Synthesis, characterization and C-H amination reactivity of nickel iminyl complexes

Article abstract of DOI:10.1039/c9sc04879k

Metalation of the deprotonated dipyrrin (^AdFL)Li with NiCl₂(py)₂ afforded the divalent Ni product (^AdFL)NiCl(py)₂ (1) (^AdFL: 1,9-di(1-adamantyl)-5-perfluorophenyldipyrrin; py: pyridine). To generate a reactive synthon on which to explore oxidative group transfer, we used potassium graphite to reduce 1, affording the monovalent Ni synthon (^AdFL)Ni(py) (2) and concomitant production of a stoichiometric equivalent of KCl and pyridine. Slow addition of mesityl- or 1-adamantylazide in benzene to 2 afforded the oxidized Ni complexes (^AdFL)Ni(NMes) (3) and (^AdFL)Ni(NAd) (4), respectively. Both 3 and 4 were characterized by multinuclear NMR, EPR, magnetometry, single-crystal X-ray crystallography, theoretical calculations, and X-ray absorption spectroscopies to provide a detailed electronic structure picture of the nitrenoid adducts. X-ray absorption near edge spectroscopy (XANES) on the Ni reveals higher energy Ni 1s → 3d transitions (3: 8333.2 eV; 4: 8333.4 eV) than Ni^I or unambiguous Ni^II analogues. N K-edge X-ray absorption spectroscopy performed on 3 and 4 reveals a common low-energy absorption present only for 3 and 4 (395.4 eV) that was assigned via TDDFT as an N 1s promotion into a predominantly N-localized, singly occupied orbital, akin to metal-supported iminyl complexes reported for iron. On the continuum of imido (i.e., NR^2-) to iminyl (i.e., ²NR^-) formulations, the complexes are best described as Ni^II-bound iminyl species given the N K-edge and TDDFT results. Given the open-shell configuration (S = 1/2) of the iminyl adducts, we then examined their propensity to undergo nitrenoid-group transfer to organic substrates. The adamantyl complex 4 readily consumes 1,4-cyclohexadiene (CHD) via H-atom abstraction to afford the amide (^AdFL)Ni(NHAd) (5), whereas no reaction was observed upon treatment of the mesityl variant 3 with excess amount of CHD over 3 hours. Toluene can be functionalized by 4 at room temperature, exclusively affording the N-1-adamantyl-benzylidene (6). Slow addition of the organoazide substrate (4-azidobutyl)benzene (7) with 2 exclusively forms 4-phenylbutanenitrile (8) as opposed to an intramolecular cyclized pyrrolidine, resulting from facile β-H elimination outcompeting H-atom abstraction from the benzylic position, followed by rapid H₂-elimination from the intermediate Ni hydride ketimide intermediate.

Full text of DOI:10.1039/c9sc04879k

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Synthesis, characterization and C–H amination
reactivity of nickel iminyl complexes†
Cite this: DOI: 10.1039/c9sc04879k
a
b
a
a
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Yuyang Dong,
Kyle M. Lancaster
James T. Lukens,
Ryan M. Clarke,
Shao-Liang Zheng,
b
a
*
and Theodore A. Betley
Metalation of the deprotonated dipyrrin (AdFL)Li with NiCl2(py)2 afforded the divalent Ni product (AdFL)
NiCl(py)2 (1) (AdFL: 1,9-di(1-adamantyl)-5-perfluorophenyldipyrrin; py: pyridine). To generate a reactive
synthon on which to explore oxidative group transfer, we used potassium graphite to reduce 1, affording
the monovalent Ni synthon (AdFL)Ni(py) (2) and concomitant production of a stoichiometric equivalent of
KCl and pyridine. Slow addition of mesityl- or 1-adamantylazide in benzene to 2 afforded the oxidized Ni
complexes (AdFL)Ni(NMes) (3) and (AdFL)Ni(NAd) (4), respectively. Both 3 and 4 were characterized by
multinuclear NMR, EPR, magnetometry, single-crystal X-ray crystallography, theoretical calculations, and
X-ray absorption spectroscopies to provide a detailed electronic structure picture of the nitrenoid
adducts. X-ray absorption near edge spectroscopy (XANES) on the Ni reveals higher energy Ni 1s / 3d
transitions (3: 8333.2 eV; 4: 8333.4 eV) than NiI or unambiguous NiII analogues. N K-edge X-ray
absorption spectroscopy performed on 3 and 4 reveals a common low-energy absorption present only
for 3 and 4 (395.4 eV) that was assigned via TDDFT as an N 1s promotion into a predominantly N-
localized, singly occupied orbital, akin to metal-supported iminyl complexes reported for iron. On the
continuum of imido (i.e., NR2ꢀ) to iminyl (i.e., 2NRꢀ) formulations, the complexes are best described as
NiII-bound iminyl species given the N K-edge and TDDFT results. Given the open-shell configuration (S
¼ 1/2) of the iminyl adducts, we then examined their propensity to undergo nitrenoid-group transfer to
organic substrates. The adamantyl complex 4 readily consumes 1,4-cyclohexadiene (CHD) via H-atom
abstraction to afford the amide (AdFL)Ni(NHAd) (5), whereas no reaction was observed upon treatment of
the mesityl variant 3 with excess amount of CHD over 3 hours. Toluene can be functionalized by 4 at
room temperature, exclusively affording the N-1-adamantyl-benzylidene (6). Slow addition of the
organoazide substrate (4-azidobutyl)benzene (7) with 2 exclusively forms 4-phenylbutanenitrile (8) as
opposed to an intramolecular cyclized pyrrolidine, resulting from facile b-H elimination outcompeting
H-atom abstraction from the benzylic position, followed by rapid H2-elimination from the intermediate
Ni hydride ketimide intermediate.
