Synthesis and electronic structure of tetrakis(η3- phenylpropargyl)zirconium

Article abstract of DOI:10.1021/om100405n

Tetrakis(η³-phenylpropargyl)zirconium (3) was synthesized from ZrCl₄ and (phenylpropargyl)magnesium bromide. The crystallographically determined structure of 3 exhibits D_2d symmetry, consistent with the ¹H and <

Full text of DOI:10.1021/om100405n

5252 Organometallics 2010, 29, 5252–5256
DOI: 10.1021/om100405n
Synthesis and Electronic Structure of Tetrakis(η3-phenylpropargyl)zirconium†
Dan R. Denomme, Seth M. Dumbris, I. F. Dempsey Hyatt, Khalil A. Abboud,
Ion Ghiviriga, and Lisa McElwee-White*
Department of Chemistry and Center for Catalysis, University of Florida, Gainesville,
Florida 32611-7200
Received May 1, 2010
Tetrakis(η3-phenylpropargyl)zirconium (3) was synthesized from ZrCl4 and (phenylpropargyl)magnesium
bromide. The crystallographically determined structure of 3exhibits D2d symmetry, consistent with the 1Hand
13C NMR spectra. The electronic structure of 3 was analyzed using DFT calculations, and a HOMO-
LUMO gap of 5.3 eV was calculated. The η3 coordination of the propargyl ligands and steric hindrance
around the Zr center prevent the coordination of additional ligands, resulting in a homoleptic complex.
Alkylzirconium compounds have been used as single-
source precursors for chemical vapor deposition (CVD) of
zirconium carbide (ZrC) thin films1-5 as an alternative to
growth from ZrCl4 and methane under a reducing H2 atmo-
sphere at high temperatures (>1500 °C).6-8 The best estab-
lished single-source precursor for the CVD of ZrC is
tetraneopentylzirconium (Np4Zr).1,3,4,9 Successful CVD
from Np4Zr is possible because the lack of β-hydrogen atoms
on the alkyl ligands renders it stable enough for volatilization
and transport in a CVD reactor.
candidates, as they contain no heteroatoms and no β-H
atoms.11-14 We thus undertook a study of zirconium propargyl
complexes as possible precursors for the CVD of ZrC.
To our knowledge, no homoleptic propargyl complex has
been reported. However, propargyl derivatives of zirconocene
have been previously reported in the literature.11,13,15-18 The
18-electron bis(phenylpropargyl)zirconocene complex 1 con-
tains one η1-propargyl ligand, with the second propargyl
coordinated in the η3-bonding mode.13 The signals of the
1
methylene protons of the propargyl ligand in the H NMR
The range of compounds that have been used as single-
source precursors for early-metal carbides is very small.
Ligands that contain heteroatoms are undesirable, as incor-
poration of the additional element into the resulting thin
films can be an issue. Early-transition-metal alkyls with
β-H’s are known to undergo β-H elimination under mild
conditions,10 making them unsuitable for CVD. The result
is that few ligands meet the necessary criteria for use in single-
source ZrC precursors. Propargyl ligands are potential
spectrum are characteristic of the bonding mode and were
assigned at δ 1.9 for the η1 ligand, while the corresponding
protons in the η3 ligand were observed at 3.3 ppm. The
16-electron phenylpropargyl methylzirconocene complex 2
was subsequently described, and a crystal structure confirmed
η3 coordination of the phenylpropargyl ligand.16 The 1H
NMR spectrum of this compound also showed the methylene
protons of the η3 ligand at the expected value of 3.37 ppm.
† Part of the Dietmar Seyferth Festschrift.
*To whom correspondence should be addressed. Tel: þ1-3523928768.
Fax: þ1-3528460296. E-mail: lmwhite@chem.ufl.edu.
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Results and Discussion
In an effort to prepare homoleptic propargylzirconium com-
pounds for use in the CVD of ZrC, we first reacted ZrCl4 with
CH3CtCCH2MgBr. Since we were unable to isolate tetrakis(η3-
methylpropargyl)zirconium from the oligomeric material that
Reiswig, R. D. J. Nucl. Mater. 1976, 62, 221–228.
