Isovalent and mixed-valent diruthenium complexes [(acac)2Ru (II) (μ-bpytz)Ru(II)(acac)2] and [(acac) 2Ru(II)(μ-bpytz)Ru(III)(acac) 2](CI04) (acac = acetylacetonate and bpytz = 3,6-bis(3,5-dimethylpyrazolyl)-1,2,4,5-tetrazine): Synthesis, spectroelectrochemical, and EPR investigation

Article abstract of DOI:10.1021/ic049346r

The title compounds involving the structurally characterized bridging ligand bpytz were characterized, showing very strong electrochemical stabilization of the mixed-valent Ru^IIRu^III state (K _c = 10^13.9) but no detectable (ε < 20 M ^-1 cm^-1) intervalence charge-transfer band in the infrared region. In situ reduction of the neutral precursor produces a diruthenium(II) complex of the bpytz radical anion according to EPR spectroscopy, whereas oxidation of the mixed-valent form leads to a diruthenium(III) species.

Full text of DOI:10.1021/ic049346r

Inorg. Chem. 2004, 43, 6108−6113
Isovalent and Mixed-Valent Diruthenium Complexes [(acac)2RuII
(µ-bpytz)RuII(acac)2] and [(acac)2RuII(µ-bpytz)RuIII(acac)2](ClO ) (acac )
4
Acetylacetonate and bpytz )
3,6-Bis(3,5-dimethylpyrazolyl)-1,2,4,5-tetrazine): Synthesis,
Spectroelectrochemical, and EPR Investigation
,‡
Srikanta Patra,† Biprajit Sarkar,‡ Sandeep Ghumaan,† Jan Fiedler,§ Wolfgang Kaim,* and
,†
Goutam Kumar Lahiri*
Department of Chemistry, Indian Institute of Technology-Bombay, Powai, Mumbai-400076, India,
Institut fu¨r Anorganische Chemie, UniVersita¨t Stuttgart, Pfaffenwaldring 55,
D-70550 Stuttgart, Germany, and J. HeyroVsky Institute of Physical Chemistry, Academy of
Sciences of the Czech Republic, DolejsˇkoVa 3, CZ-18223 Prague, Czech Republic
Received May 19, 2004
The title compounds involving the structurally characterized bridging ligand bpytz were characterized, showing very
II
III
strong electrochemical stabilization of the mixed-valent Ru Ru state (K ) 1013.9) but no detectable (ꢀ < 20 M-1
c
cm-1) intervalence charge-transfer band in the infrared region. In situ reduction of the neutral precursor produces
a diruthenium(II) complex of the bpytz radical anion according to EPR spectroscopy, whereas oxidation of the
mixed-valent form leads to a diruthenium(III) species.
Introduction
in those complexes, on the basis of the electronic nature of
ancillary ligands (Kc): 1 × 1015 (NH3); 3 × 108 (bpy); 1 ×
1013 (acac); 1.4 × 108 ([9]ane S3). In addition, modified bis-
bidentate forms of tetrazine ligands such as 3,6-bis-(3,5-
dimethylpyrazolyl)-1,2,4,5-tetrazine(bpytz),10 3,6-bis-(2-
thienyl)-1,2,4,5-tetrazine (bttz),11 3,6-bis(4-methyl-2-pyridyl)-
1,2,4,5-tetrazine (bmptz),11 and 3,6-bis(dicarboxylic acid)-
1,2,4,5-tetrazine (bctz)12 have been utilized later on in
developing diruthenium complexes [(bpy)2Ru(tz)Ru(bpy)2]n+
incorporating π-acidic bpy coligands.
The observed effect of the electron-rich acac coligands
on the comproportionation constant value (Kc ) 1 × 1013)7
of the bptz-bridged mixed-valent RuIIRuIII species in relation
to that with the π-acidic bpy ligand (Kc ) 3 × 108)6 has
prompted us to scrutinize the effect of the acac ancillary
function in the bpytz-bridged diruthenium complex.
In this report, we describe the synthesis of isovalent
[(acac)2RuII(µ-bpytz)RuII(acac)2] and mixed-valent [(acac)2RuII-
(µ-bpytz)RuIII(acac)2](ClO4) together with their spectroelec-
trochemical and EPR investigation.
