C?H Bond Activation by an Imido Cobalt(III) and the Resulting Amido Cobalt(II) Complex

Article abstract of DOI:10.1002/anie.201914718

The 3d-metal mediated nitrene transfer is under intense scrutiny due to its potential as an atom economic and ecologically benign way for the directed amination of (un)functionalised C?H bonds. Here we present the isolation and characterisation of a rare, trigonal imido cobalt(III) complex, which bears a rather long cobalt–imido bond. It can cleanly cleave strong C?H bonds with a bond dissociation energy of up to 92 kcal mol^?1 in an intermolecular fashion, unprecedented for imido cobalt complexes. This resulted in the amido cobalt(II) complex [Co(hmds)₂(NH^tBu)]^?. Kinetic studies on this reaction revealed an H atom transfer mechanism. Remarkably, the cobalt(II) amide itself is capable of mediating H atom abstraction or stepwise proton/electron transfer depending on the substrate. A cobalt-mediated catalytic application for substrate dehydrogenation using an organo azide is presented.

Full text of DOI:10.1002/anie.201914718

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Accepted Article
Title: C−H bond activation by an Imido Cobalt(III) and the Resulting
Amido Cobalt(II) Complex
Authors: Alexander Reckziegel, Clemens Pietzonka, Florian Kraus,
and Gunnar Werncke
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201914718
Angew. Chem. 10.1002/ange.201914718
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C−H bond activation by an Imido Cobalt(III) and the Resulting
Amido Cobalt(II) Complex
A. Reckziegel,[a] C. Pietzonka,[a] F. Kraus,[a] C. G. Werncke*[a]
Abstract: The 3d-metal mediated nitrene transfer is under intense
scrutiny due to its potential as an atom economic and ecologically
benign way for the directed amination of (un)functionalised C−H
bonds. Here we present the isolation and characterisation of a rare,
trigonal imido cobalt(III) complex, which bears a rather long cobalt-
imido bond. It can cleanly cleave strong C−H bonds with an bond
dissociation energy of up to 92 kcal/mol in an intermolecular fashion,
unprecedented for imido cobalt complexes. This resulted in the amido
cobalt(II) complex [Co(hmds)2(NHtBu)]−. Kinetic studies on this
reaction revealed an H atom transfer mechanism. Remarkably, the
cobalt(II) amide itself is capable of mediating H atom abstraction or
stepwise proton/electron transfer depending on the substrate. A
cobalt mediated catalytic application for substrate dehydrogenation
using an organo azide is presented.
acquire a imido metal complex in a higher spin state the use of
weak-field ancillary ligands, that support the metal nitrogen
multiple bond, is thought to be beneficial. This was show-cased
by King et al. using a bidentate pyrromethane ligand (Scheme 1,
left).[9] Respective imido cobalt complexes could perform the
intramolecular C−H activation and amination of the ancillary or
imido ligand (e. g. benzylic positions), the latter even on a catalytic
scale when using an appropriate organo azide.[21] However, no
intermolecular C−H bond activation by imido cobalt complexes
has been observed so far. Further, the knowledge on the
behaviour of the cobalt amides, which inevitably result from the
initial H atom abstraction, towards these substrates beyond
deprotonation is absent in the literature. However, knowledge
thereof is crucial to fully understand the (catalytic) nitrene transfer .
Imido metal complexes of the late 3d-transition metals are of
principal interest as they feature a multiply bonded [N−R]
functionality that can be potentially transferred to unreactive
substrates. As such they are key intermediates in the metal
catalyzed aziridation of olefins and amination of substrates
bearing (un-)functionalized C−H bonds.[1] An important factor that
determines their reactivity is the electronic situation of the imido
bound metal ion, which is impacted by coordination number and
electronic spin as well as oxidation state. Whereas imido iron
complexes have been under intense scrutiny[2,3] lesser is known
about isolable heavier 3d-metal imido complexes of cobalt to
copper. This can be rationalized by a weaker, more reactive metal
nitrogen double bond of late 3d-transition metals which is inter alia
due to a higher d-electron count and reduced ligand-to-metal
back-bonding.[4] This is reminiscent of observations in the metal
oxido chemistry, where terminal late transition metal oxido
complexes are inherently labile for the same reasons. Therefore,
isolable imido complexes of copper,[5] nickel,[6] and cobalt[3,7–19]
are scarce and knowledge on the structure, electronic situation as
well as their reactivity is still largely lacking.
