Panchromatic dirhodium photocatalysts for dihydrogen generation with red light

Article abstract of DOI:10.1039/d0sc03114c

A series of three dirhodium complexes cis-[Rh2(DPhB)2(bncn)2](BF4)2 (1, DPhB = diphenylbenzamidine; bncn = benzocinnoline), cis-[Rh2(DPhTA)2(bncn)2](BF4)2 (2, DPhTA = diphenyltriazenide), and cis-[Rh2(DPhF)2(bncn)2](BF4)2 (3, DPhF = N,N′-diphenylformamidinate) shown to act as single-molecule photocatalysts for H2 production was evaluated. Complexes 1-3 are able to generate H2 in the absence of any other catalyst in homogenous acidic solution upon irradiation with red light in the presence of the sacrificial electron donor BNAH (1-benzyl-1,4-dihydronicotinamide). The excited state of each complex is reductively quenched by BNAH, producing the corresponding one-electron reduced complex. The latter is also able to absorb a photon and oxidize another BNAH molecule, producing the doubly-reduced, activated form of the catalyst that is able to generate H2. The present work shows the effect of substitution on the bridging ligands on the driving force for reductive quenching and hydricity of the proposed active intermediate, both of which affect the efficiency of hydrogen production. Complexes 1-3 operate following a double reductive quenching mechanism and, importantly, are active with red light. This work lays the foundation for the design of single-molecule photocatalysts that operate from the ultraviolet to the near-infrared, such that solar photons throughout this entire range are harnessed and utilized for solar energy conversion.

Full text of DOI:10.1039/d0sc03114c

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Panchromatic dirhodium photocatalysts for
dihydrogen generation with red light†
Cite this: DOI: 10.1039/d0sc03114c
*
All publication charges for this article
have been paid for by the Royal Society
of Chemistry
Jie Huang, Judith C. Gallucci and Claudia Turro
A series of three dirhodium complexes cis-[Rh2(DPhB)2(bncn)2](BF4)2 (1, DPhB ¼ diphenylbenzamidine;
bncn ¼ benzocinnoline), cis-[Rh2(DPhTA)2(bncn)2](BF4)2 (2, DPhTA ¼ diphenyltriazenide), and cis-
[Rh2(DPhF)2(bncn)2](BF4)2 (3, DPhF ¼ N,N0-diphenylformamidinate) shown to act as single-molecule
photocatalysts for H2 production was evaluated. Complexes 1–3 are able to generate H2 in the absence
of any other catalyst in homogenous acidic solution upon irradiation with red light in the presence of the
sacrificial electron donor BNAH (1-benzyl-1,4-dihydronicotinamide). The excited state of each complex
is reductively quenched by BNAH, producing the corresponding one-electron reduced complex. The
latter is also able to absorb a photon and oxidize another BNAH molecule, producing the doubly-
reduced, activated form of the catalyst that is able to generate H2. The present work shows the effect of
substitution on the bridging ligands on the driving force for reductive quenching and hydricity of the
proposed active intermediate, both of which affect the efficiency of hydrogen production. Complexes
1–3 operate following a double reductive quenching mechanism and, importantly, are active with red
light. This work lays the foundation for the design of single-molecule photocatalysts that operate from
the ultraviolet to the near-infrared, such that solar photons throughout this entire range are harnessed
and utilized for solar energy conversion.
Received 3rd June 2020
Accepted 27th July 2020
DOI: 10.1039/d0sc03114c
rsc.li/chemical-science
losses during each of these charge transfer processes. Multi-
component systems featuring covalently linked chromophores
Introduction
and catalysts aim to circumvent these losses, however, their
synthesis and characterization is oen challenging.18–20 More-
over, these systems oen suffer from fast charge recombina-
tion, thus reducing their ability to effectively relay redox
equivalents to the catalyst.21,22 Nearly half of the solar spectrum
falls into the red to near-IR light, a spectral region that has been
underutilized by most light absorbers developed to date.23,24
Single-molecule photocatalysts are important to circumvent the
shortcomings of multi-component systems,25 however, it is
critical for the photocatalyst is activated by light from the
ultraviolet to the red/near-IR light to better harness the solar
photons that reach the earth.
Photons from the sun represent an abundant and free source of
energy, making it critical to develop systems that are able to
harness and transform sunlight into chemical bonds, or fuels,
for long-term storage.1–8 Carbon-free energy sources obtained
from water and sunlight, such as H2, represent a desirable clean
fuel generated from abundant sources.1–12 Traditional photo-
catalytic H2 evolution systems include a number of compo-
nents, including a light-absorbing chromophore, an electron
relay, and a catalyst, and these molecular systems have been
coupled to semiconductors and/or electrodes in dye-sensitized
photoelectrosynthesis cells.9–15 For optimal performance, the
chromophore should efficiently absorb incident photons and
its spectral prole should be well-matched with that of the solar
irradiance spectrum, such that as many wavelengths of sunlight
as possible are utilized to electronically excite the chromo-
phore.5,13–17 Following excitation, the excited state of the light
absorber must be able undergo desirable charge transfer reac-
tions, such as reducing the relay molecule so that it can shuttle
redox equivalents to a catalytic center or semiconductor.1–17
However, such multi-component systems suffer from energy
In the eld of single-chromophore photocatalysts, those for
the reduction of CO2 and for the evolution of H2 have been re-
ported. Re(I)(bpy)(CO)3Br (bpy ¼ 2,20-bipyridine) is able to
reduce CO2 to CO with a turnover number (TON) of 27 aer 4 h
(lirr > 400 nm).26 Two-electron mixed-valence dirhodium
complexes have been shown to reduce halogen acids, HX, to
produce H2 through the activation of the RhII–X bond and the
formation of RhII–H species, with the highest TON ¼ 7 aer 72 h
of irradiation with ultraviolet/blue light.25,27 In addition, the
unsymmetrically substituted diiron hydride [(m-H)Fe2(-
pdt)ꢀ(CO)4(dppv)]+ (dppv ¼ cis-1,2-C2H2(PPh2)2) can generate
H2 with TON ¼ 4 from a triic acid solution with >400 nm
Department of Chemistry and Biochemistry, The Ohio State University, Columbus, OH
43210, USA. E-mail: turro.1@osu.edu
† Electronic supplementary information (ESI) available. CCDC 1980420. For ESI
and crystallographic data in CIF or other electronic format see DOI:
10.1039/d0sc03114c
irradiation.28
A
mononuclear tungsten photocatalyst,
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Fig. 1 Schematic representation of the structures of 1–3 with the bncn ligands colored blue.
