Ruthenium(III)-aminopolycarboxylato complexes active for the reduction of the N-N bond of hydrazine and phenylhydrazine in aqueous acidic media

Article abstract of DOI:10.1039/a908338c

Interactions of hydrazines N₂H₄X⁺ (X = H or Ph) with tri-, tetra- and penta-chelated ruthenium(III)-aminopolycarboxylic acid complexes giving the respective monomeric hydrazinium (Ru^III-N₂H₄X⁺) adducts have been investigated by potentiometry, spectrophotometry and voltammetry in aqueous acidic solution at 25°C. The deprotonation and metal hydrolysis constants of the complexes and their N₂H₄X⁺ adducts in 0.1 M Na₂SO₄ solution were determined. At pH 2.8, the complexes exhibited a quasi-reversible one-electron reduction wave of Ru^III→ Ru^II in sampled dc in the potential range between -0.16 and -0.37 V vs. SCE, while their hydrazinium adducts obtained in situ by adding an excess of N₂H₄X⁺ showed an additional two-electron reduction wave assigned to Ru^III-N₂H₄X⁺ → Ru^I-N₂H₄X⁺ in the potential range of -0.02 to -0.35 V vs. SCE. The species Ru^I-N₂H₄X⁺ on successive decomposition and hydrolysis give one mole of each of NH₃, NH₂X and ruthenium(III) species. Further, the Ru^III-N₂H₄X⁺ complexes have been used as electro-catalysts for the reduction of N₂H₄X⁺ to NH₃ and NH₂X at a mercury pool cathode in acidic solutions of pH 1.9 and 2.8. The quantity of ammonia produced in all cases is linear with time. The E_1/2 of Ru^III-N₂H₄X⁺ → Ru^I-N₂H₄X⁺ and the turnover number are correlated with the sigma basicity (∑pK_a) of the aminopolycarboxylic acids and the results are discussed in terms of the hydrolytic tendency of the metal, the number of co-ordinating groups and the steric repulsion caused by the increase in size of the aminopolycarboxylic acid. The Royal Society of Chemistry 2000.

Full text of DOI:10.1039/a908338c

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Ruthenium(III)–aminopolycarboxylato complexes active for the
reduction of the N–N bond of hydrazine and phenylhydrazine in
aqueous acidic media
Raju Prakash and Gadde Ramachandraiah*
Discipline of Reactive Polymers, Central Salt and Marine Chemicals Research Institute,
Gijubhai Badheka Marg, Bhavnagar 364 002, India
Received 4th October 1999, Accepted 29th October 1999
Interactions of hydrazines N2H4Xϩ (X = H or Ph) with tri-, tetra- and penta-chelated ruthenium()–
aminopolycarboxylic acid complexes giving the respective monomeric hydrazinium (RuIII–N2H4Xϩ) adducts have
been investigated by potentiometry, spectrophotometry and voltammetry in aqueous acidic solution at 25 ЊC.
The deprotonation and metal hydrolysis constants of the complexes and their N2H4Xϩ adducts in 0.1 M Na2SO4
solution were determined. At pH 2.8, the complexes exhibited a quasi-reversible one-electron reduction wave of
RuIII → RuII in sampled dc in the potential range between Ϫ0.16 and Ϫ0.37 V vs. SCE, while their hydrazinium
adducts obtained in situ by adding an excess of N2H4Xϩ showed an additional two-electron reduction wave assigned
to RuIII–N2H4Xϩ → RuI–N2H4Xϩ in the potential range of Ϫ0.02 to Ϫ0.35 V vs. SCE. The species RuI–N2H4Xϩ on
successive decomposition and hydrolysis give one mole of each of NH3, NH2X and ruthenium() species. Further,
the RuIII–N2H4Xϩ complexes have been used as electro-catalysts for the reduction of N2H4Xϩ to NH3 and NH2X
at a mercury pool cathode in acidic solutions of pH 1.9 and 2.8. The quantity of ammonia produced in all cases is
linear with time. The E1/2 of RuIII–N2H4Xϩ → RuI–N2H4Xϩ and the turnover number are correlated with the
sigma basicity (ΣpKa) of the aminopolycarboxylic acids and the results are discussed in terms of the hydrolytic
tendency of the metal, the number of co-ordinating groups and the steric repulsion caused by the increase in
size of the aminopolycarboxylic acid.
the reduction of N2H4 to NH3 with good turnovers by CoCp2
Introduction
and 2,6-dimethylpyridine hydrochloride.13,19 Moreover, the
N2H4-bridged double cubane [{MoFe3S4Cl3(Cl4C6O2)}2-
(µ-NH2NH2)] was isolated and its catalytic activity studied.20
However, the parallel evolution of hydrogen gas which reduces
the yield of NH3 was reported as the major limiting step in
almost all the above systems.
Studies on the reactivity of hydrazine at the potential co-
ordination site of an active metal centre in the absence and
the presence of reducing agents have immense value, since the
reduction of dinitrogen to ammonia catalysed by transition
metals present in microorganisms1–7 or simple metal complexes
or clusters has long been proposed to proceed via metal bound
hydrazine. In the recent past intensive research has been carried
out to evolve a simple mechanism for dinitrogen fixation. In
this respect, numerous transition metals, cofactors and model
compounds containing hydrazine were synthesized and charac-
terized.8–10 Structural analyses of a few of these complexes have
also been made.8,11 Meanwhile, reduction studies were also
performed on hydrazine or its derivatives to ammonia and/or
amine in both aqueous and non-aqueous solutions either in the
absence or in the presence of external proton and electron
sources or reducing agents.12–14 Although, fairly good turn-
overs of ammonia have reportedly been obtained in organic
solvents, most of these reactions did not meet much success in
aqueous solutions. However, Schrauzer et al.15 reported the
reduction of hydrazine to ammonia using K2[Mo(O)(H2O)(CN)4]
as a catalyst and NaBH4 as reducing agent with a turnover
rate of 4.2 in aqueous solutions. Later catalytic conversion of
hydrazine into ammonia was obtained by [Mo2Fe6S8L9]3Ϫ or
Recently, we have reported the electrochemical reduction of
hydrazine and its phenyl derivative to NH3 and/or NH2X at
high turnover rates and high coulombic efficiency21–24 without
the liberation of hydrogen gas using terminally co-ordinated
RuIII–edta–N2H4Xϩ complexes as electro-catalysts in aqueous
acidic solutions. In continuation, herein we describe the
interaction of N2H4Xϩ (X = H or Ph) with tri-, tetra- and
penta-chelated aminopolycarboxylates of ruthenium(),
viz. K2[Ru(imda)Cl3]ؒ2H2O 1, K2[Ru(himda)Cl2]ؒH2O 2, K2-
[Ru(nta)Cl2]ؒH2O 3, K[Ru(hedta)Cl] 4, K[Ru(Hedta)Cl]ؒ2H2O
5, K[Ru(Hpdta)Cl]ؒ2H2O 6, K[Ru(Hcdta)Cl]ؒH2O
7 and
K[Ru(H2dtpa)Cl] 8 by potentiometry, spectrophotometry and
voltammetry in aqueous solution. The function of the resulting
hydrazinium complexes as catalysts for the reduction of hydra-
zine and its phenyl derivative electrochemically in acidic solu-
tion is elucidated. The effects of the sigma basicity (ΣpKa) of
the aminopolycarboxylic acid in complexes 1–8 on the half-
wave potential (E1/2) of the two-electron reduction wave of
ϩ
[Fe4S4L4]2Ϫ (L = SPh or SCH2CH2OH) with
a
moderate
RuIII–N2H5 complexes and the turnover number of NH3 pro-
turnover number.16 In another report,17 [WCp*Me3(η2-
NH2NH2)][O3SCF3] prepared from [WCp*Me3(O3SCF3)] and
N2H4 was shown to undergo reduction by Na–Hg to give NH3.
