η1 versus η5 bonding modes in Cp*Al(I) adducts of 9-borafluorenes

Article abstract of DOI:10.1021/om0209935

The reactivity of highly Lewis acidic perfluorinated borafluorenes C₁₂F₈BR (R = C₆F₅, 1a; CH₃, 1b) and the nonfluorinated 9-phenyl-9-borafluorene (2) toward [Cp*Al]₄ was investigated. The reaction of 1 with [Cp*Al]₄ leads to the formation of thermally robust η¹ Lewis acid-base adducts 3a,b as the thermodynamically favored products. Use of the less Lewis acidic 2 does not alter the mode of reactivity, with the η¹ Lewis acid-base 4 formed preferentially. Reduction of 2 to the 9-boratafluorene 2·Li₂(THF)_n is readily accomplished in THF solution. However, reaction of 2·Li₂(THF)_n with [Cp*AlCl₂]₂ or Cp*AlCl₂(THF), 5, affords aluminum metal, 2·THF, and Cp*H as the main identifiable products. Compounds 3a, 3b, 4, and 5 were fully characterized including their X-ray structures. A DFT computational study was conducted to probe the reason for the strong preference for η¹ bonding, which essentially stems from the localization of aromaticity in the flanking phenyl rings in the 9-borafluorene ring system.

Full text of DOI:10.1021/om0209935

1266
Organometallics 2003, 22, 1266-1274
η1 ver su s η5 Bon d in g Mod es in Cp *Al(I) Ad d u cts of
9-Bor a flu or en es
Patricio E. Romero,† Warren E. Piers,*,† Stephen A. Decker,‡ Dan Chau,‡
Tom K. Woo,*,‡ and Masood Parvez†
Department of Chemistry, University of Calgary, 2500 University Drive, N.W.,
Calgary, Alberta, Canada, T2N 1N4, and Department of Chemistry,
University of Western Ontario, London, Ontario, Canada, N6A 5B7
Received December 10, 2002
The reactivity of highly Lewis acidic perfluorinated borafluorenes C12F8BR (R ) C6F5, 1a ;
CH3, 1b) and the nonfluorinated 9-phenyl-9-borafluorene (2) toward [Cp*Al]4 was investi-
gated. The reaction of 1 with [Cp*Al]4 leads to the formation of thermally robust η1 Lewis
acid-base adducts 3a ,b as the thermodynamically favored products. Use of the less Lewis
acidic 2 does not alter the mode of reactivity, with the η1 Lewis acid-base 4 formed
preferentially. Reduction of 2 to the 9-boratafluorene 2‚Li2(THF)n is readily accomplished
in THF solution. However, reaction of 2‚Li2(THF)n with [Cp*AlCl2]2 or Cp*AlCl2(THF), 5,
affords aluminum metal, 2‚THF, and Cp*H as the main identifiable products. Compounds
3a , 3b, 4, and 5 were fully characterized including their X-ray structures. A DFT
computational study was conducted to probe the reason for the strong preference for η1
bonding, which essentially stems from the localization of aromaticity in the flanking phenyl
rings in the 9-borafluorene ring system.
Ch a r t 1
In tr od u ction
Boron-containing rings, such as borabenzenes,1 bo-
ratabenzenes,2 and boroles,3 are a versatile class of
ligands for transition metals since they are isoelectronic
to the ubiquitous cyclopentadienyl (Cp) ancillary. Unlike
Cp ligands, however, the reactivity of complexes bearing
the boron-based donors, particularly boratabenzenes
and borollides, can be finely adjusted via variation of
the substituent on boron. Thus, while providing steric
coverage similar to the Cp anion, the range of electronic
* Corresponding author: S. Robert Blair Professor of Chemistry
(2000-2005); E.W.R. Steacie Memorial Fellow (2000-2002). Phone:
(403) 220-5746. Fax: (403) 289-9488. E-mail: wpiers@ucalgary.ca.
† University of Calgary.
‡ University of Western Ontario.
(1) (a) No¨th, H.; Schmidt M. Angew. Chem., Int. Ed. Engl. 1996,
35, 292. (b) Hoic, D. A.; Davis W. M., Fu G. C. J . Am. Chem. Soc. 1995,
117, 8480. (c) Boese, R.; Finke, N.; Henkelmann J .; Maier G.; Paetzold,
P.; Reisenahuer, H. P.; Schmid, G. Chem. Ber. 1985, 118, 1644. (d)
Hoic, D. A.; Wolf, J . R.; Davis, W. M.; Fu, G. C. Organometallics 1996,
15, 1315.
(2) (a) Service, R. F. Science 1996, 271, 1363. (b) Bazan, G. C.;
Rodriguez, G.; Ashe, A. J ., III; Al-Ahmad, S.; Muller, C. J . Am. Chem.
Soc. 1996, 118, 2291. (c) Herberich, G. E.; Ohst H. Adv. Organomet.
Chem. 1986, 25, 199.
(3) (a) Herberich, G. E. Boron Rings Ligated to Metals. In Compre-
hensive Organometallic Chemistry II; Stone, E. W., Wilkinson, F. G.
A., Eds.; Pergamon Press: Oxford, 1995; p 197. (b) Eisch, J . J .; Galle,
J . E.; Kozima S. J . Am. Chem. Soc. 1986, 108, 379. (c) Pindado, G.;
Lancaster, S. J .; Thornton-Pett, M.; Bochmann, M. J . Am. Chem. Soc.
1998, 120, 6816. (d) Herberich, G. E.; Eckenrath, H. J .; Englert, U.
Organometallics 1997, 16, 4800. (e) Herberich, G. E.; Eckenrath, H.
J .; Englert, U. Organometallics 1997, 16, 4292. (f) Braunstein, P.;
Herberich, G. E.; Neuschutz, M.; Schmidt, M. U.; Englert, U.; Lecante,
P.; Mosset, A. Organometallics 1998, 17, 2177. (g) Herberich, G. E.;
Eckenrath, H. J .; Englert, U. Organometallics 1998, 17, 519. (h)
Woodman, T. J .; Thornton-Pett, M.; Hughes, D. L.; Bochmann, M.
Organometallics 2001, 20, 4080. (i) Woodman, T. J .; Thornton-Pett M.;
Bochmann, M. Chem. Commun. 2001, 329. (j) Braunstein, P.; Englert,
U.; Herberich, G. E.; Neuschutz, M.; Schmidt, M. U. J . Chem. Soc.,
Dalton Trans. 1999, 2807. (k) Sperry, C. K.; Cotter, W. D.; Lee, R. A.;
Lachicotte, R. J .; Bazan, G. C. J . Am. Chem. Soc. 1998, 120, 7791.
variation possible is significantly wider. This feature has
been exploited most elegantly within the context of
olefin polymerization catalysis by metallocene mimics
containing these ligands.3k,4
While boron-containing heterocycles have found util-
ity as Cp- replacements, such analogues of benzannu-
lated Cp donors, like the indenyl ligand,5 are less
common. Similarly, the dianions of 9-borafluorenes,
which would be dianionic replacements for the fluorenyl
donor,6 are a relatively unexplored class of boracyclic
ligands (Chart 1). These benzannulated structural rela-
tives of boroles have been known since an early report
by Ko¨ster and Benedict in 1963,7 but reports of their
(4) (a) Herberich, G. E.; Ohst, H. Naturforsch. 1983, B38, 1388. (b)
Quan, R. W.; Bazan, G. C.; Schaefer, W. P.; Bercaw, J . E.; Kiely, A. F.
