O-H bond oxidation by a monomeric MnIII-OMe complex

Article abstract of DOI:10.1039/c4dt03546a

Manganese-containing, mid-valent oxidants (Mn^III-OR) that mediate proton-coupled electron-transfer (PCET) reactions are central to a variety of crucial enzymatic processes. The Mn-dependent enzyme lipoxygenase is such an example, where a Mn^III-OH unit activates fatty acid substrates for peroxidation by an initial PCET. This present work describes the quantitative generation of the Mn^III-OMe complex, [Mn^III(OMe)(dpaq)]⁺ (dpaq = 2-[bis(pyridin-2-ylmethyl)]amino-N-quinolin-8-yl-acetamidate) via dioxygen activation by [Mn^II(dpaq)]⁺ in methanol at 25 °C. The X-ray diffraction structure of [Mn^III(OMe)(dpaq)]⁺ exhibits a Mn-OMe group, with a Mn-O distance of 1.825(4) ?, that is trans to the amide functionality of the dpaq ligand. The [Mn^III(OMe)(dpaq)]⁺ complex is quite stable in solution, with a half-life of 26 days in MeCN at 25 °C. [Mn^III(OMe)(dpaq)]⁺ can activate phenolic O-H bonds with bond dissociation free energies (BDFEs) of less than 79 kcal mol^-1 and reacts with the weak O-H bond of TEMPOH (TEMPOH = 2,2′-6,6′-tetramethylpiperidine-1-ol) with a hydrogen/deuterium kinetic isotope effect (H/D KIE) of 1.8 in MeCN at 25 °C. This isotope effect, together with other experimental evidence, is suggestive of a concerted proton-electron transfer (CPET) mechanism for O-H bond oxidation by [Mn^III(OMe)(dpaq)]⁺. A kinetic and thermodynamic comparison of the O-H bond oxidation reactivity of [Mn^III(OMe)(dpaq)]⁺ to other M^III-OR oxidants is presented as an aid to gain more insight into the PCET reactivity of mid-valent oxidants. In contrast to high-valent counterparts, the limited examples of M^III-OR oxidants exhibit smaller H/D KIEs and show weaker dependence of their oxidation rates on the driving force of the PCET reaction with O-H bonds. This journal is

Full text of DOI:10.1039/c4dt03546a

Dalton
Transactions
PAPER
O–H bond oxidation by a monomeric MnIII–OMe
complex†
Cite this: Dalton Trans., 2015, 44,
3295
Gayan B. Wijeratne, Victor W. Day and Timothy A. Jackson*
Manganese-containing, mid-valent oxidants (MnIII–OR) that mediate proton-coupled electron-transfer
(PCET) reactions are central to a variety of crucial enzymatic processes. The Mn-dependent enzyme
lipoxygenase is such an example, where a MnIII–OH unit activates fatty acid substrates for peroxidation
by an initial PCET. This present work describes the quantitative generation of the MnIII–OMe complex,
[MnIII(OMe)(dpaq)]+ (dpaq = 2-[bis(pyridin-2-ylmethyl)]amino-N-quinolin-8-yl-acetamidate) via dioxygen
activation by [MnII(dpaq)]+ in methanol at 25 °C. The X-ray diffraction structure of [MnIII(OMe)(dpaq)]+
exhibits a Mn–OMe group, with a Mn–O distance of 1.825(4) Å, that is trans to the amide functionality of
the dpaq ligand. The [MnIII(OMe)(dpaq)]+ complex is quite stable in solution, with a half-life of 26 days in
MeCN at 25 °C. [MnIII(OMe)(dpaq)]+ can activate phenolic O–H bonds with bond dissociation free ener-
gies (BDFEs) of less than 79 kcal mol−1 and reacts with the weak O–H bond of TEMPOH (TEMPOH = 2,2’-
6,6’-tetramethylpiperidine-1-ol) with a hydrogen/deuterium kinetic isotope effect (H/D KIE) of 1.8 in
MeCN at 25 °C. This isotope effect, together with other experimental evidence, is suggestive of a con-
certed proton-electron transfer (CPET) mechanism for O–H bond oxidation by [MnIII(OMe)(dpaq)]+.
A kinetic and thermodynamic comparison of the O–H bond oxidation reactivity of [MnIII(OMe)(dpaq)]+ to
other MIII–OR oxidants is presented as an aid to gain more insight into the PCET reactivity of mid-valent
oxidants. In contrast to high-valent counterparts, the limited examples of MIII–OR oxidants exhibit smaller
H/D KIEs and show weaker dependence of their oxidation rates on the driving force of the PCET reaction
with O–H bonds.
Received 18th November 2014,
Accepted 19th December 2014
DOI: 10.1039/c4dt03546a
www.rsc.org/dalton
S4 state, coordinated water molecules within the cluster have
been proposed to be deprotonated as a part of sequential
Introduction
Manganese-dependent enzymes that mediate proton coupled PCET processes, producing reducing equivalents.2–6 In light of
electron transfer (PCET) reactions perform vital processes in this proposal, as well as numerous other biological examples
humans and numerous other organisms.1 PCET reactivity of metal-mediated PCET,7,8 model complexes that can facilitate
within the tetranuclear manganese active site of the oxygen PCET have been extensively studied to understand the thermo-
evolving complex (OEC) of photosystem II represents a classic dynamics and kinetics of these processes.9–17
example. This active-site cluster consists of a Mn3CaO4 cubane
In general, PCET reactions can be categorized as concerted
with a dangler Mn atom linked to the cubane through bridging or sequential, based on whether the proton and electron are
ligands.2 In a more recent structure, a portion of the cluster is being transferred in a single step or multiple steps, respecti-
capped with four terminal water ligands.2 During the oxidation vely.18 Single-step processes where the proton and electron
of the OEC from its least oxidized S0 state to the most oxidized share the same orbital/bond following the reaction are often
referred to as hydrogen atom (H• = H+ + e−) transfer (HAT) reac-
tions. This is a common reaction in organic chemistry, and
Department of Chemistry and Center for Environmentally Beneficial Catalysis,
has been the subject of numerous detailed studies over the
University of Kansas, 1251 Wescoe Hall Drive, Lawrence, KS 66045, USA.
past century.13 On the other hand, when transition metal com-
E-mail: taj@ku.edu
plexes are involved, most often the proton and electron are
†Electronic supplementary information (ESI) available: Crystal data and struc-
ture refinement table for [MnIII(OMe)(dpaq)](OTf); ESI-MS plots; yields of
4-t-butylArO• from 4-t-butylArOH oxidation by [MnIII(OMe)(dpaq)]+; cyclic voltamme-
try data for [MnIII(OMe)(dpaq)]+; X-band EPR data for oxidation reaction pro-
ducts of [MnIII(OMe)(dpaq)]+ with TEMPOH and 4-t-butylArOH; kinetic data for
reactions of [MnIII(OMe)(dpaq)]+ with xanthene and substituted phenols. CCDC
1032682. For ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/c4dt03546a
transferred to separate entities (i.e. a ligand and the metal),
and no longer share the same bond. These metal-mediated
PCET reactions can also occur either in a single (concerted
proton–electron transfer; CPET) or multiple (proton transfer
followed by electron transfer or vice versa; PT/ET or ET/PT)
kinetic steps.19 Synthetic transition metal complexes that can
This journal is © The Royal Society of Chemistry 2015
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Paper
Dalton Transactions
Scheme 1 PCET reactivity of a mid-valent oxidant (MIII–OR) with an H•
donor substrate (R–H, where R is C or O).
facilitate PCET reactions are of great interest not only because
they mimic metalloenzyme reactions, but also because PCET is
central to many industrial processes.1 Recently, more work has
been focused on developing PCET mediators using greener,
less expensive metals, such as high-valent and mid-valent
Fig. 1 Representative Structures for PCET Mediating MIII–OR Adducts
of (A) [MnIII(OR)(dpaq)]+,29 (B) [MIII(OR)(PY5)]+,26 and (C) [MnIII(OR)-
(SMe N4(tren))]+,25 series of complexes.
