A new twist in anion binding: Metallo-helicate hosts for anionic guests

Article abstract of DOI:10.1039/b514580e

The condensation of 5-chlorocarbonyl-2,2′-bipyridine with a variety of rigid aromatic diamines, L, gave a series of new bisamido-2,2′- bipyridine based ligands (L = 4,4′-methylenediamine, L1; L = 1,1-bis(4-aminophenyl)cyclohexane, L2; L = 1,1-bis(4-amino-

Full text of DOI:10.1039/b514580e

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www.rsc.org/dalton | Dalton Transactions
A new twist in anion binding: metallo-helicate hosts for anionic guests
Sandrine Goetz and Paul E. Kruger*
Received 13th October 2005, Accepted 13th December 2005
First published as an Advance Article on the web 3rd January 2006
DOI: 10.1039/b514580e
The condensation of 5-chlorocarbonyl-2,2ꢀ-bipyridine with a variety of rigid aromatic diamines, L, gave
a series of new bisamido-2,2ꢀ-bipyridine based ligands (L = 4,4ꢀ-methylenediamine, L1; L = 1,1-bis(4-
aminophenyl)cyclohexane, L2; L = 1,1-bis(4-amino-3,5-dimethylphenyl)cyclohexane, L3) capable of
forming dinuclear triple helicate complexes on coordination to Fe(II). The reaction of various Fe(II)
salts with L1–L3 gave: {[Fe2(L1)3](BF4)4, 1; [Fe2(L1)3](ClO4)4, 2; [Fe2(L1)3]Cl4, 3; [Fe2(L1)3](SO4)2, 4;
[Fe2(L2)3](BF4)4, 5; [Fe2(L2)3]Cl4, 6; [Fe2(L3)3](BF4)4; 7; [Fe2(L3)3]Cl4, 8; and [Fe2(L3)3](SO4)2, 9, as
determined by UV-Vis, IR and 1H NMR spectroscopy, electrospray mass spectrometry (ESMS) and
elemental analyses. A UV-Vis complexometric titration experiment between L3 and Fe(BF4)2
established conclusively a [Fe2(L3)3]4+ product species. 1H NMR spectroscopy showed that the
complexes exist as both rac-(helical) and meso-(non-helical) isomers in DMSO-d6 solution at 298 K.
L1–L3 were designed such that following complexation, six amide hydrogen atoms would line an
inter-strand intrahelical cavity of sufficient size to facilitate the binding of guest species within it.
Indeed, ESMS studies showed characteristic peaks typical of complex–anion species in solution.
Furthermore, 1H NMR titration experiments showed that anions bind within the intrahelical cavity as
titration of 1, 5 and 7 with Bu4NCl showed significant downfield shifts in the amide and bipyridyl H6
proton resonances to yield a species of 1 : 2 host to guest stoichiometry. Moreover, addition of Bu4NCl
to 1, 5 and 7 shifted the rac-/meso-species distribution from 1 : 2 in favour of the meso- to 100% in
favour of the rac-isomer, showing that Cl− ions favour the formation of the triple helicate species in
DMSO-d6 solution.
tively. Moreover, Beer et al. have shown that [Ru(5,5ꢀ-diamido-
Introduction
2,2ꢀ-bipyridine)3]2+ and related species are capable of binding
anions.8 However, these systems all employ exo-receptors and,
to the best of our knowledge, no metallo-helicate system has
employed inter-strand (intrahelical) receptors to specifically bind
anions.
There is considerable current interest in the development of
molecular and supramolecular systems that have the ability to
bind, identify, and signal the presence of negatively charged
ions.1 The motivation behind such studies is the recognition that
anions enjoy an important role in biology, medicine, and the
environment.2 In the field of supramolecular chemistry, anion
receptors typically rely on hydrogen bonding as the dominant
force driving their interaction with anions. Ideally, a number of
judiciously placed hydrogen bond donors will maximise binding
to a specific anionic species. Numerous studies involving the self-
assembly of various metallo-helicates have been conducted over
the last 20 years.3 Whilst anions may template their synthesis and
in some instances have been structurally identified as being bound
by these species,4 they have not, to the best of our knowledge, been
specifically designed to act as anion receptors. Nevertheless, Rice
and co-workers have recently reported dinuclear triple helicate
complexes, [M2L3], of pyridyl–thiazole ligands appended periph-
erally with amide functionalities that, upon metal coordination,
form ‘pockets’ capable of binding anions at each end of the
helicate.5 Similarly, Janiaket al.6 and Williams et al.7 have observed
anion binding behaviour within ‘clefts’ formed by mononuclear
[FeL3]2+ complexes where L is either 5,5ꢀ-diamido-2,2ꢀ-bipyridine
or 5,5ꢀ-amino-acido-2,2ꢀ-bipyridine functionalised ligands, respec-
Consequently, we designed three new amide-based bisbipyridine
ligands, L1–L3, with the intention of forming [M2L3] complexes
upon their coordination to Fe(II) and, in doing so, generating an
inter-strand (intrahelical) cavity of sufficient size to bind anionic
guests, Fig. 1. The [M2L3] complexes pre-organise six amide
groups around a cavity that is reminiscent of the hexa-amide
cryptand ligands exploited with great effect in anion binding
by Bowman-James et al.9 The amide groups also provide a
convenient 1H NMR spectroscopic handle by which to probe
the contents of the cavity. Furthermore, employment of Fe(II) as
the template in the formation of [Fe2L3]4+ complexes introduces a
positive charge into the ligand scaffold, increasing the electrostatic
contribution to prospective anion binding. Low spin, diamagnetic
Fe(II) also provides an effective means of introducing helical
chirality into the host molecule without greatly perturbing any 1H
NMR spectroscopic investigations, and serves as a useful UV-Vis
marker for complex formation as bipyridine–Fe(II) compounds
are intensely coloured. We report here the synthesis of L1–L3
and detail the formation of [Fe2L3]4+ host complexes by UV-
Vis, IR and 1H NMR spectroscopy and ESMS and include
details on their ability to bind anionic guests within their
cavities.
