Coordination chemistry of Re complexes with 2(2′-pyridyl) benzimidazole

Article abstract of DOI:10.1016/j.ica.2005.06.019

Two new Re(III) and Re(IV) complexes with 2(2′-pyridyl)benzimidazole (pbimz) were prepared and their crystal and molecular structures established by single-crystal X-ray diffraction. Reaction of [ReOCl₂(OEt)(PPh ₃)₂] with

Full text of DOI:10.1016/j.ica.2005.06.019

Inorganica Chimica Acta 358 (2005) 3914–3918
www.elsevier.com/locate/ica
Coordination chemistry of Re complexes with
2(20-pyridyl)benzimidazole
a,
a,b
Maxim N. Sokolov *, Natalia E. Fedorova a,b, Eugenia V. Peresypkina
,
b
a
b
Richard Patow , Vladimir E. Fedorov , Dieter Fenske
¨
a
Nikolayev Institute of Inorganic Chemistry SB RAS, pr. Lavrentyeva 3, 630090 Novosibirsk, Russian Federation
b
Institut fu¨r Anorganische Chemie, Karlsruhe, Engesserstrasse Geb. 30.45, Karlsruhe 76128, Germany
Received 21 April 2005; accepted 13 June 2005
Available online 26 July 2005
Abstract
Two new Re(III) and Re(IV) complexes with 2(20-pyridyl)benzimidazole (pbimz) were prepared and their crystal and molecular
structures established by single-crystal X-ray diffraction. Reaction of [ReOCl2(OEt)(PPh3)2] with the ligand gave red cis(Cl),
trans(P)-[ReCl2(PPh3)2(pbimz)]Cl (1), while red [ReCl4(pbimz)] Æ OPPh3 (2) was obtained from [ReCl3(PhC(O)C(O)Ph)(PPh3)]
and pbimz in the presence of perchlorate. The compounds were characterized by elemental analysis, FAB-MS, UV–Vis, IR,
NMR spectroscopy and magnetic susceptibility measurements.
Ó 2005 Elsevier B.V. All rights reserved.
Keywords: Rhenium; 2(20-pyridyl)benzimidazole; Crystal structure; Cyclic voltammetry; Paramagnetism
1. Introduction
benzimidazol-2-yl)pyridine), was prepared and was
shown to be able to function as a bright orange–red
emitter and an electron transport material in an elec-
troluminescent device [8]. 2(20-pyridyl)benzimidazole
can be regarded as another close analog of 2,20-bipyri-
dine where one pyridine ring is substituted by an imid-
azole. In the present work, we report the preparation
of the first Re complexes with this ligand.
Transition metal complexes with polypyridyl ligands
have received much attention for their potential appli-
cations to the fields of photoinitiated energy transfer
and redox catalysis [1–3]. Re(I) complexes with 2,20-
bipyridine [Re(bpy)(CO)3X] are of considerable interest
for their potential to serve as catalysts for CO2 reduc-
tion [4]. Even in the absence of strong p-acids such as
CO and RNC, the polypyridyl ligands such as 2,20-
bipyridine, 2,20:60,200-terpyridine and tris(2-pyridylm-
ethyl)amine stabilize the lower oxidation states of Re
(Re(III), Re(II), Re(I)) [5]. Modification in the polypyr-
idyl ligands has been used to tune their spectroscopic,
photophysical, photochemical, and electrochemical
properties [6,7]. Recently, a complex with a substituted
pyridylbenzimidazole, [Re(CO)3ClL] (L = 2-(1-ethyl-
2. Experimental
2.1. General
All reactions were performed in air in reagent-grade
solvents. The chemicals were used as purchased. Infra-
red spectra (4000–400 cmꢀ1) were recorded on a IFS-
85 Fourier spectrometer (Bruker) in KBr pellets.
NMR spectra were obtained on a Bruker AC 250 spec-
trometer with TMS as external standard. Electronic
*
Corresponding author.
E-mail address: caesar@che.nsk.su (M.N. Sokolov).
