Synthesis and properties of Re(I) tricarbonyl complexes of 6,6′-disubstituted-4,4′-bipyrimidines with high energy excited states suitable for incorporation into polynuclear arrays

Article abstract of DOI:10.1016/j.ica.2005.09.042

A series of complexes [(N-N)Re(CO)₃X]ⁿ (N-N = 6,6′-diaryl-4,4′-bipyrimidine, axial ligand X = Cl, MeCN, 4-phenyl-pyridine, or t-Bu-pyridine) have been synthesized and characterized. The substituent aryl on the bipyrimidine, as well as the axial ligand X, has important effects on the properties of these complexes. The co-planarity of the 4,4′-bipyrimidine core and its substituents contributes to the extent of π-electron delocalization and, hence, to the redox and spectroscopic properties of the complexes. The complexes exhibit Re-to-bpm metal-to-ligand charge transfer absorptions in the range of 379-464 nm, which are red-shifted with the increase in the delocalization in the substituted bpm ligand and the increase in the donor character of the axial ligand. The electrochemical data support metal-based oxidations (from +1.07 to +1.40 V) and ligand-based reductions (from -0.62 to -0.75 V) and correlate well with those obtained by UV-Vis spectroscopy. The X-ray crystal structures of two of the complexes have also been investigated.

Full text of DOI:10.1016/j.ica.2005.09.042

Inorganica Chimica Acta 359 (2006) 2599–2607
www.elsevier.com/locate/ica
Synthesis and properties of Re(I) tricarbonyl complexes of
6,60-disubstituted-4,40-bipyrimidines with high energy excited states
suitable for incorporation into polynuclear arrays
*
Elena Ioachim, Elaine A. Medlycott, Garry S. Hanan
De´partement de Chimie, Universite´ de Montre´al, 2900 Edouard-Montpetit, Pavillon Roger-Gaudry, Montre´al, Que., Canada H3T-1J4
Received 29 June 2005; accepted 24 September 2005
Available online 8 November 2005
Dedicated to Brian James.
Abstract
A series of complexes [(N–N)Re(CO)3X]n (N–N = 6,60-diaryl-4,40-bipyrimidine, axial ligand X = Cl, MeCN, 4-phenyl-pyridine, or
t-Bu-pyridine) have been synthesized and characterized. The substituent aryl on the bipyrimidine, as well as the axial ligand X, has
important effects on the properties of these complexes. The co-planarity of the 4,40-bipyrimidine core and its substituents contributes
to the extent of p-electron delocalization and, hence, to the redox and spectroscopic properties of the complexes. The complexes exhibit
Re-to-bpm metal-to-ligand charge transfer absorptions in the range of 379–464 nm, which are red-shifted with the increase in the delo-
calization in the substituted bpm ligand and the increase in the donor character of the axial ligand. The electrochemical data support
metal-based oxidations (from +1.07 to +1.40 V) and ligand-based reductions (from ꢀ0.62 to ꢀ0.75 V) and correlate well with those
obtained by UV–Vis spectroscopy. The X-ray crystal structures of two of the complexes have also been investigated.
Ó 2005 Elsevier B.V. All rights reserved.
Keywords: Rhenium(I) complexes; N ligands; Spectroscopy; X-ray crystal structures; Electrochemistry
1. Introduction
electron transfer studies that can be performed is in turn
limited due to the lower driving forces involved [8,9].
The discovery that (bpy)Re(CO)3Cl can act both as a
photosensitizer and as a homogenous catalyst in the reduc-
tion of carbon dioxide has led to the development of a class
of complexes of type [(N–N)Re(CO)3X]n+ (N–N = a-dii-
mine and X varying from halide to different organic donor
ligands), which absorb and emit at relatively high energies
as compared to their Ru(II) and Os(II) analogues [10,11].
The spectroscopic and redox behaviour of the Re(I) com-
plexes are ligand dependent and can, therefore, be tuned
by varying the identity of their chelate and/or axial ligands.
Significant research has focussed on common chelating li-
gands such as bipyridine (bpy) and phenanthroline (phen)
in which differences in electronic and steric properties of
the ligands may lead to changes in the properties of their
metal complexes. Substitution on the ligand, which may in-
crease the delocalization in the acceptor ligand [12,13], or
The a-diimine complexes of d6 metals such as Ru(II) and
Os(II) have been widely used in connection with the con-
version of solar energy into storable chemical energy
[1,2]. Their spectroscopic, redox and excited-state proper-
ties make them suitable for use as photosensitizers for this
purpose [3–5]. However, the lowest energy excited-state of
these complexes is generally metal-to-ligand charge-trans-
fer (MLCT) in nature and is relatively low in energy, in
particular for Os(II) complexes [6,7]. This limits the type
of p-accepting a-diimine ligands that can be used as the
MLCT excited state and is more readily deactivated at
low energy according to the energy gap law [3b]. As the ex-
cited states are of lower energy, the range of energy and
*
Corresponding author. Tel.: +1 514 343 7056; fax: +1 514 343 2468.
E-mail address: garry.hanan@umontreal.ca (G.S. Hanan).
0020-1693/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2005.09.042

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E. Ioachim et al. / Inorganica Chimica Acta 359 (2006) 2599–2607
Chart 1. Various 6,60-disubstituted-4,40-bipyrimidine complexes of Re(I).
the use of electron-withdrawing and -donor substituents
[14–16] represents different approaches to create photosen-
sitizers with different photophysical properties.
