Compounds (2‐(3,5‐dimethyl‐1H‐pyrazol‐1‐yl)ethyldiphenylphosphinite (L1), 2‐(3,5‐di‐tert‐butyl‐1H‐pyrazol‐1‐yl)ethyldiphenylphosphinite (L2), and 2‐(3,5‐diphenyl‐1H‐pyrazol‐1‐yl)ethyldiphenylphosphinite (L3) were prepared using the synthetic routes reported in literature. These compounds were reacted with [NiCl2(DME)2] or [NiBr2(DME)2] under appropriate reaction conditions to afford six new nickel(II) compounds ([NiCl2(L1)] (1), [NiCl2(L2)] (2), [NiCl2(L3)] (3), [NiBr2(L1)] (4), [NiBr2(L2)] (5) and [NiBr2(L3)] (6)). The new nickel(II) pre‐catalysts catalyze the oligomerization of ethylene, in the presence of ethylaluminium dichloride as co‐catalyst, to produce butenes, hexenes, octenes and higher carbon chain ethylene oligomers with very little Friedel‐Crafts alkylation products when the reactions were run in toluene.
The mechanisms of oxidation of olefins with MoO 2 Cl 2 and WO 2 Cl 2 are studied with DFT. The formation of epoxide from these reactions is not very feasible by any of the postulated paths. If the epoxide precursor will form at all, it will arise via initial [3+2] O,Cl addition of ethene to MoO 2 Cl 2 and WO 2 Cl 2 to form an intermediate, followed by rearrangement to form the precursor, from which the epoxide can be generated by hydrolysis. The chlorohydrin precursor was also found to originate from [3+2] O,Cl addition of ethene to MO 2 Cl 2. The results also indicate that a dichloride is not a likely product in the oxidation of ethylene by molybdyl chloride. However, in the case of WO 2 Cl 2 , the formation of a dichloride may not be precluded. The formation of acetaldehyde and vinyl alcohol from the oxidation of ethylene does not appear energetically feasible with MoO 2 Cl 2 , but appears thermodynamically plausible with WO 2 Cl 2. Thus, the oxidation of ethylene with MoO 2 Cl 2 will most likely lead to the formation of chlorohydrins predominantly via [3+2] O,Cl addition; oxidation with WO 2 Cl 2 may also form chlorohydrins, but only extremely slowly. The oxyhalides MO 2 Cl 2 become weaker oxidants in the order CrO 2 Cl 2 >>MoO 2 Cl2>WO 2 Cl 2. Corresponding to this, reactions involving reduction of the metal [3 + 2] and [2+1] show a sharp increase in barrier going from Cr to W; reactions without a change in metal oxidation state ([2 + 2]) show much smaller variations, which are possibly mainly determined by sterics.
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