Heats of formation of diphosphene, phosphinophosphinidene, diphosphine, and their methyl derivatives, and mechanism of the borane-assisted hydrogen release

The heats of formation of diphosphene (cis- and trans-P(2)H(2)), phopshinophosphinidene (singlet and triplet H(2)PP) and diphosphine (P(2)H(4)), as well as those of the P(2)H and P(2)H(3) radicals resulting from PH bond cleavages, have been calculated by using high-level ab initio electronic structure theory. Energies were calculated using coupled-cluster theory with a perturbative treatment for triple excitations (CCSD(T)) and employing augmented correlation consistent basis sets with additional tight d-functions on P (aug-cc-pV(n+d)Z) up to quadruple- or quintuple-zeta, to perform a complete basis set extrapolation for the energy. Geometries and vibrational frequencies were determined with the CCSD(T) method. Core-valence and scalar relativistic corrections were included, as well as scaled zero-point energies. We find the following heats of formation (kcal/mol) at 298 [0] K: (P(2)H) = 53.4 [54.4]; (cis-P(2)H(2)) = 32.0 [33.9]; (trans-P(2)H(2)) = 28.7 [30.6]; (H(2)PP) = 53.7 [55.6]; ((3)H(2)PP) = 56.5 [58.3]; (P(2)H(3)) = 32.3 [34.8]; (P(2)H(4)) = 5.7 [9.1] (expt, 5.0 +/- 1.0 at 298 K); and (CH(3)PH(2)) = -5.0 [-1.4]. We estimate these values to have an accuracy of +/- 1.0 kcal/mol. In contrast to earlier results, we found a singlet ground state for phosphinophosphinidene (H(2)PP) with a singlet-triplet energy gap of 2.8 kcal/mol. We calculated the heats of formation of the methylated derivatives CH(3)PPH, CH(3)HPPH(2), CH(3)PPCH(3), CH(3)HPP, (CH(3))(2)PP, (CH(3))(2)PPH(2), and CH(3)HPPHCH(3) by using isodesmic reactions at the MP2/CBS level. The calculated results for the hydrogenation reactions RPPR + H(2) -> RHPPHR and R(2)PP + H(2) -> R(2)PPH(2) show that substitution of an organic substituent for H improves the energetics, suggesting that secondary diphosphines and diphosphenes are potential candidates for use in a chemical hydrogen storage system. A comparison with the nitrogen analogues is given. The mechanism for H(2)-generation from diphosphine without and with BH(3) as a catalyst was examined. Including tunneling corrections, the rate constant for the catalyzed reaction is 4.5 x 10(15) times faster than the uncatalyzed result starting from separated catalyst and PH(2)PH(2).