Dalton
Transactions
PAPER
Cite this: Dalton Trans., 2013, 42, 11589
Received 10th April 2013,
Accepted 11th June 2013
DOI: 10.1039/c3dt50954k
www.rsc.org/dalton
Structural and thermodynamic properties of molecular
complexes of aluminum and gallium trihalides with
bifunctional donor pyrazine: decisive role of Lewis
acidity in 1D polymer formation†
Tatiana N. Sevastianova,
a
Michael Bodensteiner,
b
Anna S. Lisovenko,
a
Elena I. Davydova,
a
Manfred Scheer,
b
Tatiana V. Susliakova,
a
Irina S. Krasnova
a
and
Alexey Y. Timoshkin*
a
Solid state structures of group 13 metal halide complexes with pyrazine (pyz) of 2 : 1 and 1 : 1 compo-
sition have been established by X-ray structural analysis. Complexes of 2 : 1 composition adopt molecular
structures MX
3
·pyz·MX
3
with tetrahedral geometry of group 13 metals. Complexes of AlBr
3
and GaCl
3
of
1 : 1 composition are 1D polymers (MX
3
·pyz)
∞
with trigonal bipyramidal geometry of the group
13 metal, while the weaker Lewis acid GaI
3
forms the monomeric molecular complex GaI
3
·pyz, which is
isostructural to its pyridine analog GaI
3
·py. Tensimetry studies of vaporization and thermal dissociation of
AlBr
3
·pyz and AlBr
3
·pyz·AlBr
3
complexes have been carried out using the static method with a glass
membrane null-manometer. Thermodynamic characteristics of vaporization and equilibrium gas phase
dissociation of the AlBr
3
·pyz complex have been determined. Comprehensive theoretical studies of
(MX
3
)
n
·(pyz)
m
complexes (M = Al, Ga; X = Cl, Br, I; n =1,2;m =1–3) have been carried out at the B3LYP/
TZVP level of theory. Donor–acceptor bond energies were obtained taking into account reorganization
energies of the fragments. Computational data indicate that the formation of (MX
3
·pyz)
∞
polymers with
coordination number 5 is only slightly more energetically favorable than the formation of molecular com-
plexes of type MX
3
·pyz for X = Cl, Br. It is expected that on melting (MX
3
·pyz)
∞
polymers dissociate into
individual MX
3
·pyz molecules. This dovetails with low melting enthalpies of the (MX
3
·pyz)
∞
complexes.
Polymer stability decreases in the order AlCl
3
> AlBr
3
> GaCl
3
> AlI
3
> GaBr
3
> GaI
3
. For MI
3
·pyz complexes
computations predict that the monomeric structure motif is more energetically favorable compared to
the catena polymer. These theoretical predictions agree well with the experimentally observed mono-
meric complex GaI
3
·pyz in the solid state. Thus, the Lewis acidity of the group 13 halides may play a deci-
sive role in the formation of 1D polymeric networks.
Introduction
Group 13 element trihalides are strong Lewis acids which form
stable donor–acceptor complexes with nitrogen-containing
bases.
1–3
Volatile group 13–15 donor–acceptor complexes are
prospective single-source precursors (SSP) for the chemical
vapor deposition (CVD) of binary and composite nitrides.
4,5
Volatility and the strength of the donor–acceptor bond are
the two key characteristics of a successful SSP. Volatility of
the solid adduct is determined by its sublimation enthalpy,
which in turn depends on the structural properties of the
compounds. Complexes which exhibit isolated molecules in
their crystal structures have lower sublimation enthalpies
and are usually more volatile than polymeric and ionic com-
pounds.
2
Complexes with large donor–acceptor bond energies
and sufficient volatility, such as the pyridine adducts MX
3
·Py,
reveal a significant concentration in vapors even at elevated
(600–800 K) temperatures.
2,6
Usually, complexes with monodentate donors, for example
AlCl
3
·NH
2
t
Bu,
5
are used as SSP for the synthesis of binary
† Electronic supplementary information (ESI) available: Crystal structure infor-
mation for studied complexes, results of quantum chemical computations (total
energies, BSSE energies, standard entropies and enthalpies, optimized struc-
tures and xyz coordinates for all studied compounds obtained at the B3LYP/
TZVP level of theory), summary of tensimetry experiments (26 pages). CCDC
927394–927398. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c3dt50954k
a
Inorganic Chemistry Group, Department of Chemistry, St. Petersburg State
University, University Pr. 26, Old Peterhof, St. Petersburg, 198504, Russia.
