Aldehyde deformylation and catalytic C-H activation resulting from a shared cobalt (II) precursor

Article abstract of DOI:10.1021/acs.inorgchem.6b02127

The tetradentate ligand N,N'-dibenzyl-N,N'-bis(2-pyridylmethyl)- 1,2-cyclohexanediamine (bbpc) was used to prepare cobalt(II) diacetonitrilo and cobalt(III) peroxo complexes, the latter of which was structurally characterized. The cobalt(III) peroxo compound forms from reactions between the cobalt(II) complex, hydrogen peroxide, and a base, and it stoichiometrically reacts with aldehydes to yield mixtures of alkenes and ketones. The cobalt(II) precursor is capable of catalyzing the activation of weak C-H bonds by either iodosobenzene or m-chloroperbenzoic acid. This chemistry differs from most previously characterized cobalt-mediated C-H activation in that (1) it is catalytic, rather than stoichiometric, with respect to the cobalt and (2) it does not need a second Lewis acid metal ion in order to proceed.

Full text of DOI:10.1021/acs.inorgchem.6b02127

Article
pubs.acs.org/IC
Aldehyde Deformylation and Catalytic C−H Activation Resulting
from a Shared Cobalt(II) Precursor
Qiao Zhang, Angela Bell-Taylor, Fraser M. Bronston, John D. Gorden, and Christian R. Goldsmith*
Department of Chemistry and Biochemistry, Auburn University, Auburn, Alabama 36849, United States
*
S Supporting Information
ABSTRACT: The tetradentate ligand N,N′-dibenzyl-N,N′-bis(2-pyridylmeth-
yl)-1,2-cyclohexanediamine (bbpc) was used to prepare cobalt(II) diacetonitrilo
and cobalt(III) peroxo complexes, the latter of which was structurally
characterized. The cobalt(III) peroxo compound forms from reactions between
the cobalt(II) complex, hydrogen peroxide, and a base, and it stoichiometrically
reacts with aldehydes to yield mixtures of alkenes and ketones. The cobalt(II)
precursor is capable of catalyzing the activation of weak C−H bonds by either
iodosobenzene or m-chloroperbenzoic acid. This chemistry differs from most
previously characterized cobalt-mediated C−H activation in that (1) it is
catalytic, rather than stoichiometric, with respect to the cobalt and (2) it does
not need a second Lewis acid metal ion in order to proceed.
INTRODUCTION
Scheme 2
■
The interactions between dioxygen (O ) and metal ions
continue to fascinate chemists even after decades of study.
2
Metal−O adducts have been characterized in a wide variety of
2
biomolecules, including several classes of oxido-reductase
enzymes, as well as their functional small-molecule
1
−10
mimics.
Such species are also highly relevant to those
studying the metal-catalyzed oxidation of water (H O) to
2
1
1−16
dihydrogen and O
substrates by O2.
and the oxidation of hydrocarbon
In the recently reported cobalt-catalyzed
2
1
7−23
29,31−36
have been found to behave as nucleophiles,
but a recent
H O oxidation, binuclear cobalt(IV) oxo species are proposed
2
report noted that protonating a cobalt(III) peroxo complex to a
cobalt(III) hydroperoxo species enables the oxidation of weak
12,13
as key intermediates for O2 generation;
a binuclear
35
cobalt(III) peroxo species could conceivably result upon
formation of the first of the two O−O bonds (Scheme 1). A
prior report implicated the agency of a mononuclear Co(IV)-
C−H bonds.
Metal oxo complexes have been found to participate in
oxygen-atom-transfer and hydrogen-atom-abstraction reac-
2
4,37−43
oxo species in cobalt-catalyzed O production from tris(2,2′-
2
tions.
Ray’s research group recently reported a cobalt-
15
44
bipyridine)ruthenium(III) and H O.
2
(IV) oxo species with a tripodal tetradentate ligand, but
subsequent research from the Borovik group suggested that this
may instead be better assigned as a Co -OH complex. The
debate about the identity of this species highlights that extreme
caution must be taken when making such assignments for metal
III
45
Scheme 1
16,46,47
ions close to the “oxo wall”.
The higher-valent oxidants
generated from the cobalt(II) precursors by the aforemen-
tioned groups require Lewis acid additives to form and react
stoichiometrically with weak C−H bonds, such as the benzylic
In recent years, many advances have been made with respect
to the preparation of coordination complexes with ligands that
44,45
ones in 9,10-dihydroanthracene.
A cobalt(II) complex
can potentially be derived from O . Nam, Que, and co-workers
2
reported by the Tilley group reacts with O and other terminal
2
have used a series of tetramethylated macrocycles (TMCs,
Scheme 2) to stabilize a widening range of complexes relevant
to O2 activation that include metal oxo, peroxo, and
oxidants to activate the much stronger C−H bonds in
48
acetonitrile (MeCN). A cobalt(IV) oxo species was proposed
to be responsible for the C−H activation; the cobalt(III)
superoxo complex initially formed from the reaction between
24−35
hydroperoxo compounds.
The stabilities and reactivities
of these with organic molecules have been found to depend on
2
8−30
the identity of the ligand,
but clarification of the
structure−function relationships remains an ongoing process.
The metal peroxo complexes with TMCs and other ligands
Received: September 6, 2016
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.6b02127
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
cobalt(II) and O is unlikely to be a strong enough oxidant to
abstract a hydrogen atom from MeCN.
of exposures (frames). Each set had a different φ angle for the crystal,
and each exposure covered a range of 0.5° in ω. A total of 1464 frames
were collected with an exposure time per frame of 20−60 s, depending
on the crystal. The SAINT software was used for data integration
including Lorentz and polarization corrections. Semiempirical
absorption corrections were applied using the program SADABS or
TWINABS. Selected crystallographic information is listed in Tables 1
and 2. Atomic coordinates and additional structural information are
provided as Supporting Information.
