Spectroscopic capture and reactivity of S = 1/2 nickel(iii)-oxygen intermediates in the reaction of a NiII-salt with mCPBA

Article abstract of DOI:10.1039/c2cc30716b

Ni(iii)-intermediates are trapped by EPR and UV/Vis spectroscopy in the reaction of a Ni(ii) salt with mCPBA. On the basis of their oxo-transfer and hydrogen-atom abstraction abilities the intermediates are assigned as the elusive terminal Ni(iii)-oxo/hydroxo species. The findings suggest that Ni(iii)-O(H) moieties are viable reactants in oxidation catalysis. The Royal Society of Chemistry 2012.

Full text of DOI:10.1039/c2cc30716b

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COMMUNICATION
Spectroscopic capture and reactivity of S = 1/2 nickel(III)–oxygen
intermediates in the reaction of a NiII-salt with mCPBAwz
Florian Felix Pfaff,y Florian Heims,y Subrata Kundu, Stefan Mebs and Kallol Ray*
Received 1st February 2012, Accepted 23rd February 2012
DOI: 10.1039/c2cc30716b
Ni(III)-intermediates are trapped by EPR and UV/Vis spectro-
scopy in the reaction of a Ni(II) salt with mCPBA. On the basis
of their oxo-transfer and hydrogen-atom abstraction abilities the
intermediates are assigned as the elusive terminal Ni(III)–oxo/
hydroxo species. The findings suggest that Ni(III)–O(H) moieties
are viable reactants in oxidation catalysis.
Metal complexes with terminal oxo or hydroxo ligands have
been implicated in a variety of oxidative transformations. For
instance, monomeric oxoiron units are proposed to be the
competent oxidants in many mono- and dioxygenases.1,2
Similarly, numerous synthetic oxo– or hydroxo–metal complexes
have been shown to be involved in either C–H bond cleavage
or O-atom transfer reactions.2,3 High-valent nickel–oxo or
–hydroxo complexes have also attracted much attention as
Scheme 1
they have shown promise as plausible intermediates in a
number of nickel based oxidation reactions.4,5 For example,
in the [NiII(TPA)]2+ [TPA: tris(2-pyridylmethyl)amine] based
catalytic alkane hydroxylation reaction in the presence of
mCPBA (meta-chloroperbenzoic acid),4a–c a [NiIII–O]+ species
is proposed as the active oxidant. Additionally, the [NiIII–O]+
species is also suggested as the most potent oxidant for the
chemically challenging conversion of methane to methanol in
the gas phase based on experimental6 and theoretical studies.7
However, no direct spectroscopic evidence for a high-valent
nickel–oxo species has been reported till date, leaving the
assignment ambiguous.
which has recently found application in the successful stabilization
of superoxocopper(II),9 high-spin (S = 2){FeIVQO},10 and
(S = 3/2){CoIV–O(Sc3+)}11 complexes. We have found that
reaction of [NiII(TMG3tren)]2+ with mCPBA at low tempera-
ture allows the unprecedented trapping of nickel(III)–oxygen
species in approximately 15% yield. In what follows, we
detail the spectroscopic and kinetic characterization of the
metastable nickel(III)–oxygen complexes.
Reactions of equimolar amounts of TMG3tren and
NiII(OTf)2 in dichloromethane/toluene afforded the brown
[NiII(TMG3tren) (OTf)]+ (1-OTf) complex in 65% yield; the
crystal structure (Scheme 1) exhibits the expected TBP geometry
(t = 0.82). 1-OTf is paramagnetic (S = 1) as demonstrated by
Our understanding of the nickel based C–H hydroxylation and
oxo-transfer reactions in the presence of mCPBA has stagnated
for want of a system in which high valent nickel–oxygen inter-
mediates could be trapped in significant yields. Here, we report a
progress on this front. We have used the tetradentate tripodal
ligand TMG3tren8 (tris[2-(N-tetramethylguanidyl)ethyl] amine),
1
the H NMR resonances spread over a chemical shift range of
ꢀ50 to +50 ppm (Fig. S1, ESIw). Reaction of 1-OTf (4 mM)
with 1 eq. mCPBA at ꢀ30 1C resulted in the generation of two
metastable intermediates (from EPR analysis) 2a and 2b
(t1/2 E 1 h at ꢀ30 1C) with absorption maxima centered at
464 nm, 520 nm and 794 nm (Fig. 1 top). Interestingly, the
spectral changes associated with the formation of 2a (and 2b)
are found to be similar to that previously observed during
the formation of the high-valent terminal metal–oxo complexes
of iron10 and cobalt11 supported by the TMG3tren ligand
(Fig. S2, ESIz). The formation of the oxoiron(IV) complex
involved development of bands at 400 nm and 825 nm and
for that of the oxocobalt(IV) species the corresponding bands
were observed at 489 and 810 nm, respectively.
Humboldt Universitat zu Berlin, Department of Chemistry,
¨
Brook-Taylor Strasse 2, 12489, Berlin, Germany.
E-mail: kallol.ray@chemie.hu-berlin.de; Fax: +49 3020937387;
Tel: +49 3020937385
w This article is part of the ChemComm ‘Porphyrins and phthalocya-
nines’ web themed issue.
z Electronic supplementary information (ESI) available: Experimental
section and X-ray crystallographic data. CCDC 865217. For ESI and
crystallographic data in CIF or other electronic format see DOI:
10.1039/c2cc30716b
y Equal contribution of the two authors.
c
3730 Chem. Commun., 2012, 48, 3730–3732
This journal is The Royal Society of Chemistry 2012

