Unsymmetrical dirhodium single molecule photocatalysts for H2production with low energy light

Article abstract of DOI:10.1039/d0cc08248a

New axially blocked unsymmetrical dirhodium complexes photocatalyze the production of H₂under red light irradiation with a turnover number (TON) of 23 ± 3 in the presence of acid and a sacrificial donor. The presence of multiple metal/ligand-to-ligand charge transfer transitions improves their absorption of light into the near-IR.

Full text of DOI:10.1039/d0cc08248a

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Unsymmetrical dirhodium single molecule
photocatalysts for H2 production with
low energy light†
Agustin Millet,a Congcong Xue,b Claudia Turro *b and Kim R. Dunbar
Cite this: Chem. Commun., 2021,
57, 2061
Received 19th December 2020,
Accepted 22nd January 2021
a
*
DOI: 10.1039/d0cc08248a
rsc.li/chemcomm
New axially blocked unsymmetrical dirhodium complexes photo- is difficult to maintain at the surfaces of semiconductors and in
catalyze the production of H2 under red light irradiation with a supramolecular systems. Consequently, the design of single
turnover number (TON) of 23 Æ 3 in the presence of acid and a molecule photocatalysts can aid in the development of more
sacrificial donor. The presence of multiple metal/ligand-to-ligand efficient photoinduced H2 production.
charge transfer transitions improves their absorption of light into
the near-IR.
Dirhodium complexes are excellent candidates for photo-
catalysis since they display panchromatic absorption profiles,
are known to catalyze proton reduction, and are water and air
The quest for more efficient carbon-neutral fuels and renewable stable. The Nocera group reported several two-electron mixed-
energy sources is of paramount importance for addressing the valence dirhodium complexes in which the catalytic center and
challenges of increasing global energy demands.1–6 Over the the sensitizer are combined in the same unit.30–34 For example,
last several decades, solar energy applications, such as dye- Rh20,II(tfepma)2(CNAd)2Cl2 (tfepma
=
CH3N[P(OCH2CF3)2]2,
sensitized solar cells (DSSCs)7–11 and artificial photosynthesis CNAd = 1-adamantylisocyanide) produces H2 upon irradiation
systems,12–16 have made great progress to begin addressing this with a TON of 7 after 144 hours of UV irradiation (lirr 4 305 nm).35
challenge. Traditional homogeneous artificial photocatalytic This compound, however, does not absorb beyond 550 nm and is
systems mimic the natural photosystem II reaction center, sensitive to moisture and oxygen.
which features separate chlorophyll derivative light absorbing
Recently, our groups developed air-stable, axially-blocked pan-
units as the photosensitizer (PS) coupled to a catalytic center chromatic dirhodium complexes sensitizers, cis-[Rh2(DTolF)2(np-
(CAT),17 and often employ an electron donor (ED) and electron COO)2] (DTolF = p-ditolylformamidinate, npCOOÀ = 2-carboxylate-
relay (ER) molecules to effect charge separation.
1,8-naphthyridine) and cis-[Rh2(DTolF)2(qnnp)2][BF4]2 (qnnp =
Conventional multicomponent systems for hydrogen gen- 2-(quinolin-2-yl)-1,8-naphthyridine),36,37 that exhibit relatively
eration feature Ru(II) and Ir(III) complexes as the PS,18–26 long-lived 3ML-LCT (triplet metal/ligand-to-ligand charge transfer)
combined with Co,18–20 Fe,21–23 or Ni24 units as the CAT, and excited states.
