Controlling metal-ligand-metal oxidation state combinations by ancillary ligand (L) variation in the redox systems [L2Ru(μ-boptz)RuL 2]n, boptz = 3,6-bis(2-oxidophenyl)-1,2,4,5-tetrazine, and L = acetylacetonate, 2,2′-bipyridine, or 2-phenylazopyridine

Article abstract of DOI:10.1002/chem.200500295

The new compounds [(acac)₂Ru(μ-boptz)Ru(acac)₂] (1), [(bpy)₂Ru(μ-boptz)Ru(bpy)₂](ClO₄) ₂ (2-(ClO₄)₂), and [(pap)₂Ru(μ- boptz)-Ru(pap)₂](ClO₄)₂ (3-(ClO ₄)₂) were obtained from 3,6-bis(2-hydroxyphenyl)-1,2,4,5- letrazine (H₂boptz), the crystal structure analysis of which is reported. Compound 1 contains two antiferromagnetically coupled (J = -36.7cm¹) Ru^III centers. We have investigated the role of both the donor and acceptor functions containing the boptz^2- bridging ligand in combination with the electronically different ancillary ligands (donating acac^-, moderately π-accepting bpy, and strongly π-accepting pap; acac = acetylacetonate, bpy = 2,2′-bipyridine pap = 2-phenylazopyridine) by using cyclic voltammetry, spectroelectrochemistry and electron paramagnetic resonance (EPR) spectroscopy for several in situ accessible redox states. We found that metal-ligand-metal oxidation state combinations remain invariant to ancillary ligand change in some instances: however, three isoelectronic paramagnetic cores Ru(μ-boptz)-Ru showed remarkable differences. The excellent tolerance of the bpy coligand for both Ru^III and Ru^II is demonstrated by the adoption of the mixed-valent form in [L₂Ru(μ-boptz)-RuL₂]³⁺, L = bpy, whereas the corresponding system with pap stabilizes the Ru ^II states to yield a phenoxyl radical ligand and the compound with L = acac^- contains two Ru^III centers connected by a tetrazine radical-anion bridge.

Full text of DOI:10.1002/chem.200500295

FULL PAPER
DOI: 10.1002/chem.200500295
Controlling Metal–Ligand–Metal Oxidation State Combinationsby Ancillary
Ligand (L) Variation in the Redox Systems [L2Ru(m-boptz)RuL2]n,
boptz=3,6-bis(2-oxidophenyl)-1,2,4,5-tetrazine, and L=acetylacetonate, 2,2’-
bipyridine, or 2-phenylazopyridine
Srikanta Patra,[a] Biprajit Sarkar,[b] Somnath Maji,[a] Jan Fiedler,[c]
[d]
Francisco A. Urbanos,[d] ReyesJimenez-Aparicio,* Wolfgang Kaim,*[b] and
Goutam Kumar Lahiri*[a]
Abstract:
[(acac)2Ru(m-boptz)Ru(acac)2]
The
new
compounds
(1),
(donating acacꢀ, moderately p-accept-
ing bpy, and strongly p-accepting pap;
acac=acetylacetonate, bpy=2,2’-bipyr-
idine pap=2-phenylazopyridine) by
using cyclic voltammetry, spectroelec-
trochemistry and electron paramagnet-
ic resonance (EPR) spectroscopy for
several in situ accessible redox states.
We found that metal–ligand–metal oxi-
dation state combinations remain in-
variant to ancillary ligand change in
some instances; however, three isoelec-
tronic paramagnetic cores Ru(m-boptz)-
Ru showed remarkable differences.
The excellent tolerance of the bpy co-
ligand for both RuIII and RuII is dem-
onstrated by the adoption of the
mixed-valent form in [L2Ru(m-boptz)-
RuL2]3+, L=bpy, whereas the corre-
sponding system with pap stabilizes the
RuII states to yield a phenoxyl radical
ligand and the compound with L=
acacꢀ contains two RuIII centers con-
[(bpy)2Ru(m-boptz)Ru(bpy)2](ClO4)2
(2-(ClO4)2), and [(pap)2Ru(m-boptz)-
Ru(pap)2](ClO4)2 (3-(ClO4)2) were ob-
tained from 3,6-bis(2-hydroxyphenyl)-
1,2,4,5-tetrazine (H2boptz), the crystal
structure analysis of which is reported.
Compound 1 contains two antiferro-
magnetically coupled (J=ꢀ36.7 cmꢀ1)
RuIII centers. We have investigated the
role of both the donor and acceptor
functions containing the boptz2ꢀ bridg-
ing ligand in combination with the
electronically different ancillary ligands
Keywords: bridging ligands · EPR
spectroscopy · magnetic properties ·
N,O ligands · ruthenium
nected by
bridge.
a tetrazine radical-anion
Introduction
[a] S. Patra, S. Maji, Prof. Dr. G. K. Lahiri
Department of Chemistry, Indian Institute of Technology Bombay
Powai, Mumbai 400076 (India)
Intramolecular electron transfer is of importance for redox
reactivity[1,2] and for some concepts of molecular electron-
ics.[2] Molecule-bridged dinuclear complexes, especially
mixed-valent compounds,[1,3] have played an important role
in understanding intramolecular electron transfer and its
control through external factors such as the ancillary (termi-
nal) ligands L in systems of the general composition [LnMm-
(m-BL)Mm+1Ln]k (BL: bridging ligand). However, beyond
the tuning of the extent of electronic coupling between
mixed-valent metal centers[3] through electron- or hole-
transfer mechanisms,[4,5] one may also envisage full partici-
pation of the bridge in electron exchange by forming ligand-
reduced species [LnMm+1(m-BLCꢀ)Mm+1Ln]k or ligand-oxi-
dized forms [LnMm(m-BLC+)MmLn]k.
