Consecutive butylations on the cyclopentadienyl ring of the [(η5-C5H5)Fe(CO)2PPh 3]+ cation

Article abstract of DOI:10.1021/om960819e

The replacement of acidic hydrogen atoms in cyclopentadiene by alkyl groups would normally involve a repetitive deprotonation and then electrophilic alkylation. By means of η⁴-Fe complexation, one can reverse the polarity of cyclopentadiene and utilize multiple hydride abstraction Inucleophilic alky lotion cycles in the preparation of the polybutylcyclopentadienes in the neutral η⁴-Fe form and the corresponding cyclopentadienyl complexes in the cationic η⁵-Fe form.

Full text of DOI:10.1021/om960819e

Organometallics 1996, 15, 5263-5265
5263
Con secu tive Bu tyla tion s on th e Cyclop en ta d ien yl Rin g
of th e [(η5-C5H5)F e(CO)2P P h 3]+ Ca tion
Ling-Kang Liu* and Lung-Shiang Luh
Institute of Chemistry, Academia Sinica, Nankang, Taipei, Taiwan 11529, ROC, and
Department of Chemistry, National Taiwan University, Taipei, Taiwan 10767, ROC
Received September 25, 1996X
Summary: The replacement of acidic hydrogen atoms in
cyclopentadiene by alkyl groups would normally involve
a repetitive deprotonation and then electrophilic alky-
lation. By means of η4-Fe complexation, one can reverse
the polarity of cyclopentadiene and utilize multiple
hydride abstraction/ nucleophilic alkylation cycles in the
preparation of the polybutylcyclopentadienes in the
neutral η4-Fe form and the corresponding cyclopentadi-
enyl complexes in the cationic η5-Fe form.
hydride abstraction/nucleophilic butylation in the se-
quential preparations of di-, tri-, tetra-, and pentabu-
tylcyclopentadienes in the neutral η4-Fe form and the
corresponding cyclopentadienyl complexes in the cat-
ionic η5-Fe form.
The complete synthesis is depicted in Scheme 1.
Compound 1H, (η4-exo-BuC5H5)Fe(CO)2PPh3, was pre-
pared in a ring alkylation reaction in which 1:1 (η5-
C5H5)Fe(CO)2I and PPh3 were treated at -78 °C with
equimolar n-BuLi6 or by treatment of [(η5-C5H5)Fe(CO)2-
PPh3]I with n-BuLi following an early literature pro-
cedure.7 The endo-H atom of the (η4-BuC5H5) ring is
hydridic and could be abstracted with protic of Lewis
acids,8 e.g., HBF4‚OEt2 or Ph3C+PF6-, resulting in
[1]+X-, (η5-BuC5H4)Fe(CO)2PPh3+X- (X ) BF4, PF6),9
which could in turn be treated with n-BuLi at -78 °C
to give again the ring alkylation product 2H, (η4-exo-
Bu2C5H4)Fe(CO)2PPh3. HBF4‚OEt2 worked better than
The cyclopentadienyl group, C5H5, is one of the most
important ligands in organometallic chemistry.1 Highly
substituted cyclopentadienyls have also been attracting
much attention because of the effects of their increased
steric bulk and modified electronic properties.2 The
replacement of acidic hydrogen atoms in a cyclopenta-
diene by alkyl groups involves multiple deprotonation/
electrophilic alkylation cycles, e.g., in the synthesis of
tri-, tetra-, and penta(ethyl or isopropyl)cyclopenta-
dienes via repetitive treatments with NaNH2 and then
EtBr or i-PrBr.3 For metal-bound cyclopentadienyl
rings, most functionalization reactions involve either
Friedel-Crafts electrophilic acylation or alkylation or
proton abstraction by base, followed by reaction of the
resulting anion with an alkyl halide.4,5 By means of η4-
Fe complexation, cyclopentadiene can be reversed in its
polarity so that hydride abstraction is applicable in-
stead. The resulting cationic cyclopentadienyl-Fe spe-
cies can undergo nucleophilic butylation as demon-
strated here by application of the novel repetitive
-
Ph3C+PF6 because H2 is volatile but Ph3CH remains
in solution. The second hydride abstraction/butylation
yielded [2]+ and 3H, the third [3]+ and 4H, and the
fourth [4]+ and 5H. Further hydride abstraction of 5H
with HBF4‚OEt2 afforded [5]+.10
The hydride abstraction of the neutral series by
HBF4‚OEt2 was followed spectroscopically. The moni-
toring of IR νCO absorption bands during the treatment
of 3H with HBF4‚OEt2 revealed only decreasing con-
centrations of 3H [1953 (s), 1892 (s) cm-1] along with
increasing concentrations of cation [3]+ [2042 (s), 1998
(s) cm-1]. Yet after seemingly complete disappearance
of 3H to the cation by IR, the 31P{1H} NMR spectra of
the reaction mixture showed the disappearance of
resonances at δ 73.8, 72.2, 70.5, and 68.1 (corresponding
to the four isomers of 3H) and formation of new
resonances at δ 53.3 and 52.6 that later were replaced
by resonances at δ 62.5 and 61.7 (corresponding to the
* To whom correspondence should be addressed at the Academia
Sinica.