require photoinduced electron transfer from PS to CAT to drive
These longer lifetimes are attributed to the axial coordination
a reaction. Therefore, the efficiency of catalysis is largely depen- that results in an increase in energy of the Rh2(s*) orbital and,
dent on the charge transfer steps making covalently linked consequently, a higher energy 3MC (metal-centered) excited state,
macromolecular assemblies an attractive alternative.27–29 These making the latter less accessible for fast non-radiative decay and
tethered systems typically involve multi-step synthesis, making allowing them to serve as panchromatic PSs in multi-component
them difficult to scale for practical use. In addition, heteroge- photocatalytic hydrogen evolution systems.38 These molecules,
neous assemblies with PS–CAT interactions at specific distances however, are not catalytically active due to the blocked axial
and orientations are essential for maximizing electron transfer sites which render the bimetallic core inaccessible for substrate
and minimizing charge recombination. Such precision, however, binding, a situation that was remedied by preparing cis-[Rh2
(form)2(bncn)2](BF4)2 (form = diphenylformamidinate, bncn =
benzo[c]cinnoline) with open axial sites. This molecule features
a Department of Chemistry, Texas A&M University, College Station, Texas 77843,
a shorter Rh–Rh bond afforded by the bncn bridging ligands,
which also raises the energy of the Rh2(s*) orbital.39 Particularly,
USA. E-mail: dunbar@mail.chem.tamu.edu
b Department of Chemistry and Biochemistry, The Ohio State University, Columbus,
cis-[Rh2(form)2(bncn)2](BF4)2 is a single-molecule photocatalyst
Ohio 43210, USA. E-mail: turro.1@osu.edu
that produces H2 with a TON = 170 Æ 5 in acidic solution upon
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
d0cc08248a
red light irradiation, 670 nm, as well as with 735 nm irradiation.
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Fig. 1 Electronic absorption spectra for 1 (green, dashed) and 2 (blue,
solid).
greater electron-withdrawing character for the qxnp and qnnp
ligands in 1 and 2 as compared to the np ligand in 3. A cathodic
scan in the presence of acetic acid (Fig. S4 and S5, ESI†) revealed
that the complexes are catalytically active, producing H2 at
potentials more negative than À1.2 V vs. Ag/AgCl. This activity
of 1 and 2 is attributed to the presence of an open axial position,
in contrast to the fully blocked complexes 4 and 5 which do not
exhibit an electrocatalytic current under similar experimental
conditions.
The steady state absorption of 1 and 2 is strong throughout
the ultraviolet to visible ranges and extends into the near-IR
region (Fig. 1 and Table 1). The complexes exhibit ligand-
centered pp* transitions at 252 nm, also observed in the
256 – 259 nm range in 4 and 5 (Table 1), consistent with the
absorption of the free ligands in solution.37 The features at 365
and 352 nm for 1 and 2, respectively, are likely to be Rh2-to-
ligand in character since they are red-shifted compared to the
absorption at 300 nm in 3, as expected given the more positive
ligand reduction potentials for these complexes. In the case of
1, the broad absorption bands at 584 nm and 790 nm are
assigned to Rh2/DTolF - np and Rh2/DTolF - qxnp 1ML-LCT
transitions, respectively, which are slightly red-shifted relative
to the peaks observed for the corresponding symmetric com-
plexes, at 566 nm for 3 and at 758 nm for 4.37 A similar
assignment can be made for the bands at 582 nm and
712 nm for 2.
Scheme 1 Synthetic scheme for the synthesis of 1 and 2 as well as the
previously published compound 6.39
The present work focuses on a new design for panchromatic
single-molecule photocatalysts that involves the use of unsym-
metrical bridging ligands. The dirhodium compounds cis-
[Rh2(DTolF)2(L)(np)][BF4]2 (np = 1,8-naphthyridine; Scheme 1),
where L = qxnp (2-(1,8-naphthyridin-2-yl)quinoxaline) (1) and
qnnp (2-(quinolin-2-yl)-1,8-naphthyridine) (2), were prepared
which feature one axial site blocked by a p-accepting ligand,
while the other axial site remains available for substrate binding
required for photocatalytic hydrogen production (Fig. S1 and S2,
