Four homologous zirconium 2,2′-biphenyldiyls: Synthesis, structure, and reactivity

Article abstract of DOI:10.1021/om060270b

A homologous family of zirconohydrocarbons bearing the 2,2′-biphenyldiyl ligand (biphe) has been prepared. One of the homologues is a tetraanionic zirconate, two are examples of homoleptic σ-bound zirconohydrocarbons, and three are anionic ate complexes.

Full text of DOI:10.1021/om060270b

4058
Organometallics 2006, 25, 4058-4061
Four Homologous Zirconium 2,2′-Biphenyldiyls: Synthesis,
Structure, and Reactivity
Cameron L. Hilton and Benjamin T. King*
Department of Chemistry, UniVersity of NeVada, Reno, Reno, NeVada 89557
ReceiVed March 27, 2006
Scheme 1. Synthesis and Interconversions
Summary: A homologous family of zirconohydrocarbons bearing
the 2,2′-biphenyldiyl ligand (biphe) has been prepared. One of
the homologues is a tetraanionic zirconate, two are examples
of homoleptic σ-bound zirconohydrocarbons, and three are
anionic “ate” complexes.
of Complexes 1-4a
We report the preparation, characterization, and preliminary
reactivity studies of a homologous family of zirconohydrocar-
bons bearing one, two, three, or four 2,2′-biphenyldiyl (biphe)
ligands. In all cases, the biphe ligands are bidentate, forming
doubly benzannulated zirconacyclopentadienes. One of the ho-
mologues is a tetraanionic zirconate, two are examples of ho-
moleptic σ-bound zirconohydrocarbons, three are anionic “ate”
complexes, and all four are easily prepared on a 0.5 g scale.
Perhaps most importantly, members of this family exhibit
unusual aryl-aryl bond forming reactivity that may provide new
routes to aryl-aryl bond formation.
The organometallic chemistry of zirconium has been studied
for more than 50 years1 and is dominated by zirconocenes,2
which are widely used as catalysts for olefin polymerization3
and in organic synthesis.4 The ability of alkyl or aryl zir-
conocenes to transmetalate stoichiometrically (Cu, Hg, Sn, B,
and Al) or catalytically (Ni and Pd) is synthetically useful.2 In
particular, the formal [4 + 2] cycloaddition of zirconacyclo-
pentadienes via their transmetalated derivatives with acetylenes
is an increasingly popular route to highly substituted benzenes5
and heterocycles.6 Sharp has extended this concept to the formal
[3 + 2] cycloaddition of polycyclic aromatic hydrocarbons with
acetylenes.7
Our initial motivation was to use the hitherto unreported bis-
(cyclopentadienyl)zirconium 2,2′-biphenyldiyl (Cp2Zr(biphe), 1)
in a [4 + 2] route to phenanthrene derivatives, by a path similar
to that of Eisch.8 In 1971, Rausch and co-workers prepared the
titanium analogue of 1, Cp2Ti(biphe), in 12% yield by treating
Cp2TiCl2 with Li2(biphe);9 the crystal structure has been
reported.10
a Legend: (a) 1.0 Li2(biphe), THF, -30 °C, 76%; (b) 4.0 Li2(biphe),
THF, -30 °C, 87%; (c) 4.0 Li2(biphe), Et2O, -30 °C, 56%; (d) 2.0
Li2(biphe), THF, -30 °C; (e) 1.0 Li2(biphe), Et2O, -30 °C, 93%; (f)
excess CpH, f1 67%, f2 86%, f3 10%; (g) 2.0 Li2(biphe), THF, -30
°C, 92%.
instead produced [Li‚(THF)4][CpZr(biphe)2] (2) by loss of a Cp-
ligand (Scheme 1a). The reactivity of Cp2ZrCl2 with Li2(biphe)
continued to surprise us. Treatment of Cp2ZrCl2 with excess
Li2(biphe) in THF gave the homoleptic complex [Li‚(THF)4]2-
[Zr(biphe)3] (3) in 87% yield. An identical reaction, except using
Et2O in place of THF, gave [Li‚Et2O]4[Zr(biphe)4] (4) in 56%
yield.
