Chromogenic behaviour of a family of hydrazine and hydrazone metal complexes

Article abstract of DOI:10.1016/j.ica.2015.01.012

A family of five tridentate hydrazone-based ligands with an N,N,N-, N,N,O- or N,N,S-donor environment is reported. The tridentate ligands are derived from either 2-(1-methylhydrazinyl)pyridine or 2-hydrazinylpyridine and have been obtained by condensation with a series of aldehydes and ketones carrying one potential metal-binding site. To study their metal complexation behaviour, the five ligands and one of the bidentate precursor compounds have been reacted with ten different transition and main group metal ions. An interesting chromogenic behaviour has been found: ligands 1, 3 and H6 (with an N,N-, N,N,N- and N,N,O-donor environment) give very specific colour changes in the presence of Fe²⁺, whereas ligand H7 specifically forms a dark brown coloured complex with Co²⁺ only. All ligands form yellow to orange complexes with Pd²⁺. Three of these metal complexes were characterized by single-crystal X-ray diffraction analysis: the molecular structures of [Pd(1)Cl₂], [Cu(1)Cl₂]₂, and [Cu(6)Cl] give an insight into the metal-binding behaviour of the ligands and nicely confirm the bidentate binding of ligand 1 and the tridentate binding of the deprotonated ligand 6^-. It is proposed that the chromogenic behaviour of the ligands can be useful for the development of selective metal ion sensors.

Full text of DOI:10.1016/j.ica.2015.01.012

Inorganica Chimica Acta 428 (2015) 14–20
Contents lists available at ScienceDirect
Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Chromogenic behaviour of a family of hydrazine and hydrazone metal
complexes
⇑
Christian Radunsky, Jutta Kösters, Jens Müller
Westfälische Wilhelms-Universität Münster, Institut für Anorganische und Analytische Chemie, Corrensstr. 28/30, 48149 Münster, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
A family of five tridentate hydrazone-based ligands with an N,N,N-, N,N,O- or N,N,S-donor environment is
reported. The tridentate ligands are derived from either 2-(1-methylhydrazinyl)pyridine or 2-hydrazinyl-
pyridine and have been obtained by condensation with a series of aldehydes and ketones carrying one
potential metal-binding site. To study their metal complexation behaviour, the five ligands and one of
the bidentate precursor compounds have been reacted with ten different transition and main group
metal ions. An interesting chromogenic behaviour has been found: ligands 1, 3 and H6 (with an N,N-,
N,N,N- and N,N,O-donor environment) give very specific colour changes in the presence of Fe2+, whereas
ligand H7 specifically forms a dark brown coloured complex with Co2+ only. All ligands form yellow to
orange complexes with Pd2+. Three of these metal complexes were characterized by single-crystal
X-ray diffraction analysis: the molecular structures of [Pd(1)Cl2], [Cu(1)Cl2]2, and [Cu(6)Cl] give an insight
into the metal-binding behaviour of the ligands and nicely confirm the bidentate binding of ligand 1 and
the tridentate binding of the deprotonated ligand 6ꢀ. It is proposed that the chromogenic behaviour of
the ligands can be useful for the development of selective metal ion sensors.
Received 23 October 2014
Received in revised form 20 December 2014
Accepted 5 January 2015
Available online 12 January 2015
Keywords:
Chromogenic behaviour
Hydrazone
Metal complex
UV–Vis spectroscopy
Ó 2015 Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental
Since Emil Fischer established the use of hydrazones in chemis-
try in 1884, a wide range of applications has been reported [1]. The
Wolff–Kishner reduction or aza-Nenitzescu indole cyclisation are
examples for the importance of hydrazones in chemical synthesis
[2,3]. Further examples for an application of these Schiff base-type
compounds include their use as antimicrobial agents or in the field
of molecular recognition, e.g., in self-assembling nanoclusters or as
selective sensors [4–6]. Because of their well-known affinity
towards metal ions, we recently reported the use of hydrazone
ligands as potential artificial nucleobases for metal-mediated base
pairs, i.e., in the context of enabling the site-specific introduction of
metal-based functionality into artificial DNA double helices [7]. In
the past few years, hydrazone-based motifs also gained importance
as chromogenic agents for the specific detection of transition met-
als with the naked eye or by UV–Vis spectroscopy, respectively
[8,9]. Hence, we set out to investigate the chromogenic behaviour
of our series of bi- and tridentate hydrazine- and hydrazone-based
2.1. Measurements
Mass spectra were recorded on a Bruker Daltonics Micro TOF
spectrometer with the electron ionisation (EI) or electrospray ion-
isation (ESI) method. The elemental analyses were recorded on a
Vario EL III CHNS analyser. 1H and 13C NMR spectra were recorded
on Bruker Avance (I) 400 and Avance (III) 400 spectrometers with
an internal standard relative to TMS (d = 0 ppm) for measurements
in CDCl3. In case of DMSO-d6 and D2O/DMSO-d6 mixtures, the
residual solvent peak was used as internal standard. The UV–Vis
spectra were recorded on a Varian Cary 100 Bio spectrometer.
The data were collected from 800 nm to 200 nm with a data inter-
val of 1 nm and a scan rate of 600 nm minꢀ1 at 15 °C in methanol. A
manual baseline correction was applied. IR spectra were recorded
on a Bruker Vector 22 IR spectrometer. Single-crystal X-ray diffrac-
tion data were collected with graphite-monochromated Mo K
radiation (k = 0.71073 Å) on Bruker Quazar diffractometer
a
a
ligands in the presence of various metal ions (Cr3+, Mn2+, Fe2+, Co2+
Ni2+, Pd2+, Cu2+, Zn2+, Hg2+ and Al3+).
