N,N′-Bis(diphenylphosphino)diaminophenylphosphine Ligands for Chromium-Catalyzed Selective Ethylene Oligomerization Reactions

Reaction of 1 or 2 equiv of Ph 2 PCl with PhP(N(H)R) 2 (R = n -propyl) yields Ph 2 PN(R)P(Ph)N(R)H ( 1 ) or Ph 2 PN(R)P(Ph)N(R)PPh 2 ( 2 ), respectively. In contrast, reaction of 1 or 2 equiv of Ph 2 PCl with PhP(N(H)R) 2 (R = isopropyl) yields exclusively Ph 2 PN(R)P(Ph)N(R)H ( 3 ), even under more forcing conditions. Low-temperature NMR spectroscopy and a conformational analysis of Ph 2 PN( i Pr)P(Ph)N( i Pr)H ( 3 ) reveal the lowest energy conformer to have a close N−H···P interaction of 2.95 Å, which we speculate may hinder further reactivity of this molecule. Reaction of 3 with [Cr(CO) 6 ] yields [Cr( 3 )(CO) 4 ] ( 5 ), which has been structurally characterized. Coordination of ligand 3 facilitates its conversion to Ph 2 PN( i Pr)P(Ph)N( i Pr)PPh 2 ( 4 ) while bound to chromium, yielding the complex [Cr( 4 )(CO) 4 ] ( 6 ), which has also been structurally characterized. Ligands 1 and 2, when reacted in situ with [Cr(acac) 3 ] (acac = acetylacetonate) and modified methylalumoxane, and complexes 5 and 6, when activated with Ag[Al(OC 4 F 9 ) 4 ] and triethylaluminum, are moderately active and selective catalysts for the selective oligomerization of ethene to 1-hexene and 1-octene.