2
49
Previous work from our group used the ligand N,N′-
dibenzyl-N,N′-bis(2-pyridylmethyl)-1,2-cyclohexanediamine
III
(
bbpc; Scheme 2) to prepare an Fe -OOH complex from both
1
8,50
O and hydrogen peroxide (H O ).
The steric bulk of the
2
2
2
ligand modulates the regioselectivity of the iron-catalyzed
oxidation of hydrocarbons by H O and sufficiently stabilizes
2
2
III
18,50,51
the Fe -OOH species for extensive characterization.
Kinetic analysis of the formation of this compound suggests
Table 1. Selected Crystallographic Data for 2
that the O chemistry may proceed through an iron(III)
2
1
8,51
empirical formula
formula
[Co(bbpc)(O )](ClO )
superoxo species.
In the current work, we use the bbpc
2
4
C H Cl Co N O
ligand to prepare stable cobalt(II) and cobalt(III) peroxo
complexes. We subsequently interrogated the reactivities of
these species with various terminal oxidants and small-molecule
substrates and found that the bbpc ligand, much like 12-
68 72
2
2
10 13
MW
1426.12
cryst syst
space group
a (Å)
triclinic
̅
P1 (No. 2)
3
5
13.7667(11)
15.0930(13)
17.3669(15)
97.363(2)
105.117(2)
103.647(2)
3315.4(5)
4
TMC, could support the formation of both electrophilic and
nucleophilic metal-based oxidants from a shared cobalt(II)
precursor.
b (Å)
c (Å)
α (deg)
β (deg)
γ (deg)
EXPERIMENTAL SECTION
■
V (Å3)
Materials. Cobalt(II) perchlorate hexahydrate (Co(ClO ) ·6H O),
4
2
2
triethylamine (Et N), benzaldehyde, 4-chlorobenzaldehyde, 1,2-
3
Ζ
dichlorobenzene, xanthene, 9,10-dihydroanthracene (DHA), anthra-
cene, 1,4-cyclohexadiene (CHD), acetophenone, cyclohexanone,
cyclohexene, and m-chloroperbenzoic acid (MCPBA) were purchased
from Sigma-Aldrich. Acetonitrile (MeCN), cyclohexanecarboxalde-
cryst color
red
T (K)
198
reflns collected
unique reflns
42371
10837
hyde (CCA), and triphenylphosphine (PPh ) were purchased from
3
a
R1(F) [I > 2σ(I)]
0.0948
Acros Organics. 2-Phenylpropionaldhyde (2-PPA), iodosobenzene
2
a
wR2(F ) (all data)
0.2774
(
PhIO), 4-fluorobenzaldehyde, and p-tolualdehyde were obtained
a
R1 = ∑||F | − |F ||/∑||F |; wR2 = [∑w(F − F ) /∑wF ]1/2.
2
2
2
4
o
from TCI America. All deuterated solvents were purchased from
Cambridge Isotopes. Methanol (MeOH) and ethanol were acquired
from Pharmaco-Aaper and EMD Chemicals, respectively. Hydrogen
peroxide (H O ; 50 wt %) was bought from Fisher Chemical and
o
c
o
o
c
Synthesis. Caution! Perchlorate salts of metal complexes are
2
2
potentially explosive and should be handled with care. Precautionary
measures include working with small quantities and low concentrations of
these reagents. Appropriate safety measures, such as protective shields,
should be employed during their syntheses and subsequent handling.
calibrated by titration of KMnO and H SO in H O. N,N′-Dibenzyl-
4
2
4
2
N,N′-bis(2-pyridylmethyl)-1,2-cyclohexanediamine (bbpc) was syn-
50
thesized as described previously. PhIO was kept in a 253 K freezer
when not in use. H O and MCPBA were kept in a 277 K refrigerator
2
2
−1
bbpc. IR data were collected for this compound. IR (KBr, cm ):
002 (w), 2937 (w), 2805 (w), 1963 (w), 1734 (w), 1587 (m), 1567
when not in use.
3
1
Instrumentation. H NMR spectra were recorded on either a 400
MHz or a 250 MHz AV Bruker NMR spectrometer at 294 K. A Varian
Cary 50 spectrophotometer was used to collect optical data, which
were processed and analyzed using software from the WinUV Analysis
Suite. IR data were obtained using a Shimadzu IR Prestige-21 FT-IR
spectrophotometer. A Thermo Scientific Trace GC ultra gas
chromatograph and Thermo Scientific TR-1 and TG-WAXMS
columns were used for gas chromatography (GC). A Johnson Matthey
magnetic susceptibility balance (model MK I#7967) was used to
measure the magnetic moments of solid samples. Pascal’s constants
were used to estimate the diamagnetic correction to the suscepti-
(
m), 1494 (m), 1473 (m), 1451 (m), 1362 (m), 1307 (m), 1264 (m),
1
9
6
212 (m), 1147 (s), 1114 (s), 1092 (s), 1073 (s), 1045 (s), 1027 (s),
97 (s), 969 (s), 945 (s), 918 (m), 850 (s), 824 (s), 768 (s), 702 (s),
54 (m), 618 (m), 567 (s), 535 (s), 478 (s), 443 (m).