Centre for Synthesis and Chemical Biology, School of Chemistry, University
of Dublin, Trinity College, Dublin 2, Ireland. E-mail: paul.kruger@tcd.ie
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Synthesis of N,Nꢀ-bis(2,2ꢀ-bipyridyl-5-yl)carbonyl-4,4ꢀ-methyl-
enediamine (L1). A CH2Cl2 solution containing 4,4ꢀ-methyl-
enediamine (0.093 g, 0.45 mmol) and 0.9 ml triethylamine
(6.6 mmol) was added dropwise to a CH2Cl2 solution of 5-
chlorocarbonyl-2,2ꢀ-bipyridine (0.2 g, 1.0 mmol) at room tem-
perature. The reaction mixture was heated at reflux for 15 h
and filtered to retrieve a brown solid that was washed with
CH2Cl2 and hot EtOH. Yield 71%. Found C, 67.0; H, 4.3; N,
12.7%. C35H26N6O2·CH2Cl2 requires C, 66.8; H, 4.4, N, 13.0%. dH
(400 MHz; DMSO-d6): 3.93 (2H, s, CH2), 7.25 (4H, d, J = 8.2 Hz,
ꢀ
H8/8 ), 7.52 (2H, ddd, J = 7.5, 4.9 and 1.4 Hz, H5ꢀ ), 7.73 (4H, d,
ꢀ
J = 8.2 Hz, H7/7 ), 8.00 (2H, ddd, J = 7.6, 7.5, 1.4 Hz, H4ꢀ ), 8.50
ꢀ
(6H, m, H3/3 /4), 8.74 (2H, d, J = 4.1 Hz, H6ꢀ ), 9.20 (2H, s, H6),
10.51 (2H, s, NH). mmax/cm−1 (KBr): 3333 (br, N–H), 1528 (N–H),
=
1646 (C O).
Synthesis of N,Nꢀ-bis(2,2ꢀ-bipyridyl-5-yl)carbonyl-1,1-bis(4-
aminophenyl)cyclohexane (L2). L2 was prepared by an anal-
ogous procedure to L1 by employing 1,1-bis(4-aminophenyl)-
cyclohexane (0.121 g, 0.45 mmol) as the diamine precursor and
filtered after 15 h reflux. The filtrate was subsequently washed
with 20 mL of water (× 4) and dried over anhydrous MgSO4.
The residue obtained after the removal of solvent under reduced
pressure was taken into MeCN, sonicated and filtered to give
an off-white powder in 53% yield. Found C, 74.9; H, 5.1; N,
14.3%. C40H34N6O2·MeCN requires C, 75.1; H, 5.5; N, 14.6%.
dH (400 MHz; DMSO-d6): 1.49 (6H, br, cyclohex-H), 2.29 (4H,
Fig. 1 Structure of the ligands L1–L3 from the current study with the
labelling scheme used for 1H NMR spectroscopy shown on L1 (top), and
a molecular model of a prospective [Fe2(L1)3]4+ complex showing the six
amide hydrogen atoms lining the intrahelical cavity (bottom).
ꢀ
br, cyclohex-H), 7.33 (4H, d, J = 8.5 Hz, H8/8 ), 7.53 (2H, ddd, J =
ꢀ
7.5, 4.9 and 1.5 Hz, H5ꢀ ), 7.70 (4H, d, J = 8.5 Hz, H7/7 ), 8.00 (2H,
ꢀ
ddd, J = 7.6, 7.5, 1.5 Hz, H4ꢀ ), 8.48 (6H, m, H3/3 /4), 8.75 (2H, d,
J = 4.5 Hz, H6ꢀ ), 9.20 (2H, s, H6), 10.46 (2H, s, NH). mmax/cm−1
=
(KBr): 3314 (br, N–H), 1522 (N–H), 1648 (C O).
Experimental
Synthesis of N,Nꢀ-bis(2,2ꢀ-bipyridyl-5-yl)carbonyl-1,1-bis(4-
amino-3,5-di-methylphenyl)cyclohexane (L3). L3 was prepared
by an analogous procedure to L1 by employing 1,1-bis(4-amino-
3,5-dimethylphenyl)cyclohexane (0.146 g, 0.45 mmol) as the
diamine precursor and filtered after 15 h reflux. The filtrate was
subsequently washed with 20 mL water (× 4) and dried over
anhydrous MgSO4. The residue obtained after the removal of
solvent under reduced pressure was recrystallised from MeOH
to give a cream crystalline solid in 50% yield. Found C, 75.0; H,
6.2; N, 11.5% C44H42N6O2.CH3OH requires C, 75.2, H, 6.4; N,
11.7%. dH (400 MHz; DMSO-d6): 1.50 (6H, br, cyclohex-H), 2.19
Materials and methods
5-Methyl-2,2ꢀ-bipyridine was prepared by the traditional Kro¨hnke
procedure from 2-acetylpyridine and methacrolein following
an adapted procedure according to Balzani et al.10 The
method of Fletcher et al. was followed in the synthesis
of 2,2ꢀ-bipyridine-5-carboxylic acid and 5-chlorocarbonyl-2,2ꢀ-
bipyridine was prepared from it in situ following reaction
with an excess of thionyl chloride under an Ar atmosphere.11
1,1-Bis(4-amino-3,5-dimethylphenyl)cyclohexane and 1,1-bis(4-
aminophenyl)cyclohexane were prepared following the method
of Hunter.12 The syntheses of L1–L3 were performed in dry
CH2Cl2 solution under an Ar atmosphere. Standard chemicals
and solvents were of reagent grade and purchased from Aldrich,
Merck, Riedel de Hae¨n, Fluka or local solvent suppliers and used
as received. Dichloromethane was dried before use by distillation
from calcium hydride. Chemical analyses were performed at the
Microanalytical Laboratories, University College Dublin, Ireland.
1H NMR spectra were acquired on a Bruker DPX 400 machine and
referenced to the residual proton shifts in the internal deuterated
solvent. UV-Vis spectra were carried out using a Cary 300 UV-
Vis spectrophotometer in spectroscopic grade CH3CN or MeOH
operating between 200–800 nm. IR spectra were recorded as KBr
pellets on a Mattson Genesis II FTIR in the 4000–400 cm−1 region.