However, from the point of view of maintaining a rela-
tively high excited state energy, Re(I) complexes can
accommodate more p-accepting a-diimine ligands than
their Ru(II) or Os(II) counterparts. Thus, Re(I) displays
greater flexibility in possible ligands combinations for the
assembly of complexes for electron and energy transfer
studies. The introduction of bidiazine ligands into the coor-
dination sphere of the metal ion gives access to a redox-ac-
tive MLCT excited state at lower energy than bpy or phen
derivatives, due to the better p acceptor properties of the
coordinated bidiazine ligands [17,18]. The most appealing
among the series of four isomeric bidiazine (e.g., 2,20-bipyr-
imidine, 3,30-bipyridazine, 2,20-bipyrazine and 4,40-bipyr-
imidine) appears to be 4,40-bipyrimidine (bpm) [19,20]. It
has been demonstrated that this particular ligand has the
lowest LUMO energy, without being the poorest r-donor
in the set of the bidiazine ligands [19]. However, Re(I) com-
plexes of bpm have not been extensively studied due to the
inaccessibility of the ligand and the difficulty in directly
functionalizing it.
Scheme 1. Synthesis of the complexes [Re(N–N)(CO)3X]n+. Reagents and
conditions: (a) toluene, 2 h, yield: 39–84%; (b) 1 equiv. AgPF6, acetoni-
trile, reflux, overnight, yield: 45–76%; and (c) 1 equiv. ppy or t-Bupy,
respectively, yield: ppy: 63–84%, t-Bupy: 70%.
As part of our programme of examining energy transfer
in multinuclear assemblies, we required high energy visible
light absorbers to play the lead role in antenna complexes
[21]. We report herein on the synthesis, the electrochemical
and spectroscopic properties of a novel series of high-energy
light absorbing Re(I) complexes of new 6,60-disubstituted-
4,40-bipyrimidine ligands of type [(N–N)Re(CO)3X]n
(Chart 1), in which N–N = 6,60-diphenyl-4,40-bipyrimidine
(dpb) (L1), and 6,60-bis(p-methoxyphenyl)-4,40-bipyrimi-
dine (dmpb) (L2), 6,60-bis-(l-naphthyl)-4,40-bipyrimidine
(dnb) (L3); and where for n = 0, X = Cl (a), and for
n = +1, X = MeCN (b), 4-phenylpyridine (ppy) (c), 4-t-
Bu-pyridine (tBupy) (d).
established method [18]. All of the complexes were recrys-
tallized from hexane and their composition was confirmed
by high-resolution mass spectrometry. The intermediate
ionic complexes [(N–N)Re(CO)3(MeCN)](PF6) (1b–3b)
were synthesized by allowing the halide carbonyls 1a–3a
to react with 1 equiv. of AgPF6 in acetonitrile overnight
[22]. After cooling, the AgCl that formed during the reac-
tion was removed by filtration. The addition of a satu-
rated solution of NH4PF6 to the filtrate precipitated the
complexes as their hexafluorophosphate salts. The partial
removal of the solvent and addition of water resulted in
the formation of corresponding complexes 1b–3b in good
yields.
Complexes 1b–3b were converted into their pyridine
derivatives 1c–3c and 2d by refluxing them with the appro-
priate 4-substituted pyridine in THF overnight. The addi-
tion of water precipitated the desired compounds, which
were further purified by diffusion of hexane into solutions
of the respective complexes in chloroform.
2. Results and discussion
2.1. Synthesis
The new [(N–N)Re(CO)3X]n complexes were obtained
via substitution reactions, as outlined in Scheme 1. In
the first step (a), the chloride Re(I) carbonyls 1a–3a were
prepared in good yield by refluxing equivalent amounts of
Re(I) pentacarbonyl chloride and the appropriate ligands
L1–L3, respectively, in toluene following a previously
1
2.2. H NMR spectroscopy
All of the compounds were characterized by 1H
NMR spectroscopy, whereas only the complexes with the

E. Ioachim et al. / Inorganica Chimica Acta 359 (2006) 2599–2607
2601
Table 1
1H NMR resonances (ppm) for 6,60-substituted-4,40-bpm in [Re(N–N)(CO)3X]n+ complexesa
Compound
2
5
20
30
40
50
60
70
80
O-Me
dpb (L1)
dmpb (L2)
dnb (L3)
bpm
9.42
9.35
9.51
9.35
9.82
9.12
9.90
9.59
9.52
9.77
9.72
9.60
9.90
9.58
9.81
8.93
8.89
9.10
8.46
8.77
8.73
8.51
8.97
8.88
8.77
9.13
8.94
8.89
8.83
8.26
8.27
8.25
7.97
7.58
7.19
7.60
7.58
7.58
7.19
7.84
8.27
8.25
7.60
3.93
8.05
7.71
7.69
8.32
Re(dpb)(CO)3Cl (1a)
Re(dmpb)(CO)3Cl (2a)
Re(dnb)(CO)3Cl (3a)
[Re(dpb)(CO)3MeCN](PF6) (1b)
[Re(dmpb)(CO)3MeCN](PF6) (2b)
[Re(dnb)(CO)3MeCN](PF6) (3b)
[Re(dpb)(CO)3(ppy)](PF6) (1c)
[Re(dmpb)(CO)3(ppy)](PF6) (2c)
[Re(dnb)(CO)3(ppy)](PF6) (3c)
[Re(dmpb)(CO)3(t-Bupy)](PF6) (2d)
Re(bpm)(CO)3Clb
8.54
8.29
8.11
8.41
8.43
8.08
8.50
8.52
8.06
8.50
7.71
7.14
7.65
7.68
7.11
7.59
7.70
7.10
7.64
7.00
7.71
7.14
7.85
7.68
7.11
7.97
7.70
7.10
8.00
7.00
8.54
8.29
7.65
8.41
8.43
7.59
8.50
8.52
7.64
8.50
4.02
3.93
8.11
7.68
7.65
7.67
7.64
8.31
8.46
8.08
7.70
3.94
3.93
8.06
8.68
8.68
a
CDCl3 at 400 MHz, referenced to residual CHCl3.
