of 143
1
Electronic
s
tructures of nickel(II)
-
bis(indanyloxazoline)
-
dihalide catalysts: Understanding
ligand field contributions
that
promote
C(sp
2
)
C(sp
3
)
cross
-
coupling
Brendon J. McNicholas
†,
1
, Z. Jaron Tong
†,
2
, Daniel Bím
1
, Raymond F.
Turro
2
, Nathanael P.
Kazmierczak
1
, Jakub Chalupsk
ý
3
, Sara
h
E
. Reisman
2
, and Ryan G. Hadt
1,
*
1
Division of Chemistry and Chemical Engineering, Arthur Amos Noyes Laboratory of Chemical
Physics, California Institute of Technology, Pasadena,
California 91125, United States
2
Division of Chemistry
and Chemical Engineering
, The Warren and Katherine Schlinger
Laboratory for Chemistry and Chemical Engineering,
California Institute of Technology, Pasadena,
California 91125, United States
3
J. Heyro
vský Institute of Physical Chemistry, The Czech Academy of Sciences, Dolejškova 3,
Prague 8, Czech Republic
4
Institute of Organic Chemistry and Biochemistry, Academy of Sciences of the Czech Republic,
Flemingovo náměstí 2, 166 10 Prague 6, Czech
Republic
Co
-
first author
*Corresponding Author:
rghadt@caltech.edu
Table of Contents
Page Number
S1. General
2
S2. NMR Spectra
5
S
3
. UV
-
vis
-
NIR Spectra
7
S
4
.
Circular Dichroism and
Magnetic Circular Dichroism
Spectra
19
S
5
.
Electrochemistry
3
3
S
6
. Spectroelectrochemistry Spectra
5
3
S
7
. X
-
Ray Crystallographic Data
6
7
S
8
. DFT/CASSCF+NEVPT2 Inputs
and Results
7
0
S
9
. References
1
4
1
2
S1. General
Anhydrous zinc(II) chloride (Millipore Sigma) and electrochemical grade
tetrabutylammonium
hexafluorophosphate (Millipore Sigma)
were used as received.
Anhydrous
N,N
-
dimethylacetamide (Millipore Sigma)
and
d
2
-
dichloromethane (Cambridge Isotope Laboratories,
Inc.) were stored
in a nitrogen
-
filled glove box over
activated 3 Å
mo
lecular sieves
.
Acetonitrile
and dichloromethane were taken from degassed, dry solvent systems and stored over activated 3
Å molecular sieves in a nitrogen
-
filled glove box
.
Nickel(II)
di
chloride (3a
R
,3a'
R
,8a
S
,8a'
S
)
-
2,2'
-
(cyclopropane
-
1,1
-
diyl)bis(3a,8a
-
di
hydro
-
8
H
-
indeno[1,2
-
d
]oxazole) [
Ni(IB)Cl
2
] and nickel(II)
di
bromide (3a
R
,3a'
R
,8a
S
,8a'
S
)
-
2,2'
-
(cyclopropane
-
1,1
-
diyl)bis(3a,8a
-
dihydro
-
8
H
-
indeno[1,2
-
d
]oxazole) [
Ni(IB)Br
2
] were synthesized according to literature precedent
and doubly
recrystallized from a m
ixture of DCM/hexanes
.
1,2
All solvents were dried and stored over activated
3 Å molecular sieves in a
nitrogen
-
filled glove box. All NMR spectra were collected on a Varian
400 MHz or Bruker 400 MHz spectrometer (
in ppm, m: multiplet).
13
C NMR spectra were
1
H
decoupled.
UV
-
vis
-
NIR spectra were acquired using a Varian Cary 500 spectrophotometer, while
ele
ctronic
CD and MCD spectra were acquired using
a Jasco J1700 CD
spectrometer
and
1.4 T permanent
magnet. MCD spectra were generated by taking the difference between
spectra
with field direction
parallel and
field direction
antiparallel to the direction
of
light propagation
.
Vibrational circular
dichroism data (VCD) were collected using a Chiral
IR
-
2X (BioTools, Inc.) spectrometer. Spectra
were background
-
corrected for both cell and solvent signals. All spectra were collected in a 0.1
mm path length calcium f
luoride cell.
Variable temperature UV
-
vis
-
NIR spectra were acquired
using a Varian Cary 50 spectrophotometer equipped with a USP
-
203 series cryostat (UNISOKU
Co.)
cooled with
liquid nitrogen. Samples were equilibrated at each temperature for five minutes
p
rior to acquisition. Spectroelectrochemical measurements
were performed in
a nitrogen
-
filled
glovebox with a quartz spectroelectrochemical cell with a 0.17
c
m path length from Pine Research
Instrumentation (AKSTCKIT3), a gold honeycomb electrode (Pine Inst
ruments), and a platinum
wire counter electrode
. Measurements were recorded using an Analytical Instrument Systems, Inc.