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A. A.; Xue, Z. L. Organometallics 1999, 18, 2081–2090.
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(11) Wojcicki, A. Inorg. Chem. Commun. 2002, 5, 82–97.
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Organomet. Chem. 1995, 37, 39–130.
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r
pubs.acs.org/Organometallics
Published on Web 09/08/2010
2010 American Chemical Society

Article
Organometallics, Vol. 29, No. 21, 2010 5253
Scheme 1. Synthesis of Complex 3
Table 1. Crystallographic Structural Data for 3
empirical formula
formula wt
temp
wavelength
cryst syst
space group
C36H28Zr
551.80
173(2) K
0.710 73 A
monoclinic
C2/c
˚
unit cell dimens
a
b
˚
20.7551(14) A
˚
8.6203(6) A
˚
17.4685(11) A
c
R
90°
β
116.0410(10)°
90°
2808.1(3) A
γ
3
˚
V
Z
4
calcd density
abs coeff
F(000)
cryst size
θ range for data collection
index ranges
1.305 Mg/m3
0.413 mm-1
1136
Figure 1. 1H and 13C chemical shifts for one of the four equivalent
phenylpropargyl ligands of 3. NMR spectra were obtained at
-60 °C in THF-d8.
0.19 ꢀ 0.11 ꢀ 0.04 mm3
2.18-27.50°
-20 e h e 26, -11 e k e 11,
-22 e l e 15
9334
resulted, we synthesized tetrakis(η3-phenylpropargyl)zirconium
(3) as a model compound for the preparation of more volatile
homoleptic propargylzirconium species. The phenylpropargyl
Grignard reagent was synthesized as described in the literature
(Scheme 1).19,20 The commercially available phenylpropargyl
alcohol was reacted with PBr3, and the resulting bromide was
then converted to the corresponding Grignard reagent. Reaction
with ZrCl4 afforded 3 in 74% crude yield. Single crystals of the
pure material can be obtained by recrystallization, but continued
handling of the complex resulted in decomposition, rendering it
unsuitable for CVD studies.
no. of rflns collected
no. of indep rflns
completeness to θ =27.50°
abs cor
3226 (R(int) = 0.0272)
99.8%
integration
max and min transmissn
refinement method
0.9861 and 0.9103
full-matrix least squares on F2
3226/0/168
no. of data/restraints/params
goodness of fit on F2
final R indices (I >2σ(I))a
R indices (all data)a
largest diff peak and hole
1.065
R1 = 0.0244, wR2 = 0.0676 (2730)
R1 = 0.0312, wR2 = 0.0700
-3
˚
0.300 and -0.346 e.A
P
P
P
P
a R1 = (||Fo|- |Fc||)/ |Fo|. wR2 = [ [w(Fo2 - Fc2)2]/ [w(Fo ) ]]1/2
.
2 2
After recrystallization, the 1H NMR of 3 at room tempera-
ture in toluene-d8 showed only a single aliphatic resonance at
δ 3.21 ppm, consistent withη3-phenylpropargyl ligands. All of
the ligandsweresymmetry-equivalent by NMR, and although
the complex was prepared in ethereal solvents, no other
signals corresponding to additional ligands such as coordi-
nated solvent were observed in the 1H NMR spectrum.
2
P
S = [ [w(Fo - Fc2)2]/(n - p)]1/2. w= 1/[σ2(Fo2) þ (mp)2 þ np];
p = [max(Fo2,0) þ 2Fc2]/3, and m and n are constants.
meta, and para carbons at 127.5, 128.8, and 126.7 ppm,
respectively. The quaternary carbon on the phenyl moiety
(Cipso), at 129.1 ppm, coupled with the meta protons. The ortho
protons coupled with a quaternary carbon at 113.5 ppm, which
was assigned as R to the phenyl. The methylene protons, on the
carbon at 38.7 ppm, coupled with this latter carbon, with
another quaternary carbon at 129.4 ppm, assigned as β to the
phenyl, and, surprisingly, to the ortho carbons on the phenyl
ring. These 13C shifts are consistent with those previously
reported for the (η3-phenylpropargyl)zirconium compounds 1
and 2. The quaternary carbons of 1 were shown to be located at
120.5 and 114.1 ppm with the methylene shift at 55.5 ppm.
