Reactive sigma-aryliron complexes or iron-promoted coupling of two Phenyl Anions to one Bis(cyclohexadienylidene) ligand: Synthesis, structure, mass spectrometry, and DFT calculations

Article abstract of DOI:10.1021/om900870j

A reaction of the bulky alkylcyclopentadienyliron(II) high-spin complex [Cp?Fe(μ-Br)]₂ (la) (Cp? = C₅H ₂(CMe₃)₃-1,2,4) with phenylmagnesium bromide produced the deep blue dinuclear complex [(Cp?Fe)

Full text of DOI:10.1021/om900870j

806 Organometallics 2010, 29, 806–813
DOI: 10.1021/om900870j
Reactive Sigma-Aryliron Complexes or Iron-Promoted Coupling of Two
Phenyl Anions to One Bis(cyclohexadienylidene) Ligand: Synthesis,
Structure, Mass Spectrometry, and DFT Calculations
Mark W. Wallasch, Daniel Weismann, Christoph Riehn, Stefan Ambrus,
€
Helmut Sitzmann*
#
Gotthelf Wolmershauser, Anita Lagutschenkov, Gereon Niedner-Schatteburg, and
€
FB Chemie der TU Kaiserslautern, Erwin-Schrodinger-Strasse 54, D-67663 Kaiserslautern, Germany.
# FB Chemie der TU Kaiserslautern and Landesforschungszentrum OPTIMAS.
Received October 7, 2009
A reaction of thebulky alkylcyclopentadienyliron(II) high-spin complex[Cp000Fe(μ-Br)]2 (1a) (Cp000=
C5H2(CMe3)3-1,2,4) with phenylmagnesium bromide produced the deep blue dinuclear complex
[{Cp000Fe}2(μ,η5:η5-H5C6dC6H5)] (2) with a bridging bis(cyclohexadienylidene) ligand. Its structural
analysis shows a centrosymmetric dimer. Each tri(tert-butyl)cyclopentadienyliron fragment is η5-
coordinated to a cyclohexadienylidene moiety in which one carbon atom is bent out of the plane by
˚
˚
0.39 A, exhibiting a bond length of 1.370 A to its symmetry equivalent. Electrospray ionization mass
spectra (ESI-MS) from acetonitrile solution confirm nicely the elemental composition of 2 by way of
their isotope patterns. Reaction of 1a or its tetraisopropylcyclopentadienyl analogue [4CpFe(μ-Br)]2
(1b) (4CpdC5H(CHMe2)4) with 2,6-diisopropylphenylmagnesium bromide affords the extremely
air-sensitive, paramagnetic σ-aryl complexes [Cp000Fe(C6H3 Pr2)] (3a) or [4CpFe(C6H3 Pr2)] (3b),
whose Cp-Fe distance of 1.92 A is typical for cyclopentadienyliron high-spin complexes. In
i
i
4
˚
reactions with copper(I) halides 3a is rearranged to a diamagnetic π complex and coordinated via the
ipso carbon atom of the six-membered ring to copper(I) halide fragments to form heterodinuclear
i
i
complexes [Cp000Fe(μ,η5:η1-C6H3 Pr2)CuCl] (4-Cl) and [Cp000Fe( μ,η5:η1-C6H3 Pr2)CuBr] (4-Br). ESI
mass spectra of complexes 4 do not show the molecular cations, but fragmentation to cyclopenta-
dienyliron arene cations and formation of the hexa(tert-butyl)ferrocenium cation on one hand and
fusion of complex fragments to oligonuclear complexes with or without inclusion of oxygen or
fragments of solvent molecules on the other hand. Three of these oligonuclear complexes formed
under the conditions of the ESI-MS experiment, whose elemental composition could be derived from
isotope patterns, have been interpreted as [Cp000Fe(μ,η5:η1-C6H3 Pr2)Cu(μ,η1:η5-OC6H3 Pr2)FeCp000]þ
i
i
and [{Cp000Fe(μ,η5:η1-C6H3 Pr2)Cu}2X]þ (X=Cl, Br). DFT calculations support the structural analysis
i
of 2 and predict the structure of the dication 22þ. The crystal structures obtained by X-ray diffraction for
2 and 3b are reported.
Introduction
modeled by DFT calculations to possess 88% aryl and
12% cyclohexadienyl-ylidene character.2
In a recent publication a reaction of the half-sandwich
iron(II) bromide [Cp000Fe(μ-Br)]2 with phenylmagnesium
bromide was briefly mentioned, and the product from a
reaction of the same starting compound with mesitylmagne-
sium bromide was described as a σ aryl complex [Cp000Fe-
(C6H2Me3-2,4,6)] with four unpaired electrons. This finding
was based on proton NMR spectra, a magnetic moment of
5.50 μB, and a weak set of X-ray data, which could not be
refined.1 With copper(I) chloride the novel σ mesityl complex
rearranged to the dinuclear π complex [Cp000Fe(μ-C6H2Me3-
2,4,6)CuCl]. For the hypothetical unsubstituted derivative
[CpFe(μ-C6H5)CuCl] the bridging aryl ligand has been
We are interested in the reactivity of high-spin cyclopenta-
dienyliron complexes in general and in the behavior of the
novel type of σ aryl complexes, whose hypothetical σ/π
rearrangement products carry about 30% carbene character
based on DFT calculations.2 In this publication full details
on the preparation and characterization of the coupling
product [(Cp000Fe)2(C12H10)] (2, Cp000 =1,2,4-tri(tert-butyl)-
cyclopentadienyl) obtained from [Cp000Fe(μ-Br)]2 and
phenylmagnesium bromide will be given, including a
crystal structure determination, ESI mass spectra, and
DFT calculations. Furthermore, with [4CpFeC6H3(CH-
Me2)2-2,6] (3b, 4Cp=tetraisopropylcyclopentadienyl), with
€
*Corresponding author. E-mail: sitzmann@chemie.uni-kl.de.
