The 18-electron, metal - Metal-bonded dimers [η5-C5Ph5Fe(CO)2]2 and [η5-C5Ph4(p-tolyl)Fe(CO)2] 2: Their proclivity to undergo spontaneous, thermal homolysis to the corresponding 17-electron monomers

Article abstract of DOI:10.1021/om960323x

The known compound [Cp^?Fe(CO)₂]₂ and the new, more soluble [Cp^?Fe(CO)₂]₂ (Cp^? = η⁵-C₅Ph₅; Cp^? = η⁵-C₅Ph₄(p-tolyl)) are prepared via a new, very effective route involving hydridic hydrogen atom abstraction from the hydrides, Cp^?Fe(CO)₂H and Cp^?Fe(CO)₂H, by the trityl radical. The dimers dissociate thermally to the corresponding 17-electron monomers, Cp^?-Fe(CO)₂ and Cp^?Fe(CO)₂, which probably assume planar (C_2ν) structures with OC-Fe-CO bond angles of ～90°.

Full text of DOI:10.1021/om960323x

Organometallics 1996, 15, 4755-4762
4755
Th e 18-Electr on , Meta l-Meta l-Bon d ed Dim er s
[η5-C5P h 5F e(CO)2]2 a n d [η5-C5P h 4(p-tolyl)F e(CO)2]2: Th eir
P r oclivity To Un d er go Sp on ta n eou s, Th er m a l Hom olysis
to th e Cor r esp on d in g 17-Electr on Mon om er s
Inga Kuksis and Michael C. Baird*
Department of Chemistry, Queen’s University, Kingston, Ontario, Canada K7L 3N6
Received April 29, 1996X
The known compound [Cp‡Fe(CO)2]2 and the new, more soluble [Cp†Fe(CO)2]2 (Cp‡ ) η5-
C5Ph5; Cp† ) η5-C5Ph4(p-tolyl)) are prepared via a new, very effective route involving hydridic
hydrogen atom abstraction from the hydrides, Cp‡Fe(CO)2H and Cp†Fe(CO)2H, by the trityl
radical. The dimers dissociate thermally to the corresponding 17-electron monomers, Cp‡-
Fe(CO)2 and Cp†Fe(CO)2, which probably assume “planar” (C2v) structures with OC-Fe-
CO bond angles of ∼90°.
There has in recent years been considerable interest
in the preparation and characterization of 17-electron,
organotransition metal compounds (metal-centered
radicals).1a,b This class of compounds may be synthe-
sized in a variety of ways1 and may often be stabilized
with respect to dimerization to the 18-electron, metal-
metal-bonded analogues by substitution of small ligands
by more sterically demanding ligands (e.g. CO by
tertiary phosphines,1gi,2 η5-C5H5 by η5-C5Me5 and η5-C5-
Ph51d,j,3). For many classes of metal-metal-bonded
compounds,4 it has been found that there is a correlation
between the proclivity of an 18-electron dimer to un-
dergo thermal homolysis to the corresponding 17-
electron species and the length of the metal-metal
bond. Thus the chromium-chromium bond of [CpCr-
(CO)3]2 (Cp ) η5-C5H5), which dissociates to the extent
of a few percent in solution at room temperature,1c,d is
much longer (3.281 Å)1e than the metal-metal bond of
the analogous, much more homolytically stable molyb-
denum compound (3.235 Å)1f but much shorter than the
chromium-chromium bonds of [CpCr(CO)2P(OMe)3]2
(3.343 Å)1g and [Cp*Cr(CO)3]2 (3.311 Å, Cp* ) η5-C5-
Me5),1d both of which dissociate much more extensively
in solution.1d,g Consistent with the apparent role of
steric effects on extent of homolysis, the compounds
CpCr(CO)2PPh31h,i and η5-C5Ph5Cr(CO)31j are completely
monomeric in solution and the solid state.
X Abstract published in Advance ACS Abstracts, October 1, 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; p 49 and
references therein. (c) McLain, S. J . J . Am. Chem. Soc. 1988, 110, 643.
(d) 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,
96, 749. (f) Adams, R. D.; Collins, D. E.; Cotton, F. A. Inorg. Chem.
1974, 13, 1086. (g) Goh, L.-Y.; D’Aniello, M. J .; Slater, S.; Muetterties,
E. L.; Tavanaiepour, I.; Chang, M. I.; Frederich, M. F.; Day, V. W.
Inorg. Chem. 1979, 18, 192. (h) Cooley, N. A.; Watson, K. A.; Fortier,
S.; Baird, M. C. Organometallics 1986, 5, 2563. (i) Fortier, S.; Baird,
M. C.; Preston, K. F.; Morton, J . R.; Ziegler, T.; J aeger, T. J .; Watkins,
W. C.; MacNeil, J . H.; Watson, K. A.; Hensel, K.; Le Page, Y.; Charland,
J .-P.; Williams, A. J . J . Am. Chem. Soc. 1991, 113, 542. (j) Hoobler, R.
J .; Hutton, M. A.; Dillard, M. M.; Castellani, M. P.; Rheingold, A. R.;
Rieger, A. L.; Rieger, P. H.; Richards, T. C.; Geiger, W. E. Organome-
tallics 1993, 12, 116.
(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.; Watson, K. A.; Fortier, S.;
Baird, M. C. Organometallics 1986, 5, 2563. (g) Cooley, N. A.;
MacConnachie, P. T. F.; Baird, M. C. Polyhedron 1988, 7, 1965. (h)
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.
(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.
Soc., Dalton Trans. 1987, 467. (e) Lane, G. A.; Geiger, W. E.; Connelly,
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.
We have recently begun an investigation into the pos-
sibility of forming persistent iron-centered radicals of
the type η5-Cp′Fe(CO)L (Cp′ ) Cp, substituted Cp; L )
bulky phosphines).6 The 17-electron compounds CpFe-
(CO)2 and Cp*Fe(CO)2 can be generated photochemi-
cally from the corresponding dimers and have been
investigated using time-resolved IR spectroscopy, but
are exceedingly reactive and recombine rapidly to form
the corresponding 18-electron dimers (eq 1).7 Attempts
hν
[Cp′Fe(CO)2]2 y
\
z 2Cp′Fe(CO)2
(1)
to stabilize substituted monomeric iron compounds with
sterically demanding phosphines have generally failed.
For instance, while abstraction of the hydridic hydrogen
atom from CpFe(CO)(PPh3)H with the trityl radical
almost certainly forms CpFe(CO)(PPh3), the latter and
(5) (a) Kova´cs, I.; Baird, M. C. Organometallics 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. Organometallics 1996, 15, 3588. (e)
McGovern, P. A.; Vollhardt, K. P. C. Synlett 1990, 493. (f) McGovern,
P. A.; Vollhardt, K. P. C. J . Chem. Soc., Chem. Commun. 1996, 1593.
(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.
(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.
