A series of silylium cations [(R1R2R3Si)2H]+[B(C6F5)4]– (1–6) (1: R1 = R2 = R3 = Et; 2: R1 = R3 = H, R2 = Ph; 3: R1 = R3 = Me,...

A scandium dication active species [(IPr)­Sc­(μ-CH2SiMe3)­(μ-CH2CHMe2)­Al i Bu2]2+[B­(C6F5)4]2 – (IPr = (2,6-C6H3 i Pr2NCH)2C) in situ generated from the reaction of an N-heterocyclic carbene-ligated scandium trialkyl complex (IPr)­Sc­(CH2SiMe3)3 (2) with 1–3 equiv of cocatalyst [Ph3C]­[B­(C6F5)4] and an excess of Al i Bu3 exhibits unprecedentedly high activity (up to 2.2 × 106 g·molSc –1·h–1) and syndiotactic selectivity (rrrr > 99%) in the polymerization of o-methoxystyrene (oMOS) and its silyloxy- or fluorine-substituted derivatives, affording syndiotactic poly­(oMOS)­s and their silyloxy or fluorine-substituted derivatives with ultrahigh molecular weight (M n up to 4.6 × 106) and moderate molecular weight distributions (M w/M n = 1.37–2.21) unavailable by the traditional catalysts. Based on in situ NMR spectroscopy, matrix-assisted laser desorption/ionization-time of flight (MALDI-TOF) spectroscopy, and density functional theory (DFT) calculations, a plausible coordination polymerization mechanism has been proposed for the syndiotactic polymerization of oMOS by such a Sc dication active species.

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The reversible transformation of a nanohelix is one of the most exquisite and important phenomena in nature. However, nanomaterials usually fail to twist into helical crystals. Considering the irreversibility of the previously studied twisting forces, the reverse process (untwisting) is more difficult to achieve, let alone the retwisting of the untwisted crystalline nanohelices. Herein, we report a new reciprocal effect between molecular geometry and crystal structure which triggers a twisting-untwisting-retwisting cycle for tri-cobalt salicylate hydroxide hexahydrate. The twisting force stems from competition between the condensation reaction and stacking process, different from the previously reported twisting mechanisms. The resulting distinct nanohelices give rise to unusual structure elasticity, as reflected in the reversible change of crystal lattice parameters and the mutual transformation between the nanowires and nanohelices. This study proposes a fresh concept for designing reversible processes and brings a new perspective in crystallography.