Unexpected formation of chiral pincer CNN nickel complexes with β-diketiminato type ligands via C-H activation: Synthesis, properties, structures, and computational studies

Article abstract of DOI:10.1021/ic3021904

Reaction of lithiated chiral, unsymmetric β-diketimine type ligands HL^2a-e containing oxazoline moiety (HL^2a-e = 2-(2′-R₁NH)-phenyl-4-R₂-oxazoline) with trans-NiCl(Ph)(PPh₃)₂ afforded a series of new chiral CNN pincer type nickel complexes (3a-3e) via an unexpected cyclometalation at benzylic or aryl C-H positions. Single crystal X-ray diffraction analysis established the pincer coordination mode and the strained conformation. Chirality, and in one case, racemization of the target nickel complexes were confirmed by circular dichroism (CD) spectroscopy. Electronic structure and band assignments in experimental UV-vis and CD spectra were discussed on the basis of Density Functional Theory (DFT) and time-dependent (TD) DFT calculations.

Full text of DOI:10.1021/ic3021904

Article
pubs.acs.org/IC
Unexpected Formation of Chiral Pincer CNN Nickel Complexes with
β‑Diketiminato Type Ligands via C−H Activation: Synthesis,
Properties, Structures, and Computational Studies
Zhengliang Lu,†,§ Srinivas Abbina,† Jared R. Sabin,‡ Victor N. Nemykin,*,‡ and Guodong Du*,†
†Department of Chemistry, University of North Dakota, 151 Cornell Street Stop 9024, Grand Forks, North Dakota 58202, United
States
‡Department of Chemistry & Biochemistry, University of Minnesota Duluth, 1039 University Drive, Duluth, Minnesota 55812,
United States
S
* Supporting Information
ABSTRACT: Reaction of lithiated chiral, unsymmetric β-diketimine type
ligands HL2a−e containing oxazoline moiety (HL2a−e = 2-(2′-R1NH)-phenyl-4-
R2-oxazoline) with trans-NiCl(Ph)(PPh3)2 afforded a series of new chiral CNN
pincer type nickel complexes (3a−3e) via an unexpected cyclometalation at
benzylic or aryl C−H positions. Single crystal X-ray diffraction analysis
established the pincer coordination mode and the strained conformation.
Chirality, and in one case, racemization of the target nickel complexes were
confirmed by circular dichroism (CD) spectroscopy. Electronic structure and band assignments in experimental UV−vis and CD
spectra were discussed on the basis of Density Functional Theory (DFT) and time-dependent (TD) DFT calculations.
Among these variations, the unsymmetric pincer ligands of
the DXD′ type have received increasing attention.4,5 The two
donor groups (D and D′) can be markedly different, which may
result in unique and novel properties in the pincer complexes.
Transition metal complexes based on the CNN pincer have
been synthesized and employed in the catalytic cross coupling,6
hydrogenation of esters,5a and transfer hydrogenation of
ketones.7 The carbon donors in these systems are typically
based on aryl (1a−b) or N-heterocyclic carbene (1c−f)
carbons (Chart 2). The introduction of chiral substituents in
the pincer framework constitutes a common strategy for
enantioselective catalysis.8,9 However, pincer complexes with
both unsymmetric and chiral ligands have been relatively less
developed,10 presumably because of lack of a general synthetic
strategy, and only a few complexes incorporating chiral CNN
pincer ligands, derived from 1a and 1e, have been reported.11
Nickel is one of the first metals incorporated in the pincer
complexes,12 and numerous pincer nickel complexes have
appeared in the literature.13 Their applications in bond
activation and catalysis such as C−C coupling, dehydrogen-
ation, and hydroamination have been extensively
studied.6a,b,14,15 The potential exhibited by these complexes
has encouraged further development of ligand precursors
bearing analogous chelating systems and isoelectronic fea-
tures.16 In this report we describe the synthesis and
characterization of a series of rare chiral CNN pincer nickel
complexes with C1-symmetry β-diketiminato type ligands, in
which the carbon donor arm is formed via an intramolecular
INTRODUCTION
■
Ligands play an essential role in catalysis, especially when regio,
stereo, and enantioselectivities are concerned, as they can
provide appropriate stereochemical and electronic environ-
ments around the active metal centers. Among the various
chelating ligands available in the literature, tridentate pincer
ligands are one of the most widely applied systems.1 The
prototypical DXD-type pincer ligands feature two donor atoms
(D) such as tertiary phosphine or amine linked through an
aromatic or aliphatic skeleton encompassing a carbon- or
nitrogen-bound anionic anchor (X) (Chart 1). In particular,
Chart 1. General Representation of Pincer Ligands
NCN and PCP pincer complexes with overall C2 symmetry are
widespread, and this design, with the carbon−metal bond
located between two lateral arms, leads to stabilization of the C-
M bond and could improve the robustness of catalytic systems.2
It is also important that such framework can be fine-tuned to
allow rational design of catalysts. Variations of donor and
anchor atoms have greatly expanded the range of pincer ligands,
in which D can be N, P, S, O, C, etc., and X can be C, Si, N, P,
B, etc.1,3
Received: October 8, 2012
© XXXX American Chemical Society
A
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a
and isolated as a yellow crystalline solid in good yields. The 1H
NMR indicates that the coordination environment of the
lithium center is completed with two tetrahydrofuran (THF)
solvent molecules. It was further noted that isolation of lithium
salts was not necessary, and the subsequent metalation
reactions were carried out using in situ generated lithium
compounds without further purification.
Chart 2. Some Examples of CNN Pincer Ligands
Preparation of Pincer Nickel Complexes 3a−3d.
Treatment of lithium salts of ligand 2a−d with trans-
NiCl(Ph)(PPh3)2 at room temperature resulted in an
immediate color change, and dark-red crystals 3a−d were
consistently formed after allowing solutions standing for 2−5
days. The isolated compounds appeared rather sensitive to air,
as the color blackened within minutes upon exposure to the air,
but could be stored under an inert atmosphere for months.
They are quite soluble in THF and toluene and have been
characterized by various spectroscopic and analytic techniques
including 1H, 13C, and 31P NMR spectroscopy. In the 1H NMR
of 3a in benzene-d6 (Figure 1), the most striking features
include the six-proton dimethyl group of the free ligands
becoming a three-proton singlet, and the appearance of two
new 1-proton multiplets at 2.76 and 1.38 ppm. The two
multiplets are coupled with each other and connected to the
same carbon atom, as indicated by 2D NMR analysis.22 These
observations suggest the metalation of one methyl group of the
aniline moiety, leading to a coordinated methylene group
(NiCH2) with two diastereotopic protons riding on the same
carbon (Scheme 2). Because of coupling with the phosphorus
nuclei, the methylene protons are both multiplets, and this is
a
Donor atoms are in bold.
