Metal and halogen dependence of the rate effect in hydroamination/cyclization of unactivated aminoalkenes: Synthesis, characterization, and catalytic rates of CCC-NHC hafnium and zirconium pincer complexes

1,3-Bis(3′-butylimidazol-1′-yl)benzene dibromide (2b) or 1,3-bis(3′-butylimidazol-1′-yl)benzene dichloride (2c) was reacted with a stoichiometric amount of Zr(NMe2)4 or Hf(NMe2)4 yielding four new early transition metal CCC-N-heterocylic carbene (CCC-NHC) pincer complexes. Two of the CCC-NHC pincer complexes were synthesized via a new methodology, which allowed for a room temperature reaction, shorter reaction times, and slightly less Zr(NMe2)4 or Hf(NMe2)4. The molecular structure of 2-(1,3-bis-3′-butylimidazol-1′-yl-2′-ylidene)phenylene)(dimethylamido)(dibromo)zirconium(IV) (3b) was determined by X-ray crystallography. The complexes were evaluated for hydroamination/cyclization of unactivated aminoalkenes yielding pyrrolidines or piperidines. The hydroamination/cyclization rates were dependent upon the halogen or the metal in the complex. The complexes with iodide ligands gave the fastest hydroamination/cyclization rates (I > Br > Cl). The Zr complexes provided faster hydroamination/cyclization rates than the Hf analogs. Use of CCC-NHC ZrI3 complex 5 enhanced the rate of reaction.