THF Exchange and Molecular Dynamics in the Series (BDI)MgX(THF), Where X = Bu^<i>n</i>, NEt₂, and OBu^<i>t</i> and BDI = 2‑[(2,6-Diisopropylphenyl)amino]-4-[(2,6-diisopropylphenyl)imino]pent-2-ene

Abstract

The complexes (BDI)­MgX­(THF), where X = Bu^<i>n</i>, NEt₂, and OBu^<i>t</i>, are shown to undergo THF exchange at low added concentrations of THF by a dissociative mechanism: X = Bu^<i>n</i>, Δ<i>H</i>^# (kcal mol^–1) = 13.4 ± 0.4 and Δ<i>S</i>^# (cal mol^–1 K^–1) = 6.3 ± 1.6; X = NEt₂, Δ<i>H</i>^# (kcal mol^–1) = 15.2 ± 0.5 and Δ<i>S</i>^# (cal mol^–1 K^–1) = 11.4 ± 2.3; X = OBu^<i>t</i>, Δ<i>H</i>^# (kcal mol^–1) = 16.4 ± 0.3 and Δ<i>S</i>^# (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 = Bu^<i>n</i> and OBu^<i>t</i>, but not for X = NEt₂, the coalescence temperatures for apparent aryl group rotation are depressed from those of the pure compounds in toluene-<i>d</i>₈, and evidence is presented that this correlates with an associative interchange process which becomes dominant in neat THF. We estimate the I_a mechanism to have activation parameters: Δ<i>H</i>^# (kcal mol^–1) = 5.4 ± 0.1 and Δ<i>S</i>^# (cal mol^–1 K^–1) = −20.9 ± 0.3 for X = Bu^<i>n</i> and Δ<i>H</i>^# (kcal mol^–1) = 8.3 ± 0.8 and Δ<i>S</i>^# (cal mol^–1 K^–1) = −19.8 ± 3.0 for X = OBu^<i>t</i>. For the complex (BDI)­MgBu^<i>n</i>(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: Δ<i>H</i>^# (kcal mol^–1) = 12.8 ± 0.5 and Δ<i>S</i>^# (cal mol^–1 K^–1) = 8.6 ± 1.8. At very low temperatures in toluene-<i>d</i>₈, restricted rotation about the Mg–O­(THF) bond is observed for the complexes where X = NEt₂ and OBu^<i>t</i> but not for the complex where X = Bu^<i>n</i>. These observations, which have been determined from dynamic NMR studies, are correlated with the reactivities of these complexes in solution