Fax : (+91)022-2572-3480
E-mail: lahiri@chem.iitb.ac.in
[b] Dr. B. Sarkar, Prof. Dr. W. Kaim
Institut für Anorganische Chemie, Universität Stuttgart
Pfaffenwaldring 55, 70550 Stuttgart (Germany)
Fax : (+49)711-685-4165
E-mail: kaim@iac.uni-stuttgart.de
[c] Dr. J. Fiedler
´
J. Heyrovsky Institute of Physical Chemistry
ˇ
Academy of Sciences of the Czech Republic, Dolejskova 3
18223 Prague (Czech Republic)
[d] Dr. F. A. Urbanos, Dr. R. Jimenez-Aparicio
Departamento de Química Inorgµnica
Facultad de Ciencias Químicas, Universidad Complutense
Ciudad Universitaria, 28040 Madrid (Spain)
Fax : (+34)91-394-4352
Bridging ligands that are capable of both reversible and
facile oxidation and reduction are uncommon. Here we
present the analysis of diruthenium systems [L2Ru(m-boptz)-
E-mail: qcmm@quim.ucm.es
Supporting information for this article is available on the WWW
under http://www.chemeurj.org/ or from the authors.
Chem. Eur. J. 2006, 12, 489 – 498
ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
489

RuL2]n,
boptz2ꢀ =3,6-bis(2-oxidophenyl)-1,2,4,5-tetrazine
Although a fairly large number of diruthenium complexes
have been synthesized by using tetrazine-based neutral
spacers, analogous complexes of corresponding anionic de-
rivatives are not known. The present work deals with the
doubly deprotonated form of 3,6-bis(2-hydroxyphenyl)-
1,2,4,5-tetrazine (H2boptz). The observed substantial effects
of ancillary ligands on the mixed-valent properties of bptz-
and bpytz-bridged RuIIIRuII species has prompted us to ex-
amine the effect of three electronically different ancillary
functions, namely, acacꢀ (s-donating), bpy (moderately p-
acidic), and pap (strongly p-acidic), on the extent of inter-
molecular electron-exchange processes in boptz2ꢀ-bridged
diruthenium species.
and L=acacꢀ (acetylacetonate, donor), bpy (2,2’-bipyridine,
weak p-acceptor), or pap (2-phenylazopyridine, strong p-ac-
ceptor).[6]
Here we report the syntheses of 3,6-bis(2-hydroxyphenyl)-
1,4-dihydro-1,2,4,5-tetrazine (H4boptz), its oxidized form
3,6-bis(2-hydroxyphenyl)-1,2,4,5-tetrazine (H2boptz), and of
the
complexes
[(acac)2Ru(m-boptz)Ru(acac)2]
(2-(ClO4)2)
(1),
and
[(bpy)2Ru(m-boptz)Ru(bpy)2](ClO4)2
[(pap)2Ru(m-boptz)Ru(pap)2](ClO4)2 (3-(ClO4)2). We de-
scribe the crystal structure of the free ligand (H2boptz), and
have investigated the role of boptz2ꢀ in combination with
the electronically different three ancillary ligands for the
tuning of oxidation state configurations by using spectro-
electrochemistry and EPR spectroscopy.
The new symmetrically bis(bidentate) bridging ligand
boptz2ꢀ contains two phenolate donors and a central tetra-
zine p-acceptor function, each bonding to both metals in a
dinuclear configuration. Phenolates are capable of coordina-
tion-supported oxidation to biochemically relevant phenoxyl
radicals that are known to be stabilized by metal bonding.[7]
On the other hand, 3,6-disubstituted-1,2,4,5-tetrazine moie-
ties have become popular as efficient electronic spacers in
dinuclear and polynuclear systems.[8] This is primarily due to
the fact that the tetrazine-based low-lying p* orbital conveys
strong p-accepting characteristics, leading to excellent elec-
tronic communication between the metal termini. This dis-
covery has spurred the design of a number of dinuclear
ruthenium complexes with a wide variation of molecular
frameworks.[9] 3,6-Bis(2-pyridyl)-1,2,4,5-tetrazine (bptz) in
particular has been used extensively in synthesizing diruthe-
nium complexes [LnRu(m-bptz)RuLn]k in combination with
ancillary ligands L of varying electronic nature such as
NH3,[9a,b] bpy,[9c] acacꢀ,[9d] [9]aneS3 (1,4,7-trithiacyclonon-
ane),[9e] or arenes.[9t] Considerable variation has been ob-
served for the comproportionation constants (Kc) of the
RuIIIRuII mixed-valent intermediates in those complexes,
based on the electronic properties of the ancillary ligands:
Kc: 1”1015 (L=NH3); 3”108 (L=bpy); 1”1013 (L=acacꢀ);
1.4”108 (L=[9]aneS3). Similarly, the modified framework of
the tetrazine-based spacer 3,6-bis(3,5-dimethylpyrazolyl)-
1,2,4,5-tetrazine (bpytz) also exhibits a substantial difference
in Kc, depending on the ancillary ligands (Kc =107.6 or 1013.9
for L=bpy[9n] or acacꢀ,[9s] respectively). In addition, other
3,6-disubstituted tetrazine-based bridging ligands such as
3,6-bis(2-thienyl)-1,2,4,5-tetrazine (bttz),[9m] 3,6-bis(4-methyl-
2-pyridyl)-1,2,4,5-tetrazine (bmptz),[9m] and 3,6-bis(carbo-
methoxy)-1,2,4,5-tetrazine (bctz)[9c] have also been utilized
in developing diruthenium complexes incorporating p-acidic
bpy co-ligands.
Results and Discussion
The free ligands 3,6-bis(2-hydroxyphenyl)-1,4-dihydro-
1,2,4,5-tetrazine (H4boptz) and its oxidized form 3,6-bis(2-
hydroxyphenyl)-1,2,4,5-tetrazine (H2boptz) were synthesized
through the reaction of 2-hydroxybenzonitrile with hydra-
zine hydrate in refluxing ethanol and through the oxidation
of H4boptz by using NO gas, respectively.
The formation of H2boptz was confirmed by single-crystal
X-ray diffraction structure analysis (Figure 1). Selected crys-
tallographic and bond parameters are listed in Tables 1 and
2, respectively. The structural parameters match well with
reported values for related compounds.[9u] There is intramo-
lecular hydrogen bonding between the hydroxy function
ꢀ
O1 H1 and the neighboring tetrazine nitrogen atom N1 (d-
o
ꢀ
(O···N), 2.631(2) Š and O H···N, 145(3) , Figure S1 and
490
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ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2006, 12, 489 – 498

Redox Chemistry
FULL PAPER
Figure 1. Crystal structure of H2boptz. Ellipsoids are drawn at the 50%
probability level.