C−H activation. Besides the unexpected C−H activation for
both sp3- and sp2-hybridized carbon atoms, these complexes are
of interest in catalysis given that the chiral ligand precursors are
readily available and tunable. The C1-symmetric systems have
received increasing attention in recent years,17 and the
unsymmetric donor sets could be advantageous when two
donor groups influence the reactivity and selectivity in different
manners.18 The presence of sp3-C in the pincer framework may
also lead to structural and electronic versatilities that can open
up new opportunities in catalysis.19
further supported by a doublet of NiCH2 at 26.43 ppm (2JC−P
=
23
1
25.3 Hz) in the13C NMR. The H NMR spectra for 3b−d
reveal the similar features that coordinated methylene protons
exhibit, two signals at 2.66 and 1.45 ppm for 3b, 2.47 and 1.64
ppm for 3c, and 2.48 and 1.56 ppm for 3d, respectively; the
NiCH2 signals appear as doublets at 26.38 ppm for 3b, 26.61
for 3c, and 26.52 for 3d, respectively, in 13C NMR (2JC−P= 25.2
−26.5 Hz) due to coupling with the phosphorus nuclei.
The proposed structures of the Ni complexes were further
verified by single-crystal X-ray diffraction experiments. The X-
ray crystal data, data collection, and refinement parameters are
summarized in Table 1. A single crystal X-ray structure of 3a is
presented in Figure 2. In agreement with the NMR data, one of
the aniline methyl substituents is metalated with nickel, forming
a five-membered metallacycle. This, along with the imine
nitrogen atom, resulted in an unsymmetric, CNN pincer type
coordination mode of the supposedly bidentate ligand. The
triphenylphosphine ligand completed the distorted square-
planar environment around nickel center. The Ni−P bond
length of 2.1336(6) Å is in the typical range for similar
compounds,24 while the Ni−C bond distance of 1.930(2) Å is
considerably shorter than the Ni−C(sp3) bond (1.97 Å) in a
PCP pincer complex,25 but longer than the Ni−C(sp2) bonds
(1.88 Å) seen in the other pincer complexes.26
RESULTS
■
Synthesis of Ligands. The chiral, unsymmetric anilido-
imine ligands, 2a−e, have been obtained as analogues of
conventional β-diketiminato framework, via a palladium
catalyzed Buchwald-Hartwig amination reaction (Scheme
1).20 In the case of 2e, because of the low and inconsistent
Scheme 1. Synthesis of Ligands 2a−2e via Amination
Reaction
Complexes 3a−3c are isomorphic with similar structural
parameters; selected bond lengths and bond angles are listed in
Table 2. The bond distances of Ni−Nimino (1.920−1.935 Å) are
slightly longer than that of Ni−Namido (1.908−1.920 Å),
presumably because of the stronger interaction with anionic
amido nitrogens. Both of them are in the normal range
compared with other reported nickel compounds.27 The
coordination plane around nickel, however, appears to be
severely distorted. While the Nimino−Ni−C bond angles of
∼163° are not unusual for the trans angles involving the lateral
yields, an alternative, Cu-catalyzed amination reaction proto-
col21 was employed. This protocol seems to be more consistent
and reliable for alkyl amines, although yields are still generally
moderate (∼40%).
These ligands can be deprotonated with a strong base such as
nBuLi at low temperature. Thus, the lithium salt of ligand 2d
was prepared by lithiation with stoichiometric amount of nBuLi
B
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1
Figure 1. H NMR of complex 3a in C6D6 with partial assignment.
Scheme 2. Synthesis of Pincer Complexes 3a−d
Table 1. X-ray Crystal Data, Data Collection Parameters, and Refinement Parameters
3a
3b
3c
3d
3e
2d
4
formula
FW
C38H37N2NiOP
627.41
C39H39N2NiOP
641.43
C39H39N2NiOP
641.43
C41H35N2NiOP
661.39
C93H84ClLiN2NiO5P4
1534.70
C23H22N2O
342.43
C75H73Ni2P5
1366.74
crystal system
space group
a, Å
orthorhombic
P212121
orthorhombic
P212121
orthorhombic
P212121
triclinic
orthorhombic
P212121
orthorhombic
P212121
triclinic
P1
̅
P1
̅
9.7830(2)
9.7903(2)
9.84280(10)
15.0952(3)
21.8779(15)
9.2807(2)
11.2227(3)
17.2906(4)
108.2600(10)
92.2040(10)
103.0970(10)
1654.02(7)
2
11.4071(3)
20.7893(7)
33.113(2)
10.7615(11)
10.8922(11)
15.7770(16)
12.7404(4)
12.8703(4)
24.2765(7)
77.185(2)
88.302(2)
61.962(2)
3413.08(18)
2
b, Å
15.0674(2)
21.6014(15)
15.3617(4)
21.5085(15)
c, Å
α, deg
β, deg
γ, deg
V, Å3
3184.1(2)
4
3234.8(2)
4
3250.6(2)
4
7852.6(6)
4
1849.3(3)
4
Z
d
calc, g cm−3
1.309
1.317
1.311
123
1.328
1.298
1.230
173
1.330
T/K
μ, mm−1
123
123
173
123
173
0.692
0.683
0.680
3.094−27.484
23395
7369
0.670
0.418
0.076
2.29−28.22
15880
4274
0.716
2θ range, deg
data collected
unique data
Rint
3.118−27.484
22147
7309
3.111−27.482
11925
7097
1.25−33.09
25654
3.003−24.710
32597
13331
0.101
0.86−28.30
86941
10575
16805
0.025
0.036
0.021
7355
0.0215
0.0249
4274
0.1086
data in refinement
7294
7083
10575
13292
8596
16805
data with I > 2.0σ(I) 6875
5413
6970
8835
4021
9748
variables
389
398
398
415
965
237
813
a
R(Fo)
0.0246
0.0438
0.9425
−0.002(7)
0.0456
0.0748
0.9724
−0.01(2)
0.0276
0.0553
0.9765
0.0351
0.0596
0.0818
0.9568
−0.036(16)
0.0381
0.1014
1.038
0.0561
b
2
Rw(Fo )
0.0951
0.1373
GOF
1.047
0.933
Flack
0.004(9)
2
a
b
R = ∑||Fo| − |Fc||/∑|Fo| for Fo > 2σ(Fo ). Rw = [∑w(Fo − Fc2)2/∑w(Fo )2]1/2
.
