Synthesis, structural, and photophysical investigation of diimine triscarbonyl Re(I) tetrazolato complexes

Article abstract of DOI:10.1021/ic1015516

The synthesis, structural, and photophysical properties of a novel family of neutral fac-[Re(N^∧N)(CO)₃(L)] complexes, where N^∧N is either 2,2′-bipyridine or 1,10-phenanthroline and L is a para functionalized 5-aryltetrazol

Full text of DOI:10.1021/ic1015516

Inorg. Chem. 2011, 50, 1229–1241 1229
DOI: 10.1021/ic1015516
Synthesis, Structural, and Photophysical Investigation of Diimine Triscarbonyl
Re(I) Tetrazolato Complexes.
Melissa V. Werrett,† Daniel Chartrand,‡ Julian D. Gale,† Garry S. Hanan,‡ Jonathan G. MacLellan,§
Massimiliano Massi,*,† Sara Muzzioli,|| Paolo Raiteri,† Brian W. Skelton,^ Morry Silberstein,† and Stefano Stagni||
†Department of Chemistry, Curtin University, GPO Box U1987, Perth, Western Australia 6845, Australia,
‡
ꢀ
ꢀ
ꢀ
Department of Chemistry, D-600 Universite de Montreal, 2900 Edouard-Montpetit Montreal, Quebec,
Canada, §School of Chemistry, Monash University, Box 23, Clayton 3800, Victoria, Australia, ||Department of
Physical and Inorganic Chemistry, University of Bologna, Viale Risorgimento 4, 40126 Bologna, Italy, and
^School of Biomedical, Biomolecular and Chemical Sciences, The University of Western Australia, Stirling
Highway, Crawley, Western Australia 6009, Australia
Received August 2, 2010
The synthesis, structural, and photophysical properties of a novel family of neutral fac-[Re(N∧N)(CO)3(L)] complexes,
where N∧N is either 2,20-bipyridine or 1,10-phenanthroline and L is a para functionalized 5-aryltetrazolate [namely,
5-phenyltetrazolate (Tph-), 4-(tetrazolate-5-yl)benzaldehyde (Tbdz-), 5-(4-acetylphenyl)tetrazolate (Tacy-), and
methyl 4-(tetrazolate-5-yl)benzoate (Tmeb-)] are reported. The complexes were prepared by direct addition of the
corresponding tetrazolate anion to the acetonitrile solvated fac-[Re(N∧N)(CO)3]þ precursor. NMR data demonstrate
that the coordination of the metal fragment is regiospecific at the N2 atom of the tetrazolate ring. These conclusions
are also supported by X-ray structural determinations. Photophysical data were obtained in diluted and deaerated
dichloromethane solutions displaying broad and structureless profiles with emission maxima ranging from 566 to 578
nm. The absorption profiles indicate the presence of higher energy intraligand (IL) π-π* transitions and lower
energies ligand-to-ligand charge transfer (LLCT) and metal-to-ligand charge transfer (MLCT). As the last two
transitions are mixed, they are better described as a metal-ligand-to-ligand charge transfer (MLLCT), a result that is
also supported by density functional theory (DFT) calculations. The complexes show excited state lifetime values
ranging from 102 to 955 ns, with associated quantum yield between 0.012 and 0.099. Compared to the parent neutral
chloro or bromo [Re(N∧N)(CO)3X], the complexes show a slightly improved performance because of the π accepting
nature of the tetrazolato ligand. The metal-to-ligand backbonding is in fact depleting the Re center of electron density,
thus widening the HOMO-LUMO gap and reducing the non-radiative decay mechanism in accordance with the
energy gap law. Finally, the electron-withdrawing or donating nature of the substituent on the phenyltetrazolato ligand
allows the fine-tuning of the photophysical properties.
Introduction
π* system such as polypyridines),6,7 these complexes exhibit
luminescence properties generally arising from their triplet
metal-to-ligand charge transfer (3MLCT) excited states.8 The
possibility of fine-tuning both the chemical and the photo-
physical properties of these compounds has made them
extremely attractive from an applicative point of view in a
variety of areas such as the fabrication of organic light emitting
devices (OLEDs),9 light emitting electrochemical cells
(LEECs),10 and solar cells.11,12 Moreover, the phosphorescent
nature of their emission (triplet to singlet radiative decay) is
The synthesis and photophysical investigation of nd6 metal
complexes,1 such as those of Re(I),2 Ru(II),3 Os(II),4 and
Ir(III),5 is still a very active area of research. When coupled
with appropriately conjugated ligands (e.g., with an accessible
*To whom correspondence should be addressed. E-mail: m.massi@curtin.
edu.au. Phone: þ61-8-92662838. Fax: þ61-8-9266-2300.
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r
2011 American Chemical Society
Published on Web 01/26/2011
pubs.acs.org/IC

1230 Inorganic Chemistry, Vol. 50, No. 4, 2011
Werrett et al.
characterized by large Stokes shifts and longer excited state
lifetimes (τ). These properties, which help in preventing self-
quenching and reducing autofluorescence in biological tissues
with the use of time-gated spectroscopy, also make lumines-
cent nd6 complexes promising candidates for novel biological
markers and in immunoassays.13-16
and the lowest unoccupied molecular orbital (LUMO), in
agreement with the energy gap law.34
Tetrazoles are considered the nitrogen-based heterocyclic
surrogate of the carboxylic acid group, which stems from the
fact that the two functional groups possess very similar pKa
values.35 Tetrazole containing compounds have been exten-
sively studied as carboxylic acid analogues for the design of
pharmaceuticals, antifungal, antibacterial formulations, and
also very recently as crystal growth modifiers.36,37 The coor-
dination chemistry of the tetrazolate anion has been the focus
of numerous investigations involving the preparation of
metal complexes and hybrid inorganic-organic bi- and tri-
dimensional systems (MOFs).38-43 The majority of the work
in these areas has been carried out with transition metals
and, more recently, also with lanthanoid elements.44-48 The
variety of structural features that have been discovered upon
coordinating tetrazolate anions to metal complexes is mainly
due to the versatility of the heterocycle itself, exposing four σ
bonding lone pairs, an aromatic donor π system, and an
accepting π* level.35,49 Also, the development of the synthetic
methods for the synthesis of tetrazoles has made possible the
use of a large number of substrates for the conversion of ni-
triles (R-CN) into the corresponding R-CN4H derivatives.50,51
Surprisingly, the coordination chemistry of the tetrazolate
anion with Re is almost virtually unexplored, with only one
example reported by Szczepura et al. of linkage between
The most popular Re(I) family of complexes studied for
their luminescent properties is the fac-[Re(N∧N)(CO)3-
(L)]nþ 2,17-22 where N∧N represents a neutral conjugated
,
diimine ligand such as 2,20-bipyridine (bipy) or 1,10-phenan-
throline (phen), L is an ancillary ligand, and n = 0 or 1
depending on the respective anionic or neutral charge of the L
ligand. Some of these complexes exhibit exceptional lumines-
cence properties with quantum yield (Φ) values up to 0.6 and
extended excited state lifetimes τ ranging from the nano- to
microsecond scale.2 Manipulation of the photophysical
properties is achieved by changing the relative energy of the
π* orbitals of the diimine system with the introduction of
electron withdrawing (EWGs) or donating groups (EDGs)
and/or by varying the crystal field strength of the ancillary
ligand L.2,23-33 Generally, cationic fac-[Re(N∧N)(CO)3(L)]þ
complexes, for example, when L is a pyridine-type ligand,
possess superior photoluminescence properties compared to
their analogous neutral ones where L = Cl-, Br-.2 These
improved properties have been rationalized in terms of a
reduced non-radiative decay constant, knr, due to the larger
gap between the highest occupied molecular orbital (HOMO)
2þ
5-methyltetrazolate and a hexanuclear Re6Se8 cluster
core.52 The compound was obtained by addition of N3- with
a coordinated molecule of acetonitrile, and no direct reaction
of the tetrazole anion with Re precursors has been described
to date. With the aim to expand the chemistry of Re tetrazolato
complexes, we have prepared a series of compounds wherþe
the tetrazolato ligand is coordinated to the Re(N∧N)(CO)3
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Article
Inorganic Chemistry, Vol. 50, No. 4, 2011 1231
Figure 1. Synthetic pathway for the preparation of the Re tetrazolato complexes (the first reaction of [Re(CO)5Cl] with the diimine ligand N∧N is
not shown).
fragment, and have investigated their structural and photo-
physical properties using a variety of experimental and com-
putational techniques. In this work, the investigated synthetic
method involved the direct attachment of differently para
substituted 5-aryltetrazolate anions to the acetonitrile sol-
vated Re(I) precursor, as illustrated in Figure 1. The motiva-
tion that prompted us to undertake this study was the
exploration of the luminescent properties of fac-[Re(N∧N)-
(CO)3(L)] complexes, where L represents a family of 5-ar-
yltetrazolate ligands. While, in general, cationic triscarbonyl
diimine Re(I) complexes have a better photophysical perfor-
mance,2 the advantage of neutral complexes lies in the pos-
sibility of device fabrication (e.g., OLEDs) by sublimation.53
Devices obtained with this method have shown improved
output efficiency. The tetrazole ligand, because of its σ donor
and π acceptor nature, could be a promising candidate to
improve the photo- and electroluminescent properties of
neutral fac-[Re(N∧N)(CO)3(L)] compounds.
emission bands (excitation spectra). For quantum yields, an
aerated solution of fac-[Re(bipy)(CO)3(NCCH3)][PF6] was used
as a reference, and its quantum yield (0.25) was measured with
an Edinburgh Instruments FLS-920 fluorimeter equipped with
an integrating sphere coated with barium sulfate. The remaining
complexes were measured by the dilute solution method, with an
excitation wavelength at 355 nm at constant absorption (0.26).54
Lifetime measurements were done with an Edinburgh Instru-
ments Mini Tau lifetime fluorimeter with an EPL 405 laser
(exciting at 405 nm). Melting points were measured with a BI
Barnsted Electrothermal 9100 apparatus and were quoted re-
ferencing the decomposition temperature.