(4) Exceptions are fulvalene complexes, many of which exhibit no
inclination to undergo homolysis in spite of containing extraordinarily
long metal-metal bonds.5
S0276-7333(96)00323-8 CCC: $12.00 © 1996 American Chemical Society

4756 Organometallics, Vol. 15, No. 22, 1996
Kuksis and Baird
Ta ble 1. IR Da ta
νCO (cm-1
its dimer are both unstable and only the diiron com-
pound Cp2Fe2(CO)3(PPh3) is formed (eq 2).6a,c
compd
)
CpqFe(CO)2H
2008, 1951 (benzene)
2033, 1993 (THF)
2007, 1951 (benzene)
2033, 1992 (THF)
CpqFe(CO)2Br
Cp†Fe(CO)2H
Cp†Fe(CO)2Br
[CpFe(CO)2]2 (cis)
[CpFe(CO)2]2 (trans)
[Cp*Fe(CO)2]2 (cis)
[Cp*Fe(CO)2]2 (trans)
[CpqFe(CO)2]2 (trans)
[Cp†Fe(CO)2]2 (trans)
2004, 1792 (cyclohexane)7a,b
1960, 1792 (cyclohexane)7a,b
1981, 1756 (cyclohexane)7a,b
1928, 1765 (cyclohexane)7a,b
∼1951, 1781/1788 (benzene)
1957, 1780 (benzene)
In view of the effectiveness with which the η5-C5Ph5
ligand stabilizes the persistent metal-centered radicals
η5-C5Ph5M(CO)3 (M ) Cr, Mo),1j,3g we turned to com-
pounds in the analogous η5-C5Ph5-iron system.8 Of
these, relevant here are the known [η5-C5Ph5Fe-
(CO)2]2,8a-c,j η5-C5Ph5Fe(CO)2Br,8a,c η5-C5Ph5Fe(CO)2H,8a
K[η5-C5Ph5Fe(CO)2],8f η5-C5Ph5Fe(CO)2Me,8b η5-C5Ph5-
Fe(CO)2Et,8f [η5-C5Ph5Fe(CO)2L]+ (L ) CO, phosphines,
C2H4),8h and η5-C5Ph5Fe(CO)LX (X ) Br, Et, EtCO; L
) phosphine).8h An analogous series of substituted
derivatives of iron, containing the η5-C5Ph5-nArn (Ar )
p-alkylphenyl) ligands, has also been reported,8d,g,i as
has an analogous ruthenium compound.8k
2004, 1938 (cyclohexane)7a
1984, 1915 (cyclohexane)7b
1990, 1921 (benzene)
•
CpFe(CO)2
•
Cp*Fe(CO)2
Cp‡Fe(CO)2
•
Cp†Fe(CO)2
1989, 1921 (benzene)
•
CpFe(µ-CO)3FeCp
Cp*Fe(µ-CO)3FeCp*
Cp(CO)2Fe-Fe(CO)2Cp
1823 (cyclohexane)7a
1790 (cyclohexane)7b
2015, 1973, 1938 (heptane)15
Infrared spectra were acquired on Bruker 85 IFS FT-IR and
Bruker IFS 25 FT-IR spectrometers; IR data are presented in
Table 1. Raman spectra were run on a Bruker RFS-100 FT-
Raman spectrometer utilizing a near-infrared Nd:YAG laser,
while 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}) NMR spectrometers. The residual proton and
the carbon resonances of deuterated solvents served as internal
references for 1H and 13C resonances, respectively. 31P NMR
spectra were run on an AM 400 (162 MHz) NMR spectrometer
and were referenced to external 85% H3PO4.
Ultraviolet-visible spectra were acquired on a Hewlett
Packard 8452A diode array spectrophotometer in quartz
sample cells with a 1 cm path length. Mass spectra were
acquired on a Fisons VG Quattro instrument. EPR spectros-
copy was conducted by Dr. K. F. Preston at the National
Research Council, Ottawa. Elemental analyses for carbon and
hydrogen were carried out by Canadian Microanalytical
Services, Delta, British Columbia, Canada.
We now describe an improved synthetic route to [η5-
8a-c,j
C5Ph5Fe(CO)2]2
and to the new, more soluble ana-
logue, [η5-C5Ph4(p-tolyl)Fe(CO)2]2 (henceforth the η5-
C5Ph5 and η5-C5Ph4(p-tolyl) groups will be denoted Cpq
and Cp†, respectively). We also describe routes to the
hydrides, CpqFe(CO)2H and Cp†Fe(CO)2H, and to the
new series of substituted hydrides Cp†Fe(CO)LH (L )
PMe3, PMe2Ph, PMePh2, PPh3), all potential precursors
to new 17-electron compounds of the type Cp′Fe(CO)L.
We find that both [CpqFe(CO)2]2 and [Cp†Fe(CO)2]2
undergo slight thermal homolysis in solution to give the
monomeric species CpqFe(CO)2 and Cp†Fe(CO)2, respec-
tively, identified by spectroscopy and by chemical reac-
tions, characteristic of 17-electron compounds,1a,b which
will be described in a subsequent paper. A preliminary
report of aspects of this work has appeared.6a
Solutions of trityl in benzene were prepared by the zinc
reduction of Ph3CCl,9 CpqFe(CO)2Br,8d CpqFe(CO)2H,8a and Cp†-
Fe(CO)2Br8d by literature methods.
[Cp ‡F e(CO)2]2.8a A mixture of 0.845 g of Ph3CCl (3.04 ×
10-3 mol) and 0.672 g of zinc metal (1.03 × 10-2 mol) in 65
mL of benzene was sonicated for 1 h and then stirred overnight
for 16 h. Stirring was stopped, the mixture was allowed to
settle for 1 h, and 10 mL of benzene was added to a flask
containing 0.422 g (7.60 × 10-4 mol) of CpqFe(CO)2H. About
62 mL of the trityl solution was cannulated onto the hydride,
and the reaction was complete within 56 min (IR). A dark
green, cloudy solution had formed, and strong IR bands were
observed at 1952 and 1788 cm-1. The green suspension was
collected by filtration, washed, and dried under reduced
pressure. Yield: 0.32 g (76%). IR of the green product
(Nujol): 1953 (s), 1788 (s) cm-1 (lit. IR 1954, 1774 cm-1
(Nujol)8b).
Exp er im en ta l Section
Except where noted, experiments were conducted under an
inert atmosphere of oxygen-free nitrogen, further purified
through columns containing heated BASF catalyst and mo-
lecular sieves. Manipulations of air-sensitive materials fol-
lowed 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, Fisher,
Strem, and Fluka and were used as received.
A similar experiment, run under conditions of greater
dilution, was monitored by IR spectroscopy. A yellow solution
of 0.55 g (9.83 × 10-4 mol) of CpqFe(CO)2H in 200 mL of
benzene (IR bands 2008 and 1951 cm-1) was cannulated into
an aluminum foil wrapped flask containing 0.96 g (1.97 × 10-3
mol) of trityl dimer. Approximately 5 min after addition, the
reaction mixture was a clear, emerald green. The solution still
exhibited bands in the IR spectrum corresponding to CpqFe-
(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. L.; 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. L.; Lindall, C. M.; Masters, A. F. J . Chem.
Soc., Dalton Trans. 1991, 1499. (h) Bre´gaint, P.; Hamon, J .-R.; Lapinte,
C. J . 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.
(k) Colbran, S. B.; Harrison, W. M.; Saadeh, C. Organometallics 1994,
13, 1061.
(CO)2H, as well as new, weak bands at 1990 and 1921 cm-1
.
An intense band at 1781 cm-1, attributable to the bridging
carbonyls of [CpqFe(CO)2]2,8b,j was also present, although the
terminal CO band of this compound overlaps a CO band
(9) (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.

[η5-C5Ph5Fe(CO)2]2 and [η5-C5Ph4(p-tolyl)Fe(CO)2]2
Organometallics, Vol. 15, No. 22, 1996 4757
of the hydride and a band of the benzene solvent at 1960 cm-1
,
zene in hexanes and using increasing proportions of benzene
in hexanes as eluent, first a yellow solution containing trityl
products and then a green fraction was obtained. On removal
of solvent, there was obtained 0.11 g (7%) of benzene-soluble
[Cp†Fe(CO)2]2. Addition of 0.02 g of this material to 10 mL of
benzene resulted in ready dissolution to give a clear, emerald
green solution exhibiting IR bands at 1957, 1780, 1989, and
1921 cm-1, all of comparable intensity. The material was
recrystallized from benzene/n-hexane with mixed success.
