Lariat ethers with fluoroaryl side-arms: A study of CF...metal cation interaction in the complexes of N-(o-fluoroaryl)azacrown ethers

Article abstract of DOI:10.1039/b807199c

New lariat ethers, N-(o-fluorophenyl)aza-15-crown-5 (F-A15C5) and N,N′-bis(o-fluorophenyl)diaza-18-crown-6 (F₂-A ₂18C6), were prepared by the N-arylation of the corresponding azacrown ethers. The interaction of the ligands with metal

Full text of DOI:10.1039/b807199c

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Lariat ethers with fluoroaryl side-arms: a study of CF ◊ ◊ ◊ metal cation
interaction in the complexes of N-(o-fluoroaryl)azacrown ethers†
Petr K. Sazonov,*a Lidiya Kh. Minacheva,b Andrei V. Churakov,b Vladimir S. Sergienko,b
Galina A. Artamkina,a Yuri F. Oprunenkoa and Irina P. Beletskaya*a
Received 28th April 2008, Accepted 10th October 2008
First published as an Advance Article on the web 4th December 2008
DOI: 10.1039/b807199c
New lariat ethers, N-(o-fluorophenyl)aza-15-crown-5 (F-A15C5) and N,N¢-bis(o-fluorophenyl)diaza-
18-crown-6 (F2-A218C6), were prepared by the N-arylation of the corresponding azacrown ethers.
The interaction of the ligands with metal cations was studied in solution by 1H and 19F NMR (in
acetone-d6) and UV spectroscopy (MeOH) confirming the formation of complexes of F2-A218C6 with
K+, Na+, Ag+, Ba2+, Pb2+ and of F-A15C5 with Na+ and giving evidence of CF ◊ ◊ ◊ metal cation
interaction. Cation binding constants (b, evaluated by UV titration method), demonstrate that
F-A15C5 and F2-A218C6 form more stable complexes than their fluorine-free analogs. The effect
depends on the nature of the metal cation and is at a maximum for hard, singly charged cations (up to
3 logb units for K+ complex of F2-A218C6). The X-ray structures of complexes [Pb(F2-A218C6)-
˚
˚
(H2O)](ClO4)2 (1) and [Ba(F2-A218C6)(ClO4)2] (2) reveal short Pb–F (2.805 A) and Ba–F (2.965 A)
contacts. Complex 2 is centrosymmetric (Ci), while complex 1 has C2 symmetry with one-side
coordination of o-fluorophenyl groups to Pb2+. This “one-side” coordination mode of Pb2+ is indicative
of a partial localization of the Pb2+ lone pair.
Existing fluorine-containing ligands utilize the endocyclic flu-
Introduction
orophenyl groups bonded though CH2-bridges as fluorine donor
centres, organized in a polycyclic cage framework. Thus, the effect
of C–F ◊ ◊ ◊ M coordination is achieved through a very high degree
of ligand preorganization. A lariat ether with fluorine-containing
side-arms would be a simpler and readily available model for
the study of C–F ◊ ◊ ◊ M interaction. However, ligands with o-
fluorobenzyl side arms (CH2-bridges) are known to show only
weak5 or no signs of C–F ◊ ◊ ◊ M+ interactions.8
If the o-fluorophenyl group is bound directly to the nitrogen
atom of the azacrown ring it will be more conformationally rigid.
At the same time the basicity of fluorine will be enhanced due
to the direct conjugation between nitrogen and fluorine centres.
In the present paper we have synthesized N-(o-fluorophenyl)aza-
15-crown-5 and N,N¢-bis(o-fluorophenyl)diaza-18-crown-6 ethers
and studied their interaction with metal cations, both in solution
and in solid state.
The importance of cation coordination with C–F units of
fluorocarbons and even the very fact of the existence of C–
F ◊ ◊ ◊ Mn+ interactions have only been recognized in relatively
recent times. It was only in the early 1980s that Glusker and
Murray-Rust et al. pointed to a widespread occurrence of short
CF–Mn+ contacts in crystal structures and concluded that “. . .C–
F bond is capable of significant, if not prominent, interactions
with both alkaline metal cations and proton donors”.1 However,
the coordination chemistry of fluorocarbons is now a well
established field, primarily due to the works of the Plenio2–6 and
Takemura7,8 groups on fluorocryptands and fluorine-containing
cage compounds. Understanding of the importance of Zr ◊ ◊ ◊ F–C
interactions in zirconocenium based Ziegler–Natta olefin poly-
merization catalysts gave another stimulus to the development
of this field.3,9 The incorporation of fluorine donor centres into
macrocyclic organic ligands enable the use of 19F NMR for
the detection of complexation phenomena, thanks to the high
sensitivity, comparable with protons, and large signal dispersion
of 19F NMR. Another, less explored, aspect concerns the effect of
fluorine, as the hardest possible donor centre, on the coordination
properties of the ligand, such as its selectivity.
It emerged that fluorine in these ligands can effectively par-
ticipate in the coordination of the metal cations increasing the
stability of the complexes. Moreover, the incorporation of fluorine
produces a marked effect on the selectivity of N-arylazacrown
ethers.
