Energy Transfer to Ni-Amine Complexes in Dual Catalytic, Light-Driven C-N Cross-Coupling Reactions

Article abstract of DOI:10.1021/jacs.9b11049

Dual catalytic light-driven cross-coupling methodologies utilizing a Ni(II) salt with a photocatalyst (PC) have emerged as promising methodologies to forge aryl C-N bonds under mild conditions. The recent discovery that the PC can be omitted and the Ni(II) complex directly photoexcited suggests that the PC may perform energy transfer (EnT) to the Ni(II) complex, a mechanistic possibility that has recently been proposed in other systems across dual Ni photocatalysis. Here, we report the first studies in this field capable of distinguishing EnT from electron transfer (ET), and the results are consistent with F?rster-type EnT from the excited state [Ru(bpy)₃]Cl₂ PC to Ni-amine complexes. The structure and speciation of Ni-amine complexes that are the proposed EnT acceptors were elucidated by crystallography and spectroscopic binding studies. With the acceptors known, quantitative F?rster theory was utilized to predict the ratio of quenching rate constants upon changing the PC, enabling selection of an organic phenoxazine PC that proved to be more effective in catalyzing C-N cross-coupling reactions with a diverse selection of amines and aryl halides.

Full text of DOI:10.1021/jacs.9b11049

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Article
Energy Transfer to Ni-amine Complexes in Dual
Catalytic, Light-driven C–N Cross-Coupling Reactions
Max Kudisch, Chern-Hooi Lim, Pall Thordarson, and Garret M. Miyake
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b11049 • Publication Date (Web): 12 Nov 2019
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Energy Transfer to Ni-amine Complexes in Dual Catalytic, Light-driven
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Max Kudisch†, Chern-Hooi Lim†‡, Pall Thordarson§*, and Garret M. Miyake†*
†Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523, United States.
‡New Iridium LLC, Boulder, Colorado 80303, United States.
§School of Chemistry, The Australian Centre for Nanomedicine and the ARC Centre of Excellence in Convergent Bio-Nano Science &
Technology, The University of New South Wales, Sydney, NSW 2052, Australia.
Supporting Information Placeholder
ABSTRACT: Dual catalytic light-driven cross coupling methodologies utilizing a Ni(II) salt with a photocatalyst (PC) have emerged
as promising methodologies to forge aryl C–N bonds under mild conditions. The recent discovery that the PC can be omitted and the
Ni(II) complex directly photoexcited suggests that the PC may perform energy transfer (EnT) to the Ni(II) complex, a mechanistic
possibility that has recently been proposed in other systems across dual Ni photocatalysis. Here, we report the first studies in this
field capable of distinguishing EnT from electron transfer (ET), and the results are consistent with Förster type EnT from the excited
state [Ru(bpy)3]Cl2 PC to Ni-amine complexes. The structure and speciation of Ni-amine complexes that are the proposed EnT ac-
ceptors were elucidated by crystallography and spectroscopic binding studies. With the acceptors known, quantitative Förster theory
was utilized to predict the ratio of quenching rate constants upon changing the PC, enabling selection of an organic phenoxazine PC
that proved to be more effective in catalyzing C–N cross coupling reactions with a diverse selection of amines and aryl halides.
complexes that can be directly photoexcited with 365 nm LEDs
INTRODUCTION
for the formation of the desired aryl C–N product (Fig. 1C).12
Dual catalytic, light-driven C–N,1-2 C–O,3 C–C,4 C–P,5 and
The existence of a direct Ni(II) irradiation route supports the
C–S6 cross-coupling methodologies utilizing a Ni(II) catalyst
along with a photocatalyst (PC) have recently emerged as prom-
ising systems for synthetic chemistry with advantages over tra-
ditional Pd catalysis in terms of sustainability, cost, and mild-
ness of reaction conditions.7 As such, mechanistic understand-
ing of these reactions is essential in order to rationally design
effective catalysts and unlock new reactivity.
In 2016, two dual catalytic, C–N cross-coupling systems
were independently developed. In both cases, a wide range of
amines and aryl halides were coupled under blue light irradia-
tion employing the same Ni(II) precatalyst (i.e. NiBr2·glyme)
and Ir(III) PC (Fig 1A).1-2 The Ir(III) PC used was
[Ir(dF(CF3)ppy)2(dtbbpy)]PF6 which is a strong photo-oxidant.