2
manganese and iron complexes.20–29 High-valent (MIV or MV; with the most electron-withdrawing –OPhp-NO ligand, dis-
2
M = Mn or Fe) oxidants, typically containing terminal oxo played the highest E° along with the lowest pKa. In contrast,
+
ligands, have been the subject of a large number of studies, as [MnIII(OH)(SMe N4(tren))] , with one of the most electron-
2
these exhibit efficient reactivity with stronger, unactivated C–H donating –OR ligands, showed the lowest E° and highest pKa.25
or O–H bonds.20–24 In contrast, only several examples of mid- Thus, in this series, E° and pKa display an inverse correlation.
valent metal (MIII) oxidants have been reported to-date,25–29 As
a
net result, only [MnIII(OH)(SMe N4(tren))]
and
+
2
although these compounds exhibit unique reactivity patterns [MnIII(OMe)(SMe N4(tren))] showed the ability to oxidize the
+
2
that can be used to gain detailed understanding into the funda- weak O–H bond of TEMPOH, with similar second-order rate
mental thermodynamic and kinetic parameters that govern constants of 2.1 × 103 M−1 s−1 and 3.6 × 102 M−1
s
−1, respecti-
PCET reactions. Among the mid-valent oxidants reported, vely. The other two MnIII–OR species with the most electron-
MIII–OR (M = Mn or Fe; R = H, CH3, Ph) species are the most withdrawing –OR ligands were inert toward TEMPOH. There-
common. In PCET reactions with MIII–OR species, MII–O(H)R fore, it was concluded that under similar entropic factors, the
Me2
+
more electron-withdrawing [MnIII(OPhp-NO )(S N4(tren))]
2
is generated as the final metal based product (Scheme 1).
The principal thermodynamic parameters controlling PCET and [MnIII(OPh)(SMe N4(tren))] did not have a sufficiently
reactivity are the reduction potential of the oxidized metal large E° to overcome the substantially small pKa to provide the
center (E°) and the acidity of the reduced product (i.e., the pKa requisite driving force for PCET.25 In the MnIII and FeIII com-
of the MII–O(H)R species).13 The relationship between these plexes of Stack and Goldsmith (Fig. 1), interesting trends are
values and the free energy change of the reaction (ΔG) is evident when comparing the Mn versus Fe complexes, where
described by the Bordwell relationship (ΔG° = −23.06E° − [MnIII(OH)(PY5)]2+ was found to be the overall best oxidant of
1.37pKa − C).10,30 This relationship can, in principle, be the series, with the highest E° and pKa.28
+
2
applied to the design of better oxidants as both the reduction
This present work describes the formation and PCET reac-
potential and pKa parameters are tunable by the ligand frame- tivity of the new complex [MnIII(OMe)(dpaq)]+, which is formed
work. E° can be controlled through the electronic properties of in quantitative yields when [MnII(dpaq)]+ is reacted with dioxy-
the supporting and –OR ligands and the pKa results from a gen in methanol at 25 °C. The [MnIII(OMe)(dpaq)]+ complex
combination of supporting ligand effects and intrinsic pro- reacts with TEMPOH and with phenolic substrates with O–H
perties of the MII–O(H)R group. In this regard, variation of the bond free energies up to 78.5 kcal mol−1, and represents the
R group of mid-valent M–OR oxidants has the potential to only example of a monomeric MnIII–OMe species reported to
tune PCET reactivity by a means unavailable for high-valent activate phenolic O–H bonds. In contrast to that observed for
MvO species. However, given the limited examples of mid- the recently reported [MnIII(OH)(dpaq)]+ complex (Fig. 1),29
valent M–OR oxidants with different R groups,25,27,28 it is phenol oxidation by [MnIII(OMe)(dpaq)]+ does not show satur-
difficult to elaborate broadly on how ligand properties affect ation behavior at high phenol concentrations, suggesting the
the thermodynamic and kinetic properties of the oxidant, and absence of an accumulating intermediate. However, mechanis-
how these can be modified to design better PCET mediators.
tic studies suggest that [MnIII(OMe)(dpaq)]+ activates substrate
Current examples of PCET-mediating MIII–OR complexes O–H bonds by a CPET process. In light of the reactivity of
with multiple
R
groups are found in the [MnIII(OH)- [MnIII(OMe)(dpaq)]+ and other mid-valent Mn oxidants, a
+
+
(SMe N4(tren))] and [MnIII(OMe)(SMe N4(tren))] complexes of detailed discussion of thermodynamic and kinetic parameters
2
2
Kovacs and co-workers (SMe N4(tren) = 3-((2-(bis(2-aminoethyl)- dictating the feasibility of PCET is presented, and we discuss
2
amino)ethyl)imino)-2-methylbutane-2-thiolate),25
and the characteristic differences in the oxidative reactivity of mid-
[MIII(OH)(PY5)]2+ and [MIII(OMe)(PY5)]2+ complexes of Stack valent versus high-valent oxidants.
and Goldsmith (M = Mn and Fe; PY5 = 2,6-bis(bis(2-pyridyl)-
methoxymethane)pyridine) (Fig. 1).26–28 The change in thermo-
dynamic properties as a function of the –OR group can be
Experimental
clearly seen in the experimental E° values of the [MnIII(OR)-
+
(SMe N4(tren))] series (Fig. 1; R = H, –OMe, –OPh and
2
All procedures, including the generation of organic substrates
and kinetic experiments, were carried out under an argon
25
Me2
+
–OPhp-NO ).
The [MnIII(OPhp-NO )(S N4(tren))] complex,
2
2
3296 | Dalton Trans., 2015, 44, 3295–3306
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Dalton Transactions
Paper
atmosphere, unless otherwise stated. Acetonitrile, methanol analysis [MnIII(OMe)(dpaq)](OTf)·0.3H2O: C25H23.6F3MnN5O5.3
S
and ether were degassed and dried using a Pure Solv Micro calc. (%): C 48.21, H 3.82, N 11.24; found (%): C 47.78, H 3.38,
(2010) solvent purification system. These solvents were N 11.25.
degassed in air-tight solvent reservoirs (4 L) by bubbling Ar gas
through the solvent for 20 min at room temperature. Aceto-
nitrile and ether were dried using air-tight alumina columns,
Preparation of [MnIII(OMe)(dpaq)](OTf) from
[MnIII(OH)(dpaq)](OTf)
and methanol was dried using a drierite column. Anhydrous A 2.5 mM solution of [MnIII(OH)(dpaq)](OTf) (3.2 mg in 2 mL)
dichloromethane was purchased from Acros Organics (99.9% was prepared as previously described,29 and the solvent was
purity) and was degassed by four freeze–pump–thaw cycles removed under reduced pressure. The solid residue was stirred
using Schlenk techniques. All solvents were taken into an in MeOH for 10 min. The final solution color was deep red.
argon-filled glovebox immediately after dispensing from the The electronic absorption spectrum and ESI-MS of this solu-
solvent purification system, or following the freeze–pump– tion are identical to that of [MnIII(OMe)(dpaq)](OTf). This solu-
thaw cycles, and were stored in tightly-sealed Schlenk glass- tion was then evaporated to dryness under reduced pressure
ware. The purity of O2 gas used was >99% and was further pur- and recrystallized using MeOH–Et2O. The final product was
ified by passage through drierite and 5 Å molecular sieves isolated as a solid in excellent yield (>98%). The resulting
prior to use. The amide-containing N5 ligand H-dpaq, [MnII(d- [MnIII(OMe)(dpaq)](OTf) complex is quantitatively converted
paq)](OTf), and [MnIII(OH)(dpaq)](OTf) were synthesized back to [MnIII(OH)(dpaq)](OTf) (>98%) when dissolved in H2O,
according to previously reported methods.29,31 TEMPOH and as observed by ESI-MS experiments. Prominent ion peak in the
TEMPOD were also prepared according to literature pro- mass spectrum of the resultant aqueous solution corresponds
cedures,9,32 and >99% deuteration of TEMPOD was confirmed to m/z = 454.0902, as expected for [MnIII(OH)(dpaq)]+ (calc. m/z
1
by H NMR experiments. All phenols, with >99% purity, were = 454.1076; Fig. S1†).
purchased from commercial sources.