Received 26th September 2019
Accepted 6th December 2019
DOI: 10.1039/c9sc04879k
rsc.li/chemical-science
resulting group transfer reactivity.1–6,8,9,14–18,35–43 Our group has
focused on the generation of highly electrophilic MLMBs by
employing weak-eld dipyrrin ancillary ligands.37,39,40,42,44 Ferric
1. Introduction
Late 3d transition metal complexes bearing metal–ligand
multiple bonds (MLMBs) have been intensely scrutinized as
potential reagents to deliver the multiply-bonded functional-
ities to organic substrates.1,2,4,5,8–43 One method to endow the
metal–ligand multiply bonded species with the capacity to
undergo functional group transfer is to attenuate the metal–
ligand bond order. We and others have pursued the synthesis of
late transition metal, low-coordinate MLMBs to investigate their
imido complexes bearing the dipyrrin ligand feature high-spin
(S ¼ 5/2) electronic structures,37–40 while the CoIII analogues
thermally access open-shell excited states.41,42 The open-shell
electronic structures for the Fe37–40 and Co41,42 complexes
provide access to reactivity along the MLMB vector, where both
inter- and intramolecular amination transfer to C–H bond
substrates were observed. In both cases, however, the MLMBs
were best formulated as imido functionalities. Oxidation of the
FeIII imidos elicits the formation of an open-shell iminyl, where
oxidation is borne out at the NR functionality, not the metal
center, leading to faster oxidative group transfer than their high
spin imido analogues.39,40
aDepartment of Chemistry and Chemical Biology, Harvard University, 12 Oxford
Street, Cambridge, Massachusetts 02138, USA. E-mail: betley@chemistry.harvard.edu
bDepartment of Chemistry and Chemical Biology, Baker Laboratory, Cornell
University, Ithaca, New York 14853, USA
† Electronic supplementary information (ESI) available. CCDC 1953729–1953733.
For ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/c9sc04879k
Formation of later transition metal MLMBs can also
diminish MLMB bond order in two ways: (1) higher d-electron
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counts necessarily populate more M–L antibonding orbitals; product were obtained by storꢁing a concentrated solution of the
(2) progression through the 3d transition metal series should material in hexanes at ꢀ35 C. Single-crystal X-ray diffraction
lead to diminished energetic overlap between the transition analysis revealed the product as the trigonal bipyramidal, bis-
metal and N valence orbitals, potentially impacting the elec- pyridine complex (AdFL)NiCl(py)2 (1, Fig. 2a), which exhibits
tronic structure and reactivity of the resultant MLMB.17 a paramagnetically-shied 1H NMR spectrum, suggesting
Indeed, four Ni imido systems have been thus far reported by a triplet ground state at room temperature (Scheme 1).
