Electron-deficient iron alkyl complexes supported by diimine ligand (Ph2CN)2C2H4: Evidence for reversible ethylene binding

Article abstract of DOI:10.1021/om8006773

Reaction of diimine ligands (Ph₂CN)₂C _nH_2n (n = 2, a; n = 3, b) with FeCl₂ or FeBr₂ results in formation of the corresponding high-spin ferrous complexes {(Ph₂CN)₂C₂H₄}FeX ₂ (1a, X = C1; 2a, X = Br) and {(Ph₂CN)₂C ₃H₆}FeCl₂ (1b). Dialkyl {(Ph ₂CN)₂C₂H₄}Fe(CH₂SiMe ₃)₂ (3a) was prepared by treatment of (py) ₂Fe(CH₂SiMe₃)₂ with diimine ligand a. Addition of B(C₆F₅)₃ to 3a at -30 °C resulted in Me₃SiCH₂ abstraction, affording [{(Ph ₂CN)₂C₂H₄)Fe(CH₂SiMe ₃)][Me₃SiCH₂B(C₆F₅) ₃] (5a). ¹⁹FNMR spectroscopy revealed that this compound exists as a contact ion-pair in toluene solution. Compound 5a decomposes at room temperature in bromobenzene-d₅ or toluene-d₈, affording dication [{(Ph₂CN)₂C₂H4}₂Fe] ²⁺ (6a); at elevated temperatures in toluene-d₈ the formation of [{(Ph₂CN)₂C₂H₄} Fe(CH₂SiMe₃)C₆F₅] (7a) was observed as well. Neither ferrous chloride {(Ph₂CN)₂C ₂H₄}FeCl₂ activated with methylaluminoxane nor contact ion-pair 5a is active in the polymerization of ethylene. Instead we were able, for the first time, to observe reversible ethylene binding to a cationic iron alkyl complex.

Full text of DOI:10.1021/om8006773

Organometallics 2009, 28, 209–215
209
Electron-Deficient Iron Alkyl Complexes Supported by Diimine
Ligand (Ph2CN)2C2H4: Evidence for Reversible Ethylene Binding
Jeroen Volbeda, Auke Meetsma, and Marco W. Bouwkamp*
Molecular Inorganic Chemistry Department, Stratingh Institute for Chemistry, UniVersity of Groningen,
Nijenborgh 4, 9747 AG Groningen, The Netherlands
ReceiVed July 17, 2008
Reaction of diimine ligands (Ph2CN)2CnH2n (n ) 2, a; n ) 3, b) with FeCl2 or FeBr2 results in formation
of the corresponding high-spin ferrous complexes {(Ph2CN)2C2H4}FeX2 (1a, X ) Cl; 2a, X ) Br) and
{(Ph2CN)2C3H6}FeCl2 (1b). Dialkyl {(Ph2CN)2C2H4}Fe(CH2SiMe3)2 (3a) was prepared by treatment of
(py)2Fe(CH2SiMe3)2 with diimine ligand a. Addition of B(C6F5)3 to 3a at -30 °C resulted in Me3SiCH2
abstraction, affording [{(Ph2CN)2C2H4}Fe(CH2SiMe3)][Me3SiCH2B(C6F5)3] (5a). 19F NMR spectroscopy
revealed that this compound exists as a contact ion-pair in toluene solution. Compound 5a decomposes
at room temperature in bromobenzene-d5 or toluene-d8, affording dication [{(Ph2CN)2C2H4}2Fe]2+ (6a);
at elevated temperatures in toluene-d8 the formation of [{(Ph2CN)2C2H4}Fe(CH2SiMe3)C6F5] (7a) was
observed as well. Neither ferrous chloride {(Ph2CN)2C2H4}FeCl2 activated with methylaluminoxane nor
contact ion-pair 5a is active in the polymerization of ethylene. Instead we were able, for the first time,
to observe reversible ethylene binding to a cationic iron alkyl complex.
Scheme 1
Introduction
Ever since the initial publications by the groups of Brookhart1
and Gibson,2 in which they independently reported the use of
pyridine-diimine complexes of iron as efficient catalysts for the
polymerization of ethylene, many research groups have prepared
related iron catalysts in order to tune catalyst performance, and
with that the properties of the polymer obtained.3 Electron-
deficient, cationic iron(II) alkyl complexes are often implied as
the active species,4 although zerovalent5 and trivalent6 iron
species have been proposed as well. It has been shown that the
pyridine-diimine iron dialkyl complex (PDI)Fe(CH2SiMe3)2
(PDI ) 2,6-{2,6-i-Pr2C6H3NC(Me)}2C5H3N)7 can be converted
into a monoalkyl cation by treatment with tris(pentafluorophe-
nyl)borane and that the resulting well-defined iron alkyl cation
[(PDI)Fe(CH2SiMe2CH2SiMe3)][MeB(C6F5)3] is indeed active
in the polymerization of ethylene.8 On the other hand, it is
known that treatment of PDI iron complexes with alkyl
aluminum species can result in ligand redistribution reactions;9
thus the exact nature of the active species in the MAO-activated
system remains under debate.
We are thus interested in the reactivity of cationic iron alkyl
complexes supported by neutral imine derived ligands. Our
initial focus has been on the synthesis and reactivity of iron
complexes with nonconjugated diimine ligands (Ph2CN)2CnH2n
(n ) 2, a; n ) 3, b; Scheme 1),10 and the results have been
compared to those previously obtained with the well-known
R-diimine ligand (2,6-i-Pr2C6H3N)2(C2Me2) (c, Scheme 1).11
Included is the synthesis of neutral and cationic iron alkyl
complexes with the C2-bridged diimine ligand a. The alkyl
cation [{(Ph2CN)2C2H4}Fe(CH2SiMe3)]+ is inactive in the
polymerization of olefins, but was found to reversibly bind
ethylene.
* Corresponding author. E-mail: M.W.Bouwkamp@rug.nl.
(1) Small, B. L.; Brookhart, M.; Bennett, A. M. A. J. Am. Chem. Soc.
1998, 120, 4049.
(2) Britovsek, G. J. P.; Gibson, V. C.; Kimberely, B. S.; Maddox, P. J.;
McTavish, S. J.; Solan, G. A.; White, A. J. P.; Williams, D. J. Chem.
Commun. 1998, 849.
(3) For recent reviews on iron-catalyzed olefin polymerization catalysis,
see: Bianchini, C.; Giambastiania, G.; Guerrero Rios, I.; Mantovani, G.;
Meli, A.; Segarra, A. M. Coord. Chem. ReV. 2006, 250, 1391. Gibson, V. C.;
Redshaw, C.; Solan, G. A. Chem. ReV. 2007, 107, 1745.