0020-1693/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2005.06.019

M.N. Sokolov et al. / Inorganica Chimica Acta 358 (2005) 3914–3918
3915
spectra were recorded on a Shimadzu UV 2101 PC spec-
trometer. Mass spectra were obtained on a MS 8230
(Finnigan) spectrometer. Cyclic voltammetry was done
using a Potentiostat/Galvanostat Model 263 (EG&G
INSTRUMENTS) with a Ag/AgCl reference electrode
and 0.1 M Bu4NClO4 as supporting electrolyte. In the
conditions employed the potential of Fc+/Fc couple
was 0.44 V. Magnetic susceptibility was measured on a
SQUID-magnetometer MPMS-5S (Quantum Design).
The diamagnetic Pascal corrections were applied. Ele-
mental analyses were done in the Laboratory of Micro-
analysis, Novosibirsk Institute of Organic Chemistry,
Russia.
X-ray analysis. Yield: 0.02 g (20%). FAB-MS; m/e: 765
(5%, [ReCl3(pbimz)(OPPh3)]+), 429 (37%, [ReOCl-
(pbimz)]+), 277 (75%, OPPh3þ), 195 (43%, pbimz+).
2.3. Crystal structure determinations
The crystal structures of 1 and 2 were determined by
X-ray diffraction. The diffraction data (Table 1) were
collected at 203 K on a Stoe IPDS2 Image Plate diffrac-
Table 1
Crystallographic data and structural refinements for 1 and 2
1
2
The starting complexes [ReOCl2(OEt)(PPh3)2] and
[ReCl3(PhC(O)C(O)Ph)(PPh3)] were prepared according
to the literature [9,10].
Formula
C51.70H42.70Cl14.10N3P2Re C30H24Cl4N3OPRe
Formula weight
Temperature (K)
Space group
1453.97
203(2)
801.49
203(2)
P21/n
16.032(3)
8.8590(18)
21.559(4)
90
98.23(3)
90
3030.4(11)
4
ꢀ
P1
2.1.1. Preparation of cis(Cl),trans(P)-[ReCl2(PPh3)2-
(pbimz)]Cl (1)
˚
a (A)
12.685(3)
15.395(3)
16.587(3)
100.38(3)
93.81(3)
104.68(3)
3060.7(11)
2
˚
b (A)
[ReOCl2(OEt)(PPh3)2] (0.200 g, 0.24 mmol) and 2(20-
pyridyl)benzimidazole (0.093 g, 0.48 mmol) were re-
fluxed in CHCl3 for 3 h. The hot solution was filtered
from a dark-blue precipitate (complex A), the brown fil-
trate evaporated to dryness and redissolved in a small
volume of chloroform. Ether vapor diffusion into this
solution gave red crystals of chloroform solvate
1 Æ 3.7CHCl3 (1a), which, however, loses the solvent eas-
ily. Yield: 0.040 g (20%). Calc. for C48H39Cl3N3P2Re: C,
56.95; H, 3.88; N, 4.15; Cl, 10.51. Found: C, 56.12; H,
3.68; N, 4.18; Cl, 10.22% (unsolvated complex). IR
(cmꢀ1): 3374 m, 3053 m, 1608 w, 1481 m, 1455 m,
1434 s, 1333 w, 1159 w, 1092 m, 998 w, 742 m, 695 s,
519 s, 494 m. Electronic spectrum (kmax, nm; e,
˚
c (A)
a (°)
b (°)
c (°)
3
˚
V (A )
Z
Dcalc (g/cm3)
1.578
2.690
1.757
4.445
l (mmꢀ1
)
Reflections
24455/11099
10052/5254
collected/unique
Goodness-of-fit
R [I > 2r(I)]
1.056
1.738
R1 = 0.0563;
wR2 = 0.1462
R1 = 0.0630;
wR2 = 0.1552
R1 = 0.0945;
wR2 = 0.2800
R1 = 0.1340;
wR2 = 0.2910
R (all data)
M
ꢀ1 cmꢀ1) in CHCl3: 490 (2958), 560 (sh, 2158); in
Table 2
˚
Main bond lengths (A) and angles (°) for complex 1
CH3CN: 488 (2940), 550 (1880); in CH3OH/CH3CN
(95:5 v/v): 322 (1970), 339 (1670), 473 (4100), 550 sh.