Anal. Calcd for C76H54Fe2O4: C, 79.87; H, 4.76. Found: C,
77.62; H, 4.99. A chemical ionization MS investigation using
isobutane as carrier gas was carried out on a benzene solution
of [Cp†Fe(CO)2]2 (MW 1143.97 g/mol). While the molecular ion
could not be observed at 298, 423, or 673 K, a peak at m/z
571.7 is attributed to [Cp†Fe(CO)2]+ (MW 571.5 g/mol). IR
and thus its intensity could not be estimated with certainty.
A green precipitate developed after ∼0.5 h, and an IR spectrum
of the solution exhibited only two strong bands, the terminal
CO band at 1951 cm-1 of [CpqFe(CO)2]2 and a bridging CO
band at 1787 cm-1, the latter of equal intensity to and ∼6 cm-1
higher in frequency than the band which had originally been
observed at 1781 cm-1. The bands of CpqFe(CO)2H and those
initially at 1990 and 1921 cm-1 had essentially disappeared.
The green precipitate was collected by filtration, washed four
times with cold benzene and then with n-hexane and pentane,
and dried. Yield: 0.36 g (66%). Anal. Calcd for C74H50
Fe2O4: C, 79.72; H, 4.52. Found: C, 79.12; H, 4.48.
-
On stirring of a suspension of the dimer in benzene, a small
amount dissolved and the CO bands at 1951 and 1781 cm-1
reappeared.
(Nujol): 1953, 1777 cm-1
. IR (benzene): 1955, 1780, 1989,
1
1921 cm-1. IR (THF): 1956, 1781, 1988, 1920 cm-1. The H
NMR spectrum in C6D6 was difficult to assess. Although
multiplets in the phenyl region (δ 6.5-7.5) were obvious, there
were no resonances in the aryl methyl region which could be
unambiguously attributed to the tolyl methyl group.
Cp †F e(CO)2H. Synthesis of this hydride was based on the
method of McVey and Pauson for their preparation of CpqFe-
(CO)2H.8a A solution of 5.02 g of Cp†Fe(CO)2Br8d (7.71 × 10-3
mol) in 80 mL of THF was treated with a solution of 0.50 g
(1.32 × 10-2 mol) of NaBH4 in 25 mL of distilled water over 7
min. The red solution turned lighter in color, some bubbling
was noticed, and eventually the solution appeared green due
to the presence of a black solid in the yellow liquid. The
mixture was stirred for 75 min (covered with aluminum foil)
and then filtered through Celite to give an orange-yellow
solution. The solvent was removed in vacuo, and the yellow
solid was dissolved in 60 mL of benzene, washed 3 times with
distilled water, and dried over MgSO4. After filtration, the
solvent was removed in vacuo to give 3.62 g (82%) of product.
Anal. Calcd for C38H28FeO2: C, 79.73; H, 4.93. Found: C,
1
In a reaction monitored by H NMR spectroscopy, 0.01 g of
Cp†Fe(CO)2H (1.92 × 10-5 mol) was dissolved in a small
amount of C6D6 and an NMR spectrum was run; the tolyl
methyl resonance at δ 1.90 (s) and the hydride resonance at δ
-10.30 were clearly observed. The sample was then treated
with 0.01 g of trityl dimer (2.05 × 10-5 mol), and the solution
turned green within a few minutes. The NMR spectrum
showed a significant decrease in the intensity of the methyl
resonance at δ 1.90 and strong growth of a new resonance at
δ 5.42, attributable to Ph3CH. Distinguishable trityl dimer
peaks were also present at δ 6.48 (d, J HH 2 Hz), 6.42 (br), 5.95
(d, J HH 4.0 Hz), 5.90 (d, J HH 4.0 Hz), and 4.92 (s, br),10 and a
rather weak, broad resonance centered at δ 2.08 appeared. The
residual hydride resonance was by now very weak, and the
reaction was essentially complete. A significant amount of
green solid precipitated from solution over the next 0.5 h. In
a similar reaction carried out using a deficiency of trityl dimer,
no residual trityl dimer resonances were observed when all of
the hydride had reacted.
80.44; H, 5.09. IR (benzene): 2008, 1951 cm-1
2006, 1952 cm-1 1H NMR (C6D6): δ 7.34-7.26 (m, 8H, aryl
. IR (THF):
.
H), 7.20 (d, J HH 7.96, 2H, aryl H), 6.91-6.77 (m, 12 H, aryl
H), 6.65 (d, J HH 7.98, 2H, aryl H), 1.90 (s, 3H, Me), -10.35 (s,
1H, Fe-H). 13C NMR (C6D6): δ 21.0 (s, Me), 102.7, 102.8,
103.0 (Cp† ring C), 129.2, 127.5-132.9 (other peaks at ∼δ 128
may be obscured by solvent resonances), 132.4, 132.5, 137.4
(aryl C), 216.4 (CO).
[Cp †F e(CO)2]2. This compound was synthesized in the dark
as described above for [CpqFe(CO)2]2, using variable amounts
of Cp†Fe(CO)2H and approximately 2-fold excesses of trityl
dimer. After addition of the solvent, the reactions were
monitored by IR spectroscopy. In a typical preparative reac-
tion, 160 mL of benzene was added to a mixture of 1.56 g of
Cp†Fe(CO)2H (2.72 × 10-3 mol) and 0.97 g of trityl dimer (1.99
× 10-3 mol) in an aluminum foil wrapped flask. The mixture
turned to an emerald green solution within 15 min, with some
green solid present. The IR spectrum exhibited a band at 2007
cm-1 (m) attributable to Cp†Fe(CO)2H (the other CO band was
obscured by a product band at 1956 cm-1), as well as bands at
1989 (w), 1921 (w), 1956 (vs), and 1779 cm-1 (s), the latter
two attributable to [Cp†Fe(CO)2]2. The reaction was essentially
complete after 45 min. The final IR spectrum, taken after 75
min, exhibited CO bands at 1957 (s), 1779 (s), 1989 (w), and
1921 cm-1 (w). The reaction mixture at this point was a dark
green solution containing some solid material. The solvent
was removed in vacuo, the residue was stirred with 20 mL of
cold benzene, and then the mixture was filtered to give a green
solid and a clear, very dark emerald green filtrate. The green
solid was washed 5 times with cold benzene and dried in vacuo
to give 1.21 g (78%) of product, [Cp†Fe(CO)2]2. This material
exhibits rather low solubility in benzene, but an IR spectrum
of a suspension of 0.02 g in 10 mL of benzene (pale green
solution) exhibited CO bands at 1957 (s), 1780 (s), 1989 (w),
On heating of a suspension of 0.036 g of [Cp†Fe(CO)2]2 in
10 mL of benzene (CO bands at 1956 (s), 1780 (s), 1989 (vw),
and 1919 cm-1 (vw) at room temperature) to ∼70 °C, all of
the solid dissolved to give a dark yellow solution. A sample
syringed out of the hot flask appeared green in the syringe
barrel, but an IR spectrum acquired immediately exhibited
very strong CO bands at 1989 and 1919 cm-1, in addition to
those at 1956, and 1780 cm-1. When the solution in the flask
was cooled to room temperature, the color changed to a clear,
emerald green. The bands at 1989, 1919, 1956 and 1780 cm-1
were still present.
The temperature-dependent changes in the [Cp†Fe(CO)2]2
system were also investigated by UV-visible spectroscopy. A
clear, green solution of 0.003 g (2.62 × 10-6 mol) of the soluble
form of [Cp†Fe(CO)2]2 in 3 mL of benzene was placed in a
quartz cell equipped with a stirring bar and allowed to
equilibrate at 298 K. The instrument was equipped with a
no. 3 filter in the beam to absorb light of wavelengths <295
nm. Before and after measurements, the beam was blocked
completely. The spectrum of [Cp†Fe(CO)2]2 at 298 K was
obtained, and the cell was covered with a black cloth as the
temperature was increased to 328 K, where the spectrum was
run again. The temperature of the sample was then decreased,
still covered with a black cloth, to 298 K. The experiment was
repeated using the insoluble form of [Cp†Fe(CO)2]2, dissolved
in hot benzene.