cis-Diacetonitrilo[N,N′-dibenzyl-N,N′-bis(2-pyridylmethyl)-1,2-
II
cyclohexanediamine]cobalt(II) Perchlorate ([Co (bbpc)(MeCN) ]-
2
(
6
ClO ) , 1). The bbpc ligand (0.476 g, 1.00 mmol) and Co(ClO ) ·
4 2 4 2
H O (0.366 g, 1.00 mmol) were dissolved in 10 mL of MeCN and
2
stirred for 2 h. After this time, the solvent was removed through
rotavaporation. The red residue was dissolved in 2.5 mL of fresh
MeCN. The gradual diffusion of Et O into the solution deposited red
2
5
2
bility. Electron paramagnetic resonance (EPR) spectra were
collected on a Bruker EMX-6/1 X-band EPR spectrometer operated
in the perpendicular mode. High-resolution mass spectrometry (MS)
data were collected at the Mass Spectrometer Center at Auburn
University on a Bruker microflex LT MALDI-TOF mass spectrometer
via direct probe analysis operated in the positive-ion mode. Crystalline
crystals of the product (0.693 g, 85%) that were not suitable for single-
crystal X-ray diffraction. Solid-state magnetic susceptibility (294 K):
−1
μeff = 3.9 μ . Optical spectroscopy [MeCN, 294 K; λ, nm (ε, M
B
−1
1
cm )]: 258 (7850), 475 (50). H NMR (CD CN, 294 K, 400 MHz):
3
δ 95.0, 86.6, 84.0, 82.4, 78.6, 75.9, 74.3, 72.4, 67.8, 66.5, 63.3, 59.4,
5
8.8, 48.3, 40.0, 38.1, 35.2, 32.2, 31.5, 27.7, 25.1, 23.2, 22.5, 21.0, 19.3,
samples were dried, stored under N , and sent to Atlantic Microlabs
2
16.4, 15.1, 14.0, 13.4, 12.4, 11.3, 10.0, −1.1, −1.5, −3.4, −4.6, −11.1,
(
Norcross, GA) for elemental analysis.
Crystallographic Studies. Single crystals of [Co(bbpc)(O )]-
−1
−
13.1, −13.7, −25.3. EPR (MeCN, 4 K): g = 4.3. IR (KBr, cm ):
2
3439 (m), 2938 (w), 2863 (w), 2026 (w), 1695 (s), 1610 (s), 1484
(m), 1449 (s), 1384 (m), 1352 (m), 1311 (w), 1297 (w), 1206 (m),
(
ClO ) (2) were mounted on CryoLoops with Krytox oil and optically
4
aligned on a Bruker APEXII Quazar X-ray diffractometer using a
digital camera. Initial intensity measurements were performed using an
IμSX-ray source and a 30 W microfocused sealed tube (Mo Kα, λ =
1
145 (s), 1122 (s), 977 (m), 930 (w), 884 (w), 828 (w), 762 (s), 708
(
m), 625 (s). Elem anal. Calcd for C H CoCl N O ·2H O: C, 50.71;
36
42
2
6
8
2
H, 5.44; N, 9.86. Found: C, 50.96; H, 5.30; N, 9.86.
0
.71073 Å) with high-brilliance and high-performance focusing Quazar
2
η -Peroxo[N,N′-dibenzyl-N,N′-bis(2-pyridylmethyl)-1,2-
III
multilayer optics. Standard APEXII software was used for determi-
nation of the unit cells and data collection control. The intensities of
the reflections of a sphere were collected by a combination of four sets
cyclohexanediamine]cobalt(III) Perchlorate ([Co (bbpc)(O )](ClO ),
2). A solution of H O (31 μL, 0.50 mmol) in 1.0 mL of MeCN was
2
4
2
2
added to a solution of 1 (82 mg, 0.10 mmol) and Et N (35 μL, 0.25
3
B
DOI: 10.1021/acs.inorgchem.6b02127
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
a
Table 2. Selected Bond Lengths (Å) and Angles (deg) for 2
bond length
cation A
cation B
bond angle
cation A
cation B
Co−N(1)
Co−N(2)
Co−N(3)
Co−N(4)
Co−Navg
Co−O(1)
Co−O(2)
Co−Oavg
1.961(6)
2.012(6)
1.982(6)
1.910(6)
1.966
1.954(6)
2.012(6)
1.984(6)
1.919(7)
1.967
O(1)−Co−O(2)
O(1)−Co−N(1)
O(1)−Co−N(2)
O(1)−Co−N(3)
O(1)−Co−N(4)
O(2)−Co−N(1)
O(2)−Co−N(2)
O(2)−Co−N(3)
O(2)−Co−N(4)
N(1)−Co−N(2)
N(1)−Co−N(3)
N(1)−Co−N(4)
N(2)−Co−N(3)
N(2)−Co−N(4)
N(3)−Co−N(4)
46.1(2)
104.9(2)
93.5(2)
157.1(2)
95.5(3)
151.0(2)
95.6(2)
111.0(2)
90.7(2)
84.3(3)
98.0(3)
93.2(3)
87.8(2)
171.0(3)
84.0(3)
45.5(2)
104.2(2)
93.6(2)
157.6(2)
95.0(3)
149.8(2)
95.5(2)
112.1(2)
90.5(2)
84.3(3)
98.1(3)
93.6(3)
87.7(3)
171.4(3)
84.3(3)
1.863(5)
1.860(5)
1.862
1.864(5)
1.849(5)
1.857
O(1)−O(2)
1.458(7)
1.437(7)
a
N(1) and N(4) correspond to pyridine nitrogen atoms; N(2) and N(3) correspond to amine nitrogen atoms. The donor atoms of cation B have
been relabeled from their CIF assignments in order to facilitate comparison to their counterparts in cation A.
Scheme 3. Proposed Structures of Co(bbpc) Complexes
mmol) in 9.0 mL of MeCN at 273 K. After 5 min, the solvent and
obtained from plots of kobs versus the concentration of the aldehyde
substrates.
