Piperazine linked salicylaldoxime and salicylaldimine-based dicopper(ii) receptors for anions

Article abstract of DOI:10.1039/c5dt02551f

The syntheses and single crystal X-ray analyses of five strapped salicylaldoxime/salicylaldimine based dicopper(ii) receptors utilising a new piperazine linker are described. The complexes 1-4 form 2 + 2 metallocycles and the molecular structures of all f

Full text of DOI:10.1039/c5dt02551f

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ARTICLE
Piperazine linked salicylaldoxime and salicylaldimineꢀbased
dicopper(II) receptors for anions
Cite this: DOI: 10.1039/x0xx00000x
Nirosha De Silva, Geoffrey B. Jameson, Ajay Pal Singh Pannu, Raphëlle Pouhet,
Marco Wenzel, and Paul G. Plieger*
Received 00th January 2012,
Accepted 00th January 2012
The syntheses and single crystal X-ray analyses of five strapped salicylaldoxime / salicylaldimine based
DOI: 10.1039/x0xx00000x
dicopper(II) receptors utilising a new piperazine linker are described. The complexes
1-4 form 2+2
metallocycles and the molecular structures of all four complexes possess a small internal cavity with the
utilisation of the short piperazine linker. The molecular structures of complexes [Cu2(L4-H)(L4-
www.rsc.org/crystengcomm
2H)⊂DMF]BF4∙DMF, 1 2 show that intramolecular H-
and [Cu2(L4-H)2Br]Br∙1.25DMSO∙H2O∙MeOH,
bonding interactions due to the presence of -OH (oxime moiety) groups lead to a Pacman-like cleft
arrangement of the two metal coordinating subunits in the metallo-macrocycle. The geometrical
constraints brought about by this constrained cleft make the receptor coordinate strongly to a bromide
anion involving both metal centres as evidenced by
excluded. Absence of the oxime moiety around the metal coordination site of the ligand as
demonstrated in the complexes [Cu2(L5)2BF4](BF4)3, and [Cu2(L5)2Br]Br3∙2MeOH,
resulted in less
2 whereas in 1 the larger tetrafluroborate anion is
3
4
constrained dicopper(II) helicate forms. For these complexes no anion size discrimination was observed.
The addition of pyridine solvent to a slightly modified piperazine-linked ligand produces an expanded 3
tube-like tricopper complex [Cu3(L4a-H)3Py3](BF4)2∙(MeOH)3∙PF6∙(H2O)3,
pyridine molecules occupying the newly
+
3
5
,
with two coordinated
formed cavity.
We are interested in the effect structure has on anion binding
ability, especially away from the binding site. Previously we
have described the anion binding properties of salicylaldoximeꢀ
and salicylaldimineꢀlinked anion receptors.9, 10, 11ꢀ13 Variations
Introduction
Sequential binding of a metal cation followed by an attendant
anion by a single receptor is an effective method of binding
anions. The development of such ditopic receptors is a
challenging task due to the specific requirements needed to be
met by both the metal coordinating site and the anion binding
site/pocket.1 Nevertheless, a number of remarkable advances
have been made recently in the areas of chiral group
recognition and anion sensing.2 One subꢀclass of these
cation/anion ditopic receptors are the cascade complexes.
These complexes rely on first coordinating a metal ion before
anion binding / encapsulation is possible. The term ‘cascade’
was first coined by Lehn when describing the ability by which
polynuclear cryptates could encapsulate guests, especially
anions.3 The work of Fabbrizzi has demonstrated that selection
of anions in dinuclear copper complexes is largely influenced
by the conformational freedom of the straps used to link the
metal coordination sites. This results in anion selectivity based
on bite length (the distance between the anion donors) rather
than anion length.4 There have been numerous examples since
based largely on variations around the bicycle5 or macrocycle6
cryptate molecules with both metals and organic cations.
BowmanꢀJames and her group have also managed to develop
the reverse cascade complex; where anions (held by hydrogen
bonding) play the traditional role of the metal centres allowing
capture of a water molecule.7 Recent examples whereby the
metallic cations have effectively been replaced with organic
cations have been reported by Jabin and coꢀworkers.8
in the linking groups for ligands L1 L2 3 (scheme 1) in terms
, , L
of length and/or conformational flexibility have led to a marked
influence in the ability of these complexes to bind anions when
coordinated with copper(II) metals. The results of these studies
have been analysed with respect to the change in geometrical
parameters (e.g cavity shape and size, position of the anion in
the cavity, CuꢀCu distance, etc.) and anion binding strengths /
preferences on complexation with metal ions. For example,
with just a slight variation in the aryl linkers, the ligands L2
(connected with a 1,4ꢀaryl spacer)11 and L3 (1,3ꢀaryl spacer)9
display striking structural differences in their respective
complexes formed and that has resulted in dramatic changes in
their binding strength/preferences under the same conditions.
In addition to these studies, deprotonation of the oxime
functionality in ligands containing this moiety has been shown
by us and others to give rise to the formation of metal
clusters.14
This journal is © The Royal Society of Chemistry 2013
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DOI: 10.1039/C5DT02551F
Campbell Microanalytical Laboratory at the University of
Otago.
Synthesis of Ligands
3,3´ꢀ(1,4ꢀpiperazineꢀbisꢀ(methylene)ꢀbisꢀ(5ꢀtertꢀbutyl)ꢀ2ꢀ
hydroxy benzaldehyde), 1(a): The compound 1a (precursor
for L4 and L5) was prepared according to the procedure of
Schröder et al.15 Et3N (4 mL) was added to compound 3ꢀ
(bromomethyl)ꢀ5ꢀtertꢀbutylꢀ2ꢀhydroxybenzaldehyde (2.52 g,
9.30 mmol) dissolved in MeCN (30 mL), followed by dropwise
addition of piperazine (0.397 g, 4.6 mmol in MeCN (30 mL)
over 1.5 hour). The yellow solution was stirred at RT
overnight. The solution was washed with distilled water (2 x 20
mL), and dried over Na2SO4. The solid was recovered by
precipitation with ethanol (35 mL). Yield = 1.318 g, 60%. δH
(500 MHz; CDCl3; Me4Si): 1.30 (18 H, s, CH3), 2.74 (8 H, s,
CH2), 3.77 (4 H, s, PhꢀCH2ꢀN), 7.46 (1 H, d, PhꢀH), 7.59 (1 H,
d, PhꢀH), 10.18 (2 H, s, CHO), 13.2 (2 H, s, OH). δC (500
MHz; CDCl3): 31.4, 34.2, 52.2, 58.4, 121.5, 123.3, 126.5,
134.1, 142.4, 158.8, 193.6. m/z (ESI) 467.93 [1(a)+H]+
υmax/cmꢀ1 1711.6 (C=O).
.
