Metal-controlled anion-binding tendencies of the thiourea unit of thiosemicarbazones

Article abstract of DOI:10.1002/chem.200800801

The terdentate ligand 3 (LH, 2-formylpyridine 4-thiosemicarbazone) forms with Fe^II and Ni^II 2:1 complexes of octahedral geometry of formula [M^II(LH)₂]²⁺. X-ray diffraction studies have shown that in b

Full text of DOI:10.1002/chem.200800801

FULL PAPER
DOI: 10.1002/chem.200800801
Metal-Controlled Anion-Binding Tendencies of the Thiourea Unit of
Thiosemicarbazones
Valeria Amendola,[a] Massimo Boiocchi,[b] Luigi Fabbrizzi,*[a] and Lorenzo Mosca[a]
Abstract: The terdentate ligand 3 (LH,
2-formylpyridine 4-thiosemicarbazone)
forms with FeII and NiII 2:1 complexes
of octahedral geometry of formula
[MII(LH)2]2+. X-ray diffraction studies
have shown that in both complexes the
thiourea moieties of the coordinated
thiosemicarbazones are exposed to the
outside and are prone to establish hy-
drogen-bonding bifurcate interactions
with oxoanions. However, spectropho-
tometric studies in CHCl3 solution
have shown that only the poorly basic
NO3 ion is able to form authentic hy-
induce deprotonation of the N-H frag-
ment. The extreme enhancement of the
thiourea acidity is based on the coordi-
native interaction of the sulphur atom
with the metal, which stabilises the
thioACHTERlUNG ate form, and it is much higher
than that exerted by any other cova-
lently linked electron-withdrawing sub-
ꢀ
drogen-bond complexes with thiourea
subunits, whereas all the other investi-
ꢀ
gated anions (CH3COOꢀ, NO2 , Fꢀ)
Keywords: anions · charge transfer ·
hydrogen bonds · receptors · supra-
molecular chemistry
ꢀ
stituent, for example, NO2.
Introduction
the more advanced the proton transfer and the stronger the
receptor–anion interaction. It is for this reason that the
rather acidic thiourea fragment (pKA 21.1 in DMSO) estab-
lishes with oxoanions stronger interactions than the distinct-
ly less acidic urea (pKA 26.9 in DMSO).[8] In a limiting situa-
tion, urea- and thiourea-based receptors, made particularly
acidic by covalently linked electron withdrawing substitu-
ents, may undergo deprotonation of one N-H fragment on
interaction with strongly basic anions.[9] This is a rather
common behaviour in the presence of an excess of fluoride,
Anion recognition by neutral H-bond donor receptors has
become a challenging theme in supramolecular chemistry
and represents an area of fervent research.[1] The most fre-
quently used H-bond donating group is the N-H fragment
from amides,[2] sulfonamides,[3] amines,[4] pyrroles,[5] ureas
and thioureas.[6] A urea/thiourea subunit can donate two hy-
drogen bonds according to a bifurcate mode and is therefore
an ideal receptor for anions that present two proximate
oxygen atoms, for example, carboxylates and inorganic oxo-
ꢀ
due to the formation of the particularly stable HF2 hydro-
A
gen bonding self-complex.[10] Also, the solvent plays a role
in controlling N-H deprotonation. In particular, deprotona-
tion and formation of the [HA2]ꢀ self-complex are favoured
by more polar media, which stabilise the negatively charged
products. For instance, derivative 1 in a DMSO solution un-
dergoes N-H deprotonation not only in presence of fluoride,
but also on addition of excess acetate, with formation of the
[CH3COOH···CH3COO]ꢀ self-complex.[11] Quantitative as-
pects of anion induced N-H deprotonation of urea–thiourea
derivatives have been clearly discussed in a recent article.[12]
less advanced proton transfer from the donor (the receptor)
to the acceptor (the anion).[7] The more acidic the receptor,
[a] Dr. V. Amendola, Prof. L. Fabbrizzi, Dr. L. Mosca
Dipartimento di Chimica Generale, Università di Pavia
via Taramelli 12, 27100 Pavia (Italy)
[b] Dr. M. Boiocchi
Centro Grandi Strumenti, Università di Pavia
via Bassi, 27100 Pavia (Italy)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200800801: Structures of the
[FeII(3)2]2+ and [NiII(3)2]2+ complexes, supplementary spectrophoto-
metric titrations and the colour photographs of the CHCl3 solutions
of the [FeII(3)2]2+ and [NiII(3)2]2+ complexes after the addition of 0,
1, and 2equiv of base.
Chem. Eur. J. 2008, 14, 9683 – 9696
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
9683

We have recently investigated the interaction of a family
of R-substituted-benzylideneamine-N’-(R1-substituted-phe-
nyl)thioureas (2) with a variety of anions in MeCN solu-
tion.[13] The formation of genuine H-bond association com-
nounced the tendency of the receptor to deprotonate in
presence of basic anions. In this study, we have chosen as a
solvent the poorly polar chloroform, with the aim of favour-
ing the formation of genuine receptor–anion H-bond com-
plexes. In order to assess anion binding tendencies of thiour-
ea-based receptors in chloroform, we carried out prelimina-
ꢀ
plexes with CH3COOꢀ and H2PO4 was observed, whereas
ꢀ
ꢀ
Fꢀ induced deprotonation. Other oxoanions (HSO4 , NO2 ,
ꢀ
NO3 ) did not form any complex even with receptors
ry investigations with the thioACHTREuUNG rea derivative 4.
equipped with powerful electron-withdrawing substituents
(e.g. NO2, CF3).
We assumed that a transition-metal ion, conveniently co-
ordinated to the benzylidene-thiourea framework, could act
as a powerful electron-withdrawing substituent, thus enhanc-
ing the acidity of the thiourea fragment and hopefully induc-
ing novel selectivity effects in anion recognition. Thus, we
have prepared derivative 3, in which the benzylidene phenyl
ring of receptor 2 has been replaced by a pyridine ring.
System 3 can behave as a terdentate ligand (LH) through an
N2S donor set (as indicated by arrows in the formula) and
can give 1:2complexes of formula [M II(LH)2]2+ with metals
prone to six-coordination and octahedral geometry, for ex-
ample, FeII, NiII, as illustrated in Scheme 1.
In 4, a 4-nitrophenyl substituent has been appended to
the thiourea unit in order i) to increase H-bond donating
tendencies and ii) to provide a powerful chromogenic re-
porter for signalling anion binding. On the other hand, the
methyl group in the meta position of the other phenyl sub-
stituent reduces the symmetry of the molecule, thus afford-
ing surprisingly high solubility in CHCl3 (up to 0.1m).
Figure 1 shows the family of spectra recorded over the
course of the titration of a solution of 4 in CHCl3 with a
standard solution of [Bu4N]CH3COO.
Scheme 1. Interaction of [MII(3)2]2+ complexes (M=Fe, Ni) with oxo-
ACHTREUNGanions.
In principle, in the [MII(LH)2]2+ metal complex, the two
ꢀ
thiourea subunits can interact with two oxoanions RO2 , ac-
cording to the equilibrium shown in Scheme 1 (constituted
by two steps), to give eventually the genuine H-bond com-
2ACHTREUNG
plex [MII(LH) (···RO2)2]2+. Moreover, the role of the coordi-
nated metal centre, as an electron-withdrawing group, on
the polarization of thiourea N-H fragments has to be consid-
ered, which may lead to stepwise deprotonation.
Figure 1. a) Spectra taken over the course of the titration of a 5.04”
We describe here an investigation on the interaction of
octahedral complexes of type [MII(LH)2]2+ (M=Fe, Ni) with
anions in a CHCl3 solution. It will be shown that most
anions promote N-H deprotonation, according to two dis-
tinct steps, signalled by noticeable colour changes. Only the
poorly basic nitrate ion is able to form stable and authentic
hydrogen bonding complexes, still in a stepwise mode, with
moderate colour modifications.
10ꢀ5
m solution of 4 in CHCl3 with a m of
solution 6.34”10ꢀ3
[Bu4N]CH3COO in CHCl3; a: refers to the spectrum of receptor 4,
!
,
before anion addition; b) titration profiles at selected wavelengths:
band centred at 300 nm, pertinent to the uncomplexed receptor 4 (AH),
!
decreasing profile; : band centred at 371 nm, pertinent to the hydrogen-
bonding complex [AH···CH3COO]ꢀ, increasing profile.
The receptor shows two overlapping bands, one centred at
270, the other at 315 nm. It is suggested that the two bands
result from charge-transfer transitions to the nitrophenyl
substituents, one from the tolyl subunit, the other from the
thiourea subunit.
Upon acetate addition, the spectrum undergoes two
major modifications: i) the intensity of the band at 270 nm
undergoes a moderate decrease; ii) the band at 315 nm ex-
periences a drastic red shift: in particular, a new band devel-
Results and Discussion
Anion affinity of thioureas in a CHCl3 solution: The com-
plexation equilibria of a variety of thiourea derivatives to-
wards anions have been investigated in media of different
polarity. On the other hand, it has been previously pointed
out that the higher the solvent polarity, the more pro-
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Chem. Eur. J. 2008, 14, 9683 – 9696

