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1. Controlling the optical and catalytic properties of artificial metalloenzyme photocatalysts using chemogenetic engineering

   https://doi.org/10.1039/D1SC05792H  

   Zubi, Yasmine S. ; Liu, Bingqing ; Gu, Yifan ; Sahoo, Dipankar ; Lewis, Jared C. ( February 2022 , Chemical Science)

   Visible light photocatalysis enables a broad range of organic transformations that proceed via single electron or energy transfer. Metal polypyridyl complexes are among the most commonly employed visible light photocatalysts. The photophysical properties of these complexes have been extensively studied and can be tuned by modifying the substituents on the pyridine ligands. On the other hand, ligand modifications that enable substrate binding to control reaction selectivity remain rare. Given the exquisite control that enzymes exert over electron and energy transfer processes in nature, we envisioned that artificial metalloenzymes (ArMs) created by incorporating Ru( ii ) polypyridyl complexes into a suitable protein scaffold could provide a means to control photocatalyst properties. This study describes approaches to create covalent and non-covalent ArMs from a variety of Ru( ii ) polypyridyl cofactors and a prolyl oligopeptidase scaffold. A panel of ArMs with enhanced photophysical properties were engineered, and the nature of the scaffold/cofactor interactions in these systems was investigated. These ArMs provided higher yields and rates than Ru(Bpy) 3 2+ for the reductive cyclization of dienones and the [2 + 2] photocycloaddition between C -cinnamoyl imidazole and 4-methoxystyrene, suggesting that protein scaffolds could provide a means to improve the efficiency of visible lightmore »photocatalysts.« less
2. Eliminating nonradiative decay in Cu(I) emitters: >99% quantum efficiency and microsecond lifetime

   https://doi.org/10.1126/science.aav2865  

   Hamze, Rasha ; Peltier, Jesse L. ; Sylvinson, Daniel ; Jung, Moonchul ; Cardenas, Jose ; Haiges, Ralf ; Soleilhavoup, Michele ; Jazzar, Rodolphe ; Djurovich, Peter I. ; Bertrand, Guy ; et al ( February 2019 , Science)

   Luminescent complexes of heavy metals such as iridium, platinum, and ruthenium play an important role in photocatalysis and energy conversion applications as well as organic light-emitting diodes (OLEDs). Achieving comparable performance from more–earth-abundant copper requires overcoming the weak spin-orbit coupling of the light metal as well as limiting the high reorganization energies typical in copper(I) [Cu(I)] complexes. Here we report that two-coordinate Cu(I) complexes with redox active ligands in coplanar conformation manifest suppressed nonradiative decay, reduced structural reorganization, and sufficient orbital overlap for efficient charge transfer. We achieve photoluminescence efficiencies >99% and microsecond lifetimes, which lead to an efficient blue-emitting OLED. Photophysical analysis and simulations reveal a temperature-dependent interplay between emissive singlet and triplet charge-transfer states and amide-localized triplet states.
3. Water-soluble dinuclear iridium( iii ) and ruthenium( ii ) bis-terdentate complexes: photophysics and electrochemiluminescence

   https://doi.org/10.1039/D2DT02104H  

   Liu, Bingqing ; Yang, Xin ; Jabed, Mohammed ; Kilina, Svetlana ; Yang, Zhengchun ; Sun, Wenfang ( September 2022 , Dalton Transactions)

   The synthesis, photophysics, and electrochemiluminescence (ECL) of four water-soluble dinuclear Ir( iii ) and Ru( ii ) complexes (1–4) terminally-capped by 4′-phenyl-2,2′:6′,2′′-terpyridine (tpy) or 1,3-di(pyrid-2-yl)-4,6-dimethylbenzene (N^C^N) ligands and linked by a 2,7-bis(2,2′:6′,2′′-terpyridyl)fluorene with oligoether chains on C9 are reported. The impact of the tpy or N^C^N ligands and metal centers on the photophysical properties of 1–4 was assessed by spectroscopic methods including UV-vis absorption, emission, and transient absorption, and by time-dependent density functional theory (TDDFT) calculations. These complexes exhibited distinct singlet and triplet excited-state properties upon variation of the terminal-capping terdentate ligands and the metal centers. The ECL properties of complexes 1–3 with better water solubility were investigated in neutral phosphate buffer solutions (PBS) by adding tripropylamine (TPA) as a co-reactant, and the observed ECL intensity followed the descending order of 3 > 1 > 2. Complex 3 bearing the [Ru(tpy) 2 ] 2+ units displayed more pronounced ECL signals, giving its analogues great potential for further ECL study.
4. Multicenter interactions and ligand field effects in platinum( ii ) tripyrrindione radicals

   https://doi.org/10.1039/D3DT00894K  

   Tomat, Elisa ; Curtis, Clayton J. ; Astashkin, Andrei V. ; Conradie, Jeanet ; Ghosh, Abhik ( May 2023 , Dalton Transactions)

