Chemistry of tricarbonylrhenium(I) chelates of azopyrimidines, azoimidazoles and anion radicals thereof

Article abstract of DOI:10.1016/j.jorganchem.2004.02.034

The azopyrimidine and azoimidazole ligands (general abbreviations, RL) used in the present work are 2-(p-R-C₆H₄NN) C₄H₃N₂, RL_pm (R=H, Cl) and 2-(p-R-C₆H₄ NN)-1-(Me)C₃H₂ N₂, RL_im (R=Me, Cl), respectively. The reaction of Re(CO)₅Cl with a slight excess of RL in boiling benzene has furnished blue-violet complexes of type Re(CO)₃Cl(RL) which have been spectrally characterized. In Re(CO)₃Cl (HL_pm) and Re(CO)₃Cl(ClL_im) the Re-N^h, Re-N^a distances are 2.173(6), 2.136(6) ? and 2.150(5), 2.166(5) ?, respectively (N^h and N^a are heterocyclic and azo N atoms, respectively). Their N-N lengths (1.271(8), 1.281(7) ?) implicate relatively weak Re-azo(π^*) back-bonding. In the lattice of Re(CO)₃Cl(HL_pm), pair-wise C-H?O hydrogen bonding between symmetry related molecules is present (C?O; 3.264(9) ?, H?O; 2.460(10) ?;C-H?O; 130.6(5)°). The lattice of Re(CO)₃Cl(ClL_im) also consists of centrosymmetric dimers held by aromatic π-π stacking between parallely placed pendant aryl rings (centroid?centroid distance, 3.781(9) ?). Extended Hückel calculations reveal that the LUMO of Re(CO)₃Cl(RL) is ～60% azo in character. One-electron quasireversible electrochemical reduction occurs near -0.1 and -0.4 V vs. SCE in the cases of Re(CO)₃Cl(RL_pm) and Re(CO)₃Cl(RL_im), respectively. The redox orbital is believed to be to the above noted LUMO. Electrogenerated Re(CO)₃Cl(RL^?-) underwent spontaneous solvolytic chloride displacement in MeCN furnishing Re(CO)₃(MeCN)(RL^?-) which has been isolated. The latter in turn reacted with imidazole and triphenyl phosphine furnishing Re(CO)₃(C₃H₄ N₂)(RL^?-) and Re(CO)₃ (PPh₃)(RL^?-), respectively. The pattern of carbonyl stretching frequencies of these radical anion complexes is similar to that of Re(CO)₃Cl(RL) but for shifts to lower frequencies by 10-40 cm^-1. All the three radical anion systems are one-electron paramagnets (1.7-1.8 μ_B). The unpaired electron is primarily localized in a predominantly azo-π* orbital. A small metal contribution (^185,187Re, I=5/2) is present and both Re(CO)₃(MeCN) (RL^?-) and Re(CO)₃(C₃ H₄N₂) (RL^?-) display six-line EPR spectra (A～28 G). The line shapes and intensities are characteristic of the presence of g-strain. In the case of Re(CO)₃(PPh₃) (RL^?-) seven nearly equispaced lines are observed due to virtually equal coupling with metal and ³¹P (I=1/2) nuclei. The g values of the radical species span the range 2.0033-2.0066.

Full text of DOI:10.1016/j.jorganchem.2004.02.034

Journal of Organometallic Chemistry 689 (2004) 1780–1787
www.elsevier.com/locate/jorganchem
Chemistry of tricarbonylrhenium(I) chelates of azopyrimidines,
azoimidazoles and anion radicals thereof q
a
Bikash Kumar Panda , Suman Sengupta , Animesh Chakravorty
a
a,b,
*
a
Department of Inorganic Chemistry, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700 032, India
b
Jawaharlal Nehru Centre for Advanced Scientific Research, Bangalore 560 064, India
Received 2 February 2004; accepted 21 February 2004
Abstract
The azopyrimidine and azoimidazole ligands (general abbreviations, RL) used in the present work are 2-(p-R-C6H4NN)C4H3N2,
RLpm (R ¼ H, Cl) and 2-(p-R-C6H4NN)-1-(Me)C3H2N2, RLim (R ¼ Me, Cl), respectively. The reaction of Re(CO)5Cl with a slight
excess of RL in boiling benzene has furnished blue–violet complexes of type Re(CO)3Cl(RL) which have been spectrally charac-
h
a
ꢀ
ꢀ
terized. In Re(CO)3Cl(HLpm) and Re(CO)3Cl(ClLim) the Re–N , Re–N distances are 2.173(6), 2.136(6) A and 2.150(5), 2.166(5) A,
a
h
ꢀ
respectively (N and N are heterocyclic and azo N atoms, respectively). Their N–N lengths (1.271(8), 1.281(7) A) implicate rela-
tively weak Re-azo(pꢀ) back-bonding. In the lattice of Re(CO)3Cl(HLpm), pair-wise C–Hꢁ ꢁ ꢁO hydrogen bonding between symmetry
ꢀ
ꢀ
related molecules is present (Cꢁ ꢁ ꢁO; 3.264(9) A, Hꢁ ꢁ ꢁO; 2.460(10) A; C–Hꢁ ꢁ ꢁO; 130.6(5)°). The lattice of Re(CO)3Cl(ClLim) also
consists of centrosymmetric dimers held by aromatic p–p stacking between parallely placed pendant aryl rings (centroidꢁ ꢁ ꢁcentroid
ꢀ
€
distance, 3.781(9) A). Extended Huckel calculations reveal that the LUMO of Re(CO)3Cl(RL) is ꢂ60% azo in character. One-
electron quasireversible electrochemical reduction occurs near )0.1 and )0.4 V vs. SCE in the cases of Re(CO)3Cl(RLpm) and
Re(CO)3Cl(RLim), respectively. The redox orbital is believed to be to the above noted LUMO. Electrogenerated Re(CO)3Cl(RLÅꢃ
)
underwent spontaneous solvolytic chloride displacement in MeCN furnishing Re(CO)3(MeCN)(RLÅꢃ) which has been isolated. The
latter in turn reacted with imidazole and triphenyl phosphine furnishing Re(CO)3(C3H4N2)(RLÅꢃ) and Re(CO)3(PPh3)(RLÅꢃ), re-
spectively. The pattern of carbonyl stretching frequencies of these radical anion complexes is similar to that of Re(CO)3Cl(RL) but
for shifts to lower frequencies by 10–40 cmꢃ1. All the three radical anion systems are one-electron paramagnets (1.7–1.8 lB). The
unpaired electron is primarily localized in a predominantly azo-pꢀ orbital. A small metal contribution (185; 187Re, I ¼ 5=2) is present
and both Re(CO)3(MeCN)(RLÅꢃ) and Re(CO)3(C3H4N2)(RLÅꢃ) display six-line EPR spectra (A ꢂ 28 G). The line shapes and in-
tensities are characteristic of the presence of g-strain. In the case of Re(CO)3(PPh3)(RLÅꢃ) seven nearly equispaced lines are observed
due to virtually equal coupling with metal and 31P (I ¼ 1=2) nuclei. The g values of the radical species span the range 2.0033–2.0066.
