Reactions of the 17-electron iron-centered radicals {η5-C5Ph5Fe(CO)2. and {η5-C5Ph4(p-tolyl)}Fe(CO)2 . with organic halides and Lewis bases

Article abstract of DOI:10.1021/om960596s

The 18-electron dimers [Cp′Fe(CO)₂]₂ (Cp′ = η⁵-C₅Ph₅, η⁵-C₅Ph₄(p-tolyl)), which undergo only very slight thermal dissociation to the corresponding 17-electron monomers Cp′Fe(CO)₂^., nonetheless exhibit reactivity patterns which reflect the chemistry of the monomers. Thus, reactions with several organic halides RX give Cp′Fe(CO)₂R and Cp′Fe(CO)₂X, consistent with initial halogen atom abstraction by Cp′Fe(CO)₂^. followed by coupling of the resulting carbon-centered radical with a second molecule of Cp′Fe(CO)₂^.. Reactions of [(η⁵-C₅Ph₄(p-tolyl)}Fe(CO) ₂]₂ with small phosphines L result in displacement of η⁵-C₅Ph₄(p-tolyl) radicals rather than formation of isolable 17-electron compounds [{η⁵-C₅Ph₄(p-tolyl)}Fe(CO)L^., while reaction with the isonitrile t-BuNC results in disproportionation to the salt [{η⁵-C₅Ph₄(p-tolyl)}Fe(t-BuNC) ₃][{η⁵-C₅Ph ₄(p-tolyl)}Fe(CO)₂], possibly via the substituted species (η⁵-C₅Ph₄(p-tolyl)Fe(CO)(t-BuNC) ^.. Reactions of [{η⁵-C₅Ph₄(p-tolyl)}Fe(CO) ₂]₂ with phosphites P(OR)₃ do give the substituted radical species {η⁵-C₅Ph₄(p-tolyl)}Fe(CO){P(OR) ₃}^., but Arbuzov products such as {η⁵-C₅Ph₄(p-tolyl)}Fe(CO)₂Me, {η⁵-C₅Ph₄(p-tolyl)}Fe(CO) ₂{P(=O)-(OMe)₂}, and {η⁵-C₅Ph₄(p-tolyl)}Fe(CO){P(OMe) ₃}{P(=O)(OMe)₂} are the major products formed when R = Me and, to a lesser extent, Et. The Arbuzov reaction is hindered when R is large, and the compound {η⁵-C₅Ph₄(p-tolyl)}Fe(CO){P(O-i-Pr) ₃}^. is a persistent radical which has been characterized by, inter alia, EPR spectroscopy. The EPR spectrum of a frozen solution in benzene can be analyzed in terms of a uniaxial g matrix (g_∥ = 1.993, g_⊥ = 2.103) with isotropic ³¹P hyperfine coupling of 37 G (3.7 mT), typical of d⁷ organometallic radicals in which the unpaired electron is essentially confined to a metal d_z2 orbital.

Full text of DOI:10.1021/om960596s

Organometallics 1996, 15, 4991-5002
4991
Rea ction s of th e 17-Electr on Ir on -Cen ter ed Ra d ica ls
•
•
{η5-C5P h 5}F e(CO)2 a n d {η5-C5P h 4(p-tolyl)}F e(CO)2 w ith
Or ga n ic Ha lid es a n d Lew is Ba ses
Inga Kuksis, Istvan Kova´cs,† and Michael C. Baird*
Department of Chemistry, Queen’s University, Kingston, Ontario, Canada K7L 3N6
Keith F. Preston
Steacie Institute for Molecular Sciences, National Research Council, Ottawa,
Ontario, Canada K1A 0R9
Received J uly 17, 1996X
The 18-electron dimers [Cp′Fe(CO)2]2 (Cp′ ) η5-C5Ph5, η5-C5Ph4(p-tolyl)), which undergo
•
only very slight thermal dissociation to the corresponding 17-electron monomers Cp′Fe(CO)2 ,
nonetheless exhibit reactivity patterns which reflect the chemistry of the monomers. Thus,
reactions with several organic halides RX give Cp′Fe(CO)2R and Cp′Fe(CO)2X, consistent
•
with initial halogen atom abstraction by Cp′Fe(CO)2 followed by coupling of the resulting
carbon-centered radical with a second molecule of Cp′Fe(CO)2 . Reactions of [(η5-C5Ph4(p-
•
tolyl)}Fe(CO)2]2 with small phosphines L result in displacement of η5-C5Ph4(p-tolyl) radicals
rather than formation of isolable 17-electron compounds [{η5-C5Ph4(p-tolyl)}Fe(CO)L•, while
reaction with the isonitrile t-BuNC results in disproportionation to the salt [{η5-C5Ph4(p-
tolyl)}Fe(t-BuNC)3][{η5-C5Ph4(p-tolyl)}Fe(CO)2], possibly via the substituted species (η5-
C5Ph4(p-tolyl)Fe(CO)(t-BuNC)•. Reactions of [{η5-C5Ph4(p-tolyl)}Fe(CO)2]2 with phosphites
P(OR)3 do give the substituted radical species {η5-C5Ph4(p-tolyl)}Fe(CO){P(OR)3}•, but
Arbuzov products such as {η5-C5Ph4(p-tolyl)}Fe(CO)2Me, {η5-C5Ph4(p-tolyl)}Fe(CO)2{P(dO)-
(OMe)2}, and {η5-C5Ph4(p-tolyl)}Fe(CO){P(OMe)3}{P(dO)(OMe)2} are the major products
formed when R ) Me and, to a lesser extent, Et. The Arbuzov reaction is hindered when R
is large, and the compound {η5-C5Ph4(p-tolyl)}Fe(CO){P(O-i-Pr)3}• is a persistent radical
which has been characterized by, inter alia, EPR spectroscopy. The EPR spectrum of a
frozen solution in benzene can be analyzed in terms of a uniaxial g matrix (g| ) 1.993, g )
2.103) with isotropic 31P hyperfine coupling of 37 G (3.7 mT), typical of d7 organometallic
2
radicals in which the unpaired electron is essentially confined to a metal dz orbital.
Major advances have been made, in recent years, in
gaining an understanding of the chemical and physical
properties of 17-electron organotransition-metal com-
pounds (metal-centered radicals).1a,b This class of com-
pounds may often be stabilized with respect to dimer-
ization to the 18-electron metal-metal-bonded analogues
by substitution of small ligands by more sterically
demanding ligands, and of relevance here is substitution
of CO by tertiary phosphines1,2 and of η5-C5H5 by η5-
C5Ph5.3 In some cases, such substitutions result in
facile thermal homolysis of the metal-metal bond of an
18-electron dimer to the corresponding 17-electron spe-
cies,4 and thus [CpCr(CO)3]2 (Cp ) η5-C5H5) dissociates
to the extent of a few percent in solution at room
temperature1c,d while [CpCr(CO)2P(OMe)3]2 and [Cp*Cr-
(CO)3]2 (Cp* ) η5-C5Me5)1d dissociate much more
• 1h,i
extensively1d,g and CpCr(CO)2PPh3
and (η5-C5Ph5)-
(2) (a) McCullen, S. B.; Brown, T. L. J . Am. Chem. Soc. 1982, 104,
7496. (b) Kidd, D. R.; Cheng, C. P.; Brown, T. L. J . Am. Chem. Soc.
1978, 100, 4103. (c) Walker, H. W.; Rattinger, G. B.; Belford, R. L.;
Brown, T. L. Organometallics 1983, 2, 775. (d) Herrick, R. S.;
Herrinton, T. R.; Walker, H. W.; Brown, T. L. Organometallics 1985,
4, 42. (e) Hanckel, J . M.; Lee, K.-W.; Rushman, P.; Brown, T. L. Inorg.
Chem. 1986, 25, 1852. (f) Cooley, N. A.; MacConnachie, P. T. F.; Baird,
M. C. Polyhedron 1988, 7, 1965.
(3) (a) Broadley, K.; Lane, G. A.; Connelly, N. G.; Geiger, W. E. J .
Am. Chem. Soc. 1983, 105, 2486. (b) Connelly, N. G.; Geiger, W. E.;
Lane, G. A.; Raven, S. J .; Rieger, P. H. J . Am. Chem. Soc. 1986, 108,
6219. (c) Connelly, N. G.; Raven, S. J . J . Chem. Soc., Dalton Trans.
1986, 1613. (d) Connelly, N. G.; Raven, S. J .; Geiger, W. E. J . Chem.
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N. G. J . Am. Chem. Soc. 1987, 109, 402. (f) DeGray, J . A.; Geiger, W.
E.; Lane, G. A.; Rieger, P. H. Inorg. Chem. 1991, 30, 4100. (g) Fei, M.;
Sur, S. K.; Tyler, D. R. Organometallics 1991, 10, 419.
(4) Exceptions are fulvalene complexes, many of which exhibit no
inclination to undergo homolysis in spite of containing extraordinarily
long metal-metal bonds. See: (a) Kova´cs, I.; Baird, M. C. Organo-
metallics 1995, 14, 4074 and references therein. (b) Kova´cs, I.; Baird,
M. C. Organometallics 1995, 14, 4084. (c) Kova´cs, I.; Baird, M. C.
Organometallics 1995, 14, 5469. (d) Kova´cs, I.; Baird, M. C. Organo-
metallics 1996, 15, 3588. (e) McGovern, P. A.; Vollhardt, K. P. C. Synlett
1990, 493. (f) McGovern, P. A.; Vollhardt, K. P. C., submitted for
publication.
† NATO Science Fellow; Research Group for Petrochemistry of the
Hungarian Academy of Sciences, Veszpre´m, Hungary.
X Abstract published in Advance ACS Abstracts, October 15, 1996.
(1) (a) Baird, M. C. Chem. Rev. 1988, 88, 1217 and references
therein. (b) Trogler, W. C., Ed. Organometallic Radical Processes; J .
Organomet. Chem. Library 22; Elsevier: New York, 1990, and refer-
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Watkins, W. C.; J aeger, T.; Kidd, C. E.; Fortier, S.; Baird, M. C.; Kiss,
G.; Roper, G. C.; Hoff, C. D. J . Am. Chem. Soc. 1992, 114, 907. (e)
Adams, R. D.; Collins, D. E.; Cotton, F. A. J . Am. Chem. Soc. 1974,
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metallics 1993, 12, 116.
S0276-7333(96)00596-1 CCC: $12.00 © 1996 American Chemical Society

