Metal-induced reductive ring opening of 1,2,4,5-tetrazines: Three resulting coordination alternatives, including the new non-innocent 1,2- diiminohydrazido(2-) bridging ligand system

Article abstract of DOI:10.1021/ic051532p

Reaction of 3,6-diaryl-1,2,4,5-tetrazines (aryl = R = phenyl, 2-furyl or 2-thienyl) with 2 equiv of Ru(acac)₂(CH₃-CN)₂ results in reductive tetrazine ring opening to yield diruthenium complexes [(acac)₂Ru^III(dih-R^2-)Ru^III(acac) ₂] bridged by the new 1,2-diiminohydrazido(2-) (dih-R^2- = HNC(R)NNC(R)NH^2-) ligands. rac/meso diastereoisomers could be detected and separated for the compounds with R = phenyl and 2-thienyl, all species are diamagnetic and were characterized by ¹H NMR spectroscopy. Crystal structure determination of the meso isomers with R = phenyl and 2-thienyl confirmed the 1,2-diiminohydrazido formulation through long N-N (≈1.40 A) and short C= N(H) bonds (≈1.31 A), implying two bridged ruthenium(III) centers at about 4.765 A distance with strong antiferromagnetic coupling. The complexes undergo two reversible and well-separated one-electron reduction and oxidation processes, respectively. EPR Spectroscopy of the paramagnetic intermediates with comproportionation constants K_c > 1012 and UV-vis-NIR spectroelectrochemistry were used to identify the accessible redox states as [(acac)₂Ru ^II(dih-R^2-)Ru^II(acac)₂] ^2-,[(acac)₂Ru^II(dih-R^2-)Ru ^III(acac)₂]^-,[(acac)₂Ru ^III(dih-R^2-)Ru^III(acac)₂], [(acac)₂Ru^III-(dih-R^?-)Ru ^III(acac)₂]⁺, and [(acac)₂Ru ^III(dih-R)Ru^III(acac)₂]²⁺. While the UV-vis-NIR spectroscopic response of [(acac)₂Ru(dih-R)Ru(acac) ₂]^0/-/2- is very similar to that of [(bpy) ₂Ru(adc-R)Ru(bpy)₂]^4+/3+/2+, adc-R^2- = 1,2-diacylhydrazido(2-), the EPR result indicating ligand-centered spin for [(acac)₂Ru^II(dih-R^?-)Ru ^II(acac)₂]^- despite deceptive NIR absorptions around 1400 nm reveals distinct differences in the electronic structures.

Full text of DOI:10.1021/ic051532p

Inorg. Chem. 2006, 45, 1316−1325
Metal-Induced Reductive Ring Opening of 1,2,4,5-Tetrazines: Three
Resulting Coordination Alternatives, Including the New Non-Innocent
1,2-Diiminohydrazido(2 ) Bridging Ligand System
−
Somnath Maji,† Biprajit Sarkar,‡ Srikanta Patra,† Jan Fiedler,§ Shaikh M. Mobin,† Vedavati G. Puranik,|
Wolfgang Kaim,*,‡ and Goutam Kumar Lahiri*,†
Contribution from the 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, J. HeyroVsky´ Institute of Physical Chemistry, Academy of
Sciences of the Czech Republic, DolejsˇkoVa 3, CZ-18223 Prague, Czech Republic, and Center for
Materials Characterization, National Chemical Laboratory, Pune, Maharashtra-411008, India
Received September 8, 2005
Reaction of 3,6-diaryl-1,2,4,5-tetrazines (aryl
) R ) phenyl, 2-furyl or 2-thienyl) with 2 equiv of Ru(acac)2(CH3-
CN)2 results in reductive tetrazine ring opening to yield diruthenium complexes [(acac)2RuIII(dih-R2-)RuIII(acac)2]
-
bridged by the new 1,2-diiminohydrazido(2
−
) (dih-R2
)
HNC(R)NNC(R)NH2-) ligands. rac/meso diastereoisomers
phenyl and 2-thienyl, all species are diamagnetic
could be detected and separated for the compounds with R
and were characterized by 1H NMR spectroscopy. Crystal structure determination of the meso isomers with R
phenyl and 2 thienyl confirmed the 1,2-diiminohydrazido formulation through long N N ( 1.40 Å) and short C
N(H) bonds (
)
)
−
−
≈
d
≈
1.31 Å), implying two bridged ruthenium(III) centers at about 4.765 Å distance with strong
antiferromagnetic coupling. The complexes undergo two reversible and well-separated one-electron reduction and
oxidation processes, respectively. EPR Spectroscopy of the paramagnetic intermediates with comproportionation
constants Kc > 1012 and UV
− −NIR spectroelectrochemistry were used to identify the accessible redox states as
vis
[(acac)2RuII(dih-R2-)RuII(acac)2]2-, [(acac)2RuII(dih-R•-)RuII(acac)2]-, [(acac)2RuIII(dih-R2-)RuIII(acac)2], [(acac)2RuIII-
(dih-R•-)RuIII(acac)2]+, and [(acac)2RuIII(dih-R)RuIII(acac)2]2+. While the UV
vis NIR spectroscopic response of
is very similar to that of [(bpy)2Ru(adc-R)Ru(bpy)2]4
), the EPR result indicating ligand-centered spin for [(acac)2RuII(dih-R•-)RuII(acac)2]- despite
deceptive NIR absorptions around 1400 nm reveals distinct differences in the electronic structures.
−
−
[(acac)2Ru(dih-R)Ru(acac)2]0
+, adc-R2
) 1,2-
/-/
2-
+/
3+/
2
-
diacylhydrazido(2
−
Introduction
charge-transfer, electron-transfer, and spin-carrier properties
in combination with coordinated metals or other organic
molecules can be used.1 However, new metal binding or even
metal-metal bridging redox systems have become rare,2
especially in the area of small molecules which are expected
to exert the strongest effects at the metal-ligand interface.
1,2,4,5-Tetrazine and their substituted forms have been
established during the past two decades as fairly stable
acceptor ligands3 which do not readily decompose in their
coordination compounds, e.g., through photolysis. Nonethe-
“Non-innocent” ligands1a-c which are redox active and thus
do not occur commonly in only one oxidation state are
attractive components of functional materials where their
* To whom correspondence should be addressed. E-mail: kaim@
iac.uni-stuttgart.de.
† Indian Institute of Technology-Bombay.
‡ Institut fu¨r Anorganische Chemie.
§ J. Heyrovsky´ Institute.
| National Chemical Laboratory.
(1) (a) Jørgensen, C. K. Oxidation Numbers and Oxidation States;
Springer: Berlin, 1969; p 213. (b) Ward, M. D.; McCleverty, J. A. J.