Results and discussion
aChemistry Department of the M. V. Lomonosov Moscow State University,
119992 Moscow. E-mail: beletska@org.chem.msu.ru; Fax: +7 495 939 3618
Synthesis of ligands
bN. S. Kurnakov Institute of General and Inorganic Chemistry RAS 119991
Moscow. E-mail: min@igic.ras.ru; Fax: +7 495 954 1279
Conventional methods for the preparation of N-aryl azacrown
ethers use rather tedious multistep cyclizations starting from the
corresponding anilines. Progress in Pd-catalyzed amination reac-
tions allowed the development of a more straightforward approach
based upon the direct arylation of the free azacrown ether with
the corresponding aryl halide. We used this direct method to
† Electronic supplementary information (ESI) available: 2D NOESY
NMR spectra and NMR simulation for F2-A218C6, an example of UV-
spectrum of F2-A218C6 as a function of metal salt concentration and
absorbance fitting for stability constant determination. CCDC reference
numbers 681113 and 681114. For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/b807199c
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prepare the N-o-fluorophenyl derivative of aza-15-crown-5 by
its reaction with o-bromofluorobenzene (Scheme 1) utilizing 2-
dimethylamino-2¢-dicyclohexylphosphinobiphenyl as a ligand for
palladium. This ligand showed the best performance in palladium-
catalyzed arylation of bulky secondary amines and azacrown
ethers in particular.10 The reaction gave the desired product (F-
A15C5), but only in meagre yield (20%) and was accompanied
by hydrodebromination of the substrate giving fluorobenzene.
Using other phosphine ligands (P(tBu)3, rac-BINAP, rac-1-[2-
(diphenylphosphino)ferocenyl]ethylmethyl◦ether) or lowering the
and soft (Ag+ and Pb2+), singly and doubly charged cations cause
significant changes in the 1H NMR spectrum of F2-A218C6. Some
of the signals of the CH2-groups of the crown ring (OCH2CH2O)
are shifted downfield, but the signals of the NCH2 groups are
shifted upfield. The upfield shift upon complexation may seem
unusual, but actually it is characteristic for NCH2 groups in N-
aryl azacrown ethers and can be attributed to the shielding effect
of the aromatic ring current brought about by the perpendicular
orientation of the aromatic ring to the ring of the macrocycle
in the complex.11,12 The cation field effect acts in the opposite
direction, resulting in smaller upfield shifts for doubly charged
cations. A downfield shift is observed for all four aromatic protons.
It indicates that both nitrogen and fluorine centres are involved
in metal coordination since in N-aryl azacrown ethers without
additional ortho-donor centres only protons ortho- to nitrogen
exhibit a strong downfield shift upon complexation.12
◦
reaction temperature (from 100 C to 70 C) only worsened the
yields.
The direct arylation of diaza-18-crown-6 ether under the con-
ditions in Scheme 1 gave the monoarylated product in low yield.
Therefore, we had to resort to a three-step procedure beginning
with nucleophilic aromatic substitution on activated 1,2-difluoro-
4-nitrobenzene for the synthesis of bis-o-fluorophenylated diaza-
18-crown-6 ether (Scheme 2). The nitro group was then reduced
to an amino group with H2 and 10% Pd/C at atmospheric
pressure and the amino group was removed by a standard
hydrodediazonation procedure utilizing H3PO2. The target bis(o-
fluorophenyl)diaza-18-crown-6 ether (F2-A218C6) was obtained
in 50% overall yield in three steps.
Similar trends (upfield shift of the NCH2 group signal and
downfield shift of the aromatic protons) are found in the effect
of the Na+ cation on the 1H NMR spectrum of F-A15C5.
The shifting of the 19F NMR signals to higher field upon the
addition of group I cations (Table 1) is another indication of
cation–fluorine interaction in the complexes of F-A15C5 and F2-
A218C6. Interaction with the cation through the nitrogen centre
only would have resulted in a downfield shift of the fluorine reso-
nance. The upfield shift of 19F signals is characteristic of cation–
fluorine coordination according to the literature data,4,9,13 and it
is also predicted by ab initio calculations.13,14 In conformationally
relaxed macrocycles, the 19F resonances are shifted upfield upon
binding of singly charged cations, but with doubly charged cations
Metal cation binding to F-A15C5 and F2-A218C6 in solution
1
We examined the effect of the addition of metal salts on the H
and 19F NMR spectra of F-A15C5 and F2-A218C6. As can be
seen from the data presented in Table 1 both hard (I and II group)
Scheme 1 Synthesis of F-A15C5 by Pd-catalyzed arylation of aza-15-crown-5 ether.
Scheme 2 Synthesis of F2-A218C6.
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Table 1 The metal cation induced change in 1H and 19F NMR chemical shifts (Dd, ppm)a of N-(o-fluorophenyl)aza-15-crown 5 (F-A15C5) and
N,N¢-bis(o-fluorophenyl)diaza-18-crown-6 ether (F2-A218C6)b
NCH2
NCCH2O
OCH2CH2O H3
H4
H5
H6
19F
Ligand
M+
d/ppm (multiplicity)
F-A15C5
F2-A218C6
—
—
3.47 (t)
3.54 (t)
3.67 (t)
3.64 (t)
3.60 (s)
3.56 (s)
6.98 (m)
7.01 (m)
6.81(m)
6.84 (m)
7.03 (m)
7.05 (m)
7.10 (t)
7.12 (t)
-122.9
-122.5
Dd/ppma
F-A15C5
Na+
K+
-0.13
-0.23
-0.12
-0.06
-0.09
-0.02
-0.08
-0.19
-0.01
0.10
0.08
0.34
0.19
0.01
0.11
0.16
0.23
0.39
0.28
0.29
0.10
0.10
0.36
0.36
0.41
0.42
0.34
0.44
0.50
0.49
0.23
0.25
0.18
0.07
0.32
0.32
0.45
0.41
0.34
0.41
0.54
0.54
-1.7
-1.2
-3.5
4.2
2.5
3.5
F2-A218C6
F2-A218C6
F2-A218C6
F2-A218C6
F2-A218C6
Na+
Ag+
Ba2+
Pb2+
a Dd = d(complex) - d(crown ether). b Solvent acetone-d6, 20 ◦C, c ª 5 ¥ 10-3 mol L-1, 3 equivalents of metal salt, counterion – [SCN]- for K+, I- for Na+
and [ClO4]- for Ag+, Ba2+ and Pb2+. Signal assignment is based on NMR simulation (aromatic region) and 2D NOESY correlations (Fig. S1–S3 of the
ESI†).
