Chemistry of monovalent and bivalent rhenium: Synthesis, structure, isomer specificity and metal redox of azoheterocycle complexes

Article abstract of DOI:10.1039/b208338h

The reaction of [Re^vO(OEt)X₂(PPh₃) ₂] (X = Cl, Br, I) with 2-(aryIazo)-1-methylimidazoles (aryl = Ph(L¹), C₆H₄Me-p(L²) or C ₆H₄Cl-p(L³)) as well as 2-(p-tolylazo)-1-benzylimidazole (L⁴) in toluene has afforded the orange coloured bis chelates of type [Re¹¹X₂(L) ₂]. The green coloured tris chelates [Re^I(L) ₃]ReO₄ are formed when the ligand is used in excess. Similar bis and tris chelates have also been synthesized by reacting [ReOCl ₃(PPh₃)₂] with 2-(arylazo)pyridines (aryl = Ph(L⁵), C₆H₄Cl-_p(L⁶)). Structure determination of [ReCl₂(L²)₂], [ReI₂(L⁴)₂] and [ReCl₂(L ⁶)₂] has revealed that the isomeric geometry for the XX-N^hN^h-N^aN^a donor sites (N ^h, heterocyclic nitrogen; N^a, azo nitrogen) is uniformly cis-trans-cis. In the structure of [Re(L)₃]ReO₄ the tris chelate has facial geometry. The isomer preference of both families is exclusive, no other isomer having been observed in any of the preparations. The ¹H NMR spectra of the tris chelates are consistent with the facial geometry. The bis chelates are one-electron paramagnets and display well-resolved EPR sextets in fluid solutions. The cyclic voltammetric Re ^III/Re^II response of [Re^IIX₂(L) ₂] occurs in the range 0.20-0.50 V vs. SCE in the case of the azoimidazole chelates and in the range 0.60-0.70 V in the case of the azopyridine chelates. In the case of [Re^I(L)₃] ⁺ the Re^II/Re^I couple is observed near 0.50 and 0.90 V for the azoimidazole and azopyridine species respectively. The average Re-N^a distance is generally shorter than the Re-N ^h distance by ～0.1 A and the average N-N length is longer by ～0.1 A compared to that of uncoordinated azo function. Strong d(Re)-π*(azo) back-bonding characterize the present systems. Both back-bonding and steric factors stabilize the cis-trans-cis isomer of [ReX ₂(L)₂]. In facial [Re(L₃)]⁺ the net back-bonding is strong enough to offset the disadvantage of steric crowding. The Royal Society of Chemistry 2003.

Full text of DOI:10.1039/b208338h

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Chemistry of monovalent and bivalent rhenium: synthesis,
structure, isomer specificity and metal redox of azoheterocycle
complexes
Indranil Chakraborty,a Suman Sengupta,a Samir Das,a Sangeeta Banerjeea and
Animesh Chakravorty*a,b
a Department of Inorganic Chemistry, Indian Association for the Cultivation of Science,
Kolkata 700 032, India. E-mail: icac@mahendra.iacs.res.in
b Jawaharlal Nehru Centre for Advanced Scientific Research, Bangalore 560064, India
Received 27th August 2002, Accepted 7th November 2002
First published as an Advance Article on the web 3rd December 2002
The reaction of [ReVO(OEt)X2(PPh3)2] (X = Cl, Br, I) with 2-(arylazo)-1-methylimidazoles (aryl = Ph(L1), C6H4Me–
p(L2) or C6H4Cl-p(L3)) as well as 2-(p-tolylazo)-1-benzylimidazole (L4) in toluene has afforded the orange coloured
bis chelates of type [ReIIX2(L)2]. The green coloured tris chelates [ReI(L)3]ReO4 are formed when the ligand is used in
excess. Similar bis and tris chelates have also been synthesized by reacting [ReOCl3(PPh3)2] with 2-(arylazo)pyridines
(aryl = Ph(L5), C6H4Cl-p(L6)). Structure determination of [ReCl2(L2)2], [ReI2(L4)2] and [ReCl2(L6)2] has revealed that
the isomeric geometry for the XX–NhNh–NaNa donor sites (Nh, heterocyclic nitrogen; Na, azo nitrogen) is uniformly
cis–trans–cis. In the structure of [Re(L)3]ReO4 the tris chelate has facial geometry. The isomer preference of both
families is exclusive, no other isomer having been observed in any of the preparations. The 1H NMR spectra of the
tris chelates are consistent with the facial geometry. The bis chelates are one-electron paramagnets and display well-
resolved EPR sextets in fluid solutions. The cyclic voltammetric ReIII/ReII response of [ReIIX2(L)2] occurs in the range
0.20–0.50 V vs. SCE in the case of the azoimidazole chelates and in the range 0.60–0.70 V in the case of the
azopyridine chelates. In the case of [ReI(L)3]ϩ the ReII/ReI couple is observed near 0.50 and 0.90 V for the
azoimidazole and azopyridine species respectively. The average Re–Na distance is generally shorter than the Re–Nh
distance by ∼0.1 Å and the average N–N length is longer by ∼0.1 Å compared to that of uncoordinated azo function.
Strong d(Re)–π*(azo) back-bonding characterize the present systems. Both back-bonding and steric factors stabilize
the cis–trans–cis isomer of [ReX2(L)2]. In facial [Re(L3)]ϩ the net back-bonding is strong enough to offset the
disadvantage of steric crowding.
Introduction
Results and discussion
This work stems from our interest in variable valent rhenium
chemistry1 and deals specifically with the bivalent and mono-
valent states of the metal. Compared to trivalent osmium and
to a lesser extent tetravalent iridium, bivalent rhenium is a
rarely observed 5d5 oxidation state.2 Excluding metal–metal
bonded species3 most structurally characterized rhenium()
complexes incorporate nitrosyl/thionitrosyl4 or tertiary phos-
phine5–7 binding. The only non-nitrosyl and non-phosphine
species of known structure are the tris bipyridine8 and trithia-
cyclononane chelates.9,10 In case of rhenium(), π-acidic ligands
such as carbon monoxide, isonitriles, and tertiary phosphines
are required for stabilization2,11 and the chemistry of the
[Re(CO)3]ϩ moiety is attracting significant activity.12 In the
design of rhenium containing radiopharmaceuticals, higher
oxidation states (specially ϩ5) of the metal are commonly
employed, but the lower states (ϩ1, ϩ2) are receiving increasing
attention.10,13
Synthesis
The azoheterocycle ligands (general abbreviation, L) employed
in the present work are imidazoles14,18,19 L1–L4 and pyridines,16
L5 and L6. Twelve bis chelates (types 1 and 2) and five tris
chelates (types 3 and 4) have been synthesized. The lone benzyl
complex 1j is included in this study as it afforded single crystals
unlike the other iodo species.
