C-H functionalization reactivity of a nickel-imide

Article abstract of DOI:10.1021/ja302149k

We report bifunctional reactivity of the β-diketiminato Ni(III)-imide [Me₃NN]Ni=NAd (1), which undergoes H-atom abstraction (HAA) reactions with benzylic substrates R-H (indane, ethylbenzene, toluene). Nickel-imide 1 competes with the nickel-amide HAA product [Me₃NN]Ni-NHAd (2) for the resulting hydrocarbyl radical R^? to give the nickel-amide [Me₃NN]Ni-N(CHMePh)Ad (3) (R-H = ethylbenzene) or aminoalkyl tautomer [Me₃NN]Ni(η²-CH(Ph)NHAd) (4) (R-H = toluene). A significant amount of functionalized amine R-NHAd is observed in the reaction of 1 with indane along with the dinickel imide {[Me₃NN]Ni} ₂(μ-NAd) (5). Kinetic and DFT analyses point to rate-limiting HAA from R-H by 1 to give R^?, which may add to either imide 1 or amide 2, each featuring significant N-based radical character. Thus, these studies illustrate a fundamental competition possible in C-H amination systems that proceed via a HAA/radical rebound mechanism.

Full text of DOI:10.1021/ja302149k

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Article
C-H Functionalization Reactivity of a Nickel-Imide
Stefan Wiese, Jason L. McAfee, Dale R. Pahls, Claire L.
McMullin, Thomas Richard Cundari, and Timothy H. Warren
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/ja302149k • Publication Date (Web): 22 May 2012
Downloaded from http://pubs.acs.org on May 28, 2012
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Journal of the American Chemical Society
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C-H Functionalization Reactivity of a Nickel-Imide
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Stefan Wiese,† Jason L. McAfee,‡ Dale R. Pahls,‡ Claire L. McMullin,‡ Thomas R. Cundari,*,‡ and
Timothy H. Warren*,†
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†Department of Chemistry, Georgetown University, Box 571227-1227, Washington, D. C. 20057
United States
‡Department of Chemistry, Center for Advanced Scientific Computing and Modeling (CASCaM),
University of North Texas, Denton, TX 78203, United States
Email: *,†thw@georgetown.edu; *,‡t@unt.edu
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RECEIVED DATE (to be automatically inserted after your manuscript is accepted if required
according to the journal that you are submitting your paper to)
ABSTRACT: We report bifunctional reactivity of the -diketiminato Ni(III)-imide [Me3NN]Ni=NAd
(1) which undergoes H-atom abstraction (HAA) reactions with benzylic substrates R-H (indane,
ethylbenzene, toluene). Nickel-imide 1 competes with the nickel-amide HAA product [Me3NN]Ni-
NHAd (2) for the resulting hydrocarbyl radical R• to give the nickel-amide [Me3NN]Ni-N(CHMePh)Ad
(3) (R-H = ethylbenzene) or aminoalkyl tautomer [Me3NN]Ni(2-CH(Ph)NHAd) (4) (R-H = toluene).
A significant amount of functionalized amine R-NHAd is observed in the reaction of 1 with indane
along with the dinickel imide {[Me3NN]Ni}2(-NAd) (5). Kinetic and DFT analyses point to rate-
limiting HAA from R-H by 1 to give R•, which may add to either imide 1 or amide 2, each featuring
significant N-based radical character. Thus, these studies illustrate a fundamental competition possible
in C-H amination systems that proceed via a HAA / radical rebound mechanism.
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Introduction
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Metal-imide (nitrene) intermediates, [M]=NR’,1 have been implicated in many catalytic C-H
amination reactions that allow the atom economical introduction of nitrogen atoms into organic
molecules without traditional functional group manipulations.2 A number of synthetically useful
protocols have been developed for both intramolecular and intermolecular C-H amination. Most
syntheses that operate through putative nitrene intermediates typically employ electron-withdrawing N-
sulfonyl or N–carbamoyl substituents on the nitrene nitrogen to achieve catalytic activity. Dirhodium3
or ruthenium4,5 catalysts are common, though catalyst systems utilizing inexpensive first row metals
such as Mn,6 Fe,7,8 Co,9 and Cu10,11 are known. Regardless of the metal, only in a limited number of
cases may discrete metal-nitrene intermediates be isolated in these C-H functionalization reactions. For
example, Che et al. identified (porphyrin)Ru(=NSO2R)2 species and described their kinetics in the
functionalization of benzylic and allylic C-H bonds.4 Later, Cenini and coworkers isolated the N-aryl
analogue (porphyrin)Ru(=NArF)2 (ArF = 3,5-(CF3)2C6H3) that reacts with C-H bonds of modest strength
such as the allylic C-H bond of cyclohexene.12
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Nickel imides have played key roles in the recent renaissance of late metal imido chemistry1 ignited
by the 2001 report of the three coordinate terminal imide (dtbpe)Ni=NAr (Ar = 2,6-iPr2C6H3)13 by
Mindiola and Hillhouse which engages in
stoichiometric nitrene group transfer to isocyanides,14
+
t
t
t
t
Bu
Bu
Bu
Bu
Bu
R'
R'
Mes
CO,14 and ethylene15 (Figure 1). Formed via 2-
R'
Ni N
R'
P
P
Ni
Ni
NR
NR
azide adducts (dtbpe)Ni(2-N3R),16 the nickel(II)
imides (dtbpe)Ni=NR (R = 1-adamantyl (Ad), 2,4,6-
Me3C6H2 (Mes))16 may be oxidized to cationic their
cationic nickel(III) analogues {(dtbpe)Ni=NR}+
which exist in both S = 1/2 (R = Ad and Mes) and S =
3/2 states (R = Ad) (Figure 1).17 The cationic N-
adamantyl species is especially reactive, undergoing
P
P
Mes
t
t
t
t
Bu
Bu
Bu
R' = CHPh
2
+
(dtbpe)Ni=NR
{(dtbpe)Ni=NR}
(IPr*)Ni=N(dmp)
2
R
[Me NN]Ni=NAd (1)
Ad
N
3
1
R
1
1
2
[Ni]
[Ni]
R = R = Me
R
N
Ni
NAd
i
[ Pr NN]Ni=NAr'
{[Me NN]Ni} (-NAd)
2 2
1
N
1
2
R
R
1
i
2
1
2
R = Pr, R = H
R = Me, R = H
2
R
-diketiminato nickel imides
Figure 1. Nickel imides.
