Mechanistic insight into the formation of phosphaferrocene

Article abstract of DOI:10.1021/om060025s

The thermal reaction between [CpFe(CO)₂]₂ and 1-tert-butyl-3,4-dimethylphosphole leading to phosphaferrocene has been studied with respect to its mechanistic characteristics. Isobutene was identified as the only product originating from the ^tBu group, suggesting a nonradical pathway which involves β-H elimination from the intermediately formed [CpFe(CO)₂-^tBu].

Full text of DOI:10.1021/om060025s

2394
Organometallics 2006, 25, 2394-2397
Mechanistic Insight into the Formation of Phosphaferrocene
Joachim Bitta, Stefan Fassbender, Guido Reiss,† and Christian Ganter*
Institut fu¨r Anorganische Chemie, Heinrich-Heine-UniVersita¨t Du¨sseldorf, UniVersita¨tsstr. 1,
D-40225 Du¨sseldorf, Germany
ReceiVed January 10, 2006
Scheme 1
Summary: The thermal reaction between [CpFe(CO)2]2 and
1-tert-butyl-3,4-dimethylphosphole leading to phosphaferrocene
has been studied with respect to its mechanistic characteristics.
Isobutene was identified as the only product originating from
the tBu group, suggesting a nonradical pathway which inVolVes
â-H elimination from the intermediately formed [CpFe(CO)2-
tBu].
Introduction
Phosphaferrocenes are among the best investigated hetero-
metallocenes. The first compound of this familiy, 3,4-dimeth-
ylphosphaferrocene (1), was prepared as early as 1977 by
Mathey.1 Since then, phosphaferrocene derivatives have been
studied intensely, due to their interesting properties.2 The
propensity to coordinate to metal fragments via the P atom
makes them unique ligands with strong π-acceptor character-
istics. Recently, substituted phosphaferrocenes with planar
chirality were incorporated into chelate ligand structures which
have found application in asymmetric catalysis.3
Scheme 2
Two main synthetic routes are available for the preparation
of phosphaferrocenes, one of them involving the reaction of a
phospholide anion with a reagent delivering a CpFe+ synthon
and the other being the thermal reaction between a neutral
phosphole and [CpFe(CO)2]2 ()Fp2) in a high-boiling-point
solvent such as xylene. Although the latter method was the first
one to be discovered and is frequently used, little is known about
the mechanism of this transformation. We have therefore studied
this reaction regarding its mechanistic features, and the results
are presented in this paper.
formation of 2-phenyl-3,4-dimethylphosphaferrocene as a byprod-
uct in the synthesis of 3,4-dimethylphosphaferrocene (1) from
1-phenyl-3,4-dimethylphosphole and Fp2 was originally ac-
counted for by the intermediate formation of free phenyl
radicals, which bring about a substitution on the phosphafer-
rocene 1.
However, it became clear later that the phenyl-substituted
product arises from a sigmatropic phenyl shift at the phosphole
prior to the reaction with Fp2 (Scheme 2).4 This sigmatropic
shift occurs typically at elevated temperatures with aromatic
groups bound to the phosphorus in the phosphole and has been
exploited in an elegant way for the synthesis of phosphamet-
allocenes carrying respective groups at the 2-position.5 On the
other hand, P-bound alkyl groups show no appreciable migratory
aptitude; therefore, 1-tert-butyl-3,4-dimethylphosphole is the
preferred starting material for the synthesis of phosphaferrocene
1.6
Results and Discussion
A possible mechanism for the formation of phosphaferrocene
from a phosphole and Fp2 was proposed by Mathey (Scheme
1).1 It involves a thermal dissociation of Fp2 as the initial step
and a subsequent coordination of the phosphole to the Fe atom
to form the 19-electron species [CpFe(CO)2(1-R-phosphole)].