Table 1. Crystallographic data for H2boptz.
formula
Mr
C14H10N4O2
266.26
crystal size [mm]
0.40”0.35”0.30
monoclinic
P21/c
crystal system
space group
Z
Scheme 1.
2
a[Š]
b[Š]
c[Š]
a[8]
4.5260 (8)
10.5570 (7)
12.4990 (11)
90
bridges two units of the metal complex fragments [RuIII-
(acac)2]+, [RuII(bpy)2]2+, or [RuII(pap)2]2+ in complexes 1,
22+, or 32+, respectively, through the phenolate Oꢀ and tet-
razine N-donor centers. While the +2 oxidation state of
ruthenium in the precursors is retained in complexes 22+
and 32+, the RuII state of the precursor is oxidized to the
+3 state in complex 1, presumably by air. The presence of
electron-rich acacꢀ ancillary ligands in complex 1 as op-
posed to the moderately p-acidic bpy in complex 22+ or
strongly p-acidic pap in complex 32+ facilitates the stabiliza-
tion of the RuIII state in complex 1 and this is also reflected
in the redox potential data (see later).
b[8]
96.279 (10)
90
593.63 (12)
1.490
293 (2)
0.105
g[8]
V[Š3]
1calcd [gcmꢀ3
T[K]
]
m[mmꢀ1
F(000)
]
276
hkl range
q range [8]
measured reflections
unique reflections
observed reflections [I>2s(I)]
parameters
R1
wR2
R(all)
h: 0–5; k: 0–12; l: ꢀ14–14
2.53–24.92
1189/1050
1050 [R(int)=0.0114]
1050
95
0.0419
0.1115
0.0618
We identified complexes 1, 2-(ClO4)2, and 3-(ClO4)2 by
microanalysis, molar conductance, and electrospray (ESI)
mass spectrometry (see Experimental Section). They were
further investigated in various oxidation states (see below)
by using cyclic voltammetry, spectroelectrochemistry, and
EPR spectroscopy. We studied the magnetism of paramag-
netic 1 by using superconducting quantum interface device
(SQUID) susceptometry.
ꢀ3
residual electron density [eŠ
]
0.290/ꢀ0.20
Table 2. Selected bond lengths [Š] and angles [8] for H2boptz.
Bond lengths Bond angles
ꢀ
The diamagnetic complexes 22+and 32+ displayed compli-
O1 C1
1.340(3)
1.313(2)
1.342(2)
1.340(2)
N2-N1-C7
N1-N2-C7#1
O1-C1-C2
119.68(14)
117.46(15)
116.65(18)
124.20(17)
122.86(15)
118.39(16)
118.75(15)
ꢀ
N1 N2
1
cated H NMR spectra due to overlap of 40 and 44 signals,
ꢀ
N1 C7
respectively, with rather similar chemical shifts in the aro-
ꢀ
N2 C7#1
O1-C1-C6
1
matic region. The H NMR spectrum of complex 22+ indi-
N2#1-C7-N1
N2#1-C7-C6
N1-C7-C6
cates the presence of a mixture of two isomers (meso and
rac),[9h,10] in a ratio of approximately 2:1 that we failed to
separate even by preparatory TLC. Whereas redox poten-
tials and absorption spectra are only marginally different for
such isomers,[10] the EPR characteristics of paramagnetic
forms are more sensitive in that respect (see EPR Section
below).
We carried out variable-temperature (2–300 K) magnetic
studies of a powder sample of complex 1. The magnetic sus-
ceptibility curve versus temperature shows a broad maxi-
mum at 64 K (Figure 2), implying bridging-ligand-mediated
antiferromagnetic interaction between the RuIII centers. In
Table S1 in the Supporting Information). The paramagnetic
complex [{(acac)2RuIII}2(m-boptz2ꢀ)] (1) and the diamagnetic
complexes [{(bpy)2RuII}2(m-boptz2ꢀ)](ClO4)2 (2-(ClO4)2) and
[{(pap)2RuII}2(m-boptz2ꢀ)](ClO4)2 (3-(ClO4)2) were prepared
from reactions of [Ru(acac)2(CH3CN)2], [Ru(bpy)2-
(EtOH)2]2+
, , respectively, with
or [Ru(pap)2(EtOH)2]2+
H2boptz in 2:1 molar ratio in the presence of excess
NaOOCCH3 (Scheme 1). Dianionic boptz2ꢀ symmetrically
Chem. Eur. J. 2006, 12, 489 – 498
ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
491

G. K. Lahiri, R. Jimenez-Aparicio, W. Kaim et al.
(acac)2](ClO4)2
(J=ꢀ0.45 cmꢀ1)
or
[(acac)2Ru(m-
OC2H5)2Ru(acac)2] (J=ꢀ0.63 cmꢀ1).[13] In contrast, the J for
complex 1 is much lower than that observed in several (m-
alkoxo)bis(m-carboxylato)diruthenium(iii) complexes (J=
ꢀ310 to ꢀ728 cmꢀ1).[14]
Electrochemistry and EPR spectroscopy: In CH3CN, the
paramagnetic RuIIIRuIII complex 1 exhibits two successive
one-electron oxidation waves (Figure 3a, Table 3) and a
*
( ) and
Figure 2. Temperature-dependence of the molar susceptibility cM
meff (&) for 1. Solid lines are the products of a least-squares fit to the
model mentioned in the text.
addition, we observe a tail in the low-temperature end that
suggests the presence of a paramagnetic impurity.
The gradual decrease of the magnetic moment of complex
1 from 2.38 mB at 300 K to 0.55 mB at 10 K confirms the exis-
tence of bridging-ligand-mediated antiferromagnetic interac-
tion between the RuIII centers. The slight decrease of m from
0.55 to 0.51 mB in the temperature range of 10–2 K suggests
the presence of a paramagnetic species without antiferro-
magnetic coupling (Figure 2). The magnetic behavior of
complex 1 can be explained with a model that takes into ac-
count the effects of an exchange spin Hamiltonian h=
ꢀ2JS1·S2 where S1 =S2 =1= , with intramolecular antiferro-
2
magnetic coupling between the RuIII centers.[11] In addition,
temperature-independent paramagnetism (TIP) has been in-
cluded as is usual in ruthenium complexes [Eq. (1)]. More-
over, a dimeric species with two uncoupled RuIII centers
(S=1, g=2) has been considered as a possible source for
paramagnetic impurities.
