Tetrazine derived mononuclear RuII(acac)2(L) (1), [RuII(bpy)2(L)](ClO4)2 (2) and [RuII(bpy)(L)2](ClO4)2 (3) (L = 3-amino-6-(3,5-dimethylpyrazol-1-yl)-1,2,4,5-tetrazine, acac = acetylacetonate, bpy = 2,2′-bipyridine): Syntheses, structures, spectra and redox properties

Article abstract of DOI:10.1016/j.poly.2004.11.019

Mononuclear ruthenium complexes of tetrazine derived L, Ru ^II(acac)₂(L) (1), [Ru^II(bpy) ₂(L)](ClO₄)₂ (2) and [Ru^II(bpy)(L) ₂](ClO₄)₂ (3) (L = 3-amino-6-(3,5- dimethylpyrazol-1-yl)-1,2,4,5-tetrazine, acac = acetylacetonate and bpy = 2,2′-bipyridine) were prepared. The free L exists as a dimeric entity in the solid state via hydrogen bonding interactions involving L and water molecules present in the crystal lattice. 1 exhibits unusually strong bonds from Ru^II to coordinating pyrazolyl-N (2.040(2) ?) and especially to tetrazine-N (1.913(2) ?). The Ru^III/Ru^II couples of 1-3 appeared at 0.28, 1.34 and 1.50 V versus SCE, respectively. The tetrazine and bpy-based reductions were observed at -1.33 (1); -0.55 and -1.55/-1.75/-1.98 (2); -0.47/-0.78 and -1.80/-2.02 V (3), respectively. 1, 2 and 3 displayed two MLCT bands each, corresponding to dπ(Ru^II) → π* (L, tetrazine) and dπ(Ru^II) → π* (acac or bpy or L) transitions. 1⁺ and 2⁺ showed rhombic EPR spectra at 110 and 4 K, respectively and 1^-, 2^- and 3 ^- exhibited multiple line EPR spectra at 300 K. 1-3 exhibited moderately strong emission spectra in EtOH-MeOH glass at 77 K.

Full text of DOI:10.1016/j.poly.2004.11.019

Polyhedron 24 (2005) 333–342
www.elsevier.com/locate/poly
Tetrazine derived mononuclear RuII(acac)2(L) (1),
[RuII(bpy)2(L)](ClO4)2 (2) and [RuII(bpy)(L)2](ClO4)2 (3)
(L = 3-amino-6-(3,5-dimethylpyrazol-1-yl)-1,2,4,5-tetrazine,
acac = acetylacetonate, bpy = 2,20-bipyridine):
syntheses, structures, spectra and redox properties
a
Animesh Nayak , Srikanta Patra , Biprajit Sarkar , Sandeep Ghumaan ,
a
b
a
c
b,
a,
*
Vedavati G. Puranik , Wolfgang Kaim *, Goutam Kumar Lahiri
a
Department of Chemistry, Indian Institute of Technology – Bombay, Powai, Mumbai 400 076, India
b
Institut fu¨r Anorganische Chemie, Universita¨t Stuttgart, Pfaffenwaldring 55, D-70550 Stuttgart, Germany
Center for Materials Characterization, National Chemical Laboratory, Pune, Maharashtra 411 008, India
c
Received 13 October 2004; accepted 24 November 2004
Abstract
Mononuclear ruthenium complexes of tetrazine derived L, RuII(acac)2(L) (1), [RuII(bpy)2(L)](ClO4)2 (2) and [RuII(bpy)(L)2]-
(ClO4)2 (3) (L = 3-amino-6-(3,5-dimethylpyrazol-1-yl)-1,2,4,5-tetrazine, acac = acetylacetonate and bpy = 2,20-bipyridine) were pre-
pared. The free L exists as a dimeric entity in the solid state via hydrogen bonding interactions involving L and water molecules
present in the crystal lattice. 1 exhibits unusually strong bonds from RuII to coordinating pyrazolyl-N (2.040(2) A) and especially
˚
to tetrazine-N (1.913(2) A). The RuIII/RuII couples of 1–3 appeared at 0.28, 1.34 and 1.50 V versus SCE, respectively. The tetrazine
˚
and bpy-based reductions were observed at ꢀ1.33 (1); ꢀ0.55 and ꢀ1.55/ꢀ1.75/ꢀ1.98 (2); ꢀ0.47/ꢀ0.78 and ꢀ1.80/ꢀ2.02 V (3),
respectively. 1, 2 and 3 displayed two MLCT bands each, corresponding to dp(RuII) ! p* (L, tetrazine) and dp(RuII) ! p* (acac
or bpy or L) transitions. 1+ and 2+ showed rhombic EPR spectra at 110 and 4 K, respectively and 1ꢀ, 2ꢀ and 3ꢀ exhibited multiple
line EPR spectra at 300 K. 1–3 exhibited moderately strong emission spectra in EtOH–MeOH glass at 77 K.
ꢀ 2004 Elsevier Ltd. All rights reserved.
Keywords: Ruthenium–tetrazine; Synthesis; Structure; Electrochemistry; Redox; Spectra
1. Introduction
gands have become more and more popular because
of their special electronic and structural properties.
The electronic properties are related to the strong
p-accepting nature of the tetrazine moiety due to the
low-lying p* MO, while the structural features are
essentially based on the efficient metal–metal bridging
capacity.
The ability of tetrazine ligand systems to function as
efficient electronic spacers between individual metal cen-
ters is reflected in a series of diruthenium complexes
Among the well established polynucleating ligands
in coordination chemistry are the 3,6-disubstituted
1,2,4,5-tetrazines [1]. During the last decade these li-
*
Corresponding authors. Tel.: +91 22 2576 7159; fax: +91 22 2572
3480.
E-mail addresses: kaim@iac.uni-stuttgart.de (W. Kaim), lahiri@
chem.iitb.ac.in (G.K. Lahiri).
