Polyamine receptors containing dipyridine or phenanthroline units: Clues for the design of fluorescent chemosensors for metal ions

Article abstract of DOI:10.1002/chem.200900283

The synthesis of the macrocyclic receptor L1, which contains a tetraamine chain linking the 6,6'-positions of a 2,2'-dipyridine moiety, is reported. Its basicity properties and complexation features toward Cu^II, Zn ^II, Cd^II

Full text of DOI:10.1002/chem.200900283

FULL PAPER
DOI: 10.1002/chem.200900283
Polyamine Receptors Containing Dipyridine or Phenanthroline Units:
Clues for the Design of Fluorescent Chemosensors for Metal Ions
Carla Bazzicalupi, Andrea Bencini,* Silvia Biagini, Antonio Bianchi,* Enrico Faggi,
Claudia Giorgi, Melania Marchetta, Federico Totti, and Barbara Valtancoli[a]
Dedicated to the Centenary of the Italian Chemical Society
Abstract: The synthesis of the macro-
cyclic receptor L1, which contains a
tetraamine chain linking the 6,6’-posi-
tions of a 2,2’-dipyridine moiety, is re-
ported. Its basicity properties and com-
plexation features toward CuII, ZnII,
CdII and PbII have been studied in
aqueous solutions by means of poten-
tiometric, UV/Vis spectroscopy and
fluorescence emission measurements
and compared with those of ligand L2,
in which a 1,10-phenanthroline moiety
replaces the dipyridine unit of L1. In
metal coordination, L1 shows a marked
selectivity toward CdII over ZnII and
PbII. The crystal structures of its metal
complexes shows that L1 possesses a
preferential tetradentate binding site
for metal cations, composed of the di-
pyridine unit and the two adjacent ben-
zylic amine groups. This binding site
has the proper dimension and confor-
mation to selectively coordinate the
CdII ion, as confirmed by DFT calcula-
tions carried out on the complexes.
This coordinative zone is lost in L2.
The rigidity of phenanthroline does not
allow the simultaneous binding of both
the benzylic amine groups to ZnII and
CdII and, in fact, one benzylic amine is
not coordinated to these metal cations.
The fluorescence emission properties
of the L1 and L2 complexes are strong-
ly pH dependent. Only the ZnII and
CdII complexes with L1 display fluores-
cence emission at neutral pH. This fea-
ture is related to the formation in solu-
tion at pH 7 of emissive protonated
complexes of the type [MACHTNUTGRNEUNG
(HxL)](2+x)+
(x=1–3), in which all the nitrogen
donors are involved in metal or proton
binding. The emissive characteristics of
these protonated complexes are con-
firmed by the fluorescence emission
spectra collected on the [Zn
ACHTUNGTRENN(UNG HL1)Br]-
A
R
ACHTUNGTRENNUNG
compounds dissolved in CH3CN. Con-
versely, the ZnII and CdII complexes
with L2 are not emissive; in fact, they
contain a benzylic amine group not in-
volved in metal or proton binding that
can quench the fluorescence emission
of the fluorophore, thanks to a photo-
induced electron-transfer process.
Keywords: chelates · fluorescence ·
host–guest systems · macrocycles ·
sensors
their potential use in medicinal or analytical chemistry.[1] In
particular, metal cations are one of the most targeted sub-
strates due to their biological and environmental rele-
vance.[1–21] The most common approach to the realisation of
synthetic chemosensors is the design of molecules consisting
of a binding and a signalling unit separated by a spacer
(conjugate chemosensors) and a number of chemosensors of
this type for different metal ions have been recently
Introduction
There is current interest in the development of luminescent
chemosensors for analytes in aqueous solution because of
[a] Dr. C. Bazzicalupi, Prof. A. Bencini, Dr. S. Biagini, Prof. A. Bianchi,
Dr. E. Faggi, Dr. C. Giorgi, Dr. M. Marchetta, Dr. F. Totti,
Prof. B. Valtancoli
AHCTUNGTREGaNNNU chieved. Among possible binding units, polyamine macro-
Department of Chemistry
cycles are undoubtedly versatile receptors for metal cat-
ions.[22–26] In fact, depending on their structural features, they
can form stable metal chelates in aqueous solution and/or
act as selective complexing agents for metal cations.[11,12]
From this point of view, polyamines are known for their
ability to form stable complexes with metal cations of envi-
ronmental or toxicological relevance, such as CdII or PbII. Fi-
University of Florence
Via della Lastruccia 3
50019 Sesto Fiorentino, Firenze (Italy)
Fax : (+39)0554573364
E-mail: andrea.bencini@unifi.it
antonio.bianchi@unifi.it
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200900283.
Chem. Eur. J. 2009, 15, 8049 – 8063
ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
8049

nally, the insertion of hydrophilic polyamine units within a
structure containing a fluorogenic unit, most often a hydro-
phobic moiety, can ensure the resulting receptor of good sol-
ubility in aqueous media, a relevant condition for the ach-
ievement of molecular chemosensors for analytes of biologi-
cal or environmental interest. Therefore, polyamines repre-
sent, in principle, optimal candidates for the synthetic as-
sembly of chemosensors for metal cations. On the other
hand, polyamines are also double-faced ligands: if they give
stable complexes with many toxic metal cations, they often
present scarce selectivity. For instance, polyamines generally
form complexes with ZnII, CdII and PbII with a similar stabil-
ity in aqueous solutions.[22,23] Furthermore, the ability of a
chemosensor to bind and signal a selected metal cation de-
pends on the characteristics of the medium in which the che-
mosensor is used, such as temperature and pH. In particular,
the sensing ability of polyamine-based receptors can be
strongly pH dependent.[8–10,13,15,17] In fact, polyamines can
easily protonate in aqueous solution. Protonation of poly-
amine groups competes with the process of metal complexa-
tion and often leads to the formation in solution of proton-
ated metal complexes, in which acidic protons and metal
cations in solution are simultaneously bound to the recep-
tors. Even if a number of polyamine-based fluorescent che-
mosensors have been recently synthesised,[1] the possible for-
mation in solution of protonated complexes and the effect
of protonation on the ability of the receptors in metal sens-
ing have been scarcely investigated.[8–10]
ZnII, CdII and PbII triad. In this paper we extend this study
to similar phenanthroline- or dipyridine-based polyazama-
crocycles containing amine groups separated by propylenic
chains (L1 and L2 in Scheme 1). At a first glance, this struc-
tural modification may appear to be of marginal relevance
to determine the binding and sensing ability of the recep-
tors. On the other hand, it is known that the replacement of
ethylenic chains with propylenic ones may affect the selec-
tivity pattern of polyamine ligands toward transition and
post-transition metals, due to the higher flexibility of the li-
gands as well as to the larger coordination bite displayed by
amine donors separated by the longer propylenic chains.[28]
Conversely, amine groups linked by propylenic chains are
characterised by a stronger basicity in aqueous solution with
respect to amines joined by ethylenic links.[29] Finally, it is
known that dipyridine and phenanthroline display a similar
binding feature toward metal cations,[30] although the possi-
ble rotation of the two rings of dipyridine around the 2,2’-
bond makes dipyridine slightly less rigid than phenanthro-
line. In principle, this latter structural feature could also in-
fluence the coordination features and emission properties of
cyclic ligands containing these units, such as L1 and L2. The
aim of the present paper is to demonstrate that apparently
slight structural differences, such as the presence of propy-
lenic chains instead of ethylenic ones or the replacement of
a phenanthroline unit with a dipyridine moiety, can strongly
influence the selectivity and sensing properties of molecular
chemosensors, focusing our attention on the coordination of
ZnII, CdII and PbII, which often form complexes with similar
stability with polyamine receptors, and of CuII, as represen-
tative examples of transition-metal cations.
We have recently reported on the metal binding and sens-
ing ability of a series of macrocyclic ligands consisting of a
phenanthroline or a dipyridine unit and a polyamine chain
containing amine groups separated by ethylenic chains, such
as 1–6 in Scheme 1.[9,27] It was found that these receptors
generally show scarce thermodynamic selectivity among the
Results and Discussion
Protonation of the receptors: Since protonation of poly-
amine receptors is competitive with the process of metal
complexation, we preliminarily carried out a study of the ba-
sicity properties of L1 and L2 by coupling potentiometric,
spectrophotometric and fluorimetric measurements in aque-
ous solutions. The protonation constants of L2 have been
previously determined in the course of a study of anion
binding by L2 and its ZnII complex,[31] whereas the protona-
tion equilibria of L1 have been potentiometrically deter-
mined in this study. The basicity constants of both ligands
are reported in Table 1.
