Self-assembly of transition metal ion complexes of a hybrid pyrazine-terpyridine ligand

Article abstract of DOI:10.1039/c2dt31982a

A new hybrid pyrazine-terpyridine ligand L (C₃₄H ₂₂N₈) and its complexes with different transition metal ions, M (M = Mn(ii) 1, Zn(ii) 2, Fe(ii) 3, Co(ii) 4, Cu(ii) 5 and Cd(ii) 6), have been synthesised. In the presence of a nitrate counter-anion, both Cu(ii) and Cd(ii) give complexes in which the ratio M : L is 2 : 1, whereas with perchlorate, trifluoromethanesulfonate or tetrafluoroborate, the other metal ions provide solids in which this ratio is 1 : 1. From mass spectral measurements and a single crystal, X-ray structure determination for the Fe(ii) complex 3, however, all the latter species are concluded to be 2 : 2 complexes. Both the Fe(ii) complex 3 and the Co(ii) complex 4, generated from tetrafluoroborate reactant salts, have the composition [M₂L ₂F₂(H₂O)](BF₄)₂, the presence of fluoride ligands being presumed to reflect the abstraction of fluoride ions from tetrafluoroborate by the metal ions under the preparative conditions. The crystal structure of complex 3 shows the Fe(ii) centres to be inequivalent, one being high-spin and heptacoordinate with a FeN ₄F₂O coordination sphere, the other low-spin and octahedral with a FeN₆ sphere. The two ligand molecules differ markedly, one being heptadentate, the other clearly hypodentate , with only three N-donor atoms of a terpyridine-like arm coordinated, although their conformations are similar, showing significant differences from that of C₂ symmetry found for the free ligand by a crystal structure determination. Mass spectra are consistent with the Cu(ii) and Cd(ii) complexes having the composition [M₂L(H₂O)_n(NO ₃)_4-n](NO₃)_4-n, and the weak antiferromagnetic coupling observed for the Cu(ii) complex is consistent with a preliminary crystal structure determination which indicates that the two Cu(ii) centres are not bridged by a pyrazine unit. The Royal Society of Chemistry 2013.

Full text of DOI:10.1039/c2dt31982a

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Self-assembly of transition metal ion complexes of a
hybrid pyrazine–terpyridine ligand†
Cite this: Dalton Trans., 2013, 42, 1743
a,b,c
a
b
Artur R. Stefankiewicz,
Monika Wałęsa-Chorab, Jack Harrowfield,
a
a
d
a
Maciej Kubicki, Zbigniew Hnatejko, Maria Korabik and Violetta Patroniak*
22 8
A new hybrid pyrazine–terpyridine ligand L (C34H N ) and its complexes with different transition metal
ions, M (M = Mn(II) 1, Zn(II) 2, Fe(II) 3, Co(II) 4, Cu(II) 5 and Cd(II) 6), have been synthesised. In the presence
of a nitrate counter-anion, both Cu(II) and Cd(II) give complexes in which the ratio M : L is 2 : 1, whereas
with perchlorate, trifluoromethanesulfonate or tetrafluoroborate, the other metal ions provide solids in
which this ratio is 1 : 1. From mass spectral measurements and a single crystal, X-ray structure determi-
nation for the Fe(II) complex 3, however, all the latter species are concluded to be 2 : 2 complexes. Both
the Fe(II) complex 3 and the Co(II) complex 4, generated from tetrafluoroborate reactant salts, have the
2 2 2 2 4 2
composition [M L F (H O)](BF ) , the presence of fluoride ligands being presumed to reflect the abstrac-
tion of fluoride ions from tetrafluoroborate by the metal ions under the preparative conditions. The
crystal structure of complex 3 shows the Fe(II) centres to be inequivalent, one being high-spin and hepta-
4 2 6
coordinate with a FeN F O coordination sphere, the other low-spin and octahedral with a FeN sphere.
The two ligand molecules differ markedly, one being heptadentate, the other clearly “hypodentate”, with
only three N-donor atoms of a terpyridine-like arm coordinated, although their conformations are
similar, showing significant differences from that of C
structure determination. Mass spectra are consistent with the Cu(II) and Cd(II) complexes having the com-
position [M L(H O) (NO 4−n](NO 4−n, and the weak antiferromagnetic coupling observed for the Cu(II)
2
symmetry found for the free ligand by a crystal
Received 29th August 2012,
Accepted 24th October 2012
2
2
n
3
)
3
)
DOI: 10.1039/c2dt31982a
complex is consistent with a preliminary crystal structure determination which indicates that the two
Cu(II) centres are not bridged by a pyrazine unit.
www.rsc.org/dalton
and geometry of the binding subunits, the presence of specific
molecules and anions, and the nature of the medium. The
Introduction
1
7
The versatility of N-heterocyclic ligands, especially derivatives abundant complexes of transition metal ions with hetero-
1
2
and analogues of 2,2′-bipyridine and of 2,2′;2′′,6′-terpyridine,
cyclic-N-donor ligands are used in many fields of science, for
1
8–20
21,22
is reflected in their utility in self-assembly processes leading to example, in medicine,
nanotechnology,
optoelectro-
2
3,24
25,26
the formation of a wide variety of relatively stable, well-defined nics
and catalysis.
