Metal Cluster Stabilized Fluorenyl, Indenyl, and Cyclopentadienyl Antiaromatic Cations: An NMR and X-ray Crystallographic Study

Article abstract of DOI:10.1021/om990292g

Treatment of fluorenone, 2,3-diphenylindenone, tetraphenylcyclopentadienone, or 2,5-diethyl-3,4-diphenylcyclopentadienone with ((trimethylsilyl)ethynyl)lithium gives, after hydrolysis, the analogous alkynol; subsequent addition of dicobalt carbonyl and then fluoroboric acid yields the corresponding fluorenyl, indenyl, or cyclopentadienyl cation stabilized by complexation to a tricarbonylcobalt moiety. Variable-temperature NMR data on these cluster cations, and on their bis(diphenylphosphino)methane derivatives, reveal that the barrier to migration of the cationic center between cobalt cluster vertices increases in the order fluorenyl < indenyl < cyclopentadienyl and suggest that the cations with more antiaromatic character have the greatest need for charge delocalization onto the metal center. Replacement of a Co(CO)₃ cationic fragment by an Fe(CO)₃ unit yields the mixed-metal species [((fluorenyl)=C=CSiMe₃)FeCo(CO)₆] (22) and [((2,3-diphenylindenyl)=C=CSiMe₃)-FeCo(CO)₆] (27). In these structural models for the cationic complexes, the Fe-C(9) distance in 22 is 2.626(11) ?, while in the indenyl system 27 the Fe-C(1) distance is 2.347(7) ?, again indicating that the 8π indenyl cation interacts more strongly with the metal center than does the 12π fluorenyl cation.

Full text of DOI:10.1021/om990292g

3372
Organometallics 1999, 18, 3372-3382
Meta l Clu ster Sta bilized F lu or en yl, In d en yl, a n d
Cyclop en ta d ien yl An tia r om a tic Ca tion s: An NMR a n d
X-r a y Cr ysta llogr a p h ic Stu d y
J ames A. Dunn,* William J . Hunks, Ralph Ruffolo, Suzie S. Rigby,
Michael A. Brook, and Michael J . McGlinchey*
Department of Chemistry, McMaster University, Hamilton, Ontario, Canada L8S 4M1
Received April 22, 1999
Treatment of fluorenone, 2,3-diphenylindenone, tetraphenylcyclopentadienone, or 2,5-
diethyl-3,4-diphenylcyclopentadienone with ((trimethylsilyl)ethynyl)lithium gives, after
hydrolysis, the analogous alkynol; subsequent addition of dicobalt carbonyl and then
fluoroboric acid yields the corresponding fluorenyl, indenyl, or cyclopentadienyl cation
stabilized by complexation to a tricarbonylcobalt moiety. Variable-temperature NMR data
on these cluster cations, and on their bis(diphenylphosphino)methane derivatives, reveal
that the barrier to migration of the cationic center between cobalt cluster vertices increases
in the order fluorenyl < indenyl < cyclopentadienyl and suggest that the cations with more
antiaromatic character have the greatest need for charge delocalization onto the metal center.
Replacement of a Co(CO)3 cationic fragment by an Fe(CO)3 unit yields the mixed-metal
species [((fluorenyl)dCdCSiMe3)FeCo(CO)6] (22) and [((2,3-diphenylindenyl)dCdCSiMe3)-
FeCo(CO)6] (27). In these structural models for the cationic complexes, the Fe-C(9) distance
in 22 is 2.626(11) Å, while in the indenyl system 27 the Fe-C(1) distance is 2.347(7) Å,
again indicating that the 8π indenyl cation interacts more strongly with the metal center
than does the 12π fluorenyl cation.
In tr od u ction
dence for the transient intermediacy of the 1-methyl-
2,4-bis(tert-butyl)cyclopentadienyl cation and has cal-
culated that the relative rates of solvolysis of fluorenyl,
indenyl, and cyclopentadienyl trifluoroacetates are ap-
proximately 3 × 104, 3.5 × 102, and 1, respectively.5
The aromatic character and ready availability of
cyclopentadienyl, indenyl, and fluorenyl anions con-
trasts with the relative inaccessibility of the correspond-
ing cations, which have been the subject of an extensive
series of laser flash photolysis studies1 and also numer-
ous mass spectrometric investigations.2 Moreover, ki-
netic measurements have revealed that hydrolyses of
9-fluorenyl esters are retarded by a factor of ap-
proximately 107 relative to those of closely analogous
benzhydryl systems which do not involve destabilizing
effects.3 Nevertheless, the debate concerning the anti-
aromatic/nonaromatic character of fluorenyl and indenyl
cations continues to attract attention.4 Very recently,
however, Tidwell has provided clear experimental evi-
A different approach invokes the stabilization of such
short-lived species as organometallic complexes;6,7 one
can then take advantage of a variety of structural and
spectroscopic techniques to probe their electronic re-
quirements. Recent examples include our X-ray crystal-
lographic characterizations of the [(endo-2-propynyl-
bornyl)Mo2(CO)4(C5H5)2]+ and (exo-2-propynylfenchyl)-
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1992, 114, 8032. (b) Allen, A. D.; Colomvakos, J . D.; Tee, O. S.; Tidwell,
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(9) For a summary of the early studies on trimetallic clusters of the
type [Co3(CO)9CCO]+, [CpMoCo2(CO)8CR2]+, and CpNiCo2(CO)6CR,
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H.; Vahrenkamp, H. Angew. Chem., Int. Ed. Engl. 1981, 20, 98. (c)
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10.1021/om990292g CCC: $18.00 © 1999 American Chemical Society
Publication on Web 07/30/1999

Metal Cluster Stabilized Antiaromatic Cations
Organometallics, Vol. 18, No. 17, 1999 3373
Co(CO)3Mo(CO)2(C5H5)2]+ cations (1 and 2, respec-
Sch em e 1. F lu xion a l P r ocesses in
[P r op a r gyl-Co2(CO)6]+ Clu ster Ca tion sa
tively).8,9 In a related study, Olah and de Meijere have
a
A signifies an antarafacial migration; R represents rota-
tion about the CdCR2 bond.
of a [Co(CO)3]+ vertex by an Fe(CO)3 moiety yields
stable fluorenyl and indenyl complexes that can be
conveniently crystallographically characterized. These
iron-cobalt systems may be viewed as models for their
corresponding dicobalt cluster cations.
Resu lts a n d Discu ssion
presented compelling NMR evidence that, unlike most
cyclopropyl cations, the ferrocenyl-substituted system
3 does not ring-open to the corresponding allyl cation
4.10
P r op a r gyl-Co2(CO)6 Ca tion s. The stabilization of
propargyl cations as dicobalt clusters, as in 7a , is well-
established;16 analogous dimolybdenum complexes (7b)-
are also known.17
We are unaware of any reports of the isolation of the
parent fluorenyl cation, C13H9+; treatment of 9-fluore-
nol, 9-chlorofluorene, or 9-bromofluorene with FSO3H,
FSO3H/SbF5, or SbF5 in SO2ClF at -120 °C immediately
yields dark, unidentifiable polymeric materials.11 Nev-
ertheless, fluorenyl cations 5, bearing an alkyl, phenyl,
chloro, ester, or hydroxy substituent at the C(9) position,
are preparable in FSO3H/SbF5 or SbF5/SO2ClF solution
1
at -78 °C, and their H and 13C NMR spectra have been
reported.11-13 It has been suggested that the corre-
+
sponding cyclopentadienyl cations, C5R5 (6),14 might
rearrange through the square-pyramidal (C4v) geometry,
that could be categorized as a nido-octahedral cluster
analogous to B5H9.15 However, attempts to detect de-
generate rearrangements of fluorenyl cations via such
pyramidal intermediates were unsuccessful.11
We here describe the preparation of a series of
fluorenyl, indenyl, and cyclopentadienyl cations stabi-
lized as (alkynyl)Co2(CO)6 complexes. Although these
dicobalt hexacarbonyl clusters are not isolable as X-ray-
quality crystals, it is shown that isolobal replacement
Moreover, as shown in Scheme 1, such molecules
exhibit two fluxional processes: antarafacial migration
(16) Nicholas, K. M. Acc. Chem. Res. 1987, 20, 207 and references
therein.
(17) (a) Meyer, A.; McCabe, D. J ,; Curtis, M. D. Organometallics
1987, 6, 1491. (b) Sokolov, V. I.; Barinov, I. V.; Reutov, O. A. Isv. Akad.
Nauk SSR, Ser. Khim. 1982, 1922. (c) Galakhov, M. V.; Bakhmutov,
V. I.; Barinov, I. V.; Reutov, O. A. J . Organomet. Chem. 1991, 421, 65.
(d) Galakhov, M. V.; Bakhmutov, V. I.; Barinov, I. V. Magn. Reson.
Chem. 1991, 29, 506. (e) Gruselle, M.; Cordier, C.; Salmain, M.; El
Amouri, H.; Gue´rin, C.; Vaissermann, J .; J aouen, G. Organometallics
1990, 9, 2993. (f) Cordier, C.; Gruselle, M.; J aouen, G.; Bakhmutov,
V. I.; Galakhov, M. V.; Troitskaya, L. L.; Sokolov, V. I. Organometallics
1991, 10, 2303. (g) Barinov, I. V.; Reutov, O. A.; Polyakov, A. V.;
Yanovsky, A. L.; Struchkov, Yu. T.; Sokolov, V. I. J . Organomet. Chem.
