Redox and spectroscopic orbitals in Ru(II) and Os(II) phenolate complexes

Article abstract of DOI:10.1021/ic0202561

A detailed spectroscopic and electrochemical study of a series of novel phenolate bound complexes, of general formulas [M(L-L)₂(box)](PF₆), where M is Os and Ru, L-L is 2,2-bipyridine or 2,2-biquinoline, and box is 2-(2-hydroxyphenyl)benzoxazole, is presented. The objectives of this study were to probe the origin of the LUMOs and HOMOs in these complexes, to elucidate the impact of metal and counter ligand on the electronic properties of the complex, and to identify the extent of orbital mixing in comparison with considerably more frequently studied quinoid complexes. [M(L-L)₂(box)](PF₆) complexes exhibit a rich electronic spectroscopy extending into the near infrared region and good photostability, making them potentially useful as solar sensitizers. Electrochemistry and spectroscopy indicate that the first oxidation is metal based and is associated with the M(II)/(III) redox states. A second oxidative wave, which is irreversible at slow scan rates, is associated with the phenolate ligand. The stabilities of the oxidized complexes are assessed using dynamic electrochemistry and discussed from the perspective of metal and counter ligand (LL) identity and follow the order of increasing stability [Ru(biq)₂(box)]⁺ < [Ru(bpy)₂(box)]⁺ < [Os(bpy)₂-(box)]⁺. Electronic and resonance Raman spectroscopy indicate that the lowest energy optical transition for the ruthenium complexes is a phenolate (π) to L-L (π*) interligand charge-transfer transition (ILCT) suggesting the HOMO is phenolate based whereas electrochemical data suggest that the HOMO is metal based. This unusual lack of correlation between redox and spectroscopically assigned orbitals is discussed in terms of metal-ligand orbital mixing which appears to be most significant in the biquinoline based complex.

Full text of DOI:10.1021/ic0202561

Inorg. Chem. 2002, 41, 5721−5732
Redox and Spectroscopic Orbitals in Ru(II) and Os(II) Phenolate
Complexes
,†
Tia E. Keyes,* Deirdre Leane,† Robert J. Forster,‡ Colin G. Coates,§ John J. McGarvey,§
,‡
Mark N. Nieuwenhuyzen,§ Egbert Figgemeier,| and Johannes G. Vos*
School of Chemistry, Dublin Institute of Technology, Dublin 8, Ireland, School of Chemical
Sciences, Dublin City UniVersity, GlasneVin, Dublin 9, Ireland, School of Chemistry, The Queens
UniVersity of Belfast, Belfast, Northern Ireland, and Department of Physical Chemistry, Uppsala
UniVersity, P.O. Box 532, S-751 21 Uppsala, Sweden
Received April 2, 2002
A detailed spectroscopic and electrochemical study of a series of novel phenolate bound complexes, of general
formulas [M(L-L)2(box)](PF ), where M is Os and Ru, L-L is 2,2-bipyridine or 2,2-biquinoline, and box is 2-(2-
6
hydroxyphenyl)benzoxazole, is presented. The objectives of this study were to probe the origin of the LUMOs and
HOMOs in these complexes, to elucidate the impact of metal and counter ligand on the electronic properties of the
complex, and to identify the extent of orbital mixing in comparison with considerably more frequently studied quinoid
complexes. [M(L-L)2(box)](PF ) complexes exhibit a rich electronic spectroscopy extending into the near infrared
6
region and good photostability, making them potentially useful as solar sensitizers. Electrochemistry and spectroscopy
indicate that the first oxidation is metal based and is associated with the M(II)/(III) redox states. A second oxidative
wave, which is irreversible at slow scan rates, is associated with the phenolate ligand. The stabilities of the oxidized
complexes are assessed using dynamic electrochemistry and discussed from the perspective of metal and counter
ligand (LL) identity and follow the order of increasing stability [Ru(biq)2(box)]+ < [Ru(bpy)2(box)]+ < [Os(bpy)2-
(box)]+. Electronic and resonance Raman spectroscopy indicate that the lowest energy optical transition for the
ruthenium complexes is a phenolate (π) to L-L (π*) interligand charge-transfer transition (ILCT) suggesting the
HOMO is phenolate based whereas electrochemical data suggest that the HOMO is metal based. This unusual
lack of correlation between redox and spectroscopically assigned orbitals is discussed in terms of metal−ligand
orbital mixing which appears to be most significant in the biquinoline based complex.
Introduction
catechols or hydroquinones, have been shown to possess
strong charge-transfer transitions in the visible and NIR
spectral regions. Typically, in such complexes, the higher
energy visible region, 400-500 nm, is dominated by bands
associated with MLCT Ru(II)-bpy(π*) transitions, whereas
at lower energy, i.e., >500 nm, transitions are ascribed to
considerably less common interligand charge-transfer, ILCT.1
There have, however, been comparably few reports of such
complexes in which the coordinated O is donated from a
phenolate.2,3 Like the quinoids, these are anticipated to be
noninnocent ligands capable of coordinating via an anionic
oxygen donor in their reduced state. In general, the oxidation
The application of metal complexes as photo- and elec-
trochromic materials requires that electronic spectral features
lie within useful, often visible ranges. The same requirement
is made for application of such species in solar energy
devices where the objective is optimal superimposition of
the electronic spectra with the solar energy flux.
There has been significant interest in Ru/Os-O- coordi-
nated quinoid ligands as a consequence of their noninnocent
redox behavior and rich visible-NIR spectroscopy. Ru(bpy)2
based complexes bound to O- bearing ligands, such as
* To whom correspondence should be addressed. E-mail: Tia.Keyes@
dcu.ie (T.E.K.).
(1) Keyes, T. E.; Jayaweera, P. M.; McGarvey, J. J.; Vos, J. G. J. Chem.
Soc., Dalton Trans. 1997, 1627.
(2) Basuli, F.; Peng, S. M.; Bhattacharya, S. Polyhedron 1998 18, 391.
(3) Dutta, S.; Peng, S. M.; Bhattacharya, S. J. Chem. Soc., Dalton Trans.
2000, 4623.
† Dublin Institute of Technology.
‡ Dublin City University.
§ The Queens University of Belfast.
| Uppsala University.
10.1021/ic0202561 CCC: $22.00 © 2002 American Chemical Society
Published on Web 10/05/2002
Inorganic Chemistry, Vol. 41, No. 22, 2002 5721

