Amino acid complexes of ruthenium: Synthesis, characterization and cyclic voltammetric studies

Article abstract of DOI:10.1016/S0277-5387(99)00302-2

Reaction of α-amino acids (HL) with [Ru(PPh₃)₃Cl₂] in the presence of a base afforded a family of complexes of type [Ru(PPh₃)₂(L)₂]. These complexes are diamagnetic (low-spin d⁶, S=0) and show ligand-field transitions in the visible region. ¹H and ³¹P NMR spectra of the complexes indicate the presence of C₂ symmetry. Cyclic voltammetry on the [Ru(PPh₃)₂(L)₂] complexes show a reversible ruthenium(II)-ruthenium(III) oxidation in the range 0.30-0.42 V vs. SCE. An irreversible ruthenium(III)-ruthenium(IV) oxidation is also displayed by two complexes near 1.5 V vs. SCE. Elsevier Science Ltd.

Full text of DOI:10.1016/S0277-5387(99)00302-2

Polyhedron 18 (1999) 3669–3673
www.elsevier.nl/locate/poly
Amino acid complexes of ruthenium: synthesis, characterization and cyclic
voltammetric studies
*
Kanchana Majumder, Samaresh Bhattacharya
Department of Chemistry, Inorganic Chemistry Section, Jadavpur University, Calcutta 700032, India
Received 21 July 1999; accepted 21 September 1999
Abstract
Reaction of a-amino acids (HL) with [Ru(PPh3)3Cl2] in the presence of a base afforded a family of complexes of type
[Ru(PPh3)2(L)2]. These complexes are diamagnetic (low-spin d6, S50) and show ligand-field transitions in the visible region. 1H and 31P
NMR spectra of the complexes indicate the presence of C2 symmetry. Cyclic voltammetry on the [Ru(PPh3)2(L)2] complexes show a
reversible ruthenium(II)–ruthenium(III) oxidation in the range 0.30–0.42 V vs. SCE. An irreversible ruthenium(III)–ruthenium(IV)
oxidation is also displayed by two complexes near 1.5 V vs. SCE.
1999 Elsevier Science Ltd. All rights reserved.
Keywords: Ruthenium; a-Amino acids; Synthesis; Characterization; Electrochemistry
1. Introduction
bidentate N,O-donor forming five-membered chelate rings
(2) [4,16]. A family of bis a-amino acid complexes of
ruthenium(II) has been synthesized, where triphenylphos-
phine has served as the coligand. The synthesis, characteri-
zation and electrochemical properties of complexes of type
[Ru(PPh3)2(L)2] are described in this paper.
Amino acids, being the building blocks of protein, are
an essential component of all living matter. Therefore, to
investigate the interaction between metals and proteins, the
necessary prerequisite is to study the interaction between
amino acids and different metals. The chemistry of transi-
tion metal complexes of amino acids has been of signifi-
cant interest in this respect [1–5]. Synthesis of different
types of complexes using various amino acids is becoming
increasingly important primarily because of the various
applications of such complexes in biological fields [6–8].
With this background we set out to explore the chemistry
of amino acid complexes of ruthenium. Among many
transition metals ruthenium has been chosen because of its
efficiency in electron-transfer reactions and particularly
because of its remarkable role in DNA intercalation
reactions [9,10]. It may be mentioned here that ruthenium
chemistry of amino acids appears to have received very
little attention [11–15]. For the present study we have
chosen a-amino acids (1), abbreviated in general as HL
where H stands for the dissociable carboxylic proton, as
the principal ligand. The a-amino acids are known to bind
to metal ions, via dissociation of the acidic proton, as
2. Experimental
2.1. Materials
Commercial ruthenium trichloride (Arora Matthey, Cal-
cutta, India) was converted to RuCl3?3H2O by repeated
evaporation with concentrated hydrochloric acid. Tri-
phenylphosphine (PPh3) was purchased from SD Fine
Chemicals, Mumbai, India. [Ru(PPh3)3Cl2] was synthes-
ized, starting from RuCl3?3H2O, by following a reported
procedure [17]. The a-amino acids were purchased from
Loba Chemie, India. HPLC-grade dimethylsulfoxide for
electrochemical work was purchased from Spectrochem,
*Corresponding author. Tel.: 191-33-483-8393.
E-mail address: icsg@mahendra.iacs.res.in (S. Bhattacharya)
0277-5387/99/$ – see front matter
PII: S0277-5387(99)00302-2
1999 Elsevier Science Ltd. All rights reserved.

