Tricarbonyl rhenium(I) complexes containing a bridging 2,5-diphenyl-1,3,4- oxadiazole ligand: Structural, spectroscopic, electrochemical, and computational characterization

Article abstract of DOI:10.1021/ic801447z

The three complexes [Re₂(μ-X¹)(μ-X ²)(CO)₆(μ-ppd-κN³:κN ⁴)] (X¹, X² = H, 1; X¹ = H, X ² = Cl, 2; X¹, X² = Cl, 3; ppd = 2,5-diphenyl-1,3,4-oxadiazole) have been synthesized by different routes, involving the reaction of [Re₄(μ₃-H) ₄(CO)₁₂] with ppd for 1, the reaction of 1 with HCl for 2, and the reaction of [ReCl(CO)₅] with ppd for 3. The three complexes possess a different number of valence electrons, so the formal Re-Re bond order varies from 2 to 1 to 0 in complexes 1, 2, and 3, respectively. This is reflected in the Re-Re bond distance (277.9, 297.9, and 358.5 pm in the same series) and in the stability of the complexes in the coordinating solvent acetonitrile (t_1/2 for ppd displacement 13.6, 4.5, and 3.7 h, for 1, 2, and 3, respectively). Both experimental and calculated structures indicates that coordination induces a distortion from planarity of the diphenyloxadiazole moiety due to the interaction of the equatorial carbonyls with the bridging ppd, which increases on going from 1 to 2 to 3 (dihedral angle between the oxadiazole and the phenyl rings 18.4°, 23.3°, and 45.0°, respectively). The UV spectra show π-π* transitions of the oxadiazole ligand (which shift to higher energy on increasing the distortion from the planarity, from 252 to 267 nm) and metal-to-ligand charge transfer absorptions (from 300 to 362 nm). Upon irradiation between 340 and 380 nm, complex 2 only features a weak broad emission at 527 nm (Φ = 0.02%), whereas upon excitation at 300 nm, the emission typical of free ppd is observed, suggesting photodissociation. Cyclic voltammetry investigations in acetonitrile showed that the three complexes exhibit ligand-centered irreversible reduction peaks (from -1.83 to -1.93 V vs Fc⁺|Fc), shifted to more positive values with respect to free ppd (-2.50 V). The shift however is smaller than in the analogous derivatives containing 1,2-diazines, suggesting a smaller electron depletion of the heterocycle ligand upon coordination. The complexes also show a metal-centered, bielectronic, irreversible oxidation peak (from 1.05 to 1.37 V vs Fc⁺|Fc). A combined density functional and time-dependent density functional (TD DFT) study allowed us to understand the factors affecting the stability of the three complexes and to rationalize their electrochemical and photophysical properties in terms of their electronic structure.

Full text of DOI:10.1021/ic801447z

Inorg. Chem. 2008, 47, 11154-11165
Tricarbonyl Rhenium(I) Complexes Containing a Bridging
2,5-Diphenyl-1,3,4-oxadiazole Ligand: Structural, Spectroscopic,
Electrochemical, and Computational Characterization
Matteo Mauro,† Monica Panigati,*,† Daniela Donghi,† Pierluigi Mercandelli,*,‡ Patrizia Mussini,§,|
Angelo Sironi,‡,|,⊥ and Giuseppe D’Alfonso†,|,⊥
Dipartimento di Chimica Inorganica, Metallorganica e Analitica (DCIMA) “L. Malatesta”,
UniVersita` di Milano and INSTM, UdR di Milano, Via Venezian 21, 20133 Milano, Italy,
Dipartimento di Chimica Strutturale e Stereochimica Inorganica (DCSSI), UniVersita` di Milano
and INSTM, UdR di Milano, Via Venezian 21, 20133 Milano, Italy, Dipartimento di Chimica
Fisica ed Elettrochimica (DCFE), UniVersita` di Milano and INSTM, UdR di Milano, Via Golgi 19,
20133 Milano, Italy, Centro di Eccellenza CIMaINa dell’UniVersita` di Milano, Via Celoria 16,
20133 Milano, Italy, and Istituto di Scienze e Tecnologie Molecolari (ISTM) del CNR, Via Golgi
19, 20133 Milano, Italy
Received July 31, 2008
The three complexes [Re2(µ-X1)(µ-X2)(CO)6(µ-ppd-κN3:κN4)] (X1, X2 ) H, 1; X1 ) H, X2 ) Cl, 2; X1, X2 ) Cl, 3; ppd
) 2,5-diphenyl-1,3,4-oxadiazole) have been synthesized by different routes, involving the reaction of [Re4(µ3-H)4(CO)12]
with ppd for 1, the reaction of 1 with HCl for 2, and the reaction of [ReCl(CO)5] with ppd for 3. The three complexes
possess a different number of valence electrons, so the formal Re-Re bond order varies from 2 to 1 to 0 in complexes
1, 2, and 3, respectively. This is reflected in the Re-Re bond distance (277.9, 297.9, and 358.5 pm in the same series)
and in the stability of the complexes in the coordinating solvent acetonitrile (t1/2 for ppd displacement 13.6, 4.5, and 3.7 h,
for 1, 2, and 3, respectively). Both experimental and calculated structures indicates that coordination induces a distortion
from planarity of the diphenyloxadiazole moiety due to the interaction of the equatorial carbonyls with the bridging ppd,
which increases on going from 1 to 2 to 3 (dihedral angle between the oxadiazole and the phenyl rings 18.4°, 23.3°, and
45.0°, respectively). The UV spectra show π-π* transitions of the oxadiazole ligand (which shift to higher energy on
increasing the distortion from the planarity, from 252 to 267 nm) and metal-to-ligand charge transfer absorptions (from
300 to 362 nm). Upon irradiation between 340 and 380 nm, complex 2 only features a weak broad emission at 527 nm
(Φ ) 0.02%), whereas upon excitation at 300 nm, the emission typical of free ppd is observed, suggesting photodissociation.
Cyclic voltammetry investigations in acetonitrile showed that the three complexes exhibit ligand-centered irreversible reduction
peaks (from -1.83 to -1.93 V vs Fc+|Fc), shifted to more positive values with respect to free ppd (-2.50 V). The shift
however is smaller than in the analogous derivatives containing 1,2-diazines, suggesting a smaller electron depletion of
the heterocycle ligand upon coordination. The complexes also show a metal-centered, bielectronic, irreversible oxidation
peak (from 1.05 to 1.37 V vs Fc+|Fc). A combined density functional and time-dependent density functional (TD DFT)
study allowed us to understand the factors affecting the stability of the three complexes and to rationalize their electrochemical
and photophysical properties in terms of their electronic structure.
Introduction
transporting materials for optoelectronic devices.1 Oxadiazole
fragments have also been connected to classical chelating
ligands (such as bipyridines) in luminescent complexes, to
Diaryl-1,3,4-oxadiazoles are electron-poor luminescent
molecules, which have been widely investigated as electron
‡
*Towhomcorrespondenceshouldbeaddressed. E-mail:monica.panigati@
unimi.it (M.P), pierluigi.mercandelli@unimi.it (P.M.). Fax: +39 02 50314405
(M.P), +39 02 50314454 (P.M.). Tel: +39 02 50314352 (M.P.), +39 02
50314447 (P.M.).
Dipartimento di Chimica Strutturale e Stereochimica Inorganica
(DCSSI), Universita` di Milano and INSTM.
§ Dipartimento di Chimica Fisica ed Elettrochimica (DCFE), Universita`
di Milano and INSTM.
|
† Dipartimento di Chimica Inorganica, Metallorganica e Analitica (DCIMA)
“L. Malatesta”, Universita` di Milano and INSTM.
Centro di Eccellenza CIMaINa dell’Universita` di Milano.
Istituto di Scienze e Tecnologie Molecolari (ISTM) del CNR.
⊥
11154 Inorganic Chemistry, Vol. 47, No. 23, 2008
10.1021/ic801447z CCC: $40.75  2008 American Chemical Society
Published on Web 11/06/2008

