Ruthenium(II)/(in) bipyridine heterochelates incorporating phenolato imine functionalities. Synthesis, crystal structure, spectroscopic and electron-transfer properties and solution reactivities

Article abstract of DOI:10.1039/b001663m

A new class of ruthenium-bipyridine complexes of the type [Ru(bpy)₂(L′)]CIO4 [bpy = 2, 2′-bipyridine; L′ = OC₆H₃(R)C(R′)=NH; R = R′ = H la; R = H, R′ = CH3 lb; R = NO₂, R′ = H le] have been synthesized via cleavage of N-N and N-C (aliphatic and aromatic) bonds of binucleating imine functionalities. The formation of 1 has been authenticated by the single crystal structure determination of la. The complexes exhibit strong MLCT transitions in the visible region and intraligand transitions in the UV region and display moderately strong emission near 700 nm. The quantum yield () of the emission process at 77 K (EtOH-MeOH, 4:1 glass) varies in the range 2.2 × 10^-2-6.5 × 10^-3 depending on L′ in 1. The complexes show ruthenium(iii)-ruthenium(n) and ruthenium(iv)ruthenium(ni) oxidations in the ranges 0.52-0.77 and 1.71-1.97 V versus SCE respectively. For la and Ib, one ligand centred oxidation near 2 V and for all the complexes two bipyridine based reductions have been detected in the ranges - 1.51 to -1.56 V and -1.72 to -1.79 V versus SCE. Coulometric oxidation of 1 initially generates the unstable trivalent one-electron paramagnetic [Ru"′(bpy)2(L′)]²⁺ congener, 1⁺. The complexes 1⁺ display one broad and moderately strong LMCT band near 750 nm and intraligand transitions in the UV region. The oxidised complexes 1+ exhibit rhombic EPR spectra at 77 K which have been analysed to furnish values of distortion parameters (A = 4543-5923 cm^-1, V= 3251-5127 cm′1) and energy of the expected ligand field transitions (v, 3086-3508, V₂ 6409-8669 cm^-1) within the t2 shell. One of the ligand field transitions has been experimentally observed (v² 7092-7812 cm^-1). The oxidised species 1+ slowly changes to diamagnetic dimeric species of the type [(bpy)₂Ru^IIIrOC₆H ₃(R)C(R>NNC(R′)(R)C⁶H³Cr}Ru ^III(bpy)²]⁴⁺ 2 where the ruthenium(ni) centres are antiferromagnetically coupled. In the presence of water 1⁺ as well as 2 return to the parent bivalent species 1. The second order rate constants (k) of the conversion process 1⁺2 in dry acetonitrile and first order rate constants (k′) of 1⁺→ 1 in ordinary acetonitrile and the thermodynamic parameters (ΔH⁺ and ΔS⁺) of both have been determined spectrophotometrically in the temperature range 303-333 K. The Royal Society of Chemistry 2000.

Full text of DOI:10.1039/b001663m

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Ruthenium(II)/(III) bipyridine heterochelates incorporating
phenolato imine functionalities. Synthesis, crystal structure,
spectroscopic and electron-transfer properties and solution
reactivities†
Soma Chakraborty, Mrinalini G. Walawalkar and Goutam Kumar Lahiri*
Department of Chemistry, Indian Institute of Technology, Bombay, Powai, Mumbai-400076,
India
Received 29th February 2000, Accepted 15th June 2000
Published on the Web 25th July 2000
A new class of ruthenium–bipyridine complexes of the type [Ru(bpy)2(LЈ)]ClO4 [bpy = 2,2Ј-bipyridine; LЈ =
ϪOC H (R)C(RЈ)᎐NH; R = RЈ = H 1a; R = H, RЈ = CH 1b; R = NO , RЈ = H 1c] have been synthesized via
᎐
6
3
3
2
cleavage of N–N and N–C (aliphatic and aromatic) bonds of binucleating imine functionalities. The formation of
1 has been authenticated by the single crystal structure determination of 1a. The complexes exhibit strong MLCT
transitions in the visible region and intraligand transitions in the UV region and display moderately strong emission
near 700 nm. The quantum yield (φ) of the emission process at 77 K (EtOH–MeOH, 4:1 glass) varies in the range
2.2 × 10Ϫ2–6.5 × 10Ϫ3 depending on LЈ in 1. The complexes show ruthenium()–ruthenium() and ruthenium()–
ruthenium() oxidations in the ranges 0.52–0.77 and 1.71–1.97 V versus SCE respectively. For 1a and 1b, one ligand
centred oxidation near 2 V and for all the complexes two bipyridine based reductions have been detected in the ranges
Ϫ1.51 to Ϫ1.56 V and Ϫ1.72 to Ϫ1.79 V versus SCE. Coulometric oxidation of 1 initially generates the unstable
trivalent one-electron paramagnetic [RuIII(bpy)2(LЈ)]2ϩ congener, 1ϩ. The complexes 1ϩ display one broad and
moderately strong LMCT band near 750 nm and intraligand transitions in the UV region. The oxidised complexes
1ϩ exhibit rhombic EPR spectra at 77 K which have been analysed to furnish values of distortion parameters
(∆ = 4543–5923 cmϪ1, V = 3251–5127 cmϪ1) and energy of the expected ligand field transitions (ν1 3086–3508,
ν2 6409–8669 cmϪ1) within the t2 shell. One of the ligand field transitions has been experimentally observed
(ν2 7092–7812 cmϪ1). The oxidised species 1ϩ slowly changes to diamagnetic dimeric species of the type
[(bpy) RuIII{ϪOC H (R)C(RЈ)᎐NNC(RЈ)(R)C H OϪ}RuIII(bpy) ]4ϩ 2 where the ruthenium() centres are
᎐
2
6
3
6
3
2
antiferromagnetically coupled. In the presence of water 1ϩ as well as 2 return to the parent bivalent species 1.
The second order rate constants (k) of the conversion process 1ϩ → 2 in dry acetonitrile and first order rate
constants (kЈ) of 1ϩ → 1 in ordinary acetonitrile and the thermodynamic parameters (∆H‡ and ∆S‡) of both
have been determined spectrophotometrically in the temperature range 303–333 K.
of three such complexes, the crystal structure of one represent-
ative, their spectroscopic, electron-transfer properties and
solution reactivites including kinetic studies.
