Synthesis of (Adamantylimido)vanadium(V) Dimethyl Complex Containing (2-Anilidomethyl)pyridine Ligand and Selected Reactions: Exploring the Oxidation State of the Catalytically Active Species in Ethylene Dimerization

V­(NAd)­Me2(L) [2a, L = 2-ArNCH2(C5H4N), Ar = 2,6-Me2C6H3], prepared
from V­(NAd)­Cl2(L) (1) by reaction with LiMe
(2.0 equiv), exhibited remarkable catalytic
activities for ethylene dimerization in the presence of MAO affording
1-butene with high selectivity [TOF = 1 120 000–1 530 000
h–1 (311–425 s–1), C4′ = 97.1–98.4%], and the catalyst
performances (activity, selectivity) were similar to those by the
dichloride analogue (1) under the same conditions. The
dimethyl complex (2a) reacted with 1.0 equiv of R′OH
to yield the mono alkoxide complexes, V­(NAd)­Me­(OR′)­(L) [R′
= OC­(CF3)3 (3a), OC­(CH3)­(CF3)2 (3b), OC­(CH3)3 (3c)], and structures of these complexes
(3a–c) and 2a were determined
by X-ray crystallography. Reactions of 2a with [Ph3C]­[B­(C6F5)4] in Et2O and 3c with B­(C6F5)3 in THF afforded the corresponding cationic complexes confirmed by
NMR spectra. Both NMR and V K-edge XANES analysis of the toluene or
toluene-d8 solution of 1 and 2a did not show any significant changes in the oxidation state
upon addition of MAO, Me2AlCl, or Et2AlCl (10
equiv). Resonances ascribed to formation of the other vanadium­(V)
species were observed in the 51V NMR spectra, and no significant
differences in the XANES spectra (V–K pre-edge peaks and edge)
were observed from 1 or 2a upon addition
of Al cocatalyst. Taking into account these results and others, it
is thus suggested that cationic vanadium­(V) alkyl/hydride species
play a role in this catalysis.

配合物V­(NAd)­Me₂(L) [2a，其中L = 2-ArNCH₂(C₅H₄N)，Ar = 2,6-二甲基苯基(2,6-Me₂C₆H₃)]由V­(NAd)­Cl₂(L)（化合物1）与甲基锂（LiMe，2.0当量）反应制得。该配合物在甲基铝氧烷（MAO）存在下对乙烯二聚反应展现出优异的催化活性，可高选择性生成1-丁烯[转换频率（TOF）= 1120000~1530000 h⁻¹（311~425 s⁻¹），C4组分选择性=97.1%~98.4%]，且相同条件下该催化剂的性能（活性与选择性）与二氯化物前驱体（1）相当。二甲基配合物（2a）与1.0当量的R'OH反应，可得到单烷氧基配合物V­(NAd)­Me­(OR′)­(L) [R' = OC(CF₃)₃（3a）、OC(CH₃)(CF₃)₂（3b）、OC(CH₃)₃（3c）]；通过X射线单晶衍射测定了配合物2a以及3a~3c的晶体结构。2a在乙醚（Et₂O）中与三苯碳鎓四(五氟苯基)硼酸盐（[Ph₃C][B(C₆F₅)₄]）的反应，以及3c在四氢呋喃（THF）中与三(五氟苯基)硼（B(C₆F₅)₃）的反应，均生成了相应的阳离子配合物，相关结果经核磁共振谱（NMR）表征确认。对化合物1和2a的甲苯或氘代甲苯（toluene-d₈）溶液进行核磁共振谱与钒K边X射线吸收近边结构谱（V K-edge XANES）分析后发现，加入10当量的MAO、二甲基氯化铝（Me₂AlCl）或二乙基氯化铝（Et₂AlCl）后，钒的氧化态未发生显著变化。在51V核磁共振谱中可观察到归属于其他钒(V)物种生成的共振峰；而加入铝助催化剂后，化合物1或2a的XANES谱图（钒K边预峰与吸收边）未出现显著差异。综合上述结果及其他相关研究，推测阳离子型钒(V)烷基/氢化物物种在该催化反应中发挥关键作用。