mass spectrometry of a methanol solution of 1 and 2 shows relevant peaks at m/z = 387.0
[Mr−CH3OH−H2O+H]+ (for 1) and 423.9 [Mr−CH3OH−H2O+H]+ (for 2). Elemental analyses and
X-ray crystallography are also in agreement with the proposed formulations.
6

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1
2
Scheme 3. Schematic representation of 1 and 2.
3.2. X-ray structure description
Crystals of 1 and 2 suitable for X-ray diffraction analysis were obtained upon crystallization
from MeOH. The molecular structures are shown in Fig. 1 with H-contacts. Crystallographic data
and refinement parameters are given in Table S1 and selected bond distances and angles in Table
S2.
In both crystal structures, the copper ion has a square-pyramidal coordination sphere with
the chelating hydrazone and water (in the case of 1) or methanol ligands occupying the four
equatorial positions and the apical site being engaged with an O-methanol (in the case of 1) or O-
water atom (Fig. 1). The copper(II) cation is thus involved in two six-membered metallacycles i.e.,
CuOC3N and CuN2C2O (for 1), CuOSC2N and CuN2C2O (for 2), respectively. The Cu−Oequatorial
bond lengths in both structures are in the range of 1.9270(14) to 1.9861(15) Å, being considerably
shorter than the Cu−Oaxial ones, which assume the value of 2.548(2) (for 1) or 2.2872(16) Å (for 2).
Moreover, the Cu−N distance in 1 is shorter [1.9298(17) Å] than in 2 [1.9534(16) Å]. This can
conceivably be due to the effect of the ionic radius of carbon (70 pm) and sulphur (170 pm) atoms
in metallocycles in compounds 1 and 2, respectively. In 1 and 2, the carbonyl group acts as H-bond
acceptor towards the amide group leading to a strong intramolecular hydrogen bonding with
N(3)ꢀꢀꢀO(3) or N(3)ꢀꢀꢀO(7) distance of 2.625(3) or 2.632(2) Å, respectively. Both compounds present
multiple inter- and intramolecular hydrogen bond interactions that lead to 2D (for 1) or 3D (for 2)
supramolecular networks (Fig. S1).
7

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1
2
Fig. 1. Crystal structures of 1 and 2 with partial atom numbering scheme. H-bond interactions are
shown as dashed blue lines.
3.2. Catalytic activity of 1 and 2 in cyanosilylation reaction
The initial experiments were performed to evaluate the influence of solvent polarity and catalyst on
the cyanosilylation reaction (Scheme 2, Table 3). Benzaldehyde with trimethylsilyl cyanide
(TMSCN) were used as model substrates and methanol, tetrahydrofurane or dichloromethane as
solvents (Table 3). In the absence of any metal catalyst, a 27% maximum yield of cyanohydrin
trimethylsilyl ether was observed after 24 h in the cyanosilylation reaction of benzaldehyde with
TMSCN (entries 7–12 and 14–17, Table 3). Although the reaction is promoted by H3L1, NaH2L2
and Cu(NO3)2·2.5H2O (entries 7–13), the yield are much lower than those achieved with 1 or 2
(Table 3). The reaction in MeOH leads to higher yields in comparison to THF and CH2Cl2, and in
general the yield decreases in the order of polarity MeOH > CH2Cl2 > THF, for the studied solvents
(entries 1–13, Table 3). The catalytic activities of both complexes (1 and 2) are comparable in all
studied solvents, 2 (with sulfonic group) being slightly more active than 1 (with the carboxylate
substituent).
In the absence of catalyst, the use of THF or CH2Cl2 had an inhibiting effect, whereas
MeOH promoted the reaction (compare entries 14–17, Table 3). Thus, we furthermore studied
different effects using 2 as the catalyst and methanol as the solvent (Table 4). Increasing the
reaction time up to 12 h led to a remarkable increase in yield (Table 4, entries 1–4), which did not
increase considerable for longer times. The increase of the catalyst amount up to 5 mol % loading
resulted also in a marked yield improvement (entries 8–12, Table 4), whereas a higher catalyst
o
amount had a much lower effect. The temperature (in the 15–55 C range) had not a crucial effect
on the yield of cyanohydrin trimethylsilyl ether (entries 13–16).
8