J . Am. Chem. Soc. 1994, 116, 4489, and references therein.
(5) Ashe, A. J ., III; Yang, H.; Fang, X.; Kampf, J . W. Organometallics
2002, 21, 4578.
(6) Alt, H. G.; Samuel, E. Chem. Soc. Rev. 1998, 27, 323.
10.1021/om0209935 CCC: $25.00 © 2003 American Chemical Society
Publication on Web 02/19/2003

Cp*Al(I) Adducts of 9-Borafluorenes
Organometallics, Vol. 22, No. 6, 2003 1267
Sch em e 1
use as dianionic ligands are absent from the literature.
Like boroles, 9-borafluorenes are stronger Lewis acids8
than acyclic analogues and are readily reduced to their
corresponding dianions by treatment with an appropri-
ate reducing agent in polar solvents. Power9 and Wehm-
schulte10 have recently reported examples of this pro-
cess, and the dianionic boratafluorene species have been
structurally characterized. Despite the availability of
these synthons, attempts to ligate to a transition metal
have been characterized by facile reduction processes,
which resulted in recovery of the neutral 9-borafluorene
precursor.
We have recently reported the synthesis and charac-
terization of the novel, highly fluorinated 9-borafluo-
renes 1 (R ) C6F5, 1a ; R ) CH3, 1b), powerful Lewis
acids that serve as effective activators for group 4
metallocene polymerization catalyst precursors.11 In
addition to their use as Lewis acid activators, we were
interested in probing their capacity for use as dianionic,
but potentially electron-deficient ligands. Unlike non-
fluorinated 9-borafluorenes, direct chemical reduction
of 1a or 1b proved experimentally challenging and did
not meet with success in providing useful reagents for
metathesis with appropriate metal chlorides.12 To ex-
plore the coordination chemistry of compounds 1, we
thus chose an approach used principally by Herberich
et al. for preparing borollide complex, which involves
reaction of neutral compounds 1 with an appropriate
low-valent organometallic fragment.3a In this process,
the interaction of the electron-rich metal center with
the empty 2pz orbital localized on boron induces the
spontaneous aromatization of the borole ring with the
simultaneous oxidation of the metal.13
herein, this expectation proved erroneous, not only for
the fluorinated 9-borafluorenes 1, but also for the
related nonfluorinated species C12H8B(C6H5), 2.
Resu lts a n d Discu ssion
Syn th esis of 9-Bor a flu or en es. The syntheses of the
fluorinated compounds 1 were carried out as described
previously.11 The original procedure for preparing the
nonfluorinated counterpart of 1a , 9-phenyl-9-borafluo-
rene 2, involved the high-temperature thermolysis of a
diphenyl-2-biphenyl borane and gave only moderate
yields of 2. Thus, we adapted the synthetic route used
to prepare 1a to accommodate 2 as shown in Scheme 1.
Addition of 2 equiv of n-BuLi to C12H8Br2 and subse-
quent quenching with Me2SnCl2 afforded, after removal
of the solvent, pure C12H8SnMe2 in virtually quantita-
tive yield (98%). The 1H NMR spectrum of this com-
pound showed an upfield singlet signal for the Sn-Me
groups centered at δ 0.27 ppm with the diagnostic 117Sn/
119Sn satellites (2J Sn-H ) 57.8 Hz, 60.2 Hz). The aro-
matic zone was composed of complex multiplets as-
signed to the hydrogen atoms of the biphenyl framework,
which gave satisfactory integration. The 119Sn{1H} NMR
spectrum showed a singlet at δ -34.1 ppm.
The low-valent main group metallocene [Cp*Al]4
(Cp* ) η5-C5Me5) originally reported by Schno¨ckel and
co-workers14 behaves as a Lewis base toward B(C6F5)3,
leading to the formation of Cp*Al‚B(C6F5)3.15 Given the
tendency of Cp*Al(I) to form Al(III) derivatives,16 we
surmised that in reaction with 9-borafluorenes 1, such
an η1 adduct might convert to an η5 compound isoelec-
tronic with the aluminocenium cation.17 As described
(7) Koster, R.; Benedikt, G. Angew. Chem., Int. Ed. Engl. 1963, 2,
323.
(8) Eisch, J . J .; Galle, J . E.; Kozima, S. J . Am. Chem. Soc. 1986,
108, 379.
The stannole C12H8SnMe2 reacted with C6H5BCl2 at
-78 °C in toluene, generating a bright yellow solution
containing 2 and Me2SnCl2 as the sole products as
judged by NMR spectroscopy. Removal of Me2SnCl2 by
sublimation gave 2, which was obtained in almost
quantitative yield. Analytical and spectral data for this
compound were as expected, and the 11B{1H} NMR
spectrum gave a single broad signal centered at δ 64.5
ppm.
Although dianionic derivatives of fluorinated 9-bo-
rafluorenes 1 are not accessible via direct chemical
reduction from their neutral precursors, the less Lewis
acidic 2 is smoothly reduced in THF solution, using an
excess of lithium powder (Scheme 1). After filtering off
the excess Li and removing the THF, the product was
extracted into toluene and precipitated using pentane.
9-Boratafluorene 2Li2‚(THF)n is isolated as THF solvate
with a variable number of molecules depending on the
experimental conditions. The 11B{1H} NMR of 2Li2‚
(THF)n showed a sharp signal at δ 6.2 ppm, which is in
(9) Grigsby, W. J .; Power, P. P. J . Am. Chem. Soc. 1996, 118, 7981.
(10) (a) Wehmschulte, R. J .; Khan, M. A.; Twamley, B.; Schiemenz,
B. Organometallics 2001, 20, 844. (b) Wehmschulte, R. J .; Diaz, A. A.;
Khan, M. A. Organometallics 2003, 22, 83.
(11) Chase, P. A.; Piers, W. E.; Patrick B. O. J . Am. Chem. Soc. 2000,
122, 12911.
(12) (a) Romero, P.; Chase, P. A.; Piers, W. E.; Parvez, M. Manuscript
in preparation. Similar observations in the reduction of B(C6F5)3 vs
B(C6H5)3 have been encountered: (b) Harlan, C. J .; Hascall, T.; Fujita,
E. Norton, J . R. J . Am. Chem. Soc. 1999, 121, 7274. (c) Kwaan, R. J .;
Harlan, J .; Norton, J . R. Organometallics 2001, 20, 3818.
(13) Formation of stable diene-like complexes has been observed,
for instance, in systems where conjugation between the exocyclic
substituent and boron exists. See for example: Herberich, G. E.;
Hessner, B.; Ohst, H.; Raap, I. A. J . Organomet. Chem. 1988, 348, 305.
(14) (a) Dohmeier, C.; Robl, C.; Schno¨ckel, H. Angew. Chem., Int.
Ed. Engl. 1991, 30, 564. (b) Dohmeier, C.; Loos, D.; Schno¨ckel, H.
Angew. Chem., Int. Ed. Engl. 1996, 35, 129. (c) Schulz, S.; Roesky, H.
W.; Kocj, H J .; Sheldrick, G. M.; Stalke, D.; Kuhn, A. Angew. Chem.,
Int. Ed. Engl. 1993, 32, 1729.
(15) Gorden, J . D.; Voigt, A.; Macdonald, C. L. B.; Silverman, J . S.;
Cowley, A. H. J . Am. Chem. Soc. 2000, 122, 950.