,
([Pd(1)Cl2]), Bruker D8 Venture diffractometer ([Cu(1)Cl2]2) and a
Stoe Stadivari diffractometer ([Cu(6)Cl]). The structures were
solved by direct methods and were refined by full-matrix, least
squares on F2 by using the SHELXTL and SHELXL-97 programs [10].
Relevant crystallographic data are listed in Table 1.
⇑
Corresponding author. Fax: +49 251 8336007.
E-mail address: mueller.j@uni-muenster.de (J. Müller).
http://dx.doi.org/10.1016/j.ica.2015.01.012
0020-1693/Ó 2015 Elsevier B.V. All rights reserved.

C. Radunsky et al. / Inorganica Chimica Acta 428 (2015) 14–20
15
2.2. Ligands and complexes
(calcd. 66.0), H 5.8 (calcd. 6.0), N 27.9 (28.0) %. ESI-MS: m/
z = 201.1136 ([M+H]+ = 201.1140). IR (KBr):
m
(C@N) 1598 (vs)
~
The synthesis of the hydrazone-based compounds 3–5 has been
reported previously [7]. Compounds H6 and H7 have been synthe-
sized analogously. In the key step of this synthesis, a solution of
compound 1 (or 2) [11] in ethanol was treated with an equimolar
amount of the respective aldehyde (ketone) and acetic acid as a
catalyst. The reaction mixture was stirred for 1 h at 58 °C. The reac-
tion volume was reduced to the half, water was added and the
respective product crystallized at ꢀ26 °C overnight. Filtration and
drying at 40 °C led to the respective clean product in 54ꢀ74% yield.
Non-methylated analogues of H6 and H7 have been reported pre-
viously by other groups and are not subject of this study [12,13].
Metal complexes were obtained by the addition of a methanolic
solution of the respective ligand to a methanolic solution of an
equimolar amount of metal salt or precursor complex (Co(NO3)2ꢁ
6H2O, Cr(NO3)3ꢁ9H2O, CuCl2ꢁ2H2O, FeSO4, MnCl2ꢁ4H2O, [PdCl2
(cod)], NiCl2ꢁ6H2O, CuI, NiSO4ꢁ6H2O, ZnCl2). Stirring for 1–3 h at
ambient temperature led to the product as an amorphous solid,
which was purified by filtration and washing with a methanol/
Et2O mixture. The respective pure product was isolated after dry-
ing at 40 °C overnight in 70ꢀ92% yield. During the investigation
of the chromogenic behaviour, Pd(OAc)2 in CH3CN was used
instead of [PdCl2(cod)] because of better solubility.
cmꢀ1
.
Complex [Pd(1)Cl2]: 1H NMR (DMSO-d6): d = 8.67 (s, 2H, NH2),
8.31 (dd, 1H, H4), 7.82 (m, 1H, H6), 6.83 (dd, 1H, H5), 6.78 (d,
1H, H3), 3.14 (s, 3H, CH3) ppm. 13C NMR (DMSO-d6): d = 160.2
(C2), 145.6 (C4), 140.5 (C6), 114.0 (C5), 107.8 (C3), 38.0 (CH3)
ppm. C6H9Cl2N3Pdꢁ0.25CH3OH: C 24.6 (calcd. 24.3), H 3.3 (calcd.
3.3), N 13.9 (calcd. 13.6)%. ESI-MS: m/z = 228.9825 ([M–2Cl]+):
228.9831.
Complex [Pd(1)2]Cl2ꢁ3H2O: 1H NMR (D2O/DMSO-d6): d = 8.05 (dd,
2H, H4), 7.72 (d, 2H, H6), 7.04 (dd, 2H, H5), 6.98 (d, 2H, H3), 3.48 (s,
6H, CH3) ppm. 13C NMR (D2O/DMSO-d6): d = 162.0 (C2), 147.9 (C4),
143.6 (C6), 117.1 (C5), 110.3 (C3), 39.9 (CH3). C12H24Cl2N6O3Pd: C
30.6 (calcd. 30.2), H 4.7 (calcd. 5.1), N 17.7 (calcd. 17.6)%. ESI-
MS: m/z = 176.0305 ([Pd(1)2]2+: 176.0314).
Complex [Cu(1)Cl2]: C6H9Cl2CuN3: C 28.0 (calcd. 28.0), H 3.4
(calcd. 3.5),
N 16.2 (calcd. 16.3)%. ESI-MS: m/z = 220.9774
([MꢀCl]+: 220.9781).
Complex [Cu(3)Cl2]: C12H12Cl2CuN4: C 41.5 (calcd. 41.6), H 3.4
(calcd. 3.5),
N
16.2 (calcd. 16.2)%. ESI-MS: mꢀ/z1 = 310.0040
+
~
m
([MꢀCl] : 310.0047). IR (KBr):
(C@N) 1621 (vs) cm
.
Complex [Mn(3)Cl2(OH2)]: C12H14Cl2MnN4O: C 40.9 (calcd. 40.5),
H 3.8 (calcd. 4.0), N 15.9 (calcd. 15.7)%. ESI-MSꢀ: 1m/z = 354.9920
+
Ligand H6: 1H NMR (CDCl3): d = 11.39 (s, 1H, OH), 8.25 (dd, 1H,
H60), 7.78 (s, 1H, CH@N), 7.58 (dd, 1H, H300), 7.30 (dd, 1H, H40),
6.91–6.85 (m, 3H, H400, H30, H500), 6.82 (dd, 1H, H50), 3.93 (s, 3H,
([M] ): 354.9925. IR (KBr):
(C@N) 1615 (vs) cm
.