These reactions involve loss of Cp- ligands. Few examples
of the loss of a single Cp- from a zirconocene have been
reported: transfer to Ir,11 transfer to Mo or W,12 and displace-
ment by alkyllithium reagents.13 Excluding exhaustive proto-
nolysis or oxidation, only one example of the loss of both Cp-
ligands from a zirconocene has been reported: treatment of Cp2-
ZrMe2 with MeLi was reported to afford 2 equiv of CpLi, but
the resulting zirconium species was not characterized.14
Treatment of Cp2ZrCl2 with Li2(biphe), even in substoichio-
metric amounts, did not produce the expected product 1 but
* To whom correspondence should be addressed. E-mail:
king@chem.unr.edu.
(1) Wilkinson, G.; Birmingham, J. M. J. Am. Chem. Soc. 1954, 76, 4281.
(2) Negishi, E.; Huo, S. In Titanium and Zirconium in Organic Synthesis;
Marek, I., Ed.; Wiley-VCH: Weinheim, Germany, 2002; pp 1-49.
(3) Kaminsky, W. J. Chem. Soc., Dalton Trans. 1998, 9, 1413-1418.
(4) Ferna´ndez-Meg´ıa, E. Synlett 1999, 7, 1179.
(5) (a) Takahashi, T.; Tsai, F.-Y.; Li, Y.; Nakajima, K.; Kotora, M. J.
Am. Chem. Soc. 1999, 121, 11093-11100. (b) Takahashi, T.; Hara, R.;
Nishihara, Y.; Kotora, M. J. Am. Chem. Soc. 1996, 118, 5154-5155.
(6) (a) Takahashi, T.; Li, Y. Z.; Ito, T.; Xu, F.; Nakajima, K.; Liu, Y. H.
J. Am. Chem. Soc. 2002, 124, 1144-1145. (b) Fagan, P. J.; Nugent, W. A.
J. Am. Chem. Soc. 1988, 110, 2310-2312. (c) Hay, C.; Fischmeister, C.;
Hissler, M.; Toupet, L.; Re´au, R. Angew. Chem., Int. Ed. 2000, 39, 1812-
1815.
(9) Rausch, M. D.; Klemann, L. P. Chem. Commun. 1971, 354.
(10) (a) Rongjun, Y.; Suafeng, S.; Qiguang, W.; Ying, Z. Lanzhou Daxue
Xuebao, Ziran Kexueban 1991, 27, 59. (b) Barton, R. J.; Cullen, W. R.;
Robertson, B. E.; Luo, Y. ACA Abstr. Papers (Winter) 1985, 13, 54. (c)
Zhu, Y.; Shen, X.-M. Acta Crystallogr., Sect. C 1996, C52, 2422-2425.
(11) Holland, P. L.; Andersen, R. A.; Bergman, R. G. Organometallics
1998, 17, 433-437.
(12) Li, X.; Xi, C.; Leng, L.; Wang, R. Inorg. Chem. Commun. 2003, 6,
211-212.
(7) (a) Sharp, P. R. J. Am. Chem. Soc. 2000, 122, 9880-9881. (b) Sharp,
P. R. J. Organomet. Chem. 2003, 683, 288-294.
(13) Kondakov, D.; Negishi, E. Chem. Commun. 1996, 1996, 963-964.
Shafiq, F. A.; Richardson, D. E.; Boncella, J. M. J. Organomet. Chem.
1998, 555, 1-4.
(8) Eisch, J. J.; Piotrowski, A. M.; Han, K. I.; Kru¨ger, C.; Tsay, Y. H.
Organometallics 1985, 4, 224-231.
10.1021/om060270b CCC: $33.50 © 2006 American Chemical Society
Publication on Web 07/12/2006

Communications
Organometallics, Vol. 25, No. 17, 2006 4059
Table 2. Geometric Measures of 1-4
r(C-Zr)/Å
(av, std dev)
∑ (C(R))/deg
(av, std dev)
r(Cp centroid-Zr)/Å
1
2
3
4
2.27, 0.01
2.34, 0.02
2.36, 0.02
2.49, 0.02
360, 0.0
2.20, 2.21
2.27
359.6, 0.4
359.6, 0.2
358.5, 1.5
2-4, is similar to that between furan17 (1.32 Å) and dibenzo-
furan18 (1.38 Å).