Scheme 1. Paramagnetic imido cobalt(III) complexes (Ad = adamantyl).
We recently reported on the synthesis of two-coordinate metal(I)
complexes such as [CoI(hmds)2]−, [1]−, (hmds = −N(SiMe3)2),
which exhibit a very weak ligand field.[22] Given the lack of
chelating and highly encumbering ligands that enforce a given
coordination sphere we hypothesized the use of [1]− as an entry
into reactive yet isolable imido higher spin cobalt complexes. In
this study we present the isolation of a paramagnetic cobalt(III)
imide bearing
a
rather
long Co−N imide bond, its
unprecedentedly potent intermolecular H atom abstraction
capability as well as the multifaceted reactivity of resulting
cobalt(II) amide towards C−H bonds.
For example, isolable imido cobalt complexes showed a variety of
observable oxidation states (+II to +V), however no clear
correlation with respect to their reactivity, such as H-atom transfer,
has emerged.[3,7–20] High-spin complexes are generally ascribed a
higher reactivity, whereas low-spin complexes are found to be
more stable. This can be reasoned by reduced back bonding into
the partially filled d-orbitals in case of high-spin complexes.
Indeed, nearly all isolable cobalt complexes are found in a low-
spin state. Only two imido cobalt systems with higher spin states
at room or near room-temperature are known (Scheme 1).[9,13] To
Scheme 2. Synthesis of K{L}[2] (L = crypt.222; hmds = −N(SiMe3)2) and its H
atom abstraction ability leading to K{L}[3] (shown for 1,4-cycloexadiene).
[a]
A. Reckziegel, C. Pietzonka, Prof. Dr. F. Kraus, Dr. C. G. Werncke
Fachbereich 15/Chemie
Philipps-Universität Marburg
Hans-Meerwein-Straße 4, D-35043, Marburg, Germany
E-mail: gunnar.werncke@chemie.uni-marburg.de
Reaction of K{L}[1] (L = crypt.222) with tert-butyl azide (Scheme
2) lead to the formation the imido cobalt complex K{L}[2] (Figure
1). In the crystal structure the complex anion [2]− exhibits a cobalt
Supporting information for this article is given via a link at the end of
the document.
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10.1002/anie.201914718
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ion surrounded by two hmds ligands and the NtBu unit in a
distorted trigonal planar fashion. The Co−Nimido bond length
amounts to 1.7067(12) Å which is slightly longer than of other
reported imido cobalt complex (1.60 − 1.69 Å) which can be
connected to the anionic charge and the higher spin state of the
complex.[3,7–19] The Co−N−CtBu angle is with 160.78(12)° slightly
bent.
<85 kcal mol−1l)[25] at ambient conditions in [D8]THF, leading to
the formation of K{L}[3]. As in-situ analysis of reactions with
substrates bearing stronger C−H bonds was limited by the
concurrent reaction of the imide with [D8]THF, respective
substrate transformations (including used solvents) were
performed either under neat conditions or in 1,2-DFB (t1/2(DFB)
=
2.5 h) and subsequently analysed by 1H NMR spectroscopy.
K{L}[2] reacted at room temperature rapidly with pure THF
(92.1±1.6 kcal mol−1)[25] and Et2O (91.7 kcal mol−1),[26] giving
K{L}[3] as the main product. For toluene (88.0±1 kcal/mol)[25] and
ethylbenzene (85.4 kcal/mol)[25] the selective formation of K{L}[3]
was observed over the course of 12 h. Kinetic studies on these
reactions via UV/Vis spectroscopy showed substrate conversion
with pseudo first-order kinetics with respect to the imido cobalt
complex [2]−.