Recently, the co-ordinatively unsaturated diruthenium complex
[Cp*Ru(µ-SPri)2RuCp*] has been reported to catalyse the
reduction of hydrazine more effectively.18 More sophisticated
systems [MoFe3S4Cl3(Cl4C6O2)(MeCN)]2Ϫ and [MoFe3S4Cl3-
(citr)]3Ϫ (citr = citrate) were also modelled and used to catalyse
duced in the electrolytic reduction of N2H4Xϩ have been
investigated.
Experimental
Ruthenium trichloride (RuCl3ؒxH2O) from Arora Matthey
India, iminodiacetic acid (H2imda), N-(2-hydroxyethyl)imino-
diacetic acid (H2himda), nitrilotriacetic acid (H3nta), N-carb-
J. Chem. Soc., Dalton Trans., 2000, 85–92
This journal is © The Royal Society of Chemistry 2000
85

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oxymethyl-NЈ-(2-hydroxyethyl)ethylenediiminodiacetic
acid
The complex was precipitated with cold absolute ethanol,
(H3hedta), propylenedinitrilotetraacetic acid (H4pdta), ( )-
trans-cyclohexane-1,2-diyldinitrilotetraacetic acid (H4cdta),
and carboxymethyliminobis(ethylenenitrilo)tetraacetic acid
(H5dtpa) from Aldrich Chemical Company Inc. USA were pur-
chased. Ethylenedinitrilotetraacetate disodium salt (Na2H2edta)
from Polypharm India, hydrazine sulfate (N2H5HSO4) from
BDH Chemicals, India and phenylhydrazine hydrochloride
(N2H4PhCl) from Allied Chemicals, USA. The zero grade
Ar gas from IOLAR & Co. was used after purification with
vanadium sulfate and alkaline pyrogallol solutions. All other
reagents used were of AR grade. A mixture of 0.2 M
CH3COONa and 18 N H2SO4 solutions at a desired pH between
1 and 5 was used as supporting electrolyte in all voltammetric
studies. Stock solutions of N2H5HSO4 (0.1 M) were prepared
and standardized with potassium iodate.21 A freshly prepared
solution of 0.1 M N2H4PhCl was used to prepare the experi-
mental solutions.
washed with 9:1 acetone–water solution until free from chlor-
ide and dried under vacuum. Calc. for C14H21ClKN2O9Ru: C,
31.32; H, 4.19; N, 5.14. Found: C, 31.20; H, 4.30; N, 5.20%.
IR (KBr): 3450 (OH); 1750 (COOH); 1655 (COOϪ asym);
1375 (COOϪ sym); 1220 (CO); 885, 685 (OCO def); 520 cmϪ1
(M–N). UV-vis, λ/nm (ε/MϪ1 cmϪ1): 240 (2430); 285 (1640) and
370 (700).
K[Ru(H2dtpa)Cl] 8. This complex was isolated from K2-
[RuCl5(OH2)] and H5dtpa (0.403 g, 1.02 mmol) in 20 mL of
HClO4 by the procedure described in the case of 7. Calc. for
C16H20ClKN3O12Ru: C, 29.82; H, 3.49; N, 7.34. Found: C, 29.7;
H, 3.56; N, 7.43%. IR (KBr): 3560 (OH); 1750 (COOH); 1655
(COOϪ asym); 1375 (COOϪ sym); 1280 (CO); 885, 685 (OCO
def); 525 cmϪ1 (M–N). UV-vis, λ/nm (ε/MϪ1 cmϪ1): 235 (2235);
280 (1560) and 370 (375).
Instrumentation
Preparations
Perkin-Elmer Series II-2400, CHNS/O Analyzer for C, H,
N-data; Bio-Rad FT-40 spectrometer coupled to a SP-3200
computer for IR; Shimadzu UV-vis NIR scanning spectro-
photometer UV-3101 PC for absorption spectra. An Adair
Dutt digital pH meter ( 0.01 pH) was used to record the pH of
all experimental solutions other than those in potentiometry.
Electrochemical measurements were performed on EG&G
Princeton Applied Research (PARC) instruments. A model
PAR 174A Polarographic Analyzer and PAR 175 Universal
Programmer coupled to a high precision Houston X-Y recorder
were used to record sampled dc polarograms and cyclic vol-
tammograms. A three electrode assembly, PAR 303 SMDE/
HMDE comprising a dropping (DME, 3.85 mg sϪ1)/hanging
(HMDE, 0.021 cm2) mercury drop working, platinum wire aux-
iliary and SCE reference electrodes was employed.
The controlled potential coulometry was performed on a
EG&G PAR model 173 Potentiostat/Galvanostat coupled to a
model 179 digital coulometer provided with a three electrode
cell assembly. The cell consisted of a mercury pool (4 cm convex
diameter) working electrode in the main compartment along
with a platinum mesh as counter electrode separated by a glass
frit, SCE reference electrode and a glass disc agitator. An Orion
940 Ion Analyzer equipped with a model 9512 ammonia
sensing membrane electrode (sensitive to 10Ϫ12 M) was used for
the estimation of ammonia in the electrolysed solutions.
K2[RuCl5(OH2)]. The compound was prepared by the
method reported elsewhere,25 and used as the precursor to syn-
thesize three- to five-co-ordinated ruthenium()–aminopoly-
carboxylic acid complexes 1–8 according to the following
methods.
