Ion-pairing thermodynamics for (η5-pentadienyl)Fe(CO)2- (pentadienyl = MeCp, C5Me5, C5Ph5, C9H7) and x-ray crystal structure of [(η5-C5Ph5)Fe(CO)2][PPN]

Article abstract of DOI:10.1021/om0107340

The pentadienyl complexes [(η⁵-pentadienyl)Fe(CO)₂] [Na] (pentadienyl = MeCp, C₅Me₅, C₅Ph₅, C₉H₇) have been prepared in THF from the corresponding dimers [(η⁵-pentadienyl)-Fe(CO)₂]₂ and (η⁵-C₅Ph₅)Fe(CO)₂Br. The exact composition of the ion pairs that exist in THF solution was explored as a function of temperature by FT-IR spectroscopy. The sodium metalates bearing the pentadienyl ligands MeCp, C₅Me₅, and C₉H₇ show the presence of two species that involve carbonyl oxygen-sodium and iron-sodium contact ion pairs in THF. In contrast, the pentaphenylcyclopentadienyl complex [(η⁵-C₅Ph₅)Fe(CO)₂] [Na] reveals the existence of solvent-separated ion pairs and carbonyl oxygen-sodium contact ion pairs. The absence of iron-sodium contact ion pairs in [(η⁵-C₅Ph₅)Fe(CO)₂] [Na] is attributed to a steric shielding of the iron center by the ancillary phenyl groups. The temperature-dependent behavior of these sodium metalates was explored by variable-temperature FT-IR measurements, and the equilibrium constants for the observed ion-pairing equilibria have been used to determine values for ΔH and ΔS. The X-ray structure of [(η⁵-C₅Ph₅)Fe(CO)₂] [PPN] is reported. The reactivity of [(η⁵-C₅Ph₅)Fe(CO)₂]^- with CO₂ was investigated, and only in the case of the lithium salt was the carboxylate complex [(η⁵-C₅Ph₅)Fe(CO)₂(CO₂ )]^- observed by IR spectroscopy.

Full text of DOI:10.1021/om0107340

Organometallics 2002, 21, 373-379
373
Ion -P a ir in g Th er m od yn a m ics for
-
(η5-p en ta d ien yl)F e(CO)2 (P en ta d ien yl ) MeCp , C5Me5,
C5P h 5, C9H7) a n d X-r a y Cr ysta l Str u ctu r e of
[(η5-C5P h 5)F e(CO)2][P P N]
Bruce T. Carter and Michael P. Castellani*
Department of Chemistry, Marshall University, Huntington, West Virginia 25755
Arnold L. Rheingold*
Department of Chemistry, University of Delaware, Delaware 19716
Seonggyu Hwang, Steven E. Longacre, and Michael G. Richmond*
Department of Chemistry, University of North Texas, Texas 76203
Received August 9, 2001
The pentadienyl complexes [(η5-pentadienyl)Fe(CO)2][Na] (pentadienyl ) MeCp, C5Me5,
C5Ph5, C9H7) have been prepared in THF from the corresponding dimers [(η5-pentadienyl)-
Fe(CO)2]2 and (η5-C5Ph5)Fe(CO)2Br. The exact composition of the ion pairs that exist in THF
solution was explored as a function of temperature by FT-IR spectroscopy. The sodium
metalates bearing the pentadienyl ligands MeCp, C5Me5, and C9H7 show the presence of
two species that involve carbonyl oxygen-sodium and iron-sodium contact ion pairs in THF.
In contrast, the pentaphenylcyclopentadienyl complex [(η5-C5Ph5)Fe(CO)2][Na] reveals the
existence of solvent-separated ion pairs and carbonyl oxygen-sodium contact ion pairs. The
absence of iron-sodium contact ion pairs in [(η5-C5Ph5)Fe(CO)2][Na] is attributed to a steric
shielding of the iron center by the ancillary phenyl groups. The temperature-dependent
behavior of these sodium metalates was explored by variable-temperature FT-IR measure-
ments, and the equilibrium constants for the observed ion-pairing equilibria have been used
to determine values for ∆H and ∆S. The X-ray structure of [(η5-C5Ph5)Fe(CO)2][PPN] is
reported. The reactivity of [(η5-C5Ph5)Fe(CO)2]- with CO2 was investigated, and only in the
case of the lithium salt was the carboxylate complex [(η5-C5Ph5)Fe(CO)2(CO2)]- observed by
IR spectroscopy.
In tr od u ction
and the resulting manifestations of the greenhouse
effect on the planet’s climate.4
Numerous cyclopentadienyliron compounds have been
employed in the construction of complex organic mol-
ecules by serving as masked synthons and as chiral
auxiliaries in asymmetric syntheses.1,2 While these
reactions constitute the majority of interest in this genre
of compound because of their demonstrated practical
utility, the reactivity of [(η5-Cp)Fe(CO)2]- and related
Fp-derived [Fp ) (η5-Cp)Fe(CO)2] carbonylmetalates has
also attracted considerable interest in the area of CO2
fixation chemistry.3 The use of CO2 as a feedstock in
the production of C1-derived commodity chemicals
through homogeneous transition-metal catalysis re-
mains a global priority, given the present day concern
over motor vehicle and power plant emissions of CO2
The literature abounds with reports on the coordina-
tion of CO2 by the carbonylmetalates [(η5-pentadienyl)-
Fe(CO)2]- (pentadienyl ) Cp, MeCp, C5Me5, indenyl)
and the functionalization chemistry of the resulting
metallocarboxylate;5-8 however, no data exist on the
synthesis and reactivity of the corresponding anionic
(4) (a) Kubiak, C. P.; Ratliff, K. S. Isr. J . Chem. 1991, 31, 3. (b)
Aresta, M.; Quaranta, E.; Tommasi, I. New J . Chem. 1994, 18, 133.
