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.