A new RuIIRhIII bimetallic with a single Rh – Cl bond as a supramolecular photocatalyst for proton reduction †

Solar water splitting to generate H2 has gained considerable interest as a method to produce an alternative fuel to meet future energy demands. Robust systems that absorb visible light, facilitate electron transfer and catalyze H2 formation are required for achieving solar energy conversion. To this end, supramolecular complexes incorporating separate units with individual properties offering unique functions to the entire molecule have been designed. In contrast to the bimolecular electron transfer (ET) reactions in the multi-component photocatalytic water reduction systems developed in the 1970s, the supramolecular approach exploits intramolecular ET. Supramolecular complexes coupling metal-based chromophores to a catalytic center photocatalytically reduce water to H2 under various conditions. Impediments to engineering supramolecular complexes for solar H2 production include the small number of molecular systems capable of photochemically collecting reducing equivalents and the lack of fundamental understanding of multielectron photochemistry. In seeking an efficient and robust supramolecular photocatalyst, a series of RuRh-containing supramolecular complexes coupling polypyridyl Ru chromophores to a Rh catalytic center via a bridging ligand dpp (2,3-bis(2-pyridyl)pyrazine) were explored. The first photocatalyst of this type, [{(bpy)2Ru(dpp)}2RhCl2](PF6)5, inspired the development of Ru RhRu trimetallics with the architecture of [{(TL)2Ru(dpp)}2RhX2](PF6)5 (TL = bpy, 1,10-phenanthroline (phen), or 4,7-diphenyl-1,10-phenanthroline (Ph2phen); X = Cl or Br) to explore factors that control photoinitiated electron collection (PEC) and catalytic activity. The presence of a Rh(ds*)-based LUMO (lowest unoccupied molecular orbital) in the RuRhRu complexes is a key energetic requirement for PEC at the Rh catalytic center. PEC on the Rh center forms the proposed active species, RuRhRu, upon sequential reductive quenching of the MLCT (metal-toligand charge transfer) excited state by a sacrificial electron donor. Recent studies also show that two Ru chromophores are not required for photocatalysis. Active RuRh bimetallics require a Rh-based LUMO and steric protection around the photogenerated Rh center to prevent dimerization that leads to catalytic deactivation. These RuRhRu and RuRh motifs represent homogenous single-component photocatalysts for H2 generation. To the best of our knowledge, all reported dpp-bridged RuRh and RuRhRu photocatalysts have two labile halide ligands (Cl or Br) on the Rh center. Here we propose the replacement of one halide ligand with an N donor of a polypyridyl ligand as a means to (1) modulate the electrochemical properties of the catalytic Rh center, (2) test the hypothesis that only one labile halide ligand is needed in the dpp-bridged RuRh precatalyst for photocatalytic water reduction, (3) expand the scope of structural designs for competent Rh-containing supramolecular photocatalysts, and (4) provide a new mechanism of steric protection of the Rh center. The ligand tpy (2,20:60,200-terpyridine) has been widely used in coordination chemistry with meridional tridentate (Z) chelation being the most common coordination mode. Taking advantage of the tridentate binding mode, the new RuRh complex, [(bpy)2Ru(dpp)RhCl(tpy)](PF6)4 (Ru RhCl(tpy)), has been prepared to test the hypotheses described above. For comparative purposes, we have studied the cis-RhCl2 analogue [(bpy)2Ru(dpp)RhCl2(bpy)](PF6)3 (Ru RhCl2(bpy)). Herein we report the electrochemical, photochemical, and catalytic properties Department of Chemistry, Virginia Tech, Blacksburg, VA, 24061-0212, USA. E-mail: rowezhou@vt.edu † Electronic supplementary information (ESI) available: Detailed synthesis; coupled CVs before and after control potential electrolysis; ESI mass spectra of products after control potential electrolysis; and the profile of the photolysis of hydrogen production. See DOI: 10.1039/c5cc04123f ‡ Current address: Chemistry Department, Brookhaven National Laboratory, Upton, NY 11973-5000, USA. § Karen J. Brewer deceased October 24, 2014. Received 19th May 2015, Accepted 1st July 2015

Solar water splitting to generate H 2 has gained considerable interest as a method to produce an alternative fuel to meet future energy demands. 1 Robust systems that absorb visible light, facilitate electron transfer and catalyze H 2 formation are required for achieving solar energy conversion.To this end, supramolecular complexes incorporating separate units with individual properties offering unique functions to the entire molecule have been designed. 2In contrast to the bimolecular electron transfer (ET) reactions in the multi-component photocatalytic water reduction systems developed in the 1970s, 3 the supramolecular approach exploits intramolecular ET.Supramolecular complexes coupling metal-based chromophores to a catalytic center photocatalytically reduce water to H 2 under various conditions. 4Impediments to engineering supramolecular complexes for solar H 2 production include the small number of molecular systems capable of photochemically collecting reducing equivalents and the lack of fundamental understanding of multielectron photochemistry.
