Spectroscopic Analyses on Reaction Intermediates Formed during Chlorination of Alkanes with NaOCl Catalyzed by a Nickel Complex

Article abstract of DOI:10.1021/acs.inorgchem.5b01463

The spectroscopic, electrochemical, and crystallographic characterization of [(^Me,HPyTACN)Ni^II(CH₃CN)₂](OTf)₂ (1) (^Me,HPyTACN = 1-(2-pyridylmethyl)-4,7-dimethyl-1,4,7-triazacyclononane, OTf = CF₃SO₃) is described together with its reactivity with NaOCl. 1 catalyzes the chlorination of alkanes with NaOCl, producing only a trace amount of oxygenated byproducts. The reaction was monitored spectroscopically and by high resolution electrospray-mass spectrometry (ESI-MS) with the aim to elucidate mechanistic aspects. NaOCl reacts with 1 in acetonitrile to form the transient species [(L)Ni^II-OCl(S)]⁺ (A) (L = ^Me,HPyTACN, S = solvent), which was identified by ESI-MS. UV/vis absorption, electron paramagnetic resonance, and resonance Raman spectroscopy indicate that intermediate A decays to the complex [(L)Ni^III-OH(S)]²⁺ (B) presumably through homolytic cleavage of the O-Cl bond, which liberates a Cl^? atom. Hydrolysis of acetonitrile to acetic acid under the applied conditions results in the formation of [(L)Ni^III-OOCCH₃(S)]²⁺ (C), which undergoes subsequent reduction to [(L)Ni^II-OOCCH₃(S)]²⁺ (D), presumably via reaction with OCl^- or ClO₂^-. Subsequent addition of NaOCl to [(L)Ni^II-OOCCH₃(S)]⁺ (D) regenerates [(L)Ni^III-OH(S)]²⁺ (B) to a much greater extent and at a faster rate. Addition of acids such as acetic and triflic acid enhances the rate and extent of formation of [(L)Ni^III-OH(S)]²⁺ (B) from 1, suggesting that O-Cl homolytic cleavage is accelerated by protonation. Overall, these reactions generate Cl^? atoms and ClO₂ in a catalytic cycle where the nickel center alternates between Ni(II) and Ni(III). Chlorine atoms in turn react with the C-H bonds of alkanes, forming alkyl radicals that are trapped by Cl^? to form alkyl chlorides.

Full text of DOI:10.1021/acs.inorgchem.5b01463

Article
pubs.acs.org/IC
Spectroscopic Analyses on Reaction Intermediates Formed during
Chlorination of Alkanes with NaOCl Catalyzed by a Nickel Complex
Apparao Draksharapu,† Zoel Codola,‡ Laura Gomez,‡,§ Julio Lloret-Fillol,‡ Wesley R. Browne,*,†
̀
́
and Miquel Costas*,‡
†Stratingh Institute for Chemistry, Faculty of Mathematics and Natural Sciences, University of Groningen, Nijenborgh 4, 9747AG
Groningen, The Netherlands
‡Institut de Química Computacional i Catalisi (IQCC) and Departament de Química, University of Girona, Campus de Montilivi,
Girona 17071, Spain
§Serveis Tec
Catalonia, Spain
̀
nics de Recerca (STR), Universitat de Girona, Parc Cientıfic i Tecnolog
̀
ic de la UdG, Pic de Peguera 15, E17003 Girona,
́
S
* Supporting Information
ABSTRACT: The spectroscopic, electrochemical, and crys-
tallographic characterization of [(Me,HPyTACN)-
NiII(CH3CN)2](OTf)2 (1) (Me,HPyTACN = 1-(2-pyridylmeth-
yl)-4,7-dimethyl-1,4,7-triazacyclononane, OTf = CF3SO3) is
described together with its reactivity with NaOCl. 1 catalyzes
the chlorination of alkanes with NaOCl, producing only a trace
amount of oxygenated byproducts. The reaction was
monitored spectroscopically and by high resolution electro-
spray-mass spectrometry (ESI-MS) with the aim to elucidate
mechanistic aspects. NaOCl reacts with 1 in acetonitrile to
form the transient species [(L)NiII-OCl(S)]+ (A) (L =
Me,HPyTACN, S = solvent), which was identified by ESI-MS.
UV/vis absorption, electron paramagnetic resonance, and
resonance Raman spectroscopy indicate that intermediate A decays to the complex [(L)NiIII−OH(S)]2+ (B) presumably through
homolytic cleavage of the O−Cl bond, which liberates a Cl• atom. Hydrolysis of acetonitrile to acetic acid under the applied
conditions results in the formation of [(L)NiIII-OOCCH3(S)]2+ (C), which undergoes subsequent reduction to [(L)NiII-
−
OOCCH3(S)]2+ (D), presumably via reaction with OCl− or ClO2 . Subsequent addition of NaOCl to [(L)NiII-OOCCH3(S)]+
(D) regenerates [(L)NiIII-OH(S)]2+ (B) to a much greater extent and at a faster rate. Addition of acids such as acetic and triflic
acid enhances the rate and extent of formation of [(L)NiIII-OH(S)]2+ (B) from 1, suggesting that O−Cl homolytic cleavage is
accelerated by protonation. Overall, these reactions generate Cl• atoms and ClO2 in a catalytic cycle where the nickel center
alternates between Ni(II) and Ni(III). Chlorine atoms in turn react with the C−H bonds of alkanes, forming alkyl radicals that
are trapped by Cl• to form alkyl chlorides.
Ni(II) salts have been shown to be capable of oxidative
chlorination also, albeit requiring a gross excess (40 fold excess)
of the oxidant NaOCl.6 In contrast, Ito and co-workers7 have
shown recently that NiII(TPA) complexes (TPA = tris(2-
pyridylmethyl)amine) are good catalysts in the oxidation of
alkanes with mCPBA but not with other oxidants.8 Building on
these precedents, we considered the possibility of using nickel
coordination compounds with chemically robust triazacyclono-
nane derived ligands as catalysts for the chlorination of alkanes
using NaOCl as both oxidant and halide transfer agent. We
reasoned that the chemical robustness of the complexes, in
combination with the resistance of this class of ligands to
oxidative degradation, may lead to efficient chlorination
INTRODUCTION
■
Metal catalyzed oxidative functionalizations of alkanes play a
central role in a myriad of biological and chemical processes
with relevance in chemical synthesis.1 Of growing interest are
atom economic reactions, relying on oxidants that produce a
minimum amount of nontoxic waste. An interesting example is
the hypochlorite anion, which constitutes a versatile oxidant
that has found widespread application in metal catalyzed
oxidations employed in organic synthesis.2 Several manganese,3
iron,4 and nickel5,6 catalyzed oxidations are based on the use of
NaOCl as oxygen atom donor, producing NaCl as a byproduct.