(4) (a) Deng, L.; Margl, P.; Ziegler, T. J. Am. Chem. Soc. 1999, 121,
6479. (b) Babik, S. T.; Fink, G. J. Mol. Catal. A 2002, 188, 245. (c)
Britovsek, G. J. P.; Gibson, V. C.; Spitzmesser, S. K.; Tellman, K. P.; White,
A. J. P.; Williams, D. J. J. Chem. Soc., Dalton Trans. 2002, 1159. (d)
Bryliakov, K. P.; Semikolenova, N. V.; Zudin, V. N.; Zakharov, V. A.;
Talsi, E. P. Catal. Commun. 2004, 5, 45. (e) Castro, P. M.; Lahtinen, P.;
Axenov, K.; Viidanoja, J.; Kotiaho, T.; Leskela¨, M.; Repo, T. Organome-
tallics 2005, 24, 3664.
(8) Bouwkamp, M. W.; Lobkovsky, E.; Chirik, P. J. J. Am. Chem. Soc.
2005, 127, 9660.
(5) Scott, J.; Gambarotta, S.; Korobkov, I.; Budzelaar, P. H. M.
Organometallics 2005, 24, 6298.
(9) Scott, J.; Gambarotta, S.; Korobkov, I.; Knijnenburg, Q.; De Bruin,
B.; Budzelaar, P. H. M. J. Am. Chem. Soc. 2005, 127, 17204.
(10) (a) Tripathi, S. C.; Srivastava, S. C.; Shrimal, A. K.; Singh, O. P.
Inorg. Chim. Acta 1985, 98, 19. (b) Paz-Sandoval, M. A.; Domínguez-
Durán, M. E.; Pazos-Mayen, C.; Ariza-Castolo, A.; Rosales-Hoz, M. J.;
Contretas, R. J. Organomet. Chem. 1995, 492, 1. (c) Petrovski, Z.; Pillinger,
M.; Valente, A. A.; Gonçalves, I. S.; Hazell, A.; Romão, C. C. J. Mol.
Catal. A: Chem. 2005, 227, 67. (d) Kia, R.; Mirkhani, V.; Kalman, A.;
Deak, A. Polyhedron 2007, 26, 1711.
(6) (a) Britovsek, G. J. P.; Clentsmith, G. K. B.; Gibson, V. C.;
Goodgame, D. M. L.; McTavish, S. J.; Pankhurst, Q. A. Catal. Commun.
2002, 3, 207. (b) Raucoules, R.; De Bruin, T.; Raybaud, P.; Adamo, C.
Organometallics 2008, 27, 3368-3377.
(7) (a) Bouwkamp, M. W.; Bart, S. C.; Hawrelak, E. J.; Trovitch, R. J.;
Lobkovsky, E.; Chirik, P. J. Chem. Commun. 2005, 3406. (b) Scott, J.;
Gambarotta, S.; Korobkov, I.; Budzelaar, P. H. M. J. Am. Chem. Soc. 2005,
´
127, 13019. (c) Ca´mpora, J.; Naz, A. M.; Palma, P.; Alvarez, E.; Reyes,
(11) Bart, S. C.; Hawrelak, E. J.; Schmisseur, A. K.; Lobkovsky, E.;
Chirik, P. J. Organometallics 2004, 23, 237.
M. L. Organometallics 2005, 24, 4878.
10.1021/om8006773 CCC: $40.75
2009 American Chemical Society
Publication on Web 11/26/2008

210 Organometallics, Vol. 28, No. 1, 2009
Volbeda et al.
1
Scheme 2
As expected for high-spin (S ) 2) iron complexes, the H
NMR spectra of the ferrous complexes prepared in this study
reveal paramagnetically shifted resonances. Whereas most of
these are found relatively close to the resonances observed for
the free diimines, two resonances integrating to 4 protons each
show more pronounced shifts. One of these is shifted to low
field (δ 57.5-177.3 ppm) and one to high field (δ -26.9 to
-53.3 ppm). These are tentatively assigned to the o-CH and
m-CH of the phenyl group that is directed toward the iron center.
1
See Figure 2 for a representative H NMR spectrum.
Synthesis of Neutral Iron Alkyl Complexes. Treatment of
1a or 2a with 2 equiv of LiCH2SiMe3 in benzene-d6 solution
resulted in a color change from yellow to brownish-red. 1H NMR
spectroscopy of the reaction mixture revealed the number of
signals expected for a C2V symmetric dialkyl complex {(Ph2CN)2-
C2H4}Fe(CH2SiMe3)2 (3a), taking into account that the protons
for the methylene groups that are directly bound to the
paramagnetic iron center are usually not observed.7,11 Perform-
ing these reactions on a preparative scale did not allow the clean
formation of dialkyl complex 3a. One explanation may be that
alkylation reactions of iron halides with imine-based ligands
can be susceptible to a number of side reactions, such as
reduction of the iron center7a,14 and alkylation of the ligand.7b
Compound 3a, however, could be obtained in good yield (70%)
Results and Discussion
Synthesis of Iron Halide Complexes. Ferrous dichlorides
{(Ph2CN)2C2H4}FeCl2 (1a) and {(Ph2CN)2C3H6}FeCl2 (1b) and
dibromide {(Ph2CN)2C2H4}FeBr2 (2a) were prepared by stirring
THF suspensions of anhydrous FeCl2 or FeBr2 with the
corresponding diimine ligands (Scheme 2). The compounds
could be isolated in good yields (72-90%) by removal of the
solvent and extraction with dichloromethane. After filtering the
extracts over Celite compounds 1a,b and 2a were obtained as
yellow (ligand a) or orange (ligand b) powders by evaporation
of the solvent. The compounds have a high-spin ground state,
similar to R-diimine complexes of this type.11 Both compounds
1a and 2a12 were studied by single-crystal X-ray diffraction
(see Figure 1 for an ORTEP representation of 1a and Table 1
for pertinent bond distances and angles of both structures; the
structure of 2a can be found in the Supporting Information).
Both compounds crystallize in the P21/c space group with two
independent molecules in the asymmetric unit that have similar
metrical parameters, and only one of each will be discussed
here. The geometry around the iron center in complexes 1a and
2a can be best described as a distorted tetrahedron, with
N(11)-Fe(1)-N(12) angles of 81.88(9)° and 81.7(3)° and
X-Fe(1)-X angles of 126.09(3)° and 121.39(7)°, respectively.
