1,4-dioxobenzene compounds of gallium: Reversible binding of pyridines to [{(tBu)2Ga}2(μ-OC6H 4O)]n in the solid state

Article abstract of DOI:10.1021/ja0208714

The gallium aryloxide polymer, [{(^tBu)₂Ga} ₂(μ-OC₆H₄O)]_n (1) is synthesized by the addition of Ga(^tBu)₃ with hydroquinone in a noncoordinating solvent, and reacts with pyridines to yield the yellow compound [(^tBu)₂Ga(L)]₂(μ-OC₆H ₄O) [L = py (2), 4-Mepy (3), and 3,5-Me₂py (4)] via cleavage of the Ga₂O₂ dimeric core. The analogous formation of Ga(^tBu)₂(OPh)(py) (5) occurs by dissolution of [(^tBu)₂Ga(μ-OPh)]₂ in pyridine. In solution, 2-4 undergo dissociation of one of the pyridine ligands to yield [(^tBu)₂Ga(L)(μ-OC₆H₄O)Ga( ^tBu)₂]₂, for which the ΔH and ΔS have been determined. Thermolysis of compounds 2-4 in the solid-state results in the loss of the Lewis base and the formation of 1. The reaction of 1 or [(^tBu)₂Ga-(μ-OPh)]₂ with the vapor of the appropriate ligand results in the solid state formation of 2-4 or 5, respectively. The ΔH? and ΔS? for both ligand dissociation and association for the solid-vapor reactions have been determined. The interconversion of 1 into 2-4, as well as [(^tBu) ₂Ga(μ-OPh)]₂ into 5, and their reverse reactions, have been followed by ¹³C CPMAS NMR spectroscopy, TG/DTA, SEM, EDX, and powder XRD. Insight into this solid-state polycondensation polymerization reaction may be gained from the single-crystal X-ray crystallographic packing diagrams of 2-5. The crystal packing for compounds 2, 3, and 5 involve a head-to-head arrangement that is maintained through repeated ligand dissociation and association cycles. In contrast, when compound 4 is crystallized from solution a head-to-tail packing arrangement is formed, but during reintroduction of 3,5-Me₂py in the solid state-vapor reaction of compound 1, a head-to-head polymorph is postulated to account for the alteration in the ΔH? of subsequent ligand dissociation reactions. Thus, the ΔH? for the condensation polymerization reaction is dependent on the crystal packing; however, the subsequent reversibility of the reaction is dependent on the polymorph.

Full text of DOI:10.1021/ja0208714

Published on Web 08/15/2003
1,4-Dioxobenzene Compounds of Gallium: Reversible Binding
of Pyridines to [{(tBu)2Ga}2(µ-OC6H4O)]n in the Solid State
Laura H. van Poppel,† Simon G. Bott,‡ and Andrew R. Barron*,†,§
Contribution from the Department of Chemistry and Center for Nanoscale Science and
Technology, Rice UniVersity, Houston, Texas 77005 and Department of Chemistry,
UniVersity of Houston, Houston, Texas 77204
Received June 21, 2002; E-mail: arb@rice.edu
Abstract: The gallium aryloxide polymer, [{(tBu)2Ga}2(µ-OC6H4O)]n (1) is synthesized by the addition of
Ga(tBu)3 with hydroquinone in a noncoordinating solvent, and reacts with pyridines to yield the yellow
compound [(tBu)2Ga(L)]2(µ-OC6H4O) [L ) py (2), 4-Mepy (3), and 3,5-Me2py (4)] via cleavage of the Ga2O2
dimeric core. The analogous formation of Ga(tBu)2(OPh)(py) (5) occurs by dissolution of [(tBu)2Ga(µ-OPh)]2
in pyridine. In solution, 2-4 undergo dissociation of one of the pyridine ligands to yield [(tBu)2Ga(L)(µ-
OC6H4O)Ga(tBu)2]2, for which the ∆H and ∆S have been determined. Thermolysis of compounds 2-4 in
the solid-state results in the loss of the Lewis base and the formation of 1. The reaction of 1 or [(tBu)2Ga-
(µ-OPh)]2 with the vapor of the appropriate ligand results in the solid state formation of 2-4 or 5, respectively.
The ∆Hq and ∆Sq for both ligand dissociation and association for the solid-vapor reactions have been
determined. The interconversion of 1 into 2-4, as well as [(tBu)2Ga(µ-OPh)]2 into 5, and their reverse
reactions, have been followed by 13C CPMAS NMR spectroscopy, TG/DTA, SEM, EDX, and powder XRD.
Insight into this solid-state polycondensation polymerization reaction may be gained from the single-crystal
X-ray crystallographic packing diagrams of 2-5. The crystal packing for compounds 2, 3, and 5 involve a
head-to-head arrangement that is maintained through repeated ligand dissociation and association cycles.
In contrast, when compound 4 is crystallized from solution a head-to-tail packing arrangement is formed,
but during reintroduction of 3,5-Me2py in the solid state-vapor reaction of compound 1, a head-to-head
polymorph is postulated to account for the alteration in the ∆Hq of subsequent ligand dissociation reactions.
Thus, the ∆Hq for the condensation polymerization reaction is dependent on the crystal packing; however,
the subsequent reversibility of the reaction is dependent on the polymorph.
Introduction
The interaction between host and guest may be accomplished
in a number of ways; however, two general approaches have
been studied in detail. By changing (tailoring) the size and shape
of molecular cavities in host compounds, inclusion compounds,
such as zeolites and calixarenes, have been made to trap specific
molecules or ions. Alternatively, the molecules may be involved
in specific chemical binding. For example, hydrogen bonding
often plays a role in inclusion compounds’ ability to trap solvent
molecules.7 The formation of a specific bonding interaction
between a host and guest has generally been limited to spatially
controlled hydrogen bonding networks or Lewis acid-base
interactions. The latter ordinarily requires a vacant coordination
site on a metal center8 or the ability of the metal to expand its
coordination sphere.9 A combination of these approaches is often
employed.
Although aimed at different functions, chemical sensors,1 and
chemically triggered switches2,3 have certain underlying prin-
ciples in common. First, each must contain active sites or
functional groups that bind or trap a guest molecule. Second,
when a host-guest interaction occurs, it must be accompanied
by some physical change, e.g., color,4 conductivity,5 or lumi-
nescence.6 Third, the host-guest interaction should function in
a reversible fashion, ideally without degradation of the host.
† Rice University.
‡ University of Houston.
§ URL: www.rice.edu/barron.
(1) Sensors A ComprehensiVe SurVey; Go¨pel, W., Jones, T. A., Kleitz, M.,
Lundstro¨m, J., Seiyama, T., Eds.; VCH: New York, 1991; Vol. 2, Chemical
and Biochemical Sensors Part I.
(2) (a) Sugimoto, H.; Kimura, T.; Inoue, S., J. Am. Chem. Soc. 1999, 121,
2325. (b) Matthews, O. A.; Raymo, F. M.; Stoddart, J. F.; White, A. J. P.;
Williams, D. J. New J. Chem. 1998, 22, 1131.
(3) Pease, A. R.; Jeppesen, J. O.; Stoddart, J. F.; Luo, Y.; Collier, C. P.; Heath,
J. R. Acc. Chem. Res. 2001, 34, 433.
(4) (a) Zusman, R.; Rottman, C.; Ottolenghi, M.; Avnir, D. J. Non-Cryst. Solids
1990, 122, 107. (b) Dunn, B.; Zink, J. I. J. Mater. Chem. 1991, 1, 903.
(5) (a) Murakami, T. I. Anal. Lett. 1986, 19, 1973. (b) Gotoh, M.; Tamiya, E.;
Karube, I.; Kagawa, Y. Anal. Chim. Acta 1986, 187, 287. (c) Wu, S.; Li,
F.; Zhu, Y.; Shen, J. J. Mater. Sci. 2000, 35, 2005. (d) Steenwinkel, P.;
Grove, D. M.; Veldman, N.; Spek, A. L.; van Koten, G. Organometallics
1998, 17, 5647. (e) Ding, H.; Erokhin, V.; Ram, M. K.; Paddeu, S.; Valkova,
L.; Nicolini, C. Thin Solid Films 2000, 379, 279.
(6) (a) Winder, E. J.; Moore, D. E.; Neu, D. R.; Ellis, A. B.; Geisz, J. F.;
Kuech, T. F. J. Cryst. Growth 1995, 148, 63. (b) Geisz, J. F.; Kuech, T.
F.; Ellis, A. B. J. Appl. Phys. 1995, 77, 1233. (c) Glass, T. E. J. Am. Chem.
Soc. 2000, 122, 4522.
(7) Meinhold, D.; Seichter, W.; Ko¨hnke, K.; Seidel, J.; Weber, E. AdV. Mater.
1997, 9, 958.
(8) (a) Albrecht, M.; Lutz, M.; Spek, A. L.; van Koten, G. Nature 2000, 406,
970. (b) Albrecht, M.; Lutz, M.; Schreurs, A. M. M.; Lutz, E. T. H.; Spek,
A. L.; van Koten, G. J. Chem. Soc., Dalton Trans. 2000, 3797.
(9) Bourne, S. A.; Kilkenny, M.; Nassimbeni, L. R. J. Chem. Soc., Dalton
Trans. 2001, 1176.
9
11006
J. AM. CHEM. SOC. 2003, 125, 11006-11017
10.1021/ja0208714 CCC: $25.00 © 2003 American Chemical Society

