Increased photocatalytic activity in Ru(II),Rh(III) supramolecular bimetallic complexes with terminal ligand substitution

Article abstract of DOI:10.1016/j.ica.2016.06.020

Three new Ru(II),Rh(III) supramolecular bimetallic complexes of the design [(Ph₂phen)₂Ru(dpp)RhCl₂(R₂-bpy)](PF₆)₃(R?=?CH₃(Ru-Rh(Me₂bpy)), H (Ru-Rh(bpy)), or COOCH₃(Ru-Rh(dmeb)); Ph₂phen?=?4,7-diphenyl-1,10-phenanthroline; dpp?=?2,3-bis(2-pyridyl)pyrazine; dmeb?=?4,4′-dimethyl ester-2,2′-bipyridine; bpy?=?2,2′-bipyridine; Me₂bpy?=?4,4′-dimethyl-2,2′-bipyridine) have been synthesized and analyzed to determine the impact that the polypyridyl terminal ligand (TL) coordinated to the cis-dihalide rhodium(III) metal center has on the photocatalytic activity for water reduction. The bimetallic complexes demonstrate that a correlation exists between the σ-donating ability of the substituted bipyridine ligand, the rate of chloride dissociation upon electrochemical reduction and the activity towards photocatalytic hydrogen production. The weaker σ-donating –COOCH₃substituent in Ru-Rh(dmeb) increases the rate constant for Cl^?dissociation (k_?Cl?=?0.7?s^?1) and the amount of H₂produced photocatalytically (37?±?4?μmol H₂, 63?±?7 turnovers after 20?h; turnovers?=?mol H₂/mol photocatalyst) when compared to –H and –CH₃substituted complexes Ru-Rh(bpy) (k_?Cl?=?0.2?s^?1, 21?±?2?μmol H₂, 35?±?3 turnovers) and Ru-Rh(Me₂bpy) (k_?Cl?=?0.2?s^?1, 18?±?2?μmol H₂, 30?±?4 turnovers), respectively. Varied catalytic activity with respect to the σ-donor capacity of the Rh-TL is attributed to the relative ease of ligand dissociation and the ability to afford rapid electron collection at the Rh metal center.

Full text of DOI:10.1016/j.ica.2016.06.020

Inorganica Chimica Acta xxx (2016) xxx–xxx
Contents lists available at ScienceDirect
Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Research paper
Increased photocatalytic activity in Ru(II),Rh(III) supramolecular
bimetallic complexes with terminal ligand substitution
Hannah J. Sayre a, , Travis A. White b, Karen J. Brewer a,1
⇑
a Department of Chemistry, Virginia Tech, Blacksburg, VA 24061-0212, USA
b Department of Chemistry & Biochemistry, The Ohio State University, Columbus, OH 43210-1340, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Three new Ru(II),Rh(III) supramolecular bimetallic complexes of the design [(Ph2phen)2Ru(dpp)RhCl2
(R2-bpy)](PF6)3 (R = CH3 (Ru-Rh(Me2bpy)), H (Ru-Rh(bpy)), or COOCH3 (Ru-Rh(dmeb)); Ph2phen = 4,
7-diphenyl-1,10-phenanthroline; dpp = 2,3-bis(2-pyridyl)pyrazine; dmeb = 4,40-dimethyl ester-2,20-
bipyridine; bpy = 2,20-bipyridine; Me2bpy = 4,40-dimethyl-2,20-bipyridine) have been synthesized and
analyzed to determine the impact that the polypyridyl terminal ligand (TL) coordinated to the
cis-dihalide rhodium(III) metal center has on the photocatalytic activity for water reduction. The bimetal-
Received 16 January 2016
Received in revised form 7 June 2016
Accepted 9 June 2016
Available online xxxx
Keywords:
Supramolecular photocatalysis
Water reduction
Electron-withdrawing
Ligand variation
lic complexes demonstrate that a correlation exists between the
bipyridine ligand, the rate of chloride dissociation upon electrochemical reduction and the activity
towards photocatalytic hydrogen production. The weaker -donating –COOCH3 substituent in Ru-Rh
(dmeb) increases the rate constant for Clꢀ dissociation (kꢀCl = 0.7 sꢀ1) and the amount of H2 produced
photocatalytically (37 mol H2, 63 7 turnovers after 20 h; turnovers = mol H2/mol photocatalyst)
when compared to –H and –CH3 substituted complexes Ru-Rh(bpy) (kꢀCl = 0.2 sꢀ1, 21
mol H2,
35 3 turnovers) and Ru-Rh(Me2bpy) (kꢀCl = 0.2 sꢀ1, 18
mol H2, 30 4 turnovers), respectively.
Varied catalytic activity with respect to the -donor capacity of the Rh-TL is attributed to the relative
r-donating ability of the substituted
r
4
l
2
l
2
l
r
ease of ligand dissociation and the ability to afford rapid electron collection at the Rh metal center.
Ó 2016 Elsevier B.V. All rights reserved.
1. Introduction
Photochemical molecular devices that serve to collect multiple
reducing equivalents upon photoexcitation are labeled as photoini-
The utilization of sunlight to drive thermodynamically demand-
ing reactions that generate alternative fuels (e g. H2O splitting to
H2 and O2) is an attractive solution to the rise in global energy
demands [1–4]. To store solar energy in the form of chemical
bonds, light absorbing components harness incident photons that
provide the necessary energy and subsequent electrons to reduce
H2O to H2 fuel. Significant research efforts have focused on the
design of multi-component, homogeneous photocatalytic systems
for H2O reduction that rely on bimolecular, diffusion-based colli-
sions between each component within the photocatalytic system
[5–8]. In an effort minimize the number of required bimolecular
collisions and to better understand the photocatalytic processes
at the molecular level, supramolecular complexes that function
as photochemical molecular devices (PMDs) have been developed
by covalently coupling the molecular components [9–12].
tiated electron collectors (PECs) and function by collecting elec-
trons in low-lying, unoccupied ligand- or metal-based MOs. Early
examples of complexes that collect electrons within the bridging
ligand (BL) p⁄ molecular orbitals include [{(bpy)2Ru(dpb)}2IrCl2]5+
(dpb = 2,3-bis(2-pyridyl)benzoquinoxaline) and [(phen)2Ru(BL)Ru
(phen)2]4+ (phen = 1,10-phenanthroline; BL = tatpp = 9,11,20,22-
tetraazatetrapyrido[3,2-a:20,30-c:300,200-1:2000,3000-n]pentacene; tatpq =
9,11,20,22-tetraazatetrapyrido[3,2-a:20,30-c:300,200-1:2000,3000-n]pen-
tacene-10,21-quinone), yet these systems do not serve as photo-
catalysts for H2O reduction [12–14]. Modification of the BL and
reactive metal (RM) of [{(bpy)2Ru(dpb)}2IrCl2]5+ to form [{(bpy)2-
Ru(dpp)}2RhCl2]5+ (dpp = 2,3-bis(2-pyridyl)pyrazine) generated
the first PMD for PEC at a metal site to function as a photocatalyst
for H2O reduction [15,16]. In the presence of the sacrificial electron
donor N,N-dimethylaniline (DMA) and light, the [{(bpy)2Ru(dpp)}2-
RhCl2]5+ trimetallic complex functions via successive photoexcita-
tions of the Ru-based LA to transfer electrons to the cis-RhIIICl2
site which dissociates two halide ligands and generates a coordina-
tively unsaturated, four coordinated RhI species, [{(bpy)2Ru
(dpp)}2RhI]5+, that is capable of H2O reduction to H2.
