Synthesis of ligands containing N-oxide donor atoms and their assembly into metallosupramolecular structures

Article abstract of DOI:10.1039/c3dt00090g

A series of ligands that contain both N-donor and N-oxide donor atoms have been synthesised and ligands L⁵, L⁶, L⁷ and L⁸ contain, 4, 6, 5, and 6 donor atoms respectively. The smallest ligand L⁵ forms a mononuclear complex with Cu²⁺ ([Cu(L⁵)(ClO₄)₂(MeCN)]) whereas L⁶ and L⁷ form dinuclear double helicates with Ni²⁺ and Cu²⁺ respectively ([Ni₂(L⁶)₂] ⁴⁺ and [Cu₂(L⁷)₂]⁴⁺). L⁸ forms a tetranuclear cyclic helicate upon reaction with Co ²⁺ ([Co₄(L⁸)₄]⁸⁺) and in all cases the complexes are characterised by single-crystal X-ray diffraction and ESI-MS. The N-oxide units imparts flexibility in the ligand strand and where the unoxidised ligand strand forms a cyclic helicate, the incorporation of an N-oxide unit allows the formation of the dinuclear double helicate.

Full text of DOI:10.1039/c3dt00090g

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Synthesis of ligands containing N-oxide donor atoms
and their assembly into metallosupramolecular
structures†
Cite this: Dalton Trans., 2013, 42, 5805
Samantha J. Bullock, Lindsay P. Harding, Michelle P. Moore, Andrew Mills,
Sean A. F. Piela, Craig R. Rice,* Liz Towns-Andrews* and Martina Whitehead
A series of ligands that contain both N-donor and N-oxide donor atoms have been synthesised and
ligands L5, L6, L7 and L8 contain, 4, 6, 5, and 6 donor atoms respectively. The smallest ligand L5 forms a
mononuclear complex with Cu2+ ([Cu(L5)(ClO4)2(MeCN)]) whereas L6 and L7 form dinuclear double
helicates with Ni2+ and Cu2+ respectively ([Ni2(L6)2]4+ and [Cu2(L7)2]4+). L8 forms a tetranuclear cyclic
helicate upon reaction with Co2+ ([Co4(L8)4]8+) and in all cases the complexes are characterised by single-
crystal X-ray diffraction and ESI-MS. The N-oxide units imparts flexibility in the ligand strand and where
the unoxidised ligand strand forms a cyclic helicate, the incorporation of an N-oxide unit allows the for-
mation of the dinuclear double helicate.
Received 10th January 2013,
Accepted 6th February 2013
DOI: 10.1039/c3dt00090g
www.rsc.org/dalton
these protons would be significantly emphasized in an iso-
structural dizinc(II) helicate.8
Introduction
To achieve the formation of metallosupramolecular architec-
This shows how the assembly of metallosupramolecular
tures such as linear or circular helicates careful consideration systems can be controlled by the correct positioning of metal
has to go into ligand design.1–6 These ligands must contain binding domains and spacer units. In an analogous fashion
the correct number and arrangement of donor atoms such the ligand L2 which contains bidentate and tridentate
that not only the formation of the entropically favoured mono- domains separated by a 1,3-phenylene unit forms a penta-
nuclear assembly is prevented but the binding domains are nuclear cyclic helicate with Cu2+ giving [Cu5(L2)5]10+ (Fig. 1).
