Synthesis, structure, and bridge-terminal exchange kinetics of pyrazolate-bridged digallium and diindium complexes containing bridging phenyl groups

Article abstract of DOI:10.1021/om050623r

Treatment of triphenylgallium with 3,5-dimethylpyrazole, 3,5-diphenylpyrazole, or 3,5-di-tert-butylpyrazole in a 2:1 stoichiometry afforded the phenyl-bridged complexes (C₆H₅) ₂Ga(μ-Me₂pz)(μ-C₆H₅)Ga(C ₆H₅)₂ (62%), (C₆H₅) ₂Ga(μ-Ph₂pz)(μ-C₆H₅)Ga(C ₆H₅)₂·C₇H₈ (62%), and (C₆H₅)₂Ga(μ-tBu₂pz)(μ- C₆H₅)Ga(C₆H₅)₂ (40%), respectively, as colorless or off-white crystalline solids. Treatment of triphenylindium with 3,5-di-tert-butylpyrazole in a 2:1 stoichiometry afforded the phenyl-bridged complex (C₆H₅)₂In(μ- tBu₂pz)-(μ-C₆H₅) In (C₆H ₅)₂ · (C₆H₁₄)_0.5 (40%). The molecular structures of (C₆H₅) ₂Ga(μ-Ph₂pz)(μ-C₆H₅)Ga (C ₆H₅)₂·C₇H₈, (C₆H₅)₂ Ga (μ-tBu₂-pz)(μ- C₆H₅)Ga(C₆H₅)₂ · (C₆H₁₄)_0.5, and (C₆H ₅)₂ In (μ-tBu₂pz)(μ-C₆H ₅) In (C₆H₅)₂ · (C ₆H₁₄)_0.5 consist of a 3,5-disubstituted pyrazolato ligand with a diphenylgallio or diphenylindio group bonded to each nitrogen atom. A phenyl group acts as a bridge between the two metal atoms. By contrast, treatment of triphenylgallium with 3,5-di-tert-butylpyrazole in a 1:1 stoichiometry or triphenylindium with 3,5-diphenylpyrazole or 3,5-dimethylpyrazole in 2:1 or 1:1 stoichiometry afforded the dimeric complexes [(C₆H₅)₂Ga(μ-tBuC₂pz] ₂ (63%), [(C₆H₅)₂In(μ-Ph ₂pz)]₂ (40%), and [(C₆H₅) ₂In(μ-Me₂pz)]₂ (92%), respectively, as colorless crystalline solids. The dimeric nature of these complexes was determined by X-ray crystallography. Treatment of 3,5-di-tert-butylpyrazole with excess trimethylgallium afforded the dimeric complex [Me₂Ga(μ- tBu₂pz)]₂ (82%) as the major product and Me ₂Ga(H-tBu₂pz)(μ-OCH₂)GaMe₂ (2.6%) as a minor product. There was no evidence for the formation of the methyl-bridged complex Me₂-Ga(μ-tBu₂pz)(μ-CH ₃)GaMe₂. The kinetics of bridge-terminal phenyl exchange in (C₆H₅)₂Ga-(μ-Me₂pz)(μ- C₆H₅)Ga(C₆H₅)₂, (C ₆H₅)₂Ga(μ-Ph₂pz)(μ-C ₆H₅)Ga(C₆H₅)₂· C₇H₈, (C₆H₅)₂Ga(μ- tBu₂-pz)(μ-C₆H₅)Ga(C₆H ₅)₂, and (C₆H₅)₂In(μ- tBu₂pz)(μ-C₆H₅)In(C₆H ₅)₂·(C₆H₁₄)_0.5 was determined by ¹³C NMR spectroscopy and afforded the following range of activation parameters: ΔH? = 6.0-8.9 kcal/mol, ΔS? = -23.1 to -32.0 eu, and ΔG?_{(298 K)} = 15.5-15.8 kcal/mol. The large, negative values of ΔS? imply ordered transition states relative to the ground state and rotation along the N-GaPh₃ or N-InPh₃ vector without metal-nitrogen bond cleavage. The combined results suggest that the close proximity of the metal atoms is the principal determinant of the bridging phenyl interactions and that complexes of the heavier group 13 elements with bridging hydrocarbon ligands are likely to be more accessible than the current state of literature would suggest.

Full text of DOI:10.1021/om050623r

6184
Organometallics 2005, 24, 6184-6193
Synthesis, Structure, and Bridge-Terminal Exchange
Kinetics of Pyrazolate-Bridged Digallium and Diindium
Complexes Containing Bridging Phenyl Groups
Chatu T. Sirimanne, Wenjun Zheng, Zhengkun Yu, Mary Jane Heeg, and
Charles H. Winter*
Department of Chemistry, Wayne State University, Detroit, Michigan 48202
Received July 25, 2005
Treatment of triphenylgallium with 3,5-dimethylpyrazole, 3,5-diphenylpyrazole, or 3,5-
di-tert-butylpyrazole in a 2:1 stoichiometry afforded the phenyl-bridged complexes (C6H5)2-
Ga(µ-Me2pz)(µ-C6H5)Ga(C6H5)2 (62%), (C6H5)2Ga(µ-Ph2pz)(µ-C6H5)Ga(C6H5)2‚C7H8 (62%), and
(C6H5)2Ga(µ-tBu2pz)(µ-C6H5)Ga(C6H5)2 (40%), respectively, as colorless or off-white crystalline
solids. Treatment of triphenylindium with 3,5-di-tert-butylpyrazole in a 2:1 stoichiometry
afforded the phenyl-bridged complex (C6H5)2In(µ-tBu2pz)(µ-C6H5)In(C6H5)2‚(C6H14)0.5 (40%).
The molecular structures of (C6H5)2Ga(µ-Ph2pz)(µ-C6H5)Ga(C6H5)2‚C7H8, (C6H5)2Ga(µ-tBu2-
pz)(µ-C6H5)Ga(C6H5)2‚(C6H14)0.5, and (C6H5)2In(µ-tBu2pz)(µ-C6H5)In(C6H5)2‚(C6H14)0.5 consist
of a 3,5-disubstituted pyrazolato ligand with a diphenylgallio or diphenylindio group bonded
to each nitrogen atom. A phenyl group acts as a bridge between the two metal atoms. By
contrast, treatment of triphenylgallium with 3,5-di-tert-butylpyrazole in a 1:1 stoichiometry
or triphenylindium with 3,5-diphenylpyrazole or 3,5-dimethylpyrazole in 2:1 or 1:1 stoichi-
ometry afforded the dimeric complexes [(C6H5)2Ga(µ-tBu2pz)]2 (63%), [(C6H5)2In(µ-Ph2pz)]2
(40%), and [(C6H5)2In(µ-Me2pz)]2 (92%), respectively, as colorless crystalline solids. The
dimeric nature of these complexes was determined by X-ray crystallography. Treatment of
3,5-di-tert-butylpyrazole with excess trimethylgallium afforded the dimeric complex [Me2-
Ga(µ-tBu2pz)]2 (82%) as the major product and Me2Ga(µ-tBu2pz)(µ-OCH3)GaMe2 (2.6%) as a
minor product. There was no evidence for the formation of the methyl-bridged complex Me2-
Ga(µ-tBu2pz)(µ-CH3)GaMe2. The kinetics of bridge-terminal phenyl exchange in (C6H5)2Ga-
(µ-Me2pz)(µ-C6H5)Ga(C6H5)2, (C6H5)2Ga(µ-Ph2pz)(µ-C6H5)Ga(C6H5)2‚C7H8, (C6H5)2Ga(µ-tBu2-
pz)(µ-C6H5)Ga(C6H5)2, and (C6H5)2In(µ-tBu2pz)(µ-C6H5)In(C6H5)2‚(C6H14)0.5 was determined
by 13C NMR spectroscopy and afforded the following range of activation parameters: ∆Hq )
6.0-8.9 kcal/mol, ∆Sq ) -23.1 to -32.0 eu, and ∆Gq(298 K) ) 15.5-15.8 kcal/mol. The large,
negative values of ∆Sq imply ordered transition states relative to the ground state and
rotation along the N-GaPh3 or N-InPh3 vector without metal-nitrogen bond cleavage. The
combined results suggest that the close proximity of the metal atoms is the principal
determinant of the bridging phenyl interactions and that complexes of the heavier group 13
elements with bridging hydrocarbon ligands are likely to be more accessible than the current
state of literature would suggest.
