Electrochemical, EPR and computational results on [Fe2Cp 2(CO)2-based complexes with a bridging hydrocarbyl ligand

Article abstract of DOI:10.1016/j.jorganchem.2011.07.039

The dimetallacyclopentenone complexes [Fe₂Cp₂(CO) (μ-CO){μ-η¹:η³-C_αHC _β(R)C(O)} (R = CH₂OH, 1a; R = CMe₂OH, 1b; R = Ph, 1c) were prepared by photolytic reaction of [Fe₂Cp ₂(CO)₄ with alkyne according to the literature procedure. The X-ray and the electrochemical characterization of 1c are presented. The μ-allenyl compound [Fe₂Cp₂(CO)₂(μ-CO) {μ-η¹:η²_α,β-C _αHC_βCMe₂[BF₄ ([2[BF ₄), obtained by reaction of 1b with HBF₄, underwent monoelectron reduction to give a radical species which was detected by EPR at room temperature. The EPR signal has been assigned to [Fe₂Cp ₂(CO)₂(μ-CO){μ-η¹: η²_α,β-C_αHC _βCMe₂}, [2^?. The molecular structures of [2⁺ and [2^? were optimized by DFT calculations. The unpaired electron in [2^? is localized mainly at the metal centers and, coherently, [2^? does not undergo carbon-carbon dimerization, by contrast with what previously observed for the μ-vinyl radical complex [Fe₂Cp₂(CO)₂(μ-CO){μ- η¹:η²-CHCH(Ph)}^?, [3 ^?. Electron spin density distributions similar to the one of [2^? were found for the μ-allenyl radical complexes [Fe ₂Cp₂(CO)₂(μ-CO){μ-η¹: η²_α,β-C_αHC _βC(R¹)(R²)}^? (R ¹ = R² = H, [4^?; R¹ = H, R ² = Ph, [5^?; R¹ = R² = Ph, [6^?).

Full text of DOI:10.1016/j.jorganchem.2011.07.039

Journal of Organometallic Chemistry 696 (2011) 3551e3556
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Electrochemical, EPR and computational results on [Fe2Cp2(CO)2]-based
complexes with a bridging hydrocarbyl ligand
,
,
a
Adriano Boni a b, Tiziana Funaioli a, Fabio Marchetti a , Guido Pampaloni ,
*
Calogero Pinzino c, Stefano Zacchini d
a University of Pisa, Dipartimento di Chimica e Chimica Industriale, Via Risorgimento 35, 56126 Pisa, Italy
b Scuola Normale Superiore, Piazza dei Cavalieri, I-56126 Pisa, Italy
c ICCOM-CNR UOS, Area della Ricerca, Via Moruzzi 1, 56124 Pisa (I), Italy
d University of Bologna, Dipartimento di Chimica Fisica e Inorganica, Viale Risorgimento 4, 40136 Bologna, Italy
a r t i c l e i n f o
a b s t r a c t
Article history:
The dimetallacyclopentenone complexes [Fe2Cp2(CO)(
m
ꢀCO){
m :
h1
ꢀ
h3ꢀCaH]Cb(R)C(]O)}] (R ¼ CH2OH,
Received 29 June 2011
Received in revised form
27 July 2011
1a; R ¼ CMe2OH, 1b; R ¼ Ph, 1c) were prepared by photolytic reaction of [Fe2Cp2(CO)4] with alkyne
according to the literature procedure. The X-ray and the electrochemical characterization of 1c are pre-
sented. The
m
-allenyl compound [Fe2Cp2(CO)2(
m
ꢀCO){
m
ꢀ
h1
:
h2 bꢀCaH]Cb]CMe2][BF4] ([2][BF4]),
a
,
Accepted 29 July 2011
obtained by reaction of 1b with HBF4, underwent monoelectron reduction to give a radical species
which was detected by EPR at room temperature. The EPR signal has been assigned to [Fe2Cp2(CO)2(
h1 h2 beCaH]Cb]CMe2}], [2]ꢁ. The molecular structures of [2]þ and [2]ꢁ were optimized by DFT
calculations. The unpaired electron in [2]ꢁ is localized mainly at the metal centers and, coherently, [2]ꢁ does
not undergo carbonecarbon dimerization, by contrast with what previously observed for the -vinyl
radical complex [Fe2Cp2(CO)2(
ꢀCO){
h1 h2eCH]CH(Ph)}]ꢁ, [3]ꢁ. Electron spin density distributions
m
ꢀCO)
Keywords:
{
m
ꢀ
:
a
,
Diiron complexes
Dimetallacyclopentenone complexes
Allenyl ligand
Vinyl ligand
Electrochemistry
DFT calculations
m
m
m
ꢀ
:
similar to the one of [2]ꢁ were found for the h1 h2
m-allenyl radical complexes [Fe2Cp2(CO)2(meCO){me :
a,b
eCaH]Cb]C(R1)(R2)}]ꢁ (R1 ¼ R2 ¼ H, [4]ꢁ; R1 ¼ H, R2 ¼ Ph, [5]ꢁ; R1 ¼ R2 ¼ Ph, [6]ꢁ).
Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction
see Scheme 1. Otherwise, in the case of 1c, the proton attacks the Cb
carbon yielding the
m-vinyl complex [Fe2Cp2(CO)2(mꢀCO){
m : -
h1 h2
ꢀ
The reactivity of diiron complexes based on the [Fe2Cp2(-
CaH]CbH(Ph)}][BF4] ([3][BF4]), Scheme 1 [6].
CO)(
m
ꢀCO)] core has been the object of intense investigation [1]. This
In a recent paper, we have outlined the unusual electrochemical
behavior of [3]þ, that undergoes reversible CeC dimerization upon
monoelectron reduction [7]. We present herein some novel features
metal frame may provide non conventional reactivity patterns to
bridging hydrocarbyl ligands and offer the possibility of stabilizing
coordination fashions, as consequence of the cooperativity effects of
the two iron centers [2]. The long known reaction of [Fe2Cp2(CO)4]
with alkynes under UV irradiation results in removal of one carbonyl
ligand and insertion of the alkyne into a FeeCO bond [3];thisapproach
has allowed the synthesis of a variety of dimetallacyclopentenone
regarding diiron complexes containing the [Fe2Cp2(CO)(
i) the preparation and the characterization of the unreported dime-
tallacyclopentenone compound [Fe2Cp2(CO)(
ꢀCO){
h1 3-CaH]
Cb(CH2OH)C(]O)}] (1a); ii) the X-ray and the electrochemical char-
meCO)] core:
m
mꢀ
:h
acterization of the dimetallacyclopentenone [Fe2Cp2(CO)(
m
ꢀCO)
compounds, including [Fe2Cp2(CO)(
mꢀCO){mꢀ
h1
:h
3-CaH]Cb(R)C(]
{
m
ꢀ
h1
:
h3ꢀCaH]Cb(Ph)C(]O)}] (1c); iii) a detailed investigation on
O)}] (R ¼ CMe2OH, 1b [4]; R ¼ Ph, 1c [3]), see Scheme 1.
the electrochemistry of the
m
-allenyl [Fe2Cp2(CO)2(
m
ꢀCO){
m :
ꢀ
h1 h2
a,
The chemistry of these derivatives is dominated by the CbeCO
bond cleavage promoted by proton addition, which takes place
selectively at different sites depending on the nature of R. In the case
of 1b, the protonation is addressed to the hydroxo group and leads to
bꢀCaH]Cb]CMe2][BF4] ([2][BF4]), including EPR and DFT analyses.
2. Results and discussion
formation of the m-allenyl species [2][BF4] via water removal [4,5],
The new compound [Fe2Cp2(CO)(
m
ꢀCO){
m
ꢀ
h1
:
h3ꢀCaH]
Cb(CH2OH)C(]O)}] (1a) was prepared in moderate yield by photo-
lytic reaction of [Fe2Cp2(CO)4] with propargyl alcohol, according to
the published procedure (Scheme 2) [3,4]. Compound 1a, which
* Corresponding author.
E-mail address: fabmar@dcci.unipi.it (F. Marchetti).
0022-328X/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.jorganchem.2011.07.039

3552
A. Boni et al. / Journal of Organometallic Chemistry 696 (2011) 3551e3556
+
Me
C
Me
H
Cβ
Cα
CO
OC
Fe
Fe
C
O
R = CMe2OH
−H2O
R
[2]+
H
O
C
Cβ
O
Cα
Fe
CO
OC
C
HBF4, thf
OC
HC≡CR, hν
−CO
Fe
Fe
Fe
+
C
O
Ph
Cβ
C
O
R = Ph
H
H
Cα
1b R = CMe2OH
1c R = Ph
CO
OC
Fe
Fe
C
O
[3]+
Scheme 1.
adds to the family of analogous dimetallacyclopentenone
complexes, has been characterized by IR and NMR spectroscopy, and
elemental analysis. The IR spectrum (in the solid state) exhibits the
absorptions due toterminal and bridging carbonyl ligands (1953 and
1787 cmꢀ1 respectively), and one more absorption related tothe acyl
group (1744 cmꢀ1). Moreover a broad band at 3410 cmꢀ1 accounts
for the presence of the OH unit. The 1H NMR spectrum (in CDCl3)
shows the resonance of the alkylidene proton at typical low field
C(13)eC(14) double bond is
In the second form, C(13) may be regarded as a
a ketenic substituent [C(14)]C(15)]O(1)] which is
h
2-bound to Fe(2) (Scheme 3, form I).