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facile H-atom abstraction from ether to deliver the cationic nickel(II) amide {(dtbpe)Ni-NHAd}+.
Hillhouse also demonstrated that N-heterocyclic carbene ligands may support nickel imides such as the
bridging species [(IPr)NiCl]2(-NMes) (IPr =1,3-di(2,6-di-isopropylphenyl)imidazolin-2-ylidene)
which catalyzes catalytic nitrene group transfer to isocyanides.18 Employing an especially bulky N-
heterocyclic carbene, Hillhouse observed C-H functionalization reactivity from the two coordinate
(IPr*)Ni=N(dmp) in its reaction with ethylene to form CH2=CHNH(dmp) in addition to nitrene transfer
to CO (Figure 1).19
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Warren and coworkers showed that -
Scheme 1. Reactivity of 1 with cyclohexadiene
diketiminato ancillary ligands enable the isolation of
and Cp2Co
the dinickel(II) imide {[Me2NN]Ni}(-NAd) as
+ 1/2
Ad
H
[NiII] N
[NiII] N
2
well
[Me3NN]Ni=NAd from the reaction of N3Ad with
the corresponding nickel(I) precursors
as
the
terminal
nickel(III)
imide
- 1/2 benzene
NAd
N
N
N
N
Ni
Ad
HAA
Ni
N
Ad
1
Cp2Co
2c - 3e-
 interaction
CoCp
C-N bond
formation
[MexNN]Ni(2,4-lutidine) (x = 2 or 3) (Figure 1).20 In addition to clean nitrene transfer to PMe3 and CO,
[Me3NN]Ni=NAd (1) readily abstracts a hydrogen atom from the bis-allylic C-H bond of 1,4-
cyclohexadiene (C-H BDE ≈ 77 kcal/mol)21 to form [Me3NN]Ni-NHAd (2) as well as undergoes radical
addition of cobaltocene to the imido N atom resulting in a nickel(II)-amide with a new C-N bond
(Scheme 1).20 DFT calculations revealed considerable spin density on the imido N atom of 1, suggesting
a description of 1 as a Ni(II)-imidyl species (viz NR⁻·) that results from the 2-center, 3-electron
interaction of an imido lone pair with a singly occupied d-orbital on the formally d7 NiIII center.20 Such
an electronic description also rationalizes the C-C and C-N coupling chemistry at the imido p-aryl
i
position of putative [iPr2NN]Ni=NAr’ (Ar’ = R2C6H3; R = Pr or Me) intermediates observed in the
reaction of {[iPr2NN]Ni}2(-toluene) with N3Ar’ reported by Bai and Stephan (Figure 1).22
While there are several experimental reports of hydrogen atom abstraction (HAA) reactions of C-H
substrates R-H by discrete, mid- to late-first row imido complexes [M]=NR’ (M = Fe,8,23 Co,24 Ni,17,20,25
Cu11), considerably more rare is complete functionalization of the C-H bond in R-H to give amines R-
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NHR’.8,11,19,26 Cundari and coworkers described
computational studies of Fe to Cu nitrene complexes
featuring chelating phosphines and -diketiminate
Scheme 2. Copper nitrene-based C-H amination
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stoichiometric
R-H
[Cu]2(-NAd)
[Cu]=NAd
Cl
- [Cu]
- [Cu]
cat.
[Cu]2(benzene)
Ad
Cl
Cu
Cl
N
N
N
R
H
N=N=NAd + R-H
- N2
catalytic
ligands.27,28
In
particular,
the
H
H
supporting
Cl
H
H
R-H =
9
[Cl2NN]Cu = [Cu]
thermodynamic favorability of methane C-H
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amination to give -diketiminato [M](NH(Me)R) products increases dramatically as the metal is
changed from Fe to Cu in an analogous series of neutral [M]=NR complexes.28 Consistent with
heightened C-H amination reactivity predicted for the copper complexes, the -diketiminato dicopper
nitrene {[Cl2NN]Cu}2(-NAd) ([Cl2NN] = 2,4-bis(2,6-dichlorophenylimido)pentyl) participates in the
stoichiometric amination of strong C-H bonds such as those in ethylbenzene, toluene, and even
cyclohexane, via the putative terminal nitrene [Cl2NN]Cu=NAd.11 The -diketiminato copper(I)
complex {[Cl2NN]Cu}2(-benzene) also catalytically aminates C-H bonds in hydrocarbons (R-H) to
give amines (HN(R)Ad) in high yields using N3Ad as the nitrene transfer reagent (Scheme 2).11,29
In light of the efficient stoichiometric and catalytic C-H amination by its copper analogue and the
aforementioned computational studies of nickel complexes, we reinvestigated the reactivity of
[Me3NN]Ni=NAd (1) with benzylic C-H substrates. Specifically, we were interested in the potential to
couple previously observed HAA and C-N bond
Scheme 3. Competition between [Ni]-NHAd and
forming reactions (Scheme 1) in a complete C-H
[Ni]=NAd for R• generated by HAA of R-H
I
[Ni ](NH(R)Ad)
functionalization sequence employing a single
RR
H
R-H
HAA
III
II
[Ni ]=NAd
[Ni ] N
+ R
R
Ad
substrate R-H to give the new amine R-NHAd. A
key outcome of the present study, however, reveals
RC
II
[Ni ] N
1
2
by
Ad
III
[Ni ]=NAd
that radical addition of R• to the nickel imide [Me3NN]Ni=NAd (radical capture (RC)) may successfully
compete with addition of R• to the nickel based HAA product [Me3NN]Ni-NHAd (radical rebound
(RR)) that ultimately completes C-H amination to deliver the new amine R-NHAd (Scheme 3).