The design of polynuclear metal complexes exhibiting
strong intermetallic electronic coupling in mixed-valent states
via the mediation by suitably bridging functionalities has
generated considerable research interest in recent years.1 This
has been primarily due to the relevance for biological
processes,2 molecular electronics,3 and theoretical studies of
electron-transfer kinetics.4 The observation of exceptionally
strong intermetallic electronic coupling in mixed-valent RuII-
(tz)RuIII complexes mediated by 3,6-substituted 1,2,4,5-
tetrazine (tz) ligands has initiated continuous efforts in
designing newer classes of tz-bridged diruthenium complexes.1a
Among the bridging ligands based on the tz unit, the bis-
bidentate ligand 3,6-bis(2-pyridyl)-1,2,4,5-tetrazine (bptz) has
been extensively used in framing diruthenium complexes in
combination with ancillary ligands of varying electronic
aspects such as NH3,5 2,2′-bipyridine (bpy),6 acetylacetonate
(acac),7 [9]aneS3,8 and arenes.9 Considerable variation of
comproportionation constant values (Kc) has been observed
* Authors to whom correspondence should be addressed. E-mail:
lahiri@chem.iitb.ac.in (G.K.L.); kaim@iac.uni-stuttgart.de (W.K.).
† Indian Institute of Technology-Bombay.
Results and Discussion
‡ Universita¨t Stuttgart.
The bridging ligand 3,6-bis(3,5-dimethylpyrazolyl)-1,2,4,5-
tetrazine (bpytz) was prepared according to the reported
§ J. Heyrovsky Institute of Physical Chemistty, Academy of Sciences of
the Czech Republic.
6108 Inorganic Chemistry, Vol. 43, No. 19, 2004
10.1021/ic049346r CCC: $27.50 © 2004 American Chemical Society
Published on Web 08/18/2004

IsoWalent and Mixed-Valent Diruthenium Complexes
Chart 1
procedure;13 its crystal structure( is shown in Figure 1. The
bond distances and angles are in the expected range. The
complexation reaction of bpytz was carried out with the
precursor compound RuII(acac)2(CH3CN)2 in a 1:2 molar
ratio in EtOH under nitrogen, followed by chromatography,
to yield purple and blue complexes corresponding to isova-
(1) (a) Kaim, W. Coord. Chem. ReV. 2002, 230, 126. (b) Brunschwig, B.
S.; Creutz, C.; Sutin, N. Chem. Soc. ReV. 2002, 31, 168. (c) Ward, M.
D.; McCleverty, J. A. J. Chem. Soc., Dalton Trans. 2002, 275. (d)
Demandis, K. D.; Hartshorn, C. M.; Meyer, T. J. Chem. ReV. 2001,
101, 2655. (e) Kaim, W.; Klein, A.; Glo¨ckle, M. Acc. Chem. Res.
2000, 33, 755. (f) McCleverty, J. A.; Ward, M. D. Acc. Chem. Res.
1998, 31, 842. (g) Astruc, D. Acc. Chem. Res. 1997, 30, 383. (h) Ward,
M. D. Chem. Soc. ReV. 1995, 24, 121. (i) Giuffrida, G.; Campagna,
S. Coord. Chem. ReV. 1994, 135-136, 517. (j) Kar, S.; Pradhan, B.;
Sinha, R. K.; Kundu, T.; Kodgire, P.; Rao, K. K.; Puranik, V. G.;
Lahiri, G. K. Dalton Trans. 2004, 1752. (k) Patra, S.; Sarkar, B.;
Ghumaan, S.; Fiedler, J.; Kaim, W.; Lahiri, G. K. Dalton Trans. 2004,
754. (l) Patra, S.; Sarkar, B.; Ghumaan, S.; Fiedler, J.; Zalis, S.; Kaim,
W.; Lahiri, G. K. Dalton Trans. 2004, 750. (m) Patra, S.; Miller, T.
A.; Sarkar, B.; Niemeyer, M.; Ward, M. D.; Lahiri, G. K. Inorg. Chem.
2003, 42, 4707. (n) Chanda, N.; Sarkar, B.; Fiedler, J.; Kaim, W.;
Lahiri, G. K. Dalton Trans. 2003, 3550. (o) Kar, S.; Miller, T. A.;
Chakraborty, S.; Sarkar, B.; Pradhan, B.; Sinha, R. K.; Kundu, T.;
Ward, M. D.; Lahiri, G. K. Dalton Trans. 2003, 2591. (p) Patra, S.;
Mondal, B.; Sarkar, B.; Niemeyer, M.; Lahiri, G. K. Inorg. Chem.