Catalytic halogenation of nonactivated C−H bonds, although
receiving much less attention, has been achieved also, for
example, with manganese porphyrin based catalysts and sodium
hypochlorite, the latter acting both as oxidant and source of the
halogen to be incorporated.2b Ni-salen complexes and indeed
Received: June 30, 2015
© XXXX American Chemical Society
A
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catalysts. Toward this end, herein we describe the preparation
and structural and spectroscopic characterization of
[(Me,HPyTACN)NiII(CH3CN)2](OTf)2 (1).
Catalyst 1, with NaOCl, transforms a range of alkanes to
alkyl chlorides (Scheme 1) with good chemoselectivity,
Scheme 1. Structure of the
[(Me,HPyTACN)NiII(CH3CN)2](CF3SO3)2 Complex 1, and
the Catalyzed Halogenation of Alkanes with NaOCl
Figure 2. . Structure of 1 with 50% probability ellipsoids along with
the bond distances. Hydrogen atoms and triflate anions are removed
for clarity.
those reported for the complex [(TPA)NiII(OAc)(H2O)]-
(BPh4).7
The coordination of acetonitrile ligands both in solution and
the solid state is apparent by the nitrile bands at 2293 and 2318
cm−1 observed by both the FTIR and Raman spectroscopy
(Figure S1). The Raman spectra recorded under nonresonant
conditions at λexc 785 nm of both crystalline material and of 1
(75 mM) in acetonitrile are (with the exception of solvent
bands) identical both in terms of Raman shift and relative
intensity of the bands (Figure S1). The close correlation
between FTIR and Raman spectra of 1 in the solid state with
the nonresonant Raman spectrum in acetonitrile confirms that
the structure of complex 1, i.e., the tetradentate coordination of
the PyTACN ligand to the NiII center, is retained in solution.
The cyclic voltammetry of 1 in acetonitrile shows a single quasi-
reversible redox wave for the NiIII/NiII couple at 0.48 V vs SCE
(Figure 3b and Figure S2), which is unusually negative in
comparison with other Ni(II) complexes bearing nitrogen
donor ligands.10
producing only minor amounts of oxygenated products under
optimized conditions. Fundamental aspects of the reactivity of
these species in formal chlorine atom transfer reactions are
described. UV/vis absorption, ESI-MS, resonance Raman,
NMR, and EPR spectroscopic data obtained from reaction
mixtures provide evidence for the intermediacy of [(L)NiII−
OCl(S)]+ (A), [(L)NiIII-OH(S)]2+ (B), [(L)NiIII
-
OOCCH3(S)]2+ (C), and [(L)NiII-OOCCH3(S)]2+ (D) (L =
Me,HPyTACN) in the reaction (Figure 1).
The UV/vis absorption spectrum of 1 in acetonitrile shows
bands at λmax = 275, 321, 525, 809, and 906 nm (vide infra,
Figure 3a and Figure S3). The spectrum is similar to that of
[(TPA)NiII(OAc)(H2O)](BPh4),11 TPA = tris(2-
pyridylmethyl)amine. By analogy, the bands in the UV region
are assigned to intraligand transitions, and the bands are at 525
3
3
3
3
and 906 nm are assigned to A2g → T1g (F) and A2g → T2g
(F) transitions, respectively. The band at ca. 809 nm is assigned
tentatively, as the spin forbidden A2g → E1g transition.11,12
These features are typical of Ni(II) in an octahedral
coordination environment.12
3
1
Addition of water to 1 in acetonitrile resulted in a red shift in
the band at 525−538 nm, and the bands at 321 and 906 nm
increased in intensity (Figure S3). These changes are ascribed
to the displacement of the CH3CN ligands by H2O/OH− (vide
infra).
Figure 1. Structure of 1 and proposed structures of species A, B, C,
and D (S = solvent).
1
The H NMR spectrum of 1 in acetonitrile shows (Figure
RESULTS
S4) two broad signals at 50 and 47 ppm assigned to pyridyl β-
protons and a sharp resonance at 15 ppm assigned to the
pyridyl γ-protons by comparison with [(TPA)NiII(CH3CN)-
(H2O)]2+.13 The signals of the pyridyl α-proton and PyCH2
were not observed presumably due to their proximity to the
metal center and the resulting increased broadening from fast
paramagnetic induced relaxation.14
Catalytic C−H Activation with 1 and NaOCl. The
functionalization of alkanes with aqueous sodium hypochlorite
(ca. 0.3 equiv) at room temperature in acetonitrile, catalyzed by
1 (1 mol %) is shown in Table 1. Conversion was observed
neither in the absence of 1 nor in the presence of [NiII(OAc)2].
■
Synthesis and Characterization of 1. Complex 1 was
prepared by reaction of equimolar amounts of the tetradentate
ligand Me,HPyTACN9 with [NiII(CH3CN)6](OTf)2 in acetoni-
trile and isolated by precipitation with diethyl ether. Crystals
suitable for X-ray crystallographic analysis were obtained by
slow diffusion of diethyl ether into an acetonitrile solution of 1
(Table S1). Complex 1 adopts a distorted octahedral geometry
(Figure 2). Four coordination sites are occupied by N atoms
from the Me,HPyTACN ligand and the other two by acetonitrile.
The acetonitrile ligands are cis to each other. The six Ni−N
bond distances range from 2.046 to 2.112 Å and are similar to
B
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Figure 3. (a) UV/vis absorption spectrum of 1 (1.55 mM) in acetonitrile and (b) cyclic voltammetry of 1 (1 mM) in acetonitrile (100 mM
Bu4NPF6, vs SCE).
