Ion-pair triple helicates and mesocates self-assembled from ditopic 2,2′-bipyridine-bis(urea) ligands and Ni(ii) or Fe(ii) sulfate salts

Article abstract of DOI:10.1039/c2cc33062h

NiSO₄ and FeSO₄ self-assemble with heteroditopic ligands (L) comprising 2,2′-bipyridine and o-phenylene-(bis)urea cation- and anion-binding sites, respectively, into [ML₃SO₄] (M = Ni²⁺, Fe²⁺

Full text of DOI:10.1039/c2cc33062h

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Chemical Communications
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Volume 48 | Number 60 | 4 August 2012 | Pages 7421–7528
ISSN 1359-7345
COMMUNICATION
Radu Custelcean
Ion-pair triple helicates and mesocates self-assembled from ditopic 2,2′-bipyridine-bis(urea) ligands
and Ni(II) or Fe(II) sulfate salts

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COMMUNICATION
Ion-pair triple helicates and mesocates self-assembled from ditopic
2,20-bipyridine-bis(urea) ligands and Ni(II) or Fe(II) sulfate saltsw
Radu Custelcean,* Peter V. Bonnesen, Benjamin D. Roach and Nathan C. Duncan
Received 28th April 2012, Accepted 16th May 2012
DOI: 10.1039/c2cc33062h
NiSO4 and FeSO4 self-assemble with heteroditopic ligands (L)
comprising 2,20-bipyridine and o-phenylene-(bis)urea cation- and
anion-binding sites, respectively, into [ML3SO4] (M = Ni2+, Fe2+
)
triple-stranded ion-pair helicates and mesocates.
Triple-stranded helical coordination complexes of M2L3
stoichiometry represent prototypical metallosupramolecular
architectures typically assembled from bis-bidentate ligands
wrapping around two octahedral metal cations.1 Depending
on the stereochemistry (L or D) of the two ensuing chiral metal
centres, helicates (LL or DD) or mesocates (LD) can be
formed. While such metal-directed supramolecular structures
continue to generate much interest, a new paradigm has
recently been introduced, where anion coordination chemistry2
is employed for the self-assembly of helices. Such anion-based
helical structures include foldamers and double- and triple-
stranded helicates.3 Similarly to metal coordination, anion
coordination can induce and control helicity. However,
compared to metals, the coordination numbers and geometries
of anions are not so well defined, which can often undermine
the structural design and predictability of anion-directed
architectures.
Scheme 1 Self-assembly of ion-pair helicates (H) and mesocates (M).
the 3 : 1 metal binding by 2,20-bipyridine (bpy) in classical
M2L3 helicates.3i We reasoned that by combining the charge
2ꢀ
complementary [M(bpy)3]2+ and SO4(pbu)3
metal- and
anion-coordinating motifs, respectively, triple-stranded ion-
pair helicates and mesocates could be generated. Here we
demonstrate this concept with the self-assembly of helicates
H1,2 and mesocates M1,2 from ditopic ligands bpy-CH2-pbu-R
(R = p-Ph-NO2 (L1), p-Ph-tBu (L2)), and MSO4 (M = Ni, Fe)
(Scheme 1).8
The ditopic ligands L1 and L2 were synthesized in three
steps from 5-aminomethyl-2,20-bipyridine, with 62% and 55%
overall yields, respectively (see ESIw for details). A solution
containing 3 eq. L1 and 1 eq. NiSO4 in DMF/H2O/CH3CN
yielded crystals of [Ni(L1)3SO4](DMF)2(CH3CN) (H1a), as
determined by single-crystal X-ray diffraction. The crystal
structure of H1a showed the formation of an ion-pair triple
helicate complex, with the Ni2+ cation and sulfate anion
octahedrally coordinated by 3 bpy and 3 pbu chelating groups,
respectively (Fig. 1a). The intra-helicate Ni to S distance
measures 7.08(1) A. Both the metal and sulfate centres within
the same helix adopt the same chirality (LL or DD), as
required for a helicate structure, though overall the crystal is
One anion coordination motif that has been proven rela-
nꢀ
tively reliable consists of sulfate, or other XO4 (n = 2,3)
tetrahedral oxoanions, coordinated by six chelating urea
hydrogen bond donors arranged octahedrally around the
anion. This binding motif provides 12 complementary hydrogen
bonds to the anion, which was first predicted computationally
by Hay et al.,4 and later confirmed in X-ray structural studies
by our group and others.5 As recently recognized by Wu
2ꢀ
%
racemic (P1 space group), containing both the M and P
et al.,6 a coordination number of 12 for the SO4
and
3ꢀ
helicates. The sulfate accepts 12 hydrogen bonds from the
six urea groups (d(Hꢁ ꢁ ꢁO) = 1.99(1)ꢀ2.37(1) A, av. 2.21 A;
oN–Hꢁ ꢁ ꢁO = 142(1)ꢀ171(1)1, av. 153(1)1), though the binding
geometry is distorted from the ideal coordination mode
requiring each urea to chelate an edge of the SO42ꢀ tetrahedron.
Instead, five of the six urea groups each bind an O vertex of the
anion. This asymmetrical binding mode is a result of the
inverted sulfate orientation inside the helicate, pointing an
S–O bond to the Ni centre instead of an SO3 triangular face, as
needed for idealized binding.5f
PO4 anions can be achieved with oligo-urea ligands con-
taining the o-phenylene-bis(urea) (pbu) motif, which had been
previously identified by Hay using structure-based design.7
The extension of this concept by Wu’s group led to an elegant
example of anion-coordinated triple helicate self-assembled
from a bis(pbu) ligand and phosphate anions, where the
3ꢀ
