Imidazole-Bridged Tetrameric Group(IV) Heteroleptic Complexes from the Spontaneous Metal-Ligand Assembly of a Potentially N-4-Tetradentate Ligand

The imidazole-containing N-4-tetradentate ligand N-(2-(1H-imidazol-2-yl)-3-(pyridin-2-yl)propyl)-2,6-diisopropylaniline (L2(H)) and its N-benzyl-protected variant (L2(Bn)) at the imidazole fragment have been synthesized and fully characterized. Both molecules contain an unresolved C-sp3 stereogenic center. The coordination behavior of the newly prepared ligands towards group IV metal ions (M-IV = Zr, Hf) has been examined through multinuclear H-1 and C-13{H-1} NMR spectroscopy and selected single-crystal X-ray structural analyses. The ability of the imidazole fragment to enter the metal coordination sphere as a neutral or a monoanionic system has also been investigated, unveiling quite original coordination modes as well as unexpected molecular architectures. When one N imidazole atom is blocked by a benzyl protecting group (L2(Bn)), the ligand reaction with M-IV(NMe2)(4) (M-IV = Zr, Hf) as metal precursor gives rise to discrete monometallic tris(dimethylamido) 5-coordinated compounds of general formula L2(Bn)M(NMe2)(3). The ligand chelates the metal ion as a bidentate monoanionic kappa(2){N-,N} system through the imidazole moiety and the anilido N donor while an uncoordinated picolyl arm dangles away from the metal center. Upon coordination to the metal ion, the unprotected L2(H) undergoes a unique self-assembly of the chiral racemic ligand to generate an achiral tetrameric network featuring a regularly alternating (R*,S*,R*,S*) configuration around the 6-coordinated metal centers. The resulting bis(dimethylamido) tetrameric architectures of formula [L2(H)M(NMe2)(2)](4) named "poker complexes" contain the imidazole fragment of each ligand bridging two adjacent M-IV ions in a mu-kappa{N}:kappa{N-} coordination hapticity. At the same time, the picolyl fragments of each chelating L2(H) ligand "sting" a neighboring metal center as unconventional scorpion's tails that impose further rigidity to the tetrameric structure.