Vinyl Oxidative Coupling as a Synthetic Route to Catalytically Active Monovalent Chromium

Reaction of the deprotonated form of cis-{(t-Bu)N(H)P[μ-N(t-Bu)]2PN(H)(t-Bu)} with CrCl3(THF)3 afforded the trivalent cis-{(t-Bu)NP[μ-N(t-Bu)]2PN(t-Bu)}[Li (THF)])CrCl2 (1). Subsequent reaction with 2 equiv of vinyl Grignard (CH2CH)Mg Cl gave the butadiene derivative (cis-{(t-Bu)NP[μ-N(t-Bu)]2PN(t-Bu)}[Li(THF)])Cr(cis-η4-butadiene) (3) formally containing the metal in its monovalent state. The presence of the monovalent state was thereafter confirmed by DFT calculations. The coordination of the butadiene unit appears to be rather robust since reaction with Me3P afforded cleavage of the dimeric ligand core but not its displacement. The reaction formed the new butadiene complex [(t-Bu)N−P−N(t-Bu)]Cr(cis-η4-butadiene)PMe3 (4) containing a regular NPN monoanion. In agreement with the presence of monovalent chromium, complexes 3 and 4 act as single-component self-activating catalysts for selective ethylene trimerization and dimerization, respectively.