Mass spectra were recorded on a Micromass LCT electrospray
instrument.
ꢀ
(12H, s, Me), 2.30 (4H, br, cyclohex-H), 7.12 (4H, s, H8/8 ), 7.53
(2H, ddd, J = 7.5, 4.5 and 1.5 Hz, H5ꢀ ), 8.00 (2H, ddd, J = 7.7,
ꢀ
7.5, 1.5 Hz, H4ꢀ ), 8.50 (6H, m, H3/3 /4), 8.75 (2H, d, J = 4.5 Hz,
H6ꢀ ), 9.24 (2H, s, H6), 9.92 (2H, s, NH). mmax/cm−1 (KBr): 3262 (br,
=
N–H), 1510 (N–H), 1649 (C O).
Synthesis of [Fe2(L1)3](BF4)4, 1, [Fe2(L1)3](ClO4)4, 2, [Fe2(L1)3]Cl4,
3, [Fe2(L2)3](BF4)4, 5 and [Fe2(L2)3]Cl4, 6. To a stirred suspension
of either L1 (0.1 g, 0.18 mmol) or L2 (0.1 g, 0.16 mmol) in MeOH
(80 mL) a methanolic solution of Fe(X)2·6H2O (0.12 mmol)
[X = BF4− 1 and 5; ClO4− 2; Cl− 3 and 6] was added dropwise. The
resultant deep purple mixture was heated at reflux temperature
for 22 h and allowed to cool. A deep purple precipitate was
filtered off directly for 1–3, washed with MeOH and ether and
recrystallised from MeOH or MeCN. For 5 and 6 the solution
was filtered and the deep purple filtrate evaporated to dryness
1278 | Dalton Trans., 2006, 1277–1284
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under reduced pressure. The residue was dissolved in MeCN from
which a purple microcrystalline powder was retrieved by filtration
after a few days following the slow diffusion of ether into the
MeCN solution.
Analytical data for 6. Yield 53%. Found C, 67.0; H, 4.9; N,
13.0%. [Fe2(L2)3]Cl4·3MeCN requires C, 67.5; H, 4.99; N, 13.1%.
dH (400 MHz; DMSO-d6): 1.41 (18H, br, cyclohex-H), 2.30 (12H,
ꢀ
br, cyclohex-H), 7.20 (12H, d, J = 7.5 Hz, rac-H8/8 ), 7.35 (6H, m,
ꢀ
ꢀ
rac-H5ꢀ ), 7.54 (18H, d, J = 7.5 Hz, rac-H7/7 /4 ), 8.12 (6H, s, rac-H6),
8.23 (6H, br, rac-H6ꢀ ), 8.54 (6H, d, J = 8.5 Hz, rac-H4), 8.86 (6H,
dobs, J = 7.5 Hz, rac-H3ꢀ ), 8.90 (6H, d, J = 8.5 Hz, rac-H3), 10.92
(6H, s, rac-NH). kmax/nm, (MeCN): 350 sh (24,217 M−1 cm−1),
544 (4,939 M−1 cm−1). mmax/cm−1 (KBr): 3431 (br, N–H), 1532 (N–
Analytical data for 1. Yield 51%. Found C, 58.4; H, 3.7; N,
11.7%. [Fe2(L1)3](BF4)4 requires C, 58.7; H, 3.7; N, 11.7%. dH
(400 MHz; DMSO-d6): 3.71 (2H, d, J = 14.5 Hz, meso-CH2),
3.77 (2H, d, J = 14.5 Hz, meso-CH2), 3.80 (2H, s, rac-CH2), 7.11
ꢀ
ꢀ
(4H, d, J = 8.5 Hz, rac-H8/8 ), 7.15 (8H, d, J = 8.5 Hz, meso-H8/8 ),
7.41 (4H, dobs, J = 5.5 Hz, meso-H5ꢀ ), 7.45 (2H, dobs, J = 5.5 Hz,
rac-H5ꢀ ), 7.53 (4H, d, J = 8.5 Hz, meso-H4), 7.58 (14H, m, meso- +
H), 1672 (C O). MS (ESI): m/z 500.93 [6 − (4 × Cl )] , 721.18
−
4+
=
[6·(MeOH)4 − (3 × Cl−)]3+.
ꢀ
rac-H7/7 , rac-H4), 7.86 (2H, s, rac-H6), 8.00 (4H, s, meso-H6), 8.28
Synthesis of [Fe2(L1)3](SO4)2, 4, and [Fe2(L3)3](SO4)2, 9. An
aqueous solution of FeSO4·6H2O (0.12 mmol) was added dropwise
to a stirred methanolic suspension of L1 (0.1 g, 0.18 mmol) (4) (or
solution of L3, 9) and the reaction mixture heated at reflux for 22 h
(4), or stirred at room temperature for ca. 5 h (9). After allowing
the solution to cool, it was filtered and the filtrate evaporated
to dryness under reduced pressure. The residue was dissolved in
MeOH and a purple powder retrieved by filtration after a few days
following the slow diffusion of ether into the MeOH solution.
(6H, m, meso- + rac-H3ꢀ ), 8.69 (4H, d, J = 8.5 Hz, meso-H3),
8.73 (2H, d, J = 8.5 Hz, rac-H3), 9.00 (12H, m, meso- + rac-
H4ꢀ/6 ), 10.67 (2H, s, rac-NH), 10.77 (4H, s, meso-NH). kmax/nm,
ꢀ
(MeCN:MeOH): 350 sh (19,556 M−1 cm−1), 546 (6,357 M−1 cm−1).
mmax/cm−1 (KBr): 3416 (br, N–H), 1539 (N–H), 1656 (C O), 1055
=
(br, BF4−). MS (ESI): m/z 449.86 [1 − (4 × BF4−)]4+.
