Synthesis of a “Super Bulky” Guanidinate Possessing an Expandable Coordination Pocket
Friedel–Crafts alkylation of 4-tert-butylaniline with 2 equiv of benzhydrol affords bulky 2,6-bis(diphenylmethyl)-4-tert-butylaniline (Ar*NH2) in good yield, which can be readily synthesized on a tens of grams scale. The reaction of 6 equiv of Ar*NH2 with triphosgene generates the symmetric urea (Ar*NH)2CO, which, upon dehydration with a P2O5/Al2O3 slurry in pyridine, produces the sterically encumbered carbodiimide (Ar*N)2C as an air-stable white solid. The treatment of (Ar*N)2C with LiN═CtBu2 in tetrahydrofuran cleanly gives the monomeric lithium guanidinate Li[Ar*ketguan], free of coordinating solvent, in 85% yield. Protonation of Li[Ar*ketguan] with lutidinium chloride produces the guanidine Ar*ketguanH (MW = 1112.60 g/mol), which is easily derivatized to give the monomeric alkali metal complexes M[Ar*ketguan] (M = K, Cs) in 94% and 51% yield, respectively. The solid-state molecular structures of M[Ar*ketguan] (M = Li, K, Cs) show formally two-coordinate alkali metal cations encapsulated within a hydrophobic coordination pocket formed by the peripheral diphenylmethyl substituents of the guanidinate. Remarkably, percent buried volume analyses (% VBur) of M[Ar*ketguan] [M = Li (94.8% VBur), K (92.1% VBur), Cs (81.7% VBur)] reveal a coordination cavity that adjusts to individually accommodate the variously sized metal ions despite the highly encumbering nature of the ligand. This demonstrates a flexible ligand framework that is able to stabilize low-coordinate metal centers within a “super bulky” coordination environment.