-carbene with
2-bonded to
m
h
Fe(2) through C(14)eC(15) (Scheme 3, form II). A third description
as an allylic ligand appears to have some validity (Scheme 3, form
III), however the bonding parameters suggest that the actual situ-
ation may be represented almost exclusively by I and II, with
prevalence of I. The C(15)eO(1) interaction [1.210(10) Å] possesses
an almost pure double bond character, whereas both C(13)eC(14)
[1.442(12) Å] and C(14)eC(15) [1.463(12) Å] are in between a single
and a double bond.
(d
¼ 12.16 ppm). The signals in the 13C NMR spectrum have been
attributed as follows:
d 261.6 (CObridging), 235.2 (C]O), 210.3
(COterminal), 176.8 (Ca), 87.4, 85.8 (Cp), 77.2 (CH2), 9.4 (Cb).
The reaction of 1a with HBF4, performed under experimental
conditions analogous to those employed for the protonation of 1b
(Scheme 1), failed to give cleanly the expected allenyl derivative
On account of the fact that scarce information is available in the
literature regarding the electrochemical behavior of diiron
complexes based on the [Fe2Cp2(CO)2] core [9], we carried out
electrochemical analysis on compound 1c and on the allenyl
complex [2][BF4].
[Fe2Cp2(CO)2(mꢀCO){
m :
h1 h2 b-CaH]Cb]CH2}][BF4], see Scheme
a,
ꢀ
2 [8].
In view of our interest on the chemistry of the vinyl complex [3]
Compound 1c undergoes two irreversible oxidation processes
at þ0.17 and þ0.97 V (vs FeCp2) and one reduction at ꢀ1.85 V, in
CH2Cl2/[NnBu4][PF6] solution. Analysis of the cyclic voltammo-
grams, with scan rates varying between 0.02 and 1.00 V sꢀ1, have
confirmed that the reduction is an electrochemically-reversible,
[BF4] [7], we prepared the intermediate dimetallacyclopentenone
[Fe2Cp2(CO)(
m
ꢀCO){
m
ꢀ
h1
:
h3ꢀCaH]Cb(Ph)C(]O)}] (1c). In the
course of the purification of this compound we collected some X-
ray quality crystals, which were used for a X-ray diffraction analysis.
The ORTEP molecular diagram of 1c is shown in Fig. 1, whereas
relevant bond lengths and angles are reported in Table 1.
diffusion-controlled process (the peak-to-peak separation,
D
Ep,
approaches the theoretical value of 59 mV and the (ip)red
/
n1/2
The molecular structure of 1c closely resembles that previously
remains almost constant [10]), complicated by a subsequent fast
reported for the analogous Ru-complex [Ru2Cp2(CO)(
m
ꢀCO){
m :
h1
ꢀ
chemical reaction (ipc/ipa ¼ 0.31 at 1.0 V sꢀ1).
h3ꢀCa(Ph)]Cb(Ph)C(]O)}] [3], so the former will be only briefly
Fig. 2 shows the cyclic voltammetric profile exhibited by the
allenyl complex [2][BF4] in CH2Cl2/[NnBu4][PF6] solution. Two irre-
versible oxidation (þ1.11 and þ1.48 V) and two consecutive reduc-
tion processes (ꢀ1.03 and ꢀ2.04 V) point out an electrochemical
behavior similar to that of [3][BF4] [7]. According to the cyclic vol-
tammetric response of the reductions, with scan rates varying
described here. Compound 1c contains a bridging {
mꢀ
h1
:
h3ꢀCaH]
Cb(Ph)C(]O)} ligand coordinated to the cis-Fe2Cp2(CO)(meCO)
core. The bonding of the bridging ligand can be formalized in terms
of three resonance forms. The first form consists of a dimetallacy-
clopentenone ring Fe(1)eC(13)eC(14)eC(15)eFe(2), in which the
CH2OH
BF4
CH2
CO
H
O
C
H
Cβ
Fe
Cβ
O
Cα
Fe
CO
OC
C
Cα
Fe
HBF4, thf
OC
HC≡CCH2OH, hν
−CO
OC
Fe
Fe
Fe
C
O
C
C
O
O
1a
Scheme 2.