sufficiently arranged so that a discrete self-assembled species
In this structure the copper centre is coordinated by the
(e.g. a dinuclear double helicate [M2L2]2/4+ or cyclic helicates bidentate tail of one ligand and a tridentate tail of a different
[MnLn]2n+, where n = 3, 4, 5 etc.) is formed.7 In a recent ligand giving rise to a head-to-tail cyclic helicate.8 There are a
example, we showed that incorporation of a 1,3-phenylene large number of self-assembled species resulting from the
spacer unit, within a potentially hexadentate ligand, can give coordination of ligands containing many different donor units
different self-assembled species dependent upon the ionic but surprisingly the use of the N-oxide donor unit has received
radius of the metal ion used to coordinate the ligand strand. little attention in self-assembly and its ability to control the
Reaction of L1 with Cd2+ ions gives a relatively simple dinuc- formation of metallosupramolecular species has largely been
lear double helicate [Cd2(L1)2]4+ whilst Zn2+ ions result in the overlooked. The only previously reported example incorporates
assembly of a pentanuclear cyclic helicate [Zn5(L1)5]10+. This N-oxide units within the middle of the ligand strand (L3 and
difference is attributed to the steric demands of the central
phenyl unit. In the [Cd2(L1)2]4+ complex the distance between
the two phenyl rings is ca. 4.2 Å and examination of the van
der Waals radii reveals marginal surplus space between these
inward facing protons. When smaller zinc ions are employed it
is likely that any steric and/or electrostatic repulsion between
Department of Chemical and Biological Sciences, University of Huddersfield,
Huddersfield, HD1 3DH, UK. E-mail: c.r.rice@hud.ac.uk; Fax: +44 (0) 148-447-2182;
Tel: +44 (0) 148-447-3759
†CCDC 918502–918505. For crystallographic data in CIF or other electronic
format see DOI: 10.1039/c3dt00090g
Fig. 1 Ligands L1–L4.
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L4) and this partitions the ligand into two separate binding parameters, 32 restraints. Largest peak and hole 1.180 and
domains with the N-oxide bridging the metal ion. Specifically, −0.903 e Å−3. CCDC 918505.
reaction of L4 with Cd2+ gives a dinuclear double helicate with
Crystal data for [Cu2(L7)2](ClO4)4(MeNO2)1.1(CH2Cl2)0.6
each of the Cd2+ ions coordinated by the N-oxide and pyridyl (C55.38H37.83Cl4.60Cu2N11.08O20.16S4): M = 1599.26, Triclinic, P1,
thiazole donor units of two different ligands giving the a = 12.0799(12), b = 14.7340(15), c = 17.2067(17) Å, α = 95.609
ˉ
complex [Cd2(L4)2]4+.9 Complexes of polydentate N-oxide (2), β = 94.864(2), γ = 90.453(2)°, V = 3036.5(5) Å3, Z = 2; ρcalc
=
ligands (e.g. 2,2′:6′,2″-terpyridine and 2,2′:6′,2″:6″,2′″-quaterpyr- 1.749 Mg m−3, F(000) = 1618.7; dimensions 0.9 × 0.5 ×
idine) have been reported but these gave mononuclear 0.1 mm3; μ(MoKα) = 1.129 mm−1, T = 100(2) K. A total of 52 841
species.10
reflections were measured in the range 1.92 ≤ θ ≤ 27.88° (hkl
In this work we demonstrate how an N-oxide unit can be range indices: −10≤ h ≤15, −19 ≤ k ≤ 19, −22 ≤ l ≤ 22), 14 250
incorporated into the terminal domains of the ligand strand unique reflections (Rint = 0.0400). The structure was refined on
and how this can affect the coordination ability of the ligand F2 to Rw = 0.1232, R = 0.0453 (11 126 reflections with I > 2σ(I))
strand.
and GOF = 1.018 on F2 for 978 refined parameters, 135
restraints. Largest peak and hole 1.630 and −1.276 e Å−3
.
CCDC 918503.
Crystal data for [Co4(L8)4](BF4)8(CH2Cl2)8.6(H2O)3(MeNO2)
(C142.65H116.31B8Cl19.31Co4F32N25O17S8): M = 4324.00, Triclinic,
Experimental
Crystallographic details
ˉ
P1, a = 17.9775(15), b = 21.8535(18), c = 24.415(2) Å, α =
Single crystal X-ray diffraction data were collected on a 105.483(2), β = 95.760(2), γ = 103.275(2)°, V = 8862.2(13) Å3, Z =
Bruker Apex Duo diffractometer equipped with a graphite 2; ρcalc = 1.620 Mg m−3, F(000) = 4351; dimensions 0.3 × 0.1 ×
monochromated Mo(Kα) radiation source and a cold stream of 0.1 mm3; μ(MoKα) = 0.854 mm−1, T = 150(2) K. A total of
N2 gas. In all examples the perchlorate or tetrafluoroborate 164 320 reflections were measured in the range 1.52 ≤ θ ≤
counter ions contained varying degrees of disorder and were 27.50° (hkl range indices: −23 ≤ h ≤ 23, −26 ≤ k ≤ 28, −29 ≤
modelled in two positions using the part instruction and con- l ≤ 31), 39 488 unique reflections (Rint = 0.0802). The structure
strained using DELU and SIMU instructions in the least was refined on F2 to Rw = 0.1534, R = 0.0925 (21 115 reflections
squares refinement. In the case of [Co4(L8)4]8+ the crystals lost with I > 2σ(I)) and GOF = 1.084 on F2 for 2357 refined para-
solvent extremely rapidly and examination of the X-ray data meters, 401 restraints. Largest peak and hole 2.625 and
showed that the complex contained diffuse solvent molecules −1.169 e Å−3. CCDC 918502.
