Photophysical and photochemical trends in tricarbonyl rhenium(I) N-heterocyclic carbene complexes

Article abstract of DOI:10.1021/ic403138a

A family of tricarbonyl Re(I) complexes of the formulation fac-[Re(CO) ₃(NHC)L] has been synthesized and characterized, both spectroscopically and structurally. The NHC ligand represents a bidentate N-heterocyclic carbene species where the cent

Full text of DOI:10.1021/ic403138a

Article
pubs.acs.org/IC
Photophysical and Photochemical Trends in Tricarbonyl Rhenium(I)
N‑Heterocyclic Carbene Complexes
Jamila G. Vaughan,† Brodie L. Reid,† Phillip J. Wright,† Sushil Ramchandani,† Brian W. Skelton,‡
Paolo Raiteri,† Sara Muzzioli,§ David H. Brown,† Stefano Stagni,*,§ and Massimiliano Massi*,†
†Nanochemistry Research Institute, Department of Chemistry, Curtin University, Kent Street, Bentley 6102 Western Australia,
Australia
‡Centre for Microscopy, Characterisation and Analysis, University of Western Australia, Crawley 6009 Western Australia, Australia
§Department of Industrial Chemistry “Toso Montanari”, University of Bologna, viale del Risorgimento 4, Bologna 40126, Italy
S
* Supporting Information
ABSTRACT: A family of tricarbonyl Re(I) complexes of the
formulation fac-[Re(CO)3(NHC)L] has been synthesized and
characterized, both spectroscopically and structurally. The NHC
ligand represents a bidentate N-heterocyclic carbene species where
the central imidazole ring is substituted at the N3 atom by a butyl, a
phenyl, or a mesityl group and substituted at the N1 atom by a
pyridyl, a pyrimidyl, or a quinoxyl group. On the other hand, the
ancillary L ligand alternates between chloro and bromo. For the
majority of the complexes, the photophysical properties suggest emission from the lowest triplet metal-to-ligand charge transfer
states, which are found partially mixed with triplet ligand-to-ligand charge transfer character. The nature and relative energy of
the emitting states appear to be mainly influenced by the identity of the substituent on the N3 atom of the imidazole ring; thus,
the pyridyl complexes have blue-shifted emission in comparison to the more electron deficient pyrimidyl complexes. The
quinoxyl complexes show an unexpected blue-shifted emission, possibly occurring from ligand-centered excited states. No
significant variations were found upon changing the substituent on the imidazole N3 atom and/or the ancillary ligand. The
photochemical properties of the complexes have also been investigated, with only the complexes bound to the pyridyl-substituted
NHC ligands showing photoinduced CO dissociation upon excitation at 370 nm, as demonstrated by the change in the IR and
NMR spectra as well as a red shift in the emission profile after photolysis. Temperature-dependent photochemical experiments
show that CO dissociation occurs at temperatures as low as 233 K, suggesting that the Re−C bond cleaves from excited states of
metal-to-ligand charge transfer nature rather than thermally activated ligand field excited states. A photochemical mechanism that
takes into account the reactivity of the complexes bound to the pyridyl-NHC ligand as well as the stability of those bound to the
pyrimidyl- and quinoxyl-NHC ligands is proposed.
represents a bidentate N-heterocyclic carbene ligand possessing
an imine-type N atom and a carbene-type C atom as donors,
have received considerably less attention. This is somewhat
surprising, as NHC-type ligands have been extensively used to
improve the luminescent properties in complexes of Ru(II),
Ir(III), Pt(II), and Au(I).9−13 With the intent to investigate the
effect that the exchange of a diim for a NHC ligand would have
on the luminescent properties of tricarbonyl Re complexes, our
group and others have investigated the photophysical behavior
of this class of compounds, where the NHC ligand is based
around a pyridyl-, pyrimidyl-, or quinolyl-substituted imidazole
or benzimidazole ring.14−16 The findings have revealed that
these types of NHC ligands are able to activate tunable metal-
to-ligand charge transfer (MLCT) transitions through their π*
system, upon which phosphorescent decay to the ground state
(GS) is then observed (3MLCT → GS). These types of neutral
INTRODUCTION
■
The photophysical properties of luminescent tricarbonyl Re(I)
complexes bound to π-conjugated ligands have been extensively
studied. The general formulation of this class of compounds is
fac-[Re(CO)3(diim)L]0/+, where diim represents a diimine
ligand possessing a π* system of accessible energy and L either
an anionic or neutral monodentate ancillary ligand.1 The most
common diim ligands investigated are designed around 1,10-
phenanthroline (phen) and 2,2′-bipyridine (bipy): it has been
well established how chemical variations of these systems can
be exploited to tune the photophysical properties of the Re
complexes.1 Coupled with favorable quantum yields, relatively
long-lived excited state lifetimes, and resistance to photo-
bleaching, the ability to tune the photophysical properties has
made this class of complexes amenable for a variety of applied
fields, ranging from organic light emitting devices (OLEDs) to
cellular labels.2−8
In comparison to Re complexes bound to diim ligands, the
analogous fac-[Re(CO)3(NHC)L] complexes, where NHC
Received: December 24, 2013
Published: March 25, 2014
© 2014 American Chemical Society
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complexes have highlighted a blue-shifted emission in
comparison to their analogues bound to phen and bipy, with
the blue shift being ascribed to a decrease in the overall
conjugation on passing from phen or bipy to the NHC ligand.
While investigating the photophysical properties of the fac-
[Re(CO)3(NHC)L] complexes, we have also serendipitously
discovered that photochemical CO dissociation can occur in
some instances upon excitation.17 This type of photochemical
transformation had been studied before for tricarbonyl Re(I)
complexes bound to ligands possessing a strong trans effect
such as phosphines, phosphites, and isonitriles.18−25 More
specifically, the CO dissociation has been proposed to occur
from a thermally accessible reactive state of triplet ligand field
character (3LF), following a dissociative mechanism without
rearrangement of the two remaining CO ligands.21 The
bonding properties of NHC ligands are often regarded as
analogous to P-based ligands;26 in this respect, the carbene C
atom is capable of promoting the labilization of the ligand in a
trans position due to its good σ-donation properties. However,
our preliminary investigation on the photochemical properties
of Re NHC complexes revealed that a dissociative mechanism
the photochemical reaction at 293 and 233 K in degassed
acetonitrile solutions. The intent of this investigation is to
understand how the specific chemical nature of the complex is
linked to its photophysical and photochemical properties.
EXPERIMENTAL SECTION
■
General Procedures. All reagents and solvents were purchased
from Sigma Aldrich and Alfa Aesar and used as received without
further purification. All of the reactions were performed under a
nitrogen atmosphere using standard Schlenk techniques. 1-Mesityli-
midazole,27 1-phenylimidazole,28 1-(2-pyridyl)-3-butylimidazolium
bromide,29 1-(2-pyridyl)-3-(2,4,6-trimethylphenyl)imidazolium bro-
mide,30 1-(2-pyridyl)-3-(2,4,6-trimethylphenyl)imidazolium chlor-
ide,31 and 3Cl16 were prepared according to previously published
procedures. Deactivated acidic alumina of Brockmann II activity was
prepared by adding water to Brockmann I alumina at a ratio of 3% w/
w, shaking until clumping stopped, and leaving in a sealed container
for 2 days. Nuclear magnetic resonance spectra were recorded using a
1
Bruker Avance 400 spectrometer (400.1 MHz for H; 100 MHz for
13C) at 300 K. All of the NMR spectra were calibrated to residual
solvent signals. Infrared spectra were recorded using an attenuated
total reflectance Perkin-Elmer Spectrum 100 FT-IR with a diamond
stage. IR spectra were recorded from 4000 to 650 cm−1. The
intensities of the IR bands are reported as strong (s), medium (m), or
weak (w), with broad (br) bands also specified. Melting points were
determined using a BI Barnsted Electrothermal 9100 apparatus.
Elemental analyses were obtained at the Central Science Laboratory,
University of Tasmania, using a Thermo Finnigan EA 1112 Series
Flash instrument. The imidazolium chloride salts were found to be
slightly hygroscopic, and the theoretical elemental analyses had to be
adjusted by adding water molecules.
Photophysical Measurements. Absorption spectra were re-
corded at room temperature using a Perkin-Elmer Lambda 35 UV/vis
spectrometer. Uncorrected steady-state emission and excitation spectra
were recorded on an Edinburgh FLSP920 spectrometer equipped with
a 450 W xenon arc lamp, double-excitation and single-emission
monochromators, and a Peltier cooled Hamamatsu R928P photo-
multiplier tube (185−850 nm). Emission and excitation spectra were
corrected for source intensity (lamp and grating) and emission spectral
response (detector and grating) by a calibration curve supplied with
the instrument. According to the approach described by Demas and
Crosby,32 luminescence quantum yields (Φ) were measured in
optically dilute solutions (OD < 0.1 at excitation wavelength) obtained
from absorption spectra on a wavelength scale [nm] and compared to
the reference emitter by the equation33
3
from a thermally activated LF reactive state alone does not
adequately describe the loss of CO in a position trans to the
carbene C atom upon photoexcitation.17 In fact, three distinct
photochemical products are obtained and they have been
identified as a tricarbonyl acetonitrile-solvated complex and two
different dicarbonyl complexes. Moreover, the CO dissociation
is solvent-dependent, as it was found to proceed in acetonitrile
but not in coordinating acetone or noncoordinating chloro-
form. Our studies also revealed that lowering the relative energy
of the lowest 3MLCT state, achieved by exchanging the pyridyl-
substituted NHC ligand with an analogous quinolyl-substituted
NHC ligand, renders the complexes photostable.17
To further our studies on the photochemical CO dissociation
of fac-[Re(CO)3(NHC)L] complexes with the aim of gaining
more detailed information on the photochemical mechanism
and nature of the reactive excited state, we have prepared a
family of complexes (schematized in Figure 1) by varying the
substituents bound to the N3 (butyl, phenyl, and mesityl) and
N1 (pyridyl, pyrimidyl, and quinoxyl) atoms of the NHC
imidazole ring. The ancillary ligand L was also changed
between chloro and bromo. Furthermore, we have monitored
2
⎡
⎤
⎥
⎦
⎡
⎤⎡
⎤
⎡
⎤
Ar(λr) Ir(λr) nx
D
x
D
r
⎢
Φ = Φ⎢
⎥⎢
A (λ ) I (λ )
⎥
⎢
⎣
⎥
⎦
x
r
2
r
⎣
⎦⎣
⎦ n
⎣
x
x
x
x
where A is the absorbance at the excitation wavelength (λ), I is the
intensity of the excitation light at the excitation wavelength (λ), n is
the refractive index of the solvent, D is the integrated intensity of the
luminescence, and Φ is the quantum yield. The subscripts r and x refer
to the reference and the sample, respectively. The quantum yield
determinations were performed at identical excitation wavelengths for
the sample and the reference, therefore canceling the I(λr)/I(λx) term
in the equation. All of the complexes were measured against an air-
equilibrated aqueous solution of [Ru(bpy)3]Cl2 used as reference (Φr
= 0.028), with the exception of 1Cl and 2Cl, which were measured
against an air-equilibrated ethanol solution of rhodamine 101 (Φr =
1.0).33,34 Emission lifetimes (τ) were determined with the single
photon counting technique (TCSPC) with the same Edinburgh
FLSP920 spectrometer using a pulsed picosecond diode laser “(EPL
375, fwhm < 800 ps) as the excitation source, with repetition rates
between 10 kHz and 1 MHz, and the aforementioned R928P PMT as
detector. The goodness of fit was assessed by minimizing the reduced
χ2 function and by visual inspection of the weighted residuals. To
record the 77 K luminescence spectra, the samples were put in glass
tubes (2 mm diameter) and inserted in a special quartz Dewar filled
Figure 1. General synthetic pathway for the synthesis of the
complexes. Complex 1Cl was prepared using a silver transmetalation
protocol.
