Neutral and cationic paramagnetic amino-amidinate iron(II) complexes: 19F NMR evidence for interactions with weakly coordinating anions

Article abstract of DOI:10.1021/om701155b

Two aikylarnirio-benzamidinates, [RNC(Ph)NCH₂CH ₂NMe₂]^- (^ArL, R = 2,6-iPr ₂C₆H₃; ^SiL, R = SiMe₃), have been employed as ligands for Fe(II). The ligands adopt bridging coordination modes in their diiron(II) dichloro complexes [(μ- ^RL)FeCl]₂ (1). Their structure and reactivity are strongly dependent on the nature of R. With CO, [(μ-^Ar)FeCl]₂ (1a) forms the monomelic, 18-electron dicarbonyl compound [κ³- C(O)(2,6-iPr₂C₆H₃)NC(Ph)NCH₂CH ₂NMe₂]FeCl(CO)₂ (3), in which CO has inserted into the Fe-N(amidinate) bond, whereas [(μ-^SiL)FeCl]₂ (1b) is unreactive under similar conditions. Alkylation of 1a was unsuccessful, leading to ligand redistribution and isolation of (κ²- ^ArL)₂Fe (2), in which the ligands adopt a κ²-amidoamino coordination mode, but 1b reacts with KCH2Ph to give the 14-electron diiron dibenzyl derivative [(μ-^SiL)FeCH ₂Ph]₂ (4). Benzyl abstraction by B(C₆F ₅)₃, [Ph₃C][B(C₆F₅) ₄], or [PhNHMe₂][B(C₆F₅) ₄] affords the paramagnetic monocation [(μ-^SiL) ₂Fe₂CH₂Ph]⁺ (5⁺) partnered with [PhCH₂B(C₆F₅)₃] ^-" or [B(C₆F₅)₄]^-, respectively. Interactions of these weakly coordinating anions with the paramagnetic Fe(II) centers in solution are apparent from significantly shifted and broadened resonances in the ¹⁹F NMR spectra.

Full text of DOI:10.1021/om701155b

2058
Organometallics 2008, 27, 2058–2065
Neutral and Cationic Paramagnetic Amino-amidinate Iron(II)
19
Complexes: F NMR Evidence for Interactions with Weakly
Coordinating Anions
Timo J. J. Sciarone,†,‡ Christian A. Nijhuis,† Auke Meetsma,† and Bart Hessen*,†,‡
Center for Catalytic Olefin Polymerization, Stratingh Institute for Chemistry and Chemical Engineering,
UniVersity of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands, and Dutch Polymer
Institute (DPI), P.O. Box 906, 5600 AX EindhoVen, The Netherlands
ReceiVed NoVember 15, 2007
Two alkylamino-benzamidinates, [RNC(Ph)NCH2CH2NMe2]- (ArL, R ) 2,6-iPr2C6H3; SiL, R ) SiMe3),
have been employed as ligands for Fe(II). The ligands adopt bridging coordination modes in their diiron(II)
dichloro complexes [(µ-RL)FeCl]2 (1). Their structure and reactivity are strongly dependent on the nature
of R. With CO, [(µ-ArL)FeCl]2 (1a) forms the monomeric, 18-electron dicarbonyl compound [κ3-C(O)(2,6-
iPr2C6H3)NC(Ph)NCH2CH2NMe2]FeCl(CO)2 (3), in which CO has inserted into the Fe-N(amidinate)
bond, whereas [(µ-SiL)FeCl]2 (1b) is unreactive under similar conditions. Alkylation of 1a was unsuccessful,
leading to ligand redistribution and isolation of (κ2-ArL)2Fe (2), in which the ligands adopt a κ2-amido-
amino coordination mode, but 1b reacts with KCH2Ph to give the 14-electron diiron dibenzyl derivative
[(µ-SiL)FeCH2Ph]2 (4). Benzyl abstraction by B(C6F5)3, [Ph3C][B(C6F5)4], or [PhNHMe2][B(C6F5)4] affords
the paramagnetic monocation [(µ-SiL)2Fe2CH2Ph]+ (5+) partnered with [PhCH2B(C6F5)3]- or [B(C6F5)4]-,
respectively. Interactions of these weakly coordinating anions with the paramagnetic Fe(II) centers in
solution are apparent from significantly shifted and broadened resonances in the 19F NMR spectra.
Chart 1
Introduction
Iron(II) complexes supported by neutral pyridine-2,6-diimine
ligands have emerged as highly active precatalysts for olefin
polymerization when activated with methylaluminoxane (MAO)
cocatalyst.1 Electronically unsaturated, 14 valence electron (14
VE), cationic iron alkyls have been identified as competent
catalytically active species in this transformation.2 In cationic
transition metal alkyl catalysts, the metal center is influenced
by the coordinative properties of the counterion. This can
influence, to a significant extent, activity, stability, chain transfer
characteristics, and stereoselectivity of the catalytic system.3 In
non-MAO-activated catalysts, perfluorinated arylborates, such
as [B(C6F5)4]-, are often employed as counterions, since they
are among the most weakly coordinating anions available.
Monoanionic, sterically hindered ꢀ-diketiminate ligands have
been utilized to stabilize electron-deficient (12 VE), neutral
Fe(II) monoalkyls.4,5 Although these neutral, paramagnetic
systems are inactive for olefin polymerization, we observed that
the derived cations allowed spectroscopic observations of
cation-anion interactions in solution involving weakly coor-
dinating fluorinated arylborate anions.5 The paramagnetic cation
acts as a chemical shift reagent, causing significant effects on
the 19F NMR spectra of the associated anions.
* Corresponding author. E-mail: B.Hessen@rug.nl.
† University of Groningen.
A more open coordination space around the metal center is
expected to enhance the extent of cation-anion interactions.
Since amidinates, [R′C(NR)2]-, are isoelectronic with ꢀ-diketim-
inates, but have considerably smaller N-M-N bite angles in
their transition metal complexes, these compounds ususally
feature a more accessible metal center. Whereas 12 VE Fe(II)
monoalkyls [(N^N)FeR] (N^N ) monoanionic κ2 ligand) are
readily isolated for the ꢀ-diketiminate ligand (A, Chart 1), those
supported by the smaller amidinate ligand were found to
be unstable with respect to ligand redistributions forming the
bis(amidinate) complex [(N^N)2Fe]. The mono(amidinate)
complexes could be stabilized, however, through coordination
of an additional Lewis base L, thus allowing for isolation of an
‡ DPI.
(1) (a) Britovsek, G. J. P.; Gibson, V. C.; Kimberley, B. S.; Maddox,
P. J.; McTavish, S. J.; Solan, G. A.; White, A. J. P.; Williams, D. J. Chem.
Commun. 1998, 849. (b) Small, B. L.; Brookhart, M.; Bennett, A. M. A.
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P. A.; Mu¨nck, E. J. Am. Chem. Soc. 2002, 124, 3012. (b) Smith, J. M.;
Lachicotte, R. J.; Holland, P. L. Organometallics 2002, 21, 4808. (c) Vela,
J.; Smith, J. M.; Lachicotte, R. J.; Holland, P. L. Chem. Commun. 2002,
2886. (d) Vela, J.; Vaddadi, S.; Cundari, T. R.; Smith, J. M.; Gregory, E. A.;
Lachicotte, R. J.; Flaschenriem, C. J.; Holland, P. L. Organometallics 2004,
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Commun. 2002, 1580. (b) Sciarone, T. J. J.; Meetsma, A.; Hessen, B. H.
Inorg. Chim. Acta 2006, 359, 1815.