DT2000 deuterium
-
tungsten UV
-
vis
-
NIR light source coupled to Stellarnet Black Comet UV
-
vis
and DWARF
-
Star NIR spectrometers. All room
-
t
emperature UV
-
vis
-
NIR spectra and low
-
temperature magnetic circular dichroism spectra were deconvolved simultaneously using Gaussian
3
functions with fixed absorption maxima if possible or by letting values float within 10% for each
set of spectra. All decon
volutions were performed in Matlab 2018b.
Homogeneous voltammetry experiments were conducted in a nitrogen
-
filled glove box using
either a 3 mm diameter glassy carbon (CH Instruments) or an 11 μm carbon fiber microdisk
working electrode (Gamry
Instruments). The real surface areas of these electrodes were
determined using averaged values of decay currents from chronoamperometry (macro
-
disk) or
steady
-
state currents from low scan rate linear sweep voltammetry (micro
-
disk) in acetonitrile with
0.1
M TBAPF
6
and ferrocene as the redox
-
active standard (
D
0
= 2.24 x 10
−5
cm
2
s
−1
).
3
Based on
the Cottrell equation (macro
e
lectrode) or the steady
-
state current equation (microelectrode), the
real surface areas were determined to b
e 0.0877 cm
2
and 1.79 x 10
−6
cm
2
, respectively.
4
A 0.01 M
Ag
+/0
non
-
aqueous reference electrode and platinum wire counter electrode were used for all
voltammetry experiments. 0.01 M AgNO
3
/0.1 M TB
APF
6
in MeCN was used as the filling solution
for the non
-
aqueous reference electrode (Bioanalytical Systems, Inc.). Linear fitting to the
intercept of a Nyquist plot obtained from potentiostatic electrochemical impedance spectroscopy
at the open circuit p
otential was used to determine the uncompensated resistance, and
9
5% of this
value was compensated.
All voltammetry was internally referenced to the Fc
+/0
redox couple.
All
experiments
utilized
a
Gamry
Reference
600
or
Biologic
SP
-
200 potentiostat.
Low
-
temperature voltammetry
was conducted using a temperature
-
controlled
50:50 ethylene glycol:water bath. Solutions were degassed with N
2
prior to measurement. A silver
wire pseu
do
-
reference electrode was used in place of Ag
+/0
.
Zinc(II) bis(chloride) (3aR,3a'R,8aS,8a'S)
-
2,2'
-
(cyclopropane
-
1,1
-
diyl)bis(3a,8a
-
dihydro
-
8H
-
indeno[1,2
-
d]oxazole)
[
Zn(IB)(Cl)
2
]:
Previous syntheses of Ni(II) complexes were used as a template.
(3a
R
,3a'
R
,8a
S
,8a'
S
)
-
2,2'
-
(cyclopropane
-
1,1
-
diyl)bis(3a,8a
-
dihydro
-
8
H
-
indeno[1,2
-
d
]
-
oxazole) (0.126 g, 0.354 mmol) and
zinc(II) chloride (0.056 g, 0.412 mmol)
were dissolved in 6.5 mL MeCN/0.1
mL H
2
O in a 20 mL
scintillation vial under N
2
. The solution was heated
at
80
o
C for six hours and then cooled to room
temperature. The solvent was evaporated
in
vacuo
, and the resulting solid was redissolved in
dichloromethane. The solution was filtered th
rough a microfiber pipet filter, and the complex was
precipitated with excess pentane. The solid was filtered, dried, and weighed to yield the product
as a colorless
powder (137.8 mg, 79% yield).
13
C NMR (100 MHz, CD
2
Cl
2
)
d
169.0, 139.5, 138.7,
4
130.1, 128.
3, 127.5, 125.5, 86.1, 74.9, 39.3, 21.8, 19.3;
1
H NMR (400 MHz, CD
2
Cl
2
)
d
7.95 (m,
2H), 7.34 (m, 6H), 5.89 (m, 2H), 5.52 (m, 2H), 3.48 (m, 2H), 3.28 (m, 2H), 1.96 (m, 2H), 1.76 (m,
2H).
