Diphenylphosphide-bridged diiron derivatives of [Fe 2(η5-C5H5)2(μ-H) (μ-PPh2)(CO)2]

Article abstract of DOI:10.1021/om049574t

The complex trans-[Fe₂Cp₂(μ-H)(μ-PPh ₂)(CO)₂] is obtained in 91% yield by refluxing toluene solutions of [Fe₂Cp₂(CO)₄] (Cp = μ⁵-C₅H₅) and the secondary phosphine PPh₂H. This compound isomerizes upon irradiation with visible - UV light under a CO atmosphere to yield cis-[Fe₂Cp₂(μ-H) (μ-PPh₂)(CO)₂]. The above hydride complexes react under photochemical conditions with 1 equiv of secondary phosphines PR₂H (R = Et, Ph) to give the corresponding monocarbonyl compounds [Fe ₂Cp₂(μ-PPh₂)(μ-PR₂)(μ-CO)] via the hydride intermediates [Fe₂Cp₂(μ-H)(μ- PPh₂)(CO)(PR₂H)] (detected and isolated for R = Et). Deprotonation of trans[Fe₂Cp₂(μ-H)(μ-PPh ₂)(CO)₂] with LiBu gives the binuclear anion [Fe ₂Cp₂(μ-PPh₂)(CO)₂]. This highly nucleophilic carbonylate reacts rapidly with [AuCl(PⁱPr₃)] or MeI to give the corresponding gold diiron cluster [AuFe₂Cp ₂(μ-PPh₂)(CO)₂(PⁱPr₃)] or methyl derivative [Fe₂Cp₂(Me)(μ-PPh ₂)(μ-CO)(CO)₂], respectively. Both hydrides cis- and trans- [Fe₂Cp₂(μ-H)(μPPh₂)(CO) ₂] can be reversibly oxidized at low temperature to the corresponding cation radicals cis- and trans- [Fe₂Cp₂(μ-H)(H- PPh₂)(CO)₂]^-. At room temperature, however, the trans dicarbonyl cation isomerizes to its cis isomer, which in turn experiences a degradation process involving the reductive elimination of the bridging groups. The structures of the new complexes are analyzed on the basis of the corresponding IR and NMR (¹H, ³¹P and ¹³C) spectroscopic data. The nature of the new radical cations is analyzed also on the basis of cyclic voltammetry and ESR measurements.

Full text of DOI:10.1021/om049574t

4750
Organometallics 2004, 23, 4750-4758
Dip h en ylp h osp h id e-Br id ged Diir on Der iva tives of
[F e2(η5-C5H5)2(µ-H)(µ-P P h 2)(CO)2]
Celedonio M. Alvarez, M. Esther Garc´ıa, and Miguel A. Ruiz*
Departamento de Quı´mica Orga´nica e Inorga´nica/ IUQOEM, Universidad de Oviedo,
E-33071 Oviedo, Spain
Neil G. Connelly
School of Chemistry, University of Bristol, Bristol BS8 1TS, U.K.
Received J une 11, 2004
The complex trans-[Fe2Cp2(µ-H)(µ-PPh2)(CO)2] is obtained in 91% yield by refluxing toluene
solutions of [Fe2Cp2(CO)4] (Cp ) η5-C5H5) and the secondary phosphine PPh2H. This
compound isomerizes upon irradiation with visible-UV light under a CO atmosphere to
yield cis-[Fe2Cp2(µ-H)(µ-PPh2)(CO)2]. The above hydride complexes react under photochemical
conditions with 1 equiv of secondary phosphines PR2H (R ) Et, Ph) to give the corresponding
monocarbonyl compounds [Fe2Cp2(µ-PPh2)(µ-PR2)(µ-CO)] via the hydride intermediates [Fe2-
Cp2(µ-H)(µ-PPh2)(CO)(PR2H)] (detected and isolated for R ) Et). Deprotonation of trans-
[Fe2Cp2(µ-H)(µ-PPh2)(CO)2] with LiBu gives the binuclear anion [Fe2Cp2(µ-PPh2)(CO)2]-. This
highly nucleophilic carbonylate reacts rapidly with [AuCl(PiPr3)] or MeI to give the
corresponding gold diiron cluster [AuFe2Cp2(µ-PPh2)(CO)2(PiPr3)] or methyl derivative [Fe2-
Cp2(Me)(µ-PPh2)(µ-CO)(CO)2], respectively. Both hydrides cis- and trans-[Fe2Cp2(µ-H)(µ-
PPh2)(CO)2] can be reversibly oxidized at low temperature to the corresponding cation radicals
cis- and trans-[Fe2Cp2(µ-H)(µ-PPh2)(CO)2]+. At room temperature, however, the trans
dicarbonyl cation isomerizes to its cis isomer, which in turn experiences a degradation process
involving the reductive elimination of the bridging groups. The structures of the new
complexes are analyzed on the basis of the corresponding IR and NMR (1H, 31P and 13C)
spectroscopic data. The nature of the new radical cations is analyzed also on the basis of
cyclic voltammetry and ESR measurements.
In tr od u ction
studies we have concluded that the diphenylphosphide
ligand acts as a robust and flexible bridge, able to keep
the integrity of the dimetal center through a great
variety of elemental reactions including decarbonyl-
ation,3a oxidation,3b and protonation,3c thus leading to
new and reactive metal-metal-bonded substrates. We
thus considered the possibility of exploring the potential
of the hydride 2 as a synthetic precursor of new
diphenylphosphide-bridged diiron complexes. Notice-
ably, only a few related hydride-phosphide diiron
complexes have been described so far, these being
limited to several alkylphosphide complexes [Fe2Cp2(µ-
H)(µ-PR2)(CO)2] (R ) menthyl and related hydrocarbon
chains)4 and the diphenylphosphide-bridged [Fe2Cp2(µ-
H)(µ-PPh2)(µ-dppm)].5 Moreover, the chemistry of these
phosphide compounds has not been explored in detail.
First, we faced the problem of finding an efficient
synthetic route to cis-2, because the photochemical
reaction between 1 and dppm was clearly inappropriate
(the yield was only 15%).2 Different methods to prepare
cyclopentadienyliron complexes having phosphide bridg-
ing ligands have been described in the literature. The
classical cleavage of a P-P bond in the reactions of
We have recently reported our results on the thermal
and photochemical reactions of the metal-metal-bonded
dimer [Fe2Cp2(CO)4] (1; Cp ) η5-C5H5) with the biden-
tate phosphorus ligands (EtO)2POP(OEt)2 (tedip)1 and
Ph2PCH2PPh2 (dppm).2 In these reactions, several bond
ligand activations (C-H, C-O, P-O, and P-C) have
been discovered, thus explaining the formation of a wide
variety of binuclear iron complexes exhibiting very
different bridging groups such as P(OEt)2, OP(OEt)2, H,
PPh2, and C5H4CH2PPh2.1,2 Among those complexes we
noticed the presence of two simple complexes exhibiting
bridging diphenylphosphide ligands, the hydride cis-
[Fe2Cp2(µ-H)(µ-PPh2)(CO)2] (cis-2) and the carbonyl-
bridged [Fe2Cp2(µ-PPh2)2(µ-CO)] (3). During the past
few years we have been studying the reactions of related
dimolybdenum and tungsten complexes [M2Cp2(µ-H)-
(µ-PPh2)(CO)4] and [M2Cp2(µ-PPh2)2(µ-CO)].3 From these
* To whom correspondence should be addressed. E-mail: mara@
fq.uniovi.es.
(1) Alvarez, C. M.; Gala´n, B.; Garc´ıa, M. E.; Riera, V.; Ruiz, M. A.;
Bois, C. Organometallics 2003, 22, 3039-3048.
(2) Alvarez, C. M.; Gala´n, B.; Garc´ıa, M. E.; Riera, V.; Ruiz, M. A.;
Vaissermann, J . Organometallics 2003, 22, 5504-5512.
(3) (a) Garc´ıa, M. E.; Riera, V.; Rueda, M. T.; Ruiz, M. A.; Saez, D.
Organometallics 2002, 21, 5515-5525. (b) Garc´ıa, M. E.; Riera, V.;
Rueda, M. T.; Ruiz, M. A.; Lanfranchi, M.; Tiripicchio, A. J . Am. Chem.
