Correlation between density functional studies and experimental data of three new 19-electron metal sandwich complexes containing amido, ester, and thioester cyclopentadienyl substituents

Article abstract of DOI:10.1021/om7012875

Three 18-electron complexes of the type [CpFe^II(η ⁶-C₆Me₆)]⁺ containing an amido, an ester, and a thioester group directly attached to the Cp ring were synthesized and reduced to Fe^I19-electron complexes. These new 19-electron stable complexes were characterized by UV/vis spectroscopy, showing significant differences in the obtained spectra. In order to better understand the structural and optical properties of the three 19-electron complexes (η⁵-C₅H₄CONHR) Fe^I(η⁶-C₆Me₆), (η⁵-C₅H₄COOR) Fe^I(η⁶-C₆Me₆), and (η⁵-C₅H₄COSR)Fe(η⁶-C ₆Me₆), we have carried out DFT calculations on these compounds, as well as on their diamagnetic 18-electron cations [(η⁵-CpCONHR)Fe^II(η⁶-C ₆Me₆)][PF₆], [(η⁵-CpCOOR)Fe ^II(η⁶-C₆Me₆)][PF₆], and [(η⁵-CpCOSR)Fe^II(η⁶-C ₆Me₆)] [PF₆] for the sake of comparison. Full geometry optimizations under C₁ symmetry have been carried out on the six complexes, and their optical transitions have been computed on the optimized geometries at the time-dependent DFT (TD-DFT) level. The comparison of the data obtained for the three 19-electron complexes gives us significant information about the influence of the functionalized cyclopentadienyl ring on the electronic structure.

Full text of DOI:10.1021/om7012875

Organometallics 2008, 27, 3693–3700
3693
Correlation between Density Functional Studies and Experimental
Data of Three New 19-Electron Metal Sandwich Complexes
Containing Amido, Ester, and Thioester Cyclopentadienyl
Substituents
Samia Kahlal,† Ca´tia Ornelas,‡ Jaime Ruiz,‡ Didier Astruc,*,‡ and Jean-Yves Saillard*,†
Sciences Chimiques de Rennes, UMR CNRS No. 6226, UniVersite´ de Rennes 1, 35042 Rennes Cedex,
France, and Institut des Sciences Mole´culaires, UMR CNRS No. 5255, UniVersite´ de Bordeaux I,
351 Cours de la Libe´ration, 33405 Talence Cedex, France
ReceiVed January 7, 2008
Three 18-electron complexes of the type [CpFeII(η6-C6Me6)]+ containing an amido, an ester, and a
thioester group directly attached to the Cp ring were synthesized and reduced to FeI 19-electron complexes.
These new 19-electron stable complexes were characterized by UV/vis spectroscopy, showing significant
differences in the obtained spectra. In order to better understand the structural and optical properties of
the three 19-electron complexes (η5-C5H4CONHR)FeI(η6-C6Me6), (η5-C5H4COOR)FeI(η6-C6Me6), and
(η5-C5H4COSR)Fe(η6-C6Me6), we have carried out DFT calculations on these compounds, as well as on
their diamagnetic 18-electron cations [(η5-CpCONHR)FeII(η6-C6Me6)][PF6], [(η5-CpCOOR)FeII(η6-
C6Me6)][PF6], and [(η5-CpCOSR)FeII(η6-C6Me6)][PF6] for the sake of comparison. Full geometry
optimizations under C1 symmetry have been carried out on the six complexes, and their optical transitions
have been computed on the optimized geometries at the time-dependent DFT (TD-DFT) level. The
comparison of the data obtained for the three 19-electron complexes gives us significant information
about the influence of the functionalized cyclopentadienyl ring on the electronic structure.
with a large spin density on the metal atom.3 Even if the SOMO
localization is lower in the arene ligand, several variations were
made in the arene ring in order to study the stability of the
complexes, and it was found that hexamethylbenzene is the most
adequate arene to stabilize the 19-electron complex.3 Besides
the C5H5, C5Me5, and C5H4CO2-, no other variations were made
to the Cp ring in order to study their influence on the stability
and properties of the 19-electron complexes.
Introduction
A series of 19-electron metal sandwich complexes based on
CpFeI(η6-C6R6) (Cp ) η5-C5H5, R ) Me or Et), called
“electron-reservoir” complexes, was described in 1979.1 These
complexes were characterized by UV/vis spectroscopy, cyclic
voltammetry, EPR, and Mo¨ssbauer spectroscopy, and the most
commun one, CpFeI(η6-C6Me6), was characterized by X-ray
diffraction.1 This stable 19-electron electron-reservoir complex
gave rise to a particularly rich chemistry, especially in the field
of electron-transfer reactions.2 The electron-reservoir properties
of this family of complexes (η5-C5R5)FeI(η6-C6R′6) (R ) H or
Me; R′ ) Me or Et) rely on the localization of the extra electron
on the metal center protected by the ligand shell. Several
theoretical investigations were carried out, and they all suggested
that the unpaired electron is mainly localized on the metal center.
Indeed, theoretical and experimental data agree on a SOMO
localization following the order Fe > -C5R5 > arene, associated
Besides electron-transfer chemistry,1,2,3h the [CpFe(η6-arene)]
complexes are useful for dendritic construction3i subsequent to
photolytic ligand substitution.3j The electron-transfer properties
can be transferred to dendrimers if the appropriate linkage is
used to connect the sandwich compound to the dendrimer
branches. Therefore, the amido and ester functionalities have
been successfully used for this purpose, as indicated here by
the thermal stability of the presently reported functional
sandwich complexes of both 18- and 19-electron forms. We
have also been intrigued by the variation of color of the 19-
(3) (a) Lacoste, M.; Rabaˆa, H.; Astruc, D.; Le Beuze, A.; Saillard, J.-
Y.; Pre´cigoux, G.; Courseille, C.; Ardoin, N.; Boywer, W. Organometallics
1989, 8, 2233. (b) Lacoste, M.; Rabaˆa, H.; Astruc, D.; Ardoin, N.; Varret,
F.; Saillard, J.-Y.; Le Beuze, A. J. Am. Chem. Soc. 1990, 112, 9548. (c)
Ruiz, J.; Ogliaro, F.; Saillard, J.-Y.; Halet, J.-F.; Varret, F.; Astruc, D. J. Am.
