Far-red luminescent ruthenium pyridylimine complexes; Building blocks for multinuclear arrays

Article abstract of DOI:10.1039/b518027a

Ruthenium(ii) pyridylimine complexes are explored for their potential as units that might be incorporated into electronic or photonic arrays. The complexes [Ru(bipy)₂(L)][PF₆]₂ (1) and [Ru(tpy)(L)Cl][BF₄] (2) wi

Full text of DOI:10.1039/b518027a

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PAPER
www.rsc.org/dalton | Dalton Transactions
Far-red luminescent ruthenium pyridylimine complexes; building blocks for
multinuclear arrays†
Anna C. G. Hotze, Jonathan A. Faiz, Nikolaos Mourtzis, Gabriel I. Pascu, Philip R. A. Webber,b
a,b
a
b,c
a,b
b
c
a
a,b
Guy J. Clarkson, Konstantina Yannakopoulou, Zoe Pikramenou* and Michael J. Hannon*
Received 20th December 2005, Accepted 6th April 2006
First published as an Advance Article on the web 13th April 2006
DOI: 10.1039/b518027a
Ruthenium(II) pyridylimine complexes are explored for their potential as units that might be
incorporated into electronic or photonic arrays. The complexes [Ru(bipy)
Ru(tpy)(L)Cl][BF
characterised using X-ray diffraction analysis and (2D) NMR spectroscopy. 1 displays emission in the
2
(L)][PF
6
] (1) and
2
[
4
] (2) with L = phenylpyridin-2-ylmethylene-amine are synthesized and fully
far-red area of the spectrum at room temperature. The emission is significantly shifted to longer
2
+
wavelength with respect to [Ru(bpy) ] indicating that the lowest MLCT state is localised on the
3
pyridylimine ligand. 2 is non-emissive at room temperature and at 77 K.
3
Introduction
architectures, might also be of benefit in design of multinuclear
photoactive ruthenium complexes for nano-electronics and pho-
tonics and we have embarked on a programme to explore such
application. We were surprised to discover that, despite the many
1
Pyridylimine units are attractive ligands as they have similar
properties to the much explored bipyridine ligands (N
2
2
diimine
donor sets and available p-bonding orbitals) yet are much simpler
to prepare. We have recently been exploring the potential of
pyridylimine ligands for metallo-supramolecular assembly of
mononuclear ruthenium polypyridyl complexes that have been
13–17
investigated, there are few reports
of ruthenium pyridylimine
complexes and so to initiate our investigations we chose first to ex-
plore the properties of simple mononuclear complexes to confirm
their potential photoactivity. This is particularly pertinent since
the addition of aryl rings adjacent to the nitrogens in bipyridines
3
complex architectures. The ease-of-preparation of these ligands
allows ready access to a wide variety of architectures and we
have shown that the ligand design can be used to encode subtle
features which determine the precise micro-architecture of the
arrays. Moreover we have demonstrated that this control of
supramolecular architecture can be used to encode functions such
18,19
and phenanthrolines leads to a dramatic loss of photoactivity.
We report herein the synthesis and characterisation (in both solid
state and solution) of mononuclear ruthenium(II) pyridylimine
complexes containing bipyridine and terpyridine co-ligands, and
demonstrate that the ruthenium bipyridine complex displays
emission in the far-red area of the spectrum at room temperature,
whilst the terpyridine complex displays no emission at room
temperature or 77 K.
4
as recognition of the DNA major groove.
While architecture assembly is a fascinating application of
polypyridine ligands, their enduring popularity arises primar-
ily from their ruthenium complexes, such as the prototypi-
2
+
cal [Ru(bpy)
3
]
cation, which exhibit exciting photochemical
5
and photophysical properties. Indeed the field of inorganic
supramolecular photochemistry remains dominated by multin-
uclear ruthenium polypyridyl complexes linked either covalently
or non-covalently. Such complexes are being employed in light-
harvesting devices and designed into nano-scale electronic and
photonic arrays. The complexes have also been developed as
DNA binding agents, particularly when an intercalating agent is
Several examples of mononuclear ruthenium complexes con-
6
20–23
ꢀ
taining terpyridine and phenylazopyridine ligands
or 2,2 -
7
24
azobispyridine are known in literature. Only a few reports
8,9
are known of ruthenium complexes with both terpyridine and
10
15,16
pyridylimine-based ligands.
Previous reports of ruthenium(II)
1
1
tris-diimine complexes containing bipyridine and pyridylimine lig-
25
ands are few and occasionally appear contradictory. Meyer et al.
12
incorporated into the molecular design.
reported what was probably the first example by electrochemical
oxidation of the corresponding pyridylmethylamine complex to
The advantages that our readily prepared pyridylimine ligand
systems have brought to design of functional supramolecular
2+
26
yield [Ru(bpy)
2
(C
5
H NCHNH)] . Belser and von Zelewsky
5
13
and Maruyama and Kaizu described a similar complex with
the alkyl pyridylimine ligand prepared from methylamine. Dose
and Wilson studied the pyridylimine ligand derived from 4-
a
School of Chemistry, The University of Birmingham, Edgbaston, Birm-
27
ingham, UK B15 2TT. E-mail: m.j.hannon@bham.ac.uk, z.pikramenou@
tolylamine but reported that mixed ligand complexes could not be
bham.ac.uk; Fax: +44 121 414 7871; Tel: +44 121 414 2527/2290
b
prepared by direct reaction with [Ru(bpy) Cl ]. However, Choud-
Centre for Supramolecular and Macromolecular Chemistry, Department of
2
2
17
Chemistry, University of Warwick, Gibbet Hill Road, Coventry, UK CV4
hury et al., using a silver(I) mediated synthesis, subsequently
obtained this mixed complex in low yield, after chromatographic
purification to remove side-products. They report the complex
to be non-emissive at room temperature. More recently Yam and
7
AL
c
Institute of Physical Chemistry, National Centre for Scientific Research
“
Demokritos”, 15310, Aghia Paraskevi, Greece
†
Electronic supplementary information (ESI) available: Fig. S1 and S2.