X Abstract published in Advance ACS Abstracts, November 1, 1996.
(1) (a) Wilkinson, G., Stone, F. G. A., Abel, W., Eds. Comprehensive
Organometallic Chemistry, The Synthesis, Reactions, and Structures
of Organometallic Compounds; Pergamon: Oxford, U.K., 1982; Vols.
3-7. (b) Adv. Organomet. Chem. 1964-1989, 1-29.
(2) (a) King, R. B.; Bisnette, M. B. J . Organomet. Chem. 1967, 8,
287. (b) King, R. B. Coord Chem. Rev. 1976, 20, 155. (c) Maitlis, P. M.
Acc. Chem. Res. 1978, 11, 301. (c) Wolzanski, P. T.; Bercaw, J . E. Acc.
Chem. Res. 1980, 13,121. (d) Campbell, A. C.; Lichtenberger, D. L. J .
Am. Chem. Soc. 1981, 103, 6389. (e) Pez, G. P.; Armor, J . N. Adv.
Organomet. Chem. 1981, 19, 1. (f) Marks, T. J . Science 1982, 217, 989.
(g) Miller, E. J .; Landon, S. J .; Brill, T. B. Organometallics 1985, 4,
533. (h) J utzi, P. Pure Appl. Chem. 1989, 61, 1731. (i) J aniak, C.;
Schumann, H. Adv. Organomet. Chem. 1991, 33, 291. (j) Okuda, J .
Top. Curr. Chem. 1991, 160, 99.
(3) (a) Sitzmann, H. Z. Naturforsch. 1989, 44B, 1293. (b) Sitzmann,
H. J . Organomet. Chem. 1988, 354, 203. (c) Riemschneider, R. Z.
Naturforsch. 1963, 18B, 641. (d) Alder, K.; Ache, H. J . Chem. Ber. 1962,
95, 503.
(4) (a) Watt, W. E. In Comprehensive Organometallic Chemistry, The
Synthesis, Reactions, and Structures of Organometallic Compounds;
Wilkinson, G., Stone, F. G. A., Abel, E. W., Eds.; Pergamon: Oxford,
U.K., 1982; Vol. 8, pp 1013-1071. (b) Collman, J . P.; Hegedus, L. S.;
Norton, J . R.; Finke, R. G. Principles and Applications of Organo-
transition Metal Chemistry; University Science Books: Mill Valley, CA,
1987. (c) Elschenbroich, C.; Salzer, A. Organometallics, A Concise
Introduction, 2nd ed.; VCH: Weinheim, Germany, 1992.
(5) One of the alternative ways is the Stille coupling reaction using
a Pd catalyst: (a) Lo Sterzo, C.; Stille, J . K. Organometallics 1990, 9,
687 and references cited therein. (b) Bunz, U. H. F.; Enkelman, V.;
Raeder, J . Organometallics 1993, 12, 4745.
1
two isomers of [3]+). In the H NMR spectrum of the
reaction mixture there were broad peaks at δ -2.8 and
-8.0 which could not be assigned to either the neutral
η4-Fe or the cationic η5-Fe species. Overall, the spec-
(6) Liu, L.-K.; Luh, L.-S. Organometallics 1994, 13, 2816.
(7) Treichel, P. M.; Shubkin, R. L. Inorg. Chem. 1967, 6, 1328.
(8) Liu, L.-K.; Luh, L.-S.; Wen, Y.-S.; Eke, U. B.; Mesubi, M. A.
Organometallics 1995, 14, 4474.
