Quadruply Bonded Dimetal Units Supported by 2,4,6-Triisopropylbenzoates MM(TiPB)4 (MM = Mo2, MoW, and W2): Preparation and Photophysical Properties

The preparation and characterization (elemental analysis, H-1 NMR, and cyclic voltammetry) of the new compounds MM(TiPB)(4) where MM = MOW and W-2 and TiPB = 2,4,6-triisopropylbenzoate, are reported. Together with Mo-2(TiPB)(4), previously reported by Cotton et al. (Inorg. Chem. 2002, 41, 1639), the new compounds have been studied by electronic absorption, steady-state emission, and transient absorption spectroscopy (femtosecond and nanosecond). The compounds show strong absorptions in the visible region of the spectrum that are assigned to MM delta to arylcarboxylate pi* transitions, (MLCT)-M-1. Each compound also shows luminescence from two excited states, assigned as the (MLCT)-M-1 and (MM)-M-3 delta delta* states. The energy of the emission from the (MLCT)-M-1 state follows the energy ordering MM = Mo-2 > MOW > W-2, but the emission from the (MM)-M-3 delta delta* state follows the inverse order: MM = W-2 > MOW > M02. Evidence is presented to support the view that the lower energy emission in each case arises from the (MM)-M-3 delta delta* state. Lifetimes of the (MLCT)-M-1 states in these systems are similar to 0.4-6 ps, whereas phosphorescence is dependent on the MM center: Mo-2 similar to 40 mu s, MOW similar to 30 mu s, and W-2 similar to 1 mu s.