Abstraction of endo-H of (η4-BuC5H5)Fe(CO)2PPh 3-nPyn (n = 1 and 2) using Lewis acids

Article abstract of DOI:10.1081/SIM-100104846

Dropwise addition of an excess amount of HBF₄ · OEt₂ through a pressure equalizing funnel to (η4-BuC₅H₅) Fe(CO)₂PPh_3-nPy_n resulted in [(η⁵-BuC₅H₄)Fe(CO)₂PPh _3-n Py⁺_n][BF^-₄]. The smooth abstraction of the endo-H atom of (η⁴-BuC₅H₅)Fe(CO)₂PPh _3-nPy_n is a result of the hydridic nature of this hydrogen. HBF₄·OEt₂ which worked better than an other Lewis acid, Ph₃C⁺PF^-₆, because the H₂ produced in the course of the reaction could be removed, whereas Ph₃CH remains in the solution along with a catonic species which is partially soluble in THF. The hydride abstraction of the neutral complexes was followed spectroscopically.

Full text of DOI:10.1081/SIM-100104846

Synthesis and Reactivity in Inorganic and Metal-Organic
Chemistry
ISSN: 0094-5714 (Print) 1532-2440 (Online) Journal homepage: http://www.tandfonline.com/loi/lsrt19
ABSTRACTION OF ENDO-H OF (η4-
BuC5H5)Fe(CO)2PPh3−nPyn (n = 1 AND 2)USING
LEWIS ACIDS
O. G. Adeyemi & L.-K. Liu
To cite this article: O. G. Adeyemi & L.-K. Liu (2001) ABSTRACTION OF ENDO-H OF (η4-
BuC5H5)Fe(CO)2PPh3−nPyn (n = 1 AND 2)USING LEWIS ACIDS, Synthesis and Reactivity in
Inorganic and Metal-Organic Chemistry, 31:5, 737-742, DOI: 10.1081/SIM-100104846
To link to this article: http://dx.doi.org/10.1081/SIM-100104846
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Date: 18 August 2017, At: 12:33

SYNTH. REACT. INORG. MET.-ORG. CHEM., 31(5), 737–742 (2001)
ABSTRACTION OF ENDO-H OF
(Z4-BuC5H5)Fe(CO)2PPh3ꢀnPyn (n ¼ 1 AND 2)
USING LEWIS ACIDS
O.G.Adeyemi a,b,* and L.-K. Liub,c
aDepartment of Chemistry, University of Ilorin,
P.M.B.1515 Ilorin, Nigeria
bInstitute of Chemistry, Academia Sinica,
Taipei, Taiwan
cDepartment of Chemistry, National Taiwan
University, Taipei, Taiwan
ABSTRACT
Dropwise addition of an excess amount of HBF4 Á OEt2
through
a
pressure equalizing funnel to (Z4-BuC5H5)
Fe(CO)2PPh3ꢀnPyn resulted in [(Z5-BuC5H4)Fe(CO)2PPh3ꢀn
Pyþn ][BFꢀ4 ]. The smooth abstraction of the endo-H atom of
(Z4-BuC5H5)Fe(CO)2PPh3ꢀnPyn is a result of the hydridic
nature of this hydrogen. HBF4ÁOEt2 which worked better
than an other Lewis acid, Ph3CþPF6ꢀ, because the H2 pro-
duced in the course of the reaction could be removed, whereas
Ph3CH remains in the solution along with a catonic species
which is partially soluble in THF. The hydride abstraction of
the neutral complexes was followed spectroscopically.
*Corresponding author. E-mail:gabisola@ilorin.skannet.com
737
Copyright # 2001 by Marcel Dekker, Inc.
www.dekker.com