1
and 1921 cm-1 (w), while a H NMR spectrum (C6D6) indicated
Rea ction s of [Cp †F e(CO)2]2 w ith 13CO a n d P Me3.
A
that no trityl dimer was present. A Nujol mull IR spectrum
solution of 0.032 g of [Cp†Fe(CO)2]2 (2.8 × 10-5 mol) in 10 mL
of benzene, exhibiting CO bands 1955 (s), 1780 (s), 1989 (w)
and 1921 (w) cm-1, was treated with 13CO for 30 s at 1 atm.
The IR spectrum run after ∼30 min exhibited CO bands at
of the solid exhibited strong bands at 1956 and 1780 cm-1
.
Raman spectra of both pure, powdered samples and samples
in Nujol exhibited CO bands at 1971 (m) and 1599 (vs) cm-1
.
The filtrate was taken down to dryness and then suspended
in benzene, and the resulting mixture was placed on a column
of hexanes-equilibrated silica gel. Beginning with 10% ben-
(10) (a) Smith, W. B. J . Chem. Educ. 1970, 47, 535. (b) Lankamp,
H.; Nauta, W. T.; MacLean, C. Tetrahedron Lett. 1968, 249.

4758 Organometallics, Vol. 15, No. 22, 1996
Kuksis and Baird
1911 (s), 1740 (s), 1942 (w), and 1876 (w) cm-1
experiment using [CpFe(CO)2]2 resulted in no change in the
IR spectrum.
.
A similar
combined and chromatographed on an alumina column equili-
brated with hexanes. Gradually increasing quantities of
diethyl ether in hexanes resulted in elution of an orange band
of the product; yield 53%. The product was recrystallized from
ethyl ether-hexanes. Anal. Calcd for C45H39FeOP: C, 79.18;
H, 5.76. Found: C, 78.49; H, 5.73. IR (ethyl ether, hexanes):
1915, 1890 cm-1 (sh). IR (benzene, THF): 1914, ∼1887 (sh)
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)2.11 An additional 8 mL of PMe3 were added to consume
all remaining dimer, and ultimately the IR spectrum exhibited
only strong bands attributable to Fe(CO)3(PMe3)2 and Cp†Fe-
(CO)2H.
Rea ction of Cp †F e(CO)2H w ith P Me3. The compound Fe-
(CO)3(PMe3)2 was also obtained on treating Cp†Fe(CO)2H with
PMe3 and trimethylamine oxide in THF. There was no IR
evidence in this reaction for the formation of Cp†Fe(CO)-
(PMe3)H, although a 1H NMR spectrum of the crude solid
products exhibited a weak hydride doublet resonance at δ
-13.35 (J PH 88 Hz).
cm-1
. .
Raman (solid): 1916, 1882 cm-1 1H NMR (C6D6): δ
7.54-7.49 (m, 2H, aryl H), 7.41-7.34 (m, 10H, aryl H), 7.27
(d, J HH 7.97 Hz, 3H, aryl H), 7.01-6.94 (m, 4H, aryl H), 6.93-
6.82 (m, 14H, aryl H), 6.69 (d, J HH 7.97 Hz, ∼2H, aryl H), 1.94
(s, 3H, aryl-Me) 1.43 (d, J PH 8.62 Hz, 3H, PMe), 1.38 (d, J PH
8.9 Hz, 3H, PMe), -13.17 (d, J PH 86.3 Hz, ∼1H, Fe-H). 13C-
{1H} NMR (C6D6): δ 18.9 (d, J PC 28.7 Hz, PMe), 21.0 (s, aryl-
Me), 23.0 (d, J PC 35.2 Hz, PMe), 99.2, 99.4, 99.6, 99.7 (Cp† ring
C), 128.9 (d, PPh), 128.9-133.4 (aryl C; some resonances may
be partially obscured by solvent), 130.7 (d, J PC 8.90 Hz, PPh),
131.5, 134.7, 134.8, 134.9, 136.3 (Cp† phenyl C), ∼141 (d, J PC
27 Hz, PPh), 222.5 (d, J PC 30.6 Hz, CO). 31P{1H} NMR
(toluene-d8): δ 39.7 at 298 K. δ 39.4, 39.3 at 240 K; both
resonances with shoulders.
Cp †F e(CO)(P MeP h 2)H. A solution of 0.80 g of Cp†Fe-
(CO)2H (1.4 × 10-3 mol) and 2.0 mL of PMePh2 (4.3 × 10-3
mol) in 180 mL of THF was photolyzed as above for 11 h to
give an orange solution. The final IR spectrum exhibited a
new band at 1915 cm-1, as well as bands attributable to the
Cp †F e(CO)(P Me3)Br . A solution of 0.10 g of Cp†Fe(CO)2Br
(1.54 × 10-4 mol) in acetone (νCO 2035, 1992 cm-1) was treated
with 32 µL of PMe3 (3.09 × 10-4 mol), the color of the solution
changing from dark brown-orange to dark yellow-brown and
new bands appearing in the IR spectrum at 2040 and 1996
hydride at 2006 and 1950 cm-1
. The reaction mixture was
cm-1
.
On the addition of 0.018 g (1.62 × 10-4 mol) of
worked up as for Cp†Fe(CO)(PMe2Ph)H to give 0.205 g (18%)
of dark red product which was recrystallized from ethyl ether/
hexanes. Anal. Calcd for C50H41FeOP: C, 80.64; H, 5.50.
trimethylamine oxide‚2H2O, the solution changed to a brown
yellow color and the IR spectrum exhibited a new band at 1941
cm-1, corresponding to Cp†Fe(CO)(PMe3)Br (compare CpqFe-
(CO)(PMe3)Br: νCO 1942 cm-1 in CH2Cl2).8h In a more direct
procedure, Cp†Fe(CO)(PMe3)Br was synthesized via the pro-
cedure for CpqFe(CO)(PMe3)Br.8h A solution of 0.50 g of Cp†-
Fe(CO)2Br (7.7 × 10-4 mol) in THF was treated with 100 µL
of PMe3 (9.66 × 10-4 mol). The solution changed color quickly
from red to dark yellow-black, a single strong band being
observed in the IR spectrum at 1942 cm-1. Various approaches
to substitution of the bromide by hydride gave mixtures of
products.
Found: C, 79.90; H, 5.62. IR (THF): ∼1914 (br) cm-1
. IR
(Nujol): 1919, 1890 (sh) cm-1 1H NMR in benzene-d6: δ 7.74-
.
7.70 (m, ∼1H, aryl H), 7.67-7.62 (m, 2H, aryl H), 7.43-7.37
(m, ∼8H, aryl H), 7.29 (d, J HH 7.55 Hz, ∼2H, aryl H), 7.08-
6.95 (m, ∼6H, aryl H), 6.87-6.81 (m, ∼12H, aryl H), 6.67 (d,
J HH 8.13 Hz, 2H, aryl H), 1.93 (s, 3H, aryl Me), 1.67 (d, J PH
8.40, 3H, PMe), -12.48 (d, J PH 82.68 Hz, ∼1H, Fe-H). 13C-
{1H} NMR (C6D6): δ 20.08 (d, J PC 28.4 Hz, PMe), 21.0 (s, Ph
Me), 99.4, 99.6, 99.7, 99.8, 100.0 (Cp† ring C), 129.3 (PPh),
129.5 (PPh), 132.4 (d, J PC 8.56 Hz, PPh), 132.8 (d, J PC 8.96
Hz, PPh), 136.3-127.4 (aryl, some resonances may be obscured
by solvent), 138.3 (d, J PC 35.62 Hz, PPh), 141.4 (d, J PC 47.5
Hz, PPh), 222.0 (d, J PC 32.20 Hz, CO). 31P{1H} NMR (C6D6):
δ 60.8.
