Halogen Photoelimination from Monomeric Nickel(III) Complexes Enabled by the Secondary Coordination Sphere

Article abstract of DOI:10.1021/acs.organomet.5b00568

Endothermic halogen elimination reactions, in which molecular halogen photoproducts are generated in the absence of chemical traps, are rare. Inspired by the proclivity of mononuclear Ni(III) complexes to participate in challenging bond-forming reactions in organometallic chemistry, we targeted Ni(III) trihalide complexes as platforms to explore halogen photoelimination. A suite of Ni(III) trihalide complexes supported by bidentate phosphine ligands has been synthesized and characterized. Multinuclear NMR, EPR, and electronic absorption spectroscopies, as well as single-crystal X-ray diffraction, have been utilized to characterize this suite of complexes as distorted square pyramidal, S = 1/2 mononuclear Ni(III) complexes. All complexes participate in clean halogen photoelimination in solution and in the solid state. Evolved halogen has been characterized by mass spectrometry and quantified chemically. Energy storage via halogen elimination was established by solution-phase calorimetry measurements; in all cases, halogen elimination is substantially endothermic. Time-resolved photochemical experiments have revealed a relatively long-lived photointermediate, which we assign to be a Ni(II) complex in which the photoextruded chlorine radical interacts with a ligand-based aryl group. Computational studies suggest that the observed intermediate arises from a dissociative LMCT excited state. The participation of secondary coordination sphere interactions to suppress back-reactions is an attractive design element in the development of energy-storing halogen photoelimination involving first-row transition metal complexes.

Full text of DOI:10.1021/acs.organomet.5b00568

Article
pubs.acs.org/Organometallics
Halogen Photoelimination from Monomeric Nickel(III) Complexes
Enabled by the Secondary Coordination Sphere
Seung Jun Hwang,† Bryce L. Anderson,† David C. Powers, Andrew G. Maher, Ryan G. Hadt,
and Daniel G. Nocera*
Department of Chemistry and Chemical Biology, Harvard University, 12 Oxford Street, Cambridge, Massachusetts 02138,
United States
S
* Supporting Information
ABSTRACT: Endothermic halogen elimination reactions, in
which molecular halogen photoproducts are generated in the
absence of chemical traps, are rare. Inspired by the proclivity of
mononuclear Ni(III) complexes to participate in challenging
bond-forming reactions in organometallic chemistry, we
targeted Ni(III) trihalide complexes as platforms to explore
halogen photoelimination. A suite of Ni(III) trihalide com-
plexes supported by bidentate phosphine ligands has been
synthesized and characterized. Multinuclear NMR, EPR, and
electronic absorption spectroscopies, as well as single-crystal X-ray diffraction, have been utilized to characterize this suite of
complexes as distorted square pyramidal, S = 1/2 mononuclear Ni(III) complexes. All complexes participate in clean halogen
photoelimination in solution and in the solid state. Evolved halogen has been characterized by mass spectrometry and quantified
chemically. Energy storage via halogen elimination was established by solution-phase calorimetry measurements; in all cases,
halogen elimination is substantially endothermic. Time-resolved photochemical experiments have revealed a relatively long-lived
photointermediate, which we assign to be a Ni(II) complex in which the photoextruded chlorine radical interacts with a ligand-
based aryl group. Computational studies suggest that the observed intermediate arises from a dissociative LMCT excited state.
The participation of secondary coordination sphere interactions to suppress back-reactions is an attractive design element in the
development of energy-storing halogen photoelimination involving first-row transition metal complexes.
INTRODUCTION
■
The small-molecule activation reactions that lie at the heart of
energy storage and conversion, such as H2 and H2O oxida-
tions and H+, O2, and CO2 reductions, are all multielectron/
multiproton transformations. Development of catalysts to
effectively mediate these energy conversion transformations
relies on the careful choreography of proton and electron
Figure 1. Functionality in the secondary coordination sphere that has
been useful for promoting multielectron catalysis and stabilization of
delivery.1,2 Examples abound in both biological and synthetic
catalysis of secondary coordination sphere effects that synchro-
nize the electron and proton. Biological H2 oxidation in Fe-only
hydrogenase is accomplished via cooperation of second-
coordination-sphere bases, which enforce selective heterolytic
cleavage of H2.3−5 Similarly, installation of biomimetic proton
relays in the secondary coordination sphere of transition metal
complexes has emerged as a powerfully enabling design ele-
ment for the development of selective catalysts based on earth-
abundant first-row transition elements.6 Introduction of a basic
functionality, as either pendant carboxylates (i.e., hangman
porphyrin 1)7−9 or tertiary amines (i.e., hydrogenase mimic 2),10,11
in the secondary coordination sphere of electrocatalysts allows
controllable coupling of proton and electron delivery to sub-
strates during electrocatalysis. This strategy has enabled the
development of O2 (Figure 1, 1) and H+ (Figure 1, 1 and 2)
reduction reactions. H-bonding from basic groups in the
secondary coordination sphere has also been exploited in
energetic intermediates.
stoichiometric small-molecule reactions to generate isolable
metal oxido complexes, such as 6 (Figure 1, 3).12,13
Against this backdrop, HX-splitting photochemistry (X =
Cl, Br) has been advanced as an approach to solar-to-fuels
conversion in which proton reduction is coupled to halide
oxidation to provide carbon-neutral, closed energy conversion
cycles.14−16 Halogen photoelimination reactions are multi-
electron chemical transformations in which two electrons must
be simultaneously managed in the formation of X−X bonds;
Special Issue: Gregory Hillhouse Issue
Received: July 1, 2015
© XXXX American Chemical Society
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this bond-forming transformation represents the energy-storing
step in HX-splitting cycles. Whereas selective two-electron
halogen elimination reactions have been realized for 4d and 5d
Chart 1
metal complexes17−26 and some main-group compounds,27,28
a
similar chemistry is underdeveloped for earth-abundant 3d
metal complexes.29−31 The inherently shorter excited-state life-
times of 3d metal complexes, as well as their attendant pro-
clivity to participate in one-electron photoredox reactions,
contribute to the challenges that must be overcome to achieve
energy storage via halogen elimination from first-row com-
plexes.32−34
We have turned our attention to energy-storing HX photo-
chemistry that is driven by first-row transition metal complexes
under the rubric of two design principles. First, we have
targeted mononuclear Ni(III) complexes based on the seminal
work of Hillhouse that has demonstrated the selective two-
electron elimination of C−heteroatom bonds from high
oxidation state Ni complexes (Figure 2).35−41 C−N and C−O
the elimination reactions (Figure 3). The complexes also
participate in high-yielding trap-free halogen elimination, which
we have established to be substantially endothermic by
solution-phase calorimetry. The excited states responsible for
the observed halogen elimination reactions are ligand-to-metal
in character, and the ability of these complexes to promote
halogen elimination is attributed to the proclivity of the photo-
eliminating halogen atom to be guided from the primary
coordination sphere via a secondary coordination sphere ligand
effect arising from an aromatic−halogen interaction.
