The Ullmann coupling reaction: A new approach to tetraarylstannanes

Article abstract of DOI:10.1021/om060456a

Several iodobenzenes form tetraarylstannanes, in addition to other products, under reaction conditions typically employed for the Ullmann reaction, i.e., activated copper bronze (a copper-tin alloy) and 7 days at 230°C. The isolated yields of the tetraarylstannanes were low to good (8-58%). Significantly higher yields (54-64%) of tetraphenylstannane were obtained by the direct reaction of iodobenzene with an excess of tin powder (iodobenzene: tin = 1:1 w/w) under the same conditions. Crystal structure analysis reveals that tetrakis(4-carbomethoxyphenyl)stannane crystallizes in a tetragonal space group and has 4 symmetry, which is the case for many symmetrical tetraarylstannanes. However, tetrakis(2,4-dichlorophenyl)stannane, tetrakis(3,4-dichlorophenyl)stannane, and tetrakis(2,4-dimethylphenyl)stannane do not crystallize in a tetragonal space group and do not have real 4 symmetry.

Full text of DOI:10.1021/om060456a

Organometallics 2006, 25, 4207-4214
4207
The Ullmann Coupling Reaction: A New Approach to
Tetraarylstannanes
Nadim S. Shaikh,† Sean Parkin,‡ and Hans-Joachim Lehmler*,†
Department of Occupational and EnVironmental Health, The UniVersity of Iowa, College of Public Health,
100 Oakdale Campus, 124 IREH, Iowa City, Iowa 52242, and Department of Chemistry, UniVersity of
Kentucky, Lexington, Kentucky 40506
ReceiVed May 24, 2006
Several iodobenzenes form tetraarylstannanes, in addition to other products, under reaction conditions
typically employed for the Ullmann reaction, i.e., activated copper bronze (a copper-tin alloy) and 7
days at 230 °C. The isolated yields of the tetraarylstannanes were low to good (8-58%). Significantly
higher yields (54-64%) of tetraphenylstannane were obtained by the direct reaction of iodobenzene
with an excess of tin powder (iodobenzene:tin ) 1:1 w/w) under the same conditions. Crystal structure
analysis reveals that tetrakis(4-carbomethoxyphenyl)stannane crystallizes in a tetragonal space group and
has 4h symmetry, which is the case for many symmetrical tetraarylstannanes. However, tetrakis(2,4-
dichlorophenyl)stannane, tetrakis(3,4-dichlorophenyl)stannane, and tetrakis(2,4-dimethylphenyl)stannane
do not crystallize in a tetragonal space group and do not have real 4h symmetry.
to synthesize several PCB congeners of interest. To our surprise,
with some starting materials, a chlorinated tetraarylstannane was
isolated as a product along with the desired PCB congener. To
the best of our knowledge, the formation of chlorinated
tetraarylstannanes as a product of the Ullmann reaction has not
been reported previously. Therefore, the present study investi-
gates the formation of tetraarylstannanes by the reaction of
functionalized iodobenzenes (Cl-, MeO-, F-, MeOCO-, Me-, and
O2N-) with copper bronze or elemental tin under typical
Ullmann reaction. The crystal structures of several of the
synthesized tetraarylstannanes are discussed.
Introduction
The Ullmann reaction, initially reported in 1901,1 was a
breakthrough in the area of aryl-aryl and carbon-heteroatom
bond formation. Several reviews have discussed this methodol-
ogy in great detail.2-4 Typically, 2 mol of aryl halides react
with 1 equiv of copper at high temperature (above 200 °C) to
form biaryls and copper halides.5 Several modifications and
improvements of the Ullmann reaction employ copper and its
derivatives,6-9 but the most frequently used form of copper is
activated copper bronze (or bronze powder), a 9:1 mixture of
copper with tin.10 Typically, the copper bronze is activated prior
to the coupling reaction using iodine in acetone followed by
washing the bronze with concentrated HCl in acetone.11 This
classical Ullmann reaction is still the only reaction that allows
the synthesis of certain highly ortho-substituted biaryls,2,12 for
example environmentally relevant polychlorinated biphenyls
(PCBs) with g3 ortho chlorine substituents.
Results and Discussion
Ullmann Coupling Reactions of Chlorinated Iodoben-
zenes. The Ullmann coupling reaction is the only reaction that
allows the synthesis of PCB congeners with g3 ortho chlorine
substituents in good yields. During a synthesis of 2,2′,4,4′-
tetrachlorobiphenyl (2a) via the Ullmann reaction we isolated
two fractions with similar Rf-values that both resembled our
target molecule according to preliminary characterization by 1H
NMR spectroscopy. Further characterization revealed that the
isolated products were tetrakis(2,4-dichlorophenyl)stannane (3a)
and the desired PCB congener 2a, respectively. In contrast, no
stannane 3d was isolated during the synthesis of 2,2′,3,3′,6,6′-
hexachlorobiphenyl, 2d. Because it is important for toxicological
studies to have access to highly pure test compounds free of
possible tetraarylstannane impurities, this study investigates
experimental conditions (e.g., activation of the copper bronze,
ratio of aryl halide to metal, workup procedure) that result in
the formation of significant amounts of chlorinated tetraaryl-
stannanes.
As a part of our efforts to investigate the disposition and
toxicity of ortho-substituted PCB congeners,13-16 we employed
the copper-bronze-mediated classical Ullmann coupling reaction
* Corresponding author. Phone: (319) 335-4211. Fax: (319) 335-4290.
E-mail: hans-joachim-lehmler@uiowa.edu.
† Department of Occupational and Environmental Health.
‡ Department of Chemistry.
(1) Ullmann, F.; Bielecki, J. Chem. Ber. 1901, 34, 2174-2185.
(2) Hassan, J.; Sevignon, M.; Gozzi, C.; Schulz, E.; Lemaire, M. Chem.
ReV. 2002, 102, 1359-1469.
(3) Sainsbury, M. Tetrahedron 1980, 36, 3327-3359.
(4) Bringmann, G.; Walter, R.; Weirich, R. Angew. Chem. 1990, 102,
1006-1019.
(5) Lewin, A. H.; Cohen, T. Tetrahedron Lett. 1965, 4531-4536.
(6) Meyers, A. I.; Himmelsbach, R. J. J. Am. Chem. Soc. 1985, 107,
682-685.
