GROUND-STATE GUEST-HOST CHEMISTRY IN THE HYDROPHOBIC CAVITY OF THE UNSATURATED CYCLIC PD3(DPPM)3CO2+ CLUSTER

The guest-host chemistry of the Pd3(dpPM)3CO2+ cluster (dppm = ((C6H5) 2P)2CH2) has been investigated in some detail for a series of inorgani c and organic substrates by X-ray crystallography (in two cases), UV-v isible (and IR) spectroscopy, and molecular mechanics. The two X-ray c rystallography characterized complexes are the [Pd3(dppm)3-CO](PF6)2.( CH3)2CO and [Pd3(dppm)3CO](CF3CO2)(PF6).2(CH3)2CO, which have been use d to obtain model structures of the empty cavity (formed by the six up per dppm-phenyl groups) and filled cavity (by the CF3CO2-anion), respe ctively. The latter was also used as a starting structure for the comp utations. The binding constants (K11) were measured spectroscopically using the Benesi-Hildebrand (B.-H.), Scatchard (Scat), and Scott (Scot ) methods for about 20 different substrates. The stoichiometry of the association is found to be 1:1 where the K11 values range from 0.07 to 10 000 M-1. The substrate-cluster associations are competitive and re versible for most studied systems. In some cases (nitro, cyano, and di azonium derivatives), very slow thermal reactions have been observed. Further molecular mechanic calculations on the cluster-aromatic associ ated complexes showed that agostic interactions are possible at the mi nimum computed energy configuration for the aromatic compounds. Overal l, these studies show that the binding strength of a substrate (both o rganic and inorganic) into the bifunctional cavity (metallic center an d hydrophobic section) is found to be related to a combination of para meters. These properties are the substrate charge and ligand strength, and the size and hydrophobic properties of the substrates. Crystal da ta: [Pd3(dppm)3CO](PF6)2, 173 K, triclinic (P1BAR), a = 13.640(4) angs trom, b = 14.0639(17) angstrom, c = 22.4835(15) angstrom, alpha = 104. 298(7)-degrees, beta = 105.143(17)-degrees, gamma = 99.301(18)-degrees , V = 3914.7(13) angstrom3, Z = 2, R = 0.039, R(W) = 0.037; [Pd3(dppm) 3CO](CF3CO2)(PF6),293 K, monoclinic (P2(1)), a = 11.0189(10) angstrom, b = 26.6515(19) angstrom, c = 14.4746(14) angstrom, beta = 99.786(9)- degrees, V = 4188.9(6) angstrom3, Z = 2, R = 0.057, R(W) = 0.053.