Figure 2. Oxidatively induced C−N and C−O reductive elimination
reactions from Ni(II) amide and alkoxide complexes pioneered by
Hillhouse. The reactions are proposed to proceed via single-electron
oxidation to generate Ni(III) complexes followed by reductive
elimination.
EXPERIMENTAL SECTION
reductive elimination reactions are sluggish from Ni(II)
complexes (i.e., 4 to 6), but become facile upon one-electron
oxidation to Ni(III) (5) via single electron transfer to an
external oxidant. Given the short excited-state lifetimes of first-
row complexes, we reasoned that their generation by MLCT
excitation would be insufficient to drive the targeted chemistry.
Accordingly, we turned our attention to installing Ni(III) into
the ground state of the putative photocatalyst. Second, owing to
our success in oxygen- and hydrogen-based chemical trans-
formation, we sought to utilize secondary coordination sphere
effects to enforce selective halogen elimination from complexes
with inherently short excited-state lifetimes to circumvent the
exothermic back-reaction.
■
General Considerations. All reactions were carried out in an
N2-filled glovebox. Anhydrous solvents were obtained from drying
columns and stored over activated molecular sieves.42 NiCl2(dme)
(dme = 1,2-dimethoxyethane), NiBr2(dme), dppb (dppb = bis(diphenyl-
phosphino)benzene), dppey (dppey = cis-bis(diphenylphosphino)-
ethylene), and NiCl2(dppe) (dppe = 1,2-bis(diphenylphosphino)ethane)
were obtained from Strem Chemicals. Bromine, dppe, and dcpe
(dcpe = 1,2-bis(dicylohexylphosphino)ethane) were obtained from
Sigma-Aldrich. PhICl2,43 dppe-OMe (dppe-OMe = 1,2-bis(di-4-metho-
xyphenyl)phosphine)ethane), and dppe-Cl (dppe-Cl = 1,2-bis(di-4-
chlorophenyl)phosphine)ethane) were prepared according to literature
procedures.44 Ni(III) complexes (7) and Ni(II) complexes (9) that are
discussed herein are enumerated based on the halide (X) and supporting
phosphine (LL) ligand: X = Cl, LL = dppe, 7a, 9a; X = Cl, LL = dppe-
OMe, 7a-OMe, 9a-OMe; X = Cl, LL = dppe-Cl, 7a-Cl, 9a-Cl; X = Cl,
LL = dppey, 7b, 9b; X = Cl, LL = dppb, 7c, 9c; X = Cl, LL = dcpe, 7d,
9d; X = Br, LL = dppe, 7e, 9e; X = Br, LL = dppe-Cl, 7e-Cl, 9e-Cl; X =
Br, LL = dppe-OMe, 7e-OMe, 9e-OMe; X = Br, LL = dppey, 7f, 9f; X =
Br, LL = dppb, 7g, 9g. Complexes 7a,45 7b,45 7c,45 7e,45 7f,45 7g,45 9a,46
9a-OMe,44 9b,47 9c,48 9d,49 9e,46 9e-OMe,50 and 9f51 were prepared
according to literature procedures. NMR spectra were recorded at the
Harvard University Department of Chemistry and Chemical Biology
NMR facility on a Varian Unity/Inova 600 spectrometer operating at
Figure 3. Photoextrusion from Ni(III) trichloride 7a proceeds via
initial chlorine radical extrusion, which is stabilized by an arene-to-
chlorine-atom charge transfer interaction (8a). Subsequent evolution
of chlorine affords Ni(II) complex NiCl2dppe (9a).
1
600 MHz for H acquisitions and 162 MHz for 31P{1H} acquisitions.
NMR chemical shifts are reported in ppm with the residual solvent
resonance as internal standard. 31P{1H} NMR chemical shifts were
referenced to an external 85% H3PO4 standard. Solution magnetic
moments were determined using the Evans method in CH3CN or
CH2Cl2 and measured using 19F NMR (hexafluorobenzene added);
diamagnetic corrections were estimated from Pascal constants.52 UV−vis
spectra were recorded at 293 K in quartz cuvettes on a Spectral Instru-
ments 400 series diode array blanked against the appropriate solvent. EPR
spectra were recorded on a Bruker ELEXIS E-500 or E-580 spectrometer
equipped with a Bruker ER4122 SHQE-W1 resonator and an Oxford
We report here a suite of Ni(III) trihalide complexes
(Chart 1) supported by bidentate phosphine ligands, NiX3(LL)
(LL = bidentate phosphine), that engage in halogen elimination
photoreactions both in solution and in the solid state. Time-
resolved photochemical studies have identified halogen atom
adducts of ligand-based aromatic groups to be intermediates in
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Instruments ESR 900 cryostat. EPR spectra were simulated using
EasySpin 4.5.5 to obtain g-values and hyperfine constants. Isotopic
abundance was accounted for by averaging the simulated spectra of all
naturally occurring isotopes scaled by their natural abundance. The
hyperfine coupling constants presented are that of the most abundant
isotope.53 The electrochemical experiment was recorded at ambient
temperature in a glovebox with a CH Instrument 760D or 730C
potentiostat. Cyclic voltammetry of a 1 mM CH3CN solution of Ni
complex 7a was measured with a scan rate of 100 mV/s in the dark
(0.1 M nBu4PF6 as supporting electrolyte). A glassy carbon working elec-
trode, BASi Ag/AgCl reference, and Pt wire counter electrode were used.