0277-5387/$ - see front matter ꢀ 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2004.11.019

334
A. Nayak et al. / Polyhedron 24 (2005) 333–342
encompassing a variety of 3,6-disubstituted tetrazine
derivatives [2]. Their structural, spectroelectrochemical,
EPR, magnetic and luminescence aspects have been
thoroughly investigated. However, only a limited num-
ber of mononuclear ruthenium–tetrazine derivatives are
known so far, e.g., [(trpy)Ru(bphtz)](PF6)2 [3a],
[(CO)(PPh3)2(H)Ru(bptz)]PF6 [3b], [(NH3)4Ru(bptz)]-
(PF6)2 [3c,d], [(bpy)2Ru(bptz)](PF6)2 [3d], [{(CH3)2-
SO}2(Cl)2Ru(bptz)] Æ mH2O [3e] and [(bpz)2Ru(bptz)]-
(PF6)2 [3d,f] [bptz = 3,6-bis(2-pyridyl)-1,2,4,5-tetrazine;
bphtz = 3,6-bis(1,10-phenanthroline)-1,2,4,5-tetrazine;
trpy = 2,20:60,200-terpyridine; bpz = 2,20-bipyrazine].
This situation has prompted the present program of
exploring the metallation aspects of 3-amino-6-(3,5-dim-
ethylpyrazol-1-yl)-1,2,4,5-tetrazine (L), comprising of a
five-membered pyrazolyl ring on one side of the tetr-
azine and an –NH2 group on the other side. Although
L has been synthesized previously [4], its structural
and metallation aspects have not been investigated so
far. As part of our systematic approach, we report in
this paper the synthesis of three ruthenium complexes,
viz., [RuII(acac)2(L)] (1), [RuII(bpy)2(L)](ClO4)2 (2) and
[RuII(bpy)(L)2](ClO4)2 (3), incorporating ancillary li-
gands such as r-donating acetylacetonato (acacꢀ) or
p-acidic 2,20-bipyridine (bpy). The crystal structures of
L, 1 and 2, their spectro-electrochemical and lumines-
cence behavior, and the EPR characteristics of the in
situ generated RuIII derivatives (1+, 2+, 3+) and the re-
duced radical species (1ꢀ, 2ꢀ, 3ꢀ) are being reported
here.
2.2. Physical measurements
UV–Vis spectra were recorded with a JASCO V-570
spectrophotometer. FT-IR spectra were taken on a Nic-
olet spectrophotometer with samples prepared as KBr
pellets. Solution electrical conductivity was checked
using a Systronic 305 conductivity bridge. Magnetic sus-
ceptibility was checked with a CAHN electrobalance
7550 model sample magnetometer. 1H NMR spectra
were obtained with a 300 MHz Varian FT spectrometer.
The elemental analyses were carried out with a Perkin–
Elmer 240C elemental analyzer. The EPR measurements
were made in a two-electrode capillary tube [9] with an
X-band Bruker system ESP300, equipped with a Bruker
ER035M gaussmeter and a HP 5350B microwave detec-
tor. Cyclic voltammetric, differential pulse voltammetric
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 elec-
trolyte was [NEt4]ClO4 and the solute concentration
was ꢁ10ꢀ3 M. The half-wave potential E0298 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 and
were uncorrected for junction potentials. Electrospray
mass spectra were recorded on a Micromass Q-ToF
mass spectrometer. Steady state emission experiments
were made using a Perkin–Elmer LS 55 luminescence
spectrometer fitted with a cryostat. The quantum yields
of the complexes were with reference to the reported
quantum yield of [Ru(bpy)3](PF6)2 in EtOH–MeOH
(4:1) solution (Uem,77 K = 0.35) [10].
2.3. Preparation of L and complexes 1, 2 and 3
2.3.1. Preparation of L, [3-amino-6-(3,5-dimethyl
pyrazol-1-yl)-1,2,4,5-tetrazine]
2. Experimental
The ligand L was synthesized via a modification
(Scheme 1) of the method reported by Glidewell et al.
[4]. The ligand bpytz (300 mg, 1.11 mmol) in 20 ml
toluene was treated dropwise with 0.3 ml of ammonia
at 298 K. The stirring was continued for 2.5 h. The or-
ange solid thus obtained was filtered and washed with
toluene and then dried in vacuum. Yield: 170 mg
2.1. Materials
The starting complexes Ru(acac)2(CH3CN)2 [5],
Ru(bpy)2Cl2 Æ 2H2O [6] and Ru(bpy)Cl3 [7] were pre-
pared according to the reported procedures. The ligand
3-amino-6-(3,5-dimethylpyrazol-1-yl)-1,2,4,5-tetrazine
was prepared via a modification of the procedure
developed by Glidewell et al. [4]. Other chemicals and
solvents were reagent grade and were used as received.
For spectroscopic and electrochemical studies HPLC
grade solvents were used. Commercial tetraethylammo-
nium bromide was converted into pure tetraethylam-
monium perchlorate following a published procedure
[8].
1
(80%). H NMR in (CD3)2SO d (ppm), 2.20 (s, CH3),
2.37 (s, CH3), 6.17 (s, CH), 8.2 (s, NH2).
2.3.2. [RuII(acac)2(L)] (1)
The starting complex Ru(acac)2(CH3CN)2 (100 mg,
0.26 mmol) and the ligand L (50 mg, 0.26 mmol) were
dissolved in 20 ml of ethanol, and the mixture was
heated under reflux for 8 h in air. The initially orange

A. Nayak et al. / Polyhedron 24 (2005) 333–342
335
Scheme 1. NH3, toluene.
solution gradually changed to blue. The solvent was
evaporated to dryness under reduced pressure and the
solid mass thus obtained was purified by using a silica
gel column. Initially, a red compound corresponding
to Ru(acac)3 was eluted by CH2Cl2–CH3CN (5:1). With
CH2Cl2–CH3CN (1:1), a blue compound corresponding
to 1 was separated later. Evaporation of solvent under
reduced pressure yielded complex 1. Yield: 1 (52 mg,
40%). Anal. Calc. for complex 1: C, 41.54; H, 4.69;
N, 19.96. Found: C, 41.14; H, 4.33; N, 19.81%. IR,
m/cmꢀ1: 3432 and 3293 (–NH2).
ice-cold water. Yield: (80 mg, 43%). Anal. Calc. for
complex 3: C, 34.38; H, 3.13; N, 26.73. Found: C,
34.53; H, 3.59; N, 22.27%. (Repeated measurements
showed the lower ꢀNꢁ value, typical of compounds
containing tetrazine). KM/Xꢀ1 (cm2 molꢀ1) in MeOH
at 298 K: 220. IR, m/cmꢀ1: 3429/3318 (–NH2) and
1097/629 ðClO4ꢀÞ.
