Ruthenium-(il)/-(in) terpyridine complexes incorporating imine functionalities. Synthesis, structure, spectroscopic and electrochemical properties 1

Article abstract of DOI:10.1039/b000257g

A new class of ruthenium terpyridine complexes of the type [Ru¹ trpyXL Cl] 1-6 (trpy = 2, 2′ :6′, 2"-terpyridine; L₁₃ = o-OC₆H₃(R)C(R′)=NCH2C6H5andL′M = o-OC₆Hj(R)CH(R′)N=NC6Hj;vhereR = Horp-NO₂ and R′ = Hor CH₃) have been synthesized. The "free" ligands incorporating a NH spacer 0-~OC6H3(R)C(R′)=NNHC6H5 (HL46) have undergone imine to azo tautomerism on co-ordination to the ruthenium terpyridine moiety in the complexes 4-6, whereas those having a CH₂ spacer (HL^-3) remain unaltered on co-ordination. The diamagnetic, neutral complexes 1-6 exhibit strong MLCT transitions in the visible region and intraligand transitions in the UV region. A significant shift in MLCT band energy has been observed depending on the ligand field strength of the co-ordinated L. The complexes display a reversible ruthenium(in)-ruthenium(ii) couple in the potential range of 0.12-0.63 V and a quasi-reversible ruthenium(iv)-ruthenium(iii) couple in the range of 1.21-1.85 V versus SCE. The higher ligand field strength of the co-ordinated L4 compared to the co-ordinated L is reflected in the observed metal redox processes. The reduction of the co-ordinated terpyridine has been observed near -1.3 V. The complexes exhibit moderately strong emissions from the lowest energy MLCT bands in the range 661-690 nm in EtOH-MeOH (4:1 v/v) at 77 K. The quantum yields of the complexes (0 = 0.006-0.09) are found to be reasonably sensitive to the nature of the coordinated L. The oxidised complexes 3⁺ and 6⁺ have been isolated in the solid state as their perchlorate salts. The crystal structure of 3⁺ exhibits pseudo-octahedral trans geometry with regard to the relative disposition of the imine nitrogen (N4) of L3 and the central pyridyl group of the trpy ligand. The one-electron paramagnetic complexes show 1:1 conductivity and display ligand-to-metal charge transfer bands near 600 and 400 nm and intraligand transitions in the UV region. The observed rhombic EPR spectra at 77 K corresponding to the distorted octahedral geometry have been analysed to furnish values of axial (A) and rhombic (F) distortion parameters as well as the energies of the two expected ligand field transitions (v,: and v2 within the t2 shell. The Royal Society of Chemistry 2000.

Full text of DOI:10.1039/b000257g

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Ruthenium-(II)/-(III) terpyridine complexes incorporating imine
functionalities. Synthesis, structure, spectroscopic and
electrochemical properties†
Biplab Mondal, Soma Chakraborty, Pradip Munshi, Mrinalini G. Walawalkar and
Goutam Kumar Lahiri*
Department of Chemistry, Indian Institute of Technology, Bombay, Mumbai-400076, India
Received 12th January 2000, Accepted 15th May 2000
Published on the Web 22nd June 2000
A new class of ruthenium terpyridine complexes of the type [RuII(trpy)(L1–6)Cl] 1–6 (trpy = 2, 2Ј:6Ј, 2Љ-terpyridine;
L1–3 = o-ϪOC H (R)C(RЈ)᎐NCH C H andL4–6 = o-ϪOC H (R)CH(RЈ)N᎐NC H ;whereR = Horp-NO andRЈ = Hor
᎐
᎐
6
3
2
6
5
6
3
6
5
2
CH ) have been synthesized. The “free” ligands incorporating a NH spacer o-ϪOC H (R)C(RЈ)᎐NNHC H (HL4–6
)
᎐
3
6
3
6
5
have undergone imine to azo tautomerism on co-ordination to the ruthenium terpyridine moiety in the complexes
4–6, whereas those having a CH2 spacer (HL1–3) remain unaltered on co-ordination. The diamagnetic, neutral
complexes 1–6 exhibit strong MLCT transitions in the visible region and intraligand transitions in the UV region. A
significant shift in MLCT band energy has been observed depending on the ligand field strength of the co-ordinated
L. The complexes display a reversible ruthenium()–ruthenium() couple in the potential range of 0.12–0.63 V and a
quasi-reversible ruthenium()–ruthenium() couple in the range of 1.21–1.85 V versus SCE. The higher ligand field
strength of the co-ordinated L4–6 compared to the co-ordinated L1–3 is reflected in the observed metal redox processes.
The reduction of the co-ordinated terpyridine has been observed near Ϫ1.3 V. The complexes exhibit moderately
strong emissions from the lowest energy MLCT bands in the range 661–690 nm in EtOH–MeOH (4:1 v/v) at 77 K.
The quantum yields of the complexes (Φ = 0.006–0.09) are found to be reasonably sensitive to the nature of the co-
ordinated L. The oxidised complexes 3ϩ and 6ϩ have been isolated in the solid state as their perchlorate salts. The
crystal structure of 3ϩ exhibits pseudo-octahedral trans geometry with regard to the relative disposition of the imine
nitrogen (N4) of L3 and the central pyridyl group of the trpy ligand. The one-electron paramagnetic complexes show
1:1 conductivity and display ligand-to-metal charge transfer bands near 600 and 400 nm and intraligand transitions
in the UV region. The observed rhombic EPR spectra at 77 K corresponding to the distorted octahedral geometry
have been analysed to furnish values of axial (∆) and rhombic (V) distortion parameters as well as the energies of the
two expected ligand field transitions (ν1 and ν2) within the t2 shell.