Preparation of Ni(II) Dihalide Complexes (NiX3(LL)). Ni(II)
complexes NiCl2(dppe-Cl) (9a-Cl), NiBr2(dppe-Cl) (9e-Cl), and NiBr2-
(dppb) (9g) were prepared by sequential treatment of NiX2(dme) (X =
Cl and Br) with an equimolar amount of the appropriate phosphine in
THF, precipitation with pentane, and isolation by vacuum filtration.
Preparation of Ni(III) Trichloride Complexes (NiCl3(LL)).
The following preparation of complex 7a-OMe is representative of
other members of this family. In a scintillation vial was added 60 mg
(0.093 mmol) of NiCl2(dppe-OMe) (9a-OMe) followed by 2 mL of
CH3CN. In a separate vial were added 15.3 mg (0.055 mmol,
0.6 equiv) of PhICl2 and 2 mL of CH3CN. PhICl2 was added dropwise
to the solution of 9a-OMe, during which time a color change from
orange to green was observed. The solution was stirred with the
exclusion of light for 5 min at 23 °C, at which time the solvent was
removed in vacuo to yield a green solid. The residue was washed with
pentane and dried in vacuo to afford 55 mg of the title complex as a
green solid (86% yield). EPR (1:1 CH3CN/PhCH3) g-value: g1 =
2.258; g2 = 2.169; g3 = 2.017. μeff = 2.17 μB. Complex 7a-Cl was
prepared analogously: green solid; 81% yield. EPR (1:1 CH3CN/
PhCH3) g-value: g1 = 2.241; g2 = 2.165; g3 = 2.01. μeff = 2.13 μB.
Complex NiCl3(dcpe) (7d) was prepared analogously except CH2Cl2
was used as solvent and Et2O was used to wash: green solid; 60% yield.
EPR (1:1 CH3CN/PhCH3) g-value: g1 = 2.26; g2 = 2.28; g3 = 2.01.
μeff = 2.10 μB.
CCD was 100 to 200 ns. For single-wavelength kinetics experiments,
the entrance and exit slits were set to 3 nm resolution, and 1.0 kV bias
was used for the photomultiplier tube (PMT). PMT outputs were
collected and averaged with a 1 GHz oscilloscope (LeCroy 9384CM).
The full spectra are averages of 50−200 four-spectrum sequences, and
the single-wavelength kinetics traces are averages of 500−1000 acqui-
sitions. CH3CN solutions of complexes 7a−7d were prepared in
100 mL Schlenk flasks in an N2-filled glovebox. During TA acqui-
sitions the solutions were flowed without recirculation through a 3 mm
diameter, 1 cm path length flow cell (Starna, type 585.2) using a
peristaltic pump and positive N2 pressure.
X-ray Crystallographic Details. Single crystals of 7c were ob-
tained from a CH2Cl2 solution layered with toluene. Crystals of 7g
were obtained from a CH2Cl2/PhCH3 solution of the complex layered
with a small amount of Br2 in CH2Cl2 solution. Diffraction data were
collected either on a Bruker three-circle platform goniometer equipped
with an Apex II CCD and an Oxford cryostream cooling device
(100 K) with radiation from a graphite fine focus sealed tube Mo Kα
(0.710 73 Å) source or on a vertically mounted Bruker D8 three-circle
platform goniometer equipped with an Apex II CCD and an Oxford
Diffraction Helijet cooling device (15 K) with synchrotron radiation
(0.413 28 Å) supplied to ChemMatCARS located at Advanced Photon
Source (APS), Argonne National Laboratory (ANL). Crystals were
mounted on a glass fiber pin using Paratone N oil. Data were collected
as a series of φ and/or ω scans. Data were integrated using SAINT and
scaled with multiscan absorption corrections using SADABS.56 The
structures were solved by intrinsic phasing using SHELXT (Apex2
program suite v2014.1) and refined against F2 on all data by full matrix
least-squares with SHELXL-97.57 All non-H atoms were refined aniso-
tropically. H atoms were placed at idealized positions and refined using
a riding model. Crystal data and refinement statistics are sum-
marized in Table 1, and thermal ellipsoid plots are shown in Figure 4.
Table 1. Crystal Data and Structure Refinement for 7c
and 7g
Preparation of Ni(III) Tribromide Complexes (NiBr3(LL)). The
following preparation of complex 7e-OMe is representative of other
members of this family. A sample of NiBr2(dppe-OMe) (9e-OMe)
(60 mg, 0.081 mmol) was dissolved in 2 mL of CCl4. A solution of
0.194 M Br2 (206 μL, 0.041 mmol, 0.5 equiv) in CH2Cl2 was added
dropwise to give a black solution, which was stirred at 23 °C for
30 min in the dark. The volatiles were removed in vacuo to give the
product as a dark brown powder. The residue was washed with
pentane and dried in vacuo (80% yield). EPR (1:1 CH2Cl2/PhCH3)
g-value: g1 = 2.208; g2 = 2.208; g3 = 2.025. μeff = 1.95 μB. Complex
7e-Cl was prepared analogously: dark brown solid; 77% yield. EPR
NiCl3(dppb) (7c)
NiBr3(dppb) (7g)
formula
CCDC no.
fw, g/mol
temp, K
cryst syst
space group
a, Å
C30H24Cl3NiP2
1040260
611.49
C16.5H15Br1.5Cl3Ni0.5
1040259
499.83
P
100(2)
15(2)
monoclinic
P2(1)
monoclinic
P2(1)/m
8.7809(4)
21.401(1)
10.6362(5)
90
9.7441(9)
14.730(1)
10.2714(9)
90
b, Å
(1:1 CH3CN/PhCH3) g-value: g1 = 2.201; g2 = 2.191; g3 = 2.025. μeff
1.90 μB.