Introduction
The discovery of important photo-redox activities of
ruthenium–bipyridine complexes has generated continuous
effort in the direction of designing new classes of these deriv-
atives in order to develop efficient redox catalysts and photo-
sensitisers.1 The present trend in this direction has been
tilted towards the framing of multinuclear to supramolecular
ruthenium–bipyridine assemblies.2 In that context we had
begun a programme of co-ordinating two Ru(bpy)2 units
(bpy = 2,2Ј-bipyridine) through simple bridging ligands having
anionic imine functionalities, H2L1–12, with varying lengths and
electronic natures of the spacers. Although the bridging ligands
H2L are stable enough both in the solid and solution states, in
contact with the precursor metal complex [Ru(bpy)2(EtOH)2]2ϩ
the N–N and N–C (aliphatic and aromatic) bonds of the linking
units of H2L1–12 are selectively cleaved. This in turn leads to the
formation of a new class of ruthenium–bipyridine derivatives,
[RuII(bpy)2(LЈ)]ϩ 1, where LЈ corresponds to the stable imine
Results and discussion
Synthesis
A group of twelve bridging ligands H2L1–12 have been chosen
for the present study. The ligands primarily differ with respect
to the nature of the substituents (R/RЈ) present in the aldehydic
and ketonic functions and the nature of the spacers (X). They
are designed with the intention of holding two Ru(bpy)2 units
through the terminal O, N donors of L2Ϫ. However, the reac-
tion of H2L with [RuII(bpy)2(EtOH)2]2ϩ in ethanol solvent
under a N2 atmosphere results in monomeric complexes of the
type [RuII(bpy)2(LЈ)]ϩ 1 where LЈ corresponds to an imine
function of the type ϪOC H (R)C(RЈ)᎐NH. During the course
᎐
6
3
of the reaction the N–N and N–C (aliphatic and aromatic)
bonds of the bridging ligands H2L are cleaved and the resulting
monomeric fragment takes up a proton from the reaction
medium which eventually leads to the formation of 1 (Scheme
1). The cleavage of the N–N and N–C bonds makes the four
ligands in each set of R and RЈ equivalent, thus the overall
reactions essentially lead to the formation of three different
complexes 1a–1c (Scheme 1) which have been isolated as their
perchlorate salts, [RuII(bpy)2(LЈ)]ClO4.
functions OC H (R)C(RЈ)᎐NH incorporating the rare C᎐NH
᎐
᎐
6
3
fragment. To the best of our knowledge this work demonstrates
the first example of a stable imine moiety LЈ having a (R)C᎐NH
᎐
fragment attached to a metal centre, which so far is known only
as an unstable intermediate.3 Herein we report the synthesis
† Electronic supplementary information (ESI) available: microanalyti-
cal data for ligands. See http://www.rsc.org/suppdata/dt/b0/b001663m/
DOI: 10.1039/b001663m
J. Chem. Soc., Dalton Trans., 2000, 2875–2883
This journal is © The Royal Society of Chemistry 2000
2875

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Table 1 Microanalytical,a conductivity,b IRc and electronicb spectral data
Microanalysis (%)
IR/cmϪ1
ΛM/ΩϪ1
cm2 molϪ1
νC
νClO
UV/vis λ/nm(ε/dm3 molϪ1 cmϪ1
)
Ϫ
Compound
C
H
N
᎐
N
᎐
4
1a
51.02
(51.22)
51.88
(51.97)
47.97
3.40
(3.48)
3.67
(3.71)
3.17
(3.10)
—
11.21
(11.06)
10.95
(10.83)
12.54
145
148
140
—
1602
1605
1598
1091
623
1105
631
1110
630
578(2550),d 498(5050), 375(6300), 298(25850), 245(231500), 194(39300)
574(5500),d 507(8500), 375(9300), 295(46950), 245(32800), 194(49900)
1b
1c
534(6900),d 477(14400), 401(14300), 293(55850), 245(36550),
194(48300)
1410(105), 790(1950), 358(4650), 314(11100), 271(19100), 210(41450),
194(54550)
1280(85), 748(2450), 373(6600), 302(22750), 271(27050), 238(31450),
195(68650)
(47.82)
(12.40)
1aϩ
1bϩ
1cϩ
—
—
—
—
—
—
—
1400(140), 680(2150), 364(10100), 313(31350), 244(41450), 202(82300)
a Calculated values are in parentheses. b In acetonitrile solution. c As KBr discs. d Shoulder.
(i)
[RuII(bpy)2(EtOH)2]2ϩ ϩ H2L
[RuII(bpy)2(LЈ)]ϩ
H2L1–4 → 1a
H2L5–8 → 1b
H2L9–12 → 1c
Scheme 1 (i) EtOH, NaOCH3, N2, stirring, 298 K.
Under identical reaction conditions (Scheme 1) but in the
absence of metal precursor the binucleating identity of H2L
remains unaltered even upon heating. This implies direct
2ϩ
involvement of the Ru(bpy)2 core in driving the reaction
equilibrium in the direction of products 1.
The complexes 1 are highly soluble in CH3CN, DMF, DMSO
and moderately soluble in CH2Cl2, CHCl3 and slightly in water.
Their microanalytical data are in good agreement with the cal-
culated values (Table 1). The complexes act as 1:1 electrolytes
in acetonitrile solution (Table 1) and are diamagnetic at room
temperature. Selected IR bands are listed in Table 1. The
Fig. 1 Crystal structure of complex 1a. The perchlorate anion and
acetonitrile molecule are removed for clarity.
C᎐N stretching frequencies of the “free” ligands (≈1640 cmϪ1
)
᎐
have been shifted to ≈1600 cmϪ1 in the complexes in accord-
ance with co-ordination of the azomethine (CH᎐N) function.
᎐
value of 90.06(14)Њ. The average trans angle is 174.41(14)Њ. The
distortion of the co-ordination sphere is primarily caused
by the two acute (≈78Њ) bite angles of the juxtaposed two
bipyridine chelate rings. The bite angle of the phenolato
imine chelate, 91.70(13)Њ, is very close to the ideal value.
The ruthenium–phenolato moiety is essentially planar. The
RuII–O distance is observed to be 2.060(3) Å which agrees well
with the RuII–O (phenolato) distance 2.064(4) Å observed
in [RuII(bpy)2(NC5H4C6H4O)]ϩ 4 but slightly longer than the
2.022(5) and 2.042(4) Å observed in [RuII(tap)2(cat)]5 (tap =
tolylazopyridine, cat = catecholate) and [RuII(bpy)2(sal)]6 (sal =
salicylate) respectively. However, it is much shorter than the
RuII–O distances found in RuIIO(H2O), 2.122(16) Å,7 and RuII–
The bands due to ionic perchlorate and NH function are
observed near 1100/625 and 3300 cmϪ1 respectively (Table 1).
