Electron localisation in electrochemically reduced mono- and bi-nuclear rhenium(I) complexes with bridged polypyridyl ligands

Article abstract of DOI:10.1039/b110730p

A number of mono- and bi-nuclear rhenium(I) complexes have been prepared and their physical properties, including the infrared spectra of the reduced complexes, have been studied. These compounds have the general formula [Re(CO)₃Cl(L)] and [Cl(CO)₃Re(μ-L)Re(CO)₃Cl], where L can be 2,3-(2′,2″)-diquinolylquinoxaline, 6,7-dimethyl-2,3-(2′,2″)-diquinolylquinoxaline, 2,3-(2′,2″)-diquinolylbenzoquinoxaline, 6,7-dichloro-2,3-(2′,2″)-diquinolylquinoxaline, 2,3-(2′,2″)-diquinoxalylquinoxaline, 6,7-dimethyl-2,3-(2′,2″)-diquinoxalylquinoxaline, 2,3-(2′,2″)-diquinoxalylbenzoquinoxaline and 6,7-dichloro-2,3-(2′,2″)-diquinoxalylquinoxaline. The electrochemical studies show that the first reduction potential of the free ligands correlates with the reductions of the corresponding mono- and bi-nuclear complexes. The properties of the complexes have been modelled using semi-empirical methods. These show linear correlations between: (a) the energy of the MLCT transitions versus the difference in energy between the LUMO and the HOMO and (b) the change in carbonyl force constant with reduction vs. the wavefunction amplitude of the π* LUMO at the site of coordination. The experimental data and calculations point to significant alterations in the π* LUMO with substitution at the ligand and with the chelation of a second Re(I) center.

Full text of DOI:10.1039/b110730p

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Electron localisation in electrochemically reduced mono- and
bi-nuclear rhenium(I) complexes with bridged polypyridyl ligands†
Simon E. Page, Amar Flood and Keith C. Gordon*
Department of Chemistry, Union Place, University of Otago, Dunedin, New Zealand.
E-mail: kgordon@alkali.otago.ac.nz
Received 22nd November 2001, Accepted 29th January 2002
First published as an Advance Article on the web 19th February 2002
A number of mono- and bi-nuclear rhenium() complexes have been prepared and their physical properties,
including the infrared spectra of the reduced complexes, have been studied. These compounds have the general
formula [Re(CO) Cl(L)] and [Cl(CO) Re(µ-L)Re(CO) Cl], where L can be 2,3-(2Ј,2Љ)-diquinolylquinoxaline,
3
3
3
6
,7-dimethyl-2,3-(2Ј,2Љ)-diquinolylquinoxaline, 2,3-(2Ј,2Љ)-diquinolylbenzoquinoxaline, 6,7-dichloro-2,3-(2Ј,2Љ)-
diquinolylquinoxaline, 2,3-(2Ј,2Љ)-diquinoxalylquinoxaline, 6,7-dimethyl-2,3-(2Ј,2Љ)-diquinoxalylquinoxaline,
,3-(2Ј,2Љ)-diquinoxalylbenzoquinoxaline and 6,7-dichloro-2,3-(2Ј,2Љ)-diquinoxalylquinoxaline. The electrochemical
2
studies show that the first reduction potential of the free ligands correlates with the reductions of the corresponding
mono- and bi-nuclear complexes. The properties of the complexes have been modelled using semi-empirical methods.
These show linear correlations between: (a) the energy of the MLCT transitions versus the difference in energy
between the LUMO and the HOMO and (b) the change in carbonyl force constant with reduction vs. the
wavefunction amplitude of the π* LUMO at the site of coordination. The experimental data and calculations
point to significant alterations in the π* LUMO with substitution at the ligand and with the chelation of
a second Re() center.
Introduction
Metal polypyridyl complexes have been widely studied, in part,
1
because of their potential utility in photocatalysis, including
2
3
CO2 remediation, solar energy systems5 and molecular
4
electronics, including light-emitting diodes. In almost all of
these complexes the key excited state is MLCT in nature.
1–3
One way to model part of the excited state is to electro-
chemically reduce the complex and establish the nature of the
6
–11
MO populated by the reducing electron,
the so-called redox
12
MO. The properties of metal complexes, with respect to
MLCT photo-excitation and in electron transfer schemes will
be mitigated and controlled by the nature of the redox MO.
Fig. 1 Ligands used in this study, with numbering scheme for NMR
data. Ligands are labelled: 1 (R = H, X = CH); 2 (R = CH , X = CH);
(R = benzo, X = CH); 4 (R = Cl, X = CH); 5 (R = H, X = N); 6
R = CH , X = N); 7 (R = benzo, X = N); 8 (R = Cl, X = N).
13
3
3
(
We have studied a series of mono- and bi-nuclear rhenium()
complexes with a variety of bridging ligands in an attempt to
understand how coordination of a 2Њ metal and substitution at
the ligand alter the nature of the redox MO. Complexes with
3
{
Re(CO) Cl} moieties tend to interact strongly with bridging
3
ligands and this can lead to unexpected polarisation of the
14
redox MO.
The ligands used, depicted in Fig. 1, may be considered as
having arms (quinoline or quinoxaline, labelled 1Ј–8Ј and 1Љ–8Љ
in Fig. 1) and a bridge (quinoxaline, labelled 1–8, Fig. 1). The
bridging ligands fall into two classes, diquinoline–quinoxaline
Fig. 2 Calculated structures for Re1 and Re 1. For clarity only the
2
bonding rings are shown and H atoms have been removed. The internal
(
dqq, 1–4) and diquinoxaline–quinoxaline (dqxq, 5–8).
coordinates for CO vibrations are also shown (r through r ) with the
1
6
We have utilised simple semi-empirical calculations in order
to assist data interpretation.
numbering scheme for the bonding N atoms.
show that the mononuclear complexes with quinoline and
quinoxaline armed ligands have the unbound arm almost
perpendicular to the bound ring systems. The bound ring
systems are distorted, bowing in an arc and displaying a 26Њ
dihedral angle with respect to the Re–N linkage and the
bridging C–C bond (Fig. 1, C2, C2Ј) of the ligand. No crystallo-
graphic data are available for the complexes reported herein
Results and discussion
The structure of each of the complexes was optimised
using semi-empirical calculations with PM3 parameterization.
Representative examples are shown in Fig. 2. The calculations
but a structure is available for the related [Re(CO) (dpq)Cl]
complex. This also reveals the bowing of the bound ligand
rings to the Re–N bond (ca. 23Њ). A PM3 calculation of
3
†
Electronic supplementary information (ESI) available: force constants
15
of the mono- and bi-nuclear [Re(CO) Cl] complexes of 1–8 in CH Cl
solution. See http://www.rsc.org/suppdata/dt/b1/b110730p/
3
2
2
1
180
J. Chem. Soc., Dalton Trans., 2002, 1180–1187
This journal is © The Royal Society of Chemistry 2002
DOI: 10.1039/b110730p

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Table 1 Electrochemical reduction potentials for 1–8 and their
Re(CO) Cl} complexes in CH Cl (1 mM where possible) with 0.1 M
[
Re(CO) (dpq)Cl] overestimates the angle between the unbound
3
{
pyridine ring and the bound quinoxaline; it is probable this
overestimation also occurs in the dqq and dqxq complexes.
The calculated structure of the binuclear complexes are
similar. Re 1 (Fig. 2) shows a twisting of the bridging ligand
with a dihedral angle of 42Њ between the two arms of the ligand
3
2
2
TBAP as supporting electrolyte. A Ag/AgCl reference electrode was
used with ferrocene (ϩ0.4 V vs. NHE) added as an internal standard.
