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

I. jy Chang, Daniel G. Nocera

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30 Citations (Scopus)


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* (1A2u1Alg) 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)43_/4_and Mo2(HP04)42-/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* (2Blu2B2g) 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, gl{= 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* (!A2u4«- 1Alg) 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.

Original languageEnglish
Pages (from-to)4901-4907
Number of pages7
JournalJournal of the American Chemical Society
Issue number16
Publication statusPublished - 1987 Aug 1
Externally publishedYes

ASJC Scopus subject areas

  • Catalysis
  • General Chemistry
  • Biochemistry
  • Colloid and Surface Chemistry


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