Cation recognition and pseudorotaxane formation of tris-dipyrrin BF 2 macrocycles

Article abstract of DOI:10.1039/c0cc01717e

Macrocyclic planar tris-dipyrrin BF₂ complexes exhibit strong alkali-metal recognition and pseudorotaxane formation ability with a secondary ammonium salt through BF₂-cation interactions.

Full text of DOI:10.1039/c0cc01717e

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Cation recognition and pseudorotaxane formation of tris-dipyrrin BF2
macrocyclesw
Naoya Sakamoto, Chusaku Ikeda and Tatsuya Nabeshima*
Received 3rd June 2010, Accepted 29th July 2010
DOI: 10.1039/c0cc01717e
Macrocyclic planar tris-dipyrrin BF2 complexes exhibit strong
alkali-metal recognition and pseudorotaxane formation
ability with a secondary ammonium salt through BF2–cation
interactions.
Fluorinated organic compounds exhibit unique non-covalent
interactions1 such as CFꢀ ꢀ ꢀH-X (X: C, N, O),2 CFꢀ ꢀ ꢀFC,3
CFꢀ ꢀ ꢀM+,4 and perfluorobenzeneꢀ ꢀ ꢀbenzene,5 which are very
important in medicinal1a and separation chemistry6 as well.
planar triangular structure with a 4 A cavity.10z The electro-
Although fluorine-assisted interactions of a B–F moiety in
static potential surfaces (EPSs) based on the structure indicated
neutral organic compounds would be effective due to significant
the fluorine atoms are negatively charged (Fig. 1). The X-ray
difference in electronegativity between boron and fluorine
structure of 2 showed that the phenylene rings are tilted with
atoms, such interactions have not been employed for molecular
an average dihedral angle of 521. The DFT calculations
showed the structure with the less tilted phenylene rings in 1
recognition. Our DFT calculation suggests that the BF2
(321)11 compared to 2 (551) because the methoxy groups in 2
moiety is electronically delocalized along the B–F bond to
make the fluorine partially negative7 due to the large electro-
face outward to decrease the steric repulsion (see ESIw). The
CPK model also supported the finding that the OMe moieties
negativity difference and small atomic radius of the fluorine
(see ESIw). Thus, the fluorine atom should interact with metal
cations or cationic molecules. We conceived that 4,4-difluoro-
4-bora-3a,4a-diaza-s-indecene (BODIPY)8 is an ideal candi-
date for a host building block to provide efficient B–Fꢀ ꢀ ꢀM+
interactions because (1) the BF2 moiety can work as either a
monodentate or bidentate chelating recognition site, (2) the
interaction can be followed by the change in the optical
properties, and (3) the BODIPY framework can be easily
modified to allow the interaction with the BF2 site to be fine
tuned. Neverthless, the BF2 moiety of BODIPY has not been
used for supramolecular complexation.
in 2 are not oriented inward. However, the methoxy protons in
the 1H NMR spectrum of 2 appeared as a sharp singlet at
3.80 ppm, indicating that the phenylene rings flip fast on the
1H NMR time scale.
UV-vis spectroscopy confirmed strong interactions of 1 and
2 with cationic guests. We employed a superweak and hydro-
phobic base, tetrakis(3,5-bis(trifluoromethyl)phenyl)borate
(TFPB), as an counter anion of the guests because these salts
are soluble in usual organic solvents. The addition of caesium
tetrakis(3,5-bis(trifluoromethyl)phenyl)borate (CsTFPB) to 1
and 2 in CHCl3–CH3OH (10 : 1) decreased the absorption at
552 and 531 nm with a concomitant increase in the absorption
at 540 and 522 nm (Fig. 2). UV-vis titration suggested the
formation of a 1 : 1 complex. The log Ka (Mꢁ1) value for Cs+
was determined to be 4.9 ꢂ 0.1 by a nonlinear-least-squares
regression. 1 and 2 showed size-selective binding, and the
largest Ka values were observed in Cs+ (Table 1) probably