Keyes et al.
Scheme 1. Structure of box Ligand Illustrating Numbering Scheme
Absorption spectra were measured using a Shimadzu 3500 UV-
vis-NIR spectrophotometer. Spectra were fitted to Gaussian curves
using standard iterative algorithms.8
1
for H NMR Data
Electrochemical studies were conducted in HPLC grade solvents,
dried over molecular sieves, 3 Å; 0.1 M tetraethylammonium
tetrafluoroborate (TeaBF4) was used as the supporting electrolyte.
Cyclic voltammetry was carried out using a CH instruments
model 660 electrochemical workstation. Cells were of conventional
design and were thermostated to within (0.2 °C using a Julabo
F10-HC refrigerated circulating bath. All potentials are quoted with
respect to a BAS Ag/AgCl organic reference electrode. Under the
conditions employed, the oxidation potential of ferrocene was
+0.312 V versus Ag/AgCl. All solutions were degassed using
nitrogen, and a blanket of nitrogen was maintained 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.9 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 deionized water for at least 5
min. The polished electrodes were electrochemically cleaned by
cycling in 0.1 M HClO4 between potential limits chosen first to
oxidize and then to reduce the surface of the platinum electrode.
Finally, the electrode was cycled between -0.300 and 0.700 V in
0.1 M NaClO4 until hydrogen desorption was complete.
potentials of phenols are anticipated to be higher than those
of their quinoid counterparts.
We are interested in phenolate donors for three reasons:
First, as strong σ-donors, they are capable of increasing the
photochemical stability of Ru(II) complexes while simulta-
neously extending the visible absorbance range of the
complex through ILCT transitions. Second, significant
research has been dedicated to catecholate and quinone based
Ru and Os O-O and O-N bound complexes. However, the
influence of the reduced ligand on the metal redox levels is
not apparent in these systems because the ligand oxidation
is more facile than the metal. The higher oxidation potentials
of the phenolate described here allows us to directly study
the M-O- behavior from the perspective of the metal. Such
studies provide us with the opportunity of investigating
directly the redox behavior of the metal when bound to
anionic oxygen. Finally, a feature of O bound catecholate
and quinoid ligands seems to be significant metal-ligand
orbital mixing. We were interested in investigating if this
behavior is limited to quinoid ligands or typical of aryl O-
donor ligands in general.
In this paper, we report on three complexes, [Ru(bpy)2-
(box)]+, [Os(bpy)2(box)]+, and [Ru(biq)2(box)]+, where bpy
is 2,2-bipyridine, biq is 2,2-biquinoline, and box is 2-(2-
hydroxyphenyl)benzoxazole (Scheme 1). The use of bipyridyl
and biquinoline derivatives allows the influence of the
identity of both the metal and counter ligand on optical
transitions and electrochemistry of these phenolate based
complexes to be addressed. We provide a detailed assignment
of the electrochemistry and optical spectroscopic properties
of these complexes on the basis of potential controlled
electronic and resonance Raman spectroscopy. We discuss
the influence of acceptor ligands and metal on optical
transitions and illustrate how, by judicious choice of ligand,
we can manipulate the spectral region over which the
transition will occur. We illustrate and discuss an interesting
lack of agreement between redox assigned and spectroscopi-
cally assigned location of the HOMO on the bipyridyl based
complex and ascribe this behavior to strong ligand-metal
orbital mixing.
Spectroelectrochemistry was carried out using a homemade 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 previously.
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.
Resonance Raman experiments were recorded using an Ar+ laser
as the excitation source in the region 355 to 528 nm and a Ti-
sapphire laser pumped by an Ar+ laser for experiments carried out
at wavelengths beyond 700 nm. A backscattering geometry was
employed for the spectral accumulations using a liquid nitrogen
cooled CCD multichannel detector coupled to a single stage
monochromator (JY640HD). A holographic notch filter was used
at the entrance slit to minimize Rayleigh scattering. Alternatively,
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 nm) 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 cm-1 per pixel was achieved using a
grating of 1800 lines/mm. The applied potential was controlled with
respect to an SCE reference electrode using a CH instruments model
(5) Sullivan, B. P.; Salmon, D. J.; Meyer, T. J. Inorg. Chem. 1978, 17,
3334.
Experimental Section
(6) Lay, P. A.; Sargeson, A. M.; Taube, H.; Chou, M. H.; Creutz, C. Inorg.
Synth. 1986, 24.
(7) Keyes, T. E.; Vos, J. G.; Kolnaar, J. A.; Haasnoot, J.; Reedijk, J.;
Hage, R. Inorg. Chim. Acta 1996, 237, 245.
(8) Diamond, D.; Hanratty, V. Excel for Chemists; John Wiley & Sons:
VCH, 1997.
(9) (a) Faulkner, L. R.; Walsh, M. R.; Xu, C. Contemporary Electroana-
lytical Chemistry; Plenum Press: New York, 1990. (b) Xu, C. Ph.D.
Thesis, University of Illinois at Urbana-Champaign, 1992. (c) Forster,
R. J.; Faulkner, L. R. J. Am. Chem. Soc. 1994, 116, 5444.
Materials. d8-bpy was synthesized according to published
methods.4 Dichloride complexes, cis-Ru(bpy)2Cl2, Ru(d8-bpy)2Cl2,
Os(bpy)2Cl2, Ru(d8-bpy)2Cl2, Ru(biq)2Cl2, were synthesized by
standard methods.5-7 All other reagents, purchased from Sigma-
Aldrich, were used as received.
(4) Keyes, T. E.; Weldon, F.; Muller; Pechy, E. P.; Gratzel, M.; Vos, J.
G. J. Chem. Soc., Dalton Trans. 1995, 2705.
5722 Inorganic Chemistry, Vol. 41, No. 22, 2002

Ru(II) and Os(II) Phenolate Complexes
600A potentiostat or oxidation was achieved chemically using
cerium sulfate.
Complex Synthesis. [Ru(bpy)2(box)](PF6)‚(CH3CN). [Ru-
(bpy)2Cl2]‚2H2O (0.676 g, 1.3 mmol) was dissolved in ethanol/
water (1:1 v/v, 40 cm3) containing diethylamine (2%, v/v). This
solution was deoxygenated with argon and heated, and box
dissolved (0.12 g, 0.6 mmol) in ethanol/water (1:1 v/v, 40 cm3)
(2% DEA) was added slowly over 20 min. The mixture was heated
under reflux in an argon atmosphere for 4 h after which time the
burgundy solution was reduced in volume to approximately 10 cm3
and neutralized with sulfuric acid. After neutralization, the solution
was allowed to stand for several hours and then filtered to remove
any salt formed. Several drops of saturated aqueous NH4PF6 were
subsequently added to the solution, and the resulting burgundy
crystals were collected by filtration. The product was recrystallized
twice from acetonitrile/water 3:2 v/v. Yield 65%. Anal. Calcd for
[Ru(bpy)2(box)](PF6)‚CH3CN, RuC35H28N6O2PF6: C, 51.66; H,
Figure 1. Crystal Structure of [Ru(bpy)2(box)](PF6) showing numbering
scheme and counterion.
1
3.44; N, 10.32. Found: C, 51.58; H, 3.47; N, 10.02. H NMR of
box related resonances (400 MHz, CD3CN): H3, H4, 7.13 (m 2H);
H5, H6, H7, 7.39 (m, 3 H); H1, H2, 7.75 (m, 2H); H8, 7.93 (d, 1H).
[Ru(d8-bpy)2(box)](PF6). [Ru(bpy)2(box)](PF6) was synthesized
and purified as described for [Ru(bpy)2(box)](PF6) starting with
Ru(d8-bpy)2Cl2; the complex was obtained as a dark burgundy
powder. Yield 68%. H NMR box related resonances (400 MHz,
CD3CN): H3, H4, 7.10 (m 2H); H5, H6, H7, 7.39 (m, 3 H); H1, H2,
7.73 (m, 2H); H8, 7.93, (d, 1H).
[Os(bpy)2(box)](PF6). [Os(bpy)2(box)](PF6) was synthesized and
purified as described for [Ru(bpy)2(box)](PF6) starting with Os(bpy)2-
Cl2, except the reflux was conducted over 8 h; the complex was
obtained as a dark purple-brown powder. Yield 60%. Anal. Calcd
for OsC33H25N5O2PF6: C, 46.04; H, 2.91; N, 8.13. Found: C, 45.38;
H, 3.11; N, 8.21.
1H NMR box related resonances (400 MHz, CD3CN): H3, H4,
7.03 (m 2H); H5, H6, H7, 7.39 (m, 3 H); H1, H2, 7.71 (m, 2H); H8,
7.93 (d, 1H).
for data collection, and Lorentz and polarization corrections were
applied. An empirical absorption correction using ψ scans was
applied to [Ru(bpy)2(box)](PF6).
The structure was solved using direct methods with the SHELX-
TLPC and SHELXL-93 program packages,11 and the non-hydrogen
atoms were refined with anisotropic thermal parameters. Hydrogen
atom positions were located from difference Fourier maps, and a
riding model with fixed thermal parameters (Uij ) 1.2Ueq for the
atom to which they are bonded) was used for subsequent refine-
1
2
2
ments. The function minimized was ∑[w(|Fo| - |Fc| )] with
2
reflection weights w-1 ) [σ2 |Fo| + (g1P)2 + (g2P)] where P )
2
2
[max|Fo| + 2|Fc| ]/3.
Crystal data for [Ru(bpy)2(box)](PF6)‚(CH3CN): MW ) 809.67,
monoclinic, space group P21/c, a ) 8.959(2) Å, b ) 13.930(2) Å,
c ) 26.737(5) Å, â ) 92.07(2)°, U ) 3334.6 (11) Å-3, Z ) 4, µ
) 0.595 mm-1, Rint ) 0.0155 A total of 6261 reflections were
measured for the angle range 4 < 2θ < 50, and 5860 independent
reflections were used in the refinement, transmission range (max,
min) ) 0.9712, 0.8935. The final parameters were wR2 ) 0.0724
and R1 ) 0.0291 [I > 2σI].
[Os(d8-bpy)2(box)](PF6). [Os(bpy)2(box)](PF6) was synthesized
and purified as described for [Os(bpy)2(box)](PF6) starting with
Os(d8-bpy)2Cl2; the complex was obtained as a dark purple-brown
Semiempirical calculations were performed using the program
package ZINDO within the INDO/S-CI framework, which had been
parametrized to reproduce spectroscopic properties of ruthenium
complexes.12 The geometries for [Ru(bpy)2(box)]+ were taken from
the crystal coordinates, and only single point calculations were
carried out, whereas for [Ru(biq)2(box)]+ molecular mechanics
geometry optimizations were conducted prior to calculation.
1
powder, yield, 58%. H NMR box related resonances (400 MHz,
CD3CN): H3, H4, 7.06 (m 2H); H5, H6, H7, 7.40 (m, 3 H); H1, H2,
7.71 (m, 2H); H8, 7.92 (d, 1H).
[Ru(biq)2(box)](PF6). [Ru(biq)2(box)](PF6) was synthesized as
described for [Ru(bpy)2(box)](PF6) starting with Ru(biq)2Cl2. The
complex was obtained as a purple powder. Yield 60%. Calcd for
RuC49H33N5O2PF6: C, 61.5; H, 3.51; N, 7.22. Found: C, 60.8; H,
1
3.47; N, 7.96. H NMR (400 MHz) box related resonances (400
Results and Discussion
MHz, CD3CN): H3, 6.58 (d 1H); H5, H6, H7, 7.28 (m, 3 H); H1,
H2, H4, 7.58 (m, 2H); H8, 8.14, (d, 1H).
Structural Characterization. The complexes were syn-
thesized according to standard techniques; addition of base
was necessary to deprotonate the hydroxyl group and render
it a better coordination site. Complexes were obtained in high
yield and were found to be greater than 98% pure on HPLC
after two recrystallizations. The structure of the [Ru(bpy)2-
(box)]PF6 complex was determined by X-ray crystallography
and is shown in Figure 1. The asymmetric unit contains one
[Ru(bpy)2(box)](PF6) ion pair and a molecule of acetonitrile.
The box anion is coordinated to ruthenium as a bidentate
N,O-donor forming a five-membered chelate ring, and the
Crystal Structure Determination. Crystals of [Ru(bpy)2(box)]-
(PF6) were grown by slow evaporation of an acetonitrile/water
solution, 2:1 v/v. Data were collected on a (0.54 × 0.21 × 0.18
mm3) crystal.
Crystallographic data were collected on a Siemens P4 diffrac-
tometer using the XSCANS10 software with graphite monochro-
mated Mo KR radiation using ω scans. A crystal was mounted onto
the diffractometer at low temperature under dinitrogen at ca. 120
K. Crystal stability was monitored by measuring standard reflections
every 100 reflections, and there were no significant variations
(<(1%). Cell parameters were obtained from 35 accurately
centered reflections in the 2θ range 10-25°. ω scans were employed
(11) Sheldrick, G. M. SHELXLPC, A System for Structure Solution and
Refinement; Bruker-AXS: Madison, WI, 1998.
(10) Fait, J. XSCANS, System for Data Collection; Bruker-AXS: Madison,
(12) Rensmo, H.; Lunell, S.; Siegbahn, H. J. Photochem. Photobiol., A
1998, 114, 117.
WI, 1993.
Inorganic Chemistry, Vol. 41, No. 22, 2002 5723

