Substituted pyridazines as ligands in homoleptic (fac and mer) and heteroleptic Ru(ii) complexes

Article abstract of DOI:10.1039/c1dt10340g

This article reports the preparation of a range of phenyl, pyridyl and pyrazinyl substituted pyridazines via the inverse electron demand [2 + 4] Diels-Alder reaction between 3,6-di(2-pyridyl)-1,2,4,5-tetrazines (bptz) and 3,6-di(2-pyrazinyl)-1,2,4,5-tetra

Full text of DOI:10.1039/c1dt10340g

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PAPER
Substituted pyridazines as ligands in homoleptic (fac and mer) and
heteroleptic Ru(II) complexes†
´
Gareth Cooke, Gearo´id M. O Ma´ille, Roberto Quesada, Longsheng Wang, Sunil Varughese and
Sylvia M. Draper*
Received 28th February 2011, Accepted 25th May 2011
DOI: 10.1039/c1dt10340g
This article reports the preparation of a range of phenyl, pyridyl and pyrazinyl substituted pyridazines
via the inverse electron demand [2 + 4] Diels–Alder reaction between 3,6-di(2-pyridyl)-1,2,4,5-tetrazines
(bptz) and 3,6-di(2-pyrazinyl)-1,2,4,5-tetrazines (bpztz) and suitable dienophiles including
acenaphthalene. The resulting polyaromatic compounds vary systematically in the number of aromatic
substituents and the number and position of N-heteroatoms. For four of these compounds, the effect of
the molecular changes on the solid-state structures were investigated using single crystal X-ray
crystallography. The pyridazines were used as bidentate ligands in {M(II)(bipy)2} and tris(homoleptic)
complexes (M = Fe, Ru). The optical and electrochemical properties of these complexes reflect the
electron accepting character of the new ligands. The facial and meridional isomers of the tris complexes
could be separated by column chromatography (on silica), thus allowing a spectral comparison of their
absorption and emission properties. The solid-state structures of several of the metal complexes are
discussed, including that of the facial isomer of the tris Ru(II) complex of 3,6-bis(2-pyridyl)-4,5-bis(4-
pyridyl)pyridazine—a potential preformed geometric motif for the predirected construction of
supramolecular assemblies.
resulting pyridazine-derivatives, thus providing access to a variety
Introduction
of transition metal complexes6 and a useful set of systematic
building blocks for the construction of supramolecular systems.7–9
3,6-Di(2-pyridyl)pyridazine, for example, is a popular bidentate
chelating ligand in coordination chemistry, and complexes a
wide range of metals including iridium and palladium.10,11 As
highly adaptable ligands, pyridazines have also been used for
the construction of supramolecular frameworks containing sil-
ver and copper.12,13 Constable and co-workers have prepared
pyridazyl-centred ligands by reacting bptz with a variety of
substituted alkynyl dienophiles; reporting the supramolecular
self-assembly of some of these pyridazyl ligands with silver
salts.14
The work herein describes the synthesis of both homoleptic
ruthenium and iron and heteroleptic ruthenium bis(bipyridine)
complexes of the form [Ru(L)(bpy)2]2+. Such systems possess
valuable chemical and spectral properties, e.g. metal to ligand
charge transfer (MLCT) in the visible region, and long lived excited
states. The inertness of the Ru(II) centre also allows the study
of redox behaviour without complications arising from ligand
substitution or exchange.15 The tris(homoleptic) complexes of the
systems presented offer considerable possibilities as supramolecu-
lar building blocks due to the ease with which multiple secondary
coordination sites can be built into the ligand. However, ideally
in order to exploit this potential, the separation of the meridional
and facial isomers should be demonstrated.