Crystal structure of [RuII(bpy)2(LЈ)]ClO4ؒCH3CN 1a
The formation of complexes 1 has been authenticated in one
case, 1a, by single-crystal structure determination. The struc-
ture is shown in Fig. 1 and selected bond distances and angles
are listed in Table 2. The single crystal contains acetonitrile of
crystallisation in the ratio 1a:CH3CN = 1:1. The RuN5O co-
ordination sphere is distorted octahedral as can be seen from
the angles subtended at the metal. The cis angles around the
metal ion range from 78.58(15) to 100.50(13)Њ with an average
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Table 2 Selected bond distances (Å) and angles (Њ) and their estimated
standard deviations for [Ru(bpy)2(LЈ)]ClO4ؒCH3CN 1a
Ru–N(1)
Ru–N(2)
Ru–N(3)
Ru–N(4)
Ru–N(5)
2.046(4)
2.009(4)
2.050(4)
2.033(4)
2.039(4)
Ru–O(4)
2.060(3)
1.280(6)
1.302(6)
1.356(9)
1.353(6)
C(222)–N(5)
C(21)–O(4)
C(1)–C(2)
N(1)–C(1)
N(2)–Ru–N(4)
N(2)–Ru–N(1)
N(4)–Ru–N(1)
N(2)–Ru–N(5)
N(4)–Ru–N(5)
N(1)–Ru–N(5)
N(2)–Ru–N(3)
N(4)–Ru–N(3)
N(1)–Ru–N(3)
92.92(15)
78.94(14)
97.53(15)
174.41(13)
89.96(15)
95.94(15)
100.50(13)
78.58(15)
176.06(15)
N(5)–Ru–N(3)
N(2)–Ru–O(4)
N(4)–Ru–O(4)
N(1)–Ru–O(4)
N(5)–Ru–O(4)
N(3)–Ru–O(4)
C(222)–N(5)–Ru
C(21)–O(4)–Ru
84.76(15)
86.05(15)
172.77(14)
89.30(14)
91.70(13)
94.55(14)
124.0(3)
125.2(3)
O(phenolato) in four-membered metallocycles, 2.235(4)8 and
2.205 Å.9 It may be noted that only a limited number of authen-
tic RuII–O(phenolato) distances are available in the literature.
The observed three ruthenium–nitrogen (bipyridine) bond
lengths of 2.033(4), 2.046(4) and 2.050(4) Å agree well with
values found in other bivalent ruthenium–bipyridine com-
plexes, however Ru–N(2) (2.009 Å) is significantly shorter.6
The Ru–N(5) (imine nitrogen) distance in complex 1a is
2.039(4) Å, which merits scrutiny. No information on Ru–N
(imine nitrogen) bond lengths is available in the literature, but
RuII–N (oxime nitrogen) distances have been observed to be
1.989(7) and 1.986(7) Å.10 The present relatively long Ru–N(5)
distance may account for the significantly short Ru–N(2) bond
length. The bond distance of N(5)–C(222) is 1.280(6) Å, which
is normal for a C᎐N double bond.8 The four oxygen atoms of
᎐
the perchlorate anion are thermally disordered and were refined
Ϫ
anisotropically giving partial occupancy for each. The ClO4
anion is tetrahedral with an average Cl–O distance of 1.337 Å
and an average O–Cl–O angle of 109.45Њ.
Fig. 2 1H NMR spectra of (a) H2L3, (b) H2L4 and (c) complex 1a in
(CD3)2SO.
The protonation of the imine function in complex 1a is
confirmed by the 1H NMR spectrum (see later).
the individual signals, but with the help of 1H correlation spec-
troscopy (COSY) the observed twenty aromatic signals for 1a
and 1b could be separated into five groups of four each, and for
1c four groups of four each and one group of three protons
corresponding to H3, H4, H5 and H6 of the aromatic ring (usual
numbering).
1H NMR spectra
The 1H NMR spectra of one set of ligands (H2L1–H2L4, where
R = RЈ = H) have been recorded in (CD3)2SO solvent. The
data are listed in Table 3 and representative spectra are shown
in Fig. 2. The observed H NMR spectra of H2L1–3 (Fig. 2a)
1
indicate that each half of the ligand is equivalent due to
internal symmetry, therefore it may be assumed that the trans
configuration of the ligands is predominant in solution or that
there is a fast equilibrium between the cis and trans isomers of
H2L1–3. In the case of H2L4 the preferred trans configuration of
the two phenyl rings of the biphenyl group11 makes each half of
the ligand different. However, except for H-9/10 and H-9Ј/10Ј
all aromatic protons are found to overlap (Table 3, Fig. 2b).
In the complexes the equatorial plane contains one pyridine
ring from each of the two bipyridine ligands (b,d), the oxygen
atom and the azomethine nitrogen atom of the ligand (LЈϪ). The
axial positions are occupied by the other pyridine ring of the
bipyridine ligands. The signals of the pyridine ring (d) which is
trans to the phenolato oxygen are likely to be different from the
rest since they experience the trans effect of the σ-donor phen-
olato function. Therefore the signal which appears most up-
field for the individual H6, H5, H4, H3 may be assigned to the
corresponding d ring. The order of the pyridine ring protons in
increasing field strength appears to be 6 > 3 > 4 > 5 for rings
a and b but for pyridine rings c and d it is 6 > 4 > 3 > 5. It may
be noted that similar distribution patterns of pyridine ring
protons have been observed for other [Ru(bpy)2L] systems
where L corresponds to an asymmetric ligand.4 Of the four
bipyridine pyridine rings (a–d), except for the d-ring protons,
the other sets cannot be assigned unambiguously to particular
pyridine rings. The phenolato imine function protons follow the
order 4 > 6 > 5 > 3 for 1a and 1c but for 1b it is 6 > 4 > 5 > 3.
The effects of the presence of the σ-donating phenolato func-
tion and of the electron withdrawing NO2 group in the ring ‘e’
are directly reflected in the relative chemical shifts of the ring
‘e’ signals.
