Linkage and redox isomerism in ruthenium complexes of catecholate, semiquinone, and o-acylphenolate ligands derived from 1,2-dihydroxy-9,10-anthracenedione (Alizarin) and related species: Syntheses, characterizations, and photophysics

Article abstract of DOI:10.1021/ic000529x

The complexes Ru(CO)₂L₂(AL-2H) (AL = alizarin; L = PPh₃, PCyc₃, PBu₃, P(m-NaSO₃C₆H₄)₃), Ru(CO)(dppe)-(PBu₃)(AL-2H), and RuH(CO)L₂(AL-H

Full text of DOI:10.1021/ic000529x

Inorg. Chem. 2000, 39, 5807-5816
5807
Linkage and Redox Isomerism in Ruthenium Complexes of Catecholate, Semiquinone, and
o-Acylphenolate Ligands Derived from 1,2-Dihydroxy-9,10-anthracenedione (Alizarin) and
Related Species: Syntheses, Characterizations, and Photophysics
Melvyn Rowen Churchill,* Kim M. Keil, Frank V. Bright,* Siddharth Pandey,
Gary A. Baker, and Jerome B. Keister*
Department of Chemistry, University at Buffalo, the State University of New York,
Buffalo, New York 14260-3000
ReceiVed May 19, 2000
The complexes Ru(CO)2L2(AL-2H) (AL ) alizarin; L ) PPh3, PCyc3, PBu3, P(m-NaSO3C6H4)3), Ru(CO)(dppe)-
(PBu3)(AL-2H), and RuH(CO)L2(AL-H) (L ) PPh3, PCyc3), and Ru(CO)2L2(AR-2H) (AR ) anthrarobin; L )
PBu3) were prepared by reactions of Ru3(CO)12, L, and AL, and the complexes RuH(CO)(PPh3)2(AL-H), RuH-
(CO)(PPh3)2(QN-H) (QN ) quinizarin), and RuH(CO)(PPh3)2(LQN-H) (LQN ) leucoquinizarin) are prepared
by reactions of RuH2(CO)(PPh3)3 with AL or QN. The AL-2H and AR-2H ligands act as 1,2-catecholates, whereas
the AL-H, QN-H, LQN-H ligands are 1,9-o-acylphenolate ligands. RuH(CO)(PPh3)2(AL-H) is characterized by
X-ray crystallography. The electrochemistry of these complexes is examined, and the semiquinone complexes
[Ru(CO)2L2(AL-2H)]+ (L ) PPh3, PCyc3, PBu3) and [Ru(CO)(dppe)(PBu3)(AL-2H)]+ are generated by chemical
oxidation and were characterized by EPR and IR spectroscopy. The photophysical properties are also reported.
tris(m-sulfonatophenyl)phosphine (TPPTS) was provided by Dr. Laura
Francisco and Professor Jim Atwood. Other chemicals were of reagent
grade purity and were used as received.
Introduction
Metal complexes of alizarin1 have been used as dyes for
centuries. Ancient Egyptians extracted the compound from the
root of the madder plant, and the discovery of a synthetic route
in 1869 was a significant factor in the development of the
German chemical industry.2 Although the coordination chem-
istry of alizarin has received some attention, very few organo-
metallic complexes have been reported. As ligands, alizarin and
its substituted derivatives offer three features of interest to
organometallic chemistry: (1) Alizarin can exhibit linkage
isomerism, acting as a chelate via the 1,2- or the 1,9-oxygen
atoms. (2) As a 1,2-chelate, it displays ligand-based redox
chemistry. (3) The complexes are deeply (and beautifully)
colored, allowing for applications of these compounds as optical
sensor components or as molecular recognition elements. In this
paper, we present the syntheses and characterizations of
ruthenium complexes of the alizarinate ligand, demonstrating
the ability of this ligand to act as a 1,2-catecholate, a 1,2-
semiquinone, and an o-acylphenolate ligand. In a subsequent
paper, we will describe analogous organometallic complexes
of tri- and tetrahydroxy-9,10-anthracenediones.
Physical Methods of Characterizations. Infrared spectra were
recorded on a Nicolet Magna 550 spectrophotometer. 1H NMR spectra
were obtained on a Varian Associates Gemini 300 or VXR-400S
instruments. 13C NMR spectra were recorded either on the Gemini 300
or the VXR-400S instrument in deuteriochloroform and referenced to
TMS. 31P NMR spectra were recorded on the VXR-400S instrument
in deuteriochloroform, and chemical shifts are reported relative to
orthophosphoric acid. UV/visible spectra were acquired by using a
Hewlett-Packard 8452A diode array spectrophotometer. EPR spectra
were recorded on an IBM/Bruker ESP 300 X-band ESR spectrometer
in dichloromethane solution; g values and hyperfine coupling constants
were determined by spectral simulations using Bruker WINEPR
SimFonia Version 1.25 software.
Electrochemistry. Cyclic voltammetric experiments were performed
with Bioanalytical Systems (West Lafayette, IN) BAS-100W electro-
chemical analyzer. Measurements were made in accordance with
standard techniques which included purging of solutions with nitrogen
to exclude oxygen. Dichloromethane was freshly distilled from calcium
hydride and stored under nitrogen. The supporting electrolyte was 0.1
M tetrabutylammonium tetrafluoroborate, TBATFB (Kodak; also
prepared by ion exchange of tetrabutylammonium bromide with sodium
tetrafluoroborate), which had been recrystallized three times from
ethanol/water and vacuum-dried. The working electrode for experiments
at relatively low scan rates was a platinum disk electrode (diameter 3
mm). For work at higher scan rates, and for some of the steady-state
investigations, smaller platinum disks (BAS; diameters 100 and 10 µm)
were employed. A platinum wire served as the auxiliary (counter)
electrode. The concentrations of the analyte were 10-3 M unless stated
otherwise. The reference electrode used was a Ag wire, and potentials
were referenced to ferrocene or decamethylferrocene, added to the
solutions. Compensation for resistive losses (iR drop) was employed
for all measurements. All potentials are reported relative to the
ferrocene/ferrocenium couple as 0 V.
Experimental Section
Starting Materials. Dichloromethane was distilled under nitrogen
from calcium hydride before use. Alizarin (AL; 97%), quinizarin (QN),
anthrarobin (AR technical grade, 85% purity), and tributylphosphine
were purchased from Aldrich, and Ru3(CO)12 and 1,2-bis(diphenylphos-
phino)ethane (dppe) were obtained from Strem Chemical Co. Sodium
(1) Throughout this paper the following abbreviations are used: alizarin
(1,2-dihydroxy-9,10-anthracenedione), AL; anthrarobin (9,9-dihydro-
1,2-dihydroxy-10-anthracenone), AR; quinizarin (1,4-dihydroxy-9,-
10-anthracenedione), QN; leucoquinizarin (2,3-dihydro-9,10-dihydroxy-
1,4-anthracenedione), LQN. The removal of one hydroxyl proton is
indicated by “-H” and that of two hydroxyl protons as “-2H”.
(2) Fieser, L. F. J. Chem. Educ. 1930, 7, 2609.
2+
Photophysical Measurements. (a) Materials. Ru(bpy)3 and
rhodamine 6G were obtained from GFS Chemicals and Acros Organics,
10.1021/ic000529x CCC: $19.00 © 2000 American Chemical Society
Published on Web 11/17/2000

5808 Inorganic Chemistry, Vol. 39, No. 25, 2000
Churchill et al.
respectively. The solvents used were as follows: dichloromethane
(Fisher Scientific; 99.9%), toluene (Aldrich; 99.8%, anhydrous),
acetonitrile (Sigma-Aldrich; 99.9%, HPLC grade), chloroform (Fisher
Scientific; 99.9%), cyclohexane (Acros Organics; 99+%, spectropho-
tometric grade), ethanol (Pharmco, dehydrated), dimethyl sulfoxide
(Fisher Scientific; 99.9%), and doubly distilled deionized water
(Millipore).
(b) Methods. All samples were subjected to multiple freeze-pump-
thaw cycles to remove dissolved oxygen prior to data acquisition.
Absorbance measurements were carried out on a Spectronic 1201
spectrophotometer (Spectronic Instruments, Rochester, NY) with a
spectral band-pass of (0.5 nm. The scan rate was typically 200 nm/
min, and all spectra were blank-corrected.