X-ray diffraction data collection and analysis for
Preparation of [MnIII(OMe)(dpaq)](OTf) from [MnII(dpaq)]-
(OTf) and dioxygen
[MnIII(OMe)(dpaq)](OTf)
Synthesis of [MnIII(OMe)(dpaq)](OTf) on a large scale to obtain
[MnIII(OMe)(dpaq)](OTf) was generated by reacting a 2.5 mM suitable material for X-ray crystallographic and kinetic experi-
[MnII(dpaq)](OTf) solution in MeOH with excess O2 gas at ments was carried out starting from [MnIII(OH)(dpaq)](OTf). In
room temperature. The formation of [MnIII(OMe)(dpaq)](OTf) this procedure, [MnIII(OH)(dpaq)](OTf) (20 mg in 5 mL) was
was monitored by electronic absorption spectroscopy, as it has dissolved in MeOH and was stirred for 10 min. The conversion
characteristic features at 510 and 760 nm. These features show of [MnIII(OH)(dpaq)](OTf) to [MnIII(OMe)(dpaq)](OTf) was
no significant shifts in terms of their energies when compared monitored by ESI-MS and electronic absorption spectroscopy.
to [MnIII(OH)(dpaq)](OTf) but are more defined for Upon complete conversion, the resulting solution was evapor-
[MnIII(OMe)(dpaq)](OTf) in methanol. In addition, a solvent- ated to dryness under reduced pressure, and the solid residue
dependent broadening of the absorption features of was repeatedly recrystallized by vapor diffusion using MeOH
[MnIII(OMe)(dpaq)](OTf) was observed for spectra collected in and Et2O under ambient conditions. Red racemically-twinned
MeCN. A representative formation reaction is as follows. A crystals of the partially hydrated salt, [Mn(OCH3)(C23H20N5O)]–
2.5 mM [MnII(dpaq)](OTf) solution (2.9 mg in 2 mL of MeOH) [CF3SO3]2, (1) are, at 100(2) K, orthorhombic, space group
was prepared under an inert atmosphere and transferred to a Pna21 – C2v (no. 33)33 with a = 13.6388(4) Å, b = 26.1547(7) Å,
9
gas-tight cuvette sealed with a pierceable septum. An excess of c = 15.1412(4) Å, V = 5401.1(3) Å3 and Z = 8 [Mn(OCH3)-
O2 gas was then delivered to the solution by means of a (C23H20N5O)][CF3SO3]·0.37H2O formula units (dcalcd = 1.533 g
syringe, and the formation of [MnIII(OMe)(dpaq)](OTf) was cm−3; μa(CuKα) = 5.306 mm−1). A full set of unique diffracted
monitored by electronic absorption spectroscopy. The for- intensities (5238 frames with counting times of 2 to 5 seconds
mation was complete in ∼4000 min and the resulting dark red and an ω- or ϕ-scan width of 0.50°) was measured34 for a
solution was evaporated to dryness under reduced pressure. single-domain specimen using monochromated CuKα radi-
The solid residue was then recrystallized using MeOH–Et2O ation (λ = 1.54178 Å) on a Bruker Proteum Single Crystal Diffr-
(3.0 mg/98% yield). [MnIII(OMe)(dpaq)](OTf) was further action System equipped with Helios multilayer optics, an APEX
characterized by ESI-MS and magnetic susceptibility. ESI-MS: II CCD detector and a Bruker MicroSTAR microfocus rotating
{[MnIII(OMe)(dpaq)]+} m/z = 468.1219 (calc. 468.1232; Fig. S1†). anode X-ray source operating at 45 kV and 60 mA. Lattice con-
The solution phase magnetic susceptibly found was 5.53μB (at stants were determined with the Bruker SAINT software
25 °C in CD3CN), which compares well with the expected μeff
=
package using peak centers for 9745 reflections. A total of
4.89μB for a monomeric high-spin d4 system.25 Cyclic voltam- 50 470 integrated reflection intensities having 2θ(CuKα) <
metry of [MnIII(OMe)(dpaq)](OTf) was recorded in MeCN 140.02° were produced using the Bruker program SAINT;35
(12.4 mg in 10 mL) at 25 °C under an argon atmosphere. A 0.1 8511 of these were unique and gave Rint = 0.025. The data were
M Bu4N(PF6) solution was used as the supporting electrolyte corrected empirically for variable absorption effects using
along with a glassy carbon working electrode, platinum auxili- equivalent reflections; the relative transmission factors ranged
ary electrode and a AgCl/Ag reference electrode. Elemental from 0.715 to 1.000. The Bruker software package SHELXTL
This journal is © The Royal Society of Chemistry 2015
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Dalton Transactions
was used to solve the structure using “direct methods” tech- (2 mL) within an argon-filled glovebox, transferred to a gas-
niques. All stages of weighted full-matrix least-squares refine- tight cuvette sealed with a pierceable septum. Solutions of
2
ment were conducted using Fo data with the SHELXTL phenolic substrate were prepared in dichloromethane (300 μL)
Version 2010.3-0 software package.36
and sealed in a 4 mL glass vial with a pierceable septum.
The asymmetric unit contains two cationic metal com- Then, the cuvette containing the [MnIII(OMe)(dpaq)]+ solution
plexes, two anions, and a water molecule of crystallization. A was inserted into a temperature-controlled cryostat (Unisoku),
substantial portion of the structure is disordered. The quino- held at 50 °C, coupled to an Agilent 8453 UV/Visible spectro-
line ring for the second metal complex is 56%/44% disordered photometer. Upon achieving thermal equilibrium (10 min),
with two slightly different orientations in the crystal. Mild data collection was started, and 100 μL of the substrate solu-
restraints were eventually applied to the anisotropic thermal tion was injected into the cuvette using a gas-tight syringe.
parameters for 13 of the disordered quinoline nonhydrogen Data collection times ranged from 300 to 6000 s. An aliquot of
atoms. The nonhydrogen bond lengths and angles for the final reaction mixture was analyzed by electron paramag-
both orientations were restrained to have values similar to netic resonance (EPR) spectroscopy at 5 K. The rates of the
those for the quinoline ring of the first metal complex. reactions were calculated by applying the initial rate approxi-
This disorder produces slightly elongated or flattened anisotropic mation in order to prevent any interference by side reactions at
thermal ellipsoids for atoms in these quinoline ligands. The longer time scales. These initial rates (in absorbance units/
second triflate anion is also disordered with two slightly time) were converted into s−1 units by using the extinction
different (58%/42%) orientations and the water solvent mole- coefficient (ε800 = 130 M−1 cm−1) and initial concentration
cule of crystallization is present only 74% of the time. Both (1.25 mM) of [MnIII(OMe)(dpaq)]+. For reactions involving the
orientations for the second (disordered) triflate anion were 2,4,6-tri-t-butylphenol, the kinetic data were collected to five
restrained to have bond lengths and angles similar to the first half-lives and fit directly to obtain a pseudo-first-order rate
(ordered) triflate.
constant. In these cases, the directly obtained pseudo-first-
The final structural model incorporated anisotropic order rate constants were identical, within error, to those
thermal parameters for all nonhydrogen atoms of both metal obtained using the initial rate method. The rates of reactions
complexes and the ordered triflate as well as the water oxygen involving 4-methoxy-2,6-di-t-butylphenol as the substrate were
atom and the sulfur atoms for both orientations of the dis- directly fit to obtain pseudo-first-order rate constants as they
ordered triflate. All other nonhydrogen atoms were incorpor- were significantly faster (data collection times 300–3000 s),
ated into the structural model with isotropic thermal and the data exhibited pseudo-first-order behavior up to at
parameters. The hydrogen atoms for the metal complexes were least five half-lives.
included in the structural model at idealized positions (sp2- or
sp3-hybridized geometry and C–H bond lengths of
0.95–0.99 Å). The methoxy methyl groups were incorporated in
Kinetic studies of [MnIII(OMe)(dpaq)](OTf) with TEMPOH(D)
the structural model as rigid groups (using idealized sp3-hybri- Similar to the kinetic experiments with phenols, a 1.25 mM
dized geometry and C–H bond lengths of 0.98 Å) with idealized [MnIII(OMe)(dpaq)]+ (1.5 mg, 2.5 × 10−3 mmol) solution was
“staggered” geometry. The hydrogen atoms were assigned prepared in acetonitrile (2 mL) within an argon-filled glovebox,
fixed isotropic thermal parameters with values 1.20 (non- and was sealed in a gas-tight cuvette with a pierceable septum.
methyl) and 1.50 (methyl) times the equivalent isotropic The TEMPOH (or TEMPOD) solution was prepared in
thermal parameter of the carbon atom to which they are acetonitrile (100 μL) and sealed in a 300 μL glass vial with a
bonded.