This work forms part of a long-standing interest in N-containing
polyphenylenes both as synthons in the formation of heteroatomic
graphenes and as ligands in co-ordination chemistry.1–3 In our
synthetic development of N-containing polyaromatic compounds
we have established protocols for the [2 + 4] cycloaddition of ap-
propriately substituted alkynes and nitriles to cyclopentadienone
derivatives. In order to introduce higher levels of N-doping in the
resulting systems our attention has turned to substituted tetrazines
as precursors to pyridazines. Such compounds were attractive
because tetrazines, particularly 3,6-di(2-pyridyl)-1,2,4,5-tetrazine
(bptz) have been used as dienes in inverse electron demand
Diels–Alder reactions for many years. Indeed pyridazine-derived
assemblies generated synthetically in this manner have been used
previously by the authors to demonstrate the importance of C–
H ◊ ◊ ◊ p, p ◊ ◊ ◊ p and numerous weak C–H ◊ ◊ ◊ N interactions in the
stabilisation of supramolecular architectures.4
Another favourable aspect of this chemistry is the retention
of the rich co-ordination chemistry of the tetrazine5 in the
School of Chemistry, University of Dublin, Trinity College, Dublin 2, Ireland.
E-mail: smdraper@tcd.ie; Fax: (+353)-1-671-2826
† Electronic supplementary information (ESI) available: full experimen-
tal details. CCDC reference numbers 812335–812342. For ESI and
crystallographic data in CIF or other electronic format see DOI:
10.1039/c1dt10340g
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The presence of only one isomer leads to a more predictable
geometric platform on which to build supramolecular structures.
In general, fac isomers of coordination complexes have proven
most valuable in this regard due to their inherent C3 symmetry,
which gives greater directional control in the formation of assem-
blies such as helicates. Unfortunately the formation of fac isomers
in tris complexes is statistically disfavoured, and fac selective
syntheses have only been achieved via tethering the chelating
groups, as demonstrated by Weizman et al.16 and further developed
by Fletcher.17
In the case of alkene-derived dienophiles, the resulting dihy-
dropyridazinyl Diels–Alder adduct must be oxidized to furnish
the aromatic pyridazine product. The reactions of bptz with
trans-stilbene, 1,2-di(4¢-pyridyl)ethene and 3,3¢,5,5¢-tetramethoxy
stilbene led to yields of 92%, 75% and 61%, respectively. After
purification by column chromatography these were oxidised using
nitrous oxide gas to yield the pyridazine-derived ligands 3a, 4a
and 5a in yields of 74%, 50%, and 75%, respectively. 6 has been
reported elsewhere6 and 3a has been synthesised previously from
diphenylacetylene by Constable and co-workers.14
The separation of facial and meridional isomers is a challenging
undertaking, and consequently few examples are found in the
literature. Of the reported methods for such isomeric separa-
tion (including sublimation,18 HPLC19 and ion-exchange assisted
Sephadex chromatography20,21), it was found that conventional
preparative chromatography18,22,23 was sufficient to effect the
separation of the mer and fac isomers reported herein.
The same procedures were used to generate compounds 1b–4b.
The yields of these reactions were generally lower and their metal
complexes deviate little from their bptz-derived counterparts.
Heteroleptic complexes [Ru(1a–6)(bpy)2][PF6]2
The ruthenium(II) bis(bpy) complexes of 1a–6 were prepared by
refluxing the ligands with [Ru(bpy)2Cl2] in ethylene glycol and
water for 4–6 h (Scheme 2). The complexes were isolated in good
yield as their PF6 salts and purified by column chromatography.
Results and discussion
The Diels–Alder reaction of bptz and bpztz (synthesised according
to literature procedures24,25) and suitable dienophiles gave rise
to novel pyridazine centred ligands (Scheme 1). In the reaction
of 2-acetyl pyridine with bptz, the ketone functional group was
activated using a 10% methanolic solution of KOH, allowing for
an 80% yield of 1a. Diels–Alder reactions involving acetylene-
derived dienophiles required more extreme reaction conditions:
5-ethynylpyrimidine and bptz were refluxed for 24 h in toluene to
give a 65% yield of 2a. In both reactions the products were purified
by column chromatography.
Ruthenium(II) and iron(II) tris(homoleptic) complexes
The ruthenium(II) tris(homoleptic) complexes of 3a, 4a and 5a
were prepared on reflux with RuCl3 and N-ethyl morpholine
in ethylene glycol for 72 h. Following precipitation with sat-
urated KPF6, the two geometric isomers were separated using
preparative TLC plates. The particularly low yield of the complex
[Ru(5a)3][PF6]2 (mer 5%, fac 3%) arose as a result of the need to
further purify the mer isomer via preparative TLC.