1991, 418, C24. (h) Leberre-Cosquer, N.; Kergoat, R.; L’Haridon, P.
Organometallics 1992, 11, 721. (i) Cordier, C.; Gruselle, M.; Vaisser-
mann, J .; Troitskaya, L. L.; Bakhmutov, V. I.; Sokolov, V. I.; J aouen,
G. Organometallics 1992, 11, 3825. (j) El Amouri, H.; Vaissermann,
J .; Besace, Y.; Vollhardt, K. P. C.; Ball, G. E. Organometallics 1993,
12, 605. (k) El Amouri, H.; Vaissermann, J .; Besace, Y.; J aouen, G.;
McGlinchey, M. J . Organometallics 1994, 13, 4426. (l) McClain, M.
D.; Hay, M. S.; Curtis, M. D.; Kampf, J . W. Organometallics 1994, 13,
4377.
(10) Prakash, G. K. S.; Buchholz, H.; Reddy, V. P.; de Meijere, A.;
Olah, G. A. J . Am. Chem. Soc. 1992, 114, 1097.
(11) Olah, G. A.; Prakash, G. K. S.; Liang, G.; Westerman, P. W.;
Kunde, K.; Chandrasekhar, J .; Schleyer, P. v. R. J . Am. Chem. Soc.
1980, 102, 4485.
(12) Korchagina, D. V.; Derendyaev, B. G.; Shubin, V. G.; Koptyug,
V. A. Zh. Org. Khim. 1971, 7, 2582.
(13) J ohnston, L. J .; Kwong, P.; Shelemay, A.; Lee-Ruff, E. J . Am.
Chem. Soc. 1993, 115, 1664.
(14) (a) Masamune, S.; Sakai, M.; Ona, H. J . Am. Chem. Soc. 1972,
94, 8955. (b) Stohrer, W. D.; Hoffmann, R. J . Am. Chem. Soc. 1972,
94, 1661.
(15) Wade, K. Adv. Inorg. Chem. Radiochem. 1976, 18, 1.

3374 Organometallics, Vol. 18, No. 17, 1999
Sch em e 2. Syn th eses of Alk yn yl-F lu or en yl Clu ster Com p lexes
Dunn et al.
of the cationic carbon between the two metal vertices
equilibrates the ML, moieties but maintains the differ-
ence between the exo and endo environments (defined
relative to the metal-metal bond). A second, higher
energy, process allows rotation about the C-CR′R′′ bond
and so interconverts the exo and endo substituents.18
The activation energy for the antarafacial migration
process falls from approximately 18 kcal mol-1 for
primary cations to about 11 kcal mol-1 for tertiary
cations, indicating that the more stable tertiary centers
have less need for electronic assistance from the metal
than do secondary or primary cations. Indeed, for the
tertiary cations, the barriers for the antarafacial migra-
tion process and for the exo/endo interconversion are
normally identical within experimental error.6,17
perature, as anticipated for the unsymmetrical structure
10, shown in Scheme 2. Selected variable-temperature
13C NMR spectra of the cobalt-stabilized fluorenyl cation
10 are shown in Figure 1, and they reveal peak
coalescences between symmetry-related resonances. The
Gutowsky-Holm approximation19 yields a ∆Gq value of
12.2 ( 0.5 kcal mol-1 for the fluxional process that
equilibrates the exo and endo six-membered-ring envi-
ronments; this activation energy is in the normal range
for cobalt-stabilized tertiary carbocations.18 Interest-
ingly, the resonance attributable to C(9), the formally
cationic carbon, exhibits a noticeable temperature-
dependent chemical shift.
In this particular system, there is no probe (such as
diastereotopic isopropyl methyl groups) for the simple
antarafacial migration between the metal centers, and
it was necessary to incorporate a chelating bis(diphe-
nylphosphino)methane (dppm) ligand to label the cobalt
vertices, as in 11. The variable-temperature NMR data
on the corresponding cation [(C13H8CtCSiMe3)Co2(CO)4-
(dppm)]+ (12) yield ∆Gq values of 11.4 ( 0.5 kcal mol-1
and 12.0 ( 0.5 kcal mol-1 for the antarafacial migration
and exo/endo interconversion processes, respectively.
The former was obtained from the temperature-depend-
ent equilibration (see Figure 2) of the phosphorus
environments (31P NMR at 203 K shows peaks at 24.4
F lu or en yl- a n d In d en yl-Co2(CO)6 Ca tion s. With
the aim of isolating a metal-stabilized fluorenyl cation,
fluorenone was converted to 9-((trimethylsilyl)ethynyl)-
9-fluorenol (8) and then treated with Co2(CO)8 to give
the tetrahedral dicobalt cluster 9. Protonation of 9 with
HBF4 in CD2Cl2 at -78 °C resulted in the immediate
development of a deep red coloration, indicating the
formation of the cation 10, whose 13C NMR spectrum
at -78 °C exhibits 13 resonances for the fluorenyl
skeleton. Eight of these peaks represent proton-bearing
1
carbons, and the H and 13C spectra reveal that these
2
are grouped into two independent sets, each made up
of four CH units. It is evident from these data that the
two six-membered rings are nonequivalent at low tem-
and 21.3 ppm, with J (P-P) ) 69.7 Hz), while the latter
was derived from the coalescence behavior of the fluo-
renyl ring carbon resonances. As is commonly the case
(18) (a) Schreiber, S. L.; Klimas, M. T.; Sammakia, S. J . Am. Chem.
Soc. 1987, 109, 5749. (b) Padmanabhan, S.; Nicholas, K. M. J .
Organomet. Chem. 1983, 268, C23.
(19) (a) Sandstrom, J . Dynamic NMR Spectroscopy; Academic
Press: London, 1982; pp 77-109. (b) Gutowsky, H. S.; Holm, C. H. J .
Chem. Phys. 1956, 25, 1228.

Metal Cluster Stabilized Antiaromatic Cations
Organometallics, Vol. 18, No. 17, 1999 3375
Sch em e 3. Syn th eses of Alk yn yl-In d en yl Clu ster
Com p lexes
F igu r e 1. 75 MHz variable-temperature 13C NMR spectra
showing coalescence of the fluorenyl resonances in 10: (()
aromatic CH’s; (b) aromatic C’s; (*) C(9). Spectrum a is
for the fluorenol 9, spectra b-e are spectra of the cation.
available. In an analogous fashion to the generation of
the cluster-stabilized fluorenyl cation 10, the 1-((tri-
methylsilyl)ethynyl)-2,3-diphenylindenol-Co2(CO)6 com-
plex (13) was protonated to yield the indenyl cationic
cluster 14, as shown in Scheme 3. The variable-tem-
perature 13C NMR spectra of 14 exhibit only four CH
resonances assignable to the indenyl moiety; further-
more, the spectra do not change with temperature until
the onset of decomposition as one approaches 273 K.
Thus, the cation appears to exist as a single isomer, but
it is not possible to differentiate between structures 14a
and 14b in which the six-membered ring is oriented
endo or exo, respectively, relative to the cobalt-cobalt
bond; we shall return to this point presently. In an
attempt to detect fluxional behavior in an indenyl
cluster cation, the bis(diphenylphosphino)methane com-
plex 15 was prepared. The stereogenic center at C(1)
renders diastereotopic the two cobalt vertices, and their
attached phosphorus atoms (31P NMR peaks at 34.6 and
30.4 ppm, with 2J (P-P) ) 114.6 Hz). Subsequent
protonation to form the cation 16 gives rise to a 31P
NMR spectrum exhibiting only two equally intense
2
doublet resonances (at 38.8 and 15.2 ppm, with J (P-
P) ) 57.2 Hz). Once again, there is no change over the
range 213-268 K, suggesting that a single isomer (with
the benzo ring either exo or endo) is formed. Above this
temperature, there is gradual peak broadening and loss
of the phosphorus-phosphorus coupling constant; from
these observations, one can estimate the barrier for
indenyl migration between cobalt vertices to be at least
13.5 kcal mol-1, and probably somewhat higher.
F igu r e 2. 121 MHz variable-temperature 31P NMR spec-
tra of 12. Spectrum a is for the fluorenol 11; spectra b-e
are spectra of the cation.
with tertiary cations, within experimental error these
two barriers do not differ significantly.
Although indenone itself is not stable except at very
low temperatures,20 the 2,3-diphenyl derivative is readily
(20) Marvel, C. S.; Hinman, C. W. J . Am. Chem. Soc. 1954, 76, 5435.

3376 Organometallics, Vol. 18, No. 17, 1999
Dunn et al.
Sch em e 4. Isoloba l Rep la cem en t of Co(CO)3
Cyclop en ta d ien yl-Co2(CO)6 Ca tion s. The prepa-
ration of cyclopentadienyl analogues of the fluorenyl
cation 10 and the indenyl cation 14 require the avail-
ability of cyclopentadienones, and such molecules only
resist Diels-Alder dimerization when bulky substitu-
ents are present.21 Thus far, we have prepared the
cyclopentadienyl clusters 18a ,b and the corresponding
cations 19a ,b derived from tetraphenylcyclopentadi-
Ver tices by F e(CO)3 F r a gm en ts
interactions between the peripheral phenyls of the five-
membered ring and the PPh2 substituents of the dppm
ligand.