a
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with liquid nitrogen. The dichloromethane solvent used in the
preparation of the solutions for the photophysical investigations was of
spectrometric grade. All of the prepared solutions were filtered
through a 0.2 μm syringe filter before measurement. Degassed samples
were prepared by the freeze−pump−thaw technique. Temperature-
dependent lifetime measurements were obtained with an Edinburgh
FLS980-stm spectrometer equipped with a temperature-controlled
cuvette holder, for measurements between 293 and 333 K, or a liquid
nitrogen cooled Oxford Instrument OptiscanDN cryostat, for
measurements below room temperature. Experimental uncertainties
are estimated to be 8% for lifetime determinations, 20% for
quantum yields, and 2 and 5 nm for absorption and emission
peaks, respectively.
imidazole, and the reaction temperature was set to 140 °C to afford a
light brown solid (87%). Mp: 224−227 °C. Anal. Calcd for
C13H11ClN4·H2O: C, 56.42; H, 4.74; N, 20.25. Found C, 56.29; H,
4.58; N, 20.56. 1H NMR (DMSO-d6): δ 10.70 (1H, app t, splitting 2.0
Hz, NCHN), 9.12 (2H, d, J = 4.8 Hz, pyrimidyl H4, H6), 8.75 (1H,
app t, splitting 2.0 Hz, imidazoyl CH), 8.57 (1H, app t, splitting 2.0
Hz, imidazoyl CH), 7.99−7.96 (2H, m, phenyl ortho-H), 7.84 (1H, t, J
= 4.8 Hz, pyrimidyl H5), 7.72−7.65 (3H, m, phenyl meta-H, para-H)
ppm. 13C NMR (DMSO-d6): δ 160.2 (2 × pyrimidyl CH; C4, C6),
152.2 (pyrimidyl quat C1), 135.0 (NCHN), 134.5 (phenyl quat C),
130.4 (phenyl CH), 130.1 (2 × phenyl CH), 122.9 (imidazoyl CH),
122.8 (pyrimidyl CH; C5), 122.6 (2 × phenyl CH), 120.0 (imidazoyl
CH) ppm. ATR-IR: ν 3140 m, 3015 m, 2324 w, 2163 w, 2105 w, 1660
w, 1587 m, 1561 w, 1536 s, 1496 w, 1467 w, 1418 s, 1387 m, 1351 w,
1337 w, 1311 w, 1275 w, 1242 m, 1190 w, 1165 w, 1144 w, 1119 w,
1057 w, 1001 w, 977 w, 951 w, 914 w, 816 w, 788 w, 763 m, 737 w,
684 w cm−1.
Photolysis Experiments. The complexes (∼5 mg) were dissolved
1
in CD3CN (1.5 mL), and an initial H NMR spectrum was acquired.
The solution was then transferred to a quartz cuvette, and 99
consecutive emission spectra with an excitation wavelength of 370 nm
were collected on a Hitachi F-4600 luminescence spectrophotometer
for approximately 4 h. The following parameters were set: sweep width
200−800 nm, excitation slit width 20 nm, and emission slit width 5
Synthesis of 1-(2-Pyrimidyl)-3-phenylimidazolium Hexa-
fluorophosphate. A saturated aqueous solution of potassium
hexafluorophosphate was added to an aqueous solution of 1-(2-
pyrimidyl)-3-phenylimidazolium chloride until precipitation ceased.
The resulting solid was used in the following reaction without further
purification or characterization.
1
nm. A second H NMR spectrum was acquired of the photolyzed
solutions. An IR spectrum was then collected by slow addition of small
droplets of the solution onto the diamond stage of the Perkin-Elmer
Spectrum 100 FT-IR spectrometer. The solution was allowed to
evaporate, and the dropping procedure was repeated until a sample
concentrated enough to allow collection of an IR spectrum was
obtained. The IR spectrum of the photolyzed sample was overlaid with
the IR spectrum of the same rhenium compound in DMSO, as the
poor solubility of the starting material in acetonitrile produced poor-
quality spectra. Temperature-dependent photolysis studies were
obtained in a similar procedure on an Edinburgh FLSP980-stm
spectrometer equipped with a temperature-controlled cuvette holder,
for experiments between 293 and 333 K, or liquid nitrogen cooled
Oxford Instrument OptiscanDN cryostat, for experiments below room
temperature.
Synthesis of 1-(2-Pyrimidyl)-3-(2,4,6-trimethylphenyl)-
imidazolium Chloride. The same procedure used for the
preparation of 1-(2-pyridyl)-3-(2,4,6-trimethylphenyl)imidazolium
chloride was followed with the following modifications: 2-chloropyr-
imidine was substituted for 2-chloropyridine, and the reaction
temperature was set to 140 °C to afford a light brown solid (61%).
Mp: 272−275 °C. Anal. Calcd for C16H17ClN4·1.15H2O: C, 59.77; H,
6.05; N, 17.44. Found C, 59.34; H, 5.85; N, 17.88. 1H NMR (DMSO-
d6): δ 10.48 (1H, app t, splitting 1.6 Hz, NCHN), 9.10 (2H, d, J = 5.2
Hz, pyrimidyl H4, H6), 8.77 (1H, app t, splitting 2.0 Hz, imidazoyl
CH), 8.19 (1H, app t, splitting 2.0 Hz, imidazoyl CH), 7.82 (1H, t, J =
4.8 Hz, pyrimidyl H5), 7.18 (2H, s, phenyl meta-H), 2.35 (3H, s,
CH3), 2.11 (6H, s, CH3) ppm. 13C NMR (DMSO-d6): δ 160.0 (2 ×
pyrimidyl CH; C4, C6), 152.3 (pyrimidyl quat C1), 140.4 (phenyl quat
C1), 137.2 (NCHN), 134.3 (phenyl quat C2, C6), 131.2 (phenyl quat
C4), 129.2 (2 × phenyl CH), 125.3 (imidazoyl CH), 122.6 (pyrimidyl
CH; C5), 120.1 (imidazoyl CH), 20.6 (CH3), 17.0 (CH3), 16.9 (CH3)
ppm. ATR-IR: ν 3421 w, 3363 w, 3255 w, 3189 w, 3160 m, 3122 m,
3049 m, 2978 m, 2918 s, 2753 w, 2324 w, 2287 w, 2193 w, 2162 w,
2152 w, 2103 w, 2067 w, 2050 w, 1823 w, 1628 w, 1605 w, 1584 s,
1561 w, 1521 s, 1489 w, 1450 w, 1412 s, 1377 m, 1344 w, 1327 w,
1307 w, 1291 w, 1248 w, 1191 w, 1120 w, 1101 w, 1073 w, 1051 w,
998 w, 971 w, 934 w, 902 w, 884 w, 852 w, 841 w, 790 w, 767 w, 738
w, 674 w cm−1.
Synthesis of 1-(2-Quinoxyl)-3-phenylimidazolium Chloride.
The same procedure used for the preparation of 1-(2-pyridyl)-3-(2,4,6-
trimethylphenyl)imidazolium chloride was followed with the following
modifications: 2-chloroquinoxaline was substituted for 2-chloropyr-
idine, 1-phenylimidazole was substituted for 1-(2,4,6-trimethylphenyl)-
imidazole, and the reaction temperature was set to 140 °C to afford a
black powder (78%). Mp: 287−289 °C. Anal. Calcd for C17H13N4Cl·
0.3H2O: C, 64.99; H, 4.36; N, 17.83. Found: C, 64.90; H, 4.03; N,
1
17.70. H NMR (DMSO-d6): δ 11.20 (1H, app t, splitting 1.6 Hz,
NCHN), 9.93 (1H, s, quinoxyl CH), 8.96 (1H, app t, splitting 1.8 Hz,
imidazoyl CH), 8.74 (1H, app t, splitting 1.8 Hz, imidazoyl CH),
8.30−8.19 (1H, m, quinoxyl CH), 8.22−8.19 (1H, m, quinoxyl CH),
8.09−8.01 (4H, m, 2 × phenyl CH, 2 × quinoxyl CH), 7.77−7.65
(3H, m, 3 × phenyl CH) ppm. 13C NMR (DMSO-d6): δ 120.2
(imidazoyl CH), 121.9 (phenyl CH), 122.3 (imidazoyl CH), 128.6
(quinoxyl CH), 129.2 (quinoxyl CH), 130.0 (phenyl CH), 130.2
(phenyl CH), 130.4 (quat phenyl C), 131.6 (quinoxyl CH), 132.4
(quinoxyl CH), 134.5 (quat quinoxyl C), 135.5 (imidazoyl NCHN),
138.9 (quinoxyl C), 141.2 (quat quinoxyl C), 141.8 (quat quinoxyl C)
ppm. ATR-IR: ν 3150 w, 3066 w, 3016 w, 2949 w, 2814 w, 1657 w,
1598 w, 1584 w, 1548 m, 1499 m, 1433 w, 1400 w, 1373 w, 1349 w,
1318 m, 1289 m, 1274 m, 1234 m, 1206 w, 1149 w, 1133 w, 1116 w,
1104 w, 1062 w, 1013 w, 1001 w, 959 w, 942 w, 895 w, 853 w, 771 w,
752 m, 677 m cm−1.
Synthesis of 1-(2-Quinoxyl)-3-phenylimidazolium Hexa-
fluorophosphate. A saturated aqueous solution of potassium
hexafluorophosphate was added to an aqueous solution of 1-(2-
quinoxyl)-3-phenylimidazolium chloride until precipitation ceased.
The resulting solid was used in the following reaction without further
purification or characterization.
Synthesis of 1Cl. A suspension of 1-butyl-3-(2-pyridyl)-
imidazolium bromide (408 mg, 1.45 mmol) and Ag2O (375 mg,
1.62 mmol) in dichloromethane (20 mL) was stirred in darkness at
room temperature for 48 h, after which [Re(CO)5Cl] (346 mg, 0.95
mmol) was added and the reaction mixture was heated at reflux under
an inert atmosphere for 4 days. The resulting green solution was
filtered through a short plug of deactivated acidic alumina and washed
with dichloromethane (50 mL) followed by acetonitrile (50 mL). The
combined fractions were concentrated in vacuo and washed with
diethyl ether (50 mL) to afford a yellow solid (178 mg, 37%). Crystals
suitable for a single-crystal X-ray diffraction study were grown by slow
diffusion of diethyl ether into a solution of the compound in
chloroform. Mp: 198 °C dec. Anal. Calcd for C15ClH15N3O3Re: C,
Synthesis of 1-(2-Pyrimidyl)-3-(2,4,6-trimethylphenyl)-
imidazolium Hexafluorophosphate. A saturated aqueous solution
of potassium hexafluorophosphate was added to an aqueous solution
of 1-(2-pyrimidyl)-3-(2,4,6-trimethylphenyl)imidazolium chloride
until precipitation ceased. The resulting solid was used in the
following reaction without further purification or characterization.
