Electronic properties of Ru(II) complexes bound to a bisphenolate bridge with low lying π* orbitals

Article abstract of DOI:10.1039/b312641b

The synthesis and a detailed investigation into the electronic properties of mononuclear and dinuclear Ru(II) complexes of the ligand bis(2-hydroxyphenyl)-2,5-dihydropyrazine (H₂BHD) is described. In these complexes the Ru(II) moieties are boun

Full text of DOI:10.1039/b312641b

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Electronic properties of Ru(II) complexes bound to a bisphenolate
bridge with low lying ꢀ* orbitals†
Ian Brady,a Deirdre Leane,b Helen P. Hughes,a Robert J. Forsterb and Tia E. Keyes*b
a Dept of Chemical and Life Science, Waterford Institute of Technology, Co Waterford, Ireland
b School of Chemical Sciences, National Centre for Sensors Research, Dublin City University,
Glasnevin, Dublin 9, Ireland. E-mail: tia.keyes@dcu.ie
Received 10th October 2003, Accepted 20th November 2003
First published as an Advance Article on the web 5th December 2003
The synthesis and a detailed investigation into the electronic properties of mononuclear and dinuclear Ru()
complexes of the ligand bis(2-hydroxyphenyl)-2,5-dihydropyrazine (H2BHD) is described. In these complexes the
Ru() moieties are bound through O,N coordination to an anionic phenolate and the pyrazine bridge. Relatively few
reports are available on the dinuclear complexes bridged across a phenolate and this study provides an opportunity
to examine the impact of reduced oxygen donor ligands on metal–metal communication. The results presented here
reveal some very unusual behavior whereby the apparent location of the LUMO changes between the mononuclear
and dinuclear complexes. The lowest energy optical transition appears to involve the peripheral bipyridine ligand as
acceptor in the mononuclear complex, whereas this ligand is not involved in the lowest energy optical transition in the
dinuclear complex. The origin of this difference is not clear, however, significant changes in the electronic properties
of the mononuclear complex are observed on coordination of the second metal, reflected in significant alterations in
the electrochemistry of the bridge and metals as well as changes in the optical spectroscopy. The BHD2Ϫ bridge is
shown to support weakly coupled class II behavior according to the Robin and Day classification, reflected in a Kc
of 335.
In this contribution, we present novel Ru() mononuclear
and dinuclear coordination compounds bound across the
Introduction
Significant effort has been dedicated in recent years to under-
standing the optical and electronic properties of Ru() com-
deprotonated bis(2-hydroxyphenyl)-2,5-dihydropyrazine bridge
(BHD2Ϫ) (Chart 1).
plexes bound to oxygen donor groups based on hydroquinone
This is the first report of a dinuclear Ru() complex in
and quinone ligands.1 Original interest in these species stemmed
which both metals are O,N coordinated to a phenolate donor in
from the importance of quinones in biological processes, in
which the lowest lying empty molecular orbitals lie on the
particular their participation in the photosynthetic reaction.
bridge. The bridge contains a coordinating pyrazine moiety, a
The non-innocent redox behavior and strong metal orbital
strong π-acceptor, which is expected to possess the LUMO in
mixing of these ligands has produced complexes with rich and
this complex.
interesting optical and electronic properties. Less studied, but
In creating these complexes, we seek to investigate the impact
also of significant interest, have been phenolate ligands. Like
of the phenol donor on the extent of electronic coupling
the quinonoids, phenol plays an important role in photosystem
between metals in a dinuclear phenolate complex where the
II whereby tyrosine acts as a mediator of electron transport in
bridge also contains a pyrazine which has been shown to medi-
ate inter-metal communication. In addition, by combining a
strong σ-donor with a strong π-acceptor on a single bridge, we
the photo-induced oxidation of the manganese cluster.2
In complexes containing phenolate donors, the strong
σ-donating nature of the ligand increases electron density at the
probe the impact of low lying bridge orbitals on the electronic
metal site, creating complexes, like the quinonoids, with rich
behavior of the metals and phenolates in a variety of oxidations
redox chemistry and electronic spectroscopy with transitions
states.
frequently extending to the NIR. Typically in Ru hydroquinone
coordination chemistry, the facile electrochemistry of the
ligand means that the hydroquinone is the site of first oxid-
ation. In such instances the metal is oxidized last and any
investigation of metal to metal communication in a dinuclear
hydroquinone bridged system will be across the oxidized
quinone bridge.3
Although redox active like the quinones, the higher oxidation
potential of phenol permits study of the redox and spectro-
scopic behavior of ruthenium() bound to a reduced OϪ
donor.4 In particular, employing a bridging system containing
coordinating phenolates permits the impact of reduced OϪ sites
on metal–metal communication to be examined. As well as
being of fundamental interest, studies of the ability of phenol-
ate to mediate electronic communication are of interest because
of the key role phenol plays in the mechanisms of a wide range
of metalloenzymes in biochemical systems.5
Experimental
Materials
D8-bpy was synthesized according to published methods,6
Dichloride complexes, cis-Ru(bpy)2Cl2, Ru(d8-bpy)2Cl2,
Os(bpy)2Cl2, Ru(d8-bpy)2Cl2 and Ru(biq)2Cl2 were synthesized
by standard methods.7–9 All other reagents, purchased from
Sigma-Aldrich, were used as received.
