A cobalt(ii) iminoiodane complex and its scandium adduct: Mechanistic promiscuity in hydrogen atom abstraction reactions

Article abstract of DOI:10.1039/c6dt01815g

In addition to oxometal [Mⁿ⁺O] and imidometal [Mⁿ⁺NR] units, transient metal-iodosylarene [M^(n-2)+-OIPh] and metal-iminoiodane [M^(n-2)+-N(R)IPh] adducts are often invoked as a possible second oxidant responsible for the oxo and imido group transfer reactivity. Although a few metal-iodosylarene adducts have been recently isolated and/or spectroscopically characterized, metal-iminoiodane adducts have remained elusive. Herein, we provide UV-Vis, EPR, NMR, XAS and DFT evidence supporting the formation of a metal-iminoiodane complex 2 and its scandium adduct 2-Sc. 2 and 2-Sc are reactive toward substrates in the hydrogen-atom and nitrene transfer reactions, which confirm their potential as active oxidants in metal-catalyzed oxidative transformations. Oxidation of para-substituted 2,6-di-tert-butylphenols by 2 and 2-Sc can occur by both coupled and uncoupled proton and electron transfer mechanisms; the exact mechanism depends on the nature of the para substituent.

Full text of DOI:10.1039/c6dt01815g

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A cobalt(II) iminoiodane complex and its scandium
adduct: mechanistic promiscuity in hydrogen atom
abstraction reactions†
Cite this: DOI: 10.1039/c6dt01815g
Received 9th May 2016,
Accepted 11th July 2016
Subrata Kundu,a Petko Chernev,b Xenia Engelmann,a Chan Siu Chung,c Holger Dau,b
DOI: 10.1039/c6dt01815g
Eckhard Bill,d Jason England,*c Wonwoo Nam*e and Kallol Ray*a
www.rsc.org/dalton
In addition to oxometal [Mn+vO] and imidometal [Mn+vNR] units, group transfer reactivity. Nam et al. reported the first isolation
transient metal–iodosylarene [M(n−2)+–OvIPh] and metal– and spectroscopic characterization of an iodosobenzene
iminoiodane [M(n−2)+–N(R)vIPh] adducts are often invoked as a adduct, namely [(porph)FeIII(OIPh)]+, from the reaction of
possible “second oxidant” responsible for the oxo and imido group [(porph+•)FeIVO]+ with iodobenzene.3c Very recently, McKenzie
transfer reactivity. Although a few metal–iodosylarene adducts et al. provided the molecular structure of a non-heme iron(III)
have been recently isolated and/or spectroscopically character- iodosylarene adduct by X-ray crystallography.3d Fujii et al.3e,f
ized, metal–iminoiodane adducts have remained elusive. Herein, also reported structural and spectroscopic evidence in support
we provide UV-Vis, EPR, NMR, XAS and DFT evidence supporting of bis(iodosylarene) coordination to a manganese(IV)–salen
the formation of a metal–iminoiodane complex 2 and its scandium center. Notably, the aforementioned isolated [M(n−2)+–OvIPh]
adduct 2-Sc. 2 and 2-Sc are reactive toward substrates in the adducts were all found to be reactive toward substrates in both
hydrogen-atom and nitrene transfer reactions, which confirm their hydrogen- and oxygen-atom transfer reactions, which confirms
potential as active oxidants in metal-catalyzed oxidative trans- their potential as active oxidants in metal-catalyzed oxidative
formations. Oxidation of para-substituted 2,6-di-tert-butylphenols transformations.3 In contrast, isoelectronic [M(n−2)+–N(R)vIPh]
by 2 and 2-Sc can occur by both coupled and uncoupled proton complexes have remained elusive, and their presence as transi-
and electron transfer mechanisms; the exact mechanism depends ent reactive intermediates has been forwarded on the basis of
on the nature of the para substituent.