absorption spectra (Fig. 1 top inset). This indicates that the
absorption spectral changes associated with the reaction of
1-OTf with mCPBA can be attributed to the formation of the
Ni(III) products. Thus, homolytic O–O cleavage (pathway A,
Scheme 1) takes place in 1-mCPBA to yield the Ni(III)
products, 2a and 2b, as proposed before4a,c for the corres-
ponding [(TPA)NiII(OOC(O)C6H4Cl)]1+ complex. This notion
is also corroborated by gas chromatographic analysis of the
peracid derived products in the reaction of 1-OTf with 1 eq.
mCPBA at ꢀ30 1C in CH2Cl2. Observed were 0.25 eq. chloro-
benzene, 0.10 eq. 1,3-dichlorobenzene and 0.1 eq. 3-chlorophenol.
These products are formed from the chlorophenyl radical
(pathway A, Scheme 1) that is derived from the O–O bond
homolysis process.
The electrospray mass (ESI-MS) spectrum of the reaction
mixture of 1-OTf with mCPBA at ꢀ30 1C in CH2Cl2 shows a
peak at m/z = 513.352 (Fig. S3, ESIz). By simulation of
the observed isotope distribution patterns this peak is
formulated as {[NiII(O)(TMG3tren-H)]}+ and is derived from
the alkoxonickel(II) product resulting from the self-hydroxyla-
tion of the ligand. Such a decay pathway has also been
reported previously for the corresponding [(TMG3tren)-
FeIV(O)]2+ and [(TMG3tren)CoIV(O)]2+ complexes.10,11 The
ESI-MS spectrum also features two intense peaks at m/z =
663.321 and 803.328 corresponding to {[NiIV(O)(TMG3-
tren)](OTf)}+ and [(TMG3tren)NiII(HOC(O)C6H4Cl)](OTf)+,
respectively. We propose that these peaks derive from
the heterolytic cleavage of the residual [(TMG3tren)NiII-
(OOC(O)C6H4Cl)]+ species under the conditions of ESI-MS
experiments (pathway B, Scheme 1). In solution only homolysis
occurs as evident from the GC-MS analysis of the peracid
derived products; no chlorobenzoic acid was obtained, which is
expected to result from O–O bond heterolysis (Scheme 1).
Compounds 2a and 2b display the signature reactivity
features2,3,12 of oxidizing metal–oxo (or –hydroxo) complexes.
Rate constants for oxo-transfer to PPh3 or C–H activation of
9,10-dihydroanthracene (DHA), 1,4-cyclohexadiene, xanthene,
and 1-benzyl-1,4-dihydronicotinamide (BNAH)14 in CH2Cl2
at ꢀ30 1C were obtained from the pseudo-first order fit
(Fig. S4–S8, ESIz) of the decay of the absorption band at
520 nm. Second order rate constants (Table 1) were then
determined from the dependence of the first-order rate constants
on substrate concentrations, which were found to decrease with
an increase in the C–H bond dissociation energy (BDE) of the
substrates (Fig. 2A). Additionally, a deuterium kinetic isotope
effect (KIE) of 3.9 was obtained in the oxidation of 9,10-
dihydroanthracene-d4 (Fig. 2B). Interestingly, a comparable
KIE of 2.8 has been reported4a for the [NiII(TPA)]2+ catalyzed
oxidation of cyclohexane, for which a Ni(III)–oxo intermediate
has been proposed as the active oxidant in the catalytic cycle.
These confirm H-atom abstraction as the rate-determining step
in the C–H activation reactions. Furthermore, the involvement of
the Ni(III)-oxidant in the oxo-transfer and H-atom abstraction
Fig. 1 Top: UV/vis spectral changes associated with the reaction of
1-OTf (4 mM) with 1 eq. mCPBA at ꢀ30 1C in CH2Cl2 and the time
trace for the development of the absorption band at 520 nm (inset);
bottom: the X-band EPR spectrum (black trace) of the reaction
product of 1-OTf with mCPBA in CH2Cl2 at ꢀ30 1C and then frozen
to 10 K (frequency 9.43479 GHz, power 0.20 mW, modulation
0.75 mT). The simulated spectrum is shown in blue. Simulation parameters
for 2a (85% yield; red trace): g1 = 2.05, g2 = 2.16, g3 = 2.31; 2b
(15% yield, green trace): g1 = 2.13, g2 = 2.17, g3 = 2.26. In the inset is
shown the time trace for the development of the S = 1/2 EPR signals.
The reactions of peracids with transition metal centers in the
+2 oxidation state have been studied in detail before.4,12 The
initially formed acylperoxometal(II) intermediate (1-mCPBA
in Scheme 1) can undergo O–O bond homolysis or heterolysis
to generate the oxometal(III) or oxometal(IV) complexes,
respectively (Scheme 1). The EPR spectrum of a sample frozen
to 10 K after mixing 1-OTf with mCPBA at ꢀ30 1C shows
signals of two rhombic S = 1/2 species; a major species
(2a, 85%) with g1 = 2.05, g2 = 2.16, and g3 = 2.31 and a
minor species (2b, 15%) with g1 = 2.13, g2 = 2.17, and g3 = 2.26
(Fig. 1B). These EPR properties, i.e. gav = 2.17 for 2a and 2.19
for 2b, and gJ 4 g>, are indicative of Ni(III) with distorted
square planar, trigonal bipyramidal, or compressed octahedral
coordination with the unpaired electron in d(xy) or d(x2 ꢀ y2)
orbital.13 The maximum concentration of Ni(III) amounts to
15% of the total nickel concentration. The time trace of the
development of the S = 1/2 Ni(III) EPR signals (Fig. 1 bottom
inset) in the time-range of 0–2000 seconds is found to be similar
to the time trace for the development of the 520 nm band in the
Table 1 Second order rate constants determined for the different substrates by kinetic studies using UV-Vis spectroscopy
Substrate
PPh3 (ꢀ30 1C)
—
1,4-CHD (ꢀ30 1C)
78
DHA (ꢀ30 1C)
77
Xanthene (ꢀ30 1C)
75.5
BNAH (ꢀ60 1C)
67.9
1.53
BDE/kcal molꢀ1
k2/Mꢀ1 sꢀ1
5.49 ꢁ 10ꢀ3
7.26 ꢁ 10ꢀ3
1.25 ꢁ 10ꢀ2
1.31 ꢁ 10ꢀ2
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 3730–3732 3731