For the C−H activation reactions, the cobalt(II) compound 1,
unreactive internal standard, and hydrocarbon substrate were dissolved
in 2.5 mL of MeCN, with initial concentrations of 1.0 and 50 mM,
respectively. A total of 25 equiv of either PhIO or MCPBA were added
as a solid, after which the reaction vessel was sealed. During the
reaction, aliquots were removed via syringe, diluted with ether, filtered
through silica gel, and analyzed via GC.
excess Et N were removed through rotavaporation to yield a dark-red
3
solid. Crystalline product was obtained from the diffusion of Et O into
2
a solution of the crude in MeCN at 253 K (49 mg, 74%). Optical
−1
−1
spectroscopy [MeCN, 294 K; λ, nm (ε, M cm )]: 320 (3100), 505
1
(
350). H NMR (CD CN, 294 K, 400 MHz): δ 8.08 (1H, t, J = 7.8
3
Hz), 7.78 (2H, d, J = 7.2 Hz), 7.67 (1H, d, J = 8.0 Hz), 7.45 (8H, m),
.18 (1H, d, J = 5.2 Hz), 7.09 (4H, m), 6.49 (1H, d, J = 7.6 Hz), 5.19
1H, d, J = 12 Hz), 4.63 (1H, d, J = 12 Hz), 4.58 (1H, d, J = 16 Hz),
.02 (2H, dd, J = 16 Hz, J = 5.8 Hz), 3.79 (1H, d, J = 16 Hz), 3.68
7
(
4
1
2
(
2
1H, d, J = 14 Hz), 3.44 (1H, t, J = 5.4 Hz), 3.32 (1H, d, J = 14 Hz),
.57 (1H, t, J = 12 Hz), 2.09 (substantial overlap with the MeCN peak
prevents accurate integration, t, J = 4.8 Hz), 1.67 (1H, d, J = 14 Hz),
.31 (1H, d, J = 12 Hz), 1.1−0.79 (5H, m). IR (KBr, cm ): 3424 (m),
938 (w), 2861 (w), 1605 (s), 1448 (m), 1384 (m), 1352 (m), 1295
RESULTS
■
Synthesis and Spectroscopic Characterization. The
cobalt(II) species 1 (Scheme 3) forms readily from mixtures of
bbpc and Co(ClO ) ·6H O. The complex can be isolated in
−1
1
2
(
(
5
4
2
2
w), 1261 (w), 1146 (s), 1122 (s), 1119 (s), 941 (w), 929 (w), 803
high yields through precipitation from MeCN/Et O mixtures.
2
+
m), 762 (m), 706 (m), 625 (s). MS (ESI). Calcd for M : m/z
Attempts to crystallize this species have thus far been
unsuccessful, even with alternative counteranions. The material
is highly hygroscopic and needs to be dried under a vacuum in
order to remove most of the adventitious H O. The magnetic
susceptibility of solid-state samples is consistent with a μ value
67.2170. Found: m/z 567.2093. Elem anal. Calcd for
C H ClCoN O ·1.5H O: C, 55.38; H, 5.66; N, 8.07. Found: C,
5
3
2
36
4
6
2
5.10; H, 5.37; N, 8.04.
Reactivity. Unless stated otherwise, all reactivity studies were done
2
at 298 K in MeCN under air. The yields of organic products were
determined by GC; all reported values are the average of at least three
independent measurements. The listed errors represent one standard
deviation. The identities and yields of products were confirmed by
comparing the GC retention times with those of authentic compounds
and by comparing the peak integrations with that of a nonreactive 1,2-
dichlorobenzene internal standard.
eff
7
1
of 3.9 μ and a high-spin d metal center. As anticipated, the H
B
NMR spectrum of 1 is consistent with a paramagnetic species,
with peaks ranging from −30 to +100 ppm (Figure S1). The
number of peaks exceeds the number of hydrogen atoms on the
bbpc ligand and suggests that multiple conformational or
solvate isomers are present. Complex 1 is EPR-silent at 77 K
but displays a feature at g = 4.3 at 4 K (Figure S2), similar to
that observed for other high-spin cobalt(II) complexes.
Unlike its iron-containing analogue and some cobalt(II)
complexes with other pyridylamine ligands, 1 does not react
rapidly with O
For the reactions between 2 and aldehydes, complex 2 was made in
situ from 2.0 mM 1, 10 mM H O , and 5.0 mM Et N in MeCN, as was
2
2
3
53,54
2
9
done for an earlier study of cobalt(III) peroxo reactivity. The
disappearance of 2 was followed by monitoring the local absorbance
maximum at 505 nm; pseudo-first-order rate constants, kobs, were
calculated from these data. At least three values of kobs were measured
, even in the presence of substrates with weak
2
18,55
per concentration of the aldehyde substrate. The listed k values were
allylic and benzylic C−H bonds.
The cobalt(II) complex
2
C
DOI: 10.1021/acs.inorgchem.6b02127
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
can be stored for several weeks under air without any evidence
of degradation, but a sample that was stored under air for
approximately one year partly decomposed over this time, as
evidenced by changes to its UV/vis spectrum. The MeCN
ligands exchange readily, and markedly different UV/vis spectra
are obtained when the compound is dissolved in other
coordinating solvents, such as MeOH (Figure S3).
Complex 1 can be oxidized to 2 (Scheme 3) upon reaction
with slight excesses of H O and Et N. Upon oxidation, the
2
2
3
dark-red solutions of 2 persist at room temperature with no
noticeable discoloration for several hours. Although the
compound can be precipitated in high yields, we limited the
scale of the reaction to under 100 mg because of the potential
explosive hazards associated with the peroxides and perchlorate
+
Figure 1. ORTEP representation of cation A, [Co(bbpc)(O )] , in the
1
2
salts. The isolated 2 is diamagnetic, as assessed by H NMR
crystal structure of 2. Hydrogen atoms, solvent molecules, perchlorate
counteranions, and cation B have been omitted for clarity. All thermal
ellipsoids are drawn at 50% probability. The cobalt atom has been
relabeled from its original CIF designation.
(Figure S4) and the absence of an EPR signal. The UV/vis
absorption shoulder at 505 nm clearly distinguishes 2 from the
other cobalt species in this paper and gives the solid a dark-red
color (Figure S3). Attempts to obtain resonance Raman data
capable of providing further insight into the strength and
electronic character of the O−O bond were unsuccessful. Both
with the greatest distortions deriving from the small bite angles
associated with the κ -peroxo ligand (Figure 1 and Table 2).