E-mail: alextim@AT11692.spb.edu
b
Department of Inorganic Chemistry, University of Regensburg, 93040 Regensburg,
Germany. E-mail: manfred.scheer@chemie.uni-regensburg.de
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13–15 compounds. For generating ternary and composite
nitrides SSP should have different group 13 elements in the
same molecule. This may be achieved by the introduction of
the bifunctional donors LL. Complexes with LL = ethylenedi-
amine (en), tetramethylethylenediamine (tmen) containing
organometallic derivatives MR
3
adopt molecular structures
MR
3
·LL·MR
3
,
7
but have weak donor–acceptor (DA) bonds and
dissociate upon heating.
4
Both theoretical and experimental
studies
2,3,8,9
show that the substitution of organometallic
acceptors by group 13 element halides strongly increases the
DA bond energy of complexes with monodentate donors. It is
natural to assume that such a trend will hold for the com-
plexes with bifunctional donors as well. Our previous study of
group 13 metal halide complexes with en and tmen
10
showed
that such complexes adopt ionic structures [M(LL)X
2
]
+
[M′X
4
]
−
,
in which en and tmen serve as chelating bidentate ligands. It
is expected that the use of non-chelating rigid bifunctional
donors will be suitable for the formation of molecular mixed
metal precursors.
In continuation of our studies on structures, volatility, and
gas-phase stability of group 13 element halides with mono-
dentate nitrogen-containing donors,
2
we turned our attention to
complexes with rigid bifunctional donor pyrazine (pyz). Poly-
meric structures of GaCl
3
·pyz and GaBr
3
·pyz in the solid state
were established in 2007 by Richards and co-workers.
11
Pre-
vious mass spectrometry and tensimetry studies of the
complex formation in the GaCl
3
-pyz system
12
confirmed the
existence of the individual molecules (GaCl
3
)
2
·pyz and
GaCl
3
·pyz in the gas phase. Both complexes undergo reversibly
thermal dissociation in the gas phase; GaCl
3
·pyz is the domi-
nant form in vapors, while the (GaCl
3
)
2
·pyz content is very low
(0.2% at 383 K and only 0.05% at 673 K).
12
It is expected that
the substitution of GaCl
3
by the stronger Lewis acid AlBr
3
8
will
stabilize complexes of 2 : 1 composition in vapors. To test this
hypothesis, vaporization and thermal stability of (AlBr
3
)
2
·pyz
and AlBr
3
·pyz complexes have been studied by the static tensi-
metric method. The structures of both complexes, as well as
their GaCl
3
analogs, and the GaI
3
·pyz adduct have been deter-
mined by X-ray structural analysis. In addition, results of com-
parative theoretical DFT studies of (MX
3
)
n
·(pyz)
m
(M = Al,Ga;
X = Cl,Br,I; n = 1,2; m =1–3) are also reported.
Results and discussion
I. Structural studies
Let us first consider results of structural investigation of the
complexes. Expected structural types of the molecular com-
plexes are presented in Fig. 1 (M – group 13 metal, X –
halogen, L – monodentate, LL – bifunctional donor ligand).
Note that the group 13 metal can adopt both tetrahedral
(Fig. 1a–c) and trigonal bipyramidal environments (Fig. 1d,e).
LL serves either as a terminal monodentate ligand (Fig. 1b) or
as a bridging ligand with formation of distinct molecules MX
3
-
LL-MX
3
(Fig. 1c) or infinite polymeric chains -LL-MX
3
-LL-MX
3
-
(Fig. 1e).
We have been able to grow single crystals of complexes of
the bifunctional donor pyz with 2 : 1 composition (AlBr
3
)
2
·pyz
(1), (GaCl
3
)
2
·pyz (2), and 1 : 1 composition AlBr
3
·pyz (3),
GaCl
3
·pyz (4), GaI
3
·pyz (5). Experimental details of all com-
plexes are presented in Table S1.† We will start our discussion
with structural features of the complexes with a 2 : 1
composition.
