Trap-Free Halogen Photoelimination from Mononuclear Ni(III) Complexes

Article abstract of DOI:10.1021/jacs.5b03192

Halogen photoelimination reactions constitute the oxidative half-reaction of closed HX-splitting energy storage cycles. Here, we report high-yielding, endothermic Cl₂ photoelimination chemistry from mononuclear Ni(III) complexes. On the basis of time-resolved spectroscopy and steady-state photocrystallography experiments, a mechanism involving ligand-assisted halogen elimination is proposed. Employing ancillary ligands to promote elimination offers a strategy to circumvent the inherently short-lived excited states of 3d metal complexes for the activation of thermodynamically challenging bonds.

Full text of DOI:10.1021/jacs.5b03192

Communication
pubs.acs.org/JACS
Trap-Free Halogen Photoelimination from Mononuclear Ni(III)
Complexes
Seung Jun Hwang,† David C. Powers,† Andrew G. Maher,† Bryce L. Anderson,† Ryan G. Hadt,†
Shao-Liang Zheng,† Yu-Sheng Chen,‡ and Daniel G. Nocera*,†
†Department of Chemistry and Chemical Biology, Harvard University, Cambridge, Massachusetts 02138, United States
‡ChemMatCARS, The University of Chicago, Argonne, Illinois 60439, United States
S
* Supporting Information
ligand design strategies aimed at destabilizing low-lying ligand-
ABSTRACT: Halogen photoelimination reactions con-
stitute the oxidative half-reaction of closed HX-splitting
energy storage cycles. Here, we report high-yielding,
endothermic Cl2 photoelimination chemistry from mono-
nuclear Ni(III) complexes. On the basis of time-resolved
spectroscopy and steady-state photocrystallography experi-
ments, a mechanism involving ligand-assisted halogen
elimination is proposed. Employing ancillary ligands to
promote elimination offers a strategy to circumvent the
inherently short-lived excited states of 3d metal complexes
for the activation of thermodynamically challenging bonds.
field states and thus removing nonradiative pathways for rapid
excited-state relaxation.21 We report an energy-storing, photo-
induced halogen elimination reaction from molecular Ni(III)
complexes that utilizes a new strategy based on secondary
coordination sphere effects to overcome the challenges of 3d
photochemistry. Nanosecond-resolved transient absorption
spectroscopy, steady-state photocrystallography, and computa-
tional methods have allowed for the observation and character-
ization of the M−X bond activation process.
Ni(III) trichloride complex NiCl3(dppe) (1a) (dppe =
bis(diphenylphosphino)ethane) has been prepared by treating
NiCl2(dppe) (2a) with 0.5 equiv of PhICl2, as previously
described.22 Complex 1a displays no resonances in the 1H NMR
or {1H}31P NMR spectrum. The solid-state structure of complex
1a was evaluated using its bromide congener NiBr3(dppe) (1b)
because diffraction-quality crystals of 1a were not accessible. The
structure of complex 1b is illustrated in Figure 1 and features a
distorted square pyramidal geometry; the basal Ni(III)−Br
bonds (2.351(1) and 2.374(1) Å) are shorter than the apical
Ni(III)−Br bond (2.400(1) Å). Evans method and variable-
hotodriven HX-splitting (X = Cl, Br, and OH) reactions
P
underpin chemical strategies for solar-to-fuels energy
conversion.1−3 In closed, carbon-neutral HX-splitting cycles,
both proton reduction and anion oxidation half-reactions must
be accomplished. Despite substantial work toward halogen
elimination photoreactions with 4d and 5d metal complexes,
such as Pt,4−8 Au,9 Rh,10 Ir, and related heterobimetallics,11−13
halide oxidation remains the kinetic bottleneck for the majority
of developed HX-splitting photocycles.14,15 In many cases,
chemical traps are required to promote halogen extrusion, which
inevitably mitigates the utility of these reactions in energy-storing
catalysis. Although trap-free halogen eliminations have begun to
be developed,4,6,9 little progress has been made toward energy-
storing halogen elimination chemistry with earth-abundant 3d
metal complexes. Here, we report a family of mononuclear
Ni(III) complexes that participate in trap-free X2 elimination
photoreactions. Both time-resolved spectroscopy and steady-
state photocrystallography have established critical ligand-to-
chlorine-atom charge-transfer intermediates in the observed
photoreactions.