Introduction
13 elements gallium and indium. Structurally charac-
terized examples for gallium are limited to [Me2Ga(µ-
The bridging of monoanionic hydrocarbon groups such
as alkyl,1,2 aryl,3 acetylide,4 and alkenyl4 between two
metal centers is well established for the lighter main
group elements and is particularly broadly appreciated
for aluminum. Classic aluminum complexes with bridg-
ing hydrocarbon ligands include dimeric trimethylalu-
minum1 and dimeric triphenylaluminum.3a Despite the
broad occurrence of bridging interactions in aluminum
chemistry, there are very few examples of similar
bridging hydrocarbon ligands between the heavier group
5a
CCPh)]2 and a gallole dimer containing bridging
alkenyl groups.5b For indium, the only example is [Me2-
In(µ-2-C4H3S)]2.6 Several classes of cyclopentadienyl
complexes also contain formal cyclopentadienyl bridges
between gallium or indium and other metal centers.7,8
A small number of gallium-substituted ferrocenes have
been structurally characterized.7 Indium(I) cyclopenta-
dienyl complexes contain various bridging cyclopenta-
dienyl ligands.8 In addition, a few group 13 heteronu-
clear complexes have been structurally documented in
which a mesityl group bridges between boron and
gallium or boron and indium,9 and an alkenyl group
bridges between boron and indium.10
(1) Crystal structure determinations of dimeric trimethylalumi-
num: (a) Huffman, J. C.; Streib, W. E. Chem. Commun. 1971, 911. (b)
Byram, S. K.; Fawcett, J. K.; Nyburg, S. C.; O’Brien, R. J. Chem.
Commun. 1970, 16. (c) Vranka, R. G.; Amma, E. L. J. Am. Chem. Soc.
1967, 89, 3121. (d) Lewis, P. H.; Rundle, R. E. J. Chem. Phys. 1953,
21, 986. (e) Evans, W. J.; Anwander, R.; Ziller, J. W. Organometallics
1995, 14, 1107.
Some striking differences exist between aluminum
and gallium complexes containing similar hydrocarbon
10.1021/om050623r CCC: $30.25 © 2005 American Chemical Society
Publication on Web 11/04/2005

Pyrazolate-Bridged Digallium and Diindium Complexes
Organometallics, Vol. 24, No. 25, 2005 6185
ligands. For example, triphenylaluminum is dimeric in
the solid state with two bridging phenyl groups,3a while
triphenylgallium and triphenylindium are monomeric
in the solid state and have only long, weak intermo-
lecular contacts between the metal atoms and phenyl
ligand carbon atoms.11 Trimethylaluminum is dimeric
in the solid state and in solution with a well-known
methyl-bridged structure,1 while the methyl groups in
solid trimethylgallium have only very weak bridge-like
interactions between neighboring gallium atoms.12a,b
Within this perspective, we report the synthesis, struc-
tural characterization, and bridge-terminal exchange
kinetics of a series of gallium and indium pyrazolato
complexes that contain bridging phenyl ligands. The
observation of bridging phenyl ligands in these com-
plexes is particularly surprising, since triphenylgallium
and triphenylindium are monomeric. The results imply
that bridging interactions in the heavier elements may
be induced by appropriate choice of ancillary ligands
that dispose two metal centers in close proximity. Thus,
complexes of the heavier group 13 elements with bridg-
ing hydrocarbon ligands are likely to be more accessible
than the current literature may suggest. We also
describe attempts to prepare a bridging methyl complex
of gallium. A portion of this work was communicated.13
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Results
Synthesis of Phenyl-Bridged Gallium and In-
dium Pyrazolato Complexes. Treatment of triphen-
ylgallium (2 equiv) with 3,5-dimethylpyrazole, 3,5-
diphenylpyrazole, or 3,5-di-tert-butylpyrazole afforded
(C6H5)2Ga(µ-Me2pz)(µ-C6H5)Ga(C6H5)2 (1, 62%), (C6H5)2-
Ga(µ-Ph2pz)(µ-C6H5)Ga(C6H5)2‚C7H8 (2‚C7H8, 62%), and
(C6H5)2Ga(µ-tBu2pz)(µ-C6H5)Ga(C6H5)2 (3, 40%), respec-
tively, as colorless or off-white crystalline solids (eq 1).
Similar treatment of triphenylindium with 3,5-di-tert-
butylpyrazole afforded (C6H5)2In(µ-tBu2pz)(µ-C6H5)In-
(C6H5)2‚(C6H14)0.5 (4‚(C6H14)0.5, 40%) as colorless crys-
tals. The structural assignments for 1, 2‚C7H8, 3, and
4‚(C6H14)0.5 were based on spectral and analytical data
and X-ray crystal structure determinations. The 1H
NMR spectra of 1, 2‚C7H8, and 3 at ambient tempera-
ture showed three broad multiplets for the bridging
phenyl groups and sharper multiplets for the terminal
phenyl groups. The 13C{1H} NMR spectra at ambient
temperature exhibit broad ipso-carbon resonances for
the bridging and terminal phenyl rings in the range
157.33-158.33 and 146.45-148.26 ppm, respectively.
In the NMR spectra of 4‚(C6H14)0.5 at ambient temper-
ature, broad resonances for one type of phenyl ligand
were observed, which suggested that the coalescence
temperature for phenyl site exchange was below ambi-
ent temperature.
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Stammler, H.-G. Organometallics 2002, 21, 3018. (b) Scholz, S.; Green,
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Kucht, A.; Go¨rlitz, F. H.; Dietrich, A. Z. Naturforsch. 1992, 47B, 1241.
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6186 Organometallics, Vol. 24, No. 25, 2005
Sirimanne et al.
Synthesis of Dimeric Pyrazolate-Bridged Gal-
lium and Indium Complexes Containing Terminal
Phenyl Groups. Treatment of triphenylgallium (1
equiv) with 3,5-di-tert-butylpyrazole (1 equiv) afforded
bis(µ-3,5-di-tert-butylpyrazolato)tetraphenyldigallium (5,
63%) as an off-white crystalline solid (eq 2). Similarly,
treatment of triphenylindium (1-2 equiv) with 3,5-
diphenylpyrazole or 3,5-dimethylpyrazole afforded bis-
(µ-3,5-diphenylpyrazolato)tetraphenyldiindium (6, 40%)
and bis(µ-3,5-dimethylpyrazolato)tetraphenyldiindium
(7, 92%), respectively, as colorless crystalline solids. The
structural assignments for 5-7 were based on spectral
and analytical data and X-ray crystal structure deter-
minations. Treatment of 3,5-dimethylpyrazole or 3,5-
diphenylpyrazole with large excesses of triphenylindium
as a possible route to analogues of 4‚(C6H14)0.5 gave
several products that appeared from 1H NMR analyses
to correspond to mixtures of 6 or 7, the analogues of
4‚(C6H14)0.5, and the unreacted triphenylindium. It was
not possible to isolate the analogues of 4‚(C6H14)0.5 as
pure samples by fractional crystallization, so these
reactions were not pursued further.
°C/0.01 Torr of the residue left after fractional crystal-
lization of 8. An X-ray crystal structure established that
9 corresponds to the bridging methoxo complex and not
the bridging methyl complex. The methoxo ligand in 9
probably originates through insertion of dioxygen into
a gallium-methyl bond. Such an oxidation could either
occur in trimethylgallium prior to formation of 9 or could
occur in a putative bridging methyl complex. If great
care was not taken to exclude air from the reaction
vessels, much higher yields of 9 were obtained. Variable-
1
temperature H NMR spectra between -80 and 20 °C
of a mixture of trimethylgallium (5 equiv) and 3,5-di-
tert-butylpyrazole (1 equiv) in toluene-d8 showed no
evidence for new complexes other than 8 and 9. Thus,
there was no evidence for the formation of a bridging
methyl complex similar to the bridging phenyl com-
plexes 1, 2‚C7H8, 3, and 4‚(C6H14)0.5
.
X-ray Structural Characterization. To establish
the solid-state geometries, the X-ray crystal structures
of 1, 2‚C7H8, 3‚(C6H14)0.5, 4‚(C6H14)0.5, 5-7, and 9 were
determined. The crystal structure of 1 was described
in our preliminary communication.13 In this section, the
structures of 2‚C7H8, 3‚(C6H14)0.5, 4‚(C6H14)0.5, 6, and 9
are discussed. The molecular structures of 5 and 7 are
similar to those of 6, and structural data for these
complexes are contained in the Supporting Information.
Experimental crystallographic data are summarized in
Table 1, selected bond lengths and angles are given in
Tables 2-6, and perspective views are presented in
Figures 1-5.
The molecular structure of 2‚C7H8 is shown in Figure
1. Consistent with the NMR analysis, the molecule
consists of a diphenylpyrazolato ligand with a diphen-
ylgallio group bonded to each nitrogen atom. A phenyl
group acts as a bridge between the gallium atoms. The
geometry about the gallium centers is distorted tetra-
hedral. The two gallium atoms and the two nitrogen
atoms occupy an approximate plane, while the ipso-
carbon atom of the bridging phenyl group is situated
0.963(4) Å above this plane. The bridging phenyl group
is slightly canted toward Ga(2), giving a distinct tri-
phenylgallium unit containing Ga(1). The gallium-
nitrogen bond lengths are 2.021(4) and 1.983(4) Å. The
gallium-carbon bond lengths lie in the range 1.958(4)-
1.972(4) Å for the terminal phenyl groups and are 2.051-
(5) (to Ga(1)) and 2.254(5) Å (to Ga(2)) for the bridging
phenyl group. The gallium-carbon-gallium angle is
95.82(18)°.
