Heteroatom Derivatives of Cyclopentadienylaluminum: X-ray Crystal Structure of (η5-C5H5)(2,6-t-Bu-4-Me-C6H 2O)2Al

Article abstract of DOI:10.1021/ic970376m

The cyclopentadienylaluminum aryloxide derivatives bis(cyclopentadienyl)(2,6-di-tert-butyl-4-methylphenoxy)-aluminum (1) and (η ⁵-cyclopentadienyl)bis(2,6-di-tert-butyl-4-methylphenoxy)aluminum (2) have been prepared via the alcoholysis of tricyclopentadienylaluminum with 2,6-di-tert-butyl-4-methylphenol. The X-ray crystal structure of 2 was determined. The molecule crystallizes in the monoclinic space group C2/c with a = 15.4870-(6) A?, b = 11.5404(5) A?, c = 18.3294(7) A?, β = 103.0990(10)°, Z = 4, and V = 3190.7(2) A?³ (R[I > 2σ(I)] = 0.0755, R_w = 0.1489). In the solid state, the cyclopentadienyl ring is bound η⁵ to the aluminum atom. Ab initio calculations on model compounds indicate that the pentahapto geometry of the cyclopentadienyl ring is due to the electron-withdrawing nature of the aryloxide ligands which allows greater π-interaction between the aluminum center and the cyclopentadienyl ligand.

Full text of DOI:10.1021/ic970376m

Inorg. Chem. 1998, 37, 1295-1298
1295
Heteroatom Derivatives of Cyclopentadienylaluminum: X-ray Crystal Structure of
(η5-C5H5)(2,6-t-Bu-4-Me-C6H2O)2Al
James D. Fisher,‡,§ Pamela J. Shapiro,*,‡ P. M. H. Budzelaar,*,† and Richard J. Staples‡,|
Department of Chemistry, University of Idaho, Moscow, Idaho 83844-2343, and Department of
Inorganic Chemistry, University of Nijmegen, P.O. Box 9010, 6500 GL, Nijmegen, The Netherlands
ReceiVed April 4, 1997
The cyclopentadienylaluminum aryloxide derivatives bis(cyclopentadienyl)(2,6-di-tert-butyl-4-methylphenoxy)-
aluminum (1) and (η5-cyclopentadienyl)bis(2,6-di-tert-butyl-4-methylphenoxy)aluminum (2) have been prepared
via the alcoholysis of tricyclopentadienylaluminum with 2,6-di-tert-butyl-4-methylphenol. The X-ray crystal
structure of 2 was determined. The molecule crystallizes in the monoclinic space group C2/c with a ) 15.4870-
(6) Å, b ) 11.5404(5) Å, c ) 18.3294(7) Å, â ) 103.0990(10)°, Z ) 4, and V ) 3190.7(2) Å3 (R[I > 2σ(I)] )
0.0755, Rw ) 0.1489). In the solid state, the cyclopentadienyl ring is bound η5 to the aluminum atom. Ab initio
calculations on model compounds indicate that the pentahapto geometry of the cyclopentadienyl ring is due to
the electron-withdrawing nature of the aryloxide ligands which allows greater π-interaction between the aluminum
center and the cyclopentadienyl ligand.
Introduction
characterize a monomeric alkoxide analogue of Cp2AlMe (Cp
) C5H5), for which we have characterized an unusual bis(η2-
Cyclopentadienyl compounds of the main group elements
exhibit a broad range of ring-coordination geometries, fluxional
behaviors, and reactivities that reflect the degree of covalent
versus ionic character in their bonding.1-3 The structure and
reactivity of cyclopentadienylaluminum compounds are par-
ticularly sensitive to the nature of the other ligands on the
aluminum center, as well as the substitution of the cyclopen-
tadienyl ring.4-6 The bonding in cyclopentadienylaluminum
compounds appears to be delicately balanced between the ionic
bonding characteristic of alkali and alkaline earth metal cyclo-
pentadienide compounds and the covalent bonding exhibited
by cyclopentadienyl compounds of Si, Ge, Sn, and Hg. Thus,
besides the monohapto, σ-type bonding typical of the latter
elements, cyclopentadienylaluminum compounds exhibit η2-,
η3-, and η5-ring coordination geometries in the solid state. Ab
initio calculations indicate that the energy differences between
these geometries are very small (<2 kcal/mol).4 This is
consistent with the highly fluxional nature of these compounds
in solution and our inability to “freeze out” their rearrangments
on the NMR time scale.