3-Nitro-4-hydroxycoumarin
9.2 cm3 of concentrated nitric acid dissolved in 10 cm3 of acetic
acid was added to a suspension of 20.5 g of 4-hydroxycoumarin
in 50 cm3 acetic acid. The solution was placed on a steam bath
and the temperature raised slowly to 85–90 ЊC to initiate
nitration. The flask was then cooled in ice. The yellow crystals
of 3-nitro-4-hydroxycoumarin were collected by filtration and
the product was recrystallised once from hot ethanol. Yield:
14 g (52%), mp 177 ЊC (lit. 177 ЊC)
† Electronic supplementary information (ESI) available: Raman spec-
trum of solid BHD ligand excited at 514 nm. See http://www.rsc.org/
suppdata/dt/b3/b312641b/
D a l t o n T r a n s . , 2 0 0 4 , 3 3 4 – 3 4 1
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4
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Chart 1 (a) H2BHD ligand, (b) [Ru(bpy)2(BHD)]ϩ and (c) [((Ru(bpy)2)2(BHD)]2ϩ
.
3-Amino-4-hydroxycoumarin
1.1 mmol (572 mg) of [Ru(bpy)2Cl2]ؒH2O in 100 cm3 ethanol–
water (2 : 1 v/v) for 8 h; 2–3 drops of concentrated ammonia
were also added. The hot solution was again filtered and the
solvent removed by rotary evaporation to near dryness. The
residue was isolated as the PF6 salt and purification was by
repeated chromatography on alumina column with acetonitrile
as eluent.
3 g of 3-nitro-4-hydroxycoumarin was suspended in 450 cm3 of
methanol. To this solution 0.5 g of the catalyst (5% palladium
on activated charcoal) was added along with 2–3 cm3 of 1%
methanolic hydrogen chloride. The mixture was hydrogenated
at atmospheric pressure for 3 h and then filtered to give a
colourless soloution. The filtrate was reduced under vacuum to
give a colourless solid. This was washed thoroughly with cold
water. Recrystallisation was from hot methanol and the com-
pound was dried in a dessicator overnight. Yield: 1.6 g (65%),
mp 222–224 ЊC (lit. 221–224 ЊC)
Ϫ
Yield: 720 mg (52%). Found: C, 48.76, H, 2.88, N, 9.36. Calc.
for C56H42N10O2Ru2P2F12: C, 48.6; H, 3.04; N, 10.13%.
[Ru(d8-bpy)2(BHD)]PF6 and [Ru(d8-bpy)2(BHD)Ru(d8-bpy)2]-
(PF6)2
Bis(2-hydroxyphenyl)-2,5-dihydropyrazine (H2BHD)
The perdeuteriated analogues were prepared using the same
procedures described for [Ru(bpy)2(BHD)]PF6. and [Ru(bpy)2-
(BHD)Ru(bpy)2](PF6)2 above.
1.5 g of 3-amino-4-hydroxycoumarin was heated at reflux in
100 cm3 of 2 M NaOH for 2 h. The pH of the solution was
lowered to about 6 by the controlled addition of a 1 M solution
of HCl. Bis(2-hydroxyphenyl)-2,5-dihydropyrazine precipitated
out as a yellow powder. The precipitate was collected by fil-
tration and washed thoroughly with cold water. Yield: 1.3 g
Physical measurements
Electronic absorption spectra were measured using a Shimadzu
3500 UV-vis/NIR spectrophotometer. Spectra were decon-
voluted into Gaussians using standard iterative algorithms.10
Electrochemical studies were conducted in HPLC grade
1
(67%), mp 264 ЊC (lit. 263–265 ЊC), H NMR, δ 6.85 (d, 2H,
H3/H3Љ), 6.76 (m, 2H, H4/H4Љ), 7.17 (t, 2H, H5/H5Љ), 7.95 (d, 2H,
H6/H6Љ), 9.47 (s, 2H, H2Ј/H5Ј).
solvents, dried over molecular sieves, 0.1
M tetraethyl-
ammonium tetrafluoroborate (TEABF4) was used as the
supporting electrolyte.
[Ru(bpy)2(BHD)]PF6ؒH2O
Cyclic voltammetry was carried out using a CH instruments
Model 660 electrochemical workstation. Cells were of con-
ventional design and were thermostated to within 0.2 ЊC using a
Julabo F10-HC refrigerated circulating bath. All potentials are
quoted with respect to a BAS Ag/AgCl organic reference elec-
trode. Under the conditions employed, the oxidation potential
of ferrocene was ϩ0.312 V vs. Ag/AgCl. All solutions were
degassed using nitrogen, and a blanket of nitrogen was main-
tained over the solution during all experiments. Platinum
microwires (Goodfellow Metals Ltd.) of radii between 5 and 25
µm were sealed in soft glass as described previously.11 Microdisk
electrodes were exposed by removing excess glass using 600 grit
emery paper followed by successive polishing with 12.5, 5, 1, 0.3
and 0.05 µm alumina. The polishing material was removed
between changes of particle size by sonicating the electrodes in
deionised water for at least 5 min. The polished electrodes were
electrochemically cleaned by cycling in 0.1 M HClO4 between
potential limits chosen to first oxidize and then to reduce the
surface of the platinum electrode. Finally, the electrode was
H2BHD (1.0 mmol, 0.25 g) was dissolved in 50 cm3 ethanol–
water (2 : 1 v/v) and 2–3 drops of concentrated NaOH was
added to the solution. The solution was then heated under
reflux with [Ru(bpy)2Cl2]ؒ2H2O (0.8 mmol, 0.43 g) for 6 h. The
hot solution was filtered and the solvent removed by rotary
evaporation to near dryness. 10 cm3 of water was added to the
filtrate and the red–brown product was subsequently precipi-
tated by the addition of a saturated solution of NH4PF6 to
give the PF6 salt. The compound was purified by repeated
chromatography on alumina column with methanol as
eluent.
