Synthesis, structures, and alkenehydrosilation activities of neutral tripodal amidozirconium alkyls

Abstract

Aminolysis of Zr(CH₂C₆H₅)₄ with CH[(CH₃)₂SiNHAr]₃ (Ar = para-tolyl and para-fluorophenyl) affords CH[(CH₃)₂SiNAr]₃ZrCH₂C₆H₅1a and 1b (a: Ar = para-tolyl; b: Ar = para-fluorophenyl). Solution and solid state X-ray structural data suggest an η²-bonding mode for the benzyl group in 1a and 1b. Treating 1a and 1b with N,N-dimethylanilinium chloride or triphenylmethyl chloride yields CH[(CH₃)₂SiNAr]₃ZrCl (2a and 2b). Complex 2a exhibits a fast dimer–monomer equilibrium in benzene. Single-crystal X-ray diffraction of 2a reveals a dimeric structure bridged by two μ-Cl atoms. Metathesis of 2a and 2b with n-butyllithium in toluene and then in hexane yields CH[(CH₃)₂SiNAr]₃ZrⁿBu (3a and 3b). The crystal structures and the IR spectra of 3a and 3b in Nujol mulls reveal weak β-agostic interactions, but solution NMR spectra of 3a and 3b give no evidence for such a motif. Further, the larger Zr–Cα–Cβ angles and the longer Zr–Cβ distances suggest the β-agostic interactions in 3a and 3b are weaker than those in the previously reported cases of Group 4 metals. Complexes 1a, 1b, 3a, and 3b are active alkene hydrosilation catalysts. The catalytic activities exhibited by these complexes are in contrast to the inertness of the previously reported neutral molecules of the type X₃MR (X = amide, alkoxide, or siloxide; M = Group 4 metal; R = alkyl or hydride). We attribute the improved activity to three factors: the small ancillary groups of the ligand, the accessibility of the coordination sites cis to the Zr-alkyl group due to the small “bite angle” of the ligand, and the weakened N(p) → Zr(d) π donation due to the coordination geometry imposed by the tripodal ligand.