Syntheses of Heteroleptic Amidinate Strontium Complexes Using a Superbulky Ligand
journal contributionposted on 22.01.2018, 14:19 by Benjamin Freitag, Jürgen Pahl, Christian Färber, Sjoerd Harder
Strontium complexes are presented with two different bulky amidinate ligands (Am): tBuC(N-DIPP)2 (DIPP = 2,6-diisopropylphenyl), abbreviated here as tBuAmDIPP, and (p-tolyl)C(N-Ar‡)2 (Ar‡ = 2,6-Ph2CH-4-iPr-phenyl) abbreviated here as pTolAmAr‡. The amidine tBuAmDIPP-H was deprotonated by Sr[N(SiMe3)2]2 in benzene at 60 °C. Although the product, tBuAmDIPPSrN(SiMe3)2, could be characterized by NMR, attempts to isolate it led to ligand scrambling via a Schlenk equilibrium. Reaction of in situ prepared tBuAmDIPPSrN(SiMe3)2 with PhSiH3 gave PhH2SiN(SiMe3)2 and presumably the intermediate tBuAmDIPPSrH, but the latter is not stable and the homoleptic complex (tBuAmDIPP)2Sr was isolated and structurally characterized. Deprotonation of the bulkier amidine pTolAmAr‡-H with Sr[N(SiMe3)2]2 needed forcing conditions, inevitably giving rise to deprotonation of the Ph2CH substituent as well. Reaction of pTolAmAr‡-H with the less bulky and less basic Sr[N(SiHMe2)2]2, however, gave the heteroleptic product pTolAmAr‡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 pTolAmAr‡SrH decomposed even at −30 °C. Instead, an amidinate complex with a deprotonated Ph2CH substituent was isolated and structurally characterized (7). The latter catalyzed the intramolecular alkene hydroamination.