Metal-Metal Quadruple Bonds Supported by 5-Ethynylthiophene-2-carboxylato Ligands: Preparation, Molecular and Electronic Structures, Photoexcited State Dynamics, and Application as Molecular Synthons

From the reaction between M-2((TPB)-P-i)(4) and 2 equiv of 5-ethynylthiophene-2-carboxylic acid (H-ThCCH) in toluene, the complexes trans-M-2((TPB)-P-i)(2)(ThCCH)(2), where M = Mo (I) or W (II) and (TPB)-P-i = 2,4,6-triisopropyl benzoate, have been isolated and characterized by H-1 NMR, IR, MALDI-TOF MS, UV-vis, steady-state emission, transient aborption, and time-resolved infrared (TRIR) spectroscopies and single-crystal X-ray crystallography for I. The molecular structure of I confirms the trans-substitution pattern and the extended conjugation of the ethynylthienyl ligands via interaction with the Mo-2 delta orbital. The HOMO of both I and II is the M-2 delta orbital, and the intense color of the compounds (I is red and II is blue) is due to the M-2 delta-to-ThCCH (MLCT)-M-1 transition. The S-1 states for I and II are (MLCT)-M-1. The T-1 state is (MLCT)-M-3 for II, but (MoMo)-Mo-3 delta delta* for I. The TRIR spectra of the V(C C) stretch in the MLCT states are consistent with the delocalization of the electron over both ThCCH litands. Compound I is shown to be a synthon for the preparation of trans-Mo-2((TPB)-P-i)(2)(ThCCPh)(2) (III) and trans-Mo-2((TPB)-P-i)(2)(ThCCAuPPh3)(2) (IV). Both III and IV have been characterized spectroscopically and by single-crystal X-ray diffraction. The structure of III indicates the extended pi-conjugation of the trans-ethynyl-thienyl units extends to the added phenyl rings.