Anion Binding
FULL PAPER
ops at 370 nm. It is suggested that the band at 315 nm origi-
nates from a charge-transfer transition from the thiourea
subunit to the nitrobenzene subunit and that the urea–ace-
tate interaction stabilises the excited state resulting from
such a transition, thus inducing a significant bathochromic
shift of the absorption band. The tolyl-to-nitrophenyl charge
transfer is not perturbed by the thiourea–acetate interaction
and the corresponding band, centred at 270 nm, does not
undergo any red shift. Moreover, multi-wavelength treat-
ment of titration data over the 250–550 nm range, by Hyper-
Quad program,[14] indicated the formation of a receptor-
anion complex of 1:1 stoichiometry, [AH···CH3COO]ꢀ, ac-
cording to Equilibrium (1):
ꢀ
AH þ Xꢀ Ð ½AH Á Á Á XŠ
ð1Þ
Figure 2. a) Spectra obtained on addition of a large anion excess to a so-
lution of 4 in CHCl3; b) plot of the logK values for the receptor–anion
equilibrium vs. the wavelength of the absorption band of the pertinent re-
ceptor-anion complex, as taken from limiting spectra in Figure 2a.
with a logK=4.44Æ0.03. It has to be noted that semiempiri-
cal studies, using PM3 method, suggested that the uncom-
plexed receptor 4 should exist in the trans conformation il-
lustrated by formula 6. Therefore, anion binding should
induce a geometrical rearrangement of 4, from the trans
conformation 6 to the cis conformation 7, required for es-
tablishing bifurcate hydrogen-bonding interaction with ace-
tate and pictorially illustrated in Scheme 2.
Table 1. logK values for the formation of H-bond complexes in CHCl3
solution, at 258C, according to the equilibrium: AH + Xꢀ Q [AH···X]ꢀ.
ꢀ
logK2 for Fꢀ refers to the equilibrium: [AH···F]ꢀ + Fꢀ Q Aꢀ + HF2
.
Anion
4 (AH)
5
CH3COOꢀ
4.44Æ0.03
2.7Æ0.1
ꢀ
NO2
3.54Æ0.01
–
Fꢀ
HSO4
H2PO4
NO3
logK1 =5.31Æ0.01; logK2 =4.29Æ0.01
2.74Æ0.01
ꢀ
3.91Æ0.01
4.41Æ0.01
3.49Æ0.01
–
–
–
ꢀ
ꢀ
tion. Quite interestingly, also the magnitude of the red shift
decreases according to the same sequence: CH3COOꢀ
Scheme 2. Formation of the H-bond complex 7 induces a conformational
rearrangement of receptor 6, from the stable trans arrangement (the ni-
trophenyl substituent is trans with respect to the thiourea sulphur atom)
to the cis arrangement, suitable for bifurcate hydrogen bonding interac-
tion with the acetate ion.
>
ꢀ
ꢀ
ꢀ
ꢀ
H2PO4 > HSO4 > NO2 > NO3 . In particular, a rough
linear relationship exists between logK and the wavelength
of the absorption band of the [AH···X]ꢀ complex (see Fig-
ure 2b). Such a correlation is not surprising, if one considers
that the interaction with the oxoanion stabilizes the charge
transfer excited state of the receptor: the stronger anion ba-
sicity, the more pronounced the stabilization of the excited
state, the lower the energy of the optical transition(s). In
any case, it is demonstrated once again that the stability of
the H-bond complexes does not reflect any geometrical
complementarity between the N-H fragments of the urea/
thiourea subunit and two consecutive oxygen atoms of the
anion, but simply depends upon the basic tendencies of the
oxoanion.
Halide ions showed a markedly different behaviour. In
fact, addition of a large excess of tetraalkylammonium chlo-
ride, bromide and iodide salts to a CHCl3 solution of 4 did
not induce any spectral change, indicating no or very poor
receptor–anion interaction. On the other hand, fluoride ex-
hibited a rather intricate behaviour. Figure 3 displays the
family of spectra taken over the course of the titration of a
A similar behaviour was observed on spectrophotometric
ꢀ
ꢀ
titration of 4 with a variety of oxoanions: H2PO4 , HSO4 ,
ꢀ
ꢀ
NO2 , NO3 . In particular, a red shift of the band at 315 nm
was observed on anion addition and titration data could be
interpreted on assuming the formation of a 1:1 receptor–
anion complex, [AH···X]ꢀ, according to Equation (1). Limit-
ing spectra obtained on addition of a large anion excess are
shown in Figure 2a. logK values for Equation (1) involving
the investigated anions are reported in Table 1.
The stability of the [AH···X]ꢀ complex decreases along
ꢀ
ꢀ
ꢀ
the series CH3COOꢀ > H2PO4 > HSO4 > NO2
>
ꢀ
NO3 , which reflects the decreasing basicity of the anion, a
behaviour typically observed in the interaction of urea- and
thiourea-based receptors with anions. Hydrogen bonding
has been defined as a more or less advanced (and “frozen”)
proton transfer from the donor (the receptor) to the accept-
or (the anion).[7] The more basic the anion, the more ad-
vanced the proton transfer, the stronger the H-bond interac-
solution of
4 in CHCl3 with a standard solution of
[Bu4N]F·3H2O.
Chem. Eur. J. 2008, 14, 9683 – 9696
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9685