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RESULTS
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Reaction of [Me3NN]Ni=NAd (1) with C-H Substrates – Kinetic Analysis. While 1 cleanly reacts
with 1,4-cyclohexadiene (C-H BDE ≈ 77 kcal/mol)21 via HAA to form [Me3NN]Ni-NHAd (2),20 we
find that [Me3NN]Ni=NAd (1) also reacts with hydrocarbon substrates R-H = indane, ethylbenzene, and
toluene that possess significantly stronger C-H bonds (C-H BDE = 85, 87 and 90 kcal/mol,
respectively).21 The addition of excess benzylic C-H substrate (200 – 3000 equiv.) to 1 in benzene at 25
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– 65 °C results in the smooth, first-order decay of 1 as monitored by its strong optical band at max
=
596 nm (Figure 2). Specific variation of the concentration of ethylbenzene indicates first-order
dependence in substrate R-H to give the rate expression: rate = k[[Ni]=NAd][R-H]. Comparison of the
rates of reaction of 1 with ethylbenzene and ethylbenzene-d8 leads to a KIE of 4.6(4) at 35 °C,
consistent with HAA of R-H by 1 to initially form [Me3NN]Ni-NHAd (2) and R• (Scheme 4). The
decreasing second-order rate constants at 35 °C for consumption of 1 by indane (2.1(2) × 10-4 M-1s-1),
ethylbenzene (2.4(2) × 10-5 M-1s-1), and toluene
Scheme 4. Kinetic data for reaction of
(4.5(2) × 10-6 M-1s-1) are also consistent with rate
[Me3NN]Ni=NAd (1) with R-H via initial HAA
limiting HAA considering the increasing C-H BDEs
Ad
HAA
III
II
+ R-H
+ R
[Ni ] NAd
[Ni ] N
of these hydrocarbon substrates. Bearing in mind
experimental uncertainties, the free energies of
activation also track the increasing C-H BDEs of
2
1
H
‡
‡
‡
C-H BDE G (298 K)
(kcal/mol)
S
H
(kcal/mol)
(kcal/mol)
R
(cal/mol K)
indanyl
CH(Me)Ph
85
87
90
-31(2)
-42(2)
-50(3)
23.0(15)
24.2(8)
25.1(13)
13.5(13)
11.6(7)
10.0(10)
CH Ph
2
the C-H substrates and suggest a more ordered transition state (e.g., shorter C…H…N separation and
earlier transition state, vide infra) as the C-H BDE increases (Scheme 4).
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Figure 2. UV-vis trace of reaction of 1 ([1]0 = 2.32 mM) with 2000 equiv. ethylbenzene in benzene at
45 °C (scan interval = 300s) monitoring the absorbance of 1 at  = 596 nm. Inset: linear plot of
ln[(A-A∞)/(A0-A∞)] vs. time.
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Isolation of New Nickel Products. In contrast to the clean HAA reaction between ½ equiv. 1,4-
cyclohexadiene and 1 that leads to [Me3NN]Ni-NHAd (2) in > 90% spectroscopic yield (Scheme 1),
product mixtures that result from the reaction of 1 with benzylic C-H substrates are rather complex and
sensitive to reaction conditions. Radicals R• that result from HAA of ethylbenzene, toluene, and indane
are better suited for C-N bond formation in comparison to the doubly allylic radical derived from 1,4-
cyclohexadiene which is primed for loss of an H-atom to give benzene. For 1 to convert a substrate R-
H to the corresponding C-H amination product R-NHAd, the radical R• must combine with the nickel-
based HAA product [Me3NN]Ni-NHAd (2) with high fidelity. A key finding of our studies, however, is
that the nickel(III)-imide 1 can successfully compete with nickel(II)-amide 2 for capture of the radical
R• (Scheme 3).
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The stoichiometric reaction of [Me3NN]Ni=NAd (1) with 100 equiv. ethylbenzene in Et2O at RT (3 h)
gave the nickel(II) secondary amido complex [Me3NN]Ni-N(CHMePh)Ad (3) as brown crystals from
Et2O at -35 °C in 49% yield based on total moles of Ni (Scheme 5; max yield = 50%). Similarly,
reaction of [Me3NN]Ni(2-pic) with 1 equiv. N3Ad in ethylbenzene gave an identical result: a 49%
isolated yield of 3. Thus, the nickel(III)-imide 1 rather efficiently captured the ethylbenzene radical
generated via HAA by another equivalent of 1. The X-ray structure of the three-coordinate nickel(II)-
amide 3 reveals a short Ni-Namido distance of 1.790(5) Å along with relatively short Ni-N-dik distances
of 1.879(4) and 1.876(4) Å (Figure 3). These parameters are comparable to those in the related
nickel(II)-amide [Me3NN]Ni-NAd(η4-C5H5)CoCp (Ni-Namido = 1.812(3) Å, Ni-N-dik =1.881(3) and
1.879(3) Å) illustrated in Scheme 1.20 The two Ni-
Scheme 5. Reactivity of 1 with ethylbenzene
Namido-C angles of 122.5(4) and 123.7(5)° in 3 along
and toluene to form 3 and 4
with the sum of the angles about Namido (359.5(4)°)
Ad
R
Ad
R-H
[NiIII] NAd
[NiII]
N
[NiIII] NAd
[NiII]
H
N
.
+ R
H
are consistent with a planar, sp2-hybridized Namido
donor. We note that the nickel(II)-amide
[Me3NN]Ni-NAd(η4-C5H5)CoCp forms by addition
RC
HAA
1
2
H
3 R = CH(Me)Ph
R-H =
AdN
Ad
-H elim
CHPh
[NiII]
N
CHPh
[Ni]
[NiII]
NHAd
CH2Ph
H
4 R = CH2Ph
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of the cobaltocene radical to 1 to give a new C-N bond (Scheme 1) just as the new nickel(II)-amide 3
forms by capture of an ethylbenzene radical by 1 (Scheme 5).
