Redox-Active Tripodal Aminetris(aryloxide) Complexes of Titanium(IV)

• Published in: Inorganic chemistry 2010-05, Vol.49 (10), p.4687-4697
• Main Authors: Lionetti, Davide, Medvecz, Andrew J, Ugrinova, Vesela, Quiroz-Guzman, Mauricio, Noll, Bruce C, Brown, Seth N
• Format: Article
• Language: English
• ISSN: 0020-1669; 1520-510X
• DOI: 10.1021/ic100347h

New sterically encumbered tripodal aminetris(aryloxide) ligands N(CH2C6H2-3- t Bu-5-X-2-OH)3 ( tBu,XLH3) with relatively electron-rich phenols are prepared by Mannich condensation (X = OCH3) or by a reductive amination/Hartwig−Buchwald amination sequence on the benzyl-protected bromosalicylaldehyde (X = N[C6H4-p-OCH3]2), followed by debenzylation using Pearlman’s catalyst (Pd(OH)2/C). The analogous dianisylamino-substituted compound lacking the tert-butyl group ortho to the phenol (H,An2NLH3) is also readily prepared. The ligands are metalated by titanium(IV) tert-butoxide to form the five-coordinate alkoxides LTi(O t Bu). Treatment of the tert-butoxides with aqueous HCl yields the five-coordinate chlorides LTiCl, and with acetylacetone gives the six-coordinate diketonates LTi(acac). The diketonate complexes tBu,XLTi(acac) show reversible ligand-based oxidations with first oxidation potentials of +0.57, +0.33, and −0.09 V (vs ferrocene/ferrocenium) for X = t Bu, MeO, and An2N, respectively. Both dianisylamine-substituted complexes R,An2NLTi(acac) (R = t Bu, H) show similar electrochemistry, with three one-electron oxidations closely spaced at ∼0 V and three oxidations due to removal of a second electron from each diarylaminoaryloxide arm at ∼ + 0.75 V. The new electron-rich tripodal ligands therefore have the capacity to release multiple electrons at unusually low potentials for aryloxide groups.