In contrast, our group recently found that organic PCs (e.g. di-
hydrophenazines8 and phenoxazines9) that are strong photo-re-
ductants also functioned efficiently for dual catalytic C–N
cross-coupling under similar conditions (Fig. 1B).10 These re-
sults suggest two potential scenarios. In one case, the Ni(II)
precatalyst is sufficiently robust that C–N cross-coupling can
occur through both reductive and oxidative electron transfer
(ET) cycles utilizing a photo-oxidant and a photo-reductant, re-
spectively. Alternatively, the Ni(II) precatalyst might be acti-
vated via energy transfer (EnT) from the PC; notably, the pos-
sibility of an EnT cycle has seldom been considered to date in
C–N cross coupling, with only one reported example describing
the coupling of sulfonamides with aryl halides.11
Figure 1. Reported C–N cross coupling systems utilizing (A) a
photo-oxidant, (B) photo-reductants, or (C) no added PC. This
work (D) on C–N bond formation promoted by energy transfer
from a PC to the Ni-amine complex. In (A), the Ir PC used was
[Ir(dF(CF3)ppy)2(dtbbpy)]PF6, where dF(CF3)ppy = 2-(2,4-difluor-
ophenyl)-5-(trifluoromethyl)pyridine and dtbbpy = 4,4’-ditertbu-
tyl-2,2’-bipyridine. The organic PCs used in (B) were 3,7-di([1,1'-
biphenyl]-4-yl)-10-(naphthalen-1-yl)-10H-phenoxazine and 5,10-
di(naphthalen-2-yl)-5,10-dihydrophenazine. PC = photocatalyst;
DMAc = N,N-dimethylacetamide. ArBr = aryl bromide.
Furthermore, we recently discovered that a PC can be omitted
from the dual catalytic C–N cross-coupling system. Specifi-
cally, upon amine addition to a solution containing Ni(II) and
aryl halide, we observed in situ formation of Ni-amine
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possibility of an EnT quenching mechanism as elucidated
herein (Fig. 1D). In the absence of an added PC, the Ni excited
state is accessed directly through photo-excitation. Similarly, in
a dual catalytic system, an analogous (but possibly distinct) Ni
excited state can be accessed through EnT from a PC.
Page 2 of 9
The bimolecular quenching step (Fig. 2B) between the ex-
cited state of PC 1 and the Ni-amine complex was further elu-
cidated via nanosecond TA experiments. We note that the spe-
ciation of the Ni-amine complexes formed in situ is detailed
later in this work. Laser irradiation at λpump = 532 nm in N,N-
dimethylacetamide (DMAc) solvent containing PC 1 produced
the long-lived (i.e. 870 ± 40 ns in DMAc, Fig. S4) metal-to-
ligand charge-transfer (MLCT) triplet excited state character-
ized by an excited state absorption (ESA) feature at λ = 370 nm
and a prominent ground state bleach (GSB) at λ = 450 nm (See
SI, Fig. S3). The MLCT excited state lifetime measured here is
consistent with previous reported values ranging from 800-
1000 ns in polar, aprotic solvents.29
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This discovery is complemented by the finding that a PC can
be omitted for dual catalytic C–O13 or C–C14 systems as the Ni
complex can similarly be directly photoexcited, suggesting the
possibility of an EnT pathway. More broadly, methods across
photocatalysis which involve the direct excitation of transition
metal complexes15-19 suggest systems which may be conducive
to EnT upon addition of a PC. Importantly, EnT pathways have
been proposed to be operative with Ni(II) or Cu(I) complexes
serving as the acceptor in several systems across light-driven
dual catalysis.20-23 However, to date, no study has utilized time-
resolved techniques capable of distinguishing between electron
and energy transfer to support that the excited state PC does in-
deed react via energy transfer in these systems.
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We note that the MLCT state can be approximated as a for-
mal reduction of one bpy ligand to the radical anion along with
formal oxidation of the Ru center to Ru(III). As such, the ESA
at λ = 370 nm has been previously assigned to a transition in-
volving the radical anion of one bpy ligand on the basis of com-
parison to spectroelectrochemical measurement of the radical
anion of free bpy and is thus diagnostic of MLCT state for-
mation.26 In addition, the GSB at λ = 450 nm was attributed to
the presence of Ru(III) in the MLCT state which lacks a transi-
tion in this region. As such, quenching through ET is character-
ized by persistence of the ESA at λ = 370 nm in the case of
reductive quenching since [Ru(bpy)3]Cl2 is formally reduced,
while oxidative quenching leads to persistence of the GSB at λ
= 450 nm.27 However, in the case of EnT, both signals fully re-
turn to baseline since the ground state is recovered, and a de-
crease in the excited state lifetime is observed.27
Notably, obtaining spectroscopic evidence of an EnT
pathway can unlock pivotal practical advances for
a
methodology as illustrated by the C–S cross coupling of
alkenes/alkynes with disulfides in which replacement of the
precious metal Ir(III) PC with an organic PC of higher triplet
energy both accelerated the rate of product formation and
alleviated sustainability concerns.24 Furthermore, dual catalysis
enables mild visible light irradiation while 365 nm light was
required previously to directly excite the Ni complex. Thus,
addition of a PC can enable use of UV-sensitive substrates.
Herein, we provide spectroscopic evidence in support of EnT
from an excited state PC to Ni-amine complexes under condi-
tions relevant for dual catalytic C–N cross-coupling driven by
visible light (Fig. 1D). In particular, using nanosecond transient
absorption (TA) spectroscopy, we observed the excited state of
[Ru(bpy)3]Cl2 (bpy = 2,2′-bipyridine) reacting with Ni-amine
complexes formed in situ in C–N cross coupling reaction mix-
tures. The spectral data is consistent with an EnT pathway pro-
ceeding primarily through a Förster type mechanism, a result
that is notably distinct from the Dexter type pathway typically
invoked in the literature in catalytic cycles involving EnT that
results in substrate sensitization.25 Next, speciation studies elu-
cidated the Ni-amine complexes that serve as EnT acceptors (or
as light absorbers in the direct excitation method).12 Finally,
these mechanistic insights were utilized in conjunction with
quantitative Förster theory to select an organic phenoxazine PC
(the same shown in Fig 1C) that proved to be more effective
than [Ru(bpy)3]Cl2 in the C–N coupling of 13 substrate pairs.