L. Fabbrizzi et al.
Figure 4. Lines (left vertical axis): concentration of the species which
form over the course of the titration illustrated in Figure 4 (AH=4,
a). Symbols (right vertical axes): absorbance at selected wavelengths
Figure 3. Spectra recorded over the course of the titration of a 5.08”
10ꢀ5 solution 5.14”10ꢀ3
solution of in CHCl3 with of
[Bu4N]F·3H2O in CHCl3. Spectra taken on solutions containing 0 equiv
of fluoride (g), 1 equiv of fluoride (a); 8 equiv of fluoride (a) are
indicated.
m
4
a
m
~
over the course of the titration; the band at 300 nm ( ) pertains to the
!
uncomplexed receptor AH, the band at 360 nm ( ) to the H-bond com-
plex [AH···F]ꢀ (c), the band at 450 nm ( ) to the deprotonated form
!
A
ꢀ (b).
Addition of fluoride first induces a moderate red shift of
the band at 315 nm (thiourea-to-nitrophenyl charge-transfer
transition), which indicates the formation of a genuine H-
bond complex [AH···F]ꢀ, according to Equation (1). In
Figure 3, the spectrum taken upon addition of 1 equiv of
fluoride has been drawn as a black dashed line. However,
on further anion addition, a more pronounced red shift of
the absorption band is observed. Such a spectrum, drawn as
a grey dashed line in Figure 3, clearly displays a two-transi-
tion nature. It is suggested that such a spectrum pertains to
the deprotonated form of the receptor, Aꢀ, which results
from Equation (2)
deprotonation, the receptor moves again to a trans confor-
mation, as represented by structural formula 8 in Scheme 3.
It is suggested that form Aꢀ is stabilised by the circum-
stance that the negative charge left on the thiourea nitrogen
atom upon proton release can delocalize, through a p mech-
anism, onto the NO2 fragment of the nitrophenyl substitu-
ent, as pictorially illustrated by the resonance formula 9 in
Scheme 3. Notice that the excess of electronic charge on the
NO2 fragment increases the intensity of the dipole of both
charge transfer transitions (at 270 and 315 nm in the neutral
form AH), which are bathochromically shifted at 385 and
435 nm in the deprotonated form Aꢀ.
ꢀ
ꢀ
½AH Á Á Á FŠ þ Fꢀ Ð Aꢀ þ ½HF2Š
ð2Þ
Deprotonation of the N-H fragment of urea and thiourea
derivatives on addition of excess fluoride has been observed
for a variety of anion receptors and has to be ultimately as-
cribed to the high stability of the [HF2]ꢀ self-complex;[7] in
fact, one Fꢀ is not an especially strong base, but two Fꢀ are.
Best fitting of titration data using the HyperQuad program
was obtained on assuming the occurrence of stepwise Equili-
bria (1) and (2), to which the following values of K1 and K2
correspond: logK1 =5.31Æ0.01, logK2 =4.29Æ0.02.
Scheme 3. Resonance representation of the deprotonated form of recep-
tor 4. The electronic charge left on the thiourea nitrogen atom following
ꢀ
N-H deprotonation (limiting formula 8) delocalizes onto the NO2 group
of the nitrophenyl substituent, according to limiting formula 9.
Figure 4 reports the concentration profiles of the three
species present at the equilibrium over the course of the ti-
tration: AH, [AH···F]ꢀ, Aꢀ. It is observed that the H-bond
complex, [AH···F]ꢀ, reaches its maximum concentration,
about 60%, on addition of 1 equiv of fluoride. Interestingly,
the absorbance at 360 nm, which refers to the maximum of
the band drawn as a black dashed line in Figure 4, superim-
poses onto the ascending branch of the concentration profile
of the H-bond complex [AH···F]ꢀ. Likewise, the absorbance
at 450 nm, which refers to the higher wavelength component
of the spectrum drawn as a grey dashed line in Figure 4, par-
allels the increase of the concentration profile of the depro-
tonated receptor Aꢀ. It is also suggested (and corroborated
by semi-empirical calculations) that on fluoride release and
The major role played by the nitrophenyl substituent in
enhancing the acidity of the proximate N-H fragment has
been further demonstrated by the behaviour of the thiourea
ꢀ
derivative 5, in which the NO2 group has been removed.
Figure 5 displays the family of spectra recorded over the
course of the titration of a solution of 5 in CHCl3 with a
standard solution of [Bu4N]F·3H2O.
It is observed that even large excess addition of fluoride
induces only a moderate shift of the charge transfer band of
the receptor, which indicates the formation of a hydrogen-
bonding complex of low thermodynamic stability. In particu-
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Anion Binding
FULL PAPER
Figure 6. a) Spectra obtained over the course of the titration of a 5.00”
Figure 5. a) Spectra obtained over the course of the titration of a 4.79”
10ꢀ5
m
solution of
3
in CHCl3 with
a
solution 9.62”10ꢀ2
m of
10ꢀ5
m
solution of
5
in CHCl3 with
a
solution 5.04”10ꢀ3
m of
!
[Bu4N]CH3COO in CHCl3; b) absorbance at 330 nm ( ).
!
[Bu4N]F·3H2O in CHCl3; b) absorbance values at 320 nm ( , e right ver-
tical axis) superimpose well on the concentration profile of the H-bond
complex [A-H···F]ꢀ (c, left vertical axis).
lar, a logK=2.74Æ0.01 was calculated from multiwave-
length least-squares treatment of titration data. The devel-
opment of a new band at higher wavelength was not ob-
served even on addition of 20 equiv and more of Fꢀ, ruling
out the deprotonation of one of the N-H fragments of the
thiourea subunit. It is therefore confirmed that proton re-
lease from thiourea (and urea) subunits in non-aqueous
media is driven both by the high stability of the [HF2]ꢀ self-
complex and by the presence of powerful electron withdraw-
Scheme 4. Intramolecular hydrogen bonding interactions stabilize the
conformation of receptor illustrated by formula 10, which disfavours the
association with oxoanions (e.g., acetate, to form complex 11).
by the presence of intramolecular hydrogen-bonding inter-
actions involving the N-H fragments of the thiourea moiety
and pyridine and imine nitrogen atoms, as outlined in
Scheme 4. It is suggested that the endothermic contribution
associated to the breaking of such intramolecular interac-
tions is responsible for the poor stability of complex 11 and
analogues.
Most interestingly, 3 (LH) behaves as a terdentate ligand
and forms with divalent transition metals six-coordinate
complexes of formula [MII(LH)2]2+. In particular, on diffu-
sion of diethyl ether on a MeCN solution containing [FeII-
ꢀ
ing substituents, in particular NO2. Receptor 5 formed a
poorly stable H-bond complex also with CH3COOꢀ (logK=
2.7Æ0.1), as obtained from a spectrophotometric titration
experiment. On titration of 5 with all the other investigated
anions, no significant spectral modifications were observed,
indicating the formation of association complexes of very
low stability or no formation at all.
Studies on thiourea derivatives 4 and 5 have provided
firm bases, which may be useful for the investigation of the
anion binding tendencies of 3 (LH) and of its metal com-
plexes [MII(LH)2]2+ (M=Fe, Ni).
AHCTREUNG
AHCTREUNG
The interaction of 3 with anions and metal cations: Anion-
binding tendencies of 3 towards anions in CHCl3 are poor
or nil. Figure 6a shows the family of spectra recorded over
the course of the titration of a solution 5.00”10ꢀ5 m in 3
with a rather concentrated solution of [Bu4N]CH3COO
(0.1m).
Minor spectral modifications are observed on acetate ad-
dition (up to 120 equiv). In particular, a very slight curva-
ture is observed in the titration profile shown in Figure 6b,
indicating a binding constant value logK ꢁ1. Less pro-
nounced or no spectral modifications at all were observed
on titration of 3 with other anions. Semiempirical investiga-
tions have shown that the uncomplexed receptor 3, in ab-
sence of anions, exists in the conformation illustrated in
Scheme 4 by formula 10. Such an arrangement is stabilised
diffraction studies were obtained. An ORTEP view of the
complex salt is shown in Figure 7.
The FeII complex exhibits a distorted octahedral coordina-
tion, with the two ligands arranged according to a meridia-
nal coordination. Selected geometrical features are shown in
Table S1 of Supporting Information.
No structures of bis(thiosemicarbazone) FeII complexes
ꢀ
have been previously reported. However, the short Fe N
bond lengths, lying in the range 1.88(2)–1.97(2) Š, are typi-
cal for low-spin FeII octahedral complexes with ligands con-
taining sp2 nitrogen atoms. In particular, the FeII–N-
AHCTREUNG
AHCTREUNG
AHCTREUNG
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9687