⇑
Corresponding author at: Department of Chemistry & Biochemistry, The Ohio
State University, Columbus, OH 43210-1340, USA.
E-mail address: sayre.166@osu.edu (H.J. Sayre).
Deceased October 24, 2014.
1
http://dx.doi.org/10.1016/j.ica.2016.06.020
0020-1693/Ó 2016 Elsevier B.V. All rights reserved.
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2
H.J. Sayre et al. / Inorganica Chimica Acta xxx (2016) xxx–xxx
Structural and component modifications of systems that couple
LA subunits to a RM center through a polypyridyl BL have resulted
in an array of photocatalytically-active architectures for H2O
reduction to H2 [17–23]. Within the Ru(II),Rh(III),Ru(II) trimetallic
architecture [{(TL)2Ru(dpp)}2RhX2]5+ (TL = bpy, phen, Ph2phen;
X = Clꢀ, Brꢀ, OHꢀ), bidentate and monodentate ligand variation
has produced a library of complexes with varied electrochemical
and photophysical properties and photocatalytic H2O reduction
activity [24–28]. The light absorbing and excited state properties
of the Ru-based LA subunit are strongly impacted by the choice
of polypyridyl TL, while the catalytic activity at the Rh-based RM
center is tuned by monodentate X identity. The trimetallic complex
[{(Ph2phen)2Ru(dpp)}2RhBr2]5+ is a highly active H2O reduction
photocatalyst when compared with the [{(phen)2Ru(dpp)}2-
RhBr2]5+ trimetallic analogue, producing 1640 vs. 180
lmol H2,
respectively, after 20 h photolysis in a N,N-dimethylformamide
(DMF) solution containing DMA electron donor [27]. This minor
change to the polypyridyl TL structure significantly enhances pho-
tocatalytic activity and thereby emphasizes the importance of
design considerations when developing new H2O reduction
photocatalysts.
Fig. 1. Structural representation of [(Ph2phen)2Ru(dpp)RhCl2(R2-bpy)](PF6)3 com-
plexes (R = CH3, H, or COOCH3).
Substitution of one LA subunit with a polypyridyl TL in the Ru
(II),Rh(III),Ru(II) trimetallic architecture has produced Ru(II),Rh
(III) bimetallic complexes of the general formula [(TL)2Ru(BL)
RhX2(TL0)]3+ in an effort to better understand the role and necessity
of the second LA subunit [29]. The judicious choice of molecular
components throughout the Ru(II),Rh(III) bimetallic architecture
dictates photocatalytic activity towards H2O reduction through a
careful balance of sterics, electronics, light absorption, and ion
pairing throughout the photocatalytic cycle [30–34]. The bimetallic
complexes [(Ph2phen)2Ru(dpp)RhX2(Ph2phen)]3+ (X = Clꢀ or Brꢀ)
represent a systematic study undertaken to determine the influ-
2. Experimental
2.1. Materials and methods
All materials for syntheses and characterizations were used as
received unless otherwise stated. RuCl3ꢂxH2O, RhCl3ꢂxH2O, 4,7-
diphenyl-1,10-phenanthroline (Ph2phen), 2,3-bis(2-pyridyl)pyra-
zine (dpp), 2,20-bipyridine (bpy), and SephadexÒ LH-20 resin were
purchased from Aldrich. 4,40-Dimethyl-2,20-bipyridine (Me2bpy)
was purchased from Atomax Chemicals. Neutral, activated alumina
was purchased from Alfa Aesar and deactivated using methanol.
Spectrophotometric grade acetonitrile was purchased from Spec-
trum Chemicals. 4,40-Dimethyl ester-2,20-bipyridine (dmeb) [38],
[(Ph2phen)2RuCl2] [39], [(Ph2phen)2Ru(dpp)](PF6)2 [40] and [Rh
(bpy)Cl3(MeOH)](MeOH) [41] were synthesized as previously
reported.
ence that the
r-donating ability and degree of ion pairing imparted
by the monodentate halide ligands have on the photocatalytic H2O
reduction activity [30]. Previous reports that vary the electronic
character (r-donor/p-acceptor) of the bidentate polypyridyl ligand
coordinated to Rh within a monometallic complex, [Rh(R2-bpy)2-
Cl2]+ (R = H or C(CH3)3) [35] and [Rh(BL)2Br2]+ (BL = dpp, dpq, or
dpb) [36], display how differences in electron density at the Rh
metal center can impact the observed catalysis. Conversely, a sys-
tematic study to decipher the electronic contributions at the Rh
2.2. Syntheses
site via variation of the polypyridyl TL’s
r-donating ability has
yet to be reported for the [(TL)2Ru(BL)RhX2(TL0)]3+ bimetallic archi-
tecture and necessitates such analysis.
2.2.1. [Rh(Me2bpy)Cl3(DMF)]
The new Rh monometallic complex was synthesized using a
similar procedure reported for [Rh(bpy)Cl3(DMF)] (DMF = N,N-
dimethylformamide) [34]. RhCl3ꢂxH2O (0.20 g, 0.75 mmol) and
4,40-dimethyl-2,20-bipyridine (0.017 g, 0.92 mmol) were heated at
60 °C in 2 mL DMF for 2 h with rapid stirring. The resultant
orange-yellow solution was allowed to cool to room temperature
and was added dropwise to stirring diethyl ether to induce precip-
itation. The orange precipitate was collected by vacuum filtration,
dissolved in chloroform, and filtered into a clean flask. The solvent
was removed from the yellow filtrate by rotary evaporation to pro-
duce a yellow solid (yield = 67%, 0.24 g, 0.51 mmol). ESI-MS:
[MꢀClꢀ]+ m/z = 430.85.
Herein we report the synthesis, characterization, and photo-
catalytic activity of a series of Ru(II),Rh(III) bimetallic complexes
of the design [(Ph2phen)2Ru(dpp)RhCl2(R2-bpy)]3+ (R = CH3 (Ru-
Rh(Me2bpy)),
H
(Ru-Rh(bpy)), or COOCH3 (Ru-Rh(dmeb);
Fig. 1) to investigate what impact electronic variation of the Rh-
TL
r-donating ability will have on the observed photocatalytic
reduction of H2O to H2. The Ru-based LA subunit, (Ph2phen)2RuII
(dpp), is not altered in an effort to minimize any contributions
that the light absorber component would have on the Rh-TL elec-
tronic comparisons. The pKa values for similarly substituted pyri-
dinium
pyridinium = 5.23; 4-methylpyridinium = 5.97) [37] can provide
an approximation for the -bonding character of the polypyridyl
TL (
-donor ability: Me2bpy ꢁ bpy > dmeb). Inclusion of stronger
(–CH3) vs. weaker (–COOCH3) -type electron-donating sub-
compounds
(4-methoxycarbonylpyridinium = 3.26;
r
2.2.2. [Rh(dmeb)Cl3(DMF)]
r
The new Rh monometallic complex was synthesized as
described above using RhCl3ꢂxH2O (0.20 g, 0.76 mmol) and 4,40-
dimethyl ester-2,20-bipyridine (0.26 g, 0.92 mmol) Yield = 79%,
0.34 g, 0.61 mmol. ESI-MS: [M+NH+4]+ m/z = 570.98.