Soc. 1999, 121, 4060-4061. (c) Garc´ıa, M. E.; Riera, V.; Rueda, M. T.;
Ruiz, M. A.; Halut, S. J . Am. Chem. Soc. 1999, 121, 1960-1961.
(4) (a) Brunner, H.; Peter, H. J . Organomet. Chem. 1990, 393, 411-
422. (b) Brunner, H.; Ro¨tzer, M. J . Organomet. Chem. 1992, 425, 119-
124.
(5) Raubenheimer, H. G.; Scott, F.; Cronje, S.; Van Rooyen, P. H. J .
Chem. Soc., Dalton Trans. 1992, 1859-1863.
10.1021/om049574t CCC: $27.50 © 2004 American Chemical Society
Publication on Web 08/25/2004

Diphenylphosphide-Bridged Diiron Derivatives
Organometallics, Vol. 23, No. 20, 2004 4751
Ta ble 1. IR a n d 31P NMR Sp ectr oscop ic Da ta for Com p ou n d s 2-10
c
compd
νst(CO)a
δ(P)b
J PP
trans-[Fe2Cp2(µ-H)(µ-PPh2)(CO)2] (trans-2)
cis-[Fe2Cp2(µ-H)(µ-PPh2)(CO)2] (cis-2)f
[Fe2Cp2(µ-PPh2)2(µ-CO)] (3)f
1912 (vs)d
1950 (vs)
1747 (vs)
1936 (s), 1731 (vs)
1746 (vs)
194.6e
187.3
116.9
[Fe2Cp2(µ-CO)2(CO)(PPh2H)] (4)
60.3e
[Fe2Cp2(µ-PEt2)(µ-PPh2)(µ-CO)] (5)
[Fe2Cp2(µ-H)(µ-PPh2)(CO)(PEt2H)] (6)
Li[Fe2Cp2(µ-PPh2)(CO)2] (7)
142.7, 119.2*
180.2*, 59.9
109.2g,h
208.4, 80.0
201.3*, 77.9
205.0e,i
34
41
1896 (vs)
1821 (vs)g
trans-[AuFe2Cp2(µ-PPh2)(CO)2(PiPr3)] (trans-8)
cis-[AuFe2Cp2(µ-PPh2)(CO)2(PiPr3)] (cis-8)
[Fe2Cp2(Me)(µ-PPh2)(µ-CO)(CO)] (9)
trans-[Fe2Cp2(µ-H)(µ-PPh2)(CO)2]PF6 (trans-10)
cis-[Fe2Cp2(µ-H)(µ-PPh2)(CO)2]PF6 (cis-10)
1899 (vs)
1945 (vs)
1953 (m), 1787 (vs)
1998 (w, sh), 1982 (vs)
2020 (vs), 2003 (w, sh)
14
12
a
b
Recorded in CH2Cl2 solution, unless otherwise stated; ν in cm-1
.
Recorded in CD2Cl2 solution at 290 K and 121.50 MHz, unless
otherwise stated; δ in ppm relative to external 85% aqueous H3PO4. An asterisk denotes the resonance assigned to the µ-PPh2 group
when two resonances are present. c Coupling constants in hertz. 1932 (sh), 1923 (vs) cm-1 in petroleum ether solution. e Recorded at
d
80.01 MHz. f Data from ref 2. Recorded in THF. Recorded at 165.97 MHz. Recorded in C6D6.
g
h
i
Ch a r t 1
tion of the carbonyl ligands in these binuclear com-
plexes. As usually found, these absorption frequencies
are lower for trans isomers than for the corresponding
cis isomers.2,4 The 31P{1H} NMR spectrum of trans-2
exhibits a strongly deshielded resonance at 194.6 ppm
(Table 1), as expected for arylphosphide ligands bridging
metal-metal bonds.9 This shift is similar to the values
found for cis-2,2 [Fe2Cp2(µ-H)(µ-PPh2)(µ-dppm)],5 and
[Fe2Cp2(µ-H)(µ-PMen2)(CO)2],4b which are 187.3, 172.3,
1
and 191.7 ppm, respectively. The H NMR spectrum of
trans-2 displays a high-field doublet at -18.89 ppm (J HP
) 40 Hz) assigned to the bridging hydride ligand. The
chemical shift and coupling constant of this resonance
are in agreement with the proposed structure and the
data originally reported for the complex (δ -18.67 ppm,
J HP ) 41 Hz in CS2)6 or related complexes. The 13C{1H}
NMR spectrum of trans-2 provides definitive proof for
the stereochemistry of the complex. Thus, the presence
of just one set of phenyl carbon resonances for the
diphenylphosphide bridge is only compatible with the
trans arrangement of the terminal carbonyl and cyclo-
pentadienyl ligands.
diphosphines P2R4 with carbonyl complexes has been
successfully used to obtain bis(phosphide) complexes of
the type [Fe2Cp2(µ-PR2)2(CO)2] (R ) Ph,6 Me,6 CF37) and
might be considered as an useful synthetic entry to
complex 3.2 However, the only useful synthetic route to
hydride 2 has been found to rely on the P-H bond
cleavage occurring in the thermal reaction between
PPh2H and 1, thus paralleling the synthetic method
used in the above-mentioned diiron hydrides4,5 or di-
molybdenum and ditungten complexes.3a The thermal
reaction, however, produces the trans isomer of 2. In
any case this product has been found to be a suitable
precursor for new neutral, anionic, and cationic phos-
phide-bridged diiron complexes, as discussed below.
As stated above, several related alkylphosphide com-
plexes of the type [Fe2Cp2(µ-H)(µ-PR2)(CO)2] were previ-
ously synthesized through the thermal reaction between
PR2H and 1.4 These reactions also led to the trans
isomer as the major product. Surprisingly, the yields
obtained were high only when the phosphorus atom was
part of a five-membered ring derived from phospholane
[P(C4H8)H], whereas the use of the related acyclic ligand
µ-PMen2 (Men ) menthyl, 2-isopropyl-4-methylcyclo-
hexyl) led to low yields of the hydride complex (40%).
This suggests that the above reactions are governed by
the steric rather than electronic properties of the
phosphorus ligands. By considering the cone angle of
model phosphines,10 we can estimate the steric demands
on the involved phosphines to follow the order P(C4H8)H
(similar to PMe2H, 108°) < PPh2H (126°) < PMen2H
(similar to PCy2H, 143°). From these figures we can
conclude that the steric demands of the PR2H ligands
might hinder the oxidative addition of the P-H bond
to the metal during reaction with 1.
Resu lts a n d Discu ssion
Th er m a l Rea ction of Com p ou n d 1 w ith P P h 2H.
Compound 1 reacts readily with the secondary phos-
phine PPh2H in refluxing toluene to give the complex
trans-[Fe2Cp2(µ-H)(µ-PPh2)(CO)2] (trans-2; Chart 1) as
a major product, along with trace amounts of its isomer
cis-[Fe2Cp2(µ-H)(µ-PPh2)(CO)2] (cis-2;2 Chart 1). IR moni-
toring of the above reaction allowed us to detect the
tricarbonyl complex [Fe2Cp2(µ-CO)2(CO)(PPh2H)] (4;
Chart 1) as an intermediate (see below).
trans-2 was first identified by Hayter in 1963 as a
low-yield (5%) byproduct in the reaction between P2Ph4
and 1,6 although its structure was not accurately
determined at the time. The IR spectrum of trans-2
exhibits two bands (1932 (sh) and 1923 (vs) cm-1 in
petroleum ether) in the region of C-O stretches,8 with
the relative intensities expected for a trans configura-
(8) Braterman, P. S. Metal Carbonyl Spectra; Academic Press:
London, 1975.
(9) Carty, A. J .; McLaughlin, S. A.; Nucciarone, D. In Phosphorus-
31 NMR Spectroscopy in Stereochemical Analysis; Verkade, J . G., Quin,
L. D., Eds.; VCH: Deerfield Beach, FL, 1987; Chapter 16.
(10) Tolman, C. A. Chem. Rev. 1977, 77, 313-348.
(6) Hayter, R. G. J . Am. Chem. Soc. 1963, 85, 3120-3124.