2
2
2
C
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orthorhombic for 3a−c and 3e) that contains a pair of
enantiomers in the unit cell related by an inversion center.22
The geometrical parameters are similar to those in 3a−c, but
the distortion appears to be less severe. The racemization of 3d
is further supported by the CD measurement, which showed no
observable signals. In comparison, the CD spectrum of ligand
2d showed distinctive features. Moreover, the single crystal X-
ray structure of 2d was determined, which is in accordance with
the ligand chirality with the same absolute configuration (R) at
the 4-oxazoline position as the starting (R)-2-phenylglycinol.
Additionally, the free ligand itself takes on a planar
configuration with the N−H proton located between amido
and imino nitrogens, forming an intramolecular N−H···N
hydrogen bond. The dimethyl phenyl unit resides nearly
perpendicular to the above-mentioned plane. Comparison with
the ligand parameters in the nickel complex further confirmed
the distortion upon coordination (Figure 4). Particularly, the
nearly coplanar oxazoline ring and the central phenyl ring in the
free ligand are now twisted at 25.20(5)°, and the dihedral angle
between the aniline phenyl ring and the central phenyl skeleton
ring is 59.01(5)° in the complex.
Figure 2. Molecular structure of compound 3a with thermal ellipsoids
drawn at the 50% probability level.
Synthesis and Structure of 3e. Inspired by the formation
of complexes 3a−d, we were interested to see if C−H bonds
other than the benzylic one can be activated and form pincer
complexes within this type of ligands. Thus, ligand 2e, in which
a chiral alkyl moiety was introduced adjacent to the amine
nitrogen, was examined. Following a similar procedure, a dark
red crystal was obtained from reaction of trans-NiCl(Ph)-
(PPh3)2 and in situ generated lithium salt of ligand 2e. The 1H
NMR spectrum in benzene-d6 showed the absence of the N−H
signal of the free ligand. The benzylic proton, however, is still
observed at 4.99 ppm as a quartet, shifted downfield compared
with free ligand (4.55 ppm). These observations are consistent
with a proposed structure with C1 symmetry (Scheme 3).
X-ray crystal structure analysis confirmed the pincer
coordination mode of the ligand, in which the cyclometalation
takes place on the ortho-phenyl position of the amine arm to
form a five-membered chelation ring (Figure 5). Selected bond
lengths and bond angles are listed in Table 2. Unlike complexes
3a−d, one Cl atom is coordinated to Ni atom instead of PPh3.
The nickel atom resides in a distorted square planar geometry
constructed by N3, N14, C17, and Cl2 with the bond distances
of N3−Ni1 = 1.942(4), N14−Ni1 = 1.887(4), C17−Ni =
1.887(5), and Ni−Cl2 = 2.210(2) Å. Clearly the Ni−C17 bond
distance is much shorter than those in complexes 3a−d.
Presumably, this is because the PPh3 group was replaced by a
smaller Cl atom, which reduces steric crowdedness around the
metal center. In addition, PPh3 has a much stronger trans
influence than chloride, further lengthening the Ni−C distance
in 3a−d. Absence of PPh3 ligand makes the nickel coordination
environment more planar, and the deviation of Ni from the
donors in pincer systems, the Namido−Ni−P bond angles of
∼153° are much smaller than the typical linear arrangement for
the trans angles involving the central atom of the pincer
ligand.28 Presumably, this reflects a rather strong steric strain of
the nickel coordination environment. Consistent with this, the
two adjacent chelating rings deviate significantly from
coplanarity, with dihedral angles of 22.7(1)° (3a), 23.9(2)°
(3b), and 23.6(1)° (3c), respectively. The six member ring
adopts an envelope-like conformation with the Ni atom in the
flap position. The Ni atoms are displaced by 0.6571(2) (3a),
0.6650(6) (3b), and 0.6984(3) (3c) Å from the plane through
other five atoms of the six member rings. The dihedral angles
between the aniline phenyl ring and the central phenyl skeleton
ring are in the range 46.9(1)° (3a), 48.6(2)° (3b), and
50.0(1)° (3c).
Comparison of structural features of complexes 3b and 3c
suggests that the absolute configuration at the 4-oxazoline
position has a profound influence on the overall configuration
of the complexes. A side-by-side comparison of structures of 3b
and 3c, roughly along the C−Ni−Nimino, is shown in Figure 3.
The 2,6-disubstituted aniline phenyl moiety bends toward the
same direction of the substituent at the oxazoline chiral center,
while the backbone aromatic ring, as well as the PPh3 group,
points toward the opposite direction, to minimize the steric
interactions.
Racemization and Structure of 3d. When ligand 2d, with
a phenyl substituent at the 4-oxazoline position, was employed,
similar benzylic C−H activation occurred and CNN pincer
complex was readily obtained. However, the chiral center at the
4-oxazoline position somehow racemized. The compound
crystallized in a different crystal system (triclinic for 3d vs
+
coordination plane is only 0.0354(6) Å. Li(OPPh3)4 is found
Table 2. Selected Bond Distances (Å) and Bond Angles (deg) in Complexes 3a−e Determined by X-ray Crystallography
3a
3b
3c
3d
3e
Ni−C
1.9320 (15)
1.9197 (14)
1.9199 (14)
2.1335(4)
1.934(4)
1.9303(18)
1.9177(16)
1.9343(17)
2.1333(5)
162.74(8)
152.84(5)
1.9308(15)
1.9081(11)
1.9358(12)
2.1631(4)
171.60(6)
167.39(4)
1.887(5)
1.887(4)
1.942(4)
2.2100(15)
176.1(2)
174.92(12)
Ni−Namido
Ni−Nimino
Ni−P/Cl
Nimino−Ni−C
Namido−Ni−P/Cl
1.914(4)
1.926 (4)
2.1327(12)
164.05(19)
153.91(12)
162.77(7)
152.69(4)
D
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Figure 3. Side view of the structures 3b (left) and 3c (right).
Figure 4. Molecular structure of ligand 2d (left) and its nickel complex 3d (right).
observed at ∼500 nm. The CD spectra of the Ni complexes
3a−3d are shown in Figure 6 and agree well with the UV−vis
spectra. All (R)-isomers have a strong negative signal, which
corresponds to the low-energy transition observed in UV−vis
spectra at ∼650 nm. This band follows the low-intensity
negative signal at ∼510 nm, which correlates with the position
of shoulder observed in UV−vis spectra of corresponding
complexes. Absorption band at ∼440 nm has a positive
amplitude for all (R)-isomers and fits well with a position of
intense band observed in UV−vis spectra of the target nickel
complexes. Finally, one low intensity (∼380 nm) and one high-
intensity negative CD signal dominate in the UV region of the
CD spectra of complexes 3a−3d. In agreement with the
expectations, the CD spectrum of the (S)-isomer of complex 3c
is a mirror image of the CD spectrum of the (R)-isomer. As it
has already been mentioned above, complex 3d has no signals
in CD spectrum, which confirms its racemization during a
metal-insertion reaction.