r
stituents on 2,20-bipyridine modulates electron density on the
Rh center and not only impacts the rate constant for halide disso-
ciation upon electrochemical reduction (kꢀCl: Ru-Rh(dmeb) > Ru-
Rh(Me2bpy) ꢁ Ru-Rh(bpy)), but also the ability to catalyze H2O
reduction (
(bpy)).
lmol of H2: Ru-Rh(dmeb) > Ru-Rh(Me2bpy) ꢁ Ru-Rh
2.2.3. [(Ph2phen)2Ru(dpp)RhCl2(Me2bpy)](PF6)3 (Ru-Rh(Me2bpy))
The new Ru(II),Rh(III) bimetallic complex was synthesized
using
a similar procedure reported for [(bpy)2Ru(dpp)RhCl2-
Please cite this article in press as: H.J. Sayre et al., Inorg. Chim. Acta (2016), http://dx.doi.org/10.1016/j.ica.2016.06.020

H.J. Sayre et al. / Inorganica Chimica Acta xxx (2016) xxx–xxx
3
(phen)](PF6)3 [29]. [(Ph2phen)2Ru(dpp)](PF6)2 (0.48 g, 0.37 mmol)
and [Rh(Me2bpy)Cl3(DMF)] (0.24 g, 0.51 mmol) were heated at
reflux in 30 mL 2:1 ethanol/water (v/v) for 3 h with rapid stirring.
The dark red solution was cooled to room temperature and precip-
itated in aqueous NH4PF6. The dark red precipitate was collected by
vacuum filtration and rinsed with water and diethyl ether. The
crude product was purified using size exclusion chromatography
with SephadexÒ LH-20 resin as the stationary phase and a 2:1
ethanol/acetonitrile (v/v) solvent mixture as the mobile phase.
Multiple fractions were collected and analyzed by electronic
absorption and emission spectroscopies to ensure the absence of
the emissive [(Ph2phen)Ru(dpp)](PF6)2 monometallic (kem = 680 -
nm) when excited at kabs = 520 nm. Fractions which did not emit
at 680 nm were combined and the solvent removed by rotary
evaporation. The product was dissolved in minimal acetonitrile
and added to stirring ether to induce precipitation. The precipitate
was collected by vacuum filtration and dried in air to produce a
dark red solid (yield = 15%, 0.10 g, 0.074 mmol). ESI-MS: [MꢀPFꢀ6 ]+
m/z = 1647.12.
counter with a 508.6 nm excitation wavelength and monitoring
790 nm emission.
Photocatalytic hydrogen production experiments were per-
formed similar to a previously reported method [30]. The photoly-
sis setup consisted of a locally designed LED array [44] and H2scan
HY-OPTIMA 700 hydrogen sensors (Valencia, CA) to permit real
time monitoring of hydrogen evolution. Empty, air-tight photolysis
reaction cells were connected to hydrogen sensors and deoxy-
genated using argon gas. Stock catalyst solutions prepared in
DMF were combined with acidified deionized water (pH = 2 with
CF3SO3H) in the air-tight photolysis cells and were further deoxy-
genated. The N,N-dimethylaniline (DMA) electron donor was
purged separately with argon gas and injected into the catalyst
solutions immediately prior to photolysis. The total solution vol-
ume was 4.5 mL, consisting of 130 lM catalyst, 1.5 M DMA,
0.62 M H2O and 0.11 mM [DMAH+][CF3SO3ꢀ] with a 15.5 mL head-
space. The solutions were photolyzed with 470 nm light (light
flux = 2.36 0.05 ꢃ 1019 photons/min). The reported value for H2
production is the average of three experiments.
2.2.4. [(Ph2phen)2Ru(dpp)RhCl2(dmeb)](PF6)3 (Ru-Rh(dmeb))
The new Ru(II),Rh(III) bimetallic complex was synthesized as
described above using [(Ph2phen)2Ru(dpp)](PF6)2 (0.48 g,
0.37 mmol) and [Rh(dmeb)Cl3(DMF)] (0.29 g, 0.51 mmol).
Yield = 18%, 0.12 g, 0.083 mmol. ESI-MS: [MꢀPFꢀ6 ]+ m/z = 1735.10.
3. Results and discussion
3.1. Synthesis
The title Ru(II),Rh(III) bimetallic complexes were synthesized
using a building block approach outlined in Fig. 2. The [(Ph2phen)2-
RuCl2] starting material was prepared by reacting two equivalents
of the Ph2phen terminal ligand with RuCl3ꢂxH2O, followed by alu-
mina column chromatography to remove the highly emissive [Ru
(Ph2phen)3]2+ impurity. The [(Ph2phen)2Ru(dpp)]+ light absorber
subunit was prepared by substituting the labile Clꢀ ligands with
the bis-bidentate dpp BL. Separately, the [Rh(R2-bpy)Cl3(DMF)]+
monometallic complexes were synthesized by gently heating a
solution of RhCl3ꢂxH2O and one equivalent of the bidentate TL in
DMF, followed by precipitation with diethyl ether and rinsing with
H2O to remove unreacted RhCl3ꢂxH2O starting material. The Ru(II)
and Rh(III) monometallic precursors were heated at reflux in 2:1
(v/v) EtOH/H2O solution and purified using size-exclusion chro-
matography to produce the new bimetallic complexes Ru-Rh(Me2-
bpy), Ru-Rh(bpy), and Ru-Rh(dmeb). The Ru(II),Rh(III) bimetallic
complexes were characterized using ESI-MS, electrochemical anal-
ysis, and steady-state and time-resolved spectroscopic techniques.
2.2.5. [(Ph2phen)2Ru(dpp)RhCl2(bpy)](PF6)3 (Ru-Rh(bpy))
The new Ru(II),Rh(III) bimetallic complex was synthesized as
described above using [(Ph2phen)2Ru(dpp)](PF6)2 (0.47 g,
0.37 mmol) and [Rh(bpy)Cl3(MeOH)](MeOH) (0.21, 0.50 mmol).
Yield = 35%, 0.23 g, 0.17 mmol. ESI-MS: [MꢀPFꢀ6 ]+ m/z = 1619.10.
2.3. Physical measurements
Cyclic voltammograms (CV) were performed at room tempera-
ture using a BAS Epsilon potentiostat with a glassy carbon disk
working electrode, platinum wire auxiliary electrode and Ag/AgCl
reference electrode (3 M NaCl) in 0.1 M nBu4NPF6/CH3CN elec-
trolyte. Solutions were purged with argon prior to measurement
and blanketed with argon during CV scans. The rate constants for
halide dissociation (kꢀCl) were determined by comparing experi-
mental voltammograms with those of simulated voltammograms
using BASi DigiSimÒ version 3.0 simulation software for cyclic
voltammetry [42,43]. Simulated voltammograms were fit using
the formulae for quasi-reversible couples that possess an electron
transfer step (E) followed by a chemical reaction step (C) as shown
in Eqs. (1) and (2), respectively.
