1 : 2 : 1 (Fig. 3(c)). A further change in the distribution of
products is not observed upon addition of more Ba2+. Here,
two regions of the 1H NMR are useful in determining the
distribution of species. The signals due to the thiazolyl groups
are all of similar intensity, as would be expected if a 1 : 2 : 1
mixture is formed (8.60 [Zn2(L2)2]4+ and 8.72 [Zn2(L1)2]4+; 8.62
and 8.80 ppm [Zn2(L2)(L1)]4+). Furthermore, a ‘‘doubling-up’’
of the methylene signals at 4.12 and 4.19 ppm is also observed,
indicating two environments for the crown ether region in the
ligand L1; this is not observed upon addition of Ba2+ to just
[Zn2(L1)2]4+. Unfortunately, due to the amount of Ba2+ re-
quired to form the mixture of helicates ESI-MS was uninforma-
tive as any ions of interest are suppressed by the barium.
Generation of such a distribution indicates that there is now
a statistical mixture of species present, with both homo- and
heteroleptic species present, and that all self-recognition be-
tween the ligand strands is lost. There are two possible reasons
for the formation of a mixture of species. First an allosteric
affect, where upon coordination of the barium ions to the
crown ether unit the ligand partitioning is altered and conse-
quently the ligand acts as a bis-bidentate donor unit in a
similar fashion to L2. This would create a helicate with two
four-coordinate Zn(II) ions allowing formation of the hetero-
leptic species. To form a ‘‘matching’’ set of two bidentate
coordination domains found in the unsubstituted ligand L2, a
decrease in the torsion angle would be required. However, as
can be seen in the solid-state structure of [Zn2(L1)2Ba2]8+ this
does not occur and the inter-domain torsion angle increases
and correspondingly the ligand still acts as a bis-terdentate
donor. Nor is it an effect of the change in pitch length of the
helicate, as to match the pitch length of the complex formed by
the unsubstituted ligand (L2: 4.609 A) the pitch length ob-
served in the zinc complex of L1 (4.812(1) A) would have to
decrease upon addition of Ba2+. However, the opposite effect
was observed and the pitch length increases to 5.142(1) A.
The second possibility is an electrostatic effect, where in the
presence of excess Ba2+ the [Zn2(L1)2]4+ complex will form a
highly charged 8+ ion. The high charge on this helicate
species destabilises the complex favouring formation of the
lower charged mixed ligand system [Zn2(L1)(L2)Ba]6+. Thus
Avance 500 spectrometer. ESI-MS were obtained on either a
Bruker MicroTOF or Micromass Quattro II mass spectro-
meter.
3.6 Synthesis of L1
The 6,60-dithioamide derivative10 (1) (0.25 g, 0.49 mmol) and
2-(a-bromoacetyl)pyridine (0.55 g, 1.96 mmol) were dissolved
in ethanol (25 cm3) and heated at reflux for 8 h. After this time
the solvent was removed and the resulting residue was dis-
solved in water, neutralised with NaHCO3(aq) and extracted
into CH2Cl2. The organic solvent was evaporated giving crude
L1 as a pale oil. The oil was then twice triturated with diethyl
ether (5 cm3), giving L1 as a tan solid (0.2 g, 0.28 mmol, 57%).
Further purification can be achieved by column chromatogra-
phy (1% MeOH in CH2Cl2, Al2O3). 1H NMR [400 MHz,
(CDCl3)]: d (ppm) 8.65 (2H, d, J = 4.50; py), 8.37 (2H, d, J =
8.52; py), 8.26 (2H, d, J = 7.81; py), 8.13 (2H, s, th), 7.81 (2H,
dd, J = 1.33, 7.77 py), 7.48 (2H, m, py), 7.25 (2H, m, py), 4.25
(4H, m, py), 3.76 (4H, m, py), 3.70–3.20 (12H, m, cr). 13C
NMR [400 MHz, (CDCl3)]: d (ppm) 169.1, 155.9, 154.8, 152.8,
149.4, 146.0, 143.7, 136.9, 122.6, 121.1, 120.7, 120.1, 118.7,
70.6, 70.5, 70.3, 70.1 and 69.9. ESI-MS: m/z 711.3 (M + H+).