1
The H NMR spectra of complexes 1a–1c were recorded in
(CD3)2SO, and a representative is shown in Fig. 2(c). The
absence of the OH proton of the “free” ligands H2L (δ ≈ 11–12,
Table 3) in the spectra of the complexes suggests metallation
through the phenolato oxygen. The presence of asymmetric
ligands LЈϪ assures non-equivalence of all aromatic rings. The
molecules 1a/1b and 1c thus possess twenty and nineteen
non-equivalent aromatic protons respectively. Since the elec-
tronic environments of many aromatic hydrogen atoms are very
similar, their signals may appear in a narrow chemical shift
range. In fact the aromatic regions of the spectra are com-
plicated, however direct comparisons of the intensity of the
aromatic proton signals with that of the clearly observable
singlet due to the CH᎐N proton for complexes 1a and 1c (δ ≈ 9)
᎐
and the singlet of the methyl group (δ ≈ 2.39) for 1b reveal the
presence of the calculated number of aromatic protons. The
partial overlapping of the signals makes it difficult to assign all
The protonation of the azomethine function in complex 1
1
is confirmed by H NMR spectra. The NH proton signal is
J. Chem. Soc., Dalton Trans., 2000, 2875–2883
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Table 3 1H NMR spectral data of ligands H2L1–H2L4 in (CD3)2SO
δ(J/Hz)a
Ligand
H(2)
H(3)
H(4)
H(5)
H(7)
H(8)
H(9)
H(10)
CH2
CH᎐N
OH
᎐
H2L1
H2L2
H2L3
H2L4
7.03
(8.0)b
6.96
7.39
(8.0)c
7.46
6.97
(7.5)c
6.99
7.35
(8.5)b
7.67
—
—
—
—
—
—
—
—
—
8.70d
8.98d
8.87d
9.05d
9.01d
11.40d
12.90d
13.41d
10.74d
10.26d
7.41
(7.9)b
7.36
7.41
(7.9)b
7.36
—
(7.8)b
6.87
(7.2)c
7.33
(8.1)c
6.91
(8.1)b
7.48
4.8d
(7.5)b
6.96
(7.8)c
7.41
(7.7)c
7.0
(6.8)b
7.85
(7.1)b
7.69
(7.1)b
7.65
7.54
(8.3)b
7.46
7.54
(8.3)b
7.46
(7.9)b
(8.2)c
(7.7)c
(8.4)b
(8.1)b
(7.6)b
(8.5)b
(8.5)b
a Tetramethylsilane as internal standard. b Doublet. c Triplet. d Singlet.
Table 4 Emission dataa
observed near δ 16 (for 1a, 16; 1b, 15.8; 1c, 15.87) and inte-
grates to one proton, which as expected disappears upon
D2O treatment.8,9,12
λmax/nm
Compound
excitation
emission
Quantum yield (φ)b
Electronic spectra
1a
1b
1c
488
506
475
688
703
676
6.3 × 10Ϫ3
2.2 × 10Ϫ2
6.5 × 10Ϫ3
Electronic spectral data of the complexes 1 in acetonitrile
solvent are given in Table 1 and spectra are shown in Fig. 3.
Multiple transitions in the UV-visible region are due to the
presence of different acceptor levels. In the visible region the
complexes display two transitions near 500 and 400 nm. The
lower energy transition (≈500 nm) is associated with a shoulder
at lower energy.
a In MeOH–EtOH 1:4 at 77 K. b Calculated by using eqn. (2).
The two visible bands have been assigned on the basis
of reported spectra of [Ru(bpy)2]2ϩ complexes having other
types of chelated third ligands.13 The bands near 500 and
400 nm are considered to be dπ(RuII) → π*(Ψ) and the
dπ(RuII) → π*(χ) MLCT transitions respectively where π*(Ψ)
and π*(χ) are believed to be antisymmetric and symmetric
bipyridine acceptor orbitals respectively. The bands in the UV
region are of intraligand π–π* type or charge transfer transi-
tions involving levels which are higher in energy than those of
the ligand lowest unoccupied molecular orbital (LUMO).
The lowest energy MLCT transition of [RuII(bpy)3]2ϩ
appears at 450 nm,14 therefore replacement of one bpy ligand
by an asymmetric ligand (LЈϪ) results in a red-shift of the same
transition. The lower ligand-field strength of σ-donating LЈϪ
compared to the π-acceptor bpy ligand and overall lowering of
the molecule symmetry on moving from Ru(bpy)32ϩ to 1 are the
contributing factors for the observed trend. Other known
ruthenium()–bis-bipyridine systems having RuN5O chromo-
phores, [RuII(bpy)2L]ϩ, L = pyridine-2-phenolate or pyridine-2-
olate, exhibit lowest-energy MLCT transitions at 570 and 500
nm respectively.4,15 Thus the present set of complexes 1 appear
to be similar to pyridine-2-olate complexes as far as the ligand
field is concerned.
Fig. 3 Electronic spectra of complexes 1a (-----), 1b (——–) and 1c
(–·–·–·–) in acetonitrile. The inset shows the emission spectrum of
1b in EtOH–MeOH 4:1 (v/v) at 77 K.
φem,s = φem,r(Ar/As)(Is/Ir)(ns/nr)2
(1)
sample and reference, Is, Ir their emission intensities and ns, nr
their refractive indices. Since As and Ar are equal and the
refractive indices are assumed to be similar, eqn. (1) can be
modified to (2). The calculated quantum yields for the com-
Emission spectra
Excitation of complexes 1 at the lowest energy MLCT bands
in 1:4 MeOH:EtOH glass at 77 K results in emissions near
700 nm (Table 4, Fig. 3). The origin of the emission spectra is
further confirmed by the excitation spectra of corresponding
solutions. For all three cases one luminescence band and a
single exponential decay are observed as expected from triplet
MLCT emission of ruthenium()–bipyridine complexes.16
The quantum yields of the complexes were determined by
comparison with the reported quantum yield of [Ru(bpy)3]2ϩ
in 1:4 MeOH:EtOH glass at 77 K (φem,s = 0.35).17 For the calcu-
lations, the excitation wavelengths were chosen such that the
standard reference and sample absorptions are equal. The
quantum yield φ was calculated by following the reported
method,18 eqn. (1), where As, Ar are the absorption values of
φem,s = φem,r(Is/Ir)
(2)
plexes are listed in Table 4. Complex 1b having a keto-based
imine function exhibits the strongest emission, 1b > 1a > 1c.
