M. E. Crestoni and F. Lanucara
IV
+
Whereas the reactivity of [(tpfpp)+ Fe O] was found to
C
established. Table 1S in the Supporting Information presents
the kinetics and thermodynamics data of the addition re-
A
increase in parallel with the oxophilic character of the
actions of [(tpfpp)MnIII]+, which are limited to the associa-
heteroACTHNGUTERNNUG
atom (S<N<P), the reactivity of [(tpfpp)MnVO]+
tion of just one ligand molecule, under the prevailing experi-
mental conditions. Only when the sampled neutral is a
strong nucleophile, L=C6H5N, OPACTHNUTRGNEUNG(OCH3)3, a sequential ad-
correlates with the ionization energy of the substrates.
The binding ability of the reduced species [(tpfpp)MnIII]+
has been examined towards numerous ligands with different
functionalities and found to favor harder ligands, endowed
with high dipole moments.
dition
step
yielding
six-coordinated
complexes,
[(tpfpp)Mn(L)2]+ (2l2) is also attained (Figure 5S in the
Supporting Information illustrates the reaction with OP-
ACHTUNGTRENNUNG(OCH3)3). The lower efficiency of this route in comparison
with the first association step is likely related to the elec-
tron-donating performance of the first ligand which reduces
the metal reactivity. As to the lack of a second addition
route when confronting compounds of higher basicity, like
Experimental Section
Materials: meso-tetrakis(pentafluorophenyl)porphyrinatomanganeseACHTUNGTRENNUNG(III)
chloride, [(tpfpp)MnIII]Cl (1-Cl) was prepared according to the litera-
ture.[48] Iodosylbenzene (C6H5IO) was prepared according to a published
procedure and stored at ꢀ208C.[49] p-Methoxy-N,N-dimethylaniline was
prepared from p-methoxyaniline and trimethylorthophosphate;[50] p-(tri-
fluoromethyl)-N,N- dimethylaniline was obtained from p-(trifluoromethy-
l)aniline and methyliodide[28a] p-Substituted thioanisoles were synthesized
by the reaction of the corresponding thiophenols with methyl iodide in
basic methanolic solution.[51] The reaction mixtures were purified by
preparative GLC using a 3 m column filled with Chromosorb 80/100 W
AW coated with a base deactivated polyethylene glycol stationary phase,
mounted on a gas chromatograph (FRACTOVAP Mod. ATC/f series
410, Carlo Erba). The end products were analyzed by GLC-MS on a
Hewlett–Packard 5890 gas chromatograph in series with a 5989B quadru-
pole mass spectrometer, using a 50 m long, 0.2 mm i.d. fused silica capil-
lary column, coated with cross-linked methylsilicone film. All the sol-
vents were analytical grade. NO and NO2 were high purity gases from
Matheson Gas Products Inc. Pyridine, p-methylaniline, and all other
chemicals were research grade products obtained from commercial sour-
ces (Sigma–Aldrich) and used as received.
L=NACHTUNGTRENNUNG(CH3)3, but significantly lower dipole moment, it may
be likely accounted for by the electrostatic contribution to
the coordinative bonding.