28
See DOI: 10.1039/b518027a
Lee have prepared a mixed ligand complex from the pyridylimine
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Dalton Trans., 2006, 3025–3034 | 3025

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2
9
ligand of an arylamino azacrown and Meade et al. have prepared
silver(I) to remove the chloride nor chromatography to purify
the product. The IR spectrum of 1 shows peaks corresponding
to the imine and aromatic stretches and the hexafluorophos-
complexes of alkyl linked pyridylimine-nucleotides. Both report
30
room temperature emission. Brunner et al. have described the
formation of diastereomeric mixed ligand complexes from a
chiral pyridylimine ligand and chromatographically separated
the two diastereoisomers, one of which was crystallographically
characterised.
phate counterion. The FAB-MS data for (1) shows peaks at
+
m/z = 741 and 596, corresponding to {[Ru(bipy)
2
L](PF
6
)}
+
and {[Ru(bipy)
2
L]} respectively, consistent with the expected
formulation [Ru(bipy)
2
L](PF ) ; partial elemental analysis is also
6
2
consistent with this proposed formulation. Recrystallisation of the
complex by slow diffusion of diisopropylether into an acetonitrile
solution of the salt afforded crystals suitable for X-ray crystallog-
raphy.
Results and discussion
Synthetic considerations of ligand L
The crystal structure (Fig. 2) reveals the expected six-coordinate
ruthenium(II) centre surrounded by five pyridine nitrogens and one
imino nitrogen. The bond lengths to the bipyridine nitrogens (Ru–
The ligand L (Fig. 1) was selected since it contains an aryl residue
linked directly into the pyridylimine binding unit. Although the
previous literature appeared to indicated that ruthenium com-
plexes of alkyl substituted pyridylimines were both more accessible
and more likely to be photoactive, we specifically investigated
this aryl substituted pyridylimine as in our ultimate multinuclear
designs we plan to incorporate conjugated links for effective
electronic and photonic communication. Moreover conjugation
of an aryl with the pyridylimine unit can dramatically enhance
the stability of the imine bond and this can affect the chemistry.
The ligand was prepared by stirring pyridine-2-carbaldehyde and
phenylamine in benzene at room temperature in the presence of
˚
N 2.057–2.077 A) are unremarkable and similar to those observed
2+
31
˚
in the [Ru(bipy)
the pyridylimine ligand is also similar (2.056 A) while that to the
3
] cation (2.056 A). The bond to the pyridine of
˚
˚
imine nitrogen is slightly shorter (2.045 A). The bipyridine units
are essentially planar (torsion angles between the rings 7.5 and
◦
◦
4
.5 ) as is the pyridylimine unit (torsion angle 1.5 ). The phenyl
◦
ring is twisted through 45 with respect to the pyridylimine unit.
This phenyl unit is positioned above a pyridyl ring of an adjacent
bipyridine, however the geometric constraints at the imine nitrogen
mean that it is not stacked coplanar with (or perpendicular to) that
pyridyl ring. The tris chelate cation is chiral and both enantiomers
are observed in the solid-state structure. In the structure there are
columns of cations with the same helicity, and adjacent columns
alternate in helicity. The hexafluorophosphate anions are packed
between these columns and form a number of short contacts to the
protons on the ligands. There are no face–face p–p interactions
within or between the cations and, somewhat surprisingly, nor
are there extensive face–edge (CH · · · p) interactions between the
cations.
˚
3
A molecular sieves (to remove water, one of the condensation
products). The molecular sieves were removed by filtration and
the solvent removed in vacuo to yield the crude ligand as an oil
which was reacted on without further purification.
Fig. 1 The structure of pyridylimine ligand L.
The FAB mass spectrum of ligand L shows a single peak
+
corresponding to {LH} and the IR spectrum shows peaks
1
corresponding to imine and aromatic stretches. The H NMR
spectrum was recorded in acetone-d
6
and shows a singlet at
8
.58 ppm corresponding to the imino proton and confirming the
formation of the desired imine bond. The rest of the peaks follow
the typical patterns of a pyridyl and a monosubstituted phenylene
moiety, and are readily assigned from their splitting patterns,
the coupling constants and the integration. The assignment was
confirmed by 2D COSY NMR spectroscopy. There are some
minor peaks in the spectra corresponding to about ∼5% of starting
amine and aldehyde; imine bonds are susceptible to hydrolysis in
the free ligands and this observation is unsurprising.
Synthesis, characterisation and luminescent properties of
[
Ru(bipy)
2
(L)][PF
6
]
2
(1). The complex [Ru(bipy)
Cl
2
(L)][PF
] in methanol
6
] (1)
2
Fig. 2 Structure of the cation 1.
was prepared by reacting L with cis-[Ru(bipy)
2
2
solution under reflux, to afford a red-brown solution. After
cooling to room temperature, the solution was treated with excess
methanolic ammonium hexafluorophosphate to yield a red-brown
It is instructive to compare the coordination environment ob-
served in this pyridylimine complex with that observed in the crys-
27
ꢀ
2+
precipitate of 1. Despite the reports of Dose and Wilson and
tallographically characterised [Ru(bipy)
2
(tpy-N,N )] cation (in
17
Choudhury et al. with the tolyl analogue, with this ligand the
which the terpyridine ligand acts as a bidentate ligand with a non-
2
+
reaction proceeded cleanly in good yield with need neither for
coordinated pyridyl ring); the structure of [Ru(bipy)
2
(Phbpy)] is
3
026 | Dalton Trans., 2006, 3025–3034
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1
2 6 2 6
Fig. 3 500 MHz H NMR spectrum of [Ru(bipy) (L)][PF ] in acetone-d solution at 298 K.
19
expected to be analogous. In that structure the non-coordinated
any certainty, but they do not show NOEs to the other H
3
protons
ring was stacked coplanar above another pyridine ring, however
the steric constraints needed to accommodate the non-coordinated
ring resulted in elongation of the Ru–N bonds to the tpy. This is not
the case in this pyridylimine containing cation whose coordination
or the imino or phenyl protons. The comparison of the chemical
2
+
shifts of the bpy rings with those observed for the [Ru(bipy)
3
]
cation is revealing. The chemical shifts of the protons in rings A
2
+
and B are very similar to those of [Ru(bipy)
3
]
except for H6A
2
+
environment is more like that of [Ru(bipy)
3
] .
The H NMR spectrum of the diamagnetic complex 1 was
recorded in acetone-d solutions at 298 K and is illustrated in
which is shifted dramatically downfield by 0.8 ppm. NOESY cross
peaks are observed between this proton and weakly to the imino
proton (Him) and strongly to the ortho proton of the phenyl ring
confirming that this proton is the one located above the imine unit
and this establishes the absolute assignment of rings A and B. The
dramatic downfield shift is accounted for by the absence of the
1
6
Fig. 3. Connectivity within the pyridyl and phenyl rings was
established using 2D COSY NMR experiments. There are five
different pyridine moieties; one derived from ligand L and four
from the two non-equivalent bipy ligands. The chemical shifts
of the pyridyl protons are collected in Table 1 together with
pyridyl ring current shift experienced by H of the other rings.