Ó 2004 Elsevier B.V. All rights reserved.
Keywords: Tricarbonylrhenium(I) chelates; Azoligands and radical anions thereof; Azopyrimidines and azoimidazoles
1. Introduction
particular case of the monovalent state, homoleptic sta-
bilization requires tris chelation thus satisfying the large
back-bonding demand of the 5d6 ion via multiple dp(Re)–
ppꢀ(azo) interactions [4]. On the other hand, in the pres-
ence of a strong p-accepting coligand, bis/mono chelation
of azoheterocycles to rhenium(I) should be feasible. In
this context the ReI(CO)3 moiety would be a good choice
and indeed its monochelates with 2,20-azobispyridine [5]
and 2-(phenylazo)pyridine [6] have been structurally
characterized. We also note that ReI(CO)3 complexes are
of general interest in terms of their potential photophys-
ical [7] and radiopharmaceutical [8] properties.
In combination with appropriate coligands, azohet-
erocycles have been shown from this laboratory to act as
versatile ligands suited for stable binding of all oxidation
states of rhenium in the interval þ1 to þ5 [1–4]. In the
q Supplementary data associated with this article can be found, in the
online version, at doi: 10.1016/j.jorganchem.2004.02.034.
* Corresponding author. Address: Department of Inorganic Chem-
istry, Indian Association for the Cultivation of Science, Jadavpur,
Kolkata 700 032, India. Tel.: +91-33-24724436; fax: +91-33-24732805.
E-mail address: icac@mahendra.iacs.res.in (A. Chakravorty).
0022-328X/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2004.02.034

B.K. Panda et al. / Journal of Organometallic Chemistry 689 (2004) 1780–1787
1781
Table 1
Carbonyl stretching frequenciesa
In the present work we explore the Re(CO)3 chemistry
of azopyrimidine and azoimidazole ligands (RL). The
synthesis and structure of species of type ReI(CO)3Cl(RL)
are reported. Reductive halide elimination in the presence
of neutral monodentate donors (D) have furnished radi-
cal anion organometallics of type ReI(CO)3(D)(RLÅꢃ)
which have been isolated and characterized.
Complexes
mCO cmꢃ1
Re(CO)3Cl(HLpm
Re(CO)3Cl(ClLpm
Re(CO)3Cl(ClLim
Re(CO)3Cl(MeLim
Re(CO)3(CH3CN)(ClLpÅꢃm
Re(CO)3(CH3CN)(ClLiÅmꢃ
Re(CO)3(PPh3)(ClLpÅꢃm
Re(CO)3(PPh3)(ClLiÅꢃm
Re(CO)3(C3H4N2)(ClLpÅꢃm
Re(CO)3(C3H4N2)(ClLiÅmꢃ
)
2032s 1924s;br
2022s 1928s;br
2026s 1931s 1902s
2023s 1934s 1899s
2010s 1888s;br
2020s 1924s 1880sh
2010s 1900s;br
2010s 1898s;br
2005s 1890s;br
)
)
)
)
)
)
)
2. Results and discussion
)
)
2021s 1925sh 1901s;br
2.1. Synthesis and characterization of Re(CO)3Cl(RL)
a In KBr disc. s Strong peak, s;br Strong and broad, sh Shoulder.
The azoheterocycles (general abbreviation RL) used
in the present work are 2-(arylazo)pyrimidine (RLpm), 1
and 2-(arylazo)-1-methylimidazole (RLim), 2. The reac-
tion, Eq. (1), of Re(CO)5Cl with slight excess of RL
proceeds smoothly in boiling
2.2. Structure
The X-ray structures of Re(CO)3Cl(HLpm) and Re(-
CO)3Cl(ClLim) have been accurately determined. Mo-
lecular views are shown in Figs. 1 and 2 and selected
bond parameters are listed in Tables 2 and 3. In the
distorted octahedral ReC3N2Cl coordination sphere the
three carbon monoxide ligands are facially disposed to
maximize Re–CO back-bonding. In Re(CO)3Cl(HLpm),
the Re(HLpm) fragment excluding the pendant phenyl
N
R
N
N
N
N
N
R
N
N
Me
RLim(R=Me, Cl)
RLpm(R=H, Cl)
ꢀ
ring constitutes a good plane (mean deviation 0.03 A)
2
1
and the C11O1 and C12O2 molecules also lie on this
plane which makes a dihedral angle of 37.1° with the
pendant phenyl ring. The behaviour of Re(CO)3Cl
(ClLim) is very similar; the dihedral angle between the
corresponding plane and the pendant chlorophenyl
group is 45.1°.
