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DOI: 10.1039/C5CC04123F (Communication) Chem. Commun., 2015, 51, 12966-12969

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

Rongwei Zhou *, Gerald F. Manbeck‡ , Dexter G. Wimer and Karen J. Brewer§
Department of Chemistry, Virginia Tech, Blacksburg, VA 24061-0212, USA. E-mail: rowezhou@vt.edu

Received 19th May 2015 , Accepted 1st July 2015

First published on 1st July 2015

Abstract

A new RuIIRhIII structural motif [(bpy)2Ru(dpp)RhCl(tpy)]4+ with one halide on the RhIII center demonstrates light-driven proton reduction ability, establishing that two halide ligands are not mandatory despite all prior systems containing a cis-RhCl2 catalytic site. This new design provides a novel approach to modulate RhIII redox behavior and catalytic activity with insight into catalytic intermediates.

Solar water splitting to generate H2 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 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.2 In 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 H2 under various conditions.4 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 RuIIRhIII-containing supramolecular complexes coupling polypyridyl RuII chromophores to a RhIII 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 RuIIRhIIIRuII 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.4f,5 The presence of a Rh(dσ*)-based LUMO (lowest unoccupied molecular orbital) in the RuIIRhIIIRuII complexes is a key energetic requirement for PEC at the RhIII catalytic center.6 PEC on the RhIII center forms the proposed active species, RuIIRhIRuII, upon sequential reductive quenching of the 3MLCT (metal-to-ligand charge transfer) excited state by a sacrificial electron donor.6b Recent studies also show that two RuII chromophores are not required for photocatalysis. Active RuIIRhIII bimetallics require a Rh-based LUMO and steric protection around the photogenerated RhI center to prevent dimerization that leads to catalytic deactivation.4g These RuIIRhIIIRuII and RuIIRhIII motifs represent homogenous single-component photocatalysts for H2 generation.

To the best of our knowledge, all reported dpp-bridged RuIIRhIII and RuIIRhIIIRuII photocatalysts have two labile halide ligands (Cl or Br) on the RhIII 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 RuIIRhIII pre-catalyst 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 RhI center.

The ligand tpy (2,2′:6′,2′′-terpyridine) has been widely used in coordination chemistry with meridional tridentate (η3) chelation being the most common coordination mode. Taking advantage of the tridentate binding mode, the new RuIIRhIII complex, [(bpy)2Ru(dpp)RhCl(tpy)](PF6)4 (RuIIRhIIICl(tpy)), has been prepared to test the hypotheses described above. For comparative purposes, we have studied the cis-RhIIICl2 analogue [(bpy)2Ru(dpp)RhCl2(bpy)](PF6)3 (RuIIRhIIICl2(bpy)). Herein we report the electrochemical, photochemical, and catalytic properties of these two RuRh supramolecular complexes. It was found that RuIIRhIII(tpy) is an active photocatalyst for H2 production. The results suggested that two Rh–Cl bonds were not required for photocatalysis. All synthetic details, including 1H 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 RhIII center. Fig. 1 shows that both complexes possess a reversible RuIII/II couple at a similar potential (ca. 1.60 V vs. Ag/AgCl) indicating that the Ru(dπ) orbital energy is insensitive to the variation of the remote TL on RhIII. Reductively, RuIIRhIIICl2(bpy) shows a quasi-reversible RhIII/II couple at −0.43 V (ΔE = 60 mV), an irreversible RhII/I couple at Ecp = −0.79 V, and a reversible dpp0/− at −1.01 V. The assignments are confirmed by coulometric experiments and consistent with [(bpy)2Ru(dpp)RhCl2(phen)]3+.7 In RuIIRhIIICl(tpy), the first reduction appears as an irreversible wave at Ecp = −0.35 V vs. Ag/AgCl and comprises 2e−/molecule. The appearance of the 2Cl−/Cl2 oxidation couple in the CV of the reduced solution (Fig. S4, ESI†) suggests the dissociation of the Cl− ligand from RhIII upon reduction, providing an assignment of RhIII/II/I for the first reduction. Unlike RuIIRhIIICl2(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−1 (Fig. S5, ESI†), establishing that Cl− loss is faster in the tpy complex than the bpy complex. The RuIIRhIIICl(tpy) geometry requires the Cl− ligand to be trans to dpp whereas RuIIRhIIICl2(bpy) has one Cl−trans to dpp and one Cl−trans to bpy. This uncovers an important consideration in controlling the rate of halide loss critical to providing an active site in the reduced RhI species. The single RhIII/II/I couple in RuIIRhIIICl(tpy) is in marked contrast to two separate Rh reductions in RuII(cis-RhIIICl2) bimetallics. The effect is traced to rapid halide loss and instability of the 1e− reduced species of RuIIRhIIICl(tpy) toward disproportionation whereas the 1e reduced RuIIRhIIICl2(bpy) likely possesses dpp˙− character and is comparatively more stable. The Ru-based first oxidation and Rh-based first reduction establish the Ru(dπ) HOMO and the Rh(dσ*) LUMO in both complexes with a lowest-lying MMCT (metal-to-metal charge transfer) excited state predicted to undergo PEC at the RhIII center producing active photocatalysts.


image file: c5cc04123f-f1.tif
Fig. 1 CVs and structures of RuIIRhIIICl2(bpy) (blue dashed) and RuIIRhIIICl(tpy) (red solid) in 0.1 M Bu4NPF6 acetonitrile.

