Preparation, characterization and reactivity of 1-benzylpyridinium-4-aldoxime chloride and 1-phenacylpyridinium-4-aldoxime chloride and their complexes with the aquapentacyanoferrate(II) ion

Article abstract of DOI:10.1016/j.ica.2010.01.041

A detailed structural characterization of the biologically active 1-benzylpyridinium-4-aldoxime chloride and 1-phenacylpyridinium-4-aldoxime chloride was performed using NMR and vibrational and electronic spectroscopy, as well as X-ray diffraction. The co

Full text of DOI:10.1016/j.ica.2010.01.041

Inorganica Chimica Acta 363 (2010) 1425–1434
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Preparation, characterization and reactivity of 1-benzylpyridinium-4-aldoxime
chloride and 1-phenacylpyridinium-4-aldoxime chloride and their complexes
with the aquapentacyanoferrate(II) ion
a,
a
b
a
*
ˇ
´
´
Blazenka Foretic , Igor Picek , Ivica Ðilovic , Nicoletta Burger
a Department of Chemistry and Biochemistry, Faculty of Medicine, University of Zagreb, Šalata 3, 10000 Zagreb, Croatia
b Department of Chemistry, Faculty of Science, University of Zagreb, Horvatovac 102a, 10000 Zagreb, Croatia
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 7 July 2009
Received in revised form 22 January 2010
Accepted 26 January 2010
Available online 4 February 2010
A detailed structural characterization of the biologically active 1-benzylpyridinium-4-aldoxime chloride
and 1-phenacylpyridinium-4-aldoxime chloride was performed using NMR and vibrational and elec-
tronic spectroscopy, as well as X-ray diffraction. The complexes of these compounds with the aquapent-
acyanoferrate(II) ion were examined in solution, isolated as solids and characterized by elemental
analysis, electronic, FT-IR and NMR spectral data. They were found to be mononuclear substituted pent-
acyanoferrates(II) containing the aldoxime group coordinated to the iron through the nitrogen atom. The
complexes were also precipitated in the form of the respective zinc salts; the analysis of these complexes
revealed a molar Fe/Zn ratio of 1, thus confirming the charge of the complex anions to be ꢀ2. The ioni-
zation constants of the aldoxime group in the free ligands and in the respective cyano complexes were
also determined. Despite the presence of two donor sites in 1-phenacylpyridinium-4-aldoxime chloride,
only the aldoxime group was found to be reactive.
Keywords:
Substituted pentacyanoferrates(II)
Oximes
Pyridinium-4-aldoximes
Carbonyl compounds
Spectroscopy
Ionization constants
Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction
1-Benzylpyridinium-4-aldoxime chloride (BPA4-Cl) and 1-
phenacylpyridinium-4-aldoxime chloride (FEPA4-Cl) are represen-
Compounds that have an amphoteric aldoxime group (>C@N–
OH), including the slightly basic nitrogen and the mildly acidic hy-
droxyl group, are known to form complexes with a great number of
metal ions [1–3], as well as with the aquapentacyanoferrate(II) ion,
[Fe(CN)5OH2]3ꢀ [4–12]. Many of these compounds and their metal
complexes have shown significant and versatile bioactivities,
which are evidently closely related to their chelating ability [13].
They can generally assume the form of two different configuration
isomers (E and Z). The mono- and bis-pyridinium type aldoximes
are particularly potent reactivators of the human blood acetylcho-
linesterase, and thus they are efficient protectors of this enzyme
upon phosphorylation by poisons such as pesticides and warfare
agents [13]. The cytotoxic and antiproliferative effects of one of
the ligands of interest (1-phenacylpyridinium-4-aldoxime chlo-
ride) in the SH-SY5Y human neuroblastoma cell line, which is char-
acterized by a high expression of acetylcholinesterase, have been
previously observed [14]. The substituted low-spin species of iron,
[Fe(CN)5L]nꢀ, in the mononuclear and polynuclear forms, were the
subject of numerous investigations mainly as models for active
sites in biological macromolecules (porphyrines, cytochromes)
and as representatives of supramolecular systems [15–18].
tative of the quaternized derivatives of pyridine-4-aldoxime
(Scheme 1). Despite their common aldoxime group, considerable
differences concerning the reactivity of pyridinium-4-aldoximes
and pyridine-4-aldoxime toward [Fe(CN)5OH2]3ꢀ have been found
[7,10–12]. Molecules of the 1-phenacylpyridinium type can be sub-
ject to keto-enol tautomerism, but ketones in their solutions ap-
pear in equilibrium with an insignificant amount of the enol
form [19]. Regarding the reactivity of the carbonyl group, previ-
ously performed examinations of the reactions of [Fe(CN)5OH2]3ꢀ
with oxo-oximes, more precisely naphthoquinone oximes, re-
vealed participation of both the carbonyl and the oxime group in
complex formation [8]. Recently, certain ketones of the phenacyl-
and benzoylethyl-pyridinium type were also found to react with
[Fe(CN)5OH2]3ꢀ [9,20].
To recognize the reactive forms and the stability of the cited li-
gands in aqueous solutions, the previous scarce characterization
[21] was reexamined and extended. Thus, a comparative study
concerning the coordination ability of the ligands of interest
(BPA4-Cl and FEPA4-Cl) toward the pentacyanoferrate(II) moiety
was performed, with special regard to establish which of the
two potential donor sites of FEPA4-Cl actually reacts with
[Fe(CN)5OH2]3ꢀ. A detailed structural characterization of the two
ligands and their complexes with [Fe(CN)5OH2]3ꢀ in solution and
in the solid state was performed using UV–Vis, NMR (1H and 13C)
* Corresponding author. Tel.: +385 1 4566 760; fax: +385 1 4590 236.
E-mail address: bforetic@mef.hr (B. Foretic´).