Chem. Soc. 1998, 120, 11693. (d) Ogliaro, F.; Halet, J.-F.; Astruc, D.;
Saillard, J.-Y. New J. Chem. 2000, 24, 257. (e) Bendjaballah, S.; Boucek-
kine, A.; Saillard, J.-Y. Inorg. Chim. Acta 2005, 358, 1305. (f) For another
DFT approach of this family of 19-electron complexes, see: Braden, D. A.;
Tyler, D. R. Organometallics 2000, 19, 1175. (g) For DFT studies of other
19-electron complexes, see: Braden, D. A.; Tyler, D. R. Organometallics
1998, 17, 4060. Braden, D. A.; Tyler, D. R. Organometallics 2000, 19,
3762. (h) Desbois, M.-H.; Astruc, D.; Guillin, J.; Varret, F.; Trautwein,
A. X.; Villeneuve, G. J. Am. Chem. Soc. 1989, 111, 5800. (i) Moulines, F.;
Astruc, D. Angew. Chem., Int. Ed. Engl. 1988, 27, 1347. Nlate, S.; Ruiz,
J.; Blais, J.-C.; Astruc, D. Chem. Commun. 2000, 417. Astruc, D; Daniel,
M.-C.; Ruiz, J. Chem. Commun. 2004, 2637. (j) Catheline, D.; Astruc, D. J.
Organomet. Chem. 1983, 248, C9.
* Corresponding authors. E-mail: d.astruc@lcoo.u-bordeaux1.fr;
saillard@univ-rennes1.fr.
† Universite´ de Rennes 1.
‡ Universite´ de Bordeaux I.
(1) (a) Astruc, D.; Hamon, J.-R.; Althoff, G.; Roman, E.; Batail, P.;
Michaud, P.; Mariot, J.-P.; Varret, F.; Cozak, D. J. Am. Chem. Soc. 1979,
101, 5445. (b) Astruc, D.; Roman, E.; Hamon, J.-R.; Batail, P. J. Am. Chem.
Soc. 1979, 101, 2240. (c) Hamon, J.-R.; Astruc, D.; Michaud, P. J. Am.
Chem. Soc. 1981, 103, 758. (d) Astruc, D.; Hamon, J.-R.; Roman, E.;
Michaud, P. J. Am. Chem. Soc. 1981, 103, 7502. (e) Hamon, J.-R.; Astruc,
D.; Roman, E.; Batail, P.; Mayerle, J. J. J. Am. Chem. Soc. 1981, 103,
2431–2432. (f) Ruiz, J.; Guerchais, V.; Astruc, D. J. Chem. Soc., Chem.
Commun. 1989, 812. (g) Ruiz, J.; Lacoste, M.; Astruc, D. J. Chem. Soc.,
Chem. Commun. 1989, 813.
(2) (a) Astruc, D. Acc. Chem. Res. 1986, 19, 377. (b) Astruc, D. Chem.
ReV 1988, 88, 1189. (c) Astruc, D. Bull. Soc. Chim. Jpn. 2007, 80, 1658.
10.1021/om7012875 CCC: $40.75
2008 American Chemical Society
Publication on Web 07/18/2008

3694 Organometallics, Vol. 27, No. 15, 2008
Kahlal et al.
electron complexes bearing these different linkages. The color
change between the two easily switchable redox forms of
dendritic materials can be very useful for sensing, and tuning
the choice of color, which varies as a function of the link, is
important in this sensor context. Since the colors are connected
to the electronic structure of these redox systems, theoretical
calculations concerning the spectrocopies properties are timely.
In the present article, we describe the first theoretical calculations
concerning the spectroscopic properties of Cp-substituted
complexes (η5-C5H4X)FeI(η6-C6Me6) (X ) CONHR, COOR,
COSR), including the crucial influence of the amido, ester, and
thioester groups, in their two most easily accessible redox forms,
FeII and FeI. We report the synthesis, stability, spectroscopic
properties, and theoretical calculations of three complexes of
the type (η5-C5H4R)FeI(η6-C6Me6) containing an amido, an
ester, and a thioester group R directly attached to the cyclo-
pentadienyl ring. The comparison of the properties of the three
complexes, especially using UV/vis spectroscopy, will shed light
on the influence of the functionalized Cp ring on the localization
of the unpaired electron.
complexes. The chemical stability of the 19-electron complexes
was confirmed by 1H NMR after a complete reduction-oxidation
cycle.
2. Cyclic Voltammetry Data. The redox stability of the three
18-electron complexes 3, 4, and 5 was also studied by cyclic
voltammetry in order to compare the influence, on the redox
potential, of the functional group that is directly attached to the
Cp ring. The three complexes present one single fully reversible
redox wave in DMF. The complex 3, which contains the amido
group, is the one that presents the more negative redox potential
(-1.360 V vs FeCp2*) (Table 1).
