A tetrameric allyl complex of sodium, and computational modeling of the Na-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.