Yield: 460 mg (56%). Found: C, 49.62; H, 3.32; N, 9.45. Calc.
for C36H29N6O3RuPF6: C, 51.37; H, 3.45; N 9.98%.
Ϫ
[Ru(bpy)2(BHD)Ru(bpy)2](PF6)2
This compound was prepared using the same procedure as for
[Ru(bpy)2(BHD)]PF6ؒH2O, except that 0.5 mmol (143 mg)
of the ligand H2BHD was added to a refluxing mixture of
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Scheme 1 Synthesis of H2BHD.
cycled between Ϫ0.300 and 0.700 V in 0.1 M NaClO4 until
hydrogen desorption was complete.
Spectroelectrochemistry was carried out using a home-made
Pyrex glass thin layer cell (1 mm). The optically transparent
working electrode was a platinum gauze which was inserted
fully into the cell, the auxiliary electrode was a platinum wire
and the reference was Ag/AgCl. Solvent and electrolyte were as
described above. The working electrode was held at the required
potential throughout the measurement using an EG&G PAR
model 362 scanning potentiostat. Absorption spectra were
obtained on a Shimadzu 3500 UV-vis/NIR spectrophotometer.
Raman spectroscopy was conducted on a Dilor.Jobinyvon.
Spex Labram. The exciting 20 mW helium–neon laser (632.8
nm) or Ar ion laser (514, 488 and 457.9 nm), laser was focused
through the thin layer electrochemical cell onto the surface of
the optically transparent platinum/rhodium gauze surface using
a 10× objective lens. Focusing was confirmed by using an
imaging video camera. A spectral resolution of 1.5 cmϪ1 per
pixel was achieved using a grating of 1800 lines mmϪ1. The
applied potential was controlled with respect to an SCE refer-
ence electrode using a CH instruments Model 600A potentio-
stat or exhaustive oxidation was achieved chemically using
cerium() sulfate. Solid samples where used, were dispersed as
5% w/w in KBr.
Fig. 1 COSY 45Њ 1H NMR spectrum of [Ru(bpy)2(BHD)]ϩ in
d6-DMSO.
observed for each phenolate ring and the pyrazine. The more
substantial shifts are observed on the coordinated site, for
example, the pyrazine singlet, H5Ј, adjacent to the coordinated
pyrazine nitrogen observed at 9.47 ppm in H2BHD shifts
upfield by 1.57 to 7.90 ppm on coordination. H2Ј on the other
hand shifts by only Ϫ0.55 ppm to 8.92 ppm. Resonances due to
the uncoordinated phenolate were also influenced by co-
ordination. The largest shift was observed for the H4 proton
that was shifted downfield by 0.42 ppm. However, the reson-
ances due to the protons on coordinated phenolate ring were
shifted upfield by up to 0.75 ppm. In the dinuclear complex,
Results and discussion
Synthesis and structure
The ligand, bis(2-hydroxyphenyl)-2,5-dihydropyrazine, H2BHD,
was prepared according to Scheme 1 using a slightly modified
literature method.12 The mononuclear and dinuclear complexes
were then synthesized according to standard techniques. It was
essential to conduct the complex synthesis at high pH, in order
to ensure the phenolate sites were deprotonated to permit
coordination.
1
the symmetry of the ligand is restored and in the H NMR
of the deuteriated dinuclear complex only four resonances are
observed. These are shifted significantly by comparison with
the free ligand, and like the mononuclear complex, the pyrazine
ring is most strongly affected by coordination, with a shift in
the H2Ј and H5Ј protons of greater than 2.5 ppm.
1H NMR spectroscopy confirmed that the complexes were
diamagnetic, whereby under ambient conditions, as Fig. 1 illus-
trates, sharp well-defined resonances were observed for both
complexes. In addition, elemental analysis confirmed the charge
on the complexes to be 1ϩ for [Ru(bpy)2(BHD)]ϩ and 2ϩ for
[(Ru(bpy)2)2(BHD)]2ϩ further confirming that the metal is in a
reduced state.