1,4-Dioxobenzene Compounds of Gallium
A R T I C L E S
Group 13 alkoxides are known to form dimeric compounds
with the general formula [R2M(µ-OR′)]2 (M ) Al, Ga, In).10
With the addition of a Lewis base, this M2O2 core can be
cleaved, and monomeric compounds can be observed in solution
in equilibrium with the dimeric species. With more sterically
bulky R′ groups, monomeric compounds have been isolated and
characterized.11 Despite various kinetic and thermodynamic
measurements for these reactions in solution, and the determi-
nation of dissociation energies in vapor phase, there has been a
paucity of studies on solid-vapor reactions. With this in mind,
a polymeric gallium aryloxide polymer [{(tBu)2Ga}2(µ-OC6H4O)]n
has been synthesized in which a Ga2O2 core is cleaved by a
strong Lewis base to afford a monomeric species in solution,
as well as in the solid state. This gallium aryloxide polymer
also exhibits the characteristics of a molecular sensor and/or
switch, where the polymer acts as a “host” to pyridine (the
“guest”), but instead of relying on a vacant coordination site,
the Lewis base cleavage of a Ga2O2 core is the key for the host-
guest interaction.
pyridines allow for the dissolution of 1. Only the products from
the interactions with pyridine, 4-picoline (4-Mepy), and 3,5-
lutadine (3,5-Me2py) have been characterized; however, based
1
on the H and 13C NMR spectra of compound 1 dissolved in
d8-THF and d3-MeCN, similar species are formed with these
Lewis bases. A similar reaction has been previously reported
for aluminum 1,4-dioxybenzene compounds.16,17
Dissolution of compound 1 in pyridine (or methyl substituted
pyridine) results in cleavage of the Ga2O2 dimeric core and
yields the yellow monomeric compound [(tBu)2Ga(L)]2(µ-
OC6H4O), where L ) py (2), 4-Mepy (3), and 3,5-Me2py (4).
Compounds 2-4 may also be prepared by the direct reaction
of Ga(tBu)3 with hydroquinone in the presence of pyridine,
4-Mepy or 3,5-Me2py, respectively. The aluminum analogues
of compounds 2 and 4, as well as the THF complex have been
isolated.17 Compounds 2-4 have been characterized by solution
and solid-state NMR spectroscopy and single-crystal X-ray
diffraction. In addition, crystals of compound 2 grown from
pyridine solution contain a solvent of crystallization of pyridine,
i.e., 2.py. The analogous formation of Ga(tBu)2(OPh)(py) (5)
by dissolution of [(tBu)2Ga(µ-OPh)]2 in pyridine (see the
Experimental section) further supports the proposed structure
of compound 1.
The solid-state structure of compounds 2-4 are shown in
Figure 1; selected bond lengths and angles are given in Table
1. The centrosymmetric molecular structures consist of a 1,4-
dioxybenzene ligand bridging two Ga(tBu)2(L) moieties, with
all bond lengths and angles within the ranges for related
compounds.18 The gallium atoms are positioned on either side
of the plane of the 1,4-dioxybenzene ligand. This is in contrast
to the solid-state structure of the mono-gallium phenoxide
analogue, Ga(tBu)2(OPh)(py) (5) (Table 1) in which the pyridine
and phenoxide ligands are coplanar (Figure 2). The presence
of a pyridine of solvation in 2.py does not appear to significantly
affect the structure, see Table 1.
Results and Discussion
The gallium aryloxide polymer, [{(tBu)2Ga}2(µ-OC6H4O)]n
(1), is synthesized by the addition of Ga(tBu)3 to hydroquinone
at -78 °C in a noncoordinating solvent. Elemental analysis and
mass spectra fragmentation patterns are consistent with this
formulation. Compound 1 is stable in air for short periods of
time and is insoluble in solvents in which it does not react.
The 13C CPMAS NMR of compound 1 (see the Experimental
section) shows only a single resonance for each carbon
environment concordant with a highly symmetrical species in
which the 1,4-dioxobenzene ligand is bridging. The chemical
shifts for the tert-butyl groups and the aryl carbons of the
polymer are similar to the equivalent peaks found for the
phenoxide dimer, [(tBu)2Ga(µ-OPh)]2, see the Experimental
section, suggesting a similar structure. We have previously
shown that the 13C NMR shift for the alkyl substituents on
Group 13 metals is dependent on the coordination number
around the metal.12 The 13C CPMAS NMR shift for the tert-
butyl in compound 1 (δ ) 32.7 ppm) is within the range
previously observed for dimeric gallium aryloxide compounds,
[(tBu)2Ga(µ-OR)]2, (δ ) 30.7-32.9 ppm),13 and distinct from
the analogous shift in three-coordinate gallium alkoxides, e.g.,
δ ) 28.98 ppm for (tBu)2Ga(OCPh3).14 On this basis, we
propose that the gallium aryloxide polymer has a Ga2O2 dimeric
core (I) and structure typically observed for gallium alkoxides.15
As is common with other gallium aryloxides, compounds 2-4
are moisture sensitive, but only slightly air sensitive. Each
compound is soluble in its respective Lewis base, and also
sparingly soluble in C6H6 and CHCl3. Unlike compound 1, the
Lewis base adducts are yellow in color showing two absorptions
at 302-303 and 327-329 nm (see the Experimental section)
which may be assigned to a ligand-to-ligand charge transfer
from a Ga-Cσ to the low lying π* orbital of the pyridine.19
1
Ligand Dissociation in Solution. The solution H and 13C
NMR spectra of compounds 2-4 shows two sets of resonances;
the first can be assigned to [(tBu)2Ga(L)]2(µ-OC6H4O), whereas
(12) (a) Barron, A. R. J. Chem. Soc., Dalton Trans. 1988, 3047. (b) Rogers, J.
H.; Apblett, A. W.; Cleaver, W. M.; Tyler, A. N.; Barron, A. R. J.Chem.
Soc., Dalton Trans. 1992, 3179.
(13) Cleaver, W. M.; Barron, A. R.; McGuffey, A. R.; Bott, S. G. Polyhedron
1994, 13, 2831.
(14) Cleaver, W. M.; Barron, A. R. Organometallics 1993, 12, 1001.
(15) (a) Dembowski, U.; Pape, T.; Herbst-Irmer, R.; Pohl, E.; Roesky, H. W.;
Sheldrick, G. M. Acta Cryst. 1993, C49, 1309. (b) Keys, A.; Barbarich,
T.; Bott, S. G.; Barron, A. R. J. Chem. Soc., Dalton Trans. 2000, 577.
(16) Kaul, F. A. R.; Tschinkl, M.; Gabba¨ı, F. P. J. Organomet. Chem. 1997,
539, 187.
(17) van Poppel, L. H.; Bott S. G.; Barron, A. R. J. Chem. Soc., Dalton Trans.
2002, 3327.
(18) Tuck, D. G. ComprehensiVe Organometallic Chemistry; Wilkinson, G.,
Stone, F. G. A., Abel, E. W., Eds.; Pergamon Press: Oxford, 1982; Vol.
1, Ch. 7.
Compound 1 is insoluble in noncoordinating solvents;
however, coordinating solvents, such as THF, MeCN, and
(19) (a) Beumgarten, J.; Bessenbacher, C.; Kaim, W.; Stahl, T. J. Am. Chem.
Soc. 1989, 111, 2126. (b) Lichtenberger, D. L.; Hogan, R. H.; Healy, M.
D.; Barron, A. R. Organometallics 1991, 10, 609. (c) Mason, M. R.; Smith,
J. M.; Bott, S. G.; Barron, A. R. J. Am. Chem. Soc. 1993, 115, 4971.
(10) Healy, M. D.; Ziller J. W.; Barron, A. R. Organometallics 1991, 10, 597.
(11) Healy, M. D.; Power, M. B.; Barron, A. R. Coord. Chem. ReV. 1994, 94,
63.
9
J. AM. CHEM. SOC. VOL. 125, NO. 36, 2003 11007

