Rhenium(I) complexes of readily functionalized bidentate pyridyl-1,2,3-triazole "click" ligands: A systematic synthetic, spectroscopic and computational study

Article abstract of DOI:10.1016/j.poly.2012.05.003

A family of electronically tuned fac-Re(CO)₃Cl pyridyl-1,2,3-triazole complexes have been synthesized by refluxing methanol solutions of [Re(CO)₅Cl] and the substituted 2-(1-R-1H-1,2,3-triazol- 4-yl)pyridine ligands (pytri-R). The re

Full text of DOI:10.1016/j.poly.2012.05.003

Polyhedron 52 (2013) 1391–1398
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Polyhedron
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Rhenium(I) complexes of readily functionalized bidentate pyridyl-1,2,3-triazole
‘
‘click’’ ligands: A systematic synthetic, spectroscopic and computational study
a
a,b
a
a
a,b,
⇑
Tae Y. Kim , Anastasia B.S. Elliott , Karl J. Shaffer , C. John McAdam , Keith C. Gordon
,
a,
⇑
James D. Crowley
a
Department of Chemistry, University of Otago, P.O. Box 56, Dunedin, New Zealand
MacDiarmid Institute for Advanced Materials and Nanotechnology, New Zealand
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Available online 15 May 2012
3
A family of electronically tuned fac-Re(CO) Cl pyridyl-1,2,3-triazole complexes have been synthesized by
refluxing methanol solutions of [Re(CO)
5
Cl] and the substituted 2-(1-R-1H-1,2,3-triazol-4-yl)pyridine
ligands (pytri-R). The resulting rhenium(I) complexes were characterised by elemental analysis, HR-ESMS,
This paper is dedicated to Alfred Werner on
the 100th Anniversary of his Nobel prize in
Chemistry in 1913.
1
13
IR, H and C NMR and in one case the molecular structure was confirmed by X-ray crystallography. The
electronic structure of this family of fac-[(pytri-R)Re(CO) Cl] complexes was probed using UV–Vis, Raman
3
and emission spectroscopy and cyclic voltammetry techniques. The complexes show intense absorptions
⁄
in the visible region, comprising strong
p
?
p
and metal-to-ligand charge-transfer (MLCT) transitions,
Keywords:
‘
‘Click’’ chemistry
which were modelled using time-dependent density functional theory (TD-DFT). Interestingly, the MLCT
transition energy and the emission maxima are unaffected by the nature of the R substituent on the 2-(1-
R-1H-1,2,3-triazol-4-yl)pyridine ligand indicating that the 1,2,3-triazoyl unit is acting as an electronic
insulator. The emission lifetimes of the complexes are modestly dependent on the nature of the 1,2,3-tri-
azole substituent, with the conjugated complexes displaying longer lifetimes than the non-conjugated
ones. The shorter lifetimes for complexes with non-conjugated ligands are attributed to the ‘‘free-rotor’’
effect which allows molecules to relax through non-radiative pathways. In this case, the freely rotating
Rhenium
Emission lifetimes
Resonance Raman
Electrochemistry
2
CH group located between the triazole and the R group causes the decrease in excited lifetime. The elec-
trochemistry of all examples is defined by irreversible Re oxidation and triazole based ligand reduction
processes. The nitro substituted complexes show additional nitrobenzene type reduction features. Simi-
larly, the ferrocenyl substituted complex displays the expected reversible one electron oxidation process.
Consistent with the spectroscopic data, the position of the oxidation and reduction processes are essen-
tially unaffected by the electronic nature of the 2-(1-R-1H-1,2,3-triazol-4-yl)pyridine substituent.
Ó 2012 Elsevier Ltd. All rights reserved.
1
. Introduction
Rhenium was not officially known when Alfred Werner was
yl)pyridine (pytri-R) family of ligands have recently emerged as
0
tunable 2,2 -bipyridyl analogues [11–13], because they can be
I
readily synthesized using the Cu -catalyzed 1,3-cycloaddition of
organic azides with terminal alkynes (the CuAAC reaction) [14–
awarded his Nobel Prize in 1913. In the intervening century the
element rhenium was discovered and the coordination chemistry
for Re was developed. Complexes of rhenium are extensively used
I
I
II
II
18]. Complexes with Ag [19–21], Cu [21], Cu [19,22,23], Pd
I
II
II
II
II
III
[24–27], Pt [24], Zn [28], Pb [29], Ru [30–34], and Ir [34–
38] have been synthesized and structurally characterized.
in catalysis [1,2] and recently there has been considerable interest
in fac-[(
a
-diimine)Re(CO)
3
Cl] complexes, especially those contain-
A number of these studies have examined electronic structure
and photophysical properties of the 2-(1-R-1H-1,2,3-triazol-4-
yl)pyridine complexes. Many of these studies have focused on
the effect of the triazole N1-substituent on the properties of the
0
ing 2,2 -bypyridine (bpy) and 1,10-phenanthroline, as they have
been shown to have interesting photophysical properties [3–5].