W(pyNHC)(CO)4 (pyNHC ¼ (pyridyl)-N heterocyclic carbene), 1-benzyl-1,4-dihydronicotinamide (BNAH) were obtained from
was recently reported to absorb with maximum at 400 nm and TCI. Cobaltocene was stored in a glove box preventing it from
to produce H2 with TON of 17 aer 3 h of irradiation.29 In exposure to moisture and oxygen. Synthetic procedures were
general, mechanistic studies of H2-producing catalysts have performed under a N2 atmosphere and the purication
revealed that terminal metal-hydrides act as important inter- processes were opened to air; the general procedure is shown in
mediates in product formation.30,31
Dirhodium complexes of the type cis-[Rh(form)2(L)2](BF4)2 products are shown in Fig. S1–S6 (ESI†). The ligands diphe-
chelating/bridging diimine nylbenzamidine (DPhB) and diphenyltriazenide (DPhTA), as
Scheme S1† and 1H NMR spectra of the starting materials and
(form ¼ formamidinate,
L
¼
ligands) feature an intense metal/ligand to ligand charge well as the precursor cis-[Rh2(DPhTA)2(CH3CN)6][BF4]2 were
transfer (ML-LCT) transition in the visible to near-IR range, synthesized according to literature procedures.37,38
making them good candidates as panchromatic photosensi-
tizers.32–35 The present work focuses on the comparison of the Rh2(DPhB)3(OAc)
photocatalytic H2 production by cis-[Rh2(DPhB)2(bncn)2](BF4)2
Rh2(OAc)4 (50 mg, 0.11 mmol) and 3 eq. of diphenylbenzami-
dine ligand were dissolved in 8 mL chlorobenzene from a sealed
(1, DPhB ¼ diphenylbenzamidine; bncn ¼ benzocinnoline) and
cis-[Rh2(DPhTA)2(bncn)2](BF4)2 (2, DPhTA
¼
diphenyl-
bottle under N2 and reuxed for 2 days. The solution turned
dark-green. The reaction mixture was then cooled to room
temperature and concentrated to dryness. The residue was
washed with methanol to give a green solid. Yield: 80 mg
(0.074 mmol, 66%).
triazenide), shown in Fig. 1, and a comparison of their activity to
that of cis-[Rh2(DPhF)2(bncn)2](BF4)2 (3; DPhF ¼ N,N0-diphe-
nylformamidinate). Complex 3 was recently shown by our group
to act as a single-chromophore photocatalyst for H2 generation
with TON of 170 over 24 h under red light irradiation (lirr ¼ 670
nm), also operative with lirr ¼ 735 nm as well as with higher
energy light.36 Herein, the electron donating ability of the
bridging formamidine ligands is tuned in 1 and 2 to improve
the excited state lifetime and photocatalytic H2 evolution
activity of this bimetallic paddlewheel architecture. These
Rh2(II,II) complexes represent a new class of single-molecule,
single-chromophore photocatalysts that are able to act as both
the light absorber and the catalytic center with absorption
proles that make them operative from the ultraviolet to the
near-IR.
cis-[Rh2(DPhB)2(CH3CN)6][BF4]2
Rh2(DPhB)3(OAc) (100 mg, 0.093 mmol) was dissolved in 30 mL
CH2Cl2/CH3CN (1 : 1, v/v), was heated to 45 ꢁC, and 0.75 mL of
0.1 M (C2H5)3O(BF4) was added to the solution and allowed to
stir at this temperature for 6 h, during which time it changed
color from the initial green to brown/red. The reaction mixture
was then cooled to room temperature and concentrated to
dryness. The remaining solid was redissolved in ꢂ2 mL CH3CN,
was added dropwise to 150 mL of rapidly stirring diethyl ether,
was allowed to stir 1 h, and was then ltered through a medium
frit under vacuum. The ltrate was pink and was conrmed to
be Rh2(DPhB)4. The solid was washed with excess diethyl ether,
resulting in a light green solid. Yield: 83% (90 mg, 0.077 mmol).