ESI†). The properties of 1 and 2 are compared to those of the
previously reported compounds cis-[Rh2(DTolF)2(np)2][BF4]2 (3),
cis-[Rh2(DTolF)2(qxnp)2][BF4]2 (4), and cis-[Rh2(DTolF)2(qnnp)2]
[BF4]2 (5), shown in Scheme 1.37,40
Complexes 1 and 2 exhibit a one-electron reversible oxida-
tion couple at +0.82 and +0.80 V vs. Ag/AgCl, respectively
(Table 1 and Fig. S3, ESI†), assigned to the removal of an
electron from the highest occupied molecular orbital (HOMO)
of Rh2(d*)/DTolF character, as reported for related dirhodium
formamidinate complexes.36,37,39 In addition, a reversible one-
electron reduction was observed for 1 and 2 at À0.60 and
À0.69 V vs. Ag/AgCl, assigned to the reduction of the qxnp and
qnnp ligands, respectively. These complexes are more easily
reduced than 3, consistent with more extended p-systems and
Density functional theory (DFT) calculations show that the
HOMOs of 1–5 are DTolF(p*)/Rh2(d*) in character, consistent
with related formamidinate-bridged dirhodium complexes
Table 1 Electronic absorption maxima with molar absorptivities, reduction potentials, singlet (tS) and triplet (tT) lifetimes for 1–5 in CH3CN
l
abs/nm (e  103 MÀ1 cmÀ1
)
E1/2a/(V)
t1/ps
t2/ns
Complex
1
252 (98), 365 (29), 584 (3.9), 790 (1.9)
252 (109), 352 (31), 582 (4.3), 712 (2.4)
235 (55), 300 (25), 436 (1.8), 566 (3.6)
259 (83), 369 (32), 554 (2.2), 758 (3.2)
256 (90.4), 366 (32), 583 (2.5), 704 (3.4)
+0.82, À0.60
+0.80, À0.69
+0.87, À0.94
+1.08, À0.43
+0.99, À0.62
6b
10b
14
8
1.5b
2
4c
3d
4d
5d
0.64
7
7
8
a
b
c
vs. Ag/AgCl, 0.1 M Bu4NPF6. From transient experiments at 298 K (lexc = 720 nm, 2.5 mJ, IRF = 85 fs); error B10%. From transient absorption
experiments at 298 K (lexc = 650 nm, 5 mJ, IRF = 6 ns); error B1 ns. From ref. 34.
d
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able to undergo bimolecular charge transfer with low energy red
excitation.
Given the recent report of photocatalytic H2 production by
cis-[Rh2(form)2(bncn)2](BF4)2 (6),39 it was hypothesized that 1
and 2 may also act as single-molecule photocatalysts. Upon
irradiation with 655 nm, 1 and 2 generate H2 in DMF upon in
the presence of 0.1 M TsOH as the proton source and 1-benzyl-
1,4-dihydronicotinamide (0.12 M; BNAH, EOX = +0.62 V vs.
Ag/AgCl) as the sacrificial electron donor (Table S5, ESI†).39
A turnover number (TON) of B23 was measured for 2 after 24 h
of irradiation. This value, along with the use of low energy
Fig. 2 fsTA spectra of 1 recorded 0.6, 2.1, 4.3, 5.3, 18, 68, 911, and 2753 ps
after the excitation pulse (lexc = 720 nm, 2.5 mJ, IRF = 85 fs) in CH3CN;
black trace at À20 ps.
0,II
visible light, represents a notable improvement over Rh2
(tfepma)2(CNtBu)2Cl2.35
A lower TON of B2 was observed for 1 under the same
3
conditions which is attributed to the shorter ML-LCT lifetime
(Fig. S6–S8 and Tables S1–S4, ESI†).41 Interestingly, in the compared to 2, reducing its ability to participate in electron
presence of acid, a slight blue-shift of the 1ML-LCT band is transfer reactions hindering the production of H2. Complexes
observed for 1 and 2 (Fig. S9, ESI†). Titrations of 1 and 2 with 3–5 were irradiated under similar conditions but no H2 was
tosylic acid, TsOH, followed by electronic absorption spectro- detected. Additional control experiments revealed that H2
scopy reveal a 1 : 1 protonation stoichiometry with each production by 2 requires light, BNAH, a 5–10 ns lifetime, and
complex (Fig. S10, ESI†). In addition, Time Dependent DFT TsOH. Moreover, the fact that 4 and 5 are not photocatalytic,
(TD-DFT) calculations (Fig. S11 and S12, ESI†) and 1H NMR acid points at the need of an open axial site for reactivity. It is noted
titrations (Fig. S13 and S14, ESI†) indicate that protonation that 2 is not as active as the recently reported photocatalyst 6,
occurs at the N atom of the formamidinate ligand trans to the owing to its shorter triplet lifetime, lower excited state driving
axial blocking ligand. This protonation, however, does not lead force, the presence of only one active open site for catalysis, and
to ligand dissociation on the timescale of days.
lower oscillator strength at the irradiation wavelength.
