Tuning coordination environments through ligand redox chemistry: The thiol-disulfide reaction

Article abstract of DOI:10.1071/CH10105

Oxidative coupling of 6-(pyridin-2-yl)pyridine-2(1H)-thione yields 1,2-bis(2,2′-bipyridin-6-yl)disulfide (4), which can act as a bis(chelate) to a single zinc(ii) centre. The effects on the solid-state structure of introducing a methyl substituent into each 6-position of 4 have been examined. Ligand 4 functions as a bridging ligand in [Cu₂(-4)(-6)]⁴⁺ in which ligand 6 is 1,2-bis(2,2′:6′,2″-terpyridin-4′- yl)disulfide; [Cu₂(-4)(-6)]⁴⁺ self-assembles from the components according to the preference shown by copper(ii) for a five-coordinate {Cu(bpy)(tpy)} environment. Reaction of 4 with [Cu(NCMe)₄][PF ₆] leads to a product, tentatively formulated as {[Cu(4)][PF ₆]}_n, which, in air, undergoes oxidation of both copper and ligand to yield [Cu(5)₂] (H5 ≤ 2,2′-bipyridine-6- sulfonic acid), the solid state structure of which is presented. CSIRO 2010.

Full text of DOI:10.1071/CH10105

Full Paper
CSIRO PUBLISHING
Aust. J. Chem. 2010, 63, 1334–1341
www.publish.csiro.au/journals/ajc
Tuning Coordination Environments Through Ligand Redox
Chemistry: the Thiol–Disulfide Reaction
Edwin C. Constable,A Catherine E. Housecroft,A,B Markus Neuburger,A
Jason R. Price,A and Jennifer A. ZampeseA
ADepartment of Chemistry, University of Basel, Spitalstrasse 51, CH–4056 Basel, Switzerland.
BCorresponding author. Email: catherine.housecroft@unibas.ch
Oxidative coupling of 6-(pyridin-2-yl)pyridine-2(1H)-thione yields 1,2-bis(2,2ꢀ-bipyridin-6-yl)disulfide (4), which can act
as a bis(chelate) to a single zinc(ii) centre. The effects on the solid-state structure of introducing a methyl substituent into
each 6-position of 4 have been examined. Ligand 4 functions as a bridging ligand in [Cu2(μ-4)(μ-6)]4+ in which ligand
6 is 1,2-bis(2,2ꢀ:6ꢀ,2ꢀꢀ-terpyridin-4ꢀ-yl)disulfide; [Cu2(μ-4)(μ-6)]4+ self-assembles from the components according to the
preference shown by copper(ii) for a five-coordinate {Cu(bpy)(tpy)} environment. Reaction of 4 with [Cu(NCMe)4][PF6]
leads to a product, tentatively formulated as {[Cu(4)][PF6]}n, which, in air, undergoes oxidation of both copper and ligand
to yield [Cu(5)2] (H5 = 2,2ꢀ-bipyridine-6-sulfonic acid), the solid state structure of which is presented.
Manuscript received: 3 March 2010.
Manuscript accepted: 19 May 2010.
Introduction
metal ion in a bis(chelate) mode, or to act as a bridging ligand.
Although compound 3 has been previously reported,[13,14,17] the
oxidative coupling to 4 and the coordination chemistry of this
attractive ligand remain unexplored.
We have recently become interested in the development of self-
replicating systems based on coordination chemistry. The basis
of our approach is three-fold: (i) the preference of copper(ii) cen-
tres for five-coordinate tpy-bpy environments; (ii) the oxidation
of thiols to disulfides by copper(ii) with concomitant reduction
to copper(i); and (iii) the preference of copper(i) for four-
coordinate bpy-bpy environments.[1–3] This has led us to investi-
gate fundamental aspects of the interactions of thiol ligands with
copper species, and in this paper, we report studies on oligopy-
ridine thiols (thiones) and disulfides. It is well established that
pyridinethiols show a strong preference for the thione tautomeric
form both in the solid state and in polar solvents.[4–7] Although
1,10-phenanthrolinethiols (thiones)[8–10] are well known, thiol
(thione) derivatives of 2,2ꢀ-bipyridine[3,11–14] and 2,2ꢀ:6ꢀ,2ꢀꢀ-
terpyridine[15,16] have received relatively little attention. Of
significant interest is the facile oxidative coupling of thiols
to disulfides (Eqn 1). Thiol(thione)–disulfide interconversion
(Eqn 1) has been used to establish dynamic combinatorial
libraries,[17–19] and several recent reports have appeared con-
cerning the in situ formation of dipyridyl disulfides and their
incorporation into coordination complexes.[20–23]
Results and Discussion
The yellow thione 3 was prepared by treating 6-chloro-2,2ꢀ-
bipyridine[25] with NaSH·xH2O in DMF followed by workup
under aqueous conditions at pH 7. The 1H and 13C NMR
spectra[14] were acquired in the presence of dithiothreitol to
prevent oxidative coupling of the thione,[26] and have been
reassigned[14] using NOESY, COSY, HMQC, and HMBC tech-
niques. The broadened signal assigned to HA6 (see Scheme 1)
is indicative of hindered rotation of the 2,2ꢀ-bipyridine (bpy)
unit, and is consistent with the presence of an NH· · ·N hydro-
gen bond in the thione tautomer. Recrystallization of 3 from
Et2O gave X-ray-quality crystals and structural analysis con-
firmed the thione form of the compound (Fig. 1). The bpy
unit in each of the two independent molecules of 3 is in a
cis-conformation constrained by the NH· · ·N hydrogen bond,
and is planar; the angles between the least-squares planes
of the two pyridine rings in the two molecules are 5.11(9)