In a C–N cross-coupling reaction mixture consisting of the
same molar ratio of components detailed in Fig. 2A, we moni-
tored excited state quenching of PC 1 using λpump = 532 nm,
consistent with the green LED used in cross coupling reactions.
We note that under these conditions, Ni-morpholine complexes
are formed in situ (vide infra) and are active in quenching PC
1’s excited state. Importantly, consistent with an EnT pathway,
signals corresponding to PC 1’s excited state return fully to the
baseline at all wavelengths from λ = 300-800 nm (See SI, Fig.
S11), indicative of recovery of the ground state of PC 1. In par-
ticular, kinetic traces of PC 1 at both λprobe = 370 nm and λprobe
= 450 nm returned fully to baseline (Fig. 2D) and thus neither
oxidized nor reduced PC 1 indicative of an ET mechanism was
observed.
We further note that cyclic voltammetry experiments suggest
that an ET quenching mechanism (either oxidative or reductive)
is unlikely to occur (See SI, Section 4). Specifically, E0*
([Ru(bpy)3]*2+/[Ru(bpy)3]3+) = –1.19 V vs. Fc/Fc+ (–0.74 V vs.
SCE) as measured in this work in DMAc, while the first reduc-
tion of a Ni-morpholine complex occurs at Ep = –1.71 V vs.
Fc/Fc+ (–1.26 V vs. SCE); thus, an oxidative quenching path-
way is thermodynamically unfavorable. Similarly, reductive
quenching is unlikely as E0* ([Ru(bpy)3]*2+/[Ru(bpy)3]+) = 0.27
V vs. Fc/Fc+ (0.72 V vs. SCE), and the first oxidation of a Ni-
morpholine complex occurs at Ep/2 ≈ 0.48 V vs. Fc/Fc+ (0.93 V
vs. SCE).
RESULTS AND DISCUSSION
[Ru(bpy)3]Cl2 was chosen as the PC for spectroscopic studies
due to the extensive body of photophysical literature describing
its spectral changes upon EnT or ET26-27 and the precedence for
its use as a PC in related dual catalytic C–N cross coupling sys-
tems.1,28 Here, we initially confirmed that [Ru(bpy)3]Cl2 (PC 1)
is an effective PC for C–N cross coupling involving 4-bromo-
benzotrifluoride and morpholine, achieving 88% conversion as
measured by 19F NMR after 22 hours irradiation with a green
(i.e. λmax = 523 nm) LED (Fig. 2A). Importantly, no product was
observed either in the absence of light or PC under these condi-
tions, indicating that the direct excitation of the Ni complex was
not a valid pathway, and thus the observed reactivity can be
completely ascribed to the role of PC 1.
Furthermore, the presence of Ni-morpholine complexes led
to significant reduction in the excited state lifetime of PC 1 from
870 ± 40 ns to 360 ± 20 ns, while free morpholine did not lead
to quenching (See SI, Fig. S5-6). Further control experiments
showed that electron transfer products were not observed even
under high laser power (See SI, Fig. S9-10). We note that sub-
traction of spectra did not yield any signals that could be
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Figure 2. (A) Dual catalytic C–N cross coupling control reactions. Reaction details: 0.4 mmol in aryl bromide in DMAc, 22 h reaction time.
(B) Simplified catalytic cycle highlighting the focus of this work: the PC quenching step probed by transient absorption (TA) experiments
and the speciation of the Ni catalyst. For the full proposed cycle, see Fig. S71. (C) Förster EnT efficiency approximated by overlap between
PC 1 emission and Ni-amine complex absorption. (D) TA single wavelength kinetic traces acquired with λpump = 532 nm and λprobe = 450 or
370 nm for PC 1 and a mixture (80:1 molar ratio of Ni:PC) containing PC 1 (0.1 mM) with Ni-morpholine complexes. (E) Stern-Volmer
plot and extracted quenching rate constants. Quenchers are mixtures of Ni-amine complexes formed in situ. The species formed are deter-
mined later in this work. ‡We note that debromination was not observed (See SI, Fig. S72); only unreacted aryl bromide was detected by
19F NMR. EnT = energy transfer. ET = electron transfer. kq = quenching rate constant. k0 = decay rate constant of PC 1. kobs = observed
decay rate constant in the presence of quencher. Kn = equilibrium binding constant. See SI, Section 2 for TA experimental details and spectra.
assigned to an excited state Ni complex, likely due to the ex-
cited state lifetime being too short to measure (See SI, Fig. S12);
for example, a square planar Ni(II) aryl halide complex was ob-
served to have a 4.2 ns lifetime,13 too short to be detectable with
our setup. Overall, these results suggest that C–N cross-cou-
pling reactivity is derived from excited Ni-amine complexes,
which can be accessed through either EnT from a PC as in this
work or through direct photo-excitation as in our previous
work.12 As such, we propose that mechanistic steps following
the EnT step will mirror those we proposed previously (See SI,
Fig. S71 for the catalytic cycle).
