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 (N2)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…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 (N2)2(HB), nor with the MEP maxima for PXH2. For fixed X, the binding energy of (N2)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 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)2(HB):PXH2, 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 P…B bonds in these complexes are stabilized by relatively weak interactions with little covalent character. The chemical shieldings of 11B are essentially unaffected by complex formation. In contrast, the shieldings of 31P 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 1pJ(P-B) increase quadratically with decreasing P-B distance. For fixed X, 1pJ(P-B) is greater for (CO)2(HB):PXH2 compared to (N2)2(HB):PXH2.