Tandem redox mediator/Ni(II) trihalide complex photocycle for hydrogen evolution from HCl

Article abstract of DOI:10.1039/c4sc02357a

Photoactivation of M-X bonds is a challenge for photochemical HX splitting, particularly with first-row transition metal complexes because of short intrinsic excited state lifetimes. Herein, we report a tandem H₂ photocycle based on combination of a non-basic photoredox phosphine mediator and nickel metal catalyst. Synthetic studies and time-resolved photochemical studies have revealed that phosphines serve as photochemical H-atom donors to Ni(II) trihalide complexes to deliver a Ni(I) centre. The H₂ evolution catalytic cycle is closed by sequential disproportionation of Ni(I) to afford Ni(0) and Ni(II) and protolytic H₂ evolution from the Ni(0) intermediate. The results of these investigations suggest that H₂ photogeneration proceeds by two sequential catalytic cycles: a photoredox cycle catalyzed by phosphines and an H₂-evolution cycle catalyzed by Ni complexes to circumvent challenges of photochemistry with first-row transition metal complexes.

Full text of DOI:10.1039/c4sc02357a

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Tandem redox mediator/Ni(II) trihalide complex
photocycle for hydrogen evolution from HCl†
Cite this: DOI: 10.1039/c4sc02357a
a
Seung Jun Hwang,a David C. Powers,a Andrew G. Maherab and Daniel G. Nocera
*
Photoactivation of M–X bonds is a challenge for photochemical HX splitting, particularly with first-row
transition metal complexes because of short intrinsic excited state lifetimes. Herein, we report a tandem
H2 photocycle based on combination of a non-basic photoredox phosphine mediator and nickel metal
catalyst. Synthetic studies and time-resolved photochemical studies have revealed that phosphines serve
as photochemical H-atom donors to Ni(II) trihalide complexes to deliver a Ni(I) centre. The H2 evolution
catalytic cycle is closed by sequential disproportionation of Ni(I) to afford Ni(0) and Ni(II) and protolytic
H2 evolution from the Ni(0) intermediate. The results of these investigations suggest that H2
Received 4th August 2014
Accepted 7th October 2014
photogeneration proceeds by two sequential catalytic cycles:
a photoredox cycle catalyzed by
DOI: 10.1039/c4sc02357a
phosphines and an H2-evolution cycle catalyzed by Ni complexes to circumvent challenges of
www.rsc.org/chemicalscience
photochemistry with first-row transition metal complexes.
To overcome the short excited state lifetimes typical of rst-
row complexes, we have pursued a photoredox strategy for H2
Introduction
evolution from HCl in which the photochemistry and H2
evolution roles are separated between a photoredox mediator
and a hydrogen-evolution catalyst, respectively.23 We were
attracted to this strategy because it does not rely on molecular
excited states of rst-row metal complexes. In our rst foray into
photoredox catalysis for H2 evolution, we employed bipyridines
as photoredox mediators and Ni polypyridyl complexes as H2
evolution catalysts. H-atom abstraction (HAA) by the excited
state of the bipyridine afforded a pyridinyl radical, which
engaged with a Ni(II) halide complex to generate a Ni(I) inter-
mediate via halogen radical abstraction. The resulting Ni(I)
complex underwent disproportionation to a Ni(II) complex and a
Ni(0) species, which subsequently engaged in protolytic H2
evolution. While these efforts demonstrated a synthetic cycle for
H2 evolution, H2-evolution catalysis was not observed because
the basic bipyridyl photoredox mediator was passivated in the
presence of HCl.