Table 1. Oxidative Halogenation of Aliphatic Hydrocarbons
b
c
without acetic acid
TON
with acetic acid
TON
a
a
substrate
products
chlorocyclohexane
yield Ox (%)
remarks
yield Ox (%)
remarks
e
f
cyclohexane
16.8
3.8
11
2
C:K = 4.4:1
36.7
24
C:K = 14.7:1
cyclohexanone
2.5
1.7
24
d
adamantane
1-chloroadamantane
2-chloroadamantane
1-adamantanol
11.7
4.1
7.5
2.6
4.4
2
3°:2° = 8.6:1
37.3
14.5
3°:2° = 7.7:1
6.3
6.9
2-adamantanol + adamantanone
chlorocyclooctane
cyclooctanone
3.1
cyclooctane
hexane
10.7
0.7
6.9
27.7
5.6
18
3.6
7
C:K = 5:1
2-chlorohexane
5.8
3.7
3.7
traces of ketone
N/A
10.7
11.3
36.1
traces of ketone
3-chlorohexane
5.8
7
indaneg
indanone
N/A
N/A
23
ketone product only
a
b
c
d
Yield based on oxidant. Conditions: (1/oxidant/substrate) 1:68:165. Conditions: (1/oxidant/substrate/acetic acid) 1:68:165:27. In acetonitrile
e
f
except for adamantane (1:1 CH2Cl2 and CH3CN). Conversion was observed neither in the absence of 1 nor in the presence of [NiII(OAc)2]. 33%
conversion was observed in the absence of 1. All the yields are determined by GC except “g” (NMR yield with dichlorobenzene as internal standard).
C and K are chloro and ketone products, respectively.
Figure 4. Conversion of cyclohexane to chlorinated products monitored inline by Raman spectroscopy (λex 532 nm) with dichlorobenzene as
internal standard in the (a) absence and (b) presence of acetic acid. The area under the bands 804 cm−1 (cyclohexane) normalized to the band at
662 cm−1 (internal reference, 1,2-dichlorobenzene) was used to estimate conversion. Ratio; 1: NaOCl:cyclohexane:acetic acid is 1:68:165:27.
Chlorocyclohexane and chlorocyclooctane were obtained in
17% and 12% yields, respectively, with respect to the terminal
oxidant and 5.7% and 4% yields, with respect to the substrate
cyclohexane and cyclooctane, respectively. For both substrates,
the only mono-oxygenated products obtained were the
corresponding ketones, which appeared in minor amounts. In
the case of n-hexane, the methylene groups were not
discriminated with the mono chlorinated products 2-
chlorohexane and 3-chlorohexane obtained as the major
products in a roughly 1:1 ratio. A competition reaction
between cyclohexane and cyclohexane-d12 yielded a kinetic
isotopic effect of 2, consistent with the involvement of a highly
reactive hydrogen atom abstracting reagent. Furthermore,
chlorination of cis-1,2-dimethylcyclohexane and decalins
provided complex mixtures of chlorinated products, indicative
of a poorly discriminating C−H oxidizing agent. In contrast,
however, chlorination of adamantane proceeds regioselectively
toward chlorination at the tertiary C−H position (3°/2° = 8.6),
which is inconsistent with the involvement of hydroxyl
radicals,15 but is consistent with the involvement of chloride
radicals.16 The combination of a relatively moderate 3°/2° site
selectivity and low K.I.E. is inconsistent with the reactivity
patterns of high valent metal-oxo species and instead suggest
C
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the involvement of highly reactive chlorine species as C−H
bond cleaving agents.17
Addition of acetic acid (27 equiv with respect to catalyst)
resulted in a 2-fold increase in the conversion to chlorocyclo-
hexane (to 37%), and the chloro/ketone ratio increased from
4.4 to 14.7. Increased conversion in favor of the corresponding
chlorinated products in the presence of acetic acid (Table 1)
was observed for adamantane, cyclooctane, and n-hexane also.
With indane, which has a relatively weak benzylic C−H bond,
indanone was the only product observed, in 36% yield with
respect to the terminal oxidant. Reactions did not proceed in
either acetone or dichloromethane, although trace amounts of
chlorinated products were observed in the latter case upon
t
addition of a phase transfer catalyst, Bu4NCl.
In Situ Reaction Progress Monitoring by Raman
Spectroscopy. The oxidation of cyclohexane was monitored
by Raman spectroscopy with conversion manifested in a
decrease in the relative intensity of the cyclohexane band at 804
cm−1 and the appearance of a band at 731 cm−1 due to
chlorocyclohexane with respect to a band of the internal
standard 1,2-dichlorobenzene at 662 cm−1. The time depend-
ence of the catalyzed oxidation of cyclohexane shows that the
reaction continues for ca. 60 min with ca. 30%18 decrease in the
intensity of the cyclohexane band. Conversion was observed
neither in the absence of 1 nor in the presence of [NiII(OAc)2]
(Figure 4a).
Addition of acetic acid results in an increase in reaction rate
and ca. 45% conversion was observed within 1 h (Figure 4b). It
is important to note, however, in the absence of 1, 33%
conversion over 2 h was still observed when acetic acid was
added, although at a lower reaction rate. Hence a key challenge
in determining the catalytic activity of 1 with NaOCl is the
concurrent reaction catalyzed by Brønsted acids (see section 12
in Supporting Information).19
Figure 5. (a) Changes in the UV/vis absorption spectrum of 1 (1.55
mM, 1 mL CH3CN, with stirring) over time (in s) with 10.7 equiv of
NaOCl (∼10 μL, 1.66 M in H2O). The baseline rises as a result of
scattering from particulates. The spectra are offset corrected for clarity
by setting the absorbance at 650 nm to zero. (b) Change in the UV/vis
absorption spectrum of 1 (1.55 mM) in CH3CN (with stirring) before
and after addition of NaOCl (10.7 equiv) together with triflic acid (8.6
equiv). After 5 min a second addition of of NaOCl (10.7 equiv in
water) was made. The spectra were offset corrected by setting the
absorbance at 1000 nm to zero.
In the presence of an alkane, which has relatively strong C−
H bonds, the initially purple solution turned colorless
immediately upon addition of oxidant followed by a change
to pale orange within a few seconds (Figures S5 and S6). After
approximately 3 min a fine black suspension appeared, and at
the end of the reaction a white solid was obtained identified as
NaCl containing NiII(OAc)2 by Raman spectroscopy as (Figure
S7). It should be noted that although the reaction ceased in
terms of conversion of cyclohexane, at ca. 60 min a black
precipitate formed and persisted for a further 30 min indicating
that this species is not in fact the active catalyst but rather a
catalyst degradation product. Indeed, when NiII(OAc)2 is used
instead of 1, the black particles formed also, but significant
substrate conversion is not observed. Notably, NiII(OAc)2
served to decrease conversion under acidic conditions
compared with that observed with acetic acid alone (Figure 4).
