Experimental Evidence for the Aggregation of [(Phen)₂Pd₂(μ-H)(μ-CO)]⁺ in Solution

Abstract

The aggregation in solution of the dinuclear palladium(I) complex [(Phen)2Pd2(μ-H)(μ-CO)]+BArF 4 - (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 Å. The dependence of the 1H 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.

Ion pairing using PGSE diffusion methods

“…The corresponding radii are 6.78 and 8.01 Å and, in this case, Song et al [20] suggest an aggregation number of 3.12 (see Table 3). There are now several examples of this type of D-value dependence, where charged species are concerned [21,22], Table 4 shows selected data for the dinuclear arsine palladium complex 2, as well as the mononuclear cationic ruthenium arene complex 3.…”

“…Moreover, it is not possible to use conductivity data, even qualitatively, to estimate the extent of ion pairing. The diffusion data from the NMR experiments, when taken together with (NOESY or HOESY) Overhauser studies, which allow one to place the anion in three-dimensional space relative to the cation [110][111][112][113][114][115][116][117][118][119][120][121][122][123], frequently determine the structural details for the ion pair. This is yet another advantage of this NMR approach to ion pairing.…”

A New Series of Homologous Cluster Complexes [Mo3(M'EPh3)Q4Cl4(H2O)5] (M' = Ni, Pd; E = P, As, Sb; Q = S, Se)

“…The complexes are air stable, soluble in common organic solvents, and slowly, but completely soluble in water, apparently undergoing a hydrolytic decomposition. Electrospray mass spectrometry (ES-MS) is becoming a valuable tool for the investigation of coordination compounds allowing the study of solution speciation, [21] formation of aggregates, [22,23] or identification of major and minor products in solutions on a microscale, [24] since it allows pre-existing ions to be transferred into the gas phase with minimal fragmentation. Applications to a number of inorganic and organometallic systems have demonstrated the versatility of the technique.…”

PGSE diffusion, 1H–19F HOESY and NMR studies on several [Rh(1,5‐COD)(Biphemp)]X complexes: detecting positional anion effects

“…This approach is slowly winning support as the method of choice for recognizing anion-cation interactions. 34,35 Fig. 2 shows HOESY data for 2a, from which an interesting selectivity is observed.…”