Trianionic NCN3– Pincer Complexes of Chromium in Four Oxidation States (CrII, CrIII, CrIV, CrV): Determination of the Active Catalyst in Selective 1-Alkene to 2-Alkene Isomerization

• Published in: Organometallics Vol. 30; no. 18; pp. 4949 - 4957
• Main Authors: McGowan, Kevin P, Abboud, Khalil A, Veige, Adam S
• Format: Journal Article
• Language: English
• Published: American Chemical Society 26.09.2011
• ISSN: 0276-7333; 1520-6041
• DOI: 10.1021/om200547u

The synthesis and characterization of a series of four Cr complexes in the +2, +3, +4, and +5 oxidation states supported by an NCN trianionic pincer ligand are reported. Treating CrMeCl2(THF)3 with the dilithio salt pincer ligand precursor {[2,6- i PrNCHN]Li2}2 provides the CrIII complex [2,6- i PrNCN]CrIII(THF)3 (1), CrIV complex [2,6- i PrNCN]CrIVMe(THF) (2), and CrII complex [2,6- i PrNHCN]CrII(THF)2 (3). Complexes 2 and 3 are the result of disproportionation. Treating 1 with 1 equiv of styrene oxide in THF converts the CrIII complex to the CrV(O) species [2,6- i PrNCN]CrV(O)(THF) (4). Complex 2, characterized by single-crystal X-ray diffraction, is a rare CrIV methyl complex that is kinetically stable at 25 °C; at 85 °C, Cr–Me bond homolysis occurs. The homolytic cleavage results in CH4 formation and biphenyl via a radical mechanism. The metal-containing product from thermolysis is the same CrII species formed during metalation, except one of the protons is substituted for a deuterium from C6D6 (3- d ). Complex 2 is a precatalyst for the selective isomerization of 1-hexene and 1-octene to the corresponding 2-alkenes. An induction period occurs during the catalytic isomerization, and the active catalyst was determined to be the CrII complex 3, not 2.