Strong Inhibition of O-Atom Transfer Reactivity for MnIV(O)(π-Radical-Cation)(Lewis Acid) versus MnV(O) Porphyrinoid Complexes

Article abstract of DOI:10.1021/jacs.5b00875

The oxygen atom transfer (OAT) reactivity of two valence tautomers of a Mn^V(O) porphyrinoid complex was compared. The OAT kinetics of Mn^V(O)(TBP₈Cz) (TBP₈Cz = octakis(p-tert-butylphenyl)corrolazinato^3-) reacting with a series of triarylphosphine (PAr₃) substrates were monitored by stopped-flow UV-vis spectroscopy, and revealed second-order rate constants ranging from 16(1) to 1.43(6) × 10⁴ M^-1 s^-1. Characterization of the OAT transition state analogues Mn^III(OPPh₃)(TBP₈Cz) and Mn^III(OP(o-tolyl)₃)(TBP₈Cz) was carried out by single-crystal X-ray diffraction (XRD). A valence tautomer of the closed-shell Mn^V(O)(TBP₈Cz) can be stabilized by the addition of Lewis and Bronsted acids, resulting in the open-shell Mn^IV(O)(TBP₈Cz^?+):LA (LA = Zn^II, B(C₆F₅)₃, H⁺) complexes. These Mn^IV(O)(π-radical-cation) derivatives exhibit dramatically inhibited rates of OAT with the PAr₃ substrates (k = 8.5(2) × 10^-3 - 8.7 M^-1 s^-1), contrasting the previously observed rate increase of H-atom transfer (HAT) for Mn^IV(O)(TBP₈Cz^?+):LA with phenols. A Hammett analysis showed that the OAT reactivity for Mn^IV(O)(TBP₈Cz^?+):LA is influenced by the Lewis acid strength. Spectral redox titration of Mn^IV(O)(TBP₈Cz^?+):Zn^II gives E_red = 0.69 V vs SCE, which is nearly +700 mV above its valence tautomer Mn^V(O)(TBP₈Cz) (E_red = -0.05 V). These data suggest that the two-electron electrophilicity of the Mn(O) valence tautomers dominate OAT reactivity and do not follow the trend in one-electron redox potentials, which appear to dominate HAT reactivity. This study provides new fundamental insights regarding the relative OAT and HAT reactivity of valence tautomers such as M^V(O)(porph) versus M^IV(O)(porph^?+) (M = Mn or Fe) found in heme enzymes. (Figure Presented).

Full text of DOI:10.1021/jacs.5b00875

Article
pubs.acs.org/JACS
Strong Inhibition of O‑Atom Transfer Reactivity for MnIV(O)(π-
Radical-Cation)(Lewis Acid) versus MnV(O) Porphyrinoid Complexes
Jan Paulo T. Zaragoza, Regina A. Baglia, Maxime A. Siegler, and David P. Goldberg*
Department of Chemistry, The Johns Hopkins University, 3400 North Charles Street, Baltimore, Maryland 21218, United States
S
* Supporting Information
ABSTRACT: The oxygen atom transfer (OAT) reactivity of
two valence tautomers of a MnV(O) porphyrinoid complex was
compared. The OAT kinetics of MnV(O)(TBP8Cz) (TBP8Cz
= octakis(p-tert-butylphenyl)corrolazinato3−) reacting with a
series of triarylphosphine (PAr3) substrates were monitored by
stopped-flow UV−vis spectroscopy, and revealed second-order
rate constants ranging from 16(1) to 1.43(6) × 104 M−1 s−1.
Characterization of the OAT transition state analogues
MnIII(OPPh3)(TBP8Cz) and MnIII(OP(o-tolyl)3)(TBP8Cz)
was carried out by single-crystal X-ray diffraction (XRD). A
valence tautomer of the closed-shell MnV(O)(TBP8Cz) can be
stabilized by the addition of Lewis and Brønsted acids,
resulting in the open-shell MnIV(O)(TBP8Cz•+):LA (LA =
ZnII, B(C6F5)3, H+) complexes. These MnIV(O)(π-radical-cation) derivatives exhibit dramatically inhibited rates of OAT with the
PAr3 substrates (k = 8.5(2) × 10−3 − 8.7 M−1 s−1), contrasting the previously observed rate increase of H-atom transfer (HAT)
for MnIV(O)(TBP8Cz•+):LA with phenols. A Hammett analysis showed that the OAT reactivity for MnIV(O)(TBP8Cz•+):LA is
influenced by the Lewis acid strength. Spectral redox titration of MnIV(O)(TBP8Cz•+):ZnII gives Ered = 0.69 V vs SCE, which is
nearly +700 mV above its valence tautomer MnV(O)(TBP8Cz) (Ered = −0.05 V). These data suggest that the two-electron
electrophilicity of the Mn(O) valence tautomers dominate OAT reactivity and do not follow the trend in one-electron redox
potentials, which appear to dominate HAT reactivity. This study provides new fundamental insights regarding the relative OAT
and HAT reactivity of valence tautomers such as MV(O)(porph) versus MIV(O)(porph•+) (M = Mn or Fe) found in heme
enzymes.
observed in several synthetic porphyrin models. Early work
showed that coordination of methoxide, a strong π-donor
ligand, causes FeIII(TMP•+)(ClO4)2 (TMP = 5,10,15,20-
tetramesitylporphyrinato2−) to convert to its valence tautomer
FeIV(TMP)(OMe)2.16 A similar finding with iron corroles was
observed for FeIV(TPFC)(Cl) (TPFC = 5,10,15-tris-
(pentafluorophenyl)corrolato3−), in which the replacement of
INTRODUCTION
■
Much attention has been given to the mechanistic aspects of
biological oxidation reactions involving high-valent metal-oxo
porphyrin species due to their invaluable roles in synthetic
organic chemistry and heme enzyme mechanisms.1−9 Due to
the noninnocent nature of porphyrinoid ligands in these
biomimetic complexes, they often undergo an electronic
redistribution between metal and ligand, which has been
characterized as valence tautomerism. Valence tautomers are of
fundamental interest because of their distinct optical, electronic,
and magnetic properties.10,11 Heme-containing enzymes such
as peroxidases, catalases, and cytochrome P450s, take advantage
of the facile valence tautomerism inherent to iron porphyrins in
order to access formally high oxidation state species. In the case
of P450, a wide range of spectroscopic methods was employed
to conclusively show that the reactive Compound I (Cpd I)
intermediate is in the FeIV(O)(porph•+)] form, as opposed to
the FeV(O)(porph) valence tautomer.12−15
the axial Cl− ligand with the weaker donor ClO4 forms
−
FeIII(TPFC•+)(ClO4).17 Recently, Nakamura18 has shown that
there is an equilibrium between FeIII(p-X-TPP•+)(N3)2 (TPP =
5,10,15,20-tetraphenylporphyrinato2−) and FeIV(p-X-TPP)-
(N3)2, and the position of the equilibrium is dependent on
the nature of the para-X substituent of the meso-phenyl groups.
