8.3 C−H Bond Activation
131
of the 6 s orbital of Pt
− is ~20% [118], the constructive overlap of the 16σ bonding
region is a mimic of the Pt 6 s orbital contraction, manifested by the c
2
s = 0.19
coefficient [106].
8.3.2 Reactivity of VIII Group Metal Cluster Ions
There are abundant investigations concerning the reactivity of group VIII metals
(i.e., Fe, Co, Ni series) with hydrocarbons [120–127]. The gas-phase reactivity of
such metal clusters leading to the activation of C–H bonds (producing hydrogen)
attracts reasonable research interest involving a variety of interesting questions [9].
For example, early studies showed that the Mn 2
+ and Co 2
+ dimers do not directly react
with alkanes [128], but Co 2 (CO)
+ reacts with butane (C 4 H 10 ) to form Co 2 (CO)C 4 H 8
+
with a loss of hydrogen [129]. Similarly, Re 3 (CO) n
+ , Re 4 (CO) n
+ , and Ir 4 (CO) n
+ were
also observed to support dehydrogenation at the presence of cyclohexane (C 6 H 12 )
providing n was not too large, and a critical value of n for each case was rationalized
in terms of frontier orbital theory [130, 131].
Figure 8.9 lays out the rate constants for reaction of Co n (CO) n
+ and Ir n (CO) n
+
with cyclohexane. For bare metal Co clusters, their reactions toward cyclohexane
(C 6 H 12 ) were demonstrated to follow:
Co
+
n + C 6 H 12 → Co n (C 6 H 12−2m )
+
+ mH 2
(8.1)
This reaction pathway was obviously seen for Co clusters in the case n = 1 & m
= 1–3, and n = 3–4 & m = 2–3. Note that Con(C 6 H 12–2m )
+ may undergo subsequent
reactions. In comparison, the reactivity for Ir clusters with cyclohexane was noted
as:
Ir
+
n + C 6 H 12 → Ir n (C 6 H 6 )
+
m + 3mH 2
(8.2)
where the Ir n
+ with n = 2–4 & m = 1–3 were found to be highly reactive in this
way, with a total rate constant up to 7.2, 10.1, and 11.8 × 10
–10 cm
3 s
−1 respectively
[120].
Also clusters Pt n (n = 2–8) and Nb n (n = 4–13) were found to dehydrogenate
small alkanes in a fast flow [132–135], and showed a trend that the extent of dehydrogenation and the number of molecules attached increases with the cluster size [9].
Sputtered ionic clusters of Cu n , Pt n , Pd n , and Ni n were also found to dehydrogenate
small alkanes [136, 137], and it appeared that Cu clusters are more reactive than Ag
clusters, while Pt clusters are more reactive than Pd clusters which are more reactive
than Ni clusters [9].
These reactions aim at remarkable dehydrogenation reactions which generally
need an initial oxidative addition of the C–H bond onto the metal. In atomic metal
cations, the empty and low-energy s orbitals facilitate the oxidative addition owing to
131
of the 6 s orbital of Pt
− is ~20% [118], the constructive overlap of the 16σ bonding
region is a mimic of the Pt 6 s orbital contraction, manifested by the c
2
s = 0.19
coefficient [106].
8.3.2 Reactivity of VIII Group Metal Cluster Ions
There are abundant investigations concerning the reactivity of group VIII metals
(i.e., Fe, Co, Ni series) with hydrocarbons [120–127]. The gas-phase reactivity of
such metal clusters leading to the activation of C–H bonds (producing hydrogen)
attracts reasonable research interest involving a variety of interesting questions [9].
For example, early studies showed that the Mn 2
+ and Co 2
+ dimers do not directly react
with alkanes [128], but Co 2 (CO)
+ reacts with butane (C 4 H 10 ) to form Co 2 (CO)C 4 H 8
+
with a loss of hydrogen [129]. Similarly, Re 3 (CO) n
+ , Re 4 (CO) n
+ , and Ir 4 (CO) n
+ were
also observed to support dehydrogenation at the presence of cyclohexane (C 6 H 12 )
providing n was not too large, and a critical value of n for each case was rationalized
in terms of frontier orbital theory [130, 131].
Figure 8.9 lays out the rate constants for reaction of Co n (CO) n
+ and Ir n (CO) n
+
with cyclohexane. For bare metal Co clusters, their reactions toward cyclohexane
(C 6 H 12 ) were demonstrated to follow:
Co
+
n + C 6 H 12 → Co n (C 6 H 12−2m )
+
+ mH 2
(8.1)
This reaction pathway was obviously seen for Co clusters in the case n = 1 & m
= 1–3, and n = 3–4 & m = 2–3. Note that Con(C 6 H 12–2m )
+ may undergo subsequent
reactions. In comparison, the reactivity for Ir clusters with cyclohexane was noted
as:
Ir
+
n + C 6 H 12 → Ir n (C 6 H 6 )
+
m + 3mH 2
(8.2)
where the Ir n
+ with n = 2–4 & m = 1–3 were found to be highly reactive in this
way, with a total rate constant up to 7.2, 10.1, and 11.8 × 10
–10 cm
3 s
−1 respectively
[120].
Also clusters Pt n (n = 2–8) and Nb n (n = 4–13) were found to dehydrogenate
small alkanes in a fast flow [132–135], and showed a trend that the extent of dehydrogenation and the number of molecules attached increases with the cluster size [9].
Sputtered ionic clusters of Cu n , Pt n , Pd n , and Ni n were also found to dehydrogenate
small alkanes [136, 137], and it appeared that Cu clusters are more reactive than Ag
clusters, while Pt clusters are more reactive than Pd clusters which are more reactive
than Ni clusters [9].
These reactions aim at remarkable dehydrogenation reactions which generally
need an initial oxidative addition of the C–H bond onto the metal. In atomic metal
cations, the empty and low-energy s orbitals facilitate the oxidative addition owing to