and scattering contribution to these diffuse elements and elec-
tron density equating to two molecules of dichloromethane
General details
were removed using the Squeeze routine in Platon. These Chemicals were purchased and used without further purifi-
additional solvents are included in the moiety formula.
cation. 1H NMR spectra were recorded on a 400 MHz Bruker
Crystal data for [Cu(L5)(ClO4)2(MeCN)][Cu(L5)(ClO4)2(H2O)] Avance DPX400. Mass spectra were obtained on a Bruker
ˉ
(C38H29Cl6Cu2N9O27S2):
M
=
1447.60, Triclinic, P1,
a
=
MicroTOF-q LC mass spectrometer. Extreme care should be
9.4086(8), b = 16.9828(14), c = 18.2559(16) Å, α = 67.070(2), β = taken when using TMSCN and it should only be used in a well
82.993(2), γ = 74.439(2)°, V = 2587.5(4) Å3, Z = 2; ρcalc
=
ventilated fume cupboard. The materials 2,2′-bipyridine-1,1′-
1.858 Mg m−3, F(000) = 1456; dimensions 0.35 × 0.3 × bis-N-oxide, 2,2′-bipyridine-1-oxide, and 6-cyano-2,2′-bipyridine
0.1 mm3; μ(MoKα) = 1.314 mm−1, T = 100 K. A total of 15 306 have all been prepared previously.11
reflections were measured in the range 1.21 ≤ θ ≤ 26.02° (hkl
range indices: −11 ≤ h ≤ 7, −20 ≤ k ≤ 20, −21 ≤ l ≤ 22), 10 037
Synthesis of 6′-cyano-2,2′-bipyridine-1-oxide (method 1). To
solution of 2,2′-bipyridine-N,N′dioxide (1) (0.20 g,
a
unique reflections (Rint = 0.0645). The structure was refined on 1.06 mmol) and benzoyl chloride (0.15 g, 1.06 mmol, 0.12 ml)
F2 to Rw = 0.1326, R = 0.0581 (5604 reflections with I > 2σ(I)) in DCM (50 ml) was added trimethylsilyl cyanide (0.10 g,
and GOF = 0.986 on F2 for 777 refined parameters, 25 1.06 mmol, 0.13 ml) and the reaction heated under reflux. The
restraints. Largest peak and hole 0.592 and −0.687 e Å−3
.
reaction was continually monitored by TLC (2% MeOH in
DCM, Al2O3) until it was assumed that the maximum amount
CCDC 918504.
Crystal
data
for
[Ni2(L6)2](ClO4)4(MeNO2)5(H2O) of mono-cyano compounds had formed (between four and six
ˉ
(C55H40Cl4N15Ni2O26S4): M = 3384.92, Triclinic, P1, a = 10.7357 hours). The reaction was then cooled to room temperature, fil-
(5), b = 13.4575(6), c = 23.2619(11) Å, α = 82.9930(10), β = tered and the solvent was reduced by rotary evaporation result-
78.8390(10), γ = 82.3710(10)°, V = 3252.0(3) Å3, Z = 2; ρcalc
=
ing in a viscous oil containing a mixture of both mono- and
1.728 Mg m−3, F(000) = 1719; dimensions 0.2 × 0.2 × bis-cyano compounds and also any unreacted starting
0.05 mm3; μ(MoKα) = 0.968 mm−1, T = 150(2) K. A total of material. Purification of the crude product via column chrom-
57 654 reflections were measured in the range 1.70 ≤ θ ≤ atography (Al2O3, 1% MeOH in DCM until the first fraction
30.53° (hkl range indices: −15≤ h ≤15, −19 ≤ k ≤ 18, −33 ≤ l ≤ eluted, then 2% methanol in DCM,) gave (2) as a sandy solid
1
33), 19 730 unique reflections (Rint = 0.0610). The structure was (0.12 g, 0.61 mmol, 57%). H NMR (500 MHz, CDCl3) δ (ppm)
refined on F2 to Rw = 0.1773, R = 0.0641 (11 017 reflections 9.19 (dd, J = 8.3, 1.0, 1H), 8.25 (dd, J = 6.2, 1.0, 1H), 8.20 (dd,
with I > 2σ(I)) and GOF = 1.036 on F2 for 983 refined J = 8.1, 2.1, 1H), 7.91 (t, J = 7.7, 1H), 7.68 (dd, J = 7.7, 1.0, 1H),
5806 | Dalton Trans., 2013, 42, 5805–5811
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7.35 (td, J = 7.7, 1.3, 1H), 7.28 (dt, J = 7.7, 2.2 Hz, 1H). ESI-MS ESI-MS m/z 333 (M + H+). Found: C, 64.7; H, 3.8; N, 16.5%;
m/z 220 (M + Na+). HR ESI-MS found 220.0480 C11H7N3NaO C18H12N4OS requires C, 65.0; H, 3.6; N, 16.9%.