Synthetic Methods. fac-[Re(N∧N)(CO)3Cl], fac-[Re(N∧N)-
(CO)3(NCCH3)][BF4] (N∧N = bipy and phen),55 TphH,51
TbdzH,51 TacyH,51 and TmebH51 were prepared according to
previously published procedures. fac-[Re(N∧N)(CO)3(NCCH3)-
][BF4] was treated as a reactive intermediate and was never
isolated. fac-[Re(N∧N)(CO)3(NCCH3)][BF4] was dissolved in
acetonitrile (30 mL). To this, an acetontrile solution containing
1.2 equiv of the corresponding tetrazole and 1.2 equiv of triethyl-
amine was added. The resulting suspension was refluxed for 16 h
and then filtered over Celite. The solvent was then reduced to
approximately 5 mL, and the solution cooled in an ice bath,
causing the precipitation of a yellow solid, which was filtered,
washed with cold acetonitrile (2 ꢀ 5 mL), and dried under
reduced pressure.
Experimental Section
General Considerations. All the reagents and solvents were
obtained from Sigma Aldrich and used as received without any
further purification, unless otherwise specified. Although not
strictly required, all the reactions involving Re complexes were
carried out under a nitrogen atmosphere following Schlenk pro-
tocols. When possible, the purification of the Re complexes was
performed via column chromatography with the use of alumina
as the stationary phase (Fluka, Brockmann activity I). IR
spectra were recorded on solid samples using a diamond ATR
Perkin-Elmer Spectrum 100 FT-IR. Nuclear magnetic reso-
nance spectra (consisting of 1H, 13C, 1D NOE, HSQC, and
HMBC experiments) were recorded using a Bruker Avance 400
fac-[Re(bipy)(CO)3(Tph)]. The title compound was purified
via column chromatography using a 4:6 mixture CH2Cl2/CH3-
CN as eluent (second fraction, yellow). Yield 0.113 g (44%). Mp
253 ꢀC (dec.). Elemental analysis for C20H13N6O3Re: calcd: C
42.03, H 2.29, N 14.70; found: C 42.13, H 2.41, N 14.81. IR
(ν, cm-1): 3115 w, 3090 w, 3065 w, 3038 w, 2018 s (CO, A0(1)),
1893 s br (CO, A0(2)/A00), 1604 m (tetrazole CN), 1572 w,
1496 w, 1474 m, 1445 m, 1359 w, 1319 w, 1287 w, 1246 w,
1222 w, 1174 w, 1161 w, 1125 w, 1110 w, 1073 w, 1031 w, 1009 w,
971 w, 929 w, 899 w, 788 w, 767 m, 731 m, 690 m. 1H NMR (δ,
1
spectrometer (400.1 MHz for H, 100 MHz for 13C) at room
temperature. 1H and 13C chemical shifts were referenced to
residual solvent resonances. Elemental analyses were performed
by CMAS (Melbourne, Australia) on the desolvated bulk
samples. Photophysical measurements were done in deoxyge-
nated (by bubbling of argon gas) freshly distilled dichloro-
methane, using a borosilicate cell. Absorption and emission
spectra were recorded at room temperature using a Cary 500i
UV-Vis-NIR spectrophotometer and a Cary Eclipse 300
fluorimeter, respectively. The wavelengths for the emission
and excitation spectra were determined using the absorption
maxima of the metal-ligand-to-ligand charge transfer (MLLCT)
transition bands (emission spectra) and at the maxima of the
0
ppm, DMSO-d6): 9.19 (2H, d, J = 5.6 Hz, bipy H6,6 ), 8.80 (2H,
0
d, J = 8.2 Hz, bipy H3,3 ), 8.39 (2H, app. t, J = 7.2 Hz, bipy
0
0
H
4,4 ), 7.84-7.77 (2H, m, bipy H5,5 ), 7.67-7.65 (2H, m,
CN4-C6H5 Ho), 7.36-7.32 (3H, m, CN4-C6H5 Hm, Hp). 13C
NMR (δ, ppm, DMSO-d6): 197.9, 195.1, 162.3, 156.8, 154.4,
141.5, 130.3, 129,5, 129.3, 128.9, 126.5, 124.9.
fac-[Re(phen)(CO)3(Tph)]. The title compound was purified
via column chromatography using a 4:6 mixture CH2Cl2/
CH3CN as eluent (second fraction, yellow). Yield 0.104 g
(42%). Mp 284 ꢀC (dec.). Elemental analysis for C22H13N6-
O3Re: calcd: C 44.37, H 2.20, N 14.11; found: C 44.74, H 2.40, N
(54) Crosby, G. A.; Demas, J. N. J. Phys. Chem. 1971, 75, 991.
(55) Amoroso, A. J.; Coogan, M. P.; Dunne, J. E.; Fernandez-Moreira,
V.; Hess, J. B.; Hayes, A. J.; Lloyd, D.; Millet, C.; Pope, S. J. A.; Williams, C.
Chem. Commun. 2007, 3066.
(53) Mauro, M.; Procopio, E. Q.; Sun, Y. H.; Chien, C. H.; Donghi, D.;
Panigati, M.; Mercandelli, P.; Mussini, P.; D’Alfonso, G.; De Cola, L. Adv.
Funct. Mater. 2009, 19, 2607.

1232 Inorganic Chemistry, Vol. 50, No. 4, 2011
Werrett et al.
14.23. IR (ν, cm-1): 3092 w, 3067 w, 2019 s (CO, A0(1)), 1909 s
(CO, A0(2)) 1884 s (CO, A00), 1631 w, 1605 w (tetrazole CN),
1585 w, 1519 m, 1496 w, 1459 w, 1426 m, 1362 w, 1347 w, 1313 w,
1280 w, 1225 w, 1147 w, 1126 w, 1112w, 1073 w, 1036 w, 1007 w,
998 w, 970 w, 930 w, 844 m, 788 w, 774 w, 733 m, 721 m, 692 m.
1H NMR (δ, ppm, DMSO-d6): 9.61 (2H, d, J = 5.2 Hz, phen
H2,9), 9.02 (2H, d, J = 8.0 Hz, phen H4,7), 8.34 (2H, s, phen
fac-[Re(phen)(CO)3(Tmeb)]. The title compound was purified
via filtration from hot acetonitrile. Yield 0.081 g (41%). Mp
243 ꢀC (dec.). Elemental analysis for C24H15N6O5Re: calcd: C
44.10, H 2.31, N 12.86; found: C 44.19, H 2.58, N 13.07. IR (ν,
cm-1): 3049 w, 2957 w, 2020 s (CO, A0(1)), 1912 s (CO, A0(2)),
1887 (CO, A00), 1713 s (ester CO), 1678 w, 1615 m (tetrazole CN),
1584 w, 1534 w, 1519 wm 1449 w, 1427 m, 1307 w, 1277 m, 1226 w,
1212 w, 1190 w, 1174 w, 1150 w, 1141 w, 1108 w, 1098 w, 1037 w,
1005 w, 975 w, 864 w, 851 m, 829 w, 777 w, 737 m, 721 m, 698 m.
1H NMR (δ, ppm, DMSO-d6): 9.58 (2H, d, J = 4.0 Hz, phen
H2,9), 8.98 (2H, d, J = 7.4 Hz, phen H4,7), 8.31 (2H, s, phen H5,6),
8.16-8.09 (2H, m, phen H3,8), 7.87 (2H, d, J = 8.4 Hz,
CN4-C6H4-CO2CH3 Hm), 7.66 (2H, d, J = 8.4 Hz, CN4-
C6H4-CO2CH3 Ho), 3.80 (3H, s, CO2CH3). 13C NMR (δ, ppm,
DMSO-d6): 197.7, 196.4, 166.7, 161.5, 155.2, 147.3, 140.7, 134.3,
131.2, 130.5, 130.2, 128.6, 127.6, 126.5, 53.0.