A complete assignment of NMR resonances was achieved
for both mono- and dinuclear complexes using a combination
of COSY 45 techniques and perdeuteriation of the bipyridyl
moieties in the complex. The latter resulted in considerable
simplification of the NMR spectra, by eliminating the contri-
bution from the bipyridyl protons. 1H NMR spectra of the
complexes were recorded in DMSO and the NMR shifts for the
coordinated BHD2Ϫ ligand are shown in Table 1; although all
bipyridyl shifts were identified they were not individually
assigned. In each instance, the spectra of the coordinated
BHD2Ϫ was compared with that of the free ligand. For the
mononuclear complex, coordination of the Ru(bpy)2 unit des-
troys the symmetry of the ligand and shifts in resonances are
Electronic spectroscopy
Fig. 2 shows the electronic spectra of the mono- and dinuclear
complexes [Ru(bpy)2(BHD)]ϩ and [(Ru(bpy)2)2(BHD)]2ϩ. In
each instance, the visible portion of the spectra extends to
>700 nm, estimates of λmax and extinction coefficients for over-
lying bands were determined by fitting the electronic spectra to
a multiple Gaussian model. [Ru(bpy)2(BHD)]ϩ shows a main
absorbance feature centered around 495 nm (ε = 3.05 × 104 MϪ1
cmϪ1) with a shoulder evident around 550 nm. The analogous
features in the binuclear complex are red-shifted by com-
parison, the main feature for [(Ru(bpy)2)2(BHD)]2ϩ is observed
at 512 nm (ε = 6.18 × 104 MϪ1 cmϪ1) with two bands super-
imposed on this feature centered around 550 and 630 nm. The
most intense feature in each instance is ascribed to a metal
to ligand charge transfer (MLCT) transition and the lower
energy transitions, from comparable complexes, to an inter- or
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Fig. 2 Electronic spectra of (a) [Ru(bpy)2(BHD)]ϩ, 1.5 × 10Ϫ5 M and
(b) [(Ru(bpy)2)2(BHD)]2ϩ, 5 × 10Ϫ6 M, in acetonitrile.
intraligand charge transfer transition, ILCT.4,3 The red shift
in λmax of the MLCT for the dinuclear complex is typical of
the behavior expected where the bridging ligand is a strong
π-acceptor site which is participating in the MLCT.13 Binding
of the second metal to this ligand reduces the energy of the π*
orbital leading to the observed shift of the MLCT to lower
energy. That the red shift is observed in this instance confirms
that the pyrazine π* level is the acceptor in the MLCT. The
ILCT transitions have also shifted to the red in the dinuclear
complex. Again, this observation suggests that the pyrazine site
participates in these transitions and that formally; the low
energy transitions may be an intra- rather than interligand
charge transfer.
Resonance Raman spectroscopy
In order to identify the origin of the transitions observed in the
electronic spectra and to confirm the origin of the HOMO and
LUMO in these complexes resonance Raman spectroscopy was
conducted. Coincidence of the wavelength of the Raman excit-
ation source with an optical transition in the analyte material
leads to enhancement of Franck–Condon vibrational modes by
up to seven orders of magnitude. Therefore, only modes associ-
ated with the chromophoric units in the material are expected
to be observed, providing a valuable means of identifying the
origin of optical transitions. Fig. 3(b), (c) and (d) show the
resonance Raman spectra of [Ru(bpy)2(BHD)]ϩ excited at 488,
514 and 632.8 nm, respectively, and Fig. 3(a) shows the Raman
spectrum of the perdeuteriated mononuclear complex [Ru-
(d8-bpy)2(BHD)]ϩ excited at 632.8 nm.
Exciting at 488 nm, signature bipyridyl features appear at
1602, 1556, 1482, 1318, 1264, 1170 and 665 cmϪ1; the Ru–N
vibration typically observed for Ru–bpy MLCT transitions, is
obscured here by an acetonitrile mode. Comparison of this
spectrum with that of [Ru(d8-bpy)2(BHD)]ϩ excited at the same
wavelength confirms that these features are due to bipyridyl,
and consequently, the underlying electronic transition is
attributed to a Ru() (dπ)–bipyridyl (pπ*) MLCT transition.
Exciting at 514 nm, similar features are observed, with bpy sig-
natures again apparent at 1602, 1556, 1482, 1318, 1264, 1170,
1020 and 665 cmϪ1. However, in addition, new features appear
at 1416 cmϪ1, which is partially obscured by solvent bands, and
at 750 and 605 cmϪ1. These features are likely to be due to
pyrazine which is anticipated to participate in the lower energy
portion of the MLCT. Exciting at 632.8 nm, these latter
features appear relatively stronger while the bipyridyl features
remain evident, and an additional feature also appears at
465 nm. As seen in Fig. 3(a), perdeuteriation of the bipyridyl
group leads to shifts of between 30 and 60 cmϪ1 in features
associated with this ligand. However, features at 1416, 1006,
750, 990 and 605 cmϪ1 are unaffected by deuteriation and are
therefore confirmed to be due to the BHD2Ϫ ligand. The per-
sistence of bipyridyl features in transitions extended beyond
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Table 2 Electrochemical dataa
Compound
Metal E /mV (∆E/mV)
Pyrazine E /mV (∆E/mV)
Phenol E /mV (∆E/mV) Bipyridyl E /mV
½
½
½
½
H2BHD
–
–
1300 (irrev.)