FAB-MS; m/e: 976 ([ReCl2(PPh3)2(pbimz)]+, 40%), 714
([ReCl2(PPh3)(pbimz)]+, 70%), 452 ([ReCl2(pbimz)]+,
65%). leff (B.M.): 0.87 (80 K); 0.95 (100 K); 1.09
(140 K); 1.22 (180 K); 1.32 (220 K); 1.44 (260 K); 1.48
(300 K).
Bond distance
Re(1)–P(1)
Re(1)–P(2)
Re(1)–Cl(1)
Re(1)–Cl(2)
Re(1)–N(1)
Re(1)–N(2)
2.4864(16)
2.4934(16)
2.3704(15)
2.3686(15)
2.111(5)
2.141(4)
Recrystallization of 1 from 0.1 M Bu4NClO4 in
methanol (ꢀ5 °C) gives red needles of trans(Cl),
trans(P)-[ReCl2(PPh3)2(pbimz)]ClO4 (1b). Calc. for
C48H39Cl3N3O4P2Re: C, 53.56; H, 3.66; N, 3.90. Found:
C, 53.58; H, 3.62; N, 3.91%.
Bond angle
P(1)–Re(1)–P(2)
Cl(1)–Re(1)–P(1)
Cl(1)–Re(1)–P(2)
Cl(2)–Re(1)–P(1)
Cl(2)–Re(1)–P(2)
Cl(2)–Re(1)–Cl(1)
N(1)–Re(1)–P(1)
N(1)–Re(1)–P(2)
N(1)–Re(1)–Cl(1)
N(1)–Re(1)–Cl(2)
N(1)–Re(1)–N(2)
N(2)–Re(1)–P(1)
N(2)–Re(1)–P(2)
N(2)–Re(1)–Cl(1)
N(2)–Re(1)–Cl(2)
C(13)–P(1)–Re(1)
174.06(4)
90.74(6)
93.07(6)
87.43(6)
87.64(6)
96.07(5)
2.2. Preparation of [ReCl4(pbimz)] Æ OPPh3 (2)
88.17(14)
88.95(14)
168.94(12)
94.88(13)
76.83(18)
92.77(14)
91.63(14)
92.23(13)
171.69(13)
108.63(18)
A benzene solution of [ReCl3(PhC(O)C(O)Ph)(PPh3)]
(0.100 g, 0.13 mmol) was refluxed with 2(20-pyridyl)-
benzimidazole (0.076 g, 0.39 mmol) for 1 h. After cool-
ing down, the solution was filtered and the filtrate was
taken to dryness, dissolved in CH3OH, followed by
addition of 0.10 g of NaClO4. Slow evaporation of the
solution gave red crystals of 2, which were suitable for

3916
M.N. Sokolov et al. / Inorganica Chimica Acta 358 (2005) 3914–3918
tometer (compound 1) and a Stoe IPDS1 Image Plate
diffractometer (compound 2) with Mo Ka radiation
(k = 0.71073 A). Both structures were solved by direct
[(O)Cl4Re–O–Re(O)(pbimz)2]. Unfortunately, low solu-
bility of this product in common organic solvents pre-
cluded its purification and further characterization.
The second product is the complex 1 and it is remark-
able that its formation is accompanied by reduction of
Re(V) into Re(III) even though the synthesis is done
in air. Both PPh3 and EtOꢀ ligands can act as reducing
agents here, though disproportionation into Re(III) and
Re(VII) (similar to that taking place in the synthesis of
[Re(bipy)3](ReO4)2 from K2ReF6 and bipy [12]) cannot
be excluded. This shows that pbimz, similar to its dii-
mine analogs (bipy and phen), stabilizes Re in lower oxi-
dation states. Similarly, the complexes with biimidazole
[ReX2(PPh3)2(biimH2)]+ are obtained from [ReOX2
(OEt)(PPh3)2] or [ReOX3(PPh3)2] and the ligand [14].
