Synthesis and characterization of low-spin and cation radical complexes of ruthenium(III) of a tridentate pyridine bis-amide ligand

Article abstract of DOI:10.1021/ic034356y

Using a tridentate bis-amide ligand 2,6-bis(N-phenylcarbamoyl)pyridine (H₂L), in its deprotonated form, a new mononuclear ruthenium(III) complex [Et₄N][RuL₂]·H₂O (1) has been synthesized. Structural analysis reveals that the RuN₆ coordination comprises four deprotonated amide-N species in the equatorial plane and two pyridine-N donors in the axial positions, imparting a tetragonally compressed octahedron around Ru. To the best of our knowledge, this is the first time that a ruthenium(III) complex coordinated solely by two tridentate deprotonated peptide ligands has been synthesized and structurally characterized. When examined by cyclic voltammetry, complex 1 displays in MeCN/CH₂Cl ₂ solution three chemically/electrochemically reversible redox processes: a metal-centered reductive Ru^III-Ru^II couple (E_1/2 = -0.84/-0.89 V vs SCE) and two ligand-centered oxidative responses (E_1/2 = 0.59/0.60 and 1.05/1.05 V vs SCE). Isolation of a dark blue one-electron oxidized counterpart of 1, [RuL₂]· H₂O (2), has also been readily achieved. The complexes have been characterized by analytical, solution electrical conductivity, IR, electronic absorption and EPR spectroscopy, and temperature-dependent magnetic susceptibility measurements, For complex 1, a weak and broad transition within the t_2g level has been identified at ～1400 nm and supported by EPR spectral analysis (S = 1/2). Temperature-dependent magnetic susceptibility data provide unambiguous evidence that in 2 strong antiferromagnetic coupling of the S = 1/2 ruthenium atom with the S = 1/2 ligand π-cation radical leads to an effectively S = 0 ground state (¹H NMR spectra in CDCl ₃ solution).

Full text of DOI:10.1021/ic034356y

Inorg. Chem. 2003, 42, 6497−6502
Synthesis and Characterization of Low-Spin and Cation Radical
Complexes of Ruthenium(III) of a Tridentate Pyridine Bis-Amide Ligand
Akhilesh Kumar Singh, V. Balamurugan, and Rabindranath Mukherjee*
Department of Chemistry, Indian Institute of Technology, Kanpur 208 016, India
Received April 2, 2003
Using a tridentate bis-amide ligand 2,6-bis(N-phenylcarbamoyl)pyridine (H L), in its deprotonated form, a new
2
mononuclear ruthenium(III) complex [Et4N][RuL2]‚H O (1) has been synthesized. Structural analysis reveals that
2
the RuN coordination comprises four deprotonated amide-N species in the equatorial plane and two pyridine-N
6
donors in the axial positions, imparting a tetragonally compressed octahedron around Ru. To the best of our
knowledge, this is the first time that a ruthenium(III) complex coordinated solely by two tridentate deprotonated
peptide ligands has been synthesized and structurally characterized. When examined by cyclic voltammetry, complex
1 displays in MeCN/CH Cl2 solution three chemically/electrochemically reversible redox processes: a metal-centered
2
III
II
reductive Ru −Ru couple (E1/2 ) −0.84/−0.89 V vs SCE) and two ligand-centered oxidative responses (E
)
1/2
0.59/0.60 and 1.05/1.05 V vs SCE). Isolation of a dark blue one-electron oxidized counterpart of 1, [RuL2]‚H O(2),
2
has also been readily achieved. The complexes have been characterized by analytical, solution electrical conductivity,
IR, electronic absorption and EPR spectroscopy, and temperature-dependent magnetic susceptibility measurements.
For complex 1, a weak and broad transition within the t2g level has been identified at ∼1400 nm and supported
1
by EPR spectral analysis (S ) /2). Temperature-dependent magnetic susceptibility data provide unambiguous
evidence that in 2 strong antiferromagnetic coupling of the S ) 1/2 ruthenium atom with the S ) 1/2 ligand π-cation
radical leads to an effectively S ) 0 ground state (1H NMR spectra in CDCl3 solution).
Introduction
of the one-electron oxidized form of six-coordinate low-spin
iron(III) complexes of type [(bpb/Me6bpb)Fe(CN)2]- [H2bpb
) 1,2-bis(pyridine-2-carboxamido)benzene; H2Me6bpb )
1,2-bis(3,5-dimethylpyridine-2-carboxamido)-4,5-dimethyl-
benzene]. For such oxidized complexes, strong intramolecular
antiferromagnetic coupling [|-2J| g 450 cm-1; singlet-
triplet energy gap is expressed in terms of 2J] between the
metal unpaired d electron (S ) 1/2) and the unpaired electron
There has been a growing interest in the development of
coordination chemistry of deprotonated peptide ligands
derived from 2,6-pyridinedicarboxylic acid toward transition
metal ions.1-5 Recently, we6 and others7 reported isolation
* To whom correspondence should be addressed. E-mail: rnm@iitk.ac.in.
(1) Marlin, D. S.; Mascharak, P. K. Chem. Soc. ReV. 2000, 29, 69.
(2) (a) Chavez, F. A.; Olmstead, M. M.; Mascharak, P. K. Inorg. Chem.
1996, 35, 1410. (b) Chavez, F. A.; Olmstead, M. M.; Mascharak, P.