(16) (a) Rao, M. N. S.; Roesky, H. W.; Anantharaman, G. J .
Organomet. Chem. 2002, 646, 4. (b) Himmel, H.-G.; Vollet, J . Orga-
nometallics 2002, 21, 5972.
(17) Dohmeier, C.; Schno¨ckel, H.; Robl, C.; Schneider, U.; Ahlrichs,
R. Angew. Chem., Int. Ed. Engl. 1993, 32, 1655.

1268 Organometallics, Vol. 22, No. 6, 2003
Romero et al.
accord with values reported for related compounds.9,10
The symmetry of the signals in the aromatic region
observed in the 1H NMR spectrum of 2Li2‚(THF)n
resembled the pattern reported for the related 9-phenyl-
9-borataanthracene.18
Rea ction s of 9-Bor a flu or en es w ith [Cp *Al]4. The
reaction between [Cp*Al]4 and 1a took place instanta-
neously in toluene solution as judged by the change in
color of the solutions from bright yellow to orange (eq
1). 1H NMR analysis showed that the signal for the
F igu r e 1. Molecular structure of 3a . Selected bond
distances (Å): B(1)-Al(1), 2.1147(15); Al(1)-Cp*(average),
2.1560(32); B(1)-C(11), 1.6137(19); B(1)-C(28), 1.6038(19);
B(1)-C(17), 1.6054(19); C(28)-C(23), 1.4228(18); C(17)-
C(22), 1.4213(18); C(12)-F(1), 1.3611(16); Al(1)-F(1), 2.800;
C(16)-F(5), 1.3461(15). Selected bond angles (deg): B(1)-
Al(1)-Cp*(centroid), 160.95; Al(1)-B(1)-C(11), 113.64(9);
C(11)-C(12)-F(1), 120.12(12); C(11)-C(16)-F(5), 119.76-
(11).
in Figure 1. The Cp* group is bonded in a η5 fashion to
aluminum, and the average Al(1)-Cp* distance of
2.1560(32) Å compares well with those of Cp*Al‚B(C6F5)3
(2.171(5) Å) or [Cp*2Al]+ (2.155 Å),17 but it is consider-
ably shorter than that found in tetrameric [Cp*Al]4
(2.344 Å)14a or in monomeric Cp*Al (2.388(7) Å) (deter-
mined by gas-phase electron diffraction).20 The distance
Al(1)-B(1) (2.1147(15) Å), however, is shorter when
compared to Cp*Al‚B(C6F5)3 by about 0.05 Å.
protons of the Cp* shifted upfield from δ 1.90 ppm to δ
1.11 ppm upon formation of product 3a , while the19F
NMR spectrum showed seven new signals. Inspection
of the resonances due to the -C6F5 ring indicated the
difference between meta- and para-fluorine shifts of
∆δm,p ) 5.6 ppm.19 The 11B{1H} NMR spectrum showed
a single resonance at δ -25.4 ppm. These data compare
The angle B(1)-Al(1)-Cp*(centroid) of 160.95° con-
trasts with the virtually linear angle found for Cp*Al‚
B(C6F5)3 (172.9°). Apparently the bending allows for a
stabilizing dative interaction between an ortho-fluorine
atom and the Lewis acidic aluminum center (Al(1)-F(1)
) 2.800(13) Å). Although direct comparative data are
absent from the literature, values of between 2.414(3)
and 2.267(5) Å for related dative C-FfZr interactions21
and contacts in the range of 2.54-2.72 Å for C-FfIn(I)
bonds22 suggest that the donation in 3a is quite weak.
Nonetheless, variable-temperature 19F NMR experi-
ments performed on 3a in CD2Cl2 solution revealed
restricted rotation around the B-C bond on the -C6F5
substituent. At room temperature, a broad signal was
observed at δ -129.8 ppm corresponding to the ortho-
fluorines of the -C6F5 ring, which split into two
resonances at -125.9 and -132.5 ppm upon cooling to
200 K. Similar behavior was observed in the meta-
fluorine resonances. Analysis of the spectra yields a
favorably with that found for Cp*Al‚B(C6F5)3 ( ∆δm,p
)
4.9 ppm; 11B{1H} δ -32.9 ppm 15) and is thus consistent
with the formation of an η1 Lewis acid-base adduct.
27Al NMR measurements, however, were ambiguous.
Product 3a exhibits an extremely upfield resonance at
δ -114.3 ppm that could indicate the presence of an
Al(I) center, but is also consistent with an η5 structure,
since the [Cp*2Al]+ cation resonates in this region of the
spectrum as well (δ -114.5 ppm). The upfield shift for
[Cp*2Al]+ has been rationalized in terms of a diamag-
netic ring current effect of the Cp* ligands.17
To fully characterize the bonding mode in 3a , a single-
crystal X-ray diffraction study was carried out. Yellow
crystals of the product 3a could be isolated by mixing
the reactants in toluene solution and layering with
hexanes at -35 °C. This experiment confirmed that 3a
is an η1 Lewis acid-base adduct; a thermal ellipsoid
diagram and selected metrical parameters are provided
∆Gq
of 42.2(5) kJ mol-1 at a coalescence temper-
rotation
(18) Lee, R. A.; Lachicotte, R. J .; Bazan, G. C. J . Am. Chem. Soc.
1998, 120, 6037.
ature of 243 K. This spectral behavior is possibly
(19) The charge and coordination environment of a boron atom
attached to a pentafluorophenyl ring (C6F5) is found to correlate directly
with the chemical shift difference between the para- and meta-fluorine
(∆δm,p) in the 19F NMR spectrum, which is a particularly useful tool
for reactions with B(C6F5)3. See: Horton, A. D.; de With, J . Organo-
metallics 1997, 16, 5424. We have found that 1a , which possesses a
-C6F5 substituent, behaves similarly.
(20) Haaland, A.; Martinsen, K.; Shlykov, S. A.; Volden, H. V.;
Dohmeier, C.; Schno¨ckel, H. Organometallics 1995, 14, 3116.
(21) Sun, Y.; Spence, R. E. v. H.; Piers, W. E.; Parvez, M.; Yap, G.
P. A. J . Am. Chem. Soc. 1997, 119, 5132.
(22) Wright, R. J .; Phillips, A. D.; Hardman, N. J .; Power, P. P. J .
Am. Chem. Soc. 2002, 124, 8538.

Cp*Al(I) Adducts of 9-Borafluorenes
Organometallics, Vol. 22, No. 6, 2003 1269
indicative of a low-energy solution structure akin to that
seen in the solid state.
Adduct 3a is highly thermally stable. Heating samples
to 110 °C for 2 days resulted in little change in either
1
the H or 19F NMR spectra, negating the possibility of
a kinetic barrier between the η1/η5 bonding modes.
Furthermore, no substantial change was observed when
the solution was monitored by high-temperature NMR
spectroscopy. Rationalizing that the preference of an η1
over an η5 bonding mode in 3a might be due to
unfavorable steric interactions between the Cp* ring
and the C6F5 ring lying nearly orthogonal to the bo-
rafluorene plane,23 we carried out the reaction of
[Cp*Al]4 with 1b. In addition to being less sterically
significant, the methyl substituent should result in a
less Lewis acidic boron center.
