Synthesis and characterisation of [6]-azaosmahelicenes: The first d 4-heterometallahelicenes

Article abstract of DOI:10.1039/c2cc30356f

[6]-Azaosmahelicenes, the first d⁴-heterometallahelicenes, have been synthesised and fully characterised. Their optical properties (UV-Vis absorption and luminescence) are reported.

Full text of DOI:10.1039/c2cc30356f

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Synthesis and characterisation of [6]-azaosmahelicenes: the first
d4-heterometallahelicenesw
a
a
a
b
´
Olga Crespo, Beatriz Eguillor, Miguel A. Esteruelas,* Israel Fernandez,
a
b
b
Jorge Garcıa-Raboso, Mar Gomez-Gallego, Mamen Martın-Ortiz,
´ ´
´
a
´
Montserrat Olivan and Miguel A. Sierra*
b
Received 16th January 2012, Accepted 5th April 2012
DOI: 10.1039/c2cc30356f
[6]-Azaosmahelicenes, the first d4-heterometallahelicenes, have
been synthesised and fully characterised. Their optical properties
(UV-Vis absorption and luminescence) are reported.
Heterometallahelicenes constitute a class of [n]heterohelicenes1 in
which a CH group has been formally replaced by a transition metal
and its associated ligands.2 These interesting structures3 would
allow the study of the effect of the incorporation of the metal d
electrons to a helical p-backbone, and hence, on the aromaticity
and all the properties that rely on electronic coupling along the
system. The first compounds of this type, which are d8-, d7-, d6-
azaplatinahelicenes4 and d6-azairidahelicenes,4a have been reported
just a few months ago. Reported herein are the first d4-hetero-
metallahelicenes, which are also the first heteroosmahelicenes.
The organic fragments 1a and 1b used to build these structures
were prepared by means of a Wittig CQC bond formation–Stille
coupling–photocyclisation sequence starting from 5-bromo-2-
methyl-benzaldehyde, 2-(naphthyl)methyltriphenyl phosphonium
bromide, and 2-(tributylstannyl)pyridine or 2-(tributylstannyl)-
pyrazine, respectively (Scheme 1). The 1-methyl-4-(2-pyridyl)-
benzo[g]phenanthrene 1a has been characterised by X-ray
diffraction analysis (Fig. 1). The structure reveals a substantial
distortion of the A/D ring planes with an angle between the
terminal aromatic rings of 36.12(5)1. This helical curvature
resembles that of 1,4-dimethylbenzo[c]phenanthrene (36.651).5
The separation between the pyridyl substituent and the poly-
cyclic core (C(5)–C(6)) of 1.4903(18) A is more than 0.04 A
longer than the longest C–C distance within the aromatic
system (C(12)–C(16), 1.4587(18) A), which indicates low electron
delocalization between the polycycle and the aromatic substituent.
As expected, the nucleus-independent chemical shifts (NICS),
computed at the different [3, +1] ring critical points of the electron
density, show that the terminal rings are the most aromatic.6
Scheme 1 Synthesis of 1a and 1b.
The UV-Vis spectra of 1a and 1b in dichloromethane contain
bands between 250 and 380 nm, corresponding to the poly-p-
conjugated system. A similar pattern is observed in the Diffuse
Reflectance UV-Vis (DRUV) spectra, indicating that the
absorption properties of the carbohelicenes do not depend
upon the aggregation state. Furthermore, both 1a and 1b
display emission in the blue region at about 400 nm in
dichloromethane solution and in the solid state, at 77 K and
at room temperature. Other emissions are not observed.
The activation of C–H bonds is usually promoted by low-valent
compounds.7 These types of reactions with high-valent complexes,
in particular hydride derivatives, are rare. However, previous work
has proven that the osmium(VI) complex OsH6(PiPr3)2 (2) effi-
ciently activates C–H bonds of different organic substrates.8 In
agreement with this, the treatment of toluene solutions of 2 with
1.2 equiv. of 1a and 1b (3 h, reflux) led to the corresponding
d4-[6]azaosmahelicene derivatives 3a and 3b, which were isolated as
racemic orange (3a) and dark red (3b) solids in 56% and 63% yield,
a
´
Departamento de Quımica Inorganica, Instituto de Sıntesis Quımica
y Catalisis Homogenea, Universidad de Zaragoza-CSIC,
´
´
´
´
´
50009 Zaragoza, Spain. E-mail: maester@unizar.es
b Departamento de Quı´mica Orga´nica, Facultad de Quı´mica,
Universidad Complutense, 28040 Madrid, Spain.
