Direct Manipulation of Metal Imido Geometry: Key Principles to Enhance C-H Amination Efficacy

Article abstract of DOI:10.1021/jacs.9b09015

We report the catalytic C-H amination mediated by an isolable Co^III imido complex (^TrL)Co(NR) supported by a sterically demanding dipyrromethene ligand (^TrL = 5-mesityl-1,9-(trityl)dipyrrin). Metalation of (^TrL)

Full text of DOI:10.1021/jacs.9b09015

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Article
Direct Manipulation of Metal Imido Geometry: Key
Principles to Enhance C–H Amination Efficacy
Yunjung Baek, Elisabeth T. Hennessy, and Theodore A. Betley
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b09015 • Publication Date (Web): 24 Sep 2019
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Direct Manipulation of Metal Imido Geometry: Key Principles to
Enhance C–H Amination Efficacy
Yunjung Baek, Elisabeth T. Hennessy, and Theodore A. Betley*
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Department of Chemistry and Chemical Biology, Harvard University, 12 Oxford Street, Cambridge, Massachusetts 02138,
United States
ABSTRACT: We report the catalytic C−H amination mediated by an isolable CoIII imido complex (TrL)Co(NR) supported by a
sterically demanding dipyrromethene ligand (TrL = 5-mesityl-1,9-(trityl)dipyrrin). Metalation of (TrL)Li with CoCl2 in THF afforded
a high-spin (S = 3/2) three-coordinate complex (TrL)CoCl. Chemical reduction of (TrL)CoCl with potassium graphite yielded the
high-spin (S = 1) CoI synthon (TrL)Co which is stabilized through an intramolecular η6-arene interaction. Treatment of (TrL)Co with
a stoichiometric amount of 1-azidoadamantane (AdN3) furnished a three-coordinate, diamagnetic CoIII imide (TrL)Co(NAd) as con-
firmed by single-crystal X-ray diffraction, revealing a rare trigonal pyramidal geometry with an acute Co−Nimido−C angle 145.0(3)°.
Exposure of 1-10 mol% of (TrL)Co to linear alkyl azides (RN3) resulted in catalytic formation of substituted N-heterocycles via
intramolecular C−H amination of a range of C−H bonds, including primary C−H bonds. The mechanism of the C−N bond for-
mation was probed via initial rate kinetic analysis and kinetic isotope effect experiments [kH/kD = 38.4(1)], suggesting a stepwise
H−atom abstraction followed by radical recombination. In contrast to the previously reported C−H amination mediated by
(ArL)Co(NR) (ArL = 5-mesityl-1,9-(2,4,6-Ph3C6H2)dipyrrin), (TrL)Co(NR) displays enhanced yields and rates of C−H amination
without the aid of a cocatalyst, and no catalyst degradation to a tetrazene species was observed, as further supported by the pyridine
inhibition effect on the rate of C−H amination. Furthermore, (TrL)Co(NAd) exhibits an extremely low one-electron reduction poten-
tial (E°red = − 1.98 V vs. [Cp2Fe]+/0) indicating that the highly basic terminal imido unit contributes to the driving force for H−atom
abstraction.
1. INTRODUCTION
hydroxylation, assessing redox potentials and basicity of rele-
vant species (e.g., imides or amides) involved in C−H amina-
tion processes operating via H−atom abstraction (HAA) is
essential to comprehend the thermodynamic and kinetic prop-
erties of the desired transformations.47-53
The development of a cost-effective and efficient transi-
tion-metal catalyst for direct C−H functionalization has been
of interest to synthesize value-added commodity chemicals
from simple hydrocarbon feedstocks.1-2 To this end, the use of
bioinspired transition metal complexes featuring metal-ligand
multiple bonds (MLMBs) has emerged as an appealing C–H
functionalization methodology.3-5 In this regard, the iron-oxo
complex (compound I) of Cytochrome P450 demonstrates
nature’s key strategy to render the iron-oxo multiple bond
highly reactive, and thereby enhancing its O−atom transfer
efficacy.6 Specifically, the open-shell electronic configuration,
high oxidation state of compound I, and highly basic oxo unit
of its one-electron reduced form have been proposed as lead-
ing features enabling facile C−H hydroxylation at ambient
conditions.7-8
From a synthetic perspective, these properties (E°red and
pKa) can be tuned by various elements such as identity or oxi-
dation state of a metal center(s),54 electronic configuration of
metal-nitrenoid complexes,19, 32, 55 and the local geometry of
the terminal nitrenoid moiety.56 Previously, we have demon-
strated that manipulation of the spin state of the metal imide32
and the spin density at the nitrogen atom of the nitrenoid unit19
can serve as synthetic approach to weaken the MLMBs and
enable facile C−H amination.