Tricarbonyl Re(I) Complexes Containing a Bridging ppd Ligand
Chart 1. Structures of the Three Oxadiazole Complexes Here Investigated, Showing Their Formal Re-Re Bond Order
Scheme 1. The Synthetic Routes to the Novel Oxadiazole Complexes
1, 2, and 3
obtain multifunctional (emitting and charge transporting)
molecular species.2 In very few cases, oxadiazoles have been
used as ligands by themselves. Complexes of iridium(III)
containing chelating cyclometalated (κ2N,C) oxadiazolates
have been reported recently3-5 and have been tested in the
fabrication of electroluminescent devices. Other researchers
addressed the use of intact oxadiazoles (containing different
substituents on the aryl rings) as rigid bridging ligands in
the construction of supramolecular frameworks and reported
on several coordination polymers of silver containing the
Ag(µ-oxadiazole-κN3:κN4)Ag building block.6
We turned our attention toward oxadiazoles as ligands on
the basis of their similarity to 1,2-diazines, which we have
recently used for the preparation of a new family of
luminescent tricarbonyl rhenium(I) complexes.7,8 We have
therefore prepared the series of dinuclear rhenium(I) com-
plexes shown in Chart 1, which contain a bridging µ-2,5-
diphenyl-1,3,4-oxadiazole-κN3:κN4 (ppd) and ancillary ligands
(H, Cl) with different steric and electronic properties.
Information on the effectiveness of oxadiazole as ligand and
on the stability of the complexes on varying the (formal)
Re-Re bond order and the Re-Re distance has been
obtained. The effect of the coordination on the reduction
potential of oxadiazole has been determined and the photo-
physical properties of the new species have been investigated.
The complex stability, the nature of the frontier orbitals, and
the electronic transitions involved in the absorption spectrum
have been studied by means of density functional and time-
dependent density functional calculations.
(1) (a) Shirota, Y.; Kageyama, H. Chem. ReV. 2007, 107, 953–1010. (b)
Kulkarni, A. P.; Tonzola, C. J.; Babel, A.; Jenekhe, S. A. Chem. Mater.
2004, 16, 4556–4573.
Results and Discussion
(2) Si, Z.; Li, J.; Li, B.; Zhao, F.; Liu, S.; Li, W. Inorg. Chem. 2007, 46,
6155–6163. (b) Wong, Y. P.; Xie, W. F.; Li, B.; Li, W. L. Chin. Chem.
Lett. 2007, 18, 1501–1504. (c) Liu, C. B.; Li, J.; Li, B.; Hong, Z. R.;
Zhao, F. F.; Liu, S. Y.; Li, W. L. Chem. Phys. Lett. 2007, 435, 54–
58. (d) Lundin, N. J.; Blackman, A. G.; Gordon, K. C.; Officer, D. L.
Angew. Chem., Int. Ed. 2006, 45, 2582–2584. (e) Xiang, N. J.; Leung,
L. M.; So, S. K.; Wang, J.; Su, Q.; Gong, M. L. Mater. Lett. 2006,
60, 2909–2913. (f) He, Z.; Wong, W.-Y.; Yu, X.; Kwok, H.-S.; Lin,
Z. Inorg. Chem. 2006, 45, 10922–10937. (g) Wong, W.-Y.; He, Z.;
So, S.-K.; Tong, K.-L.; Lin, Z. Organometallics 2005, 24, 4079–4082.
(h) Ng, P. K.; Gong, X.; Chan, S. H.; Lam, L. S. M.; Chan, W. K.
Chem.sEur. J. 2001, 7, 4358–4367. (i) Kim, Y.; Vanhelmont,
F. W. M.; Stern, C. L.; Hupp, J. T. Inorg. Chim. Acta 2001, 318,
53–60. (j) Gong, X.; Ng, P. K.; Chan, W. K. AdV. Mater. 1998, 10,
1337–1340.
(3) (a) Chen, L.; You, H.; Yang, C.; Ma, D.; Qin, J. Chem. Commun.
2007, 1352–1354. (b) Chen, L.; Yang, C.; Li, M.; Qin, J.; Gao, J.;
You, H.; Ma, D. Cryst. Growth Des. 2007, 7, 39–46.
(4) An aluminum derivative, analogous to Alq3, had been reported some
times ago, which contains a κ2N,O cyclometalated phenolate oxadia-
zole: Wang, J. F.; Jabbour, G. E.; Mash, E. A.; Anderson, J.; Zhang,
Y.; Lee, P. A.; Armstrong, N. R.; Peyghambarian, N. AdV. Mater.
1999, 11, 1266–1269.
(5) A cyclomanganated complex has also been reported: Michon, C.;
Djukic, J.-P.; Pfeffer, M.; Gruber-Kyritsakas, N.; De Cian, A. J.
Organomet. Chem. 2007, 692, 1092–1098.
(6) (a) Dong, Y.; Xu, H.; Ma, J.; Huang, R. Inorg. Chem. 2006, 45, 3325–
3343. (b) Dong, Y.; Zhang, Q.; Wang, L.; Ma, J.; Huang, R.; Shen,
D.; Chen, D. Inorg. Chem. 2005, 44, 6591–6608.
(7) Panigati, M.; Donghi, D.; D’Alfonso, G.; Mercandelli, P.; Sironi, A.;
D’Alfonso, L. Inorg. Chem. 2006, 45, 10909–10921.
(8) Donghi, D.; D’Alfonso, G.; Mauro, M.; Panigati, M.; Mercandelli,
P.; Sironi, A.; Mussini, P.; D’Alfonso, L. Inorg. Chem. 2008, 47, 4243–
4255.
Syntheses of the New Complexes. Different pathways
have been followed for the synthesis of the [Re2(µ-X1)(µ-
X2)(CO)6(µ-ppd-κN3:κN4)] complexes (X1, X2 ) H, Cl; ppd
) 2,5-diphenyl-1,3,4-oxadiazole), as summarized in Scheme
1. In all cases, the nature of the products has been established
on the basis of the spectroscopic data and confirmed by
single-crystal X-ray diffractometric analysis.
The dinuclear hydrido complex [Re2(µ-H)2(CO)6(µ-ppd)]
(1) was obtained in high yields by a route analogous to that
used for the synthesis of the corresponding pyridazine
derivative [Re2(µ-H)2(CO)6(µ-pydz-κN1:κN2)] (pydz ) py-
ridazine),7 namely, the reaction of the unsaturated tetrahedral
cluster [Re4(µ3-H)4(CO)12] with 2 equiv of ppd at room
temperature in dichloromethane solution. NMR monitoring
revealed complete disappearance of the starting cluster in
about 4 h to give mainly the dinuclear complex 1, which is
poorly soluble in dichloromethane and slowly precipitates
from the reaction mixture (isolated yield ca. 70%).9
The mixed hydridochloro complex [Re2(µ-H)(µ-Cl)(CO)6(µ-
ppd)] (2) was prepared by reaction of 1 with a saturated
dichloromethane solution of gaseous HCl, according to eq
1. The reaction was selective and went to completeness in
about 4 h at room temperature.
Inorganic Chemistry, Vol. 47, No. 23, 2008 11155