3
tially from the small energy gap between the emitting MLCT
state and the upper lying 3MC state, an increase in energy
gap would therefore be desirable. This can be achieved by
introducing either suitable substituents in the terpyridine
framework itself or by using ancillary ligands, L along with the
ruthenium monoterpyridine core.8
Introduction
The development of new photo-redox active ruthenium poly-
pyridine derivatives has been the subject of continuous research
activity.1 Facile electron-transfer properties, strong metal to lig-
and charge-transfer absorptions and long lived 3MLCT excited
states of this class of complexes make them attractive for the
development of photochemical and electrochemical devices.2 In
this regard a variety of ruthenium bipyridine and phenanthro-
line complexes have been synthesized and studied extensively
over the last three decades in order to modulate their photo-
chemical and photophysical properties.3 However, the corre-
sponding terpyridine complexes have not been explored to that
extent.4 Although the terpyridine type of tridentate ligand
has structural advantages over bidentate bipyridine and phen-
anthroline ligands, it has a serious drawback from the photo-
physical point of view.5 As compared to bipyridine ligands the
bite angles of terpyridine ligands are not ideally suited for
octahedral co-ordination and this in turn results in a relatively
weak ligand field and low energy metal centred states.6 In con-
sequence ruthenium terpyridine complexes exhibit relatively
short lived MLCT states and behave as weak emitters compared
to the analogous bipyridine complexes (τ = 890 ns, 250 ps and
The present work originates from our interest in introducing
phenolic Schiff base ligands HL having CH2 and NH spacers
[HOC H (R)C(RЈ)᎐NCH C H (A) and HOC6H3(R)C(RЈ) =
᎐
6
3
2
6
5
NNHC6H5 (B)] as ancillary ligands to develop mixed ligand
ruthenium–terpyridine complexes of the type [Ru(trpy)(L)(Cl)].
Although both types of ligands (A and B) are known to be
stable enough either in the free state or on co-ordination, here
the ligands having the NH spacer (B) undergo internal imine to
azo tautomerism on chelation to the ruthenium–terpyridine
moiety. To the best of our knowledge this work demonstrates
the first example of ruthenium–terpyridine complexes incorpor-
ating imine functionalities. Herein we report the synthetic
aspects of complexes of type [RuII/III(trpy)(L)(Cl)], their spectro-
scopic and electron-transfer properties, the crystal structure of
one trivalent complex [RuIII(trpy)(L3)(Cl)]ClO4, the spectro-
electrochemical correlation and solution electronic structure of
the trivalent complexes.
Φ = 0.054, ≤ 5 × 10Ϫ6 for Ru(bpy)3 and Ru(trpy)2 respect-
ively at room temperature).7 Since the short lifetime of
ruthenium terpyridine complexes is known to originate essen-
2ϩ
2ϩ
Results and discussion
Synthesis and spectral properties
† Electronic supplementary information (ESI) available: IR spectra;
analytical, spectral, electrochemical and emission data. See http://
www.rsc.org/suppdata/dt/b0/b000257g/
The six phenolic Schiff base ligands used are classified into two
groups, HL1–HL3 (A) and HL4–HL6 (B), depending on the
DOI: 10.1039/b000257g
J. Chem. Soc., Dalton Trans., 2000, 2327–2335
This journal is © The Royal Society of Chemistry 2000
2327

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(Scheme 1). This implies that the tautomerism of LϪ takes place
particularly on co-ordination to the Ru(trpy)Cl3 moiety. The
observed imine (D) → azo (E) tautomerism of the co-
ordinated LϪ in complexes 4–6 may be compared with the
allylic rearrangement (F–G) of alkene derivatives.9 The inert-
nature of the spacer (CH2 or NH) present in between the imine
nitrogen and the phenyl ring and within each group the three
ligands differ with respect to the substituents either on the
phenolic ring or on the imine fragment. The complexes [RuII-
(trpy)(L)(Cl)] 1–6 (trpy = 2,2Ј:6Ј,2Љ-terpyridine) have been
synthesized from [Ru(trpy)Cl3] following the general dechlorin-
ation synthetic route as shown in Scheme 1. The dark coloured
ness of the “free” ligands H2L4–6 with respect to tautomerism
compared with their behaviour in the complexes 4–6 is presum-
ably due to the basic nature of the NH nitrogen atom. On
chelation the nitrogen centre becomes less basic which in turn
facilitates migration of the acidic NH proton from the nitrogen
atom to the carbon concerned.
The unsymmetrical nature of LϪ leads to the possibility of
two isomeric forms, H and I, of the complexes [Ru(trpy)(L)-
Scheme 1 (i) MeOH, CH3CO2Na, heat.
pure neutral complexes [Ru(trpy)(L3)(Cl)] 3 and [Ru(trpy)-
(L6)(Cl)] 6 are precipitated directly from the reaction mix-
ture, whereas the other four complexes (1, 2, 4 and 5) are
obtained on removal of solvent followed by chromatographic
purification using a silica gel column. The microanalytical data
of the complexes confirm the gross composition of the mixed
chelates [RuII(trpy)(L)(Cl)] 1–6. The diamagnetic complexes are
non-conducting in acetonitrile solution. The FAB mass spectra
of two representative complexes, 3 and 6 (one from each class),
have been recorded. The maximum molecular peaks are
observed at m/z 625 and 626.7 for 3 and 6 respectively, which
match well with the corresponding calculated masses (625.8 and
626.9).
(Cl)].10 In practice both isomers have been noticed in solution as
an intimate mixture for the complexes 1, 2, 4 and 5 (see NMR
part). All our attempts to separate them by column chromato-
graphy have failed. Therefore, data are reported here only for
the isomerically pure complexes, 3 and 6.