the opposite downfield shift was observed.5 In F2-A218C6 the
effect of cations on the 19F signal shift (except for Ag+) follows
the same trend (Table 1).
of both bands gradually decreases (Fig. S4 and S5 of the
ESI†).
As can be seen from the data given in Table 2, one ortho-fluorine
centre in F-A15C5 makes the Na+ complex forty-fold more stable
compared to H-A15C5, while two ortho-fluorines in F2-A218C6
increase the binding constant with K+ by 3 orders of magnitude.
As a result, F2-A218C6, in contrast to H2-A218C6, forms quite a
stable complex with K+. The magnitude of complex stabilization
caused by the ortho-fluorine donor centres depends on the nature
of the metal cation. For the softer Pb2+ cation the increase of
the binding constant in F2-A218C6 compared to H2-A218C6 is
much smaller than for hard K+. It means that incorporation of
fluorine changes the selectivity of the ligand. Interestingly, F2-
A218C6 forms a more stable complex with a singly charged alkaline
metal cation (K+) than with a doubly charged alkaline earth cation
(Ba2+) of a similar ionic radii, an opposite trend to the increase of
the charge density of the cation and rather unusual for a crown
ether. Stronger solvation of a highly charged cation, which is not
1
No changes in the H and 19F NMR spectra of F-A15C5 and
F2-A218C6 were observed upon the addition of Y(NO3)3·6H2O to
F-A15C5 and La(NO3)3·6H2O or La(ClO4)3·6H2O to F2-A218C6,
indicating the absence of any complexation between these ligands
and the rare earth metal salts.
Next, the effect of fluorine on the stability of the complexes
was estimated by measuring the cation binding constants for F-
A15C5 and F2-A218C6 and comparing them with the literature
data for the corresponding fluorine-free ligands N-phenylaza-15-
crown-5 (H-A15C5)15 and N,N¢-diphenyldiaza-18-crown-6 (H2-
A218C6).16 Binding constants (b) determined from the results of
UV spectrophotometric titrations are given in Table 2. The ligands
F-A15C5 and F2-A218C6 have identical UV spectra consisting
of a strong band at lmax = 252 nm with a weaker shoulder at
275–295 nm. Upon the addition of metal salts the absorbance
b
Table 2 The cation binding constants (
ꢀꢀꢀꢁ
) for N-(o-fluorophenyl)aza-15-crown 5 (F-A15C5) and N,N¢-bis(o-fluorophenyl)diaza-18-
ML
M+L
ꢂꢀꢀꢀ
crown-6 (F2-A218C6) ethers in comparison with the literature data for N-phenylaza-15-crown-5 (H-A15C5), N,N¢-diphenyldiaza-18-crown-6 (H2-A218C6)
and N,N¢-bis(o-methoxyphenyl)diaza-18-crown-6 [(MeO)2-A218C6 ] ethers in MeOH, 22 ◦
C
logb
Crown ether
K+
—
Na+
Ba2+
Mg2+
—
Pb2+
—
F-A15C5
2.5 ( 0.1)
0.9
—
H-A15C5b
F2-A218C6
H2-A218C6d
(MeO)2-A218C6f
4.0 ( 0.1)
1.1
3.9
2.3 ( 0.1)c
<1
2.9 ( 0.1)
<1
1.2 ( 0.1)
<1
4.0 ( 0.1)
2.7e
4.0
a Counterion – I- for K+, Br- for Na+ and [ClO4]- for Mg2+, Ba2+ and Pb2+
.
b Data from ref. 15. c When NaClO4 was used instead of NaBr logb = 2.45( 0.1)
was obtained. d Data from ref. 16 e Data from ref. 18. f Data from ref. 17, 50 : 50 MeOH–H2O mixture.
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◦
a
˚
compensated by the interaction with weak and weakly polarizable
fluorine donor centres, seems a natural explanation.
It is worth noting that F2-A218C6 and F-A15C5 are the first
examples of monocyclic ligands where fluorine exerts such a
marked effect on the complex stability, approaching the maximum
values reported for fluorocryptands.3,4,6
It is also instructive to compare the cation selectivity of
F2-A218C6 with that of its ortho-methoxy analog (N,N¢-bis(o-
methoxyphenyl)diaza-18-crown-6, ((MeO)2-A218C6).17 (MeO)2-
A218C6 has the same binding constant for K+ as F2-A218C6
(Table 2), though in a more polar medium, but it has no K/Na
selectivity, while F2-A218C6 has a fifty-fold preference for binding
K+ over Na+.