Reaction between [Cp*Al]4 and perfluorinated 9-meth-
yl-9-borafluorene 1b in bromobenzene afforded complex
3b in moderate yields (eq 2). Crystals could be obtained
F igu r e 2. Molecular structure of 3b showing the two
different arrangements found in the unit cell. Selected bond
distances (Å): B(1)-Al(1), 2.149(7); Al(1)-Cp*(average),
2.185(13); B(1)-C(1), 1.634(9); B(1)-C(2), 1.602(9); B(1)-
C(13), 1.614(9). Selected bond angles (deg): B(1)-Al(1)-
Cp*(centroid), 162.76; Al(1)-B(1)-C(1), 103.4(4); C(2)-
B(1)-C(13), 98.7(5); C(2)-B(1)-C(1), 117.3(5).
by cooling the solution at -35 °C overnight. The
spectroscopic features exhibited by 3b were similar to
those of η1 adduct 3a , and X-ray analysis confirmed a
similar bonding arrangement in 3b. A thermal ellipsoid
diagram and pertinent metrical parameters are pro-
vided in Figure 2. The solid structure of 3b consists of
an array of two independent molecules in the unit cell,
which differ in that one of the adducts shows an
intermolecular interaction through C-H‚‚‚F hydrogen
bonds (Figure 2b). The F-C distance of 3.208 Å is
shorter than those reported by Desiraju for a series of
fluorobenzene derivatives.24 Intramolecular metrical
parameters for the two molecules are almost identical,
and key data compare well with that found for the
adduct 3a . This is the case even for the B(1)-Al(1)-
Cp*(centroid) angle, which is 162.76° in 3b. In this
instance, the bend is likely a consequence of packing
forces, since the orientation of the Cp*Al fragment with
respect to the Lewis acid is different from that seen in
3a . The boron center shows a distorted tetrahedron
environment with an angle C(2)-B(1)-C(13) of 98.7-
(5)°, which is virtually identical to that reported for the
ion pair Cp2ZrMe(µ-Me)Me-B(C12F8) (97.2(2)°).11
[Cp*Al]4 with the nonfluorinated 9-phenyl-9-borafluo-
rene 2 was explored, the idea being that a less Lewis
acidic boron center might disfavor the η1 bonding mode.
The reaction of 2 and [Cp*Al]4 was not as clean as the
previously described reactions, as precipitation of col-
loidal aluminum metal accompanied product formation.
After heating the mixture for a short period of time, the
aluminum metal was separated by filtration and yellow
crystals of a new compound, 4, were obtained from the
filtrate in moderate yields (eq 3). Microanalysis of
Attempts to thermally convert η1-3b to an η5 structure
were similarly unfruitful, indicating that even for the
less sterically bulky borafluorene 1b, the η1 structure
is thermodynamically favored. In a final attempt to
produce an η5 aluminocenium analogue, the reaction of
(23) Cowley et al. have demonstrated that in the case of decameth-
ylborocenium cation, [Cp*2B]+, the observed η1(σ)/η5(π) arrangement
can be explained in part by steric congestion: Voigt, A.; Filliponi, S.;
Macdonald, C. L. B.; Gorden, J . D.; Cowley, A. H. Chem. Commun.
2000, 911.
(24) Thalladi, V. R.; Weiss, H.-C.; Blaser, D.; Boese, R.; Nangia, A.;
Desiraju, G. R. J . Am. Chem. Soc. 1998, 120, 8702.

1270 Organometallics, Vol. 22, No. 6, 2003
Romero et al.
Sch em e 2
F igu r e 3. Molecular structure of 4. Selected bond dis-
tances (Å): B(1)-Al(1), 2.1347(13); Al(1)-Cp*(average),
2.1792; B(1)-C(23), 1.5963(16); B(1)-C(11), 1.6133(17);
B(1)-C(22), 1.6091(17). Selected bond angles (deg): B(1)-
Al(1)-Cp*(centroid), 164.12; Al(1)-B(1)-C(23), 114.92(8);
C(11)-B(1)-C(22), 110.19(9); C(11)-B(1)-C(23), 119.46-
(10); C(22)-B(1)-C(23), 123.86(10).
the crystals confirmed a 1:1 ratio between the Cp*Al
and 2. The 1H NMR spectrum showed a singlet at δ 1.19
ppm and the expected signals in the aromatic region.
Sharp signals in the 1H NMR spectrum were dependent
on the absence of even traces of [Cp*Al]4 reagent; when
added to samples of 4, the resonance for the Cp* protons
broadened considerably, suggesting that exchange be-
tween bound and free Cp*Al is facile in this system. This
contrasts with the nonlability of the Lewis base in
adducts 3, but perhaps explains why precipitation of
colloidal Al is observed in this case. It has been
observed, for example, that biasing the tetramer/
monomer equilibrium present in solutions of [Cp*Al]4
toward the monomer, either by heat or by addition of a
Lewis acid such as AlCl3,25 leads to substantial deposi-
tion of aluminum. Furthermore, 4 is much less ther-
mally stable in solution than its fluorinated counter-
parts, slowly decomposing at room temperature over a
period of days, to yield aluminum metal as the main
observable byproduct, along with Cp*H and some
unidentified organometallic Al(III) species. These ob-
servations are more consistent with an η1 bonding mode
for 4; the 11B{1H} NMR (δ -4.9 ppm) and 27Al NMR (δ
-70.3 ppm) spectra are also indicative of such a
structure, which was again confirmed crystallograpi-
cally.
A thermal ellipsoid diagram of η1-4 along with se-
lected metrical parameters are provided in Figure 3.
There are no unusually short contacts in the crystal
structure, and, as in the previous examples, the Cp* ring
is bonded in a η5 mode to the aluminum center. The
angle B(1)-Al(1)-Cp*(centroid) of 164.12° is in the
same range as those observed for 3a ,b, while the
distance B(1)-Al(1) of 2.1347(13) Å is also very close to
the distances observed for the other adducts. The boron
center in 4 shows a distorted tetrahedral environment
with an angle C(11)-B(1)-C(22) of 110.19(9)°, which
is about 11° wider compared with the same angles in
3a ,b.
Rea ction of 2Li2‚(THF )n w ith Cp *AlCl2(THF ). The
availability of the dilithium salt of 2 suggested an
alternative route to the putative η5 9-boratafluorene
aluminum compound via metathesis with [Cp*AlCl2]2,26
which is the starting material for the synthesis of
[Cp*Al]4. Since reaction of 2Li2‚(THF)n and [Cp*AlCl2]2
leads initially to the THF-stabilized complex Cp*AlCl2-
(THF), 5, via transfer of a THF molecule from lithium
to aluminum, we prepared 5 separately for use in
subsequent reactions (Scheme 2). The identity of 5 was
confirmed by X-ray analysis. A thermal ellipsoid dia-
gram is provided in Figure 4. The solid structure
confirmed the presence of a coordinated molecule of THF
with an Al(1)-O(1) distance of 1.8332(15) Å. This
distance is slightly shorter than the range of 1.852(1)-
1.902(5) Å found for other RAlCl2‚THF adducts.27 The
compound features an η1 σ-bonded Cp* ligand with a
distance between Al(1) and the cyclopentadienyl carbon
C(1) of 2.009(2) Å, within the range observed for other
η1-bonded cyclopentadienyl derivatives of aluminum.28
In solution, the presence of a singlet for the Cp* methyls
1
in the H NMR spectrum even at 200 K is consistent
(26) Koch, H.; Schulz, S.; Roesky, H. W.; Noltemeyer, M.; Schmidt,
H.; Heine, A.; Herbst-Irmer, R.; Stalke, D.; Sheldrick, G. M. Chem.