Dynamic exchange between η3 and η1 coordination of the
phenylpropargyl ligand of 2 in solution gave shifts of 112.9,
98.8, and 30.7 ppm.15 The methylene protons displayed an
NOE with the ortho protons in the NOESY spectrum. Exam-
ination of the X-ray crystal structure (vide infra) leads to the
conclusion that the NOE is from the methylene group on one
propargyl ligand to the phenyl group of an adjacent ligand.
1
The H spectrum in THF-d8 at -60 °C displays distinct
signals for the aromatic protons of the four equivalent
phenyl rings at 7.35 (t, 7.7 Hz, 8H), 7.23 (t, 7.7 Hz, 4H),
and 6.96 (d, 7.7 Hz, 8H), which were assigned as meta, para,
and ortho, correspondingly, on the basis of their multiplicity
and intensity (Figure 1).
The intensity was referenced to the signal for the four CH2
groups at 3.10 ppm. The 13C chemical shifts were measured
in the gHMBC spectrum, which was acquired with two
different spectral windows in f1 to detect possible folding.
One-bond couplings with the protons identified the ortho,
ꢀ
(19) Andre, V.; Robin, S.; Rousseau, G. Tetrahedron Lett. 2008, 49,
5059–5062.
(20) Lappin, G. R. J. Am. Chem. Soc. 1949, 71, 3966–3968.

5254 Organometallics, Vol. 29, No. 21, 2010
Denomme et al.
with the C1-C2-C3 plane of 2 canted so that the methylene
carbon C1 is farther from the metal center. For both complexes,
an assignment of the phenylpropargyl bonding as intermediate
between the η3-propargyl and allenyl limiting resonance struc-
tures (A and B) is supported by the bond lengths and angles. A
similar assignment of the bonding in Cp*(TBM)Zr(η3-CH2Ct
CCH3) was made on the basis of the crystal structure.17
The bonding of the propargyl ligands to the metal center in
3 was further analyzed by density functional theory calcula-
tions. Geometry optimizations and single-point calculations
were performed using the DFT B3LYP23,24 functional and
the lanl2dz25,26 basis set utilized in the Gaussian 03 program
package.27 Compositions of molecular orbitals were calcu-
lated using the AOMix program.28,29 Molecular orbital
pictures were generated from Gabedit.30 Initial calculations
were performed on 3 itself; however, the presence of the
phenyl rings complicated the interpretation by delocalizing
the molecular orbitals to such an extent that visualization
was difficult. In order to simplify the analysis, calculations
were carried out on the parent tetrapropargylzirconium
complex 4, in which the phenyl rings were replaced with
hydrogen to provide a computational model structure. The
crystallographically determined structure of 3 was used for
the positions of the non-H atoms of 4. Hydrogen atoms were
placed by geometry optimization, and the D2d symmetry of 3
was enforced in 4.
Figure 2. Thermal ellipsoids drawing of the molecular structure
of 3. Thermal ellipsoids are drawn at the 50% probability level.
Hydrogens on the phenyl rings are omitted for clarity.
˚
Table 2. Selected Bond Angles (deg) and Distances (A) for 3
Zr-C1
Zr-C2
Zr-C3
2.4955(2)
2.4043(1)
2.4474(2)
C1-C2
C2-C3
C3-C4
1.3760(2)
1.2490(0)
1.4500(2)
C2-Zr-C12
C2-Zr-C2A
C2-C1-Zr
C3-C2-C1
C3-C2-Zr
98.49(5)
C4-C3-Zr
C1-Zr-C1A
C11-Zr-C1
C2-Zr-C11
C3-Zr-C11
137.87(1)
128.88(8)
131.95(5)
108.31(6)
82.21(5)
96.11(7)
70.08(9)
154.38(2)
77.01(1)
16
˚
Table 3. Selected Bond Distances (A) for 2
C1-C2a
C2-C3
Zr-C1
Zr-C2
Zr-C3
1.344(5)
1.259(4)
2.658(4)
2.438(3)
2.361(3)
a The numbering system of the propargyl ligand is as shown for
compound 3 in Figure 2.