2003, 42, 1322. (q) Chanda, N.; Laye, R. H.; Chakraborty, S.; Paul,
R. L.; Jeffery, J. C.; Ward, M. D.; Lahiri, G. K. J. Chem. Soc., Dalton
Trans. 2002, 3496. (r) Chakraborty, S.; Laye, R. H.; Munshi, P.; Paul,
R. L.; Ward, M. D.; Lahiri, G. K. J. Chem. Soc., Dalton Trans. 2002,
2348. (s) Chakraborty, S.; Laye, R. H.; Paul, R. L.; Gonnade, R. G.;
Puranik, V. G.; Ward, M. D.; Lahiri, G. K. J. Chem. Soc., Dalton
Trans. 2002, 1172. (t) Passaniti, P.; Browne, W. R.; Lynch, F. C.;
Hughes, D.; Nieuwenhuyzen, M.; James, P.; Maestri, M.; Vos, J. G.
J. Chem. Soc., Dalton Trans. 2002, 1740. (u) Mosher, P. J.; Yap, G.
P. A.; Crutchley, R. J. Inorg. Chem. 2001, 40, 1189. (v) Laye, R. H.;
Couchman, S. M.; Ward, M. D. Inorg. Chem. 2001, 40, 4089. (w)
Meyer, W. E.; Amoroso, A. J.; Horn, C. R.; Jaeger, M.; Gladysz, J.
A. Organometallics 2001, 20, 1115. (x) Ritchie, J. E.; Murray, R. W.
J. Am. Chem. Soc. 2000, 122, 2964. (y) Weyland, T.; Coustas, K.;
Toupet, L.; Halet, J. F.; Lapinte, C. Organometallics 2000, 19, 4228.
(z) Launay, J.-P.; Fraysse, S.; Coudret, C. Mol. Cryst. Liq. Cryst. 2000,
344, 125. (aa) Baitalik, S.; Florke, U.; Nag, K. J. Chem. Soc., Dalton
Trans. 1999, 719. (bb) Crutchley, R. J. AdV. Inorg. Chem. 1994, 41,
273.
Figure 1. Crystal structure of bpytz.
lent (acac)2RuII(µ-bpytz)RuII(acac)2, 1, and mixed-valent
[(acac)2RuII(µ-bpytz)RuIII(acac)2](ClO4), [1](ClO4) (Chart 1).
Alternatively, the chemical oxidation of 1 by an aqueous
CeIV solution also resulted in 1+. This mixed-valent complex
can be reduced to the isovalent 1 by using reducing agents
such as hydrazine hydrate. All attempts to synthesize a
mononuclear derivative (acac)2RuII(bpytz) using a 1:1 molar
ratio of {Ru(acac)2} and bpytz have failed so far, confirming
the propensity of such systems for charge-transfer-supported
coordinative saturation.1a,5,6 On every occasion 1 and 1+ were
obtained exclusively. The formation of the stable mixed-
valent RuIIRuIII species [1]+ along with the isovalent RuII-
RuII state (1) can be explained in terms of the low RuIIRuIII
h RuIIRuII redox potential (-0.15 V versus SCE) and the
(6) (a) Kaim, W.; Kasack, V. Inorg. Chem. 1990, 29, 4696. (b) Ernst, S.;
Kasack, V.; Kaim, W. Inorg. Chem. 1988, 27, 1146.
(7) Chellamma, S.; Lieberman, M. Inorg. Chem. 2001, 40, 3177.
(8) Roche, S.; Yellowlees, L. J.; Thomas, J. A. Chem. Commun. 1998,
1429.
(9) Singh, A.; Singh, N.; Pandey, D. S. J. Organomet. Chem. 2002, 642,
48.
(10) Sarkar, B.; Laye, R. H.; Mondal, B.; Chakraborty, S.; Paul, R. L.;
Jeffery, J. C.; Puranik, V. G.; Ward, M. D.; Lahiri, G. K. J. Chem.
Soc., Dalton Trans. 2002, 2097.
(11) Sarkar, B.; Kaim, W.; Klein, A.; Schwederski, B.; Fiedler, J.; Duboc-
Toia, C.; Lahiri, G. K. Inorg. Chem. 2003, 42, 6172.
(12) Kasack, V.; Kaim, W.; Binder, H.; Jordanov, J.; Roth, E. Inorg. Chem.
1995, 34, 1924.
(13) (a) Coburn, M. D.; Buntain, G. A.; Harris, B. W.; Hiskey, M. A.;
Lee, K. Y. Ott, D. G. J. Heterocycl. Chem. 1991, 28, 2049. (b)
Glidewell, C.; Lightfoot, P.; Royles, B. J. L.; Smith, D. M. J. Chem.