Solvent
-
dependent Yields for Reducti
ve Heterocoupling
Procedure: To an oven
-
dried 1 dram vial equipped with a stir bar was added (
E
)
-
1
-
(2
-
bromovinyl)
-
4
-
methoxybenzene (21.3 mg, 0.10 mmol, 1 equiv), Mn
0
powder (7.5 mg, 0.30 mmol, 3 equiv), and
Ni
II
(IB)
Cl
2
(4.9 mg, 0.010 mmol, 0.1 equiv). The vial was then brought into a N
2
-
filled glovebox
where NaI (7.5 mg, 0.050 mmol, 0.5 equiv) was added followed by a careful (as to not disturb Mn
0
powder) addition of 500
μ
L (0.2 M) of the appropriate solvent. 1
-
chloroet
hyl)benzene (13.3
μ
L,
0.10 mmol, 1 equiv) was then added followed by
n
-
dodecane internal standard. The vial was sealed
with a
T
eflon
-
lined cap and further sealed with electrical tape then removed from the glovebox
where it was allowed to stir at 1500 rpm f
or 24 h. Upon completion, the reaction was quenched
with 1 mL of H
2
O and extracted with 1 mL EtOAc then filtered through a MgSO
4
plug where the
filtrate was further diluted with EtOAc and analyzed by GC
-
FID. Procedure was repeated 3x for
each solvent.
MeO
Br
+
Ph
Me
Cl
(IB)NiCl
2
(10 mol%)
NaI (0.5 equiv)
Mn
0
(3 equiv)
solvent (0.2 M)
21ºC, 24h
MeO
Me
Me
Ph
Ph
Me
+
BnBn
1:1 dr
(±)
PDT
5
S2. NMR Spectra
Figure S
1
.
1
H NMR of
Zn
(IB)Cl
2
in
d
2
-
dichloromethane
.
-
3
-
2
-
1
0
1
2
3
4
5
6
7
8
9
1
0
1
1
1
2
1
3
1
4
1
5
1
6
C
h
e
m
i
c
a
l
S
h
i
f
t
(
p
p
m
)
2
.
0
5
2
.
4
8
2
.
0
1
2
.
4
9
2
.
1
2
2
.
1
6
7
.
0
7
2
.
0
0
1
.
7
6
1
.
9
6
3
.
2
8
3
.
4
8
5
.
5
2
5
.
8
9
7
.
3
4
7
.
9
5
C
D
2
C
l
2
A
c
e
t
o
n
e
H
2
O
H
G
r
e
a
s
e
6
Figure S
2
.
13
C NMR of
Zn
(IB)Cl
2
in
d
2
-
dichloromethane
.
-
1
0
0
1
0
2
0
3
0
4
0
5
0
6
0
7
0
8
0
9
0
1
0
0
1
1
0
1
2
0
1
3
0
1
4
0
1
5
0
1
6
0
1
7
0
1
8
0
1
9
0
2
0
0
2
1
0
C
h
e
m
i
c
a
l
S
h
i
f
t
(
p
p
m
)
1
9
.
2
6
2
1
.
7
8
3
9
.
2
6
7
4
.
8
7
8
6
.
0
5
1
2
5
.
5
1
1
2
7
.
5
4
1
2
8
.
3
2
1
3
0
.
0
7
1
3
8
.
6
7
1
3
9
.
4
7
1
6
9
.
0
1
C
D
2
C
l
2
A
c
e
t
o
n
e
A
c
e
t
o
n
e
7
S3. UV
-
vis
-
NIR Spectra
and Expanded Main Text Discussion
Briefly,
MCD spectroscopy relies on the differential absorption of left
-
and right
-
circularly
polarized light in the presence of a longitudinal magnetic field. The general intensity of an MCD
spectrum can be defined by
Equation 1
:
%
!
'
휕푓
(
)
휕퐸
.
+
0
"
+
"
푘푇
5
(
)
6
(1)
where
A
-
terms and
B
-
terms are temperature independent, while
C
-
terms are temperature dependent.
Based on the magnitude of low
-
symmetry distortions from ideal
T
d
, consideration of first
-
order
spin
-
orbit coupling on the Ni
II
center, and acquisition temperature, the observed MCD intensity is
tentatively assigned to
B
-
te
rm intensity due to magnetic field
-
induced mixing of excited states. A
future study utilizing variable
-
temperature variable
-
field MCD will be used to define this, along
with the nature of the ground state zero
-
field splittings in these and other Ni
II
cross
-
coupling
catalysts.
Full Gaussian resolutions obtained from collective fits of the absorption, CD, and MCD
spectra of
Ni
II
(IB)Cl
2
and
Ni
II
(IB)Br
2
in DCM are given in
Figure 1
and summarized in
Table 1
.
Both complexes are pseudo
-
T
d
, a geometry that has
been studied extensively for high
-
spin Ni
II
complexes using ligands spanning a range of ligand field strengths.