~
m
Complex [Zn(3)Cl2]: 1H NMR (DMSO-d6): d = 11.78 (s, 1H, NH),
8.67 (m, 1H, H40), 8.19 (m, 2H, H600, H400), 8.06 (d, 1H, H60), 7.92
(dd, 1H, H500), 7.73 (m, 1H, H30), 7.16 (m, 2H, H50, H300), 2.59 (s,
13
OCH3), 3.68 (s, 3H, NCH3) ppm. C NMR (CDCl3): d = 156.5 (C20),
13
148.2 (C200), 147.3 (C60), 146.3 (C600), 138.1 (C300), 137.7 (CH@N),
121.6 (C400), 119.4 (C100), 119.0 (C500), 116.2 (C50), 111.9 (C30),
108.6 (C40), 56.0 (OCH3), 29.1 (NCH3) ppm. C14H15N3O2: C 65.4
(calcd. 65.4), H 5.8 (calcd. 5.9), N 16.4 (calcd. 16.3) %. ESI-MS: m/
~
3H, CH3) ppm. C NMR (DMSO-d6): d = 162.9 (C200), 152.1 (C20),
145.7 (C40), 142.0 (C400), 135.5 (C600, C60), 115.3 (H3CC@N), 52.4
(C300, C500), 31.2 (C50), 25.0 (C30), 4.9 (CH3) ppm. C12H12Cl2N4
Znꢁ0.5H2O: C 40.4 (calcd. 40.3), H 3.3 (calcd. 3.7), N 15.7 (calcd.
~
z = 258.1236 ([M+H]+ = 258.1243). IR (KBr):
m
(C@N) 1589 (vs)
15.7)%. ESI-MS: m/z = 311.0037 ([MꢀCl]+: 311.0042). IR (KBr):
m
cmꢀ1
.
(C@N) 1599 (vs) cmꢀ1
.
Ligand H7: 1H NMR (CDCl3): d = 9.01 (s, 1H, NH), 8.20 (ddd, 1H,
H60), 7.59 (s, 1H, CH@N), 7.55 (m, 2H, H50, H40), 6.86 (dd, 1H, H500),
6.73 (ddd, 1H, H30), 6.38 (ddd, 1H, H300), 6.26 (dd, 1H, H400), 3.62 (s,
Complex [Cu(4)Cl2]: C11H11Cl2CuN3Oꢁ0.33H2O: C 38.8 (calcd.
38.7), H 3.2 (calcd. 3.4),
N
12.3 (calcd. 12.3)%. ESI-MS: m/
z = 298.9880 ([MꢀCl]+: 298.9887). IR (KBr):
m
(C@N) 1612 (vs)
~
13
3H, CH3) ppm. C NMR (CDCl3): d = 157.6 (C20), 147.0 (C60), 137.3
cmꢀ1
.
(C200), 129.7 (CH@N), 127.0 (C40), 119.6 (C500), 114.9 (C300), 110.6
Complex [Pd(4)Cl2]ꢁ2H2O: 1H NMR (DMSO-d6): d = 8.56 (d, 1H,
H60), 8.37 (s, 1H, CH@N), 8.29 (m, 1H, H40), 8.11 (dd, 1H, H50),
(C500), 109.7 (C400), 109.3 (C30), 29.4 (CH3) ppm. C11H12N4: C 65.6
Table 1
Crystallographic data for compounds [Pd(1)Cl2], [Cu(1)Cl2]2, and [Cu(6)Cl].
[Pd(1)Cl2]
[Cu(1)Cl2]2
[Cu(6)Cl]
Empirical formula
Formula weight
Crystal system
Space group
a (Å)
C6H9Cl2N3Pd
300.46
monoclinic
C2/c
16.175(1)
8.8970(5)
13.0950(8)
90
C12H18Cl4Cu2N6
515.20
C14H14ClCuN3O2
355.27
monoclinic
P21/n
triclinic
ꢀ
P1
6.456(5)
9.011(7)
9.022(8)
65.54(2)
6.636(1)
b (Å)
c (Å)
17.981(4)
11.492(2)
90
a
(°)
b (°)
98.542(1)
90
1863.5(2)
8
2.142
2.511
84.13(3)
72.48(2)
455.4(7)
1
1.878
2.929
98.31(3)
90
1356.8(5)
4
1.739
c
(°)
V (Å3)
Z
qcalc (g cm–3
)
l
(Mo K
a
) (mmꢀ1
)
1.813
Crystal size (mm)
Temperature (K)
hmin, hmax (°)
Dataset
Total unique data
0.70 ꢂ 0.30 ꢂ 0.20
0.25 ꢂ 0.17 ꢂ 0.08
0.66 ꢂ 0.32 ꢂ 0.08
153(2)
153(2)
153(2)
2.79, 30.0
ꢀ22:22, ꢀ12:12, ꢀ15:18
7950, 2706
2508
2706, 110
0.0206, 0.0500, 1.077
0.57, ꢀ0.45
2.48, 27.7
ꢀ8:8, ꢀ11:11, ꢀ11:11
5742, 2129
2011
2129, 116
0.0226, 0.0617, 1.117
0.33, ꢀ0.39
2.17, 32.6
ꢀ10:10, ꢀ27:27, ꢀ17:17
33500, 5269
2767
5269, 192
0.0274, 0.0487, 0.625
0.40, ꢀ0.39
Observed data [I > 2
Nref, Npar
r(I)]
a
R, wR2, S [I > 2
r(I)]
Minimum and maximum residual density (e Åꢀ3
)
P
P
P
P
a
R1
=
||Fo| ꢀ |Fc||/ ||Fo|, wR2 = [ w(F2o ꢀ F2c)2/ w(Fo2)2]1/2
.

