Heteroleptic dipyrrin/bipyridine complexes of ruthenium(ll)

Article abstract of DOI:10.1021/ic8016497

The synthesis and characterization of a series of heteroleptic dipyrrinato/2, 2'-bipyridine complexes of ruthenium(ll) are reported. Spectroscopic analysis, including resonance Raman, indicates that the complexes are only weakly emissive and that the dipy

Full text of DOI:10.1021/ic8016497

Inorg. Chem. 2009, 48, 13-15
Heteroleptic Dipyrrin/Bipyridine Complexes of Ruthenium(II)
Serena J. Smalley, Mark R. Waterland, and Shane G. Telfer*
MacDiarmid Institute for AdVanced Materials and Nanotechnology, Institute of Fundamental
Sciences, Massey UniVersity, PriVate Bag 11222, Palmerston North, New Zealand
Received August 29, 2008
Scheme 1. Synthetic Route to Complexes [1a]+ and 1b
The synthesis and characterization of a series of heteroleptic
dipyrrinato/2,2′-bipyridine complexes of ruthenium(II) are reported.
Spectroscopic analysis, including resonance Raman, indicates that
the complexes are only weakly emissive and that the dipyrrin and
Ru f bipyridine (metal-to-ligand charge transfer) chromophores
are uncoupled.
The coordination chemistry of dipyrromethene (dipyrrin)
ligands, first studied several decades ago, is undergoing a
rapid resurgence.1,2 Meso-substituted dipyrrins are easily
accessible from arylaldehydes via condensation with pyrrole
followed by oxidation.3 They typically coordinate as monoan-
ionic dipyrrinato chelates. Various functional groups may
be incorporated on the periphery of complexes of dipyrrinato
ligands by substitution on the aryl or pyrrole rings. On
kinetically inert complexes, these functional groups can be
interconverted using standard synthetic methodologies.4
Dipyrrin ligands possess a conjugated π system, analogous
to porphyrins, which can endow their complexes with useful
optical properties including intense absorption bands in the
visible region of the spectrum and photoluminescence
(particularly BF2 complexes or BODIPYs).5 As such, dipyrrin
complexes hold promise as functional components of light-
harvesting and energy transfer systems.
ruthenium-dipyrrin complexes could conceivably combine
these properties with those of dipyrrins, they represent a very
attractive synthetic target. We herein report the first dipyrrin
complexes of ruthenium(II), namely, heteroleleptic dipyrrin/
2,2′-bipyridine (bipy) complexes.
Ligand L was synthesized from methyl 4-formylbenzoate
via a slightly modified literature procedure.6 Complex
[1a]PF6 was obtained as a green solid in good yield by
reacting equal equivalents of L and [Ru(bipy)2Cl2] in
ethylene glycol under microwave irradiation in the presence
of a base (Scheme 1). Subsequent hydrolysis of the ester
moiety in aqueous base provided complex 1b. When the
carboxyl group is deprotonated, complex 1b is neutral and
conveniently precipitates from the aqueous solution. It was
found necessary to employ microwave irradiation in the
synthesis of [1a]+, as conventional heating methods failed
to provide the desired product. The use of ethylene glycol
as a solvent resulted in a significant degree of trans-
esterification (giving [1a′]PF6), although this did not appear
to have any adverse effect on the subsequent hydrolysis step.
Complex 1b, which is green in the solid state but
Although dipyrrinato complexes of many transition metal
ions have been reported,1 ruthenium does not feature on this
list. Ruthenium(II) complexes are generally stable, diamag-
netic, and kinetically inert and have justifiably received great
attention for their unparalled photophysical properties. As
* Author to whom correspondence should be addressed. E-mail: s.telfer@
massey.ac.nz.
(1) Wood, T. E.; Thompson, A. Chem. ReV. 2007, 107, 1831–1861.
(2) For example, see: Telfer, S. G.; Wuest, J. D. Chem. Commun. 2007,
3166–3168. Garibay, S. J.; Stork, J. R.; Wang, Z.; Cohen, S. M.; Telfer,
S. G. Chem. Commun. 2007, 4881–4883. Salazar-Mendoza, D.;
Baudron, S. A.; Hosseini, M. W. Inorg. Chem. 2008, 47, 766–768.
(3) Laha, J. K.; Dhanalekshmi, S.; Taniguchi, M.; Ambroise, A.; Lindsey,
J. S. Org. Proc. Res. DeV. 2003, 7, 799–812.
(4) Bruckner, C.; Zhang, Y.; Rettig, S. J.; Dolphin, D. Inorg. Chim. Acta
1997, 263, 279–286.
(5) Ulrich, G.; Ziessel, R.; Harriman, A. Angew. Chem., Int. Ed. 2008,
47, 1184–1201. Loudet, A.; Burgess, K. Chem. ReV. 2007, 107, 4891–
4932. Stork, J. R.; Thoi, V. S.; Cohen, S. M. Inorg. Chem. 2007, 46,
11213–11223.
red-orange in solution, was characterized by 1H NMR, 13
C
NMR, ESI-MS, UV/vis spectroscopy, elemental analysis, and
X-ray crystallography. All data are consistent with the
structure depicted in Scheme 1.
The UV-visible spectrum of 1b is of note, particularly
the two distinct peaks in the visible region (Figure 1). The
(6) Rohand, T.; Dolusic, E.; Ngo, T. H.; Maes, W.; Dehaen, W. ARKIVOC
2007, 10, 307–324.
10.1021/ic8016497 CCC: $40.75  2009 American Chemical Society
Inorganic Chemistry, Vol. 48, No. 1, 2009 13
Published on Web 12/02/2008