In contrast, the direct manipulation of the geometry of a
metal nitrenoid and its effect on C−H amination efficacy is
relatively less explored. Nonetheless, Holland and co-workers
have proposed that the geometry of the terminally bound im-
ido unit can alter the basicity of the metal imido complex.
They found enhanced C−H activation reactivity from the
β−diketiminate FeIII imido complex upon pyridine coordina-
tion.56-58 The pyridine-bound iron imide was proposed as more
bent and basic compared to the pyridine-free iron imide. How-
ever, structural characterization of the bent imide is absent,
and thus, a direct correlation between the bent imido geometry
and its basicity is lacking.
As such, the isoelectronic synthetic analogues, metal-
nitrenoid complexes (e.g., imido, M(NR2–);4,
iminyl,
9-11
M(2NR–);12-19 nitrene adducts, M(3NR)20-22) have received
great attention as viable N−group transfer reagents into unac-
tivated C−H bonds. In particular, late, first-row transition met-
al-based nitrenoid complexes have been extensively studied
for direct C−N bond formation (Mn,23 Fe,18-19,
Co,34-42
24-33
Ni,10-11, 43-44 and Cu45-46). Among the structurally characterized
metal-nitrenoid complexes, however, only a few manifest pro-
ductive C−H amination due to the typically strong MLMBs
between the metal ion and nitrene unit.32-33, 37-38, 43, 45 Thus, in
order to improve the efficacy of C−H amination, understand-
ing the key factors governing the lability of the terminal
nitrenoid moiety is necessary. Akin to the enzymatic C−H
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Scheme 1. Ligand and Metal Complex Syntheses
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Recently, our group showcased that the geometry of the
imido unit can also affect the rate of C−H amination.37 We
have demonstrated the intramolecular C−H amination mediat-
2. RESULTS AND DISCUSSTION
2.1. Synthesis and Characterization of Cobalt Synthons.
As discussed in the introduction, the ligand design principle to
synthesize a bent CoIII imide was informed by the previously
studied ligand-accelerated C−H amination processes mediated
ed
by
(ArL)Co(NR)
(ArL
=
5-mesityl-1,9-(2,4,6-
Ph3C6H2)dipyrrin) and found that addition of pyridine im-
proves both yields and rates of C−H amination. Exogenous
pyridine coordinates to the trigonal planar (ArL)Co(NR) to
generate the pyridine-bound imide (ArL)Co(NR)(py) providing
steric protection to circumvent formation of a catalytically
inactive cobalt tetrazido complex. More importantly, the rate
of C−H amination was accelerated in the presence of pyridine
as cocatalyst. We have ascribed the enhanced rate of C−H
amination from (ArL)Co(NR)(py) to the bent and attenuated
MLMB (Co–NR) as suggested by density functional theory
(DFT) calculations, yet isolation of the bent (ArL)Co(NR)(py)
was hampered due to the weak binding affinity of pyridine.
by
(ArL)Co(NR)
(ArL
=
5-mesityl-1,9-(2,4,6-
Ph3C6H2)dipyrrin).37 Specifically, we sought to emulate the
primary coordination environment of (ArL)Co(NR)(py) by
modifying the flanking groups of the dipyrrin scaffold. To this
end, trityl moieties were considered to be more voluminous
ligand flanking units than the 2,4,6-triphenylphenyl substitu-
ents of (ArL). Thus, we envisioned that the steric profile of the
trityl groups may enforce a bent geometry on the nitrenoid unit
and eliminate formation of cobalt tetrazido complexes even in
the absence of exogenous pyridine. Additionally, we proposed
that one of the phenyl rings of the trityl substituents may stabi-
lize the low-coordinate cobalt center via an intramolecular η6-
interaction affording a roboust CoI synthon akin to the previ-
ously reported iron and cobalt synthons such as (ArL)Fe32 and
(ArL)Co.37
Motivated by these precedents, we postulated that generat-
ing a metal-bound nitrenoid unit in a bent geometry can be an
alternative strategy to engender a more reactive MLMB and
thus improve C−H amination efficacy both kinetically and
thermodynamically.51 Towards this, we sought to design a
ligand that would enable synthesis of a cobalt imido complex
featuring a significantly bent nitrenoid unit in the absence of
exogenous ligands (e.g., pyridine). Specifically, we sought to
investigate the following questions: (1) can we synthesize a
dipyrrinato CoIII imido complex featuring a bent nitrenoid unit
by modifying the substituents on the dipyrrin scaffold? If pos-
sible, (2) is the imide competent for catalytic C−H amination
to afford substituted N-heterocycles? And (3) how does the
geometry of the metal-bound nitrenoid moiety alter the effica-
cy and reactivity profile for C−H amination with respect to
yields, rates, and reaction scope?
