Mono- and dinuclear osmium N,N′-di- and tetraphenylbipyridyls and extended bipyridyls. Synthesis, structure and electrochemistry

Article abstract of DOI:10.1039/c2dt32548a

The efficient synthesis of mono- and dinuclear Os(iv) bipyridyl complexes is reported. These compounds show a two-step oxidation process leading to notable structural changes, which are reflected in their emission properties. During the second oxidation process a tetracation with a hydride-dihydrogen structure (instead of a trihydride) is formed. This results in a significant bathochromic shift of the emission band, accompanied by a moderate increase in intensity.

Full text of DOI:10.1039/c2dt32548a

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Mono- and dinuclear osmium N,N’-di- and
tetraphenylbipyridyls and extended bipyridyls.
Cite this: Dalton Trans., 2013, 42, 3597
Synthesis, structure and electrochemistry†
Miguel A. Esteruelas,a Israel Fernández,b Mar Gómez-Gallego,b
Mamen Martín-Ortíz,b Pedro Molina,c Montserrat Oliván,a Francisco Otón,c
Miguel A. Sierra*b and Marta Valenciaa
The efficient synthesis of mono- and dinuclear Os(IV) bipyridyl complexes is reported. These compounds
show a two-step oxidation process leading to notable structural changes, which are reflected in their
emission properties. During the second oxidation process a tetracation with a hydride-dihydrogen struc-
ture (instead of a trihydride) is formed. This results in a significant bathochromic shift of the emission
band, accompanied by a moderate increase in intensity.
Received 24th October 2012,
Accepted 4th December 2012
DOI: 10.1039/c2dt32548a
www.rsc.org/dalton
Introduction
The interest in dinuclear and polynuclear metal-complexes
in which the metal centers are linked by bridging bi- and tri-
dentate ligands remains unabated. They are ideal structures
to study electron transfer mechanisms,1 provide the perfect
ground to disentangle the origin of the factors affecting charge
localization and, due to their electronic and magnetic proper-
ties, are also useful in the design of molecular electronic or
photonic devices.2 Properties in these systems are also related
to the nature of the ligands and the spacers, in many cases
biaryls, for which the influence of geometry and torsion
angles3 is of particular relevance in the design of electron
transport-based devices.3,4 Furthermore, changes in the oxi-
dation state of the metals often produce a substantial altera-
tion of their photophysical properties, which has found
application in different areas of chemistry, from materials to
biochemistry.5 All these reasons make the flexible access to
new dinuclear and polynuclear metal-complexes a very attrac-
tive field of research.
Fig. 1 Bridged dinuclear Os(IV) complexes.
Taking advantage of the ability of osmium(VI) hydride
complex OsH6(PiPr3)2 to efficiently activate C–H bonds6 we
devised the synthesis of two types of bridged Os(IV) capped
4,4′-binuclear complexes I and II (Fig. 1) as novel substrates
of study. In this regard, we have recently described the use
of OsH6(PiPr3)2 to build metallapolycycles by reaction with
2-phenylazines.7 The reactions involve the activation of two or
four C–H bonds, yielding compounds having five and eight
fused cycles, respectively. This paper reports the synthesis,
structural and electrochemical study of a series of mono- and
binuclear Os(IV)-N,N′-di- and tetraphenyl bipyridyls with
different conjugated and non-conjugated spacers. In addition,
the dramatic influence of the redox processes on the structure
aDepartamento de Quíımica Inorgánica, Instituto de Síntesis Química y Catálisis
Homogénea, Universidad de Zaragoza-CSIC, 50009 Zaragoza, Spain.
E-mail: maester@unizar.es
bDepartamento de Química Orgánica, Facultad de Ciencias Químicas, Universidad
Complutense, 28040-Madrid, Spain. E-mail: sierraor@quim.ucm.es;
Fax: (+) 34-91-3944310
cDepartamento de Química Orgánica, Facultad de Química, Universidad de Murcia,
Campus de Espinardo, E-30100 Murcia, Spain
†Electronic supplementary information (ESI) available: Full synthetic pro-
cedures, NMR spectra, electrochemical and computational details (in a.u., non-
corrected zero-point vibrational energies included). CCDC 870543 and 870544.
For ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/c2dt32548a
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of the complexes and their emission properties are also
discussed.
Results and discussion
N,N′-Di- and tetraphenyl bipyridines 2–4 were prepared start-
ing from 4-bromo-2-phenylpyridine (1) as depicted in
Scheme 1. Nickel-catalyzed homocoupling of 1 using the mild
conditions reported by Iyoda and col.8 (NiBr2(PPh3)2/Zn/Et4NI,
in tetrahydrofuran at 50 °C) afforded 2,2′-diphenyl-4,4′-bipyri-
dine (2), which was isolated in 43% yield. The phenyl and
anthranyl derivatives 3 and 4 were obtained by Suzuki coupling
between 1 and 1,4-phenylboronic acid and anthracene-9,10-
diboronic acid bis(pinacol) ester in 82% and 35% yields,
respectively. In turn, 2-phenyl-4-(2-(2-phenylpyridin-4-yl)-
ethynyl)pyridine (5) was prepared by means of two successive
Sonogashira couplings (Scheme 2). Thus, the reaction of 1
with trimethylsilyl acetylene in the presence of Pd(PPh3)4 led
to 4-ethynyl-2-phenylpyridine (6), which after a second coup-
ling with 1 under similar conditions afforded 5 in 68% yield.