unambiguous assignment of 2a and 2b. Thus, despite the nature
of these Ni(III)–oxygen species is not entirely clear, their key role
in oxidation reaction has been clearly established.
We gratefully acknowledge financial support of this work
from the Cluster of Excellence ‘‘Unifying Concepts in Catalysis’’
(EXC 314/1), Berlin. We also thank Dr E. Bill, Mr F. Reikowski,
and Prof. Dr R. Stober for measurement and interpretation of
¨
EPR data.
Notes and references
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Fig. 2 Reactivity of 2 in oxo-transfer and C–H activation reactions.
(A) Plot of log k02 of 2a (2b) at ꢀ30 1C against C–H BDE of substrates.
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equivalent target C–H bonds of substrates. The reaction with BNAH
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s
ꢀ1); (C) decay of the Ni(III) EPR feature at
ꢀ30 1C upon addition of PPh3 {[1-OTf]: 4 mM; [mCPBA]: 4 mM;
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and 2b with a pseudo-first order decay (Fig. 2C and Fig. S9,
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Ni(III)–oxygen intermediates, 2a and 2b, during the reaction
of [NiII(TMG3tren)]2+ with mCPBA at low temperature.
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C–H activation with a rate-determining H-atom abstraction
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14 The reaction with BNAH was found to be too fast to be followed
at ꢀ30 1C. Thus k2 for BNAH was determined at ꢀ60 1C and
adjusted for ꢀ30 1C by multiplying k2 by 8. Rate is approximately
considered to be doubled for every 10 1C rise in temperature.
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c
3732 Chem. Commun., 2012, 48, 3730–3732
This journal is The Royal Society of Chemistry 2012