ReðCOÞ Cl þ RL ! ReðCOÞ ClðRLÞ þ 2CO
ð1Þ
5
3
benzene furnishing blue–violet crystalline complexes of
composition Re(CO)3Cl(RL) belonging to the coordi-
nation type 3 (Na and Nh are azo and heterocyclic N
atom, respectively) in excellent yields.
The structure of ReI(CO)3Cl(ClLim) provides an op-
portunity to observe the effect of rhenium oxidation
states on metal–ligand bond lengths via comparison
with ReIICl2(MeLim)2 [4a] and ReIIICl3(OPPh3)(ClLim)
[2a]. The average Re–N(imidazole), Re–N(azo) and Re–
CO
Na
Nh
CO
CO
I
Re
ꢀ
Cl lengths are 2.052(8), 2.003(8) and 2.389(3) A, re-
spectively in ReIICl2(MeLim)2 and 1.999(14), 1.978(15)
Cl
3
III
ꢀ
and 2.356(6) A, respectively in Re Cl3(OPPh3)(ClLim).
Upon comparing with the lengths 2.150(5), 2.166(5) and
Spectral characterization data of the diamagnetic
(idealized t62g) complexes are collected in Section 3. In 1H
NMR, well resolved lines corresponding to RL pro-
tons are observed. The complexes are characterized by
a moderately intense band near 550 nm in Re(CO)3-
Cl(RLpm) and near 510 nm in Re(CO)3Cl(RLim). The
band is assigned to MLCT transition of the idealized
type t2g(Re) ! pꢀ(azo). The observed band energy order
Re(CO)3Cl(RLpm) < Re(CO)3Cl(RLim) is consistent with
the more negative reduction potential of the azo group in
the latter species (vide infra). In IR three strong and sharp
CO stretching frequencies are resolved near 2025, 1930
and 1900 cmꢃ1 (Table 1) in the case of Re(CO)3Cl(RLim).
However in Re(CO)3Cl(RLpm) only two strong bands
occur near 2025 and 1925 cmꢃ1 (Table 1). The latter band
is broader and more intense than the former and is
believed to incorporate two unresolved components.
Fig. 1. Perspective view and atom-labeling scheme for Re(CO)3
Cl(HLpm).

1782
B.K. Panda et al. / Journal of Organometallic Chemistry 689 (2004) 1780–1787
trend is observed between ReI(CO)3Cl(HLpm) and Re-
IIICl3(OPPh3)(HLpm) [3]. In contrast to the Re–N and
ꢀ
Re–Cl distances, the azo N–N length (1.27–1.28 A) in
the present ReI(CO)3Cl(RL) complexes are significantly
ꢀ
shorter than those (1.32–1.34 A) [2a,3,4a] in the bivalent
and trivalent complexes noted above.
This happens because in Re(CO)3Cl(RL) the CO li-
gand act as a strong back-bonder, reducing the demand
for Re-azo back-bonding.
Significant non-covalent interactions characterize
both the lattices. In the case of Re(CO)3Cl(HLpm) where
all the H atoms were directly located in difference
Fourier maps, C(1)–H(1)ꢁ ꢁ ꢁO(3) hydrogen bonding be-
tween symmetry related molecules generate centrosym-
metric pairs as depicted in Fig. 3(a). The relevant bond
parameters (Table 2) are well within the acceptable
Fig. 2. Perspective view and atom-labeling scheme for Re(CO)3
Cl(ClLim).
I
ꢀ
2.449(2) A, respectively in Re (CO)3Cl(ClLim), the pro-
gressive contraction of the metal radius upon oxidation
is neatly revealed. Between ReI(CO)3Cl(ClLim) and
ReIIICl3(OPPh3)(ClLim) the above three distances de-
ꢀ
ranges for such hydrogen bonding: Cꢁ ꢁ ꢁO, 3.0–3.5 A;
ꢀ
Hꢁ ꢁ ꢁO, 2.3–3.0 A and C–Hꢁ ꢁ ꢁO, 100–180° [9]. The lat-
tice of Re(CO)3Cl(ClLim) also consists of dimers which
incorporate aromatic p–p stacking of the pendant aryl
rings (Fig. 3(b)). The perpendicular distance between the
ꢀ
crease by as much as 0.1–0.2 A. A similar bond length
ꢀ
two parallel six-membered rings is 3.598(9) A, the offset
angle [10] being 17.9(6)°. The distance between the
Table 2
ꢀ
Selected bond parameters (bond lengths in A; bond angles in (°)) for
Re(CO)3Cl(HLpm
)
ꢀ
centroids of the two rings is 3.781(9) A. The p–p
Bond lengths
Re–N(1)
stacking is associated with a Cl(2)ꢁ ꢁ ꢁO(3) contact of
2.173(6)
2.467(2)
2.136(6)
1.916(8)
1.920(8)
1.923(8)
O(1)–C(11)
O(2)–C(12)
O(3)–C(13)
N(3)–N(4)
H(1)ꢁ ꢁ ꢁO(3)
C(1)ꢁ ꢁ ꢁO(3)
1.167(10)
1.150(10)
1.128(9)
1.271(8)
2.460(10)
3.264(9)
ꢀ
length 3.188(7) A which is slightly shorter than the Van
Re–Cl(1)
Re–N(4)
ꢀ
der waals sum of ꢂ3.3 A.
