The tetramethylene-bridged PMes2/B(C6F5)2 frustrated Lewis pair (FLP) 8 was prepared by hydroboration of Mes2P-(CH2)2CH═CH2 with HB(C6F5)2. It is an active FLP that splits dihydrogen under mild conditions and, consequently, serves as a metal-free hydrogenation catalyst for a variety of substrates. It also reacts typically with terminal acetylenes. The C4-bridged FLP 23 was prepared by HB(C6F5)2 hydroboration of 1-dimesitylphosphino-2-vinylferrocene. It represents a rare example of a FLP where the equilibrium between the open form and the closed internal P/B adduct form is experimentally observable. It also shows a variety of typical FLP reactions, including dihydrogen splitting. The FLPs 8 and 23 (open form) and many precursors and products were characterized by X-ray diffraction.
Ring-opening
polymerization (ROP) is a powerful approach to prepare
well-defined polymers. Herein, a one-component Lewis pair strategy
was adopted, and two delicate Lewis pairs 1–2 were rationally designed and conveniently synthesized. Lewis
pairs 1–2 featured an electropositive
phosphonium cation, two electrophilic boron centers involving two
9-borabicyclo[3.3.1]nonane moieties, and a nucleophilic halide (Br– or I–). Lewis pair 1–2-mediated ROP of propylene oxide (PO) exhibited
activity (turnover frequency = 3600 h–1) with a
living characteristic and low catalyst feeding (0.003 mol %) and afforded
an extremely high-molecular-weight α-halide/ω-OH polypropylene
oxide (PPO, M
n = 489.5 kg/mol) under mild
conditions (−30 to 0 °C). We highlight the success of
the one-component Lewis pair strategy to achieve living PO polymerization
and extremely high-molecular-weight PPO attributing to two important
factors: better control (less transfer reaction) and rapid polymerization
kinetics, which takes full advantage of the unique structures and
the synergistic effect of 1–2. A
preliminary investigation demonstrated that water can be added to
the polymerization setup and served as a chain transfer reagent, leading
to a controlled polymerization and producing α-hydroxy/ω-OH
PPO. The present study provides an insightful mechanistic understanding
of the designed one-component Lewis pair systems in epoxide homopolymerization
based on spectroscopic data analysis and density functional theory
calculations.
A series of chloride-bridged unsymmetrical Fe(ii)-HS/Fe(ii)-LS binuclear complexes has been developed, which can efficiently catalyze isoprene polymerization with 0.00025 mol% loading, delivering ultra-high molecular weight polymers.
Aminopyridine iron(ii) chloride complexes coordinating a type of flexible ligand are reported as precursors for controlled polymerization of isoprenes.
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