requires 220.0481 (error = 0.46 ppm).
Synthesis of L6. To a suspension of 1-N-oxide-2,2′-bipyri-
Synthesis of 6′-cyano-2,2′-bipyridine-1-oxide (method 2). To dine-6′-thioamide (3) (0.1 g, 0.43 mmol) in EtOH (20 ml) was
a solution of 6-cyano-2,2′-bipyridine (5) (0.20 g, 1.06 mmol) in added 1,4-dibromo-2,3-dione (7) (0.05 g, 0.20 mmol) and the
DCM (25 ml) was slowly added, over a period of 2 hours, reaction refluxed for 8 hours. The reaction was allowed to cool
mCPBA (77%, 0.24 g, 1.06 mmol). After complete addition the and the precipitate was filtered and washed with EtOH (2 ×
reaction was allowed to stir for 8 h after which the solvent was 2 ml) and Et2O (2 × 2 ml) and dried in vacuo to give L6·HBr.
evaporated and the resulting solid purified in an analogous The hydrobromide salt was then suspended in aqueous
fashion to method 1 (0.15 g, 0.76 mmol, 72%). The analytical ammonia (0.88 sp.gr., 5 ml) for 12 hours, filtered and washed
data was identical to the product which was obtained from with water (2 × 2 ml), EtOH (2 × 2 ml) and Et2O (2 × 2 ml)
2,2′-bipyridine-N,N′dioxide.
Synthesis of 1-N-oxide-2,2′-bipyridine-6′-thioamide. To
giving L6 as a pale yellow solid (0.08 g, 0.16 mmol, 74%).
a
ESI-MS m/z 509 (M + H+). The solubility of the ligand was very
solution of the cyano compound (2) (0.70 g, 3.55 mmol) in poor, even in d6-DMSO at 80 °C and precluded H NMR analy-
ethanol (20 ml), triethylamine (1.0 g, 9.9 mmol, 1.38 ml) was sis. Found: C, 61.6; H, 3.5; N, 16.0%; C26H16N6O2S2 requires C,
added and H2S was slowly bubbled through the solution for 61.4; H, 3.2; N, 16.5%.
1
15 minutes, during which time the solution turned yellow. The
Synthesis of L7. To a suspension of 1-N-oxide-2,2′-bipyri-
yellow solution was allowed to stand for 48 hours during dine-6′-thioamide (3) (0.1 g, 0.43 mmol) in EtOH (20 ml) was
which time a yellow solid slowly precipitated. Collection via fil- added the thiazole-containing α-bromoacetyl (8) (0.15 g,
tration gave pure (3) as a yellow solid (0.7 g, 3.03 mmol, 88% 0.42 mmol) and the reaction refluxed for 8 hours, during
yield). 1H NMR (400 MHz, CDCl3) δ (ppm) 9.40 (s, 1H, NH), which time all the reactants dissolved. The reaction was
8.97 (dd, J = 7.9, 0.80, 1H), 8.78 (dd, J = 7.9, 0.76, 1H), 8.36 (dd, allowed to stand at room temperature for 12 hours and the
J = 6.3, 0.80, 1H), 8.04 (dd, J = 8.0, 2.1, 1H), 8.00 (t, J = 7.9, 1H), resulting precipitate was filtered and washed with EtOH (2 ×
7.76 (s, 1H, NH), 7.41 (dt, J = 7.6, 1.2, 1H), 7.35 (dt, J = 6.6, 2.2 2 ml) and Et2O (2 × 2 ml) and dried in vacuo to give L7·HBr.
