Reaction of Ru(PPh3)3Cl2 with 2-(arylazo)pyrimidines. Spectral and redox characterisation of the products. Part V. Single crystal X-ray structure of Ru(PPh3)2(papm)Cl2 (papm = 2-(phenylazo)pyrimidine)

Article abstract of DOI:10.1016/S0277-5387(00)00655-0

Reaction of 2-(arylazo)pyrimidine (aapm) with Ru(PPh₃)₃Cl₂ in CH₂Cl₂ solution affords [Ru(PPh₃)₂(aapm)Cl₂] (2) while the reaction under refluxing conditions in EtOH isolates [Ru(PPh₃)₂(aapm)₂] (ClO₄)₂·H20 (3/4). Single crystal X-ray diffraction study of dichloro-bis(triphenylphosphine) (2-(phenylazo)pyrimidine)ruthenium(II) has assigned a cis-Ru(PPh₃)₂ motif to the complex. Isomers of [Ru(PPh₃)₂(aapm)₂] (ClO₄)₂ have been characterised by ¹H NMR data and they exist in cis-trans-cis and cis-cis-cis configurations in which coordination is considered with reference to three pairs of sequence of P, P (PPh₃ abbreviated as P), N, N (N is N(pyrimidine)) and N′, N′ (N′ is N(azo)). The complexes exhibit MLCT transitions in the visible region. Redox studies show the Ru(III)/Ru(II) couple is in the range 0.8-1.2 V vs. SCE and [Ru(PPh3)2(aapm)2](ClO₄)₂ exhibits a higher potential value than that of [Ru(PPh₃)₂(aapm)Cl₂].

Full text of DOI:10.1016/S0277-5387(00)00655-0

Polyhedron 20 (2001) 599–606
www.elsevier.nl/locate/poly
Reaction of Ru(PPh3)3Cl2 with 2-(arylazo)pyrimidines.
Spectral and redox characterisation of the products.
Part V. Single crystal X-ray structure of Ru(PPh3)2(papm)Cl2
(papm=2-(phenylazo)pyrimidine)
Prasanta Kumar Santra a, Chittaranjan Sinha a,*, Wen-Jyh Sheen b, Fen-Ling Liao c,
Tian-Huey Lu b
a Department of Chemistry, The Uni6ersity of Burdwan, Burdwan 713104, India
b Department of Physics, National Tsing-Hua Uni6ersity, Hsinchu 300, Taiwan, ROC
c Department of Chemistry, National Tsing-Hua Uni6ersity, Hsinchu 300, Taiwan, ROC
Received 23 August 2000; accepted 7 November 2000
Abstract
Reaction of 2-(arylazo)pyrimidine (aapm) with Ru(PPh3)3Cl2 in CH2Cl2 solution affords [Ru(PPh3)2(aapm)Cl2] (2) while the
reaction under refluxing conditions in EtOH isolates [Ru(PPh3)2(aapm)2](ClO4)2·H2O (3/4). Single crystal X-ray diffraction study
of dichloro-bis(triphenylphosphine){2-(phenylazo)pyrimidine}ruthenium(II) has assigned a cis-Ru(PPh3)2 motif to the complex.
1
Isomers of [Ru(PPh3)2(aapm)2](ClO4)2 have been characterised by H NMR data and they exist in cis–trans–cis and cis–cis–cis
configurations in which coordination is considered with reference to three pairs of sequence of P, P (PPh3 abbreviated as P), N,
N (N is N(pyrimidine)) and N%, N% (N% is N(azo)). The complexes exhibit MLCT transitions in the visible region. Redox studies
show the Ru(III)/Ru(II) couple is in the range 0.8–1.2 V vs. SCE and [Ru(PPh3)2(aapm)2](ClO4)2 exhibits a higher potential value
than that of [Ru(PPh3)2(aapm)Cl2]. © 2001 Elsevier Science B.V. All rights reserved.
Keywords: Azopyrimidines; High potential Ru(II) complexes; Phosphine; Isomer; Structures
1. Introduction
plexes where one of the chelates is constituted by
catecholato/dithiolato coordination.
This work stems from our interest in the exploration
of chemical reactivities of arylazoheterocycles [1–9].