(7) Meyers, A. I.; Flisak, J. R.; Aitken, R. A. J. Am. Chem. Soc. 1987,
109, 5446-5452.
(13) Lehmler, H.-J.; Price, D. J.; Garrison, A. W.; Birge, W. J.; Robertson,
L. W. Fresenius EnViron. Bull. 2003, 12, 254-260.
(8) Meyers, A. I.; Willemsen, J. J. Chem. Commun. 1997, 1573-1574.
(9) Nelson, T. D.; Meyers, A. I. Tetrahedron Lett. 1993, 34, 3061-
3062.
(14) Kania-Korwel, I.; Hornbuckle, K. C.; Peck, A.; Ludewig, G.;
Robertson, L. W.; Sulkowski, W. W.; Espandiari, P.; Gairola, C. G.;
Lehmler, H.-J. EnViron. Sci. Technol. 2005, 3513-3520.
(15) Lehmler, H.-J.; Robertson, L. W.; Garrison, A. W.; Kodavanti, P.
R. S. Toxicol. Lett. 2005, 156, 391-400.
(10) Fanta, P. E. Synthesis 1974, 9-21.
(11) Kleiderer, E. C.; Adams, R. J. Am. Chem. Soc. 1933, 55, 4219-
4225.
(16) Kania-Korwel, I.; Garrison, A. W.; Avants, J. K.; Hornbuckle, K.
C.; Robertson, L. W.; Sulkowski, W. W.; Lehmler, H.-J. EnViron. Sci.
Technol. 2006, 40, 3704-3710.
(12) Hassan, J.; Penalva, V.; Lavenot, L.; Gozzi, C.; Lemaire, M.
Tetrahedron 1998, 54, 13793-13804.
10.1021/om060456a CCC: $33.50 © 2006 American Chemical Society
Publication on Web 07/22/2006

4208 Organometallics, Vol. 25, No. 17, 2006
Table 1. Ullmann Coupling Reaction of Chlorinated Iodobenzenesa
Shaikh et al.
a Unless otherwise stated, all the reactions were carried out at 230 °C using 3 g of 1a-f and 7 g of purified copper bronze. b Yields of isolated (crude)
products. c Reaction was performed using tin powder (5 molar equiv). d Yield of impure product. nd: not detected. ni: a tetraphenyl stannane was formed
according to 119Sn NMR spectroscopy but could not be isolated by column chromatography.
Initial experiments investigated the Ullmann reaction with a
3:7 (w/w) ratio of 2,4-dichloroiodobenzene (1a) to copper bronze
at 230 °C (entry 1-1 in Table 1). This model reaction was
reproduced several times (see Supporting Information) and,
according to GC-MS and 119Sn NMR analysis of the crude
reaction mixtures, always resulted in the formation of biphenyl
2a, 1,3-dichlorobenzene (the dehalogenated product of 1a),
tetraarylstannane 3a, and several other organotin compounds.
Unfortunately, these organotin species could not be isolated by
column chromatography. Complete conversion of 1a was
observed after 7 days. Tetraarylstannane 3a was also formed,
in addition to the desired biphenyl 2a, when nonactivated copper
bronze or different batches of copper bronze were used.
Significantly more 119Sn signals were observed in the 119Sn
NMR spectra of reactions using nonactivated copper bronze.
When the same reaction was carried out with a smaller
amount of copper bronze (3:3 w/w instead of 3:7 w/w), only
the 119Sn NMR signal of 3a at -143.6 ppm was observed.
Similarly, 3a was formed when a 3:1 weight ratio of 1a to
copper bronze was employed. We also investigated the reaction
of 1a with activated tin powder (entry 1-2 in Table 1). After 7
days more than 50% of starting iodide was converted to 1,3-
dichlorobenzene and neither biphenyl 2a or tetraarylstannane
3a was formed. However, a 119Sn NMR signal at -175.6 ppm
indicates the formation of a small quantity of an unknown
organotin compound. This organotin compound could not be
isolated by column chromatography. However, when untreated
tin powder was employed, the reaction resulted in the formation
of 8% of tetraarylstannane 3a. No biphenyl 2a was detected
under these conditions.
In addition to 1a, the Ullmann coupling of chloroiodoben-
zenes 1b-f was investigated. The respective target compounds
2b-f are environmentally relevant PCB congeners. The results
of these experiments are summarized in Table 1. Similar to 1a,
the Ullmann reaction of these chloroiodobenzenes resulted in
the formation of the biphenyls 2b-f, respectively. We were
also able to detect significant amounts of the respective
deiodinated chlorobenzene by GC-MS. In addition, one or more
signals indicative of the formation of organotin compounds were
observed in the 119Sn NMR spectrum of all crude reaction
mixtures. Isolation of the tetraarylstannanes 3 was successful
for 1b and 1c (entries 1-2 and 1-3), but not for the two
trichloroiodobenzenes 1d and 1e (entries 1-5 and 1-6). Stannane
3f was isolated in impure form and was found to decompose
during our purification attempts (entry 1-7).
These finding suggest that some tetrakis(trichlorophenyl)-
stannanes may be unstable, which is surprising because the
highly chlorinated tetrakis(pentachlorophenyl)stannane, Sn(C6-
Cl5)4, is apparently stable under ambient conditions.17 However,
(17) Ara, I.; Fornies, J.; Garcia-Monforte, M. A.; Martin, A.; Menjon,
B. Chem. Eur. J. 2004, 10, 4186-4197.

The Ullmann Coupling Reaction
Organometallics, Vol. 25, No. 17, 2006 4209
Table 2. Ullmann Reaction of Functionalized Iodobenzenes
a Yields of isolated (crude) products. ni: 2l was the only product formed according to GC-MS analysis; its yield was not determined. nd: not detected
by 119Sn NMR spectroscopy. nf: not formed according to GC-MS analysis.