Chem. Soc., Dalton Trans. 2002, 275. (c) Herebian, D.; Bothe, E.;
Bill, E.; Weyhermu¨ller, T.; Wieghardt, K. J. Am. Chem. Soc. 2001,
123, 10012. (d) Oh, M.; Carpenter, G. B.; Sweigart, D. A. Acc. Chem.
Res. 2004, 37, 1. (e) Fourmigue´, M. Acc. Chem. Res. 2004, 37, 179.
(f) Drain, C. M.; Hupp, J. T.; Suslick, K. S.; Wasielewski, M. R.;
Chen, X. J. Porphyrins Phthalocyanines 2002, 6, 243.
(2) (a) Ghumaan, S.; Sarkar, B.; Patra, S.; van Slageren, J.; Fiedler, J.;
Kaim, W.; Lahiri, G. K. Inorg. Chem. 2005, 45, 3210. (b) Kar, S.;
Sarkar, B.; Ghumaan, S.; Janardanan, D.; van Slageren, J.; Fiedler,
J.; Puranik, V. G.; Sunoj, R. B.; Kaim, W.; Lahiri, G. K. Chem. Eur.
J. 2005, 11, 4901.
(3) Kaim, W. Coord. Chem. ReV. 2002, 230, 127.
1316 Inorganic Chemistry, Vol. 45, No. 3, 2006
10.1021/ic051532p CCC: $33.50
© 2006 American Chemical Society
Published on Web 12/23/2005

Metal-Induced ReductiWe Ring Opening of 1,2,4,5-Tetrazines
Scheme 1
Scheme 2
less, despite the formal Hu¨ckel aromaticity, the presence of
four electronegative nitrogen atoms within a six-membered
ring makes 1,2,4,5-tetrazines so electron deficient that facile
one-electron reduction to radical intermediates and, at
significantly more negative potentials, to two-electron re-
duced (and thus re-oxidizable) “dihydro” forms can take
place.3,4 Although the two-electron reduction was frequently
observed to occur irreversibly,3 no detailed studies of the
follow-up products have been reported so far. Two-electron
reduction can involve added charges in the 1,4- or 1,2-
position, the latter, higher-energy alternative causing the
weakening of one N-N bond. Further reduction of 1,2-
dihydro-1,2,4,5-tetrazine should then result in N-N bond
cleavage; the acyclic six-atom product can be described as
an N,N′-coupled bis-amidinato or as a 1,2-diiminohydrazido-
(2-) system capable of various metal binding opportunities
if one includes potentially coordinating C substituents
(Scheme 1).
Scheme 3
In fact, a report by Bu and co-workers has recently
described that copper-induced (reductive) cleavage of 3,6-
bis(2-pyridyl)-1,2,4,5-tetrazine (bptz) can lead to tetranuclear
metal complexes with N,N′-bis(R-picolinoyl)hydrazido(2-)
bridges;5a a 1,3,4-oxadiazole intermediate was proposed.5
Subsequently, the use of chlorogold(III) compounds was
shown to effect a related tetrazine ring cleavage of bptz;
however, the final product following imine hydrolysis
contained AuIII bound by the basic 2-pyridyl and hydrazido
N atoms (Scheme 1, lower right).6 In a different kind of
reaction, the combination of Ru(acac)2(CH3CN)2, acac- )
acetylacetonato ) 2,4-pentanedionato, with 3,6-bis(3,5-
dimethylpyrazolyl)-1,2,4,5-tetrazine produced another coor-
dination mode, involving the pyrazolyl N and the negatively
charged NH functions in a ketazine situation of the backbone
(Scheme 1, lower left), i.e., leaving the N-N-bonded
nitrogen centers uncoordinated.7
(4) Sauer, J. In ComprehensiVe Heterocyclic Chemistry II; Katritzky, A.
R., Rees, C. W., Scriber, E. F. V., Eds; Elsevier Science Ltd.: Oxford,
1996; Vol. 6, p 901.
Using the three differently substituted derivatives 3,6-
diphenyl-1,2,4,5-tetrazine (phtz), 3,6-bis(2-furyl)-1,2,4,5-
(5) (a) Bu, X. H.; Liu, H.; Du, M.; Zhang, L.; Guo, Y.-M.; Shionoya,
M.; Ribas, J. Inorg. Chem. 2002, 41, 1855 and 5634. (b) See also
Wignacourt, J. P.; Sueur, S.; Lagrene´e, M. Acta Crystallogr., Sect. C
1990, 46, 394. (c) Lagrene´e, M.; Sueur, S.; Wignacourt, J. P. Acta
Crystallogr., Sect. C 1991, 47, 1158.
(6) Dogan, A.; Schwederski, B.; Schleid, Th.; Lissner, F.; Fiedler, J.; Kaim,
W. Inorg. Chem. Commun. 2004, 7, 220.
(7) Patra, S.; Miller, T. A.; Sarkar, B.; Niemeyer, M.; Ward, M. D.; Lahiri,
G. K. Inorg. Chem. 2003, 42, 4707.
Inorganic Chemistry, Vol. 45, No. 3, 2006 1317

Maji et al.
Scheme 4
Scheme 5
tetrazine (bftz), and 3,6-bis(2-thienyl)-1,2,4,5-tetrazine (bttz)
(Scheme 2), we can show in this work that non-coordinating
(phtz) or poorly metal binding substituents in the 3,6
positions of tetrazines (bftz, bttz) result in a third coordination
alternative (Scheme 1, center), involving the four N atoms
of the original tetrazine in the form of imino π-acceptor and
hydrazido(2-) π-donor centers.
The resulting rigid arrangement with edge-sharing five-
membered chelate rings (“S frame configuration”) is known
to bring the coordinated metals together at a rather close
distance of less than 5 Å with several possibilities of
π-electron delocalization.8-10 The related 1,2-diacylhydrazido-
(2-) ligands were previously shown to be oxidized to stable
radical ion complexes8 and to stabilize mixed-valent inter-
mediates (Scheme 3).9
The replacement of acyl by the much better metal-binding
imino functions can be expected to result in a significantly
different situation, both energetically and with respect to the
electronic structures. Herein, we describe the rather general
applicability of the reductive ring-opening reactions of diaryl-
substituted 1,2,4,5-tetrazines (Scheme 1) and the properties
of the resulting new kinds of dinuclear complexes with [Ru-
(acac)2]. Particular emphasis is given to the accessible stable
redox states (two one-electron reductions and oxidations,
respectively) which could be characterized by EPR and UV-
vis-NIR spectroelectrochemistry.