Cp) structure in the solid state.7,8 Since Cp2Al(O-i-Pr) is
dimeric,9,10 we needed an alkoxide derivative that would be
sufficiently bulky to prevent dimer formation. The 2,6-di-tert-
butyl-4-methylphenoxy group (BHT) has been a popular ligand
for this purpose.11-14 Described herein are our efforts to prepare
and structurally characterize Cp2Al(BHT) (1). Although we
have been unsuccessful at determining the molecular structure
of this compound, we have obtained the structure of the
monocyclopentadienyl compound CpAl(BHT)2 (2) which is
obtained as a coproduct in the synthesis of Cp2Al(BHT). The
η5-Cp coordination geometry exhibited by CpAl(BHT)2 in the
solid state is interesting, for it has been characterized for only
one other CpAl compound, [CpAlN(2,6-i-Pr2C6H3)]2.15 Ab
initio calculations on model hydroxy, alkoxy, amido, and imido
derivatives of cyclopentadienylaluminum offer an explanation
for why both electron-withdrawing and electron-donating groups
can promote η5 Cp-Al bonding.
Experimental Section
General Procedures and Methods. All manipulations were
performed using a combination of glovebox, high-vacuum, or Schlenk
techniques. All solvents were distilled under nitrogen over sodium
To determine the effects of potentially π-bonding alkoxide
ligands on the coordination geometry between the cyclopenta-
dienyl ring and aluminum, we set out to prepare and structurally
(7) Fisher, J. D.; Wei, M.-Y.; Willett, R.; Shapiro, P. J. Organometallics
1994, 13, 3324-3329.
(8) Fisher, J. D.; Wei, M.-Y.; Willett, R.; Shapiro, P. J. Organometallics
1995, 14, 4030.
(9) Kunicki, A.; Sadowski, R.; Zachara, J. J. Organomet. Chem. 1996,
508, 249-250.
(10) Fisher, J. D.; Golden, J. T.; Shapiro, P. J.; Yap, G. P. A.; Rheingold,
A. L. Main Group Met. Chem. 1996, 19, 521-530.
(11) Shreve, A. P.; Mulhaupt, R.; Fultz, W.; Calabrese, J.; Robbins, W.;
Ittel, S. D. Organometallics 1988, 7, 409-416.
(12) Petrie, M. A.; Olmstead, M. M.; Power, P. P. J. Am. Chem. Soc. 1991,
113, 8704-8708.
(13) Healy, M. D.; Mason, M. R.; Gravelle, P. W.; Bott, S. G.; Barron, A.
R. J. Chem. Soc., Dalton Trans. 1993, 441-454.
(14) Healy, M. D.; Wierda, D. A.; Barron, A. R. Organometallics 1988, 7,
2543-2548.
(15) Fisher, J. D.; Shapiro, P. J.; Yap, G. P. A.; Rheingold, A. L. Inorg.
Chem. 1996, 35, 271-272.
† University of Nijmegen.
‡ University of Idaho.
§ Current address: Department of Chemistry, University of Louisville,
Louisville, KY 40292.
| Current address: Department of Chemistry and Chemical Biology,
Harvard University, Cambridge, MA 02138.
(1) Jutzi, P. AdV. Organomet. Chem. 1986, 26, 217-295.
(2) Jutzi, P. Chem. ReV. 1986, 86, 983-996.
(3) Jutzi, P. J. Organomet. Chem. 1990, 400, 1-17.