10.1021/om701155b CCC: $40.75
2008 American Chemical Society
Publication on Web 04/11/2008

Paramagnetic Amino-amidinate Iron(II) Complexes
Organometallics, Vol. 27, No. 9, 2008 2059
Scheme 1
Fe(II) alkyl of the type [(N^N)FeR(L)] (B, Chart 1).6 Enhanced
stability is therefore expected for Fe(II) monoalkyls chelated
by amidinate ligands functionalized with a pendant Lewis base
(C, Chart 1). Amidinates functionalized with amine or pyridine
functionalities on one of the nitrogen atoms have been reported
as ancillary ligands for various transition metals, lanthanides,
and main group elements.7–11
Figure 1. Crystal structure of 1a. Thermal ellipsoids are drawn at
50% probability. Hydrogen atoms have been omitted for clarity.
Selected interatomic bond distances (Å) and angles (deg): Fe-Fe_a
2.9242(6), Fe-N1 2.090(2), Fe-N2 2.136(2), Fe-N3 2.160(2),
Fe-N2_a 2.390(2), Cl-Fe-N1 106.84, Cl-Fe-N2 136.40(7),
Cl-Fe-N3101.92(6),Cl-Fe-N2_a91.12(5),N1-Fe-N2115.19(9),
N1-Fe-N2_a 60.02(8), N1-Fe-N3 105.10(9), N2-Fe-N3
(78.46(8), N2_a-Fe-N2 99.71(8), N2_a-Fe-N3 162.94(8).
Here we describe the chemistry of two dimethylaminoethyl-
substituted benzamidinates, [RNC(Ph)NCH2CH2NMe2]- (ArL,
R ) Ar, and SiL, R ) SiMe3) with Fe(II), including the isolation
of a 14 VE diiron dibenzyl complex. Subsequent benzyl
abstraction affords ion pairs, which provide an opportunity to
observe very weak cation-anion interactions between a para-
magnetic diiron cation and (perfluoroaryl)borate anions by
means of 19F NMR spectroscopy.
Results and Discussion
Synthesis and Characterization of Amidinate-amine Iron-
(II) Chlorides. The lithium amidinates Li[RL] react with 1 equiv
of FeCl2 in THF solvent to afford the paramagnetic diiron
complexes [(µ-ArL)FeCl]2 (1a) and [(µ-SiL)FeCl]2 (1b) in 40%
and 64% isolated yields, respectively (Scheme 1). Complex 1a
is soluble in THF and toluene, while 1b is sparingly soluble in
THF or dichloromethane.
The molecular structures of both diiron complexes were
determined by X-ray diffraction (Figures 1 and 2). Dimer 1a is
lying on a crystallographic inversion center, whereas 1b has
crystallographic C2-symmetry.
In both complexes, the iron centers are bridged by the
amidinate part of the ligand, and the dimethylamino donor forms
a five-membered chelate ring including the amidinate nitrogen
atom carrying this functionality. The amidinate nitrogen atoms
Figure 2. Crystal structure of 1b. Thermal ellipsoids are drawn at
50% probability. Hydrogen atoms have been omitted for clarity.
Selected interatomic bond distances (Å) and angles (deg): Fe-Fe_a
3.0792(10), Fe-N1 2.193(4), Fe-N2 2.061(3), Fe-N3 2.024(3),
Cl-Fe-N1 105.18(10), Cl-Fe-N2 101.84(10), Cl-Fe-N3
122.32(8), N1-Fe-N2 80.94(12), N1-Fe-N3 106.38(11), N2-
Fe-N3 129.85(12).
) 3 or 4), showing Fe-Fe bonds in the range 2.23-2.46 Å,12,13
the Fe-Fe separations in dimers 1 (1a: 2.9242(6) Å, 1b:
3.0792(10) Å) do not indicate the presence of a direct Fe-Fe
interaction. Despite the overall similarities, the different R
carrying the R-subsituents are essentially planar (1a: ∑N1
)
358.58°, 1b: ∑N3 ) 359.95°). Whereas bridging amidinates often
form so-called lantern complexes (e.g., [µ-HC(NPh)2]nFe2, n
(6) Sciarone, T. J. J.; Nijhuis, C. A.; Meetsma, A.; Hessen, B. Dalton
Trans. 2006, 4896.
(7) Brandsma, M. J. R.; Brussee, E. A. C.; Meetsma, A.; Hessen, B.;
Teuben, J. H. Eur. J. Inorg. Chem. 1998, 1867.
(8) Doyle, D.; Gun’ko, Y. K.; Hitchcock, P. B.; Lappert, M. F. J. Chem.
Soc., Dalton Trans. 2000, 4093.
(9) (a) Bambirra, S.; Brandsma, M. J. R.; Brussee, E. A. C.; Meetsma,
A.; Hessen, B.; Teuben, J. H. Organometallics 2000, 19, 3197. (b) Bambirra,
S.; Otten, E.; van Leusen, D.; Meetsma, A.; Hessen, B. Z. Anorg. Allg.
Chem. 2006, 632, 1950.
(12) (a) Cotton, F. A.; Daniels, L. M.; Falvello, L. R.; Matonic, J. H.;
Murillo, C. A. Inorg. Chim. Acta 1997, 256, 269. (b) Cotton, F. A.; Daniels,
L. M.; Falvello, L. R.; Murillo, C. A. Inorg. Chim. Acta 1994, 219, 7.
(13) Cotton, F. A.; Daniels, L. M.; Matonic, J. H.; Murillo, C. A. Inorg.
Chim. Acta 1997, 256, 277.
(10) Kincaid, K.; Gerlach, C. P.; Giesbrecht, G. R.; Hagadorn, J. R.;
Whitener, G. D.; Shafir, A.; Arnold, J. Organometallics 1999, 18, 5360.
(11) Boyd, C. L.; Guiducci, A. E.; Dubberley, S. R.; Tyrrell, B. R.;
Mountford, P. J. Chem. Soc., Dalton Trans. 2002, 4175.

2060 Organometallics, Vol. 27, No. 9, 2008
Sciarone et al.
1
Figure 3. H NMR spectrum of 1b (500 MHz, CD2Cl2, RT). Asterisk (inset) indicates CDHCl2.
Scheme 2
substituents in the ligands result in some significant differences
in molecular geometry for the dinuclear complexes. In 1b each
amidinate nitrogen is bound to only one iron atom, whereas in
1a the amidinate nitrogen atom bearing the dimethylaminoethyl
substituent (N2) is bridging the iron centers, with one shorter
(2.136(2) Å) and one longer (2.390(2) Å) Fe-N bond. In 1a
the two bridging nitrogen atoms and the iron atoms thus form
a planar Fe2N2 diamond motif. A bonding situation similar to
that in 1a was found in the dinuclear aminoamidinate lithium
complex {[µ-Me3SiNC(Ph)N(CH2)3NMe2]Li}2.8 For Fe(II), a
related structure is present in the dimeric bis(amidinate) complex
{[κ2-PhC(NPh)2][µ-PhC(NPh)2]Fe}2.13 While the amidinate
NCN planes of the two ligands in 1a have parallel orientations,
those in 1b are inclined at 82.3(5)°. A final significant structural
difference between the dimers 1a and 1b is that in 1a the
chlorides display a trans-arrangement relative to the Fe2(µ-
amidinate)2 unit, whereas in 1b they display a cis-arrangement.
The cis-orientation found in 1b is likely to result in a higher
dipole moment, which is reflected in the low solubility of this
compound in apolar solvents.
protons is not observed, presumably due to their proximity to
the paramagnetic Fe(II) centers.