The dearth of photoeliminations from 3d metal complexes is a
reflection of the intrinsically short excited-state lifetimes of these
complexes.16−18 Whereas long-lived charge-transfer excited
states are frequently encountered in the photochemistry of 4d
and 5d transition-metal complexes, 3d transition-metal com-
plexes typically possess extremely short-lived charge-transfer
states. These short lifetimes are a consequence of the relatively
small crystal field splitting, which results in low-energy ligand-
field excited states that provide a mechanism for rapid
nonradiative relaxation.17,18 Strategies to overcome the short
lifetimes of 3d complexes include photoredox chemistry, in
which the chromophore and catalyst are decoupled,19,20 and
Figure 1. Photolysis of 1a affords 2a and Cl2. Endothermic energy
shown for the overall transformation from 1a to 2a was determined from
calorimetry measurements. Thermal ellipsoid plot (50% probability) of
representative Ni(III) trihalide complex NiBr3(dppe) (1b) with solvent
molecules and H atoms removed for clarity.
Received: March 26, 2015
© XXXX American Chemical Society
A
DOI: 10.1021/jacs.5b03192
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Communication
1
temperature EPR indicated the complexes are S = /2, as
expected for a mononuclear d7 complex (Figure S1). The EPR
spectra display pseudoaxial signals with gx = 2.245, gy = 2.164 and
gz = 2.012 and a four-line superhyperfine coupling pattern in the
gz direction. The superhyperfine coupling is a direct probe of the
coupling of the 3dz2 unpaired electron with the nuclear magnetic
3
moment of the apical halide ligand (I = /2 for Cl), and the
magnitude is a measure of the degree of electron delocalization of
the unpaired electron along the apical Ni(III)−X bond.23
The absorption spectrum of 1a, given in Figure 2, exhibits two
prominent bands, which are assigned to ligand-to-metal charge-
Figure 3. Mass spectrometry analysis of the gas evolved from solid-state
photolysis of complex 1a. Traces correspond to 35Cl (black), 37Cl
(blue), 35Cl35Cl (pink), 35Cl37Cl (red), and 37Cl37Cl (green) mass
fragments. Inset: relative abundance of 35Cl (black) and 37Cl (red).
S7).29,30 No reduction of 1a was observed by H NMR, 31P
1
NMR, or UV−vis spectroscopy after heating a powdered sample
of 1a at 70 °C in the exclusion of light and moisture, confirming
that halogen elimination is photochemically, and not thermally,
driven (Figure S8).
The energy-storage capacity of Cl2 photoelimination from 1a
was established by solution calorimetric measurements of
chlorine addition to 2a to afford 1a (i.e., the reverse of halogen
elimination). Using the known heat of reaction of PhICl2 → PhI
+ Cl2,31 ΔH for Cl2 extrusion from 1a was determined to be
+23.7 kcal mol−1 (Figure S9). DFT calculations at the B3LYP
level of theory with the SDD basis set for Ni, P, and Br and 6-
311++G** for all other atoms are consistent with our
experimental values (Table S8). Computationally, the elimi-
nation of Cl2 from 1a is +20.3 and +13.4 kcal mol−1 for ΔH and
ΔG, respectively). The energy stored in Cl2 elimination from 1a
is similar to the most endothermic halogen elimination reactions
that have been achieved with noble metal complexes.7
Figure 2. Photolysis of a 0.2 mM solution of 1a in CH3CN (λ > 430 nm)
at 25 °C.
transfer (LMCT) transitions on the basis of time-dependent
density functional theory (TD-DFT) calculations (vide infra).