Table 3 Selected bond lengths (A) and angles ( ) for 1
Pb(1)-O(3)
Pb(1)-O(1)
Pb(1)-O(2)
Pb(1)-N(1)
Pb(1)-F(1)
F(1)-C(12)
O(2A)Pb(1)O(2)
2.431(14)
2.626(7)
2.639(9)
2.843(9)
2.805(10)
1.334(9)
O(3)Pb(1)O(1A)
71.4(5)
74.1(5)
145.2(4)
89.1(4)
113.4(3)
76.2(4)
61.7(3)
O(3)Pb(1)O(1)
O(1A)Pb(1)O(1)
O(3)Pb(1)O(2A)
O(1)Pb(1)O(2A)
O(3)Pb(1)O(2)
O(1)Pb(1)O(2)
165.3(5)
a Symmetry transformation A: -x + 1,y,-z + 1/2.
the atoms O(1), O(1A), O(2) and O(2A) occupying equatorial
positions and the water atom O(3) lying in an axial position. The
˚
metal atom is displaced from the equatorial plane of TP by 0.562 A
in the direction opposite to the axial atom O(3).
X-Ray structure of F2-A218C6 complexes with Pb(ClO4)2 and
Ba(ClO4)2
It is of interest to compare the structure of complex 1
with the previously reported dihydrated perchlorate complex of
Pb with N,N¢-bis(tetrafluoropyridin-4-yl)diaza-18-crown-6 (L¢),
[Pb(L¢)(H2O)2](ClO4)2 (3).18
The change of the tetrafluoropyridyl substituent on the crown
ether nitrogen atom in 3 to an o-fluorophenyl moiety in 1
leads to a change of complex stoichiometry (the loss of one
coordinated water molecule and the decrease of the CN of lead
from 10 to 9). The latter resulted in the noticeable ordering of
the CP of lead. Contrary to 1, in the structure of 3 the bond
distances formed by Pb and crown oxygen atoms are significantly
The composition of crystals formed by the reaction of Pb(ClO4)2
and F2-A218C6 corresponds to a monohydrated complex with
the formula [Pb(F2-A218C6)(H2O)](ClO4)2 (1). Reaction with
Ba(ClO4)2 gave an anhydrous complex [Ba(F2-A218C6)(ClO4)2]
(2).
The structure of 1 consists of complex cations [Pb(F2-
A218C6)(H2O)]2+ and ClO4- anions. In the cation, the metal atom
lies on the two-fold axis (Fig. 1). The metal centre is bonded to four
oxygen atoms of the F2-A218C6 macrocycle with approximately
˚
equal Pb–O distances (2.626(7) and 2.639 (9) A) and to the water
˚
different (2.558(6)–2.727(6) A). The Pb–O(H2O) distances also
˚
atom O(3) (d(Pb–O) = 2.429(14) A). The coordination number
˚
differ seriously (2.450(8) and 2.704(8) A). The Pb–N(1) and Pb–
(CN) of the lead atom is increased to nine by more distant nitrogen
F(1) bond lengths in the structure of 1 are shorter than in 3 (d(Pb–
N)–2.978(8) and 3.108(8); d(Pb–F)–2.890(10) and 3.123(9) A, cor-
respondingly). The latter indicates a stronger bonding interaction
of nitrogen and fluorine atoms with the metal centre in 1 and is in
accordance with the greater stability of compound 1 in solutions.
In the structure of 1, the Pb–F(1) distance is slightly longer
atoms of the macrocycle and two ortho-fluorine atoms (d(Pb–
˚
˚
N) = 2.843(9), d(Pb–F) = 2.805(10) A). Selected bond lengths
and angles are listed in Table 3.
than the shortest known Pb ◊ ◊ ◊ F contacts (organic molecule)
3,20
˚
(2.761(2), 2.784(8) A).
The main structural feature of complexes 1 and 3 is the trend to
“one-side” coordination of lead atom. For 1, in the irregular nine-
vertex polyhedron five oxygen atoms O(1), O(1A), O(2), O(2A)
and O(3) lie on one side of metal atom at relatively short distances
˚
(2.429(14)–2.639(9) A), while the other four atoms (2 N + 2F)
occupy opposite side at longer distances. Similar to 1, the ten-
vertex polyhedron PbO6N2F2 in 3 comprises from three oxygen
˚
atoms lying at one side (d(Pb-O)–2.450(8)–2.608(6) A) and other
seven ligands (3O + 2 N + 2F) occupying the opposite side.
The Pb(II) atom represents the p-element in low-valent state.
In general, the CP geometry of these elements depends on the
stereochemical behavior of the lone electron pair (LEP).19 The
observed “one-side” coordination of Pb atom in 1 and 3 confirms
the partial stereochemical activity (localization) of LEP. In both
compounds the LEP is localized at the side occupied by more
distant ligands (2 N + 2F for 1 and 3O + 2 N + 2F for 3).
The crystals of 2 comprise neutral centrosymmetrical molecules
[Ba(F2-A218C6)(ClO4)2] (Fig. 2). Selected bond lengths and angles
are listed in Table 4. As for the lead atom in 1, the Ba atom in 2
is coordinated by four oxygen atoms at slightly different distances
Fig. 1 The structure of complex cation [Pb(F2-A218C6)(H2O)]2+. All
hydrogen atoms have been omitted for clarity.
˚
Taking into account the first coordination sphere of the metal
atom only, the coordination polyhedron (CP) of lead may be
considered as a highly distorted tetragonal pyramid (TP) with
(av. 2.804 A) and by the nitrogen and fluorine atoms of ligand L
˚
(d(Ba–N) = 2.991(4), d(Ba–F) = 2.965(3) A). The CP of the Ba
atom is supplemented with four oxygen atoms of unsymmetrically
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Fig. 2 Molecular structure of [Ba(F2-A218C6)(ClO4)2]. All hydrogen atoms have been omitted for clarity.