Both compounds show a strong puckering in the Fe(1)-N(11)-
C(114)-C(115)-N(12) ring, with one of the two carbon atoms
of the C2-spacer deviating from the ligand plane. As a result,
one of the imine moieties is pushed out of the Fe(1)-N(11)-
N(12) plane (Figure 1). The (Cipso)2CNCH2 moieties are virtually
planar, resulting in two sets of distinct phenyl groups: one that
is directed toward the metal center and one that is pointing away.
The metrical parameters of 1a are similar to the corresponding
ferrous chloride with an R-diimine ligand, {(2,6-i-Pr2C6H3N)2-
(C2Me2)}FeCl2 (1c),13 with slightly larger Fe-N and Fe-Cl
bond distances for the first (average Fe-N: 2.130(4) Å for 1a
and 2.1016(14) for 1c; average Fe-Cl: 2.2588(13) Å for 1a
and 2.2222(10) Å for 1c) and a larger bite angle (81.88(9)° for
1a and 76.50(5)° for 1c).
7c
by treatment of the iron-dialkyl reagent (py)2Fe(CH2SiMe3)2
with 1 equiv of the ligand. When the reaction was performed
on an NMR tube scale and the volatiles of the reaction mixture
were transferred into a tube with a known amount of ferrocene
internal standard, it was found (by integration) that only 1 equiv
of pyridine was released during the reaction, suggesting the
formation of the monopyridine adduct {(Ph2CN)2C2H4}-
Fe(CH2SiMe3)2(py) (3a · py). In the initial reaction mixture no
free pyridine was observed, suggesting a fast (on the NMR time
scale) exchange between free and bound pyridine. Recrystalli-
zation of 3a · py from pentane at -30 °C afforded single crystals
of the pyridine-free compound {(Ph2CN)2C2H4}Fe(CH2SiMe3)2
(3a). The geometry of 3a (see Figure 1 for an ORTEP
representation of 3a and Table 1 for pertinent bond distances
and angles) is very similar to that of complexes 1a and 2a, with
slightly longer Fe-N bond distances. The Fe-N and Fe-C
bond distances in 3a are slightly elongated compared with those
found in {(2,6-i-Pr2C6H3N)2(C2Me2)}Fe(CH2SiMe3)2 (3c).11 As
expected, iron dialkyl 3a has a high-spin ground state (µeff
4.9 µB).
)
Treatment of 1b with LiCH2SiMe3 in benzene-d6 in an NMR
tube resulted in a reddish-brown, intractable mixture of products.
Furthermore, dissolving (py)2Fe(CH2SiMe3)2 and the C3-bridged
diimine ligand (b) in benzene-d6 did not result in formation of
the corresponding dialkyl complex. Instead, a mixture of the
starting materials was observed by 1H NMR spectroscopy, even
after shaking the NMR tube for 3 days at room temperature.
Synthesis of Cationic Iron Alkyl Complexes. THF-solvated
cation [{(Ph2CN)2C2H4}Fe(CH2SiMe3)(THF)][BPh4] (4a′) was
prepared in reasonable yield (53%) by reaction of a 1:1 mixture
of dialkyl 3a and Brønsted acid [PhNMe2H][BPh4] in THF. The
compound was isolated as yellow crystals by diffusion of
pentane into a THF solution of the compound. X-ray analysis
confirms its formulation, and an ORTEP representation is
depicted in Figure 3 (see Table 1 for pertinent bond distances
and angles). The geometry of the compound is similar to the
other ferrous complexes described in this study, with slightly
(12) A small amount of crystals of compound 2a was obtained during
the decomposition of compound 5a in bromobenzene-d5. These crystals
were used for a single-crystal X-ray analysis.
(13) Kervern, G.; Pintacuda, G.; Zhang, Y.; Oldfield, E.; Roukoss, C.;
Kuntz, E.; Herdtweck, E.; Basset, J. M.; Cadars, S.; Lesage, A.; Cope´ret,
C.; Emsley, L. J. Am. Chem. Soc. 2006, 128, 13545.
(14) (a) Scott, J.; Gambarotta, S.; Korobkov, I.; Budzelaar, P. H. M.
Organometallics 2005, 24, 6298. (b) Ferna´ndez, I.; Trovitch, R. J.;
Lobkovsky, E.; Chirik, P. J. Organometallics 2008, 27, 109.

Electron-Deficient Iron Alkyl Complexes
Organometallics, Vol. 28, No. 1, 2009 211
Figure 1. ORTEP representations at the 30% probability level of 1a and 3a. Hydrogen atoms are omitted for clarity.
Table 1. Pertinent Bond Distances and Angles for 1a, 2a, 3a, 4a′, 6a, and 7a
1a
2a
3a
4a′
6a
7a
Fe(1)-N(11)a
Fe(1)-N(12)b
Fe(1)-Xc
2.149(3)
2.112(2)
2.2431(9)
2.2744(9)
1.296(3)
1.291(4)
2,114(8)
2.129(8)
2.443(2)
2.374(2)
1.308(13)
1.295(12)
2.214(3)
2.176(3)
2.078(4)
2.098(4)
1.287(5)
1.285(5)
2.120(3)
2.154(3)
2.055(2)
2.024(4)
1.301(5)
1.292(5)
2.123(6)
2.095(6)
2.073(6)
2.085(6)
1.306(9)
1.300(9)
1.301(11)
1.304(11)
84.5(2)
2.186(5)
2.149(5)
2.132(7)
2.077(6)
1.293(7)
1.292(7)
Fe(1)-Yd
C(11)-N(11)a e
,
C(116)-N(12)b f
C(135)-N(13)
C(150)-N(14)
,
N(11)-Fe(1)-N(12)a b
81.88(9)
126.09(3)
81.7(3)
121.39(7)
80.09(11)
131.66(16)
81.52(11)
137.06(12)
80.34(17)
124.1(2)
,
X-Fe(1)-Yc d
83.5(2)
,
a Fe and N(1) for 3a and 7a. b Fe and N(2) for 3a and 7a. c X ) Cl11 (1a); Br11 (2a); C(29) (3a); O(11) (4a′); N(13) (6a); C(29) (7a). d Y ) Cl12
(1a); Br12 (2a); C(29) (3a); C(129) (4a′); N(14) (6a); C(35) (7a). e C(1) for 3a and 7a; C(17) for 6a. f C(16) for 3a and 7a; C(122) for 6a.