Experimental
Materials
The solvents acetonitrile, N,N-dimethylformamide (DMF),
dichloromethane, diethyl ether, and chlorobenzene were
cis-[Rh2(DPhB)2(bncn)6][BF4]2 (1)
purchased from Fisher Scientic or Sigma Aldrich. Prior to use, Complex 1 was prepared from the reaction of cis-[Rh2(-
the solvents were distilled over calcium hydride or ltered over DPhB)2(CH3CN)6][BF4]2 (50 mg, 0.043 mmol) and benzo[c]cin-
dried alumina and stored over molecular sieves. Benzo[c]cin- noline (15.4 mg, 0.086 mmol) in 20 mL dry CH3CN and reuxed
noline (bncn) and p-toluenesulfonic acid (TsOH) were overnight under N2 to give a green-blue solution. The reaction
purchased from Sigma-Aldrich, Rh2(OAc)4 (OAc ¼ acetate) and mixture was then cooled to room temperature and concentrated
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to dryness. The remaining solid was dissolved in ꢂ2 mL of electrode under a N2 atmosphere. The sweep rate was constant
CH3CN and was added dropwise to 150 mL of rapidly stirring at 200 mV sꢀ1, the potential was referenced to ferrocene (E1/2
¼
diethyl ether, and allowed to stir for 1 h. The solution was then +0.55 V vs. Ag/AgCl in DMF or +0.44 V vs. Ag/AgCl in CH3CN),
ltered through a medium frit under vacuum the solid product and 0.1 M nBu4NPF6 (TBAPF6) was used as the supporting
was washed with excess diethyl ether on the lter frit. Yield: electrolyte. For electrocatalysis experiments, aliquots of 3 mL of
73% (42 mg, 0.031 mmol). 1H NMR (CD3CN, 400 MHz, 298 K) d/ 2 M TsOH, salicylic acid, or acetic acid in CH3CN or DMF
ppm (int., mult., coupl., assignment): 9.57 (4H, d, J ¼ 8.6, bncn), solution were titrated into the solution containing the photo-
8.85 (4H, d, J ¼ 7.8, bncn), 8.19 (8H, m, bncn), 7.16 (2H, d, J ¼ catalyst (3 mL total volume). Before each scan, the working
7.6, Phenyl), 6.91 (13H, m, DPhB phenyl), 6.73 (3H, t, J ¼ 7.6, electrode was polished to eliminate the possibility of the accu-
Phenyl), 6.53(10H, t, J ¼ 7.8 Phenyl), 6.09 (2H, d, J ¼ 7.7, Phenyl). mulation of decomposition products or aggregation on the
ESI-MS: m/z ¼ 554.4, z ¼ 2+.
surface. Spectroelectrochemistry experiments were recorded
using a homemade H-shape two-compartment cell. The glassy
carbon rod working electrode and Ag/AgCl reference electrode
were placed in the working compartment equipped with
a quartz cuvette, and the carbon mesh counter electrode was
positioned in the auxiliary compartment. Spectroelec-
trochemistry solutions containing 0.1 M TBABF4 as the elec-
trolyte were held ꢂ200 mV more negative potential than the
one- and two-electron couples to generate the respective species
until the electronic absorption spectrum no longer changed.
Emission spectra were recorded on a FluoroMax-4 spec-
trometer (Horiba Jobin Yvon) at 77 K in CH3CN using a home-
made Dewar. An appropriate long-pass lter was used at the
entrance slit of the detection monochromator to eliminate
scattered excitation light. A PMT single wavelength detector was
used to record l < 800 nm signals and an iCCD was used for l >
800 nm emission; the reported results are the average of ve
trials. Femtosecond transient absorption spectroscopy was
performed on a homebuilt system previously described in
detail.41 The excitation (2–3 mJ) and white light probe beams are
focused onto a 1 mm pathlength ow cell that contained the
cis-[Rh2(DPhTA)2(bncn)6](BF4)2 (2)
Complex 2 was prepared by reuxing a solution of 20 mL of dry
CH3CN containing cis-[Rh2(DPhTA)2(CH3CN)6](BF4)2 (50 mg,
0.05 mmol) and bncn (18 mg, 0.10 mmol) in 20 mL overnight
under N2, resulting in a purple-blue reaction mixture. The
solution was then cooled to room temperature and dried under
vacuum. The solid was redissolved in ꢂ2 mL CH3CN and this
concentrated solution was added dropwise to 150 mL of rapidly
stirring diethyl ether. The mixture was stirred for 1 h and was
then ltered through a medium frit under vacuum. The residue
was washed with excess diethyl ether to obtain dry product.
1
Yield: 59% (35 mg, 0.029 mmol). H NMR (CD3CN, 400 MHz,
298 K) d/ppm (int., mult., coupl., assignment): 9.70 (4H, d, J ¼
8.4, bncn), 8.83 (4H, d, J ¼ 7.7, bncn), 8.33 (4H, t, J ¼ 7.9, bncn),
8.24 (4H, t, J ¼ 7.2, bncn), 7.34 (12H, m, J ¼ 7.3, DPhTA phenyl),
7.05 (8H, dd, J ¼ 7.6, J ¼ 1.9 Phenyl). ESI-MS: m/z ¼ 479.3, z ¼ 2+.
Instrumentation and methods
1H NMR spectra were recorded on a Bruker 400 MHz or 250 sample. Nanosecond transient absorption data was collected
MHz spectrometer and the peaks were referenced to the residue using a basiScan tunable optical parametric oscillator (Spectra-
CD3CN solvent peak at 1.94 ppm. Electronic spectra were Physics) pumped by a frequency-tripled Quanta-Ray INDI
collected on a Hewlett–Packard 8454 diode array spectrometer Nd:YAG laser (Spectra-Physics, ꢂ6 ns pulses at 10 Hz), and an
in 1 cm ꢃ 1 cm quartz cuvettes. Electrospray ionization mass LP980 spectrometer system (Edinburgh Instruments) equipped
spectroscopy (ESI-MS) was performed on a Bruker MicroTOF with a 150 W Xe arc lamp as the white light probe. Spectral
spectrometer in positive low ion mode with samples dissolved measurements were collected using an iCCD camera (iStar,
in CH3CN.