These fac-[(
as photocatalysts for the reduction of CO
and as bioimaging agents [9,10]. The 2-(1-R-1H-1,2,3-triazol-4-
a-diimine)Re(CO)
3
Cl] complexes have been exploited
2
[6], as sensors [5,7,8]
complex. Felici et al. [34] reported the emission properties of com-
+
plexes of [Ir(ppy)
2
(pytri-R)] where the triazole–pyridine ligand
contained either adamantyl or cyclodextrin (CD) substituents.
The emission from these compounds originated from the Ir(ppy)
luminophore. The complex with the CD-substituent showed a very
high quantum yield for emission; which was attributed to the pro-
tection afforded to the complex by this bulky group. In a detailed
⇑
Corresponding authors. Tel.: +64 3 4797731; fax: +64 3 4797906 (J.D. Crowley).
E-mail addresses: Kgordon@chemistry.otago.ac.nz (K.C. Gordon), jcrowley@
chemistry.otago.ac.nz (J.D. Crowley).
0
277-5387/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.poly.2012.05.003

1
392
T.Y. Kim et al. / Polyhedron 52 (2013) 1391–1398
+
study of [Ir(F
2
ppy)
2
(pytri-R)] complexes (where F
2
ppy is difluoro-
2
Ruthenium complexes with the DNA active moiety [Ru(TAP) (-
pytri-R)] (where TAP = 1,4,5,8 tetraazaphenanthracene) have also
been reported [43]. The most notable effect of pytri-R ligand is that
2
+
phenyl-pyridine based and pytri-R is N1 substituted with adaman-
tyl, benzyl, biphenyl and phenyl substituents) Mydlak et al. [38]
show that emission for each of these complexes is predominantly
ppy) based. The triazole substituent however does have an ef-
fect on the photophysics of the complex. It is noted that the emis-
sion lifetime for the biphenyl substituted complex is almost three
the Ru d
p
orbitals are stabilised by ꢁ 50 mV relative to L = 1,10-
Ir(F
2
phenanthroline; presumably a consequence of poorer overlap be-
tween the d
⁄
p
and
p
(pytri-R) MOs.
In a study of pytri-OBu it was found that presence of alkaline
and selected first row transition metals resulted in varying levels
of emission intensity but no tuning of the emission which was ob-
served at 500 nm [44].
times longer than for the other complexes. This is attributed to the
⁄
lowering in energy of the
p
MO on the pytri-R ligand that results
3
in greater mixing of states and an increase in the LC character of
the lowest excited state and thus an increased lifetime. It was pos-
sible to use these compounds in the fabrication of OLEDs with very
Altering the N1 substituent does not appear to radically shift
the emission spectrum for [(pytri-R)Re(CO)
example the emission maxima for substituted [(pytri-R)Re(CO)
(where R = (CH CO H) lies at 526 nm in DMF/MeOH [45] and
3
Cl] complexes. For
blue emission properties. Liu et al. [37] report the photophysics of
3
Cl]
(pytri-R)]+ and [Ir(F
ppy)
(pytri-R)] complexes with R
+
)
2
[
Ir(ppy)
2
2
2
2
2
being fluoroalkyl, fluorophenyl and fluorotolyl substituents. The
photophysical properties reveal emissive states of Ir(ppy) nature.
Happ et al. [32] showed that for ruthenium(II) complexes with
with R = (2-methoxyphenyl)piperazine it is reported at 522 nm
(in MeOH) [46]. It is notable that the DFT calculations conducted
on these systems show modest contribution from the N1 substitu-
ent in the LUMOs.
Herein we extended the work of Obata [39] and Benoist [45,46]
3
and generate a family of electronically tuned fac-Re(CO) Cl 2-(1-R-
1H-1,2,3-triazol-4-yl)pyridine complexes. Efficient safe one-pot
CuAAC ‘‘click’’ methodologies were exploited to synthesize a series
of electronically modified 2-pyridyl-1,2,3-triazole ligand architec-
tures which were readily converted into the corresponding family
0
0
4
,4 -dimethyl-2,2 -bypyridine (dmbpy) and pytri-R ligands, of the
2+
form [Ru(dmbpy)3ꢀn(pytri-R)
Ru MLCT chromophore unless n = 3, when emission from the Ru
pyridyl-1,2,3-triazole MLCT is blue-shifted by up to 29 nm in n-
n
]
, emission occurs from the dmbpy
3
3
butyronitrile glass at 77 K. These studies provide evidence that
the pyridyl-1,2,3-triazole and the substituents borne on the N1 po-
sition can affect the electronic structure and emission properties of
complexes. A more direct way to probe this interaction is to study
emissive complexes in which the chromophore and luminophore
are metal (pyridyl-1,2,3-triazole) based. This may be explored
of fac-[Re(CO)
tronic structure of a series of fac-[(pytri-R)Re(CO)
in which the substituents are capable of conjugative coupling with
the triazole and insulated via a CH group from such communica-
3
Cl] complexes. In this systematic study the elec-
3
Cl] complexes
using [Re(CO)
CO) Cl] complexes several studies have been conducted. Obata
et al. [39] synthesized [(pytri-R)Re(CO) Cl] where R = benzyl or a
3
Cl] as a binding unit. In the case of [(pytri-R)Re(-
2
3
tion are examined. In order to probe the nature of the lowest en-
ergy transition resonance Raman spectroscopy was utilised in
concert with DFT calculations. The nature of the lowest excited
state was established using time-resolved emission spectroscopy.