(7) Dobbie, R. C.; Mason, P. R. J . Chem. Soc., Dalton Trans. 1973,
1124-1128.

4752 Organometallics, Vol. 23, No. 20, 2004
Alvarez et al.
Sch em e 1. P r op osed Rea ction P a th w a y in th e
F or m a tion of th e Dip h en ylp h osp h id e Com p lex
tr a n s-2
Syn th esis a n d Str u ctu r a l Ch a r a cter iza tion of
Com p ou n d 4. As stated above, complex 4 is detected
as an intermediate during the IR and NMR spectro-
scopic monitoring of the thermal reaction of 1 with
PPh2H. To prepare this compound selectively, we con-
sidered the same strategy used before to obtain the
isostructural complexes [Fe2Cp2(µ-CO)2(CO)(κ1-L)] (L )
tedip,1 dppm2), based on the fact that the acetonitrile
ligand in [Fe2Cp2(CO)3(NCMe)]11 is easily displaced by
two-electron-donor ligands. As expected, [Fe2Cp2(CO)3-
(NCMe)] reacts rapidly with PPh2H in toluene even at
0 °C to give compound 4 in very good yield.
When a toluene solution of pure compound 4 was
heated under reflux for 1 h, clean transformation into
trans-2 occurred. This proves that complex 4 is a
genuine intermediate in the formation of 2 from 1 and
PPh2H.
(µ-CO)2(µ-L2)] (L2 ) dppm,14 tedip1). We note that the
oxidative addition of P-H bonds to multiple metal-
metal bonds is a process well established at dimolyb-
denum, ditungsten, and diosmium centers.3a,15,16 Fur-
thermore, a precedent involving diiron complexes can
be found in the reaction of the unsaturated [Fe2Cp*2-
(µ-H)4] with PPh2H.17 Moreover, it is noteworthy that
the “agostic” bridging coordination mode of a PR2H
ligand proposed for intermediate B has been found at
dipalladium centers.18
P h ot och em ica l R ea ct ion s of t h e Com p ou n d
tr a n s-2. The irradiation with UV-visible light of a THF
solution of compound trans-2 at low temperature (-15
°C) causes its transformation to the cis isomer cis-2.
However, when the photolysis is carried out at room
temperature, the bis(phosphide) compound [Fe2Cp2(µ-
PPh2)2(µ-CO)] (3; Chart 1) is obtained in medium yield.
As stated in the Introduction, both cis-2 and 3 have been
previously obtained in low yields (15% and 14%, respec-
tively) through the photochemical reaction of complex
1 with dppm.2 Interestingly, yields above 80% can be
obtained for these complexes by appropriate adjustment
of the photolytic conditions of trans-2. Thus, when
irradiation of trans-2 is carried out at low temperature
under a CO atmosphere, cis-2 is obtained in high yield
(83%). On the other hand, when irradiation of trans-2
is carried out in the presence of 1 equiv of PPh2H at
room temperature, then the yield of compound 3 in-
creases up to 80%. Noticeably, the monitoring of this
reaction by IR revealed that substantial amounts of cis-2
are first formed, which then gradually transform into
the final product 3. From this we conclude that the
photochemical trans to cis isomerization of compound
2 is not suppressed by the presence of PPh2H to a
significant extent.
The presence of the PPh2H ligand in 4 is easily
deduced from its 31P{1H} NMR spectrum, which exhibits
a singlet at 60.3 ppm (Table 1). This chemical shift is
in the usual region observed for arylphosphines bonded
to iron complexes.12 The 1H NMR spectrum of 4 exhibits,
as expected, two signals due to the nonequivalent
cyclopentadienyl ligands. The resonance of the P-bonded
hydrogen atom appears at 4.70 ppm (CD2Cl2) as a
strongly coupled doublet (J PH ) 353 Hz). The IR
spectrum of 4 in THF exhibits three bands in the C-O
stretching region at 1935 (m), 1777 (w), and 1731 (vs)
cm-1 with the pattern expected for this geometry,
having one terminal and two transoid bridging carbo-
nyls. In agreement with this, the 13C{1H} NMR spec-
trum of this complex exhibits two carbonyl resonances
at δ 280.0 ppm (J CP ) 15 Hz, two equivalent bridging
CO ligands) and 214.6 ppm (J CP ) 11 Hz, terminal CO).
Compound 4 is thus another member of the large family
of phosphine derivatives of compound 1 having the
general formula [Fe2Cp2(µ-CO)2(CO)(PR3)].13 For these
compounds, cis and trans isomers are possible, depend-
ing on the relative positions of the terminal ligands with
respect to the average plane defined by the Fe atoms
and bridging CO ligands. When both isomers of the
same compound are known, it is always found that the
cis isomer exhibits ν(CO) stretching frequencies around
20 cm-1 higher than the trans isomer. Therefore, by
recalling that trans-[Fe2Cp2(µ-CO)2(CO)(PEtPh2)] ex-
hibits a terminal C-O stretch at 1940 cm-1 (cyclohexane
solution)13 and that the terminal C-O band of 4 appears
at 1935 cm-1 (THF solution), we can safely assign a
trans structure to the latter complex.
It should be noted that, once formed, cis-2 and trans-2
do not interconvert in refluxing toluene. Therefore, the
stereoselectivity in the formation of these hydride
derivatives must be kinetic in origin. It can be imagined
that decarbonylation of tricarbonyl 4 would initially
generate the unsaturated intermediate A (Scheme 1),
which would induce the oxidative addition of a P-H
bond of the phosphine through the “agostic” intermedi-
ate B to yield specifically trans-2 as a consequence of
the trans arrangement of the bridging carbonyls in the
latter species (Scheme 1). Actually, intermediate B could
be isostructural with the diphosphine-bridged [Fe2Cp2-
The reaction leading to the phosphide complex 3
turned out to be a useful synthetic method to prepare
mixed bis(phosphide) complexes. Thus, when trans-2 is
irradiated at room temperature in the presence of the
(14) Haines, R. J .; Du Preez, A. L. J . Organomet. Chem. 1970, 21,
181-193.
(15) Woodward, S.; Curtis, M. D. J . Organomet. Chem. 1992, 439,
319-325.
(16) Ebsworth, E. A. V.; Mcintosh, A. P.; Schroder, M. J . Organomet.
Chem. 1986, 312, C41-C43.
(17) Ohki, Y.; Suzuki, H. Angew. Chem., Int. Ed. 2000, 39, 3120-
(11) Labinger, J . A.; Madhaven, S. J . Organomet. Chem. 1977, 134,
381-389.
(12) King, R. B.; Raghuveer, K. S. Inorg. Chem. 1984, 23, 2482-2491.
(13) Haines, R. J .; Du Preez, A. L. Inorg. Chem. 1969, 8, 1459-1464.
3122.
(18) Leoni, P.; Pasquali, M.; Sommovigo, M.; Laschi. F.; Zanello, P.;
Albinati, A.; Lianza, F.; Pregosin, P. S.; Ru¨egger, H. Organometallics
1993, 12, 1702-1713.