Andor Technology) and single wavelength kinetic traces were
A single crystal of 1 was obtained by slowly diffusing collected using a PMT and digital oscilloscope (Tektronix
diethyl ether over a CH3CN solution containing the complex. MDO3022, 200 MHz, 2.5 GS sꢀ1). The instrument response
The diffraction pattern was measured on a Bruker D8 Venture function (IRF) in CH3CN was measured to be 12 ns (fwhm).42
diffractometer with a Photon II detector, which indicated The samples were purged with N2 for 15 min prior to each
a monoclinic crystal system conducted at 150 K using an measurement and the electronic absorption spectrum were
Oxford Cryosystems Cryostream Cooler. The data collection recorded before and aer the experiment to ensure no sample
strategy was set up to measure a quadrant of reciprocal space degradation had taken place.
with a redundancy factor of 6, such that 90% of the reec-
Photolysis experiments were performed using four light
tions were measured at least 6 times. Omega and phi scans emitting diodes (LEDs, Luxeon Star) centered at 655 nm as the
with a frame width of 0.5ꢁ and a frame time of 20 seconds light source. The sum of the irradiation intensity was kept at 800
were used and additional details are listed in Table S1 (ESI†). mW. Photolysis solutions contained 30 mM BNAH, 0.1 M TsOH
The data frames were collected using the program APEX3 and (p-toluenesulfonic acid) and 64 mM 1 or 100 mM 2 in 3 mL DMF;
processed with the SAINT program within APEX3.39 Absorp- the samples were prepared in the glove box using a cuvette with
tion and beam corrections were made with the multiscan a side arm equipped with a Kontes stopcock. The amount of H2
technique in SADABS.40
produced in head space was quantied by a Shimadzu GC-2014
Cyclic voltammetry was conducted using a BASi CV-50 W gas chromatography with He as the carrier gas. Absorption
potentiostat with glassy carbon disc as the working electrode, spectra were collected at time intervals until no further spectral
Ag/AgCl as the reference electrode, and Pt wire as the counter changes were observed.
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DPhB(p*) in 1 and Rh2(d*)/DPhTA(p*) in 2, to the bncn(p*)
LUMO in each complex.33,34 The blue shi of the charge-transfer
band in 2 relative to that in 3 is attributed to the better electron
withdrawing ability of the DPhTA ligand which lowers the
energy of the HOMO. Complexes 1–3 also exhibit absorption in
the 330–344 nm range attributed to a bncn-centered pp* tran-
sition and peaks in the 401–433 nm range believed to arise from
1ML-LCT transitions, as previously reported for 3, as well as
other dirhodium complexes with similar structures.33,34,36
The redox potentials obtained from cyclic voltammetry for 1
and 2 are summarized in Table 1 and shown in Fig. S8,† and are
compared to those previously reported for 3. The rst reversible
oxidation is observed at +1.33 V for 1 and +1.57 V for 2 in CH3CN
vs. Ag/AgCl, which compare well to that measured for 3 in the
same solvent, +1.28 V vs. Ag/AgCl.36 In 3, this oxidation event is
assigned to the removal of an electron from the Rh2(d*)/for-
m(p*) HOMO.36 A similar assignment of this couple can be
made for 1 and 2, with HOMOs of mixed Rh2(d*)/DPhB(p*) and
Rh2(d*)/DPhTA(p*) character, respectively. Both DPhB and
DPhTA ligands are expected to be more difficult to oxidize than
DPhF in 3, attributed to the more electron withdrawing phenyl
substituent on the bridgehead carbon atom in DPhB in 1 and
the presence of a nitrogen atom in DPhTA in 2. Two reversible
reduction couples are observed for 1 and 2, the rst at ꢀ0.44 V
and ꢀ0.35 V vs. Ag/AgCl in CH3CN, at potentials slightly more
positive than that of 3 at ꢀ0.46 V under similar conditions
(Table 1).36 A similar trend is observed for the second reversible
reduction waves in 1 and 2, which are comparable to that of 3
(Table 1). These two reduction events in 1–3 are ascribed to
placement of two consecutive electrons on each of the bncn
ligands.36 It should be noted that HOMO–LUMO gap calculated
from the difference between the rst oxidation and rst
reduction waves in 2, 1.92 V, are greater than those of 1 (1.77 V)
and 3 (1.74 V), by 0.15 V and 0.18 V, respectively (Table 1). This
difference is consistent with the higher energy LM-LCT transi-
tion observed for 2 at 575 nm (17 390 cmꢀ1, 2.16 eV), as
compared to those of 1 and 3, at 645 nm (15 500 cmꢀ1, 1.92 eV)
and 624 nm (16 030 cmꢀ1, 1.99 eV), respectively (Table 1).
Results and discussion
X-ray crystal structure
The single crystal X-ray structure of 1 is depicted in Fig. 2 and
additional details are provided in Table S1 (ESI†). The cis
disposition of the two diphenylbenzamidine (DPhB) anions and
the two neutral benzo[c]cinnoline (bncn) ligands that bridge the
two Rh(II) centers is readily evident in Fig. 2, resulting a heter-
oleptic paddlewheel structure. Each axial site is coordinated by
an acetonitrile solvent molecule, as is typical for this class of
ꢀ
complexes,43 along with two BF4 counterions and a free
acetonitrile molecule in the outer coordination sphere (not
shown). The shorter Rh–Rh bond distance determined for 1,
2.3878(4) A, as compared to that of 3, 2.4049(5) A,36 is attributed
to steric strain afforded by central phenyl ring on the bridging
DPhB ligand, not present in diphenylformamidinate (DPhF) in
3. The torsion angle between the coordinating N atoms on the
formamidinate ligand with the two rhodium atoms increases
from 4.7ꢁ in 3 to 9.4ꢁ in 1, a measure that is also indicative of the
additional steric strain in the latter. Although crystals of 2 were
not suitable for X-ray diffraction, the corresponding homoleptic
complex, Rh2(DPhTA)4, was reported to feature a Rh–Rh
˚
˚
˚
distance of 2.377(3) A, shorter than that of Rh2(DPhB)4 as
44,45
˚
2.389(1) A.
Based on the these results, it may be hypothe-
sized that the Rh–Rh bond in 2 is similar to or shorter than that
in 1.