3. UV/Vis Spectroscopy. The 19-electron complexes 3a, 4a,
and 5a present significantly different colors, 3a being deep blue,
4a turquoise blue, and 5a deep green-gray. These differences
in color are reflected in the completely different UV/vis spectra
obtained for the three complexes (see Figures 1–4). This fact
suggests that the functional group directly attached to the Cp
ring has a significant influence on the localization of the unpaired
electron. The values of absorption bands obtained for the six
complexes are gathered in Table 2.
4. Theoretical Calculations. 4.1. Geometric and Electronic
Structures of the 18-Electron Cations. The electronic structure
of a typical sandwich compound such as ferrocene or CpFe(η6-
C6H6)+ is that of a pseudo-octahedral 18-electron ML6 species.7
The three highest occupied MOs are nonbonding and of very
large 3d(Fe) character. They constitute the so-called “t2g” set.7
The two lowest unoccupied MOs are metal-ligand antibonding
and of dominant 3d(Fe) character. They constitute the so-called
“eg*” set.7 Approximating the Cp and benzene ligands in
CpFe(η6-C6H6)+ as simple circles of different sizes, the C∞V
pseudosymmetry can be considered for the complex. The
qualitative MO diagram of CpFe(η6-C6H6)+ is sketched in
Figure 5, based on the interaction between the Fe(II) metal and
the π(C-C) orbitals of the benzene and Cp- ligands. The levels
are labeled in both the approximate C∞V (where a1, e1, and e2
stand for σ, π, and δ, respectively) and exact Cs symmetries.
Note that all the previous published calculations on CpFe(η6-
C6H6)+ and CpFe(η6-C6Me6)+ found a very strong C∞V pseu-
dosymmetry for these compounds, i.e., an almost exact degen-
eracy for the levels labeled e1 and e2 in Figure 5.3,7,8
As one may expect, the major features of the CpFe(η6-C6H6)+
diagram shown in Figure 5 are maintained in the substituted
3-5 derivatives, although the e1 and e2 pseudodegeneracy is
of course split by the substituent effect. The optimized
geometries of the three cations are shown in Figure 6, and their
major metrical and electronic data are given in Table 3. These
data are consistent with those previously calculated on related
species.3,8 The MO diagrams of the three cations are shown in
Figure 7. As stated above, they are related to that of their simple
CpFe(C6H6)+ relative.
In order to provide better insight into the structural and optical
propertiesofthethree19-electroncomplexes(η5-C5H4CONHR)FeI(η6-
C6Me6), 3a, (η5-C5H4COOR)FeI(η6-C6Me6), 4a, and (η5-
C5H4COSR)FeI(η6-C6Me6), 5a, we have carried out DFT
calculations on these species, as well as on their diamagnetic
18-electron cations [(η5-C5H4CONHR)FeII(η6-C6Me6)][PF6], 3,
[(η5-C5H4COOR)FeII(η6-C6Me6)][PF6],4,and[(η5-C5H4COSR)FeII(η6-
C6Me6)][PF6], 5, for the sake of comparison. Full geometry
optimizations under C1 symmetry have been carried out on the
six compounds with the help of the ADF 2006 package.4 In a
second step, the optical transitions of the considered complexes
have been computed on the optimized geometries at the time-
dependent DFT (TD-DFT) level.5
Results and Discussion
1. Synthesis of the [(η5-C5H4COR)FeII(η6-C6Me6)][PF6]
Complexes and Their Reduction to 19-Electron Form. The
complex [CpFeII(η6-C6Me6)][PF6] was functionalized with
amido, ester, and thioester groups directly attached to the Cp
ring.
The synthesis starts with the transformation of the acid
derivative [(η5-C5H4COOH)FeII(η6-C6Me6)][PF6],6a 1, to acyl
chloride by refluxing complex 1 in SOCl2 overnight. The
resulting complex [(η5-C5H4COCl)FeII(η6-C6Me6)][PF6], 2, was
then dissolved in dry dichloromethane and added to a dichlo-
romethane solution of triethylamine and propyl amine (for
complex 3), phenoltriallyl dendron (for complex 4), or de-
canethiol (for complex 5). The resulting complexes were isolated
as orange powders and fully characterized as 18-electron d6 FeII
complexes (Scheme 1).
A comparison of the electronic effects of the amido, ester,
and thioester groups in these complexes is not straightforward.
It is generally assumed that an ester group has a larger electron-
withdrawing effect than an amido group, and little is known
about the comparative electronic effect of a thioester group. It
turns out that in the case of complex 4 the electronic effect of
The 18-electron complexes 3, 4, and 5 were reduced to their
19-electron form upon reaction with the complex CpFeI(η6-
C6Me6), in THF, at room temperature for 5 min. The resulting
19-electron complexes 3a, 4a, and 5a are stable and were
characterized by UV/vis spectroscopy in order to understand
the visible difference of the coloration among the three
(7) van Gisbergen, S. J. A.; Snijders, J. G.; Baerends, E. J. Comput.
Phys. Commun. 1999, 118, 119.
(4) See for example: Fillaut, J.-L.; Andries, J.; Perruchon, J.; Desvergne,
J.-P.; Toupet, L.; Fadel, L.; Zouchoune, B.; Saillard, J.-Y. Inorg. Chem.
2007, 46, 5922.
(5) (a) Vlcˇek, A. Jr.; Za´lisˇ, S. Coord. Chem. ReV. 2007, 251, 258. (b)
Barone, V.; Improta, R.; Rega, N. Acc. Chem. Res. 2008, 41, 605–616. (c)
Ciofini, I.; Adamo, C J. Phys. Chem. 2007, 111, 5549, and references therein.
(6) ADF Version 2006.01; Theoretical Chemistry Vrije Universiteit:
Amsterdam, The Netherlands, SCM.
(8) (a) Roma´n, E.; Dabard, R.; Moinet, C.; Astruc, D. Tetrahedron Lett.
