Nitro-ruthenium(II)-arylazoimidazoles: Synthesis, spectra, crystal structure and electrochemistry of dinitro-bis{1-alkyl-2-(arylazo)imidazole}ruthenium(II). Nitro-nitroso derivatives and reactivity of the electrophilic {Ru-NO}6 system

Article abstract of DOI:10.1039/b204442k

Silver ion assisted aquation of blue cis,trans,cis-RuCl₂(RaaiR′)₂ (4-6) leads to solvento species, blue-violet cis,trans,cis-[Ru(OH₂)₂(RaaiR′)₂](ClO ₄)₂ [RaaiR′ = p-R-C₆H₄-N=N-C₃H₂-NN-1-R′ (1-3), abbreviated as N,N′-chelator where N(imidazole) and N(azo) represent N and N′, respectively; R = H (a), Me (b), Cl (c) and R′ = Me (1/4/7/10), CH₂CH₃(2/5/8/11), CH₂Ph (3/6/9/12)] that have been reacted with NO₂^-in warm ethanol resulting violet dinitro complexes of the type, Ru(NO₂)₂(RaaiR′)₂ (7-9). The structure in one case, [Ru(NO₂)₂(HaaiCH₂Ph)₂] (9a), has been established by X-ray diffraction as the cis-Ru(NO₂)₂ motif along with trans-N,N and cis-N′,N′ dispositions of the chelator N atoms around the coordination sphere. The nitrite complexes are useful synthons of electrophilic nitrosyls, and on triturating the compounds 7b-9b with conc. HClO₄ nitro-nitrosyl derivatives, [Ru(NO₂)(NO)(MeaaiR′)₂](ClO₄) ₂ (10b-12b), are isolated. The solution structure and stereoretentive transformation in each reaction step have been established by ¹H NMR results. All the complexes exhibit strong MLCT transitions in the visible region. They are redox active and display one metal-centred oxidation and successive ligand-based reductions. The redox potentials of Ru(III)/Ru(II) (E_1/2^M) of 10b-12b are anodically shifted by ～0.2 V as compared to those of dinitro precursors 7b-9b. The ν(NO) > 1900 cm^-1 strongly suggests the presence of linear Ru-N-O bonding. The electrophilic behaviour of metal bound nitrosyl has been proved in one case (12b) by reacting with a bicyclic ketone, camphor, containing an active methylene group and an arylhydrazone with an active methine group, and the heteroleptic trischelates thus formed are characterised.

Full text of DOI:10.1039/b204442k

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Nitro–ruthenium(II)–arylazoimidazoles: synthesis, spectra, crystal
structure and electrochemistry of dinitro-bis{1-alkyl-2-
(arylazo)imidazole}ruthenium(II). Nitro–nitroso derivatives
and reactivity of the electrophilic {Ru-NO}6 system
P. Byabartta,a Sk. Jasimuddin,a B. K. Ghosh,a C. Sinha,*a A. M. Z. Slawinb and J. D. Woollinsb
a
Department of Chemistry, The University of Burdwan, Burdwan 713104, India.
E-mail: c_r_sinha@yahoo.com
Department of Chemistry, The University of St-Andrews, St-Andrews Fife, UK KY16 9ST
b
Received (in London, UK) 7th May 2002, Accepted 29th July 2002
First published as an Advance Article on the web 9th September 2002
Silver ion assisted aquation of blue cis,trans,cis-RuCl2(RaaiR0)2 (4–6) leads to solvento species, blue–violet
cis,trans,cis-[Ru(OH2)2(RaaiR0)2](ClO4)2 [RaaiR0 ¼ p-R–C6H4–N=N–C3H2–NN-1-R0 (1–3), abbreviated as
N,N0-chelator where N(imidazole) and N(azo) represent N and N0, respectively; R ¼ H (a), Me (b), Cl (c) and
ꢀ
R0 ¼ Me (1/4/7/10), CH2CH3(2/5/8/11), CH2Ph (3/6/9/12)] that have been reacted with NO2 in warm
ethanol resulting violet dinitro complexes of the type, Ru(NO2)2(RaaiR0)2 (7–9). The structure in one case,
[Ru(NO2)2(HaaiCH2Ph)2] (9a), has been established by X-ray diffraction as the cis-Ru(NO2)2 motif along with
trans-N,N and cis-N0,N0 dispositions of the chelator N atoms around the coordination sphere. The nitrite
complexes are useful synthons of electrophilic nitrosyls, and on triturating the compounds 7b–9b with conc.
HClO4 nitro–nitrosyl derivatives, [Ru(NO2)(NO)(MeaaiR0)2](ClO4)2 (10b–12b), are isolated. The solution
1
structure and stereoretentive transformation in each reaction step have been established by H NMR results.
All the complexes exhibit strong MLCT transitions in the visible region. They are redox active and display one
M
metal-centred oxidation and successive ligand-based reductions. The redox potentials of Ru(III)/Ru(II) (E1/2
of 10b–12b are anodically shifted by ꢁ0.2 V as compared to those of dinitro precursors 7b–9b. The
)
n(NO) > 1900 cmꢀ1 strongly suggests the presence of linear Ru–N–O bonding. The electrophilic behaviour of
metal bound nitrosyl has been proved in one case (12b) by reacting with a bicyclic ketone, camphor, containing
an active methylene group and an arylhydrazone with an active methine group, and the heteroleptic tris-
chelates thus formed are characterised.
philicity of bound NO has been established by reacting with
Introduction
nucleophiles such as camphor (cmp), a bicyclic ketone with
an active methylene group and an arylhydrazone (ahz) having
an active methine group. The details of synthesis and charac-
terisation are presented below.
For the last few years, nitric oxide (NO) has been the focus of
discussion because of the discovery of its role in the immune
defense mechanism, neuronal signaling process, cardiovascular
system and in environmental chemistry.1,2 The chemistry of
transition metal nitrosyl complexes3 in general and group 8
metal nitrosyls4,5 in particular is of great interest in this regard.
The search for suitable precursors to synthesize NO-complexes
is a challenging domain3 and nitrite compounds are found to
be useful.6
Experimental
Measurements
Recently, we have developed the arylazoimidazole chemistry
of ruthenium and synthesised dichloro componds
RuCl2(RaaiR0)2 (4–6)7 and diaquo species8 [Ru(OH2)2-
UV–VIS (in MeCN) and IR (KBr discs, 4000–300 cmꢀ1) spec-
tra were run with JASCO UV–VIS–NIR model V-570 and
JASCO FTIR model 420 spectrophotometers, respectively.
Electrical conductivities (in MeCN, solute concentration
ꢁ10ꢀ3 M) were recorded using a Systronics conductivity
bridge. Magnetic susceptibilities were measured with a PAR
155 vibrating sample magnetometer. The 1H NMR spectra
in CDCl3 were carried on a Brucker 300 and 500 MHz
FTNMR spectrometer using SiMe4 as internal standard.