The three notable features of the infrared spectra are: (i) The
“free” ligands HL4–HL6 display one sharp band near 3300 cmϪ1
corresponding to the νN-H frequency11 which is systematically
absent for the corresponding complexes (4–6); (ii) the formation
of the N᎐N bond in the ligand framework present in complexes
᎐
4–6 is evidenced by the appearance of a new strong and sharp
band near 1300 cm ;
Ϫ1 12 thus the formation of a new N᎐N bond
᎐
The anionic form of the ancillary ligands of type A is
observed to be retained in complexes 1–3 whereas the ligands of
type B (L4–L6) have undergone imine → azo tautomerism
during the reaction (Scheme 1) which eventually leads to the
formation of complexes of type C (see NMR part). It may be
at the expense of the νN-H frequency supports the structure C of
4–6; (iii) complexes 1–6 exhibit one sharp band near 335 cmϪ1
due to the RuII–Cl stretching frequency, as expected.13
The 1H NMR spectra of two representative complexes 1 and
4 are shown in Fig. 1. The absence of the O–H proton of the
“free” ligands HL1–HL6 in the spectra of the complexes sug-
gests co-ordination through the phenolato oxygen of LϪ. The
methylene protons (CH2) of L1–3 in complexes 1–3 appear
near δ 5.95 as a singlet (Fig. 1a) which is reasonably deshielded
as compared to that of the “free” ligands (δ 4.85).14 The methyl
signal of 2 appears at δ 2.39 as a singlet. The N–H proton of the
“free” ligands HL4–HL6 appears near δ 7.7 as a broad singlet,11
however, this signal is absent in the spectra of complexes 4–6.14
On the other hand a new singlet corresponding to two protons
for 4 and 6 and a multiplet corresponding to one proton for 5
appeared near δ 3.8, which are assigned to the H CN᎐N and
᎐
2
HC(CH )N᎐N protons respectively. The methyl protons of
᎐
3
complex 5 are observed at δ 2.35 as a doublet.
noted that the anionic form of ligands (L4–6) is found to
be stable in other ruthenium complexes and also remains
unchanged in the presence of sodium acetate, but in absence
of [Ru(trpy)Cl3], under hot methanolic reaction conditions
The presence of asymmetric ligands LϪ in the complexes
make all the five aromatic rings inequivalent. The complexes 1,
2, 4, 5, and 3, 6 thus possessing twenty and nineteen non-
equivalent aromatic protons respectively. Since the electronic
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Table 1 Electronic spectral data in dichloromethane
Complex
UV/VIS λmax/nm (ε/dm3 molϪ1 cmϪ1
)
3
583 (10800), 429 (13090), 324 (34370), 282 (24660), 240
(47250)
6
530 (5320), 404 (4250), 316 (15960), 275 (19680), 234
(22130)
1600 (90), 600 (5170), 420 (8890), 310 (25850), 275
(22960), 230 (41360)
1080 (70), 570 (4300), 400 (7820), 307 (20220), 272
(23580), 217 (32320)
3ϩ
6ϩ
Fig. 1 1H NMR spectra of (a) [RuII(trpy)(L1)Cl] 1 and (b) [RuII-
(trpy)(L4)Cl] 4 in CDCl3.
environments of many aromatic hydrogen atoms are similar,
their signals may appear in a narrow chemical shift range. In
fact the aromatic regions of the spectra of complexes 1–3 are
complicated due to overlapping of several signals (Fig. 1a)
which has precluded the identification of the individual pro-
tons. However, for 4–6 the peaks are reasonably separated and
all the doublets, triplets and singlet have clearly been assigned
(Fig. 1b). The direct comparison of the intensity of the
aromatic proton signals with that of the clearly observable
aliphatic (CH2) protons reveals the presence of the calculated
number of aromatic protons in the complexes.
Fig. 2 Electronic spectra of [RuII(trpy)(L3)Cl] 3 (------) and [RuII-
(trpy)(L6)Cl] 6 (——) in dichloromethane. The inset shows the emission
spectrum of 3 in EtOH–MeOH, 4:1 (v/v) at 77 K.
transitions in the higher energy region which are believed to be
the ligand centred π → π* transitions.16
The lowest energy MLCT band of [Ru(trpy)2]2ϩ appears at
478 nm.17 Thus the replacement of one strong π-acidic terpyr-
idine ligand by LϪ decreases the energy of the same transition,
which is in consonance with the greater σ-donor and weaker
π-acceptor property of LϪ relative to terpyridine. This red shift
compared to [Ru(trpy)2]2ϩ is observed to be more prominent in
3 (having a CH2 spacer) than in complex 6 and implies ligand
L6 exerts a greater ligand field strength compared to L3. The
presence of the azo group in the framework of co-ordinated
L6 might be responsible for the observed difference in field
strength.18 The observed difference in ligand field strength
between the two types of ligands (L3 and L6) is also reflected in
the metal redox processes (see later).
The azomethine (CH᎐N) proton of complexes 1 and 3 has
᎐
clearly been observed near δ 9.0 as a singlet (Fig. 1a) which
is considerably deshielded relative to that of the “free” ligands
(δ ≈ 8.4) as a consequence of electron donation to the metal
centre.15 The azomethine proton (CH᎐N) of the “free ligands”
᎐
HL4 and HL6 is found to be absent in the NMR spectra of the
corresponding complexes 4 and 6 (Fig. 1b). Therefore this
absence as well as the absence of the N–H proton in all the three
complexes 4–6 and the simultaneous appearance of the new
C–H proton signal in the upfield region (δ 3.8) strongly support
the internal imine → azo tautomerism (D → E) of the
ligands L4–L6 in these complexes.
The presence of an intimate mixture of the two isomers H
and I for complexes 1, 2, 4 and 5 in a ratio of ≈ 2:1 in solution
has been revealed from the well resolved two distinct upfield
methylene (CH2) proton signals and the direct comparison of
the intensity of the methylene protons with that of the aromatic
protons (Fig. 1).
In dichloromethane solution the complexes exhibit two mod-
erately intense transitions in the visible region and three intense
transitions in the uv region (Table 1). The lowest energy transi-
tion is associated with a shoulder to even further lower energy
(Table 1, Fig. 2). The visible region transitions are assigned to
dπ(RuII) → π*(trpy) MLCT transitions,16 their energy vary-
ing depending on the nature of the ancillary ligands (L). In
addition each complex exhibits an assortment of three intense
Electron-transfer properties
The complexes are electroactive with respect to metal as well as
ligand centres and display the same three responses in the
potential range of 2.0 V versus SCE. The reduction potential
data are listed in Table 2 and the representative voltammograms
are shown in Fig. 3.