Our previous work on azoheterocycle complexes of
rhenium14–17 has prompted us to scrutinize the possible
stabilization of bivalent and monovalent states by such ligands.
Herein we report the synthesis and characterization of bis and
tris chelates of the coordination types [ReIIX2(NN)2] (X = Cl,
Br, I) and [ReI(NN)3]ϩ which, to our knowledge, are
unprecedented in rhenium chemistry. The structure, mag-
netism, and spectra of the two families are examined and the
systematics of metal reduction potentials analyzed. A remark-
able feature of each family is its exclusive occurrence in a
single isomeric form. The origin of this geometrical selectivity
is scrutinized.
The orange coloured type 1 complexes are afforded in good
yield upon reacting [ReVO(OEt)X2(PPh3)2] (X = Cl, Br, I) with L
in 1 : 2.5 molar ratio in toluene followed by chromatographic
workup of the reaction mixture. In this synthesis the initial
reduction of the metal (ReV
ReIII) occur via oxygen atom
transfer from ReVO to PPh3, eqn. (1).
[ReVO(OEt)X2(PPh3)2] ϩ L
[ReIII(OPPh3)(OEt)X2(L)] ϩ PPh3 (1)
D a l t o n T r a n s . , 2 0 0 3 , 1 3 4 – 1 4 0
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 3
134

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Species of types [ReIII(OPPh3)(OEt)X2(L)] have been isolated
as intermediates and these are found to react with L in presence
of PPh3 (reducing agent) furnishing 1. Details of one case are
given in the Experimental section. In the direct synthesis from
[ReVO(OEt)X2(PPh3)2] it is not necessary to add PPh3 because it
is already present in the reaction mixture, eqn. (1). The green
coloured tris chelates were isolated as perrhenate salts, 3, by the
reaction of [ReVO(OEt)X2(PPh3)2] (X = I affords the best yields)
Table 1 Cyclic voltammetric formal potentials at 298 K in acetonitrile
(0.1 M Et4NClO4) at a platinum electrodea
Compound
E1/2/V (∆Ep/mV)
Compound
E1/2/V (∆Ep/mV)
1a
1b
1c
1d
1e
1f
0.46(80)
0.44(80)
0.48(80)
0.43(120)
0.41(120)
0.46(120)
0.25(100)
0.20(60)
0.27(60)
0.21(100)
2a
2b
0.61(160)
0.70(160)
3a
3b
3c
0.48(80)
0.46(60)
0.50(80)
Ϫ
with excess (1 : 6) L in acetonitrile. It is unclear how the ReO4
anion originate from the starting material but this behaviour
has precedence.4c,8 For the synthesis of the azopyridine chelates
2 and 4 the convenient starting material is [ReOCl3(PPh3)2]. The
chelation reactions were carried out in benzene for both bis and
tris chelates.
1g
1h
1i
4a
4b
0.93(60)
0.95(80)
1j
a ReIII/ReII couple for 1 and 2 and ReII/ReI couple for 3 and 4. Scan rate
50mV sϪ1. E1/2 = ½(Epa ϩ Epc), where Epa and Epc are the anodic and
cathodic peak potentials respectively. ∆Ep = Epa Ϫ Epc. Reference
electrode, SCE.
The geometrical disposition of the XX–NhNh–NaNa donor
sites (Nh is heterocyclic nitrogen and Na is azo nitrogen) in both
1 and 2 is cis–trans–cis as proven by representative structure
determination. The other possible geometrical isomers are
cis–cis–cis, trans–cis–cis, cis–cis–trans and trans–trans–trans.
Careful chromatographic experiments did not reveal the
formation of any of these isomers in the synthetic methods
developed
here.
Dihaloosmium()20–22
and
dihalo-
ruthenium()23–26 bis chelates of L are known to occur in
isomeric forms: cis–cis–cis and/or trans–cis–cis in addition to
cis–trans–cis.
were recorded for all three compounds. X = Cl (315, 335 cmϪ1);
X = Br (204, 227 cmϪ1); X = I (174, 188 cmϪ1).
The bis chelates are paramagnetic (t2g 5, S = ½) with room
temperature (300 K) magnetic moments in the range 1.94–2.10
µB. In fluid solution (300 K) these display well-resolved EPR
spectra with six hyperfine lines due to the I = 5/2 nuclei 185Re
and 187Re. There is a systematic increase in the separation
between the adjacent hyperfine lines in going towards higher
fields due to second order effects29 (range of variable
separation, 200–400 G). Center field g values and the average
hyperfine splitting A for the complexes span the ranges
2.05–2.13 and 240–315 G respectively (see Experimental
section). Variable hyperfine spacing has been observed1b,c
among ReVINAr(t2g1) chelates of picolinamides where the metal
oxidation state is higher and the magnitude and span of A
(300–700 G) are also larger. The [Re(L)3]ReO4 complexes are
In 3 and 4 the tris chelates geometry is exclusively facial as
proven by structure determination and H NMR results. The
1
facial geometry has a potential threefold axis which can make
the three chelate rings exactly equivalent. The meridional
isomer which lacks such symmetry has not been observed either
in the solid state or in solution. Osmium() is isoelectronic
with rhenium() but tris chelates of type [Os(L)3]2ϩ occur pre-
dominantly in the meridional form.20,27
Spectra and magnetism
The bis chelates display a moderately intense band near 500 nm
while the tris chelates display two such bands near 600 and 450
1
diamagnetic and display well-resolved H NMR spectra. The
three chelate rings are equivalent and give rise to a single set of
resonances consistent with facial geometry (3 and 4).
nm. These are presumably of MLCT origin. In IR an N᎐N
᎐
stretch occurs near 1300 and 1230 cmϪ1 in the bis and tris
chelates respectively. The latter shows a characteristic strong
Metal redox: chelate ring number and oxidation state
Ϫ
band near 910 cmϪ1 assigned to the ν3 vibration of the ReO4
anion.28 Consistent with the cis-ReCl2 disposition, the
[ReCl2(L)2] species uniformly display two Re–Cl stretches near
320 and 330 cmϪ1. In the case of [ReX2(L3)2] the ReX2 stretches
All the complexes are electroactive in acetonitrile solution
(Table 1). The azoimidazole bis species display a quasireversible
one-electron cyclic voltammetric oxidative response assignable
D a l t o n T r a n s . , 2 0 0 3 , 1 3 4 – 1 4 0
135

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to the ReIII/ReII couple in the range 0.20–0.50 V vs. SCE. For a
given X there is a small but systematic shift of the reduction
potential to more positive values as the electron withdrawing
power of the RЈ substituent increases (Me < H < Cl). For a
given RЈ the potential decreases as the X ligand becomes softer
and its ligand field strength decreases (Cl > Br > I), the effect
being more pronounced between the bromo and iodo species.