Cp †F e(CO)(P Me3)H. A solution of 0.49 g of Cp†Fe(CO)2H
(8.50 × 10-4 mol in 125 mL of THF was treated with 132 µL
of PMe3 (1.28 × 10-3 mol). The solution, in a water-cooled
quartz vessel, was photolyzed with a 250 W Hanovia UV lamp
for 4.5 h, during which time an additional 50 µL of PMe3 (4.83
× 10-4 mol) was added. IR spectra showed the gradual
disappearance of the CO bands of Cp†Fe(CO)2H (2006, 1952
cm-1) and the growth of new bands at 1915 (s) and 1888 cm-1
(sh). Following the photolysis, the solvent was removed in
vacuo, and the residue was chromatographed on hexanes-
equilibrated alumina using increasing concentrations of diethyl
ether in hexanes to elute Cp†Fe(CO)(PMe3)H as a yellow band.
The yield was 0.10 g (17.4%), which was recrystallized from
ethyl ether/hexanes. Increasing the concentrations, using a
2.5-fold excess of PMe3 and irradiating for a longer period,
resulted in a yield of 55%. Anal. Calcd for C40H37FeOP: C,
77.42; H, 6.01. Found: C, 76.84; H, 6.14. IR (THF): 1915,
Cp †F e(CO)(P P h 3)H. A solution of 0.72 g of Cp†Fe(CO)2H
(1.3 × 10-3 mol) and 0.41 g of PPh3 (1.6 × 10-3 mol) in THF
was photolyzed as above for 14 h to give an orange solution.
The final IR spectrum exhibited a new band at 1920 cm-1, as
well as weak bands attributable to the hydride at 2006 and
1950 cm-1
. The solvent was removed in vacuo, and the
resulting solid was extracted with ethyl ether, filtered off, and
dried. The yield of crude product was 0.20 g (18.3%), which
was recrystallized from toluene-hexanes. Anal. Calcd for
C
55H43FeOP: C, 81.99; H, 5.25. Found: C, 79.90; H, 5.62. IR
(THF): 1920 (br) cm-1 IR (Nujol): 1924, 1900 (sh) cm-1
Raman (solid): 1928, 1897 cm-1 1H NMR in benzene-d6: δ
.
.
.
7.64-7.59 (m, ∼10H, aryl H), 7.38-7.29 (m, 8H, aryl), 7.23
(d, J PH 8.06 Hz, 2H, aryl H), 7.12-6.77 (m, ∼25H, aryl H, some
resonances may be obscured by solvent), 6.63 (d, J PH 8.04 Hz,
2H, aryl H) 2.10 (s, 2.4 H, aryl Me), 1.91 (s, 3H, aryl Me),
-11.98 (d, J PH 83.53 Hz, 1H, Fe-H). A variable-temperature
1H NMR study showed that the resonances at δ 1.91 and 2.10
neither coalesced nor moved together between 298 and 343
K. Also no loss of the phosphorus-hydride coupling was
observed. 31P{1H} NMR (C6D6): δ 57.7.
1888 (sh) cm-1
.
(Nujol): 1919, ∼1880 (sh) cm-1
. Raman
(solid): 1916, 1877 cm-1 1H NMR (benzene-d6): δ 7.46-7.41
.
(m, 8H, aryl H), 7.32 (d, J HH 8.3 Hz, 2H, aryl H), 6.92-6.8 (m,
12H, aryl H), 6.71 (d, J HH 7.63 Hz, 2H, aryl H), 1.94 (s, 3H,
aryl Me), 1.15 (d, J PH 8.6 Hz, ∼9H, PMe), -13.36 (d, J PH 85
Hz, 1H, Fe-H). 13C{1H} NMR (C6D6): δ 21.0 (s, aryl Me), 21.6
(d, J PC 29.0 Hz, PMe), 99.2, 99.4 (Cp† ring C), 127.5-133.4,
131.9, 135.1, 135.3, 136.3 (aryl C; some resonances may
be partially obscured by solvent), 221.8 (d, J PC 32 Hz, CO).
31P{1H} NMR (C6D6): δ 29.4.
Cp †F e(CO)(P Me2P h )H. A solution of 0.23 g of Cp†Fe-
(CO)2H (3.97 × 10-4 mol) and 114 µL of PMe2Ph (8.01 × 10-4
mol) in 100 mL of THF was photolyzed as above for 2.5 h to
give an orange solution. The solvent was removed in vacuo,
and two batches of crude product prepared in this way were
Resu lts a n d Discu ssion
Pentaarylcyclopentadienyl ligands have been much
investigated as extensions of the better known η5-C5H5
(Cp) and η5-C5Me5 (Cp*) ligands, and there has in recent
years been increased interest in their coordination to
metal ions.12 Their exceptional size provides consider-
(11) Bigorgne, M. J . Organomet. Chem. 1970, 24, 217.

[η5-C5Ph5Fe(CO)2]2 and [η5-C5Ph4(p-tolyl)Fe(CO)2]2
Organometallics, Vol. 15, No. 22, 1996 4759
Fe(CO)2]2 has previously been synthesized via the
reduction of CpqFe(CO)2Br with sodium amalgam8a and
cobaltocene,8b but the very low solubility and apparently
high reactivity of this compound seem to have discour-
aged further investigations. Aroney et al.8j also inad-
vertently synthesized the compound during synthesis
of CpqFe(CO)2Br, but the procedure was not reproduc-
ible and was not pursued. In contrast, although a new
route to [CpqFe(CO)2]2 was not our original intention,
the chemistry of eq 3 does indeed provide a very
convenient, high-yield method for its synthesis.
F igu r e 1. One possible enantiomeric structure of the C5-
Ph5 group.
IR spectroscopy has been used to monitor hydrogen
atom abstraction from CpqFe(CO)2H and clearly shows
diminution of the CO bands of the hydride and the
concomitant growth of a strong bridging CO band at
1781 cm-1. The reaction mixture initially changes from
yellow to a clear emerald green solution, but within 0.5
h, a green suspension forms, the solution turns light
able steric protection, thus shielding otherwise reactive
metal centers from other reagents and conferring kinetic
stability to complexes such as 17-electron compounds.1j,3
In addition, their chiral structures, in which the aryl
rings are canted relative to the five-membered ring
(Figure 1),13 have provided much impetus for the
investigation of ring rotation dynamics.14 The neutral,
purple pentaphenylcyclopentadienyl species is itself a
persistent radical which can be observed in solution and
exhibits ESR resonance at {g} ) 2.003 ( 0.001.3g
The first report of metal coordination by a pentaaryl-
cyclopentadienyl ligand involved the synthesis of Cpq-
Fe(CO)2Br;8a the crystal structure of this compound was
then reported in 1989.8c The compound assumes the
expected piano stool structure, with Br-Fe-CO and
OC-Fe-CO angles of 86.6(1) and 89.8(2)°, respectively.
The C5 ring of the Cpq ligand is planar, but the ipso-
carbon atoms of each of the phenyl rings sit 0.05-0.23
Å above the plane. The phenyl rings are oriented
between 49.09 and 142.84° to the C5 frame in a
conformation of much less regular structure than that
depicted in Figure 1, while the Fe atom is not found
directly below the centroid of the Cp ring but is rather
shifted slightly off-center.