˚
Table 5 Selected torsion angles (◦) for 1 and 2
◦
Table 4 Selected bonds distances (A) and angles ( ) for 2
1
2
Ba(1)–O(2)
Ba(1)–O(1)
Ba(1)–O(3)
Ba(1)–F(1)
Ba(1)–N(1)
Ba(1)–O(4)
F(1)–C(12)
O(1)Ba(1)F(1)
O(3)Ba(1)F(1)
O(2)Ba(1)N(1)
O(1)Ba(1)N(1)
2.785(3)
2.822(3)
2.916(4)
2.965(3)
2.991(4)
3.196(5)
1.368(6)
58.89(9)
119.17(9)
117.83(11)
60.35(10)
O(3)Ba(1)N(1)
F(1)Ba(1)N(1)
O(2)Ba(1)O(4)
O(1)Ba(1)O(4)
O(3)Ba(1)O(4)
F(1)Ba(1)O(4)
N(1)Ba(1)O(4)
O(2)Ba(1)O(1)
O(2)Ba(1)O(3)
O(1)Ba(1)O(3)
O(2)Ba(1)F(1)
70.14(11)
54.93(10)
65.40(10)
65.06(10)
44.20(10)
123.48(9)
102.73(10)
59.85(10)
106.80(11)
73.26(11)
80.64(9)
C(6A)N(1)C(1)C(2)
N(1)C(1)C(2)O(1)
C(1)C(2)O(1)C(3)
C(2)O(1)C(3)C(4)
O(1)C(3)C(4)O(2)
C(3)C(4)O(2)C(5)
C(4)O(2)C(5)C(6)
O(2)C(5)C(6)N(1A)
C(5)C(6)N(1A)C(1A)
168.7
60.3
176.1
172.6
55.7
169.5
171.5
57.8
165.0
63.0
178.5
171.6
59.4
172.2
177.4
68.6
73.9
169.6
side. In 1 the lead atom is displaced from the mean plane of the
˚
bonded perchlorate anions (d(Ba–O) = 2.916(4)–3.196(5) A). In 2
the CN of the Ba atom is equal to 12. High coordination numbers
are not unusual for Ba atoms, but, in most cases the CNs of Ba
atoms are not greater than 11.20 The increased CN (for 2) may
be attributed to the bidentate behavior of perchlorate anions and
to the presence of fluorine donor centres in the side chains of the
ligands.
˚
˚
macrocyclic oxygen atoms ([4O], D = 0.224 A) by 0.557 A, while
in 2 the barium atom is sited in the plane [4O]. In 2 the aromatic
rings are approximately perpendicular to the mean plane of the
macrocycle (torsion angle is 96.2◦), while in 1 that angle is equal
to 64◦.‡
Compounds 1 and 2 differ in the F2-A218C6 ligand conforma-
tions around the C–N bonds. Selected torsion angles for 1 and 2
are listed in Table 5. In both complexes, all eight C–O bonds adopt
a more stable trans-conformation, all six C–C bonds have a gauche-
conformation. However, in 1, two C–N bonds adopt a trans-
conformation, and the other two have a gauche-conformation.
In 2 all four C–N bonds adopt a trans-conformation. Obviously,
these differences are the result of different arrangements of
fluorophenyl ligands around the plane of the macrocycle. Thus,
the conformational formulae of the macrocycles are as follows
(starting from the N(1)–C(1) bond):
Many examples of short Ba ◊ ◊ ◊ F contacts have been reported
previously (2.76–3.287 A). However, the majority of these contacts
˚
were found in the complexes of hexafluoroacetylacetone (Hfa) and
hexafluoro-tert-butylate.3,20 Two complexes of barium perchlorate
with fluoro-containing crown ethers were investigated, namely
1,3-(2-fluorobenzo)-18-crown-5 (FO5)2 and 1,3-(2-fluorobenzo)-
21-crown-6 (FO6).13 In 2 the Ba ◊ ◊ ◊ F distance is comparable with
that found for Ba(FO6)(ClO4)2 – 2.990(3) A. However, it is 0.186 A
longer than the value reported for Ba(FO5)(ClO4)2 – 2.799(8) A.
In the last case, the surprisingly short contact may be attributed
to the small size of the cavity in the FO5 ligand.
˚
˚
˚
1–TNGT TGT TGGN TNGT TGT TGGN
2–TNGT TGT TGTN TNGT TGT TGTN
The coordination mode of the F2-A218C6 ligand is similar in
structures 1 and 2. However, the symmetry of these complexes is
different (C2 for 1 and Ci for 2). In 1 the fluorophenyl substituents
lie on one side of the macrocyclic plane, while in 2 one lies to each
(G = gauche, T = trans)
‡ The mean displacements of N and O atoms from the plane of the
macrocyclic ligand (D) are 0.425 A for 1 and 0.247 A for 2.
˚
˚
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The average geometrical parameters of the F2-A218C6 cycles
in 1 and 2 are listed below: bond lengths C–O - 1.419(16) vs
1.430(6); C(sp3)–N - 1.505(19) vs. 1.475(6); C(sp2)–N - 1.453(13)
vs. 1.440(6); C–F - 1.334(9) vs. 1.368(6); C–C - 1.50(2) vs. 1.499(7)
Experimental
NMR spectra were recorded using Bruker Avance spectrometer
operating at 400.13 (1H), 100.62 (13C) and 376.46 (19F) MHz,
respectively, with TMS as a reference for H and 13C and C6F6
(d = -162.9 ppm) as a reference for 19F. MALDI MS spectra were
obtained on Bruker Daltonics Autoflex II spectrometer equipped
with N2 laser (337 nm), accelerating voltage 19 kV, in positive
ion mode using dithranol as a matrix. UV-spectrophotometric
titrations were carried out on a Hewlett-Packard 8452 UV-Visible
spectrophotometer.
