Selective metal-to-ring alkyl migration during irradiation of CpFe(CO)2[CHPh(OSiMe3)]

Article abstract of DOI:10.1021/om980872y

UV-vis irradiation of CpFe(CO)₂[CHPh-(OSiMe₃)] (1b) in the presence of PPh₃ produces η⁴-{exo-C₅H₅[CHPh(OSiMe ₃]}Fe(CO)₂PPh₃ (3b), the product of alkyl migra

Full text of DOI:10.1021/om980872y

Organometallics 1999, 18, 1569-1570
1569
Selective Meta l-to-Rin g Alk yl Migr a tion d u r in g
Ir r a d ia tion of Cp F e(CO)2[CHP h (OSiMe3)]
Nancy E. Carpenter,† Masood A. Khan, and Kenneth M. Nicholas*
Department of Chemistry and Biochemistry, University of Oklahoma,
Norman, Oklahoma 73019
Received October 22, 1998
Summary: UV-vis irradiation of CpFe(CO)2[CHPh-
(OSiMe3)] (1b) in the presence of PPh3 produces η4-{exo-
C5H5[CHPh(OSiMe3)]}Fe(CO)2PPh3 (3b), the product of
alkyl migration from iron to the Cp ring, in good yield.
rather than the one expected for 2b. Furthermore, the
1H NMR spectrum of 3b did not show the anticipated
single Cp resonance (plus two from the CHPh(OSiMe3)
and CHPh(OSiMe3) groups), but rather, seven nonaro-
matic resonances were present. Similarly, the 13C NMR
spectrum of 3b exhibited five resonances in the 45-85
ppm region, further indicating disruption of the η5
coordination mode and a lack of molecular symmetry.
To establish the structure of 3b unambiguously, single
crystals were obtained from pentane/CH2Cl2 and sub-
jected to X-ray diffraction. The resulting molecular
structure is shown in Figure 1.
Indeed, the structure of 3b consists of an -Fe-
(CO)2PPh3 fragment coordinated in an η4 fashion to a
cyclopentadiene ligand, substituted with the original
alkyl ligand exo to the iron fragment. Aside from a
disorder associated with the oxygen of the TMS group,
no unusual bond distances or angles are observed. The
exo relationship of the iron and migrated alkyl group
strongly suggests a dissociative pathway for the 1b to
3b conversion. The stereocenter of the -CHPh(OSiMe3)
group renders the â- and γ-pairs of cyclopentadienyl
carbons (and hydrogens) inequivalent (diastereotopic),
accounting for the unsymmetrical NMR spectra of 3b.
As part of our investigations of carbon-centered
organometallic radicals1 we sought to prepare the iron
alkyl complex (η5-C5H5)Fe[CHPh(OSiMe3)](CO)2PPh3
(2b). On the basis of the well-established photosubsti-
tution reactions of CpFe(CO)2R with PR3,2 including the
conversion of methoxymethyl complex 1a to 2a ,2f the
reaction of siloxyalkyl complex 1b3 with PPh3 appeared
to offer a convenient route to 2b (eq 1). We report here
on the unexpected outcome of this reaction.
Irradiation of a 0.2 M benzene/pentane (1:2) solution
of 1b containing 1.1 equiv of PPh3 (400 W Hg-vapor