0
E Ј was measured as the average position between the cathodic and
2
anodic peak; this was estimated in the cases with limited reversibility
0
(
Fig.1, C2Ј, C2, C3, C2Љ). The bound rings at each chelating
E Ј/V (vs. NHE)
site show a bowed structure similar to that observed in the
mononuclear complex.
a
1
2
3
4
5
6
7
8
Ϫ1.54b
Ϫ1.64
Ϫ1.29
Ϫ1.37
Ϫ1.35
Ϫ1.42
Ϫ1.14
Ϫ1.20
Ϫ0.73
Ϫ0.82
Ϫ0.59
Ϫ0.59
Ϫ0.36
Ϫ0.46
Ϫ0.24
Ϫ0.21
Ϫ0.50
Ϫ0.57
Ϫ0.40
Ϫ0.37
Ϫ0.14
Ϫ0.23
Ϫ0.09
Ϫ0.07
a
The binuclear complexes produced have the ability to form as
isomers, with the Cl groups cis or trans to one another. This has
a
16
been observed and commented on by Kaim et al. and Lehn
17
et al. in previous studies. These studies suggest that there is
little difference in terms of electronic properties between the
2ϩ
two isomers. This is in contrast to recent work on {Ru(bpy)2}
binuclear complexes which can show significant differences
Re1
Re2
Re3
Re4
Re5
Re6
Re7
Re8
Re21
Re22
Re23
18
with diastereoisomers. In this work a substantial part of the
isomer effect is attributed to ion-pairing which would not occur
with the neutral complexes described herein.
The PM3 calculations provide a vibrational output and this
may be used to give some insight into which of the binuclear
complexes’ isomers are present. The PM3 predictions for ν(CO)
are generally between 1–3% too high but the observed relative
pattern, one high wavenumber band and two lower wave-
number bands, is predicted for the mononuclear complexes. The
lack of absolute accuracy in the frequencies calculated is
unsurprising in view of the fact that semi-empirical calculations
use restricted basis sets and neglect atomic orbital overlap
c
Ϫ1.25
Ϫ1.04b
Ϫ1.11b
Ϫ0.71
Re 4
Re25
2
b
Re26
Re27
Re28
Ϫ0.74b
Ϫ0.70b
Ϫ0.72
19
between adjacent nuclei. The parameters used for PM3 calcu-
lations have been optimised to reproduce reliable equilibrium
a
b
Previously published results. See ref. 18. Indicates quasi-reversible
c
electrode behaviour. Indicates irreversible electrode behaviour.
20
geometries, but recently PM3 calculations have been utilised
to provide insight into vibrational and electronic spectral prop-
21
erties. In the case of the binuclear complexes structures may
be optimised in cis or trans configurations, with respect to Cl
atoms. The IR frequencies predicted for each isomer have
LUMO on going from dqq to dqxq type ligands and mono- to
bi-nuclear Re complexes changes. Calculations on the com-
plexes can provide MO plots that can be related to these
findings. The behaviour of each LUMO may be parameterised
by cN , this is the square of the wavefunction at each of the
chelating N-atom sites. These parameters are presented in
Table 2.
Ϫ1
the following features; for the trans-Cl isomer a 10 cm
2
splitting is predicted for the high wavenumber mode, the lower
wavenumber modes also show splitting but of only a few wave-
numbers. All of the IR data are given in Table 4 (see later). The
25
22
Ϫ1
2
cis-isomer also shows a 10 cm splitting of the high wave-
The cN parameters show that the mononuclear dqq-type
number mode, but additionally one of the lower wavenumber
modes is predicted to split by 12 cm . The experimental data
on these systems does not show cis-isomer behaviour and this
suggests the bulk of the samples studied herein are in a trans
configuration.
complexes, Re1–Re4 have asymmetric LUMOs in which the
Ϫ1
greater proportion of the MO resides at the quinoxaline ring.
2
On arm substitution to the dqxq complexes the value of cN for
each of the chelating N atoms is closer (i.e. the LUMO is less
asymmetric). In both cases the unbound ring shows no π* MO
wavefunction amplitude. A more dramatic effect is evident for
2
the cN of the binuclear complexes. These show large amplitude
Molecular orbital energies
at the bridged ring system but almost none on the arms of the
ligands.
The electronic absorption and electrochemical data provide
experimental probes of the energetics of the dπ and ligand π*
MOs. The electrochemical data (Table 1) show that the substi-
tution of an electron-withdrawing group, such as chloro- or
benzo-, makes the ligand easier to reduce by ca. 100 mV.
Furthermore, the substitution of quinoline arms for quin-
oxaline also results in easier reduction, i.e. the π* acceptor
redox MO is stabilised, by ca. 100–200 mV. Arm substitution
The UV–Vis data shows absorptions due to ligand-based
π–π* transitions and MLCT transitions (Table 3). MLCT
transitions correlate with the energetics of the oxidation of
0
the metal and reduction of the ligand. A plot of νMLCT vs. E Ј
(1st reduction), for complexes with a homologous series of
26,27
0
ligands, should appear linear.
The νMLCT vs. E Ј (1st reduc-
tion) parameters for the complexes may be fitted by the linear
0
0
2
affects the E Ј for reduction, this suggests that the three-ring
relationship, νMLCT = 0.7625E Ј(1st reduction) ϩ 2.1539 (R =
0.97) for the mononuclear complexes, and by ν
systems of the ligands have some level of communication and
are not orthogonal to each other. Note, the three-ring systems
cannot be co-planar due to steric interactions between the
=
MLCT
0
2
0.5685E Ј(1st reduction) ϩ 1.812 (R = 0.95) for the binuclear
complexes. The linearity of these plots suggest the lowest
MLCT transition is occupying the lowest π* MO on the ligand
and that the Re dπ energies are reasonably invariant through
the series of complexes.
3
Ј and 3Љ protons. The crystal structure of a related copper
complex [(PPh ) Cu(µ-4)Cu(PPh ) ](BF ) has a torsion angle
3
2
3
2
4 2
23
between the bridge and the arms of ca. 30Њ.
The binding of {Re(CO) Cl} to the ligands to form the
The energy of the observed MLCT transition, EMLCT, obsd
(eV), may be related to a calculated parameter, namely
E(LUMO) Ϫ E(HOMO) (EMLCT, calc). The absolute values of
EMLCT, calc are much higher than the observed values, ca. 5 eV
and the absolute values are clearly inaccurate; however a plot of
3
respective mononuclear complexes significantly alters the
0
electrochemistry. The E Ј (1st reduction) is stabilised by 700–
0
8
00 mV (∆E Ј). This may be compared to the dpq ligand
24
which is stabilised by 800 mV on chelation to {Re(CO) Cl}.
3
Addition of a second {Re(CO) Cl} unit further stabilises the
E Ј by ca. 300 mV. These data suggest that the nature of the
EMLCT, obs vs. E
is linear with EMLCT, obs = 1.44EMLCT, calcd
3
MLCT, calcd
0
2
Ϫ 5.77 (R = 0.98), suggesting that the calculations are
J. Chem. Soc., Dalton Trans., 2002, 1180–1187 1181

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2
Table 2 Squared MO coefficients cN at the coordinating N centres (labelling from Fig. 1) for the LUMO, the predicted energies of HOMO and
LUMO and the observed MLCT transition energies
2
cN
Energy/eV
HOMO
2
Complex
4
1Ј
1Љ
1
ΣcN
LUMO
MLCT obsd.