Electronic absorption spectroscopy and electrochemistry
The lowest energy absorption exhibit maxima at 645 nm (3 ¼
8900 Mꢀ1 cmꢀ1) for 1 and 575 nm (3 ¼ 7660 Mꢀ1 cmꢀ1) for 2
(Fig. S7†). This band is observed at 624 nm (3 ¼ 8600 Mꢀ1 cmꢀ1
)
in 3 and has been previously attributed to a metal/ligand-to-
ligand charge transfer (ML-LCT) from the Rh2(d*)/DPhF(p*)
HOMO (highest occupied molecular orbital) to the bncn LUMO
(lowest unoccupied molecular orbital).36 Given the similarity in
structure and absorption proles of 1 and 2 to that of 3, the
lowest energy transition is also ascribed to the HOMO, Rh2(d*)/
Excited state properties
Emission is observed for 1 (lexc ¼ 645 nm) and 2 (lexc ¼ 585 nm)
at 77 K in CH3CN with maxima at 830 nm and 750 nm,
respectively, with excitation spectra that are consistent with the
absorption prole of each complex (Fig. S9†). Similar results
were previously reported for 3, with emission at 800 nm (lexc
¼
515 nm) under similar experimental conditions.36 No emission
is observed from 1–3 at room temperature in CH3CN or DMF
solutions. It should be noted, however, that luminescence has
not been previously reported for other Rh2(II,II) complexes. The
emission from 1–3 is attributed to the presence of a low-lying
3ML-LCT state, together with a deactivating metal-centered
(MC) state(s) with electron density on the Rh2(s*) orbital. In
these complexes, it is hypothesized that the MC state(s) are
located at a sufficiently high energy to avoid fast deactivation,
afforded by the short distance of two rhodium atoms that raise
the energy of the Rh2(s*) MO.43 The onset (5% of maximum
Fig.ꢀ2 Crystal structure of 1; hydrogen atoms, solvent molecules, and
BF4 counterions have been omitted for clarity (ellipsoids drawn at
50% probability).
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Table 1 Electronic absorption maxima, labs, and extinction coeffi-
cients, 3, in DMF and reduction potentials, E1/2, in CH3CN for 1–3
that correspond to the ground state absorption at ꢂ350 nm and in
the 570–750 nm range. The ultrafast transient absorption spectra
of 1 recorded following 650 nm excitation (IRF ¼ 85 fs) is shown in
Fig. S10,† and exhibit proles that parallel those in Fig. 3 at longer
timescales; similar spectral features are observed for 2 (Fig. S10†).
The positive signals compare well to those previously assigned to
the 1ML-LCT and 3ML-LCT states of 3.36 The 1ML-LCT states of 1–3
Complex
labs/nm (3/ꢃ103 Mꢀ1 cmꢀ1
)
E1/2a/V
1
344 (13.3), 422 (6.2), 645 (8.9)
330 (18.1), 433 (4.8), 575 (7.7)
332 (2.0), 401 (5.6), 624 (8.6)
+1.33, ꢀ0.44, ꢀ0.73
+1.57, ꢀ0.35, ꢀ0.57
+1.28, ꢀ0.46, ꢀ0.63
2
3b
1
a
b
Potentials vs. Ag/AgCl with 0.1 M nBu4NPF6 under N2. From ref. 36.
decay with lifetimes, s, of 13–15 ps, whereas the corresponding
triplet 3ML-LCT lifetimes, 3s, were measured to be 33 ns in 1 and
ꢂ8 ns in 2, respectively (Table 2 and Fig. S11†). The lifetime of 2
was estimated based on the lack of excited state signal observed in
the nsTA experiment (IRF ¼ 12 ns) and the remaining excited state
absorption at 416 nm at 2.7 ns in the fsTA (Fig. S11b†). The 3ML-
LCT state of 2, ET00 in Table 2, lies at a higher in energy than the
Table 2 Singlet (1s) and triplet (3s) ML-LCT excited state lifetimes,
triplet excited state energy, E0T0, and ground and triplet excited state
reduction potentials, E1/2[Rh2]0/ꢀ and E1/2[Rh2]*2+/+, respectively, for
1–3 in CH3CN
3
same state in 1 and 3, placing it closer to the deactivating MC
state, thus resulting in a shorter triplet lifetime in the former.
1/3ML-LCT
Complex 1s/ps 3s/ns E1/2[Rh2]0/ꢀa/V ET00b/eV E1/2[Rh2]*2+/+a/V
Electro- and photocatalysis
1
13
15
15
33
ꢂ8
19
ꢀ0.44
ꢀ0.35
ꢀ0.46
1.49
1.67
1.55
+1.05
+1.32
+1.09
2
In the presence of excess p-toluenesulfonic acid (TsOH), the
application of a negative bias beyond the second reduction
couple of 1 and 2 results in a catalytic current that is dependent
on the concentration of acid (Fig. S12†). Bulk electrolysis
measurements show that the catalytic current in these experi-
ments is due to the production of H2. These results indicate that
the doubly-reduced forms of complexes 1 and 2 is able to
undergo H2 generation (Fig. S12†). Similar results were previ-
ously obtained for 3.36 The ability of 1 and 2 to undergo elec-
trocatalytic H2 production in acidic solutions, together with the
3c
a
Potentials vs. Ag/AgCl with 0.1 M nBu4NPF6 under N2. b From emission
at 77 K. c From ref. 36.