4992 Organometallics, Vol. 15, No. 23, 1996
Kuksis et al.
Cr(CO)3• 1j are completely monomeric in solution and the
solid state. Similarly, the compound [(η5-C5Ph5)Mo-
(CO)3]2 also dissociates in solution, unusual among
metal-metal-bonded dimers of the heavier transition
metals.3g
ences between the two being whether the relative
canting of the five aryl groups on the five-membered
rings is the same or different.6d We have previously
shown that both [Cp‡Fe(CO)2]2 and [Cp†Fe(CO)2]2 un-
dergo slight thermal homolysis in solution to the cor-
responding dicarbonyl radical species,6d and we now
describe experiments designed to probe reactions of
these 17-electron species with organic halides.
Until recently,5 the iron triad had yet to yield ex-
amples of persistent metal-centered radicals, and we
therefore initiated an investigation into the chemistry
of iron-centered radicals of the type η5-Cp′′Fe(CO)L•
(Cp′′ ) Cp, substituted Cp; L ) bulky ligands).6 It was
known that the 17-electron compounds CpFe(CO)2• and
As has been previously established, many metal-
centered radicals readily abstract halogen atoms from
alkyl halides (eq 3).1a,b,2d,e,9 When LnM is present in
•
Cp*Fe(CO)2 may be formed photochemically from the
LnM• + RX f LnMX + R•
(3)
corresponding dimers as in eq 1, but they are exceed-
ingly reactive and recombine rapidly to form the corre-
sponding 18-electron dimers (eq 1).7 Our attempts to
sufficiently high concentrations that bimolecular cou-
pling reactions of LnM and R become competitive, the
corresponding alkylmetal compounds LnMR may also
be formed. Then the net overall reaction is
hν
•
[Cp′Fe(CO)2]2 y
\
z 2Cp′Fe(CO)2
(1)
stabilize substituted monomeric iron compounds with
sterically demanding phosphines have also failed. For
instance, abstraction of the hydridic hydrogen atom
from CpFe(CO)(PPh3)H with trityl radical yielded not
the persistent CpFe(CO)(PPh3)• but the diiron compound
Cp2Fe2(CO)3(PPh3), presumably via CpFe(CO)(PPh3)•
(eq 2).6a,c
2LnM• + RX f LnMX + LnMR
(4)
In general, rates of reaction increase as the strength of
the R-X bond decreases, i.e. RI > RBr > RCl, t-BuX >
i-PrX > EtX > MeX; benzylic and allylic halides also
react relatively quickly.
We also describe experiments designed to probe
reactions of [{η5-C5Ph4(p-tolyl)}Fe(CO)2]2 with Lewis
bases, which are expected to involve CO substitution
via bimolecular processes.1a,b We anticipated enhancing
the extent of homolysis of the dimers on substitution of
a CO ligand with sterically more demanding ligands
such as phosphines, isonitriles, and phosphites, thus
possibly resulting in the formation of persistent metal-
centered radicals. Although ultimately successful (see
below), this line of research has been complicated, since
every reaction attempted has resulted in different types
of products. Preliminary reports of aspects of this work
have appeared.6a,b
2CpFe(CO)(PPh3)H + 2Ph3C• f
2CpFe(CO)(PPh3)• f Cp2Fe2(CO)3(PPh3) (2)
In view of the effectiveness with which the η5-C5Ph5
ligand stabilizes the persistent metal-centered radicals
•
(η5-C5Ph5)M(CO)3 (M ) Cr, Mo),1j,3g we have more
recently turned our attention to compounds in the
analogous η5-C5Ph5-iron system.8 As entries to the iron
system, we have developed an improved synthetic route
to the dimer [(η5-C5Ph5)Fe(CO)2]2 (henceforth the η5-
C5Ph5 group will be denoted Cp‡)8a-c,j and a route to the
new analogue [{η5-C5Ph4(p-tolyl)}Fe(CO)2]2 (henceforth
the η5-C5Ph4(p-tolyl) group will be denoted Cp†).6d The
latter was expected to be more soluble than [(η5-C5Ph5)-
Fe(CO)2]2 but in fact exists in unusual high- and low-
solubility, diastereomeric forms, the apparent differ-
Exp er im en ta l Section
Experiments were conducted under an inert atmosphere of
oxygen-free-grade nitrogen, further purified through a heated
BASF catalyst and molecular sieves. Manipulations of air-
sensitive materials followed standard Schlenk line techniques
and included the use of a Vacuum Atmospheres glovebox. The
solvents benzene, toluene, tetrahydrofuran, hexanes, and
diethyl ether were dried and distilled over alkali metals.
CH2Cl2 was dried and distilled over CaH2. Solvents were
thoroughly deoxygenated prior to use by saturation with N2
or repeated freeze-thaw cycles. Chromatographic separations
were typically carried out in a cold-water-jacketed column
using alumina or silica gel. Chemicals were obtained from
Aldrich, BDH, Fischer, Strem, and Fluka and were used as
received.
Infrared spectra were acquired on Bruker 85 IFS FT-IR and
Bruker IFS 25 FT-IR spectrometers; IR data are presented in
Table 1. NMR spectra were obtained on Bruker ACF 200
(200.1 MHz, 1H; 50.3 MHz, 13C{1H}) and AM 400 (400.1 MHz,
1H; 100.6 MHz, 13C{1H}) FT-NMR spectrometers using benzene-
d6 unless stated otherwise. The residual proton and the carbon
resonances of deuterated solvents served as internal references
(5) See, for instance: (a) Ruiz, J .; Lacoste, M.; Astruc, D. J . Am.
Chem. Soc. 1990, 112, 5471. (b) Giese, B.; Thoma, G. Helv. Chim. Acta
1991, 74, 1135. (c) Giese, B.; Thoma, G. Helv. Chim. Acta 1991, 74,
1143. (d) Hill, D. H.; Parvez, M. A.; Sen, A. J . Am. Chem. Soc. 1994,
116, 2889. (e) J onas, K.; Klusmann, P.; Goddard, R. Z. Naturforsch.
1995, 50B, 394. (f) Hamon, P.; Toupet, L.; Hamon, J .-R.; Lapinte, C.
Organometallics 1996, 15, 10.
(6) (a) Kuksis, I.; Baird, M. C. Organometallics 1994, 13, 1551. (b)
Kuksis, I.; Baird, M. C. J . Organomet. Chem. 1996, 512, 253. (c) Kuksis,
I.; Baird, M. C. J . Organomet. Chem., in press. (d) Kuksis, I.; Baird,
M. C. Organometallics, in press.
(7) (a) Moore, B. D.; Simpson, M. B.; Poliakoff, M.; Turner, J . J . J .
Chem. Soc., Chem. Commun. 1984, 972. (b) Moore, B. D.; Poliakoff,
M.; Turner, J . J . J . Am. Chem. Soc. 1986, 108, 1819. (c) Bloyce, P. E.;
Campen, A. K.; Hooker, R. H.; Rest, A. J .; Thomas, N. R.; Bitterwolf,
T. E.; Shade, J . E. J . Chem. Soc., Dalton Trans. 1990, 2833. (d) Dixon,
A. J .; George, M. W.; Hughes, C.; Poliakoff, M.; Turner, J . J . J . Am.
Chem. Soc. 1992, 114, 1719.
(8) (a) McVey, S.; Pauson, P. L. J . Chem. Soc. 1965, 4312. (b)
Connelly, N. G.; Manners, I. J . Chem. Soc., Dalton Trans. 1989, 283.
(c) Field, L. D.; Hambley, T. W.; Lindall, C. M.; Masters, A. F.
Polyhedron 1989, 8, 2425. (d) Field, L. D.; Ho, K. M.; Lindall, C. M.;
Masters, A. F. Webb, A. G. Aust. J . Chem. 1990, 43, 281. (e) Brown,
K. N.; Field, L. D.; Lay, P. A.; Lindall, C. M.; Masters, A. F. J . Chem.
Soc., Chem. Commun. 1990, 408. (f) Bre´gaint, P.; Hamon, J .-R.;
Lapinte, C. J . Organomet. Chem. 1990, 398, C25. (g) Field, L. D.;
Hambley, T. W.; Lay, P. A.; Lindall, C. M.; Masters, A. F. J . Chem.
Soc., Dalton Trans. 1991, 1499. (h) Bre´gaint, P.; Hamon, J .-R.; Lapinte,
C. Organometallics 1992, 11, 1417. (i) Field, L. D.; Masters, A. F.;
Gibson, M.; Latimer, D. R.; Hambley, T. W.; Buys, I. E. Inorg. Chem.
1993, 32, 211. (j) Aroney, M. J .; Buys, I. E.; Dennis, G. D.; Field, L. D.;
Hambley, T. W.; Lay, P. A.; Masters, A. F. Polyhedron 1993, 12, 2051.
(9) (a) Brown, T. L. In ref 1b. (b) Chock, P. B.; Halpern, J . J . Am.
Chem. Soc. 1969, 91, 582. (c) Halpern, J .; Maher, J . P. J . Am. Chem.
Soc. 1964, 86, 2311. (d) Halpern, J .; Maher, J . P. J . Am. Chem. Soc.
1965, 87, 5361. (e) Halpern, J .; Phelan, P. F. J . Am. Chem. Soc. 1972,
94, 1881. (f) MacConnachie, C. A.; Nelson, J . M.; Baird, M. C.
Organometallics 1992, 11, 2521. (g) Huber, T. A.; Macartney, D. H.;
Baird, M. C. Organometallics 1993, 12, 4715. (h) Huber, T. A.;
Macartney, D. H.; Baird, M. C. Organometallics 1995, 14, 592.

Reactions of 17-Electron Iron-Centered Radicals
Organometallics, Vol. 15, No. 23, 1996 4993
Ta ble 1. IR Da ta for th e Com p ou n d s Cp ‡F e(CO)2X
a n d Cp †F e(CO)2X
material had disappeared and new bands at 2028, 2001, and
1988 cm-1 had appeared. The solvent was removed in vacuo,
and the residue was dissolved in methylene chloride. The IR
spectrum exhibited CO bands attributable to Cp‡Fe(CO)2I and
Cp‡Fe(CO)2Me.8b See Table 1.
Cp‡Fe(CO)2X
νCO (cm-1
Cp†Fe(CO)2X
νCO (cm-1
)
X
)
X
Eth yl Iod id e. A suspension of 0.01 g of [Cp‡Fe(CO)2]2 (9.0
× 10-6 mol) in 5 mL of benzene was treated with 3.1 µL of EtI
(3.9 × 10-5 mol), and the reaction was monitored by IR
spectroscopy. Within 10 min, the bands of the starting
material had disappeared and new bands attributable to
Cp‡Fe(CO)2I and Cp‡Fe(CO)2Et8f (Table 1) had appeared. 1H
NMR (C6D6): δ 1.71 (t, CH2CH3) and 2.37 (q, CH2CH3).
Isop r op yl Iod id e. A suspension of 0.014 g of [Cp‡Fe(CO)2]2
(1.3 × 10-5 mol) in 5 mL of benzene was treated with 5.0 µL
of i-PrI (5.0 × 10-5 mol), and the reaction was monitored by
IR spectroscopy. Within 15 min, the bands of the starting
material had been replaced by two bands attributable to
Cp‡Fe(CO)2I and one band tentatively attributable to Cp‡Fe-
(CO)2(i-Pr) (the other CO band would be obscured by the band
at 1988 cm-1; see Table 1). The solvent was removed in vacuo,
and the residue was dissolved in C6D6. 1H NMR (C6D6): δ
1.78 (d, CHMe2), 3.43 (m, CHMe2). The NMR assignments
were corroborated by an NMR experiment involving 0.017 g
of [Cp‡Fe(CO)2]2 (1.5 × 10-5 mol) and 6.2 µL of i-PrI (6.2 ×
10-5 mol) in ∼1.5 mL of C6D6. 1H NMR (C6D6): δ 1.77 (d,
CHMe2), ∼3.42 (m, CHMe2). Resonances of propane at δ 0.85
(m, Me) and 1.25 (m, CH2) and of propylene at δ 1.54 (d, Me),
∼4.95 (m), ∼5.02 (s), and ∼5.7 (m) were also observed. An IR
spectrum of the NMR solution exhibited strong CO bands
attributable to Cp‡Fe(CO)2I at 2027 and 1988 cm-1 and a
significantly weaker band attributable to Cp‡Fe(CO)2CHMe2
H
Br
I
Me
Et
i-Pr
allyl
benzyl
2008, 1951
2033, 1993c
2027, 1988
2002, 1948b
1994, 1942
∼1989, 1938
∼1997, 1948
∼1998, 1949
H
Br
I
Me
Et
i-Pr
allyl
2007, 1951
2033, 1992c
2026, 1986
2000, 1947
1993, 1941
1993, 1940
1998, 1948
1998, 1949
2027, 1981
2025, 1980
2021, ∼1972
benzyl
PO(OMe)2
PO(OEt)2
PO(O-i-Pr)2
a
All spectra were recorded in benzene unless otherwise noted.
Recorded in CH2Cl2. c Recorded in THF.
b
1
for H and 13C resonances, respectively. 31P NMR spectra were
run on the AM 400 (162 MHz) spectrometer on solutions in
benzene-d6 and were referenced externally to 85% H3PO4. EPR
spectra were recorded on a Varian E-12 X-band spectrometer
equipped with a Bruker ER035M gauss meter for magnetic
field measurements and a Systron-Donner microwave fre-
quency counter. A thermostated Dewar assembly held samples
at the center of the resonant rectangular cavity for spectra
run at 96 K.
Mass spectra were acquired on
a Fisons VG Quattro
instrument utilizing a direct insertion probe in the CI mode
with isobutane as reagent gas and flow injection analysis with
pneumatically assisted electrospray ionization. FAB positive
measurements utilizing m-NBA as matrix were acquired on
an Autospec Fisons high-resolution spectrometer at Fisons.
Elemental analyses for carbon, hydrogen, and nitrogen were
carried out by Canadian Microanalytical Services, Delta,
British Columbia, Canada.
at 1938 cm-1
.
Ben zyl Br om id e. A suspension of 0.015 g of [Cp‡Fe(CO)2]2
(1.35 × 10-5 mol) in 5 mL of benzene was treated with 6.5 µL
of benzyl bromide (5.5 × 10-5 mol), and the reaction was
monitored by IR spectroscopy. Within 6 min, the bands of the
starting material had been replaced by two bands attributable
to Cp‡Fe(CO)2Br8d and a weak band tentatively attributable
to Cp‡Fe(CO)2(CH2Ph) (Table 1; the other CO band would be
obscured by the band at 1998 cm-1). The solvent was removed
in vacuo, and the residue was dissolved in C6D6. 1H NMR
(C6D6): δ 3.65 (s, CH2). The NMR assignments were confirmed
by an NMR experiment involving 0.014 g of [Cp‡Fe(CO)2]2 (1.3
× 10-5 mol) and 6 µL of PhCH2Br (5.0 × 10-5 mol) in ∼2 mL
of C6D6. 1H NMR (C6D6): δ 3.65 (s, FeCH2), 2.73 (s, bibenzyl).
Allyl Br om id e. A suspension of 0.02 g of [Cp‡Fe(CO)2]2
(1.6 × 10-5 mol) in 5 mL of benzene was treated with 6 µL of
allyl bromide (6.9 × 10-5 mol), and the reaction was monitored
by IR spectroscopy. Within 34 min, the bands of the starting
material had been replaced by new bands attributable to
Cp‡Fe(CO)2Br8d and a weak band tentatively attributable to
Cp‡Fe(CO)2(η1-C3H5) (Table 1; the other CO band would be
obscured by the band at 1998 cm-1). The solvent was removed
in vacuo, and the residue was dissolved in C6D6. 1H NMR
(C6D6): δ 3.06 (d, J HH 9 Hz, 2H, FeCH2), 5.03 (dd, J HH 10 Hz,
1H, dCH2), 5.37 (dd, J HH 17 Hz, 1H, dCH2), 6.57 (m, 1H,
CHd). The NMR assignments were confirmed by an NMR
experiment involving 0.011 g of [Cp‡Fe(CO)2]2 (1.3 × 10-5 mol)
and 3.5 µL of allyl bromide (4.0 × 10-5 mol) in ∼2 mL of C6D6.
The above-mentioned resonances of Cp‡Fe(CO)2(η1-C3H5) were
all observed, in addition to resonances of 1,5-hexadiene at δ
1.99 (s) and 4.95 (m); the third resonance was obscured by a
resonance of allyl bromide.
Solutions of trityl radical in benzene were prepared by the
zinc reduction of Ph3CCl,10,11 while Cp‡Fe(CO)2Br,8d Cp‡Fe-
6d
(CO)2H,8a Cp†Fe(CO)2Br,8d [Cp‡Fe(CO)2]2,6d and [Cp†Fe(CO)2]2
were prepared by literature methods, the dimers by hydridic
hydrogen abstraction by trityl radical from the corresponding
hydrides.6d
Rea ction s of [Cp ‡F e(CO)2]2 w ith Or ga n ic Ha lid es. IR
scale reactions of organic halides with [Cp‡Fe(CO)2]2 were
normally conducted in the dark using 10-35 mg of the dimer
suspended in 5 mL of benzene and 3-7 µL (4× excess) of
organic halide, which were injected directly into the solution.
IR spectra of the solutions were taken prior to organic halide
addition and at various time intervals afterwards. Usually,
the reactions were complete within 35 min (IR), at which time
the dark green cloudy suspension had largely disappeared. The
solvent was then removed in vacuo, and the residue was
dissolved in C6D6 and filtered through Celite into an NMR
tube, and a 1H NMR spectrum was recorded. NMR scale
reactions of [Cp‡Fe(CO)2]2 with organic halides were normally
conducted using 11-17 mg of the dimer, suspended in ∼1.5-
2.0 mL of C6D6, and a 4× excess of organic halide in an NMR
tube. Spectra of the occasionally shaken reaction mixtures
were run periodically as required, and IR spectra of the
solutions were obtained after the final NMR spectra were
acquired to confirm completion of the reactions.
Meth yl Iod id e. A suspension of 0.035 g of [Cp‡Fe(CO)2]2
(3.1 × 10-5 mol) in 7 mL of benzene was treated with 0.1 mL
of MeI (1.61 × 10-3 mol), and the reaction was monitored by
IR spectroscopy. Within 30 min, the bands of the starting
Rea ction s of [Cp †F e(CO)2]2 w ith Or ga n ic Ha lid es.
Reactions of [Cp†Fe(CO)2]2 with organic halides were carried
out with both suspensions of the insoluble form and solutions
of the soluble form, all in benzene, following the procedures
described above for [Cp‡Fe(CO)2]2.
Meth yl Iod id e. A solution of 0.05 g of the soluble form of
[Cp†Fe(CO)2]2 (4.5 × 10-5 mol) in 125 mL of benzene was
treated with 111 µL of MeI (1.8 × 10-3 mol). After 100 min,
the solution was a clear, dark yellow-brown, and an IR
spectrum exhibited only bands attributable to Cp†Fe(CO)2I and
(10) (a) Drake, P. R.; Baird, M. C. J . Organomet. Chem. 1989, 363,
131. (b) Koeslag, M. D.; Baird, M. C. Organometallics 1994, 13, 11. (c)
Ungva´ry, F.; Marko´, L. J . Organomet. Chem. 1980, 193, 383. (d) Turaki,
N. N.; Huggins, J . M. Organometallics 1986, 5, 1703. (e) Edidin, R. T.;
Hennessy, K. M.; Moody, A. E.; Okrasinski, S. J .; Norton, J . R. New J .
Chem. 1988, 12, 475. (f) Eisenberg, D. C.; Lawrie, C. J . C.; Moody, A.
E.; Norton, J . R. J . Am. Chem. Soc. 1991, 113, 4888.
(11) (a) Smith, W. B. J . Chem. Educ. 1970, 47, 535. (b) Lankamp,
H.; Nauta, W. T.; MacLean, C. Tetrahedron Lett. 1968, 249.