Scheme 3. Synthesis of Pincer Complexes 3e
as the countercation in the crystal structure; presumably oxygen
comes from adventitious air during the reaction process.
UV−vis and CD Spectroscopy of Ni Complexes 3a−
3d. UV−vis and CD spectra of Ni complexes 3a−3d are
presented in Figure 6 and summarized in the Experimental
Section. In general, all complexes exhibit four features in their
UV−vis spectra. The first low intensity band (ε ∼ 200 M−1
cm−1) is located at ∼650 nm, which follows by two intense
bands at ∼440 (ε ∼ 2500 M−1 cm−1) and ∼320 (ε ∼ 9000 M−1
cm−1) nm bands with one low-intensity broad shoulder
DFT-PCM and TDDFT-PCM Calculations. A tentative
interpretation of the UV−vis and CD spectra of nickel(II)
E
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transition-metal complexes29 including nickel-containing com-
pounds.30 Since UV−vis and CD spectra of all investigated
nickel complexes are very close to each other, we have only
calculated electronic structure, UV−vis, and CD spectra of (R)-
and (S)-isomers of complex 3c. As shown in the Supporting
Information, Table S1, the predicted geometries from DFT-
PCM calculations are in good agreement with the X-ray
experimental parameters. The DFT-PCM predicted MO energy
diagram for 3c is presented in Figure 7, while an analysis of the
Figure 5. X-ray crystal structure of compound 3e. Cationic counterion
is omitted for clarity.
Figure 7. Molecular energy diagram and frontier orbitals of complex
3c calculated using DFT-PCM approach and X3LYP exchange-
correlation functional. HOMO−LUMO energy gap is indicated by the
dotted line.
orbital compositions is provided in Figure 8 and Supporting
Information, Table S2. In addition, the frontier orbitals of the
complex 3c are also pictured in Figure 7.
Figure 6. UV−vis and CD spectra of 3a−3d in CH2Cl2.
Figure 8. Molecular orbitals contribution analysis of complex 3c
calculated at DFT-PCM level using X3LYP exchange-correlation
functional. Black bars are the contribution of Ni ion, red bars are the
contribution of PPh3 ligand, blue bars are the contribution of oxazoline
part of the pincer ligand, bluegray bars are the contribution of PhNPh
part of the pincer ligand.
complexes 3a−3d is quite challenging. In particular, the
following questions should be addressed: (i) taking into
consideration the “soft base” character of new CNN pincer
ligand, is the highest occupied molecular orbital (HOMO)
predominantly nickel- or CNN π-centered MO? (ii) is the
lowest unoccupied molecular orbital (LUMO) π* MO centered
on the pincer CNN ligand or the soft PPh3 fragment? (iii) are
the low-intensity band at∼665 nm and a shoulder at ∼510 nm
classic nickel(II) d-d transitions? (iv) is the intensive band
observed at ∼440 nm charge-transfer or π−π* in nature? Thus,
the further insight into the electronic structure and UV−vis as
well as CD spectroscopy of the target nickel complexes 3a−3d
was gained on the basis of DFT-PCM and TDDFT-PCM
calculations which have been shown to provide accurate
energetic and spectroscopic parameters for a large variety of
The X3LYP/6-31G(d) DFT-PCM calculations predict that
the HOMO in the complex 3c is a predominantly π-orbital with
an electron density delocalized over diphenylamide fragment of
the ligand with the metal contribution of ∼10%. This orbital is
energetically well-separated (∼0.8 eV) from the closely spaced
predominantly nickel-centered HOMO-1 to HOMO-3 MOs.
2
HOMO-1 is dominated by a nickel dz AO contribution, while
HOMO-2 and HOMO-3 have prominent nickel dxz and dyz
characters, respectively. The other set of MOs, which is
F
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important for understanding of the UV−vis and CD spectra of
3c (HOMO-4 to HOMO-9), is predominantly localized over
PPh3 and chiral pincer ligands (Figures 7 and 8 and Supporting
Information, Table S2). For instance, HOMO-4 has distinct π-
character and is localized over C6H3CH2 fragment. Similarly,
HOMO-5 and HOMO-6 are π-orbitals delocalized over the
pincer ligand, while HOMO-7 and HOMO-8 have distinct
PPh3 localization. Except LUMO+2, which has ∼10% of nickel
dx −y character, LUMO to LUMO+10 MOs are dominated
either by PPh3 (LUMO+1 to LUMO+6) or pincer ligand
(LUMO, LUMO+7 to LUMO+9) contributions and could be
characterized as π* MOs.
These transitions have pure MLCT character and are
dominated by HOMO-1 (Ni dz ) → LUMO (∼40%, pincer
2
π* MO), LUMO+2 (∼25%, PPh3 π* MO) for excited state 2
or HOMO-2 (Ni dxz) → LUMO, LUMO+2 (∼25%, pincer π*
MO), LUMO+2 (∼20%, PPh3 π* MO) for excited state 3
transitions. In addition, ∼22% of excited state 3 could be
described as HOMO-3 (Ni dyz) →LUMO, LUMO+1, LUMO
+7, and LUMO+8 single-electron excitations. Again, in
agreement with experimental data, TDDFT-PCM predicted
rotary strengths of the excited states 2 and 3 are positive and
significantly smaller compared to that in the first excited state.
According to TDDFT-PCM calculations, excited state 4 is the
main contributor to the 440 nm band observed in UV−vis
spectrum of 3c. This excited state consists of five major single-
electron contributions, has ∼90% of π−π* character, and
dominated by HOMO → LUMO (∼64%) and HOMO →
LUMO+2 (∼26%) transitions. In agreement with experimental
CD spectrum, TDDFT-PCM calculations predict strong
negative signal associated with this excited state. Excited state
5 could be associated with the higher-energy shoulder of the
440 nm band, a positive signal in the CD spectrum observed at
∼375 nm. This excited state has 17 single-electron contribu-
tions, has ∼83% of π−π* and ∼17% of MLCT character, and
has no dominant contribution (the largest single-electron
contribution is ∼11% for HOMO → LUMO+2 transition).