2
cobalt complexes and the free bbpc ligand have bands between
Each bbpc ligand wraps around a single cobalt atom in a cis-β
conformation, which places the two pyridine rings cis to each
other. Three previously characterized coordination complexes
with the bbpc ligand instead displayed either the cis-α
conformation, in which the terminal binding groups of the
tetradentate ligand are trans to each other, or the trans
conformation, in which the donors from the tetradentate ligand
8
00 and 900 cm− in their IR spectra (Figures S5−S7),
1
preventing us from definitively assigning any of these peaks to
O−O stretches.
The H NMR spectrum of 2 indicates that the ligand is
1
bound asymmetrically to the metal center (Figure S4). The
pyridine rings are inequivalent, as are the methylene groups
connecting the benzyl and pyridine rings to the diamine.
Because free motion of the benzyl groups would be anticipated
to coalesce the benzylic peaks into singlets, the splitting
patterns associated with these peaks suggest that their mobility
is limited, even in solution.
5
0
+
are approximately coplanar. In both [Co(bbpc)(O )]
2
cations, one of the benzyl rings from the bbpc ligand orients
roughly parallel to a metal-bound pyridine ring, possibly
indicating intramolecular aromatic interactions between the two
(Figure 1). For both cations, the centroids of the phenyl and
pyridine rings are 3.78 Å apart. The metal−ligand bond
distances are consistent with low-spin cobalt(III) metal centers
Complex 1 appears to be oxidized to another higher-valent
1
species upon reaction with PhIO. H NMR analysis of the
29,56−60
reaction mixture shows that much of PhIO has reacted to form
PhI, demonstrating that the reaction does not halt at a simple
(Table 2).
The 1.46 and 1.44 Å O−O bond lengths
from the two molecules in the unit cell are both consistent with
single bonds between the oxygen atoms and are similar to
II
1
Co -PhIO adduct (Figures S8 and S9). The H NMR spectrum
otherwise lacks well-resolved diamagnetic features. MS analysis
of the reaction between 1 and PhIO revealed a m/z 296.6
feature consistent with [Co(bbpc)(MeCN)(OH)]2+ (3;
predicted m/z 296.6; Scheme 3), but this is a minor portion
of the spectrum (Figures S12 and S13). When PhIO is
29,56−59
values measured for other cobalt(III) peroxo complexes.
1
The H NMR and MS data (Figure S11) collected for 2 suggest
that the structure is maintained in solution.
Reactivity of the Cobalt(III) Peroxo Species. Complex 2
was tested for its ability to participate in a variety of different
reactions. The cobalt(III) peroxo species is not a competent
electrophile, and it is unable to abstract hydrogen atoms even
from benzylic and allylic substrates, such as xanthene, DHA,
and cyclohexene. The lack of reactivity was confirmed by both
UV/vis and GC analyses of the reaction mixtures, the latter of
which showed no substrate oxidation. Complex 2 was likewise
unable to convert alkenes, such as cyclooctene and cyclohexene,
18
premixed with O-labeled H O, the peak at m/z 297.6
2
1
8
intensifies, consistent with the incorporation of O into the
complex. To our chagrin, attempts to isolate this species in a
solid form failed. Removal of the solvent resulted in a brown
oil, as did the introduction of nonpolar solvents into MeOH
and MeCN solutions. Other features observed by MS are
consistent with ligand oxidation, specifically the loss of either a
benzyl or phenyl group and/or dehydrogenation of the C−N
bonds to CN groups (Figure S12). The data suggest that the
metal-based oxidant does not accumulate to a high enough level
to allow for its isolation.
into epoxides.
III
The Co -O species, conversely, is a competent nucleophile.
2
III
Much like previously characterized M -O2 compounds, 2
2
8,29,31,36
reacts with aldehydes in MeCN at 298 K (Table 4).
In
Crystal Structure of the Cobalt(III) Peroxo Complex.
The cobalt(II) complex 1 precipitates as a microcrystalline
material, but we were unable to obtain sufficiently high-quality
crystals for characterization by X-ray diffraction. The cobalt(III)
peroxo complex 2, conversely, was successfully characterized by
this technique (Table 1).
these studies, the cobalt(III) peroxo complex was generated in
61
situ, largely to facilitate comparisons to earlier studies. CCA is
converted to a mixture of cyclohexene and cyclohexanone in
yields of 26 (±6)% and 63 (±10)%, respectively. With a large
excess of CCA, the chromophore associated with 2 undergoes
first-order decay (Figure S14). The reaction is first-order with
respect to CCA (Figure 2), and analysis of the relationship
Each unit cell in the structure contains two independent
+
cations with the formula [Co(bbpc)(O )] . In each cation, the
between kobs and the concentration of the aldehyde yields a k
2
2
value of 2.4 (±0.2) × 10−
2
M
−1 −1
s . Complex 2 converts 2-PPA
cobalt is coordinated in a highly distorted octahedral geometry,
D
DOI: 10.1021/acs.inorgchem.6b02127
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
DHA is converted exclusively into anthracene; the time-
dependent yields of anthracene suggest that the reaction
completes in under 30 min. The oxygenated products anthrone
and anthroquinone are not observed, and performing the
reaction under N has no significant impact on either the
2
product distribution or the yield of anthracene. 1,4-Cyclo-
hexadiene (CHD) is oxidized exclusively to benzene under
both air and N . As with the DHA reactions, the CHD
2
reactions are complete within 30 min and yield the same results
under air and N . Xanthene is oxidized to xanthone. Under air
2
but not N , the yields of xanthone are higher and continue to
2
increase past 30 min. Neither fluorene, cyclohexane, cyclo-
hexene, cumene, nor toluene react with 1 and PhIO to
observable degrees. No oxidation of cyclohexane, cumene, or
toluene is detected even when 500 mM of these substrates is
added instead of our standard 50 mM.
