Elucidating the Mechanism of Excited State Bond Homolysis in
Nickel–Bipyridine Photoredox Catalysts
David A. Cagan
a
,
Daniel Bím
a
,
Breno Silva
a,b
,
Nathanael P. Kazmierczak
a
,
Brendon J.
McNicholas
a
,
Ryan G. Hadt
a,*
a
Division of Chemistry and Chemical Engineering, Arthur Amos Noyes Laboratory of Chemical
Physics, California Institute of Technology, Pasadena, California 91125, United States
b
Department of Chemistry and Biochemistry, Suffolk University, Boston, Massachusetts 02108,
United States
Abstract
Ni 2,2’–bipyridine (bpy) complexes are commonly employed photoredox catalysts of bond-
forming reactions in organic chemistry. However, the mechanisms by which they operate are
still under investigation. One potential mode of catalysis is via entry into Ni(I)/Ni(III) cycles,
which can be made possible by light-induced, excited state Ni(II)
─
C bond homolysis. Here we
report experimental and computational analyses of a library of Ni(II)-bpy aryl halide complexes,
Ni(
R
bpy)(
R
′
Ph)Cl (R = MeO, t-Bu, H, MeOOC; R
′
= CH
3
, H, OMe, F, CF
3
), to illuminate
the mechanism of excited state bond homolysis. At given excitation wavelengths, photochemical
homolysis rate constants span two orders of magnitude across these structures and correlate
linearly with Hammett parameters of both bpy and aryl ligands, reflecting structural control over
key metal-to-ligand charge transfer (MLCT) and ligand-to-metal charge transfer (LMCT) excited
state potential energy surfaces (PESs). Temperature- and wavelength-dependent investigations
reveal moderate excited state barriers (ΔH
‡
~4 kcal mol
−1
) and a minimum energy excitation
threshold (~55 kcal mol
−1
, 525 nm), respectively. Correlations to electronic structure calculations
further support a mechanism in which repulsive triplet excited state PESs featuring a critical
aryl-to-Ni LMCT lead to bond rupture. Structural control over excited state PESs provides a
rational approach to utilize photonic energy and leverage excited state bond homolysis processes
in synthetic
chemistry.
Graphical Abstract
*
Corresponding Author: rghadt@caltech.edu.
SUPPORTING INFORMATION
The Supporting Information is available free of charge on the ACS Publications website at DOI: . Experimental and computational
methods, UV-vis/photochemical data, X-ray crystallography, NMR spectra, calculated spectra/properties, global analysis modeling,
and additional comments.
HHS Public Access
Author manuscript
J Am Chem Soc
. Author manuscript; available in PMC 2023 March 02.
Published in final edited form as:
J Am Chem Soc
. 2022 April 13; 144(14): 6516–6531. doi:10.1021/jacs.2c01356.
Author Manuscript
Author Manuscript
Author Manuscript
Author Manuscript
1. INTRODUCTION
1.1. Ni(II)
─
bpy Photoredox Catalysis.
Merging thermal catalysis with photochemistry (i.e., photoredox catalysis) has had a
profound influence within organic chemistry, including coupling reactions forging sp
2
─
sp
3
and sp
3
─
sp
3
C
─
C or C
─
X bonds and their applications to medicinal chemistry.
1
-
9
By
leveraging photonic energy to drive key catalytic processes and utilizing earth-abundant
transition metals, photoredox catalysis provides an attractive and sustainable means to
replace precious metal catalysts.
10
-
14
The disparate electron transfer properties of first-
row transition metal catalysts can also provide pathways to new reactive intermediates
and/or excited state avenues that can unlock synthetic possibilities for drug development
and discovery. However, while methodological studies have demonstrated the power of
photoredox approaches in achieving bond-forming reactivity, the mechanisms that underlie
these processes are largely unknown. In response, recent research has taken key steps
towards a deeper mechanistic understanding, utilizing a combination of experiment and
theory.
4
,
5
,
15
-
19
Mechanistic survey of photoredox catalysts requires thorough exploration due to the
numerous possible photophysical pathways present. For example, reactive molecular excited
states can be generated photochemically through photosensitized energy transfer
17
,
20
-
22
or direct excitation.
16
,
23
-
24
In either case, the ensuing transition metal photophysics will
strongly influence the overall catalytic efficacy by directing the photonic energy to specific
pathways, only some of which may be productive to the target reaction. This complexity
motivates highly detailed studies of the excited state potential energy surfaces (PESs) that
govern the important photophysics underlying photoredox catalysis.