3
,4
5
complexes like grids, squares, double, triple (and higher)
The physical properties of pyrazine derivatives, such as
6
,7
8–10
11–14
and circular helicates, racks,
cages
and coordination dipole moment, π-stacking interactions and base strength, are
1
5,16
polymers.
tures depends on many factors, such as the nature, flexibility aza-aromatics.
ligands have found applications as light-harvesting anten-
The formation of such supramolecular architec- usually quite different from those of other electron-deficient
2
7,28
Metal ion complexes of pyrazine-based
2
9,30
31
32,33
nae,
liquid crystals and molecular wires,
and brid-
a
Faculty of Chemistry, Adam Mickiewicz University, Grunwaldzka 6, 60780 Poznań,
ging pyrazine units have often proved to be efficient mediators
3
4,35
Poland. E-mail: violapat@amu.edu.pl
b
of intermetallic coupling.
Magnetic interactions between
Institut de Science et d’Ingénierie Supramoléculaires, Université de Strasbourg, 8,
pyrazine-bridged paramagnetic centers take place mainly via
allée Gaspard Monge, 67083 Strasbourg, France
3
6
c
the pyrazine ring and while this is usually associated with
University Chemical Laboratory, Lensfield Road, Cambridge CB2 1EW, United
3
7–39
antiferromagnetic exchange,
some examples of copper(II)
Kingdom
d
Faculty of Chemistry, University of Wrocław, 14 Joliot-Curie, 50383 Wrocław,
complexes which show ferromagnetic coupling are also
4
0
Poland
†
known. Exchange interactions mediated by the π-system of
Electronic supplementary information (ESI) available: A preliminary structure
41
bridging pyrazine may differ depending on the relative orien-
determination of the Cu(II) complex 5. CCDC 856604 (L) and 757398 (3). For
tations of the orbitals containing unpaired electrons on the
two metal centres, so that a truly orthogonal arrangement
ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/c2dt31982a
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Fig. 1 Perspective view of the ligand L. Ellipsoids are drawn at the 50% prob-
ability level, hydrogen atoms are shown as spheres of arbitrary radius.
Scheme 1
gives rise to ferromagnetic coupling, whereas antiferromag-
4
2
netic coupling is expected when this is not so.
Following on from our successful synthesis of certain qua-
4
3–45
terpyridine transition metal complexes,
ested in the ligand L 2,3-bis(6-(6-(pyridin-2-yl)pyridin-2-yl)-
pyridin-2-yl)pyrazine (Scheme 1) containing two N -donor sub-
units as an essentially double-quaterpyridine. The purpose of
we became inter-
4
this work was to establish some of the coordination modes Fig. 2 Planar arrangements (conformations) of the ligand L for which metal
ion chelation would be possible.
possible for L which might bind metal ions in various ways
(Fig. 2). We have investigated physicochemical properties and
characterised complexes with different transition metal ions.
This characterisation involved the crystal structure determi-
nations of the ligand and its Fe(II) complex, as well as both
luminescence and magnetic susceptibility measurements on
various other transition metal ion complexes.
Compound D was prepared in the same way as compound B
and then used in a Stille-type coupling reaction with 2,3-
dichloropyrazine under Pd(0) catalysis to obtain the ligand L.
All steps of this procedure resulted in good yields, so that the
ligand L can be considered a readily accessible potentially
octadentate species. The overall yield for the synthesis of the
ligand L starting from A is 47%.
Results and discussion
Synthesis and characterisation of ligand L
Confirmation of the expected structure of L was provided by
a single crystal, X-ray structure determination (Fig. 1), suitable
The ligand L was obtained using a multistep Stille-type coup- crystals being obtained by vapour diffusion of diethyl ether
4
6
ling reaction as outlined in Scheme 1.
-Bromo-2,2′-bipyridine A was prepared from 2-trimethyl- pyridyl ligand of this type, the conformation adopted by the
stannylpyridine and 2,6-dibromopyridine by an adaptation of a terpyridine arms in the crystal is one where all N–C–C–N units
into a nitromethane solution. As would be expected for a poly-
2
6
4
7
literature method. Compound B was obtained in a catalytic have a transoid form and each set of three linked rings is
reaction of A with hexamethylditin and was treated with 2,6- essentially planar. It is not possible, however, to attain a con-
dibromopyridine under Pd(0) catalysis to obtain terpyridine C. formation where the ligand overall is planar and the pyrazine-
1
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N atoms can also be included in true transoid arrangements.
(This would involve complete overlap of the central pyridine
units.) The observed structure is nonetheless consistent with
an approach to this situation achieved by a twisting of approxi-
mately 35° of the terpyridine unit mean planes with respect to
the pyrazine ring plane, giving the molecule essentially C
2
symmetry in the solid state. The molecule is thus chiral and
the conical entities fit together, one within the next, to form
columns of homochiral species running parallel to the b axis.
These columns lie adjacent in an undulating fashion to form
homochiral sheets parallel to the ab plane, with the absolute
configuration alternating from one sheet to the next, so that
the lattice is overall racemic (Fig. S1†). The structure of the
ligand L is only the fourth crystal structure of a seven-ring
4
8–50
compound not containing metal ions.