H), 6.58 (d, J ) 8.4 Hz, 1 H), 1.71 (m, 12 H), 1.49 (br d, J ) 12 Hz),
1.35 (pent, J ) 7 Hz, 12 H), 0.87 (t, J ) 7.0 Hz, 18 H) ppm; 13C{1H}
NMR (CDCl3) 198.3 (1 C), 198.0 (1 C), 182.8 (1 C), 182.6 (1 C),
173.1 (1 C), 167.3 (1 C), 137.1 (1 C), 135.3 (1 C), 132.8 (1 C), 132.2
(1 C), 127.0 (1 C), 126.7 (1 C), 123.5 (1 C), 121.6 (1 C), 120.4 (1 C),
117.3 (1 C), 25.9 (6 C), 25.1 (6 C), 23.7 (6 C), 14.2 (6 C) ppm; 31P-
{1H} NMR (CDCl3) 20.0 (s) ppm. Anal. Calcd for C40H60O6P2Ru: C,
60.06; H, 7.56. Found: C, 59.90; H, 7.69.
Ru(CO)2(PBu3)2(AR-2H): IR (hexanes) 2025.3 s, 1959.7 s cm-1
;
1H NMR (CDCl3) 8.32 (d, J ) 8.0 Hz, 1 H), 7.58 (d, J ) 8.0 Hz, 1 H),
7.41 (m, 3 H), 6.59 (d, J ) 8.4 Hz, 1 H), 4.06 (s, 2 H), 1.68 (br, 12 H),
1.48 (br, 12 H), 1.34 (m, 12 H), 0.89 (t, J ) 7 Hz, 18 H) ppm; 13C-
{1H} NMR (CDCl3) 198.8 (1 C), 198.4 (1 C), 184.2 (1 C), 167.2 (1
C), 157.2 (1 C), 142.0 (1 C), 134.2 (1 C), 131.6 (1 C), 129.1 (1 C),
127.9 (1 C), 127.3 (1 C), 126.7 (1 C), 122.1 (1 C), 118.5 (1 C), 115.9
(1 C), 28.5 (1 C), 26.1 (6 C), 25.2 (6 C), 23.6 (6 C), 14.4 (6 C) ppm;
31P{1H} NMR (CDCl3) 19.4 (s) ppm. Anal. Calcd for C40H62O5P2Ru:
C, 61.13; H, 7.95. Found: C, 61.31; H, 7.96.
Steady-state and time-resolved fluorescence experiments were
performed with an SLM-AMINCO model 48000 MHF phase-modula-
tion spectrofluorometer (Spectronic Instruments). The instrument and
its capabilities have been described in detail elsewhere.3
Fluorescence quantum yields (Φ) were determined relative to an
optically dilute reference fluorophore solution that exhibits a well-
known fluorescence quantum yield (Φr).4,5 The quantum yield standard
(b) Ru(CO)2(PPh3)2(AL-2H) and RuH(CO)(PPh3)2(AL-H). A
solution of Ru3(CO)12 (110.0 mg, 173 µmol), PPh3 (296 mg, 1130
µmol), and alizarin (128 mg, 533 µmol) in toluene (30 mL) was heated
at reflux under an argon atmosphere for 9 h. The resulting solution
was evaporated to dryness, and the residue was applied as a dichlo-
romethane solution to silica gel preparative TLC plates. Elution with
dichloromethane gave a yellow leading band, followed by one green,
one brown, and two purple bands. Extraction of the green band with
ethyl acetate gave 77.8 mg, 16.9%, of RuH(CO)(PPh3)2(AL-H).
Extraction of the purple bottom purple band gave 197.3 mg, 41.5%, of
Ru(CO)2(PPh3)2(AL-2H).
2+
used in this study was a degassed aqueous Ru(bpy)3 solution (Φ )
0.042 ( 0.002 at 25 °C).6 Because there are characteristic MLCT
absorbance bands that overlap the fluorescence, we corrected all
emission profiles for secondary inner-filter effects.7,8
Magic angle polarization conditions were used for all excited-state
intensity decay kinetic experiments to eliminate bias stemming from
fluorophore rotational reorientation. Rhodamine 6G dissolved in water
was used as the reference lifetime standard; its lifetime was assigned
a value of 3.85 ns.9 The excited-state fluorescence lifetime, τ, was
recovered from the phase-modulation data by using a nonlinear least-
squares software package purchased from Globals Unlimited (Urbana,
IL). In all data analyses, we used the true uncertainty in each datum as
the frequency weighting factor. Further details on phase-modulation
fluorescence can be found elsewhere.10
For all results reported here, the excited-state fluorescence lifetimes
were rigorously single exponential.
For any fluorophore, the radiative (kr) and nonradiative (∑knr) decay
rates describe the deactivation kinetics following electronic excitation.11
These decay rates are related to the fluorescence quantum yield, Φ,
and the excited-state fluorescence lifetime, τ, by
1
RuH(CO)(PPh3)2(AL-H): IR (CH2Cl2) 1920.7 vs cm-1; H NMR
(CDCl3) 8.12 (dd, J ) 2, 7 Hz, 1 H), 8.02 (dd, J ) ∼2, 7 Hz, 1 H), 7.6
(m, H), 7.2 (m, 19 H), 6.99 (s, 1 H), 6.49 (d, J ) 8 Hz, 1 H), -13.99
(t, J ) 19 Hz, 1 H) ppm, minor isomer hydride at -14.13 (t, J ) 19
Hz) ppm, major/minor ∼12; 31P{1H} NMR (CDCl3) 45.0 (s) ppm. Anal.
Calcd for C51H38O5P2Ru: C, 68.53; H, 4.28. Found: C, 68.21; H, 4.20.
Ru(CO)2(PPh3)2(AL-2H): IR (CH2Cl2) 2043.6 s, 1981.5 s cm-1; 1H
NMR (CDCl3) 8.34 (d, J ) 7.6 Hz, 1 H), 8.14 (d, J ) 6.8 Hz, 1 H),
7.66 (t, J ) 7.6 Hz, 1 H), 7.58 (t, J ) 8.4 Hz), 7.54 (m, 12 H), 7.28
(m, 18 H), 6.97 (d, J ) 8.4 Hz, 1 H), 5.83 (d, J ) 8.0 Hz, 1 H) ppm;
31P{1H} NMR (CDCl3) 21.0 (s) ppm. Anal. Calcd for C52H36O6P2Ru:
C, 67.90; H, 3.94. Found: C, 67.68; H, 3.82.
(c) Ru(CO)2(PCyc3)2(AL-2H) and RuH(CO)(PCyc3)2(AL-H). A
solution of Ru3(CO)12 (110 mg, 172 µmol), PCyc3 (303 mg, 1018 µmol),
and alizarin (131 mg, 546 µmol) in toluene (30 mL) was heated at
reflux under an argon atmosphere for 12 h. The resulting solution was
evaporated to dryness, and the residue was applied as a dichloromethane
solution to silica gel preparative TLC plates. Elution with dichlo-
romethane gave two major bands. Extraction of the red-brown top band
with ethyl acetate gave 152 mg, 32%, of RuH(CO)(PCyc3)2(AL-H).
Extraction of the blue-purple bottom band gave 135 mg, 27%, of Ru-
(CO)2(PCyc3)2(AL-2H).
kr ) φ/τ
(1)
(2)
knr ) (1 - φ)/τ
∑
Syntheses. (a) Ru(CO)2(PBu3)2(AL-2H) and Ru(CO)2(PBu3)2(AR-
2H). A solution of Ru3(CO)12 (54.6 mg, 85 µmol), PBu3 (120 µL,481
µmol), and alizarin (62.8 mg, 262 µmol) in toluene (20 mL) was heated
at reflux under an argon atmosphere for 10 h. The resulting purple
solution was evaporated to dryness and the residue was applied as a
dichloromethane solution to a silica gel preparative TLC plate. Elution
with first dichloromethane and then 8% acetone in dichloromethane
gave a beet-colored leading band, closely followed by an orange band.
A second TLC separation with 8% acetone in dichloromethane yielded
pure compounds, which were extracted with acetone/dichloro-
methane: Band 1, Ru(CO)2(PBu3)2(AL-2H), 77.3 mg, 97 µmol, 40%;
band 2, Ru(CO)2(PBu3)2(AR-2H), 58.0 mg, 74 µmol, 31%.
RuH(CO)(PCyc3)2(AL-H): IR (CH2Cl2) 1897.2 cm-1 1H NMR
;
(CDCl3) 8.23 (m, 1 H), 8.16 (m, 1 H), 7.68 (m, 2 H), 7.66 (s, 1 H),
7.48 (d, J ) 7.6 Hz, 1 H), 6.90 (d, J ) 7.6 Hz, 1 H), 2.1-0.8 (66 H),
-15.15 (t, J ) 19.6 Hz, 1 H) ppm; 31P{1H} NMR (CDCl3) 43.5 (s)
ppm. Anal. Calcd for C51H74O5P2Ru: C, 65.86; H, 8.02. Found: C,
65.86; H, 8.25.
Ru(CO)2(PCyc3)2(AL-2H): IR (CH2Cl2) 2028.6 s, 1962.1 s cm-1
;
Ru(CO)2(PBu3)2(AL-2H): IR (hexanes) 2029.5 s, 1965.5 s cm-1
.