=
c, Å
α, deg
β, deg
γ, deg
Photochemistry. Steady-state photochemical reactions were per-
formed using a 1000 W high-pressure Hg/Xe arc lamp (Oriel), and the
beam was passed through a water-jacketed filter holder containing
the appropriate long-pass filter, an iris, and a collimating lens. Samples
were photolyzed in a constant-temperature circulating water bath.
Photochemical quantum yields (Φp) were determined using 370 nm
monochromatic light, which was generated with a 370 nm Hg line
filter on a 1000 W Hg/Xe arc lamp. Potassium ferrioxalate solution
(0.006 M) was used as a chemical actinometer.54 The photo flux was
determined from the average of actinometric measurements before
and after irradiation of the reaction samples. Each Φp determination
was carried out in triplicate on solutions of the complex in CH2Cl2
or CH3CN with benzene. Nanosecond resolved transient absorption
(TA) spectroscopy experiments were performed using a system
described previously,55 but with an iHR320 spectrometer. The 355 nm
laser pulses (8−10 ns at fwhm) used for excitation were generated
using the third harmonic of a 10 Hz Nd:YAG pulsed laser. The white-
light continuum was provided by a Xe-arc lamp set to 2.0 ms pulses
with 30 A current. A 250 or 1000 nm blaze grating (300 grooves/mm)
was used for spectral acquisitions centered below 500 nm and above
500 nm, respectively. For full-spectrum TA acquisitions, the entrance
slit was set to 0.16 mm (3.0 nm resolution) and the gate time for the
113.297(2)
90
113.3720(7)
90
V, A3
1354.1(2)
2
1834.8(2)
4
Z
a
R1
0.0525
0.0187
b
wR2
0.0861
0.0439
c
GOF (F2)
0.983
1.039
Rint
0.0678
0.0210
a
b
2
2
R1 = ∑∥Fo − |Fc|/∑|Fo|. wR2 = (∑(w(Fo − Fc )2)/
c
2
2
∑(w(Fo )2))1/2. GOF = (∑w(Fo − Fc2)2/(n − p))1/2 where n is
the number of data and p is the number of parameters refined.
X-ray crystal structures of 7c, 7g, 9c, and 9e are available from the
Cambridge Crystal Structure Database (1040260, 1040259, 1401868,
and 1401864, respectively).
Solution-Phase Calorimetry. Solution-phase calorimetry meas-
urements were carried out using a Thermometric 2225 Precision
Solution calorimeter. In the glovebox, a CH2Cl2 solution of Ni(II)
dichloride was prepared. This solution was transferred to the glass cell
of the calorimeter, which was equipped with an ampule-breaking rod.
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Oxidation of Ni dichloride complexes 9a−9d with PhICl2
afforded the corresponding monomeric Ni trichloride com-
plexes NiCl3(dppe) (7a), NiCl3(dppe-OMe) (7a-OMe),
NiCl3(dppe-Cl) (7a-Cl), NiCl3(dppey) (7b), NiCl3(dppb)
(7c), and NiCl3(dcpe) (7d). Using a procedure adapted from
Levason,45 oxidation of Ni(II)Br2(LL) complexes was accom-
plished by treatment with 0.5 equiv of Br2 in CCl4 to afford
NiBr3(dppe) (7e), NiBr3(dppe-OMe) (7e-OMe), NiBr3(dppe-
Cl) (7e-Cl), NiBr3(dppey) (7f), and NiBr3(dppb) (7g). Cyclic
voltammetry of Ni(III) compound 7a in CH3CN showed a
quasi-reversible wave at 0.40 V vs Fc+/Fc assigned to the
Ni(II)/Ni(III) couple (Figure S3).
Ni(III) complexes display no 1H NMR or 31P NMR spectra.
Evans’ method and variable-temperature EPR indicated the
complexes are S = 1/2, as expected for mononuclear d7 com-
plexes. The EPR spectrum of NiCl3(dppey) (7b), shown in
Figure 5, is exemplary of the series; the EPR spectra of remaining
Figure 4. Thermal ellipsoid plots of Ni(III) complexes 7c and 7g in
which solvent molecules and H atoms have been removed for clarity.
Ellipsoids are drawn at 50% probability.
A small glass ampule containing PhICl2 was added, sealed, and loaded
into the calorimeter. Following temperature equilibration (achieved
standard deviation of 2 μK after 2 h), the reaction was initiated by
breaking the ampule and rotating the calorimeter to ensure complete
mixing. Electrical calibrations were run before and after breaking the
ampule. The enthalpy of chlorination of Ni(II) was determined by
subtracting half the enthalpy of chlorination of PhI from the experi-
mentally measured reaction enthalpy. Measured data were corrected
for the enthalpy of solution of the solid PhICl2, which was measured
separately. Reported data are the average of three independent mea-
surements.
Computational Details. All DFT calculations were performed
using the Gaussian 09, revision D.01, software suite.58 For all calcu-
lations, the B3LYP59−61 hybrid exchange−correlation functional was
used in combination with a split basis set (TZVP62 for Ni, P, and Br;
TZV63 for C and H). Starting geometries for gas-phase optimization
were obtained from crystallographic coordinates; frequency calcu-
lations were carried out to ensure structures represented energetic
minima. Time-dependent DFT (TD-DFT) single-point calculations
were carried out on the gas-phase geometry optimized models using
the same combination of functional and basis sets. Where used,
solvation effects were included using the polarized continuum model
(PCM). Ground- and excited-state potential energy surfaces (PESs)
for metal−apical halide bond loss were constructed using constrained
geometry optimizations coupled to subsequent TD-DFT calculations.