H
5,6), 8.19-8.12 (2H, m, phen H3,8), 7.51-7.48 (2H, m,
CN4-C6H5 Ho), 7.26-7.24 (3H, m, CN4-C6H5 Hm, Hp). 13C
NMR (δ, ppm, DMSO-d6): 197.9, 194.6, 162.1, 155.2, 147.5,
140.7, 131.2, 130.1, 129.5, 129.4, 128.6, 127.6, 126.4.
fac-[Re(bipy)(CO)3(Tbdz)]. The title compound was purified
via filtration from hot acetonitrile. Yield 0.092 g (62%). Mp
265 ꢀC (dec.). Elemental analysis for C21H13N6O4Re: calcd: C
42.07, H 2.19, N 14.02; found: C 42.00, H 2.26, N 14.07. IR (ν,
cm-1): 3067 w, 3059 w, 2823 w, 2731 w, 2021 s (CO, A0(1)), 1885
s br (CO, A0(2)/A00), 1696 m (aldehyde CO), 1603 m (tetrazole
CN), 1573 w, 1531 w, 1494 w, 1470 m, 1446 m, 1388 w, 1312 w,
1302 w, 1281 w, 1265 w, 1244 w, 1205 m, 1159 w, 1119 w, 1103 w,
1071 w, 1037 w, 1004 w, 977 w, 833 m, 767 m, 756 m, 731 m, 693 w.
fac-[Re(phen)(CO)3(Tbdz)]. The title compound was purified
via column chromatography using a 4:6 mixture CH2Cl2/
CH3CN as eluent (second fraction, yellow). Yield 0.056 g
(45%). Mp 268 ꢀC (dec.). Elemental analysis for C23H13N6-
O4Re: calcd: C 44.30, H 2.10, N 13.48; found: C 44.02, H 2.23, N
13.21. IR (ν, cm-1): 3107 w, 3069 w, 2020 s (CO, A0(1)), 1916 s
(CO, A0(2)), 1892 s (CO, A00), 1697 (aldehyde CO), 1631 w, 1609
m (tetrazole CN), 1574 w, 1519 w, 1444 w, 1426 m, 1397 w, 1347
w, 1303 w, 1224 w, 1204 w, 1166 w, 1145 w, 1119 w, 1040 w, 1003
w, 854 w, 757 w, 722 w. 1H NMR (δ, ppm, DMSO-d6): 9.92 (1H,
s, CN4-C6H4-CHO), 9.59 (2H, d, J = 3.8 Hz, phen H2,9), 8.99
(2H, d, J = 8.2 Hz, phen H4,7), 8.31 (2H, s, phen H5,6),
8.16-8.09 (2H, m, phen H3,8), 7.82 (2H, d, J = 8.4 Hz, CN4-
C6H4-CHO Hm), 7.73 (2H, d, J = 8.3 Hz, CN4-C6H4-CHO
Ho). 13C NMR (δ, ppm, DMSO-d6): 197.7, 194.9, 193.3, 161.5,
155.2, 147.4, 140.7, 136.7, 135.4, 131.2, 130.9, 128.6, 127.6, 126.8.
fac-[Re(bipy)(CO)3(Tacy)]. The title compound was purified
via filtration from hot acetonitrile. Yield 0.052 g (30%). Mp
260 ꢀC (dec.). Elemental analysis for C22H15N6O4Re: calcd: C
43.06, H 2.46, N 13.70; found: C 42.91, H 2.52, N 13.62. IR (ν,
cm-1): 3082 w, 3058 w, 3035 w, 2017 s (CO, A0(1)), 1898 s br (CO,
A0(2)/A00), 1679 m (acetyl CO), 1604 m (tetrazole CN), 1570 w,
1530 w, 1497 w, 1475 m, 1445 w, 1418 w, 1359 w, 1320 w, 1301 w,
1283 w, 1254 m, 1175 w, 1161 w, 1123 w, 1108 w, 1076 w, 1038 w,
1006 w, 960 w, 913 w, 841 w, 776 m, 759 m, 730 m, 720 w, 662 w.
fac-[Re(phen)(CO)3(Tacy)]. The title compound was purified
via filtration from hot acetonitrile. Yield (bulk, not optimized)
0.029 g (22%). Mp 245 ꢀC (dec.). Because of the very low
solubility of the title compound, apart from in hot DMSO,
purification of the bulk material proved particularly challen-
ging. The bulk material was purified several times before an
elemental analysis consistent with the formulation could be ob-
tained. Elemental analysis for C24H15N6O4Re: calcd: C 45.21, H
2.37, N 13.18; found: C 45.52, H 2.43, N 13.25. IR (ν, cm-1):
3046 w, 3006 w, 2016 s (CO, A0(1)), 1913 s (CO, A0(2)), 1887
(CO, A00), 1675 m (acetyl CO), 1630 w, 1612 m (tetrazole CN),
1585 w, 1572 w, 1531 w, 1519 w, 1427 m, 1359 w, 1300 w, 1285 w,
1263 m, 1226 w, 1175 w, 1159 w, 1147 w, 1123 w, 1107 w, 1040 m,
1024 w, 1006 w, 958 w, 847 m, 772 w, 758 w, 720 m.
X-ray Crystallography. Unless otherwise specified, diffrac-
tion data were collected at T = 100(2) K on an Oxford Diffrac-
tion Gemini diffractometer fitted with graphite-monochro-
˚
mated Mo KR radiation (λ = 0.71073 A). Following analytical
absorption corrections and solution by direct methods, the
structures were refined against F2 with full-matrix least-squares
using the program SHELXL-97.56 All H-atoms were added at
calculated positions and refined by use of riding models with
isotropic displacement parameters based on those of the parent
atoms. Anisotropic displacement parameters were employed
throughout for the non-hydrogen atoms.
X-ray Crystal Data for fac-[Re(bipy)(CO)3(Tph)]. Diffraction
data were collected on a Bruker X8 Apex KAPPA CCD
diffractometer fitted with graphite-monochromated Mo KR
X-ray radiation at 123(2) K and corrected for absorption using
the SADABS package. Empirical formula C20H13N6O3Re; for-
mula weight 571.56; crystal system: orthorhombic; space group
Pbca; unit cell dimensions a = 11.5585(4), b = 9.2000(3), c =
3
˚
˚
35.8140(14) A; volume 3808.4(2) A ; Z = 8; density (calculated)
1.994 Mg/m3; absorption coefficient 6.418 mm-1; F(000) 2192;
crystal size 0.19 ꢀ 0.19 ꢀ 0.14 mm3; θ range for data collection
2.10 to 27.87ꢀ; reflections collected 14294; independent reflec-
tions 4328 [R(int) = 0.1412]; completeness to θ = 25.00ꢀ 95.4%;
data/restraints/parameters 4328/108/271; goodness-of-fit on F2
0.998; final R indices [I > 2σ(I)] R1 = 0.0656, wR2 = 0.1210; R
indices (all data) R1 = 0.1750, wR2 = 0.1593.
X-ray Crystal Data for fac-[Re(phen)(CO)3(Tph)]. Empirical
formula C22H13N6O3Re; formula weight 595.58; crystal system:
triclinic; space group P1; unit cell dimensions a = 9.2520(3), b =
˚
9.3821(3), c = 12.9643(4) A, R = 96.860(2), β = 105.448(3),
3
˚
γ = 110.008(3)ꢀ; volume 991.29(5) A ; Z = 2; density (calcu-
lated) 1.995 Mg/m3; absorption coefficient 6.169 mm-1; F(000)
572; crystal size 0.39 ꢀ 0.25 ꢀ 0.06 mm3; θ range for data
collection 3.83 to 40.36ꢀ; reflections collected 54643; indepen-
dent reflections 12252 [R(int) = 0.0516]; completeness to θ =
40.00ꢀ 99.2%; data/restraints/parameters 12252/0/289; good-
ness-of-fit on F2 1.003; final R indices [I > 2σ(I)] R1 = 0.0370,
wR2 = 0.0873; R indices (all data) R1 = 0.0461, wR2 = 0.0901.
X-ray Crystal Data for fac-[Re(bipy)(CO)3(Tbdz)]. Since the
crystals were found to be invariably twinned, a number of
samples were cleaved in attempts to obtain a suitable crystal.
Also, since there appeared to be some crystal deterioration at
lower temperatures, diffraction data were collected at 220(2) K.