intensity) of the 77 K emission of each complex was used to
estimate the energy of the triplet excited state, ET00, and the
resulting values range from 1.49 eV in 1 to 1.67 in 2 (Table 2). As
listed in Table 2, the triplet excited state reduction potentials of
1–3, E1/2[Rh2]*2+/+, range from +1.05 V in 1 to +1.32 V in 2, vs. Ag/
AgCl, making them signicantly better oxidants than the
prototypical [Ru(bpy)3]Cl2 (bpy ¼ 2,20-bipyridine) complex,
+0.82 V vs. Ag/AgCl in CH3CN.46
The transient absorption spectrum of 1 collected upon excita-
tion with 640 nm pulse (IRF ¼ 12 ns) is shown in Fig. 3 and
exhibits a sharp positive signal in the 390–550 nm region previ-
ously attributed to reduced bncn ligand,36 as well as bleach signals
3
presence of ML-LCT excited states with nanosecond lifetimes
that are powerful oxidants, led to the investigator of their
potential as single-molecule photocatalysts. Importantly, this
class of complexes absorbs light strongly from the ultraviolet to
the near-IR range and possess excited states that can engage in
charge transfer reactions, making them ideal systems to effi-
ciently harvest sunlight for solar energy applications.33,34
The irradiation of 64 mM of 1 with 655 nm light in DMF in the
presence of 0.1 M TsOH and 30 mM BNAH (1-benzyl-1,4-
dihydronicotinamide) as a sacricial electron donor results in
the evolution of H2 gas with a turnover number (TON) of 180 in
24 h, as shown in Fig. 4 (black symbols). No H2 is observed in
the absence of any single component, complex, light, acid, or
sacricial donor (Table S2†). At early photolysis times, up to 4 h,
the generation of H2 is linear resulting in an initial turnover
frequency (TOF) of 18 hꢀ1. These ndings are similar to those
previously observed for 3, which resulted in TON ¼ 170 in 24 h
and initial TOF ¼ 28 hꢀ1 (lirr ¼ 670 nm).36 Fig. 4 also shows that
under these conditions, the production of H2 begins to plateau
at longer irradiation times. In order to eliminate the possibility
of the consumption of substrate and/or sacricial donor as the
cause of the deviation from linearity in Fig. 4, aliquots of 300 mL
DMF solution containing 0.16 M BNAH and 0.53 M TsOH were
added to the reaction mixture every 4 h (Fig. 4, red trace). It is
evident from a comparison of the data in Fig. 4 that there is
a
marked improvement in the activity when additional
substrate is present in solution, resulting in an initial TOF of 22
Fig.
3 Transient absorption spectra of 1 at various delay times
following excitation in CH3CN (640 nm, IFR ꢂ 6 ns).
hꢀ1 and TON ¼ 250 aer 24 h. However, it should be noted that
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Fig. 4 Plot of turnover number (TON) as a function of irradiation time
(lirr ¼ 655 nm) of 64 mM 1, 30 mM BNAH, 0.1 M TsOH in DMF without
the addition of any acid or BNAH during the course of the reaction
(black squares) and upon the addition of 1.5 mL of a DMF solution
containing 0.16 M BNAH and 0.53 M TsOH every four hours (red
circles).
deviations from linearity are still observed in the red curve in
Fig. 4, which could be indicative of catalyst degradation over
long irradiation times. In contrast, the irradiation of 180 mM of
2 under similar experimental conditions results in a TON value
of only 33 aer 24 h (lirr ¼ 655 nm, 0.1 M TsOH, 30 mM BNAH).
A higher concentration of 2 was used in order to compensate for
the lower molar absorption of 2 at 655 nm. The lower activity of
2 compared to those of 1 and 3 can be explained by its less
negative reduction potentials, as discussed below. The simi-
larity in the activities of 1 and 3 can be rationalized by their
nearly identical molar absorptivities at the irradiation wave-
length, excited state lifetimes, and excited state redox potentials
(Table 1).
Fig. 5 Electronic absorption spectra of (a) 1, [1]ꢀ, and [1]2ꢀ in DMF
obtained through the addition of 1 eq. and 2 eq. of cobaltocene,
respectively, to 1 and (b) changes to the electronic spectrum of [1]ꢀ,
obtained by the addition of 1 eq. cobaltocene to 1, as a function of
irradiation time in the presence of 30 mM BNAH in DMF (lirr ¼ 655 nm).
species, [1]ꢀ in DMF in the presence of 30 mM BNAH result in
the quantitative production of the corresponding doubly-
reduced complex, [1]2ꢀ (lirr ¼ 655 nm). Similar results are
observed for the irradiation of [2]ꢀ with excess BNAH in DMF
(Fig. S16†). The reduction of [2]ꢀ under these conditions was
occurred at signicantly lower irradiation times as compared to
that for [1]ꢀ, indicating that [2]ꢀ functions as a better excited
state oxidant than [1]ꢀ under similar conditions, as expected
from the more positive E1/2[Rh2]*2+/+ for 2 as compared to 1
(Table 2). These results parallel those from spectroelec-
trochemistry, where the generation of [1]ꢀ from 1 required
a longer time period, 83 min, as compared to [2]ꢀ from 2, 3 min
(Fig. S14 and S15†). No spectral changes are observed upon the
irradiation of [1]ꢀ and [2]ꢀ in DMF in the absence of the
sacricial electron donor under similar conditions and no
reactivity with BNAH is observed in the dark. These results
indicate that [1]ꢀ and [2]ꢀ possess an excited state able to
oxidize BNAH. The addition of excess TsOH to solutions con-
taining chemically-generated [1]ꢀ and [2]ꢀ in DMF result in the
regeneration of the corresponding initial complex, 1 and 2,
respectively, as well as the production of H2 (Fig. S17†). Similar
results were also observed for [1]2ꢀ and [2]2ꢀ, which also
produce H2 and regenerate 1 and 2 when excess TsOH is added
The addition of one and two equivalents of cobaltocene to
a DMF solution of 1 results in the generation of the corre-
sponding one- and two-electron reduced species, [1]ꢀ and [1]2ꢀ
,
respectively. The electronic absorption spectra of [1]ꢀ and [1]2ꢀ
are shown in Fig. 5a, and the corresponding species for 2 are
shown in Fig. S13.† The absorption spectra of the one- and two-
electron reduced species for 1 and 2 are consistent with those
obtained from spectroelectrochemistry in DMF at applied
potentials slightly more negative than the rst and second
reduction couples of each complex, respectively (Fig. S14 and
S15†). The lowest energy transition of [1]ꢀ is observed at 760 nm
and the intensity of the absorption at l > 600 nm decreases
signicantly in [1]2ꢀ (Fig. S14a†). It should be noted that in
Fig. 5 the maximum of [1]ꢀ is slightly blue-shied relative to
that recorded in the spectroelectrochemistry in Fig. S14,†
attributed to the incomplete conversion from 1 to [1]ꢀ. These
results parallel those previously reported for [3]ꢀ and [3]2ꢀ 36 as
,
well as those recorded herein for [2]ꢀ and [2]2ꢀ, as shown in
Fig. S13 and S15† for the chemically and electrochemically
reduced complex, respectively.