A homolytic scission of the exocyclic P-C bond is postulated
as the next step, leading to the radical R• and the 18-electron
complex [CpFe(CO)2(phospholyl)], which leads to phospha-
ferrocene on thermal elimination of two molecules of CO. The
The enthalpy required for the homolytic dissociation of Fp2
has been estimated by various methods to be in the range of
110 ( 20 kJ/mol, and therefore, a small concentration of
monomeric [CpFe(CO)2] radicals exists at elevated temperature.
* To whom correspondence should be addressed. E-mail: christian.ganter@
uni-duesseldorf.de.
† X-ray structure determination.
(1) Mathey, F. J. Organomet. Chem. 1977, 139, 77.
(2) (a) Mathey, F. Coord. Chem. ReV. 1994, 137, 1. (b) Mathey, F. Top.
Curr. Chem. 2002, 220, 27.
(3) (a) Ganter, C.; Brassat, L.; Glinsbo¨ckel, C.; Ganter, B. Organome-
tallics 1997, 16, 2862. (b) Ganter, C. Chem. Soc. ReV. 2003, 32, 130. (c)
Ganter, C. Dalton Trans. 2001, 3541. (d) Bitta, J.; Fassbender, S.; Reiss,
G.; Frank, W.; Ganter, C. Organometallics 2005, 24, 5176. (e) Shintani,
R.; Fu, G. C. J. Am. Chem. Soc. 2003, 125, 10778. (f) Shintani, R.; Fu, G.
C. Angew. Chem. 2003, 115, 4216. (g) Shintani, R.; Fu, G. C. Org. Lett.
2002, 4, 3699.
(4) Mercier, F.; Mathey, F. J. Organomet. Chem. 1984, 263, 55.
(5) (a) Review: Mathey, F. Acc. Chem. Res. 2004, 37, 954. (b) Holand,
S.; Jeanjean, M.; Mathey, F. Angew. Chem. 1997, 109, 117. (c) Carmichael,
D.; Ricard, L.; Seeboth, N.; Brown, J. M.; Claridge, T. D. W.; Odell, B.
Dalton Trans. 2005, 2173. (d) Carmichael, D.; Klankermayer, J.; Ricard,
L.; Seeboth, N. Chem. Commun. 2004, 1144.
(6) (a) Mathey, F. J. Organomet. Chem. 1978, 154, C13. (b) See also
ref 3a.
10.1021/om060025s CCC: $33.50 © 2006 American Chemical Society
Publication on Web 03/21/2006

Notes
Organometallics, Vol. 25, No. 9, 2006 2395
Scheme 3
At room temperature in cyclohexane the equilibrium constant
for the dissociation of Fp2 is K ) 2 × 10-17 mol/L.7 The
i
sterically demanding derivative [(C5 Pr5)Fe(CO)2] is a stable
monomeric species in solution but a dimer in the solid state.8
The photochemical dissociation of Fp2 is also a well-known
process.9 To the best of our knowledge, 19-electron species of
the type [CpFe(CO)2L] have not been isolated, due to their
strong reducing character. However, they are considered to play
a role as transient species in the disproportionation reaction of
Fp2 with various donor ligands.10 It was shown that the rate of
recombination of photochemically generated [CpFe(CO)2] radi-
cals in solution is not affected by the presence of added PPh3,11
while the reaction of Fp radicals with P(OMe)3 proceeds via
the short-lived substitution product [CpFe(CO){P(OMe)3}] to
form the mono- and disubstituted dimers [Cp2Fe2(CO)3P(OMe)3]
and [Cp2Fe2(CO)2{P(OMe)3}2] as the final products.9b The
quoted experimental observations render the original mechanistic
suggestion for the formation of phosphaferrocene unlikely, and
we suggest a more plausible pathway based on the following
experimental results.