components of the quenching mixture must be elucidated in or-
der to determine the identity and molar absorptivity of the EnT
acceptor(s) to utilize Förster theory quantitatively in examining
this hypothesis. Turning to classic reports30-35 describing mono-
dentate primary and secondary amine ligands binding to Ni(II),
Next, we observed that when using different types of amines
and the Ni:amine ratio is held at 1:70, the same molar ratio used
in C–N reactions, PC 1 was quenched to different degrees. To
explore this relationship, a Stern-Volmer quenching study was
performed with ratios of Ni:PC 1 increasing from 10:1 to 80:1.
Notably, use of morpholine gives kq = (2.3 ± 0.1)*108 M-1 s-1 at
λprobe = 450 nm while propylamine quenched with a signifi-
cantly reduced rate constant of kq = (3.5 ± 0.4)*107 M-1 s-1 (Fig.
2E), consistent with the lower cross-coupling performance of
primary amines relative to secondary amines observed herein
and previously under direct excitation with 365 nm irradia-
tion.12 Since the same PC was used throughout and none of the
other reaction components were suitable ET or EnT quenchers
(See SI, Fig. S5-8), this variation in kq must arise from changes
in electronic structure of the Ni-amine complex, and thus the
speciation of the complexes formed must be determined.
Unlike typical organic substrates for which the contribution
of Förster type EnT can be deduced to be minimal a priori on
account of lacking significant spectral overlap with the excited
PC,27 Ni-amine complexes absorb significantly in the wave-
length range of PC 1 phosphorescence (Fig. 2C). Further, the
overlap area for absorption of Ni-morpholine and Ni-propyla-
mine mixtures appears correlated with the rate of quenching,
supporting the hypothesis of Förster type EnT. However, the
Figure 3. (A) Crystal structures of [NiBr2(propylamine)4],
[NiBr2(morpholine)3], and [NiBr2(quinuclidine)2] shown at 50%
thermal ellipsoids with hydrogens omitted for clarity. (B) Left axis:
molar absorptivity of Ni-amine complexes in DMAc solution.
Right axis: solid-state UV-visible absorption spectra of single crys-
tals of complexes shown in (A). Inset: photographs of crystals of
each complex at 40X magnification. (C) Selected bond distances
and angles. See SI, Section 4 for further experimental details.
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the available information is insufficient due to the lack of both
crystallographic characterization and speciation studies in
DMAc solution.
observed complexes from the NiBr2·3H2O precatalyst. These
binding equilibria were directly observed through UV-vis iso-
thermal titrations in which mixtures were analyzed with in-
creasing ratios of the amine ligand:Ni.
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To address this challenge, we first grew single crystals from
concentrated Ni-amine mixtures, and analysis by single crystal
X-ray diffraction (XRD) revealed for the first time that propyl-
amine, morpholine, and quinuclidine form 6-, 5-, and 4-coordi-
nate Ni(II) bromide complexes, respectively (Fig. 3A). Im-
portantly, these complexes are likely the EnT acceptors, but it
must be confirmed that the structures obtained in solid-state can
serve as reasonable approximations of the geometry of com-
plexes formed in situ in DMAc solution. To confirm that this
assumption is reasonable, single crystals were examined
through solid state UV-visible absorption spectroscopy (Fig.
3B; see SI, Section 8 for details). Qualitatively, similar absorp-
tion features are observed in solution as are found in the single
crystals for all complexes, suggesting that structures obtained
from XRD are not changed significantly upon DMAc solvation.
Further, a control UV-vis experiment supports that the same
complexes also exist in the full C–N coupling reaction mixtures
(i.e. with aryl halide and PC added; See SI, Fig. S58). As such,
some bands in the UV-vis spectra of the quenching mixtures
used in TA experiments can now be assigned to specific com-
plexes, namely [NiBr2(morpholine)3] (i.e. λmax,1 = 427 nm, λmax,2
= ~740 nm) and [NiBr2(propylamine)4] (λmax,1 = 379 nm, λmax,2
= ~620 nm).
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UV-vis of these mixtures shows the evolution and demise of
species with increasing numbers of amine ligands in the Ni-
morpholine mixture (Fig. 4A). Initially, we note that the DMAc
solvent forms the salt [Ni(DMAc)6][NiBr4] prior to amine addi-
tion (See SI, Fig. S51) and that multiple pathways exist for the
first two amine additions (See SI, Fig. S69-70). However, all
pathways converge to form the tetrahedral [NiBr2(morpho-
line)2] as the product of K2, which can be assigned to the signal
with λmax,1 = ~550 nm and λmax,2 = ~850 nm. To precisely deter-
mine the ratio of these species in the Ni-morpholine quenching
mixture, the titration data was fitted to four variants (flavors)40
of a 1:3 (metal:ligand or host:guest) binding model. This anal-
ysis was performed using a Matlab code based on the analytical
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Notably, the structure obtained for [NiBr2(morpholine)3]
closely matches the density functional theory (DFT) optimized
ground state geometry in DMAc solution described in our pre-
vious work,12 supporting this assignment as well as the accuracy
of our reported DFT calculations. Further, the formation of
[NiBr2(propylamine)4] suggests the general trend that primary
amines form 6-coordinate NiBr2 complexes as supported by the
isolation of [NiBr2(aniline)4] and [NiBr2(cyclohexylamine)4]
(see SI, Fig. S48-49). Ni-amine complexes of the type charac-
terized here are proposed to form across Ni catalysis in systems
lacking exogenous ligand and employing amines in conjunction
with NiBr2, regardless of the NiBr2 source used (e.g.
