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New Pincer-Type Diphosphinito (POCOP) Complexes of Nickel

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posted on 03.04.2020, 15:47 by Valerica Pandarus, Davit Zargarian
This report describes the synthesis, characterization, and reactivities of a new series of pincer-type nickel complexes based on the diphosphinito (POCOP) ligands 1,3-(i-Pr2PO)2C6H4, 1, and (i-Pr2POCH2)2CH2, 2. Reacting these ligands with (THF)1.5NiCl2, (THF)2NiBr2, or (CH3CN)nNiX2 (X = Br, n = 2; I, n = 3) gives pincer-type complexes by metalation of the central carbon atom. The yields of these (POCOP)NiX complexes vary with the type of ligand and Ni precursor used, as well as the reaction conditions. In general, the aromatic ligand 1 was metalated more readily to give excellent yields of the pincer complexes {2,6-(i-Pr2PO)2C6H3}NiX (X = Cl, 1a, 85% yield; X = Br, 1b, 95% yield; X = I, 1c, 85% yield), especially when the reaction mixture was heated to 60 °C for 1 h in the presence of 1 equiv of 4-dimethylaminopyridine (DMAP). The analogous reactions of ligand 2 were more sluggish and required refluxing in toluene to give {(i-Pr2POCH2)2CH}NiX (X = Cl, 2a, 33% yield; X = Br, 2b, 93% yield; X = I, 2c, 70% yield). Displacement of Br from 1b and 2b by Ag(O3SCF3), acetonitrile, or acrylonitrile gave, respectively, the neutral Ni−O3SCF3 derivatives 1d and 2d and the cationic adducts of CH3CN (1e and 2e) and CH2CHCN (1f and 2f). Reacting 1b with MeMgCl or EtMgCl gave the corresponding Ni−alkyl derivatives 1g and 1h, respectively, whereas alkylation of complexes 2ac was unsuccessful. The POCsp3OP-based complexes 2a and 2b could be oxidized to paramagnetic, 17-electron species {(i-Pr2POCH2)2CH}NiIIIX2 (X = Cl, 2i; Br, 2j). Solid-state structures are reported for Ni−halide derivatives, the neutral complexes 2d, 1g, and 1h, the cationic adduct 1e, and the Ni(III) derivative 2i. The cationic acrylonitrile derivative 1f promotes the Michael addition of morpholine, cyclohexyl amine, or aniline to acrylonitrile, methacrylonitrile, or crotonitrile, whereas the paramagnetic NiIII complex 2j promotes the addition of CCl4 to methyl acrylate, methyl methacrylate, styrene, 4-methylstyrene, acrolein, and acrylonitrile (Kharasch reaction).