Axial ligand-dependent valence tautomerism has also been
observed in Mn porphyrins, where the addition of strong axial
donors such as CH3O− to MnIII(TPP•+)(Cl)(SbCl6) induces
the formation of MnIV(TPP)(OCH3)2.19
Although the former complexes provide clear evidence for
the propensity of porphyrin ligands to allow for valence
tautomerism, there are few examples of metal-oxo porphyrinoid
Valence tautomers in metalloporphyrins involve electron-
transfer between the bound metal and the aromatic π system of
the ligand, and can be induced both by chemical and
nonchemical (temperature changes, irradiation and pressure)
means. Chemically driven valence tautomerization has been
Received: January 26, 2015
© XXXX American Chemical Society
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Scheme 1. Oxygen Atom Transfer Reaction between MnVO(TBP8Cz) and a Series of Phosphine Derivatives
complexes for which both valence tautomers are known. Fujii
and co-workers showed that protonation of the oxo ligand of
FeIV(O)(TPFP•+)(L) (TPFP = 5,10,15,20-tetrakis(penta-
fluorophenyl)porphyrinato2−) results in the formation of the
electronic isomer FeIII(TPFP++)(L)2 by intramolecular electron
transfer from the porphyrin π-radical cation to the Fe center.
Subsequent addition of Cl− forms the FeIII meso-chloro-
isoporphyrin, an excellent chlorinating agent.20 Acid-dependent
valence tautomerization was observed in Mn(salen) complexes,
where MnIII(H2O)(salen•+) was reacted with base to
sequentially form MnIV(OH)(salen) and MnIV(O)(salen).
Among the three species, MnIV(O)(salen) was observed to
be the most reactive in H-atom transfer (HAT) and O-atom
transfer (OAT) reactions.21 Besides these few studies, there is
little known about the comparative reactivity of isoelectronic
high-valent metal-oxo complexes that constitute valence
tautomers.
catalytic rate enhancements for OAT produced by addition of
Lewis acids to manganese cross-bridged cyclam complexes.
Fukuzumi, Nam, and co-workers30 found that there is a 2,200-
fold rate increase in OAT for [MnIV(O)(N4Py)]2+ to sulfide
substrates upon binding of ScIII ions. These previous results
point to the ability of the Lewis acid to increase the
electrophilicity of the metal-oxo complex. In all of these former
cases, however, valence tautomerism was not observed and the
oxidation state of the Mn ion remained unchanged upon
addition of Lewis acids.
Herein we provide a comparative study on the reactivity of
two isoelectronic valence tautomers of a high-valent Mn-oxo
porphyrinoid complex. The MnV(O)(TBP8Cz) and MnIV(O)-
(TBP8Cz•+):LA (LA = ZnII, B(C6F5)3, H+) valence tautomers
were examined for their O-atom transfer reactivity with a wide
range of triarylphosphine derivatives. Both valence tautomers
react with PAr3 to give the respective two-electron reduced
MnIII complexes. The MnV(O) species shows relatively rapid
reaction kinetics, but remarkably, MnIV(O)(π-radical-
cation):LA shows dramatically slower reaction rates. Kinetic
analyses, including Hammett plots and substrate steric effects,
provide insights into the origins of the difference in rate
constants for the different valence tautomers. The reduction
potential of MnIV(O)(TBP8Cz•+):ZnII was also determined by
redox titration, and gives additional insight into the observed
OAT and HAT reactivities. This comparison of O-atom
transfer reactivity for Mn(O) valence tautomers can be
considered as analogous to a comparison of heme Cpd I-type
(FeIV(O)(porph•+) versus FeV(O)(porph)) valence tautomers.
We have shown previously that reaction of MnV(O)-
(TBP8Cz) with Lewis acids (LA) (LA = ZnII, B(C6F5)3)
leads to stabilization of the valence tautomer in dilute solution,
in which an electron from the Cz ligand transfers to the metal
and gives a metastable MnIV(O) π-radical-cation complex. The
data suggested that the Lewis acids were most likely bound to
the terminal oxo group, although direct structural information
has not been obtained.22,23 The new species, MnIV(O)-
(TBP8Cz•+):ZnII, exhibited enhanced reactivity toward the
one-electron oxidation of ferrocene, and the abstraction of
hydrogen atoms from phenol O−H substrates. However, the
reactivity of MnIV(O)(TBP8Cz•+):LA in two-electron, O-atom
transfer reactions has not been investigated.
Model systems have been used to investigate the effect of
Lewis acids on manganese-oxo complexes, including their
influence on HAT, dioxygen activation,24 redox potential,25 and
OAT reactivity. The addition of Lewis acids (e.g., ZnII, ScIII) to
a nonheme MnV(O)(TAML) complex, in which a remote
pyridyl-based LA binding site was incorporated, led to an
increase in OAT reaction rates with PPh3 as substrate.26
Significant rate enhancements were obtained by addition of LAs
to MnO4− in the oxidation of alcohol27 and alkane28 substrates.
Yin and co-workers29 have also reported stoichiometric and
RESULTS AND DISCUSSION
■
Reactivity of MnV(O)(TBP8Cz) in O-Atom Transfer to
Phosphines. Previously, we showed that the two-electron
oxidation of PPh3 by MnV(O)(TBP8Cz) is rapid and high-
yielding, resulting in OPPh3 (83%) and MnIII(TBP8Cz).31
Evidence for a direct O-atom transfer mechanism between the
terminal oxo ligand and PPh3 came from the isotopically
labeled MnV(18O) complex, which reacted to give 18OPPh3. It
was found that the stable MnV(O) complex could be oxidized
by strong one-electron oxidants to give a novel [MnV(O)-
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(TBP8Cz)]+ species in which the Cz ring was oxidized to a π-
radical-cation. The O-atom transfer reactivity of the latter
complex was compared to the starting MnV(O) complex, and
exhibited a 125-fold rate enhancement for OAT with dimethyl
sulfide to give the sulfoxide product. An Eyring analysis showed
that unfavorable entropic factors attenuated the rate enhance-
ment, but the highly favorable enthalpic term still gave a
significant increase in the O-atom transfer reactivity for the π-
radical-cation complex. This study was the first example of a
direct comparison of the reactivity of a MnV(O) versus
MnV(O)(π-radical-cation), in which two high-valent Mn-oxo
complexes with the same structure differed by only one unit of
charge.32 In other work, MnV(O)(TBP8Cz) was modified
through addition of anionic axial donors (X = CN−, F−) which
dramatically enhanced OAT reactivity for thioether sub-
strates.33 However, the kinetics for the oxidation of phosphine
substrates by MnV(O) corrolazine has not been reported.
Herein stopped-flow UV−vis spectroscopy was used to
measure the rate constants of O-atom transfer between
MnV(O)(TBP8Cz) and a wide range of triarylphosphine
derivatives. This series of substrates allowed for systematic
variation of both steric and electronic properties.
constants (kobs). The kobs values for PPh3 and the other
triarylphosphines shown in Scheme 1 increased linearly with
increasing concentration of triarylphosphine. Second-order rate
constants were obtained from the linear plots shown in Figures
1b-c for the various phosphine derivatives, and can be
compared in Table 1.