16
C. Radunsky et al. / Inorganica Chimica Acta 428 (2015) 14–20
7.76 (d, 1H, H30), 7.50 (d, 1H, H300), 7.29 (dd, 1H, H400), 6.98 (m, 1H,
13
H500), 3.39 (s, 3H, CH3) ppm. C NMR (DMSO-d6): d = 161.8 (C20),
150.7 (C40), 146.0 (C60), 145.7 (C300), 144.0 (C400), 141.6 (CH@N),
140.5 (C50), 119.5 (C30), 114.5 (C200), 112.8 (C500), 44.6 (CH3) ppm.
C
11H11Cl2N3OPd: C 31.5 (calcd. 31.9), H 3.5 (calcd. 3.7), N 9.9 (calcd.
10.1)%. ESI-MS: m/z = 341.9612 ([M–Cl–2H2O]+: 341.9625). IR
ꢀ1
~
m
(KBr):
(C@N) 1600 (vs) cm
.
Complex [Ni(4)Cl2]: C11H11Cl2N3NiO: C 39.9 (calcd. 39.9), H 3.3
(calcd. 3.4),
N
12.7 (calcd. 12.7)%. ESI-MS: m/z = 293.9937
+
ꢀ1
~
([MꢀCl] : 293.9944). IR (KBr):
m
(C@N) 1603 (vs) cm
.
Complex [Co(4)(NO3)2(OH2)]: C11H13CoN5O8: C 32.7 (calcd. 32.9),
H 3.2 (calcd. 3.3), N 17.1 (calcd. 17.4)%. ESI-MS: m/z = 322.0107
+
ꢀ1
~
m
([M–NO3–H2O] : 322.0112). IR (KBr):
(C@N) 1607 (vs) cm
.
Complex [Co(5)(NO3)2(OH2)]: C11H13CoN5O7S:
C
31.7 (calcd.
31.6), H 3.1 (calcd. 3.1),
N
16.8 (calcd. 16.7)%. ESI-MS: m/
z = 337.9881 ([M–NO3–H2O]+: 337.9884). IR (KBr):
m
(C@N) 1610
~
(vs) cmꢀ1
.
Complex [Cu(5)Cl2]: C11H11Cl2CuN3S: C 37.6 (calcd. 37.6), H 3.1
(calcd. 3.2),
N
12.0 (calcd. 12.0)%. ESI-MS: m/z = 314.9651
+
ꢀ1
~
m
([MꢀCl] : 314.9658). IR (KBr):
(C@N) 1611 (vs) cm
.
1
Complex [Cu(5)I]: H NMR (DMSO-d6): d = 8.29 (ddd, 1H, H60),
8.13 (s, 1H, CH@N), 7.76 (dd, 1H, H500), 7.54 (dd, 1H, H40), 7.47
(m, 2H, H300, H400), 7.07 (dd, 1H, H50), 6.89 (dd, 1H, H30), 3.64 (s,
Scheme 1. Synthesis of the hydrazone-based ligands 3–H7. Conditions: EtOH,
500
l
L HOAc, 40–60 min, 50–60 °C, 68–80% yield.
13
3H, CH3) ppm. C NMR (DMSO-d6): d = 154.5 (C60), 146.2 (C20),
+
~
m (C@N) 1609
MS: m/z = 568.1514 ([Fe(6)2] : 568.1521). IR (KBr):
139.1 (C200), 138.0 (C40), 133.1 (CH@N), 129.6 (C500), 127.4 (C400),
127.0 (C300), 115.5 (C30), 108.5 (C50), 31.4 (CH3) ppm. C11H11CuIN3S:
C 32.6 (calcd. 32.4), H 2.6 (calcd. 2.7), N 10.0 (calcd. 10.3)%. ESI-MS:
~
(vs) cmꢀ1
.
Complex [Co(6)2]NO3ꢁ3H2O: C28H34CoN7O10: C 49.2 (calcd. 48.9),
H 4.8 (calcd. 5.0), N 14.4 (calcd. 14.3)%. ESI-MS: m/z = 571.1497
m/z = 279.9964 ([MꢀI]+: 279.9970). IR (KBr):
m (C@N) 1600 (vs)
([M–NO3–3H2O]+: 571.1504). IR (KBr):
(C@N) 1614 (vs), 1605
~
m
cmꢀ1
.
(s) cmꢀ1
.
Complex [Pd(5)Cl2]: 1H NMR (DMSO-d6): d = 8.71 (s, 1H, CH@N),
8.60 (dd, 1H, H60), 8.41 (dd, 1H, H30), 8.21 (m, 1H, H40), 8.16 (m, 1H,
H50), 7.66 (dd, 1H, H500), 7.41 (m, 1H, H300), 7.37 (m, 1H, H400), 3.39
(s, 3H, CH3) ppm. 13C NMR (DMSO-d6): d = 162.5 (CH@N), 156.5
(C20), 146.7 (C40), 142.0 (C60), 140.2 (C300), 130.0 (C400), 128.5
(C500), 128.2 (C200), 121.1 (C50), 114.2 (C30), 46.8 (CH3) ppm. C11H11-
Cl2N3PdS: C 33.5 (calcd. 33.5), H 2.8 (calcd. 2.8), N 10.5 (calcd.