With these design principles in mind, we first prepared 2-
trityl-1H-pyrrole via Friedel-Crafts alkylation of excess pyr-
role with trityl chloride to avoid bis-alkylation. Following
literature procedures,18 subsequent condensation with mesit-
ylaldehyde dimethyl acetal followed by oxidation with 2,3-
dichloro-5,6-dicyano-1,4-benzoquinone
(DDQ)
afforded
(TrL)H (Scheme 1a). Subsequent deprotonation of (TrL)H with
PhLi yielded the lithiated scaffold (TrL)Li. Metalation of
(TrL)Li with CoCl2 in THF for 20 hours at room temperature
afforded a three-coordinate complex (TrL)CoCl (1) (Scheme
1b). Single crystals of 1 suitable for X-ray diffraction were
grown from a layered 1:2 benzene/hexanes solution at room
temperature. In contrast to the previously reported trigonal
planar CoII complex (ArL)CoBr, 1 adopts a trigonal pyramidal
geometry, displaying an η1-arene interaction between the co-
balt and one of the ortho carbons from the trityl substituents
Towards this goal, we report herein the synthesis of a
trityl-substituted dipyrromethene platform (TrL) (TrL = 5-
mesityl-1,9-(trityl)dipyrrin) that provides access to a robust
CoI catalyst (TrL)Co and the corresponding bent CoIII imido
complex (TrL)Co(NR), enables a facile CoI/III catalytic C−H
amination. Detailed mechanistic studies on the C–H amination
processes establish the improved C−H amination efficacy of
(TrL)Co(NR) as compared to the previously reported linear
(ArL)Co(NR), demonstrating the importance of modulating the
primary coordination sphere to effect more reactive and syn-
thetically applicable C−H amination catalysis.
(Figure 1a). The discrete η1-arene interaction [Co–Cortho
=
2.345(7) Å] is of note as it occupies the equatorial position of
the dipyrrin plane positioning the chloride on the axial site.
Solid-state magnetic susceptibility of 1 was measured with
superconducting quantum interference device magnetometry
(SQUID) and is consistent with a high-spin ground state (S =
3/2) (Figure 2a).
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Figure 1. Solid-state structures of (a) (TrL)CoCl (1), (b) (TrL)Co (2), and (TrL)Co(NAd) (3) at 100 K with thermal ellipsoids set at 50%
probability (hydrogen atoms and solvent molecules are omitted for clarity; Co aquamarine, C gray, N blue, Cl green). Selected bond
lengths (Å) and angles (deg) for 1: Co–Cortho, 2.345(7); 3: Co–Cortho, 2.756; Co–N, 1.636(3); ∠Co−N−C = 145.0(3).
Figure 2. Variable-temperature susceptibility data of (a) (TrL)CoCl (1) and (b) (TrL)Co (2) collected at 1.0 T from 5 to 300 K. (Inset) Re-
duced magnetization data collected at three fields (1, 4, 7 T) over the temperature range 1.8−10 K. Magnetization fit parameters obtained
with PHI:59 for 1, S = 3/2, g = 2.4, D = − 48.1 cm−1, |E/D|= 0.11 and for 2, S = 1, g = 2.03, D = 25.98 cm−1, |E/D|= 0.085. (c) Variable-
temperature susceptibility data of (TrL)Co(NAd) (3) collected at 0.5 T and 1.0 T upon heating and cooling (5 to 300 K and 300 to 5 K).
Chemical reduction of 1 with KC8 in a thawing benzene
solution cleanly furnished the reduced species (TrL)Co (2),
which exhibits an intramolecular η6-interaction between the
cobalt and one of the phenyl groups of the trityl substituents
(Scheme 1b, Figure 1b). Solid-state magnetic susceptibility of
2 is consistent with an S = 1 high-spin ground state (Figure
2b). The cyclic voltammogram of 2 measured in a non-
coordinating solvent 1,2-difluorobenzene reveals two
sequential one-electron oxidations (E1/2 = – 0.80, 0.75 V vs.