Ng2b2 2 expð2J=kTÞ
ð1Þ
c ¼
þ TIP
kT
1 þ 3 expð2 J=kTÞ
The fit of the experimental data with Equation (2) gives
excellent agreement of the calculated and experimental
magnetic moment and susceptibility curves (Figure 2). The
parameters obtained in the best fits are: g=2.0, J=
ꢀ36.7 cmꢀ1, TIP=1.1”10ꢀ4 emumolꢀ1, P=3.7%, and s2 =
3.4”10ꢀ5 (s2 =ꢀ(meffcalcdꢀmeffexptl)2/ꢀme2ff exptl). The calculated g
and TIP values are typical for ruthenium complexes.[12–14]
Figure 3. Cyclic voltammograms of: a) 1 in CH2Cl2, b) 2-(ClO4)2, and
c) 3-(ClO4)2 in CH3CN at 298 K.
2 Ng2b2
3 kT
weak rhombic EPR signal at 4 K. Both the g anisotropy
g1ꢀg3 =0.636 and the average gav =2.240 from EPR spec-
troscopy are indicative of RuIII-based spin (Table 4).[15] The
half-field EPR signal expected for a triplet state was not ob-
served under those conditions. This and the low intensity of
the EPR response already suggest that the RuIII centers in
complex 1 are rather strongly coupled with antiparallel
alignment of spins from the low-spin d5 configurations. In
c0 ¼ ð1ꢀPÞc þ P
ð2Þ
The J value of ꢀ36.7 cmꢀ1 for complex 1 is similar to that
reported for [L(acac)Ru(m-O)Ru(acac)L](PF6)2 (L=1,7-tri-
methyl-1,4,7-triazacyclononane) (J=ꢀ53 cmꢀ1) that has a
Ru-O-Ru angle of 1808,[12] but much higher than that ob-
served in [(acac)2Ru(m-(NC5H4)2N-C6H4-N-(NC5H4)2)Ru-
492
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Chem. Eur. J. 2006, 12, 489 – 498

Redox Chemistry
FULL PAPER
Table 3. Redox potentials of complexes[a]
.
complex can be best described
by
a
three-spin situation,[19]
[c]
[c]
[c]
n/nꢀ1[b]
Couple
E1/2 (DEpp
)
Couple
E1/2 (DEpp
)
Couple
E1/2 (DEpp)
RuIII(boptzC3ꢀ)RuIII, in which
4/3
24+/23+
23+/22+
22+/2+
2+/2
0.84 (70)
0.48 (80)
ꢀ0.75 (60)
34+/33+
33+/32+
32+/3+
3+/3
1.28[e]
3/2
2/1
1/0
1.14[e]
the (S=1= ) ground state in-
2
12+/1+
1+/1
1.19 (90)
0.96 (90)
ꢀ0.10(70)
volves dominant antiferromag-
netic coupling between one of
ꢀ1.76 (100)[d]
ꢀ0.56 (60)
ꢀ0.67(60)
ꢀ1.17(60)
ꢀ1.30(60)
0/ꢀ
1/1ꢀ
ꢀ0.18 (80)
3/3ꢀ
the
RuIII
centers
and
ꢀ/2ꢀ
2ꢀ/3ꢀ
1ꢀ/12ꢀ
ꢀ0.89 (90)
3ꢀ/32ꢀ
boptzC3ꢀ,[19] leaving one metal-
centered spin active for EPR.
With bpy as ancillary ligands
the corresponding RuIIRuII
compound 22+ exhibits two
successive one-electron oxida-
tion processes at 0.48 and
3
2ꢀ/33ꢀ
[a] From cyclic voltammetry in CH2Cl2 for 1 and in CH3CN/0.1m Et4NClO4 for 2 and 3, at 50 mVsꢀ1
.
[b] Change in charge of the [Ru(m-boptz)Ru](n/nꢀ1)+ core. [c] In V versus SCE; peak potential differences
DEpp [mV] (in parentheses). [d] Further bpy-based reductions at ꢀ1.76 (100) and ꢀ2.02 V (80 mV). [e] Anodic
peak potential (Epa), process not fully reversible.
Table 4. EPR data of paramagnetic states.[a]
0.84 V versus SCE with Kc =1.3”106 (Figure 3b, Table 3).
The first-step oxidized species 23+ displays two sets of rhom-
bic EPR spectra at 4 K (Figure 4b, Table 4). The corre-
sponding g anisotropies g1ꢀg3 are 0.67 and 0.45, respective-
ly; the resulting gav values are 2.180 and 2.130, respectively.
[b]
g1
g2
g3
gav
Dg=g1ꢀg3
Kc[c]
1[d]
2.543
2.220
2.499
2.224
2.220
2.160
1.907
1.743
1.828
2.240
2.073
2.180
0.636
0.477
0.671
0.450
7.9”103
1.1”1012
1.3”106
1ꢀ[d]
23+[d,e]
2.328
2.160
1.878
2.130
[f]
[f]
[f]
2+[g]
2.0034
2.0007[h]
1.9946
33+[d,g]
2.022
1.999
1.999
[f]
[f]
[f]
3+[g]
[a] From EPR spectroelectrochemical data in CH3CN/0.1m Bu4NPF6,
except for 1, g tensor components determined at 4 K. [b] gav =(1/3(g21 +
g22 +g23))1/2. [c] Comproportionation constant from RTlnKc =nF(DE); for
DE see Table 3. [d] EPR measurements at 4 K. [e] Two isomers (meso
and rac). [f] No g anisotropy measured. [g] EPR measurements at 298 K,
14N hyperfine coupling of 0.5 mT. [h] giso =2.0046 at 298 K.
fact, SQUID susceptibility measurements reveal antiferro-
magnetic coupling.