Hz, 1H). ESI-MS m/z 254 (M + Na+). HR ESI-MS found 254.0371 The hydrobromide salt was then suspended in aqueous
C11H9N3NaOS requires 254.0359 (error −4.79 ppm).
ammonia (0.88 sp.gr., 5 ml) for 12 hours, filtered and washed
Synthesis of 1,3-di(α-bromoacetyl)cresol. A solution of 1,3- with water (2 × 2 ml), EtOH (2 × 2 ml) and Et2O (2 × 2 ml)
diacetylcresol (0.50 g, 2.60 mmol) in acetic acid (20 ml) was giving L7 as a pale yellow solid (0.11 g, 52%). 1H NMR
heated to 80 °C after which time a solution of bromine (0.83 g, (500 MHz, d6-DMSO) δ (ppm) 8.86 (d, J = 7.87, 1H, py), 8.75 (t,
0.27 ml, 5.20 mmol) in acetic acid (1 ml) was added drop-wise J = 0.8, 1H, Ph), 8.67 (m, 1H, py), 8.49 (s, 1H, tz), 8.42–8.46 (m,
over a period of 1 hour. Once added the reaction was heated overlapping, 3H), 8.34 (d, J = 7.8, 1H, py), 8.24 (dd, J = 9.0, 2.4,
further for 30 minutes and then cooled to room temperature. 1H, py), 8.18 (t, J = 7.8, 1H, py), 8.13–8.09 (m, overlapping, 2H,
The solvent was removed and the crude material was Ph), 8.04 (dt, J = 7.7, 1.6, 1H, py), 7.63 (t, J = 7.6 Hz, 1H, Ph),
purified by column chromatography (SiO2, 1% hexane in 7.60–7.55 (m, overlapping, 3H, py). ESI-MS m/z 492 (M + H+).
DCM) giving the dibromoacetyl species (9) as a light yellow Found: C, 66.4; H, 3.6; N, 13.9%; C27H17N5OS2 requires C,
solid (0.37 g, 1.05 mmol, 41%). 1H NMR (400 MHz, CDCl3) 66.0; H, 3.5; N, 14.2%.
δ (ppm) 12.79 (s, 1H, –OH), 7.91 (s, 2H, Ar), 4.61 (s, 4H,
Synthesis of L8. To a suspension of 1-N-oxide-2,2′-bipyri-
–CH2–), 2.40 (s, 3H, –CH3). ESI-MS m/z 372 (M + Na+). Hr dine-6′-thioamide (3) (0.1 g, 0.43 mmol) in dimethylform-
ESI-MS found 370.8889 C11H10Br2NaO3 requires 370.8889 amide (10 ml) was added 1,3-di(α-bromoacetyl)cresol (9)
(error −0.07 ppm).
(0.068 g, 0.19 mmol) and the reaction heated at 80 °C for
Synthesis of L5. To a suspension of 1-N-oxide-2,2′-bipyri- 8 hours. The reaction was allowed to cool and the precipitate
dine-6′-thioamide (3) (0.1 g, 0.43 mmol) in EtOH (20 ml) was was filtered and washed with EtOH (4 × 2 ml) and Et2O (2 ×
added 2-(α-bromoacetyl)-pyridinium hydrobromide (6) (0.14 g, 2 ml) and dried in vacuo to give L8·HBr. The hydrobromide salt
0.5 mmol) and the reaction refluxed for 8 hours, during which was then suspended in aqueous ammonia (0.88 sp.gr., 5 ml)
time all the reactants dissolved. The reaction was allowed to for 12 hours, filtered and washed with water (2 × 2 ml), EtOH
stand at room temperature for 12 hours and the resulting pre- (2 × 2 ml) and Et2O (2 × 2 ml) giving L8 as a pale yellow solid
cipitate was filtered and washed with EtOH (2 × 2 ml) and (0.065 g, 49%). As with L6 the solubility of the ligand was very
1
Et2O (2 × 2 ml) and dried in vacuo to give L5·HBr. The hydro- poor, even in d6-DMSO at 80 °C and precluded H NMR analy-
bromide salt was then suspended in aqueous ammonia sis. ESI-MS m/z 616 (M + H+). Found: C, 64.8; H, 3.9; N, 13.2%;
(0.88 sp.gr., 5 ml) for 12 hours, filtered and washed with water C33H22N6O3S2 requires C, 64.5; H, 3.6; N, 13.7%.