[Cp2Fe]+/0) and a single one-electron reduction (– 2.1 V,
Figure 3a). The observed redox potentials are almost identical
with the ones observed for its congener (ArL)Co, suggesting
that 2 is a suitable alternative CoI synthon to further explore
two-electron oxidative group transfer reactivity (Figure 3b).
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coordinate tetrazido complex was detected upon heating a
benzene-d6 solution of 3 with excess amount of AdN3 (Scheme
1
2
2).
3
4
We note that only a handful of structurally characterized
metal imides display a substantially bent nitrenoid unit. For
late, first-row transition metals,
a
four-coordinate
5
bis(pyrrolyl)pyridine FeIV imide60 and five-coordinate FeIV
imide61 supported by a tetradentate N-heterocyclic (NHC)
ligand display an Fe−N−C angle of 140.5(3)° and 150.07(10)°,
respectively. Hillhouse and co-workers have shown the syn-
thesis of a three-coordinate nickel imide featuring a bent
nitrenoid moiety [∠Ni−N−C = 127.3(3)°] using a sterically
encumbered aryl azide (e.g., 2,5-dimesitylphenylazide, 2,6-
bis(2,6-diisopropylphenyl)phenylazide).44 Given that those
bent M−N−C linkages were induced by either multidentate
chelating ligands or steric properties of the R substituents on
the metal-bound imido (NR) group, furnishing the bent imido
complex 3 via direct manipulation of the primary coordination
enviroment with the low-coordinate bidentate dipyrrin ligand
is significant.
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Figure 3. Cyclic voltammograms of (a) 2 and (b) (ArL)Co
obatined in 1,2-difluorobenzene at room temperature using 0.1 M
[nBuN4][PF6] electrolyte. Potentials were referenced to [Cp2Fe]+/0
couple. Scan rate = 100 mV/s.
2.2. Synthesis and Characterization of (TrL)Co(NAd).
With the (TrL)CoI synthon in hand, we sought to investigate its
Having isolated the bent imido 3, we sought to understand
its electronic structure. As discussed, the diamagnetic spin
ground state of 3 is further supported by the temperature-
independent susceptibility data (Figure 2c), which is consistent
with the vast majority of CoIII imides including the analogous
dipyrrin alkyl CoIII imide (ArL)Co(NR) reported by our
group.36-42 However, unlike (ArL)Co(NR), no thermal popula-
tion of spin excited states (e.g., S = 0 → 2) was observed (Fig-
ure 2c inset).36
reactivity towards organoazides. Treating
a benzene-d6
solution of with stoichiometric amount of 1-
2
a
azidoadamantane (AdN3) immediately furnished a diamagnetic
1
species as verified by H and 13C NMR spectroscopies. Single
crystals of the product obstained from a 1:1 benzene/hexanes
mixture stored at room temperature were analyzed by single
crystal X-ray diffraction to reveal formation of a three-
coordinate CoIII imido complex (TrL)Co(NAd) (3) (Scheme 2
and Figure 1c-d).
To further elucidate the frontier molecular orbital interac-
tions in 3, DFT calculations were conducted (SI-59).62-63 Inter-
estingly, owing to the bent geometry, HOMO, HOMO–1, and
HOMO–2 of 3 display almost non-bonding or significantly
disrupted π* interactions with respect to the imido unit. In-
stead, σ* or π* contributions from the dipyrrin become domi-
Scheme 2. Reactivity Assessment of 2.
2
2
nant. The non-bonding interaction with 3dx –y and the slight
π-overlap between the 2p orbitals of the Nimido and Co 3dz2,
3dyz and 3dxy are illustrated in Figure 4.
Notably, 3 exhibits a unique distorted trigonal pyramidal
geometry with a significantly bent imido moiety [∠Co−N−C =
145.0(3)°]. Akin to 1, a releatively long η1-arene interaction
between the cobalt and one of the ortho carbons from the trityl
groups is present (Co–Cortho = 2.756 Å) orienting the nitrenoid
unit orthogonal to the dipyrrin plane. Furthermore, the sterical-
ly encumbered trityl substituents shield the trans position of
the nitrenoid moiety, enforcing an acute ∠L−Co−NAd
(119.7°) as well as blocking the fourth-coordination site of 3
as further evidenced by the space-filling model (Figure S-28).