F e-Co Com p lexes a s Mod els of Clu ster Ca tion s.
Cobalt-stabilized cations 7a have long been thought to
adopt a molecular geometry such that the vinylidene
capping group leans towards one of the metal vertices,
as in 7a , rather than the vertical orientation 7c. This
ground-state structure in which the cationic center is
bonded to a single cobalt vertex gains strong support
from variable-temperature NMR studies22 and from
molecular orbital calculations;23 in addition, crystal-
lographic data are available for numerous molybdenum-
stabilized cluster cations 7b.17 Moreover, in a very
recent and important study, Melikyan has obtained the
X-ray crystal structure of the doubly complexed prop-
argyl cation [(t-BuCtC)3C(Co2(CO)6)2]+, in which there
is preferential coordination of the cationic center with
one of the metal atoms in each cluster.24
enone and from 2,5-diethyl-3,4-diphenylcyclopentadi-
enone, respectively. The 13C NMR spectra of 19a are
complicated by the presence of many overlapping phenyl
resonances, but the diethyl cation 19b provides a more
tractable system. In this latter case, the carbon reso-
nances of the two ethyl groups are distinct (CH3’s at
14.8 and 16.8 ppm, CH2’s at 18.2 and 18.5 ppm) over
the range 188-263 K, at which temperature decomposi-
tion becomes evident. That is, there is no rotation about
the C-C5R4 bond that would equilibrate the exo and
endo ethyl environments on the 13C NMR time scale;
however, this provides no indication as to whether there
is antarafacial migration of the cation between cobalt
vertices.
We have previously suggested that isolobal substitu-
tion of a Co(CO)3+ vertex in [Co2(CO)6(HCtCCR2)]+ by
an Fe(CO)3 unit to give [CoFe(CO)6(HCdCdCR2)] should
serve as an excellent model for the structure of the
cluster cation.25,26 Indeed, molecular orbital calculations
yield almost identical energy-minimized geometries for
such pairs of clusters. For this reason, we chose to
synthesize and structurally characterize the neutral
cluster [(Me3SiCdCd(fluorenyl))FeCo(CO)6] (22). The
+
neutral Fe(CO)3 analogues of the Co(CO)3 cationic
As was done for the fluorenyl and indenyl analogues,
the dppm-substituted cyclopentadienyl clusters 20a ,b
and cations 21a ,b were examined; the 31P NMR spectra
of the cations at 193 K showed in each case the expected
two doublets with 2J (P-P) values of 60.5 Hz and 58.9
Hz, respectively. In the diethyl cation 21b , these
resonances remain distinct until 248 K, at which tem-
perature the onset of decomposition is evident. These
data can only indicate a minimum ∆Gq value of ∼12 kcal
mol-1, but the true barrier toward migration of the
cyclopentadienyl ligand between cobalt vertices is pre-
sumably considerably higher. In contrast, the 31P reso-
nances of the tetraphenylcyclopentadienyl cationic com-
plex 21a coalesce at 233 K, indicating a barrier to
antarafacial migration of ∼10 kcal mol-1. This surpris-
ingly low barrier is perhaps attributable to steric
clusters can be prepared directly from cobalt precursors.
Thus, as depicted in Scheme 4, treatment of (propargyl
alcohol)Co2(CO)6 with Fe(CO)5 in refluxing acetone
yields (HCdCdCH2)FeCo(CO)6 (23); this same reaction
(22) (a) Edidin, R. T.; Norton, J . R.; Mislow, K. Organometallics
1982, 1, 561. (b) D’Agostino, M. F.; McGlinchey, M. J . Polyhedron 1988,
7, 807.
(23) (a) Schilling, B. E. R.; Hoffmann, R. J . Am. Chem. Soc. 1979,
101, 3456. (b) D’Agostino, M. F.; Mlekuz, M.; Kolis, J . W.; Sayer, B.
G.; Rodger, C.A.; Halet, J .-F.; Saillard, J .-Y.; McGlinchey, M. J .
Organometallics 1986, 5, 2345. (c) D'Agostino, M. F.; Frampton, C. S.;
McGlinchey, M. J . J . Organomet. Chem. 1990, 394, 145.
(24) Melikyan, G. G.; Bright, S.; Monroe, T.; Hardcastle, K. I.;
Ciurash, J . Angew. Chem., Int. Ed. Engl. 1998, 37, 161.
(25) (a) Osella, D.; Dutto, G.; J aouen, G.; Vessieres, A.; Raithby, P.
R.; De Benedetto, L.; McGlinchey, M. J . Organometallics 1993, 12,
4545. (b) Ruffolo, R.; Brook, M. A.; McGlinchey, M. J . Organometallics
1998, 17, 4992.
(26) One cannot claim a similar correspondence in susceptibility to
nucleophilic attack, since the dicobalt cluster bears a positive charge,
whereas the iron-cobalt complex is neutral.
(21) Allen, C. H. F. Chem. Rev. 1962, 62, 653.

Metal Cluster Stabilized Antiaromatic Cations
Organometallics, Vol. 18, No. 17, 1999 3377
F igu r e 3. Molecular structure of 22 (30% probability
ellipsoids). Selected bond lengths (Å) and angles (deg):
Fe(1)-C(9) ) 2.626(11), C(9)-C(10) ) 1.351(8), C(10)-
C(11) ) 1.348(9); Fe(1)-C(10)-C(9) ) 102.4(3).
F igu r e 4. Molecular structure of 27 (30% probability
ellipsoids). Selected bond lengths (Å) and angles (deg):
Fe(1)-C(1) ) 2.346(2), C(1)-C(20) ) 1.387(4), C(20)-C(21)
) 1.355(4); Fe(1)-C(20)-C(1) ) 85.4(2).
is also applicable to the diol (HOCH2CtCCH2OH)Co2-
(CO)6, which, upon treatment with Fe(CO)5 in acetone,
gives (CH2dCdCdCH2)Fe2(CO)6 (24).27
logue. Indeed, treatment of the 2,3-diphenylindenol-
dicobalt cluster 13 with Fe(CO)5 in acetone yielded
X-ray-quality crystals of the mixed-metal cluster 27, and
the structure appears in Figure 4. The orientation of
the six-membered ring in the endo position is readily
apparent (suggesting that the corresponding dicobalt
cationic cluster exists as the endo isomer 14a ), but the
most important observation is the Fe-C(1) distance in
27, which is now only 2.347(7) Å, markedly shorter than
the corresponding Fe-C(9) value in the fluorenyl ana-
logue 22. Furthermore, the Fe(1)-C(20)-C(1) angle is
85.4(2)°, considerably smaller than the corresponding
angle in 22, implying that the indenyl ligand has more
need for electronic assistance from the neighboring
metal center. As with 22, the alkyne-derived unit
exhibits allene-like character; the C(1)-C(20) and C(20)-
C(21) distances are 1.387(4) and 1.355(4) Å, respectively.
Attempted syntheses of the analogous iron-cobalt
clusters bearing cyclopentadienyl moieties, i.e. neutral
analogues of cations 19a ,b, yield instead complexes of
the type (Co2(CO)6[TMS-CtCC5R4]Fe(CO)2H, in which
the incoming iron carbonyl moiety binds in an η5 fashion
to the cyclopentadienyl ring. The X-ray structures of
these trimetallic systems and their mechanistic implica-
tions are deferred to another paper.
It is clearly relevant to compare the structures of the
iron-cobalt fluorenyl cluster 22 and the diiron fluore-
nylidene complex 25. The latter determination dates
back more than 25 years and was obtained from film
data; moreover, no absorption correction was applied.29
Nevertheless, the essential features of the molecular
geometry are clear. The triene backbone in 25 is no
longer linear, and the fluorenyl moieties lean toward
the iron atoms. The quoted Fe‚‚‚CR2 distances are
approximately 2.40 Å.
When we compare the structures of the known vi-
nylidene clusters in which the capping group leans
towards an Fe(CO)3 vertex, it is clear that the Fe‚‚‚CR2
distance is a sensitive probe of the strength of this
interaction. To put this in perspective, we note that a
comprehensive survey of the Mo‚‚‚C+ distances in
One might suggest that these (butatriene)Fe2(CO)6
complexes28 may be regarded as models for the corre-
sponding dicationic dicobalt clusters. In particular, [bis-
(biphenylene)butatriene]Fe2(CO)6 (25),29 which has been
prepared directly from the cumulene, may be regarded
as a model for the dication 26.
The reaction of (9-((trimethylsilyl)ethynyl)-9-fluorenol)-
Co2(CO)6 (9) with Fe(CO)5 in refluxing acetone gave,
after chromatographic separation, crystals of [FeCo-
(CO)6(TMS-CdCd(fluorenyl))] (22), whose structure
appears in Figure 3; the bending of the fluorenyl unit
toward the iron atom is evident. The Fe(1)-C(9) dis-
tance is 2.626(11) Å, and the Fe(1)-C(10)-C(9) angle
is 102.4°; these may be compared with the EHMO-
calculated values of 2.73 Å and 108° (see below). The
C(11)-C(10) and C(10)-C(9) distances are 1.351(8) and
1.348(9) Å, respectively, and these values appear to be
more indicative of an allenyl CdCdC13H8 system than
of an acetylenic CtCC13H8 linkage. It is also noteworthy
that the peripheral six-membered rings are each bent
away from the central five-membered ring through
approximately 5°.