binding of each PO4 by three chelating pbu groups mimics
Oak Ridge National Laboratory, Oak Ridge, TN 37831-6119, USA.
E-mail: custelceanr@ornl.gov
w Electronic supplementary information (ESI) available: Synthetic
procedures, NMR, DOSY, ESI-MS results, and X-ray crystallo-
graphic details. CCDC 879402–879406. See DOI: 10.1039/c2cc33062h
Crystallization of L1 with NiSO4 in a 3 : 1 molar ratio, from a
DMF/H2O/1,4-dioxane solution yielded crystals of [Ni(L1)3SO4]-
(DMF)2(1,4-dioxane)1.5 (H1b). Single crystal X-ray diffraction
c
7438 Chem. Commun., 2012, 48, 7438–7440
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Fig. 2 Crystal structure of M1a. Left: mesocate structure; Right:
sulfate-binding site. Included solvent molecules are not shown.
and 2.00(1)–2.40(1) A (av. 2.14 A), and the oN–Hꢁ ꢁ ꢁO angles
are 157(1)–172(1)1 (av. 1601) and 157(1)–172(1)1 (av. 1631),
respectively.
The self-assembly of the H2 and M2 complexes was
also studied in solution by electrospray ionization mass
spectrometry (ESI-MS) and NMR spectroscopy. The ESI-MS
analysis indicated that both H2 and M2 persist in solution, as
evidenced by the peaks corresponding to [H2H]+ and [M2H]+,
for which the observed isotopic distributions match the
predicted ones (see ESIw).9
Fig. 1 Crystal structures of H1a (a) and H1b (b). Top: helicate
structures; Bottom: sulfate-binding sites. Included solvent molecules
are not shown.
revealed a triple-stranded ion-pair helicate isomeric with H1a
(Fig. 1b), with a slightly longer Ni to S intramolecular
separation of 7.21(1) A. The most significant difference from
the previous structure was observed in the orientation of the
sulfate anion, which points an SO3 triangular face toward the
Ni centre. This leads to more favourable hydrogen bonding
interactions with the urea groups, with observed NHꢁ ꢁ ꢁO
contacts of 1.94(1)–2.40(1) A (av. 2.13 A). However, one of
the terminal urea groups is slightly twisted away, hydrogen
bonding to a DMF solvent molecule, which results in only
11 hydrogen bonds to sulfate. A similar helicate structure was
observed for the crystals of the Fe2+ analogue (H2a) crystal-
lized as [Fe(L1)3SO4](DMF)5.5(H2O)1.5 from DMF/H2O/
CH3CN. In this case, though, the hydrogen bonding to sulfate
is disrupted by water solvent molecules acting as both hydrogen
bond acceptors from one of the terminal urea groups, and
hydrogen bond donors to the sulfate anion (see ESIw).
In contrast to L1, L2 formed ion-pair triple mesocates with
either NiSO4 or FeSO4 (3 : 1 molar ratio) when crystallized
from DMF/H2O/CH3CN or DMF/H2O/dioxane. The resulting
crystalline complexes [Ni(L2)3SO4](DMF)2(CH3CN) (M1a)
and [Fe(L2)3SO4](DMF)2(dioxane)2 (M2a) have very similar
structures, with intramolecular metal to S separations of
7.40(1) and 7.30(1) A, respectively, and the sulfate oriented
with an SO3 triangular face toward the metal centre (Fig. 2).
The metal and sulfate centres within the same complex adopt
opposite chirality (LD), with both M and P mesocates coexisting
in racemic crystals. The sulfate hydrogen bonding is more
symmetrical than in the helicate structures, with each of the
6 ureas chelating an edge of the anion, resulting in a close to
optimal 12-coordination environment. The observed NHꢁ ꢁ ꢁO
contacts for M1a and M2a are 2.00(1)–2.60(1) A (av. 2.16 A)
Further supporting evidence for the formation of H2 and
M2 was provided by 1H NMR in DMF-d7/D2O (95 : 5)
(Fig. 3). Addition of a slight excess of FeSO4 in D2O to
a solution of L1 or L2 in DMF-d7 led to an immediate
appearance of an intense red colour, characteristic to
Fe(bpy)3 formation, and complete disappearance of the reso-
nances corresponding to the free ligands. The initial NMR
spectra of the resulting solutions displayed broad and poorly
defined peaks, suggesting the formation of either low-symmetry
structures or complex mixtures of species. Nevertheless, after
10–12 min, the formation of highly symmetrical complexes
became evident, which accounted for about 45–50% of the
initial ligands. Of particular diagnostic value was the t-Bu
peak of L2, initially observed at 1.29 ppm, which converted
into four peaks after 12 min: a major peak at 1.15 ppm
corresponding to the symmetrical complex, and three smaller
peaks of approximately equal intensities at 1.27, 1.24, and
1.20, apparently corresponding to a low-symmetry complex.
The exact nature of this complex is difficult to determine,
though we note that different types of isomers are possible for
these structures, involving the M(bpy)3 configuration (fac vs. mer),
the orientation of sulfate (vide supra), or the conformation of
the ligand. The relative concentration of the high-symmetry
complex continued to grow, until it became predominant
(495%) after a few hours. Notably, the ꢀCH2– resonances
of L1 and L2, initially found at 4.53 ppm, split into AB
patterns consisting of two doublets centred at 4.52 and 3.50 ppm,
and 4.52 and 3.40 ppm, respectively. These changes are
indicative of the two methylene protons becoming diastereo-
topic, as expected in the formation of C3-symmetrical com-
plexes. Three of the four urea NH resonances of L1 and L2
c
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Chem. Commun., 2012, 48, 7438–7440 7439