1979, 16, 1433. (b) Ruiz, J.; Astruc, D C. R. Acad. Sci. Paris, Ser. 2 1998,
21. (c) Ruiz, J.; Daniel, M.-C.; Astruc, D. Can. J. Chem. 2006, 84, 288.
(9) Albright, T. A.; Burdett, J. K.; Whangbo, M. W. Orbital Interactions
in Chemistry; John Wiley and Sons: New York, 1985.
(10) Le Beuze, A.; Lissillour, R.; Weber, J. Organometallics 1993, 12,
47.

19-Electron Metal Sandwich Complexes
Organometallics, Vol. 27, No. 15, 2008 3695
Scheme 1. Synthesis of [(η5-C5H4COR)FeII(η6-C6Me6)][PF6] and Their Reduction to 19-Electron Formsa
a
(i) SOCl2, reflux, 18 h; (ii) propylamine, CH2Cl2, triethylamine, rt, 4 h; (iii) phenoltriallyl dendron, CH2Cl2, triethylamine, rt, 4 h; (iv) decanethiol,
CH2Cl2, triethylamine, rt, 4 h; (v) FeICp(C6Me6), THF, rt, 5 min; (vi) ferricinium hexafluorophosphate, ether, rt, 5 min.
Table 1. Redox Potentials of the Complexes
[(η5-CpCOR)Fe(C6Me6)][PF6] Obtained by Cyclic Voltammetry
complex
E1/2 (V)a
3
4
5
-1.360
-1.160
-1.140
a E1/2 ) (Epa + Epc)/2 vs FeCp2* (in V). Electrolyte: [nBu4N][PF6]
0.1 M; working and counter electrodes: Pt; reference electrode: Ag;
internal reference: FeCp2*; scan rate: 0.200 V s-1; 20 °C. Solvent:
dimethylformamide. Compare [CpFe(η6-C6Me6][PF6], whose E1/2 value
is -1.425 V, to Cp*2Fe in DMF.6b
the conjugated R substituent of the CO2R group is at least as
important as that of the ester function. Indeed, a striking feature
of 4 is that its “t2g” set is not the highest occupied MO set as
in most of the other relative compounds, but lies below a group
of five MOs that are associated with π(C-C) orbitals of the
conjugated R substituent. A similar but much weaker effect
occurs in the thioester complex 5, for which the “t2g” set lies
just below the HOMO, which can be identified as the highest
of the σ(C-C) orbitals of the aliphatic C10H21 chain on the sulfur
atom, with some metal admixture. In the cases of 3 and 5, the
electron-withdrawing effect of the amido and thioester substit-
uents has a small stabilizing effect on the HOMO energy, which
is slightly higher (-8.63 eV) in the unsubstituted CpFe(C6Me6)+
relative. Similarly, the LUMO energies of 3, 4, and 5 are slightly
lower than those of CpFe(C6Me6)+, which is -6.12 eV.
4.2. Geometric and Electronic Structures of the
Neutral 19-Electron Complexes. Calculations have shown that
reducing the 18-electron CpFe(η6-C6H6)+ and CpFe(η6-C6Me6)+
Figure 1. UV/vis spectra of the 18-electron complexes 3, 4, and 5
(c ) 2.00 × 10-3 M in acetone).
complexes leads to the occupation of one of the “eg*” orbitals
without any significant Jahn-Teller distortion,3,8 in agreement
with all the experimental data reported on this type of
complexes.1,3 The only significant structural change is a
lengthening of the Fe-C distances upon reduction, due to the
occupation of a metal-ligand antibonding nature of one “eg*”
MO. The major computed data given in Table 3 confirm this
general trend. Apart from these small differences, the optimized
structures of the 18- and 19-electron complexes are very similar.

3696 Organometallics, Vol. 27, No. 15, 2008
Kahlal et al.
Table 2. UV/Vis Data Obtained for the Six Complexes
complexa
18-e-
λmax1 (ε1)
λmax2 (ε2)
λmax3 (ε3)
3
4
5
3a
4a
5a
416 (151)
413 (394)
430 (233)
340 (2017)
468 (451)
486 (65)
462 (131)
482 (298)
484 (206)
581 (1012)
630 (561)
691 (158)
19-e-
690 (487)
776 (240)
a The UV/vis spectra were obtained in acetone for the 18-electron
complexes and in ether for the 19-electron complexes.
Figure 2. UV/vis spectrum of the 19-electron complex 3a (c )
3.65 × 10-4 M in ether).
Figure 3. UV/vis spectrum of the 19-electron complex 4a (c )
4.65 × 10-4 M in ether).
Figure 5. Qualitative MO interaction diagram for CpFe(C6H6)+
(C∞V: pseudosymmetry; Cs: exact symmetry).
Figure 4. UV/vis spectra of the 19-electron complex 5a (c ) 2.10
× 10-3 M in ether).
The computed spin-orbital diagrams of 3a, 4a, and 5a are
shown in Figure 8. One can see that, contrary to the cation cases,
the three highest occupied levels correspond to the “t2g” set in
the three species. The stabilization of the highest occupied ligand
levels with respect to the “t2g” set when going from 4 to 4a
should be associated with some inductive and electrostatic
effects of the CO2R substituent, which stabilizes the orbitals of
the electron-poor Fe(II) center in 4 and destabilizes that of the
electron-rich Fe(I) center in 4a. The effect of the substituent
on the cyclopentadienyl ring is stronger for the neutral series
as compared to the cationic series. In the three compounds, the
Figure 6. Optimized geometries of the cations 3, 4, and 5.