Soc., Perkin Trans. 1997, 1167. Crystal data for bpytz: C12H14N8,
Mr ) 270.31, monoclinic, space group P21a, Z ) 4, a ) 7.2310(13)
Å, b ) 16.6690(11) Å, c ) 11.0570(7) Å, â ) 103.335(9)°, V )
1296.8(3) Å3, T ) 293(2) K, R ) 0.0386, and Rw ) 0.0935.
(2) Solomon, E. I.; Brunold, T. C.; Davis, M. I.; Kemsley, J. N.; Lee, S.
K.; Lehnert, N.; Neese, F.; Skulan, A. J.; Yang, Y. S.; Zhou, J. Chem.
ReV. 2000, 100, 235.
(3) (a) Paul, F.; Lapinte, C. Coord. Chem. ReV. 1998, 178-180, 431. (b)
Ward, M. D. Chem. Ind. 1996, 568. (c) Ward, M. D. Chem. Ind. 1997,
640.
(4) (a) Brunschwig, B. S.; Sutin, N. Coord. Chem. ReV. 1999, 187, 233.
(b) Bencini, A.; Ciofini, I.; Daul, C. A.; Ferretti, A. J. Am. Chem.
Soc. 1999, 121, 11418.
(5) Poppe, J.; Moscherosch, M.; Kaim, W. Inorg. Chem. 1993, 32, 2640.
Inorganic Chemistry, Vol. 43, No. 19, 2004 6109

Patra et al.
Table 1. Redox Potentials and Comproportionation Constants (Kc) for Corresponding Diruthenium Complexes
compd
RuIIIRuII-RuIIRuII couple E°1/V
RuIIIRuIII-RuIIIRuII couple E°2/V
Kc
ref
(acac)2Ru(bpytz)Ru(acac)2 (1)a
-0.15
1.25
0.17
1.52
0.69
0.67
1.70
0.97
2.02
1.58
1013.9
107.6
1013.6
108.5
1015
present work
[(bpy)2Ru(bpytz)Ru(bpy)2]4+ a
10
7
6, 14
5
b
(acac)2Ru(bptz)Ru(acac)2
[(bpy)2Ru(bptz)Ru(bpy)2]4+ c
[(NH3)2Ru(bptz)Ru(NH3)2]4+ d
a CH3CN-(TEA)ClO4, E/V versus SCE. b CH2Cl2-(TBA)BF4, E/V versus Ag/AgCl. c CH3CN-(TBA)ClO4, E/V versus SCE. d CH3CN-(TBA)ClO4,
E/V versus Ag/AgCl.
Figure 3. 1H NMR spectrum of 1 in CDCl3.
Figure 2. Electrospray mass spectrum of 1 in CH3CN. The inset shows
the expansion in the range m/z ) 800-400.
high comproportionation constant (Kc) value of 1013.9 in the
mixed-valent state (see later). Earlier we have observed that
the reaction of the precursor complex Ru(acac)2(CH3CN)2
with the dihydrotetrazine form, i.e., 3,6-bis(3,5-dimeth-
ylpyrazolyl)-1,4-dihydro-1,2,4,5-tetrazine (H2bpytz), unex-
pectedly led to the formation of a diruthenium(III) complex,
(acac)2RuIII(µ-L2-)RuIII(acac)2 (2), where the preformed
bridging ligand H2bpytz underwent a tetrazine ring-opening
process to a new class of dianionic bridging ligand, L2-.1m
The complex 1 is diamagnetic and neutral whereas [1]-
ClO4 exhibits paramagnetism due to one unpaired electron
and shows 1:1 conductivity. Both products yield satisfactory
microanalyses (see Experimental Section). The formation of
1 and 1+ was confirmed by their electrospray mass spectral
data. Compound 1 exhibits signals centered at m/z values of
869.08, 771.04, 670, and 569.96 (Figure 2), corresponding
to {1}+ (calculated molecular weight 868.87), {1 - acac}+
(769.76), {1 - 2acac}+ (670.66), and {1 - 3acac}+ (571.55),
respectively. Expectedly, [1]ClO4 yielded a molecular mass
at m/z, 869.07 corresponding to [1]+.
Figure 4. Cyclic voltammograms (s) and differential pulse voltammo-
grams (---) of 1 in CH3CN. Scan rate ) 50 mV sec-1
.