Attempts to Prepare a Bridging Methyl Complex
of Gallium. Encouraged by the stability of 1, 2‚C7H8,
3, and 4‚(C6H14)0.5, we explored the synthesis of the
bridging methyl analogue. Treatment of 3,5-di-tert-
butylpyrazole with trimethylgallium (2 equiv) in hexane
at ambient temperature, followed by fractional crystal-
lization from hexane, afforded bis(µ-3,5-di-tert-butylpyra-
zolato)tetramethyldigallium (8, 82%) as a colorless
crystalline solid (eq 3). The structure of 8 was estab-
lished from spectral and analytical data. While 8 was
not structurally characterized, due to the inability to
grow single crystals of sufficient quality, several similar
dimeric gallium and indium pyrazolato complexes have
been reported.14 Inspection of the 1H NMR spectrum of
crude 8 indicated a 95:5 mixture of 8 and a new complex
9 with resonances that could possibly correspond to the
bridging methyl complex Me2Ga(µ-tBu2pz)(µ-Me)GaMe2.
In particular, the 1H NMR spectrum of 9 revealed
resonances with a ratio of 4:1 at δ 0.07 for the gallium-
bound methyl groups and at δ 3.25 for another methyl
group. In Me2Al(µ-R2pz)(µ-Me)AlMe2 (R ) Me, Ph,
tBu),2g the bridging methyl groups exhibited a downfield
chemical shift of δ 0.3-0.4 compared to the terminal
methyl groups, which is quite different than the down-
field shift of δ 3.18 observed in 9. It was found that pure
9 could be isolated in 2.6% yield by sublimation at 60
Complex 3‚(C6H14)0.5 crystallized as a hexane solvate,
and its molecular structure is shown in Figure 2. The
overall molecular structure is similar to that of 2‚C7H8.
The synthesis of bulk 3 afforded hexane-free material,
and drying experiments revealed that the hexane sol-
vate is lost upon standing at ambient temperature for
(14) (a) Rendle, A. F.; Storr, A.; Trotter, J. Can. J. Chem. 1975, 53,
2930. (b) Rendle, A. F.; Storr, A.; Trotter, J. Can. J. Chem. 1975, 53,
2944. (c) Hausen, H.-D.; Locke, K.; Weidlein, J. J. Organomet. Chem.
1992, 429, C27.

Pyrazolate-Bridged Digallium and Diindium Complexes
Organometallics, Vol. 24, No. 25, 2005 6187
Table 1. Experimental Crystallographic Data for 2‚C7H8, 3‚(C6H14)0.5, 4‚(C6H14)0.5, 6, and 9
2‚C7H8
3‚(C6H14
)
4‚(C6H14
)
6
9
0.5
0.5
empirical formula
C
52H44Ga2N2
C
44H51Ga2N2
C
44H51In2N2
C
54H42In2N4
C
16H34Ga2N2O
fw
836.33
P1h
747.31
P1h
837.51
P1h
976.56
409.89
space group
P2(1)/c
P2(1)/c
a (Å)
10.384(7)
12.324(9)
18.056(14)
74.324(16)
85.822(19)
71.84(2)
2114(3)
2
10.4027(9)
11.5593(12)
18.4424(19)
89.757(2)
87.640(2)
64.239(2)
1995.3(3)
2
10.210(3)
11.723(3)
18.786(6)
89.051(5)
86.619(7)
66.667(7)
2061.0(10)
2
10.226(3)
12.212(4)
18.225(6)
12.4949(19)
9.1385(12)
18.617(3)
b (Å)
c (Å)
R (deg)
â (deg)
101.009(9)
108.656(2)
γ (deg)
V (Å3)
2234.0(13)
2
2014.1(5)
4
Z
T (K)
295(2)
295(2)
295(2)
295(2)
0.71073
1.452
1.073
3.38
213(2)
0.71073
1.352
2.676
3.12
λ (Å)
0.71073
1.314
0.71073
1.244
0.71073
1.350
density calcd (g cm-3
)
µ (mm-1
)
1.312
1.381
1.149
R(F)a (%)
4.88
3.88
5.25
Rw(F)b (%)
10.02
8.42
10.97
6.48
8.14
2
2
a R(F) ) ∑||Fo| - |Fc||/∑|Fo|. b Rw(F)2 ) [∑w(Fo - Fc )2/∑w(Fo2)2]1/2
.
Table 2. Selected Bond Lengths (Å) and Angles
Table 4. Selected Bond Lengths (Å) and Angles
(deg) for 2‚C7H8
(deg) for 4‚(C6H14)0.5
Ga(1)-N(1)
2.021(4)
In(1)-N(1)
2.238(5)
2.219(5)
2.298(7)
2.145(7)
2.156(7)
2.331(6)
2.152(7)
2.164(7)
1.382(7)
100.9(2)
110.1(2)
101.6(2)
110.7(3)
101.5(3)
128.5(3)
96.2(2)
Ga(2)-N(2)
1.983(4)
In(2)-N(2)
Ga(1)-C(16)
2.051(5)
In(1)-C(12)
Ga(1)-C(22)
1.972(4)
In(1)-C(18)
Ga(1)-C(28)
1.967(5)
In(1)-C(24)
Ga(2)-C(16)
2.254(5)
In(2)-C(12)
Ga(2)-C(34)
1.971(5)
In(2)-C(30)
Ga(2)-C(40)
1.958(4)
In(2)-C(36)
N(1)-N(2)
1.377(4)
N(1)-N(2)
N(1)-Ga(1)-C(16)
N(1)-Ga(1)-C(22)
N(1)-Ga(1)-C(28)
C(16)-Ga(1)-C(22)
C(16)-Ga(1)-C(28)
C(22)-Ga(1)-C(28)
N(2)-Ga(2)-C(16)
N(2)-Ga(2)-C(34)
N(2)-Ga(2)-C(40)
C(16)-Ga(2)-C(34)
C(16)-Ga(2)-C(40)
C(34)-Ga(2)-C(40)
Ga(1)-C(16)-Ga(2)
97.42(17)
109.39(18)
108.42(17)
113.69(18)
110.21(19)
115.99(19)
94.74(16)
111.09(19)
111.59(17)
110.82(18)
107.39(18)
118.6(2)
N(1)-In(1)-C(12)
N(1)-In(1)-C(18)
N(1)-In(1)-C(24)
C(12)-In(1)-C(18)
C(12)-In(1)-C(24)
C(18)-In(1)-C(24)
N(2)-In(2)-C(12)
N(2)-In(2)-C(30)
N(2)-In(2)-C(36)
C(12)-In(2)-C(30)
C(12)-In(2)-C(36)
C(30)-In(2)-C(36)
In(1)-C(12)-In(2)
101.8(2)
124.2(3)
110.6(3)
105.5(3)
116.6(3)
94.6(3)
95.82(18)
Table 3. Selected Bond Lengths (Å) and Angles
Table 5. Selected Bond Lengths (Å) and Angles
(deg) for 6a
(deg) for 3‚(C6H14)0.5
Ga(1)-N(1)
2.013(2)
In-N(1)
2.217(3)
Ga(2)-N(2)
2.004(2)
In-N(2)′
2.233(3)
Ga(1)-C(12)
2.184(2)
In-C(16)
2.137(3)
Ga(1)-C(18)
1.971(3)
In-C(22)
2.138(4)
Ga(1)-C(24)
1.969(3)
N(1)-N(2)
1.371(4)
Ga(2)-C(12)
2.167(3)
N(1)-In-N(2)′
N(1)-In-C(16)
N(1)-In-C(22)
N(2)′-In-C(16)
N(2)′-In-C(22)
C(16)-In-C(22)
103.48(10)
101.96(14)
104.52(12)
104.86(13)
101.22(12)
136.94(16)
Ga(2)-C(30)
1.978(3)
Ga(2)-C(36)
1.970(3)
N(1)-N(2)
1.393(3)
N(1)-Ga(1)-C(12)
N(1)-Ga(1)-C(18)
N(1)-Ga(1)-C(24)
C(12)-Ga(1)-C(18)
C(12)-Ga(1)-C(24)
C(18)-Ga(1)-C(24)
N(2)-Ga(2)-C(12)
N(2)-Ga(2)-C(30)
N(2)-Ga(2)-C(36)
C(12)-Ga(2)-C(30)
C(12)-Ga(2)-C(36)
C(30)-Ga(2)-C(36)
Ga(1)-C(12)-Ga(2)
98.78(9)
103.14(10)
122.37(11)
110.65(11)
105.08(11)
115.44(12)
103.52(9)
105.05(10)
113.08(10)
102.87(11)
109.24(11)
121.28(11)
92.37(10)
a Primed atoms are transformed by 1-x, 2-y, and 1-z.
lengths lie in the range 1.969(3)-1.978(3) Å for the
terminal phenyl groups and are 2.184(2) and 2.167(3)
Å for the bridging phenyl group. The gallium-carbon-
gallium angle for 3‚(C6H14)0.5 is 92.37(10)°.