Being catalytically active via photosensitization or direct excitation, Ni(II) complexes
featuring the bidentate 2,2
′
–bipyridine (bpy) ligand have received a great deal of attention
due to their many applications in photoredox catalysis. For example, MacMillan et
al. demonstrated a photosensitized, energy transfer mediated approach to enable Ni(II)
─
bpy catalyzed coupling of aryl halides with carboxylic acids.
20
In particular, an Ir(III)
photosensitizer enabled triplet energy transfer to a ground state Ni(II)
─
bpy aryl acetate
complex (formed
in situ
from a Ni(II)
─
bpy aryl halide). Energy transfer from the Ir(III)
complex generates a long-lived triplet excited state of the Ni(II)-bpy complex, which can
Cagan et al.
Page 2
J Am Chem Soc
. Author manuscript; available in PMC 2023 March 02.
Author Manuscript
Author Manuscript
Author Manuscript
Author Manuscript
subsequently undergo reductive elimination of the aryl and acetate ligands, forming a new
C
─
O bond (Figure 1A, top).
25
The mechanism of this photosensitized, energy transfer-
mediated reaction is still being investigated. However,
ab initio
calculations have suggested
a triplet metal-to-ligand charge transfer (
3
MLCT) state may be active for excited state
C
─
O bond formation.
26
Furthermore, chemical oxidation of the ground state Ni(II)
─
bpy
complex also facilitates reductive elimination.
25
These ground and excited state pathways
are consistent with earlier research from the Hillhouse group demonstrating ground state
chemical oxidation of Ni(II) complexes to Ni(III) can trigger reductive elimination and the
formation of new C
─
X bonds.
27
-
28
In addition to energy transfer pathways, photocatalytic
cross-couplings can also be driven by direct excitation and can circumvent the need for
external photosensitizers, which often contain precious metals. For example, irradiation
of the Ni(II)-bpy aryl halide complex in the presence of ancillary ligands enables the
downstream formation of new C
─
O bonds (Figure 1A, bottom).
23
-
24
Previous research has noted that direct excitation of the Ni(II)
─
bpy complex homolytically
cleaves the Ni(II)
─
C(aryl) bond, generating aryl radicals and a formal Ni(I) species. This
reduced Ni species may allow access to catalytically active Ni(I)/Ni(III) cycles.
16
,
23
-
24
While the use of light-induced homolysis to generate reactive Ni species has broad
implications for photoredox catalysis, the precise mechanism of this critical bond rupture
step is not yet well understood and is the main subject of this study.
1.2. Mechanistic Hypotheses for Excited State Ni(II)
─
C Bond Homolysis.
There are two proposed excited state Ni(II)
─
C bond homolysis mechanisms in Ni(II)
─
bpy
aryl halide complexes. Using a combination of transient optical and IR spectroscopies,
Doyle et al. demonstrated that excitation of Ni(II)
─
bpy singlet metal-to-ligand charge
transfer (
1
MLCT) (
λ
pump
= 530 nm) resulted in the formation of triplet Ni(II) ligand field
excited states (
3
(d-d)).
16
Intersystem crossing occurs in ~5-10 ps, and the
3
(d-d) state has
a lifetime of ~4 ns. Subsequent correlation to density functional theory (DFT) calculations
led to the proposal that Ni(II)
─
C homolysis occurs thermally from this photochemically
formed Ni(II)
3
(d-d) state (Figure 1B), which features a tetrahedral coordination geometry
and a weakened Ni(II)
─
C bond. With DFT, the calculated homolytic bond dissociation
energy (BDE) is ~25 kcal mol
−1
. However, no direct experimental evidence was provided to
demonstrate homolysis from the
3
(d-d) state.
Ab initio
multiconfigurational/multireference calculations suggested an alternative
mechanism of excited state Ni(II)
─
C bond homolysis that is also consistent with the
experimental data provided by Doyle et al. (Figure 1B).
29
This approach yielded larger
homolytic BDEs (~90 kcal mol
−1
from the S = 0 geometry, ~70 kcal mol
−1
from S = 1
geometry) than DFT and highlighted a putative one-photon, two-electron process leading to
Ni(II)
─
C bond homolysis. In this mechanism, initial excitation of the S = 0 complex forms
a
1
MLCT state (Ni(II)-to-bpy). From this PES (blue curve in Figure 1B), a ligand-to-metal
(aryl-to-Ni(III)) charge transfer (LMCT) PES can be accessed. Critically, this LMCT results
in the population of the antibonding d(x
2
-y
2
)/C(sp
2
)* orbital (Figure 1B, right), which
reduces the bond order and results in a repulsive triplet PES, leading to homolytic bond
rupture (red curve in Figure 1B, right).