Fp2 and tert-butyl- and phenylphospholes react in benzene
under reflux to give the monosubstituted dimers 2a,b (Scheme
3). 31P NMR spectra of the reaction mixtures indicate a virtually
complete conversion of the phenylphosphole after 10 h, whereas
the conversion of the tert-butylphosphole is ca. 50% after the
same reaction time. The selectivity of the transformation toward
the complexes 2a,b is greater than 90% in both cases, and the
products are isolated as deep green powders after chromatog-
raphy.12
Figure 1. ORTEP view of the molecular structure of 2a. H atoms
have been omitted for clarity. Selected bond lengths (pm): Fe1-
Fe2 ) 253.75(7), Fe1-P1 ) 220.8(1), Fe1-C13 ) 189.9 (3), Fe1-
C14 ) 188.1(4), Fe2-C13 ) 195.0(4), Fe2-C14 ) 195.6(3), Fe2-
C15 ) 174.2(4), C13-O13 ) 117.8(4), C14-O14 ) 118.4(4),
C15-O15 ) 114.7(4). Selected angles (deg): C13-Fe1-C14 )
98.1(2), C13-Fe2-C14 ) 93.9(1), Fe1-C13-Fe2 ) 82.5(1),
Fe1-C14-Fe2 ) 82.8(1), Fe1-Fe2-C15 ) 104.3(1), Fe2-Fe1-
P1 ) 103.7(1), C15-Fe2-Fe1-P1 ) 7.1(1).
Bz)]14 (B), [Cp2Fe2(CO)3{P(OPh)3}]15 (C), and [Cp2Fe2(CO)2-
(1,1′-diphenyl-3,3′,4,4′-tetramethyl-2,2′-diphosphole)]16 (D). In
detail, the Fe-Fe distance of 253.75(7) pm is almost identical
with the values found in A (253.1 pm), B (254.0 pm), and the
P,P-bridged complex D (252.0 pm). The Fe-CO bonds to the
bridging carbonyl ligands are slightly shorter for the P-
substituted iron atom Fe1 (189.9(3) and 188.1(4) pm) than for
the other atom Fe2 (195.0(4) and 195.6(3) pm). A similar
asymmetry was found in complex B (averaged values 187.3
and 193.6 pm), while the related value in the parent compound
A is 191.7 pm. There is only a small deviation of 7° from an
ideal eclipsed arrangement of the terminal CO and the phosphole
P atom. The two Fe1-Fe2-C planes containing C13 or C14,
respectively, form an interplanar angle of 163°, identical with
the value in Fp2 and close to those of the derivatives B (157°)
and C (159°).
The phenyl derivative 2a, which has been prepared earlier
by Mathey,1 could be characterized by X-ray diffraction. The
molecular structure of the complex is depicted in Figure 1
together with selected bond lengths and angles. The structure
is quite typical for a monosubstituted Fp2 derivative, featuring
a cis arrangement of the two CpFe fragments with two bridging
carbonyl groups. The structural parameters all lie within the
range typically observed for related derivatives such as the
parent compound cis-[CpFe(CO)2]213 (A), [Cp2Fe2(CO)3(PPh2-
If the above reaction is carried out in refluxing xylene (160
°C), phosphaferrocene 1 is formed. There is no evidence for a
disubstituted Fp2 dimer where both terminal CO ligands have
been replaced by phosphole. Indeed, such disubstituted dimers
are rare but some examples are known, including [CpFe(CO)-
{P(OMe)3}]2 and the 2,2′-diphosphole-bridged dimer D.16
17
Interestingly, the latter complex does not yield diphosphafer-
rocene on heating.
(7) (a) Tenhaeff, S. C.; Covert, K. J.; Castellani, M. P.; Grunkemeier,
J.; Kunz, C.; Weakley, T. J. R.; Koenig, T.; Tyler, D. R. Organometallics
1993, 12, 5000. (b) Pugh, J. R.; Meyer, T. J. J. Am. Chem. Soc. 1992, 114,
3784. (c) Cutler, A. R.; Rosenblum, M. J. Organomet. Chem. 1976, 120,
87.
(8) Sitzmann, H.; Dezember, T.; Kaim, W.; Baumann, F.; Stalke, D.;
Ka¨rcher, J.; Dormann, E.; Winter, H.; Wachter, C.; Kelemen, M. Angew.