Hillhouse1,3–6,10–12 and Warren.2,13 The seminal work by Hill-
Chemical reduction of 1 with KC8 in thawing THF solution
house and coworkers rst described the synthesis and reac- cleanly generated the pyridine adduct (AdFL)Ni(py) (2, Scheme 1)
tivity of phosphine-supported NiII imidos (Fig. 1a and b),10–12 as a dark brown solid. Crystals of 2 suitable for single-crystal X-
later establishing both higher valent imido cations (Fig. 1a ray diffraction were obtained by storing a concentrated hexanes
and b)4 and lower-coordinate, carbene supported imido solution at ꢀ35 ꢁC overnight (Fig. 2b). The solid-state structure
complexes (Fig. 1c and d).3,5,6 Warren and coworkers reported of 2 unveils a pyramidally distorted T-shape geometry around
b-diketiminate supported neutral, nominally NiIII imides the Ni center with an NL–Ni–Npy angle of 146.4(2)ꢁ, similar to
(Fig. 1e).2,13 Of note, Hillhouse's two-coordinate, carbene the previously reported three-coordinate NiI b-diketiminate
supported NiII imido5 and Warren's NiIII imido complexes2,13 complex by Warren and coworkers (Fig. 1e).2 The EPR spectrum
are competent for nitrene transfer into vinylic and benzylic of 2 collected at 77 K in toluene indicates that the unpaired
C–H bonds, respectively. Each example represents a highly electron occupies a rhombic environment (g1 ¼ 2.438, g2
electrophilic imido complex capable of functional group 2.121, g3 ¼ 2.064; Fig. 2c).
transfer. Given the promising capability Ni imido species
¼
exhibit towards C–H bond functionalization, we were inter-
ested in investigating the synthesis of dipyrrin-supported Ni
analogues. We report herein the synthesis, characterization,
2.1 Isolation and structural characterization of terminal
nitrenoid complexes
The addition of 1 equiv. of either MesN3 or AdN3 to a benzene
solution of 2 resulted in instantaneous color change from dark
brown to dark pink accompanied by vigorous effervescence due
to N2 release from the azide (Scheme 1). The 19F NMR spectra of
the crude reaction mixtures showed total consumption of 2 with
clean conversion to new paramagnetic products. Single crystals
and reactivity of two Ni iminyl complexes bearing aryl and
alkyl N-substituents.
2. Results and discussion
The synthesis of a modied adamantyl-substituted dipyrrin
scaffold proceeded cleanly via substitution of penta-
uorobenzaldehyde
for
2-(dimethoxymethyl)-1,3,5-
trimethylbenzene in the original procedure.37 The uorinated
aryl meso-substituent on the dipyrrin anodically shis the
bound transition metal and provides a useful 19F handle for 19
F
NMR characterization.45 Metalation of (AdFL)H with Ni followed
previously reported protocols for preparing Fe and Co conge-
ners.37,42,44 Lithiation of (AdFL)H is carried out by adding PhLi as
a solid to a frozen benzene solution of (AdFL)H.37,44 The addition
of resultant (AdFL)Li in THF to a frozen solution of NiCl2(py)2 in
THF afforded a dark-red, paramagnetic product. Crystals of the
Fig. 2 Solid-state molecular structure for (a) (AdFL)NiCl(py)2 (1) and (b)
(
AdFL)Ni(py) (2) with thermal ellipsoids at 50% probability level. Color
scheme: Ni, pink; N, blue; C, gray; Cl, yellowgreen; F, green. H atoms
omitted for clarity. (c) Frozen solution EPR spectrum of (AdFL)Ni(py) (2)
collected at 77 K in toluene (red). Blue line represents a fit of the data
using the program EasySpin.7 Fitting parameters: S ¼ 1/2, g1 ¼ 2.438, g2
¼ 2.121, g3 ¼ 2.064.
Fig. 1 All previously reported Ni imides.1–6
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Scheme 1 Synthesis of (AdFL)NiCl(py)2 (1), (AdFL)Ni(py) (2), (AdFL)Ni(NMes) (3), and (AdFL)Ni(NAd) (4).
of the respective reaction products were obtained by storing strongly deviates from linearity [:(Ni–Nim–CMes): 146.5(2)ꢁ] in
concentrated hexanes solutions at ꢀ35 ꢁC overnight. The solid- contrast to its linear bisphosphine supported analogue [(dtbpe)
state structures of the reaction products (Fig. 3a and d) Ni(NAr)]+ [:(Ni–Nim–CAr): 178.4(3)ꢁ]4 but similar to the bis-NHC
conrmed N2 extrusion from the organoazides to install system [(NHC2)Ni(NAr), :(Ni–Nim–CAr), Ar: 127.3(3)ꢁ, Mes;
terminal NMes and NAd functionalities onto Ni in (AdFL) 170.0(2)ꢁ, DIPP] (Fig. 1 and S7†).6 The deviation from linearity
Ni(NMes) (3) and (AdFL)Ni(NAd) (4), respectively.