In seeking an efficient and robust supramolecular photocatalyst, a series of Ru II Rh III -containing supramolecular complexes coupling polypyridyl Ru II chromophores to a Rh III catalytic center via a bridging ligand dpp (2,3-bis(2-pyridyl)pyrazine) were explored.The first photocatalyst of this type, [{(bpy) 2 Ru-(dpp)} 2 RhCl 2 ](PF 6 ) 5 , inspired the development of Ru II Rh III Ru II trimetallics with the architecture of [{(TL) 2 Ru(dpp)} 2 RhX 2 ](PF 6 ) 5 (TL = bpy, 1,10-phenanthroline (phen), or 4,7-diphenyl-1,10-phenanthroline (Ph 2 phen); X = Cl or Br) to explore factors that control photoinitiated electron collection (PEC) and catalytic activity.4f,5 The presence of a Rh(ds*)-based LUMO (lowest unoccupied molecular orbital) in the Ru II Rh III Ru II complexes is a key energetic requirement for PEC at the Rh III catalytic center. 6PEC on the Rh III center forms the proposed active species, Ru II Rh I Ru II , upon sequential reductive quenching of the 3 MLCT (metal-toligand charge transfer) excited state by a sacrificial electron donor.6b Recent studies also show that two Ru II chromophores are not required for photocatalysis.Active Ru II Rh III bimetallics require a Rh-based LUMO and steric protection around the photogenerated Rh I center to prevent dimerization that leads to catalytic deactivation.4g These Ru II Rh III Ru II and Ru II Rh III motifs represent homogenous single-component photocatalysts for H 2 generation.
To the best of our knowledge, all reported dpp-bridged Ru II Rh III and Ru II Rh III Ru II photocatalysts have two labile halide ligands (Cl or Br) on the Rh III center.Here we propose the replacement of one halide ligand with an N donor of a polypyridyl ligand as a means to (1) modulate the electrochemical properties of the catalytic Rh center, (2) test the hypothesis that only one labile halide ligand is needed in the dpp-bridged Ru II Rh III precatalyst for photocatalytic water reduction, (3) expand the scope of structural designs for competent Rh-containing supramolecular photocatalysts, and (4) provide a new mechanism of steric protection of the Rh I center.
The ligand tpy (2,2 0 :6 0 ,2 00 -terpyridine) has been widely used in coordination chemistry with meridional tridentate (Z 3 ) chelation being the most common coordination mode.Taking advantage of the tridentate binding mode, the new Ru II Rh III complex, [(bpy) 2 Ru(dpp)RhCl(tpy)](PF 6 ) 4 (Ru II Rh III Cl(tpy)), has been prepared to test the hypotheses described above.For comparative purposes, we have studied the cis-Rh III Cl 2 analogue [(bpy) 2 Ru-(dpp)RhCl 2 (bpy)](PF 6 ) 3 (Ru II Rh III Cl 2 (bpy)).Herein we report the electrochemical, photochemical, and catalytic properties of these two RuRh supramolecular complexes.It was found that Ru II Rh III (tpy) is an active photocatalyst for H 2 production.The results suggested that two Rh-Cl bonds were not required for photocatalysis.All synthetic details, including 1 H NMR spectra, are provided in the ESI † (Fig. S1-S3).