labelling experiments alone.4a
Herein, we describe the spectroscopic characterization of
a novel [(TMG3tren)CoII–(sPhINTs)]2+ (2) complex [sPhINTs =
{N-(p-toluenesulfonyl)imino}(2-tert-butylsulfonyl)phenyliodinane;5
TMG3tren = tris[2-{N-tetramethylguanidyl}ethyl]amine] and its
scandium adduct [(TMG3tren)CoII–sPhINTs(Sc(OTf)3)]2+ (2-Sc,
OTf = CF3SO2−), which are able to oxidize the C–H and O–H
bonds of a variety of substrates. Additionally, we demonstrate
that the presence of Sc3+ in 2-Sc initiates a two-step reaction
mechanism for the oxidation of benzyl alcohol (PhCH2OH),
involving a [CoII–(sPhINTs)⋯Sc3+⋯O(H)–CH2Ph] association
event prior to C–H bond cleavage. In the case of para-substi-
tuted phenols two different mechanistic pathways have been
established for reactions with 2 and 2-Sc: a concerted proton-
coupled electron transfer (PCET), and a proton transfer fol-
lowed by electron transfer (PT-ET). The nature of the phenol
para-substituent is shown to control the mechanism of
oxidation.
Iodosobenzene (PhIvO) and iminoiodanes (PhIvNR) are an
important class of group transfer reagents in organic syn-
thesis, and they are often used in conjunction with transition
metal-based catalysts.1 High-valent oxometal [Mn+vO] and
imidometal [Mn+vNR] units are generally accepted to be key
reactive intermediates in these reactions.2 However, transient
metal–iodosylarene [M(n−2)+–OvIPh]3 and metal–iminoiodane
[M(n−2)+–N(R)vIPh]4 adducts have also been suggested as a
possible “second oxidant” responsible for this oxo and imido
aHumboldt-Universität zu Berlin, Institut für Chemie, Brook-Taylor-Straße 2,
D-12489 Berlin, Germany. E-mail: kallol.ray@chemie.hu-berlin.de;
Fax: +49 30 2093 7387; Tel: +49 30 2093 7385
bFreie Universität Berlin, FB Physik, Arnimallee 14, D-14195-Berlin, Germany
cDivision of Chemistry and Biological Chemistry, School of Physical and
Mathematical Sciences, Nanyang Technological University, 21 Nanyang Link,
Singapore 637371. E-mail: jengland@ntu.edu.sg
dMax-Plank-Institut für Chemische Energiekonversion, Stiftstraße 34-36,
D-45470 Mülheim an der Ruhr, Germany
eDepartment of Chemistry and Nano Science, Center for Biomimetic System,
Ewha Womans University, Seoul 120-750, Korea. E-mail: wwnam@ewha.ac.kr
†Electronic supplementary information (ESI) available. See DOI:
10.1039/c6dt01815g
s
A combination of [(TMG3tren)CoII–OTf]+ (1) with PhINTs5
in acetone at −40 °C led after 6000 seconds to the formation of
an orange complex 2 with absorption maxima λmax [εmax] cen-
tered at 420 nm [1150 M−1 cm−1] and 820 nm [130 M−1 cm−1
]
(Fig. S1†). The absorption spectrum of 2 is clearly distinct
from 1 + sPhINTs (Fig. 1), and titration experiments (Fig. 1
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rhombic EPR spectrum (E/D = 0.15 0.01) with split effective
g⊥ = 5.60, 3.60, and gk = 1.96 was observed for the unusual
{(TMG3tren)CoIV–O–Sc3+} (3-Sc) core containing a cobalt center
with a formal oxidation state of +4.6 Taken together, the near-
identical EPR spectra of 2, 2-Sc, and 1 indicate that the cobalt
centers in 2 and 2-Sc retain the +2 oxidation state of 1.
2 and 2-Sc were further characterized by X-ray absorption
spectroscopy (XAS). The positions of the Co K-edges of these
complexes do not differ from that of 1 (Fig. S7†), which indicates
s
that the reaction of 1 with PhINTs does not cause a change in
the CoII oxidation state. This result is in agreement with our
EPR study and is in stark contrast to 3-Sc,6 whose Co K-edge
energy was found to be blue-shifted by 1.3 eV relative to 1.
