Chromium Complexes Supported by the Multidentate Monoanionic N2P2 Ligand: Reduction Chemistry and Reactivity with Ethylene

Chromium complexes supported by the multidentate monoanionic ligand [N2P2] {H[N2P2] = (BuN)-Bu-t(H)SiMe2N((CH2CH2PPr2)-Pr-i)(2)} are presented, and the activity of these complexes toward ethylene oligomerization/polymerization is examined. The complexes [N2P2]CrCl2 (1) and [N2P2]CrCl (2) polymerized ethylene after activation with MAO. Derivatives of I and 2 were synthesized in order to gain insights about the active species in the ethylene oligomerization/ polymerization processes. The alkyl complexes [N2P2]CrMe (3), [N2P2]CrCH2SiMe3 (4), and [N2P2]Cr(Cl)CH2SiMe3 (5), the cationic species {[N2P2]CrCl}BF4 (7), {[N2P2]CrCl}BPh4 (8), and [N2P2]CrCH2SiMe3}BF4 (9), and the Cr(II) complex [N2P2]CrOSO2CF3 (11) were not active ethylene oligomerization/polymerization catalysts in the absence of an activator. Reaction of I with 2 equiv of MeLi led to reduction to 3; however, with 1 equiv of MeLi the stable mixed alkyl halide derivative [N2P2]Cr(Cl)Me (6) was obtained. Reaction of 2 with Red-Al produced the bridging di-hydride {([N2P2]Cr)(mu-H)}(2) (10), which reacted with CO to produce the Cr(1) complex [N2P2]Cr(CO)(2) (12). Reduction of 2 with KC8 in the presence of p-tolyl azide produced the dimeric cis mu-imido {([N2P2]Cr)(mu-NC7H7)}(2) (13). A similar reduction in the presence of ethylene resulted in the isolation of the Cr(III) metallacyclopentane compound [N2P2]CrC4H8 (14).