Spectroscopic Monitoring of the Reaction of 1 with
NaOCl. The UV/vis absorption spectrum of 1 changes upon
addition of increasing amounts of NaOCl (2.7−10.7 equiv,
Figure S8). The initial weak visible absorbance is replaced by a
stronger absorption in the visible region (Figure S8). Addition
of 10.7 equiv of NaOCl (∼10 μL, 1.66 M in H2O) to a solution
of 1 in acetonitrile resulted in the formation an orange colored
species (B, λmax 470 nm) over ca. 3 min. The visible absorption
spectrum shows a broad increase over the entire spectrum due
to scattering from precipitated salts as well as the appearance of
absorption bands, at λmax 367 and 470 nm, Figure 5a. Over time
the two absorption bands decrease in intensity and the solution
becomes yellow and, ultimately, colorless (with the remaining
absorbance due to attenuation by precipitated salts).
Reaction of 1 with 10.7 equiv of NaOCl and 8.6 equiv of
CF3SO3H, which is stronger than acetic acid but a weaker
ligand, does not lead to the appearance of a band at 470 nm
even after 300 s. Instead, an absorption band at 316 nm was
observed to grow in over 180 s (Figure 6), which is assigned to
a ClO2.20 Indeed in the absence of Ni(II), this band appears
rapidly upon addition of acid to NaOCl in acetonitrile (Figure
Figure 6. Change in the UV/vis absorption spectrum of 1 (1.55 mM)
in CH3CN (with stirring) before and up to 180 s after addition of
NaOCl (10.7 equiv) together with triflic acid (8.6 equiv). See Figure
5b for changes over longer times.
D
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S21). The addition, after 300 s, of a further 10.7 equiv of
NaOCl, led to a substantial increase in absorbance at 470 nm
corresponding to species B along with a broad band at ca. 750
nm. The two bands reach a maximum absorbance within 1 min;
followed by a slower decrease (Figure 5b). The maximum
absorbance at 470 nm reached in this case is substantially
greater than in the absence of acid.
The change from 1 to B occurred also upon addition21 of
aqueous Ca(OCl)2 both in the presence and absence of acetic
acid. In the absence of acid, with 17 equiv of Ca(OCl)2 the
absorbance at 470 nm reached a maximum after 450 s followed
by a decrease in absorbance and eventual decolouration of the
solution. Addition of a second batch (17 equiv) of Ca(OCl)2 to
the colorless solution regenerated the absorbance at 470 nm
more rapidly than occurred upon the initial addition (Figure 7
and Figure S9).
Figure 8. 1H NMR spectra of 1 (1.55 mM, bottom), after addition of
10.7 equiv of NaOCl and after becoming colorless (after ca. 15 min,
top) in CD3CN.
complexes and together with EPR and ESI-MS data (vide
infra) confirm that several nickel species are present in various
oxidation states (e.g., NiII and NiIII complexes). For the
corresponding spectra obtained with Ca(OCl)2, see Figure S10.
EPR Spectroscopy. 1 is EPR silent in acetonitrile at 77 K.
In contrast B, generated with 10.7 equiv of NaOCl, shows a
signal with g⊥ = 2.13, and g// = 2.03 characteristic of a S = 1/2
Ni(III) species,23 which decreases and ultimately disappears
concomitant with the increase and decrease in absorbance at
470 nm (Figure 9a). With acetic acid and NaOCl a signal with
g⊥ = 2.15 and g// = 2.04 was observed (Figures S11b, S12, and
S13).23
Figure 7. Change in the UV/vis absorption spectrum of 1 in
acetonitrile upon addition of (a) 17 equiv Ca(OCl)2 (with stirring)
and (b) at 815 s an additional 17 equiv of Ca(OCl)2 was added.
Conditions: 1 (1.55 mM, 1 mL CH3CN) and 17 + 17 equiv of
Ca(OCl)2 (each 17 equiv contains 57.6 μL water). Spectra are offset
corrected by setting the absorbance at 650 nm to zero.
Figure 9. EPR spectra of 1 (3.1 mM) with (a) 10.7 equiv NaOCl and
(b) 8.6 equiv of triflic acid and 21.4 equiv of NaOCl in CH3CN at 77
K. “B” indicates spectrum recorded from a sample flash frozen when
the absorbance at 470 nm reached a maximum, i.e., when a mixture of
species B and C are present at highest total concentration.
Addition of 17 equiv of Ca(OCl)2 to 1 generates B in the
presence of 8.6 equiv of acetic acid within 150 s; i.e., formation
is accelerated by acetic acid (vide supra). Regeneration of B is
observed over several cycles; three cycles are shown in Figure
S9. The absorption at 470 nm is greater after the second
addition of Ca(OCl)2 compared to the first addition.22
Resonance Raman spectroscopy (vide infra) confirms that the
more rapid increase in absorption is indeed due to more rapid
formation of B.
The EPR spectra obtained in the presence of triflic acid were
similar to those obtained with acetic acid (Figure 9b); however,
the signals were considerably more intense, which is consistent
with the increased absorbance at 470 nm (Figure 5b).
Interestingly, the shape of the EPR spectrum of the sample
after 10 min is slightly different from the initial strong NiIII
signal. This observation may suggest that the species
responsible for this signal could be different from species B,
i.e., species C (vide infra). EPR spectra of the species generated
with Ca(OCl)2 in the presence and absence of acetic acid were
essentially the same as those obtained with NaOCl (Figures
S13 and S14). As in the absence of acid, the Ni(III) species
decay over time toward an EPR silent species.
1H NMR Spectroscopy. The 1H NMR spectra after
addition of 10.7 equiv of NaOCl to 1, obtained immediately
after appearance of B and after 15 min, i.e., when the solution is
colorless, are shown in Figure 8. The reaction mixture shows
signals at 49, 43, 15, 14.5, and 13 ppm, and several signals
between 0−10 ppm. The signals are assigned to Ni(II)
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16
Figure 10. ESI-MS spectra of 1 (1.55 mM) in acetonitrile of (a) solution after addition of 17 eq Ca(OCl)2 (dissolved in H2 O) and (b) solution
18
after addition of 17 eq Ca(OCl)2 (dissolved in H2 O).
Figure 11. Generation of B and C from 1 in acetonitrile (with stirring) upon addition of (a) 10.7 equiv of NaOCl and (b) a second batch addition of
NaOCl (10.7 equiv) at 782 s monitored by Raman spectroscopy at λexc 473 nm. Conditions: 1 (1.55 mM, 1 mL CH3CN) and 10.7 equiv of NaOCl
(∼10 μL, 1.66 M in H2O). Spectra were normalized to the acetonitrile bands (#) at 1375 and 756 cm−1 and offset for clarity.