~
3. Results and discussion
3.1. Ligand synthesis
The reaction of 2-chloropyridine with methylhydrazine led to
the formation of 2-(1-methylhydrazinyl)pyridine 1 and 2-hydrazi-
nylpyridine 2, respectively. The hydrazone-based ligands 3–H7
with N,N,O- (4, 6ꢀ), N,N,S- (5) and N,N,N-donor motifs (3, 7ꢀ) were
formed by an acid-catalysed condensation reaction of 1 and 2 with
different heterocyclic carboxaldehydes and ketones, respectively
(Scheme 1). It is expected that compounds 3, 6ꢀ and 7ꢀ are the best
ligands among the tridentate ligands investigated here. In particu-
lar, ligand 3 and closely related compounds (based on pyridine-2-
carboxaldehyde rather than acetylpyridine) have been thoroughly
10.7)%. ESI-MS: m/z = 359.9391 ([MꢀCl]+: 359.9401). IR (KBr):
m
(C@N) 1610 (vs) cmꢀ1
.
Complex [Ni(5)Cl2]: C11H11Cl2N3NiS: C 37.7 (calcd. 38.1), H 3.1
(calcd. 3.2),
N
11.8 (calcd. 12.1)%. ESI-MS: mꢀ/z1= 309.9710
+
~
m
([MꢀCl] : 309.9716). IR (KBr):
(C@N) 1609 (vs) cm
.
Complex [Cu(6)Cl]: C14H14ClCuN3O2: C 47.3 (calcd. 47.3), H 4.1
(calcd. 4.0),
N
11.7 (calcd. 11.8)%. ESI-MS: mꢀ/z1= 319.0375
+
~
m
([MꢀCl] : 319.0382). IR (KBr):
(C@N) 1616 (vs) cm
.
Complex [Ni(H6)SO4]: C14H15N3NiO6S: C 40.8 (calcd. 40.8), H 3.6
(calcd. 3.7), N 10.2 (calcd. 10.2)%. ESI-MS: m/z = 314.0434 ([M–H–
+
ꢀ1
~
SO4] ): 314.0439. IR (KBr):
m (C@N): 1619 (vs), 1606 (s) cm .
1
Complex [Pd(6)Cl]: H NMR (DMSO-d6): d = 8.71 (d, 1H, H60),
8.45 (s, 1H, CH@N), 8.01 (dd, 1H, H40), 7.42 (d, 1H, H30), 7.28 (m,
1H, H50), 7.07 (m, 1H, H400), 6.92 (d, 1H, H300), 6.64 (m, 1H, H500),
3.85 (s, 3H, OCH3), 3.76 (s, 3H, NCH3) ppm. C14H14ClN3O2Pd: C
42.2 (calcd. 42.2), H 3.5 (calcd. 3.5), N 10.5 (calcd. 10.6)%. ESI-
+
~
m
MS: m/z = 362.0120 ([MꢀCl] : 362.0121). IR (KBr):
(C@N) 1615
(vs), 1607 (vs) cmꢀ1
.
Complex [Mn(6)Cl(OH2)2]ꢁ2H2O: C14H22ClMnN3O6: C 40.6 (calcd.
40.2),
H
3.5 (calcd. 5.3),
N
9.9 (calcd. 10.0)%. ESI-MS: m/
z = 311.0471 ([M–Cl–2H2O]+: 311.0467). IR (KBr):
m
(C@N) 1610
~
(vs) cmꢀ1
.
Complex [Cr(6)2]NO3ꢁ2H2O: C28H32CrN7O9: C 50.4 (calcd. 50.8), H
4.6 (calcd. 4.9), N 14.9 (calcd. 14.8)%. ESI-MS: m/z = 564.1568 ([M–
+
ꢀ1
~
m (C@N) 1612 (vs) cm .
NO3–2H2O] : 564.1577). IR (KBr):
Scheme 2. General synthesis of metal complexes containing the ligands 1, 3 to H7
(3: R1 = H, R2 = CH3, R3 = pyridyl; 4: R1 = CH3, R2 = H, R3 = furyl; 5: R1 = CH3, R2 = H,
R3 = thienyl; 6: R1 = CH3, R2 = H, R3 = ortho-vanillin; 7: R1 = CH3, R2 = H, R3 = pyrr-
olyl; M = Cu2+, Pd2+, Hg2+, Cr3+, Ni2+, Co2+, Mn2+, Fe2+; X = counter ion/co-ligand (Clꢀ,
NOꢀ3 , SO24ꢀ)).
Complex [Fe(6)(OH2)2(SO4)]: C14H18FeN3O8S: C 37.5 (calcd. 37.9),
H 4.4 (calcd. 4.1), N 9.4 (calcd. 9.5)%. This elemental analysis is also
in agreement with the composition [Fe(6)2]SO4ꢁFeSO4ꢁ4H2O, which
would be supported by the following mass spectrometric data. ESI-

C. Radunsky et al. / Inorganica Chimica Acta 428 (2015) 14–20
17
Fig. 1. Visual analysis of methanolic metal salt solutions (c = 0.05 mM) (a) in the absence of a hydrazine- or hydrazone-based ligand and in the presence of equimolar
amounts of the ligands (b) 1, (c) 3, (d) H6, and (e) H7. Specific colour changes are observed. The corresponding Vis spectra are shown on the right.
investigated in terms of their metal-binding capabilities [14–18].
Moreover, several reports exist on the interesting structural diver-
sity of metal complexes of a ligand closely related to H6 [19–21].
The furyl oxygen atom in 4 and the thienyl sulfur atom in 5 are
probably rather weak donors [7]. Nonetheless, several studies have
shown that ligands with a pendant furyl or thienyl moiety can
indeed lead to complexes with a comparatively strong coordina-
tion via the respective oxygen or sulfur atom [22], hence it is cer-
tainly worth including 4 and 5 in the screen for a chromogenic
behaviour.