K2[Ru(imda)Cl3]ؒ2H2O 1. To a hot solution of 10 mL K2-
[RuCl5(OH2)] (0.375 g, 1 mmol) in 1 mM HClO4, H2imda
(0.136 g, 1.02 mmol) dissolved in 10 mL of 1 mM HClO4 was
added slowly with constant stirring. The resultant mixture
was refluxed (two hours) till the reddish brown colour changed
to pale yellow. Later, the volume of the solution was reduced
(5 mL) in vacuo on a Rotovapour. It was then precipitated with
coldabsolutealcohol,filteredoff,washedwith9:1acetone–water
till free of chloride and dried under vacuum. Calc. for
C4H9Cl3K2NO6Ru: C, 10.46; H, 1.92; N, 3.07. Found: C, 10.60;
H, 2.01; N, 3.09%. IR (KBr): 1635 (COOϪ asym); 1365 (COOϪ
sym); 1210 (CO); 840 (OCO def); 535 cmϪ1 (M–N). UV-vis,
λ/nm (ε/MϪ1 cmϪ1): 240 (2270); 285 (1400) and 365 (445).
K2[Ru(himda)Cl2]ؒH2O 2. The compound 2 was prepared
from K2[RuCl5(OH2)] (0.375 g, 1 mmol) and H2himda (0.180 g,
1.02 mmol) by the procedure described for 1. Calc. for
C6H10Cl2K2NO6Ru: C, 16.04; H, 2.32; N, 3.09. Found: 16.29;
H, 2.28; N, 3.17%. IR (KBr): 1640 (COOϪ asym); 1365 (COOϪ
sym); 1235 (CO); 825 (OCO def); 535 cmϪ1 (M–N). UV-vis,
λ/nm (ε/MϪ1 cmϪ1): 245 (2595); 285 (2360) and 360 (640).
Analytical methods
All acid–base titrations were conducted in a jacketed glass-
walled cell of about 150 mL capacity, connected to a Metrohm
Swiss model 682 Auto Titroprocessor coupled with a 665
Dosimat and E-649 magnetic stirrer, through a combined glass-
reference electrode. The cell also had inlet and outlet tubes for
passing Ar gas through the solution. The electrode system was
calibrated in terms of Hϩ concentration by means of a titration
of HCl solution with NaOH29 in 0.1 M Na2SO4 at 25 0.1 ЊC.
The experimental data ranged over pH values between 1.6 and
11.2.
K2[Ru(nta)Cl2]ؒH2O 3. This compound was prepared from
K2[RuCl5(OH2)] (0.375 g, 1 mmol) and H3nta (0.195 g, 1.02
mmol) in the same manner as described above for 1. Calc. for
C6H8Cl2K2NO7Ru: C, 15.85; H, 1.68; N, 3.13. Found: C, 15.79;
H, 1.76; N, 3.07%. IR (KBr): 1635 (COOϪ asym); 1358 (COOϪ
sym); 1230 (CO); 850 (OCO def); 525 cmϪ1 (M–N). UV-vis,
λ/nm (ε/MϪ1 cmϪ1): 285 (2420) and 360 (735).
K[Ru(HmL)Cl]ؒxH2O {m ؍ 0, x ؍ 0 for L ؍ hedta 4; m ؍ 1,
x ؍ 2 for edta 5; m ؍ 1, x ؍ 2 for pdta 6}. These compounds
were prepared from K2[RuCl5(OH2)] and the respective
aminopolycarboxylic acid according to the procedures reported
earlier26–28 and subsequently characterized by physico-chemical
methods.
In a typical experiment, a weighed quantity of a complex
1–8, N2H5HSO4 or N2H4PhCl (1 mM) was dissolved in 50 mL
of 0.1 M Na2SO4 and titrated independently against carbonate
free (0.1 M) NaOH solution in the region 2 < pH < 10 under Ar
at 25 ЊC. Similar titrations were conducted in the presence of
1 equivalent of N2H5HSO4/N2H4PhCl. The resulting data were
used to calculate the acid dissociation (pKa) constants cor-
responding to N2H4Xϩ, 1–8 and adducts of both, and the metal
hydrolysis (pKOH) constants of the last two systems. The titra-
tion data at pH > 10 in the case of 1–8 and at pH > 9.0 in the
case of N2H4Xϩ adducts were not considered in the present
investigations. The acid dissociation constants of N2H4Xϩ and
the unco-ordinated COOH/CH2CH2OH in complexes 1–8 and
K[Ru(Hcdta)Cl]ؒH2O 7. This complex was obtained by treat-
ing K2[RuCl5(OH2)] (0.375 g, 1.0 mmol) in 10 mL 1 mM HClO4
with Na2H2cdta (0.400 g, 1.02 mmol) in 20 mL 1 mM HClO4
under a nitrogen atmosphere, initially for 30 min with stirring at
room temperature followed by refluxing for about two hours.
The greenish yellow solution was evaporated to small volume.
86
J. Chem. Soc., Dalton Trans., 2000, 85–92

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in their 1:1 adducts of N2H4Xϩ, where the dissociation of pro-
tons takes place in well defined buffer regions, were evaluated29
with the help of Kw (1.008 × 10Ϫ14 at 25 ЊC) and suitable mass
and charge balance equations fitting to the generalized eqn. (1).
Kna
HnL
H
n Ϫ 1L ϩ Hϩ
Kna = [Hn Ϫ 1L][Hϩ]/[HnL]
(1)
Charges on the species are omitted for the sake of clarity. The
stepwise first, second or third metal hydrolyses of 1–8 and their
N2H4Xϩ adducts were considered according to reactions (2).
KOH
M(OH)n Ϫ 1 ϩ H2O
M(OH)n ϩ Hϩ
Fig. 1 Potentiometric titration curve of (a) K2[Ru(imda)Cl3]ؒ2H2O,
1 mM; (b) K2[Ru(imda)Cl3]ؒ2H2O, 1 mM and N2H5ϩ, 1 mM; (c)
K2[Ru(imda)Cl3]ؒ2H2O, 1 mM and N2H4Phϩ, 1 mM at 25 ЊC. I = 0.1 M
(Na2SO4), a = mols of base added per mol of complex or N2H4Xϩ.