ESI-MS Analysis. The ESI-MS spectrum of 1 in acetonitrile
shows major peaks (Figure S15) at [(Me,HPyTACN)NiII]2+ (m/
z 153.0699), [(Me,HPyTACN)NiII(H2O)]2+ (m/z 162.50725),
[(Me,HPyTACN)NiII(CH3CN)]2+ (m/z 173.5823), and
[(Me,HPyTACN)NiII(OTf)]+ (m/z 455.0897). Addition of
10.7 equiv of NaOCl to 1 in acetonitrile results in the
appearance of several major ions [(Me,HPyTACN)NiII(Cl)]+
(m/z 341.1055), [(Me,HPyTACN)NiII(OCl)]+ (m/z 357.1014)
The intensity of the peak assigned to [(Me,HPyTACN)-
NiII(OCl)]+ (m/z 357.1014), formed immediately upon
reaction of 1 with NaOCl, decreases concomitant with the
increase in the intensity of the absorption of B. Ultimately,
once the solution has become colorless, the signals observed
correspond to [(Me,HPyTACN)NiII(OOCCH3)]+ (m/z
365.1502) and [(Me,HPyTACN)NiII(Cl)]+ (m/z 341.1055).
The use of Ca(OCl)2, which is available as a solid, allows for
H2O and H218O (Figure 10) to be used for 18O labeling studies
avoiding interference from the H216O present in aqueous
NaOCl. The spectrum obtained after addition of 17 equiv of
Ca(OCl)2 (in H2O) to 1 shows major signals of
[(M e , H PyTACN)NiI I (OCl)]+ (m/z 357.101) and
[(Me,HPyTACN)NiII(OTf)]+ (m/z 455.091). As expected, the
signal at m/z 357.101 increased by two m/z units when
Ca(18OCl)2 was used (Figure 10b), further supporting its
assignment. Over time the intensity of this signal decreases
relative to intensity of the signal at m/z 341.105
[(Me,HPyTACN)NiII(Cl)]+. Notably, evidence for the presence
of Ni(III) species was not obtained by ESI-MS. The absence of
as well as [(Me,HPyTACN)NiII(OTf)]+ (m/z 455.0897).24
A
signal assigned to [(Me,HPyTACN)NiII(OOCCH3)]+ (m/z
365.1502) increases in intensity over time at the expense of
[(Me,HPyTACN)NiII(OCl)]+ (m/z 357.1014). When analogous
experiments were performed in CD3CN, a shift in the signal at
m/z 365.1502 by 3 m/z units was observed, which confirms its
assignment as [(Me,HPyTACN)NiII(OOCCD3)]+ (m/z
368.1689) and that the acetate originates from in situ hydrolysis
of CH3CN (Figure S16). The signal at m/z 357.1014 increased
by two mass units when using Na18OCl (10 μL of NaOCl in
H216O + 30 μL of H218O),25 which supports the assignment of
this ion as [(Me,HPyTACN)NiII(OCl)]+ (Figure S17).
F
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Table 2. Summarized Vibrational Frequencies of species B and C
species
cm−1
cm−1
observed shift (cm−1
)
expected shift (cm−1
)
[(L)NiIII-OH]+ (B)
[(L)NiIII-O(CO)CH3]2+ (C)
724 (Ni−O16)
699 (Ni−O18)
25
27
28
41
1682 (CO16)
1655 (CO18)
Ni(III) species in the MS spectra, even where EPR confirms
their presence in solution, strongly suggests that these species
are reduced under the conditions of the electrospray, through a
process that is not fully understood but is nevertheless
precedented.26
Resonance Raman Spectroscopy. Resonance Raman
spectroscopy has been applied widely in probing the electronic
origin of visible absorption bands and in characterizing species
formed in solution at low concentrations. The absence of
resonance enhancement of the spectrum of 1 at 785 nm (using
the Raman scattering of triflate as an internal reference, Figure
S1) is consistent with the assignment of the absorption bands
in that region as metal centered transitions.
Time-dependent resonance Raman spectra following the
formation of B and C with 10.7 equiv of NaOCl were shown in
Figure 11. At the concentrations (1.55 mM) employed for
catalysis (vide infra) bands related to the parent Ni(II) complex
are too weak to be observed by Raman spectroscopy at λexc 473
nm. Two bands appear at 1682 and 724 cm−1 and reach a
maximum intensity at ca. 180−240 s after addition of NaOCl,
and decrease in intensity again over time (420−480 s, Figure
11a). Complete disappearance of the band at 1682 cm−1 was
observed after 540 s. These data are consistent with changes
observed by UV/vis absorption spectroscopy. Addition of a
second batch of NaOCl (10.7 equiv) to regenerate B resulted
in the band at 724 cm−1 reaching a maximum intensity after
888 s, but the band at 1682 cm−1 does not appear
concomitantly confirming that the 1682 cm−1 band originates
from a distinct species, i.e., C (Table 2). Furthermore, the band
at 1682 cm−1 reaches a maximum intensity at ca. 995 s at which
point there has already been a decrease in the intensity of the
band at 724 cm−1 (Figure 11b). Regeneration of B occurs more
rapidly and precedes the reappearance of C, upon a second
addition of NaOCl consistent with observations by UV/vis
absorption spectroscopy (Figure S9). That the formation of B
is followed by the appearance of C after each addition of
NaOCl is more evident when 5.3 eq of NaOCl were added
each time (Figure S18). The resonance enhancement of the
1682 cm−1 band of C is direct evidence that it absorbs at 470
nm also, i.e., resonant enhancement is not possible otherwise
and shows that C is not present at high concentrations and has
a lifetime not much longer than that of B (vide supra, see EPR
spectroscopy).
Figure 12. Generation of B and C from 1 (1.55 mM) in 1 mL of
acetonitrile by adding 10.7 equiv of NaOCl (40 μL; 10 μL of NaOCl
in H216O + 30 μL of H218O), monitored by Raman spectroscopy at λexc
473 nm. Spectra were normalized to the acetonitrile bands (#) at 1375
and 756 cm−1.
Figure 13. Resonance Raman spectra of B and C generated from 1
(1.55 mM) with acetic acid (8.6 equiv) and (top) NaOCl or (bottom)
NaOBr (10.7 equiv) in acetonitrile at λmax 473 nm. Artifacts due to
imperfect solvent subtraction are masked by a white box. 18O shifts are
noted for each band in parentheses.