The chromophore is –NꢀN–CꢀN–. It stabilises low
valent metal redox states [2,7,8]; forms oxometal com-
plexes [10]; assists metal mediated organic transforma-
tion to incorporate atoms/groups at the ortho-C–H
function of the pendant aryl ring [11–13]. The com-
plexes exhibit different charge transfer transitions like
MLCT (metal-to-ligand charge transfer) [2,7,8] and
LLCT (ligand-to-ligand charge transfer) [3,9] and the
latter is observed usually in the mixed chelate com-
2-(Arylazo)pyrimidines (aapm) are a newly designed
system and some of their coordination compounds have
been reported by our group [7,8]. In the present work
we wish to report two classes of mixed-ligand rutheniu-
m(II)–arylazopyrimidine complexes incorporating one
and two 2-(arylazo)-pyrimidine (aapm) ligands.
Triphenylphosphine and Cl− act as coligands.
Ru(PPh3)3Cl2 is used as starting complex of ruthenium.
In dichloromethane, the mixing of Ru(PPh3)3Cl2 and
aapm at ambient condition gives Ru(PPh3)2(aapm)Cl2
whereas the reaction in EtOH under reflux yields
[Ru(PPh3)2(aapm)2]2+ as the major species. The com-
plexes are characterised by spectroscopic studies and in
one case, Ru(PPh3)2(aapm)Cl2, the structure is estab-
lished by X-ray crystallography.
* Corresponding author. Tel.: +91-342-60810; fax: +91-342-
64452.
E-mail address: bdnuvlib@giascl01.vsnl.net.in (C. Sinha).
0277-5387/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved.
PII: S0277-5387(00)00655-0

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P.K. Santra et al. / Polyhedron 20 (2001) 599–606
2. Experimental
enced to the saturated calomel electrode (SCE) in ace-
tonitrile and are uncorrected for junction potential.
2.1. Material
2.3. Preparation of complexes dichloro-
bis(triphenylphosphine){2(phenylazo)pyrimidine}
ruthenium(II), Ru(PPh3)2(papm)Cl2 (2a)
Commercial RuCl3 was purchased from Arora
Matthey, Calcutta, India and was converted into
RuCl3·3H2O by repeated evaporation to dryness with
conc. HCl. PPh3 was obtained from E. Merck.
Ru(PPh3)3Cl2 [14] and 2-(arylazo)pyrimidines [7] were
obtained as described earlier. [Bu4N][ClO4] and MeCN
for electrochemical work were purified by a reported
procedure [2].
The ligand papm (1a) (0.04 g, 0.23 mmol) in dry
CH2Cl2 (10 ml) was added dropwise to a stirred solu-
tion (10 ml) of Ru(PPh3)3Cl2 (0.2 g, 0.21 mmol) at
room temperature in CH2Cl2. The pink–violet solution
was stirred for 1/2 h. The solution was then allowed to
diffuse into hexane which was layered on the CH2Cl2
solution. The dark crystalline product that separated
was filtered off, washed with hexane and recrystallised
from dichloromethane–hexane layer, the crystals were
dried in vacuo, yield 0.15 g (78%). All other ruthenium
complexes were prepared by the same procedure and
the yields varied from 65–75%.
2.2. Physical measurements
Microanalyses (C, H, N) were performed using a
Perkin–Elmer 2400 CHNO/S elemental analyser. Spec-
troscopic measurements were carried out using the fol-
lowing instruments: UV–Vis spectra, JASCO
UV–Vis/NIR model V-570; IR spectra (KBr disk,
4000–200 cm−1), JASCO FT-IR model 420; H NMR
1
2.4. bis[2-(Phenylazo)pyrimidine]-bis-
(triphenylphosphine)ruthenium(II) perchlorate
monohydrate, [Ru(PPh3)2(aapm)2](ClO4)2. H2O (3/4)
spectra, Bruker 300 MHz FT-NMR spectrometers.
Molar conductances (\M) were measured in a Systron-
ics conductivity meter 304 model using ca. 10−3
M
2(Phenylazo)pyrimidine was added to a suspension of
Ru(PPh3)3Cl2 (0.2 g, 0.21 mmol) in EtOH (20 ml) ,
papm (1a) (0.08 g, 0.43 mmol) and the mixture was
refluxed for 2 h. The resulting brown–red solution was
cooled to room temperature and a saturated aqueous
solution of NaClO4 (10 ml) was added. The dark solid
that precipitated was collected by filtration, washed
with cold H2O and dried in vacuo over P4O10. The
dried product was dissolved in a small volume of
CH2Cl2 and chromatographed on a silica gel column. A
small greenish–blue band was eluted by C6H6–MeCN
(4:1, v/v) and the desired pink–violet band was eluted
with MeCN. The solution was evaporated in air and
dried over P4O10. Yield: 0.16 g, (63%). All other ruthe-
nium complexes were prepared following similar proce-
dures and the yields varied from 55–65%.