E-mail: sierraor@quim.ucm.es
w Electronic supplementary information (ESI) available: Experimental,
X-ray collecting data, photophysical studies and computational details.
CCDC 859551 (1a) and 859552 (3a). For ESI and crystallographic data in
CIF or other electronic format see DOI: 10.1039/c2cc30356f
Fig. 1 Molecular diagram of 1a and NICS (0) values for the aromatic
rings.
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and À6.30 (3b), which were assigned to HA, HB, and HC,
respectively, on the basis of the NOESY spectra and T1(min) values.
The hydride resonances are temperature dependent. The
coalescence of the HB and HC resonances occurs between
243 K and 263 K for 3a and 235 K and 245 K for 3b, whereas
a single hydride resonance is observed at temperatures higher
than 343 K for 3a and 335 K for 3b. This is consistent with the
operation of two thermally activated site exchange processes, in
agreement with the behaviour of related OsH3-derivatives.11
The exchange mechanism implies Os–H stretching, H–H short-
ening, and subsequent rotation of the resulting dihydrogen
ligand. Since the activation barriers for the HB–HC exchanges
(11 kcal molÀ1 for 3a and 10 kcal molÀ1 for 3b) are lower than
those for the HA–HB exchanges (15 kcal molÀ1 for 3a and
12 kcal molÀ1 for 3b), the disposition trans of the metallated
carbon atom to the dihydrogen ligand, in the hydride–dihydrogen
transition state, appears to be favoured with regard to the
disposition trans of the nitrogen atom. As expected for an easily
accessible Os(Z2-HB–HC) species, HB and HC undergo quantum
exchange coupling12 in addition to the thermally activated site
exchange. Thus, for both 3a and 3b, the JHB–HC value decreases
with the temperature; from 30 to 18 Hz as the temperature
decreases from 213 K to 183 K, for 3a and from 50 to 32 Hz as
the temperature decreases from 215 K to 205 K, for 3b.
Scheme 2 Synthesis of complexes 3a and 3b.
Fig. 2 Molecular diagram of 3a and NICS (0) values for the aromatic
rings.
respectively (Scheme 2). Complexes 3a and 3b result from the
ortho-CH bond activation of the substituted ring of the starting
[4]-carbohelicene, directed by the nitrogen atom of the aromatic
substituent. Complex 3a has been characterised by X-ray diffrac-
tion analysis and its structure proves the [6]-azaosmahelicene
nature of the new d4-species (Fig. 2).
As expected, the involvement of the metal fragment in the
helicene moiety provokes dramatic changes in the UV-Vis spectra
of complexes 3a and 3b compared to those of the free ligands 1a
and 1b. Thus, the spectra are characterised by the presence of a
significant absorption around 500 nm (l = 500 nm for 3a and
l = 520 nm for 3b). Our gas-phase TD-DFT calculations
accurately assign this band to the HOMO - LUMO vertical
transition (lcalc = 489 nm for 3a and lcalc = 508 nm for 3b). As
readily seen in Fig. 3 for 3a, the HOMO is mainly located in a dp
orbital of the metal whereas the LUMO is clearly a p* molecular
orbital delocalized in the helicene ligand. Therefore, this absorp-
tion can be assigned to a MLCT(p - p*) type transition.
Luminescence studies of 3a and 3b in the solid state exhibit
emissions in the red region at 77 K (l = 624 nm for 3a and l =
683 nm for 3b) and at room temperature (l = 624 nm for 3a and
l = 676 nm for 3b). Emission bands in the red region are also
observed in dichloromethane, toluene, tetrahydrofuran, acetone,
and methanol at 77 K. However, in solution at room temperature
a dual behaviour is observed (Fig. 12a, ESIw). The dominant
bands are intense blue emissions (l = 406 nm for 3a and l =
405 nm for 3b) accompanied by weak bands in the red region
(l = 634 nm for 3a and l = 625 nm for 3b). According to the
lifetimes, Stokes shift and the published data for helicene platinum
complexes,4 the blue fluorescence can be explained in terms of
emissions from the singlet excited state (S1), whereas red
Compared to 1a, the interplanar angle between the external
rings in 3a increases by about 121. This angle of 48.23(2)1 agrees
well with that of 9-methylphenanthro[4,3-a]dibenzothiophene
(47.50(10)1),9 which has also a five-membered ring between a
six-membered cycle and a methylbenzophenanthrene moiety.