€
(1) Wallasch, M. W.; Rudolphi, F.; Wolmershauser, G.; Sitzmann,
H. Z. Naturforsch. 2009, 64b, 11–17.
(2) Wallasch, M. W.; Vollmer, G. Y.; Kafiyatullina, A.; Wolmershauser,
G.; Jones, P. G.; Mang, M.; Meyer, W.; Sitzmann, H. Z. Naturforsch. 2009,
64b, 18–24.
r
pubs.acs.org/Organometallics
Published on Web 01/21/2010
2010 American Chemical Society

Article
Organometallics, Vol. 29, No. 4, 2010 807
a σ-diisopropylphenyl ligand, the second example of a σ aryl
cyclopentadienyliron complex will be presented along with a
well-resolved crystal structure analysis. The copper(I) bro-
mide complex [Cp000Fe(μ,η5:η1-C6H3(CHMe2)2-2,6)CuBr]
(4) obtained from [Cp000FeC6H3(CHMe2)2-2,6] (3a) and
copper(I) bromide reacts under the conditions of ESI mass
spectrometry with fragmentation, but also with agglomera-
tion to form complexes of higher nuclearity, as will be shown
in the following section.
Results and Discussion
Synthesis, X-ray Analysis, and ESI Mass Spectrometry of
Complex 2. When one equivalent of phenylmagnesium bro-
mide was added to a solution of tri(tert-butyl)cyclopenta-
dienyliron(II) bromide3 [{Cp000Fe(μ-Br)}2] (1a) in tetrahy-
drofuran, the mixture quickly turned light green and deve-
loped a deep blue color within two hours at room tempera-
ture. Dark blue blocks of the dinuclear complex [{Cp000Fe}2-
(μ,η5:η5-H5C6dC6H5)] (2) could be obtained from pentane
in good yield. The solid compound can be handled briefly in
air, but turns brown and starts to decompose after several
minutes in air. In an inert atmosphere it can be stored at
room temperature. Its pentamethylcyclopentadienyl (Cp*)
analogue [{Cp*Fe}2(μ,η5:η5-H5C6dC6H5)] was obtained by
two-electron reduction of the dicationic biphenyl complex
Figure 1. Crystal structure of the bis(cyclohexadienyl-ylidene)
˚
complex 2. Selected distances/A: Fe1-C1 2.101(3), Fe1-C2
2.099(3), Fe1-C3 2.058(3), Fe1-C4 2.097(3), Fe1-C5 2.064(3),
Fe1-C6 2.148(3), Fe1-C7 2.044(4), Fe1-C8 2.029(3), Fe1-C9
2.041(3), Fe1-C10 2.138(3), Fe1 C11 2.537(4), Fe-Cp ring
3 3 3
plane (C1-C5) 1.70, Fe-dienyl plane (C6-C10) 1.58, C6-C7
1.397(5), C7-C8 1.400(5), C8-C9 1.372(5), C9-C10 1.403(5),
C10-C11 1.442(5), C11-C6 1.450(5), C11-C110 1.370(6),
C11-dienyl plane (C6-C10) 0.548, fold angle along C6 C11
[{Cp*Fe}2(μ,η6:η6-H5C6-C6H5)]2þ 4,5
.
Characteristic 1H
3 3 3
27.1°. For details on the crystal structure determinations see
Table 1.
NMR signals of 2 are due to the hydrogen atoms of the
coupled six-membered rings in ortho (2.87 ppm), meta (4.46
ppm), and para (6.29 ppm) position, which compare well
with the signals (2.6, 4.1, and 5.6 ppm) reported for the
Cp*Fe derivative [{Cp*Fe}2(μ,η5:η5-H5C6dC6H5)].5 In the
13C NMR spectra the signals of the ipso carbon atoms of
the six-membered rings are found at 105.4 ppm, and those of
the ortho carbon atoms at 47.2 ppm. More data can be found
in the Experimental Section.
The dinuclear molecule resides on a crystallographic inver-
sion center and exhibits a typical CdC double bond length of
˚
1.37 A between the ipso carbon atoms of the two six-mem-
bered rings, which arebentout oftheir respectivering plane by
Table 1. Details on the Crystal Structure Determination of
Complexes 2 and 3b
2
3b
formula
C
732.40
46H68Fe2
C29H46Fe
450.51
fw/g mol-1
crystal size/mm
space group
unit cell
0.40 ꢀ 0.37 ꢀ 0.09 0.24 ꢀ 0.33 ꢀ 0.08
P1
triclinic
P1
triclinic
˚
a/A
9.0560(11)
9.8081(12)
12.6486(18)
103.924(15)
91.979(16)
110.895(15)
1009.7(2)
1
8.5442(13)
9.047(2)
19.018(4)
95.835(18)
100.718(15)
107.787(17)
1355.6(5)
2
˚
b/A
˚
c/A
˚
0.55 A. Due to the different ring sizes, the distance of the iron
R/deg
β/deg
γ/deg
˚
atom to the dienyl plane (1.58 A) is shorter than the
iron-Cp000 distance (1.70 A), as expected. The conformation
3
˚
˚
V/A
Z
T (K)
of the molecule with pairs of adjacent tert-butyl substituents
symmetrically facing the CdC double bond between the six-
membered rings can be seen in Figure 1, and the fold angle
293(2)
1.205
7.48
150(2)
1.104
11.04
Dcal./g cm-3
μ/cm-1
along the line C6 C11 is 27.1°, in good agreement with the
3 3 3
transmission
theta limits/deg
reflns
0.75217-0.93498
2.76-25.68
14 182
0.0258-0.9559
2.81-33.38
20 312
25° angle of the Cp* derivative.5 Coupling of two phenyl
anionsto a bis(cyclohexadienylidene) ligand is a processrarely
encountered in transition metal chemistry, but has been
documented for yttrium6 and holmium complexes.7
unique reflns
solution
program
refinement
data/restraints/params
R1
3582
9508
direct methods
SIR97
SHELXL-97
9508/0/283
0.0371
direct methods
SHELXS-97
SHELXL-97
43582/0/226
0.0420
ESI mass spectra of 2 exhibit three signals originating from
iron-containing species, as could be concluded from the
wR2 (all data)
GooF (all data)
max./min. diff peak/e A
0.0995
0.861
0.351, -0.276
0.0868
0.996
0.429, -0.269
€
(3) (a) Wallasch, M.; Wolmershauser, G.; Sitzmann, H. Angew.