For azopyridine bis chelates the ReIII/ReII reduction potentials
is higher, 0.60–0.70 V. Attempted isolation of the oxidized
species, [ReIIIX2(L)2]ϩ, via coulometry has not succeeded so far
because of their instability. In the tris azoimidazole chelates
the oxidative ReII/ReI couple occur at ∼ 0.5 V and is nearly
reversible with peak-to-peak separation in the range 60–80 mV.
Again the azopyridine species have significantly higher poten-
tials ∼0.9 V.
The E1/2 value of a given ReZϩ1/ReZ couple increases progres-
sively with the number of L chelate rings present. Consequently
variation of this number along with coligand control provides
an excellent handle for stabilizing a range of oxidation states of
rhenium under ambient conditions. With monochelation,
rhenium() and rhenium() were found to be stabilized as
in [ReVOCl3(L)], [ReV(NAr)Cl3(L)], [ReIII(OPPh3)Cl3(L)] and
[ReIII(PPh3)Cl3(L)].14,16 Bis and tris chelation are now shown to
make the bivalent and monovalent states readily accessible in
the form of [ReIIX2(L)2] and [ReI(L)3]ReO4, respectively.
Fig. 2 A perspective view of [ReI2(L4)2] 1j. The atoms are represented
by their 30% thermal probability ellipsoids.
Structures
Among bis chelates the structures of [ReCl2(L2)2], 1b;
[ReI2(L4)2], 1j and [ReCl2(L6)2], 2b have been determined.
Perspective views are shown in Fig. 1–3 and selected bond
parameters are listed in Tables 2 and 3.
Fig.
3
A
perspective view of [ReCl2(L6)2] 2b. The atoms are
represented by their 30% thermal probability ellipsoids.
crystallographic threefold axis. In mobile solution, the three
chelate rings become exactly equivalent (1H NMR). The
complex [Re(L3)3]ReO4, 3c also afforded single crystals. The
X-ray results revealed the overall connectivity and the facial
geometry. However, the structure did not refine very well due
to crystallographic complications.
Fig.
1
A
perspective view of [ReCl2(L2)2] 1b. The atoms are
represented by their 30% thermal probability ellipsoids.
The distorted octahedral ReX2N4 coordination spheres have
cis–trans–cis geometry. Whereas the angular dimensions in 1b
and 1j are similar, subtle differences related to halide bulk do
occur. Thus, the X1–Re–X2 and N1–Re–N5 angles are 3–4Њ
larger in 1j than in 1b resulting in associated changes in the
other angles. In all three molecules the chelate ring along with
the heterocyclic ring constitute a satisfactory plane.
Bond length, back-bonding and isomer specificity
The average Re–Na distance is significantly shorter (by 0.05–0.1
Å) than the Re–Nh distance in all the four structures. On the
other hand the average N–N length is uniformly longer (by
0.07–0.1 Å) than that (1.25 Å)30 in uncoordinated azo com-
pounds. Clearly strong d(Re)–π*(azo) back-bonding charac-
terizes the present compounds. The excellent π-acceptor
character of azoheterocycles originates from low lying π*(azo)
orbitals.14,16,20,30–32 The average Re–Cl length in 1b and 2b is
2.39 Å which is significantly shorter than that (2.42 Å) in
compounds of type trans-[ReCl2(diphosphine)2].7
Among tris chelates [Re(L5)3]ReO4, 4a afforded good single
crystals. A perspective view is shown in Fig. 4 and selected bond
parameters are listed in Table 4.
The ligands are facially disposed in the distorted octahedral
ReN6 coordination sphere. Each chelate ring along with the
pyridine ring constitute a satisfactory plane. The three chelate
rings are very closely similar although there is no formal
D a l t o n T r a n s . , 2 0 0 3 , 1 3 4 – 1 4 0
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Table 2 Selected bond distances (Å) and angles (Њ) for compounds 1b
and 1j
Table 4 Selected bond distances (Å) and angles (Њ) for compound 4a
Re1–N1
Re1–N3
Re1–N4
Re1–N6
Re1–N7
Re1–N9
N2–N3
2.111(7)
1.979(7)
2.092(7)
1.991(8)
2.103(7)
1.986(7)
1.337(10)
N5–N6
N8–N9
Re2–O1
Re2–O2
Re2–O3
Re2–O4
1.313(10)
1.321(10)
1.705(9)
1.702(8)
1.709(9)
1.702(8)
1b
1j
Re–N1
Re–N4
Re–N5
Re–N8
Re–X1
Re–X2
N4–N3
N8–N7
2.057(7)
2.007(8)
2.046(8)
1.999(8)
2.381(3)
2.398(3)
1.328(11)
1.345(11)
2.065(8)
2.041(8)
2.053(8)
2.000(9)
2.7214(10)
2.7412(12)
1.339(10)
1.366(11)
N3–Re1–N9
N9–Re1–N6
N9–Re1–N4
N3–Re1–N7
N6–Re1–N7
N3–Re1–N1
N6–Re1–N1
N7–Re1–N1
O4–Re2–O1
O4–Re2–O3
O1–Re2–O3
96.7(3)
96.7(3)
95.8(3)
96.5(3)
164.7(3)
74.3(3)
96.6(3)
94.5(3)
109.9(6)
109.7(6)
109.3(5)
N3–Re1–N6
N3–Re1–N4
N6–Re1–N4
N9–Re1–N7
N4–Re1–N7
N9–Re1–N1
N4–Re1–N1
O4–Re2–O2
O2–Re2–O1
O2–Re2–O3
96.5(3)
165.2(3)
74.1(3)
73.9(3)
94.6(3)
164.7(3)
95.1(3)
109.0(5)
109.4(6)
109.5(6)
N8–Re–N4
N4–Re–N5
N4–Re–N1
N8–Re–X1
N5–Re–X1
N8–Re–X2
N5–Re–X2
X1–Re–X2
N8–Re–N5
N8–Re–N1
N5–Re–N1
N4–Re–X1
N1–Re–X1
N4–Re–X2
N1–Re–X2
89.4(3)
112.0(3)
74.5(3)
93.6(3)
87.8(2)
160.4(2)
86.3(2)
92.04(12)
75.2(3)
109.0(3)
172.6(3)
160.1(2)
85.9(2)
91.7(2)
90.1(2)
85.7(3)
109.7(3)
74.4(3)
93.8(2)
87.6(2)
160.6(2)
88.0(3)
95.75(4)
75.6(3)
106.8(3)
175.5(3)
161.8(2)
88.4(2)
90.4(2)
90.3(2)
The cis (as opposed to trans) disposition of two Na donor
sites ensures lack of competition between them for the same
metal d-orbital thus maximizing back-bonding. This happens
in the cis–trans–cis geometry of 1 and 2. This geometry is also
sterically more favourable than the other two possible but
unobserved isomers (cis–cis–cis and trans–cis–cis) having the cis
ReNaNa disposition. The various isomers of [OsX2(L)2]20–22 and
[RuX2(L)2]23–26 undergo spontaneous thermal conversion to the
thermodynamically most stable cis–trans–cis form.23b,26,33 In the
case of [ReX2(L)2] this happens to be the only isomer observed.