3670
K. Majumder, S. Bhattacharya / Polyhedron 18 (1999) 3669–3673
Mumbai, India. Purification of dichloromethane and prepa-
ration of tetrabutylammonium perchlorate (TBAP) for
electrochemical work and was performed as reported in the
literature [18,19]. All other chemicals and solvents were
commercial reagent-grade and were used as received.
Perkin-Elmer 240C elemental analyzer. IR spectra were
obtained on a Perkin-Elmer 783 spectrometer with samples
prepared as KBr pellets. NMR spectra were recorded on a
Brucker DRX-500 NMR spectrometer. Electronic spectra
were recorded on a Shimadzu UV 240 spectrophotometer.
Magnetic susceptibilities were measured using a PAR 155
vibrating sample magnetometer fitted with a Walker sci-
entific L75FBAL magnet. Electrochemical measurements
were made using a PAR model 273 potentiostat. A
platinum-disc (area 0.04 cm2) working electrode, a
platinum wire auxiliary electrode and an aqueous saturated
calomel reference electrode (SCE) were used in a three-
electrode configuration. A RE 0089 X–Y recorder was
used to trace the voltammograms. Electrochemical mea-
surements were made under a dinitrogen atmosphere. All
electrochemical data were collected at 298 K and are
uncorrected for junction potentials.
2.2. Preparations
2.2.1. [Ru(PPh3 )2(L1)2 ]
To a solution of glycine (HL1) (17 mg, 0.22 mmol) in
warm ethanol (30 cm3) were added [Ru(PPh3)3Cl2] (100
mg, 0.10 mmol) and triethylamine (0.03 cm3, 0.22 mmol)
with constant stirring. Stirring was continued for 1.5 h
keeping the solution warm (|408C). Within 30 min, a
yellow microcrystalline compound began to separate out.
The reaction mixture was then allowed to settle at room
temperature (|258C). The precipitated solid was collected
by filtration, washed thoroughly with water followed by
hexane and acetonitrile and dried in vacuo over P4O10. The
yield was 66% (53 mg).
3. Results and discussion
2.2.2. [Ru(PPh3 )2(L2)2 ]
Five different amino acids have been used in the present
work. The ligands and their respective abbreviations are
shown in 3. Reaction of these ligands with [Ru(PPh3)3Cl2]
in the presence of a base afforded the desired complexes in
decent yields. [Ru(PPh3)3Cl2], our starting material, has
served as a useful source of ruthenium(II) because of the
considerable substitutional lability of the chlorides and one
PPh3 ligand. The composition of these complexes have
been verified by their elemental (C, H, N) analytical data
(Table 1). All the [Ru(PPh3)2(L)2] complexes are diamag-
netic which corresponds to the bivalent state of ruthenium
(low-spin d6, S50) in these complexes.
This complex was synthesized following the above
procedure using alanine (HL2) instead of glycine (HL1),
methanol instead of ethanol and sodium hydroxide instead
of triethylamine. The yield was 62% (52 mg).
2.2.3. [Ru(PPh3 )2(L3)2 ]
This complex was synthesized following the same
procedure for [Ru(PPh3)2(L1)2] using phenylalanine
(HL3) instead of glycine (HL1). The yield was 64% (64
mg).
2.2.4. [Ru(PPh3 )2(L4)2 ]
To a solution of tyrosine (HL4) (40 mg, 0.22 mmol) in
warm ethanol (30 cm3) were added [Ru(PPh3)3Cl2] (100
mg, 0.10 mmol) and triethylamine (0.03 cm3, 0.22 mmol)
with constant stirring. Stirring was continued for 1.5 h
keeping the solution warm (|408C). A greenish-yellow
solution was obtained, which upon evaporation afforded a
solid. This was washed with water followed by hexane and
dried in vacuo over P4O10. Purification was achieved by
chromatography through a silica gel (60–120 mesh) col-
umn. Using 1:1 benzene–acetonitrile as the eluent, a deep
yellow band came out which was collected and evapora-
tion of the eluate gave [Ru(PPh3)2(L4)2] as a yellow solid.
The yield was 60% (62 mg).
As a-amino acids are unsymmetrical bidentate ligands,
the [Ru(PPh3)2(L)2] complexes may exist in five different
geometric isomeric forms (4–8). Complexes of ruthenium
having the Ru(PPh3)2 moiety are known to prefer the two
PPh3 ligands in the cis position for better dp–dp inter-
action [20,21]. Assuming a similar cis disposition of the
triphenylphosphines in these [Ru(PPh3)2(L)2] complexes,
the number of possible stereoisomers reduces to three
(4–6). Out of these three isomers, two (4 and 5) have a C2
2.2.5. [Ru(PPh3 )2(L5)2 ]
This complex was synthesized following the same
procedure for [Ru(PPh3)2(L4)2] using leucine (HL5) in-
stead of tyrosine (HL4). The yield was 61% (56 mg).
2.3. Physical measurements
Microanalyses (C, H, N) were performed using a