Ber. 1992, 125, 1107.
(27) (a) Petrie, M. A.; Power, P. P.; Rasika Dias, H. V.; Ruhlandt-
Senge, K.; Waggoner, K. M.; Wehmschulte, R. J . Organometallics 1993,
12, 1086. (b) Wehmschulte, R. J .; Power, P. P. Inorg. Chem. 1996, 35,
3262. (c) Schnitter, C.; Klimek, K.; Roesky, H. W.; Albers, T.; Schmidt,
H.-G.; Ro¨pken, C.; Parisini, E. Organometallics 1998, 17, 2249. (d)
Eaborn, C.; Hitchcock, P. B.; Smith, J . D.; So¨zerli, S. E. Organometallics
1998, 17, 4322.
(25) Gauss, J .; Schneider, U.; Ahlrichs, R.; Dohmeier, C.; Schno¨ckel,
H. J . Am. Chem. Soc. 1993, 115, 2402.

Cp*Al(I) Adducts of 9-Borafluorenes
Organometallics, Vol. 22, No. 6, 2003 1271
Ta ble 1. Su m m a r y of Op tim ized Geom etr ic
P a r a m eter s
parameter
I-CpAl I-Cp*Al II-CpAl II-Cp*Al
4
Al-B
2.165
2.104
2.216
2.294
1.553
1.451
1.415
124
2.177
2.114
2.223
2.253
1.550
1.451
1.414
123
2.210
2.850
3.535
2.286
1.602
1.424
1.475
109
2.180
2.890
3.623
2.251
1.605
1.424
1.474
109
2.135
2.819
3.510
2.180
1.611
1.419
1.468
115
Al-CR
Al-Câ
Al-CCp
B-CR
CR-Câ
Câ-Câ′
H-B-Al
H-B-CR
H-B-CR-Câ
129
1
129
1
124
34
123
39
120
41
Ch a r t 2
F igu r e 4. Molecular structure of 5. Selected bond dis-
tances (Å): Al(1)-C(1), 2.009(2); Al(1)-Cl(1), 2.1392(8);
Al(1)-Cl(2), 2.1439(9); Al(1)-O(1), 1.8332(15); C(1)-C(6),
1.530(3); C(1)-C(2), 1.475(3); C(1)-C(5), 1.467(3); C(2)-
C(3), 1.370(3); C(5)-C(4), 1.355(3); C(3)-C(4), 1.445(3).
Selected bond angles (deg): C(1)-Al(1)-O(1), 113.81(8);
Cl(1)-Al(1)-Cl(2), 106.91(4); Cl(1)-Al(1)-O(1), 103.79(6);
Cl(1)-Al(1)-C(1), 111.99(7); C(1)-Al(1)-Cl(2), 116.77(7);
Cl(2)-Al(1)-O(1), 102.33(6); Al(1)-C(1)-C(6), 109.34(15);
C(2)-C(1)-C(5), 104.77(17).
adducts. By imposing a number of geometric con-
straints, it was possible to partially optimized the
structures of η1-I-AlCp and η5-II-AlCp, thereby provid-
ing an estimate, albeit an upper limit, of the energy
difference between these complexes and the preferred
coordination complexes of I-AlCp and II-AlCp, which
was predicted to be 13.2 and 18.6 kcal/mol, respectively.
Selected metrical parameters for the four optimized
structures, along with experimental data from 4, are
shown in Table 1. Overall, the agreement between the
experimental data and the calculated values for the η1-
borafluorene adducts is quite good. It should be added
that the same coordination preferences were predicted
for complexes of AlCp with I and II when the H
substituent on the B atom was replaced with bulkier
CH3 and phenyl groups.
Examination of the π-symmetric molecular orbitals
of the borole and 9-borafluorene rings provides insight
as to why the latter strongly prefers η1 coordination.
The π orbitals of I used to bond with the CpAl fragment
orbitals are Cp-like, with two nondegenerate π-sym-
metric orbitals due to the broken symmetry of the
boracyclic ring. In II, however, the π orbitals of the
flanking phenyl rings mix with the π orbitals of the
central borole ring, through both in-phase and out-of-
phase combinations. In-phase mixing lowers the energy
of the MO with respect to that in I, while out-of-phase
mixing results in a slight increase in the orbital energy.
The MO correlation diagram displayed in Figure 5
illustrates how the π-symmetric MOs of the 9-bora-
fluoorene ring arise from those of the parent borole ring
via conjugation onto the phenyl rings. As evident in
Figure 5, the aromaticity in the 9-borafluorene system
is localized in the phenyl rings, leaving the LUMO
largely associated with the boron center.
with a highly fluxional structure, which is a common
feature of cyclopentadienyl derivatives of aluminum.28
The reaction of 2Li2‚(THF)n and 5 was carried out at
-78 °C in THF and slowly warmed to room tempera-
ture, during which time the dark red-brown color
corresponding to 2Li2‚(THF)n disappeared and an in-
tense blue color was observed at about 0 °C. Once the
reaction reached room temperature, the blue color
disappeared and a pale yellow solution was obtained
along with a deposit of colloidal aluminum metal.
Analysis of the solid obtained after filtration and
removal of the solvent indicated the presence of Cp*-H
and the THF adduct of the neutral 9-phenyl-9-borafluo-
rene, 2‚(THF), as the main identifiable products (Scheme
2). The latter was positively identified by treating 2 with
a stoichiometric amount of THF.
It thus seems that, even by this metathesis route, the
η5-boratafluorene structure is not accessible. Rather, the
dilithio salt of 2 functions as a reducing agent, a notion
supported by the observation of the transient deep blue
color, which is suggestive of a boron-based radical
anion12c,29 Our results are also in accord with those of
Wehmschulte,10 who observed reduction behavior when
sterically encumbered 9-boratafluorenes were reacted
with metal halides. These compounds thus exhibit a
strong tendency toward η1 adducts when bonding to the
Cp*Al fragment. To understand this phenomenon in
more detail, DFT computations were undertaken.
Com p u ta tion a l Stu d ies. DFT computations at the
B3LYP/6-31G** level were performed on CpAl and
Cp*Al adducts of the parent borole (C4H5B) and 9-bo-
rafluorene (C12H9B) Lewis acids (Chart 2). Geometry
optimizations were initiated from both η1 and η5 struc-
tures, and it was found that the borole minimized to η5
structures, while the 9-borafluorene minimized to η1
To gain further insight into the preferred bonding
modes of these compounds, the fragment orbital analy-
sis scheme of Ziegler and Rauk30 was employed to
examine the nature of the interaction between CpAl and
the borole and 9-borafluorene rings in I-CpAl and II-
CpAl. In this approach the MOs of a complex AB (I-
AlCp or II-AlCp) are reconstructed in terms of the MOs
(28) (a) Shapiro, P. J . Coord. Chem. Rev. 1999, 189, 1. (b) Fisher, J .
D.; Budzelaar, P. H. M.; Shapiro, P. J .; Staples, R. J .; Yap, G. P. A.;
Rheingold, A. L. Organometallics 1997, 16, 871. (c) Pietryga, J . M.;
Gorden, J . D.; Macdonald, C. L. B.; Voigt, A.; Wiacek, R. J .; Cowley,
A. H. J . Am. Chem. Soc. 2001, 123, 7713.
(29) For example, the radical anion of BPh3 is reported to be blue
in color: (a) Dupont, T. J .; Mills, J . L. J . Am. Chem. Soc. 1975, 97,
6375. (b) Schulz, A.; Kaim, W. Chem. Ber. 1989, 122, 1863.