Compound 1 displays two quasi-reversible RuII/RuIII
processes in CH3CN at E°1 ) -0.15 V (∆Ep ) 80 mV) and
E°2 ) 0.67 V (∆Ep ) 85 mV) versus SCE (Figure 4; Table
1) which are assigned as successive RuIIRuIII h RuIIRuII and
RuIIIRuIII h RuIIRuIII couples. The analogous bipyridine
complex [(bpy)2RuII(µ-bpytz)RuII(bpy)2]4+ exhibited the same
RuII/RuIII based couples at a much higher potential10 (Table
1). This destabilization of the RuII state in 1 relative to the
bpy derivative can account for the preferential formation of
the mixed-valent RuIIRuIII species [1]+ along with 1. A
similar trend has also been observed in the bptz series for
the acac and bpy complexes (Table 1).6,7,14 The observed
820 mV separation in the potentials between the successive
RuII/RuIII couples (E°2-E°1) in 1 corresponds to a compro-
portionation constant (Kc) value of 1013.9 [calculated using
the equation RT lnKc ) nF(∆E)15]. The observed high Kc
value indicates strong electrochemical coupling, suggesting
a delocalized mixed-valent RuIIRuIII state in 1+. The Kc values
of the corresponding other bpytz and bptz complexes are
set in Table 1 which reveal a dramatic increase in the Kc
value while changing from the combination bpy-tz to acac-
tz. The lowering of the positive charge of the complex
1
The H NMR spectrum of 1 in CDCl3 showed only one
set of signals (Figure 3), which implies the presence of either
the Cs-symmetrical meso complex or a 1:1 mixture of C2-
symmetrical ∆,∆ and Λ,Λ enantiomers.6b,14 The observed
three CH singlets (δ at 5.95 and 5.45 and 5.30 ppm
corresponding to bpytz and acac, respectively) and six
distinct CH3 signals (δ/ppm: 2.78, 2.31, 2.20, 2.04, 2.00,
1.90; four from acac and two from bpytz) suggest that half
of the molecule is essentially representative of 1 due to
internal symmetry.
(14) Ernst, S. D.; Kaim, W. Inorg. Chem. 1989, 28, 1520.
6110 Inorganic Chemistry, Vol. 43, No. 19, 2004
(15) Creutz, C. Prog. Inorg. Chem. 1983, 30, 1.

IsoWalent and Mixed-Valent Diruthenium Complexes
electron-transfer transformations during spectroelectrochemi-
cal experiments.
The starting RuIIRuII complex 1 exhibits two intense
transitions in the visible region at 895 nm (ꢀ ) 25 900 M-1
cm-1) and 557 nm (ꢀ ) 6400 M-1 cm-1). Ligand-based
strong transitions are observed in the UV region. The long-
wavelength bands at 895 and 557 nm are assigned as RuII
f bpytz and RuII f acac based MLCT transitions, respec-
tively.10 This is consistent with the observation that the tetra-
zine function in the bridging ligands (bptz, bpytz) undergoes
facile reduction.1a,7,10 The energy of the bpytz-based MLCT
transition of 1 can be estimated with the help of eqs 1 and
2.17
ν(MLCT) ) 8065(∆E1/2) + 3000 cm-1
∆E1/2 ) E1/2(RuIII-RuII) - E1/2(ligand)
(1)
(2)
where E1/2(RuIII-RuII) is the formal potential in V of the
reversible first RuIIIRuII couple, E1/2 is the potential of the
bpytz-based reduction, and ν(MLCT) is the predicted wave-
number of the charge-transfer band in cm-1; 8065 cm-1/V
is the conversion factor, and the term 3000 cm-1 is of
empirical origin. Considering the value of 1.06 V for ∆E1/2,
the calculated MLCT energy is 11 549 cm-1, well compa-
rable to the observed lowest energy MLCT transition of
11 173 cm-1.
The RuII f π*(tetrazine) transition in the corresponding
bipyridine complex [(bpy)2RuII(µ-bpytz)RuII(bpy)2]4+ ap-
peared at 757 nm.10 The red-shift of 138 nm for the RuII f
π*(tetrazine) transition on moving from bpy to acac ancillary
ligands implies substantial destabilization of the RuII state
in 1 relative to the bpy derivative, as confirmed by their metal
redox potentials stated above. A less pronounced red-shift
of the RuII f π*(tetrazine) transition has been reported in
the bptz series, going from [(bpy)2RuII(µ-bptz)RuII(bpy)2]4+
(683 nm)14,18 to [(acac)2RuII(µ-bptz)RuII(acac)2] (707 nm).7
On one-electron oxidation to the mixed-valent RuIIRuIII
species [1]+, the RuII f π*(tetrazine) transition is blue-
shifted to 617 nm with appreciable reduction in intensity (ꢀ
) 21 000 M-1 cm-1), in accord with a decrease in the
number of RuII centers in [1]+. The mixed-valent state [1]+
did not exhibit a detectable IVCT band in the near-or mid-
IR region. The whole spectral range of the near- and even
mid-IR was explored in the search for the IVCT band, using
UV-vis-NIR and FTIR instruments. In contrast, the cor-
responding [(bpy)2RuII(µ-bpytz)RuIII(bpy)2]5+ showed one
narrow IVCT band at 1534 nm (ꢀ ) 1800 M-1 cm-1) with
the characteristics of class III mixed-valent species.10
In view of the very large Kc value of 1013.9, the absence
of the typically expected IVCT transition in [1]+ appears
puzzling at first. However, a look at bptz-bridged diruthe-
nium(II,III) complexes reveals that the intensity of the IVCT
Figure 5. UV-vis-NIR spectroelectrochemistry of the conversions (a)
1 f 1+, (b) 1+ f 12+, and (c) 1 f 1- in CH2Cl2/0.1 M Bu4NPF6.