Charge decomposition analyses (CDA) were carried out using the
AOMix program (revision 6.82).64 The degree of phosphine ligand
donation was calculated using NiX3 and phosphine ligands as
fragments; the degree of phosphine ligand donation was calculated
as the difference in total electron donation (sum of all occupied α and
β molecular orbitals) from the phosphine ligand to NiX3 and back-
donation between NiX3 and the phosphine ligand. Wave function
contours were generated using the β-LUMO program.65
Figure 5. EPR spectrum of NiCl3(dppey) (7b) recorded at 8.0 K in
1:1 CH3CN/toluene glass (−, black) and simulated (− red).
members of the series are shown in Figures S4−S14. The EPR
spectra display pseudoaxial doublet signals with gx, gy > 2 and
gz ≈ 2.000 and a four-line superhyperfine coupling pattern in the
gz direction (Table S3). The superhyperfine coupling is a direct
2
probe of the coupling of the dz unpaired electron with the
nuclear magnetic moment of the apical halide ligand (I = 3/2
for both Br and Cl) and is a measure of the degree of elec-
tron delocalization of the unpaired electron along the apical
Ni(III)−X bond.66 The superhyperfine coupling (Az) varies depend-
ing on the phosphine ligand backbones and arene substituents
on the bidentate phosphine ligands; superhyperfine coupling
constants are tabulated in Table S3.
Single-crystal X-ray structures of representative Ni(III) com-
plexes were obtained and indicate a distorted square pyramidal
geometry about the Ni center (Figure 4), with shorter basal
Ni(III)−X bonds and a longer apical Ni(III)−X bond (Table S4).
For example, the apical Ni−Cl in complex 7c is 2.287(1) Å,
whereas the basal Ni−Cl bonds are 2.224(2) and 2.219(2) Å,
respectively.
The electronic absorption spectra of Ni complexes 7c and 9c
are shown in Figure 6a; spectra of complexes 7 and 9 are shown
in Figures S15−S21. The spectrum of Ni(II) complex 9c is
representative of the Ni(II) complexes discussed herein and
is dominated by an intense band at 285 nm (ε = 2.2 ×
104 M−1 cm−1) that is flanked by a shoulder at 335 nm (ε =
3.0 × 104 M−1 cm−1) and a much weaker band at 463 nm (ε =
1.7 × 103 M−1 cm−1). Oxidation of complex 9c results in a
bathochromic shift of the 335 nm absorption feature to 348 nm
(ε = 5.2 × 103 M−1 cm−1) and a slight hypsochromic shift
of the 463 nm absorption feature to 440 nm (ε = 1.8 ×
103 M−1 cm−1). In addition, a pronounced absorption at 657 nm
RESULTS
■
Synthesis and Characterization of Mononuclear Ni(III)
Complexes. Monomeric Ni(II) dihalide complexes were
accessed by treatment of Ni(II) halide starting materials
(NiX2(dme) (X = Cl and Br)) with an equimolar amount of
the appropriate bidentate phosphine ligand. Thermal ellipsoid
plots of Ni(II) complexes 9c and 9e are illustrated in Figures S1
and S2, and crystallographic data are summarized in Table 1.
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NiCl3(dcpe) is Φp(7d) = 20.1%, whereas that for the dppe
complex is Φp(7a) = 76.4% and near unity for the dppey
complex 7b. We note that the presence of an aromatic solvent
increases the Φp of 7d. The addition of benzene to CH3CN
solutions causes Φp to increase monotonically until 30%
mixtures are attained, after which Φp levels off at ∼60%.
Addition of 30% CCl4 showed no improvement in the
photoefficiency (Φp = 20.8%), suggesting that a change in
solvent dielectric (ε(C6H6) = 2.27 and ε(CCl4) = 2.24) is not
the source of increased quantum yield for CH3CN/benzene
mixtures. A similar change in Φp was also observed for the
addition of benzene to solutions of NiCl3(dppb) (7c) (Φp =
8% in CH3CN vs Φp = 35% with 30% benzene). Halogen
photoelimination from substituted dppe-supported NiX3(LL)
complexes was also examined to probe the effects of the
electronic properties of the phosphine without modifying the
steric environment about the metal center. The quantum yields
for para-OMe and para-Cl substituted dppe derivatives were
determined to be 35.0% and 5.6% for 7a-OMe and 7a-Cl,
respectively.
Time-Resolved Photochemistry. Figures S33−S42 show
the transient absorption full spectra and single-wavelength
kinetics of samples containing Ni(III)Cl3(LL) complexes 7a−c
following laser excitation at 355 nm in CH3CN. For each
sample, the earliest achievable TA difference spectrum acquired
(time delay = 40−70 ns) is a superposition of the bleach signal
of the starting complex, the absorption of the final product,
Ni(II)Cl2(LL), and any transient species present promptly.
The single-wavelength kinetics measurements show changes in
absorbance with lifetimes ranging from 100 ns to 10 μs,
depending on the complex, indicating the decay of a transient
species to the final Ni(II)Cl2(LL) photoproduct, as confirmed
by the full TA spectra taken at 50 μs. In the case of high
quantum yields for halogen elimination, the back-reaction in
which the transient decays directly back to the original
Ni(III)Cl3(LL) complex is necessarily negligible. Therefore,
to a good approximation subtraction of the 50 μs TA spectrum
from that of the early spectrum yields the absorption spec-
trum of the transient species. Accordingly, Figure 7a shows the
resulting absorption spectrum (blue) for the transient
generated from complex 7a. The spectrum of the transient
displays an absorption maximum at 366 nm as well as a broad
band with a maximum at 530 nm. These spectral features are
similar to those of the chlorine atom charge transfer adduct of
benzene (Figure 7a, black), generated by either laser flash
photolysis or pulse radiolysis.67−70 This suggests that the
observed transient intermediate is a Cl atom adduct formed
with the ligand aryl group. A similar result is obtained for each
Ni(III)Cl3(LL) complex 7a−c described herein, suggesting that
each complex forms a similar transient intermediate.
Figure 6. (a) Extinction spectra of complex 7c (−, black) and complex
9c (−, red). (b) Photolysis of a 0.08 mM solution of complex 7c in
CH3CN (λ > 400 nm) at 25 °C.
(ε = 1.4 × 103 M−1 cm−1) for complex 7c is notably absent in
Ni(II) complex 9c. The profiles of the bromide and chloride
Ni(III) complexes are similar, but the former is red-shifted,
indicating that the absorption spectra of the nickel halide
complexes reported here are largely determined by X → Ni(III)
charge transfer.