Ruthenium(III)-ruthenium(II) couple. In dichloromethane
solution complex
3 displays a reversible ruthenium()–
ruthenium() couple at 0.34 V whereas for 6 the same couple
appears at 0.63 V.14 The one-electron nature of the responses
is established by constant potential coulometry (Table 2). The
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Table 2 Electrochemical data at 298 Ka
EЊ298/V(∆Ep/mV)
νMLCT/cmϪ1
ligand
reduction
Complex
RuIII–RuII
RuIV–RuIII
nb
∆EЊc/V
observedd
calculatede
3
6
0.34 (70)
0.63 (70)
1.71 (120)
1.85f
Ϫ1.32 (80)
Ϫ1.38 (100)
1.04
1.03
1.66
2.01
17152
18867
16388
19210
a Solvent, dichloromethane; supporting electrolyte, [NEt4][ClO4]; reference electrode, SCE; solute concentration, 10Ϫ3 mol dmϪ3; working electrode,
platinum wire. Cyclic voltammetric data; scan rate, 50 mV sϪ1; EЊ298 = 0.5 (Epa ϩ Epc) where Epa and Epc are the anodic and cathodic peak potentials,
b
respectively. n = Q/QЈ, where QЈ is the calculated coulomb count for 1eϪ transfer and Q is the coulomb count found after exhaustive electrolysis
of a ≈10Ϫ2 M solution of the complex. c Calculated by using eqn. (2) of text. d In dichloromethane solution. e Calculated by using eqn. (1) of text.
f Epa value is considered due to the irreversible nature of the voltammogram.
same height as that of the first couple (Fig. 3). The oxidised
species [Ru(trpy)(L)(Cl)]2ϩ is found to be unstable on the coulo-
metric timescale. The second oxidation process can tentatively
be assigned to the RuIII → RuIV oxidation as the “free” lig-
ands (HL) or their sodium salts (NaL) do not show any redox
activity within the experimental potential range ( 2 V). Here
the potential difference between the two successive oxidation
processes is observed to be in the range 1.2–1.4 V which
matches well with the average potential difference (1.1–1.5 V)
between the two successive redox processes of ruthenium
(RuII/III–RuIII/IV) observed in other mononuclear ruthenium
complexes.23 However, the possibility of the oxidation of
co-ordinated ligands cannot be ruled out.
Ligand reduction. The complexes exhibit one reversible
reduction near Ϫ1.3 V (Table 2, Fig. 3). The one-electron
stoichiometry of this process has been verified by differential
pulse voltammetry. Since the “free” ligands (HL or NaL) do
not show any reduction and the reduction of co-ordinated
terpyridine is well documented,24 the observed reduction is
assigned to be that of co-ordinated terpyridine.
Fig. 3 Cyclic voltammograms of ≈ 10Ϫ3 mol dmϪ3 solutions of the
complexes [RuII(trpy)(L2)Cl] 3 (——) and [RuII(trpy)(L6)Cl] 6 (------)
in dichloromethane at 298 K. The reduction of co-ordinated terpyr-
idine and differential pulse voltammograms are shown only for 3.
All our attempts to make suitable single crystals for X-ray
characterisation of the complexes have failed. However, elem-
ental analysis and FAB-mass data are consistent with the gross
composition and IR and NMR data provide strong support
in favour of the internal transformation of the ligand L6 in
complex 6.
presence of trivalent ruthenium in the oxidised solution is
confirmed by the characteristic rhombic EPR spectra of
the ruthenium() congeners (see later).19 Although here the
stability of the ruthenium() state is found to be greater in 6
as compared to 3, the unaltered ligand L6 (having NH spacer)
results in a lower ruthenium()–ruthenium() potential com-
pared to that with L3 in [Ru(bpy)2L] complexes.14 The observed
reverse order of stability of the ruthenium() state in the pres-
Trivalent ruthenium(III) congeners [RuIII(trpy)(L)(Cl)]ϩ 3ϩ and
6ϩ
Coulometric oxidations of the complexes at a potential 150 mV
positive of the corresponding Epa of RuIII–RuII couple in
dichloromethane solution at 298 K result in green oxidised
species 3ϩ and 6ϩ. The observed Coulomb count corresponds to
1eϪ transfer for both complexes (Table 2). The oxidised com-
plexes display voltammograms which are superposable on those
of the corresponding bivalent complexes (3 and 6), implying
that the oxidation here may be stereoretentive in nature. The
trivalent complexes can quantitatively be reduced to the parent
bivalent complexes 3 and 6.
ent set of complexes might be due to the presence of the N᎐N
᎐
group in the modified ligand framework of L6 in 6, as the
π-acidic nature of the azo function helps to stabilise the lower
oxidation state of the metal ion preferentially.20 A potential
shift of >300 mV has been observed while moving from 3 to 6.
The ruthenium()–ruthenium() potential of [Ru(trpy)2]2ϩ
appears at 1.30 V.21 Therefore a decrease in 0.7–1.0 V of this
couple depending on the nature of L has been observed while
moving from [RuII(trpy)2]2ϩ to [RuII(trpy)(L)(Cl)]. The σ-donor
nature of the phenolato group of LϪ provides electrostatic
stabilisation of the RuIII–L species which has originated from
the reduction of the overall charge of ϩ2 in [Ru(trpy)2]2ϩ to 0
in the present complexes, [Ru(trpy)(L)(Cl)].22 The decrease in
potential of the RuIII–RuII couple and its reversible nature
(Fig. 3) facilitates the isolation of the trivalent congeners,
[RuIII(trpy)(L)(Cl)]ϩ with the present ligand set up.
Chemical oxidations of complexes 3 and 6 by aqueous
ammonium cerium() sulfate and ammonium cerium()
sulfate in 0.1 M aqueous HClO4 respectively result in the same
green oxidised complexes (3ϩ and 6ϩ). That incorporating the
ligand of type A, 3ϩ, is stable both in the solid and solution
states whereas the complex having the ligand of type B, 6ϩ, is
found to be reactive in the solution state. The observed stability
order of 3ϩ and 6ϩ in the solution state is in accord with the
RuII → RuIII oxidation potentials of the complexes (Table 2).
The complexes 3ϩ and 6ϩ can be quantitatively reduced back to
the parents 3 and 6 by hydrazine hydrate.
Ruthenium(IV)–ruthenium(III) couple. The complexes display
a second quasi reversible oxidation process near 2 V (Fig. 3).
