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Syntheses of Heteroleptic Amidinate Strontium Complexes Using a Superbulky Ligand

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journal contribution
posted 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.