K. Inorg. Chem. 1997, 36, 6323. (c) Marlin, D. S.; Olmstead, M. M.;
Mascharak, P. K. Inorg. Chem. 1999, 38, 3258. Noveron, J. C.;
Olmstead, M. M.; Mascharak, P. K. J. Am. Chem. Soc. 1999, 121,
3553. (d) Marlin, D. S.; Olmstead, M. M.; Mascharak, P. K. Inorg.
Chim. Acta 2000, 297, 106. (e) Marlin, D. S.; Olmstead, M. M.;
Mascharak, P. K. J. Mol. Struct. 2000, 554, 211. (e) Noveron, J. C.;
Olmstead, M. M.; Mascharak, P. K. J. Am. Chem. Soc. 2001, 123,
3247. (f) Marlin, D. S.; Olmstead, M. M.; Mascharak, P. K. Inorg.
Chem. 2001, 40, 7003.
(3) (a) Ray, M.; Ghosh, D.; Shirin, Z.; Mukherjee, R. Inorg. Chem. 1997,
36, 3568. (b) Patra, A. K.; Mukherjee, R. Inorg. Chem. 1999, 38, 1388.
(4) (a) Redmore, S. M.; Rickard, C. E. F.; Webb, S. J.; Wright, L. J.
Inorg. Chem. 1997, 36, 4743. (b) Yano, T.; Tanaka, R.; Nishioka, T.;
Kinoshita, I.; Isobe, K.; Wright, L. J.; Collins, T. J. Chem. Commun.
2002, 1396.
1
of ligand π-cation radical (S ) /2), yielding an effectively
S ) 0 ground state for the iron(III) complex, was observed.
Che et al. reported that low-spin iron(III) complexes of type
[Fe(bpc)(L)2][ClO4] [H2bpc ) 1,2-bis(pyridine-2-carbox-
amido)-4,5-dichlorobenzene, L ) Bu3P, Im, 1-MeIm, tBu(py)]
exhibit well-behaved one-electron ligand-centered oxidation
processes.8a They observed similar ligand-centered oxidation
processes with Co(III),8b Rh(III),8c and Ir(III)8c complexes
(6) (a) Ray, M.; Mukherjee, R.; Richardson, J. F.; Buchanan, R. M. J.
Chem. Soc., Dalton Trans. 1993, 2451 and references therein. (b) Patra,
A. K.; Ray, M.; Mukherjee, R. Inorg. Chem. 2000, 39, 652 and
references therein.
(5) Kawamoto, T.; Hammes, B. S.; Ostrander, R.; Rheingold, A. L.;
Borovik, A. S. Inorg. Chem. 1998, 37, 3424.
(7) Dutta, S. K.; Beckmann, U.; Bill, E.; Weyhermu¨ller, T.; Wieghardt,
K. Inorg. Chem. 2000, 39, 3355.
10.1021/ic034356y CCC: $25.00 © 2003 American Chemical Society
Published on Web 09/12/2003
Inorganic Chemistry, Vol. 42, No. 20, 2003 6497

Singh et al.
bamoyl)pyridine and tetra-n-butylammonium perchlorate (TBAP)
were prepared as before.3,11 [Ru(DMSO)4Cl2] was prepared fol-
lowing a reported procedure.12
of bpb(2-) and bpc(2-). But in either case, the oxidized
products were not isolated, and their full characterization was
not attempted.8
Synthesis of Metal Complexes. (a) [Et4N][RuL2]‚H2O (1). The
ligand H2L (0.1 g, 0.315 mmol) was dissolved in dinitrogen-flushed
N,N′-dimethylformamide (DMF) (15 mL), and to it was added solid
NaH (0.015 g, 0.628 mmol), resulting in a light yellow solution.
To it was added solid [Ru(DMSO)4Cl2] (0.076 g, 0.157 mmol)
under dinitrogen atmosphere. The resulting solution was stirred for
10 min and refluxed for 6 h during which a color change to dark
red was observed. To this was added solid [Et4N]Cl‚xH2O (0.052
g, 0.314 mmol), and stirring was continued for 10 h. Removal of
the solvent was followed by addition of MeCN (5 mL) and filtration.
Slow evaporation afforded crystalline reddish-brown precipitate,
which was filtered and washed with 5 mL of MeCN-Et2O (1:5
v/v). The solid thus collected was dried in air (yield: 80 mg,
∼58%).