The principal feature of the inner-sphere scattering peaks
in the extended X-ray absorption fine structure (EXAFS) spectra
of 2 and 2-Sc (Fig. 2, S8 and Table S1†) at R′ ∼ 1.67 Å (similar
to that previously reported for 1)6 can be fitted in both cases
with a single shell of 5 O/N scatterers at a distance of ∼2.05 Å,
and corresponds to the donor atoms of TMG3tren and
sPhINTs. Notably, in contrast to 3-Sc,6 no evidence of a short
Co–N/O distance could be obtained from the fit of the EXAFS
Fig. 1 Main: Electronic spectra of 1 (solid trace), sPhINTs (dash-dotted
trace), 2 (dashed trace), and 2-Sc (dotted trace) in acetone at −40 °C.
Inset: Plot of the absorbance of the 420 nm band of 2 against the
equivalents of sPhINTs added to 1. Notably, 2 and 2-Sc can also be gen-
erated in CH2Cl2 with λmax [εmax] values identical to that observed in
acetone.
inset) show that addition of only 1 equiv. sPhINTs to 1 is data of either 2 or 2-Sc. The outer-shell features in the EXAFS
necessary to maximize the yield of 2. Similarly, the 1H-NMR spectra of 2 and 2-Sc can be satisfactorily accounted for by con-
s
spectrum of 2 is distinct from that of 1 + free PhINTs (Fig. S2 sidering single scattering paths involving 8 C atoms at
and S3†), and does not show the presence of any unreacted 2.9–3.0 Å and 6 C/N atoms at 3.4–3.5 Å. Satisfactory fitting of
starting materials in significant amounts. Moreover, in the the EXAFS data of 2 requires inclusion of an additional outer-
1H-NMR spectrum of 2 the complete set of signals expected for shell scatterer at 3.78 Å corresponding to a single sulfur or
sPhINTs are observed (Fig. S3†), which are broadened and iodine atom (fits 4 and 5 in Table S2†). The removal of this
s
shifted to slightly higher fields relative to free PhINTs; this shell significantly lowers the quality of the fit (fit 3 in
can be attributed to binding to the paramagnetic cobalt Table S2†). The best fit of the EXAFS data of 2-Sc (fit 8 in
centre.
Upon addition of scandium triflate (Sc(OTf)3) at −40 °C, 2 can be assigned to a Sc atom, in addition to the Co–S scatterer
is instantaneously converted to a new species 2-Sc (λmax [εmax
at 3.66 Å.
= 450 nm [1350 M−1 cm−1] and 885 nm [180 M−1 cm−1])
Taken together, the XAS, EPR, NMR, and UV-Vis data of 1, 2
Table S3†) requires an outer-shell scatterer at 3.62 Å, which
]
(Fig. 1). As was the case for sPhINTs, only 1 equiv. Sc3+ is and 2-Sc conclusively indicate that a CoII oxidation state is
required for complete conversion of 2 to 2-Sc (Fig. S4†). retained throughout this series. Complex 2 is, therefore, for-
Notably, complex 1 can also be converted directly to 2-Sc by mulated as the five-coordinate cobalt(II)–iminoiodane adduct
reacting with sPhINTs in the presence of 1 equiv. Sc(OTf)3. [(TMG3tren)CoII–(sPhINTs)]2+ (Scheme 1), and 2-Sc is proposed
Both 2 and 2-Sc are meta-stable intermediates with half-lives to be [(TMG3tren)CoII–(sPhINTs)(Sc(OTf)3)]2+, wherein a single
(t1/2) of 3600 and 720 seconds, respectively, at 25 °C.
