Tungsten–alkylidyne complexes of the form W(CAr)L4X (Ar = aryl; L = neutral ligand; X = halide) are known to possess widely tunable redox, optical, and photophysical properties and include examples that are powerful photoreductants competent for photoredox catalysis. We report the synthesis of a series of isoelectronic d2 molybdenum-alkylidyne and tantalum-imido complexes with general formula M(EPh)(R2PCH2CH2PR2)2Cl (R = Et, depe; R = Ph, dppe; E = C, N), which has allowed elucidation of the effects of the nature of the metal and multiply bonded ligand on the ground and excited-state properties of these chromophores. Single-crystal X-ray diffraction studies show that these compounds are isostructural, allowing direct comparison of their relevant properties. Cyclic voltammetry of the complexes shows that they possess quasireversible one-electron oxidations to the corresponding d1 complex, with oxidation potentials that depend strongly on M and E. The electronic-absorption spectra of the compounds manifest similarly strong perturbations, with the molybdenum complexes showing broad 1(d®π*) transitions deep in the visible region and those from the tantalum complexes extending into the near infrared. DFT calculations on these compounds relate the shifts in these properties to M- and E-derived variations in their frontier-orbital energies. Long-lived room temperature phosphorescence is observed from Mo(CPh)(depe)2Cl, from which an estimated excited-state oxidation potential of E*/+ = –2.6 V vs. Fc0/+, highlighting its potential use as a photoreductant in photocatalytic reactions.

Metal dependent properties of a series of isoelectronic multiply metal-ligand bonded photochemical super-reductants

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