Re1
Re2
Re3
Re4
Re5
Re6
Re7
Re8
Re21
Re22
Re23
Re24
Re25
Re26
Re27
Re28
0.140
0.136
0.125
0.130
0.166
0.168
0.153
0.156
0.032
0.032
0.034
0.030
0.035
0.035
0.042
0.033
0.197
0.199
0.207
0.199
0.172
0.168
0.186
0.177
0.217
0.216
0.197
0.215
0.215
0.213
0.191
0.212
0.337
0.335
0.332
0.329
0.338
0.336
0.339
0.333
0.499
0.497
0.463
0.490
0.500
0.497
0.467
0.491
Ϫ8.50
Ϫ8.46
Ϫ8.48
Ϫ8.54
Ϫ8.52
Ϫ8.48
Ϫ8.51
Ϫ8.56
Ϫ8.78
Ϫ8.74
Ϫ8.72
Ϫ8.79
Ϫ8.77
Ϫ8.73
Ϫ8.85
Ϫ8.78
Ϫ2.64
Ϫ2.58
Ϫ2.66
Ϫ2.75
Ϫ2.74
Ϫ2.68
Ϫ2.76
Ϫ2.84
Ϫ3.41
Ϫ3.32
Ϫ3.39
Ϫ3.48
Ϫ3.41
Ϫ3.32
Ϫ3.59
Ϫ3.48
2.71
2.77
2.67
2.58
2.51
2.60
2.46
2.44
2.02
2.07
1.93
1.95
1.89
1.95
1.86
1.85
0.217
0.216
0.198
0.215
0.215
0.213
0.191
0.212
0.032
0.032
0.034
0.030
0.035
0.035
0.042
0.033
Table 3 Electronic absorption bands of 1–8 and their {Re(CO) Cl} complexes measured in CH Cl at various concentrations
3
2
2
3
3
Ϫ1
Ϫ1
λ/nm (ε/10 dm mol cm
)
π–π*
π–π*
π–π*
π–π*
a
a
1
2
3
4
5
6
7
8
255 (89)
261 (87)
284 (87)
260 (88)
264 (62)
270 (56)
262 (74)
267 (69)
334 (27)
324 (26)
319 (73)
349 (24)
336 (36)
340 (36)
334 (72)
340 (33)
354 (sh) (25)
382 (20)
a
π–π*
π–π*
MLCT
MLCT
420 (17)
420 (25)
MLCT
Re1
Re2
Re3
Re4
Re5
Re6
Re7
Re8
273 (34)
277 (43)
293 (37)
275 (43)
279 (37)
266 (45)
272 (51)
285 (46)
320 (17)
325 (29)
328 (33)
325 (24)
332 (28)
283 (sh) (41)
348 (48)
393 (14)
391 (sh) (21)
352 (31)
458 (4.1)
448 (sh) (4.8)
465 (5.2)
480 (3.7)
494 (3.8)
477 (4.7)
505 (5.5)
508 (4.1)
405 (22)
402 (15)
410 (19)
403 (19)
390 (17)
334 (35)
402 (sh) (24)
420 (sh) (21)
424 (24)
335 (34)
π–π*
π–π*
MLCT
384 (sh) (23)
MLCT
422 (sh) (20)
416 (25)
MLCT
MLCT
Re21
Re22
Re23
Re24
Re25
Re26
Re27
Re28
244 (41)
247 (43)
264 (53)
257 (sh) (49)
256 (54)
266 (43)
261 (sh) (42)
256 (47)
288 (31)
292 (28)
360 (27)
366 (30)
368 (36)
355 (sh) (32)
384 (38)
387 (sh) (34)
392 (38)
383 (33)
511 (7.9)
476 (sh) (9.8)
523 (9.3)
532 (10)
510 (13)
489 (sh) (13)
530 (10)
615 (sh) (3.4)
598 (sh) (3.4)
641 (sh) (4.4)
637 (sh) (5.2)
657 (sh) (5.4)
636 (sh) (3)
666 (sh) (3.5)
672 (sh) (3.6)
305 (sh) (30)
293 (46)
285 (sh) (34)
303 (sh) (24)
311 (sh) (28)
299 (sh) (30)
410 (38)
364 (35)
406 (38)
414 (35)
438 (30)
430 (sh) (32)
431 (sh) (33)
433 (45)
408 (33)
428 (31)
451 (35)
522 (11)
a
Previously published results. See ref. 23.
reproducing the changes caused by ligand substitution and the
Re centres.
complexes the lowest energy band, ν , was in the range 1904–
A
Ϫ1
Ϫ1
1926 cm . The next band, ν , was in the range 1928–1953 cm
B
and the higher energy band, ν , was in the range 2022–2027
C
Ϫ1
cm . For the binuclear complexes, a shoulder is also observed
in the range 2030–2037 cm . These data are consistent with
other fac-{Re(CO) Cl} complexes.
Infrared spectroscopy
Ϫ1
29,30
The IR spectra of the parent complexes and their reduced
counterparts offer a direct probe of the extent of mixing
between the ligand π* MO and the dπ MOs of the metal. The
wavenumber of these bands are presented in Table 4. The shifts
3
For the mononuclear complexes, the three band pattern of
one high wavenumber mode and two lower wavenumber modes,
close together, are reproduced from the PM3 frequency calcu-
lation. It is possible to invoke scaling factors to better match
the predicted frequencies to experimental values, however, our
interest lies in a qualitative understanding of this spectral
region and how substitution alters this region. We therefore use
the unscaled computational outputs.
in the ν(C᎐O) bands for fac-{Re(CO) Cl} moieties are well
᎐
3
31
documented and the nature of the vibrational modes is quite
28
well understood.
Infrared absorption spectra in the carbonyl region (1850–
Ϫ1
2
100 cm ) for the {Re(CO) Cl} complexes of 1–8 were
3
measured for both the parent complexes and after photo-
reduction by triethylamine (Table 4). A set of typical spectral
changes is shown in Fig. 3. In the spectra of the parent
The mononuclear complexes all show three bands in their
experimental infrared spectra in the CO region. The general
form of the predicted wavenumbers correlate well with those
1
182
J. Chem. Soc., Dalton Trans., 2002, 1180–1187

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Table 4 Infrared absorption data for {Re(CO) Cl} complexes of 1–8 in CH Cl (ground state) and CH Cl –Et N (4 : 1) for the photoreduced
3
2
2
2
2
3
complexes. Typically 1 mM solutions were used depending on solubility. Band positions were estimated as gaussian functions using curve fitting
software
Ϫ1
Ϫ1
ν/cm
ν/cm
b
Ϫ1
c
Ϫ1
Compound
Experimental
Calculated
kCO /N m
Reduced species
∆k /N m
a
Re(bpy)
1899
1905
1904
1905
1909
1911
1909
1911
1914
1915
1917
2023
2024
2023
2023
2027
2026
2025
2025
2027
2023
1922
1921
1921
1922
1922
1931
1930
1932
1931
1924
1967
1923
1965
1924
1966
1924
1969
1932
1962
1931
1958
1933
1971
1932
1962
1960
1956
1954
1955
1958
1951
1950
1950
1953
1925
2114
1925
2114
1926
2113
1926
2114
1934
2114
1933
2114
1935
2112
1934
2114
2123
2119
2118
2118
2119
2119
2119
2118
2119
1965
2123
1962
2122
1963
2122
1966
2122
1958
2123
1955
2122
1968
2121
1959
2122
1531
1542
1539
1540
1548
1548
1545
1548
1552
1553
1867
1871
1869
1881
1895
1894
n.d.
1893
n.d
1906
1903
1908
1912
1996
2003
2002
49
47
46
n.d
48
n.d.