3.2. Electrochemistry
Electrochemistry provides information pertaining to the fron-
tier molecular orbital energies and the subsequent chemical reac-
tivity following heterogeneous electron transfer [45]. Cyclic
voltammograms (CVs) of the title Ru(II),Rh(III) complexes (Fig. 3)
display similar redox-activity and features as those for previously
reported polyazine-bridged Ru(II),Rh(III) bimetallic complexes that
possess a cis-RhIIIX2 center [29–33,46]. Anodic scans display a
A þ e ! B
ð1Þ
B ! C
ð2Þ
Other pertinent parameters included
m
= 100 and 1000 mV/s,
resistance = 100–300 ohms, T = 298 K, E0 = ꢀ0.38 V for Ru-Rh
(dmeb) and E0 = ꢀ0.41 for Ru-Rh(Me2bpy) and Ru-Rh(bpy),
a/
reversible, one-electron RuII/III oxidation at E = +1.60 V vs. Ag/AgCl
½
k = 0.5 eV and Keq = 1 ꢃ 1010
.
for all three bimetallic complexes, indicative of minimal-to-negli-
gible electronic communication between the Ru(II) metal center
and the terminal ligand coordinated to Rh(III). Similarly, RuII/III oxi-
dation of the previously reported [(Ph2phen)2Ru(dpp)RhX2(Ph2-
A Hewlett-Packard 8452A diode array spectrophotometer was
employed for electronic absorption spectroscopy and spectra were
collected in spectrophotometric grade CH3CN using a 1 cm quartz
cuvette (Starna Cells; Atascadero, CA). Extinction coefficient values
were measured in triplicate. Steady-state luminescence spec-
troscopy was performed with a water-cooled 150 W xenon arc
lamp with a 540 nm excitation using a Quanta Master Model QA-
200-45E fluorimeter from Photon Technologies International and
a thermo-electric cooled Hamamatsu R2658 photomultiplier tube.
Time-resolved luminescence spectroscopy was carried out with an
Edinburgh Instruments FLS920 time-correlated single photon
phen)]3+ (X = Clꢀ or Brꢀ) bimetallic complexes occurs at E
=
½
+1.59 V vs. Ag/AgCl, providing further evidence of electronic isola-
tion between Ru(II) and Rh-TL [30].
Cathodic scans present somewhat complicated electrochem-
istry given the nearly isoenergetic nature of the dpp(p⁄) and Rh
(dr⁄) unoccupied MOs and the competitive electrochemical mech-
anisms upon reduction [32]. The bimetallic complexes display a
reversible reduction at ꢀ0.41 V vs. Ag/AgCl for Ru-Rh(Me2bpy)
Please cite this article in press as: H.J. Sayre et al., Inorg. Chim. Acta (2016), http://dx.doi.org/10.1016/j.ica.2016.06.020

4
H.J. Sayre et al. / Inorganica Chimica Acta xxx (2016) xxx–xxx
1. LiCl, DMF a
Δ, 6 h
N
N
Ru Cl
Cl
N
RuCl3·xH2O
2
+
N
N
N
2. acetone
Ph2phen
[(Ph2phen)2RuCl2]
2+
1. EtOH/H2O b
Δ, 3 h
N
N
N
N
Ru Cl
Cl
N
N
N
N
N
N
N
N
Ru
N
N
+
+
+
N
2. NH4PF6(aq)
N
dpp
[(Ph2phen)2RuCl2]
[(Ph2phen)2Ru(dpp)](PF6)2
Cl
Cl
1. DMF
N
R
O
Rh
N
R
60 ºC, 2 h
N
N
RhCl3·xH2O
Cl
N
2. Et2O
R
R
R2-bpy
[(R2-bpy)Rh(DMF)Cl3]
3+
2+
1. EtOH/H2O
Δ, 2 h
Cl
Cl
N
N
N
N
N
Cl
Cl
N
N
N
N
N
O
Rh
N
R
N
N
Ru
N
N
Rh
N
R
N
Ru
N
N
Cl
2. NH4PF6 (aq)
N
N
R
R
[(R2-bpy)Rh(DMF)Cl3]
[(Ph2phen)2Ru(dpp)RhCl2(R2-bpy)](PF6)3
phen) Ru(dpp)](PF )
[(Ph2
2
6 2
Fig. 2. General building block synthetic approach for the preparation of Ru-Rh(Me2bpy), Ru-Rh(bpy), Ru-Rh(dmeb) bimetallic complexes (see Section 2 for details).
aReference [39]. bReference [40].
mated k
value of 0.2 sꢀ1 for Ru-Rh(Me2bpy) and Ru-Rh(bpy)
ꢀCl
and 0.7 sꢀ1 for Ru-Rh(dmeb). The second cathodic couple is
labeled an EECC process whereby a portion of the one-electron
reduced species, RhIICl2, does not dissociate chloride rapidly and
undergoes further reduction prior to dissociation of both chloride
ligands [32]. The second cathodic couple shifts from Ecp = ꢀ0.82 V
vs. Ag/AgCl for Ru-Rh(Me2bpy) and Ecp = ꢀ0.80 V vs. Ag/AgCl for
Ru-Rh(bpy) to Ecp = ꢀ0.68 V vs. Ag/AgCl for Ru-Rh(dmeb) which
supports more rapid chloride dissociation with the methyl ester-
substituted TL. The faster rate of chloride dissociation following
reduction for Ru-Rh(dmeb) suggests increased formation of a
coordinatively-unsaturated Rh metal center for potential catalysis,
an important factor in the generation of a highly active catalyst.
Importantly, the frontier molecular orbitals within this Ru(II),Rh
(III) architecture are described as primarily Ru(II) HOMO and Rh
(III) LUMO in character.
50 μA
a
b
c
1.8
1.4
1.0
0.6
0.2
-0.2
-0.6
-1.0
-1.4
(Potential (V vs. Ag/AgCl)
Fig. 3. Cyclic voltammograms of Ru-Rh(Me2bpy) (a), Ru-Rh(bpy) (b) and Ru-Rh
(dmeb) (c) measured at RT under Ar atm in 0.1 M nBu4NPF6 supporting electrolyte
in CH3CN solvent at a scan rate of 100 mV/s.
3.3. Electronic absorption spectroscopy
The light absorbing ability of the Ru(II),Rh(III) bimetallic com-
plexes was measured using electronic absorption spectroscopy
and are tabulated in Table 1. As observed with similar complexes
[29–31,33,46], Ru-Rh(Me2bpy), Ru-Rh(bpy) and Ru-Rh(dmeb)
are strong light absorbers throughout the UV and visible with poly-
and Ru-Rh(bpy) and a quasi-reversible reduction at ꢀ0.38 V vs.