Halogen Photoelimination Chemistry. Irradiation of
CH2Cl2 solutions of NiX3(LL) (7) with visible light (λ > 400 nm)
leads to clean conversion to the corresponding reduced
NiX2(LL) (9) as determined by both UV−vis and 31P{1H}
NMR. Well-anchored isosbestic points are observed in most
cases studied (Figures 6b and S15−S21), and no P-containing side-
products were observed by 31P{1H} NMR (Figures S22−S32).
Halogen photoelimination quantum yields (Φp) from each of
the Ni(III) complexes were measured actinometrically (against
potassium ferrioxalate) and are summarized in Table 2.
Table 2. Quantum Yields of NiX3(LL) Halogen Elimination
a
Photochemistry
LL
dppe
NiCl3(LL)
7a
Φp
NiBr3(LL)
Φp
0.764
0.230
0.128
0.960
0.077
0.201
0.330
0.410
0.490
0.590
0.620
7e
0.169
0.352
0.056
0.401
0.047
dppe-OMe
dppe-Cl
dppey
7a-OMe
7a-Cl
7b
7e-OMe
7e-Cl
7f
Further support for the intermediacy of a Cl-atom|aromatic
adduct formed along the pathway of the halogen elimination
photochemistry of the NiX3(LL) complexes was obtained by
investigating substituted dppe-supported Ni(III) complexes.
The λmax of the transient intermediate is perturbed by the pres-
ence of substituents on the dppe ligand. As shown in Figure 7b−d,
the λmax of the transient spectrum red shifts with donor sub-
dppb
7c
7g
dcpe
7d
b
w/5% C6H6
b
b
b
b
w/10% C6H6
w/20% C6H6
w/30% C6H6
w/40% C6H6
a
b
stitution on the aryl ring (7a, λmax = 530 nm; 7a-Cl, λmax
=
In CH2Cl2, λexc = 370 nm bandpass filter. In CH3CN with percent
540 nm; 7a-OMe, λmax = 610 nm). This observed red shift in the
energy of the Cl-atom|aromatic adduct with increasing donor
properties of the substituent group is consistent with the spectral
trend previously reported for Cl-atom|aromatic adducts as ob-
served by pulse radiolysis experiments.67
addition of benzene.
The presence of an aromatic group in the second coordi-
nation sphere significantly enhances Φp. The photochemical
quantum yield, Φp, for cyclohexyl phosphine complex
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Table 3. Reaction Enthalpy for Chlorination of Ni(II)
Complexes
LL
ΔH (kcal mol−1
)
dppe
dppey
dppb
dcpe
−23.7
−20.8
−16.7
−7.0
the average of three independent measurements. All complexes
confront endothermic barriers to halogen elimination, with
complex 7a possessing significant energy storage capacity for
the Cl2 photoelimination reaction.
Computational Results. DFT geometry optimization of
7f was performed as described in the Experimental Section.
The DFT-based ground-state wave function is consistent with a
Ni(III) center and an unpaired electron/hole residing in an
2
antibonding Ni dz /halide pz orbital, which results in a
delocalized metal-to-apical-ligand bonding description as
indicated by EPR spectroscopy (e.g., DFT-based spin densities
of ∼0.81 and ∼0.23 for Ni(III) and the apical halide, respec-
tively, for 7f). Additionally, the β-Mayer bond order for this
complex is calculated to be ∼0.44. To gain insight into the
nature of the excited states of the Ni(III) compounds, TD-DFT
calculations were performed using the optimized geometry of 7f.
The calculated absorption spectrum largely reproduces the
spectral features observed in the experimental data (Figure S54
and Table S19). On the basis of the TD-DFT results, the lower
energy absorption feature (∼575 nm) arises from an apical
Figure 7. (a) Comparison between the absorption spectra of the
transient species generated by laser flash photolysis of 7a (−, blue) and
the chlorine atom charge transfer adduct of benzene (−, black), also
generated by laser flash photolysis (data adapted from refs 76 and 70,
respectively). The blue spectrum is generated by subtracting the TA
difference spectrum of a 0.44 mM solution of 7a, NiCl3(dppe), in
CH3CN acquired at a 50 μs time delay from that acquired at a 40 ns
time delay (λexc = 355 nm). The low-energy transient feature shifts to
lower energy with the introduction of electron-donating substituents
of the phenyl ring of dppe along the series (b) 7a, (c) 7a-Cl, and
(d) 7a-OMe.
2
2
Br(p(π)) → Ni(dx −y ) LMCT, while the higher energy
absorption feature (∼400−500 nm) largely consists of intense
2
2
phenyl ring π → Ni(dx −y ) charge transfer (CT) transitions and
Given that Φp for halogen photoelimination increases
with the addition of benzene (Table 2), we examined the TA
spectrum of complex 7d in the presence of 30% benzene. The
nanosecond-resolved TA spectrum of cyclohexyl phosphine
Ni(III) complex 7d exhibits a weak feature with a maximum
at 430 nm. The signal intensity of the observed transient
intermediate at 430 nm increases substantially with the addition
of benzene to the solution and it slightly red shifts; the time
evolution of the transient decay remains essentially unchanged.