Chem. 1996, 108, 3013.
(9) For the photochemistry of Fp2 see: (a) Vitale, M.; Lee, K. K.;
Hemann, C. F.; Hille, R.; Gustafson, T. L.; Bursten, B. E. J. Am. Chem.
Soc. 1995, 117, 2286 and references cited therein. (b) Dixon, A. J.; George,
M. W.; Hughes, C.; Poliakoff, M.; Turner, J. J. J. Am. Chem. Soc. 1992,
114, 1719. (c) Scherer, O. J.; Schwarz, G.; Wolmersha¨user, G. Z. Anorg.
Allg. Chem. 1996, 622, 951.
(10) (a) Tyler, D. R.; Mao, F. Coord. Chem. ReV. 1990, 97, 119. (b)
Castellani, M. P.; Tyler, D. R. Organometallics 1989, 8, 2113.
(11) (a) Review: Meyer, T. J.; Caspar, J. V. Chem. ReV. 1985, 85, 187.
(b) Caspar, J. V.; Meyer, T. J. J. Am. Chem. Soc. 1980, 102, 7794.
(12) For the preparation and properties of complexes of the type Cp2-
Fe2(CO)3(PR3) see: (a) Zhang, S.; Brown, T. L. Organometallics 1992,
11, 4166. (b) Haines, R. J.; DuPreez, A. L. Inorg. Chem. 1969, 8, 1459.
t
Heating the pure Bu compound 2b in boiling xylene yields
selectively phosphaferrocene 1, Fp2, and isobutene. A slow
stream of nitrogen was passed through the reaction flask during
the heating period, and low-boiling-point volatiles were isolated
from the gas stream in a cooling trap at liquid-nitrogen
temperature. Isobutene was the only isolated species and was
identified by GC-MS and NMR analysis. In particular, there
(13) Bryan, R. F.; Greene, P. T.; Newlands, M. J.; Field, D. S. J. Chem.
Soc. A 1970, 3068.
(14) Klasen, C.; Lorenz, I.-P.; Schmid, S.; Beuter, G. J. Organomet.
Chem. 1992, 428, 379.
(15) Cotton, F. A.; Frenz, B. A.; White, A. J. Inorg. Chem. 1974, 13,
1407.
(16) Dupuis, A.; Gouygou, M.; Daran, J.-C.; Balavoine, G. G. A. Bull.
Soc. Chim. Fr. 1997, 134, 357.
(17) Tyler, D. R.; Schmidt, M. A.; Gray, H. B. J. Am. Chem. Soc. 1983,
105, 6018.

2396 Organometallics, Vol. 25, No. 9, 2006
Notes
Scheme 4
compound 3 was independently prepared according to the
literature by starting from Na[CpFe(CO)2] and methallyl
chloride23 and its behavior in refluxing xylene was investi-
gated: the only products observed under these conditions were
Fp2 and isobutene, confirming that â-elimination to [CpFe-
(CO)2H] and subsequent release of H2 were indeed operative
under preparative conditions. The identification of H2 in the
volatiles was not attempted.
An alternative potential pathway from the phosphole-
substituted dimer 2b to the phosphaferrocene 1 might involve
fragmentation of 2b to give the two 17-electron species [CpFe-
(CO)2] and [CpFe(CO)(tert-butylphosphole)], respectively. While
dimerization of the former fragment could straightforwardly
account for the formation of Fp2, the selective transformation
of the latter species into phosphaferrocene and isobutene does
not seem very convincing, due to its radical character. Instead,
was no isobutane and no other butene isomers. This result makes
the intermediate presence of free tert-butyl radicals very
unlikely, because they certainly would not selectively decay to
isobutene but also to isobutane by H-atom abstraction. Under
the conditions of the preparative phosphaferrocene synthesis,
i.e., Fp2 and tert-butylphosphole in a 1:2 ratio in refluxing
xylene, isobutene is again the only detectable species in the
volatiles. (The identification of H2 was not attempted.) Under
optimized conditions, phosphaferrocene can be isolated in 65%
yield.3a
t
the formation of products expected to arise from Bu radicals
would be anticipated: e.g., isobutane or dimeric tBu-tBu. Since
those species could not be detected, we consider the mechanism
outlined in Scheme 4 the more plausible.