2.4. X-ray structure determination
Single crystals for L and 1 were grown by slow diffu-
sion of a dichloromethane solution into n-hexane, fol-
lowed by slow evaporation. For 2, an acetonitrile
solution was allowed to diffuse slowly into benzene, fol-
lowed by slow evaporation. X-ray data of L and 1/2
2.3.3. [RuII(bpy)2(L)](ClO4)2 (2)
A mixture of [Ru(bpy)2Cl2] Æ 2H2O (100 mg, 0.19
mmol) and AgClO4 (100 mg, 0.5 mmol) in 20 ml ethanol
was heated with constant stirring for 2 h. The removal of
the AgCl precipitate using a sintered-glass funnel re-
were collected on
a PC-controlled Enraf–Nonius
CAD-4 (MACH-3) and on a Bruker SMART APEX
CCD single crystal X-ray diffractometer, respectively.
The structures were solved and refined by full-matrix
least-squares techniques on F2 using the SHELX-97
sulted in
a clear solution containing [Ru(bpy)2-
(EtOH)2](ClO4)2. To this solution the ligand (L) (39
mg, 0.21 mmol) was added and the mixture was heated
under reflux for 5 h in air. The initially red solution
gradually changed to purple. The volume of the solution
was reduced to about 5 ml under reduced pressure and
kept overnight at 268 K. A dark precipitate thus ob-
tained was filtered and washed with cold water followed
by cold ethanol. The dried product was then purified by
using an alumina column. The purple compound 2 was
eluted by CH3CN. Evaporation of solvent under re-
duced pressure yielded complex 2. Yield: 2 (100 mg,
60%). Anal. Calc. for complex 2: C, 40.35; H, 3.14; N,
19.18. Found: C, 40.68; H, 2.87; N, 19.55%. KM/Xꢀ1
(
SHELXTL program package) [11]. SADABS correction
was applied for 1 and 2. All the data were corrected
for Lorentz, polarization and absorption effects.
3. Results and discussion
3.1. Synthesis and characterization of L, 1, 2 and 3
The formation of L was authenticated by its single
crystal X-ray structure (Fig. 1). The asymmetric unit
contains two molecules of L and two water molecules,
all crystallographically independent. The crystal struc-
ture of L reveals that it crystallizes as a dimer which is
held together via hydrogen bonding interactions involv-
ing L and H2O molecules present in the lattice. Impor-
tant crystallographic data are summarized in Table 1.
Selected bond distances and bond angles for one mole-
cule (molecule A) are listed in Table 2, as bond distances
and angles of molecule B are close to those for molecule
A. The bond parameters are comparable with standard
values [2b,12].
(cm2 molꢀ1) in MeCN at 298 K: 210. IR, m/cmꢀ1
:
ꢀ
3436/3322 (–NH2) and 1088/618 ClO4
.
2.3.4. [RuII(bpy)(L)2](ClO4)2 (3)
A mixture of Ru(bpy)Cl3 (100 mg, 0.27 mmol) and
the ligand (L) (130 mg, 0.68 mmol) in ethanol was
heated to reflux in air for 12 h. The initially brownish
solution gradually changed to purple. The solvent of
the reaction mixture was then evaporated to dryness
under reduced pressure and the solid mass thus ob-
tained was purified by using a silica gel column. A
purple compound containing [Ru(bpy)(L)2](ClO4)2
was eluted with 4:1 acetonitrile–methanol mixture.
The solvent was evaporated to dryness under reduced
pressure, the solid mass was dissolved in the minimum
volume of acetonitrile, and an aqueous solution of ex-
cess NaClO4 was added to the solution. The dark
complex 3 was filtered and washed thoroughly with
In order to synthesize ruthenium complexes of L
incorporating ancillary ligands of a differing electronic
nature we chose metal precursors encompassing
r-donating acetylacetonate {Ru(acac)2} and p-acidic
bipyridine fRuðbpyÞ 2þg and {Ru(bpy)3+} cores. The
2
reaction of L and RuII(acac)2(CH3CN)2 in ethanol, fol-
lowed by chromatographic purification using a silica gel

336
A. Nayak et al. / Polyhedron 24 (2005) 333–342
failed altogether. On all occasions the monomeric spe-
cies 1–3 were obtained.
The neutral compound 1 is soluble in non-polar sol-
vents such as CH2Cl2 or CHCl3 whereas 2 and 3 are sol-
uble in polar solvents such as CH3CN and CH3OH. The
ꢀ
NH2 vibrations of coordinated L and the ClO4 vibra-
tions appear near 3300 and 1100/630 cmꢀ1, respectively
(see Section 2). The electrospray mass spectra of 1 in
CH2Cl2 and of 2 and 3 in CH3CN show maximum
molecular ion peaks (m/z) at 492.23, 704.24 and
739.12, respectively (Fig. S1), corresponding to [1]+ (cal-
culated molecular mass: 491.08), {[2]-ClO4}+ (calculated
molecular mass: 704.09) and {[3]-ClO4}+ (calculated
molecular mass: 739.10).
3.2. Crystal structures of the complexes
Fig. 1. Asymmetric unit of L.
The formation of 1 and 2 was confirmed by single
crystal X-ray structures (Figs. 2 and 3). Important crys-
tallographic parameters are given in Table 1, selected
bond distances and bond angles are listed in Table 2.
The single crystal of 1 contains 0.5 equivalents of H2O
of crystallization. The RuO4N2 coordination sphere is
slightly distorted octahedral as can be seen from the an-
gles at the metal (Table 2). Although the bite angles
involving the acac ligands in 1 are close to the ideal value
at 90.95(8)ꢁ and 93.19(8)ꢁ, the bite angle involving L is
only 80.11(9)ꢁ, reflecting the smaller chelate ring size.
The average trans angle is 177.41(11)ꢁ. The RuII–O dis-
column, yielded the blue diamagnetic complex [RuII-
(acac)2L] (1) (Scheme 2).
Similarly, the reactions of L with [RuII(bpy)2-
(EtOH)2]2+ and RuIII(bpy)Cl3 in ethanol under atmo-
spheric conditions, followed by chromatographic
purification, afforded the 1:2 conducting diamagnetic
complexes [RuII(bpy)2L](ClO4)2 (2) and [RuII(bpy)(L)2]-
(ClO4)2 (3), respectively (Scheme 2). During the course
of the reaction the +3 oxidation state of the metal ion
in the precursor complex RuIII(bpy)Cl3 was reduced to
the RuII state in 3, even under atmospheric conditions.
