Ground and excited state properties of photoactive platinum(IV) diazido complexes: theoretical considerations

Recently synthesized by the group of Sadler, the platinum(IV) diazido complexes [Pt(N(3))(2)(OH)(2)(L')(L'')] (L' and L'' are N-donor ligands) have potential to be used as photoactivatable metallodrugs in cancer chemotherapy. In the present study optimized structures and UV-Vis electronic spectra of trans,trans,trans- and cis,trans,cis-[Pt(N(3))(2)(OH)(2)(NH(3))(2)] (1t and 1c, respectively) as well as cis,trans,cis-[Pt(N(3))(2)(OH)(2)(L)(2)] (L = NH(3), NH(2)CH(3), NF(3), PH(3), PF(3), H(2)O, CO, OH(-), CN(-), py, imid; 2c-11c) and cis,trans-[Pt(N(3))(2)(OH)(2)(bpy)] (12c) complexes were predicted using density functional theory (DFT). The ground state electronic structures of all complexes were analyzed with the help of the natural bond orbital analysis (NBO). The electronic spectra of 1c and 1t were computed using time-dependent density functional theory (TDDFT) with five different density functionals and the ab initio CASSCF/CASPT2 method (for the five lowest energy transitions). The best agreement with available experiments was found in the case of the long-range corrected ωB97X functional. The electronic transitions were characterized by the analysis of the natural transition orbitals (NTO). The low-lying excited singlet states of 1t and 1c have significant azide-to-platinum(IV) charge-transfer character (LMCT). Geometry optimization of the three lowest singlet excited states performed using TDDFT results in the simultaneous dissociation of two azide ligands with the formation of the azidyl radicals N(3)˙ and photoreduction of Pt(IV) to Pt(II). Variation of the ligand L does not strongly affect the nature and the relative energies of the low-lying states. It is shown that the replacement of the OH(-) groups in 1c by OPh(-) ligands results in the red shift of the intense N(3)(-)→Pt LMCT band and the appearance of transitions with significant intensity in the visible region of the spectrum. The dissociative nature of the low-lying unoccupied orbitals remains unaffected. These theoretical results may suggest new experimental routes for the improvement of the photochemical activity of Pt(IV) diazido complexes.