THF Exchange and Molecular Dynamics in the Series (BDI)MgX(THF), Where X = Bun, NEt2, and OBut and BDI = 2-[(2,6-Diisopropylphenyl)amino]-4-[(2,6-diisopropylphenyl)imino]pent-2-ene
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Department of Chemistry and Biochemistry, The Ohio State University, 100 West 18th Avenue, Columbus, Ohio 43210, United States
Inorg. Chem., Article ASAP
DOI: 10.1021/ic401559b
Publication Date (Web): September 25, 2013
Copyright © 2013 American Chemical Society
*E-mail: chisholm@chemistry.ohio-state.edu., *E-mail:fraenkel@mps.ohio-state.edu.
Abstract
The complexes (BDI)MgX(THF), where X = Bun, NEt2, and OBut, are shown to undergo THF exchange at low added concentrations of THF by a dissociative mechanism: X = Bun, ΔH#(kcal mol–1) = 13.4 ± 0.4 and ΔS# (cal mol–1 K–1) = 6.3 ± 1.6; X = NEt2, ΔH# (kcal mol–1) = 15.2 ± 0.5 and ΔS# (cal mol–1 K–1) = 11.4 ± 2.3; X = OBut, ΔH# (kcal mol–1) = 16.4 ± 0.3 and ΔS# (cal mol–1 K–1) = 9.5 ± 1.3. The apparent aryl group rotations involving the BDI ligands have, within experimental error, the same activation parameters as the THF dissociation, which suggests that the two are correlated involving a three coordinate reactive intermediate akin to what is well-known for related (BDI)ZnR compounds involving three-coordinate trigonal planar Zn2+. At higher concentrations of THF for X = Bun and OBut, but not for X = NEt2, the coalescence temperatures for apparent aryl group rotation are depressed from those of the pure compounds in toluene-d8, and evidence is presented that this correlates with an associative interchange process which becomes dominant in neat THF. We estimate the Iamechanism to have activation parameters: ΔH# (kcal mol–1) = 5.4 ± 0.1 and ΔS# (cal mol–1 K–1) = −20.9 ± 0.3 for X = Bun and ΔH# (kcal mol–1) = 8.3 ± 0.8 and ΔS# (cal mol–1 K–1) = −19.8 ± 3.0 for X = OBut. For the complex (BDI)MgBun(2-MeTHF), the dissociative exchange with added 2-MeTHF occurs more readily than for its THF analogue, as expected for the more sterically demanding Lewis base O-donor: ΔH# (kcal mol–1) = 12.8 ± 0.5 and ΔS# (cal mol–1 K–1) = 8.6 ± 1.8. At very low temperatures in toluene-d8, restricted rotation about the Mg–O(THF) bond is observed for the complexes where X = NEt2 and OBut but not for the complex where X = Bun. These observations, which have been determined from dynamic NMR studies, are correlated with the reactivities of these complexes in solution.
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