contracted metal-ligand bond distances, a result of the increased
(C2Me2)}Fe(CH2SiMe3)C6F5 (7c) was obtained, the result of a
fast C6F5 transfer in the initially generated ion-pair (Vide infra).11
The 19F NMR spectrum of 5a in toluene-d8 (at a concentration
of 30.2 mM) revealed three resonances at δ -98.5 (m-F),
-118.1 (o-F), and -152.3 (p-F) ppm for the pentafluorophenyl
groups, indicative of a coordinated borate anion.15,16 Interest-
ingly, the chemical shifts are sensitive to the concentration of
the ion-pair, with the signals shifting to lower field upon dilution
(Figure 4). In general a concentration dependency could suggest
the presence of an equilibrium between a solvent-separated and
a contact ion-pair, though in that case a shift in the opposite
direction is expected. The concentration dependency observed
here may be the result of the formation of higher aggregates at
higher concentrations, which will in turn affect the strength of
the ion-pairing.17 Addition of a drop of THF to a diluted toluene
solution of 5a (9.19 mM) results in a shift of the three fluorine
resonances to δ -124.8 (o-F), 162.2 (m-F), and 162.5 (p-F)
ppm. This suggests that there is still an interaction between the
ions in THF adduct 4a in toluene, although this interaction is
much weaker compared to that in the contact ion-pair 5a. When
dissolving compound 5a in THF-d8, resonances are observed
at δ -129.7 (o-F), -164.7 (p-F), and -166.8 (m-F) ppm, as
expected for a noncoordinating anion.18
electron deficiency of the metal center.
A base-free iron alkyl complex (5a) was prepared by addition
of a cold (-30 °C) toluene-d8 solution of B(C6F5)3 to a cooled
solution of 3a in toluene-d8. This resulted in a color change
from purple to yellow. After replacing the solvent with THF-d8
1
a species was observed with a similar H NMR spectrum to
that of 4a′, suggesting the initial formation of ion-pair
[{(Ph2CN)2C2H4}Fe(CH2SiMe3)][Me3SiCH2B(C6F5)3] (5a, Scheme
3). The ESI-MS of a solution of 4a in THF revealed a peak at
m/z ) 603.3 and 531.1 g/mol (calcd for the cation of 4a, 603.3;
for the cation of 5a, 531.2) in the positive-ion spectrum and at
m/z ) 599.1 (calcd for [Me3SiCH2B(C6F5)3], 599.1) in the
negative-ion spectrum. Hence, similar to the R-diimine case,
the coordination sphere of the iron center in 3a is sufficiently
open to allow abstraction of a Me3SiCH2 group,11 and there
is no indication for Me3SiCH2-methyl group abstraction.8 It
should be noted that, in the case of the R-diimine complex,
the iron alkyl cation [{(2,6-i-Pr2C6H3N)2(C2Me2)}Fe(CH2Si-
Me3)]+ could not be observed. Instead, {(2,6-i-Pr2C6H3N)2-
During the synthesis of the contact ion-pair (5a) at room
temperature in toluene-d8 or bromobenzene-d5, a secondary iron
species (6a) was observed as well. In bromobenzene-d5 species
6a was observed exclusively after standing for 24 h at room
temperature. The compound was characterized as [{(Ph2CN)2-
(C2H4)}2Fe][Me3SiCH2B(C6F5)3]2 (6a) by single-crystal X-ray
analysis (see Figure 3 for an ORTEP representation and Table
1 for selected bond distances and angles).19 Iron dication 6a
can be prepared purposely by treating dialkyl complex 3a with
(15) (a) Sciarone, T. J. J.; Meetsma, A.; Hessen, B.; Teuben, J. H. Chem.
Commun. 2002, 1580. (b) Sciarone, T. J. J.; Meetsma, A.; Hessen, B. Inorg.
Chim. Acta 2006, 359, 1815. (c) Sciarone, T. J. J.; Nijhuis, C. A.; Meetsma,
A.; Hessen, B. Organometallics 2008, 27, 2058.
1
Figure 2. H NMR spectrum of 1a. The numbers represent the
relative integrations; * marks the CDCl3 solvent peak.

212 Organometallics, Vol. 28, No. 1, 2009
Volbeda et al.
Figure 3. ORTEP representations at the 30% probability level of 4a′, 6a, and 7a. Hydrogen atoms and counterions are omitted for clarity.
Scheme 3
2 equiv of B(C6F5)3 in the presence of an additional equivalent
of the ligand in toluene solution (Scheme 3). A closer inspection
of the structure of 6a reveals that, in contrast to the other
structures described in this paper, the iron center no longer
adopts a distorted tetrahedral geometry. Instead its geometry is
in between tetrahedral and square planar, with an angle between
the N(11)-Fe(1)-N(12) and N(13)-Fe(1)-N(14) planes of
48.8(3)°. A similar ligand redistribution reaction was observed
in the neutral ferrous amidinate complex [FeCl]2{µ-(2,6-i-
Pr2C6H3NC{Ph}NC2H4NMe2)}15c and in the attempted synthesis
of a cationic amidinate complexes of the type [{PhC(N-2,6-i-
Pr2C6H3)2}Fe(CH2SiMe3)]+.20 The fate of the side product in
the disproportionation reaction observed here is as yet unknown,
though analysis of the reaction mixture by GC-MS reveals the
formation of (Me3SiCH2)2, suggesting concomitant formation
of [Fe(CH2SiMe3)2].
Thermolysis of 5a in toluene (2 days, 50 °C) resulted in
formation of a mixture of compound 6a, which precipitates as
a yellow solid, and a new iron species, {(Ph2CN)2C2H4}Fe-
(CH2SiMe3)C6F5 (7a). Compound 7a is highly soluble in
aromatic and aliphatic solvents and could be isolated by
(16) Lee, H.; Hong, S.-D.; Park, Y.-W.; Jeong, B.-G.; Nam, D.-W.; Jung,
H. Y.; Jung, M. W.; Song, K. H. J. Organomet. Chem. 2004, 689, 3402.
(17) (a) Beck, S.; Lieber, S.; Schaper, F.; Geyer, A.; Brintzinger, H.-H.
J. Am. Chem. Soc. 2001, 123, 1483. (b) Stahl, N. G.; Zuccaccia, C.; Jensen,
T. R.; Marks, T. J. J. Am. Chem. Soc. 2003, 125, 5256. (c) Zuccaccia, C.;
Stahl, N. G.; Macchioni, A.; Chen, M.-C.; Roberts, J. A.; Marks, T. J. J. Am.
Chem. Soc. 2004, 126, 1448.
(18) Horton, A. D.; De With, J.; Van der Linden, A. J.; Van de Weg,
H. Organometallics 1996, 15, 2672.
(19) Single crystals were also obtained from a thermolysis experiment
(50 °C) of 5a in toluene. This afforded a similar structure, but with
cyclohexane rather than bromobenzene cocrystallized in the lattice. The
cif file of the crystals structure determination can be found in the Supporting
Information.
