Unique tripodal chiral tertiary amine, 2,6-trans-1,2,6-trisubstituted piperidine with pyridine and bis(phenol) donor groups: Its stereoselective coordination to titanium(IV) ion

A new optically active functionalized tertiary amine, (2R,6R)-2,6-bis(2-hydroxyphenyl)-1-(2-pyridylmethyl)piperidine {(R,R)-1}, was synthesized and its complexation behavior as a tripodal ligand was elucidated with titanium(IV) ion. Protonolysis of Ti(OPr-i)4 with (R,R)-1 and H2O in a stepwise manner led to the formation of C3-symmetric tris(μ-oxo)trititanium(IV,IV,IV) complex [Ti3L3(μ-O)3] {L = diphenolato anion of (R,R)-1} with unified chiralities induced at the metal center (A) and by the conformations of the N∧O chelate loops (λ) as well as the six-membered non-planner chair-like (Ti-μ-O)3 ring core as defined by X-ray crystallography. These stereochemical issues are principally regulated by the stereogenic centers on the ligand and a chair conformation of the piperidine skeleton, which settle an enantiotopos-differentiating cis(Ophenolato–Ophenolato) binding mode by unequivocally locating one Ophenolato in an axial position and the other Ophenolato in an equatorial plane.

Graphical abstractA new chiral tertiary amine (H2L) based on 2,6-trans-disubstituted piperidine scaffold appended with pyridine and bis(phenol) functionalities was synthesized and demonstrated to work as a tetradentate tripodal ligand, forming [Ti3L3(μ-O)3] with unified chiralities induced at the metal center by the N∧O chelate and the chair-like (Ti-μ-O)3 ring conformations.Figure optionsDownload full-size imageDownload as PowerPoint slide