In facial [Re(L)3]ϩ the three Na atoms are mutually cis to one
another representing a model situation for back-bonding (three
cis ReNaNa interactions). This geometry is, however, more
crowded (pendant aryl groups) than the unobserved meridional
form which can have only two ReNaNa interactions. Evidently
the strong back-bonding more than offsets the steric disadvan-
tage of the facial form which alone is observed for [Re(L)3]ϩ.
A comparison of isoelectronic [Re(L)3]ϩ and [Os(L)3]2ϩ is in
order. The latter occurs primarily in the meridional form.20
That back-bonding is significantly weaker in OsII–Na than in
ReI–Na can be indirectly inferred from bond parameter data
of bis chelates. In cis–trans–cis [OsCl2(L2)2] the average N–N
distance is 1.32 Å22 as compared to 1.35 Å in [ReCl2(L2)2], 1b.
Similarly the N–N distances in azopyridine bis chelates are
1.3120 and 1.33 Å respectively. The π-basicity order is thus
OsII < ReII which in turn implies the order OsII << ReI.
Table 3 Selected bond distances (Å) and angles (Њ) for compound 2b
Re1–N1
Re1–N3
Re1–N4
Re1–N6
2.069(11)
2.006(11)
2.066(10)
1.964(12)
Re1–Cl1
Re1–Cl2
N2–N3
N5–N6
2.382(4)
2.380(4)
1.319(14)
1.33(2)
N6–Re1–N3
N3–Re1–N4
N3–Re1–N1
N6–Re1–Cl2
N4–Re1–Cl2
N6–Re1–Cl1
N4–Re1–Cl1
Cl2–Re1–Cl1
91.3(4)
104.5(4)
74.1(4)
89.1(3)
88.7(3)
166.5(3)
92.4(3)
90.5(2)
N6–Re1–N4
N6–Re1–N1
N4–Re1–N1
N3–Re1–Cl2
N1–Re1–Cl2
N3–Re1–Cl1
N1–Re1–Cl1
74.1(4)
100.4(4)
174.4(4)
166.4(3)
92.5(4)
92.3(3)
93.1(3)
Concluding remarks
The synthesis and structural characterization of the hitherto
unreported bivalent and monovalent rhenium coordination
types [ReIIX2(NN)2] and [ReI(NN)3]ReO4 have been successfully
achieved. An increase in the number of azoheterocycle chelate
rings causes a relatively large increase in metal reduction
potentials, thus stabilizing lower oxidation states: bis chelation
affords rhenium() and tris chelation rhenium().
The bis and tris chelates occur exclusively in the cis–trans–cis
and facial isomeric forms respectively. The presence of strong
d(Re)–π*(azo) back-bonding in both families is revealed by
bond distance data. In the bis chelates the cis–trans–cis isomer
has the advantage of both maximized back-bonding and mini-
mized steric crowding. In the tris chelates the net back-bonding
is strong enough to offset the disadvantage of steric crowding in
the facial geometry. The high π-acidity of azoheterocycles is
neatly revealed by the present work.
Experimental
[ReOCl3(PPh3)2],34 [ReO(OEt)X2(PPh3)2],35 2-(arylazo)imid-
azoles14,18,19 and 2-(arylazo)pyridines16 were prepared by
reported methods. For electrochemical work HPLC grade
Fig. 4 A perspective view of [Re(L5)3]ReO4 4a. The atoms are
represented by their 30% thermal probability ellipsoids.
D a l t o n T r a n s . , 2 0 0 3 , 1 3 4 – 1 4 0
137

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acetonitrile was used. All other chemicals and solvents were of
reagent grade and were used as received. Spectral measure-
ments were made with following equipment: UV-vis, Shimadzu
UV-1601 PC spectrophotometer; IR (KBr and polyethylene
disk), Nicolet Magna IR 750 Series II and 550 FAR IR spectro-
[ReBr2(L3)2] 1f. (Found: C, 30.46; H, 2.27; N, 14.30. Calc. for
C20H18N8Cl2Br2Re: C, 30.51; H, 2.30; N, 14.23.) UV-vis [λmax
/
nm (ε/dm3 molϪ1 cmϪ1), CH2Cl2 solution]: 485 (3700) and 347
(7900). IR (cmϪ1): 204, 227 (Re–Br), 1300 (N᎐N). µ /µ , 1.96;
᎐
eff
B
g(center), 2.098; Aav/G, 285.