L. Fabbrizzi et al.
[FeII(3)2]
complex salt is shown in Figure 8.
ACHTREU(GN CF3SO3)2·H2O analogue. An ORTEP view of the
Figure 7. ORTEP view of the [FeII(3)2]
ACHTRE(UNG CF3SO3)2·H2O molecular complex
(ellipsoids are drawn at the 30% probability level; the water molecule
was omitted for clarity, as were hydrogen atoms not belonging to thio-
ACHTREUNGurea groups).
Figure 8. ORTEP view of the [NiII(3)2]
ACHTRE(UNG CF3SO3)2·MeCN molecular com-
plex (ellipsoids are drawn at the 30% probability level; the MeCN mole-
cule was omitted for clarity, as were hydrogen atoms not belonging to
thioACHTREuUNG rea groups). Hydrogen bonds are drawn as dashed lines.
stance, with the complex [FeII(N,N’-dimethylethylenedi-
ACHTREUNG
imine)3]2+: 1.95 Š.[16] It is suggested that the unusually short
FeII–N distance is a consequence of the steric constraints as-
sociated to the coordination of the terdentate ligand. The
sulphur atoms are placed 2.28(1) and 2.30(1) Š from the
metal centre. Such distances are distinctly lower than ob-
Actually, relevant geometrical features are similar for
both compounds, as shown by values in Tables 1 and 2.
Metal–donor atom distances of the NiII complex are larger
than for FeII, due to the closed shell electronic configuration
(t2g6) of the latter metal centre (see d values in Table 1).
Geometrical features and distances are similar to those ob-
served in previously investigated NiII bis(thiosemicarba-
zones) complexes.[19,20] Also in the present case, an unusually
served in the octahedral [FeII(N,N’-dimethylthiourea)6]2+
,
II
ꢀ
which has a high-spin nature and Fe S bond lengths range
over the interval 2.49–2.61 Š.[17] The two mer-terdentate li-
gands are placed in an almost perpendicular setting and the
dihedral angle between the two thiosemicarbazone moieties
is 89.8(4).
Very interestingly, with respect to the anion-recognition
properties of the complex, each thiourea subunit establishes
a bifurcate hydrogen-bonding interaction with a triflate ion.
Selected features of the hydrogen-bonding interactions are
shown in Table 2. Donor···acceptor distances are those ob-
served for moderate-to-weak interactions.[18]
short metal–NAHCTRE(UNG imine) distance is observed, which is again
ascribed to the constraints associated to the terdentate coor-
dination mode of each thiosemicarbazone ligand (mean
NiII–N
ACHTRE(UNG imine)=2.01(1) Š; compare with 2.11 Š in the com-
[21]
plex [NiII(acetaldoxime)4Cl2]2+
,
or 2.10 Š in the complex
[NiII(pyrazole)6]2+).[22] Finally, structural features of the bi-
AHCTREUNG
Notwithstanding the presence of a different solvent mole-
furcate hydrogen interactions of the two thiourea subunits
are totally analogous to those of the FeII complexes, as
shown by structural parameters in Table 2. Moreover, the
perpendicular setting of the two
cule in the crystals, MeCN instead of H2O, the [NiII(3)2]-
ACHTREUNG(CF3SO3)2·MeCN complex salt is isostructural with the
mer-terdentate moieties, with a
dihedral angle 89.3(2)8, is total-
ly similar to that observed for
the FeII analogue.
Table 2. Features of the hydrogen bonds involving the thiourea subunits of 3 and the triflate counterions in
FeII and NiII complexes.
Donor N-H fragment
N(3)–H(3N)
D···A [Š]
H···A [Š]
D-H···A [8]
Acceptor atom
O(6)
FeII
NiII
FeII
NiII
FeII
NiII
FeII
NiII
FeII
NiII
2.81(4)
3.00(1)
3.21(4)
3.38(1)
3.24(7)
3.06(1)
2.84(3)
2.83(1)
2.95(4)
2.99(1)
1.9(2)
2.2(1)
2.4(2)
2.6(1)
2.4(3)
2.1(1)
1.9(2)
1.9(1)
2.1(2)
2.1(1)
168(17)
145(3)
145(20)
138(3)
144(20)
164(3)
157(15)
172(3)
147(12)
161(2)
The [FeII(LH)2]2+ metal com-
plex as an anion receptor:
Structural studies have demon-
strated that octahedral metal
complexes of 3 (=HL) expose
their thiourea subunits towards
the outside and are therefore
inclined to establish hydrogen
N(4)–H(4N)
N(4)–H(4N)
N(7)–H(7N)
N(8)–H(8N)
O(6)
O(4)
O(1)
O(3)
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Chem. Eur. J. 2008, 14, 9683 – 9696

Anion Binding
FULL PAPER
bonding
interactions
with
Figure S3 in the Supporting Information. The [FeII(LH)2]2+
complex has a violet colour, which originates from rather in-
tense MLCT transitions. In particular, the band results from
two transitions: one centred at 510 nm, the other centred at
570 nm. MLCT bands in this spectral portion are usually ob-
served in FeII (d6, low-spin) octahedral complexes with
ligand containing sp2 hybridized nitrogen atoms (polypyri-
dines, imines), and result from the transitions from metal
centred p orbitals (pM, t2g in origin) to p* orbitals, essential-
ly centred on the ligands. In particular, the model 1:3 com-
plex of FeII with the pyridine-imine ligand N-phenyl-pyri-
dine-2-yleneamine shows a similar two-transition MLCT
spectrum, with one band centred at 495 nm (shoulder, e=
2100mꢀ1 cmꢀ1) and the other centred at 535 nm (e=
2630mꢀ1 cmꢀ1, see Figure S4, Supporting Information). Base
addition induces a dramatic red shift of the two-transition
band of the [FeII(LH)2]2+ complex. However, the change
does not take place gradually, but discontinuously. In partic-
ular, on addition of the first equivalent of base, the MLCT
band of the [FeII(LH)2]2+ complex (solid line in Figure 9b,
responsible for the violet colour) decreases in intensity,
while a new band forms and develops, featuring two transi-
tions at 535 and 607 nm (while a dark blue colour appears,
see Figure S3 in the Supporting Information). On addition
of the second equivalent of base, such a two-transition band
decreases and a new one develops at higher wavelengths
(568 and 643 nm). Such a band reaches a limiting value after
the addition of 2equiv of base, while the solution takes an
emerald green colour. This behaviour can be accounted for
by assuming the occurrence of the two stepwise acid–base
Equilibria (3) and (4):
anions, without experiencing
any preliminary endothermic
conformational rearrangement
(as the uncomplexed thiosemi-
carbazone receptor does). Our
first interest was to determine
the intrinsic tendencies of thio-
urea N-H fragments to release
protons and to characterize the
Scheme 5. Neutralization of
monoprotic acid HA by the
Proton Sponge.
a
deprotonated species that form, if any. Thus, we carried out
a preliminary titration experiment on a CHCl3 solution of
the [FeII(LH)2]2+ complex with the Proton Sponge (12,
N1,N1,N8,N8-tetramethylnaphthalene-1,8-diamine,
Scheme 5).
Compound 12 guarantees the highest proton affinity in
non-aqueous media and is not hygroscopic (like, for in-
stance, the strong base currently used in such circumstances,
[Bu4N]OH). Figure 9 shows the family of spectra recorded
over the course of the titration of a CHCl3 solution 9.74”
10ꢀ5 m in [FeII(3)2]2+ with a solution of 12 (1.03”10ꢀ2 m). In
particular, Figure 9a shows the UV portion of the spectrum,
displaying the ligand-to-metal charge transfer (LMCT)
band; Figure 9b shows the visible part of the spectrum, with
metal-to-ligand charge transfer (MLCT) bands.
On addition of base, drastic colour changes and spectral
modifications are observed. Colour changes are shown in
2þ
þ
½FeIIðLHÞ2Š þ PS Ð ½FeIIðLHÞðLފ þ PSHþ
ð3Þ
ð4Þ
½FeIIðLHÞðLފ þ PS Ð ½FeIIðLފ þ PSHþ
þ
In particular, it is suggested that each proton is released
from an N-H fragment of the thiourea moiety of each coor-
dinated thiosemicarbazone. Moreover, the electron pair left
on the thiourea nitrogen atom on deprotonation, formula
14, is transferred onto the entire ligand framework, accord-
ing to the p-conjugative mechanism illustrated in Scheme 6.
Scheme 6. Resonance formulae of the monodeprotonated complex
[FeII(LH)(L)]+. The lone pair left on the thiourea nitrogen atom, follow-
ing N-H deprotonation, delocalizes both on the pyridine nitrogen atom
(grey arrows) and on the sulphur atom (black arrows), even if to a differ-
ent extent.
Figure 9. Spectra obtained over the course of the titration of a 9.74”
10ꢀ5 m solution of [FeII(3)2]2+ in CHCl3 with a solution 1.03”10ꢀ2 m of 12
in CHCl3 (LH=3). a) UV portion, displaying LMCT bands; b) visible
portion, showing MLCT bands. Thick lines refer to spectra taken after
the addition of 0, 1, 2equiv of base.
In particular, electron charge can be relocated onto the
sulphur atom, according to the pathway illustrated by black
Chem. Eur. J. 2008, 14, 9683 – 9696
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9689