solutions in MeOH. Electrochemical measurements
were carried out with the use of computer controlled
EG&G PARC VersaStat model 270 electrochemical
instrument using a glassy carbon disk working elec-
trode. The solution was IR compensated and the results
were collected at 298 K. The reported results are refer-
Table 1
Crystallographic data for Ru(PPh3)2(papm)Cl2 (2a)
Formula
M
C46H38N4P2Cl2Ru
880.71
Crystal system
Space group
Unit cell dimensions
monoclinic
P21/c
,
a (A)
9.988(2)
17.318(4)
22.830(5)
98.055(3)
3910.0(14)
0.71073
1.496
,
b (A)
,
c (A)
i (°)
V (A )
2.5. X-ray crystal structure and analysis
3
,
,
u (A)
zcalc (gm cm−3
)
A single crystal of Ru(PPh3)2(papm)Cl2 (2a), suitable
for X-ray diffraction, was grown by slow diffusion of
hexane into dichloromethane solution at 298 K. The
crystal size was 0.15×0.40×0.50 mm3. Diffraction
measurements were carried out on a Siemens SMART
CCD diffractometer with graphite-monochromated
Z
4
T (K)
295(2)
0.660
497
0.0410
0.1119
0.639
v(Mo Ka) (mm−1
)
Refined parameters
a
R
[I\2|(I)]
b
wR2
GOF c
,
(Mo Ka) radiation (u=0.71073 A) at 295(2) K. The
unit cell was determined and refined using setting an-
gles of 25 reflections with 2q angles in the range of
3–56°. A summary of the crystallographic data and
structure refinement parameters are given in Table 1.
Of 9420 unique reflections 3780 with I\2|(I) were
a R=([Fo−Fc]/(Fo).
b wR2=[(w(Fo2−Fc2)/(wFo4]1/2
[max(F2o, 0)+2F2c]/3.
,
w=1/[(2F2o+(0.0712P)2],
P=
c GOF is defined as [w(Fo−Fc)/(no−nv)]1/2, where no and nv denote
the numbers of data and variables, respectively.

P.K. Santra et al. / Polyhedron 20 (2001) 599–606
601
Scheme 1.
used for the structure solution. Data corrections for Lp
effects and for linear-decay on „-scans were applied
[15]. Data reductions and structure refinement were
performed using SHELXS-97 [16] and successive differ-
ence Fourier synthesis. The structure was solved by the
conventional heavy-atom method and refined by the
full-matrix least-squares method on all F2o data using
the SHELXL-97 [17] package on a digital-ALFA 200
computer. All non-hydrogen atoms were refined an-
isotropically. The hydrogen atoms were fixed geometri-
cally and refined using the riding model. In the final
is isolated as the perchlorate salt, [Ru(PPh3)2(aapm)2]-
(ClO4)2·H2O (3/4) (Eq. (2)).
Ru(PPh3)3Cl2+aapm
ꢁꢁꢁꢁrꢁ.tꢁ. ꢁꢁꢁꢂ [Ru(PPh3)2(aapm)Cl2]+PPh3
(1)
(2)
CH Cl −Hexane
2
2
Ru(PPh3)3Cl2+2 aapm
EtOH
ꢁꢁꢁꢁꢁꢁꢁꢁꢂ [Ru(PPh3)2(aapm)2(ClO4)2]·H2O
reflux, NaClO
4
difference Fourier map the residual maxima and min-
2-(Arylazo)pyrimidine (aapm, 1) is an unsymmetrical
N,N%-donor chelating ligand. Ru(PPh3)2(aapm)Cl2 may
exist in cis-RuCl2 (i and ii) and trans-RuCl2 (iii)
configurations. Only one isomer is isolated and is estab-
lished structurally as cis-configuration. The structure of
Ru(PPh3)2(papm)Cl2 (2a) has been confirmed by single
crystal X-ray structure.
−3
,
ima were 0.394 and −0.464 e A
.