Although an odd-electron system, the one-electron oxi-
dized intermediate 1+ did not show an EPR signal, even at
4 K. The rapid EPR relaxation suggests metal-centered
spin(s) and close-lying excited states as may be anticipated
for a RuIVRuIII (d4/d5) mixed-valent situation with possibly
close-lying singlet and triplet states of the d4 center. This
EPR silence is in contrast to EPR observations made for
other bridged bis(bis(acetylacetonato)ruthenium) com-
plexes, which allow for a RuIV(L2ꢀ)RuIII$RuIII(L)RuII reso-
nance.[16] The 230 mV separation between the oxidation cou-
ples for complex 1 leads to a comproportionation constant
Kc of 7.9”103 (calculated by using the equation RTlnKc =
nF(DE))[17] for the monocationic intermediate. This low
value and the absence of an EPR signal for RuIV-
(boptz2ꢀ)RuIII suggest a Class II mixed-valent state.[18]
Complex 1 exhibits two one-electron reduction processes
at ꢀ0.18 and ꢀ0.89 V versus SCE (Figure 3a, Table 3). The
Kc value of 1.1”1012 for the 1ꢀ complex is much higher than
that for the 1+ complex, suggesting either tetrazine-centered
reduction[8,9] or the formation of a strongly coupled RuIIIRuII
mixed-valent state.[4,18] The one-electron reduced species 1ꢀ
displays an axial EPR signal with an average gav factor of
2.073 and a g anisotropy g1ꢀg3 =0.48 (Figure 4a, Table 4),
indicative of metal-centered spin. However, complex 1ꢀ
does not show the NIR transition expected for a RuIII-
(boptz2ꢀ)RuII intermediate (see later). Thus, the reduced 1ꢀ
Figure 4. EPR spectra of: a) 1ꢀ in CH2Cl2 at 4 K, b) 23+ at 4 K (left) and
2+ at 298 K (right), and c) 33+ at 298 K (left) and 33+ at 4 K (right) in
CH3CN (* denotes artifact signals due to the EPR cavity).
These observations reveal: 1) the presence of two isomeric
forms (meso and rac), in the oxidized 23+ state with distinct
electronic structures; 2) a metal-based oxidation correspond-
ing to a RuIII(boptz2ꢀ)RuII (or, better, Ru2.5(boptz2ꢀ)Ru2.5)
formulation for the intermediate 23+; and 3) the Class III
category for that intermediate. The RuIII/RuII couples for
complexes with the corresponding neutral tetrazine-based
spacers bptz, bpytz, bmptz, and bttz appeared at 1.52, 2.2;[9c]
1.25, 1.70;[9n] 1.34, 1.87,[9m] and 0.68, 1.68 V,[9m] respectively.
The dianionic boptz2ꢀ ligand substantially destabilizes the
RuII state in the 22+ complex. It should be noted that the
combination of bpy ancillary ligands and neutral tetrazine-
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493

G. K. Lahiri, R. Jimenez-Aparicio, W. Kaim et al.
based spacers also produced delocalized RuIIIRuII
states.[9c,n,m] Unlike these compounds with neutral tetrazine-
centered spacers and mostly higher Kc values, however, the
mixed-valent 23+ does not show a well-defined intervalence
charge-transfer (IVCT) band in the NIR region, but only
significantly enhanced absorption around and beyond
2000 nm (see later, Figure 6).
zine ring of the bridge in complex 3+, formulated according
to RuII(boptzC3ꢀ)RuII. The subsequent two sets of closely
spaced reduction waves at ꢀ0.56, ꢀ0.67 and ꢀ1.17, ꢀ1.30 V
versus SCE (Figure 3c and Table 3) can be tentatively as-
signed as successive reduction processes involving the 2-phe-
nylazopyridine ancillary ligands.[6]
The tetrazine-based one-electron reduction of complex
22+ occurs at ꢀ0.75 V versus SCE in CH3CN (Figure 3b).
For the analogous bptz, bpytz, bmptz, and bttz complexes
this process is observed at ꢀ0.03,[9c] ꢀ0.13,[9n] ꢀ0.21,[9m] and
ꢀ1.02 V,[9m] respectively. Thus, the introduction of dianionic
boptz2ꢀ ligand in complex 22+ in place of neutral tetrazines
results in an appreciably destabilized LUMO.
Spectroelectrochemistry: We investigated the absorption
spectra of reversibly accessible states in the UV, visible, and
NIR regions with an optically transparent thin-layer elec-
trolysis (OTTLE) cell[23] in order to confirm the above as-
signments based on EPR data, to assign oxidation state
combinations for the EPR-silent species, and to obtain infor-
mation on the electronic structures in general. Due to the
combined (phenolate) donor and (tetrazine) acceptor char-
acter of the bridging ligands, and because of the ancillary li-
gands containing conjugated-p systems, a large number of
charge-transfer transitions can be expected at rather low en-
ergies. Assignments are therefore tentative and will have to
be confirmed by quantum chemical calculations at a later
stage. Unless stated otherwise, the results involve fully re-
versible transitions as confirmed by 100% spectra regenera-
tion and the occurrence of isosbestic points. The spectra are
shown in Figures 5–7, and spectral data are summarized in
Table 5.
The one-electron-reduced species 2+ exhibits a typically
resolved radical-type EPR spectrum[20] in CH3CN at 298 K
due to hyperfine splitting of approximately 0.5 mT from
four 14N atoms of the tetrazine ring of boptzC3ꢀ. The spec-
trum is centered at g=2.0034 (Figure 4b, Table 4); both
ruthenium atoms remain in the divalent state in complex 2+.
In addition to a second reduction of the bridging ligand,
complex 22+ also displays the expected multiple bpy-based
reductions in the range between ꢀ1.76 and ꢀ2.02 V (Fig-
ure 3b, Table 3).[21]
The analogous complex 32+ with the stronger p-accepting
pap[6] terminal ligands exhibits two closely spaced oxidation
waves at 1.14 and 1.28 V versus SCE (Figure 3c, Table 3).
