β-diketiminato nickel imides in catalytic nitrene transfer to isocyanides

Article abstract of DOI:10.1021/om300909n

The β-diketiminato nickel(I) species [Me₃NN]Ni(2-picoline) (1) serves as an efficient catalyst for carbodiimide (RN=C=NR′) formation in the reactions of a range of organoazides N₃R with isocyanides R′NC. [Me₃NN]Ni(CNR)₂ (R = ^tBu, Ar (Ar = 2,6-Me₂C₆H₃)) species provide carbodiimides RN=C=NAr′ upon reaction with Ar′N₃ (Ar′ = 3,5-Me₂C₆H₃). Nitrene transfer takes place via the intermediacy of nickel imides. Reaction of [Me_xNN]Ni(2-picoline) (x = 2 or 3) with Ar′N₃ gives the new dinickel imides {[Me _xNN]Ni}₂(μ-NAr′) (4 (x = 3) and 5 (x = 2)) as deep purple, diamagnetic substances. The X-ray structure of {[Me ₂NN]Ni}₂(μ-NAr′) (5) features short Ni-N _imide distances of 1.747(2) and 1.755(2) A along with a short Ni-Ni distance of 2.7210(3) A. These dinickel imides 4 and 5 react stoichiometrically with ^tBuNC to provide the corresponding carbodiimides ^tBuN=C=NAr′ in good yield. Azide transfer takes place upon reaction of 1 with TMS-N₃ to give the square planar nickel(II) azide [Me₃NN]Ni(N₃)(2-picoline) (7). Stoichiometric reaction of dinickel dicarbonyl {[Me₃NN]Ni} ₂(μ-CO)₂ with organoazides such as Ar′N ₃ is sluggish, indicating that 1 is not an efficient catalyst for nitrene transfer from organoazides to CO to form isocyanates RN=C=O.

Full text of DOI:10.1021/om300909n

Article
pubs.acs.org/Organometallics
β‑Diketiminato Nickel Imides in Catalytic Nitrene Transfer to
Isocyanides
Stefan Wiese, Mae Joanne B. Aguila, Elzbieta Kogut, and Timothy H. Warren*
Department of Chemistry, Georgetown University, Box 571227-1227, Washington, D.C. 20057 United States
S
* Supporting Information
ABSTRACT: The β-diketiminato nickel(I) species
[Me3NN]Ni(2-picoline) (1) serves as an efficient catalyst for
carbodiimide (RNCNR′) formation in the reactions of a
range of organoazides N3R with isocyanides R′NC. [Me3NN]-
Ni(CNR)2 (R = tBu, Ar (Ar = 2,6-Me2C6H3)) species provide
carbodiimides RNCNAr′ upon reaction with Ar′N3 (Ar′
= 3,5-Me2C6H3). Nitrene transfer takes place via the
intermediacy of nickel imides. Reaction of [MexNN]Ni(2-picoline) (x = 2 or 3) with Ar′N3 gives the new dinickel imides
{[MexNN]Ni}2(μ-NAr′) (4 (x = 3) and 5 (x = 2)) as deep purple, diamagnetic substances. The X-ray structure of
{[Me2NN]Ni}2(μ-NAr′) (5) features short Ni−Nimide distances of 1.747(2) and 1.755(2) Å along with a short Ni−Ni distance of
t
2.7210(3) Å. These dinickel imides 4 and 5 react stoichiometrically with BuNC to provide the corresponding carbodiimides
tBuNCNAr′ in good yield. Azide transfer takes place upon reaction of 1 with TMS-N3 to give the square planar nickel(II)
azide [Me3NN]Ni(N3)(2-picoline) (7). Stoichiometric reaction of dinickel dicarbonyl {[Me3NN]Ni}2(μ-CO)2 with
organoazides such as Ar′N3 is sluggish, indicating that 1 is not an efficient catalyst for nitrene transfer from organoazides to
CO to form isocyanates RNCO.
Scheme 1. Carbodiimide and Isocyanate Formation from
Organoazides via Metal Imides
INTRODUCTION
■
Late metal imido complexes [M]NR have been implicated in
a range of nitrene [NR] transfer reactions to unsaturated
substrates and C−H bonds.1 Especially in conjunction with
nitrene transfer reagents bearing electron-poor N-substituents
such as N-tosyl imidoiodinanes PhINSO2R and sulfonyl
azides NNNSO2R, catalytic alkene aziridination (addition
to CC double bonds)2,3 and C−H amination (formal
insertion into C−H bonds)3,4 represent particularly valuable
C−N bond-forming reactions. Synthetic routes to many
isolable late transition metal imido complexes [M]NR,
however, often employ N-aryl and N-alkyl organoazides N3R in
conjunction with a reduced metal precursor [M].1 Perhaps
somewhat surprisingly, a limited number of discrete metal
imides [M]NR participate in stoichiometric C−H amina-
tion5,6 or alkene aziridination.7
Reactions with isocyanides CNR′ or carbon monoxide (CO)
with metal imides [M]NR to give carbodiimides RNC
NR′ and isocyanates RNCO represent additional
stoichiometric nitrene transfer pathways. Especially given the
wide synthetic availability of organoazide precursors RNN
N,8 the synthesis of carbodiimides and isocyanides from
organoazides represents a promising class of C−N bond-
forming reactions to provide synthetically versatile unsaturated
products9 that proceed with loss of environmentally benign N2
(Scheme 1).