Consequently, no bimolecular reactivity affording the four-
Figure 4. Frontier molecular orbital description of 3.
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Figure 5. (a) Double-reciprocal plot with 10 mol% of 2 at varying concentrations of pyridine-d5 at 25 °C. (b) Job plot analysis between 2
and pyridine in benzene-d6 at 25 °C. (c) Solid-state structures of (TrL)Co(py) (4) at 100 K with thermal ellipsoids set at 50% probability
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(hydrogen atoms and solvent molecules are omitted for clarity; Co aquamarine, C gray, N blue). Selected bond lengths (Å): Co–Cortho
2.050(8); Co–Cmeta: 2.239(8).
,
Figure 6. (a) Full reaction progression with 10 mol% of 2 at 50 °C in benzene-d6. (b) Zeroth-order dependence on the concentration of 5 at
25 °C in benzene-d6. (Inset) Initial stages of reaction showing the concentration of 7 is constant. A dashed line indicates the region less
than 10% of 6. (c) First-order dependence on the concentration of 7 at 25 °C benzene-d6.
2.3. Catalytic C−H Amination.
Indeed, unlike (ArL)Co(NR), which binds one equivalent of
pyridine in solution,37 3 showed no binding affinity towards
pyridine. Moreover, at 25 °C, slower rates of C−H amination
were observed when exposing 5 to 10 mol% of 2 at higher
2.3.1. Steric protection from (TrL) leads to higher yields of
C−H amination. Encouraged by the distorted MLMB of 3 and
its steric shielding to prevent the catalyst degradation, we
speculated that analogous (TrL)Co(NR) species featuring a
linear alkyl moiety would greatly benefit the intramolecular
C–H amination to afford N-hetetrocycles. To test this
hypothesis, (4-azido-4-methylpentyl)benzene (5) was exposed
to 10 mol% of 2 in benzene-d6. Gratifyingly, the desired
concentrations of pyridine-d5 (Figure S-7).
A double-
reciprocal plot suggests that pyridine-d5 and 5 compete for the
same coordination site of 2,64 and thus, the desired C−H ami-
nation is hampered in the presence of pyridine-d5 (Figure 5a
and Figure S-9). Job plot analysis further supports that 2 readi-
ly binds pyridine in benzene-d6 to furnish the (TrL)Co(py) (4)
adduct (Scheme 2 and Figure 5b).65 Furthermore, single-
crystal X-ray diffaction analysis of 4 verifies that pyridine
occupies the same axial position as the imido moiety of 3
(Figure 5c). The solid-state structure of 4 displays an η2-arene
interaction with one of the ortho and meta carbons from the
trityl groups [Co–Cortho = 2.0520(8) Å and Co–Cmeta = 2.239(8)
Å]. These results are in line with the aforementioned steric
enviroment of 3 suggesting that the fourth coordination site of
(TrL)Co(NR) is no longer available.
product
2,2-dimethyl-5-phenylpyrrolidine
(6)
was
quantitatively obtained at room temperature along with
regenerated catalyst 2 (Figure S-2). We note that under the
same conditions ([5] = 85.0 mM at 25 °C), 10 mol% of
(ArL)Co yielded only 13% of 6 due to concomitant formation
of a catalytically inactive tetrazido complex (ArL)Co(κ2-N4R2,
R = C(CH3)2(CH2)3Ph),37 highlighting that absence of the off-
cycle pathway is critical to improve the yields of C−H
amination. As a result, even with 1 mol% of 2 in benzene-d6, 6
was obtained in 91% yield at 80 °C.
Thus, by simply switching the flanking groups of the CoI
catalyst from the 2,4,6-triphenylphenyl to trityl, pyridine be-
comes a competitive inhibitor as opposed to a cocatalyst for
the desired C−H amination. These results demonstrate that the
pyridine ligation effect precluding tetrazene formation, which
is observed for the previously studied amination catalysis with
The steric protection of the trityl substituents is further
supported by the effect of pyridine on the rates of catalytic C–
H amination. Given that the fourth-coordination site of 3 is
blocked by the trityl substituents, we hypothesized that pyri-
dine would behave as an inhibitor as opposed to an accelerant
in contrast with the C–H amination mediated by (ArL)Co(NR).