Complexes (MX
3
)
2
·pyz. In contrast to en and tmen, which
form ionic complexes [M(LL)X
2
]
+
[MX
4
]
−
,
10
pyrazine serves as a
bridging ligand, coordinating two molecules of MX
3
with for-
mation of molecular complexes MX
3
·pyz·MX
3
. 1 and 2 are iso-
structural, molecular structure of 1 is shown in Fig. 2,
structure of 2 is available in the ESI.† In these complexes the
central atom M adopts a usual tetrahedral environment with
coordination number 4. However, DA bond distances in 1 and
2 (1.999 and 2.044 Å, respectively) are noticeably larger com-
pared to M–N distances in complexes with monodentate donor
Py (1.935(3) and 1.966(2) Å for AlBr
3
·Py and GaCl
3
·Py,
respectively
13
).
Complexes MX
3
·pyz. The molecular structures of the com-
pounds (AlBr
3
·pyz)
∞
(3) and GaI
3
·pyz (5) are given in Fig. 3–5.
Data for (GaCl
3
·pyz)
∞
4 are in good agreement with previously
reported values by Samanamu et al.
11
(in their work
11
the
complex was synthesized in tetrahydrofuran solution and
recrystallized from diethyl ether). Note that in 3 and 4 pyrazine
serves as a bridging ligand with formation of a polymeric
chain in which the group 13 metal possesses the coordination
number five. The halogen atoms always occupy equatorial, and
the nitrogen atoms – axial positions. The MX
3
fragment
remains essentially planar. Compounds 3 and 4 are isostruc-
tural and the bond distance M–Nin3 (2.133 Å) is by 0.07 Å
shorter than in 4 (2.203 Å). Such a trend agrees well with the
Fig. 1 Expected structural types of the molecular complexes of group 13
element halides MX
3
with monodentate (a,d) and bifunctional (b,c,e) donor
ligands L and LL. Definition of α and β angles for the determination of τ values.
Fig. 2 Molecular structure of complex AlBr
3
·pyz·AlBr
3
(1) in the crystal.
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changes of the covalent radii of Al and Ga.
14
Note that the
M–N bond distances are elongated (by 0.13–0.16 Å) on going from
complexes of 2 : 1 to 1 : 1 composition due to the change in the
tetrahedral environment in compounds 1 and 2 to trigonal
bipyramidal in 3 and 4. The packing of the polymer chains in
a crystal of 3 is shown in Fig. 4. The closest inter-polymer
H⋯Br contacts are 3.01 and 3.12 Å.
In contrast to the polymers 3 and 4, GaI
3
·pyz (5) exists in
the solid state only as an individual molecule GaI
3
·pyz (Fig. 5),
isostructural to the molecular complex GaI
3
·Py.
23
The Ga–N
bond distance in 5 is by 0.027 Å longer than in GaI
3
·Py, which
indicates lower donor ability of pyrazine compared to pyridine.
Coordination polymers of group 13 metal halides. Examples
of coordination polymer networks based on group 13 element
halides are known. An interesting 3D polymer network was
recently reported for InF
3
·4,4′bipy.
15
The formation of 1D poly-
meric chains with pyrazine was reported in 2007 for gallium
halides,
11
while indium and thallium trihalides prefer ladder-
type one-dimensional polymers.
11,16
In the present report we
show that AlBr
3
with pyz in a 1 : 1 stoichiometry also forms a
chain polymer (AlBr
3
·pyz)
∞
, while GaI
3
·pyz exists in the form
of individual molecules. Zig-zag chain polymers with 1,3-bis-
(dimethylamino)propane were previously reported for AlH
3
17
and GaH
3
.
18
Other 1D polymers include catena complexes of
AlCl
3
and GaCl
3
with the O-containing bidentate donor
dioxane.
19,20
Interestingly, the complex AlCl
3
·2diox adopts the
polymeric structure (AlCl
3
·diox)
∞
·diox with “free” dioxane
solvate molecules in between the polymeric chains.