F or m a t ion a n d Ch a r a ct er iza t ion of [Cp ‡F e-
(CO)2]2 a n d [Cp †F e(CO)2]2. Although the reactions
of CpqBr and Cp†Br with Fe(CO)5 to produce CpqFe-
(CO)2Br8a,c,d and Cp†Fe(CO)2Br8a,d are the only proce-
dures used to date in the literature to synthesize these
compounds, we have found that the use of Fe2(CO)9
instead of Fe(CO)5 greatly improved the efficiency of the
reaction as reaction times were reduced to 2-3 h and
product yields were generally higher (54% for CpqFe-
(CO)2Br and as high as 83% for Cp†Fe(CO)2Br). The
compound Cp†Fe(CO)2H is new and was prepared as
was the Cpq analogue, reported in 1965.8a
green, and the bridging CO band shifts to 1787 cm-1
.
The terminal CO band of the product at ∼1951 cm-1
overlaps the hydride band at 1951 cm-1 as well as a
benzene solvent band at 1960 cm-1, and thus the
relative intensity and exact position of this band cannot
be discerned over the course of the reaction. Since [Cpq-
Fe(CO)2]2 was the sole product formed and exhibits very
low solubility, the compound was readily filtered and
washed to remove trityl products and residual hydride.
Its low solubility precluded recrystallization, but el-
emental analyses were found to be satisfactory.
The bands at ∼1951 and 1781-1788 cm-1 are con-
sistent with a centrosymmetric trans isomer of the
dimer,8b, j in accord with the trend to the trans config-
uration with increasing size of the η5-ligands in these
iron compounds. Thus while [CpFe(CO)2]2 forms a
mixture of cis and trans isomers in solution, [Cp*Fe-
(CO)2]2 is predominantly found in the trans con-
figuration,7a,b,15 and it is thus anticipated that [CpqFe-
(CO)2]2 will also exist as the trans isomer. Furthermore,
since IR spectra of cis-trans mixtures of isomers exhibit
differences of 40-50 cm-1 between the terminal CO
stretching frequencies of the cis and trans forms (Table
1), our observation of two different bridging CO bands
in spectra of [CpqFe(CO)2]2 clearly cannot be rationalized
in these terms.
However, the phenyl groups of the C5Ph5 ligands are
expected to be canted relative to the five-membered ring
(Figure 1), and thus the C5Ph5 ligands are chiral.8j,13,14
It therefore seems reasonable that the bands at 1781
and 1787 cm-1 are attributable to different diastereo-
mers in which the pairs of C5Ph5 ligands in each molcule
of dimer have the same or opposite chirality. Although
dynamic processes involving the Cpq ligand in rather
similar CpqFe compounds have been studied,14 thefac-
tors influencing the conversion of the bridging CO band
between positions at 1781 and 1788 cm-1 are not
understood and the phenomenon we describe has not
been noted previously.8a,b,j
Hydrogen atom abstraction reactions of Cp′′Fe(CO)2H
(Cp′′ ) Cpq, Cp†) by the trityl radical proceed in benzene
as found for other metal hydrides,9 yielding triphenyl-
methane and, presumably, the 17-electron compounds
Cp′′Fe(CO)2. The latter rapidly couple to give the green
dimers [Cp′′Fe(CO)2]2, which are generally the major
iron-containing products of the reactions (eq 3). [Cpq-
2Cp′′Fe(CO)2H + 2Ph3C• f [Cp′′Fe(CO)2]2 +
2Ph3CH (3)
In an effort to find more soluble compounds of the type
[{C5(aryl)5}Fe(CO)2]2, Field et al.8i have synthesized
[{C5(p-tolyl)5}Fe(CO)2]2 by reduction of {C5(p-tolyl)5Fe-
(CO)2Br with zinc metal but have commented only
briefly on its properties and chemistry. We prepared
(12) J aniak, C.; Schumann, H. Adv. Organomet. Chem. 1991, 33,
291.
(13) (a) Chambers, J . W.; Baskar, A. J .; Bott, S. G.; Atwood, J . L.;
Rausch, M. D. Organometallics 1986, 5, 1635. (b) Adams, H.; Bailey,
N. A.; Browning, A. F.; Ramsden, J . A.; White, C. J . Organomet. Chem.
1990, 387, 305. (c) Ramsden, J . A.; Milner, D. J .; Hempsread, P. D.;
Bailey, N. A.; White, C. J . Chem. Soc., Dalton Trans. 1995, 2101.
(14) 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.
(15) (a) Fischer, R. D.; Vogler, A.; Noack, K. J . Organomet. Chem.
1967, 7, 135. (b) Bullitt, J . G.; Cotton, F. A.; Marks, T. J . Inorg. Chem.
1972, 11, 671. (c) Noack, K. J . Organomet. Chem. 1967, 7, 151. (d)
Bullitt, J . G.; Cotton, F. A.; Marks, T. J . J . Am. Chem. Soc. 1970, 92,
2155.

4760 Organometallics, Vol. 15, No. 22, 1996
Kuksis and Baird
[Cp†Fe(CO)2]2 for the same reason but via the reaction
of Cp†Fe(CO)2H with trityl radical. The dimeric [Cp†-
Fe(CO)2]2 was readily formed as in eq 3, and its CO
bands were observed at 1958 and 1780 cm-1, neither
shifting as the reaction progressed. It is possible either
that the two diastereomers have coincident CO bands
or that only one diastereomer is formed during the
reaction. Interestingly, however, [Cp†Fe(CO)2]2 was
obtained in two forms, one which exhibits very low
solubility in cold benzene or toluene but does dissolve
when heated, and another which is sufficiently soluble
in benzene at room temperature that it can be purified
by column chromatography. The insoluble variety is
generally the major product of eq 3, especially if formed
in large-scale reactions (∼1 g). Nujol mull IR spectra
of the two forms exhibit identical CO stretching bands,
leading to the suggestion that the origin of the solubility
differences is again the type of diastereoisomerism that
is apparently observed for [CpqFe(CO)2]2, although it is
strange that the two forms do not interconvert readily.
IR spectra of the solid materials exhibited strong CO
bands at 1956 and 1780 cm-1, while Raman spectra of
both pure, powdered samples and samples in Nujol
exhibited CO bands at 1971 (m) and 1599 (vs) cm-1. The
noncoincidence of the IR and Raman spectra provides
clear evidence for the centrosymmetric trans structure
suggested above. The Raman spectra also exhibited
bands at 215 and 115 cm-1, either of which may
correspond to the Fe-Fe bond stretching mode which
has been reported at 225 cm-1 in the Raman spectrum
F igu r e 2. Nujol mull IR spectrum of [Cp†Fe(CO)2]2.
16a
of [CpFe(CO)2]2 but at 140-154 cm-1 in the spectrum
F igu r e 3. IR spectrum of [Cp†Fe(CO)2]2 (1.8 × 10-3 M) in
of [Cp*Fe(CO)2]2.16b
benzene.
Using purified trityl dimer, it was possible to monitor
the synthesis of [Cp†Fe(CO)2]2 in C6D6 by 1H NMR
spectroscopy and to show that the reaction does indeed
proceed with the stoichiometry shown in eq 3. It was
found that, on addition of trityl dimer to a C6D6 solution
of Cp†Fe(CO)2H, the yellow color changed to green with
the consequent decrease in the intensity of the tolyl
methyl resonance of the hydride at δ 1.90 and the
growth of the Ph3CH resonance at δ 5.42.
Also noted during the course of the hydrogen atom
abstraction reactions of Cp′′Fe(CO)2H was the appear-
ance of relatively weak IR bands at ∼1990 and ∼1920
cm-1 (Table 1), which were not present in spectra of the
solid products ultimately isolated. Although not noted
previously by workers with these types of compounds,
the bands reappeared when solid dimers [Cp′′Fe(CO)2]2
were redissolved and were eventually attributed to the
F igu r e 4. IR spectrum of [Cp†Fe(CO)2]2 (1.3 × 10-2 M) in
benzene.