Figure 1. Sections of the crystal structures of KL}[2] (left) and K{L}[3] (right).
Cations and unnecessary H atoms are omitted for clarity.
1H-NMR spectroscopic examination in [D8]THF showed two
paramagnetically shifted signals at −2.68 and 32.8 ppm for the
hmds as well as the tBu group. The paramagnetic nature of K{L}[2]
was confirmed in solution (eff = 3.75 B, Evans method) as well
as in solid state via VT-dc susceptibility measurements (eff = 3.52
B at 300 K), which suggested a triplet configuration (S = 1) as the
electronic ground state. These values are similar to the cobalt (III)
imido complex AMes (eff = 3.68 B, Scheme 1)[9] for which a triplet
ground state was proposed, too, and slightly above values found
for
other
low-coordinate
X-Band
cobalt(III)
perpendicular-mode
complexes
(e.g.
EPR-
[CoIII(SAr)4]−[23]).
measurements at 12 K gave no useful signal, expected for an
integer spin system.
K{L}[2] is surpisingly incompetent in nitrene transfer reactions (e.g.
to phosphines or alkenes), contrasting the behavior of most imido
cobalt complexes. We then analysed the reactivity of K{L}[2]
concerning H atom abstraction (HAA) (Scheme 2). When K{L}[2]
1
Figure 2. Correlation between the reaction constants kobs and the C−H BDE (■)
(and their acidity (x)) for different substrates (shown for BDE < 85 kcal mol-1) for
their reaction with [2]−.
was treated with 1,4-cyclohexadiene (1,4-CHD) in [D8]THF H-
NMR spectroscopic examination revealed the full conversion of
K{L}[2], formation of benzene and new paramagnetic signals at
76.2 and −15.5 ppm. Layering the reaction mixture with pentane
gave the formal
H
atom abstraction (HAA) product
The observed reaction rates correlated well with the BDE of the
involved C−H bond(s) (Figure 2 and S27) but not their acidity. The
use of partially deuterated [D4]9,10-DHA revealed a kinetic
isotope effect of KIEH/D = 6.5(2). This is lower than the semi
classical limit at that temperature (≈13), pointing to a non-linear
C∙∙H∙∙N trajectory. An Eyring analysis of the temperature
dependence of the rate constants of the reaction of complex [2]−
with 1,4-CHD in a range of −70 °C to −30 °C gave a rather small
activation enthalpy (H‡ = 19(3) kJ mol−1). The H‡ value is similar
to the one for H atom abstraction of C−H or O−H bonds by metal
oxido or hydroxido complexes.[27] Overall, these findings clearly
indicate the partition of the C−H bond cleavage in the rate
determining step and suggest an H atom transfer mechanism.
K{L}[Co(hmds)2(HNtBu)], K{L}[3], which in the crystal structure
exhibits a three coordinate cobalt ion with a clearly bent tert-butyl
amide unit (134.79(10)°). The Co−N1 bond length of 1.8832(12)
Å is in line with a cobalt amide single bond.[24] The Co−Nhmds
bonds in complex [3]− are elongated by about 0.05 Å in
comparison with complex [2]−, indicative of a lower oxidation state
of the metal ion in complex [3]−. Its magnetic moment in solution
amounts to eff = 4.58 B, expected for a high spin cobalt(II)
complex (4.3 – 5.3 B).
Given the unprecedented intermolecular HAA by an isolable imido
cobalt complex, we were interested in the HAA potential of K{L}[2].
K{L}[2] reacted rapidly with stoichiometric amounts of substrates
bearing rather weak C−H bonds (9,10-dihydroanthracene (9,10-
DHA), xanthene (XAN), indene (IND), 9-fluorene (9-FLU), Ph2CH2,
Ph3CH and ethylbenzene (EtPh); bond dissociation energies BDE
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view of this equilibrium and the comparison of the speed of
homolytic Co-C cleavage by [7]− with the reaction of the amide
[3]− with xanthene, a stepwise PT/ET mechanism can be
assumed for this substrate. In contrast, for 9,10-DHA such a
pathway is not plausible, as the substrate conversion by the cobalt
amide [3]− exceeded dramatically the formation of [1]− from
complex [6]−. Therefore, a direct H atom transfer is hypothesized
for this substrate. Such a mechanism is also likely for the reaction
of the amide [3]− with 1,4-CHD (pka ≈ 34).