–
[Ru(bpy)2(BHD)]ϩ
189 (84) 1 eϪ
Ϫ1200 (100), 1eϪ
Ϫ1316 (86), 1eϪ
1414 (95), 1eϪ
1390 (90), 1eϪ
Ϫ1653, 1eϪ; Ϫ1791, 1eϪ
Ϫ1653, 1eϪ; Ϫ1852, 1eϪ
[Ru(bpy)2(BHD)Ru(bpy)2]2ϩ 585 (83), 1 eϪ; 734 (86), 1 eϪ
a All cyclic voltammetery conducted at 2 V sϪ1 in acetonitrile, potentials quoted vs. Ag/Agϩ.
attributed to the bridging ligand by comparison with the free
ligand (ESI†) and comparable O-coordinated ruthenium com-
plexes.4,14 In addition, the Raman spectrum shown in Fig. 4 (c),
excited at 632.8 nm, shows significantly weaker contribution
from the bipyridyl modes. The most recognizable bpy features
at 1602 and 1556 cmϪ1 have essentially disappeared. This sug-
gests little participation of bpy in the electronic transition
resonant with 632.8 nm. This is further confirmed in examining
the resonance Raman spectrum of the dinuclear complex con-
taining perdeuteriated bipyridine, which is relatively unaffected.
This suggests that, as expected, the lowest energy optical
transition involves the bridging ligand. Absence of Ru–N
modes in the low energy region of the Raman spectrum
and comparison with other coordinated phenolate complexes
suggest it may be a phenolate to pyrazine π–π* transition.
This suggests, as expected, that the LUMO lies on the bridg-
ing pyrazine and not the bipyridine moieties in the dinuclear
complex.15 The discrepancy between the location of LUMO in
the mononuclear complex and dinuclear complex is unclear.
However, significant changes in the molecular orbitals of the
bridge occur on addition of the second metal, this is reflected in
comparative electronic spectroscopy of the mononuclear com-
plex and dinuclear complex and also in the electrochemistry of
the complexes.
Fig. 3 Resonance Raman spectrum of (a) mononuclear [Ru(d8-
bpy)2(BHD)]ϩ dispersed in KBr, excitation wavelength, 632.8;
(b) [Ru(bpy)2(BHD)]ϩ in acetonitrile, excitation wavelength, 488 nm;
(c) [Ru(bpy)2(BHD)]ϩ in acetonitrile, excitation wavelength, 514 nm;
(d) [Ru(bpy)2(BHD)]ϩ in acetonitrile, excitation wavelength, 632.8 nm;
* indicates solvent bands.
632.8 nm, is surprising, as based on the electrochemical data
presented vide infra, and comparable systems, the LUMO in
this complex is expected to lie on the pyrazine bridge. However,
that both bpy and bridge features are evident at long wave-
length excitation, suggests that both participate in the lowest
energy electronic transition, therefore this transition is attri-
buted to phenolate-to-bpy ILCT. That this transition is evident,
but phenolate to pyrazine is not seems very surprising and is
thought to be due to the mixing of metal and phenolate states,
which promote bpy–phenolate communication.14
Electrochemistry
Fig. 5 shows the cyclic voltammetry for the complexes discussed
and the electrochemical data is presented in Table 2.
Fig. 4 shows resonance Raman spectra of the dinuclear com-
plex excited at (a) 488, (b) 514 and (c) 632.8 nm. Exciting
at 457.9 nm, the spectrum is very similar to that observed for
the mononuclear complex at the same wavelength. The eight
signature bipyridyl features at 1602, 1556, 1482, 1318, 1264,
1170, 1020 and 665 cmϪ1 are evident, typical of resonance with
Ru() to bpy (π*) MLCT transition. Exciting at 488 nm, the
bipyridyl features are evident, but in addition new features
appear at 1416, 1006 and 750 cmϪ1. In exciting at 514 and 632.8
nm, these features become more prevalent with additional
features appearing at 605 and 990 cmϪ1. These features are
Fig. 5 Cyclic voltammogram of (a) (—) [Ru(bpy)2(BHD)]ϩ and
(b) (—) [(Ru(bpy)2)2(BHD)]2ϩ at a 10 um glassy carbon electrode; scan
rate 2 V sϪ1 in 0.1 M TEABF4–acetonitrile.
[Ru(bpy)2(BHD)]ϩ exhibits a quasi-reversible one electron
oxidation at 189 mV vs. Ag/Agϩ. A second, quasi-reversible
oxidation is observed for this complex at 1414 mV, the revers-
ibility of this process is highly scan rate dependent and at scan
rates <1 V sϪ1 it becomes irreversible. The former process, on
the basis of comparison with similar behavior observed in a
Ru()–phenolate complex is attributed to the metal Ru()/()
process.4 The more anodic process is attributed to the phenolate
oxidation on the basis of comparison with the electrochemistry
of the free ligand and also the irreversibility of this process,
which exhibits a tendency to adsorb onto to the electrode
surface which is typical of phenol.
Fig.
4
Resonance Raman spectrum of [(Ru(bpy)2)2(BHD)]2ϩ in
The first reduction of this complex occurs at Ϫ1200 mV.