Keyes et al.
Table 1. Selected Bond Distances (Å) and Angles (deg) for
[Ru(bpy)2(box)](PF6)‚CH3CN
Ru(1)-N(12B)
Ru(1)-N(12A)
Ru(1)-N(1B)
2.031(2) Ru(1)-N(1A)
2.041(2) Ru(1)-O(28)
2.046(2) Ru(1)-N(13)
2.048(2)
2.0569(18)
2.093(2)
N(12A)-Ru(1)-N(13) 171.95(8) O(28)-Ru(1)-N(13)
89.30(8)
N(1B)-Ru(1)-N(13)
N(1A)-Ru(1)-N(13) 94.87(8) N(12B)-Ru(1)-N(1B) 79.20(8)
87.21(8) N(12B)-Ru(1)-N(12A) 88.65(8)
N(12A)-Ru(1)-N(1B) 99.19(8) N(12B)-Ru(1)-N(1A) 99.21(8)
N(12A)-Ru(1)-N(1A) 78.85(8) N(1B)-Ru(1)-N(1A) 177.54(8)
N(12B)-Ru(1)-O(28) 170.48(8) N(12A)-Ru(1)-O(28)
85.37(8)
86.90(8)
N(1B)-Ru(1)-O(28)
N(12B)-Ru(1)-N(13)
94.43(8) N(1A)-Ru(1)-O(28)
97.39(8)
RuN5O coordination sphere is distorted octahedral, typical
of these bidentate ligands, Table 1. The [Ru(bpy)2(box)]
cations form an extended array via a combination of
C-H‚‚‚π and π‚‚‚π interactions and C-H‚‚‚O hydrogen
bonds. This leads to the formation of channels along the a
axis in which the anions and solvent molecules are located,
Figure 2.
In [Ru(bpy)2(box)](PF6), the PF6 anions within the chan-
nels are involved in a significant number of short contacts
(2.3-2.9 Å) with the C-H moieties of the [Ru(bpy)2(box)]
cations and the methyl groups of the acetonitrile molecules.
This is consistent with the formation of C-H‚‚‚F hydrogen
bonds between the cations and anions and in agreement with
a database study on organometallic compounds containing
Figure 2. View down the a axis showing the channels formed by the
cations in 1. The anions and solvent molecules have been removed for
clarity.
-
fluorinated anions by Bragga et al.13 showing that both BF4
-
and PF6 anions can act as conventional hydrogen bond
acceptors. They concluded that C-H‚‚‚F hydrogen bonds
to BF4- and PF6- anions are significant for the formation of
many crystalline materials.
This structural configuration, where anions exist within
channels in the crystal, is particularly interesting from the
perspective of the solid state behavior of similar materials.
We recently reported on the solid state electrochemistry of
O,N coordinated Ru and Os complexes bound to hydro-
quinone; the crystallography of these materials was not
available, but it was speculated that such a porous structure
might account for the excellent conductivity of these materi-
als in their solid states.14
Figure 3. UV-vis absorption spectra of [M(L-L)2(box)]+ complexes in
acetonitrile solution.
cm-1), and a weaker shoulder, which from Gaussian fitting,8
appears to be centered at 550 nm (ꢀ ) 1150 M-1 cm-1);
this broader band extends to nearly 750 nm. The main
absorbance feature for [Os(bpy)2(box)]+ is at 520 nm (ꢀ )
9244 M-1 cm-1); this band is also broadened and would
appear to overlay the same broad feature observed in the
analogous Ru complex. A broad, non-Gaussian absorbance
with a λmax of 720 nm (ꢀ ) 3031 M-1 cm-1) is also observed
The complexes are diamagnetic, and accordingly, sharp,
1
well defined H NMR resonances were observed for all
complexes, consistent with the presence of both M(II) and
the phenolate as reduced species. A complete assignment of
all box based resonances could be made by employing a
combination of COSY 1H NMR experiments and deuteriation
of bipyridine;4 these are presented in the synthetic section.
Identification of individual biquinoline and bipyridine was
not undertaken; however, integration of NMR spectra was
correct for each complex.
Spectroscopy. Electronic Spectroscopy. All three com-
plexes reveal strong CT transitions across the entire visible
region, as shown in Figure 3. [Ru(bpy)2(box)]+ exhibits a
strong absorbance with a λmax of 488 nm (ꢀ ) 8695 M-1
3
and is most likely associated with the MLCT (Os to bpy
π*) transition rendered allowed by spin-orbit coupling of
the heavy Os atom.15 [Ru(biq)2(box)]+ exhibits intense visible
absorbances which extend beyond 800 nm. The main feature,
with a λmax of 600 nm (ꢀ ) 3031 M-1 cm-1), is superimposed
on a broad underlying shoulder identified from spectral fitting
as centered at 670 nm (ꢀ ) 1583 M-1 cm-1).
Raman Spectroscopy. Raman spectroscopy was carried
out to elucidate the origin of the optical transitions, in each
(13) Grepioni, F.; Cojazzi, G.; Draper, S. M.; Scully, N.; Braga, D.
Organometallics 1998, 17, 296.
(14) Bond, A. M.; Marken, F.; Williams, C. T.; Beattie, D. A.; Keyes, T.
E.; Forster, R. J.; Vos, J. G. J. Phys. Chem. B 2000, 104, 1977.
(15) Hupp, J. T.; Neyhart, G. A.; Meyer, T. J. J. Am. Chem. Soc. 1986,
108, 5349.
5724 Inorganic Chemistry, Vol. 41, No. 22, 2002

