Bond shift and charge transfer dynamics in methylene- and dimethylsilyl-bridged dicyclooctatetraene dianions

Abstract

The rate constants for bond shift in dicyclooctatetraenylmethane (1), dicyclooctatetraenyldimethylsilane (2), and their dianions (1 2Ϫ and 2 2Ϫ ) in [ 2 H 8 ]THF, have been determined from the temperature dependence of their 13 C NMR linewidths. The corresponding parameters for intramolecular electron and cation transfer (charge transfer) between the dinegative and neutral rings have been measured by 13 C NMR spin saturation transfer experiments for the dipotassium salts of 1 and 2. Selected structural features of the neutral compounds and the dianions are discussed on the basis of 13 C NMR chemical shifts and ab initio molecular orbital calculations at the HF/6-31G* and HF/3-21G(*) levels of theory. Energy contributions to the ring flattening in the bond shift process are calculated by molecular mechanics methods. The measured rate constants for both bond shift and charge transfer are larger for the methylene-bridged dianion. Approximately half of this difference is due to the greater ease of "gating" (i.e., ring flattening and distortion to the bond shift transition state) in 1 2Ϫ -2K + . A significant portion of the remainder is attributed to a greater inter-ring through-space interaction in 1 2Ϫ , although mediation by the cation and/or throughbridge interactions probably also contribute to some extent. A temperature-dependent differential 13 C NMR line broadening is observed for the dianion ring carbons of the dipotassium salts. Possible mechanisms for this counterion-specific line broadening, which occurs only for carbons with large HOMO coefficients, are discussed.

Conformational Analysis and Kinetics of Ring Inversion for Methylene- and Dimethylsilyl-Bridged Dicyclooctatetraene

“…d Weighted average for the two diastereomers. e Ref . f Extrapolated using ln( k / T ) = 29.52 − 9.388 × 10 3 T -1 .…”

“… The factor of 2 in front of the brackets is required because inversion of either of the COT rings in 1 or 2 results in an exchange of diastereomers. The value of k BS was obtained from the rate of pairwise exchange of C 2 , C 3 and C 4 with C 6 , C 7 and C 8 , respectively . The denominator of k BS /2 reflects the fact that each BS results in inversion one-half of the time.…”

“…Ring inversion (RI) and bond shift (BS) (Figure ) are two fundamental dynamic processes of cyclooctatetraene (COT) that are closely related to each other . In addition to their intrinsic interest, they are important for understanding the role of ring flattening in the “gated” charge transfer (CT) that occurs in bridged dicyclooctatetraenes (Figure ). − The bridge could, in principle, affect CT both at the ring flattening and bond equalization (“gating”) steps and during the actual transfer of charge (electron and counterion) from one planar COT ring to the other (“intrinsic” CT). The clarification of this issue would allow a better understanding of intrinsic CT, i.e., of the actual charge conduction pathways in these dianions.…”

“…The dipotassium salts of dicyclooctatetraenylmethane ( 1) and dicyclooctatetraenyldimethylsilane ( 2) have recently been the subject of several studies of CT (Figure ). , Compounds 1 and 2 are also appropriate for the study of both RI and BS (Figure ). The CH 2 COT group is a weak donor, whereas Si(CH 3 ) 2 COT is an acceptor …”

Structure of an Unsymmetrical Isomer from the Attempted Synthesis of 1,8-Dicyclooctatetraenylnaphthalene

“…We have been interested in the “gated” transfer of charge (electrons and counterions) in bridged dicyclooctatetraene dianions. − Reversible charge transfer (CT) in these mixed-valence systems occurs on the NMR time scale as a consequence of a high-energy gating step, i.e., ring flattening and bond length equalization in the neutral cyclooctatetraenyl (COT) ring, that must precede CT.…”