(4) Fisher, J. D.; Budzelaar, P. H. M.; Shapiro, P. J.; Staples, R. J.; Yap,
G. P. A.; Rheingold, A. L. Organometallics 1997, 16, 871-879.
(5) Koch, H.-J.; Schulz, S.; Roesky, H. W.; Noltemeyer, M.; Schmidt,
H.-G.; Heine, A.; Herbst-Irmer, R.; Stalke, D.; Sheldrick, G. M. Chem.
Ber. 1992, 125, 1107-1109.
(6) Schonberg, P. R.; Paine, R. T.; Campana, C. F.; Duesler, E. N.
Organometallics 1982, 1, 799-807.
S0020-1669(97)00376-5 CCC: $15.00 © 1998 American Chemical Society
Published on Web 02/24/1998

1296 Inorganic Chemistry, Vol. 37, No. 6, 1998
Fisher et al.
benzophenone ketyl (toluene, methylcyclohexane, petroleum ether). The
dried solvents were stored in line-pots from which they were either
vacuum-transferred from sodium benzophenone ketyl or cannulated
directly. NMR solvents benzene-d6 and chloroform-d were dried over
activated 4-Å molecular sieves. Argon was purified by passage over
oxy tower BASF catalyst (Aldrich) and 4-Å molecular sieves. Alu-
minum trichloride (Aldrich) was sublimed prior to use. 2,6-Di-tert-
butyl-4-methylphenol (Aldrich) was used as received. Cp3Al was
prepared as described previously.7
Data were collected using a Siemens SMART CCD (charge-coupled
device) based diffractometer equipped with a LT-2 low-temperature
apparatus operating at 193 K. Data were measured using ω scans of
0.3°/frame for 30 s, such that a hemisphere was collected. A total of
3232 independent reflections were collected. The first 50 frames were
collected at the end of the data collection to monitor for decay. Cell
parameters were retrieved using SMART software and refined using
SAINT on all reflections. Data reduction was performed using the
SAINT software which corrects for Lorentz polarization and decay.
Absorption corrections were applied using XEMP supplied by Siemens
in their SHEXTL-PC software.
The systematic absences in the diffraction data were uniquely
consistent with C2/c. The structure was solved using direct methods,
completed by subsequent difference Fourier syntheses and refined by
full-matrix least-squares procedures on F2. All non-hydrogen atoms
were refined anisotropically. Hydrogen atoms were treated as idealized
contributions.
1
NMR spectra were recorded on an IBM NR-300 (300.13-MHz H,
74.43-MHz 13C, 78.206-MHz 27Al) and an IBM NR-200 (200.13-MHz
1H, 50.327-MHz 13C, 52.148-MHz 27Al). All chemical shifts are
1
reported in parts per million and referenced to solvent (13C, H) or
Al(OH)3 (27Al, external reference, δ 0 ppm). Elemental analyses were
performed by Desert Analytics (Tucson, AZ).
Preparation of Al(C5H5)2(BHT) (1). To a solution of AlCp3 (1.02
g, 4.6 mmol) in 50 mL of toluene was added 2,6-di-tert-butyl-4-
methylphenol (1.01 g, 4.6 mmol), and the reaction mixture was heated
overnight at 50 °C. All volatiles were removed in vacuo, and the
remaining white, solid residue was transferred to a sublimator.
Sublimation of the solid (90-100 °C, 10-2 Torr) afforded pure (C5H5)2-
(BHT)Al as a waxy solid. (Yield ) 0.516 g, 30%; mp ) 114-117
All software and sources of the scattering factors are contained in
either the SHELXTL (5.1) or the SHELXTL PLUS (4.2) program
libraries (G. Sheldrick, Siemens XRD, Madison, WI).
Results and Discussion
1
°C, uncorrected.) H NMR (C6D6): δ 7.10 (s, 2, aryl-H), 6.03 (s, 10,
C5H5), 2.29 (s, 3, aryl-CH3), 1.47 (s, 18, aryl-C(CH3)3). 13C NMR
(C6D6): δ 152.7, 137.4, 126.1, 125.4 (aryl-C), 109.8 (C5H5), 34.1
(C(CH3)3), 30.8 (C(CH3)3), 20.4 (CH3). 27Al NMR (C7D8): δ -14.