Microanalyses (C, H and N) were done with a Perkin–Elmer
2400 CHNS/O elemental analyser. Electrochemical studies
(in MeCN) were performed with a PARC electrochemistry sys-
tem as described elsewhere.9 In cyclic voltammetry (CV) the
following parameters and relation were used: scan rate, 50
mV sꢀ1; formal potential E0 ¼ 0.5(Epa + Epc) where Epa and
Epc are anodic and cathodic peak potentials, respectively;
(RaaiR0)2]2+
[RaaiR0 ¼ p-R–C6H4–N=N–C3H2–NN-1-R0
(1–3), R ¼ H, Me, Cl and R0 ¼ Me, CH2CH3 , CH2Ph abbre-
viated as N,N0-chelator where N(imidazole) and N(azo) repre-
sent N and N0, respectively]. Syntheses of hetero-tris-chelates,
[Ru(bpy)n(RaaiR0)3 ꢀ n](ClO4)2 [bpy ¼ 2,20-bipyridine; n ¼ 2,9
n ¼ 18) from the solvento complexes [Ru(OH2)2(bpy)2]2+
/
[Ru(OH2)2(RaaiR0)2]2+ containing labile reaction centres are
reported by us.8 In this paper, we examine the reactivity of
ꢀ
NO2 towards [Ru(OH2)2(RaaiR0)2]2+ and the reactions of
the complexes derived therefrom. The nitrites Ru(NO2)2-
(RaaiR0)2 (7b–9b) are useful synthons of nitrosyls, and mono-
nitrosyl compounds of type [Ru(NO2)(NO)(MeaaiR0)2](ClO4)2
(10b–12b) are afforded in perchloric acid medium. The electro-
DOI: 10.1039/b204442k
New J. Chem., 2002, 26, 1415–1424
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DEp is the peak–to–peak separation. For differential pulse vol-
tammetry (DPV): scan rate, 10 mV sꢀ1; modulation amplitude
(DE), 25 mV; E0 ¼ Ep + 0.5(DE) where Ep is the DPV peak
potential. The agreement between E0 data obtained by the
two techniques was good (within ꢂ5 mV). In constant poten-
tial coulometry experiments (oxidation done at potential
E0 + 200 mV); n ¼ Q/Q0 where Q0 is the calculated coulomb
count for 1e transfer and Q is coulomb count found after
exhaustive electrolysis of 0.01 mmol of solute. All experiments
were done under a dry N2 atmosphere at 295(2) K in a three
electrode configuration by using a Pt-disk milli working elec-
trode and a Pt-wire auxiliary electrode. The potentials are
referenced to a saturated calomel electrode (SCE) and are
uncorrected for the junction contributions.
C20H20N10O4Ru (7a): C, 42.47; H, 3.54; N, 24.77. Found: C,
42.34; H, 3.41; N, 24.98%. Calc. for C22H24N10O4Ru (7b): C,
44.51; H, 4.05; N, 23.60. Found: C, 44.34; H, 3.95; N,
23.68%. Calc. for C20H18N10O4Cl2Ru (7c): C, 37.85; H, 2.84;
N, 22.08. Found: C, 37.64; H, 2.75; N, 22.17%. Calc. for
C22H24N10O4Ru (8a): C, 44.51; H, 4.05; N, 23.60. Found: C,
44.64; H, 3.99; N, 23.56%. Calc. for C24H28N10O4Ru (8b): C,
46.37; H, 4.51; N, 22.54. Found: C, 46.48; H, 4.58; N,
22.49%. Calc. for C22H22N10O4Cl2Ru (8c): C, 39.87; H, 3.32;
N, 16.92. Found: C, 40.14; H, 3.21; N, 16.78%. Calc. for
C32H28N10O4Ru (9a): C, 53.55; H, 3.90; N, 19.52. Found: C,
53.39; H, 3.85; N, 19.59%. Calc. for C34H32N10O4Ru (9b): C,
54.76; H, 4.29; N, 18.69. Found: C, 54.64; H, 4.35; N,
18.58%. Calc. for C32H26N10O4Cl2Ru (9c): C, 48.85; H, 3.31;
N, 17.81. Found: C, 48.74; H, 3.45; N, 17.63%.
Materials
Ctc-[Ru(NO2)(NO)(MeaaiMe)2](ClO4)2 (10b). ctc-Ru(NO2)2-
(MeaaiMe)2 (7b) (0.1 g, 0.17 mmol) and conc. HClO4 (3 ml)
were taken in a small beaker, and the mixture was triturated
with a glass rod for 1 h. The colour of the mixture immediately
changed from violet to deep red. The pasty mass was extracted
with CH2Cl2 (4 ꢃ 5 ml) and concentrated under reduced pres-
sure. The dark red crystalline product was separated by filtra-
tion, washed with chilled water containing a few drops of dil.
HClO4 and dried in vacuo over P4O10 to yield analytically pure
10b. The yield was 0.092 g (70%). 11b [yield, 0.078 g (60%)] and
12b [yield 0.081 g (65%)] were prepared similarly starting with
8b (0.1 g, 0.16 mmol) and 9b (0.1 g, 0.13 mmol), respectively.
Calc. for C22H24N10O11Cl2Ru (10b): C, 34.02; H, 3.09; N,
18.04. Found: C, 34.14; H, 3.20; N, 18.16%. Calc. for
C24H28N10O11Cl2Ru (11b): C, 35.82; H, 3.48; N, 17.41. Found:
C, 35.74; H, 3.61; N, 17.36%. Calc. for C34H32N10O11Cl2Ru
(12b): C, 43.96; H, 3.45; N, 15.08. Found: C, 43.64; H, 3.56;
N, 15.24%.
Published methods were used to prepare RaaiR0,7 ctc-
RuCl2(RaaiR0)2 ,7 ctc-[Ru(OH2)2(RaaiR0)2](ClO4)2 ,8 alkylide-
nearylhydrazone, ArCH=NNHPh (Ar ¼ Ph, p-MeC6H4),10
arylazooxime ArN=NC(=NOH)Ph (Ar ¼ Ph, phaaoH;
Ar ¼ p-MeC6H4 , taaoH)10 and camphorquinone monoxime
(cmpoH).10 All other chemicals and organic solvents used for
preparative work were of reagent grade (SRL, India). The pur-
ification of MeCN and preparation of tetrabutylammonium
perchlorate [Bun4N][ClO4], respectively, used as solvent and
supporting electrolyte in electrochemical experiments were
done following a literature method.8
Preparation of the complexes
CAUTION! Perchlorates of heavy metal ions with organic
ligands are potentially explosive. The syntheses involve in
some cases the use of perchlorate ions. Due care must be exer-
cised to avoid explosion hazards, although we have not
encountered any problem using a small quantity at a time.