The replacement of electron rich Clꢀ ligands by the
strongly p-acidic L stabilizes the metal ion in the +2
state, reflected also by the high redox potential of 3
(see later).
˚
tances vary between 2.0312(19) and 2.048(2) A. These
are comparable to reported RuII–O(acac) distances
[2d]. The RuII–N(1)(pyrazolyl) and RuII–N(4)(tetrazine)
˚
distances in 1 are 2.040(2) and 1.913(2) A, respectively,
All our attempts to bind a second metal complex
fragment via the other tetrazine nitrogen donor center
(N4) and the pendant NH2 group or its anionic form
showing the expected stronger bonding to the better
p-accepting tetrazine ring. The RuII–N(pyrazolyl) dis-
tance in 1 is distinctly shorter than that found in
Table 1
Crystallographic data for L Æ H2O, [RuII(acac)2L] Æ 0.5H2O (1) and [RuII(bpy)2L](ClO4)2 (2)
L
1
2
Formula
M
2(C7H9N7) Æ 2H2O
418.46
RuC17H24N7O4.50
499.50
RuC27H25Cl2N11O8
803.55
Radiation
T (K)
Mo Ka
298(2)
Mo Ka
293(2)
Mo Ka
293(2)
Crystal symmetry
Space group
monoclinic
P21/n
monoclinic
C2/c
monoclinic
P21/n
˚
a (A)
8.791(1)
16.992(2)
13.516(1)
91.201(9)
2018.5(4)
4
18.624(6)
17.083(6)
15.571(5)
123.557(4)
4128(2)
8
14.6380(13)
15.5770(14)
14.8802(13)
108.968(2)
3208.7(5)
4
˚
b (A)
˚
c (A)
b (ꢁ)
3
˚
V (A )
Z
l (mmꢀ1
)
0.102
1.377
0.801
1.607
0.723
1.663
Dcalc (g cmꢀ3
R1
)
0.062
0.118
0.912
0.0275
0.0678
1.099
0.0680
0.1446
1.039
wR2
Goodness-of-fit

A. Nayak et al. / Polyhedron 24 (2005) 333–342
337
Table 2
II
II
˚
Selected bond distances (A) and bond angles (ꢁ) and their standard deviation for L, [Ru (acac)2L] Æ 0.5H2O (1) and [Ru (bpy)2L](ClO4)2 (2)
L
1
2
˚
Bond distances (A)
N(1a)–N(2a)
N(2a)–C(6a)
N(3a)–C(6a)
N(3a)–N(4a)
N(4a)–C(7a)
N(6a)–C(7a)
N(5a)–N(6a)
N(5a)–C(6a)
1.384(4)
1.398(5)
1.336(5)
1.311(5)
1.348(5)
1.342(6)
1.336(5)
1.319(5)
Ru–N(1)
Ru–N(4)
Ru–O(1)
Ru–O(2)
Ru–O(3)
Ru–O(4)
N(3)–N(4)
N(5)–N(6)
2.040(2)
1.913(2)
Ru(1)–N(1)
Ru(1)–N(4)
Ru(1)–N(8)
Ru(1)–N(9)
Ru(1)–N(10)
Ru(1)–N(11)
N(3)–N(4)
2.077(6)
1.996(5)
2.060(6)
2.048(6)
2.074(5)
2.055(6)
1.334(7)
1.302(8)
2.0346(19)
2.0419(18)
2.048(2)
2.0312(19)
1.345(3)
1.357(3)
N(5)–N(6)
Bond angles (ꢁ)
C(2a)–N(1a)–N(2a)
C(4a)–N(2a)–N(1a)
C(4a)–N(2a)–C(6a)
N(1a)–N(2a)–C(6a)
N(4a)–N(3a)–C(6a)
N(3a)–N(4a)–C(7a)
C(6a)–N(5a)–N(6a)
N(5a)–N(6a)–C(7a)
N(1a)–C(2a)–C(3a)
N(1a)–C(2a)–C(1a)
C(3a)–C(2a)–C(1a)
C(4a)–C(3a)–C(2a)
C(3a)–C(4a)–N(2a)
C(3a)–C(4a)–C(5a)
N(2a)–C(4a)–C(5a)
104.5(4)
N(4)–Ru–N(1)
N(4)–Ru–O(4)
N(1)–Ru–O(4)
N(4)–Ru–O(3)
N(1)–Ru–O(3)
O(4)–Ru–O(3)
N(4)–Ru–O(2)
O(4)–Ru–O(2)
O(3)–Ru–O(2)
N(4)–Ru–O(1)
N(1)–Ru–O(1)
N(1)–Ru–O(2)
O(4)–Ru–O(1)
O(3)–Ru–O(1)
O(2)–Ru–O(1)
80.11(9)
89.95(8)
92.42(9)
178.61(8)
98.79(8)
90.95(8)
95.82(8)
86.24(8)
85.29(8)
92.40(8)
88.30(8)
175.73(8)
177.62(7)
86.70(8)
93.19(8)
N(4)–Ru(1)–N(11)
N(9)–Ru(1)–N(11)
N(4)–Ru(1)–N(8)
N(9)–Ru(1)–N(8)
N(11)–Ru(1)–N(8)
N(4)–Ru(1)–N(10)
N(9)–Ru(1)–N(10)
N(11)–Ru(1)–N(10)
N(8)–Ru(1)–N(10)
N(4)–Ru(1)–N(1)
N(9)–Ru(1)–N(1)
N(11)–Ru(1)–N(1)
N(8)–Ru(1)–N(1)
N(10)–Ru(1)–N(1)
N(4)–Ru(1)–N(9)
97.8(2)
94.9(2)
87.1(2)
79.3(2)
172.7(2)
173.3(2)
91.8(2)
78.2(2)
97.4(2)
78.4(2)
170.7(2)
91.4(2)
94.9(2)
96.2(2)
93.9(2)
111.3(4)
131.0(4)
117.7(4)
117.5(4)
116.9(4)
116.8(4)
117.0(4)
111.3(4)
120.3(5)
128.4(5)
107.3(5)
105.6(4)
129.6(5)
124.9(4)
ported data of similar species [13]. The RuII–N(1)(pyraz-
olyl) and RuII–N(4)(tetrazine) distances are at 2.077(6)
N
N
N
N
N
NH2
N
˚
and 1.996(5) A, respectively. Thus, the back bonding
(i)
II
2+
II
III
Ru (bpy) (C H OH)
Ru (acac) (CH CN)
Ru (bpy)(Cl)
2
2
5
2
2
3
2
3
2+
2+
N
N
N
N
N
N
N
N
N
N
N
NH2
N
NH2
N
NH2
N
N
N
N
N
N
O
N
Ru
N
Ru
N
N
O
Ru
O
N
N
N
O
N
N
N
N
H2N
1
[2]2+
[3]2+
Scheme 2. (i) EtOH, D.