6
For ring C, the shifts of protons 6 and 4 are unremarkable
while proton 3 and all the protons of ring D are shifted upfield
2
+
those of [Ru(bipy)
3
]
(also a numbering scheme is given). 2D
2
+
NOESY NMR experiments enabled the connectivity between the
imine, pyridyl and phenyl units to be established with cross peaks
with respect to the corresponding protons in [Ru(bipy) ] . These
3
are the protons over which the phenyl ring is located and they
experience the ring current of this residue. This assignment is
confirmed by NOESY cross peaks from H6D to the ortho proton
of the phenyl ring, which establish the absolute assignment
of rings D and C. The protons of the phenyl ring are sharp
indicating that the phenyl ring is rotating freely in solution at room
temperature. This is in contast to the non-coordinated rings in the
observed between Him–H3py; Him–HoPh;
H3C–H3D. H3A and H3B are
too close in chemical shift to observe an NOE between them with
1
Table 1 H NMR chemical shift (d) data for the pyridyl protons in
2+
2+
[
2 3 6
Ru(bipy) (L)] and [Ru(bipy) ] in acetone-d solution at 298 K
ꢀ
2+
2+
19
[
Ru(bpy)
2
(tpy-N,N )] and [Ru(bpy) (Phbpy)] cations which
2
experience restricted rotation and again emphasises the difference
between the complexes of this aryl substituted pyridylimine and
those of analogous aryl substituted bipyridines. This assigment
using 2D NOESY NMR is in agreement with the assignment for
the structural related [Ru(bpy)
2
(azpy)](PF
6
)
2
compound (azpy =
32
2
-phenylazopyridine).
The electrochemical behaviour of the complex in acetonitrile
H
6
H
5
H
4
H
3
solution has been examined by cyclic voltammetry. Solutions of
2
+
[
+
Ru(bpy)
2
(L)] show a reversible ruthenium(II)/(III) oxidation at
2+
[
Ru(bipy)
2
3
(L)]
py of pyim
bpy A
bpy B
bpy C
bpy D
8.17
8.94
8.09
8.04
7.94
8.07
7.67
7.76
7.57
7.71
7.40
7.59
8.25 8.59
8.31 8.87
8.25 8.86
8.25 8.55
7.94 8.34
8.23 8.83
+
0.96 (vs. Fc/Fc ). This is slightly higher than the value of +0.85 V
5
,19
2+
2+
reported for the [Ru(bpy)
3
]
and [Ru(bpy) (Phbpy)] cations
2
ꢀ
2+
and +0.91 V for the [Ru(bpy) (tpy-N,N )] cation and implies that
2
the pyridylimine ligand slightly stabilises the ruthenium(II) oxida-
2+
[
Ru(bipy)
]
tion state (compared to bipyridine) which may reflect enhanced
This journal is © The Royal Society of Chemistry 2006
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p-back donation to this ligand, which would be consistent with the
slightly shorter Ru–N bond to the imino nitrogen observed in the
X-ray crystal structure. Four reversible reductions are observed at
+
−
1.41 V and −1.92 V and −2.11 V and −2.23 V (vs. Fc/Fc ). The
reductions in such systems are known to be ligand centred rather
than ruthenium centred and comparing these reduction potentials
ꢀ
2+
with those of [Ru(bpy)
2
(tpy-N,N )] (−1.69 V and −1.91 V) and
2
+
19
[
Ru(bpy)
2
(Phbpy)] (−1.76 V and −1.91 V) it is apparent that
the pyridylimine ligand is more easily reduced than the bipyridines
and that the first ligand centred reduction takes place at L.
The absorption spectrum of an acetonitrile solution of
2
+
[
Ru(bpy)
2
(L)] shows a very broad absorption band between 400–
5
30 nm with a structure that indicates two overlapping bands with
3
−1
−1
maxima around 440 nm (e ∼ 9 000 dm mol cm ) and 480 nm (e
3
−1
−1
5,19
∼
9 000 dm mol cm ) (Fig. 4). This may be compared with
3
−1
−1
the absorption maxima at k = 452 nm (e = 13 000 dm mol cm )
2
+
3
−1
−1
for [Ru(bpy)
Ru(bpy)
(tpy-N,N )] and at 447 nm (e = 10 000 dm mol cm )
for [Ru(bpy) (Phbpy)] . A red shift of the MLCT absorption
band in Ru(II) complexes on replacing pyridine ligands with
3
]
at k = 450 nm (e = 12 000 dm mol cm ) for
Fig. 5 Luminescence spectra of 1 in CH CN at RT (dotted line), and in
3
ꢀ
2+
3
−1
−1
[
2
BuCN–MeCN (4 : 1) (solid line) kexc= 440 nm, recorded with a 750 nm
2
+
1
grating on the emission monochromator. The RT spectrum is not corrected
for PMT response. Intensities are not normalised.
2
13,17,33
imines has been previously noted
and is consistent with the
electrochemical results that suggest that the pyridylimine ligand
has a lower energy p* level than the bipyridine ligand. In addition
to the broad 400–530 nm band a shoulder at ∼350 nm (e ∼
1
3,33
on the imine ligand L.
The 140 nm red shift compared to
2
+
[Ru(bpy)
3
]
might be attributed to C=N bond distortion in the
3
−1
−1
13
7
000 dm mol cm ) is evident on the edges of the 280 nm
excited state. The samples used for the emission studies were
recrystallised and were single-crystal samples. The luminescence
lifetimes of the 770 nm band in aerated or degassed solutions are
shorter than the instrument response (50 ns). The short lifetime
ligand-based band.