1
meters; H NMR, Bruker 300 MHz FT spectrometer (proton
[ReI2(L1)2] 1g. The diiodo complexes were synthesized by the
same general procedure as described for 1a. Only [ReO(OEt)-
Cl2(PPh3)2] was replaced by [ReO(OEt)I2(PPh3)2] for the above
synthesis. (Found: C, 29.67; H, 2.58; N, 13.70. Calc. for
C20H20N8I2Re: C, 29.57; H, 2.48; N, 13.79.) UV-vis [λmax/nm
(ε/dm3 molϪ1 cmϪ1), CH2Cl2 solution]: 525 (3600) and 374
numbering scheme used is the same as in crystallography);
X-band EPR, Varian E-109C spectrometer. Electrochemical
measurements were performed under nitrogen atmosphere
using a PAR 370–4 electrochemistry system with platinum
working electrode.36 The supporting electrolyte was tetraethyl-
ammonium perchlorate (TEAP), and potentials are referenced
to the saturated calomel electrode (SCE) without junction
correction. Magnetic susceptibilities were measured on a PAR
155 vibrating-sample magnetometer, and the microanalyses
(C, H, N) were performed using a Perkin-Elmer 2400 Series II
elemental analyzer.
(7600). IR (cmϪ1): 1290 (N᎐N). µ /µ , 1.95; g (center), 2.117;
᎐
eff
B
Aav/G, 254.
[ReI2(L2)2] 1h. (Found: C, 31.35; H, 2.83; N, 13.40. Calc. for
C22H24N8I2Re: C, 31.44; H, 2.88; N, 13.33.) UV-vis [λmax/nm
(ε/dm3 molϪ1 cmϪ1), CH2Cl2 solution]: 533 (1900) and 355
(7500). IR (cmϪ1): 1290 (N᎐N). µ /µ , 1.94; g(center), 2.131;
᎐
eff
B
Synthesis of complexes
Aav/G, 248.
Synthesis of azoimidazole [ReX2(L)2] 1 complexes
[ReI2(L3)2] 1i. (Found: C, 27.17; H, 2.10; N, 12.80. Calc. for
C20H18N8Cl2I2Re: C, 27.26; H, 2.06; N, 12.71.) UV-vis [λmax/nm
(ε/dm3 molϪ1 cmϪ1), CH2Cl2 solution]: 516 (2000) and 353
These complexes were prepared by the same general pro-
cedure based on the reaction of [ReO(OEt)X2(PPh3)2] with L
in toluene. Details are given below for a representative case.
(7200). IR (cmϪ1): 174, 188 (Re–I), 1300 (N᎐N). µ /µ , 1.94;
᎐
eff
B
g(center), 2.117; Aav/G, 255.
[ReCl2(L1)2] 1a. To a suspension of [ReO(OEt)Cl2(PPh3)2]
(100 mg, 0.119 mmol) in 25 cm3 of toluene was added 56 mg
(0.298 mmol) of L1. The resulting mixture was heated to reflux
for 45 min affording a reddish orange solution. The solvent was
then removed under reduced pressure, and the mass thus
obtained was dissolved in minimum amount of dichloro-
methane and subjected to chromatography on a silica gel
column (20 × 1 cm, 60–120 mesh). Following elution with pure
toluene, reddish orange band was eluted out with toluene–
acetonitrile (50 : 3) mixture. Solvent removal from the eluate
under reduced pressure afforded [ReCl2(L1)2] in pure form
which was dried under vacuo over fused calcium chloride. Yield:
52 mg (70%). (Found: C, 38.21; H, 3.25; N, 17.76. Calc. for
C20H20N8Cl2Re: C, 38.16; H, 3.20; N, 17.80.) UV-vis [λmax/nm
(ε/dm3 molϪ1 cmϪ1), CH2Cl2 solution]: 484 (5600) and 373
[ReI2(L4)2] 1j. (Found: C, 41.22; H, 3.35; N, 11.18. Calc. for
C34H32N8I2Re: C, 41.14; H, 3.25; N, 11.29.) UV-vis [λmax/nm
(ε/dm3 molϪ1 cmϪ1), CH2Cl2 solution]: 489 (7000) and 351
(12400). IR (cmϪ1): 1280 (N᎐N). µ /µ , 1.95; g(center), 2.110;
᎐
eff
B
Aav/G, 253.
Synthesis of the intermediate [Re(OPPh3)(OEt)Cl2(L3)]. To a
suspension of [ReO(OEt)Cl2(PPh3)2] (100 mg, 0.119 mmol) in
25 cm3 of toluene was added 26 mg (0.119 mmol) of L3. The
resulting mixture was stirred magnetically for 1 h at room
temperature, affording an yellowish orange solution. The
solvent was then removed under reduced pressure, and the
mass thus obtained was dissolved in a minimum amount of
dichloromethane and subjected to chromatography on a silica
gel column (20 × 1 cm, 60–120 mesh) in the same manner as in
the case of 1a. Solvent removal from the eluate under reduced
pressure afforded [Re(OPPh3)(OEt)Cl2(L3)] in pure form
which was dried under vacuo over fused calcium chloride. Yield:
62 mg (65%). (Found: C, 44.87; H, 3.60; N, 7.11. Calc. for
C30H29N4Cl3O2PRe: C, 44.96; H, 3.65; N, 7.00.) UV-vis
[λmax/nm (ε/dm3 molϪ1 cmϪ1), CH2Cl2 solution]: 659 (370), 459
(4500) and 343 (6200). IR (cmϪ1): 1125 (O–P), 924 (O–Et), 1437
(10200). IR (cmϪ1): 320, 330 (Re–Cl), 1300 (N᎐N). µ /µ , 2.09;
᎐
eff
B
g(center), 2.053; Aav/G, 312.
[ReCl2(L2)2] 1b. (Found: C, 40.24; H, 3.60; N, 17.09. Calc. for
C22H24N8Cl2Re: C, 40.18; H, 3.68; N, 17.04.) UV-vis [λmax/nm
(ε/dm3 molϪ1 cmϪ1), CH2Cl2 solution]: 486 (7300), and 345
(12000). IR (cmϪ1): 317, 320 (Re–Cl), 1300 (N᎐N). µ /µ , 2.10;
᎐
eff
B
g(center), 2.078; Aav/G, 304.
(N᎐N).
᎐
[ReCl2(L3)2] 1c. (Found; C, 34.49; H, 2.57; N, 16.12. Calc. for
C20H18N8Cl4Re: C, 34.39; H, 2.60; N, 16.04.) UV-vis [λmax/nm
(ε/dm3 molϪ1 cmϪ1), CH2Cl2 solution]: 489 (5700) and 337
Conversion to [ReCl2(L3)2] 1c. To a solution of [Re(OPPh3)-
(OEt)Cl2(L3)] (50 mg, 0.062 mmol) in 25 cm3 of toluene was
added 21 mg (0.093 mmol) of L3 and 3 mg (0.011 mmol) of
PPh3. The resulting mixture was heated to reflux for 30 min
affording a reddish orange solution. The solvent was then
removed under reduced pressure and chromatographic work up
in the same manner as described for 1a afforded [ReCl2(L3)2] in
pure form which was dried under vacuo over fused calcium
chloride. Yield: 27 mg (60%).
