New Rh2(II,II) Complexes for Solar Energy Applications: Panchromatic Absorption and Excited-State Reactivity

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.

Revolutionizing carbon chemistry: Solar-powered C(sp3 )-N bond activation and CO2 transformation via newly designed SBE-Y cutting-edge dynamic photocatalyst

A solvent-free sulfur-bridge-eosin-Y (SBE-Y) polymeric framework photocatalyst was prepared for the first time through an in situ thermal polymerization route using elemental sulfur (S8 ) as a bridge. The addition of a sulfur bridge to the polymeric framework structure resulted in an allowance of the harvesting range of eosin-Y (E-Y) for solar light. This shows that a wider range of solar light can be used by the bridge material's photocatalytic reactions. In this context, supercharged solar spectrum: enhancing light absorption and hole oxidation with sulfur bridges. This suggests that the excited electrons and holes through solar light can contribute to oxidation-reduction reactions more potently. As a result, the photocatalyst-enzyme attached artificial photosynthesis system developed using SBE-Y as a photocatalyst performs exceptionally well, resulting in high 1,4-NADH regeneration (86.81%), followed by its utilization in the exclusive production of formic acid (210.01 μmol) from CO2 and synthesis of fine chemicals with 99.9% conversion yields. The creation of more effective photocatalytic materials for environmental clean-up and other applications that depend on the solar light-driven absorption spectrum of inorganic and organic molecules could be one of the practical ramifications of this research.

Sun Light Responsive 2D Covalent-Organic Frameworks Platform as a Catalysts Boost C-H Bond Arylation and Dopamine Regeneration

Photocatalysis is one of the most promising methods for producing organic compounds with a renewable source of energy. Two-dimensional covalent organic frameworks (2D COFs) are a type of polymer that has developed as a potential light-harvesting catalyst for artificial photosynthesis with a design-controllable platform that might be developed into a new type of cost-effective and metal-free photocatalyst. Here, we present a two-dimensional covalent organic framework synthesis technique as a low-cost and highly efficient visible light active flexible photocatalyst for C-H bond activation and dopamine regeneration. 2D COF were synthesized from tetramino-benzoquinone (TABQ) and terapthaloyl chloride monomer through condensation polymerization reaction and the resultant photocatalyst have remarkable performance due to its visible light-harvesting capacity, appropriate band gap, and highly organized π-electron channels. The synthesized photocatalyst is capable to convert dopamine into leucodopaminechrome with a higher yield (77.08%) and also capable to activate the C-H bond between 4-nitrobenzenediazonium tetrafluoroborate and pyrrole.

Fabrication of Polyoxometalate-Based Metal-Organic Frameworks Integrating Paddlewheel Rh2(OAc)4 for Visible-Light-Driven Oxidative Coupling of Amines

The development of visible-light-responsive, environmentally friendly, and reusable photocatalysts for organic oxidation reactions is of vital significance. Herein, four polyoxometalate-based metal-organic frameworks (POMOFs) were synthesized and systematically characterized by assembling the paddlewheel complex Rh2(OAc)4 and various polyoxometalates (POMs). Single-crystal X-ray diffraction analysis revealed that the four POMOFs were isomorphic and possessed rare structural features among the POMOFs, with POMs as nodes and Rh2(OAc)4 as linkers. As expected, the activities of the four POMOFs for the photocatalytic oxidative coupling of benzylamine were better than that of Rh2(OAc)4 or POMs individually, which was ascribed to the synergistic effect between them, and the intrinsic reasons for the difference in the activity were explained via electrochemical measurements. In particular, the product imine yield reached 96.1% with NaRh-SiW12 as the catalyst and a turnover number and a turnover frequency of 480.5 and 120.5 h-1, respectively, while the product yield remained as high as 92% after three repetitions, evidencing its high stability. Moreover, the higher activities of the four POMOFs for the selective epoxidation of various alkenes reaffirm the synergistic effect between Rh2(OAc)4 and POMs.

