Exploring the Photophysics and Photocatalytic Activity of Heteroleptic Rh(III) Transition-Metal Complexes Using High-Throughput Experimentation

High-throughput synthesis and screening (HTSS) methods were used to investigate the photophysical properties of 576 heteroleptic Rh(III) transition-metal complexes through measurement of the UV-visible absorption spectra, deaerated excited-state lifetime, and phosphorescent emission spectra. While 4d transition-metal photophysics are often highly influenced by deleterious metal-centered deactivation channels, the HTSS of structurally diverse cyclometalating and ancillary ligands attached to the metal center facilitated the discovery of photoactive complexes exhibiting long-lived charge-transfer phosphorescence (0.15-0.95 μs) spanning a substantial portion of the visible region (546-620 nm) at room temperature. Further photophysical and electrochemical investigations were then carried out on select complexes with favorable photophysics to understand the underlying features controlling these superior properties. Heteroleptic Ir(III) complexes with identical ligand morphology were also synthesized to compare these features to this family of well understood chromophores. A number of these Rh(III) complexes contained the requisite properties for photocatalytic activity and were consequently tested as photocatalysts (PCs) in a water reduction system using a Pd water reduction cocatalyst. Under certain conditions, the activity of the Rh(III) PC actually surpassed that of the Ir(III) PC, uncovering the potential of this often-overlooked class of transition metals as both efficient photoactive chromophores and PCs.

Pyridinethiolate-Capped CdSe Quantum Dots for Red-Light-Driven H2 Production in Water

The utilization of low-energy sunlight to produce renewable fuels is a subject of great interest. Here we report the first example of metal chalcogenide quantum dots (QDs) capped with a pyridinethiolate carboxylic acid (pyS-COOH) for red-light-driven H2 production in water. The precious-metal-free system is robust over 240 h, and achieves a turnover number (TON) of 43910±305 (vs Ni) with a rate of 31570±1690 μmol g-1 h-1 for hydrogen production. In contrast to the inactive QDs capped with other thiolate ligands, the CdSe-pyS-COOH QDs give a significantly higher singlet oxygen quantum yield [ΦΔ (1O2)] in solution.

Ligand-Centered Photocatalytic Hydrogen Production in an Axially Capped Rh2(II,II) Paddlewheel Complex with Red Light

A new series of Rh2(II,II) complexes with the formula cis-[Rh2(DTolF)2(bpnp)(L)]2+, where bpnp = 2,7-bis(2-pyridyl)-1,8-naphthyridine, DTolF = N,N'-di(p-tolyl) formamidinate, and L = pdz (pyridazine; 2), cinn (cinnoline; 3), and bncn (benzo[c]cinnoline; 4), were synthesized from the precursor cis-[Rh2(DTolF)2(bpnp)(CH3CN)2]2+ (1). The first reduction couple in 2-4 is localized on the bpnp ligand at approximately -0.52 V vs Ag/AgCl in CH3CN (0.1 M TBAPF6), followed by reduction of the corresponding diazine ligand. Complex 1 exhibits a Rh2(δ*)/DTolF → bpnp(π*) metal/ligand-to-ligand charge-transfer (1ML-LCT) absorption with a maximum at 767 nm (ε = 1800 M-1 cm-1). This transition is also present in the spectra of 2-4, overlaid with the Rh2(δ*)/DTolF → L(π*) 1ML-LCT bands at 516 nm in 2 (L = pdz), 640 nm in 3 (L = cinn), and 721 nm in 4 (L = bncn). Complexes 2 and 3 exhibit Rh2(δ*)/DTolF → bpnp 3ML-LCT excited states with lifetimes, τ, of 3 and 5 ns, respectively, in CH3CN, whereas the lowest energy 3ML-LCT state in 4 is Rh2(δ*)/DTolF → bncn in nature with τ = 1 ns. Irradiation of 4 with 670 nm light in DMF in the presence of 0.1 M TsOH (p-toluene sulfonic acid) and 30 mM BNAH (1-benzyl-1,4-dihydronicotinamide) results in the production of H2 with a turnover number (TON) of 16 over 24 h. The axial capping of the Rh2(II,II) bimetallic core with the bpnp ligand prevents the formation of an Rh-H hydride intermediate. These results show that the observed photocatalytic reactivity is localized on the bncn ligand, representing the first example of ligand-centered H2 production.