Finally, 2,2′,6,6′-tetraphenyl-4,4′-bipyridine (8) was prepared in
36% yield by nickel-homocoupling of 4-chloro-2,6-diphenyl-
pyridine (7), using the reaction conditions described for 2
(Scheme 2).
Scheme 2 Synthesis of bipyridines 5 and 8.
reflux afforded the trihydride derivative 10, which was isolated
as a yellow solid in 77% yield according to Scheme 3.
Under the same conditions, the treatment of 9 with 1.0
equiv. of the diphenylbipyridine 2 led to a 2 : 1 mixture of the
mononuclear compound 11, related to 10, and the bimetallic
species 12 resulting from the C–H bond activation of both
phenyl substituents by different metal centers. When the reac-
tion between 9 and 2 was performed in a 1 : 3 molar ratio, the
mononuclear complex 11 was selectively formed. However, the
treatment of 9 with 0.3 equiv. of 2 produced the quantitative
transformation of the bipyridine into the bimetallic derivative
12. Complexes 11 and 12 were isolated as dark red solids in
50% and 91% yields with regard to the respective limiting
reagents. The presence of phenylidene and anthracenylidene
spacers between the pyridyl groups has no significant influ-
ence on the reactivity of the bipyridines. Thus, similarly to 2,
the addition of 0.3 equiv. of 3 and 4 to the toluene solutions of
9 gave the bimetallic derivatives 13 and 14 after 6 h and 15 h,
respectively, under reflux. Both 13 and 14 were isolated as dark
red solids in almost quantitative yield with regard to the
bipyridines.
Scheme 1 Synthesis of bipyridines 2–4.
Osmium(IV)–polyhydride complexes containing N,N′-di- and
tetraphenyl bipyridines
Complexes 10–14 were characterized by elemental analysis,
1
IR and H, 31P{1H} and 13C{1H} NMR spectroscopy. Complexes
The hexahydride complex OsH6(PiPr3)2 (9) activates an ortho-
C–H bond of the phenyl substituent of 2-phenylpyridine, in
agreement with the tendency shown by this compound to acti-
vate C(sp2)–H bonds.6,7 Thus, the treatment of toluene solu-
tions of 9 with 1.0 equiv. of 2-phenylpyridine, for 18 h, under
11 and 12 were further characterized by X-ray diffraction analy-
sis. Fig. 2 and 3 show views of their molecular geometries. The
structure of 11 (Fig. 2) proves the selective ortho-C–H bond acti-
vation of one of the phenyl substituents of the bipyridine to
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afford an osmatricycle core, which is planar (maximum devi- and the hydride ligands. The Os–C(29) distance of 2.102(3) Å is
ation 0.121(3) Å for C(23)). The geometry around the osmium similar to the Os–C bond lengths reported for osmafurans,6c,9
atom can be rationalized as a distorted pentagonal bipyramid osmapyrroles,10 and related compounds,6i,11 while the Os–N(1)
with the phosphine ligands occupying axial positions (P(1)– distance of 2.155(3) Å compares well with the Os–N bond
Os–P(2) = 162.44(3)°). The metal coordination sphere is com- lengths found in lower π-electron osmacyclic nitrogen-contain-
pleted by the C(29) and N(1) atoms of the metalated bipyri- ing compounds.10,11 In agreement with a significant π-electron
dine, which acts with a C(29)–Os–N(1) bite angle of 76.48(11)°, delocalization in the five-membered ring, the C(19)–C(24) dis-
tance of 1.466(4) Å is about 0.02 Å shorter than the C(21)–
C(30) separation of 1.494(4) Å between the osmatricycle core
and the pyridylphenyl substituent and the C(32)–C(35) sepa-
ration of 1.487(4) Å between the pyridinic ring and the phenyl
group of the latter. The pyridinic ring is almost planar with
the metallatricycle core, the dihedral angle between the planes
is 9.49(2)° whereas the dihedral angle between the pyridinic
ring and its phenyl substituent is 32.03(1)°.
The structure of 12 (Fig. 3) proves the C–H bond activation
of both phenyl substituents of the bipyridine, by different
Fig. 2 Molecular diagram of complex 11. Selected bond lengths (Å) and
angles (°): Os–P(1)
= 2.3412(9), Os–P(2) = 2.3503(9), Os–N(1) = 2.155(3),
Os–C(29) = 2.102(3), C(19)–C(24) = 1.466(4), C(21)–C(30) = 1.494(4), C(32)–
C(35) = 1.487(4); P(1)–Os–P(2) = 162.44(3), C(29)–Os–N(1) = 76.48(11).
Scheme 3 Synthesis of complexes 10–14.