Table 1. Protonation constants of L1 and L2 (298 K, 0.1m NMe4NO3).
Equilibrium
LogK
L1
L2[a]
L+H+ =[HL]+
9.94(1)[b]
8.96(2)
7.03(2)
5.95(2)
10.39
9.01
7.35
6.02
[HL]+ +H+ =[H2L]2+
AHCTUNGTRENNUNG
[H2L]2+ +H+ =[H3L]3+
AHCTUNGTRENNUNG
[H3L]3+ +H+ =[H4L]4+
Scheme 1. Drawing of the receptors with the atom numbering used in the
NMR spectroscopy experiments.
[a] From ref. [31]. [b] Values in parentheses are standard deviations on
the last significant figure.
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Fluorescent Chemosensors
FULL PAPER
Receptors L1 and L2 display similar protonation features.
As shown in Table 1, both ligands can bind up to four acidic
protons in the pH range investigated (2.5–10.5) with similar
protonation constants. Of note, the protonation constants
are higher than those observed for the corresponding proto-
nation equilibria in phenanthroline-containing macrocycle
2,[27g] in which the amine groups are separated by ethylenic
chains; this is in agreement with the larger +I inductive
effect exerted on amine groups by propylenic chains and
with the higher flexibility of L1 and L2, which allow a better
minimisation of the electrostatic repulsion between positive
charges in the polyprotonated forms of the receptors.[29] All
of the protonation constants of L1 and L2 are also higher
than those reported for 2,2’-dipyridine or 1,10-phenathroline
(logK=4.42 and 4.96, respectively),[30] which suggests that
the heteroaromatic nitrogen atoms are not directly involved
in the process of proton binding in aqueous solutions, at
least in the pH range investigated. However, slow evapora-
tion of a 0.1m HCl aqueous solution containing NaClO4
leads to crystallisation of the [(H4.5L2)
CATHGNURTNEN(UNG H2O)0.5Cl0.5]-
ACHTUNGTRENNUNG
pentaprotonated forms (see the Supporting Information for
its crystal structure, Figure S1). This points out that the phe-
nanthroline nitrogen atoms can be involved in proton bind-
ing at least in strongly acidic conditions.
A punctual analysis of the successive protonation steps of
1
L1 and L2 can be carried out by recording H NMR spectra
at different pH values. The 1H NMR spectrum of L1 at
pH 12, in which the free amine predominates in solution, is
featured by a set of nine signals, six for the aliphatic methyl-
ene protons and three for the aromatic protons; this is in
keeping with a time-averaged C2v symmetry, which is pre-
served throughout all the pH range investigated. As shown
in Figure 1, formation of the mono- and diprotonated spe-
cies of the ligand, [HL1]+ and [H2L1]2+, in the pH range
11–8 is accompanied by a marked downfield shift of the sig-
nals of the methylene protons H1, H2 and H3, adjacent to
Na of the amine groups, which indicates that the first two
protonation steps occur on the central nitrogen atoms of the
tetraamine chain. The higher proton affinity of the Na nitro-
gen atoms, in comparison with those of Nb, can be ascribed
to the electron-withdrawing effect of the heteroaromatic
unit on the adjacent Nb amine groups. A similar behaviour
was also found for L2.[31] Therefore, both ligands feature a
preferential binding zone for protons, that is, the central
amine groups of the aliphatic chain, located far from the
heteroaromatic units. The two successive protonation steps
take place on Nb of the benzylic amine groups, as testified
by the downfield shift observed for the resonances of H5
and H6 below pH 8, where the [H3L]3+ and [H4L]4+ species
are formed. The resonances of the heteroaromatic protons
do not show significant shifts in the pH range 12–2, indicat-
ing that protonation of dipyridine does not occur in this pH
range.
Figure 1. Top: pH dependence of the 1H NMR spectroscopic signals of
L1. Bottom: A distribution diagram of its protonated species (298 K,
0.1m NMe4NO3).
equal to those of unprotonated 2,2’-dipyridine and 1,10-phe-
nanthroline even at strongly acidic pH values (pH 2), where-
as it is known that protonation of 2,2’-dipyridine[9d,27a,d] and
1,10-phenanthroline[32] is accompanied by remarkable red-
shifts of their absorption bands at 290 and 270 nm, respec-
tively. Conversely, the emission spectra of L2 are strongly
pH dependent. As shown in Figure 2, the receptor is not
emissive in alkaline aqueous solutions and shows the typical
This fact is confirmed by analysis of the UV spectra of L1
and L2 recorded in aqueous solutions at different pH values.
In fact, their spectra are not pH dependent and are almost
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A. Bencini, A. Bianchi et al.
Table 2. Bond lengths [ꢁ] and angles [8] for the metal coordination envi-
ronments in [Cu
(HL1)
E
N
ACHUTNGTRNENUG(HL1)Br]ACHTUNTGREN[NGUN ClO4]2 (b),
[Cd(HL1)Br][ClO4]2 (c) and [Pb
G
E
N
ACHTUNGTRENNUNG
a
b
c
d
Cu1 (X=O5) Zn1 (X=Br2) Cd1 (X=Br1) Pb1 (X=Br1)
ꢀ
M X
ꢀ
M N1
2.414(6)
1.925(4)
1.955(6)
2.037(4)
2.041(5)
2.366(2)
2.11(1)
2.140(9)
2.145(9)
2.148(9)
2.577(1)
2.341(6)
2.349(5)
2.354(5)
2.350(6)
2.841(1)
2.509(8)
2.518(8)
2.760(8)
2.722(7)
ꢀ
M N2
ꢀ
M N3
ꢀ
M N6
X-M-N6 89.8(2)
X-M-N2 100.0(2)
X-M-N3 91.1(2)
X-M-N1 103.3(2)
N6-M-N2 162.3(2)
N6-M-N3 113.5(2)
N6-M-N1 83.1(2)
N2-M-N3 81.3(2)
N2-M-N1 80.3(2)
N3-M-N1 158.3(2)
107.6(2)
112.3(2)
103.2(3)
113.0(3)
136.3(3)
111.4(3)
75.7(3)
76.4(3)
72.8(4)
139.1(3)
103.3(1)
114.0(1)
99.6(1)
114.8(1)
132.6(2)
131.2(2)
70.5(2)
71.1(2)
67.7(2)
134.3(2)
77.1(2)
85.9(2)
87.8(2)
82.4(2)
128.9(2)
157.4(2)
64.8(2)
65.5(2)
65.4(2)
130.5(2)
The crystal structures of a, b, c, and d are composed of
[Cu(HL1) [Zn [Cd
(ClO4)]2+ (HL1)Br]2+ (HL1)Br]2+ and
[Pb
(H2L1)Br]3+ complexed cations, perchlorate anions and,
A
E
,
U
,
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
in the cases of a and d, water molecules. The a, b and c com-
plexes feature similar metal coordination environments and
overall ligand conformations (Figure 3a,b and c). In all
these cases, the metal is coordinated by the two nitrogen
atoms of dipyridine, N1 and N2, and by the amine donors
N3 and N6, adjacent to the heteroaromatic unit. The re-
maining two amine groups (N4 and N5) are not coordinated,
one of them being protonated. A perchlorate oxygen atom
(O5 in a) or a bromide anion (Br2 in b, Br1 in c) complete
the coordination sphere of the metal cations. The resulting
coordination geometry can be best described as a distorted
square pyramid, in which the four nitrogen atoms define the
basal plane (maximum deviation from the mean plane
0.184(5) ꢁ for N2 in a, 0.034(9) ꢁ for N1 and N2 in b and
0.024(5) for N1 in c) and an exogenous oxygen atom or bro-
mide anions occupy the apical position. The metal ions lie
above the basal plane (0.3817(7) ꢁ in a, 0.670(1) ꢁ in b and
0.6227(7) in c), shifted towards the apical position, whereas
the Cu1–O5, Zn1–Br2 and Cd1–Br1 axes form angles of
7.92(2)8, 5.9(2)8 and 11.4(1)8 with the basal plane. In the
Figure 2. Top: Emission spectra of L2 at selected pH values. Bottom: pH
dependence of its emission intensity at 366 nm (298 K, lexc =280 nm,
0.1m NMe4NO3).
emission of phenanthroline only at acidic pH values, that is,
upon protonation of the [H3L2]3+ species to give the
[H4L2]4+ one. This effect can be attributed to the presence,
in [H3L2]3+, of an unprotonated benzylic amine group (Nb
in Scheme 1). In fact, as already observed in other phenan-
throline-containing polyamine ligands,[9e,f] the lone pair of
the benzylic nitrogen atoms, located close to phenanthroline,
can efficiently quench the fluorescence emission of the fluo-
rophore through an electron-transfer process. Finally, proto-
nation of all amine groups of the aliphatic chain below pH 7
makes the lone pairs no longer available for the photoin-
duced eletctron-transfer (PET) process and gives rise to a
consequent renewal of the fluorescence emission of phenan-
throline.