3. Results and discussion
3.1. Synthesis
Pseudo-octahedral [Ru(PPh3)2(aapm)2]2+ may exist
in five geometrical isomeric forms (iv–viii) with consid-
eration to the coordination pair sequence P, P (PPh3 is
abbreviated as P), N, N (N(pym) refers to N), N%, N%
(N(azo) refers to N%). The isomers are trans–cis–cis
(tcc), trans–trans–trans (ttt), cis–trans–cis (ctc), cis–
cis–trans (cct) and cis–cis–cis (ccc) (Scheme 1). The
isomers have not been separated by chromatographic
A dichloromethane solution of Ru(PPh3)3Cl2 and
2-(arylazo)pyrimidine (aapm) in equimolar proportion
on slow diffusion into hexane at ambient condition
isolates a dark brown crystalline product of composi-
tion Ru(PPh3)2(aapm)Cl2 (2) (Eq. (1)). The reaction of
Ru(PPh3)3Cl2 and aapm in a 1:2 molar ratio in ethanol
solution under reflux synthesises an ionic product which

602
P.K. Santra et al. / Polyhedron 20 (2001) 599–606
1
,
separation. H NMR spectral studies have been useful
to identify the isomers that are formed; two isomers are
characterised, namely ctc- and ccc-[Ru(PPh3)2-
(aapm)2](ClO4)2·H2O.
stitutes an excellent plane (mean deviation ꢀ0.01 A).
The pendant phenyl ring (C(81)–C(86)) is inclined at an
acute angle (ꢀ44.6°) with the chelate ring. Two PPh3
are trans and P(1)–Ru–P(2) angle is 175.42(4)° and is
in agreement with the reported results [18]. The chelate
bite angle N(1)–Ru–N(2) is 77.14(15)°. The trans–cis–
cis-RuP2N2Cl2 coordination sphere shows large devia-
tions from octahedral geometry and it is assumed that
much of it originates from the acute bite angle of the
azoimine chelate.
3.2. Molecular structure
The crystal structure of Ru(PPh3)2(papm)Cl2 (2a) is
shown in Fig. 1. The bond parameters are listed in
Table 2. The coordination around ruthenium is approx-
imately octahedral. The atomic arrangement involves
sequentially trans-phosphine, cis-chlorine and azoimine
chelate within the RuP2N2Cl2 coordination sphere. The
atomic group Ru,Cl(1),Cl(2),N(1),N(2) constitute a
good plane (mean deviation B0.06 A). 2-(Phenyl-
azo)pyrimidine is chelated with Ru and the atomic
group Ru,N(1),N(4),C(41),N(2) in the chelate ring con-
The Ru–N(1) (N(azo)) bond distance is shorter than
the Ru–N(2) (N(pyrimidine)) bond distance by 0.05 A.
The shortening may be due to p-back bonding in
d(Ru)“p*(azo). The N–N distance is 1.309(4) A and
is comparable with reported results of Ru(papm)2Cl2
[8]. The N–N bond distance for the free ligand is not
available, however, the data available in some free azo
ligands suggest that is ca. 1.25 A [19]. In the present
example the N–N distance is elongated by ca. 0.06 A.
,
,
,
,
,
This may be due to strong p-back bonding d(Ru)“
p*(azo) (supported by spectral data). The Ru–P and
Ru–Cl distances are comparable with reported results
[20,21].
3.3. Spectra
IR spectra of Ru(PPh3)2(aapm)Cl2 (2) exhibit two
sharp stretches at 350 and 300 cm−1 which correspond
to w(Ru–Cl) and indicate a cis-RuCl2 configuration. A
sharp stretch at 1380–1390 cm−1 corresponds to
w(NꢀN) and this is red shifted by 40–50 cm−1 com-
pared to the free ligand value (ꢀ1430 cm−1) [8]. The
complexes (3/4) exhibit a broad band centred at 3445
cm−1, corresponding to w(H2O) and this band was
eliminated on cautious slow heating (perchlorates are
explosive) in the 90–100°C range [6]. Sharp stretches at
1395–1400 and 1575–1580 cm−1 in these complexes
(3/4) are assigned to ((NꢀN) and ((CꢀN), respectively.