38
43
41
33
Re1
Re2
Re3
Re4
Re5
Re6
Re7
Re8
Re21
1931
1928
1929
1935
1936
1933
1936
1939
1940
d
1878
2007
1881
1881
1885
1895
2006
2007
2009
2009
2017
2008
2018
2009
2022
2010
2020
2009
2020
2008
2019
2010
2025
2011
2020
2
032
2023
031
2022
030
2024
033
2027
037
2026
035
2025
036
2027
037
Re22
Re23
Re24
Re25
Re26
Re27
Re28
1913
1914
1918
1921
1922
1920
1926
1937
1938
1943
1950
1946
1950
1953
1550
1550
1557
1563
1561
1562
1568
1892
1895
1895
1900
1900
1897
1901
1909
1910
1914
1917
1916
1921
1921
33
30
35
38
36
34
39
2
2
2
2
2
2
2
a
b
c
d
Data measured in CH CN; taken from ref. 29. kCO = (kax ϩ 2k )/3. ∆k = kCO(parent species) Ϫ kCO(reduced species). n.d. = No data available.
3
eq
The binuclear complexes show four bands in the CO region.
Calculations predict six CO bands, however, in the lower wave-
number region the splitting between bands is of the order of 2
Ϫ1
cm and only the higher wavenumber mode shows appreciable
splitting. The form of the normal modes from the calculations
may be analysed and this reveals that the highest wavenumber
mode (ν ) is r ϩ r ϩ r ϩ r ϩ r ϩ r , that is the in-phase ν of
AЈ
1
2
3
4
5
6
A
each {Re(CO) Cl} unit. The next mode predicted (ν ) is r ϩ r
3
AЉ
1
2
ϩ r Ϫ r Ϫ r Ϫ r . This is the out-of-phase symmetric stretch
3
4
5
6
of each {Re(CO) Cl} unit. The internal coordinates for the CO
3
Ϫ1
ligands are given in Fig. 2. The predicted splitting is 8 cm and
Ϫ1
8
± 2 cm is observed. The next two bands (ν and ν ) are
BЈ BЉ
Ϫ1
Fig. 3 Infrared difference spectrum for Re5 in CH Cl undergoing
2
2
photoreduction. Arrows depict the spectral changes as reduction occurs.
predicted to lie within 3 cm of each other, they are described
by: r Ϫ r ϩ r Ϫ r and r Ϫ r ϩ r Ϫ r , respectively. The
2
3
6
5
3
2
5
6
Ϫ1
lowest two bands (νCЈ and ν ), predicted to lie within 2 cm of
CЉ
Ϫ1
observed. A higher wavenumber band is predicted at 2119 cm ;
each other are described by: r ϩ r Ϫ 2r ϩ 2r Ϫ r Ϫ r and 2r
2
3
1
4
5
6
1
Ϫ1
one is observed at about 2025 ± 2 cm . This band is predicted
Ϫ r Ϫ r Ϫ 2r ϩ r ϩ r respectively. The experimental differ-
2 3 4 5 6
Ϫ1
to remain constant throughout the series of mononuclear
complexes and this is observed. The calculation also provides
a normal mode associated with this transition. This is the
symmetric stretch of all three CO bonds, with an in
ence between ν and ν for Re 1–Re 8 are about 20 cm . The
C B 2 2
Ϫ1
calculations predict a larger splitting, approximately 30–40 cm .
The shifts in the ν(C᎐O) on going from the ground to MLCT
34
excited state of [Re(CO) Cl(bpy)] have been measured.
3
phase motion of the COaxial (internal coordinate r ) and two
Previous studies using time-resolved resonance Raman spectro-
scopy suggest that the charge-transfer for the MLCT state
1
COequatorial (internal coordinates r and r ) bonds. The details
2
3
of the labelling are shown on Fig. 2. The mode is described
is about ϩ0.8 charge on the metal and this leads to the shifts
32
35,36
qualitatively as r ϩ r ϩ r . At lower wavenumbers two bands
to higher wavenumber for the CO bands.
The shifts in wave-
1
2
3
Ϫ1
are predicted about 20 cm apart. The experimental data show
number of bands alone are not fully informative as to the bond-
ing changes that are occurring about the Re centre. A more
insightful approach is to determine the force constants of the
normal modes of vibration associated with the CO ligands, to
Ϫ1
two bands that vary in separation by ca. 15–25 cm . These
bands show a pattern with respect to ligand substitution. The
dqq complexes (Re1–Re4) show a lower wavenumber for νA
Ϫ1
(
1905–1909 cm ) than complexes with dqxq ligands (Re5–
give the kax and keq force constants (ax refers to C᎐O ligands
᎐
Ϫ1
Re8). The predicted wavenumber is 1921 cm for the dqq series
and 1931 cm for the dqxq series. In all cases this corresponds
trans to Cl, eq refers to C᎐O ligands cis to Cl). Using these
᎐
Ϫ1
infrared data it is possible to calculate force constants for the
to a mode described as 2r Ϫ r Ϫ r . The intermediate wave-
axial (k ) and equatorial (k ) carbonyl bonds along with inter-
1
2
3
ax
eq
number band (ν ) is predicted to have a higher wavenumber
action constants for local interactions at a metal centre (k ). In
B
l
for the dqq series than dqxq; this prediction poorly reflects
the case of the binuclear complexes an additional interaction
the experimental data. The normal mode associated with ν is
constant is invoked (k ) that represents the interaction of equa-
B
b
r2 Ϫ r . These modes are related to those reported for
torial CO ligands on adjacent metal sites. The average carbonyl
3
33
{
Re(CO) Cl} fragments by Turner and coworkers.
force constant (kCO) is given by kCO = (kax ϩ 2k )/3.
3
eq
J. Chem. Soc., Dalton Trans., 2002, 1180–1187
1183

View Article Online
Results for the complexes along with the changes upon
photoreduction are given in Table 4 and Table S1 (ESI†.
The force constant calculations followed the approach of
Conclusions
A series of rhenium() complexes with bridging polypyridyl
ligands have been synthesised. The nature of the electron
accepting π* MO has been investigated by analysis of the
electrochemical, UV–Vis and IR data. The perturbation of
the π* MO for the BL appears to be critically controlled by
the presence of the Re() centre. It is found that substituents
on the bridging quinoxaline moiety make very little difference
to the bonding/energetic properties of the complexes. The
data from this study suggests the redox MO for metal com-
plexes may possess significant dπ character in a formal π*
ligand MO; particularly for binuclear complexes in which
the energy differences between dπ HOMO and π* LUMO are
smaller.
37
38
39
Braterman, Cotton and Turner.
The average change in the carbonyl force constants
40
with reduction (∆k) is a consequence of the dπ–π*(L) back-
bonding. This interaction produces a delocalised LUMO which
is predominantly a pure ligand π* MO with a smaller percent-
age of (dπ ϩ π*CO) character contributed from the molecular
orbitals of the {Re(CO) Cl} moiety. Occupation of the LUMO
3
by photoreduction results in the population of the π*CO MO
and hence the bonding changes (∆k < 0) as observed. The
percentage of (dπ ϩ π*CO) character in the LUMO is related
to the amplitude of the π*(L) MO at the chelating N atoms
and the difference in energy between the respective MOs. The
amplitude of the π*(L) wavefunction may be obtained from the
We have also shown that semi-empirical calculations can
provide useful insight into the electronic and vibrational
properties of large polyatomic metal complexes.