Figure 1. ESI-MS spectra of (a) 1a, (b) 1b, (c) 2, (d) 3a, and (e) 3b in
CH2Cl2.
1
matographic separation and H NMR characterization (cf.
below), the diamagnetic products were formed as mixtures
of two isomers, 1a and 1b from bttz and 3a and 3b from
phtz (see Scheme 5 and Experimental Section); the bftz
precursor yielded almost exclusively one isomer, compound
2. Mass spectroscopy (Figure 1) showed a composition
involving two Ru(acac)2 complex fragments and a formally
dehydrogenated, i.e., reduced, ligand which is also confirmed
by additional D-exchangeable 1H NMR signals at low field.
In agreement with previous reports on similar â-diketonato-
ruthenium complexes12 rather narrow bands were found at
about 3340 cm-1 for the N-H stretching vibrations.
The crystallization of isomers 1b and 3b (Tables 1 and 2
and Figures 2 and 3) allowed for X-ray structure analysis
which showed that the tetrazine ring has been opened during
the reaction and that only the four former tetrazine N atoms,
but not the substituents, participate in metal coordination.
While the resulting structure with two nearly coplanar edge-
sharing five-membered chelate rings involves a new bis-
Results and Discussion
Synthesis, Structure, and Isomerism. All three substi-
tuted tetrazines, bttz, bftz, and phtz, react in the same way
with 2 equiv of Ru(acac)2(CH3CN)2. This result already
suggests that the substituents are not coordinating, even in
the case of bttz with the 2-thienyl groups positioned in a
suitable position for metal chelation.11 According to chro-
(8) Moscherosch, M.; Field, J. S.; Kaim, W.; Kohlmann, S.; Krejcik, M.
J. Chem. Soc., Dalton Trans. 1993, 211.
(9) Kasack, V.; Kaim, W.; Binder, H.; Jordanov, J.; Roth, E. Inorg. Chem.
1995, 34, 1924.
(10) Kaim, W. Coord. Chem. ReV. 2001, 219-221, 463.
(11) Sarkar, B.; Kaim, W.; Klein, A.; Schwederski, B.; Fiedler, J.; Duboc-
Toia, C.; Lahiri, G. K. Inorg. Chem. 2003, 42, 6172.
(12) Hashimoto, T.; Hara, S.; Shiraishi, Y.; Yamauchi, M.; Natarajan, K.;
Shimizu, K. Inorg. Chim. Acta 2005, 358, 2207.
1318 Inorganic Chemistry, Vol. 45, No. 3, 2006

Metal-Induced ReductiWe Ring Opening of 1,2,4,5-Tetrazines
Table 1. Crystallographic Data for 1b‚0.5H2O and 3b
1b‚0.5H2O
3b
formula
fw
C30H37N4O8.50Ru2S2
855.90
C34H40N4O8Ru2
834.84
cryst syst
space group
T (K)
monoclinic
P21/c
293(2)
monoclinic
C2/c
293(2)
λ (Å)
0.71073
0.71073
a (Å)
11.8089(9)
17.448(1)
18.646(2)
95.645(2)
3823.3(5)
0.25 × 0.09 × 0.06
1.487
25.225(2)
10.2960(4)
15.9520(19)
122.897(8)
3478.7(5)
0.30 × 0.25 × 0.20
1.594
b (Å)
c (Å)
â (deg)
V (Å3)
cryst size (mm3)
d
calcd (g cm-3
)
Z
4
4
µ (mm-1
F (000)
)
0.949
1732
0.924
1696
2θ range (deg)
no. of reflns collected
no. of ind reflns
4.18-50.00
3.84-50.00
19 197
3183
6710 [R(int) ) 0.0710]
6710/20/436
0.971
3059 [R(int) ) 0.0255]
3059/0/221
1.030
no. of data/restraints/params
GOF, F2
R1,wR2 [I > 2σ(I)]
R1,wR2 (all data)
largest diff. peak/hole, (e Å-3
0.0583, 0.1276
0.1114, 0.1491
0.925/-0.482
0.0445, 0.0882
0.1054, 0.1039
0.530/-0.869
)
Table 2. Selected Bond Distances (Å) and Bond Angles (deg) for 1b and 3b
distances
anglesb
1b
3b
1b
3b
Ru1-N1
Ru1-N3a
Ru1-O1
Ru1-O2
Ru1-O3
Ru1-O4
Ru2-N2
Ru2-N4
Ru2-O5
Ru2-O6
Ru2-O7
Ru2-O8
N1-C11
N2-C11
N2-N3a
N3-C16
N4-C16
Ru-Ru
1.948(5)
2.020(5)
2.001(4)
2.038(4)
2.062(5)
2.025(5)
2.007(5)
1.951(5)
1.990(5)
2.066(5)
2.057(5)
2.019(5)
1.314(8)
1.336(7)
1.397(7)
1.342(7)
1.296(8)
4.769
1.927(5)
2.012(4)
2.010(4)
2.058(4)
2.065(4)
2.014(4)
N1-Ru1-N3a
N1-Ru1-O1
N1-Ru1-O2
N1-Ru1-O3
N1-Ru1-O4
N3a-Ru1-O1
N3a-Ru1-O2
N3a-Ru1-O3
N3a-Ru1-O4
N2-Ru2-N4
N2-Ru2-O5
N2-Ru2-O6
N2-Ru2-O7
N2-Ru2-O8
N4-Ru2-O5
N4-Ru2-O6
N4-Ru2-O7
N4-Ru2-O8
76.6(2)
77.01(18)
90.67(18)
93.90(17)
176.47(19)
93.14(17)
84.62(17)
170.42(16)
103.90(16)
96.60(17)
90.5(2)
95.5(2)
179.2(2)
90.7(2)
87.1(2)
172.1(2)
103.8(2)
93.7(2)
76.9(2)
90.6(2)
171.5(2)
102.2(2)
91.2(2)
93.2(2)
94.6(2)
176.5(2)
88.0(2)
1.318(6)
1.334(7)
1.407(8)
4.766
a N(2′) for 3b. b Angles O-Ru-O 84-93° (cis) and 176-179° (trans).
chelate ligand system, related arrangements have been
described before for mixed bis(N,O-donor) ligands derived
from azodicarbonyl compounds or from their reduced forms,
the diacylhydrazido(2-) ligands.8,9 The edge-sharing of two
five-membered chelate rings involves an “S-frame” structure
with the ligand in s-trans/E/s-trans conformation and typically
short metal-metal distances of d < 5 Å.8-10 Accordingly,
the present two examples 1b and 3b have d(Ru-Ru) ) 4.77
Å.