Fig. 3 Molecular diagram of complex 12. Selected bond lengths (Å) and angles (°): Os–P(1) = 2.3304(15), Os–P(2) = 2.3355(15), Os–N = 2.135(4), Os–C(6) =
2.098(5); P(1)–Os–P(2) = 164.29(5), N–Os–C(6) = C6 Os N 75.60(17).
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metal centers. The molecule can be described as a symmetrical
Our calculations also suggest a significant degree of
dimer formed by two OsH3(PiPr3)2 moieties, which are joined π-delocalization within the planar (C–C–C–N angle of 0.5°)
by a bridging diorthometallated 2,2′-diphenyl-4,4-bipyridine five-membered osmacycle. This can be viewed in the computed
ligand. The geometry around the osmium atoms is as that of Wiberg-bond orders of the C–C and C–N bonds which are
11; i.e., a distorted pentagonal bipyramid with the phosphines intermediate between single and double bonds (ranging from
occupying axial positions (P–Os–P = 164.29(5)°). The metal 1.10 to 1.33) and points to a participation of the occupied dπ
coordination spheres are completed by the donor atoms of orbitals of the osmium center in the π-system of the ring. The
the metallated bipyridine (C–Os–N
=
75.60(17)°) and computed HOMO and HOMO − 1 molecular orbitals (Fig. 4b)
the hydride ligands. The Os–C and Os–N bond lengths nicely confirm this hypothesis as well as the second-order
of 2.098(5) Å and 2.135(4) Å, respectively, agree well with those perturbation theory of the Natural Bond Orbital (NBO)
of 11.
To gain more insight into the structure and bonding situ- occupied dπ (Os) atomic orbitals to π*(C6vC11) and π*(NvC)
ation of complexes 11 and 12, density functional theory (DFT) molecular orbitals (associated second-order energies of ΔE(2)
method, which clearly shows two-electron delocalizations from
=
calculations12 were carried out on model complex 12M, where −8.3 and −4.3 kcal mol−1, respectively). Moreover, the com-
the bulky isopropyl groups were replaced by hydrogen atoms. puted nuclear independent chemical shift (NICS)16 value at
As can be readily seen in Fig. 4a, the most remarkable differ- the [3,+1] ring critical point of the electron density17 is slightly
ence between the B3LYP/def2-SVP computed and the experi- negative (−1.03 ppm), thus indicating that the involvement of
mental structures is the non-planarity of the bipyridyl ligand the metal fragment in the π-system of the ring makes it slightly
in the gas-phase (computed torsion angle of 34.7°). We have aromatic.18
also re-optimized the gas-phase geometry of 12M at the M06L/
The 1H, 31P{1H} and 13C{1H} NMR spectra of 10–14 in
def2-SVP13 to take into account possible dispersion effects, but toluene-d8 are consistent with the structures shown in Fig. 2
again a non-planar structure (torsion angle of 30.9°) was and 3. As expected for three inequivalent hydride ligands, the
observed.14 Although the discrepancy could be attributed to 1H NMR spectra at 203 K contain three hydride resonances at
the use of the simple model in the calculations, this situation about −6, −11, and −12 ppm. These resonances are temp-
is similar to that of biphenyls having H atoms in all four ortho erature dependent. Fig. 5 shows the high field region of the
positions, which are especially susceptible to crystal-packing spectra of 14 as a function of the temperature. The resonances
effects and adopt a planar geometry in the solid state but are
twisted in the gas phase.3f–h In fact, the dihedral angle
between the two phenyl rings is determined by competition
between π-conjugation and steric repulsions; the former
favours a coplanar configuration, while the latter prefers a
non-planar form.15
Fig. 4 (a) B3LYP/def2-SVP fully optimized geometry of compound 12M
showing the computed torsional angle (in degrees). (b) Molecular orbitals of
Fig. 5 High field of the 1H{31P} NMR spectra (400 MHz, toluene-d8) of complex
14 as a function of temperature.
complex 12M (isosurface value of 0.035 au).
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Table 1 Coupling constants as a function of temperature for complexes 10,
11, 13, 14, 15, and 16
Complex
Temperature (K)
Coupling constant (Hz)
10
11
203
213
193
203
213
203
213
193
203
213
223
203
213
203
213
36.5
44.4
48.3
65.8
86.1
42.4
54.4
22.8
32.2
38.8
45.8
30
13
14
Scheme 4 Reactions of hexahydride 9 with 5.
15
16
41
26.1
29.2
The H, 31P{1H} and 13C{1H} NMR spectra of 15 and 16 in
toluene-d8 agree well with those of 10–14 and support the
hydrogenation of the ethynylene spacer. In agreement with the
presence of the resulting ethylidene group, both 1H NMR
spectra contain at 2.47 ppm the signal due to the CH2-
hydrogen atoms, which appear with the complex form for 15
and as a singlet for 16. Similar to 10–14, in the high field
region of the spectra, at 203 K the hydride ligands display
three resonances at −6.51, −11.21 and −12.16 (15) and −6.49,
−11.20 and −12.12 (16) ppm, whereas at around 363 K only
one hydride signal at about −10 ppm is observed. Also in
these cases, the resonances at about −6 and −11 ppm undergo
quantum exchange coupling in addition to the thermally
activated site exchange. The activation barrier for the position
exchanges is between 10 and 12 kcal mol−1. The 31P{1H}
NMR spectra at 298 K show a singlet at about 20 ppm. In the
13C{1H} NMR spectra, the CH2 resonances of the spacer appear
at 35.4 and 35.6 (15) and 35.8 (16) ppm, whereas the signal
due to the metalated carbon atoms is observed at 187.7 ppm
for both compounds.