1
2
3
4
5
6
7
8
The stoichiometric reaction of [Me3NN]Ni=NAd (1) with neat toluene at RT (3 days) yielded the
nickel(II) secondary aminoalkyl complex [Me3NN]Ni(η2-CH(Ph)NHAd) (4) as brown crystals from
Et2O at -35 °C in 38% yield based on total moles of Ni (Scheme 4; max. yield = 50%). The connectivity
of 4 was determined by X-ray crystallography and revealed an η2-aminoalkyl group with an Ni-C bond.
Complex 4 likely forms via the tautomerization of [Me3NN]Ni-N(CH2Ph)Ad via a β-H
elimination/reinsertion sequence after initial capture of a benzyl radical by the imido nitrogen of 1. The
X-ray structure of 3 (Figure 3) reveals a four-coordinate nickel(II)-aminoalkyl species 4 with Ni-Calkyl
and Ni-Namine bond distances of 1.897(4) and 1.936(3) Å, respectively, along with a highly acute Ni-
Calkyl-Nalkyl angle of 69.4(3)°. The Ni-N-dik distances are 1.917(3) and 1.872(3) Å. Such asymmetric Ni-
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Nβ-dik bond distances were also observed in the related β–agostic alkyl complex [Me2NN]Ni(2-
CH2CH3) (1.892(3) and 1.859(3) Å).30 As in the β–agostic alkyl complex, the elongated Ni-Nβ‐dik
distance in 4 is trans to the Ni-C covalent bond.
In the stoichiometric reaction of 1 and neat indane (100 equiv.) at RT (1 day) we did not isolate any
product consistent with addition of the indanyl radical to 1. Rather, we isolated the dinickel imide
C47
N1
N1
N5
C24
N3
N3
Ni
C24
C34
Ni
Ni1
Ni2
C34
N2
N2
N3
N1
N4
N2
4
3
5
Figure 3. X-ray structures of [Me3NN]Ni-N(CHMePh)Ad (3), [Me3NN]Ni(η2-CH(Ph)NHAd) (4) and
{[Me3NN]Ni}2(μ-NAd) (5).
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{[Me3NN]Ni}2(μ-NAd) (5) as brown crystals from
Scheme 6. Reactivity of 1 with indane to give
1
2
3
Et2O at -35 °C in 52% yield (Scheme 6). The X-ray
[Ni]2(-NAd) (5)
4
Ad
H
5
6
7
8
structure of 5 (Figure 3) shows Ni-NAd distances of
1.738(3) and 1.758(3) Å along with a Ni-Ni distance
of 2.4872(7) Å. This species is nearly identical in
R-H
I
III
II
.
[Ni ](NH(R)Ad)
[Ni ] NAd
[Ni ] N
+ R
RR
HAA
1
2
Ad
N
III
I
5
[Ni ] NAd
[Ni ](NH(R)Ad) +
+ HN(R)Ad
[Ni] [Ni]
H
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R-H =
I
III
[Ni ](2-pic) + [Ni ]=NAd
5
- 2-pic
structure
to
the
previously
reported
{[Me2NN]Ni}2(μ-NAd).20 These dinickel adamantylimido complexes may be directly compared to their
Cu analogue {[Me3NN]Cu}2(μ-NAd)11 which has Cu-NAd distances of 1.785(6) and 1.821(5) Å along
with a much longer Cu…Cu separation of 2.901(2) Å. EPR analysis indicates at least partial dissociation
of 5 to [Me3NN]Ni=NAd (giso = 2.05) and [Me3NN]Ni(solvent) (giso = 2.21) at RT (ca. 1 mM in
toluene). Thus, [Ni]2(-NR) intermediates may act as a reservoir for terminal [Ni]=NR species.
We propose that the dinickel species {[Me3NN]Ni}2(-NAd) (5) isolated from the reaction of 1 with
indane results from the complexation of [Me3NN]Ni=NAd (1) by a Ni(I) species in solution such as
[Me3NN]Ni(NH(indanyl)Ad) generated by the complete C-H amination of indane by 1. In support of
this proposal, addition of the nickel(I) 2-picoline adduct [Me3NN]Ni(2-pic) to 1 allows for the isolation
of dinickel imide 5 in 78% yield (Scheme 6). Direct “rebound” of the 1-indanyl radical to 2 is
substantiated by the 1H NMR identification of the C-H functionalized organic product NH(1-indanyl)Ad
in 33% yield when 1 is allowed to react with 200 equiv. indane in Et2O at RT. Separate reactivity
studies under analogous conditions show that dinuclear 5 is essentially unreactive towards indane. In
light of this observation, the maximum amount of the functionalized amine NH(1-indanyl)Ad to be
expected is ca. 50%.
Theory – H-atom Abstraction by [Me3NN]Ni=NAd (1). We employed theory to examine several
details relevant to the hydrocarbon reactivity of 1 and the formation of species 3 - 5. At the
ONIOM(B3LYP:6-311++G(d,p):UFF) level of theory, we calculated the N-H bond dissociation
enthalpy in [Me3NN]Ni-NHAd (2) to be 85.2 kcal/mol. Comparison against the value of 83.8 kcal/mol
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calculated for the BDE of a benzylic C-H bond in
Scheme 7. Calculated activation parameters for
1
2
3
4
5
6
7
8
ethylbenzene supports the contention that
[Me3NN]Ni=NAd (1) could engage in HAA
reactions with related benzylic substrates.
HAA by 1 and transition state for HAA from
indane
[Ni] NAd + R-H
1
9
To computationally evaluate the kinetic feasibility
of these HAA reactions, we identified S = ½
transition states for HAA from indane,
ethylbenzene, and toluene by [Me3NN]Ni=NAd (1).
These transition states are 26.1 to 27.9 kcal/mol
higher in free energy than separated 1 and the
‡
R
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H
C
[Ni] N
H
H
Ni
N
Ad
.