3
sugar unit. They studied the electronic structure of these systems
using DFT and electronic absorption and emission spectroscopic
methods. The TD-DFT calculations reveal that the HOMO?LUMO
transition was MLCT in nature and was well modelled. This pre-
dicted that the MLCT transition would be to the blue of the Re ?
bpy MLCT, which is observed. The emission from Re ? pytri-R
2. Results and discussion
3
MLCT (where R = benzyl) was also blue-shifted (at 538 nm in
2.1. Ligand synthesis and characterization
3
CH CN) compared to the Re(bpy) luminophore (which lies at
6
33 nm).
We have previously used efficient one-pot CuAAC ‘‘click’’ meth-
odologies to synthesize a range of substituted 2-(1-R-1H-1,2,3-
triazol-4-yl)pyridine (1a, 1b, 1e and 1f) ligands [19,20,24,25]. The
same methodologies were exploited to generate a family of elec-
tronically tuned 2-pyridyl-1,2,3-triazole ligand architectures
(Scheme 1). We have described [19,24,25] the synthesis of the 2-
pyridyl-1,2,3-triazole ligands 1a (R = octyl) and 1b (R = Bn) using
standard Fokin [47] conditions and found the same methodology
could be applied to the synthesis of the electronically modified
benzyl substituted ligands, 1c (R = 4-methoxybenzyl) and 1d
(R = 4-nitrobenzyl). Simply mixing 2-ethynylpyridine, the appro-
The ability of pytri-R to facilitate electronic communication be-
tween differing emitting centres has been investigated in an ele-
gant systematic fashion by Rowan et al. [36] In this study
0
0
[
Ir(ppy) ] and Ir(C^N ) (where C^N is 1-phenylpyrazole) units
2 2
were bound to pytri-R and these ligands in turn were connected
via the (N1) sp triazole nitrogen to form dimer and trimer struc-
tures. This study found that the photophysical properties of the
individual Ir units were unperturbed by the modulation of linker
groups through the N1 sp triazole nitrogen. Gratzel et al. [40] have
2
2
shown that an analogue to the N719 dye [41] (used in dye-sensi-
tised solar cells) with pytri-R ligand [Ru(pytri-R)(bpy
priate benzyl bromide/chloride, NaN
3
4 2 2 3
, CuSO ꢂ5H O, Na CO and
(
CO
H)
2 2
)(CNS)
2
] provides a cell with 7% efficiency. This is impres-
L-ascorbic acid in DMF/H O (4:1) then stirring at room tempera-
2
sive in view of the poor spectral coverage the pytri-R dye affords
in comparison to N719. In a DFT analysis it is shown that the LUMO
and LUMO+1 of [Ru(pytri-R)(bpy(CO H) )(CNS) ] are based on the
2 2 2
ture (1c and 1d) for 20 h provided, after a simple work up, the de-
sired ligands (86–92%, Scheme 1(i)) without the need for isolating
the potentially hazardous azide intermediates.
bpy ligand. Happ et al. [42] have carried out a systematic study of
The methods of Liang [48,49] and Guo/Aldrich [50,51] have
previously been applied for the synthesis of the aryl substituted
2-(1-R-1H-1,2,3-triazol-4-yl)pyridine (1g and 1h) ligands. As
4-iodoanisole is commercially available we elected to use the
conditions of Liang and co-workers [48,49] to generate the 4-
substituent effects on the ruthenium complexes with a substituted
+
pytri-R ligand [Ru(dmbpy)
2
(pytri-R)]2 . Substitution with ethynyl
phenyl groups occurs at either the 5-pyridyl or N1 triazole posi-
tions. Substitution of the 5-pyridyl position has a striking effect
on the emission properties – tuning the emission wavelength from
methoxyphenyl substituted ligand 1g. A mixture of 4-iodoanisole,
0
6
02 nm with 1-ethynyl-4-methoxybenzene to 674 nm with 1-
3
NaN , sodium
L
-ascorbate, N,N -dimethylethylenediamine and CuI,
ethynyl-4-nitrobenzene. However the 1-ethynyl-4-nitrobenzene
substituted at the N1 triazole nitrogen results in an emission at
were heated at 100 °C for 1.5 h in EtOH/H
arylazide intermediate in situ (Scheme 1(ii)). The reaction mixture
was cooled to room temperature, 2-ethynylpyridine, CuSO O,
ꢂ5H
-ascorbate were added and the Cu -catalysed 1,3-
cycloaddition reaction proceeded at room temperature over 16 h
2
O (2:1) to generate the
6
10 nm – effectively the potent electron withdrawing power of
4
2
I
this substituent is insulated from the Ru centre by linkage through
the triazole.
and sodium L

T.Y. Kim et al. / Polyhedron 52 (2013) 1391–1398
1393
nylazide had been shown to form in high yield (85%) [50] so the
I
problem appeared to be during the Cu -catalysed 1,3-cycloaddition
step. This could potentially be due to the electron withdrawing ni-
tro group deactivating the azide. Alternatively, the low yield could
be connected the instability of nitro substituted arylazide as has
been observed before [52].