0020-1693/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2010.01.041

´
1426
B. Foretic et al. / Inorganica Chimica Acta 363 (2010) 1425–1434
H
6
H
OH
1
5
O
N
2
3
6
13
OH
1
14
14
5
12
11
N
N+
2
7
8
Cl
13
12
C
9
Cl
4
N+
H2
3
7
C
H2
9
10
4
10
11
BPA4-Cl
FEPA4-Cl
Scheme 1. Assignment of the ligands: 1-benzylpyridinium-4-aldoxime chloride (BPA4-Cl) and 1-phenacylpyridinium-4-aldoxime chloride (FEPA4-Cl).
and FT-IR spectra, as well as X-ray diffraction in the case of the li-
gands. The isolated complexes were re-precipitated in the form of
their respective zinc salts, and the metals were analyzed by parti-
cle induced X-ray emission spectroscopy (PIXE).
mation of white precipitates that were filtered off and purified by
recrystallization from ethanol. Transparent, water-soluble single
crystals of BPA4-ClꢁH2O (m.p.: 204.9 °C, dec.) spontaneously crys-
tallized from the reaction mixture. Yield, 70.4%. Anal. Calc. for
C13H13N2OClꢁH2O: C, 58.54; H, 5.67; N, 10.50. Found: C, 58.00; H,
5.53; N, 10.75%. FT-IR (cmꢀ1):
m
(O–H)water, 3444 (vs);
m(O–H)oxime
,
,
2. Experimental
3370 (br, vs);
1609 (vs), 1522 (vs);
m
(C@N)oxime, 1644 (vs);
m
(C–C, C–N)pyridinium
ring
m
(NO), 998 (vs). Raman (cmꢀ1):
m(C@N)oxime
2.1. Materials and instruments
1646(s);
1004 (s).
m(C–C, C–N)pyridinium ring, 1612 (vs), 1526 (vs); m(NO),
Sodium amminepentacyanoferrate(II), Na3[Fe(CN)5NH3]ꢁ3H2O,
(Sigma–Aldrich) was recrystallized from a concentrated ammonia
solution. Solutions of [Fe(CN)5OH2]3ꢀ were obtained by aquation
of [Fe(CN)5NH3]3ꢀ; the solutions were freshly prepared at room
temperature and kept in the dark to minimize dimerization and
photolytic and thermal decomposition. The starting materials for
the ligand synthesis were reagent grade Aldrich products and were
used as purchased. Britton and Robinson buffers were prepared by
mixing 100 mL of phosphoric, boric and acetic acid solution (all
0.04 M) with different volumes of 0.2 M sodium hydroxide. The io-
nic strength was maintained with sodium chloride. Redistilled
water was used throughout.
A pH-meter (Mettler Toledo) with an open junction combina-
tion polymer electrode was used for pH measurements accurate
to 0.01 pH units. Melting points were determined by a Stuart
SMP3 apparatus with a temperature resolution of 0.1°. C, H and
N analyses were performed with a LECO elemental analyzer using
the ASTM D 5291 method. UV–Vis spectra were recorded on a UNI-
CAM UV 4 spectrophotometer with thermostatted cell holders and
1 cm silica-glass cells. FT-IR and FT-Raman spectra were recorded
on a Perkin Elmer Spectrum GX, Series R spectrometer in the range
of 4000–400 cmꢀ1 using KBr pellets. The 1H and 13C NMR spectra
were recorded with a Bruker AV-600 spectrometer equipped with
5 mm inverse detection and dual probes, respectively, operating at
600.133 MHz for the 1H nucleus and 150.917 MHz for the 13C nu-
cleus. The spectra were recorded in D2O and DMSO-d6 with tetra-
methylsilane as the internal standard. The HMQC spectra were
measured in pulsed field gradient mode (z-gradient). The particle
induced X-ray emission (PIXE) analysis of the solid zinc salts of
1H NMR (D2O, d): 8.84 (d, J = 6.84 Hz, H-4, H-5), 8.06 (d,
J = 6.78 Hz, H-3, H-6), 5.71 (s, H-7), 7.44 (d, J = 7.68 Hz, H-10, H-
14), 7.38 (d, J = 7.56 Hz, H-11, H-13), 7.32 (t, J = 7.35 Hz, H-12),
8.23 (s, H-11) ppm. 1H NMR (DMSO-d6, d): 13.02 (s, 1H, OH) ppm.
13C NMR (D2O, d): 144.4 (C-4, C-5), 124.9 (C-3, C-6), 148.8 (C-2),
64.2 (C-7), 132.9 (C-9), 129.7 (C-10, C-14), 129.1 (C-11, C-13),
130.0 (C-12), 145.6 (C-1) ppm.
Transparent, water soluble single crystals of FEPA4-ClꢁH2O
(m.p.: 206.8 °C) were prepared analogously. Yield, 70.4%. Anal. Calc.
for C14H13N2O2ClꢁH2O: C, 57.05; H, 5.13; N, 9.50. Found: C, 57.11;
H, 5.15; N, 9.17%. IR (cmꢀ1):
m
(O–H)water, 3465 (vs);
(C@N)oxime, 1643 (vs);
1610 (vs), 1520 (vs); (NO), 1001 (vs).
(C@O), 1704 (m), (C@N)oxime 1646(s); (C–C,
C–N)pyridinium ring, 1612 (vs), 1526 (s); (NO), 1002 (s).
m
(O–H)oxime
,
3405 (vs);
C–N)pyridinium
Raman (cmꢀ1):
m(C@O), 1698 (vs);
m
m
(C–C,
,
m
ring
m
m
m
m
1H NMR (D2O, d): 8.74 (d, J = 6.72 Hz, H-4, H-5), 8.26
(d, J = 6.75 Hz, H-3, H-6), 6.38 (s, H-7), 8.05 (d, J = 7.38 Hz, H-10,
H-14), 7.63 (d, J = 7.86 Hz, H-11, H-13), 7.80 (t, J = 7.47 Hz, H-12),
8.40 (s, H-1) ppm. 1H NMR (DMSO-d6, d): 13.10 (s, 1H, OH) ppm.
13C NMR (D2O, d): 146.0 (C-4, C-5), 124.5 (C-3, C-6), 149.5 (C-2),
66.6 (C-7), 192.0 (C-8) 132.8 (C-9), 128.4 (C-10, C-14), 129.3 (C-11,
C-13), 135.6 (C-12), 146.2 (C-1) ppm.
2.3. Preparation and isolation of BPA4 and FEPA4 substituted
pentacyanoferrate(II) complexes
An aqueous solution of 0.124 g of BPA4-Cl (0.5 mmol) in 5–
10 mL of water was gradually added to a solution containing
0.033 g of Na3[Fe(CN)5NH3]ꢁ3H2O (0.1 mmol) in 2 mL of water.
The resulting mixture had a pH of about 6. The quickly formed pre-
cipitate was filtered off, washed with water and vacuum dried in a
desiccator over phosphorus pentoxide. The resulting solid was a
dark-blue powder sparingly soluble in water, ethanol and acetone.