Ref. [18].
b
acetonitrile and pyridine ligands were soluble enough to
due to the positive charge of the complexes. The axial li-
gands only influence the H NMR chemical shift of H5
1
1
ensure characterization by 13C NMR. The 400 MHz H
NMR data for the series of Re(I) complexes are displayed
in Tables 1 and 2 for the chelate ligands and axial ligands,
respectively. The proton labelling scheme for ligands L1–
L3 is presented in Chart 2.
to a significant amount in complexes 1a–3a.
The 1H NMR spectra show that in most cases the signal
of H2 proton of the pyrimidine ring is shifted to lower field
with respect to the signal of the free ligand due to metal ion
complexation (Table 1). This effect is also noted for the 1H
chemical shifts of the substituents of the pyrimidine rings.
However, in the case of the H2 proton of complex 2a, an
opposite trend is noted; the proton is more shielded as
compared to that of its free ligand L2. The electron donat-
ing nature of the methoxy group may contribute to the
higher field resonance of H2 in 2a. In the neutral complexes
1a–3a, the second proton of the pyrimidine ring, H5, is
shifted to higher field as compared to the free ligand due
to metal ion complexation as the 4,40-bipyrimidine core
of the free ligand undergoes a trans-to-cis change in orien-
tation of the N atoms about the pyrimidine-pyrimidine
bond. Thus, the H5 proton is no longer deshielded by the
N lone pair of the other pyrimidine ring and it shifts up-
field. In 1b–3b, 1c–3c, and 2d, the effect is somewhat muted
Table 2
1H NMR resonances (ppm) for the axial ligand 4-substituted-pyridine in
complexes 1c–3c and 2da
Compound
Pyridine
R
Phenyl
t-Bu
2,6
3,5
20,60 30,40,50
4-Phenylpyridine (ppy)
4-t-Bupyridine (t-Bupy)
8.69 7.68 7.52 7.50
8.50 7.25
1.30
[Re(dpb)(CO)3(ppy)](PF6) (1c)
[Re(dmpb)(CO)3(ppy)](PF6) (2c)
[Re(dnb)(CO)3(ppy)](PF6) (3c)
8.22 7.59 7.79 7.50
8.23 7.61 7.69 7.50
8.86 7.64 7.86 7.52
[Re(dmpb)(CO)3(t-Bupy)](PF6) (2d) 8.12 7.42
1.45
Chart 2. 6,60-Disubstituted-4,40-bipyrimidines L1–L3 with their 1H NMR
labels.
a
CDCl3 at 400 MHz, referred to residual CHCl3.

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E. Ioachim et al. / Inorganica Chimica Acta 359 (2006) 2599–2607
For the pyridine axial ligands, an upfield shift of the pyr-
idine ring protons was observed in most cases (Table 2).
The downfield shift in the pyridyl resonances for 4-phenyl-
pyridine in 3c may be due to their proximity to the naph-
thyl groups on L3 as compared to the substituents on L1
0
0
and L2. The protons H2 ,6 of the phenyl substituent on
pyridine are more deshielded as compared with the free
4-substituted pyridine, while the other protons are less
influenced by coordination to the Re(I) centre.
2.3. X-ray crystal structures
The solid state structures of 3a and 2b were determined
by X-ray crystallography and are presented with their atom
labelling scheme in Figs. 1 and 2, respectively. Details of
data collection and refinement are given in Tables 3 and
4, respectively. Selected bond lengths and angles for the
molecules are listed in Tables 5 and 6, respectively. Both
complexes feature distorted octahedral coordination
around the Re(I) centre. The Re-to-pyrimidine nitrogen
(N1 and N3) bonds length range from 2.165(11) to
Fig. 2. X-ray structure of 2b as an ORTEP representation [24]. Thermal
ellipsoids are set at 30%. The hydrogen atoms, the anion and solvent of
crystallization have been omitted for clarity.
˚
2.194(2) A, which are typical for Re(I) complexes [23]. As
by the slightly shorter bond lengths in the Re–N chelate
of complex 2b as compared with 3a. For the carbonyls
trans to the chelate ligand, with which is defined the equa-
torial plane, the Re–COeq distances range between
shown in Fig. 1, the aryl groups in complex 3a exhibit
twists of 21.8° and 28.4° between each 1-naphthyl substitu-
ent and the pyrimidine ring, which breaks the planar
arrangement between them.
In the case of complex 2b, the p-methoxyphenyl substit-
uents are twisted with dihedral angles of 5.2° and 4.0°
around the pyrimidine rings. Furthermore, the two pyrim-
idine rings of the substituted bipyrimidine in 2b are almost
coplanar (0.4° between rings), which may promote better
delocalization of the p-electronic system and confer better
p-acceptor properties for this ligand, while the bpm ligand
in 3a displays an inter-ring angle of about 7.1°. Although
this view is in the solid state, the structure of 3a is clearly
more distorted than that of 2b. This is further supported
Table 3
Crystallographic data for 3a recorded at 100 K
Empirical formula
Molecular weight
Crystal system
Space group
C31H18ClN4O3Re
716.14
monoclinic
P21/c
18.6881(4)
22.4993(4)
13.0895(2)
108.7220(10)
5212.51(17)
8
˚
a (A)
˚
b (A)
˚
c (A)
b (°)
3
˚
V (A )
Z
Rint
R
0.034
0.0238
wR2
0.0678
Table 4
Crystallographic data for 2b recorded at 100 K
Empirical formula
Molecular weight
Crystal system
Space group
C27H21N5O5Re Æ 0.5(C2H3N) Æ (PF6)
847.18
triclinic
ꢀ
P1
˚
a (A)
13.1297(4)
15.3942(5)
18.9822(6)
113.6260(10)
101.3190(10)
93.9720(10)
3399.70(19)
4
˚
b (A)
˚
c (A)
a (°)
b (°)
c (°)
3
˚
V (A )
Z
Rint
R
0.043
0.0970
0.3549
Fig. 1. X-ray structure of 3a as an ORTEP representation [24]. Thermal
ellipsoids are set at 30%. The hydrogen atoms and solvent of crystalli-
zation have been omitted for clarity.