A R T I C L E S
von Poppel et al.
Table 1. Selected Bond Lengths (Å) and Angles (°) for
[(tBu)2Ga(L)]2(µ-Oc6H4O) (2-4) and (tBu)2Ga(OPh)(py) (5)
compd
L
2
py
2.py
py
3
4
5
py
4-Mepy
3,5-Me2py
Ga-O
Ga-N
Ga-C
1.836(3)
2.215(4)
1.921(5)
2.082(5)
1.858(2)
2.112(3)
1.992(4)
1.999(3)
1.849(3)
2.070(4)
1.971(5)
1.989(4)
1.861(2)
2.075(3)
1.990(5)
2.006(5)
1.875(2)
2.087(3)
1.990(4)
1.998(4)
O-Ga-N 91.3(2)
O-Ga-C 101.7(2)
119.9(2)
96.9(1)
104.1(1)
115.5(1)
103.1(1)
106.7(1)
126.0(2)
128.3(2)
95.8(1)
105.3(2)
115.2(2)
103.3(2)
106.3(2)
126.0(2)
128.7(3)
92.6(1)
110.0(2)
111.5(2)
103.3(2)
106.9(2)
126.5(2)
127.1(2)
90.5(1)
109.2(2)
113.0(2)
104.4(2)
104.6(1)
127.8(2)
131.9(2)
N-Ga-C 104.4(2)
109.3(2)
C-Ga-C 124.4(2)
Ga-O-C 125.9(3)
Figure 2. Molecular structure of [(tBu)2Ga(OPh)(py)] (5). Thermal
ellipsoids shown at the 30% level, and hydrogen atoms are omitted for
clarity.
Table 2. Thermodynamic Data for Ligand Dissociation from
[(tBu)2Ga(L)]2(µ-OC6H4O) (2-4) and (tBu)2Ga(OPh)(py) (5) in
CDCl3 Solution
Figure 1. Molecular structures of (a) [(tBu)2Ga(py)]2(µ-OC6H4O) (2), (b)
[(tBu)2Ga(4-Mepy)]2(µ-OC6H4O) (3) and (c) [(tBu)2Ga(3,5-Me2py)]2(µ-
OC6H4O) (4). Thermal ellipsoids shown at the 30% level, and hydrogen
atoms are omitted for clarity.
K
eq @25 °C
∆H
∆S
∆G @25 °C
(kJ.mol-1 b
compd
L
(mol‚dm-3 a
)
(kJ‚mol-1 b
)
(J‚K-1mol-1 b
)
)
2
3
4
5
py
4-Mepy
1.48 × 10-4
69(2)
52(2)
57(1)
160(6)
107(8)
125(2)
211.7(7)
21(2)
20(4)
19(1)
40.1(5)
2.98 × 10-4
the second is consistent with dissociation of one of the Lewis
base ligands per di-gallium unit, and the formation of the dimers
[(tBu)2Ga(L)(µ-OC6H4O)Ga(tBu)2]2 (II). A similar structure has
been previously reported for dimethylaluminum 1,4-dioxyben-
zene compounds.16
3,5-Me2py 2.11 × 10-4
py
1.45 × 10-7 103.2(3)
a Calculated from thermodynamic data. b Errors given in parentheses.
This is in contrast to most dissociation equilibria observed for
Group 13 Lewis acid-base complexes, including (tBu)2Ga-
(OPh)(py), which show a single set of resonances under
equilibrium conditions.
The temperature dependence of the equilibrium constant
allows for the determination of ∆H and ∆S using a Van’t Hoff
plot (Table 2). The ∆S values are positive, as would be expected
for a dissociative process. The ∆H values are positive as well,
indicating that this is an endothermic reaction, and the reactant
is more stable than the product.
Variable temperature NMR indicates that [(tBu)2Ga(L)]2(µ-
OC6H4O) and [(tBu)2Ga(L)(µ-OC6H4O)Ga(tBu)2]2 (II) are in
equilibrium (eq 1), although the presence of two sets of
resonances indicates that the reaction is slow on the NMR time
scale (10-5 s)
The 1H and 13C NMR spectra of (tBu)2Ga(OPh)(py) (5) only
show a single set of resonances under equilibrium conditions,
1
but the H NMR signals for the pyridine ligand of (tBu)2Ga-
(OPh)(py) have an upfield shift with increasing temperature.20
The magnitude of the shift is temperature dependent consistent
with the solution equilibrium shown in eq 2
Keq
2 [(tBu)2Ga(L)]2(µ-OC6H4O) y z
Keq
[(tBu)2Ga(L)(µ-OC6H4O)Ga(tBu)2]2 + 2 L (1)
2 (tBu)2Ga(OPh)(py) y z [(tBu)2Ga(µ-OPh)]2 + 2 py (2)
9
11008 J. AM. CHEM. SOC. VOL. 125, NO. 36, 2003