Empirical formulaC21H13N6O4Re; formulaweight599.57 g/mol;
crystal system: monoclinic; space group P21/n; unit cell dimen-
fac-[Re(bipy)(CO)3(Tmeb)]. The title compound was purified
via filtration from hot acetonitrile. Yield 0.043 g (32%). Mp
275 ꢀC (dec.). Elemental analysis for C22H15N6O5Re: calcd: C
41.97, H 2.40, N 13.35; found: C 42.05, H 2.53, N 13.49. IR
(ν, cm-1): 3075 w, 3035 w, 2949 w, 2020 s (CO, A0(1)), 1902 s br
(CO, A0(2)/A00), 1721 s (ester CO), 1603 m (tetrazole CN), 1573
w, 1496 m, 1474 m, 1447 m, 1433 m, 1379 m, 1306 w, 1271 m,
1175 m, 1123 m, 1094 m, 1036 w, 1005 w, 863 w, 779 m, 740 m,
732 m, 694 w.
˚
sions a = 8.0117(2), b = 8.1105(3), c = 31.1788(10) A, β =
3
˚
94.702(2)ꢀ; volume 2019.15(11) A ; Z = 4; density (calculated)
1.972 Mg/m3, absorption coefficient 6.061 mm-1; F(000) 1152;
crystal size 0.13 ꢀ 0.08 ꢀ 0.08 mm3; θ range for data collection
3.58 to 27.50ꢀ; reflections collected 4684; independent reflec-
tions 4616 [R(int) = 0.053]; completeness to θ = 27.50ꢀ 99.0%;
(56) Sheldrick, G. M. Acta Crystallogr. 2008, A64, 112.

Article
Inorganic Chemistry, Vol. 50, No. 4, 2011 1233
data/restraints/parameters 4616/0/291; goodness-of-fit on F2
1.183; final R indices [I > 2σ(I)] R1 = 0.0862, wR2 = 0.1983;
R indices (all data) R1 = 0.0972, wR2 = 0.2036.
SIESTA methodology57 and package. Here the Generalized
Gradient Approximation has been employed based on the
AM05 exchange-correlation functional.58 This functional has
been found to yield improved geometries relative to many other
GGA functionals, which typically show a systematic under
binding, while being comparable to parametrizations that are
explicit designed specifically for the solid-state.
X-ray Crystal Data for fac-[Re(phen)(CO)3(Tbdz)]. Empirical
formula C23H13N6O4Re; formula weight 623.59 g/mol; crystal
system: monoclinic; space group P21/n; unit cell dimensions a =
˚
9.6886(5), b = 10.7874(3), c = 20.4181(18) A, β = 95.615(7)ꢀ;
3
3
˚
Within the SIESTA methodology the core electrons and
nuclei are represented through the use of norm-conserving pseu-
dopotentials, and the modified Troullier-Martins form59 has
been used in the present work. Valence configurations consisting
of the 2s and 2p orbitals were employed for C, N, and O, while
for Re the 6s25d2 electronic state was explicitly considered. A
double-ζ polarized (DZP) basis set was employed for all ele-
ments except Re, for which no f polarization functions were
included. A split-norm of 0.15 was used to create the second zeta
for heavy elements, while a larger value of 0.5 was specified for
hydrogen. The numerical basis functions were strictly confined
within radii of 7 Bohr for all functions, except for those of Re (8
Bohr) and the explicit polarization function of H (3.5 Bohr).
Soft confinement60 was employed for all basis functions with a
potential of 100 Ry and inner radius of 0.95 times the outer
confinement value. In the case of the polarization functions the
soft confinement parameters were modified such that the inner
radii are zero and the potentials were 120 and 70 Ry for heavy
atoms and hydrogen, respectively.
volume 2123.8(2) A ; Z = 4; density (calculated) 1.950 Mg/m ;
absorption coefficient 5.767 mm-1; F(000) 1200; crystal size
0.25 ꢀ 0.14 ꢀ 0.11 mm3; θ range for data collection 3.55 to
34.96ꢀ; reflections collected 29861; independent reflections 8768
[R(int) = 0.0566]; completeness to θ = 34.00ꢀ 98.6%; data/
restraints/parameters 8768/0/307; goodness-of-fit on F2 0.786;
final R indices [I > 2σ(I)] R1 = 0.0304, wR2 = 0.0414; R indices
(all data) R1 = 0.0680, wR2 = 0.0445.
X-ray Crystal Data for fac-[Re(bipy)(CO)3(Tacy)]. Empirical
formula C22H15N6O4Re; formula weight 613.60 g/mol; crystal
system: triclinic; space group P1; unit cell dimensions a =
˚
7.5265(2), b = 8.4382(2), c = 16.4431(4) A, R = 98.986(2), β =
3
˚
96.361(2), γ = 91.788(2)ꢀ; volume 1023.88(4) A ; Z = 2; density
(calculated) 1.990 Mg/m3; absorption coefficient 5.979 mm-1
;
F(000) 592; crystal size 0.19 ꢀ 0.12 ꢀ 0.07 mm3; θ range for data
collection 3.73 to 36.39ꢀ; reflections collected 29744; indepen-
dent reflections 9550 [R(int) = 0.0381]; completeness to θ=35.50ꢀ
99.4%; data/restraints/parameters 9550/0/299; goodness-of-fit on
F2 0.968; final R indices [I > 2σ(I)] R1 = 0.0253, wR2 = 0.0538;
R indices (all data) R1 = 0.0325, wR2 = 0.0546.
The electron density was expanded in an auxiliary basis set
consisting of a Cartesian mesh with a fineness determined by the
equivalent kinetic energy cutoff of 250 Ry. In addition, to
minimize any breaking of translational invariance, grid cell
sampling was employed based on averaging of the forces over
two positions displaced by half the mesh spacing along each
axis. For calculations on the crystallographic unit cell, integra-
tion across the Brillouin zone was performed using a Mon-
khorst-Pack mesh determined according to an equivalent real
X-ray Crystal Data for fac-[Re(phen)(CO)3(Tacy)]. Diffrac-
tion data were collected at 100(2) K on an Oxford Diffraction
Xcalibur diffractometer fitted with graphite-monochromated
Mo KR radiation. Empirical formula C24H15N6O4Re; formula
weight 637.62 g/mol; crystal system: monoclinic; space group
P21/n; unit cell dimensions a = 7.46260(10), b = 17.8403(3), c =
3
˚
˚
16.7636(3) A, β = 99.457(2)ꢀ; volume 2201.49(6) A ; Z = 4;
density (calculated) 1.924 Mg/m3; absorption coefficient 5.566
mm-1; F(000) 1232; crystal size 0.17 ꢀ 0.15 ꢀ 0.05 mm3; θ range
for data collection 3.06 to 37.18ꢀ; reflections collected 79975;
independent reflections 11100 [R(int) = 0.0461]; completeness to
θ = 37.18ꢀ 98.0%; data/restraints/parameters 11100/0/317; good-
ness-of-fit on F2 0.914; final R indices [I > 2σ(I)] R1 = 0.0236,
wR2 = 0.0438; R indices (all data) R1 = 0.0427, wR2 = 0.0461.
X-ray Crystal Data for fac-[Re(bipy)(CO)3(Tmeb)]. Empirical
formula C22H15N6O5Re; formula weight 629.60 g/mol; crystal
system: triclinic; space group P1; unit cell dimensions a =
˚
space cutoff of 12 A. For supercell calculations simulating an
isolated molecule only the Γ point was sampled. Full geometry
optimization of the atomic coordinates was performed within
the crystallographic unit cell using a tight convergence threshold
˚
of 0.002 eV/A. A more stringent than usual tolerance was found
to be necessary to ensure proper convergence of the torsional
angles.
Computational Methods: Solution. To confirm the interpreta-
tion of the photophysical results we performed a series of
molecular quantum mechanical calculations using Gaussian
basis sets, as implemented within the program GAUSSIAN-
09.61 The X-ray determined geometries of all the complexes were
first relaxed in vacuum using the B3LYP functional,62,63 the
˚
7.0089(2), b = 8.4369(2), c = 18.7123(4) A, R = 102.980(2)ꢀ,
3
˚
β = 99.297(2)ꢀ, γ = 91.745(2)ꢀ; volume 1061.52(5) A ; Z = 2;
density (calculated) 1.970 Mg/m3; absorption coefficient 5.773
mm-1; F(000) 608; crystal size 0.31 ꢀ 0.12 ꢀ 0.045 mm3; θ range
for data collection 3.72 to 41.06ꢀ; reflections collected 43713;
independent reflections 13752 [R(int) = 0.0302]; completeness
to θ = 40.50ꢀ 99.8%; data/restraints/parameters 13752/0/308;
goodness-of-fit on F2 0.917; final R indices [I > 2σ(I)] R1 =
0.0193, wR2 = 0.0335; R indices (all data) R1 = 0.0277, wR2 =
0.0342.
(57) Soler, J. M.; Artacho, E.; Gale, J. D.; Garcia, A.; Junquera, J.;
Ordejon, P.; Sanchez-Portal, D. J. Phys.: Condens. Matter 2002, 14, 2745.
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2001, 64, 235111.
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M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Patersson,
G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H. P.; Izmaylov, A. F.;
Bloino, J.; Zheng, G.; Sonnengerg, 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., J. A.; 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. Gaussian 09,
Revision A.02; Wallingford, CT, 2009.