3,3´ꢀ(1,4ꢀpiperazineꢀbisꢀ(methylene)ꢀbisꢀ(2ꢀhydroxyꢀ5ꢀ
Scheme 1 Linked SALEN Ligands.
methylbenzaldehyde)), 1(b): The compound 1b (precursor for
L4a) was prepared according to the procedure of Schröder et
al.15 To a solution of Et3N (2 mL) in dry DCM (20 ml) were
added simultaneously over 30 minutes, solutions of 3ꢀ
(bromomethyl)ꢀ2ꢀhydroxyꢀ5ꢀmethylbenzaldehyde (1.270 g,
5.54 mmol) and piperazine (0.240 g, 2.77 mmol) each
dissolved in dry DCM (15 mL). The yellow solution was
stirred at RT overnight. The solution was washed with distilled
water (3 x 70 mL), and dried over anhydrous Na2SO4. The
solid was recovered by precipitation with ethanol (50 mL).
Yield = 0.901 g, 85%. δH (500 MHz; CDCl3; Me4Si): 2.29 (6
H, s, CH3), 2.66 (8 H, br, CH2), 3.70 (4 H, s, PhꢀCH2ꢀN), 7.17
In this paper, we demonstrate that a further decrease in the
linker length in the series of salicylaldoxime / salicylaldimine
complexes coupled with enhanced rigidity around the metal
binding site leads to selection via size exclusion at the anion
binding site. Specifically, by changing the linking group to a
new shorter piperazine spacer, the size of the resulting anion
binding site formed on complexation is reduced in size.
However, when the metal binding site retains some flexibility,
such is the case with the salicylaldimine complexes
[Cu2(L5)2BF4](BF4)3, , and [Cu2(L5)2Br]Br3·2MeOH,
3 4, this
binding site is capable of binding monovalent anions as large
as tetrafluoroborate. The metal binding site can be stiffened by
replacing the iminophenyl functionality with that of the oxime
functionality in ligands L4 and L4a. The secondary hydrogen
bonding surrounding the metal center is strong and restricts the
degree to which the ligand donors to the metal can move. This,
when combined with the newly implemented short spacer,
results in a characteristic Pacmanꢀlike cleft arrangement, as
(2 H, d,
J = 1.75 Hz, PhꢀH), 7.41 (2 H, d, J = 1.61 Hz, PhꢀH),
10.21 (2 H, s, CHO), 13.2 (1 H, s, OH) ppm. δC (500 MHz;
CDCl3): 20.2, 52.4, 58.5, 122.0, 123.5, 128.4, 129.5, 137.0,
158.8, 192.6 ppm. Found: C, 68.14; H, 6.74; N, 7.26. Calc. for
C22H26N2O4·0.3Ethanol: C, 68.44; H, 7.09; N, 7.04%. m/z
(ESI) 383.84 [1(b)+H]+ υmax/cmꢀ1 1678.6 (C=O).
.
6,6´ꢀ(1,4ꢀpiperazineꢀbis-(methylene)ꢀbisꢀ(4ꢀtertꢀbutylꢀ2ꢀ
(hydroxyimino)) phenol, L4: To a solution of KOH (0.40 g,
7.13 mmol) in ethanol (80 mL), a solution of hydroxylamine
hydrochloride (0.50 g, 7.19 mmol) in ethanol (80 mL) was
added. The resultant precipitate was filtered off. The filtrate
was dripped into a solution of 1(a) (1.67 g, 3.58 mmol) in
CHCl3:EtOH / 1:20 (120 mL) over 2h. The resultant pale
yellow solution was stirred overnight at RT. The solvent was
removed under reduced pressure to give a pale yellow solid
which was recrystallized from hot toluene. Yield = 65%, δH
(500 MHz; CDCl3; Me4Si): 1.33 (18 H, s, CH3), 2.61 (8 H, s,
typified
2H) DMF]BF4ꢁDMF,
H)2Br]Br·1.25DMSO·H2O·MeOH,
constraints of the complex provide a strong binding site for the
smaller bromide anion in , whereas larger anions such as
by
the
complexes
[Cu2(L4ꢀH)(L4ꢀ
[Cu2(L4ꢀ
⊂
1
,
and
The
2.
geometric
2
tetrafluoroborate are now excluded as demonstrated in 1.
Experimental Section
Unless specified, commercial regents and solvents were used
1
without purification. H and 13C NMR spectra were recorded
N(CH2)2), 3.77 (4 H, s, PhꢀCH2N), 7.40 (2 H, d,
H), 7.63 (2 H, d, = 2.53, PhꢀH), 10.27 (2 H, s, CHO). Found:
C, 65.69; H, 8.32; N, 10.13. Calc. for
J = 2.63, Phꢀ
on Bruker Avance 400 MHz and 500 MHz spectrometers; δ
values are relative to TMS or the corresponding solvent. Mass
J
spectra were obtained using
a Micromass ZMD 400
C28H40N4O4·0.3Toluene·1.5H2O: C, 65.70; H, 8.30; N,
10.11%. m/z (ESI) 497.97 (L4+H)+. υmax/cmꢀ1 1616.6 (C=N).
6,6´ꢀ(1,4ꢀpiperazineꢀbis-(methylene)ꢀbisꢀ(2ꢀ(hydroxyimino)ꢀ
4ꢀmethyl) phenol, L4a: To a solution of KOH (0.324 g, 5.76
mmol) in EtOH (60 mL), a solution of hydroxylamine
hydrochloride (0.400 g, 5.76 mmol) in EtOH (60 mL) was
added. The resultant precipitate was filtered off. The filtrate
electrospray spectrometer. IR spectra were recorded on a
Nicolet 5700 FTꢀIR spectrometer from Thermo Electron
Corporation using an ATR sampling accessory. UVꢀVis spectra
were recorded in THF using a CARY 100Bio UVꢀVis
spectrometer. Elemental analyses were determined by the
2 | J. Name., 2012, 00, 1-3
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DOI: 10.1039/C5DT02551F
was dripped into a solution of 1(b) (1.67 g, 3.58 mmol) in
further recrystallised by the slow vapour diffusion of diethyl
CHCl3:EtOH / 1:20 (120 mL) over 30 minutes. The resultant ether into a MeOH solution containing the complex. Yield =
pale yellow solution was stirred overnight at RT during which
a pale yellow precipitate was formed which was washed with
0.344 g, 82%. Found: C, 56.96, H, 5.92, N, 6.10. Calc. for
C80H96B4Cu2F16N8O4·Et2O: C, 56.61, H, 6.00, N, 6.29%. m/z
CHCl3. Yield = 0.321 g, 41%. δH (500 MHz; DMSO; Me4Si): (ESI) 617.1 (L5 H)+, 679.1 [(L5 + Cu]+, 745.9 [(L5+Cu2]+,
ꢀ
2.19 (6 H, s, CH3), 3.59 (8 H, s, N(CH2)2), 3.62 (4 H, s, Phꢀ
CH2N), 6.96 (2 H, d, = 1.92, PhꢀH), 7.23 (2 H, d,
= 1.74, [Cu2(L5)2Br]Br3·2MeOH, (4): The general method outlined
PhꢀH), 8.27 (2 H, s, CHN). δC (500 MHz; DMSO): 20.5, 52.4, above, except that diethylether was required to precipitate the
58.4, 118.5, 123.2, 126.5, 127.8, 131.6, 146.9, 153.6 ppm. complex initially from solution, was followed using copper(II)
Found: C, 63.59; H, 6.84; N, 13.58. Calc. for bromide. The green powder obtained was further recrystallised
831.9 [(L5+Cu2+BF4]+. υmax/cmꢀ1 1592 (C=N).