because the cavity sizes (ca. 4 A) of 1 and 2 are close to the
Herein we designed macrocyclic planar tris-dipyrrin BF2
complexes 1 and 2 because (1) the macrocyclic framework
should possess a well preorganized binding site with the six
fluorine atoms facing inward, (2) rotation of the p-phenylene
moieties should afford suitable flexibility required for guest
recognition, and (3) guest recognition in the cavity may affect
the optical properties of the macrocycle. Additionally, we
report cation recognition by the dipyrrin BF2 complexes and
pseudorotaxane formation with a secondary ammonium salt
through the BF2ꢀ ꢀ ꢀH hydrogen bonds in a bifurcated manner.
Treatment of the corresponding dipyrrin macrocycle9 with
BF3ꢀOEt2 in the presence of ethyl diisopropyl amine gave
tris-dipyrrin BF2 complexes 1 and 2 in 29% and 99% yields,
respectively. X-ray crystallographic analysis revealed 2 has a
Graduate School of Pure and Applied Sciences,
University of Tsukuba, Tsukuba, Ibaraki, 305-8571, Japan.
E-mail: nabesima@chem.tsukuba.ac.jp; Fax: (+81)29-853-4507;
Tel: (+81)29-853-4507
w Electronic supplementary information (ESI) available: Experimental
details and crystallographic data. CCDC 680144 and 767041. For ESI
and crystallographic data in CIF or other electronic format see DOI:
10.1039/c0cc01717e
Fig. 1 (a), (b) Molecular structure of 2. Solvent molecules are
omitted for clarity. Color: C, sky-blue; H, white; B, orange; F, green;
N, blue; O, red. (c) Electrostatic potential surfaces (EPSs) of 2
generated at the B3LYP/6-31G* level.
c
6732 Chem. Commun., 2010, 46, 6732–6734
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Cs+ ionic diameter (3.34 A).12 2 had a higher Cs+ affinity than
1 (Table 1). The stronger binding of 2 may be ascribed to the
larger tilting angle compared to the phenylene rings of 1.
Interaction of the counter anion TFPB with the hosts were
negligibly small because n-Bu4NꢀTFPB caused no change in
the absorption spectra of the hosts due to superweak basicity
of TFPB.
Table 1 log Ka values of 1 and 2
Na+
K+
Rb+
Cs+
3
1
2
2.3 ꢂ 0.1
1.9 ꢂ 0.1
3.6 ꢂ 0.1
4.04 ꢂ 0.07
4.6 ꢂ 0.1
4.9 ꢂ 0.1
5.5 ꢂ 0.1
3.1 ꢂ 0.2
4.1 ꢂ 0.1
) determined from the UV-vis spectral changes in
4.06 ꢂ 0.03
Ka (in Mꢁ1
CHCl3–CH3OH (10 : 1), assuming a 1 : 1 stoichiometry.
1H, 11B, and 19F NMR spectra confirmed the formation of
1
the metal complexes. The H NMR spectra of 1ꢀCs+ and 2ꢀ
Cs+ in CDCl3–CD3OD (10 : 1) showed significant upfield
shifts (Dd = ꢁ0.24 and ꢁ0.27 ppm, respectively) of the
p-phenylene ring proton resonances compared to those of 1
and 2.13 Notably, the change in the methoxy proton resonance
of 2 was negligible upon the addition of Cs+, indicating that
the methoxy oxygen atoms of 2 do not contribute to cation
binding. The 19F NMR spectra of 1 showed a characteristic
downfield shift (Dd = 2.1 ppm),14 which is ascribed to the
BFꢀ ꢀ ꢀCs+ interaction. The 11B signal shifted slightly upfield
upon Cs+ complexation (Dd = ꢁ0.4 ppm).
Fig. 3 (a) Ball-and-stick (side view) and (b) partial structure of the
binding site of pseudo[2]rotaxane 2ꢀ3. Phenyl groups and most of
hydrogen atoms in (a), counter anion, and solvent molecule are
omitted for clarity. Color: C, sky-blue (for 2) and pink (for 3); H,
white; B, orange; F, green; N, blue; O, red.
interact with 4 because spectral changes were not observed.
This observation well rationalizes the proposed pseudorotaxane-
type binding mode by the cavity because the phenyl ring of 4
(ca. 7 A) is even larger than the cavity diameters of 1 and 2. All
these results are consistent with pseudorotaxane formation of
1 and 2 with 3 in solution.
Next we examined the interactions of 1 and 2 with
di-n-butylammonium (3) and dibenzylammonium hexafluoro-
