A Comparative Study of π-Arene-Bridged Lanthanum Arylamide and Aryloxide Dimers. Solution Behavior, Exchange Mechanisms, and X-ray Crystal Structures of La₂(NH-2,6-ⁱPr₂C₆H₃)_6, La(NH-2,6-ⁱPr₂C₆H₃)₃(THF)₃, and La(NH-2,6-ⁱPr₂C₆H₃)₃(py)₂

Reaction of 3 equiv of 2,6-diisopropylaniline with La[N(SiMe3)2]3 produces the dimeric species La2(NHAr)6 (1). X-ray crystallography reveals a centrosymmetric structure, where the dimeric unit is bridged by intermolecular η6-arene interactions of a unique arylamide ligand attached to an adjacent metal center. Exposure of 1 to THF results in formation of the monomeric tris-THF adduct La(NHAr)3(THF)3 (2), which was shown by X-ray crystallography to maintain a fac-octahedral structure in the solid state. 1H NMR spectroscopy illustrates that the binding of THF to 1 to form 2 is reversible and removal of THF under vacuum regenerates dimeric 1. Addition of pyridine to 1 yields the monomeric bis-pyridine adduct La(NHAr)3(py)2 (3), which exhibits a distorted trigonal-bipyramidal La metal center. Solution 1H NMR, IR, and Raman spectroscopy indicate that the π-arene-bridged dimeric structure of 1 is maintained in solution. Variable-temperature 1H NMR spectroscopic investigations of 1 are consistent with a monomer−dimer equilibrium at elevated temperature. In contrast, variable-temperature 1H NMR spectroscopic investigations of the aryloxide analogue La2(OAr)6 (4) show that the bridging and terminal aryloxide groups exchange by a mechanism in which the dimeric nature of the compound is retained. Density functional theory (DFT) calculations were carried out on model compounds La2(OC6H5)6, La2(NHC6H5)6, and (C6H5R)La(XC6H5)3, where X = O or NH and R = H, OH, or NH2. The formation of η6-arene interactions is energetically favored over monomeric LaX3 (X = OPh or NHPh) with the aryloxide π-arene interaction being stronger than the arylamide π-arene interaction. Calculation of vibrational frequencies reveals the origin of the observed IR spectral behavior of both La2(OC6H5)6 and La2(NHC6H5)6, with the higher energy ν(CC) stretch due to terminal ligands and the lower energy stretch associated with the bridging ligands.