A molecular orbital diagram (see the Supporting In-
formation) was generated from the computational results
A crystallographic structure determination (Table 1) con-
firmed the identification of 3 as tetrakis(η3-phenylpropargyl)-
zirconium. The crystal structure of 3 verified the presence of
only the four propargyl ligands, all displaying η3 coordina-
tion (Figure 2). Complex 3 has an overall D2d symmetry, a
point group previously but rarely observed in other Zr
compounds.21,22 An EAN of 16 electrons for 3 results from
each phenylpropargyl ligand donating four electrons. Selected
bond angles and distances of 3 are shown in Table 2.
(23) Becke, A. D. J. Chem. Phys. 1993, 98, 5648–5652.
(24) Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. B 1988, 37, 785–789.
(25) Vosko, S. H.; Wilk, L.; Nusair, M. Can. J. Phys. 1980, 58, 1200–
1211.
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Phys. Chem. 1994, 98, 11623–11627.
(27) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.;
Robb, M. A.; Cheeseman, J. R.; Montgomery, J. J. A.; Vreven, T.;
Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.;
Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson,
G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.;
Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai,
H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken,
V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev,
O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.;
Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakr-
zewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.;
Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.;
Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov,
B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.;
Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.;
Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.;
Gonzalez, C.; Pople, J. A. Gaussian 03, Revision B.04; Gaussian, Inc.,
Wallingford, CT, 2004
Although structural data have been reported for several
η3-propargyl complexes,16 the (phenylpropargyl)methyl-
zirconocene complex 2 (Table 3) is perhaps the best model
for the geometry of the propargyl ligands of 3. The phenylpro-
pargyl ligands of complexes 2 and 3 exhibit nearly identical
C-C-C bond angles of 154.4(3) and 154.38(2)°, respectively,
indicating similar bonding of the propargyl moiety to the Zr
center. The Zr-C2 bond lengths are approximately the same in
the two structures. However, the three Zr-C bond distances in
˚
3 are roughly the same length, differing only by a net 0.09 A
˚
overall, whereas those in 2 differ by a much larger value, 0.29 A,
(28) Gorelsky, S. I. AOMix: Program for Molecular Orbital Analysis;
University of Ottawa, Ottawa, Canada, 2009.
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M. B. J. Chem. Soc., Dalton Trans. 1984, 2789–2794.
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Inorg. Chem. 2005, 44, 468–470.
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Article
Organometallics, Vol. 29, No. 21, 2010 5255
Figure 4. LUMO (top left), LUMOþ1 (top right), and degen-
erate LUMOþ2 (bottom) of propargyl complex 4.
Figure 3. Degenerate HOMO (top) and HOMO-2 (bottom) of
the propargyl complex 4.
C3 π orbital, which has its largest coefficient on C2. The
largest contributions to the LUMO from Zr are s (14.2%),
and dz2 (11.9%). The LUMOþ1 is mainly comprised of the
nonbonding dxy orbital of Zr (86.7%). The LUMOþ2 con-
sists of two degenerate orbitals, one composed primarily of
Zr dxz (25.3%) and the other dyz (25.3%).
The AO composition of the LUMO provides insight into
why this 16-electron early-transition-metal complex does not
have an open coordination site for addition of another
ligand. The section of the LUMO derived from metal AO’s
is sterically blocked by the CH2 groups of the four propargyl
ligands. Although complex 3 was recrystallized from THF
with vapor diffusion of pentanes, coordinated THF is not
detected in either the NMR or the crystal structure.
for 4 and showed a calculated HOMO-LUMO gap of 5.2
eV. This substantial splitting of the frontier orbitals is
consistent with the lack of reactivity of 3 with other species
in the reaction mixtures during synthesis. While other D2d
group 4 metal complexes have been reported in the litera-
ture,31-33 examples of computational results are rare. An
electronically similar D2d-symmetric bis(pentalene)titanium
complex had a calculated HOMO-LUMO gap of 1.93 eV,
far smaller than that of 3.31 Tetrakis(η3-allyl)zirconium is
also a known compound, described as a bright red solid
which decomposes at -20 °C.34,35 The red color and lability
of tetrakis(η3-allyl)zirconium suggest that its HOMO-LUMO
gap must also be smaller than that of 3, which is a colorless solid
that is stable for moderate periods of time at room temperature.
