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

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(Ph-2)=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(Ph-2)NH-(p-MeC6H4)}] (2a). Treatment of 1b with (DME)NiCl2 and following with NaH produces [NiCl{(1,2-C6H4)P(Ph)(NHBut)=N(8-C9H6N)}] (4). Complex 4 was also obtained by reaction of (DME)NiCl2 with [Li{(1,2-C6H4)P(Ph)(NHBut)=N(8-C9H6N)}] (5) prepared through lithiation of 1b. Reaction of 2-PyCH2P(Ph-2)=N(8-C9H6N) (6, Py = pyridyl) and PhN=C(Ph)CH2P(Ph-2)=N(8-C9H6N) (8), respectively, with (DME)NiCl2 yields two five-coordinate N,N,N-chelate nickel complexes, [NiCl2{2-PyCH2P(Ph-2)=N(8-C9H6N)}] (7) and [NiCl2{PhN=C(Ph)CH2P(Ph-2)=N(8-C9H6N)}] (9). Similar reaction between Ph2PCH2P(Ph-2)=N(8-C9H6N) (10) and (DME)NiCl2 results in five-coordinate N,N,P-chelate nickel complex [NiCl2{Ph2PCH2P(Ph-2)=N(8-C9H6N)}] (11). Treatment of [(8-C9H6N)N=P(Ph-2)](2)CH2 (12) [prepared from (Ph2P)(2)CH2 and 2 equiv. of 8-azidoquinoline] with LiBun and (DME) NiCl2 successively affords [NiCl{(8-C9H6N)NP(Ph-2)}(2)CH] (13). The new compounds were characterized by H-1, C-13 and P-31 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.