and 1.87(8)◦. In the solid-state structure of compound 1, we
observed that face-to-face π-interactions between pyridine-
2(1H)-thione rings were a primary feature of the packing,
although the steric demands of the methyl substituents precluded
π-stacking of the methylpyridine rings. In 3, the packing is
best described in terms of stacked sheets. Each sheet assem-
bles through short S· · ·H contact between pairs of independent
molecules (S1· · ·H131C13 = 2.98, S1· · ·C13 = 3.8645(18) Å,
S1· · ·H131–C13 = 156◦) and non-classical CH· · ·N hydro-
gen bonds (C12H121· · ·N2 = 2.77, C12· · ·N2 = 3.713(2) Å,
C12–H121· · ·N2 = 180◦; C2H21· · ·N4i = 2.71, C2· · ·N4i =
3.641(2) Å, C2–H21· · ·N4i = 173◦, symmetry code i = −1 + x,
2RSH → RSSR + 2H+ + 2e−
(1)
We have recently shown that the conversion of 6-(6-
methylpyridin-2-yl)pyridine-2(1H)-thione (1, Scheme 1) to
1,2-bis(6ꢀ-methyl-2,2ꢀ-bipyridin-6-yl)disulfide (2, Scheme 2)
changes the metal-binding domain from a monodentate S-donor
to an N,Nꢀ-chelate,[3] and we illustrated this phenomenon within
a series of copper(i) complexes, the stabilization of which
required the incorporation of the methyl substituent.[24] We now
report the oxidative coupling of 6-(pyridin-2-yl)pyridine-2(1H)-
thione (3) to 1,2-bis(2,2ꢀ-bipyridin-6-yl)disulfide (4), in which
the flexible disulfide backbone permits the ligand to bind a single
© CSIRO 2010
10.1071/CH10105
0004-9425/10/091334

Tuning Coordination Environments Through Ligand Redox Chemistry
1335
N
S
N
N
H
S
N
S
N
N
1
2
4
4
3
3
5
5
N
B
N
H
B
3
A
6
3
S
N
4
5
4
5
S
N
S
N
SO H
3
A
N
N
N
6
3
4
H5
N
N
N
S
S
N
N
N
6
Scheme 1. Ligand structures and atom labelling for NMR spectroscopic assignments.
N
N
N
N
N
N
N
ϩ
S
S
S
2
S
S
S
N
N
N
N
N
N
N
N
7
Scheme 2. Proposed formation of ligand 7 in the electrospray mass spectrometer.
−1 + y, z). The packing of the sheets is such that adjacent
molecules are arranged in centrosymmetric pairs (shown on
the left of Fig. 2b for molecules containing atom S2) or in
pairs in which the molecules are orthogonal to one another
(Fig. 2b, right). The distances between sheets are consistent with
π-stacking pervading the structure.
significantly affected by the thione to disulfide transformation,
shifting from δ 7.18 to 8.20 ppm on going from 3 to 4. With
the exception of the appearance of a resonance for proton HA6
1
(Scheme 1), the pattern of signals in the H NMR spectrum
of 4 is similar to that in 2. Single crystals of 4 were selected
from the bulk sample, and Fig. 3 depicts the molecular structure.
One of the interesting features of ligands containing polysul-
fide bridges is the helical twist that renders the ligand chiral in
the solid state. Compound 4 crystallizes in the centrosymmetric
space group C2/c with both the P and M forms present. There is
one independent half-molecule in the asymmetric unit, and the
bpy domain is close to planar (angle between the least-squares
planes of the two pyridine rings is 11.22(6)◦). It adopts a trans-
conformation, and the switch from cis to trans on going from 3
to 4 is consistent with the loss of the NH· · ·N hydrogen bond.
The angle between the least-squares planes of the sulfur-bonded
pyridine rings of 76.3◦ is smaller than the 88.8◦ observed in the
When solutions of 3 are left in air, the thione oxidizes to 4.
The reaction can be carried out preparatively by treating a MeCN
solution of 3 with H2O2. In the electrospray mass spectrum of 4,
peaks at m/z 397.3 and 375.3 were assigned to [M + Na]+ and
[M + H]+; the isotope distributions matched those simulated.
1
The H and 13C NMR spectra of a CDCl3 solution of 4 were
assigned by 2D-techniques. In the 13C NMR spectrum, the most
diagnostic feature of the spectrum is the change in chemical shift
for the sulfur-attached carbon atom, CB6, from δ 179.5 ppm in
1
3 to 158.6 ppm in 4. In the H NMR spectrum, the signal for
HB3 (the proton para to the position of sulfur attachment) is

1336
E. C. Constable et al.
methyl derivative 2,[3] and this is most likely associated with the
differences in packing between the two compounds. A princi-
pal packing motif in both 2 and 4 is a centrosymmetric pair of
molecules that associate through face-to-face π-stacking of bpy
units. In 2, the methyl substituents prevent efficient stacking of
these domains throughout the lattice.[3] However, in 4, the face-
to-face interactions at separations of 3.5 Å between molecules of
alternating P and M chirality generate infinite chains (Fig. 4a).
The latter are aligned in the same direction through the lat-
tice (Fig. 4b), and intermolecular interactions between adja-
cent chains are predominantly CH· · ·S (S1· · ·H10Aii = 2.94 Å,
symmetry code ii = ½ + x, −½ + y, z) and CH· · ·N contacts
(N2· · ·H8Aiii = 2.73, N1· · ·H4Aiv = 2.72 Å, symmetry codes
iii = x, 2 − y, ½ + z, iv = ½ − x, −½ + y, ½ − z). The overall
packing efficiencies of 4 and 2 are 71.3 and 69.3% respectively.