ꢀ
ꢀ
Differently from L2, L1 displays a weak fluorescent emis-
sion even at acidic pH values, in keeping with the scarcely
emissive nature of unprotonated 2,2’-dipyridine.[27a,d]
case of the a and c complexes, the Cu N and Cd N distan-
ces, ranging respectively from 1.925(4) to 2.041(5) ꢁ and
from 2.341(6) to 2.354(5) ꢁ, compare well with the mean
values obtained from a search on the Cambridge Structural
Database (CSD)[33] for the Cu N (2.029(1) ꢁ) and Cd N
(2.349(1) ꢁ) bond lengths. Conversely, three of four Zn N
bond lengths in b are longer than that generally observed in
ZnII complexes with polyaza ligands (mean value of
2.087(1) ꢁ found by a CSD search).[33]
ꢀ
ꢀ
Metal complexation—structural characteristics of the com-
plexes in the solid state: Crystals suitable for X-ray analysis
ꢀ
of the protonated complexes [Cu
(a), [Zn(HL1)Br][ClO4]2 (b), [Cd
[Pb(H2L1)Br][ClO4]3·H2O (d) were obtained by slow evapo-
A
E
N
A
E
N
ACHTUNGTRENNUNG
A
ACHTUNGTRENNUNG
ration of slightly acidic aqueous solutions containing L1 and
the appropriate metal salt in the presence of an excess of
NaClO4. Selected distances and angles for the metal coordi-
nation environments are listed in Table 2.
As far as the ligand conformation is concerned, in all
three complexes the two pyridine units are not coplanar,
with dihedral angles of 5.8(3)8 in a, 12.7(5)8 in b and 7.3(3)8
in c. The ligand assumes a bent conformation along the N3–
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Fluorescent Chemosensors
FULL PAPER
ised; however, the N4 and N5 nitrogen atoms are still hydro-
gen bonded, as testified by the short interatomic distance
(2.92(1) ꢁ).
In the [PbACTHNUTRGNEUNG
(H2L1)Br]3+ complex, the metal ion is, once
again, pentacoordinated by the heteroaromatic and benzylic
nitrogen atoms and by a bromide anion. Although the ar-
rangement of the donor atoms around the metal can be as-
similated to a square pyramid, with the nitrogen donors de-
fining the basal plane, in this case the coordination geome-
try presents strong distortions from the theoretical poly-
hedron. For instance, the N3-Pb1-N6 angle is 157.4(2)8 in-
stead of the expected value of 908. The PbII ion lies
0.3436(4) ꢁ outside the basal plane; however, differently
from the other L1 complexes, the metal is shifted in the op-
posite direction with respect to the apical position occupied
by the bromide anion. The arrangement of the five donors
around the metal leaves a large zone free from coordinated
atoms, which is likely to be occupied by the lone pair of
Pb2+. Such a “gap” in the coordination geometry of PbII has
been ascribed by several authors to the presence of a stereo-
chemically active lone pair and has been found in other
complexes with polyazamacrocycles containing heteroaro-
II
matic moieties.[27g,34] The Pb
distances span from
ꢀ
N
2.509(8) to 2.760(8) ꢁ. Two of them are around 0.2 ꢁ longer
than those generally observed in polyamine complexes with
PbII (mean value of 2.577(6) ꢁ found by a CSD search).
In the case of L2, crystals suitable for X-ray analysis were
obtained only in the case of the ZnII and PbII complexes.
The crystal structure of the [Zn
G
ACHTUNGRTENUN[NG ClO4] complex
has been previously described.[31] The [Zn
AHCTUNGTRENNUNG
(Figure 4a and Table 3) presents significant differences from
the corresponding L1 complex b. Although in [Zn-
AHCTUNGTRENNUNG
(HL2)Br]2+ the ZnII ion is once again pentacoordinated by
Figure 3. ORTEP drawings of the a) [CuACHTGNUTERN(UGNN HL1)AHCUTNGTRENNGNU , b) [Zn-
(ClO4)]2+
ACHTUNGTRENNUNG ACHTUNGTRENNUNG ACTHNUGTRENNGUN
(HL1)Br]2+, c) [Cd(HL1)Br]2+ and d) [Pb(H2L1)Br]3+ cations.
N6 axis with dihedral angles between the mean planes de-
fined, respectively, by the four aliphatic nitrogen atoms and
by the dipyridine units of 157.8(1)8 in a, 141.8(3)8 in b and
145.3(1)8 in c. In the a and c complexes, the analysis of the
Fourier difference maps allowed us to individuate the acidic
proton, which results in being localised on the N4 amine
groups. In both complexes, N4 is involved in a hydrogen
bond with the adjacent N5 nitrogen atom (N4···N5
2.935(8) ꢁ, H4···N5 2.14(5) ꢁ, N4-H4-N5 146(5)8 in a;
N4···N5 2.818(8) ꢁ, H2···N5 1.98(8) ꢁ, N4-H2-N5 148(7)8 in
c). In the ZnII complex b, the acidic proton was not local-
Figure 4. ORTEP drawings of the a) [Zn
(HL2)Br]2+ cations.
(HL2)Br]2+ and b) [Pb-
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
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A. Bencini, A. Bianchi et al.
Table 3. Bond lengths [ꢁ] and angles [8] for the metal coordination envi-
ronments in [Zn(HL2)Br][Br][ClO4] and [Pb(HL2)Br][PF6]1.5
[NO3]0.5·H2O.
throline and the plane defined by the secondary nitrogen
atoms. A strong intramolecular hydrogen bond (N4···N5
2.77(2) ꢁ) is observed between the two unbound amine
donors, one of them being protonated. In this case, however,
the acidic proton was not localised in the DF map.
G
E
E
G
-
ACHTUNGTRENNUNG
[Zn
N
N
[PbACHNUTRTGEG(NNUN HL2)Br]ACHTUNRTEGG[NNUN PF6]1.5CATHNGUTRNE[NUNG NO3]0.5·H2O
ꢀ
ꢀ
Zn1 Br1
Pb1 Br1
2.735(1)
2.540(9)
2.51(1)
2.69(1)
2.77(1)
ꢀ
ꢀ
Zn1 N1
2.370
2.047
2.276
2.072
Pb1 N1
ꢀ
ꢀ
Zn1 N2
Pb1 N2
Metal complexation in aqueous solutions—general features
and selectivity patterns: The binding ability of the two li-
gands toward CuII, ZnII, CdII and PbII was first analysed by
means of potentiometric titrations in aqueous solutions. This
was to determine the species present in solution and their
formation constants (Table 4) and to establish the possible
presence of a selectivity pattern among the different metal
cations. Both ligands form rather stable 1:1 complexes with
the metals under investigation. However, the stability con-
stants of the [ML1]2+ and [ML2]2+ (M=CuII, ZnII, CdII or
PbII) complexes are 3–4 logarithmic units lower than the cor-
responding complexes with the hexadentate ligand 2 (see
ꢀ
ꢀ
Zn1 N3
Pb1 N3
ꢀ
ꢀ
Zn1 N4
Pb1 N6
Br1-Zn1-N1
Br1-Zn1-N3
Br1-Zn1-N2
Br1-Zn1-N4
N1-Zn1-N3
N1-Zn1-N2
N1-Zn1-N4
N3-Zn1-N2
N3-Zn1-N4
N2-Zn1-N4
99.2
Br1-Pb1-N1
Br1-Pb1-N3
Br1-Pb1-N2
Br1-Pb1-N6
N1-Pb1-N3
N1-Pb1-N2
N1-Pb1-N6
N3-Pb1-N2
N3-Pb1-N6
N2-Pb1-N6
90.6(2)
76.8(2)
88.4(2)
77.3(2)
128.8(3)
65.5(3)
62.7(3)
64.7(3)
151.7(3)
125.8(3)
102.5
115.2
118.2
149.4
75.0
98.0
76.4
90.5
126.6
[a] From ref. [31].
Scheme 1),[9e,27e,g] which contains
a triethylentetraamine
the two heteroaromatic nitro-
gen atoms, two amine groups
(N3 and N4) and a bromide
anion, one of the benzylic nitro-
gen atoms (N6) is not involved
in metal binding. Such a differ-
ence from the b complex with
L1 may reside in the higher ri-
gidity of phenanthroline with
respect to dipyridine. Whereas
Table 4. Stability constants of the CuII, ZnII, CdII, and PbII complexes with L1 and L2 in aqueous solution
(0.1m NMe4NO3, 298 K).