Steady-state photolysis of a CH3CN solution of 1a with visible
light (λ > 430 nm) led to the rapid evolution of low-spin Ni(II)
complex 2a, as established by UV−vis (Figures 2 and S3) and
both 1H and 31P NMR spectra (Figures S4 and S5, respectively).
No side products were observed, and similar photochemistry also
1
proceeded in CH2Cl2. Integration of the H NMR spectrum
Nanosecond-resolved transient absorption (TA) spectroscopy
was used to investigate the mechanism of Cl2 photoelimination
from 1a in solution. Figure S10 shows the time evolution of the
TA spectrum obtained from laser flash photolysis (355 nm
pump) of a CH3CN solution of 1a. The prompt TA spectrum
observed at 40 ns evolves over the course of several microseconds
into a spectrum that corresponds to that of the formation of 2a
from 1a; single-wavelength kinetics were monitored at 540 nm
and provided a lifetime for the TA signal of 3.43 0.05 μs. The
initial 40 ns spectrum is accounted for by the superposition of the
Ni(II) difference spectrum and that of a transient species.
Accordingly, the absorption spectrum of the transient species
was obtained by subtracting the TA spectrum acquired at a 50 μs
delay, which is well after the system had fully evolved to 2a, from
that acquired at 40 ns. The resulting spectrum displays an
absorption maximum at 366 nm as well as a broad band with a
maximum at 525 nm; this spectrum of the transient species is
shown in Figure 4a. We attribute the observed spectral features to
an arene-to-chlorine charge-transfer adduct between a ligand aryl
group and a photogenerated chlorine atom. This assignment is
based on the similarity of the observed spectral features to those
observed for chlorine atom charge-transfer adducts of benzene
(Figure S11), generated by either laser flash photolysis or pulse
radiolysis.32−35 The observed spectral features are shifted ∼35
nm lower in energy than those in the benzene adduct because of
substituent effects (vide infra).32 The chlorine−benzene
complex has been proposed to be an η1 π complex on the basis
of DFT calculations.36 The assignment is bolstered by
examination of the transient species generated by laser-flash
photolysis of a derivative of 1a in which each aryl substituent is
against 1,2-dimethoxyethane as an external standard indicated
that 2a was observed in 80% yield. No olefinic traps were
required to promote halogen elimination. Cl-based products of
solution-phase photolysis were not characterized; photogen-
erated Cl radicals are expected to participate in H atom
abstraction reactions with either CH3CN24,25 or CH2Cl2.26
Wavelength-dependent quantum yields for halogen elimination
from 1a were measured in CH3CN against a potassium
ferrioxalate standard actinometer and were determined to be
77% (370 nm), 76% (434 nm), and 9% (510 nm). These data are
consistent with a common, productive excited state that is
accessed upon 370 and 434 nm irradiation,27 indicating rapid
internal conversion to a lower-energy, photodissociative state.
The substantially lower quantum yield at 510 nm indicates that
the higher-energy LMCT transition (λmax ≈ 450 nm) is
responsible for efficient halogen photoelimination.