J
J
C22H28N4O4·0.2Ethanol: C, 63.80; H, 6.98; N, 13.29%. m/z by the slow vapour diffusion of diethyl ether into a MeOH
(ESI) 451.71 (L4a
υmax/cmꢀ1 1625.2 (C=N).
6,6´ꢀ(1,4ꢀpiperazineꢀbis-(methylene)ꢀbisꢀ(4ꢀtertꢀbutylꢀ2ꢀ
(phenylimino)) phenol, L5: To 1(a) (0.303 g, 0.65 mmol)
dissolved in CHCl3 (20 mL) was added aniline (0.150 g, 1.61
mmol). The yellow solution was stirred and heated at reflux
overnight. The solvent was partially removed and EtOH (10
+
K)+, 436.68 (L4a
+
Na)+, 413.73 (L4a
+
H)+. solution containing the complex. Yield = 0.168 g, 48%. Found:
C, 56.96, H, 5.94, N, 6.92. Calc. for C80H96Br4Cu2N8O4: C,
57.18, H, 5.76, N, 6.67%. m/z (ESI) 618.09 (L5 H)+, 678.99
+
[(L5+Cu]+, 746.6 [(L5+Cu2]+, 821 [(L5+Cu2+Br]+. υmax/cmꢀ1
1600 (C=N).
[Cu3(L4aꢀH)3Py3](BF4)2·(MeOH)3·PF6ꢁ(H2O)3, (5). To the
ligand L4a (0.206 g, 0.50 mmol) suspended in MeOH (12.5
mL) was added. The solution was then sonicated (30 min). mL), was added Cu(BF4)2ꢁH2O (0.255 g, 1.00 mmol) dissolved
Finally, all solvent was removed and the solid was recovered in MeOH (12.5 mL). After full dissolution, pyridine (2 mL)
by precipitation with EtOH (35 mL). Yield = 0.279 g, 88%. δH was added to the green coloured solution followed by NaPF6
(500 MHz; CDCl3; Me4Si): 1.37 (9 H, s, CH3), 2.74 (4 H, s, (0.042 g, 0.25 mmol). The dark green coloured solution was
CH2), 3.78 (2 H, s, PhꢀCH2ꢀN), 7.28 (2 H, q, PhꢀH), 7.30 (1 H, stirred for 3h, filtered, and the filtrate was left to evaporate
s, PhꢀH), 7.42 (1 H, s, PhꢀH), 7.44 (2 H, q, PhꢀH), 7.48 (1 H, s, slowly. Square shaped, green coloured xꢀray quality crystals
PhꢀH), 8.66 (1 H, s, CHN), 13.2 (1 H, s, OH). Found: C, were produced after two weeks. The compound was then
77.24; H, 7.99; N, 9.12. Calc. for C40H48N4O2·0.33EtOH: C, purified by diffusing diethylether into the complex dissolved in
77.27; H, 7.97; N, 8.86%. m/z (ESI) 617.95 (L5+H)+. υmax/cmꢀ1
1618.4 (C=N).
Copper salt complexes (general procedure):
A suspension of Ligand L4 or L5 (0.50 mmol) in MeOH (10
mL) was stirred at 40 °C and an equimolar amount of the [Cu3(L4a
copper(II) salt in H2O (10 mL) was added. The resulting
DMF. Yield = 0.320 g, 91%. Found: C, 49.05, H, 5.32, N,
10.58. Calc. for C81H96B2Cu3F14N15O12P·DMF: C, 49.12; H,
5.05; N, 10.91%. m/z (ESI) 474.1210 [Cu3(L4a
ꢀ
H)]+, 535.0393
2H)]+, 1010.1864
ꢀ
3H)]+. υmax/cmꢀ1 1683.6 (C=N).
[Cu2(L4a
ꢀ
ꢀ
3H)]+. 949.2635 [Cu2(L4a
H)(L4a
ꢀ ꢀ
H)(L4a
XꢀRay structure determination
solution was cooled to room temperature and stirred overnight.
The solid was filtered, washed with diethyl ether and dried in Xꢀray data of complexes
1ꢀ5
were recorded at low temperature
vacuo
.
with a RigakuꢀSpider Xꢀray diffractometer, comprising a
DMF]BF4ꢁDMF, (1). The general Rigaku MM007 microfocus copper rotatingꢀanode generator,
[Cu2(L4ꢀH)(L4ꢀ2H)
⊂
method outlined above was followed using copper(II)
tetrafluoroborate monohydrate. The green powder obtained was mirror optics (Cu K
further recrystallised with slow diffusion of diethyl ether into a imageꢀplate detector. CrystalClear16 was utilized for data
DMFꢀMeOH solution of the complex. Yield = 0.116 g, 64%. collection and FSProcess in PROCESSꢀAUTO17 for cell
highꢀflux Osmic monochromating and focusing multilayer
radiation, λ = 1.5418 Å), and a curved
α
Found: C, 54.72, H, 6.78, N, 10.01. Calc. for
refinement and data reduction. All structures were solved
C56H77N8O8Cu2·BF4·2DMF: C, 55.15, H, 6.79, N, 10.37%. m/z employing direct methods and expanded by Fourier
(ESI) 497.9 (L4+H)+, 558.8 [(L4ꢀH) Cu]+, 622.1 [(L4ꢀH) Cu2]+, techniques.18 All nonꢀhydrogen atoms were refined using
1115.9 [(L4ꢀ2H)2 Cu2]+, 1204.0 [(L4ꢀ2H)2 Cu2 BF4]+. υmax/cmꢀ1
anisotropic thermal parameters. The hydrogen atoms were
included in the ideal positions with fixed isotropic U value and
1582.6 (C=N).
[Cu2(L4ꢀH)2Br]Br·1.25DMSO·H2OꢁMeOH, (2). The general were riding on their respective nonꢀhydrogen atoms. For
method outlined above was followed using copper(II) bromide. complex
The product was further recrystallised by the slow diffusion of tetrafluoroborate anion (25:75) and for one tꢀbutyl group
diethyl ether into a DMSOꢀMeOH solution of the complex. (44:56). For complex positional disorder was modelled for
Yield = 0.107 g, 94%. Found: C, 50.64, H, 6.32, N, 8.22. Calc. the tetrafluoroborate anion (25:75) and for one tꢀbutyl group
1 positional disorder was modelled for the
2
for C56H78N8O8Cu2Br2·1.25DMSO·H2O: C, 50.41; H, 6.33; N, (72:28). Disordered solvent regions in the structures 1, 2, 4 and
8.04%. m/z (ESI) 497.0 (L4)+, 558.8 [(L4ꢀH) Cu]+, 701.6 [(L4ꢀ
H) Cu2 Br]+, 1115.9 [(L4ꢀH)2 Cu2 Br]+, 1197.6 [(L4ꢀ2H)2 Cu2
Br]+. υmax/cmꢀ1 1590.6 (C=N).
