Fast and Versatile Functionalization of Glassy Carbon.

A rapid procedure for the functionalization of glassy carbon surfaces (GCSs) is disclosed. A three-step sequence of bromomethylation, azide displacement, and copper(I)-catalyzed azide-alkyne cycloaddition (CuAAC) allows ethynylated molecules to be attached covalently to the carbon surface through a methylene functional group. Redox-active ethynyl ferrocene and [RuII(Cl)(DMSO)(ethynyl-TPA)]1+ (DMSO = dimethylsulfoxide; TPA = tris(2-pyridylmethyl)amine) are attached with high coverages as assessed by cyclic voltammetry, and the elemental composition of the surface is confirmed by X-ray photoelectron spectroscopy. In less than 1 h, surface coverages of 1 × 1014 molecules/cm2 are possible that exhibit good durability in both acidic and basic media. Attached [RuII(Cl)(DMSO)(ethynyl-TPA)]1+ catalytically oxidizes alcohols, yet the currents and potentials are less impressive compared to an attachment without the intervening methylene group. The advantages of this covalent attachment procedure for GCSs are its short reaction times, mild reaction conditions, and the use of standard laboratory reagents and glassware, allowing for many types of ethynylated molecules to be attached rapidly to the surface.

Electrocatalytic alcohol oxidation by covalently immobilized ruthenium complex on carbon.

A dearth of discrete immobilized metal complexes exist that electrocatalytically oxidize methanol. Reported here is the covalent immobilization of a tris(2-pyridylmethyl)amine ruthenium complex [RuII(Cl)(DMSO)(ethynyl-TPA)]+ (ethynyl-TPA = (5-ethynyl-2-pyridylmethyl)bis(2-pyridylmethyl)amine) to a glassy carbon (GC) electrode through a CuI catalyzed azide-alkyne cycloaddition (click) reaction between the ethynyl-TPA ligand and an azide derivatized carbon surface forming [RuII(Cl)(DMSO)(GC-click-TPA)]+. Following water substitution for DMSO and proton coupled electron transfer, [RuIV(O)(Cl)(GC-click-TPA)]+ electrooxidizes alcohols, including methanol, efficiently relative to other immobilized metal complexes. A primary kinetic isotope effect suggests rate-limiting Cα-H bond cleavage of benzyl alcohol. Approximately 40% of the [RuII(Cl)(DMSO)(GC-click-TPA)]+ undergoes the DMSO for water exchange to form an active oxidant, consistent with the 40% distribution of the more labile Cl-cis-amine isomer before immobilization. Using the benchmark of benzyl alcohol electrocatalytic oxidation, [RuIV(O)(Cl)(GC-click-TPA)]+ operates at ca. 250 mV lower overpotential, with a 15% increase in faradaic efficiency, and at least an order of magnitude increase in average turnover frequency (0.7 s-1 TOFavg) compared to the previously best immobilized discrete ruthenium complexes.

Quantum mechanical transition-state analysis reveals the precise origin of stereoselectivity in chiral quaternary cinchonidinium phase-transfer catalyzed enolate allylation.

Density functional theory was used to model glycinate enolate binding and enantiomeric allylation transition states mediated by the cinchonidinium phase-transfer catalyst 2. Transition states show oxy-anion-ammonium interactions in contrast to π-face interactions in the ground states. The details of stereoselectivity are described within the quaternary ammonium-tetrahedron face model.

Unrestricted prescriptions for open-shell singlet diradicals: using economical ab initio and density functional theory to calculate singlet-triplet gaps and bond dissociation curves.

Here we present and test several computational prescriptions for calculating singlet-triplet (ST) gap energies and bond dissociation curves for open-shell singlet diradicals using economical unrestricted single reference type calculations. For ST gap energies from Slipchenko and Krylov's atom and molecule test set (C, O, Si, NH, NF, OH(+), O(2), CH(2), and NH(2)(+)) spin unrestricted Hartree-Fock and MP2 energies result in errors greater than 15 kcal/mol. However, spin-projected (SP) Hartree-Fock theory in combination with spin-component-scaled (SCS) or scaled-opposite-spin (SOS) second-order perturbation theory gives ST gap energies with a mean unsigned error (MUE) of less than 2 kcal/mol. Density functionals generally give poor results for unrestricted energies and only the ωB97X-D, the M06, and the M06-2X functionals provide reasonable accuracy after spin-projection with MUE values of 4.7, 4.3, and 3.0 kcal/mol, respectively, with the 6-311++G(2d,2p) basis set. We also present a new one parameter hybrid density functional, diradical-1 (DR-1), based on Adamo and Barone's modified PW exchange functional with the PW91 correlation functional. This DR-1 method gives a mean error (ME) of 0.0 kcal/mol and a MUE value of 1.3 kcal/mol for ST gap energies. As another test of unrestricted methods the bond dissociation curves for methane (CH(4)) and hydrofluoric acid (H-F) were calculated with the M06-2X, DR-1, and ωB97X-D density functionals. All three of these functionals give reasonable results for the methane C-H bond but result in errors greater than 50 kcal/mol for the H-F bond dissociation. Spin-projection is found to significantly degrade bond dissociation curves past ~2.2 Å. Although unrestricted Hartree-Fock theory provides a very poor description of H-F bond dissociation, unrestricted SCS-MP2 and SOS-MP2 methods give accurate results.