SOMO 11a and LUMO 12a lie much lower in energy than the
SOMO and LUMO of CpFe(C6Me6), the energies of which are
-2.14 and -1.56 eV, respectively. The SOMO and LUMO
energy order within the series is CpFe(C6Me6) > 3a > 5a ≈

19-Electron Metal Sandwich Complexes
Organometallics, Vol. 27, No. 15, 2008 3697
Table 3. Relevant Computed Data for the 18-Electron Cations 3, 4, and 5 and Their Reduced 19-Electron Relatives 3a, 4a, and 5a
3a 4a 5a
3
4
5
ionizationpotential (eV)
HOMO-LUMO gap (eV)
Fe-C(Cp) average (Å)a
Fe-C(Cp) range (Å)
Fe-C(HMB) average (Å)
Fe-C(HMB) range (Å)
C(Cp)-C(Cp) average (Å)
C(Cp)-C(Cp) range (Å)
C(HMB)-C(HMB) average (Å)
C(HMB)-C(HMB) range (Å)
Mulliken atomic spin density
Fe
4.52
4.71
4.71
2.39
2.101
2.090-2.111
2.155
2.131-2.197
1.434
1.425-1.442
1.434
1.29
2.104
2.097-2.100
2.152
2.129-2.190
1.432
1.426-1.438
1.435
2.30
2.104
2.097-2.111
2.153
2.132-2.193
1.433
1.426-1.440
1.434
2.177
2.136-2.261
2.169
2.122-2.253
1.434
1.421-1.444
1.446
2.174
2.142-2.252
2.169
2.126-2.237
1.433
1.424-1.441
1.430
2.171
2.140-2.244
2.169
2.128-2.241
1.433
1.423-1.441
1.445
1.433-1.436
1.425-1.445
1.432-1.438
1.426-1.446
1.432-1.436
1.426-1.445
0.90
0.87
0.86
SOMO composition
% Fe
91
92
92
a Cp ) η5-C5H4X (X ) CONHR, COOR, COSR), HMB ) η6-C6Me6; SOMO ) singly occupied molecular orbital.
Figure 8. Computed level diagrams of the neutral 19-electron
complexes 3a, 4a, and 5a. The energies are those of spin-orbitals
(spin-polarized calculations).
Figure 7. Computed MO diagrams of the 18-electron cations 3, 4,
and 5.
4a. This energy order follows the expected electrophilicity of
the amido, ester, and thioester groups. It should be noted that
in the three compounds the vacant level, which lies just above
the “eg*” LUMO (noted 13a in Figure 8), is derived from one
of the π*(Cp) orbitals mixed in a bonding way with the π*(CO)
orbital of the substituent on the Cp ring. In the case of 5a, this
accepting orbital has also some participation on the R group of
the ester substituent. A plot of the three spin-up 13a spin-orbitals
is shown in Figure 9. Thus, contrarily to the case of CpFe(C6H6)
and CpFe(C6Me6), where the lowest ligand vacant orbitals are
of arene major character (Figure 5), in the imido, ester, and
thioester derivatives 3a, 4a, and 5a, the lowest nonmetallic
vacant level is localized on the substituted Cp ligand, with
significant localization on the substituent.
4.3. Computed UV/Vis Transitions of the 18- and
19-Electron Species. For the sake of comparison, we first
computed the optical transitions of the simple CpFe(η6-
C6Me6)0/+ system, which we will consider as a reference. The
calculated optical spectrum of CpFe(η6-C6Me6)+ is very simple,
with a unique significant absorption band occurring above 200
nm. The corresponding computed maximum (233 nm) corre-
sponds to a π(C-C)fmetal(“eg*”) transition. This value
compares well to the reported experimental λmax (244 nm).1c
The calculated optical spectrum of the 19-electron CpFe(η6-
C6Me6) complex is very different. The absorption of lowest
energy is computed at 792 nm and corresponds to a transition
from the SOMO (“eg*” component, see Figure 1) to the lowest
unoccupied ligand level, which is of dominant arene character
(lowest e2* level in Figure 1). This is the only transition
computed above 410 nm, so that it can be unambiguously
indexed to the reported experimental λmax value of 684 nm.1b
Such discrepancy between the TDDFT-computed and experi-
mental values is not uncommon in transition-metal complexes4
and is related to the limitations of standard TDDFT calculations
in quantitatively reproducing charge-transfer transitions.5 Three
other bands are computed in the range 200-410 nm. Two of
them are associated with two metal(“t2g”)f π*(C-C) at 298
and 334 nm. Thus, the ligand-to-metal transition calculated at
233 nm in the case of CpFe(η6-C6Me6)+ is no longer present

3698 Organometallics, Vol. 27, No. 15, 2008
Kahlal et al.
For a better understanding of the simulated spectra, the major
computed transitions and their associated oscillator strengths
are reported in Table 4. As for CpFe(η6-C6Me6), the transition
of lowest energy is a metal(SOMO)-to-ligand transition, the
ligand level involved being the lowest vacant nonmetallic spin-
orbital. However, whereas in CpFe(η6-C6Me6) this ligand level
is of arene character, it is localized for the substituted cyclo-
pentadienyl ligand in the case 3a, 4a, and 5a (see discussion
above and Figure 9). Although, the first transitions of the three
compounds 3a, 4a, and 5a are of the same SOMO-
(“eg*”)fligand nature, they differ significantly from the ener-
getic point of view, due to very different substituent effects.