We have previously illustrated the ability of 2 and its trisul-
fide analogue to coordinate to copper(i) through the bpy binding
domains and to adopt bridging modes between two copper(i)
centres.[3] We decided to initiate our investigations of the metal-
binding properties of 4 by comparing the reactions of zinc(ii)
with 2 and 4. Acetonitrile solutions of 2 or 4 were treated with
Zn(ClO4)2·6H2O under reflux. The products were isolated as
colourless crystalline solids after diffusion of Et2O into the reac-
tion mixtures. Elemental analysis of each product indicated a 1:1
ratio of Zn:ligand, and dominant peak envelopes in the matrix-
assisted laser desorpton–ionization (MALDI) mass spectra at
m/z 466.53 and 538.57 respectively were assigned to the ions
[Zn(2)]+ and [Zn(4)ClO4]+. The base peak, however, in each
spectrum was consistent with loss of sulfur from each ligand
and appeared at m/z 370.52 (derived from 2) and at m/z 342.52
(derived from 4). All the observed isotope distributions matched
those simulated. The elemental and mass spectrometric analy-
ses therefore suggested the formation of [Zn(L)][ClO4]2 where
L = 2 or 4. Attempts to obtain an electrospray mass spectrum
of the complex containing ligand 2 yielded peaks assigned to
[2 + H]+ (m/z 403.2, base peak) and [2 + Na]+ (m/z 425.1), but
no zinc-containing ions were observed.
When compared with the 1H NMR spectra of the free ligands
2[3] and 4, the spectra of the complexes showed a characteristic
patterninvaluesofꢀδ[ꢀδ = δcomplex(CD3CN) − δligand(CDCl3)].
The signal for HA6 in 4 shifted to lower frequency on complex
formation (ꢀδ = −0.27). All other signals shifted to higher fre-
quency, the largest values of ꢀδ being for HA4 (ꢀδ = +0.68
for 2, and +0.69 for 4) and HB4 (ꢀδ = +0.68 for 2, and +0.60
for 4).
Single crystals of 2{[Zn(2)(OClO3)][ClO4]}·EtOH and
[Zn(4)(OClO3)][ClO4] were grown by slow diffusion of Et2O
into an EtOH solution of [Zn(2)][ClO4]2 and into an MeCN
solution of [Zn(4)][ClO4]2 respectively. In both compounds, one
perchlorate ion is coordinated to the zinc(ii) ion (Figs 5 and
6a). [Zn(4)(OClO3)][ClO4] contains two independent formula
units in the asymmetric unit. The description of the structures as
five- rather than six-coordinate zinc(ii) has been made by
C8
C3
C6
N2
C5
N1
S1i
S1
C10
C1
N1
C1
H1
C7
S1
Fig. 1. Structure of one of the two independent molecules of
C3
C9
C6
C4
3
(ellipsoids plotted at 40% probability level). Selected bond para-
meters: S1–C1 = 1.6749(18), N1–C1 = 1.368(2), N1–C5 = 1.357(2),
N2–C6 = 1.347(2), N2–C10 = 1.337(2), C1–C2 = 1.418(2), C2–C3 =
1.359(2), C3–C4 = 1.399(2), C4–C5 = 1.373(2), C5–C6 = 1.475(2),
C6–C7 = 1.381(2), C7–C8 = 1.378(3), C8–C9 = 1.378(3), C9–C10 =
N2
Fig. 3. Molecular structure of 4 (ellipsoids plotted at 40% probability
level); symmetry code i = 1 − x, y, ½ − z. Important bond parameters: S1–
C1 = 1.7806(11), S1–S1i = 2.0198(9) Å; C1–S1–S1i = 106.41(4)◦; torsion
angle C1–S1–S1i–C1i = −99.12(6)◦.
1.386(3) Å;
S1–C1–C2 = 125.65(14),
N1–C1–S1 = 120.16(12)◦.
Parameters for the second molecule are similar.
(a)
(b)
S2
S2ii
a
b
S2iii
o
S1
c
Fig. 2. Packing of molecules of 1: (a) assembly of sheets, and (b) stacking motifs between sheets. Symmetry codes ii = x, −1 + y,
z, iii = 1 − x, 2 − y, 1 − z.

Tuning Coordination Environments Through Ligand Redox Chemistry
1337
(a)
(b)
c
b
b
o
a
o
a
c
Fig. 4. Packing of molecules of 4: (a) face-to-face bpy· · ·bpy interactions between molecules of alternating P and M chirality produce chains, and
(b) alignment of chains through the lattice.
of the bpy units containing atoms N1 and N2 at a sep-
C22
aration of 3.59 Å. The additional, dominant packing forces
involve extensive CHpyridine· · ·Operchlorate and OHEtOH· · ·S short
contacts. The ethanol molecule is disordered and has been
modelled over two sites related by a two-fold axis. Crys-
tal packing motifs in [Zn(4)(OClO3)][ClO4] resemble those
in 2{[Zn(2)(OClO3)][ClO4]}·EtOH. One bpy domain in each
independent [Zn(4)(OClO3)]+ forms a π-stack across an inver-
sion centre to an adjacent cation (separations are 3.44 Å for
N1/N2· · ·N1i/Ni and 3.63 Å for N7/N8· · ·N7ii/N8ii; symmetry
codes i = 2 − x, 2 − y, 1 − z, ii = 1 − x, 1 − y, 1 − z). Extensive
CHpyridine· · ·Operchlorate interactions make a dominant contribu-
tion to the overall packing.