Buoyed by this result, we chose to attempt the
synthesis of the analogous 2,3-diphenylindenyl ana-
(27) Victor, R. J . Organomet. Chem. 1977, 127, C25.
(28) (a) Gerlach, J , N.; Wing, R. M.; Ellgen, P. C. Inorg. Chem. 1976,
15, 2959. (b) Eigemann, S-E.; Fo¨rtsch, W.; Hampel, F.; Schobert, R.
Organometallics 1996, 15, 1511.
(29) Bright, D.; Mills, O. S. J . Chem. Soc., Dalton Trans. 1972, 2465.

3378 Organometallics, Vol. 18, No. 17, 1999
Dunn et al.
clusters of the type [Cp2Mo2(CO)4(RCtCCR′R′′)]+ re-
veals that the molybdenum-to-carbocation distance is
in the range 2.44-2.55 Å for primary cations but
increases to 2.61-2.63 Å for secondary cations and can
reach 2.74-2.92 Å for tertiary carbocations.17 Moreover,
there is a very clear inverse relationship between this
distance parameter and the NMR-derived activation
energies for antarafacial migration between molybde-
num centers: the longer the Mo‚‚‚C+ bond, the lower
the barrier! It has also been shown that one can apply
the Bu¨ rgi-Dunitz structure correlation method,30
whereby these structures can be mapped onto a calcu-
lated trajectory and, in effect, provide a series of
snapshots of the migration pathway.31
We are aware of three relevant X-ray crystal struc-
tures in which (OC)3Fe‚‚‚CH2 distances have been
reported.32 In Cp2W2(CO)4Fe(CO)3(CdCH2),33 (MeCd
CdCH2)Fe(CO)3Co(CO)2PPh3,25a and (CH2dCdCdCH2)-
Fe2(CO)5PPh3,28a the iron-carbon distances are 2.21,
2.195, and 2.208 Å, respectively. Taking an extreme
view, these relatively short bonds can be considered as
+
arising from the interaction of CH2 (i.e. primary)
cations with [Fe(CO)3]- vertices. In contrast, in (Me3-
SiCdCdCMe2)FeCo(CO)6,25b (Me2CdCdCdCMe2)Fe2-
(CO)6,28b and (Me3SiCdCd(fluorenyl))FeCo(CO)6 (22),
the Fe-C distances of 2.335, 2.401 and 2.633 Å,
respectively, are a manifestation of the weaker bonding
between a tertiary cation and the formally anionic Fe-
(CO)3 moiety. The relatively short (2.347 Å) Fe-C(1)
bond in the indenyl cluster 27 supports the hypothesis
that the less stable “indenyl cation” has more need of
electronic assistance from the “[Fe(CO)3-]” unit than
does a “fluorenyl cation”. We plan to synthesize an
extended series of such iron-cobalt clusters to probe the
effect of changing the character of the vinylidene moiety.
F igu r e 5. Orbital energy diagram showing the effect of
bending a cyclopentadienyl cation toward a cobalt vertex
in 29. The HOMO is marked with an asterisk (*). The
dashed line represents the change in total electronic energy
and is drawn to a different scale.
for the analogous indenyl and cyclopentadienyl clusters
28 and 29 suggest somewhat larger θ values (28, θ )
Exten d ed Hu1 ck el Molecu la r Or bita l Ca lcu la -
tion s. Molecular orbital calculations at the extended
Hu¨ckel level have allowed us to gain some understand-
ing of the factors controlling the geometry of metal-
stabilized cluster cations.22 When the formally sp2-
hybridized carbocationic center is allowed to lean toward
a metal vertex, the enhanced overlap between the
+
vacant p orbital on the CR2 moiety and a filled metal
d orbital of suitable symmetry leads to marked stabi-
lization of the system. Figure 5 shows the result of
bending a cyclopentadienyl cation toward one of the
cobalt vertices in [(C5H4CtCR)Co2(CO)6]+; the total
electronic energy is minimized at a bend angle, θ, of
approximately 35°, after which point unfavorable steric
interactions with the carbonyl ligands begin to domi-
nate. Concomitantly, the LUMO (which arises from out-
of-phase overlap between the aforementioned carbon p
orbital and metal d orbital) rises sharply in energy, thus
markedly increasing the HOMO-LUMO gap.
30°, Co‚‚‚C+ ) 2.62 Å; 29: θ ) 35°, Co‚‚‚C+ ) 2.53 Å),
as anticipated for the enhanced antiaromatic character
of these ring systems. Doubtless, computations at a
higher level of theory would lead to better calculated
geometries, and we defer to our more knowledgeable
colleagues for further exegesis.
It is noteworthy that cationic fragments in which the
charge can be readily delocalized may have no need of
assistance from neighboring organometallic units. For
example, the pyrylium salts 30 and 31,34 and also the
ferrocenyl-tropylium salt 32,35 have been characterized
crystallographically; the cationic units retain their
planarity and there is no intimation of a metal‚‚‚C+
interaction.
For the fluorenyl cluster cation 10, EHMO calcula-
tions predict a θ value of 25° and a Co‚‚‚C+ distance of
2.71 Å for the energy-minimized geometry. Calculations
(30) Bu¨rgi, H-B.; Dunitz, J . D. Acc. Chem. Res. 1983, 16, 153.
(31) Girard, L.; Lock, P. E.; El Amouri, H.; McGlinchey, M. J . J .
Organomet. Chem. 1994, 478, 189.
(34) Malisza, K. L.; Top, S.; Vaissermann, J .; Caro, B.; Se´ne´chal-
Tocquer, M.-C.; Se´ne´chal,D.; Saillard, J .-Y.; Triki, S.; Kahlal, S.;
Britten, J . F.; McGlinchey, M. J .; J aouen, G. Organometallics 1995,
14, 5273.
(32) Another closely related molecule is Co2Ru(CO)9CdCHtBu:
Bernhardt, W.; Vahrenkamp, H. Angew. Chem., Int. Ed. Engl. 1994,
23, 141.
(33) Delgado, E.; J effery, J . C.; Stone, F. G. A. J . Chem. Soc., Dalton
Trans. 1986, 2105.
(35) Brownstein, S. K.; Gabe, E. J .; Hynes, R. C. Can. J . Chem. 1992,
70, 1011.

Metal Cluster Stabilized Antiaromatic Cations
Organometallics, Vol. 18, No. 17, 1999 3379
(CH2Cl2): νCtC at 2304 cm-1; MS (EI) m/ z (%): 278 [M]+ (30),
263 [M - CH3]+ (100), 202 (25), 165 [C13H9]+ (15), 73 [Me3Si]+
(20). HRMS (EI): calcd for C18H18OSi, 278.1127; found,
278.1132.
(Me3SiCtCC9H8OH)Co2(CO)6 (9). 9-((Trimethylsilyl)ethy-
nyl)fluoren-9-ol (0.969 g, 3.49 mmol) dissolved in THF (50 mL)
was added dropwise over a 45 min period to dicobalt octacar-
bonyl (1.26 g, 3.68 mmol) dissolved in THF (30 mL). The
solution was then stirred for 24 h at room temperature. After
removal of solvent, the product was recrystallized from a
hexane/CH2Cl2 (9:1) mixture to give dark brown-red crystals
of 9 in quantitative yield, mp 118 °C. 1H NMR (200 MHz, CD2-
Cl2): δ 7.66 (d, 2H, H1,8
H4,5
3J (1H-1H) ) 7.5), 7.39 (td, 2H, H3,6
,
3J (1H-1H) ) 7.4 Hz), 7.60 (d, 2H,
,
,
3J (1H-1H) ) 7.4,
4J (1H-1H) ) 1.3), 7.29 (td, H2,7, J (1H-1H) ) 7.4, J (1H-1H)
) 1.3), 2.74 (s, OH), 0.43 (s, 9H, Me3Si). 13C NMR (50 MHz,
CD2Cl2): δ 200.6 (Co-CO’s); 150.9 (C8a,9a), 139.3 (C4a,4b), 129.9
(C3,6), 128.0 (C2,7), 124.7 (C1,8), 120.6 (C4,5), 118.2 (CtC-TMS),
83.7 (C9), 80.4 (CtC-TMS), 0.60 (Me3Si). IR (CH2Cl2): νCO at
2089, 2053, 2027 cm-1. MS (EI) m/ z (%): 536 [M - CO]+ (10),
480 [M - 3CO]+ (40), 452 [M - 4CO]+ (10), 424 [M - 5CO]+
(25), 396 [M - 6CO]+ (40), 278 [M - Co2(CO)6]+, 263 [M -
Co2(CO)6 - CH3]+ (100), 180 [C13H8OH]+ (60), 73 [Me3Si]+ (70).
Anal. Calcd for C24H18Co2SiO7: C, 51.08; H, 3.21. Found: C,
51.28; H, 3.40.
3
4
To conclude, we report the synthesis and NMR
characterization of alkynyl-fluorenyl, -indenyl, and
-cyclopentadienyl cations stabilized by coordination to
a Co2(CO)6 moiety. In the first two instances, isolobal
replacement of one tricarbonylcobalt vertex by an Fe-
(CO)3 moiety yields stable iron-cobalt clusters whose
X-ray crystal structures correspond reasonably well with
the energy-minimized geometry predicted from ex-
tended Hu¨ckel molecular orbital calculations. The ex-
perimental extension of these concepts to the unsubsti-
tuted indenyl and cyclopentadienyl cluster cations
[(C9H7CtCR)Co2(CO)6]+ and [(C5H4CtCR)Co2(CO)6]+
(28 and 29, respectively) is presently under investiga-
tion and will be the subject of a future report.