Table 3. Direct Synthesis of Tetraphenylstannane Using
this compound was synthesized by reaction of [SnCl4(thf)2] with
Iodobenzene and Tin
LiC6Cl5 at -78 °C and recrystallized from CCl2dCCl2. By
contrast, in the present study the Ullmann reactions were
performed at 230 °C with a reaction time of 7 days, and column
chromatography was employed in our attempts to separate the
tetraarylstannanes 3d and 3e from other reaction products. The
drastic reaction conditions and/or the column chromatographic
purification may explain why 3d and 3e could not be isolated
from the Ullmann reaction mixtures. Alternatively, the tetraar-
reaction conditions
ylstannanes 3d-f may be unstable as a result of the substitution
pattern.
weight ratio temperature duration conversion
yield
(%)a
entry
(Ar-I:Sn)
(°C)
(days)
(%)a
Ullmann Coupling Reactions of Miscellaneous Iodoben-
3-1
3-2
3-3
3-4
3-5
3-6
3-7
3-8
3-9
3-10
3-11
3:7b,c
3:0.7c
1:0.7
1:0.7
1:1
230
230
230
230
230
230
300
300
7
7
4
7
7
7
2
4
2
4
6
100
20
25
52
91
97
30
98
20
48
100
67 (58)d
zenes. Tetraarylstannanes 3 were also formed with iodobenzenes
with functional groups such as methoxy, fluoro, ester, and
methyl. As shown in Tables 2 and 3, 4-methoxy-1-iodobenzene
(1h), 4-fluoroiodobenzene (1i), methyl-4-iodobenzoate (1j), 2,4-
dimethyliodobenzene (1k), and iodobenzene (1n) gave the
corresponding biphenyls 2h-k and 2n as well as the tetraaryl-
stannanes 3i-k and 3n (entries 2-1 to 2-4; entry 3-1). 2-Ni-
troiodobenzene gave the corresponding biphenyl 2l, but no
tetraarylstannane was formed according to the 119Sn NMR
spectrum of the crude reaction mixture (entry 2-5). No biphenyl
derivative 2m and no tetraarylstannanes 3m was formed during
the Ullmann reaction of iodothiophene 1m. Within this series
of starting materials, the formation of stannane 3j directly from
methyl-4-iodobenzoate (1j) is of particular interest because
tetraarylstannanes with CdO functional groups cannot be
synthesized directly using conventional approaches, for example
the reaction of Grignard or organolithium reagents with SnCl4.18
8
6
20
69 (64)d
1:2
1:1
66 (54)d
15e
1:1
1e
1:2c
1:2c
1:2c
150 (DMF)
150 (DMF)
150 (DMF)
5
16
20
a Determined by GC. All reactions were carried out in a sealed glass
tube using 1 g of 1n unless otherwise stated. b Reaction was performed
using copper bronze (3 g of 1n and 7 g of copper bronze). c Reactions were
carried out using 3 g of 1n and Sn powder. d Yields in parentheses refer to
isolated yields with purity >98% (GC). e Biphenyl (2n) was also detected
(GC-MS).
Synthesis of Tetraarylstannanes with Tin Powder. To
further optimize the synthesis of tetraarylstannanes, we inves-

4210 Organometallics, Vol. 25, No. 17, 2006
Shaikh et al.
tigated the reaction of iodobenzene (1n) with elemental tin. The
reaction of alkyl halides with tin and tin alloys is a classical
approach for the synthesis of symmetrical organotin compounds
of the type R4Sn.18,19 However, there is only one report that
describes the direct synthesis of a tetraarylstannane (i.e., Sn-
(C6F5)4) by the reaction of elemental tin with an iodobenzene
at 260 °C.20 Considering the great interest in functionalized
tetraarylstannanes, we further investigated the observation that
the reaction of untreated tin power with iodobenzene 1a results
in the formation of tetraarylstannane 3a. As part of this
investigation, iodobenzene 1n was treated with tin powder under
different reaction conditions, and the conversion and yields of
tetraphenylstannane 3n were determined by GC (Table 3).
As mentioned above, the reaction of 1n with copper bronze
gave complete conversion with 67% yield (58% isolated yields)
of 3n (entry 3-1). In this reaction approximately 0.7 g of pure
tin was present (equals 10% of 7 g of copper bronze). In a
parallel experiment we used 0.7 g of pure tin instead of copper
bronze (entry 3-2). Analogous to a similar reaction with 2a
(entry 1-2), the conversion and yields of this reaction were
surprisingly poor. Even increasing the amount of tin to a weight
ratio of 1n:tin of 1:0.7 resulted in only 25% and 52% conversion
after 4 and 7 days, respectively (entries 3-3 and 3-4). However,
when an equal amount of tin was used, a conversion of 91%
was observed with a good yield of the desired product 3n (entry
3-5). Only a slight improvement in conversion but not in yield
was observed when 2 equiv of tin was used (entry 3-6, 97%
conversion and 66% yield). The fact that copper bronze results
in a good yield of tetraphenylstannane whereas larger quantities
of tin powder are needed suggests different reaction mechanisms,
possibly involving copper intermediates in the reactions with
copper bronze.
When the reaction was carried out at 300 °C, a conversion
of 30% and a yield of 15% were observed after 2 days (entry
3-7). But 1n completely disappeared after 4 days and only a
negligible amount of 3n was detected, indicating the decom-
position of the tetraphenylstannane (3n) at a temperature of 300
°C (entry 3-8). According to GC-MS analysis, biphenyl 2n was
formed as a byproduct under these conditions, possibly from
the decomposition of 3n or its putative intermediate, e.g.,
diphenyltin diiodide (Ph2SnI2). A preliminary experiment showed
that the reaction of 1d with tin using the optimized reaction
conditions from entry 3-5 resulted in the formation of biphenyl
2d and tetraarylstannane 3d.
In a search for milder reaction conditions we also studied
the reaction of iodobenzene 1n with tin in DMF, a solvent
frequently used for Ullmann reactions. Iodobenzene (1n) was
treated with tin suspended in DMF at 150 °C, and the reaction
was monitored by GC after 2, 4, and 6 days (entries 3-9 to 3-11).
Although quantitative conversion was observed after 6 days,
the yield of tetraphenylstannane 3n was poor (20%).
molecular structures of these compounds and the respective
numbering scheme are shown in Figure 1. Crystallographic data
and structure determination details are given in Table 4, and
relevant geometrical parameters (selected bond lengths and bond
angles) are given in Table 5. As shown in Table 5, the Sn-C
bond distances of all four tetraarylstannanes are within the range
previously reported for other tetraarylstannanes [2.120(8)-
2.152(8) Å]. All four compounds show the deviation of the
phenyl rings from ideal D6h geometry that is typical for this
class of compounds. Although the sum of all six interior
C-C-C angles is 720.0° for every phenyl ring, the individual
C-C-C angles vary from 116.3(2)° to 123.4(2)° (3a), 118.3-
(2)° to 121.5(2)° (3b), 118.24(18)° to 121.36(18)° (3j), and
117.98(18)° to 122.51(18)° (3k), respectively.