The high-lying occupied orbitals of 4 are depicted in
Figure 3. The HOMO is largely comprised of two degenerate
orbitals containing the p orbitals of C1 and C3 of the
propargyl ligands. The symmetry dictates that one HOMO
orbital includes the dyz orbital of Zr (20.7%) and the p
orbitals in the yz plane of the propargyl ligands, while the
other HOMO orbital utilizes the dxz orbital of Zr (20.7%)
and the xz plane of the p orbitals on the propargyl ligand.
The HOMO-2 is comprised mainly of dz2 on Zr (21.3%) and
the xy plane of the p orbitals on the ligand. Metal-propargyl
bonding dominated by interactions of metal d orbitals with
propargyl MO’s localized on C1 and C3 is consistent with the
calculated MO diagrams for bonding in [(η3-CH2CtCPh)-
Pt(PPh3)2].36
Conclusion
The synthesis of 3 yields, to the best of our knowledge, the
first example of a homoleptic propargyl complex and has
1
been shown by H NMR spectroscopy and X-ray crystal-
lography to have all four phenylpropargyl ligands coordi-
nated to the Zr center in an η3 mode, resulting in a complex of
D2d symmetry. The π bonding in 3 was analyzed by DFT
calculations on the model compound 4. The π bonding from
the four symmetry-equivalent η3-propargyl ligands and
HOMO-LUMO gap of 5.3 eV is consistent with the stability
of the molecule and the lack of reactivity toward coordinat-
ing solvents.
The LUMO of 4 (Figure 4) has the strongest p orbital
contributions from C2 in the propargyl groups. This is
consistent with its derivation from the antibonding C1-C2-
Experimental Section
General Procedures. All chemicals were purchased in reagent
grade purity and used with no further purification unless other-
wise noted. All manipulations were carried out using standard
Schlenk and glovebox techniques under an inert atmosphere of
argon or nitrogen. All solvents, unless otherwise noted, were
purchased from Fisher and passed through an M. Braun MB-SP
solvent purification system or were distilled from sodium/ben-
zophenone prior to use. 1H and 13C NMR spectra were obtained
on Varian Gemini 300, VXR 300, Mercury 300, and Inova 500
spectrometers. Infrared spectra were measured on a Perkin-
Elmer 1600 FT-IR.
(31) Costuas, K.; Saillard, J.-Y. Chem. Commun. 1998, 18, 2047–
2048.
(32) Gleiter, R.; Bethke, S.; Okubo, J.; Jonas, K. Organometallics
2001, 20, 4274–4278.
(33) Bendjaballah, S.; Kahlal, S.; Costuas, K.; Bevillon, E.; Saillard,
ꢀ
J.-Y. Chem. Eur. J. 2006, 12, 2048–2065.
(34) Becconsall, J. K.; O’Brien, S. Chem. Commun. 1966, 302–303.
(35) Jennings, J. R. J. Mol. Catal. 1990, 58, 95–105.
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1999, 18, 837–842.

5256 Organometallics, Vol. 29, No. 21, 2010
Denomme et al.
Phenylpropargyl Bromide. A 50 mL Schlenk flask containing
5.0 mL of ether, 4.8 g (3.7 mmol) of phenylpropargyl alcohol,
and 1.0 g of pyridine was cooled to 0 °C, and 5.0 g (18 mmol) of
phosphorus tribromide was added dropwise over a 45 min
period with strong stirring under nitrogen in accordance with
the literature procedure.19 The resulting mixture was added to
25 mL of ice to quench the excess PBr3 and extracted with ether
(3 ꢀ 25 mL). The organics were then washed with NaHCO3,
dried over MgSO4, and filtered, and the ether was then removed
by reduced pressure. Yield: 6.0 g, 83%. 1H NMR (C6D6): δ 4.1
(s, 2H), 7.4 (m, 5H). 13C NMR (CDCl3): δ 15.3, 84.2, 86.6, 121.9,
128.1, 128.7, 131.7.