As a part of this program, we have developed the
coordination chemistry of the tridentate dianion of 2,6-bis-
(N-phenylcarbamoyl)pyridine (H2L), and in the process, a
number of complexes of the type [ML2]z [M ) Fe(III) and
Co(III) (z ) 1-); Ni(II) (z ) 2-); Ni(IV) (z ) 0)] have
been synthesized and characterized by X-ray crystallogra-
phy.3 Each complex contains two meridionally coordinated
tridentate ligands to impart a tetragonally compressed
octahedral geometry about the metal ion. It is worth
mentioning here that the one-electron oxidation processes
observed for [FeL2]- and [CoL2]- were arbitrarily assigned
as ligand-centered.3a
Collins et al. have shown that redox-inert macrocyclic
tetraamido-N ligands are capable of stabilizing unusual non-
oxo high-valent transition metal ions such as cobalt(IV) and
iron(IV).9 They also demonstrated that such a macrocyclic
ligand is noninnocent and structurally characterized a ligand
π-cation radical complex of Co(III).10 Therefore, we felt that,
by using a bis-ligand complex of ruthenium(III) supported
by ligand L(2-), which interestingly provides nonmacro-
cyclic tetraamido-N coordination in the equatorial plane, it
might be possible to generate and characterize [RuIVL2]
species. This hope was fueled by the added expectation that,
for ruthenium, the generation/stabilization of the non-oxo
ruthenium(IV) state would be facilitated. Our approach to
this problem has been to work out a synthetic method to
isolate a bis-ligand complex of ruthenium(III) that would
allow the oxidized species to be isolated in the analytically
pure form and to assign the correct formulation of such
species. In the present work, we have studied the ruthenium
chemistry with tridentate ligand H2L, in its deprotonated
form. Here, we report the isolation and characterization of
the ruthenium(III) complex [Et4N][RuL2]‚H2O (1), including
X-ray crystallography, and to firmly establish whether L(2-)
is noninnocent, we prepared the one-electron oxidized
complex [RuL2]‚H2O (2) and characterized it to a reasonable
level of confidence.
Characterization. Anal. Calcd for C46H48N7O5Ru: C, 62.79;
H, 5.46; N, 11.15. Found: C, 63.00; H, 5.50; N, 11.10. Conductivity
(MeCN, ∼1 mM solution at 298 K): ΛM ) 125 Ω-1 cm2 mol-1
(expected range13 for 1:1 electrolyte: 120-160 Ω-1 cm2 mol-1).
Absorption spectrum [λmax, nm (ꢀ, M-1 cm-1)]: (in MeCN) 250
sh (27 700), 300 sh (13 500), 383 (11 200), 440 sh (8300), 1410
(270); (in CH2Cl2) 254 sh (28 050), 304 sh (13 450), 388 (11 400),
440 sh (8820).
(b) [RuL2]‚H2O (2). To a magnetically stirred solution of
[Et4N][RuL2]‚H2O (1) (0.080 g, 0.09 mmol) in MeCN (3 mL) was
added a solution of (NH4)2Ce(NO3)6 (0.060 g, 0.109 mmol) in
MeCN (3 mL) dropwise. The dark blue compound that precipitated
out was collected by filtration and washed with MeCN. The solid
thus obtained was dried in vacuo (yield: 55 mg, ∼83%).
Characterization. Anal. Calcd for C38H28N6O5Ru: C, 60.87;
H, 3.74; N, 11.21. Found: C, 60.94; H, 3.80; N, 11.20. Conductivity
(DMF, ∼1 mM solution at 298 K): ΛM ) 16 Ω cm2 mol-1
(expected range13 for 1:1 electrolyte: 65-90 Ω-1 cm2 mol-1).
Absorption spectrum [λmax, nm (ꢀ, M-1 cm-1)]: (in CH2Cl2) 303
(16 000), 360 sh (11 520), 658 (11 650). 1H NMR (400 MHz,
CDCl3): δ 7.814 (2H, d, py-3,5-H), 7.655 (1H, t, py-4-H), 7.057
(3H, m, aromatic-3,4,5-H), 6.617 (2H, d, aromatic-2,6-H), 1.581
(s, H2O).
Physical Measurements. Elemental analyses were obtained at
the Department of Chemistry, Indian Institute of Technology
Kanpur, India. Conductivity measurements were done with an Elico
type CM-82T conductivity bridge (Hyderabad, India). Spectroscopic
measurements were made using the following instruments: IR (KBr,
4000-600 cm-1), Bru¨ker Vector 22; electronic, Perkin-Elmer
Lambda 2; X-band EPR, Varian 109 C (fitted with a quartz dewar
for measurements at liquid-dinitrogen temperature), the spectra were
calibrated with diphenylpicrylhydrazyl, DPPH (g ) 2.0037).
Magnetic Measurements. Temperature-dependent magnetic
susceptibility measurements on solid samples of 1 and 2 were done
(9) (a) Anson, F. C.; Collins, T. J.; Coots, R. J.; Gipson, S. L.; Richmond,
T. G. J. Am. Chem. Soc. 1984, 106, 5037. (b) Collins, T. J.; Kostka,
K. L.; Mu¨nck, E.; Uffelman, E. S. J. Am. Chem. Soc. 1990, 112, 5637.
(c) Collins, T. J.; Fox, B. G.; Hu, Z. G.; Kostka, K. L.; Mu¨nck, E.;
Rickard, C. E. F.; Wright, L. J. J. Am. Chem. Soc. 1992, 114, 8724.
(d) Kostka, K. L.; Fox, B. G.; Hendrich, M. P.; Collins, T. J.; Rickard,
C. E. F.; Wright, L. J.; Mu¨nck, E. J. Am. Chem. Soc. 1993, 115, 6746.
(10) Collins, T. J.; Powell, R. D.; Slebodnick, C.; Uffelman, E. S. J. Am.
Chem. Soc. 1991, 113, 8419.
Experimental Section
Materials and Reagents. All reagents were obtained from
commercial sources and used as received. Solvents were dried/
purified as reported previously. The ligand 2,6-bis(N-phenylcar-
(11) (a) Ray, M.; Mukerjee, S.; Mukherjee, R. J. Chem. Soc., Dalton Trans.