29
Notably, the energy difference between the MLCT/
Cagan et al.
Page 3
J Am Chem Soc
. Author manuscript; available in PMC 2023 March 02.
Author Manuscript
Author Manuscript
Author Manuscript
Author Manuscript
repulsive triplet crossing point (purple circle Figure 1B, right) and the Frank-Condon point
of the MLCT state constitutes the energy barrier (
E
a
) for bond rupture. Thus, it was reasoned
that structural and electronic control over the key MLCT/LMCT PESs, and, consequently,
the barrier for photolysis, will result in variable rates of excited state Ni(II)
─
C bond
homolysis (Figure 1C), but new experimental data are required to further elucidate the
overall mechanism.
Indeed, we demonstrate a direct correlation between experimental rates at given excitation
wavelengths and the energies of both of these excited state PESs. Furthermore, we
provide an experimental measure of the excited state energetic barrier for homolysis
in Ni(II)
─
bpy aryl halide complexes utilized as photoredox catalysts. The homolysis
rate constants are wavelength-dependent, and we have demonstrated a minimum energy
threshold for photochemical activation. Coupled to extensive computational analyses, these
data provide experimental evidence implicating high energy, repulsive aryl-to-Ni LMCT
PESs as being vital to homolytically cleaving the Ni(II)
─
C bond, a critical process
in photocatalytic C
─
X cross coupling catalysis. The dynamics of the excited states
of these Ni(II)-complexes resemble those previously associated with third-row transition
metal catalysts (e.g., Re-complexes),
30
-
39
unveiling underexplored reactivity pathways in
these earth-abundant transition metal catalysts. Beyond fundamental interest, demonstrating
structural and electronic control over the key PESs in photoredox catalysis will, for example,
allow chemists to tune the rates of formation of novel reactive intermediates and guide the
discovery of new photon-driven organic methodological approaches to coupling reactions.
2. RESULTS AND ANALYSIS
2.1. Experimental Studies.
In Sections 2.1.1-2.1.5, we detail the syntheses and spectroscopic/photochemical
characterizations of a matrix of Ni(II)
─
bpy aryl halide complexes (Figure 2). We
demonstrate direct correlations between ligand-based electronic perturbations, observable
MLCT transition energies, and rate constants of excited state bond homolysis. Temperature-
and wavelength-dependent studies provide experimental barriers and energetic thresholds for
excited state Ni(II)
─
C bond homolysis, respectively.
2.1.1. Synthetic Approach.—
To probe the mechanism of excited state Ni(II)
─
C bond
homolysis, we targeted the matrix of Ni(II)(
R
bpy)(
R
′
Ph)Cl complexes (R = CH
3
O, t-Bu, H,
CH
3
OOC; R
′
=
ortho-
CH
3
, H, CH
3
O, F, CF
3
),
1A–5D
, shown in Figure 2. Two primary
synthetic approaches were utilized: 1) oxidative addition and 2) ligand substitution (Scheme
S1). In the former, bis–(1,5–cyclooctadiene) nickel(0) was pre-stirred with a given bpy
ligand; subsequent reaction with the specific aryl halide resulted in the target complex. The
latter method called for either bis(triphenylphosphino)(2–methylphenyl)chloronickel(II) or
the independent preparation of a pre-catalyst complex, Ni(TMEDA)(
R
′
Ph)Cl, R
′
= CH
3
or CF
3
, TMEDA =
N,N,N
′
,N
′
–
tetramethyl ethylenediamine.
40
These TMEDA compounds
afforded a more labile ligand that could be substituted by bpy.
41
The pre-catalyst complexes
themselves were prepared by oxidative addition. Ligand substitution was used in cases
where oxidative addition proved slow, yielded inconsistent results, or would not produce the
Cagan et al.
Page 4
J Am Chem Soc
. Author manuscript; available in PMC 2023 March 02.
Author Manuscript
Author Manuscript
Author Manuscript
Author Manuscript
desired product. Full synthetic details for both previously prepared
16
and novel compounds
are available in Supporting Information Section S1.3.