Anal. Calcd for C25H33AlO: C, 79.75; H, 8.83. Found: C, 78.28; H,
8.79.
Synthesis of [Cp2Al(BHT)]x and CpAl(BHT)2. Alcoholysis
of Cp3Al with 1 equiv of 2,6-di-tert-butyl-4-methylphenol
produces a mixture of [Cp2Al(BHT)]x (1), CpAl(BHT)2 (2), and
unreacted Cp3Al. No Al(BHT)3 was detected, although it can
be formed by reacting 3 equiv of the alcohol with Cp3Al under
the same reaction conditions. [Cp2Al(BHT)]x can be isolated
cleanly from the product mixture as a waxy, white solid by
sublimation (90-100 °C, 10-2 Torr). The 27Al NMR chemical
shift of -15 ppm for 1 is the highest field resonance found for
any (C5H5)Al compound to date. We attribute the low frequency
of the 27Al NMR signal for 1 to the shielding effects of an η5-
Cp in the compound since a comparable 27Al NMR chemical
shift of -5 ppm is exhibited by [(η5-C5H5)AlN(2,6-i-Pr2C6H3)]2,15
and an extraordinarily high field signal of δ -114 was observed
for Schno¨ckel’s aluminocene cation, [(η5-C5Me5)2Al]+,16 which
has two pentahapto-coordinated rings. Efforts to obtain suitable
crystals of 1 for an X-ray structure have been unsuccessful.
Furthermore, the compound decomposes over several weeks,
even when stored as a solid at room temperature in a nitrogen-
filled glovebox. The decomposition products have not been
characterized. Compound 2 is more robust, showing only a
slight gray discoloration after months of storage. We did obtain
X-ray quality crystals of 2, the molecular structure of which is
described below. Compound 2 can be obtained more directly
from the reaction of Cp3Al with 2 equiv of BHT. Interestingly,
its 27Al NMR signal is above 0 ppm (though poorly resolved
due to its overlap with the probe signal and its considerable
broadness). The downfield shift of the signal relative to that
of compound 2 is probably due to the deshielding effect of the
additional electron-withdrawing BHT group.
Molecular Structure of CpAl(BHT)2. An ORTEP drawing
of the molecular structure of 2 is shown in Figure 1. Crystal-
lographic data and selected bond lengths and angles are listed
in Table 1 and Table 2, respectively. Particularly noteworthy
is the η5 geometry of the cyclopentadienyl ring. This ring
coordination geometry has been found for only one other
structurally characterized (C5H5)Al compound, [CpAlN(2,6-i-
PrC6H3)]2.15 The cyclopentadienyl ring is disordered and
appears slightly slipped due to a 2-fold rotational axis that passes
through the aluminum atom and the ring, though not quite
through the center of the ring. Indeed, the angle between the
Al-ring centroid and the mean plane of the ring is 88.4°. The
structure of CpAl(BHT)2 is very similar to that of MeAl-
(BHT)2.11 As in the structure of MeAl(BHT)2, the BHT rings
Preparation of Al (C5H5)(BHT)2 (2). To a solution of 0.52 g (2.3
mmol) of AlCp3 in 25 mL of methylcyclohexane was added 2,6-di-
tert-butyl-4-methylphenol (BHT) (1.0 g, 4.5 mmol). The reaction
mixture was heated at 50 °C overnight. The solution was concentrated
to ca. 10 cm3 and cooled to -78 °C to afford (C5H5)(BHT)2Al as a
white precipitate. (Yield ) 0.40 g, 56%; mp ) 162-164 °C,
uncorrected.) 1H NMR (C6D6): δ 7.17 (s, 4, aryl-H), 6.18 (s, 5, C5H5),
2.34 (s, 6, aryl-CH3), 1.53 (s, 36, aryl-C(CH3)3). 13C NMR (CDCl3):
δ 153.5, 138.5, 127.0, 126.0 (aryl-C), 110.4 (C5H5), 35.4 (C(CH3)3),
32.2 (C(CH3)3), 21.0 (CH3). 27Al NMR (C6D6): δ >0 (extremely broad
and overlapping with probe signal). Anal. Calcd for C35H51AlO2: C,
78.84; H, 9.42. Found: C, 79.28; H, 9.90.