NiBr2·glyme, NiBr2·3H2O, or anhydrous NiBr2, See SI, Fig.
S57). The same complexes also form in the presence of all PCs
used in C–N coupling reactions (Ir(III), Ru(II), and phenoxa-
zine; See SI, Fig. S54-56), suggesting they may more broadly
serve as mechanistically relevant species in many dual catalytic
Ni(II) cross coupling systems.
Reported magnetic moment measurements of identical34 or
related36-38 Ni(II) complexes, as well as our previous DFT cal-
culations, are consistent with [NiBr2(propylamine)4],
[NiBr2(morpholine)3] possessing triplet ground states in DMAc
solution, suggesting that the d-d absorptions in the region of
Förster overlap are of triplet to triplet nature. As such, Förster
EnT is allowed based on the conservation of spin angular mo-
menta39 from the triplet excited state of PC 1 as the donor to
form a triplet excited state Ni-amine complex. In order to em-
ploy Förster theory in modeling this excited state reactivity, the
molar absorptivity of each acceptor complex overlapping with
PC 1 emission must be known. However, UV-vis absorption
peaks are observed in the Ni-amine solutions which remain to
be assigned (e.g. the feature at ~550 nm in the Ni-morpholine
DMAc solution, Fig. 2C) that might be viable EnT acceptors.
We hypothesized that these features must originate from other
Ni-amine complexes with fewer amine ligands, since a stepwise
series of amine additions is required in the formation of the
Figure 4. (A) Selected UV-vis traces from one replicate titration
experiment showing equilibria between Ni-morpholine complexes.
Arrows indicate features that rise or fall in the forward direction of
each equilibrium. 70 eq. of amine ligand added (relative to Ni) cor-
responds to the exact conditions used in C–N cross coupling reac-
tions. (B) Calculated average molar absorptivity (n = 3 replicates)
of Ni-morpholine complexes. (C) Scheme defining stepwise series
of equilibria upon addition of amine ligands. Equilibrium constants
and molar absorptivities were extracted from titration data via a
global analysis fitting procedure (See SI, Fig. S62-65 for details).
aWe note that the NiBr2 precatalyst forms a tetrabromonickelate salt
in DMAc solution from which multiple amine addition pathways
are possible for K1 and K2. bValues in kcal mol-1. See SI for details.
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solution to the system of equations for the 1:3 equilibria, similar
donor to a transition metal acceptor,44-45 an expression was de-
rived (see SI, section 3 for details) to evaluate the ratio of
quenching rate constants (kEnT,1/kEnT,2) as the donor, PC 1, was
held constant but the acceptor was changed, from the Ni-mor-
pholine quenching mixture to the Ni-propylamine quenching
mixture, according to the following equation:
1
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8
to that described in a NMR study on a 3:1 complexation of a
bis-antimony receptor with halide anions.41 In our work we per-
formed a global analysis42 using the UV-vis binding isotherms
from λ = 395-1200 nm (See SI, Section 7, for details). Compar-
ing how the various flavors of the 1:3 binding model fitted the
data40, 43 clearly showed that the “full” 1:3 model which assumes
i) cooperativity and ii) that the 1:1, 1:2, and 1:3 stepwise com-
plexes have distinct spectra, gave a significantly better fit to the
data than the other binding model (flavors) considered.
푘
퐽
1
퐸푛푇,퐴1
=
(Eq. 4)
푘
퐽
2
퐸푛푇,퐴2
Using Equation 4, the ratio for PC 1 was calculated as
kEnT,A1/kEnT,A2 = 4.6, which compares favorably with the experi-
mental value of 6.5 obtained from the TA experiments de-
scribed above (See SI, Table S5, for J integrals used in the cal-
culation). This level of agreement compares very well with that
obtained for similar calculations in the literature,44 supporting
our assignment of the EnT quenching step as a Förster type EnT
process. Thus, based on equations 1-3, it can be seen that selec-
tion of a PC with higher radiative rate constant kr,D and higher
overlap integral J will result in a higher EnT rate.