The bands are at relatively high energy positions in 3/4
compared to Ru(PPh3)2(aapm)Cl2 (2). This may be due
to the competition between two p-acidic azoimine
groups in [Ru(PPh3)2(aapm)2]2+ compared to one
azoimine group in Ru(PPh3)2(aapm)Cl2. Besides, trans
orientation of two chelated azoimine groups in (iv)/(v)
will give rise to competition for the same metal d-or-
bital and may not perturb NꢀN stretching frequency
significantly. In [Ru(PPh3)2(aapm)2]2+ w(NꢀN) is red
shifted by 30–40 cm−1 compared to free ligand value
(ꢀ1430 cm−1) [8]. This suggests that there may be
cis-orientation, (vi)–(viii), in the complexes. The per-
chlorate vibration of the complexes is seen at 1100
cm−1 together with weak band at 615(625 cm−1 [22].
The solution electronic spectra of the complexes were
recorded in CH2Cl2 for Ru(PPh3)2(aapm)Cl2 (2) and in
MeCN for [Ru(PPh3)2(aapm)2](ClO4)2·H2O (3/4) in the
200–900 nm range. Transitions below 400 nm are
Fig. 1. Single crystal X-ray structure and the atom-labelling scheme
for Ru(P)2(papm)Cl2 (2a). For clarity, all hydrogen atoms have been
omitted.
Table 2
,
Selected bond lengths (A) and bond angles (°) for
Ru(PPh3)2(papm)Cl2 (2a) with their estimated standard deviation in
parentheses
Bond lengths
Ru–P(1)
Ru–P(2)
Ru–Cl(1)
Ru–Cl(2)
2.4244(13)
2.4303(13)
2.4276(13)
2.4283(13)
Ru–N(1)
Ru–N(2)
N(1)–N(4)
C(41)–N(2)
1.968(3)
2.015(3)
1.309(4)
1.361(5)
Bond angles
P(1)–Ru–Cl(1)
P(1)–Ru–Cl(2)
P(2)–Ru–Cl(1)
P(2)–Ru–Cl(2)
P(1)–Ru–N(1)
P(1)–Ru–N(2)
N(1)–Ru–Cl(1)
N(1)–Ru–Cl(2)
90.98(4)
85.79(4)
90.59(4)
P(2)–Ru–N(1)
P(2)–Ru–N(2)
N(1)–Ru–N(2)
Cl(1)–Ru–Cl(2)
P(1)–Ru–P(2)
N(2)–Ru–Cl(1)
N(2)–Ru–Cl(2)
93.22(10)
87.71(10)
77.14(15)
90.22(4)
175.42(4)
176.06(11)
93.33(11)
89.89(4)
90.77(10)
90.99(10)
99.42(11)
169.82(11)

P.K. Santra et al. / Polyhedron 20 (2001) 599–606
603
Table 3
Microanalytical a, UV–Vis spectral b and voltammetric data c
Compound
Elemental analyses (%)
UV–Vis spectral data
(umax/nm)
CV data
E° (V) (DEp, mV)
C
H
N
(10−3m M−1 cm−1
)
Ru(III)/Ru(II)
Ligand reduction
Ru(P)2(papm)Cl2 (2a)
Ru(P)2(o-tapm)Cl2 (2b)
Ru(P)2(m-tapm)Cl2 (2c)
Ru(P)2(p-tapm)Cl2 (2d)
62.64 (62.73)
63.00 (63.08)
63.18 (63.08)
63.20 (63.08)
4.24 (4.32)
4.38 (4.47)
4.55 (4.47)
4.55 (4.47)
6.22 (6.12)
3.87 (3.97)
4.29 (4.20)
4.11 (4.20)
4.10 (4.20)
3.50 (3.60)
6.28 (6.36)
6.18 (6.26)
6.34 (6.26)
6.35 (6.26)
4.15 (4.05)
9.35 (9.26)
9.15 (9.05)
9.14 (9.05)
8.95 (9.05)
8.85 (8.76)
770 d(0.17), 532(4.20),
378(11.00), 360 d(10.72),
285(21.01)
0.897(70)
−0.824(120),
−1.147 e
852 d(0.18), 520(3.57),
374(6.64), 342 d(5.54),
284(21.28)
0.875(80)
0.873(80)
0.870(80)
0.922(90)
−0.847(100),
−1.118 e
750 d(0.11), 532(2.79),
382(8.20), 350(8.13),
282 d(1.62)
−0.845(120),
−1.120 e
775 d(0.19), 530(4.32),
585(11.24), 358 d(10.54),
286(21.32)
−0.848(110),
−1.118 e
Ru(P)2(p-Clpapm)Cl2 (2e) 60.26 (60.36)
768 d(0.10), 521 (2.65),
375(6.43), 345 d(5.47),
280(18.74)
−0.801(110),
−1.044 e
[Ru(P)2(papm)2](ClO4)2·
H2O (3a/4a)
55.45 (55.54)
894 d(0.24), 750(0.42),
520(3.41), 378(8.25),
348 d(8.99), 262(18.54)
892 d(0.13), 744(0.31),
530(3.35), 370(6.02),
344 d(5.61), 260(15.31)
890 d(0.14), 742(0.64),
522(6.72), 378(14.09),
350 d(15.10), 262(14.11)
896 d(0.25), 745(0.47),
