Chromium(II) and Chromium(III) Complexes Supported by Tris(2-pyridylmethyl)amine: Synthesis, Structures, and Reactivity

This report describes the synthesis, structural characterization, and polymerization behavior of a series of chromium(II) and chromium(III) complexes ligated by tris(2-pyridylmethyl)amine (TPA), including chromium(III) organometallic
derivatives. For instance, the combination of TPA with CrCl2 yields monomeric (TPA)CrCl2 (1). A similar reaction
of CrCl2 with TPA, followed by chloride abstraction with NaBPh4 or NaBArF4 (ArF = 3,5-(CF3)2C6H3), provides the
weakly associated cationic dimers [(TPA)CrCl]2[BPh4]2 (2A) and [(TPA)CrCl]2[BArF4]2 (2B), respectively. X-ray
crystallographic analysis reveals that each chromium(II) center in 1, 2A, and 2B is a tetragonally elongated octahedron;
such Jahn−Teller distortions are consistent with the observed high spin (S = 2) electronic configurations for these
chromium(II) complexes. Likewise, reaction of CrCl3(THF)3 with TPA, followed by anion metathesis with NaBPh4 or
NaBArF4, yields the monomeric, cationic chromium(III) complexes [(TPA)CrCl2][BPh4] (4A) and [(TPA)CrCl2][BArF4]
(4B), respectively. Treatment of 4A with methyl and phenyl Grignard reagents produces the cationic chromium(III)
organometallic derivatives [(TPA)Cr(CH3)2][BPh4] (5) and [(TPA)CrPh2][BPh4] (6), respectively. Similar reactions of
4A with organolithium reagents leads to intractable solids, presumably due to overreduction of the chromium(III)
center. X-ray crystallographic analysis of 4A, 5, and 6 confirms that each possesses a largely undistorted octahedral
chromium center, consistent with the observed S = 3/2 electronic ground states. Compounds 1, 2A, 2B, 4A, 4B,
5, and 6 are all active polymerization catalysts in the presence of methylalumoxane, producing low to moderate
molecular weight high-density polyethylene.