Comparison with other pyrazine containing complexes suggests
acetonitrile, excitation wavelength: (a) 488 nm, (b) 514 nm, (c) 632.8
nm; * indicates solvent bands.
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this is the pyrazine reduction as reduction of bipyridyl typic-
ally occurs around Ϫ1600 mV. Correspondingly, reductions
attributed to bpy are observed at Ϫ1653 and Ϫ1791 mV.
The electrochemistry of the dinuclear complex is also shown
in Fig. 5. Two moderately well resolved oxidations occur at
Spectroelectrochemistry
Electronic spectroscopy. In order to confirm the electro-
chemical assignments and to further classify the extent of
metal–metal coupling in the dinuclear complex, spectroelectro-
chemistry was conducted on both complexes. Fig. 6 shows
electronic spectroelectrochemistry of [Ru(bpy)2(BHD)]ϩ. On
application of 300 mV vs. Ag/Agϩ, the main absorbance feature
at 495 nm is gradually lost with concomitant grow in of a broad
new feature at 900 nm. Isosbestic points are maintained
throughout the reaction at 653, 378, 347 and 327 nm, reflecting
a clean conversion from one redox state to the next with no
intermediates forming. The feature at 495 nm, was assigned
above from resonance Raman to be the Ru() dπ–pyrazine π*
MLCT transition, eventually this feature is lost completely at
this potential confirming that it is indeed the metal site that is
being oxidized. The 900 nm feature is attributed to an LMCT
between the phenol and oxidized metal. This is assigned on
the basis that the most facile oxidation in the (oxidized) [Ru-
(bpy)2(BHD)]2ϩ complex occurs on the phenol and by com-
parison with other phenol based systems.4 Application of 1.5 V,
which exceeds the oxidation potential of the phenolate, results
in irreversible loss of the main visible features of the spectrum.
This is attributed to decomposition of the Ru()–phenoxy
radical bond which is anticipated to be very electron poor.4
E
values of 585 and 734 mV. These processes are assigned to
½
Ru()/() processes, which is confirmed by spectroelectro-
chemistry, vide infra. Like the mononuclear complex, an addi-
tional oxidative process occurs at more anodic potential, at
1390 mV for the dinuclear complex, whose reversibility is scan
rate dependent. Similarly, this is assigned to the oxidation of
the phenol. Interestingly, this feature is a one-electron process,
no evidence for oxidation of the second phenolate moiety is
found in the potential window investigated at any scan rate,
despite the symmetry of the complex (both metals at this poten-
tial are in the 3ϩ state). This suggests that the pyrazine moiety
permits communication between the phenolate moieties as the
second phenolate oxidation, must occur in excess of 100 mV
positive of the initial step, which is greater than anticipated on
the basis of purely electrostatic considerations.
The pyrazine reduction in the dinuclear complex is apparent
at Ϫ1316 mV. Surprisingly, its reversibility is also scan rate
dependent, becoming irreversible at scan rates less than 500 mV
s
Ϫ1. The origin of this behavior is currently unclear, but is
unlikely to be attributable to decomposition as the subsequent
bipyridyl reduction potentials are unaffected by the scan rate.
Reversible bipyridyl reductions appear at Ϫ1653 and
Ϫ1852 mV in the dinuclear complex.
The shifts in potential between the mononuclear and di-
nuclear complexes are particularly interesting. The first metal
oxidation in the dinuclear complex occurs at a potential that is
479 mV anodic of that of the mononuclear complex, and the
pyrazine reduction potential also shifts anodically by approx-
imately 120 mV compared with the mononuclear complex.
Anodic shifts between metal oxidation potentials in analogous
mononuclear complexes and dinuclear complexes have been
proposed to be indicative of the extent of electronic coupling in
the system.16 However, when this difference is small, between 50
and 100 mV, it may be attributed to electrostatic/coulombic
effects induced by the higher charge on the complex on binding
of a second metal site.13 The shift in oxidation potential
observed here, at 479 mV, is very large and unlikely to be
attributed to purely electrostatic effects, since the charge of
the complex only increases by one unit on addition of the
second metal due to deprotonation of the second phenol site.
Interestingly, such large shifts have been observed previously in
pyrazine bridged dinuclear systems17 and attributed to the
strong π-acceptor ability of the pyrazine. Coordination of the
second metal decreases the energy of the bridge π* orbital and
reduces the energy of the Ru t2g orbital,18 this in turn increases
the π acceptor ability of the pyrazine with respect to the metals
forcing the first metal oxidation to higher potential.19
Fig.
6
Spectroelectrochemistry of the [Ru(bpy)2(BHD)]ϩ
[Ru(bpy)2(BHD)]2ϩ conversion, 3 × 10Ϫ5 M in acetonitrile containing
0.1 M TEABF4; potential 0.3 V vs. Ag/Agϩ.
Fig. 7 shows the spectroelectrochemistry of [(Ru(bpy)2)2-
(BHD)]2ϩ in acetonitrile. Holding the potential at 0.7 V vs.