Ru(II) and Os(II) Phenolate Complexes
Such an effect has been reported previously for hydroquinone
based complexes.1
The Raman spectrum of [Ru(bpy)2(box)]+ excited at 632.8
nm is shown in Figure 4a. The 632.8 nm excitation is
resonant with the shoulder centered at 550 nm, but at this
wavelength, any contribution from the MLCT is anticipated
to be minimal. Surprisingly, however, vibrations attributable
to both bipyridine and box moieties are evident. The feature
at 1526 cm-1 associated with the phenolate ligand observed
at 457.9 nm excitation is present, although at this excitation
wavelength there can be no resonance with intraligand
transitions. Furthermore, bands at 1435, 1358, 1265, and
1249 cm-1, and features between 962 and 490 cm-1, are
enhanced at this wavelength. These modes are unaffected
by deuteriation of the bipyridine ligands and are therefore
attributed to box. The Ru-N feature apparent at 377 cm-1
with 457.9 nm excitation is now barely evident. This strongly
suggests that the longest wavelength absorbance in this
complex is associated with an interligand charge-transfer
transition involving the phenolate and polypyridyl units. Such
behavior has been described previously in hydroquinone
bound Ru(bpy) units.1,17,18 For [Os(bpy)2(box)]+, similar
arguments may be made: 457.9 nm excitation leads to
enhancement of the Os-N, bpy, and box modes. Excitation
at 632.8 nm, however, leads to enhancement of broadly the
same features; however, in this instance, the Os-N vibration
remains. This is not surprising because 632.8 nm is also
3
resonant with the low energy Os(dπ)-bpy (π*) transition.
Figure 4b shows the Raman spectrum for [Ru(biq)2(box)]+
excited at 514 nm. Features at 1599, 1494, 1458, 1374, 1320,
and and 772 cm-1 are typically associated with the biquino-
line ligands,19 and the band at 372 cm-1 is attributed to the
Ru-N biquinoline vibration. This confirms that the electronic
band centered at 600 nm is Ru (dπ)-biq (π*) MLCT. Bands
at 1526 and 667 cm-1 observed in Figure 4 (inset), associated
with box excitation, are not evident here. This is because
this wavelength is too low in energy to be resonant with
any intraligand box based transitions. In contrast, the Raman
spectrum excited at 457.9 nm20 reveals only box transitions,
with no evidence for biq or metal based transitions. The 457.9
nm excitation is resonant with the box π-π* transition but
not with MLCT transitions which occur at lower energy for
the biquinoline based complex. This confirms that bands
centered around 1609, 1525, 1461, 1433, 1367, 1347, and
1312 cm-1 are attributable to the ligand.
Excitation at 632.8 nm is resonant with both the long
wavelength shoulder of [Ru(biq)2(box)]+ centered at 670 nm
and the MLCT at 600 nm. This excitation results in
enhancement of vibrational features at 1599, 1506, 1458,
1373, 1320, and 772 cm-1 associated with biquinoline but
also strong enhancement of box features at 1526 and 670
cm-1 and weaker box features at 871, 807, and 629 cm-1.
The M-L vibrations observed at 514 nm excitation are of
Figure 4. Resonance Raman of (a) [Ru(bpy)2(box)]+ excited at 632.8 nm
in dichloromethane and (b) [Ru(biq)2(box)]+ excited at 514 nm in
dichloromethane. Inset: [Ru(bpy)2(box)]+ excited at 457.9 nm in acetoni-
trile.
instance employing excitation wavelengths that were coin-
cident with the absorbances under interrogation. Under such
conditions, the Franck-Condon modes of the chromophore
are resonantly enhanced by up to 7 orders of magnitude. This
provides a valuable means of identifying the origin of optical
transitions.
Figure 4 (inset) shows the Raman spectrum of [Ru(bpy)2-
(box)]+ using 457.9 nm excitation which is resonant with
the 488 nm absorbance of this complex. The spectrum
exhibits bands typically expected for resonance with a Ru
(dπ)-bpy (π*) MLCT transition with enhancement of
bipyridyl vibrations, at 1605, 1558, 1488, 1320, 1170, and
1023 cm-1, and a Ru-N vibration is also observed at 377
cm-1.16 The relatively strong feature at 1526 cm-1 and
features at approximately 1475 and 1600 cm-1, obscured by
the bpy features at 1605 and 1558 cm-1, are attributed to
the box ligand because they are insensitive to deuteriation
of the bipyridyl moieties. Furthermore, these bands are not
observed in the resonance Raman spectrum for typical MLCT
transitions of [Ru(bpy)3]2+. The appearance of box vibrations
is attributed to the fact that the 457.9 nm excitation falls
within the tail of a box based intraligand π-π* transition.
(17) Hartl, F.; Snoeck, T. L.; Stufkens, D. J.; Lever, A. B. P. Inorg. Chem.
1995, 34, 3887.
(18) Keyes, T. E.; Forster R. J.; Jayaweera, P. M.; Vos, J. G.; McGarvey,
J. J. Inorg Chem. 1998, 22, 5925.
(16) Strekas, T. C.; Gafney, H. D.; Tysoe, S. A.; Thummel, R. P.; Lefoulon,
(19) Akyuz, S.; Akyuz, T.; Davis, J. E. D. Vib. Spectrosc. 2000, 22, 11.
(20) See Supporting Information.
F. Inorg. Chem. 1989, 28, 2964.
Inorganic Chemistry, Vol. 41, No. 22, 2002 5725

