Structures, Energies, and Spin-Spin Coupling Constants of Fluoro-Substituted 1,3-Diborata-2,4-diphosphoniocyclobutanes: Four-Member B-P-B-P Rings B2P2FnH8_n with n = 0, 1, 2, 4
An ab initio study has been carried out to determine the structures, relative stabilities, and spin−spin coupling constants of a set of 15 fluoro-substituted 1,3-diborata-2,4-diphosphoniocyclobutanes B2P2FnH8−n, for n = 0, 1, 2, 4, with four-member B−P−B−P rings. Except for B2P2F4H4 with four fluorines bonded to two borons, these rings are puckered in a butterfly conformation. For a fixed number of fluorines, the isomers with B−F bonds are significantly more stable than those with P−F bonds. As the number of fluorines increases, the energy difference between the most stable isomer and the other isomers increases. Transition structures which interconvert axial and equatorial positions present relatively small inversion barriers. Coupling constants involving 31P, namely, 1J(B−P), 1J(P−F), 2J(P−P), 2J(P−F), and3J(P−F) are large and are capable of providing structural information. They are sensitive to the number of fluorines present and can discriminate between axial, equatorial, and geminal B−F and P−F bonds, although not all do this to the same extent. 1J(B−P) and 2J(P−P) are similar in equilibrium and transition structures. Although transition structures no longer discriminate between axial and equatorial bonds, 1J(P−F) and 3J(P−F) remain sensitive to the number of fluorine atoms present.