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A series of pyridine- and lutidine-linked bis-N-heterocyclic carbene (NHC) palladium pincer complexes were electrochemically characterized and screened for CO2 reduction capability with 2,2,2-trifluoroethanol, acetic acid, or 2,2,2-trifluoroacetic acid (TFA) as proton sources. The lutidine-linked pincer complexes electrocatalytically reduce CO2 to CO at potentials as low as -1.6 V versus Ag/AgNO3 in the presence of TFA. The one-electron reduction of these complexes is shown to be chemically reversible, yielding a monometallic species, with density functional theory studies indicating charge storage on the redox-active ligand, thus addressing a major source of deactivation in earlier triphosphine electrocatalysts.

Bithiophene-based flexible Lewis pairs with P(O)R2 (R = phenyl, isopropyl) and BMes2 (Mes = 2,4,6-trimethylphenyl) functionalities are able to toggle between closed, Lewis adduct and open, unbound Lewis pair structures. The open structure is favored in strong hydrogen bond donating solvents or at higher temperatures giving rise to an intense charge-transfer (CT) luminescence, while the closed structure without this emission dominates in non-hydrogen bond donating solvents or at lower temperatures. Intermediate solvents result in an equilibrium mixture of both structures, which shows unusual mixed emission that is dependent on excitation wavelength.

A recent study of soft X-ray absorption in native and hydrogenated coronene cations, C 24 H + 12+m m = 0-7, led to the conclusion that additional hydrogen atoms protect (interstellar) Polycyclic Aromatic Hydrocarbon (PAH) molecules from fragmentation [Reitsma et al., Phys. Rev. Lett. 113, 053002 (2014)]. The present experiment with collisions between fast (30-200 eV) He atoms and pyrene (C 16 H + 10+m , m = 0, 6, and 16) and simulations without reference to the excitation method suggests the opposite. We find that the absolute carbon-backbone fragmentation cross section does not decrease but increases with the degree of hydrogenation for pyrene molecules.

The new complexes 1,1′-bis(5-(2,2′-bithienyl))ferrocene (2) and 1,1′-bis(5-(2,2′:5′,2′′-terthienyl))ferrocene (3) have been synthesized by coupling 1,1′-bis(2-thienyl)ferrocene (1) with 2-bromothiophene and 5-bromo-2,2′-bithiophene, respectively. The cyclic voltammograms of 1-3 contain a reversible Fe II/III wave with E 1/2 between 0.37 and 0.46 V vs SCE and irreversible thiophene-based oxidation waves at higher potentials. These compounds can be electrochemically oxidized to yield solutions of the monocations 1 + -3 + . The visible-near-IR spectra of these monocations all contain low-energy bands due to an oligothienyl group to Fe III charge-transfer transition. The absorption maxima and intensities of these bands correlate to the length of the conjugation in the oligothienyl group. Above the thiophene oxidation potential and by careful exclusion of water, 2 and 3 electropolymerize resulting in the deposition of electrochromic films on the electrode surface. The films are golden-red and stable when neutral and become black upon oxidation. The cyclic voltammetry of the film growth process and the formation of electroactive films indicate that the films are conductive. Spectroelectrochemical characterization of the films demonstrates that broad, low-energy absorptions appear upon oxidation of the ferrocenyl centers and that stronger and much broader bands appear upon full oxidation of the films.

The synthesis of a conducting polymer by electropolymerization of 5,5'-(2-thienyl)-2,2'bithiazole is reported. Films of this polymer exhibit electrochemical behavior typical of conducting polymers and have a conductivity of -0.2 W1 cm-' a t 25 "C in 0.1 M [ n -B a l P F d CH2C12 a t the potential of maximum conductivity (-+1.5 V vs Ag). The activation energy for conductivity for partially oxidized (-+1.3 V vs Ag) films is 0.25 eV. Reaction of these polymer films with refluxing solutions of Re(C0)SCl in CHCl3 yields polymers in which the [Re(CO)&lI moiety is bound to the bithiazolyl sites in the polymer. Subsequent reaction with AgPF6 converts the Re centers to cationic [Re(C0)3(CH3CN)I+ groups. These polymers were characterized by XPS and surface reflectance FTIR. The polymer containing the cationic Re groups is conducting (-2 x C2-I cm-' a t +1.5 V vs Ag). The potential region in (38) Paul, E. W.; Ricco, A. J.; Wrighton, M. S. J . Phys. Chem. 1985, (39) Kittlesen, G. P.; White, H. S.; Wrighton, M. S.

The series of ruthenium(II) mono(oligothienylacetylide) complexes trans-Ru(dppm) 2 (Cl)-(CtCR) (dppm ) Ph 2 PCH 2 PPh 2 ; R ) 2-thienyl (1a), 5-(2,2′-bithienyl) (1b), and 5-(2,2′:5′,2′′-terthienyl) (1c)) and bis(oligothienylacetylide) complexes trans-Ru(dppm) 2 (CtCR) 2 (R ) 2-thienyl (2a), 5-(2,2′-bithienyl) (2b), and 5-(2,2′:5′,2′′-terthienyl) (2c)) were synthesized. Complex 2c was crystallographically characterized. The cyclic voltammograms of complexes 1a-c all contain two oxidation waves, a Ru(II/III) wave and a ligand-based oxidation wave. As the length of the conjugated oligothienyl ligand increases, the thiophene-based oxidation wave becomes more chemically reversible. Complexes 2a-c all have a Ru(II/III) wave in their cyclic voltammograms, as well as multiple ligand-based oxidation waves. Complexes 2b and 2c both form films on the electrode surfaces upon repeated cycling in the range 0-1.4 V vs SCE. The UV-vis spectra of complexes 1a-c and 2a-c all contain intense absorptions due to the π-π* transition in the oligothienyl ligand, and these appear at lower energy than the π-π* transitions in the corresponding oligothiophenes. The monocations 1c + and 2c + were synthesized in solution at -20°C and were characterized by visible and near-IR spectroscopy. The π-π* transitions of the terthienyl ligand in 1c + and 2c + shift to higher energy compared with the analogous transitions in 1c and 2c, and a series of LMCT absorption bands of high intensity appear between 500 and 700 nm and between 900 and 1700 nm, respectively. These results support the conclusion that the π system of the conjugated oligothienyl ligands interacts strongly with the Ru(III) center.