18
C. Radunsky et al. / Inorganica Chimica Acta 428 (2015) 14–20
3.2. Metal complex synthesis
the bidentate binding of ligand 1, the square-planar coordination
geometry is slightly distorted towards a trapezoid-like shape, devi-
ating from the ideal 90° angle. In the solid state, molecules of
[Pd(1)Cl2] are arranged in a layered structure, with Pdꢁ ꢁ ꢁPd dis-
tances between neighbouring layers of 3.3376(3) and
3.5303(3) Å, respectively (Fig. S1, Supporting Information). These
distances are larger than twice the van-der-Waals radius of Pd
[28], indicating the absence of bonding metalꢁ ꢁ ꢁmetal interactions.
Upon vapour diffusion crystallization of [Cu(1)Cl2]2 in methanol
with diethyl ether as antisolvent, green single crystals were
obtained. X-ray diffraction analysis shows the formation of a
centrosymmetric dinuclear complex in the solid state. The Cu2+
ions are coordinated in a distorted square pyramidal fashion, with
N1p, N2h, Cl1 and Cl2 located in the basal plane and Cl1a in the
To investigate the affinity of these hydrazine- and
hydrazone-based ligands towards metal ions, complexation reac-
tions were carried out with different metal salts and precursor
complexes in methanolic solution as indicated in Scheme 2. Mass
spectrometric data and elemental analyses of the amorphous prod-
ucts confirm both the successful coordination of the ligands 1 and
3–H7 to the respective metal ions and the stoichiometry of the
metal complexes (see Section 2.2). As expected, the characteristic
IR band of the free ligand (3–H7) in the region of 1589–
~
1598 cmꢀ1, corresponding to the
m (C@N) vibration, was found to
shift to slightly higher wavenumbers upon complexation with a
metal ion [23].
axial position of Cu1 (Fig. 3). The geometrical index
s [29],
3.3. Visual analysis and UV–Vis spectroscopy
With respect to the general metal-binding affinity of the inves-
tigated ligands, their potential application in terms of a specific
metal-ion detection appears feasible provided that the complexa-
tion of specific metal ions leads to a characteristic colour change
(as is known from other compounds with a hydrazone-based motif
[8,24,25]). To investigate this possibility, colourless methanolic
solutions of ligands 1–H7 (c = 0.05 mmol/L) were mixed with solu-
tions of various metal ions (except for Pd(OAc)2 where acetonitrile
was used as solvent) directly before visually analysing them at
ambient temperature (Fig. 1). As reported previously for other
hydrazone-based systems [8], the presence of Pd2+ is indicated
by a typical yellow colour with all ligands 1–H7. Similarly, ligands
Fig. 2. Molecular structure of [Pd(1)Cl2]. Thermal ellipsoids are drawn at the 50%
probability level.
1, 3, 4 and 5 form readily detectable green complexes with Cu2+
while ligand H7 gives a brown complex.
,
Table 2
While N,N,O-donor ligand 4 and N,N,S-donor ligand 5 show no
further metal-specific colour changes, the ligands 1, 3 and H6
nicely demonstrate additional chromogenic behaviour, especially
towards Fe2+ ions (1: red, 3: blue, H6: blue-green). The hypochro-
mic effect and the red-shifted absorption bands upon complexa-
tion of the ligands indicate a more pronounced delocalization of
Selected bond lengths and angles of [Pd(1)Cl2].
Bond length (Å)
Pd1–N1p
Angle (°)
2.001(2)
2.018(2)
N1p–Pd1–Cl1
N1p–Pd1–N2h
171.17(5)
81.18(7)
Pd1–N2h
N1p–Pd1–Cl2
N2h–Pd1–Cl2
95.82(5)
175.09(5)
the
p
system upon Fe2+ binding. The additional band (1: 516 nm;
3: 603 nm; H6: 592 nm) falls into the metal-to-ligand charge-
transfer region of Fe2+ complexes. In addition to the significant col-
our changes in the presence of Pd2+ and Cu2+ ions and the slight
yellowish colour changes upon the addition of ligand H7 to the
other metal ions (except for Mn2+), a brown colour of the solution
results in the case of combining ligand H7 and Co2+ ions (Fig. 1e). In
order to get an idea about the detection limits, we probed the chro-
mogenic behaviour of the four most promising ligands (1, 3, H6,
H7) against four relevant metal ions (Fe2+, Co2+, Pd2+, Cu2+). As
can be seen from Figs. S2–S5 (Supporting Information), the limit
for detection of these metal ions by the naked eye is in the order
Pd1–Cl1
Pd1–Cl2
2.3113(5)
2.3046(5)
Cl1–Pd1–Cl2
N2h–Pd1–Cl1
91.583(2)
91.75(2)
of 5 lM.
3.4. X-ray diffraction analyses
Further investigations were carried out to gain structural infor-
mation on the coordination of the chromogenic ligand 1 towards
Pd2+ ions. The reaction of the bidentate ligand 1 with the precursor
complex [PdCl2(cod)] (cod = 1,5-cycloocadiene) [26] led to the for-
mation of a 1:1 complex of the composition [Pd(1)Cl2]. Interest-
ingly, the reaction of the corresponding non-methylated ligand 2
with PdCl2 had been reported to give the 1:2 complex [Pd(2)2]Cl2
instead [27]. Vapour diffusion crystallization of [Pd(1)Cl2] in DMSO
with toluene as antisolvent gave single crystals suitable for X-ray
diffraction analysis. The molecular structure of [Pd(1)Cl2] is given
in Fig. 2. Table 2 lists relevant bond lengths and angles. The central
Pd2+ ion is surrounded by the donor atoms N1p, N2h, Cl1 and Cl2.