A total of 826 parameters were refined using 211 restraints, TEMPOH (or TEMPOD) solution was added into the
8511 data and weights of w = 1/[σ2(F2) + (0.1429 P)2 [MnIII(OMe)(dpaq)]+ solution using
gas-tight syringe.
pierceable septum. Data collection was started, and the
+
a
2
2
(13.9982 P)], where P = [Fo + 2Fc ]/3. Final agreement factors Data collection times ranged from 300 to 4000 s. Kinetic
at convergence for [Mn(OCH3)(C23H20N5O)][CF3SO3]·0.37 H2O experiments at variable temperatures (−15 to 50 °C) were per-
are: R1(unweighted, based on F) = 0.075 for 8385 independent formed following the same procedure as described, allowing
absorption-corrected “observed” reflections having 2θ(CuKα) < the cuvette to achieve thermal equilibrium (10 min) in the
140.02° and I > 2σ(I); R1(unweighted, based on F) = 0.076 and cryostat, prior to the addition of the substrate. In order to
wR2(weighted, based on F2) = 0.213 for all 8511 independent confirm that substrate isotopic scrambling (i.e., exchange of
absorption-corrected reflections having 2θ(CuKα) < 140.02°. deuterium in TEMPOD with hydrogen due to trace H2O)
The largest shift/s.u. was 0.004 in the final refinement cycle. does not affect the H/D KIE, kinetic experiments were also
The final difference map had maxima and minima of 1.18 and performed where TEMPOD was dissolved in 1 : 1 MeCN–
−0.77 e− Å−3, respectively.
MeOD. The rates of reactions with added MeOD were
identical, within error, to those without added MeOD
(6.50(7) × 10−3 s−1 and 6.45(7) × 10−3 s−1, respectively, for 100
equiv. of TEMPOD). All reactions involving TEMPOH (or
Kinetic studies of [MnIII(OMe)(dpaq)](OTf) with substituted
phenols
For each kinetic experiment, a 1.25 mM [MnIII(OMe)(dpaq)]+ TEMPOD) followed pseudo-first-order behavior up to at least
(1.5 mg, 2.5 × 10−3 mmol) solution was prepared in acetonitrile five half-lives.
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Results and analysis
Formation of [MnIII(OMe)(dpaq)](OTf) from dioxygen
When treated with excess O2 gas at 25 °C, the light orange
colored methanol solution of [MnII(dpaq)](OTf) slowly
changed its color to dark red, as new absorption features grew
in at 510 (ε = 328 M−1 cm−1) and 760 nm (ε = 130 M−1 cm−1
)
(Fig. 2; the extinction coefficients were obtained using recrys-
tallized [MnIII(OMe)(dpaq)](OTf)). These changes in the elec-
tronic absorption spectrum are consistent with the oxidation
of the MnII center upon reacting with O2. The final product of
the oxygenation reaction, [MnIII(OMe)(dpaq)](OTf), was iso-
lated in an essentially quantitative yield of greater than 98%.
The formation of [MnIII(OMe)(dpaq)](OTf) from the oxygen-
ation of a 2.5 mM [MnII(dpaq)](OTf) solution in methanol took
over 33 hours to approach completion (Fig. 2), which is signifi-
cantly longer than that of the same reaction in MeCN
(∼40 min; see ref. 29). A half-life (t1/2) of 26 days was estimated
for [MnIII(OMe)(dpaq)](OTf) by monitoring the electronic
absorption spectrum of a 1.25 mM acetonitrile solution at
25 °C. This is identical to the half-life observed previously for
[MnIII(OH)(dpaq)](OTf) under the same conditions.29 At 50 °C,
the same concentration of [MnIII(OMe)(dpaq)](OTf) in MeCN
has an appreciable self-decay rate of 2.5(2) × 10−5 s−1 at 50 °C,
which corresponds to a half-life of 7.5(2) hours. [MnIII(OH)-
(dpaq)](OTf) showed essentially no decay at 50 °C in MeCN.
Fig. 3 ORTEP (left) and space filling (right) diagrams of [MnIII(OMe)-
(dpaq)](OTf). ORTEP diagram shows 50% probability thermal ellipsoids.
Hydrogen atoms, non-coordinating triflate counteranions and the water
molecule have been removed for clarity. Significant interatomic dis-
tances and angles are listed in Table 1.
Table 1 Bond lengths and angles for one of the two [MnIII(OMe)-
(dpaq)]+ cations in the asymmetric unita
[MnIII(OMe)(dpaq)]+ (A)
Mn–O2
Mn–N2
Mn–N1
Mn–N3
Mn–N4
Mn–N5
1.825(4)
1.979(5)
2.051(5)
2.175(5)
2.203(6)
2.212(6)
O2–Mn–N2
N4–Mn–N5
N1–Mn–N3
N4–Mn–N2
N1–Mn–N2
N3–Mn–N2
177.9(3)
154.8(2)
161.7(2)
87.8(2)
80.0(2)
81.8(2)
a Corresponding metric parameters for the other cation are in
Table S2.
Structural and physical properties of [MnIII(OMe)(dpaq)](OTf)
The X-ray structure of [MnIII(OMe)(dpaq)](OTf) shows a six-
coordinate MnIII center with a distorted octahedral geometry
(Fig. 3), with the anionic amide ligand trans to the methoxide
group. There are two molecules of [MnIII(OMe)(dpaq)](OTf)
within the asymmetric unit with very similar metric para-
meters; the metric parameters for the two cationic units are in
Table 1 and S2.† The structure of [MnIII(OMe)(dpaq)](OTf) is
virtually identical to that of the previously reported [MnIII(OH)-
(dpaq)](OTf) complex.29 Most Mn–N distances are statistically
indistinguishable from those of [MnIII(OH)(dpaq)](OTf), with
the exception of the Mn–N4 distance, which is shorter in
[MnIII(OMe)(dpaq)](OTf) (2.203(6) and 2.154(7) Å in the two
cations) than in [MnIII(OH)(dpaq)](OTf) (2.260(14) Å). The
methoxide group in [MnIII(OMe)(dpaq)](OTf) has Mn–O2 dis-
tances of 1.825(4) and 1.814(5) Å (Tables 1 and S2†), comparable
to those of other MnIII–OMe distances reported in the literature
(1.836(5) and 1.814(7)).25,37 Two triflate ions and a water mole-
cule are observed within the asymmetric unit as well but are not
associated with the Mn centers (Mn–O(triflate) distances of 6.397
and 6.936 Å, and Mn–O(water) distances of 4.760 and 8.604 Å).
Cyclic voltammetric studies of [MnIII(OMe)(dpaq)]+ were
performed in MeCN, and revealed an irreversible MnIII/MnII
couple at a cathodic peak potential (Ep,c) of −0.88 V vs. Fc+/Fc
at a scan rate of 100 mV s−1 at 25 °C (Fig. S2†). Although it is
unwise to make strong comparisons of this irreversible
reduction wave with the reversible or partially-reversible pro-
cesses observed for the MnIII/MnII couple of other mid-valent
oxidants, including [MnIII(OH)(dpaq)]+, it is interesting that
the Ep,c of [MnIII(OMe)(dpaq)]+ is 0.22 V lower than the corres-
ponding Ep,c of [MnIII(OH)(dpaq)]+. The smaller reduction
potential observed for [MnIII(OMe)(dpaq)]+ compared to
[MnIII(OH)(dpaq)]+ is in contrast with the trend observed for
several other MIII–OR oxidants,25,27,28 but is consistent with
the milder oxidizing ability of [MnIII(OMe)(dpaq)]+ (vide infra).
Fig. 2 Electronic absorption spectra of 2.5 mM [MnII(dpaq)]+ (solid
green trace) upon the addition of excess O2 at 25 °C in MeOH under
argon. Inset: Time evolution of absorption signals at 510 and 760 nm
from data collected every 5 min.
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In addition, in an electrochemical study of MnIII–OR com-
plexes supported by
a
phenolate-containing N4O− ligand
reported by Anxolabéhère-Mallart and co-workers, a slightly
lower irreversible reduction potential was observed for the MnIII–
OMe complex compared to the analogous MnIII–OH species.37
Separate frozen samples of 2 mM [MnIII(OMe)(dpaq)]+ in
MeCN and 1 : 1 ethanol–methanol (the latter of which is a glas-
sing solvent) were analyzed by EPR spectroscopy at 5 K.
Neither sample showed features in either perpendicular or par-
allel modes. This is consistent with a monomeric MnIII
complex with a moderate to large zero-field splitting.