To address the challenges of photoredox catalysis for H2
evolution from HCl, we turned our attention to identifying a
photoredox mediator that could function under acidic condi-
tions. We now examine the role of phosphines as photoredox
mediators under acidic conditions (pKa in CH3CN : PPh3 ¼ 7.64;
pyridine ¼ 12.53).24 The photochemical homolysis of P–H bonds
of 2ꢀ phosphines generates phosphinyl radicals that display
sufficient lifetime (ꢁ160 ms) to participate in halogen-atom
abstraction from a Ni(II) halide complex to furnish a reduced
Ni intermediate that participates in an H2 evolution cycle; the
phosphine photoredox mediator is regenerated by HAA from
solvent to close the photocycle.25–27 The H2-evolution cycle may
The photochemical splitting of hydrohalic acids (HX) into H2
and X2 is an approach to solar fuel synthesis1 that stores a
comparable amount of energy to water splitting. In addition to
the similar energy densities implicit in HX and H2O splitting
chemistries, HX splitting mandates management of only two
electrons and two protons, whereas H2O splitting requires
management of four protons and four electrons.2–5 Photo-
catalytic HX splitting requires accomplishing multielectron
photochemical reactions to activate strong M–X bonds. Typi-
cally, photoreduction has been the limiting step in HX splitting
photocatalysis and oen X2 elimination requires the use of
chemical traps for evolved halogen equivalents.6–17 Attempts at
promoting HX splitting with rst-row transition metal
complexes are attractive given that these metals are typically
earth abundant, but have been largely unsuccessful. The chal-
lenge of using rst-row metal complexes as HX splitting pho-
tocatalysts is that photochemical activation of Ni(I) or Ni(II)
halides frequently does not lead to photoreduction reac-
tions,18,19 likely due to the short excited state lifetimes of rst-
row transition metal complexes.20–22
aDepartment of Chemistry and Chemical Biology, 12 Oxford Street, Cambridge, MA
02138-2902, USA. E-mail: dnocera@fas.harvard.edu
bDepartment of Chemistry, Massachusetts Institute of Technology, 77 Massachusetts
Avenue, Cambridge, MA 02139-4307, USA
† Electronic supplementary information (ESI) available: Experimental procedures
and time-dependent photochemical data; transient absorption spectroscopy and
electrochemical experimental details. CCDC 992218–992221. For ESI and
crystallographic data in CIF or other electronic format see DOI:
10.1039/c4sc02357a
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eventually be closed by thermally promoted protolytic H2 2.09 (2.06). Complex 2[TEA] was prepared analogously by
evolution with HCl.
substitution of nBu4NCl with nEt4NCl in 95% yield; 1H NMR
(600 MHz, CD3CN) d (ppm): 3.48 (q, 2H), 1.45 (t, 3H). meff
(CH3CN) ¼ 4.17 mB. Anal. calcd (found) for C26H35Cl3NNiP: C,
56.01 (56.14); H, 6.33 (6.26); N, 2.51 (2.67). Crystals suitable for
single-crystal diffraction analysis were obtained from a CH3CN
solution of the complex layered with Et2O and unit cell data
matched literature reports.32
Experimental
Materials and methods
All reactions were carried out in an N2-lled glovebox. Anhy-
drous solvents were obtained by ltration through drying
columns.28 NMR chemical shis are reported in ppm with the
residual solvent resonance as internal standard. UV–vis spectra
were recorded at 293 K in quartz cuvettes on a Spectral Instru-
ments 400 series diode array and were blanked against the
Preparation of [ClPPh3]OTf
To a solution of Cl2PPh3 (127 mg, 3.81 ꢂ 10ꢃ4 mol, 1.00 equiv.)
in CH2Cl2 was added AgOTf (98.0 mg, 3.81 ꢂ 10ꢃ4 mol, 1.00
equiv.) as a suspension in CH2Cl2. White solid immediately
precipitated when AgOTf was added and the reaction mixture
was stirred at 23 ꢀC for 1 h before being ltered through Celite.
The ltrate was concentrated in vacuo and the residue was taken
up in THF and solvent was decanted, and the residue was dried
in vacuo to afford 157 mg of the title compound as a white solid
(92% yield). 31P NMR (160 MHz, CD2Cl2) d (ppm): 66.4; 19F NMR
(275 MHz, CD2Cl2) d (ppm): ꢃ78.9. The spectral data is consis-
tent with that reported for ClPPh3$AlCl4.33 Crystals suitable for
single-crystal diffraction analysis were obtained from a CH2Cl2
solution layered with Et2O.