Obviously, it is more difficult to rationalize the formation of
phosphaferrocene from 1-phenylphosphole. Very likely, the
reaction similarly proceeds via the monosubstituted dimer 2a.
However, â-elimination does not provide a suitable pathway,
due to the highly unfavorable release of benzyne.
In conclusion, we have provided a meaningful mechanistic
pathway for the formation of phosphaferrocene from 1-tert-
butylphosphole and Fp2 under thermal conditions.
To account for the selective formation of isobutene during
the reaction, we propose the reaction pathway given in Scheme
4: the initial step is the substitution of one CO ligand in Fp2
by tert-butylphosphole, leading to the substituted dimer 2b,
which is unstable under the reaction conditions (160 °C) and
reacts further under simultaneous fragmentation of the two CpFe
t
fragments and the P-CMe3 bond. Transfer of the Bu group
from phosphorus to the second Fe atom thus generates [CpFe-
t
(CO)2 Bu] (3) andsafter elimination of another CO groups1
equiv of phosphaferrocene 1. â-H elimination of isobutene from
the Bu complex 3 offers a straightforward explanation as to
Experimental Section
t
General Procedures. Reactions were carried out under an
atmosphere of dry nitrogen by means of conventional Schlenk
techniques. Solvents were dried and purified by standard methods.
Alumina was heated at 220 °C for 12 h, cooled to room temperature
under high vacuum, deactivated with 5% water, and stored under
nitrogen. NMR spectra were recorded on a Bruker Avance DRX
500 (1H, 500 MHz; 31P{1H}, 202 MHz; 13C{1H}, 126 MHz) and a
Bruker Avance DRX 200 spectrometer (1H, 200 MHz; 31P{1H},
why this is the only observed product originating from the tert-
butyl group and strongly supports the mechanistic suggestion.
The resulting hydride complex [CpFe(CO)2H] is known to be
thermally labile and to form Fp2 under concomitant release of
H2, even below room temperature.18 In contrast, the phosphine
hydride derivatives [CpFe(CO)(PR3)H] are stable species.19
â-H elimination is a well-known process for the decomposi-
tion of metal alkyl complexes, which may or may not be
isolable, depending on the nature of the metal, the coligands,
and the alkyl group. Various complexes of the type [CpFe-
(CO)2R] have been prepared, isolated, and characterized,
including the methyl, ethyl, n-propyl, long-chain alkyl, and tert-
butyl derivatives.20 Under photochemical conditions in a mo-
lecular beam, scission of the Fe-R bond in [CpFe(CO)2R]
proceeds by both â-elimination and homolytic cleavage, produc-
ing free radicals.21 The irradiation of [CpFe(CO)2Et] in hexane
solution produces Fp2 and both ethylene and ethane. However,
the formation of ethane presumably does not proceed via ethyl
radicals.22
1
81 MHz). H spectra are referenced to the residual solvent signal
and 31P spectra to external H3PO4 (85%). Mass spectra were
recorded on a Varian MAT 311A spectrometer (EI, 70 eV electron
t
energy). [CpFe(CO)2 Bu] (3) was prepared according to literature
procedures, starting from NaFp and methallyl chloride.23,24
Synthesis of 2b. A mixture of 1-tert-butyl-3,4-dimethylphos-
phole25 (1.45 g, 8.6 mmol) and [CpFe(CO)2]2 (3.05 g, 8.6 mmol)
in 50 mL of benzene was refluxed for 1 day. 31P NMR inspection
of the crude mixture indicated a 50% conversion of starting
phosphole to product 2b. Purification by chromatography on
alumina with hexane/diethyl ether (8:1) afforded the pure product
as a deep green powder (1.04 g, 25%). 1H NMR (500 MHz, acetone-
d6): 0.97 (d, 3JHP ) 14.2 Hz, 9H, tBu), 2.02 (s, 6H, CH3), 4.54 (s,
5H, Cp), 4.72 (s, 5H, Cp), 5.96 (d, 2H, 2J(HP) ) 32.6 Hz,
phospholyl R-H). 31P{1H} NMR (81 MHz, CDCl3): 83.50 ppm
(s). MS: 429 [M+ - Cp], 401 [M+ - Cp - CO]. Anal. Calcd: C,
55.91; H, 5.51. Found: C, 55.72; H, 5.25.