Qualitative analysis showed that the compound did not contain so-
dium or chlorine. Yield, 26.1%. Anal. Calc. for (C13H13N2O)2
[Fe(CN)5(C13H13N2O)]ꢁ3H2O: C, 60.07; H, 5.16; N, 17.51. Found: C,
59.74; H, 5.25; N, 17.07%.
the BPA4 and FEPA4 substituted pentacyanoferrate(II) complexes,
-
´
analyzed as thin targets, were carried out at the Ruder Boškovic
Institute. The details of the system have been described previously
[22]. The single-crystal X-ray diffraction data were collected on an
Oxford Diffraction Xcalibur 3 CCD diffractometer with graphite-
monochromated Mo Ka radiation (k = 0.71073 Å).
2.2. Preparation of BPA4-ClꢁH2O and FEPA4-ClꢁH2O
BPA4-Cl and FEPA4-Cl were prepared according to a general
procedure by mixing equimolar ethanol solutions of pyridine-4-
aldoxime with benzyl chloride and 2-chloroacetophenone, respec-
tively. The resulting mixtures were left at ambient temperature for
approximately seven days. Ether was then added, inducing the for-
The IR spectrum undoubtedly showed the occurrence of the free
BPA4 cation present as the counter ion. IR (cmꢀ1):
m
m
(OH)water
(C@N)oxime
(C–C, C–N)pyridinium ring, 1616 (s), 1518 (m); (NO),
1016 (br, vs), 998 (sh, vs).
+
(OH)oxime, 3420 (br, vs);
m
(C„N)cyano, 2048 (vs);
m
,
1640 (vs);
m
m

´
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B. Foretic et al. / Inorganica Chimica Acta 363 (2010) 1425–1434
Table 1
The NMR spectra were taken quickly after dissolving the com-
Crystallographic data for BPA4-ClꢁH2O and FEPA4-ClꢁH2O.
plex to prevent possible decomposition. The spectra were domi-
nated by intense signals of the uncoordinated BPA4 cation,
causing low or undetectable signals of coordinated BPA4 in the
1H NMR spectrum; meanwhile, the majority of 13C NMR shifts of
coordinated and uncoordinated ligands were distinguishable. 13C
NMR (D2O, d) of the complex anion: 146.2 (C-4, C-5), 126.5 (C-3,
C-6), 148.7 (C-2), 129.6 (C-10, C-14), 129.3 (C-11, C-13), 130.1
(C-12), 145.6 (C-1), 166.6 (cis-C„N), 164.0 (trans-C„N) ppm.
The other complex with FEPA4 was prepared in an analogous
way by mixing aqueous solutions containing 0.138 g of FEPA4-Cl
(0.5 mmol) and 0.033 g of Na3[Fe(CN)5NH3]ꢁ3H2O (0.1 mmol).
Yield, 32.7%. Anal. Calc. for (C14H13N2O2)2[Fe(CN)5)(C14H13N2O2)]ꢁ
3H2O: C, 58.57; H, 4.71; N, 15.99. Found: C, 58.74; H, 4.54; N,
16.33%.
BPA4-ClꢁH2O
FEPA4-ClꢁH2O
C14H15ClN2O3
294.73
Formula
C13H15ClN2O2
266.72
colorless, plate
Formula weight
Crystal color, habit
Crystal size (mm3)
Crystal system
Space group
Unit cell parameters
a (Å)
colorless, prism
0.08 ꢂ 0.26 ꢂ 0.31 0.30 ꢂ 0.31 ꢂ 0.40
monoclinic
monoclinic
P21/n
P21
8.1459(2)
11.3603(2)
14.0928(3)
90
10.2499(3)
12.9070(3)
11.4331(4)
90
105.841(3)
90
1455.10(7)
4
1.345
100
0.71073
0.271
616
14 958
2553, 0.035
b (Å)
c (Å)
a
(°)
b (°)
98.976(2)
90
1288.18(5)
4
1.375
120
c
(°)
V (Å3)
Z
Likewise, the IR spectrum undoubtedly showed the occurrence
Dcalc (g cmꢀ3
T (K)
)
of the free FEPA4 cation. IR (cmꢀ1):
m
(OH)water
(C@O), 1695 (vs), 1699 (vs);
(C–C, C–N)pyridinium ring, 1616 (s), 1520
m(NO), 1019 (s), 999 (s).
+ m(OH)oxime, 3420
(br, vs);
m(C„N)cyano, 2059 (vs);
m
q
l
(Å)
0.71073
0.292
(mmꢀ1
)
m
(C@N)oxime, 1645 (vs);
m
F(0 0 0)
560
(m);
Number of collected data
Number of unique data
13 121
6799, 0.020
The NMR spectra were of poor quality because of insufficient
solubility of the complex in water, and thus the spectra were not
reliable for interpretation.
[Fo P 4r(Fo)], Rint
Number of parameters
340
193
b
R1a, wR2 [Fo P 4
r
(Fo)]
0.0392, 0.0924
0.0480, 0.0948
0.987
0.0372, 0.0530
0.0920, 0.1004
1.003
ꢀ0.221, 0.438
R1, wR2 [all data]
2.3.1. Isolation of zinc salts of the substituted pentacyanoferrate(II)
complexes
Goodness of fit (GOF) on F2, Sc
Minimum and maximum electron
ꢀ0.213, 0.365
density (e Åꢀ3
)
The zinc salts of the substituted pentacyanoferrate(II) com-
plexes were isolated by addition of a few milliliters of an acetic
acid solution of zinc nitrate [23] to about 30 mL of a saturated
water solution of each of the complex solids. The instantaneously
formed gelatinous red-violet precipitates were then filtered off,
washed with about 100 mL of water until neutral reaction and
dried in vacuo. The mass ratio of Fe and Zn was determined in a
homogenous fine powder by PIXE analysis, and for both complexes,
the ratio was found to be 0.74 with a statistical error of less than a
few percent. This is close to the molar ratio of 1 and concurrent
with the formulas Zn[Fe(CN)5(C13H13N2O)] and Zn[Fe(CN)5(C14H13
P
P
a
b
c
R ¼ kFo ꢀ Fck= jFoj.
P
P
2
2 1=2
wR ¼ ½ ðF2o ꢀ Fc2Þ = wðF2o Þ ꢃ
.
P
2
1=2
S ¼ ½wðF2o ꢀ Fc2Þ =Nobs ꢀ NparamÞꢃ
.
structure in the P21/c space group, but it did not converge well and
gave an R value of 0.17. The chlorine ion and water molecule would
be disordered at the two sites, whereas in P21, they were clearly re-
solved. Geometrical calculations were done using PLATON [27].
Drawings of the structures were done using the PLATON and MERCURY
[28] programs. The crystallographic data are summarized in
Table 1.
