Salt elimination reactions do not always eliminate. Mechanistic study of the reaction of NdCl3 with sodium N,N-dimethylaminodiboranate

Abstract

Treatment of neodymium trichloride (NdCl3) in tetrahydrofuran with 3 equiv of sodium N,N-dimethylaminodiboranate, Na[(H3B)2NMe2], which we will refer to as NaL, followed by solvent removal and extraction with pentane, affords a solution of the trisubstituted product NdL3(thf) in excellent yield. Although this transformation appears to be a straightforward salt metathesis/elimination reaction, the reaction mixture in thf surprisingly does not contain the ultimate NdL3(thf) product; instead the only aminodiboranate species present are the monosubstituted Nd compound NdLCl2(thf)3 and two equivalents of unreacted NaL. A series of 11B NMR experiments in thf show that the monosubstituted product is formed under thermodynamic control: elimination of the first equivalent of salt is thermodynamically downhill but the subsequent two salt elimination reactions are thermodynamically uphill. Consistent with this view, the salt elimination reactions are reversible: treatment of isolated samples of NdL3(thf) with NaCl in tetrahydrofuran generates the same NdLCl2(thf)3 monosubstituted product, along with two equivalents of the aminodiboranate salt NaL. The reversibility of the salt elimination reaction can be attributed to the solvation of the sodium cations by tetrahydrofuran, and the affinity of the Lewis acidic lanthanide centers for tetrahydrofuran and chloride relative to the aminodiboranate anion. The importance of the sodium-solvent and lanthanide-solvent interactions to the thermodynamics of salt elimination reactions is confirmed by the finding that the reaction of NdCl3(thf)2 with 3 equiv of NaL in diethyl ether proceeds – even in the reaction solution – to the fully substituted NdL3(thf) product.