(9300). IR (cmϪ1): 315, 335 (Re–Cl), 1310 (N᎐N). µ /µ , 2.10; g
᎐
eff
B
(center), 2.066; Aav/G, 315.
[ReBr2(L1)2] 1d. The dibromo complexes were synthesized
by the same general procedure as described for 1a. Only
[ReO(OEt)Cl2(PPh3)2] was replaced by [ReO(OEt)Br2(PPh3)2]
in the above synthesis. (Found: C, 33.39; H, 2.73; N, 15.70.
Calc. for C20H20N8Br2Re: C, 33.44; H, 2.81; N, 15.60.) UV-vis
[λmax/nm (ε/dm3 molϪ1 cmϪ1), CH2Cl2 solution]: 464 (3400) and
Synthesis of azopyridine [ReCl2(L)2] 2 complexes
352 (6700) IR (cmϪ1): 1300 (N᎐N). µ /µ , 2.02; g(center),
᎐
eff
B
These complexes were synthesized by the same general
procedure based on the reaction of [ReOCl3(PPh3)2] with L in
benzene. Details are given below for a representative case.
2.103; Aav/G, 273.
[ReBr2(L2)2] 1e. (Found: C, 33.50; H, 3.29; N, 15.09. Calc. for
C22H24N8Br2Re: C, 35.40; H, 3.24; N, 15.01.) UV-vis [λmax/nm
(ε/dm3 molϪ1 cmϪ1), CH2Cl2 solution]: 477 (3500) and 342
[ReCl2(L5)2] 2a. To a suspension of [ReOCl3(PPh3)2] (100 mg,
0.120 mmol) in 25 cm3 of benzene was added 55 mg (0.300
mmol) of L5. The resulting mixture was warmed at 60ЊC for 10
(6300). IR (cmϪ1): 1290 (N᎐N). µ /µ , 1.98; g(center), 2.116;
᎐
eff
B
Aav/G, 295.
D a l t o n T r a n s . , 2 0 0 3 , 1 3 4 – 1 4 0
138

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Table 5 Crystal data for complexes 1b, 1j, 2b and 4a
Complex
1b
1j
2b
4a
Formula
M
C22H24Cl2N8Re
657.59
C34H32I2N8Re
992.68
C22H16Cl4N6Re
692.41
C33H27N9O4Re2
986.04
System
Space group
Monoclinic
C2/c
Monoclinic
P21/c
Monoclinic
P21/n
Monoclinic
Cc
a/Å
b/Å
c/Å
α/Њ
28.74(2)
13.454(4)
13.427(5)
90
10.946(2)
18.790(4)
17.394(4)
90
8.532(6)
22.793(8)
12.417(3)
90
12.876(10)
22.313(10)
10.938(6)
90
β/Њ
110.86(4)
90
4852(4)
8
99.60(3)
90
3527.4(12)
4
100.22(4)
90
2377(2)
4
90.08(6)
90
3143(3)
4
γ/Њ
U/Å3
Z
D/mg mϪ3
1.801
1.869
1.935
2.084
T /K
293
5.257
293
5.231
293
5.587
293
7.752
µ/mmϪ1
Unique reflections
Rint
Observed reflections [I > 2σ(I )]
R1, wR2 [I > 2σ(I )]
All data
3604
0.0476
2718
0.0399, 0.1049
0.0642, 0.1297
5257
0.0376
3631
0.0483, 0.0955
0.0876, 0.1206
4165
0.0658
2829
0.0616, 0.1386
0.1040, 0.1687
3733
0.0435
3106
0.0243, 0.0557
0.0388, 0.1160
min affording a reddish orange solution. The solvent was then
quickly removed under reduced pressure and the solid mass
thus obtained was dissolved in a small amount of dichloro-
methane and subjected to chromatography on a silica gel
column (20 × 1 cm, 60–120 mesh). Following elution with pure
benzene, a reddish orange band was eluted out with benzene–
acetonitrile (25 : 2) mixture. Solvent removal from the elute
under reduced pressure afforded [ReCl2(L5)2] in pure form
which was dried under vacuo over fused calcium chloride. Yield:
47 mg (62%). (Found: C, 42.41; H, 2.85; N, 13.41. Calc. for
C22H18Cl2N6Re: C, 42.38; H, 2.90; N, 13.48.) UV-vis [λmax/nm
(ε/dm3 molϪ1 cmϪ1), CH2Cl2 solution]: 509 (7400). IR (cmϪ1):
(multiplet, H7, H9), 5.96 (d, 6.8, H6, H10) and 7.20 (multiplet,
H8).
[Re(L2)3]ReO4 3b. (Found: C, 38.28; H, 3.40; N, 16.13. Calc.
for C33H36N12O4Re2: C, 38.22; H, 3.50; N, 16.21.) UV-vis
[λmax/nm (ε/dm3 molϪ1 cmϪ1), CH2Cl2 solution]: 579(8830), 445
(14800), 398 (10500) and 311 (13000). IR (cmϪ1): 1233 (N᎐N),
᎐
909 (Re–O, ReO4Ϫ). 1H NMR [δ (J/Hz), CDCl3 solution]:
MeC3N2, 7.23 (d, 1.6, H1), 6.50 (d, 1.7, H2) and 3.98 (s, N–Me);
C6H4Me, 6.97 (d, 8.1, H7, H9), 6.01 (d, 8.2, H6, H10) and 2.35
(s, p-Me).
[Re(L3)3]ReO4 3c. (Found: C, 32.77; H, 2.53; N, 15.22. Calc.
for C30H27N12Cl3O4Re2: C, 32.80; H, 2.48; N, 15.30.) UV-vis
[λmax/nm (ε/dm3 molϪ1 cmϪ1), CH2Cl2 solution]: 573 (7530), 447
320, 330 (Re–Cl), 1310 (N᎐N). µ /µ , 2.10; g(center), 2.104;
᎐
eff
B
Aav/G, 250.
