On the correlation of gas-phase ion-chemistry and related processes in solution – heterodinuclear transition-metal complexes as model systems

In this work, a series of heterodinuclear, formal M+IPt0 complexes (M = Au, Ag, Cu) was synthesized and characterized in order to establish a correlation between processes observed in mass-spectrometric gas-phase experiments and related processes in solution. The characterization of the model complex [(Ph3P)AuPt(PPh3)3](BArF4) revealed a high fluxionality of the ligand sphere, which manifests itself in fast intramolecular and slow intermolecular PPh3-ligand exchange-processes. While the platinum atom in [(Ph3P)AuPt(PPh3)3](BArF4) is coordinated in a square-planar fashion, a trigonal-pyramidal geometry is found in the first olefin-stabilized Au+IPt0 complex [(Ph3P)AuPt(nbe)3](BArF4). An explanation for the preferred geometry is provided by inspection of the molecular orbitals of the precursor complexes [Pt(PPh3)3] and [Pt(nbe)3]. In the series [(Ph3P)AuPt(PPh3)n(nbe)3–n](BArF4), both the Au–Pt-dissociation energy and the elementary charge shift from the respective Pt0 Lewis-bases to the Lewis-acid [(Ph3P)Au]+ decrease with an increasing number of norbornene ligands. [(Ph3P)AuPt(nbe)3](BArF4) was further probed as a precursor for other AuPt complexes. When [(Ph3P)AuPt(PPh3)3](BArF4) is heated in solution, the formation of benzene is observed. This process is preceded by reversible phenyl-group exchange between the PPh3 ligands as elucidated by deuterium-labeling studies. Both the elimination of benzene and the preceding phenyl-group exchange are also observed in mass-spectrometric fragmentation experiments. Apparently, the primary reactivity is identical both in gas phase and in solution. Based on the sum of observations, the heterometallic nature is expected to be key for the observed reactivity in both realms. In order to examine the influence of the coinage metal on the observed processes upon CID and heating of [(Ph3P)AuPt(PPh3)3](BArF4), the homologous complexes [(Ph3P)MPt(PPh3)3](BArF4) (M = Ag or Cu) were investigated as well. For both complexes, a lower PPh3-dissociation energy is observed in the course of CID and NMR experiments as well as DFT studies. For [(Ph3P)AgPt(PPh3)3](BArF4), kinetic experiments revealed a faster intermolecular PPh3-ligand exchange, when compared with [(Ph3P)AuPt(PPh3)3](BArF4). Moreover, Pt–P-bond cleavage appears to be the rate-determining step in these processes. To investigate the relative Pt–P-bond dissociation-energies for the cationic complexes [Au, Pt, L4]+ (L = PPh3, PTol3, PPhF3, PPhOMe3) and [Ag, Pt, (PPh3)4]+, breakdown graphs were acquired in gas-phase experiments. The obtained collision energies correlate well with calculated dissociation energies. For the Pt–P-bond cleavage-processes, rate constants were determined at different collision energies. These rate constants together with calculated values of ΔH and ΔS were used to estimate the effective ion temperatures in the employed ion-trap massspectrometer.