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)2afford 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)3produce 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 1aand 1band the Cr(II) atom in 1cpossess a distorted trigonal bipyramidal coordination environment with the Cipsoatom 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 2call 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. [ABSTRACT FROM AUTHOR]