An Approach to Developing Cyanines with Upconverted Photosensitive Efficiency Enhancement for Highly Efficient NIR Tumor Phototheranostics

Upconverted reactive oxygen species (ROS) photosensitization with one-photon excitation mode is a promising tactic to elongate the excitation wavelengths of photosensitive dyes to near-infrared (NIR) light region without the requirement of coherent high-intensity light sources. However, the photosensitization efficiencies are still finite by the unilateral improvement of excited-state intersystem crossing (ISC) via heavy-atom-effect, since the upconverted efficiency also plays a decisive role in upconverted photosensitization. Herein, a NIR light initiated one-photon upconversion heavy-atom-free small molecule system is reported. The meso-rotatable anthracene in pentamethine cyanine (Cy5) is demonstrated to enrich the populations in high vibrational-rotational energy levels and subsequently improve the hot-band absorption (HBA) efficiency. Moreover, the spin-orbit charge transfer intersystem crossing (SOCT-ISC) caused by electron donated anthracene can further amplify the triplet yield. Benefiting from the above two aspects, the ¹ O₂ generation significantly increases with over 2-fold improved performance compared with heavy-atom-modified method under upconverted light excitation, which obtains efficient in vivo phototheranostic results and provides new opportunities for other applications such as photocatalysis and fine chemical synthesis.

Light-Driven Hydrodefluorination of Electron-Rich Aryl Fluorides by an Anionic Rhodium-Gallium Photoredox Catalyst

An anionic Rh-Ga complex catalyzed the hydrodefluorination of challenging C-F bonds in electron-rich aryl fluorides and trifluoromethylarenes when irradiated with violet light in the presence of H₂ , a stoichiometric alkoxide base, and a crown-ether additive. Based on theoretical calculations, the lowest unoccupied molecular orbital (LUMO), which is delocalized across both the Rh and Ga atoms, becomes singly occupied upon excitation, thereby poising the Rh-Ga complex for photoinduced single-electron transfer (SET). Stoichiometric and control reactions support that the C-F activation is mediated by the excited anionic Rh-Ga complex. After SET, the proposed neutral Rh⁰ intermediate was detected by EPR spectroscopy, which matched the spectrum of an independently synthesized sample. Deuterium-labeling studies corroborate the generation of aryl radicals during catalysis and their subsequent hydrogen-atom abstraction from the THF solvent to generate the hydrodefluorinated arene products. Altogether, the combined experimental and theoretical data support an unconventional bimetallic excitation that achieves the activation of strong C-F bonds and uses H₂ and base as the terminal reductant.

In-Situ Prepared NRCPFs as Highly Active Photo-platforms for In-Situ Bond Formation Between Aryldiazonium Salts and Heteroarenes

Covalent perylene frameworks (CPFs) with melamine linkages have newly received risinginterest for a variety of applications because of nitrogen rich content and high stability. Herein we account a new simple strategy to in-situ attain nitrogen rich covalent peryleneframeworks (NRCPFs) as highly active photo-platforms for in-situ bond formation betweenaryldiazonium salts and heteroarenes (C-H bond arylation) through the controlled photoredoxroute.

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

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

In Pursuit of Panchromatic Absorption in Metal Coordination Complexes: Experimental Delineation of the HOMO Inversion Model Using Pseudo-Octahedral Complexes of Diarylamido Ligands

The ability of a compound to broadly absorb light across the incident solar spectrum is an important design target in the development of molecular photosensitizers. The 'HOMO inversion' model predicts that for [(tpy)₂Fe]²⁺ (tpy = 2,2':6',2″-terpyridine) compounds, adjusting the character of the highest occupied molecular orbital (HOMO) from metal-centered to ligand-centered can drastically improve photophysical properties by broadening absorption in the visible and increasing molar extinction coefficients. In an effort to experimentally realize strong, panchromatic absorption, a tridentate N^N^-^N diarylamido ligand bearing flanking benzannulated N-heterocyclic donors (^tBuL) was used to prepare deeply colored, pseudo-octahedral coordination complexes of a range of first-row transition and main-group metals [(^tBuL)₂M^0/+; M = Fe, Co, Ni, Zn, Ga]. While the Fe(II) congener exhibits the sought-after broad absorption, isostructural and isoelectronic complexes of other first-row transition and main-group metals show vastly different absorption and redox properties. Density functional theory (DFT) calculations point toward the relative energies of the metal d orbitals and ligand orbitals as the source of major changes in electronic structure, confirming aspects and limitations of the predictive 'HOMO inversion' model in experimentally realized systems with implications for the design of abundant transition-metal sensitizers with broad, panchromatic absorptive properties.

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.