(30) Ziegler, T.; Rauk, A. Theor. Chim. Acta 1977, 49, 143.

1272 Organometallics, Vol. 22, No. 6, 2003
Romero et al.
F igu r e 5. MO correlation diagram to illustrate how the
π-symmetric MOs of the parent borole ring change due to
conjugation onto the phenyl rings in the 9-borafluorene
ring. (Pictorial representations of the molecular structures
are also provided to give the reader a frame of reference to
interpret the MOs.)
of its constituent fragements A (I or II) and B (AlCp).
The bonding in I-CpAl is akin to that expected for a
metallocenium cation. Of particular interest is the
interaction between the CpAl HOMO and the borole
LUMO, which is positive due to the greater emphasis
of the latter orbital on CR-B-CR′; however, the overlap
is only 0.17 (Figure 6a). In II-CpAl the most significant
bonding interaction also involves mixing of the 9-bo-
rafluorene LUMO with the CpAl HOMO (Figure 6b).
In this case, since the 9-borafluorene LUMO is largely
associated with the boron atom, the overlap for this
interaction is significantly larger than the analogous
overlap for the borole ring (0.33 vs 0.17). Furthermore,
in II-AlCp the AlfB interaction is bolstered by the
interaction of the CpAl HOMO with the 9-borafluorene
HOMO-2 (Figure 6c), which arises via in-phase mixing
of the borole LUMO with the π orbitals of the flanking
phenyl rings (see Figure 5). Although, the overlap in
this second bonding orbital is smaller at 0.18, the
interaction does stablize the η1 bonding mode for the
9-borafluorene complexes. This interaction is not pos-
sible in the parent borole, thereby leading to its prefer-
ence for the η5 coordination geometry when bound to
AlCp.
F igu r e 6. Summary of the fragment orbital analyses of
the important MO interactions in I-AlCp and II-AlCp. The
MOs of the complex and the corresponding constituent
fragment orbitals are displayed, along with their respective
coefficients and the overlap between them. (Pictorial
representations of the molecular structures are also pro-
vided to give the reader a frame of reference to interpret
the MOs.)
of two electrons and formation of an η5 structure is
favorable for the parent system, this process is disfa-
vored in the 9-borafluorenes because aromatization of
the five-membered borole core comes at the expense of
the aromatic character of the phenyl rings. Thus, the
Cp*Al fragment functions only as a Lewis base electron
pair donor rather than a formal two-electron reducing
agent and forms η1 structures exclusively with 9-bora-
fluorenes. The implications of these observations for the
prospects of 9-borafluorenes serving as η5 donors for
transition metals are not clear, but the fact that dilithio
salts of 9-borafluorenes exhibit structural parameters
consistent with an aromatized C4B ring9,10 suggests that
η5 bonding modes should be possible for highly reducing
metals.
Con clu sion s
In summary, in an attempt to explore the coordination
chemistry of a series of 9-borafluorenes, we have found
a strong thermodynamic preference for η1 bonding
modes with the Lewis base Cp*Al(I). Given the propen-
sity of Al(I) to be oxidized to Al(III), and the perceived
legitimacy of an (η5-9-borafluorene)AlCp* product, the
stability of the η1 adducts was something of a surprise.
While aromatization of the borole ring via the addition
Exp er im en ta l Section
Gen er a l P r oced u r es. Argon-filled Innovative Technology
System One dryboxes were used to store air- and moisture-
sensitive compounds and for manipulation of air-sensitive
materials. Reactions were performed either on a double-
manifold vacuum line using standard Schlenk techniques or
under an argon atmosphere in the drybox for small-scale

Cp*Al(I) Adducts of 9-Borafluorenes
Organometallics, Vol. 22, No. 6, 2003 1273
reactions. C6H5BCl2, Me2SnCl2, C6H4Br2, C5Me5-H, and Li
powder (0.5% Na) were purchased from Aldrich and used
without further purification. AlCl3 (99%) was also purchased
from Aldrich and additionally purified by sublimation under
full dynamic vacuum at 140 °C. 1a ,b,11 C12H8Br2,31 Cp*AlCl2,26
(CD2Cl2): 8.20 (m, 2 H), 7.82 (m, 2 H), 7.65-7.57 (m (overlap),
3 H), 7.46 (m, 2 H), 7.38 (m, 2 H), 7.19 (m, 2 H). 13C{1H} NMR
(CD2Cl2): 154.9 (s, quaternary), 136.31 (s, 2 C), 135.42 (s, 2
C), 134.67 (s, 2 C), 132.84 (s, 2 C), 128.78 (s (overlap), 4C),
120.37 (s, 2 C). 11B{1H} NMR (C6D6): 64.5.
14c
and [Cp*Al]4 were prepared according to known literature
Syn th esis Cp *Al‚(1a ), 3a . [Cp*Al]4 (50 mg, 0.31 mmol
based on monomer) was suspended in ca. 3 mL of toluene in
a vial and stirred for 5 min. 1a (146 mg, 0.31 mmol) was
dissolved in a separate vial in ca. 3 mL of toluene and added
dropwise to the stirred suspension of [Cp*Al]4. The yellow-
orange solution obtained was stirred for 20 min and then
filtered into a vial and layered with hexanes. After cooling
overnight at -35 °C, yellow crystals of 3a were obtained. The
crystals were washed with cold hexanes and dried under
vacuum. Yield: 99 mg, 50%. Anal. Calcd for C34H15AlBF13: C,
52.85; H, 2.36. Found: C, 52.30; H, 2.40. 1H NMR (C6D6): 1.11
(s, 15 H, C5(CH3)5). 13C{1H} NMR (C6D6): 116.21 (s, C5(CH3)5),
8.28 (s, C5(CH3)5). 11B{1H} NMR (C6D6): -25.4. 27Al NMR
(C6D6): -114.3. 19F{1H} NMR (C6D6): -129.7 (2 F, o-F), -133.1
(2 F), -136.1 (2 F), -157 (1 F, p-F), -157.7 (2 F), -158.1
(2 F), -162.6 (2F, m-F).
procedures. Toluene, hexanes, and tetrahydrofuran (THF)
were purified using the Grubbs/Dow system32 and stored in
glass bombs over an appropriate indicator (titanocene for
toluene and hexanes, sodium/benzophenone ketyl for THF).
Diethyl ether (Et2O) was predried over calcium hydride
overnight and then distilled and stored over sodium/benzophe-
none ketyl in a glass bomb. All deuterated solvents were
purchased from Cambridge Isotopes. C7D8 and C6D6 were
distilled from sodium/benzophenone ketyl and stored in ap-
propriate glass bombs. d8-THF was also distilled from sodium/
benzophenone and stored over 4 Å molecular sieves. CD2Cl2
was distilled from CaH2. Where applicable, liquid reagents
were degassed by repeated freeze-pump-thaw cycles. Nuclear
magnetic resonance (NMR) spectra were obtained on Bruker
ACE-200 (1H, 200.134 MHz), AMX 300 (1H 300.138 MHz, 19F
282.371 MHz), and BAM-400 (1H 400.134 MHz, 13C 100.614
MHz, 11B 128.377 MHz, 119Sn 149.211 MHz, 27Al 104.264
MHz). All 1H and 13C spectra were referenced externally to
Me4Si at 0 ppm by referencing the residual solvent peak. 11B
NMR, 119Sn NMR, and 27Al NMR spectra were referenced
Syn th esis of Cp *Al‚(1b), 3b. In the glovebox, [Cp*Al]4 (35
mg, 0.22 mmol based on monomer) was suspended in ca. 2
mL of bromobenzene. In a separate vessel, 1b (70 mg, 0.22
mmol) was dissolved in bromobenzene and then added drop-
wise to the suspension of [Cp*Al]4. The yellow-orange suspen-
sion obtained was gently heated in a hot plate to dissolve the
unreacted Cp*Al and then quickly filtered into a vial to
separate insoluble impurities. The clear yellow solution ob-
tained was layered with pentane and cooled at -35 °C
overnight. Yellow crystals of 3b were obtained after 24 h,
which were washed with cold pentane and vacuum-dried.