N2O2)]. Zn[Fe(CN)5(C13H13N2O)] IR:
m
(C„N)cyano
(NO), 1002 (w). Zn[Fe(CN)5(C14H13N2O2)]
(C„N)cyano, 2087 (vs); (C@O), 1699 (w), (C@N)oxime
(NO), 992 (vw).
, 2086 (vs);
m
(C@N)oxime, 1635 (m); m
IR (cmꢀ1):
1635 (m);
m
m
m
m
,
3. Results and discussion
2.4. X-ray crystallographic study
3.1. The crystal structures of FEPA4-ClꢁH2O and BPA4-ClꢁH2O
The single-crystal X-ray diffraction data of BPA4-ClꢁH2O and
FEPA4-ClꢁH2O were reduced and corrected using the CrysAlis soft-
ware package [24]. Solution, refinement and analysis of the struc-
ture was performed using the programs integrated in the WinGX
system [25]. Data for BPA4-ClꢁH2O were collected at room and
low temperature using two different crystals. Only the low-tem-
perature data are presented here because it was of better quality.
Both structures were solved by direct methods. Refinement was
performed by the full-matrix least-squares method based on F2
against all reflections using SHELXL-97 [26]. The non-hydrogen
atoms were refined anisotropically. All hydrogen atoms were lo-
cated in the difference Fourier maps. However, because of poor
geometry for some of them, they were placed in calculated posi-
tions and refined using the riding model. In both structures, the
hydrogen atoms on the hydroxyl groups were placed and refined
using the rotating group refinement procedure (the torsion angles
that maximized the electron density were chosen), and the hydro-
gen atoms on the water molecules were placed as found in the map
and were refined with restraints. The crystal of BPA4-ClꢁH2O was
found to be a racemic twin (same as the crystal with room-temper-
ature data) and was refined using TWIN and BASF instructions. The
twin ratio was 0.36(4):0.64(4). An attempt was made to refine the
There were two independent cations of benzylpyridinium-4-
aldoxime, two chloride ions and two water molecules in the
asymmetric unit of the crystal structure of BPA4-ClꢁH2O (one cat-
ion, anion and a H2O molecule are shown in Fig. 1). The structure
of another BPA4 salt, anhydrous 1-benzylpyridinium-4-aldoxime
bromide (BPA4-Br), was published recently [29]. The molecular
structure of FEPA4-ClꢁH2O is shown in Fig. 2. Its asymmetric unit
contains a 1-phenacylpyridinium-4-aldoxime cation, a chloride an-
ion and a water molecule of crystallization. The selected bond dis-
tances and angles for both structures are listed in Table 2. The
arrangement of the substituents around the C@N bond in the cat-
ions of the structures of BPA4-ClꢁH2O, FEPA4-ClꢁH2O, and BPA4-Br
shows that the aldoxime is in the E configuration. There are only
four other structures that contain the pyridinium-4-aldoxime moi-
ety, and all are E isomers [30–33].
Analysis of the Cambridge Structural Database (CSD) [34] was
performed for the structures that contain the aldoxime unit (struc-
tures containing metal atoms were not included). There were 199
structures found (only two protonated on the nitrogen atom), of
which only 20 structures were in the Z configuration. One outlier
(structure EZEGUL) and the protonated structures were suppressed

´
B. Foretic et al. / Inorganica Chimica Acta 363 (2010) 1425–1434
1428
Fig. 1. Drawing of one benzylpyridinium-4-aldoxime cation, one chloride anion and one water molecule of crystallization in the structure of BPA4-ClꢁH2O with the atom
numbering scheme. An analogous labeling scheme is used for the other independent cation (O21, N21, N22, C21-C213), anion (Cl2) and water molecule (O2).
Fig. 2. Drawing of FEPA4-ClꢁH2O with the atom numbering scheme.
Table 2
Table 3
Selected bond lengths (Å) and angles (°) for FEPA4-ClꢁH2O and two independent
Hydrogen bonds (Å, °) in BPA4-ClꢁH2O and FEPA4-ClꢁH2O.
molecules in BPA4-ClꢁH2O.
D–Hꢁ ꢁ ꢁA
D–H
Hꢁ ꢁ ꢁA
Dꢁ ꢁ ꢁA
D–Hꢁ ꢁ ꢁA
FEPA4-ClꢁH2O
BPA4-ClꢁH2O
BPA4-ClꢁH2O
BPA4-ClꢁH2O
cation 1a
cation 2b
O1–H1Aꢁ ꢁ ꢁCl2i
O1–H1Bꢁ ꢁ ꢁCl1
O2–H2Aꢁ ꢁ ꢁCl2
O2–H2Bꢁ ꢁ ꢁCl1
O11–H11ꢁ ꢁ ꢁCl1ii
O21–H21ꢁ ꢁ ꢁO2iii
0.80
0.78
0.80
0.79
0.84
0.84
2.45
2.47
2.36
2.38
2.14
1.84
3.2320(15)
3.2445(14)
3.1365(14)
3.1634(2)
2.9612(17)
2.693(2)
166
172
167
175
165
174
N1–O1
C1–N1
C1–C2
C2–C6
C4–N2
C5–N2
C7–N2
C8–O2
1.3884(19)
1.277(2)
1.454(3)
1.394(3)
1.343(2)
1.348(2)
1.477(2)
1.212(2)
1.386(2)
1.293(3)
1.470(3)
1.398(3)
1.345(3)
1.348(2)
1.502(2)
1.390(2)
1.272(3)
1.469(3)
1.394(3)
1.340(3)
1.351(3)
1.479(3)
FEPA4-ClꢁH2O
O1–H1Oꢁ ꢁ ꢁCl1iv
O3–H31ꢁ ꢁ ꢁCl1iv
O3–H32ꢁ ꢁ ꢁCl1vi
0.88(4)
0.87(3)
0.90(3)
2.16(4)
2.35(3)
2.28(3)
3.0389(16)
3.2194(18)
3.1726(19)
175(3)
176(2)
171.2(19)
N1–C1–C2
N2–C7–C8
C1–N1–O1
C4–N2–C5
118.80(18)
112.30(15)
111.35(16)
120.71(16)
118.52(19)
111.11(15)
110.58(17)
121.24(17)
119.28(18)
112.03(16)
110.55(17)
120.52(18)
Symmetry transformation of the asymmetric unit: (i) 1 + x, y, z; (ii) 1 ꢀ x, y ꢀ 3/2,
1 ꢀ z; (iii) 1 ꢀ x, 1/2 + y, 1 ꢀ z; (iv) x ꢀ 1/2, 1/2 ꢀ y, z ꢀ 1/2; (v) 1/2 + x, 1/2 ꢀ y, 1/
2 + z; (vi) 1/2 ꢀ x, y ꢀ 1/2, 3/2 ꢀ z.
a
Numbering of molecule 1 of BPA4-ClꢁH2O begins with digit 1 (N1–O1 in FEPA4-
ClꢁH2O is N11–O11 in BPA-ClꢁH2O, molecule 1).
b
Numbering of molecule 2 of BPA4-ClꢁH2O begins with digit 2 (N1–O1 in FEPA4-
the BPA4 and FEPA4 cations given in Table 4, they are all close to
these mean values.