Mauro et al.
[Re2(µ-H)2(CO)6(µ-ppd)] + HCl f
[Re(µ-H)(µ-Cl)(CO)6(µ-ppd)] + H2 (1)
The lability of the dinuclear structure in the case of 3 was
further evidenced by its decomposition (with oxadiazole loss)
even in a less donating solvent such as tetrahydrofuran, with
a longer t1/2 than in acetonitrile (17 h).
The dichloro derivative [Re2(µ-Cl)2(CO)6(µ-ppd)] (3) was
obtained in high yield by treating [ReCl(CO)5] with 0.5 equiv
of ppd in toluene at reflux. Also in this case, the synthesis
parallels that of the analogous 1,2-diazine derivatives recently
reported.8 Spectroscopic analysis (IR and NMR) of the
reaction mixture assessed the good selectivity of the reaction.
The three complexes 1, 2, and 3 possess 32, 34, and 36
valence electrons, respectively. Accordingly, the formal
Re-Re bond order varies from 2 to 1 to 0 in the same series,
as evidenced in Chart 1. This is reflected in the Re-Re
distance (see below) and in the stability of the complexes.
Actually, the three complexes have been found to be rather
unstable in acetonitrile solution. In all cases, 1H NMR
monitoring showed loss of the oxadiazole ligand following
a clean pseudo-first-order behavior with a rate that decreased
on increasing the formal Re-Re bond order: t1/2 ) 3.7, 4.5,
and 13.6 h, for 3, 2, and 1, respectively. The reaction
products, only partially identified, were different in the three
cases. For 3, the reaction resulted in fragmentation of the
dinuclear unit, according to eq 2, with formation of the
mononuclear complex [ReCl(CO)3(MeCN)2], identified by
its ν(CO) IR bands.10
The relatively fast decomposition of 3 in acetonitrile
compared with the stability in the same conditions of the
related [Re2(µ-Cl)2(CO)6(µ-pydz)] derivative suggests that
ppd is a worse ligand than 1,2-diazine, in line with its lower
basicity (pKa values -2.49 for ppd14 vs +2.33 for pydz15).
The preference of rhenium for binding to pyridazine has been
confirmed by a competition experiment, in which a toluene
solution of [ReCl(CO)5] was refluxed in the presence of 0.5
equiv of ppd and of pydz. The NMR spectra showed the
almost quantitative formation of the diazine derivative.
It is also worthwhile to mention that 3 was the only
reaction product even when [ReCl(CO)5] was treated with 1
equiv (or more) of ppd, unlike the analogous reaction with
pyridazine, which under these conditions affords the mono-
nuclear species fac-[ReCl(CO)3(pydz)2], containing two
monodentate diazine ligands.8,16
Structural Characterization. The structures of the com-
plexes 1-3 have been established by single-crystal X-ray
analysis.17 Figure 1 shows a picture of the three species, and
Table 1 reports some relevant geometrical parameters,
averaged according to the idealized Cs symmetry of these
molecules. Each rhenium atom attains a distorted octahedral
coordination and bears three terminal carbonyl ligands (in a
facial arrangement), two bridging (hydrido or chloro) ligands,
and one of the nitrogen atoms of the bridging 2,5-diphenyl-
1,3,4-oxadiazole ligand. The Re-Re distance observed in
the three species (277.9, 297.9, and 358.5 pm in 1, 2, and 3,
respectively) is in agreement with their (formal) Re-Re bond
order (2, 1, and 0, respectively) and with the dimension of
the bridging hydrido or chloro ligands.
[Re2(µ-Cl)2(CO)6(µ-ppd)] + 4MeCN f
2[ReCl(CO)3(MeCN)2] + ppd (2)
In contrast, in the case of the reaction of 2 with acetonitrile,
1
the dinuclear skeleton was maintained, as the H NMR
monitoring showed that the main product exhibited a
resonance unambiguously attributable to a bridging hydrido
ligand (δ -9.13 ppm). This species has been tentatively
formulated as [Re2(µ-H)(µ-Cl)(CO)6(MeCN)2], on the basis
of its spectroscopic data.11
In the case of 1, its very slow decomposition in acetoni-
trile-d3 was poorly selective and led to the progressive
formation of a number of hydrido species, as well as to H2
evolution, as described in more detail in the Experimental
Section.
The molecular structures of 1 and 3 are strictly related to
those of the corresponding all-carbonyl derivatives [Re2(µ-
H)2(CO)8] and [Re2(µ-Cl)2(CO)8].18 The bridging coordina-
tion of an oxadiazole molecule leads, however, to major
structural perturbations. Indeed, to allow the lone pairs on
the two nitrogen atoms to effectively interact with the metal
atoms, the two rhenium-centered octahedra must bend toward
the bridging ligand, leading to a larger Re-Re-Cax angle
(+5.6° and +22.2° for 1 and 3) and to a shorter Re-Re
distance (-9.7 and -22.9 pm for 1 and 3) with respect to
the D2h-symmetric all-carbonyl species.
(9) The 1H NMR analysis (in THF-d8) of the lemon yellow precipitate
revealed that it was constituted by complex 1 only. Contrary to what
occurred in the reaction with pyridazine, no tetranuclear clusters with
spiked-triangular skeleton have been detected as intermediates. At
further variance, the tetranuclear square clusters, which were the main
products in the reactions with pyridazine, in this case were formed
only in traces. The main byproducts (less than 5%) of the reaction
with ppd have been identified, on the basis of their NMR signals, as
the cis and trans isomers (ratio ca. 1:2) of the triangular cluster [Re3(µ-
H)3(CO)9(µ-ppd)(ppd-κN3)]. The formulation of these byproducts has
been confirmed by their preparation through the reaction of the
triangular cluster [Re3(µ3-H)(µ-H)3(CO)9]- with CF3SO3H in the
presence of 2 equiv of ppd. A full characterization of these species
and the study of their properties are in progress.
(10) Farona, M. F.; Kraus, K. F. Inorg. Chem. 1970, 9, 1700–1704.
(11) Particularly diagnostic was the position of its hydrido resonance.
Indeed, the analogous [Re2(µ-H)(µ-Cl)(CO)8] complex gives a hydrido
signal at -12.7 ppm,12 and it has been previously observed that the
replacement of carbonyls by MeCN molecules in a (CO)4Re(µ-
H)Re(CO)4 fragment causes a strong downfield shift of the hydrido
resonance.13
(12) Adams, R. D.; Kuhns, J. D. Polyhedron 1988, 7, 2543–2547.
(13) Beringhelli, T.; D’Alfonso, G.; Freni, M.; Ciani, G.; Sironi, A.;
Molinari, H. Dalton Trans. 1986, 2691–2697.
(14) Trifonov, R. E.; Rtishchev, N. I.; Ostrovskii, V. A. Spectrochim. Acta,
Part A 1996, 52, 1875–1882.
(15) Dean, J. A. Lange’s Handbook of Chemistry; McGraw-Hill: New York,
1992.
(16) Abel, E. W.; Blackwall, E. S.; Heard, P. J.; Orrel, K. G.; Sik, V.;
Hursthouse, M. B.; Mazid, A. M.; Abdul Malik, K. M. Dalton Trans.
1994, 100, 445–455.
(17) CCDC-695237-95239 contain the crystallographic data for 1-3.
These data can be obtained free of charge from The Cambridge
Crystallographic Data Centre (www.ccdc.cam.ac.uk).
(18) (a) Masciocchi, N.; Sironi, A.; D’Alfonso, G. J. Am. Chem. Soc. 1990,
112, 9395–9397. (b) Vega, A.; Calvo, V.; Manzur, J.; Spodine, E.;
Saillard, J.-Y. Inorg. Chem. 2002, 41, 5382–5387.
11156 Inorganic Chemistry, Vol. 47, No. 23, 2008

Tricarbonyl Re(I) Complexes Containing a Bridging ppd Ligand
Figure 1. View of the molecular structure of the three [Re2(µ-X1)(µ-X2)(CO)6(µ-ppd)] species, as determined by X-ray diffraction, with a partial labeling
scheme.
Table 1. Experimental and Calculated Geometrical Parameters [pm and
deg] for the Complexes [Re2(µ-X1)(µ-X2)(CO)6(µ-ppd)] (1-3)a
Table 2. Absorption Data for the Complexes
[Re2(µ-X1)(µ-X2)(CO)6(µ-ppd)] (1-3) in Dichloromethane Solution at
RT
exptl
calcd
λabs (ε) [nm (103 M-1 cm-1)]
1
2
3
1
2
3
π-π*
1MLCT
Re-Re
Re-X1
Re-X2
Re-N
277.89(7) 297.92(9) 358.48(8) 280.8 300.7 361.3
ppd
1
2
283 (30)
267 (31)
261 (34)
252 (27)
185.3
185.4
185.2
249.06(11) 188.0 185.7 252.0
362 (11)a
300 (16)
n.d.b
249.52(9) 249.85(11) 189.0 252.9 253.5
219.0(9) 219.6(2)
140.6(5) 141.5(3)
222.0(3)
141.4(4)
221.8 221.8 224.6
138.4 138.2 138.5
3
N-N
a Partially overlapped less intense absorptions are observed at 310 and
416 nm. b The position of the band cannot be established with certainty
(see Figure 2).
Re-N-N
108.3(2) 110.86(14) 118.9(2)
108.7 111.5 119.7
Re-Re-Cax 97.67(15) 100.6(2)
111.49(11) 98.4 101.2 111.8
38.06(10) 15.6 20.1 39.6
eq1/eq2
18.1
20.8
C2N2O/Ph
18.4(3)
23.34(15) 45.0(2)
17.3 28.0 43.4
a For 1, X1 ) X2 ) H; for 2, X1 ) H and X2 ) Cl; for 3, X1 ) X2 )
Cl. C2N2O and Ph are the least-squares planes defined by the oxadiazole
five-membered ring and the phenyl ring, respectively; eq1 and eq2 are the
planes defined by the equatorial ligands of the two rhenium atoms. Numbers
in parentheses are the greatest standard uncertainties on individual values.
Further geometrical parameters are listed in Table S1 (Supporting Informa-
tion).
leads to a progressive loss of conjugation within the 2,5-
diphenyl-1,3,4-oxadiazole ligand and, in the case of 1 and
3, to a reduction of the idealized symmetry of the molecule
from C2V to Cs.19
Photophysical Characterization. The absorption spectra
of 1, 2, and 3 in dichloromethane solution show, in the UV
range, one band (at 267, 261, and 253 nm, respectively)
attributable to the intraligand π-π* transition of the oxa-
diazole molecule (See Table 2 and Figure 2). These absorp-
tions are blue-shifted with respect to the free oxadiazole
molecule (283 nm)20 by 16, 22, and 30 nm, respectively.
This shift can be attributed to a conformational twist of the
dihedral angle between the phenyl groups and the oxadiazole
An interesting feature of these species is the distortion from
planarity of the 2,5-diphenyloxadiazole moiety. Indeed, as
a consequence of the interaction of the equatorial carbonyl
ligands with the bridging heterocycle, the two phenyl rings
rotate in the same direction out of the plane of the oxadiazole
ring. The effect increases on going from 1 to 2 to 3, as shown
by the dihedral angle between the planes defined by the
oxadiazole five-membered ring and the phenyl ring (C2N2O/
Ph 18.4°, 23.3°, and 45.0°), in agreement with the increase
of the bending of the rhenium-centered octahedra, as
measured by the Re-Re-Cax angle (97.7°, 100.6°, and
111.5°) and by the dihedral angle between the planes defined
by the equatorial ligands of the two rhenium atoms (eq1/eq2
18.1°, 20.8°, and 38.1°). This rotation of the phenyl rings
(19) For the dichloro derivative 3, the two C2N2O/Ph dihedral angles are
significantly different, 61.6(2)° and 28.3(2)°, as can be noticed in
Figure 1. However, all the remaining geometrical parameters are
consistent with an overall idealized Cs-symmetrical structure, as
confirmed by the computational study.
Inorganic Chemistry, Vol. 47, No. 23, 2008 11157