phosphates (4). The 1 : 1 complex formation was ascertained
1
1
by H NMR titration in CDCl3 and Job plots. The H NMR
spectra of the mixtures with different mole ratios of 2 and 3
showed the time-averaged signals at room temperature.
However, at 218 K, complexation and decomplexation were
sufficiently slow on the 1H NMR time scale because a 1 : 2
mixture of 2 and 3 gave two independent sets of signals for the
2ꢀ3 complex and free 3. Furthermore, NOE correlations were
observed at 218 K between the N+–CH2 protons of 3 and the
phenylene protons of 2. The MALDI-TOF mass spectra
showed a strong peak at m/z 1336.5, which is assigned to the
2ꢀ3 ion. Additionally, complexation of 2 with 3 was monitored
by the UV-vis spectral changes with a hypsochromic shift
(11 nm). Spectroscopic titration indicated 1 and 2 have strong
affinities to 3. In addition, 2 showed an emission increase upon
binding to 3 in a significant on-off fashion (2: lmax 670 nm,
F o 1%, 2ꢀ3: 700 nm, 10%). In sharp contrast, 2 did not
X-ray crystallographic analysis clarified the pseudorotaxane
structure of 2ꢀ3 (Fig. 3).z The ammonium ion 3 was threaded
through the cavity of 2 as seen in reported pseudorotaxane.15
The ammonium nitrogen N(7) was located near B(1) and B(2).
The estimated distances between the NH proton, H(71) or
H(70), and the fluorine atoms F(1), F(2), or F(3), F(4)
were significantly less than 2.9 A (Fig. 3b).1b Therefore, the
pseudorotaxane structure is maintained by the non-classical
chelating hydrogen bonds, that is, bifurcated BF2ꢀ ꢀ ꢀH–N
interactions.
In conclusion, neutral macrocycles 1, 2, which contain the
BF2 moieties, strongly bind alkali metal ions as well as organic
cationic guest 3 through novel BF2–cation interactions. This
new interaction should be applicable in a variety of molecular
recognition systems, including rotaxanes with a highly functional
‘‘rota’’ moiety based on the conjugated macrocycle with
intriguing optical properties.
We thank Prof. T. Akasaka, Univ. of Tsukuba, for the
MALDI-TOF mass measurements. This research was financially
supported by Grants-in-Aid for Scientific Research from the
Ministry of Education, Culture, Sports, Science and Technology
of Japan.
Notes and references
z Crystallographic data for 2ꢀ3CHCl3: C72H54B3Cl9F6N6O6,
Mr = 1564.69, monoclinic, space group P21/n (no. 14), a = 16.94(3)
A, b = 20.22(4) A, c = 20.85(5) A, b = 95.87(8)1, V = 7103(26) A3,
Z = 4, rc = 1.463 gcmꢁ3, m = 0.428 mmꢁ1, MoKa radiation
(l = 0.71075 A), T = 120(2) K, 2ymax = 501, 52 087 reflections
measured, 12 381 unique (Rint = 0.1355), R1 = 0.0845 (I 4 2s(I)),
wR2 = 0.2251 (all data), GOF (F2) = 1.002, Drmax/Drmin = 0.515/
ꢁ0.446 eAꢁ3. The structure was solved by direct methods. CCDC
Fig. 2 UV-vis spectral changes of 1 upon the addition of CsTFPB.
Inset shows the binding isotherm recorded at 552 nm with the
calculated 1 : 1 binding curve (solid line): [1] = 4.0 mM, 0 r [Cs+]/
[1] r 100, CHCl3–CH3OH (10 : 1).
c
This journal is The Royal Society of Chemistry 2010
Chem. Commun., 2010, 46, 6732–6734 6733

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680144 contains the supplementary crystallographic 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.16
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Crystallographic data for 2ꢀ3ꢀPF6ꢀC7H8: C84H79B3F12N7O6P, Mr =
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GOF (F2) = 1.030, Drmax/Drmin = 0.458/–0.514 eAꢁ3. The structure
was solved by direct methods. CCDC 767041 contains the supplementary
crystallographic 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.16
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6734 Chem. Commun., 2010, 46, 6732–6734
This journal is The Royal Society of Chemistry 2010