Cyclic voltammetry (CV) was utilized to investigate the influence of TL variation on the redox properties of the Rh III center.Fig. 1 shows that both complexes possess a reversible Ru III/II couple at a similar potential (ca.1.60 V vs. Ag/AgCl) indicating that the Ru(dp) orbital energy is insensitive to the variation of the remote TL on Rh III .Reductively, Ru II Rh III Cl 2 (bpy) shows a quasi-reversible Rh III/II couple at À0.43 V (DE = 60 mV), an irreversible Rh II/I couple at E c p = À0.79V, and a reversible dpp 0/À at À1.01 V.The assignments are confirmed by coulometric experiments and consistent with [(bpy) 2 Ru(dpp)RhCl 2 -(phen)] 3+ . 7In Ru II Rh III Cl(tpy), the first reduction appears as an irreversible wave at E c p = À0.35V vs. Ag/AgCl and comprises 2e À /molecule.The appearance of the 2Cl À /Cl 2 oxidation couple in the CV of the reduced solution (Fig. S4, ESI †) suggests the dissociation of the Cl À ligand from Rh III upon reduction, providing an assignment of Rh III/II/I for the first reduction.Unlike Ru II Rh III Cl 2 (bpy), very little current is seen in the anodic wave associated with the first reduction even upon increasing the scan rate to 1.0 V s À1 (Fig. S5, ESI †), establishing that Cl À loss is faster in the tpy complex than the bpy complex.The Ru II Rh III Cl(tpy) geometry requires the Cl À ligand to be trans to dpp whereas Ru II Rh III Cl 2 (bpy) has one Cl À trans to dpp and one Cl À trans to bpy.This uncovers an important consideration in controlling the rate of halide loss critical to providing an active site in the reduced Rh I species.The single Rh III/II/I couple in Ru II Rh III Cl(tpy) is in marked contrast to two separate Rh reductions in Ru II (cis-Rh III Cl 2 ) bimetallics.The effect is traced to rapid halide loss and instability of the 1e À reduced species of Ru II Rh III Cl(tpy) toward disproportionation whereas the 1e reduced Ru II Rh III Cl 2 (bpy) likely possesses dpp À character and is comparatively more stable.The Ru-based first oxidation and Rh-based first reduction establish the Ru(dp) HOMO and the Rh(ds*) LUMO in both complexes with a lowest-lying MMCT (metal-to-metal charge transfer) excited state predicted to undergo PEC at the Rh III center producing active photocatalysts.
The electronic absorption spectra of Ru II Rh III Cl(tpy) and Ru II Rh III Cl 2 (bpy) are provided in Fig. S6 (ESI †).The UV spectrum is dominated by intense ligand centered pp* transitions.Ru II Rh III Cl(tpy) displays a higher absorption intensity (e = 74 200 M À1 cm À1 ) at 280 nm than Ru II Rh III Cl 2 (bpy) (e = 59 300 M À1 cm À1 ).A broad band between 400 and 500 nm is 1 MLCT in character with lower energies attributed to Ru(dp)dpp(p*) 1 MLCT transitions and higher energies attributed to Ru(dp)bpy(p*) 1 MLCT transitions.The spectra of the two bimetallics are nearly identical in the visible region, indicating that the structural difference at Rh does not impact the Ru II 1 MLCT transitions.