[(Me3SiCdCdC9H8)Co2(CO)6]BF4 (10). Two drops of HBF4/
Et2O were added to 9 in CD2Cl2 at -78 °C in an NMR tube,
and the sample immediately turned deep red in colour,
indicating the formation of 10. 13C NMR (75 MHz, CD2Cl2, 223
K): δ 192.7 (br, Co-CO’s), 142.8, 140.7, 140.0, 138.5 (C9),
136.8, 135.0, 134.1, 129.0, 128.5, 122.4, 122.0 (br), 121.9, 111.8
(CtC-TMS), 88.7 (CtC-TMS), 1.2 (Me3Si).
Exp er im en ta l Section
(Me3SiCtCC9H8OH)Co2(CO)4dppm (11). (Me3SiCtCC9H8-
OH)Co2(CO)6 (9; 0.356 g, 0.631 mmol) and bis(diphenylphos-
phino)methane (0.376 g, 0.979 mmol) were dissolved in
hexanes (60 mL), and the mixture was heated under reflux
for 2 h. After removal of the solvent, the residue was subjected
to flash chromatography. Elution with a 2:1 hexanes/CH2Cl2
solvent mixture yielded 11 (0.365 g, 0.409 mmol, 65%) as a
brownish red powder, mp 208-209 °C. 1H NMR (500 MHz,
CD2Cl2): δ 7.80-6.65 (m, 28H), 3.52 (br, 2H, CH2), 2.45 (s,
1H, OH), -0.37 (s, 9H, Me3Si). 13C NMR (125 MHz, CD2Cl2):
δ 208.2, 204.4 (CO’s), 152.3 (C9a,9b), 139.2 (C4a,4b), 140.1 (ipso,
br), 136.7 (ipso, br), 132.7, 131.3 (meta), 129.8, 129.4 (para),
128.9, 128.8 (ortho), 128.3, 127.5, 126.0, 119.9 (C-H’s), 114.5
(CtC-TMS), 89.7 (CtC-TMS), 86.9 (C9), 36.8 (CH2, t, 1J (31P-
13C) ) 19.9 Hz), 0.1 (Me3Si). 31P NMR (CD2Cl2): δ 35.54 (s).
IR (CH2Cl2) νCO at 2018, 1990, 1963 cm-1. MS (ES+) m/ z (%):
Gen er a l Meth od s. All reactions were carried out under an
atmosphere of dry nitrogen employing conventional benchtop
and glovebag techniques. All solvents were dried according to
standard procedures before use.36 Silica gel (particle size 20-
45 µm) was employed for flash column chromatography. 1H
and 13C solution NMR spectra were acquired on Bruker DRX
500, AM 300, and AM 200 spectrometers and were referenced
to the residual proton signal, or the 13C solvent signal. 31P
solution NMR spectra were acquired on a Bruker AM-300
spectrometer (31P at 121.5 MHz) and referenced to an external
85% H3PO4 sample. Mass spectra were determined using a
VG Analytical ZAB-E spectrometer by direct electron impact
(EI) or positive electrospray (ES+) methods. Infrared spectra
were recorded on a Bio-Rad FTS-40 spectrometer. Melting
points (uncorrected) were determined on a Thomas-Hoover
melting point apparatus. Elemental analyses were performed
by Guelph Chemical Laboratories, Guelph, Ontario, Canada.
9-((Tr im eth ylsilyl)eth yn yl)flu or en -9-ol (8). n-BuLi (11.67
mL of a 1.44 M hexane solution, 16.80 mmol) was added
dropwise to a solution of (trimethylsilyl)acetylene (1.65 g, 16.80
mmol) in ether at -78 °C via cannula over a 60 min period,
and the solution was warmed to room temperature. After it
was stirred for 1.5 h, the solution was then cooled to -78 °C
and fluorenone (3.024 g, 16.80 mmol) in ether (40 mL) was
added dropwise. The solution was warmed to room tempera-
ture, stirred for 24 h, and washed with distilled water, and
the organic layer was collected. Removal of ether yielded
9-((trimethylsilyl)ethynyl)fluorenol (8; 4.077 g, 14.67 mmol,
892 [M]+ (25), 875 [M - OH]+ (100). Anal. Calcd for C47H40
-
Co2P2SiO5: C, 63.23; H, 4.52. Found: C, 63.56; H, 4.50.
[(Me3SiCdCdC9H8)Co2(CO)4d p p m ]BF 4 (12). Protonation
of 11 with HBF4/Et2O at -78 °C resulted in an immediate deep
brown-red solution, indicating the formation of the cation 12.
13C NMR (75 MHz, CD2Cl2, 213 K): δ 203.6, 201.8, 197.3, 193.2
(Co-CO’s), 149.5, 143.1, 140.1, 139.0, 137.3 (C4a,4b,8a,9,9a), 134.1,
132.7, 131.6-128.2 (broad), 122.9, 122.0, 121.9, 114.3 (CtC-
TMS), 86.6 (CtC-TMS), 31.8 (br, CH2), 1.6 (Me3Si). 31P NMR
2
(CD2Cl2, 213 K): δ 24.37 (d, 1P), 21.31 (d, 1P), J (31P-31P) )
69.7 Hz.
2,3-Dip h en yl-l-((tr im eth ylsilyl)eth yn yl)in d en -l-ol. By
analogy to the procedure described for 8, 2,3-diphenylindenone
(2.00 g, 7.10 mmol) in THF (20 mL) was added to a 3-fold
excess of the lithium salt of (trimethylsilyl)ethyne. The solution
was quenched with water and extracted with ether, and the
solvent was removed to yield the product (2.102 g, 5.53 mmol,
78%) as a light yellow powder, mp 102.5-104 °C. 1H NMR
(500 MHz, CD2Cl2): δ 7.70-7.69 (m, 1H), 7.61-7.59 (m, 2H),
7.42-7.39 (br, 4H), 7.36-7.34 (m, 3H), 7.30-7.29 (m, 3H),
7.24-7.23 (m, 1H), 2.69 (s, OH), 0.21 (s, 9H, Me3Si). 13C NMR
(125 MHz, CDC13): δ 146.3, 144.0, 142.7, 140.2 (C2,3,4,9), 134.1,
133.6 (ipso), 129.8, 129.2 (meta, ortho), 129.1 (para), 128.5,
1
87%) as a light yellow powder, mp 120-121 °C. H NMR (500
3
MHz, CD2Cl2): δ 7.69 (d, 2H, H1,8, J (1H-1H) ) 7.4 Hz), 7.65
(d, 2H, H4,5
,
3J (1H-1H) )7.4), 7.43 (t, 2H, H3,6 3J (1H-1H) )
7.2), 7.38 (t, 2H, H2,7, J (1H-1H) ) 7.2), 2.74 (s, OH), 0.19 (s,
9H, Me3Si). 13C NMR (125 MHz, CD2Cl2): δ 147.5 (C8a,9a), 139.5
(C4a,4b), 130.1 (C3,6), 129.0 (C2,7), 124.6 (C1,8), 120.6 (C4,5), 105.6
(CtC-TMS), 88.5 (CtC-TMS), 75.3 (C9), -0.13 (Me3Si). IR
3
(36) Perrin, D. D.; Armarego, W. L. F.; Perrin, D. R. Purification of
Laboratory Chemicals, 2nd ed.; Pergamon Press: New York, 1980.

3380 Organometallics, Vol. 18, No. 17, 1999
Dunn et al.
3
4
127.91 (meta, ortho), 127.86 (para), 127.6, 127.2, 123.1, 121.1
(C5,6,7,8), 104.9 (CtC-TMS), 89.7 (CtC-TMS), 78.3 (C1), -0.3
(Me3Si). IR (CH2Cl2) νCtC at 2309 cm-1. MS (EI) m/ z (%): 380
[M]+ (80), 365 [M - CH3]+ (22), 303 [M - C6H5]+ (35), 291
CDCl3): δ 7.55 (dd, ortho, 4H, J (1Ho-1Hm) ) 7.9 Hz, J (1Ho-
lHp) ) 1.7 Hz), 7.23-7.19 (m, meta, para, 6H), 7.13-7.07 (m,
meta, para, 6H), 6.97 (dd, ortho, 4H, 3J (1Ho-1Hm)) 7.6 Hz,
4J (1Ho-1Hp) ) 1.5 Hz), 2.56 (s, OH), 0.12 (9H, Me3Si). 13C NMR
(125 MHz, CDCl3): δ 143.0, 142.6 (C1,2), 134.7, 133.8 (ipso),
129.82, 129.79 (meta), 127.81, 127.76 (ortho), 127.2, 127.1
(para), 104.9 (CtC-TMS), 91.3 (CtC-TMS), 81.4 (C5), -0.4
(Me3Si). IR (CH2Cl2): νCtC at 2306 cm-1. MS (EI) m/ z (%): 482
[M]+ (100), 467 [M - CH3]+ (10), 405 [M - Ph]+ (50), 381 (30),
315 (15), 289 (10), 178 [C2Ph2]+ (20), 73 [Me3Si]+ (40). HRMS
(EI): calcd for C34H30OSi, 482.2065; found, 482.2027.