2
PM3 calculations and is parameterised by ΣcN (Table 2)while
the energy between the respective dπ and π*(L) MOs is
provided by the electronic spectral data. A plot (Fig. 4) of ∆k vs
Experimental
Synthesis
1
,2-(2Ј,2Љ)-Diquinolylethenediol. Quinoline 2-carbaldehyde
was purchased from Aldrich and used without further purifi-
45
cation. A benzoin condensation was employed to produce the
product. In a typical preparation quinoline 2-carbaldehyde (2.5
g, 16 mmol) was refluxed in ethanol (20 mL). While stirring,
NaCN (0.14 g, 2.9 mmol) was added, and the mixture was
refluxed for 10 min. A red–brown solid was collected by filtra-
tion while the mixture was still hot. The filtrate was then
brought to reflux again at which point more quinoline
2
-carbaldehyde could be added. The solid product was washed
2
with cold water and dried under vacuum. Yield: (1.7 g, 69%).
Fig. 4 Plot of ∆k vs. ΣcN /EMLCT for mononuclear complexes Re1, Re2
1
H NMR (CDCl ): δ 7.534 (dd, J = 6.6, 6.9 Hz, 2H, 6Ј, 6Љ); 7.734
(
and Re4 (᭿) and complexes Re5–Re 8 (᭹)
3
2
dd, J = 6.9, 6.9 Hz, 2H, 7, 7Љ); 7.833 (d, J = 8.1 Hz, 2H, 5Ј, 5Љ);
2
2
8.014 (d, J = 7.5 Hz, 2H, 8Ј, 8Љ); 8.133 (d, J = 8.7 Hz, 2H, 3Ј, 3Љ);
ΣcN /EMLCT is linear (∆k = 0.0016ΣcN /EMLCT ϩ 0.069) for most
8
.293 (d, J = 8.7 Hz, 2H, 4Ј, 4Љ).
of the complexes studied herein (Re5–Re 8). The remaining
2
complexes (Re1, Re2 and Re4) also show a linear relationship
between ∆k and ΣcN /EMLCT (∆k = 0.0030ΣcN /EMLCT Ϫ 0.017)
2
2
2,2Ј-Quinadil:
Quinadil was prepared by literature methods.
(1,2-(2Ј,2Љ)-diquinolylethanedione).
2,2Ј-
45
but the slope and intercept for these complexes differs sig-
41
nificantly from that of the others. The fact that the mono-
nuclear Re–dqq complexes (Re1–Re4) appear to be much more
sensitive to substitution of the quinoxaline bridge ring substitu-
tion may be a consequence of the polarisation of the π* MO
towards the bridging ring.
Quinoxaline 2-carbaldehyde. Quinoxaline 2-carbaldehyde
46
was prepared by modifying the procedure of Kaplan.
-Methylquinoxaline was purchased from Aldrich and used
without further purification. Selenium dioxide (11.5 g, 104
2
The shifts of the observed kCO with reduction suggest that for
many of these complexes there is significant electron density of
the redox MO on the metal site. Studies of the excited state
infrared spectra of [ReCl(CO) (4,4Ј-bpy) ] show a ∆k of about
mmol) was added to a mixture of p-dioxane (120 mL) and H O
2
(5 mL) and heated to reflux. 2-Methylquinoxaline (10 g, 69
mmol) was dissolved in p-dioxane (20 mL) and added dropwise
to the refluxing solution. A colour change to red–brown was
observed. After refluxing for 4 hours the hot mixture was
filtered through Celite and washed with p-dioxane (2 × 50 mL).
The filtrate was allowed to cool. The wet red–brown solid was
extracted into hexane (500 mL) using a Soxhlet apparatus over
a period of 24 hours. After cooling the orange hexane solution
3
2
Ϫ1 44
8
0 N m . The excited state probed is formally a MLCT state
where the oxidation number of the metal is altered by ϩ1. This
∆
k can be related to changes in the bond length of the CO
42,43
ligands (rCO) through an adaptation of Badger’s rule,
where rCO = 1.647–0.184ln(kCO). For the excited state of
44
[
ReCl(CO) (4,4Ј-bpy) ] the change in CO ligand bond length
was dried under vacuum and the product was recrystallised
3
2
1
from ground state (∆rCO) is of the order of Ϫ0.010 Å. For the
complexes studied herein the change in CO bond length on
from CH OH–H O. Yield: (8.0 g, 73%). H NMR (CDCl ):
3
2
3
δ 7.930 (m, 2H, 6, 7); 8.219 (d, J = 8.1 Hz, 1H, 5); 8.265 (d, J =
7.3 Hz, 1H, 8); 9.437 (s, 1H, 3); 10.270 (s, 1H, CHO). Anal.
going from the parent species to the reduced species (∆rCO
)
Calc. for C
H N O: C, 68.35; H, 3.82; N, 17.71. Found: C,
corresponds to: ϩ0.007 Å for Re1, Re2 and Re4; ϩ0.006 Å
for Re6–Re8 and between ϩ0.004 and ϩ0.005 Å for the bi-
nuclear complexes. These changes suggest sizeable components
of the redox MO lie on the metal site. If we assume the force
constant changes are directly related to bond order, which in
turn relates to the charge at the Re centre then for the Re–dqq
complexes the charge leakage on to the Re must be of the order
of Ϫ0.5 to cause the ∆k observed. The binuclear complexes are
remarkable in that the charge leakage is close to Ϫ0.3 per
Re site; this means the majority of the redox MO is in fact
metal-based.
9
6 2
6
8.28; H, 3.95; N, 17.85.
1,2-(2Ј,2Љ)-Diquinoxalylethenediol. 1,2-(2Ј,2Љ)-Diquinoxalyl-
ethenediol was prepared, in an analogous fashion to 1,2-(2Ј,2Љ)-
diquinolylethenediol by replacing quinoline 2-carbaldehyde
with quinoxaline 2-carbaldehyde (2.0 g, 13 mmol). The product
1
was isolated as a red solid. Yield: (1.2 g, 61%). H NMR
(CDCl ): δ 7.801 (m, 4H, 6Ј, 6Љ, 7Ј, 7Љ); 8.026 (dd, J = 7.4, 2 Hz,
3
2H, 5Ј, 5Љ); 8.151 (dd, J = 7.2, 1.8 Hz, 2H, 8Ј, 8Љ); 9.566 (s, 1H,
3Ј, 3Љ).
1
184
J. Chem. Soc., Dalton Trans., 2002, 1180–1187

View Article Online
1
2
,2Ј-Quinoxadil (1,2-(2Ј,2Љ)-diquinoxalylethanedione). 2,2Ј-
cream solid. Yield: 0.3 g, 58%. H NMR (CDCl ): δ 7.488 (d,
3
Quinoxadil was prepared in an analogous fashion to 2,2Ј-
quinadil by replacing to 1,2-(2Ј,2Љ)-diquinolylethenediol with
J = 8.4 Hz, 2H, 5Ј, 5Љ); 7.608 (dd, J = 8.4, 6.8 Hz, 2H, 6Ј, 6Љ);
7.760 (dd, J = 6.8, 6.8 Hz, 2H, 8Ј, 8Љ); 8.159 (d, J = 8.1 Hz, 2H,
13
1
,2-(2Ј,2Љ)-diquinoxalylethenediol (1.0 g, 3.2 mmol). The
8Ј, 8Љ); 8.464 (s, 2H, 5, 8); 9.615 (s, 2H, 3Ј, 3Љ). C NMR
product was isolated as an orange solid. Yield: (0.54 g, 54%).
(CDCl ): (12 signals) δ 129.23, 129.36, 130.24, 130.43, 130.81,
3
1
H NMR (CDCl ): δ 7.772 (dd, J = 7.0, 6.9 Hz, 2H, 7Ј, 7Љ); 7.914
(
136.48, 139.98, 140.85, 141.84, 145.54, 150.72, 151.27. Anal.
