Syntheses of Heteroleptic Amidinate Strontium Complexes Using a Superbulky Ligand

Strontium complexes are presented with two different bulky amidinate ligands (Am): <i>t</i>BuC­(N<i>-</i>DIPP)₂ (DIPP = 2,6-diisopropylphenyl), abbreviated here as <i>t</i>BuAm^DIPP, and (<i>p</i>-tolyl)­C­(N-Ar^‡)₂ (Ar^‡ = 2,6-Ph₂CH-4-<i>i</i>Pr-phenyl) abbreviated here as <i>p</i>TolAm^Ar‡. The amidine <i>t</i>BuAm^DIPP-H was deprotonated by Sr­[N­(SiMe₃)₂]₂ in benzene at 60 °C. Although the product, <i>t</i>BuAm^DIPPSrN­(SiMe₃)₂, could be characterized by NMR, attempts to isolate it led to ligand scrambling via a Schlenk equilibrium. Reaction of in situ prepared <i>t</i>BuAm^DIPPSrN­(SiMe₃)₂ with PhSiH₃ gave PhH₂SiN­(SiMe₃)₂ and presumably the intermediate <i>t</i>BuAm^DIPPSrH, but the latter is not stable and the homoleptic complex (<i>t</i>BuAm^DIPP)₂Sr was isolated and structurally characterized. Deprotonation of the bulkier amidine <i>p</i>TolAm^Ar‡-H with Sr­[N­(SiMe₃)₂]₂ needed forcing conditions, inevitably giving rise to deprotonation of the Ph₂CH substituent as well. Reaction of <i>p</i>TolAm^Ar‡-H with the less bulky and less basic Sr­[N­(SiHMe₂)₂]₂, however, gave the heteroleptic product <i>p</i>TolAm^Ar‡SrN­(SiHMe₂)₂, which has been structurally characterized. The latter was also at 60 °C stable toward ligand scrambling. Reaction with PhSiH₃ did give hydride exchange, but the product <i>p</i>TolAm^Ar‡SrH decomposed even at −30 °C. Instead, an amidinate complex with a deprotonated Ph₂CH substituent was isolated and structurally characterized (<b>7</b>). The latter catalyzed the intramolecular alkene hydroamination