Scheme 1 Synthesis of pyridazine derivatives. (i) Stirred at R.T. in THF with KOH; (ii) toluene, reflux overnight; (iii) nitrous oxide gases, CH2Cl2.
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Scheme 2 The synthesis of [Ru(1a–6)(bpy)2][PF6]2.
The iron(II) tris complexes were prepared from 3a, 4a and 5a
with an excess of Fe(BF4)2 in acetonitrile. The mer and fac isomers
were again separated by column chromatography and/or TLC
plates. (Reaction conditions summarised in Scheme 3.)
Fig. 1 The 1H-NMR spectra of the non-deuterated bpy complex
[Ru(2a)(bpy)2][PF6]2 (bottom), compared with the deuterated bpy Ru(II)
complex [Ru(2a)(bpyd8)2][PF6]2 (top) (CD3CN, RT, 600 MHz). Full
assignment can be found in the ESI.†
through the relaxation pathways provided to the other rings by
the co-ordinated metal centre. Signals within each spin system
were again assigned using selective TOCSY experiments. The 1H
NMR spectrum of [Ru(5a)(bpy)2]2+ was assigned similarly.
UV-Vis spectroscopy
Scheme 3 Synthesis of [M(3a–5a)3]2+ (M = Fe, Ru). 1: RuCl3, N-ethyl
The UV-Vis absorption spectra of the [Ru(1a–5a)(bpy)2][PF6]2
complexes are shown in Fig. 2. They are quite similar to that
of [Ru(bpy)3]2+, though the absorbance in the visible region is
broader, consisting of two MLCT bands. The most red-shifted
morpholine, ethylene glycol, 170 ◦C, 72 h; 2: Fe(BF4)2, acetonitrile, 60 ◦C,
2 h.
~
of these ( l 485 nm) arises from the pyridazine derived ligand
=
~
Characterization of [Ru(1a–6)(bpy)2][PF6]2
(dp → p *). The other band ( l 428 nm) is ascribed to an
=
1
MLCT of the bpy ligands, or to a HOMO–SLUMO (second
In order to facilitate NMR characterisation of the ruthenium(II)
bis(bipyridyl) complexes of 1a, 2a, 3a, and 4a, [Ru(1a–4a)(bpy)2]2+,
the analogous deuterated bipyridine (bpyd8) complexes were syn-
thesised. As illustrated in Fig. 1 for [Ru(2a)(bpy)2]2+, the removal
lowest unoccupied molecular orbital) transition (dp→p2*).26 The
~
absorption maximum at l 280 nm is a ligand-centred transition
=
and varies only slightly from one complex to another. The
spectrum of [Ru(6)(bpy)2][PF6]2 is different from the others:
the introduction of the aromatic fluoranthene to the molecule
causes some structured ligand-centred absorptions to be observed
1
of the bipyridine signals vastly simplifies the H NMR spectra.
The two 2-pyridyl ring systems were differentiated on the basis
of nOe experiments. These indicated a through-space interaction
between the proton at C13 on the pyridazine ring in 1a and 2a,
(see Scheme 1 for atom labelling) and the adjacent proton on the
co-ordinated pyridyl ring. Full assignment was achieved using a
range of 2-D TOCSY NMR experiments.
1
The H NMR spectrum of [Ru(6)(bpy)2]2+ was well resolved,
allowing satisfactory characterisation without resorting to the use
1
of bpyd8. Analysis of the H–1H COSY showed two three-spin
systems (arising from the fluoranthene portion) and six four-spin
systems (the pyridyl portions of bpys and 6). ROESY experiments
revealed a single interaction between a fluoranthene and a pyridyl
spin system—indicating H3¢ of the co-ordinated (fixed) pyridine
of 6 (ring A) and its closest neighbouring proton, a¢ on ring
E (see Scheme 1). Further evidence supporting this assignment
was obtained by inversion recovery experiments—five of the four-
spin systems (excluding ring C) recovered from inversion within
0.8 s. The remainder took over 2.5 s and was assigned as the
non-coordinated ring C, the only ring which could not recover
Fig. 2 The UV-Vis absorption spectra of the complexes [Ru(1a–
6)(bpy)2][PF6]2 (2 ¥ 10-5 M in CH3CN).