K. Majumder, S. Bhattacharya / Polyhedron 18 (1999) 3669–3673
3671
Table 1
Characterization data of the [Ru(PPh3)2(L)2] complexes
Compound
Microanalytical dataa
Electronic spectral
datab
lmax (nm) [´ (M21 cm21)]
Cyclic voltammetric
datac
%C
%H
%N
E1 / 2 (V) (DEp (mV)]
[Ru(PPh3)2(L1)2]
[Ru(PPh3)2(L2)2]
62.06
(62.09)
4.94
(4.91)
3.63
(3.62)
524 (359), 424 (671),
340d (1661), 292d (3767),
268 (5712)
524 (257), 420 (485),
344d (1149), 296d (2234),
260 (3828)
0.33 (60)
0.30 (60)
0.37 (60)
62.98
(62.92)
5.20
(5.24)
3.50
(3.49)
[Ru(PPh3)2(L3)2]
[Ru(PPh3)2(L4)2]
[Ru(PPh3)2(L5)2]
67.86
(67.90)
65.83
(65.78)
65.11
5.27
(5.24)
5.10
(5.07)
6.10
2.88
(2.93)
2.86
(2.84)
3.17
420 (286), 344d (1072),
300d (2287), 270 (5289)
420 (563), 344d (1408),
302d (5582), 272 (8989)
420 (411), 344d (1643),
304d (3481), 276 (7744)
0.42 (60)e,
1.56e,f
0.39 (60)e,
1.48e,f
(65.08)
(6.10)
(3.16)
a Calculated values are within parentheses.
b In dimethylsulfoxide solution.
c Solvent, dimethylsulfoxide; supporting electrolyte, TBAP; reference electrode, SCE; E1 / 2 50.5(Epa 1Epc), where Epa and Epc are anodic and cathodic
peak potentials, respectively; DEp 5Epa 2Epc; scan rate 50 mV s21
.
d Shoulder.
e In dichloromethane solution.
f Epa value.
1
axis while the other (6) does not have C2 symmetry. H
NMR spectra of the [Ru(PPh3)2(L)2] complexes are a little
complicated. However, 31P NMR spectra of these complex-
es show only one sharp resonance near 47 ppm which
clearly indicates the presence of a C2 axis. Therefore, the
[Ru(PPh3)2(L)2] complexes have structure 4 or 5.
qualitatively similar, each complex showing several vi-
brations of different intensities below 1700 cm21. All the
[Ru(PPh3)2(L)2] complexes show strong vibrations near
530, 700 and 750 cm21 which are also displayed by
[Ru(PPh3)3Cl2]. Hence these vibrations are attributed to
the Ru(PPh3)2 moiety of [Ru(PPh3)2(L)2]. In the spectra
of all the [Ru(PPh3)2(L)2] complexes a broad and very
strong vibration is observed near 1610 cm21 and a sharp
and strong vibration near 1380 cm21, which are absent in
the spectrum of [Ru(PPh3)3Cl2] and these are assigned
[22] to the nas(CO) stretching and ns(CO) stretching of the
coordinated carboxylate groups. The distinct peaks near
3300 cm21 are attributed to the N–H stretching vibrations.
The infrared spectral data are therefore in accordance with
the composition of the complexes.
The
[Ru(PPh3)2(L1)2],
[Ru(PPh3)2(L2)2]
and
[Ru(PPh3)2(L3)2] complexes are soluble only in di-
methylsulfoxide and dimethylformamide, while the other
two
complexes,
viz.
[Ru(PPh3)2(L4)2]
and
[Ru(PPh3)2(L5)2], are soluble in dichloromethane, acetoni-
trile and chloroform as well as in dimethylsulfoxide and
dimethylformamide. All the solutions are yellow in colour.
Electronic spectra of these complexes have been recorded
in dimethylsulfoxide solution. Spectral data are presented
in Table 1 and selected spectra are shown in Fig. 1. Each
complex shows absorptions in the visible and ultraviolet
region. The intense absorptions in the ultraviolet region are
assignable to transitions within the ligand orbitals. The
absorptions in the visible region are rather weak and are
probably due to ligand-field transitions [23,24].
Further distinguishing between these two structures could
not be done from the NMR spectral results. However, all
the complexes are assumed to have structure 4, where the
nitrogens of the amino acid ligands are trans and the less
crowded carboxylate oxygens are cis and hence this
geometry is expected to be sterically more favorable.
IR spectra of all the [Ru(PPh3)2(L)2] complexes are
Electron transfer properties of the [Ru(PPh3)2(L)2]
complexes have been studied by cyclic voltammetry in
dimethylsulfoxide (for L5L1, L2 and L3) and dichlorome-