Pd–N and Pd–Cl bond lengths are in the normal range. As a result of
Fig. 3. Molecular structure of [Cu(1)Cl2]2. Thermal ellipsoids are drawn at the 50%
probability level.
Table 3
Selected bond lengths and angles of [Cu(1)Cl2]2.
Bond length (Å)
Angle (°)
Cu1–N1p
Cu1–N2h
Cu1–Cl1
Cu1–Cl2
Cu1–Cl1a
1.999(2)
2.022(2)
2.291(2)
2.271(2)
2.776(2)
N1p–Cu1–Cl1
N1p–Cu1–N2h
N1p–Cu1–Cl2
N2h–Cu1–Cl2
Cl1–Cu1–Cl2
161.94(5)
80.60(8)
96.84(6)
175.93(4)
95.06(6)

C. Radunsky et al. / Inorganica Chimica Acta 428 (2015) 14–20
19
describing the transition from an ideal square pyramid (
s
= 0) to an
interesting to note that much more complex structures have been
reported for the non-methylated analogue of ligand 6ꢀ, including a
hydrogen-bonded dimer of an Fe3+ complex [20], a trinuclear Mn2+
helicate [21], and a tetranuclear cubane-like Cu2+ complex [19].
The completely different coordination behaviour may be due to
the additional methyl group in 6ꢀ but may also be the result of dif-
ferent stoichiometric ratios of the starting compounds in the
respective syntheses.
ideal trigonal bipyramid ( = 1), amounts to 0.23. Table 3 includes
s
relevant bond lengths and angles. As expected, the axial Cu1–Cl1a
bond (2.776(2) Å) is significantly longer than the Cu1–Cl1
(2.291(2) Å) and Cu1–Cl2 (2.271(2) Å) bonds [30]. For comparison,
the formation of (distorted) tetrahedral complexes [Cu(2)2]+ had
been reported previously when using the corresponding non-
methylated ligand 2 and a Cu+ salt [31].
For an insight into the structure of the complex formed from
ligand H6 and Cu2+, [Cu(6)Cl] bearing a deprotonated ligand 6ꢀ
was synthesized and isolated. Upon vapour diffusion crystalliza-
tion in DMSO with toluene as antisolvent, single crystals were
obtained that could be analysed by X-ray diffraction. The molecu-
lar structure of [Cu(6)Cl] is given in Fig. 4, relevant bond lengths
and angles are listed in Table 4. The deprotonated ligand 6ꢀ binds
Cu2+ in a tridentate manner via N1p, N2h and the deprotonated
O1v atom. The planar coordination environment is completed by
the chlorido ligand Cl1. All four donor atoms are arranged in a pla-
nar fashion around the central metal ion, as evident from the sum
of the interior angles of 360.6(3)°. As a result of the significantly
different Cu1–N2h and Cu1–Cl1 bond lengths of 1.965(1) Å and
2.2443(6) Å, the square planar coordination environment is dis-
torted towards a deltoid-like structure. Two neighbouring mole-
cules were found to stack at a distance of 3.2723(6) Å. It is
4. Conclusions
The study of a series of related hydrazine- and hydrazone-based
ligands in the presence of different metal ions shows an interesting
chromogenic behaviour towards Pd2+ and Cu2+ ions for all analysed
ligands. Moreover, the donor systems 1, 3 and H6 show very spe-
cific colour changes to red (1), blue (3), and blue-green (H6) in
the presence of Fe2+ ions (Table 5). Further chromogenic behaviour
was observed for ligand H7 in the presence of Co2+. Three of the
complexes were analysed by single crystal X-ray diffraction analy-
sis ([Pd(1)Cl2], [Cu(1)Cl2]2, and [Cu(6)Cl]), confirming their molec-
ular structures as well as the bidentate (1) and tridentate (6ꢀ)
coordination mode of the ligands. To conclude, the analysed sys-
tems show a high potential for an application in the field of chro-
mogenic metal sensors, especially for the detection of Fe2+, Co2+
Pd2+ and Cu2+ ions.
,
Acknowledgements
Financial support by the DFG (IRTG 1143) is gratefully acknowl-
edged. We thank James Reilly and Stefan Seidel for performing
some of the syntheses.
Appendix A. Supplementary material
CCDC 1030702, 1030703 and 1030704 contain the supplemen-
tary crystallographic data for this paper. These data can be
obtained free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif. Supplementary
data associated with this article can be found, in the online version,
at http://dx.doi.org/10.1016/j.ica.2015.01.012.
Fig. 4. Molecular structure of [Cu(6)Cl]. Thermal ellipsoids are drawn at the 50%
probability level.
References
Table 4
Selected bond lengths and angles of [Cu(6)Cl].
[1] E. Fischer, Dtsch. Chem. Ges. 17 (1884) 579.
Bond length (Å)
Angle (°)
[2] H.H. Szmant, Angew. Chem., Int. Ed. 7 (1968) 120.
[3] V.M. Lyubchanskaya, L.M. Alekseeva, V.G. Granik, Chem. Heterocycl. Compd.
35 (1999) 570.
[4] M. Belicchi Ferrari, S. Capacchi, G. Pelosi, G. Reffo, P. Tarasconi, R. Albertini, S.
Pinelli, P. Lunghi, Inorg. Chim. Acta 286 (1999) 134.
