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 M(2)(T(i)PB)(4), where T(i)PB = 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-M(2)(T(i)PB)(2)(O(2)C-C≡C-Ar)(2) have been prepared where Ar = p-tolyl and M = Mo, , and M = W, , and Ar = 9-anthracenyl, where M = Mo, 2a, and M = W, 2b. Single crystal X-ray crystallographic studies of 1a and 2b confirmed the trans substitution pattern about the Mo(2)(4+) unit and the centrosymmetric molecules have structural features that indicate extensive Lπ-Mo(2)δ-Lπ conjugation involving the arylethynylcarboxylates. The compounds are intensely colored as a result of the HOMO → LUMO, metal δ-to-ligand π* charge transfer (1)MLCT 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(C≡C), ν(CO(2)) 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 S(1) and T(1) states are described. The molybdenum compounds have short lived (1)MLCT states, 1a ~ 5.0 ps and 2a ~ 10.5 ps, that undergo intersystem crossing to long lived (3)MoMoδδ* states 1a ~ 101 µs and 2a ~ 83 µs. The tungsten complexes show interesting time-resolved infrared spectra in the ν(C≡C) region when compared with their ground state. Compound 1b shows ν(C≡C) at 1975 cm(-1) for the (1)MLCT state which decays with τ ~ 0.7 ps to ν(C≡C) at 2000 cm(-1) for the (3)MLCT state. For 2b the (1)MLCT is characterized by ν(C≡C) 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 (3)MLCT state for shows no evidence of ν(C≡C) 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.