Table 1. Hammett Constants, Cone Angles, and Second-
Order Rate Constants for the Oxidation of
Triarylphosphines by MnV(O)(TBP8Cz)
a
b
c
P(X−Ph)3
3σp
cone angle
k (M−1 s−1
)
o-CH3
m-CH3
p-CH3
p-H
−0.51
−0.21
−0.51
0.00
194°
165°
145°
145°
145°
145°
1.6(1) × 101
9(1) × 103
5.8(2) × 104
1.43(6) × 104
4.7(4) × 103
6.0(6) × 102
p-Cl
+0.68
+1.62
p-CF3
a
b
c
Ref 34. Ref 35. In CH2Cl2 at 25 °C.
For the para-substituted derivatives, there is a significant
dependence on the electron-donating properties of the para-X
substituent. This dependence is seen in the Hammett plot
shown in Figure 2, where log(kX/kH) versus 3σp (where σp is
The reaction of MnV(O)(TBP8Cz) with the para-substituted
triarylphosphines shown in Scheme 1 was monitored by UV−
vis spectroscopy. The decrease in absorbance at 635 nm
corrresponding to the decay of MnV(O)(TBP8Cz), was
accompanied by an isosbestic growth at 685 nm due to the
formation of MnIII(OPPh3)(TBP8Cz) (Figure 1a). Plots of
absorbance versus time for both species fit a first-order kinetics
model (inset, Figure 1a), and yielded pseudo-first-order rate
Figure 2. Hammett plot for the OAT reactions between MnV(O)-
(TBP8Cz) and PAr3 in CH2Cl2 at 25 °C.
the Hammett substituent constant34) gave a straight line with ρ
= −0.91(5), confirming that the MnV(O) complex oxidizes
PAr3 by an electrophilic mechanism.
The influence of the steric properties of the triarylphosphines
on the OAT reactivity was examined by employing the ortho,
meta, and para-substituted tri(tolyl)phosphines. These phos-
phine derivatives vary significantly in their Tolman cone angles,
which is a measure of the steric encumbrance around the
phosphine center (Scheme 1). A significant decrease in rate of
∼3700-fold was observed upon an increase in cone angle from
145° to 194° for tri(p-tolyl)phosphine versus tri(o-tolyl)-
phosphine. This strong steric effect supports a concerted OAT
mechanism, in which a nucleophilic phosphorus substrate must
attack the electrophilic oxo group while avoiding steric clash
with the Cz ring and its peripheral substituents. It also helps to
rule out other mechanisms that include outer-sphere electron-
transfer as part of the rate-determining step. The least sterically
hindered pathway likely involves the P atom approaching in an
approximately collinear fashion with the Mn−O bond. This
pathway could then be expected to give a MnIII(OPAr3)
product with an Mn−O−P angle of ∼180°.
Figure 1. (a) Time-resolved UV−vis spectral changes observed in the
reaction of MnV(O)(TBP8Cz) (13 μM) with PPh3 (0.16 mM) in
CH2Cl2 at 25 °C. Inset: changes in absorbance vs time for the growth
of MnIII(TBP8Cz) (685 nm) (red circles) and decay of MnV(O)-
(TBP8Cz) (635 nm) (blue circles) with the best fit lines (black). Plots
of pseudo-first order rate constants (kobs) vs [triarylphosphine] for (b)
para-substituted triarylphosphines and (c) para, meta and ortho-tolyl
substituted phosphines.
Structural Characterization of MnIII(OPAr3)(TBP8Cz)
Complexes as Transition State Analogues. Further
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insights were gained from the synthesis and structural
characterization of MnIII(OPPh3)(TBP8Cz) (1) and MnIII(OP-
(o-tolyl)3)(TBP8Cz) (2), which were prepared by the addition
of excess OPAr3 to MnIII(TBP8Cz) in CH2Cl2 followed by
layering with CH3CN to yield crystals suitable for X-ray
structure determination. The displacement ellipsoid plots for
molecules 1 and 2 are shown in Figure 3, and selected bond
distances and angles are summarized in Table 2.
Table 2. Selected Bond Distances (Å) and Angles (deg) for 1
and 2a−b
1
2a
2b
Mn1−N1
Mn1−N2
Mn1−N4
Mn1−N6
Mn1−(Npyrrole
1.872(2)
1.876(2)
1.884(2)
1.879(2)
0.274
1.871(2)
1.873(3)
1.897(3)
1.884(3)
0.358
1.878(3)
1.885(3)
1.895(3)
1.879(4)
0.368
)
plane
Mn1−(23-atom)core
Cβ−Cβ (av)
0.201
0.399
0.396
1.394
1.391
1.395
Cα−Cβ (av)
1.449
1.445
1.446
Cα−Cα (C4−C5)
Cα−Npyrrole (av)
Cα−Nmeso (av)
Mn1−O1
1.439(3)
1.370
1.455(5)
1.368
1.463(5)
1.370
1.341
1.344
1.336
2.0754(19)
1.4900(19)
80.51(9)
90.27(8)
94.07(9)
90.63(9)
97.63(9)
96.81(8)
99.09(8)
98.53(9)
155.57(13)
2.084(2)
1.498(3)
80.56(11)
88.82(11)
93.86(11)
88.76(11)
102.12(11)
105.34(11)
99.83(11)
96.29(11)
158.15(17)
2.107(4)
1.496(5)
80.45(11)
88.61(11)
93.67(11)
88.85(11)
100.1(3)
101.6(2)
102.4(3)
100.0(2)
174.5(5)
P1−O1
N1−Mn1−N2
N2−Mn1−N4
N4−Mn1−N6
N6−Mn1−N1
N1−Mn1−O1
N2−Mn1−O1
N4−Mn1−O1
N6−Mn1−O1
Mn1−O1−P1
consistent with our hypothesis that the more sterically
encumbered phosphine substrate would favor a collinear attack
on the MnV(O) group. The other independent molecule 2a
exhibits a slightly larger Mn−O−P angle (158.15(17)°)
compared to 1, but much less than ∼180° seen for 2b.
Taken together, the structures for 1 and 2 are consistent with
the strong influence of the steric properties of the phosphine
substrates on the rate of O-atom transfer.
Valence Tautomerism of MnV(O)(TBP8Cz) to MnIV(O)-
(TBP8Cz•+) by Addition of Lewis and Brønsted Acids. We
previously reported the stabilization of the MnIV(O)-
(TBP8Cz•+) valence tautomer by addition of the Lewis acids
ZnII or B(C6F5)3.22,23 Reaction of Zn(OTf)2 or B(C6F5)3 with
the MnV(O) complex led to the isosbestic conversion of
MnV(O)(TBP8Cz) (λmax = 420, 635) to MnIV(O)(TBP8Cz•+)
with a weakened and broadened Soret band, and a low-intensity
band in the near-IR region (λmax = 419, 789), which are
characteristic of porphyrin,39,40 corrole,41,42 and corrolazine32
π-radical cations. The Lewis acid adducts were not stable to
isolation as solids, but the binding of the Lewis acids was
reversible in solution, and spectral titrations yielded association
constants of Ka (ZnII) = 4.0 × 106 M−1 and Ka (B(C6F5)3) = 2.0
Figure 3. Displacement ellipsoid plot (50% probability level) of (a)
MnIII(OPPh3)(TBP8Cz) (1) and (b) MnIII(OP(o-tolyl)3)(TBP8Cz)
(2b) at 110(2) K. The disorder, H-atoms and solvent molecules are
omitted for clarity.