1
˚
A; bond angles CCO - 110.6(15) vs. 109.3(4); COC - 114.9(11)
vs. 111.6(4); CNC - 108.4(12) vs. 109.0(4); NCC - 116.0(15) vs.
114.0(5)◦.
In both 1 and 2 the geometry of the nitrogen atoms indicates
that their LEPs are involved in metal atom coordination. In
both cases the nitrogen atoms adopt pyramidal configuration (the
displacements of the N atom from the plane of substituents are
˚
0.488 and 0.443 A, the sum of the valence angles at N(1) are
329.3 and 333.7◦, bond lengths N(1)–C(7) are equal 1.453(13) and
13-(2-Fluorophenyl)-1,4,7,10-dioxa-13-azacyclopentadecane
(F-A15C5)
˚
1.440(6) A in 1 and 2, respectively).
It is worth mentioning that the structures of complexes
of Ba(ClO4)2 with F2-A218C6 (2) and Ba(ClO4)2 with N,N¢-
bis(4-dimethylaminophenyl)diaza-18-crown-6 ether (4) are very
similar.12 They do not differ significantly in either Ba–O distances
Aza-15-crown-5 (110 mg, 0.50 mmol), o-bromofluorobenzene
(108 mg, 0.62 mmol), t-BuONa (72 mg, 0.75 mmol),
Pd2dba3·CHCl3 (8.6 mg, 0.0083 mmol) and 2-dimethylamino-2¢-
dicyclohexylphosphinobiphenyl (7.8 mg, 0.02 mmol) were placed
into a Schlenk reactor. After cooling in liquid N2 and evacuating
(10-2 torr), toluene (1.5 ml) was vacuum-transferred into the
reactor. The reactor was filled with argon and the mixture was
stirred at 100 ◦C for 24 h. Reaction progress was monitored by
GLS. After cooling the reaction mixture was diluted with CH2Cl2
(5 ml), filtered through a Celite pad and the solvent was removed
under reduced pressure. F-A15C5 was isolated as a colorless oil
(30 mg, 19% yield) by column chromatography on silica gel (Merck
˚
˚
(av. 2.779 0.009 A in 4) or Ba–N distances (3.004 A in 4).
In both 2 and 4, the macrocycles adopt centrosymmetric (Ci)
conformation and adopt a roughly perpendicular arrangement
of aromatic rings around the macrocyclic planes (105◦ for 4). This
similarity shows that aryl side arms in N,N¢-diaryldiaza-18-crown-
6 complexes adopt a conformation favourable for metal cation
binding by the donor centres in the ortho-positions of the aryl
groups.
In the structure of 1, the complex cation [Pb(F2-
40–63) using CH2Cl2–PE–MeOH (8 : 8 : 1) as eluent. 1H and 19
F
A218C6)(H2O)]2+ is linked with two symmetrically related ClO4
-
NMR data are given in Table 1. dC (CDCl3) 53.53, 70.02, 70.34,
70.42, 71.03, 116.46 (d, JC-F = 22 Hz), 119.07, 119.94 (d, JC-F = 8
Hz), 124.21.§
anions by weak hydrogen bonds O(3)–H(1) ◊ ◊ ◊ O(12) (x, -y, z
+ 1/2) (O(3)–H(1) - 0.85, H(1) ◊ ◊ ◊ O(12) - 1.85, O(3) ◊ ◊ ◊ O(12) -
◦
˚
2.69(2) A, O(3)H(1)O(12) - 169 ) and O(3)–H(2) ◊ ◊ ◊ O(12)¢ (-x +
1, -y, -z) (O(3)–H(2) - 0.85, H(2) ◊ ◊ ◊ O(12)¢ - 1.86, O(3) ◊ ◊ ◊ O(12)¢
7,16-Bis(2-fluoro-4-nitrophenyl)-1,4,10,13-tetraoxa-7,16-
diazacyclooctadecane
◦
˚
- 2.70(2) A, O(3)H(2)O(12)¢ - 170 ). The latter results in neutral
aggregates [Pb(F2-A218C6)(H2O)](ClO4)2 combined in the crystal
by van der Waals interactions only.
Diaza-18-crown-6 (393 mg, 1.5 mmol), 1,2-difluoro-4-
nitrobenzene (626 mg, 3.9 mmol), triethylamine (0.500 ml,
3.5 mmol) and DMF (2 ml) were stirred in a closed vessel for 2 h
at 100 ◦C. After cooling the reaction mixture was diluted with
MeOH (5 ml), the precipitated product (yellow solid, 615 mg,
76%) was collected, washed with MeOH and acetone (5 ml) and
dried in vacuo. dH (CDCl3) 3.63 (s, 8H), 3.77 (m, 16H), 7.09 (t,
J = 9 Hz, 2H), 7.85 (dd, J = 15, 2.5 Hz, 2H), 7.92 (dd, J = 9,
2.5 Hz, 2H). MS (MALDI) m/z. Expected: 539.20 [M-H]. Found:
538.8.
In the structure of 2 the stacks of complex molecules [Ba(F2-
A218C6)(ClO4)2] form a base-cantered motif and are linked by van
der Waals interactions only.
