Borylene as an electron-pair donor for P…B pnicogen bonds

Abstract

Ab initio MP2/aug’-cc-pVTZ calculations have been performed on the complexes (CO)(HB):PXH and (N)(HB):PXH, for X = F, Cl, NC, OH, CN, CCH, CH, and H, in order to investigate the properties of these complexes which are stabilized by PB pnicogen bonds, with B the electron-pair donor. The binding energies of these complexes exhibit an exponential dependence on the P-B distance, but they do not correlate with the MEP minima for (CO)(HB) and (N)(HB), nor with the MEP maxima for PXH. For fixed X, the binding energy of (N)(HB):PXH is greater than that of (CO)(HB):PXH. Charge-transfer stabilizes both series of complexes, and occurs from the B electron pair to the antibonding P-A σ orbital, with A the atom of X directly bonded to P. These charge-transfer energies also exhibit an exponential dependence on the P-B distance. In the complexes (CO)(HB):PXH, there is a second charge-transfer interaction from the lone pair on P to the antibonding π orbitals of the two C-O groups. Electron density analyses indicate that the PB bonds in these complexes are stabilized by relatively weak interactions with little covalent character. The chemical shieldings of B are essentially unaffected by complex formation. In contrast, the shieldings of P increase from 10 to 50 ppm in the four most strongly bound complexes, but decrease by −4 to −12 ppm in the remaining complexes. For each series of complexes, EOM-CCSD spin-spin coupling constants J(P-B) increase quadratically with decreasing P-B distance. For fixed X, J(P-B) is greater for (CO)(HB):PXH compared to (N)(HB):PXH.