4994 Organometallics, Vol. 15, No. 23, 1996
Kuksis et al.
Cp†Fe(CO)2Me (Table 1), all strong and all of comparable
intensity. Column chromatography, on alumina equilibrated
with hexanes, and elution with increasing concentrations of
toluene in hexanes removed a yellow fraction and a brown
fraction. An IR spectrum in benzene of the yellow fraction
exhibited CO bands attributable to Cp†Fe(CO)2Me, prepared
and characterized below, while a 1H NMR spectrum (C6D6)
exhibited resonances reasonably attributable to Cp†Fe-
(CO)2Me: δ 1.07, (s, FeMe), 1.91 (s, tolyl Me), 6.66 (d, aryl H),
7.25-6.69 (m, aryl H).
The brown fraction was recrystallized from toluene and
hexanes to give pure Cp†Fe(CO)2I; a 1H NMR spectrum
(toluene-d8) exhibited resonances at δ 1.91 (s, tolyl Me), 2.12
(s), 6.6-7.4 (aryl H). 13C{1H) NMR (CDCl3): δ 21.22 (tolyl Me),
100.18, 101.16 (Cp† ring C), 126.38, 127.56, 127.65, 128, 128.29,
128.96, 129.78, 129.99, 132.45, 132.53, 132.59, 138.33 (aryl C),
215.30 (CO). Anal. Calcd for C38H27FeIO2: C, 65.35; H, 3.90.
Found: C, 64.72; H, 4.20. The same compound was obtained
on treating [Cp†Fe(CO)2]2 with iodine in benzene.
Further confirmation of the identification of Cp†Fe(CO)2Me
was obtained by synthesizing it by treating 0.75 g Cp†Fe-
(CO)2Br (1.2 × 10-3 mol) in 25 mL of THF at 0 °C with 0.6 mL
of a 3.0 M ethyl ether solution of MeMgBr (1.8 × 10-3 mol).
After 45 min, the IR spectrum of the solution exhibited only
the bands of the product (2000, 1947 cm-1); the volume of
solvent was reduced in vacuo to ∼1.5 mL, and the resulting
mixture was transferred to the top of an n-hexane-equilibrated
alumina column with an additional 3 mL of THF. Elution with
THF in n-hexane (1:12 THF-hexane) resulted in the removal
of a yellow band containing the product. The solution was
taken to dryness, and the resulting residue was recrystallized
from n-hexane-CH2Cl2 to give 0.22 g of yellow powder (32.5%).
Anal. Calcd for C39H30FeO2: C, 79.87; H, 5.16. Found: C,
79.74; H, 5.27. 1H NMR (C6D6): δ 1.07 (s, FeMe), 1.91 (s, tolyl
Me), 6.66 (d, J HH 7.8 Hz, aryl H), ∼6.89 (m, aryl H), 7.10 (d,
J HH 8.2 Hz, aryl H), 7.20 (m, aryl H). 13C{1H} NMR (C6D6): δ
-6.87 (FeMe), 20.98 (tolyl Me), 102.14, 102.64, 103.14 (Cp† ring
C), 127.73, 128.09, 128.30, 128.87, 129.05, 129.82, 132.32,
132.43, 132.56, 132.82, 133, 137.47 (aryl C; some resonances
may have been obscured by those of the solvent), 218.47 (CO).
Eth yl Iod id e. A suspension of 0.017 g of [Cp†Fe(CO)2]2 (1.5
× 10-5 mol) in 5 mL of benzene was treated with 3.4 µL of EtI
(4.25 × 10-5 mol). The reaction was completed within 15 min
(IR), the reaction mixture becoming yellow, and the IR
spectrum exhibited CO bands attributable to Cp†Fe(CO)2I and
Cp†Fe(CO)2Et (Table 1). The solvent was removed in vacuo,
and the sample was dissolved in methylene chloride. Attempts
to isolate the ethyl compound pure were unsuccessful, but a
chemical ionization mass spectrum (isobutane as a carrier gas)
exhibited very weak peaks for Cp†Fe(CO)2Et (MW 600.5): [M]+
600.2; [M - 1]+ 599.3, [M - CO]+ (MW 572.5) 572.3.
collected, the solvent was removed, and the residue was
recrystallized from CH2Cl2-pentane at 195 K to give a small
amount of rather unstable material which exhibited CO bands
in the IR spectrum (benzene) at 1993 and 1940 cm-1
. A
chemical ionization mass spectrum using a direct-insertion
probe and isobutane as the carrier gas exhibited peaks for [M]+
and [M + H]+. Cp†Fe(CO)2CHMe2 (MW 614.6 g/mol): [M]+
)
614.4, [M + 1]+ ) 615.4. A second, darker yellow fraction was
identified as Cp†Fe(CO)2I.
1
In a H NMR experiment, a suspension of 0.016 g of [Cp†Fe-
(CO)2]2 (1.40 × 10-5 mol) in ∼1.5 mL of C6D6 was treated with
5.6 µL (5.6 × 10-5 mol) of 2-iodopropane. A 1H NMR spectrum,
run after ∼35 min, exhibited the tolyl methyl resonance of
Cp†Fe(CO)2I at δ 1.88, as well as resonances attributable to
Cp†Fe(CO)2CHMe2 at δ 3.44 (m, CH), 1.92 (s, tolyl Me), and
1.80 (d, J HH 6.4 Hz, Me). Integration of the tolyl methyl
resonances showed that Cp†Fe(CO)2I was present in ∼2.5
times greater quantity than Cp†Fe(CO)2CHMe2. The reso-
nances of propylene at δ 5.02 (m, CH2), 4.96 (m, CH), and 1.54
(d, J HH 6.4 Hz, Me) and of propane at δ 1.27 (m, CH2) and
0.86 (t, J HH 7.2 Hz, Me) were also observed, the ratio of
propylene to propane being ∼1.2:1. An IR spectrum of the
solution showed that the carbonyl band of Cp†Fe(CO)2CHMe2
at 1938 cm-1 was about half as intense as that of Cp†Fe(CO)2I
at 2026 cm-1
.
ter t-Bu tyl Iod id e. A suspension of 0.02 g of [Cp†Fe(CO)2]2
(1.84 × 10-5 mol) in 10 mL of benzene was treated with 4.5
µL (3.9 × 10-5 mol) of tert-butyl iodide. The dark green
solution turned dark golden yellow in ∼2 min, and an IR
spectrum exhibited bands of Cp†Fe(CO)2I in addition to much
weaker bands of Cp†Fe(CO)2H. In an NMR study, a suspen-
sion of 0.014 g of [Cp†Fe(CO)2]2 (1.23 × 10-6 mol) in ∼2 mL of
C6D6 was treated with 3 µL (2.6 × 10-5 mol) of tert-butyl iodide.
A
1H NMR spectrum of the resulting orange solution was
rather broad after 15 min but much sharper after 45 min, and
the tolyl methyl resonance of Cp†Fe(CO)2I was evident at δ
1.88. Isobutylene formation was also indicated by resonances
at δ 4.74 (br s) and ∼1.6 (overlapped) as was isobutane by
resonances at δ 0.86 (d) and ∼1.6 (overlapped). The proportion
of isobutylene to isobutane was found to be 3:4 on the basis of
integration. A very weak Fe-H resonance for Cp†Fe(CO)2H
was observed at δ -10.4.
Slightly different results were obtained when the reaction
was carried out using the soluble form of [Cp†Fe(CO)2]2.
A
solution of 0.024 g of [Cp†Fe(CO)2]2 (2.10 × 10-5 mol) in 10
mL of benzene was treated with 5 µL (4.2 × 10-5 mol) of tert-
butyl iodide. The solution quickly turned yellow, and an IR
spectrum exhibited strong bands of Cp†Fe(CO)2I and Cp†Fe-
(CO)2H (Table 1). The latter set was only slightly less intense
than the former.
Ben zyl Br om id e. A suspension of 0.016 g of [Cp†Fe(CO)2]2
(1.4 × 10-5 mol) in 5 mL of benzene was treated with 6.6 µL
(5.55 × 10-5 mol) of PhCH2Br. The reaction mixture turned
golden yellow within 8 min, and an IR spectrum exhibited the
bands of both Cp†Fe(CO)2Br and, tentatively, Cp†Fe(CO)2CH2-
Ph (Table 1). Electrospray, FAB, and chemical ionization mass
spectrometry experiments on the residue obtained after re-
moval of solvent yielded no useful information.
In an NMR experiment, a suspension of 0.016 g of [Cp†Fe-
(CO)2]2 (1.4 × 10-5 mol) in ∼1.5 mL of C6D6 was treated with
6.6 µL of PhCH2Br (5.55 × 10-5 mol). The reaction mixture
had become an orange-brown solution within 15 min, and a
1H NMR spectrum exhibited resonances of both Cp†Fe(CO)2Br
(tolyl methyl resonance at δ 1.88) and Cp†Fe(CO)CH2Ph (tolyl
methyl resonance at δ 1.92, CH2 resonance at δ 3.66). The
ratio of Cp†Fe(CO)2Br to Cp†Fe(CO)CH2Ph was 1.3:1, on the
basis of integration of the methyl resonances, while an IR of
the solution showed the carbonyl bands of the two products to
be of comparable intensities.
In an NMR experiment, a suspension of 0.016 g of [Cp†Fe-
(CO)2]2 (1.4 × 10-5 mol) in ∼1 mL of C6D6 was treated with
4.5 µL of EtI (5.6 × 10-5 mol), and the reaction was monitored
for 1.5 h as the mixture changed from dark green to a clear
brown. The 1H NMR spectra exhibited resonances attributable
to Cp†Fe(CO)2Et at δ 2.37 (q, -CH2CH3), 1.72 (t, -CH2CH3), and
1.92 (s, tolyl Me) and to Cp†Fe(CO)2I at δ 1.88 (s, tolyl Me).
Integration of the two tolyl methyl resonances showed that
∼17% more iodo product was present. Both ethane (δ 0.79
(s)) and ethylene (δ 5.25 (s)) were observed as minor compo-
nents. An IR spectrum of the solution exhibited approximately
equal-intensity CO bands for Cp†Fe(CO)2Et and Cp†Fe(CO)2I.
Isop r op yl Iod id e. A suspension of 0.45 g of [Cp†Fe(CO)2]2
(4.0 × 10-4 mol) in 40 mL of benzene was treated with 160 µL
of 2-iodopropane (1.6 × 10-3 mol). The mixture became a clear
brown solution within ∼10 min, and an IR spectrum exhibited
the bands of Cp†Fe(CO)2I (Table 1) in addition to a weaker
band at 1940 cm-1
,
tentatively attributable to Cp†Fe-
(CO)2CHMe2. The solvent was removed in vacuo, the residue
was dissolved in a minimum of THF, and this solution was
chromatographed on an n-hexane-equilibrated column with
25% THF in n-hexane. The yellow fraction initially eluted was
Allyl Iod id e. A solution of 0.019 g of the soluble form of
[Cp†Fe(CO)2]2 (1.66 × 10-5 mol) in 10 mL of benzene was
treated with 5 µL of CH2dCHCH2I (5.5 × 10-5 mol). The
solution instantly turned to moss green, and an IR spectrum