Tetraarylstannanes typically crystallize in a variety of tet-
ragonal space groups, for example space group P4h21c, I4h, P41/
a, or P42/n.21,22 At the same time they have 4h symmetry,21,22
which represents the lowest-energy molecular conformation for
compounds of the type Ar4Sn.23 In compounds with ring
substituents that are small and preferably in the 4-position, this
conformation also allows close packing of the quasi-spherical
molecules required to minimize free volume in the crystals.24
As shown in Table 4, only 3j crystallized in a tetragonal space
group (I41/a) and had 4h symmetry. The two C-Sn-C bond
angles â and θ are 108.26(5)° and 111.93(10)°, respectively
(the 4h symmetry axis falls through two C-Sn-C bond angles
which are referred to as â and θ; see refs 23, 24 for a detailed
explanation); that is, compound 3j deviates only slightly from
ideal tetrahedral symmetry. The dihedral angle æ, which is used
to describe the aryl ring orientation, is 19.3° (æ is defined as
the angle between the plane of the aryl ring and the C-Sn-C
plane containing the principal axis23,24), which is relatively small
compared to other tetraarylstannanes with 4h symmetry.24
To maintain both the molecular and crystal symmetry, all
four ester groups of 3j are rotated by ∼8° out of the plane of
the benzene ring, with O1 0.142(4) Å and O2 0.176(4) Å above
and below this plane, respectively. This slightly twisted
conformation of the ester group allows for efficient filling of
space and hence maintains a relatively dense packing of the 3j
molecules in the crystal, despite the relative complexity of the
ester groups in 3j compared to the chloro and methyl substituents
in 3a, 3b, and 3k. Similarly, other tetraarylstannanes with large
4-substituents, for example tetra[4-(tert-butyl)phenyl]stannane
and tetrakis[4-(trifluoromethyl)phenyl]stannane, also crystallize
with tetragonal space groups and have 4h symmetry. In contrast,
tetrakis(4-ethoxyphenyl)stannane, a compound with a functional
group of a size similar to -CO2Me, has point symmetry 1 and
crystallizes in space group P21/c.25 In the crystal structure of
tetrakis(4-ethoxyphenyl)stannane the CAr-CAr-O-CH2 torsion
angles of three of the ethoxy groups are approximately 180°
(178.4(5)-179.3(6)°); that is, the -OCH2- fragment lies in
the plane defined by the benzene ring. As a result of intermo-
lecular interactions, one ethoxy group is rotated relative to the
plane of the benzene ring with a torsion angle CAr-CAr-O-
CH2 of 169.4(6)°, thus resulting in a total loss of symmetry.
These observations suggest that an increased size of the
functional group is not necessarily causing loss of symmetry
of 4-substituted tetraarylstannanes.
Crystal Structures of Tetraarylstannanes. The crystal
structures of several tetraarylstannanes, typically with meta or
para substituents, have been reported (please see the Supporting
Information for a list of published crystal structures with the
general formula Ar4Sn). To add to the number of available
crystal structures, the crystal structures of the two ortho
substituent tetraarylstannanes 3a and 3k, and of the non-ortho-
substituted tetraarylstannanes 3b and 3j, were determined. The
(21) Wharf, I.; Belanger-Gariepy, F. Acta Crystallogr. 2003, E59, m661-
m663.
(18) Davies, A. G. Organotin Chemistry, 2nd ed.; John Wiley & Sons:
Chichester, UK, 2004.
(19) Neumann, W. P. The Organic Chemistry of Tin; John Wiley &
Sons: London, 1970.
(20) Cohen, S. C.; Reddy, M. L. N.; Massey, A. G. Chem. Commun.
1967, 451-453.
(22) Wharf, I.; Simard, M. G.; Lamparski, H. Can. J. Chem. 1990, 68,
1277-1282.
(23) Hutchings, M. G.; Andose, J. D.; Mislow, K. J. Am. Chem. Soc.
1975, 97, 4553-4561.
(24) Wharf, I.; Simard, M. G. Acta Crystallogr. 1995, B51, 973-980.
(25) Wharf, I.; Simard, M. G. Acta Crystallogr. 1991, C47, 1314-1315.

The Ullmann Coupling Reaction
Organometallics, Vol. 25, No. 17, 2006 4211
Figure 1. View of stannanes 3a, 3b, 3j, and 3k showing the atom-labeling scheme. Displacement ellipsoids are drawn at the 50% probability
level.
The stannanes 3a, 3b, and 3k do not crystallize in tetragonal
space groups. Rather, 3b and 3k crystallize in the triclinic space
group P1h, and 3a crystallizes in the orthorhombic space group
Pbca. Especially for 3a and 3b, the deviation from tetragonal
symmetry is not surprising because of the large, obtrusive
chlorine substituents, which are known to cause loss of close
packing as a result of the extramolecular steric requirements of
the Cl substituents.17,26,27 Two of the crystal structures show
evidence of π-π interactions between phenyl rings. In 3b rings
are related via an inversion center to rings on adjacent molecules
such that (for example) the plane of the C1-C6 ring is 3.483-
(3) Å from an inversion-related ring. In 3k the situation is
similar, but the interplanar distance is 3.473(3) Å. To our
knowledge, this is the first time that such π-π interactions have
been reported for tetraarylstannanes. It is not surprising that 3a,
3b, and 3k also deviate more significantly compared to 3j from
tetrahedral geometry (Table 5). 3b does not have a 4h symmetry
axis, whereas both 3a and 3k have an approximate 4h symmetry
axis. Overall, the π-π interactions, the deviations from
tetrahedral geometry around the central Sn atom, and the
orientation of the phenyl rings relative to the approximate 4h
symmetry axis (æ(3a) ) 49.3°, 49.9°, 48.0°, and 45.2° and æ-
(3k) ) -133.0°, -135.7°, 45.4°, and 46.2°, respectively) allow
a more dense packing of 3b and 3k in the solid state, thus
compensating for the extramolecular steric requirements of the
Cl and methyl substituents, respectively.
Conclusion
The present study describes the formation of biaryls 2 and
tetraarylstannanes 3 during the reaction of iodobenzenes 1 with
copper bronze under reaction conditions typically employed for
the Ullmann reaction (Tables 1, 2, and 3). Depending on the
iodobenzene starting material 1, the yields of the biaryl 2 and
the tetraarylstannane 3 are low to good (12-32% and 8-58%,
respectively). Although 119Sn NMR spectroscopy indicated the
formation of organotin compounds, no tetraarylstannane could
(26) Ng, S. W. Acta Crystallogr. 1997, C53, 273-274.