The dimensions within the new bridging ligand system
are revealing: The central N-N bonds are rather long in
both cases at about 1.40 Å, close to the expected value for
a single bond.13a The terminal CdN bonds, on the other hand,
in which the N atoms bind the added hydrogen atoms, are
short with average values of about 1.31 Å which suggests
a double bond.13b Together with the ca. 1.34 Å for the
“inner C-N” bonds this result points to alternative A in
Scheme 4 for the then-dianionic bridging ligands, thus best
formulated as C-substituted 1,2-diiminohydrazido(2-) ligands,
dih-R2- (R ) Th ) 2-thienyl, R ) Fu ) 2-furyl, R ) Ph )
phenyl).
Obviously, the reactions between two ruthenium(II) pre-
cursor complexes and the tetrazines have involved extensive
electron transfer (4e-) accompanied by double protonation,
leading to ring-opened, four-electron reduction products of
the tetrazine and, in turn, to one-electron oxidation at the
metal centers.14
Considering the established15,16 preference for the [Ru-
(acac)2]n+ complex fragment to adopt the n ) 1 state with
(13) (a) Huheey, J. E.; Keiter, E. A.; Keiter, R. L. Inorganic Chemistry,
4th ed.; HarperCollins: New York, 1993; p A-31. (b) Shriver, D. F.;
Atkins P. W.; Langford, C. H. Inorganic Chemistry; Oxford University
Press: Oxford, 1990; p 69.
(14) The formation of additional ruthenium(III) is evident from Ru(acac)3
identified as byproduct (see Experimental Section).
Inorganic Chemistry, Vol. 45, No. 3, 2006 1319

Maji et al.
1b and 3b are unremarkable, the aryl substituents are
considerably twisted with respect to the central bis-chelate
ring plane.
In contrast to recently reported metal-induced ring-opening
reactions of tetrazines (Scheme 1),5-7 the present examples
do not make use of the C substituents but rely wholly on
the donor set provided originally by the tetrazine ring.
Even symmetrically dinuclear complexes of the described
kind can exist as isomers due to the tris-chelate coordination
at the metals.17 In structurally related cases such as {(µ-abpy)-
[Ru(bpy)2]2}4+, the meso and rac isomers could be separated
and identified on the basis of NMR spectroscopy,18 the
recently obtained analogues (µ-abpy)[Ru(acac)2]2 could even
be crystallized in both forms.19 The two crystallized com-
pounds 1b and 3b in the present case contain meso-
configured molecules (Figures 2 and 3); the rac forms are
distinguished from the meso isomers mainly by a much larger
difference (∆) between the CH resonances of the acac- co-
ligands (∆meso ) 0.2 ppm vs ∆rac ) 0.7 ppm, see Figure 4
and Experimental Section). According to this criterion, the
isolated form 2 from the reaction with bftz is the rac isomer
because it has ∆ ) 0.68 ppm, Scheme 5 summarizes the
assigments.
Figure 2. ORTEP diagram of 1b. Ellipsoids are drawn at 30% probability.
Hydrogen atoms with the exception of N-H have been omitted for clarity.
Only one kind of the disordered thiophene rings is shown.
The other 1H NMR resonances, the redox potentials (Table
3) and the absorption spectra (Table 5) show very little
differences between rac and meso isomers, which is not
unexpected.17-20
Redox Activity of the New Complexes. The potential
redox reactivity of the complex fragments [Ru(acac)2]n+, n
) 0, 1, or 2,15,16 and of the new bis-chelate ligand according
to Scheme 4 invited electrochemical and spectroelectro-
chemical studies. To identify the proper oxidation state
distribution a combination of UV-vis-NIR absorption
spectroscopy and EPR was chosen.
All five isolated complexes undergo two one-electron
reductions and two one-electron oxidations in the well-
accessible potential region of acetonitrile (Figure 5, Table
3). The processes are reversible as confirmed by spectro-
electrochemistry (cf. below), except for the second reduc-
tions, 3a(-)f(2-) and 3b(-)f(2-). The results from Table 3 show
wide separations between the four waves, leading to very
large comproportionation constants Kc > 1012 for the odd-
electron intermediates (monoanions or monocations) which
may involve radical ligands or mixed-valent metal configura-
tions9 [RT ln Kc ) nF(∆E)]. There is relatively little variation
between the compounds; all potentials are shifted to slightly
more negative values when going from the thienyl via the
furyl to the phenyl-substituted systems which thus are the
easiest to oxidize and the most difficult to reduce.
The paramagnetic intermediates generated electrochemi-
cally from the five neutral complexes through oxidation or
Figure 3. ORTEP diagram of 3b. Ellipsoids are drawn at 50% probability
level. Hydrogen atoms with the exception of N-H have been omitted for
clarity.
ruthenium(III) configuration, this result is not too surprising;
however, the resulting diamagnetism of the complexes (µ-
dih-R2-)[RuIII(acac)2]2 must then be a consequence of strong
ligand-enhanced antiferromagnetic spin-spin coupling. Not
only the unusually short metal-metal distances for molecule-
bridged dinuclear species but also the electronic mediator
effects of the unsaturated and, as we shall see later, non-
innocent ligands dih-Rn- are held responsible for the
observed strong spin-spin interaction.
The Ru-N distances differ consistently: At about 2.01
Å, the RuIII-N(hydrazido) bonds are distinctly longer than
the RuIII-N(imine) bonds at about 1.93 Å. The negative
charge localized on the hydrazido N atoms according to
resonance form A of Scheme 4 is held responsible for this
difference. The remaining structural features of compounds
(17) Ernst, S.; Kasack, V.; Kaim, W. Inorg. Chem. 1988, 27, 1146.
(18) Kelso, L. S.; Reitsma, D. A.; Keene, F. R. Inorg. Chem. 1996, 35,
5144.
(15) Patra, S.; Sarkar, B.; Mobin, S. M.; Kaim, W.; Lahiri, G. K. Inorg.
Chem. 2003, 42, 6469.
(16) Kar, S.; Sarkar, B.; Ghumaan, S.; Roy, D.; Urbanos, F. A.; Fiedler,
J.; Sunoj, R. B.; Jimenez-Aparicio, R.; Kaim, W.; Lahiri, G. K. Inorg.
Chem. 2005, 44, 8715.
(19) Sarkar, B.; Patra, S.; Fiedler, J.; Sunoj, R.; Janardanan, D.; Mobin, S.
M.; Niemeyer, N.; Lahiri, G. K.; Kaim, W. Angew. Chem., 2005, 117,
5800; Angew. Chem., Int. Ed. 2005, 44, 5655.
(20) Heilmann, M.; Frantz, S.; Kaim, W.; Fiedler, J.; Duboc, C. Inorg.