5
were treated in the manner described by van der Sluis and
Spek,19 resulting in the removal of 130 eꢀ for
1
, 81 eꢀ for 2, 24
eꢀ for
4
and 80 eꢀ for
5
per cell respectively. These values
, 1xMeOH (18) for and
4 per formula unit respectively. The
[Cu2(L5)2BF4](BF4)3, (3): The general method outlined above, approximate to 2xMeOH (36) for
1
2
5
except that diethylether was required to precipitate the complex and 0.33xMeOH (6) for
initially from solution, was followed using copper(II) data measurement and other refinement parameters for crystal
tetrafluoroborate monohydrate. The green powder obtained was structures are given Table 1.
This journal is © The Royal Society of Chemistry 2012
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DOI: 10.1039/C5DT02551F
Table 1 Crystal data and structure refinement for complexes 1ꢀ 5.
1
2
3
4
5
Empirical
formula
C62H92BCu2F4N10O10
C59.33H90.67Br2Cu2N8O11S1.167
C164H204B10Cu4F32N16O12.5
C82H103Br4Cu2N8O6
C81H98B2Cu3F14N15O13P
Formula
weight
Crystal system
space group
1351.34
1416.37
3569.68
Triclinic
1743.44
1998.95
Monoclinic
P 21/a
Monoclinic
P 21/c
Monoclinic
P21/n
Monoclinic
P 21/n
ꢀ
P 1
a (Å)
b (Å)
c (Å)
α (°)
β (°)
γ (°)
16.0641(4)
21.0442(5)
22.954 (2)
90
107.834 (8)
90
15.4887(3)
28.5296(5)
17.255(1)
90.00
97.390(7)
90.00
13.8450(3)
16.2809(3)
20.972(2)
93.011(7)
100.424(7)
101.741(7)
4532.3(4)
1
19.1144(4)
24.4307(4)
19.189(1)
90.00
93.644(7)
90.00
17.433(5)
28.692(4)
18.949(3)
90
90.051(5)
90
Volume (Å3)
Z
7386.9(7)
4
7561.3(6)
4
8942.4(7)
4
9478(3)
4
Reflections
collected /
unique
65462/8961
[R(int) = 0.073]
83050/10698
[R(int) = 0.0781]
59339/14809
[R(int) = 0.0474]
98204/15072
[R(int) = 0.0457]
70527/16003
[R(int) = 0.079]
Data /
8961/1141/697
1.04
10698/610/768
0.955
14809/487/1165
1.100
13641/1237/966
1.085
16003/191/1154
restraints /
parameters
Goodnessꢀofꢀ
fit on F2
Final R indices
[I>2sigma(I)]
1.10
R1 = 0.0866
wR2 = 0.2517
R1 = 0.0780
wR2 = 0.2117
R1 = 0.0929
wR2 = 0.2686
R1 = 0.0687
wR2 = 0.2004
R1 = 0.0863
wR2 = 0.2530
R indices (all
R1 = 0.1255
R1 = 0.1012
R1 = 0.1122
R1 = 0.0797
R1 = 0.1003
data)
wR2 = 0.2806
wR2 = 0.2424
wR2 = 0.2942
wR2 = 0.2171
wR2 = 0.2700
CCDC No.
1042536
1042537
1042539
1042540
1042538
which is in contrast to our examples reported here. There is a
DMF solvent molecule occupying the newly formed cavity
with a second DMF molecule lying in the crystal lattice at the
more open end of the complex.
Results and discussion
Solidꢀstate samples of Cu(II) salt complexes 1ꢀ4 and containing
The O(phenolate)ꢀCu(II) bonds are in the range 1.896(4) to
and 1.907(4) Å in length, while the N(oxime)ꢀCu(II) bond
lengths are in the range 1.935(1) to 1.967(1) Å. Important bond
lengths and angles are listed in Table 2. The formation of this
metalloꢀmacrocycle is supported by intramolecular Hꢀbonds
between the oxime hydrogen atoms and the phenolate oxygen
atoms of the complementary ligand with (oxime)Oꢀ
H···O(phenolate) donorꢀtoꢀacceptor distances lying between
2.657(4) and 2.812(4) Å. In addition, an OꢀH···N interaction
has formed towards the piperazine linker with O223ꢀ
H223···N642 and O233ꢀH233···N612 donor to acceptor
distances being 2.944(16) Å and 2.927(16) Å, respectively.
These multiple intramolecular Hꢀbonding interactions
combined with the shorter piperazine linker have seriously
impacted the conformational freedom of the ligand resulting in
an openꢀmouthed Pacmanꢀlike cleft arrangement of the two
metalꢀcoordinating subunits in the metalloꢀmacrocycle (Figure
S1). See Table S1 for selected Hꢀbonding interactions
ꢀ
BF4 and Brꢀ anions were readily prepared by the direct
combination of L4 or L5 and the appropriate Cu(II) salt in
methanol. The complex,
5
was prepared from
a
MeOH/pyridine solution of the ligand L4a in the presence of
NaPF6 at RT. The decreases in the C=N stretching frequencies
in complexes 1ꢀ5 when compared to those in the corresponding
ligands L4 L4a and L5 indicate that coordination of the ligand
,
to the metal cation via the imine nitrogen atom has successfully
occurred. Crystals suitable for single crystal Xꢀray structure
determination were obtained for the complexes 1ꢀ5 and
structural elucidation was performed on each to investigate the
effect of incorporating the piperazine linker into the ligand
design. In addition, a direct structural comparison could be
made between the receptors having oxime functionality
(salicylaldoximeꢀbased receptors; complexes
those having imine functionality (salicylaldimineꢀbased
receptors; complexes and ). Unfortunately, the poor
1 and 2) and
3
4
solubility of these complexes excluded the possibility to
perform a comprehensive study on solutionꢀbased anion
involving complex 1.
uptake. Complex
5 was synthesised to investigate whether
coordinating solvent would have any structural influence on the
complex.
The cation of the copper complex
1 is shown in Figure 1. Both
the copper(II) centres are occupying slightly distorted square
planar geometries. Each of the two metal centres is coordinated
by two phenolate Oꢀatoms and two oxime Nꢀatoms, a set of
each from two different ligand molecules with a metalꢀtoꢀ
ligand coordination ratio of 2:2. Each ligand is acting in a
bidentate bridging fashion between the two metal centres with
the piperazine groups linking the metal coordination sites. A
search of the literature reveals that ligands with piperazine
incorporated near the metal binding site (as is the case in the
reported examples here) typically bind in a chelating fashion
such as in the examples of Kandaswamy and coꢀworkers20
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Figure 1. Molecular structure of complex
1 with atom numbering scheme,
and N612) effectively ‘tying up’ two of the amines in the
complex. The two other piperazine nitrogen atoms N622 and
N632 are pointing away from their nearby oxime moieties with
a (piperazine)N···O(oxime) distance in the range of 4.027(1) –
4.286(2) Å effectively ruling out any hydrogen bonding
interactions with the nearest neighbour oxime group. Out of
these two, N622 makes no hydrogen bonds nor accepts any and
thus is not protonated, whereas the piperazine nitrogen atom
N632 is protonated and forms a Hꢀbonding interaction with
showing the intra-molecular bonding within the complex. Some selected
hydrogen atoms, the tetrafluoroborate counterion and the non-encapsulated
DMF solvent molecule have been omitted for clarity.