KOH = [M(OH)n][Hϩ]/[M(OH)n Ϫ 1
]
(2)
The hydrolysis constants falling in the pH range 2 < pH < 10
were calculated with the help of suitable mass balance
equations and Kw at 25 ЊC.
may be due to its decomposition and hence was not considered
here. Similarly, complexes 2 and 3 showed three buffer regions
caused by metal hydrolyses at 2 < pH < 10 with two ill defined
inflections at a = 1 and 2. On the other hand, 4–7 showed the
stepwise liberation of two protons in two well defined buffer
regions 0 < a < 1 and 1 < a < 2. Of the two, the first buffer
region is assigned to the dissociation of an unco-ordinated
CO2H proton in the case of complexes 5–7 and may be the OH
(CH2CH2OH) proton in the case of 4 while the second buffer
region is considered for the metal hydrolysis.33,34 On the
contrary, the complex 8 showed one buffer region 0 < a < 2 in
acidic pH followed by another buffer region 2 < a < 3 in basic
pH. The first buffer region is assigned to the simultaneous
liberation of two protons on the dtpa ligand, the second to
metal hydrolysis.
The metal hydrolysis steps, three in the case of complex 1,
the first two in the case of 2 and 3, the single one in the case
of 4–6 and 8, and the first one in the case of 7, are accounted
for by the rapid substitution of chloride ions by H2O as veri-
fied kinetically in the case of 4–6, and the liberation of titrat-
able protons following its dissociation.26,35 The third metal
hydrolysis step in the case of 2 and 3 could be explained if
one of the loosely bound carboxylate groups of the tetra-
dentate himda and nta is substituted by H2O or the liberation
of OH (CH2CH2OH) proton on tridentate himda in 2. The acid
(pKa) and metal hydrolysis (pKOH) constants were calculated
with the help of eqns. (1)–(3) and are given in Table 1. These
showed that the complexes 1–3 are more hydrolysable because
the chelation capability of imda (tri), himda (tri or tetra) and
nta (tetra) ligands is low. All other complexes, 4–8, showed
a single hydrolysis step which occurs almost at neutral pH.
This implies that all these complexes have a potential site avail-
able to another ligand for co-ordination/substitution, though
the number of co-ordinating sites in the aminopolycarboxylic
acid ligand varies from 5 to 8. The present data also revealed
that only five co-ordination sites on the hexacovalent
ruthenium are used by all penta- or higher poly-dentate
aminopolycarboxylic acids in the process of bond formation.
The hydrolytic tendency of the metal increased in the order
8 < 4 < 5 < 6 < 7 < 2 < 3 < 1.
A graphical method30 was used to calculate the dissociation
constants, in the case of simultaneous dissociation of two pro-
tons (either by the participation of COOH/CH2CH2OH and/or
in the combination of dissociation of N2H4Xϩ proton and
metal hydrolysis) described by the generalized eqn. (3) or com-
bined eqns. (1) and (2), respectively.
Ka,K2a
H2L
L ϩ 2Hϩ
KaK2a = [L][Hϩ]2/[H2L]
(3)
The absorption spectral studies on complexes 1–8 (1 mM) in
the absence and in the presence of various concentrations of
N2H5ϩ or N2H4Phϩ (1–100 mM) were performed at pH 2.8 in a
10 mm quartz cuvette and the spectra were recorded in the
wavelength range between 200 and 700 nm at 25 0.1 ЊC.
The voltammetric responses were recorded with a weighed
quantity of complex (1 mM) in the absence or presence of a
ϩ
known concentration (0.1–100 mM) of substrate (N2H5 /
N2H4Phϩ) in 10 mL electrolyte solution at the desired pH under
Ar at 25 ЊC. The i vs. E plots were recorded between ϩ0.2 and
Ϫ0.8 V (vs. SCE) at the sweep rates 10 mV sϪ1 in sampled dc
and 20–500 mV sϪ1 in cyclic voltammetry (CV). The effect of
pH on the diffusion current (id) and half-wave potentials (E1/2)
was studied in the range 1–5. All the solutions were deaerated at
least 15 min prior to measurements and the experiments were
maintained at 25 0.1 ЊC unless otherwise stated. The criteria
for the electrochemical reversibility and diffusion current were
established by following the reported techniques.31,32
Constant potential electrolysis was performed in 25 mL of
deaerated buffer solution having the desired pH (1.9/2.8)
containing 1 mmol of complex (1–8) and 100 mmol of substrate
at predetermined potential for at least 10 h under Ar at
25 0.1 ЊC. The ammonia produced during the reaction
was estimated every hour21 while the aniline produced con-
comitantly with ammonia was tested qualitatively using
vanadium() salt.33
The titration data of N2H5HSO4 showed two buffer regions
separated by an inflection at a = 1. The buffer region 0 < a < 1
is considered for the neutralization of the strong acidic proton
Results and discussion
Interaction of N2H4X؉ with the complexes 1–8
Ϫ
ϩ
Potentiometry. Complexes 1–8 were independently titrated
against standard sodium hydroxide in the absence and in the
presence of one equivalent of N2H4Xϩ. The representative
titration data thus obtained with 1 are presented in Fig. 1.
Complex 1 (Fig. 1(a)) showed three titratable protons dissoci-
ating in three independent buffer regions, two in acidic and one
in basic pH by the metal hydrolysis. Further hydrolysis of 1
occurring beyond a > 3 resulted in a dark brown solution which
due to the HSO4 group, since the N2H6 (Ka 11)11 does not
form inder the present experimental conditions. The buffer
region corresponding to this proton was ignored in all other
studies presented below and hence not included in Fig. 1(b), but
considered in the evaluation of relevant constants. The second
buffer region 1 < a < 2 is assigned to the neutralization of pro-
ϩ
ton in the equilibrium N2H5
N2H4 ϩ Hϩ. Similarly, the
titration data of N2H4PhCl contained a single buffer region
J. Chem. Soc., Dalton Trans., 2000, 85–92 87

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Table 1 Acid dissociation (pKa) and metal hydrolysis (pKOH) constants of complexes 1–8 at 25 ЊC, I = 0.1 M (Na2SO4)
pKa
pKOH
Complex
CO2H
CH2CH2OH
First
Second
Third
1
2
3
4
5
6
7
8
—
—
—
—
—
—
—
2.42 0.02
2.86 0.02
2.47 0.01
7.96 0.03
7.64 0.02
7.60 0.02
6.20 0.03
8.28 0.02
5.78 0.03
5.72 0.02
4.80 0.03
—
—
—
—
—
8.86 0.03
8.53 0.03
9.62 0.03
—
—
—
—
—
4.82 0.03
2.34 0.02
2.38 0.02
2.55 0.03
(2.58, 3.40)
—
—
—
—
Values given in parentheses were evaluated by a graphical method.