L. Fabbrizzi et al.
arrows in Scheme 6 (to give formula 15) or on the pyridine
nitrogen atom, according to the pathway illustrated by grey
arrows (to give formula 13). In any case, negative charge is
eventually transferred from the donor atoms to the metal
centre, the reducing tendencies of which increase. As a con-
sequence, the charge transfer transition from the metal to
the empty p* orbitals centred on the pyridine–imine system
becomes easier and pertinent MLCT bands are red shifted.
Such an effect becomes more pronounced on successive de-
protonation of an N-H fragment from the other coordinated
thiosemicarbazone, which brings additional charge on the
metal, whose reducing properties are further enhanced.
Notice that in Scheme 6 it has been assumed that N-H de-
protonation induces a simultaneous conformational change,
leading to a trans arrangement of the phenyl substituent
with respect to the sulphur atom. Such an arrangement has
been observed in the 1:1 complex of CuII with a deprotonat-
ed thiosemicarbazone.[23] This would imply that the cis con-
present at the equilibrium. In any case, diagrams in
Figure 10 confirm the existence of three distinct species,
with a defined stoichiometry and with different spectral
properties.
The [FeII(LH)2]2+ complex presents also a rather intense
band at 335 nm (Figure 9a). Such a band results from a tran-
sition from a donor atom of the thiosemicarbazone ligand
(either sulphur or nitrogen, or both) to an empty antibond-
ing orbital centred on the metal (sM*, eg in origin). On base
addition, the intensity of such a band decreases, while a new
band forms and develops at 375 nm. This behaviour clearly
reflects the progress of Equilibria (3) and (4) on base addi-
tion. In particular, on N-H deprotonation, negative charge is
transferred onto the donor atoms, whose reducing tenden-
cies are increased. As a consequence, the LMCT band is sig-
nificantly red shifted.
Quite interestingly, appearance and modifications of
LMCT and MLCT bands over the course of the titration are
definitely different. Actually, looking at spectra in Figure 9,
one could assume that spectral changes in the MLCT region
(Figure 9b) proceed horizontally (as indicated by a two-step
bathochromic shift of the band) and those in the LMCT
region (Figure 9a) proceed vertically (i.e., through the simul-
taneous decrease-increase of two bands). The apparently
paradoxical behaviour results from the fact that the three
degrees of deprotonation (0, ꢀ1, ꢀ2) generate three distinct
states of charge of the metal,
formation, observed for instance in the [FeII(3)2]
ACHTER(UGN CF3SO3)2
in the solid state, is intrinsically unstable and is adopted
only in the presence of hydrogen bonding bifurcate interac-
tions of the thiourea subunit with an oxoanion (e.g.,
ꢀ
CF3SO3 ).
Figure 10 shows the profiles of the lower energy compo-
nent of the MLCT band for the three species at equilibrium,
[FeII(LH)2]2+, [FeII(LH)(L)]+, [FeII(L)2].
which give rise to three differ-
ent metal-to-ligand transitions
and to three limiting MLCT
spectra (Figure 9b: solid, short
dashed and long dashed thick
lines). On the other hand, the
thiosemicarbazone donor atoms
(either sulphur or pyridine ni-
trogen atom, or both) can exist
only in two states: either neu-
tral or negatively charged, to
which two ligand-to-metal tran-
sitions correspond: this results
Figure 10. Symbols (e, right vertical axes): titration profiles of the lower energy MLCT band (pM!p-pyridine)
for the three species at equilibrium, [FeII(LH)2]2+, [FeII(LH)(L)]+, [FeII(L)2]. Lines (left vertical axis): % con-
centration of the three complex species calculated on assuming for equilibria (3) and (4) the following values:
logK(3)=13.5, logK(4)=10.0.
in two bands, one decreasing,
the other increasing (Figure 9a).
Thus, the intermediate species
[FeII(LH)(L)]+ does not show
its specific LMCT band, but a
The profiles show discontinuity approximately at 1 and
2equiv of added base. Solid and dashed lines in Figure 10
spectrum which results from the balanced combination of
the band originating from the neutral donor atom-to-FeII
charge transfer and of that originating from the negatively
charged donor atom-to-FeII transition.
represent the concentration of the [FeII(LH)2]2+
,
[FeII(LH)(L)]+, [FeII(L)2] complexes over the course of the
titration, which have been calculated on assuming for Equi-
libria (3) and (4) the following values: logK(3)=13.5,
logK(4)=10.0. It is observed that symbols (i.e., absorbances
of the low energy charge transfer bands) superimpose, at
least partially, on the concentration curves. Poorly satisfacto-
ry fitting may depend on the fact that each absorbance
value does not pertain to a single species, but receives
minor, yet significant contributions from other complexes
The Brønsted acidic tendencies of [FeII(3)2]2+ in CHCl3
being defined, titration experiments with anions were car-
ried out. Figure 11 displays the family of spectra obtained
over the course of the titration of a 9.74”10ꢀ5 m solution of
[FeII(3)2]2+ in CHCl3 with
a m of
solution 8.03”10ꢀ3
[Bu4N]CH3COO in CHCl3 (LH=3).
Quite surprisingly, on acetate addition, the same spectral
pattern was obtained, as observed on titration with the
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Chem. Eur. J. 2008, 14, 9683 – 9696

Anion Binding
FULL PAPER
Figure 12. Spectra taken over the course of the titration of a 9.74”10ꢀ5
solution of [FeII(3)2]2+ in CHCl3 with solution 1.01”10ꢀ2
of
[Bu4N]NO3 in CHCl3 (LH=3). Thick lines refer to spectra taken after
the addition of 0 (c) and 6.5 equiv (a) of nitrate.
m
Figure 11. Spectra obtained over the course of the titration of a 9.74”
10ꢀ5 m solution of [FeII(3)2]2+ in CHCl3 with a solution 8.03”10ꢀ3 m of
Bu4N]CH3COO in CHCl3 (LH=3). Thick lines refer to spectra taken
after the addition of 0 (c), 1 (a), 2equiv ( b) of acetate.
a
m
Proton Sponge. This indicates that the following stepwise
goes a less pronounced red shift and decreases in intensity;
moreover, a continuous shift is observed, which seems to ex-
clude the formation of two species with a different degree
of deprotonation. On these bases, it is suggested that the
acid–base equilibria take place (Xꢀ =CH3COOꢀ):
2þ
þ
½FeIIðLHÞ2Š þ Xꢀ Ð ½FeIIðLHÞðLފ þ HX
ð5Þ
ð6Þ
ꢀ
NO3 establishes hydrogen-bonding interactions with the
þ
½FeIIðLHÞðLފ þ Xꢀ Ð ½FeIIðLÞ2Š þ HX
thiourea subunit of the metal coordinated thiosemicarba-
ꢀ
zone, as observed in the solid state with the CF3SO3 anion.
Thus, the investigated iron(II) complex appears as an ex-
tremely strong acid in CHCl3, which affords definitive
proton transfer from thiourea N-H fragments of the two co-
ordinated thiosemicarbazones to the base CH3COOꢀ. Strong
acidity reflects the stabilization experienced by the metal
complex on N-H deprotonation, due to the electron-pair de-
localization towards the donor atoms and to the establishing
of especially intense metal–ligand interactions. Titration ex-
In particular, the interaction should take place at the two
thiourea moieties of the [FeII(LH)2]2+ complex, according to
the stepwise Equilibria (7) and (8):
2þ
ꢀ
þ
½FeIIðLHÞ2Š þ NO3 Ð ½FeIIðLHÞ2 Á Á Á NO3Š
ð7Þ
þ
ꢀ
½FeIIðLHÞ2 Á Á Á NO3Š þ NO3 Ð ½FeIIðLHÞ2 Á Á Á ðNO3Þ2Š
ꢀ
ꢀ
periments with H2PO4 and HSO4 induced precipitation.
ð8Þ
ꢀ
On titration with NO2 , the same spectral pattern obtained
with PS and CH3COOꢀ was observed (see Figure S5, Sup-
porting Information), indicating the occurrence of acid–base
Equilibria of type (5) and (6). A similar behaviour was ob-
tained on titration with fluoride (Figure S6). Quite interest-
ingly, each deprotonation step proceeded in presence of
only 1 equiv of Fꢀ, thus obeying to Equations (5) and (6):
the diprotic acid [FeII(3)2]2+ is so strong that its stepwise de-
protonation needs not to be associated to the formation of
On non-linear least-squares treatment of spectrophotomet-
ric titration data over the 200–800 nm spectral range, the fol-
lowing stepwise constants were obtained, associated to the
binding of the first [logK1 =4.78Æ0.06, Eq. (7)] and of the
ꢀ
second NO3 ion [logK2 =4.3Æ0.1), Eq. (8)].
Figure 13 displays the concentration profiles of the species
present at the equilibrium along the titration experiment.
The absorbance at 615 nm (pertinent to the lower energy
transition of the MLCT band of the limiting spectrum,
dashed line in Figure 13) is also reported (open triangles).
The latter profile does not show any discontinuity, but
smoothly proceeds towards saturation. The above evidences
indicate that, following the hydrogen-bonding interaction of
ꢀ
HF2 .
ꢀ
Among the investigated anions, NO3 exhibits a peculiar
behaviour. Figure 12shows the spectra taken over the
course of the titration of
a m solution of
9.74”10ꢀ5
[FeII(3)2]2+ in CHCl3 with a solution 1.01”10ꢀ3 m of [Bu4N]-
ꢀ
NO3 in CHCl3. The spectral pattern is remarkably different
the NO3 ion at the N-H fragments of the thiourea subunit,
from that observed with CH3COOꢀ and involving stepwise
double deprotonation: i) LMCT region: the band centred at
335 nm on nitrate addition undergoes a less pronounced red
shift (from 335 to 350 nm, to be compared to 335-to-375 nm,
partial negative charge is transferred on the sulphur atom,
which accounts for the red shift of the LMCT band. Such an
effect is remarkably less pronounced than observed on de-
protonation of the N-H fragment, which justifies the moder-
ate magnitude of the red shift. Consequent transfer of nega-
tive charge on the FeII centre induces a red shift also of the
MLCT band. Such a transfer of electron charge on the
ꢀ
observed with CH3COOꢀ, NO2 and PS); moreover, on
ꢀ
NO3 addition, the intensity of the band decreases; ii)
MLCT region: also the two-transition MLCT band under-
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9691