Clearly, the ligand L is not preorganised, at least in the
solid state, for metal ion chelation, though of course it is well
known that appropriate rearrangements of polypyridyl ligands
2
are usually facile. Two (idealised) planar conformations are
shown in Fig. 2, one (A) being compatible with the binding of
two metal ions in equivalent sites, the other (B) being compati-
ble with the binding of up to four metal ions, and probably oli-
gomer/polymer formation. Conformation A is similar to that of
a large number of ditopic ligands known to form grid com-
Fig. 3 (a) Perspective view of the cation 3; (b) a view of the Fe(II) coordination
spheres. Ellipsoids are drawn at the 20% level, hydrogen atoms are omitted for
clarity. Relevant distances (Å): Fe1–N1A 2.016(7), Fe1–N8A 1.837(8), Fe1–N14A
3
,4
plexes but here grid formation would involve eight-coordi-
nation of the metal ion and thus be a possibility more likely to
be observed with, say, the lanthanides, rather than the pre- 2.046(6), Fe1–N26B 2.059(7), Fe1–N32B 1.828(9), Fe1–N38B 2.019(7), Fe2–N1B
2
.226(8), Fe2–N8B 2.234(7), Fe2–N14B 2.262(7), Fe2–N20B 2.304(7), Fe2–O1W
sently investigated transition metal ions.
2.033(8), Fe2–F1 1.861(4), Fe2–F2 1.869(4); and angles (°) N32B Fe1 N8A 169.8
Given that a non-planar conformation is observed for L in
the solid state, other possibilities for its coordination modes
arise, perhaps the most obvious being one where the pyrazine
(3), N1A Fe1 N14A 159.0(4), N38B Fe1 N26B 161.0(4), F1 Fe2 F2 174.7(2).
ring serves as no more than a link between two terpyridine-like complexes. An additional complication does arise, however, in
binding sites. In that the conformations shown in Fig. 2 both the case of the complexes prepared from tetrafluoroborate
involve clashes between aromatic CH groups, there is in fact a salts.
good reason to anticipate that even the bound ligand would
Thus, a single crystal structure determination (Fig. 3) on
not adopt a planar conformation. Because terpyridine is an the complex derived from hydrated iron(II) tetrafluoroborate,
excellent ligand for transition metal ions, giving rise to numer- the complex again being crystallised by vapour diffusion of
2
ous complexes of practical interest, our initial investigations diethyl ether into a nitromethane solution, showed the pres-
of the coordination behaviour of L were therefore centred on ence of two fluoride ions as ligands in the binuclear complex
its complexation by these species.
of composition [Fe L F (H O)](BF ) ·1.5H O. The fluoride
2 2 2 2 4 2 2
ligands are presumed to arise from the abstraction of fluoride
ions from tetrafluoroborate counterions in the presence of the
Reactions with various transition metal salts were carried out Lewis-acidic metal ion. Such a process is well known and has
Synthesis and characterisation of complexes of L
5
1
in acetonitrile–dichloromethane (1 : 1) at room temperature for been used in the preparation of coordination compounds con-
5
2–54
4
8 h. In all cases, formation of complexes with the metal in its taining a fluoride ion as a bridging ligand.
The coordi-
original oxidation state was efficient and the products were nation environments of the two Fe(II) centres are completely
readily isolated as stable solids. However, while in all instances different and can be seen as illustrating what might be con-
the reaction mixtures contained the metal salt and L in a sidered the ambivalent character of the ligand L. One site, to
molar ratio of 2 : 1, elemental analyses of the isolated solids which both fluoride ions and a water molecule are bound as
showed that this ratio was retained only in the case of reac- well as four N-donors from one half of a ligand molecule, con-
tions with the hydrated nitrates of Cu(II) and Cd(II). With Mn tains a high-spin, pentagonal bipyramidal Fe(II) centre, with
(
II), Fe(II), Co(II) and Zn(II) in the presence of weakly coordinat- fluoride ligands in the apical positions and water-O complet-
−
−
−
ing anions (ClO4 , CF SO or BF4 ), the products proved all ing the equatorial pentagon, as might be expected from the
3
3
to be species with M : L 1 : 1, although on the basis of their known coordination chemistry of simple quaterpyridine
4
3–45
mass spectra and a single crystal structure determination for ligands.
The other site contains Fe(II) as a low-spin,
the Fe(II) complex, it is concluded that all are, in fact, M2L2 pseudo-octahedral species, bound exclusively to pyridine-N
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Table 1 Maximum absorption bands (λmax. [nm]) and molar absorption coeffi-
4
−1
cm−1]) of the studied compounds
cients (ε [10
M
λ
max. [nm] (ε [10 M cm−1])
4
−1
Compound
L
288.0 (4.0)
[
[
[
[
[
Mn
Zn
Fe
Co
2
L
2
](ClO
](CF SO
(H O)](BF
(H O)](BF
L(NO ] 6
4
)
4
1
) 2
3 4
287.5 (2.2); 308.0 (2.2); 334.5 (2.2)
288.0 (3.0); 322.5 (3.2); 335.5 (2.7)
278.0 (5.1); 329.0 (4.1); 555.0 (0.5)
285.0 (3.2)
2
L
2
F
3
2
L
2
2
2
4 2
) 3
) 4
4 2
2
L
2
F
2
2
Cd
2
)
3 4
286.0 (3.3); 304.5 (3.1); 336.5 (2.8)
considered simply a distortion of the planar conformation A
Fig. 2) with one pyrazine-N detached from one of the Cu(II)
ions. The mass spectra of both complexes were dominated by
peaks due to M L species and magnetic measurements on the
(
2
Cu(II) complex (see below) showed very weak antiferromagnetic
coupling consistent with the absence of a pyrazine bridge
between the two centres.