When NaOCl (aq), diluted in H218O (i.e., 1:3 H216O and
H218O, Figure S19), was employed,27 an additional band was
observed at 699 cm−1 in a ca. 3:1 ratio with the band at 724
cm−1 (Figure 12). For a Ni−O and an O−Cl stretching mode a
shift of 28 and 32 cm−1, respectively, is expected based on the
two atom approximation (Table 2, Figure 13).28 In addition,
the band at 1682 shifts to 1655 cm−1, i.e., a 27 cm−1 (Figure
12). With NaOBr, two intense bands at 728 and 1679 cm−1
were observed also (Figure 13), indicating that neither mode
involves a halogen atom. The isotope shift observed for the
band at 1682 cm−1 (27 cm−1 and expected shift is 41 cm−1) was
assigned tentatively to a CO stretch of a NiIII−OCOCH3
species (C).29 The formation of acetate from CH3CN in situ
was confirmed by ESI-MS (vide supra, Figure S16).
Resonance Raman spectra obtained during the reaction of
NaOCl with 1 in the presence of an acid (triflic or acetic acid)
showed similar changes; the species that absorbs at 470 nm, i.e.,
(B) is associated with the band at 724 cm−1 assigned to a Ni−O
stretch and an EPR signal consistent with a Ni(III) oxidation
state. After decay, it can be regenerated by further addition of
NaOCl. In the presence of acid, B forms faster and to a greater
extent (Figure 14 and Figure 15).
Raman spectra recorded after addition of 17 eq Ca(OCl)2,
instead of NaOCl, resulted in the appearance of the band at 724
cm−1 both in the presence and absence of acetic acid (Figure
S20).
G
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Article
Table 3. Summary of Spectroscopic Properties of
Intermediates A−D
NiIII−16O
(Ni−18O)
λmax
“g”
frequency
ESI-MS
(m/z)
intermediate
(nm) values
(cm−1
)
[(L)NiII−OCl]+ (A)
357.1014
[(L)NiIII−OH]2+ (B)
480 2.13,
2.03
724 (699)
[(L)NiIII−OOCCH3]2+
(C)
[(L)NiII−OOCCH3]+ (D)
2.13,
2.03
1682 (1655)
365.1502
and [(L)NiII-OOCCH3]2+ (D) complexes dominating after the
initial addition of NaOCl.
Figure 14. Generation of B from 1 in CD3CN (with stirring) by
addition of 21.4 equiv of NaOCl (a) in the presence of 8.6 equiv of
triflic acid and (b) 8.6 equiv of acetic acid monitored by Raman
spectroscopy at λexc 473 nm. Conditions 1 (1.55 mM, CD3CN), triflic
acid (8.6 equiv), acetic acid (8.6 equiv) and 21.4 equiv of NaOCl (∼20
μL, in water). Spectra were normalized to the CD3CN band (#) at 686
cm−1.
The formation of Ni(III) species, which is enhanced
substantially by the presence of a Bronsted acid, is apparent
by the concomitant increase and decrease in absorbance at ca.
470 nm, resonantly enhanced bands in the Raman spectra
recorded at λexc 473 nm and EPR signals at g ∼ 2. It is apparent
from the time dependence of the spectral changes that two
Ni(III) species (B and C) are formed, with B preceding C. 18
O
labeling data and comparison with spectra obtained with
NaOBr indicate that B has the structure [(L)NiIII−OH(S)]2+.
Furthermore, the complex [(L)NiII−OCl]+ (A) is detected by
ESI-MS, preceding the appearance of B. Thus, A may undergo
homolytic O−Cl cleavage, assisted by H+ to form B and a
chloride atom (i.e., protonation of LNi(II)-OCl to form Ni(II)-
O(H)-Cl allows for homolytic cleavage of the O−Cl bond by
stabilizing the Ni(III)−OH species formed).25,30 Spectroscopic
and ESI-MS analysis also shows formation of species C which is
assigned, tentatively, as [(L)NiIII-OOCCH3]2+. Use of
deuterated acetonitrile reveals that these species originate
1
from the hydrolysis of acetonitrile. EPR, H NMR spectrosco-
py, and ESI-MS indicate that Ni(III) species are not stable
under reaction conditions, and that they are eventually reduced
to Ni(II) species, closing the catalytic cycle. The most likely
path for this process involves 2e− oxidation of the OCl− ion,
forming the corresponding chlorine dioxide.
A particularly robust aspect of this mechanistic proposal
(Scheme 2) is that none of the metal species appear to be a
Figure 15. Addition of a second batch of NaOCl (21.4 equiv) to the
solution shown in Figure 14 (a) with triflic acid and (b) with acetic
acid monitored by Raman spectroscopy at λexc 473 nm. Conditions as
in Figure 14. Spectra were normalized to the CD3CN band (#) at 686
cm−1.
Scheme 2. Proposed Mechanism for C−H Chlorination
a
Reaction
DISCUSSION
■
Solid state and solution characterization of 1 indicates that the
nickel center adopts an octahedral coordination geometry with
two acetonitrile ligands binding at the two coordination sites
not occupied by the tetradentate ligand (Figure 2).11,12
Addition of water to an acetonitrile solution of 1 leads to the
displacement of CH3CN ligands by either aqua or hydroxido
ligands; however, the overall structure of the complex is
retained (Figure S3). The complex reacts with NaOCl and
catalyzes the chlorination of alkanes, although the product
distribution and KIE imply a highly reactive, poorly
discriminating species, most likely chloride atoms. Spectro-
scopic studies provide direct evidence that addition of NaOCl
to 1 leads to the formation of a mixture of species in both the
Ni(II) and Ni(III) oxidation states, with no evidence for higher
1
a
valence nickel species (Table 3). H NMR spectroscopy and
The precise nature of reductant involved in the conversion of C to D
ESI-MS show that Ni(II) species are present during the whole
is unclear but is certain to involve interchange between the various
course of the reaction of 1 with hypohalites, with [(L)NiII-Cl]+
redox states of chlorine, e.g., ClO2 and ClO2.
−
H
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Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
(0.26 mmol, 66%). The sample for elemental analysis was crushed and
kept under vacuum for 12 h prior to the analysis during which time 1
changed color to pale violet. Anal. Calcd for (1 + MeCN)
C18H27F6N5NiO6S2: C, 33.5; H, 4.2; N, 10.8; S, 9.9%. Found: C,
33.7; H, 4.0; N, 10.7; S, 10.1 ESI-MS (m/z): 455.1 [M − OTf]+
priori competent to react with alkanes. Ni(III)-OR species have
been observed recently31 and appear to be sluggish oxidants,
only capable of undergoing hydrogen atom transfer (HAT)
with weak O−H bonds. This implies that either chlorine atoms
are responsible for activating the substrate, generating a free
diffusing carbon centered radical, with the alkyl halide forming
by subsequent reaction of this radical with a chlorinated species,
most likely Cl· or Cl2. Both product analysis, which shows only
modest C3/C2 regioselectivity in the oxidation of adamantane,
and the KIE in the case of oxidation of cyclohexane are
consistent with this interpretation. Furthermore, evidence for
long-lived carbon centered diffusing radicals is provided by the
observation of stereo scrambling in the oxidation of decalins
and cis-1,2-dimethylcyclohexane.