The molecular structure of 4‚(C6H14)0.5 is shown in
Figure 3. The molecular structure is very similar to that
of 3‚(C6H14)0.5. The two indium atoms and the ipso-
carbon atom of the bridging phenyl group define a plane,
while the two nitrogen atoms lie 0.573(10) and 1.200(9)
Å above this plane. The bridging phenyl group is slightly
canted toward In(2), suggesting a distinct triphenylin-
dium unit containing In(1). The indium-nitrogen bond
>24 h. The two gallium atoms and the ipso-carbon atom
of the bridging phenyl group define a plane, while the
two nitrogen atoms lie 1.070(4) and 0.465(4) Å above
this plane. The gallium-nitrogen bond lengths are
2.013(2) and 2.004(2) Å. The gallium-carbon bond

6188 Organometallics, Vol. 24, No. 25, 2005
Sirimanne et al.
Table 6. Selected Bond Lengths (Å) and Angles
(deg) for 9
Ga(1)-O
1.933(2)
Ga(2)-O
1.934(2)
Ga(1)-N(1)
Ga(2)-N(2)
2.011(2)
2.011(2)
Ga(1)-C(12)
Ga(1)-C(13)
Ga(2)-C(15)
Ga(2)-C(16)
O-C(14)
1.964(3)
1.964(2)
1.962(3)
1.963(3)
1.429(3)
N(1)-N(2)
1.394(2)
Ga(1)-N(1)-N(2)
Ga(2)-N(2)-N(1)
O-Ga(1)-N(1)
O-Ga(2)-N(2)
Ga(1)-O-C(14)
Ga(2)-O-C(14)
Ga(1)-O-Ga(2)
C(12)-Ga(1)-C(13)
C(15)-Ga(2)-C(16)
118.72(12)
118.40(12)
90.27(6)
90.50(6)
117.81(16)
116.80(16)
118.11(8)
124.44(12)
123.36(13)
Figure 3. Perspective view of 4‚(C6H14)0.5 with thermal
ellipsoids at the 50% probability level.
Figure 1. Perspective view of 2‚C7H8 with thermal el-
lipsoids at the 50% probability level.
Figure 4. Perspective view of 6 with thermal ellipsoids
at the 50% probability level.
Figure 2. Perspective view of 3‚(C6H14)0.5 with thermal
ellipsoids at the 50% probability level.
Figure 5. Perspective view of 9 with thermal ellipsoids
at the 50% probability level.
lengths are 2.219(5) and 2.238(5) Å. The indium-carbon
bond lengths lie between 2.145(7) and 2.164(7) Å for the
terminal phenyl groups and are 2.298(7) and 2.331(6)
Å for the bridging phenyl group.
The molecular structure of 6 is shown in Figure 4. It
is dimeric with a six-membered In2N4 ring and four
terminal phenyl groups. Two diphenylpyrazolato ligands
serve as bridges between the two indium atoms, and
the six-membered In2N4 ring consists of four nitrogen
atoms from two diphenylpyrazolato ligands and two
indium atoms. The geometry about the indium centers
is distorted tetrahedral. The indium-nitrogen bond
lengths are 2.217(3) and 2.233(3) Å, the indium-carbon
bond lengths are 2.137(3) and 2.138(4) Å, and the
nitrogen-indium-nitrogen angle is 103.48(10)°.
The molecular structure of 9 is shown in Figure 5.
The molecule consists of a 3,5-di-tert-butylpyrazolato

Pyrazolate-Bridged Digallium and Diindium Complexes
Organometallics, Vol. 24, No. 25, 2005 6189
Table 7. Activation Parameters from Eyring
Analyses and Rate Constants at 298 K for 1,
2‚C7H8, 3, and 4‚(C6H14)0.5
∆Hq
(kcal/mol)
∆Sq
(eu)
∆Gq(298 K) k(298 K)
complex
(kcal/mol)
(s-1
)
1
7.6 ( 0.4 -27.2 ( 1.4
6.0 ( 0.3 -32.0 ( 1.2
8.9 ( 0.5 -23.1 ( 1.8
6.1 ( 0.4 -30.5 ( 1.4
15.7 ( 1.8
15.5 ( 1.5
15.8 ( 2.3
15.2 ( 1.8
20.1
24.8
15.8
40.7
2‚C7H8
3
4‚(C6H14
)
0.5
mation. Eyring analyses of the rate data for 1, 2‚C7H8,
3, and 4‚(C6H14)0.5 afforded the activation parameters
given in Table 7. The enthalpies of activation (∆Hq)
range between 6 and 8.9 kcal/mol, while the entropies
of activation (∆Sq) range between -23.1 and -32.0 eu.
The free energies at 298 K for phenyl exchange range
between 15.2 and 15.8 kcal/mol. The calculated rate
constants at 298 K for phenyl site exchange in 1, 2‚C7H8,
3, and 4‚(C6H14)0.5 range between 15.8 and 40.7 s-1
.
Analysis of ∼0.05 M solutions of 1, 2‚C7H8, 3, and 4‚
(C6H14)0.5 in toluene-d8 afforded NMR spectra that were
identical to those obtained in ∼0.10 M solutions, sug-
gesting that the exchange processes are intramolecular.
Discussion
The molecular structures of 1, 2‚C7H8, 3‚(C6H14)0.5
,
and 4‚(C6H14)0.5 provide an opportunity to examine the
characteristics associated with bridging phenyl ligands
in gallium and indium complexes. In the series 1, 2‚
C7H8, and 3‚(C6H14)0.5, the gallium-carbon bond lengths
associated with the terminal phenyl ligands fall within
a narrow range of 1.958-1.978 Å. These values are very
similar to those found in GaPh3 (1.946(7), 1.968(5) Å)11
and are consistent with normal gallium-phenyl bonds.
The gallium-nitrogen bond distances are roughly simi-
lar across the series 1, 2‚C7H8, and 3‚(C6H14)0.5 and span
1.967-2.021 Å. The gallium-carbon bond lengths for
the bridging phenyl ligand, however, do not follow a
clear trend in 1, 2‚C7H8, and 3‚(C6H14)0.5. In 1 and 2‚
C7H8, these bond lengths are asymmetric, with values
of 2.114(2) and 2.219(2) Å and 2.051(5) and 2.254(5) Å,
respectively. By contrast, the related bond distances in
3‚(C6H14)0.5 (2.167(3), 2.184(2) Å) are identical within
experimental error. The differences in bridging phenyl
gallium-carbon bond lengths in 1, 2‚C7H8, and 3‚
(C6H14)0.5 could have several origins. The bond strengths
associated with the bridging phenyl ligands are probably
small, which would allow low-energy distortions in the
bond distances during crystallization to achieve opti-
mum crystal packing. The pyrazolato ligand carbon
atom substituents in 1, 2‚C7H8, and 3‚(C6H14)0.5 also
have significantly different steric profiles, which affect
the bonding to the pyrazolato ligands. In 1, the nitrogen
and carbon atoms within the Ga2N2C ring are ap-
proximately coplanar, while one gallium atom is slightly
above and the other is slightly below this plane. Since
the methyl substituents in 1 have the smallest steric
profile, this molecular structure is probably the least
distorted of the series. In 2‚C7H8, the gallium and
nitrogen atoms are approximately coplanar within the
Ga2N2C ring, and the bridging phenyl ipso-carbon atom
lies slightly out of this plane. Such an arrangement
appears to place the pyrazolato phenyl groups in the
Figure 6. Variable-temperature 13C{1H} NMR spectra of
1. The resonance denoted with an asterisk corresponds to
a pyrazolate ring carbon atom.
ligand with a dimethylgallio group bonded to each
nitrogen atom. A methoxo ligand acts as a bridge
between the two gallium atoms. The gallium and
nitrogen atoms occupy an approximate plane, while the
methoxo ligand oxygen atom lies 0.383(2) Å above this
plane. The gallium-oxygen bond lengths are 1.933(2)
and 1.934(2) Å. The gallium-nitrogen bond lengths are
both 2.011(2) Å. The gallium-carbon bond lengths are
between 1.962(2) and 1.964(3) Å. The gallium-oxygen-
gallium angle is 118.11(8)°.