appropriate solvent. PhICl2 and Ni(PPh3)2(CH2]CH2)30 were
29
prepared according to reported procedures. NiCl2dme (dme ¼
1,2-dimethoxyethane) and AgOTf (OTf ¼ triuoromethane-
sulfonate) were obtained from Strem Chemicals. Cl2PPh3,
prepared by treatment of PPh3 with PhICl2, displayed
spectral features identical to those reported in the literature.31
NiCl2(PPh3)2 (1), Ni(PPh3)4 (5), tetrabutylammonium chloride
(nBu4NCl), tetraethylammonium chloride (nEt4NCl), and tri-
phenylphosphine (PPh3) were obtained from Sigma Aldrich. All
chemicals were used without further purication. Elemental
analysis was obtained by Complete Analysis Laboratories, Inc.,
New Jersey. Evolved hydrogen was quantied by gas chroma-
tography using a calibration curve derived from adding HCl to
known quantities of NaH; over the relevant concentration
range, the gas chromatograph response was linear. This
procedure has previously been validated by comparison with
Toepler pump combustion analysis.15
Preparation of Ni(II) tetrachloride complex [NiCl4][Et4N]2
A solution of nEt4Cl (30.2 mg, 1.82 ꢂ 10ꢃ4 mol, 1.00 equiv.) in 2
mL of CH2Cl2 was added to a solution of NiCl2(dme) (40.0 mg,
1.82 ꢂ 10ꢃ4 mol, 1.00 equiv.) in 2 mL of CH2Cl2 to prompt an
immediate colour change from yellow to green. Aer stirring at
23 ꢀC for 0.5 h, the reaction mixture was concentrated to
dryness and the residue was taken up in pentane and solvent
was decanted, and the residue was dried in vacuo to afford 77.2
mg of the title compound as a green solid (92% yield). Crystals
suitable for single-crystal diffraction analysis were obtained
from a CH3CN solution layered with Et2O and unit cell data
matched literature reports.34
Preparation of Ni(II) trihalide complexes
Complex 2[ClPPh3]. A solution of PhICl2 (9.1 mg, 3.30 ꢂ 10ꢃ5
mol, 1.00 equiv.) in 1 mL of CH2Cl2 was added to a solution of
NiCl2(PPh3)2 (21.4 mg, 3.30 ꢂ 10ꢃ5 mol, 1.00 equiv.) in 2 mL of
CH2Cl2 to prompt an immediate colour change from a light
beige to blue. The solvent was removed in vacuo, and the
resulting solid was treated with pentane. The pentane was
decanted, and the resulting solid was dried in vacuo to afford
21.5 mg of title compound (90% yield). 1H NMR (600 MHz,
Preparation of Ni(I) complexes
Complex 3. To a scintillation vial was added Ni(cod)2
CD3CN) d (ppm): 7.77 (m, 9H), 7.64 (m, 6H). meff (CH3CN) ¼ 4.20 (58.0 mg, 2.10 ꢂ 10ꢃ4 mol, 1.00 equiv.) and NiCl2(dme) (46.0
mB. Anal. calcd (found) for C36H30Cl4NiP2: C, 59.63 (59.53); H, mg, 2.10 ꢂ 10ꢃ4 mol, 1.00 equiv.) as solids, followed by 3 mL of
4.17 (4.09). Crystals suitable for single-crystal diffraction anal- PhCH3. To this solution was added PPh3 (330 mg, 1.26 ꢂ 10ꢃ3
ysis were obtained from a CH3CN solution of the complex mol, 6.00 equiv.) dissolved in 2 mL PhCH3 and the reaction
layered with Et2O.
mixture was stirred at 23 ꢀC for 12 h before being ltered
Complex 2[TBA]. To a suspension of NiCl2(dme) (40.0 mg, through Celite. The ltrate was concentrated in vacuo to a
1.82 ꢂ 10ꢃ4 mol, 1.00 equiv.) in CH2Cl2 was added PPh3 (47.7 volume of 1.5 mL, layered with hexanes, and cooled to ꢃ30 ꢀC
n
mg, 1.82 ꢂ 10ꢃ4 mol, 1.00 equiv.) and Bu4NCl (50.6 mg, 1.82 to afford yellow crystalline solid (80% yield). 1H NMR (600
ꢂ 10ꢃ4 mol, 1.00 equiv.) as a solid. The reaction solution MHz, THF-d8) d (ppm): 9.51 (br, s, 20H), 5.28 (br, s, 15H), 4.20
immediately turnꢀed from yellow to blue. The reaction mixture (br, s, 10H). meff (CH3CN) ¼ 1.74 mB. Crystals suitable for single-
was stirred at 23 C for 1 h. The reaction was concentrated to crystal diffraction analysis were obtained from a PhCH3 solu-
dryness and the residue was taken up in pentane and Et2O, tion of the complex layered with n-hexane and collected unit
solvent was decanted, and the residue was dried in vacuo to cell data matched literature reports.35
afford 119 mg of the title complex as a blue solid (98% yield).