Interconversion: [Ru(NO2)(NO)(MeaaiMe)2](ClO4)2 (10b) !
Ru(NO2)2(MeaaiMe)2 (7b). To an aqueous solution of 10b (0.1
g, 0.13 mmol) was added an equivalent amount of KOH in the
same solvent. The orange–red solution immediately turned
violet, which was extracted with CH2Cl2 . The solvent was
removed by evaporation under reduced pressure and the pasty
mass, dissolved in a minimum volume of dichloromethane,
was chromatographed on an alumina (neutral) column. A deep
violet band was eluted by toluene–acetonitrile (4:1, v/v). The
identity of the product as 7b was checked by comparing its
properties with an authentic sample. The yield is almost quan-
titative.
Cis,trans,cis-Dinitrobis{1-methyl-2-(p-tolylazo)imidazole}r-
uthenium(II), ctc-Ru(NO2)2(MeaaiMe)2 (7b). Three indepen-
dent methods were employed to synthesise 7b.
Method (a). To an ethanolic blue–violet solution (15 ml) of
ctc-[Ru(OH2)2(MeaaiMe)2](ClO4)2 (0.1 g, 0.14 mmol) was
added 0.019 g (0.27 mmol) of solid NaNO2 , and the mixture
was stirred at 343–353 K for 12 h. A violet solution that
resulted was concentrated (4 ml) and kept in a refrigerator
overnight (12 h). The precipitate was collected by filtration,
washed thoroughly with water and dried in vacuo over CaCl2 .
Analytically pure 7b was obtained after chromatography over
an alumina (neutral) column on eluting the violet band with
toluene–acetonitrile (4:1, v/v) and evaporating slowly in air.
The yield was 0.088 g (80%).
Method (b). To a suspension of ctc-RuCl2(MeaaiMe)2 (4b)
(0.1 g, 0.18 mmol) in ethanol (25 ml) was added an aqueous
solution of AgNO3 and the reaction mixture stirred at room
temperature (300 K) for 2 h. AgCl so precipitated was filtered
through a G-4 sintered crucible. An ethanolic solution of
NaNO2 (0.025 g, 0.35 mmol) was added to the filtrate, and
the resulting mixture was stirred at room temperature for 8 h
under a nitrogen atmosphere. The violet solution was concen-
trated by slow evaporation and the precipitate was processed
as in method (a); yield, 0.047 g (45%).
Method (c). To a CH2Cl2–Me2CO (1:1, v/v, 30 ml) solution of
ctc-RuCl2(MeaaiMe)2 (4b) (0.1 g, 0.18 mmol) was added an
H2O–Me2CO solution of NaNO2 (0.024 g, 0.35 mmol). The
mixture was stirred at 343–353 K for 30 h. The violet solution
was processed as in method (a) to give analytically pure 7b;
yield, 0.021 g (20%).
The high yield in method (a) has prompted us to follow this
route for syntheses of the other complexes 7a, 7c, 8a–8c and
9a–9c. The yields varied in the range 65–85%. Calc. for
[Ru(cmpo)(MeaaiMe)2]ClO4 (13). Method (a): coupling with
nitroso derivative. To a solution of [Ru(NO2)(NO)(Meaai-
Me)2](ClO4)2 (0.2 g, 0.26 mmol) in 20 ml MeOH were added
0.032 g (0.26 mmol) camphor (cmp) and NaOMe (0.015 g,
0.26 mmol), and the mixture was refluxed for 2 h. During this
period the initial orange–red solution gradually turned blue–
violet. The solution was cooled and filtered. To the filtrate, a
saturated aqueous solution of NaClO4 was added and allowed
to evaporate slowly in air. The precipitate was collected by fil-
tration, washed with chilled water followed by Et2O and dried
in vacuo over P4O10 . The dry mass was dissolved in a minimum
volume of CH2Cl2 and was chromatographed through silica
gel column in toluene. A deep blue–violet band was eluted with
toluene–acetonitrile (3:1, v/v) and evaporated on a steam bath.
Analytically pure 13 was obtained after drying in vacuo over
P4O10 . The yield was 0.14 g (70%).
Method (b): direct reaction with diaquo complex. Camphorqui-
none monoxime (cmpoH) (0.049 g, 0.27 mmol) and NaOMe
(0.015 g, 0.27 mmol) were added to a solution of [Ru(OH2)2-
(MeaaiMe)2](ClO4)2 (0.2 g, 0.27 mmol) in 20 ml MeOH, and
the mixture was refluxed for 4 h. The solution was cooled
and processed as in method (a). The yield was 0.16 g (75%).
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New J. Chem., 2002, 26, 1415–1424

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Table
Ph)2]ꢄ0.5H2O
1
Crystallographic data for ctc-[Ru(NO2)2(HaaiCH2-
The product from methods (a) and (b) gave same analytical
results. Calc. for C32H38N9O6ClRu (13): C, 49.17; H, 4.87;
N, 16.13. Found: C, 49.08; H, 4.74; N, 15.99%.
Chemical formula
Formula weight
Crystal size/mm3
Crystal system
Space group
C32H29N10O4.5Ru
726.1
[Ru(taao)(MeaaiMe)2]ClO4 (14b). Method (a): Coupling
with nitroso derivative. To a methanolic solution (20 ml) of
[Ru(NO2)(NO)(MeaaiMe)2](ClO4)2 (0.2 g, 0.26 mmol) equiva-
lent amounts of p-Me–C6H4–CH=NNHPh (0.054 g, 0.26
mmol) and NaOMe (0.015 g, 0.26 mmol) were added, and
the mixture was boiled under reflux for 2 h. The orange-red
solution changed to red-violet. The volume of the solution
was reduced to one-third of its original and cooled. A satu-
rated aqueous solution of NaClO4 was added to it. The preci-
pitate that resulted was filtered and dried. Analytically pure
14b was obtained by column chromatography as described in
13. The yield was 0.15 g (70%). [Ru(phaao)(MeaaiMe)2]ClO4
(14a) was prepared (yield, 75%) similarly using
PhCH=NNHPh instead of p-Me–C6H4–CH=NNHPh.
Method (b): Direct reaction with diaquo complex. 0.065 g, (0.27
mmol) p-Me–C6H4–C(=NOH)N=NPh (taaoH) and 0.015 g
(0.27 mmol) NaOMe were added to a methanolic solution
(20 ml) of [Ru(OH2)2(MeaaiMe)2](ClO4)2 (0.2 g, 0.27 mmol)
and heated to reflux for 4 h. The reaction solution was pro-
cessed as above to give 14b [yield, 0.17 g (75%)]. 14a was pre-
pared [yield, 70%] similarly using PhC(=NOH)N=NPh
(phaaoH) instead of p-Me–C6H4–C(=NOH)N=NPh (taaoH).