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at ~ l 350 nm and the highest energy 1MLCT band is less intense.
Enhanced aromaticity also lowers the energy of the ligand’s p*
orbitals, resulting in the most red shifted 1MLCT-pyridazine
absorption in the series.
its oxidation potential. These values give an indication of the
HOMO–LUMO energy gap and correlate with the position of
the MLCT emissions. [Ru(5a)(bpy)2][PF6]2 has the highest energy
emission (lmax 670 nm) and the largest DE 1 (2.51 V). Simi-
2
The emission spectra of complexes [Ru(1a–6)(bpy)2][PF6]2 are
shown in Fig. 3, excitation into either of the two MLCT absorption
maxima exhibited by these compounds, results in the same red
emission. Complexes of ligands 3a and 5a emit at a slightly shorter
wavelength (lem = 670 nm for both) than those of 1a, 4a and 6
(lem = 700 nm, 710 nm and 710 nm, respectively). The lower energy
emission of the latter may be due to the electron-withdrawing
character of nitrogen-rich substituents on 1a and 4a, and is a
consequence of the aromaticity of the fluoranthene portion of 6.
larly [Ru(6)(bpy)2][PF6]2, having the lowest energy emission, lmax
710 nm, has the smallest DE 1 value (2.22 V). Although DE 1 values
2
can have error values of up 2to 20 mV, the results were found to
be highly reproducible.
Ruthenium(II) tris(homoleptic) complexes of ligands 3a–5a
The 1H NMR spectra of the mer isomers of the ruthenium(II) tris
complexes of ligands 3a, 4a and 5a consisted of many overlapping
signals and only the most highly shielded and deshielded signals
could be assigned (see ESI†). 2D-NMR experiments and the
C3 symmetry of the fac isomers aided spectral assignments. The
1H-NMR spectrum of fac-[Ru(3a)3][PF6]2 is shown colour coded
in Fig. 4. The H3 protons are expected to be shielded by the
neighbouring ring current; H3¢ of ring A experiences this effect
to the greatest extent as it has no freedom to rotate. On this basis
the two 2-pyridyl rings were differentiated using 2-dimensional
NMR TOCSY experiments. The two phenyl rings give rise to two
separate sets of signals, TOCSY NMR experiments show that
one ring exhibits three sharp peaks whereas the other displays two
broad signals which sharpen and shift at higher temperatures. Full
details of the NMR assignments of these complexes are given in
the ESI.†
Fig.
3
Normalised emission spectra of the complexes [Ru(1a–
6)(bpy)2][PF6]2 (2 ¥ 10-5 M in CH3CN).
Electrochemistry
The redox potentials associated with [Ru(1a–6)(bpy)2][PF6]2 are
shown in Table 1. The first ligand reduction in each case was
assigned to the pyridazine-centred ligand, as their lower energy
p* orbitals (compared to bpy) ensure that these are more easily
reduced.27 The second and third highly consistent reductions
are assigned to the bpy ligands, though there are pyrimidyl
complexes known where the second reduction is thought to occur
preferentially on the non-bpy ligand.28 The values of the oxidation
potentials indicate the effect of the pyridazine-centre on the
electrochemistry of the complexes; its lower pKa (an indicator of
lower s-bonding strength) relative to pyridine results in higher
Ru(II/III) oxidation potentials. The final column in Table 1
Fig. 4 The 1H-NMR spectrum of fac-[Ru(3a)3][PF6]2 (CD3CN, RT,
600 MHz).
UV-Vis spectroscopy
The UV-Vis absorption spectra of both isomers of [Ru(3a–5a)3]-
[PF6]2 are shown in Fig. 5. The absorption maxima are
bathochromically shifted (~l 20 nm) relative to [Ru(bpy)3]2+ 29 due
provides the DE 1 values of each of the six complexes, calculated
2
by subtracting the first reduction potential of each complex from
Table 1 The oxidation and reduction potentials obtained for [Ru(1a–6)(bpy)2][PF6]2 in CH3CN (1 mM) with 0.1 M TBAPF6 as supporting electrolyte.