Finally it should be noted that the strength of the acid has an
effect of the steady state concentration of the species observed.
The pKa of acetic acid in acetonitrile is close to 23.5, while for
CF3SO3H it is 0.7.32 Hence under general acid catalyzed
conditions the strength of the acid will determine the rate of
reaction and the maximum concentration of intermediates B
and C reached. The precise reason for this is not yet clear but is
likely to reflect a reduced driving force for reduction of the
Ni(III) species to Ni(II), i.e., changes in the chemistry of
chlorine rather than the nickel complex, as there are no obvious
changes to the spectroscopic properties of the nickel based
intermediates observed.
(100%), 153.0 [M]2+ (16%). UV/vis Absorption in acetonitrile λmax
=
275 nm (745 M−1 cm−1), 321 nm (100 M−1 cm−1), 525 nm (10 M−1
cm−1), 809 nm (20 M−1 cm−1) and 906 nm (14 M−1 cm−1).
Catalysis. 63 μL NaOCl (104.9 μmol, (10−15 active chlorine)
NaOCl) in water was added to a stirred solution of [(Me,HPyTACN)-
NiII(CH3CN)2](OTf)2 (1) (1.55 μmol), alkane (255.4 μmol) in 1 mL
of acetonitrile in a 10 mL sealed vial. The resulting solution was stirred
vigorously for 2 h. Reactions were carried out at room temperature.
After 2 h the internal standard (biphenyl) was added, and the solution
was filtered through a plug of basic alumina, which was subsequently
rinsed with 3 mL of dichloromethane. Samples were filtered over a
short silica plug and analyzed by GC (Agilent 6890 equipped with a
HP-1 dimethylpolysiloxane column). For KIE determination a
CPWax25 column was used. Products were quantified with respect
to the biphenyl internal standard.
1
Experimental. H NMR spectra (400 MHz) were recorded on a
Varian Mercury Plus. Chemical shifts are denoted relative to the
residual solvent peak (1H NMR spectra CD3CN, 1.94 ppm).
Elemental analyses were performed using a CHNS-O EA-1108
elemental analyzer from Fiscons. UV/vis absorption spectra were
recorded with a HP8453 spectrophotometer or a Specord600
(AnalytikJena) in 10 mm and 2 mm path length quartz cuvettes.
Electrochemical measurements were carried out on a model CHI760B
Electrochemical Workstation (CH Instruments). Analyte concen-
trations were typically 1 mM in acetonitrile containing 0.1 M
tetrabutylammonium hexafluorophosphate [(TBA)PF6]. Unless stated
otherwise, a 3 mm diameter Teflon-shrouded glassy carbon working
electrode (CH Instruments), a Pt wire auxiliary electrode, and an SCE
reference electrode were employed. Cyclic voltammograms were
obtained at sweep rates between 0.1 V s−1 and 1 V s−1. All potential
values are quoted with respect to the SCE. EPR spectra (X-band, 9.46
GHz) were recorded on a Bruker ECS106 spectrometer in liquid
nitrogen (77 K). Samples for measurement (300 μL) were transferred
to EPR tubes, which were flash frozen in liquid nitrogen. High
resolution mass spectra (HRMS) were recorded on a Bruker
CONCLUSIONS
■
In conclusion, the nickel(II) complex 1 has been prepared and
characterized spectroscopically, structurally, and electrochemi-
cally. 1 mediates the halogenation of alkanes with NaOCl with
notable chemoselectivity in favor of halogenated over non-
halogenated products. The complexes involved in the reaction
have been identified as [(L)NiII−OCl(S)]+ (A), [(L)NiIII−
OH(S)]2+ (B), [(L)NiIII−OOCCH3]2+ (C), and [(L)NiII−
OOCCH3]+ (D) (where S is solvent) by ESI-MS, UV/vis
absorption, EPR and Raman spectroscopy. These intermediates
are involved a catalytic cycle where chlorine atoms are
generated in a Fenton-like process (i.e., where free diffusing
reactive radical species that engage in substrate oxidation are
generated by a metal catalyst) in which the nickel center cycles
between Ni(II) and Ni(III). Chlorine atoms and chlorine
dioxide are powerful reactive species capable of homolysis of
the strong C−H bond of alkanes, forming alkyl radicals, which
are eventually trapped by chlorine atoms, or Cl2, forming
chlorinated alkanes. Unfortunately, as the alkane functionaliza-
tion reaction is not exclusively a metal-based process, the
regioselectivity of these reactions is at best modest. Efforts to
develop novel catalysts where halogenation involve metal based
C−H cleaving reagents are being pursued in our laboratories.
̀
MicrOTOF-Q II Instrumental at Serveis Tecnics of the University
of Girona. A cryospray attachment was used for CSI-MS (cryospray
mass spectrometry). The temperature of the nebulizing and drying
gases was set at 5 and 0 °C, respectively. Samples were introduced into
the mass spectrometer ion source by direct infusion using a syringe
pump. Spectra were calibrated externally using sodium formate. The
instrument was operated in positive ion mode. FTIR spectra were
recorded using a UATR (ZnSe) with a PerkinElmer Spectrum400,
equipped with a liquid nitrogen cooled MCT detector. Raman spectra
were recorded at λexc 785 nm using a PerkinElmer Raman Station at
room temperature. Raman spectra at 532 nm were recorded using a
home-built system consisting of an Andor Technology iDus-420-OE
CCD camera, a Shamrock163 spectrograph and a 532 nm (300 mW,
Cobolt) laser, both fiber coupled to an Inphotonics 532 nm Raman
probe. Raman spectra were obtained at 473 nm (100 mW at source,
Cobolt Lasers) to the sample through a 2.5 cm diameter plano-convex
lens (f = 7.5 cm) and Raman scattering collected in a 135° or 180°
backscattering arrangement. The collimated Raman scattering was
focused by a second 2.5 cm diameter plano convex lens (f = 7.5 cm)
through a long pass edge filter (Semrock) into a Shamrock300i
spectrograph (Andor Technology) with a 1200 L/mm grating blazed
at 500 nm, or 2400 L/mm blazed at 400 nm and acquired with an
DV420A-BU2 CCD camera (Andor Technology). The spectral slit
width was set to 10 or 20 μm. Each spectrum was accumulated,
typically 6 or 60 times with 3 s acquisition time, resulting in a total
acquisition time of between 18 to 180 s per spectrum. Data were
recorded and processed using Solis (Andor Technology) with spectral
calibration performed using the Raman spectrum of acetonitrile/
toluene 50:50 (v:v). Samples were held in quartz 10 mm path length
cuvettes. A multipoint baseline correction was performed for all
EXPERIMENTAL SECTION
■
All solvents and reagents were of commercial grade and used without
further purification unless stated otherwise. Solvents and reagents for
spectroscopic and electrochemistry were UVASOL grade (Merck).