Mauro et al.
Electrochemical Characterization. The three complexes
1, 2, and 3 and the free ligand ppd have been investigated
by cyclic voltammetry (CV), and the most significant results
are summarized in Table 3 and Figure 3.
Ligand. The oxadiazole ligand features a series of
reduction peaks in the potential window available in the
chosen medium (acetonitrile containing 0.1 M tetrabutylam-
monium hexafluorophosphate, TBAPF6). The first peak in
the series appears to be chemically and electrochemically
reversible and monoelectronic, consistent with the 57 mV
half-peak width, the nearly zero slope of the Ep vs log V
linear characteristics (where V indicates the scan rate), and
the presence of a symmetrical return peak. Such a peak
clearly corresponds to the formation of a stable radical anion
and strongly resembles (both in potential and features, see
Table 3 and Figure 3) the case of the ligand phthalazine
(phal) studied in our previous work concerning the rhenium
complexes with 1,2-diazines.8 Interestingly, it has been
previously shown that the ligand pyridazine, featuring a less
extended π-system than phthalazine or diphenyloxadiazole,
gives a bielectronic, chemically irreversible first reduction
peak, corresponding to an ECE mechanism.8,23
Figure 2. Absorption spectra (normalized with respect to the π-π*
transition) of the three complexes [Re2(µ-X1)(µ-X2)(CO)6(µ-ppd)] (1-3)
and the free ligand (ppd) in dichloromethane solution. A comparison with
excitation energies and oscillation strengths computed in dichloromethane
is reported (vertical lines).
ring, induced by the coordination to the metal, as discussed
in the structural and computational part of this work.21
Complex 1 shows another main absorption, at lower
energy (362 nm), which can be assigned to a metal-to-ligand
charge transfer transition (1MLCT). Indeed, on increasing
solvent polarity, the position of the band shifts to higher
energy (from 367 nm in toluene to 357 nm in ethanol), as is
typical of charge transfer transitions,22 while the position of
the band attributed to the π-π* transition remains un-
changed. For complexes 2 and 3, these charge transfer bands
are significantly blue-shifted and fall on the tail end of the
intraligand π-π* transition: the charge transfer band of 2
is still recognizable, at ca. 300 nm (Figure 2), while that of
3 is almost completely buried and appears as a broad tail of
the higher energy absorption. Density functional calculations
(see below) support this view.
The normalized current for ppd (Figure 3) is lower than
both the normalized phthalazine and ferrocene ones under
the same conditions, indicating a lower diffusion coefficient
of the oxadiazole molecule.
Complexes. First Reduction Peak. The first reduction
peaks of complexes 1-3 are still ligand-centered, as dem-
onstrated by their appearance almost unchanged on varying
the nature of the monatomic bridging ligands.24 The peak
potentials are shifted to more positive values with respect
to free ppd, as a consequence of the electron donation to the
rhenium atoms. However, this shift (0.56 V for 3) is much
less pronounced than in the analogous pyridazine (1.26 V)
and phthalazine (0.93 V) derivatives,8 suggesting a smaller
electron depletion of the heterocycle ligand upon coordina-
tion, thus a less efficient electron donation.
Upon complexation, the first reduction peak of oxadiazole
loses its chemical reversibility, showing that the radical anion
undergoes a fast chemical step in the experimental time scale.
This again resembles what was observed in the phthalazine
case (Figure 3),8 and it is quite the opposite of the pyridazine
case, where the chemically irreversible, bielectronic reduction
peak of the free ligand becomes a monoelectronic, chemically
reversible peak upon complexation.8
The photoluminescent behavior of the three complexes was
investigated at room temperature in toluene solution. No
emission was observed for 1 and 3 upon irradiation at
wavelengths in the range 340-380 nm. In contrast, for
complex 2 a weak broad emission was observed at 527 nm
(Φ ) 0.02%) upon irradiation at 370 nm. The large Stokes
3
shift agrees with an emission from a MLCT state.
When the excitation wavelength was shifted to 300 nm,
for all the complexes a structured emission appeared (λmax
) 348 nm). Its position and shape were however identical
to that of the free ppd ligand. This suggests that the
complexes are unstable in solution under UV irradiation and
dissociate the oxadiazole ligand. Actually, a progressive neat
increase of the emission intensity was observed in a series
of spectra acquired sequentially (as shown for 3 in Figure
S1 of Supporting Information).
Complexes. First Oxidation Peak. The first oxidation
peaks of the three complexes are clearly metal-centered, as
demonstrated by the variation of the peak potentials on varying
the nature of the monatomic ligands X1 and X2. They also
appear to be bielectronic, like the first oxidation peaks of the
pyridazine and phthalazine complexes.8 However, different from
the latter case, where electrochemically and chemically revers-
(20) Ballard, R. E.; Titchard, A. Spectrochim. Acta, Part A 1967, 23, 1883–
1887.
(21) See also: Jansson, E.; Jha, P. C.; Ågren, H. Chem. Phys. 2006, 330,
166–171.
(22) (a) Drago, R. S. Physical Methods in Chemistry; W. B. Saunders
Company: Philadelphia, PA, 1977. (b) Giordano, P. J.; Wrighton, M. S.
J. Am. Chem. Soc. 1979, 101, 2888–2897, and references therein.
(23) Organic Electrochemistry, 4th ed.; Lund, H., Hammerich, O., Eds.;
Dekker: New York, 2001; pp 695-696, and references therein.
(24) Moreover, the very small variation of the reduction potentials with
increasing ligand electronegativity is contrary to the trend expected
from the inductive effect of the hydrido and chloro ligands, confirming
that metal orbitals are not involved in the reduction.
11158 Inorganic Chemistry, Vol. 47, No. 23, 2008

Tricarbonyl Re(I) Complexes Containing a Bridging ppd Ligand
Table 3. Selected CV Features Obtained for the Free Diphenyloxadiazole Ligand (ppd) and Its Rhenium Complexes [Re2(µ-X1)(µ-X2)(CO)6(µ-ppd)]
(1-3)a
first reduction peak
first oxidation peak
ligands and
complexes
Ep,cI
[V]
|Ep - Ep/2
|
|dEp/d log V|
iL,conv/c
Ep,aI
[V]
Ep-Ep/2
[V]
dEp/d log V
iL,conv/c
Eg
[eV]
[V]
[V]
[A cm-2 mol-1 dm3]
[V]
[A cm-2 mol-1 dm3]
ppd
1
2
-2.495
-1.827
-1.898
-1.932
0.057
0.043
0.043
0.056
0.005
0.027
0.021
0.033
0.41
0.47
0.53
0.52
1.045
1.175
1.374
0.054
0.108
0.049
0.033
0.081
0.016
0.69
1.17
0.58
2.87
3.07
3.31
3
phal
3phal
-2.458
-1.532
0.054
0.040
0.006
0.023
0.51
0.34
1.244
0.030
0.010
0.61
2.78
a Scan rate 0.2 V s-1, performed in MeCN with 0.1 M TBAPF6 as the supporting electrolyte, at 298 K, with ohmic drop compensation. First cathodic and
anodic peak potentials (Ep,cI and Ep,aI), together with relevant half-peak widths (Ep - Ep/2), slopes of the linear Ep vs log V characteristics (dEp/d log V),
normalized limiting currents after convolution (iL,conv/c),and electrochemical energy gaps (Eg). Potentials are referred to the Fc+|Fc couple in the operating
medium. Data for phthalazine (phal) and its dichloro rhenium complex (3phal) are taken from ref 8.
transfer (particularly in the case of 2). Moreover, no return peak
is observed in the experimental time scale, pointing to a fast
chemical step following the electron transfer, as in the case of
the reduction peak.25
Comparison between the electrochemical behavior of 3
and that of 3phal (Table 3 and Figure 3) shows that for the
phthalazine complex 3phal both reduction and oxidation are
easier than for the corresponding ppd complex 3 (by 0.40
and 0.13 V, respectively), resulting in a significantly lower
HOMO-LUMO gap. Both features can be explained by the
less efficient ligand-to-metal electron donation from the
oxadiazole, resulting in a ring richer in electrons and a metal
redox site poorer in electrons with respect to the phthalazine
case.
Computational Study. The three oxadiazole rhenium
complexes [Re2(µ-X1)(µ-X2)(CO)8(µ-ppd)] (1-3) have been
studied by means of density functional and time-dependent
density functional calculations. A comparison of the three
species allows identification of the effect of the different
number of valence electrons on the geometry of the
complexes, on their electronic structure, and on the electronic
transitions responsible for their absorption spectrum. In
addition, a comparison between the dichloro derivative 3 and
thepreviouslystudiedpyridazinederivative[Re2(µ-Cl)2(CO)8(µ-
pydz)] (3pydz)8 allows assessment of the difference in the
donor ability of the two heterocycles (ppd and pydz).
Computational details can be found in the Experimental
Section.
Optimized Geometry and Stability of the Com-
plexes. The most relevant optimized geometrical parameters
are reported in Table 1 (for a complete list, see Table S1 in
the Supporting Information). For the three compounds, the
(25) For complex 3, a small, irregular reduction peak connected with the
oxidation peak is observed at a distance of about 0.3 V. Its features
suggest accumulation on the electrode surface of the product of a
chemical reaction following the electron loss. It is worthwhile
comparing this case with the analogous phthalazine complex 3phal,
which gives a perfectly reversible bielectronic peak (working at 2 V
s-1, red line in Figure 3), gradually losing its chemical reversibility
at lower scan rates. Now, the increasing competition of the chemical
step with decreasing scan rate (such as in the 0.2 V s-1 characteristics,
black line) results in a decreasing “regular” return peak and an
increasing “irregular” one (in the same position as in the ppd complex
case), corresponding to the reduction of the locally accumulated
product of the chemical reaction.
Figure 3. A synopsis of normalized CV characteristics obtained for ppd
and its rhenium complexes (1-3) at 0.2 V s-1 scan rate in MeCN with 0.1
M TBAPF6 as the supporting electrolyte, at 298 K, with ohmic drop
compensation. Normalized CV features for phthalazine (phal), its complex
3phal, and the reference redox couple Fc+|Fc, obtained in the same
conditions, are also reported. Red curves have been obtained at 2 V s-1
.
ible two-electron transfer was observed, the peaks of the
oxadiazole complexes are wider and show a significant negative
shift with increasing scan rate, indicating less facile electron
Inorganic Chemistry, Vol. 47, No. 23, 2008 11159

