Synthesis, Reactivity, and Solid State Structures of Four-Coordinate Iron(II) and Manganese(II) Alkyl Complexes

Article abstract of DOI:10.1021/om034188h

Synthesis and characterization of new, four-coordinate, high-spin iron(II) and manganese(II) complexes of the general form L₂MR₂ (L₂ = neutral chelating ligand, R = alkyl) are described. Alkylation of the α-diimine complex, [ArN=C(Me)-C(Me)=NAr]FeCl₂ (Ar = 2,6-diisopropylphenyl), as well as the enantiopure iron dichloride compounds, (-)-(sparteine)FeCl₂ and (S)-(^tBuBox)FeCl₂ ((S)-(^tBuBox) = 2,2-bis[2-[4(S)-(R′)-1,3-oxazolinyl]propane), with LiCH₂SiMe₃ afforded the corresponding dialkyl derivatives. Solution magnetic susceptibility measurements and X-ray diffraction studies reveal each of the new iron(II) bis-trimethylsilylmethyl complexes to be high-spin, S = 2, tetrahedral molecules. In addition (-)-(sparteine)Fe(CH₂CMe₃)₂, (-)-(sparteine)Fe(CH₂C₆H₅)₂, and (S)-(^tBuBox)Fe(CH₂C₆H₅)₂ were also prepared and characterized by NMR spectroscopy and elemental analysis. An enantiopure, high-spin, tetrahedral manganese(II) dialkyl complex, (-)-(sparteine)Mn(CH₂SiMe₃)₂, has also been synthesized. The catalytic activity of the new iron complexes in carbon-carbon bond forming processes has been evaluated, and stoichiometric reactions of the dialkyls with olefins, carbon monoxide, and the Lewis acid B(C₆F ₅)₃ have been examined.

Full text of DOI:10.1021/om034188h

Organometallics 2004, 23, 237-246
237
Syn th esis, Rea ctivity, a n d Solid Sta te Str u ctu r es of
F ou r -Coor d in a te Ir on (II) a n d Ma n ga n ese(II) Alk yl
Com p lexes
Suzanne C. Bart, Eric J . Hawrelak, Amanda K. Schmisseur,
Emil Lobkovsky, and Paul J . Chirik*
Department of Chemistry and Chemical Biology, Baker Laboratory, Cornell University,
Ithaca, New York 14853
Received September 23, 2003
Synthesis and characterization of new, four-coordinate, high-spin iron(II) and manganese-
(II) complexes of the general form L2MR2 (L2 ) neutral chelating ligand, R ) alkyl) are
described. Alkylation of the R-diimine complex, [ArNdC(Me)-C(Me)dNAr]FeCl2 (Ar ) 2,6-
diisopropylphenyl), as well as the enantiopure iron dichloride compounds, (-)-(sparteine)-
FeCl2 and (S)-(tBuBox)FeCl2 ((S)-(tBuBox) ) 2,2-bis[2-[4(S)-(R′)-1,3-oxazolinyl]propane), with
LiCH2SiMe3 afforded the corresponding dialkyl derivatives. Solution magnetic susceptibility
measurements and X-ray diffraction studies reveal each of the new iron(II) bis-trimethyl-
silylmethyl complexes to be high-spin, S ) 2, tetrahedral molecules. In addition (-)-
(sparteine)Fe(CH2CMe3)2, (-)-(sparteine)Fe(CH2C6H5)2, and (S)-(tBuBox)Fe(CH2C6H5)2 were
also prepared and characterized by NMR spectroscopy and elemental analysis. An enan-
tiopure, high-spin, tetrahedral manganese(II) dialkyl complex, (-)-(sparteine)Mn(CH2SiMe3)2,
has also been synthesized. The catalytic activity of the new iron complexes in carbon-carbon
bond forming processes has been evaluated, and stoichiometric reactions of the dialkyls with
olefins, carbon monoxide, and the Lewis acid B(C6F5)3 have been examined.
In tr od u ction
described as a low-spin, square-planar molecule.8 Since
this initial report, a few other examples of L2FeR2
species have been described and a range of magnetic
properties measured. Seidel has described a series of
(R3P)2Fe(Mes)2 (PR3 ) tertiary phosphine or phosphite,
Mes ) mesityl) compounds that range from high to low
spin, depending on the phosphine or phosphite donor.9
In addition, (dme)Fe(Mes)2 (dme ) 1,2-dimethoxy-
ethane) has also been synthesized, and X-ray diffraction
revealed the molecule to be tetrahedral in the solid
state.10 Sen has also observed the intermediacy of
(tmeda)Fe(CH2C6H5)2 during the iron-promoted cou-
pling of benzyl halides.11 More recently, Hermes and
Girolami12 have reported the synthesis of a family of
(dippe)FeR2 (dippe ) 1,2-bis(diisopropylphosphinoeth-
ane) compounds, all of which are tetrahedral and high
spin.
Transition metal alkyl complexes are of practical
importance due to their role in numerous catalytic
transformations such as olefin hydrogenation, hydro-
formylation, and polymerization.1 Alkyl complexes of
iron(II) are traditionally coordinatively saturated, 18-
electron species supported by cyclopentadienyl and
carbonyl ligands.2 More recently, considerable attention
has been devoted to preparing low-coordinate, unsatur-
ated Fe(II) derivatives supported by tris(pyrazolyl)-
borate,3 â-diiminate,4,5 and phosphine-amide6 ligands.
One class of iron alkyl, L2FeR2 (L ) neutral ligand,
R ) alkyl, aryl, hydride), is particularly attractive, since
these molecules are formally 14-electron species and are
relatively rare examples of open-shell organometallic
molecules that bridge classical Werner-type coordination
compounds with more modern organometallic com-
plexes.7 Chatt and Shaw provided the first example of
an L2FeR2 complex with the preparation of (PhEt2P)2-
Fe(C6Cl5)2, which on the basis of magnetic data has been
The recent independent reports by Brookhart13 and
Gibson14 using five-coordinate Fe(II) dihalide complexes
as precatalysts for the polymerization of ethylene and
R-olefins suggested to us that iron(II), when in the
(1) Spessard, G. O.; Miessler, G. L. Organometallic Chemistry;
Prentice Hall: Upper Saddle River, NJ , 1996; p 116.
(2) Kerber, R. C. In Comprehensive Organometallic Chemistry II;
Abel, E. W., Stone, F. G. A., Wilkinson, G., Eds.; Pergamon: New York,
1995; Vol. 7, Chapter 2.
(3) Kisko, J . L.; Hascall, T.; Parkin, G. J . Am. Chem. Soc. 1998, 120,
10561.
(4) (a) Holland, P. L.; Cundari, T. R.; Perez, L. L.; Eckert, N. A.;
Lachicotte, R. J . J . Am. Chem. Soc. 2002, 124, 14416. (b) Smith, J .
M.; Lachicotte, R. J .; Holland, P. L. Organometallics 2002, 4808.
(5) Sciarone, T. J . J .; Meetsma, A.; Hessen, B.; Teuben, J . H. Chem.
Commun. 2002, 1580.
(6) Fryzuk, M. D.; Leznoff, D. B.; Ma, E. S. F.; Rettig, R. J .; Young,
V. G. Organometallics 1998, 17, 2313.
(8) Chatt, J .; Shaw, B. L. J . Chem. Soc. 1961, 285.
(9) Seidel, V. W.; Lattermann, K.-J . Z. Anorg. Allg. Chem. 1982, 488,
69.
(10) Mu¨ller, H.; Seidel, W.; Go¨rls, H. Z. Anorg. Allg. Chem. 1996,
622, 756.
(11) Hill, D. H.; Sen, A. J . Am. Chem. Soc. 1988, 110, 1651.
(12) Hermes, A. R.; Girolami, G. S. Organometallics 1987, 6, 763.
(13) Small, B. L.; Brookhart, M. J . Am. Chem. Soc. 1998, 120, 7143.
(b) Small, B. L.; Brookhart, M. Macromolecules 1999, 32, 2120.
(14) (a) 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. (b) 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.
(7) Poli, R. Chem. Rev. 1996, 96, 2135.
10.1021/om034188h CCC: $27.50 © 2004 American Chemical Society
Publication on Web 12/19/2003