The red shift of the emission maximum while moving from
2ϩ
Ru(bpy)3 to the present complexes 1a–1c is consistent with
the electrochemical results (see later). The emission energy
increases with increasing redox energy, the difference in energy
between the metal centred oxidation process and the first ligand
centred reduction of the complexes. The introduction of a
σ-donor imine function (LЈϪ) instead of a strong π-acidic bpy
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Table 5 Electrochemical data in acetonitrile at 298 Ka
EЊ298/V (∆Ep/mV)
νMLCT/cmϪ1
Compound
RuIII–RuII Couple
RuIV–RuIII oxidation
Ligand oxidation
Ligand reduction
∆EЊb/V
obs.d
calc.e
1a
1b
1c
0.61 (70)
0.518 (80)
0.770 (80)
1.86c
1.71c
1.97c
2.36c
2.25c
—
Ϫ1.51 (70)
Ϫ1.74 (95)
Ϫ1.56 (70)
Ϫ1.79 (100)
Ϫ1.54 (80)
Ϫ1.72 (100)
2.12
2.08
2.31
20098
19775
21630
20576
19762
21052
a Solvent, acetonitrile; supporting electrolyte, NEt4ClO4; reference electrode, SCE; solute concentration, ≈10Ϫ3 mol dmϪ3; working electrode, plat-
inum. Cyclic voltammetric scan rate, 50 mV sϪ1; EЊ298 = 0.5(Epc ϩ Epa) where Epc and Epa are cathodic and anodic peak potentials respectively.
b Calculated by using eqn. (4) of text. c Epa values, due to irreversible nature of the voltammograms. d In acetonitrile solution. e Using eqn. (3) of text.
ruthenium() couple at 0.41 and 0.64 V respectively.4,15 The
similarity of the ruthenium()–ruthenium() potential of 1a
with that of the pyridine-2-olate system further supports the
close ligand field strengths of these two classes of complexes.
Complexes 1 also display a second irreversible oxidation pro-
cess. The one-electron nature of this is confirmed by differential
pulse voltammetry. It could be due to either RuIII → RuIV
oxidation or oxidation of the ligand. The potential difference
between the two successive oxidation processes is ≈1.2 V, which
agrees well with the average potential difference between the
redox processes of the ruthenium centre (RuII/III Ϫ RuIII/IV
)
(≈1.0–1.5 V) observed in other mononuclear complexes.19
Therefore we believe that the second redox process corresponds
to ruthenium()–ruthenium() oxidation. The formal poten-
tials are found to vary in the expected manner depending on the
electronic nature of LЈ.
Fig. 4 Cyclic voltammograms of a ≈10Ϫ3 mol dmϪ3 solution of com-
plex 1a in acetonitrile at 298 K. Differential pulse voltammograms are
shown only for positive potentials.
The complexes 1a and 1b also exhibit a third irreversible
oxidation process above 2 V. In the case of 1c the third oxidation
process could not be seen within the experimental potential
limit ϩ2.5 V possibly due to solvent cut off. Although the
anodic current height (ipa) of this irreversible process is ≈1.5
times that of the previous redox processes, RuIII–RuII and RuIV–
RuIII, the differential pulse voltammogram shows the third
oxidation wave to have the almost same height, implying a
one-electron process. The third oxidative response might be
due either to oxidation of the ligand LЈ or to RuIV → RuV
oxidation. Since the redox behaviour of LЈ either in the
free state or in other complex systems has not been investigated,
it is difficult to rule out one particular possibility exclusively.
ligand in the Ru(bpy)2 core lowers the energy of the eg orbitals
and hence narrows the energy gap between the MLCT and d–d
states. The d–d state therefore becomes thermally accessible
and decays through an alternative route. This is reflected in
2ϩ
the lower φ values of 1 as compared to Ru(bpy)3
.
Electrochemical studies
The electrochemical properties of complexes 1 have been
studied in acetonitrile solution by cyclic voltammetry and
However,
a ligand based response seems appealing as
constant-potential coulometry using
a platinum working
ruthenium() complexes are known to be rare particularly in
non-oxo environments.
electrode. The complexes are electroactive with respect to the
metal as well as the ligand centres and display five redox
processes in the potential range 2.5 V versus SCE. Represen-
tative voltammograms are shown in Fig. 4 and the data are
summarised in Table 5.
The complexes display one reversible oxidation process which
is assigned to the ruthenium()–ruthenium() couple. The one-
electron nature of the couple is confirmed by constant-potential
coulometry (Table 5). The presence of trivalent ruthenium in
the oxidised solution of 1ϩ has been confirmed by the character-
istic EPR spectra of the ruthenium() congeners (see later).15
The formal potential of the ruthenium()–ruthenium()
couple varies depending on LЈ, such that the electron-donating
methyl group in 1b decreases the potential and the electron-
withdrawing NO2 group in 1c increases it with respect to that of
complex 1a (Table 5).
Two successive one-electron reductions due to co-ordinated
bpy are also observed for all the complexes (Fig. 4, Table 5).4,15
Spectroelectrochemical correlation
The observed MLCT transitions for complexes 1 involve the
6
excitation of an electron from the filled t2g orbital of ruthen-
ium() to the lower π* orbital of the diimine function of bpy.
The energy of this band can be predicted from the observed
electrochemical data with the help of eqns. (3) and (4).20
νMLCT = 8065(∆EЊ) ϩ 3000
(3)
(4)
∆EЊ = EЊ298(RuIII–RuII) Ϫ EЊ298(LЈ)
Thus substitution of one π-acidic bpy ligand from the
[Ru(bpy)3]2ϩ core by one σ-donating LЈ in complexes 1 results in
a decrease of the ruthenium()–ruthenium() potential. This is
due to the reduction of overall charge of the complex cation
from ϩ2 in [Ru(bpy)3]2ϩ to ϩ1 in 1 which provides electrostatic
stabilisation of the oxidised RuIII–LЈ species.
EЊ298(RuIII–RuII) corresponds to formal potential (in V) of the
RuIII–RuII couple, EЊ298(LЈ) that of the first ligand reduction
and νMLCT is the frequency or energy of the charge-transfer
band in cmϪ1. The factor 8065 is used to convert the potential
difference ∆E from V into cmϪ1 and the term 3000 cmϪ1 is of
empirical origin. The calculated and experimentally observed
νMLCT values are listed in Table 5 and are in exceptionally good
agreement.