3
of the 770 nm band, attributed to the MLCT, is not surprising
due to the low energy of that state that leads to fast decay to
the ground state. However, acetonitrile solutions of 1 displayed
an additional emission band at 520 nm following excitation at
3
60 nm (see Electronic Supplementary Information, ESI†). The
emission intensity at 520 nm increased on addition of HCl or
to a lesser extent on addition of Et NCl, while the emission
at 770 nm was unaffected by this and no detectable change in
4
34
the UV-Vis absorption spectra was observed. The excitation
spectrum monitored at 520 nm reveals a band at 360 nm to be
responsible for leading to this emission (see ESI†). In previous
studies of Ru(II) complexes with imine ligands, there is one brief
17
mention of multiple band emission spectra at low temperature,
however only preliminary studies were reported and their two
emissionwavelengthswerequitesimilartoeachother. Apossibility
may be that this green emission is attributed to an intraligand
charge transfer state, although in the UV-Vis spectrum we cannot
unequivocally assign this band. To further investigate the origin
of this green emission, and its unusual sensitivity to environment
CN (2.3 × 10− M).
5
Fig. 4 Absorption spectrum of 1 in CH
3
Upon excitation at 440 or 480 nm complex 1 exhibits a broad
weak emission centred at 770 nm which is assigned to a MLCT
transition (Fig. 5). The excitation spectrum (monitored at 770 nm)
displays a profile of the two overlapping bands in the visible
we performed a series of control experiments. We can exclude
3
2+
the 520 nm emission arising from traces of [Ru(bpy)
3
]
(kem
=
630 nm) or the free ligand (which emits at 330 nm) but we cannot
exclude the formation of a species upon irradiation that may lead
to an emission at this range of the spectrum. Photosubstitution is
known for Ru polypyridylimine complexes although this is most
4
40 and 480 nm and a broad UV profile. This confirms that
1
the emission originates from population of the MLCT states.
Excitation of the complex in a rigid glass at 77 K leads to a sharper
and blue shifted emission profile with an emission maximum
at 730 nm (Fig. 5). The unusual appearance of a luminescence
signal from a Ru(II) complex at the far-red region of the visible
is consistent with the electrochemical and optical properties of
the complex, indicating that the lowest MLCT state is localised
35
usually associated with complete loss of a bidentate ligand. If
2
+
that were the case in our systems, [Ru(bpy)
2
2
(MeCN)
2
]
would be
+
formed. The complex [Ru(bpy)
2
(MeCN) ]
2
is reported to emit
5
at 77 K at 542 nm. The increased flexibility of the pyridylimine
ligand (compared to bpy) might allow a monosubstitution instead,
that would give rise to green emission upon prolonged irradiation
3
028 | Dalton Trans., 2006, 3025–3034
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and formation of species arising from partial substitution of
the pyridylimine ligand (didentate to monodentate) with one
acetonitrile ligand.
isomers predominates on precipitating the compound as BF
4
salt. To investigate this further the reaction was repeated and
the reaction mixture taken to dryness, rather than being treated
36
1
with tetrafluoroborate anions. An H NMR spectrum of the solid
Synthesis and characterisation of [Ru(tpy)(L)Cl][BF
4
] (2).
residue indicated the isomer ratio (A : B) to be approximately
3
7
[
Ru(tpy)(L)Cl][BF ] (2) was prepared by reaction of [Ru(tpy)Cl ]
4
3
4
: 1.
with one equivalent of L in refluxing methanol, in the presence
of 4-ethylmorpholine. Subsequent treatment with ammonium
tetrafluoroborate afforded the dark purple complex 2. The IR
spectrum of 2 shows peaks corresponding to the imine and
aromatic stretches and the tetrafluoroborate counterion. The
FAB mass spectrum of (2) shows a strong peak at m/z = 552,
The identity of the major isomer is readily assigned from the
H NMR spectrum. The observation of doublet at high frequency
10.26 ppm) immediately confirms this isomer to be isomer (A).
1
(
The doublet corresponds to H
located cis to, and oriented towards, a coordinated chloride. Only
6
of L and is typical for a pyridine H
6
38
in isomer (A) is there a pyridine H proton in such a position and
6
+
+
corresponding to [Ru(tpy)LCl] . While [Ru(tpy)(bpy)Cl] exists
as a single isomer, the asymmetry of the ligand L means that
two possible geometric isomers are possible depending on the
orientation of L with respect to the chloride ligand (shown
schematically in Fig. 6). The position adjacent to the chloride
might either be occupied by the pyridine (A) or the imine (B).
Steric interactions between the phenyl ring and the chloride might
favour isomer (A).
orientation (vide infra). The spectrum is readily assigned from the
coupling constants and relative integrals. 2D COSY experiments
confirm the connectivity within the rings and the expected NOEs
are observed in 2D NOESY experiments between H
the tpy ligand and between Him and both H and H
confirming the connectivity between the rings. A further NOE
is observed between H on the phenyl ring and H of the tpy
confirming that the phenyl ring is located on top of the tpy ligand
and that the isomer is (A). A further NOE between H of L and
of the tpy is also consistent with isomer (A). Moreover, the
3
and H3ꢀ in
o
3
in ligand L,
o
6
1
The H NMR spectrum of 2 (Fig. 7) reveals that although
6
both isomers are present (as shown by examining the reaction
mixture by NMR before adding the counterion) one of the two
H
6
chemical shifts of the central tpy ring are similar to those of the
+
corresponding ring in [Ru(tpy)(bpy)Cl] and in stark contrast to
+
those in [Ru(tpy)(Phbpy)Cl] which experience a dramatic upfield
38
shift as a consequence of a stacked phenyl ring. Thus in solution
the phenyl ring (which is rotating freely at room temperature) lies
above the tpy ligand but is not p-stacked and coplanar with the
tpy ligand.
The minor isomer was isolated after purification on a neutral
alumina column as a PF salt. ESI-Mass revealed the molecular
6
+
1
ion peak of [Ru(terpy)(L)Cl] at m/z 552 and H NMR imme-
diately showed the existence of isomer (B) (Fig. 8). The 6py
resonance is shifted upfield relative to isomer (A), as this H
atom is no longer deshielded by the choride atom. In addition,
6
Fig. 6 Schematic representation of the two possible isomers for complex
cation 2.
1
Fig. 7 500 MHz H NMR of [Ru(tpy)(L)Cl][BF
4
] in acetone-d
6
solution at 298 K.
Dalton Trans., 2006, 3025–3034 | 3029
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6 6
Fig. 8 300 MHz of minor isomer of [Ru(terpy)(L)Cl][PF ] in acetone-d at 298 K.
2
1
the ortho resonance shows a considerable downfield shift, as the
ortho hydrogen atoms are now close to the chloride ligand. The
full assignment has been achieved using integration values and 2D
COSY NMR spectroscopy.
methylated 2-phenylazopyridine compound the existence of the
minor isomer was also mentioned but attempts to isolate the minor
isomer failed.