carbodiimides are typically formed via aza-Wittig reactions of
phosphinimines with isocyanates,11 but may also be prepared
by addition of silylamines to isocyanates via tin(II)
intermediates.12 Importantly, carbodiimides may also be
prepared directly from amines by oxidative coupling with
isocyanides CNR′ in the presence of heterogeneous and
homogeneous precious metal catalysts such as Au surfaces13
with O2 or PdCl2 with Ag2O.14
Renewed interest in later transition metal imido complexes
initiated by the isolation and synthetic studies of the
mononuclear Ni(II) imide (dtbpe)NiN(2,6-iPr2C6H3)15
(dtbpe =1,2-bis(di-tert-butylphosphino)ethane) revealed that
such [M]NR species can undergo stoichiometric nitrene
transfer to isocyanides and carbon monoxide (Figure 1). For
instance, (dtbpe)NiN(2,6-iPr2C6H3) reacts with benzyl
isocyanide (PhCH2NC) and CO to give nickel-bound
carbodiimide or isocyanate adducts.16 Peters found that
pseudotetrahedral Co(III)17 and Fe(III)18 [M]NAr species
react with CO to give RNCO and the corresponding
metal dicarbonyl [M](CO)2. Employing a β-diketiminato
Early transition metal imido complexes [M]NR′ can
mediate the synthesis of carbodiimides from the coupling of
two isocyanates R′NCO with loss of CO2,10,11 a method
best employed for symmetric carbodiimides. Unsymmetric
Received: September 25, 2012
© XXXX American Chemical Society
A
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CNR′ and CO suffered from the competition reaction to form
diazenes RNNR23 as well as strong binding of the product to
the (dtpbe)Ni(0) center.16 In 2009 Holland reported the use of
organoazides AdN3 and p-tolylN3 with CO, CNtBu, and CNCy
to give the corresponding isocyanates and carbodiimides via
Fe(III) imido complexes generated in situ from a dinuclear,
bulky β-diketiminato iron precatalyst {LtBuFe}2(μ-N2) (Figure
1).24 Hillhouse described the use of the dinuclear N-
heterocyclic carbene {(IPr)Ni(μ-Cl)}2 (IPr = 1,3-di(2,6-
diisopropylphenyl)imidazolin-2-ylidene) in catalytic nitrene
transfer from MesNNN to CNCH2Ph and CNtBu (Figure
1).25 A very recent example involves the use of a zirconium(IV)
imide supported by a chelating, redox-active ligand that allows
26
t
catalytic nitrene transfer from AdN3 and BuN3 to CNtBu.
Since we observed efficient transfer of the [NAd] group to
CNtBu and CO from the discrete nickel(III) imide [Me3NN]-
NiNAd that may be prepared from the nickel(I) complexes
[Me3NN]Ni(L) (L = 2-picoline27 (1) or 2,4-lutidine;19 Scheme
2), we became interested in the possibility of general catalytic
nitrene transfer from organoazides RN3 to isocyanides and
carbon monoxide. We describe our efforts herein that illustrate
relatively broad organoazide substrate scope in catalytic nitrene
transfer to CNtBu and CNAr (Ar = 2,6-Me2C6H3) to form the
corresponding carbodiimides. Synthetic studies reveal new
dinickel imide intermediates {[MexNN]Ni}2(μ-NAr′) (x = 3
(4) or 2 (5), Ar′ = 3,5-Me2C6H3) as well as complete azide
transfer from TMS-N3 to 1 to give [Me3NN]Ni(N3)(2-pic)
(7).
RESULTS AND DISCUSSION
■
Figure 1. Selected first-row, late metal imides in stoichiometric and
catalytic nitrene transfer to CNR′ and CO.
Catalytic Nitrene Transfer. As probed by the commer-
cially available isocyanides CNtBu and CNAr, the monovalent
[Me3NN]Ni(2-picoline) (1) precatalyst exhibits relatively
broad azide substrate scope (Table 1). In our initial screen,
we employed a 5 mol % loading of 1 relative to the isocyanide
(ca. 0.2 M) and organoazide (ca. 0.2 M) reactants for 30 min in
Et2O at room temperature (RT). A range of aryl azides
examined in combination with tert-butylisocyanide (CNtBu)
gave excellent yields (79−99%; Table S1). In contrast, the
more electron-poor aryl isocyanide CNAr in combination with
aryl azides delivered moderate yields (46−84%), with
particularly poor performance by the sterically hindered azide
MesN3 (3%). Despite the ability of [Me3NN]NiNAd to
participate in efficient stoichiometric transfer (Scheme 2),19
attempted catalytic carbodiimide formation with the bulky
organoazide AdN3 met with little success.
supporting ligand, we demonstrated nitrene transfer from the
nickel imide [Me3NN]NiNAd to CNtBu and CO to give
AdNCNtBu and AdNCO (Scheme 2)19 as well as
Scheme 2. Synthesis and Reactivity of [Me3NN]NiNAd
with CNtBu and CO
Reactions with the aryl isocyanide CNAr typically proceed
more sluggishly and require longer reaction times (17 h) to
obtain high yields with 5 mol % 1. Thus, to allow for
comparisons between CNtBu and CNAr substrates, a standard
reaction time of 17 h at RT was employed (Table 1). While
most reactions took place readily at room temperature, mesityl
azide required heating at 60 °C in benzene for 24 h to achieve a
73% yield with CNAr. On the other hand, the more basic
isocyanide CNtBu proved uniformly more reactive, allowing a
low catalyst loading of 1 mol % 1 in most cases.
We also examined the primary alkyl azide PhCH2CH2N3 as
well as the electron-deficient sulfonylazide TsN3 and
benzoylazide PhC(O)N3, which each represent classes of
azides that have not been reported for catalytic nitrene transfer
to isocyanides. Hillhouse et al. reported the use of TsN3 for
their NHC Ni system but only in the stoichiometric reaction of
{(IPr)Ni(μ-Cl)}2 with TsN3 to form {(IPr)Ni}2(Cl)(μ-Cl)(μ-
from the dicopper nitrene {[Me3NN]Cu}2(μ-NAr′) (Ar′ = 3,5-
Me2C6H3) to CNtBu to give Ar′NCNtBu.20 Hillhouse
deployed an especially bulky N-heterocyclic carbene to stabilize
a two-coordinate imide that transfers a sterically encumbered
imido ligand to CO from (IPr*)NiN(dmp) (Figure 1).6
Other related examples include the reaction of CO with the
bridging dinuclear Ru(II) imide {Cp*Ru}2(μ-NPh)(μ-CO) to
give PhNCO21 as well as a novel example of CO and
CNtBu addition to a terminal Fe(IV) nitride to give bound
isocyanate and carbodiimate anions [NCO]− and [N
CNtBu]−.22
Early attempts to employ (dtbpe)NiNR intermediates in
catalytic nitrene transfer from organoazides RNNN to
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74% and 34% isolated yield, respectively (Scheme 3). The two
νCN bands observed in the IR spectrum of each isocyanide
Table 1. Carbodiimide Formation from Organoazides and
Isocyanides Catalyzed by 1
c
Scheme 3. Synthesis of NiI Isocyanide Adducts 2 and 3
adduct are consistent with the presence of two coordinated
isocyanide ligands owing to symmetric and antisymmetric
combinations of the individual CN oscillators. These bands
occur at νCN = 2080 and 2112 cm−1 for [Me3NN]Ni(CNtBu)2
(2) and at νCN = 1997 and 2025 cm−1 for [Me3NN]Ni(CNAr)2
(3). The νCN stretches are found at lower energy than in free
CNtBu and CNAr (2131 and 2123 cm−1, respectively),
indicating that a significant degree of back-bonding occurs in
the adducts 2 and 3. EPR glass spectra of 2 at 89 K in toluene
glass (with a drop of CNtBu) indicate a pseudoaxial
environment with g1 = 2.32 and g2 ≈ g3 = 2.17 (Figure S7).