Closely related bipy complex [ReCl2(PPhMe2)2(bipy)]+
is obtained either from [Re(bipy)Cl4] and excess PPhMe2
(with the reduction of Re(IV) to Re(III)), or from [Re
(PPhMe2)3Cl3] and bipy [13]. Our attempts to prepare
[Re(pbimz)2Cl2]+ or [Re(pbimz)3]2+ from [Re(ben-
zil)(PPh3)Cl3] and the ligand, by analogy with the prep-
aration of corresponding bipy and phen complexes [5],
did not give a tractable product, but unexpectedly, after
adding perchlorate to the reaction mixture, a neutral
Re(IV) complex 2 was obtained in a low yield. The per-
chlorate can be the oxidant in this reaction. As can be
seen, concomitant oxidation of PPh3 into OPPh3 took
place, the latter forming a hydrogen-bonded 1:1 adduct
with the [ReCl4(pbimz)] molecule. The related bipy com-
plexes, [ReCl4(bipy)] and [ReBr4(bipy)], were obtained
by thermolysis of (bipyH2)[ReCl6] [13] and by reaction
of mer-[ReOBr3(bipy)] with PPh3 in bromoform [16],
respectively.
˚
methods and refined by full-matrix least-squares method
against |F|2 in anisotropic approximation using SHELXTL
programs set [11]. Hydrogen atom positions were refined
with geometry constraints. Absorption corrections for 1
and 2 were applied empirically. In 1 site occupancy fac-
tors for solvate CHCl3 molecules were refined to give a
0.7 occupancy factor for one of CHCl3 molecules, result-
ing in 1 Æ 3.7CHCl3 composition. Some bond lengths and
bond angles for 1 and 2 are listed in Tables 2 and 3.
Crystallographic data for 1 and 2 have been deposited
to Cambridge Structural Database as CCDC Nos.
267812 and 267813, respectively.
3. Results and discussion
3.1. Synthesis
The complex 1 is formed by reaction of [ReO-
Cl2(OEt)(PPh3)2] with pbimz in a rather moderate yield.
The main product of the reaction is a deep-blue
precipitate (A) whose elemental analysis corresponds
equally satisfactorily to its formulation, both as [ReO-
Cl2(OEt)(pbimz)] and [Re2O3Cl4(pbimz)2]. However, in
its FAB-MS the heaviest peak corresponds to [ReOCl(p-
bimz)2]+ which could be explained either by non-homo-
geneity of the sample, or by a rearrangement upon
ionization. It can even reconcile with the binuclear for-
mulation by ascribing an unsymmetrical structure of
Table 3
3.2. Crystal and molecular structures
˚
Main bond lengths (A) and angles (°) for complex 2
Bond distance
The view of the complex cation of 1, cis(Cl),trans(P)-
[ReCl2(PPh3)2(pbimz)]+ is shown in Fig. 1, and Table 2
lists main geometric parameters of the complex. Re has
Re(1)–N(2)
Re(1)–N(1)
Re(1)–Cl(4)
Re(1)–Cl(1)
Re(1)–Cl(3)
Re(1)–Cl(2)
2.141(15)
2.168(13)
2.311(5)
2.331(4)
2.335(5)
2.366(4)
Bond angle
N(2)–Re(1)–N(1)
N(2)–Re(1)–Cl(4)
N(1)–Re(1)–Cl(4)
N(2)–Re(1)–Cl(1)
N(1)–Re(1)–Cl(1)
Cl(4)–Re(1)–Cl(1)
N(2)–Re(1)–Cl(3)
N(1)–Re(1)–Cl(3)
Cl(4)–Re(1)–Cl(3)
Cl(1)–Re(1)–Cl(3)
N(2)–Re(1)–Cl(2)
N(1)–Re(1)–Cl(2)
Cl(4)–Re(1)–Cl(2)
Cl(1)–Re(1)–Cl(2)
Cl(3)–Re(1)–Cl(2)
77.4(5)
96.2(4)
173.3(4)
86.7(4)
88.4(4)
93.3(2)
169.7(4)
92.4(4)
94.0(2)
91.50(18)
90.1(4)
85.6(4)
92.43(18)
173.73(19)
90.68(18)
Fig. 1. The complex cation of 1. A.d.p. ellipsoids of 50% probability
level. Hydrogen atoms are omitted for clarity.