Solid-state irradiation of 1a (∼20 Torr, 25 °C, λ > 350 nm)
results in the formation of 2a (87% yield on the basis of recovered
starting material determined by 1H NMR versus external
standard 1,2-dimethoxyethane; Figure S6) with concurrent
evolution of half an equivalent of Cl2. Evolved Cl2 was collected
at 77 K and subsequently characterized by mass spectrometric
analysis of gaseous reaction products: m/z corresponding to
35,35Cl2, 37,37Cl2, 35,37Cl2, 35Cl·, and 37Cl· were observed, and the
measured ratio of 35Cl and 37Cl was in good agreement with the
natural isotopic abundance for chlorine (Figure 3).28 The yield of
evolved Cl2 (82%) was quantified colorimetrically by trapping
evolved halogen with N,N-diethyl-1,4-phenylenediamine sulfate
(DPD) to afford the intensely pink radical cation DPD•+ (Figure
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(Figure S14). The photodifference map shows that the apical
Ni−Br bond elongates from 2.464(2) to 3.70(4) Å, whereas two
basal Ni−Br bonds are crystallographically unchanged
(2.3615(7) Å (dark), and 2.35(5) Å (photoinduced)), indicating
that the apical halide is photochemically labile. The observed
structural changes are neither photochemically nor thermally
reversible, consistent with the photoinduced component arising
from irreversible Ni−X scission in the solid state. Crystallinity of
the samples is destroyed upon further photolysis, also consistent
with the operation of a chemically irreversible solid-state
reaction. The metrical parameters of 1c do not exhibit significant
temperature dependence, confirming that the photodifference
map arises from photochemical, not thermal, M−X activation
(Table S3;38 evaluation of the extent of laser heating on the
crystal samples, in Figure S15).
To better understand the ground- and excited-state properties
of 1a, a DFT geometry optimization was performed at the
B3LYP level of theory with TZV (C and H) and TZVP (Ni, Cl,
and P) basis sets.39 The ground state is consistent with the
delocalized metal-to-apical-ligand bonding description indicated
by EPR spectroscopy (e.g., a single unpaired electron in the Ni
3dz2 orbital and spin densities of ∼0.88 and ∼0.17 for Ni(III) and
the apical Cl (Clap), respectively (Table S9)). The β-Mayer bond
order (MBO) for the Ni(III)−Cl apical bond is calculated to be
∼0.42. TD-DFT calculations were performed using the
optimized geometry of complex 1a, and the calculated
absorption spectrum reproduces the major spectral features
observed in the experimental data (Figures 2 (expt) and S16, and
Table S10 (calc)). The feature centered ∼620 nm largely consists
of apical Cl pσ- and pπ-based transitions to the unoccupied
Figure 4. (a) Transient absorption spectrum generated by subtracting
the difference spectrum of a 0.44 mM solution of 1a in CH3CN acquired
at a 50 μs time delay from that acquired at a 40 ns time delay (λexc = 355
nm). Inset: single-wavelength kinetics trace monitored at 540 nm fit
with a monoexponential lifetime of 3.43
0.05 μs (red line). (b)
Thermal ellipsoid plot (50% probability) of photocrystallographic
analysis of NiBr3(dppey) 1c; photoinduced structure (solid; Ni(1)−
Br(1) 3.70(4) Å) superimposed on dark structure (faded; Ni(1)−Br(1)
2.464(2) Å). This plot was generated by normalizing the positions of the
P centers in the dark and irradiated structures. H atoms and solvent
molecules are omitted for clarity.
2
2
3dx − y orbital. The absorption feature in the ∼500−400 nm
2
2
region largely consists of intense phenyl-ring π → Ni(3dx − y
)
charge-transfer transitions and apical/basal Cl(pπ) → Ni-
2
2
(3dx − y ) LMCTs. It is of interest to note that two components
functionalized with a para-methoxy group (4-MeO-1a; EPR
spectrum in Figure S2). The observed TA line shape is similar to
that shown in Figure 4a, but the λmax is red-shifted ∼75 nm
compared to that of 1a (Figure S12). Charge-transfer complexes
between arenes and halogen atoms are known to display a linear
relationship between the absorption energy and the ionization
potential of the donor.29 We propose that the formation of the
relatively long-lived (3.43 μs) transient species (4) from a
dissociative LMCT excited state (3) (vide infra) affords a
mechanism to circumvent facile exothermic back reaction of
photogenerated Cl radicals with Ni(II) complexes.