Other RuN5O chromophoric systems, [RuII(bpy)2(LЉ)]
(LЉ = pyridine-2-phenolate or -olate) exhibit the ruthenium()–
J. Chem. Soc., Dalton Trans., 2000, 2875–2883
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Table 6 EPR g values,a distortion parametersb and NIR transitionsc
Compound
gx
gy
gz
k
∆/λ
V/λ
ν1/λ
ν2/λ
ν2/λ (obs.c)
1aϩ
1bϩ
1cϩ
Ϫ2.340
Ϫ2.333
Ϫ2.300
Ϫ2.080
Ϫ2.061
Ϫ2.051
1.854
1.867
1.812
0.594
0.586
0.484
5.391
5.923
4.543
Ϫ4.167
Ϫ5.127
Ϫ3.251
3.456
3.508
3.086
7.676
8.669
6.409
7.092
7.812
7.142
a Measurements were made in acetonitrile at 77 K. b Symbols have the same meaning as in the text. c In acetonitrile.
Fig. 6 X-Band EPR spectrum and t2 splittings of complex 1aϩ in
acetonitrile–toluene at 77 K.
Fig. 5 Electronic spectra of complexes 1aϩ (-----), 1bϩ (——–) and
1cϩ (–·–·–·–) in acetonitrile. The inset shows the expanded part of
the spectra in the range 500–900 nm.
Electrogeneration of trivalent ruthenium congeners 1؉, distortion
parameters and solution properties
Coulomeric oxidations of complexes 1a–1c in dry acetonitrile
solvent at a potential of ≈200 mV above the Epa of the first
oxidation couple generate a deep green oxidised solution. The
observed coulomb count corresponds to one-electron transfer
for each case (Table 5). The resulting oxidised solution shows
cyclic voltammograms which are identical to those of the
starting bivalent complexes, [RuII(bpy)(LЈ)]ϩ, this may be due to
the stereoretentive nature of the oxidation process. The oxidised
solution can quantitatively be reduced to the parent bivalent
state. In the visible region the complexes 1ϩ exhibit one
moderately intense broad band near 750 nm (Table 1, Fig. 5)
corresponding to π(bpy) → t2g(RuIII) LMCT transition.4
The X-band EPR spectra of 1ϩ in acetonitrile (produced
coulometrically followed by quick freezing in liquid N2, 77 K)
are rhombic characteristic of low-spin ruthenium() com-
plexes in distorted octahedral environments (Table 6, Fig. 6).21
The EPR spectra were analysed using the g-tensor theory
for low-spin d5 ions.22 This afforded values of the axial
distortion (∆) which splits the t2 shell into e ϩ b and of the
rhombic distortion (V) which splits e further into two non-
degenerate components. The value of the orbital reduction
factor k is also furnished by analysis. Two ligand field transi-
tions ν1 and ν2 among the Kramer’s doublets are expected.
Their energies, along with values of ∆ and V calculated from
the g parameters with help of the theory, are listed in Table 6.
The spin–orbit coupling constant, λ, of RuIII was taken as 1000
Fig. 7 Qualitative electronic spectra of complexes 2a (–––), 2b (——–)
and 2c (—·—) in acetonitrile.
The coulometrically oxidised complexes 1aϩ–1cϩ are found
to be unstable. In dry acetonitrile solvent the oxidised deep
green one-electron paramagnetic 1ϩ slowly changes to an EPR
silent diamagnetic species. The intensity of the LMCT band
of 1ϩ near 750 nm gradually decreases with the concomitant
growth of a new band near 430 nm and eventually it transforms
to a yellowish green species having a completely different
spectral profile (Table 1, Fig. 7). On the other hand in the pres-
ence of a small amount of water (ordinary acetonitrile) the
oxidised 1ϩ gradually reduces to the parent bivalent species 1.
However, the direct addition of water to a dry acetonitrile
solution of 1ϩ accelerates the reduction process. Thus the
rate of conversion of 1ϩ → 1 is dependent on the water
content in acetonitrile. The transformed yellowish green
species exhibits two closely spaced reversible ruthenium()–
ruthenium() reduction couples almost at the same potential as
that of the starting deep green 1ϩ (Fig. 8). Considering all the
facts together it may be reasonable to believe that in dry
acetonitrile solution the electrochemically generated trivalent
monomeric species 1ϩ dimerises to a diamagnetic species of
cm .
Ϫ1 23 In near-IR spectra the ν2 band is indeed detected in the
expected position (Table 6). In view of the approximation
involved in the theory, the agreement between the experi-
mentally observed and calculated ν2 value is excellent. Owing to
the instrumental wavelength scan limitation (maximum up to
2200 nm) it has not been possible to check the ν1 band. The
presence of a high degree of distortion in the complexes 1ϩ is
reflected in the ∆ and V values (Table 6). The complexes behave
as model ruthenium() species possessing rhombic symmetry.
2880
J. Chem. Soc., Dalton Trans., 2000, 2875–2883

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Fig. 8 Cyclic voltammogram and differential pulse voltammogram of
coulometrically generated complex 2a in acetonitrile at 298 K (only
RuIII–RuII couples are shown).
Fig. 9 Time evolution of the electronic spectra of a changing solution
of 1bϩ → 2b in dry acetonitrile at 303 K. The arrows indicate increase
or decrease in band intensities as the reaction proceeds.
Table 7 Rate constants and activation parameters of 1ϩ → 2 and
1ϩ → 1
104 k/
∆H‡/
∆S‡/
Compound
T/K
dm3 molϪ1 sϪ1
kJ molϪ1
J KϪ1 molϪ1
(a) 1ϩ → 2 in dry acetonitrile solution
1aϩ
1bϩ
1cϩ
303
318
333
303
318
333
303
318
333
1.6
5.4
18.2
2.1
7.0
24.4
2.4
62.79
64.79
54.30
Ϫ47.47
Ϫ43.40
Ϫ58.03
7.6
20.6
(b) 1ϩ → 1 in acetonitrile solution (104 kЈ/sϪ1
)
1aϩ
1bϩ
1cϩ
303
318
333
303
318
333
303
318
333
0.3
0.7
1.3
0.25
0.52
0.90
0.7
1.2
1.9
35.9
Ϫ91.9
Ϫ97.93
Ϫ104.37
32.55
25.53
Scheme 2
type 2 (Scheme 2) where the two ruthenium() centres are anti-
ferromagnetically coupled.24 In contact with water the N–N
bond of 2 is cleaved which in turn leads to the formation of the
parent monomeric species 1. The observed instability of 1ϩ and
2 in the presence of water may account for the formation
of mononuclear complexes 1 starting from the binucleating
ligands H2L (Scheme 1). The two closely spaced cyclic
voltammetric responses of 2 (Fig. 8) are assigned to successive
reductions of two ruthenium() centres, eqns. (5) and (6).