Table 2 Acid dissociation (pKa) and metal hydrolysis (pKOH) constants of N2H4Xϩ and their adducts with complexes 1–8 at 25 ЊC, I = 0.1 M
(Na2SO4)
Adduct
pKa
pKOH
N2H4Xϩ
Complex
N2H4Xϩ
CO2H/CH2CH2OH
First
Second
ϩ
N2H5
—
1
8.05 0.01
7.67 0.02
7.42 0.03
7.36 0.02
7.65 0.02
7.52 0.02
7.28 0.03
7.15 0.02
7.30 0.03
5.27 0.02
(5.24)
5.25 0.02
5.34 0.02
(5.32)
5.20 0.02
5.22 0.02
5.31 0.02
5.30 0.02
—
—
—
—
—
2.43 0.02
2.82 0.02
2.45 0.02
—
—
—
—
—
5.82 0.03
2
3
—
—
—
—
—
—
—
(5.84)
—
—
—
—
—
—
4
4.83 0.02
2.40 0.02
2.42 0.02
2.58 0.02
(2.60, 3.43)
—
—
—
—
(4.62)
2.35 0.02
2.41 0.02
2.60 0.02
(2.64, 3.49)
5
6
7
8
N2H4Phϩ
—
1
—
2.45 0.02
2.89 0.03
2.50 0.03
—
—
—
—
—
2
3
4
5
6
7
8
—
Values given in parentheses were evaluated by a graphical method.
0 < a < 1, representing the equilibrium N2H4Phϩ
Ph ϩ Hϩ.
N2H3-
The acid dissociation constants and metal hydrolysis con-
stants corresponding to free N2H4Xϩ and its adducts with com-
plexes 1–8 were calculated with the help of eqns. (1)–(3) and
presented in Table 2. The values of Ka (CO2H/CH2CH2OH) and
KOH are closely comparable to the corresponding values seen
in Table 1. Moreover, the values obtained for N2H4Xϩ
N2H3X ϩ Hϩ in the presence of 1–8 are in close agreement
with those obtained in their absence. However, the Ka value is
The titration curves of complexes 1–8 in the presence of
N2H5ϩ possessed buffer regions, four in the case of 1 (Fig. 1(b))
and three in the case of 2–8. The last buffer region (fourth in the
case of 1, and third in the case of other complexes), where RuIII–
ϩ
N2H5 species were decomposed to give ruthenium() com-
plexes, as found by sampled dc at pH > 9, was not considered in
the present study. The third buffer region in the case of 1 and
the second in the case of 2–8, which were assigned earlier to
ϩ
reduced to only 0.38–1.0 logarithmic units in the case of N2H5
and negligibly affected in the case of N2H4Phϩ after their inter-
action with the ruthenium complex. On the basis of these
observations together with the disappearance of metal hydrol-
ysis steps (third in the case of 1, second in the of 2 and 3,
and first in the other complexes as seen in Table 1) in the pres-
ence of N2H4Xϩ, the co-ordination of N2H4Xϩ to ruthenium
occurs at the vacant site where the poorly dissociable water was
attached. Further, the small changes in pKa values of the
N2H4Xϩ ligand after its interaction with ruthenium suggests
that its co-ordination takes place through the weakly basic
ϩ
metal hydrolysis in the absence of N2H5 , are assigned to the
ϩ
equilibrium N2H5
N2H4 ϩ Hϩ in the vicinity of the metal
co-ordination sphere. The first and second buffer regions in the
case of 1 and the first in the case of 2 and 3 are assigned to the
metal hydrolysis, while the first in the case of 4–7 and the first
and second buffer regions in the case of 8 are assumed for the
deprotonation of the CO2H/CH2CH2OH group on the amino-
ϩ
polycarboxylic acid as found above in the absence of N2H5 .
The potentiometric data obtained in the presence of N2H4-
PhCl showed two buffer regions: a = 0–1, 1–3 in the case of
complex 1 (Fig. 1(c)), a = 0–1, 1–2 in the case of 2, 3 and 5–7 and
a = 0–2, 2–3 in the case of 8, while one buffer region a = 0–2
in the case of 4. It was noticed that the hydrolysis steps, third in
the case of 1, second in the case of 2 and 3, and first in the case
of 5–8 which were observed in the absence of N2H4Phϩ, were
not seen. Thus, the deprotonation of N2H4Phϩ was assumed in
the buffer region a = 1–3 in the presence of 1 along with the
second metal hydrolysis, 1–2 in the presence of 2, 3, 5–7, 0–2 in
the case of 4 together with the deprotonation of the OH
(CH2CH2OH) proton and 2–3 in the case of 8.
ϩ
nitrogen (NH2 of N2H5 or NHPh of N2H4Phϩ) leaving the
other nitrogen in the protonated form, which is further con-
firmed by the voltammetric data presented at pH < 3.0 in the
latter part of this paper.
Absorption spectra. The electronic absorption spectra of
complexes 1–8 were recorded under identical conditions in the
absence and presence of N2H4Xϩ salts at different compositions
at pH 2.8 to verify the interaction of the latter (N2H4Xϩ)
through the weakly basic nitrogen with ruthenium. In the
absence of N2H4Xϩ the ligand charge transfer between 240 and
88
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Table 3 Absorption spectral data of complexes 1–8 in the absence and in the presence of N2H4Xϩ at pH 2.8
λmax/nm (εmax/MϪ1 cmϪ1
)
Presence of N2H4Phϩ
ϩ
Complex
Absence
Presence of N2H5
1
2
3
4
5
6
7
8
240(2270); 285(1400); 365(445)
245(2595); 285(2360); 360(640)
245(2330); 285(2200); 360(735)
240(2270); 285(1400); 360(445)
245(2600); 285(2370); 360(640)
245(2565); 285(2420); 360(630)
240(2430); 285(1630); 360(700)
235(2240); 280(1560); 360(375)
240(2240); 285(1340)
245(2370); 285(2320)
245(2290); 285(2120)
240(2240); 285(1375)
245(2570); 285(2360)
245(2560); 285(2380)
240(2390); 285(1580)
235(2230); 280(1540)
420(1070); 540(350)
425(1450); 540(350)
420(1360); 540(350)
400(1400); 540(260)
420(2250); 540(480)
400(1350); 540(130)
430(1230)
450(1270)
Table 4 Half wave potentials of the RuIII → RuII wave and those of
complexes 1–8 observed in the presence of 100 equivalents of N2H4Xϩ
at pH 2.8
ϪE1/2/V vs. SCE
Presence of
N2H5
Presence of
N2H4Phϩ
ϩ
Complex
Absence
1
2
3
4
5
6
7
8
0.372
0.364
0.354
0.248
0.238
0.251
0.286
0.165
0.346, 0.475
0.328, 0.450
0.300, 0.440
0.030, 0.256
0.025, 0.230
0.034, 0.250
0.045, 0.230
0.028, 0.250
0.360, 0.482
0.340, 0.462
0.322, 0.456
0.165, 0.248, 0.335
0.150, 0.225, 0.346
0.156, 0.235, 0.340
0.168, 0.260, 0.386
0.154, 0.250, 0.395
Fig. 3 Sampled dc polarogram of (a) K2[Ru(imda)Cl3]ؒ2H2O, 1 mM;
(b) K2[Ru(imda)Cl3]ؒ2H2O, 1 mM and N2H5ϩ, 100 mM; (c) K2[Ru-
(imda)Cl3]ؒ2H2O, 1 mM and N2H4Phϩ, 100 mM. The pH in each case
was maintained at 2.8.
