Titanium(IV) Catecholate-Grafted Mesoporous Silica KIT-6: Probing Sequential and Convergent Immobilization Approaches

Molecular Ti-IV catecholates were obtained as dimeric complexes from protonolysis reactions of Ti(NMe2)(4) with pyrocatechol (H(2)CAT), 4-tert-butylcatechol (H(2)CATtBu-4), 3,5-di-tert-butylcatechol (H(2)CATtBu(2)-3,5), 2,3-dihydroxynaphthalene (H2DHN) and 3,6-di-tert-butylcatechol (H(2)CATtBu(2)-3,6). The heteroleptic monocatecholate amide complexes were characterized by X-ray structure analysis but only [Ti(CAT)(NMe2)(2)](2) and [Ti(DHN)(NMe2)(2)](2) were obtained as pure compounds. The solid-state structures revealed Ti-IV centers bridged by two catecholato ligands adopting a [kappa(2)(O,mu-O ')] coordination mode except for complex [Ti(CATtBu(2)-3,6)(NMe2)(2)] featuring one bridging catecholato and one bridging amido ligand. Heteroleptic complex [Ti(DHN)(NMe2)(2)](2) was grafted onto large-pore periodic mesoporous silica KIT-6 (convergent approach) and the resulting Ti-IV surface species compared with the established sequential approach, i.e., grafting of Ti(NMe2)(4) onto the KIT-6 material and subsequent ligand exchange with H2DHN. Dimeric complex [Ti(DHN)(NMe2)(2)](2) proved to be sufficiently reactive for a direct grafting yielding the same dominant surface species as obtained according to the sequential approach, as revealed by nitrogen physisorption, elemental analysis, and DRIFT/C-13 CP MAS spectroscopy.