Chem. 2005, 117, 2653–2655. (b) Angew. Chem., Int. Ed. 2005, 44,
2597-2599.
(4) Rabaa, H.; Lacoste, M.; Delville-Desbois, M.-H.; Ruiz, J.;
-3
˚
^
Gloaguen, B.; Ardoin, N.; Astruc, D.; Le Beuze, A.; Saillard, J.-Y.;
Linares, J.; Varret, F.; Dance, J.-M.; Marquestaudt, E. Organometallics
1995, 14, 5078–5092.
almost perfect coincidence of the experimentally determined
isotope pattern with theoretical expectation (Figure 2).
The parent signal was detected at an m/z ratio of 366.2
and corresponds to the dication 22þ derived from 2 by
ionization of neutral complex 2 either in acetonitrile solution
by traces of air or during the electrospray process. The
corresponding monocation 2þ was not detected. The signal
^
(5) Lacoste, M.; Rabaa, H.; Astruc, D.; Ardoin, N.; Varret, F.; A.;
Saillard, J.-Y.; Le Beuze, A. J. Am. Chem. Soc. 1990, 112, 9548–9557.
(6) Fryzuk, M. D.; Love, J. B.; Rettig, S. J. J. Am. Chem. Soc. 1997,
119, 9071–9072.
(7) Fryzuk, M. D.; Jafarpour, L.; Kerton, F.; Love, J. B.; Patrick, B.;
Rettig, S. J. Organometallics 2001, 20, 1387–1396.

808 Organometallics, Vol. 29, No. 4, 2010
Wallasch et al.
Figure 2. ESI mass spectrum of 2. Comparison of experiment and simulation of isotope patterns. (a) Overview spectrum. Experiment
(black) and simulation for three peaks (colored). (b) Enlarged region of mass peaks assigned to 22þ. Experiment (black) and simulation
(red). Spectra obtained with an ESI-FT-ICR mass spectrometer (see Experimental Section).
at m/z 443.2 is most probably due to loss of an undetected
Cp000Feþ cation from 22þ, and the m/z 522.4 peak has
been assigned to the hexa(tert-butyl)ferrocenium cation
[Cp0002Fe]þ, which is probably formed under the conditions
of the ESI-MS experiment. It is interesting to note that the
two fragment peaks at m/z 443.2 and 522.4 appeared also as
the strongest and second strongest peak in the electron
impact mass spectrum.
have been performed at the B3LYP/6-311G (Fe) and B3LYP/
cc-pVDZ levels of theory as suggested in refs 8-10.
The NBO analysis was implemented similarly to the ana-
lysis provided by Meyer and Mang.2 Moreover the calcula-
tions have also been performed with the B3LYP/SDD basis
sets similar to ref 11. The detailed results of the calculations
are given in the Supporting Information. Here we would like
to point out only the salient features of the structures and the
changes imposed by the increased positive charge.
DFT Calculations on Complex 2 and the Cations 2þ and
22þ. In order to gain insight into the structural changes upon
ionization, we have investigated the structure of compound 2
as well as those of the related monocation 2þ and dication
22þ by ab initio calculations on the DFT level. Using the
Gaussian 03 program package chemical structure optimizations
The structural results of the DFT calculations are com-
pared in Table 2 with data from X-ray diffraction. Although
(9) P. J. Hay, P. J. J. Chem. Phys. 1977, 66, 4377.
(10) Dunning, T. H., Jr. J. Chem. Phys. 1989, 90, 1007.
(11) Kirgan, R. A.; Rillema, D. P. J. Phys. Chem. A 2007, 111, 13157–
13162.
(8) Wachters, J. H. J. Chem. Phys. 1970, 52, 1033.

Article
Organometallics, Vol. 29, No. 4, 2010 809
Table 2. Comparison of Structural Data for 2, 2þ, and 22þ from X-ray Analysis and DFT Calculations
neutral complex 2
DFT B3LYP/6-311G,
cc-pVDZ (B3LYP/SDD)
cation 2þ
dication 22þ
DFT B3LYP/6-311G,
cc-pVDZ (B3LYP/SDD)
DFT B3LYP/6-311G,
cc-pVDZ (B3LYP/SDD)
˚
distances/A and angles/deg
X-ray
2.084
Fe1-CX (X = 1-5) average
Fe1-C6
2.103 (2.136)
2.146 (2.177)
2.070 (2.099)
2.065 (2.095)
2.071 (2.105)
2.146 (2.183)
2.590 (2.604)
1.69 (1.75)
2.116 (2.150)
2.146 (2.186)
2.096 (2.138)
2.092 (2.133)
2.107 (2.153)
2.160 (2.206)
2.370 (2.401)
1.71 (1.83)
2.103 (2.125)
2.124 (2.157)
2.115 (2.150)
2.130 (2.168)
2.127 (2.162)
2.133 (2.166)
2.183 (2.213)
1.75 (1.72)
2.148(3)
2.044(4)
2.029(3)
2.041(3)
2.138(3)
2.537(4)
1.70
Fe1-C7
Fe1-C8
Fe1-C9
Fe1-C10
Fe1-C11
Fe-Cp ring plane C1-C5
Fe-dienyl plane C6-C10
C6-C7
1.58
1.60 (1.65)
1.60 (1.65)
1.60 (1.63)
1.397(5)
1.400(5)
1.372(5)
1.403(5)
1.442(5)
1.450(5)
1.370(6) 1.37a
0.548
1.413 (1.422)
1.424 (1.436)
1.422 (1.435)
1.414 (1.422)
1.470 (1.478)
1.470 (1.477)
1.367 (1.373)
1.412 (1.420)
1.416 (1.428)
1.417 (1.429)
1.410 (1.418)
1.442 (1.450)
1.442 (1.451)
1.433 (1.435) 1.49a
1.414 (1.424)
1.412 (1.424)
1.411 (1.423)
1.414 (1.422)
1.422 (1.432)
1.425 (1.432)
1.498 (1.495)
C7-C8
C8-C9
C9-C10
C10-C11
C6-C11
C11-C110
C11-dienyl plane (C6-C10)
fold angle between C6-C10-C11
and dienyl plane
27.1 25a
33.6 (30.2)
13.5 (12.7) 5.5a
3.2(2.2)
a Data from the X-ray analysis of [Fe2(μ2,η10-biphenyl)Cp2]nþ (n = 0, 1) taken from lit.12 and references therein.