Reaction
LogK[a]
CuII
ZnII
CdII
PbII
M2+ +L1=[ML1]2+
15.2(1)
9.5(1)
5.9(1)
3.9(1)
2.7(1)
–
10.85(1)
8.01(1)
6.10(1)
4.5(1)
4.5(1)
3.5(1)
13.9(1)
7.04(8)
6.05(7)
3.93(5)
4.01(1)
–
8.7(1)
8.7(1)
7.5(1)
–
ACHUTNGRENNUG CAHTUNGTRENNUNG
[ML1]2+ +H+ =[M
[M
[M
(HL1)]3+ +H+ =[M
ACHUTGTNRENNUG CAHTUNGTRENNUNG
A
ACHTUNGTRENNUNG
[ML1]2+ +OHꢀ =[M{(OH)L1}]+
–
[M{(OH)L1}]+ +OHꢀ =[M{(OH)2L1}]
7.4(1)
in [ZnACHTUNGTRENNUNG
(HL2)Br]2+ the rigidity
M2+ +L2=[ML2]2+
13.90(3)
9.04(2)
5.97(3)
5.46(2)
–
9.83[b]
8.41
7.15
5.30
3.90
2.47
10.21(1)
8.66(1)
7.20(1)
–
–
–
8.6(3)
9.26(2)
7.65(1)
–
3.77(3)
–
A
U
of the phenanthroline moiety
does not allow the simultaneous
coordination of both benzylic
amine groups to this metal, in
[M
[M
U
N
N
A
[M{(OH)L2}]+ +OHꢀ =[M{(OH)2L2}]
–
[ZnACHTUNGTRENNUNG
(HL1)Br]2+ the loss of co-
[a] Values in parentheses are standard deviations on the last significant figure. [b] From ref. [31].
planarity between the two pyri-
dine rings enables the ligand to
assume an appropriate conformation for the simultaneous
binding of both benzylic amine groups to the metal.
chain linking the 2,9-position of 1,10-phenanthroline. This
effect can be attributed to the replacement of the ethylenic
chains that link the amine groups in 2 by propylenic ones. In
fact, the increased N-M-N bond angle, as a result of the
larger bite of propylenic chains, may reduce the stability of
the complexes.[28] As a consequence of the reduced binding
ability of amine groups joined by propylenic bridges, some
amine groups are probably weakly bound or not bound to
the metal cations. This structural hypothesis is supported by
the marked tendency of the [ML1]2+ and [ML2]2+ com-
plexes to bind acidic protons in aqueous solutions, affording
mono-, di- and, in some cases, even triprotonated species. In
particular, the constants for the addition of the first two pro-
tons to the [ML]2+ complexes are remarkably high, in most
cases greater than 6 logarithmic units. These protonated spe-
cies are of particular relevance in the solution chemistry of
the present complexes because of their presence in a wide
pH range, including the neutral pH region, as shown in
Figure 5 for the CuII and ZnII complexes with L1.
The crystal structure of [Pb
ACHTUNGTERNNGNU(HL2)Br]ACHUTNRTGEG[NNUN PF6]1.5ACTHUNTGRENNUGN
[NO3]0.5·H2O
(e) (see Figure 4b and Table 3) consists of [PbACHTUNGTRENNUNG
(HL2)Br]2+
ꢀ
ꢀ
cations, PF6 and NO3 anions and water solvent molecules.
ꢀ
One PF6 and the nitrate anions lie on crystallographic C2
axes and are in consequence only partially contained in the
asymmetric unit. Both the coordination geometry and ligand
conformation are very similar to those found for the PbII
complex with L1, d (Figure 3d). In fact, the coordination en-
vironment is best described as a strongly distorted square
pyramid, in which the basal plane is defined by the four ni-
trogen atoms (N1, N2, N3 and N6) and the apical position is
occupied by the bromide anion. Once again, a zone “free”
from coordinated donor atoms is clearly observable in the
metal coordination environment, due to the presence of the
stereochemically active lone pair of PbII.[27g,34] The ligand
displays a conformation folded along the N3–N6 axis, with a
dihedral angle of 143.0(2)8 between the plane of phenan-
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Fluorescent Chemosensors
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Figure 5. Distribution of the complexed species of L1 in the presence of
1 equiv of CuII (top) and ZnII (bottom) ([L]=[CuII]=[ZnII]=1ꢂ10ꢀ3 m,
298 K, 0.1m NMe4NO3).
Figure 6. Top: Selected absorption spectra of L1 in the presence of
To corroborate the potentiometric study, metal complexa-
tion was also followed by recording UV spectra at different
pH values. In fact, the UV absorption band of 2,2’-dipyridine
and 1,10-phenanthroline is markedly affected by metal coor-
dination. In particular, metal binding to dipyridine is accom-
panied by the disappearance of its original band at 293 nm
and the simultaneous formation of a new structured band at
approximately 305 nm, whereas metal coordination by 1,10-
phenanthroline may result in either a decrease (in the case
of ZnII, CdII and PbII complexation) or an increase (in the
case of CuII) of the absorbance of its UV band at
270 nm.[9,27] These spectral changes can be used as a diagnos-
tic tool to assess the involvement of the heteroaromatic ni-
trogen atoms in metal binding. As shown in Figure 6 for the
ZnII complexes with L1, the appearance of a new band with
a maximum at 305 nm is observed above pH 4. In conse-
quence, the absorbance monitored at 305 nm increases from
pH 4 to pH 7, where the process of metal complexation
II
&
1 equiv of Zn . Bottom: Absorbance at 305 nm ( , right y axis) as a func-
tion of pH compared to the distribution curves of the complexes (c,
left y axis) for a system containing L1 and ZnII in 1:1 molar ratio ([L1]=
5ꢂ10ꢀ5 m, 298 K, 0.1m NMe4NO3).
complexed species formed by L1 or L2, as is displayed by
all of the crystal structures obtained for their metal com-
plexes (see above).
The crystal structures of the complexes also show that the
metal cations are coordinated by a single (in [Zn
or both benzylic nitrogen atoms (in [Cu(HL1)
[Zn
(HL1)Br]2+, [Cd(HL1)Br]2+, [Pb(H2L1)Br]3+ and [Pb-
(HL2)Br]2+). The remaining couple of amine donors, L1
A
)
A
R
,
A
R
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
and L2, far from the heteroaromatic moieties and character-
ised by a higher basicity, are probably not involved or
weakly involved in metal binding in aqueous solution; they
can therefore act as binding sites for the acidic protons in
the protonated forms of the complexes.
If we compare the binding ability of the two ligands,
Table 4 shows that the [ML1]2+ complexes are generally
more stable than the corresponding [ML2]2+ ones, with the
only exception being the PbII complexes, which display sta-
bility constants equal to those within the experimental error.
The higher stability of the complexes with L1 cannot be at-
tributed to the different binding ability of the dipyridine ni-
trogen atoms with respect to the phenanthroline ones, since
2,2’-dipyridine and 1,10-phenanthroline give metal chelates
with almost equal stability (for instance, logK=9.00 and
9.25 for the equilibrium Cu2+ +L=[CuL]2+ with L=2,2’-di-
pyridine and 1,10-phenanthroline, respectively).[30] On the
occurs to form the [Zn
nally, the band does not show significant changes in the al-
(HxL1)](2+x)+ (x=1–2) complexes. Fi-
ACHTUNGTRENNUNG
kaline pH region, that is, with the formation of the [ZnL1]2+
,
[Zn{(OH)L1}]+ and [Zn{(OH)2L1}] complexes. Complexa-
tion of ZnII by L2 to give the protonated complexes [Zn-
ACHTUNGTRENNUNG ACHTUNGTRENNUNG ACTHNUGTRENNGUN
(H3L2)]5+, [Zn(H2L2)]4+ and [Zn(HL2)]3+ in the pH range
4–7 is accompanied by a decrease of the absorbance and by
a slight redshift (around 6 nm) of the phenanthroline band
at 270 nm (Figure 7). No further changes are then observed
in the alkaline pH region. Similar results are also obtained
with the other metal ions (see the Supporting Information,
Figures S2 and S3). These data account for the involvement
of the heteroaromatic nitrogen atoms in all of the different
Chem. Eur. J. 2009, 15, 8049 – 8063
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8055

A. Bencini, A. Bianchi et al.
Scheme 2. Proposed binding mode of L1 in the [ML1]2+ and [M-
AHCTUNGTRENNUNG
(H2L1)]4+ complexes (M=CuII, ZnII, CdII).
(Figure 3d and Figure 4b), gives complexes of equal stability
with both ligands.
