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.