[(bpy)2RuII(l-bpytz)RuII(bpy)2]4+ at 2.114(16) A [2b].
The RuII–N(tetrazine) distances found in dinuclear com-
plexes such as [(bpy)2RuII(l-bpytz)RuII(bpy)2]4+ [2b]
and [(acac)2RuII(l-bptz)RuII(acac)2] [2d] are 1.976(14)/
˚
˚
1.993(11) and 1.946(6)/1.943(6) A, respectively, which is
˚
appreciably longer than the 1.912(3) A observed for 1.
The RuN6 center in 2 is more distorted octahedral.
The distortion of the coordination sphere is primarily
caused by the small bite angles, viz., 79.3(2)ꢁ (bpy),
78.2(2)ꢁ (bpy) and 78.5(2)ꢁ (L). The average trans angle
is 172.23(2)ꢁ. The RuII–N(bpy) distances lie between
Fig. 2. ORTEP diagram of the complex [RuII(acac)2L] Æ 0.5H2O (1).
Ellipsoids are drawn at 50% probability. Water of crystallization has
been removed for clarity.
˚
2.048(6) and 2.074(5) A which is comparable with re-

338
A. Nayak et al. / Polyhedron 24 (2005) 333–342
Fig. 3. ORTEP diagram of the complex cation [RuII(bpy)2L]2+ of (2).
Ellipsoids are drawn at 50% probability.
effect dp(RuII) ! p*(tetrazine) is also reflected in the
structure of 2. The presence of p-acidic bpy ancillary
ligands competing with L in 2 is held responsible for
the observed Ru–N bond lengthening, leading to appre-
ciable decreasing RuII ! p* back donation on moving
from 1 to 2.
The strong RuII ! p*(tetrazine) back bonding in 1 is
also reflected in the appreciable lengthening of the tetr-
azine bond lengths involving the coordinated nitrogen
donor center [N(4)], C(16)–N(4) 1.377(7) and N(3)–
˚
N(4) 1.345(3) A, as compared to those of the free ligand
˚
(L), C(6)–N(3), 1.336(5) and N(3)–N(4), 1.311(5) A.
However, in the case of the corresponding bipyridine
complex (2) only a slight increase in bond distances
has been observed [C(26)–N(4), 1.340(8) and N(3)–
Fig. 4. 1H NMR spectra of (a) [RuII(acac)2L] (1) in CDCl3,
(b) [RuII(bpy)2L](ClO4)2 (2) in (CD3)2SO, inset shows the expanded
aromatic region (d, 7.4–9.0 ppm), (c) [RuII(bpy)(L)2](ClO4)2 (3) in
(CD3)2SO (solvent peaks are substracted for clarity).
˚
N(4), 1.334(7) A], which is certainly due to the existence
of much weaker RuII ! p*(tetrazine) back bonding in 2
compared to 1 as stated above.
1
3.3. H NMR spectra
Similarly, the four pyridine rings of two bpy ligands
in 2 are inequivalent. Complex 2 should thus exhibit
16 non-equivalent ‘‘aromatic’’ proton signals. Since the
electronic environment of many such hydrogen atoms
is similar, their signals may appear in a narrow chemical
shift range. In fact, the ‘‘aromatic region’’ of the spec-
trum of 2 is complicated due to the overlapping of sev-
eral signals, which has essentially precluded the
identification of individual resonances (Fig. 4(b)). How-
ever, a direct comparison of the intensity of these signals
with that of the clearly observable CH (6.53 ppm) and
CH3 (2.76 and 1.47 ppm) protons of coordinated L in
the upfield region revealed the presence of the expected
16 aromatic protons. The NH2 protons of L appear at
7.4 ppm.
1H NMR spectra of the complexes 1, 2 and 3 were re-
corded in CDCl3 (1) and (CD3)2SO. The spectra are
shown in Fig. 4. The unsymmetrical nature of L makes
the protons associated with the chelate rings magneti-
1
cally different. Therefore, the H NMR spectrum of 1
displays three distinct singlets corresponding to two acac
CH protons (d, 5.57 and 5.20 ppm) and one CH proton
associated with L (d, 5.92 ppm) (Fig. 4(a)). The NH2
protons of L appear as a slightly broad singlet at 4.92
ppm. In addition, six singlets are observable in the up-
field region at 2.85, 2.24, 2.18, 2.16, 2.11 and 1.92
ppm, corresponding to four and two CH3 groups asso-
ciated with acac and L, respectively.

A. Nayak et al. / Polyhedron 24 (2005) 333–342
339
In 3 two distinct CH signals at 6.69 and 6.54 ppm and
four CH3 signals at 2.77, 2.74, 1.79 and 1.47 ppm, cor-
responding to two L, were observed (Fig. 4(c)). The
eight protons of bpy and the NH2 protons of L appear
between 7.4 and 9.0 ppm as partially overlapping
signals.
ile reduction at relatively low potentials due to the low
lying p* orbital [1–3], the observed reduction is therefore
considered to involve the tetrazine function of coordi-
nated L.
3.4. Metal oxidation
Compounds 1–3 exhibit one quasi-reversible RuIII
/
RuII couple each at E1/2 = 0.28, 1.34 and 1.50 V versus
SCE, respectively (Table 3, Fig. 5). The data reveal that
a redox potential shift of >1.0 V has taken place on
switching from the acac ancillary ligands in [Ru-
(acac)2(L)] (1) to the bpy co-ligands in [Ru(bpy)2(L)]-
(ClO4)2 (2). The presence of electron-rich acac ligands
in 1 clearly stabilizes the RuIII state [14].