Keyes et al.
Table 2. Electrochemical Data
E1/2
M(II)/(III)
a
complex
solvent
E
ox phenol
E1/2 L-L
kf (s-1
)
kb (s-1
)
Keq
box (free ligand)
[Ru(bpy)2(box)]+
[Ru(bpy)2(box)]+
[Os(bpy)2(box)]+
[Os(bpy)2(box)]+
[Ru(biq)2(box)]+
[Ru(biq)2(box)]+
methanolb
acetonitrile
methanol
acetonitrile
methanol
0.58c
1.15c
0.28
0.23
-0.13
-0.08
0.49
-1.85, -2.11
-1.80, -2.10
-1.3, -1.60
2 × 106
9 × 102
4 × 106
1386
1500
30
1.00c
1.36c
30.1
1925
acetonitrile
methanol
1900
0.44
a Keq of M(III)-O (phenoxy) bond after second oxidation process, obtained from kf/kb. b Electrochemistry carried out in methanol containing 0.1 M
NaOH. c Irreversible.
lower relative intensity than those of the bipyridine based
complexes. Therefore, longer wavelength excitation results
in the appearance of box features indicating that the lowest
energy transitions involve both the phenolate and biquinoline
ligands.
An interesting conclusion derived from these spectroscopic
studies is that the lowest energy electronic transition is an
interligand charge transfer originating from the phenolate in
the ruthenium complexes. The lowest energy transition in
[Os(bpy)2(box)]+ appears to be the triplet MLCT transition
which is superimposed on the ILCT.
The energies of the ILCT bands are strongly dependent
on the nature of the counter ligands but are less influenced
Figure 5. Scan rate dependent cyclic voltammetry of [Ru(bpy)2(box)]+
conducted in acetonitrile containing 0.1 M tetraethylammonium tetrafluo-
roborate on a 5 µm electrode, potentials are versus Ag/AgCl electrode.
by the identity of the metal. Comparing [Ru(bpy)2(box)]+
to [Ru(biq)2(box)]+, the ILCT charge transfers are centered
at approximately 550 and 670 nm, respectively. The lower
The electrochemical data from the discussed complexes
are shown in Table 2. Figure 5 illustrates the voltammetry
of [Ru(bpy)2(box)]+, in acetonitrile at scan rates of 1 and
3000 V s-1. Two oxidations are observed for this complex,
a reversible one-electron oxidation at 0.28 V and an
irreversible one-electron process at 1.15 V. Two reversible
one-electron reductions are observed at -1.85 and -2.11 V
versus Ag/AgCl and are assigned, from consideration of
comparable materials, to the reduction of the bipyridine
moieties. The spectroelectrochemistry of the complexes
discussed is described in detail in the following section, and
suggests, surprisingly, on the basis of the spectroscopy of
the reduced complex, that the first oxidation is metal based.
We attribute the second wave to the oxidation of the phenol
site, on the basis of the spectroelectrochemistry described
vide infra and the oxidation potential of the free deprotonated
box ligand which exhibits an irreversible oxidation in
methanol at 0.58 V at a scan rate of 0.1 V s-1.
energy of the latter transition is a result of the greater
π-accepting ability of the biquinoline ligand compared with
bpy and is consistent with the more anodic reduction potential
of this site discussed vide infra.
The influence of the metal site on the ILCT is more
difficult to assess as a result of difficulty in deconvoluting
the electronic transitions from the [Os(bpy)2(box)]+ spectrum.
However, it would appear that the ILCT in the Os based
complex is similarly placed compared to that of its Ru
analogue.
Electrochemistry. Spectroscopic studies indicate that the
lowest energy singlet electronic transitions in the reduced
complexes are phenolate (π)-bpy (π*). This suggests that
the site of most facile oxidation in these complexes should
be the phenolate ligand. Such spectroscopy has been observed
in Ru(II) catecholate and hydroquinone based systems, and
indeed, the ligands are the sites of the most facile oxida-
tion.1,32
The behavior of the phenolate oxidation wave in [Ru-
(bpy)2(box)]+ is dependent on scan rate, being irreversible
at slower scan rates, and becoming reversible only at scan
rates in excess of 500 V s-1. Repetitive scans and scan rate
dependence show no indication of adsorption or other surface
phenomena common in phenols, which may be associated
with this irreversible behavior. At 1.15 V, the metal is already
(21) As the decomposition reaction is first order, t1/2 ) ln 2/kf.
(22) Goldsby, K. A.; Meyer, T. J. Inorg. Chem. 1984, 23, 3002.
(23) Hage, R.; Haasnoot, J. G.; Nieuwenhuis, H. A.; Reedijk, J.; DeRidder,
D. J. K.; Vos, J. G. J. Am. Chem. Soc. 1990, 112, 9245.
(24) Nazeeruddin, M. K.; Zakeeruddin, S. M.; Kalyanasundaram, K. J. Phys.
Chem. 1993, 97, 9607.
(25) Tang, J.; Albrecht, A. C. Raman Spectroscopy; Szynanski, H., Ed.;
Plenum Press: New York, 1970; Vol. 2.
(26) See Supporting Information.
(27) Manuscript in preparation.
(28) (a) Vogler, A.; Kunkely, H. Comments Inorg. Chem. 1990, 9, 201 (b)
Vogler, A.; Kunkely, H. Comments Inorg. Chem 1997, 19, 283.
(29) (a) Cummings, S. D.; Eisenberg, R. J. Am. Chem. Soc. 1996, 118,
1949. (b) Connick, W. B.; Gray, H. B. J. Am. Chem. Soc. 1997, 119,
11620. (c) Paw, W.; Lachicotte, R. J.; Eisenberg, R. Inorg. Chem.
1998, 37, 4141
(30) Da Silva, R. S.; Gorelsky, S. I.; Dodsworth, E. S.; Tfouni, E.; Lever,
A. B. P. J. Chem. Soc., Dalton Trans. 2000, 4078.
(31) Masui, H.; Lever, A. B. P., Dodsworth, E. S. Inorg. Chem. 1993, 32,
258.
(32) Lever, A. B. P.; Masui, H.; Metcalfe, R. A.; Stufkens, D. J.; Dodsworth,
E. S.; Auburn, P. R. Coord. Chem. ReV. 1993, 125, 317.
5726 Inorganic Chemistry, Vol. 41, No. 22, 2002

Ru(II) and Os(II) Phenolate Complexes
in the 3+ state, and irreversibility in the following oxidation
wave is attributed to the decomposition of the electron poor
Ru(III)-O (phenoxy) bond. Such behavior has been reported
for Ru(III)-O (quinone) bonds.18,1 At faster scan rates, the
phenol oxidation becomes reversible, suggesting relatively
slow decomposition of the complex, which is outrun at scan
rates in excess of 500 V s-1.
The equilibrium constant for formation of the decomposed
material provides a useful means of quantifying and compar-
ing the relative influences of counter ligand and metal on
the stability of the M(III)-O (phenoxy) bond. We have,
therefore, fitted the scan rate dependent cyclic voltammo-
grams to a mechanism in which electron transfer, E, is
followed by a chemical reaction, C. In fitting this EC
mechanism, we assume that the product of the following
reaction is not electroactive at potentials where the parent
complex is being oxidized. This fitting approach yields rate
constants for the forward and backward steps that are
consistent across a wide range of scan rates. Table 2 contains
these rate constants for the following chemical reactions and
ligands23 and is therefore unlikely to precede the phenolate
oxidation in these complexes. Second, as described, the
potential difference in the metal based oxidation waves of
[Ru(bpy)2(box)]2+ and [Os(bpy)2(box)]2+ is 410 mV com-
pared with a 150 mV difference between the second
oxidation processes. The former potential difference is typical
of that observed for other Ru/Os analogues, and if the second
wave were due to a metal process, a comparable potential
difference between Ru and Os would be anticipated.
[Ru(biq)2(box)]+ exhibits the most anodic oxidations of
the three complexes studied. The first, reversible, oxidation
wave is apparent at 0.49 V. The first reduction step for this
complex, sited at the biquinoline, is observed at -1.30 V.
The strong π-accepting ability of this ligand is reflected in
the ease with which it is reduced and is responsible for the
reduced electron density at the metal and phenolate sites
resulting in the anodic shift in the associated oxidations. The
second oxidation at 1.36 V is anodically shifted and
considerably less reversible by comparison with [Ru(bpy)2-
(box)]+ and [Os(bpy)2(box)]+, requiring scan rates in excess
of 3000 V s-1 for a reversible response. Keq for decomposi-
estimates of Keq obtained from these data whereby Keq
)
kf/kb, where kf is the forward and kb is the reverse rate
constant. Fitting the CV of [Ru(bpy)2(box)]+ to an EC
mechanism yields a decomposition half-life of approximately
5 × 10-4 s-1 and an estimate of Keq of 1500.21
tion of [Ru(biq)2(box)]2+ was determined to be 1900 (t1/2
)
3.6 × 10-4 s-1), the largest equilibrium constant for the three
complexes reflecting the lability of its oxidized state. The
assignments of the first and second redox steps in this
complex, on first examination, appear to be metal and phenol
based, respectively, as for the [M(bpy)2(box)]+ based com-
plexes. Spectroelectrochemical data, provided vide infra,
indicate strong mixing, and the formal assignment of the
sequence of oxidation steps is less clear. However, whether
it is the metal or ligand which is oxidized first, the final
oxidized product should remain the same, and the irrevers-
ibility of the second oxidative process is still attributed to
decomposition of the electron poor Ru(III)-O• bond.
The reversibility of the phenolate oxidation wave is a
useful guide to the electron density on metals and ligands in
these systems. [Os(bpy)2(box)]+ exhibits the most cathodic
oxidations on both metal and ligand sites. The phenolate
oxidation shows the greatest reversibility in this complex
and the slowest rate of decomposition when the system is
oxidized. The greater electron density available to the Os
site and its increased propensity for π-back-donation is likely
to contribute to the relative ease of oxidation of the phenolate
and the enhanced stability of the Os(III)-phenoxy radical
species. This contrasts with the [Ru(biq)2(box)]+ complex
which exhibits a large anodic shift in both oxidation
processes and significantly less reversibility in the oxidation
step. In this instance, the strong π-accepting ability of the
biquinoline moieties forces oxidations to higher potentials
and strips electron density from the Ru(III)-O, reducing the
stability of this bond.
The electrochemical data for [Os(bpy)2(box)]+ are pre-
sented in Table 2. [Os(bpy)2(box)]+ exhibits an initial
oxidation at -0.13 V versus Ag/AgCl which is in excess of
400 mV cathodic of its Ru analogue. This would appear to
confirm the metal based nature of these first oxidations. Such
shifts are typically observed across many groups of analogous
Ru and Os complexes and are associated with the higher
energy of the 5d orbitals in Os compared with the 4d orbitals
in Ru.22 Bipyridyl reductions are observed at -1.8 and -2.10
V. For the Os complex, the phenolate oxidation appears at
a potential 150 mV cathodic of the Ru analogue. This also
appears to confirm the assignment of the second oxidation
wave for these two complexes because the 150 mV difference
is less than expected between Ru and Os metal oxidations,
but it reflects the impact of metal identity on the phenol site.
The phenol oxidation exhibits greater chemical reversibility
for the osmium complex than its Ru analogue, exhibiting
full reversibility at a scan rate of 20 V s-1. Keq is estimated
to be 30, and this material has shows a relatively slow
decomposition of the Os(III)-O• bond with a half-life for
decomposition of approximately 0.023 s-1. This implies
greater M(III)-O stability in the Os complex.
There are relatively few reports of phenolate bound Ru
and Os(II) complexes in the literature. Where complexes with
comparable electrochemistry have been reported, the second
oxidation wave has been attributed to the M(IV)/(III)
couple.2,3 In the complexes discussed here, we conclude that
the second oxidation is attributed to the phenol. First, because
in basic media the deprotonated box ligand is oxidized
irreversibly at 0.58 V, and in neutral media the potential of
this irreversible wave is approximately 1.1 V. Conversely,
the M(III)/M(IV) couple is rarely observed in normal
potential windows, even for complexes bearing strong donor
Spectroelectrochemistry. To investigate the electrochemi-
cal processes further, potential controlled electronic and
resonance Raman spectroscopy was conducted. The Raman
spectroelectrochemistry of the complexes is shown in dichlo-
romethane, because this solvent interfered least with the
M-L transitions which appear between 350 and 400 cm-1
and are useful in assessing the involvement of the metal in
Inorganic Chemistry, Vol. 41, No. 22, 2002 5727