Calc. for C H N Cl : C, 63.31; H, 2.66; N, 18.46. Found: C,
3
dd, J = 8.3, 7.0 Hz, 2H, 6Ј, 6Љ); 7.966 (d, J = 8.5 Hz, 2H, 5Ј, 5Љ);
24
12
6
2
8
.221 (d, J = 8.8 Hz, 2H, 8Ј, 8Љ); 9.705 (s, 2H, 3Ј, 3Љ).
63.35; H, 2.61; N, 18.54%.
Pentacarbonylchlororhenium(I). Pentacarbonylchlororhen-
ium() was prepared using literature methods.
Rhenium(I) complexes. Tricarbonylchlororhenium() com-
plexes were prepared using established literature methods.
47
50
If purification was necessary the crude product was dissolved
Ligands. Ligands were prepared by the Schiff base condens-
ation of a 1,2-diaminobenzene derivative and 2,2Ј-quinadil
in CH Cl2 (∼200 mL) and then chromatographed using
a silica column. The binuclear complex was eluted first
2
48
or 2,2Ј-quinoxadil. 1,2-diaminobenzene, 4,5-dimethyl-1,2-
diaminobenzene, 4,5-dichloro-1,2-diaminobenzene, and 2,3-
diaminonapthalene were all purchased from Aldrich and used
without further purification. In a typical preparation 2,2Ј-
quinadil (0.3 g, 1.0 mmol) and the appropriate diaminobenzene
with CH Cl ; the mononuclear complex was then eluted with
2
2
CH CN.
3
Cl(CO) Re(1) (Re1). Re1 was isolated as an orange solid.
3
Yield: 0.2 g, 65%. Anal. Calc. for C H N O ClRe: C, 50.47; H,
29
16
4
3
2.34; N, 8.12. Found: C, 50.08; H, 2.26; N, 8.24%.
(
1.0 mmol) were suspended in ethanol (100 mL, freshly distilled
Cl(CO) Re(2) (Re2). Re2 was isolated as a light-orange
3
49
from Mg/I2) and refluxed for one hour. An orange–yellow
colour change was observed in the solution phase. After cool-
ing, the solvent was removed under vacuum and the ligand was
solid. Yield: 0.1 g, 71%. Anal. Calc. for C H N O ClRe: C,
31
20
4
3
51.84; H, 2.81; N, 7.80. Found: C, 51.71; H, 2.78; N, 7.78%.
Cl(CO) Re(3) (Re3). Re3 was isolated as an orange solid.
3
recrystallised from ethanol–H O.
Yield: 0.15 g, 76%. Anal. Calc. for C H N O ClRe(CH OH):
2
33 18
4
3
3
The synthesis of 2,3-(2Ј,2Љ)-diquinolylquinoxaline (1), 6,7-
dimethyl-2,3-(2Ј,2Љ)-diquinolylquinoxaline (2) and 6,7-dichloro-
C, 52.90; H, 2.87; N, 7.26. Found: C, 52.74; H, 2.60; N, 7.37%.
Cl(CO) Re(4) (Re4). Re4 was isolated as an orange solid.
3
2
,3-(2Ј,2Љ)-diquinolylquinoxaline (4) has been previously
Yield: 0.11 g, 69%. Anal. Calc. for C H N Cl O Re: C, 45.89;
H, 1.86; N, 7.38. Found: C, 46.21; H, 1.82; N, 7.61%.
29
14
4
3
3
23
described.
2
,3-(2Ј,2Љ)-Diquinolylbenzoquinoxaline (3). The product was
Cl(CO) Re(5) (Re5). Re5 was isolated as a light-red solid.
3
1
isolated as a light brown solid. Yield: 0.2 g, 63%. H NMR
CDCl ): δ 7.499 (m, 6H, 5Ј, 5Љ, 6Ј, 6Љ, 7Ј, 7Љ); 7.619 (dd, J = 6.6,
Yield: 0.14 g, 81%. Anal. Calc. for C H N O ClRe: C, 46.85;
27
14
6
3
(
H, 2.04; N, 12.15. Found: C, 46.94; H, 1.85; N, 12.30%.
3
3
Hz, 2H, 7, 8); 7.813 (dd, J = 6.6, 2.7 Hz, 2H, 8Ј, 8Љ); 8.170 (dd,
Cl(CO) Re(6) (Re6). Re6 was isolated as a red solid. Yield:
3
1
J = 6.3, 3.3 Hz, 2H, 6, 9); 8.215 (d, J = 8.7 Hz, 2H, 3Ј, 3Љ); 8.285
0.1 g, 63%. H NMR (CDCl ): δ 2.639 (s, 3H, –CH (7)); 2.704
3
3
13
(
d, J = 8.8 Hz, 2H, 4Ј, 4Љ); 8.865 (s, 2H, 5, 10). C NMR
(s, 3H, –CH (6)); 7.815 (dd, J = 6.8, 6.8 Hz, 1H, 6Ј); 7.911 (dd,
3
(
CDCl ): (15 signals) δ 121.61, 126.88, 127.08, 127.38, 127.44,
28.06, 128.71, 129.40, 129.40, 134.44, 136.32, 137.73, 147.19,
J = 6.8, 6.8 Hz, 1H, 7Ј); 7.980 (d, J = 7.1 Hz, 1H, 5Ј); 7.992 (dd,
J = 6.6, 7 Hz, 1H, 6Љ); 8.083 (dd, J = 6.8, 7 Hz, 1H, 7Ј); 8.112 (d,
J = 6.8 Hz, 1H, 5Љ); 8.163 (s, 1H, 8); 8.254 (d, J = 7.3 Hz, 1H, 8Ј);
8.590 (s, 1H, 5); 8.777 (s, 1H, 3Љ) 8.929 (d, 7.8 Hz, 1H, 8Љ); 9.853
(s, 1H, 3Ј). Anal. Calc. for C H N O ClRe: C, 48.36; H, 2.52;
3
1
1
1
53.26, 157.32. Anal. Calc. for C H N : C, 82.9; H, 4.2; N,
30
18
4
2.9. Found: C, 83.2; H, 4.0; N, 13.0%.
,3-(2Ј,2Љ)-Diquinoxalylquinoxaline (5). The product was
2
29
18
6
3
1
isolated as a orange–brown solid. Yield: 0.4 g, 86%. H NMR
CDCl ): δ 7.539 (d, J = 8.4 Hz, 2H, 5Ј, 5Љ); 7.624 (dd, 8.4, 6.7
N, 11.67. Found: C, 48.45; H, 2.42; N, 11.84%.
(
Cl(CO) Re(7) (Re7). Re7 was isolated as a dark-red solid.
3
3
Hz, 2H, 6Ј, 6Љ); 7.750 (dd, J = 6.7, 6.7 Hz, 2H, 7Ј, 7Љ); 7.949 (dd,
Yield: 0.11 g, 61%. Anal. Calc. for C H N O ClRe: C, 50.17;
H, 2.17; N, 11.33. Found: C, 50.30; H, 2.02; N, 11.46%.
31
16
6
3
J = 6.5, 3.5 Hz, 2H, 6, 7); 8.153 (d, J = 8.1 Hz, 2H, 8Ј, 8Љ); 8.345
13
(
dd, J = 6.4, 3.5 Hz, 2H, 5, 8); 9.643 (s, 2H, 3Ј 3Љ). C NMR
Cl(CO) Re(8) (Re8). Re8 was isolated as a dark-red solid.
3
(
CDCl ): (12 signals) δ 129.15, 129.37, 129.72, 130.24, 130.51,
Yield: 0.1 g, 65%. Anal. Calc. for C H N Cl O Re: C, 42.61;
3
27 12
6
3
3
1
31.61, 140.92, 141.35, 141.70, 145.77, 150.18, 151.35. Anal.