Sc3+ ion binds directly to the [(TMG3tren)CoII–(sPhINTs)]2+
The electronic structures of 2 and 2-Sc were probed by EPR complex.8 Consistent with this assignment the electrospray
spectroscopy. Surprisingly, in spite of the significant differ-
ences in their UV-Vis spectra, the X-band EPR spectra of 2 and
2-Sc (Fig. S5†) exhibit identical axial S = 3/2 signals, with
effective g values of g⊥ = 4.40 and gk = 2.09 that are strikingly
similar to the previously reported6 axial EPR signals of 1. Fur-
thermore, the EPR spectra of the series of [(TMG3tren)CoIIX]+
(X = OTf, Cl, SCN, N3, CH3COO, CN) complexes all exhibit axial
S = 3/2 signals identical to 2, with effective g values of g⊥ = 4.40
and gk = 2.09, even though their UV-Vis spectra are distinct
from one another (Fig. S6†). This shows that the EPR para-
meters of the {(TMG3tren)CoII} moiety are effectively indepen-
dent of the nature of the fifth ligand, and appear to be
Fig. 2 Fourier-transformed Co K-edge EXAFS spectra of 2 (A) and 2-Sc
(B) [experimental data: dotted line, simulation: solid line]. Insets show
the corresponding EXAFS data in a wave-vector scale before the Fourier
dictated solely by the strong binding of the TMG3tren ligand to
the CoII ion. The EPR parameters change only when the
formal oxidation state of cobalt is altered. For example, a transformations.
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the more sterically disfavoured N-bound sulfonamide and
O-bound sulfone isomers being higher by 15.3 and 26.9 kcal
mol−1 respectively. Consistent with this conclusion, the DFT
calculated average Co–N/O distance of 2.10 Å (corresponding
s
to four N-donors of TMG3tren and one O-donor of PhINTs) in
an S = 3/2 O-bound sulfonamide [(TMG3tren)CoII–(sPhINTs)]2+
complex (Tables S1 and S4†) best matches the EXAFS deter-
mined Co–N/O distance of 2.05 Å for 2. The calculated average
Co–N distances for the corresponding N-bound sulfonamide
and O-bound sulfone structures are significantly longer at
2.17 Å and 2.14 Å, respectively.
We then compared the reactivities of 2 and 2-Sc in hydro-
gen atom transfer (HAT) reactions from the hydrocarbon
substrates xanthene, 9,10-dihydroanthracene (DHA), 1,4-cyclo-
hexadiene (CHD), benzyl alcohol (PhCH2OH) and 1-benzyl-1,4-
dihydronicotinamide (BNAH). In all cases, addition of a
substrate led to significantly accelerated decay of the chromo-
phore associated with 2 and 2-Sc. In the presence of an
excess substrate this proceeded with first-order kinetics and
Scheme 1 Products from reactions of 1 with sPhINTs, in the presence
and absence of Sc3+ ions. The DFT calculated possible structures of 2
and their relative energies in the S = 3/2 ground state are shown in the
inset. Color code: C, grey; O, red; N, blue; Co, brown; sulfur, yellow;
iodine, violet.
mass spectrum (ESI-MS; Fig. S9†) of both 2 and 2-Sc exhibits a the resulting effective rate constants (keff) were found to be
prominent peak at m/z = 496.15, with a mass and isotope distri- linearly dependent upon substrate concentration, thereby
bution pattern corresponding to [(TMG3tren)CoII–(sPhINTs)]2+.
yielding second-order rate constants. Such reaction kinetics
DFT calculations were also performed in order to obtain are consistent with the direct reaction of 2 and 2-Sc with a sub-
further insights into the electronic and geometric structures of strate or alternatively a rapid pre-equilibrium yielding an
s
2.7 The binding of PhINTs to the cobalt center in 2 can in alternative active oxidant. Intermediacy of a more traditional
principle occur either via the sulfonamide group (O-bound or imidometal [(TMG3tren)CoIV–(NTs)]2+ oxidant would, there-
N-bound) or alternatively via one of the O-atoms of the sulfone fore, require rapid reversible N–I bond scission. Given that
group (Scheme 1 and Table S4†). The most stable structure of there is no precedence for this and the high energy calculated
the experimentally observed spin-quartet ground (S = 3/2) state for the N-bound isomer, which would be a necessary inter-
was calculated to contain the O-bound sulfonamide binding mediate in its formation, this seems highly improbable. As a
s
mode of PhINTs (Scheme 1, inset), with the respective ener- consequence, we postulate that the O-bound sulphonamide
gies (Table S5†) of the geometry optimized structures of isomer is the active oxidant.