The electron-transfer process 1ϩ → 1 in the presence of
ordinary acetonitrile has also been monitored spectrophoto-
metrically in the temperature range 303–333 K (Fig. 10). The
conversion process is first order with respect to [1ϩ]. The rate
constant (kЈ) values and activation parameters (∆H‡ and ∆S‡)
are listed in Table 7. The observed rate constant (kЈ) varies
systematically depending on the nature of LЈ in 1ϩ, following
the order 1cϩ > 1aϩ > 1bϩ (Table 7). This implies superior
stability of the trivalent ruthenium() state in 1bϩ. The EЊ298
values of the ruthenium()–ruthenium() couple also follow
the order 1c > 1a > 1b. Thus the trends of k and EЊ298 values are
internally consistent. The observed high negative entropy value
(≈100 J) is not clearly understood, however the participation of
a solvent molecule in the electron-transfer process (1ϩ → 1)
may account for the observed kinetic behaviour.
RuIII–RuIII ϩ eϪ → RuIII–RuII
RuIII–RuII ϩ eϪ → RuII–RuII
(5)
(6)
Rate of conversion of 1ϩ → 2 and 1ϩ → 1
The conversion 1ϩ → 2 has been followed spectrophoto-
metrically in dry acetonitrile solution in the temperature range
303–333 K (Fig. 9). The reaction is second order with respect to
[1ϩ]. Variable temperature rate constants (k) and activation
parameters (∆H‡ and ∆S‡) are listed in Table 7. The observed
rate constant (k) values vary systematically with respect to LЈ
and follow the order 1cϩ ≈ 1aϩ > 1bϩ (Table 7). The observed
large enthalpy values may be ascribed to the breaking of
two N–H bonds of two monomeric units (1) and simultaneous
formation of one N–N bond in the dimer, 2. The large negative
∆S‡ value is suggestive of an association process.25
Conclusion
We have observed the cleavage of N–N and N–C (aliphatic and
aromatic) bonds of the binucleating imine functionalities
(H2L1–H2L12) in contact with a Ru(bpy)2 core. This in turn
leads to the formation of complexes of the type [Ru(bpy)2LЈ]-
ClO4 1 where LЈ corresponds to the stable imine functions
J. Chem. Soc., Dalton Trans., 2000, 2875–2883
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Table 8 Crystallographic data for [RuII(bpy)2(LЈ)]ClO4ؒCH3CN 1a
Formula
M
C29H25ClN6O5Ru
674.07
Crystal symmetry
Space group
Monoclinic
P21/n
a/Å
b/Å
c/Å
9.220(10)
19.906(3)
15.722(3)
90.760(10)
2885.3(8)
4
β/Њ
U/Å3
Z
T/K
296
0.686
µ/mmϪ1
Reflections collected
Unique reflections (Rint
R1
5405
5052 (0.0062)
0.052
)
wR2
0.200
Treatment of EPR data
Fig. 10 Time evolution of the electronic spectra of a changing
solution of 1bϩ → 1b in ordinary acetonitrile at 303 K. The arrows
indicate increase or decrease in band intensities as the reaction
proceeds.
An outline of the procedure can be found in our recent publi-
cations.28 We note that a second solution also exists that is
different from the chosen one, having small distortions and ν1
and ν2 values. The experimentally observed near-IR results
clearly eliminate this solution as unacceptable.
ϪOC H (R)C(RЈ)᎐NH incorporating the rare C᎐NH fragment.
᎐
᎐
6
3
The complexes are susceptible to successive metal and ligand
based redox processes and display moderately strong emission
from the lowest energy MLCT bands. The electrochemically
oxidised one-electron paramagnetic ruthenium() species,
[RuIII(bpy)2LЈ]2ϩ 1ϩ, is found to be unstable, slowly changing
Kinetic measurements
The conversions 1ϩ → 2 and 1ϩ → 1 were monitored
spectrophotometrically in thermostatted cells. For the deter-
mination of k, the decrease in absorptions (At) at 790 nm for
complex 1aϩ, 748 nm for 1bϩ and 680 nm for 1cϩ were recorded
as a function of time (t). A∞ were measured when the intensity
changes levelled off. Values of the second order rate constant, k,
for the process 1ϩ → 2 were obtained from the slopes of
linear least-squares plots of x/(a0 Ϫ x) against t, where a0 is the
initial concentration of 1ϩ and x the amount of 1 that has
disappeared in time t. Values of first order rate constant, kЈ, for
the process 1ϩ → 1 were obtained from the slopes of linear
least-squares plots of Ϫln(A0 Ϫ At) against t. The activation
parameters ∆H‡ and ∆S‡ were determined from the Eyring
plot.29
to a dinuclear species of the type [(bpy) RuIII{ϪOC H (R)C(RЈ)᎐
᎐
2
6
3
NN᎐C(RЈ)C(R)C H OϪ}RuIII(bpy) ]4ϩ 2 where the ruthen-
᎐
6
3
2
ium() centres are antiferromagnetically coupled. In contact
with a water source both complexes 1ϩ and 2 return to the
parent bivalent species 1.
Experimental
Materials
The starting complex cis-[Ru(bpy)2Cl2]ؒ2H2O was prepared
according to the reported procedures.26 Benzidine and α,αЈ-
diamino-p-xylene were obtained from Fluka, Switzerland.
Other chemicals and solvents were of reagent grade used as
received. For electrochemical studies HPLC-grade acetonitrile
was used. Commercial tetraethylammonium bromide was con-
verted into pure tetraethylammonium perchlorate by following
an available procedure.27
Preparation of ligands H2L1–12 and complexes [Ru(bpy)2(LЈ)]-
ClO4 1a–1c
The ligands H2L1–12 were prepared by condensing an appro-
priate salicylaldehyde with an appropriate diamine in a 2:1 mol
ratio in dry ethanol with stirring and recrystallised from hot
ethanol. The microanalytical data of the ligands are given in the
Supplementary Table. The complexes 1a–1c were synthesized
by following a general procedure and yields varied in the range
60–65%. Details are given for one representative, 1a.
