The molecular, electronic, bonding, and photophysical features of the [(c-Pt3)Tl(c-Pt3)]+ inorganic metallocenes

Abstract

The molecular, electronic, bonding and photophysical properties of a series of inorganic metallocenes with the general formula {[Pt₃(μ₂-L)₃(L′)₃]₂(μ₆-Tl)}⁺ (L = CO, CH₃CN, PH₂, C₆F₅, or SO₂ and L′ = CO, PH₃, CH₃CN, C₆F₅) have been studied by means of DFT electronic structure calculations. The estimated Tl–cd distances between Tl⁺ cations and the centroids (cd) of the trimetallic Pt₃(μ₂-L)₃(L′)₃ {3 : 3 : 3} decks were found in the range 2.932–3.397 Å. The predicted bond dissociation energy, D₀, of the (c-Pt₃)⋯Tl⁺ bonds was found to lie within the range −31.5 up to −77.5 kcal mol⁻¹ at the B3LYP/LANL2TZ(f)(Pt) ∪ 6-31G(d,p)(E) ∪ SRLC(Tl) level of theory. Most of the [(c-Pt₃)Tl(c-Pt₃)]⁺ inorganic metallocenes adopt a bend titanocene-like structure. The Localized Orbital Locator (LOL) contour maps along with the 3D contour plots of the Reduced Gradient Density (RDG) mirror the composite nature of the interaction of Tl⁺ with the triangular Pt₃ metallic ring cores consisting of electrostatic, covalent and dispersion interaction components. The Pt₃⋯Tl⁺⋯Pt₃ bonding mode was further validated by Energy Decomposition Analysis (EDA) calculations which demonstrated that the electrostatic and covalent components of the interaction contribute almost equally to the bonding interactions. Furthermore, Charge Decomposition Analysis (CDA) and Natural Bond Orbital Analysis (NBO) calculations indicated that charge transfer from the Tl⁺ cation to the Pt₃(0) {3 : 3 : 3} decks also occurs. The {[Pt₃(μ₂-L)₃(L′)₃]₂(μ₆-Tl)}⁺ sandwiches absorb in the UV-Vis region (300–500 nm) and emit in the visible-near IR region (600–1000 nm). The absorption bands are mainly of MLCT/MC character while phosphorescence is predicted to occur via the first triplet excited state, T₁, since the spin density of this excited state could be described as a SOMO − 1/SOMO combination. Generally, no significant distortions occur upon excitation of these systems from their S₀ ground state to the first triplet excited state, T₁. The S₀ → T₁ transition causes only an elongation of the Tl–cd distances by up to 0.7 Å accompanied in a few cases by a change in the <cd–Tl–cd bond angle as well.