Long-Lived Radical Cation Salts Obtained by Interaction of Monocyclic Arenes with Niobium and Tantalum Pentahalides at Room Temperature: EPR and DFT Studies

The 1:3 reactions of the alkoxy arenes 1,4-(MeO)(2)C6H4 and 1,4-F-2-2,5-(MeO)(2)C6H2 with TaF5 in chloroform at 40-50 degrees C resulted in formation in about 35% yield of the long-lived radical cation salts [1,4-(MeO)(2)C6H4][Ta2F11] (2a) and [1,4-F-2-2,5-(MeO)(2)C6H2][Ta2F11] (2b), respectively. The non-alkoxy-substituted [arene][M2X11] [M=Ta, X=F: arene=C6H5Me (2c), 1,4-C6H4Me2 (2d), C6H5F (2e), C6H5NO2 (2f); M=Nb, X=F: arene=C6H5Me (4a), 1,4-C6H4Me2 (4b), C6H5F (4c), C6H5NO2 (4d); M=Ta, X=Cl: arene=1,4-C6H4Me2 (5)] were obtained from the 3:1 reactions of MX5 with the appropriate arene in chloroform at temperatures in the range 40-90 degrees C. Compounds 2-5 were detected by EPR spectroscopy (in CHCl3) at room temperature, and their gas-phase structures were optimized by DFT calculations. Formation of the M-IV species [MX4(NCMe)(2)] [M=Ta, X=F (3a); M=Nb, X=F (3b); M=Ta, X=Cl (3c)] was ascertained by EPR spectroscopy on solutions obtained by treatment of the reaction mixtures with acetonitrile. Non-selective reactions occurred upon combination of 1,4-F-2-2,5-(MeO)(2)C6H2 with AgNbF6 (in CH2Cl2) and 1,4-(MeO)(2)C6H4 with SbF5.