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 La-2(NHAr)(6) (1). X-ray crystallography reveals a centrosymmetric structure, where the dimeric unit is bridged by intermolecular eta(6)-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. H-1 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 H-1 NMR, IR, and Raman spectroscopy indicate that the pi-arene-bridged dimeric structure of 1 is maintained in solution. Variable-temperature H-1 NMR spectroscopic investigations of 1 are consistent with a monomer-dimer equilibrium at elevated temperature. In contrast, variable-temperature H-1 NMR spectroscopic investigations of the aryloxide analogue La-2(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 La-2(OC6H5)(6), La-2(NHC6H5)(6), and (C6H5R)La(XC6H5)(3), where X = O or NH and R = H, OH, or NH2. The formation of eta(6)-arene interactions is energetically favored over monomeric LaX3 (X = OPh or NHPh) with the aryloxide pi-arene interaction being stronger than the arylamide T-arene interaction. Calculation of vibrational frequencies reveals the origin of the observed IR spectral behavior of both La-2(OC6H5)(6) and La-2(NHC6H5)(6), with the higher energy nu(C=C) stretch due to terminal ligands and the lower energy stretch associated with the bridging ligands.