Yield: 35 mg, 33%. Anal. Calcd for C23H18AlBF8: C, 57.05; H,
3+
externally relative to BF3‚Et2O, Me4Sn, and Al(H2O)6 at 0
ppm, respectively. 19F NMR were referenced externally to C6F6
at -163 ppm relative to CFCl3 at 0 ppm. Elemental analyses
were performed by Dorothy Fox or Roxanna Simank using a
Control Equipment Corporation 440 elemental analyzer.
Syn th esis of C12H8Sn Me2. C12H8Br2 was placed in a 100
mL round-bottom flask (1.0 g, 3.2 mmol), and diethyl ether
(60 mL) was condensed onto the solid at -78 °C with stirring.
At this temperature n-BuLi (1.6 M in hexanes, 4.0 mL, 6.4
mmol) was added dropwise via syringe. Once the addition was
completed, the solution was stirred for 3 h at room tempera-
ture. Me2SnCl2 (0.71 g, 3.2 mmol) dissolved in 15 mL of diethyl
ether was then added dropwise via syringe at room temper-
ature and the mixture was stirred overnight. After this period
a cloudy solution was obtained and the solvent was removed
in vacuo. Hexane was vacuum transferred into the flask, and
the milky solution obtained was sonicated for 10 min. The
solution was filtered through Celite, and the solvent was
removed in vacuo, affording C12H8SnMe2 as a white solid.
Yield: 0.93 g, 96%. Anal. Calcd for C14H14Sn: C, 55.87; H, 4.66.
1
3.72. Found: C, 57.02; H, 3.72. H NMR (C6D6): 1.85 (15 H,
C5(CH3)5), 0.29 (3 H, B-CH3). 13C{1H} NMR (C6D6): 115.77
(s, C5(CH3)5), 8.48 (s, C5(CH3)5). 11B{1H} NMR (C6D6): -14.7.
27Al NMR (C6D6): -114.3. 19F{1H} NMR (C6D6): -135.7 (2 F),
-137.0 (2 F), -159.8 (2 F), -160.2 (2 F).
Syn th esis of Cp *Al‚(2), 4. [Cp*Al]4 (68 mg, 0.42 mmol
based on monomer) and 2 (100 mg, 0.42 mmol) were mixed in
a 50 mL glass bomb in the glovebox, and ca. 5-10 mL of
toluene was added at room temperature. A black suspension
was obtained because of the formation of metallic aluminum.
The vessel was then heated in an oil bath at 60-70 °C for 1 h
to dissolve the unreacted [Cp*Al]4. In the glovebox, the solution
was filtered directly into a small vial and a clear yellow
solution was obtained. The solution was layered with pentane
and cooled at -35 °C overnight. Yellow crystals of 4 were
obtained after this period. The crystals were washed with cold
pentane and dried under vacuum. Yield: 45 mg, 27%. Anal.
Calcd for C28H28AlB: C, 83.63; H, 6.97. Found: C, 81.05; H,
7.29. Carbon analyses were consistently low for this compound.
1H NMR (C6D6): 8.00-7.98 (m, 2 H), 7.88-7.85 (m, 2 H), 7.77-
7.75 (m, 2 H), 7.31-7.25 (m, 6 H), 7.13-7.10 (m, 1 H), 1.19 (s,
15 H, C5(CH3)5). 13C{1H} NMR (C6D6, 20000 scans): 155.4 (s,
br), 146. 0 (s), 135.9 (s), 132.7 (s), 126.4 (s), 126.2 (s), 125.9
(s), 120.8 (s) 115.3 (s), 8.7 (s). 11B{1H} NMR (C6D6): -5.0. 27Al
NMR (C6D6): -70.3.
Syn th esis of 2Li2‚(THF )n . 2 (300 mg, 1.25 mmol) and an
excess of Li powder (44 mg, 5 equiv) were weighed together
into a 50 mL round-bottom flask, and the system was con-
nected to a swivel frit in the glovebox. In the vacuum line,
THF (20 mL) was vacuum transferred into the flask at -78
°C. The reaction mixture was warmed at room temperature
and stirred overnight. The excess lithium powder was sepa-
rated by filtration from the dark red-brown solution, and the
solvent was vacuum evaporated. The pasty solid residue was
extracted into toluene (2 × 10 mL) and decanted to remove
insoluble impurities. The solution was layered with pentane
and cooled at -35 °C overnight to yield dark green crystals.
The crystals were washed with cold pentane and dried slowly
1
Found: C, 55.12; H, 4.67. H NMR (C6D6): 0.27 (s, 6 H, 117Sn
2
(7.7%), and 119Sn (8.4%) satellites J ) 57.8 Hz, 60.2 Hz), 7.85
(m, 2 H), 7.53 (m, 2 H), 7.27 (m, 2 H), 7.20 (m, 2 H). 13C{1H}
NMR (CD2Cl2): 148.79 (s, quaternary), 141.65 (s, quaternary),
136.92 (s, 2 C), 129.67 (s, 2 C), 128.07 (s, 2 C), 122.93 (s, 2 C),
-8.27 (s, 2C, 117Sn (7.7%) and 119Sn (8.4%) satellites 1J )
348.33 Hz, 364.62 Hz). 119Sn{1H} NMR (C6D6): -34.1.
Syn th esis of 9-P h en yl-9-bor a flu or en e, 2. C12H8SnMe2
(1.9 g, 6.3 mmol) was placed in a 50 mL flask, and toluene (25
mL) was vacuum transferred into the flask at -78 °C. C6H5-
BCl2 (1.0 g, 6.3 mmol) also dissolved in toluene (10 mL) was
added dropwise via syringe at -78 °C. The mixture was slowly
warmed to room temperature and stirred overnight. A bright
yellow solution was obtained, and after removal of the solvent
under vacuum a waxy yellow solid was obtained. The solid was
transferred to a sublimator, and the Me2SnCl2 byproduct was
fully removed under static vacuum at 40 °C for 2 days. The
yellow solid obtained as a residue contained pure 2 as judged
by NMR spectroscopy. Yield: 1.5 g (99%). Anal. Calcd for
C
18H13B: C, 90.08; H, 5.42. Found: C, 85.97; H, 5.67. Carbon
analyses were consistently low for this compound. 1H NMR
(31) Dougherty, T. K.; Lau, K. S. Y.; Hedberg, F. L. J . Org. Chem.
1983, 48, 5273.
(32) Pangborn, A. B.; Giardello, M. A.; Grubbs, R. H.; Rosen, R. K.;
Timmers, F. J . Organometallics 1996, 15, 1518.

1274 Organometallics, Vol. 22, No. 6, 2003
Romero et al.