+ R
[Ni] NHAd
[1 + indane]‡
S‡
2
G‡(298 K)
(kcal/mol) (kcal/mol) (cal/mol K)
H‡
N
C
(Å)
H
R
(Å)
indanyl
CH(Me)Ph
CH2Ph
-56.3
-56.3
-61.3
1.305 1.406
1.305 1.346
1.279 1.361
26.1
26.3
27.9
9.4
9.5
9.6
corresponding benzylic substrate R-H (Scheme 7). Principally due to a tighter transition state (shorter
C…H…N separation, Scheme 7) with increasing substrate C-H bond strength, the calculated G‡ values
at 298 K increase with increasing C-H bond strength and track well with the experimentally determined
values (Scheme 4). In each HAA transition state, the imido N atom attacks the C-H bond in an “axial”
direction relative to the trigonal imide 1, with the incipient N-H bond roughly positioned as it appears in
the product nickel-amide [Me3NN]Ni-NHAd (2) (Scheme 7). Comparison of the C…H and N…H
distances among the three transition states reveals that the T.S. becomes more ordered (e.g., tighter
C…H…N linkage) as the C-H bond strength of the hydrocarbon substrate increases from indane to
toluene. Prompted by the S = 1 state identified for [Me3NN]Ni-NHAd (see below), the S = 3/2 transition
state for HAA of indane was found to be 3.4 kcal/mol higher in free energy than the corresponding S =
½ transition state.
Theory – Nickel(II)-Amido Complexes. Experimentally, [Me3NN]Ni-NHAd (2) possesses a
solution magnetic moment of ca. 1.0 B.M.,20 suggesting contributions from both singlet and triplet spin
states. On the other hand, amide 3 and aminoalkyl 4 have solution moments of 0.0 B.M. QM/MM
calculations on amides 2 and 3 suggests that the triplets are close in energy to the singlet, located 3.2
and 2.3 kcal/mol (free energy) below the singlet in the gas phase, respectively. For Ni(II)-aminoalkyl
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species 4, however, calculations place the triplet state significantly higher than the singlet by ca. 9
kcal/mol. Calculations also indicate that the Ni(II)-aminoalkyl 4 is 5.2 kcal/mol lower in free energy
than the Ni(II)-amide tautomer [Me3NN]Ni-N(CH2Ph)Ad that directly results from capture of the
PhCH2• radical by 1 (Scheme 4). Consistent with experiment, the free energy ordering of the
aminoalkyl and amido tautomers from capture of an ethylbenzene radical by 1 is reversed: amide 3 is
favored over the tertiary aminoalkyl [Me3NN]Ni(η2-CMe(Ph)NHAd) by 4.2 kcal/mol.
1
2
3
4
5
6
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As summarized in Scheme 8, the addition of a benzylic radical R• derived from indane, ethylbenzene,
or toluene to [Me3NN]Ni=NAd (1) is significantly more thermodynamically favored than “rebound” of
this radical R• to the metal-amide [Me3NN]Ni-NHAd (2), the initial product of HAA by 1. Nonetheless,
it is not straightforward to computationally predict the
Scheme 8. Calculated thermochemistry for
outcome of these competing reactions because these
addition of benzylic radicals R• to 1 and 2-S=1
radical combination reactions are likely to have low
Ad
R
Ad
H
.
.
[NiI](NH(R)Ad)
R
R
[NiIII] NAd
[NiII] N
G
[NiII] N
kinetic barriers, especially when considering the
triplet state of [Me3NN]Ni-NHAd (2-S=1).
2
1
.
R
H
G (kcal/mol)
H
indanyl
-23.3
-22.1
-29.3
-21.0
-21.2
-26.5
-41.3
-40.8
-46.0
-3.1
-2.9
-9.5
CH(Me)Ph
CH2Ph
Scheme 8 also indicates that radical capture is
thermodynamically favored by an average of ~20 kcal/mol for addition of R• to Ni-imide versus
addition of the radical to Ni-amide. The calculated thermodynamics in Scheme 8 are consistent with the
hypothesis that radical capture by the Ni-imide 1 may compete with radical rebound to the Ni-amide 2,
especially as the former may be present in significant concentration in solution. Finally, we note that
HAA by [NiII]-N(R)Ad is calculated to be significantly endergonic (G’s > 18 kcal/mol) for the
substrates studied here, implying that the Ni-amides are not expected to compete with Ni-imides for
substrate C—H bond activation (Scheme S1).
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Both 1 and 2-S=1 possess significant unpaired electron density at the imido or amido N atom (Figure
4 and S15). The triplet state of amide [Me3NN]Ni-NHAd (2-S=1) features a “twisted” amido group that
is only ca. 30° out of planarity with the -diketiminate backbone whereas both the calculated geometry
for singlet 2 as well as the X-ray crystal structure of 2 indicates an orthogonal arrangement of the amide
relative to the -diketiminate backbone. A lengthening of the Ni-Namide distance calculated for triplet 2-
S=1 (1.831 Å) relative to singlet 2 (1.786 Å) and found in its X-ray structure (1.742(4) Å) accompanies
this conformational change.
1
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[Me NN]Ni=NAd (1)
[Me NN]Ni-NHAd (2)
3
3
-
-
N
:
0.60 e
0.32 e
N
: 0.41 e
1.25 e
imido
amido
-
-
Ni:
Ni:
Figure 4. Spin density plots of 1 and 2-S=1 (0.003 isospin value; ADF 2007.1/BP/ZORA/TZ2P(+)).
DISCUSSION AND CONCLUSIONS
The relatively high N-H bond strength in [Me3NN]Ni-NHAd (2) facilitates HAA reactions of
[Me3NN]Ni=NAd (1) with indane, ethylbenzene, and toluene under mild conditions at rates that
decrease with increasing C-H bond strength. A key outcome of this study reveals that the nickel imide
1 can capture the radical R• generated in the HAA reaction of 1 with R-H to provide the diamagnetic
nickel amido [Me3NN]Ni-N(CHMePh)Ad (3) and alkylamine [Me3NN]Ni(2-CH(Ph)NHAd) (4)
products from the reaction of 1 with ethylbenzene and toluene, respectively (Scheme 3). Thus, radical
addition to 1 competes with capture of R• by [Me3NN]Ni-NHAd (2) required to complete “insertion” of
the NAd group of 1 into the C-H bond of R-H to give coordinated R-NHAd, the amine product of C-H
functionalization (Scheme 3).