738
ADEYEMI AND LIU
INTRODUCTION
The cyclopentadienyl group, C5Hꢀ5 , is one of the most important
ligands in organometallic chemistry1. Highly substituted cyclopentadienyls
also have attracted much attention because of the effects of their increased
steric bulk and modified electronic properties2. The replacement of acidic
hydrogen atoms in cyclopentadiene by alkyl groups involves multiple
deprotonation=electronic alkylation cycles. This is evidenced in the synthesis
of tri-, tetra-, and penta(ethyl or isopropyl)cyclopentadiene via repetitive
treatments with NaNH2 and then EtBr or i-PrBr3. For metal7bonded
cyclopentadiene rings most functionalization reactions involve either
Friedel-Crafts electrophilic acylation or alkylation or proton abstraction
by base. This could be followed by reaction of the resulting anion with an
alkyl halide4,5
.
As a continuation of our work on the series of arylpyridylpho-
sphines6,7, we are reporting here that, by means of Z47Fe complexation,
cyclopentadiene can be reversed in its polarity with the applicability of
hydride abstraction by Lewis acid.
EXPERIMENTAL
General Procedure
All experimental manipulation were performed under an atmosphere
of pre-purified nitrogen utilizing standard Schlenk techniques and a double
manifold vacuum line. Also a nitrogen atmosphere glove box was employed.
All solvents were freshly distilled from appropriate drying agents8. For
instance, THF, Et2O and n-hexane were dried over sodium benzophenone
ketal. CaH2 was used for CH2Cl2.
The 31P NMR spectra were obtained on Bruker AC 200=AC 300
spectrometers employing 85% H3PO4 as an external standard (31P, d 0.0).
Infrared spectra were recorded in CH2Cl2 using CaF2 optics on a Perkin-
Elmer (FT-IR) Paragon 1000 spectrophotometer. The 1H NMR spectra
were obtained on Bruker AC 200=AC 300=AMX 400 spectrometers.
Chemical shifts are reported in d values relative to the residual solvent
resonance of CDCl3 (1H, d 7.24). Monopyridyldiphenylphosphine9, dip-
yridylphenylphosphine10, tripyridylphosphine11 and (Z5-C5H5)Fe(CO)2I12
were prepared according to the known procedures. All other reagents were
obtained from commercial sources e.g., Aldrich Chemical Co., TCI Co.,
Merck Co., Across Chemical, and Strem Chemical Co and were used
without further purification.

ABSTRACTION OF ENDO-H
Reaction of (Z4-BuC5H5)Fe(CO)2PPh2Py with HBF4
739
An excess amount of HBF4 Á OEt2 (2 mL, 2 mmol) in THF (3 mL) was
added slowly through a pressure equalizing funnel to (Z4-BuC5H5)
ꢁ
Fe(CO)2PPh2Py (0.1 g, 0.2 mmol) at ꢀ5 C in the course of 5 min. The
yellow solution changed to orange and the reaction mixture was allowed
to stand for 45 min at room temperature until an IR spectrum of the crude
product gives the characteristic Z5 signal. The solvent and excess HBF4
were removed under vacuum to give a yellowish-orange oil. The oil was
dissolved in ether and excess water was added to crystallise out a yellow
solid, which was filtered on a frit and washed with ether (365 mL).
The yellow solid was then dried under vacuum to obtain pure
[(Z5-BuC5H4)Fe(CO)2PPh2Pyþ] [BF4ꢀ]. Yield: 0.039 g (33%); IR (CH2Cl2)
1
n(C¼O) 2056 (s), 2014 (s); 31P NMR (acetone-d6) d 69.00 (s); H NMR
(acetone-d6) d 8.95 (b, 1H, Py), 7.4777.96 (m, 13H, Ph and Py), 5.55 (b, 2H,
Cp), 5.36 (b, 2H, Cp), 0.8872.39 (m, 9H, Bu).
Reaction of (Z4-BuC5H5)Fe(CO)2PPhPy2 with HBF4
This reaction was carried out in a manner similar to the preceding
PPh2Py analogue.Yield: 0.034 g (29%); IR (CH2Cl2) n(C¼O) 2057 (s), 2015
1
(s); 31P NMR (acetone-d6) d 72.38 (s); H NMR (acetone-d6) d 8.98 (b, 2H,
Py), 7.4677.98 (m, 11H, Ph & Py), 5.54 (b, 2H, Cp), 5.37 (b, 2H, Cp),
0.8872.37 (m, 9H, Bu).
Attempted Reaction of (Z4-BuC5H5)Fe(CO)2PPy3 with HBF4
This compound was synthesiꢁsed in a manner similar to that used for
the PPh2Py analogue, but at ꢀ5 C in the course of 15 min. An orange
precipitate was obtained instead of an oil. The orange precipitate gradually
turned to an orange liquid when exposed to air. The orange compound
could not be identified. IR (CH2Cl2) n(C¼O) 2059 (s), 2019; 31P NMR
(acetone-d6) d ꢀ31.67 (s). Utilizing Ph3CþPF6ꢀ instead of HBF4, similar
spectroscopic data were obtained.
RESULTS AND DISCUSSION
Treatment of an equimolar amount of [(Z5-C5H5)Fe(CO)2I] and
ꢁ
PPh3-nPyn in THF at ꢀ78 C by dropwise addition of 1.2 equivalents of