(51), 252 (10), 73 [Me3Si]+ (100). HRMS (El): calcd for C26H24
OSi, 380.1596; found, 380.1547.
-
{2,3-Dip h en yl-(1-(t r im et h ylsilyl)et h yn yl)in d en -l-ol}-
Co2(CO)6 (13). 2,3-Diphenyl-1-((trimethylsilyl)ethynyl)inden-
l-ol (0.511 g, 1.34 mmol) dissolved in THF (20 mL) was added
to a solution of Co2(CO)8 (0.803 g, 2.34 mmol) in THF (20 mL).
The solution was stirred for 24 h, the solvent was removed on
a rotary evaporator, and the product was purified by flash
chromatography using a solvent mixture of 1:1 hexanes/CH2-
Cl2, yielding 13 (0.761 g, 1.14 mmol, 85%) as a brown-red
powder, mp 153-154 °C. 1H NMR (500 MHz, CD2Cl2): δ 7.67-
7.60 (br, 3H), 7.39-7.08 (br, 11 H), 2.86 (s, 1H, OH), 0.17 (s,
9H, Me3Si). 13C NMR (125 MHz, CD2Cl2): δ 200.6, 200.2 (Co-
CO’s), 151.4, 146.4, 142.9, 141.1 (C2,3,4,9), 135.5, 134.6 (ipso),
130.9, 129.5 (2C), 129.2, 128.3, 128.2 (o, m, p), 128.1, 126.7,
123.3, 21.4 (C5,6,7,8), 114.3 (CtC-TMS), 88.2 (C1), 80.8 (Ct
C-TMS), 1.1 (Me3Si). IR (CH2Cl2): νCO at 2087, 2051, 2022
cm-1. MS (El) m/ z (%): 582 [M - 3CO]+ (45), 498 [M - 6CO]+
(70), 422 [M - Co(CO)6 - OH]+ (50), 380 [M - Co2(CO)6]+ (65),
365 (40), 291 (35), 282 (70), 252 (20), 73 [Me3Si]+ (100). Anal.
Calcd for C32H24Co2SiO7: C, 57.66; H, 3.63. Found: C, 57.34;
H, 3.42.
[{2,3-Dip h en yl-l-((tr im eth ylsilyl)eth yn yl)in d en yl}Co2-
(CO)6]BF 4 (14). Protonation of 13 in CD2Cl2 with 2 drops of
HBF4/Et2O at -78 °C resulted in an immediate deep brown-
red solution, indicating the formation of the cation 14. 1H NMR
(300 MHz, CD2Cl2, 233 K): 7.41-7.29 (m, 10H), 7.20-7.13 (m,
4H), 0.69 (s, 9H, Me3Si). 13C NMR (75 MHz, CD2Cl2, 233 K):
δ 192.7 (br., Co-CO’s), 156.5, 142.1, 140.7, 140.4, 138.5, 132.6,
132.4, 131.8, 130.2, 130.0, 129.7, 129.2, 128.7 (2C’s), 128.5,
124.1, 121.2, 117.6 (CtC-TMS), 89.2 (CtC-TMS), 1.8 (Me3-
Si).
{5-((Tr im eth ylsilyl)eth yn yl)-1,2,3,4-tetr a p h en ylcyclo-
p en ta d ien -5-ol}Co2(CO)6 (18a ). Compound 17a (1.207 g,
2.50 mmol) dissolved in THF (20 mL) was added dropwise over
a 30 min period to dicobalt octacarbonyl (1.084 g, 3.17 mmol)
dissolved in THF (15 mL) and the solution stirred for 24 h at
room temperature. The product was recrystallized from hex-
ane/CH2Cl2 (1:1) to give dark red crystals of 18a in quantitative
yield, mp 169-171 °C. 1H NMR (300 MHz, CD2Cl2): δ 7.66
(br, 4H), 7.12 (br, 12H), 7.04 (br, 4H), 2.95 (s, 1H, OH), 0.11
(s, 9H, Me3Si). 13C NMR (75 MHz, CD2Cl2): δ 200.3 (Co-CO’s),
147.0, 144.9 (C1,2), 136.7, 134.8 (ipso), 130.7, 130.2 (meta), 128.2
(br, ortho), 128.0, 127.2 (para), 110.8 (CtC-TMS), 92.9 (C5),
81.5 (CtC-TMS), 1.3 (Me3Si). IR (CH2Cl2): νCO at 2086, 2050,
2020 cm-1. MS (EI) m/ z (%): 580 [M - Co(CO)4 - OH]+ (5),
524 [M - Co(CO)6 - OH]+ (15), 178 [C2Ph2]+ (20), 73 [Me3Si]+
(100). Anal. Calcd for C40H30Co2SiO7: C, 62.50; H, 3.94.
Found: C, 62.74; H, 3.84.
[{5-((Tr im eth ylsilyl)eth yn yl)-1,2,3,4-tetr a p h en ylcyclo-
p en ta d ien yl}Co2(CO)6]BF 4 (19a ). Protonation of 18a in CD2-
Cl2 with HBF4/Et2O at -78 °C resulted in an immediate
deepening in color, indicating formation of the cation 19a . 13
C
NMR (75 MHz, CD2Cl2, 233 K): δ 198.4 (br, Co-CO’s), 151.9,
147.8, 141.6, 137.4 (C1,2,3,4), 94.1 (CtC-TMS), 83.6 (CtC-
TMS), 0.2 (Me3Si). The 13C NMR spectrum of 19a exhibited a
marked increase of complexity in the aromatic region with
respect to 18a .
{2,3-Dip h en yl-l-((tr im eth ylsilyl)eth yn yl)in d en ol}Co2-
(CO)4d p p m (15). 13 (0.491 g, 0.737 mmol) and bis(diphe-
nylphosphino)methane (0.432 g, 1.13 mmol) were heated under
reflux in hexanes (60 mL) for 2 h. Removal of the solvent, and
flash chromatography of the residue using 3:1 hexanes/CH2-
Cl2 as eluent, yielded 15 (0.412 g, 0.414 mmol, 56%) as a
{5-((Tr im eth ylsilyl)eth yn yl)-1,2,3,4-tetr a p h en ylcyclo-
p en ta d ien -5-ol}Co2(CO)4d p p m (20a ). 18a (0.250 g, 0.326
mmol) and dppm (0.188 g, 0.490 mmol) were dissolved in THF
(50 mL), and the mixture was stirred at room temperature
for 40 h. Removal of the solvent and flash chromatography of
the residue with 2:1 hexanes/CH2Cl2 as eluent yielded 20a
(0.126 g, 0.115 mmol, 35%) as a brown-red powder, mp 193-
1
brown-red powder, mp 215-216 °C. H NMR (300 MHz, CD2-
Cl2): δ 7.67-7.00 (m, 30H), 4.97 (br, 1H, methylene-H), 3.55
(br, 1H, methylene-H), 2.78 (s, OH), 0.03 (s, 9H, Me3Si). 13C
NMR (75 MHz, CD2Cl2): δ 208.2, 206.1, 205.6, 201.6 (Co-
CO’s), 152.4, 148.3, 143.1, 139.6 (C2,3,4,9), 139.0, 137.3, 136.1,
134.9 (ipso C's), 132.6, 132.5, 132.0, 131.9, 131.6, 130.6, 129.8,
129.5, 128.8, 128.3, 128.2, 127.5, 127.3, 125.9, 125.1, 120.5,
114.3 (CtC-TMS), 91.0 (C1), 84.7 (CtC-TMS), 36.8 (t, CH2,
1J (13C-31P) ) 20.9 Hz), 1.3 (Me3Si). 31P NMR (CD2Cl2): δ 34.56
(d), 30.41 (d), 2J (31P-31P) ) 114.6 Hz). IR (CH2Cl2): νCO at
2018, 1991, 1963 cm-1. MS (ES+) m/ z (%): 994 [M]+ (30), 977
[M - OH]+. Anal. Calcd for C55H46Co2P2SiO5: C, 66.39; H, 4.66.
Found: C, 66.33; H, 4.62.
1
194 °C. H NMR (500 MHz, CD2Cl2): δ 7.70-6.98 (m, 40H),
3.57 (q, 1H, methylene-H), 3.43 (q, 1H, methylene-H), 3.30
(s, OH), 0.38 (s, 9H, Me3Si). 13C NMR (125 MHz, CD2Cl2): δ
208.7, 202.4 (Co-CO’s), 148.3, 144.4 (C1,2), 138.5 (t, ipso,
1J (31P-13C) 3J (31P-13C) ) 24.0 Hz), 137.2, 135.9 (ipso C’s),
134.4 (t, ipso, J (31P-13C) ≡ 3J (31P-13C) ) 17.6 Hz), 132.9,
131.3, 131.2, 130.6, 130.0, 129.5, 128.6, 128.2, 127.8, 127.6,
127.2, 126.8 (o, m, p C-H’s), 120.1 (CtC-TMS), 94.8 (C5), 73.1
(CtC-TMS), 40.1 (t, CH2, J (31P-13C) ) 18.1 Hz), 3.1 (Me3-
1
Si). 31P NMR (CD2Cl2): δ 31.78 (s). IR (CH2Cl2): νCO at 2019,
1994, 1964 cm-1. MS (ES+) m/ z (%): 1096 [M]+ (100). Anal.
