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RSC Advances
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In the recent literature supramolecular, non-covalent inter-
actions have been employed in order to direct the selective
oxidation of particular C–H bonds among many others present
in the same substrate.7 In one case7a–c two crown-ether receptors
were implanted in the ligand of a catalyst based on the Mn or
Fe(pdp) core,1i,5b for the recognition of an ammonium
anchoring group8 in the substrate (see Fig. 1, complex 2).
Recognition of the ammonium head of the substrate by the
crown-ether receptors allowed the selective oxidation of C–H
bonds located at the right distance from the anchoring group
(8–9 simple bonds). Application of the same strategy to the
imine-based catalytic core 1 (see Fig. 1, complex 3), did not
afford comparable results in terms of selectivity.9 In the latter
case, the difference between the selectivity properties of
complexes 1 and 3, devoid of and endowed with the crown-ether
receptors, respectively, towards the oxidation of the C–H bonds
present in the tested substrates was not evident enough, and
when appreciable, it was ascribed to a mere steric hindrance
rather than to recognition by the crown-ether moieties. In fact,
steric hindrance around the catalyst is known to affect the
reaction selectivity, favouring oxygenation of the most acces-
sible sites (C–H or C]C bonds) on the substrate.1k,10,11
Scheme 2 In situ preparation of complexes 4 and 5. Within 45 and
75 min, respectively, from the addition of the precursors, the
complexes are completely formed and the solution becomes deep
violet.
based on 1H and 13C NMR monitoring, HSQC 2D NMR, HR-MS
and UV-Vis spectroscopy, showed the high similarity of
complexes 4 and 5 with the parent complex 1 (compare, for
example, trace at t ¼ 44 min in Fig. S6† with the 1H NMR
spectrum of complex 1 reported in Fig. S1†). The UV-Vis spectra
reported in Fig. S22† show the near resemblance among
complexes 1, 4 and 5. Apart from a modest bathochromic effect
due to the TIPS groups, the shape of the bands related to the
iron core from 400 to 700 nm remains practically unchanged.
Eventually, the 2 : 2 : 1 stoichiometry of the new complexes 4
and 5 is denitely demonstrated by the Job's plots reported in
the ESI (Fig. S11 and S21,† respectively). Thus, the only differ-
ence among complexes 1, 4 and 5 should lie in the increasingly
limited access to the metal centre.
The results obtained in the H2O2 oxidation of a series of
aromatic compounds carried out in the presence of complexes 4
and 5 are reported in Table 1 together with those obtained with
catalyst 1 under the same conditions for the sake of compar-
ison. In all cases hardly detectable trace amounts or no trace of
products with oxidized lateral chain were found, in accordance
with the well-known preference of this imine based catalytic
core for aromatic C–H oxidation with respect to aliphatic C–H
oxidation.2d
When catalyst 1 is taken into account, total yield of oxidation
generally increases on increasing the size of the lateral alkyl
chain of the substrate as previously observed.2d When
complexes 4 and 5 are used as catalysts under the same
conditions, quite astonishingly and in contrast with what was
found with catalyst 3,9 the yields of phenol products remain
more or less the same for each substrate in comparison with
catalyst 1 in the limit of experimental errors. Even more
surprising is the fact that the presence of two or four, very bulky
TIPS groups in the catalyst backbone does not inuence at any
extent the (meta + para)/ortho ratio in the reaction products.
Such ratio remains the same along each series 1, 4 and 5.
Although the exact nature of the active species involved in
the reaction is still unknown, a series of clues collected in
previous investigations2 prompted us to hypothesize a mecha-
nism in which, aer an initial outer-sphere oxidation of FeII to
FeIII, the temporary detachment of one of the pyridine arms
would unmask the iron centre, which could be available to take
In order to shed light on the role and importance of steric
hindrance on the action of catalyst 1 we investigated in detail
the effect of the presence of bulky substituents in the catalyst
structure. Here below we report the results of such
investigation.
Results and discussion
Like complexes 1 and 3, new complexes 4 and 5, which are
characterized by an increased steric hindrance around the
catalytic core due to the presence of triisopropylsilyl (TIPS)
groups, were prepared in situ by self-assembly of the parent
compounds added in solution in the proper ratios (see Scheme
ꢀ
2), in acetonitrile at 25 C.
1H NMR monitoring of the related solutions showed that
complexes 4 (see ESI, Fig. S6†) and 5 (Fig. S18†) are completely
formed within 45 min and 75 min, respectively, in contrast with
complex 3, whose assembly requires 60 h at the same temper-
ature. Formation of complex 1 under the same conditions is
completed in 5 min.2a Characterization of the new complexes,
Fig. 1 Catalysts endowed with crown-ether receptors for substrate
decorated with ammonium head. Catalyst 2 is based on the M(pdp)
catalytic core (amino-pyridine catalyst) while catalyst 3 on complex 1
(imino-pyridine catalyst).
538 | RSC Adv., 2021, 11, 537–542
© 2021 The Author(s). Published by the Royal Society of Chemistry