Complexes 1 and 2 are both 5-coordinate MnIII complexes
with one OPAr3 molecule bound in the axial position. Complex
2, which contains the OP(o-tolyl)3 axial ligand, crystallized with
two crystallographically independent molecules (2a−b) in the
asymmetric unit. The MnIII−N distances for 1 and 2 are
comparable with those found in other MnIII corrolazines,36,37
and the Mn−O distances (2.075(2)−2.107(4) Å) are slightly
elongated compared to the FeIII analogue FeIII(OPPh3)-
(TBP8Cz) (Fe−O = 2.001(2) Å).38 The Mn ion in 1 is
displaced by ca. 0.20 Å from the plane described by the 23-
atom Cz core, while this displacement is ca. 0.40 Å in both 2a
and 2b. The latter displacement is consistent with the o-tolyl
derivative having a larger steric demand, forcing the Mn ion
further out of plane to allow for good binding of the phosphine
oxide axial donor. The out-of-plane distance for Fe in the
FeIII(OPPh3) analogue is 0.26 Å, close to that seen for 1. The
Mn−O−P angle in 1 is 155.57(13)°, nearly identical to that
seen for the FeIII derivative (154.4(1)°). However, this angle
deviates significantly for the major component of 2b, which
shows Mn−O−P = 174.5(5)°. This nearly linear angle may be
1
× 107 M−1. These species exhibited paramagnetic H NMR
spectra, and Evans method yielded μeff(ZnII) = 4.11 μB and
μeff(B(C6F5)3) = 4.19 μB, both falling in between the predicted
values for S = 1 (2.83 μB) and S = 2 (4.90 μB). A high spin MnIV
3
1
(S = /2) π-radical cation (S = /2) could couple in either a
ferromagnetic (Stotal = 2) or antiferromagnetic (Stotal = 1)
manner. These complexes were also EPR silent, consistent with
an integer-spin (Stotal = 1 or 2) assignment. Although the
instability of these species precluded characterization by X-ray
crystallography, the B(C6F5)3 adduct was characterized by ESI-
MS (Mn(O)(TBP8Cz):B(C6F5)3), 1939.7619 m/z, (M+)).
Without XRD characterization, we are unable to conclusively
assign the binding site for Lewis acids on the Mn(O) complex.
However, other metal-oxo-Lewis acid adducts,30 including an
D
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isoelectronic ReV(O)−B(C6F5)3 complex,43 bind Lewis acids at
the terminal oxo ligand. In addition, the most likely way to
stabilize the MnIV(O)(TBP8Cz•+) electronic configuration is by
weakening the metal-oxo π-bonding, which can occur only if
LA/H+ binds to the terminal oxo group. Our data, together
with the literature precedent, suggests that binding of Lewis
acids (or H+) occurs at the oxo position.
In this study, we sought to compare the OAT reactivity of
the open-shell Lewis acid adducts with the closed-shell MnV(O)
complex. We also wanted to examine the influence of Brønsted
acids, and thus the strong H+ donor HBF4 was examined.
Reaction of MnV(O)(TBP8Cz) and 1 equiv of HBF4 in CH2Cl2
led to the isosbestic conversion of the green MnV(O)(TBP8Cz)
to brown MnIV(O)(TBP8Cz•+):H+ (Supporting Information
Figure S12). A similar conversion was observed when
H+[B(C6F5)4]− was used.44 The reversibility of this reaction
was tested by addition of 2,6-lutidine, which led to 75%
recovery of the starting MnV(O) complex (Supporting
1
Information Figure S13). The H NMR spectrum following
addition of HBF4 was paramagnetic, and Evans method gave
μeff = 3.96 μB (Supporting Information Figure S14). Thus, the
spectroscopic data are similar to the data seen for the Lewis
acid adducts, providing strong evidence that the Brønsted acid
HBF4 also stabilizes the open-shell valence tautomer MnIV(O)-
(TBP8Cz•+):H+.
Figure 4. (a) Time-resolved UV−vis spectral changes for the reaction
of MnIV(O)(TBP8Cz•+):ZnII (13 μM) and PPh3 (0.02 M) in 100:1
(v/v) CH2Cl2/CH3CN at 25 °C. Inset: changes in absorbance vs time
for the growth of MnIII(TBP8Cz):ZnII (725 nm) (red circles) and
decay of MnIV(O)(TBP8Cz•+):ZnII (789 nm) (blue circles) with the
best fit lines (black). (b) UV−vis spectral changes for the decay of
MnIII(TBP8Cz) (green solid line) upon titration with Zn(OTf)2 (0−
2.25 equiv), forming MnIII(TBP8Cz):ZnII (red solid line). Inset: plot
of A − Ao at 725 nm vs total equiv of Zn(OTf)2, showing maximal
formation at 1 equiv of ZnII.
O-Atom Transfer Reactivity of MnIV(O)(TBP8Cz•+):ZnII.
Product Analysis. The valence tautomer MnIV(O)-
(TBP8Cz•+):ZnII was generated as described,22 by addition of
Zn(OTf)2 to MnV(O)(TBP8Cz) (13 μM) in a 1:1 ratio in
CH2Cl2/CH3CN (100:1 v/v). The previously measured
association constant for ZnII (Ka = 4 × 106 M−1) indicates
that MnIV(O)(TBP8Cz•+):ZnII should be quantitatively formed
under these conditions. Addition of PPh3 initiated the OAT
reaction, and caused the spectrum for the MnIV(O) π-radical-
cation to undergo isosbestic conversion to a new spectrum with
λmax = 443, 725 nm (Figure 4a). The final spectrum matches
that seen for an independently generated sample from the
addition of Zn(OTf)2 to MnIII(TBP8Cz) in CH2Cl2/CH3CN
(100:1 v/v) (Figure 4b). The MnIII oxidation state for the λmax
= 443, 725 nm species was supported by the absence of an EPR
signal (13 K, 9.44 GHz) for this product (Supporting
Information Figure S5). A reasonable binding site for ZnII on
the MnIII complex is one of the Lewis basic meso-N-atoms of
the Cz ligand. Both porphyrazine and phthalocyanine
compounds are known to coordinate Lewis and Brønsted
acids at the meso-N positions.45−48 In recent work, we provided
structural evidence by XRD that the meso-N positions of
MnIII(H2O)(TBP8Cz) can be protonated.44
Scheme 2
Direct O-atom transfer from MnIV(O)(TBP8Cz•+):ZnII to
PPh3 was confirmed by 31P{1H} NMR and GC-MS, which
revealed the phosphine oxide as the only product in 73% yield
(Supporting Information Figure S6). The use of 18O-labeled
starting material MnV(18O)(TBP8Cz) (70% 18O) followed by
treatment with Zn(OTf)2 and PPh3 led to substantial labeling
of the OPPh3 product (88% 18O incorporation from GC-MS)
(Supporting Information Figure S7). A summary of these
observations is shown in Scheme 2.