1
at about −6 and −11 ppm coalesce between 235 K and 253 K,
whereas a single hydride signal at about −10 ppm is observed
at a temperature higher than 363 K. This is consistent with the
operation of two thermally activated site exchange processes,
in agreement with the behaviour of related OsH3-deriva-
tives.6,19 The exchange mechanism implies Os–H stretching,
H–H shortening, and subsequent rotation of the resulting
dihydrogen ligand. Since the activation barrier of both
exchanges is similar, about 10 and 11 kcal mol−1, the tran-
sition states containing the dihydrogen ligand trans disposed
to the nitrogen or carbon atoms of the metallated bipyridines
are similarly favoured. The hydrogen atoms of 10, 11, 13, and
14 corresponding to the resonances at about −6 and −11 ppm
undergo quantum exchange coupling20 in addition to the
thermally activated site exchange. Thus, the JH–H values
decrease with the temperature (Table 1). In agreement with the
presence in these compounds of equivalent phosphines, their
31P{1H} NMR spectra show a singlet at about 21 ppm, which
is temperature invariant from 298 K to 203 K. In the 13C{1H}
NMR spectra at room temperature, the most noticeable
resonance is that due to the metallated carbon atoms of the
bipyridines that is observed at about 188 ppm as a triplet with
a C–P coupling constant of about 6 Hz. The ethynylene spacer
is not an inert group under the reaction conditions, in contrast
to phenylidene and anthracenylidene, since the hexahydride
precursor promotes its reduction into ethylidene. Thus, the
treatment under reflux of toluene solutions of 9 with 3.0 equiv.
Complex 9 activates an ortho-C–H bond of both phenyl sub-
stituents of 2,6-diphenylpyridine. Treatment of toluene solu-
tions of this d2-hexahydride complex with 1.0 equiv. of the
organic molecule for 15 h, under reflux, afforded the dihydride
derivative 17, which was isolated as a dark orange solid in 64%
yield according to Scheme 5. Under the same conditions the
treatment of 9 with 1.0 equiv. of 2,2′,6,6′-tetraphenyl-4,4′-bipyri-
dine (8) gave a 1 : 1 mixture of the mononuclear species 18,
related to 17, and the bimetallic derivative 19. The latter
results from the C–H bond activation of the four phenyl substi-
tuents by two different metal centers. Each osmium atom
metallates both phenyl groups of a pyridinic unit. When 9 was
treated with 3.0 equiv. of 8, its quantitative transformation
into 18 occurred. However, the latter could not be separated
from the excess of 8 due to the similar solubility of both
compounds in the usual organic solvents. The addition of
0.2 equiv. of 8 to 9 selectively led to the bimetallic species
19, which was isolated as a dark red solid in 88% yield with
regard to 8.
of
5 gave after 10 min the mononuclear complex 15
(Scheme 4), as a result of the C–H bond activation of a phenyl
substituent and the hydrogenation of the C–C triple bond of
the organic molecule. As expected, the addition of 0.5 equiv. of
5 to the toluene solutions of 9 yielded the dinuclear species
16 after 4 h, under reflux. The latter is a consequence of the
C–H activation of both phenyl substituents, by different
metal centers, and the hydrogenation of the spacer. Com-
plexes 15 and 16 were isolated as dark green solids in
16% (15) and 63% (16)21 yield with regard to 9 and 5,
respectively.
The 1H, 31P{1H} and 13C{1H} NMR spectra of 17–19 in
toluene-d8, at room temperature, strongly support the struc-
tures proposed for these compounds in Scheme 5. In the
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1H NMR spectra, the equivalent hydride ligands display at
The electrochemical study of the mononuclear complexes
about −8 ppm a triplet with a H–P coupling constant of about 11 and 18 shows two quasi-reversible processes at about 0
16 Hz. As expected for equivalent phosphines, the 31P{1H} and 1 V (Table 2), which agree with the values found in the
NMR spectra contain a singlet at about 0 ppm. In the 13C{1H} literature for the Os(IV)–Os(V) and Os(V)–Os(VI) processes,
NMR spectra, the resonance due to the metallated carbon respectively.22 Fig. 7 shows the CV and OSWV of 18.
atoms of the pyridinic organic moiety appears at about
The electrochemistry of the bimetallic complexes 12, 13
178 ppm as a triplet with a C–P coupling constant of about and 19 was also studied. In the three cases two quasi-reversible
7 Hz.
waves at about 0 and 1 V were observed (Table 2). No splitting
of the waves due to electronic communication between the
metal centers was found. It is worth to mention that all mono-
metallic and bimetallic complexes behave similarly.