In the crystal structure of 2, the Li‚(THF)4 cation is separated
(7.27 Å) from the formally anionic zirconium atom. A disor-
dered THF molecule fills a void. This zirconate possesses a
four-legged piano-stool geometry. In the crystal structure of 3,
the two Li‚(THF)4 cations are separated (8.02 Å) from the
formally dianionic zirconium atom. The three biphe ligands are
disposed about the Zr atom with a geometry closer to prismatic
than octahedral (θ ) 22.4°, where θ is the angle between the
medians of the triangles formed by the C atoms bound to Zr; θ
) 0° for a prism and θ ) 60° for an octahedron). Similar
structures with bisphosphinine or 2,2′-bipyridyl ligands have
been reported.19 In the crystal structure of 4, a single Li‚Et2O
cation nests in each of the four clefts defined by the biphe
ligands. The Li cations are in van der Waals contact with
roughly six C atoms (average, 2.33 Å, standard deviation 0.13
Å). The C atoms remain essentially planar, as demonstrated by
the sum of the angles around C(R) exceeding 358° in all cases
(cf. sp3 328.5° and sp2 360°). Hence, the Li-C interactions are
ionic, not covalent. The four biphe ligands are disposed about
Zr with a geometry closer to square antiprismatic than square
prismatic (θ ) 32.2°, where θ is the angle of offset between
the squares formed by the C atoms bound to Zr; θ ) 45° for a
square antiprism and θ ) 0° for a square prism). The distance
between the C atoms of the biphe ligands and the Zr atom
increases with the number of biphe ligands (Table 2).
The inability to prepare 1 from the direct reaction of Cp2-
ZrCl2 with Li2(biphe) demonstrates that the addition of the se-
cond biphe and loss of Cp- is faster than the addition of the
first biphe.20 Indeed, the stepwise addition of biphe ligands is
effective. Both complexes 1 and 2 can be converted to 3 by
treatment with Li2(biphe) in THF in >90% yield. Likewise,
complex 3 is converted to 4 by treatment with excess Li2(biphe)
in Et2O in 97% yield. The conversion of 1 to 2 has not been
realized.
Figure 1. Single-crystal X-ray diffraction analysis structures of
1-4. Hydrogens, counterions, and solvate molecules are omitted
for clarity.
Table 1. X-ray Crystallographic Data
1
2
3
4
formula
C22H18Zr
C47H53Li-
O4Zr
C
68H86Li2-
O8Zr
C
64H72Li4-
O4Zr
fw
373.58
788.05
1136.47
monoclinic
C2/c
1024.20
monoclinic
P21/n
12.9268(5)
17.6970(6)
23.7283(8)
90.00
95.9130
90.00
5399.3
cryst syst
space group
a, Å
b, Å
c, Å
R, deg
â, deg
γ, deg
V, Å3
rhombohedral monoclinic
R3h
19.1732(16)
19.1732(16)
24.043(4)
90.00
P21/c
19.2821(17) 28.632(10)
14.6363(13) 13.128(5)
15.0442(13) 21.206(7)
90.00
106.650
90.00
90.00
131.001(8)
90.00
6015(4)
4
90.00
120.00
7653.6
18
4067.7(6)
4
Z
4
temp, K
total no. of
rflns
no. of unique
rflns (Rint
100(2)
33 084
100(2)
69 947
100(2)
38 854
100(2)
69 988
3919
(0.1915)
55
0.0398,
0.0865
14 180
(0.0217)
65.08
0.0339,
0.0937
6914
(0.0660)
55
0.0559,
0.1253
12 393
(0.0411)
55
0.0350,
0.0887
)
2θ max (deg)
R1, wR2
(I > 2σ(I))
Compounds 1-4 can all be prepared on a greater than 500
mg scale as crystalline solids. The simplest to prepare is 3, which
immediately precipitates as a pure, crystalline solid. Treatment
of 3 with Li2(biphe) in Et2O is our preferred method to prepare
4, which crystallizes from the reaction mixture at -30 °C.
The reactivity of 1-4 varies drastically with the number of
biphe ligands. Both the neutral species 1 and monoanion 2 are
indefinitely stable at 25 °C as a solid or in Et2O or THF solution.
The dianion 3, which is poorly soluble, is indefinitely stable at
-35 °C but decomposes at room temperature over 12 h in THF
under an Ar atmosphere. In benzene, a good solvent for 3, the
decomposition occurs within 1 h. The tetraanion 4 is extraor-
dinarily reactivesit decomposes rapidly at room temperature
as a solid or in solution but can be stored for days at -35 °C
under Et2O.