Among the investigated metal ions, CuII forms the most
stable complex (Table 4), as expected considering the effect
of the crystal-field stabilisation energy (CFSE) in CuII coor-
dination. The most interesting feature in Table 4, however,
is the higher stability of the [CdL]2+ complex with respect
to the [ZnL]2+ and [PbL]2+ ones (L=L1 or L2). This effect
is of particular relevance in the case of L1, in which the
[CdL1]2+ complex is more than 3 logarithmic units more
stable than the corresponding ZnII and PbII complexes, a
rather uncommon feature with polyamine ligands, including
phenanthroline or dipyridine-containing polyamine macro-
cycles.[9,27] However, the comparison between the binding
ability of L1 or L2 toward different metal cations is compli-
cated by the presence of overlapping protonation and de-
protonation equilibria involving the [ML]2+ complexes
(Table 4). This problem can be conveniently overcome by
considering a competitive system containing ligand and
metal cations in equimolecular concentrations and calculat-
ing the overall percentages of the different complexed cat-
ions over a wide pH range. Plots of the percentages versus
pH produces species distribution diagrams from which the
binding ability of a ligand can be interpreted in terms of se-
lectivity.[35] Figure 8 displays similar plots obtained for com-
petitive systems containing L1 or L2 and ZnII, CdII and PbII.
Whereas L2 shows a poor ability to recognise a single metal
cation, L1 is characterised by a marked selectivity for CdII
over PbII and ZnII. For instance, at pH 7 more than 95% of
CdII is complexed by L1, whereas ZnII is complexed at less
than 5%. PbII is complexed in percentages lower than 1%
all over the pH range investigated. As discussed above, in
L1 the metal cations are preferentially bound by the coordi-
nation site defined by the dipyridine nitrogen atoms and the
two adjacent benzylic amine groups. Most likely, selective
coordination of CdII over ZnII and PbII is due to a better di-
Figure 7. Top: Selected absorption spectra of L2 in the presence of
II
&
1 equiv of Zn . Bottom: Absorbance at 270 nm ( , right y axis) as a func-
tion of pH compared to the distribution curves of the complexes (c,
left y axis) for a system containing L2 and ZnII in 1:1 molar ratio ([L2]=
3ꢂ10ꢀ5 m, 298 K, 0.1m NMe4NO3).
other hand, previous work on phenanthroline-containing
macrocycles, such as 1–3, has shown that the rigidity of the
heteroaromatic unit does not allow the simultaneous coordi-
nation of the phenanthroline donors and of both the adja-
cent benzylic amine groups to relatively small metal cations
such as CuII or ZnII.[1p,9e,f,27e] This seems to be the case of L2
also. In fact, the crystal structure of the [ZnACTHNUTRGNEUNG
(HL2)]3+ com-
plex shows that only one benzylic group is bound to the
metal ion. Conversely, in the complexes with L1 the possible
rotation of the two pyridine rings along the 2,2’-axis of di-
pyridine may confer to the macrocyclic framework a some-
what higher flexibility, which allows both the benzylic amine
groups to bind to the metal cations, as is shown by the crys-
tal structures of the CuII, ZnII and CdII complexes with L1.
It seems likely that L1 contains a preferred binding site for
metals, consisting of the dipyridine nitrogen atoms and the
two adjacent benzylic amine groups (Scheme 2). This tetra-
dentate site is lost in L2, in which one benzylic nitrogen
atom is not coordinated, due to the rigidity of phenanthro-
line. This structural difference may lead to the higher stabili-
ty observed for the L1 complexes with CuII, ZnII and CdII
with respect to the corresponding complexes with L2. Of
note, the larger PbII cation, which appears to be coordinated
in
a
similar tetradendate mode by both L1 and L2
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Fluorescent Chemosensors
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ꢀ
Table 5. M X (M=Zn, Cd and Pb; X=N, O) distances computed at the
B3LYP level for the [ML1ACHTNUTGRNEUNG
(H2O)]2+ complexes.
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
Complex
[ZnL1
[CdL1
[PbL1
(H2O)]2+ 2.507 2.507 2.563 2.564 4.543 4.543 2.313
M N1 M N2 M N3 M N6 M N4 M N5 M O
(H2O)]2+ 2.118 2.144 2.196 2.112 4.803 3.226 2.065
G
E
E
(H2O)]2+ 2.342 2.351 2.319 2.279 4.372 3.552 2.251
ACHTUNGTRENNUNG
G
ACHTUNGTRENNUNG
Figure 8. Overall percentages of the L1 (top) and L2 (bottom) complexed
species with ZnII, CdII and PbII as a function of pH in competitive sys-
tems containing ZnII, CdII and PbII and L1 or L2 in equimolecular ratio
([L1]=[L2]=[ZnII]=[CdII]=[PbII]=1ꢂ10ꢀ3 m).
ꢀ[M
(H3L)]5+ +[M{(OH)L}]+
[M{(OH)2L}] with L=L1 or L2 and M=Zn, Cd or Pb.
(HxL)](xꢀ1)+
ACHTUNGTERNNUNG =
[ML]2+ +[M(HL)]3+ +[M
G
R
+
mensional and stereochemical matching between CdII and
this tetradentate coordination site, which possesses the opti-
mal dimension and conformation to host CdII. This sugges-
tion is supported, once again, by the crystallographic analy-
sis of the [MACHTUNGTRENNUNG
(HL1)]3+ complexes (M=ZnII, CdII and PbII) in
the solid state. In fact, while CdII forms four strong Cd N
ꢀ
II
II
bonds, ZnII shows weaker M N interactions and Pb fea-
tures a strongly distorted coordination environment, with
two of four bond lengths being remarkably longer.
ꢀ
Figure 9. Optimised geometries computed at the B3LYP level for [ML1-
AHCTUNGTRENNUNG
(H2O)]2+ complexes: a) M=Zn, b) M=Cd, c) M=Pb.
To verify the presence of a tetradentate cavity well suited
to host CdII that is also in non-protonated complexes, we op-
the dipyridine nitrogen atoms and by the adjacent benzylic
amine donors, whereas the central amine groups of the ali-
phatic chain do not show interactions with the metals. The
data in Table 5 point out that the most favourable stereo-
chemical matching is found for the CdII ion. In this case, the
L1 ligand acts as a perfect tetradentate ligand. In fact, three
timised the [ML1ACHTUNGTRENNUNG
(H2O)]2+ species within the density func-
tional theory (DFT) framework. For all complexes, we used
as starting coordinates those derived from the crystal struc-
tures of the protonated complexes, by removing the acidic
protons and replacing the halogen ion with a water mole-
cule. In fact, halogen ions are generally weakly bound by
the present metal cations and therefore they can be easily
replaced by a water molecule in aqueous solution. The ob-
ꢀ
of the four closest Cd N distances are perfectly in agree-
II
ꢀ
ment with the Cd N distances generally observed in Cd
ꢀ
complexes with polyaza ligands. The short Cd N6 should be
ꢀ
tained M N (M=Zn, Cd or Pb) distances and complete ge-
induced by the non-symmetric L1 folding through the hy-
drogen bond between the N6 proton and N5. The [ZnL1-
ometries are reported in Table 5 and Figure 9, respectively.
Similarly to the crystal structures of the protonated com-
plexes, the metal cations are unequivocally coordinated by
AHCTUNGTRENNUNG
(H2O)]2+ geometry instead presents fairly longer distances
than those usually found in ZnII complexes with polyamine
Chem. Eur. J. 2009, 15, 8049 – 8063
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8057

A. Bencini, A. Bianchi et al.
ligands (mean value 2.087(1) ꢁ, see above), confirming the
ZnII ion to be too small to fit into the tetradentate binding
(H2O)]2+, the Zn N6 bond is
ꢀ
site of L1. Similarly to [CdL1
the shortest one, probably due to the formation of a hydro-
gen bond between the N6 proton and N5. The [PbL1-
ACHTUNGTRENNUNG
(H2O)]2+ cation is characterised by a water molecule inter-
acting through hydrogen bonding with the amine groups not
involved in metal coordination. Similarly to the crystal struc-
ture of [PbACHTUNGTRENNUNG
(H2L1)Br]3+, the metal cation is shifted in the op-
posite direction with respect to the apical position occupied
by the water molecule. The two dipyridine nitrogen atoms
interact with the metal at very short distances, whereas the
two benzylic amine groups are coordinated more weakly.
These structural features suggest a not optimal fitting of PbII
within the tetradendate binding site of L1.
