244011-68-5Relevant articles and documents
Photochemical disproportionation of Mn2(CO)10. Nineteen-electron intermediates and ligand and intensity dependence
Stiegman, Albert E.,Goldman, Alan S.,Philbin, Cecelia E.,Tyler, David R.
, p. 2976 - 2979 (2008/10/08)
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 2MnL62+ + Mn(CO)5- + 4CO The net reaction is thus 3Mn2(CO)10 + 12L hv → 2MnL62+ + 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).
