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
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Abstract
X-ray crystallographic NMR and calculational modeling
studies using B3LYP/6-311G* of selected dilithium derivatives of the
1,3-butadiene dianion including <i>cis</i>-dilithio-1,4-bis(TMS)-2-butene·(TMEDA)<sub>2</sub> <b>2</b>, internally solvated <i>cis</i>-dilithio-1,4-bis[bis(2-methoxyethyl)aminomethyldimethylsilyl]-2-butene <b>5</b>, and using only modeling, 1,4-dilithio-2-butene·(TMEDA)<sub>2</sub> <b>9</b> reveal remarkably similar structural and NMR
parameters. In the solid, <b>5</b> 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 <sup>13</sup>C 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 <sup>13</sup>C NMR provides activation parameters for nitrogen inversion
in <b>2</b> and <b>5</b>, overall molecular inversion
of <b>5</b>, and conformational interconversion of <b>2</b>