Similarly, 3 also shows an axial environment at 89 K in toluene
glass (with excess CNAr) with g1 = 2.27 and g2 = g3 = 2.17
(Figure S9). Isotropic solution or frozen glass EPR measure-
ments of pure 2 and 3 in the absence of added isocyanide give
more complex spectra. This is likely a result of facile partial
dissociation of these isocyanides from 2 and 3 indicated by the
presence of free isocyanide in NMR spectra of 2 and 3 at RT in
benzene-d6. While this may reflect the dissociation of one
isocyanide from the bis(isocyanide) complexes 2 and 3 in
solution to give a mixture of the corresponding mono- and
bis(isocyanide) complexes, β-diketiminato Ni(I) arene com-
plexes are also known.30
The X-ray structure of [Me3NN]Ni(CNAr)2 (3) (Figure 2)
shows a distorted tetrahedral coordination at the Ni center with
a
b
c
60 °C in benzene, 17 h. 5 mol % 1, 17 h. General conditions: 1 or 5
mol % 1 in ether at RT for 17 h.
N,κ1-O:NSO2Tol).25 Despite the possibility of primary aliphatic
azides to form metal imines via α-H migration,28 PhCH2CH2N3
cleanly forms the corresponding carbodiimides. Tosylazide
requires a higher catalyst loading with CNtBu and gives the
highest yield with the aryl isocyanide (95%; entry e). Benzoyl
azides are a substrate class that has not been heavily
investigated in late metal nitrene chemistry, perhaps due to
the thermal Curtius arrangement that gives PhNCO and N2.29
Nonetheless we demonstrate high and moderate carbodiimide
yields employing this nitrene source with CNtBu and CNAr,
respectively (Table 1, entry g).
Insights into Catalytic Mechanism: Isocyanide Com-
plexes. We performed a series of synthetic investigations
involving the catalyst [Me3NN]Ni(2-pic) (1) and both
isocyanide and azide reagents to probe new nickel species
that could reasonably participate in the catalytic reactions. The
addition of 2.2 equiv of CNtBu or 3.7 equiv of CNAr to
[Me3NN]Ni(2-pic) (1) gives [Me3NN]Ni(CNtBu)2 (2) or
[Me3NN]Ni(CNAr)2 (3) as brown crystals from pentane in
Figure 2. X-ray structure of [Me3NN]Ni(CNAr2,6Me2)2 (3).
symmetric Ni−CNAr bonds shown by the nearly identical Ni−
C bond distances of 1.863(3) and 1.866(3) Å. The Ni−Nβ‑dik
bond distances are 1.956(2) and 1.963(2) Å. The twist angle of
67.6° between Nβ‑dik−Ni−Nβ‑dik and Cisocy−Ni−C planes
illustrates the distorted tetrahedral geometry at nickel.
We find that both [Me3NN]Ni(CNtBu)2 (2) and [Me3NN]-
Ni(CNAr)2 (3) react at RT in ether with the representative
azide N3Ar′ (Ar′ = 3,5-Me2C6H3) to give the corresponding
carbodiimides (Scheme 4). For instance, [Me3NN]Ni(CNtBu)2
C
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Scheme 4. Reaction of NiI Isocyanide Adducts with N3Ar′
(2) reacts with 2 equiv of N3Ar′ to give Ar′NCNtBu in
97% yield as quantified by 1H NMR spectroscopy (Scheme 4).
Monitored by UV−vis spectroscopy under catalytically relevant
conditions in ether, the dinickel imide 4 forms upon addition of
N3Ar′ to 2 or 3. Addition of 20 equiv of CNR′ to
[Me3NN]Ni(pic) (1) results in the formation of the isocyanide
adducts [Me3NN]Ni(CNR′)2 (2 and 3), which react
immediately to produce the dinickel imide {[Me3NN]Ni}2(μ-
NAr′) (4) upon subsequent addition of N3Ar′ to these
solutions (Figures S17, S18). The dinickel imide 4 is also
observed as the major Ni-containing species after standing 1 h
at RT.
Figure 3. X-ray structure of {[Me2NN]Ni}2(μ-NAr′) (5).
possesses modestly longer Cu−Nnitrene distances of 1.794(5)
and 1.808(5) Å.
Synthesis of New Dinickel Imides. On the basis of
stoichiometric studies between CNtBu and [Me3NN]Ni
NAd,19 we anticipated that related nickel imides might be
generally active for nitrene group transfer. To probe the nature
of the nickel nitrene species formed upon reaction of 1 with
aryl azides, we added N3Ar′ to 2 equiv of 1, which gives
{[Me3NN]Ni}2(μ-NAr′) (4), as substantiated by NMR and
elemental analysis (Scheme 5). Despite the diamagnetic nature
We find that {[Me2NN]Ni}2(μ-NAr′) (5) exhibits related
1
temperature-dependent H NMR chemical shifts in toluene-d8.
For instance, at 20 °C the imido o-H, backbone C-Me, and
backbone C−H resonances appear at δ 1.87, 0.30, and 4.74
ppm, which shift downfield with cooling to −60 °C to δ 3.88,
1.16, and 5.00 ppm. In contrast, the nitrene p-Ar-H signal shifts
upfield from δ 7.34 ppm at 20 °C to δ 6.78 pm at −60 °C
(Figures S12, S13).
Both {[Me3NN]Ni}2(μ-NAr) (4) and {[Me2NN]Ni}2(μ-
NAr′) (5) are rather reactive toward isocyanides. Each reacts
Scheme 5. Synthesis of Dinickel Imides 4 and 5
t
quickly with excess CNtBu to give the carbodiimide BuN
CNAr′ in 88% and 69% yield, respectively (Scheme 6).