Analytical data for 2. Yield 46%. Found C, 57.0; H, 3.5; N,
11.3%. [Fe2(L1)3](ClO4)4·MeOH requires C, 57.1; H, 3.7; N, 11.3%.
dH (400 MHz; DMSO-d6): 3.73 (2H, d, J = 14.5 Hz, meso-CH2),
3.76 (2H, d, J = 14.5 Hz, meso-CH2), 3.81 (2H, s, rac-CH2), 7.09
Analytical data for 4. Yield 42%. Found C, 62.5; H, 4.0;
N, 12.0%. [Fe2(L1)3](SO4)2·2MeOH requires C, 62.5; H, 4.2; N,
12.3%. dH (400 MHz; DMSO-d6): 3.69 (2H, d, J = 14.1 Hz, meso-
CH2), 3.74 (2H, d, J = 14.1 Hz, meso-CH2), 3.77 (2H, s, rac-CH2),
ꢀ
ꢀ
(4H, d, J = 8.5 Hz, rac-H8/8 ), 7.15 (8H, d, J = 8.5 Hz, meso-H8/8 ),
7.43 (4H, dobs, J = 5.5 Hz, meso-H5ꢀ ), 7.45 (2H, dobs, J = 5.5 Hz,
rac-H5ꢀ ), 7.55 (4H, d, J = 8.5 Hz, meso-H4), 7.60 (14H, m, meso- +
ꢀ
7.08 (4H, d, J = 8.0 Hz, rac-H8/8 ), 7.14 (8H, d, J = 8.0 Hz, meso-
ꢀ
H8,8 ), 7.38 (4H, dobs, J = 5.0 Hz, meso-H5ꢀ ), 7.41 (2H, dobs, J =
ꢀ
rac-H7/7 , rac-H4), 7.84 (2H, s, rac-H6), 8.03 (4H, s, meso-H6), 8.26
5.0 Hz, rac-H5ꢀ ), 7.53 (4H, dobs, J = 8.5 Hz, meso-H4), 7.61 (14H,
(6H, m, meso- + rac-H3ꢀ ), 8.70 (4H, d, J = 8.5 Hz, meso-H3),
ꢀ
ꢀ
m, meso- + rac-H7/7 /4 ), 7.95ꢀ(2H, s, rac-H6), 8.11 (4H, s, meso-H6),
8.28 (6H, m, meso- + rac-H3 ), 8.47 (4H, d, J = 8.5 Hz, meso-H3),
8.73 (2H, d, J = 8.5 Hz, rac-H3), 9.01 (12H, m, meso- + rac-
H4ꢀ/6 ), 10.65 (2H, s, rac-NH), 10.75 (4H, s, meso-NH). kmax/nm,
ꢀ
ꢀ
8.53 (2H, d, J = 8.5 Hz, rac-H3), 8.96 (12H, m, meso- + rac-H4/6 ),
(MeCN:MeOH): 350 sh (19,061 M−1 cm−1), 545 (6,204 M−1 cm−1).
mmax/cm−1 (KBr): 3409 (br, N–H), 1535 (N–H), 1650 (C O), 1092
=
10.77 (2H, s, rac-NH), 10.89 (4H, s, meso-NH). kmax/nm, (MeOH):
350 sh (12,833 M−1 cm−1), 546 (3,053 M−1 cm−1). mmax/cm−1 (KBr):
(br, ClO4−), 624 (ClO4−). MS (ESI): m/z 449.50 [2 − (4 × ClO4−)]4+.
3432 (br, N–H), 1535 (N–H), 1664 (C O), 1132 (br, SO42−). MS
=
(ESI): m/z 578.24 [4·(MeOH)4 − (2 × SO42−)]4+.
Analytical data for 3. Yield 61%. Found C, 62.4; H, 4.5; N,
11.8%. [Fe2(L1)3]Cl4·5MeOH requires C, 62.9; H, 4.7; N, 12.0%.
dH (400 MHz; DMSO-d6): 3.69 (6H, s, rac-CH2), 6.98 (12H, d, J =
Analytical data for 9. Yield 26%. Found C, 66.1; H, 5.52;
N, 10.0%. [Fe2(L3)3](SO4)2·2MeOH requires C, 66.3; H, 5.6; N,
10.4%. dH (400 MHz; DMSO-d6): signals are broad with many
overlapping resonances, which renders the identification and
assignment impossible. mmax/cm−1 (KBr): 3430 (br, N–H), 1535
ꢀ
8.5 Hz, rac-H8/8 ), 7.32 (6H, dobs, J = 5.5 Hz, rac-H5ꢀ ), 7.55 (6H,
ꢀ
ddobs, J = 5.5, 2.5 Hz, rac-H4ꢀ ), 7.61 (12H, d, J = 8.5 Hz, rac-H7/7 ),
8.22 (6H, s, rac-H6), 8.51 (6H, d, J = 7.5 Hz, rac-H6ꢀ ), 8.91 (18H,
ꢀ
m, rac-H3/3 /4), 11.05 (6H, s, rac-NH). kmax/nm, (MeCN:MeOH):
(N–H), 1668 (C O), 1128 (br, SO42−). MS (ESI): m/z 575.06
=
350 sh (19,227 M−1 cm−1), 543 (4,898 M−1 cm−1). mmax/cm−1 (KBr):
[9·(MeOH)4 − (2 × SO42−)]4+.
=
3433 (br, N–H), 1533 (N–H), 1670 (C O). MS (ESI): m/z 653.07
[3·(MeOH)4 − (3 × Cl−)]3+; 934.66 [3 − (2 × Cl−)]2+.
Synthesis of [Fe2(L3)3](BF4)4, 7, and [Fe2(L3)3]Cl4, 8.
A
methanolic solution of Fe(X)2 (0.097 mmol) (X = BF4−, 7; Cl−,
8) was added dropwise to a stirred methanolic solution of L3
(0.1 g, 0.15 mmol). The solution became deep purple and was
allowed to stir at room temperature for ca. 5 h. It was then filtered
and reduced to dryness. The resulting powder was taken up in
dichloromethane and sonicated for 5 min, filtered and washed
with dichloromethane.
