58
4 Halogenation of Metal Clusters
4.1 Reaction with Alkyl Halide
Several halogenation pathways have been addressed in metal cluster reactivity, analogous to those demonstrated in organic synthesis, including free radical halogenation
[31–40], electrophilic halogenation [41–43], and halogen addition reaction [44–47],
etc. An early investigation regarding clusters by Bergeron and Castleman [48] found
that the aluminum cluster anions readily react with methyl iodide vapor in a fastflow tube apparatus, yielding primarily I
− and clusters series of Al n I
− , as shown in
Fig. 4.1. With thermodynamic considerations, it was found that abundant I
− ions
were produced associated with a neutralization reaction to form Al n CH 3 clusters,
read as,
Al
−
n + CH 3 I ↔ Al n CH 3 I
−
(4.1)
Al n CH 3 I
−
→ Al n CH 3 + I
−
(4.2)
Al n CH 3 I
−
→ Al n I
−
+ CH 3
(4.3)
Al n CH 3 I
−
→ Al n CH
−
3 + I
( 4 . 4 )
It is noteworthy that, unavailable dissociation reactions have been ascertained, in
particular, the reaction “Al 13
−
+ CH 3 I → Al 13 + CH 3 + I
− ” requires approximately
~140 kJ/mol energy of interaction between the cluster and the methyl group; even the
cluster of smallest electron-affinity, Al 2
− , requires ~45 kJ/mol of interaction energy
to make such a reaction favourable as “Al 2
−
+ CH 3 I → Al 2 + CH 3 + I
− ”. This is
also consistent with the conclusion in subsequent investigations by Kim et al. [49].
Fig. 4.1 a Mass spectra of Al n
− clusters reacting with 100 sccm helium-diluted MeI. The I − peak
grows rapidly upon introduction of MeI, and Al n I − peaks can be seen between Al n
− series; b
Comparison of relative rates of disappearance for Al n
− (dash line linked dots) to relative inverse
vertical detachment energies (solid line linked dots) as measured by Cha et al. [50]. Identical trends
are interpreted as evidence of the formation of covalently bonded Al n CH 3 species
4 Halogenation of Metal Clusters
4.1 Reaction with Alkyl Halide
Several halogenation pathways have been addressed in metal cluster reactivity, analogous to those demonstrated in organic synthesis, including free radical halogenation
[31–40], electrophilic halogenation [41–43], and halogen addition reaction [44–47],
etc. An early investigation regarding clusters by Bergeron and Castleman [48] found
that the aluminum cluster anions readily react with methyl iodide vapor in a fastflow tube apparatus, yielding primarily I
− and clusters series of Al n I
− , as shown in
Fig. 4.1. With thermodynamic considerations, it was found that abundant I
− ions
were produced associated with a neutralization reaction to form Al n CH 3 clusters,
read as,
Al
−
n + CH 3 I ↔ Al n CH 3 I
−
(4.1)
Al n CH 3 I
−
→ Al n CH 3 + I
−
(4.2)
Al n CH 3 I
−
→ Al n I
−
+ CH 3
(4.3)
Al n CH 3 I
−
→ Al n CH
−
3 + I
( 4 . 4 )
It is noteworthy that, unavailable dissociation reactions have been ascertained, in
particular, the reaction “Al 13
−
+ CH 3 I → Al 13 + CH 3 + I
− ” requires approximately
~140 kJ/mol energy of interaction between the cluster and the methyl group; even the
cluster of smallest electron-affinity, Al 2
− , requires ~45 kJ/mol of interaction energy
to make such a reaction favourable as “Al 2
−
+ CH 3 I → Al 2 + CH 3 + I
− ”. This is
also consistent with the conclusion in subsequent investigations by Kim et al. [49].
Fig. 4.1 a Mass spectra of Al n
− clusters reacting with 100 sccm helium-diluted MeI. The I − peak
grows rapidly upon introduction of MeI, and Al n I − peaks can be seen between Al n
− series; b
Comparison of relative rates of disappearance for Al n
− (dash line linked dots) to relative inverse
vertical detachment energies (solid line linked dots) as measured by Cha et al. [50]. Identical trends
are interpreted as evidence of the formation of covalently bonded Al n CH 3 species