Analyses from both methods are same. Calc. for
C35H34N11O5ClRu (14a): C, 50.77; H, 4.13; N, 18.89. Found:
C, 50.68; H, 4.23; N, 18.94%. Calc. for C36H37N11O5ClRu
(14b): C, 51.49; H, 4.21; N, 18.36. Found: C, 51.32; H, 4.12;
N, 18.21%.
0.10 ꢃ 0.10 ꢃ 0.04
Monoclinic
C2/c
Dc/g cmꢀ3
1.379
11.6651(3)
˚
a/A
˚
b/A
18.8847(2)
16.8101(1)
˚
c/A
b/ꢅ
V/A
101.261(1)
3631.7(4)
3
˚
m(Mo-Ka)/mmꢀ1
0.487
0.71073
˚
l/A
T/K
293(2)
4 < 2y < 47
4
7867
2y range/ꢅ
Z
Reflections collected
Unique reflections (I > 2s(I))
hkl range
2565
ꢀ12 p h p 12, ꢀ13 p k p 20,
ꢀ18 p l p 17
7.68
18.89
Ra (%)
wRb (%)
GOFc
1.068
a
b
2
4 1/2
R ¼ S|Fo ꢀ Fc|/SFo .
wR ¼ [Sw(Fo ꢀ Fc2)/SwFo
]
w ¼ 1/
[s2(Fo2) + (0.1514P)2 + 0.00P] where P ¼ (F02 + 2Fc2)/3. Goodness
of fit is defined as [w(Fo ꢀ Fc)/(no ꢀ nv)]1/2, where no and nv denote
the numbers of data and variables, respectively.
c
RuCl2(MeaaiMe)2 by AgNO3 followed by the reaction with
NaNO2 [method (b); yield, 45%]. The low yield in method
(c) may be due to the inertness of the Ru–Cl bond in such reac-
tion conditions while Ag+-assisted halide displacement
enhances the yield in method (b). The high yield in method
(a) results from the use of the solvento complex containing
labile reaction centres.8 The composition of the complexes is
supported by microanalytical results. Room temperature solid
state magnetic susceptibility measurements show that the com-
plexes are diamagnetic (t2g6, S ¼ 0). The structure of the dini-
tro derivative has been examined in one case (9a) by X-ray
crystallography. Trituration6 of solid Ru(NO2)2(MeaaiR0)2
(7b–9b) in concentrated HClO4 at ambient condition gives an
orange–red solution from which the nitrosyl complexes
[Ru(NO2)(NO)(MeaaiR0)2](ClO4)2 (10b–12b) are isolated. In
alkaline media nitrosyl complexes, 10b–12b, regenerate the
corresponding nitrite precursors (7b–9b). The reaction of H+
with the coordinated NO2 group in nitrite complexes and the
nucleophilic attack of OHꢀ on the NO+ in nitrosyls are the
key steps for such reactions.11
The violet dinitrites 7–9 are soluble in common organic sol-
vents but insoluble in H2O whereas the orange–red nitrosyls
10b–12b are soluble in water and in a range of common
organic solvents viz., methanol, ethanol, acetone, acetonitrile,
chloroform, dichloromethane. In MeCN, 10b–12b behave as
1:2 electrolytes (LM ¼ 140–160 Oꢀ1 cm2 molꢀ1) whereas
non-electrolytic behaviour is seen for type 7–9 complexes as
indicated by their very low LM values (10–20 Oꢀ1 cm2 molꢀ1).
X-Ray structure determination of Ru(NO2)2(HaaiCH2Ph)2 (9a)
Crystals suitable for the X-ray diffraction study of dinitro-
bis{1-benzyl-2-(phenylazo)imidazole}ruthenium(II),
Ru(NO2)2(HaaiCH2Ph)2 (9a) were grown by slow diffusion of
hexane into a dichloromethane solution of the complex at 298
K. X-Ray diffraction data were collected at 293 K with a Sie-
mens SMART CCD diffractometer using graphite-monochro-
˚
mated Mo-Ka radiation (l ¼ 0.71073 A). A summary of the
crystallographic data and structure refinement parameters
are given in Table 1. Data were corrected for Lorentzian polar-
ization effects and for linear decay. Semi-empirical absorption
corrections based on C-scans were applied. The structure was
solved by the direct method using SHELXS-86 and successive
difference Fourier syntheses. All non-hydrogen atoms were
refined anisotropically. The hydrogen atoms were fixed geome-
trically and refined using a riding model. There is one water
molecule per half of the complex. The solvate water was
refined in three 1/3 occupancy sites; the oxygen positions were
refined isotropically and water hydrogen-atoms were not
included in the refinement.
CCDC reference number 190870. See http://www.rsc.org/
suppdata/nj/b2/b204442k/ for crystallographic data in CIF
or other electronic format.
Results and discussion
Synthesis and formulation
Molecular structure
The synthetic routes are shown in Scheme 1. Diaquo com-
plexes, ctc-[Ru(OH2)2(RaaiR0)2](ClO4)2 , prepared by Ag+-
assisted aquation of ctc-RuCl2(RaaiR0)2 (4–6), were reacted
with NaNO2 (excess amount > 3 mol) under stirring at 343–
353 K in aqueous alcohol to give Ru(NO2)2(RaaiR0)2 (7–9)
in good yield [method (a), 65–85%]. The dinitrites 7–9 were
also synthesized either directly on stirring RuCl2(MeaaiMe)2
with NaNO2 in dichloromethane–acetone for 30 h [method
(c); yield, 20%) or in situ synthesis of the aquo complex from
The single crystal X-ray structure of Ru(NO2)2(HaaiCH2Ph)2
(9a) is shown in Fig. 1, and the bond parameters are collected
in Table 2. The structure shows a distorted octahedron around
the metal centre with six N donor set (RuN6) arising from two
N(azo), two N(imidazole) and two N(nitro) donor centres. A
crystallographic 2-fold rotation axis passes through Ru. Two
atomic groups Ru, N(1a), N(6a), N(1), N(20a) (plane 1) and
Ru, N(1a), N(20), N(1), N(6) (plane 2) constitute excellent
New J. Chem., 2002, 26, 1415–1424
1417

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Scheme 1 Reagents and conditions: (i) AgNO3 , EtOH–H2O (1:1, v/v), reflux, NaClO4 ; (ii) NaNO2 , EtOH–H2O, stir, 343–353 K; (iii) conc.