Mauro et al.
Table 4. Calculated Dissociation (De), Interaction (∆Eint), and
Preparation (∆Eprep) Energies [kJ mol-1] for the Complexes
[Re2(µ-X)2(CO)6(µ-ppd)] (1-3) and for [Re2(µ-Cl)2(CO)6(µ-pydz)]
(3pydz)
1
2
3
3pydz
De
∆Eint
∆Eprep,M
∆Eprep,L
237
-263
13
212
-263
34
192
-264
40
240
-283
42
13
17
32
1
optimized geometry compares well to the corresponding
solid-state structure as determined by X-ray diffraction.26
The minimum energy structure optimized for compounds
1-3 belongs to the Cs point group.27 Deviation from the
highest possible symmetry (C2V) for species 1 and 3 is
associated with the rotation of the two phenyl rings (in the
same direction) out of the plane of the oxadiazole ring, in
order to avoid their interaction with the equatorial carbonyl
ligands. For compound 2, the presence of two different
bridging ligands (H and Cl) allows differentiation of the
equatorial carbonyls. In particular, the carbonyl groups cis
to the chloro ligand, due to the larger size of chlorine with
respect to hydrogen, are more pushed toward the oxadiazole
moiety. As a result the two phenyl rings rotate in such a
way to avoid their interaction with these specific ligands.
The distortion from planarity of the 2,5-diphenyloxadiazole
moiety connected with the bending of the rhenium-centered
octahedra toward the nitrogen atoms of the bridging hetero-
cyclic ligand increases on going from 1 to 2 to 3, as already
described in the X-ray structure characterization section. It
is important to note, however, that the unfavorable steric
interactions between the equatorial carbonyl ligands and
oxadiazole does not lead to a restricted rotation of the phenyl
rings. Indeed, the rotational barriers estimated for 1 and 3
(14.5 and 10.3 kJ mol-1, respectively)28 are lower than that
computed for the free ligand (28.0 kJ mol-1). This seemingly
unexpected finding can be understood considering that steric
hindrance affects the minimum energy portion of the ro-
tational profile only while it has almost no effect on the
transition structure. This result is in agreement with the
Figure 4. Partial molecular orbital diagrams for the Cs-symmetric oxa-
diazole complexes 1-3. Blue arrows highlight HOMO-LUMO gaps (DFT
energy values in eV are given).
fragment from its equilibrium geometry to the geometry
adopted in the complex.
The computed dissociation energy decreases on going from
1 to 2 to 3, in agreement with the observed kinetic stability
of the complexes in acetonitrile solution. However, the
interaction energy is almost identical for the three species
and the difference in stability can be entirely attributed to
differences in the preparation energy of both the metal and
the 2,5-diphenyloxadiazole fragments. These findings are in
accord with the trend observed in the above-described
geometrical distortions (i.e., the bending of the rhenium-
centered octahedra and the loss of planarity of the ppd ligand)
and is consistent with a similar donation from the ppd ligand
to the metal atoms in the three compounds.
As far as the two dichloro derivatives 3 and 3pydz are
concerned, the greater dissociation energy computed for the
pyridazine complex is in agreement with its experimental
higher stability in acetonitrile solution. The stability of 3pydz
can be attributed both to a higher interaction energy and to
a lower preparation energy for the ligand fragment (almost
zero for 3pydz). This is expected, since pyridazine, being
richer in electrons than oxadiazole, acts as a better ligand
toward the metal fragment and no substantial difference
between the geometry of the free and the coordinated
pyridazine can be detected. The preparation energy for the
metal fragment in the two complexes is similar, since the
bite of the two bridging ligands, albeit not identical, leads
to a superimposable geometry of the [Re2(µ-Cl)2(CO)6]
fragment in the two complexes.
1
dynamic symmetry shown by the phenyl rings in the H
NMR spectra of the complexes.
The dissociation energy De calculated for the complexes
1-3 and 3pydz is reported in Table 4, along with its partition
into three components: the (instantaneous) interaction energy,
29
∆Eint, and the preparation energies, ∆Eprep,M and ∆Eprep,L
.
The interaction energy is the energy difference between the
complex (ML) and the constituting fragments (M and L)
computed employing the frozen geometry of the complex.
The preparation energy (also known as deformation energy)
is the energy necessary to promote the (metal or ligand)
(26) Mean and maximum absolute deviations in bond distances (1.7 and
3.7 pm) and bond angles (0.7° and 2.3°) are small, particularly if the
standard deviations of the experimental data are taken into account.
(27) Higher energy conformers belonging to different point groups have
been characterized for 1 and 3 (see the Experimental Section).
(28) Rotational barriers have been estimated optimizing a Cs-symmetric
structure in which one of the two phenyl rings lays on a plane
orthogonal to the oxadiazole ring.
Electronic Structure and Transitions. Partial molecular
orbital diagrams for the Cs-symmetric oxadiazole complexes
here investigated are reported in Figure 4, while isodensity
surface plots of some relevant molecular orbitals are depicted
in Figures 5 and 6.
The two lowest unoccupied molecular orbitals (LUMO and
LUMO + 1) correspond to the two lowest-lying π* orbitals
(29) The dissociation energy is the opposite of the sum of the three terms:
-De ) ∆Eint + ∆Eprep,M + ∆Eprep,L. Zhang, J.; Frenking, G. Chem.
Phys. Lett. 2004, 394, 120–125.
11160 Inorganic Chemistry, Vol. 47, No. 23, 2008