Reactions of 17-Electron Iron-Centered Radicals
Organometallics, Vol. 15, No. 23, 1996 4995
run within 2 min exhibited only bands, of comparable inten-
sity, attributable to Cp†Fe(CO)2I and (tentatively) to Cp†Fe-
(CO)2CH2CHdCH2 (Table 1). A similar experiment, involving
a suspension of 0.019 g of the insoluble form of [Cp†Fe(CO)2]2
and 5 µL of CH2dCHCH2I in 10 mL benzene, turned moss
green within 2 min but orange-brown within 6 min. While
strong IR bands of Cp†Fe(CO)2I were present, only a weak,
high-frequency shoulder at 1986 cm-1 and a weak band at 1949
cm-1 indicated the presence of Cp†Fe(CO)2CH2CHdCH2. No
IR bands corresponding to [Cp†Fe(CO)2]2 were evident.
In an NMR experiment, a suspension of 0.014 g of [Cp†Fe-
(CO)2]2 (1.22 × 10-5 mol) in 2 mL of C6D6 was treated with
solvent was removed in vacuo to give [Cp†Fe(t-BuNC)3][Cp†Fe-
(CO)2]. Difficulties were experienced trying to purify the
compound, and the suggested formulation is based on spec-
troscopic and chemical evidence. IR (benzene): 2178, 2147,
1863, 1794 cm-1
.
1H NMR (C6D6): δ 2.04 (s, 3H, tolyl Me),
1.95 (s, 3H, tolyl Me), 1.30 (br s, 27H, t-Bu).
A yellow-orange solution of 0.027 g of [Cp†Fe(t-BuNC)3]-
[Cp†Fe(CO)2] (1.99 × 10-5 mol) in 5 mL of benzene was treated
with 5 µL (8.03 × 10-5 mol) of MeI, turning pale yellow
instantly. The IR spectrum showed that all starting material
had reacted, with new bands appearing at 2180, 2149, 2000
and 1946 cm-1
. The last two bands are attributed to the
1
4.5 µL (4.9 × 10-5 mol) of allyl iodide. After ∼20 min, the H
known compound Cp†Fe(CO)2Me (Table 1) and those at 2180
and 2149 cm-1 to the complex [Cp†Fe(t-BuNC)3]I. Volatiles
were removed in vacuo, but attempts at recrystallization from
toluene/pentane/hexane were unsuccessful. 1H NMR (C6D6)
of the reaction mixture exhibited the resonance at δ 1.07 (s,
3H, Fe-Me) in addition to a resonance at δ 1.5 (s, 9H, t-Bu).
A synthesis on a large scale resulted in precipitation of [Cp†Fe-
(t-BuNC)3]I, which was washed with benzene and pentane. 1H
NMR (C6D6): δ 1.52 (s, 27H, Me3C), 1.92 (s, 3H, tolyl Me), 6.70
(d, J HH 8.0 Hz, aryl H), 6.90-6.80 (m, aryl H), 7.16 (d, J HH 1.6
Hz, aryl H), 7.26-7.22 (m, aryl H). 13C{1H} NMR (CDCl3): δ
21.21 (tolyl Me), 30.66 (Me3C), 59.18 (Me3CNC), 98.94, 99.30,
99.80 (Cp† ring C), 127.44, 127.69, 128.16, 128.30, 128.41,
130.81, 130.90, 131.78, 131.97, 138.16 (aryl C), 157.55
(Me3CNC). Elemental analyses were inconclusive, but high-
resolution mass spectral measurements confirmed the forma-
tion of the complex [Cp†Fe(t-BuNC)3]+ (MW 764.85). Fast
atom bombardment resulted in the appearance of peaks
corresponding to the calculated isotopic distribution for the
molecular ion.
NMR spectrum of the brown solution exhibited resonances of
Cp†Fe(CO)2I, the major product, at δ 1.88 (s, tolyl Me) and of
Cp†Fe(CO)2CH2CHdCH2, the minor product, at δ 1.92 (s, tolyl
Me), ∼3.06 (d, -CH2) and ∼5.02 (m, dCH2). Additional
resonances for the allyl group, expected at ∼δ 5.34 and ∼6.57
compared with those observed for Cp‡Fe(CO)2CH2CHdCH2
(see above), were either barely discernable at ∼δ 5.35 or
completely obscured by aryl resonances at ∼δ 6.61. 1,5-
Hexadiene was identified by resonances at δ 2.00 (m, CH2),
4.95 (m, CdCH2), 5.00 (s, CdCH2), and ∼5.72 (m, partially
overlapped, CHd). An IR spectrum of the solution verified
that the major product was Cp†Fe(CO)2I, only weak bands
being attributable to Cp†Fe(CO)2CH2CHdCH2. A chemical
ionization mass spectrometry experiment yielded no useful
information.
At t em p t s To F or m Su b st it u t ed Com p ou n d s Cp †F e-
(CO)L: Rea ction s of [Cp †F e(CO)2]2 w ith 13CO, P Me3, a n d
t-Bu NC. Rea ction s of [Cp †F e(CO)2]2 w ith 13CO. A stream
of 13CO was bubbled through a solution of [Cp†Fe(CO)2]2 in
benzene for 15 s. The solution was stirred for 30 min and then
treated with a stream of 13CO for a further 15 s. An IR
spectrum of the reaction mixture showed that the 12CO bands
at 1955, 1780, 1989, and 1921 cm-1 had been completely
Rea ction s of [Cp †F e(CO)2]2 w ith P h osp h ites. P (OMe)3.
A suspension of 0.053 g of [Cp†Fe(CO)2]2 (4.6 × 10-5 mol) in 5
mL of benzene was treated with 12 µL (1.0 × 10-4 mol) of
P(OMe)3. Much of the solid dissolved within 2 min, and strong
bands at 2027, 2000, 1981, and 1946 cm-1 were present in the
IR spectrum as well as a weak band at 1897 cm-1; the bridging
CO band of [Cp†Fe(CO)2]2 (1780 cm-1) had diminished in
intensity by ∼50%. The reaction was complete within 15 min
(yellow solution), and the solvent was removed in vacuo. The
IR spectrum of the residue exhibited the strong CO bands
mentioned above, attributable to Cp†Fe(CO)2Me and, tenta-
tively, Cp†Fe(CO)2{P(dO)(OMe)2} (Table 1), but the weak band
at 1897 cm-1, attributable to the substituted radical Cp†Fe-
replaced by bands at 1911, 1740, 1942, and 1876 cm-1
respectively.
,
Rea ction s of [Cp †F e(CO)2]2 w ith P Me3. A solution of
0.10 g of [Cp†Fe(CO)2]2 (8.8 × 10-5 mol) in 20 mL of benzene
was treated with 36 µL (3.48 × 10-4 mol) of PMe3 at room
temperature, and the reaction mixture was monitored by IR
spectroscopy for 2.5 h. Within 10 min, the IR spectrum showed
a significant decrease in the intensity of the bridging CO band
of the dimer at 1780 cm-1 and the growth of a band at 1874
cm-1 attributable to Fe(CO)3(PMe3)2.12 An additional 8 mL of
PMe3 was added to consume all remaining dimer, and ulti-
mately the IR spectrum exhibited only strong bands attribut-
able to Fe(CO)3(PMe3)2 and Cp†Fe(CO)2H (2007, 1954 cm-1).
The same products were obtained on treating Cp†Fe(CO)2H
with PMe3 and trimethylamine oxide in THF. There was no
IR evidence in the latter reaction for the formation of Cp†Fe-
(CO)(PMe3)H, although a 1H NMR spectrum of the products
formed exhibited a weak hydride doublet resonance at δ
-13.35 (J PH 88 Hz).
(CO){P(OMe)3} (see below), had disappeared.
A
1H NMR
spectrum of the reaction mixture (C6D6) exhibited three strong
singlet tolyl methyl resonances (δ 1.91, 1.89, 1.95; ∼1:1:0.5
ratio of intensities, respectively), the iron-methyl resonance
of Cp†Fe(CO)2Me (δ 1.07), and a doublet at δ 3.64 (J PH 11.0
Hz, OMe), tentatively attributed to Cp†Fe(CO)2{P(dO)(OMe)2}.
Also observed were doublet resonances at δ 3.42 (J PH 10 Hz,
OMe), 3.56 (d, J PH 11.0 Hz) and δ 3.77 (d, J PH 10 Hz),
tentatively attributed to Cp†Fe(CO){P(OMe)3}{P(dO)(OMe2)}.
The 31P{1H} NMR spectrum exhibited a singlet at δ 103.8,
tentatively attributed to Cp†Fe(CO)2{P(dO)(OMe)2}, and a
doublet of doublets at δ 116.7 and 170.8 (J PP 155 Hz),
tentatively attributed to Cp†Fe(CO){P(OMe)3}{P(dO)(OMe2)}.
If we assume similar extinction coefficients for the CO bands
of Cp†Fe(CO)2Me and Cp†Fe(CO)2{P(dO)(OMe)2} and utilize
the 31P integrations, the ratio of the reaction products formed
is ∼1:1:0.7 for Cp†Fe(CO)2Me, Cp†Fe(CO)2{P(dO)(OMe)2}, and
Cp†Fe(CO){P(OMe)3}{P(dO)(OMe2)}, reasonably consistent
with the tolyl methyl integrations in the 1H NMR spectrum.
A solution of 0.05 g of Cp†Fe(CO)(PMe2Ph)H (7.46 × 10-5
mol) in 10 mL of benzene was wrapped with aluminum foil
and treated with a 4-fold excess of trityl radical in 7 mL of
benzene solution. The reaction was monitored by IR spectros-
copy, but there was no apparent reaction after stirring for 16
h at room temperature followed by 3 h of refluxing.
Rea ction s of [Cp †F e(CO)2]2 w ith ter t-Bu NC. A suspen-
sion of 0.09 g of the insoluble form of [Cp†Fe(CO)2]2 (8.0 × 10-5
mol) in 10 mL of benzene was treated with 20 µL of t-BuNC
(1.77 × 10-4 mole, 2.2 fold excess). An IR spectrum taken 25
min after addition exhibited new bands at 2178, 2147, 1863,
and 1794 cm-1. After 84 min, the solution had turned brown,
but bands attributable to [Cp†Fe(CO)2]2 were still evident and
an additional 5 µL of t-BuNC (4.42 × 10-5 mol) was added.
The solution turned orange-red after another 30 min, and an
IR spectrum showed that the reaction was complete. The
A minor species present in the reaction mixture gave rise
to an apparent virtual triplet in the OMe region at δ 3.26 and
1
a partially obscured triplet at δ 1.12 (J PH 6 Hz) in the H NMR
spectrum, as well as a singlet at δ 172.4 in the 31P{1H} NMR
spectrum.
In an attempt to separate the products by column chroma-
tography, a reaction mixture of 0.20 g of [Cp†Fe(CO)2]2 and
45 µL of P(OMe)3 in 20 mL of benzene was stirred for 30 min,
by which time the mixture had become a yellow solution. The
(12) Bigorgne, M. J . Organomet. Chem. 1970, 24, 211.