Reactivity of [MnIII(OMe)(dpaq)]+ with TEMPOH
The PCET reactivity of [MnIII(OMe)(dpaq)]+ was investigated
using TEMPOH as the substrate to permit comparison with
other MnIII–OR oxidants.25,29 Treatment of [MnIII(OMe)-
(dpaq)]+ with 100 equiv. of TEMPOH under an argon atmos-
phere in MeCN at 25 °C lead to the rapid disappearance of the
electronic absorption bands of [MnIII(OMe)(dpaq)]+ (Fig. 4).
The final electronic absorption spectrum appears essentially
identical to that of [MnII(dpaq)](OTf) dissolved in MeCN,
showing a very weak band at 510 nm. Using the intensity and
the extinction coefficient of this band, the final yield of
[MnII(dpaq)]+ was calculated to be over 99%. The X-band EPR
spectrum (perpendicular-mode) of the final reaction mixture
at 5 K shows an intense feature centered at g = 2.04 that is
characteristic of the TEMPO radical (Fig. S3†). Together these
data demonstrate that the reaction between TEMPOH and
[MnIII(OMe)(dpaq)]+ approaches completion under these con-
ditions, generating TEMPO radical and [MnII(MeOH)(dpaq)]+.
The reaction of [MnIII(OMe)(dpaq)]+ and TEMPOH was
studied over a range of TEMPOH concentrations (10–200
equiv. with respect to initial [MnIII(OMe)(dpaq)]+ concen-
tration), and the pseudo-first-order rate constants were
Fig. 5 Pseudo-first order rate constants, kobs (s−1) versus TEMPOH and
TEMPOD concentration for a 1.25 mM solution of [MnIII(OMe)(dpaq)]+ at
25 °C. The second-order rate constant, k2 (M−1 s−1), was calculated from
the linear correlation of the observed rate constant and substrate
concentration.
observed to increase linearly with the concentration of
TEMPOH (Fig. 5). In all cases, the disappearance of
[MnIII(OMe)(dpaq)]+ showed pseudo-first-order behavior up to
at least five half-lives. A second-order rate constant of 8.0(1) ×
10−2 M−1 s−1 was calculated based on these data, which is
slightly lower than the rate observed for TEMPOH oxidation by
[MnIII(OH)(dpaq)]+ (1.3(1) × 10−1 M−1 s−1). Using the deuter-
ated substrate, TEMPOD, a KIE of 1.8 was calculated for this
PCET process, which suggests a rate-limiting O–H/D bond
breaking step. Given the exceptionally high pKa of TEMPOH in
MeCN (pKa = 41),7 and the highly negative reduction potential
of [MnIII(OMe)(dpaq)]+, these results are consistent with a rate-
limiting CPET reaction. An Eyring analysis of the reaction
between [MnIII(OMe)(dpaq)]+ and TEMPOH from −15 to 50 °C
(258 to 323 K) yielded activation parameters ΔH≠ and ΔS≠ of
11.4(5) kcal mol−1 and −27(2) cal mol−1 K, respectively (Fig. 6).
Reactivity of [MnIII(OMe)(dpaq)]+ with phenols
[MnIII(OMe)(dpaq)]+ also reacted with 2,4,6-tri-t-butylphenol
(
4-t-butylArOH) in MeCN at 50 °C. The formation of the corres-
ponding phenoxyl radical (λmax = 628 nm; ε = 400(10) M−1
cm−1 in MeCN38 at 25 °C; see Fig. 7) and the decay of
[MnIII(OMe)(dpaq)]+ were concomitant, which suggests the
absence of secondary reactions involving [MnIII(OMe)(dpaq)]+
and the radical. The phenoxyl radical was formed in ∼75%
yield relative to the initial [MnIII(OMe)(dpaq)]+ concentration,
a yield similar to that previously reported for the reaction of
4-t-butylArOH with [MnIII(OH)(dpaq)]+.29 The perpendicular-
mode X-band EPR spectrum of the final products of
4-t-butylArOH oxidation provide evidence for the formation of
the phenoxyl radical (Fig. S3†). The ∼75% yield of the phenoxyl
radical was observed regardless of the initial [MnIII(OMe)-
(dpaq)]+: 4-t-butylArOH ratio (Table S3†).
Fig. 4 Electronic absorption spectra of 1.25 mM [MnIII(OMe)(dpaq)]+ upon
the addition of 100 equiv. TEMPOH at 25 °C in MeCN under argon. Inset:
Decay of the 550 nm absorption signal from data collected every 0.3 s. The
electronic absorption bands of [MnIII(OMe)(dpaq)]+ are broadened in
MeCN as compared to data collected for a sample in MeOH (see Fig. 2).
When [MnIII(OMe)(dpaq)]+ was treated with an excess of
4-t-butylArOH (10–125 equiv.) in MeCN, the decay of
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Fig. 8 Observed pseudo-first-order rate constants (kobs) as a function
of 4-t-butylArOH concentration for [MnIII(OMe)(dpaq)]+.
Fig. 6 Eyring plot showing ln(k/T) versus 1/T (K−1) for the reaction of
1.25 mM [MnIII(OMe)(dpaq)]+ with TEMPOH from −15 to 50 °C (258 to
323 K).
[
4-t-butylArOH] as well, with a second order rate constant of
3.7(1) × 10−3 M−1 s−1 (Fig. S4†). The observation of similar
rates in both protic and non-protic solvents for oxidation of
the same substrate suggests the absence of charged intermedi-
ates during the course of the reaction.27 Thus, the similar rates
in MeOH and MeCN are supportive of a single step CPET
process rather than a stepwise PCET, where the proton and
electron transfer occur in separate kinetic steps.
To further investigate the mechanism of phenol oxidation,
we explored the reactivity of [MnIII(OMe)(dpaq)]+ with other
phenolic substrates, including 4-methoxy-2,6-di-t-butylphenol
(
4-MeOArOH), 4-methyl-2,6-di-t-butylphenol (4-MeArOH), 2,6-di-t-
butylphenol
4-CNArOH). A linear dependence between the oxidation rate
(
4-HArOH), and 4-cyano-2,6-di-t-butylphenol
(
and the O–H bond strength would provide stronger evidence
for a CPET process for phenolic O–H oxidation by [MnIII(OMe)-
(dpaq)]+. With the exception of 4-CNArOH, all of the phenols
reacted with [MnIII(OMe)(dpaq)]+ to give phenoxyl radicals and
MnII species, and in each case, a linear dependence between
the rate of the reaction and the substrate concentration was
observed (Fig. S5†). The second order rate constants calculated
from these kinetic data are collected in Table 2. In the reaction
Fig. 7 Electronic absorption spectra of 1.25 mM [MnIII(OMe)(dpaq)]+
upon the addition of 100 equiv. 2,4,6-tri-t-butylphenol (4-t-butylArOH) at
50 °C in MeCN under argon. Inset: Time evolution of single-wavelength
traces at 628, 550, and 800 nm.
[MnIII(OMe)(dpaq)]+ and the formation of 4-t-butylArO• followed
pseudo-first-order kinetics up to at least three half-lives.
[MnIII(OMe)-(dpaq)]+ exhibited linear dependence of the rate
of the reaction (kobs) on phenol concentration up to 0.15 M
4-t-butylArOH (Fig. 8). The second order rate constant found
from these data is 5.2(1) × 10−3 M−1 s−1. The y-intercept of the
linear fit of kobs versus [4-t-butylArOH] corresponds to a self-
decay rate of 3.3(1) × 10−5 s−1, which is in good agreement
with the observed self-decay rate of [MnIII(OMe)(dpaq)]+ of
2.5 × 10−5 s−1 in MeCN at 50 °C (vide supra).