530(3.51), 370(10.64),
345 d(11.24), 264(18.65)
889 d(0.12), 742(0.35),
520(2.98), 372(8.07),
342 d(8.64), 260(17.31)
1.21(100)
1.084(90)
−0.93(110),
−1.15(100)
[Ru(P)2(o-tapm)2](ClO4)2· 57.32 (56.22)
H2O (3b/4b)
1.13(130)
0.973(100)
−1.00(120),
−1.28(140)
[Ru(P)2(m-tapm)2](ClO4)2· 56.33 (56.22)
H2O (3c/4c)
1.11(110)
0.958(80)
−1.01(110),
−1.30(140)
[Ru(P)2(p-tapm)2](ClO4)2· 56.10 (56.22)
H2O (3d/4d)
1.13(100)
0.960(90)
−1.00(100),
−1.30(130)
[Ru(P)2(p-Clpapm)2]-
(ClO4)2·H2O (3e/4e)
52.45 (52.54)
1.27(100)
1.118(100)
−0.88(110),
−1.10(100)
a Calculated values are in parentheses.
b Solvent: CH2Cl2, for Ru(P)2(aapm)Cl2; CH3CN, for [Ru(P)2(aapm)2](ClO4)2·H2O.
c Solvent in CH3CN, supporting electrolyte, TBAP (0.01 M); solute concentration, 10−3 M; scan rate, 0.05 V s−1
.
d Shoulder.
e Cathodic peak potential, Epc (V).
assigned to intra-ligand charge transfer (n“p* and
p“p*) and are not considered further. The spectral
data are summarised in Table 3. The complexes 2
exhibit two consecutive transitions, one high intense
band (mꢀ104) at 520–530 nm and a shoulder at longer
wave length, 750–850 nm. [Ru(PPh3)2(aapm)2]2+ show
three transitions in the visible region: one high intense
transition (mꢀ104) at 520–530 nm and two shoulders at
740–750 and 885–895 nm. Diamagnetic octahedral d6
transition metal complexes in principle, exhibit mainly
singlet–singlet transition and the low energy shoulders
1
1
refer to A1g“1T1g, A1g“1T2g transitions. Assignment
of the spectral transition to the stereochemistry of the
complex is very difficult.
The 1H NMR spectra of the complexes were ob-
tained in CDCl3 and have well defined aromatic and
aliphatic regions. The proton numbering pattern is
shown in Scheme 1. The spectral data are collected in
Table 4. The aromatic zone of the spectra is very
complex due to the phenyl protons in PPh3. The NMR
spectra of the ligands and their ruthenium(II) com-
plexes reported elsewhere [7,8] are helpful in assigning
the resonance of protons in present series of the com-
plexes. The proton-numbering pattern is shown in
aapm (1) and assignments are made on the basis of
spin–spin interactions and changes therein upon substi-
1
1A1g“1T1g and A1g“1T2g transitions in the visible
region. The high intensity band does not conform to
the simple d–d transition. Ruthenium(II) complexes of
p-acidic azoimine systems exhibit metal-to-ligand
charge transfer transitions [23]. Thus, we conclude that
high intensity (mꢀ104) bands are due to a spin allowed

604
P.K. Santra et al. / Polyhedron 20 (2001) 599–606
Table 4
1H NMR data for the complexes a
Compound l/ppm (J/Hz)
4-Hd
5-Ht
6-Hd
8-H
12-Hd
9-H
11-H
10-H
R
(2a)
(2b)
(2c)
(2d)
(2e)
(3a/4a)
(3b/4b)
8.65 (6.0)
8.63 (6.2)
8.62 (6.2)
8.63 (6.0)
8.68 (6.0)
8.68 (6.0)
8.66 (6.2)
7.53 (7.4)
7.50 (7.4)
7.50 (7.4)
7.51 (7.4)
7.56 (7.0)
7.89 (7.4)
7.88 (7.4)
8.29 (6.0)
8.26 (6.0)
8.27 (6.0)
8.26 (6.2)
8.31 (6.0)
8.19 (6.2)
8.14 (6.2)
7.14d (8.0)
7.14 (8.0)
7.12 (8.0)
7.10 (8.0)
7.10 (8.0)
7.27 (7.4)
7.13 (7.4)
7.10 (8.0)
6.81t (8.0)
6.64d (8.0)
6.81t (8.0)
6.78t (8.0)
6.74t (8.0)
6.54d (8.0)
6.89d (8.0)
6.92t (8.0)
6.72t (8.0)
6.42 (8.0)
6.45t (8.0)
6.40d (8.0)
2.68
2.55
2.48
6.82s
7.10d (8.0)
