Journal of the American Chemical Society
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100.4
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February 15, 1978
Production of Hydrogen by Ultraviolet Irradiation of Binuclear Molybdenum(11) Complexes in Acidic Aqueous Solutions. Observation of Molybdenum Hydride Intermediates in Octahalodimolybdate(11) Photoreactions William C. Trogler, David K. Erwin, Gregory L. Geoffroy, and Harry B. Gray* Contribution No. 5598from the Arthur Amos Noyes Laboratory of Chemical Physics. California Institute of Technology, Pasadena, California 91 125. Received June 17, 1977
Abstract: Irradiation (254 nm) of M02(S04)4~-in aqueous H2S04 at 25 " C yields 0.5 mol of H2 and M02(S04)4~-. The disappearance quantum yield for this photoreaction (0.17 f 0.02) is independent of acid concentration (0.001-5 M H2S04). Similar irradiation of M02(aq)~+in I M H03SCF3 produces 1 mol of hydrogen and M o 2 ( ~ - O H ) 2 ( a q ) ~The + . quantum yield for this process is 0.035 (254-nm excitation). When Mo2Cls4- is irradiated a t 254 nm in 3 M HCI, Mo2C18H3- is formed with a quantum yield of 0.13. In a subsequent step, Mo2C18H3- thermally decomposes to yield 1 mol of hydrogen gas and Moz( y - O H ) ~ ( a q ) ~ +A. similar reaction occurs for Mo2Brs4- in 3 M HBr (a254 = 0.043). At 313 nm the quantum yields for Mo2XgH3- formation from Mo2Xs4- in 3 M H X are 0.008 ( X = CI) and 0.007 (X = Br). It is proposed that Mo2XsH3- ( X = CI, Br) is formed from the reaction of H 2 0 with a ligand to metal charge transfer (LMCT) excited state of MoZXs4-. No reaction is observed when M O ~ ( O ~ C C H Z N Hin~0.25 ) ~ ~M+ HCI is subjected to UV irradiation.
Introduction a I .O- or 0. I-cm quartz cuvette attached to a Pyrex tube, which in turn was connected to a Teflon needle valve and a IO-mL Pyrex bulb. A Photooxidation reactions have been observed for a variety 24/20 ground glass joint after the needle valve allowed attachment of metal ions i n aqueous Frequently, molecular hyto a vacuum line. Solid samples, weighed in room atmosphere, were drogen is one product of the photoredox reaction. For example, transferred to the cuvette side arm and the appropriate solvent was ultraviolet excitation of Cr(aq)*+, Fe(aq)2+, Eu(aq)2+, and added to the Pyrex bulb. The cell was then sealed, attached to the vacuum line, and freeze-pump-thaw degassed for three to four cycles; Ce(aq)3+ results in a one-electron oxidation of the metal center at this point the solution was mixed with the solid in the side arm. For along with formation of molecular hydrogen.] Recently, we H2SO4, HCI, and HBr solutions, 5 m L of the acid mixture was pireported4 the first examples of analogous reactions involving petted into the IO-mL bulb. Solutions of M02(aq)~+were transferred binuclear Mo(I1) and Rh(1) complex ions. In order to test the in a nitrogen-filled glovebag. Extinction coefficients were determined generality of this process for binuclear Mo(I1) complexes, we in 0.1-cm cells so that a sufficiently large amount (0.02 g) of material have examined the photochemistry of M02(aq)~+,Mo2Xg4could be used for an accurate weighing. N o correction for sample ( X = C1, Br), and M O ~ ( O Z C C H ~ N Hin~acidic ) ~ ~ +aqueous volume was made, as it is an insignificant source of error. Molar exsolutions. tinction coefficients were reproducible to f5% and are believed to be Although photooxidation reactions in aqueous media are precise within &IO%. often loosely described as the result of charge transfer to solvent Irradiations at 546,505, and 436 nm were obtained from the output of a 1000-W Hg-Xe arc lamp that was collimated before passing (CTTS) excitation, solvated electron production has only been through the appropriate interference filter. For some 546-nm irraestablished for those complex ions where independent specdiations, light from a 450-W Hanovia medium-pressure H g lamp was troscopic studies also indicate a low-lying CTTS absorption (e.g., Cr(aq)2+, Fe(CN)h4-, W(CN)g4-, and M O ( C N ) ~ ~ - ) . ' - ~passed through a Corning 4-102 glass filter combination. A 30-W low-pressure H g lamp (Hanovia Germicidal) with a Vycor envelope This should be compared with the fact that solvated electrons was used as a source of 254-nm radiation. Owing to competing thermal have not been observed subsequent to flash photolysis of side reactions, it often was vital to have an intense source of 254-nm aqueous ferrous ~ o l u t i o n sMechanistic .