were recorded in the absence and in the presence of various
equivalents of N2H4Xϩ. In the absence, each complex exhibited
a well defined one-electron cathodic wave in the potential range
between Ϫ0.150 and Ϫ0.400 V (E1/4 Ϫ E3/4 55–65 mV; I (dif-
fusion current constant) 1.2–1.5) vs. SCE assignable to the
RuIII → RuII process. The height of this wave proportionately
enhanced with increase in complex concentration, while the
plots Ede (potential at the electrode) vs. log [i/(id Ϫ i)] were linear
with a slope of about 70 mV, indicating the electrode reactions
are one-electron, diffusion controlled and quasi-reversible
processes. The wave shifted to a measurable extent (60 mV per
pH unit) in the case of 1–3, less in the case of other complexes
with the change in pH between 1.0 and 5.0. The polarogram
of 1 representing this one-electron wave is shown in Fig. 3(a)
while the half-wave potential (E1/2) data of all the complexes
1–8 are listed in Table 4. It decreased in the order
8 > 5 > 4 > 6 > 7 > 3 > 2 > 1 which is in good agreement with
the order of sigma basicity of the co-ordinated aminopoly-
carboxylic acid.37 However, it is noted that the E1/2 values for
1–3 are relatively more negative than those of other complexes.
This may be attributed to the strong hydrolytic tendency
of these complexes when dissolved in aqueous solution.
Accordingly, the hydrolytic tendency of these complexes may be
given as 3 < 2 < 1. On the other hand, the lower E1/2 value for
complex 8 than those of 4–7 may be accounted for by its
destabilization due to steric repulsion because of the large size
of dtpa, despite its large sigma basicity.
Fig. 2 Absorption spectra of (a) K[Ru(hedta)Cl], 1 mM; (b) K[Ru-
(hedta)Cl], 1 mM and N2H5ϩ, 1 mM; (c) K[Ru(hedta)Cl], 1 mM and
N2H4Phϩ, 1 mM. In all cases the pH was 2.8.
250 nm and the ligand to metal charge transfer bands in the
regions 280–290 and 360–370 nm were observed for all com-
plexes. The absorption band at 360 nm, characteristic of a
Ru–OH2 bond,36 was reduced in intensity on introduction of
N2H5ϩ due to the replacement of bound water molecule by the
latter. In the case of addition of N2H4Phϩ two new absorption
bands at 400–450 and 540 nm appeared rapidly. The band at
540 nm did not occur in the case of complexes 7 and 8. Con-
comitantly, the yellow colour of these complexes instantly
changed to reddish brown accounting for the formation of
N2H4Phϩ adducts in solution. Further, plots of change in
absorbance (∆A) vs. the ratio [N2H4Xϩ]:[complex] confirmed
the formation of 1:1 N2H4Xϩ adducts as suggested above with
1–8. Fig. 2 shows the typical spectral changes observed with 4
in the absence and in the presence of N2H4Xϩ (1 mM) at pH
2.8. The spectral features observed with all complexes 1–8 are
summarized in Table 3.
Voltammetry. Sampled dc polarograms of complexes 1–8
J. Chem. Soc., Dalton Trans., 2000, 85–92
89

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When 0.1–5.0 equivalents of N2H5ϩ were introduced into the
complex solution the RuIII → RuII wave was enhanced while
its plateau extended to anodic potentials in cases of complexes
1–8. As a result, the E1/2 value apparently shifted to anodic
potentials. A plot of the enhanced current increased linearly as
ϩ
the number of equivalents of added N2H5 approached unity
and remained constant thereafter indicating the instant form-
ϩ
ation of monomeric N2H5 complexes in acidic solutions, as
suggested above. The plateau of the enhanced wave gradually
split into two as [N2H5ϩ] reached twenty times more than that
of the complex and the wave finally appeared as two dis-
tinguishable waves (w1 and w2) at all other compositions of
ϩ
N2H5 . The polarographic responses of 1 in the presence of 100
ϩ
equivalents of N2H5 are shown in Fig. 3(b). In all cases, the
wave w1 (E1/4 Ϫ E3/4 32 mV; I 1.9) shifted anodically with the
increase in [N2H5ϩ] while the wave w2 (E1/4 Ϫ E3/4 65 mV; I 1.0)
remained steady. On the other hand, the diffusion currents of
w1 and w2 were directly proportional to [complex]. The plots Ede
vs. log [i/(id Ϫ i)] for w1 and w2 were linear with slopes of 33 and
68 mV, respectively. The diffusion current (id) of the composite
wave or of the wave w1 or w2 was maximum in the pH range
2.5–3.5 and somewhat reduced when the ionic strength was
raised beyond 0.5 M. Besides, the wave w1 shifted negligibly
with increase in pH between 1 and 4. The E1/2 data (Table 4)
revealed that the wave w1 is 0.02–0.05 V less negative in the
case of 1–3, while it is 0.13–0.22 V less negative in the case of
4–8 compared to that of the corresponding one-electron
Fig. 4 Cyclic voltammograms of (a) K[Ru(Hcdta)Cl]ؒH2O, 1 mM; (b)
K[Ru(Hcdta)Cl]ؒH2O, 1 mM and N2H5ϩ, 100 mM; (c) K[Ru(Hcdta)-
Cl]ؒH2O, 1 mM and N2H4Phϩ, 100 mM. The pH in each case was
maintained at 2.8.
to ruthenium. Again, the large negative values in the case of
1–3 and negligible dependence of E1/2 in the case of 4–8 are
accounted for by the reasons given before.
ϩ
RuIII → RuII wave observed before the addition of N2H5 .
Similarly, the wave w2 is about 0.09–0.10 V more negative in the
case of 1–3 and 8 and similar in the case of 4–7 to that of the
initial one-electron RuIII → RuII wave. The wave w1 in the
case of 1–3 is more negative than that of the complexes 4–7.