2.1.2. Steady-state UV-vis Spectroscopy.—
The steady-state UV-vis spectra of the
Ni(II)
─
bpy aryl halide series in tetrahydrofuran (THF) are given in Figure 3A. Molar
absorptivity plots in both THF and toluene are given in Figures S5-S6 and are consistent
with previous spectral assignments of dominantly MLCT intensity across the UV-vis range
(Table S1). The spectral assignments are discussed further in Section 2.2.1 and given
explicitly in Table S10. It is also noted that the MLCT transition energies are generally
solvatochromic, with transition energies being lower in toluene relative to THF (Figures
S5-S6).
Increasing the electron withdrawing effect of the bpy substituents (proceeding down the
columns in Figure 2) generally decreases the energies of the
1
MLCT transitions. The MLCT
λ
max
for spectra in Figure 3A (extrema denoted by dashed lines) correlate linearly with
the Hammett parameter
42
(
σ
p
) for each bpy substituent (blue and red curves in Figure
3B; Δ
ν
max
= 3000 cm
−1
). Similar Hammett relationships have been shown for Cu and Re
bipyridine complexes.
43
-
44
Variation in the aryl ligand (rows in Figure 2) also modulates
λ
max
(Figure 3A) (Δ
ν
max
= 1500 cm
−1
), with increases in electron withdrawing group strength leading to increases
in the energy of the
1
MLCT transitions. While the aryl ligands are all modified at the
ortho-
position with respect to the Ni(II)
─
C bond, the MLCT
λ
max
correlates with the
corresponding
meta
-Hammett parameter (
σ
m
) (orange line in Figure 3B). This demonstrates
a larger contribution of electrostatic and inductive effects over resonance effects upon
variation of the aryl substituent relative to bpy.
45
-
50
Accordingly, this series (
1B–5B
) also
trends with Taft’s field parameter,
σ
F
(Figure S7);
42
for consistency, we use
σ
m
in the
main text of this manuscript. No linear trend was observed when using Taft’s steric (
E
s
)
parameter,
42
as the aryl ligand and its substituent are rotated orthogonal to the plane of the
molecule (Figure S7 and S55).
45
,
50
2.1.3. Photochemical Investigations.—
We first sought to confirm the formation of
aryl radicals upon irradiation (see Supporting Information S1.2 for experimental setup).
Irradiating well-characterized
1B
and analogous
5B
at 390 nm results in distinctive
1
H NMR
peaks assigned to aryl radical products, 2–(
o
–tolyl)tetrahydrofuran, and 2,2
′
–dimethyl-1,1
′
-
biphenyl (Figure S11-12).
23
Using
19
F NMR,
5B
revealed new peaks associated with the
free aryl ligand, concomitant with a loss of aryl peaks in the
1
H NMR after 390 nm
irradiation (Figure S14). As demonstrated previously, homolysis does not occur in the
absence of light; only minor degradation is observed when the complex is heated to 55
°C for 60 minutes (Figure S13), but no radical products are observed.
23
-
24
We also noted
the formation of radical products upon extended irradiation of the analogous precatalyst
complexes, Ni(TMEDA)(
R
′
Ph)Cl (R
′
= CH
3
, CF
3
), implicating ligand-field excited states as
operative for photolysis in the diamine complexes.
51
More detailed discussion regarding this
result relative to the photochemistry of Ni(II)
─
bpy complexes is available in Supporting
Information Section S1.6.
Cagan et al.
Page 5
J Am Chem Soc
. Author manuscript; available in PMC 2023 March 02.
Author Manuscript
Author Manuscript
Author Manuscript
Author Manuscript
Time-dependent absorption spectra were obtained during photolyses (390 nm) in THF of
all complexes
1A–5D
. Photolysis kinetics were monitored at two wavelengths (arrows in
Figure 4 and S19-S20). From these kinetics, the observed rate constant (
k
obs,1
) of excited
state Ni(II)
─
C bond homolysis can be obtained. For a representative compound,
1B
,
concentration dependence studies found negligible change in
k
obs,1
across the absorbance
window tested (Figure S44). The data for series
1A–1D
, which varies only the bpy
substituent, are provided in Figure 4. Note this series of complexes (
1A–1D
) has previously
been investigated using transient absorption spectroscopy (the only difference being EtOOC
versus MeOOC in
1D
here).
16
Clear rate constant changes are observed upon variation
of the bpy ligand
(Figure 4 and Table 1). The largest rate constant was found for
1D
,
which underwent photolysis with an observed rate constant of
k
obs,1
= (17.0 ± 0.7) x 10
−2
min
−1
. Compounds
1A–1C
presented smaller, but noticeable, differences in their decay
rate constants (Table 1). For
1A
, background scattering from precipitation precluded clear
observation of the decay of starting material.