Scheme 6. Isocyanide Transfer to Nickel Imides
of 4 in solution (μeff = 0.0(2) μB at RT), variable-temperature
1H NMR spectra of 4 in toluene-d8 show somewhat unusual
chemical shifts. For instance, at 20 °C the imido o-H, backbone
C-Me, and backbone C−H resonances appear at δ 0.16, −0.40,
and 4.71 ppm, which shift downfield with cooling to −60 °C to
δ 3.26, 0.94, and 5.08 ppm. In contrast, the nitrene p-Ar-H
signal shifts upfield from δ 8.01 ppm at 20 °C to 7.18 ppm at
−60 °C (Figures S10, S11).
To better establish the nature of this putative dinickel nitrene
species, we sought the crystal structure of 4. Unfortunately,
numerous attempts were unsuccessful, which motivated the
synthesis of the analogue {[Me2NN]Ni}2(μ-NAr′) (5), which
lacks p-Me groups on the β-diketiminato N-aryl rings. Reaction
of 2 equiv of [Me2NN]Ni(2-pic) with N3Ar′ in Et2O results in
an immediate color change from red to dark purple. The
product {[Me2NN]Ni}2(μ-NAr′) (5) may be crystallized from
Et2O as dark purple crystals suitable for X-ray diffraction in 80%
yield (Scheme 5). The X-ray structure of 5 (Figure 3) reveals
the dinuclear nature of this imido species with a Ni−Ni′
separation of 2.7120(3) Å and Ni−Nimide distances of 1.747(2)
and 1.755(2) Å. While the Ni−Nimide distances are quite similar
to the related dinickel alkylimide {[Me3NN]Ni}2(μ-NAd)
(1.738(3) and 1.758(3) Å),27 the Ni−Ni distance is
significantly longer (2.4872(7) Å). Nonetheless, the Ni−Ni
distance is shorter than in the related dicopper arylnitrene
{[Me3NN]Cu}2(μ-NAr′)20 (Cu···Cu = 2.911(1) Å), which
Monitored by UV−vis spectroscopy in ether, sequential
addition of 0.5 equiv of N3Ar′ to 1 to give in situ generated
4 followed by addition of excess CNAr cleanly gives the
isocyanide adduct [Me3NN]Ni(CNAr)2 (3), while addition of
excess CNtBu does not lead to [Me3NN]Ni(CNtBu)2 (2)
(Figures S19, S20). It is possible that the more electron-rich
carbodiimide product interacts with the [Me3NN]Ni fragment,
hindering association with CNtBu. Nonetheless, the high
catalytic yield in nitrene transfer from N3Ar′ to CNtBu (99%,
Table 1, entry b) would not suggest appreciable product
inhibition during catalysis.
DFT Studies: Insight into Dinickel Imides. DFT studies
(ADF2007.1−BP/ZORA/TZ2P(+)) conducted on a model of
{[Me3NN]Ni}2(μ-NAr′) (4) obtained through the optimiza-
tion of the crystal structure of {[Me2NN]Ni}2(μ-NAr′) (5)
predict the S = 0 and S = 1 forms to be close in energy, favoring
the S = 0 form by only 0.7 kcal/mol in electronic energy.
Experimentally, this energy difference is higher (>1.5 kcal/mol)
based on the magnetic moment of 0.0(2) μB measured for 4
and 5 in benzene-d6 at RT. These two forms are predicted to be
very similar in structure (Figure S17), with the only substantial
D
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difference being slightly longer Ni−Nimide distances in 4-S=1
(Ni−N: 1.782 and 1.785 Å) compared to 4-S=0 (Ni−N: 1.738
and 1.739 Å).
Scheme 7. Synthesis and X-ray Structure of
[Me3NN]Ni(N3)(2-picoline) (6)
The distribution of unpaired electron density in the S = 1
spin state of 4 (Figure 4) is correlated with the directions of the
somewhat out of the normal diamagnetic range for related
square planar Ni(II) β-diketiminato compounds. Significantly,
the peaks for bound 2-picoline are essentially broadened into
the baseline at RT. These observations suggest that 2-picoline
reversibly dissociates from 6, giving the presumably para-
magnetic [Me3NN]Ni(N3). A solution magnetic moment
measurement in benzene-d6 in the presence of a large excess
of 2-picoline (ca. 100 equiv) gives μeff = 0.81 μB. Supporting
this suggestion that the three-coordinate [Me3NN]Ni(N3)
would be high spin (S = 1), a related three-coordinate
nickel(II) chloride complex [β-dik]Ni−Cl bearing an excep-
tionally bulky β-diketiminate ligand is high spin with a solution
magnetic moment μeff = 3.1 μB.35 Strong π-donor ligands such
as amides −NR1R2 are typically required to give low-spin,
three-coordinate β-diketiminato Ni(II) complexes.19,27,31
Cooling to −80 °C results in sharpening of broad peaks to
Figure 4. Spin density plot of 4-S=1. Excess spin α in blue; spin β in
red (0.001 isovalue).
temperature-dependent 1H NMR chemical shifts observed for 4
and 5. There is excess spin α in the vicinity of the imide N-aryl
o-C-H units as well as the β-diketiminato backbone C-H, which
each shift downfield with decreasing temperature. On the other
hand, there is excess spin β at the nitrene N-aryl p-C atom
correlated with an upfield shift of the corresponding p-C-H
resonance with decreasing temperature. The chemical shifts of
the imido o-H resonances for 4 and 5, which appear at δ 0.16
and 1.87 ppm at 20 °C, respectively, clearly move toward their
“diamagnetic” positions (ca. δ 7 ppm) with decreasing
temperature, suggesting that this temperature-dependent
behavior is likely due to contact shifts from a minute amount
of the S = 1 form that increasingly contributes at elevated
temperature. Related behavior has been seen in the monomeric,
low-spin nickel(II) amide [Me3NN]Ni-NPh2, which is
calculated to possess a relatively low lying triplet state.31
Transfer of the Azido Group to Nickel. Reaction of
[Me3NN]Ni(2-picoline) (1) with trimethylsilyl azide demon-
strates an extreme case in which the whole azide functionality is
transferred to the metal center.32−34 This reaction generates the
square planar nickel(II) azide species [Me3NN]Ni(N3)(2-pic)
(6), which may be isolated from ether in 44% yield as red
crystals. The X-ray crystal structure of 6 (Scheme 7) shows a
four-coordinate, pseudo square planar coordination environ-
ment at nickel with Ni−Nβ‑dik distances of 1.912(2) and
1.900(2) Å. The Ni−N distances to the azide and 2-picoline N-
donors are just slightly longer at 1.939(2) and 1.923(2) Å,
respectively. The square planar geometry is only slightly
distorted, shown by the slight twist angle between the N1−Ni−
N2 and N3−Ni−N4 planes of 17.3°. The linear azide moiety is
mildly unsymmetrical, with N4−N5 and N5−N6 distances of
1.213(3) and 1.168(3) Å, though the shorter N5−N6 distance
may also be due to greater librational motion at the free
terminus of the azide group.
