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═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.

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] (1), which can be isolated as a dark blue solid in 40% yield. Complex 1 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 1 with a variety of electrophiles and oxidants, including benzophenone, 1-azidoadamantane, and benzonitrile, was also explored. Reaction of 1 with 2 equiv of benzophenone affords the insertion product [Li]­[U­(2-C6H3CH2NMe2-3-COPh2)2(2-C6H4CH2NMe2)] (3) as a red-orange solid in 61% yield, concomitant with formation of 1 equiv of 2-Li-C6H4CH2NMe2. Reaction of 1 with 2 equiv of PhCN also affords an insertion product, [Li]­[Li­(Et2O)]­[U­(2,3-C6H3CH2NMe2)­(2-C6H3CH2NMe2-3-C­(Ph)N)­(2-C6H4CH2NMe2)2] (4), as a green-brown crystalline solid in 21% yield. In an attempt to oxidize 1 to U­(VI), the reaction of 1 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-(N-NN-Ad))2(2-C6H4CH2NMe2)2] (5) 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.

The synthesis and spectroscopic and structural characterization of well-defined organometallic Ni(II) and Ni(III) complexes bearing the PyNMe3 ligand - a tetradentate N-based macrocyclic ligand which coordinates to the metal center...

The last two decades have experienced a dramatic development of nickel-catalyzed cross-coupling reactions involving alkyl substrates, including a wide range of stereoselective transformations. In these reactions, nickel complexes supported by N-donor ligands have been proposed to involve paramagnetic Ni(I) and Ni(III) species as the active intermediates during catalysis. Herein, we report the use of a bulky 1,4,7-triisopropyl-1,4,7-triazacyclononane (iPr3TACN) ligand that allowed the detection and isolation of uncommon organometallic Ni(I) and Ni(III) complexes, which are involved in well-defined oxidative addition, transmetallation, and reductive elimination steps of the proposed catalytic cycle. Moreover, (iPr3TACN)Ni(II) complexes were shown to be efficient catalysts for the alkyl-alkyl Kumada cross-coupling, while a beneficial effect of acetonitrile on the yield of the desired cross-coupled products was observed, likely due to the stabilization of transient Ni(I) species. Overall, these studies provide unambiguous evidence for the involvement of both Ni(III) and Ni(I) organometallic species in Ni-catalyzed alkyl-alkyl cross-coupling reactions.

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