218
A. V. Pomogaeva and A. Y. Timoshkin
as well as reactions of monomeric Me 2 AlNH 2 with excess of AlMe 3 and ammonia
has been computationally studied by Lisovenko et al. [126]. It was shown that the
TS for the first methane elimination from the Me 3 AlNH 3 complex is the highest
point on the reaction pathway and the only one lying above the isolated AlMe 3 and
NH 3 . Subsequent stepwise substitution of methyl groups in AlMe 3 by amido groups
NH 2 is exothermic, and despite the sizable activation energies of the intermediate
steps, the overall reaction profile is essentially downhill. It was also shown [126]
that formation of [Me 2 AlNH 2 ] 2 dimer is very favorable thermodynamically and
affordable kinetically. This dimer can rearrange to the trimer [Me 2 AlNH 2 ] 3 , which
is a smallest open needle cluster and a potential candidate for the growth of needleshaped Me 3 [MeAlNH] 3n H 3 oligomers.
Enthalpy of the dimer-trimer equilibrium reaction [Me 2 AlNH 2 ] 2 = 2[Me 2 Al
NH 2 ] 3 is −38 kJ mol −1 according to temperature-dependent NMR studies
in benzene-d 6 solution [127]. Since rearrangement of [Me 2 AlNH 2 ] 2 dimer to
[Me 2 AlNH 2 ] 3 trimer is exothermic in solution, formation of trimeric species is also
expected. Indeed, trimeric [Me 2 AlNH 2 ] 3 has been synthesized and characterized
by Interrante et al. [128] and successfully used as single source precursor to AlN.
Numerical simulations of processes in MOCVD reactors [129] and thermodynamic
analysis of gas phase reactions [123] underline the importance of oligomeric species
in the chemical reactivity model. In 2003, trimeric [Me 2 AlNH 2 ] 3 was identified as
one of the important intermediates in AlN formation by X-ray powder diffraction
[130]. Vaporization and gas phase thermal decomposition of the deuterated
analog [Me 2 AlND 2 ] 3 were monitored by time-of-flight mass spectrometry [131,
132]. Dimeric and trimeric molecules [Me 2 AlND 2 ] 2,3 , deuterated methane
CH 3 D, and oligomers with a pentameric structure were detected in the vapor
[132]. Experimental mass spectrometry observations reveal formation of cluster
compounds in the gas phase upon laser irradiation of AlMe 3 –NH 3 mixtures at low
(below room) temperatures [34–36].
Reaction energy profiles starting from trimers [Me 2 AlNH 2 ] 3 and leading to
hexamer amido-imino [Me 9 Al 6 N 6 H 9 ] and imino [MeAlNH] 6 compounds have been
computationally explored by Davydova et al. at B3LYP/def2-SVP, B3LYP-D3/def2TZVPP//B3LYP/def2-SVP, and M06-2X/def2-TZVPP//B3LYP/def2-SVP levels of
theory [133]. Three alternative reaction pathways of transformations of cyclic
[Me 2 AlNH 2 ] 3 have been explored (Figs. 6.9, 6.10, and 6.11). Activation energies for
the first methane elimination are very close both for mono- and bimolecular reaction
pathways (Figs. 6.9 and 6.10). Intermolecular methane elimination is always
exothermic, while intramolecular methane elimination steps are endothermic. Thus,
generation of [MeAlNH] 6 from [Me 2 AlNH 2 ] 3 at low temperatures is expected
to proceed via a bimolecular pathway. Formation of [Me 9 Al 6 N 6 H 9 ] from two
[Me 2 AlNH 2 ] 3 trimers is expected to be less favorable, as it involves higher barriers
and lower endothermicity.