Recrystallisation of isomer (A) by slow diffusion of benzene
into an acetone solution of the salt afforded crystals suitable for
X-ray crystallography. The cation is of isomer type (A) and the
ruthenium(II) centre occupies a distorted octahedral geometry,
coordinating to one terdentate tpy ligand, one didentate L ligand
and one chloride (Fig. 9). The tpy ligand is constrained to occupy
three mer coordination sites. The imine of the pyridylimine unit is
located trans to the chloro group, and the phenyl ring is located
above the central ring of the tpy ligand. The ruthenium–nitrogen
The isolation and characterisation of the minor isomer type
ꢀ
(
B) is unusual. From all complexes of the type {Ru(terpy)(L )X},
ꢀ
ꢀ
with L = 2-phenylazopyridine, 2,2 -azobispyridine, pyridylimine
or phenolic Schiff base, the main isomer type (A) (i.e. the isomer
with the central pyridine ring of terpy trans to the pyridine ring of
14,20,24
ꢀ
ligand L) is obtained.
Only in one case of a {Ru(terpy)(L )X}
ꢀ
complex with L = 2-phenylazopyridine has the minor isomer has
23
been isolated, but was not fully characterised. In the case of a
˚
bond lengths to the central ring of the tpy ligand (1.959 A) are
˚
shorter than those to the terminal rings (2.065 and 2.064 A) and
these bond lengths and the associated bond angles are comparable
8,37,39
with those observed in other ruthenium(II) terpyridyl systems.
The phenyl ring is twisted with respect to the pyridylimine units
◦
(
55 ) and positioned above the central ring of the tpy ligand. As
in complex 1 the geometric constraints at the imine nitrogen mean
that it is not stacked coplanar with (or perpendicular to) that
pyridyl ring. The terpyridyl unit is essentially planar (torsion an-
◦
gles between rings 0–5 ) as is the pyridylimine units (torsion angle
◦
3
). The bond from the ruthenium to the pyridine nitrogen of the
˚
pyridylimine ligand (2.073 A) is similar to those in 1 and again that
˚
to the imine nitrogens is slightly shorter (2.020 A). These distances
ꢀ
are in correspondence with an analogous [Ru(terpy)(L )Cl]ClO
4
ꢀ
15
complex (L = NC
5
H
4
C(H)=N(C
6
H
4
)NH ).
2
The cations are packed into chains through p–p and CH · · · p
interactions as illustrated in Fig. 10. Each cation forms two types
of intra-chain links. On one side of the cation a double ring
Fig. 9 Structure of the cations in the crystal structure of 2 (isomer A).
Fig. 10 Packing of the cations in the crystal structure of 2 (isomer A) through p–p and CH · · · p interactions. For clarity only the imine hydrogens are
shown.
3
030 | Dalton Trans., 2006, 3025–3034
This journal is © The Royal Society of Chemistry 2006

View Article Online
p-stack is formed between the tpy ligand and the corresponding
aryl ring is positioned in such a way that it does not face–face
p-stack with the other aryl rings in the complex; it is thus able
to rotate freely in solution. The results confirm that units based
on pyridylimine ligands may indeed be suitable for incorporation
into nano-scale electronic or photonic arrays as we hoped. Both
compounds 1 and 2 display reversible redox behaviour similar to
the corresponding bipyridine analogues. 1 displays room temper-
ature emission in the far-red region of the visible spectrum and
shifted to longer wavelength than the corresponding bipyridine
analogues. Pyridylimine units are thus attractive because of their
ease of synthesis and because of their inherent emission profiles.
We are currently exploring the incorporation of these units into
polynuclear arrays.
˚
ligand on the adjacent cation (centroid–centroid 3.78 A). On the
other side two CH · · · p interactions are formed involving the imine
CH and the pyridine ring of the tpy which was not involved in the
˚
double ring p-stack (CH · · · centroid 3.12, 3.23 A; C · · · centroid
◦
˚
3
.96, 4.06 A; ∠CH · · · centroid 149, 145 ). The chains are packed
together to form 2D sheets with disordered solvent and anions
between these sheets. The anions make short contacts with cation
protons and some additional (but less extensive) face–edge p
contacts are observed between chains.
The electrochemical behaviour of the complex in acetonitrile so-
lution has been examined by cyclic voltammetry. The complex ex-
+
hibits a reversible oxidation process close to +0.50 V (vs. Fc/Fc )
corresponding to the ruthenium(II)/(III) couple. As in complex
1
this value is slightly higher than the observed values in the
38
+
corresponding bpy complexes of 0.45 V for [Ru(tpy)(Phbpy)Cl]
and 0.42 V for [Ru(tpy)(bpy)Cl] , consistent with the ligand acting
Experimental
+
as a more effective p-acceptor. A reversible reduction is observed at
General
−
1.60 V, and two semi-reversible reductions at −1.85 and −2.15 V
4
0
37
The complexes cis-[Ru(bipy)
2
Cl
2
]·2H
2
O
and [Ru(tpy)Cl ] were
3
+
(
vs. Fc/Fc ). As in complex 1, the first reduction (pyridylimine
prepared according to literature methods. Commercially available
solvents and reagents were used without further purification.
NMR spectra were recorded on Bruker DPX 300 and DRX
based) is again more facile than in the corresponding bpy complex.
1
The MLCT absorption of 2 has a peak maximum at 520 nm
3
−1
−1
(
e = 12 000 dm mol cm ) which is around 15 nm red
5
00 instruments using standard Bruker software. FAB mass
shifted in comparison with other mixed ligand Ru(II) terpyridyl
spectra were recorded on a Micromass Autospec spectrometer
using 3-nitrobenzyl alcohol as matrix. ESI-MS spectra were
recorded on a Bruker Esquire 2000 in an acetonitrile solution.