Diphenylphosphide-Bridged Diiron Derivatives
Organometallics, Vol. 23, No. 20, 2004 4753
Ch a r t 2
Sch em e 2. P r op osed Rea ction P a th w a ys in th e
P h otoch em ica l Rea ction s of tr a n s-2
different phosphine PEt2H, the mixed-phosphide com-
plex [Fe2Cp2(µ-PEt2)(µ-PPh2)(µ-CO)] (5; Chart 2) is
formed in high yield. This complex is isostructural with
compound 3. For example, its IR spectrum displays a
single C-O stretching band at 1746 cm-1 in CH2Cl2
solution, this indicating the bridging character of the
carbonyl ligand, as observed for 3 (Table 1). Two doublet
resonances at 142.7 and 119.2 ppm (Table 1) in the
31P{1H} NMR spectrum indicate the presence of the
two different phosphide ligands bridging identical metal
fragments, the latter being denoted by the chemical
equivalence of the cyclopentadienyl groups in the 1H
NMR spectrum. As is the case for 3, the chemical shifts
of the phosphorus atoms in 5 can be considered as
unusually low for metal-metal-bonded phosphide
bridges, and this is thought to be an indication of strain
in the Fe2P cycle or steric congestion around the dimetal
unit.2 Although complexes 3 and 5 are not very stable,
it should be noted that no other carbonyl-bridged
phosphide complexes of the type [Fe2Cp2(µ-PX2)2(µ-CO)]
appear to have been reported in the literature. Interest-
ingly, similar thiolate-bridged complexes [Fe2Cp2(µ-SR)2-
(µ-CO)] have been recently detected as transient inter-
mediates in the photolytic isomerization cis- to trans-
[Fe2Cp2(µ-SR)2(CO)2].19
If the irradiation of compound trans-2 in the presence
of PEt2H is carried out at -20 °C, the hydride trans-
[Fe2Cp2(µ-H)(µ-PPh2)(CO)(PEt2H)] (6; Chart 2) is ob-
tained in high yield. This compound formally results
from substitution of carbonyl by phosphine at trans-2
and is an important intermediate in the formation of
the mixed-phosphide complex 5. Indeed, a separate
experiment showed that photolysis of 6 at room tem-
perature yields 5 as the major product. Analysis of the
spectroscopic data for 6 (Table 1 and Experimental
Section) indicates that this compound is isostructural
with the monocarbonyl complex trans-[Fe2Cp2(µ-H)(µ-
PPh2)(CO)(PPh2Me)], previously prepared through the
photochemical reaction of cis-2 with PPh2Me.2 The
structure of trans-[Fe2Cp2(µ-H)(µ-PPh2)(CO)(PPh2Me)]
has been discussed in detail previously, and therefore
no further comments are needed in the case of com-
pound 6.
tion to a significant extent, as the presence of neither
CO nor PR2H suppresses this isomerization in a notice-
able way. Therefore, we propose that reversible opening
of the hydride bridge occurs at the transient stage C,
this being followed by rotation of the FeCp(CO) moiety
around an Fe-P bond, allowing the formation of the
isomer cis-2. This bridge-opening mechanism is similar
to that proposed to explain cis/trans isomerization in
the thiolate-bridged dimolybdenum complexes [Mo2Cp2-
(µ-H)(µ-SR)(CO)4].20 We note, however, that Brunner et
al. also reported photochemical trans to cis isomeriza-
tion at the alkylphosphide diiron complexes [Fe2Cp2(µ-
H)(µ-PR2)(CO)2]. A CO-dissociative mechanism was
envisaged there4 and cannot be completely excluded in
the case of the trans to cis isomerization of compound 2
(dashed arrow in Scheme 2). However, the fact that this
isomerization is not suppressed to a significant extent
by the presence of PR2H leads us to think that the
nondissociative pathway must be dominant. In this
context, it is important to note that a recent and detailed
mechanistic study on the photochemical cis to trans
isomerization in the thiolate complexes [Fe2Cp2(µ-SR)2-
(CO)2] led to the conclusion that both CO-loss and non-
CO-loss pathways were operative.19
Rea ction P a th w a ys in th e P h otoch em ica l Rea c-
tion s of th e Ir on Com p lex tr a n s-2. As discussed
above, depending on the experimental conditions the
irradiation of trans-2 with UV-visible light can induce
several processes such as isomerization, CO substitu-
tion, and P-H bond cleavage. Our synthetic data
suggest that the dominant reaction pathway should be
close to that depicted in Scheme 2. The fastest photo-
chemical process in all cases would be the trans to cis
isomerization of the terminal ligands of the starting
material. This process seems not to involve CO dissocia-
The fact that the cis geometry is favored for compound
2 under UV-visible light irradiation might be a more
general property of diiron dicarbonyls having two
bridges and a metal-metal bond. For example, a similar
(photochemically favored) trans to cis isomerization has
been reported for the silylene-bridged complex [Fe2(η-
(20) Schollhammer, P.; Pe´tillon, F. Y.; Pichon, R.; Poder-Guillou, S.;
Talarmin, J .; Muir, K. W.; Girdwood, S. U. J . Organomet. Chem. 1995,
486, 183-191.
(19) Bitterwolf, T. E.; Scallorn, W. B.; Li, B. H. Organometallics
2000, 19, 3280-3287.

4754 Organometallics, Vol. 23, No. 20, 2004
Alvarez et al.
Sch em e 3. Rea ction s of An ion 7 w ith Differ en t
Electr op h iles
water, which makes its manipulation very difficult in a
relatively hydrophilic solvent such as tetrahydrofuran.
The IR spectrum of the anion 7 exhibits only one C-O
stretching band at 1821 cm-1, some 100 cm-1 lower than
that in its neutral precursor trans-2 (Table 1). This large
decrease in the C-O stretching frequency is character-
istic of metal carbonylates and is similar to the ca. 90
cm-1 decrease observed for the anion [Mo2Cp2(µ-PPh2)-
(CO)4]- when obtained from neutral [Mo2Cp2(µ-H)(µ-
PPh2)(CO)4].25 The 31P{1H} NMR spectrum of 7 exhibits
only one signal at 109.2 ppm (Table 1), which appears
considerably upfield with respect to the usual values
found for bridging diphenylphosphide ligands. However,
analysis of the spectroscopic data for different dinuclear
phosphide-bridged complexes indicates that a change
in the overall charge of a compound, as happens in
protonation/deprotonation reactions of hydride-bridged
complexes, can induce either upfield or downfield changes
in the chemical shift of the phosphide 31P nucleus.9
Unfortunately, no additional spectroscopic data could
be obtained for 7 because of its easy protonation during
manipulation. Nevertheless, the structures of some of
its derivatives, discussed next, suggest that anion 7
would exist in solution as an equilibrium mixture of cis
and trans isomers.
Despite the relatively simple nature of compound 7,
we note that this anion is a rare example of a binuclear
iron carbonylate having cyclopentadienyl ligands. In
fact, there are only relatively few diiron anions, none
of them having Cp ligands as, for example, the diphenyl-
phosphide-bridged anions [Fe2(µ-PPh2)(CO)n]- (n ) 6,
7),26 the related alkenyl and thiolate anions [Fe2(µ-X)-
(µ-CO)(CO)6]- (X ) RCCHR,27 SR),28 and the octocar-
bonyl complexes [Fe2(CO)8]2- and [Fe2(µ-H)(CO)8]-.29
Rea ction of An ion 7 w ith [Au Cl(P iP r 3)]. As ex-
pected, compound 7 easily displaces chloride ions from
[AuCl(PiPr3)]. However, this reaction yields almost
instantaneously a mixture of the isomers cis- and trans-
[AuFe2Cp2(µ-PPh2)(CO)2(PiPr3)] (8; Scheme 3) in a 1:4
ratio. These two isomers could not be separated by using
either chromatography or crystallization techniques.
The IR spectrum of the mixture of isomers in the C-O
stretching frequency region is very similar to those of
isomers 2 but is displaced by ca. 10 cm-1 toward lower
frequency (Table 1). This is expected, due to the lower
electronegativity of the Au(PiPr3) fragment with respect
to the H atom.30 The 31P{1H} NMR spectrum shows two
doublets for each isomer (Table 1). The signal of the
bridging phosphide atom (ca. 200 ppm, Table 1) appears
in the expected region for this kind of ligand.9 Again,
as was observed for hydrides 2, the trans isomer gives
rise to a more deshielded resonance. Finally, the reso-
nances of the phosphorus atoms coordinated to Au
atoms appear around 80 ppm, as expected.
C5Me5)2{µ-SiH(C6H4Me)}(µ-CO)(CO)2].21 The isomer cis-2
is relatively stable under UV-visible light, although the
low yield of monocarbonyl 3 upon extended photolysis
at room temperature suggests that some slow decarbo-
nylation is induced under these conditions. In fact, this
can be put to good use by performing the photolysis of
2 in the presence of 1 equiv of PR2H, in which case the
bis(phosphide) monocarbonyls 3 and 5 are formed in
good yield. It is assumed that initial decarbonylation
would give the unsaturated intermediate D, which then
would rapidly add a molecule of phosphine to give
compound 6 or its PPh2H analogue (not detected). This
is a well-established reaction pathway for the photo-
chemical substitution of CO by phosphines at [Fe2Cp2-
(µ-CO)2(CO)2]22 and [Fe2(η-C5Me5)2(µ-CO)(µ-CHMe)-
(CO)2].23
The final steps connecting the hydrides of type 6 and
the bis(phosphide) derivatives 3 and 5 are less straight-
forward. Under photochemical conditions, a hydride-
bridge-opening process would generate a 16-electron
metal center (intermediate E), on which the oxidative
addition of a P-H bond would readily occur to give the
dihydride intermediate F . This step would be similar
to the addition of P-H bonds from coordinated phos-
phines to coordinatively unsaturated PtL2 centers to
afford phosphide-bridged heterometallic derivatives.24
Finally, dehydrogenation from the overcrowded inter-
mediate F would yield the bis(phosphide) complexes 3
and 5 after rearrangement.