led to a second order plot (Supporting Information Figure S8),
which showed a linear dependence of kobs on [PPh3] and
yielded a second-order rate constant k = 0.99(1) M−1 s−1
(Table 3). This rate constant can be compared to that for the
MnV(O) complex in the absence of ZnII (Table 1), which
shows that MnIV(O)(TBP8Cz•+):ZnII reacts at a rate 14,000-
fold slower than the parent MnV(O) complex. The series of
para-substituted phosphine derivatives (Scheme 1) was reacted
with the MnIV(O) π-radical-cation complex, and each derivative
Kinetics of Oxygen Atom Transfer for MnIV(O)-
(TBP8Cz•+):(LA) and Phosphine Substrates. As seen in
Figure 4a, the spectral changes versus time for the reaction
between MnIV(O)(TBP8Cz•+):ZnII and PPh3 fit a single
exponential model, yielding pseudo-first-order rate constants,
kobs. The concentration of phosphine substrate was varied and
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number increases for the Lewis acids. The influence of the
strength of the Lewis acid is most evident for the p-CF3
derivative, where the rate constants vary over 2 orders of
magnitude from ZnII (lowest) to H+ (highest). These trends
clearly follow the reactivity/selectivity principle, where the least
reactive MnIV(O) π-radical-cation complex, which is generated
from the weakest Lewis acid (ZnII), is the most selective toward
O-atom transfer for the phosphine derivatives.
The influence of the steric bulk of the phosphine derivatives
on reaction rates for OAT was also examined. The methyl-
substituted tri(tolyl)phosphines in Scheme 1 were employed,
and rate constants are given in Table 3. These data show that
the MnIV(O)(π-radical-cation) formed with the Lewis acid
B(C6F5)3 is most sensitive to the steric demand of the
phosphine substrate, with an ∼10-fold rate decrease for ortho-
versus para-substituted tri(tolyl)phosphine. For the less
sterically bulky Lewis acids ZnII and H+, the steric nature of
the substrate has little influence on the rate of OAT. These data
are consistent with the Lewis acid being coordinated to the
terminal oxo group, where the relatively large B(C6F5)3 can be
expected to have the largest steric clash with the incoming
phosphine nucleophile.
Redox Titration. Assessment of the redox potential of
MnIV(O)(TBP8Cz•+):ZnII could be expected to shed light on
its reactivity in the oxidation of organic substrates. Addition of
the one-electron reductant acetylferrocene (Eox = 0.62 V vs
SCE, where Eox is the redox potential of the species being
oxidized)55 and monitoring by spectral redox titration resulted
in ring reduction and isosbestic conversion of MnIV(O)-
(TBP8Cz•+):ZnII to MnIV(O)(TBP8Cz):ZnII, as seen previously
with ferrocene.22 The titration curve (Supporting Information
Figure S18) was fit to a 1:1 electron-transfer equilibrium model,
yielding a KET value of 18.3. The Nernst equation (eq 1) was
employed to obtain a redox potential of Ered = +0.69 V vs SCE
for MnIV(O)(TBP8Cz•+):ZnII. In comparison, Ered (Ered is the
redox potential of the species being reduced) for the valence
tautomer MnV(O)(TBP8Cz) is −0.05 V vs SCE.36
Table 3. Second-Order Rate Constants for the Oxidation of
Triarylphosphines with MnIV(O)(TBP8Cz•+):(LA) (LA =
ZnII, B(C6F5)3 or HBF4)
k (M−1 s−1
)
a
b
b
P(X−Ph)3
Zn(OTf)2
B(C6F5)3
HBF4
o-CH3
m-CH3
p-CH3
p-H
1.9(2)
0.6(1)
2.2(1)
0.9(1)
3.3(2)
2.5(3)
2.2(2)
1.12(3)
1.8(1)
1.2(1)
8.7(4)
6.5(4)
0.99(1)
0.109(4)
1.65(3)
0.525(5)
p-Cl
p-CF3
0.0085(2)
0.126(9)
a
b
In 100:1 (v/v) CH2Cl2/CH3CN at 25 °C. In CH2Cl2 at 25 °C.
gave good overall second-order kinetics. The second-order rate
constants obtained for each of the PAr3 derivatives (Table 3)
are dramatically slower than those seen for the MnV(O)
complex, on the order of a 104-fold reduction in rate.
To gain further insights regarding the influence of the Lewis
acid, the ZnII ion was replaced with B(C6F5)3, as well as the
Brønsted acid HBF4. Reaction of MnIV(O)(TBP8Cz•+):LA (LA
= B(C6F5)3, H+) with the para-substituted phosphine
derivatives led to good second-order kinetics and production
of the two-electron reduced MnIII complexes. The log k values
for the MnIV(O)(TBP8Cz•+):LA complex were plotted versus
Hammett σ parameters as shown in Figure 5. Linear trends are
Ered = Eox + (RT/F) ln KET
(1)
The increased redox potential for MnIV(O)(TBP8Cz•+):ZnII
suggests that this complex could show enhanced rates for one-
electron oxidations such as H-atom abstraction, and indeed this
complex, as well as a borane analog, exhibits faster rates of HAT
with phenol O−H substrates.22,23 However, the redox potential
for PPh3 is 2.08 V vs Cp2Co+/0 (converted to 2.97 V vs SCE),56
and therefore, one-electron oxidation of PPh3 by the MnIV(O)
complex remains strongly endergonic, helping to rule out either
a pure ET or ET-OT type mechanism.30,57,58
Figure 5. Plots of log k (second-order rate constants) versus Hammett
σ values for para-substituted triarylphosphines for the reactions with
MnIV(O)(TBP8Cz•+):(LA) (LA = ZnII (red), B(C6F5)3 (green), HBF4
(blue)).
revealed for all three complexes, with a negative ρ value found
for ZnII (−1.4) that is larger than the parent complex (Figure
2). The negative ρ value (−0.82) for B(C6F5)3 is also consistent
with an electrophilic mechanism, but is smaller in magnitude
than the ρ value seen for ZnII. In contrast, the small value of ρ =
−0.13 for H+ shows that, for this complex, there is little
correlation of reaction rate with the electron-rich nature of the
phosphine substrates.