wR2

E. Ioachim et al. / Inorganica Chimica Acta 359 (2006) 2599–2607
2603
Table 5
Selected interatomic distances (A) for (dnb)Re(CO)3Cl (3a)
Table 7
Carbonyl ligand stretching frequencies of [(N–N)Re(CO)3X]n complexes
in acetonitrile solution
˚
Re–N(1)
Re–N(3)
Re–Cl(1)
Re–C(1)
Re–C(2)
Re–C(3)
C(1)–O(1)
2.194(2)
2.181(2)
2.4742(7)
1.934(3)
1.933(3)
1.910(3)
1.137(4)
C(2)–O(2)
C(3)–O(3)
O(1)–C(1)–Re
O(2)–C(2)–Re
O(3)–C(3)–Re
N(1)–Re–N(3)
1.141(4)
1.150(4)
178.9(3)
179.0(3)
176.2(3)
74.50(9)
Compound
Re(dpb)(CO)3Cl (1a)
Re(dmpb)(CO)3Cl (2a)
Re(dnb)(CO)3Cl (3a)
2033
2032
2029
2053
2041
2034
2038
2033
2025
2031
2032
2050
1933(br)
1926
1922
1945(br)
1937
1924
1923
1916(br)
1918
1921
1928
1907
1906
[Re(dpb)(CO)3MeCN](PF6) (1b)
[Re(dmpb)(CO)3MeCN](PF6) (2b)
[Re(dnb)(CO)3MeCN](PF6) (3b)
[Re(dpb)(CO)3(ppy)](PF6) (1c)
[Re(dmpb)(CO)3(ppy)](PF6) (2c)
[Re(dnb)(CO)3(ppy)](PF6) (3c)
[Re(dmpb)(CO)3(t-Bupy)](PF6) (2d)
Re(bpm)(CO)3Cla
1925
1908
1905
Table 6
1908
1916
1911
˚
Selected interatomic distances (A) and bond angles (°) for [(dmpb)Re-
(CO)3MeCN](PF6) (2b)
[Re(bpm)(CO)3MeCN] (PF6)a
1948(br)
Re–N(1)
Re–N(3)
Re–N(5)
Re–C(1)
Re–C(2)
Re–C(3)
N(5)–C(4)
C(1)–O(1)
2.179(10)
2.165(11)
2.074(13)
1.931(15)
1.909(13)
1.99(2)
C(2)–O(2)
C(3)–O(3)
1.164(15)
1.106(19)
175.70(14)
178.30(14)
171.90(17)
178.90(14)
74.2(4)
a
From [22].
O(1)–C(1)–Re
O(2)–C(2)–Re
O(3)–C(3)–Re
N(5)–C(4)–C(5)
N(1)–Re–N(3)
other hand, the p-donor Cl ligand increases the Re ! CO
back-bonding, which decreases the energy of the m(CO).
1.149(17)
1.167(17)
2.5. Mass spectrometry
˚
1.909(13) and 1.934(3) A, which agrees with the corre-
sponding values found for other Re-a-diimine carbonyls
[23]. In the case of the carbonyl in cis position to the chelate
ligand and referred to as the axial ligand, the distance Re–
COax is affected by the nature of the axial ligand, X. The
chlorine group is a good r-donor and favours p-back
donation from rhenium to the carbonyl in a trans configu-
Mass spectrometry analyses of all of the substituted pyr-
idine tricarbonyl complexes, as well as those with the ace-
tonitrile axial ligand, only displayed the signal
corresponding to the expected molecular mass. For the
chloride as axial ligand, the signals detected in the MS
spectra of the metal complexes corresponded to those of
the complexes in which the chloride ion had been substi-
tuted by an acetonitrile molecule. The latter solvent mole-
cule originates from the conditions used in the MS
technique.
˚
ration. Hence, the Re–C(3) in complex 3a (1.910(3) A) is
shorter than in 2b (1.99(2) A). The replacement of the chlo-
rine with MeCN in 2b also leads to a shortening of the
C(3)–O(3) bond (1.106(19) versus 1.150(4) A for chlorine
˚
˚
as the axial ligand) and a decrease of the Re–C(3)–O(3) an-
gle value (171.90(17)° versus 176.2(3)°), due to the weaker
donor and better acceptor properties of acetonitrile com-
pared to chlorine.
2.6. Electrochemistry
The electrochemical properties of the newly prepared
complexes have been studied by cyclic voltammetry. The
redox potentials are collected in Table 8. All of the
complexes display one oxidation and two reduction pro-
cesses in the potential region from +2.00 to ꢀ2.00 V.