The new ligands (1c and 1d) and the known ligands synthesized
using in situ methodologies (1g and 1h) have been characterized by
1
13
elemental analysis, HR-ESMS, IR, H and C NMR spectroscopy. For
1
example, the diagnostic singlet, in the H NMR spectra of the li-
gands, corresponding to the triazole unit was found between 8.7
and 8.4 ppm (see Supporting Information).
2.2. Synthesis of Re(I) complexes
Obata [39] and Benoist [45,46] have independently synthesized
a series of fac-[Re(CO)
,2,3-triazol-4-yl)pyridines [R = Bn (2b), COOCH
glucose, and CH CH -(2-methoxyphenyl)piperazine] by simply
refluxing methanol solutions of [Re(CO) Cl] and the substituted
-(1-R-1H-1,2,3-triazol-4-yl)pyridine ligands. Exploiting the same
3
Cl] complexes of substituted 2-(1-R-1H-
1
3
, COOH, CH CH
2
2
-
2
2
5
2
methodology we generated the family of electronically tuned rhe-
nium(I) complexes (2a–h). One of the ligands (1a–h, 1 equiv) was
added to a methanol solution of [Re(CO)
resulting reaction mixture was heated at reflux for 6–20 h
Scheme 1). This provided complexes (2a–h) in good to excellent
yields (63–86%) [39,45,46]. The rhenium(I) compounds have been
5
Cl] (1 equiv) and the
(
characterized by elemental analysis, HR-ESMS, IR, 1H and
13
C
NMR spectroscopy. Elemental analysis confirmed that the isolated
powders were pure and indicated that the bidentate ‘‘click’’ ligands
(1a–h) formed rhenium(I) complexes with a 1:1 metal/ligand ratio.
Consistently, the HR-ESMS spectra (dissolution in DMSO followed
by methanol elution) showed the presence of the neutral com-
+
+
1
plexes ionised as Na adducts (fac-[(pytri-R)Re(CO)
NMR spectra of the rhenium(I) complexes were recorded at room
temperature in d -DMSO (see Supporting Information) and when
3
Cl](Na) ). H
6
compared with the spectra of the corresponding ligands, the pyri-
dyl and triazole proton signals of the complexes were shifted
downfield indicative of metal complexation (Fig. S1 in Supporting
Information). Three strong
in the region of 2029–1882 cm , indicative of the presence of
the fac-[Re(CO) Cl] core [39,45,46]. Unequivocal proof of this facial
fac) coordination geometry was obtained from X-ray crystallogra-
phy (vide infra).
m(CO) stretching bands are observed
ꢀ1
3
(
The X-ray crystal structure of 2c was obtained using single crys-
tals grown by vapour diffusion of diethyl ether into an acetonitrile
solution of the complex. The complex crystallised in the triclinic
space group and showed the expected coordination geometry about
the rhenium(I) centre (Fig. 1, Table S1 in Supporting Information).
The bond lengths and angles for 2c are similar to those previously
observed for 2b [39] (Table S2 in Supporting Information).
Scheme 1. (i) NaN
3
, CuSO
5 °C, 20 h; (ii) (a) NaN , sodium
00 °C, 1.5 h, (b) CuSO ꢂ5H
5 °C, 2 h, (b) sodium
4
ꢂ5H
2
O, ascorbic acid, Na
2
CO
3
, DMF/H
2
O (4:1), RT or
0
9
1
5
3
L
-ascorbate, N,N -dimethylethylenediamine, CuI,
4
2
O, sodium
L
-ascorbate; (iii) (a) NaN
3 2
, Cu(OAc) , MeOH,
2.3. Electronic structure
L
-ascorbate, RT, 16 h; (iv) [Re(CO) Cl], 70 °C, 6 h or 20 h.
5
3
The electronic structure of the fac-[(pytri-R)Re(CO) Cl] com-
to give 1g in modest yield (45%). The method of Aldrich [50] was
exploited to convert 4-nitrophenylboronic acid into the nitro-
phenyl substituted ligand 1h (Scheme 1(iii)). Stirring 4-nitro-
plexes was investigated using both computational and spectro-
scopic methods. The veracity of computational methods may be
ascertained by comparison of calculated parameters with experi-
mental data. We have used X-ray crystallographic data, where
available, to compare to the calculated structures and FT-Raman
spectra compared to simulated spectra obtained through fre-
quency calculations.
DFT calculations of a number of bond lengths and angles are
compared to experimental values for 2c in Table 1. They show that
DFT models the molecular geometry with all but one bond length
differing by <0.0024 Å and angles by <3°.
phenylboronic acid, NaN
5 °C resulted in the in situ generation of the azido intermediate.