1
allow for assignment of the H and 13C NMR spectra, though
reversible dissociation/association of 2-pic may not be
completely frozen out (Figure S14). For instance, the
diastereotopic sets of backbone Me and N-Ar p-Me peaks are
not resolved and appear at δ 1.37 and 2.12 ppm, and the N-Ar
o-Me groups appear as a broad resonance centered at δ 2.66
ppm. The 2-Me group of the bound 2-pic ligand appears at δ
4.166 ppm (Figure S14).
Scouting Reactions with CO. Scouting experiments
seeking to prepare isocyanates RNCO from organoazides
N3R and CO catalyzed by 1 unfortunately met with little
success. We believe that the poor performance of 1 in catalytic
nitrene transfer to CO is due to the formation of a rather stable
dinuclear nickel carbonyl complex. Addition of excess CO to
[Me3NN]Ni(2-picoline) (1) quickly gives the d9−d9 dimer
{[Me2NN]Ni}2(μ-CO)2 (7), which may be isolated in 62%
yield as black crystals from ether (Scheme 8). This bridging
carbonyl had been previously prepared in the addition of CO to
[Me3NN]NiNAd to give OCNAd and 7 in 76% yield.19
The X-ray structure of {[Me2NN]Ni}2(μ-CO)2 (7) reveals a
rather short Ni−Ni distance of 2.401(2) Å in which metal
center may be viewed as distorted trigonal pyramidal to
accommodate bulky aryl groups on the N-donor atoms
(Scheme 8). The two carbonyl groups are not symmetrically
bound to the metal center, with Ni−C bond distances of
1.759(4) and 2.078(4) Å. Both CO ligands are coordinated in a
bent fashion with M−C−O angles of 160.8(3)o and 121.8(3)o,
one being significantly more obtuse that the other. The
relatively high CO stretching frequencies νCO = 1931 and 1893
cm−1, especially for a bridging species, do not suggest an
extremely strong back-bonding interaction.
[Me3NN]Ni(N3)(2-pic) (6) is not stable in solution when
redissolved after isolation: excess 2-picoline must be added for
spectroscopic characterization. Broad 1H NMR signals are
observed at RT in toluene-d8 spread over δ −2 to 9 ppm,
E
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employed with low (1 mol %) catalyst loadings of 1 to give
good to excellent yields of the corresponding carbodiimides.
The scope of catalytic carbodiimide formation established for 1
is considerably broader than initial reports employing Fe(I) β-
diketiminato and bridging NHC Ni(I) catalysts: the former
examined the azides AdN3 and TolN3 with the alkyl isocyanides
CNtBu and CNCy,24 and the later examined MesN3 with
CNCH2Ph and CNtBu.25
Scheme 8. Synthesis and X-ray Structure of
{[Me3NN]Ni}2(μ-CO)2 (7)
Synthetic studies focusing on understanding the interaction
of the isocyanide and organoazide reagents with the nickel(I)
precatalyst [Me3NN]Ni(2-pic) (1) reveal the formation of
bis(isocyanide) complexes [Me3NN]Ni(CNR′)2 (R′ = tBu (2),
R′ = Ar2.6‑Me2 (3)) as well as bridging dinickel arylimides
{[MexNN]Ni}2(μ-NAr′) (x = 3 (4), x = 2 (5)). Importantly,
the bis(isocyanide) species 2 and 3 react with the organoazide
N3Ar, and the dinickel arylimides 4 and 5 react with the
t
isocyanide BuNC employed in this study to deliver the
corresponding carbodiimides. Thus, these studies indicate that
bis(isocyanide) and nickel imido complexes may participate in
catalytic carbodiimide formation upon reaction with organo-
azides and isocyanides, respectively.
The 1H NMR spectrum of 7 in toluene-d8 at −90 °C (Figure
S15) is consistent with its solid-state structure (Scheme 8),
giving rise to six methyl signals (eight expected) since the two
Ni fragments can be interconverted by a C2-axis in solution;
separation of two backbone-Me signals was not achieved.
Warming to −75 °C results in the coalescence of the p-Me
signals at δ 2.37 and 2.13 ppm in the low-temperature limit,
consistent with a concerted twisting motion of each [Me3NN]
Ni fragment about its respective Ni−Ni vector, corresponding
to an activation barrier ΔG⧧ = 9.4(4) kcal/mol at this
temperature (Figure S16). All o-Me resonances coalesce at
−23 °C, likely a result of reversible dissociation/reassociation
of a [Me3NN]Ni moiety from the {[Me3NN]Ni(μ-CO)}2
Some electron-poor organoazides did not perform as well as
their electron-rich counterparts, and the use of N3TMS with
isocyanides led to no carbodiimide formation. It is possible that
the lower nucleophilicity of electron-poor organoazides makes
it difficult for them to compete against isocyanide for access to
the catalyst’s coordination sphere. In the case of N3TMS,
synthetic studies reveal that the azide group is oxidatively
transferred to the nickel center to form the nickel(II) azide
complex [Me3NN]Ni(N3)(2-picoline) (6), which is inactive in
catalytic nitrene transfer to isocyanides with organoazides. Such
reactivity appears to be not entirely uncommon, with related
examples resulting in metal-azide formation starting from β-
diketiminato Fe(II)32 and Ge(II)34 hydrides as well as with an
Al(I) species.33 The picoline ligand of 6 appears to be rather
labile, requiring added 2-picoline to enhance its stability in
solution.
The relative difficulty of employing CO in catalytic nitrene
transfer reactions employing precatalyst 1 is likely due to the
bridging nature of the nickel(I) carbonyl species {[Me3NN]-
Ni}2(μ-CO)2 (7) that results from exposure of 1 to CO.
