Application of a Stable and Soluble Dibenzylbarium Reagent in the Synthesis of a Barium Imido Cluster

The complex (DMAT)2Ba·(THF)2 (DMAT =
2-dimethylamino-α-trimethylsilylbenzyl) was prepared by a salt
metathesis reaction between (DMAT)K and BaI2 in THF, and
crystals were isolated in 83% yield. This monomeric complex dissolves
well in aromatic or ethereal solvents and is also stable at higher
temperatures in solution. Structural details and NMR parameters reveal
a high degree of charge delocalization in the DMAT aryl ring. Comparison
with identical Ca and Sr complexes, (DMAT)2Ae·(THF)2 (Ae = Ca and Sr), shows that the extent of delocalization
increases down group 2: Ca < Sr < Ba. Due to chirality at the
benzylic carbon, in solution the heterochiral R,S-isomer and homochiral R,R/S,S-isomers exist in equilibrium.
A temperature-dependent NMR study gives an estimate of the Ae–C
and Ae–N bond energies which decrease down the group: Ca >
Sr > Ba. Although the DMAT carbanion is stabilized by resonance
and
negative hyperconjugation, (DMAT)2Ba·(THF)2 is still a very potent deprotonation reagent. Double deprotonation
of Ph3SiNH2 gave the first Ba imido complex.
A similar reaction using (DMAT)2Sr·(THF)2 gave the analogue Sr imido complex. Comparison with reported structures
shows trends for the full series [(Ph3SiN)­Ae·(THF)]4 (Ae = Mg, Ca, Sr, and Ba) which crystallize as tetramers
with very stable, ionically bound heterocubane Ae4N4 cores.