Table 1 Substrate reactivity studies for 2 and 2-Sc
k2 (10−2 M−1 s−1) at −40 °C
Substrate
BDEC–H (kcal mol−1
)
Product (% yield)a
2
2-Sc
BNAH
Xanthene
CHD
DHA
PhCH2OH
Ph3P
67.6
74.0
76.0
76.3
80.0
—
—
—
210.1
2.06
1.32
0.74
0.20
53.1
190.5
1.14
1.38
1.17
0.37b
3.21
Benzene (65)
Anthracene (60)
PhCHO (52)
Ph3PvNTs (100)
k2 (10−2 M−1 s−1) at −20 °C
X-DTBP X =
BDEO–H (kcal mol−1
)
σ
pKa
2
2-Sc
MeO
Me
78.3
81
−0.78
−0.31
−0.26
0
0.40
0.48
0.55
0.66
14.82
14.77
14.82
14.22
9.33
10.27
—
10.15
100.8
2.42
1.97
208.7
0.70
1.06
0.05
6.61
4.32
2.84
2.67
tBu
81.2
82.1
83.1
83.1
83.4
84.2
H
0.43
CHO
MeC(O)
COOH
CN
40.12
31.29
63.47
35.82
a Yields are for complex 2 and calculated based on the amount of the starting complex 1; yields for reactions with 2-Sc are comparable to 2.
b The rate was determined using less than 150 equiv. of benzyl alcohol.
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Product analysis of the reaction mixtures of DHA, CHD and p-toluenesulfonamide (ArSO2NH2; see Fig. 3 and 4) is also
PhCH2OH reactions revealed the formation of the corres- detected as a reaction product (80–85% yield) in each case.
s
ponding dehydrogenated products (Table 1) in 52–65% yields; Control reactions of the substrate with PhINTs alone or with
sPhINTs + Sc3+ did not lead to any oxidation products. Thus,
sPhINTs is activated upon binding to the {(TMG3tren)CoII}
core. Interestingly, the second order rate constants (k2) deter-
mined at −40 °C are comparable both in the absence and pres-
ence of Sc3+ ions (Table 1). This is in contrast to the previous
report of the enhancement of the oxidizing capabilities of the
corresponding metal-oxo intermediates6,8 in the presence of
various Lewis-acids. Furthermore, the k2 values form a linear
correlation (Fig. S10†) with the C–H bond dissociation ener-
9
gies (BDEC–H
)
of the substrates, which indicates that C–H
bond homolysis is the rate determining step (r.d.s) in these
reactions.
In order to obtain additional mechanistic insights, deuter-
ium kinetic isotope effects (KIE) on the second-order rate con-
stant k2(C–H)/k2(C–D) were measured for reactions of xanthene
and DHA with 2. Values of 1.87 and 1.66 were determined at
−40 °C for xanthene and DHA (Fig. S11A and B†), respectively,
indicating that proton transfer is involved in the rate determin-
ing step. These KIE values are significantly smaller than those
typically seen for high-valent metal-oxo and metal-imido
mediated HAT reactions (3–25).1,2 This is again consistent with
the [(TMG3tren)CoII–(sPhINTs)]2+ core in 2 not evolving to a
reactive high-valent [(TMG3tren)CoIV–(NTs)]2+ species prior to
the reaction, and supports the suggestion that it is able to act
directly as the reactive species in these oxygenation reactions.
Although the presence of Sc3+ did not significantly perturb
the rate of C–H bond homolysis mediated by the [(TMG3tren)-
CoII–(sPhINTs)]2+ core, it does affect a change in the mechan-
ism in the reaction with PhCH2OH. As expected, the addition
of large excess of PhCH2OH (30–1000 equivalents) to pre-
formed solutions of 2 at −40 °C caused pseudo-first order
decay (Fig. S12†) of the absorption bands associated with this
complex at 420 and 820 nm. Furthermore, the rate of decay
was found to increase linearly with increasing PhCH2OH con-
centration (Fig. 3A), thereby affording a second order rate con-
Fig. 3 Proposed mechanisms for hydrogen atom abstraction from
benzyl alcohol based on the dependence of the kobs values of com-
plexes 2 (A) and 2-Sc (B) at −40 °C on the concentrations of benzyl
alcohol. B inset: expansion of the highlighted region for low concen-
trations of PhCH2OH. r.d.s = rate determining step; HAT = hydrogen
atom transfer.