As stated above, the trend in the SOMO energy is 3a > 5a ≈
4a. The trend in the energy of the 13a spin-orbital involved in
the transition of lowest energy is 3a > 4a ≈ 5a. Due to the
presence of the substituent on the Cp ligand, the energy variation
of the 13a spin-orbital is dominating. Therefore the energy of
the SOMO(“eg*”)f13a transition follows the order 4a ≈ 5a >
3a. This is the major reason for their different colors. The
particularly large discrepancy between experiment and theory
observed for the lowest band of 4 (482 vs 621 nm) should be
related to the delocalization on the π system of its unoccupied
spin-orbital involved in the transition (Figure 9), which increases
the spatial range of the charge transfer.5 As in the case of
CpFe(η6-C6Me6), the other computed transitions involve the
“t2g” HOMO.
Figure 9. The unoccupied 13a spin-orbitals involved in the
transitions of lowest energies (SOMO(“eg*”)fligand) computed
for 3a, 4a, and 5a.
Conclusion
Three new 18-electron complexes of the type CpFe(η6-
C6Me6) containing an amido, an ester, and a thioester group
directly attached to the Cp ring were synthesized and reduced
to FeI 19-electron complexes. These new 19-electron complexes
are thermally stable and were characterized by UV/vis spec-
troscopy, showing significant differences in the spectra. Thus,
it is important to consider that such functionalizations do not
provoke destabilization of the 19-electron complexes. This
makes feasible a strategy consisting in the synthesis of functional
dendritic reservoir complexes. The influence of the function-
alized Cp ring on the localization of the unpaired electron was
studied by DFT calculations, and it was found that the first
transitions of the three 19-electron complexes are of the same
SOMO(“eg*”)fligand nature as in the case of CpFe(η6-C6Me6),
the involved ligand level being localized on the substituted
cyclopentadienyl ring, contrarily to the CpFe(η6-C6Me6) case,
for which it is of arene nature. The other computed transitions
involve the “t2g” HOMO as in CpFe(η6-C6Me6). All these
transitions differ significantly from the energetic point of view,
due to the different substituent effects of the imido, ester, and
thioester groups. This is the major reason for their different
colors.
Figure 10. Simulated UV/vis spectra of 3, 4, and 5 from TD-DFT
calculations.
in the computed window in the case of the neutral CpFe(η6-
C6Me6) species. The third one, at 407 nm, is weaker and is a
SOMOfligand transition involving the highest arene π*(C-C)
level. A similar general trend, more or less tuned by the
substituent effect, is computed when comparing 3, 4, and 5 with
their reduced relatives 3a, 4a, and 5a, respectively, as described
in detail below. The simulated spectra of 3, 4, and 5 (Figure
10) appear quite different from the experimental ones (Figure
1). Such a quantitative discrepancy is not surprising owing to
the TDDFT limitations discussed above and also to the fact that
our calculations do not take into account the effects of the
solvent and counter anion, which are expected to be particularly
important in the case of these polar species. Nevertheless, the
three experimental bands reported in Figure 1 and Table 2 for
each of the 3, 4, and 5 cations can be qualitatively assigned to
the three bands of lowest energy appearing on the simulated
spectra of Figure 10. In the three cases, the less energetic band
is a ligandfmetal (“eg*”) transition as in the case of CpFe(η6-
C6Me6)+. In the case of 3 and 5, two other low-energy bands
are also associated with ligandfmetal (“eg*”) transitions, but
involve deeper ligand levels, with in the case of 3 an additional
metal (“t2g“)fligand transition. In the case of 4, the two other
bands are ligandfligand transitions involving the highest
occupied ligand MOs, which, in that particular case, are
associated with the conjugated substituent of the ester group
and situated above the “t2g” set (Figure 7).
Experimental Section
Computational Methods. Density functional theory (DFT)
calculations were carried out on the studied compounds using the
Amsterdam Density Functional (ADF) program,6 developed by
Baerends and co-workers.11 Electron correlation was treated within
the local density approximation (LDA) in the Vosko-Wilk-Nusair
(11) (a) Baerends, E. J.; Ellis, D. E.; Ros, P. Chem. Phys. 1973, 2, 41.
(b) te Velde, G.; Baerends, E. J. J. Comput. Phys. 1992, 99, 84. (c) Fonseca
Guerra, C.; Snijders, J. G.; te Velde, G.; Baerends, E. J. Theor. Chim. Acc.
1998, 99, 391. (d) Bickelhaupt, F. M.; Baerends, E. J. ReV. Comput. Chem.
2000, 15, 1. (e) te Velde, G.; Bickelhaupt, F. M.; Fonseca Guerra, C.; van
Gisbergen, S. J. A.; Baerends, E. J.; Snijders, J. G.; Ziegler, T. J. Comput.
Chem. 2001, 22, 931.

19-Electron Metal Sandwich Complexes
Organometallics, Vol. 27, No. 15, 2008 3699
Table 4. Relevant Computed Transitions and Their Associated Oscillator Strengths for 3, 4, and 5 and 3a, 4a, and 5a (HMB ) η6-C6Me6)
3
4
5
transitions
(major
transitions
(major
transitions
(major
wavelength oscillator
wavelength oscillator
wavelength oscillator
(nm)
strength
components)
(nm)
strength
components)
(nm)
strength
components)
402
0.009
0.014
0.027
0.087
0.161
CONHRf“eg*”
“t2g ”fπ*(HMB+Cp)
CONHRf“eg*”
CONHRf“eg*”
CONHRfπ*(HMB)
621
0.067
0.004
0.004
0.075
π
π
π
π
CC(R) f“eg*”
434
311
286
0.018
0.056
0.043
COSRf“eg*”
π(HMB+Cp)f“eg*”
π(HMB+Cp)f“eg*”
279
270
241
240
454
448
336
CC(R)fπ*(HMB+Cp)
CC(R) fπ*(HMB+Cp)
CC(R)fπ*(HMB)
3a
4a
5a
wavelength
(nm)
oscillator
strength
transitions
(major
components)
wavelength
(nm)
oscillator
strength
transitions
(major
components)
wavelength
(nm)
oscillator
strength
transitions
(major
components)
696
545
400
344
0.027
0.010
0.006
0.009
“eg*”fπ*(Cp+CO)
“t2g ”fCONHR
“t2g ”fCONHR
“t2g ”fCONHR
742
576
413
0.074
0.027
0.022
“eg*” fπ*(Cp+COOR)
736
572
408
345
0.041
0.022
0.018
0.025
“eg*”fπ*(Cp+CO)
“t2g ”f COSR
“t2g ”f COSR
“t2g ”f “eg*”
“t2g ”fCOOR
“t2g ”fCOOR
parametrization.12 The nonlocal corrections of Becke and Perdew
were added to the exchange and correlation energies, respectively.13,14
The numerical integration procedure applied for the calculations
was developed by te Velde et al.11e The atom electronic configura-
tions were described by a triple-ꢀ Slater-type orbital (STO) basis
set for C 2s and 2p, N 2s and 2p, O 2s and 2p, and S 3s and 3p
augmented with a 3d single-ꢀ polarization for C, N, O, and S atoms.