1H NMR (CDCl3) 8.23 (t, J ) 7 Hz, 1 H), 8.22 (t, J ) 7 Hz, 1 H),
7.62 (d, J ) 8 Hz, 1 H), 7.58 (m, 2 H), 6.52 (d, J ) 8 Hz, 1 H),
2.2-1.0 (m, 66 H) ppm; 31P{1H} NMR (CDCl3) 37.2 (s) ppm. Anal.
Calcd for C52H72O6P2Ru: C, 65.32; H, 7.59. Found: C, 65.05; H, 7.88.
(d) Ru(CO)2(P(O-i-Pr)3)2(AL-2H): A solution of Ru3(CO)12 (100
mg, 156 µmol) and alizarin (115 mg, 479 µmol) in toluene (25 mL)
was heated at reflux under an argon atmosphere for 4.5 h. Then P(O-
i-Pr)3 (180 µL) was added, and the resulting solution was heated at 90
°C for 1 h. After standing overnight, the solution was evaporated to
dryness and the residue was applied as a dichloromethane solution to
silica gel preparative TLC plates. Elution with dichloromethane gave
three bands. Extraction of the purple third band with ethyl acetate gave
260 mg of a mixture containing the desired product. A second TLC
1H NMR (CDCl3) 8.22 (m, J ) 6.6 Hz, 2 H), 7.60 (m, J ) 7.5 Hz, 3
(3) Wang, R.; Sun, S.; Bekos, E. J.; Bright, F. V. Anal. Chem. 1995, 67,
149.
(4) Demas, J. N.; Crosby, G. A. J. Phys. Chem. 1971, 75, 991.
(5) Parker, C. A.; Rees, W. T. Analyst 1962, 87, 83.
(6) Mills, A.; Lepre, A. Anal. Chem. 1997, 69, 4653.
(7) Yappert, M. C.; Ingle, J. D. Appl. Spectrosc. 1989, 43, 759.
(8) Tucker, S. A.; Amszi, V. L.; Acree, W. E., Jr. J. Chem. Educ. 1992,
69, A11.
(9) Heitz, M. P.; Bright, F. V. Appl. Spectrosc. 1995, 49, 20.
(10) Bright, F. V. Appl. Spectrosc. 1995, 49, 14A.
(11) Barltrop, J. A.; Coyle, J. D. Principles of Photochemistry, John Wiley
& Sons: New York, 1978.

Linkage and Redox Isomerism in Ruthenium Complexes
Inorganic Chemistry, Vol. 39, No. 25, 2000 5809
separation using ca. 2% ethyl acetate in dichloromethane gave 39 mg,
RuH(CO)(PPh3)2(QN-H): IR (CH2Cl2) 1915 cm-1; 1H NMR (CDCl3)
14.06 (s, 1 H), 8.14 (d, J ) 8 Hz, 1 H), 8.05 (d, J ) 8 Hz, 1 H), 7.58
(m, 14 H), 7.18 (m, 20 H), 6.61 (d, J ) 9.2 Hz, 1 H), 6.31 (d, J ) 9.2
Hz, 1 H), -13.92 (t, J ) 20 Hz, 1 H) ppm, minor isomer hydride at
-13.83 (t, J ) 19 Hz) ppm, major/minor ∼12; 31P{1H} NMR (CDCl3)
43.6 (s) ppm.
10%, of Ru(CO)2(P(O-i-Pr)3)2(AL-2H) by extraction of the beet red
1
third band: IR (CH2Cl2) 2062.3 s, 2000.3 s cm-1; H NMR (CDCl3)
8.21 (t, J ) 8 Hz, 2 H), 8.76 (m, 3 H), 6.61 (d, J ) 8 Hz, 1 H), 4.80
(sept, J ) 6 Hz, 12 H), 1.23 (dd, JHH ) 6 Hz, JPH ) 28 Hz, 36 H)
ppm; 31P{1H} NMR (CDCl3) 110.7 (s) ppm.
RuH(CO)(PPh3)2(LQN-H): IR (CH2Cl2) 1915.8 cm-1 1H NMR
;
(e) Ru(CO)2(TPPTS)2(AL-2H). A solution of Ru3(CO)12 (35 mg,
55 µmol), TPPTS (204 mg, ca. 324 µmol), and alizarin (43 mg, 179
µmol) in DMSO (15 mL) was heated at 80 °C under an argon
atmosphere for 8 h. Then the atmosphere was changed to CO and the
solution was heated at 90 °C for 2.5 h. The resulting solution was
evaporated on a rotavap, and the purple residue was dissolved in
methanol. This solution was filtered, and the filtrate was then treated
with 2-propanol to yield a purple solid, 212 mg, contaminated with
16% OTPPS, as evidenced by the 31P NMR spectrum. Characteriza-
(CDCl3) 14.06 (s, 1H), 8.14 (d, J ) 8 Hz, 1 H), 7.94 (d, J ) 8 Hz, 1
H), 7.62 (m, 13 H), 7.38 (t, J ) 7.4 Hz, 1 H), 7.2 (m, 18 H), 2.29 (t,
J ) 7 Hz, 2 H), 2.16 (t, J ) 7 Hz, 2 H), -13.84 (t, J ) 20 Hz, 1 H);
31P{1H} NMR (CDCl3) 43.6 (s) ppm.
(j) [Ru(CO)2(PPh3)2(AL-2H)]SbCl6, [Ru(CO)2(PCyc3)2(AL-2H)]-
SbCl6, and [Ru(CO)2(PBu3)2(AR-2H)]SbCl6. Semiquinone complexes
were prepared by adding a stoichiometric amount of tris(4-bromo-
phenyl)ammoniumyl hexachloroantimonate (Aldrich) to a dichlo-
romethane solution of the appropriate complex. IR and EPR spectra
were then immediately recorded. The semiquinone complexes decom-
posed within a few hours. IR (CH2Cl2): [Ru(CO)2(PPh3)2(AL-2H)]-
SbCl6, 2071.3 s, 2019.5 m cm-1; [Ru(CO)2(PCyc3)2(AL-2H)]SbCl6,
2055.9 vs, 2001.6 s cm-1; [Ru(CO)2(PBu3)2(AR-2H)]SbCl6, 2057.9 vs,
1
tion: IR (H2O): 2060.0 s, 2002.8 s cm-1; H NMR (D2O) 8.07 (br d,
J ) 8 Hz, 1 H), 7.97 (m, 11 H), 7.69 (br d, J ) 8 Hz, 12 H), 7.63 (m,
1 H), 7.51 (m, ca. 15 H), 6.86 (d, J ) 8 Hz, 1 H), 6.00 (d, J ) 8 Hz,
1 H) ppm; 31P{1H} NMR (D2O) 35.5 (s, 16%, OTPPS), 25.3 (s, 84%)
ppm.
2000.3 s cm-1
.
(f) Ru(CO)(dppe)(PBu3)(AL-2H). A solution of Ru3(CO)12 (103.1
mg, 161 µmol), dppe (194.6 mg, 489 µmol), and alizarin (125.3 mg,
522 µmol) in toluene (25 mL) was heated at reflux under an argon
atmosphere for 4 h. Then PBu3 (120 µL, 481 µmol) was added, and
refluxing was continued for 6 h. The resulting solution was evaporated
to dryness and the residue was recrystallized from dichloromethane/
(k) [Ru(CO)(dppe)(PBu3)(AL-2H)]PF6. Ferrocenium hexafluoro-
phosphate (36 mg) was added as a dichloromethane suspension to a
solution of Ru(CO)(dppe)(PBu3)(AL-2H) (103 mg) in dichloromethane
(20 mL). After 20 min, the IR spectrum indicated complete reaction.
The solution was reduced in volume on a rotavap, followed by the
addition of hexanes. The purple solution was pipetted off, and remaining
purple solid was washed with hexanes and dried. Yield: 43 mg, 36%.
Subsequent crops were contaminated with starting material. IR (CH2-
Cl2) 1976.6 cm-1. Anal. Calcd for C53H57O5F6P4Ru: C, 57.20; H, 5.16.
Found: C, 55.20; H, 5.16. Ion exchange with sodium tetraphenylborate
and recrystallization from dichloromethane/toluene gave a small amount
of [Ru(CO)(dppe)(PBu3)(AL-2H)]BPh4. Anal. Calcd for C77H77O5BP3-
Ru: C, 71.85; H, 6.03. Found: C, 71.17; H, 6.36.
Crystal Structure Analysis of trans-RuH(CO)(PPh3)2(AL-H). The
crystal chosen for the analysis, a well-formed red parallelepiped of
dimensions 0.2 × 0.25 × 0.65 mm, was sealed in a thin-walled glass
capillary and mounted on a Siemens R3m/V diffractometer. Cell
constants were derived from a least-squares fit of 50 carefully centered
reflections with 2θ(Mo KR) ) 25-30°, which were well dispersed
throughout reciprocal space. Data were collected as described previ-
ously12 and were corrected for Lorentz and polarization effects and for
absorption.