stant (k2) of 0.0020 M−1
s
−1. The pseudo-first order rate
constants (kobs) for the reaction of 2-Sc with 30–150 equivalents
of PhCH2OH were also observed to follow a linear correlation,
and a second order rate constant (k2) of 0.0037 M−1 s−1 at
−40 °C was measured (Table 1; Fig. 3B inset). However, at
higher concentrations (>150 equivalents) the correlation
became non-linear with saturation behavior being obtained
(Fig. 3B). The simplest model to account for these findings is
to invoke the presence of a relatively fast equilibrium preced-
ing the rate-determining step.10 More specifically, PhCH2OH
reversibly binds to 2-Sc (Fig. 3) to yield a substrate bound inter-
mediate [(TMG3tren)CoII–(sPhINTs)⋯Sc3+⋯O(H)–CH2Ph], which
then undergoes C–H bond cleavage by a rate determining
H-atom abstraction process. At high PhCH2OH concentrations
(>150 equivalents), the equilibrium shifts completely towards
the substrate-bound species and the rate becomes independent
of PhCH2OH concentration, thereby explaining the saturation
kinetics.
Fig. 4 Proposed mechanisms for the hydrogen atom transfer reactions
from different substituted phenols based on the dependence of the kobs
values of complex 2 on the OZH bond dissociation energies (A), as well
as, the pKa (B) of the phenols. PCET = proton coupled electron transfer;
PT = proton transfer; ET = electron transfer; r.d.s = rate determining step.
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This notion is further reinforced by the absence of satur- determined for the reactions of 2 and 2-Sc with PPh3 are 0.531
ation kinetics for the other substrates, which do not possess a and 0.0321 M−1 s−1, respectively, demonstrating that the
ligating atom, and 2, which lacks a Lewis acidic binding site.
group-transfer reactivity of 2 is decelerated by factors of ∼15
Mechanistic studies for the reaction of 2 and 2-Sc with upon binding of Sc3+ ions (Table 1; Fig. S16 and S17†).
para-substituted 2,6-di-tert-butylphenols [X-DTBP; X = –OMe,
–Me, –tBu, –H, –CN, –CHO, –COOH, –MeC(O)] reveal that the
second-order rate constants depend markedly on the electron-
donating/-withdrawing properties of the para-substituents
Conclusions
(Table 1). For all the substituted phenols studied, second order In conclusion, we have provided spectroscopic evidence sup-
kinetic behavior (Fig. S13†) and identical reaction products, porting formation of a metal–iminoiodane complex 2 and its
s
namely ArSO2NH2 and two equivalents of the corresponding scandium adduct 2-Sc. A novel binding mode of PhINTs via
phenoxyl radical, were observed. However, when the rates the sulfonamide O-atom is predicted in 2 based on DFT calcu-
obtained for both 2 and 2-Sc are plotted as a function of σ of lations. Both 2 and 2-Sc possess cobalt in a +2 oxidation state.
the para-substituents a linear correlation with a negative This is in sharp contrast to the analogous oxo chemistry,6
Hammett slope (Fig. S14†), as previously reported for the reac- where the presence of scandium led to the stabilization of an
tion of metal-oxo11a and -superoxo11b intermediates with unusual {CoIV–O–Sc3+} core. The formation of the isoelectronic
phenol substrates, is obtained only for the electron-donating {CoIV–N(Ts)–Sc3+} core in 2-Sc, which would require a short
substituents (X = –OMe, –tBu, –Me, and –H). The k2 values for Co–N(Ts) distance, is presumably prevented by the additional
the electron withdrawing substituents [X = –CN, –CHO, steric demands of the tosyl group, which is absent in the oxo
–COOH, –MeC(O), –Br] appear significantly above the trend chemistry. The reaction of 2 and 2-Sc with 4-substituted 2,6-di-
line, thereby hinting at a change in the mechanism for these tert-butylphenols is shown to proceed via a stepwise PT-ET
substrates.