The E1/2 of w1 is correlated with ΣpKa of the primary ligand,37
aminopolycarboxylic acid. The large negative E1/2 for com-
plexes 1–3 is responsible for their hydrolytic tendency, while the
negligible dependence of E1/2 in complexes 4–8 shows that there
is no considerable change in the redox properties of the metal
with the increase in sigma basicity of the aminopolycarboxylic
acid. Probably, this could be due to identical binding of penta-,
hexa- or hepta-dentate aminopolycarboxylic acids to ruthenium
through the same number of co-ordinating groups.
The CV responses of complexes 1–8 in the absence and in the
presence of N2H4Xϩ were run subsequently for the above solu-
tions. The results in the absence and in the presence of 100
equivalents of N2H4Xϩ are discussed below. All the complexes,
in the absence of N2H4Xϩ, yielded a pair of cathodic and anodic
peaks corresponding to the quasi-reversible RuIII–RuII couple.
Fig. 4(a) shows a typical cyclic voltammogram of 4, which
peaked at Ϫ0.320 V in the forward scan and at Ϫ0.252 V in the
reverse scan. The peak separation which varied between 70 and
65 mV at 0.1 V sϪ1 increased with increase in scan speed, while
the peak width Ep/2 Ϫ Ep tended to 62–65 mV which is close to
the theoretical value for a one-electron process. On the other
hand, the peak currents increased linearly with increase in the
square root of the scan speed, while the peak to peak ratio
(ipa :ipc) was less than unity.
The polarographic responses of complex 1 in the presence
of 0.1–100 equivalents of N2H4Phϩ were probed. The response
in 100 equivalents of N2H4Phϩ is depicted in Fig. 3(c). It dis-
played two closely separated waves (w1 and w2) at potentials
close to that of the RuIII → RuII wave with a doubled diffu-
sion current. The responses in the presence of other concen-
trations (0.1–100 equivalent of N2H4Phϩ) overlapped those in
Fig. 3(c) except for a negligible negative shift in E1/2 and a minor
change in the overall diffusion current. Complexes 2 and 3 also
exhibited two cathodic waves replacing the RuIII → RuII wave
while the other complexes (4–8) showed an additional low
intensity wave poorly resolved from w1 and w2. The E1/2 data
pertaining to these waves are summarized in Table 4. The meas-
ured data (E1/4 Ϫ E3/4 34 mV and I 2.0 for w1; 65 mV and 0.9 for
w2) indicated w1 to be a two-electron process while w2 and the
additional wave are one-electron processes. This was further
substantiated by plots of Ede vs. log [i/(id Ϫ i)] which were linear
having slopes of 32–35 mV for w1 and 67–70 mV for both w2
and the additional wave. The overall diffusion current at Ϫ0.5 V
was proportional to [1] while the enhanced current (∆id) was a
maximum in the pH range 2.0 to 3.5. The data in Table 4
revealed that, for a given complex, the E1/2 of both w1 and w2
All the complexes, in the presence of 100 equivalents of
ϩ
N2H5 , exhibited two well resolved cathodic peaks (p1 and p2
corresponding to the polarographic waves w1 and w2, respec-
tively) and a single anodic peak (p3, counterpart of p2). The
anodic counterpart of p1 did not appear under the present
experimental conditions. Representative CV responses of com-
plex 4 are shown in Fig. 4(b). The complexes 1–3 exhibited two
well resolved cathodic peaks (p1 and p2) in the presence of 100
equivalents of N2H4Phϩ. Contrarily, they showed a single
anodic peak (p4) as counterpart of p1 and no peak for p2. Com-
plexes 4–8 behaved similarly in the presence of 100 equivalents
of N2H4Phϩ. Additionally, they showed a well defined cathodic
peak and its counterpart (Fig. 4(c)) corresponding to the
additional wave observed in sampled dc between w1 and w2.
The peak analyses data for p1 and p2, confirmed them to be a
two- and one-electron reduction process, respectively. Peak p1
shifted anodically with increase in [N2H4Xϩ] but cathodically
with increase in scan speed, but plots of ipc vs. ν1/2 were
linear for both p1 and p2.
ϩ
are more negative with N2H4Phϩ than with N2H5 , indicating
that the former hydrazine binds to ruthenium more strongly
than the latter can through a sigma bond. This may due to
the effect of phenyl ring substitution on the co-ordinating
nitrogen in the former hydrazine. The E1/2 values of w1 obtained
with complexes 1–8 in the presence of N2H4Phϩ are corre-
lated with ΣpKa of the aminopolycarboxylic acid37 attached
Mechanism of electrode reactions
The complexes 1–8 were reduced in solution in the absence and
in the presence of one equivalent of N2H4Xϩ at pH 1.9 and 2.8
by holding the potential at a mercury pool cathode at 0.1 V
away from the E1/2 of RuIII → RuII wave as given in Table 4. In
90
J. Chem. Soc., Dalton Trans., 2000, 85–92

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the absence, all the complexes consumed one faraday of charge
and produced the corresponding ruthenium() complexes. In
the presence of N2H4Xϩ all the complexes consumed three
faradays. The electrolysed solutions contained an ammonia
concentration two times that of [N2H5ϩ] but equal to that of
[N2H4Phϩ] taken initially. On the basis of these data and the
data in Figs. 3 and 4, it is proposed that the RuIII–N2H4Xϩ
complexes take two electrons from the electrode at w1 and pro-
duce RuI–N2H4Xϩ species (reaction (4), charges on the species
RuIII–N2H4Xϩ ϩ 2eϪ
RuI–N2H4Xϩ ϩ Hϩ
RuI–N2H4Xϩ
(4)
(5)
RuIIINH2X ϩ NH3
When X = H
RuIII–NH3 ϩ eϪ
RuIII–NH3 ϩ H2O
RuII–NH3 ϩ H2O
RuIII–H2O ϩ eϪ
RuII–NH3
(6)
(7)
(8)
(9)
RuIII–H2O ϩ NH3
RuII–H2O ϩ NH3
RuII–H2O
Fig. 5 Plots of mols of NH3 produced per mol of complex (1–8)
during the reduction of N2H5ϩ at pH 2.8 vs. electrolysis time.
When X = Ph
RuIII–NH2Ph ϩ eϪ
RuIII–NH2Ph ϩ H2O
RuII–NH2Ph ϩ H2O
RuIII–H2O ϩ eϪ
RuII–NH2Ph
(10)
RuIII–H2O ϩ NH2Ph (11)
RuII–H2O ϩ NH2Ph (12)
RuII–H2O
(13)
are omitted for clarity). These ruthenium() species are less
stable and hence rapidly decompose as shown in reaction (5)
to RuIII–NH2X and one mole of NH3 by abstracting one
equivalent of Hϩ from the medium. The species RuIII–NH2X
are reduced by one electron in a subsequent step at w2 (reactions
(6) and (10)) to give RuII–NH2X which on aquation produces
the corresponding RuII–H2O species and NH2X (reactions (8)
and (12)), or hydrolyse to RuIII–OH2 producing an equivalent
amount of NH2X (reactions (7) and (11)). The RuIII–OH2
species thus produced may be reduced along with RuIII–NH2X
at w2 in the case of N2H5ϩ (reaction (9)) or at potentials where a
new reduction step appeared between w1 and w2 in the case of
N2H4Phϩ (reaction (13)). The RuII–NH2Ph complexes produced
at w1 are not as stable as the corresponding RuIII–NH3 due to
the presence of the Ph group and hence these species and their
reduced forms RuII–NH2Ph quickly hydrolyse to the corre-
sponding aqua species and NH2Ph (reactions (11) and (12)).