The oxidation is associated with a metal-centred reaction
Re(I) ! Re(II) and is irreversible in all cases. As noticed
for other bidiazine Re(I) complexes [22], the formation
of the Re(II) species would allow for the loss of a
carbonyl ligand. However, the nature of the bidiazine
affects the oxidation potential values. The increasing in
p-acceptor properties of the bidiazine ligand leads to a
stabilization of the Re(I). Hence, the abstraction of the
electron from the metal centre occurs at more positive
potentials for complex 1a as compared with 2a or 3a.
Additionally, the replacement of the chloride with a
neutral axial ligand results in an anodic shift of Eox,
due to the positive charge of the complex and the weaker
donor nature of these ligands. The nature of the N-based
axial ligand affects the ease with which the metal is
oxidized: a stronger r-donor facilitates the oxidation
process in the following order: t-Bupy > ppy > MeCN.
2.4. Infrared spectroscopy
All of the newly synthesized complexes were character-
ized by vibrational spectroscopy in acetonitrile solutions
and the data are listed in Table 7. The infrared spectra
for all complexes exhibit three absorption bands in the car-
bonyl stretching region: a sharp intense band at about
2030–2040 cmꢀ1 and two lower-energy bands closely
spaced, typical of Re(I) a-diimine tricarbonyl complexes
[18,23]. A decrease in the splitting of the low-energy band
with the p-acceptor character of the axial ligand has been
noted. As a general trend, the shift of the CO stretching
bands to higher frequencies as the chelate ligand becomes
a better p-acceptor could be noted. The nature of the axial
ligand influences also, to some extent, the position of the
absorption bands. The m(CO) energy is slightly higher for
the better p-acceptor phenylpyridine (2c) as compared to
t-butylpyridine (2d). The m(CO) is further increased with
the weakly donating acetonitrile ligands in 1b–3b. On the

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E. Ioachim et al. / Inorganica Chimica Acta 359 (2006) 2599–2607
Table 8
Table 9
Half-wave potentials for [Re(N–N)(CO)3X]n+ complexesa
Spectroscopic and photophysical data at 298 K
Complex
E1/2(ox) E1/2(red)
Compound
Absorption kmax/nm Emission
(e/Mꢀ1 cmꢀ1
)
kmax (nm)
Re(dpb)(CO)3Cl (1a)b
1.08c
1.07c
1.07c
1.44c
1.47c
1.49c
1.42c
1.43c
1.45c
ꢀ0.66(111) ꢀ1.38(105)
Re(dmpb)(CO)3Cl (2a)b
ꢀ0.72(113) ꢀ1.35(100)
ꢀ0.75(105) ꢀ1.36(69)
Re(dpb)(CO)3Cl (1a)a
464 (2380)
460 (790)
635
561
529
520
509
504
528
523
519
550
662
Re(dnb)(CO)3Cl (3a)b
Re(dmpb)(CO)3Cl (2a)a
[Re(dpb)(CO)3MeCN](PF6) (1b)
[Re(dmpb)(CO)3MeCN](PF6) (2b)
[Re(dnb)(CO)3MeCN](PF6) (3b)
[Re(dpb)(CO)3(ppy)](PF6) (1c)
[Re(dmpb)(CO)3(ppy)](PF6) (2c)
[Re(dnb)(CO)3(ppy)](PF6) (3c)
ꢀ0.62(99)
ꢀ0.65(82)
ꢀ0.67(94)
ꢀ0.65(95)
ꢀ0.67(61)
ꢀ0.68(86)
ꢀ0.68(51)
ꢀ0.70(75)
ꢀ0.68(90)
ꢀ1.29c
Re(dnb)(CO)3Cl (3a)a
431 (2790)
388 (2000)
379 (11800)
380 (6000)
396 (3560)
386 (8590)
383 (3000)
ꢀ1.19c
[Re(dpb)(CO)3MeCN](PF6) (1b)b
[Re(dmpb)(CO)3MeCN](PF6) (2b)b
[Re(dnb)(CO)3MeCN](PF6) (3b)b
[Re(dpb)(CO)3(ppy)](PF6) (1c)b
[Re(dmpb)(CO)3(ppy)](PF6) (2c)b
[Re(dnb)(CO)3(ppy)](PF6) (3c)b
ꢀ1.21(101)
ꢀ1.31c
ꢀ1.21(105)
ꢀ1.19(99)
ꢀ1.26(65)
ꢀ1.31
[Re(dmpb)(CO)3(t-Bupy)](PF6) (2d) 1.40c
Re(bpm)(CO)3Cld
[Re(dmpb)(CO)3(t-Bupy)](PF6) (2d)b 388 (10000)
Re(bpm)(CO)3Clc
[Re(bpm)(CO)3MeCN](PF6)d
ꢀ1.26
458
a
a
From cyclic voltammetry in acetonitrile/0.1 M in TBAP, unless stated
otherwise. Potentials are in volts vs. SCE at a sweep rate of 200 mV/s. The
difference between cathodic and anodic peak potentials (mV) is given in
parentheses.
In deaerated chloroform solutions.
In deaerated CH3CN solutions.
From [18].
b
c
b
In DMF solutions.
Irreversible; potential is given for the cathodic wave.
From [22].
c
1
ligand and, thus, lowers the MLCT energy of its com-
plexes. The electron-releasing substituent –OMe on the
phenyl ring in the complexes 2a–d destabilizes the LUMO
orbitals of the bidiazine L2 to some extent, giving rise to
absorptions at slightly higher energies. In the case of the
1-naphthyl substituted compounds 3a–c, the naphthyl ring
does not favour a co-planar alignment of the naphthyl
and pyrimidyl rings, which limits the extent of p-electron
delocalization and consequently shifts the MLCT bands
to higher energy as compared to the other substituted
bidiazines.