After cooling to room temperature, sodium -ascorbate and 2-eth-
3 2
, and Cu(OAc) in dry MeOH for 2 h at
5
L
ynylpyridine were added and the resulting suspension was stirred
at room temperature for a further 16 h, yielding the desired ligand
1
h in modest yield (40%), but without the need for isolating the
potentially hazardous azide intermediate. The yield of 1h (40%)
was depressed relative to 1e (84%). The corresponding 4-nitrophe-

1
394
T.Y. Kim et al. / Polyhedron 52 (2013) 1391–1398
Fig. 1. ORTEP drawing of 2c with thermal ellipsoids shown at 50% probability.
Selected bond lengths (Å) and angles (°) for 2c: N1–Re1 2.2002 (54), N2–Re1 2.1557
(
1
(
54), Cl1–Re1 2.5051 (18), C22–Re1 1.9271 (87), C21–Re1 1.9236 (74), C20–Re1
.9141 (73); N1–Re1–N2 73.88 (20), N1–Re1–Cl1 84.98 (16), N2–Re1–C22 96.37
25), C22–Re1–Cl1 176.49 (21), C20–Re1–C21 89.39 (29), C21–Re1–C22 88.67 (29).
Fig. 2. Comparison of experimental (upper, black trace) FT-Raman spectrum of 2b
with the simulated spectrum (lower, red trace). (Colour online.)
Table 1
Comparison of bond lengths (Å) and angles (°) for 2c obtained from X-ray crystal
structure data and DFT calculations using the B3LYP method and LANL2DZ basis set
for Re and the 6-31g(d) basis set for the remaining molecules. Numbering is given in
Fig. 1.
and observed spectral behaviour is that density functional theory
systematically overestimates conjugative effects. Thus, in this case
the effect of the conjugated group is predicted to be stronger than
observed [66]. It should be noted that TD-DFT calculations are of-
ten shifted compared to experimental observation by up to
0.4 eV [67].
Bond/angle
X-ray
Calculation
N1–Re1
N2–Re1
Cl1–Re1
C22–Re1
C21–Re1
C20–Re1
N1–Re1–N2
N1–Re1–Cl1
N2–Re1–C22
C22–Re1–Cl1
C20–Re1–C21
C21–Re1–C22
2.2002 (54)
2.1557 (54)
2.5051 (18)
1.9271 (87)
1.9236 (74)
1.9141 (73)
73.88 (20)
84.98 (16)
96.37 (25)
176.49 (21)
89.39 (29)
88.67 (29)
2.24
2.18
2.53
1.92
1.93
1.93
74.1
82.6
94.2
175.5
90.8
91.7
The nature of the MLCT band at 330 nm differs subtly between
the complexes with conjugated and non-conjugated ligands. In all
cases the donor orbital is Re dp in nature. The acceptor orbital
however differs in a systematic way; for the unconjugated com-
plexes (2a–d) it is localised solely on the triazole unit while the
conjugated complexes display an MO that is spread over the R sub-
stituent in addition to the triazole moiety (Fig. 4). Some of this dif-
ference is a consequence of the overweighting of the conjugation
interactions associated with the DFT method, this may be tested
experimentally by measuring the resonance Raman spectroscopy.
Resonance Raman spectroscopy may be used to probe the nat-
ure of electronic transitions, this is because the resonance effect
causes an enhancement of bands that reflect the structural changes
In comparing the Raman spectra (simulated and experimental)
we have used the mean absolute deviation (MAD) of wavenum-
ꢀ1
bers, for bands between 400 and 1650 cm that have greater than
2
0% maximum band intensity. 1A typical comparison is shown in
ꢀ1
Fig. 2. For these systems MADs of <10 cm are obtained; this is con-
sidered to give a useful model of the electronic structure of the com-
plex of interest [53–65].
Electronic absorption spectra of 2a–h are shown in Fig. 3. The
spectra show
a lowest energy transition at approximately
3
30 nm that is unaffected by substitution whether it be unconju-
gated with the triazole ring (2a–d) or conjugated (2e–h).
The band at 330 nm is assigned to a metal-ligand charge-trans-
fer (MLCT) based on previous literature assignments [39]. Further-
more this assignment is supported by the TD-DFT data which is
summarised in Table 2. At shorter wavelengths there are additional
transitions associated with the ligand (275 nm) and higher energy
MLCT transitions (290 nm).
The TD-DFT data predict that the lowest energy transition in the
complexes with non-conjugated substituents (2a–d) is MLCT in
nature and is unshifted in energy through the series (predicted at
3
58, observed at 330 nm) – this is what is observed. In the case
of the complexes with conjugated substituents on the triazole
2e–h) shifts are predicted and these are not observed in the exper-
(
imental data. One reason for this discrepancy between calculated
1
The carbonyl bands are not used in this MAD comparison as they are more
ꢀ
1
ꢀ5
anharmonic than the skeletal modes in the 300–1650 cm
region [56]; for this
Fig. 3. Electronic absorption spectra of complexes obtained at 10 M in CH
3
CN.
reason they require a differing scaling factor and as there are only three bands they
are not included.
Unconjugated molecules (2a–d) are shown as solid traces while conjugated ones
are dashed (2e–h).