The combination of two bpy ligands and one L ligand
around the RuII center in 2 shifted the RuIII/RuII couple
to 1.34 V which is even higher than that of [RuII(b-
py)3]2+ (E1/2 = 1.29 V versus SCE) [15]. This implies that
the donor strength of L is lower than that of bpy. It may
be noted that the first RuIII/RuII couple of the related
diruthenium complex [(bpy)2Ru(l-bpytz)Ru(bpy)2]4+
appears at 1.25 V [2b].
On the other hand, the replacement of two bpy li-
gands from [RuII(bpy)3]2+ by two ligands L in 3 in-
creases the RuII ! RuIII oxidation potential further to
1.50 V. Thus, successive exchange of bpy by L from
via Ru(bpy)2(L)2+ to RuðbpyÞðLÞ
en-
2þ
2þ
RuðbpyÞ
3
2
hances the stability of the RuII state.
3.5. Ligand reduction
Fig. 5. Cyclic (–––) and differential pulse (------) voltammograms of
(a) [RuII(acac)2L] (1) in CH2Cl2, (b) [RuII(bpy)2L](ClO4)2 (2) and
(c) [RuII(bpy)(L)2](ClO4)2 (3) in CH3CN at 298 K.
Complex 1 shows one reversible reduction at ꢀ1.33 V
versus SCE. Since tetrazines are known to undergo fac-
Table 3
Electrochemical,a electronic spectraa and spectroelectrochemical correlation datab
Compound
RuIII–RuII couple,
E0298=V ðDEp; mVÞ
Ligand reductions,
E0298=V ðDEp; mVÞ
DE1/2 (V)
UV–Vis, k/nm
(e/Mꢀ1 cmꢀ1
m(MLCT) (cmꢀ1
)
c
)
L-based
bpy-Based
Observed
16 666
Calculatedd
15 984
1
2
0.28(110)
1.34(80)
ꢀ1.33(110)
1.61
600 (13 650), 366 (9700),
268 (71 250), 218 (30 100)
ꢀ0.55(100)
1.89
2.89
538 (8000), 400 (4800),
278 (52 400), 214 (23 000)
18 587
25 000
18 242
26 307
ꢀ1.55(60)
ꢀ1.75(100)
ꢀ1.98(60)
3
1.50(70)
ꢀ0.47(100)
ꢀ0.78(100)
1.97
2.28
540 (10 380), 480 (7230),
276 (47 260), 206 (56 400)
18 518
20 833
18 888
21 388
ꢀ1.80(60)
ꢀ2.02(60)
a
1 in CH2Cl2 and 2/3 in CH3CN.
As stated in the text.
b
c
Calculated by Eq. (2) in the text.
Calculated by Eq. (1) in the text.
d

340
A. Nayak et al. / Polyhedron 24 (2005) 333–342
Similarly, the reduction of the tetrazine in 2 appears
exhibits one main transition at 540 nm with a high-en-
ergy shoulder at 480 nm. The lowest energy bands for
all the three complexes are attributed to MLCT transi-
tions dpðRuIIÞ ! p1ꢂL (where pꢂ1L is essentially domi-
nated by the tetrazine moiety) [2]. The higher energy
transitions are assigned as dp(RuII) ! p* (acac),
dp(RuII) ! p*(bpy) and dp(RuII) ! p* L for 1, 2 and
3, respectively. This is based on the observation that
the reduction of L takes place at a much lower potential
than that of bpy. The above assignments find justifica-
tion through the spectro-electrochemical correlations
stated below.
at ꢀ0.55 V. This substantial increase in the reduction
potential by 0.78 V on moving from strongly r-donating
acac as a co-ligand in 1 to p-acidic bpy in 2 parallels to
some extent the stabilization of the RuII state in 2 as
compared to 1.
In addition to the tetrazine-based reduction of L, 2
also shows three more bpy-based quasi-reversible one-
electron reduction processes in the range from ꢀ1.55
to ꢀ1.98 V. Each bpy can accommodate two electrons,
therefore, a total of four such reduction waves is ex-
pected. Three such waves are detected within the exper-
imental potential limit (ꢀ2.0 V versus SCE). The
potentials at which the bpy reductions of 2 are observed
3.7. Spectro-electrochemical correlation
2þ
is consistent with the reduction range of RuðbpyÞ
3
(ꢀ1.3 to ꢀ1.8 V versus SCE) [16].
The energies of the MLCT transitions at the band
maximum can be reproduced with the help of empirical
Eqs. (1) and (2) [17]:
Compound 3 contains two tetrazine units from two
L ligands, therefore the expected two tetrazine-based
one-electron reduction waves were observed at ꢀ0.47
and ꢀ0.78 V. Two bpy-based reductions appear at
ꢀ1.80 and ꢀ2.02 V. The first tetrazine reduction po-
tential in 3 is slightly less negative relative to 2, which
correlates with the enhanced stabilization of the RuII
state in 3.
mðMLCTÞ ¼ 8065ðDE1=2Þ þ 3000 cmꢀ1
DE1=2 ¼ E1=2ðRuIII=RuIIÞ ꢀ E1=2ðLÞ:
;
ð1Þ
ð2Þ
Here E1/2(RuIII/RuII) is the potential of the reversible
RuIII/RuII couple, E1/2(L) is that of the first ligand-based
reduction and m(MLCT) is the wave number of the
charge transfer band in cmꢀ1. The factor 8065 is used
to convert the potential difference DE from V into
cmꢀ1 and the term 3000 cmꢀ1 is of empirical origin
(reorganization energy of polypyridine ruthenium
MLCT transitions). The calculated and experimentally
observed MLCT energies are given in Table 3. Using
Eqs. (1) and (2) and DE1/2 values of 1.61, 1.89/2.89
and 1.97/2.28, for 1, 2 and 3, respectively, the calculated
m(MLCT) values agree rather well with the experimental
values (Table 3).
3.6. Spectral properties
Electronic absorption spectra of 1, 2 and 3 were re-
corded in CH2Cl2 (1) and CH3CN. The spectral data
are given in Table 3 and the spectra are shown in
Fig. 6. The complexes exhibit strong ligand-based
p ꢀ p* transitions in the UV region. In the visible re-
gion, 1 and 2 display two well separated broad bands
at 600/366 and 538/400 nm, respectively, whereas 3
3.8. EPR spectra
The in situ generated species 1+ in CH2Cl2 at
110 K exhibits a rhombic EPR signal (g1 = 2.260,
g2 = 2.170, g3 = 1.880, Fig. 7(a)), typical of low-spin
RuIII. The g anisotropy g1 ꢀ g3 = 0.380 and the aver-
age
g
factor of Ægæ = 2.11), derived from
1=2
hgi ¼ ½1=3ðg21 þ g22 þ g23Þꢃ indicate a slightly distorted
octahedral arrangement around the ruthenium center
[18] as consistent with the molecular structure of 1.
