FULL PAPER
strated that, through the functionalization of the trisurea
binding sites with redox-active (ferrocenyl) or fluorescent
1H NMR titration experiments of ortho-phenylene-bridg-
2À
ed ligand L2 with SO4
anions in [D6]DMSO/water
(quinolinyl or [Ru
N
(99.5:0.5) solution showed two slow-exchange processes (see
the Supporting Information, Figure S4a). The first step was
similar to that observed previously for ligand L1 on the addi-
ical and/or optical recognition of sulfate and phosphate
anions could be achieved.[16] Herein, we decorated the tripo-
dal hexaurea backbone with ferrocenyl redox groups[17] to
afford three electrochemical tripodal hexaurea sensors with
different bridges (ortho-phenylene L2, meta-phenylene L3,
and para-phenylene L4; Scheme 1). These bridges exhibit
different angles between the two urea groups of each bis-
2À
tion of 0–1.0 equivalents of SO4 ions,[14] during which, new,
sharp signals were observed downfield of all of the urea NH
protons, as well as the peaks of the free ligand. The changes
in the shifts of the “inner” NH groups (NHa Dd=1.16 ppm,
NHb Dd=0.97 ppm with 1.0 equiv SO42À) are larger than
those of the “outer” NH groups (NHc Dd=0.70 ppm, NHd
ACHTUNGTRENNUNGurea arm and may form different spaces to accommodate
2À
anions. The anion-binding properties of ligands L2–L4 have
been studied in detail. Interestingly, the meta-phenylene-
bridged ligand (L3), owing to its trigonal bipyramidal struc-
Dd=0.31 ppm), thus implying that the bound SO4 ion is
closer to the NHa and NHb atoms than the NHc and NHd
atoms. In the second slow-exchange process (>1.0 equiv of
sulfate ions), a new set of obviously broadened signals ap-
peared even further downfield, which may be assigned to
the formation of a 1:2 host/guest complex, whilst the NH
protons of the 1:1 complex gradually disappeared. However,
unlike the case of ligand L1, these changes were not com-
plete with the addition of 2.0 equivalents of sulfate ions, at
which point the 1:1 and 1:2 sulfate complexes may co-exist.
ture, shows the distinct stepwise encapsulation of two SO4
ions. A crystal structure of the sulfate capsular complex with
the ortho-ligand (L2), (TBA)
2ACHTUNGTRENNUNG
rabutylammonium ion) reveals excellent complementarity
between the receptor and sulfate ion as in the case of nitro-
phenyl analogue L1.
2À
Upon the addition of 3.0 equivalents of SO4 ions, the sig-
Results and Discussion
nals of the 1:1 complex completely disappeared and the
spectrum reached saturation. In the second process, all of
the urea NH groups displayed downfield shifts, but the
changes in the shifts of the outer NH protons (Hc Dd=
1.07 ppm, Hd Dd=1.12 ppm) were much larger than those
of the inner NH protons (Ha Dd=0.16 ppm, Hb Dd=
0.50 ppm). The overall downfield shifts of the inner NHa
(Dd=1.32 ppm) and NHb protons (Dd=1.47 ppm) are
somewhat smaller than those of the outer NHc (Dd=
1.77 ppm) and NHd protons (Dd=1.43 ppm). The associa-
Ferrocenyl-functionalized tripodal hexaurea receptors L2–L4
were readily synthesized as yellow powders from the reac-
tion of N,N’,N’’-(nitrilotri-2,1-ethanediyl)tris-(N’-aminophe-
nylurea)[14] with three equivalents of ferrocenyl isocyanate
under a nitrogen atmosphere in 80% yield. These new re-
ceptors are soluble in DMSO and DMF, whilst they are
hardly soluble in other common organic solvents.
1H NMR studies of the anion-binding properties of receptor
L2: The solution-state binding properties of receptors L2, L3,
and L4 with different anions were investigated by perform-
ing 1H NMR titration experiments in [D6]DMSO/water
(99.5:0.5). The C3-symmetric ligand molecules displayed
four NH resonances in their NMR spectra. For the ortho-
phenylene-bridged ligand (L2), when one equivalent of vari-
tion constants can be estimated from the H NMR titration
1
data to be larger than 104 mÀ1.[18,19] Notably, the formation of
a 1:2 complex was also observed for nitrophenyl-appended
ligand L1, which had the same ortho-phenylene bridges on
the three bisurea arms as in ligand L2. Although there is no
direct evidence for the exact structure of such species, we
propose that some of the “outer” urea groups may be twist-
ed towards the outside of the tripodal cavity to bind the
second sulfate ion in multiple binding conformations, as in-
dicated by the largely broadened NMR peaks. This conclu-
sion may also be supported by the crystal structure of the
free L1 ligand, which shows an “umbrella” conformation in
which the three outer arms are folded outwards.[14]
À
À
À
À
ous anions (SO42À, AcOÀ, H2PO4 , HSO4 , NO3 , ClO4 ,
ClÀ, BrÀ, and IÀ) were separately added as their TBA salts
to solutions of the receptor, the SO4 ion induced the larg-
2À
est downfield shifts of all of the urea groups. The addition
of AcOÀ and ClÀ anions also resulted in some downfield
À
shifts of the NH protons; with the HSO4 anion, the NH sig-
nals broadened severely, which may be due to the rapid
equilibration of multiple binding modes. A competitive-
binding experiment demonstrated that receptor L2 could se-
lectively bind sulfate ions in the presence of equal amounts
of various competitive anions (see the Supporting Informa-
tion, Figure S3a). The meta-phenylene- and para-phenylene-
bridged receptors (L3 and L4, respectively) showed similar
selectivities for the sulfate ions to those for the other anions
that were examined (see the Supporting Information, Fig-
ure S3b,c). However, detailed 1H NMR titrations of these
three receptors revealed different anion-binding behavior,
as discussed below.
Crystal structure of (TBA)2ACHTUNGTRENNUNG
[SO4ꢀL2]·2H2O (1): Anion com-
plex 1 was obtained by the slow evaporation of a solution of
L2 in water/DMSO in the presence of excess (TBA)2SO4.
Complex 1 crystallizes in the monoclinic space group P21/c.
As expected, a cage structure with an encapsulated sulfate
ion was formed. The hexaurea ligand adopts a folded cone
conformation in which all of the NH groups of the six urea
groups point towards the inside of a distorted tetrahedral
cavity to bind the sulfate ion through 12 N H···O hydrogen
bonds (Figure 1 and Table 1). The binding mode is the same
as that of the sulfate complex of para-nitrophenyl-appended
À
Chem. Eur. J. 2013, 19, 9034 – 9041
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
9035