All calculations were performed under the SHELXTL PLUS (Release
4.11 VMS) program package.13 The analytical scattering factors for
neutral atoms14a were corrected for anomalous dispersion.14b The
structure was solved by direct methods and difference Fourier
techniques and refined by least-squares procedures. All non-hydrogen
atoms were refined anisotropically. All organic hydrogen atoms were
included in optimized positions with d(C-H) ) 0.96 Å.15 Atoms H(1)
(the hydride ligand) and H(2) (bonded to O(2) in the AL-H ligand)
were located on a difference Fourier map, and their positions were
refined. Data for the crystallographic study are collected in Table 1.
1
methanol: yield 370 mg, 79%; IR (CH2Cl2) 1942.7 s cm-1; H NMR
(CDCl3) 8.97 (dd, J ) 8, 10 Hz, 2 H), 8.38 (d, J ) 7.6 Hz, 1 H), 8.20
(d, J ) 7.2 Hz, 1 H), 8.07 (m, 2 H), 7.85 (br t, J ) 7 Hz, 2 H), 7.68
(t, J ) 7.2 Hz, 1H), 7.6-7.4 (m, 10 H), 7.37 (d, J ) 8.4 Hz, 1 H),
6.91 (dd, J ) 7.2, 10.8 Hz, 2 H), 6.66 (td, J ) 7.6, 1.6 Hz, 2 H), 6.60
(dd, J ) 6.4, 1.6 Hz, 1 H), 6.40 (d, J ) 8.0 Hz, 1 H), 3.01 (dm, JPH
)
41 Hz, 1 H), 2.61 (dm, JPH ) 39 Hz, 1 H), 2.42 (m, 1 H), 2.03 (m, 1
H), 1.27 (m, 6 H), 1.14 (m, 6 H), 1.06 (m, 6 H), 0.69 (t, J ) 7.2 Hz,
9 H) ppm; 31P{1H} NMR (CDCl3) 59.7 (dd, 1 Pa,), 43.4 (dd, 1 Pb),
17.9 (dd, 1 Pc) ppm, Jab ) 12 Hz, Jac ) 23 Hz, Jbc ) 350 Hz. Anal.
Calcd for C53H57O5P3Ru: C, 65.76; H, 5.94. Found: C, 65.55; H, 5.95.
(g) Ru(CO)2(PBu3)2(AR-2H). A solution of Ru3(CO)12 (97 mg, 150
µmol), PBu3 (240 µL, 960 µmol), and anthrarobin (Aldrich technical
grade, 85% purity, 105 mg, 465 µmol) in toluene (30 mL) was heated
at reflux under an argon atmosphere for 14 h. The resulting solution
was evaporated to dryness, and the residue was applied as a dichlo-
romethane solution to a silica gel preparative TLC plate. Elution with
first dichloromethane and then 6% acetone in dichloromethane gave
red, beet red, orange, purple, purple, and yellow bands, in order of
decreasing Rf. Extraction of the yellow band with ethyl acetate gave
135 mg (172 µmol, 38%) of a material whose IR and NMR data were
identical to those of the yellow product from alizarin. Extraction of
the beet red band gave 87.3 mg of Ru(CO)2(PBu3)2(AL-2H), (109 µmol,
24%); this product was presumed to arise from alizarin impurity in the
starting material.
(h) RuH(CO)(PPh3)2(AL-H). A solution of RuH2(CO)(PPh3)3 (175
mg, 191 µmol) and alizarin (58 mg, 242 µmol) in 2-methoxyethanol
(10 mL) was heated at reflux under an argon atmosphere for 30 min.
The resulting dark solution was evaporated to dryness, and the residue
was applied as a dichloromethane solution to a silica gel preparative
TLC plate. Elution with dichloromethane gave one green, and two
purple bands. Extraction of the green top band with ethyl acetate and
recrystallization from dichloromethane/methanol gave the product as
green crystals. Yield: 108 mg, 63%.
(i) RuH(CO)(PPh3)2(QN-H) and RuH(CO)(PPh)2(LQN-H). A
solution of RuH2(CO)(PPh3)3 (196 mg, 214 µmol) and quinizarin (52
mg, 217 µmol) in 2-methoxyethanol (10 mL) was heated at reflux under
an argon atmosphere for 30 min. The resulting green solution was
evaporated to dryness, and the residue was applied as a dichloromethane
solution to silica gel preparative TLC plates. Elution with 1:1
dichloromethane/hexanes gave a yellow, a green, and another yellow
band. The green band was extracted with ethyl acetate; evaporation
gave RuH(CO)(PPh3)2(QN-H) (96 mg, 49%). Recrystallization from
dichloromethane/methanol gave green crystals. Extraction of the yellow
third band gave RuH(CO)(PPh3)2(LQN-H) (71 mg, 36%).
Results and Discussion
The coordination chemistry of hydroxyanthraquinones has not
received much recent attention. Although complexes of alizarin
have been used for centuries as dyes, structural information
appears to be slight. A search of the Cambridge Structural
Database found only three references containing crystal struc-
tures, all of coordination complexes involving bridging between
(12) Churchill, M. R.; Lashewycz, R. A.; Rotella, F. J. Inorg. Chem., 1977,
16, 265.
(13) Sheldrick, G. M. SHELXTL PLUS (Release 4.11 VMS); Siemens
Analytical Instruments Inc.; Madison, WI, 1990. (See also Siemens
SHELXTL PLUS Manual, 2nd ed., 1990).
(14) International Tables for X-ray Crystallography; Kynoch Press: Bir-
mingham, England, 1974; Vol. 4 (a) pp 99-101, (b) pp 149-150.
(15) Churchill, M. R. Inorg. Chem., 1973, 12, 1213.

5810 Inorganic Chemistry, Vol. 39, No. 25, 2000
Churchill et al.
Scheme 1
complexes of [RuII(bpy)2]. A Ru complex of alizarin has been
shown to undergo interconversion between 1,2- and 1,9-
chelating forms, induced by deprotonation.22 Dimetallic 1,9-
quinizarin coordination complexes of Fe have been crystallo-
graphically characterized, as well as a dimetallic complex
derived from 5,8-dihydroxynaphthoquinone.16b,c
Table 1. Crystal Data for RuH(CO)(PPh3)2((AL-H)
empirical formula
fw
C51H38O5P2Ru
893.8
wavelength, Å
crystal system
space group
a, Å
0.710 730
monoclinic
C2/c (No. 15)
32.862(8)
b, Å
c, Å
13.377(3)
22.841(6)
124.39(2)
8285(4)
Reactions of Ru3(CO)12 with alizarin and phosphine ligands
L ) PBu3, PCyc3, and PPh3 in refluxing toluene give as the
major products RuH(CO)L2(AL-H), Ru(CO)2L2(AL-2H), and
Ru(CO)2L2(AR-2H), with the relative amounts depending upon
L and the reaction time (see Scheme 1); the analogous reaction
with L ) P(m-NaSO3C6H4)3 (TPPTS) in DMSO gives Ru(CO)2-
(TPPS)2(AL-2H). The reaction most likely proceeds by oxidative
addition of the catechol, which serves as the hydrogen source
for reduction of the quinone, with RuH(CO)L2(AL-H) as an
intermediate complex. Consistent with this, RuH(CO)(PPh3)2-
(AL-H) reacts with CO in refluxing toluene to give Ru(CO)2-
(PPh3)2(AL-2H) within 1 h. However, Ru(CO)2(PPh3)2(AL-2H)
does not form RuH(CO)(PPh3)2(AL-H) under a hydrogen
atmosphere.
â, deg
V, Å3
Z
d
8
calcd, Mg/m3
1.433
3664
0.493
5.0-45.0
5842
5442 (Rint ) 1.08%)
3829
F(000)
µ, mm-1
2θ range, deg
no. of reflns collected
no. of unique reflns
no. of reflns >2σ(I)
abs. corr
ψ scans
max and min transm
final R indices [I > 2σ(I)]
R indices (all data)
largest diff peak and hole, (e/Å3)
0.9584 and 0.9360
R1 ) 3.74%, wR2 ) 3.38%
R1 ) 6.52%, wR2 ) 3.77%
0.36, -0.27
Under similar conditions, the reactions of alizarin, Ru3(CO)12,
and L ) P(OPh)3 and P(O-i-Pr)3 did not give the analogous
products. A low yield of Ru(CO)2(P(O-i-Pr3)2(AL-2H) was
two metals.16 The bridged complexes Ca2Al2(OH)2(AL-2H)4,
its purpurin analogue, and Ca2Ti2O2(AL-2H)4 contain 1,2-
coordination to Al or Ti and 1,9-coordination to Ca.16a A number
of studies on coordination complexes of dihydroxyanthraquino-
nes and related ligands have appeared, most suggesting 1,9-
coordination.17-22 The most complete studies have concerned
(18) Bulatov, A. V.; Khidekel, M. L.; Egorochkin, A. N.; Panicheva, M.
