Two new ligand sets, pipMeLH2 and NO2LH2 (pipMeL = N,N′-bis(2,6-diisopropylphenyl)-1-methylpiperidine-2,6-dicarboxamide, NO2L = N,N′-bis(2,6-diisopropyl-4-nitrophenyl)pyridine-2,6-dicarboxamide), are reported which are designed to perturb the overall electronics of the copper(III)–hydroxide core and the resulting effects on the thermodynamics and kinetics of its hydrogen-atom abstraction (HAT) reactions. Bond dissociation energies (BDEs) for the O–H bonds of the corresponding Cu(II)–OH2 complexes were measured that reveal that changes in the redox potential for the Cu(III)/Cu(II) couple are only partially offset by opposite changes in the pKa, leading to modest differences in BDE among the three compounds. The effects of these changes were further probed by evaluating the rates of HAT by the corresponding Cu(III)–hydroxide complexes from substrates with C–H bonds of variable strength. These studies revealed an overarching linear trend in the relationship between the log k (where k is the second-order rate constant) and the ΔH of reaction. Additional subtleties in measured rates arise, however, that are associated with variations in hydrogen-atom abstraction barrier heights and tunneling effciencies over the temperature range from −80 to −20 °C, as inferred from measured kinetic isotope effects and corresponding electronic-structure-based transition-state theory calculations.

Oxomanganese(V) species have been implicated in a variety of biological and synthetic processes, including as a key reactive center within the oxygen-evolving complex in photosynthesis. Nearly all mononuclear MnV–oxo complexes have tetragonal symmetry, producing low-spin species. A new MnV–oxo complex that is high-spin is now reported, which was prepared from a well-characterized oxomanganese(III) complex having trigonal symmetry. Titration experiments with [FeCp2]+ were monitored with optical and electron paramagnetic resonance (EPR) spectroscopies and support a high-spin oxomanganese(V) complex formulation. The parallel-mode EPR spectrum has a distinctive S = 1 signal at g = 4.01 with a six-line hyperfine pattern having Az = 113 MHz. The presence of an oxo ligand was supported by resonance Raman spectroscopy, which revealed O-isotope sensitive peaks at 737 cm−1 and 754 cm−1 assigned as a Fermi doublet centered at 746 cm−1(Δ18O = 31 cm−1). Kβ Mn X-ray emission spectra showed Kβ' and Kβ1,3 bands at 6475.92 and 6490.50 eV, which are characteristic of a high-spin MnV center.

Insight into copper-oxygen species proposed as intermediates in oxidation catalysis is provided by the identification of a Cu(II)-superoxide complex supported by a sterically hindered, pyridinedicarboxamide ligand. A tetragonal, end-on superoxide structure is proposed based on DFT calculations and UV-vis, NMR, EPR, and resonance Raman spectroscopy. The complex yields a trans-1,2-peroxodicopper(II) species upon reaction with [(tmpa)Cu(CH 3 CN)]OTf, and, unlike other known Cu(II)-superoxide complexes, acts as a base rather than an electrophilic (Hatom abstracting) reagent in reactions with phenols.An important first step in copper-promoted aerobic oxidations in biology1 and catalysis2 is the formation of a 1:1 Cu/O 2 adduct, in which the O 2 molecule is activated for subsequent reactions, either with substrate or to form different copper-oxygen species. In one approach aimed at understanding such adducts, synthetic 1:1 Cu/O 2 complexes have been targeted for detailed structural, spectroscopic, and reactivity studies.3 To date, three types have been identified: (a) end-on, triplet Cu(II)-superoxos supported by tetradentate tripodal4 or, in one case, tridentate5 N-donor ligands, (b) side-on, singlet Cu(II)-superoxos supported by facially coordinating tris(pyrazolyl)hydroborates,6 and (c) side-on, singlet Cu(III)-peroxos supported by strongly electron-donating, bidentate β-diketiminates or anilido-imines.3 , 7 A key finding from reactivity studies of type (a) compounds is that they are electrophilic, with the ability to perform biologically relevant H-atom abstractions from phenols and weak C-H bonds; 4b , 5 investigations of the reactivity of type (b) compounds have not been reported, and type (c) compounds are relatively unreactive with external organic substrates. Herein we report that in seeking to expand the repertoire of available 1:1 Cu/O 2 structures for comparative evaluations, we have discovered an end-on Cu(II)-superoxide complex that displays unique characteristics, including a tetragonal geometry and non-electrophilic reactivity.Inspired by a recent report,8 we prepared complex 1 (Scheme 1) by treating N,N′-bis(2,6-diisopropylphenyl)-2,6-pyridinedicarboxamide9 with NaOMe followed by CuCl 2 in the presence of CH 3 CN.10 In methanol or THF solution, 1 is green, whereas it is red-brown in the presence of CH 3 CN or pyridine. Consistent with this solvatochromism (Figure S2), attempts to obtain crystals of 1 suitable for X-ray crystallographic analysis were complicated by apparent CH 3 CN lability. The addition of 4-tBu-pyridine, however, yielded X-ray quality dark red crystals of 2 (Scheme 1). The complex is square planar with a geometry similar to other known 2,6-pyridinedicarboxamide Cu(II) complexes.11 wtolman@umn.edu. Supporting Information Available: Experimental procedures, spectra, computational details (PDF), and CIF. This material is available free of charge via the Internet at http://pubs.acs.org. NIH Public AccessAuthor Manuscript J Am Chem Soc. Author manuscript; available in PMC 2011 Nov...