Table 3. Development of NBO Charge Distribution As a Function
of Overall Charge Statea
charge being delocalized over both iron centers. Our
computational results (structure, partial charges) for 2
compare very well with the results obtained in ref 2 on a
compound closely related to 4-Cl. Moreover, our results
agree also quite well with the X-ray structural results
obtained by Astruc and co-workers5,12 for similar com-
pounds. The model of stepwise electron transfer accom-
panied by a structural rearrangement between 2 and 2þ, as
put forward by Astruc et al.5 based on cyclic voltammetry,
2
2þ
22þ
tri(tert-butyl)cyclopentadienyl
metal center
half biphenyl
C17H29
Fe
-0.37
þ0.84
-0.47
-0.94
-0.47
þ0.84
-0.37
-0.09
þ0.62
-0.03
-0.06
-0.03
þ0.62
-0.09
þ0.13
þ0.87
0.00
C6H5
C12H10 0
C6H5
Fe0
complete biphenyl
half biphenyl0
0.00
-0.01
þ0.87
þ0.12
metal center0
tri(tert-butyl)cyclopentadienyl0
C17H29
0
€
X-ray, and Mossbauer data, is clearly confirmed by our
calculations.
a Point charges are given in units of electron charge e. All structures
were fully optimized at the B3LYP level of theory, using the 6-311G
basis on Fe and cc-pVDZ on all other atoms.
Synthesis, X-ray Analysis, and ESI Mass Spectrometry of
Complexes 3 and 4. In order to suppress carbon-carbon
coupling during the reaction of the aryl Grignard reagent
with the dinuclear iron bromide complex 1a and in order to
sterically protect the envisaged reaction product, 2,6-diiso-
propylphenylmagnesium bromide was employed. The
the calculated absolute bond lengths for the neutral complex
2 differ from the X-ray data by þ2% on average, the overall
structure is described very well. In particular, the double-
bond character of C11-C110, the relatively long Fe-C11
length (indicating η5-coordination of the six-membered
ring), and the fold angle of ca. 25-30° are nicely reproduced.
These features develop upon increase of the overall charge
(þ1, þ2) in the following way: the distances of Fe to the C
atoms of the six-membered rings equalize (η6-coordination),
the carbon-carbon bond between the two six-membered
rings is elongated towards a single bond, and the fold angle is
strongly reduced to approximately 5°. The increasing biphe-
nyl character of the bridging ligand is also reflected in the
change of the charge distribution as obtained by NBO
analysis of the optimized structures (Table 3). A note of
caution is mandatory for the interpretation of the results of
such an arbitrary scheme for the partition of the total
electron density. The calculated partial charges reflect
neither an experimentally observable quantity nor formal
charges or oxidation states of chemical reasoning. As a
result of the NBO analysis, the neutral 2 contains one extra
elemental charge in the bridging ligand equally distributed
over the two rings. This charge is reduced nearly to zero for
22þ, whereas the positive charge on the iron atoms remains
the same. The intermediate state 2þ, which we have not
observed here experimentally, already displays an almost
neutral bridging ligand of biphenyl character, its positive
i
paramagnetic σ aryl complex [Cp000Fe(C6H3 Pr2-2,6)] (3a)
resulting from this reaction is extremely sensitive, like the
corresponding mesityl derivative [Cp000Fe(C6H2Me3-2,4,6)]
described recently.1 During the isolation procedure decom-
position is already visible as a darkening process of the
brownish-orange precipitate, which looks crystalline in the
beginning and quickly turns sticky. For this reason 3a could
not be isolated nor analyzed. NMR spectra of the paramag-
netic material showed contamination with diamagnetic im-
purities probably arising from the decomposition process.
The same problems were encountered with the tetraisopro-
i
pylcyclopentadienyl derivative [4CpFe(C6H3 Pr2-2,6)] (3b)
obtained from bromide [{4CpFe(μ-Br)}2] (1b).13 3b was
somewhat more stable than 3a, showing less, but still sig-
nificant contamination with diamagnetic material in the 1H
NMR spectrum. 3b could, however, be crystallized from
pentane solution and characterized by an X-ray crystal
structure analysis at low temperature.
(12) Astruc, D. Acc. Chem. Res. 1997, 30, 383–391.
(13) (a) Sitzmann, H.; Dezember, T.; Kaim, W.; Baumann, F.; Stalke,
€
D.; Karcher, J.; Dormann, E.; Winter, H.; Wachter, C.; Kelemen, M.
Angew. Chem. 1996, 108, 3013–3016. (b) Angew. Chem., Int. Ed. Engl.
1996, 35, 2872-2874.