It is interesting to note that only in the reaction of [Me3NN]Ni=NAd (1) with excess indane (200
equiv. in ether) may an appreciable amount the C-H amination product be observed ((1-indanyl)NHAd,
33% yield).
Under analogous conditions with ethylbenzene or toluene, less than < 5% of the
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corresponding functionalized amine R-NHAd is observed. Since there is little calculated energetic
difference between the addition of the indanyl or ethylbenzene radical to either 1 or 2 (though addition
to 1 is strongly favored), this finding is somewhat striking. Bearing in mind the larger size and greater
rigidity of the indanyl radical vs. that of the ethylbenzene (and benzyl) radical, it appears that steric
accessibility to the nitrogen atom of imide 1 vs. amide 2 plays an important role.31 The longer Ni-N
bond in amide 2, especially in its triplet form (1.831 Å (calc.)) compared to imide 1 (1.662(2) Å (X-
ray); 1.670 (calc.)) should allow greater access of a more sterically encumbered radical to the nickel-
bound amido N atom of 2. Furthermore, the “twisted” nature of the nickel-amide linkage in 2-S=1
1
2
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orients N-based spin-density differently than in nickel-
Scheme 9. Calculated thermochemistry for
imide 1 (Figure 3), perhaps allowing for smoother C-N
addition of benzylic radicals R• to
bond formation with the rigid indanyl radical.
[Cl2NN]Cu=NAd and [Cl2NN]Cu-NHAd
Ad
Ad
.
R
.
R
I
II
II
[Cu ](NH(R)Ad)
Regarding the role of late transition metal-imides
[M]=NR’ in C-H amination, these studies illustrate that
more is required for successful C-H amination than
[Cu] NAd
[Cu ] N
[Cu ] N
R
H
H
-35.1
-31.3
-34.5
.
G (kcal/mol)
-17.2
-14.7
R
G
H
indanyl
-47.4
-45.0
-50.8
-66.4
-63.1
-67.7
CH(Me)Ph
CH Ph
2
-17.9
simply a strong N-H bond in the corresponding metal-amide [M]-NHR’. While a strong N-H bond
certainly provides driving force for HAA from the C-H substrate R-H, it should also thermodynamically
favor the addition of the radical R• to the metal-imide [M]=NR’ to give the metal-amide [M]-NRR’, a
key outcome observed in this study. Both the amount of unpaired electron density at the N atom in
[M]=NR’ and [M]-NHR’ species as well as the relative accessibility of these different types of N atoms
are likely important determinants of the site of R• attack.
In this light, the relatively unselective C-H functionalization reactivity of 1 should be contrasted with
the highly selective stoichiometric and catalytic C-H amination of benzylic substrates R-H by
{[Cl2NN]Cu}2(-NAd).11 Proceeding through the presumed S = 0 intermediate [Cl2NN]Cu=NAd, likely
present in only small amount in solution via the dissociation of a [Cl2NN]Cu fragment from
{[Cl2NN]Cu}2(-NAd) (Scheme 2),11 a related HAA reaction with R-H would result in the copper(II)-
amide [Cl2NN]Cu-NHAd.32 The copper nitrene possesses significantly more driving force for HAA
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reflected in the N-H bond dissociation enthalpy of 98.4 kcal/mol calculated for [Cl2NN]Cu-NHAd
compared to 85.2 kcal/mol for [Me3NN]Ni-NHAd (2). Comparison of the addition of R• to
[Me3NN]Ni-NHAd (2) (G = -2.9 to -9.5 kcal/mol; Scheme 8) and [Cl2NN]Cu-NHAd (G = -14.7 to -
17.9 kcal/mol; Scheme 9) reveals a notable difference in driving force favoring radical addition to the
copper(II)-amide that possesses a significant amount of spin density at the amido N atom (0.49 e-).32
While radical addition of R• to both [Cl2NN]Cu=NAd and [Cl2NN]Cu-NHAd is highly favored
thermodynamically (Scheme 9), clean C-H amination with substrates R-H by {[Cl2NN]Cu}2(-NAd)
may also be promoted by the presumably low concentration of the putative [Cl2NN]Cu=NAd active
intermediate generated by dissociation of a [Cl2NN]Cu fragment from {[Cl2NN]Cu}2(-NAd) (Scheme
2).
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EXPERIMENTAL SECTION
General Procedures. All experiments were carried out in a dry nitrogen atmosphere using an
MBraun glovebox and/or standard Schlenk techniques. 4A molecular sieves were activated in vacuo at
180 ºC for 24 h. Diethyl ether and tetrahydrofuran (THF) were first sparged with nitrogen and then
dried by passage through activated alumina columns. Pentane was first washed with conc. HNO3/H2SO4
to remove olefins, stored over CaCl2, sparged and the passed through activated alumina columns.
Benzene, toluene and ethylbenzene were purchased anhydrous and stored over 4A molecular sieves. All
deuterated solvents were sparged with nitrogen, dried over activated 4A molecular sieves and stored
13
under nitrogen. Celite was dried overnight at 200 °C under vacuum. 1H and C NMR spectra were
recorded on a Varian MR 400 MHz Spectrometer (400 and 100.47 MHz, respectively). All NMR
spectra were recorded at room temperature unless otherwise noted and were indirectly referenced to
residual solvent signals or TMS as internal standards. UV-vis spectra were measured on a Varian Cary
50 or 100 spectrophotometer, using air-tight quartz cuvettes with screw-cap tops or Teflon stoppers.
Solution EPR spectra were recorded on a JEOL continuous wave spectrometer JES-FA200 equipped
with an X-band Gunn oscillator bridge and a cylindrical mode cavity employing a modulation frequency
of 100 kHz. GC-MS spectra were recorded on a Varian Saturn 3900 and elemental analyses were
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performed on a Perkin-Elmer PE2400 microanalyzer at Georgetown. IR measurements were performed
on Perkin Elmer Spectrum One FT-IR Spectrometer. All reagents were obtained commercially unless
otherwise noted. [Me3NN]Ni=NAd20 (1) and [Me3NN]Tl20 were prepared by literature methods.