Emission spectroscopy was used to investigate the photophysical properties of these Ru II Rh III complexes.The emission spectra of Ru II Rh III Cl 2 (bpy), Ru II Rh III Cl(tpy), and a model complex [{(bpy) 2 Ru} 2 (dpp)](PF 6 ) 4 were recorded at room temperature (Fig. S7, ESI †) and 77 K (Table S1, ESI †).Both Ru II Rh III complexes are weak emitters at room temperature from the 3 MLCT state (l max em = 750 nm, F em = 1.3 Â 10 À4 , t = 40 ns for Ru II Rh III Cl 2 (bpy); F em = 6.5 Â 10 À5 , t = 30 ns for Ru II Rh III Cl(tpy)) and are dramatically quenched compared to the model complex [{(bpy) 2 Ru} 2 (dpp)](PF 6 ) 4 (F em = 8.9 Â 10 À4 , t = 130 ns).The quenching is due to intramolecular electron transfer from dpp to Rh to populate the 3 MMCT excited state.Using k r and k nr from this model complex, k et was obtained as 2.6 Â 10 7 s À1 for Ru II Rh III Cl(tpy) and 1.7 Â 10 7 s À1 for Ru II Rh III Cl 2 (bpy).Intramolecular ET is impeded at 77 K and results in a long-lived 3 MLCT emission.At 77 K in an ethanol/methanol (4 : 1, v/v) glass matrix, the emissions of Ru II Rh III Cl(tpy) and Ru II Rh III Cl 2 (bpy) are blue-shifted to 715 nm (l max ) with a dramatic increase in the intensity and lifetime (t = 2.3 ms for Ru II Rh III Cl 2 (bpy) and 2.1 ms for Ru II Rh III Cl(tpy)) comparable to 2.4 ms for the model.
Spectrophotochemical and spectroelectrochemical analyses of the title Ru II Rh III complexes demonstrated PEC at the Rh III center.PEC is essential for Ru II Rh III systems to provide active photocatalysts.Fig. 2 and Fig. S8 (ESI †) illustrate spectroscopic changes which accompany reduction at À0.65 V for Ru II Rh III Cl(tpy) and À0.95 V vs. Ag/AgCl for Ru II Rh III Cl 2 (bpy) to generate the Rh I species.The changes upon reduction are analogous to the changes upon the photolysis of Ru II Rh III Cl(tpy) and Ru II Rh III Cl 2 (bpy) in the presence of N,N-dimethylaniline (DMA), establishing both complexes as molecular devices for PEC.Upon reduction, the Ru(dp)dpp(p*) 1 MLCT transitions blue shift, consistent with dpp bound to electron rich Rh I .Reduction of Rh III is  accompanied by halide loss as the Rh I (d 8) reduces its coordination number to adopt a square planar geometry.This demonstration of PEC establishes Ru II Rh III Cl(tpy) as the first Ru II Rh III system that undergoes PEC where the Rh III is coordinated to a single halide, removing the previously employed design constraint that two coordinated halides are needed to promote PEC in Ru II Rh III supramolecules as well as greatly expanding the potential supramolecular motifs available as single-component photocatalysts for proton reduction to produce H 2 fuel. 5eductive quenching of the 3 MLCT excited state by DMA (E(DMA +/0 ) = 0.86 V vs. Ag/AgCl) is reported as the primary pathway to generate Ru II Rh I during the photolysis of Ru II Rh III . 8sing the ground state reduction potential, E(CAT n+ /CAT (nÀ1)+ ), of 0.35 V for Ru II Rh III Cl(tpy) and 0.43 V for Ru II Rh III Cl 2 (bpy), and E 0,0 estimated from l max em (77 K) as 1.73 eV, the thermodynamic driving force for reductive quenching, E redox , is determined to be 0.52 V for Ru II Rh III Cl(tpy) and 0.44 V for Ru II Rh III Cl 2 (bpy).5c This driving force demonstrates that reduction of Ru II Rh III to Ru II Rh II using DMA is thermodynamically favorable.Quenching of the new Ru II Rh III Cl(tpy) is more favorable than Ru II Rh III Cl 2 (bpy) and [{(bpy) 2 Ru(dpp)} 2 RhCl 2 ](PF 6 ) 5 (0.49 V). 5c Greater driving force for reductive quenching facilitates the formation of the Rh I active species and is hypothesized to enhance the photochemical reactivity for proton reduction.