4996 Organometallics, Vol. 15, No. 23, 1996
Kuksis et al.
(CO)2]2 and [Cp†Fe(CO)2]2 with various reagents give
comparable products, and we therefore combine discus-
sion of analogous chemistry of the two iron compounds.
Hereafter we use Cp′ to denote Cp‡ and Cp† collectively.
Rea ction s of [Cp ‡F e(CO)2]2 a n d [Cp †F e(CO)2]2
w ith Or ga n ic Ha lid es. It has been shown previously
that the chemistry of the 18-electron, dimeric compound
[CpCr(CO)3]2 reflects the relatively high reactivity of the
solvent was removed under reduced pressure, and the residue
was dissolved in a minimum of toluene and this solution placed
on a silica gel column. Elution with hexanes resulted in the
removal of Cp†Fe(CO)2Me (νCO 2000, 1948 cm-1; δ 1.91 (s, 3H,
tolyl Me), 1.07 (s, 3 H, FeMe)), contaminated with small
amounts of other materials. Elution with 1:1 hexanes-toluene
then removed a second yellow band containing a mixture of
materials, including Cp†Fe(CO)2Me, and a previously un-
detected compound (νCO 1920 (br), 1605 (m) cm-1; δ 3.24 (d,
J HP 10.2 Hz, POMe), 2.10 (s, tolyl Me)), while elution with
toluene removed a small amount of Cp†Fe(CO)2{P(dO)(OMe)2}
(νCO 2027, 1981 cm-1; δ 3.63 (d, J HP 10.8 Hz, 6 H, POMe), 1.89
(s, 3 H, tolyl Me)). Considerable colored material remained
at the top of the column, even after an attempt to elute with
acetone.
•
17-electron monomer CpCr(CO)3 , even though homoly-
sis of the dimer is slight and the monomer constitutes
only a few percent of the total chromium in solution at
room temperature.1c,d,9f-h The two dimeric iron com-
pounds discussed behave similarly; [Cp‡Fe(CO)2]2 and
[Cp†Fe(CO)2]2 both dissociate slightly in solution,6d and
both react with organic halides to give the products
anticipated on the basis of halogen abstraction by the
corresponding 17-electron radical monomers, as in eq
5.1a,b,9
P (OEt)3. A suspension of 0.052 g (4.55 × 10-5 mol) of
[Cp†Fe(CO)2]2 in 5 mL of benzene was treated with 16 µL (9.33
× 10-5 mol) of P(OEt)3, and the reaction was monitored by IR
spectroscopy. Within ∼2 min, a band at 1894 cm-1 became
strong relative to the bands of the dimer and, after ∼3 h, there
had also appeared very weak bands at 2025, 1994, 1980, and
[Cp′Fe(CO)2]2 + RX f Cp′Fe(CO)2R + Cp′Fe(CO)2X
1941 cm-1
.
Addition of more P(OEt)3 (∼50 µL) over 1 h
resulted in formation of a dark yellow solution and in a
dramatic increase of the band at 1894 cm-1. Essentially all
the starting dimer had disappeared after 9 h.
(5)
Thus, reactions of [Cp‡Fe(CO)2]2 and [Cp†Fe(CO)2]2
with methyl iodide resulted in the formation of Cp′Fe-
(CO)2Me and Cp′Fe(CO)2I. The compound Cp‡Fe-
(CO)2Me (νCO 2002, 1948 cm-1) has been reported
previously and was characterized by comparisons with
spectroscopic data in the literature.8b The compound
Cp†Fe(CO)2Me, however, is new and was characterized
by elemental analyses and IR (νCO 2000, 1947 cm-1) and
1H NMR (δCMe 1.91, δFeMe 1.07) spectroscopy. The same
compound was prepared via reaction of Cp†Fe(CO)2Br
with MeMgBr. The iodo compound Cp†Fe(CO)2I is also
new and was characterized by elemental analysis, IR
spectroscopy (νCO 2026, 1986 cm-1), and comparison
with an authentic sample formed by reaction of [Cp†Fe-
(CO)2]2 with iodine.
By analogy with the P(OMe)3 system, the weak bands at
1941 and 1994 cm-1 are tentatively attributed to Cp†Fe(CO)2Et
and those at 2025 and 1980 cm-1 to Cp†Fe(CO)2{P(dO)(OEt)2}.
The strong band at 1894 cm-1, on the other hand, is reasonably
attributed to the substituted radical Cp†Fe(CO){P(OEt)3}• (see
below). The solvent was removed in vacuo, and a 1H NMR
spectrum run in C6D6 was too complex to be interpreted. The
31P{1H} spectrum (C6D6) of a similar reaction mixture indicated
the presence of products similar to those formed in the reaction
with P(OMe)3, i.e. a singlet at δ 100.3 tentatively attributed
to Cp†Fe(CO)2{P(dO)(OEt)2}, a doublet of doublets at δ 112.4
and 163.8 (d, J PP 156 Hz) tentatively attributed to Cp†Fe(CO)-
{P(OEt)3}{P(O)(OEt)2}, and a very weak singlet at δ 165.0
tentatively attributed to Cp†Fe{P(OEt)3}2Et. A relatively
strong, very broad resonance was observed at ∼δ 142; the
31P{1H} resonance of free P(OEt)3 is observed at δ 139.4.
P (O-i-P r )3. A suspension of 0.052 g of [Cp†Fe(CO)2]2 (4.55
× 10-5 mol) in 5 mL of benzene was treated with 23 µL (9.32
× 10-5 mol) of P(O-i-Pr)3. Reaction was very slow, and even
after several hours, the only new band of significant intensity
was observed at 1886 cm-1, tentatively attributed to Cp†Fe-
(CO){P(O-i-Pr)3}•. Resonances in the 1H NMR spectrum (C6D6)
were broad, as was a very strong resonance at δ 139.8 in the
31P{1H} NMR spectrum (C6D6); the 31P{1H} resonance of free
P(OEt)3 is observed at δ 139.4.
The chemistry of eq 5 undoubtedly proceeds via two
steps, iodine atom abstraction from methyl iodide by
Cp′Fe(CO)2• followed by coupling of the resulting methyl
radical with a second molecule of Cp′Fe(CO)2• (eqs 6 and
7).
A
1H NMR study in C6D6, using [Cp†Fe(CO)2]2,
Cp′Fe(CO)2• + MeI f Cp′Fe(CO)2I + Me• (6)
Cp′Fe(CO)2• + Me• f Cp′Fe(CO)2Me
(7)
The EPR spectrum of a similar reaction mixture in benzene
at 96 K exhibited the spectrum shown below, but no resonance
was detected in liquid benzene at 293 K. Although the
presumed Cp†Fe(CO){P(O-i-Pr)3} remained in solution for
several hours, all attempts to grow crystals failed.
showed that the methyl and iodo products formed to
comparable extents on the basis of integrations of the
respective tolyl methyl resonances, but neither methane
nor coupling products derived from the carbon-centered
radicals were observed. These results are as previously
found for reactions of [CpCr(CO)3]2 with methyl iodide.9f-h
In a similar fashion, reactions of Cp′Fe(CO)2• and EtI
resulted in the formation of comparable quantities of
Cp′Fe(CO)2Et and Cp′Fe(CO)2I, on the basis of relative
intensities of the carbonyl stretching bands in IR spectra
of reaction mixtures. The ethyl compounds could not
be obtained analytically pure, but Cp‡Fe(CO)2Et has
been reported,8f and the carbonyl stretching bands are
not only very similar but compare well with data for
the methyl analogues (Table 1).
Resu lts a n d Discu ssion
Pentaarylcyclopentadienyl ligands have been receiv-
ing increased attention in recent years as extensions of
the better known η5-C5H5 (Cp) and η5-C5Me5 (Cp*)
ligands,13 their exceptional size often providing consid-
erable steric hindrance and thus providing kinetic
stability to complexes such as 17-electron compounds.1j,3
In addition, their chiral structures, in which the aryl
rings are canted (see above), have provided much
impetus for the investigation of ring rotation dynam-
ics.13,14 In general, we find that the reactions of [Cp‡Fe-
1
A H NMR investigation indicated that Cp†Fe(CO)2I
formed to a somewhat greater extent (∼17%) than did
the ethyl complex, on the basis of integrations of the
tolyl resonances. Interestingly, small quantities of
ethylene and ethane were also detected, as was observed
(13) J aniak, C.; Schumann, H. Adv. Organomet. Chem. 1991, 33,
291.
(14) (a) Li, L.; Decken, A.; Sayer, B. G.; McGlinchey, M. J .; Bregaint,
P.; Thepot, J .-Y.; Toupet, L.; Hamon, J .-R.; Lapinte, C. Organometallics
1994, 13, 682. (b) McGlinchey, M. J ., private communication.

Reactions of 17-Electron Iron-Centered Radicals
Organometallics, Vol. 15, No. 23, 1996 4997
previously with [CpCr(CO)3]2.9f-h The ethylene may not
have been formed via â-elimination from the ethyl
compound, as ethane but no hydride, Cp†Fe(CO)2H, was
observed. Instead, the hydrocarbon products may have
been formed via the secondary reactions shown in eqs
8-10.
hydrogen abstraction reaction from Cp′Fe(CO)2H (eq
12), resulting in the formation of a higher proportion of
hydride.
The proportions of alkyl-iron products relative to iodo
product decrease in the order Me > Et > CHMe2
>
CMe3. This trend is probably a result (a) of increasing
steric hindrance to coupling of the very bulky iron-
centered radical with the organic radicals as the size of
the latter increases (Me < Et < CHMe2 < CMe3), (b) of
decreasing Fe-C bond strengths in the order Me > Et
> CHMe2 > CMe3, and (c) of competition from increas-
ingly more favorable hydrogen atom abstractions from
aliphatic fragments with increasing numbers of avail-
able â-hydrogen atoms. Since the expected coupling
products of the organic radicals Me•, Et•, i-Pr•, and t-Bu•
were not observed, olefin disproportionation reactions15
were not a factor.
Cp′Fe(CO)2• + EtI f Cp′Fe(CO)2I + Et•
(8)
Cp′Fe(CO)2• + Et• f Cp′Fe(CO)2H + C2H4 (9)
Cp′Fe(CO)2H + Et• f Cp′Fe(CO)2• + C2H6 (10)
Thus, while some ethyl compound is formed via direct
•
coupling of Et• and Cp′Fe(CO)2 , the ratio of ethyl to iodo
product is less than 1 because some of the ethyl groups
are diverted as in eqs 9 and 10.
Consistent with halogen abstraction being the initial
step, treatment of [Cp′Fe(CO)2]2 with n-butyl chloride
and bromide did not result in the type of chemistry
described above. The carbon-halogen bonds in these
cases are relatively strong,16 making halogen abstrac-
tion reactions energetically much less feasible.
Formation of alkane and alkene was much more
extensive in the reactions of Cp′Fe(CO)2• with isopropyl
iodide and, although a significant amount of iodo
compound formed, a much smaller quantity of isopropyl
compound was produced. The latter could not be
isolated pure but was readily identified by the carbonyl
stretching bands in IR spectra of reaction mixtures,
which are similar to those of the iron methyl and ethyl
complexes (Table 1), and by its 1H NMR spectrum,
which exhibited isopropyl resonances at δ 3.44 (m, CH)
and 1.80 (d, J HH 6.4 Hz, Me) and a tolyl methyl
resonance at 1.92 (s, tolyl Me). A 1H NMR spectrum of
a reaction mixture involving [Cp†Fe(CO)2]2 clearly ex-
hibited resonances of both propane and propylene but
not of Cp†Fe(CO)2H, and a sequence of reactions analo-
gous to those of eqs 8-10 again seems to be occurring.
Reaction of the low-solubility form6d of [Cp†Fe(CO)2]2
with tert-butyl iodide produced Cp†Fe(CO)2I as the
major iron-containing product; in addition, a small
amount of Cp†Fe(CO)2H was evident in the IR spectrum
of a reaction mixture. An NMR experiment showed that
isobutane and isobutylene were also formed, in compa-
rable amounts, suggesting a sequence of reactions as
outlined above for the ethyl and isopropyl systems.
There were no resonances which could be attributed to
Cp†Fe(CO)2CMe3. In contrast, a reaction of the soluble
form6d of [Cp†Fe(CO)2]2 with tert-butyl iodide turned
yellow very quickly and an IR spectrum showed that
comparable amounts of Cp†Fe(CO)2I and of Cp†Fe-
(CO)2H had formed. The latter result implies that the
relative contributions of eqs 9 and 10 have altered. In
contrast to the reaction with a suspension of the low-
solubility form of [Cp†Fe(CO)2]2, where the steady-state
concentration of Cp†Fe(CO)2• would be very low, reaction
of tert-butyl iodide with the high-solubility form of
[Cp†Fe(CO)2]2 would proceed in the presence of a
relatively high steady-state concentration of Cp†Fe-
In contrast, reaction of suspensions of [Cp′Fe(CO)2]2
with benzyl bromide readily yielded the products Cp′Fe-
(CO)2Br and Cp′Fe(CO)2CH2Ph. The IR data obtained
for Cp′Fe(CO)2Br (Table 1) are comparable with those
obtained previously for these compounds,8d while IR
data for Cp′Fe(CO)2CH2Ph (1998 and 1949 cm-1) com-
pare favorably with literature values for the Cp ana-
logue (2009 and 1958 cm-1 17
and for the compounds
)
Cp′Fe(CO)2Me (Table 1). The benzyl methylene reso-
nances were identified (δ ∼3.65) in 1H NMR experi-
ments, integration of the tolyl methyl resonances in the
Cp† system indicating that ∼32% more Cp†Fe(CO)2Br
(δ 1.88) than Cp†Fe(CO)2CH2Ph (δ 1.92) had formed,
although an IR spectrum of the same NMR solution
exhibited product bands which were of comparable
1
intensities. In a H NMR experiment carried out with
[Cp‡Fe(CO)2]2, some bibenzyl was shown to have also
formed as a result of coupling of the benzyl radicals
formed during reaction.
Similar to the benzyl system, reaction of a suspension
of [Cp‡Fe(CO)2]2 with allyl bromide yielded both Cp‡Fe-
(CO)2Br8d and Cp‡Fe(CO)2CH2CHdCH2. Although the
latter, a very minor product, could not be isolated pure,
its carbonyl stretching bands in IR spectra compare well
with those of the methyl and ethyl analogues (Table 1)
1
and a H NMR spectrum exhibited resonances at δ 3.06
(d, J HH 9 Hz, CH2), 5.03 (dd, J HH 10 Hz, dCH2), 5.37
(dd, J HH 17 Hz, dCH2), and 6.57 (m, CHd). These
chemical shifts and coupling constants are very similar
to those of CpCr(CO)3CH2CHdCH2, formed via reaction
of allyl halides with [CpCr(CO)3]2,9f and are character-
istic of a σ-allyl group. Resonances of 1,5-hexadiene,
the product of allyl radical coupling,9f were also ob-
served.
Similar results were found in reactions of [Cp†Fe-
(CO)2]2 with allyl iodide although, interestingly, reaction
of the insoluble form of the dimer produced significantly
•
(CO)2 (eqs 11-13). In the latter case, the rate of
Cp′Fe(CO)2• + t-BuI f Cp′Fe(CO)2I + t-Bu• (11)
Cp′Fe(CO)2• + t-Bu• f Cp′Fe(CO)2H + Me2CdCH2
(12)
(15) Alfassi, Z. B. In Chemical Kinetics of Small Organic Radicals;
Alfassi, Z. B., Ed.; CRC Press: Boca Raton, FL, 1988; Vol. III, p 129.
(16) (a) Egger, K. W.; Cocks, A. T. Helv. Chim. Acta 1973, 56, 1516.
(b) McMillen, D. F.; Golden, D. M. Annu. Rev. Phys. Chem. 1982, 33,
493.
Cp′Fe(CO)2H + t-Bu• f Cp′Fe(CO)2• + Me3CH (13)
hydrogen atom abstraction by iron from the t-Bu• radical
(eq 12) might well be greater than the rate of the
(17) Blaha, J . P.; Wrighton, M. S. J . Am. Chem. Soc. 1985, 107, 2694.

