Angewandte
Communications
Chemie
functional theory (DFT) calculations were performed on 2–9
by using the Gaussian09 package, and the optimized struc-
tures were obtained after geometry optimizations at the
B3LYP/6-31G(d) level (Figure S10-1).[15] The calculated
structures are in good agreement with the crystal structures,
thereby supporting the above arguments.
The electrochemical properties of the subporphyrin B-
peroxides were examined by cyclic voltammetry (Figure S9).
The first one-electron reductions of the B-peroxides are all
quasi-irreversible, thus suggesting that the one-electron
À
reduction induces O O bond cleavage. The first reduction
Figure 2. UV/Vis absorption and fluorescence spectra recorded in
CH2Cl2.
potentials are positively shifted in the order 4 (À1.97 V) < 7
(À1.94 V) < 2 (À1.81 V) < 8 (À1.70 V), again reflecting the
electron-donating abilities of the peroxy substituents. These
differences are also supported by calculations of the molec-
ular orbital energies (Figure S10-2).
2.8 ns, which was quite similar to that of 3 (tf = 2.95 ns),
while a slightly shorter lifetime constant of tf = 2.3 ns was
obtained for 4. These observations are consistent with the
results obtained by femtosecond transient absorption meas-
urements, where the excited species of 2 and 4 were found to
decay with time constants of 2.8 and 2.3 ns, respectively.
The reactivity of the subporphyrin B-peroxides was
briefly examined (Scheme 3). Subporphyrin B-hydroperoxide
The infrared and Raman spectra of subporphyrin B-
peroxides were measured. DFT frequency calculations were
conducted to simulate the infrared spectra, thereby allowing
the B-O stretching vibrations of 2, 4, 7, and 8 at 966, 972, 992,
and 970 cmÀ1, respectively, to be assigned (Figures S7-1 and
S7-2). These B-O bands are at lower frequencies than that of 5
(1105 cmÀ1), but at higher frequencies than those of 3
(954 cmÀ1) and 9 (909 cmÀ1). These results indicate that the
B-O stretching vibrational frequencies increase in the order
B-O-R < B-O-O-R < B-O-H, as the size of the substituents
on the oxygen atom decreases. The O-H stretching band of 2
was observed at 3373 cmÀ1, which is lower than that of 5
(3634 cm ), which suggests that the O H bond of 2 is weaker
À1
À
than that in 5. The Raman spectra of the subporphyrin B-
peroxides were measured to directly observe the O-O
stretching vibration. As a consequence of the relatively
intense fluorescence and facile decomposition of 2, 4, and 8
under the measurement conditions, their Raman spectra were
difficult to observe. However, a rather clear Raman spectrum
was obtained for the tert-butylperoxide 7. The frequencies of
the experimentally observed Raman feature of 7 is linearly
correlated with the calculated Raman-active vibration modes
Scheme 3. Reactions of subporphyrinatoboron(III) peroxides. [a] Yields
were determined by 1H NMR spectroscopy, using 1,1,2,2-tetrachloro-
ethane as an internal standard.
À
(Figure S7-3). The s-bond character of O O bond leads to
2 smoothly oxidized triphenylphosphine to triphenylphos-
phine oxide in good yields with concurrent formation of 5.
However, 2 could not oxidize phenyldodesylsulfide or do-
desylmethylsulfide, thus indicating its weak oxidation ability
compared to tert-butylhydroperoxide and 3-chloroperoxy-
benzoic acid. The other B-peroxides 4, 7, and 8 could not
oxidize triphenylphosphine. Finally, it was found that the
reaction of 7 with Ph3C[B(C6F5)4] produced triphenylmethyl-
tert-butylperoxide and the borenium cation in good yields.[11d]
In summary, the subporphyrin B-peroxides 2, 7, and 8
were synthesized effectively in good yields by nucleophilic
substitution of the boron methoxide 3 with the corresponding
hydroperoxides. Thermal dimerization of 2 provided peroxo-
bridged subporphyrin dimer 4 quantitatively. Single-crystal
X-ray diffraction analysis revealed the first solid-state struc-
tures of a boron hydroperoxide, an acyclic boron organylper-
oxide, and a neutral peroxo-bridged diboron. As the electron-
a small polarizability of the O-O stretching modes and hence
the almost suppressed Raman peaks near 900–1000 cmÀ1,
where O-O stretching is predicted to be observed. However,
as a result of coupling with the backbone vibration of the
subporphyrin macrocycle, slightly allowed Raman peaks of 7
have been observed as broad signals around 900 nm.
The UV/Vis absorption and fluorescence spectra of 2, 7,
and 8 in CH2Cl2 are quite similar to those of 3 and 5, with their
Soret-like bands at 372–374 nm, Q-like bands at 460–461 and
484–485 nm, and fluorescence maxima at 519–521 nm
(Figure 2, see also Figures S5-1 and S5-2). On the other
hand, peroxo-bridged dimer 4 and m-oxo-bridged diboron 6
revealed blue-shifted Soret-like bands at 364 and 363 nm,
respectively. Since the two subporphyrin subunits of 4 and 6
are in proximity, the energy shifts of the Soret bands were
attributed to excitonic coupling between the subporphyrin
moieties. We performed time-correlated single-photon count-
ing (Figure S5-4) to obtain the lifetimes of the excited singlet
state. The fluorescence decay profile of 2 fitted well with
a single exponential function with a time constant of tf =
À
donating ability of the peroxy substituents increases, the O O
bond length increases while the s-orbital character of the
central boron atoms decreases. The electrochemical proper-
ties of the peroxides can be modulated by exchanging the
2598
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2016, 55, 2596 –2599
Tsurumaki, Eiji