(2 × 2 ml), EtOH (2 × 2 ml) and Et2O (2 × 2 ml) giving L5 as a
In all cases the complexes were synthesized in a similar
pale yellow solid (0.081 g, 0.24 mmol, 57%). 1H NMR manner and the ligand (5 mg) was reacted with 1.1 equivalents
(400 MHz, d6-DMSO) δ (ppm) 8.85 (d, J = 7.84, 1H, py), 8.67 of the relevant metal ion in MeCN (L5) or MeNO2 (L6–L8) and
(m, 1H, py), 8.47 (s, 1H, tz), 8.44 (d, J = 5.64, 1H), 8.35 (d, J = the reaction heated and sonicated until complete dissolution.
7.8, 1H, py), 8.26–8.15 (m, overlapping, 3H), 7.98 (t, J = 7.6 Hz, Slow diffusion of dichloromethane ([Cu2(L7)2](ClO4)4 and
1H, py), 7.57–7.56 (m, overlapping, 2H), 7.42 (m, 1H, py). [Co4(L8)4](BF4)8), diethyl ether (Cu(L5)(ClO4)2) and ethyl acetate
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([Ni2(L6)2](ClO4)4) gave crystalline materials which were iso-
lated by filtration, washed with the diffusion solvent and dried
under vacuum. [Cu(L5)(ClO4)2(MeCN)][Cu(L5)(ClO4)2(H2O)]
(yield = 43%). Found: C, 36.4; H, 2.6; N, 9.8%. Calculated for
C38H29N9O19S2Cu2Cl4: C, 36.6; H, 2.3; N, 10.1%. [Ni2(L6)2]-
(ClO4)4 (yield = 65%) Found: C, 40.4; H, 1.8; N, 10.8%. Calcu-
lated for C52H32N12O20S4Ni2Cl4: C, 40.8; H, 2.1; N, 11.0%.
[Cu2(L7)2](ClO4)4 (yield = 53%) Found: C, 42.8; H, 2.4; N, 9.1%.
Calculated for C54H34N10O18S4Cu2Cl4: C, 43.0; H, 2.3; N, 9.3%.
[Co4(L8)4](BF4)8 (yield = 47%) Found: C, 44.8; H, 2.2; N, 9.1%.
Calculated for
H, 2.6; N, 9.5%.
C132H88N24O12S8Co4B8F32·C2H4Cl4: C, 45.2;
Results and discussion
Ligand synthesis
The principal material for the formation of these N-oxide
ligands is 6′-cyano-2,2′-bipyridine-1-oxide (2) which can be pre-
pared via two different routes. The first route involves the reac-
tion of 2,2′-bipyridine-bis-1,1′-oxide (1) with benzoyl chloride
and tetramethylsilylcyanide in dichloromethane; the reaction
monitored by TLC until it was judged that the maximum quan-
tity of 6′-cyano-2,2′-bipyridine-1-oxide (2) had formed. Alter-
natively, 6′-cyano-2,2′-bipyridine-1-oxide can be prepared by
reaction 2,2′-bipyridine-1-oxide (4) with TMSCN to give
6-cyano-2,2′-bipyridine (5) and then reaction with mCPBA
which gives 6′-cyano-2,2′-bipyridine-1-oxide (2), the selectivity
in oxidation presumably controlled by the electronic and steric
effects of the cyano group (Scheme 1). Although the second
route requires more synthetic steps we found it the most
reliable method of preparation as the yields starting from 2,2′-
bipyridine-bis-1,1′-oxide were variable. Reaction of the thio-
amide (3) with α-bromoacetyls (6)–(9) either in EtOH or DMF
followed by deprotonation with concentrated ammonia gave
the corresponding ligands L5–L8 (Scheme 2).
Scheme 2 Synthesis of ligands L5–L8.