(27) Wharf, I.; Lebuis, A. M. Acta Crystallogr. 2003, E59, m794-m796.

4212 Organometallics, Vol. 25, No. 17, 2006
Table 4. Crystallographic Data and Structure Determination Details for Compounds 3a, 3b, 3j, and 3k
Shaikh et al.
3a
3b
3j
3k
C32H36Sn
539.30
molecular formula (M)
fw
C24H12Cl8Sn
702.63
C24H12Cl8Sn
702.63
C32H28O8Sn
659.23
cryst type
cryst size (mm)
a (Å)
b (Å)
c (Å)
colorless block
0.30 × 0.25 × 0.20
16.7318(2)
16.5324(2)
18.7277(3)
90.00
90.00
90.00
orthorhombic (Pbca)
1.802
colorless block
0.17 × 0.13 × 0.10
9.8948(1)
11.2574(2)
11.7771(2)
97.9527(6)
98.0515(6)
94.0355(7)
triclinic (P1h)
1.822
colorless plate
0.30 × 0.27 × 0.03
8.5731(1)
8.5731(1)
39.7548(7)
90.00
90.00
90.00
tetragonal (I41/a)
1.499
yellow block
0.22 × 0.18 × 0.12
10.9038(1)
12.0001(1)
12.4461(1)
66.4013(4)
69.3562(4)
70.6943(4)
triclinic (P1h)
1.315
R (deg)
â (deg)
γ (deg)
symmetry (space group)
D
expt (g cm-3
)
abs coeff (µ)
1.825
Mo KR
5180.40(12); 8
90.0(2)
1.845
Mo KR
1281.02(3); 2
90.0(2)
0.925
Mo KR
2921.90(7); 4
90.0(2)
0.955
Mo KR
1362.00(2); 2
90.0(2)
radiation
cell volume (Å3); Z
T (K)
θ-range (deg)
no. of indep reflns
no. of params
R1, wR2 (>2σ(I))
R1, wR2 (all data)
1.00-27.48
1.00-27.48
1.00-27.48
1.00-27.48
45 671
29 569
13 588
31 543
299
298
94
306
0.0371; 0.0256
0.0585; 0.0546
0.0339; 0.0258
0.0573; 0.0537
0.0319; 0.0245
0.0551; 0.0517
0.0263, 0.0225
0.0545, 0.0526
Table 5. Selected Bond Lengths (Å) and Bond Angles (deg)
for Compounds 3a, 3b, 3j, and 3ka
509504 (copper 89.59%, tin 10.41%, lube 0.75%, ∼100 mesh,
99.0% metals basis). The copper bronze was activated prior to use
as described below. Tin powder was purchased from Alfa Aesar
and used without any further treatment. Aryl iodides were purchased
from Lancaster (Pelham, NH) or Alfa Aesar and used as received.
A sample of tetraphenyl stannane 3n was also purchased from Alfa
Aesar. 2,3,6-Trichloroiodobenzene was prepared from 2,6-dichlor-
oiodobenzene using a literature procedure.28
parameter
3a
3b
3j
3k
Sn1-C1
2.142(2)
2.138(2)
2.143(2)
2.139(2)
2.138(2) 2.1351(19)
2.137(2) or
2.139(2) 2.1352(19)
2.143(2)
2.1482(17)
2.1433(17)
2.1473(17)
2.1494(17)
Sn1-C9
Sn1-C13
Sn1-C19
C1-Sn1-C9
108.43(8) [â] 104.40(9) 108.26(5) [â] 112.64(7) [θ]
1H, 13C, and 119Sn{1H} NMR analyses were carried out on a
Bruker DRX 400 MHz instrument. 1H and 13C NMR chemical shifts
were determined relative to CDCl3. In the 13C NMR spectra of each
tetraarylstannane Sn-C satellites are observed due to the spin-
spin coupling between 117Sn and 119Sn with 13C nuclei. 119Sn{1H}
NMR chemical shifts were determined using tetramethylstannane
as an internal standard. 19F NMR was carried out on a Bruker AC
300 MHz instrument using CFCl3 as a reference. Elemental analyses
were obtained from Atlantic Microlab Inc. (Norcross, GA). Crude
reaction mixtures and, if possible, purified products were analyzed
using a GC 6890 series gas chromatograph (Agilent Technologies,
Palo Alto, CA) equipped with a J&W Scientific HP-1 capillary
column (Agilent Technologies, Wilmington, DE) and a flame
ionization detector. The following program was employed: initial
temperature 50 °C, initial time 1 min, rate 10 °C/min, final
temperature 250 °C, final time 2 min. Quantitative GC analysis
was carried out by using the following, modified temperature
program: initial temperature 45 °C, initial time 2 min, rate 14°/
min, final temperature 260 °C. Mass spectral analysis (EI-MS) was
carried out at 70 eV on a Voyager GC-MS (ThermoFinigan, San
Jose, CA) instrument in solid probe mode. GC-MS analysis was
carried out on the same instrument in GC-MS mode using a ZB-1
column (15 m, 0.35 mm i.d.) at 50 °C, rate 10 °C/min, final
temperature 250 °C, final time 8 min. Melting points were
determined in capillary tubes using a MelTemp apparatus and are
uncorrected. High-resolution mass was carried out on a Micromass
Autospec by electron impact.
C1-Sn1-C13 113.97(8) [θ] 111.00(8) 111.93(10) [θ] 108.34(6) [â]
C1-Sn1-C19 106.69(8) [â] 106.84(9)
C9-Sn1-C13 107.38(8) [â] 114.30(9)
C9-Sn1-C19 112.48(8) [θ] 110.22(9)
C13-Sn1-C19 108.00(8) [â] 109.73(9)
107.59(7) [â]
109.31(7) [â]
107.64(7) [â]
111.34(7) [θ]
a [â] and [θ]: see text for explanation.
be isolated with 2,3,6-trichloro- or 2,3,5-trichloroiodobenzene
as starting material. The reaction of iodobenzene 1n with tin
powder was investigated to further improve the yield of the
tetraarylstannanes (Table 3). In this model reaction, tetraaryl-
stannane 3n was obtained in good yield (54-64%) at weight
ratios of 1n:tin ) 1:1 or higher. Despite the high reaction
temperature and the long reaction time, this reaction may be an
alternative route to symmetrical tetraarylstannanes with func-
tional groups that are sensitive to the reaction conditions of
other, conventional approaches to tetraarylstannanes.