Keyes et al.
electrolysis of the complexes at their second oxidation
potentials results in irreversible decomposition.
Resonance Raman spectroscopy of the oxidized complexes
was carried out to confirm the identity of the new optical
transition at 850 nm. Figure 6b shows the Raman spectrum
of [Ru(bpy)2(box)]2+ excited at 785 nm which is resonant
with this absorption.
The spectroscopy is rich; enhanced features appear at 1601,
1551, 1523, 1470, 1426, 1331, 1310, 1264, 1235, 1158, 1132,
970, 918, 874, and 808 cm-1 which are attributed to the box
ligand C-C and ring stretch modes. Vibrations at 674, 652,
590, and 561 cm-1 are strongly enhanced and are attributed
to Ru-O bridge coupled modes; similar vibrations have been
described in other oxygen coordinated Ru complexes.1,18 The
relative intensity of these features suggests that the optical
transition results in significant distortion around the Ru-O
bond.25 Bands at 365 and 461 cm-1 are attributed to Ru-
(III)-N(box) and Ru-O(box), respectively; the shift of the
Ru-N from 372 cm-1 in [Ru(bpy)2(box)]+ to 365 cm-1 for
[Ru(bpy)2(box)]2+ is indicative of the oxidation of the metal.
Clearly, the appearances of metal-ligand and metal-ligand
bridge coupled modes indicate the metal is participating in
the electronic transition. There is, however, no evidence for
the participation of the bipyridyl moieties in this transition.
Therefore, the 850 nm band is attributed to a phenolate (π)
to Ru(III) (dπ), LMCT, transition. Finally, the Raman
spectrum of [Ru(bpy)2(box)]2+ was also investigated exciting
at 457.9 nm, where formerly the MLCT transition had been
identified in the reduced species.26 Oxidation results in
complete loss of the bpy bands associated with the MLCT
transition. However, bands associated with the box ligand
do remain, because of some degree of resonance with the
π-π transition of the ligand. This observation again confirms
the metal-based nature of the first oxidation of [Ru(bpy)2-
(box)]+. The electronic spectroelectrochemistry of [Os(bpy)2-
(box)]+ is similar to that of [Ru(bpy)2(box)]+. Holding the
potential at 0.4 V results in loss of the MLCT transition and
formation of a new electronic transition at 830 nm. Figure
6c shows the resonance Raman spectrum of [Os(d8-bpy)2-
(box)]2+ excited at 785 nm. The similarities between this
spectrum and that in Figure 6b are remarkable. Vibrations
of common origin for both complexes are observed at 562,
591, 672, 871, 975, 1019, 1132, 1158, 1235, 1263, 1316,
1330, 1429, 1474, 1555, and 1603 cm-1. The small differ-
ences that are observed correspond to the different metal
identities; for example, the M(III)-N vibration occurs at 355
cm-1 for Os compared with 365 cm-1 for Ru, and M-O
bridge coupled modes occur at 645 cm-1 for Os and 652
cm-1 for Ru. The similarities in Figure 6b,6c confirm two
points: First, the electrochemical behavior of [Ru(bpy)2-
(box)]2+ and [Os(bpy)2(box)]2+ is analogous; that is, the metal
is the site of primary oxidation. Second, the Raman spectrum
shown in Figure 6c corresponds to the complex in which
the bipyridyl units are perdeuteriated. Despite this, the spectra
in Figure 6b,c are essentially identical. If bipyridyl were
involved in the NIR transition for the oxidized complex,
shifts of up to 60 cm-1 in the bpy based vibrations would
be expected on deuteriation.18 That no shifts are observed
Figure 6. Controlled potential spectroscopy of (a) [Ru(bpy)2(box)]+
electronic spectroscopy, (b) [Ru(bpy)2(box)]+ resonance Raman spectros-
copy in dichloromethane, excited at 785 nm, and (c) resonance Raman of
[Os(d8-bpy)2(box)]+ in dichloromethane, excited at 785 nm. The applied
potential in each instance was 0.5 V vs Ag/AgCl, and the electrolyte was
0.1 M tetraethylammonium tetrafluoroborate.
optical transitions. Changing the solvent in this way does
not alter the spectra significantly.
The electronic spectroelectrochemistry of [Ru(bpy)2(box)]+
is shown in Figure 6a. Holding the applied potential at 0.4
V, the main feature at 488 nm is lost with concomitant
growth of an intense new band centered at 850 nm. A new
feature also grows in at 310 nm which is typical of Ru(III)
formation The 850 nm band is thought to be an LMCT
transition. Because the most facile oxidation for this complex
lies on the phenolate group in this instance and not on a
bipyridine, it seems likely that the transition observed is
phenolate (n) to Ru(III) (dπ*). In polypyridyl systems such
as [Ru(bpy)3]3+, the LMCT, bpy (π) to Ru (dπ*), is a very
weak transition centered at approximately 675 nm.24 The
intensities of such transitions are strongly influenced by the
reducing ability of the donor ligand. The LMCT transition
reported here is intense and red-shifted by comparison with
bipyridine based transitions.
The optical changes associated with the first oxidation step
for all complexes described are entirely reversible. Consistent
with the electrochemical studies described previously, bulk
5728 Inorganic Chemistry, Vol. 41, No. 22, 2002