X-ray Crystal Data for fac-[Re(phen)(CO)3(Tmeb)]. Empiri-
cal formula C24H15N6O5Re; formula weight 653.62 g/mol;
crystal system: monoclinic; space group P21/c; unit cell dimen-
˚
sions a = 15.2843(4), b = 19.4828(4), c = 15.8578(3) A, β =
3
˚
102.071(3)ꢀ; volume 4617.74(18) A ; Z = 8; density (calculated)
1.88 Mg/m3; absorption coefficient 5.313 mm-1; F(000) 2528;
crystal size 0.35 ꢀ 0.19 ꢀ 0.02 mm3; θ range for data collection
3.77 to 30.0ꢀ; reflections collected 47138; independent reflections
13451 [R(int) = 0.0764]; completeness to θ = 30.00ꢀ, 99.8%;
data/restraints/parameters 13451/0/651; goodness-of-fit on F2
0.786; final R indices [I > 2σ(I)] R1 = 0.0406, wR2 = 0.0685; R
indices (all data) R1 = 0.0920, wR2 = 0.0743.
Computational Methods: Solid-State. All solid-state quantum
mechanical calculations have been performed within the Kohn-
Sham density functional theory, as implemented within the
(62) Becke, A. D. J. Chem. Phys. 1993, 98, 5648.
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1234 Inorganic Chemistry, Vol. 50, No. 4, 2011
Werrett et al.
Stuttgart-Dresden (SDD) Effective Core Potential64 for the Re
atoms, and the 6-31G** basis set for the C, H, N, and O atoms.
The ground state structures were then reoptimized in the pre-
sence of the PCM solvation model65 with parameters set appro-
priate to mimic dichloromethane as a medium. To check the
convergence of the calculations with respect to the basis set, the
final geometries were reoptimized in dichloromethane with the
6-311G** basis set. In the latter case, no significant differences
Table 1. Stretching Frequency Values (cm-1) for the Carbonyl Bands of the
Prepared Complexesa
compound
A0(1)
A0(2)
A00
fac-[Re(bipy)(CO)3Cl]
fac-[Re(phen)(CO)3Cl]
fac-[Re(bipy)(CO)3(NCCH3)][BF4]
fac-[Re(phen)(CO)3(NCCH3)]þ
2013
2014
2031
2040c
1891
1923
1872
1885
1909b
1938c
˚
have been observed in the geometries (atomic rmsd <0.02 A).
fac-[Re(bipy)(CO)3(Tph)]
fac-[Re(phen)(CO)3(Tph)]
fac-[Re(bipy)(CO)3(Tbdz)]
fac-[Re(phen)(CO)3(Tbdz)]
fac-[Re(bipy)(CO)3(Tacy)]
fac-[Re(phen)(CO)3(Tacy)]
fac-[Re(bipy)(CO)3(Tmeb)]
fac-[Re(phen)(CO)3(Tmeb)]
2018
2019
2021
2020
2017
2016
2020
2020
1893b
1909
The electronic absorption spectra for the singlet-singlet transi-
tions were then computed using Time-Dependent Density
Functional Theory (TDDFT) at the B3LYP/6-311þþG**//
B3LYP/6-311G** level again in the presence of the PCM
continuum solvent model. The nine lowest lying excitation
energies were determined in all cases, except for one ligand
(Tph), where an additional three transitions were computed to
locate the strong transition at the lower bound of the observed
experimental spectrum.
1884
1892
1887
1887
1885b
1898b
1902b
1916
1913
1912
a Spectra recorded in solid state. b The A0(2) and A00 modes appear as
one broad signal. c Previously published data obtained from acetonitrile
solution.66
Results and Discussion
Therefore, the specific solubility of each compound de-
termined the purification method used, with the more
soluble complexes isolated via alumina filled column chro-
matography. Generally, three yellow fractions were ob-
tained: (i) the first fraction eluted with a CH2Cl2/CH3CN
8:2 mixture, always present in very low amount, was
identified as the previously unreacted fac-[Re(N∧N)-
(CO)3Cl] from the halogen extraction reaction; (ii) the
second fraction eluted with a CH2Cl2/CH3CN 4:6 mix-
ture corresponded to the targeted tetrazolato complex;
(iii) the third fraction, eluted with CH3CN/MeOH 8:2
was not clearly identified and it contained a mixture of Re
complexes, with analysis suggesting positively charged
species. The more insoluble complexes were purified via
filtration from hot acetonitrile to remove dissolved by-
products.
IR and NMR Spectroscopy. All the synthesized fac-[Re-
(N∧N)(CO)3(L)]-type species display IR spectra consis-
tent both with the facial configuration of the carbonyl
ligands and with the Cs symmetry of the complexes, as
witnessed by the presence of three intense absorptions
which are observed in the 2022 and 1880 cm-1 region
(Table 1). According to previous reports on similar com-
pounds, the sharp band at higher frequency, about 2020
cm-1, was attributed to the A0(1) mode (totally symme-
tric in-phase stretching of the three CO ligands), whereas
the remaining two bands at intermediate and lower
frequencies were assigned to the A0(2) (totally symmetric
out-of-phase stretching) and A00 modes (asymmetric
stretching of the equatorial CO ligands).66-69 As some-
times observed in fac-[Re(N∧N)(CO)3(L)]-type com-
plexes, in the case N∧N = bipy, the A0(2) and A00
bands are superimposed into a single broad band.34,66,70
The relative frequencies of the carbonyl peaks are con-
sistent with the neutral nature of the complexes. The
values are lowered with respect to their corresponding
cationic precursors,66 confirming an increase in electron
Synthesis of the Complexes. The four tetrazole ligands
used in this work were prepared by addition of NaN3 to
the corresponding nitrile in toluene and in the presence of
triethylammonium chloride, using a method reported by
Koguro and co-workers.51 The cationic precursor fac-
[Re(N∧N)(CO)3(NCCH3)][BF4] was prepared following
previously published procedures:55 [Re(CO)5Cl] was first
treated with the corresponding diimine to displace two
carbonyl ligands, followed by chloride extraction with
AgBF4 in refluxing acetonitrile. fac-[Re(N∧N)(CO)3-
(NCCH3)][BF4] was finally reacted with the correspond-
ing tetrazole ligand in the presence of triethylamine in
refluxing acetonitrile to yield the desired products. Fol-
lowing this procedure (Figure 1) we were able to obtain all
the Re complexes in moderate to good yields. The chlor-
ide extraction reaction was always performed by strictly
using 1 equiv of AgBF4 with respect to fac-[Re(N∧N)-
(CO)3Cl]. If an excess of silver salt was used, we noticed
the formation of an off-white precipitate when the catio-
nic precursor was then reacted with the tetrazole ligand
and the triethylamine base. In this case the yield of the
final Re complexes was significantly lowered. This un-
wanted byproduct was light sensitive as its color darkened
upon prolonged exposure to light. Its IR spectrum was
very similar to that of the tetrazolate ligand; hence, we
concluded that the precipitate was mainly composed of
silver tetrazole salts. Using a 1:1 stoichiometric ratio
between AgBF4 and the Re precursor avoided the forma-
tion of the byproduct and improved the overall yields.
Although the complexes have a similar chemical struc-
ture, with small variations only in the nature of the diimine
ligand N∧N and the para substituent on the aryltetrazo-
lato ligand, their solubilities are quite different. The majo-
rity of the complexes are in fact fairly insoluble, some of
them, namely, fac-[Re(bipy)(CO)3(Tbdz)], fac-[Re(bipy)-
(CO)3(Tacy)], fac-[Re(phen)(CO)3(Tacy)], and fac-[Re-
(bipy)(CO)3(Tmeb)], are not completely soluble even in
hot DMSO. On the other hand, the three complexes
fac-[Re(bipy)(CO)3(Tph)], fac-[Re(phen)(CO)3(Tph)], and
fac-[Re(phen)(CO)3(Tbdz)] are soluble in acetonitrile,
chlorinated and more polar solvents such as DMSO.
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Article
Inorganic Chemistry, Vol. 50, No. 4, 2011 1235
Table 2. NMR Chemical Shift Data (ppm, DMSO-d6) Relative to Ho, Hm, C5 for the Tetrazole Ligands and the Re Complexes
Re complexes
corresponding 5-aryl-1H-tetrazole
compound
δHo
δHm
δC5
δHo
δHm
δC5
fac-[Re(bipy)(CO)3(Tph)]
fac-[Re(phen)(CO)3(Tph)]
fac-[Re(phen)(CO)3(Tbdz)]
fac-[Re(phen)(CO)3(Tmeb)]
7.67-7.65a
7.51-7.48a
7.73
7.36-7.32a,b
7.26-7.24a,b
7.82
162.3
162.1
161.5
161.5
8.05-8.03a
7.61-7.58a,b
155.2
8.25
8.17
8.11
8.13
155.5
155.3
7.66
7.87
a Multiplet range. b The peak includes the resonances of Hm and Hp.
density for the Re centers when the anionic tetrazolato
ligand displaces the coordinated acetonitrile molecule.