Synthesis of 1-(2-Pyrimidyl)-3-phenylimidazolium Chloride.
The same procedure used for the preparation of 1-(2-pyridyl)-3-(2,4,6-
trimethylphenyl)imidazolium chloride was followed with the following
modifications: 2-chloropyrimidine was substituted for 2-chloropyr-
idine, 1-phenylimidazole was substituted for 1-(2,4,6-trimethylphenyl)-
1
35.54; H, 2.98; N, 8.29. Found: C, 35.19; H, 2.50; N, 7.90. H NMR
(DMSO-d6): δ 8.83 (1H, d, J = 5.2 Hz, pyridyl H6), 8.45 (1H, d, J =
2.0 Hz, imidazoyl CH), 8.35−8.25 (2H, m, 2 × pyridyl CH), 7.74 (1H,
d, J = 2.4 Hz, imidazoyl CH), 7.53−7.49 (1H, m, pyridyl CH), 4.23
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1
(2H, t, J = 7.2 Hz, NCH2), 1.88−1.83 (2H, m, CH3CH2CH2), 1.40−
1.35 (2H, m, CH3CH2), 0.93 (3H, t, J = 7.6 Hz, CH3) ppm. 13C NMR
(DMSO-d6): δ 198.9 (CO), 198.0 (CO) 190.0 (NCN), 189.2 (CO),
153.2 (pyridyl CH), 152.6 (pyridyl quat C), 142.3 (pyridyl CH), 124.0
(pyridyl CH), 124.0 (imidazoyl CH), 117.5 (imidazoyl CH), 112.8
(pyridyl CH), 51.0 (NCH2), 32.7 (CH2CH2CH3), 19.1 (CH2CH3),
13.6 (CH3) ppm. ATR-IR: ν 3937 w, 3775 w, 3164 w, 3114 w, 3092 w,
3060 w, 3030 w, 2964 w, 2935 w, 2864 w, 2017 s (CO), 1918 s (CO),
1871 s (CO), 1710 w, 1615 w, 1575 w, 1558 w, 1487 m, 1464 w, 1450
w, 1429 w, 1384 w, 1370 w, 1331 w, 1288 w, 1263 w, 1249 w, 1197 w,
1159 w, 1146 w, 1128 w, 1107 w, 1091 w, 1033 w, 1022 w, 1001 w,
910 w, 886 w, 856 w, 778 w, 754 w, 702 w, 691 w cm−1.
39.16; H, 2.79; N, 6.85. Found: C, 39.22; H, 2.62; N, 6.79. H NMR
(CDCl3): δ 8.92 (1H, d, J = 6.0 Hz, pyridyl CH), 8.08−8.03 (1H, m,
pyridyl CH), 7.77 (1H, d, J = 2.4 Hz, imidazoyl CH), 7.64 (1H, d, J =
8.0 Hz, pyridyl CH), 7.33−7.31 (1H, m, pyridyl CH), 7.07−7.06 (3H,
m, imidazoyl CH, mesityl CH), 2.37 (3H, s, 4-mesityl CH3), 2.29 (3H,
s, 2,6-mesityl CH3), 2.13 (3H, s, 2,6-mesityl CH3) ppm. 13C NMR
(CDCl3): δ 196.9 (CO), 195.9 (CO), 194.2 (NCN), 188.2 (CO),
154.3 (pyridyl CH), 153.2 (pyridyl quat C), 140.7 (pyridyl CH), 140.3
(4-mesityl C), 136.2 (2,6-mesityl C), 134.8 (2,6-mesityl C), 129.8 (2,5-
mesityl CH), 129.3 (2,5-mesityl CH), 124.4 (imidazoyl CH), 123.5
(pyridyl CH), 116.3 (imidazoyl CH), 111.7 (pyridyl CH), 21.3 (4-
mesityl CH3), 19.0 (2,6-mesityl CH3), 17.8 (2,6-mesityl CH3) ppm.
ATR-IR: ν 3919 w, 3169 w, 3148 w, 3117 w, 3091 w, 3039 w, 2977 w,
2923 w, 2856 w, 2407 w, 2011 s (CO), 1918 s (CO), 1870 s (CO),
1616 m, 1575 w, 1558 w, 1484 m, 1453 w, 1439 w, 1419 w, 1378 w,
1342 w, 1314 w, 1266 w, 1159 w, 1135 w, 1111 w, 1093 w, 1083 w,
1038 w, 977 w, 953 w, 934 w, 872 w, 848 w, 768 w, 747 w, 720 w, 700
w cm−1.
Synthesis of 1Br. A suspension of [Re(CO)5Br] (445 mg, 1.09
mmol), 1-butyl-3-(2-pyridyl)imidazolium bromide (305 mg, 1.08
mmol), and triethylamine (1.5 mL, 10.8 mmol) in toluene (ca. 10
mL) was heated at reflux for 2 days. The resulting mixture was cooled
to room temperature, water (ca. 5 mL) and hexanes (ca. 7 mL) were
added, and eventually the hexanes layer was removed. The aqueous
layer was then extracted with dichloromethane, and the organic phase
was collected, dried over MgSO4, and evaporated to dryness. The
crude product was then purified by flash column chromatography on
deactivated acidic alumina with dichloromethane as the eluting solvent
(327 mg, 55%). Mp: 192 °C dec. Anal. Calcd for C15H15N3O3ReBr: C,
Synthesis of 3Br. The target complex was prepared by following
the same procedure as for 1Br, using 1-(2-pyrimidyl)-3-(2,4,6-
trimethylphenyl)imidazolium hexafluorophosphate as the starting
imidazolium salt (46 mg, 58%). Mp: 277 °C dec. Anal. Calcd for
C19H16BrN4O31Re: C, 37.14; H, 2.62; N, 9.12. Found: C, 37.45; H,
2.66; N, 8.94. H NMR (DMSO-d6): δ 9.24−9.22 (1H, m, pyrimidyl
H6), 9.15−9.13 (1H, m, pyrimidyl H4), 8.45 (1H, d, J = 2.0 Hz,
imidazoyl CH), 7.76 (1H, d, J = 2.4 Hz, imidazoyl CH), 7.64 (1H, t, J
= 5.2 Hz, pyrimidyl H5), 7.13 (1H, s, mesityl CH), 7.10 (1H, s, mesityl
CH), 2.33 (1H, s, CH3), 2.17 (1H, s, CH3), 2.07 (1H, s, CH3) ppm.
13C NMR (100 MHz, DMSO-d6): δ 196.7 (CO), 194.9 (CO), 191.2
(NCN), 188.1 (CO), 163.1 (pyrimidyl CH), 161.0 (pyrimidyl CH),
157.7 (pyrimidyl quat C), 139.2 (phenyl quat C), 135.3 (phenyl quat
C), 134.8 (phenyl quat C), 134.4 (phenyl quat C), 129.0 (phenyl CH),
128.8 (phenyl CH), 125.4 (imidazoyl CH), 120.8 (pyrimidyl CH; C5),
118.6 (imidazoyl CH), 20.7 (CH3), 18.1 (CH3), 17.1 (CH3) ppm.
ATR-IR: ν 3926 w, 3685 w, 3676 w, 3182 w, 2988 m, 2973 m, 2912 m,
2902 m, 2747 w, 2410 w, 2324 w, 2489 w, 2012 s (CO), 1921 s (CO),
1883 s (CO), 1595 m, 1568 w, 1557 w, 1512, 1470 m, 1449 m, 1411
w, 1394 w, 1379 w, 1343 w, 1306 w, 1257 w, 1197 w, 1168 w, 1139 w,
1058 w, 1028 w, 978 w, 963 w, 934 w, 848 w, 809 w, 788 w, 744 w,
699 w, 655 w cm−1.
1
32.67; H, 2.74; N, 7.62. Found: C, 32.69; H, 2.71; N, 7.65. H NMR
(CDCl3): δ 8.90 (1H, d, J = 5.6 Hz, pyridyl H6), 8.00 (1H, dd, J = 1.6
Hz, J = 8.0 Hz, pyridyl CH), 7.57−7.56 (2H, m, imidazoyl CH and
pyridyl CH), 7.29 (1H, dd, J = 1.6 Hz, J = 6.8 Hz, pyridyl CH), 7.08
(1H, d, J = 2.4 Hz, imidazoyl CH), 4.35−4.23 (2H, m, NCH2), 2.04−
1.92 (2H, m, CH2CH2CH2), 1.51−1.46 (2H, m, CH3CH2) 1.01 (3H,
t, J = 7.6 Hz, CH3) ppm. 13C NMR (CDCl3): δ 197.3 (CO), 196.8
(CO), 193.0 (NCN), 188.0 (CO), 153.9 (pyridyl CH), 152.9 (pyridyl
quat C), 140.7 (pyridyl CH), 123.2 (pyridyl CH), 123.1 (imidazoyl
CH), 115.8 (imidazoyl CH), 111.5 (pyridyl CH), 52.4 (NCH2), 33.0
(CH2CH2CH3), 19.8 (CH2CH3), 13.7 (CH3) ppm. ATR-IR: ν 3905 w,
3748 w, 3164 w, 3118 w, 3095 w, 3067 w, 3039 w, 2964 w, 2942 w,
2867 w, 2014 s (CO), 1915 s (CO), 1887 s (CO), 1681 w, 1615 m,
1578 w, 1489 m, 1456 m, 1430 m, 1386 w, 1375 w, 1364 w, 1331 m,
1314 w, 1299 w, 1257 w, 1197 w, 1166 w, 1144 w, 1130 w, 1103 w,
1091 w, 1022 w, 1006 w, 958 w, 908 w, 892 w, 780 w, 747 w, 732 w,
704 w, 690 w cm−1.
Synthesis of 2Cl. The target complex was prepared following the
same procedure reported for 1Br, but starting from [Re(CO)5Cl],
using 1-(2-pyridyl)-3-(2,4,6-trimethylphenyl)imidazolium chloride as
the starting imidazolium salt (32 mg, 49%). Crystals suitable for a
single-crystal X-ray diffraction study were grown from slow
evaporation of an acetonitrile solution of 2Cl. Mp: 235 °C dec.
Anal. Calcd for C20H17N3O3ReCl: C, 42.21; H, 3.01; N, 7.39. Found:
Synthesis of 4Cl. The target complex was prepared following the
same procedure as for 1Br, but starting from [Re(CO)5Cl], with 1-(2-
pyrimidyl)-3-(phenyl)imidazolium chloride as the starting imidazolium
salt (43 mg, 58%). Crystals suitable for a single-crystal X-ray diffraction
study were grown from slow evaporation of an acetonitrile solution of
4Cl. Mp: 249 °C dec. Anal. Calcd for C16H10ClN4O3Re: C, 36.40; H,
1.91; N, 10.61. Found: C, 36.10; H, 1.94; N, 10.42. 1H NMR (DMSO-
d6): δ 9.27−9.25 (1H, m, pyrimidyl H6), 9.17−9.16 (1H, m, pyrimidyl
H4), 8.42 (1H, d, J = 2.0 Hz, imidazoyl CH), 8.01 (1H, d, J = 2.0 Hz,
imidazoyl CH), 7.71−7.61 (6H, m, pyrimidyl H5, phenyl ortho-H,
meta-H, para-H) ppm. 13C NMR (DMSO-d6): δ 197.3 (CO), 196.5
(CO), 190.4 (NCN), 188.6 (CO), 162.8 (pyrimidyl CH), 161.2
(pyrimidyl CH), 157.7 (pyrimidyl quat C), 138.6 (phenyl quat C),
129.7 (phenyl CH), 129.6 (2 × phenyl CH), 126.0 (2 × phenyl CH),
125.4 (imidazoyl CH), 121.0 (pyrimidyl CH), 118.3 (imidazoyl CH)
ppm. ATR-IR: ν 3175 w, 3145 w, 3067 w, 2020 s (CO), 1913 s (CO),
1865 s (CO), 1596 m, 1568 m, 1500 w, 1470 m, 1446 m, 1416 m,
1378 m, 1346 m, 1320 m, 1301 w, 1263 m, 1199 w, 1144 w, 1097 w,
1070 w, 1022 w, 979 w, 951 w, 817 w, 787 w, 761 w, 746 m, 690 m,
633 w, 618 w, 527 w cm−1.