Excited state homolysis of the Ni(II)
─
C bond yielded a new product with absorbance
in the visible region for each compound. Isosbestic points are observed in the photolysis
data for all compounds studied here except
1A
, where light scattering contributes to the
time-dependent spectra. While the general absorption profiles of these new species are
similar, the primary low energy features shift from ~650 nm in
1A
to 805 nm for
1D
(Figure
4). The spectral shift over this series suggests the bpy ligand is present in the new species.
The analogous time-dependent UV-vis spectra for compounds
5A–5D
are given in Figure
S19. There are significant changes in the rate constants of excited state Ni(II)
─
C bond
homolysis across these compounds, with
k
obs,1
varying over an order of magnitude (Figure
S19, Table 1). However, these compounds generally exhibit much smaller rate constants
than the complementary
1A
─
1D
series.
Thus, the electron withdrawing effect of the aryl
ligand also impacts the rate.
The growth of a new species was also observed for these
complexes (~650 nm in
5A
to 805 nm in
5D
), albeit at significantly lower quantities.
To further investigate the dependence of
k
obs,1
on variation of the aryl ligand, analogous
time-dependent UV-vis data were obtained for complexes
2B–4B
(Figure S20). From these
data, the full trend is revealed: increasing the electron withdrawing nature of the aryl ligand
(left to right in the row of Figure 2) again resulted in smaller rate constants across the series.
This behavior is opposite to that observed for variations in the electron withdrawing effect
of the bpy ligand, hence the opposite slopes in Figure 3B. Note also the primary low energy
absorption feature of the new species is not dependent on the aryl ligand (
λ
= 660 nm for
1B–5B
, Figures S41-S43).
We note that in certain regions, the absorption spectrum of the photolysis product overlaps
with that of the starting material, including where decay kinetics are measured (blue
arrows in Figures 4 and S19-S20). Furthermore,
k
obs,1
is in most cases less than
k
obs,2
. To
deconvolute the spectral overlap and rationalize these differences, global kinetics modeling
was carried out (full discussion and details of the kinetic modeling are available in
Supporting Information Section S1.9). Good agreement is seen between the observed rate
constants and those obtained from the global fits, and the kinetic trends across the matrix
of compounds are preserved (Figure S40). Comparison between
k
obs,1
and rate constants
Cagan et al.
Page 6
J Am Chem Soc
. Author manuscript; available in PMC 2023 March 02.
Author Manuscript
Author Manuscript
Author Manuscript
Author Manuscript
from global fitting (
k
p
) are also given in Table 1. Additionally, using the method developed
by Gescheidt et al.,
52
we calculated the quantum yields for each complex to account for
differential absorbance at the 390 nm excitation wavelength (see Supporting Information
Section S1.11 for complete details). We found good linear agreement between the observed
photolysis rates and the calculated quantum yields (R
2
= 0.9730, Figure S53), further
supporting our kinetic analysis.
Furthermore, the rate constants of excited state Ni(II)
─
C bond homolysis correlate linearly
with specific Hammett parameters of the bpy and aryl ligands. As shown in Figure 5, linear
relationships are observed upon plotting log(
k
obs,1
/
k
obs,1
(H)) versus
σ
p
or
σ
m
(for
R
bpy or
R
′
Ph, respectively) (R
2
≥ 0.95) (
ρ
= ~1.4 for
R
bpy and
ρ
= ~−2.6 for
R
′
Ph).
Thus, the rate of
excited state Ni(II)
─
C homolysis is sensitive to electronic structure perturbations from both
the bpy and aryl ligands.
Electronically stabilizing the Ni(II)-to-bpy MLCT transition energies by increasing the bpy-
based electron withdrawing effect accelerated the rate of photolysis. Conversely, increasing
the aryl-based electron withdrawing effect resulted in increased MLCT transition energies
and slower rates of photolysis (see the oppositely signed slopes in Figure 3B). These data
reflect competing effects on the excited state PESs involved in homolysis and are further
described in the computational sections below.
2.1.4. Preliminary Investigations of the Photochemically Generated Species.
—
The immediate product of Ni(II)
─
C bond homolysis has been proposed to be a three–
coordinate Ni(I)
R
bpy(Cl) complex. A recent study by Bird et al. on the related Ni(I)
(
t-Bu
bpy)Br reported its UV-vis spectrum as generated by pulse radiolysis or electrolysis.