A. Boni et al. / Journal of Organometallic Chemistry 696 (2011) 3551e3556
3553
Ph
Cβ
Ph
Cβ
Ph
H
H
O
H
Cβ
O
O
C
Cα
Cα
C
Cα
C
OC
OC
Cp
OC
Cp
Fe
Fe
Fe
Fe
Fe
Fe
Cp
Cp
Cp
Cp
C
O
C
C
O
O
I
II
III
Scheme 3.
The reaction of [2][BF4] (ca. 0.60 mmol) with equimolar amount
of CoCp2 (in THF or CH2Cl2) afforded a mixture of non identified
products, with prevalence of [Fe2Cp2(CO)4] (isolated in ca. 30 yield).
This evidence suggests that the dinuclear complex [2]ꢁ is the initial
product of the monoelectron reduction of [2][BF4], but cleavage of
the diiron frame may then take place to give, inter alia, the Fe(I)
radical [FeCp(CO)2]ꢁ, dimerizing to [Fe2Cp2(CO)4]. This observation
puts some doubts on the attribution of the EPR spectrum (Fig. 4).
Nevertheless it should be noted that the radical complex
[FeCp(CO)2]ꢁ appeared EPR silent, except at low temperature in the
presence of O2 or radical trappers [11]. In these conditions, the EPR
resonance was seen at giso value not corresponding to that of the
spectrum in Fig. 4.
Furthermore, it is noteworthy that diiron complexes similar to
[2]þ, based on the [Fe2Cp2(CO)2] core, undergo monoelectronic
reduction affording still dinuclear compounds detected by EPR at
room temperature [9a].
According to these considerations, we conclude that the EPR
signal, detected in the course of the monoelectron reduction of [2]
[BF4], could be reasonably assigned to a dinuclear Fe(I)eFe(II)
complex ([2]ꢁ), rather than to a following mononuclear Fe(I) product.
The main geometric parameters calculated for [2]þ and [2]ꢁ are
listed in Table 2, together with those obtained experimentally for
[2]þ in the solid state [4].
Fig. 1. Molecular structure of [Fe2Cp2(CO)(
with key atoms labeled. Thermal ellipsoids are at the 30% probability level. Only the
main image of the disordered Cp bound to Fe(2) is drawn.
m
ꢀCO){
m :
h1 h3eC(H)]C(Ph)C(]O)}] (1c),
ꢀ
between 0.02 and 1.00 V sꢀ1, the first reduction is an electrochem-
ically-reversible, diffusion-controlled process complicated by
a subsequent chemical reaction (ipc/ipa ¼ 0.73 at 0.10 V sꢀ1). This is
testified by the appearance of oxidation processes in the back scan
toward positive potentials, and by the deposition, after several
cycles, of insoluble by-products onto the electrode surface.
The reduction process occurring at the more negative potential
(ꢀ2.04 V) appears as a quasi-reversible one (Fig. 2).
The gas phase structure of the allenyl complex [2]þ and that of
[2]ꢁ, which is the formal product of the monoelectron reduction of
[2]þ, were optimized by DFT calculations (the calculated structure
of [2]þ is shown in Fig. 3).
EPR experiment was carried out at room temperature as follows:
a 5.0ꢂ10ꢀ3 M solution of [2][BF4] in CH2Cl2/[NnBu4][PF6] was
introduced into a EPR spectroelectrochemical cell under a deoxy-
genated argon atmosphere and the solution was electrolyzed at
constant potential (Ew ¼ ꢀ1.2 V vs FeCp2).
A reading of Table 2 indicates the good agreement between the
values related to [2]þ in the gas phase and the corresponding X-ray
values obtained experimentally for the solid state [4]. Furthermore,
the salient bond distances and angles concerning the
m-allenyl
ligand do not significantly vary on going from [2]þ to [2]ꢁ. In other
words, the allenyl ligand comes almost unaltered, in terms of both
geometric features and coordination fashion, after monoelectron
addition to the cation [2]þ. In fact the only noticeable variation
seems to be the elongation of the FeꢀFe interaction [from
2.6063(14) Å in [2][BF4] (exp.) to 2.843 Å in [2]ꢁ (calc.)]. These data
The obtained EPR spectrum (Fig. 4) consists of a broad signal
(line width ¼ 6.6 G) at giso ¼ 1.9773, and is in good agreement with
the spectrum computed for [2]ꢁ (Fig. 5A), corresponding to a situ-
ation in which the electron density is almost localized at the two
iron atoms Fe1 (0.4299) and Fe2 (0.6681).