The present study also investigates structural features of
tetraarylstannanes. Symmetric tetraarylstannanes are generally
expected to display 4h symmetry in the crystalline state. Out of
the four compounds investigated by crystal structure analysis,
only 3j crystallizes in a tetragonal space group (I41/a) and has
4h symmetry, whereas 3a, 3b, and 3k do not crystallize in
tetragonal space groups and do not have true 4h symmetry. These
findings suggest that, for example, ortho chlorine or methyl
substituents (3a and 3k) and/or several large chlorine substit-
uents (3a and 3b) strongly influence molecular point symmetry
in tetraaryl stannanes.
Activation of Copper Bronze.11 The copper bronze (100 gm)
was treated with 1000 mL of a solution of iodine (2%) in acetone
for 5-10 min. The product was then collected in a Buchner funnel,
removed, washed by stirring with 500 mL of concentrated HCl in
acetone (1:1, v/v), and filtered again. The residue was washed with
acetone (3 × 150 mL) and dried under vacuum to yield 82 g of
activated copper bronze.
Experimental Section
General Information. Three different batches of copper bronze
(also referred to as bronze powder) were purchased from com-
mercial sources. Batch A: Alfa Aesar (Ward Hill, MA), lot no.
F26M27 (copper 90.25%, tin 9.75%, lube 0.75%, ∼100 mesh,
99.0% metals basis). Batch B: Alfa Aesar, lot no. K05P10 (copper
89.99%, tin 10.01%, lube 0.75%, ∼100 mesh, 99.0% metals basis).
batch C: Atlantic Equipment Engineers (Bergenfield, NJ), lot no.
General Procedure for Ullmann Coupling. A mixture of aryl
iodide (3 gm) and activated copper bronze (7 gm) in a sealed glass
(28) Waller, S. C.; Mash, E. A. Org. Prep. Proc. Int. 1997, 29, 679-
685.

The Ullmann Coupling Reaction
Organometallics, Vol. 25, No. 17, 2006 4213
ampule flushed with nitrogen was heated in a sand bath at 230 °C.
The reaction mixture was heated for 7 days and allowed to cool to
room temperature. The ampule was opened, and the contents were
extracted with boiling CH2Cl2 (3 × 100 mL). The combined extracts
were dried over anhydrous Na2SO4, filtered, and evaporated to give
a dark brown, viscous liquid. Column chromatography on silica
gel eluted with hexanes gave the corresponding biphenyl derivative
2 and the tetraphenyl stannane 3, respectively.
Tetrakis(4-methoxyphenyl)stannane (3h).29-31 Biphenyl 2h and
stannane 3h were separated by fractional crystallization from
dichloromethane and methanol: yellow solid, mp 127-129 °C (lit.
1
134.8 °C30 and 135.5-136.5 °C31), Rf 0.48 (hexanes). H NMR
(CDCl3): δ 3.83 (s, 12H, -OMe), 6.93-7.00 (m, 4 × H-3/H-5),
7.44-7.60 (m, 4 × H-2/H-6). 13C{1H} NMR (CDCl3): δ 55.02 (4
× OCH3), 114.43 (4 × C-3/C-5, 3J(13C-117Sn) ) 56.2 Hz, 3J(13C-
1
119Sn) ) 53.9 Hz), 128.73 (4 × C-1, J(13C-117Sn) ) 540.9 Hz,
2
1J(13C-119Sn) ) 535.2 Hz), 138.26 (4 × C-2/C-6, J(13C-117Sn)
Tetrakis(2,4-dichlorophenyl)stannane (3a). This compound
was purified by column chromatography over silica gel using
hexanes. Recrystallization from hot methanol gave colorless crystals
suitable for X-ray crystal structure analysis: white solid, mp 217-
) 43.6 Hz, 2J(13C-119Sn) ) 42.1 Hz), 160.41 (4 × C-4, 4J(C-Sn)
) 11.8 Hz). 119Sn{1H} NMR (CDCl3): δ -116.84 (s). IR (cm-1
,
CHCl3): 817, 1025, 1073, 1179, 1244, 1276, 1494, 1584, 2924.
Anal. Calcd for C28H28O4Sn: C 61.46, H 5.15. Found: C 61.31, H
5.15.
1
219 °C, Rf 0.59 (hexanes). H NMR (CDCl3): δ 7.29 (dd, J ) 8
Hz, J ) 2 Hz, 4 × H-5), 7.44 (d, J ) 2.0 Hz, 4 × H-3), 7.49 (d,
Tetrakis(4-fluorophenyl)stannane (3i).29,32,33 This compound
was purified by column chromatography over silica gel using 10-
15% ethyl acetate in hexanes. The compound was further purified
by recrystallization from a mixture of hexane and ethyl acetate (1:
1, v/v) to give colorless crystals with 99% purity (by GC): mp
130-132 °C (lit. 144-145 °C33), Rf 0.48 (12% ethyl acetate in
hexanes). 1H NMR (CDCl3): δ 7.00-7.11 (m, 4 × H-3/H-5), 7.37-
7.54 (m, 4 × H-2/H-6). 13C{1H} NMR (CDCl3): δ 116.2 (4 ×
C-3/C-5, 3J(13C-117Sn) ) 57.7 Hz, 3J(13C-119Sn) ) 55.3 Hz, 2J(C-
J ) 8.0 Hz, 4 × H-6). 13C{1H} NMR (CDCl3): δ 127.51 (4 ×
3
3
C-5, J(13C-117Sn) ) 55.3 Hz, J(13C-119Sn) ) 54.3 Hz), 128.83
3
4
(4 × C-3, J(C-Sn) ) 34.1 Hz), 137.01 (4 × C-4, J(C-Sn) )
12.9 Hz), 137.31 (4 × C-1, J(13C-117Sn) ) 614.5 Hz, J(13C-
1
1
119Sn) ) 587.2 Hz), 139.41 (4 × C-6, 2J(C-Sn) ) 38.8 Hz), 141.99
(4 × C-2, J(C-Sn) ) 13.8 Hz). 119Sn{1H} NMR (CDCl3): δ
2
-143.81 (s). IR (cm-1, KBr): 437, 547, 791, 810, 1020, 1088,
1101, 1355, 1450, 1543, 1561. Anal. Calcd for C24H12Cl8Sn: C
41.02, H 1.72. Found: C 41.24, H 1.64.