9
The full model allowed us not only to extract the stepwise
equilibrium binding constants but also the molar absorptivity
(Fig 4B) of each species, giving K1 = (6 ± 3) *103 M-1, K2 = 130
± 30 M-1, and K3 = 2.6 ± 0.5 M-1 with the corresponding free
energies (after correcting for statistical factors) ΔG1 = –2.0 ±
0.3 kcal mol-1, ΔG2 = –1.3 ± 0.1 kcal mol-1, and ΔG3 = –0.5 ±
0.1 kcal mol-1, respectively (Fig. 4C). The increasing ΔG values
indicate negative binding cooperativity, or less favorable addi-
tion of morpholine to Ni with each successive association. No-
tably, ΔG3 = –0.5 kcal mol-1 is reasonably close to the value of
ΔG3 = -1.1 kcal mol-1 calculated by DFT in our previous work.12
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As such, we hypothesized that changing the donor PC from
PC 1 to the organic phenoxazine PC 2 would impart improved
performance in C–N cross coupling reactions given that PC 2
covers increased spectral range in its emission compared to PC
1 (See SI, Fig. S33) and has a radiative rate constant reported46
to be 4 ± 1 * 106 s-1 which issignificantly higher than that of PC
1 (i.e. 7 * 104 s-1).29 First, we confirmed that PC 2 also reacts via
EnT as opposed to ET through spectroscopic (See SI, Fig. S24-
32) and electrochemical (See SI, Section 4) control experi-
ments. To theoretically probe the hypothesis that PC 2 will in-
crease the rate of EnT, a second equation was derived (See SI,
Section 3) to predict the ratio of quenching rate constants as the
PC is changed but the Ni-morpholine mixture is kept constant
as the acceptor:
Furthermore, we calculate that under C–N coupling condi-
tions (i.e. 70:1 amine:Ni molar ratio), the Ni-morpholine
quenching mixture consists of 73 % [NiBr2(morpholine)3], 27
% [NiBr2(morpholine)2], and < 0.3 % other complexes. As
shown in Fig. 2C, both [NiBr2(morpholine)2] and [NiBr2(mor-
pholine)3] demonstrate absorptions that overlap significantly
with PC 1’s emission. While [NiBr2(morpholine)2] shows the
largest overlap, it is present in lower concentration. Interest-
ingly, [NiBr2(morpholine)3] contains a morpholine bound in the
apical position (Fig. 3A) with a significantly shortened Ni–N
bond of 2.050(4) Å compared the other morpholines (i.e.
2.099(4) and 2.101(4)); thus, this morpholine is the strongest
donor and may facilitate the subsequent proposed mechanistic
step, intramolecular ET to generate a Ni(I) center and a mor-
pholino radical cation (See SI, Fig. S71 for our proposed mech-
anism). As such, both [NiBr2(morpholine)2] and [NiBr2(mor-
pholine)3] show positive features for EnT catalysis and we con-
clude that both are the EnT acceptors.
푘
푘
퐽
푟,퐷1 퐷1
퐸푛푇,퐷1
=
(Eq 5)
푘
푘
퐽
퐸푛푇,퐷2
푟,퐷2 퐷2
We note that the photophysics of PC 2 are more complicated
than PC 1 in that both singlet and triplet excited states are pop-
ulated. The triplet state is unlikely to react at a kinetically sig-
nificant rate via a Förster type pathway given the extremely low
phosphorescence radiative decay rates of organic PCs in solu-
tion, typically on the order of 100-103 s-1.47 Thus, the fluores-
cence spectrum and fluorescence radiative decay constant were
used in conjunction with Eq. 5 to calculate the ratio of kEnT,PC
With the acceptors and their respective molar absorptivities
known, we turned to classical Förster theory in which the theo-
retical energy transfer rate constant, kEnT, can be calculated as
follows:27
2:kEnT,PC = 20.4, which is within a factor of 3 of the value of
1
6
R
0
푘퐸푛푇 = 푘푟,퐷 ( ) 푤ℎ푒푟푒
(Eq. 1)
(Eq. 2)
(Eq. 3)
12.7 determined experimentally from quenching studies (see
above for PC 1 and SI, section 3 for PC 2). Agreement between
these values suggests that the observed EnT occurs through pri-
marily a Förster type pathway in the case of both PCs and with
a much higher rate constant for PC 2 of (2.9 ± 0.2) *109 M-1 s-1
(See SI, Fig. S36) as compared to that for PC 1 of (2.3 ± 0.1)
*108 M-1 s-1.
R
−25
2
8.79∗10
휅 퐽
6
R0
=
푎푛푑
4
휂
∞ 퐹 (휈)휀
퐽 =
(휈) 푑휈
퐷
퐴
∫
4
0
휈
In these equations, R is the donor-acceptor distance, R0 is the
critical Förster distance defined in Eq. 2, kr,D is the radiative de-
cay constant of the donor in absence of acceptor, κ is the dipolar
orientation factor, η is the refractive index of the solvent, and J
is the spectral overlap integral defined in Eq. 3 which involves
FD, the area-normalized emission spectrum of the donor, and εA,
the molar absorptivity of the acceptor as a function of fre-
quency. In order to apply this equation to the reaction between
excited state PC 1 and a Ni-amine complex, we needed to elim-
inate variables R and κ which are difficult to measure in solution
with freely tumbling donors and acceptors.
With these results in hand, we compared the performance of
PCs 1 and 2 in C–N cross coupling reactions (Fig. 5; see SI,
section 9 for experimental details and product characterization).