Solution-Phase Calorimetry. The chlorine photoelimina-
tion enthalpies were determined by solution calorimetry, which
was accomplished by addition of PhICl2 as a solid to solutions
of Ni(II)Cl2(LL) complexes (9) (Figures S49−S51). The
enthalpy for chlorination was obtained based on the equations:
2
2
weak basal Br(p(π)) → Ni(dx −y ) LMCTs. These assignments
are consistent with the photochemical results in that the lower
energy CT feature decays upon irradiation, while the higher
energy feature persists. Interestingly, an apical Br(p(σ)) →
2
Ni(dz ) LMCT is predicted within the higher energy absorption
envelope (∼440 nm, state 7, SI Figure S54 and Table S19). In a
2
molecular orbital picture, transitions to the unoccupied Ni(dz )
orbital (i.e., the β-LUMO) eliminate any formal Ni−apical
halide bond order and would be expected to result in significant
destabilization. We therefore explored the nature of the ground-
and excited-state PES for elongation of the Ni−apical halide
bond for 7f. The CH2Cl2-corrected PESs for Ni−Br bond loss
are given in Figure 8a, in which the TD-DFT calculated σ → σ*
excited-state energy is plotted as a function of ligand−metal
bond distance. Note that this transition involves excitation of a
2
2
β pz/dz bonding electron to the β pz/dz antibonding MO,
β156 and β171 in Figures S52−S53. This LMCT state is
clearly dissociative,71 as can be seen from the TD-DFT ground-
and excited-state surfaces, resulting in the formation of a one-
electron-reduced Ni(II) S = 0 metal center and a Br radical.
In addition to the PES, we have geometry optimized the
resulting Cl-atom|aromatic charge transfer intermediate for
both 8a and 8e. DFT frequency calculations indicate local
energetic minima for both complexes, which is consistent with
the TA spectroscopic results and previous reports of the
energetics of halogen|arene complexes.72 The preferred halogen
atom|arene interaction is roughly η1 (C−X distances of ∼2.6
and 2.9 Å for the Cl and Br radicals, respectively). Mulliken
spin densities are consistent with significant charge transfer,
The heat of reaction was measured for eq 1. The known heat
of reaction of chlorine release from solid PhICl2 was corrected
for the enthalpy of solution of the solid PhICl2, which was
measured separately. The enthalpies of Cl2 addition to com-
pounds of 9, listed in Table 3, were obtained from eq 3 and are
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DISCUSSION
■
The significant oxidizing power of Ni(III) suggested the
possibility of developing a halogen elimination photochemistry
based on Ni(III) complexes and thus the possibility of develop-
ing HX-splitting energy-storing cycles. To this end, we targeted
NiX3(LL) complexes (X = Cl, Br; LL = bidentate phosphine)
(7), which are furnished from the oxidation of corresponding
Ni(II) square planar NiX2(LL) complexes (9) with halogen
sources. All of the Ni(III) compounds are paramagnetic and
possess a square pyramidal primary coordination sphere (Figure 4).
Steady-state photolysis of Ni(III) complexes 7 results in their
clean conversion to Ni(II) complexes 9. The solution photo-
chemistry is characterized by tightly anchored isosbestic points,
and both UV−vis and 31P NMR indicate that the photo-
elimination proceeds cleanly (Figure 6). All of the complexes
demonstrate an efficient halogen elimination photochemistry
with quantum yields reaching 96%. In general, Cl2 photo-
elimination proceeds at higher quantum efficiencies than Br2
elimination from homologous complexes (Table 2). The origin
of this difference in efficiencies between Cl and Br complexes is
unclear and under current investigation. In solution, halogen is
trapped by H atom abstraction from solvent, and consequently
the photoevolved halogen equivalents are not observed.74,75
The need to trap the eliminated halogen is circumvented when
the photochemistry is carried out in the solid state. Solid-state
photolysis of thin films of Ni(III) complexes 7 affords the
Ni(II) complexes 9 and molecular halogen, which has been
isolated, characterized, and quantified. Solution-phase calorim-
etry indicates that halogen addition to Ni(II) is substantially
exothermic (Table 3), and thus photoelimination from
NiX3(LL) compounds represents an energy-storing trans-
formation.
Preliminary transient spectroscopic experiments suggested
that halogen photoelimination from NiCl3(dppe)76 was assisted
by a secondary coordination sphere effect involving an arene-
to-chlorine-radical charge transfer intermediate. The generality
of the proposed intermediate has now been established for
the class of Ni(III)X3(LL) complexes. TA spectra of the
intermediate generated by NiCl3(LL) photoexcitation display
two distinct spectral features: an absorption in the UV region
and a broad absorption in the visible region. The photointer-
mediate observed in the TA spectrum of complexes with un-
substituted aryl phosphines exhibits a UV feature at ∼350 nm
and a band in the visible region at λmax ≈ 550 nm. As shown in
Figure 7a, the spectrum of the intermediates is consistent with
that of halogen|aryl CT complexes, although it is red-shifted as
compared to simple CT adducts owing to electron donation
from the phosphine group. The CT absorption band further
red shifts with the introduction of donor groups on the
phenyl phosphine. The TA intermediate of 7a-Cl and 7a-OMe
(Figure 7b and c) shifts to 540 and 600 nm, respectively, con-
sistent with the observed shift of CT bands in arene|Br com-
plexes resulting from aryl substitution with electron donors.77
To probe the influence of aromatic residues on the halogen
elimination photochemistry, the phenyl groups of the dppe
were replaced with cyclohexyl groups. Complex 7d participates
in solution-phase halogen elimination but with considerably
attenuated quantum yield. A transient intermediate of 7d is
observed, but it is weak and considerably blue-shifted from 7a.
Halogen atom adducts of cyclohexane are known, and as
observed here, the signal is weak and the charge transfer band is
blue-shifted (λmax = 435 nm) due to an increase in ionization
Figure 8. (a) CH2Cl2-corrected PESs (S = 1/2) of 7f as a function of
Ni−Br(ap) bond cleavage. Ground-state energies are given as black
squares, and the energies of the β(pσ(Br(ap))) → β(Ni(d(z2)))
LMCT excited state (7 in Table S19) are given as red squares. Note
that the PES of this σ → σ* excited state is repulsive and can, upon
excitation, result in homolytic cleavage of the Ni(III)−Br bond to a Br
radical and a low-spin, S = 0 Ni(II) metal center. (b) Computational
results of the charge transfer transitions of NiX3(dppe) complexes.