4
F) ) 19.5 Hz,), 132.1 (4 × C-1, J(C-F)) 4.1 Hz), 138.6 (4 ×
Tetrakis(3,4-dichlorophenyl)stannane (3b). This compound
was purified by column chromatography over silica gel using
hexanes. Recrystallization from dichloromethane and methanol gave
colorless crystals suitable for X-ray crystal structure analysis: white
C-2/C-6, 2J(13C-117Sn) ) 43.9 Hz, 3J(C-F) ) 7.1 Hz), 164.04 (4
× C-4, 1J(C-F) ) 247.5 Hz). 119Sn{1H} (CDCl3): δ -119.64 (m).
19F (300 MHz, CDCl3): δ -111.14 to -111.02 (m). IR (cm-1
,
CHCl3): 758, 820, 1019, 1066, 1161, 1227, 1385, 1489, 1581, 3023.
MS (EI): m/z 499 (M+), 405 (M+ - Ar), 309 (M+ - 2Ar), 215
(M+ - 3Ar), 120.59 (Sn)+, 95, 75.
1
solid, mp 172-174 °C (methanol), Rf 0.34 (hexanes). H NMR
(CDCl3): δ 7.32 (dd, J ) 7.8 Hz, J ) 1.3 Hz, 4 × H-6), 7.51-
7.57 (m, 4 × H-2/H-5). 13C{1H} NMR (CDCl3): δ 131.30 (4 ×
Tetrakis(4-carbomethoxyphenyl)stannane (3j). This compound
was purified by column chromatography using 5-10% ethyl acetate
in hexanes. Further purification was carried out by recrystallization
from dichloromethane and methanol to yield crystals suitable for
X-ray crystal structure analysis: colorless crystals, mp 178-180
3
3
C-5, J(13C-117Sn) ) 58.7 Hz, J(13C-119Sn) ) 56.3 Hz), 134.08
(4 × C-3, J(13C-117Sn) ) 74.3 Hz, J(13C-119Sn) ) 71.0 Hz),
135.03 (4×C-4, 4J(C-Sn) ) 13.1 Hz), 135.17 (4 × C-1, 1J(13C-
117Sn) ) 541.0 Hz, 1J(13C-119Sn) ) 517.0 Hz), 135.64 (4 × C-2*,
3
3
2J(C-Sn) ) 38.7 Hz), 138.01 (4 × C-6*, J(C-Sn) ) 46.3 Hz).
2
1
°C, Rf 0.09 (45% ethyl acetate in hexanes). H NMR (CDCl3): δ
119Sn{1H} NMR (CDCl3): δ -122.11 (s). IR (cm-1, CHCl3): 670,
812, 1026, 1130, 1357, 1461, 1510. Anal. Calcd for C24H12Cl8Sn:
C 41.02, H 1.72. Found: C 40.86, H 1.52.
3.85 (s, 12H, -CO2CH3), 7.54-7.70 (m, 4 × H-2/H-6), 7.98-
8.07 (m, 4 × H-3/H-5). 13C{1H} NMR (CDCl3): δ 52.11 (4 ×
-CO2CH3), 129.39 (4 × C-2/C-6, 2J(13C-117Sn) ) 53.2 Hz,
2J(13C-119Sn) ) 51.2 Hz), 131.25 (4 × C-4, 4J(C-Sn) ) 11.7 Hz),
Tetrakis(2,4,5-trichlorophenyl)stannane (3c). This compound
was purified by column chromatography over silica gel using
hexanes. Recrystallization from chloroform gave colorless crys-
3
137.03 (4 × C-3/C-5, J(C-Sn) ) 38.7 Hz), 142.74 (4 × C-1,
1J(13C-117Sn) ) 528.2 Hz, 1J(13C-117Sn) ) 504.8 Hz), 166.77 (4
1
tals: white solid, mp 254-256 °C, Rf 0.62 (hexanes). H NMR
× -COOCH3). 119Sn{1H} NMR (CDCl3): δ -133.47 (s). IR (cm-1
,
(CDCl3): δ 7.54 (s, 4 × H-6), 7.57 (s, 4 × H-3). 13C{1H} NMR
CHCl3): 693, 745, 1015, 1131, 1278, 1435, 1592, 1720, 2952. MS
(EI): m/z 660 (M+• + 1, Ar4Sn + H), 523 (M+ - Ar, 100%), 390
(M+ - 2Ar), 253 (M+ - 3Ar), 181, 103, 76. HRMS (EI): m/z
660.0803 (M+•, exact mass calculated for C32H35Sn 660.0806).
Tetrakis(2,4-dimethylphenyl)stannane (3k).34 This compound
was purified by column chromatography using 5% ethyl acetate in
hexanes. Further purification was carried out by recrystallization
from mixture of dichloromethane and methanol to give a pale
yellow solid, mp 228-230 °C, Rf 0.14 (hexanes). 1H NMR
(CDCl3): δ 2.35 (s, 12H, -Mepara), 2.44 (s, 12H, -Meortho), 7.05-
7.25 (m, 4 × H-5/H-6), 7.42-7.62 (m, 4 × H-3). 13C{1H} NMR
(CDCl3): δ 21.31 (4 × Mepara), 24.9 (4 × Meortho), 126.49 (4 ×
(CDCl3): δ 130.59 (4 × C-3, 3J(C-Sn) ) 36.6 Hz), 132.63 (4 ×
4
C-5), 135.86 (4 × C-4, J(C-Sn) ) 12.8 Hz), 138.12 (4 × C-1),
139.02 (4 × C-6, 2J(C-Sn) ) 42.7 Hz), 139.50 (4 × C-2). 119Sn-
{1H} NMR (CDCl3): δ -149.6 (s). IR (cm-1, CHCl3): 770, 1020,
1088, 1140, 1256, 1357, 1450, 1586, 1639, 2349. Anal. Calcd for
C24H8Cl12Sn: C 34.3, H 0.96. Found: C 34.16, H 0.75.
Tetrakis(2,3,6-trichlorophenyl)stannane (3d). This compound
could not be isolated by column chromatography. However, a strong
119Sn{1H} signal was observed in the crude reaction mixture at δ
-187.4 (s).
