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

Friedel–Crafts alkylation of 4-<i>tert</i>-butylaniline with 2 equiv of benzhydrol affords bulky 2,6-bis­(diphenylmethyl)-4-<i>tert</i>-butylaniline (<b>Ar*NH</b>_<b>2</b>) in good yield, which can be readily synthesized on a tens of grams scale. The reaction of 6 equiv of <b>Ar*NH</b>_<b>2</b> with triphosgene generates the symmetric urea <b>(Ar*NH)</b>_<b>2</b><b>CO</b>, which, upon dehydration with a P₂O₅/Al₂O₃ slurry in pyridine, produces the sterically encumbered carbodiimide <b>(Ar*N)</b>_<b>2</b><b>C</b> as an air-stable white solid. The treatment of <b>(Ar*N)</b>_<b>2</b><b>C</b> with LiNC^<i>t</i>Bu₂ in tetrahydrofuran cleanly gives the monomeric lithium guanidinate <b>Li­[</b>^<b>Ar</b><b>*ketguan]</b>, free of coordinating solvent, in 85% yield. Protonation of <b>Li­[</b>^<b>Ar</b><b>*ketguan]</b> with lutidinium chloride produces the guanidine ^<b>Ar</b><b>*ketguanH</b> (MW = 1112.60 g/mol), which is easily derivatized to give the monomeric alkali metal complexes <b>M­[</b>^<b>Ar</b><b>*ketguan]</b> (M = K, Cs) in 94% and 51% yield, respectively. The solid-state molecular structures of <b>M­[</b>^<b>Ar</b><b>*ketguan]</b> (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 (% <i>V</i>_Bur) of <b>M­[</b>^<b>Ar</b><b>*ketguan]</b> [M = Li (94.8% <i>V</i>_Bur), K (92.1% <i>V</i>_Bur), Cs (81.7% <i>V</i>_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

Synthesis and Reactivity of a U(IV) Dibenzyne Complex

Reaction of UCl₄ with 6 equiv of 2-Li-C₆H₄CH₂NMe₂ affords the U­(IV) dibenzyne complex [Li]₂[U­(2,3-C₆H₃CH₂NMe₂)₂(2-C₆H₄CH₂NMe₂)₂] (<b>1</b>), which can be isolated as a dark blue solid in 40% yield. Complex <b>1</b> represents a rare example of a structurally characterized dibenzyne complex, and its solid-state metrical parameters suggest that the two benzyne ligands are best described with the dianionic metallacyclopropene resonance form. The reactivity of <b>1</b> with a variety of electrophiles and oxidants, including benzophenone, 1-azidoadamantane, and benzonitrile, was also explored. Reaction of <b>1</b> with 2 equiv of benzophenone affords the insertion product [Li]­[U­(2-C₆H₃CH₂NMe₂-3-C<i>O</i>Ph₂)₂(2-C₆H₄CH₂NMe₂)] (<b>3</b>) as a red-orange solid in 61% yield, concomitant with formation of 1 equiv of 2-Li-C₆H₄CH₂NMe₂. Reaction of <b>1</b> with 2 equiv of PhCN also affords an insertion product, [Li]­[Li­(Et₂O)]­[U­(2,3-C₆H₃CH₂NMe₂)­(2-C₆H₃CH₂NMe₂-3-C­(Ph)<i>N</i>)­(2-C₆H₄CH₂NMe₂)₂] (<b>4</b>), as a green-brown crystalline solid in 21% yield. In an attempt to oxidize <b>1</b> to U­(VI), the reaction of <b>1</b> with 2 equiv of AdN₃, in the presence of 2 equiv of 12-crown-4, was probed. This reaction only yields the U­(IV) insertion product [Li­(12-crown-4)₂]­[Li]­[U­(2-C₆H₃CH₂NMe₂-3-(<i>N</i>-N<i>N-</i>Ad))₂(2-C₆H₄CH₂NMe₂)₂] (<b>5</b>) as a red crystalline solid in 42% yield. No evidence for the formation of a U­(VI) imido complex is observed in the reaction mixture

Synthesis and Reactivity of a U(IV) Dibenzyne Complex

Reaction of UCl₄ with 6 equiv of 2-Li-C₆H₄CH₂NMe₂ affords the U­(IV) dibenzyne complex [Li]₂[U­(2,3-C₆H₃CH₂NMe₂)₂(2-C₆H₄CH₂NMe₂)₂] (<b>1</b>), which can be isolated as a dark blue solid in 40% yield. Complex <b>1</b> represents a rare example of a structurally characterized dibenzyne complex, and its solid-state metrical parameters suggest that the two benzyne ligands are best described with the dianionic metallacyclopropene resonance form. The reactivity of <b>1</b> with a variety of electrophiles and oxidants, including benzophenone, 1-azidoadamantane, and benzonitrile, was also explored. Reaction of <b>1</b> with 2 equiv of benzophenone affords the insertion product [Li]­[U­(2-C₆H₃CH₂NMe₂-3-C<i>O</i>Ph₂)₂(2-C₆H₄CH₂NMe₂)] (<b>3</b>) as a red-orange solid in 61% yield, concomitant with formation of 1 equiv of 2-Li-C₆H₄CH₂NMe₂. Reaction of <b>1</b> with 2 equiv of PhCN also affords an insertion product, [Li]­[Li­(Et₂O)]­[U­(2,3-C₆H₃CH₂NMe₂)­(2-C₆H₃CH₂NMe₂-3-C­(Ph)<i>N</i>)­(2-C₆H₄CH₂NMe₂)₂] (<b>4</b>), as a green-brown crystalline solid in 21% yield. In an attempt to oxidize <b>1</b> to U­(VI), the reaction of <b>1</b> with 2 equiv of AdN₃, in the presence of 2 equiv of 12-crown-4, was probed. This reaction only yields the U­(IV) insertion product [Li­(12-crown-4)₂]­[Li]­[U­(2-C₆H₃CH₂NMe₂-3-(<i>N</i>-N<i>N-</i>Ad))₂(2-C₆H₄CH₂NMe₂)₂] (<b>5</b>) as a red crystalline solid in 42% yield. No evidence for the formation of a U­(VI) imido complex is observed in the reaction mixture