HClO4 ; (iv) KOH, MeCN.
values due to acute chelate bite angles.7,9 The N–N distance is
˚
planes (max. deviation from mean plane by p0.03 A). The
˚
1.293(9) A and is longer than the free ligand value [1.250(1)
atomic group Ru, N(20), N(20a), N(6), N(6a) (plane 3) consti-
˚
12
˚
A]. The increase in bond length supports charge delocalisa-
tutes a good plane and deviate by 0.13 A from the mean plane.
Planes 1 and 2 are orthogonal (88ꢅ) and plane 3 inclines at an
angle 97ꢅ with other two planes separately. Two chelate planes
constituting the set of atoms Ru, N(1a), C(2a), N(2a), N(6a),
and Ru, N(1), C(2), N(2), N(6), respectively (mean
deviation < 0.01 A) are mutually orthogonal (86 ) to each
other. The pendant phenyl group to each chelate ring is no
longer planar and makes dihedral angle of 54ꢅ with the respec-
tive chelate ring. The bite angle N(1)–Ru–N(6)/N(1a)–Ru–
N(6a) is 76.2(3)ꢅ and is deviated from regular pentagonal
angle. This may cause distortion to octahedral geometry.
The angles N(6)–Ru–N(20)/N(6a)–Ru–N(20a), 170.91(4)ꢅ
and N(1)–Ru–N(1a), 172.4(4)ꢅ are reduced from trans angular
tion, dp Ru(II) ! p* ligand (N=N). The coordination of
N(azo) with Ru can lead to decrease in the bond order due
to both s-donor and p-acceptor character of the ligand—the
latter having a more pronounced effect.7,13 The Ru–N(azo)
[N(6)/N(6a)] and Ru–N(imidazole) [N(1)/N(1a)] bond dis-
ꢅ
˚
˚
tances are equivalent at 2.035 A which is unusual. In general,
complexes of ruthenium(II)–1-alkyl-2-(arylazo)imidazoles7
show shorter Ru–N(azo) distances than Ru–N(imidazole)—
supporting metal-to-ligand charge transfer to be localized on
the azo bond (N=N) (vide infra). This strengthens the p-acidic
nature of the ligand system. The electron withdrawing cisoid
NO2 groups compete with azo N atoms to withdraw metal
d-electrons. Again the azo groups lie trans to –NO2 groups,
thus they are in competition for the same metal dp orbitals.
This may cause elongation of the Ru–N(azo) distance and
the accidentally equivalence to the Ru–N(imidazole) bond
length, which is not7–9 the case for RuCl2(RaaiR0)2 and [Ru(b-
py)n(RaaiR0)3 ꢀ n](ClO4)2 . The NO2 groups display some dis-
order of the oxygen atoms—one oxygen [O(1)] was refined
with 100% occupancy, the remaining two sites [O(2) and
O(3), assigned in Fig. 2) were refined with 50% occupancies.
Fig. 1 omits O(3) for clarity. There is one half molecule of
water per molecule of the complex.
A molecular packing view is shown in Fig. 2. An interesting
hydrogen bonded one-dimensional (1D) wave is observed
where every molecular unit present as W or M motif. Two 1-
CH2Ph group act as two wings in the motif and two (N=N)-
Ph groups cross each other. An intramolecular p–p interaction
between two 1-CH2Ph groups is noticed. Molecular units are
hydrogen bonded to constitute one-dimentional (1D) chain.
Imidazolyl-H(4) is hydrogen bonded with disordered O(2)/
O(3)–N and –CH2– of 1-CH2Ph forms hydrogen bonding with
the O–N(NO2) group of neighbouring molecule. Thus, H-
bonding constructs a 1D chain. The pendant phenyl and (1-
CH2)Ph are in efficient C–Hꢄ ꢄ ꢄp interaction with optimum dis-
tance between the centre of gravity of the phenyl ring and
H(8a)/H(14a) (range of 2.94–3.27 A).14 The hydrogen bonding
˚
parameters are listed in Table 2.
Fig. 1 Single crystal X-ray structure of Ru(NO2)2(HaaiCH2Ph)2
(9a).
1418
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ꢅ
˚
Table 2 Selected bond distances (A) and angles ( ) for ctc-[Ru(NO2)2(HaaiCH2Ph)2]ꢄ0.5H2O
Angles/ꢅ
˚
Distances/A
Ru–N(1)
Ru–N(6)
2.035(7)
2.035(6)
2.137(13)
1.293(9)
1.01(2)
N(1)–Ru–N(6)
76.2(3)
89.8(8)
172.4(4)
98.3(3)
94.8(4)
90.6(4)
94.8(4)
88.3(3)
91.6(4)
170.9(4)
N(20)–Ru–N(20A)
N(1)–Ru–N(1A)
N(1)–Ru–N(6A)
N(1)–Ru–N(20)
N(1A)–Ru–N(20)
N(1A)–Ru–N(20A)
N(6)–Ru–N(6A)
N(6A)–Ru–N(20)
N(6)–Ru–N(20)
Ru–N(20)
N(2)–N(6)
N(20)–O(1)
N(20)–O(3)
1.32(4)
Hydrogen bonding parameters
D–H–A/ꢅ
˚
D–H/A
˚
H–A/A
˚
D–A/A
Donor (D)
H
Acceptor (A)
C(4)
C(4)
H(4A)
H(4A)
H(10A)
H(12A)
O(1)
O(2)
O(3)
O(2)
0.9300
0.9300
0.9300
0.9699
2.4880
2.3882
2.3294
2.4872
3.3771
3.1235
3.0621
3.3495
160.05
135.85
135.41
148.01
C(10)
C(12)
X–Hꢄ ꢄ ꢄCg (p-ring) interactions
X–Hꢄ ꢄ ꢄCg/ꢅ
˚
X–Cg/A
˚
H-perpendicular to Cg (p-ring)/A
C(8)–H(8A)ꢄ ꢄ ꢄ(C-6)
C(14)–H(14A)ꢄ ꢄ ꢄ(C-5)
C(18)–H(18A)ꢄ ꢄ ꢄ(C-5)
3.2012
2.9367
3.2767
3.181
2.724
3.109
138.14
128.30
123.37
Cg ¼ centroid of (C-5) or (C-6). The (C-5) plane contains C6C7C8C9C10C11; (C-6) plane contains C13C14C15C16C17C18; (C-4) plane contains
˚
N1C2N3C4C5. Distances among the planes (A), (C-5)–(C-5) 5.535(3), (C-5)–(C-4) 4.269(2), (C-4)–(C-4) 3.890(1), (C-6)–(C-5) 5.151(5), (C-6)–(C-4)
˚
5.683(6) A.