Table 2. UV-Vis-NIR Data for 1n+ (n ) 2, 1, 0, -1) from
Spectroelectrochemistrya
compd
λ )
max/nm (ꢀ/M-1 cm-1
12+
1+
1
808 (22 950), 390 (9360), 300 (54 100)
617 (21 000), 455 (br sh), 320 (sh), 285 (54 800)
895 (25 900), 557 (6400), 353 (13 680), 277 (67 500)
872 (9860), 450 (13 800), 340 (sh), 275 (68 100)
1-
a In CH2Cl2/0.1 M Bu4NPF6.
molecule from +4 to 0 essentially facilitates the intermetallic
electrochemical coupling process to a large extent in the
mixed-valent state.
The tetrazine-based one-electron reduction was observed
at E° ) -1.21 V (∆Ep ) 80 mV) versus SCE in CH3CN.
For the corresponding bpy-bpytz complex this process
appeared at -0.13 V.10 Therefore, the introduction of the
acac function in 1 in place of bpy effects substantial
destabilization of both the metal-based highest occupied
molecular orbitals and of the tetrazine-based LUMO.
UV-vis-NIR spectroelectrochemical studies of [1]n+ (n
) 2, 1, 0, -1) were performed in dichloromethane at 298 K
using an OTTLE setup.16 The spectra are shown in Figure
5, and the data are listed in Table 2. The presence of distinct
isosbestic points during the oxidation and reduction processes
(Figure 5) and the electrochemical regeneration without
significant degradation support the reversibility of the
(17) (a) Ghosh, B. K.; Chakravorty, A. Coord. Chem. ReV. 1989, 95, 239.
(b) Santra, B. K.; Lahiri, G. K. J. Chem. Soc., Dalton Trans. 1997,
129. (c) Dosworth, E. S.; Lever, A. B. P. Chem. Phys. Lett. 1986,
124, 152.
(16) Krejcik, M.; Danek, M.; Hartl, F. J. Electroanal. Chem. 1991, 317,
(18) Gordon, K. C.; Burrel, A. K.; Simpson, T. J.; Page, S. E.; Kelso, G.;
Polson, M. I. J.; Flood, A. Eur. J. Inorg. Chem. 2002, 554.
179.
Inorganic Chemistry, Vol. 43, No. 19, 2004 6111

Patra et al.
isotropic g value of 2.15 agrees with g ) 2.17 reported for
the bptz-bridged analogue.7 That spectral profile clearly
indicates a distorted octahedral arrangement around the
ruthenium center(s) in [1]+ with negligible ligand contribu-
tion and a valence-averaged situation.1c,d,22,23 The one-
electron-reduced species [1]- exhibited an unstructured EPR
signal in CH2Cl2 at 4 K with g ) 2.0058. The obviously
small g anisotropy as well as the value close to g (electron)
) 2.0023 confirms that the additional electron in the reduced
state [1]- is largely localized on the bridging ligand bpytz.
Figure 6. EPR spectrum of 1+ in CH2Cl2 at 4 K.