(9) For example, (η4-BuC5H5)Fe(CO)2PPh3 (2.48 g, 5.0 mmol), dis-
solved in THF (100 mL), was stirred at 0 °C while Ph3C+PF6- (1.91 g,
5.0 mmol) in CH2Cl2 (30 mL) was added dropwise. The yellow
precipitate was collected, dried under vacuum, and then recrystallized
from CH2Cl2/n-hexane to yield (η5-BuC5H4)Fe(CO)2(PPh3)+PF6 in
-
quantitative yield. (η5-BuC5H4)Fe(CO)2(PPh3)+PF6-: Mp 172-174 °C;
IR (CH2Cl2) νCO: 2052 s, 2010 s cm-1 31P NMR (CDCl3) δ 62.08 (s),
;
-143.01 (sept, 1J PF ) 711 Hz); 1H NMR (CDCl3) δ 7.10-7.55 (m, 15H,
Ph), 5.18 (s, 2H, Cp′-â), 4.98 (s, 2H, Cp′-R), 2.33, 1.48, 1.35 (b, 6H,
CH2CH2CH2Me), 0.87 (s, 3H, CH2CH2CH2Me). Anal. Calcd for C29H28F6-
FeO2P2: C, 54.40; H, 4.41. Found: C, 54.01; H, 4.38.
(10) Satisfactory spectroscopic data and elemental analyses were
obtained as described in the Supporting Information.
S0276-7333(96)00819-9 CCC: $12.00 © 1996 American Chemical Society

5264 Organometallics, Vol. 15, No. 25, 1996
Communications
Sch em e 1
Ta ble 1. Sp ectr oscop ic Da ta
troscopic data were suggestive of an unisolated, cationic
transient species, likely involving an Fe-hydrogen
interaction in nature.11 Similar intermediates could
also be seen during the conversions of 4H to [4]+ (an
extra resonance at δ 53.0 in the 31P{1H} NMR spectrum
IR νCO
31P{1H} NMR
(CDCl3), δ
compd
1H
2H
3H
(CH2Cl2), cm-1
1962 (s), 1902 (s)
1958 (s), 1897 (s)
1953 (s), 1892 (s)
73.32 (s)
74.70 (s), 72.91 (s)
73.84 (s), 72.19 (s),
70.54 (s), 68.11 (s)
71.76 (s), 67.96 (s)
66.75 (s)
1
and an extra broad signal at δ -11.3 in the H NMR
spectrum). Experimentally, the hydride abstraction
steps in Scheme 1 required increasingly more forcing
conditions. The optimum condition generally was de-
cided by constant monitoring by 31P{1H} NMR spec-
troscopy.
4H
5H
1949 (s), 1889 (s)
1945 (s), 1887 (s)
2052 (s), 2010 (s)
2047 (s), 2004 (s)
2042 (s), 1998 (s)
2040 (s), 1994 (s)
2039 (s), 1997 (s)
[1]+BF4
[2]+BF4
[3]+BF4
[4]+BF4
[5]+BF4
62.14 (s)
-
-
-
-
-
62.90 (s), 61.93 (s)
62.50 (s), 61.71 (s)
62.45 (s)
The two series, neutral η4-Fe complexes (1H-5H) and
cationic η5-Fe complexes ([1]+-[5]+), have been char-
acterized. The number of butyl groups on the ring
61.09 (s)
increases from 1H to 5H. Hence, the donor capability
of cyclopentadiene increases from 1H to 5H, red-shifting
the IR νCO aborption bands by 4-5 cm-1 per butyl group,
as shown in Table 1. For the cationic series [1]+-[5]+,
(11) Although not fully characterized, the intermediate could likely
be an Fe-(hydride) or an Fe-(molecular hydrogen) species or a mixture
of both. For the nature and stabilization of the transition metal-
molecular hydrogen bond see: (a) Kubas, G. J . Comments Inorg. Chem.
1988, 7, 17. (b) Kubas, G. J . Acc. Chem. Res. 1988, 21, 120. (c) Crabtree,
R. H. Acc. Chem. Res. 1990, 23, 95. (d) J essop, P. G.; Morris, R. H.
Coord Chem. Rev. 1992, 121, 155. (e) Crabtree, R. H. Angew. Chem.,
Int. Ed. Engl. 1993, 32, 789. (f) Heinekey, D. M.; Oldham, W. J . Chem.
Rev. 1993, 93, 913. (g) Cappellani, E. P.; Drouin, S. D.; J ia, G.; Maltby,
P. A.; Morris, R. H.; Schweitzer, C. T. J . Am. Chem. Soc. 1994, 116,
3375. (h) Scharrer, E.; Chang, S.; Brookhart, M. Organometallics 1995,
14, 5686.
a similar trend, but with less regular differences in νCO
,
also could be observed. The cationic series in the IR
spectra exhibits higher νCO aborption frequencies than
the neutral series by 90-110 cm-1 for the symmetric
and asymmetric bands, respectively. With the exception
of 1H, the 31P NMR data of 2H-5H suggest that the
PPh3 phosphorus nucleus in the neutral series becomes

Communications
Organometallics, Vol. 15, No. 25, 1996 5265
more shielded as butyl groups accumulate on the ring.
