Multidentate Pyridyl-Aminophosphinite and Pyridyl-Phosphoramidite Ruthenium(II) Complexes: Synthesis, Structure and Application as Levulinic Acid Hydrogenation Pre-Catalysts: Multidentate Pyridyl-Aminophosphinite and Pyridyl-Phosphoramidite Ruthenium(II) Complexes: Synthesis, Structure and Application as Levulinic Acid Hydr

Deciphering the Role of Crown-ether Receptor Orientation in C-H Oxidation Catalyzed by Supramolecular Nonheme FeIV(O) Complexes

The outstanding efficiency and selectivity of enzymatic reactions, such as C-H oxidation by nonheme iron oxygenases, stems from a precise control of substrate positioning inside the active site. The resulting proximity between a specific moiety (a certain C-H bond) and the reactant (a FeIV(O) active species) translates into higher rates and selectivity, that can be in part replicated also with artificial supramolecular catalysts. However, structural modification of the position and orientation of the binding site both in enzymes and in artificial catalysts often leads to significant variations in reactivity that can be difficult to rationalize due to the system's complexity. Herein, we quantitatively analyzed the impact of such a structural modification (namely receptor orientation) on the C-H oxidation reactivity (kinetics, Effective Molarity) and selectivity by comparing simple supramolecular FeIV(O) models. Overall, we did not observe significant differences in reaction rates, but we noticed slight changes in the selectivity profile. These results indicate that, when a crown-ether is employed as a recognition site, the key ingredient for enhanced reactivity is the presence of the supramolecular receptor itself rather than its exact orientation, providing a guide for the rational design of supramolecular catalysts.

Comparison of mononuclear and dinuclear copper(II) biomimetic complexes: spectroelectrochemical mechanistic study of their catalytic pathways

Two catecholase-like biomimetic catalysts, namely, two dinuclear copper complexes [Cu₂(L1)(OH)(H₂O)(EtOH)][ClO₄]₂ (C1) and [Cu₂Ac₂O(L1)ClO₄] (C2) with the 2,6-bis(4-methyl piperazin-1-yl-methyl)-4-formyl-phenoxy ligand (L1) together with the mononuclear complex Cu(ClO₄)₂(L2) (C3) containing ligand 1,2-(C₅H₄N-6-OCH₃-2-CHN)₂CH₂CH₂ (L2), were synthesized. Their catalytic pathways were investigated and compared. The evaluation of the catalytic activity of compound C1 (and C2, C3) using the Michaelis-Menten model was represented by values of K_M = 272.93 (223.02; 1616) μmol L^-1 and V_max of 0.981 (1.617; 1.689) μmol L^-1 s^-1. The role of water content in the solvent is also discussed. The dinuclear complexes C1 and C2 were found to be more efficient catalysts than mononuclear complex C3. The mode of catalytic action was characterized via cyclic voltammetry, spectrophotometry, and UV-Vis spectroelectrochemistry. The catalytic mechanism of 3,5-di-tert butyl catechol oxidation in the presence of oxygen was proposed. The reaction circle was proved by the confirmation of the chemical reversibility of complex reduction. The advantage of the in situ spectroelectrochemical measurement enabled to control the reduction of quinone formed by the chemical reaction of catechol with oxygen in solution. At this step, the simultaneous change in the absorption spectrum indicated a change in the copper redox state of the catalyst.

N-Donor stabilized tin(II) cations as efficient ROP catalysts for the synthesis of linear and star-shaped PLAs via the activated monomer mechanism

α-Iminopyridine ligands L¹ (2-(CHN(C₆H₂-2,4,6-Ph₃))C₅H₄N), L² (2-(CHN(C₆H₂-2,4,6-tBu₃))C₅H₄N) and L³ (1,2-(C₅H₄N-2-CHN)₂CH₂CH₂) differing by the steric demand of the substituent on the imine CHN group and by the number of donating nitrogen atoms were utilized to initiate a Lewis base mediated ionization of SnCl₂ in an effort to prepare ionic tin(II) species [L^1-3 → SnCl][SnCl₃]. The reaction of L¹ and L² with SnCl₂ led to the formation of neutral adducts [L¹ → SnCl₂] (2) and [L² → SnCl₂] (3). The preparation of the desired ionic compounds was achieved by subsequent reactions of 2 and 3 with an equivalent of SnCl₂ or GaCl₃. In contrast, ligand L³ containing four donor nitrogen atoms showed the ability to ionize SnCl₂ and also Sn(OTf)₂, yielding [L³ → SnCl][SnCl₃] (7) and [L³ → Sn(H₂O)][OTf]₂ (8). The study thus revealed that the reaction is dependent on the type of the ligand. The prepared complexes 4-8 together with the previously reported [{2-((CH₃)CN(C₆H₃-2,6-iPr₂))-6-CH₃O-C₅H₃N}SnCl][SnCl₃] (1) were tested as catalysts for the ROP of L-lactide, which could operate via an activated monomer mechanism. Finally, a DFT computational study was performed to evaluate the steric and electronic properties of the ionic tin(II) species 1 and 4-8 together with their ability to interact with the L-lactide monomer.