Again, TDDFT-PCM calculations predict positive amplitude
for the CD signal associated with this excited state. Finally, the
shoulder at ∼350 nm observed in the UV−vis spectrum of 3c
and a weak negative signal observed in its CD spectrum in this
region can be assigned to the excited state 6. This excited state
has pure π−π* character and could be described as almost pure
HOMO → LUMO+1 (PPh3, π* MO) single electron
transition (∼96%). TDDFT-PCM calculations predict that
the higher energy regions of UV−vis and CD spectra of 3c
consist of numerous overlapping excited states and thus it is
impossible to provide a clear assignment for intense 320 nm
and higher energy bands.
2
2
The further interpretation of the UV−vis and CD spectra of
complex 3c was solidified on the basis of TDDFT-PCM
calculations (Figure 9 and Supporting Information). TDDFT-
Figure 9. Experimental UV−vis and CD data (top) and TDDFT-
PCM predicted UV−vis and CD spectra (bottom) of complex 3c. Blue
lines represent the (S)-isomer and red lines represent the (R)-isomer
of the chiral complex.
Overall, TDDFT-PCM calculations are in a good agreement
with the experimental UV−vis and CD data and allow to assign
the observed spectra in the 400−900 nm region to four excited
states with predominantly π−π* character and two excited
states with predominantly MLCT character.
PCM predicted vertical excitation energies, oscillator strengths,
and rotary strengths of 3c calculated with and without solvent
equilibration are virtually identical. UV−vis and CD spectra in
the 400−900 nm range could be described using the first six
low-energy excitations. The first low-intensity band exper-
imentally observed as a weak band at ∼665 nm in UV−vis
spectrum and as a strong positive signal in the CD spectrum of
3c is associated with the first transition predicted by the
TDDFT-PCM method. This excited state (Supporting
Information) consists of 10 significant single-electron con-
tributions, has ∼74% of intra- and interligand π−π* character,
and ∼26% of metal-to-ligand charge-transfer (MLCT)
character, and dominated by HOMO → LUMO (∼47%) and
HOMO → LUMO+2 (∼14%) transitions. The first transition
has intra(pincer)-ligand character, while the second one can be
described as the charge-transfer transition from the pincer
ligand to PPh3 fragment. In agreement with experimental data,
the oscillator strength of this transition is small, while rotary
strength is positive and large. The second and third excited
states are responsible for the broad, low intensity shoulder
observed in UV−vis spectrum of 3c between 500 and 600 nm
and weak positive CD signal observed in the same region.
DISCUSSIONS
■
A few anilido imine complexes of nickel have been reported as
analogues of conventional β-diketimine or α-diimine based
complexes, mostly for applications in catalystic olefin polymer-
ization.31 Usually they are obtained as mono- or dinuclear
Ni(II) species by reaction of free or deprotonated ligands with
a nickel precursor such as Ni(OAc)2, NiCl2, NiBr2,
NiCl2(THF)1.5, NiCl2(py)4, and Ni(acac)2, with or without
the presence of a base. When trans-NiCl(Ph)(PPh3)2 was
employed as the precursor, formation of a three coordinate
Ni(I) complex (NN)NiI(PPh3) was observed.27a,32 Analogous
results were obtained for the conventional β-diketiminato
ligand, leading to reduction of Ni(II) and formation of three
coordinate Ni(I) complexes.33 However, when less bulky
ketiminato and salicylaldiminato ligands were allowed to react
with trans-NiCl(Ph)(PPh3)2, square planar Ni(II) complexes
from simple metathesis were obtained as the main
products.33,34 It should be emphasized that in none of these
reactions C−H activation of ligands has been observed.
Therefore, it is surprising to note that in the present system,
G
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one of the benzylic or aryl C−H bonds on the aniline side arm
was cyclometalated, and the ligand functioned as a tridentate,
dianionic chelate and led to the formation of unsymmetric
CNN pincer type complexes.
complexes via C−H activation may have a similar steric origin.
Presumably, the ligand is first coordinated to nickel(II) in an
N,N-bidentate fashion, with PPh3 in the less congested side and
phenyl group adjacent to the aniline moiety. Because of the
proper steric interaction with the environment, particularly the
chiral oxazoline quadrant, the dimethylphenyl group was forced
out of its normal perpendicular position, with one methyl
leaning close toward the Ni center. This may lead to an agostic
interaction or a σ-complex that eventually resulted in the
elimination of benzene and the formation of the carbon nickel
bond, possibly through a conventional concerted σ-bond
metathesis or a σ-complex assisted metathesis pathway
(Scheme 5). Such mechanisms are common for electrophilic
A large number of pincer complexes with a C backbone or
arm have been prepared; the vast majority of them are
introduced through direct metalation, transmetalation, or
cyclometalation of C(sp2)-H bonds.8 In comparison, examples
with C(sp3)-H bonds, either benzylic or aliphatic, are relatively
less common,12,35 although the coordination-assisted C(sp3)-H
bond activation by palladium is well-documented.36 It also
appears that the metalation occurs only when the metal center
is easily accessible to the CH bond so that substitution at the
sp3 carbon is feasible.37 Thus, the results here are even more
striking, considering the typical orientation that the ortho-
dimethyphenyl group adopts and the strong distortion the
ligand would have to go through to form the observed
complexes. We have described the distorted coordination
environment around the Ni center. The sensitivity of these
complexes toward air may also be a reflection of the strain in
the system.
Scheme 5. Possible Pathway for Complexes 3a−d
Another puzzling yet important aspect is the observation of
racemization of chiral oxazolines in compound 3d, apparently
during the complex formation. Oxazoline and its derivatives
have been employed extensively in transition metal asymmetric
catalysis,38 but racemization of chiral oxazolines upon metal
coordination or during catalysis is rarely reported. Gabbai and
co-workers noted that a chiral oxazoline palladium complex,
(S,S)-di-μ-(acetate)-bis[2-[2-(4-carbomethoxy) oxazolinyl]-
phenyl-C,N]-dipalladium(II), underwent racemization reaction
when serving as a catalyst for the hydrolysis of organo-
phosphorus.39 However, the mechanism of racemization is not
well-understood. One possibility is that deprotonation of
hydrogen at 4-oxazoline may occur because of the enhanced
acidity when a phenyl (as in 3d) or carbomethoxy (as in the
dipalladium complex) group is attached. Isomerization to 3-
oxazoline or nonselective recombination of proton and
carbanion resulted in racemization (Scheme 4). It is unclear
early transition metal systems,40 but it can occur with late
transition metals as well.41 The observation of the nickel
product in the same oxidation state and the absence of biphenyl
from phenyl coupling in the products are in agreement with
this mechanism. An oxidative addition pathway involving a high
valent nickel, formally NiIV, seems less likely, but could not be
ruled out.42 Further studies are required to elucidate the
reaction mechanism and to utilize this C−H activation
chemistry.