M.N. Sokolov et al. / Inorganica Chimica Acta 358 (2005) 3914–3918
3917
a distorted octahedral environment. The two Cl atoms,
Re and the whole pbimz ligand are in the same plane
˚
(Re–Cl 2.3686(15)–2.3704(15) A; Re–N 2.111(5)–
˚
2.141(4) A). The PPh3 ligands are trans to each other
and take axial position with respect to the above-defined
˚
plane (Re–P 2.4864(16)–2.4934(15) A). The same ligand
arrangement is found in [ReCl2(PPhXMe3 ꢀ x)2(bipy)]+,
[ReCl2(PPh3)2(biimH2)]+ (biimH2 is 2,20-biimidazole)
and [TcCl2(PR3)2(L)]0/+ (L = bipy, phen) [13–15].
The crystal packing employs hydrogen bonds be-
tween the outer-sphere Clꢀ and the imidazolic N–H
˚
group (Clꢁꢀꢁ ꢁH 2.202 A), as well as much weaker con-
tacts of Cl with C30 and C40 atoms of the pyridine ring
˚
Fig. 3. The molecule of 2. A.d.p. ellipsoids of 50% probability level.
Hydrogen atoms are omitted for clarity.
(Clꢁ ꢁ ꢁH 2.692–2.788 A). In addition, weak contacts be-
tween Cl, coordinated to Re, and hydrogen atom of
˚
the nearest Ph group (Clꢁ ꢁ ꢁH 2.789 A) assist the forma-
tion of weakly bound centrosymmetric dimers along the
c-axis direction (Fig. 2).
In the structure of 2 (Fig. 3, Table 3), a distorted
octahedral Re environment is formed by four chloride
˚
ligands (Re–Cl 2.311(4)–2.366(4) A) and two nitrogen
atoms of the heterocyclic ligand (Re–N 2.141(15)–
˚
2.168(13) A). It is tempting to ascribe the longer Re–
Cl(3) bond to the trans-effect of the imidazolic nitrogen,
but the variation of Re–Cl bond lengths of the same
magnitude is observed on the Cl–Re–Cl axis, and may
be imposed by packing forces. Each pair of complex
molecules is in a slipped p–p stacking by their benzimi-
dazolic rings. The distance between the planes of their
3.54 Å
1.77 Å
˚
aromatic systems is 3.54 A. In 2, the coordinated chlo-
ride ligands cannot prevent stacking in contrast to 1,
where the benzimidazolic rings are kept well apart from
each other by bulky PPh3 ligands, and molecular associ-
ation proceeds via weak Clꢁ ꢁ ꢁH–C contacts. Co-crystal-
lized OPPh3 molecule forms a strong hydrogen bond to
˚
N(3) atom of the benzimidazolic ring (Oꢁ ꢁ ꢁH 1.77 A,
Fig. 4). The complex and OPPh3 molecules are accom-
Fig. 4. Fragment of crystal packing of 2. Hydrogen bonds N–Hꢁ ꢁ ꢁO
are shown by dashed lines. Benzimidazolic rings of Re(IV) complexes
are in slipped p–p stacking.
modated in their respective alternating layers along the
a-axis.