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new prospects for ion-pair separations, as metal sulfates could
potentially be extracted selectively from aqueous solutions by
more lipophilic derivatives of L1 and L2.
This research was sponsored by the Division of Chemical
Sciences, Geosciences, and Biosciences, Office of Basic Energy
Sciences, U.S. Department of Energy.
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Fig. 3 Monitoring the self-assembly of M2 by 1H NMR. (a) Aromatic
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after 12 min; Bottom: final M2. See ESIw for the similar analysis of H2.
shifted downfield by 0.67, 0.08, 1.11 ppm and 0.73, 0.25,
1.08 ppm, respectively, suggesting hydrogen bonding to sulfate.
Taken together, all these observations are consistent with the
formation of C3-symmetrical ion-pair helicates or mesocates
in solution, though the existing data cannot distinguish between
the two types of structures. Furthermore, 2D diffusion order
spectroscopy (DOSY) showed that, for either solution, all the
peaks belonged to a single supramolecular aggregate, and
yielded diffusion coefficients (D) of 2.35(11) ꢂ 10ꢀ10 and
2.26(12) ꢂ 10ꢀ10 m2 sꢀ1, for the two complexes. For comparison,
the measured D values for L1 and L2 were 3.87(3) ꢂ 10ꢀ10 and
4.14(3) ꢂ 10ꢀ10 m2 sꢀ1, respectively.
In conclusion, a new class of ion-pair triple helicates and
mesocates was self-assembled from heteroditopic ligands L1
and L2, and NiSO4 or FeSO4, which involved octahedral
coordination of metals and sulfate by bpy and pbu, respec-
tively. While this study provided the preliminary proof of
concept, future work will address the thermodynamics and
kinetics of helicate/mesocate self-assembly, the factors deter-
mining the formation of helicates vs. mesocates (e.g., the role
played by the R group), and the metal and anion specificities in
the self-assembly of these structures. This study also opens
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9 The analogous H1 and M1 complexes could not be studied by NMR
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[H1H]+ and [M1H]+ peaks could not be detected by ESI-MS.
c
7440 Chem. Commun., 2012, 48, 7438–7440
This journal is The Royal Society of Chemistry 2012