The spectroelectrochemical study of bimetallic complexes
12, 13 and 19 was subsequently performed. Electrochemical
oxidations of these complexes did not exhibit bands that could
be assigned to intervalence charge transfer (IVCT) processes.
In all cases, it was found an increase of the band around
280 nm (probably associated with a ligand π–π* band) and the
vanishing of the charge-transfer band around 450 nm as a con-
sequence of the oxidation of the metal center. Spectroscopic
data are detailed in Table 3 and UV-vis spectra are shown in
ESI.† In contrast to the mononuclear complex 11, changes in
the emission spectra of the binuclear complexes 12, 13 and 19
were found upon oxidation. In the three cases the transform-
ation from Os(IV) to Os(V) at 0.05 V vs. Fc+/Fc produced small
changes in the emission spectra of the complexes (λexc
=
280 nm). When the excitation wavelength is chosen to match
the charge-transfer band that is present near λ = 450 nm, no
luminescence was detected. It is worth mentioning that the
Scheme 5 Reactions of hexahydride 9 with 2,6-diphenylpyridine and 8.
absorbance of the sample at a concentration of c = 2 × 10−6
M
is of near 1 × 10−3 and self-absorption processes do not affect
Electrochemical study
Table 2 Electrochemical data (V vs. Fc+/Fc)
Cyclic Voltammetry (CV) and Osteryoung Square Wave Voltam-
metry (OSWV) experiments were carried out in order to study
the electrochemical properties of the complexes 11–13, 18 and
19. 2,2′,6,6′-Tetraphenyl-4,4′-bipyridine (8) was used as a
model for the electrochemical behaviour of the coordinated
bipyridine ligands. In the voltammograms of 8 only an irre-
versible reduction process is observed at Ep = −1.8 V (Fig. 6).
Therefore, no oxidation processes associated with the ligands
are expected in the complexes.
Complex
1st wave potentiala
2nd wave potentiala
11
12
13
18
19
0.0
−0.02
0.00
−0.03
−0.02
0.97
0.94
0.99
0.95
0.95
a Half-wave potentials in CH2Cl2–CH3CN (1 : 1) (c = 5 × 10−4 M).
Fig. 6 CV (a) and OSWV (b) of 8 (c = 5 × 10−4 M) in dichloromethane–acetonitrile (1 : 1) with 0.1 M of tetrabutylammonium hexafluorphosphate as a supporting
electrolyte.
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Fig. 7 CV (a) and OSWV (b) of mononuclear complex 18 (c = 5 × 10−4 M) in dichloromethane–acetonitrile (1 : 1) with 0.1 M of TBAHP as supporting electrolyte.
The small irreversible wave that appears at around 0.80 V could be due to an irreversible oxidation of an impurity present in the sample.
Table 3 UV-vis-NIR spectra data of the studied complexes
large background currents, around 150% of the current origi-
nated from the oxidation of the metal centers, found when the
oxidations were carried out at 1.10 V vs. Fc (above the potential
of the second electrochemical reversible wave) make the calcu-
lations of the number of electrons of this oxidation process
less accurate. However, a charge near to 0.5 C, when the back-
ground charge is subtracted, points again to a bi-electronic
process.
Complex
λ/nm (ε × 10−3/M−1 cm−1
)
Isosbestic points
12
267 (17.83), 483 (6.94), 544 (sh)
252 (30.87), 449 (2.57)
283 (7.30), 448 (1.85), 519 (sh)
260 (28.96), 388 (sh)
285 (7.07), 454 (2.73), 529 (sh)
253 (26.77), 393 (sh)
—
122+
13
270, 315, 618
—
575
—
590
132+
19
192+
The geometries of the corresponding oxidized species
12M2+ and 12M4+ were also optimized by means of compu-
the emission of the sample. The analysis of the excitation tational methods. The two electron oxidation process on 12M
spectra (see ESI†) also supports that the emission is originated leads to the open-shell singlet biradical 12M2+ where the
in a ligand-centered transition. The slight increase of the lumi- unpaired electrons are mainly located at the osmium atoms
nescence intensity may be due to the increase in the molar (computed spin densities of 0.56 e and −0.56 e, respectively,
absorptivity of the sample. However, when a potential of see Fig. 10). As expected, a significant structural change is
1.10 V is applied to oxidize the metal centers of the complexes observed upon oxidation. In fact, the hydride ligand transoid
from Os(V) to Os(VI), a bathochromic shift of the band is to the metallated nitrogen atom and the central one approach
observed, accompanied by a moderate but significant increase each other as a consequence of the oxidation of the metal
in the luminescence intensity (Fig. 8).
center. Thus, whereas a H⋯H bond length of 1.666 Å was com-
Quantum yields of the different species are summarized in puted for 12M, a shorter bond length (1.338 Å) is found for
Table 4. The quantum yield of organic molecule 2 is higher 12M2+. This shortening is even more remarkable in the 12M4+
than those of the organometallic species. For the bimetallic species formed as a consequence of the second 2-electron oxi-
complexes the oxidation from Os(V) to Os(VI) produces a signifi- dation process. As can be readily seen in Fig. 10, the computed
cant bathochromic shift from about 360 nm to 405 nm and an H⋯H bond length for the latter complex was 0.899 Å, a value
increase of the emission.