Theoretical Calculations. All calculations were of the all-electron
restricted Hartree-Fock17a and MP217b types, using the 3-21G(*) basis
set,19-21 and were carried out using the GAMESS program18 on IBM
SP2, Silicon Graphics Challenge, and O2 workstations. All structures
were fully optimized at the RHF and RMP2 levels; geometry differences
between RHF and RMP2 structures were small, in agreement with
earlier work.4
X-ray Crystallographic Procedures. Block-shaped crystals of
AlCp(BHT)2 were grown from a saturated toluene solution cooled at 0
°C. A suitable crystal was selected and mounted in a thin-walled,
nitrogen-flushed, glass capillary. The unit-cell parameters were
obtained by the least-squares refinement of 120 reflections.
(16) Dohmeier, C.; Schno¨ckel, H.; Robl, C.; Schneider, U.; Ahlrichs, R.
Angew. Chem., Int. Ed. Engl. 1993, 32, 1655-1657.
(17) (a) Roothan, C. C. J. ReV. Mod. Phys. 1951, 23, 69. (b) GAMESS
MP2 implementation: Rice, J. E.; Amos, R. D.; Handy, N. C.; Lee
T. J.; Schaefer, H. F. J. Chem. Phys. 1986, 85, 963. Watts, J. D.;
Dupuis, M. J. Comput. Chem. 1988, 9, 158.
(18) GAMESS-UK is a package of ab initio programs written by M. F.
Guest, J. H. van Lenthe, J. Kendrick, K. Scho¨ffel, P. Sherwood, and
R. J. Harrison, with contributions from D. Amos, R. J. Niessen, V. R.
Saunders, and A. J. Stone. The package is derived from the original
GAMESS due to M. Dupuis, D. Spangler, and J. Wendoloski, NRCC
Software Manual: Guest, M. F.; Fantucci, P.; Harrison, R. J.; Kendrick,
J.; van Lenthe, J. H.; Scho¨ffel, K.; Sherwood, P. GAMESS-UK User’s
Guide and Reference Manual 1, Revision C.0; Computing for Science
(CFS) Ltd.: Daresbury, U.K., 1993.
(19) Binkley, J. S.; Pople, J. A.; Hehre, W. J. J. Am. Chem. Soc. 1980,
102, 939.
(20) Gordon, M. S.; Binkley, J. S.; Pople, J. A.; Hehre, W. J. J. Am. Chem.
Soc. 1982, 104, 2797.
(21) Pietro, W. J.; Francl, M. M.; Hehre, W. J.; DeFrees, D. J.; Pople, J.
A.; Binkley, J. S. J. Am. Chem. Soc. 1982, 104, 5039.

Heteroatom Derivatives of Cyclopentadienylaluminum
Inorganic Chemistry, Vol. 37, No. 6, 1998 1297
Figure 2. PLUTO drawings of calculated structures (RMP2/3-21G-
(*)) for CpAlH2 (A), CpAl(OH)2 (B), CpAl(OCF3)2 (C), and [CpAlNH]2
(D).
Figure 1. Molecular structure of (η5-C5H5)(2,6-t-Bu-4-Me-C6H2O)2-
Al (2).Thermal ellipsoids are shown at 50% probability.
We started with CpAl(OH)2 as the simplest model compound
for our alkoxide species. This compound was found to prefer
an η2-Cp geometry. In fact, the structure calculated for CpAl-
(OH)2 is similar to the structure calculated earlier for CpAlH2
(Figure 2). Bond distances for the two compounds are similar,
and there was no obvious shift to an η1 structure. Curiously,
one of the OH bonds is aligned with the Cp ring plane, whereas
the other points away from the plane; this is the only arrange-
ment of the OH groups that corresponds to a local minimum.