Ta ble 2. Su m m a r y of Da ta Collection a n d Str u ctu r e Refin em en t Deta ils for Com p lexes
3a
3b
4
5
formula
fw
cryst syst
a, Å
b, Å
c, Å
R, deg
â, deg
γ, deg
V, Å3
C
41H23AlBF13
C
23H18AlBF8
C
28H28AlB
C14H23AlCl2O
305.20
monoclinic
8.5509(2)
17.8743(6)
11.4519(4)
728.32
triclinic
10.0810(1)
11.3355(1)
14.7178(1)
100.320(1)
90.371(1)
108.127(1)
1569.04(2)
P1h
484.16
402.29
monoclinic
16.6916(7)
13.9942(6)
18.0524(9)
monoclinic
11.0625(1)
12.7668(2)
17.3434(3)
90.647(2)
108.228(1)
110.9044(13)
4216.5(3)
P21/ n
8
1.525
0.18
0.093
0.218
0.228
1.02
2326.54(6)
P21/ n
4
1.149
0.099
0.044
0.112
0.126
1.03
1635.11(9)
P21/ n
4
1.240
0.44
0.052
0.126
0.149
1.04
space group
Z
2
d
calc, mg m-3
1.542
0.17
0.043
0.106
0.117
1.02
µ, mm-1
R1
wR2
wR2 (all data)
gof
at room temperature. Under vacuum, solvent is removed and
the crystals become a pasty solid difficult to handle. 2Li2‚
(THF)n crystallizes with a different number of coordinated THF
molecules depending on the experimental conditions, with
n ) 2-4. Yield: 50-55%. Anal. Calcd for C26H29BLi2O2 (2
molecules of THF): C, 78.40; H, 7.29. Found: C, 77.99; H, 7.35.
1H NMR (d8-THF): 8.24 (d, 2 H, 3J H-H ) 8.3 Hz), 8.10 (d, 2 H,
with the 6-31G** basis set.35 To optimize the geometries of
the unstable η1-I-AlCp and η5-II-AlCp complexes, it was
necessary to fix the Câ-CR-B-Al dihedral angles for η1-I-AlCp
and the CR-Al, Câ-Al, and B-Al bond distances for η5-II-AlCp.
The J aguar program36 was employed for all of the geometry
optimization calculations. The ADF package37 was employed
for the fragment orbital analysis calculations. These calcula-
tions were carried out at the DFT level of theory with the PBE
exchange-correlation functional.38 The core electrons of all
heavy atoms were kept frozen, and a double-ú slater valence
basis set, augmented with a d polarization function on all
heavy atoms and a p polarization function on H, was employed.
In the fragment orbital analysis calculations the geometries
of I-AlCp and II-AlCp were fixed at their respective B3LYP/
6-31G** optimized structures.
3
3J H-H ) 7.8 Hz), 8.01 (d, 2 H, J H-H ) 8.1 Hz), 7.01 (t, 2 H,
3J H-H ) 7.7 Hz), 6.60-6.54 (m, 3 H), 6.35 (m, 2H), 3.62 (m,
variable), 1.79 (m, variable). 13C{1H} NMR (C6D6): 135.42 (s,
quaternary), 132.64 (s, 2 C), 129.82 (s, 2 C), 127.27 (s, 2 C),
121.07 (s, 2 C), 119.21 (s, 2 C), 118.77 (s, 1 C), 116.53 (s, 2 C),
110.19 (s, 2 C). 11B{1H} NMR (d8-THF): 6.3.
Syn t h esis of Cp *AlCl2‚THF , 5. [Cp*AlCl2]2 (0.58 g, 2.5
mmol based on monomer) was dissolved in 5 mL of toluene in
a vial in the glovebox. THF (0.20 mL, 2.5 mmol) was then
syringed into the vial, and a pale yellow solution was obtained.
The solution was filtered through Celite to eliminate insoluble
impurities, and the clear solution obtained was layered with
pentane and cooled at -35 °C overnight. Colorless crystals of
5 were obtained after 24 h, which were washed with cold
pentane and dried under vacuum. Yield: 0.66 g, 87%. 1H NMR
(C6D6): 3.47 (m, 4 H), 1.96 (s, 15 H, C5(CH3)5), 0.83 (m, 4 H).
13C{1H} NMR (C6D6): 117.53 (s, C5(CH3)5), 71.73 (br, O-CH2-
CH2), 24.87 (s, C5(CH3)5), 11.94 (m, O-CH2CH2). 27Al NMR
(C6D6): 82.4.
Rea ction betw een 2Li2‚(THF )2 a n d 5. In the glovebox,
2Li2‚(THF)2 (170 mg, 0.427 mmol) and 5 (130 mg, 0.427 mmol)
were weighed into a 50 mL round-bottom flask and connected
to a swivel frit. In the vacuum line, THF (25 mL) was
condensed onto the solids at -78 °C, and the dark brown
solution obtained was stirred for 20 min. The system was
slowly warmed to room temperature, and at 0 °C the solution
presents an intense blue color. After 5 min at room temper-
ature a clear yellow solution was obtained along with ap-
preciable amounts of aluminum metal. The solvent was
evaporated to dryness, the residue was extracted into bro-
mobenzene or CH2Cl2 (10 mL), and the white insoluble solid
was filtered along with the aluminum metal. The solvent was
removed in vacuo, and NMR analysis indicated the presence
of Cp*-H and 2‚(THF) as the main identifiable products.
Com p u ta tion a l Deta ils. The geometric structures of all
borole and 9-borafluorene complexes were optimized at the
DFT level of theory, using the B3LYP hybrid functional
(comprised of Becke’s hybrid exchange functional33 and the
correlation functional of Lee, Yang, and Parr34) in conjunction
X-r a y Cr ysta llogr a p h y. Crystal data and refinement
details are given in Table 2; full details can be found in the
Supporting Information.
Ack n ow led gm en t. Funding for this work came
from the Natural Sciences and Engineering Research
Council of Canada in the form of a Discovery Grant, an
E. W. R. Steacie Fellowship (2001-2003) (to W.E.P.),
and the Province of Ontario in the form of a Premiers’
Research Excellence Award (to T.K.W.). Funding for
computing resources was provided by the Canadian
Foundation for Innovation, the Ontario Innovation
Trust Fund, the Natural Sciences and Engineering
Research Council of Canada, and the University of
Western Ontario Academic Development Fund.
Su p p or tin g In for m a tion Ava ila ble: Full experimental
details concerning the X-ray crystallography, tables of atomic
coordinates, anisotropic displacement parameters, and com-
plete bond distances and angles for the structurally character-
ized molecules are presented. This material is available free
of charge via the Internet at http://pubs.acs.org.
OM0209935
(34) Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. B 1988, 37, 785.
(35) (a) Harihan, P. C.; Pople, J . A. Theor. Chim. Acta 1973, 28,
213. (b) Francl, M. M.; Petro, W. J .; Hehre, W. J .; Binkley, J . S.; Gordon,
M. S.; DeFrees, D. J .; Pople, J . A. J . Chem. Phys. 1982, 77, 3654.
(36) J aguar 4.1; Schrodinger, Inc.: Portland, OR, 2000.
(37) ADF 2002.02; Scientific Computing and Modeling NV: Am-
sterdam, The Netherlands, 2002.
(38) Perdew, J . P.; Burke, K.; Ernzerhof, M. Phys. Rev. Lett. 1996,
77, 3865.
(33) Becke, A. D. J . Chem. Phys. 1993, 98, 5648.