New Pincer-Type Diphosphinito (POCOP) Complexes of Nickel

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 2a−c 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).