t
To corroborate the assumption of the Bu complex 3 being
an intermediate in the reaction which decays via â-elimination,
(18) (a) Green, M. L. H.; Street, C. N.; Wilkinson, G. Z. Naturforsch.
1959, B14, 738. (b) Davison, A.; Green, M. L. H.; Wilkinson, G. J. Chem.
Soc. 1961, 3172. (c) Brown, D. A.; Glass, W. K.; Ubeid, M. T. Inorg. Chim.
Acta 1984, 89, L47.
(19) Reger, D. L.; Culbertson, E. C. J. Am. Chem. Soc. 1976, 98, 2789.
(20) (a) Short chain: Emeran, A.; Gafoor, M. A.; Goslett, J. K. I.; Liao,
Y.-H.; Pimble, L.; Moss, J. R. J. Organomet. Chem. 1991, 405, 237. (b)
Long chain: Hill, R. O.; Marias, C. F.; Moss, J. R.; Naidoo, K. J. J.
Organomet. Chem. 1999, 587, 28.
Synthesis of 2a. 2a (2.37 g) was obtained in a manner analogous
to that for 2b from 1-phenyl-3,4-dimethylphosphole26 and Fp2 in
66% yield after chromatography. Crystals suitable for X-ray
diffraction were obtained by cooling a hexane solution to 4 °C. 1H
(21) (a) Bartz, J. A.; Barnhart, T. M.; Galloway, D. B.; Huey, L. G.;
Glenewinkel-Meyer, T.; McMahon, R. J.; Crim, F. F. J. Am. Chem. Soc.
1993, 115, 8389. (b) See also: Gerhartz, W.; Ellerhorst, G.; Dahler, P.;
Eilbracht, P. Liebigs Ann. Chem. 1980, 1296.
(22) Alt, H. G.; Herberhold, M.; Rausch, M. D.; Edwards, B. H. Z.
Naturforsch. 1979, 34b, 1070.
(23) (a) Giering, W. P.; Rosenblum, M. J. Organomet. Chem. 1970, 25,
C71. (b) Pannell, K. H.; Giasolli, T.; Kapoor, R. N. J. Organomet. Chem.
1986, 316, 315.
(24) Faller, J. W.; Johnson, B. V. J. Organomet. Chem. 1975, 88, 101.
(25) Mathey, F. Tetrahedron 1972, 28, 4171.
(26) Breque, A.; Mathey, F.; Savignac, P. Synthesis 1981, 983.

Notes
Organometallics, Vol. 25, No. 9, 2006 2397
NMR (500 MHz, acetone-d6): 1.81 (s, 6H, 2 × CH3), 4.34 (s, 5H,
Cp), 4.61 (s, 5H, Cp), 6.04 (d, 2H, 2J(HP) ) 33,1 Hz, phospholyl
R-H), 7.56 (m, 5H, phenyl). 31P{1H} NMR (81 MHz, CDCl3):
72.70 (s). Anal. Calcd: C, 58.41; H, 4.51. Found: C, 58.53; H,
4.90.
Thermal Treatment of 2a,b. 2b (1 mmol) was heated in xylene
under reflux for 1 h, and the volatiles were removed under vacuum.
