Wrapping an Organic Reducing Reagent in a Cationic Boron Complex and Its Use in the Synthesis of Polyhalide Monoanionic Networks

The reaction between BF3 center dot OEt2 and one of two guanidines, 1,8-bis(tetramethylguanidinyl) naphthalene (btmgn) and 1,2,4,5-tetrakis(tetramethylguanidinyl) naphthalene (ttmgn), yields the salts [(btmgn) (BF2)]BF4 and [(ttmgn) (BF2)(2)](BF4)(2). NMR spectroscopic data show that the boron atoms in the cation and anion exchange in the case of [(ttmgn) (BF2)(2)](BF4)(2), but not in the case of [(btmgn) (BF2)]BF4. The rate constant for this exchange was estimated to be 4 s(-1) at 80 degrees C for solutions in CH3CN. These salts were subsequently used for the reduction of dihalides Br-2 or I-2 to give polyhalide salts. We report the synthesis and first complete characterization (including structural analysis) of salts that contain pentabromide monoanions. In these salts, the Br-5(-) anions interact to give dimeric units or polymeric chains. Our results are compared to previous quantum chemical calculations on the gas-phase structure of the Br-5(-) anion. The possible pathways that lead to the polyhalides are evaluated. In the case of [(ttmgn)(BF2)(2)](BF4)(2), reduction is accompanied by ttmgn oxidation, whereas in the case of [(btmgn)(BF2)]BF4 reduction is initiated by aromatic substitution.