(20) Sciarone, T. J. J.; Nijhuis, C. A.; Meetsma, A.; Hessen, B. Dalton
Trans. 2006, 4896.
Figure 4. From bottom up: 19F NMR spectra of the contact ion-
pair 5a in toluene-d8 at 30.2, 19.4, 14.3, 11.4, and 9.19 mM,
respectively, and of the THF adduct 4a in toluene-d8.

Electron-Deficient Iron Alkyl Complexes
Organometallics, Vol. 28, No. 1, 2009 213
Scheme 4
extraction with pentane. Cooling of the pentane solution to -30
°C afforded reddish-orange crystals suitable for X-ray analysis
(see Figure 3 for an ORTEP representation of the molecule and
Table 1 for pertinent bond distances and angles). Its structure
is very similar to that of dialkyl 3a, with shorter metal-nitrogen
bond distances, a result of the increased electrophilicity of the
metal center imparted by the electron-withdrawing pentafluo-
rophenyl group. Again, a comparison of bond distances to the
R-diimine analogue {(2,6-i-Pr2C6H3N)2(C2Me2)}Fe(CH2SiMe3)-
C6F5 (7c) shows longer iron-nitrogen bond distances in 7a.11
Performing the reaction in an NMR tube shows the expected
formation of Me3SiCH2B(C6F5)2, which could be identified using
19F NMR spectroscopy. The 19F NMR spectrum of 7a revealed
two resonances at 162 and -7.3 ppm for the pentafluorophenyl
group attached to the iron center. One signal, presumably the
o-F resonance, could not be observed.
Reactivity toward Ethylene. Well-defined iron alkyl cation
5a was treated with ethylene, but no polymerization of the olefin
was observed. Likewise, treatment of ferrous chloride 1a with
MAO (MAO ) methylaluminoxane) in toluene in the presence
of ethylene (5 bar, 30 min) did not result in ethylene uptake,
and no formation of a substantial amount of polyethylene was
observed. Following the reaction of 5a with ethylene by NMR
spectroscopy, however, revealed that compound 5a is not
unreactive toward the olefin (Scheme 4). After the addition of
1 equiv of ethylene to a 30.2 mM toluene-d8 solution of 5a, the
1H NMR spectrum of the reaction mixture reveals a broad signal
at 14.5 ppm (∆ν1/2 ) 306 Hz) for the olefin (Figure 5). This
suggests formation of an ethylene adduct, in which the
coordinated molecule of ethylene is in fast exchange, relative
to the NMR time scale, with free ethylene in solution. As
expected, the signal shifts upfield upon increasing the ethylene
concentration. The 19F NMR spectrum of the reaction mixture
corroborates such an equilibrium, as an increase of the ethylene
pressure results in an upfield shift of the fluorine resonances
toward the values expected for a ligand-separated ion-pair
(Figure 5). Unfortunately, during the course of the reaction the
formation of a yellowish-brown oil was observed, frustrating
quantification of the equilibria involved. The yellowish oil was
identified as [{(Ph2CN)2(C2H4)}2Fe][Me3SiCH2B(C6F5)3]2.
Conclusions
Diimines (Ph2CN)2CnH2n (n ) 2, 3) are suitable ligands for
the stabilization of electron-deficient iron complexes, and in case
of the ligand with the C2 bridge, both neutral and cationic iron
alkyl complexes were prepared. Ligand binding is more labile
compared to the R-diimine ligand framework, as the cationic
ironalkylcomplex[{(Ph2CN)2C2H4}Fe(CH2SiMe3)][Me3SiCH2B-
(C6F5)3] rearranges to dication [{(Ph2CN)2C2H4}2Fe]2+, the result
of a ligand redistribution reaction. In toluene, at 50 °C a
secondary thermolysis product was observed as well, which was
identified as {(Ph2CN)2C2H4}Fe(CH2SiMe3)C6F5. Whereas we
were able, for the first time, to observe an ethylene adduct of
an iron alkyl cation, the compound is not an active catalyst for
the polymerization of olefins. This suggests that the barrier for
ethylene insertion is the determining factor in this system.
Unfortunately, a comparison with the R-diimine system is not
possible, as in that case pentafluorophenyl abstraction from the
borate anion is instantaneous and an iron alkyl cation cannot
be observed.
1
Figure 5. From top to bottom: 19F NMR (left) and H NMR (right) spectra of the reaction mixture containing compound 5a and 1.1, 2.1,
4.2, 6.4, 9.5, and 13 equiv of ethylene in toluene-d8, respectively.

214 Organometallics, Vol. 28, No. 1, 2009
Volbeda et al.
(Ph2CN)2C3H6 (b). A three-necked flask equipped with a reflux
condenser was charged with benzophenone (4.07 g; 22.3 mmol),
diaminopropane (0.92 mL; 11 mmol), a catalytic amount of p-
toluenesulfonic acid, and xylenes (60 mL). The reaction mixture was
warmed to reflux and stirred overnight, affording a yellow solution.
