Synthesis, X-ray Structures, and Spectroscopic and Electrochemical Properties of (μ-Oxo)bis(μ-carboxylato)diruthenium Complexes Having Six Imidazole Bases as Terminal Ligands

Complexes
[Ru2O(O2CR)2(1-MeIm)6](ClO4)2
(1a−c),
[Ru2O(O2CR)2(ImH)6](ClO4)2
(2a,b), and
[Ru2O(O2CR)2(4-MeImH)6](ClO4)2
(3a,b) with a
(μ-oxo)bis(μ-carboxylato)diruthenium(III) core
have been prepared by reacting
Ru2Cl(O2CR)4 with
the corresponding imidazole base, viz. 1-methylimidazole
(1-MeIm), imidazole (ImH), and
4-methylimidazole (4-MeImH) in methanol, followed by treatment with
NaClO4 in water (R:  Me, a;
C6H4-p-OMe, b; C6H4-p-Me,
c). Diruthenium(III,IV) complexes
[Ru2O(O2CR)2(1-MeIm)6](ClO4)3
(R:  Me, 4a; C6H4-p-OMe, 4b;
C6H4-p-Me, 4c) have been
prepared by one-electron oxidation of 1 in MeCN with
K2S2O8 in water.
Complexes 1a, 2a·3H2O,
and 4a·1.5H2O have been structurally
characterized. Crystal data for the complexes
are as follows:  1a, orthorhombic,
P212121, a
= 7.659(3) Å, b = 22.366(3) Å, c
= 23.688(2) Å, V = 4058(2)
Å3,
Z = 4, R = 0.0475, and
Rw = 0.0467 for 2669 reflections with
Fo > 2σ(Fo);
2a·3H2O, triclinic, P1̄,
a = 13.735(3)
Å, b = 14.428(4) Å, c = 20.515(8)
Å, α = 87.13(3)°, β = 87.61(3)°, γ =
63.92(2)°, V = 3646(2) Å3,
Z = 4, R
= 0.0485 and Rw = 0.0583 for 10 594
reflections with Fo >
6σ(Fo);
4a·1.5H2O, triclinic, P1̄,
a = 11.969(3) Å,
b = 12.090(6) Å, c = 17.421(3) Å,
α = 108.93(2)°, β = 84.42(2)°, γ =
105.97(2)°, V = 2292(1) Å3,
Z = 2, R
= 0.0567, and Rw = 0.0705 for 6775
reflections with Fo >
6σ(Fo). The complexes have a
diruthenium unit held
by an oxo and two carboxylate ligands, and the imidazole ligands occupy
the terminal sites of the core. The
Ru−Ru distance and the Ru−Ooxo−Ru angle in
1a and 2a·3H2O are
3.266(1), 3.272(1) Å and 122.4(4),
120.5(2)°,
while in 4a·1.5H2O these values are
3.327(1) Å and 133.6(2)°. The diruthenium(III)
complexes 1−3 are blue
in
color and they exhibit an intense visible band in the range 560−575
nm. The absorption is charge transfer in
nature involving the Ru(III)-dπ and Ooxo-pπ
orbitals. The diruthenium(III,IV) complexes are red in
color and
show an intense band near 500 nm. The diruthenium(III) core
readily gets oxidized with K2S2O8
forming
quantitatively the diruthenium(III,IV) complex. The
visible spectral record of the conversion shows an
isosbestic
point at 545 nm for 1 and at 535 nm for 2 and
3. Protonation of the oxide bridge by HClO4
in methanol yields
the
[Ru2(μ-OH)(μ-O2CR)2]3+
core. The hydroxo species shows a visible band at 550 nm. The
pKa value for 1a
is 2.45. The protonated species are unstable. The 1-MeIm
species converts to the diruthenium(III,IV)
core,
while the imidazole complex converts to
[Ru(ImH)6]3+ and some uncharacterized
products. Complex [Ru(ImH)6](ClO4)3 has been structurally characterized.
The diruthenium(III) complexes are essentially diamagnetic
and show
characteristic 1H NMR spectra indicating the presence of
the dimeric structure in solution. The
diruthenium(III,IV)
complexes are paramagnetic and display rhombic EPR spectral features.
Complexes 1−3 are redox
active.
Complex 1 shows the one-electron reversible
RuIII2/RuIIIRuIV,
one-electron quasireversible
RuIIIRuIV/RuIV2,
and
two-electron quasireversible
RuIII2/RuII2 couples
near 0.4, 1.5, and −1.0 V vs SCE in MeCN−0.1 M
TBAP,
respectively, in the cyclic and differential pulse voltammetric
studies. Complexes 2 and 3 exhibit only
reversible
RuIII2/RuIIIRuIV
and the quasireversible
RuIIIRuIV/RuIV2
couples near 0.4 and 1.6 V vs SCE, respectively.
The
observation of a quasireversible one-step two-electron transfer
reduction process in 1 is significant
considering
its relevance to the rapid and reversible
FeIII2/FeII2 redox
process known for the tribridged diiron core in the oxy
and deoxy forms of hemerythrin.