4998 Organometallics, Vol. 15, No. 23, 1996
Kuksis et al.
Cp†Fe(CO)2• + 213CO f Cp†Fe(13CO)2• + 2CO (18)
less of the σ-allyl product relative to the amount of iodo
product than did the soluble form. The mechanism of
these reactions is expected to occur as in eqs 14-16.
2 Cp†Fe(13CO)2• a [Cp†Fe(13CO)2]2
(19)
Cp′Fe(CO)2• + XCH2CHdCH2 f
Cp′Fe(CO)2X + •CH2CHdCH2 (14)
lar but slower 13CO substitution has also been observed
•
in CpCr(CO)3 , which is in thermal equilibrium with
[CpCr(CO)3]2.1c,d Substitution is expected to occur via
an associative mechanism, and it is possible, in spite of
the bulky Cp† group, that the iron of Cp†Fe(CO)2• is less
Cp′Fe(CO)2• + •CH2CHdCH2 f
Cp′Fe(CO)2CH2CHdCH2 (15)
•
sterically hindered than is the metal of CpCr(CO)3 , with
three coordinated carbonyl groups.
2•CH2CHdCH2 f CH2dCHCH2CH2CHdCH2 (16)
At t em p t s To P r ep a r e P h osp h in e-Su b st it u t ed
Ra d ica ls. In an attempt to form more stable iron-
centered radicals, phosphine substitution reactions of
The relative proportions of the Cp′Fe(CO)2CH2CHdCH2
•
Cp†Fe(CO)2 were attempted. It was found, however,
and 1,5-hexadiene formed therefore depend on the
that treatment of suspensions of [Cp†Fe(CO)2]2 in
benzene or toluene with PMe3 at room temperature
resulted primarily in the slow formation of a small
•
concentration of the radical species Cp′Fe(CO)2 avail-
able in solution. When the steady-state concentration
of iron-centered radical is low, as when [Cp‡Fe(CO)2]2
or the insoluble form of [Cp†Fe(CO)2]2 was used, then
the bimolecular process involved in coupling of the iron-
centered and allyl radicals is not competitive with allyl
radical self-coupling to form 1,5-hexadiene. On the
other hand, the greater steady-state concentration of
12
amount of Fe(CO)3(PMe3)2 as a consequence of Cp†
ring displacement from the metal atom. Very small
amounts of Cp†Fe(CO)2H were also formed, possibly via
hydrogen atom abstraction by Cp†Fe(CO)2• from decom-
position products. The same products were obtained
when the reaction was carried out using a solution
containing the monomer and dimer in equilibrium in
hot benzene.
•
Cp′Fe(CO)2 obtained through use of the soluble dimer
permits more extensive coupling of the iron and allyl
radicals, giving rise to a higher proportion of the
allyliron product. Radical coupling is observed only in
the allyl and benzyl systems because of the greater
stabilities of the benzyl and allyl radicals.16
In a variation of this approach, a mixture of Cp†Fe-
(CO)2H and PMe3 was treated with trityl radical to give
[Cp†Fe(CO)2]2, Fe(CO)3(PMe3)2, and unidentified prod-
ucts exhibiting two bands in the IR spectrum at 1943
and 2035 cm-1. Neither of these latter bands may be
attributed to the anticipated product Cp†Fe(CO)(PMe3),
which would be expected to exhibit a carbonyl stretching
band at a frequency comparable with that reported for
Su bstitu tion Reaction s of Cp†Fe(CO)2: Attem pted
Syn th eses of Cp †F e(CO)L (L ) P h osp h or u s Do-
n or ). The substitutional lability of 17-electron com-
plexes has long been recognized, and it is well-known
that many neutral metal-centered radicals undergo
facile substitution reactions with CO and with phos-
phines and phosphites, L, to produce substituted
radicals.1a,b The products undergo a variety of bi-
molecular secondary reactions, including coupling, un-
less stabilized by substitution with sterically demanding
ligands.1
CpFe(CO){P(OMe)3} (1907 cm-1 20a
or for compounds
)
of the type CpFe(CO)LX (L ) phosphine, phosphite; X
) H, Cl), the CO stretching bands of which also occur
at much lower frequencies.20b
Attempts at hydrogen atom abstraction from Cp†Fe-
(CO)(PMe2Ph)H using trityl radical were also un-
successful in producing Cp†Fe(CO)(PMe2Ph)•. To our
surprise and disappointment, Cp†Fe(CO)(PMe2Ph)H did
not react with trityl radical, even on extended stirring
and heating. Hydrogen atom abstraction was probably
ineffective because of steric hindrance imposed on the
metal center by the large, coordinated ligands, which
would not allow access of the central carbon atom of the
organic radical, itself a large molecule, sufficiently close
to the coordinated hydrogen atom.
Rea ction s of [Cp †F e(CO)2]2 w ith 13CO. In order
to establish substitutional lability of the 17-electron
compounds under consideration here, the reaction of
[Cp†Fe(CO)2]2 with 13CO was investigated. As antici-
pated, rapid and complete substitution into both radical
and dimer occurred, as indicated by the 40-47 cm-1
decrease in the position of the IR bands from the
nonsubstituted 12CO species. Thus, the CO bands of
Cp†Fe(12CO)2 and [Cp†Fe(12CO)2]2, at 1989, 1921 cm-1
and 1955, 1780 cm-1, respectively, were converted to
bands at 1942, 1876 cm-1 and 1911, 1740 cm-1, respec-
tively. The changes are as predicted theoretically18 and
are also as observed in the analogous Cp iron system.19
Interestingly, we find that [CpFe(CO)2]2 is inert to CO
substitution under thermal conditions, although pho-
tochemical substitution, presumably via the monomer,
is facile.19 Thus, typical of the behavior of many 17-
electron metal-centered radicals,1,19 13CO substitution
Rea ction s of [Cp †F e(CO)2]2 w ith Ison itr iles. Al-
though reactions of [Cp†Fe(CO)2]2 with t-BuNC in
benzene were expected to result in simple substitution
to form the neutral, 17-electron compound Cp†Fe(CO)-
(t-BuNC)•, the result was, instead, the ionic product of
disproportionation, [Cp†Fe(t-BuNC)3][Cp†Fe(CO)2]. At-
tempts to recrystallize the compound failed, and it was
therefore characterized spectroscopically and chemi-
cally. IR spectra of clear, orange-red reaction mixtures
exhibited only the CN stretching bands of the cation
(2178, 2147 cm-1) and the CO stretching bands of the
anion (1863, 1794 cm-1), consistent with reported IR
data for similar Cp iron salts such as [CpFe(t-BuNC)3]-
•
occurred readily into Cp†Fe(CO)2 , present in relatively
small concentrations in the solution (eqs 17-19). Simi-
•
[Cp†Fe(CO)2]2 a 2Cp†Fe(CO)2
(17)
(20) Dixon, A. J .; Gravelle, S. J .; van de Burgt, L. J .; Poliakoff, M.;
Turner, J . J .; Weitz, E. J . Chem. Soc., Chem. Commun. 1987, 1023.
(b) Treichel, P. M.; Shubkin, R. L.; Barnett, K. W.; Reichard, D. Inorg.
Chem. 1966, 5, 1177.
(18) Braterman, P. S. Metal Carbonyl Spectra; Academic Press: New
York, 1975; pp 36, 146.
(19) Zhang, S.; Brown, T. L. J . Am. Chem. Soc. 1993, 115, 1779.