mechanism for electron withdrawing substituents. Although
In response, we also plotted the rate constants against the PT-ET mechanisms have previously been invoked for the oxi-
O–H bond dissociation energy (BDE).9 Once again, a linear dation of phenols by organic radicals,13 this is the first experi-
correlation with a negative slope comparable to that obtained mental evidence for a metal complex mediated variant.
for the metal-oxo and -dioxygen intermediates was observed Additionally, the presence of Sc3+ in 2-Sc is shown to promote
for the phenols with electron-donating substituents, but the the formation of a precursor complex during the oxidation of
data for the phenols with electron-withdrawing groups benzyl alcohol, thereby demonstrating the cooperativity of two
appeared outside the correlation (Fig. 4A and S15A†). Plotting metal centers in promoting substrate oxidation reactions.
12
log k2 versus pKa
(Fig. 4B and S15B†) revealed that for Taken together, the present study expands our understanding
X-DTBP with X = –CHO, –C(O)Me, –CN, –Br, and –H, the rate of metal-mediated oxidation reactions using iminoiodanes,
(log k2) decreased linearly with decreasing acidity (increasing with metal–iminoiodane adducts being demonstrated to be a
pKa), whereas the rates for the electron donating substituents second plausible oxidant, in addition to the often invoked
–MeO, –Me, and –tBu scatter irregularly. From the above high-valent metal-imido reactive intermediates,2b,c,e in hydro-
studies we can conclude that 2 and 2-Sc oxidize acidic phenols gen atom abstraction and group transfer reactions.
(X-DTBP; X = –CN, –CHO, –COOH, –C(O)Me) via a stepwise
We gratefully acknowledge financial support of this work
proton transfer and electron transfer mechanism, with the from the Cluster of Excellence “Unifying Concepts in Catalysis”
proton transfer (PT) being effectively rate determining. This is (EXC 314/2), Berlin. K. R. also thanks the Heisenberg-Pro-
likely to involve an initial proton transfer to solvent (acetone) gramm of the Deutsche Forschungsgemeinschaft for financial
to give a phenolate anion, which may bind to the Co(II) center support. W. N. acknowledges financial support from the NRF
and is much easier to oxidize than the phenol itself. The of Korea through the CRI (NRF-2012R1A3A2048842) and GRL
resulting rate acceleration, therefore, parallels the extent of (NRF-2010-00353). JE is thankful to the NAP fellowship of the
ionization (acidity) of the phenol. In contrast, less acidic Nanyang Technological University.
phenols like MeO-DTBP, Me-DTBP, and tBu-DTBP, proceed
via
a concerted proton-coupled electron transfer (PCET)
mechanism. Interestingly, data for 2,6-di-tert-butylphenol
(H-DTBP) falls in all the plots of the second-order rate constant
(log k2) versus the thermochemical parameters BDEO–H and pKa
(Fig. 4A, B and S15A, B†), thereby locating H-DTBP on the
mechanistic borderline between the concerted PCET and step-
wise PT-ET reaction pathways.
Notes and references
1 (a) D. Ostovic and T. C. Bruice, Acc. Chem. Res., 1992, 25,
314; (b) M. Costas, Coord. Chem. Rev., 2011, 255, 2912;
(c) R. T. Gephart and T. H. Warren, Organometallics, 2012,
31, 7728.
The reaction of excess triphenylphosphine (30–60 equi-
valents) with preformed 2 and 2-Sc at −40 °C leads to
the quantitative formation of triphenylphosphonium tosyl-
aza-ylide (Ph3PvNTs) and regeneration of the starting CoII
complex 1. Interestingly, the second order rate constants (k2)
2 (a) W. Nam, Acc. Chem. Res., 2007, 40, 522;
(b) J. Hohenberger, K. Ray and K. Meyer, Nat. Commun.,
2012, 3, 720; (c) K. Ray, F. Heims and F. F. Pfaff,
Eur. J. Inorg. Chem., 2013, 3784; (d) V. Lyaskovskyy,
A. I. O. Suarez, H. Lu, H. Jiang, X. P. Zhang and B. de
This journal is © The Royal Society of Chemistry 2016
Dalton Trans.