Found: C, 61.1; H, 5.0; N, 11.4. C36H34N6O6S2 requires: C,
60.8; H, 4.8; N, 11.8%.
3.7 [Zn2(L1)2](ClO4)4
Reaction of [Zn(H2O)6](ClO4)2 (0.008 g, 0.021 mmol) and L1
(0.015 g, 0.021 mmol) in MeCN (2 cm3) resulted in a colourless
solution. Filtration followed by layering with diethyl ether (10
cm3) afforded small colourless crystals (11 mg, 54%). 1H
NMR [400 MHz, (CDCl3)]: d (ppm) 8.58 (s, 2H, th), 8.19 (d,
J = 8.89, 2H), 8.13 (d, J = 7.89, 2H), 8.01 (m, 2H), 7.56 (d, J
= 8.78 Hz, 2H), 7.22 (m overlapping, 4H), 3.79 (m, 2H), 3.64
(m, 2H), 3.48 (m, 2H), 3.40 (m, 8H), 3.23 (m, 2H), 3.75 (m, m,
2H), 2.96 (m, 2H). ESI-MS m/z 1851 {[Zn2(L1)2(ClO4)3]}+.
Found: C, 44.1; H, 3.8; N, 8.8. C72H68Cl4Zn2N12-
O28S4 ꢁ MeCN requires: C, 44.6; H, 3.6; N: 9.1%.
3.8 [Zn2(L1)2Ba2](ClO4)8
Reaction of [Zn(H2O)6](ClO4)2 (0.008 g, 0.021 mmol) and L1
(0.015 g, 0.021 mmol) in MeCN (2 cm3) resulted in a colourless
solution, to which was added Ba(ClO4)2 (0.071 g, 0.21 mmol).
Filtration followed by layering with diethyl ether (10 cm3)
afforded small colourless crystals (17 mg, 62%).1H NMR [400
MHz, (CDCl3)]: d (ppm) 8.69 (s, 2H, th), 8.26 (d, J = 8.90,
2H), 8.17 (d, J = 7.90, 2H), 8.03 (m, 2H), 7.63 (d, J = 8.87
Hz, 2H), 7.26 (m overlapping, 4H), 4.10 (m, 2H), 3.62–3.33
(m, 14H), 3.19 (m, 2H), 2.93 (m, 2H). ESI-MS m/z 1213
{[Zn2(L1)2Ba2](ClO4)6}2+ and 2187 {[Zn2(L1)2Ba](ClO4)5}+.
Found: C, 32.8; H, 2.9; N, 6.4. C72H68Ba2Cl8Zn2-
N12O44S4 ꢁ 2H2O ꢁ MeCN requires: C: 33.0, H: 2.8, N: 6.7%.
the
previously
unfavourable
heteroleptic
species
[Zn2(L1)(L2)Ba]6+ would be stabilised with respect to the
highly charged homoleptic species [Zn2(L1)2Ba2]8+. The abil-
ity of electrostatic factors to control the ligand recognition
properties is also supported by studies with other s-block
metal ions. Upon addition of K+, which is virtually identical
in size to the barium ion but is only monocationic, to the
homoleptic system no change in the proportion of the hetero-
leptic species is observed. Although other factors cannot be
ignored, it would seem that the electrostatic effects would be
the basic driving force in the formation of a statistical mixture
of species.
3.9 Crystallography
[Zn2(L1)2](ClO4)4. X-Ray single-crystal diffraction data
were collected on a Bruker APEX CCD area-detector diffract-
ometer under a stream of cold nitrogen.
3
Experimental
3.1 General methods
All reagents were used as supplied. The NMR spectra were
recorded on either a Bruker Avance DPX400 or Bruker
Crystal data: [Zn2(L1)2](ClO4)4 ꢁ 3MeCN ꢁ H2O;
M =
ꢀ
2082.33, triclinic, P1, a = 13.641(3), b = 17.738(4), c =
ꢀc
This journal is the Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2007
1528 | New J. Chem., 2007, 31, 1525–1529