Coordination chemistry
Reaction of ligand L5 with Cu(ClO4)2·6H2O. Reaction of L5
with Cu(ClO4)2·6H2O in MeCN gave a green solution from
which crystals were obtained by slow diffusion of ethyl acetate.
Fig. 2 X-ray crystal structure of [Cu(L5)(ClO4)2(MeCN)] (top) and [Cu(L5)-
(ClO4)2(H2O)] (bottom).
In the solid state there are two different molecules in the unit
cell, each of which contains a 6-coordinate metal centre
formed by coordination of one ligand strand, two perchlorate
counter ions and either a water molecule or a molecule of
acetonitrile giving the simple mononuclear species [Cu(L5)-
(ClO4)2(sol)] where sol = H2O or MeCN (Fig. 2) (Cu–N:
Scheme 1 Synthesis of 6’-cyano-2,2’-bipyridine-1-oxide. Reagents and con-
ditions: (i) TMSCN, BzCl, CH2Cl2, reflux; (ii) H2S, Et3N, EtOH, RT; (iii) TMSCN, BzCl,
CH2Cl2, reflux; (iv) mCPBA, CH2Cl2, RT.
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1.96–2.03 Å; Cu–ON: 1.92 Å). In both cases the ligand acts as a
tridentate donor, coordinating the metal centre by thiazole
and pyridine N-donor units and the terminal O-donor N-oxide.
The terminal N-donor unit does not coordinate the metal
centre which can be attributed to the divergent nature of the
pyridyl-thiazole-pyridyl domains which prevents the ligand
acting as a tetradentate chelate.
In the ESI-MS an ion is observed at m/z = 494 which corre-
sponds to {[Cu(L5)]ClO4}+, however higher molecular weight
ions at m/z = 1088, 1683, 2280 and 2873 which correspond to
{[Cun(L5)n](ClO4)2n−1}+, where n = 2, 3, 4 and 5 are also
observed. It is likely that these higher nuclearity ions are due
to aggregation of the mononuclear species during the ESI-MS
process.
Reaction of ligand L6 with Ni(ClO4)2·6H2O. Reaction of
ligand L6 with Ni(ClO4)2·6H2O in MeNO2 gives a green solution
from which light green crystals are produced after slow
diffusion of tetrahydrofuran. ESI-MS analysis gave a remark-
ably simple spectrum with ions at m/z = 1433, 922 and 665 cor-
responding to {[Ni2(L6)2](ClO4)3}+, {[Ni2(L6)](ClO4)3}+ and
{[Ni2(L6)2](ClO4)2}2+. Solid-state analysis shows the formation
of a dinuclear double helicate [Ni2(L6)2]4+; each of the ligand
strands partitions into two tridentate domains comprising
thiazole-pyridyl-pyridyl-N-oxide donor units (Fig. 3). Each of
the Ni2+ ions adopts a six-coordinate geometry and is coordi-
nated by two tridentate domains from different ligands (Ni–N:
2.04–2.05 Å; Ni–O: 2.02–2.05 Å).
Reaction of ligand L7 with Cu(ClO4)2·6H2O. Reaction of
ligand L7 with Cu(ClO4)2·6H2O in MeNO2 gives a green solu-
tion from which green crystals were produced upon slow
diffusion of CH2Cl2. As with the Ni2+ complex, the ESI-MS
analysis was straightforward with ions at m/z = 1407 and 915
corresponding to {[Cu2(L7)2](ClO4)3}+ and {[Cu2(L7)](ClO4)3}+.
Solid-state analysis shows the formation of a head-to-tail
dinuclear double helicate [Cu2(L7)2]4+ where the ligand par-
titions into a tridentate and a bidentate domain separated by a
1,3-phenylene spacer (Fig. 4). Each of the Cu2+ ions is coordi-
nated by N-oxide-pyridyl-thiazole unit from one ligand and a
pyridyl-thiazole unit from a different ligand giving a 5-coordi-
nate metal centre (Cu–N: 1.98–2.43 Å; Cu–O: 1.96 Å). It is
somewhat surprising that this ligand gives a dinuclear double
helicate with Cu2+ as we have previously shown that this type
of ligand (i.e. containing a 1,3-phenylene spacer) usually pro-
duces a pentanuclear cyclic helicate upon coordination with
first-row transition metal ions. These cyclic helicates result
due to unfavourable steric interactions between the spacer
units. However, the inclusion of an N-oxide must allow the
ligand to flex so that the two phenyl units are more remote,
reducing the steric interaction. Indeed, the distance between
the phenyl carbon atoms is 4.2 Å (cf. [Cd2(L1)2]4+ inter-ligand
phenyl separation 4.2 Å).