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Table 3. Selection of catalyst for the cyanosilylation of benzaldehyde with TMSCN.a
Amount
of catalyst
Entry
Catalyst
1
Solvent
Yieldb (%)
(mol%)
1
2
5
5
5
5
5
5
5
5
5
5
5
5
5
-
THF
CH2Cl2
MeOH
THF
CH2Cl2
MeOH
THF
CH2Cl2
MeOH
THF
CH2Cl2
MeOH
MeOH
-
25.8
28.1
78.5
26.4
28.9
79.8
18.3
19.9
26.7
20.4
22.3
27.1
32.9
20.0
14.0
14.4
24.7
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
2
H3L1
NaH2L2
Cu(NO3)2·2.5H2O
-
-
-
-
-
-
-
THF
CH2Cl2
MeOH
a
Reaction conditions: solvent (2 mL), TMSCN (0.6 mmol) and aldehyde (0.4 mmol), in air at room
temperature, 24 h reaction time. b Determined by 1H NMR analysis of crude products.
Table 4. Optimization of the parameters of the cyanosilylation reaction between
benzaldehyde and TMSCN with catalyst 2.a
Amount of
catalyst
Entry
Time (h)
T (˚C)
Yieldb (%)
(mol%)
1
2
3
4
5
6
7
1
4
8
12
16
20
24
5
5
5
5
5
5
5
25
25
25
25
25
25
25
38.9
47.5
63.4
78.2
79.0
79.6
79.8
8
9
10
11
12
12
12
12
12
12
1
3
5
7
9
25
25
25
25
25
41.1
62.4
78.2
80.0
80.4
13
14
15
16
12
12
12
12
5
5
5
5
15
35
45
55
76.3
80.9
81.5
82.7
a Reaction conditions: MeOH (2 mL), TMSCN (0.6 mmol) and benzaldehyde (0.4 mmol), in air.
b Determined by 1H NMR analysis of crude products (see Experimental part).
On the next stage, we studied the influence of different substituents at benzaldehyde and of
different alkyl chains at aliphatic aldehydes (Table 5). All the aromatic aldehydes reacted with
moderate to good yields (Table 5), but the benzaldehydes with electron-withdrawing groups gave
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higher yields than those with electron-donating substituents (entries 1–6, Table 5). The substrate
scope was not limited to aromatic aldehydes since aliphatic ones (acetaldehyde, propionaldehyde
and hexanal) can also be used as substrates, generating cyanohydrin trimethylsilyl ethers in good
yields (90.3, 89.8 and 88.5 %, respectively) (entries 7–9, Table 5).
Table 5. Cyanosilylation of various aldehydes with TMSCN using catalyst 2a
Entry
Substrate
4-Nitrobenzaldehyde
4-Chlorobenzaldehyde
4-Bromobenzaldehyde
Benzaldehyde
Yield, %b
87.4
1
2
3
4
5
6
7
8
9
82.2
81.5
78.2
4-Methylbenzaldehyde
4-Methoxybenzaldehyde
Acetaldehyde
77.4
76.9
90.3
Propionaldehyde
Hexanal
89.8
88.5
a
Reaction conditions: 5 mol% of catalyst 3, MeOH (2 mL), TMSCN (0.6 mmol) and aldehyde (0.4 mmol). Reaction
time: 12 h. b Determined by 1H NMR analysis of crude products (see Experimental part).
As mentioned in the introduction section, in comparison to the heterogeneous systems, the
homogeneous catalysis of aldehyde cyanosilylation with TMSCN is not usual and only scant
examples have been reported [6]. The catalytic activity of our catalyst 2 is higher than those
recently reported for Zn(II) (30%) [25], Cu(II) (27%) [26], etc. complexes. Moreover, in some
o
o
systems, a longer reaction time (96 h) [27], higher (40 C) [26] or lower (even negative) (-50 C)
[28] temperatures are required, what can be considered as disadvantages in comparison with our
system.
As reported for similar systems [6], the reaction conceivable involves the Cu-assisted
activation of C=O group of benzaldehyde towards the nucleophilic addition of the C≡N group,
followed by migration of the silyl group to the oxygen and subsequent product liberation.
3. Conclusions
We have achieved simple and effective syntheses of two mononuclear copper(II) complexes,
[Cu(HL1)(H2O)(CH3OH)] (1, H3L1
=
(Z)-2-(2-(1,3-dioxo-1-(phenylamino)butan-2-ylidene)
hydrazinyl)benzoic acid) and [Cu(HL2)(H2O)(CH3OH)] (2, NaH2L2 = sodium (Z)-2-(2-(1,3-dioxo-
1-(phenylamino)butan-2-ylidene)hydrazinyl)benzenesulfonate). The coordination geometries of
both complexes are identical, of the square-pyramidal type, with the arylhydrazone ligand adopting
the same binding mode (tridentate). Both complexes show high-dimensional supramolecular
architectures constructed via non-covalent interactions involving intra- and intermolecular hydrogen
bonding. These complexes were applied for the cyanosilylation of both aromatic and aliphatic
aldehydes with trimethylsilyl cyanide. Complex 2 was found to be the more efficient catalyst for the
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cyanosilylation reaction in methanol, providing cyanohydrin trimethylsilyl ethers in high yields
(77−90%) at room temperature in 12 h.
Acknowledgements
Authors are grateful to the Fundação para a Ciência e a Tecnologia: FCT (project
UID/QUI/00100/2013), Portugal, for financial support. The publication was supported by the
Ministry of Education and Science of the Russian Federation (the Agreement number
02.a03.21.0008). Authors are thankful to the Portuguese NMR Network (IST-UL Centre) for access
to the NMR facility and the IST Node of the Portuguese Network of mass-spectrometry for the ESI-
MS measurements.
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Graphical abstract synopsis
Mononuclear copper(II) complexes bearing carboxylic- or sulfonic-functionalized arylhydrazones
of acetoacetanilide ligands act as homogeneous catalysts for the cyanosilylation of aldehydes with
trimethylsilyl cyanide leading to cyanohydrin trimethylsilyl ethers

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Highlights
► Copper(II) complexes bearing carboxylic- or sulfonic-functionalized arylhydrazones of
acetoacetanilide ligands were prepared
► Both complexes showed moderate catalytic activity in homogeneous cyanosilylation of
aldehydes
► High yield of cyanosilylation product is found in methanol at room temperature