740
ADEYEMI AND LIU
n-BuLi, changed the bonding mode in the precursor [(Z5-C5H5)Fe(CO)2I] to
an (Z4-BuC5H4) bonding mode in [(Z4-BuC5H5)Fe(CO)2PPh3ꢀnPyn]
(n ¼ 173), instead of the formation of acetyl compounds6.
[Z4-BuC5H5)Fe(CO)2PPh3ꢀnPyn] (n ¼ 173) was the major product,
with
a
trace amount of [(Z5-C5H5)Fe(CO)(I)PPh3ꢀnPyn] and
[(Z5-C5H5)Fe(CO)2]2. But for the ligands PPhPy2 and PPy3, especially at
high concentrations, [(Z5-C5H5)Fe(CO)IPPh3ꢀnPyn] (n ¼ 273) was not
observed at all. And indeed, CO alkylation leading to the formation of
[(Z5-C5H5)Fe(CO)C(O)BuPPh3ꢀnPyn] (n ¼ 173) was not observed as well,
perhaps there is considerable reluctance of the ligands to undergo an
attack directly on the carbonyl because of steric hindrance due to the large
phosphine ligand6. [Z4-BuC5H5)Fe(CO)2PPh3-nPyn] (n ¼ 1 or 2) complexes
were characterized by spectroscopic methods, elemental analysis and X-ray
crystallography6.
The endo H atom of [(Z4-BuC5H5)Fe(CO)2PPh3-nPyn] (n ¼ 1 and 2) is
hydridic (equation 1) and could be abstracted with Lewis acids like
HBF4ÁOEt2 resulting in [(Z5-BuC5H4)Fe(CO)2PPh3-nPynþ][BFꢀ4 ]. Here,
HBF4ÁOEt2 worked better than Ph3CþPF6ꢀ because the H2 produced in the
course of the reaction could be removed in vacuo, whereas Ph3CH remains
in the solution along with cationic species which are partially soluble
in THF. The hydride abstraction of the neutral series [Z4-BuC5H5)Fe
(CO)2PPh3ꢀnPyn] (n ¼ 172) by HBF4ÁOEt2 was followed spectroscopically.
The IR n(C¼O) absorption bands during the treatment of the
neutral species with excess HBF4 Á OEt2 revealed only a decreasing con-
centration of the neutral species along with an increasing concentration of
the cations. After 40 min of reaction at room temperature, the seemingly
complete disappearance of the neutral complex as determined by the IR
prompted worked-up. The IR n(C¼O) absorption consist of two strong
peaks at 2056 and 2014 cmꢀ1 for n ¼ 1 and 2057 and 2015 cmꢀ1 for n ¼ 2.
The 2:1:2 integration ratio observed in the 1H NMR spectra for the
neutral species gave way to two broad signals corresponding to 2H each

ABSTRACTION OF ENDO-H
741
for the [(Z5-BuC5H4) ring, which shifted a little downfield. Also, on the
butyl side, the -CH2 attached to the aromatic ring shifted significantly
downfield. The 31P NMR spectra corroborate these findings. Surprisingly,
the elemental analysis and melting point determination of the complexes
did not give any reproducible result. This is due to the unstable nature of
the complexes.
An attempt was made to abstract the endo-H of [(Z4-BuC5H5)Fe
-(CO)2PPy3] using a similar procedure, but an unidentified complex was
obtained (Equation 2).
ðZ4-BuC5H5ÞFeðCOÞ2PPy3 þHBF4
½ðZ5 ꢀBuC5H4ÞFeðCOÞ PPyþꢂ½BFꢀ4 ꢂ
ð2Þ
THF
!
3
2
?
The IR n(C¼O) exhibits Z5-signals at 2059 and 2019 cmꢀ1 of
[(Z5-BuC5H4)Fe(CO)2PPyþꢀ][BFꢀ4 ], but the 31P NMR reveals protonated
1
HPPyþ3 and the H NMR spectrum of the complex is unresolved. This may
be due to one P and three N donor sites which could complicate the
resolution of the spectrum.
ACKNOWLEDGMENTS
Financial support from the Institute of Chemistry and National Sci-
ence Council Taipei, Taiwan, ROC is acknowledged. OGAthanks the
University of Ilorin for staff development and a Senate Research Grant
award.
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4. Elschenbroich, C.; Salzer, A. Organometallics, A Concise Introduction;
2nd Ed.; VCH, Weinheim: Germany, 1992.

742
ADEYEMI AND LIU
5. Stille, J.K. Use of Palladium-catalyzed Coupling Reactions in the
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Received: August 19, 2000
Accepted: March 2, 2001
Referee I: W. D. Jones
Referee I: M. E. Draganjac