5
7
Chloride and bromide are
weak σ donors and π donors, while the bidentate
IB
is a moderate σ donor. Thus,
these four
-
coordinate complexes are expected to follow a weak
-
field excited state ordering. Band assignments
in idealized
T
d
and
C
2v
symmetry are provided in
Figure 1
and
Table 1
.
By group theory, the
3
T
1
(F) ground state (in
T
d
) will split due to low symmetry distortions.
Descending in symmetry, and depending on the specific distortion angles and bond
compression/elongation, the ground state can be
3
A
2
,
3
B
1
, or
3
B
2
in
C
2v
.
8
Based on
multiconfigurational calculations and previous assignments for complexes with similar primary
ligand coordination spheres, the low symmetry distorted ground state is tentatively assigned as
3
B
1
in both complexes. For this assignment, transitions
to
3
B
2
excited states are forbidden by group
theory; transitions to
3
A
1
,
3
A
2
, or
3
B
1
excited states are electric dipole allowed.
5
Based on
calculations and assigned
3
B
1
ground state, the two holes lie in the
d
(x
2
y
2
) and
d
(xz) 3
d
orbitals,
consistent with previous angular overlap calculations on similar complexes.
9
We assign st
ates for
Ni
II
(IB)Cl
2
from low to high energy; these assigned states correlate directly with those in
Ni
II
(IB)Br
2
. Bands 2, 3, and 4 fall in the ~5000
11 000 cm
-
1
region (
Figure 1
). Note band 1 is
8
observed at low energy in vibrational CD (
Main Text
,
Section 2.2
). Bands 2 and 3 are assigned as
the
3
B
1
and
3
A
1
components (in
C
2v
), respectively, of the parent
3
T
1
(F)
3
T
2
(in
T
d
) excited state.
One component from the
3
T
1
(F)
3
T
2
(i.e., the
3
B
2
) does not seem to be observed, consistent with
its
electric dipole forbidden nature and proximity to other transitions. Assignment of these
transitions can be made with more certainty based on calculations (
Section
8
.2
), which suggest
both states should yield negative differential CD and MCD intensity. This is the case for band 3.
Since
3
B
1
3
B
2
is electric dipole forbidden, the higher oscillator strength observed for band 3 is
consistent with calculated values, suppor
ting assignment of this band as the
3
B
1
3
A
1
transition.
Band 4, the most intense transition in this lower energy manifold, is assigned to the
3
A
2
(F) term
(in
T
d
) (
3
B
1
(F)
3
A
2
in
C
2v
), consistent with previously reported spectra for a large number of
high
-
spin, pseudo
-
tetrahedral Ni
II
complexes.
5
A sharp spin
-
forbidden ligand field transition is observed at ~12 100 cm
-
1
(band i) and can
be assigned to a component of the
3
T
1
(F)
1
E,
1
T
2
(in
T
d
) spin
-
flip transitions. This region consists
of additional intensity (band ii), which is tentatively ascribed to additional overlapping spin
-
forbidden components that are broadened
due to low
-
symmetry distortions. This assignment is
also consistent with the additional structure present in this spectral region in the MCD spectrum
(
Figure 1C
).
The higher energy manifold of components (~14 000
~22 000 cm
-
1
) is assigned to the
next orbital triplet,
3
T
1
(F)
3
T
1
(P) (in
T
d
), which is comprised of bands at 14 940 cm
-
1
[band 5
(
3
B
1
(F)
3
B
1
(P))], 18 210 cm
-
1
[band 6 (
3
B
1
(F)
3
B
2
(P))], and 20 130 cm
-
1
[band 7 (
3
B
1
(F)
3
A
2
(P))].
The oscillator strengths for
these bands track with electric dipole selection rules predicted
by theory (i.e., bands 7, 6 > band 5).
The similarity in the signs of vibrational CD (
vide infra
), CD, and MCD signals between
Ni
II
(IB)Cl
2
and
Ni
II
(IB)Br
2
lead us to conclude that they exhibit the same state orderings for
observed spin
-
allowed ligand field transitions. All assignments are also self
-
consistent based on
differential intensity in CD and MCD. For example,
3
B
1
3
A
2
transitions all display posit
ive CD
intensity and negative MCD intensity. For
Ni
II
(IB)Cl
2
, additional bands are present at 12 400 cm
-
1
(band ii, UV
-
vis
-
NIR/CD), 15 890 cm
-
1
(band iii, UV
-
vis
-
NIR/CD/MCD), 19 050 cm
-
1
(band iv,
CD), and 23 740 cm
-
1
(band v, UV
-
vis
-
NIR/CD/MCD). As partia
lly described above, these are
tentatively ascribed to the geometric and spin
-
orbit splitting of spin
-
forbidden transitions that
broaden components (12 400 and 19 050 cm
1
) and small amounts of trimer formation (23 740