One of our initial goals is to prepare nickel(II) complexes
incorporating chiral, monoanionic β-diketimine type ligands.
Therefore, we explored a number of commonly used nickel
precursors listed above in the synthesis. Though signs of
reactions were noted in several occasions, only the procedure
with trans-NiCl(Ph)(PPh3)2 afforded the isolable and identifi-
able nickel complexes, leading to the formation of pincer
complexes via unexpected C−H activation. However, the yields
are generally low; and the highest so far obtained is ∼35% with
3a, despite numerous attempts to improve the reactions.
Efforts were also made to isolate and characterize other Ni-
containing products formed in the reaction. The paramagnetic
species were often observed in the crude reaction mixture, as
indicated by the appearance of 1H NMR signals in the +50 and
−50 ppm range. Another type of byproduct features two nickel
centers without incorporation of the anilido imine ligands. One
of them was isolated as dark-green crystals and characterized by
X-ray diffraction crystallography, which revealed a dinuclear
structure of (PPh3)2Ni(μ-PPh2)2Ni(PPh3), 4.22 These obser-
vations may explain, at least in part, the low yields generally
obtained, and they also indicate the complexity of the process.
Scheme 4. Possible Pathways for Racemization of 3d
what could serve as a base to deprotonate the 4-H. Fortunately,
no racemization is observed when aliphatic substituents are
present at the 4-oxazoline position, as seen in 3a−c in our
study.
It has been suggested that the steric bulk of the chelating NN
ligands in combination of the bulky PPh3 played a key role in
the reduction of Ni(II) and the formation of three-coordinated
Ni(I).27a We suspect that unexpected formation of CNN pincer
CONCLUSIONS
In summary, we have synthesized and characterized a series of
chiral and unsymmetrical CNN pincer nickel complexes with
■
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133.28, 133.21, 133.17, 129.23, 128.87, 127.27, 121.59, 115.68, 109.10,
105.69, 73.27, 70.09, 68.13 (THF), 25.68 (THF), 19.31, 19.23.
Synthesis of 3a. The following procedure is typical: 2a (30.8 mg,
0.1 mmol) was dissolved in 5 mL of THF which was cooled to −78
°C. At this temperature 50 μL of BuLi (0.1 mmol) was added, and the
resulting yellow solution was stirred at low temperature for 1 h and
then was allowed to warm to room temperature. All volatiles were
removed under vacuum. The yellow residue was dissolved in 5 mL of
toluene, and then mixed with the orange solution of NiCl(Ph)(PPh3)2
(0.1 mmol) in 5 mL of toluene. The red to dark-red solution was
stirred overnight at ambient temperature. After filtration and removal
of solvent, the residue was dissolved in a small amount of toluene and
layered up with hexanes. After a few days, the red crystals formed and
were collected. The yield is 22 mg (35%).1H NMR (500 MHz, C6D6):
δ 7.92 (d, J = 7.9 Hz, 1H), 7.79−7.65 (m, 6H, o-PPh3), 7.12−7.06 (m,
1H), 7.06−6.99 (m, 3H), 6.99−6.89 (m, 9H, m,p-PPh3), 6.76 (t, J =
7.3 Hz, 1H), 6.52 (t, J = 7.2 Hz, 1H), 3.57 (dd, J = 8.6, 2.2 Hz, 1H,
NCH(R)CH2O), 3.31 (t, J = 8.6 Hz, 1H, NCH(R)CH2O), 2.76 (dd, J
= 13.5, 5.0 Hz, 1H, NiCH2), 2.54−2.42 (m, 1H, NCH(R)CH2O), 2.19
(s, 3H, ArCH3), 2.04−1.96 (m, 1H, CHMe2), 1.38 (dd, J = 14.3, 14.3
Hz, 1H, NiCH2), 0.74 (d, J = 6.7 Hz, 3H, CHMe2), 0.28 (d, J = 6.9 Hz,
3H, CHMe2). 13C NMR (126 MHz, C6D6): δ 163.57, 156.54, 155.37,
145.66, 134.66, 133.79, 133.48, 133.15, 130.34, 129.53−128.08,
C1 symmetric ligands via a coordination assisted cyclo-
metalation process. Both C(sp3)-H and C(sp2)-H bonds may
be activated, showing the diversity it may bring. The absolute
configuration of chiral groups exerts considerable influence on
the overall structural arrangement. The fact that both benzylic
and aryl C−H bonds are activated with similar ease suggests
that the geometries of the intermediates favor activation,
regardless of the energetics of the process. These findings open
a new possibility for a pincer ligand design based on the anilido
imine framework and appear promising for further inves-
tigations. Current efforts aim to establish the general
applicability of the synthetic approach, further probe the origin
of the observed activity by varying substituent groups on both
arms, and explore if the activity can be harnessed for practical
C−H activations. In addition, these nickel complexes are chiral
with easily tunable substituents, and their potential applications
in asymmetric catalysis will be investigated.
EXPERIMENTAL SECTION
■
General Procedures. All air- or moisture-sensitive reactions were
carried out under a dry nitrogen atmosphere, employing standard
Schlenk line and drybox techniques. Tetrahydrofuran, toluene, and
diethyl ether were dried over potassium hydroxide and distilled over
Na/benzophenone prior to use. Dimethylformamide (DMF) was
distilled over CaH2. Deuterated solvents were purchased from
Cambridge Isotope Laboratory, dried over sodium or calcium hydride,
degassed, and distilled by vacuum transfer. trans-NiCl(Ph)(PPh3)2 was
prepared according to a literature procedure.43
126.92, 124.68, 120.53, 119.87, 112.90, 111.24, 69.81 (NCH(R)-
2
CH2O), 67.73 (NCH(R)CH2O), 33.44 (CHMe2), 26.43 (d, JC−P
=
25.3 Hz, NiCH2), 21.05 (ArCH3), 18.56 (CHMe2), 15.40 (CHMe2).
31P NMR (202 MHz, C6D6): δ 32.04. UV−vis (DCM, ε M−1 cm−1):
319 (8010), 437 (3310), 514sh (622), 665 (220).
Synthesis of 3b. The procedure is the same as 3a while ligand 2b
(32.2 mg, 0.1 mmol) was used. The yield is 15.2 mg (24%). Elemental
analysis: Calc. C39H39N2NiOP, C, 73.03; H, 6.13; N, 4.37. Found: C,
All 1H and 13C NMR spectra were recorded on a Bruker AVANCE-
500 NMR spectrometer and referenced to TMS or the residue peaks
in CDCl3 or C6D6. 31P NMR was referenced to P(OEt)3 at 137 ppm.