Microanalyses were conducted on a Leeman Labs CE44 CHN
analyser by Warwick Analytical Service. Infrared spectra were
recorded on a Perkin-Elmer Paragon 1000 instrument with the
sample as solid pellets. UV-Vis spectra were recorded on a Perkin-
Elmer Lambda 25 instrument. The luminescence studies were
performed on a Photon Technology International QM-1 steady
state spectrometer employed with a dual grating 500/750 nm
emission monochromator and equipped with a 75 W xenon
arc lamp and a model 810 photon counting detection system
with a red-sensitive R928 photomultiplier tube. The data were
collected and analysed with Felix software. Electrochemical
measurements were performed using an EcoChemie lAutolab
electrochemical workstation and standard GPES software. A
conventional three-electrode configuration was used, with plat-
3
−1
−1
complexes (k = 502 nm (e = 11 000 dm mol cm ) for
+
3
−1
−1
[
Ru(tpy)(bpy)Cl] and at k = 508 nm (e = 10 700 dm mol cm )
+ 38
for [Ru(tpy)(Phbpy)Cl] ). However, this may be due to a non-
symmetrical spectral envelope which indicates the presence of
two overlapping bands. The complex does not emit at room
temperature, indicating that the pyridylimine ligand L does not
alter the energy of the MC state responsible for deactivation of the
3
MLCT emission in Ru(II) terpyridyl complexes. The samples used
for the emission studies were single-crystal samples and did not
show emission at 77 K in ethanol–methanol (4 : 1) frozen glass. In
one non-single-crystalline sample we did observe an emission at
7
5
7 K in ethanol–methanol (4 : 1) frozen glass with a maximum at
95 nm. Emission at this wavelength has been reported for a similar
15
terpyridine–imine complex. However, the 595 nm emission is
strongest on irradiation at 475 nm (not 520 nm; the absorption
maximum of complex 2). Since [Ru(tpy)(bpy)Cl] itself does emit
at low T but at much longer wavelength (692 nm), we propose that
this 595 nm emission is not inherent emission from 2 but rather
+
5
+
inum working and auxiliary electrodes and a Ag/Ag reference.
The measurements were conducted in acetonitrile solution with
2
+
from a trace impurity of [Ru(tpy)
2
]
(kem= 596 nm; kabs = 470/
0
.1 M [n-Bu
4
N][PF ] base electrolyte. Potentials are quoted vs.
6
5
4
75 nm) in that particular non-single-crystalline sample.
+
ferrocene/ferrocenium couple (Fc/Fc = 0.0 V) and all potentials
were referenced to internal ferrocene added at the end of each
experiment.
Conclusion
We have demonstrated that N-aryl pyridylimine units can be
used as structural alternatives to bipyridine units for construct-
ing ruthenium(II) complexes. The crystal structures reveal that
Phenyl-pyridin-2-ylmethylene-amine (L). Phenylamine (0.536 g,
5 mmol) and pyridine-2-carbaldehyde (0.466 g, 5 mmol) were
stirred in benzene (15 ml) at room temperature for 3 h, in the
these N-arylpyridylimines are quite different from the 6-aryl-
ꢀ
˚
2
,2 -bipyridines because the aryl group does not interfere with
presence of 3 A molecular sieves. After cooling and removal of the
coordination of the imino nitrogen to the metal centre. They
also do not introduce the cyclometallation possibilities associated
molecular sieves, the solvent was evaporated in vacuo. The product
is not pure and is a yellow oil. Yield 0.84 g (92%). FAB-MS: m/z =
ꢀ
+ 1
with 6-aryl-2,2 -bipyridines and which complicates the chemistry
183 [MH] . H NMR 300 MHz (acetone-d
6
): d = 8.71 (d, 1 H, H
6
),
), 7.49 (t, 1 H,
), 7.45 (t, 2 H, m-Ph), 7.32 (d, 2 H, o-Ph), 7.29 (t, 1 H, p-Ph)
38
of those ligands (a disfavoured 4-membered chelate ring would
8.58 (s, 1 H, Him), 8.22 (d, 1 H, H
3
), 7.94 (t, 1 H, H
4
be required for cyclometallation of an N-aryl pyridylimine). In
H
5
ꢀ
13
this sense the N-aryl pyridylimines are structurally more like 2,2 -
ppm. C NMR (acetone-d
6
): d = 207.1 (br), 162.7, 151.6, 138.5,
ꢀ
bipyridines than 6-aryl-2,2 -bipyridines. On coordination the N-
131.1, 128.5, 127.2, 122.9, 122.8
This journal is © The Royal Society of Chemistry 2006
Dalton Trans., 2006, 3025–3034 | 3031

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[
Ru(bipy)
2
L](PF
6
)
2
(1). A mixture of cis-[Ru(bipy)
2
Cl
2
]·2H
2
O
7.84 (t, 1H, H4py), 7.77 (d, 1H, H6py), 7.53 (m, 5H, m-Ph, o-Ph,
H5tpy).
(
(
0.104 g, 0.2 mmol) and phenyl-pyridin-2-ylmethylene-amine (L)
0.055 g, 0.3 mmol) in methanol (65 ml) was heated at reflux
under nitrogen for 4 h. After cooling at room temperature,
the solution was filtered through celite and the filtrate was
X-Ray crystal structure determination of 1 and 2
X-Ray data for 1 and 2 were collected with a Siemens SMART
treated with an excess of methanolic NH
4
PF solution. The
6
three-circle system with a CCD area detector, with crystals held at
solvent was removed in vacuo to approximately 20 ml and the
41
◦
180 K with the Oxford Cryostream Cooler.
solution was cooled to 5 C overnight. The brown-red product
¯
There were no systematic absences. Space groups (both P1)
were chosen on the basis of intensity statistics and shown to be
correct by successful refinement. The structures were solved by
was collected on a porosity 4 sintered-glass filter and air dried.
Recrystallisation from acetonitrile by slow diffusion of diisopropyl
ether afforded crystals suitable for X-ray analysis. Yield 0.104 g
42
direct methods with additional light atoms found by Fourier
methods. Hydrogen atoms were added at calculated positions and
refined using a riding model. Anisotropic displacement parameters
were used for all non-H atoms; H atoms were given isotropic dis-
placement parameters equal to 1.2 times the equivalent isotropic
displacement parameter of the atom to which the H atom was
attached. Pertinent data for each of the structure determinations
are given in Table 2, and bond lengths and angles in Tables 3
and 4.