Thermal Stability. The two dinuclear amidinate-amine iron
complexes 1a and 1b show a significant difference in thermal
stability. Whereas the Me3Si-substituted 1b is stable in solution
at ambient temperature for days, monitoring solutions of 1a in
C6D6 or PhMe-d8 by H NMR spectroscopy shows that the
complex is fully converted into a new paramagnetic species
within one day at room temperature.
1
It turned out that this thermolysis product is the mononuclear
2:1 complex (ArL)2Fe (2), which was also prepared directly by
the reaction of FeCl2 with 2 equiv of Li[ArL] in THF (Scheme
1
2). Complex 2 was characterized by microanalysis, H NMR
Dimers 1 are paramagnetic with room-temperature magnetic
moments of 5.3 µB (1a) and 5.0 µB (1b) per iron ion, indicating
a high-spin (S ) 2) configuration for Fe(II). The high-spin nature
and IR spectroscopy, and single-crystal X-ray diffraction.
The molecular structure of 2 (Figure 4) shows that the amino-
amidinate ligands are bound in a κ2-amino-amido fashion,
leaving the arylimine moiety uncoordinated. Consistently, the
1
of the chloro complexes is also evident from their H NMR
IR spectrum shows a strong CdN absorption at 1578 cm-1
.
spectra. The spectrum of 1a in C6D6 displays nine resonances
in a window from δ 110 to -40 ppm with linewidths ranging
from 30 to 4500 Hz. Complex 1b shows 10 signals from 350
to 0 ppm with linewidths (∆ν1/2) ranging from 47 to 9600 Hz
in CD2Cl2. For both complexes, some resonances are too broad
to be observed, and extreme broadness of some of the observable
peaks makes their integration unreliable. This precludes unam-
biguous assignment of the resonances for 1a, but the spectrum
is nevertheless useful as a fingerprint. For the spectrum of 1b
(Figure 3) however, some tentative assignments could be made.
Apparently, decoordination of the imine is more favorable than
that of the pendant amine, possibly due to reduction of steric
hindrance around the metal center. The iron center is coordinated
in a distorted tetrahedral fashion, with the N-Fe-N coordina-
tion planes inclined at 85.42(8)°. The average N-Fe-N bite
angle of the amino-amido ligands is 82°.
Complex 1b decomposes upon heating at 50 °C for several
days (or 24 h at 80 °C), but the identity of the decomposition
product(s) could not be established. The 1H NMR spectrum of
the mixture shows an intense signal at δ 0.00 ppm, suggesting
loss of the SiMe3 group.
Reaction with CO. In spite of the formal electron deficiency
of complexes 1 (14 VE per Fe), their high-spin nature leaves
no empty valence orbitals available. Thus, complexes 1 are
unreactive toward simple Lewis bases such as THF or PMe3.
Nevertheless, complex 1a does react with the typical π-acceptor
ligand CO. Exposure of 1a to excess CO (1 atm) in toluene
solution cleanly affords the 18 VE carbonyl derivative [κ3-
C(O)(2,6-iPr2C6H3)NC(Ph)NCH2CH2NMe2]FeCl(CO)2 (3), which
was isolated in 81% yield (Scheme 3). The identity of 3 was
established by X-ray diffraction.
The resonance at 5.1 ppm is assigned to the protons of the
silyl groups (18H). The appearance of two 6H resonances at
115.3 and 68.5 ppm for the NCH3 protons suggests coordination
of the amino groups. Amino coordination also makes the four
methylene protons of the ligand diastereotopic. Two pairs of
resonances (2H each) from these protons are found at δ 327.0
(∆ν1/2 ) 2130 Hz), 175.1 (1820 Hz), 110.5 (990 Hz), and 42.4
(790 Hz) ppm. The meta protons of the phenyl group are
observed as two signals at 35.1 and 18.1 ppm (both 490 Hz),
each integrating for 2H, suggesting that rotation of this group
is slow on the NMR time scale. Finally, the para protons
resonate at 2.2 ppm (47 Hz). A resonance for the phenyl ortho

Paramagnetic Amino-amidinate Iron(II) Complexes
Organometallics, Vol. 27, No. 9, 2008 2061
Figure 4. Xray structure of 2. Thermal ellipsoids are drawn at 50%
probability. Selected interatomic bond distances (Å) and angles
(deg): Fe-N1 2.1861(14), Fe-N2 2.0018(13), Fe-N4 2.1905(15),
Fe-N5 1.9924(13), N2-C5 1.357(2), N3-C5 1.300(2), N5-C28
1.348(2), N6-C28 1.303(2), N1-Fe-N2 82.75(5), N1-Fe-N4
105.96(5), N1-Fe-N5 127.31(6), N2-Fe-N4 118.55(6), N2-
Fe-N5 140.22(6), N4-Fe-N5 81.18(5).
Figure 5. X-ray structure of 3. Thermal ellipsoids are drawn at
50% probability. Hydrogens and cocrystallized THF molecules have
been omitted for clarity. Selected interatomic distances (Å) and
angles (deg): Fe-C24 1.942(3), Fe-C25 1.790(3), Fe-C26
1.845(3), Fe-N1 2.146(3), Fe-N2 1.935(2), C24-N3 1.453(3),
N3-C5 1.393(3), N2-C5 1.283(3).
Scheme 3
Scheme 4
Complex 3 results from the insertion of a CO molecule into
one of the amidinate-N-Fe bonds, which converts the amidinate
into a carbamoyl functionality, and saturation of the Fe(II) center
with two CO ligands. Related CO insertions have been reported
in the solid state. The spectra give evidence of hindered rotation
of the 2,6-iPr2C6H3 ring about the Cipso-N bond. Coordination
of the amino arm in solution is indicated by the presence of
two resonances for the NMe groups. The three carbonyl
functions show resonances at δ 208.1, 207.9, and 207.5 ppm
(absolute assignments were not made).
6
for [PhC(NAr)2]FeCl(µ-Cl)Li(THF)3 and [FcC(NCy)2]CpFe-
(CO) (Fc ) ferrocenyl).14 In contrast to 1a, complex 1b does
not react with CO under identical conditions. Bis(aminoamidi-
nate) complex 2 also does not react with CO under the applied
conditions, whereas the homoleptic amidinate complexes
[tBuC(NR)2]2Fe (R ) Cy, iPr) react with CO only to give the
bis(carbonyl) adducts [tBuC(NR)2]2Fe(CO)2 without subsequent
CO insertion.15 This is also the case for the dinuclear amidinate
complex {[µ-1,2-[NC(Ph)NSiMe3]2-c-C6H10]Fe}2.16
Alkylation. Alkylation of chloro complex 1a was investigated
using LiMe, LiCH2SiMe3, LiCH(SiMe3)2, KCH2Ph, and
PhCH2MgBr in THF at -80 °C. Nevertheless, in all cases, dark
brown to black reaction mixtures were formed, and pentane
extraction resulted only in the isolation (in 20-50% yield based
on Fe) of (ArL)2Fe (2), as identified by 1H NMR. GC-MS
analysis of the reaction mixture indicated the formation of 1,2-
diphenylethane when KCH2Ph was employed as the alkylating
agent. These observations suggest that an initially formed benzyl
complex decomposes via ligand redistribution to form 2 and
“Fe(CH2Ph)2”. In the absence of strong monodentate17 or
chelating bidentate18 donor ligands, the latter is unstable with
respect to Fe(0) and PhCH2CH2Ph. Similar decomposition
products were found previously upon attempted synthesis of
Fe(II) monoalkyls supported by the nonfunctionalized amidinate
ligand [PhC(NAr)2]-.6
The X-ray structure of 3 (Figure 5) shows a pseudo-
octahedrally coordinated iron center, with the amino-carbamoyl
ligand occupying a meridional coordination mode.