As a consequence of the formation of the heterometallaring, the
pyridyl group and the benzophenanthrene moiety approach.
Thus, the C(5)–C(6) distance of 1.467(7) A, which is about
0.02 A shorter than that in 1a, is statistically identical to the
C(13)–C(20) bond length of cycle B (1.460(7) A). This suggests
some aromatic character for the five-membered metallacycle E.
In agreement with this, the NICS values10 computed at the
[3, +1] ring critical point (NICS (0)) and at 1.0 A above this
point (NICS (1)) are À1.21 and À4.51 ppm, respectively.
Furthermore, the reduction of the aromatic character of the
pyridyl group and the rings D and B of the benzophenanthrene
moiety also occurs, which is revealed by the decrease of the
NICS (0) values of these rings of 3a with regard to those of 1a.
As a consequence of the reduction of the aromaticity of the
pyridyl group (cycle F), in contrast to 1a, the terminal rings of
the helicene are not now the most aromatic.
The coordination geometry around the osmium atom of 3a can
be rationalised as a distorted pentagonal bipyramid with trans
phosphines (P(1)–Os–P(2) = 163.97(7)1). The 13C{1H}, 31P{1H}
1
and H NMR spectra of 3a and 3b in toluene-d8 at 193 K are
consistent with this ligand distribution. The 13C{1H} NMR spectra
show the C-metallated resonances at 191.9 (3a; t, JC–P = 6.7 Hz)
and 193.6 (3b; t, JC–P = 5.7 Hz) ppm. In agreement with the
asymmetry introduced by the screw of the polycycles, the 31P{1H}
NMR spectra contain AB spin systems centered at about 24.5 ppm
and defined by Dn = 902 Hz and JAB = 242 Hz (3a) and Dn =
1115 Hz and JAB = 228 Hz (3b). As expected for three inequi-
valent hydrides, the 1H NMR spectra show three high field region
resonances at À11.78, À10.89 and À6.35 (3a) and À10.87, À10.10,
Fig. 3 Frontier molecular orbitals of 3a (isosurface value of 0.04 au).
Chem. Commun., 2012, 48, 5328–5330 5329
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phosphorescence could derive from the first excited triplet state (T1)
and seems to be governed by the concentration of the solutions and
not by the solvent polarity. Thus, a study of the emission spectra of
3a in dichloromethane at room temperature has shown that
the intensity ratios between blue and red bands decrease as the
concentration of 3a increases, leading to the disappearance of the
blue fluorescence at concentrations of ca. 10À3 M. At high
concentrations only the red emission is observed with low intensity,
suggesting an efficient intersystem crossing (ISC) process.13 Thus,
molecules would emit from the T1 state. The intensity of these red
emissions is not high and no blue emission is observed, which could
indicate quenching of the S1 state. At low concentrations, the blue
emission (emission from S1) governs the luminescence. Solvent
effects could also account for these facts.14
In summary, the first d4-heterometallahelicenes have been
prepared by reaction of the hexahydride OsH6(PiPr3)2 with pyridyl
and pyrazinyl substituted benzo[g]phenanthrenes. The participa-
tion of the d-orbitals of the metal in the helical p-backbone of the
resulting [6]-azaosmahelicenes produces significant perturbations
in the aromaticity of the six-membered rings compared to that in
the starting [4]-carbohelicenes, which gives rise to notable differ-
ences between the optical properties of the [6]-azaosmahelicene
products and the [4]-carbohelicene reagents.
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Financial support from the Spanish MICINN Projects
CTQ2010-20414-C02-01, CTQ2010-20500-C02-01, CTQ2011-
23459, and Consolider Ingenio 2010 (CSD2007-00006), DGA
(E35), Comunidad de Madrid (CCG07-UCM/PPQ-2596) and
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Cajal Fellow.
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This journal is The Royal Society of Chemistry 2012