The elemental analysis was performed by Midwest Microlab,
Indianapolis, IN. UV−vis data were obtained on Jasco-720 or Cary
17 spectrophotometers. Circular Dichroism (CD) data were recorded
using OLIS DCM 17 CD spectropolarimeter. GC-MS analyses were
performed on an HP 5890 GC/HP 5971/B MSD system with electron
impact ionization (70 eV).
1
72.74; H, 5.99; N, 4.26. H NMR (500 MHz, C6D6): δ 7.93 (dd, J =
7.9, 1.6 Hz, 1H), 7.75−7.66 (m, 6H, o-PPh3), 7.16 (s, 2H), 7.12−7.01
(m, 3H), 6.94 (m, 9H, m,p-PPh3), 6.77 (t, J = 7.3 Hz, 1H), 6.53 (ddd, J
= 7.9, 6.7, 1.2 Hz, 1H), 3.58 (dd, J = 8.7, 2.9 Hz, 1H, NCH(R)CH2O),
3.27 (t, J = 8.7 Hz, 1H, NCH(R)CH2O), 2.66 (dd, J = 13.8, 5.5 Hz,
1H, NiCH2), 2.60 (dt, J = 8.6, 2.9 Hz, 1H, NCH(R)CH2O), 2.20 (s,
3H, ArCH3), 1.96−1.90 (m, 1H, CHCH3(Et)), 1.45 (dd, J = 14.2, 14.2
Hz, 1H, NiCH2), 0.70 (d, J = 6.8 Hz, 1H, CHCH3(Et)), 0.49 (t, J =
6.0 Hz, 3H, CHCH3(CH2CH3)), 0.47−0.40 (m, 2H,
CHCH3(CH2CH3)). 13C NMR (126 MHz, C6D6): δ 163.62, 156.48,
155.31, 145.55, 145.50, 134.69, 134.61, 133.58, 133.26, 133.11, 130.37,
130.23, 126.91, 124.83, 120.47, 119.86, 112.88, 111.10, 69.23
(NCH(R)CH2O), 67.63 (NCH(R)CH2O), 40.47 (CHCH3(Et)),
Ligand 2e by Cu-Catalyzed Amination.21 An oven-dried
Schlenk flask was charged with a magnetic stir bar, CuI (10 mg,
0.05 mmol, 5 mol %) and K3PO4 (2 mmol, 425 mg), then evacuated
and backfilled with nitrogen three times. Under a counter-flow of
nitrogen, R-(+)-α-methylbenzylamine (181 mg, 1.5 mol), oxazoline
derivative (302 mg, 1 mmol), and DMF (0.5 mL) were added by
syringe. Finally, 2-isobutyrylcyclohexanone (34 mg, 0.2 mmol, 20 mol
%) was added via syringe, the flask was sealed, and the mixture was
heated at the 110 °C for 24 h. Upon completion of the reaction, the
mixture was allowed to cool to room temperature, diluted with ethyl
acetate, and passed through a fritted glass filter to remove the
inorganic salts. The solvent was removed with the aid of rotary
evaporator. The residue was purified by column chromatography on
silica gel, and the product was dried under vacuum for at least 1 h.
Colorless crystals of the product could be obtained in ethyl acetate by
slow evaporation. The typical yield was ∼40%. The identity of 2e was
2
26.38 (d, JC−P= 25.3 Hz, NiCH2), 26.29 (CHCH3(CH2CH3)),
21.08 (ArCH3 ), 12. 41 (CHCH3 (CH2 CH3 )), 11.85
(CHCH3(CH2CH3)). 31P NMR (202 MHz, C6D6): δ 33.02. UV−vis
(DCM, ε M−1 cm−1): 441 (3490), 510sh (390), 647 (210).
Synthesis of 3c. The procedure is the same as 3a. 2c (32.2 mg, 0.1
mmol) was used. Yield: ∼15%. 1H NMR (500 MHz, C6D6): δ 7.99 (d,
J = 8.5 Hz, 1H), 7.74 (dd, J = 9.4, 7.7 Hz, 6H, o-PPh3), 7.12−7.07 (m,
3H), 6.99−6.89 (m, 10H, m,p-PPh3+ArH), 6.79 (t, J = 7.3 Hz, 1H),
6.56 (ddd, J = 7.9, 5.5, 2.3 Hz, 1H), 3.50 (dd, J = 8.2, 2.7 Hz, 1H,
NCH(R)CH2O), 3.23 (t, J = 8.1 Hz, 1H, NCH(R)CH2O), 2.52−2.44
(m, 2H, NCH(R)CH2O + NiCH2), 2.21 (s, 3H, ArCH3), 1.89−1.80
(m, 1H, CH2CHMe2), 1.64 (dd, J = 13.8, 13.8 Hz, 1H, NiCH2), 1.18
(ddd, J = 13.8, 11.1, 4.9 Hz, 1H, CH2CHMe2), 1.03−0.94 (m, 1H,
CH2CHMe2), 0.61 (d, J = 6.6 Hz, 3H, CH2CHMe2), 0.28 (d, J = 6.6
Hz, 3H, CH2CHMe2). 13C NMR (126 MHz, C6D6): δ 163.96, 156.19,
155.27, 145.13, 134.92, 134.83, 133.37, 133.09, 132.99, 130.40, 130.06,
128.89, 128.79, 127.01, 124.94, 120.35, 120.01, 112.91, 110.95, 71.35
(NCH(R)CH2O), 63.89 (NCH(R)CH2O), 45.83 (CH2CHMe2),
compared with literature20a and confirmed by H NMR and GC-MS.
1
Lithium Salt L2dLi(THF)2. Ligand 2d (1 mmol) was dissolved in 10
mL of THF and cooled to −78 °C. To it was added an n-butyl lithium
solution in hexane (0.625 mL, 1.6 M) at low temperature. The
solution changed from colorless to dark-green and then orange. It was
allowed to stir for 2 h at −78 °C and then warm to room temperature
with stirring. THF was then removed under vacuum, and the yellow
residue was washed with hexane. Light yellow needle-like crystals
could be obtained by diffusion of hexane into a THF solution of the
lithium salt. The yield is 0.39 g (80%). 1H NMR (500 MHz, C6D6): δ
8.40 (d, 1 H, J = 8.41 Hz), 7.21 (m, 3 H), 7.13 (m, 1 H), 6.99 (m, 5
H), 6.58 (d, 1 H, J = 6.55 Hz), 6.48 (t, 1 H, J = 6.47 Hz), 4.78 (t, 1 H,
J = 4.78 Hz), 4.16 (t, 1 H, J = 4.15 Hz), 3.72 (t, 1 H, J = 3.70 Hz), 3.11
(m, 8 H, THF), 2.36 (s, 3 H), 2.31 (s, 3 H), 1.13 (m, 8 H, THF) ppm.
