The mechanism of oxidative coupling of two naphthol molecules to form binaphthol catalyzed by Cu(OH)ClTMEDA (TMEDA=N,N,N',N'-tetramethylethylenediamine) was approached by means of a gas-phase model system. Concise evidence is provided that the coupling reaction proceeds in clusters with two Cu(II) centers, whereby the intermediacy of free naphthoxy radicals in the coupling step is avoided. In the absence of TMEDA, the cluster is bound via a bridging counterion and the coupling reaction is followed by cluster cleavage. The coordination of one or two TMEDA molecules to the reactive complex results in more efficient coupling of naphthol molecules, and moreover, the binuclear cluster is also conserved after the reaction is completed. The effect of TMEDA is twofold: First, it supports clustering of copper and, second, as a ligand bound to a copper center in the reactive complex, it weakens the bond between copper and the naphtholato ligand such that the naphtholato unit is more prone to undergo C--C coupling. Furthermore, a pronounced counterion effect is found that correlates well with condensed-phase data: weakly bridging counterions (e.g., NO3(-)) yield less stable dicopper clusters and the coupling reaction hardly occurs, whereas better bridging counterions (e.g., Cl(-) or Br(-)) provide more stable clusters that make the coupling reaction more efficient.
The bimolecular reactions of several hydrocarbon dications C(m)H(n)(2+) (m = 6-10, n = 4-9) with neutral benzene are investigated by tandem mass spectrometry using a multipole instrument. Not surprisingly, the major reaction of C(m)H(n)(2+) with benzene corresponds to electron transfer from the neutral arene to the dication resulting in the pair of monocationic products C(m)H(n)(+) + C(6)H(6)(+). In addition, also dissociative electron transfer takes place, whereas proton transfer from the C(m)H(n)(2+) dication to neutral benzene is almost negligible. Interestingly, the excess energy liberated upon electron transfer from the neutral arene to the C(m)H(n)(2+) dication is not equally partitioned in the monocationic products in that the cations arising from the dicationic precursor have a higher internal energy content than the monocations formed from the neutral reaction partner. In addition to the reactions leading to monocationic product ions, bond-forming reactions with maintenance of the two-fold charge are observed, which lead to a condensation of the C(m)H(n)(2+) dications with neutral benzene under formation of intermediate C(m+6)H(n+6)(2+) species and then undergo subsequent losses of molecular hydrogen or neutral acetylene. This reaction complements a recently proposed dicationic route for the formation of polycyclic aromatic hydrocarbons under extreme conditions such as they exist in interstellar environments.
The bimolecular reactivity of molecular dications in the gas phase is reviewed from an experimental point of view. Recent research has demonstrated that in addition to the ubiquitous occurrence of electron transfer in the reactions of gaseous dications with neutral molecules, bond-forming reactions play a much larger role than anticipated before. Thus, quite a number of hydrogen-containing dications show proton transfer to neutral reagents as an abundant or even as the major pathway, and also the nature of the neutral reagent itself is decisive for the amount of proton transfer which takes place. Further, several hydrocarbon dications C(m)H(n)(2+) of medium size (m = 6-14, n = 6-10) undergo bond-forming reactions with unsaturated hydrocarbons such as acetylene or benzene, thereby offering new routes for the formation of larger aromatic compounds under extreme conditions such as interstellar environments. Likewise, recent results on the bimolecular reactivity of multiply charged metal ions have revealed the occurrence of a number of new bond-forming reactions which open promising prospects for further research.
- MeSH
- acetylen chemie MeSH
- benzen chemie MeSH
- cyklotrony přístrojové vybavení MeSH
- financování organizované MeSH
- fotochemie MeSH
- hmotnostní spektrometrie metody přístrojové vybavení MeSH
- kationty chemie MeSH
- kovy chemie MeSH
- plyny chemie MeSH
- protony MeSH
- teplota MeSH
- termodynamika MeSH
- transport elektronů MeSH
- uhlovodíky chemie MeSH
- vodík MeSH