The complex we initially sought, 1, could not be prepared
by the direct reaction of Cp2ZrCl2 with Li2(biphe), even when
using substiochiometric Li2(biphe)sonly 2-4 were obtained.
The elusive complex 1 was ultimately prepared by protonolysis
of biphe ligands from 2-4 using the mild acid CpH with con-
comitant interception of a Zr intermediate by the conjugate base,
Cp-, a method reported by Scott et al.15 Thus, all homologous
zirconium 2,2′-biphenyldiyls, 1-4, have been prepared.
The structures of complexes 1-4 were determined by single-
crystal X-ray diffraction analysis (Figure 1 and Table 1; see
the Supporting Information for figures showing the counterions
and solvent). Complex 1 is similar to other zirconacyclopen-
tadienes, except that the C(R)-C(â) bonds, which are common
to both the five- and six-membered rings, are longer (1.42 Å;
cf. Cp2Zr(C4Ph4), 1.36 Å).16 This difference, also observed in
(17) Fourme, R. Acta Crystallogr., Sect. B 1972, B28, 2984.
(18) Dideberg, O.; Dupont, L.; Andre, J. M. Acta Crystallogr., Sect. B
1972, 28, 1002.
(19) Rosa, P.; Me´zailles, L. R.; Mathey, F.; LeFloch, P. Angew. Chem.,
Int. Ed. 2000, 39, 1823-1826.
(20) Rempala, P.; Kroul´ık, J.; King, B. T. J. Am. Chem. Soc. 2004, 126,
15002-15003.
(14) Razuvaev, G. A.; Vyshinkskaya, L. I.; Vasil’eva, G. A.; Malysheva,
A. V. Dokl. Akad. Nauk. USSR 1978, 243, 1212-1214.
(15) Scott, F.; Cronje, S.; Raubenheimer, H. G. J. Organomet. Chem.
1987, 326, C40-C42.
(16) Jones, B. S.; Petersen, J. L. Organometallics 1985, 4, 966-971.

4060 Organometallics, Vol. 25, No. 17, 2006
Communications
Thermolysis of 2 (70 °C, toluene) provided biphenyl, biphen-
ylene, and quaterphenyl as the only detectable organic products.
Thermolysis of 3 (70 °C, toluene) provided biphenylene as the
main organic product. Thermolysis of 4 (20 °C, Et2O) provided
biphenyl and biphenylene. Decomposition over 10 days at -35
°C provided quaterphenyl as well. The inorganic or organome-
tallic products from these reactions were intractable and were
not characterized.
a solution of 2,2′-dibromobiphenyl (3.120 g, 10.0 mmol) in 100
mL of THF at -80 °C. After the mixture was warmed to -30
°C, Cp2ZrCl2 (0.730 g, 2.5 mmol) was added. The temperature
was maintained for 18 h, and fine yellow crystals (2.47 g, 87%)
precipitated from solution. 1H NMR (500 MHz, C6D6): δ 8.41
(m, J ) 6.5 Hz, 6H, Ar H), 7.75 (m, J ) 7.5 Hz, 6H, Ar H),
7.21 (m, J ) 7 Hz, 6H, Ar H), 7.15 (m, J ) 7 Hz, 6H, Ar H),
3.34 (m, 48H, THF), 1.31 (m, 44H, THF). 13C NMR (125 MHz)
(C6D6): δ 190.9, 157.7, 140.7, 128.9, 125.9, 121.0, 67.8, 25.6.
Tetrakis(biphenyl-2,2′-diyl)zirconate(4-), Li Salt (4). The
reaction sequence was identical with that used to prepare 3,
except that Et2O replaced THF and the reaction was scaled by
0.57. Blocklike crystals (0.825 g, 56%) were formed. 1H NMR
(500 MHz, d8-toluene): δ 7.98 (m, J ) 8 Hz, 16H, Ar H), 7.13
(m, J ) 7.5 Hz, 8H, Ar H), 6.88 (m, J ) 7.5 Hz, 8H, Ar H),
3.38 (q, 7 Hz, 138H, Et2O), 1.12 (t, 7 Hz, 208H, Et2O). 13C
NMR (125 MHz, C6D6): δ 153.7, 144.8, 128.7, 126.4, 126.0,
122.8, 66.1, 15.7.
These compounds are sensitive to acids and oxidants.