Scheme 3. H atom transfer reactivity of [3]– as well as the behaviour of resulting
Co(hmds)2/substrate anion complexes.
As during the reaction of substrates with K{L}[2] follow-up
reactions could be observed, the reaction of the resulting cobalt
amide complex [3]− was examined more closely via 1H NMR
spectroscopy and X-ray diffraction analysis of reaction products.
For fluorene and indene simple deprotonation of the substrate by
the amide [3]− occurred (Scheme 3). This gave either the cobalt(II)
fluorenyl compound K{L}[Co(hmds)2(9-FLU)], K{L}[4] (Figure 3,
left), or a mixture of the indenyl anion and Co(hmds)2. The organo
cobalt complex [4]− is rather stable and as only upon heating
(60 °C, 1 h) marginal amounts of complex [1]−, the result of a
formal homolytic Co−C cleavage, were detected. For the reaction
of triphenyl methane with [3]− in 1,2-DFB immediate formation of
Figure 3. Sections of the crystal structure of K{L}[4] (left) and K{L}[6] (right).
Cations and unnecessary H atoms are omitted.
These C−H atom abstraction reactions by [3]− are, besides being
so far unreported for exogeneous substrates in case of an amido
cobalt complex, striking for several reasons: (a) The amide [3]−
can deprotonate substrates bearing little acidic C−H bonds which
potentially poses an unproductive pathway for catalytic nitrene
transfer into C−H bonds. (b) This can be followed by reduction of
the cobalt(II) complex by the formed substrate anion, which
overall constitutes a stepwise proton/electron transfer. c) The
formal H atom transfer can also occur in an synchronous fashion.
This behaviour resembles the H atom abstraction behavior of a
copper or a rhodium aminyl radical complex.[28] The reaction
trichotomy can be loosely correlated to an interplay between the
BDE and pKA values of the C−H bond of the substrate. Thereby a
very low pka value (<22) of the C−H bond lead to simple
protonation, whereas a moderate pkA value and a higher C−H
BDE lead to a stepwise PT/ET. In contrast, for higher C−H pkA
and moderate BDE values of the substrate a direct HAT pathway
is seemingly preferred. The divergent behavior of amide [3]− is
reminiscent of related terminal metal hydroxido complexes, which
– besides deprotonation – can also facilitate H atom abstraction
of C−H bonds.[29] Next, we wanted to assess the N−H bond
strength of the amide [3]−. In view of the rather slow reaction of
[2]− with PH3C−H, [3]− was reacted with a fivefold excess of the
trityl radical (Gomberg dimer). This lead to the detection of small
amounts of PH3C−H; which however could not be quantified due
to subsequent reactions. Together with the results of the substrate
scope of [2]−, this implicates a N−H bond strength range of
approximately 85−90 kcal/mol for [3]−.
t
[1]− and BuNH2 is observed. Intriguingly, the same reaction in
THF led to the formation of mainly Co(hmds)2 and the
precipitation of K{L}[CPh3], K{L}[5], due to its low solubility of the
latter in this solvent. When Co(hmds)2 was then subjected to the
methanide anion in 1,2-DFB, the rapid formation of [1]− and the
Gomberg dimer was observed, which overall implied a stepwise
proton/electron transfer for Ph3CH in its reaction with the cobalt
amide. Remarkably, the reaction of Ph3CH with [3]− proceeded
significantly faster than the analogous reaction with the imide [2]−.