Variable-Temperature NMR Behavior of 1, 2‚
C7H8, 3, and 4‚(C6H14)0.5. The bridge-terminal phenyl
site exchange processes that occur in 1, 2‚C7H8, 3, and
4‚(C6H14)0.5 were studied using NMR spectroscopy. In
the 13C{1H} NMR spectra in toluene-d8, the terminal
and bridging phenyl groups in 1, 2‚C7H8, 3, and 4‚
(C6H14)0.5 (0.106 M) undergo site exchange between -30
and 30 °C. To gain further insight into the dynamic
process, the line-broadening kinetics was studied. Fig-
ure 6 shows a selected region of the 13C{1H} NMR
spectrum for 1 taken at various temperatures, showing
the ipso-carbon atoms of the phenyl ligands. These
resonances broaden with increasing temperature and
eventually coalesce, which is consistent with bridge-
terminal phenyl exchange. Complexes 2‚C7H8, 3, and
4‚(C6H14)0.5 show similar 13C{1H} NMR spectra and
undergo similar phenyl site exchange processes. The
phenyl site exchange processes were modeled as AB4
systems. Rate constants were determined by simulating
13C{1H} NMR spectra at various temperatures using the
program gNMR.15 Full kinetics data for 1, 2‚C7H8, 3,
and 4‚(C6H14)0.5 are contained in the Supporting Infor-
sterically least demanding positions. In 3‚(C6H14)0.5
,
(15) gNMR; Version 4; Cherwell Scientific Publishing, 1997.
severe steric crowding between the pyrazolato tert-butyl

6190 Organometallics, Vol. 24, No. 25, 2005
Sirimanne et al.
Scheme 1. Proposed Transition State for Phenyl
Ligand Exchange
substituents and the terminal phenyl ligands leads to
considerable twisting of the pyrazolato ligand with
respect to an approximate Ga2C plane. Such twisting
of the pyrazolato ligand may force the gallium atoms to
form similar bond distances to the bridging phenyl
ligand. In 1 and 2‚C7H8, the pyrazolato ligands are not
as sterically demanding as in 3‚(C6H14)0.5, and asym-
metric gallium-carbon distances are observed to the
bridging phenyl ligand. The molecular structures of 3‚
(C6H14)0.5 and 4‚(C6H14)0.5 are very similar due to their
identical ligand framework.
There are few structurally characterized gallium and
indium complexes that contain bridging phenyl or
similar unsaturated ligands. Gabba¨ı has reported 1,8-
disubstituted naphthalene complexes that contain one
dimesitylboron group and either a dichlorogallio or
dichloroindio substituent.9 The ipso-carbon atom of one
of the boron-bound mesityl groups forms a bridge to the
heavier group 13 metal. In the gallium complex, the
bridging phenyl gallium-carbon distance is 2.279(4) Å,
which is much longer than all but one of the related
bond distances in 1, 2‚C7H8, and 3‚(C6H14)0.5. In the
indium complex, the bridging phenyl indium-carbon
distance is 2.442(6) Å, which is longer than the related
distances of 2.297(7) and 2.331(6) Å in 4‚(C6H14)0.5. In
1, 2‚C7H8, 3‚(C6H14)0.5, and 4‚(C6H14)0.5, the contiguous
nitrogen atoms in the pyrazolato ligands hold the MPh2
and MPh3 units closer together than the three-carbon
bridges in Gabba¨ı’s 1,8-disubstituted naphthalene com-
plexes. Hence, the metal-carbon bond distances are
longer in Gabba¨ı’s complexes than in 1, 2‚C7H8, 3‚
(C6H14)0.5, and 4‚(C6H14)0.5. In the solid-state structure
of InPh3,11 there are intermolecular contacts of 3.07-
3.40 Å between the indium atoms and ipso-, ortho-, and
meta-carbon atoms of adjacent phenyl ligands. In the
solid-state structure of triphenylgallium,11 there are
similar contacts of 3.42-3.81 Å between the gallium
atoms and the ortho-, meta-, and para-carbon atoms of
adjacent phenyl ligands. The closest homodinuclear
comparison to 1, 2‚C7H8, 3, and 4‚(C6H14)0.5 is [Me2In-
(µ-2-C4H3S)]2,6 in which 2-thienyl ligands serve as
bridges between two indium atoms. This complex has
terminal indium-carbon bond lengths between 2.139
and 2.143 Å, highly asymmetric bridging indium-
carbon bond lengths of 2.194(6) and 2.849(3) Å, and an
indium-carbon-indium angle of 89.4° within the four-
membered ring. The related angle for 4‚(C6H14)0.5 (96.88-
(9)°) is larger due to geometric constraints imposed by
the five-membered In2N2C ring. The bonding associated
with the bridging 2-thienyl ligand is clearly very dif-
ferent than the bonding of the bridging phenyl ligands
ground states. Since the metal-nitrogen bond energies
should be much stronger than the bridging phenyl bond
energies, it is very likely that the transition state is
obtained by rotation of a GaPh3 unit along a gallium-
nitrogen bond vector. Rotation by 120° would place a
terminal phenyl ligand into the bridging position and
would convert the bridging phenyl group to a terminal
phenyl ligand. Such a transition state is outlined in
Scheme 1 and is similar to what we proposed earlier
for bridge-terminal methyl exchange in Me2Al(µ-R2pz)-
(µ-Me)AlMe2.2g The large negative ∆Sq values lead to
free energies at 298 K for phenyl exchange that range
between 15.2 and 15.8 kcal/mol. Finally, the calculated
rate constants at 298 K for phenyl site exchange in 1,
2‚C7H8, 3, and 4‚(C6H14)0.5 range between 15.8 and 40.7
s-1. Thus, the pyrazolato carbon atom substituents and
the identity of the metal center have small effects on
the rates of exchange.
Complexes 5-8 form upon treatment of triphenylgal-
lium, triphenylindium, or trimethylgallium with one or
more equivalents of 3,5-dimethylpyrazole, 3,5-diphen-
ylpyrazole, or 3,5-di-tert-butylpyrazole. The complexes
have dimeric structures containing two pyrazolato
ligands that bridge between the metal atoms and two
terminal phenyl or methyl ligands per metal atom.
Several dimeric gallium and indium pyrazolato com-
plexes of the formula [Me2M(pz)]2 (pz ) pyrazolato or
substituted pyrazolato ligands) have been structurally
characterized and have structures that are very similar
to those of 5-8.14 Complexes 5-8 appear to be very
stable thermodynamically and once formed are unre-
active toward excess MPh3 or MMe3. In the case of 6
and 7, these dimeric complexes are the major products
even when two or more equivalents of triphenylindium
are employed. The failure to form the analogues of 4‚
(C6H14)0.5 in high yields upon treatment of triphenylin-
dium with 2 equiv of 3,5-dimethylpyrazole or 3,5-
diphenylpyrazole suggests either that 6 and 7 form
rapidly and do not convert to the phenyl-bridged com-
plexes in the presence of excess triphenylindium or that
the analogues of 4‚(C6H14)0.5 are unstable and decom-
pose rapidly to 6 and 7. We were also unsuccessful in
our attempts to prepare a bridging methyl complex of
gallium through treatment of 3,5-di-tert-butylpyrazole
with excess trimethylgallium. The major product was
the dimeric complex 8, and the minor product, 9, was
determined to be a bridging methoxo complex by X-ray
in 1, 2‚C7H8, 3‚(C6H14)0.5, and 4‚(C6H14)0.5
.
The activation parameters for 1, 2‚C7H8, 3, and 4‚
(C6H14)0.5 allow insight into the transition states that
lead to interconversion of the bridging and terminal
phenyl ligands. The activation parameters and calcu-
lated rate constants at 298 K shown in Table 7 are very
similar for all four bridging phenyl complexes and
suggest similar exchange mechanisms. The ∆Hq values
range between 6 and 8.9 kcal/mol and represent small
differences in energy between the ground states and the
transition states for phenyl site exchange. The ∆Sq
values range from -23.1 to -32.0 eu and suggest
transition states that are highly ordered relative to the

Pyrazolate-Bridged Digallium and Diindium Complexes
Organometallics, Vol. 24, No. 25, 2005 6191
Chemical shifts (δ, ppm) are given relative to residual protons
or the carbon atoms of the indicated solvent. Infrared spectra
were obtained using Nujol as the medium. Microanalyses were
performed by Midwest Microlab, Indianapolis, IN. Melting
points were determined in sealed capillary tubes under argon
on a Haake-Buchler HBI digital melting point apparatus and
are uncorrected.
crystallography. It is likely that in a methyl-bridged
complex with a structure similar to those of 1, 2‚C7H8,
3, and 4‚(C6H14)0.5, a methyl bridge does not provide
enough bond energy to stabilize the complex relative
to 8.