Synthesis of 4Br. The target complex was prepared following the
same procedure as for 1Br using 1-(2-pyrimidyl)-3-phenylimidazolium
hexafluorophosphate as the starting imidazolium salt (28 mg, 35%).
Crystals suitable for a single-crystal X-ray diffraction study were grown
from slow evaporation of an acetonitrile solution of 4Br. Mp: 200 °C
dec. Anal. Calcd for C16H10BrN4O3Re·0.5CH2Cl2: C, 32.23; H, 1.80;
N, 9.11. Found: C, 32.63; H, 1.80; N, 8.77 (despite several attempts,
the complex was always isolated as a solvated species; the presence of
dichloromethane was also confirmed via 1H NMR). 1H NMR
(DMSO-d6): δ 9.27−9.25 (1H, m, pyrimidyl H6), 9.16−9.14 (1H,
1
C, 42.03; H, 2.89; N, 7.21. H NMR (CDCl3): δ 8.92 (1H, d, J = 7.8
Hz, pyridyl CH), 8.07 (1H, dd, J = 1.6 Hz, J = 8.0 Hz, pyridyl CH),
7.75 (1H, d, J = 2.4 Hz, imidazoyl CH), 7.64 (1H, d, J = 8.4 Hz,
pyridyl CH), 7.64 (1H, d, J = 8.4 Hz, pyridyl CH), 7.34 (1H, dd, J =
1.2 Hz, J = 6.6 Hz, pyridyl CH), 7.07−7.06 (3H, m, imidazoyl CH,
mesityl CH), 2.38 (3H, s, 4-mesityl CH3), 2.25 (3H, s, 2,6-mesityl
CH3), 2.14 (3H, s, 2,6-mesityl CH3) ppm. 13C NMR (CDCl3): δ 197.7
(CO), 196.8 (CO), 194.7 (NCN), 188.7 (CO), 154.2 (pyridyl CH),
153.3 (pyridyl quat C), 140.8 (pyridyl CH), 140.3 (4-mesityl quat C),
136.3 (2,6-mesityl quat C), 135.2 (1-mesityl quat C), 134.8 (2,6-
mesityl quat C), 129.8 (3,5-mesityl CH), 129.3 (3,5-mesityl CH),
124.3 (imidazoyl CH), 123.6 (pyridyl CH), 116.3 (imidazoyl CH),
111.7 (pyridyl CH), 21.3 (4-mesityl CH3), 18.5 (2,6-mesityl CH3),
17.8 (2,6-mesityl CH3) ppm. ATR-IR: ν 3089 w, 2924 w, 2010 s
(CO), 1909 s (CO), 1867 s (CO), 1615 m, 1544 w, 1515 w, 1483 m,
1453 w, 1421 m, 1379 w, 1343 w, 1315 w, 1265 w, 1237 w, 1160 w,
1135 w, 1036 w, 910 w, 850 w, 771 w, 703 w cm−1.
Synthesis of 2Br. The target complex was prepared following the
same procedure as for 1Br, using 1-(2-pyridyl)-3-(2,4,6-
trimethylphenyl)imidazolium bromide as the starting imidazolium
salt (199 mg, 66%). Crystals suitable for a single-crystal X-ray
diffraction study were grown from slow evaporation of an acetonitrile
solution of 2Br. Mp: 243 °C dec. Anal. Calcd for C20H17N3O3ReBr: C,
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m, pyrimidyl H4), 8.43 (1H, d, J = 2.4 Hz, imidazoyl CH), 8.00 (1H,
d, J = 2.0 Hz, imidazoyl CH), 7.72−7.59 (6H, m, pyrimidyl H5, phenyl
ortho-H, meta-H, para-H) ppm. 13C NMR (DMSO-d6): δ 196.6 (CO),
195.9 (CO), 189.4 (NCN), 188.1 (CO), 163.0 (pyrimidyl CH), 161.2
(pyrimidyl CH), 157.7 (pyrimidyl quat C), 138.6 (phenyl quat C),
129.7 (phenyl CH), 129.6 (2 × phenyl CH), 126.1 (2 × phenyl CH),
125.5 (imidazoyl CH), 120.9 (pyrimidyl CH), 118.4 (imidazoyl CH)
ppm. ATR-IR: ν 3146 w, 2014 s (CO), 1913 s (CO), 1860 s (CO),
1594 m, 1566 m, 1498 w, 1469 m, 1423 m, 1381 m, 1345 m, 1303 w,
1263 w, 1142 w, 1074 w, 978 w, 952 w, 816 w, 788 w, 768 w, 744, 691
w, 634 w cm−1.
Cambridge CB2 1EZ, U.K.: fax, (+44) 1223-336-033; e-mail,
deposit@ccdc.cam.ac.uk.
X-ray data refinement for 1Cl: empirical formula
C15H15ClN3O3Re, formula weight 506.95, λ = 0.71073 Å, monoclinic,
space group P21/c, a = 18.0965(4) Å, b = 10.2473(2) Å, c = 9.2381(2)
Å, β = 90.810(2)°, V = 1712.94(6) Å3, Z = 4, Dcalcd = 1.966 Mg/m3, μ
= 7.266 mm−1, crystal size 0.26 × 0.17 × 0.05 mm3, 2θmax = 76.00°,
66202 reflections collected, 9297 independent reflections (Rint
=
0.0471), Tmax/Tmin = 0.704/0.362, 9297/24/228 data/restraints/
parameters, goodness of fit on F2 1.092, final R indices (I > 2σ(I))
R1 = 0.0309 and wR2 = 0.0569, R indices (all data) R1 = 0.0453 and
wR2 = 0.0603, largest difference peak and hole 2.619 and −1.293 e/Å3,
CCDC 974868. The two terminal atoms of the butyl chain are
disordered over two sets of sites with occupancies constrained to 0.5
after trial refinement.
Synthesis of 5Cl. The target complex was prepared following the
same procedure as for 1Br, but starting from [Re(CO)5Cl], with 1-(2-
quinoxyl)-3-(phenyl)imidazolium chloride as the starting imidazolium
salt (176 mg 65%). Mp: 247 °C dec. Anal. Calcd for
C20H12ClN4O3Re: C, 41.56; H, 2.09; N, 9.69. Found: C, 41.72; H,
X-ray data refinement for 2Cl: empirical formula
C20H17ClN3O3Re, formula weight 569.02, λ = 0.71073 Å. monoclinic,
space group P21/n, a = 8.1644(7) Å, b = 17.6255(17) Å, c =
13.6343(15) Å, β = 97.645(9)°, V = 1944.6(3) Å3. Z = 4, Dcalcd = 1.944
1
1.91; N, 9.49. H NMR (DMSO-d6): δ 9.99 (1H, s, quinoxyl CH),
9.01 (1H, d, J = 2.4 Hz, imidazoyl CH), 8.55 (1H, d, J = 8.0 Hz,
quinoxyl CH), 8.35 (1H, dd, J = 1.2 Hz, J = 8.2 Hz, quinoxyl CH),
8.20 (1H, app t, splitting 7.2 Hz, quinoxyl CH), 8.13 (1H, d, J = 2.4
Hz, imidazoyl CH), 8.05 (1H, app t, splitting 7.6 Hz, quinoxyl CH),
7.73−7.64 (5H, m, phenyl CH) ppm. 13C NMR (DMSO-d6): δ 197.9
(CO) 194.9 (CO), 194.0 (NCN), 188.1 (CO), 148.6 (quat quinoxyl
C), 141.0 (quat quinoxyl C), 138.9 (quat quinoxyl C), 138.4 (quinoxyl
CH), 133.3 (quinoxyl CH), 130.8 (quinoxyl CH), 130.3 (quinoxyl
CH), 129.8 (phenyl CH), 128.1 (quinoxyl CH), 126.2 (phenyl CH),
125.7 (imidazoyl CH), 119.1 (imidazoyl CH) ppm. ATR-IR: ν 3164 w,
3114 w, 3024 w, 2013 (CO) s, 1915 (CO) s, 1902 (CO) s, 1851 m,
1675 w, 1597 w, 1569 w, 1544 w, 1497 m, 1478 w, 1442 m, 1428 m,
1392 w, 1359 m, 1327 w, 1293 m, 1261 w, 1234 w, 1219 w, 1173 w,
1157 w, 1137 w, 1115 w, 1101 w, 1023 w, 994 w, 964 w, 944 w, 931 w,
874 w, 855 w, 840 w, 774 w, 759 w, 734 w, 701 w, 688 w, 677 w cm−1.
Synthesis of 5Br. The target complex was prepared following the
same procedure as for 1Br using 1-(2-quinoxyl)-3-(phenyl)-
imidazolium hexafluorophosphate as the starting imidazolium salt
(30 mg, 46%). Crystals suitable for a single-crystal X-ray diffraction
study were grown from slow evaporation of an acetonitrile solution of
5Br. Mp: 250 °C dec. Anal. Calcd for: C20H12BrN4O3Re: C, 38.59; H,
1.94; N, 9.00. Found C, 38.44; H, 1.73; N, 8.71. 1H NMR (CDCl3): δ
9.31 (1H, s, quinoxyl CH) 8.76 (1H, d, J = 8.8 Hz, quinoxyl CH), 8.28
(1H, d, J = 8.4 Hz, quinoxyl CH), 8.08−8.03 (2H, m, quinoxyl CH,
imidazoyl CH), 7.94 (1H, app t, splitting 7.6 Hz, quinoxyl CH), 7.73−
7.62 (5H, m, phenyl CH), 7.41 (1H, d, J = 2.0 Hz, imidazoyl CH)
ppm. 13C NMR (CDCl3): δ 197.2 (CO), 196.4 (CO), 193.2 (NCN),
186.8 (CO), 147.7 (quat quinoxyl C), 142.1 (quat quinoxyl C), 140.3
(quat quinoxyl C), 139.1 (quat phenyl C), 134.7 (quinoxyl CH), 134.0
(quinoxyl CH), 131.2 (quinoxyl CH), 130.7 (quinoxyl CH), 130.5
(phenyl CH), 130.2 (2 × phenyl CH), 129.7 (quinoxyl CH), 126.8 (2
× phenyl CH), 125.6 (imidazoyl CH), 116.5 (imidazoyl CH) ppm.