T.Y. Kim et al. / Polyhedron 52 (2013) 1391–1398
1395
Table 2
Comparison of experimental electronic absorption data (from Fig. 3) with calculated TD-DFT data for 2a–h.
Complex
Experimental
k (nm)
Calculated
ꢀ
1
ꢀ1
e
(L M cm
)
k (nm)
f
Configuration (coefficient)
2
a
333
11600
9400
4100
358
280
273
0.070
0.017
0.18
H-1 ? LUMO (0.98)
HOMO ? L+2 (0.28) HOMO ? L+4 (0.52)
H-3 ? LUMO (0.67)
288
73
2
2
b
336
12300
9500
358
296
0.070
0.015
H-1 ? LUMO (0.97)
290
HOMO ? L+1 (ꢀ0.12), HOMO ? L+2 (0.50),
HOMO ? L+3 (ꢀ0.14)
270
3800
272
0.20
H-5 ? LUMO (0.66)
2
c
333
3900
9500
358
282
0.072
0.030
H-2 ? LUMO (0.64), H-1 ? LUMO (ꢀ0.34)
H-2 ? L+1 (ꢀ0.13), H-1 ? L+1 (ꢀ0.20),
HOMO ? L+2 (0.19),
290
HOMO ? L+4 (0.25)
272
13000
272
0.19
H-5 ? LUMO (0.66)
2
d
333
4200
12700
22600
358
294
279
0.065
0.024
0.22
H-1 ? L+1 (0.95)
2
2
92
65
H-1 ? L+2 (0.45), H-1 ? L+3 (0.12), H-1 ? L+4 (ꢀ0.16)
H-6 ? LUMO (ꢀ0.26), H-4 ? LUMO (0.68)
2
2
2
2
e
330
78
5500
19700
317
284
0.055
0.14
H-1 ? L+1 (0.94)
H-3 ? LUMO (0.71), H-1 ? L+2 (ꢀ0.17)
H-3 ? L+1 (0.90)
H-7 ? LUMO (0.57), H-5 ? L+1 (ꢀ0.27)
H-1 ? L+1 (0.87)
H-2 ? L+1 (0.86)
2
f
330
82
5600
17300
307
272
0.06
0.160
2
g
h
330
77
6500
39300
309
288
0.127
0.460
2
330
77
11000
50300
363
277
0.0641
0.2659
H-2 ? LUMO (0.20), H-1 ? L+1 (0.77)
H-3 ? L+1 (0.74)
2
Fig. 4. Molecular orbital pictures of the main acceptor orbitals of 2b (a) and 2e (b) obtained from theoretical calculations.
that are associated with the photoexcitation [62,63,65,68–70].
Thus the pattern of band enhancements, coupled with knowledge
of the nature of the normal modes of vibration which is obtained
from the frequency calculations, can provide an insight into the
structural nature of the donor and acceptor MOs [71]. The normal
Raman spectra for the compounds are shown in Fig. 5. The spectra
differ as the complexes have different substitution on the triazole
ring; for example the spectra of 2d and 2h both show a distinctive
phenyl R group as well as the pytri. In the spectra of 2f and 2g
the lower frequency mode attributed to stretching of the R-triazole
ꢀ1
moiety shifts to 1515 and 1522 cm – consistent with a mode that
involves the conjugated aryl group. The spectrum of 2h shows
ꢀ1
2
enhancement of the NO group at 1349 cm [72,73] consistent
with an acceptor MO that has some nitro character and thus in-
volves the group appended to the triazole.
ꢀ1
band at 1349 cm associated with the NO
2
functionality [72,73].
2.4. Excited state spectroscopy
The resonance Raman spectra are shown in Fig. 6. A striking fea-
ture of these data is the similarity between the spectra for the com-
plexes 2a–d which all show bands at 1285, 1571, 1587 and
The emission data for the complexes 2a–2h are presented in Ta-
ble 3. These data show that the R group substitution has no effect
on the emission spectral profile; the lifetimes however are altered
by these substituents. It is interesting to note that the nitro substi-
tuted compounds also show cyclic voltammetry traces that differ
dramatically from those of the other compounds (vide infra).
The emission spectra of Re complexes of the type [(pytri-R)Re(-
ꢀ1
1
625 cm ; indeed these spectra appear almost identical. The fre-
quency calculations provide normal modes for these bands; these
are based on the pytri moiety (Fig. 7(a)). For the conjugated sys-
tems, 2e–h small spectral differences are observed. In the case of
2
e the pytri modes observed in the non-conjugated systems are
t
present; in addition there are weak bands at 1508 and
CO) Cl], with R = COOMe or COO Bu, show maxima at 526 nm,
3
ꢀ
1
1
601 cm which are due to modes that involve stretching of the
while [(bpy)Re(CO) Cl] has an emission band at 598 nm [45].
3

1
396
T.Y. Kim et al. / Polyhedron 52 (2013) 1391–1398
Table 3
3
Emission maxima, obtained in CH CN at 364 nm excitation, and emission lifetimes,
3
obtained in CH CN at 354.7 nm, of 2a–h at room temperature.