The electrochemically generated 2+ in CH3CN failed
to show any EPR signals at 110 K, however, at 4 K
it displays a poorly resolved rhombic type EPR signal
with g1 = 2.50, g2 = 2.360 and an undetectable g3 com-
ponent (probably < 1.8). On the other hand, the in
situ generated 3+ did not show any EPR response
even at 4 K, which signifies rapid relaxation due to
excited paramagnetic states lying close to the doublet
ground state.
Fig. 6. Electronic spectra of (a) [RuII(acac)2L] (1) (–––), (b)
[RuII(bpy)2L](ClO4)2 (2) (------) and (c) [RuII(bpy)(L)2](ClO4)2 (3)
(-ꢄ-ꢄ-ꢄ-) in CH3CN.

A. Nayak et al. / Polyhedron 24 (2005) 333–342
341
Fig. 8. Emission spectra of (a) 1 (kexcitation, 592 nm), (b) 2 (kexcitation
,
530 nm), (c) 2 (kexcitation, 413 nm) and (d) 3 (kexcitation, 524 nm) in
ethanol–methanol (4:1) glass at 77 K.
Fig. 7. EPR spectra of (a) 1+ in CH2Cl2 at 100 K and (b) 1ꢀ in CH2Cl2
at 300 K.
In situ generated reduced species 1ꢀ (in CH2Cl2), 2ꢀ
and 3ꢀ (in CH3CN) exhibit multiple line EPR spectra
at 300 K due to hyperfine coupling from four different
14N atoms of the tetrazine ring of L, the signals are
centered at g = 1.9962, 1.9991 and 1.9984, respectively
(Fig. 7(b)) [2,19].
4. Conclusion
Although tetrazine based ligands are quite popular
in developing polynuclear complexes for varying per-
spectives, rather rare mononuclear complexes 1, 2
and 3 have been synthesized using hitherto unexplored
tetrazine
derived
ligand,
3-amino-6-(3,5-dim-
3.9. Emission spectra
ethylpyrazol-1-yl)-1,2,4,5-tetrazine (L) in combination
with r-donating acetylacetonate and p-accepting
bipyridine co-ligands. Structural studies reveal that
the presence of electron-rich acetylacetonate co-ligands
attributes to a strong RuII ! p* (tetrazine) back bond-
ing in 1 which is much stronger than that in the cor-
responding mononuclear bipyridine derivative (2) as
well as that in the analogous dinuclear complexes
[(bpy)2RuII(l-bpytz)RuII(bpy)2]4+ and [(acac)2RuII(l-
bptz)RuII(acac)2].
Emission properties of the complexes were studied
in EtOH–MeOH (4:1) at 77 K. The spectra are shown
in Fig. 8. Excitations of the complexes 1, 2 and 3 at
592, 530/413 and 534 nm, respectively, result in emis-
sions with maxima at 735 nm (quantum yield,
U = 0.65 · 10ꢀ2), 682/586 nm (quantum yield,
U = 1.8 · 10ꢀ1, 1.1 · 10ꢀ1) and 734 nm (quantum
yield, U = 1.15 · 10ꢀ2), respectively, with vibrational
structures characteristic of emissions from a 3MLCT
excited state [20]. The origin of the emissions was fur-
ther confirmed by the excitation spectra of the same
solutions in each case. The observed moderately
strong emissions for 1 and 3 presumably involve pri-
marily L. On the other hand, 2 appears to exhibit
strong emissions from both the bpy and L-based
MLCT transitions.
Acknowledgements
Financial support received from the Council of Sci-
entific and Industrial Research, New Delhi (India), the
DAAD, the DFG and the FCI (Germany) is grate-
fully acknowledged. We are grateful to Professor Seik

342
A. Nayak et al. / Polyhedron 24 (2005) 333–342
(f) T.S. Akasheh, Z.M. El-Ahmed, Chem. Phys. Lett. 152 (1988)
414.
Weng Ng, Institute of Postgraduate Studies, Univer-
sity of Malaya, Kuala Lumpur, Malaysia for solving
the crystal structure of L. Special acknowledgement
is made to the Sophisticated Analytical Instrument
Facility, Indian Institute of Technology, Bombay, for
providing the NMR facility. X-ray structural studies
of L were carried out at the National Single Crystal
Diffractometer Facility, Indian Institute of Technol-
ogy, Bombay.
[4] C. Glidewell, P. Lightfoot, B.J.L. Royles, D.M. Smith, J. Chem.
Soc., Perkin Trans. (1997) 1167.
ˆ
[5] T. Kobayashi, Y. Nishina, K. Shimizu, G.P. Sato, Chem. Lett.
(1988) 1137.
[6] B.P. Sullivan, D.J. Salmon, T.J. Meyer, Inorg. Chem. 17 (1978)
3334.
[7] S. Anderson, K.R. Seddon, J. Chem. Res. (S) (1979) 74.
[8] D.T. Sawyer, A. Sobkowiak, J.L. Roberts Jr., Electrochemistry
for Chemists, Wiley, New York, 1995.
[9] W. Kaim, S. Ernst, V. Kasack, J. Am. Chem. Soc. 112 (1990)
173.
[10] (a) E.F. Godefroi, E.L. Little, J. Org. Chem. 21 (1956) 1163;
(b) P. Chen, R. Duesing, D.K. Graff, T.J. Meyer, J. Phys. Chem.
95 (1991) 5850.
Appendix A. Supplementary data
Crystallographic data for the structural analysis have
been deposited with the Cambridge Structural Data-
base, CCDC Nos. 242646, 242644 and 242645, for com-
pounds L, 1 and 2, respectively. Supplementary data are
available from CCDC, 12 Union Road Cambridge CB2
1EZ, UK (fax: +44 1223 336033; E-mail: deposit@ccdc.-
cam.ac.uk or http://www.ccdc.cam.ac.uk) on request,
quoting the deposition numbers. Electrospray mass
spectral data of 1 (Fig. S1a), 2 (Fig. S1b) and 3 (Fig.