Calcd for C63H52Co2P2SiO5: C, 68.97; H, 4.78. Found: C, 69.06;
H, 4.80.
[{2,3-Dip h en yl-l-((tr im eth ylsilyl)eth yn yl)in d en yl}Co2-
(CO)4d p p m ]BF 4 (16). Protonation of 15 in CD2Cl2 at 195 K
with HBF4/Et2O at -78 °C resulted in an immediate deep
brown-red solution, indicating the formation of the cation 16.
13C NMR (75 MHz, CD2Cl2, 213 K): δ 202.3, 200.7, 197.3, 193.7
(Co-CO’s), 90.5 (CtC-TMS), 35.7 (br, CH2), 2.5 (Me3Si). 31P
NMR (CD2Cl2, 213 K): δ 38.78 (d), 15.16 (d), 2J (31P-31P) )
57.2 Hz. A marked increase in complexity of the aromatic
region in the 13C NMR spectrum with respect to 15 was
observed.
[{5-((Tr im e t h ylsilyl)e t h yn yl)-1,2,3,4-t e t r a p h e n ylcy-
clop en ta d ien yl}Co2(CO)4d p p m ]BF 4 (21a ). Protonation of
20a in CD2Cl2 with HBF4/Et2O at -78 °C in an NMR tube
resulted in an immediate deepening in color of the solution,
indicating formation of the cation 21a . Upon protonation, the
13C NMR spectrum exhibited a marked increase in complexity
in the aromatic region relative to 20a . 31P NMR (CD2Cl2, 193
K): δ 39.00 (d), 18.14 (d), J (31P-31P) ) 60.5 Hz.
2
5-((Tr im et h ylsilyl)et h yn yl)-1,2,3,4-t et r a p h en ylcyclo-
p en ta d ien -5-ol (17a ). By the procedure described for 8,
2,3,4,5-tetraphenylcyclopentadienone (6.630 g, 17.3 mmol)
dissolved in THF (30 mL) was added to ((trimethylsilyl)-
ethynyl)lithium (2 equiv) to yield 17a (7.594 g, 15.8 mmol,
91%) as a yellow powder, mp 179-181 °C. 1H NMR (500 MHz,
5-((Tr im eth ylsilyl)eth yn yl)-1,4-d ieth yl-2,3-d ip h en ylcy-
clop en ta d ien -5-ol (17b). By analogy to the procedure de-
scribed for 8, diethyldiphenylcyclopentadienone (4.987 g, 17.31
mmol) in THF (50 mL) was added to a 2-fold excess of the
lithium salt of (trimethylsilyl)ethyne. The solution was quenched
with water and extracted with ether and the solvent removed

Metal Cluster Stabilized Antiaromatic Cations
Organometallics, Vol. 18, No. 17, 1999 3381
to yield 17b (5.709 g, 14.79 mmol, 85%) as a yellow oil. 1H
NMR (500 MHz, CD2Cl2): δ 7.25-7.22 (m, 6H, m and p H’s),
Ta ble 1. Su m m a r y of Cr ysta l Da ta a n d Str u ctu r e
Refin em en t
3
7.06 (d, 4H, ortho, J (1Ho-1Hm) ) 7.7 Hz), 2.58-2.46 (m, 4H,
22
27
CH2), 2.37 (s, OH), 1.31 (t, 6H, CH3, 3J (1H-1H) ) 7.6 Hz), 0.28
(s, 9H, Me3Si). 13C NMR (125 MHz, CD2Cl2): δ 145.8 (C1,4),
141.4 (C2,3), 135.8 (ipso), 129.5 (ortho), 128.2 (meta), 127.3
(para), 105.5 (CtC-TMS), 89.5 (CtC-TMS), 81.9 (C5), 19.6
empirical formula
Mr
C
24H17CoFeO6Si
C32H23CoFeO6Si
646.40
544.25
T (K)
293(2)
0.710 73
red plate
300(2)
λ (Å)
0.710 73
red plate
descripn
cryst size (mm)
cryst syst
space group
a (Å)
(CH2), 14.8 (CH3), 0.2 (Me3Si). IR (CH2Cl2): νCtC at 2305 cm-1
.
0.14 × 0.22 × 0.41 0.06 × 0.25 × 0.50
MS (EI) m/ z (%): 386 [M]+ (10), 371 (12) [M - CH3]+, 357 (8)
triclinic
P1h
triclinic
P1h
[M - CH2CH3]+, 343 (5), 327 (5), 73 (100) [Me3Si]+.
9.293(2)
15.865(2)
18.697(3)
106.462(10)
104.059(14)
102.17(2)
2445.7(7), 4
1.478
11.58980(10)
12.57340(10)
12.63060(10)
65.97(10)
73.21(10)
75.940(10)
1592.74(2), 2
1.496
{5-((Tr im eth ylsilyl)eth yn yl)-1,4-d ieth yl-2,3-d ip h en yl-
cyclop en ta d ien -5-ol}Co2(CO)6 (18b). Compound 17b (0.900
g, 2.33 mmol) dissolved in THF (15 mL) was added dropwise
over a 15 min period to dicobalt octacarbonyl (1.114 g, 3.34
mmol) dissolved in THF (40 mL). The solution was stirred for
24 h at room temperature. The residue was purified by flash
chromatography using 3:1 petroleum ether/CH2Cl2 eluent and
recrystallized from a hexane/CH2Cl2 (9:1) mixture to give dark
red crystals of 18b (1.311 g, 1.95 mmol, 84%), mp 123-125
b (Å)
c (Å)
R (deg)
â (deg)
γ (deg)
V (Å3), Z
Fcalcd (g cm-3
)
abs coeff (mm-1
)
1.356
1.223
θ range/index ranges (deg) 2.14-22.50
1.79-26.35
-14 < h < +14,
-15 < k < +14,
-15 < 1 < +14
12 901
limiting indices
0 < h < +9,
-15 < k < +15,
-20 < 1 < +19
6684
1
°C. H NMR (200 MHz, CD2Cl2): δ 7.20-7.16 (m, 6H), 7.03-
no. of rflns collected
no. of indep rflns
R(int)
6.99 (m, 4H), 2.61-2.39 (m, 4H, CH2), 2.00 (s, 1H, OH), 1.13
(t, 6H, CH3, 3J (1H-1H) ) 7.4 Hz), 0.41 (s, 9H, Me3Si). 13C NMR
(50 MHz, CD2Cl2): δ 200.6 (Co-CO’s) 149.9 (C1,4), 141.5 (C2,3),
136.0 (ipso), 129.3, 128.1 (ortho, meta C's), 127.1 (para), 112.0
(CtC-TMS), 91.6 (C5), 83.1 (CtC-TMS), 20.7 (CH2), 14.9
6220
6000
0.0304
0.893
0.0243
1.045
R1 ) 0.0444
wR2 ) 0.1171
R1 ) 0.0631
wR2 ) 0.1308
goodness of fit on F2
final R indices (I > 2σ(I))a R1 ) 0.0500
wR2 ) 0.1022
R indices (all data)a
R1 ) 0.0961
(CH3), 2.0 (Me3Si). IR (CH2Cl2): νCO at 2087, 2050, 2020 cm-1
.
wR2 ) 0.1165
MS (EI) m/ z (%): 588 [M - 3CO]+ (5), 532 [M - 5CO]+ (30),
504 [M - 6CO]+ (55), 428 [M - Co(CO)6 - OH]+ (12), 386 [M
- Co2(CO)6]+ (10), 371 [M - Co2(CO)6 - CH3]+ (15), 348 (25),
73 [Me3Si]+ (100). Anal. Calcd for C32H30Co2SiO7: C, 57.14;
H, 4.50. Found: C, 56.81; H, 4.37.
R1 ) ∑(||Fo| - |Fc||)/∑|Fo|; wR2 ) [∑[w(Fo2 - Fc2)2]/∑[w(Fo2)2]]0.5
.
a
solution of 9 (0.784 g, 1.39 mmol) dissolved in acetone (35 mL),
and the mixture was heated under reflux for 24 h. After
removal of solvent, the residue was subjected to flash chro-
matography on silica gel. Elution with hexanes gave light red
crystals of 22 (96 mg, 0.018 mmol, 13%), mp 300 °C dec. 1H
NMR (500 MHz, CD2Cl2): δ 7.98 (H5)§, 7.81 (H8)§, 7.75 (H4)(,
7.69 (H1)(, 7.48 (H7)§, 7.37 (H2)(, 7.32 (H6)§, 7.24 (H3)(. 13C NMR
(125.72 MHz, CD2Cl2): δ 209.7 (CO’s), 145.3 (C4a), 141.4 (C4b)§,
141.1 (C9a)(, 140.6 (C8a)§, 129.6 (C7)§, 129.0 (C2)(, 127.4 (C6)§,
127.1 (C3)(, 121.3 (C1)(, 120.9 (C8)§, 120.6 (C5)§, 120.5 (C4)(,
106.7 (CtC-TMS), 70.0 (CtC-TMS), 2.8 (Me3Si). ( and §
denote environments shown by the 1H-13C shift-correlated and
1H-1H COSY spectra to be in the same ring; assignment of
the exo and endo rings is arbitrary. IR (CH2Cl2): νCO at 2078,
2039, 2019 cm-1. MS (EI) m/ z (%): 460 [M - 3CO]+ (5), 432
[M - 4CO]+ (5), 404 [M - 5CO]+ (10), 376 [M - 6CO]+ (10),
261 [Me3SiCtCC9H8]+ (100), 73 [Me3Si]+ (66). Anal. Calcd for
[{5-((Tr im eth ylsilyl)eth yn yl)-1,4-d ieth yl-2,3-d ip h en yl-
cyclop en ta d ien yl}Co2(CO)6]BF 4 (19b). Protonation of 18b
in CD2Cl2 with HBF4/Et2O at -78 °C in an NMR tube resulted
in an immediate deepening in color, indicating formation of
the cation 19b. 13C NMR (75 MHz, CD2Cl2, 223 K): δ 192.4
(br, Co-CO’s), 152.8, 145.7, 141.3, 135.3 (C1,2,3,4), 134.1 (C5),
133.4, 133.1 (ipso), 128.8, 128.6 (2C’s), 128.2, 128.0, 127.8 (o,
m, p), 126.4, 93.3 (CtC-TMS), 83.5 (CtC-TMS), 18.5 (CH2),
18.2 (CH2), 16.8 (CH3), 14.8 (CH3), 1.6 (Me3Si).