1990, 3635. (b) Gupta, N.; Mukerjee, S.; Mahapatra, S.; Ray, M.;
Mukherjee, R. Inorg. Chem. 1992, 31, 139.
(12) Evans, I. P.; Spencer, A.; Wilkinson, G. J. Chem. Soc., Dalton Trans.
1973, 204.
(8) (a) Che, C.-M.; Leung, W.-H.; Li, C.-K.; Cheng, H.-Y.; Peng, S.-M.
Inorg. Chim. Acta 1992, 196, 43. (b) Mak, S.-T.; Wong, W.-T.; Yam,
V. W.-W.; Lai, T.-F.; Che, C.-M. J. Chem. Soc., Dalton Trans. 1991,
1915. (c) Mak, S.-T.; Yam, V. W.-W.; Che, C. M.; Mak, T. C. W. J.
Chem. Soc., Dalton Trans. 1990, 2555.
(13) Geary, W. J. Coord. Chem. ReV. 1971, 7, 81.
6498 Inorganic Chemistry, Vol. 42, No. 20, 2003

Ru(III) Pyridine Bis-Amide Complexes
Table 1. Data Collection and Structure Refinement Parameters for
[Et4N][RuL2]‚H2O (1)
in the ranges 81 < T < 300 K and 54 < T < 300 K, respectively,
using a locally built Faraday balance. The setup6 consists of an
electromagnet with constant-gradient pole caps (Polytronic Cor-
poration, Mumbai, India), Sartorius balance M-25-D/S (Go¨ttingen,
Germany), a closed cycle refrigerator, and a Lake Shore temperature
controller (Cryo Industries, USA). All measurements were made
at a fixed main field strength of ∼6 kG. The calibration of the
system and details of measurements are already reported in the
literature.3
param
chem formula
1
C46H48N7O5Ru
879.98
fw
temp/K
298
λ/Å
0.710 73
monoclinic
0.3 × 0.2 × 0.2
C2/c
cryst syst
cryst size/mm × mm × mm
space group
Solution-state magnetic susceptibilities were obtained by the
NMR technique of Evans14 in MeCN with a PMX-60 JEOL (60
MHz) NMR spectrometer. Corrections underlying diamagnetism
were applied with use of appropriate constants.15
a/Å
b/Å
c/Å
9.809(7)
22.876(5)
19.734(2)
90.0
103.67(4)
90.0
4302(3)
4
0.418
3009
2814 (Rint ) 0.0180)
1.301
R1 ) 0.0945
wR2 ) 0.2663
R1 ) 0.1151
wR2 ) 0.2900
R/deg
â/deg
Electrochemical Measurements. Cyclic voltammetric experi-
ments were performed at 298 K by using a PAR model 370
electrochemistry system consisting of M-174A polarographic
analyzer, M-175 universal programmer, and RE 0074 X-Y recorder.
The cell contained a Beckman (M 39273) platinum-inlay working
electrode, a Pt wire auxiliary electrode, and a saturated calomel
electrode (SCE) as reference electrode. Details of the cell config-
uration are as described before.11 For coulometry, a platinum-wire-
gauze electrode was used as the working electrode. The solutions
were ∼1.0 mM in complex and 0.2 M in supporting electrolyte,
TBAP.
Crystal Structure Determination. A reddish brown needle-
shaped crystal of [Et4N][RuL2]‚H2O (1) (dimensions: 0.3 × 0.2 ×
0.2 mm3) was used for data collection. Diffracted intensities were
collected on an Enraf Nonius CAD-4-Mach four-circle diffrac-
tometer using graphite-monochromated Mo KR radiation. Intensity
data were corrected for Lorentz polarization effects; analytical
absorption corrections were also applied. The structure was solved
by SHELXS-86 (Patterson heavy atom method), expanded by
Fourier-difference syntheses, and refined with the SHELXL97
package incorporated in WINGX 1.64 crystallographic collective
package.16 The linear absorption coefficients, neutral atom scattering
factors for the atoms, and anomalous dispersion corrections for non-
hydrogen atoms were taken from ref 17. The positions of the
hydrogen atoms were calculated assuming ideal geometries, but
not refined. All non-hydrogen atoms were refined with anisotropic
thermal parameters by full-matrix least-squares procedure on F2.
We could not locate hydrogen atoms of the Et4N+ cation and water
molecule from the electron density map. In fact, for this complex
some disorder was observed with the Et4N+ cation. Two positions
were identified as possible methylene carbon atoms, and they were
refined with a site occupation factor of 0.5/0.5 and 0.6/0.4. The
terminal methyl carbon sites C(20) and C(23) showed large
displacement parameters indicating some degree of disorder but
could not be resolved. An unassigned peak (2.471 Å3) was found
near Ru atom at a distance of 0.949 Å, which may be due to the
poor quality of crystal chosen for data collection. Pertinent
crystallographic parameters are summarized in Table 1.
γ/deg
V/Å3
Z
µ/mm-1
no. reflns collected
no. indep reflns
GOF on F2
final R indices [I > 2σ(I)]
R indices (all data)
Na2L with [Ru(DMSO)4Cl2] in MeCN at 298 K followed
by reflux at ∼325 K forming a reddish-brown solution.