The stability of this conformation may be due to a favorable
arrangement of charges (positive H closest to the negatively
charged Cp ring). CpAl(OMe)2 was found to prefer a similar
η2 geometry, instead of the η5 geometry found for CpAl(BHT).
Since using complete phenoxide groups would make the
calculation rather large, CpAl(OCF3)2 was examined to deter-
mine the effects of a more electron-withdrawing alkoxide on
the Cp-Al structure. CpAl(OCF3)2 was indeed found to prefer
an η5 structure like that of CpAl(BHT)2. Furthermore, both
O-C bonds are nearly parallel to the Cp, just like in the
molecular structures of MeAl(BHT)2 and CpAl(BHT)2. From
these results we conclude that π-donation from O to Al in CpAl-
(OR)2 is not strong enough to affect the Cp-Al bonding, but
that the electron-withrawing character of the aryloxy groups can
result in a shift toward an η5 structure by increasing the amount
of π-bonding in the Al-Cp interaction. Similar ligand effects
on cyclopentadienyl ring geometry have been characterized in
Cp*Al compounds. For example, whereas both [Cp*Al(Me)-
(µ-Cl)]2 and [Cp*Al(i-Bu)(µ-Cl)]2 have η3-bound Cp* rings,6
[Cp*Al(Cl)(µ-Cl)]2 has η5 Cp* rings.5 This shift toward η5-
bonding can be understood on the basis of an MO description.
The aluminum atom always has one p-orbital available for Cp-
Al π-bonding; the other is used in the formation of the two
Al-X σ-bonds. When the Al-X bonds become more ionic
(in the sense Alδ+-Xδ-), these σ-bonds become more localized
on the X atoms, and hence the Al p-orbital originally involved
in the Al-X bonds also becomes available for Cp-Al π-bond-
ing, resulting in a more covalent Al-Cp bond and a trend toward
higher hapticity. In the limit of the purely ionic CpAl2+ species,
one would expect an η5 arrangement with two strong Cp-Al
π-bonds.
Table 1. Summary of Crystallographic Data and Structural
Analysis for AlCp(BHT)2 (2)
formula
fw
a, Å
C35H51AlO2
530.74
15.4870(6)
b, Å
c, Å
11.5404(5)
18.3294(7)
R, deg
â, deg
γ, deg
Z
90
103.0990(10)
90
4
3190.7(2)
C2/c
V, Å3
space group
cryst size
0.2 × 0.2 × 0.4 mm
D
calc, g/cm3
1.105
µ, mm-1
0.091
θ range for data collection
2.22°-27.51°
GOF on F2
1.182
final R indices [I > 2σ(I)]
Ra) 0.0755, Rwb ) 0.1489
a R ) ∑|Fo - Fc|/∑|Fo|. b Rw ) {[∑ω(Fo - Fc)2]/[∑w(Fo)2]}1/2; ω
) 1/[σ2(Fo)2 + (xP)2 + (yP)2], where P ) (Fo + 2Fc2)/3.
2
Table 2. Selected Bond Lengths (Å) and Angles (deg) for
AlCp(BHT)2 (2)
Al-O(1)
Al-C(1)
Al-C(2)
Al-C(3)
Al-C(2′A)
Al-C(3′A)
1.736(2)
2.274(8)
2.259(11)
2.246(8)
2.238(10)
2.278(9)
Al-cnt
1.920(9)
C(1)-C(2)
C(2)-C(3)
C(2′A)-C(3)
C(1)-C(3′A)
1.418(12)
1.403(12)
1.426(11)
1.412(10)
O(1)-Al-O(1A)
101.59(13)
C(4)-O(1)-Al
134.8(2)
are almost coplanar, and the Al-Cp normal is almost perpen-
dicular to the plane defined by the two BHT groups. At 1.736-
(2) Å, the Al-O bonds in 2 are significantly longer than the
1.685(2)- and 1.687(2)-Å Al-O bonds of the methyl analogue.
The O-Al-O bond angle in 2 is also narrower (101.59(13)°
vs 111.9(1)°). The narrower O-Al-O angle in 2 is consistent
with its longer Al-O bonds and might be attributable to the
greater steric bulk of the cyclopentadienyl ring relative to a
methyl group.
