The synthesis and characterization of various bis-(ligand)dihydroboron(n+) salts

Abstract

Salts containing bis-(ligand)dihydroboron(1+) ions of the type LL'BH2+ where L and L' are electron-donating ligands coordinated to boron through different Group V or VI elements, including nitrogen, phosphorus, arsenic, and sulfur, have been prepared by displacement of halide ion from a (trialkyl donor)-haloborane adduct by the appropriate neutral ligand. Halide and hexafluorophosphate salts of the compounds have been characterized. The method is quite general for the preparation of a wide variety of boronium ions, as illustrated by the synthesis of chelated boron(l+) ions from the reaction of N,N,N',N'-tetramethylethylenediamine with mono- and dihaloborane adducts of trimethylamine, and from the reaction of N,N'-dimethylpiperazine with the monoiodoborane adduct of trimethylamine. Furthermore, dinuclear boron(2+) ions have been prepared from the reaction of N,N,N',N'-tetramethylethylenediamine and triethylenediamine with trimethylamine monoiodoborane, and a trinuclear boron(3+) ion prepared from the reaction of N,N,N',N",N"-pentamethyldiethylenetriamine with trimethylamine monoiodoborane. Various attempts to synthesize larger polycations linked by N-B-N groups were not successful. Polymers which contained Lewis base sites capable of reaction with the monoiodoborane adduct of trimethylamine were used in attempts to produce a polyboronium cation, but the product could not be purified sufficiently for meaningful characterization because of the presence of impurity sites on the resulting polymer product. Boronium cations were synthesized and characterized which contained a polyfunctional amine capable of polymerization. One such cation, 4-vinylpyridine-trimethylaminedihydroboron(1+), was polymerized in the hexafluorophosphate form and in the tetraphenylborate form to give the first polyboronium cation. A study of the tendency of diamines to form chelates rather than polyions when reacted with trimethylamine monoiodoborane and a deuterium exchange study of (ligand)(ligand')dihydroboron(l+) salts are also included.