13C NMR (126 MHz, C6D6): δ 170.32, 158.76, 153.66, 144.70, 133.88,
2
26.61 (d, JC−P = 26.5 Hz, NiCH2), 25.69 (CH2CHMe2), 24.08
(ArCH3), 22.02 (CH2CHMe2), 21.38 (CH2CHMe2). 31P NMR (202
MHz, C6D6): δ 34.20. UV−vis (DCM, ε M−1 cm−1): 320 (8620), 440
(3620), 516sh (673), 663 (240). The complex 3c′ was prepared
analogously, starting from ligand 2c′ with a (S)-iBu substituent at the
4-oxazoline position. The 1H NMR data are virtually identical with 3c.
Synthesis of 3d. The procedure is the same as 3a. 2d (34.2 mg,
0.1 mmol) was used. Yield: ∼20%. 1H NMR (500 MHz, C6D6): δ 7.94
I
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(d, J = 8.5 Hz, 1H), 7.53 (m, 6H, o-PPh3), 7.20−7.12 (m, 4H), 7.01−
6.96 (m, 5H), 6.95 (m, 3H), 6.86 (td, J = 7.9, 5.0 Hz, 6H), 6.76 (t, J =
7.5 Hz, 1H), 6.55 (m, 1H), 3.58 (m, 1H, NCH(R)CH2O), 3.50 (m,
2H, NCH(R)CH2O), 2.48 (dd, J = 13.8, 5.5 Hz, 1H, NiCH2), 2.29 (s,
3H, ArCH3), 1.56 (m, 1H, NiCH2). 13C NMR (126 MHz, C6D6): δ
165.71, 156.11, 155.66, 145.13, 145.07, 143.01, 134.88, 134.79, 133.33,
133.28, 133.02, 130.29, 129.32, 126.97, 126.80, 124.90, 124.88, 120.47,
120.22, 120.21, 113.05, 110.55, 74.99 (NCH(R)CH2O), 68.50
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Financial support from ND EPSCoR through NSF Grant EPS-
0814442 and the University of North Dakota is greatly
appreciated. We are grateful to Dr. L. Stahl and Dr. Q. Chu
for help with X-ray crystallography, Dr. K. Thomasson for a CD
measurement, and Dr. P. Binda for initial investigations.
Generous support from the NSF MRI CHE-0922366 (X-ray
diffractometer), Minnesota Supercomputing Institute to V.N.,
and University of Minnesota UROP Grant to J.R.S. is greatly
appreciated.
2
(NCH(R)CH2O), 26.52 (d, JC−P = 25.2 Hz, NiCH2), 21.43
(ArCH3).31P NMR (202 MHz, C6D6): δ 34.27. UV−vis (DCM, ε
M−1 cm−1): 441 (3590), 515sh (550), 656 (200).
Synthesis of 3e. The procedure is the same as 3a. 2e (0.1 mmol)
was used. Yield: ∼10%. 1H NMR (500 MHz, C6D6) δ 8.16 (t, J = 8.4
Hz, 2H), 7.82−7.56 (m, 24H), 7.20−6.96 (m, 40H), 6.48 (t, J = 7.2
Hz, 1H), 6.44 (t, J = 6.7 Hz, 1H), 4.99 (q, J = 5.8 Hz, 1H,
NCH(Ar)CH3), 4.85 (t, 1H, NCH(R)CH2O), 3.73 (dd, J = 7.5, 2.5
Hz, 1H, NCH(R)CH2O), 3.60 (t, J = 8.0 Hz, 1H, NCH(R)CH2O),
2.38 (t, J = 10.6 Hz, 1H, CH2CHMe2), 1.63 (d, J = 6.1 Hz, 3H,
NCH(Ar)CH3), 1.45−1.39 (m, 1H, CH2CHMe2), 1.00 (m, 1H,
CH2CHMe2), 0.97 (d, J = 6.4 Hz, 3H, CH2CHMe2), 0.65 (d, J = 6.5
Hz, 3H, CH2CHMe2). 31P NMR (202 MHz, C6D6): δ 25.35. 13C
NMR was not obtained because of its low solubility.
X-ray Crystallography. Single crystal X-ray diffraction of
compounds 2d, 3d, and 4 were collected on a Bruker Apex
diffractometer equipped with an Oxford Cryosystems 700 Series
Cryostream cooler, and that of compounds 3a−c and 3e were
collected on a Rigaku RAPID II diffractometer equipped with XStream
Cryosystem. Mo Kα radiation (λ = 0.71073 Å) was used on both
instruments. X-ray crystal data, data collection parameters, and
refinement parameters are summarized in Table 1 and more
crystallographic details can be found in the Supporting Information.
DFT-PCM and TDDFT-PCM Calculations. The initial geometry
of complex 3c was taken from the X-ray data and optimized at the
DFT level, using a hybrid X3LYP exchange-correlation functional. We
choose this exchange-correlation functional after 12 exchange-
correlation functional were compared for CD intensities calculations
on chiral model nickel(II) complexes (full comparison on all tested
exchange-correlation functional will be published elsewhere). Equili-
brium geometries were confirmed by frequency calculations and
specifically by the absence of the imaginary frequencies. Solvation
effects were modeled using the polarized continuum model (PCM)
approach.44 DCM was used as the solvent in all calculations to match
with experimental data. All single-point DFT-PCM and TDDFT-PCM
calculations were conducted using a X3LYP functional.45 The first 70
states were considered in all PCM-TDDFT calculations to cover UV
and visible range of the spectrum. In all cases, 6-31G(d) basis set was
used for all atoms.46 All calculations were performed using Gaussian03
or 09 software. Molecular orbital analysis was conducted using the
QMForge program.47
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ASSOCIATED CONTENT
■
S
* Supporting Information
Selected NMR, UV−vis, and CD spectra of 3a−d, crystal
packing of 3d, molecular structure of 4, and computational
details in pdf, and crystal data for compound 2d, 3a−e, and 4 in
cif. This material is available free of charge via the Internet at
http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*Tel: +1-701-777-2241 (G.D.), +1-218-726-6729 (V.N.N.).
Fax: +1-701-777-2331 (G.D.). E-mail: gdu@chem.und.edu
(G.D.), vnemykin@d.umn.edu (V.N.N.).
Present Address
§School of Chemistry and Chemical Engineering, University of
Jinan, Jinan 250022, P.R. China.
J
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