Probing the factors that influence the conformation of a guanidinato ligand in [(η5-C5Me5)M(NN)X] (NN = chelating N,N′,N′′-tri(o-substituted aryl)guanidinate(1−); X = chloro, azido and triazolato)

Abstract

Three types of half sandwich complexes, namely, [(η⁵-Cp*)M(NN)X] (Cp* = C₅Me₅; NN = chelating N,N′,N′′-tri(o-substituted aryl)guanidinate(1−) ligand; M = Rh/Ir; X = Cl (9–13), N₃ (14–18) and N₃C₂(C(O)OR)₂ (19–21)), were prepared and isolated in moderate to good yields. The new complexes were fully characterized and further molecular structures of key complexes from each type were determined by single crystal X-ray diffraction (SCXRD). The new complexes revealed syn–syn (10, 12, 15 and 20), anti–anti (13 and 16) and anti–syn (14, 17, 19·CHCl₃ and 21) conformations. DFT calculations were performed on four conformers of 14, which revealed the stability order as syn–syn (0.00 kcal mol⁻¹) < anti–syn (0.23 kcal mol⁻¹) ≪ syn–anti (3.15 kcal mol⁻¹) ≪ anti–anti (5.15 kcal mol⁻¹). An intra-ligand repulsive interaction between the o-substituent in two proximal aryl rings in guanidinate ligands of 14 and 17 was invoked as a driving force for the observance of the anti–syn conformer in the solid state, which is likely formed from a hypothetical syn–syn conformer via a guanidine centered rearrangement. The influence of packing forces upon the anti–syn conformation of the guanidinate ligand in 14 was also discussed. NBO analysis was carried out on the N₃ moiety of the syn–syn conformer of 14 in order to identify the likely resonance form responsible for [3+2] cycloaddition with the alkyne. Several new complexes that contain an o-substituted arylguanidinate ligand were shown to exist as a mixture of conformers in solution as revealed by NMR spectroscopy, and these conformers were shown to arise from a solid state conformer via guanidinine centered rearrangement.