Complex 4. To a scintillation vial was added Ni(PPh3)2-
1H NMR (600 MHz, CD3CN) d (ppm): 3.32 (q, 2H), 1.83 (m, 2H), (CH2]CH2) (20.0 mg, 3.30 ꢂ 10ꢃ5 mol, 1.00 equiv.) and
1.53 (m, 2H), 1.08 (t, 3H). meff (CH3CN) ¼ 4.02 mB. Anal. calcd NiCl2(PPh3)2 (21.4 mg, 3.30 ꢂ 10ꢃ5 mol, 1.00 equiv.) as solids
(found) for C34H51Cl3NNiP: C, 60.97 (60.93); H, 7.68 (7.58); N, followed by 4 mL of Et2O. The reaction mixture was stirred at
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23 ꢀC for 0.5 h, during which time a yellow precipitate was presence of excess HCl, H2 evolution continues and 9 turnovers
observed. The mixture was concentrated to dryness and the were achieved aer 44 h with no signs of deactivation.
residue was taken up in pentane and dried in vacuo to afford
During the photolysis of NiCl2(PPh3)2, the blue-coloured
solution that initially develops persists throughout subse-
quent H2 evolution and based on UV-vis measurements,
appears to represent the photoresting state of the catalyst
system. The pale blue photoproduct was identied as Ni(II) tri-
halide complex 2[ClPPh3] by single-crystal X-ray diffraction
analysis.‡ In addition, comparison of the in situ UV-vis spec-
trum obtained during H2-evolving catalysis with a spectrum
obtained from an authentic sample of 2[ClPPh3], prepared by
treatment of NiCl2(PPh3)2 with 1.0 equiv. of PhICl2 (Fig. S15†),
conrmed the identity of the catalyst resting state. Indepen-
dently isolated complex 2[ClPPh3] is chemically competent at
H2 generation under above photoreaction conditions.
Catalyst resting state 2[ClPPh3] is constituted of a Ni tri-
chloride anion and a phosphonium cation. In order to establish
the roles of both the Ni complex and phosphine in the observed
H2 evolution reaction, 2[TBA] and phosphonium cation [ClPPh3]
OTf were independently prepared. As summarized in Table 1,
Ni(II) complex 2[TBA] showed a similar activity toward HCl with
5.0 TON in 18 h. Complex 2[TBA] also participates in a minor
equilibrium with free PPh3 (vide infra) and thus both catalyst and
photoredox mediator are present when 2[TBA] is employed as the
photocatalyst. In contrast, phosphonium salt [ClPPh3]OTf does
not produce H2 under the same reaction conditions. Addition-
ally, neither PPh3 or [NiCl4][TEA]2 is a competent H2-evolution
catalyst, conrming the necessity of both Ni complex and phos-
phine for productive H2 evolution chemistry.
18.4 mg of the title complex as a yellow solid (90% yield).36
Results and discussion
We targeted phosphines as potential photoredox mediators for
a tandem photoredox/transition metal catalysed H2-evolution
photocycle from HCl based on their demonstrated ability to
serve as photochemical H-atom donors.27 To evaluate the
viability of the proposed phosphine-mediated photoredox
approach for H2 evolution, we photolyzed Ni(II) complex
NiCl2(PPh3)2 (1) in THF (l > 295 nm) in the presence of 1.0
equiv. PPh3 and 15 equiv. of HCl. The light beige reaction
solution turned pale blue upon photolysis. Analysis of the
headspace by gas chromatography (GC) conrmed H2 as the
exclusive gaseous product under these conditions; integration
of the chromatogram and comparison to a H2 calibration curve
generated from the reaction of NaH with HCl revealed that 3.1
turnovers had been achieved in 18 h. NiCl2(PPh3)2 participates
in ligand dissociation equilibria to release PPh3 (vide infra),37
and thus H2-evolving photocatalysis was also observed in the
absence of exogenous PPh3. Evaluation of the amount of H2
evolved as a function of time (Fig. 1), showed that in the
Fig. 2 illustrates a tandem catalytic cycle that accounts for
the photogeneration of H2 from HCl catalyzed by the Ni phos-
phine complexes and phosphine photoredox mediators.