The electronic absorption spectra of RuIIRhIIICl(tpy) and RuIIRhIIICl2(bpy) are provided in Fig. S6 (ESI†). The UV spectrum is dominated by intense ligand centered π → π* transitions. RuIIRhIIICl(tpy) displays a higher absorption intensity (ε = 74[thin space (1/6-em)]200 M−1 cm−1) at 280 nm than RuIIRhIIICl2(bpy) (ε = 59[thin space (1/6-em)]300 M−1 cm−1). A broad band between 400 and 500 nm is 1MLCT in character with lower energies attributed to Ru(dπ) → dpp(π*) 1MLCT transitions and higher energies attributed to Ru(dπ) → bpy(π*) 1MLCT 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 RuII 1MLCT transitions.

Emission spectroscopy was used to investigate the photophysical properties of these RuIIRhIII complexes. The emission spectra of RuIIRhIIICl2(bpy), RuIIRhIIICl(tpy), and a model complex [{(bpy)2Ru}2(dpp)](PF6)4 were recorded at room temperature (Fig. S7, ESI†) and 77 K (Table S1, ESI†). Both RuIIRhIII complexes are weak emitters at room temperature from the 3MLCT state (λmaxem = 750 nm, Φem = 1.3 × 10−4, τ = 40 ns for RuIIRhIIICl2(bpy); Φem = 6.5 × 10−5, τ = 30 ns for RuIIRhIIICl(tpy)) and are dramatically quenched compared to the model complex [{(bpy)2Ru}2(dpp)](PF6)4 (Φem = 8.9 × 10−4, τ = 130 ns). The quenching is due to intramolecular electron transfer from dpp to Rh to populate the 3MMCT excited state. Using kr and knr from this model complex, ket was obtained as 2.6 × 107 s−1 for RuIIRhIIICl(tpy) and 1.7 × 107 s−1 for RuIIRhIIICl2(bpy). Intramolecular ET is impeded at 77 K and results in a long-lived 3MLCT emission. At 77 K in an ethanol/methanol (4[thin space (1/6-em)]:[thin space (1/6-em)]1, v/v) glass matrix, the emissions of RuIIRhIIICl(tpy) and RuIIRhIIICl2(bpy) are blue-shifted to 715 nm (λmax) with a dramatic increase in the intensity and lifetime (τ = 2.3 μs for RuIIRhIIICl2(bpy) and 2.1 μs for RuIIRhIIICl(tpy)) comparable to 2.4 μs for the model.

Spectrophotochemical and spectroelectrochemical analyses of the title RuIIRhIII complexes demonstrated PEC at the RhIII center. PEC is essential for RuIIRhIII systems to provide active photocatalysts. Fig. 2 and Fig. S8 (ESI†) illustrate spectroscopic changes which accompany reduction at −0.65 V for RuIIRhIIICl(tpy) and −0.95 V vs. Ag/AgCl for RuIIRhIIICl2(bpy) to generate the RhI species. The changes upon reduction are analogous to the changes upon the photolysis of RuIIRhIIICl(tpy) and RuIIRhIIICl2(bpy) in the presence of N,N-dimethylaniline (DMA), establishing both complexes as molecular devices for PEC. Upon reduction, the Ru(dπ) → dpp(π*) 1MLCT transitions blue shift, consistent with dpp bound to electron rich RhI. Reduction of RhIII is accompanied by halide loss as the RhI(d8) reduces its coordination number to adopt a square planar geometry. This demonstration of PEC establishes RuIIRhIIICl(tpy) as the first RuIIRhIII system that undergoes PEC where the RhIII is coordinated to a single halide, removing the previously employed design constraint that two coordinated halides are needed to promote PEC in RuIIRhIII supramolecules as well as greatly expanding the potential supramolecular motifs available as single-component photocatalysts for proton reduction to produce H2 fuel.5


image file: c5cc04123f-f2.tif
Fig. 2 Electronic absorption spectra generated from the electrochemical reduction (A, reduced at −0.65 V vs. Ag/AgCl) and photochemical reduction (B) of RuIIRhIIICl(tpy) in deoxygenated acetonitrile at room temperature.