(Phenylpropargyl)magnesium Bromide. An addition funnel
was charged with 12.0 g (61.6 mmol) of phenylpropargyl
bromide and 30 mL of ether, and the mixture was added
dropwise to a three-neck flask cooled to 0 °C containing 1.80 g
(75.0 mmol) of activated Mg turnings with a few crystals of
HgCl2 in ether over a 4 h period, in accordance with the
literature preparation.20 After the addition, the reaction mixture
was refluxed for 1 h. The resulting mixture was filtered through a
1 cm pad of Celite (previously dried and evacuated) to yield a dark
yellow solution. Yield: 30 mL of a 1.85 M solution of the Grignard
reagent, 90.1%. 1H NMR (C6D6): δ 2.11 (s, 2H), 6.8 (m, 5H).
Tetrakis(η3-phenylpropargyl)zirconium (3). An addition fun-
nel was charged with 20.0 mL of 1.85 M phenylpropargylmag-
nesium bromide (37.0 mmol) and added dropwise into a three-
neck flask containing 2.16 g (9.25 mmol) of ZrCl4 slurried in
100 mL of ether over a 1 h period and stirred overnight at room
temperature. Volatiles were then removed via reduced pressure
to afford a brown solid. The solid was extracted with 150 mL of
toluene and filtered through a fine glass frit. The filtrate was
concentrated to afford a solution of 3, from which the com-
pound was then precipitated by the addition of hexanes. The
resulting suspension was filtered through a fine glass frit to
collect the solid precipitate of 3. Yield: 3.44 g, 74%. The product
was tan in color. Single crystals could be obtained by repeated
vapor diffusion recrystallization using THF and pentanes until a
colorless to white solid remained. 1H NMR (THF-d8, -60 °C): δ
3.10 (s, 8H), 6.96 (d, 7.7 Hz, 8H), 7.23 (t, 7.7 Hz, 4H), 7.35 (t, 7.7
Hz, 8H). 13C NMR (THF-d8, -60 °C): δ 38.7, 113.5, 126.7,
127.5, 128.8, 129.1, 129.4.
Crystallographic Structure Determination of 3. X-ray experi-
mental data for 3 were collected at 173 K on a Siemens SMART
PLATFORM equipped with a CCD area detector and a gra-
phite monochromator utilizing Mo KR radiation (λ = 0.710 73
˚
A). Cell parameters were refined using up to 8192 reflections. A
full sphere of data (1850 frames) was collected using the ω-scan
method (0.3° frame width). The first 50 frames were remeasured
at the end of data collection to monitor instrument and crystal
stability (maximum correction on I was <1%). Absorption
corrections by integration were applied on the basis of measured
indexed crystal faces.
The structure was solved by direct methods in SHELXTL637
and refined using full-matrix least squares. The non-H atoms
were treated anisotropically, whereas the hydrogen atoms were
calculated in ideal positions and were riding on their respective
carbon atoms. The complexes are located on 2-fold rotation
axes; thus, a half-complex occupies the asymmetric unit. A total
of 168 parameters were refined in the final cycle of refinement
using 2730 reflections with I>2σ(I) to yield R1 and wR2 values
of 2.44%and 6.76%, respectively. Refinement was done using F2.
Acknowledgment. We thank the Office of Naval Re-
search for partial support of this work under the project
number N00014-03-1-0605. Computational resources and
support were provided by the University of Florida High-
Performance Computing Center. K.A.A. acknowledges the
National Science Foundation and the University of Florida
for funding the purchase of the X-ray equipment.
Supporting Information Available: Tables and a CIF file
giving crystallographic data for 3 (including atomic coordinates,
bond lengths and angles, and anisotropic displacement para-
meters) and figures, text, and tables giving a calculated molec-
ular orbital diagram for 4 and 1H and gHMBC NMR spectra of
3. This material is available free of charge via the Internet at
http://pubs.acs.org.
(37) SHELXTL6; Bruker-AXS, Madison, WI, 2000.