Electronic absorption spectra and luminescence properties
Absorption and emission spectra of ligand L and its metal
−5
complexes were recorded at concentrations of 1 × 10 M in
Fig. 4 Views (stick representations), from a similar perspective with respect to
the pyrazine ring, of the conformation found for ligand L in the various environ- acetonitrile for the complexes and in dichloromethane for L at
ments described in the present work: (a) in the free ligand crystal; (b) in the hep-
room temperature. Maximum absorption bands and molar
absorption coefficients are listed in Table 1.
tadentate form found in complex 3; (c) in the tridentate form found in complex
3; (d) in the heptadentate form found in the Cu(II) complex 5 (see below).
The electronic absorption spectra in the UV region showed
5
7
bands typical for π → π* ligand-based transitions. In the case
of the Fe(II) complex 3, the broad absorption band in the
visible region at 555 nm is assigned to an MLCT electronic
donor atoms derived from two different ligands L. Thus, it is
2
+
an analogue of [Fe(terpy) ] . While its binding to two metal
2
5
8
transition. As expected where the metal ion present is essen-
tially incapable of oxidative quenching of the excited state, the
Zn(II) and Cd(II) complexes 2 and 6 display broad luminescence
ions means that all but one pyrazine-N of the eight N-donors
of one ligand are coordinated, for the other ligand but three
pyridine-N donors are bound, making it a genuinely hypoden-
5
5
bands with maxima at λem. = 364 nm (λex. = 320 nm) and λem.
=
tate species. The conformations of the two ligand molecules
Fig. 4) are nevertheless quite similar, with, in both cases, one
382 nm (λex. = 330 nm) respectively, which are red shifted com-
(
pared to the band in the emission spectrum of the ligand L,
λem. = 346 nm (λex. = 288 nm) (Fig. 5). In the other complexes,
emission is completely quenched by the binding of the metal
ion.
While emission occurs from both the Zn(II) and Cd(II) com-
plexes, it is approximately 40 times more intense from the
former than from the latter, consistent with stronger spin–
orbit coupling in the heavier metal ion providing a more
efficient pathway for excited state deactivation. For the Zn(II)
complex, the quantum yield was determined, relative to
anthracene as standard, as 0.07.
terpyridine unit being rotated so as to be almost perpendicular
to the remaining terpy–pyrazine unit.
As in the lattice of the free ligand, there are multiple con-
tacts between the aromatic ring components of 3 indicative of
the full range of interactions (CH⋯pi, CH⋯N, partial charge
5
6
attractions) which fall under the descriptor of “π-stacking”. It
may be that both inter- and intra-molecular interactions of
this type (Fig. S2†) are important in the stabilisation, in the
solid state, of the markedly hypodentate form of one ligand
molecule in the binuclear cation of 3, since it is somewhat sur-
prising that a 2 : 2 M : L complex with some ligand donors
unbound should have crystallised from a medium in which
the ratio was 2 : 1. It is also true, however, that once adapted to
Magnetic properties
the binding of a metal ion, the conformation of the ligand is Susceptibility measurements on the di-iron complex 3 are
not markedly dependent upon other factors (Fig. 4).
shown in Fig. 6 and 7. At room temperature the effective mag-
To establish the exact form of the complexes of 2 : 1 M : L netic moment is 5.17 BM, consistent with the presence of one
stoichiometry, efforts were made to obtain crystals suitable for high-spin (S = 2) and one (diamagnetic) low-spin (S = 0) Fe(II)
an X-ray structure determination. Only for the Cu(II) complex centre. The effective magnetic moment decreased monotoni-
were adequate crystals obtained and unfortunately their diffr- cally with decreasing temperature, falling to a value of 2.27 BM
action data did not provide a satisfactory structure solution, at 1.8 K. Fe–N bond distances observed in the crystal structure
although the low-quality result (ESI†) again showed the ligand of 3 are completely consistent with the assignment of a high-
to adopt a non-planar conformation (Fig. 4), one that could be spin state to the heptacoordinate FeN F O centre and a low
4
2
1
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Fig. 8 Temperature dependences of experimental χ
m
and μeff for complex 5,
calculated per copper ion. The solid lines represent the calculated curves.
Fig. 5 The emission spectra of ligand L and complexes 2 and 6.
magnetic field measurements at 2 K (Fig. 7). The experimental
5
9
curve lies only a little above the Brillouin function solid line,
for g = 2 and S = 1/2.
Susceptibility measurements on the dicopper complex 5 are
shown in Fig. 8. The effective magnetic moment at room temp-
erature is 1.8 BM and remains almost unaffected by changing
temperature down to 25 K, after which there is a small
decrease to 1.58 BM at 1.8 K. This slight temperature depen-
dence indicates that any antiferromagnetic interactions
between the Cu(II) centres must be weak and this conclusion is
supported by an analysis of the susceptibility curve in terms of
the Bleaney–Bowers equation (with modification to include
allowance for extremely weak intermolecular interactions; see
−
1
ESI†) providing a very low J value of J = −0.87 ± 0.02 cm
.
m
Fig. 6 Temperature dependences of experimental χ and μeff for complex 3.