Conclusion
New azacrown lariat ethers, F-A15C5 and F2-A218C6, bearing
florophenyl side arms, were prepared and their interaction with
metal cations was examined. Both the NMR data (1H and
19F) obtained in solution and the X-ray analysis of F2-A218C6
complexes with Ba and Pb perchlorates show that fluorine in
these ligands is coordinated to the metal cation. Relatively short
7,16-Bis(2-fluorophenyl)-1,4,10,13-tetraoxa-7,16-
diazacyclooctadecane (F2-A218C6)
A
suspension of 7,16-bis(2-fluoro-4-nitrophenyl)-1,4,10,13-
metal ◊ ◊ ◊ F–C contacts are found in both the Ba2+ (2.965 A) and the
˚
tetraoxa-7,16-diazacyclooctadecane (575 mg, 1.06 mmol) and
Pd/C (10%, 133.4 mg) in EtOH (50 ml) was purged with argon
and refluxed for 2.5 h with a slow flow of H2 bubbled through the
reaction mixture until the solution turned colorless. After purging
with argon (Caution! Hydrogen may explode on contact with air in
the presence of Pd/C) the reaction mixture was passed through a
short pad of alumina to separate it from Pd/C, the pad was washed
with EtOH (3 ¥ 5 ml) and the combined filtrate was evaporated
Pb2+ (2.805 A) complexes, but the structural motifs of Ba(ClO4)2
˚
(Ci) and Pb(ClO4)2 (C2) complexes are different. The observed one-
sided coordination of Pb2+ with fluorophenyl side arms reflects
the tendency of Pb2+ to localize its lone pair. The cation–fluorine
interaction results in a significant stabilization of F-A15C5 and
F2-A218C6 complexes as shown by the measurement of cation
binding constants (Table 2). Fluorine also modifies the selectivity
of the ligands, increasing their affinity towards hard singly charged
alkaline metal cations, which should be attributed to the hardness
of fluorine as donor centre.
§ The weaker signals of quaternary carbons were not observed.
848 | Dalton Trans., 2009, 843–850
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to dryness. The solid residue was dissolved in H3PO2 (50%, 8 ml)
which required stir◦ring for 30 min at r.t. The resulting solution
was cooled to -10 C and NaNO2 solution (170 mg, 2.5 mmol)
in H2O (1 ml) was added dropwise under stirring (15 min). The
reaction solution was allowed to warm to r.t. in 4 h, neutralized
with NaOH and extracted with CH2Cl2 (3 ¥ 20 ml), the extract
dried over Na2SO4. F2-A218C6 was isolated as a pale solid (308 mg,
65% yield) by column chromatography on silica gel (Merck 40–63)
using CH2Cl2–MeOH (10 : 1) as eluent. 1H and 19F NMR data are
given in Table 1. MS (MALDI) m/z: Expected: 451.2 [M + H],
473.2 [M + Na], 489.2 [M + K]. Found: 451.1, 473.0, 489.0. EA:
Calc. for C24H32F2N2O4: C 63.98, H 7.16, N 6.22. Found: C 63.74,
H 7.20, N 6.41%.
Cation binding constants
Working solutions were prepared in MeOH (≤0.5%H2O) in a 10 ml
measuring flask by mixing weighed amounts of the solutions of the
ligand and the corresponding metal salt(s) (KI, NaBr, Mg(ClO4)2,
M(ClO4)2·3H2O, M= Ba, Pb). Total concentration of the ligand
(~5 ¥ 10-5 mol L-1) was kept constant in a series while the total
concentration of the metal salt was variable (5 ¥ 10-5–10-1 mol L-1).
UV-spectra were recorded at 22 ◦C in a 1 cm quartz cell and
the obtained absorbance values (Ai, in 250–290 nm range) were
fitted to equation (1) to give the concentrational metal binding
constant (b, Table 2) and the absorbance of the complex (A•) as
objective variables. Good fits (r2 > 0.99) were obtained in all cases
confirming the complex formation model (metal : ligand = 1 : 1).
b[M ]i A•
b[M ]i +1
Preparation of the complexes
AL - Ai =
(1)
Crystals of the complexes suitable for X-ray diffraction were ob-
tained by diffusion crystallization. The acetone solution (~0.15 M,
~0.2 ml) of M(ClO4)2·3H2O (M = Ba, Pb) was carefully placed into
a tube (~4 mm internal diameter) containing the solution of F2-
A218C6 (~0.15 M, ~0.2 ml) in CH2Cl2 (avoiding the mixing of
the two layers) and left overnight. Pale-coloured crystals formed
and were separated from the mother liquor and dried in the open
air. Solution 1H and 19F NMR data of the complexes are given in
Table 1.
where AL is the absorbance of the starting ligand solution and [M]i
is the molar concentration of the metal salt.
X-Ray crystallography
The main crystallographic data for compounds 1 and 2 are given in
Table 6. The experimental intensities for the needle-shaped light-
brown crystals of 1 were collected on a Bruker SMART APEX
II diffractometer, and for the irregular-shaped light-violet crystals
of 2 on an Enraf-Nonius CAD4 machine at ambient temperature
[Pb(F2-A218C6)(H2O)](ClO4)2 (1). MS (MALDI) m/z: Ex-
pected: 757.2 [M-H2O,-ClO4]. Found: 756.9. EA: Calc. for
C24H34Cl2F2N2O13Pb: C 32.96, H 3.92, N 3.20. Found: C 33.77, H
3.84, N 3.43%.