•
17-electron monomers Cp′′Fe(CO)2 , the initial products
of hydrogen atom abstraction and presumably in equi-
librium with the dimers (eq 4). IR spectra of the
between those of the Cp and Cp* analogues,3a,13 as is
the case here.
It was found that the relative intensities of the
carbonyl stretching bands at ∼1990 and ∼1922 cm-1
,
•
[Cp′′Fe(CO)2]2 a 2Cp′′Fe(CO)2
(4)
absent in the spectra of the solid (Figure 2), are strongly
concentration dependent in benzene solution. Thus the
intensities of the monomer bands were relatively high
in dilute solutions (1.8 × 10-3 M, Figure 3), comparable
with the dimer band at 1780 cm-1 (the dimer band at
1958 cm-1 coincides with a solvent peak, and its relative
intensity cannot be determined reliably) but relatively
low in more concentrated solutions (1.3 × 10-2 M,
Figure 4), consistent with the thermal equilibrium of
eq 4. Also consistent with this hypothesis, it was found
that the relative intensities of the monomer bands
increased significantly in the more polar solvent THF
(1.8 × 10-3 M, Figure 5), again as expected for the
•
analogous species CpFe(CO)2 (2004, 1939 cm-1) and
•
Cp*Fe(CO)2 (1984, 1915 cm-1) have been observed by
time-resolved IR spectroscopy and matrix isolation
studies,7a-c and the CO stretching bands of both are in
close correspondence with those observed for Cp′′Fe-
(CO)2 . The assignments seem particularly reasonable
since IR bands for C5Ph5 compounds normally lie
•
(16) (a) Markwell, R. D.; Butler, I. S.; Gao, J . P.; Shaver, A. J .
Raman Spectrosc. 1993, 24, 423. (b) Vitale, M.; Lee, K. K.; Hemann,
C. F.; Hille, R.; Gustafson, T. L.; Bursten, B. E. J . Am. Chem. Soc.
1995, 117, 2286.

[η5-C5Ph5Fe(CO)2]2 and [η5-C5Ph4(p-tolyl)Fe(CO)2]2
Organometallics, Vol. 15, No. 22, 1996 4761
EPR inactive. As in other systems,1i,j,3g the ESR reso-
nance is broadened in liquid solution, in part because
of rapid exchange between the paramagnetic monomer
and the diamagnetic dimer, while cooling shifts the
equilibrium to the thermodynamically more stable
dimer.
The above-mentioned temperature dependence of the
changes in the [Cp†Fe(CO)2]2 system were also inves-
tigated in a UV-visible spectroscopic study. Consistent
with the equilibrium of eq 4, a dark green solution of
[Cp†Fe(CO)2]2 in benzene turned yellow on heating from
298 to 328 K, although the solution returned only to a
somewhat lighter green on cooling. Similar behavior
was observed on heating suspensions of the insoluble
form of [Cp†Fe(CO)2]2, the mixtures initially turning
yellow and then emerald green on cooling. In the latter
cases, some precipitation occurred on cooling. Analo-
gous thermochromic behavior has been reported for
other systems involving equilibria between monomers
and deeply colored dimers.1c,d,3g (see below).
F igu r e 5. IR spectrum of [Cp†Fe(CO)2]2 (1.8 × 10-3 M) in
THF.
monomer-dimer equilibrium of eq 4 since the cen-
trosymmetric trans dimer would not possess a perma-
nent dipole moment.
Consistent with these observations, the UV-visible
spectrum of a benzene solution of the soluble form of
[Cp†Fe(CO)2]2 in a quartz cell at 298 K exhibited very
intense, high-energy bands (>22 200 cm-1), a shoulder
Assuming comparable extinction coefficients for the
terminal CO stretching bands of the monomer and
bridging CO band of the dimer, as is found for the Cp
analogue,7d Keq was estimated to be ∼10-3 M in benzene
at room temperature. Unfortunately, attempts to carry
out variable-temperature IR studies on solutions of [Cp†-
Fe(CO)2]2 were inconclusive, resulting only in significant
decomposition as was shown by weakening of the CO
bands. In other solvents, e.g. acetone and CH2Cl2,
decomposition of the dimer was observed at room
temperature.
The magnetic susceptibility of a benzene solution of
the soluble form of [Cp†Fe(CO)2]2 was measured using
the Evans method,17 although difficulties in obtaining
reproducible results were experienced initially because
the monomer concentrations were of necessity very low
and the compound is extremely susceptible to oxidation
to highly paramagnetic species. Indeed, intentional
exposure of solutions of [Cp†Fe(CO)2]2 to air resulted
in ∼8-fold increases in the magnetic susceptibilities of
the solutions, and thus even slight oxidation could result
in misleading values. Reasonable precision was ulti-
mately achieved, however, and reproducible susceptibil-
at ∼21 000 cm-1 and a broad, weak band at 16 200 cm-1
.
On increase of the temperature to 328 K, it was found
that both the shoulder and the band at 16 200 cm-1
weakened significantly. However, on decrease of the
temperature of the sample to 298 K, the bands at
∼21 000 cm-1 and 16 200 cm-1 returned only to intensi-
ties intermediate between those of the previous two
spectra. Consistent with this observation, the solution
changed to a lighter green than it had been initially.
Repetition of this experiment using the insoluble form
of [Cp†Fe(CO)2]2, dissolved in hot benzene, yielded
similar results, and it would seem that the system is
thermally unstable under the conditions of the experi-
ments.
To rationalize these results, we note that, by analogy
with other systems, the shoulder at ∼21 000 cm-1 may
be attributed to a metal-metal σ f σ* transition and
the band at 16 200 cm-1 to a dπ f σ* transition.1c,3g,7c,18
The corresponding σ f σ* bands of [CpFe(CO)2]2 and
[Cp*Fe(CO)2]2 are found at ∼29 40018a,b and ∼27 80019
cm-1, respectively, at higher energies than for [Cp†Fe-
(CO)2]2 as expected for stronger Fe-Fe bonds (26-32
kcal/mol20 ) which do not undergo thermal dissociation
to monomers. The σ f σ* band of [CpCr(CO)3]2, which
does undergo spontaneous thermal dissociation in solu-
ity shifts of 8.2 ( 0.2 Hz were obtained for ∼10-3
M
solutions of [Cp†Fe(CO)2]2. After diamagnetic correc-
tions for ligands and solvent, and assuming an equilib-
rium constant of 10-3 for eq 4 (see above), the calculated
magnetic moment of the monomer was found to be 2.3
µB. This is marginally higher than the expected value
for one unpaired electron,17 possibly because of slight
oxidation.
tion,1 is observed at 22 200 cm-1 1c
Replacement of Cp
.
by Cp† or Cpq would be expected to further decrease the
energy of the σ f σ* transition, and a difference of
almost 2800 cm-1 is observed between the absorbances
of [(C5H4Me)Mo(CO)3]2 (25 400 cm-1, does not dissoci-
ate) and [CpqMo(CO)3]2 (22 600 cm-1, dissociates sig-
nificantly).3g
1
A useful H NMR spectrum of the insoluble form of
[Cp†Fe(CO)2]2 could not be obtained, as is the case with
1
[CpqFe(CO)2]2, but the H NMR spectrum of the soluble
form of [Cp†Fe(CO)2]2 (C6D6) was unfortunately also
rather uninformative. Although phenyl multiplet reso-
nances were readily apparent, there was no resonance
which could be attributed to the methyl of the tolyl
group. Since aryl methyl resonances were readily
assigned in the spectra of Cp†Fe(CO)2H and Cp†Fe-
(CO)2Br, it would seem that rapid monomer-dimer
exchange in the Cp† system must broaden and perhaps
shift the tolyl methyl resonance sufficiently that it
cannot be recognized. For presumably the same reason,
both liquid and frozen solutions of [Cp†Fe(CO)2]2 are also
Thus it is reasonable that the σ f σ* transition and
the low-energy dπ f σ* transition at 16 200 cm-1, which
gives rise to the green color of solutions of [Cp†Fe(CO)2]2,
(18) (a) Geoffroy, G. L.; Wrighton, M. S. Organometallic Photochem-
istry; Academic Press: New York, 1979; p 143. (b) Meyer, T. J .; Caspar,
J . V. Chem. Rev. 1985, 85, 187. (c) Abrahamson, H. B.; Palazzotto, M.