Table 2 Selected bond lengths and angles for complexes 1, 2, and 5.
Atoms
Bond length (/Å)
XꢀCuꢀX
Bond angle (/°)
ꢀ
1
atom F13 and F14 of the BF4 counterion (Figure 2) and is
Cu1 O11
Cu1 O13
Cu1 N232
Cu1 N212
Cu2 O12
Cu2 O14
Cu2 N222
Cu2 N242
1.907(4)
1.903(4)
1.947(1)
1.967(1)
1.903(4)
1.896(4)
1.935(1)
1.942(1)
O11 Cu1 O13
O11 Cu1 N212
O11 Cu1 N232
O13 Cu1 N212
O13 Cu1 N232
N212 Cu1 N232
O12 Cu2 O14
O12 Cu2 N222
O12 Cu2 N242
O14 Cu2 N222
O14 Cu2 N242
N222 Cu2 N242
2
O11 Cu1 O13
O11 Cu1 N232
O13 Cu1 N232
O11 Cu1 N212
O13 Cu1 N212
N212 Cu1 Br1
N232 Cu1 N212
O11 Cu1 Br1
O13 Cu1 Br1
N232 Cu1 Br1
5
177.5(2)
89.6(1)
87.2(1)
91.5(1)
91.7(1)
176.73(7)
176.1(2)
93.0(1)
88.9(1)
88.1(1)
90.0(1)
177.86(7)
therefore protonated. Tasker et al. reported metal complexes of
salicylaldoximeꢀbased ligands where the tertiary amines are
similarly involved in oximeꢀtoꢀphenolate hydrogen bonds
(hydrogenꢀtoꢀacceptor distances 2.052(2) Å and 2.154(2) Å)
and oximeꢀtoꢀpiperidino hydrogen bonds (hydrogenꢀtoꢀ
acceptor distance 2.119(3) Å and 2.249(2) Å).21 In the Tasker
example all the tertiary nitrogen atoms are Hꢀbond acceptors
with the oxime hydroxyl groups; none of them are protonated.
However, when the orientation of the structure precludes
(tertiary)N···H(oxime) contacts and interactions with nearby
acceptors (e.g., counterions such as Xꢀ, NO3 , BF4 ) are
possible, then the tertiary nitrogen atoms are generally
protonated and the system is stabilised by (tertiary)Nꢀ
H···Acceptor(counter ion) hydrogenꢀbonding interactions.
ꢀ
ꢀ
Cu1 O11
Cu1 O13
Cu1 N232
Cu1 N212
Cu1 Br1
1.899(5)
1.911(5)
1.942(5)
1.972(6)
2.871(1)
2.833(1)
1.928(5)
1.902(5)
1.950(6)
1.961(6)
168.5(2)
87.3(2)
92.8(2)
89.6(2)
88.6(2)
95.8(2)
170.9(2)
95.8(1)
95.6(1)
93.0(2)
Such interactions have been observed in the complexes bis(5ꢀtꢀ
butylꢀ2ꢀoxyꢀ3ꢀpiperidinꢀ1ꢀylmethylbenzaldehyde oximeꢀN,O)ꢀ
copper(II) dinitrate [(tertiary)NH···O(nitrate), 2.015(2) Å and
Cu2 Br1
Cu2 O12
Cu2 O14
Cu2 N222
Cu2 N242
2.441(3)
Å]
and
bis(ꢀ2ꢀ5ꢀtꢀbutylꢀ2ꢀoxyꢀ3ꢀpiperidinꢀ1ꢀ
ylmethylbenzaldehyde
oximeꢀN,O,O)ꢀtetrachloroꢀdiꢀzinc(II)
chloroform solvate [(tertiary)NꢀH···Cl distances 2.484(2) Å
and 2.557(3) Å].22
In 1
, the short piperazine linker of L4 leads to an observed
Cu···Cu distance of 5.604(1) Å, which is significantly shorter
than the corresponding distance in similar complexes.9, 10, 11, 12
Thus, a smaller cavity size coupled with the Pacmanꢀlike cleft
leads to an exclusion of the tetrahedral anion, preventing it
from entering the cavity and coordinating to the metal centres.
Cu1 O4
Cu1 O5
Cu1 N1P
Cu1 N242
Cu1 N252
Cu2 O2
1.947(4)
1.930(4)
2.279(5)
1.964(5)
1.960(5)
1.897(4)
1.903(4)
2.184(5)
2.019(5)
2.027(5)
1.925(4)
1.936(4)
2.292(5)
1.975(5)
1.982(5)
O4 Cu1 O5
174.6(2)
93.2(2)
90.0(2)
88.0(2)
92.2(2)
88.2(2)
91.2(2)
103.7(2)
104.1(2)
152.2(2)
179.4(2)
88.7(2)
90.3(2)
89.4(2)
90.8(2)
89.9(2)
90.9(2)
121.0(2)
112.7(2)
126.3(2)
O4 Cu1 N1P
O4 Cu1 N242
O4 Cu1 N252
O5 Cu1 N1P
O5 Cu1 N242
O5 Cu1 N252
N1P Cu1 N242
N1P Cu1 N252
N242 Cu1 N252
O2 Cu2 O6
O2 Cu2 N2P
O2 Cu2 N222
O2 Cu2 N262
O6 Cu2 N2P
O6 Cu2 N222
O6 Cu2 N262
N222 Cu2 N2P
N262 Cu2 N2P
N222 Cu2 N262
Cu2 O6
Cu2 N2P
Cu2 N222
Cu2 N262
Cu3 O1
This is clearly observed in the crystal packing of complex
1
where a nonꢀcoordinated DMF molecule is located within the
cavity of the metalloꢀmacrocycle (Figure 1) and the BF4ꢀ anion
is located on the periphery (Figure 2). Expanding the packing
Cu3 O3
Cu3 N3P
Cu3 N212
Cu3 N232
ꢀ
reveals that in fact each of the BF4 anions is linked by a
bifurcated intramolcular NꢀH···F interaction from an
ammonium group of one complex molecule and by weak Cꢀ
H···F interaction from an adjacent molecule (Figure 2). The Nꢀ
H···F donor to acceptor distances are 2.827(4) and 2.980(4) Å
(Table S1).
The ligand L4 for complex
1 is present in a partial zwitterionic
form; all four of the phenol oxygen atoms are deprotonated;
however, just one of the tertiary amine nitrogen atoms is
protonated. Thus, the overall charge from the two ligands is ꢀ3,
ꢀ
this combined with the single BF4 ion present balances the +4
charge created by the two copper ions. The partial protonation
of the amines is not unexpected but it is something that had not
been observed before in related complexes with longer straps.
Presumably the reason for a single protonation on the amine
groups is due not only to the close proximity of both the
piperazine amine groups to each other within the ligand strap,
but also to the presence of weak intramolecular O(oxime)ꢀ
H···N(piperazine) hydrogen bonds (involving the atoms N642
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-
Figure 2. Showing the H-bonding interactions involving BF4 anions present in
the crystal lattice in complex . The hydrogen atoms other than those involved
1
in H-bonding have been removed for clarity.