typical for dihydrogen species.24 The increase in the corres-
To calculate the number of electrons that are involved in ponding H⋯H bond order upon oxidation (0.09 < 0.17 < 0.48)
each process, chrono-coulombimetry experiments on com- clearly shows the structural change from a trihydride complex
plexes 19 and 12 were performed (Fig. 9). When a voltage of to a hydride-dihydrogen compound. In addition, the planarity
0.05 V vs. Fc+/Fc pair is applied in the electrochemical cell (c = of the five-membered osmacycle is also affected upon oxi-
2.5 × 10−4 M; V = 10 mL; n = 2.5 × 10−6 mol) a current of dation (Os–C–C–C dihedral angle ranging from 0.0° to 7.1°
0.427 C is obtained for 19 and a charge of 0.496 C for 12, and to −19.0° from 12M to 12M2+ and to 12M4+, respectively).
which noticeably coincides with the theoretical charge for a As a result, an increase in the NICS values computed at the
two-electron process (q = 0.482 C, note that the charge of the [3,+1] ring critical point of the electron density was also
background is around 1%). We also carried out an oxidation observed (−1.03 < +2.76 < +3.35 ppm, respectively). Thus, both
applying a potential of E = −0.2 V vs. Fc to take into account oxidized species are noticeably less aromatic than the parent
the contribution of the first irreversible process to the total planar complex 12M. From these data, it becomes obvious that
current. For complex 19 a charge of 0.062 C was found, while a the oxidation process leads to dramatic changes in the struc-
value of 0.095 C was obtained in the case of 12. Therefore, ture of the complexes which are reflected in their electronic
even when the charge of the first process is subtracted to the structures and their emission properties.
charge of the oxidation at 0.05 V, these values clearly indicate
Similar structural changes were found in the two-step
the oxidation of two electrons for the first reversible wave. The oxidation of mononuclear complex 11M, a model complex of
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Fig. 8 Emission spectra of 11 (a), 12 (b), 13 (c), 19 (d). Blue: Os(IV); red: Os(V); green: Os(VI), c = 2 × 10−6 M, dichloromethane–acetonitrile 1 : 1.
reaction of OsH6(PiPr3)2 and the appropriate di- or tetra-
Table 4 Emission quantum yields in the different oxidation states
phenylbipyridines in high yields. The structures of some rep-
resentative mono- and binuclear compounds have been
determined by X-ray diffraction analysis and in the gas
phase by DFT-calculations. The spectroelectrochemical
study of bimetallic complexes 12–14 having different spacers
showed significant changes in the emission spectra upon
oxidation. While the transformation from Os(IV) to Os(V) pro-
duced small changes in the emission spectra of the complexes,
the oxidation from Os(V) to Os(VI) provokes a notable batho-
chromic shift of the emission band, accompanied by a
moderate but significant increase in intensity. DFT calcu-
lations carried out in the dication and tetracation model
species 11M and 12M showed important structural modifi-
cations during the sequential oxidation. Thus, the final tetra-
cation, formally an Os(VI) species, has a hydride-dihydrogen
structure (instead of a trihydride) and it should be better
considered an Os(IV) species. Similar structural changes
were found in the oxidation process of the mononuclear com-
plexes which were also reflected in their photophysical
properties.
a
Compound
λemi
ϕF
2
377
367
470
470
366
365
407, (sh 465)
363
363
404
363
363
405
0.140
0.005
0.006
0.005
0.005
0.013
0.030
0.020
0.026
0.074
0.020
0.025
0.110
11
11+
112+
12
122+
124+
13
132+
134+
19
192+
194+
a Quantum yields were calculated using anthracene as the standard.23
11 where the isopropyl groups were replaced by methyl groups.
As seen in Fig. 11, the ground state structure of 11M resembles
that of binuclear complex 12M, i.e. a trihydride complex (com-
puted H⋯H bond length of 1.668 Å), with a planar and aro-
matic five-membered osmacycle (Os–C–C–C angle of 0.5° and
NICS of −1.06 ppm). As expected, the one-electron oxidation
leads to the radical-cation 11M˙+, where the unpaired electron
is mainly located at the osmium atom (computed spin density
of 0.64e), which exhibits a shorter H⋯H bond length (1.452 Å)
and a less planar, and less aromatic, five-membered osmacycle
(Os–C–C–C angle of −6.9° and NICS of +3.40 ppm).
Experimental section
General information, synthetic procedures, electrochemical
and computational details are included in ESI.† 4-Bromo-
2-phenylpyridine (1),25 4-chloro-2,6-diphenylpyridine (7)26
and OsH6(PiPr3)2 (9)27 were prepared as previously
described.
Conclusions
d4-Mono and dinuclear Os-N,N′-di- and tetraphenylbipyridyls
Procedures for the preparation of mono- and dinuclear
and extended bipyridyls have been synthesised by the complexes 11 and 12 are representative.