3.3. Spectroscopy, cyclic voltammetry and magnetic
properties
Complex 1 in organic solvents gives characteristic
red–orange solutions with well-defined absorption max-
ima, very similar to the [ReCl2(PPhxMe3 ꢀ x)2(bipy)]+
(x = 1–3) [13]. Compared to the electronic spectrum of
[ReCl2(PPh3)2(biimH2)]Cl [14], these peaks are red-
shifted. Given the similarity between all these diimine
a
b
Fig. 2. Packing of centrosymmetric dimers of 1, viewed along the c-
axis. Hydrogen atoms as well as solvate CHCl3 molecules are omitted
for clarity.

3918
M.N. Sokolov et al. / Inorganica Chimica Acta 358 (2005) 3914–3918
ligands, the origin of the visible absorption is no doubt
in dp(Re) ! p(L) charge-transfer transitions. On chang-
ing solvent, a solvatochromism is observed. The solu-
tions in acetonitrile are red, but addition of methanol
gradually changes the color to orange. Up to the 1:1
(v/v) CH3CN/CH3OH ratio the changes are negligible,
but with the increasing concentration of methanol the
peak at 550 nm becomes a shoulder, and the other peak
at 488 nm undergoes an hypsochromic shift by 15 nm.
The solvatochromism can be explained by the formation
of hydrogen bonds between the N–H bond of the imi-
dazolic ring and solvent molecules, in the expected order
CHCl3 < CH3CN < CH3OH. In the solid, the pbimz
ring forms a hydrogen bond with Clꢀ. It is possible that
the N–Hꢁ ꢁ ꢁCl bonded ionic pairs persist in CHCl3 solu-
tion, while in more polar CH3CN and especially in
CH3OH, they dissociate and Clꢀ is replaced by a solvent
molecule. The ionic pairing was detected in solutions of
[ReCl2(PPh3)2(biimH2)]Cl and [ReCl2(PPh3)2(biimH2)]-
(RCO2) [14]. The complex is paramagnetic, showing a
temperature-dependent paramagnetism, in the range of
0.87 B.M. (80 K)–1.48 B.M. (300 K). These data are in
agreement with the presence of high-spin d4-Re(III) cen-
ters. Typical values for octahedral Re(III) complexes are
1.3–2.1 B.M. For example, the complexes with biimidaz-
ole [ReX2(PPh3)2(biimH2)]X have the room-temperature
magnetic moments 1.30 (X = Cl), 1.64 (X = Br) and
1.59 (X = I) B.M. [14]. This paramagnetism makes
impossible observation of the signal in 31P NMR spec-
tra, but PMR spectrum was obtained, displaying large
paramagnetic shifts between ꢀ0.4 and 15.8 ppm. The
protons in o-, m- and p-position of the phenyl ring in
PPh3 are observed at 8.3, 13.1 and 8.5 ppm, respectively.
Similar paramagnetic shifts are observed in the PMR
spectra of the biimH2 and bipy complexes [13,14]. The
observation of only one set of phenylic peaks indicates
that no isomerization occurs in the solution. Cyclic vol-
tammetry of 1 in CHCl3 (with Bu4NPF6 as supporting
electrolyte) shows a quasi-reversible one electron reduc-
tion (E1/2 = 920 mV, DE = 134 mV). It is well known
that pyridine-containing chelates stabilize Re(II) and
Tc(II) oxidation states. The cyclic voltammetry of [Re-
Cl2(PPhMe2 ꢀ x)2(bipy)]+ shows reversible one-electron
reduction at a very close potential [13]. The Tc(II) com-
plexes, cis(Cl),trans(P)-[TcCl2(PPh3)2(L–L)] are easily
obtained by reduction of the corresponding monoca-
tions. This reduction wave was not observed with Bu4N-
ClO4 as supporting electrolyte. This may be explained
by electrocatalysis: the Re(II) complex, once formed, re-
duces ClO4ꢀ. Electrocatalytic reduction of perchlorate is
of interest, since it has become one of the industrial pol-
lutants. Other Re complexes are efficient in this process
[17].
couples. Alternatively, the more anodic peak may corre-
spond to one-electron reduction of [ReCl3(OPPh3)-
(pbimz)]ꢀ, which could form in the solution by ligand
replacement. In any case, 2 is much more difficult to
reduce than [ReCl4(bipy)] which shows two reversible
one-electron reductions with E1/2 = ꢀ40 and ꢀ1260 V
(in DMF-CH3CN, versus SCE) [13].