Analytical data for 5. Yield 40%. Found C, 61.1; H, 4.1;
N, 11.5%. [Fe2(L2)3](BF4)4·3MeCN requires C, 61.1; H, 4.5; N,
11.8%. dH (400 MHz; DMSO-d6): 1.45 (18H, br, cyclohex-H),
2.26 (12H, br, cyclohex-H), 7.20 (4H, d, J = 8.5 Hz, rac-H8ꢀ ),
ꢀ
7.23 (8H, d, J = 8.5 Hz, meso-H8,8 ), 7.44 (4H, dobs, J = 6.5 Hz,
meso-H5ꢀ ), 7.48 (2H, dobs, J = 6.5 Hz, rac-H5ꢀ ), 7.54 (18H, m,
ꢀ
ꢀ
meso- + rac-H7/7 /4 ), 7.69 (2H, s, rac-H6), 7.89 (4H, s, meso-H6),
8.30 (6H, m, meso- + rac-H3ꢀ ), 8.77 (6H, m, meso- + rac-H3),
Analytical data for 7. Yield 51%. Found C, 60.0; H, 5.8,
N, 8.8%. [Fe2(L3)3](BF4)4·10MeOH requires C, 60.1; H, 5.9; N,
8.9%. dH (400 MHz; DMSO-d6): signals are broad with many
overlapping resonances, which renders the identification and as-
signment impossible. kmax/nm, (MeCN): 350 sh (13,379 M−1 cm−1),
544 (10,099 M−1 cm−1). mmax/cm−1 (KBr): 3419 (br, N–H), 1523
ꢀ
9.01 (12H, m, meso- + rac-H4/6 ), 10.67 (2H, s, rac-NH), 10.77
(4H, s, meso-NH). kmax/nm, (MeOH): 350 sh (24,390 M−1 cm−1),
545 (6,398 M−1 cm−1). mmax/cm−1 (KBr): 3434 (br, N–H), 1522 (N–
H), 1663 (C O), 1083 (br, BF4−). MS (ESI): m/z 500.99 [5 − (4 ×
=
BF4−)]4+.
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(N–H), 1655 (C O), 1059 (br, BF4−). MS (ESI): m/z 542.93 [7-
the void between them. Furthermore, by incorporating amide
functionalities within the ligand strands prospective binding of
anions within the cavity would be assisted and easily monitored
=
(4 × BF4−)]4+.
Analytical data for 8. Yield 49%. Found C, 66.4; H, 5.9; N,
10.1%; [Fe2(L3)3]Cl4·5MeOH requires C, 66.5; H, 5.9; N, 10.2%. dH
(400 MHz; DMSO-d6): 1.44 (18H, br, cyclohex-H), 1.78 (36H, s,
Me), 2.19 (12H, br, cyclohex-H), 7.12 (12H, d, J = 7.5 Hz, rac-
1
through H NMR spectroscopy. These design criteria were met
by L1–L3 and molecular modelling studies supported their ability
to prospectively form dinuclear [Fe2L3]4+ complexes potentially
capable of accommodating anions within their cavity, Fig. 1. From
this modelling study a cavity bound by [Fe(‘bipy’)3] head groups
ꢀ
ꢀ
ꢀ
H8,8 ), 7.36 (6H, br, rac-H5ꢀ ), 7.54 (18H, d, J = 7.5 Hz, rac-H7/7 /4
)
ꢀ
(6H, br, rac-H4ꢀ ), 7.57 (12H, d, rac-H7/7 ), 8.26 (2H, d, J = 7.5 Hz,
˚
separated by ca. 18.9 A (Fe–Fe distance) and ligand sides with
ꢀ
rac-H6ꢀ ), 8.45 (6H, s, rac-H6), 8.95 (18H, br, rac-H3/3 /4), 10.40
˚
ca. 8.9 A separation between each central methylene spacers was
(6H, s, rac-NH). kmax/nm, (MeOH): 350 sh (13,284 M−1 cm−1),
˚
identified. This compares with ca. 11.3 A M–M separations and
542 (6,036 M−1 cm−1). mmax/cm−1 (KBr): 3425 (br, N–H), 1530 (N–
˚
ca. 7.3 A methylene–methylene distances in the smaller metallo-
−
4+
=
H), 1671 (C O). MS (ESI): m/z 542.9 [8-(4 × Cl )] , 777.25
helicate complexes based on ligands with pyridylimino head
groups and diphenylmethane spacers.3f ,g
[8·(MeOH)4 − (3 × Cl−)]3+.
The amide-based ligands L1–L3 were readily synthesised via
the condensation of 5-chlorocarbonyl-2,2ꢀ-bipyridine with either
4,4ꢀ-methylenediamine, L1, 1,1-bis(4-aminophenyl)cyclohexane,
L2, or 1,1-bis(4-amino-3,5-dimethylphenyl)cyclohexane, L3, in
dichloromethane solution. L1 and L2 were isolated as poorly
soluble pale brown and off-white solids, respectively, whereas L3
was isolated as a cream coloured solid which, as a result of its
greater substitution, was fully soluble in common laboratory sol-
vents. Each ligand was characterised through elemental analyses,
1H NMR and IR spectroscopy, which were fully consistent with
their proposed structures. Of particular note within the 1H NMR
spectra of L1–L3 are the characteristic singlet resonances due to
the amide protons at d 10.51, 10.46 and 9.92, and H6-bipyridine
protons at d 9.20, 9.20 and 9.24, respectively, along with the readily
identifiable resonances from the remainder of the ligand backbone,
Table 1. The ca. 0.6 ppm shift within the NH resonance on moving
from L1 to L3 arises from the increased substitution on the ligand
framework and reflects the influence this has upon the basicity of
the NH proton. Similarly, within the IR spectra of L1–L3 broad
UV-Vis spectrophotometric complexiometric titration of L3 with
Fe(BF4)2·6H2O
Sequential 10 lL aliquots of a Fe(BF4)2·6H2O stock solution
(MeOH, 5.2 × 10−4 M) were added to a methanolic solution
of L3 (5.8 × 10−5 M) at room temperature and the UV-Vis
spectrum recorded after each addition such that no more than
10% additional volume was added in total.
1H NMR titration experiments of 1, 5 and 7 with Bu4NX (X = Cl−,
F−, Br−, HSO4
)
−
DMSO-d6 solutions of Bu4NX (0.064 M) were added in 10 lL
aliquots to DMSO-d6 solutions of either [Fe2(L1)3](BF4)4, 1,
1
[Fe2(L2)3](BF4)4, 5 or [Fe2(L3)3](BF4)4, 7, (5.4 mM) and the H
NMR spectrum of the resultant solution recorded. This was
repeated until no changes were noted within the spectra.