Addition of [Et4N]Cl‚xH2O and usual workup afforded
reddish-brown air-stable crystals. Clean one-electron chemi-
cal oxidation of 1 in MeCN solution by (NH4)2Ce(NO3)6
generates a deep blue solution, which on usual workup led
to the isolation of the product as a microcrystalline moisture
sensitive blue solid, [RuL2]‚H2O (2). Therefore, all manipu-
lations with 2 were performed in dry atmosphere. Unfortu-
nately, all attempts to grow single crystals of 2 for structural
analysis failed so far.
The absence of ν(N-H) in the IR spectra of these
complexes indicates that the ligands are coordinated in the
deprotonated form.18 A strong split band at 1602 and 1573
cm-1 in the spectrum of 1 is assigned to a C-O stretching
frequency of ν(amide I) vibration; in the spectrum of 2, two
bands at 1650 and 1629 cm-1 (Figure S1, Supporting
Information) indicate an increase in the C-O bond strength
that we associate with the different oxidation levels of L(2-)
ligands in blue oxidized complex 2 (vide infra). We are
convinced that this behavior is diagnostic of ligand oxidation
(vide infra).6,7,10 A similar shift in the C-O stretching
frequencies has been observed for low-spin Fe(III) and low-
spin Fe(III) cation-radical complexes, recently reported from
this laboratory.6 In MeCN solution, while complex 1 behaves
as a 1:1 electrolyte, complex 2 is nonconducting, as
expected.13 Elemental analyses, IR, and solution electrical
conductivity data are in good agreement with the described
formulations.
Results and Discussion
Synthesis. The synthesis of the ruthenium(III) compound
[Et4N][RuL2]‚H2O (1) involved initial anaerobic reaction of
Description of the Structure of [Et4N][RuL2]‚H2O (1).
A view of the metal environment in the anion of 1 is
presented in Figure 1. The Ru atom sits on an imposed C2
axis and is coordinated by four deprotonated amide nitrogens
in the equatorial plane [N(1) and N(3) and their symmetry
related] and two pyridyl nitrogens [N(2) and its symmetry
(14) Evans, D. F. J. Chem. Soc. 1959, 2003.
(15) O’Connor, C. J. Prog. Inorg. Chem. 1992, 32, 233.
(16) Farrugia, L. J. WINGX Version 1.64, An Integrated System of Windows
Programs for the Solution, Refinement and Analysis of Single-Crystal
X-ray Diffraction Data; Department of Chemistry, University of
Glasgow: Glasgow, 2003.
(17) International Tables for X-ray Crystallography; Kynoch Press: Bir-
mingham, England, 1974; Vol. IV.
(18) Chapman, R. L.; Vagg, R. S. Inorg. Chim. Acta 1979, 33, 227.
Inorganic Chemistry, Vol. 42, No. 20, 2003 6499

Singh et al.
Figure 2. UV-vis spectra (in CH2Cl2) of [Et4N][RuL2]‚H2O (1) (s) and
[RuL2]‚H2O (2) (‚‚‚). The low-energy band of [Et4N][RuL2]‚H2O (1) (in
MeCN) is shown as an inset.
[Ru(bbpc)(PPh3)Cl] [H2bbpc ) 1,2-bis(4-tert-butylpyridine-
2-carboxamido)-4,5-dichlorobenzene]20 are 2.055 and 2.088
Å, respectively. The Ru-Namide distance (2.054 Å) is appre-
ciably longer than that found in above-mentioned complexes
cis-([Ru(pb)(bpy)Cl2], 2.01 Å, and [Ru(bbpc)(PPh3)Cl], 1.980
Å. A similar trend was observed in all “[ML2]” complexes
[M ) Fe(III), Co(III), Ni(II), and Ni(IV)].3
Figure 1. View of the structure of [RuL2]- in the crystal of its Et4N+
salt, [Et4N][RuL2]‚H2O (1). Hydrogen atoms are omitted for clarity.
Unlabeled atoms are related to labeled atoms by the crystallographic 2-fold
axis.
Absorption Spectra. The absorption spectral studies are
the easiest means of characterization of ruthenium(III)
complex 1 as well as its oxidized counterpart 2. As was
observed for other complexes of L(2-), in the visible region
the absorption spectrum of complex 1 is dominated by
ligand-to-metal charge-transfer (LMCT) transitions. In MeCN/
CH2Cl2 solution, an intense absorption at ∼440 nm justifies
its color. Oxidized product 2 is clearly identifiable by its
characteristic strong absorption feature at ∼650 nm, with
enhancement in extinction coefficients compared to its
ruthenium(III) counterpart. A similar trend was observed
before.6b The absorption spectra of 1 and 2 in CH2Cl2 are
displayed in Figure 2. It is worth noting here that the strongly
allowed LMCT band at 386 nm for activated iron(III)
bleomycins, which has a deprotonated amide coordination,
was shown to be a deprotonated amide-to-iron π-LMCT
band; the weakly allowed transition originating from a
delocalized pyrimidine/amide σ-orbital occurs at somewhat
lower energy.21 We believe that for 1 this transition is
observed at ∼440 nm.