Fig. 3 X-ray crystal structure of [Ni2(L6)2]4+ (top) and as a space-filling model
(bottom).
Solid state analysis shows the formation of a tetranuclear
cyclic helicate [Co4(L8)4]8+, where each of the ligands has separ-
ated into tridentate thiazole-pyridyl-pyridyl-N-oxide donor
units separated by a 1,3-phenol unit (Fig. 5). All four cobalt
ions are six-coordinate arising from the coordination of two of
the N, N, O-donor units (Co–N: 2.11–2.20 Å; Co–O:
2.00–2.07 Å). The phenol spacers bridge each of the domains
in an “over and under” conformation, giving rise to a helical
cyclic oligomer. The four –OH units do not coordinate to the
metal centres but hydrogen bond to one another.
Interestingly, analogous complexes that contain ligands
with a 1,3-phenyl spacer unit give pentanuclear cyclic helicates,
whereas with a 1,3-phenol spacer the lower tetranuclear species
is observed. It seems likely that this can be attributed to the
steric bulk of the methyl group on the central phenol spacer.
In a tetranuclear assembly the distance between adjacent
Reaction of ligand L8 with Co(BF4)2·6H2O. Ligand L8 con-
tains two thiazole-pyridyl-pyridyl-N-oxide donor units separ-
ated by a 1,3-phenol spacer. Upon reaction of this ligand with
Co(BF4)2·6H2O a pale orange solution results from which crys-
tals were produced upon slow diffusion of dichloromethane.
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Fig. 4 X-ray crystal structure of [Cu2(L7)2]4+ (top) and as a space-filling model
(bottom).
tridentate binding domains will be longer than the corre-
sponding pentanuclear cyclic helicate and the steric demands
of the methyl groups will thus prevent the pentanuclear assem-
bly. Measurement of the centriods of the central pyridine rings
in adjacent ligand strands show that the distance is substan-
tially longer in [Co4(L8)4]8+ (∼8.0 Å) than the pentanuclear
species [Zn5(L1)5]10+ (∼7.0 Å). ESI-MS shows a doubly charged
ion at m/z = 1607 corresponding to the tetranuclear cyclic heli-
cate {[Co4(L8)4](ClO4)6}2+. Also an ion at m/z = 2454 correspond-
ing to {[Co3(L8)3](ClO4)5}+ was observed, the presence of this
the lower nuclearity species is probably an artefact of the
ESI-MS process and has been observed in other cyclic helicate
systems.8 Whereas with L7 the N-oxide units allow the ligand
to flex and form the double helicate, here the steric bulk of the
–OH unit is sufficiently large that even with the added flexi-
bility that the N-oxide unit imparts a double helicate cannot
be formed.
Fig. 5 X-ray crystals structure of [Co4(L8)4]8+ (top) and as a space-filling model
(bottom).
polydentate ligands which produce, upon coordination with
different d-block metal ions, a mononuclear single stranded
complex as well as a dinuclear double stranded and tetra-
nuclear circular helicates. The incorporation of this unit
within the ligand strand can change the behaviour of the
ligand as it increases the flexibility of the donor units. How
the ligand would behave with other metal ions is unknown.
However, we have found that for the most part, ligands that
can partition into two tridentate domains, give similar results
with all the divalent 1st row transition metal ions.3,6,8,11 For
example, reaction of L6 with other metals such as Co2+, Fe2+ or
Zn2+ would all give the dinuclear double helicate species
[M2(L6)2]4+ however despite attempts we cannot obtain crystals
of sufficient quality to confirm this.
Conclusions
We have developed a method that allows incorporation of
N-oxide units as the terminal donor units in a series of
5810 | Dalton Trans., 2013, 42, 5805–5811
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Paper
J. C. Jeffery, R. L. Paul and M. D. Ward, Chem. Commun.,
2000, 1529.
Acknowledgements
This work was supported by the University of Huddersfield.
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