C21
C11
N3
C18
C8
S1
N2
N1
C4
Zn1
C1
O1
N4
C2
S2
C12
The manner in which ligands 2 and 4 envelop a zinc(ii)
ion contrasts with the bridging mode that we have previously
observed for 2 and its trisulfide analogue when they are bound
to copper(i).[3] Despite the fact that the {Cu(bpy)2}+ coordina-
tion sphere usually requires the presence of substituents in 6ꢀ- or
6,6ꢀ-positions to stabilize copper(i),[28] we decided to investigate
the coordination of ligand 4 to copper(i). The reaction of 4 with
[Cu(NCMe)4][PF6] in MeCN led, after workup, to a red-orange
solid. The 1H NMR spectrum of a CD3CN solution of the com-
pound exhibited seven broadened resonances between δ 8.5 and
7.5 ppm, consistent with diamagnetic copper(i) and the presence
of 4 in a symmetrical environment. However, we cannot rule out
the presence of the coordinated thione 3. All attempts to isolate
a pure sample of the product were unsuccessful. It is significant
that when [Cu(NCMe)4][PF6] reacts with ligand 2, a similarly
O4
CI1
O2
O3
Fig. 5. Structure of the [Zn(2)(OClO3)]+ cation in 2{[Zn(2)(OClO3)]
[ClO4]}·EtOH (ellipsoids plotted at 40% probability level); H atoms
are omitted. Selected bond parameters: Zn1–N1 = 2.064(5), Zn1–N2 =
2.051(5), Zn1–N3 = 2.138(5), Zn1–N4 = 2.018(5), Zn1–O1 = 2.324(4),
S1–S2 = 2.042(2) Å; N1–Zn1–N2 = 81.81(18), N1–Zn1–N3 = 98.93(18),
N2–Zn1–N3 = 128.99(18), N1–Zn1–N4 = 160.52(19), N2–Zn1–N4 =
114.18(18), N3–Zn1–N4 = 79.94(18), N1–Zn1–O1 = 81.32(16), N2–Zn1–
O1 = 106.52(16), N3–Zn1–O1 = 124.13(16), N4–Zn1–O1 = 83.32(16),
Zn1–O1–Cl1 = 139.7(2), C11–S1–S2 = 104.0(2), C12–S2–S1 = 100.6(2)◦.
1
broadened H NMR spectrum is obtained and in this case, we
comparing the sum of the Shannon ionic radii[27] with the
observed Zn· · ·O contact distances to the second perchlo-
rate anion (3.639(10) Å in 2{[Zn(2)(OClO3)][ClO4]}·EtOH;
2.6232(15) and 3.0039(19) Å for the two independent cations in
[Zn(4)(OClO3)][ClO4]). In each structure, the disulfide ligand
acts as a bis(chelate), coordinating to the zinc(ii) ion through
the four nitrogen donors. Each of the [Zn(2)(OClO3)]+ and
[Zn(4)(OClO3)]+ cations is chiral, and both P and M forms
are present in the crystal lattice of each complex. The pitch
of the helix (Fig. 6b) in each complex can be defined by
the angle between the least-squares planes through the two
chelate rings. In [Zn(2)(OClO3)]+, this angle is 51.3(5)◦, and
in the two independent [Zn(4)(OClO3)]+ cations, the corre-
sponding angles are 35.59(12) and 37.27(12)◦. The change on
going from [Zn(2)(OClO3)]+ to [Zn(4)(OClO3)]+ is attributed
to the steric repulsion between the two methyl substituents
in the former. In 2{[Zn(2)(OClO3)][ClO4]}·EtOH, centrosym-
metric pairs of cations engage in face-to-face π-stacking
tentatively proposed the formation of {[Cu(2)][PF6]}n.[3] By
analogy, we propose the formation of {[Cu(4)][PF6]}n.
The product of the reaction of [Cu(NCMe)4][PF6] and 4 was
left to stand in the NMR tube for several months, after which
time X-ray-quality green crystals had grown. Structural anal-
ysis revealed the formation of [Cu(5)2]·2.5MeCN containing
an unusual sulfonate ligand, confirming the aerial oxidation of
both the metal and ligand (see Scheme 1 for the structure of
H5). The structure of [Cu(5)2] is presented in Fig. 7. The oxida-
tive cleavage of ligand 4 is similar to that observed by Delgado
et al. during the reaction of 1,2-di(pyridin-2-yl)disulfide with
copper(ii) nitrate. In this case, however, the conjugate bases
of both pyridine-2-sulfonic acid and pyridine-2-sulfinic acid
were produced.[29] The copper(ii) centre in [Cu(5)2] is in an
octahedral environment with the two N,Nꢀ,O-donors adopting a
mer-arrangement. The chiral complex crystallizes in the P−1
space group with both enantiomers in the unit cell. This pair
of enantiomers associates through face-to-face stacking of bpy

1338
E. C. Constable et al.
O7
(a)
(b)
O6
O8
CI2
S1
C10
N2
O5
S2
C11
N3
N4
Zn1
C20
O1
N1
C1
O4
CI1
O3
O2
Fig. 6. (a) Structure of one of the independent [Zn(4)(OClO4)]+ cations in [Zn(4)(OClO4)][ClO4] showing close con-
tact to the second perchlorate ion; ellipsoids plotted at 40% probability level and H atoms are omitted. Selected bond
parameters: Zn1–N1 = 2.1082(14), Zn1–N2 = 2.0592(14), Zn1–N3 = 2.1140(14), Zn1–N4 = 2.0764(14), Zn1–O1 = 2.3059(13),
Zn1–O5 = 2.6232(15), S1–S2 = 2.0388(7) Å; N1–Zn1–N2 = 80.07(5), N1–Zn1–N3 = 156.68(5), N2–Zn1–N3 =116.90(5), N1–
Zn1–N4 =94.16(5), N2–Zn1–N4 = 150.26(5), N3–Zn1–N4 = 78.54(5), N1–Zn1–O1 = 82.43(5), N2–Zn1–O1 = 91.36(5), N3–
Zn1–O1 = 81.35(5), N4–Zn1–O1 = 116.97(5), O1–Zn1–O5 = 164.59(5)◦. For the second independent [Zn(4)(OClO4)]+
cation: Zn2–N5 = 2.0855(14), Zn2–N6 = 2.0791(13), Zn2–N7 = 2.0927(13), Zn2–N8 = 2.0989(14), Zn2–O9 = 2.2594(16), S3–
S4 = 2.0388(6) Å; N5–Zn2–N6 = 80.05(5), N5–Zn2–N7 = 158.67(6), N6–Zn2–N7 = 116.70(5), N5–Zn2–N8 = 94.17(6), N6–
Zn2–N8 = 146.32(6), N7–Zn2–N8 = 78.89(5), N5–Zn2–O9 = 84.27(6), N6–Zn2–O9 = 89.99(5), N7–Zn2–O9 = 82.72(5), N8–
Zn2–O9 = 122.67(5)◦. (b) Space-filling diagram of the same [Zn(4)(OClO4)][ClO4] unit shown in (a); the lower perchlorate
ion contains O1 and is bound to the zinc ion.