Table 2. Selected Bond Lengths (Å) and Angles (deg) in the Anionic
Part of [Et4N][RuL2]‚H2O (1)
Ru-N(1)
Ru-N(2)
Ru-N(3)
2.066(7)
1.945(9)
2.041(7)
N(1)-Ru-N(2)
N(1)-Ru-N(2′)
N(1)-Ru-N(3)
N(1)-Ru-N(3′)
N(2)-Ru-N(3)
N(2)-Ru-N(3′)
N(1)-Ru-N(1′)
N(2)-Ru-N(2′)
N(3)-Ru-N(3′)
79.5(3)
103.6(3)
158.3(3)
93.4(3)
78.9(3)
98.1(3)
89.4(3)
175.7(3)
91.9(3)
related] in the axial positions. The dihedral angle between
the coordinating planes N(1)-Ru-N(2) and N(2)-Ru-N(3)
is ∼178°, revealing meridional coordination of L(2-), as
that observed with its Fe(III), Co(III), Ni(II), and Ni(IV)
analogues.3 The geometry of the RuN6 coordination poly-
hedra is appreciably compressed octahedral (Table 2), as
before.3 Significant deviation from 90° of the bond angles
involving the chelation is observed (Table 2), presumably
due to formation of five-membered chelate rings with
extended conjugation, as observed for the bis-chelates
reported previously.3 Further evidence of strain in the chelate
rings arises from the following observation. Although in each
ligand the two N-phenyl rings and the pyridine ring are
planar, the two phenyl rings, however, make an angle of
∼80°, and they make angles of ∼60° and ∼90° with the
central pyridine ring.
EPR Spectra. The EPR spectra of low-spin ruthenium(III)
complex 1 were recorded as a microcrystalline solid and in
MeCN solution, at 300 K and at 77 K, and also as a glass in
MeCN-toluene (1:1.2 v/v) to probe the electronic ground
state and distortion parameters. For 1, in the polycrystalline
state the spectral parameters are 2.221 and 1.924 (300 K);
2.204 and 1.917 (77 K). In MeCN solution, the signal is
isotropic: 2.116 (300 K) and 2.168 (77 K). In MeCN-
toluene glass (1:1.2 v/v) the signal is also isotropic (2.168).
Thus, in the solid state the spectral data show axial symmetry
with two principal g values. It should be noted here that the
A few comments on metal-ligand bond distances are in
order. The Ru-Npy bond distance of 1.945 Å in 1 is
appreciably shorter than that observed for ruthenium(III)
complexes of deprotonated pyridine amide ligand systems.
For example, the Ru-Npy distances in cis-[Ru(pb)(bpy)Cl2]
[Hpb ) 2-(N-(4-nitro)-phenylcarbamoyl)pyridine]19 and in
(19) Dutta, S.; Bhattacharya, P. K.; Tieknik, E. R. T. Polyhedron 2001,
20, 2027.
(20) Ko, P.-H.; Chen, T.-Y.; Zhu, J.; Cheng, K.-F.; Peng, S.-M.; Che, C.-
M. J. Chem. Soc., Dalton Trans. 1995, 2215.
(21) Neese, F.; Zaleski, J. M.; Zaleski, K. L.; Solomon, E. I. J. Am. Chem.
Soc. 2000, 122, 11703 and references therein.
6500 Inorganic Chemistry, Vol. 42, No. 20, 2003

Ru(III) Pyridine Bis-Amide Complexes
nitrogen species coordinating. In fact, the E1/2 values (V vs
SCE) for the MIII-MII couple follow the following trend:
Co (-1.10) < Fe (-0.91) < Ru (-0.84) < Ni (0.05).3 Thus,
the Fe(III) state is better stabilized than the Ru(III) state by
L(2-) in their bis-ligand complexes. A trend in a similar
2+
direction was found for M(terpy)33+/M(terpy)3 (terpy )
2,2′:6′,2′′-terpyridine; M ) Fe or Ru; E1/2 (V vs SSCE) )
1.09 for Fe and 1.27 for Ru) redox processes in MeCN.24 In
both systems, the spin-state properties of bivalent and
trivalent species are expected to remain invariant, due to the
strong field nature of the ligands.
Figure 3. Cyclic voltammogram (100 mV/s) of a 1.02 mM solution of
[Et4N][RuL2]‚H2O (1) at a platinum electrode in CH2Cl2 (0.2 M in TBAP).
Indicated potentials (in V) are vs SCE.
(b) Oxidative Response of Ruthenium(III) Complex.
When scanned anodically, complex 1 displays in MeCN
solution two reversible one-electron oxidative responses at
E1/2 of 0.59 V (∆Ep ) 60 mV) and 1.05 V (∆Ep ) 100 mV)
versus SCE (Figure S3, Supporting Information). The anodic
CV scan of 1 in CH2Cl2 (Figure 3) displays two responses
at E1/2 of 0.60 V (∆Ep ) 80 mV) and 1.05 V (∆Ep ) 100
mV) versus SCE. These couples are of considerable interest
with regard to the oxidation state of ruthenium. For the less
positive oxidative response, two alternative formal descrip-
tions are possible: (1) It is a RuIV-RuIII couple, and
therefore, in the oxidized complex 2 ruthenium is present in
the tetravalent state. (2) It is a ligand-centered oxidative
response, and hence, complex 2 is a ruthenium(III)-stabilized
ligand π-cation radical complex. Admittedly, few genuine
homoleptic complexes of ruthenium(IV) are known, and
those reported invariably have strongly σ-donating anionic
ligands. To our knowledge, among pyridine amide complexes
of trivalent metal ions, barring that observed for [FeL2]- and
[CoL2]- complexes, no examples of the latter description
exist in the literature. As we have shown (vide infra), the
observed redox responses are due to ligand-centered oxida-
tions.