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the (ArL) scaffold is successfully incorporated within the (TrL)
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propose the following CoI/III catalytic cycle (Scheme 3): fast
oxidation of 2 by an alkyl azide affords a CoIII imide which
undergoes intramolecular HAA; subsequent radical recombi-
nation of the resulting CoII amide affords the cyclized prod-
ucts. However, we note that a direct C−H insertion pathway
from the Co imido complex cannot be fully excluded.
1
2
3
4
5
6
7
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ligand. Furthermore, considering that many other attempted
N−group transfer reactions utilizing organoazides as a nitrene
source are hampered by the reductive coupling of organo-
azides furnishing tetrazene36-37, 66 or hexazene67 complexes, we
believe that the study described herein provides a ligand de-
sign to circumvent loss of reactive metal-nitrenoid intermedi-
ates.
Scheme 3. Proposed Catalytic Cycle.
2.3.2. Mechanism of the C−H amination. Next, we sought
to investigate the mechanism of C−H amination mediated by
(TrL)Co(NR) and explore how the unusual geometry of the
cobalt imide affects its C−H amination reactivity as compared
to that of (ArL)Co(NR).
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2.3.3. Enhancement in rates of C−H amination. Surpris-
ingly, the rate of C–H amination mediated by (TrL)Co(NR) is
significantly faster compared to (ArL)Co(NR). Specifically, the
rate constant (k) for the benzylic C−H amination mediated by
7 to afford 6 is 0.063 min–1 at 25 °C, whereas the same magni-
tude of k = 0.062 min–1 was observed for the analogous
(ArL)Co(NR) at 50 °C.37 We propose the faster conversion is a
result of the bent imido moiety of 7 (as observed for 3) that
likely resembles the transition state geometry for HAA more
closely than the linear (ArL)Co(NR). Therefore, the enhanced
C−H amination rate of 7 supports our original hypothesis,
where a bent nitrenoid unit may enhance the rate of C–H ami-
nation as less structural rearrangement is required for HAA.
Overall, these results highlight that a bent MLMB linkage,
which is enforced by the (TrL) scaffold, is critical to enhance
the rate of C−H amination, thus kinetically improving the
overall C–H amination process.
Figure 7. Kinetic isotope effect experiments with 10 mol% of 2 at
25 °C in benzene-d6.
1
To this end, the reaction progression was monitored by H
NMR spectroscopy (Figure 6a). In the presence of 10-fold
excess amount of azide substrate 5, (TrL)Co (2) immediately
converted to the corresponding CoIII imide (7) at 25 °C as veri-
2.3.4. Scope of C−H amination. In addition to the en-
hanced rates, (TrL)Co(NR) is capable of functionalizing a
range of C−H bonds, even primary C−H bonds in a catalytic
fashion (Table 1). Previously, we were only able to functional-
ize benzylic C–H bonds from the analogous three-coordinate
metal imides M(NR2–): (ArL)Fe(NAd)32 and (ArL)Co(NR).37
Thus, the ability of (TrL)Co(NR) to functionalize C–H bonds
stronger than a benzylic C–H bond catalytically is noteworthy.
As such, we sought to address the elements rendering
(TrL)Co(NR) a more potent oxidant relative to the analogous
(ArL)Co(NR). Given that both imides (TrL)Co(NR) and
(ArL)Co(NR) accomplish intramolecular C–H amination via
an initial rate-limiting HAA step, as suggested by the meas-
ured KIE values,37 we attempted to evaluate thermodynamic
parameters such as E°red and pKa as described in eq 1, where
the constant C represents the standard potential for proton
reduction in aprotic solvent.48 We hypothesized that the differ-
ence in driving force for HAA from the respective imides is
proportional to the difference in bond dissociation energies
1
fied by the diagnostic H NMR chemical shifts (Figure S-2).
At early stages of the reaction, imido 7 is the resting species as
the concentration of azide 5 is sufficient to continuously oxi-
dize 2 as soon as 7 generates pyrrolidine 6 (Figure 6b inset).
Indeed, the rate of the azide consumption (−d[5]/dt) is almost
identical to the rate of product formation (d[6]/dt) at initial
statges of the catalytic reaction (SI-17). Thus, while the con-
centration of 7 is constant at early stages of the reaction (6 <
10%), the rate of formation of 6 is zeroth order in [5] and first
order in [7], corroborating that 7 is competent for C−H amina-
tion in a unimolecular fashion (Figure 6b-c).
