Nanoclusters of phenylphosphonium cations and cyanoferrate anions in the gas phase, and the principles of association of these ions in crystals

Abstract

Electrospray Fourier transform mass spectrometry of combinations of the cations MePh3P+ or Ph4P+ with the anions [Fe(CN)6]3− or [Fe(CN)5(NO)]2− in methanol yields a rich collection of aggregates, mainly with net negative charge. 23 anionic aggregates of MePh3P+ and [Fe(CN)6]3− range in size from [(MePh3P+)2[Fe(CN)6]3−]− to [(MePh3P+)37{[Fe(CN)6]3−}14]5−, while 23 anionic aggregates of Ph4P+ and [Fe(CN)6]3− range up to [(Ph4P+)38{[Fe(CN)6]3−}15]7−. The compositions of the anionic aggregates tend towards cation/anion ratios that yield a charge density of −0.15 per component species. Structural principles which may be applicable to these gaseous nanoclusters have been investigated via analysis of the crystal packing in four new crystal structures of salts containing phenylphosphonium cations and cyanoferrate anions which have stoichiometries that span the same cation/anion ratios. They are (Ph4P)3[Fe(CN)6](H2O)(BuOH)(hexane), (Me2Ph2P)3[Fe(CN)6](H2O)2, (MePh3P)2[Fe(CN)5(NO)](H2O)2 and (Ph4P)2[Fe(CN)5(NO)]. In the solids there is general dispersal of oppositely charged ions, consistent with the overall electrostatics, but also numerous and prevalent hydrogen bonds between phenyl C–H of the cations and coordinated CN− (and NO) of the anionic metal complexes. Multiple phenyl embraces, often formed by these cations, are not influential in these crystals. Density functional calculation of the energies of the charge-assisted C–H⋯NC hydrogen bonds (ca. 4 kcal mol−1), and estimates of ion⋯ion electrostatic energies, lead to the general conclusion that in the gas-phase ion-aggregates the net electrostatic energies and the net hydrogen bonding energies are dominant and of similar magnitude. Models of the clusters of molecular ions indicate that their diameters are 3–4 nm for [(Ph4P+)19{[Fe(CN)6]3−}8]5− and 4–5 nm for [(Ph4P+)38{[Fe(CN)6]3−}15]7−.