Experimental Section
The starting complex Ru(acac)2(CH3CN)224 and L13 were prepared
according to the reported procedures. Other chemicals and solvents
were reagent grade and used as received. For spectroscopic and
electrochemical studies HPLC grade solvents were used. UV-vis-
NIR spectroelectrochemical studies were performed in CH2Cl2/0.1
M-1 cm-1 Bu4NPF6 at 298 K using an optically transparent thin
layer electrode (OTTLE) cell16 mounted in the sample compartment
of a Bruins Instruments Omega 10 spectrophotometer. FT-IR spectra
were taken on a Nicolet spectrophotometer with samples prepared
as KBr pellets. Solution electrical conductivity was checked using
a Systronic 305 conductivity bridge. Magnetic susceptibility was
band in the near-infrared decreases considerably by replacing
π-accepting ancillary ligands in [(bpy)2Ru(µ-bptz)Ru-
(bpy)2]5+ (ꢀ ) 2800 M-1 cm-1)18 with an “innocent” neutral
ammine in [(H3N)5Ru(µ-bptz)Ru(NH3)5]5+ (ꢀ ) 500 M-1
cm-1)5 and further with anionic acac- in [(acac)2Ru(µ-bptz)-
Ru(acac)2]+ (ꢀ ) 20 M-1 cm-1).7 Starting from a lower ꢀ of
1800 M-1 cm-1 for the bpy/bpytz/bpy combination,10 perhaps
due to less favorable chelate arrangements involving the five-
membered heterocycles, the absence of a detectable IVCT
band for [1]+ (ꢀ < 20 M-1 cm-1) is thus not completely
unexpected. It seems unreasonable to assume what would
be an enormous high-energy shift of the IVCT transition to
about 620 nm where the first long-wavelength band is
observed. At this point we cannot offer a simple explanation
of this substantial band intensity effect which occurs for well-
delocalized species according to the very large Kc values;
however, it should be noted that very weak to undetectable
IVCT bands despite reasonably large Kc values have been
reported previously for a number of diruthenium and
triruthenium mixed-valent systems.1e,k,5,19a,b We, therefore
conclude that large Kc values are really independent of the
IVCT band intensity.
On further oxidation to the RuIIIRuIII species, [1]2+, a new
intense transition appeared at 808 nm (ꢀ ) 22 950 M-1cm-1)
which is assigned to a ligand f RuIII LMCT transition
involving the bridging ligand bpytz and/or the terminal acac
ligands.20
On one-electron reduction to [1]-, the RuII f bpytz MLCT
transition was found to be slightly blue-shifted from 895 nm
(ꢀ ) 25 900 M-1 cm-1) to 872 nm (ꢀ ) 9860 M-1 cm-1)
with a substantial drop in intensity. This is a consequence
of placing an electron in the LUMO (which thus becomes a
singly occupied MO, SOMO).21 On the other hand, the
transition ascribed to a RuII f acac MLCT is appreciably
blue-shifted from 557 nm (ꢀ ) 6400 M-1 cm-1) to 450 nm
(ꢀ ) 13 800 M-1 cm-1) with a reasonable enhancement in
intensity.
1
checked with CAHN electrobalance 7550. H NMR spectra were
obtained with a 300 MHz Varian FT spectrometer. The EPR
measurements were made in a two-electrode capillary tube25 with
a X-band Bruker system ESP300, equipped with a Bruker ER035M
gaussmeter and a HP 5350B microwave counter. Cyclic voltam-
metric, differential pulse voltammetric, and coulometric measure-
ments were carried out using a PAR model 273A electrochemistry
system. Platinum wire working and auxiliary electrodes and an
aqueous saturated calomel reference electrode (SCE) were used in
a three-electrode configuration. The supporting electrolyte was
[NEt4]ClO4, and the solute concentration was ∼10-3 M. The half-
wave potential E°298 was set equal to 0.5(Epa + Epc), where Epa
and Epc are anodic and cathodic cyclic voltammetric peak potentials,
respectively. A platinum wire-gauze working electrode was used
in coulometric experiments. All experiments were carried out under
a dinitrogen atmosphere. The elemental analysis was carried out
with a Perkin-Elmer 240C elemental analyzer. Electrospray mass
spectra were recorded on a Micromass O-ToF mass spectrometer.
Synthesis of 1 and [1](ClO4). The starting complex Ru(acac)2-
(CH3CN)2 (100 mg, 0.26 mmol), and the ligand L (36 mg, 0.13
mmol) were added to 20 mL of ethanol, and the mixture was heated
to reflux for 12 h under a dinitrogen atmosphere. The initial orange
color of the solution gradually changed to dark blue. The solvent
of the reaction mixture was reduced to 5 mL and kept in deep freeze
overnight. It was then filtered, and the violet solid mass thus
obtained was washed thoroughly with cold ethanol. The blue filtrate
was then evaporated to dryness under reduced pressure. The violet
precipitate was dissolved in minimum volume of CH2Cl2 and
purified by using a silica gel column. Initially, a red compound
corresponding to Ru(acac)3 was eluted with CH2Cl2-CH3CN (20:
1). With CH2Cl2-CH3CN (3:1), a violet compound corresponding
The isolated mixed-valent RuIIRuIII species [1]+ displays
a rhombic EPR spectrum in CH2Cl2 glass at 4 K with g1 )
2.442, g2 ) 2.146, and g3 ) 1.818 (Figure 6). The calculated
(22) (a) Bag, N.; Lahiri, G. K.; Basu, P.; Chakravorty, A. J. Chem. Soc.,
Dalton Trans. 1992, 113. (b) Bag, N.; Pramanik, A.; Lahiri, G. K.;
Chakravorty, A. Inorg. Chem. 1992, 31, 40.