3604 | Dalton Trans., 2013, 42, 3597–3608
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Fig. 9 Chronocoulombimetric curves of the complexes 12 (a) and 19 (b) at E = 0.05 V vs. Fc+/Fc.
Fig. 10 B3LYP/def2-SVP fully optimized geometries of complexes 12M2+ and
12M4+. Values in brackets indicate the computed spin densities.
Reaction of OsH6(PiPr3)2 with 2,2′-diphenyl-4,4′-bipyridine (2):
synthesis of complex 11
3 equiv. of 2,2′-diphenyl-4,4′-bipyridine (2) (71 mg, 0.23 mmol)
was added to
Fig. 11 B3LYP/def2-SVP fully optimized geometries of complexes 11M and
11M˙+.
a
solution of OsH6(PiPr3)2 (9) (40 mg,
0.077 mmol) in toluene (5 mL) and heated under reflux for
0.5 hours, changing the color from pale yellow to dark red.
The resulting solution was dried in vacuo. Methanol was added
to afford a dark red solid which was washed with further
portions of methanol and dried in vacuo. Yield: 32 mg (50%).
Anal. calcd for C40H61N2OsP2: C, 58.44; H, 7.48; N, 3.41.
Found: C, 58.10; H, 7.21; N, 3.51. IR (neat compound, cm−1):
ν(Os–H) 2128 (w), 1980 (w); ν(CvN) ν(CvC) 1592 (m),
(t, JH–H = 7.2, 1H, CH-arom), 6.87 (d, JH–H = 4.9, 1H, CH-arom),
6.49 (d, JH–H = 6, 1H, CH-arom), 1.91 (m, 6H, PCH(CH3)2), 1.01
(dvt, JH–H = 6.5, N = 12, 18H, PCH(CH3)2), 0.98 (dvt, JH–H = 6.5,
N = 12, 18H, PCH(CH3)2), −8.36 (br, 2H, OsH), −11.65 (br, 1H,
OsH). 1H NMR (300 MHz, C7D8, 193 K, high field region):
δ −6.40 (d, JH–H = 48.3, 1H, Os–H), −10.87 (d, JH–H = 48.3, 1H,
Os–H), −11.82 (s, 1H, Os–H). 13C{1H} NMR (75.48 MHz, C6D6,
293 K): δ 188.4 (t, JP–C = 6.6, Os–C), 168.5 (s, Cipso), 159.4
(s, CH-arom), 158.6 (s, Cipso), 150.8, 147.2 (both s, CH-arom),
146.0, 144.3, 142.5, 139.7 (all s, Cipso), 129.4, 129.0, 128.4,
127.5, 125.5, 119.6, 118.9, 117.9, 117.6, 115.7 (all s, CH-arom),
1
1459 (m). H NMR (300 MHz, C6D6, 293 K): δ 9.80 (d, JH–H = 6,
1H, CH-arom), 8.76 (d, JH–H = 7.2, 1H, CH-arom), 8.62 (d, JH–H
= 4.9, 1H, CH-arom), 8.12 (d, JH–H = 7.3, 2H, CH-arom), 8.03 (s,
1H, CH-arom), 7.93 (d, JH–H = 7.2, 1H, CH-arom), 7.74 (s, 1H,
CH-arom), 7.28 (t, JH–H = 7.3, 2H, CH-arom), 7.25 (t, JH–H
=
7.3, 1H, CH-arom), 7.17 (t, JH–H = 7.2, 1H, CH-arom), 7.12
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27.7 (vt, N = 24, PCH(CH3)2), 20.2, 19.9 (both s, PCH(CH3)2). Crystal data for 11
31P{1H} NMR (121.5 MHz, C6D6, 293 K): δ 20.7 (s).