€
Extended Huckel calculations on Re(CO)3Cl(HLpm
using crystallographic coordinates reveals that the
)
Re–C(11)
Re–C(12)
Re–C(13)
Bond angles
N(1)–Re–Cl(1)
N(1)–Re–N(4)
83.0(2)
73.0(2)
170.6(2)
101.0(3)
94.1(3)
83.8(2)
92.6(2)
93.3(2)
Cl(1)–Re–C(13)
N(4)–Re–C(11)
N(4)–Re–C(12)
N(4)–Re–C(13)
C(11)–Re–C(12)
C(11)–Re–C(13)
C(12)–Re–C(13)
C(1)–H(1)ꢁ ꢁ ꢁO(3)
177.1(2)
98.3(3)
173.6(3)
94.9(3)
87.5(4)
90.1(3)
87.7(3)
130.6(5)
N(1)–Re–C(11)
N(1)–Re–C(12)
N(1)–Re–C(13)
Cl(1)–Re–N(4)
Cl(1)–Re–C(11)
Cl(1)–Re–C(12)
Table 3
ꢀ
Selected bond parameters (bond lengths in A; bond angles in (°)) for
Re(CO)3Cl(ClLim
)
Bond lengths
Re–N(1)
2.150(5)
2.449(2)
2.166(5)
1.916(7)
1.917(7)
Re–C(13)
1.972(8)
1.138(8)
1.138(8)
1.052(9)
1.281(7)
Re–Cl(1)
Re–N(3)
O(1)–C(11)
O(2)–C(12)
O(3)–C(13)
N(2)–N(3)
Re–C(11)
Re–C(12)
Bond angles
N(1)–Re–Cl(1)
N(1)–Re–N(3)
86.4(2)
73.0(2)
169.5(2)
101.0(2)
89.5(2)
85.8(2)
94.2(2)
90.7(2)
Cl(1)–Re–C(13)
N(3)–Re–C(11)
N(3)–Re–C(12)
N(3)–Re–C(13)
C(11)–Re–C(12)
C(11)–Re–C(13)
C(12)–Re–C(13)
175.8(2)
96.5(2)
173.3(2)
93.8(2)
89.5(3)
89.9(3)
89.2(3)
N(1)–Re–C(11)
N(1)–Re–C(12)
N(1)–Re–C(13)
Cl(1)–Re–N(3)
Cl(1)–Re–C(11)
Cl(1)–Re–C(12)
Fig. 3. Lattice interactions in (a) Re(CO)3Cl(HLpm) and (b) Re(CO)3
Cl(ClLim).

B.K. Panda et al. / Journal of Organometallic Chemistry 689 (2004) 1780–1787
1783
LUMO is mostly localized on the HLpm ligand, the
major contributor (61%) being the azo function; the
LUMO in Re(CO)3Cl(ClLim) is of similar nature with
57% azo contribution. For further details see Section 3.
2.2.1. One-electron reduction: isolation of radical anion
species
In acetonitrile solution the complexes display a
quasireversible one-electron cyclic voltammetric re-
sponse near )0.1 V in the case of Re(CO)3Cl(RLpm) and
near )0.4 V vs. SCE in the case of Re(CO)3Cl(RLim).
The redox orbital is believed to be the LUMO noted
above. The added electron is thus localized in the
RL ligand, primarily in the azo function. Reduction
potential data for the couple of Eq. (2) are listed in the
Section 3.
ReðCOÞ ClðRLÞ þ e ꢀ ½ReðCOÞ ClðRLÅꢃÞꢄꢃ
ð2Þ
3
3
The RLim ligand is more difficultly reducible than RLpm
and this is also reflected in the trend of MLCT band
energies, vide supra. In addition to the above couple,
the complexes also display irreversible metal oxidation
at positive potentials. The observed electrochemical
pattern of the Re(CO)3Cl(RL) species is qualitatively
similar to those of 2-(arylazo)pyridine [6] and 2,20-az-
obispyridine [5] chelates. Reduced species of the latter
chelate, generated in solution, have been characterized
by EPR and other spectral data [11].
The Re(CO)3Cl(RLÅꢃ) species generated coulometri-
cally could not be isolated in pure form. However upon
leaving them in acetonitrile solution chloride dissocia-
tion occurs furnishing Re(CO)3(MeCN)(RLÅꢃ) species
which have been successfully isolated. Treatment of
Re(CO)3(MeCN)(RLÅꢃ) with triphenyl phosphine or
imidazole in dichloromethane solution has afforded
Re(CO)3(D)(RLÅꢃ) (D ¼ PPh3, C3H4N2). These radical
complexes are red to brown in colour and are noncon-
ducting in solution. In the solid state these are stable for
many hours. In IR the D ¼ MeCN and D ¼ C3H4N2
species respectively display mCBN and mN–H stretches near
2230 and 2985 cmꢃ1. A quasireversible cyclic voltam-
metric response due to coordinated RLÅꢃ occurs at a
potentials close to that of the parent chloro complex
consistent with localization of the redox process on the
RL ligand.
Fig. 4. X-band EPR spectrum of (a) Re(CO)3(C3H4N2)(ClLÅimꢃ), (b)
Re(CO)3(C3H4N2)(ClLiÅmꢃ) simulated and (c) Re(CO)3(PPh3)(ClLpÅꢃm).