The treatment of ligand L4 with one equivalent of copper(II)
bromide in MeOH and subsequent recrystallisation from
DMSO/EtOH by slow diffusion of Et2O resulted in the bromide
encapsulated complex
shown in Figure 3. The coordination environment around each of
the two copper(II) centres is very similar to , being coordinated by
2. The molecular structure of complex 2 is
1
two deprotonated phenol Oꢀatoms and two oxime Nꢀatoms from the
two ligand molecules. In addition to this, the two copper(II) centres
are linked to each other via a bromide bridge. Each copper(II) centre
sits, therefore, within a distorted squareꢀpyramidal geometry. The
central bromide anion has bond distances of 2.870(2) and 2.834(3)
Å to Cu(1) and Cu(2) respectively with a corresponding CuꢀBrꢀCu
angle of 138.71(4)º. The Cu···Cu distance is shorter by 0.267 Å than
that in
1 at 5.337(3) Å. In a related 2:2 (metal: ligand) complex (ꢂ2ꢀ
Figure 3. The molecular structure of complex
2
showing the coordinated
bromo)ꢀtetrakis(1,3ꢀbis(1ꢀ(3ꢀ(2ꢀpyridyl)ureido)ꢀ1ꢀmethylethyl)
benzeneꢀN,O)ꢀdiꢀcopper(II) tribromide, where the two Cu(II)
centres are also coordinated by N and O donors in distorted squareꢀ
pyramidal environments, the Cu···Cu distance is longer at 5.536(3)
Å, yet the CuꢀBr distances are shorter at 2.769(4) Å and 2.767(4) Å
and the CuꢀBrꢀCu angle is almost linear at 178.7(3)°.23 Therefore,
the piperazine···oxime hydrogen bonds create the Pacmanꢀlike cleft
with a short MꢁꢁꢁM separation that imposes a nonꢀlinear bridging
angle for the bromo complex. The geometry of the ligand is
therefore strongly influencing the coordination of the bromide
anion, in fact, this anion barely fits the complex cavity at all. The
coordination sphere of the Cu(II) centres is square pyramidal with τ
= 0.04 and 0.01 for Cu1 and Cu2, respectively.24 The protonation of
the piperazine nitrogen atoms also follows a similar trend as
bromine, the DMSO solvent molecule and the intra- and inter-molecular
hydrogen bonds. Selected hydrogen atoms, water and a methanol molecule
have been omitted for clarity.
As mentioned earlier, for each of our previously published
complex salts of the (2 : 2) metalꢀtoꢀligand type that contain
four potentially protonatable tertiary amine groups within the
straps, in every case all four tertiary nitrogen atoms are
protonated and most are involved in NꢀH···Acceptor hydrogen
bonding interactions with the encapsulated counter ion. The
hydrogenꢀtoꢀacceptor distances in these cases varies within the
range 1.889(4) Å to 2.909(4) Å depending upon the size of the
10, 11, 12
linker involved or counterion used.9,
In the soꢀcalled
‘metalꢀonly’ complexes where the complexes have been
crystallised in the absence of a counterion (where effective
competition with a basic anion has rendered the amines free of
protons) the tertiary nitrogen atoms in the ligand straps are
observed in 1. The singly protonated amine Nꢀatoms and doubly
deprotonated phenolate Oꢀatoms in each ligand molecule generate a
total ꢀ2 charge. A further ꢀ2 charge arises from the two Brꢀ
counterions and is balanced by the +4 charge from two copper ions.
The two nitrogen atoms, N642 and N612, are involved in similar
involved in (oxime)OꢀH···N hydrogen bonding interactions.11,
12
Complex
2 has an encapsulated bromide (counter ion) but in
this case either the piperazine nitrogen atoms are too far away
(N642···Br1, 4.557(2) Å; N612···Br2, 4.539(2) Å) or pointing
away from the bromide anion (N632 and N622) thereby ruling
out any possibility of Hꢀbonding interactions with the
coordinated bromide.
(
oxime)OꢀH···N(piperazine) hydrogen bonds with O223ꢀ
H223···N642 and O233ꢀH233···N612 distances being 2.975(6) Å
and 2.926(7) Å. The third one (N622) points away from the oxime
moiety makes a hydrogen bond to the nonꢀcoordinated oxygen atom
from a solvate methanol molecule and is protonated (Figure 3). The
piperazine nitrogen atom N632 is also protonated as it is involved in
an NꢀH···Br hydrogenꢀbonding interaction with the bromide ions
present in the crystal lattice (Figure 3). This bromide anion Br2 is
located in the periphery and bound by a moderate NꢀH···Br and
weaker CꢀH···Br hydrogenꢀbonding interactions. The NꢀH···Br
distance is 2.301(5) Å while the CꢀH···Br interaction (CꢀH···Br
distance 2.891(3) Å; N···Br separation 3.838(2) Å) is formed
towards a neighbouring complex molecule. This results in a
bromideꢀbridged 1ꢀD Hꢀbonded polymer along the crystallographic
bꢀaxis in the crystal packing (Figure 4). The important Hꢀbonding
Figure 4. Showing the (piperazine)N-H···Br and C-H···Br H-bonding one
dimensional chain along the b axis in case of complex 2.
interactions and parameters for
2 are listed in Table S1.
One equivalent of the ligand, L4a (a modification of the ligand L4 by
the replacement of the tꢀBu group with a methyl group at the R2
position) and one equivalent of the copper salt in a MeOH/pyridine
solution were reacted together, treated with NaPF6 and then
crystallised, resulting in the isolation of complex
5. The new
complex contains three copper cations each of which is bound to
two ligands forming a tubular structure. Each of the three ligands is
6 | J. Name., 2012, 00, 1-3
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present as [H3L4a]ꢀ where (within each ligand) both phenol oxygen
atoms are deprotonated and one of the two tertiary amine nitrogen
atoms is protonated. Thus, a total charge of ꢀ3 is created by three
accompanied by two Cu(II) ions and the corresponding counter
ꢀ
anion. It was reported that the anions, Clꢀ and NO3 in the dicopper
complexes, [Cu2L4a(Cl)]H2O and [Cu2L4a(NO3)]H2O, are
ꢀ
ꢀ
ligand molecules with a further ꢀ3 charge from two BF4 and one
coordinated, while in the [Cu2L4a]ClO4.CH3OH complex, ClO4 ion
PF6 anions. The total negative charge of ꢀ6 is balanced by three
remains uncoordinated.20
ꢀ
copper(II) cations. These anions sit outside the complex and two of
these form moderate hydrogen bonds with the protonated amines in
the arms of the ligands. Two of the three copper cations lie in a
slightly distorted squareꢀpyramidal geometry with the third
occupying
a
distorted trigonalꢀbipyramidal geometry. The
coordination sphere around each Cu atom is comprised of two
phenolate Oꢀatoms and two oximic Nꢀatoms from two of the
neighbouring ligands and the fifth coordination site is occupied by a
pyridine molecule in all cases. See Table 2 for selected bond lengths
and angles around the metal centres. The pyridine molecule
coordinated to the Cu2 sits outside the spherical unit while the other
two pyridine molecules coordinated to Cu1 and Cu3 sit inside the
tube. The Cu(II)ꢀO(phenolate) bond distances are in the range
1.897(4)ꢀ1.948(4), while the Cu(II)ꢀN(oxime) bond distances lie in
the range 1.960(5)ꢀ2.026(5). Axial positions on Cu1 and Cu3 are
taken up by a pyridine molecules whereas both axial positions on
Cu2 are occupied by phenolate Oꢀatoms (O2 and O6, Figure 5) from
two of the neighbouring ligands. Thus, the geometry of Cu1 and
Cu3 is square pyramidal while Cu2 is located in a trigonal
bipyramidal environment despite the fact that all three metal centres
share the same five donor set from two of the adjacent ligands. The
cause of this change in coordination geometry for Cu2, may be due
to a number of factors, for instance the cage complex is not capable
of accommodating a third pyridine molecule within. This, in
addition to the hydrogen bond mediated preorganised coordination
site [see Figure 5 for NꢀOHꢁꢁꢁN(amine) hydrogen bonds around Cu2]
Figure 5. Exhibiting the molecular structure and the H-bonds of the cation,
[Cu3(L4a-H)3Py3]3+ of
5 along with atom labelling scheme. Hydrogen atoms (other
than those on the piperazine nitrogen atoms and involved in H-bonding) have
been omitted for clarity.