C40H60N2OsP2, MW 821.04, dark red, needle (0.20 × 0.05 ×
ˉ
0.05), triclinic, space group P1, a: 8.5681(6) Å, b: 14.7204(10) Å,
c: 15.3568(11) Å, α: 80.2450(10)°, β: 76.4900(10)°, γ: 80.7930
(10)°, V = 1841.4(2) Å3, Z = 2, Dcalc: 1.481 g cm−3, F(000): 840,
T = 100(2) K, μ 3.580 mm−1. 20 686 measured reflections (2θ:
3–57°, ω scans 0.3°), 7671 unique (Rint = 0.0407); minimum/
maximum transmission factors 0.596/0.768. Final agreement
factors were R1 = 0.0278 (6571 observed reflections, I > 2σ(I))
Reaction of OsH6(PiPr3)2 with 2,2′-diphenyl-4,4′-bipyridine (2):
synthesis of complex 12
2,2′-Diphenyl-4,4′-bipyridine (29 mg, 0.10 mmol) was added to
a solution of OsH6(PiPr3)2 (9) (150 mg, 0.29 mmol) in toluene
(8 mL) and heated under reflux for 24 hours, changing the
color from pale yellow to dark red. The resulting solution was
dried in vacuo. Methanol was added to afford a dark red solid
which was washed with further portions of methanol and
dried in vacuo. Yield: 117.6 mg (91%). Anal. calcd for
C58H106N2Os2P4: C, 52.15; H, 8.00; N, 2.10. Found: C, 51.8; H,
8.29; N, 1.62. IR (neat compound, cm−1): ν(Os–H) 2098 (w),
1961 (w); ν(CvN) ν(CvC) 1578 (m), 1456 (m). 1H NMR
(400 MHz, C6D6, 293 K): δ 9.77 (d, JH–H = 6.0, 2H, CH-arom),
8.75 (d, JH–H = 7.2, 2H, CH-arom), 8.21 (d, JH–H = 1.4, 2H, CH-
arom), 7.82 (d, JH–H = 7.2, 2H, CH-arom), 7.16 (t, JH–H = 7.2,
2H, CH-arom), 7.09 (t, JH–H = 7.2, 2H, CH-arom), 6.64 (dd, JH–H
= 6.0, JH–H = 1.4, 2H, CH-arom), 1.87 (m, 12H, PCH(CH3)2),
0.98 (dvt, JH–H = 6.6, N = 13.3, 36H, PCH(CH3)2), 0.95 (dvt, JH–H
= 6.8, N = 13, 36H, PCH(CH3)2), −8.38 (br, 4H, OsH), −11.66
(br, 2H, OsH). 1H NMR (300 MHz, C7D8, 213 K, high field
and wR2
= 0.0453; data/restraints/parameters 7671/3/428;
GoF = 0.897. Largest peak and hole 1.111 and −1.342 e Å−3
.
Crystal data for 12
C58H104N2Os2P4·2(C7H8), MW 1517.98, violet, prism (0.10 ×
0.08 × 0.03), monoclinic, space group P2(1)/c, a: 17.258(4) Å, b:
8.775(2) Å, c: 23.919(6) Å, α: 90.00°, β: 103.864(5)°, γ: 90.00°,
V = 3516.5(15) Å3, Z = 2, Dcalc: 1.434 g cm−3, F(000): 1556, T =
100(2) K, μ 3.741 mm−1. 43 143 measured reflections (2θ:
3–58°, ω scans 0.3°), 8774 unique (Rint = 0.0867); minimum/
maximum transmission factors 0.655/0.825. Final agreement
factors were R1 = 0.0425 (5956 observed reflections, I > 2σ(I))
and wR2
= 0.0701; data/restraints/parameters 8774/3/384;
GoF = 0.852. Largest peak and hole 1.909 and −1.260 e Å−3
.
region): δ −6.43 (d, JH–H = 49.8, 2H, Os–H), −10.96 (d, JH–H
=
49.8, 2H, Os–H), −11.85 (br, 2H, Os–H). 13C{1H} NMR
(100.63 MHz, C6D6, 293 K): δ 188.3 (t, JP–C = 6.5, Os–C), 168.3,
144.3, 141.3 (all s, Cipso), 159.3, 147.1, 128.3, 125.6, 118.8,
117.3, 114.8 (all s, CH-arom), 27.5 (vt, N = 24.0, PCH(CH3)2),
20.1, 19.9 (both s, PCH(CH3)2). 31P{1H} NMR (161.99 MHz,
C6D6, 293 K): δ 20.8 (s).
Acknowledgements
Financial support from grants CTQ-2010-20414-C02-01/BQU
(to MAS), CTQ2011-23459 (to MAE) and Consolider Ingenio
2010 (CSD2007-00006) from the MINECO, Diputación General
de Aragón (E35), Comunidad de Madrid (P2009/PPQ1634-
AVANCAT) (UCM group), and European Social Fund is acknowl-
edged. I.F. and F.O. are Ramón y Cajal and Juan de la Cierva
fellows, respectively. M.M.O is an FPI fellow.
Structural analysis of complexes 11 and 12
Crystals suitable for X-ray diffraction were obtained by slow
diffusion of methanol into solutions of the complexes in
toluene. X-ray data were collected on a Bruker Smart APEX
diffractometer equipped with a normal focus, 2.4 kW sealed
tube source (Mo radiation, λ = 0.71073 Å) operating at 50 kV
and 30 (11) or 40 (12) mA. Data were collected over the com-
plete sphere by a combination of four sets. Each frame
exposure time was 10 s covering 0.3° in ω. Data were corrected
for absorption by using a multiscan method applied with the
SADABS program.28 The structures were solved by the Patter-
son (Os atoms of 11 and 12) method and conventional Fourier
techniques and refined by full-matrix least squares on F2 with
SHELXL97.29 Anisotropic parameters were used in the last
cycles of refinement for all non-hydrogen atoms. The hydrogen
atoms were observed or calculated and refined freely using a
restricted riding model. Hydride ligands were observed in the
difference Fourier maps but refined with the restrained Os–H
bond length (1.59(1) Å, CSD). For both structures the highest
electronic residuals were observed in the close proximity of the
Os centers and make no chemical sense. CCDC-870543 (11)
and 870544 (12) contain the supplementary crystallographic
data for this paper.
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288.
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3608 | Dalton Trans., 2013, 42, 3597–3608
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