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

Abstract

© 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim The imidazole-containing N4-tetradentate ligand N-(2-(1H-imidazol-2-yl)-3-(pyridin-2-yl)propyl)-2,6-diisopropylaniline (L2H) and its N-benzyl-protected variant (L2Bn) at the imidazole fragment have been synthesized and fully characterized. Both molecules contain an unresolved Csp3 stereogenic center. The coordination behavior of the newly prepared ligands towards group IV metal ions (MIV = Zr, Hf) has been examined through multinuclear 1H and 13C{1H} 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 (L2Bn), the ligand reaction with MIV(NMe2)4 (MIV = Zr, Hf) as metal precursor gives rise to discrete monometallic tris(dimethylamido) 5-coordinated compounds of general formula L2BnM(NMe2)3. The ligand chelates the metal ion as a bidentate monoanionic κ2{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 L2H 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 [L2HM(NMe2)2]4 named “poker complexes” contain the imidazole fragment of each ligand bridging two adjacent MIV ions in a µ-κ{N}:κ{N–} coordination hapticity. At the same time, the picolyl fragments of each chelating L2H ligand “sting” a neighboring metal center as unconventional scorpion's tails that impose further rigidity to the tetrameric structure.