(c) J essop, P. G.; Ikariya, T.; Noyori, R. Chem. Rev. 1995, 95, 259. (d)
Hileman, B. Chem. Eng. News 1999, 77(32), 16 and references therein.
(5) Evans, G. O.; Walter, W. F.; Mills, D. R.; Streit, C. A. J .
Organomet. Chem. 1974, 144, C34.
(6) (a) Forschner, T.; Menard, K.; Cutler, A. J . Chem. Soc., Chem.
Commun. 1984, 121. (b) Giuseppetti, M. E.; Cutler, A. R. Organome-
tallics 1987, 6, 970. (c) Vites, J . C.; Steffey, B. D.; Giuseppetti-Dery,
M. E.; Cutler, A. R. Organometallics 1991, 10, 2827. (d) Pinkes, J . R.;
Cutler, A. R. Inorg. Chem. 1994, 33, 759. (e) Pinkes, J . R.; Masi, C. J .;
Chiulli, R.; Steffey, B. D.; Cutler, A. R. Inorg. Chem. 1997, 36, 70.
(7) Lee, G. R.; Cooper, N. J . Organometallics 1985, 4, 794, 1467.
(8) For related reactions involving Fp-derived metalloacids and
metallocarboxylates, see: (a) Grice, N.; Kao, S. C.; Pettit, R. J . Am.
Chem. Soc. 1979, 101, 1627. (b) Gibson, D. H.; Ong, T.-S. Organome-
tallics 1984, 3, 1911. (c) Gibson, D. H.; Ong, T.-S.; Ye, M.; Franco, J .
O.; Owens, K. Organometallics 1988, 7, 2569. (d) Gibson, D. H.; Ong.
T.-S.; Ye, M. Organometallics 1991, 10, 1811. (e) Gibson, D. H.; Franco,
J . O.; Harris, M. T.; Ong, T.-S. Organometallics 1992, 11, 1993.
(1) Collman, J . P.; Hegedus, L. S.; Norton, J . R.; Finke, R. G.
Principles and Applications of Organotransition Metal Chemistry;
University Science Books: Mill Valley, CA, 1987.
(2) (a) Davies, S. G. Pure Appl. Chem. 1988, 60, 13. (b) Davies, S.
G. Chem. Brit. 1989, 268.
(3) For reviews dealing with CO2 reduction and functionalization
studies with Fp- and other carbonylmetalates, see: (a) Cutler, A. R.;
Hanna, P. K.; Vites, J . C. Chem. Rev. 1988, 88, 1363. (b) Gibson, D. H.
Chem. Rev. 1996, 96, 2063.
10.1021/om0107340 CCC: $22.00 © 2002 American Chemical Society
Publication on Web 12/21/2001

374 Organometallics, Vol. 21, No. 2, 2002
Carter et al.
pentaphenylcyclopentadienyl analogue [(η5-C5Ph5)Fe-
(CO)2]-.9 Moreover, with the exception of the parent
cyclopentadienyl compound [(η5-Cp)Fe(CO)2]-,10 the ion-
pairing phenomena exhibited by the other cyclopendi-
enyl derivatives in this family are not well-docu-
mented,11 despite the fact that ion pairing plays a
critical role in the stabilization of the metallocarboxy-
lates that are obtained from the reaction of these iron
anions with CO2.6e,12,13 Herein we report our data on
the temperature-dependent IR behavior of the com-
pounds [(η5-pentadienyl)Fe(CO)2][Na] (pentadienyl )
MeCp, C5Me5, C5Ph5, C9H7) in THF solution and the
thermodynamics for the various ion pairs present in
THF solution. The X-ray crystal structure of [(η5-C5Ph5)-
Fe(CO)2][PPN] has been determined, and the reactivity
of [(η5-C5Ph5)Fe(CO)2]- with CO2 was investigated and
found to afford the carboxylate complex [(η5-C5Ph5)Fe-
(CO)2(CO2)]- when lithium was employed as the gegen-
cation.
resulting [(η5-pentadienyl)Fe(CO)2][Na]. These reactions
proceed without complications, provided moisture and
oxygen are rigorously excluded, and only in the case of
the highly substituted compounds [(η5-C5Me5)Fe(CO)2]2
and [(η5-C5Ph5)Fe(CO)2]2 is the reduction found to occur
slowly (up to several hours). Qualitatively, the pen-
taphenyl iron complex affords the expected carbonyl-
metalate more slowly than the pentamethyl derivative.
Use of (η5-C5Ph5)Fe(CO)2Br in place of the parent dimer
also furnishes [(η5-C5Ph5)Fe(CO)2][Na] without any
noticeable complications.
