Nickel complexes supported by quinoline-based ligands: synthesis, characterization and catalysis in the cross-coupling of arylzinc reagents and aryl chlorides or aryltrimethylammonium salts

Abstract

Lithium and nickel complexes bearing quinoline-based ligands have been synthesized and characterized. Reaction of 8-azidoquinoline with Ph₂PNHR (R = p-MeC₆H₄, Bu^t) affords N-(8-quinolyl)iminophosphoranes RNHP(Ph₂)N(8-C₉H₆N) (1a, R = p-MeC₆H₄; 1b, R = Bu^t. C₉H₆N = quinolyl)). Reaction of 1a with (DME)NiCl₂ generates a nickel complex [NiCl₂{N(8-C₉H₆N)P(Ph₂)NH(p-MeC₆H₄)}] (2a). Treatment of 1b with (DME)NiCl₂ and following with NaH produces [NiCl{(1,2-C₆H₄)P(Ph)(NHBu^t)N(8-C₉H₆N)}] (4). Complex 4 was also obtained by reaction of (DME)NiCl₂ with [Li{(1,2-C₆H₄)P(Ph)(NHBu^t)N(8-C₉H₆N)}] (5) prepared through lithiation of 1b. Reaction of 2-PyCH₂P(Ph₂)N(8-C₉H₆N) (6, Py = pyridyl) and PhNC(Ph)CH₂P(Ph₂)N(8-C₉H₆N) (8), respectively, with (DME)NiCl₂ yields two five-coordinate N,N,N-chelate nickel complexes, [NiCl₂{2-PyCH₂P(Ph₂)N(8-C₉H₆N)}] (7) and [NiCl₂{PhNC(Ph)CH₂P(Ph₂)N(8-C₉H₆N)}] (9). Similar reaction between Ph₂PCH₂P(Ph₂)N(8-C₉H₆N) (10) and (DME)NiCl₂ results in five-coordinate N,N,P-chelate nickel complex [NiCl₂{Ph₂PCH₂P(Ph₂)N(8-C₉H₆N)}] (11). Treatment of [(8-C₉H₆N)NP(Ph₂)]₂CH₂ (12) [prepared from (Ph₂P)₂CH₂ and 2 equiv. of 8-azidoquinoline] with LiBuⁿ and (DME)NiCl₂ successively affords [NiCl{(8-C₉H₆N)NP(Ph₂)}₂CH] (13). The new compounds were characterized by ¹H, ¹³C and ³¹P NMR spectroscopy (for the diamagnetic compounds), IR spectroscopy (for the nickel complexes) and elemental analysis. Complexes 2a, 4, 7, 9, 11 and 13 were also characterized by single-crystal X-ray diffraction techniques. The nickel complexes were evaluated for the catalysis in the cross-coupling reactions of arylzinc reagents with aryl chlorides and aryltrimethylammonium salts. Complex 7 exhibits the highest activity among the complexes in catalyzing the reactions of arylzinc reagents with either aryl chlorides or aryltrimethylammonium bromides.