Ag/Agϩ, which corresponds to the mid-point between the two
metal oxidation waves from the CV, the intensity of the main
visible absorbance at 510 nm decreases to about 50% of its
original intensity and blue shifts slightly to 504 nm. This
behavior is consistent with the oxidation of a single metal
center. In addition, a new feature grows in at 950 nm which by
Despite the large ∆E between the metal oxidation of the
mononuclear complex and first metal oxidation of the dinuclear
complex, the ∆E between the two metals in [(Ru(bpy)2)2(BHD)]2ϩ
is quite small, at 149 mV.
A more appropriate measure of the extent of metal–metal
communication across a bridge is the comproportionation
constant, Kcom, which corresponds to the stability of the
mixed valence complex [2,3] described by eqn. (1), which may
be estimated from eqn. (2).
[2,3] ϩ [2,3] ⇔ [2,2] ϩ [3,3]
(1)
(2)
logKcom = ∆E /59 mV
½
The value of Kcom for [(Ru(bpy)2)2(BHD)]2ϩ, is estimated to
be 335, which is relatively small and consistent with a weakly
coupled class II system according to the Robin–Day
classification.20
Fig.
[(Ru(bpy)2)2(BHD)]4ϩ conversion, 1.5
containing 0.1 M TEABF4; potential 0.9 V vs. Ag/Agϩ.
7
Spectroelectrochemistry of the [(Ru(bpy)2)2(BHD)]2ϩ
10Ϫ5
in acetonitrile
×
M
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339

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comparison with the mononuclear complex, is attributed to a
phenol to Ru() transition. Spectra were recorded out to 3000
nm in the NIR to investigate the presence of any intervalence
charge transfer transition. An exceedingly weak feature cen-
tered around 2600 nm was observed just at the detection limit
of the instrument.
Increasing the applied potential to 1.1 V results in complete
loss of the MLCT and the IVCT transitions, concomitantly, the
950 nm LMCT feature increases in intensity.
LUMO is highly unusual. Resonance Raman spectroscopy of
[(Ru(bpy)2(BHD)]ϩ and [(Ru(bpy)2)2(BHD)]2ϩ and their per-
deuteriated analogues suggests that the bipyridyl participates in
the lowest energy transition in the mononuclear complex but
not in the dinuclear complex. In both instances, the absence of
Ru–N vibration and studies of comparable complexes suggests
that the metal does not participate in the lowest energy transi-
tion. Therefore, in the mononuclear complex this transition is
tentatively assigned as a phenolate to bpy ILCT and for the
dinuclear complex a phenolate to pyrazine transition.
As for the mononuclear complex, holding the potential
beyond the first phenolate oxidation results in irreversible
changes to the spectroscopy of the complex, which is attributed
The origin of this disparity is unclear, but from spectroscopic
and electrochemical data provided above it is clear that co-
ordination of a second metal has a profound effect on the elec-
tronic properties of the complex, influencing both the pyrazine
reductions and the metal oxidations.
ؒ
to decomposition at the Ru()–O bond.
Resonance Raman spectroelectrochemistry. Raman spectro-
electrochemistry was conducted for both [Ru(bpy)2(BHD)]ϩ
and [(Ru(bpy)2)2(BHD)]2ϩ by exciting at 488, 514 and 632.8 nm.
In each instance the loss of the MLCT is confirmed on oxid-
ation of the metal with loss of associated vibrational features.
For example, the Raman spectroelectrochemistry is shown for
the mononuclear complex excited at 632.8 nm in Fig. 8. The
potential was gradually moved between 0.6 and 0.75 V, corre-
sponding approximately to the potentials lying between the two
metal oxidations. As expected the ligand features are gradually
reduced in intensity on addition of increasing potential. At 0.9
V the two metals are anticipated to be completely oxidized and
correspondingly, the features associated with the ILCT are
completely lost. Resonance with the new optical transition at
950 nm, seen in the electronic spectroelectrochemistry, vide
supra, is limited at 632.8 nm. Nonetheless, some new features
are observed at 0.9 V, at 1710, 1462, 1445, 1420, 1080, 972, 880,
800, 606 and 576 cmϪ1. In addition, features associated with the
BHD ligand in the reduced complex at 1580 and 491 cmϪ1
remain evident in the oxidized spectrum. These features are,
however, weak due to poor resonance and are thought to be due
to the bridging ligand, by comparison with the Raman spec-
trum of the free H2BHD ligand (ESI†). This suggests, unsurpris-
ingly, that the new optical transition at 950 nm is due to a
phenolate to Ru() LMCT transition. We noted a similar
LMCT transition for a comparable mononuclear Ru() com-
plex on oxidation, in which the phenolate ligand was 2-(2-
hydroxyphenyl)benzoxazole.4 Again, it is important to note that
none of these new modes correspond to bipyridyl. Similar
changes are observed for the mononuclear complex excited at
632 nm, loss of bpy and BHD modes occur on oxidation of the
metal and there is some evidence for the weak appearance of
BHD features after complete metal oxidation.
Metal–metal communication
In a symmetric bimolecular system like [(Ru(bpy)2)2(BHD)]2ϩ
,
if the discrete metal oxidations occur simultaneously, then a
single two-electron step is observed and little or no electronic
communication between metal centers exists. This is referred to
as class I behaviour under the Robin and Day classification.21
Inter-metal communication, manifest in the resolution of the
metal oxidations in a symmetric system, is reflected in the mag-
nitude of the comproportionation constant, but also in particu-
lar, in the presence and characteristics of any inter-valence
charge transfer transitions.