810 Organometallics, Vol. 29, No. 4, 2010
Wallasch et al.
complexes 4 have been discussed in detail.2 According to
theoretical calculations and crystal structure determinations,
these complexes contain a bridging six-membered-ring
ligand belonging to an aryl(chloro)cuprate π-coordinated
to a cyclopentadienyliron moiety. There is, however, some
admixture of cyclohexadienyl-ylidene character (ca. 11%),
corresponding to a small participation of a carbene complex
of copper(I) halide as a second resonance structure.2
ESI mass spectra of 4-Br reveal a number of complexes
of different nuclearity emerging from reactions occurring
during or after the electrospray process. Only singly posi-
tively charged species are observed. In mass spectra of 4-Br
two groups of signals have been observed (m/z 450-550 and
m/z 950-1150, see Figure 4), whereas the molecular cation of
4-Br is not observed.
The first group of peaks can be assigned from their isotope
patterns to fragments containing only one metal, i.e., iron
atom. The signal with the lowest m/z ratio (451.66) corres-
ponds to the cationic sandwich complex [Cp000Fe(C6-
Figure 3. Crystal structure of the σ-aryl complex 3b. Selected
distances/A and angles/deg: Fe1-C1 2.2496(13), Fe1-C2
2.2781(12), Fe1-C3 2.2866(13), Fe1-C4 2.2711(13), Fe1-C5
2.2688(13), Fe1-C60 2.0317(13), Fe-Cp ring plane (C1-C5)
1.915, angle between cyclopentadienyl ring plane and line
Fe1-C60 84.3, angle between two ring planes 89.8. For details
on the structure determination see Table 1.
˚
i
H4 Pr2)]þ, where the copper bromide fragment of the starting
material has been replaced by a proton. Presumably the
copper halide is lost during the spray process, and the
strongly basic ipso carbon atom picks up a proton, e.g., from
impurities present in the 10-6 molar acetonitrile solution or
from the solvent itself. The parent peak at m/z 467.70
corresponds to the cationic phenol complex [Cp000Fe(C6-
3b is a rare example of an iron half-sandwich complex with
pogo stick structure.14 The distance between the iron atom
˚
and the cyclopentadienyl ring plane of 1.915 A is almost
˚
identical with the 1.92 A found for 1a and indicates a high-
i
H3 Pr2OH)]þ, which may have been formed in the presence
of traces of oxygen or water. Along this line the peaks
detected at m/z 483.75 and 499.84 could be tentatively
assigned to higher oxidized species of composition
spin electron configuration at the iron center, as has been
shown by magnetic susceptibility measurements on the
mesityl complex [Cp000Fe(C6H2Me3-2,4,6)]1 and on 1a.3
i
i
[Cp000Fe(C6H3 Pr2O2H)]þ and [Cp000Fe(C6H3 Pr2O3H)]þ,
respectively. However, the isotope patterns do not allow
for an unequivocal interpretation. The last peaks in this
group at m/z 532.07 clearly reveal a bromine isotope pattern
˚
The 2.032 A bond length Fe1-C60 is significantly shorter
than the iron-carbon bonds in the high-spin complex [Fe-
15
(1-naphthyl)4]2- (2.147(7) and 2.104(7) A). This relatively
˚
˚
short bond approaches the bond length of 1.979 A found for
and can thus nicely be fitted with
a composition
i
[Cp000Fe(C6H3 Pr2)HBr]þ (see Supporting Information).
The second group of peaks in the mass spectrum of 4-Br
can be assigned from their isotope patterns to bridged species
containing additional oxygen or halogen atoms and at least
two iron and one or two copper atoms. The peaks at m/z
979.57 can be nicely fitted to a composition [(Cp000)2Fe2Cu-
the diamagnetic cycloheptatrienylidene complex [CpFe-
(CO)2(C7H6)]þ,16 where a substituted tropylium ion or an
iron-carbene system may be discussed. The conformation of
the molecule seen in Figure 3 places one isopropyl substituent
of the aryl group underneath the substitution gap of the
tetraisopropylcyclopentadienyl ligand. The second alkyl is
accommodated between two adjacent isopropyl substituents
of the cyclopentadienyl ligand. The bond Fe-C60 extends at an
angle of 84.3° against the cyclopentadienyl plane. By bending
out of a perpendicular direction steric strain is reduced so much
that the two nearby isopropyl substituents in 2- and 3-position
of the cyclopentadienyl ligand are not even rotated away from
each other, as has been found in other cases like the dinuclear
carbonyl complexes [{4CpM(μ-CO)}2] (M=Co17 or Ni18).
The σ aryl complex 3a was reacted with copper(I) halides
to furnish good yields of the corresponding copper
i
(C6H3 Pr2)2O]þ. Although the chemical structure of this
species is not obvious, the composition points toward a
condensation of two units of 4-Br. This formation of bridged
species is more clearly seen from the analysis of the peaks at
m/z 1063.31 and 1107.22. The former is adequately assigned
by a superposition of two isotope patterns, i.e., [{Cp000Fe-
i
i
(C6H3 Pr2)Cu}2Cl - 4H]þ and [{Cp000Fe(C6H3 Pr2)Cu}2-
Cl]þ, whereas the latter can be perfectly explained by
i
[{Cp000Fe(C6H3 Pr2)Cu}2Br]þ (Supporting Information). We
assume that these species possess halogen-bridged structures
i
complexes [Cp000Fe(η5:η1-C6H3 Pr2-2,6)CuX] (X=Cl, 4-Cl;
i
i
of type [Cp000Fe(C6H3 Pr2)Cu-X-Cu(C6H3 Pr2)FeCp000]þ (X=
Cl, Br), as shown in Figure 5.
X = Br, 4-Br). The corresponding mesityl derivatives of
Conclusions
(14) Siemeling, U.; Vorfeld, U.; Neumann, B.; Stammler, H.-G.
Organometallics 1998, 17, 483.
(15) Bazhenova, T. A.; Lobkovskaya, R. M.; Shibaeva, R. P.;
Shilova, A. K.; Gruselle, M.; Leny, G.; Deschamps, E. J. Organomet.