[5] M. Ruben, J. Rojo, F.J. Romero-Salguero, L.H. Uppadine, J.-M. Lehn, Angew.
Chem., Int. Ed. 43 (2004) 3644.
[6] C. Lodeiro, F. Pina, Coord. Chem. Rev. 253 (2009) 1353.
[7] C. Radunsky, D.A. Megger, A. Hepp, J. Kösters, E. Freisinger, J. Müller, Z. Anorg.
Allg. Chem. 639 (2013) 1621.
Cu1–N1p
Cu1–N2h
Cu1–O1v
Cu1–Cl1
1.967(1)
1.965(1)
1.884(1)
2.2443(6)
N1p–Cu1–N2h
N1p–Cu1–O1v
N1p–Cu1–Cl1
Cl1–Cu1–O1v
N2h–Cu1–O1v
N2h–Cu1–Cl1
80.72(6)
171.90(6)
96.16(4)
91.21(4)
92.50(6)
170.84(5)
[8] S. Mukherjee, S. Chowdhury, A.P. Chattopadhyay, H. Stoeckli-Evans,
Polyhedron 29 (2010) 1182.
Table 5
[9] P. Wang, J. Liu, X. Lv, Y. Liu, Y. Zhao, W. Guo, Org. Lett. 14 (2012) 520.
[10] G.M. Sheldrick, Acta Crystallogr., Sect. A64 (2008) 112.
[11] A.A. Alhaider, M.A. Abdelkader, E.J. Lien, J. Med. Chem. 29 (1985) 1394.
[12] G.M. de Lima, E.R.T. Tiekink, J.L. Wardell, S.M.S.V. Wardell, Acta Crystallogr.,
Sect. E65 (2009) o3241.
[13] M. Mohan, N.S. Gupta, L. Chandra, N.K. Jha, R.S. Prasad, Inorg. Chim. Acta 141
(1988) 185.
[14] J.F. Geldard, F. Lions, Inorg. Chem. 2 (1963) 270.
[15] C. Paul Sebli, S.E. Howson, G.J. Clarkson, P. Scott, Dalton Trans. 39 (2010) 4447.
[16] E.W. Ainscough, A.M. Brodie, S.L. Ingham, J.M. Waters, Inorg. Chim. Acta 249
(1996) 47.
Overview of chromogenic behaviour of ligands 1–H7 in the presence of Fe2+, Co2+
,
Pd2+ and Cu2+ ions.
Ligand
Specific colour change in methanolic metal ion solution
a
Fe2+
Co2+
Pd2+
Cu2+
1
3
4
5
H6
H7
Red
Blue
—
—
Blue-green
—
yellow-brown
Yellow
—
—
Yellow
Intensive brown
Yellow
Orange
Yellow
Yellow
Yellow
Orange
Greenish
Greenish
Greenish
Greenish
Green
[17] T. Rojo, J.L. Mesa, M.I. Arriortua, J.M. Savariault, J. Galy, G. Villeneuve, D.
Beltrán, Inorg. Chem. 27 (1988) 3904.
Brown
[18] A.-M. Stadler, F. Puntoriero, F. Nastasi, S. Campagna, J.-M. Lehn, Chem. Eur. J. 16
(2010) 5645.
a
CH3CN solution was used for Pd2+
.

20
C. Radunsky et al. / Inorganica Chimica Acta 428 (2015) 14–20
ˇ
[19] J. Tang, J. Sánchez Costa, A. Pevec, B. Kozlevcar, C. Massera, O. Roubeau, I.
Mutikainen, U. Turpeinen, P. Gamez, J. Reedijk, Cryst. Growth Des. 8 (2008)
1005.
[25] C. Basu, S. Chowdhury, H. Stoeckli-Evans, S. Mukherjee, J. Chem. Sci. 122
(2010) 217.
[26] J. Wiedermann, K. Mereiter, K. Kirchner, J. Mol. Catal. A: Chem. 257 (2006) 67.
_
_
[20] J. Tang, J. Sánchez Costa, S. Smulders, G. Molnár, A. Bousseksou, S.J. Teat, Y. Li,
G.A. van Albada, P. Gamez, J. Reedijk, Inorg. Chem. 48 (2009) 2128.
[21] J. Tang, J. Sánchez, J. Reedijk, Eur. J. Inorg. Chem. (2007) 4119
[22] T.S. Lobana, P. Kumari, R. Sharma, A. Castineiras, R.J. Butcher, T. Akitsu, Y.
Aritake, Dalton Trans. 40 (2011) 3219.
[23] R. Abu-el-Halawah, B.F. Ali, S.F. Kayed, H. Baker, M. Qandil, M. Al-Refai, M.
Ibrahim, Z. Juddeh, K.H. Al-Obaidi, J. Coord. Chem. 57 (2004) 1139.
[24] X. Su, S. Voskian, R.P. Hughes, I. Aprahamian, Angew. Chem., Int. Ed. 52 (2013)
10734.
[27] P. Drozdzewski, M. Musiała, M. Kubiak, Aust. J. Chem. 59 (2006) 329.
[28] A. Bondi, J. Phys. Chem. 68 (1964) 441.
[29] A.W. Addison, T.N. Rao, J. Reedijk, J. van Rijn, G.C. Verschoor, J. Chem. Soc.,
Dalton Trans. (1984) 1349
[30] G.R. Desiraju, H.R. Luss, D.L. Smith, J. Am. Chem. Soc. 100 (1978) 6375.
[31] P.C. Healy, J.D. Kildea, B.W. Skelton, A.H. White, Aust. J. Chem. 41 (1988) 623.