Fluorimetric chemosensing of metal cations with L1 and L2:
The chemosensing ability of L1 and L2 was studied by
means of spectrofluorimetric measurements on aqueous sol-
utions containing the ligand and the selected metal ion in
equimolecular ratio at different pH values. In fact, both li-
gands present a marked pH dependence of their emission
spectra. In the case of L1, the emission of the ligand is com-
pletely quenched from slightly acidic to alkaline pH values
in the presence of CuII or PbII. Superimposition of the distri-
bution diagram of the CuII or PbII complexes with the maxi-
mum of the emission intensity of dipyridine at 345 nm at dif-
ferent pH values clearly shows that the fluorescence quench-
ing is simply due to the formation of the metal complexes
(see Figure 10a for PbII), as generally observed for complex-
ation of paramagnetic ions, such as CuII, or heavy metal cat-
ions, such as PbII. A different behaviour is observed in the
case of ZnII and CdII, in which a less common pH depend-
ence of the emission spectra is observed. As shown in
ACHTUNGTRENNUNG
Figure 10b and c for CdII, the fluorescence emission displays
an OFF–ON–OFF pH profile, with a maximum of the emis-
sion intensity at pH 6.2. Figure 10c points out that, among
the different complexes present in solution, only the proton-
&
Figure 10. a) Fluorescence emission intensity at 345 nm ( , right y axis)
as a function of pH compared to the distribution curves of the complexes
(c, left y axis) for a system containing L1 and PbII in 1:1 molar ratio
([L1]=[PbII]=1ꢂ10ꢀ5 m, lexc =298 nm, 298 K, 0.1m NMe4NO3); b) se-
ated complexes [CdACTHNUTRGNEUNG
(HxL1)](x+2)+ (x=1–3) are emissive,
whereas [CdL1]2+ and its hydroxo derivatives do not show
any fluorescence emission. This behaviour, which is also
found in the case of the ZnII complexes (see the Supporting
Information, Figure S4) can be interpreted in consideration
of the fact that the [ZnL1]2+ and [CdL1]2+ complexes are
characterised by the central amine groups of the aliphatic
chain not involved or weakly involved in metal coordination
(Scheme 2), which may quench the emission of the complex
thanks to a PET process involving their lone pairs. As a con-
sequence, the [ML1]2+ complexes and their hydroxo deriva-
tives, formed in the alkaline pH region, are not emissive.
Protonation of the complexes in the pH range 4–8 takes
place on these unbound amine groups (Scheme 2). Binding
of a couple of protons or of a single acidic proton, which
can be shared by the two central amine groups of the ali-
phatic chain through hydrogen bonding, can inhibit the PET
process, renewing the fluorescence emission of the ZnII and
CdII complexes. Finally, the decrease of the emission ob-
lected fluorescence emission spectra of L1 in the presence of 1 equiv of
II
&
Cd ; c) fluorescence emission intensity at 345 nm ( , right y axis) as a
function of pH compared to the distribution curves of the complexes
(c, left y axis) for a system containing L1 and CdII in 1:1 molar ratio
([L1]=[CdII]=1ꢂ10ꢀ5 m, 298 K, lexc =298 nm, 0.1m NMe4NO3).
served below pH 4 is simply due to complex decomposition,
which affords the free ligand in its tetraprotonated form
([H4L1]4+), an almost non-emissive species. The emissive
characteristics of the ZnII and CdII protonated complexes
are also confirmed by the fluorescence emission spectra of
[ZnACTHUNGTNRE(UNNG HL1)Br]ACHUTTGNRENNUG[ClO4]2 and [CdACHTUNGRETG(NNUN HL1)Br]ACHTNUGRTEN[NGUN ClO4]2 solid com-
pounds dissolved in anhydrous CH3CN (see the Supporting
Information, Figure S5). In fact, both compounds display a
marked fluorescence emission, with a maximum of the in-
tensity at 352 nm, slightly redshifted with respect to the
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Fluorescent Chemosensors
FULL PAPER
spectra in aqueous solutions. Conversely, the [CuACTHNUGTRNEUNG(HL1)-
A
ACHTUNGTRENNUNG[ClO4]2·H2O complex does not give any emission en-
Of note, addition of increasing amounts of ZnII or CdII to
an aqueous solution of L1, buffered at pH 6.5, leads to a
linear increase of the fluorescence emission intensity of di-
pyridine at 345 nm up to 0.8:1 (in the case of ZnII) or 1:1 (in
the case of CdII) metal to ligand molar ratio (see the Sup-
porting Information, Figure S6). The emission intensity
ACHTUNGTRENNUNGachieves a constant value for molar ratios greater than 1.2 in
the case of CdII or 1.4 in the case of ZnII. These data not
only confirm that L1 forms rather stable 1:1 metal chelates
with ZnII and CdII, but also indicate that the receptor is able
to ratiometrically sense these metal cations, thanks to chela-
tion-induced enhancement of fluorescence. Furthermore, a
competitive experiment carried out by adding increasing
amounts of PbII to a solution containing CdII and L1 at
pH 6.5 in equimolecular ratio shows that the fluorescence
emission intensity at 345 nm of the CdII complex is only
slightly affected by the presence of more than 1 equiv of
PbII (for instance, only a 5% reduction of the intensity is ob-
served in the presence of 3 equiv of PbII; see the Supporting
Information Figure S7), that is, the CdII complex is preferen-
tially formed in high percentages, even in the presence of a
large excess of PbII. Conversely, the same experiment car-
ried out on solutions containing L1 and ZnII in 1:1 molar
ratio show an 18% decrease of the emission of the ZnII
complex in the presence of only 1 equiv of PbII. These re-
sults point out that the stability of the complexes decreases
in the order CdII >ZnII >PbII, confirming, at least qualita-
tively, the selectivity pattern deduced from the potentiomet-
ric measurements (Figure 8, top). Actually, the ability to se-
lectively bind CdII over ZnII and PbII, and to give fluorescent
ZnII or CdII complexes in a narrow pH range represent pe-
culiar features of this receptor.
Figure 11. Top: Selected fluorescence emission spectra of L2 in the pres-
ence of 1 equiv of CdII. Bottom: Fluorescence emission intensity at
&
370 nm ( , right y axis) as a function of pH compared to the distribution
curves of the complexes (c, left y axis) for a system containing L2 and
CdII in 1:1 molar ratio ([L2]=[CdII]=5ꢂ10ꢀ6 m, lexc =280 nm, 298 K,
0.1m NMe4NO3).
orophore, can behave as an efficient quencher of the fluo-
rescence emission of phenanthroline.
Conclusion
The complexes with L2 are characterised by different pH
dependence. In fact, all the metal complexes, including
those with ZnII and CdII, do not show florescence emission
all over the pH range investigated; unlike L1, the ZnII and
This study shows that subtle structural parameters can deter-
mine the selectivity properties of ligands as well as the fluo-
rescence emission of their complexes. The coupling within
the same cyclic structure of a 2,2’-dipyridine unit and an ali-
phatic tetramine moiety characterised by propylenic spacers
between the nitrogen donors gives rise to a ligand able to
selectively coordinate CdII over ZnII and PbII. This charac-
teristic is due to the presence of a tetradentate cavity with
the optimal dimension to host the CdII ion. Of note, this co-
ordinative site is composed of the less basic heteroaromatic
and benzylic nitrogen atoms, whereas the most basic amine
groups, located in the middle of the aliphatic chain, are not
involved in metal binding and can easily protonate at neu-
tral pH. The simple replacement of dipyridine with phenan-
throline, a heteroaromatic unit with a donor ability almost
equal to that of dipyridine but more rigid, leads to loss of
selectivity. In fact, the rigidity of phenanthroline does not
allow the simultaneous involvement of both the heteroaro-
matic nitrogen atoms and the adjacent benzylic amine
groups and the consequent formation of a tetradentate bind-
ing site for metal cations. This slight structural difference
CdII protonated complexes [M(HL2)]3+ and [M(H2L2)]4+
ACTHNUTRGENNUG CAHTUNGTRENNUNG ,
which are the predominant species in solution in the pH
range 4–7 (see Figure 11 for CdII and the Supporting Infor-
mation, Figure S8), are not emissive. Whereas the lack of
emission of the CuII and PbII complexes with L2 can be re-
lated, once again, to the paramagnetic or heavy nature of
the metal cation, the different emission properties of the
protonated ZnII and CdII complexes with L2 with respect to
those with L1 is probably due to the different structural
characteristics of the complexes. Differently from L1, the L2
complexes with ZnII or CdII are characterised by a benzylic
amine group not bound or weakly bound to the metal cat-
ions. This amine group is less basic than the central amine
groups of the aliphatic chain; this suggests that in the [M-
ACHTUNGTRENNUNG ACHTUNGTRENNUNG
(HL2)]3+ and [M(H2L2)]4+ complexes the unbound benzylic
amine group is not involved in proton binding. As already
observed in the case of the protonation of the ligand alone,
this amine group, positioned at a short distance from the flu-
Chem. Eur. J. 2009, 15, 8049 – 8063
ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
8059

A. Bencini, A. Bianchi et al.
ture of the resulting solution. Yield: 19.3 mg (64.3%); elemental analysis
calcd (%) for C21H35N6CuCl3O13 (Mr =749.45): C 33.66, H 4.71, N 11.21;
found: C 33.4, H 4.8, N 11.2.
also determines the fluorescence emission properties of the
ZnII and CdII complexes. Whereas the dipyridine-containing
ligand gives fluorescent protonated complexes at neutral
pH, due to the involvement of all nitrogen atoms in metal
or proton binding, the phenanthroline-based analogue forms
non-emissive complexes at any pH value. In fact, in the L2
complexes, the presence of an unbound benzylic amine
group close to the fluorophore can efficiently quench the
fluorescent emission of phenanthroline through a PET pro-
cess.