ATR-IR: ν 3083 w, 2023 s (CO), 1938 s (CO), 1894 s (CO), 1596 w,
1542 w, 1495 m, 1475 w, 1438 m, 1421 w, 1359 w, 1333 w, 1300 w,
1231 w, 1212 w, 1154 w, 1119 w, 1023 w, 996 w, 964 w, 944 w, 919 w,
893 w, 856 w, 764 w, 690 w cm−1.
Mg/m3, μ = 6.413 mm−1, crystal size 0.12 × 0.08 × 0.07 mm3, 2θmax
56°, 17569 reflections collected, 4689 independent reflections (Rint
=
=
0.0629), Tmax/Tmin = 0.694/0.604, 4689/18/272 data/restraints/
parameters, final R indices (I > 2σ(I)) R1 = 0.0620 and wR2 =
0.1266, R indices (all data) R1 = 0.0758 and wR2 = 0.1326, largest
difference peak and hole 4.276 and −1.824 e/Å3, CCDC 974869. The
Cl atom is positionally disordered with the carbonyl group in the trans
position with site occupancies refined to 0.687(14) and its
complement. The geometries of the disordered carbonyl groups
were restrained to ideal values.
X-ray data refinement for 2Br: empirical formula
C20H17BrN3O3Re, formula weight 613.48, λ = 0.71073 Å, monoclinic,
space group P21/n, a = 8.2661(2) Å, b = 17.5306(3) Å, c = 13.6552(4)
Å, β = 97.690(2)°, V = 1960.97(8) Å3, Z = 4, Dcalcd = 2.078 Mg/m3, μ
= 8.261 mm−1, crystal size 0.29 × 0.21 × 0.15 mm3, 2θmax = 73°, 48086
reflections collected, 9217 independent reflections (Rint = 0.0405),
Tmax/Tmin = 0.341/0.166, 9217/6/272 data/restraints/parameters,
final R indices (I > 2σ(I)) R1 = 0.0484 and wR2 = 0.0764, R indices
(all data) R1 = 0.0761 and wR2 = 0.0807, largest difference peak and
hole 3.912 and −6.113 e/Å3, CCDC 974870. The Br atom is
positionally disordered with the carbonyl group in the trans position
with site occupancies refined to 0.595(2) and its complement. The
geometries of the disordered carbonyl groups were restrained to ideal
values.
X-ray data refinement for 4Cl: empirical formula
C18H13ClN5O3Re, formula weight 568.98, λ = 0.71073 Å, triclinic,
space group P1, a = 6.6905(4) Å, b = 10.8368(7) Å, c = 13.3444(11)
̅
Å, α = 101.192(6)°, β = 90.789(6)°, γ = 98.036(5)°, V = 938.96(11)
Å3, Z = 2, Dcalcd = 2.012 Mg/m3, μ = 6.643 mm−1, crystal size 0.13 ×
0.10 × 0.07 mm3, 2θmax = 55°, 10669 reflections collected, 4308
independent reflections (Rint = 0.0539), Tmax/Tmin = 0.664/0.517,
4308/0/255 data/restraints/parameters, final R indices (I > 2σ(I)) R1
= 0.0661 and wR2 = 0.1575, R indices (all data) R1 = 0.0734, wR2 =
0.1614, largest difference peak and hole 10.633 and −1.768 e/Å3,
CCDC 974871.
X-ray Crystallography. Crystallographic data for the structures
were collected at 100(2) K (150(2) K for 1Cl) on an Oxford
Diffraction Gemini diffractometer fitted with either Mo Kα radiation
(for 1Cl and 2Br) or Cu Kα radiation (for 5Br) and a Oxford
Diffraction Xcalibur diffractometer fitted with Mo Kα radiation (for
2Cl, 4Cl, and 4Br). Following analytical absorption corrections and
solution by direct methods, the structures were refined against F2 with
full-matrix least squares using the program SHELXL-97.35 Unless
stated below, anisotropic displacement parameters were employed for
the non-hydrogen atoms. All hydrogen atoms were added at calculated
positions and refined by use of a riding model with isotropic
displacement parameters based on those of the parent atom. Selected
collection and refinement data are given below together with CCDC
numbers. Supplementary crystallographic data can be obtained free of
charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html or from
the Cambridge Crystallographic Data Centre, 12 Union Road,
X-ray data refinement for 4Br: empirical formula
C16H10BrN4O3Re, formula weight 572.39, λ = 0.71073 Å, triclinic,
space group P1, a = 6.7096(14), b = 10.825(2), c = 12.490(3) Å, α =
̅
68.04(2)°, β = 88.674(19)°, γ = 81.954(16)°, V = 832.6(3) Å3, Z = 2,
Dcalcd = 2.283 Mg/m3, μ = 9.721 mm−1, crystal size 0.35 × 0.14 × 0.03
mm3, 2θmax = 53°, 5860 reflections collected, 3444 independent
reflections (Rint = 0.1222), Tmax/Tmin = 0.793/0.568, 3444/1/111
data/restraints/parameters, final R indices (I > 2σ(I)) R1 = 0.1267 and
wR2 = 0.2083, R indices (all data) R1 = 0.1956 and wR2 = 0.2411,
largest difference peak and hole 4.951 and −2.888 e/Å3, CCDC
974872. The data could support anisotropic refinement of the Re and
Br atoms only.
X-ray data refinement for 5Br: empirical formula
C20H12BrN4O3Re, formula weight 622.45, λ = 1.54178 Å, monoclinic,
space group P21/c, a = 7.1179(9) Å, b = 46.0879(5) Å, c = 34.7382(5)
Å, β = 90.3160(10)°, V = 11395.7(15) Å3, Z = 24, Dcalcd = 2.177 Mg/
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Figure 2. X-ray crystal structures of the complexes 1Cl, 2Cl, 2Br, 4Cl, 4Br, and 5Br.
m3, μ = 15.250 mm−1, crystal size 0.48 × 0.03 × 0.02 mm3, 2θmax
=
mixture of chloro and bromo species. On the other hand, the
synthesis of the chloro complexes fac-[Re(CO)3(NHC)Cl]
could be performed by starting with any imidazolium salt.
All the complexes display relatively similar peaks in the
2020−1880 cm−1 region of their IR spectra, and no significant
variation of the frequencies is noted upon changing the R1 and
R2 substituents on the NHC ligands (Figure 1). Also, no
differences were detected when exchanging the ancillary ligand
between chloro and bromo. The lack of variation in the
carbonyl peaks indicates that in all cases the electron density of
the Re centers remains unaltered, hinting at the fact that any
modification of the photophysical behavior of the complexes, in
terms of absorption and emission maxima, is likely to be mostly
attributed to the NHC π* acceptor ligand.
135°, 146805 reflections collected, 20461 independent reflections (Rint
= 0.0495), Tmax/Tmin = 0.762/0.286, 20461/18/1567 data/restraints/
parameters, final R indices (I > 2σ(I)) R1 = 0.0375 and wR2 = 0.0802,
R indices (all data) R1 = 0.0412 and wR2 = 0.0817, largest difference
peak and hole 1.393 and −0.970 e/Å−3, CCDC 974873.
Computational Calculations. Time-dependent density functional
theory (TD-DFT) calculations were performed with GAUSSIAN 09 in
order to calculate the absorption spectra for synthesized complexes.36
Prior to these calculations, the structures were relaxed at the B3LYP
level of theory in the presence of an implicit solvent (dichloro-
methane). The Re atoms were treated with the Stuttgart−Dresden
(SDD) effective core potential;37 the Pople 6-311++G** basis set was
used for C, H, N, O, Cl, and Br atoms, and in all calculations the effect
of the solvent was mimicked with the PCM solvation model,38 with
parameters adequate for dichloromethane. The low-lying singlet−
singlet excitation energies were calculated at the same level of theory,
and the spectra were reproduced as the superposition of Gaussian
functions with heights proportional to calculated intensities and a
variance of 11 nm.
The successful synthesis of all complexes was supported,
along with the presence of three CO stretching peaks in the IR
spectra, by the disappearance of the imidazolium H2 signal in
1
the 10−12 ppm region in the H NMR spectra, indicating
deprotonation of the NHC precursor. The 1H NMR spectra of
2Cl, 2Br, 3Cl, and 3Br, bearing the mesityl-substituted NHC
ligands, show three separate singlets around 2 ppm, which is
consistent with three nonequivalent methyl environments. The
presence of these three peaks, instead of two, indicates a
restricted rotation of the mesityl unit around the C−N bond,
which is likely to originate from the steric hindrance between
the o-methyl substituents against the adjacent CO ligand and
the imidazole H4 atom. On the other hand, the phenyl
substituent in 4Cl, 4Br, 5Cl, and 5Br presents a significantly
reduced rotational barrier. The free rotation is in agreement
with the fact that in the 13C NMR spectra the phenyl
substituents show only four C environments, as opposed to the
restricted mesityl rings that show a total of six environments.
The 13C NMR spectra of all the complexes display four signals
of weak intensity between 199 and 188 nm, corresponding to
each of the three individual C atoms of the CO ligands and to
the carbene C atom of the imidazole heterocycle.
RESULTS AND DISCUSSION
■
Synthesis and Spectroscopic Characterization of the
Complexes. The variously functionalized NHC-based ligands
were prepared according to previously published procedures.
Aside from a few exceptions, the majority of the complexes
were synthesized via a variation of the in situ method developed
in our previous work (Figure 1).14,17 Herein we found that
using triethylamine in 10× excess to facilitate the deprotonation
of the imidazole C atom, instead of potassium carbonate,
afforded in general an improved yield. For the preparation of
1Cl and 2Cl, a slight variation of the free carbene method
reported by Kaufhold et al. was initially attempted;31 however,
the yields of the isolated products were inferior in comparison
to those of the in situ method. 1Cl could not be synthesized by
either the in situ or the free carbene method: therefore, a silver
transmetalation approach was utilized, which afforded low to
moderate yields. The complex 3Cl was initially prepared using
the in situ method as compared to the silver transmetalation
previously attempted by Wang et al.;16 however, the former was
found to produce the complex in low yield. Generally, when
synthesizing the bromo complexes fac-[Re(CO)3(NHC)Br],
the imidazolium salt of the NHC precursor was never used as
the chloride to avoid the formation of the targeted complex as a
X-ray Structural Investigation. Single crystals suitable for
X-ray diffraction were successfully grown for 1Cl, 2Cl, 2Br,
4Cl, 4Br, and 5Br. The combined structures for these
complexes are shown in Figure 2. The facial arrangement of
the three carbonyl ligands was confirmed for all species. In all
cases, the structures show that the heterocylic substituent
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Article
a
Table 1. Selected Bond Distances (Å) between the Re Centers and the Three C Atoms of the CO Ligands
Re−CO bond
1Cl
2Cl
2Br
4Cl
5Br
trans to N atom
trans to C atom
trans to halogen
1.910(3)
1.957(2)
1.913(3)
1.936(8)
1.937(7)
1.949(11)
1.921(4)
1.952(4)
2.002(8)
1.907(10)
1.946(10)
1.969(14)
1.906(6)
1.964(6)
2.000(7)
a
The corresponding values for 4Br are not reported due to their low precision.