53
For direct comparison, we synthesized Ni(
t-Bu
bpy)(CH
3
Ph)Br,
1B–Br
, and subjected it to
the same photolysis conditions as above.
We found a roughly three-fold enhancement in the rate constant of photolysis for
1B–Br
(
k
obs,1
= (6.9 ± 0.4) x 10
−2
min
−1
) relative to
1B
(
k
obs,1
= (2.5 ± 0.2) x 10
−2
min
−1
)
and a change in the absorption spectrum of the product species (Figure 6A). The primary
low energy absorption feature of the product appears at higher energy when produced
from the
bromo
-complex (653 nm) versus the
chloro
-complex (660 nm). Thus, there is a
halide-dependence on the absorption spectrum of the product compound. A comparison
between the long-time spectra of the photoproducts from compounds
1B–Br
,
1B
,
3B
,
1C
,
and
1D
is given in Figure 6B, illustrating a change in peak maxima when changing the bpy
or halide ligands, but not the aryl ligand.
We also followed the photolysis of
1B–Br
in dimethylformamide (DMF), the same
solvent used by Bird et al. (Figure S51).
53
We first note the steady state UV-vis data are
solvatochromic, with the main MLCT bands being lower in energy in THF relative to DMF
(Figure 6C, blue versus orange lines, respectively). The homolysis product UV-vis spectra
are also solvatochromic (Figure 6C, dashed lines). In particular, DMF solutions exhibit
the same characteristic UV–vis features for the three-coordinate monomeric species (430,
620, and 860 nm) as observed by Bird et al. (Figure 6C, orange dashed line). We further
note that the rate constant of excited state Ni(II)
─
C bond homolysis is smaller in DMF
Cagan et al.
Page 7
J Am Chem Soc
. Author manuscript; available in PMC 2023 March 02.
Author Manuscript
Author Manuscript
Author Manuscript
Author Manuscript
relative to THF. We tentatively ascribe these differences in rate constants to changes in the
MLCT energies and overall excited state PESs (
vide infra
). These interesting solvent effects
on the excited state PESs and, thus, rates of homolysis are currently under more detailed
investigation.
Monomeric Ni(I)(
t-Bu
bpy)X, X = Cl, Br has been shown to be active towards the oxidative
addition of aryl iodides, while the dimeric form, [Ni(
tBu
bpy)X]
2
, is unreactive with the
same.
53
-
54
We irradiated a sample of
1B–Br
in THF, generating the photoproduct. Addition
of 2-
iodo
toluene to this solution revealed rapid reactivity and complete removal of the
characteristic absorption feature at 653 nm (Figure S52).
Furthermore, the absorption spectrum of the dimeric species shows peaks only in the UV-
region, further implicating the monomeric form as the photoproduct.
54
Therefore, we postulate that the new species formed here upon excited state Ni(II)
─
C
homolysis are three-coordinate Ni(I)(
R
bpy)X complexes (R = MeO, t-Bu, H, and MeOOC,
X = Cl or Br), as they have been shown by steady-state UV-vis spectroscopy to 1) contain
the bpy ligand, 2) not contain the aryl ligand, 3) contain the halide, and in the case of
1B–Br
, 4) exhibit the same absorption profile as Ni(I)(
t-Bu
bpy)Br, and 5) exhibit oxidative
addition reactivity with iodotoluene. A detailed comparative study of the reactivities and
further spectroscopic characterizations of these species is currently underway.
2.1.5. Further Examination of the Mechanism of Excited State Bond
Homolysis.—
To further investigate the mechanism of excited state Ni(II)
─
C bond
homolysis, we carried out temperature-dependent photolyses of
1B
and
1B–Br
. Among
the matrix of complexes studied here, these two are most often utilized for synthetic
applications, giving their analyses direct implications for photoredox catalysis.