1,4-Dioxobenzene Compounds of Gallium
A R T I C L E S
morphological change. Furthermore, no endotherm is observed
in the differential thermal analysis (DTA) during pyridine loss.
The solid-state loss of pyridine from compound 2 has been
followed by SEM and EDX. From point to point measurements
of a single crystal before and after heating to 150 °C, there is
a change in crystal volume with desorption of pyridine. A 26.6%
decrease in volume is observed for the conversion of compound
2 to 1. This should be compared a value of 25%, calculated
from the atomic volume and the crystal density of compound 2
obtained from single-crystal X-ray diffraction.
Shown in Figure 4 are the SEM and the gallium and nitrogen
EDX maps of a single crystal of [(tBu)2Ga(py)]2(µ-OC6H4O)
along with the equivalent images for a sample after heating to
150 °C. Although the Ga EDX map shows a uniform composi-
tion, the N EDX map clearly shows the loss of nitrogen (i.e.,
pyridine) from the sample. As may be seen from Figure 4d, a
single crystal of compound 2 appears to remain physically intact
upon loss of pyridine. Unfortunately, we have been unable at
this time to collect single-crystal diffraction data on compound
1, despite the crystalline nature of the product (Table 3). We
believe that this is associated with the formation of defects
within the individual crystal and/or micro-cracks and porosity
due to the loss of the volatiles. Support for this proposal is
provided by surface area measurements. Solid-state loss of
pyridine is accompanied by an increase in the surface area of
the sample. BET measurements on a crystalline sample of
compound 2 show a surface area of 0.098 m2‚g-1. After careful
heating to 150 °C to ensure dissociation of all the pyridine, and
the formation of [{(tBu)2Ga}2(µ-OC6H4O)]n, the surface area
Figure 3. TGA of [(tBu)2Ga(py)]2(µ-OC6H4O) (2).
Table 3. Comparison of Powder XRD Data for
[{(tBu)2Ga}2(µ-OC6H4O)]n Samples
t
reaction of Ga(Bu)3
thermal decomposition of
t
w. HOC6H OH
[(Bu)2Ga(L)] (µ-OC6H O) (1)a
4
2 4
2θ (°)
d
2θ (°)
d
11.48
12.38
13.29
14.40
18.20
25.29
7.70
7.14
6.66
6.15
4.87
3.52
11.38
12.29
13.10
14.40
18.42
25.37
7.77
7.19
6.75
6.15
4.81
3.51
a FOM ) 8.82 based upon a comparison with data from a sample
prepared by the reaction of Ga(tBu)3 with HOC6H4OH.
1
Assuming the H NMR shift of the pyridine signal is directly
proportional to the mole fraction of the total species present,
1
the H NMR chemical shift of the pyridine signals, at a given
is increased to 0.688 m2‚g-1
.
temperature, may be used to calculate the equilibrium constant,
Keq. The temperature dependence of Keq allows for the deter-
mination of ∆H and ∆S for the reaction of eq 2 (Table 2).
Ligand Dissociation in the Solid State. The thermal
decomposition of compounds 2-4 was studied in the solid state
by thermogravimetric/differential thermal analysis (TG/DTA).
Heating to 500 °C resulted in a weight loss, corresponding to
the formation of Ga2O3. Prior to this, an initial weight loss was
observed starting at ca. 124 °C, which corresponds to the loss
of 2 equiv of the Lewis base ligands; py ) 24.8% (calc. 24.9%),
4-Mepy ) 29% (calc. 28%), and 3,5-Me2py ) 28.8% (calc.
30.9%), e.g., Figure 3. A white stable material is formed after
this initial weight loss and is confirmed to be [{(tBu)2Ga}2(µ-
OC6H4O)]n (1) by 13C CPMAS NMR spectroscopy and powder
X-ray diffraction (Table 3). Furthermore, TGA/IR indicates that
the only substance detected during that initial weight loss is
the Lewis base. Thus, solid state thermolysis of [(tBu)2Ga(L)]2-
(µ-OC6H4O) yields [{(tBu)2Ga}2(µ-OC6H4O)]n, eq 3
The rate of loss of the Lewis base, and hence the conversion
of [(tBu)2Ga(L)]2(µ-OC6H4O) (2-4) to [{(tBu)2Ga}2(µ-OC6H4O)]n
(1) can be followed by TGA,and from the temperature depen-
dence of the rate of mass loss (Figure 5), the ∆Hq and ∆Sq
may be determined, Table 4. TGA-IR confirmed that pyridine
is the only volatile product released under the conditions for
which kinetic measurements were taken. The transformation of
[(tBu)2Ga(L)]2(µ-OC6H4O) to compound 1 is clearly a dissocia-
tive process with a large positive ∆Sq, and the activation energies
are within the range expected for the bond dissociation energy
of a gallium Lewis base complexes with nitrogen donor.21 Solid
state thermolysis of (tBu)2Ga(OPh)(py) (5) also results in the
loss of 1 equiv of pyridine per gallium, and the residual white
solid is confirmed to be [(tBu)2Ga(µ-OPh)]2 by 1H NMR
spectroscopy. As may be seen from Table 4, the values for ∆Hq
and ∆Sq for the reaction described by eq 4 are significantly
higher than compounds 2-4 (i.e., eq 3)
∆8
(tBu)2Ga(OPh)(py)
9
∆8 [(tBu)2Ga(µ-OPh)]2 + py (4)
[(tBu) Ga(L)] (µ-OC H O)
9
2
2
6
4
(2 - 4)
A comparison of ∆Hq (or ∆Gq) for the solid-state dissociation
of L from [(tBu)2Ga(L)]2(µ-OC6H4O) (2-4) versus either the
pKb (of L) or the Ga-N bond length, does not show any
correlation. If we consider that ∆Hq might be dependent on the
packing of the molecules, a comparison may be made with the
distance between the two molecules that will subsequently form
the dimeric core. A suitable measure is the Ga...Ga bond distance
[{(tBu)2Ga}2(µ-OC6H4O)]
(1)
n + 2 L (3)
It should be noted that the dissociation of pyridine is not
accompanied by melting. This may be observed visually in a
melting point apparatus, where the color change from yellow
to white is readily observed for a single crystal without
(20) (a) Power, M. B.; Nash, J. R.; Healy, M. D.; Barron, A. R. Organometallics
1992, 11, 1830. (b) Healy, M. D.; Gravelle, P. W.; Mason, M. R.; Bott, S.
G.; Barron, A. R. J. Chem. Soc. Dalton Trans. 1993, 441.
(21) Barron A. R.; Power, M. B. Gallium: Organometallic Chemistry. In
Encyclopedia of Inorganic Chemistry; King, R. B., Ed.; Wiley: Chichester,
1994.
9
J. AM. CHEM. SOC. VOL. 125, NO. 36, 2003 11009

A R T I C L E S
von Poppel et al.
Figure 4. SEM (a, d) with associated gallium (b, e) and nitrogen (c, f) elemental maps of single crystals of [(tBu)2Ga(py)]2(µ-OC6H4O) (2) (a-c) along with
the equivalent images for a sample after heating to 150 °C (d - f).
Figure 5. Plot of ln(k/T) as a function of 1/T for the conversion of
[(tBu)2Ga(L)]2(µ-OC6H4O) (2) to [{(tBu)2Ga}2(µ-OC6H4O)]n (1) (R )
0.996).
Figure 6. Plot of ∆Hq as a function of the intermolecular Ga...Ga distance
(R ) 0.979).
Table 4. Kinetic Data for Ligand Dissociation from
[(tBu)2Ga(L)]2(µ-OC6H4O) (2-4) and (tBu)2Ga(OPh)(py) (5) in Solid
expected when compared to the other compounds. Insight into
this discrepancy can be obtained by examination of the crystal
packing diagrams for compounds 2-5 (Figure 7).
State
‡
∆H
∆S‡
∆G‡ @25 °C
(kJ‚mol-1 a
compd
L
(kJ‚mol-1 a
)
(J‚K-1mol-1 a
)
)
Within the crystal unit cells of the pyridine (2 and 5) and
4-Mepy (3) compounds, the Lewis base ligands are arranged
in a head-to-head fashion, meaning that the pyridine of one
molecule is oriented toward a pyridine of another molecule next
to it (see Figure 7a, b, and d). This packing motif requires that
both molecules of pyridine must dissociate before dimerization
can occur between the two fragments, such a process requires
sequential dissociation of two Ga-N bonds, i.e., a dissociative-
dissociative pathway (D,D), see Scheme 1. The relationship
between the crystal packing and the ∆Hq (and ∆Sq) values for
compounds 2, 3, and 5 is thus reasonable based upon a
dissociative-dissociative (D,D), reaction pathway.
2
3
4
5
py
96(3)
66(2)
73(6)
184(6)
98(2)
125(17)
279(4)
41(1)
36(1)
35(1)
42(1)
4-Mepy
3,5-Me2py
py
126(1)
a Error given in parentheses.
between two molecules in the asymmetric unit cell. As is shown
in Figure 6, there exists a good correlation between ∆Hq (and
∆Sq) and the intermolecular Ga‚‚‚Ga distance for compounds
2, 3, and 5. However, the 3,5-Me2py compound (4) does not
seem to follow the trend of the other compounds. The ∆Hq and
∆Sq values for compound 4 are much lower than would be
9
11010 J. AM. CHEM. SOC. VOL. 125, NO. 36, 2003

1,4-Dioxobenzene Compounds of Gallium
A R T I C L E S
Figure 7. Crystal packing diagram for (a) [(tBu)2Ga(py)]2(µ-OC6H4O) (2), (b) [(tBu)2Ga(4-Mepy)]2(µ-OC6H4O) (3), (c) [(tBu)2Ga(3,5-Me2py)]2(µ-OC6H4O)
(4), and [(tBu)2Ga(OPh)(py)] (5) showing the relative position of the Ga-O units between adjacent molecules. Dashed lines indicate the vectors for Ga-O
bond formation upon loss of the pyridine ligands. Ga‚‚‚Ga bond distance 7.686 Å (2), 7.317 Å (3), 8.093 Å (4), and 8.308 Å (5).
9
J. AM. CHEM. SOC. VOL. 125, NO. 36, 2003 11011