Ab Initio Calculations. To understand the effects of
heteroatom substituents on the Al-Cp interaction and to explain
the occurrence of η5-ring coordination geometries in CpAl-
(BHT)2 and [CpAlN(2,6-i-Pr2C6H3)]2, we performed ab initio
calulations at the RHF/3-21G(*) and RMP2/3-21G(*) levels on
some model cyclopentadienylaluminum heteroatom derivatives.
In light of the structural arguments made above for cyclo-
pentadienylaluminum alkoxide species, the ionic character of
the Al-N bonds is unlikely to be an important factor in the

1298 Inorganic Chemistry, Vol. 37, No. 6, 1998
Fisher et al.
Table 3. Mulliken Charges for CpAl Compounds (RHF/3-21G(*)
and RMP2/3-21G(*))
meaningful, the relative values for the charges on the Cp groups
should be meaningful, and they do reflect the trend we describe.
Thus, for aluminum we have the curious situation that, starting
from an η2 arrangement, both increasing and decreasing the
degree of ionicity of the Cp-Al bond result in a shift to higher
hapticity. One difference between the two situations is that the
η5 structure observed with electron-withdrawing X groups
should have a significantly stronger Al-Cp bond. Indeed, the
calculated Al-Cp distance in CpAl(OCF3)2 is only 1.87 Å
compared to 1.91 Å for [CpAlNH]2. That the experimental Al-
Cp distances do not reflect these trends is probably due to the
fact that both structures have disordered Cp rings, and both
compounds are rather crowded. Overall, however, our calcula-
tions on simple model compounds appear to reproduce the trends
observed in the X-ray structures of much more crowded systems,
indicating that one does not need to invoke steric factors to
explain the observed hapticities. Our future efforts will be
directed toward expanding the database of examples of crys-
tallographically characterized cyclopentadienylaluminum com-
pounds in order to further test these theories.
q(Cp)
compound
RHF
RMP2
hapticity
CpAl(OCF3)2
CpAlH2
CpAl(OH)2
CpAl(NH2)2
[CpAlO]2
-0.33
-0.42
-0.48
-0.52
-0.58
-0.64
-0.25
-0.32
-0.39
-0.43
-0.46
-0.52
η5
η2
η2
η2
η5
η5
[CpAlNH]2
pentahapto Cp geometry exhibited by [CpAlN(2,6-i-Pr2C6H3)]2.
Indeed, the calculated structure of CpAl(NH2)2 turned out to
be η2. When [CpAlNH]2 was used as a simple model of the
dimeric iminoalane, however, the calculated structure gave a
symmetrically η5-bound Cp ring and a nearly square Al2N2 core,
just like the structure of [CpAlN(2,6-i-Pr2C6H3)]2. We believe
that the explanation for the η5-Cp geometry in the cyclopen-
tadienyliminoalane structure is effectively the opposite of that
for the structure of Cp2Al(BHT). Because the imido substituents
on each aluminum atom are strong σ- and π-donors, they
stabilize polarization of the Cp-Al bond in the sense of Alδ+
-
Acknowledgment. Support of this work by the National
Science Foundation (Grant No. CHE-9320407) is gratefully
acknowledged.
Cpδ-. In this case, the substituents push the Al 3p acceptor
orbital up in energy so that it is less available for Cp-Al
π-bonding, and the η5-Cp geometry arises from the ionic nature
of the Al-Cp interaction. To illustrate the effects of different
substituents on aluminum on the polarity of the Al-Cp bond,
Table 3 lists the Mulliken charges on Cp, determined at the
RHF and RMP2 levels, for some CpAlX2 model compounds.
Although absolute values of Mulliken charges are not very
Supporting Information Available: An X-ray crystallographic file
in CIF format for complex (η5-C5H5)(2,6-t-Bu-4-Me-C5H2O)Al (2) is
available on the Internet only. Access information is given on any
current masthead page.
IC970376M