COMMUNICATION
Figure 1. UV/vis spectra of 1b (in CH3OH), 2a (in CH3OH), and 2b (in
CH2Cl2).
intense narrow band with λmax ) 483 nm can be attributed
to a So f S1 (π-π*) transition of the dipyrrin ligand. Both
the relatively narrow bandwidth and the position of this peak,
which has been found to be relatively insensitive to the
identity of the metal center and coligands, are diagnostic of
this type of transition.7 The broader peak centered at 512
nm presumably originates from a Ru(II)-to-bipy charge
transfer (metal-to-ligand charge transfer, MLCT) transition.
This transition is red-shifted with respect to [Ru(bipy)3]2+,
where it appears at 443 nm.8 Similar shifts are seen in
analogous halide and oxalate complexes9 and can be
rationalized on the basis of anionic, weakly π-accepting
ligands raising the energies of the ruthenium(II) d orbitals.
The simple appearance of the absorption spectrum of 1b
as the superposition of a π-π* dipyrrin transition and a
MLCT transition suggests that the two chromophores are
largely uncoupled; that is, the orbitals involved in these
transitions are distinct and located in different spatial regions
of the complex.
To further probe this behavior, resonance Raman experi-
ments were conducted on complex 1b. Resonance Raman
is a useful technique for assigning electronic transitions as
vibrational modes of chromophores that resonate at the
Raman excitation wavelength are selectively enhanced.
Spectra were acquired at two excitation wavelengthss514
nm (Figure 2a) and 458 nm (Figure 2b)scorresponding to
the two bands in the above-mentioned absorption spectrum.
The observed Raman spectra are distinctly different. The
longer excitation wavelength is associated with vibrational
modes of the Ru-bipy MLCT chromophore, as evidenced
by the qualitative similarity to the resonance Raman spectrum
of [Ru(bipy)3]2+ (Table S1, Supporting Information).8 Con-
versely, the shorter excitation wavelength results in resonant
enhancement of dipyrrin vibrational modes. These observa-
tions confirm the origin of the two absorption bands and lend
Figure 2. Resonance Raman spectra of 1b in CH3OH and 2b in CH2Cl2 at
various excitation wavelengths. S denotes a solvent band, and asterisks
denote where laser lines have been removed. Tabulated data are available
in the Supporting Information.
further weight to a localized, noninteracting depiction of the
dipyrrin and MLCT chromophores.
j
[1b] · (CH3OH) · 6H2O crystallizes in the space group P1
with one enantiomer of the complex occupying the asym-
metric unit.10 As anticipated, the geometry at the ruthe-
nium(II) center is distorted octahedral (Figure 3). The
Ru-Nbipy bond lengths fall in the range 2.047(2)-2.058(2)
Å, while the Ru-Ndipyrrin bond lengths are marginally longer
(2.062(2) and 2.062(4) Å). The phenyl ring is twisted 71.5°
out of the plane of the dipyrrin chelate, while the deproto-
nated carboxylato group twists 15.0° out of the plane of the
phenyl ring. The C-O bond distances of the carboxylato
group (1.260(8) and 1.263(7) Å) are consistent with it being
deprotonated, thus rendering 1b neutral. The carboxylato
groups of neighboring complexes are linked into an infinite
network by hydrogen bonds to common H2O molecules.
Complex 2a was obtained in good yield by reacting L (2
equiv) with [Ru(dmso)4Cl2] in absolute ethanol (Scheme 2).
Purification via column chromatography on alumina afforded
a fine red-brown solid, which was orange-colored in
solution. As its monodentate dmso ligands should be readily
displaced by chelating ligands, 2a is expected to be a useful
(7) Yu, L.; Muthukumaran, K.; Sazanovich, I. V.; Kirmaier, C.; Hindin,
E.; Diers, J. R.; Boyle, P. D.; Bocian, D. F.; Holten, D.; Lindsey,
J. S. Inorg. Chem. 2003, 42, 6629–6647. Karolin, J.; Johansson, L. B.-
A.; Strandberg, L.; Ny, T. J. Am. Chem. Soc. 1994, 116, 7801–7806.
Halper, S. R.; Malachowski, M. R.; Delaney, H. M.; Cohen, S. M.
Inorg. Chem. 2004, 43, 1242–1249.
(10) Crystal data for 1b·(CH3OH)·6H2O: C37H42RuN6O9, M ) 815.83,
j
triclinic, space group P1, a ) 8.76980(10) Å, b ) 12.46720(10) Å, c
) 17.3089(2) Å, R ) 108.0700(10)°, ꢀ ) 90.1170(10)°, γ )
92.8580(10)°, V ) 1796.58(3) Å3, T ) 150(2) K, Z ) 2, 73 884
reflections measured, of which 10 474 were unique (Rint ) 0.039),
refined against 512 parameters to give R1 ) 0.0472 (I > 2σ(I)), GOF
) 1.06.
(8) Mabrouk, P. A.; Wrighton, M. S. Inorg. Chem. 1986, 25, 526–531.
(9) Klassen, D. M.; Crosby, G. A. J. Chem. Phys. 1968, 48, 1853–1858.
14 Inorganic Chemistry, Vol. 48, No. 1, 2009