In our report of bridging methyl complexes of the
formula Me2Al(µ-R2pz)(µ-Me)AlMe2,2g molecular orbital
calculations demonstrated that the bridging methyl
ligand is stabilized by through-space bonding with
pyrazolato-based π-orbitals and that these interactions
contribute to the stability of these complexes. In our
preliminary communication describing 1, 2‚C7H8, and
3, we described molecular orbital calculations of the
model complex H2Ga(µ-C3H3N2)(µ-C6H5)GaH2.13 In the
optimized structure, the gallium and nitrogen atoms
occupy an approximate plane, with the bridging phenyl
carbon atom lying above this Ga2N2 plane. The opti-
mized structure of H2Ga(µ-C3H3N2)(µ-C6H5)GaH2 and
the solid-state structure of 2 thus have similar Ga2N2C
core structures. However, the difference between a
planar Ga2N2C ring in H2Ga(µ-C3H3N2)(µ-C6H5)GaH2
and one with a bent carbon atom was calculated to be
only 1.77 kcal/mol. Thus, through-space bonding inter-
actions between the bridging phenyl ligand and pyra-
zolato-based orbitals provide very little stabilization to
H2Ga(µ-C3H3N2)(µ-C6H5)GaH2. Instead, the stability of
the bridging phenyl ligand in H2Ga(µ-C3H3N2)(µ-C6H5)-
GaH2, 1, 2‚C7H8, 3, and 4‚(C6H14)0.5 appears to arise
from strong gallium-nitrogen bonds that hold the two
metal atoms in close proximity and allow a weak
bridging interaction to exist. The results of this work
suggest that appropriate ligand design could lead to
many new examples of homometallic heavier group 13
complexes that contain bridging hydrocarbon ligands.
We have been so far unsuccessful in our attempts to
prepare a bridging methyl complex of gallium. However,
the present work was inspired by weak, long (>3.0 Å)
metal-carbon contacts in the solid-state structures of
gallium and indium phenyl11 complexes and by the
successful synthesis of heterobimetallic group 13 com-
plexes with bridging phenyl ligands.9 Reports of long
metal-carbon contacts in the solid-state structures of
the heavier group 13 alkyl complexes12 suggest that it
still may be possible to stabilize a bridging methyl
complex of gallium or indium. We continue to work
toward this and other goals with group 13 complexes.
Synthesis of (C6H5)2Ga(µ-η1:η1-3,5-Me2pz)(µ-C6H5)Ga-
(C6H5)2 (1). To a stirred solution of triphenylgallium (0.602
g, 2.00 mmol) in toluene (40 mL) was added dropwise a
solution of 3,5-dimethylpyrazole (0.096 g, 1.00 mmol) in
toluene (30 mL). The reaction mixture was stirred at room
temperature for 18 h, and the volatile components were
removed under reduced pressure to afford a white solid. The
resultant solid was extracted with hexane (280 mL), filtered
through a 2 cm pad of Celite, and concentrated. Complex 1
was isolated as colorless single crystals at room temperature
(0.385 g, 62%): mp 161-162 °C; IR (Nujol, cm-1) 3060 (w),
3037 (w), 2955 (s), 2927 (s), 2851 (s), 1571 (w), 1529 (m), 1425
(m), 1056 (w), 1038 (w), 989 (w), 702 (s), 665 (w); 1H NMR
(benzene-d6, 23 °C, δ) 8.43 (br m, 2H, bridging Ph-H), 7.48,
7.18 (m, 20 H, terminal Ph-H), 6.79 (br m, 1H, bridging Ph-
H), 6.68 (br m, 2H, bridging Ph-H), 5.78 (s, 1H, pz ring CH),
1.94 (s, 6H, CH3); 13C{1H} NMR (benzene-d6, 23 °C, ppm) 157.3
(br s, ipso-C, bridging phenyl ring), 150.85 (s, pz ring C-CH3),
146.5 (br s, ipso-C, terminal phenyl rings), 137.50 (br s, ortho-
C-H, bridging phenyl ring), 136.53 (s, ortho-C-H, terminal
phenyl rings), 128.15 (br s, meta-C-H, bridging phenyl ring),
128.00 (s, para-C-H, bridging phenyl ring), 127.92 (s, meta-
C-H, terminal phenyl rings), 127.76 (s, para-C-H, terminal
phenyl rings), 108.05 (s, pz ring CH), 13.02 (s, C-CH3). Anal.
Calcd for C35H32Ga2N2: C, 67.79; H, 5.20; N, 4.52. Found: C,
67.69; H, 5.29; N, 4.64.
Synthesis of (C6H5)2Ga(µ-η1:η1-3,5-Ph2pz)(µ-C6H5)Ga-
(C6H5)2‚C7H8 (2‚C7H8). In a fashion similar to the preparation
of 1, treatment of triphenylgallium (0.602 g, 2.00 mmol) with
3,5-diphenylpyrazole (0.220 g, 1.00 mmol) in toluene afforded
2‚C7H8 (0.521 g, 62%) as colorless single crystals after crystal-
lization from toluene at -20 °C: mp 186-187 °C; IR (Nujol,
cm-1) 3055 (w), 3041 (w), 2945 (s), 2925 (s), 2855 (s), 1572 (w),
1539 (w), 1425 (m), 1104 (m), 761 (m), 698 (s); 1H NMR
(benzene-d6, 23 °C, δ) 8.41 (br m, 2H, bridging Ph-H), 7.61 (m,
10H, pz ring Ph-H), 7.35-7.07 (br m, 20H, terminal Ph-H),
6.81 (s, 1H, pz ring CH), 6.67 (br m, 3H, bridging Ph-H); 13C-
{1H} NMR (benzene-d6, 23 °C, ppm) 157.85 (br s, ipso-C,
bridging phenyl ring), 156.50 (s, pz ring C-Ph), 146.45 (br s,
ipso-C, terminal phenyl rings), 139.2 (br s, ortho-C-H, bridging
phenyl ring), 136.54 (br s, ortho-C-H, terminal phenyl ring),
125-131 (overlapping resonances from terminal and bridging
gallium-phenyl, pyrazolato phenyl groups), 107.24 (s, pz ring
C-H). Anal. Calcd for C52H44Ga2N2: C, 74.68; H, 5.30; N, 3.35.
Found: C, 74.29; H, 5.19; N, 3.49.
Synthesis of (C6H5)2Ga(µ-η1:η1-3,5-tBu2pz)(µ-C6H5)Ga-
(C6H5)2 (3). In a fashion similar to the preparation of 1,
treatment of triphenylgallium (0.602 g, 2.00 mmol) with 3,5-
di-tert-butylpyrazole (0.180 g, 1.00 mmol) in toluene afforded
3 (0.282 g, 40%) as colorless single crystals after crystallization
from hexane at -20 °C: mp 149-151 °C; IR (Nujol, cm-1) 3052
(w), 3037 (w), 2955 (s), 2925 (s), 2855 (s), 1571 (w), 1511 (w),
1426 (w), 701 (m); 1H NMR (benzene-d6, 23 °C, δ) 8.50 (br m,
2H, bridging Ph-H), 7.32-7.15 (m, 20 H, terminal Ph-H), 6.79
(br m, 1H, bridging Ph-H), 6.69 (br m, 2H, bridging Ph-H),
6.39 (s, 1H, pz ring C-H), 1.22 (s, 18H, C(CH3)3); 13C{1H} NMR
(benzene-d6, 23 °C, ppm) 165.86 (s, pz ring C-C(CH3)3),
158. 33 (br s, ipso-C, bridging phenyl rings), 148.26 (br s, ipso-C
of terminal phenyl rings), 136.93 (br s, ortho-C-H, bridging
phenyl ring), 128.42 (br s, meta-C-H, bridging phenyl ring),
128.15 (s, ortho- or meta-C-H, terminal phenyl rings), 128.00
(s, para-C-H, bridging phenyl ring), 127.81 (s, para-C-H,
terminal phenyl rings), 127.68 (s, ortho- or meta-C-H, terminal
phenyl rings), 104.28 (s, pz ring C-H), 32.70 (s, C(CH3)3), 31.33
Experimental Section
General Considerations. All manipulations were carried
out under argon using Schlenk line or glovebox techniques.
Hexane and toluene were distilled from sodium immediately
prior to use. 3,5-Dimethylpyrazole, benzene-d6, and toluene-
d8 were purchased from Aldrich Chemical Co. 3,5-Diphen-
ylpyrazole and 3,5-di-tert-butylpyrazole were prepared accord-
ing to a published procedure.16 Triphenylgallium17 and
triphenylindium18 were prepared according to published pro-
cedures.
1H and 13C{1H} NMR spectra were recorded at 500, 300,
125, or 75 MHz in benzene-d6 or toluene-d8, as indicated.
(16) Elgue´ro, J.; Gonzalez, E.; Jacquier, R. Bull. Soc. Chim. Fr. 1968,
707.
(17) Xiushan, Z.; Xiaolin, Z.; Lei, F.; Wentian, T. Huaxue Shiji 1997,
19, 246.