The observed potentials vary depending on the type of ligands
(A and B) (Table 2). The one-electron nature of the process has
been established with the help of differential pulse voltam-
metry, which shows the second oxidation process to have the
The complexes 3ϩ and 6ϩ have been isolated in the solid state
as their monohydrated perchlorate salts, [RuIII(trpy)(L)(Cl)]-
ClO4ؒH2O. Their microanalytical data match well with the
calculated values. The complexes exhibit 1:1 conductivity in
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Table 3 Selected bond distances (Å) and angles (Њ) and their standard
deviations for [Ru(trpy)(L3)Cl]ClO4ؒ0ؒ5C6H6 3ϩ
Ru–N(1)
Ru–N(2)
Ru–N(3)
Ru–N(4)
2.070(4)
1.974(4)
2.092(4)
2.100(4)
Ru–O(1)
Ru–Cl(1)
C(22)–N(4)
C(10)–O(1)
1.982(3)
2.334(16)
1.289(6)
1.314(5)
N(2)–Ru–O(1)
N(2)–Ru–N(1)
O(1)–Ru–N(1)
N(2)–Ru–N(3)
O(1)–Ru–N(3)
N(1)–Ru–N(3)
N(2)–Ru–N(4)
O(1)–Ru–N(4)
N(1)–Ru–N(4)
88.21(4)
79.20(15)
87.18(14)
79.56(15)
91.34(14)
158.75(15)
175.96(14)
90.88(14)
104.69(14)
N(3)–Ru–N(4)
N(2)–Ru–Cl(1)
O(1)–Ru–Cl(1)
N(1)–Ru–Cl(1)
N(3)–Ru–Cl(1)
N(4)–Ru–Cl(1)
N(4)–C(22)–C(21)
C(10)–O(1)–Ru
C(22)–N(4)–Ru
96.53(14)
87.55(11)
175.19(10)
89.81(11)
90.11(11)
93.50(11)
127.6(4)
127.6(3)
122.1(3)
Fig. 5 Electronic spectrum of the oxidised complex [RuIII(trpy)(L3)-
Cl][ClO4]ؒH2O 3ϩ in acetonitrile.
the first example of structurally characterised mononuclear
ruthenium()–terpyridine complex as well as a ruthenium()–
imine function. The RuIII–O bond distance of 1.982(3) Å in 3ϩ
is significantly shorter than the RuIII–O (phenolato) distance
[2.112(5) Å] observed in the ruthenium()–orthometallated
azophenol system (C–Ru–O in trans configuration)27 but close
to the average RuIII–O(phenolato) distance, 1.981(2) Å, found
in the ruthenium() salicylaldehydato complex.23 The trans
effect of the bound carbon atom was assigned to be the reason
for the unusually long RuIII–O distance in the azophenol com-
plex. Thus this work demonstrates a good measure of a normal
RuIII–O(phenolato) distance. The RuIII–Cl bond distance of
2.334(16) Å in 3ϩ is similar to that reported for other RuIII–Cl
complexes.28,29 The RuIII–N(4) (imine nitrogen) distance in
3ϩ, 2.100(4) Å, merits scrutiny. No information on RuIII–
N(imine) bond lengths is available in the literature, but a case of
RuII–N(imine) has been examined recently.30 The latter distance
is 2.039(4) Å which is significantly shorter than the RuIII–N(4)
distance in 3ϩ. The relatively short Ru–N(2)(trpy) distance trans
to Ru–N(4)(imine) may account for the observed relatively long
RuIII–N(4)(imine) distance. The bond distance of C(22)–N(4)
Fig. 4 An ORTEP25 plot for [RuIII(trpy)(L3)Cl]ClO4 3ϩ. The benzene
and perchlorate anion are removed for clarity.
is 1.289(6) Å, which is normal for a C᎐N double bond.31 The
᎐
Ϫ
ClO4 anion is tetrahedral with an average Cl–O distance of
acetonitrile at 298 K [ΛM/ΩϪ1 cm2 molϪ1 155 for 3ϩ and 148 for
6ϩ]. The presence of perchlorate ion is evidenced by a strong
and broad band near 1100 cmϪ1 and a sharp band near 630
III
1.371(6) Å and an average O–Cl–O angle of 109.35Њ.
The complexes exhibit moderately strong ligand to metal
charge-transfer (LMCT) transitions near 600 and 400 nm and
in the uv region ligand based transitions (Table 1, Fig. 5).28 The
magnetic moment values of the complexes correspond to the
low-spin d5 configuration (µeff/µB; 1.93 for 3ϩ and 1.88 for 6ϩ)
and consequently the complexes display rhombic EPR spectra
in dichloromethane–toluene glass (77 K) with distinct three g
values (Fig. 6, Table 4).
cmϪ1 in the IR spectra of the complexes. The νRu
stretching
-Cl
frequency is observed near 340 cmϪ1 as a sharp singlet.
The molecular structure of complex 3ϩ has been established
by single-crystal X-ray diffraction. The crystal structure is
shown in Fig. 4 and selected bond lengths and angles are listed
in Table 3. The complex is monomeric and the lattice consists
of one type of molecule. The crystal consists of an array
of [Ru(trpy)(L3)Cl]ϩcations, [ClO4]Ϫ anions and benzene of
crystallisation in a 1:1:0.5 stoichiometry. The RuN4OCl co-
ordination sphere is distorted octahedral as can be seen from
the angles subtended at the ruthenium centre and composed of
a meridionally bound trpy ligand, a bidentate L3 ligand and a
chloride. The central pyridine of the trpy ligand is bound trans
to the imine nitrogen (N4) (structure I) with an observed N(4)–
Ru–N(2) angle of 175.96(14)Њ. The other two terminal pyridine
rings of trpy bind to the ruthenium ion at angles N(1)–Ru–N(2)
of 79.20(15) and N(3)–Ru–N(2) of 79.56(15)Њ, which indicates
that due to structural constraint it cannot bind to ruthenium
with an ideal (90Њ) geometry. The Ru–N(2) (central pyridine
nitrogen of trpy) distance [1.974(4) Å] is shorter than the
terminal Ru–N pyridine distances [Ru–N(1), 2.070(4) and Ru–
N(3) 2.092(4) Å] as observed in other ruthenium–terpyridine
complexes.4 The RuIII–N(trpy) bond distances found in this
complex are similar to those reported for another known
RuIII-trpy complex [trpy)(C2O4)RuIIIORuIII(C2O4)(trpy)]ؒ1.8
H2O.26 To the best of our knowledge this work represents
The EPR spectra of the complexes have been analysed in
terms of crystal field g-tensor theory of low-spin d5 ions.28,32
Among the parameters that can be derived from the theory are
the splitting due to axial distortion (∆) which transforms the t2
shell into e ϩ b (tetragonal distortion), the splitting due to
rhombic distortion (V) which splits e further into non-
degenerate components and the energies (ν1 and ν2) of the two
optical transitions among the three Kramers doublets originat-
ing from the application of ∆, V and λ (spin–orbit coupling
constant) on t2 and the orbital reduction factor (k). The good-
ness of the analysis can be assessed by independent observation
of the transitions ν1 and ν2 in optical spectra. The EPR spectra
provide only the absolute g values and so neither their signs nor
the correspondence of g1, g2 and g3 to gx, gy and gz are known.