Figure 2. Plot of observed rate constants (k ) for the pseudo-first
order decay of 2 as a function of the concentration of CCA. All
reactions were run at 298 K in MeCN under air.
obs
The reactivity with DHA, CHD, and xanthene is notable in
that it is catalytic, rather than stoichiometric, with respect to the
cobalt. With excess PhIO, up to 12 equiv of anthracene per
equiv of 1 can be produced, suggesting that the cobalt catalyst
turns over 11 times. These substrates do not react with PhIO
without a catalyst. The yield of anthracene maximizes within 30
min, which is again consistent with the catalyst degrading under
the reaction conditions. The higher and increasing yields of
xanthone under air suggests that this process partly proceeds
through the propagation of organic radicals, rather than solely
to acetophenone in 81 (±12)% yield. Styrene, the anticipated
product from deformylation, is not observed. As with the CCA
reaction, the reaction with 2-PPA is first-order with respect to
the substrate (Figure S15). The reaction occurs at about the
−2
−1 −1
same rate, with a k value of 1.7 (±0.3) × 10
M
s
(Figure
2
S16).
Complex 2 also degrades in the presence of benzaldehydes
Figure S17). Regrettably, the organic product was not isolated
(
and identified. Prior reports in this area have encountered
2
8,29,36
similar difficulties,
suggesting that the initially generated
the regeneration of a metal-based oxidant, when O is present.
2
organic products may react further. The reactivity was tested
with four para-substituted benzaldehydes (Cl, H, F, Me) in
MeCN at 298 K (Figure 3). The more electron-rich
Although Sc(III) or another Lewis acid was not needed to
enable the catalysis, the necessity of such for prior cobalt-
mediate C−H activation prompted us to investigate whether an
44,45,63
additional Lewis acid could enhance the activity.
To our
surprise, we found that the addition of Sc(OTf) inhibited,
3
rather than improved, the catalysis, and the yields of anthracene
from DHA were noticeably lower when Sc(III) was present
(
Table 4). The product distribution remained unaltered, and no
oxygenated organic products were found. The yield of
anthracene at 120 min is essentially the same as that measured
at 30 min, and it does not appear that the Sc(III) is either
stabilizing the metal-based oxidant or otherwise prolonging the
lifetime of the catalyst.
meta-Chloroperbenzoic acid (MCPBA) can substitute for
PhIO as the terminal oxidant, but the yields of the oxidized
hydrocarbons are generally lower (Table 5). MCPBA reacts
directly with alkenes to yield epoxides, and 1,4-cyclohexene
monoxide is observed in its reactions with 1 and CHD in
addition to the anticipated benzene. We also investigated
oxone, H O , tert-butylhydroperoxide, and air by itself as
terminal oxidants but found no reactivity with organic
substrates.
We explored using 1 as a catalyst for oxygen-atom-transfer
reactions but found that mixtures of the Co(II) precursor and
PhIO were unable to convert cyclohexene, cyclooctene, and α-
methyl-styrene to the corresponding epoxides. Compound 1
Figure 3. Hammett plot for the reactions between 2.0 mM 2 and 400
mM para-X-benzaldehydes (X = Cl, H, F, Me) under air in 298 K
MeCN. ρ = 1.6, R = 0.986.
62
2
62
benzaldehydes, as assessed by their σp+ values, react with 2
2
2
more slowly, consistent with the Co(III)-peroxo complex
+
serving as a nucleophile. A plot of log(k ) versus σ yields a ρ
obs
p
value of 1.6.
The identity of the initially generated cobalt-containing
product regrettably remains unresolved. MS analysis of
reactions between 2 and aldehydes show evidence of ligand
decomposition (Figure S18).
was, however, able to catalyze the oxidation of Ph P to
3
triphenylphosphine oxide (Ph PO) by PhIO. When 1.0 mM of
Reactivity of the Species Generated from PhIO. Prior
research found that certain Co(II) complexes could react with
PhIO in the presence of a Lewis acid, such as Sc(III), to yield
species capable of oxidizing DHA. Mixtures of complex 1 and
PhIO are likewise capable of activating weak C−H bonds
3
1
was used to catalyze the reaction between 25 mM PhIO and
5
0 mM Ph P in MeCN, Ph PO is formed quantitatively within
3
3
30 min (>99%, 25 TON). Unlike the hydrocarbon oxidation
1
reactions, the Ph
P reactions were followed by H NMR. As has
3
(Table 4); an additional Lewis acid beyond the Co(II),
been found for other systems, the catalyst-free reaction between
64
however, is not needed for this reactivity to proceed.
PhIO and Ph P did not yield any Ph PO.
3
3
E
DOI: 10.1021/acs.inorgchem.6b02127
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
DISCUSSION
a cobalt(III) peroxo complex with the 13-TMC ligand (Scheme
■
29
2
). The 13-TMC complex converts CCA exclusively to the
The bbpc ligand was previously used to stabilize a ferric
hydroperoxide complex,
whether the ligand could be used to stabilize other species
relevant to metal−O chemistry. Cobalt complexes with O and
related ligands, in particular, may have high mechanistic
relevance in the cobalt-catalyzed O production recently
studied by Nocera and others.
1
8,50,51
alkene. Both compounds react with 2-PPA to yield
acetophenone as the sole organic product. Despite the
dissimilarity in their ligand structures, the two cobalt(III)
peroxo compounds react with aldehydes at similar rates. This is
notable because aldehyde deformylation was not observed for a
cobalt(III) peroxo complex with the 12-TMC ligand in the
and we were curious as to
2
2
2
1
2−14,65,66
2
9,35
absence of acid.
The k values for the 13-TMC complex are
Structurally characterized cobalt(III) peroxo species are rare,
but the few that have been observed have a diverse array of
coordination spheres. Some of these adducts have been
prepared from the reactions between cobalt(I) precursors and
2
−1
−1
s−1 for CCA and 1.5 × 10
−2
−1 −1
2
.0 × 10
M
M
s
for 2-
2
9
PPA. Complex 2 reacts with CCA more slowly, with a k
2
value of 2.4 × 10−
2
M
−1 −1
s , but reacts at approximately the
5
6−58
same rate with 2-PPA as a substrate, with a k value of 1.7 ×
O2;
others have been synthesized from reactions between
2
−2
−1 −1
29,56,59
10
with derivatized benzaldehydes yields a ρ value of 1.6 (Figure
). This is both similar to the 1.7 value found for the 13-TMC
M
s . The Hammett plot derived from the reactions
cobalt(II) complexes, H O , and an added base.