Experimental
General consideration
6
-Bromo-2,2′-bipyridine A was prepared according to literature
4
7
methods. The metal salts were used without further purifi-
cation as supplied from Aldrich. LC-MS analyses of the tran-
sition metal complexes were performed using a Varian LC MS/
MS TripleQuad 320 spectrometer equipped with an ESI ionisa-
tion interface. Acetonitrile was used as a mobile phase with a
flow rate of 300 μL min and nitrogen as a blanketing gas. ESI
mass spectra for acetonitrile solutions ∼10− M were measured
using a Waters Micromass ZQ spectrometer. Microanalyses
were performed using a Perkin-Elmer 2400 CHN microanaly-
ser. IR spectra were obtained with a Perkin-Elmer 580 spectro-
−
1
4
Fig. 7 Magnetization vs. magnetic field relation at 2 K. The solid line is the Bril-
louin function curve for non-coupled ions, the g-factor is taken as 2.00, S = 1/2.
−
1
photometer and peak positions are reported in cm . All
spin state to the FeN6 centre, the strong absorption band electronic absorption spectra were recorded with a Shimadzu
giving rise to the violet colour of the complex also being UVPC 2001 spectrophotometer, between 220 and 800 nm, in
moiety. Given the 10 × 10 mm quartz cells using solutions 1 × 10− M with
5
indicative of the presence of a low-spin FeN
6
long Fe⋯Fe separation of 6.50 Å in the binuclear unit, it is respect to the metal ions. Excitation and emission spectra were
unsurprising that negligible interactions are apparent between measured at room temperature on a Hitachi 7000 spectro-
the two centres.
The presence of essentially only one unpaired electron at quantum yield for the Zn(II) complex 2 was measured using a
fluorimeter with excitation and emission slits of 2.5 nm. The
6
0
the lowest temperatures was confirmed by magnetization vs. relative method with anthracene as the standard.
As
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6
1
calculated from the equation:
SYNTHESIS OF 6-(TRIMETHYLSTANNYL)-2,2′:6′,2′′-TERPYRIDINE D. The
compound C (1.433 g, 4.5 mmol), hexamethylditin (1.504 g,
4.6 mmol) and 5% of Pd(PPh ) (0.260 g, 0.2 mmol) were
3 4
Ix As n2x
Is Ax n2s
ϕ ¼ ϕs
degassed for 10 minutes, then dry DME (60 mL) was added
and the reaction mixture was heated at 80 °C for 15 h. The
solvent was evaporated and the residue was pumped under
vacuum overnight. The crude product D was then used
immediately for the subsequent synthesis of ligand L.
where ϕ is the quantum yield, subscript s stands for the refer-
ence and x for the sample. A is the absorbance at the excitation
wavelength, n is the refractive index and I is the integrated
emission intensity. This was 0.27 in ethanol solution.
SYNTHESIS OF 2,3-BIS(6-(6-(PYRIDIN-2-YL)PYRIDIN-2-YL)PYRIDIN-2-YL)PYR-
AZINE L. To a mixture of 6-(trimethylstannyl)-2,2′:6′,2′′-terpyri-
dine D (1.818 g, 4.5 mmol), 2,3-dichloropyrazine (0.274 g,
Susceptibility measurements in the temperature range of
.8–300 K were carried out on powdered samples of complexes,
1
at a magnetic field of 0.5 T, using a Quantum Design SQUID
magnetometer (type MPMS-XL5). Corrections for diamagnet-
ism of the constituting atoms were calculated using Pascal’s
1.8 mmol), LiCl (0.402 g, 9.4 mmol) and Pd(PPh
3 4
) (0.266 g,
0.2 mmol), under an argon atmosphere, degassed toluene
5
9,62
−6
3
−1
(100 mL) was added gradually by a syringe. The reaction
mixture was stirred and heated at reflux for 24 h, then the
toluene was evaporated off under reduced pressure. The
residue was purified by column chromatography on alumina
using dichloromethane–ethyl acetate (3 : 2) as an eluent to
constants,
the value of 60 × 10 cm mol being used as
the temperature-independent paramagnetism of copper(II) ion.
The effective magnetic moments were calculated from the
expression
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
corr
1
μeff ¼ 2:83
χ
ꢀ T
m
afford 0.730 g (73%) of ligand L. H NMR (300 MHz, CDCl ):
3
δ = 8.74 (s, 2H), 8.62 (d, J = 4.8 Hz, 2H), 8.52 (d, J = 7.6 Hz, 2H),
8
7
.51 (d, J = 8.0 Hz, 2H), 8.28 (d, J = 7.7 Hz, 2H), 8.08 (d, J =
.7 Hz, 2H), 8.02 (t, J = 7.6 Hz, 2H), 7.80 (t, J = 7.6 Hz, 2H), 7.64
The best exchange parameters were obtained by fitting with
an agreement factor R defined as:
(
t, J = 7.8 Hz, 2H), 7.52 (d, J = 7.9 Hz, 2H), 7.26 (t, J = 7.5 Hz,
n
exp
calc
2
13
X
3
2H) ppm. C NMR (300 MHz, CDCl ): δ = 156.52, 156.00,
1
1
ðχi T ꢁ χi TÞ
R ¼
exp
2
55.08, 155.01, 154.78, 152.61, 148.97, 142.56, 137.47, 136.70,
ðχi TÞ
i¼0
23.61, 121.00, 120.84, 120.72, 120.31 ppm. ESI-MS: m/z = 565
+
(
100) [Na(C34
H
22
N
8
)] . Anal. calc. for C34
H
22
N
8
(542.59): C,
6
3
Standard deviations were added to calculated exchange 75.26; H, 4.09; N, 20.65; found C, 75.22; H, 4.11; N, 20.63%.