Reactions of 17-Electron Iron-Centered Radicals
Organometallics, Vol. 15, No. 23, 1996 4999
Br (νCN 2175, 2135 cm-1 21a
and [Bu4N][CpFe(CO)2] (νCO
)
1864, 1787).21b In addition, 1H NMR spectra of the salt
showed clearly the presence of two types of Cp† groups,
there being two equal-intensity tolyl methyl resonances,
at δ 2.04 and 1.95.
Also, as anticipated, a red solution of [Cp†Fe(t-
BuNC)3][Cp†Fe(CO)2] was found to react readily with
methyl iodide to give, within 10 min, the compound
Cp†Fe(CO)2Me (νCO 2000, 1946 cm-1), which has been
synthesized by other routes (see above). The byproduct
of this reaction was [Cp†Fe(t-BuNC)3]I (νCN 2180, 2149
cm-1), characterized by IR, 1H NMR and 13C NMR
spectroscopy. A high-resolution FAB mass spectrum of
[Cp†Fe(t-BuNC)3]I exhibited a very strong signal for
[Cp†Fe(t-BuNC)3]+ (MW 764.85), exhibiting the peaks
corresponding to the calculated isotopic distribution for
this ion.
F igu r e 1. Structures of the Arbuzov products A-C.
Rea ction s of [Cp †F e(CO)2]2 w ith P h osp h ites:
Ar bu zov Rea r r a n gem en ts a n d Syn th eses of Cp†F e-
(CO)L (L ) P (OMe)3, P (OEt)3, P (O-i-P r )3). In spite
of our inability to prepare monocarbonyl radicals by
direct substitution of Cp†Fe(CO)2• with phosphines and
isonitriles, we carried out a similar series of reactions
of [Cp†Fe(CO)2]2 with phosphites. We began with the
relatively small P(OMe)3 and found that the reaction
of [Cp†Fe(CO)2]2 with P(OMe)3 resulted in the formation
of three major Arbuzov25 products, Cp†Fe(CO)2Me (A),
Cp†Fe(CO)2{P(dO)(OMe)2} (B), and Cp†Fe(CO)-
{P(OMe)3}{P(dO)(OMe)2} (C) (see Figure 1), as indi-
cated by the presence of three tolyl methyl resonances
and several OMe resonances in the carefully integrated
1H NMR spectrum of the reaction mixture. While little
success was achieved in separating the products (see
below), all are known or are very similar to known
compounds, and all were satisfactorily characterized by
spectroscopic means.
The presence of A was confirmed by comparison of
its IR (νCO 2000, 1946 cm-1) and NMR (1H NMR δ 1.07
(s, FeMe), 1.91 (s, tolyl Me)) spectral data with those of
the Cp†Fe(CO)2Me formed from the reaction of [Cp†Fe-
(CO)2]2 with MeI (see above). The IR spectrum of B (νCO
2027, 1981 cm-1) is very similar to that of the known
Cp‡Fe(CO)2{P(dO)(OMe)2} (νCO 2030, 1983 cm-1), which
has been synthesized by the reaction of Cp‡Fe(CO)2Br
with P(OMe)3.14a In addition, the chemical shift and
hydrogen-phosphorus coupling constant of the OMe
resonance (δ 3.64, d, J PH 11 Hz) are very similar to those
of Cp‡Fe(CO)2{P(dO)(OMe)2} (δ 3.70, d, J PH 10 Hz).14b
A singlet 31P resonance (δ 103.8), attributable to B, was
observed in the 31P{1H} NMR spectrum of the reaction
mixture. The corresponding resonance of Cp‡Fe(CO)2-
{P(dO)(OMe)2} was observed at δ 116.4, in good agree-
ment since a different solvent (CH2ClCH2Cl) was used.14a
Compounds A and B were formed in approximately
equal amounts in this experiment, but the proportions
were found to vary in experiments carried out under
somewhat different conditions.
The mechanism of disproportionation may involve
•
either initial addition to or substitution of Cp†Fe(CO)2
by t-BuNC to give a 19-electron intermediate22a or a 17-
electron intermediate,22b respectively (eq 20 or 21). The
Cp†Fe(CO)2• + t-BuNC f Cp†Fe(CO)2(t-BuNC)•
(20)
Cp†Fe(CO)2• + t-BuNC f
Cp†Fe(CO)(t-BuNC)• + CO (21)
electron-rich 19-electron species of eq 20 could then
•
reduce a molecule of Cp†Fe(CO)2 and proceed to the
cationic product as in eqs 22 and 23, while the 17-
Cp†Fe(CO)2(t-BuNC)• + Cp†Fe(CO)2• f
[Cp†Fe(CO)2(t-BuNC)]+ + [Cp†Fe(CO)2]- (22)
[Cp†Fe(CO)2(t-BuNC)]+ + 2t-BuNC f
[Cp†Fe(t-BuNC)3]+ (23)
electron species of eq 21 could similarly reduce a
•
molecule of Cp†Fe(CO)2 and proceed to products as in
eqs 24 and 25.
Cp†Fe(CO)(t-BuNC)• + Cp†Fe(CO)2• f
[Cp†Fe(CO)(t-BuNC)]+ + [Cp†Fe(CO)2]- (24)
[Cp†Fe(CO)( t-BuNC)]+ + 2t-BuNC f
[Cp†Fe(t-BuNC)3]+ + CO (25)
No distinct CO band attributable to C was observed
in the IR spectrum of the reaction mixture, but it should
occur 10-15 cm-1 lower24,26 than for the Cp analogue
In contrast, [CpFe(CO)2]2 reacts with isonitriles under
thermal conditions to form mono-, di-, and trisubstituted
neutral dimers.23 Since the cyclopentadienyl and penta-
arylcyclopentadienyl ligands are expected to be similar
with respect to electronic properties,24 it seems possible
that the bulkiness of the pentaarylcyclopentadienyl
ligand may inhibit comproportionation, thereby stabiliz-
ing the ionic products.
(1967 cm-1 27
and thus overlaps the band of A at 1946
)
1
cm-1. The H NMR spectrum of the reaction mixture
exhibits a series of OMe doublet resonances which are
completely consistent with the presence of a compound
with a structure such as C, containing a chiral iron
atom. Thus, a methyl resonance observed at δ 3.42 (d,
J PH 10 Hz) is assigned to the phosphite OMe group and
two methyl resonances at δ 3.56 (d, J PH 11 Hz) and 3.77
(21) (a) Coville, N. J .; Albers, M. O.; Singleton, E. J . Chem. Soc.,
Dalton Trans. 1983, 947. (b) Goldman, A. S.; Tyler, D. R. Inorg. Chem.
1987, 26, 253.
(22) (a) Balla, J .; Bakac, A.; Espenson, J . H. Organometallics 1994,
13, 1073. (b) Therien, M. J .; Ni, C.-L.; Anson, F. C.; Osteryoung, J . G.;
Trogler, W. C. J . Am. Chem. Soc. 1986, 108, 4037.
(23) Ennis, M.; Kumar, R.; Manning, A. R.; Howell, J . A. S.; Mathur,
P.; Rowan, A. J .; Stephens, F. S. J . Chem. Soc., Dalton Trans. 1981,
1251.
(25) Brill, T. B.; Landon, S. J . Chem. Rev. 1984, 84, 577.
(26) (a) Chambers, J . W.; Baskar, A. J .; Bott, S. G.; Atwood, J . L.;
Rausch, M. D. Organometallics 1986, 5, 1635. (b) Ramsden, J . A.;
Milner, D. J .; Hempstead, P. D.; Bailey, N. A.; White, C. J . Chem. Soc.,
Dalton Trans. 1995, 2101.
(27) Howell, J . A. S.; Rowan, A. J .; Snell, M. S. J . Chem. Soc., Dalton
Trans. 1981, 325.
(24) Adams, H.; Bailey, N. A.; Browning, A. F.; Ramsden, J . A.;
White, C. J . Organomet. Chem. 1990, 387, 305.

5000 Organometallics, Vol. 15, No. 23, 1996
Kuksis et al.
(d, J PH 10 Hz) are assigned to the diastereotopic
phosphonate OMe groups; cf. δ 3.68 (d, J PH 12 Hz), 3.53
(d, J PH 11 Hz), 3.57 (d, J PH 11 Hz), respectively, for the
Cp analogue.27 The 31P{1H} NMR spectrum provided
clear evidence for the formation of C, as a distinctive
doublet of doublets (AB quartet) at δ 116.7 and 170.8
(J PP 155 Hz) corresponds well with data for the Cp
analogue (δ 125.7 and 180.2 (J PP 139 Hz)).27
As mentioned in the Experimental Section, the reac-
tion also yielded a minor product, the 1H NMR spectrum
of which exhibited an apparent virtual triplet in the
OMe region at δ 3.26 and a partially obscured triplet
1
at δ 1.12 (t, J PH 6 Hz). The H NMR spectrum of the
compound CpFe{P(OMe)3}2Me is reported to exhibit
OMe and FeMe resonances at δ 3.73 (t) and 0.20 (t, J PH
5.3 Hz), respectively,28 and it is tempting to suggest that
this compound is Cp†Fe{P(OMe)3}2Me, a reasonable
product. While the 31P{1H} chemical shift of the puta-
tive Cp†Fe{P(OMe)3}2Me (δ 172.4) is quite different from
that reported for CpFe{P(OMe)3}2Me (δ 11.42), the
latter is well outside the region normally found for
phosphite complexes29 and may be in error.
F igu r e 2. EPR spectrum (9456.8 MHz) of a frozen solution
of Cp†Fe(CO){P(O-i-Pr)3}• in benzene at 96 K.
lower24,26 than does the corresponding band of the Cp
analogue (1907 cm-1), which has been observed by time-
resolved IR spectroscopy.20a It is also similar in fre-
quency to the CO band of the paramagnetic isopropyl
analogue, discussed below.
The reaction of [Cp†Fe(CO)2]2 with P(OEt)3 proceeded
relatively slowly to produce low yields of the Arbuzov
products Cp†Fe(CO)2Et and Cp†Fe(CO)2{PO(OEt)2},
identified on the basis of IR and 31P{1H} NMR spectra
of reaction mixtures since the 1H NMR spectra were
very complex. The IR spectrum of the ethyl compound
exhibited CO bands at 1994 and 1941 cm-1, identical
with those of the ethyl compound formed from reaction
of [Cp†Fe(CO)2]2 with EtI (see above). The identity of
the compound Cp†Fe(CO)2{P(dO)(OEt)2} was inferred
from its IR spectrum (νCO 2025, 1980 cm-1), very similar
to that of the methyl analogue (see above), while its
31P{1H} NMR spectrum exhibited a singlet at δ 100.3.
A doublet of doublets at δ 112.36 and 163.83 (J HP ∼155.5
Hz) indicated the probable formation of Cp†Fe(CO)-
{P(dO)(OEt)2}{P(OEt)3}, while a singlet at δ 165.0 may
indicate the formation of Cp†Fe{P(OEt)3}2Et.
An attempt to separate the various products on a
silica column utilizing hexanes-toluene mixtures as
eluant resulted in mixed success. Compound A eluted
readily and was characterized by IR and 1H NMR
spectroscopy, but a band containing a previously un-
detected compound followed that containing A. This
unexpected species could not be freed of contaminants,
and thus its 1H NMR resonances could not be un-
ambiguously identified. However, the presence of car-
bonyl bands at 1920 (br) and 1605 (m) cm-1 suggest that
the compound may be Cp†Fe(CO){P(OMe)3}(COMe); the
corresponding CO bands of the Cp analogue are ob-
served at 1939 and 1603 cm-1 27
in reasonable agree-
,
ment. The 1H NMR spectrum of the eluted mixture
exhibited a single POMe doublet resonance at δ 3.63
(d, J HP 10.8 Hz), clearly different from all of the
compounds present in the original reaction mixture, but
the acetyl resonance could not be unambiguously identi-
fied. The corresponding resonances of CpFe(CO)-
{P(OMe)3}(COMe) occur at δ 3.58 (d, J PH 11.3 Hz,
CDCl3) and 2.50 (d, J PH 0.5 Hz).27
However, the extent of formation of these products
was far from complete, and the IR spectrum was
dominated by a CO band at 1894 cm-1, which remained
for several hours and which is attributable to the new,
iron-centered radical Cp†Fe(CO){P(OEt)3}•. Consistent
with this assignment, the 31P{1H} NMR spectrum also
exhibited a broad (∆δ1/2 ∼20 ppm) resonance (δ ∼142),
which may be reasonably attributed to free phosphite
(δ 139) exchanging with coordinated phosphite of the
paramagnetic Cp†Fe(CO){P(OEt)3}•.
A third compound eluted from the column was C, with
IR and 1H NMR data corresponding to those inferred
from the spectra of the reaction mixture. Although C
also could not be obtained free of impurities, it clearly
was the dominant species in this fraction and the IR
Reaction of [Cp†Fe(CO)2]2 with a 17-fold excess of P(O-
i-Pr)3 proceeded even more slowly and was still incom-
plete after stirring for 7.5 h. Again the dominant CO
band in the IR spectrum was attributable to the
1
and H NMR data of this compound are unambiguously
correlated. Thus, the chromatography experiment lends
credence to the spectroscopic assignments suggested
above. However, the appearance, as a product of facile
decomposition, of the species exhibiting CO bands at
1920 (br) and 1605 (m) cm-1 and a POMe doublet
resonance at δ 3.63 underscores the problems inherent
in the separation and isolation of the products as
analytically pure compounds.
substituted radical Cp†Fe(CO){P(O-i-Pr)3}• (1886 cm-1
)
and only traces of other products were evident. Al-
though an IR study indicated that Cp†Fe(CO){P(O-i-
Pr)3}• can persist for many hours at least, all attempts
to grow crystals failed; slow decomposition occurred
instead.
No EPR spectrum was detected in liquid benzene, but
an EPR investigation of a frozen benzene solution (96
K) of Cp†Fe(CO){P(O-i-Pr)3}• revealed the spectrum
illustrated in Figure 2. The spectrum observed is
clearly that of an electronic doublet (S ) 1/2) with a
uniaxial g matrix (g| ) 1.993, g ) 2.103) and an
isotropic 31P hyperfine coupling of 37 G (3.7 mT). The
principal g values are typical of d7 organometallic
radicals in which the unpaired electron is essentially
During the course of the reaction of [Cp†Fe(CO)2]2
with P(OMe)3, a weak CO band of an apparent inter-
mediate was observed at 1897 cm-1 but was not present
at the end of the reaction. This ephemeral band is
reasonably attributable to the anticipated substituted
radical Cp†Fe(CO){P(OMe)3}•, since it occurs 10 cm-1
(28) Ruiz, J .; Lacoste, M.; Astruc, D. J . Am. Chem. Soc. 1990, 112,
5471.
(29) Nixon, J . F.; Pidcock, A. Annu. Rev. NMR Spectrosc. 1969, 2,
346.