Removal of the solvent by rotary evaporation afforded a yellow oil,
which, upon addition of pentane and overnight cooling to -30 °C,
turned solid. The pentane was decanted, and the off-white solid was
dried in Vacuo. Recrystallization from chloroform/pentane afforded
Experimental Section
General Considerations. All manipulations of air- and moisture-
sensitive compounds were performed under a nitrogen atmosphere
using standard Schlenk and vacuum line techniques or in an MBraun
glovebox. Solvents (THF, pentane, toluene) were dried by percola-
tion under a nitrogen atmosphere over columns of alumina,
molecular sieves, and supported copper oxygen scavenger (BASF
R3-11) or by distillation from Na/K alloy (cyclohexane, benzene-
d6, THF-d8) or CaH2 (chloroform-d, bromobenzene-d5, dichlo-
romethane). Reagents were purchased from commercial providers
and used without purification unless stated otherwise. Anhydrous
FeCl2,21 (Ph2CN)2C2H4,22 (py)2Fe(CH2SiMe3)2,7c LiCH2SiMe3,23
1
2.45 g (6.08 mmol; 56%) of the title compound. H NMR (CDCl3,
RT): δ 7.12-7.53 (m, 20H, Ph), 3.45 (t, 4H, 6.85 Hz, CH2), 2.04 (q,
2H, 6.89 Hz, CH2). 13C NMR (CDCl3, RT): δ 140.9, 138.0 (2 × Ph
Cipso), 130.7, 129.4, 129.3, 129.2, 128.9, 128.8 (6 × Ph CH), 52.8
(CH2), 33.8 (CH2). Anal. Calcd for C29H26N2: C, 86.53; H, 6.51; N,
6.96. Found: C, 86.18; H, 6.48; N, 6.95.
25
[PhNMe2H][BPh4],24 and B(C6F5)3 were prepared following
literature procedures. NMR spectra were recorded on Varian Inova
500, Varian Gemini VXR 400, Varian VXR 300, and Varian
Gemini 200 instruments. 1H chemical shifts are referenced to
residual protons in deuterated solvents and are reported relative to
tetramethylsilane; 19F chemical shifts are reported relative to R,R,R-
trifluorotoluene (δ ) -65 ppm), used as an external reference. For
paramagnetic molecules, the chemical shifts are followed by the
peak width at half-height in hertz, followed by integration value
and, where possible, peak assignment. 1H NMR spectra of
paramagnetic complexes were recorded with a sweep width of
100 000 Hz, delay of 0 s, and acquisition time of 200 ms. Magnetic
moments are determined by the Evans method.26 Elemental analyses
were performed by the Microanalytical Department at the University
of Groningen. Reported values are the averages of two independent
determinations. The electrospray ionization mass spectrometry (ESI-
MS) experiments were conducted on an API III (PE SCIEX) triple
quadrupole MS system with an IonSpray (pneumatically assisted
electrospray) source equipped with a gas curtain, which is contained
in a closed chamber that can be evacuated, flushed, and maintained
under nitrogen. Typical sample analysis: in a glovebox, the sample
was taken up into a 500 mL syringe (model 1750 RNR, Hamilton)
and electrosprayed via a syringe pump operating at 10 mL/min.
The capillary voltage was 3.5 kV. Mass spectra were recorded from
m/z 50 to 900 (example) at 10 s per scan using a step size of 0.1
Da. The sampling orifice (nozzle) was at +35 V. The skimmer
was located behind the sampling. Crystals suitable for a single-
crystal X-ray analysis were grown as described in the text. A crystal
was mounted on a glass fiber inside a drybox and transferred under
an inert atmosphere to the cold nitrogen stream of a Bruker SMART
APEX CCD diffractometer. Intensity data were corrected for
Lorentz and polarization effects, scale variation, decay, and
absorption: a multiscan absorption correction was applied, based
on the intensities of symmetry-related reflections measured at
{(Ph2CN)2C2H4}FeCl2 (1a). THF (20 mL) was added to a
mixture of a (1.55 g; 3.99 mmol) and FeCl2 (0.514 g; 4.06 mmol).
The resulting suspension was stirred overnight. The solvent was
removed in Vacuo and a yellow powder was obtained. The
compound was extracted using dichloromethane. The extractions
were filtered through a pad of Celite, affording a yellow solution.
The solvent was removed in Vacuo, affording 1.85 g (3.59 mmol,
90%) of the title compound as a yellow solid. Layering a
dichloromethane solution of the solid with pentane resulted in
crystals suitable for X-ray analysis. 1H NMR (CDCl3, RT): δ 69.6
(4H, 579 Hz), 13.1 (4H, 24 Hz), 5.3 (4H, 73 Hz), 2.9 (2H, 23 Hz),
1.8 (4H, 66 Hz), -0.5 (2H, 34 Hz), -26.9 (4H, 601 Hz). Anal.
Calcd for C28H24Cl2FeN2: C, 65.27; H, 4.70; N, 5.44. Found: C,
64.64; H, 4.67; N, 5.25. µeff ) 5.3 µB.
{(Ph2CN)2C3H6}FeCl2 (1b). Compound 1b was prepared analo-
gous to 1a, by treating b (1.47 g; 3.79 mmol) with FeCl2 (0.480 g;
3.78 mmol) in THF (20 mL). This afforded 1.51 g (2.85 mmol;
1
75%) of 1b. H NMR (CDCl3, RT): δ 126.0 (4H, 916 Hz), 35.6
(4H, 1009 Hz), 10.4 (4H, 35 Hz), 9.5 (2H, 51 Hz), 7.2 (4H, 44
Hz), 4.8 (2H, 32 Hz), 3.9 (2H, 411 Hz), -0.1 (4H, 98 Hz). A sample
for analysis was recrystallized from CH2Cl2/cyclohexane. Anal.
Calcd for C29H26Cl2FeN2 · CH2Cl2: C, 58.66; H, 4.60; N, 4.56.
Found: C, 58.42; H, 4.59; N, 4.42.
{(Ph2CN)2C2H4}FeBr2 (2a). Compound 2a was prepared analo-
gous to 1a, by treating FeBr2 (0.179 g; 0.830 mmol) with a (0.324
g; 0.833 mmol) in THF (10 mL). This afforded 0.36 g (0.60 mmol;
1
72%) of 2a. H NMR (bromobenzene-d5, RT): δ 70.1 (4H, 663
Hz), 14.1 (4H, 28 Hz), 5.8 (4H, 85 Hz), 2.6 (2H, 29 Hz), 1.7 (4H,
75 Hz), -2.4 (2H, 40 Hz), -36.1 (4H, 413 Hz). Anal. Calcd for
C28H24Br2FeN2: C, 55.66; H, 4.00; N, 4.64. Found: C, 55.43; H,
3.96; N, 4.52.
Generation of {(Ph2CN)2C2H4}Fe(CH2SiMe3)2 from 1a. Ad-
dition of a solution of LiCH2SiMe3 (9.5 mg; 0.10 mmol) in THF
(0.5 mL) to 1a (26.7 mg; 0.0518 mmol) resulted in a color change
to purple. After 1 h the volatiles were pumped off. Pentane was
added and removed in Vacuo to remove residual THF. Analysis of
the reaction mixture by 1H NMR spectroscopy in benzene-d6
revealed the formation of a mixture of products, one of which was
identified as 3a (see below).
different angular settings (SADABS),27 and reduced to F0 . The
2
structures were solved by Patterson methods, and extension of the
model was accomplished by direct methods applied to difference
structure factors using the program DIRDIF.28 The positional and
anisotropic displacement parameters for the non-hydrogen atoms
were refined. Final refinement on F2 was carried out by full-matrix
least-squares techniques.
Generation of {(Ph2CN)2C2H4}Fe(CH2SiMe3)2 from 2a.
LiCH2SiMe3 (6.6 mg; 0.070 mmol) in benzene-d6 (0.4 mL) was
added to an NMR tube with 2a (21.4 mg; 0.0354 mmol). The
resulting purple reaction mixture was analyzed by 1H NMR
spectroscopy and revealed the formation of 3a (see below).