Replacement of the oxime functionality around the metal
binding site by an imine moiety relaxes the conformational
restrictions within the ligand as evidenced by the Xꢀray
structure of [Cu2(L5)2](BF4)4,
3
(Figure 6). The coordinating
atoms around each of the two copper(II) centres in complex
are essentially the same as in and the deprotonated
3
has given rise to
a change in coordination geometry. The
1
2;
transformation from square pyramidal geometry to trigonal
bipyramidal geometry has elongated the Cu2(II)ꢀN(oxime) bond
length with respect to the corresponding distances of the Cu1 and
Cu3, and it is observed that the metal centres Cu1 and Cu3 sit a
similar distance from Cu2 (Cu1···Cu2 = 7.268(1) Å, Cu2···Cu3 =
7.597(1) Å, and Cu1···Cu3 = 9.448(2) Å). The inclusion of
coordinated pyridine molecules within the structure prevents anion
encapsulation within the cavity. The oximic –OH and phenolate Oꢀ
atoms, as well as the nonꢀprotonated tertiary amine Nꢀatoms, form
phenolꢀO atoms and imine (earlier oxime) Nꢀatoms with CuꢀN
distances being in the range 1.973(2) – 1.997(2) Å and the Cuꢀ
O distances being in the range from 1.871(4) to 1.888(3) Å.
The ligands exist in a zwitterionic form with deprotonated
phenol groups balanced by protonated amines in each ligand.
Thus charge balance considerations identify one (disordered)
BF4ꢀ anion within the cavity with a further two BF4ꢀ anions and
a BF3OMeꢀ anion (positionally disordered over two sites) lying
in the lattice space. The total anion charge count of ꢀ4 thus
balances the +4 charge of the cation created by the two
copper(II) ions.
intermolecular hydrogen bonds throughout
5 (Figure 5). The
presence of these Hꢀbonds within the copper dimers of oximeꢀbased
ligands was reported in 2010 by Tasker et al.21
The important bond lengths and angles for complex
3 are
In 2002, Kandaswamy and coworkers reported the binuclear copper
and nickel complexes of L4a with a different metal coordination
summarized in Table 3. The replacement of the oxime =N–OH
groups in L4 by imine =NꢀR groups in L5, have resulted in
striking changes to the geometrical parameters such as cavity
size, shape, Cu···Cu distances and ability to encapsulate
pattern from the pattern observed in the complex 5 20
. A monocopper
complex was synthesised by reacting equimolar amounts of
copper(II) acetate and 1(b). The Cu(II) in the mononuclear complex
is coordinated by the two phenolateꢀO atoms and the tertiary amineꢀ
N atoms of the ligand. A series of binuclear copper compounds was
then obtained by treating one equivalent of the aforementioned
monocopper compound with an equal amount of the copper(II) salt
anions. These can be understood by comparing complexes
1
and as both have been synthesised under identical conditions
3
(same metal salt, solvent and reaction conditions) but using
ligands which differ by the presence and absence of the oxime
moiety.
ꢀ
ꢀ
(ClO4 , Clꢀ, and NO3
) followed by the same amount of
hydroxylamine and triethylamine in methanol. One of the Cu(II)
ions is coordinated by the two tertiary amineꢀN atoms and the two
phenolateꢀO atoms, while the other Cu(II) is coordinated by the two
oximicꢀN atoms and the same two phenolateꢀO atoms, creating two
oxygen bridges between the copper ions. Thus the ligand is present
as [HL4a]3ꢀ where both phenol oxygen atoms and one of the oximic
oxygen atoms are deprotonated. This form of the ligand is
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have deviations of 0.330, 0.324, ꢀ0.481 and ꢀ0.550 Å from this
plane. The absence of the restrictive hydrogen bonding around
the metal centre now results in more conformational freedom
of the complex 3, so the cleft arrangement as observed in 1 is
no longer present and the Cu···Cu distance has increased to
7.286(4) Å, which results in an increase in the cavity volume
and the subsequent inclusion of the tetrafluoroborate anion
(Figure 7(b)).
Figure 6. Showing the molecular structure of complex 3 along with atom
labelling scheme. Hydrogen atoms (other than those on the piperazine nitrogen
atoms) have been omitted for clarity.
Table 3 Selected bond lengths and angles for complexes 3 and 4.
Atoms
Bond length (Å)
XꢀCuꢀX
Bond angle (°)
3
Cu1 O11
Cu1 O14
Cu1 N212
Cu1 N242
Cu2 O12
Cu2 O13
Cu2 N222
Cu2 N232
1.879(3)
1.883(3)
1.985(4)
1.974(4)
1.872(3)
1.874(3)
1.975(4)
1.982(4)
O11 Cu1 O14
O11 Cu1 N212
O11 Cu1 N242
O14 Cu1 N212
O14 Cu1 N242
156.4(2)
93.0(2)
93.8()
89.5(2)
93.9(2)
N242 Cu1 N212 154.6(2)
O12 Cu1 O13
O12 Cu2 N222
O12 Cu2 N232
O13 Cu2 N222
O13 Cu2 N232
155.7(2)
93.9(2)
91.3(2)
91.9(2)
93.5(2)
Cu1 F13
Cu2 F14
Cu1 Cu2
2.83(1)
2.838(9)
7.368(1)
N222 Cu2 N232 154.6(2)
4
Cu1 O11
Cu1 O14
Cu1 N212
Cu1 N242
Cu2 O12
Cu2 O13
Cu2 N222
Cu2 N232
1.900(4)
1.892(3)
2.029(5)
2.026(4)
1.897(3)
1.871(3)
1.991(4)
1.998(4)
O11 Cu1 N212
O11 Cu1 N242
O11 Cu1 O14
O14 Cu1 N212
O14 Cu1 N242
91.3(2)
93.6(2)
168.4(2)
86.3(2)
93.3(2)
N212 Cu1 N242 155.6(2)
O12 Cu2 N222
O12 Cu2 N232
O12 Cu2 O13
O13 Cu2 N222
O13 Cu2 N232
N222 Cu2 N232
93.0(2)
95.6(2)
155.3(2)
87.8(2)
92.7(2)
158.1(2)
Figure 7
involved in H-bonding interactions and lie in the coordination plane. (b) The
coordinating nitrogen atoms (imine moiety) in complex are not involved in any
. (a) The coordinating nitrogen atoms (oxime moiety) in complex 1 are
3
Cu1 Br1
Cu1 Cu2
3.001(3)
7.286(5)
H-bonding and hence deviate considerably from the coordinating plane. The
nitrogen atoms are shown in ball-and-stick mode.