19
Major
structural parameters of known 1D polymers and nitrogen-
containing MX
3
·2L complexes of group 13 metal halides are
summarized in Table 1. The Al–N distance in 3 is in the range of
the reported values for AlX
3
·2L complexes with bidentate nitro-
gen-donor ligands (2.021–2.166 Å). In the catena polymers, as
well as in 3 and 4, the trigonal bipyramidal structure is dis-
torted. As the criterion of structure distortion from the ideal
trigonal bipyramid, the use of τ-values was proposed by
Addison et al.
21
It is defined by the formula τ =(β − α)/60,
where α, β are the largest angles in the trigonal plane and
along the principal axis (Fig. 1d,e). For the perfect trigonal
bipyramid the τ-value equals one, and for the perfect square
pyramid the τ value equals zero. For all compounds listed in
Table 1, the τ-value is larger than 0.7, indicating essentially a
trigonal bipyramidal environment. Interestingly, our computed
τ-values for the gas phase complexes MX
3
·2pyz and
(MX
3
)
2
·(pyz)
3
are very close to one (0.96–0.99), suggesting that
there is very little distortion and strain is absent in the gas
phase structures (Table 1). Structural changes are virtually
independent of the size of the complex: valence angles and
τ-values are very similar for pyz-MX
3
-pyz complexes with one
trigonal bipyramidal center and for pyz-MX
3
-pyz-MX
3
-pyz with
two trigonal bipyramidal centers. We conclude that the experi-
mentally observed inequivalence of the X–M–X angles results
from the intermolecular interactions in the solid state. The
largest Cl–Ga–Cl angle in 1D polymer 4 (GaCl
3
·pyz)
∞
(125.4
degrees) is close to 124.8 found in (GaCl
3
·diox)
∞
. The distor-
tion of 3 (largest Br–Al–Br angle is 128.1) is more pronounced
and may result from the longer Al–Br distances, which are
more affected by the packing strain. Worral and coauthors
20
noted that in catena (GaCl
3
·diox)
∞
the Ga–Cl distances are
shorter and the Ga–O distances are significantly longer than in
other compounds with coordination number 5. Our results
indicate that both Ga–N and Ga–Cl bond distances in 4 are by
0.03–0.04 Å longer compared to those in the benzotriazole
(Hbta) complex GaCl
3
·2Hbta.
30
We conclude that all studied complexes in the solid state
exist either as individual molecules (1, 2, 5) or form 1D poly-
mers (3, 4). With exception of GaI
3
·pyz (5), in all other studied
complexes pyrazine serves as a bridging ligand. Especially
noteworthy is the fact that in 3 and 4 the group 13 element
adopts a trigonal bipyramidal environment, with pyrazine
Fig. 3 Molecular structure of complex (AlBr
3
·pyz)
∞
(3) in the crystal.
Fig. 4 Packing of the polymer chains in the crystal on the example of
(AlBr
3
·pyz)
∞
(3).
Fig. 5 Molecular structure of complex GaI
3
·pyz (5) in the crystal.
Dalton Transactions Paper
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ligands occupying the axial positions. In contrast, complex
GaCl
3
·2Py adopts an ionic structure [GaCl
2
Py
4
]
+
[GaCl
4
]
−
instead of a molecular trigonal bipyramidal adduct.
31
Such
difference underlines the importance of intermolecular inter-
actions in the solid state.
II. Computational studies
In order to get insight into the stability of the 1D polymers,
quantum chemical computations have been carried out. Direct
comparison between experimental and computed values for
MX
3
·pyz complexes of a 1 : 1 composition is not possible due
to different structural environments: trigonal bipyramidal in
the solid state polymer versus tetrahedral for the gas phase
complex. In this respect, to model a polymeric chain, we opti-
mized structures of MX
3
−pyz complexes of 2 : 1, 1 : 1, 1 : 2, and
2 : 3 compositions (Fig. S6†). Structures of the considered com-
plexes, obtained structural parameters, atomic and fragment
charges, thermodynamic characteristics of the complex for-
mation are presented in full in the ESI.† Structural parameters
of the complexes are in good agreement with experimental
data (Table S2†).
