30
D.P. Day et al. / Journal of Organometallic Chemistry 770 (2014) 29e34
Table 1
to organometallic sandwich complexes such as ferrocene [17],
whilst the work conducted by Astruc has shown dendrimers can be
(n
/cmꢀ1) Bands for complexes 1e4.
Complex
n
(C^O)/cmꢀ1
n
(C]C) (triazole)/cmꢀ1
modified with metallocenes and organometallics via
a click
chemistry reaction [18].
Sym
Asym
In contrast to the behaviour of 1, and the manganese(I) cym-
antreneetriazole derivatives reported previously by our group [19],
the two new cyrhetreneetriazole complexes reported herein are
found to exhibit redox active behaviour of the triazole ligand
moiety that prevents them acting as metal-based electron-transfer
mediators.
1
2
3
4
2014
2019
2021
2025
1888
1907
n/a
n/a
1949, 1927, 1907a
1939, 1908, 1897a
1530
1621, 1565, 1498
a
An extra band is observed in the asymmetric region for samples run in the solid
state as a result of hindered rotation about the Re-Cp centroid and a subsequent
descent in symmetry.
Results and discussion
In the case of compounds 3 and 4 we observe both symmetric
and asymmetric (C^O) stretches at ~2020 cmꢀ1 and
1900e1950 cmꢀ1 respectively [9,22], in addition to characteristic
C]C stretches of the triazole group in the region of
1500e1600 cmꢀ1 [19,23].
Single crystals of 3 were grown by diffusion of petroleum ether
into a saturated CH2Cl2 solution of the compound at room tem-
perature and a X-ray crystal structure obtained (Fig. 1, bond lengths
and angles detailed in Tables S1 and S2). The structure is compa-
rable to our previously reported manganese analogue [19] with two
molecules in the asymmetric unit related by a non-crystallographic
pseudo-inversion centre. The cyclopentadienyletriazoleephenyl
rings of each molecule are approximately co-planar (the triazole
ring is twisted by ca. 12ꢁ from the co-planar cyclopentadienyl and
phenyl rings), with the aromatic ring systems of each molecule
approximately parallel, separated by ca. 3.5 Å; together this sug-
Synthesis and structural characterization of 4-substituted
cyrhetreneetriazole complexes
Following the procedure reported by Gladysz and co-workers,
we report the synthesis of 2 as an off-white solid (after purifica-
tion via flash column chromatography) in good yield {Scheme 1(i)
and (ii)}. Confirmation of the structure of 2 was provided by 1H, 13
C
NMR and IR spectroscopy {Supporting information Figs. 2 and 5b,
(n
N]N azide/cmꢀ1) 2124} [3].
Our first attempt at the copper-catalysed azide/alkyne cyclo-
addition of 3 was achieved using the conditions we have previously
reported for the manganese variant of cyrhetrene {Scheme 1, (iii)}
[3,19,20]. Reassuringly, the transformation of 2 into 3 was complete
in 8 h under these mild conditions; purification of 3 via silica gel
flash column chromatography afforded 3 as a white crystalline solid
in good yield and high purity (Scheme 1).
gests the possibility of not only a conjugated
p-system in each
The 1H NMR spectrum of 3 allows us to assign the key areas of
the complex, confirming that our product was indeed the desired
molecule but also a degree of interaction between the mole-
pep
cules. Compound 4 exhibits the same crystallographic behaviour as
its manganese analogue [19], and resisted all our exhaustive at-
tempts to grow single crystals suitable for X-ray crystallographic
characterisation (Table 2).
(h
5-[4-phenyltriazol-1-yl]cyclopentadienyl tricarbonyl rhenium(I))
product. We observe a characteristic splitting of the five protons in
the cyclopentadienyl ligand observed as a singlet in 1, into two
equivalent triplets integrating to two protons each for the cyclo-
pentadienyl protons of complex 3 (chemical shifts in the range
Electrochemical characterization
d
5.6e6.5 ppm), formed by the introduction of a single triazole
moiety. 1H COSY NMR experiments have shown the aromatic pro-
tons of the phenyl substituent attached to the triazole moiety can
We first investigated the electrochemical properties of
3
(2 mM), in dichloromethane containing 0.1 M [nBu4N][B(C6F5)4] as
supporting electrolyte, using cyclic voltammetry (CV, Fig. 2(a) and
(b)).
couple to one another around the region of
d 7.3e8.0 ppm. The
proton at the 5-position of the triazole moiety is observed as a
characteristic singlet at
Fig. 3).
d 8.86 ppm (see Supporting information
The oxidation of 3 shows only a single, irreversible oxidation
wave at ca. 1.20 V vs ferrocene at all scan rates studied. The
oxidation wave is stable over repeat voltammetric cycles at a gold
macroelectrode, whilst fouling and passivation of the working
electrode was observed at platinum and glassy carbon working
electrodes. By comparison with the voltammetry observed for the
parent cyrhetrene, 1 [9], the observed voltammetry of 4 is not
characteristic of the oxidation of the rhenium metal centre, and is
instead likely due to the oxidation of the pendant 4-substituted
triazole moiety [19]. This finding contrasts with our earlier
The UVevis spectra of the parent molecule, [CpRe(CO)3] 1, can
be directly compared with click-product 3, and we show that in
both cases, we observe MLCT bands around the region of
250e280 nm (see Supporting information, Fig. 6) [21].
The infra-red spectral data recorded for compounds 1e4, with
band assignments, are tabulated below {Table 1, See Supporting
information Fig. 5(a)e(d) for IR spectra of compounds 1e4}.
studies of the triazole-derivatised cymantrene analogue, (h
5-[4-
phenyltriazol-1-yl]cyclopentadienyl tricarbonyl manganese(I)),
where oxidation of the manganese centre occurred before any
oxidation of the triazole ligand, and likely reflects the difference in
the relative redox potentials of cymantrene and cyrhetrene, the
latter of which is oxidised at more positive potentials [19].
The voltammetric behaviour of 4 recorded under identical
conditions as described above is shown in Fig. 3 and Fig. S7.
Upon scanning in an oxidative direction an irreversible oxida-
tion wave is observed at ca. 0.64 V vs ferrocene. No oxidation wave
corresponding to oxidation of the Re metal centre is observed
beyond this first oxidation peak up to the limit of the solvent
window. Instead the voltammogram exhibits a rectangular wave
Scheme 1. Reagents and conditions: (i) 1 (1 equiv), n-BuLi (1 equiv), THF, ꢀ78 ꢁC, 2 h;
(ii) p-toluenesulfonylazide (1 equiv), THF, ꢀ78 ꢁC e rt, 14 h. (iii) 2 (1 equiv), phenyl-
acetylene (1 equiv for 3), or 4-ethynylaniline (1 equiv for 4), Cu(OAc)2$H2O (0.1 equiv),
CH3CN, 8 h.