for the Ni–N–CAr linkages occur most oen with late transition
˚
The Ni–Nim distance of 1.696(2) A in 3 is longer than the bis- metals featuring population of high-energy, antibonding (M–N)
phosphine supported cationic NiIII aryl imide [(dtbpe)Ni(NAr)]+ p* orbitals. Other factors that can contribute to distortions
II
4
˚
(1.674(3) A) but similar to those three coordinated Ni aryl away from non-linearity include imido-group constraints
1,3,4
˚
imides supported by the same ligand (1.697(2) / 1.703(4) A)
incorporated into a chelate ring and crystal-packing effects
and bis-N-heterocylic carbene (NHC) ligand ((NHC2)Ni(NAr): within the crystalline lattice.6
6
˚
1.718(2) / 1.732(4) A) (Fig. 1 and S7†). The imido linkage in 3
Fig. 3 Solid-state molecular structures for the top-view (a) and side-view and aryl moiety bond metrics (b) of (AdFL)Ni(NMes) (3); the side-view (c)
and top-view (d) of (AdFL)Ni(NAd) (4) with thermal ellipsoids at 50% probability level. Color scheme: Ni, pink; N, blue; C, gray; F, green. H atoms,
solvent molecules, and positional disorder in 4 are omitted for clarity.
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Close inspection of the C–C bond lengths within the mesityl
group of 3 (Fig. 3a and b) reveals the elongation of the Cipso
–
˚
Cortho bonds (1.423(4), 1.429(4) A), analogous to our previously
reported FeIII iminyl species37–40 as well as all the previously
reported Ni aryl imido complexes (Fig. S7†).1,3–6 The Nimido–CMes
˚
distance in 3 (1.348(4) A) is in the range of a partial C–N double
bond, indicating delocalization of electron density from Ni–
Nimido p system to the mesityl aryl moiety.17
˚
The Ni–Nim distance of 1.642(7) A in 4 is consistent with
reported three-coordinate NiIII alkyl imide supported by b-
2
˚
diketiminate ligand (1.662(2) A) and Ni alkyl imides [(dtbpe)
Ni(NAd)]n (n: +1, 1.673(2); n: 0, 1.657 (5) A); and compara-
3,4
˚
tively shorter than the Ni–aryl imido complexes (Fig. 1 and
S7†). The shortened Ni–Nim bond is consistent across all
ancillary ligand platforms, potentially due to the lack of p-
delocalization into the imido aryl moiety. The alkyl imido
linkage in 4 is more linear [:(Ni–Nim–CAd): 164.8(17)ꢁ]
compared to 3, and similar to all three reported Ni alkyl
imides [:(Ni–Nim–CAd): 163.0(2) / 165.2(4)ꢁ], regardless of
ancillary ligand or oxidation state.2–4
Fig. 5 Cyclic voltammogram of 3 obtained in THF at 25 ꢁC, with
0.1 M (nBu4N)(PF6); 100 mV sꢀ1; referenced to [Cp2Fe]+/0 couple; OCP
¼ ꢀ1.25 V; DE ¼ 310 mV.
imides with a hyperne splitting constant of A(14N, I ¼ 1) ¼
22 G.2 The S ¼ 1/2 low-spin ground states for 4 were further
veried by SQUID magnetometry (Fig. S5†) up to 300 K, in
contrast to the NiIII imide [(dtbpe)Ni(NAr)]+, which exhibits spin
transition to high-spin S ¼ 3/2 excited state.4
2.2 Spectroscopic characterization of terminal nitrenoid
complexes 3 and 4
The EPR spectrum of 3 (toluene, 77 K, Fig. 4a) displays two sets
of rhombic, S ¼ 1/2 signals (g11 ¼ 2.238, g12 ¼ 2.106, g13 ¼ 1.940;
g21 ¼ 2.302, g22 ¼ 2.128, g23 ¼ 1.962), possibly due to two
2.3 Redox stability of complex 3
conformations of the mesityl group related by rotation, forming The previously reported Ni imido complexes of the type [(dtbpe)
p-bonds with different metal based d-orbitals.1,3,12 The solid- Ni(NAr)]+/0 were accessible in both nominally NiII and NiIII
state magnetometry measurement (Fig. 4c) conrmed that 3 oxidation levels (Fig. 1a and b).4 Cyclic voltammetry (CV) was
remains low-spin (S ¼ 1/2) up to 300 K. The EPR spectrum of 4 performed to assess the redox stability of the dipyrrin aryl imido
(toluene, 77 K, Fig. 4b) reveals a single rhombic absorption, analogue 3 (Fig. 5). The CV examination of 3 in THF shows
supporting an S ¼ 1/2 formulation (g1 ¼ 2.185, g2 ¼ 2.063, g3 ¼ a quasi-reversible reduction event at ꢀ1.58 V versus [Cp2Fe]+/0
,
1.924). The signal at g2 ¼ 2.063 is split into a triplet with cathodically shied from the same formally NiII/NiIII couple of
a hyperne splitting constant of A(14N, I ¼ 1) ¼ 21.3 G. Similar the related bis-phosphine supported Ni imide (E1/2 ¼ ꢀ0.76 V,