   The tripyrrin-1,14-dione biopyrrin, which shares the scaffold of several naturally occurring heme metabolites, is a redox-active platform for metal coordination. We report the synthesis of square planar platinum( ii ) tripyrrindiones, in which the biopyrrin binds as a tridentate radical and the fourth coordination position is occupied by either aqua or tert -butyl isocyanide ligands. These complexes are stable through chromatographic purification and exposure to air. Electron paramagnetic resonance (EPR) data and density functional theory (DFT) analysis confirm that the spin density is located predominantly on the tripyrrindione ligand. Pancake bonding in solution between the Pt( ii ) tripyrrindione radicals leads to the formation of diamagnetic π dimers at low temperatures. The identity of the monodentate ligand ( i.e. , aqua vs . isocyanide) affects both the thermodynamic parameters of dimerization and the tripyrrindione-based redox processes in these complexes. Isolation and structural characterization of the oxidized complexes revealed stacking of the diamagnetic tripyrrindiones in the solid state as well as a metallophilic Pt( ii )−Pt( ii ) contact in the case of the aqua complex. Overall, the properties of Pt( ii ) tripyrrindiones, including redox potentials and intermolecular interactions in solution and in the solid state, are modulated through easilymore »accessible changes in the redox state of the biopyrrin ligand or the nature of the monodentate ligand.« less
5. Aluminium( iii ) dialkyl 2,6-bisimino-4 R -dihydropyridinates(−1): selective synthesis, structure and controlled dimerization

   https://doi.org/10.1039/C9DT00847K  

   Gallardo-Villagrán, Manuel ; Vidal, Fernando ; Palma, Pilar ; Álvarez, Eleuterio ; Chen, Eugene Y.-X. ; Cámpora, Juan ; Rodríguez-Delgado, Antonio ( June 2019 , Dalton Transactions)

   A family of stable and otherwise selectively unachievable 2,6-bisimino-4- R -1,4-dihydropyridinate aluminium (III) dialkyl complexes [AlR' 2 (4-R- i PrBIPH)] (R = Bn, Allyl; R′ = Me, Et, i Bu) have been synthesized, taking advantage of a method for the preparation of the corresponding 4- R -1,4-dihydropiridine precursors developed in our group. All the dihydropyrdinate(−1) dialkyl aluminium complexes have been fully characterized by 1 H- 13 C-NMR, elemental analysis and in the case 2′a , also by X-ray diffraction studies. Upon heating in toluene solution at 110 °C, the dimethyl derivatives 2a and 2′a dimerize selectively through a double cycloaddition. This reaction leads to the formation of two new C–C bonds that involve the both meta positions of the two 4- R -1,4-dihydropyridinate fragments, resulting the binuclear aluminium species [Me 2 Al(4-R- i PrHBIP)] 2 (R = Bn ( 3a ); allyl ( 3′a )). Experimental kinetics showed that the dimerization of 2′a obeys second order rate with negative activation entropy, which is consistent with a bimolecular rate-determining step. Controlled methanolysis of both 3a and 3′a release the metal-free dimeric bases, (4-Bn- i PrHBIPH) 2 and (4-allyl- i PrHBIPH) 2 , providing a convenient route to these potentially useful ditopicmore »ligands. When the R′ groups are bulkier than Me ( 2b , 2′b and 2′c ), the dimerization is hindered or fully disabled, favoring the formation of paramagnetic NMR-silent species, which have been identified on the basis of a controlled methanolysis of the final organometallic products. Thus, when a toluene solution of [AlEt 2 (4-Bn- i PrBIPH)] ( 2b ) was heated at 110 °C, followed by the addition of methanol in excess, it yields a mixture of the dimer (4-Bn- i PrHBIPH) 2 and the aromatized base 4-Bn- i PrBIP, in ca . 1 : 2 ratio, indicating that the dimerization of 2b competes with its spontaneous dehydrogenation, yielding a paramagnetic complex containing a AlEt 2 unit and a non-innocent (4-Bn- i PrBIP) ˙− radical-anion ligand. Similar NMR monitoring experiments on the thermal behavior of [AlEt 2 (4-allyl- i PrBIPH)] ( 2′b ) and [Al i Bu 2 (4-allyl-iPrBIPH)] ( 2′c ) showed that these complexes do not dimerize, but afford exclusively NMR silent products. When such thermally treated samples were subjected to methanolysis, they resulted in mixtures of the alkylated 4-allyl- i PrBIP and non-alkylated i PrBIP ligand, suggesting that dehydrogenation and deallylation reactions take place competitively.« less