Diphenyl phosphine is initially formed by photochemical
cleavage of the P–C bond and H-atom abstraction from
solvent.27 Photochemical cleavage of the P–H bond in HPPh2
generates an H-atom equivalent and a diphenylphosphinyl
radical.27 The H-atom participates in halogen-atom abstraction
with Ni(II) resting state 2 to generate a Ni(I) intermediate while
the accompanying diphenylphosphinyl radical participates in
C–H abstraction with solvent to regenerate the diphenylphos-
phine and close the photoredox cycle. Ni(I) intermediate 3
undergoes disproportionation to afford NiCl2(PPh3)2 (1) and
Ni(PPh3)4 (5). Protonolysis of Ni(0) complex 5 affords H2 and
regenerates Ni(II) dihalide 1, thus closing the hydrogen
Table 1 TON of H2 measured in the headspace upon photolysis of
designated compounds in the presence of 15 equiv. HCl in THF for 18 h
hv ðl . 295 nmÞ compound
HCl
H2
ƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒ!
THF; 18h
Compound
TON
Fig. 1 Photolysis of NiCl2(PPh3)2 in THF (l > 295 nm) in the presence of
15 equiv. of HCl affords H2 as well as 2[ClPPh3]. (a) Thermal ellipsoid
plot of 2[ClPPh3] drawn at the 50% probability level. The hydrogen
atoms are omitted for clarity. (b) Time-dependent turnover number
(TON) of H2 produced by a 4.5 mM THF solution of NiCl2(PPh3)2 in the
presence of 15 equiv. HCl (l > 295 nm).
2[ClPPh3]
PPh3
[NiCl4][Et4N]2
2[TBA]
2.0
0
0
5.0
0
[ClPPh3]OTf
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Fig. 2 Proposed tandem catalytic cycles for H2-generation with Ni-
based H2-evolution catalysts and phosphine-based photoredox
mediators.
Fig. 3 (a) Nanosecond transient absorption (TA) spectroscopy of Ni
complex 2[TEA] ( , red) and PPh3
(
, black) is consistent with
formation of diphenylphosphinyl radical. TA spectrum obtained by
laser flash photolysis (310 nm pump) of a 1 : 1 THF : CH3CN solution
recorded at a 1 ms delay. (b) Thermal ellipsoid of 6 drawn at the 50%
probability level. The hydrogen atoms are omitted for clarity.
evolution cycle. The number of phosphine ligands bound to
intermediates in the tandem cycle illustrated in Fig. 2 is
unknown. Isolated complexes 2[ClPPh3], 3, 4 and 5, which
display 1–4 phosphine ligands per Ni, are all competent cata-
lysts for H2 evolution and proceed with the same photoresting
state (2[ClPPh3]), demonstrating ligand dissociation equilibria37
are established during catalysis (Fig. S16†).
photolysis of either 2[TEA] (red spectrum, Fig. 3) or PPh3 (black
spectrum, Fig. 3) leads to the observation of TA signals that are
ascribed to diphenylphosphinyl radical.27 The lifetime of the
diphenylphosphinyl radical derived from PPh3 with and
without the presence of 2[TBA] in solution is the same,
consistent with no direct reaction between Ni(II) complex and
diphenylphosphinyl radical (Fig. S19†). Substantial phosphine
consumption is not required for H2 evolution because the
diphenylphosphine generated during catalysis is a competent
photoredox carrier. Nanosecond-resolved TA spectra, collected
by laser ash photolysis of diphenylphosphine in THF, display
the spectral features of diphenylphosphinyl radical (Fig. S20†),
conrming that phosphine mediators can be catalytic.27,40
That the photogenerated diphenylphosphinyl radical
engages in HAA with THF to produce diphenylphosphine is
conrmed by the isolation of complex 6, as a photo-byproduct
of the photolysis of Ni complex 2[TEA]. Complex 6 features
two C-bound tetrahydrofuranyl ligands on a phosphorus centre
and could be derived from reaction of photogenerated diphe-
nylphosphinyl radicals with furanyl radicals derived from HAA
chemistry. Additionally we observed (GC-MS) octahydro-2,20-
bifuran (homocoupled THF) as a photochemical byproduct.