Syn th esis a n d Str u ctu r a l Ch a r a cter iza tion of
th e An ion [F e2Cp 2(µ-P P h 2)(CO)2]- (7). Treatment of
a THF solution of trans-2 with 1 equiv of LiBu yields
Li[Fe2Cp2(µ-PPh2)(CO)2] (7) almost instantaneously
(Scheme 3). This anion is extremely reactive toward
(25) Hartung, H.; Walther, B.; Baumeister, U.; Bo¨ttcher, H. C.; Krug,
A.; Rosche, F.; J ones, P. G. Polyhedron 1992, 11, 1563-1568.
(26) Seyferth, D.; Brewer, K. S.; Wood, T. G.; Cowie, M.; Hilts, R.
W. Organometallics 1992, 11, 2570-2579.
(21) Kawano, Y.; Tobita, H.; Ogino, H. Organometallics 1992, 11,
499-500.
(22) (a) Caspar, J . V.; Meyer, T. J . J . Am. Chem. Soc. 1980, 102,
7794-7795. (b) Dixon, A. J .; Healy, M. A.; Poliakoff, M.; Turner, J . J .
J . Chem. Soc., Chem. Commun. 1986, 994-996. (c) Brown, T. L.;
Zhang, S. Organometallics 1992, 11, 4166-4173. (d) Brown, T. L.;
Zhang, S. J . Am. Chem. Soc. 1993, 115, 1779-1789.
(23) Burnsten, B. E.; McKee, S. D. J . Am. Chem. Soc. 1991, 113,
1210-1217.
(24) (a) Powell, J .; Fuchs, E.; Gresgg, M. R.; Phillips, J .; Stainer,
M. V. R. Organometallics 1990, 9, 387-393. (b) Powell, J .; Gregg, M.
R.; Sawyer, J . F. Inorg. Chem. 1989, 28, 4451-4460.
(27) Roser, R.; Rosell, O.; Seco, M.; Ros, J .; Yan˜ez, R.; Perales, A.
Inorg. Chem. 1991, 30, 3973-3976 and references therein.
(28) Seyferth, D.; Hoke, J . B.; Hofmann, P.; Schnellbach, M. Organo-
metallics 1994, 13, 3452-3464 and references therein.
(29) (a) Hieber, W.; Beck, W.; Braun, G. Angew. Chem. 1960, 72,
795-801. (b) Neumu¨ller, B.; Petz, W. Organometallics 2001, 20, 163-
170. (c) Collman, J . P.; Finke, R. G.; Matlock, P. L.; Wahren, R.;
Kamoto, R. G.; Brauman, J. I. J . Am. Chem. Soc. 1978, 100, 1119-1140.
(30) Riera, V.; Ruiz, M. A.; Tiripicchio, A.; Tiripicchio-Camellini, M.
Organometallics 1993, 12, 2962-2972.

Diphenylphosphide-Bridged Diiron Derivatives
Organometallics, Vol. 23, No. 20, 2004 4755
Sch em e 4. P r op osed Rea ction P a th w a ys in th e
F or m a tion of th e Meth yl Com p lex 9
The reaction of carbonyl anions with chlorogold
complexes is a general synthetic method to generate
heterometallic clusters combining Au and transition-
metal atoms.31 Its use with dinuclear anions having
M2(µ-PR2) backbones to give phosphide-bridged M2Au
clusters has been reported for different metals, including
Mo, Re, and Mn.25,32
A point yet to be addressed is the formation of a
mixture of cis and trans Fe2Au clusters apparently from
the single anion 7. In this respect, the observation that
protonation of 7 with H2O also gives a mixture of cis
and trans hydrides 2, in a ratio (ca. 1:4) identical with
the ratio observed for the gold clusters 8, is revealing.
There are two possible explanations for these observa-
tions. First, it is conceivable that two rapidly intercon-
verting (so as to give a single 31P resonance) isomers of
the anion 7 coexist in solution in a 1:4 ratio. Second, it
is possible that only one static structure for 7 is present
in solution, having two nonequivalent reaction sites,
each leading to a different isomer. In the latter case,
however, it is difficult to understand why the reaction
with two electrophiles so different in size, namely H+
and (AuPR3)+, would lead to the same ratio of isomers.
By exclusion, then, we favor the coexistence of cis and
trans isomers in the THF solutions of complex 7.
Rea ction of An ion 7 w ith MeI. Anion 7 is nucleo-
philic enough so as to react with MeI in THF to give
the methyl complex [Fe2Cp2(Me)(µ-PPh2)(µ-CO)(CO)2]
(9; Scheme 3) in good yield. The vacant site at anion 7
is located between the iron atoms as it is, presumably,
the HOMO of the molecule, as calculated for the related
dimolybdenum anion [Mo2Cp2(µ-Cl)(CO)4]-.33 The meth-
yl group is therefore expected to be incorporated as a
bridging ligand, as observed in the reaction of the
unsaturated anion [Mo2Cp2(µ-PCy2)(CO)2]- with MeI.34
Apparently, however, a bridging µ,η1 coordination for
the methyl group at this diiron center is not stable; thus,
the product would rapidly rearrange, by exchanging Me
and CO positions, to yield a CO-bridged complex with
a terminal methyl ligand (see below).
The presence of both terminal and bridging carbonyl
ligands in 9 is readily apparent from its IR spectrum
in solution (Table 1), which exhibits two C-O stretching
bands at 1953 cm-1 (terminal C-O) and at 1787 cm-1
(bridging C-O). The 31P{1H} NMR spectrum of com-
pound 9 shows a resonance at 205.0 ppm (Table 1), in
the region expected for diphenylphosphide ligands
bridging metal-metal bonds. Its 1H NMR spectrum
shows two resonances at 4.34 and 4.30 ppm correspond-
ing to the nonequivalent cyclopentadienyl ligands,
whereas the methyl protons give rise to a doublet
resonance at -1.13 ppm (J HP ) 8 Hz). The latter data
are similar to those for the methyl complex [FeCp(Me)-
(CO)(PPh3)] (δ -0.17 ppm; J HP ) 7 Hz),35 thus support-
ing the terminal coordination proposed for the methyl
ligand in complex 9.
Ch a r t 3
Early in the reaction leading to 9, a weak C-O band
is observed at 1937 cm-1 (in THF) in the IR spectrum
of the reaction mixture, this disappearing during the
reaction. It is reasonable to suggest that this unstable
species is an isomer of 9 (Scheme 4). Thus, as observed
for the protonation or “aurification” reaction of anion
7, it is likely that both cis (minor) and trans (major)
µ-methyl derivatives (G) are initially formed; they
rearrange to trans (minor) and cis (major) terminal
methyl complexes, respectively. By analogy with the IR
spectroscopic data for cis and trans complexes of the
type [Fe2Cp2(µ-CO)2(CO)(PR3)],13 the band at 1937 cm-1
can be assigned to the trans isomer, while the higher
frequency of the C-O stretching band in the final
compound 9 would identify it as the cis isomer. These
data thus imply that the trans isomer rearranges to the
cis isomer in solution at room temperature.
Syn th esis a n d Str u ctu r a l Ch a r a cter iza tion of
th e P a r a m a gn etic Ca tion s [F e2Cp 2(µ-H)(µ-P P h 2)-
(CO)2]+ (cis- a n d tr a n s-10). The hydride complexes
cis-2 and trans-2 can be oxidized with 1 equiv of [FeCp2]-
PF6 in THF at -60 °C to form the 33-electron species
cis- and trans-[Fe2Cp2(µ-H)(µ-PPh2)(CO)2]+ (cis-10 and
trans-10; Chart 3), respectively. These cations have been
characterized by IR and ESR spectroscopy and cyclic
voltammetry (CV). All the data indicate that oxidation
takes place with retention of the stereochemistry of the
neutral precursors.
