Molecular and electronic structures and photophysical properties of quadruply bonded dimetal complexes (M = Mo or W) supported by trans-arylethynylcarboxylate ligands where aryl = p-tolyl or 9-anthrancenyl

Abstract

From the reactions between M₂(TⁱPB)₄, where TⁱPB = 2,4,6-triisopropylbenzoate and M = Mo or W, in toluene and each of the respective carboxylic acids (2 equiv) the quadruply MM bonded compounds trans-M₂(TⁱPB)₂(O₂C–CC–Ar)₂ have been prepared where Ar = p-tolyl and M = Mo, 1a, and M = W, 1b, and Ar = 9-anthracenyl, where M = Mo, 2a, and M = W, 2b. Single crystal X-ray crystallographic studies of 1a and 2a confirmed the trans substitution pattern about the Mo₂⁴⁺ unit and the centrosymmetric molecules have structural features that indicate extensive Lπ–Mo₂δ–Lπ conjugation involving the arylethynylcarboxylates. The compounds are intensely colored as a result of the HOMO → LUMO, metal δ-to-ligand π* charge transfer ¹MLCT transition: 1a (orange), 1b (red), 2a (blue) and 2b (green). The compounds 1a, 2a, 1b and 2b have been characterized by UV-Vis-NIR absorption and emission spectroscopy, cyclic voltammetry, femtosecond (fs) and nanosecond (ns) transient absorption spectroscopy and by fs time-resolved infrared spectroscopy in the region of ν(CC), ν(CO₂) and from 1400–1000 cm⁻¹. Aided by density functional theory, (DFT) and time dependent DFT, the electronic structures of the ground state and the S₁ and T₁ states are described. The molybdenum compounds have short lived ¹MLCT states, 1a ∼ 5.0 ps and 2a ∼ 10.5 ps, that undergo intersystem crossing to long lived ³MoMoδδ* states: 1a ∼ 101 μs and 2a ∼ 83 μs. The tungsten complexes show interesting time-resolved infrared spectra in the ν(CC) region when compared with their ground state. Compound 1b shows ν(CC) at 1975 cm⁻¹ for the ¹MLCT state which decays with τ ∼ 0.7 ps to ν(CC) at 2000 cm⁻¹ for the ³MLCT state. For 2b the ¹MLCT is characterized by ν(CC) at 2150 cm⁻¹, τ ∼ 19 ps, and a very broad absorption with a maximum ∼1970 cm⁻¹ which is proposed to arise from a low energy electronic transition. The ³MLCT state for 2b shows no evidence of ν(CC) and is suggested to have an electron localized principally on the anthracenyl portion of the ligand, a proposal that finds support from the nature of triplet transient absorption spectrum of 2b.