A tetrameric allyl complex of sodium, and computational modeling of the 23Na–allyl chemical shift

Transmetallation of Li[A′] (A′ = [1,3-(SiMe3)2C3H3]−) with sodium tert-butoxide produces the corresponding sodium salt, which crystallizes from THF/toluene in the form of a cyclic tetramer, {Na[A′](thf)}4. The Na atoms are in a square planar arrangement, bridged with π-bound allyl ligands; the Na–C distances range from 2.591(3)–2.896(3) Å, with an average of 2.70 Å. The geometries of several model organosodium complexes containing cyclopentadienyl and allyl ligands were optimized with density functional theory methods. The optimized structures were used with the gauge-including atomic orbital (GIAO) method to calculate their 23Na NMR magnetic shielding values. Unlike the case with NaCp, the chemical shift of unsubstituted Na(C3H5) is very sensitive to the presence of coordinated THF (causing a 20 ppm upfield shift); silyl substitution has an even larger effect (30 ppm upfield shift). The observed 23Na shift of δ −3.3 ppm for Na[A′] in THF-d8, however, cannot be reliably distinguished from that calculated for the [Na(thf)4]+ cation alone.