Photochemical disproportionation of Mn2(CO)10. Nineteen-electron intermediates and ligand and intensity dependence

Albert E. Stiegman, Alan S Goldman, Cecelia E. Philbin, David R. Tyler

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Abstract

The photochemical disproportionation of Mn2(CO)10 proceeds as Mn2(CO)10 366nm → +L, -2CO Mn(CO)5 - + Mn(CO)3L3 + where L is a nitrogen or oxygen donor ligand. With many ligands, but not CH3CN, a secondary disproportionation of Mn(CO)3L3 + occurs: 3Mn(CO)3L3 + 366nm → +L 2MnL6 2+ + Mn(CO)5 - + 4CO The net reaction is thus 3Mn2(CO)10 + 12L hv → 2MnL6 2+ + 4Mn(CO)5 - + 10CO The stoichiometry of Mn(CO)5 - formation in CH3CN solution was measured and found to be as described in the initial equation above. Disproportionation of Mn2(CO)10 occurs with nitrogen- and oxygen-donor ligands but not with monodentate phosphines and phosphites; disproportionation does result, however, with the multidentate 1,2-bis(dimethylphosphino)ethane (dmpe), bis-(2-(diphenylphosphino)ethyl)phenylphosphine (triphos), and 1,1,4,7,10,10-hexaphenyl-1,4,7,10-tetraphosphadecane (tetraphos) ligands. These results are interpreted in terms of the previously proposed radical-chain pathway for Mn2(CO)10 disproportionation involving 19-electron Mn(CO)3L3 intermediates. It is proposed that steric bulk and electron-donating ability are the dominant factors in determining whether or not the dimer will photochemically disproportionate with a particular ligand. Disproportionation occurs with the chelating ligands because these ligands effectively increase the concentration of the key 19-electron intermediate. Two experiments provide additional evidence for the 19-electron intermediate: (1) Reaction of PMe3 with Mn(CO)3depe (depe = 1,2-bis(diethylphosphino)ethane) in the presence of Mn2(CO)10 in the dark gives disproportionation products. It is proposed that PMe3 attacks Mn(CO)3depe, giving the 19-electron complex Mn(CO)3(depe)(PMe3), which then reduces Mn2(CO)10. (2) The cationic product from the reaction of Mn2(CO)10 with tetraphos is Mn(CO)3(tetraphos-P,P′,P″)+. The formation of this product and not Mn(CO)2(tetraphos-P,P′,P″,P‴)+ supports the proposal that the chain reaction involves electron transfer from a 19-electron Mn(CO)3L3 species rather than from a 17-electron Mn(CO)2L3 intermediate; the latter would yield the Mn(CO)2L4 + cation product. The dependence of the disproportionation quantum yields on the exciting light intensity was investigated. In agreement with the proposed radical-chain mechanism, the quantum yields are linearly proportional to I-1/2 (I = the absorbed intensity).

Original languageEnglish
Pages (from-to)2976-2979
Number of pages4
JournalInorganic Chemistry
Volume25
Issue number17
Publication statusPublished - 1986

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ASJC Scopus subject areas

  • Inorganic Chemistry

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