The difference in Fe-C bond length (0.05 Å) for the terminal
carbonyls reflects the stronger trans influence of the amide
versus the chloride ligand. The carbamoyl chelate ring is almost
planar, as indicated by the sums of the angles around the
nonmetal atoms, which all approach 360° (highest deviation )
0.5° for N3), suggesting significant conjugation within the
chelate. Nevertheless, the C5-N distances differ by about 0.11
Å, indicating some extent of localization.
The IR spectrum of 3 is consistent with the solid state
structure, showing absorptions at 2037 and 1949 cm-1 for the
terminal carbonyls, while the carbamoyl carbonyl absorbs at
1669 cm-1. Complex 3 is diamagnetic, and shows 1H and
13C{H} NMR spectra consistent with the C1-symmetry observed
Alkylation of 1b using LiMe or LiCH2SiMe3 in THF at -40
°C also led to decomposition to give black reaction mixtures.
With PhCH2MgBr, a red solution was obtained initially, but
subsequent workup still resulted in decomposition with forma-
tion of PhCH2CH2Ph. Nevertheless, the reaction of 1b with
(14) Hagadorn, J. R.; Arnold, J. J. Organomet. Chem. 2001, 637-639,
521.
´
(15) Vendemiati, B.; Prini, G.; Meetsma, A.; Hessen, B.; Teuben, J. H.;
Traverso, O. Eur. J. Inorg. Chem. 2001, 707.
(17) Ca´mpora, J.; Naz, A. M.; Palma, P.; Alvarez, E.; Reyes, M. L.
Organometallics 2005, 24, 4878.
(16) Kawaguchi, H.; Matsuo, T. Chem. Commun. 2002, 958.
(18) Hermes, A. R.; Girolami, G. S. Organometallics 1987, 6, 763.

2062 Organometallics, Vol. 27, No. 9, 2008
Sciarone et al.
benzyl groups in tetrahedral, high-spin Fe(II) dibenzyl com-
pounds (L2)Fe(CH2Ph)2 (L ) Lewis base) exhibit comparable
chemical shifts.19 The benzyl R-protons of 4 are not observed
in the spectrum, presumably due to extreme line broadening
resulting from their proximity to the paramagnetic iron centers.
Cation-Anion Interactions. Abstraction of one of the benzyl
ligands of 4 by Lewis or Brønsted acids in C6D5Br solution
generates the monocation [(µ-SiL)2Fe2CH2Ph]+ (5+) (Scheme
5). These reactions are accompanied by a color change from
yellow to dark red. The paramagnetism of 5+ in combination
with its low symmetry results in rather uninformative 1H NMR
spectra consisting of many broad and in part overlapping
resonances. 19F NMR spectroscopy proved to be more useful
for study of the ion pairs.
Abstraction of the benzyl ligand by B(C6F5)3 was confirmed
by the presence of the [PhCH2B(C6F5)3]- ion (m/z ) -603) as
the parent ion in the negative-ion ES mass spectrum. The 19F
NMR spectum of ion pair [5][PhCH2B(C6F5)3] (Figure 7a)
shows three broad lines with 2:2:1 integral ratios. Remarkably,
the p-F resonance is found upfield from the m-F resonance, an
observation made previously with paramagnetic ꢀ-diketiminate
Fe(II) cations.4b The spectral data suggest that [5][PhCH2B-
(C6F5)3] in bromobenzene solution is at least partly present as
a contact ion pair in which 5+ acts as a paramagnetic shift
reagent for the anion. In the absence of an X-ray structure for
the ion pair, the bonding mode for the [PhCH2B(C6F5)3] anion
cannot be established, but π-coordination of the phenyl moiety
is well-precedented.3,20
Addition of excess Lewis base (THF, pyridine) induces a
color change from dark red to yellow and converts the 19F NMR
spectrum into three sharp resonances (Figure 7b), with shifts
characteristic for the free [PhCH2B(C6F5)3]- ion.21 Evidently,
the Lewis base displaces the borate anion from the Fe(II) center,
disrupting the interaction between the paramagnetic cation and
the anion.
Figure 6. X-ray structure of 4 (dominant conformer shown).
Thermal ellipsoids are drawn at 50% probability. Selected inter-
atomic distances (Å) and angles (deg): Fe-Fe_a 3.0189(6),
Fe-C152 2.087(6), Fe-N1 2.235(2), Fe-N2 2.099(2), Fe-N3
2.055(2), C152-Fe-N1 114.88(14), C152-Fe-N2 96.14(13),
C152-Fe-N3 120.50(14), N1-Fe-N2 79.23(8), N1-Fe-N3
102.35(8), N2-Fe-N3 136.71(9).
KCH2Ph in THF at -40 °C afforded the diiron dibenzyl
complex [(µ-SiL)FeCH2Ph]2 (4), which was isolated in 43% yield
as red crystals after recrystallization from toluene (Scheme 4).
The successful isolation of 4 from the reaction of 1b with
KCH2Ph shows that problems in alkylation of 1b with orga-
nolithium and Grignard reagents are not due to intrinsic
instability of 4. Possibly, the Li+ and Mg2+ cations introduced
by these reagents compete with Fe(II) for the aminoamidinate
ligands, leading to ligand redistribution. Although direct evi-
dence for the formation of Li or Mg aminoamidinate complexes
not was obtained in this case, encapsulation of Li by the amino
functions of two SiL ligands has been observed in the yttriates
Li[(µ-SiL)2Y(CH2R)2] (R ) SiMe3, Ph).9a
The crystal structure of 4 (Figure 6) resembles that of the
parent chloride 1b. The cis-orientation of the chlorides in the
starting material is translated into a cis-orientation of the benzyl
groups in 4. The iron carbon bond length (2.087(6) Å) in 4 is
only slightly shorter than that in the bis(p-tolyl) complex
(dippe)Fe(CH2C6H4Me)2 (dippe ) 1,2-(diisopropylphosphino)-
ethane) (Fe-C ) 2.120(6) Å).18
The benzyl methylene hydrogens could not be located in
X-ray structure due to disorder of the benzyl fragments, which
were refined in two orientations (see Experimental Section).
Nevertheless, the Fe-CH2-Cipso angles (114.4(3)° and 112.4(3)°)
in these conformations rule out secondary interactions between
the benzyl groups and the iron centers. This is consistent with
the absence of empty valence orbitals on the four-coordinate,
14 VE electron iron centers, given the high-spin (S ) 2) spin
state (Vide infra).
It is not clear whether contact ion pair [5][PhCH2B(C6F5)3]
in C6D5Br is in equilibrium with a solvent-separated ion pair.
The 19F NMR spectrum -35 °C still features only three signals,
indicating that, if there is reversible association/dissociation of
the anion with the metal center, the process is still fast on the
NMR time scale.