A R T I C L E S
von Poppel et al.
Scheme 1. Schematic Representation of the Thermal Conversion
(D,D mechanism) of (tBu)2Ga(py)]2(µ-OC6H4O) (2) to
[(tBu)2Ga(µ-OC6H4O)]n (1), Showing the Head-to-Head Orientation
of One Molecule with Respect to Another
Scheme 2. Schematic Representation of the Thermal Conversion
(D,A mechanism) of (tBu)2Ga(3,5-Me2py)]2(µ-OC6H4O) (4) to
[(tBu)2Ga(µ-OC6H4O)]n (1), Showing the Head-to-Tail Orientation of
One Molecule with Respect to Another
The crystal packing of compound 4 (Figure 7c) is quite
different from the other derivatives; the molecules are oriented
in a head-to-tail fashion. In this motif, the 3,5-Me2py ligand on
one molecule is oriented toward the gallium center of another
(closest) molecule, and the ligand of 3,5-Me2py on the second
molecule points away from its nearest neighbor, see Scheme 2.
This conformation allows for an alternative condensation
pathway. The dissociation of one 3,5-Me2py ligand allows for
the association of the resulting coordinately unsaturated gallium
center to the aryloxide center on an adjacent molecule. The
remaining 3,5-Me2py ligand may be substituted in an Associa-
tive (A) or Interchange (I) reaction step. It is reasonable to expect
that the D,D mechanism would a have a higher activation barrier
than a D,A (or D, I) pathway, since the full dissociation of 2
equiv of the pyridine ligands are required for a single condensa-
tion. Such a rational would explain the lower ∆Hq observed
for compound 4 in comparison to the other homologues. If
compound 4 crystallized out in a head-to-head polymorph it
would be expected to haVe a larger ∆Hq for the loss of the
3,5-Me2py ligands than obserVed in the head-to-tail polymorph.
Although we have been unable to obtain single-crystal diffrac-
tion data (i.e., the Ga...Ga distance) for a head-to-head polymorph
of compound 4, we have shown that the head-to-head polymorph
for does indeed have a significantly higher ∆Hq for the ligand
dissociation/condensation reaction (as discussed below).
To provide further comparison between the D,D and D,A
pathways, we have performed DFT calculations at the B3LYP
level (see the Experimental section) on the model compounds:
Me2Ga(OMe), Me2Ga(OMe)(NH3), Me2Ga(NH3)(µ-OMe)-
GaMe2(OMe) (III), Me2Ga(µ-OMe)(µ-Me)GaMe(OMe) (IV),
and [Me2Ga(µ-OMe)]2 (V); selected calculated bond lengths and
angles are given in Tables 5 and 6 along with comparative
experimental values for related structures. A comparison with
experimental values for tert-butyl analogues show a good
agreement between experimental and calculated structural
parameters.
Figure 8 shows a calculated reaction diagram that simulates
the dissociative-dissociative pathway proposed to occur for the
condensation/polymerization reaction of compounds 2, 3, and
5 in the solid state. It should be noted that the overall reaction
is endothermic (-65 kJ‚mol-1) rather than exothermic as is
observed experimentally for the tert-butyl/pyridine derivative,
see Table 2. This difference may be rationalized by considering
the ∆H for dimerization. Upon the basis of the data in Tables
2 and 4, the ∆H dimer for compound 5 may be estimated to be
183 kJ‚mol-1, which is significantly smaller than the value
calculated for Me2Ga(OMe), see Figure 8. This difference can
be attributed to the steric bulk of the substituents (tert-butyl
9
11012 J. AM. CHEM. SOC. VOL. 125, NO. 36, 2003

1,4-Dioxobenzene Compounds of Gallium
A R T I C L E S
Table 5. Selected Bond Lengths (Å) and Angles (deg) for
Calculated Structures of Monomeric Gallium Alkoxides in
Comparison to Experimentally Determined Values for Structural
Analogs
t
t
compd
Me2Ga(OMe) (Bu)2Ga(OCPh3)a Me2Ga(OMe)(NH ) (Bu)2Ga(OPh)(py) (5)
3
Ga-O
Ga-N
Ga-C
1.810
1.831(4)
1.880
2.132
1.998
1.998
1.875(2)
2.087(3)
1.990(4)
1.998(4)
1.920
1.970
2.003(6)
2.012(6)
O-Ga-N
O-Ga-C
81.73
114.5
114.4
105.2
105.0
125.1
123.9
90.5(1)
109.2(2)
118.2(2)
104.4(2)
104.6(1)
127.8(2)
131.9(2)
118.6
111.4
115.0(2)
113.0(2)
N-Ga-C
C-Ga-C
Ga-O-C
130.0
125.9
126.1(2)
127.5(3)
a Cleaver, W. M.; Barron, A. R. Organometallics 1993, 12, 1001.
versus methyl) and the alkoxide (OMe versus OPh).22 It is worth
noting that as it should be, the ∆H dimer calculated for the
formation of compound 1 is essentially independent of the
identity of the Lewis base, i.e., ∆H dimer ) 88 ( 3, 80 ( 2,
and 89 ( 6 kJ‚mol-1 for compounds 2, 3, and 4, respectively.
Figure 9 shows a reaction diagram representing the solid-
state dissociative-associative pathway proposed to occur for
the dimerization of compound 4. The calculated dissociative-
associative pathway shows that the activation barrier for the
rate determining step (the dissociation of pyridine) is half that
of the activation barrier for the dissociative-dissociative pathway,
The experimental value for ∆Hq for 4 is 73% of the value
expected for a dissociative-dissociative mechanism. Although
not exact reproduction of the experimental observation, this trend
is consistent with our experimental data.
Ligand Association through a Solid/Vapor Reaction. Given
that [{(tBu)2Ga}2(µ-OC6H4O)]n (1) is converted to [(tBu)2Ga-
(L)]2(µ-OC6H4O) (2-4) by dissolution in the appropriate ligand
(or reaction with the appropriate ligand in hydrocarbon solution)
and the reverse reaction occurs upon solid state thermolysis,
we have investigated whether the interconversion of that
[{(tBu)2Ga}2(µ-OC6H4O)]n and [(tBu)2Ga(L)]2(µ-OC6H4O) may
be accomplished entirely in the solid state. The TG/DTA
instrument was set up such that either N2 or N2/pyridine vapor
could be used as the flow gas.
Figure 8. Calculated reaction diagram for the dissociative-dissociative
pathway proposed to occur for the condensation/polymerization reaction
of compounds 2, 3, and 5 in the solid state. Experimental values for
compound 5 given in parentheses.
The uptake of pyridine at a vapor pressure of 22 mmHg is rather
slow, and whereas 95% uptake occurs in approximately 70 min.,
complete conversion of polymer 1 to compound 2 takes over
300 min. This observation is not surprising considering that the
pyridine would react with the surface sites first and then have
to diffuse further in the crystal lattice to find additional reactive
sites. Switching the flow to pure N2 and raising the temperature
to 124 °C results in a mass loss equivalent to dissociation of
the pyridine. The re-formation of 1 may be confirmed by powder
XRD. Cooling to 34 °C and reintroducing the pyridine vapor
again results in the formation of compound 2. This process may
be cycled repeatedly. The same result may be achieved by
starting with [(tBu)2Ga(py)]2(µ-OC6H4O) (2), e.g., Figure 10.
It should be noted that the reformation of compound 2 appears
to have a hysteresis; however, if sufficient time is allowed, then
complete formation of 2 is attained. Alternatively, higher
pyridine concentrations (through increasing the temperature of
the pyridine bubbler and thus the pyridine partial pressure)
diminish the hysteresis. It is important to note that powder XRD
and 13C CPMAS NMR indicate that decomposition is negligible
after repeated association/dissociation cycles. Similar results
have been found for compound 3.
When a sample of [{(tBu)2Ga}2(µ-OC6H4O)]n (1) is exposed
to pyridine vapor at 34 °C a weight increase is observed
equivalent to the uptake of one mole of pyridine per mole of
gallium, i.e., the conversion of 1 to 2. Confirmation of the
formation of compound 2 in this manner may be obtained from
powder XRD and solution and solid state NMR spectroscopy.
The rate of association of the Lewis base to the polymer varies
with temperature. The temperature dependence allows for the
Table 6. Selected Bond Lengths (Å) and Angles (deg) for Calculated Structures and an Experimental Dimeric Gallium Alkoxide
t
compd
Me2Ga(NH3)(µ-OMe)GaMe2(OMe)
Me2Ga(µ-OMe)GaMe2(OMe)
[Me2Ga(µ-OMe)]
[(Bu)2Ga(µ-OPh)] a
2
2
Ga-Obr
1.94, 1.99
1.90
2.07
1.95, 1.95
1.82
1.96, 1.95
2.035(1)
Ga-Oter
Ga-N
Ga-C
1.98
1.98
1.98
1.993
Obr-Ga(1)-Obr
Obr-Ga-N
Obr-Ga-C
C-Ga-C
Ga-Obr-Ga
Ga-Obr-C
Ga-Oter-C
79.84
78.3(1)
94.9
111.2, 108.5
125.8, 126.4
125.4
115.0, 120.2
119.4
109.4, 109.9
115.4, 129.2
90.48
125.2
s
109.1, 108.9
129.9
99.9, 100.3
123.7
107.2(2), 119.0(2)
119.6(5)
101.7(1)
129.1(1)
a Cleaver, W. M.; McGuffey, A. R.; Bott, S. G.; Barron, A. R. Polyhedron 1994, 13, 2831.
9
J. AM. CHEM. SOC. VOL. 125, NO. 36, 2003 11013