Ag/AgCl for Ru-Rh(dmeb) (Fig. 3). This first cathodic couple is
labeled as an ECEC process consisting of a RhIII/IICl2 reduction fol-
lowed by chloride dissociation and subsequent RhII/ICl reduction
prior to cleavage of the second Rh–Cl bond [32]. Varied reversibil-
ity of the first cathodic couple is attributed to differing rates of
chloride dissociation following the first reduction. The rate con-
stant for chloride dissociation (kꢀCl) was further investigated using
DigiSimÒ to model the first reductive couple with an EC mecha-
nism from Eqs. (1) and (2) (Fig. 4). Simulations provide an esti-
pyridyl
(d
) ? ligand(p⁄) metal-to-ligand charge transfer (1MLCT) transi-
tions in the visible (Fig. 5). For all three bimetallic complexes, the
p ?
p⁄ intraligand (1IL) transitions in the UV and Ru
p
lowest energy transition is ascribed to the Ru(dp) ? l )
-dpp(p⁄
1MLCT transition at kmax = 512 nm for Ru-Rh(Me2bpy) and Ru-
Rh(bpy) and at kmax = 520 nm for Ru-Rh(dmeb). The small 1MLCT
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H.J. Sayre et al. / Inorganica Chimica Acta xxx (2016) xxx–xxx
5
a
b
c
20 μA
20 μA
20 μA
0.0
-0.3
Potential (V)
-0.6
0.0
-0.3
Potential (V)
-0.6
0.0
-0.3
Potential (V)
-0.6
Fig. 4. Experimental CVs of the first cathodic couple (solid line) and simulated CVs using DigiSimÒ (dashed line) of Ru-Rh(Me2bpy) (a), Ru-Rh(bpy) (b) and Ru-Rh(dmeb) (c)
with a scan rate of 100 mV/s in 0.1 M nBu4NPF6/CH3CN.
Table 1
3.4. Emission spectroscopy
Light absorbing properties of Ru(II),Rh(III) bimetallic complexes.
Complexa
kabs
(nm)
e
ꢃ 10ꢀ4
Assignment
Steady-state and time-resolved emission spectroscopies pro-
vide insight into the excited state dynamics of the title Ru(II),Rh
(III) bimetallic complexes and these properties are summarized
in Table 2. A simplified Jablonski diagram is shown in Fig. 6 to pro-
vide an overview of possible excited state deactivation processes.
(Mꢀ1 cmꢀ1
)
Ru-Rh(Me2bpy)
278
426
512
10.2
1.9
Ph2phen p ?
p⁄
Ru(d
Ru(d
p
p
) ? Ph2phen(p⁄
)
1CT
2.0
) ?
l
-dpp(p⁄ 1CT
)
Ru-Rh(bpy)
278
426
512
10.2
1.9
Ph2phen
p
?
p⁄
Upon visible light photoexcitation, population of the Ru(d
dpp(p⁄ 1MLCT excited state (ES) is followed by intersystem cross-
ing to populate the weakly emissive Ru(d ) ?
-dpp(p⁄ 3MLCT
ES. The 3MLCT emission energy (kem), emission quantum yield
em), and ES lifetime ( em) are similar for all three Ru(II),Rh(III)
p) ? l-
Ru(d
Ru(d
p
p
) ? Ph2phen(p⁄
)
1CT
)
1.9
) ?
l
-dpp(p⁄ 1CT
)
p
l
)
Ru-Rh(dmeb)
Ru-Rh(Ph2phen)
278
426
520
10.2
1.9
Ph2phen p ?
p⁄
Ru(d
Ru(d
p
p
) ? Ph2phen(p⁄
)
1CT
(U
s
1.8
) ?
l
-dpp(p⁄ 1CT
)
bimetallic complexes, indicating that minimal electronic perturba-
tions are imparted on the ES dynamics upon Rh-TL variation. Weak
emission from the 3MLCT ES is due to low-lying, unoccupied Rh
(dr⁄) MOs which permit facile intramolecular electron transfer
b
278
414
514
10.7
1.7
Ph2phen p ?
p⁄
Ru(d
Ru(d
p
p
) ? Ph2phen(p⁄
)
1CT
1.6
) ?
l
-dpp(p⁄ 1CT
)
a
Measured at RT in spectrophotometric grade CH3CN using
cuvette.
a 1 cm quartz
to populate a non-emissive Ru(dp )
) ? Rh(dr⁄ 3MMCT (metal-to-
metal charge transfer) ES. The rate constant for intramolecular
electron transfer (ket) was calculated (Eqs. (3)–(5)) using the homo-
bimetallic model complex, [{(Ph2phen)2Ru}2(dpp)]4+ (labeled (Ph2-
b
Values reported from reference [30]; Ru-Rh(Ph2phen) corresponds to [(Ph2-
phen)2Ru(dpp)RhCl2(Ph2phen)]3+
.
phen)2Ru-Ru), which also emits from
a Ru(dp) ? l )
-dpp(p⁄
3MLCT but does not possess a 3MMCT ES. The rate constants for
3MLCT ES radiative (kr) and non-radiative (knr) decay of the (Ph2-
phen)2Ru-Ru model complex are used as an estimate for the Ru
(II),Rh(III) bimetallic complexes given the similar ES manifold [47].
1
sRu;Rh
¼
¼
ð3Þ
ð4Þ
ð5Þ
kr þ knr þ ket
1
sRu;Ru
kr þ knr
1
1
ket
¼
ꢀ
sRu;Rh sRu;Ru
As shown in Table 2, the ket values for the title Ru(II),Rh(III)
bimetallic complexes are similar and further imply that efficient
population of the 3MMCT ES occurs regardless of Rh-TL identity.
Equally important is the large order of magnitude (107 sꢀ1) calcu-
lated for intramolecular electron transfer that produces a formally
oxidized Ru(III) and formally reduced Rh(II) species in the absence
of a sacrificial quencher species such as N,N-dimethylaniline
(DMA). Rapid, directional charge separation is vital to the forma-
tion of photochemically-active excited states towards the photo-
catalytic reduction of H2O to H2.
Fig. 5. Electronic absorption of Ru-Rh(Me2bpy) (blue), Ru-Rh(bpy) (black) and Ru-
Rh(dmeb) (red) measured in CH3CN at RT. (For interpretation of the references to
colour in this figure legend, the reader is referred to the web version of this article.)
energy difference throughout the series of [(Ph2phen)2Ru(dpp)
RhCl2(TL)]3+ complexes (kmax = 514 nm for TL = Ph2phen [30]) is
not surprising given the same (Ph2phen)2Ru(dpp) LA subunit dom-
inates the UV–vis spectrum, as well as possessing a RhCl2(TL) RM
subunit. Modification of the Rh-TL does not strongly alter the
[(Ph2phen)2Ru(dpp)RhCl2(TL)]3+ complexes’ light absorbing ability
3.5. Photocatalysis studies
Photocatalytic H2O reduction experiments were performed in
an effort to elucidate the impact of Rh-TL identity within the Ru
(II),Rh(III) bimetallic architecture. The new bimetallic complexes
Ru-Rh(Me2bpy), Ru-Rh(bpy), and Ru-Rh(dmeb) are active photo-
and maintains the Ru(dp) ? l
-dpp(p⁄) 1MLCT as the lowest energy
transition to direct electron density towards the RhCl2(TL) RM sub-
unit upon photoexcitation.