But, the 31P NMR resonances of [1]+-[5]+ are found in
a range of less than 2 ppm, indicating that there is little
differentiation of the PPh3 phosphorus nuclei in the
cationic series.
The four remaining butyl R-carbon atoms are almost
coplanar with the diene plane. The two butyl â-carbon
atoms extending from C2 and C3 are at 1.44 and 1.53
Å to the plane, opposing Fe, whereas the other two butyl
â-carbon atoms are on the same side of Fe at 0.19 and
0.55 Å to the plane, respectively. The Fe-P length of
5H is 0.025 Å longer than the corresponding length in
1H,6 attributed to the steric hindrance. The Fe-C(CO)
lengths of 5H are shorter by 0.015-0.024 Å than those
of 1H, parallel to the red shift of IR νCO bands.
The nucleophilic addition of butyl groups to the ring
of [0]+-[4]+ is from an exo position to Fe, and thus at
the unique carbon of the cyclopentadiene there is one
endo-H that is available to undergo the next hydride
abstraction step. Following a progressive increase in
the number of butyl groups and hence a progressive
increase of charge density on the ring, the nucleophilic
butylation would be expected to be increasingly difficult.
In the butylation of [n ]+ (n ) 0-4) the yield of acyl
product (η5-BunC5H5-n)Fe(PPh3)(CO)C(O)Bu by way of
CO alkylation6 indeed increases at the expense of the
ring alkylation product (n + 1)H. The branching ratios
of CO alkylation vs ring alkylation increase with the
number of butyl groups, 0.01, 0.06, 0.11, 0.22, and 0.35.
No gem-dibutylcyclopentadiene-metal has been ob-
served in this study employing the reversed-polarity,
nucleophilic alkylation strategy. On the other hand, in
the normal electrophilic alkylation of a cyclopentadiene,
gem-dialkylcyclopentadienes have been formed in sig-
nificant quantity. Unable to be aromatized with base,
the gem-dialkylcyclopentadienes must be removed be-
fore the next deprotonation/alkylation step.3
Ack n ow led gm en t. The authors thank Academia
Sinica and the National Science Council, ROC, for the
kind financial support. We are indebted to Mr. Y.-S.
Wen for collection of X-ray intensity data.
Su ppor tin g In for m ation Available: For complexes [1]+-
[5]+ and 2H-5H, text describing the experimental procedures
and, for 5H, listings of crystallographic data and refinement
details, positional and anisotropic thermal parameters, bond
distances and angles, and structural parameters (17 pages).
Ordering information is given on any current masthead page.
OM960819E
(12) None of reported (η4-cyclopentadiene)Fe(CO)2(PPh3) structures
have substituents at the diene positions. The reported structures are
as follows. (a) (η4-exo-PhCH2C5H5)Fe(CO)2(PPh3): Sim, G. A.; Wood-
house, D. I.; Knox, G. R. J . Chem. Soc., Dalton Trans. 1979, 629. (b)
(η4-exo-BuC5H5)Fe(CO)2(PPh3): Reference 6. (c) (η4-exo-PhC5H5)Fe-
(CO)2(PPh3), Luh, L.-S.; Liu, L.-K. Bull. Inst. Chem. Acad. Sin. 1994,
41, 39. (d) (η4-exo-MeC5H5)Fe(CO)2(PMePh2) and [(η4-exo-MeC5H5)Fe-
(CO)2]2(µ-dppe): Liu, L.-K.; Luh, L.-S.; Eke, U. B. Organometallics
1995, 14, 440. (e) [(η4-exo-MeC5H5)Fe(CO)2](µ-dppe)[(η5-C5H5)Fe(CO)C-
(O)Me], Luh, L.-S.; Liu, L.-K. Organometallics 1995, 14, 1514. (f) (η4-
exo-MeC5H5)Fe(CO)2(PPh3), Liu, L.-K.; Luh, L.-S.; Chao, P.-C.; Fu, Y.-
T. Bull. Inst. Chem. Acad. Sin. 1995, 42, 1.
The X-ray structure of 5H reveals a square-pyramidal
Fe(0) core with one CO apical and the rest of ligands
basal, one olefin of the basal diene being trans to PPh3
and the second olefin trans to the second CO ligand.12
The diene plane C1-C4 (within 0.005 Å) separates Fe
and C5, C5 being displaced at 0.55 Å from the plane
and the connected exo-butyl group being further away.