(19) (a) Ward, M. D. Inorg. Chem. 1996, 35, 1712. (b) Yeomans, B. D.;
Kelso, L. S.; Tregloan, P. A.; Keene, F. R. Eur. J. Inorg. Chem. 2001,
239.
(20) (a) Bryant, G. M.; Ferguson, J. E. Aust. J. Chem. 1971, 24, 275. (b)
Williams, R. J. P. J. Am. Chem. Soc. 1953, 75, 2163.
(21) Scheiring, T.; Fiedler, J.; Kaim, W. Organometallics 2001, 20, 1437.
(23) Hoshino, Y.; Higuchi, S.; Fiedler, J.; Su, C.-Y.; Kno¨dler, A.;
Schwederski, B.; Sarkar, B.; Hartmann, H.; Kaim, W. Angew. Chem.
2003, 115, 698; Angew. Chem., Int. Ed. 2003, 42, 674.
(24) Kobayashi, T.; Nishina, Y.; Shimizu, K. G.; Satoˆ, G. P. Chem. Lett.
1988, 1137.
(25) Kaim, W.; Ernst, S.; Kasack, V. J. Am. Chem. Soc. 1990, 112, 173.
6112 Inorganic Chemistry, Vol. 43, No. 19, 2004

IsoWalent and Mixed-Valent Diruthenium Complexes
to 1 was separated later on. Evaporation of solvent under reduced
pressure yielded complex 1 in the pure state. Yield: 40 mg (35%).
Anal. Calcd (found) for 1: C, 44.24 (44.13); H, 4.87 (4.20); N,
12.90 (11.99).
structure was solved and refined by full-matrix least-squares on F2
using SHELX-97 (SHELXTL).26 Hydrogen atoms were included
in the refinement process as per the riding model.
Acknowledgment. Financial support received from the
Department of Science and Technology and Council of
Scientific and Industrial Research, New Delhi (India), the
DAAD, and the DFG and the FCI (Germany) is gratefully
acknowledged. We are grateful to Professor Seik Weng Ng,
Institute of Postgraduate Studies, University of Malaya,
Kuala Lumpur, Malaysia, for solving the crystal structure
of bpytz. Special acknowledgment is made to the Sophisti-
cated Analytical Instrument Facility, Indian Institute of
Technology, Bombay, for providing the NMR facility. X-ray
structural studies were carried out at the National Single
Crystal Diffractometer Facility, Indian Institute of Technol-
ogy, Bombay.
The blue mass obtained from the filtrate as stated above was
dissolved in minimum volume of CH3CN. To this an excess aqueous
NaClO4 solution was added and the mixture kept in deep freeze
overnight. It was then filtered and the solid mass washed with ice-
cold water followed by cold ethanol and dried under vacuum. It
was then purified using a neutral alumina column. Initially a small
amount (<3-5%) of the violet compound 1 was eluted with CH2-
Cl2-CH3CN (30:1), followed by a blue compound corresponding
to [1](ClO4), which was separated with CH2Cl2-CH3CN (8:1).
Evaporation of the solvent under reduced pressure resulted in pure
[1](ClO4). Yield: 38 mg (30%). Anal. Calcd (found) for [1](ClO4):
C, 39.69 (39.24); H, 4.37 (4.44); N, 11.57 (10.62). ΛM (Ω-1 cm2
M-1) in CH3CN at 298 K: 115. IR data [ν(ClO4-), cm-1]: 1098
and 629.
Supporting Information Available: X-ray crystallographic data
in CIF format. This material is available free of charge via the
Internet at http://pubs.acs.org.
Crystal Structure Determination. Single crystals of bpytz were
grown by slow diffusion of a dichloromethane solution of it in
hexane followed by slow evaporation. X-ray data of bpytz were
collected on a PC-controlled Enraf-Nonius CAD-4 (MACH-3)
single-crystal X-ray diffractometer using Mo KR radiation. The
IC049346R
(26) Sheldrick, G. M. SHELX-97 Program for Crystal Structure Solution
and Refinement; University of Gottingen: Gottingen, Germany, 1997.
Inorganic Chemistry, Vol. 43, No. 19, 2004 6113