Photocatalytic H 2 production from water-organic mixtures using Ru II Rh III Cl(tpy) was studied to test the hypothesis that two photolabile halides are not necessary for H 2 generation in the dpp-bridged Ru II Rh III photocatalysts.As shown in Fig. 3, in CH 3 CN Ru II Rh III Cl(tpy) produced 9.8 mmol H 2 with a TON of 33 and an overall quantum efficiency of 0.08% in 10 hours.Photocatalytic H 2 production was also observed in DMF and acetone with ca.17 mmol H 2 and a TON of 58 showing improvements relative to strongly ligating CH 3 CN. 4h Photocatalytic H 2 production by Ru II Rh III Cl 2 (bpy) and trimetallic [{(bpy) 2 Ru(dpp)} 2 RhCl 2 ](PF 6 ) 5 was also conducted in DMF and CH 3 CN for comparison (Fig. S10 and S11, ESI †).The catalytic activity of Ru II Rh III Cl(tpy) is better than Ru II Rh III Cl 2 (bpy) and comparable to [{(bpy) 2 Ru(dpp)} 2 RhCl 2 ](PF 6 ) 5 under similar conditions (Table S2, ESI †).The Ru II Rh I state is proposed as the active species for proton reduction. 6For Ru II Rh III Cl 2 (bpy), electrochemical reduction leads to the formation of [(bpy) 2 Ru-(dpp)Rh I (bpy)] 3+ following halides loss, confirmed by ESI mass spectrometry (m/z = 302.3;calcd = 302.3,M = [(bpy) 2 Ru(dpp)-Rh I (bpy)] 3+ ) (Fig. S12 and S13, ESI †).In Ru II Rh III Cl(tpy), Cl À dissociation was also observed (Fig. S4, ESI †).Electrochemical reduction of the simple model [Rh III Cl(tpy)dpp](PF 6 ) 2 showed halide loss (Fig. S14, ESI †) to form [Rh I (tpy)(dpp)] + (m/z = 570.0;calcd = 570.0,M = [(dpp)Rh I (tpy)] + Fig. S15, ESI †).The variable Z 3 -tpy or Z 2 -tpy coordination, of which the latter has been seen in some Re and Rh complexes, 9 facilitates the necessary geometry change as Rh III is reduced to Rh I to form [(bpy) 2 Ru-(dpp)Rh I (Z 2 -tpy)] 3+ .Additional support for the formation of [(bpy) 2 Ru(dpp)Rh I (Z 2 -tpy)] 3+ is provided in the detailed photolysis studies of Ru II Rh III Cl(tpy).The addition of Cl À to the photocatalytic system reduced H 2 production, whereas added tpy did not impact H 2 production, consistent with chloride, not tpy, loss occurring in the photocatalytic pathway.This Z 2 -tpy gives steric protection on the Rh I site and prevents deactivation of the catalyst by Rh I dimerization.6b Switching between Z 2 and Z 3 coordination at tpy provides a new mechanism to stabilize the supramolecule as it cycles the redox states at Rh in the catalytic cycle.Furthermore, one free pyridine of Z 2 -tpy may assist catalysis through secondary coordination sphere effects. 10The improved functionality of Ru II Rh III Cl(tpy) over Ru II Rh III Cl 2 (bpy) results from the enhanced driving force for reductive quenching by DMA, the rapid rate of halide loss and the steric protection of the photogenerated Rh I imparted by the tpy ligand.4g,6b In conclusion, a new photocatalyst, Ru II Rh III Cl(tpy), with one Cl ligand and a tridentate ligand on the Rh III center has shown lightdriven H 2 production from water.This established that two labile halide ligands on the Rh III center are not mandatory for photocatalysis.The replacement of one halide with a pyridyl ligand successfully increases the rate of halide loss and the E redox for reductive quenching of the 3 MLCT excited state by DMA.Increased driving force for intramolecular electron transfer from reduced dpp to Rh also increases photocatalytic efficiency.This study shows that photocatalytic activity can be controlled by tuning the Rh redox properties and demonstrates a new approach to design photocatalysts for H 2 generation.

Fig. 2
Fig. 2 Electronic absorption spectra generated from the electrochemical reduction (A, reduced at À0.65 V vs. Ag/AgCl) and photochemical reduction (B) of Ru II Rh III Cl(tpy) in deoxygenated acetonitrile at room temperature.