Pincer Chromium(II) and Chromium(III) Complexes Supported by Bis(imino)aryl NCN ligands: Synthesis and Catalysis on Isoprene Polymerization

Reactions of 2,6-(ArNCH)2C6H3Li with CrCl2(THF)2 afford two trinuclear bis(imino)aryl NCN pincer Cr(II) complexes {[2,6-(ArNCH)2C6H3]Cr(μ-Cl)2}2Cr [Ar = 2,6-Me2C6H3 (1a), 2,6-Et2C6H3 (1b)], and a mononuclear bis(imino)aryl NCN pincer Cr(II) complex [2,6-(ArNCH)2C6H3]Cr(μ-Cl)2Li(THF)2 [Ar = 2,6-iPr2C6H3 (1c)], respectively. Similar reactions of 2,6-(ArNCH)2C6H3Li with CrCl3(THF)3 produce only mononuclear bis(imino)aryl NCN pincer Cr(III) complexes [2,6-(ArNCH)2C6H3]CrCl2(THF) [Ar = 2,6-Me2C6H3 (2a), 2,6-Et2C6H3 (2b), 2,6-iPr2C6H3 (2c)]. X-ray crystallographic analysis reveals that the terminal Cr(II) atoms in 1a and 1b and the Cr(II) atom in 1c possess a distorted trigonal bipyramidal coordination environment with the Cipso atom of the pincer ligand and two chloride atoms in the equator and the two imine nitrogen atoms in the apical positions, while the Cr(III) complexes 2a, 2b, and 2c all adopt a distorted octahedral geometry around the chromium metal. Upon activation with trialkylaluminum and [Ph3C]+[B(C6F5)4]−, the Cr(III) complexes show high catalytic activity for isoprene polymerization and afford polyisoprene with predominately trans-1,4 units, whereas the Cr(II) complexes are inert under the same conditions.