Pyridine− and Imidazole−Phosphinimine Bidentate Ligand Complexes: Considerations for Ethylene Oligomerization Catalysts

The series of 2-substituted diphenylphosphine pyridines 1−5 were synthesized and
subsequently oxidized with silyl or aryl azides to give the series of pyridine−phosphinimine
ligands 2-(Me3SiNPPh2)C5H4N (9), 2-(2,6-Me2C6H3NPPh2)C5H4N (10), 2-(2,6-i-Pr2C6H3NPPh2)C5H4N (11), 2-(2,6-Me2C6H3NPPh2)-6-MeC5H4N (12), 2-(2,6-i-Pr2C6H3NPPh2)-6-MeC5H4N (13), 2-(2,6-Me2C6H3NPPh2)-6-BnC5H4N (14), 2-(2,6-i-Pr2C6H3NPPh2)-6-BnC5H4N
(15), 2-(2,6-Me2C6H3NPPh2)-6-SiMe3C5H4N (16), 2-(2,6-i-Pr2C6H3NPPh2)-6-SiMe3C5H4N (17),
2-(2,6-Me2C6H3NPPh2)-6-PhC5H4N (18), and 2-(2,6-i-Pr2C6H3NPPh2)-6-PhC5H4N (19). Attempts to oxidize the fluorinated phosphine 2-P(C6F5)2-6-PhC5H3N (6) were unsuccessful.
The ligand 9 reacted with PdCl2(PhCN)2 to the give the square-planar, diamagnetic compound
(L)PdCl2 (20; L = 9), while the remaining ligands were used to prepare (L)NiBr2 and (L)FeCl2 complexes 21−30 and 31−40 (L = 10-19), respectively. In these complexes the P atoms
become part of the chelate backbone. In addition, the pyridine−phosphinimines 2-(Ph3PNCH2)(C5H4N) (44), 2-(Ph3PNCH2)-6-Me(C5H3N) (45), and 2-(Ph3PNCH2)-6-Ph(C5H3N) (46)
were also prepared from the reaction of 2-azidomethyl−pyridines with PPh3. In a similar
fashion the complexes (L)PdCl2 (47, 48; L = 44, 45), (L)NiBr2 (49−51; L = 44−46), (L)FeCl2
(52, 53), and (L)CoCl2 (54, 55; L = 44, 45) were prepared. In addition, the imidazole−phosphines 1-Me-2-(PPh2)C3H2N2 (58), 1-Me-2-(PPh2)-4,5-Ph2C3N2 (59), and 1-Me-2-(PPh2)C7H6N2 (60) were prepared and oxidized to give the imidazole−phosphinimines 1-Me-2-(2,6-Me2C6H3NPPh2)C3H2N2 (61), 1-Me-2-(2,6-i-Pr2C6H3NPPh2)C3H2N2 (62), 1-Me-2-(2,6-Me2C6H3NPPh2)-4,5-Ph2C3N2 (63), 1-Me-2-(2,6-i-Pr2C6H3NPPh2)-4,5-Ph2C3N2 (64), 1-Me-2-(2,6-Me2C6H3NPPh2)C7H6N2 (65), 1-Me-2-(2,6-i-Pr2C6H3NPPh2)C7H6N2 (66) and 1-Me-2-(2,6-i-Pr2C6H3NPPh2)-4-(t-Bu)C3HN2 (67). Subsequent complexation afforded the species
(L)PdCl2 (68; L = 61), (L)NiBr2 (69−75; L = 61−67), and (L)FeCl2 (76−82; L = 61−67).
Preliminary screening for activity as catalyst precursors for ethylene polymerization indicated
that ethylene oligomerization may be occurring. In the case of complexes 30, 40, 74, and 82
activation with Et2AlCl(ClC6H5) at 35 °C under 300 psi of ethylene effected modest catalytic
dimerization of ethylene to mainly C4 alkenes. DFT computations suggested that inclusion
of P into the ligand results in diminished electrophilicity at the metal and thus a weakened
ethylene−metal interaction, accounting for the modest catalytic activity. X-ray structure
determinations were obtained for 2, 20, 26, 27, 35, 37, 40, 49−51, 54, 68, 79, and 82.