The starting complex [Ru(bpy)2Cl2]ؒ2H2O (300 mg, 0.57
mmol) and AgClO4 (240 mg, 1.17 mmol) were heated to reflux
in ethanol (25 cm3) with stirring for 1 h. The orange solution of
the resulting [Ru(bpy)2(EtOH)2]2ϩ was then cooled and filtered
through a Gooch (G-4) sintered-glass funnel. The ligand H2L1
(0.098 mg, 0.285 mmol) was then added to the above filtrate
(ethanolato species). The resulting mixture was heated to reflux
under a N2 atmosphere overnight and then cooled. The precipi-
tate thus formed was filtered off and washed thoroughly with
cold ethanol. Finally, the product was recrystallised from
acetonitrile–benzene (1:3 v/v). The yield was 60%.
Physical measurements
Solution electrical conductivity was checked using a Systronic
305 conductivity bridge, Electronic spectra were recorded using
a Shimadzu UV-2100 spectrophotometer, infrared spectra on
a Nicolet spectrophotometer with samples prepared as KBr
pellets. Magnetic susceptibility was checked with a PAR vibrat-
ing sample magnetometer. 1H NMR spectra were obtained
using a 300 MHz Varian FT-NMR spectrometer. Cyclic vol-
tammetric and coulometric measurements were carried out
using a PAR model 273A electrochemistry system. A platinum
working electrode, platinum wire auxiliary electrode and SCE
reference electrode were used in a three-electrode configuration.
The supporting electrolyte was NEt4ClO4 and the concen-
tration of the solution was ≈10Ϫ3 mol dmϪ3. The half-wave
potential EЊ298 was set equal to 0.5(Epa ϩ Epc), where Epa and
Epc are anodic and cathodic cyclic voltammetric peak poten-
tials, respectively. The scan rate used was 50 mV sϪ1. A platinum
wire gauge working electrode was used in coulometric experi-
ments. All electrochemical experiments were carried out under
a dinitrogen atmosphere and are uncorrected for junction
potentials. Elemental analyses were carried out using a Perkin-
Elmer 240C elemental analyser. Solution emission properties
were checked using a SPEX-fluorolog spectrofluorometer.
CAUTION: perchlorate salts of metal complexes are gener-
ally explosive. Care should be taken while handling them.
Crystallography
Single crystals of the complex 1 were grown by slow diffusion
of an acetonitrile solution of it in benzene followed by slow
evaporation. Significant crystal data and data collection
2882
J. Chem. Soc., Dalton Trans., 2000, 2875–2883

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7 P. Bernhard, H. B. Burgi, J. Hauser, H. Lehmann and A. Ludi,
Inorg. Chem., 1982, 21, 3936.
8 N. Bag, S. B. Choudhury, A. Pramanik, G. K. Lahiri and
A. Chakravorty, Inorg. Chem., 1990, 29, 5014.
9 N. Bag, S. B. Choudhury, G. K. Lahiri and A. Chakravorty,
J. Chem. Soc., Chem. Commun., 1990, 1626.
10 A. R. Chakravarty, A. Chakravorty, F. A. Cotton, L. R. Falvello,
B. K. Ghosh and M. Thomas, Inorg. Chem., 1983, 22, 1892.
11 D. Nasipuri, Stereochemistry of Organic Compounds, Wiley Eastern
Limited, New Delhi, 1994, p. 81.
12 M. H. Kuchama, T. Nicholson, A. Davison, W. M. Davis and A. G.
Jones, Inorg. Chem., 1997, 36, 3237.
13 B. J. Coe, T. J. Meyer and P. S. White, Inorg. Chem., 1995, 34, 593;
W. Paw, W. B. Connick and R. Eisenberg, Inorg. Chem., 1998, 37,
3919; R. Alsfasser and R. V. Eldik, Inorg. Chem., 1996, 35, 628.
14 G. M. Brown, T. R. Weaver, F. R. Keene and T. J. Meyer, Inorg.
Chem., 1976, 15, 190.
15 B. K. Santra, M. Menon, C. K. Pal and G. K. Lahiri, J. Chem. Soc.,
Dalton Trans., 1997, 1387.
16 L. M. Vogler and K. J. Brewer, Inorg. Chem., 1996, 35, 818.
17 G. A. Crosby and W. H. Elfring, Jr., J. Phys. Chem., 1976, 80,
2206.
18 J. V. Houten and R. J. Watts, J. Am. Chem. Soc., 1976, 98, 4853; E. F.
Godefroi and E. L. Little, J. Org. Chem., 1956, 21, 1163; P. Chen,
R. Duesing, D. K. Graff and T. J. Meyer, J. Phys. Chem., 1991, 95,
5850.
19 N. Bag, G. K. Lahiri, S. Bhattacharya, L. R. Falvello and
A. Chakravorty, Inorg. Chem., 1988, 27, 4396; G. K. Lahiri,
S. Bhattacharya, S. Goswami and A. Chakravorty, J. Chem. Soc.,
Dalton Trans., 1990, 561.
parameters are listed in Table 8. Absorption correction was
done by performing ψ-scan measurement.30 The data reduction
was done by using MAXUS and structure solution and refine-
ment using the program SHELXS 93 and SHELXL 97 respect-
ively.31 The metal atom was located from the Patterson map and
the other non-hydrogen atoms emerged from successive Fourier
synthesis. The structure was refined by full-matrix least squares
on F2. All non-hydrogen atoms were refined anisotropically.
Hydrogen atoms were included in calculated positions.
CCDC reference number 186/2038.
See http://www.rsc.org/suppdata/dt/b0/b001663m/ for crys-
tallographic files in .cif format.
Acknowledgements
Financial support received from the Department of Science
and Technology, New Delhi, India, is gratefully acknowledged.
The structural study was carried out at the National Single
Crystal Diffractometer Facilities, Indian Institute of Technol-
ogy, Bombay. Special acknowledgement is made to Regional
Sophisticated Instrumental Center, RSIC, Indian Institute of
Technology, Bombay for providing NMR and EPR facilities.
The suggestions of the Reviewers at the revision stage were very
helpful.
20 S. Goswami, R. N. Mukherjee and A. Chakravorty, Inorg. Chem.,
1983, 22, 2825; B. K. Ghosh and A. Chakravorty, Coord. Chem.
Rev., 1989, 95, 239.
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