The
molecular
structure
of
the
complex
[Cu2(L5)2Br]·3Br·2MeOH, (
4
) has similarities to complex
3
The OꢀH···O hydrogenꢀbonding interactions in complex
1
ensures all the coordinating atoms lie almost in the same plane
(Figure 7 (a)). A mean plane can be passed through the
coordinating atoms N212, N232, O13 and O11 with ꢀ0.012,
0.012, ꢀ0.007 and ꢀ0.007 Å being their deviation from this
mean plane, respectively. Similarly, a mean plane calculation
can be performed on the atoms N222, N242, O12 and O14 with
ꢀ0.028, ꢀ0.029, 0.0189 and 0.0178 Å being the deviations for
these atoms from that plane. These calculations show that the
oxime nitrogen atoms are very much in the plane of the
coordinating atoms.
(Figure 8); the deprotonated phenolꢀO atoms and imineꢀN
atoms coordinate to the metal centres in the metalloꢀ
macrocycle with 2:2 metalꢀtoꢀligand ratio. The ligand exists is
in a zwitterionic form with the four phenolate groups present
balanced by an equal number of ammonium groups contained
within the straps. The positive charge of the two copper(II)
cations and therefore balanced by the presence of four Brꢀ
anions. The CuꢀO bond distances lie in the range 1.871(3) to
1.899(2) Å while the CuꢀN distances range from 1.994(3) to
2.203(4) Å. There is one bromide anion present inside the
cavity and coordinated to one of the copper(II) centres with a
CuꢀBr distance of 3.001(3) Å, somewhat longer than the typical
axial CuꢀBr axial bond length in squareꢀpyramidal systems.9, 25
There are three more bromide anions present in the asymmetric
unit, thus in this complex all the piperazine nitrogen atoms are
protonated. A careful check of the various intermolecular Hꢀ
bonds confirms this analysis with hydrogen bonds to the
bromide atoms (coordinated and nonꢀcoordinated), (Figure 9)
and to the oxygen atom of one of the methanol molecules
present in the crystal lattice (not shown). The important bond
In complex 3, the intramolecular Hꢀbonds between the now
absent oxime group and the deprotonated phenol group no
longer exist. In the absence of (oxime)OꢀH···N(piperazine) Hꢀ
bonding interactions, the coordinating atoms adopt a distorted
squareꢀplanar geometry with significant outꢀofꢀplane deviations
for these atoms. For example, the mean plane calculation
through the coordinating atoms O14, N212, N242 and O11
now have large deviations (ꢀ0.319, 0.495, 0.475 and ꢀ0.338 Å,
respectively) from the plane. Similarly, the calculated mean
plane passing through the atoms O12, O13, N222 and N232
8 | J. Name., 2012, 00, 1-3
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Journal Name
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ViewAArRticTleICOLnEline
DOI: 10.1039/C5DT02551F
lengths and angles are listed in Table 3 with the hydrogen
bonding interactions and their various parameters summarised
in Table S2.
based receptors have been reported. This is the first structural
report on salicyalaldoxime/salicylaldimine based receptors
possessing a piperazine linker. In general, the incorporation of
the short piperazine linker into the receptor has led to shorter
Cu···Cu distances thereby decreasing the cavity volume. In
addition, geometrical restrictions were imposed by the presence
of the oxime groups around the metal centres through
secondary sphere interactions with the piperazine nitrogens
resulting in a Pacmanꢀlike cleft arrangement of the two metalꢀ
coordinating subunits in the metalloꢀmacrocycle. The oximeꢀ
containing receptor is still able to coordinate to bromide
(complex 2) but this receptor appears unable to encapsulate the
ꢀ
larger BF4 anion (complex
1). Removal of the oxime also
removes the conformational restrictions imposed by its
presence and the imineꢀcontaining receptors are now capable of
encapsulating both tetrafluoroborate (complex
3) and bromide
anions (complex ). We are now actively seeking ways to
4
modify these complexes for the purposes of anion sensing.
Acknowledgements
Figure 8. Showing the molecular structure of complex
labelling scheme used. The hydrogen atoms have been omitted for clarity.
4 along with atom
P.G.P. would like to acknowledge the financial assistance from
the Massey University Research Fund. A.P.S.P. would like to
acknowledge Massey University for the PostꢀDoctoral
fellowship.
A comparison between complex
dramatic influence of Hꢀbonding on the ability to accommodate
the smaller anion. In the case of complex , the presence of the
2 and complex 4 shows the
2
Notes and references
additional intramolecular hydrogen bonding between the
piperazine nitrogen atoms and the oxime hydrogen atoms
prevents the receptor from opening up fully resulting in a
shorter CuꢀCu distance of 5.337(2) Å and a correspondingly
a
Institute of Fundamental Sciences, Massey University, Private Bag 11
222,
Palmerston
North,
New
Zealand.
Eꢁmail:
p.g.plieger@massey.ac.nz; Fax: +64 6 350 5682; Tel: +64 6 356 9099,
extension 84647
smaller cavity space of
2. While this does not prevent the
†
Footnotes should appear here. These might include comments
bromide anion from becoming encapsulated within the
complex it does mean the bromide anion is now shared
between the two copper metal cations, but just barely, as
indicated by the bond length and angles of these bonds (Figure
relevant to but not central to the matter under discussion, limited
experimental and spectral data, and crystallographic data.
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/b000000x/
S2). In complex
4 the intramolecular hydrogen bonding
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as the oxime has been replaced by the imine and thus the
receptor now expands resulting in a bigger cavity space and a
larger Cu···Cu distance of 7.286(5) Å. As a consequence, only
one of the two Cu centres is able to bind with the bromide
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structures of four new dicopper(II) complexes and
a
tricopper(II) complex of salicyalaldoxime/salicylaldimine
This journal is © The Royal Society of Chemistry 2012
J. Name., 2012, 00, 1-3 | 9

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10 | J. Name., 2012, 00, 1-3
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Dalton Transactions
View Article Online
DOI: 10.1039/C5DT02551F
A series of piperazine dicopper metallo-macrocyles have been synthesised and the crystal structures
reveal the extent that intramolecular hydrogen binding influences the resulting structure and ability
to encapsulated anions.