(31) (a) Salter, I. D. Adv. Organomet. Chem. 1989, 29, 249-343. (b)
Mingos, D. M. P.; Watson, M. J . Adv. Inorg. Chem. 1992, 39, 327-399.
(32) (a) Iggo, J . A.; Mays, M. J .; Raithby, P. R.; Hendrick, K. J . Chem.
Soc., Dalton Trans. 1984, 633-641. (b) Haupt, H. J .; Heinekamp, C.;
Flo¨rke, U. Inorg. Chem. 1990, 29, 2955-2963. (c) Haupt, H. J .;
Schwefer, M.; Egold, H.; Flo¨rke, U. Inorg. Chem. 1995, 34, 5461-5467.
(33) Curtis, M. D.; Fotinos, N. A.; Han, K. R.; Butler, W. M. J . Am.
Chem. Soc. 1983, 105, 2686-2694.
Each of the isomers of compound 10 displays two C-O
stretching bands in its IR spectrum (Table 1). The
relative intensities of the two bands point respectively
to the presence of [M(CO)]2 oscillators with transoid or
(34) Garc´ıa, M. E.; Melo´n, S.; Ramos, A.; Riera, V.; Ruiz, M. A.;
Belletti, D.; Graiff, C.; Tiripicchio, A. Organometallics 2003, 22, 1983-
1985.
(35) Treichel, P. M.; Shubkin, R. L.; Barnett, K. W.; Reichard, D.
Inorg. Chem. 1966, 5, 1177-1181.

4756 Organometallics, Vol. 23, No. 20, 2004
Alvarez et al.
Sch em e 5. Red ox Tr a n sfor m a tion s of Hyd r id e
Com p lexes 2
ization might be facilitated by the presence of a weakly
bound hydride ligand, perhaps involved in a bridge-
opening mechanism similar to that operating in the
photochemical isomerization of the neutral hydride 2
(Scheme 2). In line with this, isomer cis-10 is also not
very stable at room temperature. In fact, this complex
decomposes slowly at room temperature to give the
mononuclear species [FeCp(CO)2(PPh2H)]PF6 as the
only carbonyl-containing product.41
The paramagnetic cations 10 also give mononuclear
species readily in their reactions with classical radical
traps such as I2 and S2Ph2. In both cases, mononuclear
cations of the type [FeCp(CO)2(PPh2X)]+ (X ) I, SPh)
were obtained, as judged from the IR and NMR spectra
of the corresponding reaction mixtures. No attempts
were made to characterize these products fully. How-
ever, it seems clear that the diphenylphosphide bridge
is not strong enough so as to preserve the nuclearity of
the paramagnetic cations 10.
We note finally that compounds 10 are among the
very few 33-electron diiron complexes synthesized so far.
Previous examples are limited to the phosphide-bridged
[Fe2(µ-PPh2)(CO)7]42 and the diphosphine-bridged cat-
ions [Fe2Cp2(µ-CO)2(µ-Ph2PXPPh2)]+ (X ) CnH2n; n )
1-3)43 and [Fe2Cp2(µ-CO)(µ-CH2)(µ-dppm)]+.44 Unfor-
tunately, the low stability of the diphenylphosphide
bridge in 10 precludes a detailed study of their chemical
behavior.
almost parallel CO ligands.8 In addition, the frequencies
for those bands are shifted ca. 70 cm-1 higher than those
of the neutral precursors, as expected for the change
from neutral to cationic species. The fact that both C-O
stretching bands (symmetric and asymmetric) experi-
ence a similar increase on going from compounds 2 to
10 indicates that the unpaired electron in the cations
is delocalized over both iron atoms. Thus, both cis and
trans cations 10 should be classified as class III mixed-
valence complexes.36 The room-temperature ESR spec-
tra of CH2Cl2 solutions of cations 10 exhibit in each case
a single resonance displaying no coupling to the phos-
phorus atom. This observation suggests that in both
isomers the singly occupied molecular orbital (SOMO)
has a very low participation of the phosphorus atomic
orbitals.
Cyclic voltammetry studies in CH2Cl2 indicate that
both isomers undergo reversible one-electron oxidation
at 0.24 V (cis isomer) and 0.36 V (trans isomer). In
agreement with this reversibility, both isomers 10 are
reduced by sodium amalgam in tetrahydrofuran to
regenerate the corresponding neutral precursors 2
(Scheme 5).
trans-10 is thermally unstable, and complete isomer-
ization to cis-10 takes place in ca. 15 min in CH2Cl2
solution at room temperature (Scheme 5). This process
can be monitored by either IR spectroscopy or cyclic
voltammetry. If, during cyclic voltammetry of trans-2
from -0.2 to 0.5 to -0.2 V, the potential is held for a
short time (15-30 s) at 0.5 V, an additional wave is
observed on the return scan corresponding to the
reduction cis-10 to cis-2.
Electron-transfer-induced isomerizations of organo-
metallic complexes are not unusual,37,38 but we have
found no precedents for the above isomerization in
related diiron complexes. For example, the bis(diphenyl-
phosphide) isomers cis- and trans-[Fe2Cp2(µ-PPh2)2(CO)2]
can be readily oxidized to the corresponding mono- and
dications without isomerization.39,40 This suggests that,
in the case of cations 10, the observed trans/cis isomer-
Con clu d in g Rem a r k s
The thermally induced oxidative addition of the P-H
bond of PPh2H to [Fe2Cp2(CO)4] provides a highly
efficient and stereoselective route to trans-[Fe2Cp2(µ-
H)(µ-PPh2)(CO)2] (trans-2). The diphenylphosphide bridge
in trans-2 acts as an efficient support of the dimetallic
center through a number of different reactions, includ-
ing photochemical decarbonylation, reduction, and oxi-
dation, thus allowing the synthesis of new cyclopenta-
dienyldiiron derivatives. Under photochemical conditions,
trans to cis isomerization of compound 2 is faster than
CO dissociation, but the latter generates unsaturated
intermediates able to induce new P-H bond activations
which eventually allow the synthesis of bis(phosphide)
derivatives of the type [Fe2Cp2(µ-PPh2)(µ-PR2)(µ-CO)].
Trans to cis isomerization is also thermodynamically
favored for the 33-electron cations [Fe2Cp2(µ-H)(µ-PPh2)-
(CO)2]+, which are stereospecifically generated upon
1-electron oxidation of isomers 2. These cations, how-
ever, are unstable with respect to P-H reductive
elimination and degrade to mononuclear species at room
temperature. In contrast, the diphenylphosphide ligand
behaves as a more robust bridging group under reducing
conditions, thus allowing the formation of the new
binuclear carbonylate [Fe2Cp2(µ-PPh2)(CO)2]-. The lat-
ter is a strong nucleophile, and it reacts readily with
either metal-based or carbon-based electrophiles and
(39) (a) Sinclair, J . D. Ph.D. Thesis, University of Wisconsin, 1972.
(b) Dessy, R. E.; Kornmann, R.; Smith, C.; Haytor, R. J . Am. Chem.
Soc. 1968, 90, 2001-2004.
(40) Gennett, T.; Geiger, W.; Willett, B.; Anson, F. C. J . Electroanal.
Chem. 1987, 222, 151-160.
(36) (a) Robin, M. B.; Day, P. Adv. Inorg. Chem. Radiochem. 1967,
10, 247-422. (b) Demadis, K. D.; Hartshorn, C. M.; Meyer, T. J . Chem.
Rev. 2001, 101, 2655-2686.
(41) Nakazawa, H.; Ueda, Y.; Nakamura, K.; Miyoshi, K. Organo-
metallics 1997, 16, 1562-1566.
(42) Baker, R. J . Calabrese, J . C.; Krusic, P. J .; Therien, M. J .;
Trogler, W. C. J . Am. Chem. Soc. 1988, 110, 8392-8412.
(43) Haines, R. J .; Du Preez, A. L. Inorg. Chem. 1972, 11, 330-336.