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6
H.J. Sayre et al. / Inorganica Chimica Acta xxx (2016) xxx–xxx
Table 2
Excited state properties of Ru(II),Rh(III) bimetallic complexes.
Complexa
kem
(nm)
Uem
sem
(ns)
kr
knr
ket
(10ꢀ4
)
(104 sꢀ1
)
(106 sꢀ1
)
(107 sꢀ1
)
Ru-Rh
808
5.2
20
1.0
5.9
4.4
(Me2bpy)
Ru-Rh(bpy)
Ru-Rh(dmeb)
808
787
4.7
6.4
17.3
19
19
170
1.0
1.0
1.0
5.9
5.9
5.9
4.7
4.8
–
(Ph2phen)2Ru- 754
Rub
a
Measured at RT in Ar-deoxygenated, spectrophotometric grade CH3CN using a
1 cm quartz cuvette.
b
Values reported from reference [48]; (Ph2phen)2Ru-Ru corresponds to
[{(Ph2phen)2Ru}2(dpp)]4+
.
Fig. 7. Photocatalytic H2 production and catalyst turnover numbers (TON) for Ru-
Rh(Me2bpy) (triangles), Ru-Rh(bpy) (circles), and Ru-Rh(dmeb) (squares). Condi-
catalysts for H2 production when excited with 470 nm light in DMF
solvent in the presence of H2O substrate and DMA electron donor,
Fig. 7. Although the light absorbing and excited state properties of
the modified Rh-TL complexes are similar, the photocatalytic activ-
ity displays a large disparity between the Ru-Rh(dmeb) complex
tions:
k lM, [H2O] = 0.62 M,
exc = 470 nm, solvent = DMF, [photocatalyst] = 130
[DMA] = 1.5 M, [DMAH+][CF3SO3ꢀ] = 110
lM.
Table 3
(37
and Ru-Rh(bpy) complexes (18
21
quantum yields for H2 evolution after 4 h photolysis (Max.
4 lmol H2, 63 7 TON after 20 h) and the Ru-Rh(Me2bpy)
Hydrogen evolution turnover numbers (TON) and maximum quantum yields for Ru-
Rh(Me2bpy), Ru-Rh(bpy) and Ru-Rh(dmeb).
2
lmol H2, 30
4
TON;
2 lmol H2, 35 3 TON, respectively) (Table 3). Maximum
Complex
k
ꢀCl
(sꢀ1
)
TONa
Max. U
U
) with
Ru-Rh(Me2bpy)
Ru-Rh(bpy)
Ru-Rh(dmeb)
0.2
0.2
0.7
30
35
63
4
3
7
0.002
0.003
0.004
Ru-Rh(Me2bpy), Ru-Rh(bpy) and Ru-Rh(dmeb) are 0.002, 0.003
and 0.004, respectively. Recent reports by our group [30] and Rau
and co-workers [18] have demonstrated that the halide ligand
coordinated to the reactive metal center, RhCl2 vs. RhBr2 in the for-
mer and PtCl2 vs. PtI2 in the latter, dictates photocatalytic activity
towards H2 production regardless of the similar photophysical
a
Mole H2 per mole catalyst after 20 h photolysis.
The rate constants for excited state intramolecular electron
transfer (107 sꢀ1 and deactivation (103–106 sꢀ1), as well as
properties within each series. The weaker
r
-donating Brꢀ vs. Clꢀ
)
ligand in [(Ph2phen)2Ru(dpp)RhX2(Ph2phen)]3+ is proposed to
more rapidly form the active Ru(II),Rh(I) species, as well as mini-
mize the degree of ion pairing that impedes catalyst activity
throughout the photocatalytic cycle. With regards to the weaker
bimolecular quenching by a sacrificial electron donor (4.9 ꢃ 109
Mꢀ1 sꢀ1 for [(bpy)2Ru(dpp)RhCl2(bpy)]3+ and DMA) [46], are
several orders of magnitude greater than the electrochemically
determined rate constants for chloride loss (10ꢀ1 sꢀ1). Intramolec-
ular electron transfer to populate the 3MMCT excited state and
bimolecular quenching from DMA occur rapidly to create a state
similar to the electrochemically reduced product, [(Ph2phen)2RuII
(dpp)RhIICl2(R2-bpy)]2+. In this scenario, chloride dissociation rep-
resents a potential rate limiting step in water reduction catalysis
r
-donating dmeb ligand in Ru-Rh(dmeb), more rapid Clꢀ dissoci-
ation upon photoexcitation generates the proposed [(Ph2phen)2Ru
(dpp)RhI(dmeb)]3+ more efficiently to interact with H2O and evolve
H2. The photocatalytic robustness of Ru-Rh(dmeb) is enhanced
when compared with Ru-Rh(Me2bpy) and Ru-Rh(bpy), which sug-
gests the methyl-ester substituents inhibit catalyst deactivation
until later times.
and the weaker
r-donating methyl ester substituents on bpy
facilitate more rapid chloride dissociation.
Fig. 6. Simplified Jablonski diagram depicting potential excited states and deactivation processes within the Ru(II),Rh(III) bimetallic architecture. GS = ground state,
MLCT = metal-to-ligand charge transfer, MMCT = metal-to-metal charge transfer, kisc = rate constant for intersystem crossing, kv = rate constant for vibrational relaxation,
kr = rate constant for radiative decay, knr = rate constant for non-radiative decay, ket = rate constant for intramolecular electron transfer, kꢀCl = rate constant for Clꢀ
dissociation.
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H.J. Sayre et al. / Inorganica Chimica Acta xxx (2016) xxx–xxx
7
[10] S. Rau, B. Schäfer, D. Gleich, E. Anders, M. Rudolph, M. Friedrich, H. Görls, W.
Henry, J.G. Vos, supramolecular photocatalyst for the production of
4. Conclusions
A
hydrogen and the selective hydrogenation of tolane, Angew. Chem. Int. Ed.
45 (2006) 6215–6218.
[11] H. Ozawa, M.-A. Haga, K. Sakai, A photo-hydrogen-evolving molecular device
driving visible-light-induced EDTA-reduction of water into molecular
hydrogen, J. Am. Chem. Soc. 128 (2006) 4926–4927.
The new Ru(II),Rh(III) bimetallic complexes Ru-Rh(Me2bpy),
Ru-Rh(bpy), and Ru-Rh(dmeb) were synthesized and character-
ized to investigate the influence that Rh-TL variation has on the
electrochemical, photophysical, and photocatalytic properties.
Electrochemical data indicate a Ru-based HOMO and Rh-based
LUMO for all three bimetallics. The bimetallic complexes are effi-
cient light absorbers throughout the UV and visible with the lowest
[12] S.M. Molnar, G. Nallas, J.S. Bridgewater, K.J. Brewer, Photoinitiated electron
collection in a mixed-metal trimetallic complex of the form ((Bpy)(2)Ru(dpb)
(2)IrCl2)(PF6)(5)
(bpy = 2,2’-Bipyridine
and
dpb = 2,3-bis(2-pyridyl)
benzoquinoxaline), J. Am. Chem. Soc. 116 (1994) 5206–5210.
[13] M.J. Kim, R. Konduri, H.W. Ye, F.M. MacDonnell, F. Puntoriero, S. Serroni, S.