Ionic liquids (ILs) have been established as effective promoters for the electrocatalytic upconversion of CO2 to various commodity chemicals. Imidazolium ([Im]+) cathode combinations have been reported to selectively catalyze the 2e−/2H+ reduction of CO2 to CO. Recently our laboratory has reported energy-efficient systems for CO production featuring inexpensive bismuth-based cathode materials and ILs comprised of 1,3-dialkylimidazolium cations. As part of our ongoing efforts to understand the factors that drive CO2 reduction at electrode interfaces, we sought to evaluate the catalytic performance of alternative ILs in combination with previously described Bi cathodes. In this work, we demonstrate that protic ionic liquids (PILs) derived from 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) effectively promote the electrochemical reduction of CO2 to formate (HCOO−) with high selectivity. The use of PILs comprised of the conjugate acid of DBU, [DBU-H]+, efficiently catalyzed the reduction of CO2 to HCOO− (FEFA ≈ 80%) with significant suppression of CO production (FECO ≈ 20%) in either MeCN or MeCN/H2O (95/5) solution. When they were used in combination with [DBU-H]+-based PILs, Bi-based cathodes achieved current densities for CO2 reduction (jtot ≈ 25–45 mA/cm2) that are comparable to or greater than those reported with imidazolium ILs such as [BMIM]PF6. As we demonstrate herein, the selectivity of the 2e− reduction of CO2 toward HCOO− or CO can be dictated through the choice of the IL promoter present in the electrolysis solution, even in cases in which the same electrocatalyst material is studied. These findings highlight the tunability of bismuth/IL systems for the electrochemical reduction of CO2 with high efficiency and rapid kinetics.

Into the metalloligand Cr[N(o-(NCH2P(iPr)2)-C6H4)3] (1, CrL) was inserted a second chromium atom to generate the dichromium complex Cr2L (2), which is a homobimetallic analogue of the known MCrL complexes, where M is manganese (3) or iron (4). The cationic and anionic counterparts, [MCrL]+ and [MCrL]−, respectively, were targeted, and each MCr pair was isolated in at least one other redox state. The solid-state structures of the [MCrL]+,0,− redox members are essentially the same, with ultrashort metal–metal bonds between 1.96 and 1.74 Å. The formal shortness ratios (r) of these interactions are between 0.84 and 0.74 and are interpreted as triple to quintuple metal–metal bonds with the aid of theory. The trio of (d–d)10 species [Cr2L]− (2red), MnCrL (3), and [FeCrL]+ (4ox) are S = 0 diamagnets. On the basis of M—Cr bond distances and theoretical calculations, the strength of the metal–metal bond across the (d–d)10 series increases in the order Fe < Mn < Cr. The methylene protons in the ligand are shifted downfield in the 1H NMR spectra, and the diamagnetic anisotropy of the metal–metal bond was calculated as −3500 × 10−36, −3900 × 10−36, and −5800 × 10−36 m3 molecule−1 for 2red, 3, and 4ox respectively. The magnitude of diamagnetic anisotropy is, thus, affected more by bond polarity than by bond order. A comparative vis–NIR study of quintuply bonded 2red and 3 revealed a large red shift in the δ4 → δ3δ* transition energy upon swapping from the (Cr2)2+ to the (MnCr)3+ core. Complex 2red was further investigated by resonance Raman spectroscopy, and a band at 434 cm−1 was assigned as the Cr—Cr bond vibration. Finally, 4ox exhibited a Mössbauer doublet with an isomer shift of 0.18 mm/s that suggests a primarily Fe-based oxidation to Fe(I).