parameters. [Mn L ](ClO ) (1): A mixture of Mn(ClO ) ·6H O (23.8 mg,
2
2
4 4
4 2
2
SYNTHESIS OF 6-(TRIMETHYLSTANNYL)-2,2′-BIPYRIDINE B. 6-Bromo- 66 μmol) and ligand L (17.8 mg, 33 μmol) in CH
,2′-bipyridine A
3
CN–CH
(1.433 g, 4.5 mmol), hexamethylditin 1 : 1 (20 mL) was stirred at room temperature for 48 h under
1.504 g, 4.6 mmol) and 5% of Pd(PPh ) (0.260 g, 0.2 mmol) the normal atmosphere. The complex 1 was isolated as a
2 2
Cl
2
(
3
4
were degassed for 10 minutes, dry DME (60 mL) was then yellow solid by evaporation of the solvent and recrystallisation
added and the reaction mixture was heated at 80 °C for 15 h. of the residue from the minimum volume of CH CN by the
3
The solvent was evaporated and the residue was pumped gradual addition of ether. LC-MS m/z (%) = 952 (15) [Mn L-
2
+
+
under vacuum overnight. The crude product B was then used (ClO
immediately for the subsequent synthesis of compound C. [Mn
4
)
L
3
] , 738 (30) [MnL(ClO
4
)(CH
3
CN)] , 706 (30)
O)] , 633 (45) [MnLCl] , 565 (80) [NaL] , 299
SYNTHESIS OF 6-BROMO-2,2′:6′,2′′-TERPYRIDINE C. To a mixture of (10) [MnL] . Anal. calc. for [Mn (C H N ) ](ClO ) (1592.86):
2
+
2+
+
2
2
(ClO
4
+
)
2
(H
2
2
2
34 22 8 2
4 4
6
-(trimethylstannyl)-2,2′-bipyridine B (3.339 g, 10.0 mmol), 2,6- C, 51.27; H, 2.78; N, 14.07; found: C, 51.25; H, 2.74; N, 14.08%.
dibromopyridine (2.475 g, 10.0 mmol), LiCl (1.002 g, [Zn ](CF SO (2): A mixture of Zn(CF SO (21.0 mg,
0.0 mmol) and Pd(PPh3)4 (0.600 g, 0.5 mmol), under an 58 μmol) and ligand L (15.7 mg, 29 μmol) in CH CN–CH Cl
2
L
2
3
3
)
4
3
3 2
)
2
3
2
2
argon atmosphere, degassed toluene (100 mL) was added 1 : 1 (20 mL) was stirred at room temperature for 48 h under
gradually by a syringe. The reaction mixture was stirred and the normal atmosphere. The complex 2 was isolated as a white
heated at reflux for 24 h, then the toluene was evaporated off solid by evaporation of the solvent and recrystallisation of the
under reduced pressure. The residue was purified by column residue from the minimum volume of CH
3
CN by the gradual
2
+
chromatography on alumina using dichloromethane–hexane addition of ether. LC-MS m/z (%) = 755 (30) [Zn
2 2 3 3 2
L (CF SO ) ] ,
1
2+
+
4+
(
1 : 1) as an eluent to afford 2.103 g (64%) of compound C. H 575 (20) [ZnL ] , 565 (50) [NaL] , 311 (30) [Zn L (CH OH)] ,
2
2
2
3
2
+
NMR (300 MHz, CDCl
3
): δ = 8.71 (d, J = 4.8 Hz, 1H), 8.60 (m, 303 (10) [ZnL] . Anal. calc. for [Zn
2
(C34
H
22
N
8
)
2
3 3 4
](CF SO )
J = 6.8 Hz, 2H), 8.48 (d, J = 4.3 Hz, 1H), 8.46 (d, J = 4.3 Hz, 1H), (1812.24): C, 47.72; H, 2.45; N, 12.37; S, 7.08; found: C, 47.70;
7
7
.96 (t, J = 7.9 Hz, 1H), 7.86 (t, J = 7.9 Hz, 1H), 7.71 (t, J = H, 2.44; N, 12.35; S, 7.06%.
