DOI: 10.1021/jp206801x

Structures, Energies, and Spin-Spin Coupling Constants of Methyl-Substituted 1,3-Diborata-2,4-diphosphoniocyclobutanes: Four-member B-P-B-P Rings B₂P₂(CH₃)_nH_8-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 17 methyl-substituted 1,3-diborata-2,4-diphosphoniocyclobutanes B₂P₂(CH₃)_nH_8–n, for n = 0, 1, 2, 4, with four-member B–P–B–P rings. The B–P–B–P rings are puckered in a butterfly conformation, in agreement with experimental data for related molecules. Isomers with the CH₃ group bonded to P are more stable than those with CH₃ bonded to B. If there is only one methyl group or if two methyl groups are bonded to two different P or B atoms, isomers with equatorial bonds are more stable than those with axial bonds. However, when two methyl groups are present, the gem isomers are the most stable for molecules B₂P₂(CH₃)₂H₆ with P–C and B–C bonds, respectively. Transition structures present barriers to the interconversion of two equilibrium structures or to the interchange of axial and equatorial positions in the same isomer. These barriers are very low for the isomer with two methyl groups bonded to B in axial positions for the isomer with four axial bonds and for the isomer with geminal B–C bonds at both B atoms. Coupling constants ¹J(B–P), ¹J(P–C), ¹J(B–C), ²J(P–P), and ³J(P–C) are capable of providing structural information. They are sensitive to the number of methyl groups present and can discriminate between axial, equatorial, and geminal bonds, although not all do this to the same extent. The one-bond coupling constants ¹J(B–P), ¹J(P–C), and ¹J(B–C) are similar in equilibrium and transition structures, but ³J(P–C) and ²J(P–P) are not. These coupling constants and those of the corresponding fluoro-derivatives of the 1,3-diborata-2,4-diphosphoniocyclobutanes demonstrate the great sensitivity of phosphorus coupling to structural and electronic effects.