V.; Sennikov, P. G. Transition Met. Chem. 1983, 8, 289.
(19) (a) Gooden, V. M.; Cai, H.; Dasgupta, T. P.; Gordon, N. R.; Hughes,
L. J.; Sadler, G. G. Inorg. Chim. Acta 1997, 255, 105. (b) Merrell, P.
H. Inorg. Chim. Acta 1979, 32, 99.
(16) (a) Wunderlich, C.-H.; Bergerhoff, G. Chem. Ber. 1994, 127, 1185.
(b) Pierpont, C. G.; Francesconi, L. C.; Hendrickson, D. N. Inorg.
Chem. 1978, 17, 3470. (c) Maroney, M. J.; Day, R. O.; Psyris, T.;
Fleury, L. M.; Whitehead, J. P. Inorg. Chem. 1989, 28, 173.
(17) Masoud, M. S.; Tawfik, M. S.; Zayan, S. E. Synth. React. Inorg. Met.-
Org. Chem. 1984, 14, 1
(20) Dei, A.; Gatteschi, D.; Pardi, L. Inorg. Chem. 1990, 29, 1442.
(21) (a) Tsipis, C. A.; Bakalbassis, E. G.; Papageorgiou, V. P.; Bakola-
Christianopoulou, M. N. Can. J. Chem. 1982, 60, 2477. (b) Bakola-
Christianopoulou, M. N. Polyhedron 1984, 3, 729.
(22) DelMedico, A.; Auburn, P. R.; Dodsworth, E. S.; Lever, A. B. P.;
Pietro, W. J. Inorg. Chem. 1994, 33, 1583.

Linkage and Redox Isomerism in Ruthenium Complexes
Inorganic Chemistry, Vol. 39, No. 25, 2000 5811
obtained by first refluxing Ru3(CO)12 and alizarin in toluene,
followed by treatment with P(O-i-Pr)3.
1,2-Catecholate Complexes. Characterizations of Ru(CO)2L2-
(AL-2H) are straightforward by comparison to those of previ-
ously reported catecholate complexes. The observation of two
CO stretches in the IR spectra and a single 31P resonance for
each complex indicate that all are Ru(cis-CO)2(trans-PR3)2-
1
(chelating ligand) complexes. The H NMR spectra show no
O-H resonances, and the expected six aromatic proton reso-
nances can be observed for the PBu3 and PCyc3 complexes,
although the aromatic region is largely obscured for the PPh3
complex. The AL-2H complexes each show a pair of doublets
for the 3- and 4-hydrogens, which because of the upfield shift
of the 3-hydrogen resonance (at ca. 6.5 ppm) are useful for
characterization. For complexes with Ph-substituted phosphines,
ring current effects shift the 3-hydrogen resonance upfield still
further to ca. 6.0 ppm. The complex Ru(CO)2(PBu3)2(AR-2H)
can be prepared in higher yield by the analogous reaction with
anthrarobin, thus confirming the 9,9-dihydro structure.
The product formed from dppe was too insoluble to allow
for facile characterization, so further substitution using PBu3
was performed. Ru(CO)(dppe)(PBu3)(AL-2H) is a very soluble,
deep blue compound. Its 31P NMR spectrum indicates predomi-
nantly one isomer with PBu3 occupying one site cis to the
alizarinate chelate and one end of the dppe ligand occupying
the other. The large trans P-P coupling constant of 350 Hz,
compared with the small cis values of 12 and 23 Hz, is
diagnostic.
Figure 1. Molecular geometry of the RuH(CO)(PPh3)2(AL-H) mol-
ecule.
Table 2. Selected Bond Lengths (Å) and Angles for
RuH(CO)(PPh3)2((AL-H)
(A) Ruthenium-Ligand Bond Lengths
Ru-H(1)
Ru-P(1)
Ru-P(2)
1.56(4)
Ru-O(1)
Ru-O(9)
Ru-C(21)
2.119(4)
2.180(4)
1.789(7)
2.367(1)
2.372(2)
The complexes are highly stable to heat and air. Most are
quite soluble in dichloromethane, and the PBu3 complex is
soluble in hydrocarbons. In contrast, the TPPTS complex is very
soluble in water and soluble in methanol or DMSO but insoluble
in dichloromethane. The complexes do not react with carboxylic
acids under mild conditions but release alizarin in the presence
of a large excess and at elevated temperatures. Strong acids such
as CF3CO2H rapidly cause generation of alizarin.
The presence of the 9,10-dioxo substituents suggested that
these complexes might show significant hydrogen bonding.
Direct evidence for such hydrogen bonding comes from the CO
stretching frequencies; addition of butanol to a solution of Ru-
(CO)2(PBu3)2(AL-2H) in hexanes results in the appearance of
a set of two absorbances at 2038 and 1970 cm-1, attributed to
the hydrogen-bonded complex, in addition to absorbances at
2030 and 1965 cm-1, due to the free complex. However, this
hydrogen bonding is not associated with the 9,10-dioxo sub-
stituents, since similar shifts are observed for the analogous
catecholate, Ru(CO)2(PBu3)2(O2C6H4).
1,9-Complexes. Reactions of Ru3(CO)12, PR3 (R ) Ph, Cyc),
and alizarin in the 1:6:3 ratios in refluxing toluene generate
RuH(CO)(PR3)2(AL-H), in addition to Ru(CO)2(PR3)2(AL-2H).
The former product is exclusively formed from H2Ru(CO)-
(PPh3)3 and alizarin in refluxing 2-methoxyethanol, and the
analogous reaction with quinizarin gives RuH(CO)(PR3)2(QN-
H) and RuH(CO)(PR3)2(LQN-H). The singlet 31P signal indi-
cates a trans arrangement of PR3 ligands. The 1H NMR and IR
spectra are similar to those of RuH(CO)(PPh3)2(acac).23 The
1H NMR spectra of the AL-H complexes contain an HO
resonance at ca. 7-7.5 ppm and the 3-hydrogen resonance is
not shifted as far upfield as it is for the 1,2-complexes Ru-
(CO)2L2(AL-2H). Two isomers due to the relative orientations
of the cis-(H)(CO) ligands are expected, but one predominates
by >10:1. The structure of what we assume to be the
(B) Distances within the Alizarin System
C(1)-O(1)
C(1)-C(2)
C(1)-C(12)
C(2)-O(2)
C(2)-C(3)
C(3)-C(4)
C(4)-C(11)
C(5)-C(6)
C(5)-C(13)
C(6)-C(7)
1.308(8)
1.418(7)
1.410(7)
1.349(7)
1.361(11)
1.389(9)
1.370(7)
1.373(8)
1.390(8)
1.376(12)
C(7)-C(8)
1.375(7)
1.387(7)
1.250(8)
1.447(6)
1.483(7)
1.227(10)
1.467(8)
1.471(8)
1.444(10)
1.413(11)
C(8)-C(14)
C(9)-O(9)
C(9)-C(12)
C(9)-C(14)
C(10)-O(10)
C(10)-C(11)
C(10)-C(13)
C(11)-C(12)
C(13)-C(14)
(C) Angles around the Ruthenium(II) Center
H(1)-Ru-P(1)
H(1)-Ru-P(2)
H(1)-Ru-O(1)
H(1)-Ru-O(9)
H(1)-Ru-C(21)
P(1)-Ru-P(2)
P(1)-Ru-O(1)
P(1)-Ru-O(9)
88.7(15)
88.8(15)
91.9(16)
173.5(15)
81.6(16)
177.5(1)
85.5(1)
P(1)-Ru-C(21)
P(2)-Ru-O(1)
P(2)-Ru-O(9)
P(2)-Ru-C(21)
O(1)-Ru-O(9)
O(1)-Ru-C(21)
O(9)-Ru-C(21)
91.7(2)
94.5(1)
88.8(1)
88.0(2)
82.3(1)
173.0(2)
104.3(2)
93.7(1)
predominant isomer was confirmed by a single-crystal X-ray
diffraction study for RuH(CO)(PPh3)2(AL-H). On electronic
grounds, one would expect the stronger σ donor phenoxide to
favor the position trans to the strong π acceptor CO rather than
the strong σ donor hydride, as is observed.