Recorded at RT using a glassy carbon working electrode, a Pt wire counter electrode and a SCE reference electrode
1
[Ru(L)(bpy)2][PF6]2 L =
Oxidation vs. SCE/V
Reduction vs. SCE/V
DE /V
2
1a
2a
3a
4a
5a
6
+1.48
+1.34
+1.39
+1.36
+1.43
+1.28
-0.92
-0.98
-1.04
-0.91
-1.07
-0.94
-1.39
-1.43
-1.42
-1.46
-1.42
-1.46
-1.64
-1.64
-1.66
-1.67
-1.67
-1.67
2.40
2.32
2.43
2.27
2.51
2.22
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protons appear at d 6.4 ppm, and the 2-pyridyl signals appear
between d 7.0 and 8.5 ppm. The fac isomer of [Fe(5a)3][PF6]2 is
1
drawn colour-coded along with its assigned H NMR spectrum
in Fig. 7. The proton signals were identified using a range of
2D-NMR techniques. Of particular note are the phenyl signals
appearing between d 6.0–7.0 ppm. The spectrum is similar to
that of its ruthenium counterpart, with broadening observed in
the signals associated with the aryl substituents. Assignments of
the 1H-NMR spectra of the mer and fac isomer of [Fe(3a)3][PF6]2
and [Fe(4a)3][PF6]2 were also obtained and are provided in the
ESI.†
Fig. 5 The UV-Vis absorption spectra of the fac and mer isomers of
[Ru(3a–5a)3][PF6]2 (2 ¥ 10-5 in CH3CN).
to the increased electron withdrawing character of the pyridazine
ring. The most interesting feature of the spectra is the small but
consistent bathochromic shift of the mer isomers relative to their
fac counterparts. This is as a result of the reduced symmetry
associated with the mer isomer, and the loss of degeneracy in
the ligands’ p* orbitals.
The emission spectra of [Ru(3a–5a)3][PF6]2 are shown in Fig. 6.
These spectra reflect a pattern similar to that of the absorption
spectra: there is a small consistent bathochromic shift in the
emission maxima wavelength for each mer isomer relative to its
fac counterpart. This difference is most apparent for the complex
of ligand 3a, where the maximum of the mer isomer is red-shifted
27 nm compared to the fac isomer. The fac isomer of the complex
[Ru(4a)3][PF6]2 has the lowest energy emission of the three fac
isomers (l 638 nm), probably due to the lower energy of the
p* orbitals arising from the electron-withdrawing effect of the
4-pyridyl substituents.
Fig. 7 The 1H-NMR spectrum of the fac isomer of [Fe(5a)3][PF6]2
(CD3CN, RT, 600 MHz).
The UV-Vis absorption spectra of the fac and mer isomers of
the three iron tris(homoleptic) complexes of ligands 3a, 4a and 5a
are shown in Fig. 8. The high-energy region of the spectra in each
case is dominated by a ligand-centred p→p* transition which,
for comparison appears at l 298 nm in [Fe(bpy)3]2+.30 Again, the
meridional isomer of each complex absorbs at slightly longer
wavelength than its facial analogue. The electron-withdrawing
effects of the 4-pyridyl and the electron-donating effect of the
methoxy substituents is evident in the slight shift in the absorption
Fig. 6 The normalised emission spectra of the fac and mer isomers of
[Ru(L)3][PF6]2, for ligands 3a, 4a and 5a in CH3CN.
Iron(II) tris(homoleptic) complexes [Fe(3a–5a)3][PF6]2
The meridional and facial isomers of the iron(II) complexes
were similarly characterised by 1H-NMR spectroscopy, mass
spectrometry and UV-Vis spectroscopy. The 1H-NMR spectra
of the meridional isomers were complicated due to the lack of
symmetry, however the methoxy protons were easily assigned,
being the most shielded signals (ca. d 3.6 ppm). The phenyl
Fig. 8 The UV-Vis absorption spectra of the fac and mer isomers of
[Fe(3a–4a)3][PF6]2 (2 ¥ 10-5 M in CH3CN).
