Photochemistry of Multiply Bonded Dimolybdenum Phosphate Complexes In Acidic Solution: Photoinduced Two-electron Oxidation of Mo₂(hp0₄)₄⁴~ Ion

The multiply bonded dimolybdenum phosphate dimers Mo₂(HP0₄)₄⁴“ and Mo₂(HP0₄)₄³“ have been prepared and spectroscopically and electrochemically characterized. The electronic absorption spectrum of the quadruply bonded complex mo ₂(HP0₄)₄⁴' in H₃P0₄is typical of many M—M species with the 6²-→ 66* (1A_2u¹A_lg) transition occurring at 516 nm. Cyclic voltammograms of phosphoric acid solutions of Mo₂(HP0₄)₄⁴“ are characterized by two reversible waves at -0.67 and -0.25 V vs. SCE that we have attributed to the Mo₂(HP0₄)₄^3_/4_and Mo₂(HP0₄)₄^2-/3_couples, respectively. Oxidation of Mo₂(HP0₄)₄⁴~ produces the mixed-valence dimer Mo₂(HP0₄)₄³~, which exhibits an intense near-infrared absorption band that we have assigned to the 6~6 6* (²B_lu²B_2g) transition. Solid K₃Mo₂(HP0₄)₄is paramagnetic and follows Curie law behavior Gu- 1.58 ^_B). The EPR spectrum of K₃Mo₂(HP0₄)₄at 5 K shows an axial signal (g_±= 1.894, g_l{= 1.886). Whereas Mo₂(HP0₄)₄⁴~ thermally reacts in 2 M H₃P0₄to produce Mo₂(HP0₄)₄³“ and hydrogen over a period of days, irradiation (X > 335 nm) of phosphoric acid solutions of the dimer leads to the facile production of Mo₂(HP0₄)₄²“ and hydrogen. The thermal reaction presumably results from the slow conversion of Mo₂(HP0₄)₄⁴' to yield hydrogen and Mo₂(HP0₄)₄²~ which reacts in an ensuing comproportionation reaction with Mo₂(HP0₄)₄⁴“ to produce Mo₂(HP0₄)₄³~. In contrast, the photochemical reaction mechanism is consistent with sequential oxidation of the Mo₂core [i.e,, Mo₂(II,II) —>- Mo₂(II,ITI) —* Mo₂(IIIJII)]. Electronic absorption spectra of the Mo₂(HP0₄)₄““ (n - 2–4) dimers in the ultraviolet spectral region, wavelength-dependent quantum yield measurements, and photochemical studies of Mo₂phosphate dimers under N₂0 atmospheres have led us to postulate a 7r 7r* (^!A_2u4«- 1A_lg) parentage for the photoactive state of Mo₂phosphato complexes. We propose that excitation of this transition leads to the direct production of hydrogen atoms which undergo subsequent reaction to produce hydrogen.