Spectral studies
(MeaaiR0)2](ClO4)2 (10b–12b) show a very strong stretch in
the range 1910–1925 cmꢀ1 which is conspicuously absent in
the spectra of Ru(NO2)2(MeaaiR0)2 . This is certainly due to
the stretching mode of n(NO) of the coordinated nitrosyl
group.3,4,11 The stretching frequency n(NO) qualitatively indi-
cates the stereochemistry of M–N–O bonding. Usually,
n(NO) > 1700 cmꢀ1 has been assigned to linear M–N–O
bond.11,16,17 Thus this present series of Ru–NO complexes
are assumed to be linear. Other important frequencies are
n(H2O) at 3350–3400 cmꢀ1 and n(ClO4) at 1140–1145, 1110–
1120 and 1080–1090 cmꢀ1 along with weak bands at 640 and
625 cmꢀ1. Triplet splitting pattern of ClO4 may be due to some
sort of hydrogen bonding interaction, Cl–Oꢄ ꢄ ꢄH(O/C) (vide
supra).9, 18
Infrared spectra of dinitro complexes, Ru(NO2)2(RaaiR0)2 (7–
9) show a 1:1 correspondence to the spectra of dichloro analo-
gue, ctc-RuCl2(RaaiR0)2 except the appearance of intense
stretching at 1300–1335 and 1250–1280 cmꢀ1 with concomi-
tant loss of n(Ru–Cl) at 320–340 cmꢀ1. They are assigned to
n(NO)as and n(NO2)s , respectively.5,6,15 The n(N=N) and
n(C=N) appear at 1365–1380 and 1570–1600 cmꢀ1, respec-
tively. Mono-nitro–nitroso–ruthenium(II), [Ru(NO2)(NO)-
The solution electronic spectra of these new complexes were
recorded in dry acetonitrile. Dinitro complexes 7–9 exhibit
multiple transitions in the UV–visible region (Table 3, Fig.
3). They display an intense MLCT transition in the range
550–560 nm along with weak longer wavelength absorption
at 740–750 nm. The transitions are blue shifted by ꢁ40 nm
as compared with corresponding dichloro derivatives,
RuCl2(RaaiR0)2 . In the nitroso derivatives [Ru(NO2)(NO)-
(MeaaiR0)2](ClO4)2 (10b–12b) the intense absorption bands
are further shifted to shorter wavelength, 400–420 nm along
with a weak band at 600–620 nm. This is attributed to strong
II
dp(Ru ) ! p*(NO) back bonding which stabilizes the dp level
and consequently shifts the MLCT band to lower wave-
length.5,6,15 The nitrosyl complexes are found to be stable only
in dry acetonitrile solutions. In ordinary acetonitrile or in con-
tact with water the spectrum spontaneously changes and levels
off with the spectrum of precursor dinitro derivative. The
transformation has been proved through product (7b) isolation
in one case (10b) and characterized comparing its properties
with authentic sample.
1
The H NMR spectra of Ru(NO2)2(RaaiR0)2 (7–9) com-
Fig. 2 Molecular packing view of 9a along the z-axis showing hydro-
gen bonds and the 1-D chain.
plexes were unambiguously assigned (Table 4) on comparing
New J. Chem., 2002, 26, 1415–1424
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1420
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and 5-H appear as a doublet at lower frequency side of the
spectra (7.0–7.2 ppm for 4-H; 6.9–7.1 ppm for 5-H). The
aryl-Me (R ¼ Me) in Ru(NO2)2(MeaaiR0)2 (7b–9b) appears
as a single peak at 2.30 ppm and is in consonance with stereo-
retentive reaction. Isomerisation of ctc-isomer may lead to ccc-
configuration which belongs to C1-symmetry and would give
two equally intense Ar-Me signals.7 The mononitrosyls,
[Ru(NO2)(NO)(MeaaiR0)2](ClO4)2 show a similar pattern of
signals in the aromatic region and the spectra have been shifted
to higher field by 0.03–0.15 ppm from those of respective dini-
tro derivatives. This is presumably be due to better
dp(Ru) ! p*(NO) charge transfer3,4,11 which increases the
hardness of Ru(II) leading to strong s-interaction with the che-
lator, MeaaiR0. The transformation, Ru(NO2)2(MeaaiR0)2 !
[Ru(NO2)(NO)(MeaaiR0)2]2+ reduces the overall symmetry of
the product (C2 ! C1) and is supported through the appear-
ance of two Ar-Me signals at 2.4 and 2.2 ppm in 10b–12b. This
has been indirectly proved by conversion of [Ru(NO2)(NO)-
(MeaaiMe)2](ClO4)2 (10b) to ctc-Ru(NO2)2(MeaaiMe)2 on
treating with KOH solution and the latter gives an identical
spectrum as shown by 7b.
Fig. 3 Electronic spectra of Ru(NO2)2(MeaaiCH2CH3)2 (8b) (——),
[Ru(NO2)(NO)(MeaaiCH2CH3)2](ClO4)2 (12b) (ꢄ ꢄ ꢄ), [Ru(cpmo)-
(MeaaiMe)2]ClO4 (13) (---) in dry acetonitrile.
Redox properties
The potential values of the complexes in dry acetonitrile solu-
tion are given in Table 3 and representative voltammograms
are shown in Fig. 5. The dinitrites 7–9 exhibit a quasi-reversi-
ble (DEP q 100 mV) oxidative response in the potential range
1.1–1.3 V [Table 3, Fig. 5(a)]. This is assigned to the Ru(III)/
Ru(II) couple [eqn. (1)].