The
2
2
synthetic routes originating from cobalt(I) have thus far
invariably contained softer base spectator ligands, resulting in
As O , P O , and C NO coordination spheres in the
3
29
complex and indicative of a nucleophilic reagent. The
proposed mechanism for the reactivity with aldehydes is
shown in Scheme 4; the same mechanism has been suggested
4
2
4
2
3
2
5
6−58
cobalt(III) products.
isolated from the reaction between a cobalt(I) starting material
A cobalt(II) superoxo complex
and O also merits mention; this was distinguished from the
aforementioned peroxo species on the bases of its noticeably
2
Scheme 4
shorter O−O bond and its ability to abstract hydrogen atoms
67
from weak C−H bonds. A recently characterized cobalt(III)
superoxo complex was also found to abstract hydrogen atoms
III
49
from TEMPO-H, resulting in a Co -OOH species.
A cobalt(III) peroxo complex (2) was synthesized from the
2
+
reaction between [Co(bbpc)(MeCN)2] (1), H O , and Et N
2
2
3
(
Figure 1). The Co−N and Co−O bond lengths of 2 are
2
8,34
consistent with a low-spin cobalt(III) metal center; the
oxidation- and spin-state assignments are corroborated by the
diamagnetic H NMR spectrum (Figure S4) and the absence of
an EPR signal. The O−O bond length is consistent with a
peroxo ligand, as opposed to a superoxo or a bound
for other transition-metal peroxo complexes.
Disappoint-
ingly, excesses of H and Et N do not result in catalytic
aldehyde deformylation. The sole cobalt-containing catalyst for
aldehyde deformylation instead appears to rely on a radical
O
2
2
3
1
68
cobalt(II) superoxide species as the metal-based oxidant.
2
9,56−59,67
O .
MS of samples of 2 in MeCN suggests that the
The reaction between 1 and PhIO generates another reactive
cobalt complex with a distinct reactivity profile. A previous
reaction between PhIO and another cobalt(II) complex
produced a mixture of a Co -PhIO complex and a higher-
valent species that was initially assigned as a cobalt(IV) oxo
compound. The addition of scandium triflate shifted the
equilibrium position toward the oxidized cobalt. The reaction
between PhIO, Sc(OTf) , and yet another cobalt(II) complex
resulted in a Co -OH complex with structural, spectroscopic,
2
solid-state structure is maintained in solution (Figure S11).
Much like some, but certainly not all, other isolated metal
peroxo complexes, compound 2 stoichiometrically reacts with
aldehydes (Table 3). When observed, these reactions typically
II
44
a
Table 3. Oxidation of Substrates by 2
3
b
substrate
CCA
product
yield (%)
III
cyclohexene
cyclohexanol
acetophenone
no reaction
no reaction
no reaction
26 (±6)
63 (±10)
81 (±12)
N.A.
and reactive similarities to the first higher-valent species, casting
45
doubt on its assignment as a true cobalt(IV) compound.
cobalt(IV) oxo complex with a tetraamido macrocyclic ligand
TAML) was reported from the reaction between PhIO,
A
2-PPA (CHD)
xanthene
DHA
(
6
3
N.A.
scandium(III), and a cobalt(III) precursor. The TAML
complex reacts with substrates with weak C−H bonds in a
manner reminiscent of those of the first two cobalt
cyclohexene
N.A.
a
Standard reaction conditions: All reactions were run at 298 K in 2.5
44,45,63
mL of MeCN under air for 60 min. The starting concentrations of 2
and substrate in all reactivity assays were 2.0 and 100 mM,
respectively. Complex 2 was generated in situ from the reaction
between 2.0 mM 1, 10 mM H O , and 5.0 mM Et N. The
concentrations of each organic product were calibrated relative to that
oxidants.
Tilley and co-workers reported that a cobalt(II)
complex with a tetraanionic ligand could activate the C−H
bonds of MeCN using either O or PhIO as the terminal
oxidant; the difficulty of this transformation led them to
propose that a transient cobalt(IV) species was responsible for
2
9
2
2
2
3
of an internal standard (dichlorobenzene) with a known concen-
tration. Complex 2 is the limiting reagent.
48
b
oxidation of the solvent.
The cobalt(II) complex 1 is capable of catalyzing the
oxidation of weak C−H bonds by either PhIO or MCPBA, and
the catalyst appears capable of turning over at least 10 times
(Table 4). Although complex 1 is perhaps not quite ready for
industrial use, the modest turnover is noteworthy because only
stoichiometric reactivity had thus far been reported for the
yield mixtures of alkenes and ketones; the product distribution
is highly dependent on the identities of both the coordination
28,29,31−34,36,68
complex and the aldehyde.
In the cited cases,
deformylation of the aldehyde to an alkene is sometimes found
for CCA but has never been found for 2-PPA. Compound 2
can convert CCA to cyclohexene, but not to the same extent as
44,45,48,63
cobalt-mediated oxidation of DHA by PhIO.