lamp, Pyrex) for 3.5 h followed by solvent evaporation
and chromatography over alumina afforded the yellow
solid 3b as the major product (57%), along with a small
amount of [CpFe(CO)2]2 and an unidentified green
compound. Although the FAB mass spectrum of 3b
The 1b to 3b conversion, though unusual, is not
without precedent. Although ligand substitution is the
dominant pathway in photoreactions of (C5R5)M(CO)2R′
complexes, iron-to-Cp migration of alkyl groups has
been observed.2g The substitution quantum yield has
been found to be much higher than that for metal-
carbon bond homolysis, but migration to form (η4-
cyclopentadiene)Fe(CO)3 complexes can become signifi-
cant under CO pressure, since the net CO substitution
is suppressed.2g The present result, in which migration
greatly predominates over substitution even under
nitrogen, is extraordinary when contrasted with the
exclusive photosubstitution observed for the electroni-
cally similar complex 1a (f 2a ).2f The exo relationship
of the migrated alkyl group relative to iron in 3b is
strongly suggestive of a dissociative migration pathway.
indicated incorporation of the phosphine (m/z 617, M -
1), surprisingly, its IR spectrum exhibited two absorp-
(2) (a) Treichel, P.; Shubkin, R. L.; Barnett, K. W.; Reichard, R.
Inorg. Chem. 1966, 5, 1177. (b) Nelson, G. O.; Wright, M. E. J .
Organomet. Chem. 1980, 206, C21. (c) Nesmeyanov, A. V.; Chenskaya,
T. B.; Babakhina, G. N.; Kritskaya, I. I. Bull. Acad. Sci. USSR, Div.
Chem. Sci. (Engl. Transl.) 1970, 1129. (d) Kazlauskas, R. J .; Wrighton,
M. S. Organometallics 1982, 1, 602. (e) Alt, H. G.; Herberhold, M.;
Rausch, M. D.; Edwards, B. H. Z. Naturforsch., B 1979, 34B, 1070. (f)
Flood, T. C.; DiSanti, F. J .; Miles, D. L. Inorg. Chem. 1976, 15, 1910.
(g) Blaha, J . P.; Wrighton, M. S. J . Am. Chem. Soc. 1985, 107, 2694.
(3) Theys, R. D.; Vargas, R. M.; Wang, Q.; Hossain, M. M. Organo-
metallics 1998, 17, 1333.
tions in the metal carbonyl region (1971, 1911 cm-1
)
† Permanent address: Division of Science and Math, University of
Minnesota, Morris, MN 56267.
(1) (a) Melikyan, G. G.; Vostrowsky, O.; Bauer, W.; Bestmann, H.
J .; Khan, M.; Nicholas, K. M. J . Org. Chem. 1994, 59, 222. (b) Melikyan,
G. G.; Combs, R. C.; Lamirand, J .; Khan, M.; Nicholas, K. M.
Tetrahedron Lett. 1994, 35, 363. (c) Melikyan, G. G.; Khan, M. A.;
Nicholas, K. M. Organometallics 1995, 14, 2170. (d) Salazar, K. L.;
Khan, M. A.; Nicholas, K. M. J . Am. Chem. Soc. 1997, 119, 9053.
(4) Review: Martinho Simoes, J . A.; Beauchamp, J . L. Chem. Rev.
1990, 90, 629.
10.1021/om980872y CCC: $18.00 © 1999 American Chemical Society
Publication on Web 03/20/1999