S1c). Supplementary data associated with this article
can be found, in the online version at doi:10.1016/
j.poly.2004.11.019.
[11] G.M. Sheldrick, SHELX-97 Program for Crystal Structure Solu-
tion and Refinement, University of Go¨ttingen, Go¨ttingen, Ger-
many, 1997.
[12] N. Chanda, B. Mondal, V.G. Puranik, G.K. Lahiri, Polyhedron
21 (2002) 2033.
[13] (a) S. Chakravorty, R.H. Laye, P. Munshi, R.L. Paul, M.D.
Ward, G.K. Lahiri, J. Chem. Soc., Dalton Trans. (2002)
2348;
(b) S. Chakraborty, R.H. Laye, R.L. Paul, R.G. Gonnade,
V.G. Puranik, M.D. Ward, G.K. Lahiri, J. Chem. Soc.,
Dalton Trans. (2002) 1172;
(c) S. Chakraborty, M.G. Walawalkar, G.K. Lahiri., J. Chem.
Soc., Dalton Trans. (2000) 2875.
[14] (a) S. Patra, B. Sarkar, S.M. Mobin, W. Kaim, G.K. Lahiri,
Inorg. Chem. 42 (2003) 6469;
(b) S. Patra, T.A. Miller, B. Sarkar, M. Niemeyer, M.D. Ward,
G.K. Lahiri, Inorg. Chem. 42 (2003) 4707;
References
(c) S. Patra, B. Mondal, B. Sarkar, M. Niemeyer, G.K. Lahiri,
Inorg. Chem. 42 (2003) 1322.
[1] W. Kaim, Coord. Chem. Rev. 230 (2002) 127.
[2] (a) B. Sarkar, W. Kaim, A. Klein, B. Schwederski, J. Fiedler, C.
Duboc-Toia, G.K. Lahiri, Inorg. Chem. 42 (2003) 6172;
(b) B. Sarkar, R.H. Laye, B. Mondal, S. Chakraborty, R.L. Paul,
J.C. Jeffery, V.G. Puranik, M.D. Ward, G.K. Lahiri, J. Chem.
Soc., Dalton Trans. (2002) 2097;
[15] (a) A. Ceulemansm, L.G. Vanquickenborne, J. Am. Chem. Soc.
103 (1981) 2238;
(b) M.J. Root, E. Deutsch, Inorg. Chem. 24 (1985) 1464;
(c) R. Alsfasser, R.V. Eldik, Inorg. Chem. 35 (1996) 628;
(d) B.J. Coe, T.J. Meyer, P.C. White, Inorg. Chem. 34 (1995)
593.
(c) A. Singh, N. Singh, D.S. Pandey, J. Organomet. Chem. 642
(2002) 48;
[16] (a) S. Bhattacharya, Polyhedron 12 (1993) 235;
(b) C.M. Elliott, J. Chem. Soc., Chem. Commun. (1980) 261;
(c) N.E. Tokel-Takvoryan, J. Am. Chem. Soc. 95 (1973) 6583;
(d) D.E. Morris, K.W. Hanck, M.K. DeArmond, Inorg. Chem.
24 (1985) 977.
(d) S. Chellamma, M. Lieberman, Inorg. Chem. 40 (2001) 3177;
(e) M. Glockle, W. Kaim, J. Fiedler, Z. Anorg. Allg. Chem. 627
(2001) 1141;
(f) S. Roche, L.J. Yellowlees, J.A. Thomas, Chem. Commun.
(1998) 1429;
[17] (a) B.K. Ghosh, A. Chakravorty, Coord. Chem. Rev. 95 (1989)
239;
(g) V. Kasack, W. Kaim, H. Binder, J. Jordanov, E. Roth, Inorg.
Chem. 34 (1995) 1924;
(b) B.K. Santra, G.K. Lahiri, J. Chem. Soc., Dalton Trans. (1997)
129;
(h) J. Poppe, M. Moscherosch, W. Kaim, Inorg. Chem. 32 (1993)
2640;
(c) E.S. Dosworth, A.B.P. Lever, Chem. Phys. Lett. 124 (1986)
152.
(i) W. Kaim, V. Kasack, Inorg. Chem. 29 (1990) 4696;
(j) M.H. Zaghal, H.A. Qaseer, Inorg. Chim. Acta 163 (1989) 193;
(k) S. Ernst, V. Kasack, W. Kaim, Inorg. Chem. 27 (1988) 1146.
[3] (a) D. Brown, S. Muranjan, Y. Jang, R. Thummel, Org. Lett. 4
(2002) 1253;
[18] (a) S. Patra, B. Sarkar, S. Ghumaan, J. Fiedler, W. Kaim, G.K.
Lahiri, J. Chem. Soc., Dalton Trans. (2004) 754;
(b) N. Chanda, B. Sarkar, J. Fiedler, W. Kaim, G.K. Lahiri, J.
Chem. Soc., Dalton Trans. (2003) 3550;
(c) S. Kar, T.A. Miller, B. Sarkar, B. Pradhan, T. Kundu, M.D.
Ward, G.K. Lahiri, J. Chem. Soc., Dalton Trans. (2003) 2591;
(d) B.K. Santra, M. Menon, C.K. Pal, G.K. Lahiri, J. Chem.
Soc., Dalton Trans. (1997) 1387.
(b) M. Chandra, A.N. Sahay, D.S. Pandey, M.C. Puerta, P.
Valerga, J. Organomet. Chem. 648 (2002) 39;
(c) J. Poppe, W. Kaim, A. Ben Altabef, N.E. Katz, J. Chem. Soc.,
Perkin Trans. (1993) 2105 (1972–1999);
[19] (a) W. Kaim, S. Kohlmann, Inorg. Chem. 25 (1986) 3442;
(b) W. Kaim, S. Ernst, S. Kohlmann, P. Welkering, Chem. Phys.
Lett. 118 (1985) 431.
(d) J.E.B. Johnson, C. De Groff, R.R. Ruminski, Inorg. Chim.
Acta 187 (1991) 73;
(e) T.S. Akasheh, D. Marji, Z.M. Al-Ahmed, Inorg. Chim. Acta
141 (1988) 125;
[20] A. Juris, V. Balzani, F Barigelletti, S. Campagna, P. Belser, A.
von Zelewski, Coord. Chem. Rev. 84 (1988) 85.