{(Ph2CN)2C2H4}Fe(CH2SiMe3)2(py) (3a · py). In an NMR tube
(py)2Fe(CH2SiMe3)2 (10.7 mg; 0.0275 mmol) and a (10.7 mg;
0.0275 mmol) were dissolved in benzene-d6 (0.4 mL), resulting in
(21) Kovacic, P.; Brace, N. O. Inorg. Synth. 1960, 6, 172.
(22) Ariza-Castolo, A.; Paz-Sandoval, M. A.; Contreras, R. Magn. Reson.
Chem. 1992, 30, 520.
(23) Lewis, H. L.; Brown, T. L. J. Am. Chem. Soc. 1970, 92, 4664.
(24) Eshuis, J. J. W.; Tan, Y. Y.; Meetsma, A.; Teuben, J. H.; Renkema,
J.; Evens, G. G. Organometallics 1992, 11, 362.
(25) Pohlmann, J. L. W.; Brinckmann, F. E. Z. Naturforsch. B 1965,
20, 5.
(26) Sur, S. K. J. Magn. Reson. 1989, 82, 169.
(27) Sheldrick, G. M. SADABS v2, Multi-Scan Absorption Correction
Program; University of Go¨ttingen: Germany, 2001.
(28) Beurskens, P. T.; Beurskens, G.; De Gelder, R.; Garcı´a-Granda,
S.; Gould, R. O.; Israe¨l, R.; Smits, J. M. M. The DIRDIF-99 Program
System; Crystallography Laboratory, University of Nijmegen: The Neth-
erlands, 1999.
1
a purple solution. H NMR spectroscopy indicated the formation
of 3a · py. The volatiles were transferred to a second NMR tube
charged with 4.9 mg (0.026 mmol) of ferrocene internal standard.
1H NMR analysis thus revealed that during the reaction 1 equiv of
free pyridine was generated. 1H NMR of 3a · py (C6D6, RT): δ 57.8

Electron-Deficient Iron Alkyl Complexes
Organometallics, Vol. 28, No. 1, 2009 215
(4H, 13.6 Hz), 27.6 (2H, 804 Hz), 16.6 (1H, 242 Hz), 15.1 (2H,
217 Hz), 12.7 (22H, overlapping signals), 3.7-1.3 (6H, overlapping
signals), -3.2 (2H, 51 Hz), -53.3 (4H, 1403 Hz). Two signals of
4H were not observed.
44 Hz), 165.1 (2F, 55 Hz). Anal. Calcd for C106H82BF15FeN4Si2: C,
60.18; H, 3.91; N, 2.65. Found: C, 59.57; H, 3.70; N, 2.52. µeff ) 5.8
µB. The brown solution that was obtained from the reaction above
was combined with the portions of toluene that were used to wash
compound 6a. The solvent was removed in Vacuo, and the resulting
black oil was extracted with pentane to yield a yellow solution. Cooling
{(Ph2CN)2C2H4}Fe(CH2SiMe3)2 (3a). Toluene (5 mL) was added
to a mixture of (py)2Fe(CH2SiMe3)2 (75.5 mg; 0.193 mmol) and a
(64.0 mg; 0.164 mmol), resulting in a purple solution. The reaction
mixture was stirred for 2 h, after which the solvent was removed
in Vacuo. To the resulting purple oil was added toluene (5 mL),
which was pumped off to remove residual pyridine. The solid was
dissolved in pentane and filtered through a pad of Celite. Concen-
tration of the solution and cooling at -30 °C resulted in 71.5 mg
1
to -30 °C resulted in a small amount of orange crystals of 7a. H
NMR (C6D6, RT): δ 111.2 (4H, 442 Hz), 15.3 (4H, 23 Hz), 7.1
(overlapping with solvent peak, 9H, SiMe3), 6.7 (4H, 56 Hz), 3.2 (2H,
23 Hz), 1.6 (2H, 26 Hz), -3.3 (4H, 43 Hz), -32.0 (4H, 372 Hz). 19
F
NMR (C6D6, RT): δ 162.6 (1F, p-F, 144.7 Hz), -7.3 (2F, m-F, 59.1
Hz), one peak not found. When performing the thermolysis in toluene-
d8 and monitoring the reaction mixture by 19F NMR spectroscopy,
the formation of Me3SiCH2B(C6F5)2 was observed.
1
(0.116 mmol, 70%) of purple crystals of the title compound. H
NMR (C6D6, RT): δ 57.5 (4H, 601 Hz), 13 (22H, two overlapping
signals), 2.9 (4H, 97 Hz), 2.0 (4H, 110 Hz), 0.9 (2H, 30 Hz), -3.2
(2H, 50 Hz), -53.3 (4H, 901 Hz). One signal of 4H was not
observed. Anal. Calcd for C36H46FeN2Si2: C, 69.88; H, 7.49; N,
4.53. Found: C, 68.35; H, 7.44; N, 4.39. µeff ) 4.9 µB.
Decomposition of 5a in Bromobenzene-d5. Compound 5a was
generated in an NMR tube as described above. After checking that
compound 5a had been generated cleanly, the toluene was pumped
off and bromobenzene-d5 was added to the reaction mixture. Within
10 min after the addition of bromobenzene-d5 the formation of
compound 6a was observed. The reaction was found to be complete
when the reaction mixture was left overnight at room temperature.
[{(Ph2CN)2C2H4}2Fe][Me3SiCH2B(C6F5)3]2 (6a). Toluene (20
mL) was added to a mixture of 3a (84.3 mg; 0.136 mol), a (53.0
mg; 0.136 mmol), and B(C6F5)3 (140 mg; 0.273 mmol). The color
changed to orange and a precipitate formed. The solid was washed
with toluene and recrystallized from CH2Cl2/cyclohexane, affording
103 mg (0.0497 mmol; 37%) of 6a.
[{(Ph2CN)2C2H4}Fe(CH2SiMe3)THF][B(C6H5)4] (4a′). THF (1
mL) was added to a mixture of 3a (32.9 mg; 0.0532 mmol) and
[PhNMe2H][BPh4] (23.6 mg; 0.0573 mmol). The resulting yellow-
orange solution was cooled to -30 °C for 30 min and subsequently
layered with pentane. After storing overnight at -30 °C orange
crystals had formed. The supernatant was decanted, affording 26.1
1
mg (0.028 mmol; 53% yield) of orange crystals of 4a′. H NMR
(THF-d8; RT): δ 72.3 (4H, 833 Hz), 21.3 (9H, 170 Hz, SiMe3),
13.2 (4H, 24 Hz), 10.0 (8H, 26 Hz, BPh4), 8.2 (8H, 27 Hz, BPh4),
7.6 (4H, 25 Hz, p-BPh4), 3.8 (2H, 67 Hz), 1.68 (4H, overlaps with
THF) -1.9 (2H, 33 Hz), -8.8 (4H, 61 Hz), -36.4 (4H, 726 Hz).