[Zn
ACHUTGTNRNEUG(N HL1)Br]ACHTGNUTRENNUNG
plex were prepared from ZnAHCTNUGTRENNNUG
L1·5HBr (30 mg, 0.04 mmol) by using a procedure similar to that used
for a, adjusting the pH of the solution to 6. Yield: 22.1 mg (77.5%); ele-
mental analysis calcd (%) for C21H33N6ZnBrCl2O8 (Mr =712.71): C 35.34,
H 4.66, N 11.77; found: C 35.4, H 4.7, N 11.7.
[CdACTHNUTGRENN(UG HL1)Br]AHCTUNGTRENN[UNG ClO4]2 (c): Crystals suitable for X-ray analysis of this com-
plex were prepared from CdBr2·4H2O (14.8 mg, 0.04 mmol) and
L1·5HBr (30 mg, 0.04 mmol) by using the same procedure used for com-
plex a. Yield: 22.0 mg (72.4%); elemental analysis calcd (%) for
C21H33N6CdBrCl2O8 (Mr =760.75): C 33.16, H 4.37, N 11.05; found: C
33.3, H 4.5, N 11.0.
Experimental Section
[PbACTHNUTRGENNGU(H2L1)Br]ACHTUNGTRNEN[UGN ClO4]3·H2O (d): Crystals suitable for X-ray analysis of this
complex were prepared from PbBr2·3H2O (16.8 mg, 0.04 mmol) and
L1·5HBr (30 mg, 0.04 mmol) by using the same procedure used for com-
plex a. Yield: 25.5 mg (65.4%); elemental analysis calcd (%) for
C21H36N6PbBrCl3O12 (Mr =947.12): C 25.90, H 3.73, N 8.63; found: C
25.8, H: 3.8, N 8.6.
Synthesis of ligands and their metal complexes: L2 was prepared as pre-
viously reported.[36] Crystals of [(H4.5L2)
ACHTNUGTRNNE(GU H2O)0.5Cl0.5]ACHTUNGTRENN[NUG ClO4]4·2H2O suita-
ble for X-ray analysis were obtained in 27% yield by slow evaporation of
an HCl (0.1m) aqueous solution of L2 (0.01m) in the presence of a ten-
fold excess of NaClO4. L1 was synthesised by using a modification of the
Richmam and Atkins procedure.[37] Reaction of 1,5,9,13-tetratosyl-
1,5,9,13-tetraazatridecane (1)[36] with 6,6’-bis(bromomethyl)-2,2’-bipyri-
dine[38] (2) afforded the tosylated macrocycle (3), which was then depro-
tected in a CH3COOH/HBr mixture.
Caution! Perchlorate salts of organic ligands and their metal complexes
are potentially explosive; these compounds must be handled with great
care.
[Pb
(HL2)Br]
E
E
(e):
Pb
G
(15.4 mg,
0.04 mmol) was added to a solution of L2·4HBr (28.6 mg, 0.04 mmol) in
water (10 mL). The pH of the solution was adjusted to 6 by slow addition
of a few drops of NaOH (0.1m). The solution was then stirred for 30 min
and then KPF6 (40 mg) was added. Crystals of the complex suitable for
X-ray analysis were obtained by slow evaporation at room temperature
of the resulting solution. Yield: 18.2 mg (60.7%); elemental analysis
calcd (%) for PbC23H35N6.5BrP1.5F9O2.5 (Mr =749.45): C 29.17, H 3.72, N
9.61; found: C 29.3, H 3.7, N 9.6.
6,6’-(2,6,10,14-Tetratosyl-2,6,10,14-tetraaza[15])ACTHNUTRGNEUG(N 3,3’)-2,2’-bipyridylophane
(3): A suspension of 2 (940 mg, 2.75 mmol) in dry acetonitrile (250 mL)
was added, over a period of 4 h, to a stirred suspension of 1 (2.0 g,
2.5 mmol) and K2CO3 (3.45 g, 25 mmol) in dry acetonitrile (250 mL) at
reflux, under a nitrogen atmosphere. At the end of the addition, the mix-
ture was kept under stirring at reflux for an additional 2 h. The suspen-
sion was filtered on Celite, washed with acetonitrile, and the resulting so-
lution was evaporated under reduced pressure to obtain a crude solid,
which was purified by column chromatography on neutral aluminium
oxide by using a petroleum ether/ethyl acetate 1:2 mixture as eluent.
Pure compound 3 was obtained as a white solid (862 mg, 35% yield).
1H NMR (300 MHz, [D]CHCl3, 258C, TMS): d=8.18 (d, J=7.8 Hz, 2H),
7.70 (d, J=7.8 Hz, 2H), 7.60 (d, J=8.2 Hz, 4H), 7.48 (d, J=7.0 Hz, 2H),
7.40 (d, J=8.2 Hz, 4H), 7.25 (d, J=8.3 Hz, 4H), 7.08 (d, J=8.3 Hz, 4H),
4.27 (s, 4H), 2.95–2.90 (m, 6H), 2.77–2.71 (m, 6H), 2.34 (s, 6H), 2.26 (s,
6H), 1.34–1.26 ppm (m, 6H); 13C NMR (100 MHz, [D]CHCl3, 258C,
TMS): d=159.0, 157.2, 143.7–143.3, 136.1–135.0, 137.4, 129.7–129.5,
127.1–126.8, 120.5, 119.3, 52.5, 48.3, 47.5, 46.9, 45.2, 28.7, 28.0, 21.3 ppm.
Potentiometric measurements: Equilibrium constants for protonation
and complexation reactions with L were determined by pH-metric meas-
urements at (298.1ꢁ0.1) K in 0.1m NMe4NO3, by using equipment and
procedures[39] that have been already described. Ligand and metal ion
concentrations of (1–2)ꢂ10ꢀ3 m were employed in the potentiometric
measurements, with varying of the metal to ligand molar ratio from 0.5:1
to 2:1. Three titrations (about 100 data points for each one) were per-
formed in the pH range 2–12. The computer program HYPERQUAD[40]
was used to calculate equilibrium constants from electromotive force
(emf) values.
Spectrophotometric and spectrofluorimetric measurements: Absorption
spectra were recorded on a Perkin–Elmer Lambda 25 spectrophotometer.
Fluorescence emission spectra were collected on a Perkin–Elmer LS55
spectrofluorimeter. In the measurements carried out at different pH
values, HNO3 and NaOH were used to adjust the pH values that were
measured on a Metrohm 713 pH meter. Tris(hydroxymethyl)aminome-
thane (TRIS) buffer (1 mm) was used in the titrations performed at
pH 6.5. In the competition experiments, successive readings of the emis-
sion intensity were carried out after each addition of PbII to ensure that
the equilibrium was reached.
NMR spectroscopy: 1H (300 MHz) and 13C (75 MHz) NMR spectra of
samples in CDCl3 and D2O at different pH values were recorded at
298 K on a Varian Gemini 300 spectrometer. To adjust the pD, small
amounts of NaOD (0.01m) and DCl were added to the solution contain-
ing L1. The pH was calculated from the measured pD values by using
the following formula: pH=pDꢀ0.40.[41] 1H-1H and 1H-13C 2D correla-
tion experiments were performed to assign the 1H NMR spectroscopic
signals.
6,6’-(2,6,10,14-Tetraaza[15])-2,2’-bipyridylophane
pentahydrobromide
(L1·5HBr): Compound 3 (985 mg, 1 mmol) and phenol (7.5 g, 80 mmol)
were dissolved in a HBr/AcOH 33% mixture (100 mL); the solution was
kept under stirring at 908C for 20 h, until a white precipitate was formed.