Coupled with the somewhat poor nucleophilicity of organo-
azides N3R, the dinuclear nature of 7 leads to extremely
sluggish reactivity with organoazides. To enhance reactivity, the
bridging structure of the {[Me3NN]Ni}2(μ-CO)2 resting state
should be prevented. For instance, Holland’s successful catalytic
nitrene transfer from AdN3 to CO by a very bulky β-
diketiminato Fe(I) complex results in mononuclear [Fe](CO)2
and [Fe](CO)3 species following catalysis.24 Holland et al. have
shown that a more sterically demanding β-diketiminato ligand
with N-Ar o-iPr groups can form a T-shaped Ni(I) carbonyl
complex [iPr2NN]Ni(CO).36 The use of such bulky ligands
could also enhance isocyanide dissociation from [Ni](CNR′)2
complexes during catalytic carbodiimide formation as well as
favor terminal nickel(III) imido intermediates. While this may
enhance the rate of nitrene transfer, it could also favor side
reactions involving C−C and C−N coupling reactions at the p-
aryl position of radical [NiIII]NAr intermediates observed by
Bai and Stephan.30
dimer 7 (Figure S16) with an activation barrier ΔG⧧
=
11.3(3) kcal/mol at this temperature (ca. 30 mM).
Stoichiometric reaction of {[Me3NN]Ni}2(μ-CO)2 (7) with
3,5-dimethylphenylazide gave a 13% yield of the corresponding
carbodiimide after 3 h at RT (Scheme 9). A catalytic attempt
Scheme 9. Sluggish Reactivity of 7 with N3Ar
employing 1 equiv of 3,5-dimethylphenylazide N3Ar′ and either
1 or 6 atm CO in benzene at 60 °C for 24 h with 5 mol %
[Me3NN]Ni(2-pic) (1) resulted essentially in no yield of the
anticipated isocyanate OCNAr′. Motivated by the
previously reported stoichiometric reaction of [Me3NN]Ni
NAd with excess CO resulting in a 76% yield,19 a catalytic
reaction was performed with AdN3 under similar conditions
with no observable product.
DISCUSSION AND CONCLUSIONS
■
The nickel(I) β-diketiminate [Me3NN]Ni(2-pic) (1) catalyzes
nitrene transfer from a diverse array of organoazides RN3 to
isocyanides CNR′ (R′ = tBu or Ar) to give carbodiimides RN
CNR′. Especially in conjunction with the electron-rich
isocyanide CNtBu, aryl, alkyl, tosyl, and carbonyl azides may be
EXPERIMENTAL SECTION
General Procedures. All experiments were carried out in a dry
nitrogen atmosphere using an MBraun glovebox and/or standard
■
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Article
Schlenk techniques. Molecular sieves (4A) were activated in vacuo at
180 °C 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 concentrated HNO3/
H2SO4 to remove olefins, stored over CaCl2, sparged, and then passed
through activated alumina columns. Benzene, toluene, and ethyl-
benzene were purchased anhydrous and stored over 4A molecular
sieves. All deuterated solvents were sparged with nitrogen, dried over
activated 4A molecular sieves, and stored under nitrogen. Celite was
{[Me3NN]Ni}2(μ-NAr′) (4). To a solution of [Me3NN]Ni(2-pic) (1)
(0.235 g, 0.484 mmol) in 3 mL of Et2O at −35 °C was added a
solution of 3,5-dimethylphenylazide (0.036 g, 0.242 mmol) in 2 mL of
Et2O at −35 °C. An immediate color change from red to dark purple
took place, and gas bubbles evolved. All volatiles were immediately
removed in vacuo. The remaining residue was taken up in 10 mL of
pentane, and all volatiles were again removed in vacuo. This step is
necessary to remove as much free 2-picoline as possible to maximize
the isolated yield of this substance. The remaining dark purple residue
was taken up in cold Et2O and filtered through Celite. The resulting
solution was concentrated, layered with cold pentane, and allowed to
crystallize overnight at −35 °C to afford a dark purple solid in 76%
yield (0.167 g, 0.185 mmol). 1H NMR (toluene-d8, 20 °C): δ 8.010 (s,
1H, p-NAr-H), 7.546 (s, 8H, m-Ar-H), 4.707 (s, 2H, C-H backbone),
3.411 (s, 12H, p-Ar-CH3), 3.243 (s, 24H, o-Ar-CH3), 2.597 (s, 6H, m-
NAr-CH3), 0.159 (s, 2H, o-NAr-H), −0.403 (s, 12H, backbone CH3);.
13C{1H} NMR (C6D6; RT, 400 MHz): δ 156.61, 146.81, 139.18,
126.67, 110.81, 25.05, 19.21, 15.67. μeff (C6D6) = 0.0(2) μB. UV−vis
(Et2O, 25 °C): λmax = 375 nm (ε = 18 000 M−1 cm−1) and 529 nm (ε
= 4500 M−1 cm−1). Anal. Calcd for C54H67N5Ni2: C, 71.78; H, 7.47;
N, 7.75. Found: C, 71.58; H, 7.59; N, 8.04.
1
dried overnight at 200 °C under vacuum. H and 13C NMR spectra
were recorded on a Varian MR 400 or Mercury 300 MHz
spectrometer (400 or 300 MHz; 100.47 or 75.4 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 airtight quartz cuvettes with
screw-cap tops or Teflon stoppers. Solution EPR spectra were
recorded on a JEOL JES-FA200 continuous wave spectrometer
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 performed on a Perkin-Elmer PE2400 microanalyzer at
Georgetown. IR measurements were performed on a Perkin-Elmer
Spectrum One FT-IR spectrometer using thin films of the sample on a
NaCl plate evaporated from pentane or ether. All reagents were
obtained commercially unless otherwise noted. [Me3NN]Ni(2-pico-
line)27 and [Me2NN]Ni(2-picoline)37 were prepared by literature
procedures.
