An NMR and Computational Study of a Lithium Amide and Ethereal Ligand Exchange

Abstract

The activation parameters for the exchange of an ethereal ligand in a chiral lithium amide was determined from full bandshape analysis of the dynamic NMR spectra. For diethyl ether, the activation parameters were deltaH++=11.0 kcalmol(-1) and deltaS++=12.0 cal K(-1)mol(-1). The exchange of the tetrahydrofuran ligand proceeds with a similar activation enthalpy deltaH++=11.2 kcalmol(-1); however, the entropy is close to zero. deltaS++=1.6 cal K(-1)mol(-1). The dissociation of the ethereal ligand was also modeled by means of semiempirical (PM3) and density functional (DFT) methods. The experimental 13C NMR chemical shifts for the carbons of uncoordinated and coordinated ethers were calculated with the GIAO-DFT (B3PW91/6-31G(d)) computational method.

Computational Perspectives on Organolithiums

“…Larger aggregates are most frequently monosolvated or disolvated depending on the size of the solvent (and solute), but lithium is generally considered to be tetracoordinated [36,39]. Solvent exchange between the coordinating solvent and the solvent in the bulk and flexibility due to hemilabile ligands are also processes that may take place, and these are dynamical temperature-and concentration-dependent processes, phenomena well studied by both NMR and computational techniques [28,[48][49][50][51][52][53][54][55]. Solvation energies of lithium organic compounds are routinely estimated computationally and have been compared with experimental results in several studies.…”

“…Several parallel pathways were explored and overall barriers were in agreement with experimental measurements [49].…”

Aggregation and Solvation of Chiral N,P‐Amide Ligands in Coordinating Solvents: A Computational and NMR Spectroscopic Study

“…Thus, employing a better coordinating solvent will no longer favour the eq state over the non-eq entropically, since the number of coordinated solvent molecules is the same in both forms. For the less sterically demanding ligands 1 d and 1 e, the preferred state is disolvated, and the energy difference compared to the monosolvated form of the non-eq dimer is large enough so that even shifting from THF to the slightly more entropically encumbered Et 2 O [41] will not be expected to overcome the energy penalty. It has been argued previously that in terms of the solvent used, Me 2 O (dimethoxyethane, DME) is not a suitable model for THF or Et 2 O, [42] although that study did not in- [a] 20 % dispersion, DG correction = 6.3 kJ mol À1 .…”

“…Theoretical calculations make it feasible to assign but also confirm the NMR spectra and have been used extensively for various organometallic compounds. [41,[43][44][45][46][47][48][49][50][51] In theoretical calculations, the NMR spectroscopic chemical shifts (and shieldings) are very sensitive to the basis sets and in general the highest level method and basis set should be used. [52][53][54][55] Juxtaposition of quantum-chemical-calculated NMR spectroscopic shifts together with the experimental data and various two-dimensional NMR spectroscopic approaches such as HMBC, HSQC and COSY are other ways of facilitating the assignment of the NMR spectra.…”

Complex intermediates in the NO insertion reactions into lithium amides

“…13 An enormous body of structural investigations, mostly crystallographic 14 and spectroscopic, 15 has been accumulated in the last years. Concomitantly with the experimental investigations many detailed computational studies of aggregates of lithium dialkylamides have been carried out by using ab initio and/or semiempirical methods [16][17][18] with variable degrees of accuracy, according to the size and nature of the systems examined. On the other hand, we have recently demonstrated that lithium amides formed from cyclic amines (such as piperidine) formed mixed aggregates with the precursors amine, 19 while lithium amides formed from open dialkyl amines forms homodimers.…”