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Table 2 Crystal data for complexes [Ru(bpy)2(1a)][PF6]2, [Ru(bpy)2(2a)][PF6]2, [Ru(bpy)2(6)][PF6]2 and [Ru(3a)3][PF6]2
[Ru(bpy)2(1a)][PF6]2·
[Ru(bpy)2(2a)][PF6]2·
[Ru(bpy)2(6)][PF6]2·
Compound reference
(CH3)2CO
(CH3)2CO
3CH2Cl2
[Ru(3a)3][PF6]2
Chemical formula
Formula mass
C42H35F12N9OP2Ru
1072.80
Monoclinic
11.546(1)
19.751(2)
19.714(2)
90.00
101.996(2)
90.00
4397.5(7)
173(2)
P21/c
4
35681
8172
0.0812
0.0526
0.1159
0.0877
C41H34F12N10OP2Ru
1073.79
Monoclinic
10.986(1)
20.223(2)
20.344(2)
90.00
104.637(2)
90.00
4372.9(7)
150(2)
P21/c
4
35469
8141
0.0447
0.0390
0.1009
0.0548
C47H36Cl6F12N8P2Ru
1316.55
Monoclinic
9.006(1)
43.982(3)
13.043(1)
90.00
101.319(2)
90.00
5065.9(8)
150(2)
P21/c
4
42101
9427
0.1369
0.0804
0.1461
0.1334
C78H54Cl0F12N12O0P2Ru
1550.34
Trigonal
18.443(4)
18.443(4)
12.820(3)
90.00
90.00
120.00
3776(2)
Crystal system
˚
a/A
˚
b/A
˚
c/A
a/◦
b/◦
g /◦
3
˚
Unit cell volume/A
T/K
Space group
160(2)
¯
P3
No. of formula units per unit cell, Z
No. of reflections measured
No. of independent reflections
Rint
Final R1 values (I > 2s(I))
Final wR(F2) values (I > 2s(I))
Final R1 values (all data)
Final wR(F2) values (all data)
2
9347
3936
0.0561
0.0554
0.1251
0.0942
0.1362
0.1336
0.1119
0.1636
maxima of the MLCT of complexes of 4a and 5a compared with
the complexes of 3a.
guest framework, with two acetone molecules and two PF6- anions
occupying the void space (Fig. 9). The guest species is held to the
˚
host-framework through C–H ◊ ◊ ◊ O (H ◊ ◊ ◊ O, 2.42, 2.45 A) and
˚
C–H ◊ ◊ ◊ F (with H ◊ ◊ ◊ F distance varying from 2.31 to 2.53 A)
Crystallographic studies
interactions. Unlike many such porous assemblies, the void space
The single crystal X-ray structures of the ligands 1a, 2a, 3a and
6 are provided and discussed in the ESI.† The experimental and
crystallographic data for the ruthenium complexes are detailed
here and summarised in Table 2.
is not extended further to form a channel.
Crystals of [Ru(1a)(bpy)2]2+ were grown by slow evaporation of
diethylether into an acetone solution of the complex. This mixture
yielded a complex, isostructural with that of [Ru(2a)(bpy)2]2+
(space group P21/c).
Crystals of [Ru(2a)(bpy)2][PF6]2 were grown by slow evapo-
ration of diethylether into an acetone solution of the complex.
The complex crystallised in a monoclinic space group, P21/c. The
asymmetric unit consists of the octahedral complex species along
Crystals of [Ru(6)(bpy)2]2+ were obtained via recrystallisation
from a dichloromethane/hexane mixture. The asymmetric unit
-
consists of two PF6 counter ions and three dichloromethane
-
with two PF6 counter ions and an acetone molecule. The ligands
molecules, crystallising in the monoclinic P21/c space group.