Optimized structures of individual complexes MХ
3
·pyz and
(MХ
3
)
2
·pyz reveal a tetrahedral environment at the group
13 metal. In the first complex pyrazine acts as a monodentate,
and in the second – as a bridging ligand. Upon additional
coordination of MX
3
, the M–N distances increase by
0.037–0.045 Å, indicating weaker M–N interaction in the
second complex. In the complexes of 1 : 2 and 2 : 3 compo-
sition the group 13 metal adopts a trigonal bipyramidal
environment and the M–N distance is further increased by
0.135 Å. For the complex (MX
3
)
2
(pyz)
3
the M–N distances with
terminal pyrazine ligands are by 0.08–0.09 Å shorter than
those with the bridging pyrazine. It can be concluded that the
M–N bond distance undergoes significant changes depending
on the coordination environment of the group 13 atom.
In the following the thermodynamic parameters for the dis-
sociation processes of the complexes are considered. Com-
puted proton affinities of Py and pyz are 937 and 881 kJ mol
−1
,
in good agreement with the experimental values of 929 ± 4 and
882 ± 4 kJ mol
−1
for Py and pyz, respectively.
32
Based on these
values, Py is the stronger donor compared to pyz. The com-
puted second proton affinity of pyz is much smaller (403 kJ
mol
−1
), which may result from electrostatic repulsion in the
pyrazinium dication HpyzH
2+
. Dissociation enthalpies of mole-
cular complexes with group 13 element trihalides of 1 : 1 com-
position (Table 3) are considerably lower than proton
affinities. Pyrazine complexes are by about 19 kJ mol
−1
weaker
bound than pyridine ones, in accordance with proton affinity
trends. Aluminum trichloride forms the most stable com-
plexes. Acceptor ability of Lewis acids decreases in the order
Table 1 Structural parameters of polymer compounds of aluminum and gallium trihalides and complexes with monodentate nitrogen-containing donors with
trigonal bipyramidal geometries
Compound R(M–N) (Å) Max X–M–X (°) N–M–N (°) τ Value Reference
AlCl
3
·2NMe
3
2.1580(16); 2.1662(16) 121.08(2) 178.76(5) 0.96 22
AlCl
3
·2NHMe
2
2.051(3); 2.073(3) 126.3(1) 176.5(1) 0.84 23
2.051(3); 2.057(3) 124.7(1) 176.8(1) 0.87 23
2.058(3); 2.066(3) 124.7(1) 177.6(2) 0.88 24
2.060(3); 2.078(3) 126.3(1) 176.8(2) 0.84 24
AlCl
3
·2morph
b
2.064(3); 2.093(3) 129.2(1) 175.3(1) 0.77 25
AlCl
3
·2pip
c
2.070(5); 2.070(5) 128.6(1) 176.1(3) 0.79 26
Salpen(
t
Bu)AlCl
d
2.031(8)
f
; 1.965(7)
g
126.3(3)
h
172.3(3)
a
0.77 27
AlCl
3
·2pyz
i
2.182 121.0 179.9 0.98 This work
(AlCl
3
)
2
·(pyz)
3
i
2.160; 2.226 120.5 179.8 0.99 This work
(AlCl
3
·diox)
∞
·diox 2.016(7)
k
128.7(1) 175.3(1)
j
0.78 19
Salpen(
t
Bu)AlBr 2.024(5)
f
; 1.958(5)
g
127.3(2)
h
173.5(2) 0.77 28
2.021(7)
f
; 1.962(4)
g
126.0(2)
h
172.7(2) 0.78 28
(AlBr
3
·pyz)
∞
(3) 2.133(2); 128.09(6) 173.13(13) 0.75 This work
AlBr
3
·2pyz
i
2.208 120.5 179.1 0.98 This work
(AlBr
3
)
2
·(pyz)
3
i
2.182; 2.259 120.3 178.0 0.96 This work
(AlBr
3
·diox)
∞
2.053(3)
k
129.8(1) 172.4(2)
j
0.71 29
AlI
3
·2pyz
i
2.242, 2.244 122.6 173.4 0.85 This work
(AlI
3
)
2
·(pyz)
3
i
2.215; 2.230; 2.231 120.8 176.9 0.94 This work
GaCl
3
·2Hbta
e
2.169(2); 2.169(2) 123.3(1) 177.0(1) 0.90 30
(GaCl
3
·pyz)
∞
(4) 2.2112(15) 125.17(12) 175.52(7) 0.84 11
(GaCl
3
·pyz)
∞
(4) 2.203(5) 125.36(6) 175.68(17) 0.84 This work
GaCl
3
·2pyz
i
2.276 120.8 179.95 0.99 This work
(GaCl
3
)
2
·(pyz)
3
i
2.243; 2.336 120.3 179.96 0.99 This work
(GaCl
3
·diox)
∞
2.206(8)
k
124.8(1) 175.4(2)
j
0.84 20
(GaBr
3
·pyz)
∞
2.262(6) 126.10(5) 174.2(3) 0.80 11
GaBr
3
·2pyz
i
2.326 120.6 179.7 0.99 This work
(GaBr
3
)
2
·(pyz)
3
i
2.277; 2.416 120.4 179.7 0.99 This work
GaI
3
·2pyz
i
2.424; 2.425 120.0 178.6 0.98 This work
a
N–M–Oangle.