c
O6
N2
N1
b
O4
S2
N3
a
O5
Cu1
O2
S1
O1
Fig. 8. Part of one chain of [Cu(5)2] molecules in [Cu(5)2]·2.5MeCN,
assembled through π-stacking of bpy domains. Hydrogen atoms are omitted.
N4
O3
molecule is ordered, but the remaining 1.5 solvent molecules
are disordered and have been modelled over four positions.
Not surprisingly, the isolation of copper(i) complexes con-
taining 4 proved difficult, and we therefore turned our attention
to using the known stability of the {CuII(bpy)(tpy)} motif.[1,2]
We have previously reported the synthesis of the 2,2ꢀ:6ꢀ,2ꢀꢀ-
terpyridyl analogue of disulfide 4, ligand 6 (Scheme 1) and
of the related bis(2,2ꢀ:6ꢀ,2ꢀꢀ-terpyridin-6-yl)disulfide, and their
coordination behaviour towards iron(ii) and ruthenium(ii).[15,16]
We argued that, in the presence of a mixture of ligands 4 and
6, copper(ii) would show a preference for a {Cu(bpy)(tpy)}
coordination environment over either {Cu(bpy)2} or {Cu(tpy)2}.
Equimolar amounts of Cu(ClO4)2·6H2O, 4 and 6 were heated
under microwave conditions. After workup, a solid blue product
was isolated, but attempts to purify the bulk sample were not
successful. However, X-ray-quality blue blocks grew after slow
diffusion of Et2O into a MeNO2/MeOH solution of the crude
product. The highest mass peak envelope in the electrospray
mass spectrum of a MeCN solution of the crystals appeared
at m/z 1327.0 and was assigned to [Cu2(4)(6)(ClO4)3]+; the
Fig. 7. Structure of the [Cu(5)2] molecule in [Cu(5)2]·2.5MeCN; H
atoms are omitted. Ellipsoids are plotted at the 40% probability level.
Selected bond parameters: Cu1–N1 = 1.9635(13), Cu1–N2 = 2.1276(17),
Cu1–N3 = 1.9620(13), Cu1–N4 = 2.1155(15), Cu1–O4 = 2.2215(13),
Cu1–O1 = 2.2444(15), S1–O2 = 1.4385(15), S1–O3 = 1.4452(14), S1–
O1 = 1.4650(12), S1–C1 = 1.7893(18), S2–O5 = 1.4363(15), S2–O6 =
1.4443(15), S2–O4 = 1.4670(12), S2–C11 = 1.7914(17) Å; N1–Cu1–O1 =
80.93(6), N1–Cu1–N2 = 79.23(7), N3–Cu1–O4 = 80.99(5), N3–Cu1–N4 =
79.37(6), O4–Cu1–O1 = 85.47(5)◦.
domains (separation between least-squares planes = 3.31 Å),
and further π-stacking involving the other bpy unit in the com-
plex (separation between least-squares planes = 3.35 Å) results
inthepropagationofchains(Fig. 8).Allchainsrunparalleltoone
another through the crystal lattice, with CHpyridine· · ·Osulfonate
hydrogen bonds operating between adjacent chains. Acetonitrile
molecules occupy cavities between the chains. One MeCN

Tuning Coordination Environments Through Ligand Redox Chemistry
1339
(a)
(b)
S1
S2
a
b
N2
c
N5
N1
N3
N6
N4
Cu1
S3
Cu2
S4
N7
N8
N10
N9
Fig. 9. (a) Structure of the [Cu2(μ-4)(μ-6)]4+ cation in 2{[Cu2(μ-4)(μ-6)][ClO4]4}·5MeNO2·2MeOH·6H2O (ellipsoids plotted at 40% probability
level, and H atoms omitted). The pyridine ring containing atom N10 is disordered (see text), and only one position is shown. Selected bond para-
meters: Cu1–N1 = 2.030(6), Cu1–N2 = 1.931(6), Cu1–N3 = 2.028(6), Cu1–N7 = 2.002(6), Cu1–N8 = 2.252(6), Cu2–N4 = 2.042(7), Cu2–N5 = 1.928(6),
Cu2–N6 = 2.024(7), Cu2–N9 = 2.261(7), Cu2–N10 = 2.013(15), S1–S2 = 2.036(2), S3–S4 = 2.037(5) Å; N1–Cu1–N2 = 79.6(2), N2–Cu1–N3 = 80.6(2),
N2–Cu1–N7 = 166.0(3), N2–Cu1–N8 = 117.0(2), N7–Cu1–N8 = 76.9(2), N4–Cu2–N5 = 79.7(3), N5–Cu2–N6 = 80.2(2), N5–Cu2–N9 = 116.0(2), N5–
Cu2–N10 = 166.2(5), N9–Cu2–N10 = 76.1(4)◦. (b) Packing of [Cu2(μ-4)(μ-6)]4+ cations; N and S atoms are shown in black.
calculated isotope distribution was consistent with this formu-
lation. Lower mass peak envelopes at m/z 1066.2, 690.1 (base
peak), and 536.0 were assigned to the ions [Cu(4)(6)(ClO4)]+,
[Cu(6)(ClO4)]+, and [Cu(4)(ClO4)]+. Significantly, the sec-
ond most intense peak (40% abundance) in the electrospray
ionization (ESI) mass spectrum came at m/z 613.1 and was con-
sistent with [Cu(7)(ClO4)]+, and is evidence for the cleavage
and recombination reaction shown in Scheme 2.[18]
perchlorate ion. The same coordination motif is observed when
a methyl substituent is introduced into the 6ꢀ-position of 4
(to give ligand 2) and the pitch of the single-strand helix
changes from 35.59(12)◦ and 37.27(12)◦ in the two indepen-
dent [Zn(4)(OClO3)]+ cations to 51.3(5)◦ in [Zn(2)(OClO3)]+.
