Synthesis of a “Super Bulky” Guanidinate Possessing an Expandable Coordination Pocket

• Published in: Inorganic chemistry 2014-08, Vol.53 (15), p.8155-8164
• Main Authors: Maity, Arnab K, Fortier, Skye, Griego, Leonel, Metta-Magaña, Alejandro J
• Format: Article
• Language: English
• ISSN: 0020-1669; 1520-510X
• DOI: 10.1021/ic501219q

Friedel–Crafts alkylation of 4-tert-butylaniline with 2 equiv of benzhydrol affords bulky 2,6-bis(diphenylmethyl)-4-tert-butylaniline (Ar*NH 2 ) in good yield, which can be readily synthesized on a tens of grams scale. The reaction of 6 equiv of Ar*NH 2 with triphosgene generates the symmetric urea (Ar*NH) 2 CO, which, upon dehydration with a P2O5/Al2O3 slurry in pyridine, produces the sterically encumbered carbodiimide (Ar*N) 2 C as an air-stable white solid. The treatment of (Ar*N) 2 C with LiNC t Bu2 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 (% V Bur) of M[ Ar *ketguan] [M = Li (94.8% V Bur), K (92.1% V Bur), Cs (81.7% V Bur)] 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.