Complex 1 was quantitatively converted to zirconocene dichlo-
ride by protonolysis with ethereal HCl. Oxidation of 4 with
CoCl2 in Et2O provides o-quaterphenyl, o-sexiphenyl, quater-
phenylene, and sexiphenylene (GC-MS), as well as traces of
higher oligophenyls (LD-TOF). The products of these ther-
molysis and oxidation reactions suggest that these complexes
may provide new methods to form aryl-aryl bonds.
A new family of zirconium complexes has been prepared and
characterized. All members are metallohydrocarbons. Com-
pounds 3 and 4 are homoleptic. The Zr fragment, assuming a
full positive charge on Li, varies from neutral to a remarkable
tetraanion. In all cases, the Zr atom is electronically unsatur-
ated: in complex 1, it carries 16e, in complexes 2 and 4, it
carries 14e, and in complex 3, it carries 12e. Our original intent,
the use of dibenzozirconacyclopentadienes as substrates for
formal [4 + 2] cycloaddition reactions, was realized. The
reaction of 3 with (PPh3)2NiBr2 followed by diphenylacetylene
gave 9,10-diphenylphenanthrene (47%); when followed by
3-hexyne, it gave 9,10-diethylphenanthrene. The [4 + 2]
cycloaddition chemistry of this family of molecules may,
however, be the least interesting outcome of this work. We
expect the aryl-aryl bond forming reactivity of these complexes
to be especially interesting, especially for the dianion 3 and the
reactive tetraanion 4.
3 from 2. n-Butyllithium (1.62 M in hexane, 0.4 mL, 0.66
mmol) was added to a solution of 2,2′-dibromobiphenyl (0.103
g, 0.33 mmol) in THF (10 mL) at -78 °C. The solution was
warmed to -30 °C and then added to a solution of 2 (0.250 g,
0.33 mmol) in THF (5 mL), also at -30 °C. The resulting yellow
precipitate was collected by filtration and rinsed with THF
1
(0.360 g, 96%). The H NMR spectrum and crystal unit cell
were identical with those of 3 prepared from Cp2ZrCl2.
4 from 3. n-Butyllithium (1.62 M in hexane, 0.26 mL, 0.44
mmol) was added to a stirred solution of dibromobiphenyl
(0.137 g, 0.44 mmol) in Et2O (20 mL) at -40 °C. After 0.5 h,
3 (0.500 g, 0.44 mmol) was added as a solid. The temperature
was maintained at -40 °C for 16 h. The reaction volume was
reduced by half, and crystals formed overnight (0.4191 g, 93%),
which gave 1H and 13C NMR spectra identical with those of 4
prepared from Cp2ZrCl2.
Experimental Section. (Biphenyl-2,2′-diyl)zirconocene (1).
The complex 2 or 3 (∼0.5 g) or 4 (∼30 mg) and neat
cyclopentadiene (ca. 8 mL) were stirred for 2 h. Precipitated
CpLi was removed by filtration. Removal of cyclopentadiene
under reduced pressure followed by two crystallizations from
hexanes gave (biphenyl-2,2′-diyl)zirconocene (from 2, 67%;
from 3, 86%; from 4, 2%). Anal. Calcd (found) for C22H18Zr:
3 from 1. A solution of 2,2′-dilithiobiphenyl (0.0442 g, 0.266
mmol) in THF (10 mL) was added over 5 min to 1 (0.050 g,
0.133 mmol) dissolved in THF (10 mL) at -35 °C. After 1 h,
the reaction mixture was warmed to 25 °C. The precipitate was
collected by filtration and rinsed with hexane and THF to yield
3 (0.139 g, 92%), which gave1H and 13C NMR spectra identical
with those of 3 prepared from Cp2ZrCl2.
1
C, 70.96 (70.68); H, 4.88 (4.99). H NMR (500 MHz, d8-
THF): δ 7.56 (d, J ) 7.0 Hz, 1.9H, Ar H), 6.86 (t, J ) 8.0 Hz,
2.0H, Ar H), 6.72 (t, J ) 7.1 Hz, 1.8H, Ar H), 6.49 (d, J ) 6.1
Hz, 1.6H, Ar H), 6.36 (s, 9.8 H, Cp H), 3.56 (s, solvent), 1.71
(s, solvent). 13C NMR (125 MHz, C6D6): δ 186.6, 148.0, 136.3,
126.7, 125.3, 122.4, 113.6.