A broad positive signal was observed from 400 to 675 nm Nevertheless, the structural modification of the unsymmetrical
with a maximum at B450 nm and a lower intensity feature at complexes constitutes a promising route to the design of a
B670 nm in the femtosecond transient absorption (fsTA) single-molecule photocatalysts.
spectrum for 1 (Fig. 2). It is evident from the low intensity of
In contrast with 6, protonation of 1 and 2 in the ground state
the signal at B580 nm that the ground state bleach overlaps was observed. The fsTA and nsTA spectra of the fully
with positive excited state absorption in this spectral range. The protonated compounds 1 and 2 exhibit similar 1ML-LCT and
signal at 450 nm can be fitted to a biexponential decay with 3ML-LCT spectral features and lifetimes compared to those in
lifetimes of t1 = 5.6 ps (68%) and t2 B 1.5 ns (32%) assigned to the absence of acid (Fig. S20 and S21, ESI†), indicating that
the 1ML-LCT and 3ML-LCT excited states, respectively. Complex protonation at the formamidinate bridging ligand does not
2 also exhibits a broad absorption at 470 nm attributed to the disturb the excited state properties of the complexes.
1ML-LCT state with t = 10 ps (Fig. S15, ESI†), similar to the
The chemically prepared proposed intermediates [1]1À and
symmetric analogs 4 and 5.37 The nanosecond transient [2]1À are dark green in color and, in the presence of acid, they
absorption spectra (nsTA, Fig. S16, ESI†) resemble those col- generate the purple protonated compounds with release of H2
lected at 911 ps for 1 and at B2.7 ns for 2 in the fsTA (Fig. S19 and S22, ESI†). The turnover number of H2 produced
experiments, Fig. 2 and Fig. S17 (ESI†), respectively. The decay per dirhodium complex is B0.4 as shown in Table S6 (ESI†),
of the signal of 2 at 470 nm decays with a lifetime of B4 ns suggesting a mechanism where two protonated complexes may
(Fig. S17 and S18, ESI† for 1), assigned to the decay of the
3ML-LCT state. The shorter triplet lifetimes of 1 and 2 com-
pared to their fully blocked symmetric analogues 4 and 5,
B7 ns,37 is attributed to lower energy 3MC states in the former
compounds arising from the presence of one open axial site,
thus facilitating excited state deactivation.
Complexes 1 and 2 undergo bimolecular photoinduced electron
transfer with the reversible electron donor p-phenylenediamine
(p-PD, E1/2(p-PD+/0) = +0.28 V vs. Ag/AgCl)37 upon 650 nm excitation,
resulting in nsTA spectra consistent with a p-PD radical cation
superimposed with that of the reduced dirhodium complex
(Fig. S19, ESI†), which recombine with a time constant of
B35 ms.37 Therefore, albeit shorter-lived 3ML-LCT excited states
relative to the symmetric analogues 4 and 5, complexes 1 and 2 are Fig. 3 Schematic representation of the H2 production by 1 and 2.
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In summary, two new unsymmetrical dirhodium complexes cis-
[Rh2(DTolF)2(qxnp)(np)][BF4]2 (1) and cis-[Rh2(DTolF)2(qnnp)(np)]
[BF4]2 (2) are capable of photocatalytic H2 production upon irradia-
tion with low energy visible light. This work underscores the fact
that a careful choice of ligands for the dirhodium core can lead to
lifetimes suitable for electron transfer reactions while allowing the
one of the metals to be accessed by H+ for H2 production. More
broadly, this new approach aimed at installing two different
acceptors in the coordination sphere opens unexplored possibilities
for the design of new unsymmetrical dirhodium complexes as
photocatalytic systems for H2 production.
We thank the Welch Foundation (K. R. D., A-1449) and the
U.S. Department of Energy, Office of Science, Office of Basic
Energy Sciences (C. T., DE-SC0020243) for support. The authors
¨
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Conflicts of interest
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