The IR data for each of the carbonylmetalates are
reported in Table 1. Relative to the starting dimeric
compound or (η5-C5Ph5)Fe(CO)2Br the product carbon-
ylmetalate exhibits a low-energy shifting of the CO
stretching bands consistent with an increased electron
density at the iron center. The anionic pentadienyl
compounds [(η5-MeCp)Fe(CO)2][Na] ([1][Na]), [(η5-C5-
Me5)Fe(CO)2][Na] ([2][Na]), and [(η5-C9H7)Fe(CO)2][Na]
([3][Na]) reveal significantly more complicated IR spec-
tra, as a result of the contact ion pairing (CIP) between
the sodium gegencation and the iron center and a CO
ligand.16 Here there are two symmetric and two anti-
symmetric CO stretches present in THF solution at
room temperature due to the coordination of the sodium
gegencation with the iron center and one of the CO
ligands. For example, in [(η5-MeCp)Fe(CO)2][Na] the
iron-sodium CIPs exhibit two ν(CO) bands observed at
1878 and 1806 cm-1, while the sodium-carbonyl oxygen
CIPs reveal ν(CO) bands at 1860 and 1768 cm-1. These
CO groups in both ion pairs are readily ascribed to the
symmetric and antisymmetric CO stretching modes, on
the basis of group theoretical considerations.17 Equation
1 depicts the equilibrium that exists for this carbonyl-
Resu lts a n d Discu ssion
I. IR Sp ectr a l Da ta for [(η5-p en ta d ien yl)F e(CO)2]-
[Na ]. With the exception of the anionic C5Ph5 derivative,
whose solution chemistry has only been examined with
respect to the 17-electron radical,9 the other three
carbonylmetalates have been prepared via the amalgam
route or from the direct reaction of the corresponding
Fe2 dimer with an alkali-metal reducing agent. Despite
the widespread use of the compounds [(η5-pentadienyl)-
Fe(CO)2][Na] (pentadienyl ) MeCp, C5Me5, C9H7) in a
variety of reactions,,6d,e,14,15 no detailed information has
been published regarding the IR spectral data for these
anions. Accordingly, we reduced each of the dimeric [(η5-
pentadienyl)Fe(CO)2]2 compounds with sodium amal-
gam in THF solution and recorded the IR spectra of the
metalate in THF solution. Unlike the parent cyclopen-
tadienyliron anion [(η5-Cp)Fe(CO)2][Na], which displays
an equilibrating mixture of the above two CIPs and
solvent-separated ion pairs (SSIP),10 the presence of a
single methyl substituent in [2][Na] appears to be
electron donating enough to render the formation of
SSIPs thermodynamically unfavorable. The absence of
SSIPs in [(η5-MeCp)Fe(CO)2]- was verified by the ad-
dition of excess HMPA to a THF solution containing [(η5-
MeCp)Fe(CO)2][Na]. The aforementioned ν(CO) bands
(9) For studies dealing with the radical reactivity of (η5-C5Ph5)Fe-
(CO)2, see: (a) Kuksis, I.; Baird, M. C. Organometallics 1996, 15, 4755.
(b) Kuksis, I.; Baird, M. C.; Preston, K. F. Organometallics 1996, 15,
4991.
(10) Pannell, K. H.; J ackson, D. J . Am. Chem. Soc. 1976, 98, 4443.
(11) For a report on the ion-pairing chemistry in [(η5-Me3SiCp)Fe-
(CO)2]- and related silylated-cyclopentadienyl derivatives, see: Ber-
ryhill, S. R.; Clevenger, G. L.; Burdurlu, F. Y. Organometallics 1985,
4, 1509.
(12) Fachinetti, G.; Floriani, C.; Zanazzi, P. F. J . Am. Chem. Soc.
1978, 100, 7405.
(13) For a review outlining many fine examples of the importance
of ion pairing in controlling the reaction pathways in organic and
organometallic chemistry, see: Loupy, A.; Tchoubar, B.; Astruc, D.
Chem. Rev. 1992, 92, 1141.
(15) For reports on the generation and use of [(η5-C9H7)Fe(CO)2]-,
see: (a) Forschner, T. C.; Cutler, A. R. Inorg. Chim. Acta 1985, 102,
113. (b) Forschner, T. C.; Cutler, A. R.; Kullnig, R. K. Organometallics
1987, 6, 889. (c) Levitre, S. A.; Cutler, A. R.; Forschner, T. C.
Organometallics 1989, 8, 1133. (d) Forschner, T. C.; Cutler, A. R. J .
Organomet. Chem. 1989, 361, C41. (e) Theys, R. D.; Vargas, R. M.;
Wang, Q.; Hossain, M. M. Organometallics 1998, 17, 1333.
(16) For reviews on the effect of ion pairing on the spectroscopic
properties and chemical reactivity of organometallic compounds, see:
(a) Darensbourg, M. Y. Prog. Inorg. Chem. 1985, 33, 221. (b) Darens-
bourg, M. Y.; Ash, C. E. Adv. Organomet. Chem. 1987, 27, 1. (c) Kochi,
J . K.; Bockman, T. M. Adv. Organomet. Chem. 1991, 33, 51.
(17) Lukehart, C. M. Fundamental Transition Metal Organometallic
Chemistry; Brooks/Cole: Monterey, CA, 1985.
(14) For reports on the generation and use of [(η5-C5Me5)Fe(CO)2]-,
see: (a) King, R. B.; Douglas, W. M.; Efraty, A. J . Organomet. Chem.
1974, 69, 131. (b) Ellis, J . E.; Fennell, R. W.; Flom, E. A. Inorg. Chem.
1976, 15, 2031. (c) Catheline, D.; Astruc, D. J . Organomet. Chem. 1982,
226, C52. (d) Catheline, D.; Astruc, D. Organometallics 1984, 3, 1094.
(e) Randolph, C. L.; Wrighton, M. S. Organometallics 1987, 6, 365. (f)
Guerchais, V.; Astruc, D. J . Chem. Soc., Chem. Commun. 1985, 835.
(g) McNamara, W. F.; Reisacher, H.-U.; Duesler, E. N.; Paine, R. T.
Organometallics 1988, 7, 1313. (h) Heppert, J . A.; Morgenstern, M.
A.; Scherubel, D. M.; Takusagawa, F.; Shaker, M. R. Organometallics
1988, 7, 1715.