(
58%). RuC32
H
26
N
6
P
2
F
12·1H
2
O: calc. C 42.5, H 3.1, N 9.3; found
+
C 42.4, H 2.8, N 9.2%. FAB-MS: m/z = 741 [M − PF
6
] , 596
+
[M − 2PF
6
] . UV-Vis (MeCN): 477 (e = 9 000), 442 (e = 9 000)
3
−1
−1
and 285 nm (e = 40 000 dm mol cm ). IR: 3082 m, 1603 s,
1
1
572 w, 1545 m, 1488 m, 1466 s, 1446 s, 1426 w, 1314 m, 1297 w,
274 w, 1242 m, 1200 w, 1160 m, 826 s, 757 s. H NMR 500 MHz
1
(
acetone-d
6
): d = 9.43 (s, 1H, Him), 8.94 (d, 1H, H6a), 8.87 (d, 1H,
H
(
1
H
3a), 8.86 (d, 1H, H3b), 8.59 (d, 1H, H3py), 8.55 (d, 1H, H3c), 8.34
d, 1H, H3d), 8.31 (t, 1H, H4a), 8.25 (m, 3H, H4c, H4py, H4b), 8.17 (d,
H, H6py), 8.09 (d, 1H, H6b), 8.04 (d, 1H, H6c), 7.94 (m, 2H, H6d
4d), 7.76 (t, 1H, H5a), 7.71 (t, 1H, H5c), 7.67 (t, 1H, H5py), 7.57 (t,
,
1. The asymmetric unit contains one complete complex and
two PF counterions.
6
1
6
H, H5b), 7.40 (t, 1H, H5d), 7.20 (t, 1H, o-Ph), 7.10 (t, 2H, m-Ph),
.79 (d, 2H, o-Ph).
2
(isomer A). The asymmetric unit contains one complex
comprised of an Ru bound to a terpy and a pyridine imine
ligand. There is a molecule of benzene which is disordered
over two positions (55 : 45 occupancy). Both parts were refined
[
Ru(tpy)LCl]BF
4
(2, isomer A). [Ru(tpy)Cl ] (0.044 g,
3
0
0
.1 mmol) and phenyl-pyridin-2-ylmethylene-amine (L) (0.018 g,
.1 mmol) were added in methanol (25 ml) containing a few drops
anisotropically but still have large thermal parameters. The BF
4
counter ions also disordered around the B10–F11 axis with both
positions refined anisotropically (min : major 2 : 8).
CCDC reference numbers 293457 and 293458.
For crystallographic data in CIF or other electronic format see
DOI: 10.1039/b518027a
of 4-ethylmorpholine. The mixture was heated at reflux for 4 h.
After cooling to room temperature the solution was concentrated
by rotary evaporation to approximately 15 ml. The solution was
then treated with an excess of methanolic NH
4
BF solution. The
4
resultant dark purple precipitate was collected on a porosity 4
sintered-glass filter and dried in the air. Recrystallisation from
acetone by slow diffusion of benzene afforded crystals suitable for
Table 2 Crystallographic data for crystal structure determinations of 1
and 2 (isomer A)
X-ray analysis. Yield 0.022 g (34%). RuC27
H
21
N
5
ClBF
4
·2CH
3
OH:
Compound
1
2 (isomer A)
ClRuBF
4
716.93
Triclinic
¯
P1
11.0703(12)
11.8803(13)
13.2873(15)
92.061(8)
110.294(10)
104.768(11)
1569.9(3)
1.517
calc. C 49.5, H 3.8, N 9.7; found C, 49.6; H, 4.2; N, 10.0%; FAB-
+
MS: m/z = 552 [M − BF
4
] . UV-Vis (MeCN): 274 (e = 23 000),
Formula
C
32
H
26
N
6
RuP
2
F
12
C
33
H
27
N
5
−
1
3
−1
−1
Formula weight/g mol
Crystal system
Space group
885.60
Triclinic
P1
9.0813(8)
10.6974(9)
17.7534(14)
87.438(2)
85.6830(10)
83.579(2)
1707.9(2)
1.722
3
14 (e = 31 000) and 523 nm (e = 12 000 dm mol cm )
IR: 3064 m, 3032 m, 1634 w, 1599 m, 1563 w, 1525 m, 1466 m,
¯
1
1
9
2
7
1
447 s, 1434 s, 1385 s, 1282 m, 1247 m, 1201 w, 1160 w, 1049 s,
˚
a/A
b/A˚
1
032 s, 763 s, 697 s. H NMR (acetone-d
6
): d = 10.26 (d, 1H, H6py),
c/A
˚
.10 (s, 1H, Him), 8.59 (d, 1H, H3py), 8.53 (d, 2H, H3tpy), 8.47 (d,
H, H3ꢀtpy), 8.39 (t, 1H, H4py), 8.16 (t, 1H, H5py), 8.11 (t, 2H, H4tpy),
.98 (t, 1H, H4ꢀtpy), 7.79 (d, 2H, H6tpy), 7.56 (d, 2H, H5tpy), 7.10 (t,
H, o-Ph), 6.99 (t, 2H, m-Ph), 5.94 (d, 2H, o-Ph).
◦
a/
◦
b
◦
c
3
V/A
˚
−
3
D
Z
c
/g cm
2
2
−
1
[
Ru(tpy)LCl]PF
analogous fashion, but use of NH
used for separation of the two isomers.
6
(2, isomer B). This was synthesized in an
l[Mo Ka]/mm
Crystal color
Crystal size/mm
Total data
0.653
Orange
0.640
Purple
4
PF instead of NH BF and
6
4
4
0.2 × 0.12 × 0.01
0.38 × 0.20 × 0.16
10813
10104
The solid, dissolved in acetone was put on a neutral alumina
column and acetone was used as an eluent. Fractions of ca. 10 ml
were collected. The first two fractions contained pure isomer (A),
Total unique data
7717
0.0739
478
0.0854 [3414 refl.]
0.1716
0.964
7236
0.0300
432
0.0271 [7824 refl.]
0.0631
1.020
R
int
No. of refined parameters
a
R1 [I > 2r(I)]
whereas fractions 5 and 6 contained pure isomer (B). ESI-Mass:
b
wR2 (all data)
+
m/z = 552 [M − PF
6
] .
GoF
1
H NMR 300 MHz (acetone-d
6
): d = 9.67 (s, 1H, Him), 8.76 (d,
ꢀ
||/ꢀ|F
wR2 = [ꢀ[w(F
a
b
2
2 2
R1 =
||F
o
| − |F
c
o
|.
o
− F
c
) ]/
ꢀ
2
H
H, H3ꢀtpy), 8.638 (d, 2H, H3tpy), 8.40 (d, 1H, H3py), 8.34 (d, 2H,
6tpy), 8.27 (t, 1H, H4ꢀtpy), 8.13 (d, 2H, o-Ph), 8.05 (t, 2H, H4tpy),
2
2
1/2
[
w(F
o
) ]]
.
