The rate constants for ring inversion (k
r.i.) and bond shift (k
b.s.) in 1 and 2 were determined by
dynamic NMR spectrometry while the rate constants for bond shift and intramolecular charge transfer (k
c.t.)
were determined for 1
2-/2K+ and 2
2-/2K+. These processes were modeled by HF/3-21G(*) ab initio molecular
orbital calculations of the ground states and of several transition states for 3, 4, 3
2-, 4
2-, 3
2-/2K+, and 4
2-/2K+. The results indicate that k
r.i. and k
b.s. are ca. 2.5 times greater (at 240 and 280 K, respectively) for 2
compared to 1 due to larger steric repulsions in the ground state of 2. Contrariwise, k
b.s. and k
c.t. are 1.7 and
166 times greater, respectively, at 280 K for 1
2-/2K+ than for 2
2-/2K+. These differences are attributed to
less twisting and therefore greater π delocalization between the cyclooctatetraenyl rings and the aryl ring in
the bond shift and charge-transfer transition states of 1
2- compared to 2
2-. The greater difference between
1
2- and 2
2- for k
c.t. compared to k
b.s. is postulated to result from looser ion pairing in the charge-transfer
transition state relative to the bond shift transition state.
1,4-Dicyclooctatetraenyl-1,3-butadiyne and p-, m-, and o-di(cyclooctatetraenylethynyl)benzene (1−4, respectively) and their dipotassium salts (1
2-−4
2-) in THF-d
8 have been synthesized and studied by dynamic
NMR spectroscopy. Rate constants for bond shift (k
BS) in the neutral cyclooctatetraene (COT) rings of 1−4
and 1
2-−4
2- and for intramolecular charge (electron and cation) transfer (k
CT) between the dianion and the
neutral COT rings in 1
2-−4
2- have been determined. 13C NMR chemical shifts and AM1 π-charges as well
as the values of k
CT are interpreted on the basis of a stronger through-bond interaction between the COT rings
in the order 1
2- ≫ 2
2- ≥ 4
2- > 3
2-. Similarly, k
BS decreases in the order 1
2- ≫ 4
2- > 2
2- > 3
2- on going
from 1−4 to the corresponding dianions. Analysis of the AM1 π-charge densities suggests that these differences
are primarily due to a through-bond effect with an additional through-space contribution from the electric
field of the dianion ring for bond shift in 4
2-.
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