24
Eyring plots of temperature dependent rate constants for these complexes are given in Figure
7. From these data, the enthalpy and entropy of activation for the excited state Ni(II)
─
C
bond homolysis in
1B
are ΔH
‡
= 4.4 ± 0.6 kcal mol
−1
and ΔS
‡
= −45.3 ± 1.8 cal mol
−1
K
−1
, with ΔG
‡
(298 K) = 17.9 ± 0.8 kcal mol
−1
. Similar analysis of
1B–Br
gives ΔH
‡
=
2.1 ± 0.1 kcal mol
−1
and ΔS
‡
= −49.3 ± 0.4 cal mol
−1
K
−1
, with ΔG
‡
(298 K) = 16.8 ± 0.2
kcal mol
−1
. At high temperatures (328 K), thermal decay of the starting material occurs for
1B–Br
, resulting in a downturn in the temperature-dependent rate constants (dashed yellow
line, Figure 7). Because of this, the linear fit utilized a room temperature point. As expected,
the barrier for excited state Ni(II)
─
C bond homolysis is lower in
1B–Br
than
1B
, consistent
with its larger rate constant.
In addition to being dependent on temperature, the rate constant of excited state bond
homolysis in
1B
is also highly dependent on the excitation wavelength (Figure 8A). Varying
incident wavelengths (390, 427, 456, and 525 nm, Figure S21) revealed a minimum energy
threshold for excited state Ni(II)
─
C bond homolysis of ~55 kcal mol
−1
(525 nm, 19,050
cm
−1
) in
1B
; below this incident energy, no homolysis is observed (Figure S22). Previous
optical transient absorption measurements on
1B
were carried out using
λ
pump
= 530 nm.
16
Laser excitation at this wavelength results in the formation of a Ni(II)-based triplet ligand
Cagan et al.
Page 8
J Am Chem Soc
. Author manuscript; available in PMC 2023 March 02.
Author Manuscript
Author Manuscript
Author Manuscript
Author Manuscript
field excited state, from which homolysis was proposed on the basis of DFT calculations.
Notably, however, very limited photolysis occurs here using a 525 nm excitation light
source.
These results demonstrate that the lower-energy ligand field state is not responsible
for excited state Ni(II)
─
C bond homolysis, but rather indicate the involvement of higher-
energy excited states.
To search for general trends across compounds considered here, we also conducted
wavelength dependent studies on
1D
and
5D
(Figure 8B-C). Altogether, these complexes
span a wide range of photolysis rate constants, have varying MLCT transition energies,
and feature electronic structure differences provided by the bpy and aryl ligands. In each
case, a clear wavelength dependence was observed, and high energy incident light was
required for homolysis. No appreciable decay was observed using low energy light (Figures
S23-24), again implicating high energy excited states in the mechanism of light-induced
homolysis. We also evaluated the wavelength-dependency of quantum yields for each
complex, accounting for variable LED power and complex absorbance at each wavelength,
and found their behavior mirrors the photolysis kinetics (Figure S25). In summary,
through experimental analyses of a matrix of Ni(II)
─
bpy aryl halide complexes, we have
demonstrated the following:
1.
a dependence between the MLCT
λ
max
and the Hammett parameters of the bpy
and aryl substituents over the
1A–5D
series (Figure 3B),
2.
linear correlations between the Hammett parameters of the bpy and aryl
substituents and the rate constants of excited state Ni(II)
─
C bond homolysis
over the
1A–5D
series, interestingly with oppositely signed slopes (Figure 5),
3.
the barrier for excited state bond homolysis is moderate (e.g., ΔH
‡
= 4.4 ± 0.6
kcal mol
−1
in
1B
using 390 nm excitation; Figure 7), and
4.
excited state bond homolysis is distinctly wavelength dependent (Figure 8); e.g.,
in
1B
, requiring a minimum of ~55 kcal mol
−1
(525 nm, 19,050 cm
−1
).
These experimental observations are discussed below in the context of computational
studies, which further aid in the elucidation of the mechanism of excited state Ni(II)
─
C
bond homolysis.
2.2. Computational Studies.
In the following computational Sections 2.2.1-2.2.3, we first compare the ground and excited
state properties of
1A–5D
computed at different levels of theory. We discuss the possible
photoactivation pathways that are accessible in the energy range of the external light sources
used in the photolysis experiments, as well as those pathways that are consistent with
the experimental barrier. Notably, the incident light energy required for photolysis (as
determined from wavelength dependent kinetic experiments) is substantially greater than
the energy of the
3
(d-d) bands in
1A–5D
, and the calculated barriers for homolysis from
these states are significantly larger than experiment. These points indicate thermally-driven
excited state Ni(II)
─
C bond homolysis from a spin-forbidden ligand field state is not the
operative mechanism. Instead, we focus on the possible photolysis pathways that exploit
triplet excited state LMCT-based repulsive PESs. We propose a working mechanism that
Cagan et al.
Page 9
J Am Chem Soc
. Author manuscript; available in PMC 2023 March 02.
