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.