7.27d (8.0)
7.20d
6.54d (8.0)
6.89d (8.0)
6.92t (8.0)
6.60d (8.0)
6.83m
6.70m
2.65, 250,
2.32
(3c/4c)
(3d/4d)
(3e/4e)
8.66 (6.2)
8.65 (6.0)
8.86 (6.0)
7.86 (7.0)
7.85 (7.0)
7.90 (7.0)
8.14 (6.0)
8.15 (6.0)
8.20 (6.0)
6.75s
7.05 (8.0)
7.00 (8.0)
7.18 (8.0)
6.79t (8.0)
6.88d (8.0)
6.91d (8.0)
6.60d (8.0)
2.50, 2.36,
2.17
2.50, 2.44,
2.35
7.14d (8.0)
7.27d (8.0)
6.66d (8.0)
7.18d (8.0)
a Solvent CDCl3 at 295(2) K. Ph protons of PPh3 appear at 7.1–7.2 and 7.4–7.6 ppm. d, doublet; t, triplet; s, singlet; m, multiplet.
to C1-symmetry and is expected to exhibit two –Me
signals of equal intensities. This is indeed observed (Fig.
2). The ratio of signal intensities is approximately 0.4:1
with respect to ctc-: ccc- configuration. Thus, the ccc-
isomer predominates in the mixture. The higher l in
ccc-geometry may be due to better stabilisation in the
C1-geometry and needs an extensive stereochemical re-
arrangement during complexation. Pyrimidine (4-H–6-
H) and aryl protons (8-H–12-H) (3/4) are assigned by
comparing with the spectra of Ru(PPh3)2(aapm)Cl2
(Fig. 3). Usually pyrimidine protons appear at the
downfield side and aryl protons, affected by the sub-
stituent, are at an upfield position.
Being a bulky group, PPh3 has an inherent disadvan-
tage in stabilizing the cis-configuration in cis-Ru(PPh3)2
due to steric crowding. However, there are two compet-
ing forces: steric crowding between PPh3···PPh3 and
p-back bonding between t2(Ru) and p(PPh3/aapm). The
latter effect predominates in the cis-configuration.
There are two metal dp-orbitals are available for p-ac-
tution. PPh3 protons appear as broad high intense
signals at 7.2–7.3 and 7.5–7.6 ppm.
The signals in the aromatic region are due to protons
from aapm and PPh3. Pyrimidine protons (4-H–6-H)
appear on the downfield side (7.5–8.7 ppm) while aryl
protons (8-H–12-H) exhibit resonance on the upfield
side. The latter signals are affected by substitution: 9-
and 11-H are perturbed due to change in the electronic
properties of the substituent in the tenth position
[24,25]. The –Me substituent (in p-tapm) moves the
signal upfield due to a +I effect and the 10-Cl sub-
stituent (in p-Clpapm) moves them downfield because
of the electron withdrawing effect of the group. The
9-Me substituent (in m-tapm) results in a singlet reso-
nance that corresponds to 8-H. In o-tapm, the 9-H
signal is heavily perturbed in comparison with the 9-H
signal in p-tapm.
The resonance in the aliphatic region is diagnostic
and has been used to assign the number and population
of isomers in the mixture. There are three Ar–Me
signals in [Ru(PPh3)2(tapm)2]2+ and of these two are of
equal intensities (Fig. 2). Five geometrical isomers are
possible in this series of complex, (iv)–(viii), and three
of these have cis-Ru(PPh3)2 configuration: ctc, cct and
ccc. In the series of Ru(aapm)2Cl2 complexes two iso-
mers had been characterised with cis-RuCl2 configura-
tion. This provides us with a guide to identify isomers
in the present series. Additionally, [Ru(PPh3)2-
(RaaiX)2]2+ (RaaiX=1-(alkyl-2-(arylazo)imidazole))
1
exists as two isomers and the aliphatic region of the H
NMR spectra is comparable with the present examples.