Ethylbenzene-d10 was obtained from Cambridge Isotope Laboratories, Inc. This deuterated reagent and
other deuterated solvents were degassed with N2 and stored over activated 4A molecular sieves prior to
use.
1
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Kinetic Measurements by UV-vis Spectroscopy. A stock solution containing a known amount of
freshly prepared [Me3NN]Ni=NAd (1) (typically 0.063 – 0.134 g) in benzene was prepared using a 10.0
mL volumetric flask. Aliquots of this solution of 1 were mixed with known amounts of ethylbenzene,
toluene, or indane (typically 200 – 3000 equiv. / 1) and diluted to known concentration using either a
3.0 or 10.0 mL volumetric flask. Each UV-vis run employed ca. 3 mL of a benzene solution containing
1 and C-H substrate in a thermostated quartz cell. The loss of 1 was monitored by the gradual decrease
its absorbance at  = 596 nm. In all measurements, 1 followed first-order decay as evidenced by linear
plots of ln[(A-A∞)/(A0-A∞)] vs. time (A = absorbance at  = 596 that varies with time; A∞ = final
absorbance at  = 596 nm; A0 = initial absorbance at  = 596 nm) typically collected for at least three
half-lives (ln[(A-A∞)/(A0-A∞)] = -2.08). Specific experimental details and data analysis for the
determination of the kinetic order in ethylbenzene, Eyring analysis for ethylbenzene, toluene, and
indane, and the determination of KIE for ethylbenzene may be found in the Supporting Information.
Preparation of Compounds. [Me3NN]Ni(2-picoline). To a solution of NiI2 (0.595 g, 1.91 mmol)
and 2-picoline (0.177 g, 1.91 mmol) in 5 mL THF was added [Me3NN]Tl (1.0 g, 1.91 mmol). The
reaction mixture was allowed to stir for 1 h before it was filtered through Celite to remove insoluble TlI.
To the resulting solution was added 11.0 g Na/Hg (0.5% Na by weight; 2.39 mmol Na) and the solution
was allowed to stir for 1 h before it was decanted from the amalgam and filtered through Celite. The
solution was concentrated to dryness and the remaining solid was crystallized from pentane to afford
red crystals at -35 °C in 80% yield (0.718 g, 1.52 mmol). UV-vis (Et2O, 25°C): λmax = 523 nm (ε = 1400
M-1 cm-1); μeff (benzene-d6) = 1.55 B.M. (with a drop of 2-picoline added). EPR (toluene, frozen glass,
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77 K) g1 = 2.41, g2 = 2.12, g3 = 2.06 (with a drop of 2-picoline added); Anal. Calcd for C29H36N3Ni: C,
71.77; H, 7.48; N, 8.66 Found C, 71.52; H, 7.43; N, 8.33.
1
2
3
4
5
6
7
8
[Me3NN]Ni-N(CHMePh)Ad (3). To a chilled solution of [Me3NN]Ni=NAd (1) (0.169 g, 0.312
mmol) in 4 mL Et2O was added 8 mL of a 50:50 mixture of Et2O/ethylbenzene. The reaction mixture
was allowed to stir at RT for 3 h during which a color change from green to brown occurred. All
volatiles were removed in vacuo and the remaining solid was taken up in Et2O, passed through Celite
and concentrated to afford brown crystals from Et2O at -35 °C in 49% yield based on moles of Ni (0.098
9
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60
1
g, 0.153 mmol; max yield = 50%). H NMR (C6D6, 400 MHz, RT): δ 7.02 (t, 1H, p-Ph-H), 6.92 (t, 2H,
m-Ph-H), 6.75 (overlapping s, 4H, m-Ar-H), 6.13 (d, 2H, o-Ph-H), 4.71 (s, 1H, backbone C-H), 3.06 (d,
3H, NCHMe), 2.90 (s, 6H, Ar-CH3), 2.87 (s, 6H, Ar-CH3), 2.82 (d, 3H, Ad-CH2), 2.59 (q, 1H,
NCHMe), 2.19 (s, 6H, Ar-CH3), 1.82 (br, 3H, Ad-CH), 1.65 (d, 3H, Ad-CH2), 1.51 (s, 6H, Ad-CH2),
13
1.24 (s, 6H, backbone-CH3); C{1H} NMR (C6D6): δ 156.29, 145.92, 134.04, 133.88, 133.52, 129.58,
129.31, 128.15, 127.33, 125.74, 102.44, 67.85, 65.30, 43.50, 37.25, 30.80, 22.46, 21.14, 19.55. Anal.
Calcd for C41H53N3Ni: C, 76.16; H, 8.26; N, 6.50 Found C, 76.44; H, 8.26; N, 6.55. [Me3NN]Ni-
N(CHMePh)Ad (3) may also be directly prepared from [Me3NN]Ni(2-picoline) and N3Ad in the
presence of excess ethylbenzene: To a chilled solution of [Me3NN]Ni(2-picoline) (0.240 g, 0.494
mmol) in 4 mL Et2O and 4 mL ethylbenzene was added cold a solution of 1-azidoadamantane (0.088 g,
0.494 mmol) in 2 mL Et2O. This reaction mixture was allowed to react at RT for 1 h. Bubbles evolved
immediately and the reaction mixture changed from red to green to brown. All volatiles were removed
in vacuo and remainder was taken up in Et2O and passed through Celite. This brown solution was
concentrated and allowed to crystallize at -35 °C to afford brown crystals in 49% yield based on moles
of Ni (0.159 g, 0.245 mmol; max yield = 50%) suitable for single crystal X-ray analysis.