-
Ion-Pairing Thermodynamics for (η5-pentadienyl)Fe(CO)2
Organometallics, Vol. 21, No. 2, 2002 375
Ta ble 1. IR Da ta for th e Ca r bon yl Ba n d s in
[(η5-p en ta d ien yl)F e(CO)2][Na ] a n d
[(η5-p en ta d ien yl)F e(CO)2][Na (HMP A)x]a
[(η5-MeCp)Fe(CO)2][Na]
1878, 1806
1860, 1768
1862, 1790
1860, 1791
1847, 1756
1847, 1778
1882, 1816
1866, 1780
1862, 1790
1867, 1804
1867, 1790
1872, 1807
[(η5-MeCp)Fe(CO)2][Na(HMPA)x]
[(η5-C5Me5)Fe(CO)2][Na]
[(η5-C5Me5)Fe(CO)2][Na(HMPA)x]
[(η5-C9H7)Fe(CO)2][Na]
[(η5-C9H7)Fe(CO)2][Na(HMPA)x]
[(η5-C5Ph5)Fe(CO)2][Na]
[(η5-C5Ph5)Fe(CO)2][Na(HMPA)x]
a
All spectra were recorded in THF solution at room tempera-
ture, and the quoted data are in cm-1
.
F igu r e 1. Selected IR spectra of [(η5-MeCp)Fe(CO)2][Na]
in THF as a function of temperature (-70, -25, 0 °C). All
absorbances are relative to a common scale.
were replaced by ν(CO) bands at 1862 and 1790 cm-1
,
in keeping with the trend observed in other carbonyl-
metalate anions when HMPA disrupts the ion-pairing
equilibria extant in solution.16a,18 The compounds [(η5-
C5Me5)Fe(CO)2][Na] and [(η5-C9H7)Fe(CO)2][Na] display
both types of CIPs in THF as [(η5-MeCp)Fe(CO)2][Na],
and upon the addition of excess HMPA the exclusive
formation of SSIPs is found.
Ta ble 2. Equ ilibr iu m P a r a m eter s for th e
Con ver sion of th e Ca r bon ylm eta la tes [1-3][Na ]
fr om Ca r bon yl Oxygen -Sod iu m CIP s in to
Ir on -Sod iu m CIP sa
b
b
1000/T, K- 1
ln Keq
[(η5-MeCp)Fe(CO)2][Na] ([1][Na])c
1000/T, K- 1
ln Keq
The anionic compound [(η5-C5Ph5)Fe(CO)2][Na] ([4]-
[Na]) reveals slightly different IR behavior relative to
the above carbonylmetalates. The IR spectrum of [(η5-
C5Ph5)Fe(CO)2][Na] in THF at room temperature ex-
hibits two ν(CO) bands at 1867 and 1790 cm-1 that are
assigned to the symmetric and antisymmetric CO
stretching modes of the carbonyl oxygen CIP, in addition
to a small ν(CO) band at 1804 cm-1. The absence of two
symmetric ν(CO) bands in [(η5-C5Ph5)Fe(CO)2][Na] al-
lows us to rule out the presence of an iron-sodium CIP
(vide infra) and suggests that the major species in
solution is either the carbonyl oxygen CIP or the SSIP.19
The exact nature of this particular ion pair was dem-
onstrated by the frequency response of the CO bands
to added HMPA (excess), which afforded new ν(CO)
bands at 1872 and 1807 cm-1. The fact the antisym-
metric ν(CO) band moved ca. 20 cm-1 to higher fre-
quency is fully consistent with the conversion of the
carbonyl oxygen CIP to the SSIP. Equation 2 depicts
4.93
4.69
4.48
4.29
4.12
3.51
2.53
1.66
1.08
0.67
3.80
3.66
3.53
3.41
0.15
0.51
0.78
1.00
[(η5-C5Me5)Fe(CO)2][Na] ([2][Na])d
4.93
4.48
3.95
1.90
1.31
0.40
3.66
3.37
0.04
0.33
[(η5-C9H7)Fe(CO)2][Na] ([3][Na])e
3.88
3.80
3.77
3.66
3.08
2.53
2.29
2.12
3.55
3.47
3.36
3.30
1.59
1.28
0.82
0.68
From ca. 10-2 M solutions of [1-3][Na] in THF by following
the changes in the area of the antisymmetric carbonyl bands.
a
b
Defined as [iron-sodium CIPs]/[carbonyl oxygen-sodium CIPs].
c ∆H ) 5.7 ( 0.1 kcal/mol; ∆S ) 22 ( 1 eu. ∆H ) 3.0 ( 0.2 kcal/
d
mol; ∆S ) 11 ( 1 eu. e ∆H ) 7.9 ( 0.2 kcal/mol; ∆S ) 24 ( 1 eu.
II. Ion -P a ir in g Th er m od yn a m ics in [(η5-p en ta d i-
en yl)F e(CO)2][Na ]. The temperature-dependent be-
havior of the various carbonylmetalates was explored
by VT-IR spectroscopy. Figure 1 shows selected IR
spectra and the effect of temperature on the equilibrium
involving the two CIPs of [1][Na] in THF. Lowering the
temperature from room temperature to -70 °C leads
to a shift in the equilibrium defined in eq 1 and an
increased concentration of the carbonyl oxygen-sodium
CIP. Compounds [2][Na] and [3][Na] exhibit similar be-
havior as a function of temperature. All IR spectra are
fully reproducible and have been examined by repeated
temperature cycling over a wide range of temperatures.
Band-area measurements on the overlapping antisym-
metric carbonyl stretching bands permit the calculation
of the equilibrium constant defined by eq 1 for com-
pounds 1-3. The resulting Keq values (Table 2) were
obtained by making the assumption that the area from
each deconvoluted band was proportional to the relative
amount of each CIP present in solution. Figure 2 shows
the van’t Hoff plot for these data, from which the values
of ∆H and ∆S have been computed from the slope of
the straight line and the intercept, respectively.
the pertinent ion-pairing equilibrium for [(η5-C5Ph5)Fe-
(CO)2][Na] in THF. Independent verification of the SSIP
was further shown by the IR spectrum of [(η5-C5Ph5)-
Fe(CO)2][PPN], which displayed ν(CO) bands at 1872
and 1806 cm-1, in excellent agreement with the SSIP
data obtained from the addition of HMPA to [(η5-C5Ph5)-
Fe(CO)2][Na].