C.; Reichel, C. L.; Wrighton, M. S. J . Am. Chem. Soc. 1979, 101, 4123.
(d) Abrahamson, H. B.; Wrighton, M. S. Inorg. Chem. 1978, 17, 1003.
(19) Blaha, J . P.; Wrighton, M. S. J . Am. Chem. Soc. 1985, 107, 2694.
(20) (a) Cutler, A. R.; Rosenblum, M. J . Organomet. Chem. 1976,
120, 87. (b) Pugh, J . R.; Meyer, T. J . J . Am. Chem. Soc. 1988, 110,
8245.
(17) Evans, D. F. J . Chem. Soc. 1959, 2003.

4762 Organometallics, Vol. 15, No. 22, 1996
Kuksis and Baird
decreased in intensity upon heating because of dissocia-
tion to monomer (eq 4). Similar behavior has been
reported for the compounds [CpCr(CO)3]21c and [CpqMo-
(CO)3]2.3g The homolysis reaction is at least partially
reversible, since cooling of solution resulted in dimer-
ization and a significant return of the green color.
However, the absorbance at 16 200 cm-1 recovered only
approximately half of its original intensity, presumably
because of partial thermal decomposition.
Str u ctu r e a n d Electr on ic Str u ctu r es of Cp ′F e-
(CO)2. Hofmann has carried out extended Hu¨ckel
calculations on the 16-electron fragment CpMn(CO)2,21
showing via a Walsh diagram that this species should
be “pyramidal” as in A, derived from the C3V structure
an analogous investigation of novel C-C bond forming
reactions, Sen et al. found that [CpFe(COD)]- (COD )
1,5-cyclooctadiene) undergoes a one-electron oxidation
with benzyl halide to form the neutral 17-electron
species CpFe(COD),24d which then couples with the
benzyl radical to form CpFe(COD)CH2Ph although
CpFe(COD) has been briefly claimed to be reasonably
persistent at room temperature.25 Certainly the steri-
cally hindered compounds Cp*Fe(C2H4)2 and Cp*Fe-
(COD), while substitutionally labile, are thermally very
well behaved.24d Finally, the 17-electron Cp*Fe(dppe)
(dppe ) 1,2-bis(diphenylphosphino)ethane), prepared
via reduction of Cp*Fe(dppe)(OSO2CF3), is sufficient-
ly stable that an X-ray crystal structure has been
reported.24e The crystal structure shows that the com-
pound actually assumes a structure roughly intermedi-
ate between the “pyramidal” A and the “planar” B, a
result presumably influenced by both steric and elec-
tronic factors.
Cp †F e(CO)LH (L ) P Me3, P Me2P h , P MeP h 2,
P P h 3). Since hydrogen atom abstraction from Cp†Fe-
(CO)2H resulted in efficient dimer formation, it was
anticipated that substitution of one of the carbonyl
groups in Cp†Fe(CO)2H by phosphines and subsequent
hydrogen atom abstraction would result in the forma-
tion of stabilized, substituted 17-electron compounds in
which dimerization would be sterically impeded. To this
end, several attempts were made to induce direct
thermal substitution but to no avail. In addition, the
bromo compound Cp†Fe(CO)(PMe3)Br was also prepared
and unsuccessful attempts to substitute bromide by
hydride were made.
of CpMn(CO)3, but with one CO removed. Placing
electrons in the LUMO of CpMn(CO)2, however, stabi-
lizes the “planar” structure, B, in which the Cp centroid,
the Mn, and the two CO ligands are coplanar. It is on
this basis that Hofmann rationalizes the planar struc-
ture of the 18-electron compound CpCo(CO)2. For the
same reason, then, the 17-electron molecules Cp′Fe-
(CO)2, with one electron in an orbital corresponding to
the HOMO of CpCo(CO)2, should assume a planar
structure also. Furthermore, since the SOMO of Cp′Fe-
(CO)2 is nondegenerate,21 it also follows from Hofmann’s
analysis that 17-electron molecules of the type Cp′Fe-
(CO)2 are not J ahn-Teller molecules, as is often the case
for other 17-electron compounds.1a,b
The desired products were ultimately formed in useful
yields via direct photochemical substitution of Cp†Fe-
(CO)2H with the phosphines. The products formed, Cp†-
Fe(CO)LH (L ) PMe3, PMe2Ph, PMePh2, PPh3), were
1
characterized by IR and H, 13C{1H}, and 31P{1H} NMR
As shown in Figures 3-5, the symmetric (A′) and
antisymmetric (A′′) CO stretching bands of Cp′′Fe(CO)2
are of comparable intensities, suggesting strongly that
the OC-Fe-CO bond angle of this molecule is ∼90°.22
Furthermore, and perhaps rather surprising, the fre-
quencies of the symmetric and antisymmetric CO
spectroscopy and elemental analyses. Some were also
characterized by Raman spectroscopy. In all cases, the
compounds exhibit CO bands in the IR spectra at 1915-
1920 cm-1, usually with a shoulder to lower frequencies.
Since the compounds are chiral both at iron and in the
C5(aryl)5 ligands (see above), they exist as diastereo-
mers, thus explaining the apparent doubling of the CO
stretching bands of Cp′′Fe(CO)2 (∼1989, ∼1921 cm-1
)
are ∼27 to ∼35 cm-1 below those of CpqCo(CO)2 (2016,
1956 cm-1 in CH2Cl223a). Thus the degree of back-
bonding in the 17-electron iron compound appears to
exceed that in the 18-electron cobalt compound, possibly
because of the somewhat higher electronegativity of
cobalt.23a
1
bands. The H NMR spectra all exhibit hydride reso-
nances in the region δ -12 to -13.5, to somewhat higher
field than for Cp†Fe(CO)2H (δ -10.35), and all with
spin-spin coupling to 31P (82-86 Hz). Unfortunately,
these hydrides were found not to form Cp†Fe(CO)L by
reaction with the trityl radical, presumably for steric
reasons, and they were therefore not studied further.
Several papers discussing similar 17-electron iron(I)
compounds appeared as this research was being initi-
ated or in progress. Thus Astruc et al. displaced the
arene ligands from the compounds CpFe(arene) (arene
) benzene, toluene) with PPh3 to form the persistent
17-electron compound CpFe(PPh3)2.24a Giese and
Thoma24b,c found that photochemically generated CpFe-
(CO)2 adds to activated olefins to form new carbon-
centered radicals, useful in C-C-forming reactions. In
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.K.), and Queen’s Uni-
versity (scholarships to I.K.) for financial support.
OM960323X
(24) (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.
(21) Hofmann, P. Angew. Chem., Int. Ed. Engl. 1977, 16. 536.
(22) For discussion of the relationship between bond angles and
relative intensities, see: Braterman, P. S. Metal Carbonyl Spectra;
Academic Press: New York, 1975; pp 36, 146.
(23) (a) Connelly, N. G.; Raven, S. J . J . Chem. Soc., Dalton Trans.
1986, 1613. (b) Allred, A. L. J . Inorg. Nucl. Chem. 1961, 17, 215. This
rationalization was suggested by a reviewer.
(25) J onas, K.; Schieferstein, L. Angew. Chem., Int. Ed. Engl. 1979,
18, 549.