Campagna, T. Holder, G. Kinsel, K. Rajeshwar, Dinuclear ruthenium(II)
polypyridyl complexes containing large, redox-active, aromatic bridging
ligands: synthesis, characterization, and intramolecular quenching of MLCT
excited states, Inorg. Chem. 41 (2002) 2471–2476.
[14] R. Konduri, H.W. Ye, F.M. MacDonnell, S. Serroni, S. Campagna, K. Rajeshwar,
Ruthenium photocatalysts capable of reversibly storing up to four electrons in
a single acceptor ligand: a step closer to artificial photosynthesis, Angew.
Chem. Int. Ed. 41 (2002) 3185.
energy transition displaying Ru(d
Photoexcitation results in population of a weakly-emissive, short-
lived Ru(d ) ?
-dpp(p⁄ 3MLCT excited state that undergoes
intramolecular electron transfer to populate a non-emissive, pho-
tochemically-active Ru(d
) ? Rh(dr⁄ 3MMCT excited state. The
p) ? l
-dpp(p⁄) 1MLCT character.
p
l
)
p
)
above properties are important requirements for the design of pho-
tocatalysts for H2O reduction via photoinitiated electron collection
at a Rh metal center. The new bimetallic complexes are able to cat-
alyze the reduction of H2O to H2 in the presence of light and a sac-
rificial electron donor and the Ru-Rh(dmeb) complex displayed
enhanced activity compared to the Ru-Rh(Me2bpy) and Ru-Rh
(bpy) complexes. Although the photophysical properties of the
three complexes did not vary greatly, the rate constant for chloride
[15] M. Elvington, K.J. Brewer, Photoinitiated electron collection at a metal in a
rhodium-centered mixed-metal supramolecular complex, Inorg. Chem. 45
(2006) 5242–5244.
[16] M. Elvington, J. Brown, S.M. Arachchige, K.J. Brewer, Photocatalytic hydrogen
production from water employing
a Ru, Rh, Ru molecular device for
photoinitiated electron collection, J. Am. Chem. Soc. 129 (2007) 10644–10645.
[17] G.F. Manbeck, K.J. Brewer, Photoinitiated electron collection in polyazine
chromophores coupled to water reduction catalysts for solar H-2 production,
Coord. Chem. Rev. 257 (2013) 1660–1675.
[18] M.G. Pfeffer, T. Kowacs, M. Waechtler, J. Guthmuller, B. Dietzek, J.G. Vos, S. Rau,
Optimization of hydrogen-evolving photochemical molecular devices, Angew.
Chem. Int. Ed. 54 (2015) 6627–6631.
[19] T. Stoll, C.E. Castillo, M. Kayanuma, M. Sandroni, C. Daniel, F. Odobel, J. Fortage,
M.-N. Collomb, Photo-induced redox catalysis for proton reduction to
hydrogen with homogeneous molecular systems using rhodium-based
catalysts, Coord. Chem. Rev. 304 (2015) 20–37.
[20] J.K. White, K.J. Brewer, A new Ru, Ru, Pt supramolecular architecture for
photocatalytic H-2 production, Chem. Commun. 51 (2015) 16123–16126.
[21] Y. Halpin, M.T. Pryce, S. Rau, D. Dini, J.G. Vos, Recent progress in the
development of bimetallic photocatalysts for hydrogen generation, Dalton
Trans. 42 (2013) 16243–16254.
[22] M. Schulz, M. Karnahl, M. Schwalbe, J.G. Vos, The role of the bridging ligand in
photocatalytic supramolecular assemblies for the reduction of protons and
carbon dioxide, Coord. Chem. Rev. 256 (2012) 1682–1705.
[23] H. Ozawa, K. Sakai, Photo-hydrogen-evolving molecular devices driving
visible-light-induced water reduction into molecular hydrogen: structure-
activity relationship and reaction mechanism, Chem. Commun. 47 (2011)
2227–2242.
[24] S.A. Arachchige, J. Brown, K.J. Brewer, Photochemical hydrogen production
from water using the new photocatalyst {(bpy)(2)Ru(dpp)}(2)RhBr 2 (PF6)(5),
J. Photochem. Photobiol. A 197 (2008) 13–17.
[25] S.M. Arachchige, J.R. Brown, E. Chang, A. Jain, D.F. Zigler, K. Rangan, K.J. Brewer,
Design considerations for a system for photocatalytic hydrogen production
from water employing mixed-metal photochemical molecular devices for
photoinitiated electron collection, Inorg. Chem. 48 (2009) 1989–2000.
[26] H.M. Rogers, S.M. Arachchige, K.J. Brewer, Enhancement of solar fuel
dissociation (kꢀCl) was impacted and the weaker
r
-donating –
value
COOCH3 substituted bipyridine TL possessed a larger k
ꢀCl
(0.7 sꢀ1) compared to the stronger
tuted bipyridine TL (kꢀCl = 0.2 sꢀ1). Substitution with ester groups
r-donating –CH3 and –H substi-
decreases the
r-donating ability of the bipyridine ligand, thereby
stabilizing the Rh(dr⁄) orbitals and expediting cleavage of the
Rh–Cl bond to form the active catalytic species.
Acknowledgements
Acknowledgement is made to the Chemical Sciences, Geo-
sciences and Biosciences Division, Office of Basic Energy Sciences,
Office of Sciences, U.S. Department of Energy (DE-FG02-
05ER15751) for funding of this research. The authors would also
like to thank Mr. William Bebout (Analytical Services, Department
of Chemistry, Virginia Tech) and Mr. TJ Rohrabaugh for assistance
with ESI mass spectrometry and Prof. James M. Tanko (Department
of Chemistry, Virginia Tech) for valuable discussions and helpful
electrochemical insight. We thank Prof. Claudia Turro (Department
of Chemistry and Biochemistry, The Ohio State University) for use
of the time correlated single photon counter and insightful
discussions.
production schemes by using
a Ru, Rh, Ru supramolecular photocatalyst
containing hydroxide labile ligands, Chem. Eur. J. 21 (2015) 16948–16954.
[27] T.A. White, S.L.H. Higgins, S.M. Arachchige, K.J. Brewer, Efficient photocatalytic
hydrogen production in
supramolecules containing 4,7-diphenyl-1,10-phenanthroline, Angew. Chem.
Int. Ed. 50 (2011) 12209–12213.
a single-component system using Ru, Rh, Ru
References
[28] T.A. White, K. Rangan, K.J. Brewer, Synthesis, characterization, and study of the
photophysics and photocatalytic properties of the photoinitiated electron
collector [{(phen)2Ru(dpp)}2RhBr 2](PF6)5, J. Photochem. Photobiol. A 209
(2010) 203–209.
[29] D.F. Zigler, J. Wang, K.J. Brewer, Ruthenium(II)-polyazine light absorbers
bridged to reactive cis-dichlororhodium(III) centers in a bimetallic molecular
architecture, Inorg. Chem. 47 (2008) 11342–11350.