Although even the first oxidation is not fully reversible on
the timescale of cyclic voltammetry at room temperature,
the one-electron-oxidized product could be obtained by
intra muros electrolysis, displaying a sharp EPR signal at
Table 5. UV-visible-NIR data of complexes from spectroelectrochemical
data[a]
.
lmax [nm] (e [mꢀ1 cmꢀ1])
1+ 830(sh), 690(11600), 397(12200), 304(22600), 265(21800),
242(21100)
g
iso =2.0046 (298 K) indicative of free radicals (Figure 4c,
1
815(6700), 620(sh), 560(9770), 455(10260), 390(sh), 355(14700),
297(19800), 270(19600), 240(18550)
Table 4), lying in a typical giso range for phenoxyl radi-
cals;[7,22] the small but detectable axial g component splitting
observed at 4 K (Figure 4c) is larger than that reported for
uncoordinated phenoxyl (tyrosyl) radicals.[22] This observa-
tion suggests the formation of a metal-coordinated phenoxyl
radical species RuII(boptzCꢀ)RuII (33+) instead of the other-
wise conceivable mixed-valent alternative RuII(boptz2ꢀ)RuIII
(which was observed for 23+). The formation of a phenoxyl
radical complex is not only supported by the g factor from
EPR spectroscopy and by the closeness of two less-revers-
ible oxidation processes for two sterically unprotected, spa-
tially separated, and only weakly coupled phenolate/phenox-
yl redox pairs, further evidence comes from the appearance
of a characteristic absorption band at 488 nm (see the Spec-
troelectrochemistry Section below).[7]
1ꢀ 626(10270), 540(10250), 365(17300), 290(19300), 268(19900),
242(18600)
12ꢀ 895(13300), 625(9900), 460(sh), 367(16000), 292(17600),
266(17800), 240(17500)
24+ 1350(sh), 1040(sh), 900(broadsh) (4000), 560(14300), 447(sh),
375(12000), 304(20400), 265(19000), 240(18500)
23+ 2000(broadsh), 1300(sh), 900(sh), 652(12950), 386(12750),
302(20200), 267(19200), 243(18300)
22+ 980(broadsh), 685(13300), 488(12750), 360(14900), 302(20200),
269(19700), 243(18900)
2+ 675(sh), 545(11300), 483(11600), 374(13900), 300(19750),
267(19100), 242(18400)
33+ 1105(13900), 655(15300), 488(19500), 368(28500), 275(25000),
235(25600)
32+ 890(sh), 705(sh), 617(10700), 505(15200), 349(27900), 305(29300),
240(29200)
3+ 1600(800), 725(sh), 572(8700), 485(9300), 345(25600), 300(25600),
240(29100)
Both the tetrazine[8,9] part of boptz2ꢀ and the azo function
of pap[6] are susceptible to undergo facile and often revers-
ible reduction processes. The complex ion 32+ thus shows
multiple reduction waves within the potential limit of
ꢀ2.0 V versus SCE (Figure 3c, Table 3). The first-step re-
duced species 3+ displays the familiar (see Figure 4b) nine-
line EPR spectrum with a giso value centered at 1.9946 at
298 K. This, along with the corresponding coupling constant
of about 0.5 mT from the EPR spectra simulation con-
firms[20] that the LUMO is primarily dominated by the tetra-
3
1600(800), 575(8100), 352(26800), 303(23800), 240(29100)
[a] Solvent: CH2Cl2 for 1 and CH3CN for 2-(ClO4)2 and 3-(ClO4)2.
Compound 1 exhibits intense bands at long wavelengths
due to ligand-to-metal charge-transfer (LMCT) transitions
expected for RuIII(boptz2ꢀ)RuIII. On oxidation to RuIV-
(boptz2ꢀ)RuIII the absorption in the 600–900 nm region in-
tensifies (Figure 5a) and there appears to be increased ab-
sorption in the NIR region; however, no proper IVCT band
494
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Redox Chemistry
FULL PAPER
Figure 6. UV-visible-NIR spectroelectrochemistry for the conversions:
a) 22+!23+, b) 23+!24+, and c) 22+!2+ in CH3CN/0.1m Bu4NPF6.
Figure 5. UV-visible-NIR spectroelectrochemistry for the conversions:
a) 1!1+, b) 1!1ꢀ, and c) 1ꢀ!12ꢀ in CH2Cl2/0.1m Bu4NPF6.
decreased IVCT absorption. The second oxidation to give a
RuIII(boptz2ꢀ)RuIII species 24+ produces weak absorptions
between 900 and 1500 nm and a hypsochromic shift of the
charge-transfer band in the visible region. Reduction to the
RuII(boptzC3ꢀ)RuII intermediate 2+ causes a hypsochromic
shift of the long-wavelength bands as pointed out above for
the related tetrazine radical complex 1ꢀ.
was observed in agreement[24] with a weakly coupled mixed-
valent situation (Figure 5a). The first reduction to RuIII-
(boptzC3ꢀ)RuIII results in the disappearance of the long-wave-
length band that is typical for tetrazine radical com-
pounds.[9q] The second reduced species 12ꢀ exhibits a strong
absorption band in the low-energy region (895 nm, e=
13300mꢀ1 cmꢀ1), compatible with an LMCT transition as ex-
pected for RuIII(boptz4ꢀ)RuIII. However, the alternate va-
lence description of RuII(boptz2ꢀ)RuII with a low-lying
MLCT transition[9] may not be ruled out.
The complex 32+ ion exhibits multiple MLCT bands in
the visible region in agreement with the RuII(boptz2ꢀ)RuII
formulation and with the presence of p-accepting tetrazine
and pap ligands (Figure 7). Oxidation on the timescale of
the spectroelectrochemistry experiment (approximately
1 min) shows an only partially reversible spectral change
with increasing bands at 488 and 655 nm, indicative of
phenoxyl radicals,[7] and in the NIR region at 1105 nm, as
has been observed before for transition-metal phenoxyl
compounds.[7]
The complex ion 22+ shows MLCT transitions around 700
and 480 nm to the p* orbitals of tetrazine and bpy as ex-
pected for a RuII(boptz2ꢀ)RuII situation (Figure 6).[9] The
bathochromic shift for d(Ru)!p*(bpy) results from the co-
ordination of an anionic phenolate that destabilizes the
metal d orbitals. The mixed-valent 23+ complex ion contain-
ing RuIII(boptz2ꢀ)RuII does not have a well-defined IVCT
band in the NIR region; however, the absorption band sig-
nificantly increases around and beyond 2000 nm (Figure 6a).
Bis(chelate) and especially tetrazine-bridged mixed-valent
intermediates have rather weak IVCT band intensities de-
spite very large Kc values,[4,9b] the somewhat lower Kc value
for the present case signified already attenuated metal–
metal interactions that would be compatible with a further
Reduction to a RuII(boptzC3ꢀ)RuII species 3+ decreases the
MLCT bands; however, a low-energy broad band appears
now at 1600 nm (Figure 7b) that is assigned to an interligand
charge-transfer (LLCT) transition from the singly occupied
MO (SOMO) at boptzC3ꢀ to the p* molecular orbitals
(LUMO) of pap.[25] These features remain after the second
reduction, which is likely to occur at one of the pap terminal
ligands.