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K. Majumder, S. Bhattacharya / Polyhedron 18 (1999) 3669–3673
This oxidative response is assigned to the ruthenium(II)–
ruthenium(III) oxidation. The oxidation is reversible with a
peak-to-peak separation (DEp) of |60 mV which does not
vary with variation in scan rates and the anodic peak
current (ipa) is almost equal to the cathodic peak current
(ipc). The one-electron nature of this oxidation was estab-
lished by comparing its current height with that of the
standard ferrocene/ferrocenium couple under identical
experimental conditions. The magnitudes of the oxidation
potential indicate that the bivalent state of the ruthenium is
comfortable in this N2O2P2 coordination sphere. A second
irreversible oxidation is observed in [Ru(PPh3)2(L4)2] and
[Ru(PPh3)2(L5)2] near 1.5 V which is tentatively assigned
to the ruthenium(III)–ruthenium(IV) oxidation. The one-
electron nature of this oxidation is confirmed by comparing
its current height (ipa) with that of the respective
ruthenium(II)–ruthenium(III) oxidation. This oxidation has
not been observed in the first three [Ru(PPh3)2(L)2] (L5
L1, L2 and L3) complexes because of the smaller voltage
window offered by dimethylsulfoxide.
Fig. 1. Electronic spectra of [Ru(PPh3)2(L1)2] (——) and
[Ru(PPh3)2(L4)2] (- - - - -) in dimethylsulfoxide solution.
4. Conclusions
thane (for L5L4 and L5) solutions. Voltammetric data are
given in Table 1 and a selected voltammogram is shown in
Fig. 2. Each complex shows an oxidative response on the
positive side of SCE in the range 0.30–0.42 V vs. SCE.
The present study shows that a-amino acids can form
stable complexes with ruthenium, and in the presence of
p-acid ligands such as PPh3 the 12 state of the metal
becomes stabilized. Replacement of the PPh3 ligands by
hard donors is expected to favour the higher oxidation
states of the metal and this work is currently in progress.
Acknowledgements
Financial assistance received from the Council of Sci-
entific and Industrial Research, New Delhi (Grant No.
01(1408)/96/EMR-II] is gratefully acknowledged. The
authors thank the Third World Academy of Sciences for
financial support for the purchase of an electrochemical
cell system and the Bose Institute, Calcutta 700054, for
NMR spectral measurements.
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