with RuCl2(RaaiR0)2 .7 The aryl protons (7-H–11-H) of 7–9 are
downfield shifted by 0.1–0.7 ppm as compared to those of the
parent dichloro derivatives.7–9 They are affected by substitu-
tion; 8- and 10-H are severely perturbed due to changes in
the electronic properties of the substituents in the C(9)-posi-
tion. The aryl protons 7-(70-)H and 11-(110-)H resonate
asymmetrically indicative of a magnetically anisotropic envir-
onment7 even in the solution phase. The proton movement
upon substitution (9-R) is corroborated with the electromeric
effect of R. The 1-R0 (R0 ¼ Me, CH2CH3 , CH2Ph) exhibit
usual spin–spin interaction. 1-Me appears as a singlet at 4.2
ppm for Ru(NO2)2(RaaiMe)2 ; the methylene protons, 1-
CH2(CH3) show AB type sextet (ca. 4.4, 4.6 ppm, J ¼ 10–12
Hz) and (1-CH2)CH3 gives a triplet at 1.5 ppm (8.0 Hz) for
Ru(NO2)2(RaaiCH2CH3)2 ; 1-CH2(Ph) protons appear as AB
type quartets (ca. 5.5, 5.7 ppm) with geminal coupling constant
av. 18 Hz in Ru(NO2)2(RaaiCH2Ph)2 (Fig. 4). Imidazole 4-
½RuðNOÞ2Þ2ðRaaiR0Þ2ꢆþ þ e Ð ½RuðNO2Þ2ðRaaiR0Þ2ꢆ:::::::
ð1Þ
The one-electron stoichiometry of this couple is confirmed by
constant potential electrolysis at 1.4 V vs. SCE in one case,
Ru(NO2)2(HaaiMe)2 (7a) and electron count ratio equals to
0.94. The potential E1/2M is dependent on the substituent type
M
R and R0. The present series of complexes show higher E1/2
values than that of precursor dichloro derivatives 4–6 by ꢁ0.4
V. The better electron withdrawing property of NO2ꢀ over Clꢀ
stabilises the dp shell of the metal and thus shifts the metal-
centred redox process to more anodic values.19 A similar com-
Table 4 1H NMR spectral data, d/ppm (J/Hz) of the complexes in CDCl3
Compound
9-Me
2.31
2.30
2.30
4-Hc
5-Hc
11-Hc
7-Hc
8,10-H
1-CH3
1-CH2
Others
7aa
7b
7.15 (7.5)
6.98 (7.5)
7.13 (8.1)
7.14 (7.5)
7.02 (8.1)
7.13 (8.1)
7.06 (7.8)
6.97 (8.1)
7.11 (7.8)
7.18 (8.1)
7.15 (8.1)
7.20 (7.5)
7.07 (7.8)
7.06 (7.5)
6.86 (7.5)
7.02 (8.1)
7.00 (7.5)
6.94 (8.1)
7.06 (8.1)
6.98 (7.8)
6.85 (8.1)
7.02 (7.8)
6.99 (8.1)
7.02 (8.1)
7.01 (7.5)
6.88 (7.8)
8.03 (8.1)
8.17 (8.1)
8.15 (7.8)
8.01 (7.8)
8.11 (7.5)
8.14 (7.5)
8.08 (8.1)
8.21 (8.1)
8.15 (8.1)
8.25 (8.1)
8.19 (7.8)
8.27 (7.8)
8.10 (7.5)
7.80 (8.1)
8.04 (8.1)
7.95 (7.8)
7.85 (7.8)
8.04 (7.5)
8.06 (7.5)
8.00 (8.1)
8.10 (8.1)
8.05 (8.1)
8.18 (8.1)
8.09 (7.8)
8.20 (7.8)
7.84 (7.5)
7.45 (8.1)d
7.34 (8.1)c
7.55 (7.8)c
7.45 (8.1)d
7.12 (7.5)c
7.61 (7.5)c
7.48 (8.1)d
7.10 (8.1)c
7.58 (8.1)c
7.49 (8.1)c
7.44 (7.8)c
7.52 (7.8)c
7.14 (8.1)c
4.09f
4.17f
7c
4.16f
8aa
8b
1.52 (8.1)d
1.48(8.1)d
1.50 (8.1)d
4.42, 4.55(10.0)e
4.40,4.52 (11.0)e
4.45, 4.53 (11.0)e
5.48, 5.73 (15.0)g
5.46, 5.73 (17.0)g
5.44, 5.70 (18.0)g
8c
9aa
9b
7.2–7.4h
7.2–7.5h
7.2–7.6h
9c
10b
11b
12b
13
2.44, 2.23
2.40, 2.16
2.45, 2.21
2.29, 2.38
4.36f
1.61(8.1)d
4.52, 4.65 (12.0)e
5.50, 5.74 (18.0)g
4.03b , 4.18b
1.02–1.20i ,
1.55–1.75i
7.40–7.70j
7.35–7.70j ,
2.48k
14a
14bl
2.35, 2.39
2.33, 2.35,
2.39, 2.42
6.96 (8.1)
6.95 (7.5)
6.72 (8.1)
6.74 (7.5)
8.17 (8.1)
8.15 (8.1)
7.95 (8.1)
7.92 (8.1)
7.20 (8.1)c
7.20 (8.1)c
4.02b , 4.11b
3.98, 4.02,
4.06, 4.10
a
d(9-H) 7.60 ppm(m). b d (9-Me). c Doublet. d Triplet. e AB type sextet, geminal coupling constant. f 1-Me, singlet. g AB type quartet, geminal
coupling constant. h Phenyl-H. i camphor –CH2– and –CH– signals. j Phenyl-H of Phaao. k Phenyl-Me of taao. l Four Ar-Me and 1-Me signals
appear in the intensity ratio 2:2:1:1.
New J. Chem., 2002, 26, 1415–1424
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Fig. 5 Cyclic voltammogram of (a) Ru(NO2)2(MeaaiMe)2 (7b)
(——), repeat cycles show the generation of a new couple (ꢄ ꢄ ꢄ) and
(b) [Ru(NO2)(NO)(MeaaiMe)2](ClO4)2 (10b) in MeCN using a Pt-disk
milli electrode at 298 K, SCE reference and TBAP (0.1 M) supporting
electrolyte.
metry experiments. The couples IV and V are assigned to suc-
cessive one-electron reductions of the coordinated NO+ unit;
[{Ru–NO}+ ! {Ru–NO} (couple II), {Ru–NO} ! {Ru–
NO}ꢀ (couple III). Couple III is irreversible indicating
instability of the reduced species. The assignment is based on
earlier observations of similar Ru–bipyridine21 and Ru–azo-
pyridine11 systems. The potential values of the present set of
complexes lie between those of analogous bipyridine and azo-
pyridine complexes and follow the order azopyridi-
ne > azoimidazole > bipyridine. This is in line of p-acidity
order of these different ligand systems.22 The high potential
voltammetric wave at 1.4–1.5 V (couple I) (DEP q 120 mV)
[Fig. 5(b)] is assigned to the Ru(III)/Ru(II) couple. The one-
electron stoichiometry of the couples is assigned by compari-
son of current heights in differential pulse voltammetry experi-
ments. Successive reductions on the negative side of SCE were
observable and their one-electron nature was confirmed by
comparing the current heights of these process with that of
the couple of eqn. (1) in the differential pulse voltammetry
experiments and are assigned to the reduction of coordinated
ligand. The azo group in RaaiR0 may accommodate two elec-
trons7 and hence two coordinated ligands should exhibit four
reductive responses. However, within the available potential
window only three reductions were observable.