View Article Online
Communication
Dalton Transactions
Bruin, J. Am. Chem. Soc., 2011, 133, 12264; (e) J. F. Berry,
Dalton Trans., 2012, 41, 700.
6 F. F. Pfaff, S. Kundu, M. Risch, S. Pandian, F. Heims,
I. Pryjomska-Ray, P. Haack, R. Metzinger, E. Bill, H. Dau,
P. Comba and K. Ray, Angew. Chem., Int. Ed., 2011, 50,
1711.
7 Owing to the structural complexity of 2-Sc, the actual
binding site of scandium is not clear at present.
8 (a) J. Chen, Y.-M. Lee, K. M. Davis, X. Wu, M. S. Seo,
K.-B. Cho, H. Yoon, Y. J. Park, S. Fukuzumi, Y. N. Pushkar
and W. Nam, J. Am. Chem. Soc., 2013, 135, 6388;
(b) S. Fukuzumi, Coord. Chem. Rev., 2013, 257, 1564.
9 Y.-R. Luo, Comprehensive handbook of chemical bond ener-
gies, Taylor & Francis, 2007.
3 (a) J. A. Smegal, B. C. Schardt and C. L. Hill, J. Am. Chem.
Soc., 1983, 105, 3510; (b) S. H. Wang, B. S. Mandimutsira,
R. Todd, B. Ramdhanie, J. P. Fox and D. P. Goldberg, J. Am.
Chem. Soc., 2004, 126, 18; (c) W. Nam, S. K. Choi,
M. H. Lim, J. Rohde, I. Kim, J. Kim, C. Kim and L. Que,
Angew. Chem., Int. Ed., 2003, 42, 109; (d) A. Lennartson and
C. J. McKenzie, Angew. Chem., Int. Ed., 2012, 124, 6871;
(e) C. Wang, T. Kurahashi and H. Fujii, Angew. Chem., Int.
Ed., 2012, 51, 7809; (f) C. Wang, T. Kurahashi, K. Inomata,
M. Hada and H. Fujii, Inorg. Chem., 2013, 52, 9557;
(g) B. Wang, Y.-M. Lee, M. S. Seo and W. Nam, Angew. 10 (a) I. Garcia-Bosch, A. Company, C. W. Cady, S. Styring,
Chem. Int. Ed., 2015, 54, 11740; (h) S. Hong, B. Wang,
M. S. Seo, Y.-M. Lee, M. J. Kim, H. R. Kim, T. Ogura,
R. Garcia-Serres, M. Clmancey, J.-M. Latour and W. Nam,
Angew. Chem. Int. Ed., 2014, 53, 6388.
4 (a) M. J. Zdilla and M. M. Abu-Omar, J. Am. Chem. Soc.,
2006, 128, 16971; (b) Z. Ke and T. R. Cundari, Organometal-
lics, 2010, 29, 821.
W. R. Browne, X. Ribas and M. Costas, Angew. Chem., Int.
Ed., 2011, 50, 5648; (b) J. M. Mayer, Acc. Chem. Res., 2011,
44, 36.
11 (a) D. E. Lansky and D. P. Goldberg, Inorg. Chem., 2006, 45,
5119; (b) J. Cho, J. Woo, J. Eun Han, M. Kubo, T. Ogura and
W. Nam, Chem. Sci., 2011, 2, 2057.
12 L. A. Cohen and W. M. Jones, J. Am. Chem. Soc., 1963, 81,
3397.
5 (a)
D.
Macikenas,
E.
Skrzypczak-jankun
and
J. D. Protasiewicz, J. Am. Chem. Soc., 1999, 121, 7164; 13 (a) G. Litwinienko and K. U. Ingold, J. Org. Chem., 2003, 68,
(b) S. Kundu, E. Miceli, E. Farquhar, F. F. Pfaff,
U. Kuhlmann, P. Hildebrandt, B. Braun, C. Greco and
K. Ray, J. Am. Chem. Soc., 2012, 134, 14710.
3433; (b) G. Litwinienko and K. U. Ingold, J. Org. Chem.,
2004, 69, 5888; (c) M. C. Foti, C. Daquino and C. Geraci,
J. Org. Chem., 2004, 69, 2309.
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