ClꢁH2O is N21–O21 in BPA4-ClꢁH2O, molecule 2).
The aromatic rings in the two independent BPA4 cations in the
chloride structure are inclined at angles of 70.43(11)° (molecule 1)
and 69.69(10)° (molecule 2), while the planes through the phenyl
ring (including C8) and the pyridinium-4-aldoxime ring (including
C7) in the FEPA4-ClꢁH2O structure form an angle of 50.22(9)°. In the
two crystal structures, along with the ionic interactions, a similar
hydrogen bonding pattern was formed: there were three hydrogen
for analysis. The mean value of the C@N bond in the 242 aldoxime
units was 1.271(1) Å (range 1.224–1.309 Å), and the mean value
for the N–O bond was 1.396(1) Å (range 1.333–1.434 Å). The
C@N–O angle was in the range of 104.6–121.2° with a mean value
of 111.7(1)°. As can be seen from the bond lengths and angles for

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B. Foretic et al. / Inorganica Chimica Acta 363 (2010) 1425–1434
Table 4
Ionization constants and molar absorption coefficients of the predominant ionic forms of BPA4 and FEPA4 in aqueous solution (c = 4 ꢂ 10ꢀ5 M, I = 0.1 M, t = 25 °C).
Ligand
BPA4
kmax (nm)
e
(H2L)max (Mꢀ1 cmꢀ1
)
e
(HL)max (Mꢀ1 cmꢀ1
)
e
(L)max (Mꢀ1cmꢀ1
)
pKa(aldoxime)
pKa(carbonyl)
283
342
16 194
8.76 0.02
24 280
FEPA4
283
345
440
23 330
8.72 0.07
11.40 0.20
27 800
8190
17 500
Fig. 3. Packing of BPA4 cations, chloride anions and water molecules in the unit cell
of BPA4-ClꢁH2O. Hydrogen bonds are shown by blue dashed lines. (For interpreta-
tion of the references to colour in this figure legend, the reader is referred to the
web version of this article.)
Fig. 4. Packing of FEPA4 cations, chloride anions and water molecules in the unit
cell of FEPA4-ClꢁH2O. Hydrogen bonds are shown by blue dashed lines. (For
interpretation of the references to colour in this figure legend, the reader is referred
to the web version of this article.)
bonds of the O–Hꢁ ꢁ ꢁCl type interconnecting the cations, chloride
anions and water molecules (Figs. 3 and 4, Table 3). In both struc-
tures, the water molecule was a hydrogen bond donor toward the
two chloride anions, where the two chloride anions were indepen-
dent in BPA4-ClꢁH2O and centrosymmetrically related in FEPA4-
ClꢁH2O. In BPA4-ClꢁH2O, the aldoxime hydroxyl group from one
molecule formed a hydrogen bond with one chloride ion, while
the hydroxyl group from the other molecule was a hydrogen bond
donor towards a water molecule. As a result, the anions and water
molecules are interconnected into chains through hydrophilic tun-
nels in the crystal structure (Fig. 3). Quite differently, in the anhy-
drous structure of BPA4-Br, there was only one O–Hꢁ ꢁ ꢁBr hydrogen
bond, while the rest were weak interactions including C–Hꢁ ꢁ ꢁBr
3.2. Electronic absorption spectroscopy
3.2.1. Aqueous solutions of BPA4-ClꢁH2O and FEPA4-ClꢁH2O
The absorption spectra of both ligands exhibited two intensive
*
pH-dependent bands as a result of
p?p transitions within the
pyridiniumaldoxime aromatic system [35]. The maximum at
280 nm was characteristic of the protonated pyridiniumaldoxime
group, while the maximum at about 340 nm was due to the
absorption of the deprotonated pyridiniumaldoxime group. Both
maxima are in accordance with the 1Lb absorption band of the
substituted pyridine ring. The weak band centered on 250 nm
was also attributed to the deprotonated pyridiniumaldoxime and
is in accordance with the so-called second band (2nd) of the pyr-
idiniumaldoxime aromatic system. These pH-dependent spectral
changes exhibited excellent isobesticity. The cation of FEPA4 con-
tains two isolated chromophores: acetophenone and N-methylpy-
and C–Hꢁ ꢁ ꢁ
was a hydrogen bond donor only to a chloride ion (Fig. 4). The
two aromatic rings, phenyl and pyridine, form interactions
p
[24]. In FEPA4-ClꢁH2O, the aldoxime hydroxyl group
pꢁ ꢁ ꢁp
with distances between their centroids (Cg1(N2,C2–C6)ꢁ ꢁ ꢁ
Cg2(C9–C14) [x ꢀ 0.5, 0.5 ꢀ y, z ꢀ 0.5] of 3.859(1) Å, slippage
1.443(1) Å.

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B. Foretic et al. / Inorganica Chimica Acta 363 (2010) 1425–1434
1430
ridinium-4-aldoxime. Thus its aqueous solutions exhibited the
bands compatible with the linear summation of the respective
chromophores: the characteristic intensive conjugation band (or
K band), according to the intramolecular charge transfer in aceto-
phenone, appearing at 254 nm and the bands associated with
absorptions of the pyridiniumaldoxime aromatic system appearing
at 283 nm and 345 nm (1Lb band). In basic aqueous solutions, an
additional conjugation band arose at 440 nm as a result of the
intramolecular charge transfer between the enolate form of aceto-
phenone and the deprotonated pyridiniumaldoxime part of the
molecule. Whereas molecules of the 1-phenacylpyridinium type
can be subject to keto-enol tautomerism, it must be pointed out
that there was no detectable absorption of the enol chromophore
in the examined aqueous solutions of FEPA4, as previously sug-
gested [21]. The predominance of the keto isomer in aqueous solu-
tions of FEPA4 is in accordance with the very small equilibrium
ratio of the enol to keto tautomer found for 1-phenacylpyridinium
bromide (pKT = 6.10), since it seems that substituents in the pyri-
dine ring have an insignificant effect on the tautomeric equilibria
of compounds of the 1-phenacylpyridinium type [19]. Moreover,
the low acidity (pKa > 10) of these compounds in comparison with
the 2-, 3- and 4-phenacylpyridinium isomers [19] is in accordance
with the acidity found for the FEPA4. However, the rapid base-cat-
alyzed first-order decomposition of the enolate ion observed for
the 1-phenacylpyridinium cation (k = 7.1 sꢀ1 at pH 12) [19] was
considerably slower (ꢄ1000 times) for the corresponding ion of
FEPA4, k = (7.4 0.8) ꢂ 10ꢀ4 sꢀ1 at pH 12 and 25 °C, because of res-
onance stabilization through the whole structure. The correspond-
ing schemes of the acid–base equilibria occurring in BPA4 and
FEPA4 aqueous solutions are presented in Scheme 2.