When compared with the parent neutral fac-[Re(N∧N)-
(CO)3Cl] complexes, a slight increase in the carbonyl fre-
quencies is observed. The increment was ascribed to the π
accepting nature of the tetrazole ring, which in turn cau-
ses a slight decrease of electron density around the Re
center upon backbonding involving the 5d(Re) and the
π*(tetrazole) orbitals. Although aryltetrazolato ligands
coordinated to [FeCp(CO)2]þ fragments evidenced a
strong π-accepting character,49,71 the metal to ligand
backbonding interaction seems to be somewhat reduced
for the Re complexes investigated within this work. This
effect might be explained by considering the less favorable
overlap of 5d(Re) orbitals with 2p(N) orbitals rather than
3d(Fe) with 2p(N).
Figure 2. Representationofthe N1 andN2coordinatedlinkage isomers,
with the latter also reporting the NMR assigment scheme.
tetrazole ring. The comparison of the two sets of data
evidences the presence of a stronger mesomeric electron
donating effect (þM) of the tetrazole ring in the com-
plexes, which is also consistent with an extended inter-
annular conjugation. This conjugation suggests co-
planarity between the two rings as previously shown by
the studies of Butler and co-workers.72,73 In the case of the
parent 5-aryl-1H-tetrazole species, the steric hindrance
caused by the N1-H and the adjacent phenyl C-H bond
favors the rotation of the phenyl ring with respect to the
tetrazole one, therefore diminishing the conjugation and
hence the intensity of the þM donation, thus resulting in a
predominant -I effect. Therefore, the NMR data also
support that the coordination of the Re center to the
tetrazole ring is regioselective for the N2 position.49,71,74,75
All of the complexes show in their 13C NMR spectra
two close peaks belonging to the three carbonyl ligands in
the 198-194 ppm region. This inequivalence was ascribed
to the two different N-donor ligands (bidentate diimine
and tetrazolato) in trans to the carbonyl ones, consistently
with a Cs point group.
X-ray Structural Investigation. Single crystals of all the
prepared compounds were grown from hot saturated
DMSO solutions. The most insoluble complexes were
heated in a DMSO solution (≈160 ꢀC) and any undis-
solved solid was immediately filtered through Celite. The
clear solution was then left to cool down undisturbed to
room temperature. If after a couple of days no crystals
appeared, the whole process was started again by reheat-
ing the solution. We used this crystallization method also
for the more soluble complexes, as layering the corre-
sponding dichloromethane solutions with hexane or
diethylether yielded crystals of poorer X-ray diffraction
quality. The structures of the Re complexes are shown in
Figures 3-6. All the structures contain one individual
complex per asymmetric unit, with the exception of
1H and 13C NMR data were obtained from deuterated
DMSO solutions. The number of peaks, chemical shifts,
and integration values for each spectrum are consistent
with the proposed formulation of the complexes, namely,
fac-[Re(bipy)(CO)3(Tph)], fac-[Re(phen)(CO)3(Tph)], fac-
[Re(phen)(CO)3(Tbdz)], and fac-[Re(phen)(CO)3(Tmeb)].
On the other hand, it was not possible to obtain accurate
NMR data for the remaining complexes because of their
lower solubility. Table 2 reports the chemical shift values
for Ho, Hm, and C5 of the phenyl and tetrazole rings
respectively (see Figure 2 for a referencing scheme relative
to the NMR assignment). Two sets of data were collected:
the first on the 5-aryl-1H-tetrazole species and the second
after their coordination to the Re center. In the case
of TphH, fac-[Re(bipy)(CO)3(Tph)], and fac-[Re(phen)-
(CO)3(Tph)] the chemical shift values of Ho and Hm can be
easily assigned taking into consideration the deshielding
effect caused by the tetrazole ring and the lack of an
electron withdrawing group in the para position. For the
other species the chemical shifts of the Ho and Hm were
assigned by analyzing also the 1D NOE (only for TbdzH
and fac-[Re(phen)(CO)3(Tbdz)]) and HMBC spectra. As
it can be seen from the values in Table 2, in each case there
is a slight shielding effect on both Ho and Hm upon
coordination of the aryltetrazolato ligand to the Re center.
Moreover, the Ho in fac-[Re(phen)(CO)3(Tbdz)] and
fac-[Re(phen)(CO)3(Tmeb)] is found to be more shielded
with respect to Hm, a situation which is reversed com-
pared to the parent 5-aryl-1H-tetrazole species. On the
other hand, the chemical shift of the C5 in the complexes is
deshielded of about 6-7 ppm after coordination. The
deshielding of the Ho and Hm and the shielding of C5 in
the 5-aryl-1H-tetrazole molecules are mainly caused by
the inductive electron withdrawing nature (-I) of the
(72) Butler, R. N.; Garvin, V. C.; Lumbroso, H.; Liegeois, C. J. Chem.
Soc., Perkin Trans. 2 1984, 721.
(73) Butler, R. N.; Garvin, V. C. J. Chem. Soc., Perkin Trans. 1 1981, 390.
(74) Butler, R. N.; McEvoy, T. M. J. Chem. Soc., Perkin Trans. 2 1978,
1087.
(71) Palazzi, A.; Stagni, S.; Selva, S.; Monari, M. J. Organomet. Chem.
2003, 672, 130.
(75) Palazzi, A.; Stagni, S. J. Organomet. Chem. 2005, 690, 2052.

1236 Inorganic Chemistry, Vol. 50, No. 4, 2011
Werrett et al.
Figure 3. X-ray crystal structure of fac-[Re(bipy)(CO)3(Tph)] (left) and fac-[Re(phen)(CO)3(Tph)] (right) where thermal ellipsoids have been drawn at the
50% probability level. Hydrogen atoms are omitted for clarity.
Figure 4. X-ray crystal structure of fac-[Re(bipy)(CO)3(Tbdz)] (left) and fac-[Re(phen)(CO)3(Tbdz)] (right) where thermal ellipsoids have been drawn at
the 30% and 50% probability level respectively. Hydrogen atoms are omitted for clarity.
Figure 5. X-ray crystal structure of fac-[Re(bipy)(CO)3(Tacy)] (left) and fac-[Re(phen)(CO)3(Tacy)] (right) where thermal ellipsoids have been drawn at
the 50% probability level. Hydrogen atoms are omitted for clarity.
fac-[Re(phen)(CO)3(Tmeb)] (Figure 6), which contains
two crystallographically independent complexes per asym-
metric unit. In this case, the two complexes are approxi-
also suggests that neither in solution nor in the solid
state is the N1 linkage isomer formed. This can be ascribed
to a combination of steric bulkiness of both the fac-[Re-
(N∧N)(CO)3]þ fragment and the benzene ring attached to
the tetrazole.
˚
mately related by a cell translation of 0.5 A along the
direction of the a axis, as shown in the cell projection in
Figure 7. The two complexes are similar with differences
confined to minor rotations of the ligands. The angles
between the plane of the phen and the tetrazole rings are
104.1(2)ꢀ and 103.2(2)ꢀ with those between the phenyl and
tetrazole rings being 9.1(2) ꢀ and 10.1(2)ꢀ for molecules 1 and
2, respectively.
The analysis of the structural data further confirms,
along with the 1H and 13C NMR results, that the coor-
dination of the Re center is regioselective for the N2
position of the tetrazole ring (Figure 2). This evidence
The extent of intermolecular interaction throughout
the lattice varies within each structure, with the observed
general trend that complexes bearing phen have a higher
tendency in favoring π interaction. Both fac-[Re(phen)-
(CO)3(Tph)] and fac-[Re(phen)(CO)3(Tbdz)] are arranged
to maximize π stacking between the phen units, with an
˚
average distance of 3.5 A. On the other hand, in both
fac-[Re(phen)(CO)3(Tacy)] and fac-[Re(phen)(CO)3(Tmeb)]
there is evidence of π stacking between the tetrazolato
˚
ligands and the phen units (≈ 3.5 A between the diimine

Article
Inorganic Chemistry, Vol. 50, No. 4, 2011 1237
Figure 6. X-ray crystal structure of fac-[Re(bipy)(CO)3(Tmeb)] (top) and fac-[Re(phen)(CO)3(Tmeb)] (bottom) where thermal ellipsoids have been drawn
at the 50% probability level. Hydrogen atoms are omitted for clarity.
and aryltetrazolato phenyl rings). Only fac-[Re(bipy)-
(CO)3(Tacy)] favors stacking between neighboring tetra-
zolato ligands instead of the bipy units, again with an ave-
˚
rage distance between the two planes of about 3.5 A. Fi-
nally, fac-[Re(bipy)(CO)3(Tph)],fac-[Re(bipy)(CO)3(Tbdz)],
and fac-[Re(bipy)(CO)3(Tmeb)] do not exhibit any evi-
dence of π stacking in the lattice. A correlation between
the interannular torsion (aryl and tetrazole rings) and the
intramolecular interaction seems to indicate that a higher
degree of rotation is present (dihedral angle values be-
tween 13ꢀ and 24ꢀ) when there is no evidence of π stacking
within the lattice.