A summary of HAT and OAT reactivity for the two valence
tautomers is shown in Scheme 3. A major question remains:
Scheme 3
The trends in the Hammett data in Figure 5 can be
rationalized by taking into consideration the strengths of the
Lewis acids. Gutmann-Beckett acceptor numbers, which are a
measure of the relative Lewis acidity of both Lewis and
Brønsted acids, were determined for HBF4, B(C6F5)3, and
Zn(OTf)2 by measuring their influence on the 31P chemical
shift of OPEt3 with 31P{1H} NMR spectroscopy.49−54 Acceptor
numbers of 122, 82, and 68 were obtained for HBF4, B(C6F5)3,
and Zn(OTf)2, respectively, and the magnitudes of the ρ values
for the Hammett plots in Figure 5 decrease as the acceptor
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why are the rates of OAT to PAr3 substrates dramatically
slowed for the MnIV(O)(TBP8Cz•+):LA complexes? The
increase in HAT noted for the MnIV(O) π-radical-cation
tautomer may be attributed to the large increase in redox
potential compared to the MnV(O) form, which should result
in a larger driving force for HAT.59−64 In contrast, the two-
electron OAT process involving the phosphine substrates is
dramatically slower for the MnIV(O) π-radical-cation species. If
a concerted OAT process is invoked, then the two-electron
electrophilicity of the metal-oxo unit should be a critical factor
that influences the OAT rates. We suggest that the MnIV(O) π-
radical-cation species is less electrophilic at the oxo ligand than
the MnV(O) species, despite the addition of Lewis acid, because
of the lower oxidation state at the metal. We hypothesize that
this lowering of the electrophilicity is responsible for the slower
OAT reaction rates.
It should be noted that for some metal-oxo complexes, a
correlation has been observed between the redox potentials of
the complex and OAT rates.30,57,58,65,66 In addition, DFT
calculations on two-electron atom-transfer reactions have
suggested that spin state may also be an important factor.
For example, DFT calculations performed on OAT of MnV(O)
corroles implicate a lower-barrier triplet state in the OAT
reaction with thioanisole.67 Other calculations suggested that
singlet versus triplet state energies could be responsible for the
relative rates of related nitrogen-atom-transfer reactions
involving Cu-(N-tosyl)-ScIII and Cu-(N-mesityl)-ScIII com-
plexes.68 It is clear that more work is needed to determine
the factors that control biomimetic, metal-mediated two-
electron atom-transfer reactions.
about the relative reactivities of heme-derived valence
tautomers for high-valent metal-oxo species. This work
provides fundamental information regarding the reactivity of
two biomimetic valence tautomers of a high-valent Mn-oxo
porphyrinoid complex. Determining the differences in reactivity
of valence tautomeric species may help in our understanding of
the preference for one valence tautomer over another in heme
enzymes.
EXPERIMENTAL SECTION
■
Materials. All reactions were performed using dry solvents and
standard Schlenk techniques. The complexes MnVO(TBP8Cz) and
MnIII(TBP8Cz) (TBP8Cz
= octakis(p-tert-butylphenyl)-
corrolazinato3−) were synthesized and purified according to previously
published methods.36 Dichloromethane and acetonitrile were purified
via a Pure-Solv solvent purification system from Innovative
Technologies, Inc. H218O (97% 18O) and deuterated solvents for
NMR measurements were obtained from Cambridge Isotopes, Inc.
Tri(o-tolyl)phosphine oxide (OP(o-tolyl)3) was synthesized according
to Granoth et al.70 and was recrystallized from ethanol. All other
reagents, except for tri(m-tolyl)phosphine (Alfa-Aesar, 98+%), were
purchased from Sigma-Aldrich at the highest level of purity and used
as received.
Instrumentation. Kinetics and other UV−vis measurements were
performed on a Hewlett-Packard Agilent 8453 diode-array spectro-
photometer with a 3.5 mL air-free quartz cuvette (path length = 1 cm)
fitted with a septum. For reactions with total reaction time of <10 s,
stopped-flow experiments were carried out using HiTech SHU-61SX2
(TgK scientific Ltd.) with a xenon light source and Kinetic Studio
software. Gas chromatography mass spectrometry (GC-MS) was
performed on an Agilent 6850 gas chromatograph fitted with a DB-5
5% phenylmethyl siloxane capillary column and equipped with an
electron-impact (EI) mass spectrometer. Product yields were
calculated from a dodecane internal standard. LDI-MS was conducted
on a Bruker Autoflex III TOF/TOF instrument equipped with a
nitrogen laser at 335 nm using an MTP 384 ground steel target plate.
Electrospray ionization mass spectra (ESI-MS) were collected on a
Thermo Finnigan LCQ Duo ion-trap mass spectrometer fitted with an
electrospray ionization source in positive ion mode. Samples were
infused into the instrument at a rate of 25 μL/min using a syringe
pump via a silica capillary line. The spray voltage was set at 5 kV, and
the capillary temperature was held at 250 °C. 1H NMR (400.13 MHz)
and 31P{1H} NMR (161.9 MHz) spectra were recorded on a Bruker
Avance 400 MHz NMR spectrometer at room temperature. Elemental
analyses were performed at Atlantic Microlab, Inc., Norcross, GA.
Electron paramagnetic resonance (EPR) spectra were recorded with a
Bruker EMX spectrometer equipped with a Bruker ER 041 X G
microwave bridge and a continuous-flow liquid helium cryostat
(ESR900) coupled to an Oxford Instruments TC503 temperature
controller.
SUMMARY AND CONCLUSIONS
■
The comparative O-atom transfer chemistry for MnV(O) and
MnIV(O)(π-radical-cation) porphyrinoid complexes has been
determined. The interconversion of these species is mediated
by the addition of either Lewis or Brønsted acids. The two-
electron OAT reactivity of the MnIV(O)(π-radical-cation):LA
complexes with PAr3 derivatives is dramatically inhibited in
comparison to the MnV(O) valence tautomer. This inhibition
can be related to the difference in the electronic structures for
MnIV(O)(π-radical-cation) vs MnV(O) valence tautomers, in
which the former can be anticipated to have a less electrophilic
terminal oxo group. The relative rate constants for one-electron
processes such as HAT are found to be influenced in the
opposite manner, with a significant increase in rate seen for the
open-shell MnIV(O)(π-radical-cation) tautomer. This trend can
be attributed to the large, measured increase in reduction
potential for MnIV(O)(π-radical-cation), which increases the
driving force for the HAT reaction.
Stopped-Flow UV−Vis Kinetics Studies. In a typical reaction,
MnV(O)(TBP8Cz) (13 μM, CH2Cl2) was reacted with triarylphos-
phine (0.15−0.75 mM) [tris(para-X-phenyl)phosphine (X = CH3, H,
Cl, CF3), tri(meta-tolyl)phosphine, and tri(ortho-tolyl)phosphine)].
The spectral changes showed isosbestic conversion of MnV(O)-
(TBP8Cz) (λmax = 420, 635 nm) to MnIII(TBP8Cz) (λmax = 435, 470,
685 nm). The pseudo-first-order rate constants, kobs, for these
reactions were obtained by nonlinear least-squares fitting of the
plots of absorbance at 635 nm (Abst) versus time (t) according to the
equation Abst = Absf + (Abs0 − Absf) exp(−kobst) where Abs0 and Absf
are initial and final absorbance, respectively. Second-order rate
constants (k) were obtained from the slope of the best-fit line from
a plot of kobs vs substrate concentration.