The lack of rate saturation at high phenol concentrations is
suggestive of a single-step reaction with no accumulating inter-
mediates from a pre-equilibrium step. In addition, oxidation
of 4-t-butylArOH by [MnIII(OMe)(dpaq)]+ in MeOH, instead of
MeCN, at 50 °C shows a linear correlation between kobs and
Table 2 Rate constants from kinetic experiments between [MnIII(OMe)-
(dpaq)]+ and O–H bond substrates in MeCN, and substrate BDFEO–H
values
a
Substrate
k2 (M−1 s−1
)
BDFEO–H
TEMPOH
8.0(1) × 10−2
1.1(1)
66.5(1)
73.9(1)
76.0(1)
77.1(1)
77.1(1)
78.5(1)
79.0(1)
4-MeOArOH
4-MeArOH
8.2(1) × 10−3
5.2(1) × 10−3
3.7(1) × 10−3
4-t-butylArOH
4-t-butylArOH (MeOH)b
4-HArOH
1.4(1) × 10−3
4-CNArOH
c
a kcal mol−1; from Warren et al. (ref. 7) or determined following
Waidmann et al. (ref. 14). b MeOH was used as the solvent for this
reaction. c The reaction of [MnIII(OMe)(dpaq)]+ with 4-CNArOH did not
go to completion.
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mid-valent Mn oxidants.41–45 Specifically, the active-site of Mn-
lipoxygenase is presumed to contain a MnIII–OH unit that
initiates fatty acid oxidation by a PCET step.27,45,46 Support for
this proposal comes from the H/D KIE of ∼20 at 50 °C.46
Notably, this KIE is significantly smaller than that of Fe-lipoxy-
genase (∼56 at 30 °C),47,48 and, unlike that of Fe-lipoxygenase,
the Mn-lipoxygenase H/D KIE is temperature-dependent,
increasing to ∼38 at 8 °C.46 The basis for the different KIE
behaviors of Fe- and Mn-lipoxygenases is currently unknown.
In light of this and other biological examples, there have been
several investigations of model complexes containing MIII–OR
units (M = Mn or Fe).25–29 Only a few of the synthetic MIII–OR
complexes are known to mediate PCET reactions, but these
studies have revealed interesting differences between
mid-valent MIII–OR complexes and high-valent MIV/VvO
adducts.25–29,49 Most obviously, high-valent oxidants are
capable of activating significantly stronger substrate bonds (up
to 100 kcal mol−1) than mid-valent oxidants (up to 80 kcal
mol−1). Although substrate oxidations of high-valent oxidants
have been the subject of numerous detailed studies, under-
standing of PCET reactivity mediated by low-valent oxidants is
still in its infancy.49
This present work describes the formation and PCET reac-
tivity of the new MnIII–OMe complex, [MnIII(OMe)(dpaq)]+.
This complex is formed in quantitative yields by the reaction
of [MnII(dpaq)]+ with dioxygen in methanol. Previously
reported [MnII(dpaq)]+ represents a rare example of a mono-
meric MnII complex that can activate dioxygen in solution
under ambient conditions to form a single product.29 The
mechanism of O2 activation by [MnII(dpaq)]+ is currently
under investigation. Although synthetic MnII complexes that
can activate dioxygen are unusual, the presence of an amido
ligand, as in [MnII(dpaq)]+, is known to significantly alter the
electronic properties of the metal center.50–55 [MnIII(OMe)-
(dpaq)]+ is remarkably stable in solution with a half-life of ∼26
days in MeCN at 25 °C. In addition, a distinct property of this
system is that [MnIII(OMe)(dpaq)]+ can be stoichiometrically
generated by dissolving the previously reported [MnIII(OH)-
(dpaq)]+ complex in MeOH, and this reaction can be comple-
tely reversed when [MnIII(OMe)(dpaq)]+ is placed in water
(Fig. S1†).
Fig. 9 Evans-Polanyi plot of log of PCET rate (k2) versus the BDFE of
the substrate.
of [MnIII(OMe)(dpaq)]+ with 4-CNArOH (BDFE = 79 kcal mol−1),
the reaction did not approach completion, implying that the
thermodynamic limit of the O–H bond oxidation by
[MnIII(OMe)(dpaq)]+ lies around 79 kcal mol−1
.
An Evans-Polanyi plot generated using second-order rate
constants of O–H bond oxidation reactions mediated by
[MnIII(OMe)(dpaq)]+ is shown in Fig. 9. This set of substrates
comprise a reasonable range of substrate BDFEs (66–78.5 kcal
mol−1 in MeCN),39 and a linear dependence between the log of
reaction rate versus BDFE of the substrate is observed with a
slope of −0.2. In this analysis, the oxidation of 4-MeOArOH dis-
plays a much faster rate than expected on the basis of its bond
strength, and is a clear outlier (Fig. 9). Importantly, the Polanyi
slope obtained by excluding the data point for 4-MeOArOH
is still −0.2; thus, the deviation in the oxidation rate of
4-MeOArOH does not affect the Polanyi slope. The slope seen
for this series is smaller than expected from Marcus theory for
a reaction with a small thermodynamic driving force (−0.5).
Such small slopes have been seen for most mid-valent Mn and
Fe oxidants (vide infra).26–29
The C–H bond oxidation capability of [MnIII(OMe)(dpaq)]-
(OTf) was also investigated using xanthene, which has a C–H
BDFE of 73.3 kcal mol−1 in dimethylsulfoxide. Treatment of a
MeCN solution of [MnIII(OMe)(dpaq)](OTf) with xanthene at
50 °C led to an observed decay rate of the MnIII–OMe complex
indistinguishable to the self-decay rate at this temperature
(Fig. S6†).40 Thus, unlike the corresponding MnIII–OH
complex,29 [MnIII(OMe)(dpaq)]+ does not appear to be capable
of xanthene oxidation.
PCET mechanism of [MnIII(OMe)(dpaq)]+
[MnIII(OMe)(dpaq)]+ is capable of oxidizing substrate O–H
bonds of BDFEs less than 79 kcal mol−1. To date, this is the
only MnIII–OMe species reported to activate phenolic O–H
bonds. Using this complex, we have performed the oxidation
of a series of para-substitued-2,6-di-tert-butylphenols (4-RArOH)
and TEMPOH (Table 2), which encompasses a reasonable
range of substrate O–H BDFEs (66.5–78.5 kcal mol−1). The
Evans-Polanyi plot for O–H bond oxidation of these substrates
by [MnIII(OMe)(dpaq)]+ shows a linear dependence of the log
of O–H bond oxidation rates on the BDFE of the substrate O–H
Discussion
Mn-dependent enzymes catalyze a range of biological pro- bond (Fig. 9). This is suggestive of a CPET mechanism for O–H
cesses, during which PCET reactions play a vital role.1 Active oxidation by [MnIII(OMe)(dpaq)]+. Furthermore, [MnIII(OMe)-
sites in a number of these enzymes are known to contain (dpaq)]+ reacts with TEMPOH, a substrate that is most likely to
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Paper
undergo CPET due to its remarkably high pKa (41 in MeCN) exhibited a E° of −0.45 V (vs. Fc+/Fc), but the pKa of the MnIII–
and reduction potential (+0.71 V vs. Fc+/Fc in MeCN).7 The O(H)Me form could not be precisely defined (a lower limit of
H/D KIE observed for this oxidation is 1.8, which is suggestive 16.2 was estimated). Based on these values, the MII–OH2
of a rate-limiting CPET. In addition, we carried out the oxi- complex has a stronger O–H bond by at most 5.6 kcal mol−1
dation of 4-t-butylArOH in both MeCN and MeOH, and these compared to the MII–O(H)Me species (Table 3).25 As a conse-
+
reactions proceeded with similar second-order rates of 5.2(1) × quence of these weak BDFEs, [MnIII(OH)(SMe N4(tren))] and
2
+
10−3 M−1 s−1 and 3.7(1) × 10−3 M−1 s−1 respectively (Fig. S4†). [MnIII(OMe)(SMe N4(tren))] were only able to oxidize TEMPOH.
2
Such similar rates in both protic and non-protic solvents have In contrast, Stack and Goldsmith reported a BDFE of 82 kcal
been previously attributed to the absence of charged inter- mol−1 in MeCN for the [MnII(OH2)(PY5)]2+ complex, and, as a
mediates during the reaction, which suggests that sequential result, the [MnIII(OH)(PY5)]2+ oxidant was capable of oxidizing
proton- and electron-transfer steps do not occur.27 Finally, moderately strong C–H bonds, including that of toluene (C–H
given the low Ep,c for [MnIII(OMe)(dpaq)]+ (−0.88 V vs. Fc+/Fc BDFE of 87 kcal mol−1 in MeCN),7 with a second-order rate
in MeCN at 25 °C) and the potential for the PhO•/PhO– couple constant of 2.2(5) × 10−3 M−1 s−1 26
of 4-t-butylArOH (−0.70 V vs. Fc+/Fc in MeCN at 25 °C),7
.