(18) (a) Edgell, W. F.; Barbetta, A. J . Am. Chem. Soc. 1974, 96, 415.
(b) Nitay, M.; Rosenblum, M. J . Organomet. Chem. 1977, 136, C23. (c)
Darensbourg, M. Y.; Hanckel, J . M. Organometallics 1982, 1, 82.
(19) Cf. [(η5-Cp)Mo(CO)3][Na], whose ion pairs show
a lack of
resolution in the high-frequency symmetric CO band: Darensbourg,
M. Y.; J imenez, P.; Sackett, J . R.; Hanckel, J . M.; Kump, R. L. J . Am.
Chem. Soc. 1982, 104, 1521.

376 Organometallics, Vol. 21, No. 2, 2002
Carter et al.
F igu r e 3. Selected IR spectra of [(η5-C5Ph5)Fe(CO)2][Na]
in THF as a function of temperature (-70, -30, 0 °C). All
absorbances are relative to a common scale.
F igu r e 2. Plots of ln Keq versus T-1 for the ion-pair
equilibria defined by eqs 1 and 2 in THF: (*) [(η5-MeCp)-
Fe(CO)2][Na]; (O) [(η5-C5Me5)Fe(CO)2][Na]; (b) [(η5-C9H9)-
Fe(CO)2][Na]; (4) [(η5-C5Ph5)Fe(CO)2][Na].
Ta ble 3. Equ ilibr iu m P a r a m eter s for th e
Con ver sion of th e Ca r bon ylm eta la te
[(η5-C5P h 5)F e(CO)2][Na ] ([4][Na ])a fr om SSIP s in to
Ca r bon yl Oxygen -Sod iu m CIP sb
The anionic compounds [1-3][Na] display similar
temperature-dependent processes, and as the temper-
ature is lowered, the carbonyl oxygen-sodium CIP
becomes the dominant ion pair in solution. Higher
temperatures lead to a shift in the equilibrium repre-
sented by eq 1 and to the iron-sodium CIP, in keeping
with the increased importance of the T∆S term in the
van’t Hoff expression. While ion-pairing phenomena
have been demonstrated in many carbonylmetalates, no
theory exists that can accurately predict the types of
ion pairs that might occur in solution and the effect that
temperature has on a given equilibrium. The subtle
balance between the electrostatic potential for sodium
cation solvation by the various ion pairs available and
the THF solvent molecules, coupled with steric interac-
tions between the solvated gegencation and the ancillary
ligands associated with the carbonylmetalate, serve to
define the ion-pairing chemistry observed in solution.
Qualitatively, the observed equilibrium for [1-3][Na]
could be explained by considering the number of THF
solvent molecules bound to the sodium gegencation. If
there were a higher coordination number of THF mole-
cules to the sodium gegencation in the carbonyl oxygen-
sodium CIP rather than the iron-sodium CIP, the effect
of temperature on the Keq would appear to be reason-
able. Increased temperatures would favor the release
of at least one THF molecule into solution and allow
for the partially solvated [Na(THF)x-n]+ gegencation to
further penetrate the coordination sphere of the carbo-
nylmetalate all the way to the iron center.
The perphenylated derivative [4][Na] shows reversible
temperature-dependent IR behavior, as defined by eq
2. The presence of the phenyl groups is sufficient to
change the nature of ion pairs in THF relative to
compounds [1-3][Na]. The dominant form of [4][Na] in
THF from room temperature (ca. 97%) to 173 K (ca.
68%) corresponds to the carbonyl oxygen-sodium CIP.
Lowering the temperature leads to the increased forma-
tion of SSIPs of [4][Na]. Figure 3 shows the spectral
changes as a function of temperature, while the ap-
propriate van’t Hoff data may be found in Figure 2 and
Table 3. It is interesting that the ion-pairing equilibrium
here is different from that found for [1-3][Na], which
undoubtedly has its origin in the presence of the five
c
c
1000/T, K- 1
ln Keq
1000/T, K- 1
ln Keq
5.78
5.62
5.46
5.32
5.05
4.93
4.81
0.74
0.92
1.06
1.19
1.48
1.59
1.70
4.69
4.48
4.29
4.12
3.95
3.80
3.66
1.80
2.04
2.28
2.56
2.77
2.90
3.15
∆H ) 2.2 ( 0.3 kcal/mol; ∆S ) 14 ( 0.1 eu. bFrom ca. 10-2
solutions of [4][Na] in THF by following the changes in the area
of the antisymmetric carbonyl bands.
oxygen-sodium CIPs]/[SSIPs].
M
a
b
Defined as [carbonyl
phenyl groups, which are sterically more demanding
than the substituents on the other cyclopentadienyl
rings. The attachment of five phenyl groups to the
cyclopentadienyl ring in [4][Na] is sure to disrupt and,
in this case, completely eliminate the possibility of a
direct iron-sodium CIP. The close intermolecular con-
tacts that exist between the ortho hydrogens on the
phenyl rings and the iron atom (ca. 3.15 Å) shield the
iron center from the sodium cation. The phenyl-derived
steric bias in [4][Na] retards the formation of metal
CIPs, leading to the exclusive generation of SSIPs and
carbonyl oxygen-sodium CIPs.