b
Morph – morpholine.
c
Pip – piperidine.
d
Salpen – N,N′-propylenebis(3,5-di-tert-butylsalicylideneimine).
e
Hbta – benzotriazole.
f
Axial M–Nbonddistance.
g
Equatorial M–N bond distance.
h
N–Al–Oangle.
i
Computed for the gas phase comple x at the B3LYP/TZVP lev el of theory.
j
O–Al–O angle.
k
M–O distance.
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AlCl
3
> AlBr
3
> GaCl
3
> GaBr
3
which is in line with an increase
of the DA bond distances. Stronger Lewis acids have larger
values of charge transfer (equal to charge of the acceptor MX
3
)
and a larger negative charge on the nitrogen atom of the
donor molecule (Table 4).
Another useful criterion of the complex stability in the gas
phase is the value of temperature at which the equilibrium
constant for the complex dissociation process equals one. It
may be estimated using standard dissociation enthalpies and
entropies: T
K=1
≈ Δ
diss
H
°
298
/Δ
diss
S
°
298
. This single criterion com-
bines both energetic and entropy factors. According to T
K=1
values, complexes of 1 : 1 composition are most stable in
vapors (T
K=1
are in the range 660–940 K). The T
K=1
values for
2 : 1, 1 : 2 and 2 : 3 complexes are significantly lower due to the
entropy factor. Our theoretical results are in agreement with
the experimental observations of complexes with a 1 : 1 compo-
sition in vapors.
2,33
Estimation of the donor–acceptor bond energy. In order to
make a comparison between the stability of tetrahedral and tri-
gonal bipyramidal complexes, the reorganization energy
required for the pyramidalization of the acceptor MX
3
must be
taken into account. Reorganization energies of group 13 metal
trihalides from planar to perfectly pyramidal environment (tetra-
hedral XMX angle) are generally below 90 kJ mol
−1
.
13
Since
the XMX angles in DA complexes are larger than the tetra-
hedral ones, the reorganization energies upon complex for-
mation are usually smaller (below 35 kJ mol
−1 34,35
). In the
present report we computed reorganization energies for the
donor and acceptor fragments and obtained values of DA
bond energy (Table 4): nE
DA
= Δ
diss
E°+kE
reorg
(MX
3
)+lE
reorg
-
(pyz), where n – number of the DA bonds in the molecule,
k, l – number of MX
3
and pyz fragments, respectively. The com-
parison with MX
3
·Py analogs
13
shows that DA bond energies of
MX
3
pyz complexes are by about 25 kJ mol
−1
smaller. These
data are in good agreement with the increase of the M–N bond
distances in pyz complexes compared to Py (Table 2). The for-
mation of the DA bond with a second MX
3
molecule lowers the
DA bond energy (for MX
3
·pyz·MX
3
complexes by 25 kJ mol
−1
compared to MX
3
pyz). Changes in the partial charges of MX
3
fragments follow the energetic trends, suggesting that in
MX
3
·pyz·MX
3
two acceptors compete for the transferred
charge. For complexes of 2 : 1 composition, mixed metal
compounds MX
3
·pyz·M′X
3
(M,M′ = Al, Ga; X = Cl, Br) have been
also studied theoretically (Table S5†). Dissociation enthalpies
of mixed metal (heteronuclear) complexes can be obtained
from values for homonuclear complexes using a simple addi-
tive scheme.
