Nickel complexes with functional zwitterionic N,O-benzoquinonemonoimine-type ligands:: Syntheses, structures, and catalytic oligomerization of ethylene

The zwitterionic N, N'-dialkyl-2-amino-5-alcoholate-1,4-benzoquinonemonoiminium derivatives [C6H2(-NHCH2CH2X)(2)(-O)(2)] (X = NMe2, 9; X = NHEt, 10; X = OMe, 11), previously prepared from 4,6-diaminoresorcinol by a transamination reaction, and 12 (X = N(CH2CH2)(2)O) behave as tridentate ligands when reacted with [Ni(acac)(2)] to form the corresponding octahedral Ni(II) 2:1 complexes [Ni {C6H2 (= NCH2CH2X)O (=O)(NHCH2CH2X)}(2)] (13-16), respectively. In contrast, ligand 9 reacted with NiCl2 center dot 6H(2)O in a tandemlike manner to afford the stabilized Ni(II) zwitterionic organometalate 1:1 complex [NiCl2 {C6H2 (=NCH2CH2NMe2)O(=O)(NHCH2CH2NHMe2)}] (17). The bonding parameters of complexes 13 center dot H2O center dot CH2Cl2 and 17, determined by X-ray diffraction, and the conformation of the ligands around the nickel center as well as the supramolecular arrangements are discussed and compared with those of their previously reported Zn(II) analogues. Complexes 13-17 were tested in catalytic ethylene oligomerization with MAO and AlEtCl2 as cocatalysts. Complex 17 yielded the highest turnover frequencies, with values up to 20 300 and 48 200 mol of C2H4/((mol of Ni) h), in the presence of 400 equiv of MAO and 10 equiv of AlEtCl2, respectively. Selectivities for ethylene dimers were slightly higher when using MAO: 94% (14 in the presence of 100 equiv of MAO) and 90% (14 in the presence of 6 equiv of AlEtCl2). The selectivities for 1-butene within the C-4 fraction were much higher when using MAO as cocatalyst, with values up to 68% (15 in the presence of 100 equiv of MAO). The fact that 17, which contains only one tridentate ligand per nickel center, leads to higher activities than 13-16 underlines the importance of the metal center accessibility in the catalytic process.