3.2.2. Aqueous reaction mixtures of BPA4 and FEPA4 substituted
pentacyanoferrates(II)
1-Benzylpyridinium-4-aldoxime chloride and 1-phenacylpyrid-
inium-4-aldoxime chloride reacted with [Fe(CN)5OH2]3ꢀ, forming
blue colored substituted pentacyanoferrate(II) complexes with a
1:1 molar ratio. The visible spectra of the complexes, produced in
solutions of pH from 5 to 10 and a molar ratio of reactants of 1,
were characterized by a broad, intensive maximum centered at
approximately 560 (BPA-4) and 570 nm (FEPA-4), respectively,
compatible with the metal-to-ligand charge transfer (MLCT),
*
t
2g?L . The band shifted bathochromically about 20 nm upon
p
increasing the pH of the medium over 9, due to the equilibrium:
3-
2-
H
C
H
C
Ka
H+
O-
OH
+
R
N
R
N
Fe(CN)5
Fe(CN)5
ð3Þ
where R represents the remaining part of the molecules of both li-
gands. A non-linear least-squares fit of the absorbance versus pH
data to Eq. (4) led to the curves presented in Fig. 6 and to the
pKa = 6.8 0.1 for the complex with BPA-4 and pKa = 7.2 0.1 for
the complex with FEPA-4 at 25 °C and I = 0.1 M. Calculations were
performed at the wavelengths of the corresponding MLCT maxima.
Aprotonated ꢁ ½Hþꢃ þ Adeprotonated ꢁ Ka
A ¼
ð4Þ
½Hþꢃ þ Ka
The pKa values of the coordinated aldoxime groups were re-
duced by more than one pH unit with respect to the pKa values
of the aldoxime groups in the free ligands, indicating the coordina-
tion of both ligands to the iron center through the nitrogen atom
[2,3].
The complexes produced in neutral and slightly alkaline solu-
tions exhibited an additional broad maximum or shoulder of lower
intensity centered at 450 nm (BPA-4) and 480 nm (FEPA-4). This
was also observed in the spectra of other pentacyanoferrate(II)
complexes substituted with ligands of the pyridinium aldoxime
type [5,10,11]. The band was higher in energy relative to that ob-
served in the 560–580 nm region, and this could suggest the pres-
ence of another complex type. Considering the amphoteric
character of the aldoxime group, coordination of the aldoxime oxy-
The spectral characteristics of the respective ionic forms and the
ionization constants of the aldoxime and carbonyl groups derived
from absorbance versus acidity curves (Fig. 5) are presented in Ta-
ble 4. A non-linear regression of the absorbance versus pH data
according to Eq. (1) was used. In the case of the overlapping ioni-
zation constants for the aldoxime and carbonyl group in FEPA4,
Eq. (2) was applied.
A ðHLÞ ꢁ ½Hþꢃ þ AðLÞ ꢁ KaðaldoximeÞ
A ¼
A ¼
ð1Þ
½Hþꢃ þ KaðaldoximeÞ
2
AðH LÞ ꢁ ½Hþꢃ þ AðHLÞ ꢁ KaðaldoximeÞ ꢁ ½Hþꢃ þ AðLÞ ꢁ KaðaldoximeÞ ꢁ KaðcarbonylÞ
2
2
½Hþꢃ þ KaðaldoximeÞ ꢁ ½Hþꢃ þ KaðaldoximeÞ ꢁ KaðcarbonylÞ
ð2Þ
H
H
(HL)
(L)
OH
O
N
N
Ka(aldoxime)
N+
N
H+
BPA4
H
-
H
H
(HL)
(L)
(H2L)
O
O
OH
O
N
O
N
O
N
N
N+
Ka(aldoxime)
N
Ka(carbonyl)
H+
H+
FEPA4
Scheme 2. The acid–base equilibria in BPA4 and FEPA4 solutions.

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B. Foretic et al. / Inorganica Chimica Acta 363 (2010) 1425–1434
1.1
1
1.1
a
b
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
0.9
282 nm
345 nm
283 nm
342 nm
0.8
0.7
0.6
A
A
0.5
0.4
0.3
0.2
0.1
0
4
5
6
7
8
9
10
11
12
13
14
4
5
6
7
8
9
10
11
12
13
14
pH
pH
Fig. 5. Absorbance dependence on pH at analytical wavelengths of aqueous solutions of (a) BPA4 and (b) FEPA4 (c = 4.0 ꢂ 10ꢀ5 M, t = 25 °C and I = 0.10 M).
3.3. IR spectroscopy
0.6
[Fe(CN) (FEPA4)]3-/2-; λ m= 570 nm
Fe(CN)55FEPA4
The IR spectra of BPA4-ClꢁH2O and FEPA4-ClꢁH2O exhibited
strong absorption bands centered at 3370 cmꢀ1 and 3405 cmꢀ1, as-
signed to the OH stretching of the aldoxime groups. The bands at
3444 cmꢀ1 and 3465 cmꢀ1, also appearing in that spectral region,
were the result of crystal water OH stretchings. The position and
the shape of these bands are in accordance with the hydrogen bond
networking of the aldoximic –OH group, Clꢀ and H2O established
by X-ray structure analysis. The hydrated BPA4 and FEPA4 substi-
tuted pentacyanoferrate(II) complexes displayed a strong and
broad band centered at 3420 cmꢀ1, representing an overlapping
combination of the O–H stretches of both the water and the aldox-
ime group.
Bands due to the aldoxime C@N stretchings, occurring at ca.