Furthermore, a kinetic isotope effect (KIE) value was eval-
uated by comparing the initial rates of 10 mol% of 2 with 5
and (4-azido-4-methylpentyl-1,1-d2)benzene (5-d2). The ob-
served ratio of the rate constants (kH/kD) was determined to be
38.4(1) at 25 °C (Figure 7), consistent with a rate-determining
homolytic C–H bond cleavage.68 Given all the kinetic data, we
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(BDEs) of the N–H bonds of the corresponding CoII amide
complexes (Scheme 4).
(ArL)Co(NR) arises from a more basic imido moiety of its one-
electron reduced form [(TrL)Co(NR)]−.
1
2
3
4
Table 1. Catalytic C−H amination using 2 and alkyl azidesa
5
6
7
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Figure 8. Cyclic voltammograms of 3 and 8 obatined in 1,2-
difluorobenzene at room temperature using 0.1 M [nBuN4][PF6]
electrolyte. Potentials were referenced to [Cp2Fe]+/0 couple. Scan
rate = 100 mV/s.
Unfortunately, our attempts to directly probe the pKa of the
respective amide complexes (TrL)Co(NHAd) (3a) and
(ArL)Co(NHAd) (8a) were unsuccessful due to their high pro-
pensity for subsequent HAA resulting in formation of 1-
adamantylamine.51 Instead, the difference in pKa between 3a
and 8a was estimated by accounting for the difference in
BDEs of the C–H bonds (6 kcal/mol) that can be activated by
the respective imides intramolecularly at 25 °C. Specifically at
25 °C, (TrL)Co(NR) accomplishes amination of secondary C–
H bonds (96 kcal/mol, R = C(CH3)2(CH2)3CH3) affording
2,2,5-trimethylpyrrolidine, whereas (ArL)Co(NR) can only
aIsolated yields from reactions conducted with 85.0 mM of azide
substrate. b1mol% of 2. c5 mol% of 2. d10 mol% of 2.
BDE of N–H (kcal/mol) = 23.06E°red + 1.37pKa + C (eq 1)
Scheme 4. Thermodynamic Square Scheme for HAA from
CoIII Imides 3 and 8.
functionalize benzylic C–H bonds (90 kcal/mol,
R =
C(CH3)2(CH2)3Ph) to afford 6. Accordingly, the conjugate
base of 3a [(TrL)Co(NR)]− is estimated to be about 105 times
more basic than the conjugate base of 8a [(ArL)Co(NR)]−. The
observed difference in pKa is further supported by DFT calcu-
lations,51 yet evaluation of the absolute pKa of each amide
complex needs to be further examined (SI-41).
This observed trend in basicity is in agreement with litera-
ture reports proposing that a bent imido moiety is more basic
compared to a linear one.56 Thus, these results demonstrate
that altering the local geometry of the terminally bound metal
imido unit subsequently affects its basicity as well, and there-
by enhances its oxidizing power and broadens its C–H amina-
tion reaction scope.
To this end, we used 3 and its isolable congener
(ArL)Co(NAd) (8) as model complexes to probe their one-
electron reduction potentials (E°red values for CoIII/II couples)
via electrochemical analysis. Cyclic voltammetry experiments
of 3 and 8 were conducted using 0.1 M [nBuN][PF6] electro-
lyte in the non-coordinating solvent 1,2-difluorobenzene to
obviate exogenous ligand coordination that may affect the
E°red. Interestingly, both 3 and 8 display reversible one elec-
tron reductions at E1/2 of −1.98 V and −1.93 V (vs. [Cp2Fe]+/0),
respectively (Figure 8).
3. CONCLUSIONS
The foregoing results illustrate how rational ligand design
allowed the synthesis of a robust CoI catalyst and isolation of a
bent CoIII imide that promotes facile catatlytic C–H amination.