This feature resembles to what was previously reported for diiron
m
-alkylidene complexes of the type [Fe2Cp2(CO)2(
m
ꢀCO){ ꢀC(CN)
m
(X)}]nþ (X ¼ H, C-, N-, O- or P-substituent), undergoing mono-
electron reduction to the corresponding paramagnetic, dinuclear
derivatives. In every cases, EPR spectroscopy indicated that the
unpaired electron resided mainly on the FeeFe moiety [9a].
20
10
0
Table 1
Selected bond lengths (Å) and angles (ꢃ) for 1c.
Fe(1)eFe(2)
2.561(2)
C(12)eFe(1)
1.752(10)
C(11)eFe(2)
C(13)eFe(1)
C(14)eFe(2)
C(11)eO(11)
C(13)eC(14)
C(15)eO(1)
Fe(2)eC(11)eFe(1)
Fe(1)eC(13)eFe(2)
C(13)eC(14)eC(15)
O(1)eC(15)eC(14)
1.915(12)
1.939(10)
2.114(9)
1.188(12)
1.442(13)
1.210(10)
83.2(5)
80.9(4)
112.0(8)
136.2(9)
C(11)eFe(1)
C(13)eFe(2)
C(15)eFe(2)
C(12)eO(12)
C(14)eC(15)
C(14)eC(16)
O(12)eC(12)eFe(1)
C(14)eC(13)eFe(1)
C(14)eC(15)eFe(2)
O(1)eC(15)eFe(2)
1.942(10)
2.009(10)
1.895(9)
1.148(10)
1.463(12)
1.487(13)
176.2(8)
122.5(7)
76.8(5)
-10
-20
2
1
0
-1
-2
-3
E / V vs FeCp2
Fig. 2. Cyclic voltammogram of [2][BF4] (2.0ꢂ10ꢀ3 M) recorded at a platinum electrode
146.7(8)
in CH2Cl2 solution containing [NnBu4][PF6] 0.2 M. Scan rate ¼ 0.1 V sꢀ1
.

3554
A. Boni et al. / Journal of Organometallic Chemistry 696 (2011) 3551e3556
Fig. 3. DFT-calculated structure of [2]þ
.
indicate that the unpaired electron in [2]ꢁ is localized prevalently at
the metal centers Fe1 and Fe2, coherently with the EPR evidences
(see above). The elongation of the FeꢀFe bond on going from [2]þ to
[2]ꢁ gives a possible explanation for the formation of fragmentation
products upon CoCp2 reduction of [2]þ (see above).
Fig. 5. DFT-calculated spin density (positive spin density, 0.002 electron auꢀ3) of [2]ꢁ
(A) and [Fe2Cp2(CO)2(
m
ꢀCO){
m
ꢀ
h1
:
h2 bꢀCaH]Cb]C(R1)(R2)}]ꢁ [R1 ¼ R2 ¼ H, [4]ꢁ (B);
We have recently reported that the reduction of the
m-vinyl
a
,
R1 ¼ H, R2 ¼ Ph, [5]ꢁ (C); R1 ¼ R2 ¼ Ph, [6]ꢁ (D)]. The spin density values at the two iron
complex [Fe2Cp2(CO)2(
m
ꢀCO){
m
ꢀ
h1 h2eCH]CH(Ph)}]þ (compound
:
centers are reported below each diagram, respectively.
[3]D, Scheme 4) generates a radical species with significant spin
density located at the Cb carbon of the hydrocarbyl ligand. Computer
analyses have suggested that the electron-withdrawing character of
the phenyl group is crucial in order to allocate spin density on the
vinyl chain. This leads to CꢀC coupling to give the stable [Fe]4
reactions, which normally result in the formation of mononuclear
derivatives, could proceed with initial reduction of the dinuclear
cation, causing the weakening of the FeꢀFe interaction (see above).
bisealkylidene complex [Fe2Cp2(CO)2(
m
ꢀCO){ ꢀCHCH(Ph)}]2 [7],
m
see Scheme 4.