On exposure to NO and NO2, [(tpfpp)MnIII]+ (3) proved
to be almost inert, at variance with the reactive iron com-
plex.[17a] The lack of an OAT process from NO2 may suggest
ꢀ
that the homolytic Mn O bond dissociation enthalpy for 1
is lower than 306 kJmolꢀ1 (namely, BDE for O-NO),[22] so
providing a higher limit for the MnVO bond strength. How-
ever, the existence of significant kinetic barriers due to the
interplay of different spin state surfaces may play an impor-
tant role as already revealed in the reactions of several
metal-oxo complexes.[9b,11c,i,l,26]
Conclusion
Instrumental: All experiments were carried out by using a Bruker BioA-
pex Fourier transform ion cyclotron resonance (FT-ICR) mass spectrom-
eter equipped with an Apollo I electrospray ionization (ESI) source, a
4.7 T superconducting magnet, and a cylindrical infinity cell. Analyte sol-
utions were continuously infused through a 50 mm i.d. fused-silica capilla-
ry to the ESI source at a rate of 120 mLhꢀ1 by a syringe pump, and ions
were accumulated in a rf-only hexapole ion guide for 0.8 s. The ion popu-
lation, desolvated by a heated (380 K) N2 counter current drying gas, was
driven into the ICR cell at room temperature (300 K), and the reactant
ion was isolated by ion ejection procedures and exposed to neutral re-
agents (L) admitted by a needle valve at stationary pressures in the
range of 1.0–15ꢂ10ꢀ8 mbar. The pressure was measured with a cold-cath-
ode sensor (IKR Pfeiffer Balzers S.p.A., Milan, Italy) calibrated by using
High-valent MnV-oxo porphyrins are very difficult to select
and characterize owing to their inherent instability. A suc-
cessful route to form [(tpfpp)MnVO]+ as naked ion in the
gas phase has offered the possibility to explore the oxidation
reactivity of a five coordinated MnV-oxo species towards a
variety of heteroatom-containing substrates (e. g. sulfides,
amines, phosphites) using ESI-FT-ICR mass spectrometry.
In summary, [(tpfpp)MnVO]+ performs as a versatile oxi-
dant, displaying a range of efficient oxygen transfer re-
+
the rate constant k=1.1ꢂ10ꢀ9 cm3 sꢀ1 for the reference reaction CH4
ACHTUNGTRENNUNG
actions, with a unique bias towards olefin oxidation.[18] The
+
[52,53]
C
C
+CH4!CH5 +CH3 and corrected for different response factors.
majority of the reactions of 1 with the substrates studied in
this work involve two-electron processes, as evident from
the predominance of oxygen-atom transfer, hydride transfer,
and addition routes, and from the lack of hydrogen atom
transfer products. Several lines of evidence suggest an initial
ET event with most of the sampled compounds, which may
be followed by either a release of a free radical cation inter-
mediate, for substrates with IE lower than 7.9 eV, or a fast
oxygen rebound step within the reaction complex, or a
formal hydrogen transfer, comprising stepwise proton–elec-
tron transfer. The latter route, observed only from aliphatic
tertiary amines, is comparatively less efficient with 1 than
Pseudo-first-order rate constants, obtained from the slope of the semilog
decrease with time of the parent ion abundance, were divided by the sub-
strate concentration to give bimolecular rate constants (kexp) at 300 K.
These values and product ion distribution were found to be invariant
with respect to the pressure of the neutral and of an added inert bath gas
(Ar). The reaction efficiencies (F) are percentages of the collision rate
constant (kcoll) calculated by the parameterized trajectory theory.[54]
The estimated error in the absolute rate constant values (ꢃ30%) is
mainly due to uncertainty in pressure measurements.
Low-energy collision-induced dissociation (CID) experiments were per-
formed in FT-ICR by accelerating the mass-selected reagent ions in the
presence of argon gas pulsed into the cell (peak pressure ca. 4ꢂ
10ꢀ6 mbar) for 1 s.
Sample preparation: The [(tpfpp)MnVO]+ (2) ion of interest was generat-
ed by adding iodosylbenzene (0.5 mm) to 1-Cl (10 mm) in methanol
cooled at ꢀ208C. The intermediate persisted for a few days at ꢀ208C.
The high-resolution ESI-FT-ICR mass spectra showed the resting form 1,
centered at m/z 1027, along with a cluster characterized by the same iso-
with [(tpfpp)+ Fe O] probably as a result of the lower ba-
sicity of a manganyl, [(tpfpp)MnIVO], with respect to a
ferryl, [(tpfpp)FeIVO], unit.
IV
+
C
12098
ꢁ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2011, 17, 12092 – 12100