The previously studied pyridine-accelerated C–H amination
process mediated by (ArL)Co(NR) informed the design princi-
ples for the synthesis of a dipyrromethene ligand (TrL) featur-
ing sterically encumbered trityl substituents. The subtle modi-
fication of the supporting ligand drastically altered the primary
coordination environment of a CoIII imide to force the terminal
imido unit perpendicular to the dipyrrin ligand plane, impos-
ing a bent geometry. Kinetic and thermodynamic analyses of
the C−H amination mediated by either (TrL)Co(NR) or
(ArL)Co(NR) highlight that the unique structure of
(TrL)Co(NR) is key for the significantly enhanced C−H amina-
The observed low reduction potentials suggest that the
driving force for HAA can be ascribed to the high basicity of
the one-electron reduced anionic imides [LCo(NR)]−, which
are equivalent to the conjugate base of amides [LCo(NHR)] 3a
and 8a, respectively (Scheme 4).54 Furthermore, given the
almost identical E°red values of 3 and 8, we believe that the
greater oxidizing power of (TrL)Co(NR) compared to
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Page 8 of 11
tion efficacy as manifested in yields, rates, and oxidizing pow- afford 3 as a purple powder (109 mg, 92%). Crystals suitable
1
2
3
4
5
6
7
8
er. In particular, the sterically demanding trityl substituents
prevent catalyst degradation (via tetrazene formation), con-
tributing to higher yields of C–H amination. More importantly,
the atypical trigonal pyramidal geometry of the CoIII imide
with a significantly bent imide resembles the transition state
for HAA more closely, and thereby, (TrL)Co(NR) displays
enhanced rates of C–H amination compared to the trigonal
planar (ArL)Co(NR) possessing a linear imido geometry. Last-
ly, the more bent, and thus more basic, one-electron reduced
species of (TrL)Co(NR) provides a greater thermodynamic
driving force for HAA enabling C–H amination with a broader
substrate scope at milder conditions. We believe that this study
provides an alternative synthetic strategy to generate highly
reactive metal imido complexes via direct manipulation of the
geometry of MLMBs to accomplish more applicable catalytic
transformations.
for X-ray diffraction were grown from a 1:1 n-hexane/benzene
mixture at room temperature. H NMR (600 MHz, 295 K,
1
C6D6): δ/ppm 7.94 (d, 12 H), 7.23 (d, 2 H), 6.97 (t, 6 H), 6.84
(s, 2 H), 6.67 (t, 11 H), 6.03 (d, 2 H), 2.26 (s, 5 H), 2.20 (s, 3
H), 1.80 (s, 3 H), 1.63 (t, 6 H), 1.08 (s, 6 H). 13C{1H} NMR
(125 MHz, C6D6): δ/ppm 192.23, 153.36, 151.26, 146.88,
137.28, 135.37, 134.93, 132.14, 129.57, 128.59, 125.51,
124.41, 69.27, 61.65, 61.12, 37.54, 33.30, 31.03, 21.25, 20.69.
Anal. Calc. for C66H60CoN3: C 83.08, H 6.34, N 4.40; Found:
C 82.72, H 6.52, N 4.71.
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ASSOCIATED CONTENT
Supporting Information. General experimental considerations
and procedures, multinuclear NMR data, electrochemistry data,
kinetic analysis, magnetometry data, solid state molecular struc-
tures, computational details, crystallographic CIF files, and crys-
tallography data.
This material is available free of charge via the Internet at
http://pubs.acs.org.
EXPERIMENTAL SECTION
General experimental considerations, synthesis and spec-
troscopic characterizations, X-ray diffraction techniques, ki-
netic analysis, electrochemistry measurements, and DFT re-
sults are all provided in Supporting Information.
AUTHOR INFORMATION
Corresponding Author
Preparation of (TrL)CoCl (1). In an oven-dried 20 mL vi-
al, anhydrous CoCl2 (0.105 g, 0.81 mmol, 1.2 equiv) was dis-
solved in THF and frozen in a liquid nitrogen cooled cold-
well. To a frozen solution, a solution of (TrL)Li (0.52 g, 0.68
mmol, 1 equiv) in THF was added and stirred for 20 hours at
room temperature. The resulting dark red reaction mixture was
filtered through a coarse glass frit with Celite to remove LiCl
and the excess CoCl2. The Celite was washed with an addi-
tional 20 mL benzene. The collected filtrate was frozen and
benzene was removed by sublimation in vacuo to afford 1 as a
dark red powder (0.52 g, 90%). Crystals suitable for X-ray
diffraction were grown from a 2:1 n-hexane/benzene layered
*betley@chemistry.harvard.edu.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENT
T.A.B. acknowledges this work was supported by a grant from the
NIH (GM-115815), the Dreyfus Foundation (Teacher-Scholar
Award), and Harvard University. Y.B. would like to thank a
Kwanjeong Graduate Student Fellowship.
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solution at room temperature. H NMR (600 MHz, 295 K,
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