Steric Control of Coordination Geometry in Titanium-Imido Complexes of N,N′-bis(arylimino)acenaphthylene Ligands

Titanium complexes of N,N'-bis(arylimino)acenaphthylene (BIAN) alpha-diimine ligands with varied steric profiles have been prepared. Coordination of the BIAN ligand derivatives to TiCl4 afforded the adducts (dpp-BIAN)TiCl4 (1a), (tmp-BIAN)-TiCl4 (1b), and (dmp-BIAN) TiCl4 (1c) (dpp = 2,6-diisopropylphenyl; tmp = 2,4,6-trimethylphenyl; dmp = 3,5-dimethylphenyl). While the least sterically crowded complex 1c is robust toward loss of the diimine ligand, the dpp-BIAN and tmp-BIAN ligands are readily displaced by pyridine from the more crowded derivatives 1a and 1b, respectively. The crowded profiles engendered by the tmp-BIAN and dpp-BIAN ligands result in the formation of five-coordinate titanium-imide complexes, (dpp-BIAN)TiCl2(=(NBu)-Bu-t) (2a) and (tmp-BIAN)TiCl2(=(NBu)-Bu-t) (2b), upon addition of (BuNH2)-Bu-t to solutions of 1a or 1b, respectively. Single-crystal X-ray diffraction studies reveal a square pyramidal coordination environment with an apical imide ligand and a short Ti-N distance, consistent with a Ti-N triple bond. Conversely, the less crowded dmp-BIAN ligand affords a six-coordinate titanium imido complex, (dmp-BIAN)TiCl2(=(NBu)-Bu-t)((NH2Bu)-Bu-t) (4), upon treatment of 1c with (BuNH2)-Bu-t. Surprisingly the imido ligand is coordinated trans to one arm of the diimine. This six coordinate species is fluxional in solution, and exchange and variable temperature H-1 NMR experiments suggest dissociation of the coordinated (BuNH2)-Bu-t ligand to generate a five-coordinate imido intermediate analogous to 2a and 2b.