The Ru–N bond lengths are typical, ranging from 2.030(5) A to
˚
are coordinated in an octahedral arrangement around the central
metal as shown in Fig. 9. Rings C and D are tilted at angles of
33.45◦ and 48.61◦ respectively from the plane of the coordinated
˚
2.074(5) A. The crystal packing is presented in Fig. 10.
˚
rings A and B. The Ru–N bond lengths vary between 2.014(2) A
2+
˚
and 2.068(2) A. Four molecules of [Ru(2a)(bpy)2] undergo self-
assembly through the formation of C–H ◊ ◊ ◊ N hydrogen bonds
˚
involving both the 2-pyridyl (H ◊ ◊ ◊ N, 2.38 A) and the pyrimidyl
˚
units (H ◊ ◊ ◊ N, 2.56 A). This leads to the formation of a host–
Fig. 10 Representations of the lattice structures of [Ru(6)(bpy)2][PF6]2.
A crystal of fac-[Ru(3a)3][PF6]2 suitable for single crystal
X-ray diffraction was grown by slow diffusion of diethylether
into an acetone solution of the complex (see Fig. 11). The
¯
complex crystallised in a P3 trigonal space group and the unit
cell contains two complex molecules and two [PF6]- equivalents.
The ruthenium(II) metal centre occupies a crystallographic special
position at the 3-fold centre of symmetry and the anions are
distributed across three sites (see Figure S3†). The host-framework
comprises a layered arrangement of the metal centres with the
Fig. 9 Ortep representation of the crystal structure of [Ru(2a)(bpy)2]-
[PF6]2, (ellipsoids shown at 50% probability), the host–guest interaction
observed is illustrated on the right.
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thank Dr Chris Fitchett for additional crystallographic support
and Dr Han Vos (Dublin City University) for kindly providing
the deuterated ligands. We acknowledge the funding provided by
SFI- 05PICA819 (GC, SV) and IRCSET-PG07 (GMOM), LW
EU-FP6 MKTD-011472 and SFI-PICA-1819 (RQ).
Notes and references
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Fig. 11 Left: Ortep representation of the crystal structure of
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removed for clarity). Right: A representation of the hexagonal packing
arrangement (Ru(II) centres shown as turquoise octahedra. (Counterions
removed for clarity.)
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´
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anions lying in different planes. The bond lengths between the
ruthenium metal and the pyridine and pyridazine nitrogen donor
˚
˚
atoms are similar; 2.059(4) A and 2.007(4) A respectively. An
ORTEP31 representation of the complex cation is shown in Fig. 11,
alongside a representation of the packing arrangement observed
down the c axis. The complex forms a porous assembly which is
˚
stabilised by weak intermolecular p ◊ ◊ ◊ p (3.481 A) and C–H ◊ ◊ ◊ F
˚
˚
(2.546 A–2.650 A) interactions
Conclusion
A family of substituted pyridazines, arising from inverse electron
demand [2 + 4] Diels–Alder reactions between tetrazines and suit-
able dienophiles including acenaphthalene, have been structurally
characterised and their crystal packing arrangements examined.
The optical and electrochemical properties of these species as
ligands in Fe(II) and Ru(II) bis(byridyl) and tris(homoleptic)
complexes were shown to depend on the electron withdrawing
nature of the pyridazine. The separation of both mer and fac
isomers of the tris(homoleptic) complexes was serendipitously
achieved via simple chromatographic separation on silica, avoiding
the use of more complex separation strategies. This allowed for a
spectral comparison of the properties of the geometric isomers.
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consistent red-shift in the emission and absorption spectra. In
one case the fac-isomer was structurally characterised revealing
the potential of such symmetric complexes with multiple metal
coordination modes in supramolecular frameworks. The work
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Full experimental details are available in the ESI.† CCDC 812335–
812342 contain the supplementary crystallographic data for this
paper.†.
Acknowledgements
We wish to acknowledge the technical contribution of Drs John
O’Brien, Manuel Ruether and Martin Feeney. We also wish to
8212 | Dalton Trans., 2011, 40, 8206–8212
This journal is
The Royal Society of Chemistry 2011
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