Importantly, Fig. 5b shows that the irradiation of solutions
containing the chemically-generated one-electron reduced
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to the DMF solution (Fig. S17†). These observations suggest that
both the one- and/or two-electron reduced species of these
complexes can serve as intermediates in the photocatalytic
process.
Control experiments of the photocatalysis show that no H2
product is detected in the absence of any one of the compo-
nents, light, catalyst, BNAH, or acid (Table S2†). In addition,
when 1 and 2 are irradiated in the presence of only BNAH, [1]ꢀ
and [2]ꢀ are produced within 1 min, respectively (Fig. S18†).
Fig. S18† also shows that continued irradiation of these solu-
tions generates the corresponding doubly-reduced species, [1]2ꢀ
(tirr $ 15 h) and [2]2ꢀ (tirr ¼ 1 h). This difference in the
photolysis times for the reduction 1 and 2 can be attributed to
the more positive excited state reduction potential of 2 as
compared to that of 1 (Table 2), resulting in a more negative
driving force for the oxidation of BNAH by the latter. It is
important to note that the addition of excess TsOH at the end of
the photolysis resulted in spectral changes consistent with the
regeneration of the starting catalyst in each case, 1 and 2, and
the release of H2 conrmed by gas chromatography. These
photochemical results parallel those discussed above for the
chemically-generated one- and two-electron reduced catalysts.
The mechanism of photocatalysis was further explored
through electronic absorption spectroscopy, as shown in
Fig. S19.† Upon irradiation of the photocatalytic DMF solution
of 1 (64 mM 1, 30 mM BNAH, 0.1 M TsOH, lirr ¼ 655 nm),
spectral changes consistent with the formation of [1]ꢀ were
apparent within the rst 5 min, followed by a slight decrease of
the peak at 760 nm indicative of the production of a small
amount of [1]2ꢀ. A signicantly faster generation of [2]ꢀ was
observed for 2, within 3 min of irradiation of the photocatalytic
solution (180 mM 2, 30 mM BNAH, 0.1 M TsOH, lirr ¼ 655 nm).
This fact indicates that the relatively short 3ML-LCT lifetime of 2
does not negatively impact on the bimolecular reductive
quenching by BNAH as compared to 1. The transient absorption
of spectra of the chemically-generated one-electron reduced
complexes, [1]ꢀ and [2]ꢀ, show that these species possess
excited states with a short component with s1 ꢂ 3 ps in both
complexes, as well as a longer decay, s2, of 630 ps and 1.0 ns,
respectively (Fig. S20†).
Fig. 6 Proposed catalytic cycle of photocatalytic H2 production by 1,
showing two possible pathways, one through the reaction of two
molecules [1-H] (minor pathway) and the other through the proton-
ation of [1-H]ꢀ.
then react rapidly with acid to produce H2 and regenerate 1. A
parallel mechanism is proposed for 2. Complexes 1 and 2
feature an open axial coordination site, such that the formation
of axial Rh-hydrides is possible, however, isolation of these
species has not yet been possible.30,31 A similar photocatalytic
mechanism for the production of H2 was recently proposed for 3
under similar experimental conditions.36
The photolysis experiments in the presence of BNAH show
that the rates of reductive quenching steps of 1 and 2, as well as
those of [1]ꢀ and [2]ꢀ, are not affected by the different excited
state lifetimes of these species. However the amount of photo-
catalytic H2 generated is approximately 6-fold lower for 2 as
compared to 1. Thus the hydrogen production activities were
investigated by the successive linear sweep voltammograms of
reaction mixtures in which the acid concentrations were
increased. TsOH was used as a strong acid (pKMa eCN ¼ 8.6),47 and
a catalytic current is observed for both complexes aer the
second reduction peak, but 2 requires a larger overpotential as
compared to 1 under the similar conditions (Fig. S12†). Salicylic
acid (pKMa eCN ¼ 16.7) and acetic acid (pKMa eCN ¼ 23.5) were
chosen as weaker acids to investigate the role of acid strength
on the catalytic activity.47 Using salicylic acid, the catalytic
current decreased for 2, however, complex 1 maintained high
efficiency (Fig. S21†). In contrast, the catalytic performance for 2
was hindered signicantly upon addition of acetic acid, whereas
1 remained fairly active but required a more negative potential,
ꢀ1.2 V vs. Ag/AgCl (Fig. S21†). Considering the hydride inter-
mediates required for H2 evolution shown in Fig. 6, the differ-
ence in reactivity between 1 and 2 can be explained as arising
from [1-H]ꢀ being more hydridic than [2-H]ꢀ. In that case, [2-
H]ꢀ would require a stronger acid than [1-H]ꢀ for the formation
of H2 to be thermodynamically favorable. In fact, the rst
reduction potentials of 1 and 2 associated with the placement of
an electron in the lowest unoccupied molecular orbitals
(LUMO), can also be considered as the hydride ligand acceptor
Together, the results presented herein can be used to
propose the photocatalytic cycle for 1 and 2 illustrated in Fig. 6,
using complex 1 as the example. The excitation of 1 with red
3
light places the complex in its ML-LCT excited state, which is
then reductively quenched by BNAH to form the corresponding
singly reduced complex, [1]ꢀ. Since H2 evolution is observed
when the chemically generated [1]ꢀ is treated with excess TsOH,
it is proposed that at least some fraction of the sample is
protonated during photolysis to generate the corresponding
hydride, [1-H]. However, the accumulation of the singly-reduced
complex upon irradiation of the photocatalysis solution points
at a slow reaction of [1-H] to produce H2 (Fig. S19†), making this
pathway a minor source of H2 under these conditions. Based on
the production of H2 upon the treatment of the chemically-
generated [1]2ꢀ with TsOH, it may be hypothesized that there
is an equilibrium that results in the protonation of the doubly-
reduced complex to generate [1-H]ꢀ, and that [1-H]ꢀ is able to
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orbital that is able to affect the hydride donor ability of [1-H]ꢀ
and [2-H]ꢀ. The bridging ligands in 2 are a more electron-
withdrawing as compared to those in 1, resulting in a lower
energy LUMO in the former, making 2 a poorer hydride donor
and less catalytically active than 1. A similar trend was previ-
ously reported for Ni and Co catalyst.48–50
J. M. Papanikolas and T. J. Meyer, J. Am. Chem. Soc., 2016,
138, 13085–13102.