3
032 | Dalton Trans., 2006, 3025–3034
This journal is © The Royal Society of Chemistry 2006

View Article Online
◦
Table 3 Selected bond distances ( A˚ ) and angles ( ) of 1
5 A. Juris, V. Balzani, F. Barigelletti, S. Campagna, P. Belser and A. von
Zelewsky, Coord. Chem. Rev., 1988, 84, 85.
6
7
V. Balzani and F. Scandola, Supramolecular Photochemistry, Ellis
Horwood, Chichester, 1991.
Ru(1)–N(308)
Ru(1)–N(301)
Ru(1)–N(201)
Ru(1)–N(101)
Ru(1)–N(112)
Ru(1)–N(212)
2.045(6)
2.056(6)
2.057(6)
2.061(7)
2.070(6)
2.077(6)
N(301)–Ru(1)–N(101)
N(201)–Ru(1)–N(101)
N(308)–Ru(1)–N(112)
95.2(3)
95.4(3)
98.9(2)
V. Balzani, S. Campagna, G. Denti, A. Juris, S. Serroni and M. Venturi,
Acc. Chem. Res., 1998, 31, 26; V. Balzani, A. Juris, M. Venturi, S.
Campagna and S. Serroni, Chem. Rev., 1996, 96, 759; L. DeCola,
Chimia, 1996, 50, 214; E. C. Constable, Chem. Commun., 1997, 1073;
S. Bernard, C. Blum, A. Beyeler, L. DeCola and V. Balzani, Coord.
Chem. Rev., 1999, 192, 155; A. von Zelewsky and P. Belser, Chimia,
N(301)–Ru(1)–N(112) 173.4(3)
N(201)–Ru(1)–N(112)
N(101)–Ru(1)–N(112)
N(308)–Ru(1)–N(212)
N(301)–Ru(1)–N(212)
N(201)–Ru(1)–N(212)
N(101)–Ru(1)–N(212) 172.8(3)
N(112)–Ru(1)–N(212)
84.6(2)
78.4(3)
98.8(3)
90.2(2)
79.0(3)
N(308)–Ru(1)–N(301)
N(308)–Ru(1)–N(201) 176.0(3)
N(301)–Ru(1)–N(201)
N(308)–Ru(1)–N(101)
78.6(2)
1
998, 52, 620; J.-P. Sauvage, J.-P. Collin, J.-C. Chambron, S. Guillerez
and C. Coudret, Chem. Rev., 1994, 94, 993; M. Hissler, A. El-ghayoury,
A. Harriman and R. Ziessel, Angew. Chem., Int. Ed., 1998, 37, 1717;
M. D. Ward, Chem. Soc. Rev., 1995, 24, 121.
98.0(2)
87.0(2)
96.3(3)
8
9
N. W. Alcock, P. R. Barker, J. M. Haider, M. J. Hannon, C. L. Painting,
Z. Pikramenou, E. A. Plummer, K. Rissanen and P. Saarenketo,
J. Chem. Soc., Dalton Trans., 2000, 4702.
M. Silva, J. M. Haider, R. Heck, M. Chavarot, A. Marsura and Z.
Pikramenou, Supramol. Chem., 2003, 15, 563; J. M. Haider, R. M.
Williams, L. De Cola and Z. Pikramenou, Angew. Chem., Int. Ed.,
◦
Table 4 Selected bond distances ( A˚ ) and angles ( ) of 2 (isomer A)
Ru(1)–N(108)
Ru(1)–N(208)
Ru(1)–N(114)
Ru(1)–N(101)
Ru(1)–N(201)
Ru(1)–Cl(1)
1.959(4)
2.020(4)
2.064(4)
2.065(4)
2.073(4)
N(114)–Ru(1)–N(101) 158.59(16)
N(108)–Ru(1)–N(201) 176.50(15)
N(208)–Ru(1)–N(201)
N(114)–Ru(1)–N(201) 100.31(17)
N(101)–Ru(1)–N(201) 101.10(16)
77.46(15)
2
003, 42, 1830; J. M. Haider, M. Chavarot, S. Weidner, I. Sadler, R. M.
Williams, L. De Cola and Z. Pikramenou, Inorg. Chem., 2001, 40,
3
1
912; J. M. Haider and Z. Pikramenou, Eur. J. Inorg. Chem., 2001,
89; S. Weidner and Z. Pikramenou, Chem. Commun., 1998, 1473; H.
2.4052(12) N(108)–Ru(1)–Cl(1)
N(208)–Ru(1)–Cl(1)
99.07(15) N(114)–Ru(1)–Cl(1)
79.61(16) N(101)–Ru(1)–Cl(1)
98.32(15) N(201)–Ru(1)–Cl(1)
86.13(15)
89.86(11)
170.07(12)
87.50(11)
91.36(11)
93.63(11)
Krass, E. A. Plummer, J. M. Haider, P. R. Barker, N. W. Alcock, Z.
N(108)–Ru(1)–N(208)
N(108)–Ru(1)–N(114)
N(208)–Ru(1)–N(114)
N(108)–Ru(1)–N(101)
Pikramenou, M. J. Hannon and D. G. Kurth, Angew. Chem., Int. Ed.,
2
3
001, 40, 3862; J. M. Haider and Z. Pikramenou, Chem. Soc. Rev., 2005,
4, 120.
1
1
1
0 M. Zukalova, A. Zukal, L. Kavan, M. K. Nazeeruddin, P. Liska and
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This work was supported by an EU Marie Curie Training Site
Fellowship (N.M.; HPMT-GH-01-00365-02), Marie Curie EU
Research Training Networks (UNI-NANOCUPS MRTN-CT-
003-504233; MARCY HPRN-CT-2002-00175) and the EPSRC
J.F.). We thank EPSRC and Siemens Analytical Instruments
for grants in support of the diffractometer, the Royal Society
for provision of an electrochemical workstation (M.J.H.), the
EPSRC National Mass Spectrometry Service Centre, Swansea
for recording the electrospray mass spectra and Drs A. J. Clarke
and N. Spencer for assistance with NMR studies. We thank the
referees for their helpful suggestions. This work was conducted in
the context of COST D31 (WG D31/001/04). M.J.H. is the Royal
Society of Chemistry Sir Edward Frankland Fellow 2004–5.
2
(
5
1
63; S. Satyanarayana, J. C. Dabrowiak and J. B. Chaires, Biochemistry,
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ꢀ
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◦
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4
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034 | Dalton Trans., 2006, 3025–3034
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