While HAA from THF by diphenylphosphinyl radicals is endo-
thermic (16 kcal molꢃ1 uphill based on P–H and C–H bond
In order to probe the contention that photogenerated
diphenylphosphinyl radicals could mediate the reduction of a
Ni–Cl bond from the Ni(II) trihalide complex, we carried out
time-resolved photochemical experiments. On the picosecond
timescale, a transient absorption (TA) difference spectrum
obtained by laser ash photolysis (lexc ¼ 310 nm, THF solu-
tions) of 2[TBA] exhibits a spectral growth centred at 506 nm
with a lifetime of ꢁ700 ps (Fig. S18†). An identical spectral
feature was observed during laser ash photolysis of PPh3
solutions. This feature was assigned to be that of the singlet
excited state of PPh3, and the observation of this signal in the TA
spectrum of 2[TBA] supports the presence of a minor equilib-
rium between 2[TBA] and free PPh3.38 Additional support for
this ligand dissociation equilibrium is the observation that both
2[TBA] and PPh3 both show an emission band centred at 500
nm with a 900 ps lifetime (Fig. S17†), which is well-matched to
reported PPh3 photophysics.39 The similar emission lifetimes
for both 2[TBA] and PPh3 excludes dynamic quenching of the
excited 1PPh3* species by the Ni complex and suggests that the
relatively low steady-state emission intensity observed for 2
[TBA] is due only to a low equilibrium concentration of PPh3.
The photochemistry of 2 and PPh3 were also examined at
longer time scales by nanosecond ash photolysis (Fig. 3). Flash
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dissociation energies),41 irreversible subsequent reactions, such
as radical coupling to afford homocoupled THF sequester
reactive radical intermediates.40
(1)
The rate for HAA from solvent by photogenerated diphe-
nylphosphinyl is strongly correlated with solvent C–H bond
energies.27,42 We therefore anticipated that efficiency of the
total catalytic process would be dictated by the turnover
frequency of the photoredox mediator, which depends on the
C–H BDEs of H-atom donor. The turnover frequency (TOF) of
hydrogen generation from HCl with Ni complex 2 strongly
depends on the solvent we employed, showing a positive
correlation to the BDE of solvents: 0.34 hꢃ1 TOF in THF, 0.05
hꢃ1 TOF in CH3CN, and 0.02 hꢃ1 TOF in C6H6 (92, 96, and
112 kcal molꢃ1 of C–H BDEs respectively) (see Fig. S21† for
details).43–45
To probe the potential fate of potential Ni(I) intermediates
during catalysis, we examined the chemistry of isolated Ni(I)
complexes. Ni(I) complex 3 can be isolated from the compro-
portionation reaction of NiCl2(PPh)2 with Ni(PPh3)4. Based on
the Eꢀ(NiII/NiI) and Eꢀ(NiI/Ni0) measured by cyclic voltammetry
in THF (Fig. S22†), comproportionation is thermodynamically
favored. In contrast, in the presence of exogenous chloride ion,
added as tetrabutylammonium chloride, disproportionation of
Ni(I) complex 3 to Ni(II) complex 1 and Ni(0) complex 5 is
observed, as determined by both 31P NMR and electronic
absorption spectroscopy (Fig. S8 and S14,† respectively). During
H2 evolution photocatalysis, chloride is present in large excess
(68 mM) with respect to potential Ni(I) intermediates.
To assess whether the initially produced Ni(I) complex (3) or
the Ni(0) complex (5) generated by disproportionation are active
for H2 production, we examined the stoichiometric H2-evolu-
tion reaction chemistry of Ni(I) and Ni(0) complexes with HCl.
The results of these experiments are summarized in Fig. 4.
Treatment of Ni complexes 3, 4, and 5 with 15 equiv. HCl in THF
generates H2 in 44, 33, and 89% yields, respectively, along with
Ni(II) complex 1, NiCl2(PPh3)2. To gain insight into whether H2
evolution proceeds by protonation of Ni(I) or Ni(0), generated by
disproportionation reactions, electrochemical H2 evolution was
examined using Ni(II) trihalide complex 2[TEA]. As illustrated in
Fig. 4, Ni complex 2[TEA] exhibits two electrochemically irre-
versible waves for the NiII/I and NiI/0 couples at ꢃ1.62 and
ꢃ1.95 V vs. Fc+/Fc, respectively. In the presence of excess HCl
(pKa ¼ 8.9 in CH3CN)46 these two peaks exhibit catalytic
cathodic waves. The dominant CV features of the NiI/0 wave in
the presence of excess HCl are consistent with Ni0 being
involved in the H2 generating steps in our photocatalysis.