spectroscopic features were observed in the NiIII b-diketiminate vs. [Cp2Fe]+/0, THF).4
Fig. 4 (a) Frozen solution EPR spectrum of (AdFL)Ni(NMes) (3) collected at 77 K: S1 ¼ 1/2, g11 ¼ 2.238, g12 ¼ 2.106, g13 ¼ 1.940; S2 ¼ 1/2, g21
¼
2.302, g22 ¼ 2.128, g23 ¼ 1.962, relative weight ¼ 1 : 1 (red); blue line represents a fit of the data using the program EasySpin;7 (inset) enlarged
spectrum showing hyperfine splitting attributable to two mesityl ring orientations. (b) Frozen solution EPR spectrum of (AdFL)Ni(NAd) (4) collected
at 77 K: S ¼ 1/2, g1 ¼ 2.185, g2 ¼ 2.063, g3 ¼ 1.924 (red); blue line represents a fit of the data using the program EasySpin;7,8 (inset) enlarged
spectrum showing hyperfine splitting, A ¼ 21.3 G (14N, I ¼ 1).7,9 (c) Variable-temperature susceptibility data for of 3 collected at 1.0 T, with cMT ¼
0.41 cm3 K molꢀ1 at 295 K; (inset) solid-state magnetometry data for 3 of M vs. H at 100 K, showing the absence of ferromagnetic impurities.
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The 0.8 V cathodic shi comparing [(dtbpe)Ni(NAr)]+ versus (Fig. 6a) spectra reveal pre-edge absorption features for each
neutral 3 likely largely reects the change in molecular charge of the complexes 2–5, conventionally assigned to Ni 1s / 3d
between the two species as opposed to effects originating with excitations. The maxima for the pre-edge features increase in
ligand eld strength. Chemical reduction of 3 was attempted energy across the series from NiI 2 (8332.0 eV), to formally NiII 5
using decamethylcobaltacene ðCp*2CoÞ or KC8 as a reductant in (8332.8 eV), to the terminal MLMB complexes 3 (8333.0 eV) and
frozen-thawing THF.46 In both cases, unfortunately, only 4 (8333.2 eV). The pre-edge maximum shis 0.8 eV on going
inseparable mixtures of decomposition of 3 as ascertained by from NiI 2 to NiII 5; while the terminal imidos reveal smaller
analysis of the crude reaction mixture by 19F NMR spectroscopy shis in pre-edge maxima from 5 (3, 0.2 eV; 4, 0.4 eV).
were obtained. Similar results were obtained when similar Comparing the aryl and alkyl imides (3, 4), the pre-edge
chemical reduction was attempted with alkyl imide 4.
absorption feature of the alkyl species is shied to higher
energy relative to that of the aryl congener, likely resulting from
conjugation of the aryl moiety in 3 to the singly occupied
acceptor molecular orbital. A similar observation was made
comparing aryl- and alkyl-substituted iminyl complexes on
2.4 Ni and N K-edge X-ray absorption near edge
spectroscopies of complexes 3 and 4
Given the challenges associated with metal oxidation assign- FeIII.40
ment based solely on structural metrics, we investigated the
To gain further insight into the redox states in the Ni/N pairs
suite of Ni complexes reported herein by multi-edge X-ray in 3 and 4, the nitrogen valence was probed directly using
absorption spectroscopy (XAS). Formally NiI
2
and the nitrogen K-edge XAS. As seen in Fig. 6b, pre-edge features at
formally NiII
(
AdFL)Ni(NHAd) (5) (vide infra) were chosen as 398.0 and 400.9 eV are observed for all the complexes surveyed
oxidation state references due to their similar coordination (2–5) that can be assigned as transitions into high-lying anti-
environments to those of complexes 3 and 4. The Ni K-edge bonding molecular orbitals of N 2p parentage.40,47 Two low-lying
pre-edge absorptions are present for both 3 and 4 at 395.4 eV,
accompanied by a smaller side-band at 396.3 eV. These peak
energies are consistent with those encountered for the redox-
active N-donor ligands in Fe-iminyl (394.5 and 394.8 eV),40 Ni–
aminyl (397.0 eV),47 and Cu–nitrene48 (doublet with peaks at
395.3 and 395.9 eV) complexes. This tentative assignment of
these transitions as N 1s into partially occupied, N-localized
orbitals present in 3 and 4, respectively is further supported
by time-dependent density functional theory (TDDFT) calcula-
tions (vide infra). However, quantitative covalency analysis was
precluded by sensitivity of these samples to rapid photodamage
that principally affected the low-energy pre-edge peaks of
interest (Fig. S13†).