[30] H.M. Rogers, T.A. White, B.N. Stone, S.M. Arachchige, K.J. Brewer,
Nonchromophoric halide ligand variation in polyazine-bridged Ru(II), Rh(III)
bimetallic supramolecules offering new insight into photocatalytic hydrogen
production from water, Inorg. Chem. 54 (2015) 3545–3551.
[31] J. Wang, T.A. White, S.M. Arachchige, K.J. Brewer, A new structural motif for
photoinitiated electron collection: Ru, Rh bimetallics providing insight into H2
production via photocatalysis of water reduction by Ru, Rh, Ru
supramolecules, Chem. Commun. 47 (2011) 4451–4453.
[32] T.A. White, H.E. Mallalieu, J. Wang, K.J. Brewer, Mechanistic insight into the
electronic influences imposed by substituent variation in polyazine-bridged
ruthenium(II)/rhodium(III) supramolecules, Chem. Eur. J. 20 (2014) 8265–
8268.
[33] T.A. White, B.N. Whitaker, K.J. Brewer, Discovering the balance of steric and
electronic factors needed to provide a new structural motif for photocatalytic
hydrogen production from water, J. Am. Chem. Soc. 133 (2011) 15332–15334.
[1] A.J. Bard, M.A. Fox, Artificial photosynthesis – solar splitting of water to
hydrogen and oxygen, Acc. Chem. Res. 28 (1995) 141–145.
[2] N.S. Lewis, D.G. Nocera, Powering the planet: chemical challenges in solar
energy utilization, Proc. Natl. Acad. Sci. U.S.A. 103 (2006) 15729–15735.
[3] D.G. Nocera, The artificial leaf, Acc. Chem. Res. 45 (2012) 767–776.
[4] M.G. Walter, E.L. Warren, J.R. McKone, S.W. Boettcher, Q. Mi, E.A. Santori, N.S.
Lewis, Solar water splitting cells, Chem. Rev. 110 (2010) 6446–6473.
[5] W.T. Eckenhoff, R. Eisenberg, Molecular systems for light driven hydrogen
production, Dalton Trans. 41 (2012) 13004–13021.
[6] S. Berardi, S. Drouet, L. Francas, C. Gimbert-Surinach, M. Guttentag, C.
Richmond, T. Stoll, A. Llobet, Molecular artificial photosynthesis, Chem. Soc.
Rev. 43 (2014) 7501–7519.
[7] M. Kirch, J.M. Lehn, J.P. Sauvage, Hydrogen Generation by visible-light
irradiation of aqueous-solutions of metal-complexes
– approach to the
photo-chemical conversion and storage of solar-energy, Helv. Chim. Acta 62
(1979) 1345–1384.
[8] J.M. Lehn, J.P. Sauvage, Chemical storage of light energy – catalytic generation
of hydrogen by visible-light or sunlight – irradiation of neutral aqueous-
solutions, Nouv. J. Chim. 1 (1977) 449–451.
[9] V. Balzani, L. Moggi, F. Scandola, Towards a supramolecular photochemistry:
assembly of molecular components to obtain photochemical molecular
devices, in: V. Balzani (Ed.), Supramolecular Photochemistry, Springer,
Netherlands, 1987, pp. 1–28.
Please cite this article in press as: H.J. Sayre et al., Inorg. Chim. Acta (2016), http://dx.doi.org/10.1016/j.ica.2016.06.020

8
H.J. Sayre et al. / Inorganica Chimica Acta xxx (2016) xxx–xxx
[34] R. Zhou, G.F. Manbeck, D.G. Wimer, K.J. Brewer,
A
new (RuRhIII)-Rh-II
[41] R. Bieda, I. Ott, M. Dobroschke, A. Prokop, R. Gust, W.S. Sheldrick, Structure–
activity relationships and DNA binding properties of apoptosis inducing
cytotoxic rhodium(III) polypyridyl complexes containing the cyclic thioether
[9]aneS3, J. Inorg. Biochem. 103 (2009) 698–708.
bimetallic with a single Rh-Cl bond as a supramolecular photocatalyst for
proton reduction, Chem. Commun. 51 (2015) 12966–12969.
[35] C.M. Bolinger, N. Story, B.P. Sullivan, T.J. Meyer, Electrocatalytic reduction of
carbon-dioxide by 2,20-bipyridine complexes of rhodium and iridium, Inorg.
Chem. 27 (1988) 4582–4587.
[42] M. Rudolph, D.P. Reddy, S.W. Feldberg, A simulator for cyclic voltammetric
responses, Anal. Chem. 66 (1994) 589A–600A.
[36] S.C. Rasmussen, M.M. Richter, E. Yi, H. Place, K.J. Brewer, Synthesis and
[43] BASi DigiSimÒ Simulation Software for Cyclic Voltammetry.
[44] J.R. Brown, M. Elvington, M.T. Mongelli, D.F. Zigler, K.J. Brewer, Analytical
methods development for supramolecular design in solar hydrogen
production, SPIE Optics + Photonics 6340 (2006) 634017.
characterization of
a series of novel rhodium and iridium complexes
containing polypyridyl bridging ligands – potential uses in the development
of multimetal catalysts for carbon-dioxide reduction, Inorg. Chem. 29 (1990)
3926–3932.
[45] A.J. Bard, L.R. Faulkner, Electrochemical Methods: Fundamentals and
Applications, 2nd ed., John Wiley & Sons Inc., Hoboken, NJ, 2001.
[46] A.T. Wagner, R. Zhou, K.S. Quinn, T.A. White, J. Wang, K.J. Brewer, Tuning the
photophysical properties of Ru(II) monometallic and Ru(II), Rh(III) bimetallic
supramolecular complexes by selective ligand deuteration, J. Phys. Chem. A
119 (2015) 6781–6790.
[47] M. Elvington, K.J. Brewer, Photoinitiated electron collection at a metal in a
rhodium-centered mixed-metal supramolecular complex, Inorg. Chem. 45
(2006) 5242–5244.
[37] D.D. Perrin, Dissociation Constants of Organic Bases in Aqueous Solution,
Butterworths Scientific Publications, London, 1965.
[38] A.R. Oki, R.J. Morgan, An efficient preparation of 4, 40-dicarboxy-2, 20-
bipyridine, Synth. Commun. 25 (1995) 4093–4097.
[39] L.A. Basile, J.K. Barton, Design of a double-stranded DNA cleaving agent with
two polyamine metal-binding arms: Ru(DIP)2Macron+, J. Am. Chem. Soc. 109
(1987) 7548–7550.
[40] M.T. Mongelli, K.J. Brewer, Synthesis and study of the light absorbing, redox
and photophysical properties of Ru(II) and Os(II) complexes of 4,7-diphenyl-
1,10-phenanthroline containing the polyazine bridging ligand 2,3-bis(2-
pyridyl)pyrazine, Inorg. Chem. Commun. 9 (2006) 877–881.
[48] T.A. White, S.M. Arachchige, B. Sedai, K.J. Brewer, Emission spectroscopy as a
probe into photoinduced intramolecular electron transfer in polyazine bridged
Ru(II), Rh(III) supramolecular complexes, Materials 3 (2010) 4328–4354.
Please cite this article in press as: H.J. Sayre et al., Inorg. Chim. Acta (2016), http://dx.doi.org/10.1016/j.ica.2016.06.020