Preparation and Characterization of a Reduced Chromium Complex via Vinyl Oxidative Coupling: Formation of a Self-Activating Catalyst for Selective Ethylene Trimerization

Reaction of the divalent [(t-Bu)NP(Ph)2N(t-Bu)]CrCl2Li(THF)2 (1) with 1 equiv of vinyl Grignard (CH2CH)MgCl reproducibly afforded the triangulo {π-[(t-Bu)N−P(Ph)2−N(t-Bu)]Cr}2(μ,μ′,η4,η4′-C4H4){σ-[(t-Bu)N−P(Ph)2−N(t-Bu)]Cr} (2) containing a σ-/π-bonded butadiene-diyl unit. The diene-diyl moiety was generated by an oxidative coupling and deprotonation of two vinyl anions. The crystal structure revealed that of the three chromium atoms, each bearing one NPN ligand, two are perpendicularly bonded to the two sides of the π-system of the butadiene-diyl residue in a sort of inverted sandwich type of structure. The third is instead coplanar with the doubly deprotonated C4 unit and σ-bonded to the two terminal carbon atoms. Despite the appearance as a Cr(II)/Cr(I) mixed valence species, DFT calculations have revealed that the structure of 2 consists of three divalent chromium atoms, while the additional electron resides on the π-system of the bridging organic residue. Complex 2 behaves as a single component selective catalyst for ethylene trimerization.

二价的[(叔丁基)NP(苯基)₂N(叔丁基)]氯化铬锂(四氢呋喃)₂加合物(1)与1当量的乙烯基格氏试剂((CH₂=CH)MgCl)反应，可稳定得到三角状结构的化合物(2)：{π-[(叔丁基)N−P(苯基)₂−N(叔丁基)]Cr}₂(μ,μ′,η⁴,η⁴′-C₄H₄){σ-[(叔丁基)N−P(苯基)₂−N(叔丁基)]Cr}，该化合物含有σ-/π键合的丁二烯二基单元。该丁二烯二基片段由两个乙烯基阴离子经氧化偶联与脱质子反应生成。晶体结构分析显示，三个铬原子各自配位一个NPN配体，其中两个铬原子以倒置三明治型结构垂直结合于丁二烯二基残基的π体系两侧；第三个铬原子则与双脱质子的C₄单元共平面，并通过σ键与两个末端碳原子相连。尽管该化合物表观上为Cr(II)/Cr(I)混合价态物种，但密度泛函理论（DFT）计算表明，化合物2的结构中包含三个二价铬原子，额外的电子分布于桥连有机残基的π体系上。化合物2可作为单组分选择性催化剂用于乙烯三聚反应。