III. X-r a y Diffr a ct ion St r u ct u r e of [(η5-C5P h 5)-
F e(CO)2][P P N]. The structure of [(η5-C5Ph5)Fe(CO)2]-
[PPN] was determined by single-crystal X-ray diffrac-
tion analysis in order to establish the structure and the
disposition of the ancillary phenyl groups in [4]-. Single
crystals of [(η5-C5Ph5)Fe(CO)2][PPN] were grown from
THF/hexane, and [(η5-C5Ph5)Fe(CO)2][PPN] was found
to exist in the unit cell with no unusually short inter-
or intramolecular contacts. The X-ray data collection
and processing parameters for [(η5-C5Ph5)Fe(CO)2][PPN]
are given in Table 4 with selected bond distances and
angles listed in Table 5.
Figure 4 shows the molecular structure of [(η5-C5Ph5)-
Fe(CO)2][PPN]. The bond lengths and angles found for
the ancillary pentadienyl ligand in [(η5-C5Ph5)Fe(CO)2]-
are in keeping with those values found for the isoelec-
20
tronic molecule (η5-C5Ph5)Co(CO)2 and other structur-
(20) Chambers, J . W.; Baskar, A. J .; Bott, S. G.; Atwood, J . L.;
Rausch, M. D. Organometallics 1986, 5, 1635.

-
Ion-Pairing Thermodynamics for (η5-pentadienyl)Fe(CO)2
Organometallics, Vol. 21, No. 2, 2002 377
Ta ble 4. X-r a y Cr ysta llogr a p h ic Da ta a n d
P r ocessin g P a r a m eter s for
and the phenyl ring torsion angles range from 37.7 to
61.8°, with an average torsion angle of 53.4°. This value
agrees well with the 55.8° average angle found for the
canted phenyl groups in (η5-C5Ph5)Co(CO)2.20 The Fe-
centroid distance of 1.715(3) Å is in excellent agreement
with the 1.70(1) Å distance reported for (η5-C5Ph5)Co-
(CO)2 and suggests that no unusual intramolecular
interactions exist in the carbonylmetalate [4]-.
The Fe-CO bond distances and bond angles fall
within normal ranges relative to other organometallic
iron complexes.22 The OC-Fe-CO bond angle of 88.0-
(3)° in [4][PPN] is similar to the value of 89.5(5)°
reported for (η5-C5Ph5)Co(CO)2 and is not unreasonable
when compared with the calculated solution OC-Fe-
CO bond angle of ca. 96°.23 The ca. 8° increase in this
angle in solution may reflect a repositioning of the
phenyl groups about the iron center, allowing for an
opening in the OC-Fe-CO bond angle.
[(η5-C5P h 5)F e(CO)2][P P N]
space group
a, Å
P32, trigonal
13.3229(6)
33.356(2)
c, Å
V, Å3
5127.5(5)
mol formula
C73H55FeNO2P2
fw
1095.97
3
1.065
0.710 73
3.08
formula units per cell (Z)
F, g cm-3
λ(Mo KR), Å
µ, cm-1
collecn range, deg
4.0 e 2θ e 48.0
18482
total no. of data collcd
no. of indep data, I > 3σ(I)
R
10086
0.0590
Rw
0.1492
GOF
weights
1.10
[0.04F2 + (σF)2]-1
Ta ble 5. Selected Bon d Dista n ces (Å) a n d An gles
IV. CO2 Rea ctivity w ith [(η5-C5P h 5)F e(CO)2]-.
Since the carbonylmetalates [1-3]- have been shown
to react with CO2 to afford the corresponding metallo-
carboxylates [(η5-pentadienyl)Fe(CO)2(CO2)]-, we wished
to explore the reactivity of [(η5-C5Ph5)Fe(CO)2]- with
CO2, because the phenyl groups could significantly alter
the stability of the metallocarboxylate product. Treat-
ment of [(η5-C5Ph5)Fe(CO)2][Na] in THF with CO2 at
-78 °C did not lead to any noticeable spectral changes
when examined by low-temperature IR spectroscopy.
This lack of reactivity between [(η5-C5Ph5)Fe(CO)2]- and
CO2 was a bit surprising, as it is accepted that the
anionic compounds [(η5-pentadienyl)Fe(CO)2]- generate
[(η5-pentadienyl)Fe(CO)2(CO2)]- rapidly at low temper-
atures in the presence of added CO2. We next prepared
the potassium and lithium salts of [(η5-C5Ph5)Fe(CO)2]-,
as it is known that the stability of the metallocarboxy-
late of this genre of compound is sensitive to the nature
of the gegencation. The anion of [(η5-C5Ph5)Fe(CO)2]-
[K],24 which was prepared by potassium amalgam
reduction of (η5-C5Ph5)Fe(CO)2Br, also showed no reac-
tivity with CO2 at -78 °C. Apart from a trace amout of
(η5-C5Ph5)Fe(CO)2H, only [4][K] was observed in solu-
tion -78 °C. Spectroscopic monitoring showed that the
warmup of [4][Na] and [4][K] in the presence of CO2
gradually led to (η5-C5Ph5)Fe(CO)2H without any sign
of the expected metallocarboxylate ion.
(d eg) in [(η5-C5P h 5)F e(CO)2][P P N]a
Bond Distances
Fe-C(1)
2.106(5)
2.121(5)
2.085(5)
1.715(6)
1.182(7)
1.446(6)
1.420(7)
1.439(7)
Fe-C(2)
2.133(4)
2.097(5)
1.714(6)
1.715(3)
1.186(7)
1.447(7)
1.453(6)
Fe-C(3)
Fe-C(4)
Fe-C(5)
Fe-C(7)
O(1)-C(6)
C(1)-C(2)
C(2)-C(3)
C(4)-C(5)
Fe-C(6)
Fe-centroid
O(2)-C(7)
C(1)-C(5)
C(3)-C(4)
Bond Angles
C(6)-Fe-C(7)
O(2)-C(7)-Fe
88.0(3)
178.6(5)
O(1)-C(6)-Fe
P(2)-N(1)-P(1)
178.0(5)
147.2(3)
a
Numbers in parentheses are estimated standard deviations
in the least significant digit.