{[Me2NN]Ni}2(μ-NAr′) (5). To a solution of [Me2NN]Ni(2-picoline)
(0.490 g, 1.07 mmol) in 8 mL of Et2O at −35 °C was added a cold
solution of 3,5-dimethylphenylazide (0.079 g, 0.536 mmol) in 3 mL of
Et2O at −35 °C. An immediate color change from red to dark purple
took place, and gas bubbles evolved. All volatiles were immediately
removed in vacuo. The remaining residue was taken up in 10 mL of
pentane, and all volatiles were again removed in vacuo. This step is
necessary to remove as much free 2-picoline as possible to provide
high crystalline yields. The remaining dark purple residue was taken up
in cold Et2O and filtered through Celite. The resulting solution was
concentrated, layered with cold pentane, and allowed to crystallize
overnight at −35 °C to afford a dark purple solid in 80% yield (0.362
General Procedure for Catalytic Formation of Carbodii-
mides and Characterization of Carbodiimide Products. A
solution of organoazide (0.843 mmol) along with tert-butylisocyanide
(1.2 equiv, 1.01 mmol) or 2,6-dimethylphenylisocyanide (1.1 equiv,
0.927 mmol) was prepared in 3 mL of Et2O and chilled to −35 °C. A
second solution of [Me3NN]Ni(2-picoline) (1) (0.0422 mmol = 5
mol % or 0.00843 mmol = 1 mol %) was prepared in 3 mL of Et2O
and similarly chilled to −35 °C. The two cooled solutions were added
together and allowed to react under the described conditions in Table
1 or Table S1, typically at RT for 30 min or 17 h. The reactions were
then quenched with 5 mL of CH2Cl2 to oxidize the catalyst to
insoluble {[Me3NN]Ni}2(μ-Cl)2.38 All volatiles were removed in
vacuo. The remaining oil was taken up in CH2Cl2 and filtered through
Celite. All volatiles were removed in vacuo. Quantification was
performed by addition of 1 equiv of internal standard (anthracene,
1
g, 0.428 mmol). H NMR (toluene-d8, 20 °C): δ 7.555 (d, 8H, m-Ar-
H), 7.286 (s, 1H, p-Ar-H), 6.578 (t, 4H, p-Ar-H), 4.768 (s, 2H,
backbone C-H), 2.791 (s, 24H, o-Ar-CH3), 2.294 (s, 6H, m-Ar-CH3),
1.940 (s, 2H, o-Ar-H), 0.341 (s, 12H, backbone CH3). 13C{1H} NMR
(C6D6; RT): δ 157.05, 147.38, 141.30, 127.65, 122.82, 35.46, 23.50,
17.87. μeff (C6D6) = 0.0(2). UV−vis (Et2O, 25 °C): λmax = 408 nm (ε
= 11 000 M−1 cm−1) and 527 nm (ε = 6900 M−1 cm−1). Anal. Calcd
for C50H59N5Ni2: C, 70.87; H, 7.02; N, 8.26. Found C, 71.19; H, 7.30;
N, 8.18.
[Me3NN]Ni(N3)(2-picoline) (6). A chilled (−35 °C) solution of
[Me3NN]Ni(2-pic) (1) (0.196 g (0.404 mmol) was prepared in 8 mL
of Et2O, to which a chilled (−35 °C) solution of TMSN3 (1.00 mL,
0.868 g, 7.53 mmol) was added. The solution was allowed to warm to
RT. During this time the red color of the starting material changed
hues to a magenta red. The reaction mixture was allowed to stir for 1 h
at RT, after which time all volatiles were removed in vacuo. The red
solid was taken up in Et2O and passed through Celite. Then the
intense red solution was concentrated to afford red crystals at −35 °C
suitable for single-crystal X-ray analysis in 44% yield (0.092 g, 0.176
mmol). All NMR and UV−vis spectra were taken with excess 2-
picoline (∼5−10 equiv); otherwise the compound decomposes to a
black suspension. 1H NMR (toluene-d8, −80 °C): δ 7.591 (br s, 1H, o-
pic-H), 7.171 (s, 1H, pic-H), 7.089 (s, 1H, pic-H), 6.54 (br s, m-Ar-H),
6.311 (br, 1H, pic-H), 6.007 (br, 1H, pic-H), 5.934 (br, 1H, pic-H),
4.717 (s, 1H, backbone C-H), 4.166 (s, 3H, pic-CH3), 2.661 (br, 12H,
o-Ar-CH3), 2.120 (s, 6H, p-Ar-CH3), 1.367 (s, 6H, backbone-CH3).
13C{1H} NMR (toluene-d8, −80 °C): δ 158.47, 150.46, 149.50,
148.45, 135.96, 135.50, 133.29, 132.61, 123.00, 120.60, 25.00, 24.67,
24.34, 24.00 (includes free pic). μeff (C6D6 and 100 equiv of 2-
picoline) = 0.81 μB. IR: νN3 = 2040 cm−1 (in the presence of excess 2-
pic). UV−vis (Et2O with 100 equiv of 2-picoline; 25 °C): λmax = 540
nm (ε = 3800 M−1 cm−1). Anal. Calcd for C29H36N6Ni: C, 66.05; H,
6.88; N, 15.94. Found: C, 66.34; H, 7.19; N, 15.59.
1
naphthalene, or anisole) followed by H NMR analysis in CDCl3.
Preparation of Compounds. [Me3NN]Ni(CNtBu)2 (2). A solution
of tert-butylisocyanide (0.038 g, 0.458 mmol) in 2 mL of Et2O at −35
°C was added to a solution of [Me3NN]Ni(2-pic) (1) (0.100 g, 0.206
mmol) in 5 mL of Et2O at −35 °C. An immediate color change from
red to brownish-orange took place. The reaction mixture was allowed
to stir at RT for 30 min. All volatiles were removed in vacuo, and the
resulting crude product was crystallized from pentane at −35 °C.
Brown crystals were recovered in 74% yield (0.085 g, 0.152 mmol). IR:
νCN = 2080, 2112 cm−1. μeff (C6D6) = 1.63 μB. UV−vis (Et2O, 25 °C):
λmax = 475 nm (ε = 910 M−1 cm−1). Anal. Calcd for C33H47N4Ni: C,
70.97; H, 8.48; N, 10.03. Found: C, 70.67; H, 8.35; N, 9.75. EPR of 2
t
with excess BuNC in toluene frozen glass at 89 K: g1 = 2.32 and g2 =
g3 = 2.17.