Description of the Molecular Structure for RuH(CO)-
(PPh3)2(AL-H). Figure 1 shows the molecular structure and
labeling scheme, and selected bond lengths and angles are
provided in Table 2. The central ruthenium(II) moiety has a
fairly regular octahedral coordination sphere with trans PPh3
ligands (Ru-P(1) ) 2.367(1) Å and Ru-P(2) ) 2.372(2) Å)
and mutually cis hydride (Ru-H(1) ) 1.56(4) Å) and carbonyl
(Ru-C(21) ) 1.789(7) Å) ligands. The alizarin system is linked
to ruthenium via atoms O(1) and O(9), with Ru(1)-O(1) )
2.119(4) Å (trans to the carbonyl ligand) and Ru-O(9) )
2.180(4) Å (trans to the hydride ligand). The C-O distances in
the chelate ring are C(1)-O(1) ) 1.308(8) Å and C(9)-O(9)
) 1.250(8) Å, as compared to values of C(2)-O(2) )
(23) Queiros, M. A. M.; Robinson, S. D. Inorg. Chem. 1978, 17, 310.

5812 Inorganic Chemistry, Vol. 39, No. 25, 2000
Churchill et al.
1.349(7) Å (for the C(sp2)-OH single bond) and C(10)-O(10)
) 1.227(10) Å (for the CdO double bond) associated with the
uncoordinated oxygen atoms.
The isomer identified should be preferred on electronic
grounds. The π acceptor CO ligand is oriented trans to the
stronger σ donor phenoxy end of the chelate while the stronger
σ donor hydride is trans to the π acceptor keto end.
The crystal structure as a whole is stabilized by a series of
C-H‚‚‚O hydrogen bonds and by the parallel stacking of
alizarin-H systems (see Figure 1S, Supporting Information). The
six-membered chelate ring is nonplanar, with a dihedral angle
of 13.6° between the O(1)-Ru-O(9) and C(1)-C(12)-C(9)
moieties. The nonplanarity is due to π stacking of the alizarinate
ring between a phenyl group (C(31)-C(36)) attached to P(1)
in the same molecule and on an adjacent molecule in the crystal.
The polycyclic aromatic system is closer to planarity with a
dihedral angle of only 6.0° between the two outermost six-
membered rings (viz., C(1)-C(2)-C(3)-C(11)-C(12) and
C(5)-C(6)-C(7)-C(8)-C(14)-C(13) in Figure 1).
A search of the Cambridge Structure Database found no other
examples of monometallic 1,9-alizarinate complexes, but there
are structures for Ca2Al2(OH)2(AL-2H)4(H2O)5(DMF)7 and Ca2-
Ti2O2(AL-2H)4(H2O)5(DMF)5.16a In these structures, the alizari-
nate dianion forms a 1,9-chelate with the calcium ion and a
1,2-chelate with the other metal ion. In comparison with these
structures, there is a significant difference between the two sets
of phenolic C-bond lengths, as well as a notably longer C(9)-
O(9) bond. Another related structure is that of {Fe(salen)}2-
(QN-2H), in which the quinizarinate dianion bridges the two
Fe ions, forming a 1,9:4,10-chelate.16b
Electrochemistry. An electrochemical study of Ru(bpy)2-
(1,2-AL-2H) and [Ru(bpy)2(AL-H)]+ was reported previously.22
The cyclic voltammogram (CV) of Ru(bpy)2(1,2-AL-2H) dis-
plays two reversible 1-e oxidations at +0.06 and +0.88 V
(acetonitrile) vs SCE (-0.36 and +0.46 V vs ferrocene) and
irreversible reductions at -1.58 and -1.70 V for the p-quinone/
p-semiquinone and p-semiquinone/hydroquinonate couples.
The electrochemistry of these complexes was surveyed using
cyclic voltammetry. Representative CVs are shown in Figure 2
((a) Ru(CO)2(PPh3)2(AL-2H) and (b) Ru(CO)2(PCyc3)2(AL-2H))
and Figure 3 ((a) RuH(CO)(PPh3)2(AL-H) and (b) RuH(CO)-
(PCyc3)2(AL-H)). Electrochemical data are summarized in Table
3.
In the most favorable case, the 1,2-catecholate complexes
could exhibit two 1-e oxidations associated with the catecholate
moiety and several 1-e reductions associated with the central
quinone. All of the 1,2-catecholates exhibit a reversible to quasi-
reversible 1-e oxidation, at potentials of -0.09 to +0.18 V. The
oxidation potential is not very sensitive to the other ligands in
the complex, as expected for a ligand-centered process. The
CO stretching frequencies are excellent indicators of the electron
densities on the metal centers and correlate well with metal-
centered Ru(II)/Ru(III) oxidation potentials. The dependence
of Ru(II)/Ru(III) oxidation potentials upon the ligands has been
parametrized by Lever,24 allowing a quantitative prediction of
differences in these oxidation potentials for the various com-
plexes. There is a poor correlation of the oxidation potentials
for Ru(CO)2L2(AL-2H), although the lowest oxidation potential
is associated with the most electron-rich metal center: that of
Ru(CO)(dppe)(PBu3)(AL-2H). If the 1-e oxidation were metal
centered, one would have expected the oxidation potential for
Ru(CO)2(PBu3)2(AL-2H) to be ca. 0.2 V lower than that for
Figure 2. Cyclic voltammograms for (a) Ru(CO)2(PPh3)2(AL-2H) and
(b) Ru(CO)2(PCyc3)2(AL-2H).
Ru(CO)2(PPh3)2(AL-2H); instead, its oxidation potential is
higher by 0.07 V. Furthermore, the oxidation potential for Ru-
(CO)2(PBu3)2(AL-2H) would have been expected to be ca. 0.6
V higher than that of Ru(CO)(dppe)(PBu3)(AL-2H); instead,
its potential is only 0.27 V more positive. With only one
example having a non-carbonyl ligand trans to the alizarinate
ring, it is impossible to rule out a stronger trans influence upon
the oxidation potential, but clearly there is little, if any, influence
due to the cis PR3 ligands, inconsistent with a metal-centered
redox process. On the other hand, the oxidation potential is
strongly dependent upon the catecholate ring; the alizarinate
complex Ru(CO)2(PBu3)2(AL-2H) (+0.18 V) displays a much
more positive oxidation potential than Ru(CO)2(PBu3)2(AR-2H)
(-0.056 V) and Ru(CO)2(PBu3)2(Cat) (-0.248 V), which have
the less electron-withdrawing ring substituents. In fact, there is
a strong correlation between these oxidation potentials and the
Hammett parameters25 for the C6R4O2 substituents, consistent
with a ligand-centered redox process. The second 1-e oxidation
is only accessible for complexes with stronger electron-donating
ligands. The oxidative electrochemistry of Ru(CO)2(PBu3)2(AL-
2H) is similar to the responses of other Ru(CO)2L2(Cat)
complexes previously reported.26 The CV recorded at 100 mV/s
in a 0.1 M TBATFB/dichloromethane solution displays a
reversible 1-e oxidation at +0.2 V vs Fc and a quasi-reversible
(25) Hansch, C.; Leo, A.; Taft, R. W. Chem. ReV. 1991, 91, 165.
(26) (a) Connelly, N. G.; Manners, I.; Protheroe, J. R. C.; Whiteley, M.
W. J. Chem. Soc., Dalton Trans. 1984, 2713. (b) Bohle, D. S.;
Goodson, P. A. J. Chem. Soc., Chem. Commun., 1992, 1205. (c) Bohle,
D. S.; Carron, K. T.; Christensen, A. N.; Goodson, P. A.; Powell, A.
K. Organometallics 1994, 13, 1355.
(24) Lever, A. B. P. Inorg. Chem. 1990, 29, 1271.

Linkage and Redox Isomerism in Ruthenium Complexes
Inorganic Chemistry, Vol. 39, No. 25, 2000 5813
Table 3. Cyclic Voltammetric Data
(Ep,a + Ep,c)/2
∆Ep
(mV)
(Ep,a + Ep,c)/2
[Ep,c], (V)
∆Ep
(mV)
complex
[Ep,a], (V)
ipc/ipa
ipa/ipc
Ru(CO)2(PPh3)2(AL-2H) (400 mV/s)
Ru(CO)2(PBu3)2(AL-2H) (800 mV/s)
Ru(CO)(dppe)(PBu3)(AL-2H) (100 mV/s)
Ru(CO)2(PCyc3)2(AL-2H) (100 mV/s)
+0.11
+0.18
-0.09
+0.13
81
80
88
78
0.98
1.0
1.0
-1.34
-1.95
136
102
0.69
0.79
0.66
[-2.05]
3-e process
Ru(CO)2(PBu3)2(AR-2H) (100 mV/s)
RuH(CO)(PPh3)2(AL-H) (100 mV/s)
RuH(CO)(PCyc3)2(AL-H) (100 mV/s)
-0.056
[+0.41]
+0.076
80
58
78
0.57
-1.64
-1.40
-1.67
[-1.8]
86
62
67
Ru(CO)2(TPPTS)2(AL-2H) (100 mV/s)
+0.14
0.9
are primarily metal-centered. Unfortunately, the 1-e-oxidation
products are too unstable for characterization. The reductions
of these two complexes are also different. For RuH(CO)(PPh3)2-
(AL-H), a single quasi-reversible 1-e reduction at -1.64 V is
complicated by a prepeak, which we associate with adsorbed
material. On the other hand, for RuH(CO)(PCyc3)2(AL-H), two
quasi-reversible processes are found at -1.40 and -1.67 V.