Tricarbonyl Re(I) Complexes Containing a Bridging ppd Ligand
can be found in order of increasing energy, spanning an
energy range of 1.13 eV. Moving to 2 and 3, the Re-Re
distance strongly lengthens, and as a consequence, the energy
range covered by these increasingly Re-Re nonbonding
orbitals contracts to 0.84 and 0.63 eV (for 2 and 3,
respectively). However, this general trend (the destabilization
of the a′ orbitals and the stabilization of the a′′ orbitals) is
complicated by the presence of the bridging chloro ligands
whose occupied p orbitals can interact with the d orbitals of
the metal atoms leading to a Re-Cl π-antibonding character
for some of the HOMOs in 2 and 3 (see Figure 6, second
and third columns).
In Figure 6, three orbitals are depicted, namely, the HOMO
(connected to the reduction potential of these complexes)
and the two orbitals with δ symmetry with respect to the
Re-Re interaction, indicated by δ*(a′′) and δ(a′) (involved
in the MLCT transitions). The nature of the HOMO changes
on going from the dihydrido derivative 1 (in which it shows
a substantial Re-Re antibonding character) to the hydri-
dochloro derivative 2 (in which a small Re-Cl antibonding
character is present) to the dichloro derivative 3 (which is
all-antibonding with respect to the four Re-Cl interactions).
The HOMO energy trend within this series cannot be easily
predicted since two competitive effects (the lengthening of
the Re-Re distance and the build-up of a Re-Cl antibonding
character)areatwork.However,theincreasingHOMO-LUMO
energy gap computed along the series well compares with
the trend determined by the electrochemical experiments.
The out-of-phase combination of rhenium orbitals δ*, due
to its a′′ symmetry, cannot combine with the chlorine p
orbitals, and its energy strongly lowers along the 1-3 series
as expected. Due to the antibonding contribution of the p
orbitals of the chlorine atoms, the opposite trend is shown
by the in-phase combination of rhenium orbitals δ.
In order to simulate the absorption electronic spectrum of
the oxadiazole complexes down to 250 nm, the lowest 30
singlet excitation energies were computed. However, only
singlet excitations for which the oscillator strength f is greater
than 0.01 are reported in Table 5 and depicted in Figure 2.
Computations have been done both in the gas-phase and in
dichloromethane solution (employing the conductor-like
polarizable continuum model CPCM) in order to evaluate
the solvent effect. As expected, the CPCM results are sign-
ificantly different from the gas-phase results for the MLCT
transitions only, while the π-π* transitions centered on the
ppd ligand are somehow unaffected by the solvent.
Figure 5. Isodensity surface plots of some relevant molecular orbitals of
[Re2(µ-H)2(CO)6(µ-ppd)] (1) mainly located on the diphenyloxadiazole
ligand. The corresponding molecular orbitals for molecules 2 and 3 are
very similar.
of the diphenyloxadiazole molecule. In Figures 4 and 5, they
are indicated by π*(a′) and π*(a′′), according to their
symmetry labels. As expected, as a consequence of the
coordination to the rhenium atoms, the reduced electron
density on the ligand ring leads to an overall stabilization of
the two orbitals with respect to the free ligand (in which
they lie at -1.51 and -0.69 eV, respectively).30 On going
from 1 to 2 to 3, their energy significantly rises. This effect
can be safely attributed to the increasing distortion of the
ppd ligand, the two orbitals being π-bonding with respect
to the C(oxadiazole)-C(phenyl) interaction (see Figure 5,
first row). This explanation is supported by computations
on the free oxadiazole ligand, showing an analogous raising
of the LUMO energy on displacing the phenyl groups from
the plane of the heterocyclic ring.31
Within the highest occupied molecular orbitals of the three
complexes, HOMO - 6 corresponds to the HOMO of the
free ligand, indicated by π(a′′) in Figures 4 and 5. This orbital
too is stabilized as a consequence of coordination (it lies at
-6.41 eV in free ppd). However, due to its π-antibonding
nature with respect to the C(oxadiazole)-C(phenyl) interac-
tion, its energy lowers on going from 1 to 2 to 3.
The six HOMOs correspond to the in-phase and out-of-
phase combinations of the “t2g” sets of the two rhenium atoms
in a pseudo-octahedral coordination environment. For the
unsaturated derivative 1, the two bridging hydrido ligands
give no contribution to these orbitals (see Figure 6, first
column). The three Re-Re bonding (a′ symmetry) and the
three Re-Re antibonding (a′′ symmetry) molecular orbitals
The orbitals involved in the π-π* transitions are the
HOMO - 6 π(a′′) and the LUMO π*(a′).32 The π-π*
transition for the three complexes is blue-shifted with respect
to the free ligand33 and its wavelength lowers along the series
(32) In Table 5, two entries are listed in correspondence to the transition
since, mainly for the dichloro derivative 3, a phenyl ring centered
orbital of a′′ symmetry (HOMO - 8), lying close to the HOMO - 6
(a′′), contributes to these type of transitions. A complete list of the
contribution of singly excited configurations to all the electronic
transitions described in the text can be found in the Supporting
Information (Table S4).
(33) Values computed for the diphenyloxadiazole molecule: gas-phase λπ-π*
) 289 nm (4.29 eV), f ) 0.87; dichloromethane solution (CPCM)
λπ-π* ) 294 nm (4.22 eV), f ) 0.97.
(30) A list of molecular orbital energies for the oxadiazole ligand and the
three rhenium complexes 1-3 can be found in Table S2 (Supporting
Information).
(31) The effect of phenyl ring rotation (within imposed Cs symmetry) on
the computed HOMO and LUMO energy, HOMO-LUMO energy
gap, excitation energy, and oscillator strength for the π-π* transition
can be found in the Supporting Information (Table S3 and Figures S2
and S3). Similar computations, albeit limited to the HOMO-LUMO
gap, can be found in ref 21.
Inorganic Chemistry, Vol. 47, No. 23, 2008 11161

Mauro et al.
Figure 6. Isodensity surface plots of some relevant molecular orbitals of [Re2(µ-X)2(CO)6(µ-ppd)] (1-3) mainly located on the Re2(µ-X)2 core (top row
HOMO; middle row δ*; bottom row δ).
Table 5. Computed Excitation Energies and Oscillator Strengths for the
Complexes [Re2(µ-X1)(µ-X2)(CO)6(µ-ppd)] (1-3)
than the π-π* band along the series, and for compound 3,
the MLCT component can only be detected as a shoulder of
the intraligand band (see Figure 2).
1
2
3
Gas-Phase Values
266, 4.66 (0.02) 261, 4.75 (0.03) 255, 4.87 (0.27)
272, 4.55 (0.51) 267, 4.65 (0.45) 257, 4.83 (0.13)
Conclusions
λπ-π* [nm, eV] (f)
This work provides the first detailed study of the properties
of 2,5-diphenyl-1,3,4-oxadiazole as a ligand toward transition
metal centers. It has been shown that these molecules are
able to act as bridging ligands, through the two nitrogen
atoms, toward Re2(µ-X)2(CO)6 fragments spanning a wide
range of intermetallic distances (from 278 to 358 pm). The
stability of the oxadiazole complexes is lower than that of
the corresponding 1,2-diazine derivatives, but it is high
enough to allow isolation in the solid state and also
characterization in solution. On the basis of the density
functional computations, the lower stability with respect to
the diazine complexes is attributable not only to the lower
donating ability of the more electron-poor oxadiazole but
also to the significant geometrical reorganization imposed
by this sterically demanding ligand.
λMLCT [nm, eV] (f) 354, 3.51 (0.01) 324, 3.82 (0.13) 303, 4.09 (0.13)
358, 3.46 (0.11) 350, 3.54 (0.05) 322, 3.85 (0.09)
370, 3.35 (0.11) 362, 3.42 (0.02) 339, 3.66 (0.01)
385, 3.22 (0.04) 390, 3.18 (0.02) 365, 3.40 (0.02)
Dichloromethane
Solution Values (CPCM)
λπ-π* [nm, eV] (f)
267, 4.65 (0.05) 264, 4.70 (0.04) 254, 4.89 (0.33)
273, 4.55 (0.52) 265, 4.68 (0.43) 257, 4.82 (0.15)
λMLCT [nm, eV] (f) 318, 3.90 (0.09) 307, 4.03 (0.20) 283, 4.38 (0.14)
336, 3.69 (0.02) 320, 3.87 (0.16) 295, 4.20 (0.21)
343, 3.61 (0.09) 331, 3.75 (0.02) 310, 4.00 (0.04)
357, 3.47 (0.17) 341, 3.64 (0.03) 322, 3.85 (0.03)
1-3. These findings are a consequence of the above-
described changes in energy of the two orbitals and are well
reproduced by computations on the (distorted) free oxadia-
zole molecule (see Table S3 and Figure S3 in the Supporting
Information).
The orbitals involved in the MLCT transitions are the
above-described δ(a′) and δ*(a′′) HOMOs and the two
LUMOs π*(a′) and π*(a′′). Hence, four singly excited
configurations must be taken into account, two of A′ and
two of A′′ symmetry. They are differently combined to give
the four transitions listed in Table 5. The MLCT excitation
energies show a strong blue shift along the 1-3 series.
However, since the molecular orbitals δ*(a′′) and π*(a′′),
involved in the more intense MLCT transitions, move more
than the orbitals π(a′′) and π*(a′), involved in the π-π*
transitions, the MLCT band is substantially more blue-shifted
As expected, coordination makes oxadiazole reduction
easier. However, the positive shift of the reduction potential
upon coordination is much lower than in the case of the
corresponding complexes with bridging 1,2-diazines. This
is probably a further indication of the lower donor power of
the oxadiazole system. A similar effect was previously
observed in the analogous 1,2-diazine complexes, where the
introduction of electron-withdrawing substituents lowered the
coordination shift of the reduction potential (Ered values:
pydz, free -2.60 V, bound -1.35 V, shift +1.25 V; 3,6-
Cl2pydz, free -1.99 V, bound -0.99 V, shift +1.00 V).
11162 Inorganic Chemistry, Vol. 47, No. 23, 2008