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7 R. S. Khnayzer, V. S. Thoi, M. Nippe, A. E. King, J. W. Jurss,
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Conclusion
In summary, we report a series of single-molecule dirhodium
photocatalysts that can generate hydrogen upon irradiation
with 655 nm red light. Complex 1 reaches TON ¼ 250 aer 24 h
of photolysis following two sequential reductive quenching
steps in acidic solution. Complexes 1–3 process 3ML-LCT
excited states with lifetimes that range from ꢂ8 ns to 33 ns at
room temperature, are good excited state oxidants, and are
emissive at 77 K. In addition, the ability of the complexes to
8 D. Kim, K. K. Sakimoto, D. Hong and P. Yang, Angew. Chem.,
Int. Ed., 2015, 54, 3259–3266.
9 G. Reginato, L. Zani, M. Calamante, A. Mordini and A. Dessi,
Eur. J. Inorg. Chem., 2020, 11–12, 899–917.
absorb strongly from the ultraviolet to the near-IR extends their 10 O. Bettucci, T. Skaltsas, M. Calamante, A. Dessi, M. Bartolini,
reactivity into the red, thus improving the utilization of the solar
A. Sinicropi, J. Filippi, G. Reginato, A. Mordini, P. Fornasiero
spectrum beyond traditional sensitizers. Importantly, 1–3 are
and L. Zani, ACS Appl. Energy Mater., 2019, 2, 5600–5612.
able to store two electrons upon irradiation, which facilitates 11 X. Zhang, T. Peng and S. Song, J. Mater. Chem. A, 2016, 4,
the two-electron photocatalytic generation of hydrogen. The 2365–2402.
substituents on the electron-donating ligands bridging ligands 12 S. Kampouri and K. C. Stylianou, ACS Catal., 2019, 9, 4247–
were used to tune the electronic properties of the complexes, 4270.
including the excited state lifetime and excited state reduction 13 A. Hagfeldt, G. Boschloo, L. Sun, L. Kloo and H. Pettersson,
potential, while retaining open coordination sites for catalysis.
Chem. Rev., 2010, 110, 6595–6663.
Photocatalysts with longer excited state lifetimes and the ability 14 K. R. Brereton, A. G. Bonn and A. J. M. Miller, ACS Energy
to oxidize electron donors in solution, as well as open site for Lett., 2018, 3, 1128–1136.
¨
hydride binding are expected to result in an increased efficiency 15 L. Hammarstrom, Acc. Chem. Res., 2015, 48, 840–850.
of H2 production. Complexes 1 and 2 represent new additions to 16 S. Mozaffari, M. R. Nateghi and M. B. Zarandi, Renewable
a new class of photocatalysts that act both as the light absorber
Sustainable Energy Rev., 2017, 71, 675–686.
and catalyst within the same chromophore, with absorption 17 D. L. Ashford, M. K. Gish, A. K. Vannucci, M. K. Brennaman,
throughout the visible range for enhanced harnessing of the
solar spectrum, making them useful for the direct conversion of
solar energy into H2 fuel.
J. L. Templeton, J. M. Papanikolas and T. J. Meyer, Chem.
Rev., 2015, 115, 13006–13049.
18 (a) S. J. Mora, E. Odella, G. F. Moore, D. Gust, T. A. Moore
and A. L. Moore, Acc. Chem. Res., 2018, 51, 445–453; (b)
Y. Terazono, T. A. Moore, A. L. Moore and D. Gust,
Multiporphyrin Arrays, 2012, 349–387.
Conflicts of interest
There are no conicts to declare.
19 T. Stoll, M. Gennari, J. Fortage, C. E. Castillo, M. Rebarz,
M. Sliwa, O. Poizat, F. Odobel, A. Deronzier and
M. N. Collomb, Angew. Chem., Int. Ed., 2014, 53, 1654–1658.
20 Y. Halpin, M. T. Pryce, S. Rau, D. Dini and J. G. Vos, Dalton
Trans., 2013, 42, 16243–16254.
Acknowledgements
The authors thank the Support from the Department of Energy,
Office of Science, Office of Basic Energy Sciences (DE- 21 R. Katoh and A. Furube, J. Photochem. Photobiol., C, 2014, 20,
SC0020243) and The Ohio State University for partial support 1–16.
of this work and the Center for Chemical and Biophysical 22 M. K. Gish, A. M. Lapides, M. K. Brennaman, J. L. Templeton
Dynamics (CCBD) for use of the ultrafast laser facility.
and T. J. Meyer, J. Phys. Chem. Lett., 2016, 7, 5297–5301.
23 O. S. Wenger, J. Am. Chem. Soc., 2018, 140, 13522–13533.
24 M.-Q. Yang, M. Gao, M. Hong and G. W. Ho, Adv. Mater.,
2018, 30, 1802894.
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