1625 cmꢀ1 for aliphatic aldoximes, were shifted to higher values
for aromatic aldoximes, i.e., to about 1644 cmꢀ1 in the present
cases [36]. These C@N stretches generally shift to lower frequen-
cies upon the coordination of the aldoxime nitrogen to a metal
ion [37]. This is not pronounced in the spectra of the current
complexes, since the shifts were obscured by the strong absorp-
tion of the uncoordinated ligands present in a great portion as
cationic counterparts. More illustrative are the spectra of the
respective zinc salts, where the C@N stretches appear as medium
bands at 1635 cmꢀ1, indicating the aldoxime nitrogen coordina-
tion. The relatively small shift in the frequency of the C@N band
upon ligation indicated an iron–pyridiniumaldoxime interaction
[Fe(CN)5(BPA4)]3-/2-; λ m= 560 nm
BPA4
0.5
0.4
0.3
0.2
0.1
0
4
5
6
7
8
9
10
pH
Fig. 6. Absorbance dependence on pH at MLCT wavelengths of aqueous solutions of
BPA4 and FEPA4 substituted pentacyanoferrates(II), which were produced by
mixing reactants in an equimolar ratio (c = 5.0 ꢂ 10ꢀ4 M, t = 25 °C and I = 0.10 M).
gen to the metal center is optional. The band disappeared at higher
pH values and in the presence of excess ligand. Therefore, attempts
to isolate the complex were unsuccessful.
with a predominant
r donation and weak p-back bonding. This
is in good correlation with the established low energy of the
MLCT transitions.
3.2.3. Aqueous solutions of the isolated BPA4 and FEPA4 substituted
pentacyanoferrates(II)
The ligands displayed very strong absorbances at 998 (BPA4-
ClꢁH2O) and 1001 cmꢀ1 (FEPA4-ClꢁH2O) that were assigned to the
aldoxime N–O stretchings. In the spectra of the BPA4 complex, a
The electronic spectra of the solutions of the isolated solids are
presented in Fig. 7. In the visible region, they exhibited the charac-
teristic MLCT maxima at k = 560–590 nm (pH ꢄ 7;
e
ꢄ 3010
strong band occurred at 1016 cmꢀ1 with
a shoulder at
M
ꢀ1 cmꢀ1 and
e
ꢄ 1670 Mꢀ1 cmꢀ1 for the complex with BPA4
ꢄ998 cmꢀ1, while in the FEPA4 complex, two strong bands at
1019 and 999 cmꢀ1 were distinguished. These bands corresponded
to the N–O stretchings of the coordinated and uncoordinated li-
gands, respectively. The shifts of the N–O bands of the coordinated
ligands toward higher frequencies are in accordance with the
nitrogen coordination to the iron center. In the spectra of the
respective zinc salts, the bands in that spectral region were
strongly attenuated.
and FEPA4, respectively), while the UV region was dominated by
the bands of the uncoordinated ligands. In neutral media
(pH ꢄ 7), they exhibited intraligand absorptions that were covered
at other pH values at about 340 nm (
e
ꢄ 2150 Mꢀ1 cmꢀ1 for the
BPA4 complex and
and 380–440 nm (
the complex with BPA4 and FEPA4, respectively), which was as-
e
ꢄ 1430 Mꢀ1 cmꢀ1 for the FEPA4 complex)
e
ꢄ 720 Mꢀ1 cmꢀ1 and
e
ꢄ 1500 Mꢀ1 cmꢀ1 for
*
signed to p?p transitions.

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B. Foretic et al. / Inorganica Chimica Acta 363 (2010) 1425–1434
1432
4
3.5
3
pH=4.990.3
pH=6.60
(a) [Fe(CN)5(BPA4)]3-/2-
pH=8.15
pH=9.870.2
2.5
2
0.1
1.5
1
0
320
370
420
470
520
570
620
670
720
0.5
0
220
270
320
370
420
470
520
570
620
670
720
Wavelength/nm
4
3.5
3
0.2
pH=5.03
(b) [Fe(CN)5(FEPA4)]3-/2-
pH=6.60
pH=8.32
pH=10.11
0.1
2.5
2
0
320
370
420
470
520
570
620
670
1.5
1
0.5
0
220
270
320
370
420
470
520
570
620
670
Wavelength/nm
Fig. 7. pH-dependent electronic spectra of the isolated pentacyanoferrates(II), c = 7 ꢂ 10ꢀ5 M, I = 0.1 M, t = 25 °C.
In general, the vibrational spectra of the BPA4 and FEPA4 substi-
only the keto tautomer (the C@O signal at 192.0 ppm) of FEPA4
were identified. The NMR shifts of the ligands are discussed in
comparison with those of pyridine-4-aldoxime (Scheme 3). The
13C NMR resonance signals indicate the particularities of the elec-
tronic structure of the pyridinium cation, with reduced electron
density on the nitrogen atom, in relation to pyridine-4-aldoxime.
The inductive and mesomeric effects in the electronic density dis-
tribution resulted in the large upfield shifts of the two magneti-
cally equivalent carbon atoms adjacent to nitrogen and
downfield shifts of the remaining three carbons in the pyridinium
ring of BPA4 and FEPA4. The largest deshielding (+9.2 ppm for
FEPA4 and +8.6 ppm for BEPA4) occurred at the aromatic carbon
atom bound to the aldoxime group due to the contribution of the
tuted pentacyanoferrates(II) were rather similar, except for the
strong band at 1699 cmꢀ1 arising from the uncoordinated aromatic
ketone C@O stretching, suggesting the analogous mode of coordi-
nation of both ligands. The very strong cyanide stretching frequen-
cies, centered at 2048 cmꢀ1 and 2059 cmꢀ1 for BPA4 and FEPA4
complexes, respectively, were in the range expected for octahedral
pentacyanoferrate(II) complexes [16,36].
