Switchable Polymerization Catalysts

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Synthesis and crystal structures of 5,5′-(propane-2,2-diyl)bis(2-hydroxybenzaldehyde) and 5,5′-(propane-2,2-diyl)bis(2-hydroxyisophthalaldehyde)

The title compounds 5,5′-(propane-2,2-diyl)bis(2-hydroxybenzaldehyde), C17H16O4, (1), and 5,5′-(propane-2,2-diyl)bis(2-hydroxyisophthalaldehyde), C19H16O6, (2), crystallize with one molecule in the asymmetric unit. In molecule (1), a >C(CH3)2 group bridges two nearly planar salicylaldehyde groups [r.m.s deviations = 0.010 (1) and 0.025 (2) Å], each comprising a planar phenyl ring bonded with a hydroxyl and an aldehyde group. Similarly, compound (2) has the same bridging group, but it connects two nearly planar appendants [r.m.s deviations = 0.034 (1) and 0.035 (1) Å], each comprising a phenyl ring bonded with a hydroxyl and two aldehyde groups. Molecule (1) exhibits a bridge angle of 109.5 (2)° with the salicylaldehyde planes subtending a dihedral angle of 88.4 (1)°. In contrast, molecule (2) presents a bridge angle of 108.9 (2)° with its appendants subtending a dihedral angle of 79.6 (3)°. Both molecules exhibit two intramolecular O—H...O hydrogen bonds involving the phenolic H atoms and carboxyl O-atom acceptors. In the crystal of (2), O—H...O hydrogen bonds between one of the hydroxyl H atoms and a carboxyl O atom from a symmetry-related molecule form a chain along [10\overline{1}]. In addition, (2) exhibits a strong visible luminescence when excited with ultraviolet radiation

A redox-switchable ring-closing metathesis catalyst

A Ru(II) complex ligated to a quinone-annulated N-heterocyclic carbene (NHC) was synthesized as a redox-active analogue of the Hoveyda-Grubbs II generation catalyst. The complex exhibited a single reversible reduction with a E-1/2 of -0.63 V (vs. SCE), and was successfully reduced and then oxidized with high fidelity using chemical reagents. While the catalyst facilitated a range of ring-closing metathesis (RCM) reactions in its neutral state, its activity was inhibited upon the introduction of a suitable reducing reagent. A series of density functional theory calculations revealed that the differences in catalytic activity may be attributed to the stronger donating ability of the reduced NHC ligand which stabilized a ruthenacyclobutane intermediate and thus suppressed the rate-determining retro-[2 + 2] cycloaddition step of the underlying RCM mechanism