Low-Energy and Long-Lived Emission from Polypyridyl Ruthenium(II) Complexes Having A Stable-Radical Substituent

Novel polypyridyl ruthenium­(II) complexes having a 2,2′-bipyridine (bpy) derivative which possesses a 1,5-dimethyl-6-oxoverdazyl radical (OV) group as a stable-radical substituent were designed and synthesized. The radical–ruthenium­(II) complexes showed low-energy/intense MLCT absorption and low-energy/long-lived MLCT emission, and these characteristics of the complexes were explained by the electron-withdrawing nature of the OV group. Furthermore, the radical-substituent effects were enhanced by the presence of the electron-donating methyl groups at the 4- and 4′-positions of bpy in the ancillary ligands. The detailed electrochemical, spectroscopic, and photophysical properties of the complexes were discussed in terms of the systematic modification of the second coordination sphere in the main and ancillary ligands

Anion-Controlled Assembly of Four Manganese Ions: Structural, Magnetic, and Electrochemical Properties of Tetramanganese Complexes Stabilized by Xanthene-Bridged Schiff Base Ligands

The reaction of manganese­(II) acetate with a xanthene-bridged bis­[3-(salicylideneamino)-1-propanol] ligand, H₄L, afforded the tetramanganese­(II,II,III,III) complex [Mn₄(L)₂(μ-OAc)₂], which has an incomplete double-cubane structure. The corresponding reaction using manganese­(II) chloride in the presence of a base gave the tetramanganese­(III,III,III,III) complex [Mn₄(L)₂Cl₃(μ₄-Cl)­(OH₂)], in which four Mn ions are bridged by a Cl^– ion. A pair of L ligands has a propensity to incorporate four Mn ions, the arrangement and oxidation states of which are dependent on the coexistent anions

Anion-Controlled Assembly of Four Manganese Ions: Structural, Magnetic, and Electrochemical Properties of Tetramanganese Complexes Stabilized by Xanthene-Bridged Schiff Base Ligands

The reaction of manganese­(II) acetate with a xanthene-bridged bis­[3-(salicylideneamino)-1-propanol] ligand, H₄L, afforded the tetramanganese­(II,II,III,III) complex [Mn₄(L)₂(μ-OAc)₂], which has an incomplete double-cubane structure. The corresponding reaction using manganese­(II) chloride in the presence of a base gave the tetramanganese­(III,III,III,III) complex [Mn₄(L)₂Cl₃(μ₄-Cl)­(OH₂)], in which four Mn ions are bridged by a Cl^– ion. A pair of L ligands has a propensity to incorporate four Mn ions, the arrangement and oxidation states of which are dependent on the coexistent anions

Single-Step versus Stepwise Two-Electron Reduction of Polyarylpyridiniums: Insights from the Steric Switching of Redox Potential Compression

Contrary to 4,4′-dipyridinium (i.e., archetypal methyl viologen), which is reduced by two single-electron transfers (stepwise reduction), the 4,1′-dipyridinium isomer (so-called “head-to-tail” isomer) undergoes two electron transfers at apparently the same potential (single-step reduction). A combined theoretical and experimental study has been undertaken to establish that the latter electrochemical behavior, also observed for other polyarylpyridinium electrophores, is due to potential compression originating in a large structural rearrangement. Three series of branched expanded pyridiniums (EPs) were prepared: <i>N</i>-aryl-2,4,6-triphenylpyridiniums (Ar-<b>TP</b>), <i>N</i>-aryl-2,3,4,5,6-pentaphenylpyridiniums (Ar-<b>XP</b>), and <i>N</i>-aryl-3,5-dimethyl-2,4,6-triphenylpyridinium (Ar-<b>DMTP</b>). The intramolecular steric strain was tuned via <i>N</i>-pyridinio aryl group (Ar) phenyl (Ph), 4-pyridyl (Py), and 4-pyridylium (qPy) and their bulky 3,5-dimethyl counterparts, xylyl (Xy), lutidyl (Lu), and lutidylium (qLu), respectively. Ferrocenyl subunits as internal redox references were covalently appended to representative electrophores in order to count the electrons involved in EP-centered reduction processes. Depending on the steric constraint around the <i>N</i>-pyridinio site, the two-electron reduction is single-step (Ar = Ph, Py, qPy) or stepwise (Ar = Xy, Lu, qLu). This steric switching of the potential compression is accurately accounted for by ab initio modeling (Density Functional Theory, DFT) that proposes a mechanism for pyramidalization of the N_pyridinio atom coupled with reduction. When the hybridization change of this atom is hindered (Ar = Xy, Lu, qLu), the first reduction is a one-electron process. Theory also reveals that the single-step two-electron reduction involves couples of redox isomers (electromers) displaying both the axial geometry of native EPs and the pyramidalized geometry of doubly reduced EPs. This picture is confirmed by a combined UV–vis–NIR spectroelectrochemical and time-dependent DFT study: comparison of in situ spectroelectrochemical data with the calculated electronic transitions makes it possible to both evidence the distortion and identify the predicted electromers, which play decisive roles in the electron-transfer mechanism. Last, this mechanism is further supported by in-depth analysis of the electronic structures of electrophores in their various reduction states (including electromeric forms)