Therefore, the current
a
examples of MnIII–OH complexes reveal that, even with the
stepwise PCET involving initial electron transfer between same metal and –OR ligand, the BDFE of the metal oxidant
[MnIII(OMe)(dpaq)]+ and 4-t-butylArOH is energetically up-hill by can vary considerably (17 kcal mol−1) as a function of the sup-
at least 0.18 V. Taken together, these cumulative observations porting ligand. Although the BDFEs are not known for the
strongly support a CPET process for [MnIII(OMe)(dpaq)]+- reduced, protonated forms of [MnIII(OMe)(dpaq)]+ and
mediated O–H bond oxidation.
[MnIII(OH)(dpaq)]+, these complexes show reactivity intermedi-
The thermodynamic driving force for these CPET processes ate between the SMe N4(tren)- and PY5-supported systems. That
2
is defined as the difference in O–H BDFE of the reduced-proto- is, unlike the [MnIII(OR)(SMe N4(tren))] complexes, the
+
2
nated form of the oxidant (MII–O(H)R in Scheme 1; M = Mn or [MnIII(OR)(dpaq)]+ oxidants can oxidize phenolic O–H bonds,
Fe) and that of the substrate being oxidized. When both the but cannot oxidize the stronger C–H bonds that the
pKa of the MII–O(H)R species and E° of the MIII/MII couple are [MnIII(OH)(PY5)]+ complex readily attacks.
known, the BDFE of the metal oxidant can be calculated using
Rates and activation parameters for O–H bond oxidation by
the Bordwell relationship (vide supra). Although we were able
to obtain an Ep,c of −0.88 V (vs. Fc+/Fc) for the MnIII/MnII
mid-valent Mn oxidants
couple of [MnIII(OMe)(dpaq)]+ (Fig. S2†), all attempts in deter-
TEMPOH is commonly used as a weak O–H bond substrate
mining the pKa of the MIII–O(H)R form of this oxidant were
unsuccessful.56 We have encountered similar difficulties in
determining the pKa for [MnII(OH2)(dpaq)]+. Consequently, the
BDFEs of [MnII(MeOH)(dpaq)]+ and [MnII(OH2)(dpaq)]+ are not
known.
(BDFE = 66.5 kcal mol−1 in MeCN) in metal-mediated PCET
processes and provides a good point of comparison for the
reactivity of MnIII–OR oxidants (Table 3). [MnIII(OMe)(dpaq)]+
oxidizes TEMPOH at a second-order rate of 8.0 × 10−2 M−1 s−1
in MeCN at 25 °C, which is very similar to that of [MnIII(OH)-
(dpaq)]+ (1.3 × 10−1 M−1 −1). The enthalpy of activation (ΔH≠)
s
In contrast, the thermodynamic driving forces for PCET
reactions by other MnIII–OR adducts have been determined
for TEMPOH oxidation by [MnIII(OMe)(dpaq)]+ is 11.4(5) kcal
mol−1, marginally higher than that observed for [MnIII(OH)-
(dpaq)]+ (9.9(9) kcal mol−1). In contrast, [MnIII(OMe)(dpaq)]+
shows a smaller entropic barrier to TEMPOH oxidation when
compared to [MnIII(OH)(dpaq)]+ (TΔS≠ = 8.0(6) and 10.4(9)
kcal mol−1 at 25 °C, respectively). As a net result of these
changes in ΔH≠ and TΔS≠, [MnIII(OMe)(dpaq)]+ and
[MnIII(OH)(dpaq)]+ have similar free energies of activation
using experimental BDFEs of the MII–O(H)R species.25–28 For
+
example, the BDFE of [MnII(OH2)(SMe N4(tren))] was deter-
2
mined using a modified Bordwell relationship,25 and was
found to be 70.1 kcal mol−1 in MeCN (74.0 kcal mol−1 in H2O).
In that study, [MnIII(OH)(SMe N4(tren))] exhibited a E° of
+
2
−0.60 V (vs. Fc+/Fc), and the corresponding MnII–OH2 species
had a pKa of 21.2 (Table 3). The related MnIII–OMe complex
Table 3 Thermodynamic and Kinetic Parameters Related to PCET reactivity of MnIII–OH and MnIII–OMe Oxidants with TEMPOH in MeCN at 25 °C
b
+ b
+
[MnIII(OH)(dpaq)]+ a
[MnIII(OMe)(dpaq)]+
[MnIII(OH)(SMe N4(tren))]
[MnIII(OMe)(SMe N4(tren))]
2
2
k2 (M−1 s−1
)
1.3 × 10−1
1.25
8.0 × 10−2
1.25
2.1 × 103
0.5
3.6 × 102
0.5
conc. (mM)
E (MIII/MII)
pKa MII-O(H)R
BDFE MII-O(H)Rf,g
ΔH≠ g
−0.6d
NAc
−0.88e
NAc
−0.6d
21.2
−0.45d
≥16.2
≥64.4
8.3
NAc
9.9(9)
−35(3)
20.3(1.8)
NAc
11.4(5)
−27(2)
19.5(1.1)
70.1
8.2
−25.5
15.8
ΔS≠ h
−29
16.9
ΔG≠ g,i
a Data taken from ref. 29. b Data taken from ref. 25. c Not available. d Reduction potential (E1/2) vs. Fc+/Fc in volts at 25 °C in MeCN. e Cathodic
peak potential (Ep,c) vs. Fc+/Fc in volts at 25 °C in MeCN. f In MeCN. g In kcal mol−1 h In cal mol−1 K. i At 25 °C.
.
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(ΔG≠) for TEMPOH oxidation at 25 °C (19.5(1.1) and 20.3(1.8)
kcal mol−1 respectively). This is consistent with the very
similar second-order rate constants at this temperature
(Table 3).
Table 4 Comparison of H/D KIEs and Polanyi slopes for O–H activation
by mid-valent vs. high-valent oxidants
Polanyi H/D
T
(°C)
Oxidant
Solvent slope
KIE
On the other hand, the ΔG≠ (25 °C) for TEMPOH oxidation
+
+
by [MnIII(OMe)(SMe N4(tren))] and [MnIII(OH)(SMe N4(tren))]
Mid-valent oxidants
[MnIII(OMe)(dpaq)]+
2
2
MeCN −0.2
MeCN −0.07
MeCN NRd
MeCN NRd
MeCN NRd
1.8a
25
were found to be 16.9 and 15.8 kcal mol−1 respectively, sub-
stantially lower than those of the [MnIII(OR)(dpaq)]+ com-
plexes. This is in line with the 104-greater rate of TEMPOH
[MnIII(OH)(dpaq)]+,29
1.8a, 1.4b 25, 50
[MnIII(OMe)(SMe N4(tren))] ,
2.1a
3.1a
2.0b
25
25
25
+ 25
2
[MnIII(OH)(SMe N4(tren))] ,
+ 25
2
[FeIII(OMe)(PY5)]+,27
oxidation by the SMe N4(tren)-supported complexes (Table 3).
2
High-valent oxidants
The 1.1 kcal mol−1 difference between the ΔG≠ values of
[(TBP8Cz)MnV(O)],23
DCM
DCM
−0.39
5.9b
23
−70
25
25
25
−70
[MnIII(OMe)(SMe N4(tren))]
and [MnIII(OH)(SMe N4(tren))]
+
+
2
2
[MnIV(salen)(OH)],57
−0.59e 9.5c
[FeIV(O)(TMC)(N3)]−,20
[FeIV(O)(TMC)(OOCCF3)]−,20
[FeIV(O)(TMC)(NCCH3)]−,20
MeCN −0.36
MeCN −0.42
MeCN −0.55
NRd
NRd
NRd
2.0b
results in the MnIII–OMe complex having a rate of TEMPOH
oxidation an order of magnitude slower than that of the MnIII–
OH complex (3.6 × 102 and 2.1 × 103 M−1 −1, respectively).25
s
[FeIV(β-BPMCN)(OOtBu)(OH)]2+ 59
,
PrCN
−0.4e
Notably, the enthalpic (ΔH≠) contribution to the ΔG≠ values of
a H/D KIE was studied using TEMPOH. b H/D KIE was studied using
4-t-butylArOH. c H/D KIE was studied using 4-HArOH as the O–H bond
substrate. d Not reported. e Polanyi slopes were not reported; we
calculated the slope using the reported kinetic data in ref. 57 and 59.