A R T I C L E S
von Poppel et al.
Figure 9. Calculated reaction diagram for the dissociative-associative pathway proposed to occur for the condensation/polymerization reaction of compound
4.
Table 8. Comparison of Powder XRD Data for Samples of
[(tBu)2Ga(py)]2(µ-OC6H4O) (2) Showing Retention of the Crystalline
Phase through Dissociation/Association Cycles
formation from
crystallization
from solution
vapor reaction w.
[{(Bu)2Ga}2(µ-OC6H O)] (1)a
t
4
n
2θ (°)
d
2θ (°)
d
11.10
12.37
13.02
16.10
19.22
29.96
7.96
7.15
6.79
5.51
4.62
2.98
11.21
12.29
13.06
16.11
19.22
29.99
7.88
7.19
6.77
5.50
4.61
2.98
a FOM ) 22.2 based upon a comparison with data from a sample of
compound 2 crystallized from solution.
Figure 10. TG/DTA of [(tBu)2Ga(py)]2(µ-OC6H4O) (2) cycled under N2
(124 °C) and N2/pyridine (34 °C) atmospheres, showing the reversible loss
and uptake of pyridine. The hysteresis observed in the first and second
cycles is obviated by the use of longer reaction times as is seen from the
third cycle.
through repeated dissociation/association events for the solid
state/vapor reactions.
If the addition of the Lewis base vapor to [{(tBu)2Ga}2(µ-
OC6H4O)]n (1) gives [(tBu)2Ga(L)]2(µ-OC6H4O) in the head-
to-head packing orientation, then we should see a change in
the ∆Hq for the decomposition of [(tBu)2Ga(3,5-Me2py)]2(µ-
OC6H4O) (4) between the first and subsequent cycles. As we
predicted above, the ∆Hq determined for [(tBu)2Ga(3,5-Me2py)]2-
(µ-OC6H4O) (4) formed from the vapor-solid-state reaction of
1 with 3,5-Me2py is indeed higher [88(7) kJ‚mol-1] than that
for a sample crystallized from solution [73(6) kJ‚mol-1].
Furthermore, while we have been unable to obtain crystals
suitable for single-crystal X-ray diffraction studies for the head-
to-head polymorph for compound 4, we can show that the crystal
packing does change between the “as crystallized” samples and
those formed from the solid-vapor reaction between compound
1 and 3,5-Me2py. The formation of a new morphology is
confirmed by powder XRD, see Table 9.
Table 7. Thermodynamic Data for the Formation of
[(tBu)2Ga(L)]2(µ-OC6H4O) (2 and 4) from [{(tBu)2Ga}2(µ-OC6H4O)]n
(1) Using the Solid State-Vapor Reaction
q
∆H
∆Sq
∆G @25 °C
(kJ‚mol-1 a
compd
L
(kJ‚mol-1 a
)
(J‚K-1mol-1 a
)
)
2
4
py
3,5-Me2py
-168(3)
-167(3)
-553(3)
-560(2)
-3(2)
-1(2)
determination of ∆Hq and ∆Sq of the Ga2O2 dimeric core
cleavage. These values are listed in Table 7 for the formation
of compounds 2 and 4 from compound 1.
It should be noted that with repeated cycles of association/
dissociation of pyridine, i.e., 1 T 2, the ∆Hq does not change.
This suggests that the same dissociative-dissociative (D,D)
mechanism occurs through multiple cycles, and the structure
of [(tBu)2Ga(py)]2(µ-OC6H4O) (2) is the same from one cycle
to the next. To confirm this proposal, powder XRD was taken
of [(tBu)2Ga(py)]2(µ-OC6H4O) (2) as it was crystallized out of
solution and compared to that formed from the solid state-vapor
reaction. A comparison of 2θ (°) and d spacing values for each
sample confirms that the two samples have the same morphol-
ogy (Table 8). Thus, the head-to-head polymorph is maintained
We propose, therefore, that when compound 4 is crystallized
from solution a head-to-tail packing arrangement is formed, but
during reintroduction of 3,5-Me2py in the solid state/vapor
reaction, a head-to-head polymorph is formed (i.e., the reverse
of Scheme 1). Thus, the first and subsequent cycles form two
different polymorphs with different activation barriers. If the
∆Hq for the second cycle onward, is compared to the ∆Hq versus
Ga...Ga bond distance correlation for compound 4, we see that
the head-to-head polymorph is expected to have a Ga...Ga
distance comparable to that observed for compound 2.
(22) Francis, J. A.; McMahon, C. N.; Bott, S. G.; Barron, A. R. Organometallics,
1999, 18, 4399.
9
11014 J. AM. CHEM. SOC. VOL. 125, NO. 36, 2003