In order to directly compare the PCs, we chose to use the exact
same conditions for all reactions. In particular, blue 457 nm
light irradiation was chosen since PC 2 cannot absorb green
light. Further, an irradiation time of 15 hours was chosen to al-
low for direct comparison with our previous work.12 Control re-
actions confirmed that no product is formed in the absence of
PC under blue irradiation (See SI, Table S9), and we further
note that for PC 1 blue LED irradiation accesses the same
Utilizing an approach similar to that first demonstrated in the
study of a system involving intramolecular EnT from a Re
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that quinuclidine binds more strongly than aniline to NiBr2 (See
Page 6 of 9
1
2
3
4
5
6
7
8
SI, Fig. S59), forming [NiBr2(quinuclidine)2] which has good
overlap between its absorption and PC 2’s emission (See SI,
Fig. S33). In addition, the use of 1 equivalent of KBr as an ad-
ditive improved the performance of some amines such as prop-
ylamine and unprotected piperazine (14% and 26% increase, re-
spectively; see SI, table S10 and following supplemental dis-
cussion). With propylamine, we hypothesize that KBr improves
reactivity by inhibiting the speciation equilibria as observed by
UV-vis (See SI, Fig S60-61); this inhibition increases the con-
centration of [NiBr2(propylamine)3], which has better overlap
with PC 2 than [NiBr2(propylamine)4.
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With regard to the aryl halide coupling partner, PC 2 success-
fully promotes coupling of morpholine with aryl halides con-
taining electron-withdrawing groups (EWGs, e.g. 4-bromoben-
zotrifluoride, 92%) as well as electron-donating groups (EDGs,
e.g. 4-bromoanisole, 31%). Notably, a heterocyclic aryl bro-
mide, 3-bromopyridine, could be coupled in good yield (58%).
In addition, the difficult ortho-substituted aryl halide, 2-iodotol-
uene could be coupled with morpholine in moderate yield
(27%), constituting the first example of C–N bond formation
with this substrate pair in light-driven Ni catalysis.
Figure 5. Scope of C-N coupling reactions using PC 1 and PC 2;
PC 1 = [Ru(bpy)3]Cl2 and PC 2 = 3,7-di([1,1'-biphenyl]-4-yl)-10-
(naphthalen-1-yl)-10H-phenoxazine. Yields were determined us-
ing 19F NMR for products containing fluorine. Isolated yields are
a
reported in parentheses. X = Br unless otherwise indicated. 1 eq.
b
KBr was used as an additive. 1.5 eq. of the amine was used with
c
1.5 eq. added quinuclidine. An aryl iodide was used. tr = trace
Trends in reactivity for both PCs mirror those observed in our
previous work with secondary > primary > primary aromatic
amines in terms of yield. Similarly, aryl halides containing
EWGs gave greater yields than those containing EDGs. Overall,
the similarity in these trends across both PCs to trends obtained
in our previous work12 support that a similar Ni-amine excited
state intermediate forms in both cases that can be accessed ei-
ther via EnT from a PC under visible light or via direct excita-
tion under 365 nm irradiation.
product isolated. Reactions were performed with 0.4 mmol of aryl
halide at room temperature. See SI, Section 9, for details and char-
acterizations. The blue LED emission λmax = 457 nm.
absorption band as green irradiation, forming the same lowest
MLCT excited state that we propose reacts via EnT.
Under these conditions, the performance with PC 1 and mor-
pholine is significantly worsened (e.g. 43% conversion vs. 88%
in Fig. 2A), and this can be attributed to a combination of fac-
tors, namely the lower catalyst loading which leads to a loss of
23% conversion (See SI, Table S9), the lower luminous flux of
the blue LED as compared with the green LED (1:4.4,
blue:green; See SI, Table S1), and the inner-filter effect result-
ing from increased unproductive absorption of blue vs. green
excitation light by the Ni-morpholine complexes (3:1,
blue:green; See SI, Fig. S33). Despite these worsened condi-
tions, PC 1 is effective for coupling of secondary aliphatic
amines with 4-bromobenzotrifluoride, highlighted by coupling
of unprotected piperazine (51%) and indoline (73%), nitrogen
heterocycles that are among those most frequently used in me-
dicinal chemistry.48
CONCLUSION
In sum, spectroscopic evidence supports an EnT quenching
mechanism in dual catalytic C–N cross coupling that proceeds
via a Förster type pathway. In addition, the EnT acceptors have
been identified via single crystal XRD and spectroscopic bind-
ing studies as a series of [Ni(II)Br2(amine)x] complexes formed
in situ in C–N cross coupling reaction mixtures. These com-
plexes are proposed to form more broadly across catalytic sys-
tems utilizing NiBr2 and amines and thus constitute mechanis-
tically-significant intermediates across Ni catalysis. Elucidating
the speciation enabled the use of quantitative Förster theory to
calculate EnT rate constant ratios that agreed with values deter-
mined experimentally from spectroscopic studies. Employment
of this mechanistic knowledge through selection of phenoxa-
On the other hand, PC 2 is more broadly effective, achieving
higher yields than PC 1 with almost all substrates (Fig. 5). No-
tably, PC 2 is effective in coupling difficult aliphatic (e.g. prop-
ylamine, 50%) and aromatic primary amines such as 4-
fluoroaniline (55%) and 3-aminopyridine (33%) with 4-bromo-
benzotrifluoride while PC 1 is ineffective, achieving only trace
product formation with primary aliphatic amines. PC 2’s emis-
sion extends ~100 nm further into the blue than PC 1, overlap-
ping an absorption band of [NiBr2(propylamine)4] that PC 1
cannot access (See SI, Fig. S33). Since the efficiency of EnT
has been shown to depend on specific electronic transitions,45
we hypothesize that this blue-shifted band may facilitate EnT
and thus might explain PC 2’s increased performance.