Table 2. Photophysical Properties of the Complexes from Diluted Dichloromethane Solutions (ca. 10−5 M)
emission (room temp)
emission (77 K)
absorption λabs (nm) (104ε (cm−1
λem
λem
a
b
a
b
M−1))
(nm)
τ
(ns)
τ
(ns)
Φ
Φ
(nm)
τ (μs)
c
c
1Cl
1Br
2Cl
2Br
230 (2.98), 356 (0.56)
230 (2.26), 359 (0.39)
230 (2.73),, 359 (0.33)
230 (3.04), 362 (0.44)
230 (5.09), 362 (0.82)
230 (4.34), 367 (0.61)
230 (3.81), 364 (0.67)
230 (2.09), 366 (0.32)
230 (2.62), 344 (0.90), 415(0.36)
514
520
520
510
582
577
583
575
494
432
32
43
0.007
0.006
0.008
0.001
0.013
0.013
0.016
0.019
0.038
<0.001
0.012
0.007
0.014
0.002
0.027
0.033
0.024
0.060
0.052
<0.001
462
464
470
470
508
510
504
512
610
610
5.7
c
c
9 (73%), 36 (27%) 10 (65%), 48 (35%)
1.9 (14%), 6.9 (86%,)
d
d
d
d
d
d
d
d
d
d
d
d
d
d
d
d
50
19
99
106
98
109
5
80
4.9 (27%), 9.3 (73%)
24
2.6 (17%), 7.9 (83%)
e
3Cl
3Br
4Cl
4Br
5Cl
5Br
300
373
211
397
5
10.3
8.5
7.6
6.0
3.6
230 (3.51), 344 (1.13), 427 (0.41)
2
2
5.3
a
b
c
d
From air-equilibrated solutions. From degassed solutions. Rhodamine 101 in air-equilibrated ethanol used as the reference. [Ru(bpy)3]Cl2 in air-
equilibrated water used as the reference. 3Cl was previously reported;16 however, we note slightly different values with respect to the lifetime decay
e
values.
Figure 3. Experimental absorption and emission profiles of the complexes from dilute dichloromethane solutions (ca. 10−5 M) at room temperature.
The emission profiles were obtained by exciting the complexes to their lowest energy λabs. The emission profile of 5Br is not reported due to its
extremely weak emission (see the Supporting Information, Figure S11). The same color coding (inset) is used between the absorption and emission
profiles of the same group of complexes.
(pyridyl, pyrimidyl, or quinoxyl) is virtually coplanar with the
imidazole ring. The mesityl ring in the NHC ligands of 2Cl,
2Br, and 3Br lies almost perpendicular to the imidazole ring,
confirming the high degree of steric hindrance that restricts free
rotation. In contrast, the phenyl substituent in 4Cl, 4Br, and
5Br is only twisted by ca. 40°. The data suggest that, at least in
the solid state, there is limited to no extended π conjugation
between the imidazole and the aryl (mesityl or phenyl) rings.
Across the series, the Re−C(NHC) distance is found to be
slightly shorter than the Re−N distance, in agreement with the
stronger σ donation of the carbene C atom.26,39 The longest
Re−C(NHC) bond belongs to 5Br, which could be ascribed to
the increased steric bulkiness of the quinoxyl-substituted ligand
in comparison to the pyridyl- and pyrimidyl-substituted ligands.
Notably, despite the stronger trans effect attributed to carbene-
type ligands (analogously to the corresponding tricarbonyl
Re(I) phosphine and phosphite complexes), the longest Re−
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CO bond in the complexes appears to be that trans to the
halogen ligand (Table 1). The only exception is represented by
complex 1Cl. This trend might suggest a diminished trans effect
from the NHC C atom. On analysis of the pseudo-octahedral
geometry around the Re centers, it is in fact noted that the
carbene C atom is in each case distorted from its ideal position
along the axes of the octahedron. The distortion originates as a
consequence of a bite angle lower than 90° from the five-
membered ring between the NHC ligands and the Re center;
the angles formed between the carbene C atom, the Re atom,
and the CO ligand range between 173 and 169°. See the
Supporting Information, Tables S1−S6, for complete tables of
bond lengths and angles.
Photophysical Investigation. The photophysical data of
all the complexes from dilute dichloromethane solutions (ca.
10−5 M) are summarized in Table 2, with each absorption and
emission profile shown in Figure 3 (see the Supporting
Information, Figures S1−S10, for excitation spectra). The
absorption spectra display analogous trends, highlighting
intense high-energy transitions in the 230−300 nm region
followed by red-shifted bands of lower molar absorptivity above
300 nm. The higher energy bands are associated with ligand-
centered (LC) π → π* transitions involving the NHC ligand.
In the case of the pyridyl complexes 1Cl, 1Br, 2Cl, and 2Br, the
lower energy band is interpreted as an admixture of MLCT (Re
→ NHC) and ligand-to-ligand (halogen → NHC; LLCT)
charge transfer transitions. The same band assignment was
followed in the case of the pyrimidyl complexes 3Cl, 3Br, 4Cl,
and 4Br. Finally, the region above 300 nm in the absorption
spectra of 5Cl and 5Br shows two bands: the transitions
between 300 and 360 nm are assigned to LC transitions, as
similar bands appear in the absorption profile of the
uncoordinated NHC ligand. On the other hand, the broad
band above 360 nm was attributed again to mixed MLCT and
LLCT transitions. The lowest energy CT transitions of 5Cl and
5Br are red-shifted by ca. 60 nm due to the increased π
conjugation when passing from the pyridyl or pyrimidyl to the
quinoxyl-substituted NHC ligand.
The nature of the assigned transitions in the absorption
spectra was also investigated by means of TD-DFT. The
structures of all the complexes were relaxed in dichloromethane
and were found to be in good agreement with the structures
obtained by means of X-ray diffraction, where the root-mean-
square deviation (RMSD) of atomic positions between the
crystal structures and the theoretical structures are 0.32 for 5Br
and 4Br, 3.7 for 1Cl, and 0.10 for all other complexes. The
large deviations for structures 5Br, 4Br, and 1Cl are caused by
rotations of the phenyl group or different conformations of the
butyl group. If the conformational changes are restricted/
ignored, the RMSD of these complexes is reduced to <0.17. In
all cases, the major contributors in terms of orbitals to the lower
energy transitions are HOMO-1 → LUMO and HOMO →
LUMO (see the Supporting Information, Tables S7−S16). An
analysis of the contours for these orbitals in each case reveals a
contribution to HOMO-1 and HOMO from the 5d orbitals of
the Re center and the 3p or 4p orbitals of the chloro or bromo
ligands, respectively. A relatively minor contribution to these
occupied orbitals is also visible from the CO ligands. On the
other hand, the LUMO is mainly composed by the heterocyclic
substituent connected to the imidazole ring: pyridyl, pyrimidyl,
or quinoxyl. A contribution to the LUMO is also present from
the carbene C atom of the imidazole system. These frontier
orbitals are illustrated in Figure 4 using the complex 4Cl as an
Figure 4. Selected orbital representations for 4Cl.
exemplar (see the Supporting Information, Figures S22−S41,
for all other complexes). Overall, the calculations are in
agreement with the assigned admixture of MLCT and LLCT
character for the lower excited states of the complexes.
The emission profiles of the complexes 1Cl, 1Br, 2Cl, and
2Br are all very similar and display broad and structureless
bands centered at 510−520 nm, typical of emission from
excited states of MLCT/LLCT nature (Figure 3). These
profiles are also found to be very similar, considering band
shape and λem, to those of previously reported tricarbonyl Re
NHC complexes bearing either a benzimidazole ring, instead of
an imidazole, or a phenyl substituent directly attached to the
imidazole ring, instead of a butyl or mesityl group.14,17 These
similarities confirm that only the pyridine ring strongly
influences the nature and energy of the lowest MLCT/LLCT
excited state, which is also supported when considering the
contours of the LUMO orbital for all the complexes (Figure 4
and Supporting Information, Figures S22−S41). The identical
behavior when passing from a butyl to a phenyl or mesityl
substituent suggests that very limited conjugation is present
between the imidazole and the two aryl rings, as also previously
concluded from the X-ray crystal structures. The lifetime decays
(τ) of these complexes range between 20 and 80 ns. The
complex 1Br was consistently found to decay following a
biexponential fit, a behavior that has been also reported by
others for analogous complexes.15 The τ values seem to
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elongate on degassing the dichloromethane solutions and the
same trend can be observed for the values of quantum yields
(Φ). These variations suggest that the emissive state is mainly
characterized by a triplet multiplicity (3MLCT/3LLCT).40,41
The emission profiles of the pyrimidyl complexes 3Cl, 3Br,
4Cl, and 4Br appear analogous to those of the pyridyl
complexes, although the λem bands are red-shifted to the 575−
583 nm region. While this red shift was less evident from the
absorption spectra, it is attributed to the more electron-
deficient nature of the pyrimidyl substituent. As in the previous
case, the lack of significant variations on passing from the
phenyl to the mesityl group on the NHC ligand indicates that
the π* system of the pyrimidyl substituent is the major
3
contributor to the MLCT/3LLCT excited state. Again, the
elongation of the τ and increase of Φ in degassed solutions
suggest the triplet multiplicity of the excited state.40,41 Notably,
the τ values for 3Cl, 3Br, 4Cl, and 4Br are found to be longer
than those of 1Cl, 1Br, 2Cl, and 2Br, despite the fact that the
former group emits from excited states of lower energy, thus
contradicting trends dictated by the energy gap law.42 The
shorter lifetime values for the former group might be caused by
the competing photochemical pathways triggered upon
excitation.
In a frozen matrix at 77 K (see the Supporting Information,
Figures S12−S21), all of the complexes bound to the pyridyl-
and pyrimidyl-substituted NHC ligands exhibit blue-shifted
emission bands as a consequence of the rigidochromic effect.43
The τ values are found within the 6−10 μs range.
The emission profiles of 5Cl and 5Br are found somewhat
unusual with respect to the rest of the series. A red-shifted
emission would be expected due to the lower energy π* system
of the quinoxyl substituent, as evidenced from the absorption
spectra. In fact, this red-shifted emission was previously
reported for the analogous quinolyl-NHC complexes.17 Instead,
a blue-shifted band appears in both cases with λem values at 494
and 432 nm for 5Cl and 5Br, respectively, with complex 5Br
characterized by an extremely weak emission (see the
Supporting Information, Figure S11). The lifetime decays and
quantum yields are very short and virtually insensitive to the
presence of O2. Furthermore, in the case of 5Cl, the emission
profile is found to be very similar to that of the uncoordinated
ligand. These data suggest that the emission in both cases might
be originating from LC excited states. No emission attributable
3
to a MLCT excited state is visible up to 800 nm. Reinforcing
these observations is the fact that at 77 K (see the Supporting
Information, Figures S20 and S21), both complexes exhibit an
intense, broad, and structureless emission profile centered at
610 nm and characterized by τ values of 3.6 and 5.3 μs for 5Cl
and 5Br, respectively. We attributed this band to the decay
3
from the corresponding MLCT/3LLCT states, which appear
revived within the frozen matrix, with no contribution from LC
states due to suppressed thermal population of higher excited
states.
Figure 5. IR progression of the photolysis for 1Cl, 1Br, 2Cl, and 2Br
in acetonitrile solutions. The black traces represent the starting
complexes. The photolysis was performed by irradiating each solution
at λ 370 nm for 4 h. The products of the photolysis are represented by
red traces.