Solvent used is dichloromethane (298 K) and instrument settings are
power, 30 dB; modulation, 100 kHz; sweep center, 3200 G; sweep
width, 500 G.
Table 4
EPR spectral data in dichloromethane solution and bulk magnetic
moments in solid state at 298 K
The pattern of carbonyl stretching frequencies in the
radical chelates is closely similar to that of the parent
Re(CO)3Cl(RL) species but for shifts to lower frequen-
cies by 10–40 cmꢃ1 (Table 1). The shift is consistent with
radical anion formation which logically augments Re–
CO back-bonding. Selected spectral and magnetic
properties of six representative radical species are col-
lected in Table 4. Their magnetic moments (1.7–1.8 lB)
Complexes
g
A (G)
leff ; lB
Re(CO)3(CH3CN)(ClLpÅꢃm
)
2.0066
2.0029
2.0037
2.0033
2.0058
2.0039
27.25
27.25
40.42
39.17
29.25
28.75
1.81
1.80
1.79
1.77
1.75
1.74
Re(CO)3(CH3CN)(ClLiÅmꢃ
)
Re(CO)3(PPh3)(ClLpÅꢃm
Re(CO)3(PPh3)(ClLiÅmꢃ
Re(CO)3(C3H4N2)(ClLpÅꢃm
Re(C3)(C3H4N2)(ClLiÅꢃm
)
)
)
)

1784
B.K. Panda et al. / Journal of Organometallic Chemistry 689 (2004) 1780–1787
corresponds to S ¼ 1=2. In dichloromethane solution
both Re(CO)3(MeCN)(RLÅꢃ) and Re(CO)3(C3H4N2)
(RLÅꢃ) display six-line EPR spectra due to the presence
of a small metal contribution (185Re and 187Re, I ¼ 5=2
for both) to the spin-bearing orbital (A ꢂ 28 G). The
unusual line shape and intensities which have been
simulated (Fig. 4(a) and (b)) arise from g-strain [12]. No
nitrogen hyperfine structure is resolved in the spectra; it
is known that 14N coupling constant in azo-pꢀ radicals is
small and difficult to observe [11,13]. Like monovalent
rhenium, bivalent ruthenium has d6 configuration and
structure determination of Ru(CO)Cl(PPh3)2(ClLÅimꢃ)
[14] and related species has indeed revealed the presence
of long N–N bonds corresponding to population of azo-
pꢀ orbitals [15]. However solution EPR studies failed to
reveal nitrogen hyperfine structures [14,15] as in the case
of the present azoheterocycle anion radical chelates of
rhenium(I).
In the case of Re(CO)3(PPh3)(RLÅꢃ) the coupling
with metal is somewhat stronger (A ꢂ 40 G) and seven
nearly equispaced lines (Fig. 4(c)) are observed corre-
sponding to virtually equal coupling with 185; 187Re
(I ¼ 5=2) and 31P (I ¼ 1=2). This situation has been
previously observed in diazabutadiene organometallics
[16]. The g-values in all radical species examined by us
are only slightly higher than the free-electron value of
2.0023.
PAR 155 vibrating-sample magnetometer. Microanaly-
ses (C, H, N) were performed using a Perkin–Elmer 240C
elemental analyzer. Solution electrical conductivity was
measured in acetone with a Phillips PR 9500 bridge by
using a platinized electrode (cell constant of 1.05). The
electrochemical measurements were performed at a
platinum electrode under nitrogen atmosphere in aceto-
nitrile solution by using a model 620A electrochemical
analyzer of CHI Instruments. The supporting electrolyte
was tetraethylammonium perchlorate and potentials are
referenced to the saturated calomel electrode (SCE).
3.3. Synthesis of the complexes, Re(CO)3Cl(RL)
The Re(CO)3Cl(RL) complexes were synthesized in
excellent yield (ꢂ85%) by reacting Re(CO)5Cl in boiling
benzene with slight excess of the RL ligand. Details of a
representative case are given below. The other com-
pounds were prepared analogously.
3.3.1. Re(CO)3Cl(HLpm
)
HLpm (31 mg, 0.168 mmol) was added to a hot so-
lution of Re(CO)5Cl (50 mg, 0.138 mmol) in 40 ml
benzene. On subsequent heating to reflux for 3 h the
solution colour changed from light yellow to blue–vio-
let. The solvent was then evaporated under reduced
pressure and the residue washed thoroughly with hexane
for the removal of excess ligand. It was dried, and then
recrystallized from a dichloromethane/hexane (1:3)
mixture affording a crystalline product which was dried
in vacuo over fused calcium chloride. Yield: 56 mg
(83%). Anal. Found: C, 31.81; H, 1.68; N, 11.46. Calc.
for C13H8N4O3ClRe: C, 31.87; H, 1.65; N, 11.44%. UV–
Vis (CH2Cl2, kmax (nm) (e, Mꢃ1 cmꢃ1)): 544 (2870); 386
3. Experimental
3.1. Materials
Ligands of type 2-(arylazo)pyrimidine [17] and
2-(arylazo)-1-methyl-imidazole [18] were prepared by
reported methods. Pentacarbonyl chlororhenium(I),
triphenyl phosphine and imidazole were purchased from
Aldrich (USA). The purification and drying of dichlo-
romethane and acetonitrile for synthesis as well as for
electrochemical and spectral work were done as before
[19]. Benzene was distilled over sodium before use. All
other chemicals and solvents were of analytical grade
and were used as received.