Much lower (by 60–70 kJ mol
−1
) DA bond energies are
observed for MX
3
(pyz)
2
complexes with a trigonal pyramidal
structure. Donor atoms occupy the axial positions which are
energetically less preferable. However, in this case the charge
transfer to the MX
3
fragment slightly increases, since now two
pyz donor molecules provide the electron density for the
acceptor.
Computed DA bond energies allow us to address the ques-
tion about the most preferable structure of the 1 : 1 complexes.
Values of the DA bond energies, derived from the (MX
3
)
2
(pyz)
3
compound, may be taken as a first approximation to the M–N
bond energies in the catena polymer (MX
3
·pyz)
∞
. Our compu-
tations predict that the DA bond is much stronger for the indi-
vidual molecule MX
3
·pyz (tetrahedral environment) than in the
(MX
3
·pyz)
∞
polymer with a trigonal bipyramidal environment.
However, due to the fact that in the polymer two DA bonds are
formed per one MX
3
unit, the total interaction energy slightly
favors the formation of the catena polymer. The much lower
reorganization energy of MX
3
in the polymer also facilitates
the polymer formation. Formation of the polymeric structures
in the gas phase is energetically favored by 21, 9, 11 kJ mol
−1
for AlCl
3
, AlBr
3
and GaCl
3
acceptors, respectively. In the case
of the weaker acceptor GaBr
3
computations predict almost
equal Ga–N interaction energies for the formation of an indi-
vidual molecule GaBr
3
·pyz and (GaBr
3
·pyz)
∞
polymer (the
energy difference is less than 1 kJ mol
−1
). Such small energetic
differences between molecular and polymeric forms predicted
for the gas phase structures imply that intermolecular inter-
actions in the solid state can influence the preference of one
or the other structural type.
A much lower Ga–N bond stability in the GaBr
3
·pyz polymer
may explain the relatively low melting point of (GaBr
3
·pyz)
∞
(88–90 °C
11
) compared to the isostructural compounds
(GaCl
3
·pyz)
∞
(178–180 °C
11
) and (AlBr
3
·pyz)
∞
(circa 266 °C,
present work). Derived from tensimetry studies melting enthal-
pies increase from GaCl
3
pyz (12 ± 6 kJ mol
−112
) to AlBr
3
pyz
(64 ± 3 kJ mol
−1
). Note that the melting points of polymers
Table 2 Comparison of experimental M–N and M–X bond distances in solid complexes with pyridine and pyrazine ligands
Compound M–NM–X1 M–X2 Reference
AlBr
3
·Py 1.935(3) 2.268(1) 2.277(1), 2.280(1) 13
AlBr
3
·pyz·AlBr
3
(1) 1.999(6) 2.2537(18) 2.267(2), 2.2463(16) This work
(AlBr
3
·pyz)
∞
(3) 2.133(2) 2.3099(15) 2.3257(8) This work
GaCl
3
·Py 1.966(2) 2.1503(7) 2.1587(7), 2.1598(7) 13
GaCl
3
·pyz·GaCl
3
(2) 2.044(7) 2.135(2) 2.147(2) This work
(GaCl
3
·pyz)
∞
(4)
a
2.203(5) 2.174(2) 2.1855(14) This work
(GaCl
3
·pyz)
∞
(4)
b
2.2112(15) 2.1758(8) 2.1822(6) 11
GaI
3
·Py 2.000(4) 2.5106(6) 2.5191(7), 2.5246(6) 13
GaI
3
·pyz (5) 2.027(6) 2.5056(7) 2.5041(9), 2.5091(9) This work
a
123 K.
b
293 K.
Dalton Transactions Paper
This journal is © The Royal Society of Chemistry 2013 Dalton Trans., 2013, 42, 11589–11599 | 11593
Published on 12 June 2013. Downloaded by Universitaetsbibliothek Regensburg on 03/08/2016 12:34:55.
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