8.3 C−H Bond Activation
133
of LaFe
+ displays eight “d” electrons and two “s” electrons. It is expected of a highenergy antibonding σ* LUMO would result as in the first-row transition metal dimers
such as Ni 2 . As there are only eight “d” electrons (not ten), there will be at least one
vacant d orbital even if they are not spin paired. The d orbitals are more critical in
bonding in early transition metal and in second- and third-row transition metals. In
fact, the d and s orbitals in La
+ are very close in energy, where two 5d
1 6s
1 states are
lower than the J = 4 state of the 5d
2 3 F ground state. The La
+ could use one vacant
s-d hybrid to bond to Fe while the other vacant s-d hybrid slightly perturbed from
the energy of the LUMO in the free ion, hence allowing the dimer to be reactive with
alkanes. The dimer ions with fewer than ten d electrons generally behave differently
than dimers with more than ten d electrons, just as an atomic ion with fewer than five
d electrons behaving differently compared to those with more than five d electrons
[143].
In comparison, the failure of Mn 2
+ to react was explained in a similar argument
[128]. The ground-state configurations of Mn
+ and Mn, i.e., 3d
5 4s
1 and 3d
5 4s
2 , result
in a 3d
10 4sσ
2 4sσ
*1 configuration for Mn 2
+ . As a partially occupied 4sσ
* orbital is an
effective acceptor even if it is at low energy, the alert reactivity of Mn 2
+ is reasonable. On the other hand, it was found that Mn 2
+ can react with alcohols to form
Mn(ROH)
+ together with a breaking of the metal-to-metal bond [144]. Therefore, a
singly occupied antibonding orbital not only reduces the reactivity but also weakens
the metal-to-metal bond. This is in sharp contrast with Co 2
+ which reacts with alcohols to form olefin complexes with the dimer ion, due to the difference of the bonding
on Mn 2
+ and Co 2
+ [120]. In comparison, FeCo
+ , FeV
+ , and FeCu
+ clusters react to
dehydrogenate olefins which are more energetic reagents than alkanes [138–140];
olefins have relatively weak allylic C−H bonds hence good donor HOMO S that they
can interact with the high-energy LUMOs of the metal dimers.
8.3.3 Reactivity of Neutral Metal Oxide Clusters
Moreover, reactions of neutral vanadium oxide clusters with small hydrocarbons,
namely C 2 H 6 , C 2 H 4 , and C 2 H 2 , have been investigated by experiments and DFT
calculations [145]. Both approaches of single photon ionization through extreme
ultraviolet (46.9 nm, 26.5 eV) and vacuum ultraviolet (118 nm, 10.5 eV) laser were
used to detect the neutral cluster distributions and reaction products. Under the two
ionization conditions, the results of mass spectrometry in the presence and absence
of C 2 H 6 , C 2 H 2 , and C 2 H 4 are displayed in Fig. 8.10 respectively. A few stable
vanadium oxide clusters, VO 2 , V 2 O 4,5 , and V 3 O 6,7 etc. were observed with notable
intensity in the presence of these reactants. While C 2 H 6 is stable toward reaction with
neutral vanadium oxide clusters, there were peaks observed due to an attachment of
the reactant forming correlative products V m O n C 2 H 4 and V m O n C 2 H 4 respectively
in the cases of C 2 H 4 and C 2 H 2 . Besides the dominant association products, certain
oxygen-rich clusters VO 3 (V 2 O 5 ) n = 0,1,2 …, (e.g., VO 3 , V 3 O 8 , and V 5 O 13 ) were found
to react with C 2 H 4 molecules and cause a cleavage of the C=C bond of C 2 H 4 to
133
of LaFe
+ displays eight “d” electrons and two “s” electrons. It is expected of a highenergy antibonding σ* LUMO would result as in the first-row transition metal dimers
such as Ni 2 . As there are only eight “d” electrons (not ten), there will be at least one
vacant d orbital even if they are not spin paired. The d orbitals are more critical in
bonding in early transition metal and in second- and third-row transition metals. In
fact, the d and s orbitals in La
+ are very close in energy, where two 5d
1 6s
1 states are
lower than the J = 4 state of the 5d
2 3 F ground state. The La
+ could use one vacant
s-d hybrid to bond to Fe while the other vacant s-d hybrid slightly perturbed from
the energy of the LUMO in the free ion, hence allowing the dimer to be reactive with
alkanes. The dimer ions with fewer than ten d electrons generally behave differently
than dimers with more than ten d electrons, just as an atomic ion with fewer than five
d electrons behaving differently compared to those with more than five d electrons
[143].
In comparison, the failure of Mn 2
+ to react was explained in a similar argument
[128]. The ground-state configurations of Mn
+ and Mn, i.e., 3d
5 4s
1 and 3d
5 4s
2 , result
in a 3d
10 4sσ
2 4sσ
*1 configuration for Mn 2
+ . As a partially occupied 4sσ
* orbital is an
effective acceptor even if it is at low energy, the alert reactivity of Mn 2
+ is reasonable. On the other hand, it was found that Mn 2
+ can react with alcohols to form
Mn(ROH)
+ together with a breaking of the metal-to-metal bond [144]. Therefore, a
singly occupied antibonding orbital not only reduces the reactivity but also weakens
the metal-to-metal bond. This is in sharp contrast with Co 2
+ which reacts with alcohols to form olefin complexes with the dimer ion, due to the difference of the bonding
on Mn 2
+ and Co 2
+ [120]. In comparison, FeCo
+ , FeV
+ , and FeCu
+ clusters react to
dehydrogenate olefins which are more energetic reagents than alkanes [138–140];
olefins have relatively weak allylic C−H bonds hence good donor HOMO S that they
can interact with the high-energy LUMOs of the metal dimers.
8.3.3 Reactivity of Neutral Metal Oxide Clusters
Moreover, reactions of neutral vanadium oxide clusters with small hydrocarbons,
namely C 2 H 6 , C 2 H 4 , and C 2 H 2 , have been investigated by experiments and DFT
calculations [145]. Both approaches of single photon ionization through extreme
ultraviolet (46.9 nm, 26.5 eV) and vacuum ultraviolet (118 nm, 10.5 eV) laser were
used to detect the neutral cluster distributions and reaction products. Under the two
ionization conditions, the results of mass spectrometry in the presence and absence
of C 2 H 6 , C 2 H 2 , and C 2 H 4 are displayed in Fig. 8.10 respectively. A few stable
vanadium oxide clusters, VO 2 , V 2 O 4,5 , and V 3 O 6,7 etc. were observed with notable
intensity in the presence of these reactants. While C 2 H 6 is stable toward reaction with
neutral vanadium oxide clusters, there were peaks observed due to an attachment of
the reactant forming correlative products V m O n C 2 H 4 and V m O n C 2 H 4 respectively
in the cases of C 2 H 4 and C 2 H 2 . Besides the dominant association products, certain
oxygen-rich clusters VO 3 (V 2 O 5 ) n = 0,1,2 …, (e.g., VO 3 , V 3 O 8 , and V 5 O 13 ) were found
to react with C 2 H 4 molecules and cause a cleavage of the C=C bond of C 2 H 4 to