(44) Colins, F. M. Ph.D. Thesis, University of Bristol, 1988.
(37) Pombeiro, A. J . L.; Guedes da Silva, M. F. C.; Lemos, M. A. N.
D. A. Coord. Chem. Rev. 2001, 219-221, 53-80.
(38) Connelly, N. G. Chem. Soc. Rev. 1989, 18, 153-185.

Diphenylphosphide-Bridged Diiron Derivatives
Organometallics, Vol. 23, No. 20, 2004 4757
59.55; H, 4.37. Found: C, 59.38; H, 4.29. 1H NMR (200.13
MHz, CD2Cl2): δ 7.80-7.70 (m, 4H, Ph), 7.50-7.15 (m, 6H,
Ph), 4.37 (s, 10H, Cp), -18.89 (d, J HP ) 40, 1H, µ-H). 13C{1H}
NMR (CD2Cl2): δ 216.4 (d, J CP ) 18, CO), 143.8 (d, J CP ) 28,
C1(Ph)), 132.8 (d, J CP ) 9, C2(Ph)), 127.4 (d, J CP ) 2, C4(Ph)),
126.9 (d, J CP ) 10, C3(Ph)), 80.5 (s, Cp).
P r ep a r a tion of cis-[F e2Cp 2(µ-H)(µ-P P h 2)(CO)2] (cis-2).
A THF solution (10 mL) of compound trans-2 (0.030 g, 0.062
mmol) was irradiated with visible-UV light in a quartz
Schlenk tube at -15 °C under a CO atmosphere for 1 h to
give a red solution. The latter was then filtered and the solvent
removed under vacuum to give cis-2 as a brown-red powder
(0.025 g, 83%). Spectroscopic data for cis-2 were identical with
those reported for this complex in ref 2.
P r ep a r a tion of [F e2Cp 2(µ-P P h 2)2(µ-CO)] (3). A tetrahy-
drofuran solution (10 mL) of trans-2 (0.030 g, 0.062 mmol) and
PPh2H (13 µL, 0.074 mmol) was irradiated with visible-UV
light for 90 min in a quartz Schlenk tube at 15 °C while
nitrogen was bubbled through the solution. The resulting red
solution was then filtered, the solvent removed under vacuum,
and the resulting residue washed with petroleum ether (2 ×
3 mL) to give compound 3 as a brown solid (0.032 g, 80%).
Spectroscopic data for compound 3 were identical with those
previously reported for this complex in ref 2.
might therefore have significant potential in the syn-
thesis of new diiron cyclopentadienyl derivatives.
Exp er im en ta l Section
Gen er a l Com m en ts. All manipulations and reactions were
carried out using standard Schlenk techniques under an
atmosphere of dry, oxygen-free nitrogen. Solvents were puri-
fied according to standard literature procedures45 and distilled
under nitrogen prior to use. Petroleum ether refers to that
fraction distilling in the range 60-65 °C. Compounds [Fe2Cp2-
(CO)3(NCMe)],11 and [FeCp2]PF646 were prepared as described
previously. [AuCl(PiPr3)] was prepared from [AuCl(SC4H8)]47
and PiPr3. Compounds [Fe2Cp2(CO)4], PR2H (R ) Et, Ph), LiBu,
and CH3I were obtained from the usual commercial suppliers
and used without further purification. Photochemical experi-
ments were performed using jacketed Pyrex or quartz Schlenk
tubes, refrigerated by a closed propan-2-ol circuit kept at the
desired temperature with a cryostat or by tap water. A 400 W
mercury lamp (Applied Photophysics), placed ca. 1 cm away
from the Schlenk tube, was used for these experiments. Low-
temperature chromatographic separations were carried out
using jacketed columns. Commercial aluminum oxide (alu-
mina, Aldrich, activity I, 150 mesh) was degassed under
vacuum prior to use. The latter was mixed afterward under
nitrogen with the appropriate amount of water to reach the
activity desired. NMR spectra were recorded at 300.13 (1H),
121.50 (31P{1H}), or 50.32 MHz (13C{1H}), at room temperature
unless otherwise stated. Chemical shifts (δ) are given in ppm,
relative to internal TMS (1H, 13C) or external 85% H3PO4
aqueous solution (31P), with positive values for frequencies
higher than that of the reference. Coupling constants (J ) are
given in hertz. 13C{1H} NMR spectra were routinely recorded
on solutions containing a small amount of tris(acetylacetonate)-
chromium(III) as a relaxation reagent. X-Band ESR spectra
were recorded on a Bruker ESP300 spectrometer equipped
with a Bruker variable-temperature accessory and a Hewlett-
Packard 5350B microwave frequency counter. The field cali-
bration was checked by measuring the resonance of the
diphenylpicrylhydrazyl (dpph) radical before each series of
spectra. Electrochemical studies were carried out using an
EG&G Model 273A potentiostat linked to a computer using
EG&G Model 270 Research Electrochemistry software in
conjunction with a three-electrode cell. The auxiliary electrode
was a platinum wire and the working electrode a platinum
disk. The reference was an aqueous saturated calomel elec-
trode separated from the test solution by a fine-porosity frit
and an agar bridge saturated with KCl. Solutions were 5.0 ×
10-4 or 1.0 × 10-3 mol dm-3 in the test compound and 0.1 mol
dm-3 in [NnBu4][PF6] as the supporting electrolyte in CH2Cl2.
Under the conditions used, E°′ for the one-electron oxidation
of [Fe(η5-C5H5)2], added to the test solutions as an internal
calibrant, is 0.47 V.
P r ep a r a t ion of [F e2Cp 2(µ-CO)2(CO)(P P h 2H )] (4). A
dichloromethane solution (15 mL) of [Fe2Cp2(CO)3(NCMe)]11
(0.155 g, 0.26 mmol) and PPh2H (74 µL, 0.425 mmol) was
stirred at 0 °C for 5 min to yield a green solution. The latter
was then filtered, the solvent removed under vacuum, and the
resulting residue washed with petroleum ether (2 × 3 mL) to
give compound 4 as a green powder (0.190 g, 87%). Anal. Calcd
for C25H21O3PFe2: C, 58.64; H, 4.13. Found: C, 58.39; H, 4.02.
1H NMR (200.13 MHz, CD2Cl2): δ 7.60-7.20 (m, 10H, Ph),
4.79 (s, 5H, Cp), 4.70 (d, J HP ) 353, 1H, PH), 4.61 (s, 5H, Cp).
13C{1H} NMR (CD2Cl2): δ 280.0 (d, J CP ) 15, µ-CO), 214.6 (d,
J CP ) 11, CO), 134.6-128.2 (m, Ph), 86.9 (s, Cp), 85.7 (s, Cp).
P r ep a r a tion of [F e2Cp 2(µ-P Et2)(µ-P P h 2)(µ-CO)] (5). A
tetrahydrofuran solution (10 mL) of trans-2 (0.030 g, 0.062
mmol) and PEt2H (8 µL, 0.070 mmol) was irradiated with
visible-UV light for 90 min in a quartz Schlenk tube at 15 °C
while nitrogen was bubbled through the solution. The resulting
red solution was then filtered, the solvent removed under
vacuum, and the resulting residue washed with petroleum
ether (2 × 3 mL) to give compound 5 as a brown solid (0.023
g, 77%). Anal. Calcd for C27H30OP2Fe2: C, 59.59; H, 5.56.
1
Found: C, 59.68; H, 5.60. H NMR (CD2Cl2): δ 7.80-7.15 (m,
10H, Ph), 4.50 (s, 10H, Cp), 1.64 (dq, J HP ) 12, J HH ) 7, 2H,
CH2), 1.42 (dq, J HP ) 10, J HH ) 7, 2H, CH2), 1.05 (dt, J HP
)
16, J HH ) 7, 3H, CH3), 0.66 (dt, J HP ) 14, J HH ) 7, 3H, CH3).
P r ep a r a tion of [F e2Cp 2(µ-H)(µ-P P h 2)(CO)(P Et2H)] (6).