Photochemical Investigation. The photochemical prop-
erties of the complexes were investigated by irradiating
acetonitrile solutions at 370 nm, following a previously reported
procedure,17 corresponding to excitation to the MLCT/LLCT
manifold. The photochemical CO dissociation was initially
monitored by means of IR spectroscopy (Figure 5): only the
pyridyl complexes 1Cl, 1Br, 2Cl, and 2Br were found to be
photochemically active, whereas the remaining pyrimidyl and
quinoxyl complexes were photostable under the same
experimental conditions (see the Supporting Information,
Figures S42−S53). The three peaks in the 2050−1800 cm−1
region, corresponding to the three CO ligands of the starting
tricarbonyl complexes (black traces from the spectra in Figure
5), progressively disappear while new peaks at both higher and
lower frequencies grow in intensity (red traces from the spectra
in Figure 5). The shape of the lower frequency peaks highlights
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Figure 6. 1H NMR progression of the photolysis for 1Cl, 1Br, 2Cl, and 2Br in deuterated acetonitrile solutions, obtained by monitoring the pyridyl
H6 signal. The photolysis was performed by irradiating each solution at λ 370 nm for 4 h. Initial spectra are reported at the bottom of each panel.
Figure 7. Sequential changes in the emission profiles for a ca. 10−5 M degassed acetonitrile solution of 2Br at 293 and 233 K upon constant
excitation at 370 nm.
the presence of two sets each including two individual peaks,
which indicates the formation of two distinct dicarbonyl Re
complexes. Furthermore, the presence of a higher frequency
peak at ca. 2031 cm−1 is indicative of the formation of the
cationic acetonitrile-solvated complex obtained by ligand
exchange of an acetonitrile molecule for the halogen ligand in
the starting complexes.17 The formation of these three products
was also confirmed upon monitoring the photolysis in
(Figure 6). As is more evident in the case of the bromo
complexes, the H6 signal at ca. 8.85 ppm of the pyridyl
substituent disappears while three distinct new signals arise at
ca. 8.83 ppm, corresponding to the tricarbonyl acetonitrile-
solvated complex, 8.78, and 8.75 ppm. The last two upfield
doublets correspond to the two dicarbonyl complexes, as
previously observed.17
1
The combined IR and H NMR data highlight the fact that
the photolysis of the bromo complexes appears faster than that
1
deuterated acetonitrile solutions via H NMR spectroscopy
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of the chloro complexes. Considering that a key intermediate in
this bifurcated photochemical pathway seems to be the
formation of the tricarbonyl acetonitrile-solvated complex, the
fast kinetics of the bromo complexes might be ascribed to the
more labile nature of the bromo ligand.
higher energies for Re tricarbonyl complexes with respect to the
CT manifold and,47−50 in the case of complexes of the type fac-
[Re(CO)3(diim)(PR3)]+ that undergo CO dissociation from
3
thermally activated LF states, the photochemical quantum
yield has been shown to rapidly decrease, reaching photo-
stability below 280 K.21 Furthermore, N-heterocyclic carbene
ligands generally act upon increasing the relative energy of LF
states via their strong σ donation.51 On the basis of these
observations, it would seem logical to attribute the origin of the
photochemical transformations to an excited state of MLCT
nature. When the complexes are excited to these states, a
decrease in electron density occurs at the Re center and
halogen ligand predominantly and, to a minor extent, at the CO
ligands. This conclusion is drawn upon considering the orbital
contours of the HOMO-1 and HOMO levels. On the other
hand, an increase in electron density is localized on the pyridine
ring and C atom of the NHC ligand, according to the calculated
LUMO levels (Table 3). Therefore, the Re−CO bonds and
Temperature-dependent photolysis studies were then
performed. The shift in the emission profiles of the pyridyl
complexes 2Cl and 2Br, used as exemplars in a degassed
acetonitrile solution at ca. 10−5 M, was monitored by recording
sequential emission spectra upon constant excitation at 370 nm
at temperatures of 293 and 233 K, analogously to the procedure
previously reported.17 Figure 7 shows the three-dimensional
plots at both temperatures, highlighting the sequential decrease
in intensity of the emission band of 2Br and the appearance of
the characteristic red-shifted band around 590 nm typical of
dicarbonyl species (see the Supporting Information, Figures
S54 and S55, for 2Cl). From the spectral sequences in Figure 7,
it is evident that the photochemical reaction occurs even at 233
K, although at a significantly slower rate in comparison to the
sequence at 293 K. Furthermore, in agreement with the
previously discussed IR and NMR data, the photolysis of the
bromo complex appears to occur faster than that of the chloro
complex. As competing photochemical pathways can act as
quenchers for the emissive 3MLCT/3LLCT states,21 the
observed excited state lifetime decays were recorded at various
temperatures between 233 and 333 K upon excitation at 375
nm from degassed acetonitrile solutions (ca. 10−5 M). The
observed decay constant values, kobs, were then plotted as a
function of temperature using an Arrhenius-type plot and
compared to the kobs data obtained for fac-[Re(CO)3(phen)Cl],
which is photostable when photolyzed at 370 nm (Figure 8).
Table 3. Calculated Percentage Contribution of the Re,
Halogen, and NHC C Atoms to the HOMO-1, HOMO, and
LUMO Orbitals
complex
HOMO-1
HOMO
LUMO
1Cl
1Br
2Cl
2Br
3Cl
3Br
4Cl
4Br
5Cl
5Br
Re 47; Cl 22
Re 39; Br 37
Re 48; Cl 22
Re 40; Br 35
Re 47; Cl 23
Re 40; Br 37
Re 46; Cl 23
Re 39; Br 37
Re 45; Cl 24
Re 37; Br 39
Re 47; Cl 18
Re 41; Br 30
Re 46; Cl 18
Re 40; Br 31
Re 46; Cl 19
Re 39; Br 32
Re 47; Cl 19
Re 41; Br 31
Re 47; Cl 18
Re 41; Br 30
C (NHC) 13
C (NHC) 13
C (NHC) 16
C (NHC) 16
C (NHC) 7
C (NHC) 7
C (NHC) 7
C (NHC) 7
C (NHC) 6
C (NHC) 6
Re−X bonds are weakened, while the trans effect of the NHC
ligand is increased. Whether the photochemical transformation
occurs from the lowest (and emissive) MLCT state (LE-
MLCT) or from higher energy MLCT states (HE-MLCTs) is
still unclear. Furthermore, it cannot be directly concluded
whether some photochemical pathways occur directly from the
excitation of HE-MLCT upon irradiation at 370 nm or whether
the HE-MLCTs are exclusively thermally populated from the
LE-MLCT. MLCT states have been previously shown to
undergo association of solvent molecules, and the same process
might be invoked in this case:49,52,53 when the complexes are
excited to their MLCT states, the association of an acetonitrile
molecule might be promoted, thus forming a seven-coordinated
intermediate that can eventually result in the dissociation of the
halogen ligand and formation of the solvato complex. The
dissociation of the CO ligand in a position trans to the NHC
could occur from the six-coordinated complex in the excited
state and/or from the seven-coordinated complex.
The proposed model of the photochemical pathway
originating from MLCT-type states can be also used to explain
the photostability of the pyrimidyl, quinoxyl, and previously
reported quinolyl complexes. In these complexes, the LUMO
level might be characterized by a reduced contribution of the C
atom of the NHC ligand, as the stabilization of the π* level of
the pyrimidyl, quinolyl, and quinoxyl substituents can act as a
“trap” for the electron density in the reduced ligand form of the
CT excited state. This trend is also in agreement with the
calculated percentage contribution to the LUMO orbital for
Figure 8. Dependene of ln kobs versus T−1 for 2Cl, 2Br, and fac-
[Re(CO)3(phen)Cl]. The corresponding excited-state lifetime decays
were recorded upon excitation at 370 nm.
The Arrhenius plot in Figure 8 illustrates that a temperature-
dependent nonradiative channel is responsible for the decrease
in the excited state lifetime decays of 2Cl and 2Br upon an
increase in temperature.44−46 In comparison to the photostable
fac-[Re(CO)3(phen)Cl], whose dependence of kobs versus T is
significantly less pronounced, the trend in 2Cl and 2Br might
be associated with the presence of photochemically activated
transformations. The fact that the photochemistry can be
observed at 233 K, however, suggests that the photochemistry
might not be occurring from an excited state of LF nature. In
fact, these excited states have been shown to reside at quite
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these complexes (Table 3). Therefore, the trans effect might
result in a decrease in the LE-MLCT state and photochemical
pathways might be less favored, since the energy gap between
the LE-MLCT and the reactive HE-MLCT (where the negative
charge is more localized on the carbene C atom, thus increasing
its trans effect) is increased. However, this rationalization
implies that the photochemistry proceeds from thermally
activated HE-MLCT states.
In the special case of the quinoxyl complexes 5Cl and 5Br,
whose emission at room temperature is observed at higher
energy (494 and 432 nm, respectively), the lack of photo-
chemistry is ascribed to the π−π* LC nature of the excited
state. In fact, in this excited state no electron density is
significantly withdrawn from the Re, X, or CO ligands;
therefore, the Re−CO and Re−X bonds are not weakened
and the increase in trans effect of the NHC ligand might not be
enough to promote photoreactivity.
photochemical progression of 2Cl. This material is available
free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Authors
*E-mail for S.S.: stefano.stagni@unibo.it.
*E-mail for M.M.: m.massi@curtin.edu.au.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the Australian Research Council
(FT1301000033, FT130100463, and LE1301000052). J.G.V.,
B.L.R., and P.J.W. thank Curtin University for the Australian
Postgraduate Award. The authors acknowledge the facilities and
the scientific and technical assistance of the Australian
Microscopy & Microanalysis Research Facility at the Centre
for Microscopy, Characterisation & Analysis, The University of
Western Australia, a facility funded by the University, State and
Commonwealth Governments. P.J.W. and P.R. thank the
National Computational Infrastructure for the provision of
computer time.
CONCLUSION
■
In conclusion, photophysical and photochemical variations in a
family of tricarbonyl Re(I) complexes bound to bidentate NHC
ligands have been investigated. The photophysical properties of
the complexes are found to be almost exclusively dependent on
the identity of the heterocyclic substituent bound to the
imidazole ring: pyridyl, pyrimidyl, or quinoxyl. The complexes
REFERENCES
■
(1) Kirgan, R. A.; Sullivan, B. P.; Rillema, D. P. Top. Curr. Chem.
2007, 281, 45−100.
(2) Hock, S. J.; Schaper, L.-A.; Herrmann, W. A.; Kuhn, F. E. Chem.
Soc. Rev. 2013, 42, 5073−5089.
(3) Coogan, M. P.; Fernan
exhibit phosphorescent decay from MLCT/3LLCT excited
3
states (aside from an anomalous behavior of the two complexes
bound to the quinoxyl NHC ligand), whose relative energy is
higher for the pyridyl complexes due to the increased electron
deficiency of the pyrimidyl group and extended conjugation of
the quinoxyl group. Variations of the other substituent on the
imidazole ring among butyl, phenyl, and mesityl did not affect
the photophysical properties in any appreciable manner. The
pyridyl complexes were found to be photoactive when
irradiated to their MLCT/LLCT manifold. In analogy with
previously reported complexes, the photochemical reactions
resulted in the formation of a tricarbonyl solvato complex along
with two distinct dicarbonyl complexes. The photochemistry
appears to be faster in the case of the bromo complexes, which
was ascribed to the more labile nature of the bromo ligand.