We did observe an instantaneous reaction between
[(η5-C5Ph5)Fe(CO)2][Li] and CO2 (excess) to yield the
expected metallocarboxylate ion [(η5-C5Ph5)Fe(CO)2-
(CO2)][Li] (eq 3). [4][Li] exists in solution as a mixture
F igu r e 4. ORTEP drawing of the non-hydrogen atoms of
[(η5-C5Ph5)Fe(CO)2][PPN] showing the thermal ellipsoids
at the 50% probability level. The PPN gegencation has been
omitted for clarity.
ally characterized compounds possessing an η5-C5Ph5
ligand.21 The five phenyl groups orient themselves in a
paddlewheel fashion about the cyclopentadienyl ligand,
of carbonyl oxygen-lithium CIPs and SSIPs. In the
reaction of [4][Li] with CO2 the IR bands belonging to
(22) Orpen, A. G.; Brammer, L.; Allen, F. H.; Kennard, O.; Watson,
D. G.; Taylor, R. J . Chem. Soc., Dalton Trans. 1989, S1.
(23) Cotton, F. A. Advanced Inorganic Chemistry, 3rd ed.; Wiley-
Interscience: New York, 1972.
(21) (a) Hoobler, R. J .; Hutton, M. A.; Dillard, M. M.; Castellani, M.
P.; Rheingold, A. L.; Rieger, A. L.; Rieger, P. H.; Richards, T. C.; Geiger,
W. E. Organometallics 1993, 12, 116. (b) Heeg, M. J .; Herber, R. H.;
J aniak, C.; Zuckerman, J . J .; Schumann, H.; Manders, W. F. J .
Organomet. Chem. 1988, 346, 321.
(24) Carbonyl bands indicative of the ion pairs depicted in eq 2 were
observed at 1867, 1805, and 1789 cm-1
.

378 Organometallics, Vol. 21, No. 2, 2002
Carter et al.
[4][Li] at 1870, 1802, and 1765 cm-1 are replaced with
new ν(CO) bands at 1994 and 1936 cm-1. The IR data
for [(η5-C5Ph5)Fe(CO)2(CO2)][Li] compare favorably to
the carbonyl bands reported for [(η5-Cp)Fe(CO)2(CO2)]-
[Li] at 2002 and 1942 cm-1. Allowing solutions of [(η5-
Cp)Fe(CO)2(CO2)][Li] to warm to room temperature
leads to the known hydride complex (η5-C5Ph5)Fe-
(CO)2H, as verified by IR and 1H NMR spectroscopy.
Decomposition of [(η5-Cp)Fe(CO)2(CO2)][Li] via the for-
mal homolytic cleavage of the Fe-CO2 bond would
pentadienyl)Fe(CO)2]2 complexes were synthesized by following
the basic procedure of Pauson.26 THF was distilled from
sodium/benzophenone under argon and was handled and
stored under inert atmosphere using Schlenk techniques.27 The
combustion analysis was performed by Atlantic Microlab,
Norcross, GA.
Routine infrared spectra were recorded on a Nicolet 20 SXB
FT-IR spectrometer in 0.1 mm NaCl cells. The variable-
temperature IR spectra were recorded on the same spectrom-
eter with a Specac Model P/N 21.000 variable-temperature cell
equipped with inner and outer CaF2 windows. The coolant was
either dry ice/acetone or pentane/liquid nitrogen, and the
reported cell temperatures, taken to be accurate to (1 °C, were
determined by a copper-constantan thermocouple.
P r ep a r a tion of [(η5-C5P h 5)F e(CO)2][P P N]. A 0.20 g
amount (0.31 mmol) of (η5-C5Ph5)Fe(CO)2Br dissolved in 15
mL of THF was transferred via cannula to a Schlenk flask
containing excess dilute sodium amalgam. The orange solution
immediately turned olive green with the concomitant forma-
tion of the solid dinuclear compound [(η5-C5Ph5)Fe(CO)2]2.
Stirring was continued for ca. 1 h, at which time IR analysis
revealed only the presence of the carbonylmetalate [4][Na].
The deep red solution was next transferred to a vessel
containing solid [PPN][Cl] (excess) and stirred for an additional
30 min. The solution was filtered and then layered with an
equal amount of hexane. The resulting crystals were subse-
quently isolated to yield 0.18 g (53%) of dark red [(η5-C5Ph5)-
Fe(CO)2][PPN]. IR (THF): ν(CO) 1872 (vs, symm), 1806 (s,
asymm) cm-1. Anal. Calcd for C73H55FeNO2P2: C, 79.93; H,
5.02. Found: C, 78.98; H, 5.74.
X-r a y Diffr a ction Str u ctu r e of [(η5-C5P h 5)F e(CO)2]-
[P P N]. Single crystals of [(η5-C5Ph5)Fe(CO)2][PPN] suitable
for X-ray diffraction analysis were grown from a THF solution
containing [(η5-C5Ph5)Fe(CO)2][PPN] that had been layered
with hexane. A red crystal of dimensions 0.40 × 0.30 × 0.20
mm3 was selected and found to belong to the trigonal crystal
system. Systematic absences in the diffraction data and Laue
diffraction symmetry limited the possible space groups to P31
and P32. The Flack parameter indicated that the latter option
was correct. An empirical correction for absorption (SADABS)
was applied to the data. The crystal structure was solved by
direct methods, completed from difference Fourier maps, and
refined with anisotropic thermal parameters for all non-
hydrogen atoms. Refinement converged at R ) 0.0590 and Rw
) 0.1492 for 10 086 unique reflections with I > 3σ(I). All
computations used SHELXTL 5.1 (NT) software (G. Sheldrick,
Bruker AXS, Madison, WI).