A tetrahydrofuran solution (10 mL) of trans-2 (0.030 g, 0.062
mmol) and PEt2H (8 µL, 0.070 mmol) was irradiated with
visible-UV light in a quartz Schlenk tube at -20 °C for 45
min to give a reddish solution. The latter was then filtered,
the solvent was removed under vacuum, and the resulting
residue was dissolved in petroleum ether/CH2Cl2 (19/1) and
chromatographed at 15 °C on an alumina column (activity II,
20 × 2 cm) prepared in petroleum ether. Elution with the same
solvent mixture gave a green fraction containing a very small
amount of an uncharacterized complex. Elution with petroleum
ether/CH2Cl2 (9/1) gave a brown fraction containing compound
6. Elution with petroleum ether/CH2Cl2 (3/1) gave a brown
fraction containing a small amount of cis-2. Removal of
solvents under vacuum from the second fraction gave com-
pound 6 as a brown crystalline solid (0.027 g, 81%). Anal. Calcd
for C27H32OP2Fe2: C, 59.37; H, 5.91. Found: C, 59.38; H, 5.49.
1H NMR (CD2Cl2): δ 7.70 (m, 2H, Ph), 7.44 (m, 2H, Ph), 7.30-
7.15 (m, 6H, Ph), 4.40 (d, J HP ) 1, 5H, Cp), 4.20 (d, J HP ) 1,
5H, Cp), 1.70 (m, 2H, CH2), 1.53 (m, 1H, CH2), 1.55 (d of m,
J HP ) 321, 1H, HP), 1.41 (m, 1H, CH2), 1.22 (dt, J HP ) 15, J HH
) 7, 3H, CH3), 0.71 (dt, J HP ) 13, J HH ) 7, 3H, CH3), -18.89
P r epar ation of tr a n s-[Fe2Cp2(µ-H)(µ-P P h 2)(CO)2] (tr a n s-
2). A mixture of compound 1 (0.200 g, 0.565 mmol) and PPh2H
(100 µL, 0.574 mmol) was heated under reflux in toluene (25
mL) for 1 h to give a reddish solution. Solvent was then
removed under vacuum and the residue dissolved in a mini-
mum of CH2Cl2. This solution was chromatographed at 15 °C
on an alumina column (activity II, 20 × 2 cm) prepared in
petroleum ether. After the column was washed with petroleum
ether, elution with petroleum ether/CH2Cl2 (4/1) gave a small
brown fraction containing trace amounts of cis-2 and then a
red-brown fraction containing the complex trans-2. Elution
with CH2Cl2 gave a green fraction containing a very small
amount of compound 4. Removal of solvents under vacuum
from the main fraction gave compound trans-2 (0.248 g, 91%)
as a brown crystalline solid. Anal. Calcd for C24H21O2PFe2: C,
(45) Perrin, D. D.; Armarego, W. L. F. Purification of Laboratory
Chemicals; Pergamon Press: Oxford, U.K., 1988.
(46) Connelly, N. G.; Geiger, W. E. Chem. Rev. 1996, 96, 877-910.
(47) Uson, R.; Laguna, A.; Laguna, M. Inorg. Synth. 1989, 26, 85-91.

4758 Organometallics, Vol. 23, No. 20, 2004
Alvarez et al.
(d, J HP ) 40, 1H, µ-H). 13C{1H} NMR (CD2Cl2): δ 219.8 (d, J CP
) 26, CO), 151.1 (d, J CP ) 13, C1(Ph)), 140.8 (dd, J CP ) 27, 5,
C1(Ph)), 136.3 (d, J CP ) 9, C2(Ph)), 133.0 (d, J CP ) 9, C2(Ph)),
127.8 (s, C4(Ph)), 127.7 (d, J CP ) 9, C3(Ph)), 127.3 (d, J CP ) 9,
C3(Ph)), 126.9 (d, J CP ) 2, C4(Ph)), 80.8 (s, Cp), 74.0 (s, Cp),
21.8 (dd, J CP ) 25, 4, CH2), 21.3 (d, J CP ) 25, CH2), 12.9 (d,
J CP ) 8, CH3), 11.4 (s, CH3).
P r epar ation of Tetr ah ydr ofu r an Solu tion s of Li[Fe2Cp2-
(µ-P P h 2)(CO)2] (7). Rigorously dried solvent is required for
this preparation. A petroleum ether solution of LiBu (45 µL
of an 1.6 M solution, 0.072 mmol) was added over a THF
solution (10 mL) of compound trans-2 (0.030 g, 0.062 mmol)
at room temperature. This mixture was stirred for 5 min to
give a brown solution shown by IR and 31P{1H} NMR to
contain the complex 7 as major species.
P r ep a r a tion of [F e2Cp 2(µ-P P h 2)(Me)(µ-CO)(CO)] (9). An
excess of CH3I (10 µL, 0.160 mmol) was added to a THF
solution of compound 7, prepared in situ as described above
(ca. 0.062 mmol), and the mixture was stirred at room
temperature for 5 min to give a brown solution. Solvent was
then removed under vacuum, and the resulting residue was
extracted with toluene (2 × 5 mL) and filtered. Removal of
solvent from the filtrate and washing of the residue with
petroleum ether (2 × 3 mL) gave compound 9 as a dark brown
solid (0.020 g, 66%). The solutions of this complex decompose
progressively, even at low temperature. 1H NMR (200.13 MHz,
C6D6): δ 7.90-7.08 (m, 10H, Ph), 4.34 (s, 5H, Cp), 4.30 (s, 5H,
Cp), -1.13 (d, J HP ) 8, 3H, Me).
P r ep a r a t ion of Solu t ion s of tr a n s-[F e2Cp 2(µ-H )(µ-
P P h 2)(CO)2]P F 6 (tr a n s-10). Solid [FeCp2]PF6 (0.023 g, 0.064
mmol) was added to a dichloromethane solution (8 mL) of the
compound trans-2 (0.030 g, 0.062 mmol) at -60 °C, and the
mixture was stirred at the same temperature for 5 min to give
a brown-green solution containing complex trans-10 as the
major species. This compound is quite air-sensitive and
decomposes easily at room temperature. ESR (CH2Cl2, 300
K): g ) 2.0036. E°′(trans-10/trans-2) ) 0.36 V.
P r ep a r a tion of cis- a n d tr a n s-[Au F e2Cp 2(µ-P P h 2)(CO)2-
(P iP r 3)] (cis- a n d tr a n s-8). [AuCl(PiPr3)] (0.024 g, 0.062
mmol) was added to a THF solution of compound 7, prepared
in situ as described above (ca. 0.062 mmol), and the mixture
was stirred at room temperature for 5 min to give a brown
solution. Solvent was then removed under vacuum, and the
resulting residue was extracted with CH2Cl2/petroleum ether
(1/1) and chromatographed on alumina (activity IV) at 15 °C.
After the column was washed with petroleum ether, a brown
fraction was eluted using CH2Cl2/petroleum ether (1/1). Re-
moval of solvents from this fraction gave compound 8 as a
brown solid (0.039 g, 75%). This product was shown by NMR
to be a mixture of cis and trans isomers (in a ca. 1/4 ratio),
which could not be separated by either crystallization or
chromatography. Anal. Calcd for C33H41O2P2AuFe2: C, 47.17;
P r ep a r a tion of Solu tion s of cis-[F e2Cp 2(µ-H)(µ-P P h 2)-
(CO)2]P F 6 (cis-10). The procedure is analogous to that
described for trans-10 but using cis-2 as starting material. This
compound is also quite air-sensitive and decomposes easily at
room temperature. ESR (CH2Cl2, 300 K): g ) 2.0038. E°′(cis-
10/cis-2) ) 0.24 V.
H, 4.92. Found: C, 47.29; H, 5.11. 1H NMR (CD2Cl2):
δ
Ack n ow led gm en t. We thank the Ministerio de
Ciencia y Tecnolog´ıa of Spain for financial support
(Projects BQU2000-0944 and BQU2003-05478).
7.80-7.10 (m, Ph), 4.55 (s, Cp, isomer trans), 4.33 (s, Cp,
isomer cis), 2.36 (m, 3H, CH), 1.37 (dd, J HP ) 15, J HH ) 7,
CH3, isomer trans), 1.30 (dd, J HP ) 15, J HH ) 7, CH3, isomer
cis). cis/trans ) 1/4.
OM049574T