(18) Viktorova, I. M.; Endovin, Yu. P.; Sheverdina, N. I.; Kocheshkov,
K. A. Dokl. Akad. Nauk. SSSR 1967, 177, 103.

6192 Organometallics, Vol. 24, No. 25, 2005
Sirimanne et al.
1
(s, C(CH3)3). Anal. Calcd for C41H44Ga2N2: C, 69.59; H, 6.60;
N, 4.02. Found: C, 69.92; H, 6.30; N, 3.98. Even though the
X-ray crystallographic determination was consistent with a
formulation of 3‚(C6H14)0.5, 3 was isolated as the solvent-free
material in the bulk synthesis. The hexane solvate is loosely
held and is readily lost upon standing at ambient temperature.
Synthesis of (C6H5)2In(µ-η1:η1-3,5-tBu2pz)(µ-C6H5)In-
at -20 °C: mp 228 °C; H NMR (benzene-d6, 23 °C, δ) 7.60
(m, 8H, Ph-H), 7.17 (m, 12H, Ph-H), 5.63 (s, 2H, pz ring C-H),
2.03 (s, 12H, CH3); 13C{1H} NMR (benzene-d6, 23 °C, ppm)
152.40 (s, pz ring C-CH3), 149.61 (s, ipso-C of phenyl rings),
138.35 (s, ortho-C-H of phenyl rings), 128.68 (s, meta-C-H of
phenyl rings), 128.47 (s, para-C-H of terminal phenyl rings),
106.89 (s, pz ring C-H), 13.71 (s, C-CH3). Anal. Calcd for C34H34-
In2N4: C, 56.07; H, 4.71; N, 7.69. Found: C, 54.17; H, 4.83;
N, 7.06.
(C6H5)2‚(C6H14
)
(4‚(C6H14)0.5). In a fashion similar to the
0.5
preparation of 1, treatment of triphenylindium (0.692 g, 2.00
mmol) with 3,5-di-tert-butylpyrazole (0.180 g, 1.00 mmol) in
toluene afforded 4‚(C6H14)0.5 (0.335 g, 40%) as colorless crystals
after crystallization from hexane at -20 °C: mp 159-160 °C;
IR (Nujol, cm-1) 3055 (w), 3033 (w), 1569 (w), 1506 (w), 1421
(w), 701 (m); 1H NMR (toluene-d8, 4 °C, δ) 8.28 (br m, 2H,
bridging Ph-H), 7.26-7.14 (br m, 20H, terminal Ph-H), 6.87
(br m, 3H, bridging Ph-H), 6.33 (s, 1H, pz ring C-H), 1.28 (s,
18H, C(CH3)3); 13C{1H} NMR (toluene-d8, 4 °C, ppm) 166.16
(s, pz ring C-C(CH3)3), 151.63 (br s, ipso-C, bridging, terminal
phenyl rings), 138.24 (br s, ortho-C-H, bridging, terminal
phenyl rings), 127-129 (meta- and para-C-H of bridging,
bridging phenyl rings obscured by toluene-d8 resonances),
102.12 (s, pz ring C-H), 32.32 (s, C(CH3)3), 31.32 (s, C(CH3)3).
Anal. Calcd for C44H51In2N2: C, 63.10; H, 6.14; N, 3.34.
Found: C, 62.89; H, 6.14; N, 3.20.
Synthesis of Me2Ga(µ-η1:η1-3,5-tBu2pz)2GaMe2 (8) and
Me2Ga(µ-η1:η1-3,5-tBu2pz)(µ-OMe)GaMe2 (9). A solution of
3,5-di-tert-butylpyrazole (1.80 g, 10.0 mmol) in hexane (100
mL) was cooled to -78 °C. Using a syringe, a 3.0 M solution
of trimethylgallium in hexane (20 mL, 60 mmol) was slowly
added to the 3,5-di-tert-butylpyrazole mixture. The resulting
solution was allowed to warm to ambient temperature and was
stirred for another 15 h. The volatile components were then
removed under reduced pressure. 1H NMR analysis of this
crude product indicated a 95:5 mixture of 8 and 9. The crude
product was dissolved in hexane (10 mL). This product was
allowed to stand at ambient temperature for 24 h, during
which time colorless crystals formed. Isolation of the crystals
was effected by cannulation of the mother liquor, followed by
vacuum-drying, to afford colorless crystals of 8 (2.29 g, 82%).
The volatile components were removed from the mother liquor
under reduced pressure, and the residue was then sublimed
at 60 °C/0.01 Torr to afford 9 as colorless crystals (0.11 g, 2.6%).
Spectral and analytical data for 8: mp 163 °C; IR (KBr,
cm-1) 3234 (w), 2967 (s), 2928 (s), 2870 (m), 1509 (s), 1461 (s),
1400 (m), 1361 (s), 1295 (s), 1247 (s), 1214 (s), 1200 (s), 1071
(m), 1033 (m), 1004 (m), 928 (w), 804 (s), 747 (s), 628 (s), 576
Synthesis of (C6H5)2Ga(µ-η1:η1-3,5-tBu2pz)2Ga(C6H5)2
(5). To a stirred solution of triphenylgallium (0.325 g, 0.87
mmol) in toluene (40 mL) was added a solution of 3,5-di-tert-
butylpyrazole (0.156 g, 0.87 mmol) in toluene (30 mL). The
reaction mixture was stirred at room temperature for 18 h,
and the volatile components were then removed under reduced
pressure to afford an off-white solid. This solid was dissolved
in hexane (70 mL). The resultant solution was filtered through
a 2 cm pad of Celite, and the filtrate was concentrated to a
volume of about 30 mL under reduced pressure. Complex 5
formed as colorless crystals at -20 °C and was isolated by
removal of the hexane solution using a cannula, followed by
vacuum-drying of the crystals (0.253 g, 63%): mp 176-178
°C; IR (Nujol, cm-1) 3064 (w), 3043 (w), 1516 (m), 1428 (w),
1
(s), 528 (s), 447 (m); H NMR (benzene-d6, 23 °C, δ) 6.17 (s,
2H, pz ring C-H), 1.30 (s, 36H, C(CH3)3), -0.19 (s, 12H, Ga-
(CH3)2); 13C{1H} NMR (benzene-d6, 23 °C, ppm) 169.54 (s,
C-C(CH3)3), 105.59 (s, pz ring CH), 32.53 (s, C(CH3)3), 31.14
(s, C(CH3)3), -2.42 (s, Ga(CH3)2). Anal. Calcd for C26H50-
Ga2N4: C, 55.95; H, 9.03; N, 10.04. Found: C, 55.78; H, 9.01;
N, 10.03.
1
Spectral and analytical data for 9: mp 79 °C; 1H NMR
(benzene-d6, 23 °C, δ) 6.10 (s, 1H, pz ring C-H), 3.25 (s, 3H,
Ga-OCH3), 1.25 (s, 18H, C(CH3)3), 0.07 (s, 12 H, Ga(CH3)2);
13C{1H} NMR (benzene-d6, 23 °C, ppm) 162.20 (s, C-C(CH3)3),
102.13 (s, pz ring CH), 50.21 (s, GaOCH3), 32.05 (s, C(CH3)3),
31.22 (s, C(CH3)3), -4.61 (s, Ga(CH3)2). Anal. Calcd for C16H34-
Ga2N2O: C, 46.88; H, 8.36; N, 6.83. Found: C, 47.02; H, 8.19;
N, 6.60.
Kinetics Measurements for 1. A 5 mm NMR tube was
charged with 1 (0.053 g, 0.085 mmol) and toluene-d8 (0.80 mL)
in a glovebox under argon and was sealed with a plastic cap.
The tube was transferred to the 125 MHz NMR probe, and
13C{1H} NMR spectra were recorded between -30 and 30 °C.
At -30 °C, 13C{1H} NMR resonances for the ipso-carbon of the
bridging and terminal phenyl groups were observed as sharp
signals at 157.48 and 146.17 ppm. Upon warming from -30
to 30 °C, these resonances gradually broadened, and at 30 °C
two broad peaks were observed at 157.11 and 146.49 ppm. The
dynamic process was modeled as an AB4 exchange using the
13C{1H} resonance broadening for the ipso-carbon of the
bridging and terminal phenyl groups using the program
gNMR.15 Virtually identical spectra were observed for a sample
of 1 (0.026 g, 0.042 mmol) in toluene-d8 (0.80 mL) in the same
temperature range, suggesting that the observed kinetic event
was intramolecular.