Thus, we may assume that two isomers ctc- and ccc-
[Ru(PPh3)2(tapm)2]2+ exist in the mixture. The ctc
configuration has C2-symmetry and should exhibit sin-
gle a Ar–Me signal while the ccc-configuration belongs
Fig. 2. Ar–Me Signals of [Ru(P)2(o-tapm)2]2+ denoting (a) ccc- and
(b) ctc-isomers in 1:0.4 ratio, respectively.

P.K. Santra et al. / Polyhedron 20 (2001) 599–606
605
Fig. 3. 1H NMR spectra of (a) Ru(P)2(papm)Cl2 and (b) [Ru(P)2(papm)2]2+ in CDCl3.
ceptance in the cis-geometry compared to one dp-or-
bital in the trans-geometry [26,27].
3.4. Electrochemistry
The electrochemical behaviour of the complexes was
examined in MeCN under N2 atmosphere cyclic
voltammetrically using a glassy carbon working elec-
trode with Bu4NClO4 as the supporting electrolyte. The
voltammogram displays metal oxidation at the positive
side and ligand reductions at the negative side with
respect to SCE. The results are given in Table 3 and a
representative voltammogram is shown in Fig. 4.
In the potential range +0.5–1.5 V at a scan rate 50
mV s−1 a reversible to quasi-reversible (peak-to-peak
separation, DEp=70–90 mV) oxidative response at
0.8–0.9 V versus SCE corresponding to the couple (1)
is observed for Ru(PPh3)2(aapm)Cl2.
Fig. 4. Cyclic voltammogram in MeCN (0.1 M Bu4NClO4) at a
GC-working electrode. The solute concentration and scan rate are
10−3 M and 50 mV s−1, respectively for Ru(P)2(papm)Cl2 (---) and
[Ru(P)2(papm)2](ClO4)2 (—).

606
P.K. Santra et al. / Polyhedron 20 (2001) 599–606
[Ru(PPh3)2(aapm)Cl2]++e X Ru(PPh3)2(aapm)Cl2
[Ru(PPh3)2(aapm)2]3+ +e X [Ru(PPh3)2(aapm)2]2+
1233-336033; e-mail: deposit@ccdc.cam.ac.uk or www:
http://www.ccdc.cam.ac.uk).
(3)
(4)
Acknowledgements
An identical experiment for [Ru(PPh3)2(aapm)2]-
(ClO4)2 exhibits two consecutive redox responses in the
potential range 0.90–1.00 and 1.1–1.2 V versus SCE.
They correspond to electron transfer as in couple (4).
The two responses may be ascribed to different isomers
present in the mixture. [Ru(PPh3)2(aapm)2]2+ exist in
two isomeric forms (vide supra). Thus, the Ru(III)/
Ru(II) couple at 0.9–1.0 may refer to the redox re-
sponse for ctc-[Ru(PPh3)2(aapm)2](ClO4)2 and that at
1.0–1.2 V for ccc-[Ru(PPh3)2(aapm)2](ClO4)2. The ccc-
isomer shows a higher Ru(III)/Ru(II) couple than that
of the ctc-isomer. This may be due to reduced symme-
try (C1-symmetry) in the ccc-isomer relative to the
ctc-(C2-symmetry) isomer; which may lead to better
Ru–L interaction. Besides, the couple height is very
informative in accounting the isomeric ratio in the
mixture. It varies for different complexes and lies in the
range 0.3:1–0.45:1. The ratio refers to the ratio of the
couple of the height of the ctc-isomer to the ccc-isomer.
This also supports the presence of ccc-[Ru(PPh3)2-
(aapm)2](ClO4)2 in higher amounts in the mixture (vide
supra).
Financial assistance from the University Grants
Commission, New Delhi, India is gratefully acknowl-
edged. Our sincere thanks are also due to Dr S. Chatto-
padhyay, Vidyasagar University, Midnapore for helpful
discussion.
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Crystallographic data for the structural analysis have
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Union Road, Cambridge, CB2 1EZ, UK (fax: +44-
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.