Potential Hemilabile (Imino)pyridine Palladium(II) Complexes as Selective Ethylene Dimerization Catalysts: An Experimental and Theoretical Approach

The (imino)­pyridine ligands bearing
pendant arms 2-methoxy-N-(1-(pyridin-2-yl)­ethylidene)­ethanamine
(L1), 2-methoxy-N-((pyridin-2-yl)­methylene)­ethanamine
(L2), 3-methoxy-N-((pyridin-2-yl)­methylene)­propan-1-amine
(L3), N,N-diethyl-N-((pyridin-2-yl)­methylene)­ethane-1,2-diamine (L4), and N-((6-bromopyridin-2-yl)­methylene)-2-methoxyethanamine
(L5) were synthesized by condensation of the appropriate
aldehyde and the corresponding amines. Reactions of L1–L5 with [PdCl­(Me)­(cod)] afforded the corresponding
palladium complexes [PdCl­(Me)­(L1)] (1),
[PdCl­(Me)­(L2)] (2), [PdCl­(Me)­(L3)] (3), [PdCl­(Me)­(L4)] (4),
and [PdCl­(Me)­(L5)] (5) in good yields. Treatment
of the neutral complexes 1–5 with
one molar equivalent of Na­[BAr4] (Ar = 3,5-(CF3)2C6H3) led to in situ formation of the corresponding cationic compounds [Pd­(Me)­(L1)]+ (1a), [Pd­(Me)­(L2)]+ (2a), [Pd­(Me)­(L3)]+ (3a), [Pd­(Me)­(L4)]+ (4a), and [Pd­(Me)­(L5)]+ (5a). In the presence of ethylene, the catalyst systems 1–5/MAO and 1a–5a exhibited high catalytic activities in selective ethylene dimerization
producing mainly butenes. 1H NMR spectroscopy and DFT studies
provided valuable insight into the role of ligand architecture on
ethylene-binding energies, catalytic activities, and stabilities of
the resultant catalysts.

带有悬挂臂的(亚氨基)吡啶((imino)pyridine)配体，包括2-甲氧基-N-(1-(吡啶-2-基)亚乙基)乙胺(L1)、2-甲氧基-N-((吡啶-2-基)亚甲基)乙胺(L2)、3-甲氧基-N-((吡啶-2-基)亚甲基)丙-1-胺(L3)、N,N-二乙基-N-((吡啶-2-基)亚甲基)乙烷-1,2-二胺(L4)以及N-((6-溴吡啶-2-基)亚甲基)-2-甲氧基乙胺(L5)，均通过相应醛与对应胺的缩合反应合成。将L1~L5与[PdCl(Me)(环辛二烯(cod, cyclooctadiene))]反应，以良好产率得到相应的钯配合物[PdCl(Me)(L1)](1)、[PdCl(Me)(L2)](2)、[PdCl(Me)(L3)](3)、[PdCl(Me)(L4)](4)以及[PdCl(Me)(L5)](5)。用1摩尔当量的Na[BAr4]（Ar=3,5-双(三氟甲基)苯基）处理中性配合物1~5，可原位生成相应的阳离子化合物[Pd(Me)(L1)]⁺(1a)、[Pd(Me)(L2)]⁺(2a)、[Pd(Me)(L3)]⁺(3a)、[Pd(Me)(L4)]⁺(4a)以及[Pd(Me)(L5)]⁺(5a)。在乙烯存在条件下，催化剂体系1~5/甲基铝氧烷(MAO, methylaluminoxane)与1a~5a均展现出高催化活性，可高选择性催化乙烯二聚反应，主要产物为丁烯。通过¹H核磁共振光谱(¹H NMR)与密度泛函理论(DFT, Density Functional Theory)研究，深入阐明了配体骨架对乙烯结合能、催化活性及所得催化剂稳定性的调控作用。