Chem. 1983, 244, 375–382.
(16) Petz, W. Iron-Carbene Complexes; Springer: Berlin, 1993; p 103.
€
(17) Baumann, F.; Dormann, E.; Ehleiter, Y.; Kaim, W.; Karcher, J.;
Kelemen, M.; Krammer, R.; Saurenz, D.; Stalke, D.; Wachter, C.;
The coupling of two phenyl anions to
a bis-
(cyclohexadienyl-ylidene) ligand observed upon reaction of
bis{tri(tert-butyl)cyclopentadienyliron bromide} (1a) with
phenylmagnesium bromide is unusual for transition metal
compounds and resembles reactions of lanthanide com-
plexes. One reason for such reactivity may be seen in the
unusual high-spin electron configuration of the cyclopenta-
dienyliron starting material. If the σ-aryl complex formed
€
Wolmershauser, G.; Sitzmann, H. J. Organomet. Chem. 1998, 587,
267–283.
€
(18) Sitzmann, H.; Wolmershauser, G. Z. Naturforsch. 1995, 50b,
750–756.

Article
Organometallics, Vol. 29, No. 4, 2010 811
Figure 4. ESI mass spectrum of 4-Br. Groups of peaks in the region m/z 450-550 (black: experiment; colored: simulation of isotope
pattern). Groups of peaks in the region m/z 950-1150 (black: experiment; colored: simulation of isotope pattern). Spectra obtained
with an ion-trap ESI mass spectrometer (see Experimental Section).
with 2,6-diisopropylphenylmagnesium bromide can be taken
as a model compound for the first step in the coupling
reaction, the next steps should be σ/π rearrangement fol-
lowed by dimerization or vice versa. DFT calculations
reproduce the structural parameters of the resulting complex
2 very nicely and show that upon oxidation of the dinuclear
bis(cyclohexadienyl-ylidene) complex 2 the first electron is
taken mainly from the biphenyl ligand and leads to a
pronounced structural reorganization, whereas upon further
oxidation the second electron stems from the cyclopentadie-
nyliron fragments. In our investigation electrospray ioniza-
tion mass spectrometry (ESI-MS) served as a valuable tool
for the characterization of neutral multinuclear metal or-
i
Figure 5. Suggested structure of [{Cp000Fe(C6H3 Pr2)Cu}2Br]þ
(m/z 1107.22). The structural similarity of the species related
to the mass peaks at m/z 1063.31 and 1107.22 is further
supported by the results of MS-MS experiments, leading to
ganic compounds. Their charging occurs simply during the
spraying process, and their composition could be unequi-
vocally identified by analysis of the related isotope patterns.
the same fragment ions (m/z 393.30; 449.41; 531.82) upon
isolation and subsequent collision-induced dissociation with
He.

812 Organometallics, Vol. 29, No. 4, 2010
Wallasch et al.
Moreover, the observation of new aggregates in the elec-
trospray process inspires chemists to search for new synthetic
pathways. For instance, the occurrence of tetranuclear com-
plexes [LCu-X-CuL]þ (L=Cp000FeC6H3R2) in the ESI mass
spectrometer calls for experiments aiming at halide abstrac-
tion from copper halide complexes of the [Cp000Fe(C6H3R2)-
CuX] type by means of Lewis acids in order to synthesize
such cations on a preparative scale.
magnesium the gray-black suspension was filtered through a
medium porosity glass frit, and the filtrate was evaporated to
dryness. The colorless residue was washed two times with
petroleum ether (100 mL each) and dried in an oil-pump vacuum
to yield a colorless powder (32.4 g, (96 mmol, 93%). Anal. Calcd
for the tetrahydrofuran adduct C16H25BrMgO: C, 56.93; H,
7.46. Found: C, 57.16; H, 7.79.
Tri(tert-butyl)cyclopentadienyl(2,6-diisopropylphenyl)iron (3a).
To a green solution of 1a (500 mg, 0.68 mmol) in tetrahydrofur-
an (10 mL) was added a solution of 2,6-diisopropylphenylmag-
nesium bromide bis(diethyl ether) complex (460 mg, 1.36 mmol)
in tetrahydrofuran (5 mL) with stirring at room temperature.
The solution turned from moss green to yellow. Solvent removal
in vacuo was started five minutes after the addition of the
Grignard compound. Two-fold extraction with pentane (2 ꢀ
10 mL), centrifugation, and evaporation to dryness afforded 256
mg (0.56 mmol, 42%) of product, which precipitated as an
initially brownish-orange and crystalline powder and turned
darker and sticky within minutes. Due to the instability of this
complex and its tetraisopropylcyclopentadienyl analogue 3b, we
have been unable to obtain satisfactory elemental analyses for
both of these. The compound was used immediately for the
preparation of the copper complex 4-Cl (see below).
Experimental Section
Synthetic work has been carried out under inert gas in
rigorously dried and deoxygenated solvents using Schlenk-line
techniques for the preparation of 2,6-diisopropylmagnesium
bromide and an argon-filled glovebox from MBraun (Garching)
for preparation and handling of the iron complexes. The cyclo-
pentadienyliron bromide starting compounds 1a1 and 1b13 have
been prepared according to procedures documented in the litera-
ture.
(μ,η5:η5-Dicyclohexadienylidene)bis{tri(tert-butyl)cyclopenta-
dienyliron} (2). To a solution of 1 (300 mg, 0.41 mmol) in
tetrahydrofuran (5 mL) was added a solution of phenylmagne-
sium bromide diethyl ether complex (173 mg, 0.82 mmol) in
tetrahydrofuran (5 mL) with stirring at room temperature. The
solution turned from green to deep blue within 3 h. Solvent
removal in vacuo, extraction with pentane (10 mL), centrifuga-
tion, evaporation to a residual volume of 3 mL, and cooling to
-70 °C afforded 210 mg (0.51 mmol, 63%) of blue-black
crystals. Single crystals suitable for X-ray diffraction were
grown from a toluene solution layered with pentane. Anal.