Computational Investigation of the Tetrazole and Phenyl
Rings Co-planarity. Previous studies on metal complexes
bearing 5-aryltetrazolato ligands highlighted a correla-
tion between N2 coordination, extended interannular
conjugation, and the coplanar arrangement of the tetra-
zole and phenyl rings.49,75 This correlation was obtained
by direct comparison of the 1H and 13C NMR data with
solid-state structural determinations. While in our study
both the spectroscopic and the X-ray diffraction measure-
ments confirm the regioselective N2 coordination, the
Figure 7. Unit cell content of fac-[Re(phen)(CO)3(Tmeb)] as viewed
along the a axis.
solid-state structures reveal in some cases a significant
degree of rotation between the two rings, as summarized
in Table 3. It cannot be excluded that in solution at room
temperature the torsional angle is in fact averaged to a
more coplanar configuration. Therefore we performed
theoretical calculations to predict the relative arrange-
ment of the two rings in solution, as the information is
important for the interpretation of the photophysical
behavior of the complexes from solution data (see below).
For the case of fac-[Re(bipy)(CO)3(Tph)] the interplay
between crystal packing forces and the dihedral angle
between the aryl and tetrazole rings has been investi-
gated through quantum mechanical calculations. For the
solid-state material the structure was optimized within
the constraint of the crystallographically determined unit
cell. For complex condensed phases, the most readily
applicable form of quantum mechanics is density func-
tional theory, as has been used in the present work.
However, a known limitation of most functionals is the
absence of long-range attraction due to van der Waal’s
forces. While this can be overcome through the use of
either empirical corrections,76 through explicitly adding
(76) Grimme, S. J. Comput. Chem. 2004, 25, 1463.

1238 Inorganic Chemistry, Vol. 50, No. 4, 2011
Werrett et al.
Table 3. Dihedral Angle Values between the Tetrazole and the Phenyl Rings As
Obtained from X-ray Diffraction Data for the Re Complexes
into the [Re(N∧N)(CO)3]þ coordination sphere. The
MLLCT transitions were found to generate very broad
low energy absorption bands, as found in the present
work.23,24 The distinction between the IL and the
MLLCT nature of the transition was also confirmed by
the solvatochromic nature of the latter, for which a
greater electric dipole change is associated with the elec-
tronic transition. A hypsochromic shift of about Δλ =
15-20 nm was recorded for the complexes on passing
from dichloromethane to DMSO solutions (negative
solvatochromism). On the other hand, the transitions as-
signed to IL π-π* were found to be almost insensitive to
the solvent change, as expected for transitions associated
with a minimal electric dipole change.
Although the molar absorptivities of the MLLCT
bands of the complexes are of the same order of magni-
tude, those bands incorporating phen have about 50%
higher values as compared to those containing bipy.
Differences in molar absorptivity have previously been
noted in fac-[Re(N∧N)(CO)3L]þ complexes of bipy and
phen, albeit to a lesser extent (10%).82,83 The main general
difference in the λex and λem values (Table 4) arises from
the systematically lower absorption and emission energies
of the complexes that contain the bipy ligand. As the
LUMO level is mainly localized on the diimine N∧N
ligand for all the complexes (see below), this effect is
counterintuitive to the typically lower π* levels of phen as
compared to bipy. A similar trend has been noted with
increasing donor strength in ancillary ligands connected
to the [Re(N∧N)(CO)3]þ core.21 As the N∧N ligand is the
primary contributor to the LUMO level, this red-shift is
found in both the absorption and the emission bands of
the complexes.
compound
dihedral angle (deg)
fac-[Re(bipy)(CO)3(Tph)]
fac-[Re(phen)(CO)3(Tph)]
fac-[Re(bipy)(CO)3(Tbdz)]
fac-[Re(phen)(CO)3(Tbdz)]
fac-[Re(bipy)(CO)3(Tacy)]
fac-[Re(phen)(CO)3(Tacy)]
fac-[Re(bipy)(CO)3(Tmeb)]
fac-[Re(phen)(CO)3(Tmeb)]
20.37
3.26
23.51
1.41
1.81
12.64
13.17
10.62
on damped dispersion interactions, or via the use of non-
local functionals designed to reproduce van der Waal’s in-
teractions in particular cases,77 both approaches will in-
troduce greater inaccuracy than using the known unit cell.
After optimization, the calculated acute dihedral angles
between the aryl and tetrazole rings in the crystal were
found to be 24.6 and 28.2ꢀ (the presence of two distinct
values is due to the asymmetry of the tetrazole environ-
ment). These values compare to 20.4 and 26.8ꢀ in the
experimental structure, and thus the calculated dihedral
angles are slightly higher on average and with a reduced
splitting between the two angles. However, there is qua-
litative agreement in the behavior, in that there is a
twisting of the phenyl group with respect to the plane of
the tetrazole in the crystal structure.
In the calculations it is now possible to begin to explore
the influence of intermolecular forces on this dihedral
angle. First of all we reduce the contents of the unit cell to
contain only a single molecule, such that the only inter-
actions are with its own image. Because of the anisotropic
nature of the unit cell, the images are well separated in the
c direction, while there remains some weak interaction in
the remaining two crystallographic directions. With this
transformation the dihedral angles are already substan-
tially reduced to 5.3 and 6.1ꢀ. By doubling the unit cell
dimensions in the ab plane, to further reduce the inter-
molecular interactions, the dihedral angles are now de-
creased to 0.2 and 1.4ꢀ. A clear trend is apparent that as
the intermolecular interactions decrease, so the torsional
angles are tending to values close to zero and thus the aryl
and tetrazole rings are becoming coplanar to maximize
the degree of conjugation.
Photophysical Investigation. The absorption and emis-
sion data for the complexes are summarized in Table 4.
The absorption profiles (Figure 8) are very similar for all
of the complexes, with strong intraligand (IL) π-π*
absorption bands in the high energy UV (250-310 nm)
and in the low energy UV region (350-370 nm), and
tailing into the visible region, a mix of weaker ligand-to-
ligand charge-transfer (LLCT, tetrazolato to diimine)
and MLCT (Re to diimine) bands. The close proximity
of LLCT and MLCT “states” has previously been de-
scribed as one-electron excitations that mix to give rise to
a MLLCT state,67,78-81 especially when strong donor
ancillary ligands, such as isocyanides, were incorporated
The broad and featureless 3MLLCT emission bands are
found between 566 and 578 nm (Figure 8 insets). In
general, as it can be seen from the values of τ and Φ in
Table 4, the complexes fac-[Re(phen)(CO)3(L)] show sig-
nificantly longer excited state lifetimes and greater quan-
tum yields compared to the analogous fac-[Re(bipy)-
(CO)3(L)]. The longer excited state lifetimes and quantum
yields are associated with the presence of the more rigid
phen ligands.82 In all cases, monoexponential decays were
observed and excitation spectra of the compounds were
similar to the absorption spectra, showing the same broad
MLLCT band.
The aryltetrazolato ligands offer the ability to fine-tune
the absorption and emission energies of the complexes,
and consequently the photophysical performance in
terms of lifetime τ and quantum yield Φ, by way of their
substituents. The electron-donating or withdrawing ca-
pacity of the para aryltetrazole R group will influence the
energy of any LLCT directly, and indirectly the energy of
the MLCT via the Re 5d orbitals. In this study, the
presence of EWGs lowers the energy of the HOMO, thus
blue-shifting the MLLCT. The effect, albeit very subtle in
some cases (e.g., compare the absorption wavelength of
fac-[Re(bipy)(CO)3(Tph)] at 370 nm and fac-[Re(bipy)-
(CO)3(Tbdz)] at 367 nm), is also supported by quantum
(77) Dion, M.; Rydberg, H.; Schroeder, E.; Langreth, D. C.; Lundqvist,
B. I. Phys. Rev. Lett. 2004, 92, 246401.
(78) Vlcek, A.; Zalis, S. Coord. Chem. Rev. 2007, 251, 258.
(79) Vlcek, A.; Zalis, S. J. Phys. Chem. A 2005, 109, 2991.
(80) Zalis, S.; Ben Amor, N.; Daniel, C. Inorg. Chem. 2004, 43, 7978.
(81) Gabrielsson, A.; Busby, M.; Matousek, P.; Towrie, M.; Hevia, E.;
Cuesta, L.; Perez, J.; Zalis, S.; Vlcek, A. Inorg. Chem. 2006, 45, 9789.