Two Four-Coordinate and Seven-Coordinate CoII Complexes Based on the Bidentate Ligand 1, 8-Naphthyridine Showing Slow Magnetic Relaxation Behavior

For a long time, the cobalt(II) complex ([Co(napy)₄ ](ClO₄ )₂ ) (napy=1, 8-naphthyridine) has been considered as an eight-coordinate complex without any structural proof. After careful considerations, two complexes [Co(napy)₂ Cl₂ ] (1) and [Co(napy)₄ ](ClO₄ )₂ (2) based on the bidentate ligand napy were synthesized and structurally characterized. X-ray single-crystal structural determination showed that the cobalt(II) center in [Co(napy)₂ Cl₂ ] (1) is four-coordinate with a tetrahedral geometry (T_d ), while [Co(napy)₄ ](ClO₄ )₂ (2) is seven-coordinate rather than eight-coordinate with a capped trigonal prism geometry (C_2v ). Direct-current (dc) magnetic data revealed that complexes 1 and 2 possess positive zero-field splitting (ZFS) parameters of 11.08 and 25.30 cm^-1 , respectively, with easy-plane magnetic anisotropy. Alternating current(ac) susceptibility measurements revealed that both complexes showed slow magnetic relaxation behaviour. Theoretical calculations demonstrated that the presence of easy-plane magnetic anisotropy (D>0) for complexes 1 and 2 is in agreement with the experimental data. Furthermore, these results pave the way to obtain four-coordinate and seven-coordinate cobalt(II) single-ion magnets (SIMs) by using a bidentate ligand.

One-step synthesis of ball-shaped metal complexes with a main absorption band in the near-IR region

The design of near-IR materials is highly relevant to energy and pharmaceutical sciences due to the high proportion of near-IR irradiation in the solar spectrum and the high penetration of near-IR light in biological samples. Here, we show the one-step synthesis of hexacoordinated ruthenium and iron complexes that exhibit a main absorption band in the near-IR region. For that purpose, novel tridentate ligands were prepared by condensation of two diimines and four cyanoaryl derivatives in the presence of ruthenium and iron template ions. This method was applied to a wide variety of cyanoaryl, diimine, and metal ion combinations. The relationship between the structure and the optical and electrochemical properties in the resulting complexes was examined, and the results demonstrated that these compounds represent novel near-IR materials whose physical properties can be controlled based on rational design guidelines. The intense absorption bands in the 700–900 nm region were assigned to metal-to-ligand charge transfer (MLCT) transitions, which should allow applications in materials with triplet excited states under irradiation with near-IR light.

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.

Unbridged Rh(ii)–Rh(ii) complexes of N-heterocyclic carbenes and reactions with O2 to form dirhodium(μ–η1:η1-O2) complexes

Dimeric rhodium(ii) [Rh(L)(CH₃CN)]₂(PF₆)₄ and rhodium(iii) peroxide [Rh(L)(PPh₃)]₂(μ–η¹:η¹-O₂)(PF₆)₄ and [Rh(L)(PCy₃)]₂(μ–η¹:η¹-O₂)(PF₆)₄ (L = bis(N-pyridylimidazolylidenyl)methane) were reported.

Tunable Rh2(II,II) Light Absorbers as Excited-State Electron Donors and Acceptors Accessible with Red/Near-Infrared Irradiation

A series of dirhodium(II,II) paddlewheeel complexes of the type cis-[Rh₂(μ-DTolF)₂(μ-L)₂][BF₄]₂, where DTolF = N,N'-di( p-tolyl)formamidinate and L = 1,8-naphthyridine (np), 2-(pyridin-2-yl)-1,8-naphthyridine (pynp), 2-(quinolin-2-yl)-1,8-naphthyridine (qnnp), and 2-(1,8-naphthyridin-2-yl)quinoxaline (qxnp), were synthesized and characterized. These molecules feature new tridentate ligands that concomitantly bridge the dirhodium core and cap the axial positions. The complexes absorb light strongly throughout the ultraviolet/visible range and into the near-infrared region and exhibit relatively long-lived triplet excited-state lifetimes. Both the singlet and triplet excited states exhibit metal/ligand-to-ligand charge transfer (ML-LCT) in nature as determined by transient absorption spectroscopy and spectroelectrochemistry measurements. When irradiated with low-energy light, these black dyes are capable of undergoing reversible bimolecular electron transfer both to the electron acceptor methyl viologen and from the electron donor p-phenylenediamine. Photoinduced charge transfer in the latter was inaccessible with previous Rh₂(II,II) complexes. These results underscore the fact that the excited state of this class of molecules can be readily tuned for electron-transfer reactions upon simple synthetic modification and highlight their potential as excellent candidates for p- and n-type semiconductor applications and for improved harvesting of low-energy light to drive useful photochemical reactions.

Photocatalytic H2production by dirhodium(ii,ii) photosensitizers with red light

Photocatalytic H₂evolution uponλ_irr= 655 nm with dirhodium(ii,ii) photosensitizers demonstrates tunable oxidative and reductive quenching mechanisms.