In biological systems, it has been shown that a manganese-
substituted cytochrome P450cam carries out efficient OAT to
alkenes to give epoxide through a likely MnV(O)(porphyrin)
intermediate, in line with our conclusions that MnV(O)
porphyrinoid species are good electrophiles for OAT. However,
the Mn analogue of P450 was not capable of mediating
hydroxylation, in contrast to the native FeIV(O)(porphyrin•+)
intermediate, which can perform both oxygen transfer and
hydroxylation reactions.69 Thus, in the case of Fe, the valence
tautomer containing the radical-cation appears inherently more
reactive than the MnIV(O)(π-radical-cation) observed in this
work.
Formation of MnIV(O)(TBP8Cz•+):H+. To a solution of MnV(O)-
(TBP8Cz) (20 μM, 2 mL) were added successive amounts of HBF4·
Et2O (0.2 equiv aliquots dissolved in 5 μL CH2Cl2). A color change
from green to brown was observed. Monitoring the reaction after each
addition showed isosbestic conversion of MnV(O)(TBP8Cz) (λmax
=
Valence tautomers are prevalent in nature and are of key
importance in heme enzymes, but there is still little known
420, 635 nm) to MnIV(O)(TBP8Cz•+):H+ (λmax = 419, 785 nm)
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(Supporting Information Figure S12). Full formation of the product
was observed after addition of 1 equiv HBF4. Reaction of this complex
with 2,6-lutidine (1 equiv, 25 μL in CH2Cl2) gives back ∼75% of the
starting MnV(O)(TBP8Cz) complex (Supporting Information Figure
S13).
18O Labeling Studies. To a solution of MnV(O)(TBP8Cz) in dry
CH2Cl2 (7 μmol, 6 mL) was added H218O (25 μL, 200 equiv, 97%
18O-enriched) via microsyringe under an Ar atmosphere. Stirring was
continued for 48 h and the solvent was removed under vacuum. The
product was immediately analyzed by LDI-MS, which revealed 70%
18O incorporation in MnV(O)(TBP8Cz) (Supporting Information
Magnetic Susceptibility by Evans Method. To a solution of
MnV(O)(TBP8Cz) (2.0 mM in 500 μL 0.5% TMS in CD2Cl2) was
added HBF4·Et2O (1 equiv in 20 μL CD2Cl2). Complete formation of
MnIV(O)(TBP8Cz•+):H+ was observed by UV−vis spectroscopy. The
reaction mixture was transferred to an NMR tube, together with a
second coaxial insert tube containing the solvent blank. 1H NMR
spectra were recorded at 297.2 K, and the chemical shift of the TMS
peak in the presence of the paramagnetic MnIV(O)(TBP8Cz•+):H+
complex was compared to that of the TMS peak in the inner tube
containing only the TMS standard (Supporting Information Figure
S14). The effective spin-only magnetic moment was calculated by a
simplified Evans method analysis71 according to μeff = 0.0618(sqrt-
(ΔνT/2f M)), where Δν is the difference in frequency (Hz) between
the two reference (TMS) signals, T is the temperature (K), f is the
oscillator frequency (MHz) of the superconducting spectrometer, and
M is the molar concentration of the paramagnetic metal complex. The
number of unpaired electrons was calculated using the equation μ2 =
n(n + 2). A control experiment was performed using MnV(O)-
(TBP8Cz), where no shift in the TMS peak was observed (Supporting
Information Figure S15).
Figure S7a). The 18O-enriched MnV(O) complex was combined with
Zn(OTf)2 (2 equiv) in CH2Cl2/CH3CN (10:1 v/v, 2 mL) to form
MnIV(18O)(TBP8Cz•+):ZnII. The solution was again analyzed by LDI-
MS which revealed that 40% of the 18O label was retained (Supporting
Information Figure S7b). An amount of PPh3 (10 equiv) was added,
and the reaction was monitored by UV−vis, which showed complete
conversion to MnIII(TBP8Cz):ZnII. The OPPh3 product was extracted
with CH3OH, dried, and redissolved in CH3OH. An aliquot (1 μL)
was injected directly into the GC−MS, which showed 88% 18O
incorporation (Supporting Information Figure S7c).
Lewis Acidity Measurements.49,50 In an NMR tube, a 3:1
mixture of Lewis acid (HBF4·Et2O, B(C6F5)3, Zn(OTf)2) and
triethylphosphine oxide was prepared in CD2Cl2 (CD3CN for
Zn(OTf)2). 31P{1H} NMR spectra were collected with an 85%
H3PO4 external standard. The acceptor number (A.N.) was calculated
using the relationship: A.N. = 2.21(δsample − 41.0), where δsample is the
chemical shift for the OPEt3 − Lewis acid adduct.
Measurement of Equilibrium Constant of Electron Transfer
(KET). Under strictly dry and anaerobic conditions, a solution of
MnVO(TBP8Cz) in CH2Cl2 (15 μM, 2.0 mL) was mixed with
Zn(OTf)2 (20 equiv, 20 μL in CH3CN) to generate MnIVO-
(TBP8Cz•+):ZnII. Successive aliquots of acetylferrocene (AcFc) (0−
2.25 equiv, in CH2Cl2) were added and the reaction was monitored by
UV−vis until no further change was observed. Monitoring the titration
at 725 nm resulted in the plot in Supporting Information Figure S18,
which was fitted to Supporting Information eq S3 to obtain a value for
KET. The values of KET, [MnIV(O)(TBP8Cz•+)]0, and Δε =
[ε(MnIV(O)(TBP8Cz)):ZnII − ε(MnIV(O)(TBP8Cz•+)):ZnII] were
allowed to vary during the fitting procedure either simultaneously or in
sequence, and the final values for the best fit were KET = 18.3;
[MnIV(O)Cz•+]0 = 16.4 μM; Δε = 25651; r = 0.999. The refined value
for [MnIV(O)Cz•+]0 is close to the starting concentration (15 μM),
and the refined Δε value was physically reasonable.
UV−Vis Kinetics Studies with MnIV(O)(TBP8Cz•+):LA. The
valence tautomer MnIV(O)(TBP8Cz•+):LA was generated in situ by
addition of Lewis acids (LA) [Zn(OTf)2 in CH3CN, B(C6F5)3 or
HBF4·Et2O in CH2Cl2] to an amount of MnV(O)(TBP8Cz) (13 μM,
CH2Cl2). Upon complete formation of the valence tautomer, varying
amounts of triarylphosphine (1.5−45 mM) were added to start the
reaction. The spectral change showed isosbestic conversion of
MnIV(O)(TBP8Cz•+):LA (λmax = 419, 789 nm) to MnIII(TBP8Cz):LA
(λmax = 443, 725 nm). The same kinetic analysis was used as employed
for the stopped-flow UV−vis studies, following the growth in
absorbance at 725 nm which corresponds to MnIII(TBP8Cz):LA.
Pseudo-first-order kobs values were obtained and exhibited a linear
correlation with substrate concentration for all triarylphosphine
substrates.