Molecular mechanics modelling studies
amide N–H stretches are evident between ca. 3200–3400 cm−1
Molecular mechanics calculations on [Fe2(L1)3]4+ and
[Fe2(L1)3Cl2]2+ were undertaken with Hyperchem Version 7.52.
Molecular mechanics calculations were carried out using MM+
with the Polak–Ribiere algorithm of Hyperchem. The target
configuration was arranged roughly by eye and the energy
minimised at an RMS gradient of 0.01. Molecular dynamics
was also used (simulated heating to 3000 K) to ensure that the
true energy minima had been reached. Guided by the ESMS,
−1
=
along with the amide C O stretch at ca. 1650 cm .
[Fe2(L)3]4+ host complex formation and characterisation
The formation of the [Fe2L3]4+ host complexes 1–9 was straightfor-
ward and simply involved the treatment of aqueous or methanolic
solutions of the appropriate Fe(II) salt with either a methanolic
suspension of L1 or L2 or solution of L3. Deep purple coloured
solids were obtained in each case following work-up and fully
1
UV-Vis and H NMR spectroscopic data the modelling studies
were limited to dinuclear species of general formula [Fe2L3]4+
of rac- and meso-configuration. The lowest energy structures
for rac-[Fe2(L1)3]4+ and rac-[Fe2(L1)3Cl2]2+ minimised to 146.0
and 140.7 kJ mol−1, respectively, and are shown in Fig. 1 and 5,
whereas meso-[Fe2(L1)3]4+ minimised to 164.0 kJ mol−1.
1
characterised by elemental analyses, ESMS, IR, UV-Vis and H
NMR spectroscopy. The elemental data are consistent with the
Table 1 Selected 1H NMR data for L1–L3 and 1–6 and 8a
Ligand/Complex
d H6 (rac-/meso-)
d NH (rac-/meso-)
Results and discussion
L1
9.20
10.51
[Fe2(L1)3](BF4)4 1
[Fe2(L1)3](ClO4)4 2
[Fe2(L1)3]Cl4 3
[Fe2(L1)3](SO4)2 4
L2
(7.86/8.00)
(7.84/8.03)
8.22
(7.95/8.11)
9.20
(7.69/7.89)
8.12
9.24
(10.67/10.77)
(10.65/10.75)
11.05
(10.77/10.89)
10.46
(10.67/10.77)
10.92
9.92
Ligand design strategy, synthesis and characterisation
Armed with the knowledge that flexible polypyridyl ligands readily
coordinate to Fe(II) to form multinuclear helicate complexes,13 we
set about designing new ligand types such that upon the formation
of a helicate an intrahelical cavity of sufficient size to bind guest
species within it would be formed. A more rigid spacer separating
coordination sites would therefore be required to maintain the
integrity of the cavity and to prevent the ligand strands from filling
[Fe2(L2)3](BF4)4 5
[Fe2(L2)3]Cl4 6
L3
[Fe2(L3)3]Cl4 8
8.45
10.40
a DMSO-d6, 298 K.
1280 | Dalton Trans., 2006, 1277–1284
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formation of dinuclear species of general formula [Fe2L3]4+ and
this composition was further confirmed by ESMS. Indeed, ESMS
of each complex apart from 3 displayed peaks characteristic of
[Fe2L3]4+ ions in solution with those of 4 and 9 also indicating
the association of four MeOH molecules with these ions. Inter-
estingly, the chloride containing complexes 3, 6 and 8 show peaks
350 (sh) and 540 nm, the red-shift of the 290 nm band to 300 nm
and the hypochromic red-shift of the 242 nm band to 247 nm due
to the formation of the complex species, Fig. 2. The presence of
three well defined isosbestic points at 230, 256 and 296 nm suggests
the presence of two predominant absorbing species in solution, the
uncoordinated ligand and complex species. By plotting the growth
of the 543 nm band as function of the [Fe(II)] : [L3] concentration
(Fig. 2, inset), it is clear that no further change in the intensity
of this band occurs following the addition of ∼0.67 equivalents
of Fe(II), consistent with the formation of a [Fe2(L3)3]4+ species in
solution.
3+
attributable to {[Fe2L3Cl]·MeOH4} ions, with 3 also displaying
peaks for [Fe2(L1)3Cl2]2+ ions in solution. This observation is
consistent with the chloride ion being more tightly associated with
the complex ions and possibly indicates that they are encapsulated
within the inter-strand cavity under these conditions. ESMS
did not show peaks attributable to more highly charged species
that might be associated with [Fe4L6]8+ or related complexes
being present in solution, or the presence of other compounds
1H NMR spectroscopy of 1–9
n+
(e.g. circular helicates) with variable {Mx : Ly} stoichiometry.
We can anticipate three species being present in solution for
[Fe2L3] complexes: the rac-isomers, K,K and D,D, in which both
complex units possess the same configuration and give rise to
triple helicate structures, or the corresponding meso-form, K,D,
where each complex unit has opposite configuration (meso-cate).16
Unfortunately, all attempts to grow single crystals suitable for
X-ray diffraction studies only yielded small, weakly diffracting
crystals from which no workable data could be obtained.
1
The H NMR spectra for 1–6 and 8 are well resolved whereas
UV-Vis spectroscopy of 1–9
those for 7 and 9, which incorporate the bulky L3 ligand, are
broadened possibly indicating the presence of some fluxional
behaviour.17 Each resonance within 1–9 has shifted to some degree
in comparison to those of the free ligands consistent with Fe(II)
coordination, Table 1.
Deep purple coloured solutions were obtained on reacting L1–
L3 with Fe(II) solutions in 3 : 2 stoichiometry, whose UV-Vis
spectra are dominated by three characteristic bands at ca. 300, 350
and 540 nm, indicative of the formation of tris-(2,2ꢀ-bipy)Fe(II)
type complexes.14 The former band may be assigned to the p–
p* transitions of the 2,2ꢀ-bipy moieties, whilst the latter two
are characteristic of [Fe(2,2ꢀ-bipy)3]2+ complexes and are due to
MLCT from the 3d atomic orbital to the two lowest vacant p*
molecular orbitals of the ligand. The peak at 540 nm is red-shifted
compared to that of [Fe(2,2ꢀ-bipy)3]2+ (500 nm)14 due to the electron
withdrawing nature of the amide group.