Calcd for C46H68Fe2: C, 75.27; H, 9.34. Found: C, 74.18; H,
9.33. 1H NMR (400 MHz, 298 K, C6D6): δ 6.29 (1H, para), 4.46
(2H, meta), 3.90 (2H, Cp ring H), 2.87 (2H, ortho), 1.28 (s, 18H,
C(CH3)3). 13C NMR (100 MHz, 298 K, C6D6): δ 105.4 (CdC),
99.9 (2C, ring-C-CMe3), 98.6 (1C, ring-C-CMe3), 78.8 (2C,
meta), 75.7 (1C, para), 70.3 (2C, ring-C-H), 47.2 (2C, ortho),
34.4 (2C, CMe3), 33.8 (6C, C(CH3)3), 32.7 (1C, CMe3), 31.1 (3C,
C(CH3)3).
1-Bromo-2,6-diisopropylbenzene. 2,6-Diisopropylaniline (30 g,
169 mmol) was added to concentrated hydrobromic acid
(48%, 150 mL) with vigorous stirring. The pale yellow suspen-
sion was cooled to -50 °C, and solid sodium nitrite (20 g, 289
mmol) was added with a spatula within an interval of about 10
minutes, accompanied by a color change to brownish. Stirring
was continued for one hour; then diethyl ether precooled to the
same temperature was added, and the solution was allowed to
warm slowly to -15 °C. At -22 °C the evolution of a brown gas
started, which continued for a while at -15 °C. When the gas
evolution slowed, the mixture was cooled to -50 °C again.
Water (20 mL) and sodium carbonate decahydrate (100 g) were
added, and the mixture was allowed to thaw to room tempera-
ture. Gas evolution was observed, and stirring was continued
overnight. The phases were separated, the aequeous phase was
extracted twice with diethyl ether (2 ꢀ 150 mL), and the
combined organic phases were subjected to rotary evaporation
of the diethyl ether solvent. Distillation of the remaining red-
dish-yellow oil at 0.01 mbar yielded the product fraction (29.6 g,
122.7 mmol, 73%) as a colorless liquid at a boiling temperature
of 65 °C. 1H NMR (600 MHz, 298 K, C6D6): δ 7.06 (t, 1H, para,
3JH,H=7.62), 6.96 (d, 2H, meta, 3JH,H=7.62), 3.58 (2H, CHMe2,
3JH,H=6.88), 1.14 (12H, CH3, 3JH,H=6.88). 13C{1H}NMR (150
MHz, 298 K, C6D6): δ 147.8 (1C, ipso-CBr), 127.6 (2C, meta-
CH), 126.7 (1C, para-CH), 124.4 (2C, meta-CH), 33.7 (2C,
CHMe2), 22.9 (4C, CH3).
Tetraisopropylcyclopentadienyl(2,6-diisopropylphenyl)iron (3b).
To a green solution of bis{(μ-bromo)(tetraisopropylcyclopenta-
dienyl)iron(II)} (1b) (500 mg, 0.68 mmol) in tetrahydrofuran
(10 mL) was added a solution of 2,6-diisopropylphenylmagne-
sium bromide tetrahydrofuran complex (460 mg, 1.36 mmol) in
tetrahydrofuran (5 mL) with stirring at room temperature. The
solution turned from moss green to yellow. Solvent removal in
vacuo was started after five minutes of stirring at room tem-
perature. Two-fold extraction with pentane (10 mL each),
centrifugation, and solvent evaporation afforded a yellow mi-
crocrystalline powder (496 mg, 1.10 mmol, 81%), which started
to become brownish and sticky during the workup procedure.
Pale yellow crystals suitable for X-ray diffraction were grown
1
from pentane solution. H NMR (400 MHz, 298 K, C6D6): δ
(Δν1/2) 188.0 (704), 72.2 (2216), 13.7 (577), -19.2 (1524), -28.5
(143), -118.1 (2030). Satisfactory elemental analyses could not
be obtained due to the thermal instability of the complex.
Tri(tert-butyl)cyclopentadienyliron(II)(μ,η5:η1-2,6-diisopropyl-
phenyl)copper(I) Chloride (4-Cl). The starting compound 3a was
prepared immediately before the following procedure was car-
ried out. After the addition of copper(I) chloride (22 mg, 0.22
mmol) to a magnetically stirred tetrahydrofuran solution (5 mL)
of 3a (100 mg, 0.22 mmol) at room temperature the green
solution turned intense red. Stirring was continued for about
30 min; then the solvent was removed in vacuo, and the residual
solid was extracted twice with toluene (2 ꢀ 5 mL). Toluene
removal and washing with pentane left a red, crystalline solid (85
mg, 0.15 mmol, 70%). Anal. Calcd for C29H48ClCuFe: C, 63.38;
H, 8.44. Found: C, 62.48; H, 8.21. 1H NMR (600 MHz, 298 K,
C6D6): δ 5.96 (1H, para), 5.61 (2H, meta), 4.31 (2H, Cp ring H),
3.38 (2H, CHMe2), 1.48 (6H, CH3), 1.45 (s, 9H, C(CH3)3), 1.44
(s, 18H, C(CH3)3), 1.28 (6H, CH3). 13C NMR (150 MHz, 298 K,
C6D6): δ 123.5 (s, 1C, ipso), 120.5 (s, 2C, ortho), 106.6 (s, 1C,
Cp000-C-CMe3), 99.7 (s, 2C, Cp000-C-CMe3), 81.9 (d, 1C, para,
1JC,H=170 Hz), 80.3 (d, 2C, meta, 1JC,H=165 Hz), 68.8 (dd, 2C,
Cp000-CH, 1JC,H=171 Hz, 3JC,H=6.5 Hz), 39.0 (d, 2C, CHMe2,
1JC,H=129 Hz), 33.4 (q, 6C, C(CH3)3, 1JC,H=125 Hz), 32.8 (s,
2C, CMe3), 31.6 (q, 3C, C(CH3)3, 1JC,H=125 Hz), 30.9 (s, 1C,
1
CMe3), 28.2 (q, 2C, CH3, JC,H =126 Hz), 21.6 (q, 2C, CH3,
1JC,H=127 Hz).