Author Manuscript
Author Manuscript
Author Manuscript
Author Manuscript
can ultimately be described as
1
MLCT [Ni d
→
bpy
π
*(2)] excitation followed by surface
hopping to a repulsive
3
LMCT (aryl-to-Ni) PES (
3
MLCT+LMCT). This mechanism is
in agreement with the experimentally derived reaction rates and thermodynamic barriers
determined herein.
2.2.1. DFT versus CASSCF/QD-NEVPT2 Ground and Excited States.—
To
evaluate the geometric and electronic structures of
1A–5D
, we compared their ground
and excited state properties calculated with either DFT/TD-DFT or
ab initio
complete
active space self-consistent field theory with the quasidegenerate
N
-electron valence state
perturbation theory correction (CASSCF/QD-NEVPT2)
55
-
58
; full computational details are
available in Supporting Information Section S2.1.
With DFT (B3LYP)
59
-
61
, all Ni(II) complexes are predicted to have low-spin, singlet (S
= 0) ground states with square-planar geometries (note the x-axis is directed along the
Ni
─
halide bond and the y-axis is along the Ni
─
aryl bond). The fully optimized triplet
(S = 1) ligand field excited states are in all cases ~10 kcal mol
−1
higher in energy with
pseudo-tetrahedral geometries. The valence electronic configuration of the d
8
ground state is
[d(xy)]
2
[d(yz)]
2
[d(xz)]
2
[d(z
2
)]
2
, with three unoccupied bpy-based
π
* orbitals and a highly
covalent antibonding [d(x
2
-y
2
)/C(sp
2
)*]
0
orbital (see Figure S59 for an example molecular
orbital diagram for
1D
). The orbital energies are modulated by the bpy substituents;
increasing the electron withdrawing effect of the bpy ligand (columns in Figure 2) decreases
the energies of the bpy
π
* orbitals, reducing the Ni(II)-to-bpy MLCT energy (Figure S60),
consistent with the red-shifted experimental
λ
max
features in Figure 3. On the other hand,
the HOMO and the bpy
π
* orbital energies remain essentially unchanged when modulating
the aryl substituent (rows in Figure 2), contrasting with the blue shift in Figure 3. The
correct behavior can be recovered at the TD-DFT level, which accounts for orbital mixing
in the excited states (Figure S62). Interestingly, the changes in orbital energies are not
translated into changes in the covalencies of the ground states, which remain ~51–54 % Ni d
and ~11-13 % bpy character for
1A–5D
(Table S9).
The calculated TD-DFT absorption spectra agree well with the experimental UV-vis data
(see overlaid spectra in Figure S62) and also demonstrate a similar linear relationship
with the substituent-specific Hammett
σ
parameters (Figure 9, top). The broad feature
at longer wavelengths (~400–600 nm, ~25,000–16,500 cm
−1
) encompasses all the ‘low-
energy’
1
MLCT transitions [Ni d
→
bpy
π
*(1)], with [d(yz)
→
bpy
π
*(1)] having the
highest calculated oscillator strength. The shoulder at ~350–370 nm (~28,500–27,000 cm
−1
)
apparent in most of the experimental UV-vis spectra of
1A–5D
can be similarly assigned
to a [d(yz)
→
bpy
π
*(2)] transition; other ‘high-energy’
1
MLCT [Ni d
→
bpy
π
*(2)]
transitions are predicted to fall in the ~300–450 nm (~33,000–22,000 cm
−1
) range. The
1
(d-d) transitions are calculated to be comparable in energy to the ‘low-energy’
1
MLCT
bands (~400–500 nm, 25,000–20,000 cm
−1
) and are not visible in the experimental UV-vis
spectra of
1A–5D
. This assignment is also consistent with the energy of the observable
1
(d-d) band [d(yz)
→
d(x
2
-y
2
)/C(sp
2
)*] in the Ni(II)(TMEDA)(CH
3
Ph)Cl complex that is
detected in the visible range (found at ~470 nm, 21,280 cm
−1
; calculated at 533 nm, 18,760
cm
−1
); the less intense bands observed near ~635 nm (15,750 cm
−1
) can be assigned to
the spin-forbidden triplet transitions. For example, the [d(xy)
→
d(x
2
-y
2
)/C(sp
2
)*] triplet
Cagan et al.
Page 10
J Am Chem Soc
. Author manuscript; available in PMC 2023 March 02.
Author Manuscript
Author Manuscript
Author Manuscript
Author Manuscript