1
(7210). H NMR (CDCl3, d, J/Hz): 9.28 (d, 1H, H(1),
J12 4.71, J13 2.28), 7.72 (dd, 1H, H(2), J21 4.71, J23 5.56),
9.23 (d, 1H, H(3), J32 5.56, J31 2.28), 8.07 (d, 2H, H(6),
H(10), J 9.12), 7.61 (complex multiplet, 3H, H(7), H(8),
H(9)). Epa(ReII/ReI couple): 1.60 V, E1=2(HLpm)/(HLpÅꢃm):
)0.13 V (DEp ¼ 70 mV).
3.3.2. Re(CO)3Cl(ClLpm
)
Re(CO)5Cl (50 mg, 0.138 mmol) and ClLpm (36 mg,
0.165 mmol) were employed. Yield: 61 mg (84%). Anal.
Found: C, 29.72; H, 1.36; N, 10.73. Calc. for C13H7N4
O3Cl2Re: C, 29.78; H, 1.35; N, 10.69%. UV–Vis (CH2Cl2,
kmax (nm) (e, Mꢃ1 cmꢃ1)): 552 (3850); 393 (9480). 1H
NMR (CDCl3, d, J/Hz): 9.29 (d, 1H, H(1), J12 4.56, J13
2.22), 7.65 (dd, 1H, H(2), J21 4.56, J23 5.53), 9.23 (d, 1H,
H(3), J32 5.53, J31 2.22), 8.04 (d, 2H, H(6), H(10), J 8.82),
7.62 (d, 2H, H(7), H(9), J 8.85). Epa(ReII/ReI couple): 1.70
V, E1=2(ClLpm)/(ClLÅpꢃm): )0.12 V (DEp ¼ 80 mV).
3.2. Physical measurements
Electronic and IR spectra were recorded with a Shi-
madzu UV-1601PC spectrophotometer and a Nicolet
Magna IR series II spectrometer, respectively. 1H NMR
spectra were obtained using a Bruker 300 MHz FT NMR
spectrometer. The numbering scheme used for
1H NMR is the same as in crystallography. Spin–spin
structures are abbreviated as: s, singlet; d, doublet; t,
triplet; m, multiplet. EPR spectra were recorded on a
Varian E-109C X-band spectrometer fitted with a quartz
dewar. Magnetic susceptibilities were measured on a
3.3.3. Re(CO)3Cl(MeLim)
Re(CO)5Cl (50 mg, 0.138 mmol) and MeLim (33 mg,
0.165 mmol) were used. Yield: 59 mg (84%). Anal.

B.K. Panda et al. / Journal of Organometallic Chemistry 689 (2004) 1780–1787
1785
Found: C, 33.29; H, 2.43; N, 11.09. Calc. for
C14H12N4O3ClRe: C, 33.24; H, 2.39; N, 11.07%. UV–
Vis (CH2Cl2, kmax (nm) (e, Mꢃ1 cmꢃ1)): 496 (4790); 427
2229 (mCBN). Epa(ReII/ReI couple): 1.45 V, E1=2(ClLim)/
(ClLÅimꢃ): )0.45 V (DEp ¼ 80 mV).
1
3.5. Synthesis of the complexes, Re(CO)3(D)(RLÅꢃ)
(11740). H NMR (CDCl3, d, J/Hz): 7.89 (d, 2H, H(6),
H(10), J 8.49), 7.56 (d, 1H, H(1), ill resolved), 7.40 (d,
1H, H(2), ill resolved), 7.35 (d, 2H, H(7), H(9), J 8.31),
4.20 (s, 3H, N–Me), 2.48 (s, 3H, 8-Me). Epa(ReII/ReI
The Re(CO)3(D)(RLÅꢃ) complexes were synthesized
in excellent yield (ꢂ75%) by reacting Re(CO)3(CH3CN)
(RLÅꢃ) in dichloromethane at room temperature with
slight excess of D. Details of a representative case
are given below. The other compounds are prepared
analogously.
couple): 1.41 V, E1=2(MeLim)/(MeLÅimꢃ): )0.43
(DEp ¼ 85 mV).
V
3.3.4. Re(CO)3Cl(ClLim)
Re(CO)5Cl (50 mg, 0.138 mmol) and ClLim (37 mg,
0.168 mmol) were employed. Yield: 62 mg (86%). Anal.
Found: C, 29.63; H, 1.75; N, 10.67. Calc. for
C13H9N4O3Cl2Re: C, 29.66; H, 1.72; N, 10.64%. UV–
Vis (CH2Cl2, kmax (nm) (e, Mꢃ1 cmꢃ1)): 515 (3820); 410
3.5.1. Re(CO)3(PPh3)(ClLpÅꢃm
)
To a stirred red solution of Re(CO)3(CH3CN)
(ClLÅpꢃm) (50 mg, 0.094 mmol) in dry dichloromethane
(25 ml) was added 32 mg (0.122 mmol) of PPh3. The
mixture was stirred for 1 h when the colour of the so-
lution changed from red to brown. The solvent was then
removed under reduced pressure and the brown solid
thus obtained was washed thoroughly with hexane and
finally dried in vacuo. Yield: 53 mg (75%). Anal. Found:
C, 49.62; H, 3.00; N, 7.49. Calc. for C31H22N4O3PClRe:
C, 49.57; H, 2.95; N, 7.46%. UV–Vis (CH2Cl2, kmax (nm)
(e, Mꢃ1 cmꢃ1)): 537 (3490); 369 (11430); 263 (15420).
Epa(ReII/ReI couple): 1.72 V, E1=2(ClLpm)/(ClLpÅꢃm):
)0.12 V (DEp ¼ 100 mV).