The electron-donating/electron-withdrawing character
of the axial ligand also controls the energy of the
dp(Re) ! p*(bpm) MLCT transition. The p-donor power
of the chlorine axial ligand increases the charge density
on the Re(I), making it more electron rich; this fact serves
to lower the MLCT energy which involves the bpm ligands.
An opposite effect is noticed for the p-acceptor axial ligand,
which stabilizes the t2g metal-based orbitals and shifts the
absorption maxima to increase the MLCT energy.
The emission spectra of the complexes were recorded in
acetonitrile solution and are gathered in Table 9. The emis-
d
The two reversible or quasi-reversible reduction waves
observed in the cyclic voltammograms of all of the com-
plexes are bidiazine centred. In agreement with the low-ly-
ing p* molecular orbital of the 6,60-diphenyl-4,40-bpm, the
less negative reduction potential for this ligand is due to the
more extended p-conjugation as compared with the other
substituted bipyrimidines L2 and L3. The donor effect of
the methoxy group to the bpm ligand is illustrated by a
cathodic shift for the first reduction potential in the case
of the complexes with 6,60-bis(p-methoxyphenyl)-4,40-bpm
as chelate. The first reduction potential of complexes with
1-naphthyl substituent on the bipyrimidine is more nega-
tive than that of phenyl or p-methoxyphenyl substituents,
which may be due to the non-planarity of the 1-naphthyl
ring on the pyrimidine, which limits the extent of electron
delocalization. The variation of the axial ligand changes
the reduction potentials less than the oxidation potentials
of the complexes.
2.7. Spectroscopic properties
3
sion spectra have been attributed mainly to the MLCT
The electronic absorption spectral data of the prepared
complexes are summarized in Table 9.
dp p*(bpm) state. The emission maxima are red-shifted
with the expanding conjugated structure of the chelate li-
gand in going from the complexes having identical chemi-
cal environments. This shift is similar to that observed in
the electronic absorption spectra, following the order
dpb > dmpb > dnb. The electron releasing methoxy group
in the case of ligand L2 leads to a blue-shift compared to
L1, while the non-planar structure of L3 further shifts
the emission maxima to higher energy.
The absorption spectra are dominated in the UV
region by ligand-centred p ! p* transitions. Each of the
complexes displays moderately intense absorption bands
at lower energy (379–464 nm). These transitions are as-
signed to a dp(Re) ! p*(bpm) metal-to-ligand charge
transfer. The position of the low-energy maxima is sensi-
tive to the nature of the bidiazine ligand, shifting to
longer wavelengths as the degree of p delocalization in
the acceptor ligand increases. The MLCT absorption
bands of the dpb complexes 1a–c (464, 388 and 396 nm,
respectively) are red-shifted compared with those of the
dmpb or dnb complexes, 2a–d and 3a–c, respectively. This
is in accordance with the more extended p-conjugation of
the dpb ligand, which stabilizes the p* orbitals of the dpb
The variation of the axial ligand alters the energy of the
1
lowest excited state. Consistent with the MLCT assign-
ments, the emission maxima follow the order: Cl > t-Bu-
py > ppy > MeCN. The emission spectra of complexs 2
having the same bidiazine ligand dmbp and different axial
ligands display an increase in emission energy with an in-
crease in p-acceptor ability of the axial ligand.

E. Ioachim et al. / Inorganica Chimica Acta 359 (2006) 2599–2607
2605
3. Conclusion
5. Preparation of compounds
The electrochemical and spectroscopic properties of a
new series of bidiazine-based Re(I) tricarbonyl complexes
have been investigated. Disubstitution of the 4,40-bipyr-
imidine ligand in the 6,60-positions is an efficient way
to tune the energy of the ground and excited states
and, consequently, the spectroscopic and electrochemical
properties of the complexes. The delocalization of the
p-electronic system in the chelate ligand lowers the energy
of the LUMO orbitals and red-shifts the absorption and
emission bands, and allows more facile reduction of the
complexes. Furthermore, variation of the axial ligand al-
ters the energy of the lowest excited state by modifying
the electron density on Re(I) centre. Thus, Re(I) tricar-
bonyl complexes are more flexible towards potential che-
lating ligands as they still maintain higher energy excited
states than Ru(II) or Os(II) complexes even with the
most p-accepting bidentate ligands. We are currently
investigating the incorporation of these Re(I) complexes
into larger polymetallic assemblies as the high energy vis-
ible light absorbers and will report on this work in due
course.
5.1. General procedure for the synthesis of
(N–N)Re(CO)3Cl (1a–3a)
5.1.1. Re(dpb)(CO)3Cl (1a)
A solution of 6,60-diphenyl-4,40-bipyrimidine (0.06 g,
0.19 mmol) and Re(CO)5Cl (0.07 g, 0.19 mmol) in toluene
was heated to reflux for 2 h. The precipitate that formed
on cooling was dissolved in chloroform and was reprecipi-
tated by addition of hexane and isolated by filtration.
1
Yield: 39%. H NMR (400 MHz, CDCl3, 25 °C): d 9.82
(s, 2H, H2), 8.77 (s, 2H, H5), 8.54 (d, J = 3.5 Hz, 4H,
+
0
0
0
0
0
H
2 ,6 ), 7.71 (m, 6H, H3 ,4 ,5 ). HRMS: calcd. [M] for
C25H17N5O3Re (MꢀCl+C2H3N), 622.0883; found,
622.0862.
