Molybdenum–molybdenum quadruple bonds supported by 9,10-anthraquinone carboxylate ligands. Molecular, electronic, ground state and unusual photoexcited state properties

Abstract

The preparation of trans-Mo₂(TⁱPB)₂(O₂C-2-AnQ)₂, I, and Mo₂(DAniF)₃(O₂C-2-AnQ), II, where, O₂C-2-AnQ = 9,10-anthraquinone-2-carboxylate, TⁱPB = 2,4,6-triisopropylbenzoate and DAniF = N,N′-di-(p-anisyl)formamidinate are described together with electronic structure calculations employing density functional theory on model compounds. Compounds I and II have been further characterized by ¹H NMR spectroscopy, MALDI-TOF mass spectrometry, electrochemical studies, UV-vis-NIR and steady state emission spectroscopy, time-resolved (fs and ns) transient absorption and infrared spectroscopy. Compound II has been structurally characterized by a single crystal X-ray crystallography study. Both compounds show metal-to-ligand charge transfer transitions, ¹MLCT, corresponding to Mo₂ δ to L π* (L = O₂C-2-AnQ) and have S₁ states with relatively long lifetimes (τ_I ∼ 8 ps; τ_II ∼ 2 ps). Femtosecond time-resolved IR studies, fs-TRIR indicate that for I, despite its relatively small HOMO–LUMO gap, and II the negative charge in the S₁ state resides on one anthraquinone carboxylate ligand. For II, fs-TRIR spectroscopy detects the Mo₂³δδ* state with τ ∼ 100 ns despite the presence of a lower energy ³MLCT state. This, together with earlier findings indicates a significant kinetic barrier to the interconversion of MLCT and Mo₂ δδ* states.