X-ray results have shown that the Ru atom in complex 1
has tetragonally compressed octahedral geometry and there-
fore is revealed by its axial EPR spectra (Figure S2,
Supporting Information). We have analyzed the g values
obtained in the polycrystalline state at 77 K according to
the well established theory for the low-spin d5 system.21,22
The distortion in 1 can be quantitated with the help of the
experimental g values and the g tensor theory of low-spin
d5 complexes. Details of the methods used by us can be found
elsewhere.6a,22 The axial splitting parameter (∆, expressed
in terms of spin-orbit coupling constant λ) is 6.025. Taking
λ ∼ 1000 cm-1,22 the energies of the crystal field transitions
(∆E1 and ∆E2) are predicted to be ∼5700 cm-1 (∼1750 nm)
and ∼ 6600 cm-1 (∼1500 nm). A relatively weak and broad
band is indeed observed in MeCN solution of 1 at 1400 nm
(Figure 2). The spectral result thus demonstrates that the
ground state of complex 1 is correctly represented by the
chosen solution.
Redox Properties. To investigate the extent of stabiliza-
tion of the ruthenium(III) state toward reduction and whether
a possibility corresponding to the accessibility of higher
oxidation states could be achieved, cyclic voltammetric (CV)
studies on 1 were performed. CV studies on 2 were
performed to examine whether it is the one-electron oxidized
form of 1.
On the basis of the electronic structural analysis on
activated bleomycins by Solomon et al.,21 we are inclined
to believe that highest energy ligand-based orbitals are
localized on the deprotonated amide moiety.
(c) Redox Processes of Isolated Oxidized Complex. The
CV scan of 2 in CH2Cl2 (Figure S4, Supporting Information)
displays two reductive responses: E1/2 ) -0.88 V versus
SCE (∆Ep ) 120 mV) and E1/2 ) 0.59 V (∆Ep ) 100 mV)
versus SCE. Thus, identical behavior to that observed for
complex 1 is observed, excluding the more positive response
at 1.05 V versus SCE. One-electron nature of the responses
of 2 has been confirmed by coulometric experiments [applied
potential, 0.2 V, n (the number of electrons passed per
molecule) ) 0.98; applied potential, -1.0 V, n ) 1.12]. The
clean nature of the cyclic behavior of 2 documents the purity
and authenticity of the isolated one-electron chemically
(Ce4+) oxidized form of 1. The reductive response at 0.59
V is due to [RuL2]0/- redox process (reduction of cation
radical), and the redox process at -0.88 V is due to
[RuL2]1-/2- (metal-centered reduction).
(a) Metal Reduction of Ruthenium(III) Complex. The
CV scan of 1 in MeCN at a platinum working electrode
shows (Figure S3, Supporting Information) a reversible11b
reductive response at E1/2 of -0.84 V vs SCE (∆Ep ) 80
mV) due to RuIII-RuII redox process. The one-electron nature
of this redox response has been confirmed by (i) comparison
of current height with the redox response of samples of
[MIIIL2]- (M ) Fe, Co, and Ni), [NiIIL2]2-, or [NiIVL2]
species, under the same experimental conditions, as well as
by constant-potential electrolysis (see details in a following
paragraph). The CV scan of 1 in CH2Cl223 [E1/2 of -0.89 V
vs SCE (∆Ep ) 140 mV)] is displayed in Figure 3. The
observed stabilizing potential of L(2-) toward the Ru(III)
state must be due to the presence of four deprotonated amide
(22) (a) Bhattacharya, S.; Ghosh, P.; Chakravorty, A. Inorg. Chem. 1985,
24, 3224 and references therein. (b) Taqui Khan, M. M.; Srinivas, D.;
Kureshy, R. I.; Khan, N. H. Inorg. Chem. 1990, 29, 2320 and
references therein.
In an attempt to provide proof of the existence of the
proposed ligand cation, coulometric one-electron oxidation
(23) Under our experimental conditions, the E1/2 values (V) for couple Fc+/
Fc were 0.40 (MeCN) and 0.49 (CH2Cl2) vs SCE.
(24) Morris, D. E.; Hanck, K. W.; DeArmond, M. K. J. Electroanal. Chem.
1983, 149, 115.
Inorganic Chemistry, Vol. 42, No. 20, 2003 6501

Singh et al.
of green [Et4N][CoL2]‚2H2O to generate “[CoL2]” at 1.2 V
versus SCE was attempted. However, it should be noted here
that even on the CV time-scale the oxidation peak profile,
supposedly due to formation of ligand cation,3a is not ideally
suited to generate electrochemically a stable ligand cation
species. The electrochemically generated transient solution
species (color changes from green to brownish green) very
quickly (within 10-15 s) decomposes/changes to another
species (yellow). Unlike [Et4N][CoL2]‚2H2O, this yellow
solution did not exhibit a well behaved CV response. Either
brownish green (this solution changes its color instanta-
neously to yellow even at 77 K) or yellow solutions did not
display any EPR signal. Given the results at hand, we are
not in a position to provide spectroscopic proof of the
existence of the proposed radical cation from EPR spectros-
copy on “[CoL2]”. Efforts are on to throw more light on the
nature of ligand oxidation process(es) in (i) [RuL2]-, (ii)
[FeL2]-, and (iii) [CoL2]- (with suitably ring substituted
variety of L(2-)), the results of which will be published
elsewhere.
electron spin of an incompletely filled t2g subshell of Ru(III),
rendering complex 2 diamagnetic at 300 K.
