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

Abstract

The complexes (BDI)MgX(THF), where X = Bu(n), NEt2, and OBu(t), are shown to undergo THF exchange at low added concentrations of THF by a dissociative mechanism: X = Bu(n), 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 = OBu(t), 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 Zn(2+). At higher concentrations of THF for X = Bu(n) and OBu(t), 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 Ia mechanism 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 = Bu(n) and ΔH(#) (kcal mol(-1)) = 8.3 ± 0.8 and ΔS(#) (cal mol(-1) K(-1)) = -19.8 ± 3.0 for X = OBu(t). For the complex (BDI)MgBu(n)(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 OBu(t) but not for the complex where X = Bu(n). These observations, which have been determined from dynamic NMR studies, are correlated with the reactivities of these complexes in solution.