1570 Organometallics, Vol. 18, No. 8, 1999
Notes
Ta ble 1. Cr ysta l Da ta a n d Str u ctu r e Refin em en t
Deta ils for 3b
empirical formula
fw
C35H35FeO3PSi
618.54
temp
173(2) K
wavelength
cryst syst
0.710 73 Å
triclinic
space group
unit cell dimens
P1h
a ) 10.6768(14) Å
b ) 13.001(2) Å
c ) 13.147(2) Å
R ) 100.037(8)°
â ) 105.714(9)°
γ ) 110.954(8)°
1564.1(4) Å3, 2
1.313 g/cm3
volume, Z
density (calcd)
abs coeff
0.605 mm-1
F(000)
648
cryst size
0.32 × 0.42 × 0.36 mm
θ range for data collecn
no. of rflns collected
no. of indep rflns
max and min transmissn
no. of data/restraints/params
goodness of fit on F2
final R indices (I > 2σ(I))
R indices (all data)
1.76-25.00°
5596
5249 (R(int) ) 0.0780)
0.7370 and 0.4945
5243/0/401
F igu r e 1. ORTEP diagram for 3b (50% thermal el-
lipsoids). Selected bond lengths (Å) and angles (deg): Fe1-
C20, 1.764(6); Fe1-C19, 1.776(5); Fe1-C22, 2.044(6); Fe1-
C23, 2.045(6); Fe1-C24, 2.099(6); Fe1-C21, 2.114(5); Fe1-
P1, 2.215(2); C21-C22, 1.414(7); C21-C25, 1.508(8); C22-
C23, 1.410(9); C23-C24, 1.407(9); C24-C25, 1.517(7);
C25-C26A, 1.51(2); C20-Fe1-C19, 102.2(2); C20-Fe1-
P1, 100.6(2); C19-Fe1-P1, 93.1(2); O1-C19-Fe1, 177.1-
(5); O2-C20-Fe1, 177.9(5); C22-C21-C25, 109.7(5); C23-
C22-C21, 105.9(6); C24-C23-C22, 107.0(5); C21-C25-
C24, 93.8(4); O3A-C26A-C25, 105.6(11).
1.048
R1 ) 0.0602, wR2 ) 0.1251
R1 ) 0.1147, wR2 ) 0.1728
followed by an orange band which, when concentrated, af-
forded 3b (0.17 g, 57%) as a golden foam: IR (CHCl3) 1971,
1
1911 cm-1; H NMR (C6D6) δ 7.2-7.9 (m, 20H), 5.20 (s, 1H),
5.10 (s, 1H), 3.61 (d, J ) 8, 1H), 3.45 (d, J ) 8, 1H), 2.88 (s,
1H), 2.46 (s, 1H), 0.10 (s, 9H); 13C NMR (C6D6) δ 220.1 (d, J )
12), 219.2 (d, J ) 15), 136.8, 136.4, 133.2 (d, J ) 10), 132.4 (d,
J ) 10), 131.6, 129.7, 127.0, 126.6, 83.2 (d, J ) 6), 83.0 (d, J
) 8), 64.7, 54.7, 50.3, 0.3; MS (FAB, NBA) m/z 617 (10, M -
1), 439 (100, M - CHPhOSiMe3). Elution with 20/80 ether/
petroleum ether produced a dark red band of [CpFe(CO)2]2 and
a green band which was not characterized.
X-r a y Str u ctu r e Deter m in a tion . Crystals of 3b were
obtained from hexane/CH2Cl2 at -10 °C. X-ray data were
collected at -100 °C on a Siemens P-4 diffractometer using
Mo KR (λ ) 0.710 69 Å) radiation. The data were corrected
for Lorentz and polarization effects; an empirical absorption
correction based on ψ scans was applied. The structure was
solved by the direct method using SHELXTL 5.0 and refined
by full-matrix least squares on F2 using all reflections.
Hydrogen atoms were included in the refinement with ideal-
ized parameters.The final R1 ) 0.60 is based on 3291 observed
reflections (I > 2σ(I)). Hydrogen atoms, except H21, H22, H23,
H24, and H26A, are omitted in Figure 1 for clarity. Details of
the crystal data and refinement are given in Table 1.
The efficiency of this reaction in the nonpolar medium
and the formation of [CpFe(CO)2]2 is most consistent
with Fe-C homolysis. The dominance of the metal-to-
Cp pathway in the conversion of 1b to 3b (vis a` vis 1a
f 2a ) may be the combined result of the weaker
benzylic metal bond in 1b4 and the instability of the
alternative, sterically crowded substitution product, 2b.
Exp er im en ta l Section
Gen er a l Con sid er a tion s. All reactions and manipulations
involving organometallic compounds were conducted under an
atmosphere of prepurified nitrogen using standard Schlenk
line techniques. Reagent grade tetrahydrofuran, diethyl ether,
hexane, and benzene were distilled under nitrogen from
sodium benzopheneone ketyl; dichloromethane was dried by
distillation from CaH2. All reagents were obtained com-
mercially and used without further purification. The complex
(η5-C5H5)Fe[CHPh(OSiMe3)](CO)2 (1b) was prepared as de-
scribed by Hossain.3
Ack n ow led gm en t. We are grateful for support
provided by the donors of the Petroleum Research Fund,
administered by the American Chemical Society.
P h otor eaction of (η5-C5H5)Fe[CHP h (OSiMe3)](CO)2 (1b)
w ith P P h 3. A solution of 1b (0.25 g, 0.70 mmol) and PPh3
(0.19 g, 0.74 mmol) dissolved in a mixture of 1 mL of benzene
and 2 mL of pentane was placed in a Schlenk tube under
nitrogen. The vessel was mounted next to a 400 W Hanovia
(Hg vapor) jacketed lamp, the entire assembly was cooled to 0
°C, and the reaction mixture was then irradiated for 3 h. The
mixture was then added to the top of an activity III (neutral)
alumina column under nitrogen. Elution with 5/95 ether/
petroleum ether afforded a small amount of unreacted 1b
Su p p or tin g In for m a tion Ava ila ble: Tables of atomic
coordinates and isotropic displacement factors, bond lengths
and angles, anisotropic displacement factors, hydrogen coor-
dinates, selected torsion angles, and least-squares planes and
deviations from them and an additional figure giving the
structure for 3b. This material is available free of charge via
the Internet at http://pubs.acs.org.
OM980872Y