Electrochemical studies estimate the Kc to be 335, which sug-
gests that the dinuclear complex is a weakly coupled class II
system, under the Robin and Day classification. This is further
confirmed in spectroelectrochemical experiments described
where the intervalence charge transfer band is negligible. This is
a surprising result given that the two metals are bound directly
through a phenolate bridge. Previously reported results on
phenolate bridges for Ru()–Ru() communication have
revealed that this species supports medium to very strong com-
munication. Indeed pyrazine has also demonstrated strong
intervalence communication typically ranging between class II
and III in the case of the Creutz–Taube and related com-
plexes.22 However, the Kc is comparable in this instance to that
reported for [(Ru(bpy)2Cl)2(pz)]2ϩ, where pz = pyrazine.23 As the
pyrazine is a strong π-acceptor and the phenolates strong
σ-donors it was anticipated that the bridge would provide good
communication between the mixed valence metals.
The weak coupling in this instance may be related to modu-
lation of the Ru() sites by the phenolate donor or modulation
of the bridge pryrazine by the presence of the phenolates.
Conclusions
We present novel mononuclear and dinuclear Ru() complexes
bound via a bridging ligand which contains simultaneously
strong electron donating phenolate groups and a strong
π-accepting pyrazine moiety. The results presented here reveal
some unusual behavior whereby the apparent location of the
LUMO changes between the mononuclear complex and di-
nuclear complex. The lowest energy optical transition appears
to involve the peripheral bipyridine ligand as acceptor in the
mononuclear complex, whereas this ligand is not involved in
the lowest energy optical transition in the dinuclear complex.
The origin of this difference is not clear, however, significant
changes in the electronic properties of the mononuclear com-
plex are observed on coordination of the second metal,
reflected in significant alterations in the electrochemistry of the
bridge and metals as well as changes in the optical spectroscopy.
In addition, this paper presents results on the ability of the
phenolate–pyrazine bridge to mediate electronic communication
in a dinuclear complex. In this instance, based on electrochemical
and spectroscopic evidence, [(Ru(bpy)2)2(BHD)]2ϩ is classified as
a weakly communicating class II complex.
Fig.
8
Raman spectroelectrochemistry of [Ru(bpy)2(BHD)]ϩ in
acetonitrile excited at 632.8 nm: (a) open circuit potential, (b) 0.2 V and
(c) 0.5 V vs. Ag/Agϩ.
Electronic properties of the complex
The disparity between the mononuclear and dinuclear com-
plexes in terms of location of the spectroscopically defined
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11 (a) L. R. Faulkner, M. R. Walsh, C. Xu, Contemporary
Acknowledgements
Electroanalytical Chemistry, Plenum Press, New York, 1990;
(b) C. Xu, PhD Thesis, University of Illinois at Urbana-Champaign,
1992; (c) R. J. Forster and L. R. Faulkner, J. Am. Chem. Soc., 1994,
116, 5444.
The authors acknowledge Enterprise Ireland for funding under
the operational program.
Part of this work was conducted at FOCAS, Dublin Institute
of Technology, which was funded by the Higher Education
Authority under the National Development Plan.
Mr Mick Burke, DCU, is gratefully acknowledged for NMR
spectroscopy.
T. E. K. and D. L. gratefully acknowledge Johnson Matthey
PLC for the generous loan of ruthenium trichloride. The
authors are grateful for the insightful comments provided by
the referees.
12 C. F. Huebner and K. P. Link, J. Am. Chem. Soc, 1945, 67, 99.
13 K. Kalyanasundaram and Md. K. Nazeeruddin, Inorg. Chim. Acta,
1994, 226, 213.
14 (a) C. G. Coates, T. E. Keyes, H. P. Hughes, P. M. Jayaweera,
J. J. McGarvey and J. G. Vos, J. Phys. Chem., 1998, 102, 5013;
(b) G. D. Danzer, J. A. Golus and J. R. Kincaid, J. Am. Chem. Soc.,
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15 T. E. Keyes, F. Weldon, E. Muller, P. Pechy, M. Gratzel and J. G. Vos,
J. Chem. Soc., Dalton Trans., 1995, 2705.
16 J. D. Petersen, in Supramolecular Photochemistry, ed. V. Balzani,
Reidel, Dordrecht, Netherlands, 1987, p. 135.
17 (a) R. Hage, H. E. B. Lempers, J. G. Haasnoot, J. Reedijk, F. Weldon
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18 C. H. Braunstein, A. D. Baker, T. C. Strekas and H. D. Gafney,
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19 Increasing the π-acceptor ability of the pyrazine, would be
expected to result in an anodic shift in its reduction potential on
coordination of the second metal. Surprisingly, this is not observed
here, although the oxidation potentials of the metals indicate
increasing π-accepting ability. One possible explanation for this
behavior is that despite its greater π-acceptor ability, increased
π-delocalization of electron density over Ru–O–CCC–N
metallocycles occurs in the dinuclear complex which stabilizes both
metal and bridge states.
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