There are forty eight possible combinations based on the
leveling (x, y, z) and the signs chosen for the experimentally
observed g values. For the present case we have chosen the
combination Ϫg1 > Ϫg2 > g3 as this particular set gives the
acceptable value of k (< 1.0). The value of k for all other com-
binations of g parameters does not fall within the limit of
J. Chem. Soc., Dalton Trans., 2000, 2327–2335
2331

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Table 4 The EPR g valuesa and distortion parametersb
Complex
g1
g2
g3
k
∆/λ
V/λ
ν1/λ
ν2/λ
ν2/λobs
3ϩ
6ϩ
Ϫ2.658
Ϫ2.350
Ϫ2.067
Ϫ2.054
1.691
1.887
0.778
0.639
4.258
6.987
Ϫ4.217
Ϫ6.887
2.392
3.686
6.621
10.586
6.250
9.259
a In dichloromethane at 77 K. b Meanings are given in the text.
Table 5 Emission dataa
λmax/nm
excitation
Quantum
Complex
emission
yieldb (Φ)
3
6
575
515
665
690
0.28 × 10Ϫ1
0.58 × 10Ϫ2
a In ethanol–methanol (4:1 v/v) at 77 K. b Calculated by using eqn. (4)
of text.
∆EЊ = EЊ298(RuIII–RuII) Ϫ EЊ298(L)
(2)
EЊ298(L) are the formal potentials (in V) of the ruthenium()–
ruthenium() couple and the first ligand reduction respectively
and νMLCT is the frequency of the lowest energy MLCT transi-
tion (in cmϪ1). The factor 8065 in eqn. (1) is used to convert
potential difference, ∆E, from volt to cmϪ1 and the term 3000
cmϪ1 is of empirical origin. The calculated and experimentally
observed νMLCT transition frequencies for the complexes are
listed in Table 2. The calculated values lie within 900 cmϪ1 of
the experimentally observed energies, which are in very good
agreement with the previously observed correlation in other
ruthenium complexes.35
Emission spectra
Fig. 6 X-Band EPR spectra and t2 splittings of the complexes [RuIII
-
(trpy)(L3)Cl]ClO4ؒH2O 3ϩ (——) and [RuIII(trpy)(L6)Cl]ClO4ؒH2O 6ϩ
(------) in dichloromethane–toluene glass (77 K).
The complexes display very weak emissions at room temper-
ature, therefore the emission properties of the complexes (3
and 6) have been studied in an optically dilute EtOH–MeOH
(4:1 v/v) rigid glass at 77 K. Excitations of the complexes at the
top of the lowest energy MLCT band exhibit moderately strong
emissions near 700 nm (Table 5, Fig. 2). It is known that in
polypyridine complexes, visible light absorption generally takes
k < 1.0. The orbital reduction factor (k), axial distortion (∆/λ),
rhombic distortion (V/λ), and the two ligand field transitions
(ν1/λ and ν2/λ) for complexes 3ϩ and 6ϩ are listed in Table 4. The
value of the spin–orbit coupling constant (λ) of ruthenium()
is taken as 1000 cmϪ1 33
.
1
place into a MLCT state whereas emission occurs from the
The ν2 band is detected in the expected region of the near-IR
spectra of the complexes (Table 4). In view of the approxim-
ation involved in the theory, the agreement between the experi-
mentally observed and the calculated ν2 value is excellent.
Owing to instrumental wavelength scan limitation (maximum
up to 2200 nm) it has not been possible to check the ν1 band.
The distortion parameters (∆ and V) depicted in Table 4 reveal
the presence of strong rhombic distortion in both complexes
and in fact the rhombic distortion (V) is found to be similar to
the axial distortion (∆) (Table 4, Fig. 6). Therefore the com-
plexes [Ru(trpy)(L)(Cl)]ϩ 1ϩ–6ϩ can be considered as model
ruthenium() complexes possessing a high degree of rhombic
symmetry. The distortion values (∆ and V) are observed to be
reasonably higher for 6ϩ as compared to 3ϩ. The formation of
3MLCT state. Therefore, the observed emissions are assigned to
originate from the MLCT excited sates.36 The origin of the
3
emission spectra is further confirmed by the excitation spectra
of the corresponding solutions.
The quantum yields, Φem, were measured in EtOH–MeOH
(4:1 v/v) rigid glass at 77 K relative to [Ru(bpy)3][PF6]2 for
which Φem = 0.35.37 Quantum yields are calculated using
eqn. (3) as described previously,14,38 where A is the absorbance
Φems = Φemr(Ar/As)(Is/Ir)(ns/nr)2
(3)
at the excitation wave length, I the integrated area of the
emission band and n the refractive index of the solvent for the
sample (subscript s) and the reference (subscript r), respectively.
For the calculation of the quantum yield of the complexes, the
excitation wavelengths are chosen such that standard reference
and sample absorptions are equal. Since As and Ar are equal
and the refractive indices are assumed to be similar, eqn. (3) can
be modified to (4).
the N᎐N group at the expense of the imine function of L6 in
᎐
complex 6ϩ destroys the π conjugation of the chelate ring and
this might have contributed to the observed higher degree of
molecular distortion.