Generation of cation 5+ partnered with the less nucleophilic
[B(C6F5)4]- ion was effected by reaction of 4 with the
corresponding triphenylcarbenium or N,N-dimethylanilinium
salts. The expected coproducts, 1,1,1,2-tetraphenylethane and
N,N-dimethylanilline/toluene, respectively, were detected by 1H
NMR and GC-MS. The 19F NMR spectrum of [5][B(C6F5)4]
generated from 4 and [Ph3C][B(C6F5)4] (Figure 8a) shows three
broad resonances. They are somewhat less broadened than for
[5][PhCH2B(C6F5)3], and the p-F resonance is now found
slightly downfield from the m-F resonance. A very similar
spectrum is obtained by the protonolytic route using [Ph-
NHMe2][B(C6F5)4], suggesting that the PhNMe2 coproduct does
not significantly coordinate to the cation.22 Although the 19F
Like its chloro precursor, benzyl derivative 4 is paramagnetic,
possessing a room-temperature magnetic moment of 5.8 µB per
iron ion, consistent with high-spin (S ) 2) iron centers. The
paramagnetic 1H NMR spectrum of 4 in C6D6 contains 11 broad
resonances in the δ +200 to -75 ppm window at positions
very different from those of 1b. Despite the structural similarity
to its chloride precursor, the ligand resonances of 4 could not
be assigned unambiguously. However, characteristic resonances
at δ 30.4, -41.2, and -60.0 ppm could be assigned to the meta,
ortho, and para protons, respectively, of the benzyl group. The
(19) (a) Hermes, A. R.; Girolami, G. S. Organometallics 1987, 6, 763.
(b) Hill, D. H.; Sen, A. J. Am. Chem. Soc. 1988, 110, 1650. (c) Hill, D. H.;
Parvez, M. A.; Sen, A. J. Am. Chem. Soc. 1994, 116, 2889. (d) Bart, S.;
Hawrelak, E. J.; Schmisseur, A. K.; Lobkovsky, E.; Chirik, P. J. Organo-
metallics 2004, 23, 237.
(20) (a) Pellecchia, C.; Grassi, A.; Immirzi, A. J. Am. Chem. Soc. 1993,
115, 1160. (b) Pellecchia, C.; Immirzi, A.; Grassi, A.; Zambelli, A.
Organometallics 1993, 12, 4473. (c) Lee, L. W. M.; Piers, W. E.; Elsegood,
M. R. J.; Clegg, W.; Parvez, M. Organometallics 1999, 18, 2947.
(21) (a) Horton, A. D.; de With, J.; van der Linden, A.; van de Weg, H.
Organometallics 1996, 15, 2672. (b) Horton, A. D.; de With, J. Organo-
metallics 1997, 16, 5424.

Paramagnetic Amino-amidinate Iron(II) Complexes
Organometallics, Vol. 27, No. 9, 2008 2063
Scheme 5
NMR spectra suggest looser ion pairing for [5][B(C6F5)4] than
for [5][PhCH2B(C6F5)3], interaction of [B(C6F5)4]- with the
paramagnetic cation 5+ is clearly demonstrated by the addition
of pyridine. Reaction with the Lewis base results in a color
change from dark red to bright yellow and a 19F NMR spectrum
characteristic of the free [B(C6F5)4]- ion (Figure 8b).
(e.g., in [Cp*2ThMe][B(C6F5)4] inferred from 19F,1H HOESY
NMR experiments in C6D623).
The role of the solvent was probed by stepwise dilution of a
solution of [5][B(C6F5)4] in C6D5Br. The chemical shift differ-
ence between the m-F and p-F resonances (∆(δm,p-F), 4.2 ppm
in the free anion) changes from 0.5 ppm (37 mM) to 2.4 ppm
(3.7 mM) and 3.7 ppm (0.37 mM), respectively, upon dilution.
This behavior suggests that contact ion pair [5][B(C6F5)4] is in
fast equilibrium with a solvent-separated ion pair [5 · C6D5Br]-
[B(C6F5)4].
Upon cooling a solution of [5][B(C6F5)4] in C6D5Br to -30
°C, the p-F resonance shifts upfield from the m-F resonance,
consistent with stronger ion pairing at low temperature. Nev-
ertheless, the four C6F5 rings remain magnetically equivalent
at this temperature, indicating rapid interconversion. Therefore,
the mode of the interaction between 5+ and [B(C6F5)4]- in
solution cannot be inferred with certainty from the spectra.
Whereas for [5][B(C6F5)4], the largest effect is seen on the
chemical shifts of the m-F and p-F atoms, there is also precedent
for interaction of this anion through the o-F and m-F atoms
Conclusions
The introduction of an N,N-dimethylaminoethyl substituent
on one of the nitrogen atoms of amidinate ligands reduced the
tendency of (amidinate)Fe(II) chloro complexes to decompose
through ligand redistribution. At the same time, the decreased
steric demand of the substituent has restored the tendency of
the amidinate to bridge two metal centers. Nevertheless, this
did allow the synthesis of the diiron dibenzyl complex [(µ-
SiL)FeCH2Ph]2 (4), with 14 VE per Fe center.
Abstraction of one of the benzyl groups in 4 by Lewis or
Brønsted acids gives rise to the paramagnetic diiron monocation
[(µ-SiL)2Fe2CH2Ph]+ (5+) partnered with weakly coordinating
(fluoroaryl)borate anions. Weak cation-anion interactions in
these ion pairs in C6D5Br solution were readily detected by 19
F
NMR spectroscopy, due to the paramagnetic nature of the cation,
although it is difficult to establish the precise nature of the
contact interaction between cation and anion.
Experimental Section
Figure 7. 19F NMR spectra (188 MHz, C6D5Br, RT) of 4 +
B(C6F5)3 (a) and 4 + B(C6F5)3 + pyridine-d5 (b).
Instrumentation. NMR spectra were recorded on Varian Inova
500, VXR 300, and Varian Gemini 200 instruments. H chemical
1
shifts are referenced to residual protons in deuterated solvents and
are reported relative to tetramethylsilane. IR spectra were recorded
on a Mattson 4020 Galaxy FT-IR spectrometer. GC-MS analyses
were conducted using a HP 5973 mass-selective detector attached
to a HP 6890 GC instrument. Elemental analyses were performed
by the Microanalytical Department at the University of Groningen
or by Mikroanalytisches Laboratorium H. Kolbe, Mu¨lheim an der
Ruhr, Germany. Reported values are the averages of two indepen-
dent determinations. ES-MS spectra were obtained using a Nermag
R3010 triple quadrupole MS system with a custom-built IonSpray
(pneumatically assisted electrospray) source.24 A JEOL JMS600
spectrometer was used for exact mass determinations. Magnetic
susceptibility measurements were performed on microcrystalline
samples (ca. 25 mg) using a Quantum Design MPMS-7 SQuID
(22) Addition of excess PhNMe2 to a C6D5Br solution of [5b][B(C6F5)4],
generated from 4b and [Ph3C][B(C6F5)4], does not result in a significant
increase in ∆δ(m,p-F) or in smaller linewidths in the 19F NMR spectrum.
(23) Zuccaccia, C.; Stahl, N. G.; Macchioni, A.; Chen, M.-C.; Roberts,
J. A.; Marks, T. J. J. Am. Chem. Soc. 2004, 126, 1448.
(24) Bruins, A. P.; Covey, T. R.; Henion, J. D. Anal. Chem. 1987, 59,
2642.
Figure 8. 19F NMR spectra (188 MHz, C6D5Br, RT) of 4 +
[Ph3C][B(C6F5)4] (a) and 4 + [Ph3C][B(C6F5)4] + pyridine-
d5 (b).