1
In CDCl3 solution, complex 2 displays a clean H NMR
spectrum (Figure S7, Supporting Information), closely similar
to that observed for [CoIIIL2]- and [NiIVL2],3 attesting to its
diamagnetic ground state. Assignment of the signals (Ex-
perimental Section) has been made through consideration of
relative peak areas and comparison with free ligand spectra.
Summary and Conclusions. The following are the
principal findings and conclusions of this investigation. (1)
We have succeeded in synthesizing a new bis-ligand ruthe-
nium(III) complex of meridionally coordinated tridentate
pyridine amide L(2-) ligand. Structural characterization has
revealed that the metal ion is coordinated in tetragonally
compressed geometry with four nonmacrocyclic tetradentate
N-coordination in the equatorial plane and two axial pyridine
coordination. To the best of our knowledge, this is the first
time that a ruthenium(III) complex coordinated solely by two
tridentate deprotonated peptide ligands has been synthesized
and structurally characterized. (2) Redox properties of 1
unravel the attainment of a highly stabilized ruthenium(III)
state (RuIII/RuII redox process) and a ligand-centered oxida-
tive response. (3) We have shown in this study that dipeptide
ligand L(2-) is a noninnocent ligand in the sense that it can
exist in two different oxidation levels in coordination
compounds: (i) dianion and (ii) monoanion (ligand π-radi-
cal). The data reported herein indicate that by using the
dianionic dipeptide ligand having picolinamide functionality
stable π-radical cation complexes of low-spin ruthenium(III)
can be isolated in analytically pure form for detailed
characterization. To the best of our knowledge, this report
documents the first example of such a complex with a
tridentate pyridine amide ligand system. The stabilization
of a π-radical cation form of L(2-) is achieved by an
intramolecular antiferromagnetic coupling with a low-spin
ruthenium(III) center with incompletely filled t2g subshell.
The latter description has as yet not been structurally
characterized in a complex, but electronic structure of 2 has
been established to a reasonable level of confidence.
Magnetism. (a) [Et4N][RuL2]‚H2O. The magnetism of
complex 1 was investigated over the temperature range 81-
300 K (Faraday magnetometer) to define spin-state and allow
structural inferences. It follows the Curie-Weiss law (Figure
S5, Supporting Information), which is typical for low-spin
six-coordinate ruthenium(III) complexes. Due to large spin-
orbit coupling, the observed µeff values (2.20 µB at 300 K
and 1.58 µB at 81 K) are larger than the spin-only value of
1.73 µB. Thus, the observed result clearly demonstrates the
1
S ) /2 ground state of 1, which is in excellent agreement
with the X-band EPR spectral behavior of a powdered sample
of 1 (see a following paragraph). The effective magnetic
moment of 1 in MeCN solution is 2.14 µB (300 K), in good
agreement with the solid-state value.
(b) [RuL2]‚H2O. Because we were unable to grow single
crystals for compound 2, structural information is lacking.
An attempt was made to choose between the two alternative
descriptions of the electronic structure of 2 (vide supra) by
using temperature-dependent (54-300 K) magnetic suscep-
tibility studies. The µeff value at 300 K (1.03 µΒ) reveals the
quenching of the unpaired spins, in sharp contrast to that of
1. If it were a ruthenium(IV) complex, then at room
temperature, the µeff value would have been in the range 2.7-
2.9 µB, as observed for an authentic six-coordinate ruthe-
nium(IV) complex (it is a d4 system and therefore is expected
to show paramagnetism corresponding to two unpaired
electrons) of a tetradentate salicylamide ligand.25 The
observed µeff value points toward the ligand cation radical
description. The øMT value monotonically decreases with a
lowering of the temperature (øMT ) 0.135 cm3 mol-1 K at
300 K and øMT ) 0.026 cm3 mol-1 K at 54 K) (Figure S6,
Supporting Information), which is suggestive of the existence
of strong antiferromagnetic coupling. The unpaired electron
spin of a ligand π-cation radical (see above) couples
invariably intramolecularly and strongly to the unpaired
Acknowledgment. This work is supported by grants from
the Council of Scientific and Industrial Research and
Department of Science and Technology, Government of
India, New Delhi. We sincerely thank Prof. Samaresh
Bhattacharya, Jadavpur University, Kolkata, India for his help
with low-energy absorption spectral measurements on 1.
Constructive comments of reviewers were very helpful at
the revision stage.
Supporting Information Available: IR spectra of 1 and 2
(Figure S1), EPR spectrum of 1 in MeCN-toluene glass (Figure
S2), CV scan of 1 in MeCN (Figure S3) and 2 in CH2Cl2 (Figure
S4), plot of molar magnetic susceptibility vs temperature for 1
(Figure S5) and plot of øMT versus T for for 2 (Figure S6), and 1H
NMR spectrum of 2 in CDCl3 at 300 K (Figure S7). Crystal-
lographic data in CIF format. This material is available free of
charge via the Internet at http://pubs.acs.org.
(25) Che, C.-M.; Cheng, W.-K.; Leung, W.-H.; Mak, T. C. W. J. Chem.
Soc., Chem. Commun. 1987, 418.
IC034356Y
6502 Inorganic Chemistry, Vol. 42, No. 20, 2003