Ru(II) and Os(II) Phenolate Complexes
same chromophores for each complex. Hence,the Raman
spectra, generated by excitation into these bands, would be
expected to be identical across all three compounds. Fur-
thermore, if the NIR transitions for [Ru(bpy)2(box)]2+ and
[Ru(biq)2(box)]2+ are the same, the 250 nm difference
between the band maxima seems rather large but may be
associated with the electron withdrawing nature of the
biquinoline ligand.
Enhanced vibrations are observed at 423, 537, 713, 741,
793, 848, 1268, 1346, 1570, and 1625 cm-1, for which there
are no comparable bands in the Raman spectra of [M(bpy)2-
(box)]2+. Furthermore, these bands are not ascribed to
biquinoline. Bands centered at 488, 563, 651, and 671 cm-1
are common to all three spectra and are attributed to box
and Ru-O box coupled vibrations. Similarly, bands at 916,
972, 1138, 1244, 1268, 1316, 1476, 1554, and 1605 cm-1
are tentatively attributed to C-C vibrations in the phenyl
ring of the box ligand. Given the strong similarities in the
resonance Raman spectra of the LMCT for [M(bpy)2(box)]2+,
the differences between these spectra and that of [Ru(biq)2-
(box)]2+ is unexpected and indicates that the optical transition
at 1100 nm may not be simply an LMCT transition. The
electrochemistry and Raman spectroscopy of this complex
excited at 514 nm are suggestive of metal oxidation, whereas
the vis-NIR spectroelectrochemistry and Raman spectros-
copy of [Ru(biq)2(box)]2+ excited at 810 nm suggest that
the phenol may be oxidized. These contradictory observations
may be attributed to strong mixing between phenol and
ruthenium orbitals. However, the large bandwidth of the 1100
nm transition excludes the possibility that the metal-
phenolate orbitals are entirely delocalized.
Figure 7. Controlled potential spectroscopy of (a) [Ru(biq)2(box)]+
electronic spectroscopy, (b) resonance Raman of [Ru(biq)2(box)]+ in
dichloromethane, excited at 514 nm, and (c) resonance Raman of [Ru(biq)2-
(box)]+ in dichloromethane, excited at 810 nm. The applied potential in
each instance was 0.6 V vs Ag/AgCl, and the electrolyte was 0.1 M TeaBF4.
Photochemical Stability. An issue that may be important
if these complexes are to be of utility in photovoltaic devices
is their photostability.
unambiguously confirms that the bipyridyl unit does not
participate in the this optical transition. Because the metal
is oxidized, and the phenolate is a good donor, we assign
the NIR band for M(bpy)2 containing complexes as a
phenolate (π) to M(III) (dπ), LMCT transition.
The assignments of optical transitions in [Ru(biq)2(box)]+
are somewhat less clear. The spectroelectrochemistry and
corresponding resonance Raman spectroscopy for [Ru(biq)2-
(box)]+ are shown in Figure 7. Figure 7a reveals that the
first oxidation step of this complex results in loss of the long
wavelength shoulder at 670 nm and a significant decrease
in the intensity of the transitions centered at 600 nm.
Concomitantly, an intense, broad new transition appears at
1100 nm. An isosbestic point is maintained throughout the
oxidation at 790 nm.
Photolysis studies were conducted on all three complexes
under continuous visible irradiation in the following media:
dichloromethane, dichloromethane containing 0.1 M LiCl,
acetonitrile, and acetonitrile containing 0.1 M LiCl. In all
instances, changes of less than 10% in the absorption spectra
of the complexes were observed over a 4 h period. This
photostability is most likely attributable to the fact that the
complexes have short-lived excited states27 compared with
the rate of decomposition of the complex, which electro-
chemical studies (vide supra) would indicate to be slow.
Furthermore, the strong σ-donor properties of the phenolate
may contribute to a large crystal field splitting, resulting in
3
a thermally inaccessible MC state, thus avoiding photode-
composition. Such photostability has been observed in other
Ru complexes, containing strong σ-donors such as tri-
azolates.7
Resonance Raman spectroscopy, exciting the oxidized
complex at 514 nm, reveals little change by comparison with
Figure 4b, although the relative intensity of the Ru-N
vibration at 377 cm-1 is significantly reduced. Raman spectra
were recorded using 810 nm excitation, which is resonant
with NIR absorbance of the oxidized complex at 1100 nm.
Comparison of this spectrum with the resonance Raman
spectra of [Ru(bpy)2(box)]2+ and [Os(bpy)2(box)]2+ excited
at 785 nm reveals some differences. This is surprising
because if the metal is oxidized first in [Ru(biq)2(box)]2+
the resulting optical transitions should originate from the
Comparison of Redox and Spectroscopic Orbitals.
Interligand charge-transfer transitions are relatively uncom-
mon, and it is only in recent years that they have been studied
to any significant degree.28 Observation of ILCT bands
depends on the simultaneous presence of strong donor and
acceptor ligands within the one complex. Coupling of these
sites, possibly through mediation of the metal, may result in
a charge-transfer transition. Work has focused most notably
Inorganic Chemistry, Vol. 41, No. 22, 2002 5729

Keyes et al.
Scheme 2. Schematic Illustrating Relative Energies of Metal and Ligand Orbitals of [Ru(L-L)2(box)]+ as Indicated by Electrochemical and
Spectroscopic Data
to date on complexes containing diimine and dithiolate
ligands,29 although some reports have also been made on
diimine and hydroquinone complexes. In Ru and Os O-
bound hydroquinone complexes, these transitions have been
observed at lower energy than the MLCT transition.18,30-33
In such complexes, the donating hydroquinone or catechol
ligand is oxidized at potentials that are cathodic of the
associated metal. This is not surprising because the electronic
absorption energies and ground state redox potentials have
been shown to correlate linearly across a broad range of Ru
and Os polypyridyl complexes.34-37 This strong correlation
has been attributed to the participation of the same ligand
π* orbital in the first reduction and lowest energy optical
transition. This correlation has been subsequently cited and
accepted as a useful means of predicting emission maxima
and calculating excited state redox potentials and nonradiative
decay constants for Ru and Os diimine complexes on the
basis of ground state electrochemical properties, and it has
been broadly successful in estimating such parameters.38
To our knowledge, this is the first report identifying
diimine phenolate ILCT transitions.
correlations exist, and redox and optical processes are
presumed to originate from the same orbitals. That is, in
general, the most easily oxidized ligand is identified as the
site of the LUMO, and the metal site of most facile oxidation
is the HOMO. The complexes described here appear to
provide exceptions to this general observation.
In each instance for [M(bpy)2(box)]2+, the primary oxida-
tion originates at the metal site. The ligand oxidation occurs
some 800-1000 mV positive of the metal oxidation.
However, it is important to remember that this oxidation is
artificially anodic as it corresponds to a phenolate bound to
an M(III) and not an M(II) center.
Electronic and resonance Raman spectroscopy suggests
that the lowest energy optical transition is attributable to a
phenolate (π) to bpy (π*) ILCT for each complex. Compa-
rable electronic spectroscopy has been discussed previously
for Ru and Os complexes, bound through the oxygen moiety
to hydroquinone.17,18 However, in these instances, the primary
oxidation occurred correspondingly, on the ligand.
Therefore, a clear disparity exists between what are
electrochemically and spectroscopically assigned as the
HOMO for these phenolate complexes. The origins of this
discrepancy are unclear, and to our knowledge, this is one
of the very few examples of this behavior reported. Scheme
2 provides a simple illustration of the relative orderings of
the orbitals as indicated by electrochemical and spectroscopic
data for [Ru(bpy)2(box)]2+. The diagrams are constructed
under the assumption that the bipyridine reduction potentials
are unchanged between electrochemical and spectroscopic
orbitals. On the basis of the electrochemical data, one would
expect that the MLCT would be the lowest energy transition
for these complexes. However, from spectroscopic data and
the bpy reduction potential, we estimate the phenolate
oxidation potential, when bound to Ru(II), to be ap-
proximately 0.2 V.42 As described, the potentials of the
A particularly interesting feature of the complexes dis-
cussed is that the lowest energy optical transition originates
from the phenolate whereas the first oxidation originates on
the metal. Considerable literature is available on comparative
studies of redox and spectroscopic orbitals for Ru(II) and
Os(II) complexes.39-41 In the vast majority of cases, strong
(33) Ebadi, M.; Lever, A. B. P. Inorg. Chem. 1999, 38, 467.
(34) Ernst, S.; Kaim, W. Inorg. Chem. 1989, 28, 1520.
(35) DeArmond, M. K.; Carlin, C. M. Coord. Chem. ReV. 1981, 36, 325.
(36) Vlcek, A. A. Coord. Chem. ReV. 1982, 43, 39.
(37) Dodsworth, E. S.; Lever, A. B. P. Inorg. Chem. 1990, 29, 499.
(38) Dodsworth, E. S.; Lever, A. B. P. Chem. Phys. Lett. 1986, 124, 152.
(39) Juris, A.; Barigelletti, F.; Campagna, S.; Balzani, F.; Belser, P.; von
Zelewsky, A. Coord. Chem. ReV. 1988, 84, 85.
(40) Ohsawa, Y.; Hanck, K. W.; De Armond, M. K. J. Electroanal. Chem.
Interfacial Electrochem. 1984, 175, 229.
(41) (a) Heath, G. A.; Yellowlees, L. J.; Braterman, P. S. J. Chem. Soc.,
Chem. Commun. 1981, 287. (b) Heath, G. A.; Yellowlees, L. J.;
Braterman, P. S. Chem. Phys. Lett. 1982, 92, 66.
(42) Calculated from the bipyridyl reduction, -1.85 V.
5730 Inorganic Chemistry, Vol. 41, No. 22, 2002