(82) Juris, A.; Balzani, V.; Barigelletti, F.; Campagna, S.; Belser, P.;
von Zelewsky, A. Coord. Chem. Rev. 1988, 84, 85.
(83) Kalyanasundaram, K. J. Chem. Soc., Faraday Trans. 2 1986, 82,
2401.

Article
Inorganic Chemistry, Vol. 50, No. 4, 2011 1239
Table 4. Photophysical Properties of the Re Complexes in Deoxygenated Dichloromethane at Room Temperaturea
absorption
emission
compound
λ
ex (ε) nm (cm-1 M-1
)
λem nm
τ ns
Φb ( ( 15%)
fac-[Re(bipy)(CO)3Cl]c
385(3650)
380(4000)
343(4250)
600
577
526
51
300
1201
0.005
(0.036)e
0.41
fac-[Re(phen)(CO)3Cl]d
fac-[Re(bipy)(CO)3(NCCH3)]c
fac-[Re(bipy)(CO)3(Tph)]
fac-[Re(phen)(CO)3(Tph)]
fac-[Re(bipy)(CO)3(Tbdz)]
fac-[Re(phen)(CO)3(Tbdz)]
fac-[Re(bipy)(CO)3(Tacy)]
fac-[Re(phen)(CO)3(Tacy)]
fac-[Re(bipy)(CO)3(Tmeb)]
fac-[Re(phen)(CO)3(Tmeb)]
370(2900)
358(4250)
367(3050)
357(4950)
367(3050)
357(5350)
367(2250)f
357(5200)
578
571
574
566
574
566
574
569
102
740
130
945
119
955
122
923
0.012
0.054
0.018
0.099
0.017
0.097
0.016
0.079
a The excitation wavelength for the emission spectra corresponds to the maximum absorption of the MLLCT band. The excitation wavelength for the
lifetime measurements corresponds to 405 nm, whereas for the quantum yield corresponds to 355 nm. b Quantum yields were measured by the dilute
solution method using an aerated fac-[Re(bipy)(CO)3(NCCH3)][PF6] solution as reference. c Previously published data except for emission maximum.33
d Previously published data.21 e Literature value, measured in degassed benzene.21 f As the complex is strongly insoluble, the ε value is probably
underestimated.
Figure 9. Computed absorption spectra using TDDFT for the single-
t-singlet transitions of the fac-[Re(N∧N)(CO)3(L)] complexes in dichlor-
omethane. Intensities are taken from the results of the calculations, while
Gaussian broadening is applied using an exponent of 0.005 nm-2
.
mechanical calculations (see next section). As a result of
the (slight) blue-shift of the emissions for complexes coor-
dinated with aryltetrazolato ligands bearing an EWG, the
values of both τ and Φ are increased in accordance with
the energy gap law.34 By comparing the photophysical
data of the prepared complexes fac-[Re(N∧N)(CO)3(L)]
with fac-[Re(N∧N)(CO)3Cl] and fac-[Re(bipy)(CO)3-
(NCCH3)][PF6],34 the complexes bearing the aryltetrazo-
lato ligands show strong similarities with the neutral ones
coordinated to the Cl- ligand. The blue-shifted absorption
and emission energies, as well as the increased lifetimes
Figure 8. Absorption, emission (inset, full line) and excitation (inset,
dotted line) profiles for the fac-[Re(N∧N)(CO)3(L)] complexes in deox-
ygenated dichloromethane; excitation wavelengths for the emission spec-
tra correspond to the reported absorption maximum of the MLLCT
transition of each compound as found in Table 4.

1240 Inorganic Chemistry, Vol. 50, No. 4, 2011
Werrett et al.
Figure 10. Localization of the HOMO and LUMO orbitals for the four fac-[Re(bipy)(CO)3(L)] (top) and fac-[Re(phen)(CO)3(L)] complexes (bottom).
and quantum yields, are accounted for by the π-accepting
nature (albeit not of very strong intensity as confirmed by
the IR data) of the tetrazole ring compared with the π-
donating nature of the Cl- ligand. The latter destabilizes
the metal orbitals with consequent reduction of the
HOMO-LUMO gap and increased knr. However, other
effects, such as the flexibility of the aryl-tetrazolato
ligand, might need to be taken into consideration when
comparing analogous complexes bearing ancillary li-
gands of a different nature.
Calculated Absorption Spectrum. To confirm the inter-
pretation of the photophysical results we have performed
time-dependent density functional theory calculations to
determine the energetics and absorption spectra of the
complexes. Although all the complexes display a perfectly
planar geometry, we note that different rotations of the
aryltetrazolato ligand, with respect to the diimine N∧N
ligand (Supporting Information, Figure S1), generate al-
most iso-energetic stable configurations (within 0.01 eV).
These configurations have slightly shifted electronic tran-
sitions, and often quite different intensites. To allow for
this, the absorption spectra have been calculated using the
Boltzmann-weighted average over the configurations,
with Gaussian broadening of the peaks (Figure 9). The
relative energies of the different conformations, as well as
the peak positions and intensities are given in full for each
complex in the Supporting Information.
The overall appearance of the computed absorption
spectrum agrees well with the experimental data. In
particular, the order of strong absorption maxima in
the range 300-360 nm is consistent with the experimental
data. All complexes exhibit a similar weak absorption
close to 380 nm, while those of Tph have an additional
maximum in the region of 430-440 nm. Moreover, the
energy trend displayed by the singlet-singlet transitions
agrees with the hypothesis that the HOMO-LUMO gap
is increased by the presence of an EWG in the para
position of the aryltetrazolato ligand. Quantitatively,

Article
Inorganic Chemistry, Vol. 50, No. 4, 2011 1241
there are some differences in the precise positions of peaks
relative to experiment, which are likely to be a result of
both the limitations in the exchange-correlation descrip-
tion employed and also the approximate nature of the
solvent model used to describe the surrounding environ-
ment of dichloromethane.
Although Kohn-Sham states should be treated with
caution, because of the self-interaction error, the HOMO
and LUMO provide some useful insight as to the atomic
contributions. Here the density functional calculations
indicate that for all the complexes in the singlet state the
HOMO orbital is mainly localized on the Re center and
remarkably delocalized over the entire tetrazole ligand.
This delocalization also accounts for the mixing of
LLCT and MLCT. On the other hand, the LUMO is
localized exclusively on either the bipy or the phen
ligands (Figure 10).
The spectroscopic investigation has pointed out that the
tetrazolato anion is a good σ donor but also a partial π
acceptor, thus resulting in a slightly reduced electron density
on the metal center compared with the parent fac-[Re(N∧N)-
(CO)3X] (X = Cl, Br) complexes. The absorption and emis-
sion transitions, in accordance with theoretical calculations,
result from a mixing of LLCT and MLCT, which are there-
fore more properly described as MLLCT. The effect is also
visible in the photophysical data, and a trend related to a
blue-shift is also evidenced upon addition of EWG onto the
aryltetrazolato ligand. Since this blue-shift is the consequence
of a wider HOMO-LUMO gap, an improvement of τ and Φ
is also noted. However, at the same time these parameters
might also be adversely affected by the introduction of the
flexible aryltetrazolato ligand. This very last effect is cur-
rently under investigation.
All the relaxed configurations were also excited to a
triplet state and reoptimized in dichloromethane with
spin unrestricted B3LYP. These calculations confirm that
the singlet state is the ground state in all cases, while the
singlet-triplet energy difference lies in the range 467-516
nm, placing the transition in the gap as expected. The
singlet-triplet energy difference is systematically lower
by 10 nm, on average, for phenanthroline as a ligand,
which accords with the experimentally observed shift of
5-8 nm.
Acknowledgment. This work was supported by the
Australian Research Council and Curtin University
(IRGS-46251). M.M. also wishes to thank the ARC for
the APD Fellowship DP0985481. P.R. and J.D.G. thank
the ARC for fellowships under Grant DP0986999, as well
as NCI and iVEC for provision of computing resources.
G.S.H. thanks the Natural Sciences and Engineering
Research Council of Canada. A/Prof. Philip C. Andrews
and Prof. Peter C. Junk (Monash University) are kindly
acknowledged for providing the [Re(CO)5Cl]. M.M.,
S.M., and S.S. wish to thank Prof. A. Palazzi for helpful
discussion on IR spectroscopy. The referees are also
acknowledged for providing insightful feedback.
Conclusions
In this study, the direct synthesis of neutral fac-[Re(N∧N)-
(CO)3(L)] complexes, where N∧N is either bipy or phen and L
is a para functionalized 5-aryltetrazolato ligand, has been
investigated. The complexes exist exclusively as the N2
linkage isomer, which allows for maximization of interan-
nular conjugation between the tetrazole and phenyl rings
(although some of them show distortion in the solid-state).
Supporting Information Available: X-ray crystallographic
data in CIF format. Full details of the computed singlet-singlet
transition energies and oscillator strengths. This material is
available free of charge via the Internet at http://pubs.acs.org.