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 C 6 H 4 and 1,4‐F 2 ‐2,5‐(MeO) 2 C 6 H 2 with TaF 5 in chloroform at 40–50 °C resulted in formation in about 35 % yield of the long‐lived radical cation salts [1,4‐(MeO) 2 C 6 H 4 ][Ta 2 F 11 ] ( 2 a ) and [1,4‐F 2 ‐2,5‐(MeO) 2 C 6 H 2 ][Ta 2 F 11 ] ( 2 b ), respectively. The non‐alkoxy‐substituted [arene][M 2 X 11 ] [M=Ta, X=F: arene=C 6 H 5 Me ( 2 c ), 1,4‐C 6 H 4 Me 2 ( 2 d ), C 6 H 5 F ( 2 e ), C 6 H 5 NO 2 ( 2 f ); M=Nb, X=F: arene=C 6 H 5 Me ( 4 a ), 1,4‐C 6 H 4 Me 2 ( 4 b ), C 6 H 5 F ( 4 c ), C 6 H 5 NO 2 ( 4 d ); M=Ta, X=Cl: arene=1,4‐C 6 H 4 Me 2 ( 5 )] were obtained from the 3:1 reactions of MX 5 with the appropriate arene in chloroform at temperatures in the range 40–90 °C. Compounds 2 – 5 were detected by EPR spectroscopy (in CHCl 3 ) at room temperature, and their gas‐phase structures were optimized by DFT calculations. Formation of the M IV species [MX 4 (NCMe) 2 ] [M=Ta, X=F ( 3 a ); M=Nb, X=F ( 3 b ); M=Ta, X=Cl ( 3 c )] 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 C 6 H 2 with AgNbF 6 (in CH 2 Cl 2 ) and 1,4‐(MeO) 2 C 6 H 4 with SbF 5 .