Chim. Acta, in print (web released; DOI: 10.1016/j.ica.2005.04.044).
1320 Inorganic Chemistry, Vol. 45, No. 3, 2006

Metal-Induced ReductiWe Ring Opening of 1,2,4,5-Tetrazines
slightly lower than 2, as is typical for mononuclear and
dinuclear radical complexes containing ruthenium(II).21 Start-
ing from a (dih-R2-)(RuIII)2 situation, we conclude from EPR
that reduction produces (dih-R•-)(RuII)2 instead of the
mixed-valent (dih-R2-)RuIIRuIII, despite an intense near-
infrared absorption (cf. below). This interpretation implies
that the reduction of the complex involves an oxidation of
the bridging ligand with intramolecular electron transfer
compensating for the seemingly paradoxical effect. Similar
phenomena have been observed before in dioxolene/metal
and TCNX/metal chemistry.22,23 On the other hand, one-
electron oxidation leads to metal-centered spin, either in a
mixed-valent (dih-R2-)RuIIIRuIV situation or in the three-spin
system RuIII(dih-R•-)RuIII with dominant antiferromagnetic
coupling between the radical ligand and one of the two
ruthenium(III) centers.24
Spectroelectrochemistry in the UV-vis-NIR regions
using an optically transparent thin-layer electrode (OTTLE)
cell25 (Figure 7, Table 5) was used to further analyze the
oxidation state situation, to elucidate the electronic structures
of even-electron systems, and to determine the nature of low-
lying excited states.
The neutral products (µ-dih-R)[Ru(acac)2]2 from the tet-
razine ring-opening reactions exhibit intense long-wavelength
bands around 730 nm which are attributed to ligand-to-metal
charge-transfer transitions from the electron-rich bridges to
the two ruthenium(III) centers each.26 On oxidation, this band
is decreased in intensity, yet the position remains almost
unchanged; an absorption in the near-infrared, as would be
expected16 for a mixed-valent situation (dih-R2-)RuIIIRuIV,
was not observed. This result suggests the formulation RuIII-
(dih-R•-)RuIII for the monocation, i.e., an oxidation of the
bridging ligand instead of the metal. The second oxidation
causes the long-wavelength band to increase considerably
and shift slightly to lower energies; a charge-transfer
transition, either MLCT (metal-to-ligand charge transfer) for
a formulation (dih-R°)RuIIIRuIII or LMCT in a situation (dih-
R2-)RuIVRuIV can be considered. In the absence of further
information, this question must remain open; increasing
covalency for higher charged species may also result in
considerable metal/ligand/metal orbital mixing, rendering
assignments based on localized descriptions less meaningful.
The reduction of the complexes leading to an EPR
spectroscopically established (dih-R•-)(RuII)2 situation is
accompanied by the expected disappearance of the LMCT
band, the RuIII centers are no longer available as targets of
such a transition. There is an intense broad band emerging
(21) Kaim, W.; Ernst, S.; Kasack, V. J. Am. Chem. Soc. 1990, 112, 173.
(22) Cuss, M. E.; Gordon, N. R.; Pierpont, C. G. Inorg. Chem. 1986, 25,
3962.
(23) Moscherosch, M.; Waldho¨r, E.; Binder, H.; Kaim, W.; Fiedler, J. Inorg.
Chem. 1995, 34, 4326.
(24) Ye, S.; Sarkar, B.; Lissner, F.; Schleid, Th.; van Slageren, J.; Fiedler,
J.; Kaim, W. Angew. Chem. 2005, 117, 2140; Angew. Chem., Int. Ed.
2005, 44, 2103.
(25) Krejcik, M.; Danek, M.; Hartl, F. J. Electroanal. Chem. 1991, 317,
179.
Figure 4. 1H NMR spectra of (a) 1a, (b) 1b (c) 2 (d) 3a, and (e) 3b in
(CD3)2SO.
reduction show very similar EPR behavior (Figure 6, Table
4). Oxidation produces signals in glassy frozen solutions
which are typical for ruthenium(III),15,16 i.e., rhombic g
component splitting with g > 2. Reduction, on the other
hand, yields only unresolved signals at isotropic g values
(26) The resulting excited state can be formulated as involving an anion
radical bridging two mixed-valent metal centers, *{(µ-dih-R•-)[Ru2.5
-
(acac)2]2}. Related electronic structures were recently proposed for
ground-state species (see ref 19).
Inorganic Chemistry, Vol. 45, No. 3, 2006 1321

Maji et al.
Table 3. Redox Potentialsa and Comproportionation Constants (Kc) for the Complexesb
couple
compound
I
II
III
IV
Kc (+, -)
[{(acac)2Ru}2(µ-dih-Th)] (1a)
[{(acac)2Ru}2(µ-dih-Th)] (1b)
[{(acac)2Ru}2(µ-dih-Fu)] (2)
[{(acac)2Ru}2(µ-dih-Ph)] (3a)
[{(acac)2Ru}2(µ-dih-Ph)] (3b)
1.03(90)
1.01(80)
0.92(84)
0.90(80)
0.90(80)
0.27(90)
0.25(70)
0.16(63)
0.13(85)
0.12(90)
-0.94(70)
-0.96(80)
-1.03(80)
-1.10(100)
-1.13(90)
-1.69(100)
-1.68(80)
-1.81(80)
-1.88(120)
-1.87(170)
1012.9, 1012.8
1013.0, 1012.2
1012.9, 1013.2
1013.0, 1013.2
1013.3, 1012.6
a Potentials E°298/V (∆E/mV) versus SCE. b In CH3CN/0.1 M Et4NClO4
Table 4. EPR Data for Oxidized and Reduced Complexesa
compounda
g1
g2
g3
∆gb
giso
c
1a+
1a-
1b+
1b-
2+
2.261
1.975
2.223
1.968
2.236
1.968
2.249
1.966
2.231
1.968
2.140
1.975
2.136
1.968
2.115
1.968
2.132
1.966
2.134
1.968
1.851
1.975
1.841
1.968
1.844
1.968
1.838
1.966
1.836
1.968
0.410
2.091
1.975
2.073
1.968
2.072
1.968
2.080
1.966
2.074
1.968
-
0.382
-
0.392
2-
-
3a+
3a-
3b+
3b-
0.411
-
0.396
-
a Measurements in CH3CN/0.1 M Bu4NPF6 at 110 K. b ∆g ) g1 - g3.
c giso ) x(g1 + g2 + g3 )/3.