L. Fabbrizzi et al.
metal takes place progressively, following the interaction of
ꢀ
the first and the second NO3 ion with thiourea moieties. As
a consequence, the red shift of the MLCT band is gradual
and smooth, as shown in Figure 13, and does not show the
discontinuous behaviour observed with deprotonation. No-
ꢀ
ticeably, the association of the two NO3 ions proceeds inde-
pendently, without any mutual electrostatic or electronic in-
fluence. In particular, the difference between logK1 and
logK2 (0.5Æ0.1)) has to be ascribed to the sole statistical
effect (ꢁlog 4=0.602).
Figure 14. a) spectra recorded over the course of the titration of a 1.08”
10ꢀ4 m solution of [NIII(LH)2]2+ in CHCl3 with a solution 1.03”10ꢀ2 m of
12 in CHCl3 (LH=3); b): titration profile at 385 nm.
The nature of the spectral modifications observed during
the titration has to be explained. In fact, the [NiII(LH)2]2+
complex shows a single band (thick solid line in Figure 14a),
whereas, on stepwise deprotonation, two distinctly separated
components are observed (limiting spectrum as thick dashed
line in Figure 14a). We suggest that the rather broad single
band of the [NiII(LH)2]2+ complex is the envelope of two
close ligand-to-metal transitions: one from the pyridine ni-
trogen atom, the other from the thiourea sulphur atom. On
deprotonation of the N-H fragment, charge is transferred
onto the pyridine nitrogen atom and onto the sulphur atom,
according to the p-delocalization mechanisms outlined in
Scheme 6. However, such a charge transfer is not equally
distributed over the two donor atoms. In particular, one
donor atom should receive a substantially larger amount of
negative charge, being thus responsible for the LMCT tran-
sition at higher wavelength. It is suggested that such a donor
atom is the sulphur one and that the S-to-FeII LMCT transi-
tion is that undergoing the most pronounced red-shift. Such
a hypothesis is based on structural considerations. Unfortu-
nately, we were not able to grow crystals of the [NiII(L)2]
and [FeII(L)2] complex salts suitable for X-ray diffraction
studies and no structural data are available in the literature
for complexes of NiII and FeII with analogous deprotonated
thiosemicarbazones. However, the crystal and molecular
structure has been reported for the 1:1 CuII complex of the
deprotonated form of the derivative 2-Benzoylpyridine N4-
phenylthiosemicarbazone: [CuII(L)NCS]+.[23] In this com-
plex, the CuII–S distance (2.23 Š) is distinctly shorter than
that observed in CuII complexes with a coordinated thiourea
Figure 13. Lines (left vertical axis): % concentration of the three complex
species: [FeII(LH)2]2+, [FeII(LH)2···NO3]+ and [FeII(LH)2···
ACHTRE(UNG NO3)2], which
form according to Equilibria (7) and (8), over the course of the spectro-
photometric titration illustrated in Figure 12. Symbol (e, right vertical
axis): titration profile based on the band developing at 615 nm (lower
energy of the MLCT transition pertinent to the two H-bond complexes
[FeII(LH)2···NO3]+ and [FeII(LH)2···
ACHTRE(UNG NO3)2]).
The [NiII(LH)2]2+ metal complex as an anion receptor: The
solution behaviour of the [NiII(LH)2]2+ complex is similar to
that observed with the [FeII(LH)2]2+ analogue, with respect
to both deprotonation of N-H fragments and formation of
genuine hydrogen-bonding complexes.
Figure 14a shows the family of spectra recorded over the
course of the titration of
a m in
solution 1.08”10ꢀ4
[NiII(3)2]2+ in CHCl3 with a solution of base 12 (1.03”
10ꢀ2 m). The spectrum of the [NiII(3)2]2+ complex, prior of
base addition (thick solid line in Figure 14a), shows a rather
intense band centred at 340 nm, of LMCT nature. On addi-
tion of the Proton Sponge, the LMCT band undergoes a
pronounced red shift and discloses two well distinct transi-
tions, one centred at 385 nm, the other at about 425 nm
(shoulder). The titration profile, based on the band at
385 nm and displayed in Figure 14b, increases steeply and
reaches a plateau on addition of 2equiv of base. A flex is
observed at 1 equiv addition. Such a spectral pattern can be
accounted for on assuming the occurrence of the two step-
wise deprotonation Equilibria (9) and (10), in analogy with
what observed for the corresponding FeII complex:
(e.g., [CuII (thiourea)]2+ = 2.37 Š), indicating a stron-
ACTHERNGU(bpy)2ACHTREUGN
ger coordinative interaction. Moreover, the C–S distance
(1.75 Š) is slightly, still detectably longer than that observed
in CuII-coordinated thiourea (1.72Š), which indicates a de-
crease of the bond order of C=S, due to the p-delocalization
pathway outlined in Scheme 6. On the other hand, no signif-
2þ
þ
½NiIIðLHÞ2Š þ PS Ð ½NiIIðLHÞðLފ þ PSHþ
ð9Þ
½NiIIðLHÞðLފ þ PS Ð ½NiIIðLފ þ PSHþ
þ
ð10Þ
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Chem. Eur. J. 2008, 14, 9683 – 9696