H, 1.59; N, 11.04. Found: C, 42.82; H, 1.30; N, 11.17%.
Calc. for C H N : C, 74.59; H, 3.65; N, 21.75. Found: C,
The preparations of binuclear {Re(CO) Cl} complexes
24
14
6
3
14
7
4.55; H, 3.56; N, 21.84.
,7-Dimethyl-2,3-(2Ј,2Љ)-diquinoxalylquinoxaline (6). The
followed the method previously reported. Compound purifi-
cation was achieved with column chromatography (silica, eluent
4 : 1 CH Cl –hexane solution).
6
1
product was isolated as light pink solid. Yield: 0.5 g, 85%. H
2
2
NMR (CDCl ): δ 2.599 (s, 6H, –CH ); 7.532 (dd, J = 8.3, 6.6 Hz,
Cl(CO) Re(µ-1)Re(CO) Cl (Re 1). Re 1 was isolated as a
3
3
3
3
2
1
2
2
6
8
1
1
7
H, 6Ј, 6Љ); 7.616 (d, J = 8.1 Hz, 2H, 5Ј, 5Љ); 7.735 (dd, J = 6.5,
dark-purple solid. Yield: 0.1 g, 60%. H NMR (CDCl ): δ 7.882
3
.7 Hz, 7Ј, 7Љ); 8.080 (s, 2H, 5, 8); 8.140 (d, J = 8.1 Hz, 2H, 8Ј,
(dd, J = 7.1, 7.1 Hz, 2H, 6Ј, 6Љ); 7.988 (d, J = 7.4 Hz, 2H, 5Ј, 5Љ);
8.112 (ddd, J = 6.8, 6.8, 1.7 Hz, 2H, 7Ј, 7Љ); 8.173 (dd, J = 6.6,
3.4 Hz, 2H, 6, 7); 8.377 (d, J = 8.8 Hz, 2H, 8Ј, 8Љ); 8.732 (6.6, 3.4
Hz, 2H, 5, 8); 8.773 (d, J = 8.8 Hz, 2H, 4Ј, 4Љ); 8.942 (d, J = 9
Hz, 2H, 3Ј, 3Љ). Anal. Calc. for C H N O Cl Re : C, 38.59; H,
13
Љ); 9.609 (s, 2H, 3Ј, 3Љ). C-NMR (CDCl ): (13 signals) δ 20.56,
3
28.55, 129.04, 129.26, 130.03, 130.22, 140.28, 140.87, 141.53,
42.51, 145.83, 149.10, 151.56. Anal. Calc. for C H N : C,
26
18
6
5.34; H, 4.38; N, 20.28. Found: C, 75.15; H, 4.34; N, 20.43%.
,3-(2Ј,2Љ)-Diquinoxalylbenzoquinoxaline (7). The product
32
16
4
6
2
2
2
1.62; N, 5.63. Found: C, 38.45; H, 1.74; N, 5.54%.
precipitated directly out of the reaction mixture as a bright
Cl(CO) Re(µ-2)Re(CO) Cl (Re 2). Re 2 was isolated as a
3
3
2
2
1
1
yellow solid. Yield: 0.4 g, 63%. H NMR (CDCl ): δ 7.494 (d,
dark-red solid. Yield: 0.07 g, 48%. H NMR (CDCl ): δ 2.686 (s,
3
3
J = 8.4 Hz, 2H. 5Ј, 5Љ); 7.599 (dd, J = 8.5, 6. 8 Hz, 2H, 6Ј, 6Љ);
6H, –CH ); 7.856 (dd, J = 7.1, 6.7 Hz, 2H, 6Ј, 6Љ); 7.9682 (dd,
3
7
2
.675 (dd, J = 6.5, 3.3 Hz, 2H, 7, 8); 7.747 (dd, J = 6.8, 6.7 Hz,
H, 7Ј, 7Љ); 8.167 (d, J = 9 Hz, 2H, 8Ј, 8Љ); 8.215 (dd, J = 6.3, 3.3
J = 8.1, 1.5 Hz, 2H, 5Ј, 5Љ); 8.094 (ddd, J = 6.9, 6.7, 1.8 Hz, 2H,
7Ј, 7Љ); 8.331 (d, J = 8.4 Hz, 2H, 8Ј, 8Љ); 8.464 (s, 2H, 5, 8); 8.683
(d, J = 8.7 Hz, 2H, 4Ј, 4Љ); 8.953 (d, J = 8.8 Hz, 2H, 3Ј, 3Љ). Anal.
Calc. for C H N O Cl Re : C, 39.89; H, 1.97; N, 5.47. Found:
13
Hz, 2H, 6, 9); 8.921 (s, 2H, 5, 10); 9.720 (s, 2H, 3Ј, 3Љ). C NMR
CDCl ): (14 signals) δ 127.73, 128.56, 128.89, 129.21, 129.37,
(
3
34 20
4
6
2
2
1
1
30.30, 130.57, 134.97, 137.60, 140.90, 141.80, 145.83, 150.65,
51.46. Anal. Calc. for C H N : C, 77.05; H, 3.70; N, 19.26.
C, 39.76; H, 1.79; N, 5.72%.
Cl(CO) Re(µ-3)Re(CO) Cl (Re 3). Re 3 was isolated as a
28
16
6
3
3
2
2
Found: C, 76.83; H, 3.69; N, 19.16%.
,7-Dichloro-2,3-(2Ј,2Љ)-diquinoxalylquinoxaline (8). The
product precipitated directly out of the reaction mixture as a
dark-brown solid. Yield: 0.1 g, 75%. Anal. Calc. for C H N -
O Cl Re : C, 41.34; H, 1.73; N, 5.36. Found: C, 41.63; H, 1.68;
6 2 2
N, 5.46%.
36 18 4
6
J. Chem. Soc., Dalton Trans., 2002, 1180–1187
1185

View Article Online
Ϫ1
Cl(CO) Re(µ-4)Re(CO) Cl (Re 4). Re 4 was isolated as
ca. 2 cm . The spectral breadths of these bands suggest they
would be difficult to resolve.
3
3
2
2
a dark-brown solid. Yield: 0.14 g, 83%. Anal. Calc. for
C H N Cl O Re : C, 36.10; H, 1.33; N, 5.6. Found: C, 36.24;
PM3 semi-empirical calculations were implemented using the
SPARTAN package.
32
14
4
4
6
2
52
H, 1.24; N, 5.33%.
Cl(CO) Re(µ-5)Re(CO) Cl (Re 5). Re 5 was isolated as a
3
3
2
2
dark-brown solid. Yield: 0.1 g, 67%. Anal. Calc. for
Acknowledgements
C H N O Cl Re ؒ0.25CH Cl : C, 35.65; H, 1.43; N, 8.25.
30
14
6
6
2
2
2
2
Found: C, 35.55; H, 1.20; N, 7.86%.
Support from the New Zealand Lottery Commission and
the University of Otago Research Committee is gratefully
acknowledged. A. F. would like to acknowledge the Alliance
Group for a post-graduate scholarship.
Cl(CO) Re(µ-6)Re(CO) Cl (Re 6). Re 6 was isolated as a
3
3
2
2
1
black solid. Yield: 0.04 g, 34%. H NMR (CDCl ): δ 2.707 (s,
3
6
H, –CH ); 8.114 (m, 4H, 6Ј, 6Љ, 7Ј, 7Љ); 8.311 (dd, J = 7.6, 2.2
3
Hz, 2H, 5Ј, 5Љ); 8.424 (s, 2H, 5, 8); 8.819 (d, J = 8.0 Hz, 2H, 8Ј,
8
3
Љ); 9.915 (s, 2H, 3Ј, 3Љ). Anal. Calc. for C H N O Cl Re : C,
32 18 6 6 2 2
References
7.47; H, 1.77; N, 8.19. Found: C, 37.77; H, 1.69; N, 8.46%.