This may be the reason for the irreversible nature of wave w2
and reversible nature of the poorly resolved new wave between
w1 and w2.
Fig. 6 Plot of turnover number of NH3 produced by the reduction of
N2H4Xϩ vs. ΣpKa of the aminopolycarboxylic acid: (a) N2H5ϩ, (b)
N2H4Phϩ.
ϩ
N2H5 reduction. The turnover number and mols of NH3
formed per mol of complex per hour are given in Table 5. For a
given substrate, the turnover number at pH 2.8 is nearly double
that at pH 1.9. The observed turnover number of NH3 obtained
ϩ
with N2H4Phϩ is less than half of that achieved with N2H5 .
However, the coulombic efficiency in all these cases was almost
100%. The solution at the end of electrolysis showed the same
voltammetric features as described above and produced NH3 at
the same turnover rate when an additional amount of N2H4Xϩ
was added to the electrolysed solution and the electrolysis con-
tinued after adjusting the pH to the initial value. Thus, the
catalytic efficiency of the metal was shown to be intact during
the reduction cycle.
The turnover rate is correlated in Fig. 6 with the sigma bas-
icity of the polyaminopolycarboxylic acid. The data revealed
that the turnover rate increases with the increase in sigma bas-
icity of the aminopolycarboxylic acid to nearly ΣpKa = 22 and
Electrolytic reduction of N2H4X؉ in the presence of complexes
1–8
ϩ
then decreases thereafter. With the given hydrazine N2H5 /
The hydrazines N2H4Xϩ were electrolytically reduced at pH 1.9
and 2.8 under Ar by constant potential coulometry by holding
the potential at 50 mV away from the corresponding E1/2 of w1
given in Table 5 in order to measure the catalytic ability of the
ruthenium as a function of the sigma basicity of the amino-
polycarboxylic acid on the reduction of the N–N bond in
N2H4Xϩ. In all cases the constant reduction of N2H4Xϩ to NH3
and NH2X occurred, eqn. (14), justifying the wave w1 as a
multi-electron process.
N2H4Phϩ, it increases in the order 1 < 2 < 3 < 4 < 5 ≈ 6 as the
sigma basicity of the aminopolycarboxylic acid increases in the
order 2 < 1 < 3 < 4 < 5 < 6. The lower turnover rate for com-
plex 1 than that of 2 is explained by its high hydrolytic tendency.
The decrease in the turnover rate with 5–8 with the increase in
the sigma basicity may be related to the structural constraints
developed within the aminopolycarboxylic acid due to increase
in size, affecting the overall catalytic ability of the ruthenium.
RuIII–N2H4Xϩ
Conclusion
N2H4Xϩ ϩ 2eϪ ϩ Hϩ
NH3 ϩ NH2X
(14)
Ruthenium()–aminopolycarboxylic acid complexes of tri-
(imda, himda), tetra- (himda, nta), penta- (hedtra), hexa- (edta,
pdta and cdta) and octa- (dtpa) dentate ligands were prepared
At both pH studied, the number of mols of NH3 produced
per mol of complex was linear, in all cases, as seen in Fig. 5 for
J. Chem. Soc., Dalton Trans., 2000, 85–92
91

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Table 5 Turnover number, moles of NH3 produced per mol of complex per hour, during the reduction of N2H4Xϩ
Turnover number
N2H4Phϩ
ϩ
N2H5
Complex
Potential/V
pH 1.9
pH 2.8
Potential/V
pH 1.9
pH 2.8
1
2
3
4
5
6
7
8
Ϫ0.350
1.44
1.76
2.98
3.75
9.50
7.96
4.46
1.34
2.54
4.12
6.06
Ϫ0.400
0.94
1.02
1.78
1.62
2.85
2.68
2.17
0.92
1.10
1.76
2.98
2.95
5.98
5.32
3.84
1.27
Ϫ0.200
Ϫ0.350
Ϫ0.200
Ϫ0.050
Ϫ0.100
Ϫ0.175
Ϫ0.100
Ϫ0.300
Ϫ0.400
Ϫ0.250
Ϫ0.200
Ϫ0.220
Ϫ0.300
Ϫ0.230
7.18
18.40
14.84
8.98
2.95
12 R. R. Schrock, T. E. Glassman, M. G. Vale and M. Kol, J. Am.
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and characterized. Interaction of these complexes with hydra-
zines N2H4Xϩ (X = H or Ph) was investigated by pH-metry,
voltammetry and spectrophotometry. The results confirmed
that these complexes rapidly form monomeric adducts with
N2H4Xϩ by its substitution at the less hydrolysable metal co-
ordination site in acidic media. These adducts readily accept
two electrons to form the corresponding RuI–N2H4Xϩ which on
decomposition give one mol of ammonia and an electroactive
species which in a subsequent chemical step gives one mol of
NH2X. Employing these hydrazinium adducts, N2H4Xϩ were
reduced electrolytically. Ammonia and/or amine was obtained
in catalytic amounts with nearly 100% coulombic efficiency.
The turnover rate of ammonia produced per mol of complex
per hour was evaluated and correlated with the sigma basicity
(ΣpKa) of the aminopolycarboxylic acid. The data revealed that
the turnover rate increased with increase in sigma basicity in
the order imda < himda < nta < hedtra < edta ≈ pdta. Further
increase in the sigma basicity of the aminopolycarboxylic acid
reduced the catalytic ability of the ruthenium which is attrib-
uted to the development of complexity caused by the concomi-
tant increase in size of the aminopolycarboxylic acid. The most
probable chemical and charge transfer steps were suggested for
the reactions at the electrode.
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Acknowledgements
The authors are grateful to the Department of Science and
Technology (No. SP/S1/F-12/93) New Delhi for supporting this
work. R. P. is grateful to the Council of Scientific and Industrial
Research (CSIR), New Delhi for awarding a Senior Research
Fellowship (31/28(22)/97 EMR-I and 31/28(29)/99 EMR-I).
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