The ligand Me, HPyTACN was available from earlier studies.9
[Ni(Me, HPytacn)(CH3CN)2](OTf)2 (1). A solution of [Ni-
(CH3CN)6](OTf)2 (242.7 mg, 0.40 mmol) in anhydrous THF (1
mL) was added dropwise to a vigorously stirred solution of Me,HPytacn
(99.8 mg, 0.40 mmol) in CH2Cl2 (1 mL). After 2 h, the solution
becomes cloudy and a pale violet precipitate appeared. After being
stirred for 2 h, the solution was filtered, and the solid obtained was
dried under a vacuum. The solid was dissolved in CH3CN and slow
diffusion of diethyl ether afforded 102 mg of intense violet crystals
I
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Inorg. Chem. XXXX, XXX, XXX−XXX

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Article
spectra. At 785 nm (nonresonant) Raman spectra were recorded using
0.1 M solutions.
(10) (a) El Ghachtouli, S.; Cadiou, C.; Dechamps-Olivier, I.;
Chuburu, F.; Aplincourt, M.; Patinec, V.; Le Baccon, M.; Handel, H.;
Roisnel, T. New J. Chem. 2006, 30, 392−398. (b) Jones, D. G.; Wilson,
K. R.; Cannon-Smith, D. J.; Shircliff, A. D.; Zhang, Z.; Chen, Z. Q.;
Prior, T. J.; Yin, G. C.; Hubin, T. J. Inorg. Chem. 2015, 54, 2221−2234.
(c) Batsanov, A. S.; Goeta, A. E.; Howard, J. A. K.; Maffeo, D.;
Puschmann, H.; Williams, J. A. G. Polyhedron 2001, 20, 981−986.
(d) Khusnutdinova, J. R.; Luo, J.; Rath, N. P.; Mirica, L. M. Inorg.
Chem. 2013, 52, 3920−3932.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.5b01463.
Details of X-ray structural analysis, additional FTIR,
Raman, 1H NMR, and UV/vis absorption spectral,
electrochemical, and ESI-MS data (PDF)
(11) Nagataki, T.; Ishii, K.; Tachi, Y.; Itoh, S. Dalton Trans. 2007,
1120−1128.
(12) Holman, T. R.; Hendrich, M. P.; Que, L., Jr. Inorg. Chem. 1992,
31, 937−939.
(13) Szajna, E.; Dobrowolski, P.; Fuller, A. L.; Arif, A. M.; Berreau, L.
M. Inorg. Chem. 2004, 43, 3988−3997.
(14) Hage, R.; Gunnewegh, E. A.; Niel, J.; Tjan, F. S. B.;
Weyhermuller, T.; Wieghardt, K. Inorg. Chim. Acta 1998, 268, 43−48.
(15) Fenton, H. J. H. J. Chem. Soc., Trans. 1894, 65, 899−910.
(16) See for example: Kovacic, P.; Chang, J.-H. C. J. Chem. Soc. D
1970, 1460−1461. Kovacic, P.; Chen Chang, J.-H. J. Org. Chem. 1971,
36, 3138−3145.
AUTHOR INFORMATION
Corresponding Authors
*(W.R.B.) E-mail: w.r.browne@rug.nl.
*(M.C.) E-mail: miquel.costas@udg.edu.
■
Notes
The authors declare no competing financial interest.
(17) (a) Fonouni, H. E.; Krishnan, S.; Kuhn, D. G.; Hamilton, G. A.
J. Am. Chem. Soc. 1983, 105, 7672−7676. (b) Bunce, N. J.; Ingold, K.
U.; Landers, J. P.; Lusztyk, J.; Scaiano, J. C. J. Am. Chem. Soc. 1985,
107, 5464−5472.
ACKNOWLEDGMENTS
■
Financial support comes from the European Research Council
(ERC-2011-StG-279549, W.R.B. and ERC-2009-StG-239910,
M.C.), The Netherlands Fund for Technology and Science
STW (11059, W.R.B.) the Ministry of Education, Culture and
Science (Gravity program 024.001.035, A.D., W.R.B.) and the
Ubbo Emmius Fund of the University of Groningen (AD).
M.C. thanks MINECO of Spain (CTQ2012-37420-C02-01/
BQU, CSD2010-00065) and Generalitat de Catalunya
(18) It should be noted that due to the particulates formed the
integration of peaks areas is subject to systematic errors and
conversions cannot be viewed as accurate to less than 5%.
(19) Mohrig, J. R.; Nienhuis, D. M.; Linck, C. F.; Van Zoeren, C.;
Fox, B. G.; Mahaffy, P. G. J. Chem. Educ. 1985, 62, 519−521.
(20) The formation of ClO2 upon acidification of NaOCl containing
solutions gives rise to a yellow colored solution, see Figure S21. A
recent example of such disproportionations catalyzed by manganese
complexes can be found in Hicks, S. D.; Kim, D.; Xiong, S.; Medvedev,
G. A.; Caruthers, J.; Hong, S.; Nam, W.; Abu-Omar, M. M. J. Am.
Chem. Soc. 2014, 136, 3680−3686.
̀
(2009SGR637, and ICREA Academia Award). We thank A.
Call for the synthesis of 1. The COST action CM1003
“Biological oxidation reactions−mechanism and design of new
catalyst” for funding of a short-term-scientific-mission to AD.
(21) Ca(OCl)2 is available in solid form allowing for better precision
in stoichiometry and has a different content of other chlorine species.
(22) Because of attenuation by scattering from particles that are
formed in solution, a detailed kinetic analysis by UV/vis absorption
spectroscopy is precluded.
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́
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(29) The smaller shift than expected could indicate that the mode is
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