Internally versus Externally Solvated Derivatives of Doubly Bridged 1,4-Dilithio-2-butene: Structures and Dynamic Behavior. A “T” Shaped Dimeric Cluster in the Solid State

X-ray crystallographic NMR and calculational modeling studies using B3LYP/6-311G* of selected dilithium derivatives of the 1,3-butadiene dianion including cis-dilithio-1,4-bis­(TMS)-2-butene·(TMEDA)₂ 2, internally solvated cis-dilithio-1,4-bis­[bis­(2-methoxyethyl)­aminomethyldimethylsilyl]-2-butene 5, and using only modeling, 1,4-dilithio-2-butene·(TMEDA)₂ 9 reveal remarkably similar structural and NMR parameters. In the solid, 5 consists of unusual “T” shaped dynamic clusters. In all three bridging lithiums are sited between 1.8 and 1.9 Å normal to the centroids of opposite faces of the near coplanar of the 2-butene component. Typical bond lengths of the latter are 1.458 ± 0.004, 1.385 ± 0.006, and 1.459 ± 0.003 Å, for C1–C2, C2–C3, and C3–C4, respectively. The ¹³C chemical shifts lie within the ranges δ 21 ± 0.5, 99 ± 0.7, 99 ± 0.7 and 21 ± 0.5 for C1, C2 and C3 together, and C4, respectively. Dynamic ¹³C NMR provides activation parameters for nitrogen inversion in 2 and 5, overall molecular inversion of 5, and conformational interconversion of 2.