In the FAB-MS of 1, the highest m/z peak is due to
the [ReCl2(PPh3)2(pbimz)]+ parent ion. The next two
fragments correspond to the parent ion having lost
one or two phosphines, phosphine (M ꢀ PPh3)+ and
(M ꢀ 2PPh3)+, which suggests that the coordinated
phosphines dissociate more easily than the halides. In
the case of 2, it is interesting to note that under the
FAB-MS conditions there is an interaction between
the two molecular components of the compound, giving
rise to the peaks of [ReCl3(pbimz)(OPPh3)]+ and
[ReOCl(pbimz)]+ instead of the expected molecular
peak.
Acknowledgments
A grant of Russian Science Support Foundation to
MNS is gratefully acknowledged. The work was sup-
ported by Russian Foundation for Basic Research
(Grant 04-03-32159). The authors thank Dr. V.N. Ikor-
ski for the magnetic measurements.
References
[1] J. Ferguson, F. Herren, E.R. Krauz, M. Maeder, J. Vranacic,
Coord. Chem. Rev. 64 (1985) 21.
[2] A. Juris, V. Balzani, F. Barigelletti, S. Campagna, P. Belser, A.
von Zelewsky, Coord. Chem. Rev. 84 (1988) 85.
[3] J.-P. Sauvage, J.-P. Collin, J.-C. Chambron, S. Guillerez, C.
Coudret, Chem. Rev. 94 (1994) 993.
[4] K.S. Schanze, D.B. MacQueen, T.A. Perkins, L.A. Cabana,
Coord. Chem. Rev. 122 (1993) 63.
[5] L.E. Helberg, S.D. Orth, M. Sabat, W.D. Harman, Inorg. Chem.
35 (1996) 5584.
[6] K.K.-W. Lo, W.-K. Hui, D.Ch.-M. Ng, K.-K. Cheung, Inorg.
Chem. 41 (2002) 40.
[7] K.A. Waters, Y.-J. Kim, J.T. Hupp, Inorg. Chem. 41 (2002) 2909.
[8] K. Wang, L. Huang, L. Gao, L. Jin, Ch. Huang, Inorg. Chem. 41
(2002) 3353.
[9] N.P. Johnson, C.J.L. Lock, G. Wilkinson, J. Chem. Soc. A (1964)
1054.
[10] G. Roushias, G. Wilkinson, J. Chem. Soc. A (1967) 993.
[11] G.M. Sheldrick, SHELX-97 Release 97-2, University of Goettingen,
Germany, 1998.
[12] M. Stebler, A. Gutierrez, A. Ludi, H.B. Buergi, Inorg. Chem. 26
(1987) 1449.
[13] J. Caspar, B.P. Sullivan, T.J. Meyer, Inorg. Chem. 23 (1984)
2104.
[14] S. Fortin, A.L. Beauchamp, Inorg. Chem. 40 (2001) 105.
[15] B.E. Wilcox, D.M. Ho, E. Deutsch, Inorg. Chem. 28 (1989) 3917.
[16] B. Machura, J.O. Dzie˛gielewski, R. Kruszynksi, T.J. Bartczak, J.
Kusz, Polyhedron 22 (2003) 2573.
[17] M.M. Abu-Omar, L.D. McPherson, J. Arias, V.M. Bereau,
Angew. Chem. Int. Ed. 39 (2000) 4310.
Complex 2 in chloroform shows two consecutive qua-
si-reversible reductions at ꢀ750 and ꢀ1200 mV, which
can be attributed to Re(IV)/Re(III) and Re(III)/Re(II)