ꢃ 10ꢀ
2
k
ꢃ 10 (s
5
ꢀ1
)
k
nr ꢃ 10
6
Complex
k
em (nm)
s
em (ns)
U
r
2
2
2
2
2
2
2
2
a
b
c
d
e
f
532
532
533
532
532
No emission
533
121a
103
79
7
162
–
1.2a
1.2
1.3
0.1
3.6
1.0b
1.2
1.7
1.5
2.2
8.2b
9.6
12.4
143
6.0
g
h
127
61
1.5
1.2
7.8
532
a
±
10%.
b
±20%.
2
a–d. For example 2b and 2c have lifetimes of 103 and 79 ns,
respectively, compared to their conjugate analogues 2e and 2g of
62 and 127 ns. Comparatively, the lifetimes of 2d and 2h (R = 4-
NO Bn and 4-NO Ph) also show this trend but with considerably
shorter lifetimes. Previously, the lifetime of [(bpy)Re(CO) Cl] and
1
2
2
3
Fig. 5. FT-Raman spectra of the complexes 2a–h obtained as solid samples at
064 nm.
2b was measured by Obata et al. [39] in 2-methyltetrahydrofuran
at 77 K as 3170 and 8900 ns, respectively. The shorter lifetime for
complexes with non-conjugated ligands may be due to the ‘‘free-
rotor’’ effect [74] which allows molecules to relax through non-
1
2
radiative pathways. In this case, the freely rotating CH group that
connects the triazole and the R group causes the decrease in ex-
cited lifetime.
2.5. Cyclic voltammetry
Many electrochemical studies have been performed on rhenium
tricarbonyl(a-diimine)chloride complexes [75,76]. Such systems
I
II
are characterized by an irreversible Re /Re oxidation that occurs
near the solvent limit, and ligand based reductions. As might be ex-
pected, the large variation in diimine ligand type (and substituents
they carry) is mirrored in an equally diverse reduction chemistry,
with multiple electron processes of variable reversibility, and ha-
lide exchange reactions commonly encountered.
The electrochemistry of ligands 1b, 1e [27] and 1f [24] have
previously been described. 1,2,3-triazoles typically undergo an
irreversible reduction at very high negative potentials near the sol-
vent limit [26,27]. Coordination to Pd or Pt shifts Epc of the ligand
reduction to less negative potential, but the process remains irre-
versible [24,26,27].
CN at 10 3
ꢀ
M
Fig. 6. Resonance Raman spectra of complexes 2a–h obtained in CH
concentrations using 350.7 nm excitation. The spectral region between 1370 and
1
3
ꢀ
1
490 cm is dominated by strong solvent bands and has been omitted for clarity.
The electrochemistry of the rhenium compounds 2a–h in
2 2 4 6
CH Cl solution was probed using cyclic voltammetry and Bu NPF
Lifetimes vary around 100 ns with those of the conjugated com-
as supporting electrolyte. Data are presented in Table 4 and repre-
plexes, 2e–f and 2h, being longer than the non-conjugated ones,
sentative voltammograms for 2b, 2f and 2h in Fig. 8. An internal
Fig. 7. The vibrational mode of 2b at 1282 cmꢀ1 (a) and 2e at 1288 cm (b) obtained from quantum calculations.
ꢀ1

T.Y. Kim et al. / Polyhedron 52 (2013) 1391–1398
1397
Table 4
ment error. The ferrocenyl substituted triazole complex 2f
displays the predicted reversible oxidation couple associated with
the metallocene fragment at E° = 0.85 V. This is a slight anodic shift
from the naked ligand 1f (E° = 0.78 V), and a result in line with our
previous study of Pd triazole derivatives [24]. The current mea-
sured for this process is the same as that for the Re oxidation
and thus provides supporting evidence for its one-electron
assignment.
The two nitro substituted compounds show quite different
reduction chemistry. The first two cathodic features are likely asso-
ciated with the nitrobenzyl (2d)/nitrophenyl (2h) moieties and
consist of a reversible one-electron reduction to a stable radical an-
ion, followed by irreversible multi-electron processes. This behav-
iour closely resembles that previously reported for nitrobenzene
Electrochemical data for 2a–h (1 ꢃ 10ꢀ
3
M
2 2 4 6
in CH Cl solution, 0.1 M Bu NPF ,
⁄
+/0
referenced with internal [Fc ] ).
Compound
E
pc (V)
E
pc (V)
E° (V)
E° (V)
E
pa (V)
I
(
triazole) (nitrobenzene) (nitrobenzene) (ferrocenyl) (Re )
2
2
2
2
2
2
2
2
a
b
c
d
e
f
ꢀ1.74
ꢀ1.72
ꢀ1.72
1.49
1.50
1.49
1.50
1.50
1.52
1.50
1.53
ꢀ1.53
ꢀ1.31
ꢀ0.94
ꢀ0.81
ꢀ1.66
ꢀ1.71
ꢀ1.68
ꢀ1.73
0.85
g
h
[
78,79]. The triazole reduction process for 2d falls outside the
scanned potential range.