Cl(CO) Re(µ-7)Re(CO) Cl (Re 7). The preparation of
3
3
2
1 F. Scandola, M. T. Indelli, C. Chiorboli and C. A. Bignozzi,
Top. Curr. Chem., 1990, 158, 73; V. Balzani, S. Campagna,
G. Denti, A. Juris, S. Serroni and M. Venturi, Acc. Chem. Res., 1998,
this complex yielded a mixture of the mononuclear and bi-
nuclear complexes. Separation could be achieved with column
chromatography (silica), however only small amounts of the
binuclear complex were recovered. As an alternative, 7 (0.1040
g, 0.24 mmol) was dissolved in N degassed CHCl , Re(CO) Cl
3
1, 26.
2
3
H. M. Sung-Suh, D. S. Kim, C. W. Lee and S.-E. Park, Appl.
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2
3
5
M. K. Nazeeruddin, A. Kay, I. Rodicio, R. Humphrey-Baker,
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(
0.2065 g, 0.52 mmol) was added, and the mixture refluxed
under N for 7 hours. A colour change to black was observed.
2
The reaction mixture was allowed to cool and the solvent was
removed under vacuum. The crude solid was dissolved in
CH Cl , filtered, and the product precipitated by adding Et O.
The black solid was collected by filtration. Yield: 0.19 g, 70%.
Anal. Calc. for C H N O Cl Re 0.5CH Cl : C, 38.01; H, 1.58;
2
2
2
4
V. Balzani, M. Gomez-Lopez and J. F. Stoddart, Acc. Chem. Res.,
1
998, 31, 405; M. Venturi, S. Serroni, A. Juris, S. Campagna and
34
16
6
6
2
2
2
2
V. Balzani, Top. Curr. Chem., 1998, 197, 193; R. Ziessel, M. Hissler,
A. El-Ghayoury and A. Harriman, Coord. Chem. Rev., 1998, 180,
1251; R. Ziessel and A. Harriman, Coord. Chem. Rev., 1998,
N, 7.71. Found: C, 37.76; H, 1.75; N, 7.23%.
Cl(CO) Re(µ-8)Re(CO) Cl (Re 8). Re 8 was prepared in
3
3
2
2
1
71, 331; F. Scandola, R. Argazzi, C. A. Bignozzi, C. Chiorboli,
an analogous fashion to Re 7 and was isolated as a black solid.
Yield: 0.18 g, 72%. Anal. Calc. for C H N Cl O Re : C, 33.78;
H, 1.13; N, 7.88. Found: C, 33.80; H, 1.17; N, 7.69%.
2
M. T. Indelli, M. A. Rampi, in Supramolecular Chemistry,
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Chem., 1998, 37, 3505.
30
12
6
4
6
2
5
6
Physical measurements
7
The instrumentation used in electrochemistry, UV/Vis spectro-
scopy and spectroelectrochemistry has been detailed else-
where. Infrared spectra were collected using a BIO–RAD
Digilab Division FTS-60 FT-IR with Bio-Rad Win-IR soft-
ware. Samples were prepared as ∼1 mM CH Cl solutions. For
8
M. K. DeArmond and M. L. Myrick, Acc. Chem. Res., 1989, 22,
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51
9 P. G. Bradley, N. Kress, B. A. Hornberger, R. F. Dallinger and
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2
2
1
the reductive quenching experiments, samples were prepared as
mM solutions in a freshly prepared mixture of CH Cl and
1
2
2
1
1 M. W. George, F. P. A. Johnson, J. J. Turner and J. R. Westwell,
J. Chem. Soc., Dalton Trans., 1995, 2711.
21
NEt (20%). The mononuclear complexes had in addition to
this a 0.1 M concentration of Bu NCl. The solution was
degassed with Ar and a small amount transferred to a solution
IR cell. Spectra were measured continuously, while under
irradiation from a halogen lamp, with a 4 cm resolution for a
3
4
12 C. D. Tait, T. M. Vess, M. K. DeArmond, K. W. Hanck and
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Ϫ1
maximum time of 30 minutes.
1
3 K. Kalyanasundaram, Photochemistry of Polypyridine and Porphyrin
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1
4 T. J. Simpson and K. C. Gordon, Inorg. Chem., 1995, 34, 6323.
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{
Re(CO) Cl} complexes of 1–8 were calculated by extending
3
38
16 W. Kaim and S. Kohlmann, Inorg. Chem., 1990, 29, 2909.
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assignment of Turner. An additional interaction between the
two carbonyl systems across the bridging ligand was proposed
to account for the splitting in energy of the ν band. Therefore,
four force constants were expected. Each CO stretch was either
kax or k ; interactions between COs attached to the same
rhenium centre were all assumed to be k ; remote interactions
were assumed to be non-zero only for equatorial–equatorial
2
1
7 A. Juris, S. Campagna, I. Bidd, J.-M. Lehn and R. Ziessel, Inorg.
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33
1
8 L. S. Kelso, D. A. Reitsma and F. R. Keene, Inorg. Chem., 1996, 35,
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144.
C
19 J. B. Foresman and A. Frisch, Exploring Chemsitry with Electronic
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2
0 P. Comba and T. W. Hambley, Molecular Modeling of Inorganic
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eq
l
2
1 V. Enchev, A. Ahmedova, G. Ivanova, I. Wawer, N. Stoyanov and
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interactions and these were all equal to k . It was noted that if
b
kb = 0 then the two centres would be non-interacting and the
spectrum should resemble that of a mononuclear system. The
system was solved using the linear variation principle from
37
Braterman.
It was also noted that a non-zero value of k means that the
b
two νB bands do not have equivalent frequencies. However
using the calculated k value led to a splitting of these bands of
22 For cis-Re 1, the predicted IR spectrum in the CO region has bands
2
Ϫ1
b
at 2125, 2114, 1973, 1960, 1931 and 1926 cm .
1
186
J. Chem. Soc., Dalton Trans., 2002, 1180–1187

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2
3 S. E. Page, K. C. Gordon and A. K. Burrell, Inorg. Chem., 1998, 37,
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Ϫ1
2
2
40 ∆k = {(kax ϩ 2keq)reduced species Ϫ (kax ϩ 2keq)parent species}/3 (N m ).
Ϫ4
41 The errors in the gradients of each slope is 1.5 × 10 for the
Ϫ4
1
1
997, 260, 199; J. Sherborne and K. C. Gordon, Asian J. Spectrosc.,
998, 2, 137.
Re5–Re 8 data and 1.0 × 10 for the Re1, Re2 and Re4 data.
2
42 R. M. Badger, J. Chem. Phys., 1934, 2, 128; R. M. Badger, Phys.
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2
7 Inorganic and Electronic Spectorscopy, Volume II, Applications and
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2
3
3
8 I. P. Clark, M. W. George and J. J. Turner, J. Phys. Chem. A., 1997,
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01, 8367.
Ϫ1
9 I. P. Clark, M. W. George, F. P. A. Johnson and J. J. Turner, Chem.
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63.
3
3
2 The exact nature of the normal mode may be established but our
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3
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52 PC SPARTAN Pro version 1.0.6, Wavefunction Inc., 18401 Von
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J. Chem. Soc., Dalton Trans., 2002, 1180–1187
1187