(12700), 396 (8900) and 315 (11000). IR (cmϪ1): 1232 (N᎐N),
[ReCl2(L6)2] 2b. (Found: C, 38.21; H, 2.29; N, 12.17. Calc. for
C22H16Cl4N6Re: C, 38.16; H, 2.33; N, 12.14.) UV-vis [λmax/nm
(ε/dm3 molϪ1 cmϪ1), CH2Cl2 solution]: 507 (7800). IR (cmϪ1):
᎐
910 (Re–O, ReO4Ϫ). 1H NMR [δ (J/Hz), CDCl3 solution]:
MeC3N2, 7.29 (d, 1.6, H1), 6.51 (d, 1.7, H2) and 4.01 (s, N–Me);
C6H4Cl, 7.20 (d, 7.0, H7, H9) and 6.15 (d, 7.0, H6, H10).
315, 330 (Re–Cl), 1300 (N᎐N). µ /µ , 2.09; g(center), 2.079;
᎐
eff
B
Aav/G, 248.
Synthesis of azopyridine [Re(L)3]ReO4, 4 complexes
Synthesis of azoimidazole [Re(L)3]ReO4 3 complexes
These complexes were prepared by the same general procedure
based on the reaction of [ReOCl3(PPh3)2] with excess L in ben-
zene solution. Details are given below for a representative case.
These complexes were prepared by the same general procedure
based on the reaction of [ReO(OEt)I2(PPh3)2] with excess L in
acetonitrile solution. Details are given below for a represent-
ative case.
[Re(L5)3]ReO4 4a. To suspension of [ReOCl3(PPh3)2] (100 mg,
0.120 mmol) in 15 cm3 of benzene was added 132 mg (0.720
mmol) of L5. The resulting mixture was warmed at 60ЊC for 15
min, affording a brownish green solution. The solvent was then
removed under reduced pressure, and the mass thus obtained
dissolved in a small amount of dichloromethane and subjected
to chromatography on a silica gel column (20 × 1 cm, 60–120
mesh). A deep green band was eluted out with a benzene–
acetonitrile (1 : 1) mixture. After solvent removal from the
eluate [Re(L5)3]ReO4 was afforded in pure form. It was finally
dried under vacuo over fused calcium chloride. Yield: 62 mg
(52%). (Found: C, 40.25; H, 2.72; N, 12.81. Calc. for
C33H27N9O4Re2: C, 40.20; H, 2.76; N, 12.78.) UV-vis [λmax/nm
(ε/dm3 molϪ1 cmϪ1), CH2Cl2 solution]: 596 (6750), 456 (10050)
and 314 (17200). IR (cmϪ1): 1230 (N᎐N), 910 (Re–O, ReO Ϫ).
[Re(L1)3]ReO4 3a. To a solution of [ReO(OEt)I2(PPh3)2] (100
mg, 0.097 mmol) in 15 cm3 of acetonitrile was added 108 mg
(0.582 mmol) of L1. The resulting mixture was heated to reflux
for 8 h, affording a brownish green solution. The solvent was
then removed under reduced pressure, and the mass thus
obtained was dissolved in minimum amount of dichloro-
methane and subjected to chromatography on a silica gel
column (20 × 1 cm, 60–120 mesh). A deep green band was
eluted out with a toluene–acetonitrile (1 : 1) mixture. The solid
obtained after solvent removal from the eluate under reduced
pressure was again dissolved in small amount of dichloro-
methane and the above chromatographic procedure was
repeated followed by a second repeat. After solvent removal
from the final eluate [Re(L1)3]ReO4 was afforded in pure form.
It was dried under vacuo over fused calcium chloride. Yield: 58
mg (60%). (Found: C, 36.27; H, 3.09; N, 16.80. Calc. for
C30H30N12O4Re2: C, 36.21; H, 3.04; N, 16.89.) UV-vis [λmax/nm
(ε/dm3 molϪ1 cmϪ1) CH2Cl2 solution]: 575 (6800), 448 (11300),
᎐
4
1H NMR [δ (J/Hz), CDCl3 solution]: C5N, 8.07 (d, 8.1, H1),
7.52 (t, 8.0, H2), 7.99 (t, 7.6, H3), and 7.81 (d, 5.9, H4); C6H5,
5.84 (d, 4.0, H7, H11), 7.30 (t, 7.2, H8, H10) and 7.19
(t, 7.7, H9).
393 (7800) and 313 (9900). IR (cmϪ1): 1235 (N᎐N), 910(Re–O,
[Re(L6)3]ReO4 4b. (Found: C, 36.33; H, 2.18; N, 11.61. Calc.
for C33H24N9Cl3O4Re2: C, 36.38; H, 2.22; N, 11.57.) UV-vis
[λmax/nm (ε/dm3 molϪ1 cmϪ1), CH2Cl2 solution]: 610 (8000), 459
᎐
1
ReO4Ϫ), H NMR [δ (J/Hz), CDCl3 solution]: MeC3N2, 7.28
(d, 1.8, H1), 6.57 (d, 1.8, H2) and 4.01 (s, N–Me); C6H5, 7.20
D a l t o n T r a n s . , 2 0 0 3 , 1 3 4 – 1 4 0
139

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(11800) and 315 (20200). IR (cmϪ1): 1226 (N᎐N), 907 (Re–O,
9 S. O. C. Matondo, P. Mountford, D. J. Watkin, W. B. Jones and
S. R. Cooper, J. Chem. Soc., Chem. Commun., 1995, 161.
10 G. E. D. Mullen, P. J. Blower, D. J. Price, A. K. Powell, M. J. Howard
and M. J. Went, Inorg. Chem., 2000, 39, 4093.
11 (a) M. F. C. Guedes da Silva, J. J. R. Frausto da Silva and
A. J. L. Pombeiro, J. Chem. Soc., Dalton Trans., 1994, 3299;
(b) M. L. Kuznetsov, A. J. L. Pombeiro and A. I. Dementlev,
J. Chem. Soc., Dalton Trans., 2000, 4413.
12 Representative recent papers: (a) D. R. Striplin and G. A. Crosby,
Coord. Chem. Rev., 2001, 211, 163; (b) L. A. Friedman, S. H. Meiere,
B. C. Brooks and W. D. Harman, Organometallics, 2001, 20, 1699;
(c) A. J. Di. Bilio, B. R. Crane, W. A. Wehbi, C. N. Kiser, M. M.