2
2
2
Table 5. Absorption Maxima λmax (nm)a of Diruthenium Complexes
[(acac)2Ru(dih-R)Ru(acac)2]n with Ring-Opened Tetrazines
N
1a
1b
2
3a
3b
0
734 (15.8) 718 (16.3) 744 (15.2) 729 (14.8) 716 (16.2)
350 (14.3) 353 (13.1) 347 (13.1) 352 (12.1) 353 (13.7)
270 (31.4) 268 (25.8) 273 (33.2) 270 (28.4) 267 (30.5)
240 sh
242 sh
242 sh
237 sh
241 sh
+
745 (10.3) 737 (11.2)
757 (9.3) 718 (10.3) 716 (12.2)
565 (4.9)
448 sh
572 (5.2)
443 sh
293 sh
570 (5.0)
445 sh
545sh
558 sh
430 sh
427 (4.2)
292 sh
272 (29.8) 268 (24.5) 269 (28.4)
239 (23.9) 239 (27.0)
267 (30.2) 268 (26.8)
241 sh
243 sh
2+ 803 (19.6) 815 (18.9) 800 (14.4) 794 (21.8) 800 (22.5)
268 (29.4)
242 sh
295 sh
268 (26.3)
242 (24.9)
278 (29.2)
292 sh
240 (26.8) 265 (26.7)
241 (28.2)
294 sh
-
1423 (7.2) 1385 (8.0) 1415 (9.3) 1390 (9.2) 1368 (9.4)
660 (4.2) 643 (4.1) 615 (4.3) 590 (4.2) 595 sh
430 (11.1) 428 (12.0)
269 (32.5) 268 (28.7) 271 (30.9) 269 (29.1)
Figure 5. Cyclic voltammograms of (a) 1b, (b) 2, and (c) 3a in CH3CN/
0.1 M Et4NClO4 at 298 K (scan rate 50 mV s-1).
412 (9.5) 436 (11.8) 434 (12.9)
285 sh
242 sh
241 sh
236 sh
236 sh
269 (31.1)
239 (25.4)
675 sh
2-
decb
decb
710 sh
685 sh
531 (9.9) 492 (12.4) 494 (15.1)
466 (9.8) 269 (30.8) 268 (33.0)
272 (33.5)
238 sh
285 sh
287 sh
239 (24.6) 241 (28.1)
a From OTTLE spectroelectrochemistry in CH3CN/0.1 M Bu4NPF6;
molar extinction coefficients in 103 M-1 cm-1 (in parentheses). b Partial
decomposition on the time scale of the experiment (1 min).
Figure 6. EPR Spectra of 1a- (top) and 1a+ (bottom) in CH3CN/0.1 M
Bu4NPF6 at 110 K.
in the near-infrared region around 1400 nm which, at first
sight, could be attributed to the intervalence charge transfer
(IVCT) transition of a mixed-valent situation (dih-R2-)RuII-
RuIII. However, since the EPR results unequivocally indicate
the (dih-R•-)(RuII)2 formulation to be correct, that NIR band
must be assigned otherwise, most likely to an MLCT
transition from the electron-rich RuII centers to the π-ac-
cepting (i.e., reducible) radical bridge. In other words, the
mixed-valent configuration is the close-lying excited state
of that anionic form according to eq 1.
This assignment is supported by the correspondence
between the absorption energy at about 0.88 eV and the
potential differences of ca. 0.75 V between redox couple III
and IV (Table 3).
Similar intense NIR bands at around 1600 nm have been
reported for the related paramagnetic complexes {(µ-adc-
R)[Ru(bpy)2]2}3+;9 however, there the EPR analysis of the
well-detectable g anisotropy revealed sizable, substituent-
dependent contributions from the metals according to a
continuum between situations (µ-L•-)(RuII)2 and (µ-L2-)RuII-
RuIII.9 It is thus important to note that NIR bands alone
cannot be taken per se as unambiguous eVidence for a mixed-
hν (NIR)
[(dih-R•-)(RuII)2]- MLCT 8*[(dih-R2-)(RuIIRuIII)]- (1)
1322 Inorganic Chemistry, Vol. 45, No. 3, 2006

Metal-Induced ReductiWe Ring Opening of 1,2,4,5-Tetrazines
structurally and electronically related redox systems {(µ-dih-
R)[Ru(acac)2]2}n and {(µ-adc-R)[Ru(bpy)2]2}n+4 is illustrated
in Scheme 6. In addition to the correlation between the
central two-step redox systems, the diacylhydrazido-bridged
compounds can undergo further reduction at the 2,2′-
bipyridine co-ligands while the donor effect of the acac-
ligands allows for further oxidation of (µ-dih-R)[Ru(acac)2]2.
Conclusion
In this work, we have shown that the 1,2,4,5-tetrazines
cannot only be used as unusual ligands3 with an intact six-
membered heterocyclic ring in the strongly π-accepting and
valence-exchange mediating28 neutral form or in partially
reduced (radical anion29 or 1,4-dihydro30) states but that the
products from reductive ring-opening after four-electron
addition are also highly attractive new non-innocent bridging
ligands. The form identified via structure determination of
two compounds (µ-dih-R)[Ru(acac)2]2 is best described as
1,2-diiminohydrazido(2-) (µ-dih-R2-) which can coordinate
two metal centers to form a compact structure with edge-
sharing unsaturated five-membered chelate rings and thus
rather short metal-metal distances of about 4.8 Å. These
structural and electronic effects also favor strong antiferro-
magnetic spin-spin interaction, as observed for the diru-
thenium(III) compounds (µ-dih-R)[Ru(acac)2]2. The unsat-
urated ligand system is oxidized to a radical anion in
diruthenium(II) complexes {(µ-dih-R)[Ru(acac)2]2}-, which
exhibit intense bands in the near-infrared yet are not mixed-
valent species according to comparative9,31 EPR spectros-
copy.
In contrast to previous recent reports5-7 on complexes with
ring-opened tetrazines (Scheme 1), the reaction presented
here does not require coordinative support through substit-
uents. Instead, all four nitrogen atoms from the former
tetrazine ring are involved in bis-chelation of two metal
centers. As a perspective, it may be envisaged that other
electron-rich metal compounds can induce the ring-opening
of tetrazines, either simple tetrazines such as phtz, non-
chelating functional tetrazines such as bridging 3,6-bis(4,4′-
pyridyl)-1,2,4,5-tetrazine,32 or chelating tetrazine derivatives
such as bptz.3
Figure 7. OTTLE spectroelectrochemistry of 3an in CH3CN/0.1 M Bu4-
NPF6.
Valence situation in metal complexes, especially in the
presence of non-innocent ligands.
