The cyanate and 2-phosphaethynolate anion congeners ECO- (E = N, P, As, Sb, Bi): prelude to experimental...

Pioneering synthetic research by the groups of Grutzmacher and Goicoechea have made possible the preparation of 2-phosphaethynolates (PCO(-)). The obvious question arises: can progress be made toward AsCO(-), SbCO(-), and BiCO(-)? Here the properties of all five anion congeners ECO(-) (E = N, P, As, Sb, Bi) were systematically investigated using ab initio coupled-cluster methods with correlation-consistent basis sets cc-pVXZ (X = D, T, Q). These anions exhibit linear structures with significant natural bond orbital negative charge on both the E and O atoms. These species should react with electrophiles via attack at either center. On going from nitrogen to bismuth, with the atomic radius increasing, the bond between E and C becomes weaker, while the C-O bond tends to be slightly stronger. By the time one gets to BiCO(-), the C-O bond distance is 1.181 Å, indicating a very strong double bond. Relative to the PCO(-) anion, which is reactive toward several unsaturated compounds, the As/Sb/BiCO(-) anions may undergo cycloaddition more readily with unsaturated substrates. The dissociation energy of the E-C bond, except for that of NCO(-), is predicted to be much less than that of the C-O bond. These dissociation energies are 76 kcal/mol (P(-)-CO), 58 kcal/mol (As(-)-CO), 37 kcal/mol (Sb(-)-CO), and 28 kcal/mol (Bi(-)-CO). Even the BiCO(-) anion should be achievable in the laboratory. The vibrational frequencies for these anions are predicted, and our results should assist in the experimental characterization and exploration of the heavier congeners ECO(-).