Ru(II) and Os(II) Phenolate Complexes
second, phenolate, oxidation step are artificially high as a
consequence of the preceding step. This suggests that the
metal and phenolate orbitals are closely matched energeti-
cally. Semiempirical calculations on [Ru(bpy)2(box)]+, de-
scribed later, also lend support to this conclusion, whereby
the HOMO appears to have both metal and ligand contribu-
tions. What determines which state the electron will originate
from under electrochemical and optical conditions is cur-
rently unclear but may be related to strong metal mediated
coupling of phenolate and bpy π*1 orbitals which facilitates
the optical transitions.
the HOMO. The box ligand orbital is polarized toward the
phenolate part. On the other hand, the LUMO is largely
dominated by the contributions coming from the bpy ligands,
which is consistent with the electrochemical results described
previously. The calculated electron density plots are shown
in the Supporting Information. Calculations based on the
ruthenium (III) complex were only partly successful, because
the spectroelectrochemical data were rather poorly repro-
duced partly because of the geometry changes accompanying
oxidation and also because of the absence of suitable
parameters for the Ru3+ ion.
The nature of the first oxidation in the [Ru(biq)2(box)]+
complex is somewhat ill-defined. Its electrochemical behav-
ior is reminiscent of a metal-based process; however,
spectroelectrochemistry (both UV-vis and resonance Ra-
man) suggest that the MLCT transition may be retained after
oxidation. The resonance Raman spectrum of the new optical
transition at 1100 nm is quite different from the phenolate
to Ru LMCT observed for [Ru(bpy)2(box)]2+ and [Os(bpy)2-
(box)]2+. The uncertain identity of the first oxidation in [Ru-
(biq)2(box)]+ is attributed to strong mixing of metal/phenolate
orbitals in this complex. The oxidation potential of the metal
in this complex is anodic by comparison with the bpy based
complexes as a result of the greater π-accepting ability of
the biquinoline. This is likely to narrow the energy gap
between the phenolate and metal based orbitals even further,
as the oxidation potential of [Ru(biq)2(box)]+ is 0.49 versus
0.58 V for the free phenolate. The phenol and metal based
orbitals are therefore likely to be very closely matched
energetically and hence strongly mixed, rendering formal
assignment of oxidation and spectroscopic processes difficult.
This level of strong mixing between metal and ligands has
been observed in other ligands containing O donors such as
quinones and catecholates, where mixing was similarly
attributed to comparable metal-ligand orbital energies.32,33
However, in such complexes, the ligand is itself most easily
oxidized and results in the generation of new MLCT
transitions.
A useful means of identifying the extent of localization
in the chromophores in such transitions is to study the solvent
dependence. The 1100 nm absorbance for [Ru(biq)2(box)]+
is only moderately solvent dependent showing a blue shift
of approximately 10 nm between acetonitrile and dichlo-
romethane. Previous reports of strongly mixing systems have
also identified relatively small solvent shifts. In such cases,
there is apparently a limited change in the dipole moment
between the ground and excited states.
Semiempirical Calculations. In an attempt to gain further
insight into the extent of metal-ligand orbital mixing in these
systems, the electronic spectrum of the complex was
calculated by semiempirical ZNDO (INDO/S-CI) techniques,
using the crystal data for the energy minimized structure.
The theoretical results corresponded reasonably well with
the experimental results. For [Ru(bpy)2(box)]+, a band at 465
nm and transitions with lower oscillator strengths above 500
nm were calculated. The electronic structure of [Ru(bpy)2-
(box)]+ in the ground state features a large contribution from
both the Ru atomic orbitals and the box ligand orbitals to
Conclusions
The synthesis and characterization of a series of Ru and
Os diimine complexes O,N bound to a 2-(2-hydroxyphenyl)-
benzoxazole are reported. Electronic spectroscopic and
resonance Raman studies indicate that the lowest singlet
optical transition is a phenolate to diimine interligand charge-
transfer transition in all cases. Electrochemistry indicates,
however, that the first oxidation involves a M(II)/M(III)
couple for [M(bpy)2(box)]+. This behavior contrasts with a
large range of studies on Ru and Os, which have shown that
the HOMO and LUMO assignments may be made from
electrochemical data; we discuss this apparent disparity in
terms of metal-ligand and ligand-ligand orbital mixing.
Oxidation of these complexes results in an intense NIR
transition ascribed to a LMCT phenolate (π)-M(III) (dπ).
The behavior for [Ru(biq)2(box)]+ is less well defined. The
lowest energy optical transition is assigned as ILCT in
character. However, the first oxidation step in this case would
appear to involve both metal and ligand, suggesting extensive
metal-phenolate orbital mixing in this complex. Oxidation
of the biquinoline complex results in an intense transition
centered at 1100 nm which appears to be different in nature
from those observed for [M(bpy)2(box)]2+. Strong metal-
phenol orbital mixing is implicated, and this seems to be
corroborated by the closer energy match between the
phenolate and metal redox levels. We employ dynamic
electrochemistry to assess the stability of the second oxida-
tion step in these complexes and find that it provides a useful
correlation with the electron donor and acceptor abilities of
both metal and ligands in the complex.
In all, this work illustrates some interesting anomalies in
relatively simple complexes, which are attributed to the
proximity of ligand and metal orbital energy levels. Such
ambiguity in electronic charge distribution is reminiscent of
the behavior of ruthenium quinoid complexes and suggests
that phenol based complexes may also show some of the
same complexity of behavior. This work illustrates how
simple synthetic alterations may be employed to tune the
spectroscopic range over which a number of intense visible
and NIR absorbances occur. These complexes have also
enabled examination of the behavior of the metal site when
bound to an O- donor, which remains reduced over the full
range of oxidation potentials of the metal.
Acknowledgment. T.E.K. and D.L. acknowledge the DIT
Scholarship scheme for funding. T.E.K. and R.J.F. acknowl-
Inorganic Chemistry, Vol. 41, No. 22, 2002 5731

Keyes et al.
Supporting Information Available: Resonance Raman spectra
of [Ru(biq)2(box)]+ excited at 457.9 nm. Semiempirical ZNDO
(INDO/S-CI) calculated electron density plots for the HOMO and
LUMOs of [Ru(bpy)2(box)]+. This material is available free of
charge via the Internet at http://pubs.acs.org.
edge the generous loan of Ru and Os salts from Johnson
Matthey. T.E.K. and D.L. acknowledge FOCAS, DIT, and
the Higher Education Authority under which FOCAS is
funded under the National Development Plan 2000 to 2006.
C.G.C. and J.J.McG. acknowledge the EPSRC for funding
under Grant GR-M45696.
IC0202561
5732 Inorganic Chemistry, Vol. 41, No. 22, 2002