Both the pyrimidyl and quinoxyl complexes appear to be
photostable. The photochemical studies suggest that the
transformations occur from MLCT-type excited states,
rendering this photochemical mechanism different from the
previously reported CO dissociation from thermally activated
3LF states in fac-[Re(CO)3(diim)(PR3)]+ complexes. Overall,
the results presented in this paper define the relationship
between the chemical nature and photophysical/photochemical
properties, including how these can be fine-tuned, of the
tricarbonyl Re(I) complexes bound to bidentate NHC
imidazole-based ligands.
̈
́
dez-Moreira, V. Chem. Commun. 2013, 50,
384−399.
(4) Fernan
́
dez-Moreira, V.; Thorp-Greenwood, F. L.; Coogan, M. P.
Chem. Commun. 2010, 46, 186−202.
(5) Lo, K.; Hui, W.; Chung, C.; Tsang, K.; Ng, D.; Zhu, N.; Cheung,
K. Coord. Chem. Rev. 2005, 249, 1434−1450.
(6) Mauro, M.; Procopio, E.; Sun, Y.; Chien, C.; Donghi, D.;
Panigati, M.; Mercandelli, P.; Mussini, P.; D’Alfonso, G.; De Cola, L.
Adv. Funct. Mater. 2009, 19, 2607−2614.
(7) Yu, T.; Tsang, D. P.-K.; Au, V. K.-M.; Lam, W. H.; Chan, M.-Y.;
Yam, V. W.-W. Chem. Eur. J. 2013, 19, 13418−13427.
(8) Lo, K.; Zhang, K.; Li, S. Eur. J. Inorg. Chem. 2011, 2011, 3551−
3568.
(9) Zou, T.; Lum, C. T.; Chui, S. S.-Y.; Che, C.-M. Angew. Chem., Int.
Ed. 2013, 52, 2930−2933.
(10) Unger, Y.; Zeller, A.; Ahrens, S.; Strassner, T. Chem. Commun.
2008, 3263−3265.
(11) Barbante, G. J.; Francis, P. S.; Hogan, C. F.; Kheradmand, P. R.;
Wilson, D. J. D.; Barnard, P. J. Inorg. Chem. 2013, 52, 7448−7459.
(12) Yang, C.-H.; Beltran, J.; Lemaur, V.; Cornil, J.; Hartmann, D.;
Sarfert, W.; Frohlich, R.; Bizzarri, C.; De Cola, L. Inorg. Chem. 2010,
̈
49, 9891−9901.
́
(13) Visbal, R.; Ospino, I.; Lopez-de-Luzuriaga, J. M.; Laguna, A.;
Gimeno, M. C. J. Am. Chem. Soc. 2013, 135, 4712−4715.
(14) Casson, L. A.; Muzzioli, S.; Raiteri, P.; Skelton, B. W.; Stagni, S.;
Massi, M.; Brown, D. H. Dalton Trans. 2011, 40, 11960−11967.
(15) Li, X.-W.; Li, H.-Y.; Wang, G.-F.; Chen, F.; Li, Y.-Z.; Chen, X.-
T.; Zheng, Y.-X.; Xue, Z.-L. Organometallics 2012, 31, 3829−3835.
(16) Wang, G.-F.; Liu, Y.-Z.; Chen, X.-T.; Zheng, Y.-X.; Xue, Z.-L.
Inorg. Chim. Acta 2013, 394, 488−493.
ASSOCIATED CONTENT
■
S
* Supporting Information
Tables, figures, and CIF files giving bond lengths and angles for
all of the complexes, excitation profiles for all of the complexes
at room temperature, excitation and emission profiles for 5Br at
room temperature, excitation and emission profiles for all of the
complexes at 77 K, calculated transitions for all of the
complexes, pictorial representations of the orbital contours
(17) Vaughan, J. G.; Reid, B. L.; Ramchandani, S.; Wright, P. J.;
Muzzioli, S.; Skelton, B. W.; Raiteri, P.; Brown, D. H.; Stagni, S.;
Massi, M. Dalton Trans. 2013, 42, 14100−14114.
(18) Ishitani, O.; Kanai, K.; Yamada, Y.; Sakamoto, K. Chem.
Commun. 2001, 1514−1515.
(19) Koike, K.; Tanabe, J.; Toyama, S.; Tsubaki, H.; Sakamoto, K.;
Westwell, J.; Johnson, F.; Hori, H.; Saitoh, H.; Ishitani, O. Inorg. Chem.
2000, 39, 2777−2783.
1
(HOMO-5 to LUMO+5) for all of the complexes, IR and H
NMR progression for the photostable complexes, and the
3640
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Inorganic Chemistry
Article
(20) Morimoto, T.; Nishiura, C.; Tanaka, M.; Rohacova, J.;
Nakagawa, Y.; Funada, Y.; Koike, K.; Yamamoto, Y.; Shishido, S.;
Kojima, T.; Saeki, T.; Ozeki, T.; Ishitani, O. J. Am. Chem. Soc. 2013,
135, 13266−13269.
(46) Escudero, D.; Heuser, E.; Meier, R. J.; Schaferling, M.; Thiel,
W.; Holder, E. Chem. Eur. J. 2013, 19, 15639−15644.
̈
(47) Vlce
̌
k, A. Top. Organomet. Chem. 2010, 29, 73−114.
(48) Vlce
̌
́
k, A.; Zalis, S. Coord. Chem. Rev. 2007, 251, 258−287.
(49) Lindsay, E.; Vlcek, A., Jr; Langford, C. H. Inorg. Chem. 1993, 32,
2269−2275.
(21) Koike, K.; Okoshi, N.; Hori, H.; Takeuchi, K.; Ishitani, O.;
Tsubaki, H.; Clark, I.; George, M.; Johnson, F.; Turner, J. J. Am. Chem.
Soc. 2002, 124, 11448−11455.
̌
(50) Stufkens, D.; Vlcek, A. Coord. Chem. Rev. 1998, 177, 127−179.
(51) Hudson, Z. M.; Sun, C.; Helander, M. G.; Chang, Y.-L.; Lu, Z.-
(22) Ng, C.-O.; Lo, L. T.-L.; Ng, S.-M.; Ko, C.-C.; Zhu, N. Inorg.
Chem. 2008, 47, 7447−7449.
(23) Ko, C.; Lo, L.; Ng, C.; Yiu, S. Chem. Eur. J. 2010, 16, 13773−
13782.
H.; Wang, S. J. Am. Chem. Soc. 2012, 134, 13930−13933.
(52) Sato, S.; Matubara, Y.; Koike, K.; Falkenstrom, M.; Katayama,
̈
T.; Ishibashi, Y.; Miyasaka, H.; Taniguchi, S.; Chosrowjan, H.; Mataga,
N.; Fukazawa, N.; Koshihara, S.; Onda, K.; Ishitani, O. Chem. Eur. J.
2012, 18, 15722−15734.
(24) Smithback, J.; Helms, J.; Schutte, E.; Woessner, S.; Sullivan, B.
Inorg. Chem. 2006, 45, 2163−2174.
(53) Sato, S.; Sekine, A.; Ohashi, Y.; Ishitani, O.; Blanco-Rodríguez,
(25) Pierri, A. E.; Pallaoro, A.; Wu, G.; Ford, P. C. J. Am. Chem. Soc.
2012, 134, 18197−18200.
A. M.; Vlce
3540.
̌
k, A.; Unno, T.; Koike, K. Inorg. Chem. 2007, 46, 3531−
(26) Sato, T.; Hirose, Y.; Yoshioka, D.; Oi, S. Organometallics 2012,
31, 6995−7003.
(27) Gardiner, M. G.; Herrmann, W. A.; Reisinger, C.-P.; Schwarz, J.;
Spiegler, M. J. Organomet. Chem. 1999, 572, 239−247.
(28) Xu, Z.-L.; Li, H.-X.; Ren, Z.-G.; Du, W.-Y.; Xu, W.-C.; Lang, J.-P.
Tetrahedron 2011, 67, 5282−5288.
(29) Loch, J. A.; Albrecht, M.; Peris, E.; Mata, J.; Faller, J. W.;
Crabtree, R. H. Organometallics 2002, 21, 700−706.
(30) Grundemann, S.; Albrecht, M.; Kovacevic, A.; Faller, J. W.;
̈
Crabtree, R. H. Dalton Trans. 2002, 2163−2167.
(31) Kaufhold, O.; Hahn, F. E.; Pape, T.; Hepp, A. J. Organomet.
Chem. 2008, 693, 3435−3440.
(32) Demas, J.; Crosby, G. J. Phys. Chem. 1971, 75, 991−1024.
(33) Eaton, D. Pure Appl. Chem. 1988, 60, 1107−1114.
(34) Lakowicz, J. R. Principles of fluorescence spectroscopy, 3rd ed.;
Springer: Baltimore, MD, 2009.
(35) Sheldrick, G. M. Acta Crystallogr., Sect. A 2008, 64, 112−122.
(36) Frisch, M. J.; Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.;
Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone,
V.; Mennucci, B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.;
Hratchian, H. P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J.
L.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida,
M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.;
Montgomery, J. A., Jr.; Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J.
J.; Brothers, E.; Kudin, K. N.; Staroverov, V. N.; Kobayashi, R.;
Normand, J.; Raghavachari, K.; Rendell, A.; Burant, J. C.; Iyengar, S. S.;
Tomasi, J.; Cossi, M.; Rega, N.; Millam, J. M.; Klene, M.; Knox, J. E.;
Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.;
Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.;
Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.;
Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A.
̈
D.; Farkas, O.; Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J.
Gaussian09, Revision B.01; Gaussian, Inc., Wallingford, CT, 2009.
(37) Andrae, D.; Haeussermann, U.; Dolg, M.; Stoll, H.; Preuss, H.
Theor. Chim. Acta 1990, 77, 123−141.
(38) Tomasi, J.; Mennucci, B.; Cammi, R. Chem. Rev. 2005, 105,
2999−3094.
(39) Sato, T.; Hirose, Y.; Yoshioka, D.; Shimojo, T.; Oi, S. Chem. Eur.
J. 2013, 19, 15710−15718.
(40) Stagni, S.; Colella, S.; Palazzi, A.; Valenti, G.; Zacchini, S.;
Paolucci, F.; Marcaccio, M.; Albuquerque, R. Q.; De Cola, L. Inorg.
Chem. 2008, 47, 10509−10521.
(41) Flamigni, L.; Barbieri, A.; Sabatini, C.; Ventura, B.; Barigelletti,
F. Top. Curr. Chem. 2007, 281, 143−203.
(42) Kober, E.; Caspar, J.; Lumpkin, R.; Meyer, T. J. Phys. Chem.
1986, 90, 3722−3734.
(43) Xue, W.; Chan, M.; Su, Z.; Cheung, K.; Liu, S.; Che, C.
Organometallics 1998, 17, 1622−1630.
(44) Costa, R. D.; Monti, F.; Accorsi, G.; Barbieri, A.; Bolink, H. J.;
Ortí, E.; Armaroli, N. Inorg. Chem. 2011, 50, 7229−7238.
(45) Schulze, B.; Escudero, D.; Friebe, C.; Siebert, R.; Gorls, H.;
̈
Sinn, S.; Thomas, M.; Mai, S.; Popp, J.; Dietzek, B.; Gonzal
́
ez, L.;
Schubert, U. S. Chem. Eur. J. 2012, 18, 4010−4025.
3641
dx.doi.org/10.1021/ic403138a | Inorg. Chem. 2014, 53, 3629−3641