1401 (w), 1211 (w), 1086 (m); H NMR (benzene-d6, 23 °C, δ)
7.86 (m, 8H, Ph-H), 7.26 (m, 8H, Ph-H), 7.17 (m, 4H, Ph-H),
6.10 (s, 2H, pz ring C-H), 1.17 (s, 36H, C(CH3)3); 13C{1H} NMR
(benzene-d6, 23 °C, ppm) 170.28 (s, pz ring C-C(CH3)3), 148.74
(s, ipso-C, phenyl rings), 137.01 (s, ortho-C, phenyl rings),
128.13 (s, meta-C, phenyl rings), 127.80 (s, para-C, phenyl
rings), 108.18 (s, pz ring C-H), 32.72 (s, C(CH3)3), 31.26 (s,
C(CH3)3). Anal. Calcd for C46H58Ga2N4: C, 68.51; H, 7.25; N,
6.95. Found: C, 66.72; H, 7.13; N, 7.01.
Synthesis of (C6H5)2In(µ-η1:η1-3,5-Ph2pz)2In(C6H5)2 (6).
In a fashion similar to the synthesis of 5, treatment of
triphenylindium (0.346 g, 1.00 mmol) with 3,5-diphenylpyra-
zole (0.220 g, 1.00 mmol) in toluene afforded 6 (0.205 g, 40%)
as colorless rods after crystallization from hexane at -20 °C:
mp 220-222 °C; IR (Nujol, cm-1) 3112 (w), 3052 (w), 1537 (w),
1426 (w), 1392 (w), 1273 (w), 918 (w), 698 (s); 1H NMR
(benzene-d6, 23 °C, δ) 7.51 (m, 16 H, pz ring Ph-H), 7.31 (m,
8H, terminal Ph-H), 7.26 (m, 8H, terminal Ph-H), 7.11 (m, 4H,
terminal Ph-H), 6.85 (m, 4H, pz ring Ph-H), 6.46 (s, 2H, pz
ring C-H); 13C{1H} NMR (benzene-d6, 23 °C, ppm) 159.21 (s,
pz ring C-Ph), 150.93 (s, ipso-C, terminal phenyl rings), 137.81
(s, ortho-C-H of terminal phenyl rings), 132.60 (s, ipso-C-H of
pz phenyl rings), 129.36 (s, ortho-C-H of pz phenyl rings),
128.81 (s, para-C-H of pz phenyl rings), 128.60 (s, meta-C-H
of pz phenyl rings), 127.7-127.3 (meta-C-H of terminal phenyl
rings obscured by C6D6 resonances), 107.08 (s, pz ring CH).
Anal. Calcd for C54H42In2N4: C, 66.41; H, 4.33; N, 5.74.
Found: C, 66.54; H, 4.50; N, 5.55.
Kinetics Measurements for 2‚C7H8. In a fashion similar
to the kinetics measurements for 1, a solution of 2‚C7H8 (0.071
g, 0.085 mmol) in toluene-d8 (0.80 mL) was used to record 13C-
{1H} NMR spectra between -25 and 20.6 °C. At -25 °C, sharp
13C{1H} NMR resonances for the ipso-carbon of the bridging
and terminal phenyl groups were observed at 157.93 and
146.15 ppm. Upon warming from -25 to 20.6 °C, these
resonances gradually broadened, and at 20.6 °C two broad
Synthesis of (C6H5)2In(µ-η1:η1-3,5-Me2pz)2In(C6H5)2 (7).
In a fashion similar to the synthesis of 5, treatment of
triphenylindium (0.346 g, 1.00 mmol) with 3,5-dimethylpyra-
zole (0.096 g, 1.00 mmol) in toluene afforded 7 (0.335 g, 92%)
as a colorless crystalline solid after crystallization from hexane

Pyrazolate-Bridged Digallium and Diindium Complexes
Organometallics, Vol. 24, No. 25, 2005 6193
peaks were observed at 157.92 and 146.51 ppm. Virtually
identical spectra were obtained for a sample of 2‚C7H8 (0.035
g, 0.042 mmol) in toluene-d8 (0.80 mL) in the same tempera-
ture range.
Complex 3‚(C6H14)0.5 crystallized as colorless flat rods, and
a crystal with dimensions of 0.4 × 0.3 × 0.1 mm-3 was used
for data collection; 2450 frames were collected, yielding 14 204
reflections, of which 9084 were independent. Hydrogen posi-
tions were observed or calculated. The asymmetric unit
contains one neutral molecule and 1/2 hexane solvent disor-
dered about an inversion center. The solvent exhibited large
thermal parameters but was allowed to refine.
Complex 4‚(C6H14)0.5 crystallized as colorless rods, and a
crystal with dimensions of 0.28 × 0.12 × 0.12 mm-3 was used
for data collection; 2450 frames were collected, yielding 14 967
reflections, of which 9525 were independent. Hydrogen atoms
were placed in calculated positions. The asymmetric unit
consists of one neutral binuclear complex and 1/2 equiv of
hexane solvent. The hexane occupies an inversion center.
Kinetics Measurements for 3. In a fashion similar to the
kinetics measurements for 1, a solution of 3 (0.060 g, 0.085
mmol) in toluene-d8 (0.80 mL) was used to record 13C{1H}
NMR spectra between -12 and 28 °C. At -12 °C, sharp 13C-
{1H} NMR resonances for the ipso-carbon of the bridging and
terminal phenyl groups were observed at 158.33 and 147.97
ppm. Upon warming from -12 to 28 °C, these resonances
gradually broadened, and at 28 °C two broad peaks were
observed at 158.28 and 148.24 ppm. Virtually identical spectra
were obtained for a sample of 3 (0.030 g, 0.042 mmol) in
toluene-d8 (0.80 mL) in the same temperature range.
Kinetics Measurements for 4‚(C6H14)0.5. In a fashion
Complex 6 crystallized as colorless rhomboids, and a crystal
with dimensions of 0.2 × 0.2 × 0.2 mm-3 was used for data
collection; 2450 frames were collected, yielding 15 988 reflec-
tions, of which 5322 were independent. Hydrogen positions
were calculated. The asymmetric unit consists of one-half
neutral binuclear complex occupying an inversion center.
Complex 9 crystallized as colorless flat rods, and a sample
0.6 × 0.4 × 0.1 mm-3 was used for data collection; 1850 frames
were collected, yielding 10 160 reflections, of which 4508 were
independent. Hydrogen positions were observed and refined,
except those on C16 (methyl), which were calculated. The
asymmetric unit consists of one neutral binuclear complex
without solvent.
similar to the kinetics measurements for 1, a solution of 4‚
(C6H14)0.5 (0.060 g, 0.072 mmol) in toluene-d8 (0.80 mL) was
used to record 13C{1H} NMR spectra between -30 and 4 °C.
At -30 °C, 13C{1H} NMR resonances for the ipso-carbon of the
bridging and terminal phenyl groups were observed as slightly
broad and sharp signals at 153.63 and 151.49 ppm, respec-
tively. Upon warming from -30 to 4 °C, these resonances
gradually broadened, and at 4 °C only one broad peak was
observed at 151.63 ppm.
Crystallographic Structural Determinations of 2‚
C7H8, 3‚(C6H14)0.5, 4‚(C6H14)0.5, 6, and 9. Diffraction data were
measured on a Bruker P4/CCD diffractometer equipped with
Mo radiation and a graphite monochromator at 298 K (2‚C7H8,
3, 4‚(C6H14)0.5, and 6) or 213 K (9). The samples collected at
298 K were mounted in thin-walled glass capillaries under an
inert atmosphere. A sphere of data was measured at 10 s/frame
and 0.2° between frames. The frame data were indexed and
integrated with the manufacturer’s SMART software.19 All
structures were refined using Sheldrick’s SHELX-97 soft-
ware.20
Acknowledgment. We are grateful to the National
Science Foundation (CHE-9807269, Special Creativity
Extension thereto, and CHE-0314615) for support of this
research. Support for W.Z. came from the Office of Naval
Research (grant no. N00014-21-0-866).
Complex 2‚(C7H8) crystallized as colorless rhomboids. A
crystal with dimensions of 0.2 × 0.1 × 0.02 mm-3 was used
for data collection; 2450 frames were collected, yielding 15 404
reflections, of which 9789 were independent. Hydrogen posi-
tions were calculated. The neutral complex crystallized with
1 equiv of toluene that was disordered about a crystallographic
inversion center. The disordered solvent was included in the
calculations using Spek’s SQUEEZE portion of the PLATON
software suite.21,22
Supporting Information Available: X-ray crystallo-
graphic files in CIF format for the structure determinations
of 2‚C7H8, 3, 4‚(C6H14)0.5, 5-7, and 9. This material is available
free of charge via the Internet at http://pubs.acs.org. Tables
of kinetics data for 1, 2‚C7H8, 3, and 4‚(C6H14)0.5 are also
available.
OM050623R
(19) SMART, SAINT, and SADABS collection and processing pro-
grams; Bruker AXS Inc.: Madison WI.
(20) Sheldrick, G. SHELX-97; University of Gottingen: Germany,
2003 and 1997.
(21) Spek, A. L. PLATON; Utrecht University: The Netherlands,
2003.
(22) Spek, A. L. J. Appl. Crystallogr. 2003, 36, 7.