OAT Product Analysis. In a custom-made 250 mL round-bottom
flask fitted with a 3 mL quartz cuvette under an Ar atmosphere was
combined a solution of MnV(O)(TBP8Cz) (100 μM in 15 mL of
CH2Cl2) with a solution of Zn(OTf)2 (1 equiv in CH3CN) to form
MnIV(O)(TBP8Cz•+):ZnII. An amount of PPh3 (1 equiv) was then
added and the reaction was monitored by UV−vis spectroscopy, which
showed complete conversion to MnIII(TBP8Cz):ZnII. The solution
was concentrated to dryness, redissolved in toluene and injected
directly onto the GC−MS for analysis. Yields were calculated from a
calibration curve with dodecane as an internal standard. The obtained
yield from this method (71%) is an average of three runs. Unreacted
PPh3 was also measured by GC (0.33 equiv). The yield of OPPh3 was
also independently measured by 31P{1H} NMR as follows: A solution
of MnV(O)(TBP8Cz) (2 mM) was combined with Zn(OTf)2 (1
equiv) in CH2Cl2/CH3CN 10:1 (v/v) to form MnIV(O)-
(TBP8Cz•+):ZnII. An amount of PPh3 (10 equiv) was then added,
and the reaction was monitored by UV−vis spectroscopy, which
showed complete conversion to MnIII(TBP8Cz):ZnII. An amount of
Bu4N+F− (tetrabutylammonium fluoride) (20 equip) was added to
release the OPPh3 bound to the paramagnetic MnIII species. A color
change to a dark green solution typical of [MnIII(TBP8Cz)(F)]− (λmax
= 428, 471, 680 nm) was noted.33 The solution was then concentrated
under vacuum and redissolved in CD2Cl2:CD3CN (500 μL; 10:1 v/v)
and immediately analyzed by 31P{1H} NMR (85% H3PO4 external
standard). The delay time (D1) was set to 150 s to allow for complete
relaxation of the 31P nucleus. Comparison of the integrations for the
peak assigned to OPPh3 and the peak for PPh3 gave a yield of 74% for
OPPh3, in good agreement with the GC method. Unreacted PPh3
(9.26 equiv) was also measured from the NMR spectra (Supporting
Information Figure S6).
Synthesis of MnIII(OPPh3)(TBP8Cz) (1). To a CH2Cl2 solution of
MnIII(TBP8Cz) (7 μmol, 1 mL) was added OPPh3 (10 equiv). This
solution was layered with CH3CN and set aside to undergo slow
evaporation, giving X-ray quality single crystals (black blocks) in good
yield (8 mg, 81%). UV−vis (CH2Cl2): λmax = 435, 470, 685 nm, ESI-
MS (m/z): isotopic cluster centered at 1690.1 (M + H) (Supporting
Information Figure S4). Anal. Calcd for C114H119MnN7OP: C, 81.06;
H, 7.10; N, 5.80. Found: C, 81.00; H, 7.07; N, 5.80.
Synthesis of MnIII(OP(o-tolyl)3)(TBP8Cz) (2). The complex was
prepared similarly to 1. X-ray quality single crystals (black blocks)
were obtained from slow evaporation of a CH2Cl2/CH3CN solution in
good yield (6 mg, 62%). UV−vis (CH2Cl2): λmax = 435, 470, 685 nm.
ESI-MS (m/z): isotopic cluster centered at 1732.0 (M + H)
(Supporting Information Figure S4). Anal. Calcd for
C117H125MnN7OP: C, 81.17; H, 7.28; N, 5.66. Found: C, 81.04; H,
7.21; N, 5.58.
Single Crystal X-ray Crystallography. All reflection intensities
were measured at 110(2) K using a SuperNova diffractometer
(equipped with Atlas detector) with Cu Kα radiation (λ = 1.54178
Å) under the program CrysAlisPro (Version 1.171.36.32 Agilent
Technologies, 2013). The same program was used to refine the cell
dimensions and for data reduction. The structure was solved with the
program SHELXS-201372 and was refined on F2 with SHELXL-
2013.72 Analytical numeric absorption corrections based on a
multifaceted crystal model were applied using CrysAlisPro. The
temperature of the data collection was controlled using the system
Cryojet (manufactured by Oxford Instruments). The H-atoms were
placed at calculated positions (unless otherwise specified) using the
instructions AFIX 23, AFIX 43 or AFIX 137 with isotropic
displacement parameters having values 1.2 or 1.5 times Ueq of the
attached C atoms.
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For the structure for MnIII(OPPh3)(TBP8Cz) (1) six of the eight
tert-butylphenyl (TBP) groups are found to be disordered over two
orientations (only the t-butyl part is disordered). The occupancy
factors of the six major components of the disorder refine to 0.53(2),
0.69(4), 0.768(7), 0.52(2), 0.695(6) and 0.57(3). The crystal lattice
contains some amount of solvent molecules (CH3CN and CH2Cl2).
The occupancy factors of the lattice CH3CN solvent molecules were
refined using free variables, and there are ca. 3.76 CH3CN molecules
per Mn complex. Some electron density in the asymmetric unit (i.e., a
disordered solvent CH2Cl2 molecule with at least 3 different
orientations with partial occupancies) has been taken out in the final
refinement (SQUEEZE73 details are provided in the CIF file).
Data for MnIII(OPPh3)(TBP8Cz) (1) follow: Fw = 1843.47,
irregular shaped crystal, 0.43 × 0.18 × 0.09 mm3, monoclinic, P21/c
(no. 14), a = 17.17627(19), b = 33.6006(3), c = 19.6140(2) Å, β =
101.1080(11)°, V = 11107.8(2) Å3, Z = 4, Dx = 1.102 g cm−3, μ =
1.500 mm−1, abs. corr. range: 0.640−0.882. A total of 82335 reflections
were measured up to a resolution of (sin θ/λ)max = 0.62 Å−1. Then,
21779 reflections were unique (Rint = 0.0392), of which 18304 were
observed [I > 2σ(I)]. A total of 1527 parameters were refined using
855 restraints. R1/wR2 [I > 2σ(I)]: 0.0658/0.1749. R1/wR2 [all refl.]:
0.0768/0.1841. S = 1.043. Residual electron density found between
−0.57 and 1.06 e Å−3.
ACKNOWLEDGMENTS
■
The authors gratefully acknowledge research support of this
work by the NIH (Grant GM101153 to D.P.G.). We also thank
Prof. Kenneth D. Karlin (JHU) for instrumentation (stopped-
flow UV−vis) use.
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ASSOCIATED CONTENT
■
S
* Supporting Information
UV−vis kinetics studies, X-ray crystal structure of 2a (Figure
S3), crystallographic information files (CIF for 1 and 2), EPR,
31P{1H} and 1H NMR, and MS data, and redox titration
studies. The Supporting Information is available free of charge
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AUTHOR INFORMATION
■
Corresponding Author
*dpg@jhu.edu
Notes
The authors declare no competing financial interest.
I
DOI: 10.1021/jacs.5b00875
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

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