COMMUNICATION
[Ru(bipy)3]2+ to 1b to 2b. The peak around 480 nm arises
from the intraligand dipyrrin (π-π*) transition.
This assignment of the absorption spectrum of 2b is
supported by resonance Raman spectroscopy. Remarkably
different spectra were observed upon excitation into the low-
energy band (λex ) 633 nm, Figure 2c) versus the band
around 450 nm (λex ) 458 nm, Figure 2d). The former
spectrum exhibits distinct parallels with the resonance Raman
spectrum of [Ru(bipy)3]2+. This implies that the absorption
band of 2b at 638 nm also involves a Ru f bipy MLCT
transition. The 458 nm spectrum of 2b is nearly identical to
that seen for 1b, strongly suggesting that the absorption bands
around 480 nm for 1b and 2b originate from similar
transitions, namely, dipyrrin π-π*. As for 1b, it appears
that the dipyrrin and MLCT chromophores in complex 2b
are largely uncoupled.
Preliminary measurements indicate that complexes
[1a]PF6, 1b, and 2b exhibit either very weak or no
fluorescence upon excitation into their dipyrrin absorption
bands (λex ) 460 nm). In the case of 2b, where the MLCT
absorption band is energetically well separated from the
dipyrrin band, excitation into the MLCT excited states also
does not lead to any emission. As observed for related
complexes, the excited states of these complexes may decay
via low-lying ligand field states.11 A full investigation of
the emissive properties of these complexes, including any
pH-dependent emission from 1b, is continuing.
Figure 3. Molecular structure of one of the enantiomers of 1b in the solid
state as determined by X-ray crystallography. Thermal ellipsoids are
displayed at the 30% level. Violet ) Ru; blue ) N; black ) C; red ) O;
gray ) H.
Scheme 2. The Synthetic Route to Complexes 2a and 2b
Ruthenium(II) complexes are of considerable interest in
the context of dyes in dye-sensitized solar cells.12 Mixed
dipyrrin/bipyridine ruthenium(II) complexes are anticipated
to be efficient dyes due to the light-harvesting capacity of
dipyrrins and metal-mediated energy transfer between dipyr-
rin-localized excited states and (lower-energy) MLCT excited
states. The general synthetic strategies reported herein should
provide a route to analogues of 1b and 2b that feature
carboxyl functional groups on the bipyridyl ligands, hence
enabling efficient binding and electron transfer pathways to
TiO2. Work toward this goal is continuing, along with a full
experimental and computational analysis of the fascinating
spectroscopic properties of this family of compounds.
intermediate for the synthesis of various bis- and tris(dipyr-
rinato) complexes of ruthenium(II). Indeed, 2a could be
cleanly converted into 2b via a direct reaction with 2,2′-
bipyridine at elevated temperatures. Complex 2b was isolated
Supporting Information Available: Synthetic procedures and
analytical data for 1b, 2a, and 2b. Raman spectroscopy details
including data in tabulated form. X-ray crystallographic data for
complex 1b in CIF format. This material is available free of charge
via the Internet at http://pubs.acs.org.
as a metallic green solid and characterized by 1H NMR, 13
C
NMR, ESI-MS, UV/vis spectroscopy, and elemental analysis.
The most notable feature of the absorption spectrum of
2b is the broad, intense band at 638 nm (Figure 1). We
ascribe this to a Ru f bipy (MLCT) transition on the basis
of this band being progressively red-shifted in going from
IC8016497
(11) Wacholtz, W. M.; Auerbach, R. A.; Schmehl, R. H.; Ollino, M.;
Cherry, W. R. Inorg. Chem. 1985, 24, 1758–1760.
(12) Nazeeruddin, M. K.; Gratzel, M. Comp. Coord. Chem. II 2004, 9,
719–758.
Inorganic Chemistry, Vol. 48, No. 1, 2009 15