~ studies5 have impliradiation. Therefore, the output of the low-pressure H g lamp was not cated the presence of H atoms during photolysis of Fe(aq)2+. filtered. The only other spectral line of appreciable intensity is at 578 Kinetic evidenceh for the photooxidation reaction is consistent nm.15In each instance we have shown that the compounds examined are not photoactive in this spectral region, The sample temperature with a mechanism involving hydrogen evolution from a FeH2+ was maintained a t 25.0 f 1.0 OC in all photochemical experiments. species, the latter presumably being formed from the reaction' Quantum yields were determined a t 254 nm employing the ferof H and Fe(aq)2+. Herein we present direct evidence that the rioxalate a ~ t i n o m e t e r .The ' ~ procedure was modified to adopt the production of hydrogen by ultraviolet irradiation of acidic precautions recently suggested by Bowman and Demas.16 Quantum aqueous solutions of octahalodimolybdate(I1) ions occurs via yields for the binuclear Mo(I1) compounds were obtained by moniintermediate hydride complexes. toring the decrease in intensity of the lowest energy visible absorption band. Aqueous solutions of binuclear Mo(1I) ions often decompose thermally.' 1.12,14 Correction for these thermal background reactions The compounds K~MOZCI~.~H~O ( N, *H ~ ) ~ M O ~ B Q was , ~ necessary for the quantum yield determinations. This was acK4Mo2(S04)4;i0 R ~ ~ M o ~ C I and ~ H Mo2(02CCHzNH3)4, ~ ~ . ~ ~ complished by alternating periods of time with and without irradiation for each sample. In most instances this correction was less than 5-1Wo. ( S 0 4 ) ~ 4 H @ , were prepared by published methods. Solutions of No corrections for inner filter effects were applied, as the quantum M02(aq)~+in I M H03SCF3 were prepared as described by Bowen and Taube.I4 Constant-boiling HBr was purified by distillation from yields were obtained for less than 10% photolysis. Typically, three red phosphorus at reduced pressure. Although binuclear Mo(I1) measurements were obtained, with an internal consistency of f 5 % . Except in those cases where the thermal correction was large (i.e., the compounds are air stable for short (3 h) to long (1 year) periods of time photochemical reactions in 0.001 M HzSO4 and 6 M HCI), reproin the solid state, they are exceedingly oxygen sensitive in solution. ducibility of the quantum yields was f l 5 % . The following procedures were adopted to minimize sample decomHydrogen evolution was measured for photolyzed samples in vacposition. Solids were stored under N2 prior to use. Solution spectra uum-tight I-cm spectrophotometer cells. The amount of evolved HZ were measured in special evacuable cells. The apparatus consisted of
Experimental Section
0002-7863/78/1500-1160$01 .OO/O
0 1978 American Chemical Society
Gray et al. / Irradiation of Binuclear Molybdenum(II) Complexes
1161
Table 1. Disappearance Quantum Yields for Photooxidation Reactions of Binuclear Mo(1l) Complexes in Aqueous Acids (254-nm Excitation) at 25 OC Concn X IO3, M
Complex
A,,
K~Moz(SO~)~ K4M02(S04)4 K~Moz(SO~)~ K4M02(S04)4 K4M02(S04)4 Mo2(ad4+ Mo2(ad4+
4.41 3.90 3.90 3.72 3.84 2.15 1.93
K4Mo2Cls-2H20 K4Mo2Cls-2H20 K~MO~CI~.~H~O (NH4)4MozBrs Rb3M02ClsH
1.71 1.06 0.933 1.23 0.537
= 5 1 5 nm. bValueofc,,,
Solvent
emax
5.0 M H2S04 2.5 M H2S04 0.5 M H2SO4 0.05 M H2SO4 0.001 M H 2 S 0 4 1 M HO3SCF3 1 M H03SCF3/ 0.15 M C H 3 0 H 3 M HCI 3 M HCI 6 M HCI 3 M HBr 6 M HCI
takenfromref 13; A,,
= 505 nm.
A,,
= 510nm. , , ,A
@254
150" 207 230 237 250 3376 337
0.19 0.17 0.16 0.15 0.18 0.035 0.03 1
96OC
0.14 0.13 0.092 0.043 0.040
960 1050d 1170e 1400f = 518nm. e, , ,A
= 514nm.fAm,, = 419
nm was determined by Toepler pumping the stirred photolyzed solution through three liquid nitrogen traps into a 12.2-mL volume and manometrically measuring the pressure. T o ensure that only H2 was collected, the gas was then passed over hot copper oxide, yielding water that was condensed in a liquid nitrogen trap, leaving only traces (