2064 Organometallics, Vol. 27, No. 9, 2008
Sciarone et al.
magnetometer. Samples were contained in sealed NMR tubes, which
were inserted in a plastic straw. The data were corrected for
diamagnetism using tabulated Pascal’s constants.25
Hz, 2H) ppm. IR (Nujol mull, KBr): ν˜ 1504 (m), 1484, 1461, 1388,
1379 (s), 1349 (m), 1312, 1276, 1262, 1257, 1057, 927 (w), 785
(m), 768 (w), 703 (m), 431 (w) cm-1. Anal. Calcd for C46H64-
N6Cl2Fe2 (883.65): C 62.53, H 7.30, N 9.51, Cl 8.02, Fe 12.64.
Found: C 62.55, H 7.61, N 9.40, Cl 7.50, Fe 12.46.
General Considerations. All experiments were carried out under
a dry dinitrogen atmosphere using Schlenk and drybox techniques.
Toluene, pentane, diethyl ether, and THF were distilled from Na
or Na/K alloy before use or purified by percolation under nitrogen
atmosphere over columns of alumina, molecular sieves, and
supported copper oxygen scavenger (BASF R3-11). Benzene-d6 and
THF-d8 were dried over Na/K alloy and vacuum transferred before
use. Bromobenzene-d5 was degassed and dried over CaH2.
Starting Materials. Anhydrous FeCl2,26 B(C6F5)3,27 and KCH2-
Ph28 (from nBuLi, KOtBu, and toluene) were prepared following
literature procedures. [Ph3C][B(C6F5)4] (Strem) and [PhNHMe2]-
[B(C6F5)4] (Asahi Glass Co.) were commercial products and were
used without further purification. The amidine ArLH was deproto-
nated in situ (nBuLi/THF) to give the lithium amidinate Li[ArL].
Lithium amidinate Li[SiL] was prepared according to a literature
procedure.7
N-(2,6-Diisopropylphenyl)-N′-(2-dimethylaminoethyl)benza-
midine (ArLH). N-(2,6-Diisopropylphenyl)benzimidoyl chloride29
(30 g, 0.10 mol) was added to a solution of N,N-dimethylethyl-
enediamine (22.0 mL, 0.20 mol) in toluene (150 mL). After
refluxing for 1 h the mixture was cooled and 2 N KOH (330 mL)
was added. The organic phase was washed with water, dried with
Na2SO4, and concentrated. The concentrate was distilled using a
Kugelrohr apparatus (200 °C, 0.02 mmHg) to give 28.1 g (0.080
mol, 80.0%) of ArLH (mixture of isomers) as a pale yellow, slowly
solidifying oil. 1H NMR (200 MHz, CDCl3, 25 °C): δ 6.7-7.9
(m, 8H, ArH), 4.8-5.2 (s, 1H, NH), 3.4-3.6 (s, 2H, CH2), 2.9-3.1
(s, 2H, iPr-CH), 2.4-2.6 (s, 2H, CH2), 1.9-2.3 (s, 6H, CH3, NCH3),
0.8-1.3 (m, 6H, iPr-CH3), 0.6-1.4 (m, 6H, iPr-CH3) ppm. 1H NMR
(500 MHz, PhMe-d8, 120 °C): δ 7.51 (s, 2H, ArH), 7.02-7.15 (m,
6H, ArH), 4.98 (s, 1H, NH), 3.40 (s, 2H, CH2), 3.34 (septet, J )
6.7 Hz, 2H, iPr-CH), 2.35 (s, 2H, CH2), 2.12 (s, 6H, NCH3), 1.36
(s, 6H, iPr-CH3), 1.22 (s, 6H, iPr-CH3) ppm. 13C{1H} NMR (125
MHz, CDCl3, RT): δ 156.3 (Ar-Cipso), 153.9 (Ar-Cipso), 145.3
(Ar-Cipso), 143.8 (Ar-Cipso), 139.0 (Ar-Cipso), 138.1 (Ar-Cipso),
136.2 (Ar-Cipso), 134.9 (Ar-Cipso), 129.0 (Ar-CH), 128.1 (Ar-CH),
127.7 (Ar-CH), 126.7 (Ar-CH), 122.9 (Ar-CH), 122.4 (Ar-CH),
122.0 (Ar-CH), 59.0 (CH2), 58.1 (CH2), 46.0, 45.1 (iPr-CH), 41.6
(CH2), 40.7 (CH2), 39.2 (CH2), 28.8 (iPr-CH3), 28.4 (iPr-CH3), 27.6
(iPr-CH3), 27.5 (iPr-CH3), 23.6 (iPr-CH3), 22.4 (iPr-CH3). All
signals broadened. IR (KBr): ν˜ 2959 (s), 2866, 2820, 2772 (w),
1630 (s), 1603, 1588, 1578(w), 1499, 1479, 1462, 1445 (m), 1381,
1360, 1300, 1256, 1057, 1042 (w), 774, 762, 700 (m) cm-1. HRMS
(EI): calcd for C23H33N3 351.2674; found 351.2671.
[(µ-SiL)FeCl]2 (1b). A solution of Li[SiL] (8.92 g, 33.1 mmol)
in THF (30 mL) was added dropwise to a stirred suspension of
FeCl2 (4.20 g, 33.1 mmol) in THF (20 mL) at -40 °C. The reaction
mixture was stirred for an additional 30 min at room temperature.
THF was removed under reduced pressure, and residual THF was
removed by stirring the pale yellow solid in pentane (50 mL) and
pumping off all volatiles under vacuum. Continuous extraction with
CH2Cl2 (100 mL) yields a dark yellow solution. The extract was
evaporated to dryness, and the residue was washed with THF (50
mL, 3×), yielding a pale yellow solid. Yield: 7.51 g (10.6 mmol,
64%). 1H NMR (500 MHz, CD2Cl2, RT δ (∆ν1/2, integral)): 327.0
(2130 Hz, 2H, CH2), 175.1 (1820 Hz, 2H, CH2), 115.3 (670 Hz,
6H, NMe2), 110.5 (990 Hz, 2H, CH2), 68.5 (640 Hz, 6H, NMe2),
42.4 (790 Hz, 2H, CH2), 35.1 (490 Hz, 2H, m-HPh), 18.1 (490 Hz,
2H, m-HPh), 5.1 (270 Hz, 18H, SiMe3), 2.2 (47 Hz, 2H, p-HPh)
ppm. IR (Nujol mull, KBr): ν˜ 3054 (m), 2797, 1579 (w), 1501,
1484, 1393, 1333 (s), 1281 (w), 1246 (s), 1174 (m), 1155, 1100
(w), 1076 (m), 1056 (w), 1036, 1022, 952, 939 (m), 862, 845 (s),
796, 779, 768, 737, 724, 704 (m), 653, 635, 589, 580, 530, 461,
452, 437, 427, 415 (w) cm-1. Anal. Calcd for C28H48N6Si2Cl2Fe2
(707.50): C 47.53, H 6.84, N 11.88, Cl 10.02, Fe 15.79. Found: C
47.49, H 6.70, N 11.97, Cl 10.06, Fe 15.67.
