High turnover in electro-oxidation of alcohols and ethers with a glassy carbon-supported phenanthroimidazole mediator.

Glassy carbon electrodes covalently modified with a phenanthroimidazole mediator promote electrochemical alcohol and ether oxidation: three orders of magnitude increase in TON, to ∼15 000 in each case, was observed compared with homogeneous mediated reactions. We propose the deactivation pathways in homogeneous solution are prevented by the immobilization: modified electrode reversibility is increased for a one-electron oxidation reaction. The modified electrodes were used to catalytically oxidize p-anisyl alcohol and 1-((benzyloxy)methyl)-4-methoxybenzene, selectively, to the corresponding benzaldehyde and benzyl ester, respectively

Quantum chemical studies of redox properties and conformational changes of a four-center iron CO2 reduction electrocatalyst.

The CO2 reduction electrocatalyst [Fe4N(CO)12]- (abbrev. 1-) reduces CO2 to HCO2- in a two-electron, one-proton catalytic cycle. Here, we employ ab initio calculations to estimate the first two redox potentials of 1- and explore the pathway of a side reaction involving CO dissociation from 13-. Using the BP86 density functional approximation, the redox potentials were computed with a root mean squared error of 0.15 V with respect to experimental data. High temperature Born-Oppenheimer molecular dynamics was employed to discover a reaction pathway of CO dissociation from 13- with a reaction energy of +10.6 kcal mol-1 and an activation energy of 18.8 kcal mol-1; including harmonic free energy terms, this yields ΔGsep = 1.4 kcal mol-1 for fully separated species and ΔG‡ = +17.4 kcal mol-1, indicating CO dissociation is energetically accessible at ambient conditions. The analogous dissociation pathway from 12- has a reaction energy of 22.1 kcal mol-1 and an activation energy of 22.4 kcal mol-1 (ΔGsep = 12.8 kcal mol-1, ΔG‡ = +18.1 kcal mol-1). Our computed harmonic vibrational analysis of [Fe4N(CO)11]3- or 23- reveals a distinct CO-stretching peak red-shifted from the main CO-stretching band, pointing to a possible vibrational signature of dissociation. Multi-reference CASSCF calculations are used to check the assumptions of the density functional approximations that were used to obtain the majority of the results

Dimanganese and Diiron Complexes of a Binucleating Cyclam Ligand: Four-Electron, Reversible Oxidation Chemistry at High Potentials

The reaction of a binucleating biscyclam ligand cyclam_2^iPrO [where cyclam_2^iPrO = (1,3-bis[1,4,8,11-tetraazacyclododecane]-2-hydroxypropane] with Mn(CF_3SO_3)_2 or Fe(CF_3SO_3)_2•2MeCN gives [(cyclam_2^iPrO)Mn_2(μ-CF_3SO_3)](CF_3SO_3)_2 (4) and [(cyclam_2^iPrO)Fe_2(μ-CF_3SO_3)](CF_3SO_3)_2 (6), respectively. [(cyclam_2^iPrO)Mn_2(μ-N_3)](CF_3SO_3)_2 (5) is obtained by the reaction of 4 with NaN_3. Single-crystal X-ray structural characterization indicates that in each of the bimetallic complexes the two metal centers are facially coordinated by a cyclam ligand and bridged by the isopropoxide linker of the ligand in addition to a triflate counteranion. Upon replacement of the triflate bridge with the single-atom bridge of an end-bound azide ligand in 5, the Mn—Mn distance decreases by 0.38 Å. All of the complexes are high-spin and colorless and were characterized by magnetic susceptibility measurements, electron paramagnetic resonance spectroscopy, and electrochemical methods. Magnetic susceptibility measurements indicate that 4 and 6 are weakly antiferromagnetically coupled while 5 is weakly ferromagnetically coupled. Cyclic voltammetry measurements indicate that the hard donor amine ligands impart high oxidation potentials to the metal centers and that four-electron redox activity can be accessed with a narrow potential range of 0.72 V. Upon inclusion of water in the cyclic voltammetry experiment, the oxidative waves shift to higher potentials, which is consistent with water binding the manganese centers. The diiron complex 6 displays four one-electron redox couples, of which the final two are irreversible. Inclusion of water in the cyclic voltammetry measurement for compound 6 resulted in two sets of shifted peaks, which suggests that two molecules of water bind the diiron core. In accordance with the observed reversibility of the electrochemical results, the dimanganese complex is more efficient than the diiron complex for mediating O-atom transfer to organic substrates and is an excellent hydrogen peroxide disproportionation catalyst, with the reaction proceeding for over 20 000 turnovers

A pendant proton shuttle on [Fe4N(CO)12]- alters product selectivity in formate vs. H2 production via the hydride [H-Fe4N(CO)12].

