Dinuclear Zinc Complexes Based on Parallel β-Diiminato Binding Sites: Syntheses, Structures, and Properties as CO₂/Epoxide Copolymerization Catalysts

With the background that β-diketiminato zinc complexes efficiently catalyze the CO2/epoxide copolymerization via a mechanism involving two catalyst molecules, a ligand system containing two parallel β-diiminato binding sites linked by a xanthene backbone ([RXanthdim]2- with residues R = 2,3-dimethylphenyl and 2,4-difluorophenyl at the iminato units, respectively) was investigated with respect to its zinc coordination chemistry. The corresponding diimines [RXanthdim]H2 were treated with diethylzinc to yield the complexes [Me2C6H3Xanthdim](ZnEt)2, 4, and [F2C6H3Xanthdim](ZnEt(thf))2, 5, respectively. In order to convert these compounds into polymerization catalysts, they were subsequently treated with SO2, which indeed resulted in the corresponding ethylsulfinates. Due to aggregation via intermolecular bridging of ethylsulfinate ligands, the product after the reaction of 5 represents an insoluble coordination polymer. Aggregation does take place also for the primary product obtained from the reaction of 4, as evidenced by the isolation of the tetramer [{[Me2C6H3Xanthdim]Zn2(μ-O2SEt)}μ-O2SEt]4, 6, and the precipitation of an insoluble solid on storing of such mixtures. 4, 5, and 6 display moderate activities as catalysts for the copolymerization of cyclohexene oxide and CO2. A bimodal molecular weight distribution points to two effective mechanisms, one of which probably involves two cooperating Zn centers as anticipated. Possible structural reasons for these catalytic results are discussed.基于β-二亚胺基锌配合物（β-diketiminato zinc complexes）可通过涉及双催化剂分子的反应机制高效催化CO₂与环氧化物共聚的研究背景，本研究针对一种由氧杂蒽骨架连接两个平行β-二亚胺基配位位点的配体体系（[RXanthdim]²⁻，其亚胺基单元上的取代基R分别为2,3-二甲基苯基与2,4-二氟苯基）的锌配位化学展开了探究。相应的二亚胺[RXanthdim]H₂分别与二乙基锌反应，得到配合物[Me₂C₆H₃Xanthdim](ZnEt)₂（化合物4）与[F₂C₆H₃Xanthdim](ZnEt(thf))₂（化合物5）。为将上述化合物转化为聚合催化剂，后续将其与二氧化硫（SO₂）反应，成功得到了对应的乙基亚磺酸盐产物。由于乙基亚磺酸盐配体发生分子间桥联作用，化合物5的反应产物为不溶性配位聚合物。化合物4的初始反应产物同样存在聚集现象：我们成功分离得到了四聚体[{[Me₂C₆H₃Xanthdim]Zn₂(μ-O₂SEt)}μ-O₂SEt]₄（化合物6），且该类反应混合物在储存过程中会析出不溶性固体，这也佐证了聚集现象的存在。化合物4、5与6均可作为环氧环己烷与CO₂共聚反应的催化剂，展现出中等催化活性。产物呈现双峰分子量分布，表明该共聚反应存在两种有效催化机制，其中一种正如预期那般涉及两个协同作用的锌中心。本文还对导致上述催化结果的潜在结构因素进行了讨论。