5.1.2. Re(dmpb)(CO)3Cl (2a)
1
Yield: 84%. H NMR (400 MHz, CDCl3, 25 °C): d 9.12
(s, 2H, H2), 8.73 (s, 2H, H5), 8.29 (d, J = 8.8 Hz, 4H,
0
0
0
0
H
2 ,6 ), 7.14 (d, J = 8.8 Hz, 4H, H3 ,5 ), 4.02 (s, 6H, O-
Me). HRMS: calcd. [M]+ for C27H21N5O5Re
(MꢀCl+C2H3N), 682.1094; found, 682.1114.
5.1.3. Re(dnb)(CO)3Cl (3a)
1
4. Experimental
Yield: 73%. H NMR (400 MHz, CDCl3, 25 °C): d 9.90
0
(s, 2H, H2), 8.51 (s, 2H, H5), 8.31 (d, J = 6.4 Hz, 2H, H8 ),
4.1. Materials
0
0
8.11 (d, J = 8.1 Hz, 4H, H2 ,4 ), 7.85 (d, J = 6.1 Hz, 2H,
0
0
0
0
H5 ), 7.65 (t, J = 6.5 Hz, 6H, H3 ,6 ,7 ). HRMS: calcd.
[M]+ for C33H21N5O3Re (MꢀCl+C2H3N), 722.1196;
found, 722.1191.
The ligands R2bpm were synthesized according to a
general procedure [25]. Tetrabutylammonium hexafluoro-
phosphate was purchased from Aldrich and purified by
recrystallization from toluene–methanol (20/1). Acetoni-
trile and dimethylformamide for photophysical and
electrochemical experiments were of spectroscopic grade.
High-resolution mass spectrometry analyses were
5.2. General procedure for the synthesis of
[(N–N)Re(CO)3MeCN](PF6) (1b–3b)
´
performed by Laboratoire de spectrometrie de masse de
5.2.1. [Re(dpb)(CO)3MeCN](PF6) (1b)
´
´
lꢀUniversite de Montreal.
To a solution of complex 1a (0.07 g, 0.22 mmol) in ace-
tonitrile was added 1 equiv. of AgPF6 (0.056 g, 0.2 mmol)
and the mixture was refluxed overnight. The resulting solu-
tion was filtered and a saturated aqueous solution of
NH4PF6 was added to the filtrate. Partial removal of the
solvent and addition of water gave a brown-yellow precip-
itate in about 45% yield after filtration. 1H NMR
(400 MHz, CDCl3, 25 °C): d 9.59 (s, 2H, H2), 8.97 (s, 2H,
4.2. Instrumentation
Nuclear magnetic resonance (NMR) spectra were re-
corded in CDCl3 and CD3CN at room temperature
(r.t.) on a Bruker AV400 spectrometer at 400 MHz for
1H NMR and at 100MHz for 13C NMR. Chemical shifts
are reported in part per million (ppm) relative to residual
solvent protons (7.26 ppm for chloroform-d and
1.93 ppm for acetonitrile-d3) and carbon resonance of
the solvent. Electronic absorption and emission spectra
were recorded on a Cary 500i UV–Vis–NIR spectropho-
tometer and a Cary Eclipse fluorescence spectrophotom-
eter, respectively. Electrochemistry data were collected in
deaerated acetonitrile or DMF with 0.1 M Bu4NPF6 on
a BAS CV-50 W Voltammetric Analyzer. Redox poten-
tials were corrected by internal reference ferrocene
(395 mV for acetonitrile versus SCE and 432 mV for
DMF versus SCE).
0
0
H5), 8.41 (d, J = 3.5 Hz, 4H, H2 ,6 ), 7.68 (m, 6H,
0
0
0
H
3 ,4 ,5 ). HRMS: calcd. [M]+ for C25H17N5O3Re,
622.0883; found, 622.0870.
5.2.2. [Re(dmpb)(CO)3MeCN](PF6) (2b)
1
Yield: 58%. H NMR (400 MHz, CDCl3, 25 °C): d 9.52
(s, 2H, H2), 8.88 (s, 2H, H5), 8.43 (d, J = 8.8 Hz, 4H,
0
0
0
0
H
2 ,6 ), 7.11 (d, J = 8.8 Hz, 4H, H3 ,5 ), 3.93 (s, 6H, O-
Me). 13C NMR (75 MHz, CDCl3, 25 °C): d 167.4, 164.8,
161.6, 151.8, 148.8, 130.8, 126.9, 115.5, 115.4, 56.0, 28.9,
3.46. HRMS: calcd. [M]+ for C27H21N5O5Re, 682.1094;
found, 682.1092.

2606
E. Ioachim et al. / Inorganica Chimica Acta 359 (2006) 2599–2607
4H, H3 ,5 ), 3.93 (s, 6H, O-Me), 1.45 (s, 9H, t-Bu). 13C
NMR (75 MHz, CDCl3, 25 °C): d 168.3, 165.7, 165.3,
160.7, 160.3, 151.5, 136.1, 131.7, 130.8, 126.2, 125.9,
125.0, 115.5, 56.0, 35.8, 34.6. HRMS: calcd. [M]+ for
C34H31N5O5Re, 776.18772; found, 776.18720.
0
0
5.2.3. [Re(dnb)(CO)3MeCN](PF6) (3b)
1
Yield: 76%. H NMR (400 MHz, CDCl3, 25 °C): d 9.77
0
(s, 2H, H2), 8.77 (s, 2H, H5), 8.46 (d, J = 6.4 Hz, 2H, H8 ),
0
0
8.08 (d, J = 8.1 Hz, 4H, H2 ,4 ), 7.97 (d, J = 6.1 Hz, 2H,
0
0
H5 ), 7.67 (t, J = 6.5 Hz, 2H, H7 ), 7.59 (t, J = 6.5 Hz,
0
0
4H, H3 ,6
)
13C NMR (75 MHz, CDCl3, 25 °C): 171.2,
170.8, 160.9, 159.8, 133.4, 132.7, 132.5, 130.1, 129.7,
128.4, 128.0, 126.5, 125.2, 124.1, 121.5, 13.8, 3.1. HRMS:
calcd. [M]+ for C33H21N5O3Re, 722.1196; found, 722.1185.