Thermolysis of 2 and 3. The complex 2 or 3 (200 mg) was
dissolved in toluene (50 mL) in a Schlenk flask under Ar and
then heated to 70 °C for 3 h. Solvent was removed under
vacuum, and the hexane-soluble fraction was analyzed by GC-
MS. Compound 2 yielded biphenyl, biphenylene, and quater-
phenyl, and 3 yielded primarily biphenylene with trace amounts
of biphenyl and quaterphenyl.
Bis(biphenyl-2,2′-diyl)cyclopentadienylzirconate, Li Salt
(2). n-Butyllithium (1.76 M in hexane, 6.8 mL, 12 mmol) was
added to a solution of 2,2′-dibromobiphenyl (1.807 g, 5.8 mmol)
in THF (20 mL) at -80 °C. After the mixture was warmed to
-30 °C, Cp2ZrCl2 (1.693 g, 5.8 mmol) was added. This
temperature was maintained for 1 h and then the reaction
mixture was warmed to 25 °C. Solvent was removed under
vacuum, and the residue was washed with hexane and then
dissolved in THF (20 mL). The volume of the THF was reduced
by half, and the solution was placed in the freezer. Yellow
crystals (1.736 g, 76%) formed overnight. 1H NMR (500 MHz,
C6D6): δ 7.85 (t, J ) 7.0 Hz, 8H, Ar H), 7.21 (t, J ) 7.5 Hz,
4H, Ar H), 7.12 (t, J ) 7.5 Hz, 4H, Ar H), 6.32 (s, 5H, Cp H),
3.10 (m, 16H, THF), 1.13 (m, 16H, THF). 13C NMR (125 MHz,
C6D6): δ 190.6, 155.6, 139.7, 128.7, 127.5, 125.9, 121.8, 114.0,
68.5, 25.8.
Thermolysis of 4. Complex 4 in Et2O was thermolyzed at
25 °C under Ar for 24 h. The solvent was removed under
vacuum, and the hexane-soluble fraction was analyzed by GC-
MS, which revealed biphenyl and biphenylene.
Hydrochlorolysis of 1. Complex 1 (150 mg, 0.40 mmol) was
dissolved in 1 M HCl in diethyl ether (∼15 mL), and the yellow
solution immediately turned clear. After evaporation the residue
was triturated with hexane to isolate biphenyl (60.3 mg, 100%,
GC-MS). The insoluble material was recrystallized from THF
1
to yield Cp2ZrCl2 (117 mg, 100%, H NMR).
Oxidation of 3. Complex 3 (0.250 g, 0.22 mmol), suspended
in Et2O, was mixed with CoCl2 (0.058 g, 0.22 mmol) for 6 h at
25 °C. The solvent was removed under vacuum and the toluene-
soluble fraction analyzed by GC-MS, which revealed quater-
phenyl, quaterphenylene, hexaphenyl, and hexaphenylene.
Tris(biphenyl-2,2′-diyl)zirconate(2-), Li Salt (3). n-Bu-
tyllithium (2.9 M in hexane, 6.9 mL, 20.0 mmol) was added to

Communications
Organometallics, Vol. 25, No. 17, 2006 4061
LDTOF-MS analysis showed hexaphenyl and hexaphenylene
and traces of octaphenyl, octaphenylene, and decaphenylene.
Reaction of 3 with Diphenylacetylene and Ni(PPh3)2Br2.
Complex 3 (0.125 g, 0.11 mmol) and diphenylacetylene (0.066
g, 0.33 mmol) were suspended in THF (10 mL), and Ni(PPh3)2-
Br2 (0.275 g 0.33 mmol) was added. After 12 h, aqueous HCl
(6 M, 50 mL) was added. Extraction with toluene, chromatog-
raphy, and PPh3 removal with CuCl afforded diphenylphenan-
0226402 for the diffractometer) and the Office of Naval
Research. Acknowledgment is made to the donors of the
Petroleum Research Fund, administered by the American
Chemical Society, for partial support of this research. We thank
Prof. Vincent J. Catalano for assistance with crystallography.
Supporting Information Available: Figures giving 13C and 1H
NMR spectra and thermal ellipsoid plots of 1-4 showing coun-
terions and solvent and CIF files giving crystallographic data. This
material is available free of charge via the Internet at http://
pubs.acs.org.
1
threne (0.052 g, 48%, GC-MS, H NMR).
Acknowledgment. This work was supported by the National
Science Foundation (Grant No. CHE-0449740; Grant No. CHE-
OM060270B