A triple-ꢀ STO basis set was used for Fe 3d and 4s, augmented
with a single-ꢀ 4p polarization function for atoms. A frozen-core
approximation was used to treat the core shells up to 1s for C, N,
and O, 2p for S, and 3p for Fe.14 Full geometry optimizations were
carried out using the analytical gradient method implemented by
Verluis and Ziegler.15 Spin-unrestricted calculations were performed
for all the open-shell systems. The UV-visible transitions were
calculatedbymeansoftime-dependentDFT(TDDFT)calculations,7,16
at the same level of theory. Only singlet-singlet transitions, that
is, spin-allowed transitions, have been taken into account. Moreover,
only transitions with non-negligible oscillator strengths are reported
and discussed. Representations of the molecular structures were
done using MOLEKEL4.1.17 The UV/vis spectra have been
simulated from the computed TDDFT transitions and their oscillated
strengths by using the SWizard program,18 each transition being
associated with a Gaussian function of half-height width equal to
surements were recorded under nitrogen atmosphere. Conditions:
solvent, dry dimethylformamide (DMF); temperature, 20 °C;
supporting electrolyte, [nBu4N][PF6] 0.1 M; working and counter
electrodes, Pt; reference electrode, Ag; internal reference, FeCp2*
(Cp* ) η5-C5Me5); scan rate, 0.200 V s-1
.
Synthesis of [Fe(η6-C6Me6)CpCONH(CH2)2CH3][PF6], 3. To
a dichloromethane solution of [Fe(η6-C6Me6)CpCOCl][PF6] (0.707
g, 1.44 mmol) was added 2 mL of propylamine, and the color of
the solution turned from red to brown. The solution was stirred for
4 h at rt. The solvent was removed under vacuum, and the residue
was dissolved in dichloromethane and washed with an aqueous
solution of HPF6-. The organic layer was dried with sodium sulfate
and filtrated under paper, and the solvent was removed under
vacuum. Precipitation with dichloromethane/ether yielded 0.440 g
of an orange powder in 60% yield.
1H NMR (CH3COCH3, 300 MHz), δppm: 7.76 (s, 1H, NH), 5.16
and 4.94 (s, 5H, Cp), 3.33 (m, 2H, NHCH2), 2.52 (s, 18H, C6Me6),
1.62 (m, 2H, NHCH2CH2), 0.95 (m, 3H, CH3). 13C NMR
(CH3COCH3, 62.90 MHz), δ ppm: 163.6 (CdO), 100.3 (Cq of
C6Me6), 85.9 (Cq of Cp), 81.0 and 77.3 (CH of Cp), 42.7 (NHCH2),
23.5 (NHCH2CH2), 17.0 (CH3 of C6Me6), 11.8 (CH3). MS (MALDI-
TOF; m/z): calc for C21H30OFe+ 368.31; found (M+) 368.05. Anal.
Calc for C21H30ONFePF6: C 49.14, H 5.89. Found: C 48.58, H 5.77.
3000 cm-1
.
Infrared νCdO: 1666.7 cm-1
.
General Data. Reagent-grade diethyl ether and tetrahydrofuran
(THF) were predried over Na foil and distilled from sodium-
benzophenone anion under argon immediately prior to use. Dichlo-
romethane was distilled from calcium hydride just before use. The
propylamine was distilled from AlLiH4 just before use. All
manipulations were carried out using Schlenk techniques or in a
nitrogen-filled Vacuum Atmospheres drylab. The [Fe(η6-
C6Me6)CpCOOH][PF6] and [Fe(η6-C6Me6)CpCOCl][PF6] were
Synthesis of [Fe(η6-C6Me6)CpCOOC6H4C(CH2CHdCH2)3]-
[PF6], 4. To a dichloromethane solution of HOC6H4C(CH2-
CHdCH2)3 (0.369 g, 1.62 mmol) and triethylamine (1 mL) was
added a dichloromethane solution of [Fe(η6-C6Me6)CpCOCl][PF6]
(0.530 g, 1.08 mmol), and the color of the solution turned from
red to brown. The solution was stirred for 4 h at rt. The solvent
was removed under vacuum, and the residue was dissolved in
dichloromethane and washed with an aqueous solution of K2CO3
and an aqueous solution of HPF6-. The organic layer was dried
with sodium sulfate and filtrated under paper, and the solvent was
removed under vacuum. Precipitation with dichloromethane/ether
yielded 0.623 g of a brown powder in 85% yield.