To better ascertain the nature of the species in solution, we
undertook a complexometric titration study to follow the course
of complex formation,15 tracking any changes that occurred via
UV-Vis spectroscopy. Solutions of L3 were employed for the
titration due to the poor solubility of L1 and L2 at appropriate
concentrations. To this end, sequential addition of a Fe(BF4)2
stock solution to L3 resulted in the growth of two new bands at
The spectra of complexes incorporating L1 are particularly
informative as the CH2 protons of the spacer permit easier
assignment of the resonances to either of the rac- or meso-
isomers. For the rac-form these protons are equivalent and a
single resonance is observed whereas in the meso-form they are
diastereotopic and give rise to two doublets. A mixture of the
rac- and meso-isomers with an approximate 1 : 2 abundance is
clearly evident within the spectra of 1, 2 and 4 with a singlet
resonance at ca. 3.80 ppm (rac-) and two doublets at ca. 3.70 and
3.77 ppm, respectively. This mixture of isomers is also evident in
the other resonances of the ligand backbone and particularly so
in the amide-NH and bipyridine proton H6, as they too clearly
split with 1 : 2 abundance, Fig. 3. Complex 5, which incorporates
the cyclohexane substituted ligand L2, also displays the rac/meso
Fig. 2 UV-Vis spectra resulting from the titration of L3 with Fe(BF4)2·6H2O in MeOH. The inset shows the change in the absorption band at k =
543 nm as a function of Fe(II) : L3 concentration.
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Fig. 3 1H NMR spectra of L1 (bottom), [Fe2(L1)3](BF4)4, 1 (middle), and 1 following the titration of five equivalents NBu4Cl (top) in DMSO-d6. For
the sake of clarity only the meso- and rac-NH resonances are labelled on the spectra of 1. Note the 1 : 2 abundance of the rac- and meso-isomers within
the spectrum of 1. Dashed lines track changes in NH and H6 resonances.
mix of isomers in 1 : 2 proportion under these conditions. It is
interesting to note that each of these complexes, 1, 2, 4 and 5,
contain counteranions with tetrahedral geometry, BF4− (1 and 5),
ClO4− (2) and SO42− (4), suggesting that anion geometry might play
a role in determining which species, rac- or meso-, predominates
in solution.
the titration study for solubility reasons and because ESMS and 1H
−
NMR data suggest that the BF4 counteranions are only weakly
interacting with the host molecule.20 The addition of Br− or HSO4
−
to 1, 5 or 7 afforded only minor changes in the 1H NMR spectra
and we concluded that very weak, if any, binding of these anions
occurred. The addition of F− produced slight downfield shifts (ca.
0.03 ppm) within the amide proton resonance but after about
1.5 equivalents the signals broadened considerably. We have seen
similar effects within naphthalimide based anion sensors, which is
often accompanied by deprotonation of the amide.1g,j Subsequent
additions of F− caused further broadening in all resonances until
such time that the solution changed in colour from purple to
orange consistent with complex decomposition.21 The addition
of Cl−, however, produced significant changes within the spectra,
Fig. 3. The amide and bipyridyl H6 proton resonances gradually
shifted downfield by ca. 0.30 and 0.25 ppm, respectively, indicative
of hydrogen bonding of the anion to the receptor. Furthermore,
peaks attributable to the presence of both the rac- and meso-
isomers within the receptor species coalesce to yield only the rac-
isomer in solution, Fig. 3.
This latter observation is in marked contrast to the situation in
3, 6 and 8, which contain the spherical Cl− as the counteranion.
The spectra for these complexes are consistent with the sole
presence of the rac-isomer in solution i.e. 100% triple helicate.
This dramatic effect is seen across the ligand series, 3 (L1), 6
(L2) and 8 (L3), but is particularly striking for complex 8 as
the spectra for the related complexes 7 and 9, which contain
−
2−
L3 but BF4 and SO4 counteranions, respectively, are broad
and featureless indicative of some underlying fluxional process.
We suggest that the Cl− ions are bound within the intrahelical
cavity and interact more strongly with the amide and bipyridyl H6
protons through hydrogen bonding than the other anions. These
interactions favour the formation of the helicate species, as they
possibly optimise the hydrogen bonding between them, and ‘lock’
the complexes into this conformation.18 This is fully consistent
with the observation that the resonances for these protons in 3,
6 and 8 are further downfield, indicative of stronger hydrogen
bonding than the corresponding resonances for the rac-isomers in
1, 2, 4 and 5, Table 1. This is also supported by the ESMS data for
3, 6 and 8 which showed peaks assignable to complex species
containing Cl− ions, perhaps indicating that they are trapped
within the intrahelical cavity.
Several conclusions may be drawn from this titration study: (i)
the free ligand L3 (and presumably L1 and L2) only interacts
very weakly with any anion;19 (ii) of the anions tested, only the
spherical Cl− ion elicited a significant response suggesting that the
host molecules are selective for this ion; (iii) both the amide and
bipyridyl H6 protons are equally affected by the presence of Cl−
ions suggesting that they are buried deep within the pocket created
by the ligand strands; (iv) by plotting the change in shift of the
NH and bipyridyl H6 proton resonances for meso-1 as a function
of added Cl−, it is evident that after the addition of around two
equivalents of Cl− ion no further major changes occur which is
consistent with strong binding and the formation of a 1 : 2 host-to-
guest complex, Fig. 4; (m) addition of Cl− ions causes the complexes
to adopt the helical conformation, which is consistent with that
observed within the 1H NMR spectra of the ‘legitimate’ chloride
complexes 3, 6 and 8.
Titration of 1, 5 and 7 with anions as followed by 1H NMR
spectroscopy
The ability of hosts 1, 5 and 7 to bind anions was investigated
by monitoring the changes in the 1H NMR spectra of DMSO-d6
solutions of each host upon addition of F−, Cl−, Br− and HSO4
−
(as their tetrabutylammonium salts).19 These hosts were chosen for
1282 | Dalton Trans., 2006, 1277–1284
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sensors for anions. We will report the results of this work when
they come to hand.
