Synthesis and Characterization of Ammonioundecafluoro-closo-dodecaborates(1-). New Superweak Anions

The ammonioborane monoanion H3NB12H11- was per-B-fluorinated with elemental fluorine in liquid hydrogen fluoride to yield the first member of a new class of weakly coordinating anions, H3NB12F11- (isolated as [N(n-Bu)4]2[H2NB12F11] in 41% yield). The pKa of the H3NB12F11- anion is 9.6. Several salts of the tri-N-alkylated anions Me3NB12F11- and Dd3NB12F11- (Dd = n-C12H25) were also prepared. The structure of [CPh3][Me3NB12F11] was determined by single-crystal X-ray diffraction: monoclinic, space group P21/c, a = 18.053(3) (angstrom), b = 33.139(5) (angstrom), c = 9.600(2) (angstrom), (beta)= 91.459(4) °, V = 5742(2) (angstrom)^3, Z = 8, T = 173 (2) K, R1 = 0.045. It revealed that the only direct interactions between the undecafluoroammonioborate monoanions and the trityl cations in the two independent ion pairs were long and weak BF···CPh3 interactions of 2.992(6) and 2.942(6) (angstrom). Salts of the new anions were chemically, electrochemically, and thermally stable. The conductivity of Li(Me3NB12F11) in dimethoxyethane was comparable to that of LiPF6 but less than half the value of Li(1-Me-CB11F11).