3.4. Proton and carbon NMR spectroscopy
Nuclear magnetic resonance has been previously used to char-
acterize substituted pentacyanoferrate(II) complexes and to inves-
tigate the
r
and
p
bonding Fe–ligand interactions [16]. The
positive charge formed as a consequence of p-electron resonance
assignments of NMR shifts for the free ligands were made by ana-
lyzing their 2D HMQC spectra and are found to be in good agree-
ment with literature values established for structurally similar
compounds [13,38,39]. The aldoxime protons were not observed
in D2O because of their partial ionization and subsequent exchange
with the solvent, but their shifts were observed in DMSO. The pres-
ence of exclusively one configuration isomer of both ligands and
stabilization. In the 1H NMR spectra, this effect is not pronounced
with respect to the aromatic hydrogen shifts, but it is clearly man-
ifested in the deshielding effect and downfield shift of the remote
hydrogen nucleus in the aldoxime group (13.0 ppm for BPA4,
13.1 ppm for FEPA4 and 11.8 ppm for pyridine-4-aldoxime) in-
cluded in the resonance stabilization. Such electron distribution
in pyridiniumaldoximes clearly explains their higher acidity in

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B. Foretic et al. / Inorganica Chimica Acta 363 (2010) 1425–1434
H
H
H
(13.0)
(13.1)
OH
124.5
124.9
145.6C
OH
146.2C
(11.8)
OH
120.6
146.6C
144.4
146.0
N
O
N
148.9
149.5
150.1
N
140.3
N+
N+
Ph
64.2
C
192.0
66.6
N
C
Ph
C
H2
H2
I
II
III
164.0
2-
CN
166.6
NC
166.6CN
Fe
N
NC166.6
166.6 CN
OH
126.5
148.7
146.2
N+
C145.6
H
H2C
Ph
IV
Scheme 3. Selected NMR shifts (1H shifts are given in parentheses) of the I – FEPA4, II – BPA4, III – pyridine-4-aldoxime and IV – BPA4 substituted pentacyanoferrate(II)
anion.
aqueous solution (pKa(FEPA4) = 8.72; pK (BPA4) = 8.76) in com-
4. Conclusions
a
parison with pyridine-4-aldoxime (pKa = 9.99 [40]). This is related
to the considerably higher reactivity of the aldoxime group in
pyridinium-4-aldoxime compounds toward the aquapentacyano-
ferrate(II) ions with respect to pyridine-4-aldoxime [6,7,10].
The FEPA4 substituted pentacyanoferrate(II) complex was not
sufficiently soluble in D2O, preventing a reliable spectral interpre-
tation. The 1H and 13C NMR spectra of the BPA4 substituted pent-
acyanoferrate(II) clearly indicated the presence of the coordinated
and uncoordinated ligand. The 1H NMR spectrum was dominated
by intensive signals of the uncoordinated BPA4 cation, causing
low or undetectable signals of the coordinated cation. The 13C
NMR chemical shifts of the pentacyanoferrate(II) anion were as-
signed in comparison to the shifts of the free ligand. The selected
13C NMR shifts of the BPA4 substituted pentacyanoferrate(II) com-
plex are shown in Scheme 3. The 13C NMR chemical shifts of the
coordinated BPA4 in the pentacyanoferrate(II) anion are the result
of two major effects: the deshielding effect due to the involvement
The crystalline monohydrates of 1-benzylpyridinium-4-aldox-
ime and 1-phenacylpyridinium-4-aldoxime chlorides were pre-
pared and structurally characterized. The arrangement of the
substituents around the C@N bond in the cations showed that
the aldoxime group is in the E configuration. The two presented li-
gands react with aquapentacyanoferrate(II) ion by forming mono-
nuclear complexes through the coordination of the aldoxime
nitrogen to the iron center. The new pentacyanoferrate(II) com-
plexes substituted with the biologically active ligands of interest
(BPA4-Cl and FEPA4-Cl) were synthesized and characterized on
the basis of elemental analysis and spectroscopic methods. The
pentacyanoferrate(II) moiety induced an increase in the acidity of
the pyridinium-4-aldoxime ligands, as reflected in the pKa value
of the coordinated aldoxime, which was more than 1 unit lower
than the pKa value of the free aldoxime. This conforms with an
electrostatic effect of coordination, i.e., of
r donation and weak
of
the shielding effect due to
group. The pronounced
r
electrons of aldoxime group in coordination to iron center and
-back donation of iron to the aldoxime
induction of electron density towards ir-
p
-back bonding. The weak -back bonding capability of the pyrid-
p
p
inium-4-aldoxime ligands resulted in the low energy of the MLCT
bands, the IR shifts in C@N and N–O stretching frequencies in com-
parison with free aldoximes, and the shielding effect of the carbon
nuclei in cyano groups in 13C NMR.
Spectral and elemental analyses of the synthesized complexes
were in optimal accordance with the formulas {(BPA4)2[Fe(CN)5
(BPA4)]}ꢁ3H2O and {(FEPA4)2[Fe(CN)5(FEPA4)]}ꢁ3H2O. Analysis of
the isolated zinc salts showed a Fe/Zn molar ratio of 1, confirming
the charges of the complex anions in the isolated complexes to be
ꢀ2. Within the two donor groups, aldoxime and carbonyl, present
in 1-phenacylpyridinium-4-aldoxime chloride, the aldoxime group
is obviously a more favorable and better entering species.
r
on(II) ion was manifested as the upfield shifts of the pyridinium
carbon atoms. Two cyanide resonance signals, with a ratio of rela-
tive intensities of approximately 4:1, were assigned to four cis-
(166.6 ppm) and one trans-configured cyano groups (164.0 ppm),
indicating the trans-configuration of BPA4 in the octahedral com-
plex. Since the cyano resonance in the hexacyanoferrate(II) ion oc-
curs at 177.0 ppm [16], the upfield shifts of both cyano signals
were evident. Such a shielding effect of the carbon nuclei in cya-
nide groups is a consequence of increased
p-back bonding from
iron to the cyano ligands and the very poor
p-back donation to-
wards the aldoxime group in coordinated BPA4. This observation
is in accordance with the low energy wavelength position of the
MLCT bands found not only for aqueous solutions of the examined
pentacyanoferrate(II) complexes but also for other pyr-
idiniumaldoxime substituted pentacyanoferrates(II), ranging from
500 to 620 nm [5,12].
Acknowledgments
Financial support for this research was provided by the Ministry
of Science and Technology of the Republic of Croatia (Grant Nos.
108-1193079-3070 and 119-1193079-1084).

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B. Foretic et al. / Inorganica Chimica Acta 363 (2010) 1425–1434
[17] H.E. Toma, A.A. Batsta, H.B. Gray, J. Am. Chem. Soc. 104 (1982) 7509.
[18] H. Winnischofer, F.M. Engelmann, H.E. Toma, K. Araki, H.R. Rechenberg, Inorg.
Chim. Acta 338 (2002) 27.
Appendix A. Supplementary material
CCDC 735355 and 735356 contain the supplementary crystallo-
graphic data. These data can be obtained free of charge via
www.ccdc.cam.ac.uk, or from the Cambridge Crystallographic Data
Centre, 12 Union Road, Cambridge, CB2 1EZ, UK; fax: (+44) 1223
336 033; or e-mail: deposit@ccdc.cam.ac.uk. Supplementary data
associated with this article can be found, in the online version, at
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