Acknowledgements
We thank the Programme for Research in Third Level Institutions
as coordinated through the Centre for Synthesis and Chemical
Biology and administered by the HEA for funding. We are
also grateful to Dr John O’Brien and Dr Martin Feeney for
helpful discussions concerning the NMR spectroscopy and ESMS,
respectively. Dr Anthea Lees and Dr Jennifer Keegan are thanked
for their critical assessment of the manuscript.
References and notes
Fig. 4 Changes in the chemical shift of the amide NH and bipyridine H6
protons within meso-1 upon addition of NBu4Cl in DMSO-d6.
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Conclusion
We have successfully designed and synthesised three new bis-
amido-2,2ꢀ-bipyridine ligands L1–L3 and shown them to be
capable of forming dinuclear triple helicate [Fe2L3]4+ complexes,
1
1–9, on coordination to Fe(II). H NMR spectroscopy showed
that the complexes exist as both rac-(helical) and meso-(non-
helical) isomers in DMSO-d6 solution at 298 K, although in the
presence of Cl− ions the rac-isomer is favoured. Coordination of
L1–L3 to Fe(II) was such that six amide hydrogen atoms were pre-
organised to line an intrahelical cavity of sufficient size to enable
1
the accommodation of guest species within it. Indeed, H NMR
titration experiments demonstrated that Cl− anions bind within
the intrahelical cavity as titration of 1, 5 and 7 with Bu4NCl showed
significant downfield shifts in the amide and bipyridyl H6 proton
resonances to yield a species of 1 : 2 host-to-guest stoichiometry.
Based on these observations we can propose a likely structure
for the 1 : 2 host-to-guest complex formed following the addition
of Cl− ions to 1, 5 and 7. It is clearly a helical complex in which
two Cl− ions bind within an intrahelical cavity through hydrogen
bonding with the amide and bipyridyl H6 protons. Molecular
modelling followed by geometry optimisation gave the structure
as shown in Fig. 5, which is consistent with these conclusions.
We believe this to be the first report detailing the synthesis of
metallo-helicate complexes designed specifically to bind anions
within an intrahelical cavity. We are currently extending this work
to incorporate other metal ions in conjunction with L1–L3 to see
whether these species might act as potential luminescent or redox
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Polyhedron, 1996, 15, 2983; (d) P. D. Beer, N. C. Fletcher and T. J. Wear,
Fig. 5 Molecular model of the triple helical [Fe2(L1)3(Cl)2]2+ complex
showing the proposed mode of binding of two chloride ions shown in
space-filling mode within the intrahelical cavity.
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Polyhedron, 1996, 15, 1339; (e) P. D. Beer, N. C. Fletcher and T. J. Wear,
Inorg. Chim. Acta, 1996, 251, 335.
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ligands. They showed that the helicate–mesocate equilibrium in [Al2L3]
complexes could be shifted in favour of the helicate when a solvent
molecule (H2O) is contained within the intrahelical cavity and interacts
with the host through hydrogen bonding.
19 To test whether the amide group within this ligand framework was
itself capable of interacting with anions, we first undertook a 1H
NMR titration study of L3 with Bu4NCl. L3 was chosen, as it was
sufficiently soluble at relevant concentrations compared to L1 and L2.
The sequential addition of Bu4NCl to L3 in DMSO-d6 solution of up
to six equivalents resulted in the very slight gradual downfield shift of
the amide proton resonance by ca. 0.03 ppm consistent with very weak,
if any, binding of the Cl− ion with the amide.
20 If the anions are to interact with the host molecules 1, 5 and 7 then
they must successfully compete with BF4−, although we conclude that
it is only very weakly interacting with the host molecules in the first
place. Irrespective of this, the anions F−, Cl−, Br− and HSO4− are better
−
hydrogen bond acceptors than BF4 and we would anticipate them to
successfully displace it from the cavity if it were there.
14 (a) A. B. P. Lever, Inorganic Electronic Spectroscopy, Elsevier, Ams-
terdam, 1968, p. 301; (b) I. Hanazaki and S. Nagakura, Inorg. Chem.,
1969, 8, 648.
15 B. Conerney, P. Jensen, P. E. Kruger and C. MacGloinn, Chem.
Commun., 2003, 1274.
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38, 2878; (b) M. Albrecht, Chem. Eur. J., 2000, 6, 3485.
21 It should be noted, however, that of the anions tested only F−
caused this colour change and response. Given the stability associated
with [Fe(bipy)3]2+ complexes this observation was initially surprising.
However, solvolyses of [Fe(bipy)3]2+ complexes readily occurs in non-
aqueous solvents such as DMF and DMSO as employed here (see,A. M.
Josceanu and P. Moore, J. Chem. Soc., Dalton Trans., 1998, 369).
The colour changes most probably arise from a combination of
ligand deprotonation, ligand dissociation, solvolysis and subsequent
generation of intermediate solvated species followed by F− coordination
to the Fe(II) centre which may be accompanied by oxidation to Fe(III).
Indeed, treating DMSO solutions of [Fe(bipy)3]2+ with a five-fold excess
of Bu4NF leads to solution discolouration similar to that observed here.
The [Fe(bipy)3]2+ solution was unaffected upon the addition of the other
anions tested (i. e. Bu4NX where X = Cl−, Br− and HSO4−). To the best
of our knowledge this observation is without precedent in the literature
and will be the subject of further investigation.
17 The origin of this fluxional behaviour may lie with the rate associated
with the rac-/meso-interconversion in the complexes containing the
more bulky ligand L3. It may also lie with ligand dissociation and
solvolysis combined with the effects of associated Fe(II) spin-state
equilibria experienced by the products that arise from dissociation
and solvolysis. The presence of Cl− ions clearly suppresses these
phenomena. See also ref. 21.
18 A similar effect was noted by Raymond and co-workers16a when
working with metallo-helicate complexes of bishydroxypyridinone
1284 | Dalton Trans., 2006, 1277–1284
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