The similarities of the reduction potentials suggest that these
are associated primarily with the anthracenedione units.
Semiquinone Complexes. Chemical oxidation was used to
generate the semiquinone complexes [Ru(CO)2L2(AL-2H)]+,
[Ru(CO)2(PBu3)2(AR-2H)]+, and [Ru(CO)(dppe)(PBu3)2(AL-
2H)]+. The species [Ru(CO)2L2(AL-2H)]+ (L ) PBu3, PPh3,
PCyc3) and [Ru(CO)2(PBu3)2(AR-2H)]+ were prepared by
oxidation with tris(4-bromophenyl)aminium hexachloroanti-
monate (magic blue) in dichloromethane solution. The IR spectra
of the semiquinone complexes show that the CO stretches are
shifted to higher frequency by ca. 20-30 cm-1 vs those of the
neutral precursors. In each case, the IR spectrum decays over a
period of hours to a second set of peaks (e.g., for L ) PBu3, IR
bands at 2063.6 and 2009 cm-1 decay to carbonyl stretches at
2041 and 1975 cm-1), attributed to the corresponding cis,cis,-
trans-RuCl2(CO)2L2 decomposition product, the chlorides com-
ing from the hexachloroantimonate ion. At room temperature,
the EPR spectra (Table 4) each display a strong signal near g
) 2.00 with hyperfine couplings to two equivalent 31P nuclei
(t, A(31P) ≈ 25 G) and 99Ru and 101Ru (∼4 G for each), and, in
most cases, also to two protons, assumed to be in the 3- and
4-positions. A representative EPR spectrum and a simulation,
for [Ru(CO)2(PBu3)2(AL-2H)]+, are shown in Figure 4. The
spectra are very similar to those of [Ru(CO)2(PPh3)L(o-O2C6-
Cl4)]+, reported by Connelly et al. (e.g., for L ) PPh3, g )
2.002 (t, A(31P) ) 25.2 G).26a
For Ru(CO)(dppe)(PBu3)(AL-2H), which has an oxidation
potential less positive than that of ferrocene, [FeCp2]PF6
oxidation gives a relatively stable semiquinone complex,
characterized by IR and EPR spectroscopy. The EPR spectrum
(Table 4) is a broad triplet, indicating that the hyperfine coupling
to the 31P nucleus in the plane of the alizarinate ring is too small
to resolve. The complex can be isolated as a solid (although
not in analytically pure form) but decomposes within a few days
in solution to regenerate Ru(CO)(dppe)(PBu3)(AL-2H) and
uncharacterized, non-carbonyl-containing material.
Photophysics. The color of alizarin-metal complexes was
of primary interest to the dye industry around the turn of the
last century.2 Colors of the dyes ranged from red to brown.
Recently, the structures and electronic absorption spectra of
complexes of alizarin were reexamined.16a The absorption
maximum was found to shift to shorter wavelengths with
increasing electronegativity of the metal atom, varying from
432 nm for alizarin itself to 520 nm for the Al salt to 636 nm
for the alizarinate dianion. The absorptions for the Al and Mg
Figure 3. Cyclic voltammograms for (a) RuH(CO)(PPh3)2(AL-H) and
(b) RuH(CO)(PCyc3)2(AL-H).
1-e oxidation at ca. +1.05 V. For comparison, Ru(CO)2(PPh3)2-
(o-O2C6Cl4) displays reversible 1-e oxidations at 0.58 and 1.58
V vs SCE (ca. 0.16 and 1.16 V vs ferrocene). Consistent with
other studies, both oxidations are presumed to be ligand-based.
The reduction processes are much more variable.
The two examples of RuH(CO)(PR3)2(AL-H) display very
different electrochemical behaviors. Figure 3 shows the CVs
for (a) RuH(CO)(PPh3)2(AL-H) and (b) RuH(CO)(PCyc3)2(AL-
H). The 1-e oxidation of RuH(CO)(PPh3)2(AL-H) is completely
irreversible up to 800 mV/s (Ep,a ) +0.41 V at 100 mV/s),
whereas RuH(CO)(PCyc3)2(AL-H) displays a reversible 1-e
oxidation at +0.076 V; on the basis of the Lever electrochemical
parameters, the estimated oxidation potential for the former is
0.48 V for a metal-centered process and the latter should have
an oxidation potential somewhat more than ca. 200 mV lower,
comparable with the observed difference of less than 330 mV.
Thus, it seems likely that the 1-e-oxidations in these complexes

5814 Inorganic Chemistry, Vol. 39, No. 25, 2000
Table 4. EPR Spectral Data from Dichloromethane Solutions
Churchill et al.
complex
g
A(31P), G
A(1H), G
A(101Ru, 99Ru), G
[Ru(CO)(dppe)(PBu3)(AL-2H)]PF6
2.004
23.8
25.8
1.9
1.9
1.4
4.0
4.0
[Ru(CO)2 (PBu3)2 (AL-2H)]SbCl6
[Ru(CO)2 (PPh3)2 (AL-2H)]SbCl6
[Ru(CO)2 (PCyc3)2 (AL-2H)]SbCl6
[Ru(CO)2 (PBu3)2 (AR-2H)]SbCl6
2.0056
2.0039
2.0058
2.0052
26.4
25.0
25.3
24.11
2.30
2.30
2.3
1.80 (2H)
3.10 (1H)
4.0
3.7, 3.9
4.6, 4.6
Table 5. UV/Vis Data between 350 and 820 nm
complex
λ
max (ꢀ), nm
HRu(CO)(PCyc3)2(AL-H)a
HRu(CO)(PPh3)2(AL-H)a
Ru(CO)2(PBu3)2(AL-2H)a
Ru(CO)2(PCyc3)2(AL-2H)a
Ru(CO)2(TPPTS)2(AL-2H)b
Ru(CO)(dppe)(PBu3)(AL-2H)a
Ru(CO)2(PPh3)2(AL-2H)a
Ru(CO)2(PBu3)2(AR-2H)a
486 (8.2 × 103), 600 br (1.7 × 103)
444 (6.2 × 103), 568 (2.7 × 103), 610 sh (2.1 × 103)
552 (1.3 × 104)
360 (5.1 × 103), 526 sh (9.3 × 103), 558 (1.2 × 104), 600 sh (9.1 × 103)
560 (9.2 × 103)
542 sh (9.3 × 103), 582 (1.2 × 104), 626 sh (9.0 × 103)
554 (6.6 × 103)
438 (8.8 × 103)
574 (4.0 × 103), 614 sh (3.5 × 103)
a
[Ru(CO)(dppe)(PBu3)(AL-2H)]PF6
HRu(CO)(PPh3)2(LQN-H)a
HRu(CO)(PPh3)2(QN-H)a
358 (9.4 × 103), 468 (7.1 × 103), 494 (7.1 × 103)
356 (8.2 × 103), 468 (6.6 × 103), 494 sh (5.7 × 103), 648 br (1.8 × 103)
a In dichloromethane. b In water.
Figure 4. Experimental (upper) and simulated (lower) EPR spectra
of [Ru(CO)2(PBu3)2(AL-2H)]+ in dichloromethane at room temperature.
complexes were each reported to exhibit significant charge
transfer character to a MO with a strong metal contribution.
The 1,2-complexes herein display absorptions at ca. 550 nm
(dichloromethane solution) for violet Ru(CO)2L2(AL-2H) and
at 574 nm for the more electron-rich, dark blue Ru(CO)(dppe)-
(PBu3)(AL-2H). The 1,9-complexes are quite different, with
absorption maxima at 444 nm for green RuH(CO)(PPh3)2(AL-
H) and at 486 nm for brown RuH(CO)(PCyc3)2(AL-H). Ab-
sorbance spectral data for these complexes are presented in Table
5.