C−H
activation by MCPBA has not been reported with discrete
F
DOI: 10.1021/acs.inorgchem.6b02127
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
a
Table 4. Oxidation of Hydrocarbons by PhIO Catalyzed by 1
b
c
substrate
DHA
atmosphere
air
time (min)
30
product
yield (%)
TON
d
d
d
d
d
anthracene
46 (±2), 10 (±2)
42 (±2), 8 (±2)
44 (±2), 10 (±2)
42 (±2)
12, 3.0
10, 1.8
11, 2.5
11
d
6
0
1
20
30
30
30
30
N2
air
N2
air
anthracene
benzene
CHD
64 (±4)
56 (±4)
22 (±4)
16
benzene
14
Xanthene
xanthone
6.0
6
0
34 (±8)
8.5
120
30
48 (±6)
12 (±4)
12
N2
xanthone
3.2
6
0
12 (±2)
2.9
1
20
14 (±2)
3.7
a
Standard reaction conditions: All reactions were run at 298 K in 2.5 mL of MeCN. The starting concentrations of the cobalt(II) catalyst (1) and the
substrate in all reactivity assays were 1.0 and 50 mM, respectively. A total of 25 equiv of PhIO, relative to the 1, was added at the beginning of the
reaction. For each time point, an aliquot of the reaction mixture was diluted with ether, filtered through silica gel, and analyzed via GC. The products
were identified by comparing the retention times with those of authentic samples of anthracene, xanthone, and benzene. The concentrations of each
b
organic product were calibrated relative to that of an internal standard (dichlorobenzene) with a known concentration. PhIO is the limiting reagent.
c
d
Turnover number, defined as the number of moles of oxidized organic product generated per mole of 1. Reactions contain 1.0 mM Sc(OTf) as an
3
additive.
cobalt complexes, but it has been documented with iron
appears to oxidize itself, as evidenced by MS (Figure S12). The
inability to generate the oxidant cleanly has thus far precluded
both its characterization and a more thorough kinetic analysis
of the C−H activation. Attempts to observe intermediates at
low temperatures by either EPR or UV/vis have thus far been
unsuccessful, even using the more soluble MCBPA as the
terminal oxidant.
6
9,70
compounds.
MCPBA poses a complication in that it can
react directly with alkenes to yield epoxides, and indeed this
activity is seen when CHD is used as the substrate. Despite this
background reactivity, the cobalt catalyst does sufficiently shift
the reactivity to yield benzene as the major product (Table 5).
One key difference between most prior cobalt oxidants
capable of C−H activation and our own is that a Lewis acid is
not necessary for the reactivity. The current inability to isolate a
higher-valent species from reaction mixtures of PhIO and 1
limits our ability to understand both why we observe catalysis
and why an additive is not needed to achieve C−H activation.
MS analysis of the reaction between 1 and PhIO found
Table 5. Oxidation of Hydrocarbons by MCPBA Catalyzed
by 1
a
b
c
substrate atmosphere time (min)
product
yield (%)
TON
DHA
air
N2
air
30
30
30
anthracene
anthracene
benzene
36 (±2)
28 (±4)
50 (±4)
32 (±6)
50 (±6)
26 (±4)
8.8
7.2
13
CHD
d
III
epoxide
evidence for a Co -OH species, 3, but this complex was not
1
III
N2
30
benzene
13
definitively detected by H NMR. Given that Mn -OH and
d
III
epoxide
Fe -OH species have previously demonstrated similarly mild
20,73−75
a
C−H activation chemistry,
we currently speculate that a
Standard reaction conditions: All reactions were run at 298 K in 2.5
mL of MeCN. The starting concentrations of the cobalt(II) catalyst
1) and the substrate in all reactivity assays were 1.0 and 50 mM,
III
Co -OH species could at least be partly responsible for C−H
(
activation of the external substrates (Scheme 5). This species
respectively. A total of 25 equiv of MCPBA, relative to the 1, was
added at the beginning of the reaction. For each time point, an aliquot
of the reaction mixture was diluted with ether, filtered through silica
gel, and analyzed via GC. The products were identified by comparing
the retention times with those of authentic samples of anthracene,
benzene, and 1,4-cyclohexadiene oxide. The 1,4-cyclohexadiene
monoxide standard was synthesized from the uncatalyzed reaction
between CHD and MCPBA. The concentrations of each organic
Scheme 5
product were calibrated relative to that of an internal standard
b
(
dichlorobenzene) with a known concentration. MCPBA is the
c
limiting reagent. Turnover number, defined as the number of moles
of oxidized organic product generated per mole of 1. 1,4-
d
Cyclohexadiene monoxide.
could abstract a hydrogen atom from a substrate with a suitably
II
weak C−H bond to yield a Co -OH species, which could
2
The reactivity of our system is limited to substrates with
subsequently be reoxidized to a higher-valent species by PhIO
4
4,45,48,63
III
weak C−H bonds.
The benzylic C−H bonds of DHA
or MCPBA. Co -OH could conceivably form from a reaction
and xanthene have bond dissociation energies (BDEs) of
between a hydrogen-atom donor (e.g., MeCN, bbpc ligand,
external substrate) and a more reactive cobalt(IV) oxo species
that could be generated from the direct reaction between PhIO
and 1. Unfortunately, we were unable to intercept a putative
cobalt(IV) oxidant with a large excess of cyclohexane, which
contains C−H bonds that would be too strong to be activated
−1
71
approximately 78 and 75 kcal mol , respectively. The allylic
C−H bonds of CHD have been estimated to have a BDE of 75
−1
72
kcal mol . No oxidation is observed with even moderately
stronger C−H bonds, such as the benzylic ones in fluorene and
toluene. In the absence of an external substrate, the complex
G
DOI: 10.1021/acs.inorgchem.6b02127
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
by other documented M -OH species.
attempted to generate a Co -OH species without the
intermediacy of a cobalt(IV) oxidant by sequentially reacting
Article
III
20,73−75
We also
ACKNOWLEDGMENTS
■
III
The work described was financially supported by Auburn
University and a grant from the American Chemical Society,
Petroleum Research Fund (Grant 55301-ND3).
1
with hydroxide sources and ceric ammonium nitrate. Such
+
reactions resulted in mixtures containing [Co(bbpc)(NO )]
3
and cobalt complexes with oxidized ligands. The inability to
observe the Co -OH product anticipated from aldehyde
III
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■
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Corresponding Author
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ORCID
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Inorg. Chem. 2008, 47, 1849−1861.
Qiao Zhang: 0000-0002-6326-2556
Christian R. Goldsmith: 0000-0001-7293-1267
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Notes
The authors declare no competing financial interest.
H
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