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Abstract

Photoredox catalysis has become a useful synthetic technique to make and break chemical bonds in chemical structures relevant to medicinal chemistry and renewable energy. Few general approaches, however, have been reported that can prepare synthetically useful organic radicals whose parent molecules are reduced negative of –2.6 V (vs FeCp20/+). One such approach to accessing these molecules is to design photoredox chromophores that can directly prepare these radical species. The central hypothesis investigated here is that the electronic structure of tungsten-alkylidyne (or benzylidyne) complexes lends them to serve as highly reducing, visible-light photoreductants. This work describes the electronic-structure description, optimization of synthetic routes, excited-state characterization, and net reactions of tungsten-alkylidyne complexes of the general form W(CAr)L4X (Ar = aryl, L = bidentate phosphine, X = halide/pseudo-halide). First, we elaborate the general synthetic routes to these complexes for the purpose of electronic-structure design. The electronic-structure design was accomplished via Density Functional Theory (DFT) with experimental correlations. This was used to design highly reducing tungsten photoredox catalysts. The photophysics of these complexes were studied to assess their reactivity as photoredox chromophores, and the experimental room temperature measurement of the excited-state oxidation potentials for two tungsten derivatives demonstrated the highly reducing nature of these complexes. Finally, net photoredox reactions were explored. Among those were the C–H arylation of difficult-to-reduce aryl halide (I, Br, and Cl) molecules. These reactions could be affected using low chromophore loadings and a simple industrially relevant base. A second conceptually related project stemmed from the observation that some of the electrochemical first reduction potentials of a series of molybdenum(IV)-oxo complexes of the general type, [Mo(O)L4X]+ were reversible. We were able to isolate and study an unusual example of a low-spin d3 molybdenum-oxo complex via single-electron reduction. This complex was observed to possess an elongated bond relative to the d2 redox congener by single crystal X-ray crystallography that could be corroborated by vibrational spectroscopy and DFT.

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