摘要
Ni(Ⅱ)与含氮氧配位原子的大配体形成的催化剂,可以不用助催化剂即有较高的烯烃聚合催化活性.不用助催化剂的催化机理不是阳离子催化反应机理,而是电中性催化机理.应用密度泛函理论方法,在B3LYP/LANL2MB水平上,研究了Ni(Ⅱ)催化剂催化乙烯聚合的电中性反应机理.计算结果表明,电中性催化机理类似于阳离子催化机理.电中性催化机理中,初始带空位的中性四面体络合物能量较低,是避免使用助催化剂的必要条件;由于Ni原子带正电荷较小,乙烯与催化剂共平面的的结构不是平衡几何构型,而是过渡态.在B3LYP/LANL2MB在水平上优化得到一个乙烯与催化剂共平面的二级鞍点结构.
When bulk Ni(Ⅱ)salicyladimine complexes react as a single component polyethylene catalysts, they may react via neutral reaction mechanism. Computational study was carried out for the neutral mechanism at B3LYP/LANL2MB level. The results show that the neutral mechanism is similar to a catalytic cation mechanism. The neutral mechanism starts from a neutral complex of square-planar structure with a vacant site. An ethylene molecule then attacks the vacant site perpendicular to the equatorial MeNi(NC-CO)] plane and then rotates into the plane as a transition state for insertion reaction. After insertion reaction, a β-agostic hydrogen agostic bond is formed between Ni and β-C so as to assist the regeneracy of a new vacant site and to realize chain propagation. For chain propagation, another ethylene replaces the hydrogen atom in the hydrogen agostic bond. The calculations show that agostic bonds play an important role in activating the reactant in a catalytic process. But there are some differences in the neutral mechanism as compared with the cationic mechanism. The initial square planar complex with the vacant site of the neutral reaction mechanism possesses lower energy so that it needs no cocatalyst; In contrast with cationic mechanism, there is no equilibrium geometry of ethylene in co-plane with the plane of the catalyst for the neutral mechanism because of lower positive charge on the nickel atom in the catalyst. In the neutral mechanism, the ethylene molecule and the catalyst are not coordinated in co-plane with each other as an intermediate, but as a transition state and a saddle point of second order before the insertion reaction.
出处
《分子科学学报》
CAS
CSCD
2004年第1期43-48,共6页
Journal of Molecular Science
基金
黑龙江省教育厅科学技术研究基金资助项目
关键词
电中性催化机理
Ni(Ⅱ)催化剂
乙烯
聚合
密度泛函
neutral catalytic mechanism
nickel(Ⅱ) catalyst
ethylene
polymerization
density functional theory