The difference between the partially mixed-valent com-
plexes {(µ-adc-R)[Ru(bpy)2]2}3+ and the closely related {(µ-
dih-R)[Ru(acac)2]2}-, which are diruthenium(II) radical
compounds, cannot be attributed to the ancillary ligands
because acac- favors the ruthenium(III) state15,16 whereas
bpy stabilizes ruthenium(II).27 The origin of this difference
lies rather in the bridging ligand situation where imine-
containing compounds are easier to oxidize (and harder to
reduce) than the carbonyl analogues. This effect, commonly
explained via the higher electronegativity of O versus N,
facilitates the oxidation of dih-R2- to the radical anion dih-
R•-, with concomitant results for the metal oxidation states.
Nevertheless, the spectral similarity between individual
states {(µ-dih-R)[Ru(acac)2]2}n and corresponding complexes
{(µ-adc-R)[Ru(bpy)2]2}n+4 (cf. Scheme 6) does not only
concern the paramagnetic intermediates (n ) -1) with their
intense NIR bands or the oxidized forms (n ) 0) with very
intense charge-transfer bands around 800 nm in both cases
(Table 5),9 it also extends to the reduced forms (n ) -2)
which do not exhibit prominent bands at such long wave-
lengths because the bridging ligand and the metals exist in
their reduced forms. The observed bands in the visible region
are attributed to MLCT transitions directed at the acetyl-
acetonate co-ligands.15,16,27 The correspondence between the
Experimental Section
General. The precursor complex Ru(acac)2(CH3CN)233 and the
tetrazine-based ligands 3,6-bis(2-thienyl)-1,2,4,5-tetrazine (bttz),11
3,6-bis(2-furyl)-1,2,4,5-tetrazine (bftz),4 and 3,6-diphenyl-1,2,4,5-
(28) (a) Poppe, J.; Moscherosch, M.; Kaim, W. Inorg. Chem. 1993, 32,
2640. (b) Glo¨ckle, M.; Kaim, W.; Klein, A.; Roduner, E.; Hu¨bner,
G.; Zalis, S.; van Slageren, J.; Renz, F.; Gu¨tlich, P. Inorg. Chem. 2001,
40, 2256.
(29) Schwach, M.; Hausen, H.-D.; Kaim, W. Inorg. Chem. 1999, 38, 2242.
(30) Glo¨ckle, M.; Hu¨bler, K.; Ku¨mmerer, H.-J.; Denninger, G.; Kaim, W.
Inorg. Chem. 2001, 40, 2263.
(31) Remenyi, C.; Kaupp, M. J. Am. Chem. Soc. 2005, 127, 11399.
(32) (a) Kaim, W.; Kohlmann, S. Inorg. Chem. 1990, 29, 1898. (b) Waldho¨r,
E.; Zulu, M. M.; Zalis, S.; Kaim, W. J. Chem. Soc., Perkin Trans. 2
1996, 1197. (c) Dinolfo, P. H.; Williams, M. E.; Stern, C. L.; Hupp,
J. T. J. Am. Chem. Soc. 2004, 126, 12989.
(27) Patra, S.; Sarkar, B.; Maji, S.; Fiedler, J.; Urbanos, F. A.; Jimenez-
(33) Kobayashi, T.; Nishina, Y.; Shimizu, K. G.; Satoˆ, G. P. Chem. Lett.
1988, 1137.
Aparicio, R.; Kaim, W.; Lahiri, G. K. Chem. Eur. J., in press.
Inorganic Chemistry, Vol. 45, No. 3, 2006 1323

Maji et al.
Scheme 6
tetrazine (phtz)34 were prepared according to reported procedures.
Other chemicals and solvents were reagent grade and used as
received. For spectroscopic and electrochemical studies, HPLC
grade solvents were used.
dryness under reduced pressure. The green solid thus obtained was
dissolved in the 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 green zone corresponding to a mixture of isomers
1a (rac) and 1b (meso) was eluted. The isomers were separated on
a silica gel preparatory TLC plate using CH2Cl2-CH3CN (6:1).
For 1a: Yield ) 15 mg (13.5%); Anal. Calcd for C30H36N4S2O8-
Ru2: C, 42.45; H, 4.28; N 6.61. Found: C, 42.71; H, 4.12; N 6.39.
UV-vis-NIR spectroelectrochemical studies were performed
in CH2CN/0.1 M Bu4NPF6 at 298 K using an OTTLE cell25 coupled
with a J&M TIDAS spectrophotometer. Solution electrical con-
ductivity was checked using a Systronic 305 conductivity bridge.
Magnetic susceptibility was investigated with a PAR vibrating
sample magnetometer. All neutral samples were found to be
diamagnetic. 1H NMR spectra were obtained with a 300 MHz
Varian FT spectrometer. The EPR measurements were made in a
two-electrode capillary tube21 with a X-band Bruker system ESP300
equipped with a Bruker ER035M gaussmeter and a HP 5350B
microwave counter. Cyclic voltammetric, differential pulse voltam-
metric, and coulometric measurements 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 Et4NClO4, 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
measurements were carried out under a dinitrogen atmosphere. The
elemental analyses were carried out with a Perkin-Elmer 240C
elemental analyzer. Electrospray mass spectra were recorded on a
Micromass Q-ToF mass spectrometer.
1
ESI MS (CH2Cl2): m/z ) 848.05 [M+] (calcd m/z, 848.00). H
NMR {(CD3)2SO}, δ (J/Hz): 12.08 (s, NH), 7.65 (d, 5.4), 7.50 (d,
3.6), 7.15 (t, 4.35, 4.35), 5.66 (s, CH(acac)), 4.91 (s, CH(acac)),
2.21 (s, CH3(acac)), 2.04 (s, CH3(acac)), 1.96 (s, CH3(acac)), 1.52
(s, CH3(acac)). For 1b: Yield ) 30 mg (27%); Anal. Calcd for
C30H36N4S2O8Ru2: C, 42.45; H, 4.28; N 6.61. Found: C, 42.81;
H, 4.63; N 6.97. ESI MS (CH2Cl2): m/z ) 848.33 [M+] (calcd
1
m/z, 848.00). H NMR {(CD3)2SO}, δ (J/Hz): 12.13 (NH) 7.61
(d, 5.1), 7.16 (d, 3.0), 7.10 (t, 4.8, 3.6), 5.29(s, CH(acac)), 5.11 (s,
CH(acac)), 2.18 (s, CH3(acac)), 2.14 (s, CH3(acac)), 1.97 (s, CH3-
(acac)), 1.53 (s, CH3(acac)).