2.5 Theoretical analysis for the electronic structures of
complexes 3 and 4
DFT calculations were carried out to probe the electronic
structures of 3 and 4. Calculations were carried out on the
49,50
experimental (crystallographic) structures using the BP86
generalized gradient approximation (GGA) functional as well
as the B3LYP51,52 hybrid density functional. The broken
symmetry (BS)53–55 method was applied in both cases to
ascertain whether antiferromagnetic coupling between
a high-spin d8 NiII and a ligand radical (i.e., imido oxidized to
an iminyl) could plausibly be invoked to yield the experi-
mentally observed S ¼ 1/2 ground states. No BS(2,1) solution
was obtained for either species using the BP86 functional.
Analogous calculations using B3LYP gave broken symmetry
solutions with corresponding orbital overlap values near
unity (3 ¼ 0.68, 4 ¼ 0.89) and whose total energies were
effectively identical to those obtained using a standard
unrestricted DFT approach.
To afford some justication for selecting between BP86
Fig. 6 X-ray absorption spectroscopy: (a) Ni K-edge and (b) N K-edge
and B3LYP results, TDDFT calculations47,56,57 to model the N
absorption spectra of (AdFL)Ni(py) (2) (black), (AdFL)Ni(NMes) (3) (blue),
(
AdFL)Ni(NAd) (4) (red), and (AdFL)Ni(NHAd) (5) (green).
K-edge XAS obtained for 3 and 4 were carried out starting
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Fig. 7 Experimental and TDDFT-calculated N K-edge XAS pre-edge regions for (a) (AdFL)Ni(NMes) (3), and (b) (AdFL)Ni(NAd) (4). Calculations were
initiated from spin-unrestricted B3LYP single points using the CP(PPP) basis set on Ni and the ZORA-def2-TZVP(-f) basis set on all other atoms.
Energies for the calculated spectra were adjusted using a linear correction presented as Fig. S15.† Unrestricted Kohn–Sham orbitals corre-
sponding to the acceptor MOs involved the excitation giving the ca. 395 eV pre-edge peak are plotted at an isovalue of 0.03 au.
from either unrestricted single point solution. Spectra picture that emerges for both is that the unpaired electron
calculated using BP86 gave insufficient resolved peak struc- resides in a MO of principally Ni 3d and Nim character
ture to reliably correlate to the experimental data (Fig. S14†), reecting a p* interaction. For 3, this MO comprises 58.5%
whereas B3LYP-calculated spectra adequately reproduced Ni 3d and 24.3% NMes N 2p character. For 4, corresponding
pre-edge features, with a linear t of experimental to calcu- parentages are 33.0% Ni 3d and 46% NAd N 2p. The ca. 2-fold
lated data giving R2 ¼ 0.99 (Fig. 7). Agreement between larger N 2p contribution to the SOMO of 4 is consistent with
calculated and experimental spectra is quite good in the case the resolved 14N HFC in its EPR spectrum—such ne struc-
of 3, but less so in the case of 4.
ture is not resolved in the spectrum of 3. Each complex has
The rst peak in the N K-edge XAS of 3 is predicted as an one remaining unoccupied orbital featuring Ni 3d character,
excitation from N 1s to an MO dominated by iminyl character, reecting a p* interaction with Nim 2p. This is the LUMO for
because the MO is less than 50% Ni in character.58 This MO is 3, which comprises 37.7% Ni 3d and 20.9% NMes N 2p. For 4,
delocalized over the NMes fragment and is comprised of this is LUMO+1, which features 47.3% Ni 3d and 27.7%
32.3% N 2p, 33.3% Mes arene p*, and 10.2% Ni 3d. The weaker,
second pre-edge peak is assigned as an excitation involving
a near equal mix of two acceptor MOs featuring large (10–20%)
contributions from the orthogonal Nim 2p orbitals as well as
dipyrrin p* contributions. In the case of 4, qualitatively similar
excitations are involved, although the ordering is reversed from
3: the rst excitation features contributions from 2 MOs, while
the second excitation is analogous to the rst excitation of 3.