6. Supplementary data
CCDC 263105 and 263106 contain the supplementary
crystallographic data for this paper. These data can be ob-
tained free of charge from The Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
5.3. General procedure for the synthesis of
[(N–N)Re(CO)3(ppy)](PF6) (1c–3c)
5.3.1. [Re(dpb)(CO)3(ppy)](PF6) (1c)
Acknowledgements
A solution of 1b (0.05 g, 0.06 mmol) in THF was heated
overnight with 10 equiv. of ppy (0.08 g, 0.6 mmol). After
cooling, the addition of water precipitated the complex as
hexafluorophosphate salt, which was isolated by filtration.
We thank the Natural Sciences and Engineering Re-
´
search Council (NSERC) of Canada and the Universite
´
de Montreal for financial support.
1
Yield: 84%. H NMR (400 MHz, CDCl3, 25 °C): d 9.72 (s,
References
0
0
2H, H2), 9.13 (s, 2H, H5), 8.50 (d, J = 3.5 Hz, 4H, H2 ,6 ),
0
0
8.22 (d, J = , 2H, H2,6-py), 7.79 (d, J = , 2H, H2 ,6 -phenyl-
[1] A. Juris, F. Barigelletti, S. Campagna, V. Balzani, P. Belser, A. von
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0
0
0
py), 7.70 (m, 6H, H3 ,4 ,5 ), 7.59 (d, J = , 2H, H3,5-py), 7.50
(m, 3H, H3 ,4 ,5 -phenyl-py). 13C NMR (75 MHz, CDCl3,
25 °C): d 160.5, 154.2, 152.1, 151.0, 136.1, 134.5, 129.8,
129.6, 129.5, 129.4, 125.9, 123.6, 123.1, 117.8. HRMS: calcd.
[M]+ for C34H23N5O3Re, 736.13529; found, 736.13499.
0
0
0
[3] (a) T.J. Meyer, Acc. Chem. Res. 22 (1989) 163;
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5.3.2. [Re(dmpb)(CO)3(ppy)](PF6) (2c)
1
Yield: 76%. H NMR (400 MHz, CDCl3, 25 °C): d 9.60
(s, 2H, H2), 8.94 (s, 2H, H5), 8.52 (d, J = 8.8 Hz, 4H,
0
0
0
0
H
2 ,6 ), 8.23 (d, J = , 2H, H2,6-py), 7.69 (d, J = , 2H, H2 ,6
-
phenyl-py), 7.61 (d, J = , 2H, H3,5-py), 7.50 (m, 3H,
0
0
0
0
0
H
3 ,4 ,5 -phenyl-py), 7.10 (d, J = 8.8 Hz, 4H, H3 ,5 ), 3.93 (s,
6H, O-Me). 13C NMR (75 MHz, CDCl3, 25 °C): d 168.3,
160.3, 152.6, 151.9, 150.6, 148.8, 138.5, 136.1, 131.7, 131.2,
129.5, 127.4, 124.1, 124.0, 122.1, 115.6, 56.1. HRMS: calcd.
[M]+ for C36H27N5O5Re, 796.15642; found, 796.15651.
5.3.3. [Re(dnb)(CO)3(ppy)](PF6) (3c)
1
Yield: 63%. H NMR (400 MHz, CDCl3, 25 °C): d 9.90
(s, 2H, H2), 8.89 (s, 2H, H5), 8.86 (d, J = , 2H, H2,6-py),
0
8.68 (d, J = 6.4 Hz, 2H, H8 ), 8.06 (d, J = 8.1 Hz, 4H,
0
0
0
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H
H
2 ,4 ), 8.00 (d, J = 6.1 Hz, 2H, H5 ), 7.86 (d, J = , 2H,
0
0
0
0
0
2 ,6 -phenyl-py), 7.64 (m, 8H, H3 ,6 ,7 , H3,5-py), 7.59 (t,
0
0
0
0
0
J = 6.5 Hz, 4H, H3 ,6 ), 7.52 (m, 3H, H3 ,4 ,5 -phenyl-py).
HRMS: calcd. [M]+ for C42H27N5O3Re, 836.1666; found,
836.1657.
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gna, Inorg. Chim. Acta. (2005) in press.;
5.3.4. [Re(dmpb)(CO)3(t-Bupy)](PF6) (2d)
A solution of 2b (0.05 g, 0.06 mmol) in THF was heated
overnight with 10 equiv. of t-Bupy (0.08 mL, 0.5 mmol).
After cooling, the addition of water precipitated the com-
1
plex as hexafluorophosphate salt. Yield: 70%. H NMR
(400 MHz, CDCl3, 25 °C): d 9.58 (s, 2H, H2), 8.83 (s, 2H,
(c) E. Ioachim, G.S. Hanan, Can. J. Chem. 83 (2005) 1114.
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0
0
H5), 8.50 (d, J = 8.8 Hz, 4H, H2 ,6 ), 8.12 (d, J = , 2H,
H
2,6-py), 7.42 (d, J = , 2H, H3,5-py), 7.00 (d, J = 8.8 Hz,

E. Ioachim et al. / Inorganica Chimica Acta 359 (2006) 2599–2607
2607
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