1,4-Dioxobenzene Compounds of Gallium
A R T I C L E S
Table 9. Distinguishable Peaks Found for Powder XRD Data for
Samples of [(tBu)2Ga(3,5-Me2py)]2(µ-OC6H4O) (4) Showing the
Formation of Different Polymorphs Depending on the Synthetic
Route
to the more traditional inclusion compounds and compounds
with vacant coordination sites. This study also suggests that in
order for a sensor switch to be reversible, the same morphology
(polymorph) must be maintained.
formation from
crystallization
from solution
vapor reaction w.
t
[{(Bu)2Ga}2(µ-OC6H O)] (1)
4
n
Experimental Section
2θ (°)
d
2θ (°)
d
Mass spectra were obtained on a Finnigan MAT 95 mass spectrom-
eter operating with an electron beam energy of 70 eV for EI mass
spectra. IR spectra (4000-400 cm-1) were obtained using an Nicolet
760 FT-IR infrared spectrometer. Solution NMR spectra were obtained
on a Bruker Avance 400 spectrometer. Chemical shifts are reported
relative to internal solvent resonances. 13C CPMAS NMR spectra were
recorded on a Bruker Avance 200 spectrometer. A 4 mm zirconium
dioxide rotor was used for all spectra, with the spin rates up to 8 kHz.
Microanalyses were performed by Oneida Research Services, Inc.,
Whitesboro, NY. Molecular weight measurements were made in CH2-
Cl2 with the use of an instrument similar to that described by Clark.24
Thermogravimetric/differential thermal analyses were obtained on a
Seiko 200 TG/DTA instrument using a carrier gas of either dry nitrogen
or air. TGA-IR was performed on a TA Instruments 2960 DTA-TGA
connected to a Nicolet Nexus 670 FT-IR instrument. To enable powder
XRD analysis, samples were mounted on glass slides by double sided
tape prior to analysis. Data were collected on a Siemens D5000
diffractometer. Energy-dispersive X-ray spectroscopy (EDX) and
scanning electron microscopy (SEM) were performed on a Cameca
SX50 Electron Microprobe.
10.08
10.79
11.48
11.87
16.59
28.50
8.77
8.19
7.70
7.45
5.34
3.13
9.94
10.57
11.61
14.81
17.17
28.29
28.69
8.88
8.36
7.62
5.98
5.16
3.15
3.11
a FOM ) 0.23 based upon a comparison with data from a sample of
compound 4 crystallized from solution.
In summary, the ∆Hq remains constant for the first and
subsequent cycles of the dissociation of pyridine and 4-Mepy
from [(tBu)2Ga(L)]2(µ-OC6H4O) to [{(tBu)2Ga}2(µ-OC6H4O)]n
(1) and from[(tBu)2Ga(OPh)(py)] (5) to [(tBu)2Ga(µ-OPh)]2. On
the other hand, the first ∆Hq for the dissociation of 3,5-Me2py
to [{(tBu)2Ga}2(µ-OC6H4O)]n (1) is different then that of
subsequent cycles. As a result, we can conclude that the ∆Hq
for the condensation polymerization reaction is dependent on
the crystal packing (i.e., the Ga...Ga distance), and subsequently,
the reversibility of the reaction is dependent on the polymorph.
The synthesis of Ga(tBu)3 was performed according to a modification
of the literature method.25 HOC6H4OH, pyridine, 4-methylpyridine, 2,5-
dimethylpyridine were obtained from Aldrich and (except for
HOC6H4OH) were distilled and stored over Na metal prior to use. All
manipulations were performed under an inert atmosphere of argon or
nitrogen. Solvents were dried, distilled and degassed prior to use.
Conclusions
The gallium aryloxide polymer, [{(tBu)2Ga}2(µ-OC6H4O)]n
(1) is synthesized by the addition of Ga(tBu)3 with hydro-
quinone, and reacts with pyridines, as both a solution and by a
solid-vapor reaction, to yield the yellow compound [(tBu)2Ga-
(L)]2(µ-OC6H4O) [L ) py (2), 4-Mepy (3) and 3,5-Me2py (4)].
The energetics of the dissociation of one of the pyridine ligands
in solution and solid state, and the association of the pyridine
ligands by a solid-vapor reaction, provides insight into the
reaction pathways for the interconversion of compounds 2-4
and compound 1. Compounds 1 and 2 are reversibly intercon-
verted without degredation or significant alteration in the
reactivity or structure of the two components.
The polymorphism associated with sequential cycles for the
3,5-Me2py derivative (4) is an interesting example of inducing
pseudo-polymorphs via nonsolution methods,23 for example,
formation of new crystalline phases have been achieved by
mechanical grinding and/or by thermal dehydration in thermo-
gravimetric experiments. In this case, addition of a Lewis base
vapor to a solid Lewis acid has formed a new polymorph with
different thermodynamic barriers, than that made from the same
addition in solution. Crystals of different polymorphs may be
obtained via seeding, so the synthesis of this new polymorph
might be achieved through seeding.
All density functional calculations were carried out using a Gaussian-
98 suite.26 Complete geometry optimizations were performed at B3LYP
level using the 6-31G** basis set for C and H and Stuttgart RLC ECP
basis set for Ga, N, and O.
[{(tBu)2Ga}2(µ-OC6H4O)]n (1). Hydroquinone (0.522 g, 4.74 mmol)
was suspended in degassed pentane and cooled to -78 °C. To this,
Ga(tBu)3 (3.0 mL, 12.1 mmol) was added. The solution was allowed
to warm to room temperature, and then was stirred for 1 day, giving a
white powder. Yield: 1.2 g, 42%. 1H NMR (d5-pyridine): δ 7.06 (4H,
s, C6H4), 1.24 (36H, s, CH3). 13C NMR (d5-pyridine): δ 156.7 (OC),
121.2 (OCCH), 31.4 (CH3). 13C CPMAS NMR: δ 152.9 (OC), 120.8
(OCCH), 32.7 (CH3).
[(tBu)2Ga(py)]2(µ-OC6H4O) (2). Method 1. Hydroquinone (0.423
g, 3.8 mmol) was suspended in degassed pyridine and cooled to -78
°C. To this, Ga(tBu)3 (2.0 mL, 8.0 mmol) was added. The solution
was allowed to warm to room temperature, and then was stirred for 1
day. The clear, yellow solution was cooled to -30 °C overnight,
yielding crystals suitable for single-crystal X-ray analysis. Yield:1.5
g, 62%.
(24) Clark, E. P. Ind. Eng. Chem. Anal. Ed. 1941, 13, 820.
(25) (a) Schwering, H. -U.; Jungk, E.; Weidlein, J. J. Organomet. Chem. 1975,
91, C4. (b) Kovar, R. A.; Derr, H.; Brandau, D.; Callaway, J. O. Inorg.
Chem. 1975, 14, 2809.
(26) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M.
A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, Jr., J. A.; Stratmann,
R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A. D.; Kudin,
K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi, M.; Cammi,
R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.; Ochterski, J.;
Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, K.; Malick, D. K.;
Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Cioslowski, J.; Ortiz,
J. V.; Baboul, A. G.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.;
Komaromi, I.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham,
M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.;
Johnson, B.; Chen, W.; Wong, M. W.; Andres, J. L.; Gonzalez, C.; Head-
Gordon, M.; Replogle, E. S.; Pople, J. A., Gaussian 98, ReVision A.9;
Gaussian, Inc., Pittsburgh PA, 1998.
The color change (white to yellow) accompanying the
conversion of compound 1 to 2 suggests that this or a similar
process may be used as chemically triggered switches or
chemical sensors. Although, clearly the present system is of
limited practical value (due to the rate of switching and the
application of pyridine) it does point the way to an alternative
class of chemically triggered solid-state switches as opposed
(23) Braga, D.; Grepioni, F. J. Chem. Soc., Chem. Soc. ReV. 2000, 29, 229.
9
J. AM. CHEM. SOC. VOL. 125, NO. 36, 2003 11015

A R T I C L E S
von Poppel et al.
Table 10. Summary of X-ray Diffraction Data
t
t
compd
[(Bu)2Ga(py)] (µ-OC6H O) (2)
[(Bu)2Ga(py)] (µ-OC6H O) (2.py)
2
4
2
4
emp form
C32H50Ga2N2O2
634.18
C37H55Ga2N3O2
712.29
P1h
Mw
space group
P21/c
cryst system
monoclinic
8.705(2)
8.252(2)
24.736(5)
triclinic
8.181(2)
8.525(2)
14.766(3)
106.21(3)
99.01(3)
95.64(3)
965.7(3)
1
a, Å
b, Å
c, Å
R, °
â, °
91.56(3)
γ, °
V, Å3
1776.2(6)
Z
2
no. collected
no. ind
7603
4423
2765
2453
no. obsd
(|Fo| > 4.0σ |Fo|)
2407 (|Fo| > 4.0σ |Fo|)
SHELXTL 0.0535, 0.00
0.0404
weighting scheme
SHELXTL 0.024, 0.00
0.0446
Ra
Rw
a
0.0886
0.0954
t
t
compd
[(Bu)2Ga(4-Mepy)] (µ-OC6H O) (3)
[(Bu)2Ga(3,5-Me2py)] (µ-OC6H O) (4)
2
4
2
4
emp form
C34H54Ga2N2O2
662.23
C36H58Ga2N2O2
690.29
Mw
space group
P21/c
P21/n
cryst system
monoclinic
8.683(2)
12.463(3)
16.935(3)
monoclinic
10.922(2
14.052(3)
12.791(3)
a, Å
b, Å
c, Å
R, °
â, °
95.42(3)
94.33(3)
γ, °
V, Å3
1824.5(6)
2
1957.5(7)
2
Z
no. collected
no. ind
8214
2635
8746
2825
no. obsd
1613 (|Fo| > 4.0σ |Fo|)
SHELXTL 0.01, 0
0.0426
2006 (|Fo| > 4.0σ |Fo|)
SHELXTL 0.0531, 0
0.0416
weighting scheme
Ra
Rw
a
0.0741
0.1052
t
compd
[(Bu)2Ga(OPh)(py)] (5)
emp form
Mw
cryst. syst.
space group
a, Å
b, Å
c, Å
C19H28GaNO
356.15
monoclinic
P21/n
14.972(3)
8.744(2)
16.182(3)
R, °
â, °
112.42(3)
γ, °
V, Å3
1958.5(7)
Z
4
no. collected
no. ind.
no obsd
weighting scheme
Ra
8707
2823
2120 (|Fo| > 4.0σ |Fo|)
SHELXTL 0.0661, 0
0.0379
a
Rw
0.0997
2
2
2
a R ) ∑|Fo - Fc|/∑Fo; Rw ) {∑[{w(Fo - Fc )2}/{w(Fo )2}]}1/2
.
Method 2. [{(tBu)2Ga}2(µ-OC6H4O)]n (0.15 g, 0.32 mmol) was
dissolved in degassed pyridine and stirred for 1 day. The clear, yellow
solution was cooled to -30 °C overnight, yielding yellow crystals.
Yield: 0.17 g, 87%. 1H NMR (CDCl3): δ 8.98 [4H, d, J(H-H) ) 4.0
Hz, NCH], 8.03 (2H, m, m-CH, py), 7.63 (4H, m, p-CH, py), 6.66
(4H, s, OCCH), 0.99 (36H, s, CH3). 13C NMR (CDCl3): δ 155.7 (OC),
148.5 (NCH), 140.1 (p-CH), 125.3 (m-CH), 120.8 (OCCH), 30.7 (CH3).
UV: λ ) 329 (ꢀ ) 1200 L‚mol-1cm-1), λ ) 303 (ꢀ ) 4300
L‚mol-1cm-1).
Hz, NCH], 7.43 [2H, d, J(H-H) ) 6.0 Hz, m-CH], 6.62 (2H, s, OCCH),
2.53 (3H, s, CH3), 1.04 [18H, s, C(CH3)3]. 13C NMR (CDCl3): δ 158.8
(OC), 155.7 (NCH), 147.9 (p-CH), 126.3 (m-CH), 120.5 (OCCH), 30.9
[C(CH3)3], 23.4 (CH3). UV: λ ) 302 (ꢀ ) 5950 L‚mol-1cm-1), λ )
327 (ꢀ ) 1500 L‚mol-1cm-1).
[(tBu)2Ga(3,5-Me2py)]2(µ-OC6H4O) (4). [{(tBu)2Ga}2(µ-OC6H4O)]n
(0.15 g, 0.32 mmol) was dissolved in degassed 3,5-lutidine and stirred
for 1 day. The clear, yellow solution was cooled to -30 °C overnight,
yielding yellow crystals suitable for single-crystal X-ray analysis.
Prepared in a manner similar to compound 2. Yield: 0.19 g, 87%. 1H
NMR (CDCl3): δ 8.58 (2H, s, NCH), 7.64 (4H, s, p-CH), 6.65 (4H, s,
OCCH), 2.44 (3H, s, CH3), 0.99 [36H, s, C(CH3)3].13C NMR (CDCl3):
δ 155.7 (OC), 145.7 (NCH), 141.7(p-CH), 134.9 (m-CH), 120.6
[(tBu)2Ga(4-Mepy)]2(µ-OC6H4O) (3). [{(tBu)2Ga}2(µ-OC6H4O)]n
(0.15 g, 0.32 mmol) was dissolved in degassed 3,5-lutidine and stirred
for 1 day. The clear, yellow solution was cooled to -30 °C overnight,
yielding yellow crystals suitable for single-crystal X-ray analysis.
Yield: 0.18 g, 86%. 1H NMR (CDCl3): δ 8.66 [2H, d, J(H-H) ) 6.0
9
11016 J. AM. CHEM. SOC. VOL. 125, NO. 36, 2003