The electrochemical data provide an indication of the energies
of the redox active MOs such as the HOMO and LUMO. There is a
substantial amount of literature that relates the energy separation
observed between oxidation and reduction potentials with the ob-
served optical transitions [80–82]. This correlation is particularly
successful for MLCT transitions of metal polypyridyl complexes
[
83]. The comparison between the observed MLCT transitions
and electrochemistry are consistent with this correlation, in that
they show a separation between oxidation and reduction of
approximately 3.2 eV which corresponds to a transition energy of
3
90 nm. This is slightly lower than observed (333 nm) but, impor-
tantly, is unchanged through the series. The exception is for com-
plexes with the nitro-substituted ligands – the lack of correlation
in these two cases is consistent with a redox orbital for the reduc-
tion that is unconnected to the acceptor MO in the lowest energy
optical transition – that is a redox MO on the nitrobenzene unit.
Analysis of the DFT data shows that for 2d and 2h the LUMO is
in fact nitrobenzene based and furthermore the TD-DFT calcula-
tions show that the lowest energy transition of 2d contains no
LUMO character and in the calculation of 2h the LUMO has a minor
contribution to the calculated lowest energy transition. This latter
point is consistent with the resonance Raman data of 2h that re-
veals an enhancement of the nitro vibration indicating that the
nitrobenzene group has some role in the electronic transition at
Fig. 8. Cyclic voltammograms for 2b (upper), 2h (middle) and 2f (lower) at
3
50 nm. As noted previously DFT calculations overestimate the le-
ꢀ
1
ꢀ3
1
00 mV s
(1 ꢃ 10
M
2 2 4 6
in CH Cl solution, 0.1 M Bu NPF , referenced with
⁄
+/0
vel of conjugation in p-systems [66].
equimolar [Fc ] ).
⁄
+/0
decamethylferrocene (Fc ) reference standard was used, under
3. Conclusion
+
/0
which conditions E° [FcH] = 0.55 V [77].
I
All samples display an irreversible one-electron oxidation asso-
A family of electronically tuned fac-Re (CO) Cl pyridyl-1,2,3-tri-
3
I
II
ciated with the Re /Re couple at +1.5 V. The reversibility of this
was not improved with decrease in temperature or increase in scan
rate and variations associated with the differing triazole ligand
substituents are within or close to the experimental error. For this
oxidation the decamethylferrocene chemical reference is non-
innocent and gives rise to some cathodic current on the reverse
sweep.
azole complexes have been synthesized in good to excellent yields
by refluxing methanol solutions of [Re(CO) Cl] and the substituted
2-(1-R-1H-1,2,3-triazol-4-yl)pyridine ligands. The resulting rhe-
nium(I) complexes were characterized by elemental analysis, HR-
ESMS, IR, H and C NMR and in one case the molecular structure
was confirmed by X-ray crystallography. A systematic study of the
electronic structure of this family of fac-[(pytri-R)Re(CO) Cl] com-
5
1
13
3
For the reduction chemistry two distinct behaviours are ob-
served. The voltammograms of rhenium complexes without a li-
gand nitro substituent (2a–2c, 2e–g) are characterized by an
irreversible multi-electron ligand based reduction wave. This oc-
curs at ca. ꢀ1.7 V and the reversibility does not improve with in-
creased scan rate. Between the two extremes of phenyl and octyl
substituent (2e and 2a) there is an apparent shift in Epc commen-
surate with the predicted electronic properties of the substituent.
To wit, 2e (phenyl) Epc occurs at ꢀ1.66 V, 2a (octyl) is shifted
plexes demonstrated that the MLCT transition energy, emission
maxima and the electrochemical oxidation and reduction pro-
cesses are all essentially unaffected by the electronic nature of
the R substituent on the 2-(1-R-1H-1,2,3-triazol-4-yl)pyridine li-
gand. This indicates that the 1,2,3-triazole unit acts as an electronic
insulator. The emission lifetimes of the complexes are dependent
on the nature of the 1,2,3-triazole substituent, with the conjugated
complexes displaying longer lifetimes than the non-conjugated
ones. The shorter lifetime for complexes with non-conjugated li-
gands is attributed to the ‘‘free-rotor’’ effect which allows mole-
cules to relax through non-radiative pathways. The electronic
insulation of the triazole is illustrated by the resonance Raman
I
II
8
0 mV cathodically to ꢀ1.74 V. However like Epa for the Re /Re
couple, any variability in Epc observed for the other substituted li-
gand complexes is within or close to the experimental measure-

1
398
T.Y. Kim et al. / Polyhedron 52 (2013) 1391–1398
spectroscopy which shows near identical spectral signatures for
the non-conjugated systems. This indicates that the resonant chro-
mophore is unchanged by substitution. For the conjugated systems
there are small changes with substitution reflecting modest pertur-
bation of the chromophore. The 1,2,3-triazole unit offers a substi-
tution platform that will not contaminate the electronic structure
of the unsubstituted system. This property could be useful in the
development of luminescent devices or bioprobes where the solu-
bility of the ‘‘click’’ metal complexes could be rapidly tuned
through substitution at the N1 nitrogen without affecting the
intrinsic photophysical properties of the complexes.
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