In order to understand the influence of the allenyl substituents
3. Conclusions
on the spin density distribution in the related radical complexes,
the DFT structures of [Fe2Cp2(CO)2(
m
ꢀCO){
m
ꢀ
h1
:
h2 beCaH]Cb]
In this paper, we have reported some new aspects concerning
a,
C(R1)(R2)}]ꢁ (R1 ¼ R2 ¼ H, [4]ꢁ; R1]H, R2]Ph, [5]ꢁ; R1 ¼ R2 ¼ Ph,
[6]ꢁ) were optimized for the gas phase. The spin density distribu-
tions calculated for [4]ꢁe[6]ꢁ are represented in Fig. 5. Like in
complex [2]ꢁ, the spin density in [4]ꢁe[6]ꢁ is almost localized at the
iron atoms: this means that the presence of phenyl substituents on
the terminal carbon of the allenyl chain is ineffective to stabilize
some spin density on carbon atoms, thus no CꢀC coupling reaction
should be expected for [2]ꢁ, [4]ꢁe[6]ꢁ.
the electrochemistry of dimetallacyclopentenone and
m-allenyl
complexes based on the [Fe2Cp2(CO)2] core. Cationic diiron
m
-
allenyl complexes are subjected to easy monoelectron reduction
affording initially a dinuclear derivative bearing the unpaired
Table 2
Selected bond lengths (Å) and angles (ꢃ) for [2]þ and [2]ꢁ.
+
Me
Moreover the spin density maps of [2]ꢁ and [6]ꢁ provide a possible
Me
C
explanation for the outcomes of the reactions of [Fe2Cp2(CO)2(
mꢀCO)
Me
Me
C
{mꢀ
h1
:
h2 b-CaH]Cb]C(R1)(R2)}]þ (R1 ¼ R2 ¼ Me, [2]þ; R1
¼
a
,
H
Cβ
Fe2
H
Cβ
R2 ¼ Ph, [6]þ) with strong nucleophiles (Nuꢀ) [4]. In fact these
Cα
Cα
CO
OC
CO
OC
Fe1
Fe1
Fe2
C
C
O
O
[2].
[2]+
a
[2][BF4]a
[2]þb
[2]b
2.843
Fe1eFe2
2.6063(14)
1.761(9)
1.815(9)
1.940(8)
1.766(10)
2.152(8)
2.026(8)
2.061(7)
1.332(11)
1.339(11)
124.7(6)
152.0(8)
2.649
Fe1eCO (terminal)
Fe1eCO (bridging)
Fe1eCa
1.784
1.832
1.936
1.786
2.107
2.037
2.128
1.361
1.331
128.12
153.10
1.754
1.886
1.943
1.756
2.021
2.107
2.125
1.359
1.330
130.48
152.50
b
Fe2eCO (terminal)
Fe2eCO (bridging)
Fe2eCa
Fe2eCb
CaeCb
CaeCMe2
Fe1eCaeCb
3260
3280
3300
3320
3340
3360
Gauss
CaeCbeCMe2
a
Fig. 4. EPR spectrum obtained by electrochemical monoelectron reduction of [2][BF4]
(a: experimental; b: calculated).
Experimental values for the solid state (X-ray) [4].
Calculated values for the gas phase (DFT).
b

A. Boni et al. / Journal of Organometallic Chemistry 696 (2011) 3551e3556
3555
O
C
Cp
Cp
Fe
H
Fe
Cβ
+
CO
Ph
Cβ
Ph
Cβ
OC
Cα
Ph
H
H
H
H
H
Cα
Fe
Cα
H
e−
CO
OC
Ph
CO
OC
Cβ
H
Fe
Fe
Fe
Cα
CO
Cp
OC
C
Fe
Fe
C
O
O
C
Cp
O
[3]+
Scheme 4.
electron prevalently localized at the metal centers, independently
on the nature of the allenyl substituents. This feature gives reason
4.2. Synthesis of [Fe2Cp2(CO)(
C(]O)}] (1a)
mꢀCO){
m :h
h1 3-CaH]Cb(CH2OH)
ꢀ
for the chemistry exhibited by cationic diiron m-allenyl complexes,
with particular reference to the non observed self-coupling of the
radical generated by monoelectron reduction, in contrast with
what found for analogous diiron Ph-substituted m-vinyl species.