1,4-Dioxobenzene Compounds of Gallium
A R T I C L E S
(OCCH), 30.9 [C(CH3)3], 23.4 [C(CH3)3], 18.8 (CH3). UV: λ ) 302
(ꢀ ) 4050 L‚mol-1cm-1), λ ) 327 (ꢀ ) 1600 L‚mol-1cm-1).
(tBu)2Ga(OPh)(py) (5). Phenol (0.195 g, 2.07 mmol) was suspended
in degassed pyridine and cooled to -78 °C. To this was added Ga(tBu)3
(0.5 mL, 2.1 mmol). The solution was allowed to warm to room
temperature and stir for 1 day. The clear, yellow solution was cooled
to -30 °C overnight yielding crystals suitable for single-crystal X-ray
before moving to the next temperature plateau. In all cases studied
here, the mass loss at a given temperature was linear. The slope of
each mass drop was measured and used to calculate dissociation
enthalpies.28 Association studies were performed by flowing the N2
carrier gas through a pyridine bubbler (@ 300 mL‚min-1). The sample
(3-5 mg) was heated to between 34 and 44 °C and the weight gain
measured.
Crystallographic Studies. Single-crystal diffraction data for com-
pounds 2-5 and 2.py were collected at ambient temperature on a Bruker
CCD SMART system, equipped with graphite monochromated Mo KR
radiation (λ ) 0.710 73 Å) and corrected for Lorentz and polarization
effects. The structures were solved using the direct methods program
XS29 and difference Fourier maps and refined by using full matrix least-
squares methods. All non-hydrogen atoms were refined with anisotropic
thermal parameters. Hydrogen atoms were introduced in calculated
positions and allowed to ride on the attached carbon atoms [d(C-H)
) 0.95 Å]. Refinement of positional and anisotropic thermal parameters
led to convergence (see Table 10). In compound 2.py, the lattice
pyridine resides over a center of inversion, and so is statistically
disordered. It was impossible to distinguish between different possible
models of this disorder.
1
analysis. Yield: 0.68 g, 90%. H NMR (C6D6): δ 8.58 [2H, d, J(H-
H) ) 5.0 Hz, NCH], 7.35 [2H, t, J(H-H) ) 5.0 Hz, m-CH], 6.89 [1H,
t, J(H-H) ) 7.0 Hz, p-CH], 6.71 [1H, t, J(H-H) ) 7.0 Hz, p-CH,
py], 6.40 [2H, t, J(H-H) ) 7.0 Hz, m-CH, py], 1.27 (18H, s, CH3).13C
NMR (C6D6): δ 165.5 (OC), 148.4 (NCH), 139.8 (p-CH, py), 130.4
(p-CH), 125.3 (m-CH, py), 120.8 (OCCH), 117.6 (m-CH), 31.4
[C(CH3)3].
Solution Equilibrium Studies. Because a variation in 1H NMR shifts
for the R-methylene (OCH2) is observed between different solvents,
the same solvent (toluene-d8) was used for all the variable temperature
NMR measurements. The sample of each compound was heated to the
appropriate temperature within the NMR spectrometer, and the 1H NMR
spectrum was collected. Constancy of the spectrum was taken as
evidence for the attainment of equilibrium. The temperature of the NMR
spectrometer probe was calibrated using the chemical shifts of ethylene
glycol.27 This process was repeated for a minimum of 6 temperatures
over a minimum temperature range of 80 K. Alternate points on the ln
Keq versus 1/T plot were obtained during upward and downward
passages over the temperature range spanned. Because both sets of
points fell on the same line, we consider that equilibration was achieved.
The temperature dependence of the equilibrium constant, Keq, allows
for the determination of the ∆H and ∆S. A summary of calculated
values is given in Table 2.
Solid State Ligand Dissociation and Association Studies. Purified
polycrystalline [(tBu)2Ga(L)]2(µ-OC6H4O) [L ) py (2), 4-Mepy (3),
3,5-Me2py (4)] and (tBu)2Ga(OPh)(py) (5) were analyzed for volatility
and thermal events on a thermogravimetric analyzer (Seiko TG/DTA
200). Typically 5-10 mg of sample were measured with heating rates
of 10 °C‚min-1 up to 500 °C under a 300 mL‚min-1 inert (N2 or Ar)
gas flow. Isothermal mass loss was monitored over a 10 min period
Acknowledgment. Financial support for this work is provided
by the National Science Foundation and the Robert A. Welch
Foundation, including the Bruker CCD Smart System Diffrac-
tometer of the Texas Center for Crystallography at Rice
University. The Bruker Avance 200 NMR spectrometer was
purchased with funds from ONR Grant No. N00014-96-1-1146.
Supporting Information Available: Full listings of bond
length and angles, anisotropic thermal parameters, and hydrogen
atom parameters; 13C CPMAS NMR spectrum of [{(tBu)2Ga}2-
(µ-OC6H4O)]n; van’t Hoff plots for solution equilibria; Reaction
diagram for ligand dissociation and dimerization of (tBu)2Ga-
(OR)(py); plot of ∆Sq as a function of the intermolecular
Ga‚‚‚Ga distance; calculated structures of Me2Ga(OMe), Me2Ga-
(OMe)(NH3), Me2Ga(NH3)(µ-OMe)GaMe2(OMe), Me2Ga(µ-
OMe)(µ-Me)GaMe(OMe), [Me2Ga(µ-OMe)]2. This material is
available free of charge via the Internet at http://pubs.acs.org.
(27) (a) van Geet, A. L. Anal. Chem. 1968, 40, 2227. (b) Gordon H. J.; Ford,
R. A. The Chemists Companion, Wiley, New York, 1972.
(28) Gillan, E. G.; Bott, S. G.; Barron, A. R., Chem. Mater. 1997, 9, 796.
(29) Sheldrick, G. M., SHELXTL. Bruker AXS, Inc. Madison, Wisconsin, 1997.
JA0208714
9
J. AM. CHEM. SOC. VOL. 125, NO. 36, 2003 11017