One signal of 2H was not observed. Anal. Calcd for C36H46FeN2Si2:
C, 78.08; H, 6.88; N, 3.04. Found: C, 77.40; H, 7.08; N, 2.69. µeff
) 5.2 µB.
Generation of [{(Ph2CN)2C2H4}Fe(CH2SiMe3)][Me3SiCH2B-
(C6H5)3] (5a). A solution of 3a (5.9 mg; 0.0095 mmol) in toluene-
d8 and a solution of B(C6F5)3 (4.9 mg; 0.0096 mmol) in toluene-d8
were cooled to -30 °C. The B(C6F5)3 solution was slowly added
to the solution containing 3a, affording an orange solution of 5a.
1H NMR (toluene-d8, RT): δ 177.3 (4H, 857 Hz), 17.0 (4H, 56
Hz), 11.8 (9H, 228 Hz, SiMe3), 4.6 (2H, 25 Hz), 2.3 (2H, 53 Hz),
1.1 (2H, 43 Hz), -1.68 (9H, 40 Hz), -26.3 (4H, 101 Hz), -45.2
(4H, 977 Hz). One signal of 2H was not observed. 19F NMR (30.2
mM in toluene-d8, RT): -99.1 (6F, 2202 Hz, m-F), -118.7 (6F,
969 Hz, o-F), -152.0 (3F, 318 Hz, p-F).
Reaction of 5a with C2H4. Two toluene-d8 solutions (total
amount of solvent: 0.45 g), one with B(C6F5)3 (5.3 mg; 0.010 mmol)
and the other with 3a (6.4 mg; 0.010 mmol), were cooled to -30
°C. The borane solution was added slowly to the solution of 3a in
toluene-d8, resulting in a color change to yellowish-orange. The
solution was degassed using three freeze-pump-thaw cycles, and
ethylene was added using a calibrated gas bulb (9.4 mL, 98 mmHg;
0.049 mmol). 1H and 19F NMR spectroscopy revealed spectra
similar to that of 5a, with the exception that a peak was observed
1
at 12.5 (∆ν1/2 ) 178 Hz) in the H NMR spectrum for a time-
average signal for bound and free ethylene and an upfield shift of
the resonances in the 19F NMR spectrum. In the course of the
experiment a yellow precipitate starts forming, which was identified
as dication 6a by evaporating the volatiles and dissolution of the
residue in THF-d8.
Attempted Polymerization of Ethylene Using 1a/MAO. The
polymerization experiment was performed in a temperature- and
pressure-controlled stainless steel 1 L autoclave (Medimex). The
autoclave was evacuated for 1 h at 125 °C prior to use. The reactor
was cooled to 30 °C, filled with toluene (250 mL), and pressurized
to 5 bar of ethylene. While stirring at 600 rpm, 1.33 mg of a 5 wt
% solution of PMAO in toluene (5 mmol) and 2.6 mg (0.0050
mmol) of 1a were injected. During the run the ethylene pressure
was kept constant to within 0.2 bar of the initial pressure by
replenishing flow. After 30 min reaction time the reactor was vented
and the residual aluminum alkyls were destroyed by addition of
100 mL of ethanol. No substantial amount of polymer was observed.
Generation of [{(Ph2CN)2C2H4}Fe(CH2SiMe3)(THF)][Me3-
SiCH2B(C6H5)3] (4a). A solution of 5a was prepared as described
above. The solvent was removed in Vacuo, and the residue was
dissolved in THF-d8. The product was identified by 1H NMR
spectroscopy as the title compound. 1H NMR (THF-d8, RT): δ 72.9
(4H, 701 Hz), 21.8 (9H, 177 Hz, FeCH2SiMe3), 13.6 (4H, 46 Hz),
4.1 (2H, 90 Hz), 0.72 (2H, 16 Hz, BCH2SiMe3), -0.3 (9H, 9 Hz,
BCH2SiMe3), -2.5 (2H, 54 Hz), -9.1 (4H, 76 Hz), -38.1 (4H,
593 Hz). Two signals could not be observed. 19F NMR (THF-d6,
RT): δ -129.7 (d, 22 Hz, 6F, o-F), -164.7 (ps.t., 20 Hz, 3F, p-F),
-166.8 (ps.t., 21 Hz, 6F, m-F). ESI-MS (35 eV, THF): neg-ion
mode, m/z 599.1 [Me3SiCH2B(C6F5)3]-; pos-ion mode, m/z 603.3
[{(Ph2CN)2C2H4}Fe(CH2SiMe3)(THF)]+, 531.1 [{(Ph2CN)2C2H4}-
Fe(CH2SiMe3)]+.
Acknowledgment. This investigation was supported by The
Netherlands Organization for Scientific Research (NWO). Prof.
Dr. B. Hessen (University of Groningen) is gratefully acknowl-
edged for the use of facilities and Dr. S. Bambirra (University
of Groningen) for conducting the ESI-MS measurement.
Decomposition of 5a in Toluene. A flask containing 3a (0.106
g; 0.171 mmol), B(C6F5)3 (0.087 g; 0.17 mmol), and toluene (5 mL)
was stirred for 2 days at 50 °C, resulting in a brown solution and a
yellow precipitate. The solution was filtered off and the solid was
washed with toluene. The yellow solid was recrystallized from CH2Cl2/
cyclohexane, affording 72.4 mg (0.0356 mmol; 21%) of yellow crystals
Supporting Information Available: Crystallographic data for
compounds 1a, 2a, 3a, 4a′, 6a, and 7a. This material is available
free of charge via the Internet at http://pubs.acs.org.
1
of dication 6a. H NMR (CDCl3; RT): δ 136.5 (4H, 587 Hz), 16.7
(4H, 20 Hz), 11.3 (4H, 60 Hz), 4.3 (2H, 20 Hz), 2.0 (2H, 19 Hz), 1.1
(2H, 25 Hz), 0.3 (9H, SiMe3, 12 Hz), -1.6 (4H, 46 Hz), -31.6 (4H,
467 Hz). 19F NMR (CDCl3; RT): δ -128.0 (2F, 47 Hz), -163.7 (1F,
OM8006773