For 9,10-DHA, and slower for 1,4-CHD and xanthene, the reaction
with complex [3]− lead to the formation of the starting cobalt(I)
complex [1]−, tert-butyl amine and the respective dehydrogenation
product. Thereby these reactions were slower than the analogous
ones with the imide [2]−. For xanthene an additional set of
paramagnetic signals was observed, pointing to parallel substrate
deprotonation. To elucidate a possible stepwise PT/ET process
for these substrates, Co(hmds)2(thf) was reacted with crypt.222
and K(9,10-DHA) or K(XAN), which gave the organometallic
complexes (K{L}[Co(hmds)2(9,10-DHA)], K{L}[6] (Figure 3, right)
and (K{L}[Co(hmds)2(XAN)], K{L}[7] (Figure S43, ESI). These
complexes underwent only moderate homolytic bond Co−C bond
cleavage and complex [1]− formation. When [6]− and [7]− were
t
subjected to BuNH2, the immediate deprotonation of the amine
under formation of [3]− and 9,10-DHA or xanthene occurred.
Thereby the deprotonation of the amine was not fully achieved in
case of [7]−. This pointed to a readily adjusted equilibrium
between the cobalt amide and the cobalt substrate complexes. In
Lastly, we probed selected transformations in a catalytic setting.
With 5 mol% of K{L}[2], H atom transfer from 9,10-DHA to tert-
butyl azide is observed (49% conversion, 24 h, 25 °C; Table 1).
However, examination of the reaction mixture showed
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decomposition of the employed cobalt complex although the
system remained partially catalytically active. For xanthene,
catalytic dehydrogenation was possible but gave also unidentified
side products, possibly result of C-N bond formation processes.
In case of liquid 1,4-CHD and indene the catalysis could also be
performed under neat conditions.
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Table 1. Catalytic dehydrogenation of substrates by K{L}[1] using tert-butyl
azide.
Substrate
Solvent
Time
/h
Conversion
(azide)*
Yield
(amine)*
[AH]
9,10-DHA
1,4-CHD
XAN
[D8]THF
24
36
36
36
49%
99%
61%
72%
49%
99%
/
[D8]THF
/
31%**
72 %
IND
In conclusion, we presented the synthesis, isolation and
characterization of a rare paramagnetic cobalt(III) imido complex
via reaction of a two-coordinate cobalt(I) complex with tert-butyl
azide. It exhibits a rather long Co−Nimido bond and facilitated the
rapid intermolecular H atom abstraction from C−H bonds with a
BDE of up to 92 kcal/mol under formation of a cobalt(II) amido
complex. Kinetic and mechanistic studies implicated an H atom
transfer mechanism. Remarkably, the formed cobalt(II) amide
could also abstract an H atom from the substrate, which gave free
tert-butyl amine and regenerated the cobalt(I) complex [1]−.
Depending on the substrate, the formal H atom abstraction
occurred via a concerted or a stepwise proton/electron transfer
mechanism. A first catalytic study showed the possibility of
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Acknowledgements
We thank the DFG (WE 5627/4−1) for financial support. We thank
Dr. O. Burghaus (Philipps-University Marburg), Prof. Dr. K. Ray
and B. Battistella (Humboldt-University Berlin) for EPR
measurements.
Keywords: Imido cobalt complex • H atom transfer • C−H
activation • Amido cobalt complex • Catalysis
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10.1002/anie.201914718
Angewandte Chemie International Edition
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A
trigonal planar imido cobalt(III)
A. Reckziegel, C. Pietzonka, F. Kraus,
C. G. Werncke*
complex, obtained from reaction of a
quasi-linear cobalt(I) complex with an
Page No. – Page No.
organo
azide,
is
capable
of
intermolecular H atom abstraction of C-
H bonds. The resulting cobalt(II) amide
itself can either deprotonate the
A Trigonal Imido Cobalt(III) Complex:
Synthesis and H Atom Abstraction
Reactivity
substrate, facility an
H
atom
abstraction or mediate a stepwise
proton/electron transfer. As the latter
regenerates the starting cobalt(I)
complex, a first catalytic application is
presented.
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Author(s), Corresponding Author(s)*
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Title
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