A solution of [Fe2(Cp)2(CO)4] (12.0 g, 0.0339 mol) and
HC^CCH2OH (0.31 mol) in THF (ca. 150 mL) was irradiated with UV
light for 96 h. Then the volatile materials were removed under vacuo,
and the residue was dissolved in CH2Cl2 and charged on a short
alumina pad. Elution with a mixture of acetone and methanol (1:1 v/
v) afforded a green band corresponding to 1a. The product was
obtained as a brownish-green microcrystalline solid upon removal of
the solvent. Yield: 5.30 g, 41%. Anal. Calcd. for C16H14Fe2O4: C, 50.31;
4. Experimental details
4.1. General
H, 3.69. Found: C, 49.82; H, 3.80. IR (solid state):
(CO) 1953 (vs), 1787 (s), 1744 (m) cmꢀ1. 1H NMR (CDCl3)
1 H, CaH); 5.04, 4.77 (s, 10 H, Cp); 4.46, 3.89 (m, 2 H, CH2OH). 13C{1H}
NMR (CDCl3) 261.6 (
n
(OH) 3410 (m-br),
Reactions were carried out under nitrogen atmosphere, using
standard Schlenk techniques. Glassware was oven-dried before use.
Solvents were distilled before use under nitrogen from appropriate
drying agents. Photolysis reactions were carried out in silica glass
tubes by using a 150 W mercury lamp. Chromatography separations
were carried out on columns of alumina (Fluka, Brockmann Activity
I). All reactants were commercial products (Aldrich) of the highest
purity available and used as received. Compounds [Fe2Cp2(CO)4]
(Strem), and [NnBu4][PF6] (Fluka, puriss. electrochemical grade)
were commercial products used as received. Ferrocene (FeCp2),
prepared according to the literature [12], was purified by subli-
mation. Infrared spectra were recorded at 298 K on FT-IR Per-
n
d
12.16 (s,
d
m
ꢀCO); 235.2 (C]O); 210.3 (COterminal); 176.8
(Ca); 87.4, 85.8 (Cp); 77.2 (CH2); 9.4 (Cb).
4.3. X-ray characterization of [Fe2Cp2(CO)(
C(Ph)C(]O)}] (1c)
mꢀCO){
m :
h1 h3eC(H)]
ꢀ
Crystals of 1c suitable for X-ray analysis were collected by
storing a dichloromethane solution of 1c layered with pentane
at ꢀ30 ꢃC for 48 h. Crystal data and collection details for [Fe2Cp2
(CO)(
m
ꢀCO){
m :
h1 h3eC(H)]C(Ph)C(]O)}] (1c) are reported in
ꢀ
kineElmer Spectrometer equipped with
a UATR sampling
Table 3. The diffraction experiments were carried out on a Bruker
Apex II diffractometer equipped with a CCD detector using MoeK
accessory (solid samples). NMR measurements were performed at
298 K on Bruker Avance DRX400 instrument equipped with probe
BBFO broad band. The chemical shifts for 1H and 13C NMR spectra
were referenced to the non deuterated aliquot of the solvent; the
a
radiation. Data were corrected for Lorentz polarization and
Table 3
spectra were fully assigned via DEPT experiments and 1H,13
C
Crystal data and experimental details for 1c.
correlation measured through gs-HSQC and gs-HMBC experiments
[13]. EPR analyses were recorded at 298 K by Varian (Palo Alto, CA,
USA) E112 spectrometer operating at X band, equipped with Varian
E257 temperature control unit and interfaced to IPC 610/P566C
industrial grade Advantech computer, using acquisition board [14]
and software package especially designed for EPR experiments [15].
Experimental EPR spectra were simulated by WINSIM 32 program
[16]. DFT geometry optimization and calculation of the electron
spin density distribution were performed by the parallel Linux
version of Spartan ‘10 software [17]. We adopted the B3LYP (Becke,
three-parameter, Lee-Yang-Parr) [18,19] exchange-correlation
functional formulated with the Becke 88 exchange functional
[20], the correlation functional of Lee, Yang and Parr [21] and the 6-
31G** base functions set, which is appropriate for calculations of
split-valence plus-polarization quality.
Formula
C21H16Fe2O3
Fw
T, K
428.04
294(2)
0.71073
Monoclinic
P21/n
11.811(6)
12.077(6)
12.697(7)
104.292(5)
1755.1(16)
4
1.620
1.670
872
0.18 ꢂ 0.15 ꢂ 0.11
2.11e25.02
9575
l, Å
Crystal system
Space group
a, Å
b, Å
c, Å
ꢃ
b
,
Cell volume, Å3
Z
Dc, g cme3
m
, mmꢀ1
F(000)
Crystal size, mm
ꢃ
q
limits,
Reflections collected
Apparatus and techniques for the electrochemical measure-
ments and joint EPR experiments were previously described [7].
