Experimental evidence for the aggregation of [(Phen)2Pd2(μ-H)(μ-CO)]+ in solution

The aggregation in solution of the dinuclear palladium(I) complex [(Phen)(2)Pd-2(mu-H)(mu-CO)]+BAr4F- (1c) was investigated by PGSE NMR, UV-vis, and ESI- and FAB-mass techniques. PGSE measurements showed that only the cationic moiety of 1c (1c(+)) is involved in the aggregation process, while the counteranion does not participate in it. By using compound [Pd(Phen)(Me)Cl] (2) as an external standard, it was found that, depending on the solution concentration, the volume ratio V-1c+/V-2 ranges from 7.2 to 40.2 in nitrobenzene-d(5) at 296 K. The V-1c+/V-2 value (ca. 5.5) extrapolated to infinite dilution indicates that, even at very low concentration values, at least two units of 1c(+) are associated to form a dimer. At the highest possible concentration (55.20 mM) a mean aggregate bearing about 20 1c(+) units is present in solution, having an average hydrodynamic radius of 11-12 Angstrom. The dependence of the H-1 NMR chemical shifts (for acetone-d(6) or nitrobenzene-d(5) solutions at 296 K) and UV-vis spectra (in THF or nitrobenzene at 296 K) on the concentration and of the UV-vis spectra on the temperature (from 323 to 153 K) indicates that two different aggregation processes are present, having well differentiated interaction energies. Although the latter were not quantitatively evaluated, the experimental results indicate that the interaction energy of the process occurring at low concentration (responsible for the dimer formation) is much larger than that relative to the formation of higher aggregates. As a confirmation, ESI- and FAB-mass results show the presence of peaks related to the fragmentation of the dimeric species containing four palladium atoms, indicating that such a dimer has remarkable stability.