The estimated error in peak energy prediction from our prior N
K-edge TDDFT study is on the order of 0.5 eV, so this discrep-
ancy is not unexpected. Regardless, the pre-edge of 4 also
features excitations to an MO with a large degree of Nim N 2p
vacancy. Much less delocalization is possible in the imido
fragment of 4, consequently this acceptor MO features 42.8%
Fig. 8 Frontier quasi-restricted orbitals (QROs) calculated for (a) (AdFL)
Ni(NMes) (3), and (b) (AdFL)Ni(NAd) (4). QROs were generated from
spin-unrestricted B3LYP orbitals produced using the CP(PPP) basis set
on Ni and the ZORA-def2-TZVP(-f) basis set on all other atoms.
Orbitals are plotted at an isovalue of 0.03 au.
total contribution from NAd (37.7% N 2p), and 35.0% Ni 3d.
Quasi-restricted orbitals (QROs)59 were generated from the
unrestricted B3LYP solutions to facilitate interpretation of
the ground-state electronic structures of 3 and 4 (Fig. 8). The
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Scheme 2 (Left) Reaction between 4 and 0.5 equiv. of CHD at 25 ꢁC, quantitatively generating 5 via facile H-atom abstraction; (right) reaction
between 4 and toluene as solvent at 25 ꢁC, generating 5 and 6 via C–H functionalization followed by b-hydride elimination.
ꢁ
NAd N 2p. The ground state electronic conguration for 4 was observed at 40 C in 6 hours. Interestingly, treatment of 3
appears more consistent with a formal iminyl assignment, with 100 equiv. of CHD resulted in no reaction over the course
while the larger Ni parentage of the SOMO (58.5% Ni 3d) for 3 of 3 hours at room temperature, whereas decomposition was
suggests 3 presents more imido-like character. Overall, the observed above 60 ꢁC. A similar reactivity discrepancy was also
highly covalent Ni–Nim interactions can be viewed as total observed in our previously reported FeIII adamantyl and mesityl
donation of a single electron from the Nim 2p orbitals imido species, and could be explained by the lower radical
involved in Ni–N bonding, giving rise to the iminyl character density on Nim in 3 as well as steric-shielding of the electro-
observed.
active orbital by the mesityl ortho-methyl units.39,40
Dissolving 4 in toluene resulted in slow conversion into 5
along with decomposition into ligand over the course of two
days. The amount of 5 generated is quantied by 19F NMR
spectrum to be 46% against an internal standard (tri-
uorotoluene), and the organic product is identied to be N-1-
adamantyl-benzilidene (6) as evidenced by 1H NMR spectrum of
the quenched reaction mixture (Scheme 2). The concentration
of 4 in toluene was monitored by 19F NMR spectroscopy and can
be tted as a rst-order decay (Fig. 10). An intermolecular KIE of
40 was measured over 80 days when d8-toluene was used as the
solvent. However, due to the slow rate of the reaction, decom-
position of 4 can also contribute to the large KIE value. Based on
the C–H amination reaction mediated by previously reported
2.6 Reactivity of complexes 3 and 4
With the electronic structure of 3 and 4 described in detail, we
next sought to explore their potency toward C–H amination and
inspect the effects of substituents on the Nim in group transfer
reactivity. Exposure of 4 to 0.5 equiv. of 1,4-cyclohexadiene
(CHD) at room temperature resulted in instantaneous conver-
sion into a new paramagnetic species as evidenced by the
1
paramagnetically-shied H and 19F NMR spectra (Scheme 2).
Single crystals obtained from storing a concentrated hexanes
solution of this reaction product at ꢀ35 ꢁC overnight permitted
structural determination by X-ray diffraction analysis. The
product from reacting 4 with CHD is the corresponding amido
species (AdFL)Ni(NHAd) (5, Fig. 9). The amide hydrogen atom
was located in the difference map. Similar reactivity was
observed in NiIII imides supported by b-diketiminate reported
by Warren and coworkers.2,13 Amide 5 was not stable at elevated
temperature as complete decomposition of 5 into free ligand
Fig. 10 Concentration profile of 4 during reaction between (h8/d8-)
Fig. 9 Molecular structure of (AdFL)Ni(NHAd) (5) with the ellipsoid toluene and 4 by 19F NMR spectroscopy. Trifluorotoluene is used as
probabilities set at 50%. C, gray; N, blue; H, white; Ni, pink; F green.
internal standard.