Tricarbonyl Re(I) Complexes Containing a Bridging ppd Ligand
Table 6. Spectroscopic Data for the New Complexes 1-3 and the Free
Coordination very effectively quenches emission from
oxadiazole indicating fast radiationless relaxation to low-
energy states arising from the presence of the d orbitals of
the heavy metal atoms, which also enhance, by spin-orbit
coupling, the rate of formally spin-forbidden processes.
However, the emission from the 3MLCT states is absent (or
very low), probably because the low basicity and the bulk
of diphenyloxadiazole reduce the stiffness of the molecular
structure, favoring nonradiative de-excitation pathways.
The observed photoinstability might be related to an
increased steric interaction between ppd and the metallic
fragment in the excited state with respect to the ground state.
Indeed, in the excited state a backward displacement of ppd
toward planarity is expected (due to the π bonding character
of the LUMO about the C(oxadiazole)-C(phenyl) bond),
as well as a shortening of the Re-N bond distance (due to
charge separation). This would lead to excessive steric
hindrance, and the effect should be more sizable for the
dichloro derivative, in accord with the larger deviation from
planarity of ppd in this species.
Liganda
ν(CO) [cm-1
]
δ [ppm]
ppd
1
8.19 (m, 4H), 7.60 (m, 6H)
8.54 (m, 4H),
2042m, 2016s,
1939sh, 1927vs
2048m, 2027s,
1947s, 1921s
2051mw, 2036s,
1946s, 1917s
7.90 (m, 2H), 7.79 (m, 4H)
8.30 (m, 4H),
7.88 (m, 2H), 7.78 (m, 4H)
8.17 (m, 4H),
7.89 (m, 2H), 7.79 (m, 4H)
2
3
a IR bands in CH2Cl2 and 1H NMR resonances of the ppd moiety in
CD2Cl2.
The solution was stirred overnight; then the volume was reduced
to 1/5, and the solution was treated with n-hexane, causing the
precipitation of a yellow powder, which was washed with CH2Cl2
(2 mL) to completely remove byproducts. The residue was dried
under vacuum, affording 98 mg (0.128 mmol) of pure 1 (isolated
yield 71%). Elemental Anal. Calcd for C20H12N2O7Re2: C 31.41,
H 1.58, N 3.66. Found: C 31.47, H 1.65, N 3.60. IR and NMR
data are summarized in Table 6. Crystals for photophysical
measurements and for X-ray analysis were obtained by slow
diffusion of n-hexane into a CH2Cl2 solution at 248 K.
Coordination therefore has a detrimental effect on the
emitting properties of oxadiazoles, but a beneficial outcome
on their electrochemical properties, due to the lowering of
the reduction potentials. Then the coordination of diaryloxa-
diazoles to metal centers could improve their electron
transporting properties, at least insofar as they are affected
by the LUMO energies. So these results suggest focusing
future search on the coordination to metal centers with higher
Lewis acidity (to counterbalance the low basicity of oxa-
diazoles) and with smaller coordination number than octa-
hedral rhenium(I).
Synthesis of [Re2(µ-H)(µ-Cl)(CO)6(µ-ppd)] (2). A sample of
complex 1 (53.4 mg, 0.0698 mmol) was dissolved in CH2Cl2 (15
mL), and the solution was treated with 10 mL of a saturated solution
of HCl in CH2Cl2. The mixture was stirred at room temperature
for 4 h, during which time color changed from yellow to colorless.
The solution was then concentrated, and the addition to n-hexane
afforded the precipitation of a white powder. Isolated yield: 42.8
mg (0.0535 mmol, 77%). Elemental Anal. Calcd for C20H11-
ClN2O7Re2: C 30.06, H 1.39, N 3.50. Found: C 30.31, H 1.55, N
3.52. IR and NMR data are summarized in Table 6. Crystals for
photophysical measurements and for X-ray analysis were obtained
by slow diffusion of n-hexane into a CH2Cl2 solution at 248 K.
Synthesis of [Re2(µ-Cl)2(CO)6(µ-ppd)] (3). A sample of [Re-
Cl(CO)5] (41.3 mg, 0.114 mmol) was dissolved, upon heating, in
freshly distilled toluene (20 mL). Then 0.5 equiv (12.7 mg, 0.0572
mmol) of ppd was added, and the solution was refluxed for 1.5 h. 1H
NMR showed the formation of one product only. Evaporation to
dryness under vacuum gave a white residue, which was dissolved in
CH2Cl2 and precipitated with n-hexane, affording microcrystalline pure
3. Isolated yield: 38.2 mg (0.0458 mmol, 80%) Elemental Anal. Calcd
for C20H10Cl2N2O7Re2: C 28.82, H 1.21, N 3.36. Found: C 28.90, H
1.23, N 3.30. IR and NMR data are summarized in Table 6. Crystals
for photophysical measurements and for X-ray analysis were obtained
by slow diffusion of n-hexane into a CH2Cl2 solution at 248 K.
Reaction between [ReCl(CO)5] and 1 equiv of ppd. A sample
of [ReCl(CO)5] (64.2 mg, 0.177 mmol) was dissolved, upon heating,
in freshly distilled toluene (20 mL). Then 1 equiv (39.3 mg, 0.177
mmol) of ppd was added, and the solution was refluxed for 4 h. IR
and 1H NMR showed the formation of 3 and the presence of
unreacted ppd (in 1:1 ratio). The toluene solution was further
refluxed for 8 h, but no changes in the IR spectrum were observed.
Experimental Section
All reactions were performed under N2 using the Schlenk technique.
All of the solvents were deoxygenated and dried by standard methods.
2,5-Diphenyl-1,3,4-oxadiazole (ppd) was used as received from Fluka.
[Re4(µ3-H)4(CO)12]·2C6H6 and [ReCl(CO)5] were prepared according
to literature methods.34,35 1H NMR spectra were recorded on Bruker
DRX300 or DRX400 spectrometers. IR spectra were acquired on a
Bruker Vector22 FT instrument. Electronic absorption spectra were
recorded on a Jasco V-570 spectrophotometer at room temperature.
Steady-state fluorescence measurements were performed with a Jobin-
Yvon Fluorolog-3 spectrometer equipped with double monochromators
and a Hamamatsu R928P photomultiplier tube as a detector. All the
solutions were deaerated by nitrogen bubbling for 30 min before
measurements. Emission quantum yield of 2 has been determined by
comparison with the emission of quinine sulfate in H2SO4, 0.1 M
(0.577).36
Synthesis of [Re2(µ-H)2(CO)6(µ-ppd)] (1). A solution of
[Re4(µ3-H)4(CO)12]·(C6H6)2 (112 mg, 0.090 mmol) in CH2Cl2 (20
mL) was treated at room temperature with 2 equiv of ppd (40.0
mg, 0.18 mmol). The color slowly changed from red-brown to
lemon yellow, followed by slow separation of a yellow precipitate.
1H NMR Monitoring of the Complex Stability in Ace-
tonitrile. A series of spectra was recorded on a saturated solution of
1 in CD3CN at room temperature for 21 h. The spectra showed the
slow disappearance of 1 (t1/2 ) 13.6 h) with formation of free ppd
and several unidentified hydrido species. At the beginning, the most
important hydrido resonance was at δ -4.63 ppm, tentatively attribut-
able to the derivative of substitution of ppd by CD3CN, of formula
[Re2(µ-H)2(CO)6(CD3CN)2]. This species further evolved, with partial
H2 loss and formation of many species. Among them, the known
triangular cluster [Re3(µ-H)3(CO)9(CD3CN)3] was identified.
(34) (a) Andrews, M. A.; Kirtley, S. W.; Kaesz, H. D. Inorg. Chem. 1977,
16, 1556–1561. (b) Johnson, J. R.; Kaesz, H. D. Inorg. Synth. 1978,
18, 60–62.
(35) Schmidt, S. P.; Trogler, W. C.; Basolo, F. Inorg. Synth. 1985, 23,
41–46.
(36) Lacowicz, J. R. Principles of Fluorescence Spectroscopy, 2nd ed.;
Kluwer: New York, 1999.
Inorganic Chemistry, Vol. 47, No. 23, 2008 11163