.8 Hz, 1H), 7.53 (d, J = 7.8 Hz, 1H), 7.36 (t, J = 4.8 Hz, 1H) [Fe (H O)](BF ·1.5H
): δ = 153.7, 149.2, 139.1, (20.4 mg, 60 μmol) and ligand L (16.4 mg, 30 μmol) in
38.0, 136.9, 128.0, 123.9, 121.6, 121.4, 121.1, 119.8 ppm. CH CN–CH Cl 1 : 1 (20 mL) was stirred at room temperature
2
L
2
F
2
2
4
)
2
2 4 2 2
O (3): A mixture of Fe(BF ) ·6H O
1
3
ppm. C NMR (300 MHz, CDCl
1
ESI-MS: m/z = 334 (100) [Na(C15H N Br)] . Anal. calc. for for 48 h under the normal atmosphere. The complex 3 was iso-
10 3
3
3
2
2
+
15 10 3
C H N Br (312.16): C, 57.71; H, 3.23; N, 13.46; found: C, lated as a violet solid by evaporation of the solvent and recrys-
5
7.73; H, 3.26; N, 13.43%.
tallisation of the residue from the minimum volume of CH CN
3
1
748 | Dalton Trans., 2013, 42, 1743–1751
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Paper
by the gradual addition of ether. LC-MS m/z (%) = 685 (10) Table 2 Crystal data and structural refinement parameters for ligand L and Fe
+
2+
2+
(
II) complex 3
[
6
2
FeL(BF )] , 660 (10) [Fe L (BF )F(H O)] , 652 (20) [Fe L (BF )F] ,
4
2
2
4
2
2
2
4
2
+
+
2+
17 (15) [Fe
99 (10) [FeL] . Anal. calc. for [Fe
2
L
2
F
2
] , 565 (25) [NaL] , 380 (15) [Fe
2
L(BF
4
)F] ,
O)]-
Compound
L
3
+
2
(C34
H
22
N
8
)
2
F
2
(H
2
(
BF ) ·1.5H O (1453.52): C, 56.19; H, 3.40; N, 15.42; found: C, Formula
C
34
H
22
N
8
C
68 49 2 2 16 2,5
H B F10Fe N O
4
2
2
Formula weight
Crystal system
Space group
542.60
Monoclinic
P2 /c
1453.52
Monoclinic
C2/c
5
6.22; H, 3.42; N, 15.44%.
4 2 4 2 2
Co (L) (H O)](BF ) (4): A mixture of Co(BF ) ·6H O
[
2
2
F
2
2
1
(
20.1 mg, 59 μmol) and ligand L (16.0 mg, 30 μmol) in a (Å)
9.4930(2)
20.5040(5)
14.2010(4)
104.980(1)
2670.21(11)
4
1.35
1128
0.084
27.264(4)
24.8297(19)
21.8128(16)
112.636(12)
13 629 (2)
8
1.41
5872
4.15
2.85–63.15
−31 ≤ h ≤31
−27 ≤ k ≤28
−24 ≤ l ≤24
b (Å)
3 2 2
CH CN–CH Cl 1 : 1 (20 mL) was stirred at room temperature
for 48 h under the normal atmosphere. The complex 4 was iso-
lated as a yellow solid by evaporation of the solvent and recrys- V (Å )
c (Å)
β (°)
3
Z
tallisation of the residue from the minimum volume of CH
3
CN
(g cm−
3
)
d
x
by the gradual addition of ether. LC-MS m/z (%) = 672 (10)
F(000)
Co L (BF )F(H O) ] , 663 (15) [Co L (BF )F(H O)] , 687 (20) μ (mm−1)
2
+
2+
[
[
2
2
2
2
4
4
2
2
2
2
4
2
2
+
2+
2+
Θ range (°)
hkl range
1.79–29.10
−12 ≤ h ≤12
Co
4
L
(BF
)
2
] , 654 (10) [Co
2
L
2
(BF
4
)F] , 620 (15) [Co
2
L
2
F
2
] ,
] . Anal.
calc. for [Co (C H N ) F (H O)](BF ) (1414.66): C, 57.01; H,
2
+
2+
2 4 2 2 3 2 4 2
45 (30) [Co L(BF ) (H O) ] , 415 (10) [Co L(BF )
−
21 ≤ k ≤28
−19 ≤ l ≤19
2
34 22 8 2
2
2
4 2
Reflections
Collected
Unique (Rint
3
.24; N, 15.64; found: C, 57.06; H, 3.20; N, 15.62%.
3 2 2
Cu L(NO (H O) ](NO (5): A mixture of Cu(NO ) ·5H O
12 365
7126 (0.047)
4097
68 141
10 678 (0.143)
3811
[
2
3
)
2
2
2
3
)
2
)
(
16.4 mg, 59 μmol) and ligand L (16.0 mg, 30 μmol) in With I > 2σ(I)
No. of parameters
379
906
3 2 2
CH CN–CH Cl 1 : 1 (20 mL) was stirred at room temperature
for 48 h under the normal atmosphere. The complex 5 was iso-
Weighting scheme
A
0.0592
0
0.12
0
0.094
0.222
0.219
0.265
1.00
lated as a blue solid by evaporation of the solvent and recrystal-
B
R(F) [I > 2σ(I)]
wR(F ) [I > 2σ(I)]
R(F) [all data]
0.063
0.120
0.131
0.141
1.011
0.19/−0.22
lisation of the residue from the minimum volume of CH
3
CN
2
by the gradual addition of ether. ESI-MS m/z (%) = 565 (70)
+
2+
2
[
(
(
Na(L)] , 405 (15) [Cu (L)(NO ) (H O)] , 396 (10) [Cu (L)- wR(F ) [all data]
2
3 2
2
2
2
+
2+
Goodness of fit −
NO
3
)
2
] , 303 (20) [Cu(L)] . Anal. calc. for [Cu
2 22 8
(C34H N )-
3
max/min Δρ (e Å
)
0.56/−0.30
H
2
O)
2
(NO ](NO (953.73): C, 42.82; H, 2.75; N, 17.62;
3
)
2
3 2
)
found: C, 42.80; H, 2.71; N, 17.64%.