in this energy region are predicted to have apical/basal Cl(p(π))
→ Ni(3dz ) LMCT character (transitions 4 and 6 in Figure S16
and Table S10 (red lines)). Lastly, an apical Cl(pσ) → Ni(3dz )
LMCT is predicted within the higher-energy absorption
2
2
envelope (∼360 nm, transition 11, Figure S16 and Table S10),
2
2
among additional phenyl-ring π → Ni(3dx − y ) LMCTs. These
assignments are consistent with the persistence of higher-energy
absorption features and the complete disappearance of the lower-
energy feature upon photoreduction of 1 to form 2. Interestingly,
2
LMCT to the unoccupied pz/dz antibonding Ni-based hole (e.g.,
the β-LUMO) leads to the formal elimination of any apical Ni−
Cl bond order (Figures S17 and S18). Therefore, excited states
with LMCT character to the β-LUMO may be responsible for
the photoinduced cleavage of the apical Ni(III)−Cl bond.40
Scheme 1 illustrates a halogen extrusion mechanism that is
consistent with the spectroscopic, photochemical, photo-
crystallographic, and computational experiments. Photon
absorption by 1 provides access to 3. Steady-state photo-
crystallography experiments indicate that the apical halide, which
displays a substantially longer M−X bond than the basal halide
ligands, is selectively eliminated. Participation of ligand-based
aromatic substituents via 4 stabilizes the photodissociated
chlorine atom in the secondary coordination sphere of the
metal complex. Intermediate 4 displays a microsecond lifetime,
which we propose serves to provide sufficient time to assist the
removal of halogen from the primary coordination sphere,
thereby leading to productive elimination chemistry by circum-
venting the exothermic back reaction. In solution-phase
experiments, the photodissociated chlorine atom is likely
Steady-state photocrystallography results are consistent with
the photoactivation of the apical halogen bond. As we have
previously shown for halogen photoelimination reactions from
Rh2 complexes,37 atomic displacements during solid-state
photoreactions can be directly investigated with X-ray diffraction
by comparing diffraction with and without irradiation. Such
comparison of these diffraction data sets in a photodifference
map can indicate the photoinduced atomic displacements in a
crystalline sample. 1c, NiBr3(dppey) (dppey = cis-bis-
(diphenylphosphino)ethylene), was used in these experiments
because the bromide ligands have greater electron density as
compared to chloride ligands, which makes examination of
photoinduced atomic displacement possible at low populations
of reaction progress (X-ray structure of 1c in Figure S13). 1b was
not used in these experiments because of crystallographic
disorder in the solid-state structure in the dark. The difference
map (Figure 4b), generated from diffraction data collected prior
to and during irradiation (λ = 365 nm), shows the presence of a
photoinduced structure populated at 3.5(2)% of the crystal
C
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Scheme 1. Proposed Photoreduction of Complex 1a Proceeds
via an LMCT Excited State (3) to Furnish the Arene-to-Cl
Charge-Transfer Intermediate 4
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ASSOCIATED CONTENT
* Supporting Information
■
S
Detailed experimental procedures, spectroscopic data, XYZ
coordinates of computed structures, and the full ref 39. The
Supporting Information is available free of charge on the ACS
Publications website at DOI: 10.1021/jacs.5b03192.
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AUTHOR INFORMATION
Corresponding Author
*dnocera@fas.harvard.edu
Notes
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The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
(36) Tsao, M.-L.; Hadad, C. M.; Platz, M. S. J. Am. Chem. Soc. 2003,
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We gratefully acknowledge the NSF for funding (CHE−
1332783). D.C.P. is supported by a Ruth L. Kirchenstein
National Research Service award (F32GM103211). Photo-
crystallography was carried out at ChemMatCARS, Sector 15,
APS, which is principally supported by the NSF/DOE (NSF/
CHE-1346572). Use of APS was supported by the U.S.
Department of Energy, Office of Science, Office of Basic Energy
Sciences, under contract no. DE-AC02-06CH11357. We thank
Prof. G. M. Whitesides for access to calorimetry instrumentation.
́
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L. M.; Hall, M. B.; Zheng, S.-L.; Chen, Y.-S.; Nocera, D. G. J. Am. Chem.
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