˚
(MoKa radiation, l = 0.71073 A, graphite monochromator, w-
scanning). An absorption correction based on measurements of
equivalent reflections was applied for 121; y-scanning was used
for 2.22 The low ratio of observed/unique reflections for 2 and
the low C–C bond precision in 1 are caused by poor crystal
quality. Both structures were solved by direct methods (SHELXS-
86),23 refined by full matrix least-squares on F2 (SHELXL-97)24
[Ba(F2-A218C6)(ClO4)2] (2). MS (MALDI) m/z: Expected:
687.0 [M-ClO4]. Found: 686.8. EA: Calc. for C24H32Cl2F2N2O12Ba:
C 36.64, H 4.10, N 3.56. Found: C 36.38, H 4.20, N 3.80%.
Table 6 Crystal data, data collection and refinement parameters for complexes 1 and 2
Compound
1
2
Empirical formula
M
C24H34Cl2F2N2O13Pb
874.62
0.40 ¥ 0.03 ¥ 0.02
Monoclinic
C2/c
19.423(5)
12.719(3)
13.109(3)
94.711(3)
3227.4(14)
4
1.800
5.468
1720
303(2)
3.12–26.00
12378
3154 [Rint = 0.0674]
3154/10/181
R1 = 0.0679
wR2 = 0.1773
R1 = 0.0893
wR2 = 0.1967
1.009
C24H32BaCl2 F2N2O12
786.76
0.17 ¥ 0.17 ¥ 0.11
Monoclinic
P21/n
Crystal size/mm3
Crystal system
Space group
˚
a/A
8.178(3)
˚
b/A
12.054(3)
15.072(4)
99.02(2)
1467.3(2)
2
1.781
1.613
788
293(2)
˚
c/A
b/◦
3
˚
V/A
Z
Dc/Mg m-3
m/mm-1
F(000)
T/K
q Range/◦
2.17–29.97
6568
Reflections collected
Independent reflections
Data/restraints/parameters
R indices [I > 2s(I)]
4269 [Rint = 0.0824]
4269/0/196
R1 = 0.0442
wR2 = 0.0694
R1 = 0.2164
wR2 = 0.0992
0.949
R indices (all data)
Gof on F2
Largest diff. peak, hole/eA-3
2.977, -1.006
0.786, -1.585
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anisotropically for all statistically ordered non-hydrogen atoms. In
the structure of 1 the water molecule was disordered around the
crystallographic two-fold axis (0.5/0.5). High values of isotropic
thermal parameters of perchlorate oxygen atoms, especially for
the atoms O(11) and O(13), indicate possible disorder. The ClO4
anion was refined with the same Cl–O bond distances and isotropic
thermal parameters for O(11) and O(13) atoms. The large thermal
ellipsoid for fluorine atoms and the short C(5)–C(6) distance in 1
point to possible disorder of these atoms; however, all attempts to
resolve distinct positions for these atoms failed. The phenyl ring
in 1 was refined as a rigid body.
7 H. Takemura, N. Kon, M. Yasutake, H. Kariyazono, T. Shinmyozu and
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11 K. Rurack, J. L. Bricks, G. Reck, R. Radeglia and U. Resch-Genger,
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For both structures, all hydrogen atoms were placed in calcu-
lated positions and refined using a riding model. CCDC numbers
681114 (1) and 681113 (2).†
12 L. K. Minacheva, P. K. Sazonov, V. S. Sergienko, G. A. Artamkina,
I. P. Beletskaya and A. Y. Tsivadze, Russ. J. Inorg. Chem., 2006, 51,
1071–1081.
13 H. Plenio and R. Diodone, Chem. Ber. Recl., 1997, 130, 963–968.
14 P. K. Sazonov, and I. P. Beletskaya, unpublished work.
15 M. Geringer and H. Sterk, Magn. Reson. Chem., 1989, 27, 1148–1154.
16 E. Sonveaux, Tetrahedron, 1984, 40, 793–797.
17 T. Gunnlaugsson and J. P. Leonard, J. Chem. Soc., Perkin Trans. 2,
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18 L. K. Minacheva, P. K. Sazonov, V. S. Sergienko, G. A. Artamkina,
Y. F. Oprunenko and I. P. Beletskaya, Russ. J. Inorg. Chem., 2007, 52,
1018.
Acknowledgements
Financial support of this work by the Russian Fund of Basic
Research (Project No. 05-03-32905) and Presidium RAS Program
(OX2.5) are gratefully acknowledged. The authors thank Dr Sci.
Yu. G. Gorbunova (N. S. Kurnakov IGIC) for a generous loan of
diaza-18-crown-6. Yu. F. O. thanks the Alexander von Humboldt
Foundation, Bonn, Germany for a personal computer grant.
19 R. J. Gillespie, and I. Hargittai, The VSEPR Model of Molecular
Geometry, Allyn and Bacon, Boston, 1991; R. D. Hancock and A. E.
Martell, Chem. Rev., 1989, 89, 1875–1914; R. D. Hancock, J. H.
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20 F. H. Allen, Acta Crystallogr., Sect. B, 2002, 58, 380. The Cambridge
Structural Database, www.ccdc.cam.ac.uk.
21 G. M. Sheldrick, SADABS, Program for scaling and correction of area
detector data, University of Go¨ttingen, Germany, 1997.
22 Shelxtl-Plus. Release 6.10., Bruker AXS Inc., Madison, Wisconsin,
USA, 2000.
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850 | Dalton Trans., 2009, 843–850
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The Royal Society of Chemistry 2009
©