Anion Binding
FULL PAPER
icant difference has been observed for the CuII–N
ACHTRE(UGN pyridine)
distances in the [CuII(L)Cl]+ complex (2.01 Š) and for the
model compound [CuII (H2O)2]2+ (2.00 Š).[24] All
ACHTERNGU(pyridine)2ACHTREUGN
evidence points towards a partial thiolate nature of the coor-
dinated sulphur atom in deprotonated thiosemicarbazones
and accounts for the separation of the two LMCT transi-
tions, as observed in the [NiII(LH)(L)]+ and [NiII(L)2] com-
plexes.
No MLCT bands are expected for a d8 octahedral com-
plex. This explains the less vivid colour change observed on
base addition to a CHCl3 solution of the [NiII(LH)2]2+, as
compared with the [FeII(LH)2]2+ analogue. In particular, ad-
dition of the Proton Sponge and stepwise deprotonation
caused a continuous colour variation from yellow to brown,
as illustrated in Figure S7 in the Supporting Information.
Figure 16. Spectra taken over the course of the titration of a 1.22 ”10ꢀ4
solution of [NiII(LH)2]2+ in CHCl3 with solution 2.51”10ꢀ2
of
[Bu4N]NO3 in CHCl3 (LH=3). Thick lines refer to spectra taken after
the addition of 0 (c) and 10 equiv (a) of nitrate.
m
a
m
On titration with CH3COOꢀ, NO2 and Fꢀ, the same spec-
ꢀ
tral pattern was obtained, which indicated the occurrence of
the stepwise acid–base Equilibria (11) and (12):
2þ
ꢀ
þ
½NiIIðLHÞ2Š þ NO3 Ð ½NiIIðLHÞ2 Á Á Á NO3Š
ð13Þ
þ
ꢀ
½NiIIðLHÞ2 Á Á Á NO3Š þ NO3 Ð ½NiIIðLHÞ2 Á Á Á ðNO3Þ2Š
2þ
þ
½NiIIðLHÞ2Š þ Xꢀ Ð ½NiIIðLHÞðLފ þ HX
ð11Þ
ð12Þ
ð14Þ
½NiIIðLHÞðLފ þ Xꢀ Ð ½NiIIðLފ þ HX
þ
Figure 17 shows the concentration profiles of the species
present at the equilibrium along the titration experiment il-
lustrated in Figure 16. The absorbance at 375 nm (triangles
in Figure 17) shows a continuous profile, which reaches satu-
ration on addition of a slight excess of nitrate. In particular,
absorbance values seem to result from the weighted contri-
butions of the absorbance of the hydrogen bonding com-
plexes at the equilibrium, [NiII(LH)2···NO3]+ and
As an example, Figure 15a shows the family of spectra taken
over the course of the titration of a CHCl3 solution 1.35”
10ꢀ4 m in [NiII(3)2]2+ in CHCl3 with a solution of base
[Bu4N]F·3H2O (1.58”10ꢀ2 m).
[NiII(LH)2··
ACHTRE(UNG NO3)2]. The moderate transfer of negative
charge on the donor atoms following the H-bond interaction
accounts for the moderate red shift of the LMCT band.
Figure 15. a) spectra obtained over the course of the titration of a 1.35”
10ꢀ4 m solution of [NiII(LH)2]2+ in CHCl3 with a solution 1.58”10ꢀ2 m of
[Bu4N]F·3H2O in CHCl3 (LH=3); b): titration profile at 380 nm.
Figure 17. Lines (left vertical axis):% concentration of the three complex
species [NiII(LH)2]2+ [NiII(LH)2···NO3]+ [NiII(LH)2···
ACHTRE(UNG NO3)2], which form
ꢀ
Again, only in the case of the NO3 anion, the formation
according to Equilibria (13) and (14), over the course of the spectropho-
tometric titration experiment illustrated in Figure 16. Symbol (e right ver-
tical axis): titration profile based on the developing LMCT band.
of authentic hydrogen bonding complexes was observed. In
particular, only a moderate shift of the LMCT band was ob-
served (see Figure 16).
Best fitting of titration data was obtained on assuming the
occurrence of the stepwise Equilibria (13) and (14), to
which the following association constants correspond:
logK1 =5.17Æ0.05, logK2 =4.3Æ0.1. The difference between
logK1 and logK2 slightly exceeds the statistical contribution.
On titration with [Bu4N]H2PO4 and with [Bu4N]HSO4,
precipitation was observed. Even large excess addition of
[Bu4N]ClO4 and [Bu4N]PF6 did not induce any significant
spectral modification.
Chem. Eur. J. 2008, 14, 9683 – 9696
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9693

L. Fabbrizzi et al.
chromophore and the optical pathway were adjusted to obtain spectra
with AU ꢂ 1. In the titrations with anions, the UV/Vis spectra of the
samples were recorded after the addition of aliquots of the tetraalkylam-
monium salt solution of the envisaged anion. All spectrophotometric ti-
tration curves were fitted with the HYPERQUAD program.[14] 1H NMR
spectra were obtained on a Bruker Avance 400 spectrometer (400 MHz)
operating at 9.37 T. Mass spectra were acquired by using a Thermo-Finni-
gan ion-trap LCQ Advantage MAX instrument equipped with an ESI
source.
Conclusion
The intensity of hydrogen bond is related to the acidity of
the donor (in anion coordination chemistry: the N-H con-
taining receptor) and to the basicity of the acceptor (the
anion): the higher receptorꢁs acidity and anionꢁs basicity, the
stronger the hydrogen-bonding interaction and the higher
the association constant. For these reasons, on one side, re-
ceptors are often equipped with electron-withdrawing sub-
Synthesis of ligand 3 (2-formylpyridine 4-thiosemicarbazone): 4-Phenyl-
thiosemicarbazide (1.67 g, 9.98 mmol) was dissolved in methanol (50 mL)
under vigorous stirring. As the solid was completely dissolved, a metha-
nol solution of pyridine-2-carboxaldehyde (956 mL, 9.99 mmol in 30 mL)
was added dropwise. A white precipitate formed after 2h and the reac-
tion mixture was left under stirring at room temperature for 24 h.
ꢀ
stituents (i.e., the NO2 subunit) in order to improve their
anion binding tendencies and, on the other side, in the ab-
sence of designed geometrical constraints, binding tenden-
cies parallel the basic properties of the anion: CH3COOꢀ
>
ꢀ
ꢀ
ꢀ
ꢀ
H2PO4 > HSO4 ꢁNO2 > NO3 . In the presence of
either a strong acid or a strong base, or both, the partial and
frozen proton transfer proceeds to a complete extent, with
receptorꢁs deprotonation and formation of the conjugate
acid of the anion. This work has demonstrated that coordi-
native interaction of a 3d metal centre with the sulphur
atom of the thiourea moiety of a thiosemicarbazone deriva-
tive induces an extremely high polarization of the N-H frag-
ment, which undergoes deprotonation in presence of most
anions, even if not especially basic. Such an effect had been
observed, to a moderate extent, on coordination of the thio-
urea sulphur atom to an AgI centre in a scorpionate com-
plex.[25]
The mixture was filtered under vacuum and the solid was washed with
several portions of cold methanol. The solid was dried under vacuum to
yield 3 (2.16 g, 8.43 mmol, 90%) as a white, fibrous product. 1H NMR
(400 MHz, [D6]DMSO): d=7.20 (t, 3J
(H,H)=7.9 Hz, 2H +1H), 7.53 (d, 3J
(H,H)=7.4 Hz, 1H), 8.18 (s, 1H, imine), 8.40 (d, 3J
ACHTREUNG
ACHTREUNG
A
ACHTREUNG
A
3
8.56 (d, JCAHTRE(UGN H,H)=5.0 Hz, 1H), 10.35 (brs, 1H; thioureic N-H), 12.08 ppm
(brs, 1H; hydrazone N-H); ESI-MS: m/z (+): 257.2 [M+H]+ (100),
512.9 [2M+H]+ (10); m/z (ꢀ): 255.3 [MꢀH]ꢀ (42), 291.2 [M+Cl]ꢀ (24),
301.1 [M+HCOO]ꢀ (100).
Finally, it has to be noted that, upon irradiation at l >
300 nm, the thione form of thiourea (a in Scheme 7) can be
converted to the tautomeric thiol form b, through a proton
tunnelling mechanism, characterised by a high energy barri-
er (108 kJmolꢀ1).[26] The thioneQthiol tautomeric conversion
could take place, in principle, through the classical deproto-
nation–protonation pathway illustrated in Scheme 7, a pro-
cess which is prevented by the low Brønsted acidity of the
thione form. This work has demonstrated that such an acidi-
ty can be highly enhanced through the sulphur coordination
to a 3d divalent metal centre.
Synthesis of [FeII(3)2]
ACHTRE(UGN CF3SO3)2·H2O: FeAHCTRE(UGN CF3SO3)2 (55.6 mg, 0.16 mmol)
was added to a suspension of 3 (80.4 mg, 0.31 mmol) in MeCN (50 mL)
in an N2 atmosphere and under vigorous stirring. The reaction mixture,
greenish in colour, was refluxed under a positive pressure of argon for
30 min, and allowed to cool to room temperature. The purple, limpid so-
lution was concentrated in vacuo to 15 mL. Diethyl ether (100 mL) was
added and the resulting solution was left to stand overnight under inert
atmosphere. Precipitation occurred overnight to give a dark purple crys-
talline solid, which was filtered in vacuo, rapidly washed with small por-
tions of diethyl ether, then dried in vacuo (112.3 mg, 0.13 mmol, 84%).
Single crystals suitable for X-ray structural determination were chosen
from this product. 1H NMR (400 MHz, CD3CN): d = 7.28 (t, 3J
7.2Hz, 2H), 7.39 (m, 3J(H,H)=7.8 Hz, 4H+2H), 7.46 (d, 3J
7.8 Hz, 4H), 7.80 (t, 3J(H,H)=7.2Hz, 2H), 7.93 (d, 3J
(H,H)=6.0 Hz,
2H), 8.05 (d, 3J
(H,H)=7.2Hz, 2H), 9.50 (s, 2H, imine), 9.8–10.0 (brs,
2H, thioureic N-H); 12.9–13.5 ppm (brs, 2H, hydrazone N-H).
Synthesis of [NiII(3)2]
(CF3SO3)2·MeCN: Ni(CF3SO3)2 (274.3 mg,
ACHTREUNG
A
ACHTREUNG
A
ACHTREUNG
AHCTREUNG
A
ACHTREUNG
0.77 mmol) was added to a suspension of 3 (398.8 mg, 1.56 mmol) in
MeCN (50 mL) in an N2 atmosphere and under vigorous stirring. The
pale yellow mixture was refluxed until complete dissolution (15 min), and
allowed to cool down to room temperature. The dark-yellow solution was
concentrated under vacuum to a final volume of 10 mL. Diethyl ether
(60 mL) was then added and the resulting solution was left overnight at
room temperature. A microcrystalline, brownish-yellow solid was ob-
tained. The product was filtered under vacuum and washed with small
portions of diethyl ether, then desiccated in vacuo until constant weight
(485.1 mg, 0.56 mmol, 73%). Single crystals suitable for X-ray diffraction
analysis were obtained by slow vapour diffusion of diethyl ether on a
MeCN solution of the complex salt.
Scheme 7. Thione-to-thiol tautomeric conversion of urea derivatives.
Experimental Section
Syntheses of model compounds 4 and 5
General procedures and materials: All reagents for syntheses were pur-
chased from Aldrich/Fluka and used without further purification. All re-
actions were performed under N2. UV/Vis spectra were recorded on a
Varian CARY 100 spectrophotometer, with quartz cuvettes of the appro-
priate path length (1 or 0.1 cm). In any case, the concentration of the
9694
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ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2008, 14, 9683 – 9696