Mauro et al.
A similar experiment was performed on a CD3CN solution of 2.
In this case, the decomposition reaction was faster (t1/2 ) 4.5 h)
and more selective, affording a main hydrido species only,
responsible for a resonance at δ -9.13 ppm.
Netherlands) run by a PC with GPES software. The working
electrode was a glassy carbon one (AMEL, diameter ) 1.5 mm)
cleaned by diamond powder (Aldrich, diameter ) 1 µm) on a wet
cloth (STRUERS DP-NAP); the counter electrode was a platinum
wire; the reference electrode was an aqueous saturated calomel
electrode, having in our working medium a difference of -0.385
V vs the Fc+|Fc couple (the intersolvent redox potential reference
currently recommended by IUPAC)39 and +0.032 V vs the
Me10Fc+|Me10Fc couple (an improved intersolvent reference under
investigation).40
Computational Details. Geometries were optimized by means
of density functional calculations. The parameter-free hybrid
functional PBE041 was employed along with the standard valence
double-ꢁ polarized basis set 6-31G(d,p) for C, H, Cl, N, and O.
For Re, the Stuttgart-Dresden effective core potentials were
employed along with the corresponding valence triple-ꢁ basis set.
The effect of basis set expansion was previously checked studying
an analogous pyridazine derivative8 and found to be negligible.
All the calculations were done assuming Cs symmetry. The nature
of all the stationary points was checked by computing vibrational
frequencies, and all the species were find to be true minima. Higher
energy conformers belonging to different point groups were found
for 1 and 3. In particular, a C2V-symmetric conformer in which the
diphenyloxadiazole ligand is planar was found for 1, lying only
0.3 kJ mol-1 higher in energy with respect to the Cs minimum. A
C2-symmetric conformer in which the two phenyl rings are rotated
in two opposite directions out of the oxadiazole plane was found
for 3, lying 7.7 kJ mol-1 higher in energy with respect to the Cs
minimum. For compound 2, any attempt to optimize a structure in
which one or both the phenyl rings were rotated in the opposite
direction with respect to that found in the minimum energy structure
failed, and no other conformer can be found.
In the case of 3, the progress of the reaction was monitored by
the changes in the aromatic region, which showed the progressive
decrease of concentration of 3 (t1/2 ) 3.7 h) accompanied by the
corresponding formation of free ppd. The reaction was repeated in
a Schlenk tube in MeCN at room temperature and monitored
overnight by IR. At the end, the ν(CO) bands of 3 disappeared,
replaced by bands attributable to fac-[ReCl(CO)3(MeCN)2] (2036s,
1933s, 1909s cm-1). The solvent was removed under vacuum, and
the residue dissolved in CH2Cl2. After 2 days, IR and NMR spectra
showed that most of the monomeric complex had converted back
into 3.
Reaction of 3 with Tetrahydrofuran. The reaction was per-
1
formed in a NMR tube employing THF-d8 as a solvent. The H
NMR spectra, at 298 K, showed the progressive increase of the
resonances of free ppd at the expense of 3, with a slower kinetics
than in the case of MeCN (t1/2 ) 17.2 h). The reaction was repeated
in a Schlenk tube, in THF at room temperature: after 44 h the IR
spectrum showed, besides the ν(CO) bands of 3, also absorptions
at 2026m, 1905s, and 1890s, attributable to [ReCl(CO)3(THF)2]
for the close similarity to those of the corresponding Br derivative
(2027s, 1910s, and 1892s).37 1H NMR in CDCl3 showed the
resonances of free ppd.
Competition Experiment: Reaction of [ReCl(CO)5] with
ppd and pydz. A sample of [ReCl(CO)5] (10.7 mg, 0.0295 mmol)
was treated with 3.3 mg of ppd (0.0148 mmol) and 1 µL of pydz
(0.0138 mmol) in 4 mL of freshly distilled toluene. The solution
was heated at reflux for 1.5 h. A yellow color became detectable
after 15 min. The IR and 1H NMR spectra unambiguously revealed
the formation of [Re2(µ-Cl)2(CO)6(µ-pydz)] and the presence of
unreacted ppd.
Dissociation and interaction energy values reported in Table 4
are counterpoise corrected.42
In order to simulate the absorption electronic spectrum down to
250 nm, the lowest 30 singlet excitation energies were computed
by means of time-dependent density functional calculations.
X-Ray Diffraction Structural Analysis. Data for 1: C20H12-
N2O7Re2, M ) 764.72, triclinic, P1 (No. 2), a ) 8.121(2) Å, b )
11.430(2) Å, c ) 12.526(2) Å, R ) 98.52(2)°, ꢀ ) 103.24(2)°, γ
) 109.18(2)°, V ) 1036.9(4) Å3, Z ) 2, Dx ) 2.449 g cm-3, λ(Mo
KR) ) 0.71073 Å, µ ) 11.708 mm-1, F(000) ) 704, T ) 110(2)
K, R(F) ) 0.0242, Rw(F2) ) 0.0450.
j
(39) (a) Gritzner, G.; Kuta, J. Pure Appl. Chem. 1984, 56, 461–466. (b)
Gritzner, G. Pure Appl. Chem. 1990, 62, 1839–1858.
(40) (a) Falciola, L.; Gennaro, A.; Isse, A. A.; Mussini, P. R.; Rossi, M. J.
Electroanal. Chem. 2006, 593, 47–56.
(41) Called PBE1PBE in Gaussian: (a) Adamo, C.; Barone, V. J. Chem.
Phys. 1999, 111, 6158–6170. (b) Perdew, J. P.; Burke, K.; Ernzerhof,
M. Phys. ReV. Lett. 1996, 77, 3865–3868. (c) Perdew, J. P.; Burke,
K.; Ernzerhof, M. Phys. ReV. Lett. 1997, 78, 1396.
(42) Kestner, N. R.; Combariza, J. E. In ReViews in Computational
Chemistry; Lipkowitz, K. B., Boyd, D. B., Eds.; Wiley-VCH: New
York, 1999; Vol. 13, pp 99-132.
Data for 2·CH2Cl2: C21H13Cl3N2O7Re2, M ) 884.08, triclinic,
j
P1 (No. 2), a ) 8.483(2) Å, b ) 12.283(2) Å, c ) 13.447(2) Å, R
) 63.14(2)°, ꢀ ) 80.25(2)°, γ ) 77.79(2)°, V ) 1217.3(4) Å3, Z
) 2, Dx ) 2.412 g cm-3, λ(Mo KR) ) 0.71073 Å, µ ) 10.310
mm-1, F(000) ) 820, T ) 110(2) K, R(F) ) 0.0151, Rw(F2) )
0.0404.
(43) (a) Barone, V.; Cossi, M. J. Phys. Chem. A 1998, 102, 1995–2001.
(b) Cossi, M.; Barone, V. J. Chem. Phys. 2001, 115, 4708–4717. (c)
Cossi, M.; Rega, N.; Scalmani, G.; Barone, V. J. Comput. Chem. 2003,
24, 669–681.
Data for 3: C20H10Cl2N2O7Re2, M ) 833.60, monoclinic, P21/c
(No. 14), a ) 9.100(2) Å, b ) 13.758(2) Å, c ) 18.517(2) Å, ꢀ )
93.12(2)°, V ) 2314.9(7) Å3, Z ) 4, Dx ) 2.392 g cm-3, λ(Mo
KR) ) 0.71073 Å, µ ) 10.724 mm-1, F(000) ) 1536, T ) 295(2)
K, R(F) ) 0.0157, Rw(F2) ) 0.0384.
Electrochemical Measurements. The cyclovoltammetric study
of the complexes has been performed at scan rates typically ranging
from 0.02 to 10 V s-1 in HPLC-grade MeCN solutions at
0.00025-0.001 M concentration in each substrate, deaerated by
N2 bubbling, with 0.1 M TBAPF6 (Fluka) as the supporting
electrolyte, at 298 K. The ohmic drop has been compensated by
the positive feedback technique.38 The experiments were carried
out using an AUTOLAB PGSTAT potentiostat (EcoChemie, The
(44) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin,
K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone,
V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.;
Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa,
J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene,
M.; Li, X.; Knox, J. E.; Hratchian, H. P.; 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.; Ayala,
P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.;
Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas,
O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.;
Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.;
Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.;
Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.;
Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen,
W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, revision
D.01; Gaussian Inc., Wallingford, CT, 2004.
(37) D’Alfonso, G. Unpublished results. See also: Vitali, D.; Calderazzo,
F. Gazz. Chim. Ital. 1972, 102, 587–596.
(38) Bard, A. J.; Faulkner, L. R. Electrochemical Methods. Fundamentals
and Applications; Wiley: New York, 2002; pp 648-650.
11164 Inorganic Chemistry, Vol. 47, No. 23, 2008

Tricarbonyl Re(I) Complexes Containing a Bridging ppd Ligand
Calculations were done also in the presence of solvent (dichlo-
romethane, used in most of the photophysical characterizations)
described by the conductor-like polarizable continuum model
(CPCM).43
PRIN2007, Aggregation and functionalization of clusters: an
opportunity for new materials).
Supporting Information Available: Experimental and opti-
mized geometries, molecular orbital energies, composition and
energy of the computed electronic transitions, plots of various
properties of the free ligand as a function of the phenyl ring rotation,
and the emission spectra of 3 upon successive excitations. This
materialisavailablefreeofchargeviatheInternetathttp://pubs.acs.org.
All the calculations were done with Gaussian 03.44
Acknowledgment. We thank Italy’s MIUR for financial
support (FIRB 2003, RBNE033KMA, Molecular compounds
and hybrid nanostructured material with resonant and non
resonant optical properties for photonic devices, and
IC801447Z
Inorganic Chemistry, Vol. 47, No. 23, 2008 11165