Zirconium Alkyl Complexes Supported by Ureate Ligands: Synthesis, Characterization, and Precursors to Metal−Element Multiple Bonds

A series of bis(ureate) zirconium complexes bearing reactive alkyl ligands have been prepared and fully characterized. A sterically demanding, nontethered ureate ligand was successfully employed in the synthesis of a dibenzyl complex; however synthesis can be complicated by suspected ligand disproportionation and redistribution. In contrast, tethered bis(ureate) ligands, including a chiral, C2-symmetric ligand, are reliable supports for sterically accessible, mononuclear dibenzyl and bis(neopentyl) complexes. These coordinatively unsaturated species react with pyridine to form seven-coordinate adducts that exhibit remarkable thermal and photochemical stability. The reactive nature of the dibenzyl derivative can be exploited in the synthesis of imido complexes via protonolysis with 2,6-diisopropylaniline. A monometallic imido, supported by a sterically open ureate ligand, can be prepared in this manner with the use of excess pyridine; however, NMR spectroscopy indicates that this species undergoes dimerization in solution. When the reaction is performed in the absence of pyridine, only the dimeric complex is obtained.