Spectroelectrochemical correlation
Φems = Φemr(Is/Ir)
(4)
The lowest energy MLCT transition involves excitation of the
filled t2 electron of ruthenium() to the lowest π* orbital of the
terpyridine ligand. The energy of the MLCT transition can be
predicted with the help of observed electrochemical data by
considering eqns. (1) and (2).34 Here, EЊ298(RuIII–RuII) and
The calculated quantum yields for the complexes are listed in
Table 5. The complex incorporating the ligand of type A (3) is
found to be the better emitter as compared to that having the
ligand of type B (6). The presence of additional π conjugation
νMLCT = 8065(∆EЊ) ϩ 3000
(1)
in the chelate ring of L in complex 3 (C᎐CC᎐N) [which is
᎐ ᎐
2332
J. Chem. Soc., Dalton Trans., 2000, 2327–2335

View Article Online
missing in 6 (C᎐CCN) due to internal transformation] might be
one of the important contributing factors for the observed
trend.
PAR vibrating sample magnetometer. NMR spectra were
᎐
obtained with a 300 MHz Varian FT spectrometer. 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 electrolyte was [NEt4][ClO4] and the solute con-
centration was 10Ϫ3 M. The half-wave potential EЊ298 was set
equal to 0.5 (Epa ϩ Epc), where Epa and Epc are the 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 dinitro-
gen atmosphere and uncorrected for junction potentials. The
elemental analyses were carried out with a Carlo Erba (Italy)
elemental analyser. The EPR measurements were made with a
Varian model 109C E-line X-band spectrometer fitted with a
quartz Dewar for measurements at 77 K (liquid nitrogen). The
spectra were calibrated by using tetracyanoethylene (tcne)
(g = 2.0037). The FAB mass spectra at 298 K were recorded on
a JEOL SX 102/DA-6000 mass spectrometer.
Chloride exchange reaction
The reaction of complexes [RuII(trpy)(L)Cl], with aqueous
AgClO4 has been studied in order to prepare the aqua species
[RuII(trpy)(L)(H2O)]ϩ via the chloride exchange route. However,
instead of forming the expected stable ruthenium() aqua
species the reaction leads to dimeric species of the type
[(trpy)(L)RuIII(µ-O)RuIII(L)(trpy)]2ϩ with concomitant metal
oxidation. The reaction here possibly proceeds through eqns. (5)
[(trpy)(L)RuIICl] ϩ Agϩ →
[(trpy)(L)RuII(H2O)]ϩ ϩ AgCl (5)
and (6). The source of oxidative equivalents in reaction (6) may
[(trpy)(L)RuII(H2O)]ϩ Ϫ 2eϪ →
[(trpy)(L)RuIII(µ-O)RuIII(L)(trpy)]2ϩ (6)
be either O2 or ClO4Ϫ. Further investigations with the µ-oxo
dimeric species are in progress.
Treatment of EPR data
An outline of the procedure can be found in our recent publi-
cations.40 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 the solution as unacceptable.
Conclusion
We have observed the phenolato imine functions (LϪ) incorpor-
ating CH2 and NH spacers as a co-ligand in the ruthenium
monoterpyridine [Ru(trpy)] core with respect to redox and
spectroscopic aspects. The imine functions of type B are found
to undergo internal imine → azo tautomerism. The presence
of L (both A and B) facilitates the successive reversible
ruthnium()–ruthenium() and ruthenium()–ruthenium()
processes and makes the complex environment susceptible to
moderately strong emissions from the lowest energy MLCT
transitions. The complexes possessing ligands of type B exhibit
higher ligand field strength which has been reflected well in the
metal redox processes as well as in the electronic transitions. In
general a complex having a type A ligand (3) is more effective
from the emission point of view compared to the other type of
complex (6). The distortion parameters (∆ and V) of the tri-
valent complexes (3ϩ and 6ϩ) reveal the presence of a high
degree of rhombicity in the complexes. Preliminary study indi-
cates that in the presence of aqueous Agϩ ion the complexes
undergo aquation followed by dimerisation with concomitant
metal oxidation.
Preparation of ligands and complexes
The ligands HL1–3 and HL4–6 were prepared by condensing
benzylamine and phenylhydrazine respectively with the appro-
priate 2-hydroxyaldehydes and ketone at 273 K in methanol
solvent. The recrystallised ligands were characterised by C, H,
N analyses and NMR. The synthetic details are mentioned for
one particular ligand (HL2). Yields: 80–85%.
C15H15NO, HL2. 2-Hydroxy acetophenone (2 g, 0.014 mol)
was taken in 15 ml absolute ethanol and cooled at 273 K.
Precooled benzylamine (1.5 g, 0.014 mol) was added dropwise
to the above solution and the mixture stirred at 273 K for 1 h.
The yellow precipitate thus formed was filtered off and washed
thoroughly by ice-cold ethanol and dried in vacuum over P4O10.
The product was finally recrystallised from hot ethanol. Yield:
2.64 g, 80%.
The complexes 1–6 were synthesized by following a general
procedure. Details are given for one representative (1). Yields
vary in the range 60–65%.
Experimental
Materials
[RuII(trpy)(L1)Cl] 1. A 200 mg (0.454 mmol) quantity of
[Ru(trpy)Cl3] was taken in 30 ml methanol and heated at reflux
for 5 min. The ligand HL1 (193 mg, 0.91 mmol) and NaO-
CH3ؒ3H2O (0.136 mg, 1.0 mmol) were added to the hot solution
and the mixture was heated to reflux for 6 h. The initial light
brown colour gradually turned to violet. The solvent was
removed under reduced pressure. The dried product was puri-
fied by using a silica gel column. With benzene (as eluent) a
light yellow solution due to excess of ligand was separated first
and rejected. Using dichloromethane–acetonitrile (5:1) as elu-
ent a violet band was separated, collected. Evaporation of the
solvent under reduced pressure afforded a pure solid product
which was recrystallised from dichloromethane–light petroleum
(bp 60–80 ЊC) (1:6). Yield: 164 mg (62%).
The solid complexes 3 and 6 were directly separated from
the reaction mixture. The precipitates thus formed were filtered
off and washed thoroughly with methanol followed by distilled
water. The solid complexes were dried in vacuo over P4O10.