The reaction of 4 with [Cu(NCMe)4][PF6] leads to a prod-
uct, tentatively formulated as {[Cu(4)][PF6]}n, aerial oxidation
of which gives the octahedral copper(ii) complex [Cu(5)2]
(H5 = 2,2ꢀ-bipyridine-6-sulfonic acid) in which [5]− acts as an
N,Nꢀ,O-donor. By applying a strategy that utilizes the prefer-
ence shown by copper(ii) for a five-coordinate {Cu(bpy)(tpy)}
environment, we have shown that copper(ii) reacts with a 1:1
mixture of 4 and 6 to produce [Cu2(μ-4)(μ-6)]4+, isolated and
structurally characterized as the perchlorate salt.
Single-crystal X-ray analysis of the blue crystals confirmed
the formation of [Cu2(μ-4)(μ-6)][ClO4]4, isolated as [Cu2(μ-
4)(μ-6)][ClO4]4·2MeNO2·MeOH·3H2O. Although the struc-
ture is of poor quality, it unambiguously confirms the presence
of the [Cu2(μ-4)(μ-6)]4+ cation with each copper(ii) ion in a
five-coordinate {Cu(bpy)(tpy)} environment (Fig. 9a). Each Cu
atom exhibits a weak contact to an O atom of a perchlorate
ion(Cu1· · ·O1 = 2.815(5), Cu2· · ·O5 = 2.764(8) Å)thatfillsthe
otherwise vacant position (see Fig. 9a) in the coordination sphere
of each copper(ii) centre. The pyridine ring shown in Fig. 9a to
contain atom N10 is disordered and has been modelled over two
equal occupancy sites. This disorder makes a detailed analysis
of the coordination sphere of Cu2 meaningless. However, we
note that the two tpy domains of ligand 6 bend in towards one
another, generating a V-shaped cavity that accommodates the
disulfide bridge of ligand 4. The cations pack in an interdigi-
tated assembly of rows that follow the direction of the b-axis
(Fig. 9b). Although the diagram in Fig. 9b suggests that the tpy
domains might be π-stacked, they are slipped with respect to one
another and despite an inter-ring separation of 3.2 Å, the face-
to-face interactions are inefficient. Better overlap is prevented
by the presence of the perchlorate ions, which sit over the copper
atoms (see above). All perchlorate ions are ordered, but there is
significant disorder among the solvent molecules.
Experimental
General
Room-temperature 1H and 13C NMR spectra were recorded
on a Bruker Avance DPX 500 spectrometer; Scheme 1 gives
the numbering scheme for 3 and 4. Chemical shifts for 1H
and 13C NMR spectra are referenced to residual solvent peaks
(TMS = δ 0 ppm). MALDI-time of flight and electrospray mass
spectra were recorded using PerSeptive Biosystems Voyager
and MAT LCQ mass spectrometers respectively. Microwave
reactions were carried out in a Biotage Initiator 8 reactor.
Caution! Perchlorate salts are potentially explosive and must
be handled with caution.
6-(Pyridin-2-yl)pyridine-2(1H)-thione 3
Compound 3 was prepared by an analogous route to that
reported,[14] but starting with 6-chloro-2,2ꢀ-bipyridine (0.598 g,
3.14 mmol) in place of 6-bromo-2,2ꢀ-bipyridine. Compound 3
was isolated as a yellow solid, which was recrystallized from
ethyl acetate and hexanes (0.355 g, 60%). Mp 118◦C. 1H NMR
(500 MHz, CDCl3 with dithiothreitol[26] added) δ/ppm 8.67 (br
d, J ≈4.0, 1H, HA6),7.83 (m, 2H, HA4+A3), 7.53 (d, J 8.6, HB5),
7.41 (m, 1H, HA5), 7.36 (t, J 7.9, 1H, HB4), 7.18 (d, J 7.1, 1H,
Conclusions
We have shown that 1,2-bis(2,2ꢀ-bipyridin-6-yl)disulfide 4
can by produced by the oxidative coupling of 6-(pyridin-2-
yl)pyridine-2(1H)-thione 3, using H2O2 as oxidant. Ligand 4
acts as a bis(chelate) to a single zinc(ii) centre, giving a helical
coordination environment that is completed by one O-bonded
H
B3). 13C NMR (126 MHz, CDCl3) δ/ppm 179.5 (CB6), 149.6
(CA6), 146.5 (CA2), 143.8 (CB2), 137.9 (CA4), 136.9 (CB4), 134.7

1340
E. C. Constable et al.
(CB5), 125.6 (CA5), 120.0 (CA3), 109.5 (CB3). m/z (ESI-MS;
MeOH) 211.3 [M + Na]+ (base peak), 189.2 [M + H]+.