Conclusions
As is common for rst-row transition metal complexes, nickel
halide complexes typically exhibit very short excited state life-
times. Direct photoactivation of M–X bonds using the molecular
excited states of these complexes has proven challenging owing
to their short lifetimes. To circumvent the limitations imposed
by short excited state lifetimes, we have developed a tandem
photoredox/transition metal catalysis approach to H2 evolution
in which the chromophore and the H2-evolution catalyst are
localized on different molecules. Using diaryl phosphines as
photoredox mediators, we have demonstrated that relatively
non-basic phosphines are capable of acting as photoredox
mediators under acidic conditions. Robust photocatalytic
systems have been developed by combining phosphine photo-
redox mediators and Ni phosphine H2-evolution catalysts.
Time-resolved spectroscopy has revealed that phosphines serve
as photochemical H-atom donors and activate the M–X bonds of
Ni(II) halide complexes via halogen-atom abstraction. The H2-
evolution catalytic cycle is closed by sequential disproportion-
ation of Ni(I) to afford Ni(0) and Ni(II) and protolytic H2 evolu-
tion from the Ni(0) intermediate. The described photoredox
strategy is attractive in that independent optimization of pho-
toredox mediator and H2-evolution catalyst provides multiple
handles for system optimization.
Fig. 4 (a) Protonation of Ni(I) and Ni(0) complexes afforded Ni(II)
chloride as well as H2. (b) Electrochemical response of electrolyte
background ( , black), 1 mM Ni complex 2[TEA] ( , red) to addition of
HCl 1.0 equiv. ( , blue), 5.0 equiv. ( , pink), 9.0 equiv. ( , green), 13.0
equiv. ( , dark blue) in CH3CN (0.1 M NBu4PF6; scan rate, 100 mV sꢃ1).
Glassy carbon working electrode, Ag/AgNO3 reference, and Pt wire
counter electrode.
Acknowledgements
We gratefully acknowledge Robert L. Halbach and D. Kwabena
Bediako for helpful discussions, and the funding from NSF
Grant CHE-1332783 and a Ruth L. Kirchenstein National
Research Service award (F32GM103211) for D.C.P.
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21 C. Creutz, M. Chou, T. L. Netzel, M. Okumura and N. Sutin, J.
Am. Chem. Soc., 1980, 102, 1309.
Notes and references
‡ Crystallographic data for 2[ClPPh3]: C36H30Cl4P2Ni, M ¼ 725.05, orthorhombic,
Pbca, a ¼ 17.435(4), b ¼ 15.662(3), c ¼ 24.139(9), V ¼ 6591(2), Z ¼ 8, m ¼ 1.036
mmꢃ1, T ¼ 100(2) K, R1 ¼ 0.0527, wR2 ¼ 0.0720 (based on all reections), GooF ¼
1.030, reections measured ¼ 57 380, unique reections ¼ 5858, Rint ¼ 0.0741.
Crystallographic data for 6: C40H48Cl4O4P2Ni, M ¼ 855.23, monoclinic, P21/n, a ¼
10.1261(8), b ¼ 23.4579(18), c ¼ 17.8192(14), b ¼ 106.3550(12)ꢀ, V ¼ 4061.4(5), Z ¼
4, m ¼ 0.859 mmꢃ1, T ¼ 100(2) K, R1 ¼ 0.0788, wR2 ¼ 0.1328 (based on all
reections), GooF ¼ 1.005, reections measured ¼ 44 051, unique reections ¼
7212, Rint ¼ 0.0668. Crystallographic data for Cl2PPh3: C18H15Cl2P, M ¼ 333.17,
monoclinic, P21/c, a ¼ 13.338(3), b ¼ 14.376(3), c ¼ 8.7454(17), b ¼ 102.53(3)ꢀ, V ¼
1637.0(6), Z ¼ 4, m ¼ 0.484 mmꢃ1, T ¼ 100(2) K, R1 ¼ 0.0330, wR2 ¼ 0.0710 (based
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