Gas-phase TD-DFT-calculated absorption spectra for 8a (black line)
and 8e (red line).
i.e., a closed-shell Ni(II) ion associated with a halide atom (e.g.,
spin densities of 0.00, 0.66, and 0.74 for Ni(II), Cl, and Br,
respectively). The TD-DFT-calculated absorption spectra are
given in Figure 8b. The absorption spectra are consistent with
experimental observations (Figure S39); there are relatively
intense charge transfer features predicted at ∼500 and ∼550 nm
for the Cl and Br adducts, respectively (Figure 8b). The origin of
these transitions arises from filled, bonding halide(p)|arene-
(p(π)) donor orbitals and the β-LUMO, which is an antibonding
halide(p)|arene(p(π)) molecular orbital (Figure S55 illustrates
representative orbital contours).
The amount of total electron donation from the phosphine
ligand to Ni(III) has been explored using CDA73 as described
in the Experimental Section. The values have been computed
for all NiCl3(LL) and NiBr3(LL) complexes and are compared
to the quantum yields in Figure S56. In general, a positive
correlation between Φp and electron donation of the phos-
phine to Ni(III) is observed for NiCl3(LL) and NiBr3(LL)
complexes.
Increased electron donation to Ni(III) should result in a
lower Ni(III/II) reduction potential, which would oppose
homolytic cleavage of the Ni(III)−X bond. This difference in
donation should be reflected in the solution-phase calo-
rimetry measurements. Indeed, the complexes with larger endo-
thermic barriers to halogen elimination generally have higher
quantum yields (e.g., 7a: 23.7 kcal/mol and Φp = 0.764 versus
7d: 7.0 kcal/mol and Φp = 0.201). Thus, experiment and
theory together emphasize the dual contributions of the
secondary coordination sphere and the excited-state relaxa-
tion/decay pathways in directing the outcome of the photo-
chemistry.
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Article
energy of cyclohexane as compared to an aromatic group.78
In view of the predicted blue shift coupled with the clean
conversion to starting material upon photolysis, we believe that
the intermediate observed in the TA spectrum of 7d is also a
Cl-radical complex, 8d. Upon the addition of increasing con-
centrations of benzene, the quantum yield approaches an
asymptotic value of ∼60%. We believe that the enhancement in
quantum yield is due to specific radical−π interactions, because
addition of CCl4 (similar dielectric to benzene) did not have a
measurable influence on Φp. When the TA spectrum of com-
plex 7d is measured in the presence of 30% benzene, the spec-
tral feature exhibits a 15 nm bathochromic shift (λmax = 450 nm)
and the rate of decay of the intermediate is essentially unchanged
(Figures S45−S48). The enhancement in Φp upon the addition
of benzene without a change in the time evolution of the
intermediate suggests that benzene is enhancing the initial
dissociation of the chlorine radical from the nickel center, but
does not react at a competitive rate with the generated inter-
mediate. The observed decay rate of ∼105 s−1 is considerably
slower than the dissociation rates of ∼106 s−1 observed for Cl-
benzene adducts.70 Therefore, only a small and undetectable
concentration of Cl-benzene complex would be formed during
the experiment.
The computed excited state PES of 7f (Figure 8a) provides
further support for rapid halide radical elimination. The excited-
state PESs are dissociative in both gas-phase and dielectric-
corrected (CH2Cl2) calculations, suggesting a rapid dissociation
of the halide radical from the photoreduced Ni center.79,80 The
fate of the halide radical is then dictated by its subsequent back-
reaction or sequestration by solvent. Utilization of secondary
coordination interactions with arene ligands appears to provide
an effective pathway to guide the halogen atom out of the
primary coordination sphere, thereby preventing the energeti-
cally favored back-reaction. For the resulting charge transfer
intermediate, the nature of the dominant transitions observed
in the TA spectra is predicted by TD-DFT calculations as arene
to halide charge transfer, consistent with the spectral shift of the
absorptions upon perturbation of the arene ring by the addition
of methoxy groups.
Examination of the relationship of Φp with both reaction
enthalpy and superhyperfine coupling reveals that Φp is not
necessarily correlated with reaction thermodynamics or M−L
covalency (Tables 3 and S3, respectively). However, complexes
with larger endothermic barriers do tend to show higher quan-
tum yields (Tables 2 and 3). Similarly, a positive correlation
between ligand donor strength (as determined using CDA
analyses) and Φp was observed. Generally, the increased Φp
upon increased endothermicity of ligand loss and electron
donation suggests that the important factors governing Φp
derive from the secondary coordination sphere as well as the
relative rates of the various decay pathways of the dissociative
electronic excited state. Nonradiative decay pathways have been
shown to be sensitive to changes in ligand donor ability81,82 and
appear to be suppressed in the NiX3(LL) complexes studied by
electron-rich ligands. These contributions may be unraveled by
ultrafast time-resolved spectroscopy.
stabilized by the interaction of the photoeliminating halogen
atom with an aryl ring of the secondary coordination sphere.
The efficiency of the photoelimination can be directly traced to
the stability of the charger transfer halogen|aryl adduct. Inter-
actions with aromatic groups of a coordinating ligand offer
a means to suppress rapid back-reactions and increase the
quantum yield for halogen elimination, which is the critical step
in designing photocatalytic HX splitting cycles.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.5b00568.
31P{1H} NMR analysis of photoreactions, EPR spectra,
electrochemical data, steady-state photolysis data, TA
spectra, thermograms, computational details including
Cartesian coordinates, and complete ref 58 (PDF)
Molecular geometries (XYZ)
X-ray crystallographic data (CIF)
AUTHOR INFORMATION
Corresponding Author
*E-mail: dnocera@fas.harvard.edu.
■
Author Contributions
†S. J. Hwang and B. L. Anderson contributed equally to this
work.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We gratefully acknowledge the NSF for funding (CHE-
1464232). D.C.P. is supported by a Ruth L. Kirchenstein
National Research Service award (F32GM103211). Use of APS
was supported by the U.S. DOE, Office of Science, Office
of Basic Energy Sciences, under Contract No. DE-AC02-
06CH11357. We thank Prof. G. M. Whitesides for access to his
laboratory’s calorimetry instrumentation. In memory of our
friend and colleague, Greg.
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DOI: 10.1021/acs.organomet.5b00568
Organometallics XXXX, XXX, XXX−XXX