1,2-Bis(2,2ꢀ-bipyridin-6-yl)disulfide 4
[Cu(NCMe)4][PF6] (28.0 mg, 0.075 mmol) was added while the
mixture was stirred, causing an immediate colour change to dark
red-brown. The reaction mixture was heated at reflux for 1 h and
allowed to cool to room temperature. The solution was concen-
trated to one-third of its volume, and then Et2O was allowed to
diffuse into it over a period of 2 days. A red-orange precipitate
was collected by filtration. Attempts to isolate a pure sample of
Compound 3 (84.5 mg, 0.449 mmol) was dissolved in MeCN
(∼10 mL), and H2O2 (30%, 0.25 mL) was added dropwise. The
reaction mixture was stirred vigorously at room temperature for
2 h, and was then left in the refrigerator overnight. The colour-
less crystalline product 4 was separated by filtration (71.3 mg,
85%). Mp 123◦C. 1H NMR (500 MHz, CDCl3) δ/ppm 8.64
(br d, J ∼4.0, 1H, HA6), 8.34 (d, J 7.9, 1H, HA3), 8.20 (d, J
7.1, 1H, HB3), 7.72 (m, 2H, HA4+B4), 7.65 (d, J 7.9, 1H, HB5),
7.27 (m, 1H, HA5). 13C NMR (126 MHz, CDCl3) δ/ppm 158.6
(CB6), 156.4 (CB2), 155.4 (CA2), 149.4 (CA6), 138.4 (CA4/B4),
137.2 (CA4/B4), 124.3 (CA5), 121.6 (CA3), 120.0 (CB5), 118.5
(CB3). m/z (ESI-MS; MeOH) 397.3 [M + Na]+ (base peak),
375.3 [M + H]+. Anal. calc. for C20H14N4S2 (374.48) C 64.15,
H 3.77, N 14.96. Found: C 63.67, H 3.94, N 15.00%.
1
the product were unsuccessful. H NMR (400 MHz, CD3CN)
δ/ppm 8.50 (br, 1H), 8.28 (br, 1H), 8.16 (br, 1H), 8.02 (br, 1H),
7.81 (br, 1H), 7.60–7.54 (br, overlapping, 2H). After several
months, green crystals of [Cu(5)2]·2.5MeCN had formed in the
NMR tube.
[Cu2(μ-4)(μ-6)][ClO4]4
Compounds
4 (9.2 mg, 0.0245 mmol) and 6 (13.0 mg,
0.0245 mmol) were suspended in MeCN (2 mL) and Cu(ClO4)2·
6H2O was added (18.2 mg, 0.0491 mmol). The reaction mix-
ture was sealed in a vial and heated in a microwave reactor at
120◦C for 10 min, then allowed to cool to room temperature.
The solid product was separated by filtration. A sample of the
crude product was dissolved in MeNO2 with a drop of MeOH
added, and Et2O was allowed to diffuse into the solution. Crys-
tals of [Cu2(μ-4)(μ-6)][ClO4]4·2MeNO2·MeOH·3H2O formed
overnight. m/z (ESI MS; MeCN) 1327.0 [Cu2(4)(6)(ClO4)3]+,
1066.2 [Cu(4)(6)(ClO4)]+, 690.1 [Cu(6)(ClO4)]+ (base peak),
536.0 [Cu(4)(ClO4)]+ (see text).
[Zn(2)2][ClO4]2
Compound 2 (26.5 mg, 0.0658 mmol) was suspended in MeCN
(10 mL) and Zn(ClO4)2·6H2O (24.5 mg, 0.0658 mmol) was
added while the colourless reaction mixture was stirred. After
being heated at reflux for 1 h, the reaction mixture was allowed
to cool to room temperature. It was concentrated to ∼one-third
of the volume and then Et2O was diffused into the solution
overnight. The colourless precipitate was separated by filtra-
tion and dried under high vacuum to yield [Zn(2)2][ClO4]2 as
an off-white microcrystalline solid (33.8 mg, 77.0%). 1H NMR
(500 MHz, CD3CN) δ/ppm 8.61 (d, J 8.1, 1H, HB3), 8.47 (d,
J 8.0, 1H, HA3), 8.37 (t, J 7.9, 1H, HB4), 8.29 (t, J 7.9, 1H,
Crystal Structure Determinations
Data were collected on Bruker-Nonius Kappa charge coupled
device or Stoe IPDS diffractometers; data reduction, solution
and refinement used the programs COLLECT,[30] SIR92,[31]
DENZO/SCALEPACK,[32] and CRYSTALS,[33] or Stoe IPDS
software[34] and SHELXL97.[35] Hydrogen atoms bonded to N or
Ohavebeenlocated, optimized, andthenincludedasfixedcontri-
butions. ORTEP figures were drawn with the program ORTEP-
3 for Windows.[36] Structures were analyzed using Mercury
v. 2.3.[37,38]
H
A4), 8.12 (d, J 7.8, 1H, HB5), 7.67 (d, J 7.8, 1H, HA5), 1.99 (s,
3H, HMe). 13C NMR (126 MHz, CD3CN) δ/ppm 160.4 (CA6),
158.9 (CB6), 153.5 (CB2), 148.6 (CA2), 144.5 (CB4), 143.6
(CA4), 132.2 (CB5), 130.7 (CA5), 124.4 (CB3), 122.8 (CA3),
24.1 (CMe). m/z (MALDI MS) 466.53 [Zn(2)]+. Anal. calc. for
C22H18Cl2N4O8S2Zn (666.85) C 39.63, H 2.72, N 8.40. Found
C 39.64, H 2.88, N 8.67%.
3: C10H8N2S, M 188.25, colourless plate, triclinic, space
group P−1, a 7.9706(1), b 9.5618(2), c 12.1532(2) Å, α
77.4044(9), β 76.2747(9), γ 87.3699(10)◦, U 878.11(3) Å3, Z 4,
D 1.424 Mg m−3, μ(MoKα) 0.315 mm−1, F(000) 392, T 173 K,
10187 reflections, 5146 unique (Rint 0.025), R1 0.0358 (3131
reflections, 235 parameters, I > 3.0σ(I)), wR2 0.0458, R1 0.0647
(all data), wR2 0.0564 (all data), goodness of fit (GOF) 1.0221.