Synthesis of {Ru(acac)2}2(µ-dih-Fu) (2). The starting complex
Ru(acac)2(CH3CN)2 (100 mg, 0.26 mmol) and the ligand bftz (26
mg, 0.12 mmol) were taken in 20 mL of ethanol, and the mixture
was heated to reflux for 4 h in air. The initial orange color of the
solution gradually changed to dark green. The solvent of the reaction
mixture was evaporated to dryness under reduced pressure, and
the solid mass thus obtained was dissolved in the minimum volume
of CH2Cl2 to be purified by using a silica gel column. Initially, a
red compound corresponding to Ru(acac)3 was eluted with CH2-
Cl2-CH3CN (50:1). With CH2Cl2-CH3CN (12:1), a green com-
pound corresponding to 2 was separated later on. Yield ) 35 mg
(33%). Anal. Calcd for C30H36N4O10Ru2 (816.06): C, 44.11; H,
4.45; N 6.86. Found: C, 44.52; H, 4.52; N, 6.98. ESI MS (CH2-
Cl2): m/z ) 816.06 [M+] (calcd m/z, 816.06). 1H NMR data
{(CD3)2SO}, δ (J/Hz): 12.15 (s, NH) 7.77 (d, 1.5), 7.10 (d, 4.5),
6.67 (t, 2.7, 2.7), 5.68 (s, CH(acac)), 5.00 (s, CH(acac)), 2.19 (s,
CH3(acac)), 2.06 (s, CH3(acac)), 1.99 (s, CH3(acac)), 1.72 (s, CH3-
(acac)).
Synthesis of {Ru(acac)2}2(µ-dih-Th) (1a and 1b). The precursor
complex Ru(acac)2(CH3CN)2 (100 mg, 0.26 mmol) and the ligand
bttz (30 mg, 0.12 mmol) were dissolved in 20 mL of ethanol and
the mixture was heated to reflux for 24 h under aerobic conditions.
The initial orange color of the solution gradually changed to dark
green, and the solvent of the reaction mixture was evaporated to
(34) (a) Abdel-Rahman, M. O.; Kire, M. A.; Tolba, M. N. Tetrahedron
Lett. 1968, 35, 3871. (b) Fischer, H.; Mu¨ller, Umminger, I.; Neuge-
bauer, F. A. J. Chem. Soc., Perkin Trans. II 1988, 413.
1324 Inorganic Chemistry, Vol. 45, No. 3, 2006

Metal-Induced ReductiWe Ring Opening of 1,2,4,5-Tetrazines
Synthesis of {Ru(acac)2}2(µ-dih-Ph) (3a and 3b). A mixture
of Ru(acac)2(CH3CN)2 (100 mg, 0.26 mmol) and phtz (28 mg, 0.12
mmol) in 20 mL of ethanol was heated to reflux for 5 h under
aerobic conditions. The initial orange color of the solution gradually
changed to dark green, and the solvent of the reaction mixture was
evaporated to dryness under reduced pressure. The green solid was
dissolved in the minimum volume of CH2Cl2 and purified using a
silica gel column. Initially, red Ru(acac)3 was eluted with CH2-
Cl2-CH3CN (15:1). With CH2Cl2-CH3CN (3:1), a mixture of
isomers 3a and 3b was eluted. 3a and 3b were separated on a silica
gel preparatory TLC plate using CH2Cl2-CH3CN (7:1). For 3a:
Yield ) 25 mg (23%). Anal. Calcd for C34H40N4O8Ru2: C, 48.92;
H, 4.83; N, 6.71. Found: C, 49.13; H, 4.91; N, 7.12. ESI MS (CH2-
and refined by full-matrix least-squares techniques on F2 using
SHELX-97 (SHELXTL program package) and SHELX-97, respec-
tively.35 The absorption corrections for 1b and 3b were done by
SADABS and psi-scan, respectively, and all data were corrected
for Lorentz and polarization effects. Hydrogen atoms were included
in the refinement process as per the riding model.
One of the thiophene rings in the crystal of 1b is disordered due
to 180° rotation about the C16-C17 single bond so that the S2
and C18 atoms interchange their positions. Thus, the structure was
refined considering the coincidence of S2′ with C18 and C18′ with
S2. This model led to a very good convergence. Except for the
disordered atoms of thiophene ring which were refined isotropically,
all other non-hydrogen atoms were refined anisotropically.
1
Cl2): m/z ) 836.24 [M+] (calcd m/z, 836.09). H NMR, {(CD3)2-
SO], δ (J/Hz): 12.15 (s, NH), 7.68 (multiplet for 2 phenyl ring
protons), 7.40 (multiplet for 3 phenyl ring protons), 5.47 (s, CH-
(acac)), 4.81 (s, CH(acac)), 2.10 (s, CH3(acac)), 1.92 (s, 2CH3-
(acac)), 1.26 (s, CH3(acac)). For 3b: Yield ) 30 mg (27%). Anal.
Calcd for C34H40N4O8Ru2: C, 48.92; H, 4.83; N, 6.71. Found: C,
49.25; H, 4.89; N, 6.99. ESI MS (CH2Cl2): m/z ) 836.28 [M+]
Acknowledgment. Financial support received from the
Department of Science and Technology, New Delhi (India),
from the DAAD and the DFG (Germany) is gratefully
acknowledged. X-ray structural studies for 3b were carried
out at the National Single Crystal Diffractometer Facility,
Indian Institute of Technology, Bombay. Special acknowl-
edgment is made to the Sophisticated Analytical Instrument
Facility (SAIF), Indian Institute of Technology, Bombay, for
providing facilities for NMR measurements.
1
(calcd m/z, 836.09). H NMR, {(CD3)2SO}, δ (J/Hz): 12.16 (s,
NH) 7.40 (multiplet for 5 phenyl ring protons), 5.35 (s, CH(acac)),
5.09 (s, CH(acac)), 2.16 (s, CH3(acac)), 2.08 (s, CH3(acac)), 1.98
(s, CH3(acac)), 1.32 (s, CH3(acac)).
X-ray Structure Determination. Single crystals of 1b and 3b
were grown by slow diffusion of a dichloromethane solution into
n-hexane, followed by slow evaporation (1b) or by slow evaporation
of a dimethyl sulfoxide solution (3b). The crystal of 1b contains 2
× (0.25 H2O) molecules as solvent of crystallization. X-ray data
of 1b and 3b were collected using Bruker SMART APEX CCD
and Enraf-Nonius CAD-4 (MACH-3) single-crystal X-ray diffrac-
tometers, respectively. The structures of 1b and 3b were solved
Supporting Information Available: X-ray crystallographic file
in CIF format for compounds 1b and 3b. This material is available
free of charge via the Internet at http://pubs.acs.org.
IC051532P
(35) Sheldrick, G. M. SHELX-97 program for crystal structure solution
and refinement; University of Go¨ttingen: Go¨ttingen, Germany, 1997.
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