Finally the products were recrystallised from dichloromethane–
light petroleum (1:6).
Commercial ruthenium trichloride (S.D. Fine Chemicals,
Bombay, India) was converted into RuCl3ؒ3H2O by repeated
evaporation to dryness with concentrated hydrochloric acid.
The complex [Ru(trpy)Cl3] was prepared according to the
reported procedure.7 2,2Ј:6Ј,2Љ-Terpyridine, benzylamine and
2-hydroxy-5-nitrobenzaldehyde were obtained from Aldrich,
USA. Other chemicals and solvents were reagent grade and
used as received. Silica gel (60–120 mesh) used for chrom-
atography was of BDH quality. For spectroscopic and electro-
chemical studies HPLC grade solvents were used. Commercial
tetraethylammonium bromide was converted into pure tetra-
ethylammonium perchlorate by an available procedure.39
Physical measurements
UV-visible spectra were recorded by using a Shimadzu-2100
spectrophotometer, FT-IR spectra on a Nicolet spectro-
photometer with samples prepared as KBr pellets. Solution
electrical conductivity was checked using a Systronic 305 con-
ductivity bridge. Magnetic susceptibility was checked with a
J. Chem. Soc., Dalton Trans., 2000, 2327–2335
2333

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Table 6 Crystallographic data for [RuIII(trpy)(L3)Cl]ClO4ؒ0.5C6H6 3ϩ
Regional Sophisticated Instrumental Center, RSIC, Indian
Institute of Technology, Bombay for providing NMR and EPR
facilities.
Formula
M
C32H22Cl2N5O7Ru
763.54
Crystal symmetry
Space group
a/Å
b/Å
c/Å
α/Њ
β/Њ
γ/Њ
Triclinic
P1
References
8.693(3)
13.513(7)
14.137(6)
99.88(4)
91.72(4)
102.42(3)
293(2)
1593.9(12)
2
0.046
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0.112
5596/5596
Reflections collected/unique
[R(int) = 0.0000]
Complexes [RuIII(trpy)(L3)Cl]ClO4ؒH2O 3؉ and [RuIII (trpy)-
(L6)Cl]ClO4ؒH2O 6؉. The oxidised complexes [RuIII(trpy)-
(L3)Cl]ClO4ؒH2O 3ϩ and [RuIII(trpy)(L6)Cl]ClO4ؒH2O 6ϩ were
prepared via the chemical oxidations of bivalent congeners 3
and 6 using aqueous ammmonium cerium() sulfate and
ammonium cerium() sulfate in 0.1 M HClO4 respectively.
Details are given for 3ϩ.
Complex 3 (100 mg, 0.16 mmol) was dissolved in 1:10
dichloromethane–acetonitrile (20 ml) and stirred magnetically
at room temperature (298 K). To this was added dropwise
10 ml of aqueous (NH4)4Ce(SO4)4ؒ2H2O (203 mg, 0.32 mmol)
solution. The violet solution gradually changed to green. The
stirring was continued for 1 h. The solution was filtered
through a fine frit, the volume reduced and a saturated
aqueous solution of sodium perchlorate was added. The dark
coloured solid mass thus obtained was filtered off, washed
with ice-cold water, dried in vacuo over P4O10 and subjected
to chromatography on a silica gel column. A green band
corresponding to 3ϩ was eluted with 1:1 dichloromethane–
acetonitrile. On evaporation the solid complex (3ϩ) was
obtained in 90% yield.
Crystallography
Single crystals of complex 3ϩ were grown by slow diffusion
of an acetonitrile solution of it in benzene followed by slow
evaporation. Significant crystal data and data collection
parameters are listed in Table 6. Absorption correction was
done by ψ-scan measurement.41 The data reduction was done
by using MAXUS and structure solution and refinement used
the programs SHELXS 97 and SHELXL 97 respectively.42
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 F 2. All non-hydrogen atoms were refined aniso-
tropically. Hydrogen atoms were included in calculated posi-
tions. The difference map revealed the presence of one-half
molecule of lattice benzene solvent with one of the carbon
atoms occupying a special position. This benzene molecule
was refined isotropically with the aid of rigid group refine-
ment using SHELXL 97.
12 B. K. Santra, G. A. Thakur, P. Ghosh, A. Pramanik and G. K.
Lahiri, Inorg. Chem., 1996, 35, 3050.
13 R. Samanta, P. Munshi, B. K. Santra, N. K. Lokanath, M. A.
Sridhar, J. S. Prasad and G. K. Lahiri, J. Organomet. Chem., 1999,
581, 311.
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1387.
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1437.
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McCleverty and M. D. Ward, J. Chem. Soc., Dalton Trans., 1996,
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17 C. R. Hecker, A. K. I. Gushurst and R. D. McMillin, Inorg. Chem.,
1991, 30, 538.
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139.
19 B. M. Holligan, J. C. Jeffery, M. K. Norgett, E. Schatz and
M. D. Ward, J. Chem. Soc., Dalton Trans., 1992, 3345; R. Hariram,
B. K. Santra and G. K. Lahiri, J. Organomet. Chem., 1997, 540,
155.
CCDC reference number 186/1987.
See http://www.rsc.org/suppdata/dt/b0/b000257g/ for crystal-
lographic files in .cif format.
Acknowledgements
20 A. Bharath, B. K. Santra, P. Munshi and G. K. Lahiri, J. Chem.
Soc., Dalton Trans., 1998, 2643.
Financial support received from the Department of Science
and Technology, New Delhi, India, is gratefully acknowledged.
The X-ray structural study was carried out at the National
Single Crystal Diffractometer Facilities, Indian Institute of
Technology, Bombay. Special acknowledgement is made to
21 R. P. Thummel and S. Chirayil, Inorg. Chim. Acta, 1988, 154,
77.
22 B. K. Santra and G. K. Lahiri, J. Chem. Soc., Dalton Trans., 1997,
129.
2334
J. Chem. Soc., Dalton Trans., 2000, 2327–2335

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23 N. Bag, G. K. Lahiri, S. Bhattacharya, L. R. Falvello and A.
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