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electronic structure proposed here may be applicable to those
compounds as well.2,4,13 Indeed, the short Ni–NR bonds are
commonly invoked for substantive metal–ligand multiple bond
formation. However, the presence of low-energy absorption
features in the N K-edge XAS data and corresponding assign-
ments from TDDFT strongly evidence N 2p-localized vacancies.
The C–H bond functionalization reactivities of these Ni(NR)
species were closely examined. The Ni(NR) aryl congener is
shown to be much less reactive than the alkyl variant, poten-
tially owing to the greater spin concentration on Ni in the spin-
bearing orbital. In the C–H bond amination reactions of
Ni(NAd) with toluene, the corresponding imine was determined
to be the major organic product, presumably due to recombi-
nation of the tolyl radical with the Ni(NAd) outcompeting that
with the generated Ni(NHAd) amide. A subsequent b-hydride
elimination would generate the secondary imine product.
Future work will examine the efficacy of the (dipyrrin)Ni
complexes to perform ring-closing C–H bond amination with
substrates that cannot undergo a-hydride migration as
observed here.
Scheme 3 Catalytic conversion of 7 to 8 at 25 ꢁC using 20 mol% of 2.
NiIII b-diketiminate imides, we hypothesized that 4 can
outcompete 5 (in situ) at capturing tolyl radical, giving rise to
a secondary amido intermediate. This intermediate would then
undergo b-hydride elimination and release the imine product. A
similar mechanism was proposed by Warren et al. as a rationale
towards their crystallographically characterized rearranged NiII
product.2,13 An alternative mechanism would involve H-atom
abstraction of benzyladamantylamine with a second equiva-
lent of 4, generating both the imine product observed and 5 as
a byproduct.
In order to facilitate the radical recombination with the
amido species formed in situ, the activated C–H bond was
installed onto the azide. Addition of 1 equiv. of (4-azidobutyl)
benzene (7)22 to a C6D6 solution of 2 resulted in instantaneous
color change from dark brown to dark pink along with vigorous
effervescence. The bubbling ceased aer 10 min along with the
solution reversion to a dark brown color, and 19F NMR spectrum
revealed full regeneration of 2. However, rather than the antic-
ipated cyclized pyrrolidine product,22,31,41 1H NMR analysis
revealed the major organic product to be 4-phenylbutanenitrile
(8, Scheme 3). To test the possibility of catalytically converting
azide 7 to nitrile 8, 3–6 equivalents of azide 7 were added slowly
to a C6D6 solution of 2. Full decomposition of catalyst 2 was
observed when greater than ve equivalents of the azide
substrate 7 were used, potentially due to the high concentration
of product nitrile 8. With 20 mol% catalyst loading of 2, azide 7
is catalytically converted to 4-phenylbutanenitrile 8, providing
a moderate isolated yield of 78%. We propose the product is
formed by iminyl generation following N2 extrusion from azide
7; b-H elimination (originating from the a-methylene unit)
outcompetes intramolecular H-atom abstraction from the
benzylic position to generate a Ni(hydride)(ketimide) interme-
diate. A rapid H2-elimination via deprotonation of the ketimide
Ni(hydride) furnishes the nal nitrile 8. Attempts to slow down
this pathway by deutero-substitution at the gem position were
ineffective.
Conflicts of interest
There are no conicts to declare.
Acknowledgements
This work was generously supported by a grant from the NIH
(GM-115815), the Dreyfus Foundation (Teacher-Scholar Award
to T. A. B.), and Harvard University. R. M. C. gratefully
acknowledges an NSERC Postdoctoral Fellowship. K. M. L.
acknowledges NSF (CHE-1454455) and the Alfred P. Sloan
Foundation for support. XAS data were obtained at SSRL, which
is supported by the U.S. Department of Energy, Office of
Science, Office of Basic Energy Sciences under Contract No. DE-
AC02-76SF00515. The SSRL Structural Molecular Biology
Program is supported by the Department of Energy's Office of
Biological and Environmental Research, and by NIH/NIGMS
(including P41GM103393). The authors wish to thank Dennis
Nordlund for assistance with so XAS data collection.
Notes and references
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