Tetrakis(2,3,5-trichlorophenyl)stannane (3e). This compound
could not be isolated by column chromatography. However, a strong
119Sn{1H} signal was observed in the crude reaction mixture at δ
-116.9 (s).
3
3
C-5, J(C-Sn) ) 53.0 Hz), 130.51 (4 × C-3, J(C-Sn) ) 43.8
2
Hz), 136.12 (4 × C-6, J(C-Sn) ) 42.5 Hz), 137.44 (4 × C-4,
4J(C-Sn) ) 10 Hz), 138.7 (4 × C-2, 2J(C-Sn) ) 33.3 Hz), 144.93
Tetrakis(2,4,6-trichlorophenyl)stannane (3f). An impure sample
of this compound was obtained after column chromatography over
silica gel using hexanes. Further purification by recrystallization
was not successful due to the poor solubility and, possibly,
decomposition of this compound: white solid, Rf 0.56 (hexanes).
1H NMR (CDCl3): δ 7.29 (s, 4 × H-3/H-5). 13C{1H} NMR
(29) Wharf, I. Inorg. Chim. Acta 1989, 159, 41-48.
(30) Talalaeva, T. V.; Kocheshkov, K. A. Zh. Obshch. Khim. 1942, 12,
403-408.
(31) Ohe, T.; Wakita, T.; Motofusa, S.-i.; Cho, C. S.; Ohe, K.; Uemura,
S. Bull. Chem. Soc. Jpn. 2000, 73, 2149-2155.
(32) Miller, J. M.; Luo, Y.; Wharf, I. J. Organomet. Chem. 1997, 542,
89-98.
(33) Fuchs, O.; Post, H. W. Recl. TraV. Chim. Pays-Bas 1959, 78, 566-
569.
(34) Schneider-Koglin, C.; Mathiasch, B.; Draeger, M. J. Organomet.
Chem. 1994, 469, 25-32.
3
(CDCl3): δ 127.92 (4 × C-3/C-5, J(C-Sn) ) 33.3 Hz), 136.32
(4 × C-4, 4J(C-Sn) ) 10.6 Hz), 142.94 (4 × C-2/C-6, 2J(C-Sn)
) 9.5 Hz), 144.19 (4 × C-1). 119Sn{1H} NMR (CDCl3): δ -167.6
(s).

4214 Organometallics, Vol. 25, No. 17, 2006
Shaikh et al.
(4 × C-1, 1J(13C-117Sn) ) 529.0 Hz, 1J(13C-119Sn) ) 503.7 Hz).
119Sn{1H} NMR (CDCl3): δ -120.32 (s). IR (cm-1, CHCl3): 756,
1227, 1443, 1597, 2919. MS (EI): m/z 539 (M+•, Ar4Sn), 435 (M+
- Ar), 329 (M+ - 2Ar), 225 (M+ - 3Ar), 120 (Sn), 104 (Ar), 77.
HRMS (EI): m/z 539.1771 (M+•, exact mass calculated for C32H36-
Sn 539.1761).
Lorentz/polarization and absorption corrections (SADABS36) were
applied during data reduction. The structures were solved by direct
methods (SHELXS9736) and difference Fourier (SHELXL9736).
Refinement was carried out against F2 by weighted full-matrix least-
squares (SHELXL9736). Hydrogen atoms were found in difference
maps, subsequently placed at calculated positions, and refined using
riding models with isotropic displacement parameters derived from
their carrier atoms. Non-hydrogen atoms were refined with aniso-
tropic displacement parameters. Atomic scattering factors were
taken from the International Tables for Crystallography.37
Optimization of the Synthesis of Tetraphenyl Stannane 3n
Using Tin Powder. A mixture of iodobenzene and tin powder in
a sealed glass ampule flushed with nitrogen was heated in a sand
bath using the reaction conditions shown in Table 3. The reaction
mixture was allowed to cool to room temperature. The ampule was
opened, and the contents were extracted with boiling CHCl3 (3 ×
100 mL) followed by addition of the GC standard (diethylene glycol
dibutyl ether, 100 mg). Then 1 µL of the sample was analyzed by
GC and the peaks of iodobenzene, standard, and product were
integrated. The identities of the compounds present in the solution
were confirmed by comparison with the retention times of the
respective standard. Yields were determined using diethyleneglycol
dibutyl ether as an internal standard (see Supporting Information).
After GC quantification, the combined extracts were dried over
anhydrous Na2SO4, filtered, and evaporated to give a dark brown,
viscous liquid. Tetraphenyl stannane 3n was isolated from the crude
product by recrystallization from CHCl3 and methanol (1:1, v/v)
as colorless crystals, thus allowing the determination of the isolated
yields for selected experiments (see Table 3).
Acknowledgment. Thanks to John Snyder for his help with
recording the 119Sn NMR spectra and to Vic Parcel for the
HRMS analysis (both Department of Chemistry, University of
Iowa). This research was supported by grant ES012475 from
the National Institute of Environmental Health Sciences, NIH.
Supporting Information Available: Tables and text giving
results of the 119Sn{1H} NMR analysis of all crude reaction
mixtures, the procedure for the quantitative GC analysis, NMR data
for biphenyls 2a-f and 2h-j, packing diagrams of 3a, 3b, 3j, and
3k, and a summary of published crystal structures of tetraarylstan-
nanes; crystallographic and structural data of 3a, 3b, 3j, and 3k
are also provided in CIF form. This material is available free of
charge via Internet at http://pubs.acs.org.
OM060456A
Structure Determination and Refinement. X-ray diffraction
data for 3a, 3b, 3j, and 3k were collected at 90 K on a Nonius
kappaCCD diffractometer from either block-shaped (3b, 3j, and
3k) or plate-shaped (3a) crystals, mounted in oil on a glass fiber.
Cell parameters were obtained (DENZO-SMN35) from 10 1° frames
and refined using the full data set (SCALEPACK, DENZO-SMN35).
(35) Otwinowski, Z.; Minor, W. In Methods in Enzymology Volume
276: Macromolecular Crystallography Part A; Carter, C. W., Jr., Swet,
R. M., Eds.; Academic Press: New York, 1997.
(36) Sheldrick, G. M. SHELXL97; University of Gottingen: Germany,
1997.
(37) Hahn, T. International Tables for Crystallography. Vol C: Physical
and Chemical Tables; Kluwer Academic Publishers: Holland, 1992.