Fig. 4 1H NMR spectra of (a) Ru(NO2)2(ClaaiCH2CH3)2 (8c), (b)
Ru(NO2)2(HaaiCH2Ph)2 (9a) and (c) [Ru(taao)(MeaaiMe)2]ClO4
(14b) in CDCl3at 298 K.
plex with a diimine system, [Ru(NO2)(py)(bpy)2]+ displays the
Ru(III)/Ru(II) couple at ꢁ1.0 V vs. SCE.20,21 The stronger p-
acidic nature of RaaiR0 compared to the a-diimine system
leads to better stabilisation of Ru(II) in the present series of
complexes. The cyclic voltammogram of Ru(NO2)2(RaaiR0)2
exhibits some unusual behaviour on repetitive cycles. The
reduction sweep shows a new wave that has a counter oxida-
tive wave on the second sweep. The second and consecutive
cycles increase the peak height with subsequent decrease of
the primary couple [Fig. 5(a)]. The first primary redox process
Reactivity of [Ru(NO2)(NO)(MeaaiMe)2](ClO4)2
The nitrosyl complexes exhibit a high degree of electrophilic
character3,4,11,17 of the coordinated NO [n(NO) > 1900
cmꢀ1]. The electrophilic behaviour of these new complexes
has been investigated by reacting camphor (cmp), a compound
having an active methylene group and arylhydrazone (ahz), an
organic substrate with active methine group. Methanolic solu-
tions of [Ru(NO2)(NO)(MeaaiMe)2](ClO4)2 (10b) reacted
smoothly with camphor (cmp) and arylhydrazone (ahz) in
the presence of an organic base, viz., NaOMe. The progress
of the reaction was followed by monitoring the n(NO) decrease
at ꢁ1910 cmꢀ1, and the composition of the reaction mixture
was followed by thin-layer chromatography, collecting por-
tions of the reaction solution from time to time. The product
analyses are in accordance with reactions (2) and (3). These
reactions
MI
(E1/2 ¼ 1.1–1.3 V) is assigned to the Ru(III)/Ru(II) couple
[eqn. (1)] and the newly generated wave at (E1/2M2) 0.5–0.6
2+
0
II
V
may be assigned20 to [Ru (NO2)(NO)(MeaaiR )2]
/
+
0
II
[Ru (NO2)(NO)(MeaaiR )2] . The nitrosyl complexes,
[Ru(NO2)(NO)(RaaiR0)2](ClO4)2 exhibit multiple redox
responses in the potential range +2.0 to ꢀ2.0 V vs. SCE.
Two quasi-reversible reductive responses in the potential range
0.5 to 0.6 V and ꢀ0.1 to ꢀ0.2 V are seen (Fig. 5). The one-elec-
tron stoichiometry of the reversible electron transfer process is
established by constant potential coulometry locked at 0.3 V
(n ¼ 0.93) in the case of 10b. The reduced species is unstable
even at 273 K, which has precluded its isolation and character-
ization. The one-electron nature of other couples (I and III–
VI, Table 3) has been confirmed by differential pulse voltam-
1422
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Conclusion
Dinitro complexes of ruthenium(II)–azoimidazoles, ctc-
Ru(NO2)2(RaaiR0)2 have been synthesised by stereoretentive
reaction of the diaquo complexes [Ru(OH2)2(RaaiR0)2]2+ with
nitrite ion, and characterised by spectroscopic and X-ray crys-
tallographic studies. They are useful synthons for electrophilic
mono-nitrosyls, [Ru(NO2)(NO)(RaaiR0)2](ClO4)2 . The elec-
trophilic activity of bound NO has been established through
C–N bond formation via reaction with organic substrates con-
taining active methylene (camphor)/methine groups (arylhy-
drazone). The complexes exhibit strong MLCT transitions.
Voltammetric studies show Ru(III)/Ru(II) couples along with
successive ligand-based reductions and additionally metal-
bound NO+ reduction in nitrosyl derivatives.
ð2Þ
Acknowledgements
ð3Þ
Financial support from the Council of Scientific & Industrial
Research and the Department of Science and Technology,
New Delhi, India is thankfully acknowledged.
proceed very slowly in the absence of base. However, addition
of a base, viz. NaOMe, to the reaction mixture changes the
solution colour from orange–red to red–violet. The reaction
involves electrophilic addition17,20 of the coordinated nitrosyl
to the activated methylene/methine group (formed by the
added base) of the ketone/hydrazone forming a bound oxime
(camphorquinone monoxime (13), phenylazooxime (14a) and
p-tolylazooxime (14b). Deprotonation of active –CH2–/
=CH– groups leads to the generation of a carbanion which
subsequently attacks electrophilic NO+ and results in cam-
phorquinone monoxime (13) and arylazoxime (14) chelate
rings, respectively, via expulsion of the –NO2 group. Disap-
pearance of n(NO) at ꢁ1910 cmꢀ1 and the growth of
n(N!O) at 1280 cmꢀ1 strongly support eqns. (2) and (3). To
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substantiate such nitrosation reaction
a direct synthetic
approach was adopted. The reaction of camphorquinone
monoxime (cmpoH) and arylazooxime (aaoH) separately with
the diaquo complex in boiling alcohol in presence of a base,
respectively yielded 13 and 14 through solvolytic displacement
followed by chelation of appropriate oximates. Microanalyti-
cal, spectroscopic and electrochemical results of 13 and 14
obtained in both methods match well. The heteroleptic tris-
chelates 13/14 may exist in two isomeric forms with stereore-
tentive ctc-Ru(MeaaiMe)2 geometry. However, we cannot
separate these isomers by a chromatographic purification pro-
cess. The 1H NMR spectra of the complexes suggest a
mixture of isomers. The aliphatic region (2–5 ppm) is particu-
larly useful to support the chemical reaction. There are four
Ar-Me signals at 2.33, 2.35, 2.39 and 2.42 in the intensity
ratio 2:2:1:1 and 1-Me appear at 3.98, 4.06, 4.02 and 4.10
ppm in the intensity ratio of 2:2:1:1 for [Ru(MeaaiMe)2-
(taao)]ClO4 . Two signals of 2:2 intensity ratio may be due
to Me of MeaaiMe and the signals corresponding to 1:1 inten-
sity ratio may refer to the Me group of the coordinated taao
group. The appearance of a pair of Me signals of either
MeaaiMe or taao supports two isomeric molecules in the mix-
ture. [Ru(phaao)(MeaaiMe)2]ClO4 gives a pair of Ar-Me at
2.35, 2.39 and N(1)-Me at 4.02 and 4.11 ppm of equal inten-
sity. [Ru(cmpo)(MeaaiMe)2]ClO4 shows a very complex pat-
tern of proton signals in the aliphatic region, 1.02–1.20;
1.55–1.70; 2.29; 2.38 and 4.03, 4.18 ppm. Camphor –CH2–
and –CH– may be assigned to 1.02–1.20 and 1.55–1.70 ppm
signals. The 9-Me MeaaiMe is assignable to the signals at
2.29 and 2.38 ppm and 1-Me refers to 4.03 and 4.18 ppm.
A pair of signals indicates an isomeric mixture of complexes
in solution. The aromatic region gives a very complex splitting
pattern and assignments of the signals are not made. Cyclic
voltammetric studies show an Ru(III)/Ru(II) couple at 1.2–
1.3 V and successive ligand reductions on the negative side
of SCE.
4
5
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