[MnIII(OMe)(SMe N4(tren))] and [MnIII(OH)(SMe N4(tren))] are
+
+
2
2
virtually identical (Table 3). Thus, the more sluggish reactivity
+
of [MnIII(OMe)(SMe N4(tren))] is due to the larger entropy of
2
activation (Table 3). However, the smaller ΔG≠ for TEMPOH
oxidation by [MnIII(OR)(SMe N4(tren))] complexes in compari-
+
2
Moreover, according to Marcus theory, a Polanyi slope of
−0.5 is expected for a process with a small thermodynamic
driving force and a transition state position mid-way between
reactants and products.60 The Polanyi slopes observed for O–H
oxidation reactions of mid-valent oxidants range between
−0.0729 and −0.2, which is significantly smaller than the
expected value. Albeit, at present, only a limited number of
data sets are available.61 This is in contrast with high-valent
oxidants that oxidize substrate O–H bonds with a Polanyi slope
of −0.4 to −0.6.20,23,57–59 Each Polanyi study listed in Table 4 is
based on para-substitued-2,6-di-tert-butylphenols as O–H sub-
strates. Irrespective of the identity of the metal (Mn or Fe),
high-valent oxidants show slopes closer to the expected value
of −0.5. Generally, a Polanyi slope of <0.5 indicates an early
transition state that resembles the reactants,60 with a rate less
sensitive to the driving force of the reaction. It is an open ques-
tion therefore, whether the lower sensitivity of PCET rates to
the driving force of the reaction is a characteristic property of
these mid-valent oxidants, and if they go through an early tran-
sition state regardless of the identity of the substrate. The
small KIE values observed for these oxidants could also be a
consequence of this phenomenon.
son with [MnIII(OR)(dpaq)]+ oxidants is intriguing, given that it
solely reflects differences between the dpaq and SMe N4(tren)
2
supporting ligands (Fig. 1).
When comparing the phenol oxidation behavior of
[MnIII(OMe)(dpaq)]+ and [MnIII(OH)(dpaq)]+, the most signifi-
cant difference is that the former complex lacks the rate satur-
ation behavior at high phenol concentrations.29 The saturation
behavior of [MnIII(OH)(dpaq)]+ was proposed to be due to the
equilibrium formation of a precursor complex prior to the
rate-determining CPET step. This precursor complex was pro-
posed to be a hydrogen-bonded adduct between the phenolic
substrate and the MnIII–OH unit of [MnIII(OH)(dpaq)]+. The
formation of
a similar adduct might be disfavored for
[MnIII(OMe)(dpaq)]+ due to steric clash between the methyl
moiety and the bulky phenol substrates. For most of the
phenols, the Keq for precursor complex formation with
[MnIII(OH)(dpaq)]+ was ∼20, which corresponds to a small free
energy of reaction of ∼2 kcal mol−1. Thus, a subtle steric or
electronic effect in the [MnIII(OMe)(dpaq)]+ system could be
expected to impact the equilibrium position such that the pre-
equilibrium step is no longer observed.
KIEs and Polanyi slopes of mid-valent oxidants
In comparison with high-valent oxidants, the H/D KIEs and
the Polanyi slopes for O–H bond oxidations by mid-valent oxi-
Conclusions and outlook
dants display notable differences, as summarized in Table 4. Designing metal oxidants with well-tuned properties for rapid,
The KIEs for O–H bond oxidation reactions of mid-valent selective PCET is a desirable goal in synthetic chemistry, as
M–OR oxidants are smaller (∼2–3)25,29 than those of high- such oxidants could be of significant benefit in industry, as
valent oxo and hydroxo oxidants (∼5–10),23,57,58 with the excep- well as in biomimetic studies. Among other metal oxidants,
tion of the [FeIV(β-BPMCN)(OOtBu)(OH)]2+ oxidant.59 For mid-valent M–OR adducts exhibit the potential for good tun-
example, the H/D KIE observed for 4-t-butylArOH oxidation by ability in their PCET reactivities, as both E° and pKa can be
[MnIII(OH)(dpaq)]+ is remarkably small (1.4),29 while the high- controlled by changing the R group in the M–OR unit, or
valent MnVuO complex, [(TBP8Cz)MnV(O)], has been shown to through modification of the supporting ligand. For example,
carry out the same substrate oxidation at a similar temperature the variation in MnIII/MnII potentials with respect to the pro-
with a larger KIE of 5.9.23
perties of the supporting ligands is observed in the series of
3304 | Dalton Trans., 2015, 44, 3295–3306
This journal is © The Royal Society of Chemistry 2015

Dalton Transactions
Paper
MnIII–OH complexes supported by neutral PY5 (+0.17 V vs. Fc+/
8 M. H. V. Huynh and T. J. Meyer, Chem. Rev., 2007, 107,
5004–5064.
9 E. A. Mader, E. R. Davidson and J. M. Mayer, J. Am. Chem.
Soc., 2007, 129, 5153–5166.
+
Fc), monoanionic SMe N4(tren) and dpaq (−0.60 V vs. Fc /Fc for
2
both), and trianionic H313− (H313− = [[tris(N′-tert-butylurea-
ylato)-N-ethyl]aminato]) ligands (−1.51 V vs. Fc+/Fc).25,26,62
Also, the pKa of the oxidant can be altered utilizing supporting 10 E. A. Mader, V. W. Manner, T. F. Markle, A. Wu,
ligands and –OR ligand properties, as seen in the [MnIII(OR)-
J. A. Franz and J. M. Mayer, J. Am. Chem. Soc., 2009, 131,
4335–4345.
+
(SMe N4(tren))] series of complexes (Table 3).25 As a result of
2
these differences, a range of O–H BDFEs is observed for MII– 11 E. A. Mader and J. M. Mayer, Inorg. Chem., 2010, 49, 3685–
OH2 species derived from MIII–OH oxidants (17 kcal mol−1),
along with large differences in substrate oxidation rates (104- 12 V. W. Manner and J. M. Mayer, J. Am. Chem. Soc., 2009, 131,
fold for TEMPOH as substrate; see Table 3). Additional work 9874–9875.
3687.
on mid-valent oxidants is warranted to gain further insight 13 J. M. Mayer, Acc. Chem. Res., 2010, 44, 36–46.
into the specific geometric and electronic factors governing 14 C. R. Waidmann, X. Zhou, E. A. Tsai, W. Kaminsky,
their PCET reactivities.
D. A. Hrovat, W. T. Borden and J. M. Mayer, J. Am. Chem.
Soc., 2009, 131, 4729–4743.
15 (a) J. J. Warren and J. M. Mayer, J. Am. Chem. Soc., 2008,
130, 2774–2776; (b) J. J. Warren and J. M. Mayer, J. Am.
Chem. Soc., 2011, 133, 8544–8551.
Acknowledgements
16 D. R. Weinberg, C. J. Gagliardi, J. F. Hull, C. F. Murphy,
C. A. Kent, B. C. Westlake, A. Paul, D. H. Ess,
D. G. McCafferty and T. J. Meyer, Chem. Rev., 2012, 112,
4016–4093, and references therein.
17 J. J. Warren, A. R. Menzeleev, J. S. Kretchmer, T. F. Miller,
H. B. Gray and J. M. Mayer, J. Phys. Chem. Lett., 2013, 4,
519–523.
This work was supported by the US National Science Foun-
dation (CHE-1056470 to T.A.J.). The US NSF is also acknowl-
edged for funds used for the purchase of X-ray instruments
(CHE-0079282) and the EPR spectrometer (CHE-0946883).
18 J. P. Layfield and S. Hammes-Schiffer, Chem. Rev., 2013,
114, 3466–3494.
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solvent mixture used in the phenol oxidation experiments.
TEMPOD oxidation in the mixed solvent resulted in essen-
tially identical oxidation rates as observed in pure MeCN
(6.53(4) × 10−3 s−1 and 6.45(7) × 10−3 s−1, respectively, for
100 equiv. of TEMPOD).
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in MeCN. Unfortunately, [MnIII(OMe)(dpaq)]+ reacts rapidly
with acids to generate a MnII species, as determined from
EPR spectroscopy. On the basis of these observations, we
presume that [MnIII(MeOH)(dpaq)]+ is highly unstable, and
decays by an unknown mechanism. Consequently, the
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3306 | Dalton Trans., 2015, 44, 3295–3306
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