(
ArL)2Fe (2). A mixture of FeCl2 (0.39 g, 3.03 mmol) and Li[ArL]
(2.17 g, 6.06 mmol) was dissolved in THF (25 mL). The reaction
mixture was stirred for 2 h at room temperature, and the solvent
was removed in Vacuo. The residue was extracted with diethyl ether,
and the solvent was subsequently removed in Vacuo. After addition
of pentane (15 mL) to the product a white solid precipitated. The
pentane was decanted. The product was obtained as dark yellow
crystals after recrystallization from hot toluene. Yield: 1.1 g (1.5
mmol, 50%). 1H NMR (300 MHz, C6D6, 25 °C): δ (∆ν1/2) 133 (67
Hz),118 (762 Hz), 98 (706 Hz), 83 (850 Hz), 70, 16.6, 15.7 (54
Hz), 9.4 (37 Hz), 8.8 (64 Hz), 6.9, 5.8, 5.6, 5.0, 4.1, 3.6, 2.5, 2.2,
1.2, 1.1, 0.9, -1.1 (74 Hz), -4.0 (39 Hz), -5.3 (331 Hz), -7.0
(78 Hz), -7.7 (78 Hz), -8.6 (70 Hz), -13.2 (85 Hz), -18.8 (218
Hz) ppm. IR (Nujol mull, KBr): ν˜ 1590 (w), 1578 (m), 1550 (s),
1488 (w), 1464 (m), 1318 (m), 1218 (w), 1252 (w), 1137 (w), 1073
(m), 1057 (w), 953 (w), 907 (w), 780 (m), 764 (w), 752 (w), 701
(m) cm-1. Anal. Calcd for C46H64N6Fe (756.90): C 73.00, H 8.52,
N 11.10. Found: C 72.88, H 8.49, N 10.87.
[(µ-ArL)FeCl]2 (1a). Li[ArL] (1.0 g, 2.8 mmol) and FeCl2 (0.37
g, 2.9 mmol) were stirred in THF (30 mL) for 20 min. The solvent
was removed in Vacuo. Any residual THF was removed by stirring
the reaction mixture in pentane (20 mL) and subsequently pumping
off the volatiles (2×). The resulting green-gray powder was washed
with pentane until the pentane was nearly colorless. The residue
was extracted with toluene until the toluene was colorless. The
combined toluene extracts were concentrated to ca. 50%. Warming
to 80 °C and subsequent cooling to room temperature afforded dark
[Me2NCH2CH2NC(Ph)N(2,6-iPr2C6H3)C(O)]FeCl(CO)2 (3). A
solution of 1a (0.50 g, 0.57 mmol) in toluene (15 mL) was degassed
by three freeze-pump-thaw cycles. Excess CO (1 atm) was
admitted, and the reaction mixture was stirred at room temperature
for 2 days. Toluene was removed in Vacuo, and the residue was
extracted with pentane until the pentane was colorless. Concentra-
tion of the extract and cooling to -80 °C afforded yellow needles.
1
Yield: 0.48 g (0.46 mmol, 81%). H NMR (300 MHz, C6D6, 25
3
°C): δ 7.10-6.79 (m, 8H, ArH), 4.07 (septet, JHH ) 6.6 Hz, 1H,
1
yellow crystals. Yield: 0.51 g (0.58 mmol, 40%). H NMR (500
CH), 3.07 (m, 4H, CH2 + CH), 2.36 (s, 3H, NCH3), 2.06 (s, 3H,
NCH3), 1.52 (d, 3JHH ) 6.3 Hz, 3H, CH3), 1.35 (d, 3JHH ) 6.6 Hz,
3H, CH3), 1.22 (m, 3JHH ) 6.9 Hz, 4H, CH3 and CH2), 1.07 (d, Hz
3JHH ) 6.9 Hz, 3H, CH3) ppm. 13C{1H} NMR (50 MHz, C6D6, 25
°C): δ 208.1, 207.9 and 207.5 (CO), 164.1 (NCN), 147.6 (CipsoAr),
144.6 (CipsoPh), 132.2 (CAriPr), 128.6, 127.8, 126.1, 125.6, 122.5
and 121.2 (CArH), 60.0 (CH2), 51.2 and 49.3 (NCH3), 48.0 (CH2),
27.8 and 26.7 (CH), 24.6, 24.1, 21.3, and 20.8 (CH3) ppm. IR (Nujol
mull, KBr): ν˜ 2073 (s, Ct O), 1949 (s, Ct O), 1669 (s, CdO),
1639 (m), 1463 (s), 1327 (m), 1268, 1022 (w), 990 (mw), 946 (w),
MHz, C6D6, RT): δ (∆ν1/2) 106.8 (4500 Hz, 6H), 22.7 (66 Hz,
2H), 19.7 (540 Hz, 2H), 5.7 (29 Hz, 1H), 4.4 (58 Hz, 2H), -3.5
(84 Hz, 6H), -20.9 (870 Hz, 6H), -26.3 (54 Hz, 1H), -32.6 (1100
(25) Kahn, O. Molecular Magnetism; Wiley-VCH: New York, 1993;
p 3.
(26) Kovacic, P.; Brace, N. O. Inorg. Synth. 1960, 6, 172.
(27) Pohlmann, J. L. W.; Brinckmann, F. E. Z. Naturforsch. 1965,
20b, 5.
(28) Lochmann, L.; Trekoval, J. J. Organomet. Chem. 1987, 326, 1.
(29) Nijhuis, C. A.; Jellema, E.; Sciarone, T. J. J.; Meetsma, A.;
Budzelaar, P. H. M.; Hessen, B. Eur. J. Inorg. Chem. 2005, 2089.
805 (mw), 786 (w), 769, 713 (mw), 674, 489, 467, 430 (w) cm-1
.

Paramagnetic Amino-amidinate Iron(II) Complexes
Organometallics, Vol. 27, No. 9, 2008 2065
Anal. Calcd for C26H32N3O3ClFe (525.86): C 59.39, H 6.13, N 7.99,
Cl 6.74, Fe 10.62. Found: C 59.28, H 6.20, N 8.09, Cl 6.74, Fe
10.83.
47.8 Hz, 4F, p-F), -163.6 (∆ν1/2 ) 52.7 Hz, 8F, m-F) ppm. From
this solution was taken 50 µL, which was diluted with C6D5Br (450
µL). 19F NMR (188 MHz, C6D5Br, 0.37 mM, RT): δ -132.0 (∆ν1/2
) 66.0 Hz, 8F, o-F), -161.8 (∆ν1/2 ) 80.2 Hz, 4F, p-F), -165.5
(∆ν1/2 ) 114.5 Hz, 8F, m-F) ppm.
X-ray Structure Deteminations. Crystal, collection, and refine-
ment data for complexes 1a, 1b, 2, 3, and 4 are listed in Tables S1
and S2 (Supporting Information). Illustrations were prepared with
the program PLATON.30
[(µ-SiL)FeCH2Ph]2 (4). A suspension of [(µ-SiL)FeCl]2 (1b)
(3.34 g, 4.72 mmol) and KCH2Ph (1.23 g, 9.44 mmol) in THF (30
mL) was allowed to warm from -40 °C to room temperature while
stirring. The resulting yellow solution was stirred for an additional
hour at room temperature. THF was evaporated under vacuum.
Residual THF was removed by stirring the solid residue in pentane
(50 mL) and pumping off all volatiles. The residue was extracted
(6×) with diethyl ether (150 mL), and the extract was evaporated
to dryness, yielding a yellow powder. Recrystallization from hot
(80-100 °C) toluene (20 mL) afforded red crystals. Yield: 1.68 g
1a: Crystals were obtained by slow cooling of a hot (80 °C)
toluene solution.
1b: Crystals were obtained by recrystallization from dichlo-
romethane. Refinement was complicated by rotational disorder in
the trimethylsilyl fragment (C12-C14) and conformational disorder
in the C1, C2, and C3 positions of the chelate ring. A disorder
model with site occupancy factor refinement was applied. The
C1-C3 fragment was refined to two conformations with sof’s of
0.57 and 0.43. The SiMe3 methyls were refined to two positions
with sof’s of 0.52 and 0.48. In the subsequent refinement bond
restraints for the Si-C distances were applied. A difference Fourier
synthesis resulted in the location of the hydrogen atoms of the
phenyl group. The remaining hydrogen atoms were included in the
final refinement riding on their carrier atoms.
