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

Ab initio MP2/aug'-cc-pVTZ calculations have been performed on the complexes (CO)(2)(HB):PXH2 and (N-2)(2)(HB):PXH2, for X = F, Cl, NC, OH, CN, CCH, CH3, and H, in order to investigate the properties of these complexes which are stabilized by P (aEuro broken vertical bar) B 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)(2)(HB) and (N-2)(2)(HB), nor with the MEP maxima for PXH2. For fixed X, the binding energy of (N-2)(2)(HB):PXH2 is greater than that of (CO)(2)(HB):PXH2. Charge-transfer stabilizes both series of complexes, and occurs from the B electron pair to the antibonding P-A sigma 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)(2)(HB):PXH2, there is a second charge-transfer interaction from the lone pair on P to the antibonding pi orbitals of the two C-O groups. Electron density analyses indicate that the P (aEuro broken vertical bar) B bonds in these complexes are stabilized by relatively weak interactions with little covalent character. The chemical shieldings of B-11 are essentially unaffected by complex formation. In contrast, the shieldings of P-31 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 (1p)J(P-B) increase quadratically with decreasing P-B distance. For fixed X, (1p)J(P-B) is greater for (CO)(2)(HB):PXH2 compared to (N-2)(2)(HB):PXH2.