The reaction mixture was cooled to room temperature and then CH2Cl2
(100 mL) was added to complete the precipitation and the suspension
was stirred for an additional 1 h. The solid residue was filtered and
washed several times with CH2Cl2. The hydrobromide salt was then re-
crystallised from a EtOH/water 3:1 (v/v) mixture, filtered and dried
under vacuum at 408C overnight. Yield: 470 mg, 61%; 1H NMR
(300 MHz, [D2]H2O, pH 4.5, 258C, DSS): d=8.27 (d, J=7.0 Hz, 2H),
8.12 (d, J=7.8 Hz, 2H), 7.64 (d, J=7.6 Hz, 2H), 4.56 (s, 4H), 3.33 (t, J=
7.6 Hz, 4H), 3.21 (t, J=5.4, 4H), 3.05 (t, J=7.2 Hz, 4H), 2.15 (t, J=
7.0 Hz, 4H), 2.01 ppm (t, J=7.1 Hz, 2H); 13C NMR (75 MHz, [D2]H2O,
pH 4.5, 258C, DSS): d=154.4, 150.3, 140.3, 125.1, 123.1, 50.9, 44.5, 43.3,
43.2, 22.1, 21.3 ppm; MS (ESI): m/z (%): 368.30 (100) [M++H], 184.6
(100) [M2++2H]; elemental analysis calcd (%) for C21H32N6·5HBr (Mr =
773.08): C 32.63, H 4.82, N 10.87; found: C 32.41, H 5.03, N 10.69.
Crystal structure analyses: Data for the X-ray structural analyses of
[Cu
(HL1)
E
G
ACHTUGNTRENUN(GN ClO4)2·6H2O (14.8 mg, 0.04 mmol)
[(H4.5L2)
[Zn(HL1)Br]
[ClO4]3·H2O (d) and [Pb
using an Oxford Diffraction Xcalibur3 diffractometer equipped with
CCD area detector and graphite monochromated MoKa radiation. Data
G
]
E
E
ACHUTGTNERNNUG(ClO4)]ACHTUGNTRENNNUG
was added to
a
N
R
G
G
ACHTUNGTRENNUNG
(10 mL). The pH of the solution was adjusted to 5 by slow addition of a
few drops of 0.1m NaOH. The solution was then stirred for 30 min and
then NaClO4·H2O (100 mg) was added. Crystals of the complex suitable
for X-ray analysis were obtained by slow evaporation at room tempera-
E
G
G
CHTUNGTRENNUNG
8060
www.chemeurj.org
ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2009, 15, 8049 – 8063

Fluorescent Chemosensors
FULL PAPER
Table 6. Crystal data and structure refinement for [(H4.5L2)
A
]
A
ACHTUNGENRT(NUG HL1)ACUTHNGTREN(NUNG ClO4)]ACHTNUGTRE[UNNGN ClO4]2·H2O (a), [ZnAHTCUNGERTN(NUNG HL1)Br]CAHTUNTGENRU[GN ClO4]2 (b), [Cd-
A
R
G
E
G
N
CHTUNGTRENNUNG
A
E
a
b
c
d
e
ACHTUNGTRENNUNG
formula
Mr
C23H41.5Cl4.5N6O18.5
857.65
C21H35Cl3CuN6O13 C21H33BrCl2N6O8Zn C21H33BrCdCl2N6O8 C21H36BrCl3N6O13Pb C23H35BrF9N6.5O2.5P1.5Pb
749.44
713.71
760.74
974.01
947.13
crystal system
space group
crystal size
[mm3]
a [ꢁ]
b [ꢁ]
c [ꢁ]
a [8]
b [8]
triclinic
P1
0.25ꢂ0.2ꢂ0.2
triclinic
P1
0.3ꢂ0.2ꢂ0.1
triclinic
P1
0.4ꢂ0.3ꢂ0.1
triclinic
P1
0.2ꢂ0.1ꢂ0.1
monoclinic
P21/a
0.3ꢂ0.3ꢂ0.2
orthorhombic
Pcca
0.3ꢂ0.25ꢂ0.1
¯
¯
¯
¯
7.540(5)
12.277(2)
19.118(7)
83.97(1)
79.31(2)
84.28(1)
1724(1)
2
8.335(1)
13.020(2)
15.066(2)
73.93(1)
78.90(1)
84.34(1)
1539.9(4)
2
8.262(3)
13.446(4)
13.537(4)
101.51(2)
100.12(3)
101.09(2)
1409.8(8)
2
8.503(3)
13.253(4)
13.794(3)
101.6(2)
102.7(3)
102.4(3)
1429.8(7)
2
14.161(1)
11.3706(8)
20.586(2)
90
97.520(7)
90
35.219(2)
13.5409(6)
13.0922(7)
90
90
90
g [8]
V [ꢁ3]
Z
3286.2(5)
4
3286.2(5)
4
T [K]
298
298
298
298
298
298
l [ꢁ]
0.71069
1.653
0.71069
1.616
0.71069
1.681
0.71069
1.767
0.71069
1.969
0.71069
2.015
1calcd [mgmꢀ3
]
m (MoKa
[mmꢀ1
(000)
qmax [8]
total reflns
unique reflns
(Rint
)
0.471
1.041
2.533
2.403
6.663
6.852
]
F
G
892
22
18739
5855
(0.0243)
774
728
764
1904
3672
23.25
9283
4161
23.25
11191
3986
28.52
14295
6034
23.81
14970
5012
23.25
16549
4462
)
(0.0483)
(0.0804)
(0.0538)
(0.0555)
ACHTUNGTRENN(UNG 0.0681)
observed reflns
(I>2s(I))
4339
2263
2296
4319
2995
2361
parameters
478
442
388
360
406
403
GOF
1.075
0.934
0.1360
0.0528
1.121
0.2455
0.0880
1.097
0.2048
0.0635
0.954
0.1056
0.0451
0.922
0.1316
0.0492
[a]
wR2 (all data) 0.1285
R1[b] (I>2s(I))
0.0423
1
[a] R1 =SjjFo jꢀjFc jj/SjFo j. [b] wR2 =[Sw(F2oꢀFc2)2/SwFo4] =
.
2
collection was performed using a w scan with the CrysAlis CCD pro-
gram.[42] Data reduction was carried out with the CrysAlis Red pro-
gram,[43] and an empirical absorption correction was applied using spheri-
cal harmonics, implemented with the SCALE3 ABSPACK scaling algo-
rithm.[43] Structures were solved by direct methods (SIR2004)[44] and re-
fined against F2 by using SHELXL-97,[45] with non-hydrogen atoms an-
716578 ([(H4.5L2)ACHTGUNERTNNU(G H2O)0.5Cl0.5]ACHTUGNTERN[NUNG ClO4]4·2H2O), 716577 (a), 716581 (b),
716576 (c), 716579 (d) and 716580 (e) contain the supplementary crystal-
lographic data for this paper. These data can be obtained free of charge
from the Cambridge Crystallographic Data Centre via www.ccdc.cam.
ac.uk/data_request/cif.
Computational details: GAMESS US software package[46] was used
throughout all the calculations. The default thresholds were used for all
the optimisations. Hay–Wadt pseudopotentials and basis sets[47] were
used for ZnII and CdII complexes. SBK pseudopotentials and basis sets[48]
were used for the PbII complex. The B3LYP functional[49] was used
throughout all the calculations.
ACHTUNGTRENNUNG
A
]ACHTUNGTRENNUNG
enclosed in the macrocyclic cavity, share almost the same position and
have been refined with partial occupational parameters (0.5). Attempts
performed to refine the structure in the P1 space group, considering a
doubled content of the asymmetric unit, failed; in a the H1, H2 and H4
hydrogen atoms, bound to the N4 and N5 secondary nitrogen atoms,
have been localised in the DF map, introduced in the calculation and iso-
tropically refined with a fixed thermal parameter (U=0.05 ꢁ2); in b, the
acidic proton has not been localised in the DF map, and has been intro-
duced in a calculated position with a partial population parameter (0.5)
on the secondary N4 and N5 nitrogen atoms. Disorder has been found
for one of the perchlorate anions, the oxygen atoms of which have been
found distributed over two different positions with equal occupational
factor (O41, O42, O43, O44 and O41’, O42’, O43’, O44’); in c, the H1
and H2 hydrogen atoms, bound to the N4 secondary nitrogen, have been
localised in the DF map, introduced in the calculation and isotropically
refined; in d the hydrogen atoms belonging to the water solvent molecule
have not been localised in the DF map and not introduced in the calcula-
Acknowledgements
Financial support of MIUR (PRIN 2007) is gratefully acknowledged.
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Received: February 2, 2009
Published online: June 5, 2009
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