Reactivity of a fluorine-containing dirhodium tetracarboxylate compound with proteins

Dirhodium complexes of general formula [Rh2(O2CR)4]L2 are a well-known class of bimetallic compounds that are used as efficient catalysts for a variety of reactions and have been shown to be potent antibacterial and anticancer agents. The catalytic and biological properties of these complexes largely depend on the nature of the bridging carboxylate ligands. Trifluoroacetate (tfa)-containing dirhodium compounds have been used to build artificial metalloenzymes upon reaction with peptides and have been shown to be more cytotoxic than dirhodium tetraacetate. However, there is no structural information on the interaction between these compounds and proteins. Here, cis-Rh2(μ-O2CCH3)2(μ-O2CCF3)2 ([cis-Rh2(OAc)2(tfa)2]) has been synthesized and its reaction with bovine pancreatic ribonuclease (RNase A) and hen egg white lysozyme (HEWL) was analyzed using a combination of different techniques, including Fluorine-19 nuclear magnetic resonance spectroscopy and macromolecular X-ray crystallography, with the aim to unveil the differences in the reactivity of tfa-containing dihrodium complexes with proteins when compared to [Rh2(OAc)4]. [cis-Rh2(OAc)2(tfa)2] and [Rh2(OAc)4] bind the N atoms of His side chains of RNase A at the axial position; however the fluorine-containing compound rapidly loses its tfa ligands, while [Rh2(OAc)4] can retain the acetate ligands upon protein binding. The reactivity of [cis-Rh2(OAc)2(tfa)2] with HEWL is slightly distinct when compared to that of [Rh2(OAc)4] under the same experimental conditions; however, both [cis-Rh2(OAc)2(tfa)2] and [Rh2(OAc)4] degrade when soaked within HEWL crystals. These results provide a structural-based guide for the design of new heterogenous chiral dirhodium/peptide and dirhodium/protein adducts with application in the fields of organic synthesis and asymmetric catalysis.

Digging into protein metalation differences triggered by fluorine containing-dirhodium tetracarboxylate analogues

Catalytic and biological properties of dirhodium tetracarboxylates ([Rh2(μ-O2CR)4L2], L=axial ligand, R=CH3-, CH3CH2-, etc) largely depend on the nature of the bridging carboxylate equatorial μ-O2CR ligands, which can be easily exchanged...

Unsymmetrical dirhodium single molecule photocatalysts for H2 production with low energy light

New unsymmetrical dirhodium complexes for photocatalytic H₂ production with red light.

Panchromatic dirhodium photocatalysts for dihydrogen generation with red light

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

Three dirhodium complexes cis-[Rh₂(DPhB)₂(bncn)₂](BF₄)₂, cis-[Rh₂(DPhTA)₂(bncn)₂](BF₄)₂ and cis-[Rh₂(DPhF)₂(bncn)₂](BF₄)₂ are shown to act as single-molecule photocatalysts for H₂ production.

Synthetic Strategies for Trapping the Elusive trans-Dirhodium(II,II) Formamidinate Isomer: Effects of Cis versus Trans Geometry on the Photophysical Properties

The cis- and trans-dirhodium(II,II) complexes cis-[Rh₂(μ-DTolF)₂(μ-np)(MeCN)₄][BF₄]₂ (1; DTolF = N,N'-di-p-tolylformamidinate and np = 1,8-naphthyridine), cis- and trans-[Rh₂(μ-DTolF)₂(μ-qxnp)(MeCN)₃][BF₄]₂ [2 and 3, respectively, where qxnp = 2-(1,8-naphthyridin-2-yl)quinoxaline], and trans-[Rh₂(μ-DTolF)₂(μ-qxnp)₂][BF₄]₂ (4) were synthesized and characterized. A new synthetic methodology was developed that consists of the sequential addition of π-accepting axially blocking ligands to favor formation of the first example of a bis-substituted formamidinate-bearing trans product. Isolation of the intermediates 2 and 3 provides insight into the mechanistic requirements for obtaining 4 and the cis analogue, cis-[Rh₂(μ-DTolF)₂(μ-qxnp)₂][BF₄]₂ (5). Density functional theory calculations provide support for the synthetic mechanism and proposed intermediates. The metal/ligand-to-ligand charge-transfer (ML-LCT) absorption maximum of the trans complex 4 at 832 nm is red-shifted by 1173 cm^-1 and exhibits shorter lifetimes of the ¹ML-LCT and ³ML-LCT excited states, 3 ps and 0.40 ns, respectively, compared to those of the cis analogue 5. The shorter excited-state lifetimes of 4 are attributed to the longer Rh-Rh bond of 2.4942(8) Å relative to that in 5, 2.4498(2) Å. A longer metal-metal bond reflects a decreased overlap of the Rh atoms, which leads to more accessible metal-centered excited states for radiationless deactivation. The ³ML-LCT excited states of 4 and 5 undergo reversible bimolecular charge transfer with the electron donor p-phenylenediamine when irradiated with low-energy light. These results indicate that trans isomers are a source of unexplored tunability for potential p-type semiconductor applications and, given their distinct geometric arrangement, constitute useful building blocks for supramolecular architectures with potentially interesting photophysical properties.

Electron injection into titanium dioxide by panchromatic dirhodium photosensitizers with low energy red light

Two new Rh₂(ii,ii) dyes were synthesized and anchored to TiO₂ for charge injection upon low energy irradiation.