1
(10240). H NMR (CDCl3, d, J/Hz): 7.91 (d, 2H, H(6),
H(10), J 8.91), 7.60 (d, 1H, H(1), J 1.08), 7.54 (d, 2H,
H(7), H(9), J 8.91), 7.45 (d, 1H, H(2), ill resolved), 4.22
(s, 3H, N–Me). Epa(ReII/ReI couple): 1.45 V,
E1=2(ClLim)/(ClLÅimꢃ): )0.41 V (DEp ¼ 80 mV).
3.4. Synthesis of the complexes Re(CO)3(CH3CN)
(RLÅꢃ)
Constant potential coulometry of Re(CO)3Cl(RL) at
a potential of ꢂ200 mV lower than E1=2 in dry aceto-
nitrile solution furnished the required radical species.
Details are given here for a representative case.
3.5.2. Re(CO)3(PPh3)(ClLÅimꢃ)
Re(CO)3(CH3CN)(ClLÅimꢃ) (50 mg, 0.094 mmol) and
32 mg PPh3 (0.122 mmol) were used. Yield: 55 mg
(78%). Anal. Found: C, 49.39; H, 3.25; N, 7.48. Calc. for
C31H24N4O3ClPRe: C, 49.43; H, 3.21; N, 7.44%. UV–
Vis (CH2Cl2, kmax (nm) (e, Mꢃ1 cmꢃ1)): 547 (2950); 377
(9260); 267 (12010). Epa(ReII/ReI couple): 1.48 V,
E1=2(ClLim)/(ClLÅimꢃ): )0.41 V (DEp ¼ 80 mV).
3.4.1. Re(CO)3(CH3CN)(ClLpÅꢃm
)
Re(CO)3Cl(ClLpm) (50 mg, 0.095 mmol) was taken in
30 ml dry acetonitrile. To this solution 40 mg of TEAP
was added and it was subjected to exhaustive electrolysis
at )0.33 V and the reduced solution was left to stand for
5 h during which time the colour became red. The sol-
vent was then evaporated under reduced pressure and
the residue was dissolved in benzene (excess TEAP re-
mains insoluble). The filtered solution was stripped of
solvent under reduced pressure, and the red solid com-
plex thus obtained was dried in vacuo. Yield: 37 mg
(73%). Anal. Found: C, 34.04; H, 1.95; N, 13.28. Calc.
for C15H10N5O3ClRe: C, 34.00; H, 1.90; N, 13.22%.
UV–Vis (CH2Cl2, kmax (nm) (e, Mꢃ1 cmꢃ1)): 527 (2970);
377 (10,860). IR (KBr, cmꢃ1): 2231 (mCBN). Epa(ReII/ReI
3.5.3. Re(CO)3(C3H4N2)(ClLpÅꢃm
)
Re(CO)3(CH3CN)(ClLÅpꢃm) (50 mg, 0.094 mmol) and 8
mg imidazole (0.118 mmol) were employed. Yield: 39
mg (74%). Anal. Found: C, 34.44; H, 2.04; N, 15.03.
Calc. for C16H11N6O3ClRe: C, 34.50; H, 1.99; N,
15.09%. UV–Vis (CH2Cl2, kmax (nm) (e, Mꢃ1 cmꢃ1)): 548
(3380); 374 (5850); 312 (10620). IR (KBr, cmꢃ1): 2985
(mNꢃH). Epa(ReII/ReI couple): 1.65 V, E1=2(ClLpm)/
Åꢃ
(ClL ): )0.16 V (DEp ¼ 80 mV).
pm
couple): 1.67 V, E1=2(ClLpm)/(ClLÅpꢃm): )0.13
(DEp ¼ 70 mV).
V
3.5.4. Re(CO)3(C3H4N2)(ClLÅimꢃ)
Re(CO)3(CH3CN)(ClLÅimꢃ) (50 mg, 0.094 mmol) and 8
mg imidazole (0.118 mmol) were used. Yield: 40 mg
(76%). Anal. Found: C, 34.32; H, 2.39; N, 15.08. Calc.
for C16H13N6O3ClRe: C, 34.38; H, 2.34; N, 15.03%.
UV–Vis (CH2Cl2, kmax (nm) (e, Mꢃ1 cmꢃ1)): 561 (3450);
380 (5780); 314 (11140). IR (KBr, cmꢃ1): 2988 (mN–H).
Epa(ReII/ReI couple): 1.44 V, E1=2(ClLim)/(ClLiÅmꢃ): )0.47
V (DEp ¼ 80 mV).
3.4.2. Re(CO)3(CH3CN)(ClLÅimꢃ)
Re(CO)3Cl(ClLim) (50 mg, 0.095 mmol) was em-
ployed. Yield: 36 mg (71%). Anal. Found: C, 33.82; H,
2.29; N, 13.21. Calc. for C15H12N5O3ClRe: C, 33.87; H,
2.27; N, 13.17%. UV–Vis (CH2Cl2, kmax (nm)
(e, Mꢃ1 cmꢃ1)): 544 (2960); 382 (9930). IR (KBr, cmꢃ1):

1786
B.K. Panda et al. / Journal of Organometallic Chemistry 689 (2004) 1780–1787
Table 5
Crystal data for Re(CO)3Cl(HLpm) and Re(CO)3Cl(ClLim
)
Complexes
Re(CO)3Cl(HLpm
)
Re(CO)3Cl(ClLim)
Empirical formula
Formula weight
Crystal system
Space group
C13H8ClN4O3Re
489.88
Triclinic
C13H9Cl2N4O3Re
526.34
Monoclinic
P2=c
14.365(3)
7.496(2)
14.546(3)
–
ꢁ
P1
ꢀ
a (A)
8.4010(7)
9.0220(9)
10.7310(7)
92.380(7)
106.232(6)
89.878(8)
780.21(11)
2
ꢀ
b (A)
ꢀ
c (A)
a (°)
b (°)
c (°)
96.15(3)
–
3
ꢀ
V (A )
1557.3(5)
4
8.164
Z
l (Mo Ka) (mmꢃ1
)
7.974
2.085
Dcalc (g cmꢃ1
Total reflections
)
2.245
16926
2631
2631 (0.0000)
0.0373, 0.0909
Independent reflections (Rint
)
3640 (0.0561)
0.0438, 0.1039
b
R1a; wR2 ½I > 2rðIÞꢄ
P
P
a R1 ¼ jjFoj ꢃ jFcjj= jFoj.