Electrochemical measurements were performed in 0.2 M CH2Cl2
solutions of [NnBu4][PF6] as supporting electrolyte and all the
potential values were referenced to the ferrocene/ferrocenium
couple (ferrocene was used as internal reference).
Independent reflections
Data/restraints/parameters
Goodness on fit on F2
3068 (Rint ¼ 0.0709)
3068/231/284
1.050
0.0770
0.2174
R1 [I > 2
s
(I))
wR2 (all data)
Largest diff. peak and hole, e.Åꢀ3
1.108/ꢀ0.486

3556
A. Boni et al. / Journal of Organometallic Chemistry 696 (2011) 3551e3556
(b) L. Busetto, F. Marchetti, F. Renili, S. Zacchini, V. Zanotti, Organometallics 29
(2010) 1797;
(c) L. Busetto, F. Marchetti, S. Zacchini, V. Zanotti, Organometallics 27 (2008)
5058;
(d) L. Busetto, F. Marchetti, S. Zacchini, V. Zanotti, Organometallics 26 (207)
3577;
(e) L. Busetto, F. Marchetti, S. Zacchini, V. Zanotti, Organometallics 25 (2006)
4808;
(f) L. Busetto, F. Marchetti, S. Zacchini, V. Zanotti, Organometallics 24 (2005)
2297;
(g) V.G. Albano, L. Busetto, F. Marchetti, M. Monari, S. Zacchini, V. Zanotti,
Organometallics 23 (2004) 3348.
absorption effects (empirical absorption correction SADABS) [22].
Structures were solved by direct methods and refined by full-
matrix least-squares based on all data using F2 [23]. All non-
hydrogen atoms were refined with anisotropic displacement
parameters. H-atoms were placed in calculated positions, except
H(13) which was located in the Fourier map and the C(13)eH(13)
distance restrained to 0.97 Å (s.u. 0.01) during refinement.
H-atoms were treated isotropically using the 1.2 fold Uiso value of
the parent C-atoms. The Cp ligand bound to Fe(2) is disordered.
Disordered atomic positions were split and refined isotropically
using one occupancy parameter per disordered group. Similar U
restraints (s.u. 0.01) were applied to the C and O atoms.
[3] A.F. Dyke, S.A.R. Knox, P.J. Naish, G.E. Taylor, J. Chem. Soc. Dalton Trans. (1982)
1297.
[4] A. Boni, F. Marchetti, G. Pampaloni, S. Zacchini, Dalton Trans. (2010) 10866.
[5] S.A.R. Knox, F. Marchetti, J. Organomet. Chem. 692 (2007) 4119.
[6] A.F. Dyke, S.A.R. Knox, P.J. Morris, P.J. Naish, J. Chem. Soc. Dalton Trans. (1983)
1417.
Acknowledgement
[7] A. Boni, T. Funaioli, F. Marchetti, G. Pampaloni, C. Pinzino, S. Zacchini,
Organometallics 30 (2011) 4115.
[8] The treatment of a solution of 1a with equimolar amount of HBF4/Et2O gave
a red solid after work-up. IR analysis (in CH2Cl2) of the product was coherent
We thank the Ministero dell’Università e della Ricerca Scientifica
e Tecnologica (MIUR) for financial support.
with the formation of [Fe2Cp2(CO)2(
m
-CO){
m :
h1 h2 b-CaH]Cb]CH2}][BF4]
a,
ꢀ
[bands at
n 2029vs (COterminal), 2009 m (COterminal), 1860 m (m
-CO) cmꢀ1]. 1H
and 13C NMR spectra (CDCl3, ꢀ50 ꢃC) evidenced the presence of more than
Appendix A. Supplementary Material
one set of resonances, which could not be assigned.
[9] (a) S. Bordoni, L. Busetto, F. Calderoni, L. Carlucci, F. Laschi, P. Zanello,
V. Zanotti, J. Organomet. Chem. 496 (1995) 27;
CCDC no. 819692 for 1c; contains the supplementary crystallo-
graphic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via www.
ccdc.cam.ac.uk/data_request/cif.
(b) V. Zanotti, S. Bordoni, L. Busetto, L. Carlucci, A. Palazzi, R. Serra, V.G. Albano,
M. Monari, F. Prestopino, F. Laschi, P. Zanello, Organometallics 14 (1995) 5232;
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(d) N.C. Schroeder, R.J. Angelici, J. Am. Chem. Soc. 108 (1986) 3688.
[10] P. Zanello, Inorganic Electrochemistry. Theory, Practice and Application. RSC,
Cambridge, U.K, 2003.
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