1
synthesized according to ref 6. H NMR spectra were recorded at
25 °C with a Bruker AC 300 (300 MHz) spectrometer, and 13C
NMR spectra were obtained in the pulsed FT mode at 75.0 MHz
with a Bruker AC 300 spectrometer. All chemical shifts are reported
in parts per million (δ, ppm) with reference to Me4Si (TMS).
Elemental analyses were performed by the Center of Microanalyses
of the CNRS at Lyon Villeurbanne, France. The mass spectra were
recorded with a PerSeptive Biosystems Voyager Elite (Framingham,
MA) time-of-flight mass spectrometer. All electrochemical mea-
1H NMR (CDCl3, 300 MHz), δ ppm: 7.39 and 7.23(d, 4H,
aromatic), 5.55 (m, 1H, CH ) CH2), 5.07 (t, 2H, CHdCH2), 5.02
and 4.91 (s, 4H, Cp), 2.44 (d, 2H, CH2CHdCH2), 2.24 (s, 18H,
C6Me6). 13C NMR (CDCl3, 62.90 MHz), δ ppm: 164.6 (CdO), 148.3
and 144.7 (Cq of aromatic), 134.4 (HCdCH2), 128.5 and 120.5
(CH of aromatic), 118.4 (HCdCH2), 100.2 (Cq of C6Me6), 85.9
(Cq of Cp), 81.0 and 78.2 (CH of Cp), 42.7 (CH2CHdCH2), 17.5
(CH3 of C6Me6). MS (MALDI-TOF; m/z): calc for C34H41O2Fe+
537.52; found (M+) 537.09. Anal. Calc for C34H41O2FePF6: C
(12) Vosko, S. D.; Wilk, L.; Nusair, M. Can. J. Chem. 1990, 58, 1200.
(13) (a) Becke, A. D. J. Chem. Phys. 1986, 84, 4524. (b) Becke, A. D.
Phys. ReV. A 1988, 38, 3098.
(14) (a) Perdew, J. P. Phys. ReV. B 1986, 33, 8822. (b) Perdew, J. P.
Phys. ReV. B 1986, 34, 7406.
(15) Verluis, L.; Ziegler, T. J. Chem. Phys. 1988, 88, 322.
(16) Rosa, A.; Baerends, E. J.; Gisbergen, S. J. A.; van Lenthe, E.;
Groeneveld, V; Snijders, J. G. J. Am. Chem. Soc. 1999, 121, 10356.
(17) Flu¨kiger, P.; Lu¨thi, H. P.; Portmann, S.; Weber, J. MOLEKEL4.1;
Swiss Center for Scientific Computing (CSCS): Switzerland, 2000-2001.
59.83, H 6.05. Found: C 60.81, H 6.15. Infrared νCdO: 1740 cm-1
.
Synthesis of [Fe(η6-C6Me6)CpCOS(CH2)11CH3][PF6], 5. To
a dichloromethane solution of dodecanothiol (0.286 g, 1.40 mmol)
and triethylamine (1 mL) was added a dichloromethane solution
of [Fe(C6Me6)CpCOCl][PF6] (0.462 g, 0.942 mmol), and the color

3700 Organometallics, Vol. 27, No. 15, 2008
Kahlal et al.
of the solution turned from red to brown. The solution was stirred
for 4 h at rt. The solvent was removed under vacuum, and the
residue was dissolved in dichloromethane and washed with an
aqueous solution of K2CO3 and an aqueous solution of HPF6-. The
organic layer was dried with sodium sulfate and filtrated under
paper, and the solvent was removed under vacuum. Precipitation
with dichloromethane/ether yielded 0.494 g of an orange powder
in 80% yield.
1H NMR (CH3COCH3, 300 MHz), δ ppm: 5.26 and 5.10 (s, 4H,
Cp), 3.18 (t, 2H, SCH2), 2.53 (s, 18H, C6Me6), 1.74 (m, 2H,
SCH2CH2), 1.29 (m, 18H, (CH2)9CH3), 0.88 (m, 3H, CH3). 13C
NMR (CDCl3, 62.90 MHz), δ ppm: 190.7 (CdO), 101.0 (Cq of
C6Me6), 86.5 (Cq of Cp), 82.4 and 77.4 (CH of Cp), 32.9 (SCH2),
23.6 ((CH2)9CH3), 17.3 (CH3 of C6Me6), 14.7 (CH3). MS (MALDI-
TOF; m/z): calcd for C30H47OSFe+ 511.61; found (M+) 511.21.
Anal. Calc for C30H47OSFePF6: C 54.88, H 7.22. Found: C 55.10,
H 6.97.
Figure 11. Simulated UV/vis spectra of 3a, 4a, and 5a, from TD-
DFT calculations.
General Procedure for the Reduction of Complexes 3, 4,
and 5. The cationic complex was dissolved in dry THF, and
FeICp(C6Me6) (1 equiv) was added. After agitation for 5 min at rt,
the solvent was removed under vacuum. The 19-electron complex
was extracted with dry diethyl ether.
Acknowledgment. We are grateful to Fundac¸a˜o para a
Cieˆncia e a Tecnologia (FCT), Portugal (Ph.D. grant to C.O.),
the Institut Universitaire de France (I.U.F., D.A., and J.Y.S.),
the CNRS, the Universities of Bordeaux 1 and Rennes 1,
and ANR 06-NANO-026-01 for financial support.
General Procedure for the Oxidation of Complexes 3a, 4a,
and 5a. A solution of the 19-electron complex in dry ether was
added to a suspension of ferricinium (1 equiv). After agitation for
5 min at rt, the solvent was removed under vacuum, and the product
was washed with pentane.
OM7012875
(18) Gorelsky, S. I. SWizard program; http://www.sg-chem.net/ Uni-
versity Of Ottawa, Canada, 2007.