Chromatography of the residue on alumina yielded 0.9 mmol (90%)
of phosphaferrocene 1 (hexane) and 0.45 mmol (90%) of Fp2
(hexane/ether 2/1). The NMR data of the compounds are in
agreement with the literature data. Refluxing the phenyl derivative
2a in xylene leads quantitatively to a mixture of 1 and 2-phenyl-
3,4-dimethylphosphaferrocene (9:1), as shown by in situ 31P NMR
spectroscopy.
T ) 293 K, STOE CCD, Mo KR radiation, λ ) 0.710 73 Å,
triclinic, space group P1h, a ) 8.2305(13) Å, b ) 8.7775(14) Å,
c ) 16.3203(19) Å, R ) 74.432(12)°, â ) 79.150(12)°, γ ) 79.994-
(13)°, V ) 1106.0(3) Å3, Z ) 2, Dc )1.544 Mg/m3, µ(Mo KR) )
1.409 mm-1, F(000) ) 528, total of 12 687 reflections, 4075 unique
reflections, R(int) ) 0.0481, 3421 reflections with I > 2σ(I),
structure solution by direct methods27 and subsequent ∆F syntheses,
full-matrix least-squares refinement28 on F2, 281 parameters, final
results R1 ) 0.0427, wR2 ) 0.0895 (I > 2σ(I)), R1 ) 0.0523,
wR2 ) 0.0946 (all data), GOF ) 1.046, maximum/minimum
residual electron density +0.66 /-0.46 e Å-3. Crystallographic data
(excluding structure factors) have been deposited with the Cam-
bridge Crystallographic Data Centre as supplementary publication
no. CCDC-298729. Copies of the data can be obtained free of
charge on application to The Director, CCDC, 12 Union Road,
Cambridge CB2 1EZ, U.K. (fax, int. +1223/336-033; E-mail,
teched@chemcrys.cam.ac.uk) or via the Internet at http://
www.ccdc.cam.ac.uk.
Volatile Products Formed from Fp2 and tert-Butylphosphole.
A mixture of 1-tert-butyl-3,4-dimethylphosphole25 (10.8 g, 64.6
mmol) and [CpFe(CO)2]2 (12.4 g, 35.2 mmol) in 100 mL of xylene
was refluxed for 4 h with a cooling trap attached to the reflux
condenser and kept at -78 °C to collect volatile products. About
2 mL of liquid was collected during the reaction and identified as
isobutene (36 mmol) by GC-MS and 1H NMR. Evaporation of the
solvent from the reaction mixture in the flask under vacuum and
purification of the crude product by chromatography gave 9.1 g
(39.2 mmol, 60.5%) of analytically pure 1.
Acknowledgment. Financial support of this work by the
Deutsche Forschungsgemeinschaft (DFG, SFB 380) is gratefully
acknowledged.
t
t
Supporting Information Available: A CIF file giving X-ray
structural information for compound 2a. This material is available
free of charge via the Internet at http://www.acs.org.
Thermal Treatment of [CpFe(CO)2 Bu] (3). [CpFe(CO)2 Bu]
(3; 150 mg, 0.64 mmol) was heated under reflux in 5 mL of xylene
for 1 h (until gas evolution had ceased). Ca. 35 µL of volatile
material was collected in a cooling trap (-78 °C) and identified
by GC-MS to be a mixture of xylene and isobutene. No isobutane
or 2,2,3,3-tetramethylbutane could be detected. Fp2 was identified
as the reaction product in the flask by 1H NMR and EI-MS, being
formed in almost quantitative yield.
OM060025S
(27) Sheldrick, G. M. SHELXS86: Program for the Solution of Crystal
Structures; University of Go¨ttingen, Go¨ttingen, Germany, 1985.
(28) Sheldrick, G. M. SHELXL97: Program for the Refinement of
Crystal Structures; University of Go¨ttingen, Go¨ttingen, Germany, 1997.
X-ray Structure Determination of Complex 2a. Crystal data:
C25H23Fe2PO3, Mr ) 514.10, red platelet, 0.8 × 0.5 × 0.1 mm3,