Synthesis and Structure of the Elusive [a2-B20H19]3- Anion

Abstract

The apical-apical isomer of the polyhedral borane anion [a{sup 2}-B{sub 20}H{sub 18}]{sup 4{minus}} reacts with various proton sources in dichloromethane to produce the protonated species [a{sup 2}-B{sub 20}H{sub 19}]{sup 3{minus}} in high yield. The mixed tetraethylammonium-pyridinium salt of [a{sup 2}-B{sub 20}H{sub 19}]{sup 3{minus}} crystallizes in the monoclinic space group P2{sub 1}/n with a = 11.223(3) {angstrom}, b = 10.970(3) {angstrom}, c = 15.328(4) {angstrom}, {beta} = 106.06(1){degrees}, V = 1819 {angstrom}{sup 3}, and Z = 2. Data were collected on a Syntex P1 diffractometer using Cu K{alpha} radiation, to a maximum 2{theta} = 115{degrees}, giving 2359 unique reflections, and the structure was solved by direct methods (SHELX86). The final discrepancy indices were R = 0.062 and R{sub w} = 0.086 for 1967 independent reflections with I > 3{sigma}(I). The structure of the [a{sup 2}-B{sub 20}H{sub 19}]{sup 3{minus}} anion is similar to that of its apical-apical [a{sup 2}-B{sub 20}H{sub 18}]{sup 4{minus}} precursor with the addition of a hydrogen bridge connecting the two 10-boron atom cages. The boron-hydrogen-boron bond forms an angle of 91(3){degrees} although the component B{sub 10}H{sub 9} cages a share a common axis through their B(1)-B(10) vertices.