Cd L(NO ] (6): A mixture of Cd(NO
1 μmol) and ligand L (16.6 mg, 31 μmol) in CH
: 1 (20 mL) was stirred at room temperature for 48 h under
the normal atmosphere. The complex 6 was isolated as a
yellow solid by evaporation of the solvent and recrystallisation
[
2
3
)
4
3
)
2
·4H
2
O (18.8 mg,
CN–CH Cl
2
atoms were placed in the calculated positions, and refined by a
riding model’ with the isotropic displacement parameters set
at 1.2 (1.5 for methyl groups) times the Ueq value for an appro-
priate non-hydrogen atom. Relevant crystal data are listed in
Table 2, together with refinement details.
6
1
3
2
‘
of the residue from the minimum volume of CH CN by the
3
In 3 the residual electron density was interpreted as the par-
tially disordered water molecules, weakly bound to the rest of
the structure. The positions of hydrogen atoms for these water
molecules have not been found. In this structure there still are
gradual addition of ether. ESI-MS m/z (%) = 955 (10) [CdL-
+
+
2+
(
[
NO
3 3 2 3 2
) ] , 565 (100) [NaL] , 445 (20) [Cd L(NO ) ] , 276 (70)
3
+
Cd L(NO )] . Anal. calc. for [Cd (C H N )(NO ) ] (1015.43):
2
3
2
34 22
8
3 4
3
C, 40.22; H, 2.18; N, 16.55.; found: C, 40.25; H, 2.16; N,
6.53%.
large areas devoid of any electron density (of ca. 987 Å ,
1
located around 0,1/2,0) and this can be related to the relatively
poor quality of the crystals of 3. Throughout the refinement
process, weak constraints were applied to the geometry of one
of the anions as well as to the anisotropic thermal parameters
of certain atoms (ISOR).
Single crystal structure determination
For the ligand, diffraction data were collected at −100 °C by
the φ-scan method on an Enraf-Nonius KappaCCD diffract-
ometer with graphite-monochromated MoK
.7107 Å) and for the complex 3 at room temperature, by the
same method on a SuperNova diffractometer with mirror-
monochromated CuK radiation (λ = 1.54178 Å). The data were
corrected for Lorentz-polarization and absorption effects.
α
radiation (λ =
0
Conclusions
α
6
4
The linking of two terpyridine units through a pyrazine bridge
Accurate unit-cell parameters were determined by a least- produces an octa-aza donor ligand capable of acting as either
squares fit of 5974 (L) and 2700 (3) reflections of highest inten- a quaterpyridine–terpyridine or a simple terpyridine binding
sity, chosen from the whole experiment. The structures were unit towards transition metal ions. Thus, despite its relatively
6
5
66
solved with SHELXS-97 and SIR92 and refined with the limited rotational flexibility, it is a versatile species exhibiting
2
65
full-matrix least-squares procedure on F by SHELXL97. Scat- variable denticity, this possibly being dependent on the coun-
tering factors incorporated into SHELXL97 were used. All non- teranions associated with the transition metal cation. There is
hydrogen atoms were refined anisotropically, while hydrogen some parallel in its behaviour with that of a simple
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Dalton Trans., 2013, 42, 1743–1751 | 1749

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Paper
Dalton Transactions
quaterpyridine which displays tridentate coordination in 12 W. Meng, B. Breiner, K. Rissanen, J. D. Thoburn,
forming a mixed-ligand species in the presence of 2,2′-bipyri-
dine. It would appear that repulsive interactions which would
J. K. Clegg and J. R. Nitschke, Angew. Chem., Int. Ed., 2011,
50, 3479.
9
arise in a planar conformation of L are important in limiting 13 R. Custelcean, J. Bosano, P. V. Bonnesen, V. Kertesz and
its coordinative behaviour to that of a nonplanar chelate. That B. P. Hay, Angew. Chem., Int. Ed., 2009, 48, 4025.
binuclear species, of either M L or M stoichiometry, are 14 A. Stephenson, S. P. Argent, T. Riis-Johannessen,
2
2 2
L
obtained with quite different juxtapositioning of the two metal
ion centres indicates that the complexes may be useful for
I. S. Tidmarsh and M. D. Ward, J. Am. Chem. Soc., 2011,
133, 858.
examination of bimetallic catalysis, especially where different 15 W. Lin, W. J. Rieter and K. M. L. Taylor, Angew. Chem., Int.
roles may be assigned to the two centres.
Ed., 2009, 48, 650.
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6 Yu. E. Alexseev, B. I. Kharisov, T. C. Hernandez-Garcia and
A. D. Garnovskii, Coord. Chem. Rev., 2010, 254, 794.
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Acknowledgements
We would like to express our deep gratitude to Prof Jean-Marie
Lehn for the opportunity to carry out a part of these studies in 18 S. Roy, S. Roy, S. Saha, R. Majumdar, R. R. Dighe,
his laboratory and for his precious advice. Financial support
received from the Polish National Science Centre (grant no.
E. D. Jemmis and A. R. Chakravarty, Dalton Trans., 2011,
40, 1233.
2
011/03/B/ST5/01036) is gratefully acknowledged.
19 S. L. H. Higgins, T. A. White, B. S. J. Winkel and K.
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