4: C20H14N4S2, M 374.49, colourless block, monoclinic,
space group C2/c, a 19.253(4), b 7.9777(16), c 11.636(2) Å, β
105.62(3)◦, U 1721.2(6) Å3, Z 4, D 1.445 Mg m−3, μ(MoKα)
0.321 mm−1, F(000) 776, T 173 K, 21632 reflections, 3124
unique (Rint 0.0364), R1 0.0385 (3009 reflections, 118 parame-
ters, I > 2.0σ(I)), wR2 0.1067, R1 0.0400 (all data), wR2 0.1078
(all data), GOF 1.101.
[Zn(4)2][ClO4]2
Compound 4 (21.9 mg, 0.0585 mmol) was suspended in MeCN
(10 mL) and Zn(ClO4)2·6H2O (21.8 mg, 0.0585 mmol) was
added to the stirring mixture. After being heated at reflux for
1 h, the reaction mixture was allowed to cool to room tem-
perature, and was concentrated to ∼one-third of its volume.
Slow diffusion of Et2O into the solution overnight yielded a
colourless precipitate, which was separated by filtration and
dried under high vacuum. [Zn(4)2][ClO4]2 was isolated as an
off-white microcrystalline solid (31.6 mg, 84.6%). 1H NMR
(500 MHz, CD3CN) δ/ppm 8.57 (m, 2H, HA3+B3), 8.41 (t,
J 7.8, 1H, HA4), 8.37 (d, J 4.9, 1H, HA6), 8.32 (t, J 7.9, 1H,
H
B4), 8.06 (d, J 7.8, 1H, HB5), 7.85 (m, 1H, HA5). 13C NMR
2{[Zn(2)(OClO3)][ClO4]}·EtOH: C46H42Cl4N8O17S4Zn2,
M 1379.72, colourless plate, monoclinic, space group C2/c, a
28.5683(12), b 10.4110(5), c 22.0368(10) Å, β 125.360(2)◦, U
(126 MHz, CD3CN) δ/ppm 159.0 (CB6), 153.0 (CB2), 149.8
(CA6), 149.3 (CA2), 144.2 (CB4), 143.8 (CA4), 131.3 (CB5),
129.2 (CA5), 124.7 (CA3/B3), 124.1 (CA3/B3). m/z (MALDI
MS)538.57[Zn(4)ClO4]+.Anal. calc. forC20H14Cl2N4O8S2Zn
(638.79) C 37.61, H 2.21, N 8.77. Found C 37.58, H 2.27,
N 8.90%.
5345.3(4) Å3, Z 4, D 1.714 Mg m−3, μ(MoKα) 1.334 mm−1
,
F(000) 2808, T 173 K, 33248 reflections, 5209 unique
(Rint 0.066), R1 0.0728 (3791 reflections, 377 parameters,
I > 1.0σ(I)), wR2 0.0798, R1 0.1000 (all data), wR2 0.1029 (all
data), GOF 1.1410.
[Zn(4)(OClO3)][ClO4]: C20H14Cl2N4O8S2Zn, M 638.77,
colourless block, monoclinic, space group P21/c, a 15.2359(8),
b 16.2519(9), c 19.4989(11) Å, β 109.732(3)◦, U 4544.7(4) Å3,
[Cu(5)2]·2.5MeCN
Compound 4 (28.1 mg, 0.075 mmol) was suspended in
MeCN and the mixture degassed with N2 for 15 min.

Tuning Coordination Environments Through Ligand Redox Chemistry
1341
Z 8, D 1.867 Mg m−3, μ(MoKα) 1.559 mm−1, F(000) 2576,
T 123 K, 117679 reflections, 17252 unique (Rint 0.035),
R1 0.0436 (14030 reflections, 667 parameters, I > 2.0σ(I)), wR2
0.0459, R1 0.0481 (all data), wR2 0.0615 (all data), GOF 1.0630.
[Cu(5)2]·2.5MeCN: C25H21.5CuN6.5O6S2, M 636.65, green
block, triclinic, space group P−1, a 11.0071(14), b 11.0069(13),
c 14.3152(16) Å, α 96.414(10), β 105.283(9), γ 117.951(9)◦,
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U 1420.4(3) Å3, Z 2, D 1.489 Mg m−3, μ(MoKα) 0.967 mm−1
,
F(000) 652, T 173 K, 70149 reflections, 8152 unique
(Rint 0.1101), R1 0.0359 (8024 reflections, 438 parameters,
I > 2.0σ(I)), wR2 0.1039, R1 0.0363 (all data), wR2 0.1043 (all
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[18] S. Otto, R. L. E. Furlan, J. K. M. Sanders, J. Am. Chem. Soc. 2000,
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[Cu2(μ-4)(μ-6)][ClO4]4·2MeNO2·MeOH·3H2O:C53H50Cl4
Cu2N12O24S4, M 1636.21, blue block, triclinic, space group
P−1, a 14.5596(11), b 16.0545(13), c 17.5368(14) Å, α
91.668(5), β 111.507(4), γ 109.858(4)◦, U 3531.3(5) Å3, Z
2, D 1.539 Mg m−3, μ(MoKα) 0.955 mm−1, F(000) 1648, T
123 K, 43521 reflections, 16286 unique (Rint 0.067), R1 0.1048
(8553 reflections, 1018 parameters, I > 2.0σ(I)), wR2 0.0987,
R1 0.1773 (all data), wR2 0.1530 (all data), GOF 1.0804.
The crystal data have been deposited with the Cam-
bridge Crystallographic Data Centre (www.ccdc.cam.ac.uk;
nos. CCDC 767120–767125).
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Acknowledgements
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We thank the Swiss National Science Foundation and the University of Basel
for financial support.
[28] W. W. Brandt, F. P. Dwyer, E. D. Gyarfas, Chem. Rev. 1954, 54, 959.
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