[Me3NN]Ni(2-CH(Ph)NHAd) (4). [Me3NN]Ni=NAd (1) (0.375 g, 0.693 mmol) was dissolved in 8
mL toluene and allowed to stir at RT for 72 h. All volatiles were removed in vacuo and the remaining
solid was taken up in 10 mL Et2O and all volatiles were removed in vacuo again. The remaining solid
was taken up in Et2O again and passed through Celite, concentrated, and allowed to crystallize at -35 °C
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to afford brown crystals in 38% yield based on moles of Ni (0.165 g, 0.261 mmol; max yield = 50%)
suitable for single crystal X-ray analysis. Partial assignment of 1H NMR (C6D6, 400 MHz, RT): δ 7.15 –
6.70 (m, Ar-H), 6.57 (s, 2H, m-Ar-H), 5.03 (s, 1H, backbone C-H), 2.96 (s, 3H, p-Ar-CH3), 2.92 (s, 3H,
p-Ar-CH3), 2.42 (s, 3H, o-Ar-CH3), 2.27 (s, 3H, o-Ar-CH3), 2.16 (s, 3H, o-Ar-CH3), 1.98 (d, 1H, CHPh
or NHAd), 1.81 (d, 1H, CHPh or NHAd), 1.59 (s, 3H, o-Ar-CH3 or backbone-CH3), 1.55 (s, 3H, o-Ar-
CH3 or backbone-CH3), 1.47 (s, 3H, o-Ar-CH3 or backbone-CH3), 1.35 – 1.22 (m, 9H, Ad-H), 0.87 (m,
6H, Ad-H). Despite repeated attempts, we were unable to obtain satisfactory (± 0.4%) elemental
analysis on this substance.
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{[Me3NN]Ni}2(μ-NAd) (5). To a chilled solution of [Me3NN]Ni(2-pic) (0.107 g, 0.234 mmol) in 4
mL Et2O was added a chilled solution of 1-azidoadamantane (0.021 g, 0.117 mmol) in 2 mL Et2O. The
reaction mixture immediately changed color from red to brown. All volatiles were immediately
removed in vacuo. The remaining solid was taken back up in 5 mL of pentane and all volatiles were
immediately removed in vacuo again. The remaining solid was taken up in Et2O passed through Celite,
concentrated and layered with pentane to afford brown crystals in 78% yield (0.085 g, 0.09 mmol)
suitable for single crystal X-ray analysis. 1H NMR (400 MHz, toluene) δ 7.14 (s, 8H, m-Ar-H), 2.98 (s,
24H, o-Ar-CH3), 2.77 (s, 2H, backbone C-H), 2.62 (s, 12H, p-Ar-CH3), 1.82 (br, 3H Ad-CH), 1.17 (AB
q, 6H, Ad-CH2), 0.73 (s, 6H, backbone-CH3), 0.67 (br, 6H, Ad-CH2). Calcd for C56H73N5Ni2: C, 72.04;
H, 7.88; N, 7.50 Found C, 71.91; H, 7.51; N, 7.18. {[Me3NN]Ni}2(μ-NAd) (5) may also be prepared
from [Me3NN]Ni=NAd and indane: [Me3NN]Ni=NAd (1) (0.375 g, 0.693 mmol) was dissolved in 8 mL
indane. The reaction mixture was allowed to stir at RT for 24 h during which a color change from green
to brown occurred. All volatiles were removed in vacuo by adding toluene. The remaining solid was
taken up in Et2O, filtered through Celite, and concentrated to afford brown crystals in 52% yield (0.157
g, 0.179 mmol).
Computational Details. Most computations employed the Gaussian 09 package.33 Complete
chemical models were studied via the utilization of hybrid ONIOM34 techniques. The backbone
substituents on the -diketiminate ligand (methyl and mesityl) were modeled with the Universal Force
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Field (UFF),35 and the 1-adamantyl substituent (with the exception of the -carbon). The remainder of
the system, including the entire organic substrate, was modeled at the B3LYP/6-311++G(d,p) level of
theory. ONIOM(BP86/6-311+G(d):UFF) was used to model copper complexes based on its previous
use in previously published computational studies of [Cl2NN]Cu=NAd11 and [Cl2NN]Cu-NAd.32
Stationary points were optimized without symmetry constraint, using restricted and unrestricted Kohn-
Sham formalisms for closed- and open-shell molecules, respectively. All reported enthalpies and free
energies are evaluated using unscaled vibrational frequencies determined at the ONIOM(DFT:UFF)
level of theory. All stationary points are characterized as minima (no imaginary frequencies) or
transition states (one and only one imaginary frequency) via calculation of the energy Hessian.
Spin density plots for [Me3NN]Ni=NAd (1) and triplet [Me3NN]Ni-NHAd (2 – S = 1) in Figure 3
were rendered in ADF36 by first performing a single point calculation (ADF 2007.1 BP / ZORA /
TZ2P(+)) on the optimized coordinates obtained as detailed above. Slater-type orbital (STO) basis sets
employed for H, C, and N atoms were of triple- quality augmented with two polarization functions
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(ZORA/TZ2P) while an improved triple- basis set with two polarization functions (ZORA/TZ2P+) was
employed for the Ni atom. Scalar relativistic effects were included by virtue of the zero order regular
approximation (ZORA). The 1s electrons of C and N as well as the 1s – 2p electrons of Ni were treated
as frozen core.
ACKNOWLEDGMENT. THW and SW are grateful to NSF (CHE-1012523) for support of this work.
THW thanks NSF (CHE-0840453) for the award of an EPR spectrometer to Georgetown. JAM, DRP
and TRC acknowledge research support by a grant from the Offices of Basic Energy Sciences, U.S.
Department of Energy (Grant No. DE-FG02-03ER15387).
Supporting Information Available. Complete synthetic and kinetics details, additional
characterization data for compounds 3 – 5 including EPR spectra, computational methods, X-ray details
with fully labeled thermal ellipsoid plots for 3-5 (also provided in .cif format) as well as the complete
Ref. 33. This material is available free of charge via the Internet at http://pubs.acs.org.
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Journal of the American Chemical Society
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addition of R• to [Ni]=NAd vs. [Ni]-NHAd.
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SYNOPSIS TOC (Word Style “SN_Synopsis_TOC”).
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[NiI](NH(R)Ad)
RR
H
+ R
Ad
R-H
[NiIII]=NAd
[NiII] N
R
[NiII] N
Ad
HAA
RC
by
N
Ni
[NiIII]=NAd
C
H
N
Ni
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HAA T.S.
(R-H = indane) (R-H = ethylbenzene)
[NiII]-N(R)Ad
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