Abu-Omar, R. M. Carlos, J. A. Richards, J. R. Winkler and H. B.
Gray, J. Am. Chem. Soc., 2001, 123, 3181; (d ) J. B. Cooper, M. G. B.
Drew and P. D. Beer, J. Chem. Soc., Dalton Trans., 2001, 392; (e) S.-S
Sun, D. T. Tran, O. S. Odongo and A. J. Lees, Inorg. Chem., 2002, 41,
132; ( f ) A. D. Guerzo, L. Leroy, F. Fages and R. H. Schmehl, Inorg.
Chem., 2002, 41, 359; (g) K. Wang, L. Huang, L. Gao, L. Jin and
C. Hoang, Inorg. Chem., 2002, 41, 3353.
13 (a) S. Top, H. E. Hafa, A. Vessieres, J. Quivy, J. Vaissermann, D. W.
Hughes, M. J. McGlinchey, J. P. Mornon, E. Thoreau and
G. Jaouen, J. Am. Chem. Soc., 1995, 117, 8372; (b) J. R. Dilworth
and S. J. Parrott, Chem. Soc. Rev., 1998, 27, 43.
14 I. Chakraborty, S. Bhattacharyya, S. Banerjee, B. K. Dirghangi and
A. Chakravorty, J. Chem. Soc., Dalton Trans., 1999, 3747.
15 S. Bhattacharyya, I. Chakraborty, B. K. Dirghangi and A.
Chakravorty, Chem. Commun., 2000, 1813.
16 S. Banerjee, S. Bhattacharyya, B. K. Dirghangi, M. Menon and
A. Chakravorty, Inorg. Chem., 2000, 39, 6.
17 S. Bhattacharyya, I. Chakraborty, B. K. Dirghangi and A.
Chakravorty, Inorg. Chem., 2001, 40, 286.
᎐
ReO4Ϫ). 1H NMR [δ (J/Hz), CDCl3 solution]: C5N, 8.06 (d, 8.1,
H1), 7.62 (t, 6.5, H2), 7.97 (multiplet, H3) and 7.97 (multiplet,
H4); C6H4Cl, 6.04 (d, 8.7, H7, H11) and 7.23 (d, 8.7, H8, H10).
Crystallography. Single crystals of the complexes 1b, 1j, 2b,
and 4a were grown by slow diffusion of hexane into dichloro-
methane solutions of the respective compounds. Data were
collected on a Nicolet R3m/V four circle diffractometer with
graphite monochromated Mo–Kα radiation (λ = 0.71073 Å) by
the ω-scan technique in the range 3 ≤ 2θ/Њ ≤ 47 for complexes 1b
and 1j and 3 ≤ 2θ/Њ ≤ 50 for 2b and 3 ≤ 2θ/Њ ≤ 55 for 4a. All the
data were corrected for Lorentz-polarization and absorption.37
The metal atoms were located from Patterson maps and the rest
of the non-hydrogen atoms emerged from successive Fourier
syntheses. The structures were then refined by a full-matrix least
squares procedure on F 2. All non-hydrogen atoms were refined
anisotropically. All hydrogen atoms were included in calculated
positions. Calculations were performed using the SHELXTL
V 5.0338 program package. Significant crystal data are listed in
Table 5.
CCDC reference numbers 192365 (1b), 192366 (1j), 192367
(2b) and 192368 (4a).
See http://www.rsc.org/suppdata/dt/b2/b208338h/ for crystal-
lographic data in CIF or other electronic format.
18 R. G. Fargher and L. Pyman, J. Chem. Soc., 1991, 115, 217.
19 D. Das, M. K. Nayak and C. Sinha, Transition Met. Chem., 1997,
22, 172.
20 B. K. Ghosh, A. Mukhopadhyay, S. Goswami, S. Ray and A.
Chakravorty, Inorg. Chem., 1984, 23, 4633.
21 B. K. Ghosh, S. Goswami and A. Chakravorty, Inorg. Chem., 1983,
22, 3358.
Acknowledgements
We thank the Department of Science and Technology, and
Council of Scientific and Industrial Research, New Delhi for
financial support. We are thankful to Prof. G. K. Lahiri of
Indian Institute of Technology Bombay, Mumbai.
22 P. Byabartta, S. Pal, T. K. Misra, C. Sinha, F.-L. Liao, K.
Pannerselvam and T.-H. Lu, J. Coord. Chem., 2002, 55, 479.
23 (a) R. A. Krause and K. Krause, Inorg. Chem., 1980, 19, 2600;
(b) T. Bao, K. Krause and R. A. Krause, Inorg. Chem., 1988, 27,
759.
24 (a) S. Goswami, A. R. Chakravarty and A. Chakravorty, Inorg.
Chem., 1981, 20, 2246; (b) S. Goswami, A. R. Chakravarty and
A. Chakravorty, Inorg. Chem., 1983, 22, 602.
25 A. H. Velders, H. Kooijman, A. L. Spek, J. G. Haasnoot, D. de Vos
and J. Reedijk, Inorg. Chem., 2000, 39, 2966.
26 T. K. Misra, D. Das, C. Sinha, P. Ghosh and C. K. Pal, Inorg. Chem.,
1998, 37, 1672.
27 I. Chakraborty and S. Sengupta, unpublished results.
28 K. Nakamoto, Infrared and Raman Spectra of Inorganic and
Coordination Compounds, Theory and Application in Inorganic
Chemistry, Part A, John Wiley and Sons, Inc, New York, 5th edn.,
p. 199.
29 A. Abragam and B. Bleaney, Electron Paramagnetic Resonance of
Transition Ions, Clarendon, Oxford, UK, 1970, p. 163.
30 M. Shivakumar, K. Pramanik, P. Ghosh and A. Chakravorty, Chem.
Commun., 1998, 2103.
31 (a) R. A. Krause and K. Krause, Inorg. Chem., 1982, 21, 1714;
(b) A. Seal and S. Ray, Acta Crystallogr., Sect. C, 1984, 40, 932.
32 (a) M. Shivakumar, K. Pramanik, P. Ghosh and A. Chakravorty,
Inorg. Chem., 1998, 37, 5968; (b) K. Pramanik, M. Shivakumar,
P. Ghosh and A. Chakravorty, Inorg. Chem., 2000, 39, 195; (c)
M. Shivakumar, K. Pramanik, I. Bhattacharyya and A. Chakravorty,
Inorg. Chem., 2000, 39, 4332.
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