Reductive quenching of the 3MLCT excited state by DMA (E(DMA+/0) = 0.86 V vs. Ag/AgCl) is reported as the primary pathway to generate RuIIRhI during the photolysis of RuIIRhIII.8 Using the ground state reduction potential, E(CATn+/CAT(n−1)+), of 0.35 V for RuIIRhIIICl(tpy) and 0.43 V for RuIIRhIIICl2(bpy), and E0,0 estimated from λmaxem (77 K) as 1.73 eV, the thermodynamic driving force for reductive quenching, Eredox, is determined to be 0.52 V for RuIIRhIIICl(tpy) and 0.44 V for RuIIRhIIICl2(bpy).5c This driving force demonstrates that reduction of RuIIRhIII to RuIIRhII using DMA is thermodynamically favorable. Quenching of the new RuIIRhIIICl(tpy) is more favorable than RuIIRhIIICl2(bpy) and [{(bpy)2Ru(dpp)}2RhCl2](PF6)5 (0.49 V).5c Greater driving force for reductive quenching facilitates the formation of the RhI active species and is hypothesized to enhance the photochemical reactivity for proton reduction.

Photocatalytic H2 production from water–organic mixtures using RuIIRhIIICl(tpy) was studied to test the hypothesis that two photolabile halides are not necessary for H2 generation in the dpp-bridged RuIIRhIII photocatalysts. As shown in Fig. 3, in CH3CN RuIIRhIIICl(tpy) produced 9.8 μmol H2 with a TON of 33 and an overall quantum efficiency of 0.08% in 10 hours. Photocatalytic H2 production was also observed in DMF and acetone with ca.17 μmol H2 and a TON of 58 showing improvements relative to strongly ligating CH3CN.4h


image file: c5cc04123f-f3.tif
Fig. 3 Photocatalytic H2 production of RuIIRhIIICl(tpy) (65 μM) in acetone (blue diamond), DMF (green triangle), and CH3CN (red square) solution with 1.5 M DMA and 0.6 M H2O irradiated under 470 nm LED (light flux = (2.36 ± 0.05) × 1019 photons per min).

Photocatalytic H2 production by RuIIRhIIICl2(bpy) and trimetallic [{(bpy)2Ru(dpp)}2RhCl2](PF6)5 was also conducted in DMF and CH3CN for comparison (Fig. S10 and S11, ESI†). The catalytic activity of RuIIRhIIICl(tpy) is better than RuIIRhIIICl2(bpy) and comparable to [{(bpy)2Ru(dpp)}2RhCl2](PF6)5 under similar conditions (Table S2, ESI†). The RuIIRhI state is proposed as the active species for proton reduction.6 For RuIIRhIIICl2(bpy), electrochemical reduction leads to the formation of [(bpy)2Ru(dpp)RhI(bpy)]3+ following halides loss, confirmed by ESI mass spectrometry (m/z = 302.3; calcd = 302.3, M = [(bpy)2Ru(dpp)RhI(bpy)]3+) (Fig. S12 and S13, ESI†). In RuIIRhIIICl(tpy), Cl− dissociation was also observed (Fig. S4, ESI†). Electrochemical reduction of the simple model [RhIIICl(tpy)dpp](PF6)2 showed halide loss (Fig. S14, ESI†) to form [RhI(tpy)(dpp)]+ (m/z = 570.0; calcd = 570.0, M = [(dpp)RhI(tpy)]+ Fig. S15, ESI†). The variable η3-tpy or η2-tpy coordination, of which the latter has been seen in some Re and Rh complexes,9 facilitates the necessary geometry change as RhIII is reduced to RhI to form [(bpy)2Ru(dpp)RhI(η2-tpy)]3+. Additional support for the formation of [(bpy)2Ru(dpp)RhI(η2-tpy)]3+ is provided in the detailed photolysis studies of RuIIRhIIICl(tpy). The addition of Cl− to the photocatalytic system reduced H2 production, whereas added tpy did not impact H2 production, consistent with chloride, not tpy, loss occurring in the photocatalytic pathway. This η2-tpy gives steric protection on the RhI site and prevents deactivation of the catalyst by RhI dimerization.6b Switching between η2 and η3 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 η2-tpy may assist catalysis through secondary coordination sphere effects.10 The improved functionality of RuIIRhIIICl(tpy) over RuIIRhIIICl2(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 RhI imparted by the tpy ligand.4g,6b

In conclusion, a new photocatalyst, RuIIRhIIICl(tpy), with one Cl ligand and a tridentate ligand on the RhIII center has shown light-driven H2 production from water. This established that two labile halide ligands on the RhIII center are not mandatory for photocatalysis. The replacement of one halide with a pyridyl ligand successfully increases the rate of halide loss and the Eredox for reductive quenching of the 3MLCT 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 H2 generation.

Acknowledgements were made to Professor Paul A. Deck for his helpful discussion and the Chemical Sciences, Geosciences and Biosciences Division, Office of Basic Energy Sciences, Offices of Sciences, U.S. Department of Energy DE FG02-05-ER15751 for financial support.

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Footnotes

† 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.
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