Anion Binding
FULL PAPER
1-(4-Nitrophenyl)-3-(3-methylphenyl)thiourea (4): m-Toluidine (1400 mL,
12.9 mmol) was dissolved in anhydrous dichloromethane (70 mL). 4-Ni-
trophenylisothiocyanate (2.33 g, 12.9 mmol) was then added. The solution
was refluxed for 2h. The clear solution was allowed to cool to room tem-
perature, to give a supersaturated solution which gave abundant precipi-
tate after sonication. The bright yellow precipitate was filtered off by suc-
tion and the solid was washed to the analytical grade with several, small
portions of cold diethyl ether, then dried under vacuum, to give a pale
yellow powder (2.76 g, 9.6 mmol, 74%). 1H NMR (400 MHz, CDCl3):
Table 3. Crystal data for both iron(II) and nickel(II) complexes.
[FeII(3)2] [NiII(3)2]
(CF3SO3)2·H2O (CF3SO3)2·MeCN
A
G
G
ACHTREUNG
formula
M
C28H26F6FeN8O7S4
884.70
C30H27F6N9NiO6S4
910.58
colour
dimensions [mm]
crystal system
space group
a [Š]
b [Š]
c [Š]
a [8]
b [8]
dark red
pale red
0.38”0.30”0.22
triclinic
0.80”0.60”0.20
triclinic
¯
¯
P1 (no. 2)
P1 (no. 2)
9.100(2)
11.393(3)
18.563(4)
80.02(2)
83.40(2)
75.18(2)
1827.3(8)
2
1.608
0.728
w scans
1.1–25.1
6934
9.345 (4)
11.624(5)
18.692(7)
77.67(1)
86.40(1)
75.82(1)
1923.1(14)
2
1.572
0.805
w scans
1.1–25.2
24062
d = 2.37 (s, 3H, benzylic), 7.14 (s, 1H), 7.15 (d, 3J
A
3
7.21 (d, J
U
(H,H)=8.0 Hz, 1H), 7.75 (d, 3J-
ACHTREUNG
A
MeOH]2ꢀ (45), 286.3 [MꢀH]ꢀ (58), 322.0 [M+Cl]ꢀ (18), 331.8
[M+HCOO]ꢀ (60), 400.0 [M+CF3COO]ꢀ (74), 572.9 [2M-H]ꢀ (100),
609.0 [2M+Cl]ꢀ (42), 636.1 [2M+NO3]ꢀ (15), 686.8 [2M+CF3COO]ꢀ
(34).
g [8]
V [Š3]
Z
1calcd [gcmꢀ3
]
m MoKa [mmꢀ1
scan type
]
1-(3-Methylphenyl)-3-phenylthiourea
(5):
m-Toluidine
(764 mL,
5.64 mmol) was dissolved in anhydrous dichloromethane (30 mL) under
vigorous stirring. Phenylisothiocyanate (606 mL, 5.65 mmol) was rapidly
added to the solution. The resulting solution was refluxed for 2h. Then,
the solution was allowed to cool to room temperature and left to stand
for further 12h. The solvent was removed with a rotary evaporator to
give a pale yellow oil. After trituration with diisopropyl ether, a precipi-
tate was obtained and filtered by suction, followed by washings with
small portions of cold diisopropyl ether. The solid was then dried in
q range [8]
measured reflns
unique reflns
6488
6890
Rint
strong data
5
0.0320.02
3423
5399
[I0 >2s(IO)]
refined parameters 495
518
R1, wR2(strong
0.0702, 0.1712
0.0444, 0.1195
vacuum to afford an ivory-coloured powder (1.190 g, 4.91 mmol, 87%).
data)
3
1H NMR (400 MHz, CDCl3): d=2.34 (s, 3H, benzylic), 7.12 (d, J
A
R1, wR2(all data) 0.1505, 0.2148
0.0600, 0.1317
1.039
8.1 Hz, 1H), 7.19 (s, 1H), 7.21 (d, 3J(H,H)=8.1 Hz, 1H), 7.32(t, 3J-
ACHTREUNG
GOF
1.016
(H,H)=7.7 Hz, 1H), 7.39–7.44 (m, 5H), 7.77 ppm (brs, 2H, thioureic N-
max/min residuals
1.16/ꢀ0.59
0.51/ꢀ0.37
ꢀ3
[eŠ
]
X-ray crystallographic study: Diffraction data for [FeII(3)2]-
ACHTREUNG(CF3SO3)2·H2O crystal have been collected at ambient temperature by
means of an Enraf-Nonius CAD4 four circle diffractometer, working
with graphite-monochromatized MoKa X-radiation (l=0.7107 Š). Data
reductions (including intensity integration, background, Lorentz and po-
larization corrections) were performed with the WinGX package.[27] Ab-
sorption effects were evaluated with the psi-scan method[28] and absorp-
tion correction was applied to the data (min/max transmission factors
were 0.748/0.851).
Acknowledgements
The financial support of the Italian Ministry of University and Research
(PRIN-Dispositivi Supramolecolari; FIRB-Project RBNE019H9K) is
gratefully acknowledged.
Diffraction data for [NiII(3)2]
ACHTREU(GN CF3SO3)2·MeCN crystal have been collect-
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Received: April 27, 2008
Published online: September 12, 2008
9696
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ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2008, 14, 9683 – 9696