Tri(tert-butyl)cyclopentadienyliron(II)(μ,η5:η1-2,6-diisopropyl-
phenyl)copper(I) Bromide (4-Br). Soon after the addition of
copper(I) bromide (32 mg, 0.22 mmol) to a magnetically stirred
solution of 3a (100 mg, 0.22 mmol) at room temperature the pale
yellow solution turned intense red. After 45 min the solvent was
removed in vacuo, and the residual solid was extracted twice with
toluene (2 ꢀ 5 mL). Toluene removal and washing with pentane
2,6-Diisopropylphenylmagnesium Bromide Tetrahydrofuran
Adduct. A mixture of 2-bromo-1,3-diisopropylbenzene (25 g,
104 mmol), magnesium (2.6 g, 107 mmol), tetrahydrofuran
(150 mL), and a trace of iodine was heated to 65 °C oil bath
temperature for 3 h. After almost complete consumption of

Article
Organometallics, Vol. 29, No. 4, 2010 813
left a red, crystalline solid (86 mg, 0.16 mmol, 66%). Anal. Calcd
for C29H48BrCuFe: C, 58.64; H, 7.81. Found: C, 57.56; H, 7.74.
different combinations of basis sets were applied: Either first-
row elements were described by Dunning’s correlation consis-
tent double-ζ plus polarization basis for DFT (cc-pVDZ),10
together with the Wachters-Hay 6-311G basis8,9 for the first-
row transition element Fe, or all atoms, including Fe, were
described by the SDD basis set as previously applied in ref 11.
Charge distributions were taken from natural bond orbital
(NBO) analyses carried out using the NBO 3.1 program20 as
implemented in GAUSSIAN03.
1H NMR (600 MHz, 298 K, C6D6): δ 5.95 (t, 1H, para, 3JH,H
=
5.70 Hz), 5.62 (2H, meta, 3JH,H=5.70 Hz), 4.33 (2H, Cp ring H),
2.87 (“sep”, 2H, CHMe2), 1.49 (d, 6H, CH3, 3JH,H=6.60 Hz),
1.47 (s, 9H, C(CH3)3), 1.44 (s, 18H, C(CH3)3), 1.30 (d, 6H, CH3,
3JH,H=6.54 Hz). 13C NMR (150 MHz, 298 K, C6D6): δ 123.6
(1C, ipso), 120.6 (2C, ortho), 106.7 (1C, para) 99.7 (2C, meta),
81.7 (1C, ring-C-CMe3), 80.2 (2C, ring-C-CMe3), 68.8 (2C, ring-
CH), 39.0 (2C, CHMe2), 33.4 (6C, C(CH3)3), 32.8 (2C, CMe3),
31.6 (3C, C(CH3)3), 30.9 (1C, CMe3), 28.1 (2C, CH3), 21.6 (2C,
CH3).
Acknowledgment. This work was funded by the
Deutsche Forschungsgemeinschaft (DFG grant Si 366/
12-1). H.S. is grateful to Prof. O. J. Scherer for many years
of friendly support.
Electrospray ionization mass spectrometry (ESI-MS) was
performed with two different instruments.
(A) A modified Bruker APEX III ESI-FT-ICR (7 T) mass
spectrometer equipped with a Bruker Apollo I electrospray
source used in the positive ionization mode. Nitrogen was used
as drying gas at a pressure of 10 psi and a temperature of 150 °C.
The solutions were sprayed at a nebulizer pressure of 26 psi, and
the electrospray needle was typically held at 2.0 kV. The instru-
ment was controlled by the Bruker XMASS software.
(B) A Bruker Esquire 3000plus ion trap instrument. The ion
source was used in positive electrospray ionization mode. Scan
speed was 13 000 m/z/s in maximum resolution scan mode (0.3
fwhm/m/z), scan range was 50 to 1500 m/z. All spectra were
accumulated for at least five minutes. Sample solutions in
acetonitrile at concentrations of 10-5 M were filtered through
a PVDF filter (0.45 μm-13 mm) and continuously infused into
the ESI chamber at a flow rate of 4 mL/min using a syringe
pump. Nitrogen was used as drying gas with flow rate of 5.0 L/
min at 300 °C. The solutions were sprayed at a nebulizer
pressure of 10 psi, and the electrospray needle was typically
held at 2.0 kV. The instrument was controlled by Bruker Esquire
Control 5.3 software, and data analysis was performed using
Bruker Data Analysis 3.4 software.
Supporting Information Available: Crystallographic informa-
tion files (CIF) as well as results of the ab initio calculations and
ESI mass spectra. This material is available free of charge via the
Internet at http://pubs.acs.org.
(19) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.;
Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.;
Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.;
Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson,
G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.;
Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai,
H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Adamo,
C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin,
A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma,
K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Akrzewski, V. G.;
Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.;
Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.;
Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.;
Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.;
Keith, T.; M. A. Al-Laham, Peng, C. Y.; Nanayakkara, A.; Challacombe,
M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.;
Pople, J. A. GAUSSIAN 03 (Revision C.01); Gaussian, Inc.: Wallingford,
CT, 2004.
All computations were carried out with the GAUSSIAN03
program19 by applying density functional theory (DFT) to
describe correlation effects, using the B3LYP functional. Two
(20) Glendening, E. D.; Badenhoop, J. K.; Reed, A. E.; Carpenter,
J. E.; Weinhold, F. NBO Version 3.1; Theoretical Chemistry Institute,
University of Wisconsin: Madison, 1995.