Tricoordinate Organochromium(<scp>III</scp>) Complexes Supported by a Bulky Silylamido Ligand Produce Ultra‐High‐Molecular Weight Polyethylene in the Absence of Activators

Low‐coordinate organoCr( III ) complexes supported by the silylamido ligand –N(SiMe 3 ) DIPP ( DIPP = 2,6‐diisopropylphenyl) are ethylene polymerization catalyst precursors without the need of additional cocatalyst. The reaction of CrCl 3 ( THF ) 3 with 3 or 2 equiv. of LiN(SiMe 3 ) DIPP yields either a four‐membered cyclometalated Cr complex or Cr[N(SiMe 3 ) DIPP ] 2 Cl, respectively, with no trace of Cr[N(SiMe 3 ) DIPP ] 3 . Addition of 1 equiv. of LiN(SiMe 3 ) DIPP to Cr[N(SiMe 3 ) DIPP ] 2 Cl also leads to the four‐membered metallacycle, which upon heating transforms to a new six‐membered Cr metallacycle, likely via a σ ‐bond metathesis step. Cr[N(SiMe 3 ) DIPP ] 2 Cl can be readily converted to bis(amido)Cr( III ) vinyl and alkyl complexes Cr[N(SiMe 3 ) DIPP ] 2 R (R = vinyl, Bn, and Me). All of these structurally characterized low‐coordinate Cr( III ) complexes with a Cr–C bond initiate the polymerization of ethylene in the absence of activators or cocatalysts, producing ultra‐high‐molecular weight polyethylene.