[Me3NN]Ni(CNAr)2 (3). A solution of 2,6-dimethylphenylisocyanide
(0.099 g, 0.756 mmol) in 4 mL of Et2O at −35 °C was added to a
solution of [Me3NN]Ni(2-pic) (1) (0.100 g, 0.206 mmol) in 5 mL of
Et2O at −35 °C. The reaction mixture was allowed to stir at RT
overnight, during which time a color change from red to dark brown
took place. All volatiles were removed in vacuo, and the resulting crude
product was crystallized from pentane at −35 °C. Brown crystals were
recovered in 34% yield (0.045 g, 0.0692 mmol). νCN = 1997, 2025
cm−1. μeff (C6D6) = 1.51 μB. UV−vis (Et2O, 25 °C): λmax = 543 nm (ε
= 1900 M−1 cm−1). Anal. Calcd for C41H47N4Ni: C, 75.24; H, 7.24; N,
8.56. Found: C, 75.71; H, 7.41; N, 8.44. EPR of 3 with excess CNAr in
toluene frozen glass at 89 K: g1 = 2.27 and g2 = g3 = 2.17.
{[Me3NN]Ni}2(μ-CO)2 (7). A solution of [Me3NN]Ni(2,4-lutidine)
(1) (0.250 g (0.501 mmol) was prepared in 10 mL of Et2O, to which
CO (48.9 mL @ 1 atm, 298 K; 2.00 mmol, 8 equiv) was added. The
solution immediately took on an extremely dark green hue, and the
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Article
volatiles were removed in vacuo. Recrystallization from ether at −35
°C provided black crystals in 62% yield (0.130 g, 0.155 mmol). This
species is identical to that prepared from [Me3NN]NiNAd and
excess CO.19 1H NMR (toluene-d8, −90 °C): δ 6.873 (br, 1, Ar-H),
6.767 (br, 2, Ar-H), 6.527 (br, 1, Ar-H), 4.992 (s, 1, backbone C-H),
3.365 (s, 6, o-Ar-CH3), 2.374 (s, 12, o-Ar-CH3 and p-Ar-CH3), 2.138
(s, 6, p-Ar-CH3), 2.024 (s, 6, o-Ar-CH3), 1.695 (s, 6, o-Ar-CH3), 1.601
(s, 12, backbone-CH3). 13C{1H} NMR (toluene-d8, −60 °C): δ
160.55, 149.19, 137.44, 133.45, 131.68, 129.94, 98.22 (backbone-CH),
23.70, 21.27 (backbone-CH3), 20.27, 18.79. IR: νCO = 1931, 1893
cm−1.
Experimental X-ray coordinates for {[Me2NN]Ni}2(μ-NAr′) (5)
were used as the starting point for the geometry optimization of low-
spin {[Me3NN]Ni}2(μ-NAr) (4-S=0) in a restricted (S = 0)
calculation after p-Me groups were installed on the β-diketiminato
N-aryl rings. The geometry optimization for the S = 1 form of
{[Me2NN]Ni}2(μ-NAr) (4-S=1) employed the DFT-optimized
coordinates for 4-S=0 as a starting point in an unrestricted (S = 1)
calculation specifying two unpaired electrons (spin α − spin β).
ADFview40 was used to prepare the three-dimensional representations
of the structures shown in Figure S21 as well as the spin density plot
shown in Figure 4.
Stoichiometric Carbodiimide Formation. Ar′NCNtBu
from [Me3NN]Ni(CNtBu)2 and N3Ar′. [Me3NN]Ni(CNtBu)2 (2)
(0.245 g, 0.439 mmol) was dissolved in 10 mL of Et2O and chilled
to −35 °C. To this chilled solution was added 3,5-dimethylphenylazide
(129 mg, 0.877 mmol). This reaction mixture was allowed to stir for
30 min at RT, after which all volatiles were removed in vacuo. Then the
remaining solid was taken up in Et2O and passed through Celite. All
volatiles were removed in vacuo, and anisole (95.3 μL, 0.095 g, 0.877
mmol) was added as a 1H NMR standard. The product was quantified
via 1H NMR (97% yield, essentially complete consumption of all
CNtBu initially bound to [Me3NN]Ni) as well as observed via GC/
MS: m/z (CI mode) = 203 (M + 1).
ASSOCIATED CONTENT
* Supporting Information
Complete synthetic and kinetics details, additional character-
ization data for compounds including EPR spectra, computa-
tional methods, and X-ray details with fully labeled thermal
ellipsoid plots for 3, 5, 6, and 7. This material is available free of
charge via the Internet at http://pubs.acs.org.
■
S
AUTHOR INFORMATION
Corresponding Author
*E-mail: thw@georgetown.edu.
Notes
■
Ar′NCNAr from [Me3NN]Ni(CNAr)2 and N3Ar′. [Me3NN]Ni-
(CNAr)2 (3) (0.040 g, 0.0611 mmol) was dissolved in 5 mL of Et2O
and chilled to −35 °C. To this chilled solution was added a solution of
3,5-dimethylphenylazide (0.018 g, 0.122 mmol) at −35 °C. This
reaction mixture was allowed to stir for 30 min at RT. All volatiles
were removed in vacuo. Then the remaining solid was taken up in Et2O
and passed through Celite. All volatiles were removed in vacuo, and
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
1
We are grateful to NSF (CHE-1012523) for support of this
work as well as for an award of an EPR spectrometer (CHE-
0840453).
anisole (13.2 μL, 0.013 g, 0.122 mmol) was added as a H NMR
standard. Unfortunately, overlapping peaks did not allow for ready 1H
NMR quantification. The product was clearly observed via GC/MS m/
z (CI mode) = 251 (M + 1).
Ar′NCNtBu from {[Me3NN]Ni}2(μ-NAr′) and CNtBu. To a
chilled (−35 °C) solution of {[Me3NN]Ni}2(μ-NAr′) (4) (0.048 g,
0.0531 mmol) in 5 mL of Et2O was added tert-butylisocyanide (24 μL,
0.018 g, 0.212 mmol). The reaction was allowed to stir at RT for 30
min, after which time the reaction was quenched with 2 mL of
CH2Cl2. All volatiles were removed in vacuo, and the remaining film
was taken up in Et2O and passed through Celite. All volatiles were
REFERENCES
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DFT Calculations. The DFT calculations employed the Becke−
Perdew exchange−correlation functional39 using the Amsterdam
Density Functional suite of programs (ADF 2007.01).40,41 Slater-
type orbital (STO) basis sets employed for H, C, and N atoms were of
triple-ζ quality augmented with two polarization functions (ZORA/
TZ2P), while an improved triple-ζ 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).42 The 1s electrons of C and N as well as the
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Wilk, and Nusair) functional was used for LDA (local density
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