Representative electronic absorption spectra are shown in
Figure 5. Absorbance spectral features around and below 300
nm can be attributed to various intraligand and/or interligand
transitions; the lower energy absorption bands, however, are
attributed to MLCT transitions. Figure 6 presents steady-state
fluorescence spectra for the same compounds dissolved in
dichloromethane. The remainder of our presentation here focuses
on the properties of Ru(CO)2(PBu3)2(AL-2H) exclusively
because it exhibited the strongest luminescence. The electronic
absorbance spectra of Ru(CO)2(PBu3)2(AL-2H) dissolved in
seven different solvents are presented in Figure 7. A careful
examination of the spectra reveals that the molar absorptivities,
in general, increase as the solvent dipolarity decreases. More
Figure 5. Absorbance spectra of dilute dichloromethane solutions of
Ru(CO)2(PBu3)2(AL-2H), Ru(CO)2(PBu3)2(AR-2H), and Ru(CO)(PBu3)-
(dppe)(AL-2H).
vibronic structure is also evident for the compound in the
nonpolar solvents such as cyclohexane and toluene, while these
features vanish as the solvent dipolarity increases. Furthermore,
there is also a bathochromic shift in the low-energy absorbance
maximum as the solvent dipolarity increases. Table 6 reports
the low-energy absorbance maxima and the corresponding
decadic molar absorptivities of Ru(CO)2(PBu3)2(AL-2H) dis-
solved in the solvents studied.
Figures 8 and 9 show the relative and normalized fluorescence
emission spectra, respectively, for 5 µM Ru(CO)2(PBu3)2(AL-
2H) dissolved in the degassed solvents studied. The emission
maxima are reported in Table 6. A blue shift in emission
maximum is observed with a decrease in the solvent dipolarity,
suggesting an increase in the Ru(CO)2(PBu3)2(AL-2H) dipole
moment in the excited state relative to the ground state.27,28
(27) Lippert, V. E. Z. Electrochem. 1957, 61, 962.

Linkage and Redox Isomerism in Ruthenium Complexes
Inorganic Chemistry, Vol. 39, No. 25, 2000 5815
Table 6. Absorbance and Fluorescence Maxima and Molar
Absorptivities for Ru(CO)2(PBu3)2(AL-2H) in Various Solvents
solvent,
dielectric constant
absorbance fluorescence
log ꢀ (dm3
b
max, nma
max, nma
mol-1 cm-1
)
cyclohexane, 2.02
toluene, 2.38
chloroform, 4.81
dichloromethane, 8.93
dimethyl sulfoxide, 47.24
ethanol, 25.3
522
530
547
544
552
555
542
594
600
632
635
645
641
636
4.11
4.02
4.07
3.85
3.92
3.65
3.88
acetonitrile, 36.64
a (1 nm. b (0.1.
Figure 6. Fluorescence spectra of dilute solutions of Ru(CO)2(PBu3)2-
(AL-2H) (λexc ) 543 nm; excitation and emission spectral band-passes
at 8 and 4 nm, respectively), Ru(CO)2(PBu3)2(AR-2H) (λexc ) 436 nm,
excitation and emission spectral band-passes at 16 and 16 nm,
respectively), and Ru(CO)(dppe)(PBu3)(AL-2H) (λexc ) 543 nm;
excitation and emission spectral band-passes at 16 and 8 nm, respec-
tively).
Figure 8. Fluorescence spectra of Ru(CO)2(PBu3)2(AL-2H) in seven
different solvents (λexc ) 543 nm; excitation and emission spectral band-
passes at 8 and 4 nm, respectively).
Figure 7. Molar absorptivities of Ru(CO)2(PBu3)2(AL-2H) in seven
different solvents.
If we assume that Ru(CO)2(PBu3)2(AL-2H) exhibits a cavity
radius of 6 Å,29,30 a Lippert analysis27,28 of the data presented
in Table 6 yields estimates of the difference between the excited-
and ground-state dipole moments (∆µ) of 5-6 D. This change
in dipole moment demonstrates that there is a significant change
in the electronic configuration of Ru(CO)2(PBu3)2(AL-2H) upon
optical excitation.
Figure 9. Normalized fluorescence spectra of Ru(CO)2(PBu3)2(AL-
2H) in seven different solvents (λexc ) 543 nm; excitation and emission
spectral band-passes at 8 and 4 nm, respectively).
Table 7 reports the fluorescence quantum yields, excited-state
fluorescence lifetime, and the rates of radiative and nonradiative
decays for Ru(CO)2(PBu3)2(AL-2H) dissolved in the solvents
studied. These results show that the quantum yields for
Ru(CO)2(PBu3)2(AL-2H) are between 2- and 20-fold less than
(28) Mataga, N.; Kaifu, Y.; Koizumi, M. Bull. Chem. Soc. Jpn. 1956, 29,
465.
(29) Birks, J. B. Photophysics of Aromatic Molecules, Wiley-Interscience:
New York, 1970.
(30) Lakowicz, J. R. Principles of Fluorescence Spectroscopy, Plenum
Press: New York, 1983.

5816 Inorganic Chemistry, Vol. 39, No. 25, 2000
Churchill et al.
Table 7. Fluorescence Quantum Yields (Φ), Excited-State
Lifetimes (τ), Radiative Decay Rates (kr), and Nonradiative Decay
Rates (Σknr) for Ru(CO)2(PBu3)2(AL-2H) Dissolved in Various
Solvents at Ambient Temperature
states and to tune the photophysical and photochemical proper-
ties of these sorts of Ru-based luminophores (cf. Tables 6 and
7). Many of these complexes exhibit a diversity of energetically
accessible charge-transfer, ligand-field, and intraligand excited
states. These excited states have different orbital parentages and,
thus, quite different excited-state characteristics.31-33
solvent
Φa
τ, nsb
107kr, s-1
109Σknr, s-1
cyclohexane
toluene
chloroform
dichloromethane
dimethyl sulfoxide 0.014 1.05
ethanol
0.002 0.22
0.005 0.28
0.018 1.08
0.014 1.03
0.81 ( 0.08 4.54 ( 0.41
1.74 ( 0.15 3.55 ( 0.25
1.68 ( 0.09 0.91 ( 0.02
1.33 ( 0.07 0.96 ( 0.01
1.34 ( 0.07 0.94 ( 0.01
The low-energy absorbance bands can be attributed to the
MLCT states; the fluorescence maxima above and around 600
nm are also consistent with the formation of MLCT excited
states. However, the MLCT excited states here are very different
0.010 0.84 ( 0.01 1.19 ( 0.06 1.18 ( 0.01
0.010 0.98 ( 0.01 1.07 ( 0.05 1.01 ( 0.01
2+
acetonitrile
from those of Ru(bpy)3 and other related complexes. It is
likely that the excited states for Ru(CO)2(PBu3)2(AL-2H) are
either significantly mixed versions of MLCT, ligand-field, and
intraligand states or that intersystem crossings are considerably
enhanced, perhaps due to the presence of the PBu3 ligands. From
a photophysical standpoint, Ru(CO)2(PBu3)2(AL-2H) is unique
in the origin of its behavior and in its potential bioanalytical
utility.
a (0.001. b e(0.02 ns.
those for Ru(bpy)32+. They also show that the excited-state
fluorescence lifetimes are between 2 and 3 orders of magnitude
less than those of Ru(bpy)32+. Finally, the results presented in
Table 7 show that the kr and Σkr values differ by about 1-2
orders of magnitude.
The origin of the significantly shorter (a factor of 103) excited-
state fluorescence lifetimes for Ru(CO)2(PBu3)2(AL-2H) relative
to other luminescent ruthenium complexes such as Ru(bpy)3
Acknowledgment. Research contributions from Mr. Mark
Przybylski and Ms. Sinduja Srinivasan are gratefully acknowl-
edged. This work was funded in part by a grant from the
Petroleum Research Fund (to J.B.K.), administered by the
American Chemical Society, and by grants to F.V.B. from the
National Science Foundation, the Office of Naval Research, and
the Department of Energy.
2+
,
Ru(dpp)32+, and related compounds arises from the unique
molecular structure of this complex, including the lack of
symmetry inherent to the other aforementioned ruthenium
complexes as well as the nonaromaticity of four of the ligands.
As is well-documented,31-33 the emitting-state energies and
excited-state photophysical properties are sensitive to variations
in the metal, coordinating ligand, and physicochemical properties
of the local environment. In fact, solvents and substituent choices
may be used to control the relative energetics of different excited
Supporting Information Available: Listings of crystal data,
crystallographic data collection information, structure solution, and
refinement details, atomic coordinates and equivalent isotropic displace-
ment coefficients, bond lengths, bond angles, anisotropic displacement
coefficients, hydrogen atom coordinates and isotropic displacement
coefficients, along with a packing diagram of the molecules within the
unit cell (Figure 1S). This material is available free of charge via the
Internet at http://pubs.acs.org.
(31) Xu, W.; McDonough, R. C., III.; Langsdorf, B.; Demas, J. N.; Degraff,
B. A. Anal. Chem. 1994, 66, 4133.
(32) Demas, J. N.; Degraff, B. A. Anal. Chem. 1991, 63, 829A.
(33) Juris, A.; Balzani, V.; Barigelletti, F.; Campagna, S.; Belser, P.; von
Zelewsky, A. Coord. Chem. ReV. 1988, 84, 85.
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