{5-((Tr im eth ylsilyl)eth yn yl)-1,4-d ieth yl-2,3-d ip h en yl-
cyclop en t a d ien -5-ol}Co2(CO)4d p p m (20b ). Cluster 18b
(0.495 g, 0.737 mmol) and bis(diphenylphosphino)methane
(0.385 g, 1.00 mmol) were dissolved in THF (45 mL), and the
solution was stirred for 18 h. The residue was purified by flash
chromatography using a 2:1 hexanes/CH2Cl2 solvent mixture
to yield 20b (73 mg, 0.073 mmol, 10%) as a brownish red
C
24H17CoFeSiO6: C, 52.96; H, 3.15. Found: C, 53.26; H, 3.39.
X-ray-quality crystals of 22 were grown from a 5:1 hexanes/
CH2Cl2 solvent mixture.
1
powder, mp 197-198 °C. H NMR (300 MHz, CD2Cl2): δ 7.72
(br, 4H), 7.41-7.30 (m, 10H), 7.11-6.88 (m, 16H), 4.55 (q, 1H,
methylene-H), 3.71 (q, 1H, methylene-H, {1J (1H-1H) =
2J (31P-1H) ) 11.2 Hz}), 2.06 (q, 4H, CH2), 1.64 (s, OH), 0.57
(t, 6H, CH3, 3J (1H-1H) ) 7.3 Hz), 0.23 (s, 9H, Me3Si). 13C NMR
(CD2Cl2, 75 MHz): δ 208.2, 204.9 (Co-CO’s), 151.9, 139.5
(C1,2), 139.6 (t, ipso, lJ (31P-13C) ) 3J (31P-13C) ) 23.9 Hz), 136.7
(t, ipso, 1J (31P-13C) = 3J (31P-13C) ) 16.7 Hz), 136.4 (ipso),
132.4, 131.2, 129.7, 129.5 (2C’s), 128.9, 128.3, 127.9, 126.6 (o,
m, p), 110.4 (CtC-TMS), 93.4 (C1), 86.3 (CtC-TMS), 35.8
(t, CH2, 1J (31P-13C) ) 21.4 Hz), 21.2 (CH2), 14.6 (CH3), 1.3
(Me3Si). 31P NMR (CD2Cl2): δ 32.65 (s). IR (CH2Cl2) νCO at
2015, 1985, 1959 cm-1. MS (ES) m/ z (%): 1000 [M]+ (40), 983
[M - OH]+. Anal. Calcd for C55H52Co2P2SiO5: C, 65.99; H, 5.24.
Found: C, 66.02; H, 5.40.
{2,3-Diph en yl-1-((tr im eth ylsilyl)eth yn yl)in den yl}FeCo-
(CO)6 (27). 13 (0.484 g, 0.73 mmol) dissolved in acetone (20
mL) was added to freshly distilled Fe(CO)5 (1.480 g, 6.61 mmol)
in acetone (20 mL). The solution was heated under reflux for
12 h while being monitored by TLC (1:1 hexanes/CH2Cl2). After
removal of the solvent the residue was purified by flash
chromatography using 1:1 hexanes/CH2Cl2 as the eluent. A
dark red band was collected and chromatographed twice using
hexanes solvent. X-ray-quality crystals of the dark red band,
27, were grown from a 9:1 CH2Cl2/hexanes solution (26 mg,
0.04 mmol, 6%): mp 123-124 °C. 1H NMR (500 MHz, CD2-
Cl2): δ 7.14-6.82 (m, 14H), 0.59 (s, 9H, Me3Si). 13C NMR (125
MHz, CD2Cl2): δ 210.1 (Fe-CO’s), 200.6 (Co-CO’s), 144.7,
141.3, 140.5, 133.8, 130.7, 129.8, 129.6, 129.1, 128.6, 128.4,
128.0, 127.8, 125.8, 125.6, 125.0, 121.2, 120.8, 110.2, 70.4, 2.9
(Me3Si). IR (CH2Cl2) νCO at 2079, 2036, 2020 cm-1. MS (EI)
m/ z (%): 419 [M - Co(CO)6]+ (40), 364 [M - FeCo(CO)6]+ (18),
[{5-((Tr im et h ylsilyl)et h yn yl)-1,4-d iet h yl-2,3-d ip h en -
ylcyclop en ta d ien yl}Co2(CO)4d p p m ]BF 4 (21b). The addi-
tion of HBF4/Et2O to 20b in CD2Cl2 at -78 °C in an NMR tube
resulted in an immediate darkening of the solution, indicating
the formation of the cation 21b. 31P NMR (CD2Cl2, 193 K): δ
289 (20), 207 (25), 73 [Me3Si]+ (100). Anal. Calcd for C32H23
CoFeSiO6: C, 59.46; H, 3.59. Found: C, 59.74; H, 3.28.
-
2
41.07 (d), 23.74 (d), J (31P-31P) ) 58.9 Hz.
(Me3SiCdCdC9H8)F eCo(CO)6 (22). By analogy to the
procedure previously described for (MeCdCdCH2)FeCo(CO)6,27
freshly distilled Fe(CO)5 (2.19 g, 11.18 mmol) was added to a
X-r a y Cr ysta llogr a p h ic Str u ctu r e Deter m in a tion s for
22 a n d 27. Crystal data and refinement parameters are
collected in Table 1. All crystals were grown using vapor

3382 Organometallics, Vol. 18, No. 17, 1999
Dunn et al.
diffusion techniques37 and were mounted on fine glass fibers
with epoxy cement. X-ray crystallographic data for 22 were
collected on a Siemens P4 diffractometer fitted with a rotating
anode using graphite-monochromated Mo KR radiation (λ )
0.710 73 Å). Crystallographic data for 27 were obtained using
a P4 Siemens diffractometer, equipped with a rotating anode
utilizing graphite-monochromated Mo KR radiation (λ )
0.710 73 Å) and a Siemens SMART 1K charge-coupled device
(CCD) area detector, employing the program SMART.38 The
crystal-to-detector distance was 3.991 cm, and the data col-
lection was carried out in 512 × 512 pixel mode, employing 2
× 2 pixel binning. The initial unit cell parameters were
determined by a least-squares fit of the angular settings of
the strong reflections, collected using three 4.5° scans (15
frames each) over three different parts of reciprocal space (45
frames total). All structures were solved by using the direct
methods routine contained outlined in the Siemens SHELXTL-
PLUS program library39 followed by full-matrix least-squares
refinement on F2 with anisotropic thermal parameters and
include idealized hydrogen-atom contributions. In 27, the final
refined structure was based on a rotational disorder in which
the carbon atoms of the Si(CH3)3 substituent could exist in a
minimum of five different conformations which were refined
assuming an equal occupancy of 0.2. Refinement of the data
revealed electron density in the second coordination sphere,
and the exact molecular formula could not be determined. The
solvent molecule was assumed to be CH2Cl2 and was refined
as a free variable, yielding a final occupancy of 0.64. The unit
cell for 22 contains two independent molecules whose struc-
tures differ only slightly, and only one is shown.
Ackn owledgm en t. Financial support from the Natu-
ral Sciences and Engineering Research Council of
Canada, and also from the donors of the Petroleum
Research Fund, administered by the American Chemi-
cal Society, is gratefully acknowledged. J .A.D. thanks
the NSERC for a graduate scholarship. Mass spectra
were obtained courtesy of Dr. Richard Smith of the
McMaster Regional Centre for Mass Spectrometry.
Su p p or tin g In for m a tion Ava ila ble: Tables of atomic
parameters, including fractional atomic coordinates and equiva-
lent isotropic displacement parameters, bond distances and
angles, and anisotropic displacement parameters for the
crystal structures of 22 and 27. This material is available free
of charge via the Internet at http://pubs.acs.org.
(37) Stout, G. H.; J ensen, L. H. X-ray Structure Determination, 2nd
ed.; Wiley: New York, 1989; p 76.
(38) SMART, Release 4.05; Siemens Energy and Automation Inc.,
Madison, WI 53719, 1996
(39) Sheldrick, G. M. SHELXTL PC, Release 5.03; Siemens Crystal-
lographic Research Systems, Madison, WI 53719, 1994.
OM990292G