•
furnish the metallo radical (η5-C5Ph5)Fe(CO)2 , which
would be free to react with THF by hydrogen abstraction
to furnish the iron hyride as the major end product.
The tight binding of the lithium cation with the
carboxylate oxygens accounts for the stabilization of the
metallocarboxylate ion. The sodium and potassium
cations are not expected to bind to the carboxylate
oxygens as strongly as the lithium gegencation, and the
unfavorable equilibrium for [(η5-C5Ph5)Fe(CO)2(CO2)]-
would facilitate the reversible dissociation of CO2 from
[(η5-C5Ph5)Fe(CO)2(CO2)]- to re-form [4]-. Proof of the
importance of the lithium gegencation in helping sta-
bilize the metallocarboxylate product was demonstrated
by adding LiPF6 to a solution containing [(η5-C5Ph5)-
Fe(CO)2][Na] with CO2. Immediate CO2 coordination
was confirmed by low-temperature IR spectroscopy upon
the addition of the lithium salt. Moreover, we have also
established that SSIPs of [4]- do not afford a stable
metallocarboxylate compound. Both [4][PPN] and solu-
tions of [4][K] containing 18-crown-6 showed no sign of
CO2 complexation. These data taken collectively dem-
onstrate that the presence of the lithium gegencation
is required for the stabilization of the metallocarboxy-
late [(η5-C5Ph5)Fe(CO)2(CO2)]-. While spectroscpically
observed in the case of the lithium salt, the metallocar-
boxylate ion [(η5-C5Ph5)Fe(CO)2(CO2)]- is most likely a
precursor to the iron hydride when THF solutions of the
sodium and potassium salts of [4]- are warmed to room
temperature.
Con clu sion s
The temperature-dependent behavior of several an-
ionic iron pentadienyl compounds in THF has been
studied by VT IR spectroscopy. The nature of the ion
pairs has been established, and the equilibrium con-
stants have been determined and the values of ∆H and
∆S reported. The reactivity of [(η5-C5Ph5)Fe(CO)2]- with
CO2 was examined and found to give a metallocarboxy-
late product with lithium as the gengencation. Our data
support the importance of tight ion binding between the
carboxylate oxgyens and the cation in controlling the
stability of the resulting metallocarboxylate complex.
The metallocarboxylate [(η5-C5Ph5)Fe(CO)2(CO2)]- is
thermally unstable and furnishes (η5-C5Ph5)Fe(CO)2H
at ambient temperatures.
Ba n d -Sh a p e An a lysis. Given that antisymmetric CO
stretching bands in all of the [(η5-pentadienyl)Fe(CO)2][Na]
compounds exhibit significant overlap, the IR band shapes of
these CO bands were calculated by using a numerical proce-
dure in order to determine the ratio of their areas, as
previously described,28 or by employing the commercially
available program PeakFit.
Th er m od yn a m ic Eva lu a tion of th e Ion -P a ir in g Equ i-
libr iu m in [(η5-p en ta d ien yl)F e(CO)2][Na ]. THF solutions
of ca. 10-2 M of each carbonylmetalate were examined over a
wide range of temperatures in order to establish the composi-
tion of ion pairs present in solution. From the areas associated
with each antisymmetric CO stretching band, the equilibrium
constants were calculated as defined by eq 1 for compounds
[1-3][Na] and eq 2 for [4][Na]. The thermodynamic param-
eters were obtained by plotting ln Keq vs T-1, from which the
slope and intercept gave ∆H and ∆S, respectively. The error
Exp er im en ta l Section
Gen er a l P r oced u r es. The Fe(CO)5 used in all syntheses
was purchased from Pressure Chemical Co. and was used
without further purification. (η5-C5Ph5)Fe(CO)2Br was pre-
pared by a known procedure utilizing C5Ph5H,25 while all [(η5-
(26) Hallam, B. F.; Pauson, P. L. J . Chem. Soc. 1958, 646.
(27) Shriver, D. F. The Manipulation of Air-Sensitive Compounds;
McGraw-Hill: New York, 1969.
(25) Field, L. D.; Hambley, T. W.; Lindall, C. M.; Masters, A. F.
Polyhedron 1989, 8, 2425.
(28) Nunn, C. M.; Cowley, A. H.; Lee, S. W.; Richmond, M. G. Inorg.
Chem. 1990, 29, 2105.

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Ion-Pairing Thermodynamics for (η5-pentadienyl)Fe(CO)2
Organometallics, Vol. 21, No. 2, 2002 379
limits associated with the enthalpy and entropy values were
calculated by using the available least-squares regression
program.
Society, for the research at Marshall University. The
NSF provided funds toward the University of Delaware
diffractometer.
Su p p or t in g In for m a t ion Ava ila b le: Listings giving
crystallographic data, bond distances, bond angles, and atomic
and thermal parameters of [(η5-C5Ph5)Fe(CO)2][PPN]. This
material is available free of charge via the Internet at
http://pubs.acs.org.
Ack n ow led gm en t . Financial support from The
Robert A. Welch Foundation (Grant No. B-1093; M.G.R.)
is gratefully acknowledged, as is the generous support
of the National Science Foundation (Grant No. NSF
CHE-9630094) and the donors of the Petroleum Re-
search Fund, administered by the American Chemical
OM0107340