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495

G. K. Lahiri, R. Jimenez-Aparicio, W. Kaim et al.
tors, here the terminal ligands, can influence the oxidation
state situation in the metal–bridge–metal core. Two ex-
tremes are highlighted in the following.
1) The RuII(boptz2ꢀ)RuII state is the isolation form for com-
plexes 22+ and 32+, it can also be invoked for complex
12ꢀ.
2) In contrast to the stable state RuII(boptz2ꢀ)RuII the inter-
mediate one-electron-oxidized forms differ substantially,
the oxidation state being determined by the effect of the
ancillary ligands on the metal.
The donating acetylacetonato co-ligands strongly favor
the RuIII oxidation state, leading to a species 1ꢀ best formu-
lated as RuIII(boptzC3ꢀ)RuIII.
The strongly p-accepting pap terminal ligands act to main-
tain a +2 oxidation state on ruthenium, leaving the pheno-
late-containing bridging ligand to be oxidized from 32+ to a
labile phenoxyl species 33+, formulated as RuII(boptzCꢀ)RuII.
Only the moderately p-accepting bpy ancillary ligands
allow both RuIII and RuII states to exist in the then mixed-
valent intermediate 23+, formulated as RuIII(boptz2ꢀ)RuII or,
better, as the valence-averaged Ru2.5(boptz2ꢀ)Ru2.5 species.
Thus, it appears that the ubiquitous use of the bpy co-li-
gands in ruthenium mixed-valence chemistry[3,17] relies on a
lucky choice: 2,2’-bipyridine exhibits an excellent tolerance
for both the RuIII and RuII oxidation states, as demonstrated
here by the adoption of the mixed-valent form in [L2Ru(m-
boptz)RuL2]3+ with L=bpy. In contrast, the analogous
system containing the better p-accepting co-ligand pap sta-
bilizes only the RuII states to yield a ligand containing a
phenoxyl radical, while the corresponding compound with
the donor ligand L=acacꢀ contains two RuIII centers con-
nected by a tetrazine radical-anion bridge. These unprece-
dented observations have been made possible only through
the judicious design and use of a bis(chelated) ligand that
contains both (tetrazine) acceptor and (phenolate) donor
functions. Such donor–acceptor bifunctional bridges were
shown before to have unusual metal–metal mediating prop-
erties,[9m] and developments in this direction will thus contin-
ue.
Figure 7. UV-visible-NIR spectroelectrochemistry for the conversions:
a) 32+!33+/34+, b) 32+!3+, and c) 3+!3 in CH3CN/0.1m Bu4NPF6.
Conclusions
The following Scheme 2 summarizes the oxidation state as-
signments made above for complexes [L2Ru(m-boptz)-
RuL2]n.
A comparison between the three systems 1n, 2n, and 3n is
particularly revealing for the question of how external fac-
Experimental Section
We prepared the starting complexes [Ru(acac)2(CH3CN)2],[26] [Ru-
(bpy)2Cl2]·2H2O,[27] and [Ru(pap)2Cl2][28] according to the reported proce-
dures. 2-Hydroxybenzonitrile was obtained from Aldrich. Other chemi-
cals and solvents were reagent-grade and used as received. For spectro-
scopic and electrochemical studies HPLC-grade solvents were used.
UV-visible-NIR spectroelectrochemical studies were performed in
CH3CN/0.1m Bu4NPF6 at 298 K with an OTTLE cell[23] mounted in the
sample compartment of a Bruins Instruments Omega10 spectrophotome-
ter. FTIR spectra were taken on a Nicolet spectrophotometer with sam-
ples prepared as KBr pellets. Solution electrical conductivity was checked
by using a Systronic305 conductivity bridge. 1H NMR spectra were ob-
tained with a 300 MHz Varian FT spectrometer. The EPR measurements
were made in a two-electrode capillary tube[29] with an X-band (9.5 GHz)
Bruker system ESP300, equipped with a Bruker ER035M gaussmeter
Scheme 2.
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Chem. Eur. J. 2006, 12, 489 – 498

Redox Chemistry
FULL PAPER
and a HP 5350B microwave counter. Cyclic voltammetric, differential
pulse voltammetric, and coulometric measurements were carried out by
using a PAR model 273A electrochemistry system. We used platinum-
wire working and auxiliary electrodes, and an aqueous saturated calomel
reference electrode (SCE) in a three-electrode configuration. The sup-
porting electrolyte was Et4NClO4 (0.1m) and the solute concentration
was approximately 10ꢀ3 m. The half-wave potential Eo298 was set equal to
0.5(Epa+Epc), in which Epa and Epc are anodic and cathodic cyclic voltam-
metric peak potentials, respectively. A platinum-wire gauze working elec-
trode was used in coulometric experiments. The elemental analysis was
carried out with a Perkin–Elmer 240C elemental analyser. ESI mass spec-
tra were recorded on a Micromass Q-ToF mass spectrometer.
0.30 mmol) to this purple solution and the mixture was refluxed for 4 h
under dinitrogen. The resultant purple solution was evaporated to dry-
ness under reduced pressure and the solid mass was purified by chroma-
tography with a silica gel column (60–120 mesh) with CH2Cl2/CH3CN
(5:1 v/v). Yield: 0.083 g (60%); ESI MS (in CH3CN): m/z calcd for [3-
(ClO4)]+: 1298.68; found: 1299.20 (see also Figure S2c in the Supporting
Information); elemental analysis calcd (%) for C58H44Cl2N16O10Ru2:
C 49.83, H 3.17, N 16.03; found: C 49.49, H 3.05, N 15.53.
Magnetic susceptibility measurements: The variable-temperature mag-
netic susceptibility data were measured on a Quantum Design MPMSXL
SQUID susceptometer over a temperature range of 2–300 K. Each raw
data field was corrected for the diamagnetic contribution of both the
sample holder and the complex to the susceptibility. The molar diamag-
netic corrections were calculated on the basis of Pascal constants. The fit-
ting of the experimental data was carried out by using the commercial
MATLAB V.5.1.0.421 program.
CAUTION! Perchlorate salts of metal complexes are generally explosive.
Care should be taken while handling such complexes.
