Electronic Structure Studies and Photophysics of Luminescent Th(IV) Anilido and Imido Complexes

A series of thorium anilide compounds [ThNHArR(TriNOx)] (R = para-OCH3 (1-ArOMe), para-H (1-ArH), para-Cl (1-ArCl), para-CF3 (1-Ar4-CF3), TriNOx3- = tris(2-tert-butylhydroxylaminato)benzylamine), and their corresponding imido compounds [Li(DME)][Th═NArR(TriNOx)] (2-ArR) as well as the alkyl congeners [ThNHAd(TriNOx)] (1-Ad) and [Li(DME)][Th═NAd(TriNOx)] (2-Ad), have been prepared. The para-substituents on the arylimido moiety were introduced for systematic variation of their electron-donating and withdrawing abilities, changes that were evident in measurements of the 13C{1H} NMR chemical shifts of the ipso-C atom of the ArR moiety. Room temperature, solution-state luminescence of the four new thorium imido compounds, along with the previously reported [Li(THF)2][Th═NAr3,5-CF3(TriNOx)] (2-Ar3,5-CF3) and [Li(THF)(Et2O)][Ce═NAr3,5-CF3(TriNOx)] (3-Ar3,5-CF3) have been described. Among these complexes, 2-Ar3,5-CF3 demonstrated the most intense luminescence feature with excitation at 398 nm and emission at 453 nm. The luminescence measurements, together with a time-dependent density functional theory (TD-DFT) study, helped uncover an intra-ligand n → π* transition that was assigned as the origin of the bright blue luminescence; 3-Ar3,5-CF3 has an 1.2 eV redshift in excitation energy compared with its proligand. The weak luminescence of other derivatives (2-ArR and 3-Ar3,5-CF3) was attributed to non-radiative decay from low-lying excited states originating from inter-ligand transitions (2-ArR) or ligand-to-metal charge transfer bands (3-Ar3,5-CF3). Overall, the results expand the range of the thorium imido organometallic compounds and demonstrate that thorium(IV) complexes can support strong ligand luminescence. The results also demonstrate the utility of applying a Th(IV) center for tuning the n → π* luminescence energy and intensity of an associated imido moiety.