Alternative Al-N bond breaking pathway was also considered. Obtained results
show that the energy required for Al-N bond breaking in cyclic [Me 2 AlNH 2 ] 3
(185 kJ mol −1 ) is of the same order as the activation energy (183 kJ mol −1 ) for
the first (limiting) step of methane elimination (both for mono- and bimolecular
A. V. Pomogaeva and A. Y. Timoshkin
as well as reactions of monomeric Me 2 AlNH 2 with excess of AlMe 3 and ammonia
has been computationally studied by Lisovenko et al. [126]. It was shown that the
TS for the first methane elimination from the Me 3 AlNH 3 complex is the highest
point on the reaction pathway and the only one lying above the isolated AlMe 3 and
NH 3 . Subsequent stepwise substitution of methyl groups in AlMe 3 by amido groups
NH 2 is exothermic, and despite the sizable activation energies of the intermediate
steps, the overall reaction profile is essentially downhill. It was also shown [126]
that formation of [Me 2 AlNH 2 ] 2 dimer is very favorable thermodynamically and
affordable kinetically. This dimer can rearrange to the trimer [Me 2 AlNH 2 ] 3 , which
is a smallest open needle cluster and a potential candidate for the growth of needleshaped Me 3 [MeAlNH] 3n H 3 oligomers.
Enthalpy of the dimer-trimer equilibrium reaction [Me 2 AlNH 2 ] 2 = 2[Me 2 Al
NH 2 ] 3 is −38 kJ mol −1 according to temperature-dependent NMR studies
in benzene-d 6 solution [127]. Since rearrangement of [Me 2 AlNH 2 ] 2 dimer to
[Me 2 AlNH 2 ] 3 trimer is exothermic in solution, formation of trimeric species is also
expected. Indeed, trimeric [Me 2 AlNH 2 ] 3 has been synthesized and characterized
by Interrante et al. [128] and successfully used as single source precursor to AlN.
Numerical simulations of processes in MOCVD reactors [129] and thermodynamic
analysis of gas phase reactions [123] underline the importance of oligomeric species
in the chemical reactivity model. In 2003, trimeric [Me 2 AlNH 2 ] 3 was identified as
one of the important intermediates in AlN formation by X-ray powder diffraction
[130]. Vaporization and gas phase thermal decomposition of the deuterated
analog [Me 2 AlND 2 ] 3 were monitored by time-of-flight mass spectrometry [131,
132]. Dimeric and trimeric molecules [Me 2 AlND 2 ] 2,3 , deuterated methane
CH 3 D, and oligomers with a pentameric structure were detected in the vapor
[132]. Experimental mass spectrometry observations reveal formation of cluster
compounds in the gas phase upon laser irradiation of AlMe 3 –NH 3 mixtures at low
(below room) temperatures [34–36].
Reaction energy profiles starting from trimers [Me 2 AlNH 2 ] 3 and leading to
hexamer amido-imino [Me 9 Al 6 N 6 H 9 ] and imino [MeAlNH] 6 compounds have been
computationally explored by Davydova et al. at B3LYP/def2-SVP, B3LYP-D3/def2TZVPP//B3LYP/def2-SVP, and M06-2X/def2-TZVPP//B3LYP/def2-SVP levels of
theory [133]. Three alternative reaction pathways of transformations of cyclic
[Me 2 AlNH 2 ] 3 have been explored (Figs. 6.9, 6.10, and 6.11). Activation energies for
the first methane elimination are very close both for mono- and bimolecular reaction
pathways (Figs. 6.9 and 6.10). Intermolecular methane elimination is always
exothermic, while intramolecular methane elimination steps are endothermic. Thus,
generation of [MeAlNH] 6 from [Me 2 AlNH 2 ] 3 at low temperatures is expected
to proceed via a bimolecular pathway. Formation of [Me 9 Al 6 N 6 H 9 ] from two
[Me 2 AlNH 2 ] 3 trimers is expected to be less favorable, as it involves higher barriers
and lower endothermicity.
Alternative Al-N bond breaking pathway was also considered. Obtained results
show that the energy required for Al-N bond breaking in cyclic [Me 2 AlNH 2 ] 3
(185 kJ mol −1 ) is of the same order as the activation energy (183 kJ mol −1 ) for
the first (limiting) step of methane elimination (both for mono- and bimolecular