Proton relays are known to increase reaction rates for H2 evolution and lower overpotentials in electrocatalytic reactions. In this report we describe two electrocatalysts, [Fe4N(CO)11(PPh3)]- (1-) which has no proton relay, and hydroxyl-containing [Fe4N(CO)11(Ph2P(CH2)2OH)]- (2-). Solid state structures indicate that these phosphine-substituted clusters are direct analogs of [Fe4N(CO)12]- where one CO ligand has been replaced by a phosphine. We show that the proton relay changes the selectivity of reactions: CO2 is reduced selectively to formate by 1- in the absence of a relay, and protons are reduced to H2 under a CO2 atmosphere by 2-. These results implicate a hydride intermediate in the mechanism of the reactions and demonstrate the importance of controlling proton delivery to control product selectivity. Thermochemical measurements performed using infrared spectroelectrochemistry provided pKa and hydricity values for [HFe4N(CO)11(PPh3)]-, which are 23.7, and 45.5 kcal mol-1, respectively. The pKa of the hydroxyl group in 2- was determined to fall between 29 and 41, and this suggests that the proximity of the proton relay to the active catalytic site plays a significant role in the product selectivity observed, since the acidity alone does not account for the observed results. More generally, this work emphasizes the importance of substrate delivery kinetics in determining the selectivity of CO2 reduction reactions that proceed through metal-hydride intermediates

Redox rich dicobalt macrocycles as templates for multi-electron transformations

Pyridazine-templated dicobalt macrocycles reversibly support five oxidation states with unusually positive Co^(II)/Co^I redox couples, and are also active proton reduction electrocatalysts

Penerapan Model Pembelajaran Atraktif Berbasis Multiple Intelligences Tentang Pemantulan Cahaya pada Cermin

Penelitian ini bertujuan untuk mengetahui efektivitas penerapan model pembelajaran atraktif berbasis multiple intelligences dalam meremediasi miskonsepsi siswa tentang pemantulan cahaya pada cermin. Pada penelitian ini digunakan bentuk pre-eksperimental design dengan rancangan one group pretest-post test design. Alat pengumpulan data berupa tes pilihan ganda dengan reasoning. Hasil validitas sebesar 4,08 dan reliabilitas 0,537. Siswa dibagi menjadi lima kelompok kecerdasan, yaitu kelompok linguistic intelligence, mathematical-logical intelligence, visual-spatial intelligence, bodily-khinestetic intelligence, dan musical intelligence. Siswa membahas konsep fisika sesuai kelompok kecerdasannya dalam bentuk pembuatan pantun-puisi, teka-teki silang, menggambar kreatif, drama, dan mengarang lirik lagu. Efektivitas penerapan model pembelajaran multiple intelligences menggunakan persamaan effect size. Ditemukan bahwa skor effect size masing-masing kelompok berkategori tinggi sebesar 5,76; 3,76; 4,60; 1,70; dan 1,34. Penerapan model pembelajaran atraktif berbasis multiple intelligences efektif dalam meremediasi miskonsepsi siswa. Penelitian ini diharapkan dapat digunakan pada materi fisika dan sekolah lainnya

Pre-Equilibrium Reaction Mechanism as a Strategy to Enhance Rate and Lower Overpotential in Electrocatalysis

Pre-equilibrium reaction kinetics enable the overall rate of a catalytic reaction to be orders of magnitude faster than the rate-determining step. Herein, we demonstrate how pre-equilibrium kinetics can be applied to breaking the linear free-energy relationship (LFER) for electrocatalysis, leading to rate enhancement 5 orders of magnitude and lowering of overpotential to approximately thermoneutral. This approach is applied to pre-equilibrium formation of a metal-hydride intermediate to achieve fast formate formation rates from CO2 reduction without loss of selectivity (i.e., H2 evolution). Fast pre-equilibrium metal-hydride formation, at 108 M-1 s-1, boosts the CO2 electroreduction to formate rate up to 296 s-1. Compared with molecular catalysts that have similar overpotential, this rate is enhanced by 5 orders of magnitude. As an alternative comparison, overpotential is lowered by ∼50 mV compared to catalysts with a similar rate. The principles elucidated here to obtain pre-equilibrium reaction kinetics via catalyst design are general. Design and development that builds on these principles should be possible in both molecular homogeneous and heterogeneous electrocatalysis