Reactions of 17-Electron Iron-Centered Radicals
Organometallics, Vol. 15, No. 23, 1996 5001
confined to a metal dz orbital.1a,30 In particular, they
In both types of reactions, the formation of a cationic
species results in activation of the bound phosphite with
respect to nucleophilic displacement of the (RO)2P-
(dO)R′ or LnM{P(dO)(OR)2} fragments from C-1 of the
phosphite by halide ion X-, yielding phosphorus(V)
compounds as shown.
2
resemble the g values in the 17-electron Fe(I) species
[Fe(CO)5]+ and [Fe(CO)3(PPh3)2]+.30b,31a The small iso-
tropic 31P hyperfine interaction corresponds31b to only
0.8% P 3s character and is typical of a phosphorus
nucleus vicinal to the spin-bearing metal nucleus in
phosphite- or phosphine-substituted transition metal
carbonyl radicals.30b,31c It undoubtedly arises through
bond polarization by the metal d orbital; i.e., there is
little or no direct participation of phosphorus valence
orbitals in the SOMO. The spectral parameters are
certainly consistent with Cp†Fe(CO){P(O-i-Pr)3}• as the
carrier. However, the uniaxiality of g is perplexing vis-
a-vis the molecular symmetry.
Of possibly greater relevance here, there have also
been several reports of Arbuzov reactions induced by
radicals.25,34 For instance, in a number of cases, phos-
phites P(OR)3 have been observed to react with alkyl
•
and alkoxy radicals Y as in eq 28. The reactions are
P(OR)3 + •Y f •
f P(dO)(OR) (Y) + •R
{P(Y)(OR)3}
2
D
Since the Cp† ligand is topologically equivalent to
three fac CO ligands, the radical CpFe(CO)2• is isolobal32
with [Fe(CO)5]+. It is, therefore, not too surprising that
the two such species should have similar metal contri-
butions to their SOMOs and similar principal g values,
and the unexpected symmetry of g for Cp†Fe(CO){P(O-
i-Pr)3}• may possibly be a result either of an accidental
degeneracy of filled orbitals or, alternatively, of rapid
internal motions that result in a time-averaged axial
symmetry. While extended Hu¨ckel calculations for
“planar” CpCo(CO)2 do not support the simplistic deduc-
(28)
believed to involve four-coordinated, phosphorus-cen-
tered radicals of type D. Depending on the relative
•
•
stabilities of the radical products R and OR, D can
undergo homolysis of P-O or O-C bonds (the latter as
in eq 28) to give phosphorus(V) compounds and either
alkoxy or alkyl radicals.
Among the very limited number of possibly direct
precedents for the chemistry described here is the
reaction of P(OMe)3 with [CpCr(CO)2{P(OMe)3}]2, which
exists in thermal equilibrium with its 17-electron
monomer CpCr(CO)2{P(OMe)3}•.1c,d,g Reaction of [CpCr-
(CO)3]2 with 2 molar equiv of P(OMe)3 proceeds rapidly
at temperatures below 25 °C to yield primarily CpCr-
(CO)2{P(OMe)3}•, while reaction of the latter with excess
P(OMe)3 yields CpCr(CO)2{P(OMe)3}Me and CpCr(CO)2-
{P(OMe)3}{P(dO)(OMe)2} as the major products.1g An
Arbuzov rearrangement has also been observed in the
high-temperature (refluxing xylene) reaction of Cp2-
Fe2(CO)3P(OMe)3 with P(OMe)3, which yields CpFe(CO)2-
Me and CpFe(CO){P(OMe)3}{P(dO)(OMe)2}.27 Although
spontaneous homolysis of diiron species such as [CpFe-
(CO)2]2 and Cp2Fe2(CO)3P(OMe)3 at ambient tempera-
tures has not been observed,7d radical involvement at
high temperatures seems likely.
2
tion of a dz SOMO for the 17-electron analogue CpFe-
(CO)2 , they do point to significant pyramidalization in
•
passing from the 18-electron to the 16-electron species
CpMn(CO)2.33 This, together with a very soft bending
2
mode, could result in a pz-dz hybrid SOMO.
Our inability to detect an EPR spectrum of the radical
in liquid solvent is not too surprising. A number of
metal-centered radicals have proven problematic in this
regard, notably Mn(CO)5 and CpCr(CO)3.30a The prox-
imity of the d orbitals in these species, which results in
large displacements of g values from free spin, also
provides an efficient mechanism for spin relaxation and
line broadening to the point of spectral indetectability.
Mech a n ism of th e Rea ction s w ith P h osp h ites.
As first observed and studied in organophosphorus
chemistry,34a-c the Arbuzov rearrangement normally
involves the nucleophilic dealkylation of a phosphite and
the concurrent formation of a compound containing a
phosphorus(V) compound that possesses a PO double
bond, as in eq 26. Similar processes have also been
The mechanism originally proposed for the reaction
of CpCr(CO)2{P(OMe)3}• with excess P(OMe)3 to yield
CpCr(CO)2{P(OMe)3}Me and CpCr(CO)2{P(OMe)3}-
P(dO)(OMe)2}1g involved abstraction of a methyl group
from free phosphite by the radical CpCr(CO)2{P(OMe)3}•
to form CpCr(CO)2{P(OMe)3}Me and the phosphonate
•
radical intermediate, OP(OMe)2 . The latter could then
P(OR)3 + R′X f [P(OR)3R′]X f (RO)2P(dO)R′ + RX
couple with a second chromium-centered radical (eqs 29
and 30). This mechanism was subsequently revised to
(26)
observed in reactions of phosphites with complexes of
transition metals in positive oxidation states (eq 27).25
CpCr(CO)2{P(OMe)3}• + P(OMe)3 f
•
CpCr(CO)2{P(OMe)3}Me + OP(OMe)2 (29)
[LnM{P(OR)3}]X f [LnM{P(dO)(OR)2}] + RX (27)
CpCr(CO)2{P(OMe)3}• + OP(OMe)2• f
CpCr(CO)2{P(OMe)3}{P(dO)(OMe)2} (30)
(30) (a) Baird, M. C. In Organometallic Radical Processes; Trogler,
W. C., Ed.; J . Organomet. Chem. Library 22; Elsevier: New York, 1990,
and references therein. (b) MacNeil, J . H.; Chiverton, A. C.; Fortier,
S.; Baird, M. C.; Hynes, R. C.; Williams, A. J .; Preston, K. F.; Ziegler,
T. J . Am. Chem. Soc. 1991, 113, 9834.
(31) (a) Lionel, T.; Morton, J . R.; Preston, K. F. J . Chem. Phys. 1982,
76, 234. (b) Morton, J . R.; Preston, K. F. J . Magn. Reson. 1978, 30,
577. (c) Hynes, R. C.; Preston, K. F.; Springs, J . J .; Williams, A. J . J .
Chem. Soc., Dalton Trans. 1990, 3655.
(32) Albright, T. A.; Burdett, J . K.; Whangbo, M.-H. Orbital Interac-
tions in Chemistry; Wiley: New York, 1985; Chapter 20.
(33) Hofmann, P. Angew. Chem., Int. Ed. Engl. 1977, 16, 536.
(34) (a) Cadogan, J . I. G. Adv. Free Radical Chem. 1967, 2, 203. (b)
Bentrude, W. G. In Free Radicals; Kochi, J . K. Ed.; Wiley and Sons:
New York, 1973; Vol. II, p 595. (c) Bhattacharya, A. K.; Thyagarajan,
G. Chem. Rev. 1981, 81, 415. (d) Kochi, J .; Krusic, P. J . J . Am. Chem.
Soc. 1969, 91, 3944.
make it more consistent “with the products often
encountered when alkyl phosphites react with radicals”,
as in eq 2825 (eqs 31 and 32). Here elimination of a
CpCr(CO)2{P(OMe)3}• + P(OMe)3 f
CpCr(CO)2{P(OMe)3}{P(dO)(OMe)2} + Me• (31)
CpCr(CO)2{P(OMe)3}• + Me• f
CpCr(CO)2{P(OMe)3}Me (32)

5002 Organometallics, Vol. 15, No. 23, 1996
Kuksis et al.
methyl radical from coordinated phosphite is concurrent
with or followed by coordination of free phosphite to the
16-electron phosphonate species CpCr(CO)2{P(dO)-
(OMe)2}; coupling of methyl and chromium-centered
radicals ensues. A similar mechanism was also pro-
posed for the above-mentioned high-temperature reac-
tions of Cp2Fe2(CO)3P(OMe)3 with P(OMe)3.27
The reaction under consideration here, in which
[Cp†Fe(CO)2]2 reacts with P(OMe)3 to give Arbuzov
products A-C, almost certainly involves initial forma-
tion of Cp†Fe(CO){P(OMe)3}•. The latter then under-
goes dealkylation, as in the chromium system, but
assisted, presumably, by Cp†Fe(CO)2 as in eqs 33 and
34. Direct release of a methyl radical as in eq 30 seems
assisted, bimolecular reaction (eq 34) would be retarded
for steric reasons and the iron-centered radicals con-
taining the bulkier phosphites should be more stable.
Indeed, the relative proclivities of phosphites to undergo
Arbuzov rearrangements in this system, i.e. P(OMe)3
. P(OEt)3 > P(O-i-Pr)3, parallel the relative reactivities
observed for the type of ionic process exemplified by eq
27 and, thus, the ease of attack on C-1 of an activated
phosphite, whether by nucleophile or by radical. The
same steric factors affect the relative reactivities, i.e.
Me > Et > iPr > Ph. On the other hand, as noted above,
unassisted scission of a carbon-oxygen bond as in eq
30 would not only be very slow, but it should become
more facile in the order Me < Et < i-Pr. Thus, the
compounds Cp†Fe(CO){P(OR)3}• (R ) Et, i-Pr) should
be less stable than is Cp†Fe(CO){P(OMe)3}•, the opposite
to what is observed.
Cp†Fe(CO)2• + P(OMe)3 f
Cp†Fe(CO){P(OMe)3}• + CO (33)
Su m m a r y. The compounds [Cp′Fe(CO)2]2 (Cp′ ) η5-
C5Ph5, η5-C5Ph4(p-tolyl)) dissociate slightly in solution
to give the corresponding 17-electron iron-centered
Cp†Fe(CO){P(OMe)3}• + Cp†Fe(CO)2• f
•
+ Cp†Fe(CO){P(dO)(OMe) } (34)
†
radicals Cp′Fe(CO)2 . The latter take part in a variety
Cp Fe(CO)2Me
2
of reactions characteristic of 17-electron compounds,
reacting with organic halides RX to form Cp′Fe(CO)2R
and Cp′Fe(CO)2X and, in the case of [Cp†Fe(CO)2]2,
readily with 13CO to form the fully 13CO-labeled com-
pound. The radical Cp†Fe(CO)2• is surprisingly inert to
CO substitution by phosphines, presumably for steric
reasons, because the sterically less demanding t-BuNC
reacts readily, albeit giving ultimately the product of
disproportionation [Cp†Fe(t-BuNC)3][Cp†Fe(CO)2]. The
phosphites P(OR)3 (R ) Me, Et, i-Pr) do give the
anticipated 17-electron species Cp†Fe(CO){P(OR)3}• in
apparently associative processes, since the rates of
formation vary qualitatively in the order Me > Et >
i-Pr, but the methyl analogue reacts rapidly with free
P(OMe)3 to give products of Arbuzov rearrangements.
Attempts to grow single crystals of the persistent radical
Cp†Fe(CO){P(O-i-Pr)3}• failed, but an EPR spectrum of
a frozen solution in benzene could readily be obtained
and analyzed in terms of a uniaxial g matrix (g| ) 1.993,
A
very unlikely and would lead to the prediction that
similar ethyl and isopropyl compounds would react even
more quickly. This is not so (see below). The formation
of the 16-electron intermediate Cp†Fe(CO){P(dO)-
(OMe)2} could not be verified, but it would readily
coordinate any free CO or P(OMe)3 to form Cp†Fe(CO)2-
{P(dO)(OMe)2} (B; eq 35) or Cp†Fe(CO){P(dO)(OMe)2}-
{P(OMe)3} (C; eq 36), respectively, thus rationalizing
the formation of these two products.
Cp†Fe(CO){P(dO)(OMe)2} + CO f
Cp†Fe(CO)2{P(dO)(OMe)2}
(35)
B
Cp†Fe(CO){P(dO)(OMe)2} + P(OMe)3 f
Cp†Fe(CO){P(OMe)3}{P(dO)(OMe)2}
(36)
g
) 2.103) and an isotropic 31P hyperfine coupling of
C
37 G (3.7 mT), typical of d7 organometallic radicals in
which the unpaired electron is essentially confined to a
The reactions of [Cp†Fe(CO)2]2 with P(OEt)3 and P(O-
i-Pr)3 proceed much more slowly to give the substituted
radicals Cp†Fe(CO){P(OR)3}• (R ) Et, i-Pr), as antici-
pated since the reactions are presumably associative.1
Interestingly, however, the substituted 17-electron com-
pounds are also very stable with respect to conversion
to Arbuzov products. Although IR spectra of reaction
mixtures suggest the formation of products similar to
A-C, these never appear in quantity even after many
hours when the radical species have decomposed. These
results are much more consistent with the mechanism
of eqs 33 and 34 than in that of eqs 31 and 32, as
conversion of the intermediate radical species in an
2
metal dz orbital.
Ack n ow led gm en t. We thank the Natural Sciences
and Engineering Council of Canada (Research Grant to
M.C.B.), Alcan (Fellowship to I. Kuksis), and Queen’s
University (scholarships to I. Kuksis) for financial
support. We also thank NATO for a Science Fellowship
to I. Kova´cs (administered by the Natural Sciences and
Engineering Research Council), the Hungarian Science
Fund for additional support to I. Kova´cs provided by
Grant No. OTKA F7419, and J . A. Stone for assistance
obtaining mass spectra.
OM960596S