Identification of Mn(CO)_nCF_3^- (n = 4, 5) structural isomers by IR multiphoton dissociation, collision-induced dissociation, and specific ligand displacement reactions: studies of the trifluoromethyl migratory decarbonylation reaction in the gas phase

Abstract

The trifluoromethyl migratory decarbonylation reaction which converts the (trifluoroacety1)manganese tetracarbonyl anion to the (trifluoromethy1)manganese tetracarbonyl anion is studied in the gas phase with Fourier transform ion cyclotron resonance spectroscopy. Dissociative electron attachment by (trifluoroacety1)manganese pentacarbonyl produces both Mn-(CO)_5CF_3^- and Mn(CO)_4CF_3^-. Mn(CO)_5CF_3^- slowly decomposes to yield Mn(CO)_4CF_3^- with loss of CO. In order to identify the structures of these two ions, we have employed infrared multiphoton dissociation in conjunction with collision-induced dissociation and studies of specific reactivity involving the ligand displacement process. Mn(CO)_4CF_3^- derived from dissociative electron attachment by a different precursor, (trifluoromethyl)manganese pentacarbonyl, is also used to confirm the identity of the trifluoromethyl-migration product ion. The Mn(CO)_5CF_3^- ion generated from (trifluoroacetyl)manganese pentacarbonyl does not undergo infrared multiphoton dissociation in the CO_2 laser wavelength range, which indicates the trifluoroacetyl structure CF_3COMn(CO)_4^-. Mn(CO)_4CF_3^- ions derived from the two different precursors show identical infrared multiphoton dissociation spectral features within experimental errors. Two absorption maxima at 1052 and 945 cm^(-1) are assigned as a symmetric C-F stretch of A_1-type symmetry and a degenerate C-F stretch of E-type symmetry, respectively. Collision-induced dissociation of Mn(CO)_4CF_3^- yields indistinguishable fragment mass spectra from the two different precursors. Identical rate constants within experimental error are measured for Mn(CO)_4CF_3^- from the two precursors in ligand displacement reactions with NO to yield Mn(CO)_3(NO)CF_3^- with loss of CO. Displacement of CO by ^(13)CO or PF_3 is not observed for either ion. These results support identical structures for Mn(CO)_4CF_3^- from the two different precursors, with the CF_3 ligand directly bonded to manganese, CF_4Mn(CO)_4^-. It is postulated that this ion from (trifluoromethyl)manganese pentacarbonyl is produced directly by electron attachment accompanied by loss of one equatorial CO. With (trifluoroacetyl)manganese pentacarbonyl, electron attachment leads to loss of an equatorial CO followed by the migration of CF, from the acyl carbon to the vacant equatorial site on the manganese center with loss of another CO in the equatorial position to the CF_3 ligand. The CF_3 group is an ideal infrared chromophore to investigate the infrared photochemistry of organometallic complexes, allowing studies of the structures and reaction mechanisms of coordinatively unsaturated intermediates containing metal-bonded CF_3 groups.

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