Steric factors on unsymmetrical O-hydroxyaryl N-Heterocyclic carbene ligands prevailing the stabilization of single stereoisomer of bis-ligated titanium complexes

Abstract

Bis-ligated titanium (IV) metal complexes supported by bidentate unsymmetrical o-hydroxyaryl-substituted N-heterocyclic carbene ligands were synthesized and structurally identified. While the direct addition of the doubly deprotonated bulky imidazolidinium chloride salts [Dipp,4-RNHC-H]Cl (with Dipp = 2,6-diisopropylphenyl, R = H (2-hydroxyphenyl), and R = Me (2-hydroxy-4-methyl-phenyl)) with chloro-titanium precursor favors the formation of single stereoisomer corresponding to the bis-ligated titanium complexes trans-([κ2-C,O]-Dipp,4-RNHC)2TiCl2 (R = H (2-hydroxyphenyl) for 4aH, and R = Me (2-hydroxy-4-methyl-phenyl) for 4aMe), the reactivity with sterically less hindered imidazolidinium chloride salts [Mes,HNHC-H]Cl and [Dep,HNHC-H]Cl as protio-ligands (with Mes = 2,4,6-trimethylphenyl and Dep = 2,6-diethylphenyl) did not lead to single stereoisomer, but rearranged into a tetradentate salophen-like complexes cis-([κ4-O,N,N,O]-bis(imidazolidinylidene)TiCl2 as major isomer via presumably the NHCs dimerization from bis-ligated intermediates. These results combined with topographic steric maps as well as the buried volume descriptor (%Vbur) indicate that bidentate bulky N-Dipp-substituted NHC ligands offer some level of steric protection preventing the formation of other possible bis-ligated (C,O)-NHC-titanium stereoisomers and constitutional isomers.