zine PC
2 led to increased performance relative to
[Ru(bpy)3]Cl2 PC 1 in catalyzing C–N bond formation between
diverse amines and aryl halides under mild conditions. Ulti-
mately, future work utilizing quantitative Förster theory has the
potential to both predict and discover new reactivity in energy
transfer systems across light-driven Ni catalysis.
ASSOCIATED CONTENT
Supporting Information. The supporting information consists of
transient absorption spectroscopic data, cyclic voltammetry data,
derivations of equations using Förster theory, emission spectros-
copy, single crystal X-ray crystallography procedures (for details,
see the attached .cif files), UV-visible spectroscopy and data anal-
ysis, a proposed catalytic cycle, and details regarding C–N cross
coupling reaction procedures and product characterization. This
With aromatic amines (e.g. 4-fluoroaniline), quinuclidine
was employed as a base additive based on a beneficial effect on
yield observed in our previous work.12 The effect of quinu-
clidine is likely due at least in part to its role in controlling the
speciation of the Ni catalyst; investigation by UV-vis revealed
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Journal of the American Chemical Society
material is available free of charge via the Internet at
http://pubs.acs.org.
Polymerization Using N-Aryl Phenoxazines as Photoredox
Catalysts. J. Am. Chem. Soc. 2016, 138 (35), 11399-11407.
(10) Du, Y.; Pearson, R. M.; Lim, C.-H.; Sartor, S. M.; Ryan, M.
D.; Yang, H.; Damrauer, N. H.; Miyake, G. M., Strongly Reducing,
Visible-Light Organic Photoredox Catalysts as Sustainable
Alternatives to Precious Metals. Chem. Eur. J. 2017, 23 (46),
10962-10968.
(11) Kim, T.; McCarver, S. J.; Lee, C.; MacMillan, D. W. C.,
Sulfonamidation of Aryl and Heteroaryl Halides through
Photosensitized Nickel Catalysis. Angew. Chem. Int. Ed. 2018, 57
(13), 3488-3492.
1
2
3
4
5
6
7
8
AUTHOR INFORMATION
Corresponding Author
*p.thordarson@unsw.edu.au
*garret.miyake@colostate.edu
Notes
9
The authors declare no competing financial interests.
(12) Lim, C.-H.; Kudisch, M.; Liu, B.; Miyake, G. M., C–N Cross-
Coupling via Photoexcitation of Nickel–Amine Complexes. J. Am.
Chem. Soc. 2018, 140 (24), 7667-7673.
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ACKNOWLEDGMENT
(13) Shields, B. J.; Kudisch, B.; Scholes, G. D.; Doyle, A. G., Long-
Lived Charge-Transfer States of Nickel(II) Aryl Halide Complexes
Facilitate Bimolecular Photoinduced Electron Transfer. J. Am.
Chem. Soc. 2018, 140 (8), 3035-3039.
(14) Ishida, N.; Masuda, Y.; Ishikawa, N.; Murakami, M.,
Cooperation of a Nickel–Bipyridine Complex with Light for
Benzylic C−H Arylation of Toluene Derivatives. Asian J. Org.
Chem. 2017, 6 (6), 669-672.
(15) Creutz, S. E.; Lotito, K. J.; Fu, G. C.; Peters, J. C.,
Photoinduced Ullmann C–N Coupling: Demonstrating the
Viability of a Radical Pathway. Science 2012, 338 (6107), 647.
(16) Ziegler, D. T.; Choi, J.; Muñoz-Molina, J. M.; Bissember, A.
C.; Peters, J. C.; Fu, G. C., A Versatile Approach to Ullmann C–N
Couplings at Room Temperature: New Families of Nucleophiles
and Electrophiles for Photoinduced, Copper-Catalyzed Processes.
J. Am. Chem. Soc. 2013, 135 (35), 13107-13112.
This work was supported by Colorado State University, the NIH
(R35GM119702), and the Alfred P. Sloan Foundation. PT would
like to thank the ARC for support (CE140100036 and
DP190101892). MK would like to thank Dr. Brian Newell for aid
and instruction in X-ray diffraction structure analysis and Prof.
Richard G. Finke for many insightful discussions regarding this
work. We would also like to thank the reviewers of this manuscript
for their insightful comments.
ABBREVIATIONS
PC, photocatalyst; TA, transient absorption; ET, electron transfer;
EnT, energy transfer; kq, quenching rate constant.
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