1
(2.1 mmol, 43%). H NMR (500 MHz, C6D6, RT): δ 189.7 (681,
2H, CH2), 123.4 (878, 4H, CH2), 92.8 (939, 2H, CH2), 77.3 (1269,
6H, NMe), 30.4 (388, 4H, m-HBz), 22.1 (501, 4H), 17.1 (707, 2H),
14.4 (612, 4H), 6.7 (586, 18H, SiMe3), -41.2 (601, o-HPh), -60.0
(391, 2H, p-HBz) ppm. IR (KBr, nujol): ν˜ 3066, 3049, 3013, 1590
(m), 1497, 1480 (s), 1390, 1370 (m), 1335 (s), 1299 (w), 1257,
1244, 1214, 1170, 1075 (m), 1039 (w), 1019 (m), 951, 859, 836
(s), 794, 779, 759, 744, 722, 697 (m), 637, 625, 579, 544 (w), 520
(m), 405 (w) cm-1. Anal. Calcd for C42Fe2H62N6Si2 (818.86): C
61.61, H 7.63, N 10.26, Fe 13.64. Found: C 61.04, H 7.62, N 10.26,
Fe 13.51.
2: Crystals were obtained by recrystallization from pentane.
Generation of [5][PhCH2B(C6F5)3] and Reaction with Pyri-
dine-d5. B(C6F5)3 (12.5 mg, 24.4 µmol) was added to a solution of
4 (20 mg, 24.4 µmol) in C6D5Br (0.5 mL). A dark red solution
was obtained. 19F NMR (188 MHz, C6D5Br, RT): δ -126.4 (∆ν1/2
111.4 Hz, 6F, o-F), -159.7 (∆ν1/2 ) 138.9 Hz, 6F, m-F), -161.0
(∆ν1/2 ) 85.5 Hz, 3F, p-F) ppm. Addition of 3 drops of pyridine-
d5 to the NMR tube resulted in a yellow solution. 19F NMR (188
MHz, C6D5Br, RT): δ -129.7 (d, 20 Hz, 6F, o-F), -163.3 (t, 19
Hz, 3F, p-F), -165.6 (m, 6F, m-F) ppm. ES-MS (70 eV, THF):
neg-ion mode, m/z 167 [C6F5]- (35%), 435 [PhCH2BH(C6F5)2]-
(30%), 512 [B(C6F5)3]- (15%), 603 [PhCH2B(C6F5)3]- (100%).
Generation of [5][B(C6F5)4] from 4 and [Ph3C][B(C6F5)4]
and Reaction with Pyridine-d5. An orange solution of [Ph3C]-
[B(C6F5)4] (28.2 mg, 30.5 µmol) in C6D5Br (0.5 mL) was added to
4 (25.0 mg, 30.5 µmol). A dark red solution was obtained. 19F
NMR (188 MHz, C6D5Br, RT): δ -129.2 (∆ν1/2 ) 71.8 Hz, 4F,
o-F), -159.2 (∆ν1/2 ) 78.4 Hz, 4F, p-F), -159.9 (∆ν1/2 ) 81.5
Hz, 8F, m-F) ppm. Addition of 3 drops of pyridine-d5 to the NMR
tube resulted in a yellow solution. 19F NMR (188 MHz, C6D5Br,
RT): δ -131.1 (∆ν1/2 ) 42.4 Hz, 8F, o-F), -161.4 (∆ν1/2 ) 47.3
Hz, 4F, p-F), -164.6 (∆ν1/2 ) 51.4 Hz, 8F, m-F) ppm.
Generation of [5][B(C6F5)4] from 4 and [PhNHMe2][B(C6-
F5)4] and Reaction with Pyridine-d5. Benzyl complex 4 (25.0 mg,
30.5 µmol) was dissolved C6D5Br (0.5 mL). [PhNHMe2]-
[B(C6F5)4] (24.5 mg, 30.5 µmol) was added to this solution. 19F
NMR (188 MHz, C6D5Br, RT): δ -129.7 (∆ν1/2 ) 61.1 Hz, 8F,
o-F), -160.2 (∆ν1/2 ) 62.1 Hz, 4F, p-F), -161.1 (∆ν1/2 ) 77.0
Hz, 8F, m-F) ppm. Three drops of pyridine-d5 were added to the
NMR tube. 19F NMR (188 MHz, C6D5Br, RT): δ -131.1 (∆ν1/2
) 40.7 Hz, 8F, o-F), -161.7 (∆ν1/2 ) 45.2 Hz, 4F, p-F), -165.0
(∆ν1/2 ) 49.0 Hz, 8F, m-F) ppm.
3: Crystals of composition 3 · 2THF were obtained by recrys-
tallization from THF. Refinement was complicated by disorder/
partial occupancy of both the THF solvent molecules, which did
not refine satisfactorily. The BYPASS31 procedure was used to take
into account the electron density in the potential solvent area, which
resulted in an electron count of 175 in a volume of 1072.4 Å3 in
the unit cell. (Meaning the site occupation factor for the enclosed
THF molecules is 0.55.)
4: Crystals were obtained by slow cooling of a hot (80 °C)
toluene solution to room temperature. Refinement was complicated
by conformational disorder in the benzyl fragments (C152-C212).
A disorder model with two conformations of the benzyl ligand
(refined to sof’s of 0.44 and 0.56, respectively) was introduced with
“variable metric” rigid group restraints in the refinement. A
difference Fourier synthesis resulted in the location of all the
hydrogen atoms, of which the coordinates and isotropic displace-
ment parameters were refined, except those belonging to disordered
C atoms of the benzyl ligand. The remaining hydrogen atoms were
generated by geometrical considerations and refined in the riding
mode. The adp of C152 converged to nonpositive definite displace-
ment parameters when allowed to vary anisotropically, so ultimately
this was reset to an isotropic displacement factor.
Acknowledgment. This work is part of the Research
Programme of the Dutch Polymer Institute (DPI), project
no. #110.
1
Supporting Information Available: H NMR spectrum of 4.
Magnetic susceptibility data for complexes 1 and 4. Crystallographic
data for compounds 1a, 1b, 2, 3, and 4 (CIF). This material is
available free of charge via the Internet at http://pubs.acs.org.
Dilution Experiment with [5][B(C6F5)4]. [Ph3C][B(C6F5)4] (16.9
mg, 18.3 µmol) was added to a solution of 4 (15 mg, 18.3 µmol)
in C6D5Br (0.5 mL). A dark red solution was obtained. 19F NMR
(188 MHz, C6D5Br, 37 mM, RT): δ -129.4 (∆ν1/2 ) 55.2 Hz, 8F,
o-F), -160.0 (∆ν1/2 ) 55.7 Hz, 4F, p-F), -160.5(∆ν1/2 ) 69.7
Hz, 8F, m-F) ppm. From this solution was taken 50 µL, which was
diluted with C6D5Br (450 µL). 19F NMR (188 MHz, C6D5Br, 3.7
OM701155B
(30) (a) Spek, A. L. PLATON, Program for the automated analysis of
molecular geometry (A multipurpose crystallographic tool); University of
Utrecht: The Netherlands, 2002. (b) Spek, A. L. Acta Crystallogr. A 1990,
46, C34.
mM, RT): δ -129.8 (∆ν1/2 ) 43.9 Hz, 8F, o-F), -161.2 (∆ν1/2
)
(31) Sluis, P.; Van der; Spek, A. L. Acta Crystallogr. A 2005, 46, 194.