h
i
P
P
2
2
b wR2 ¼
wðFo2 ꢃ Fc2Þ = ðFo2Þ
.
3.6. Molecular orbital calculation
cally, but for Re(CO)3Cl(ClLim) hydrogen atoms were
added at calculated positions. Calculations were per-
formed using the SHELXTL, version 5.03 [22] programme
package. Significant crystal data are listed in Table 5.
€
Extended Huckel calculations and orbital plots were
performed using the ICON software package developed
by Hoffmann and others [20]. Experimental atomic co-
ordinates of Re(CO)3Cl(HLpm) and Re(CO)3Cl(ClLim)
were used in the calculations. In the case of Re(CO)3Cl
(HLpm) significant contributors to the LUMO are azo
(61%), phenyl ring (14%), pyrimidinyl ring (13%), car-
bonyl (6%) and metal (2%). In the case of Re(CO)3(ClLim)
the contributors are azo (57%), chlorophenyl ring (7%),
imidazolyl ring (24%), carbonyl (1%), metal (2%).
4. Conclusion
Blue–violet complexes of type Re(CO)3Cl(RL) have
been synthesized and the structures of Re(CO)3
Cl(HLpm) and Re(CO)3Cl(ClLim) determined. Consis-
tent with contraction of metal radius upon oxidation,
the Re–N distances within the Re(RL) fragment de-
crease considerably in going from ReI(CO)3Cl(RL) to
ReIIICl3(OPPh3)(RL). Strong Re–CO back-bonding
makes the N–N length in the former significantly shorter
than that in the later. The crystal lattices of Re(-
CO)3Cl(HLpm) and Re(CO)3Cl(ClLim) incorporate in-
termolecular interactions: C–Hꢁ ꢁ ꢁO hydrogen bonding
in the former and aromatic p–p stacking in the latter.
The LUMO of Re(CO)3Cl(RL) is predominately azo-pꢀ
in character and coulometric reduction in the presence
of ligands has afforded the one-electron paramagnetic
(S ¼ 1=2) radical anion species Re(CO)3(MeCN)(RLÅꢃ),
Re(CO)3(C3H4N2)(RLÅꢃ) and Re(CO)3(PPh3)(RLÅꢃ) in
pure form. Their solution EPR spectra have revealed
that the unpaired electron is primarily localized on the
azoheterocycle ligand. A small metal contribution is
present.
3.7. X-ray structure determination
Single crystals of Re(CO)3Cl(HLpm) (0.40 ꢅ 0.35 ꢅ
0.30 mm3) and Re(CO)3Cl(ClLim) (0.35 ꢅ 0.30 ꢅ 0.30
mm3) were grown by slow diffusion of hexane into di-
chloromethane solutions at room temperature. Cell pa-
rameters were determined by a least-squares fit of 30
machine-centred reflections (2h ¼ 14–28°). Data were
collected by the x-scan technique on a Siemens R3m/V
four-circle diffractometer with graphite-monochromated
ꢀ
Mo Ka radiation (k ¼ 0:71073 A). Two check reflections
measured after every 198 reflections showed no signifi-
cant intensity reduction in any case. All data were cor-
rected for Lorentz polarisation effects and an empirical
absorption correction [21] was done on the basis of az-
imuthal scan of six reflections for the crystals.
The metal atom was located from Patterson maps and
the rest of the non-hydrogen atoms emerged from suc-
cessive Fourier syntheses. The structures were refined by
full matrix least-squares procedures. All non-hydrogen
atoms were refined anisotropically. The hydrogen atoms
of Re(CO)3Cl(HLpm) were located and refined isotropi-
5. Supplementary material
Crystallographic data for the structure analysis have
been deposited with the Cambridge Crystallographic

B.K. Panda et al. / Journal of Organometallic Chemistry 689 (2004) 1780–1787
1787
[9] (a) G.R. Desiraju, T. Steiner, The Weak Hydrogen Bond in
Structural Chemistry and Biology, Oxford University Press, New
York, 1999;
Data Centre, CCDC nos. 230074 and 230075 corre-
sponding to Re(CO)3Cl(HLpm) and Re(CO)3Cl(ClLim),
respectively. Copies of this information may be obtained
free of charge from The Director, CCDC, 12 Union
Road, Cambridge CB2 1EZ, UK (Fax: +44-1223-
336033; email: deposit@ccdc.cam.ac.uk or http://
www.ccdc.cam.ac.uk).
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Acknowledgements
We are thankful to the Department of Science and
Technology, the Council of Scientific and Industrial
Research, New Delhi, India, for financial support. The
help received from Professor P.T. Manoharan in simu-
lation of EPR spectra is gratefully acknowledged.
[12] J.R. Pilbrow, Transition ion Paramagnetic Resonance, Clarendon
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