Synthesis and Group 4 Complexes of Tris(pyrrolyl-α-methyl)amine

The tetradentate, trianionic ligand tris(pyrrolyl-α-methyl)amine (H3tpa) is available in 84% yield in a single step by
a triple Mannich reaction involving 3 equiv of pyrrole, 3 equiv of formaldehyde, and ammonium chloride. The new
ligand is readily placed on titanium by transamination on Ti(NMe2)4, which generates Ti(NMe2)(tpa) (1) in 73%
yield. Treating 1 with 1 equiv of 1,3-dimethyl-2-iminoimidazolidine (H-imd) in toluene provided a rare example of a
titanium 2-iminoimidazolidinide, which displays some interesting structural features. Of note is the Ti−N(imd) distance
of 1.768(2) Å, a typical Ti−N double to triple bond distance. Reaction of Zr(NMe2)4 with H3tpa gave a complex of
variable composition, probably varying in the amount of labile dimethylamine retained. However, stable discreet
compounds were available by addition of THF, pyridine, or 4,4‘-di-tert-butyl-2,2‘-bipyridine (Butbpy) to in situ generated
Zr(NMe2)(NHMe2)x(tpa). Three chloro zirconium complexes were generated using three different strategies. Treating
Zr(tpa)(NMe2)(Butbpy) (5) with ClSiMe3 afforded Zr(tpa)(Cl)(Butbpy) (6) in 92% yield. Reaction of Li3tpa with ZrCl4(THF)2 in THF gave a 72% yield of ZrCl(tpa)(THF)2 (7). In addition, treatment of ZrCl(NMe2)3 with H3tpa cleanly
generated ZrCl(NHMe2)2(tpa) (8) in 95% yield. An organometallic zirconium complex was generated on treatment
of 6 with LiC⋮CPh; alkynyl Zr(C⋮CPh)(tpa)(Butbpy) (9) was isolated in 62% yield. 1, Ti(imd)(tpa) (2), 6, and 9
were characterized by X-ray diffraction.