3,6-Bis(2-hydroxyphenyl)-1,4-dihydro-1,2,4,5-tetrazine (H4boptz): 2-Hy-
droxybenzonitrile (1.0 g, 8.39 mmol) and hydrazine hydrate (1.27 g,
1.3 mL, 25.44 mmol, 95%) were dissolved in ethanol (20 mL) and the
mixture was heated to reflux for 6 h. The reaction mixture was then
cooled at 08C overnight and the resulting orange precipitate was filtered
off and washed thoroughly with cold ethanol. The product was recrystal-
lized from hot ethanol. Yield: 0.80 g (71%); m.p. 2588C; ESI MS (in
CH2Cl2): m/z calcd for [(H4boptz)]+: 268.09; found 269.15; elemental
analysis calcd (%) for C14H12N4O2: C 62.68, H 4.51, N 20.88; found: C
62.32, H 4.61, N 19.18; 1H NMR (300 MHz, CDCl3, 298 K): d=11.34 (s,
1H; OH), 9.40 (s, 1H; NH), 7.72 (d, J=4.8 Hz, 1H), 7.36 (t, J=6.0 Hz,
1H), 6.95 ppm (m, 2H).
X-ray crystal structure analysis: Single-crystals of H2boptz were grown by
slow diffusion of a solution of the compound in dichloromethane into
hexane, followed by slow evaporation. X-ray diffraction data of H2boptz
were collected on a PC-controlled Enraf-Nonius CAD-4 (MACH-3)
single-crystal X-ray diffractometer by using MoKa radiation. The structure
was solved and refined by full-matrix least-squares on F2 by using
SHELX-97 (SHELXTL).[30] Hydrogen atoms were included in the refine-
ment process as per the riding model.
CCDC 266312 contains the supplementary crystallographic data for
H2boptz in this paper. These data can be obtained free of charge from
the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif.
3,6-Bis(2-hydroxyphenyl)-1,2,4,5-tetrazine (H2boptz): Nitric oxide (NO)
gas was purged into a solution of H4boptz (0.10 g, 0.37 mmol) in CH2Cl2
(15 mL) for 1 h. The red solution formed was evaporated to dryness. The
product was then purified by using a silica gel column (60–120 mesh)
with CH2Cl2/CH3CN (10:1 v/v ) as eluent. Yield: 0.094 g (95%); m.p.
2228C; ESI MS (in CH3OH): m/z calcd for [(H2boptz)]+: 266.08; found:
267.14; elemental analysis calcd (%) for C14H10N4O2: C 63.15, H 3.79, N
21.04; found: C 63.66, H 3.09, N 19.57; 1H NMR (300 MHz, (CD3)2SO,
298 K): d=10.73 (s, 1H; OH), 8.27 (t, J=4.5 Hz, 1H), 7.56 (t, J=5.7 Hz,
1H), 7.13 ppm (m, 2H).
Acknowledgements
We are grateful to the DST, New Delhi (India) and the DAAD and the
DFG (Germany) for financial support. The X-ray structural study was
carried out at the National Single-Crystal Diffractometer Facility, Indian
Institute of Technology Bombay (India).
[(acac)2Ru(m-boptz)Ru(acac)2]
(1):
[Ru(acac)2(CH3CN)2]
(0.10 g,
0.26 mmol), H2boptz (0.035 g, 0.13 mmol), and sodium acetate (0.03 g,
0.36 mmol) were heated to reflux in ethanol (20 mL) for 8 h. The initially
transparent orange solution gradually changed to dark brown. The solid
mass obtained on removal of the solvent under reduced pressure was dis-
solved in the minimum volume of CH2Cl2 and purified by using a silica
gel (60–120 mesh) column with CH2Cl2/CH3CN (20:1 v/v) as eluent.
Yield: 0.057 g (50%); ESI MS (in CH2Cl2): m/z calcd for [1]+: 864.05;
found: 864.07 (see also Figure S2a in the Supporting Information); ele-
mental analysis calcd (%) for C34H36N4O10Ru2: C 47.33, H 4.21, N 6.49;
found: C 47.69, H 4.21, N 6.28.
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[5] R. J. Crutchley, Adv. Inorg. Chem. 1999, 41, 273.
[(bpy)2Ru(m-boptz)Ru(bpy)2](ClO4)2 (2-(ClO4)2):
A mixture of [Ru-
(bpy)2Cl2]·2H2O (0.10 g, 0.19 mmol) and AgClO4 (0.10 g, 0.48 mmol) in
ethanol (10 mL) was heated to reflux with constant stirring for 2 h under
a dinitrogen atmosphere. The resultant AgCl precipitate was filtered off
after cooling, leaving a red solution of [Ru(bpy)2(EtOH)2]2+. We added
H2boptz (0.025 g, 0.09 mmol) and sodium acetate (0.025 g, 0.30 mmol),
and the mixture was refluxed for 6 h under dinitrogen atmosphere.
During the course of the reaction the initial red color changed to purple.
The solution was then evaporated to dryness under reduced pressure and
the obtained solid mass was purified on an alumina chromatography
column (neutral) for purification. The purple solution containing 2-
(ClO4)2 was eluted with CH2Cl2/CH3CN (2:1 v/v). Yield: 0.057 g (45%);
ESI MS (in CH3CN): m/z calcd for [2-(ClO4)]+: 1191.09; found: 1191.11
(see also Figure S2b in the Supporting Information); elemental analysis
calcd (%) for C54H40Cl2N12O10Ru2: C 50.23, H 3.12, N 13.03; found: C
49.89, H 3.18, N 12.71.
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[7] a) E. Bill, E. Bothe, P. Chaudhuri, K. K. Chlopek, D. Herebian, S.
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[(pap)2Ru(m-boptz)Ru(pap)2](ClO4)2 (3-(ClO4)2): [Ru(pap)2Cl2] (0.10 g,
0.198 mmol) and AgClO4 (0.10 g, 0.48 mmol) were heated to reflux in
ethanol (10 mL) for 2 h under a dinitrogen atmosphere. The precipitated
AgCl was filtered off leaving a purple solution of [Ru(pap)2(EtOH)2]2+
.
We added H2boptz (0.025 g, 0.09 mmol) and sodium acetate (0.025 g,
Chem. Eur. J. 2006, 12, 489 – 498
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Received: March 16, 2005
Published online: August 31, 2005
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