238 Organometallics, Vol. 23, No. 2, 2004
Bart et al.
Sch em e 1
appropriate coordination environment and spin state,
may be an active catalyst for C-C and C-H bond
forming reactions for small organic molecules. Iron is
an attractive metal for this purpose given its low cost
and relatively low toxicity. In this report, we describe
our initial efforts in this area with the synthesis and
characterization of (N-N)FeR2 (N-N ) R-diimine,
diamine) complexes and determination of their reactiv-
ity toward olefins, carbon monoxide, and the Lewis acid
B(C6F5)3.
isotopic purity can be achieved by submitting the
partially deuterated 2,3-butadione to further H/D ex-
change cycles;19 however the material obtained after
four iterations was sufficiently pure for subsequent H
NMR characterization.
2
Resu lts a n d Discu ssion
Syn th esis of Ir on (II) a n d Ma n ga n ese(II) Dih a -
lid e Com p lexes. Our studies commenced with the
preparation of R-diimine Fe(II) complexes. This class of
ligand was chosen due to its ease of synthesis, excellent
steric and electronic tunability, and precedent for
stabilizing four-coordinate Fe(II). A series of R-diimine
Fe(II) dichloride complexes of the general form (RNd
CR′-CR′dNR)FeCl2, first prepared by tom Dieck and
Dietrich, were found to be active catalysts for the
dimerization of butadiene to 4-vinyl-1-cyclohexene.15
More recently, Gibson has explored this class of mol-
ecule as catalysts for atom transfer radical polymeri-
zation.16
Complexation of the deuterated R-diimine as well as
its protio isotopomer to iron(II) was accomplished by
addition of ArNdC(CD3)-C(CD3)dNAr or ArNdC(CH3)-
C(CH3)dNAr to a THF slurry of FeCl2. In this manner,
both [ArNdC(CD3)-C(CD3)dNAr]FeCl2 (1-d 6) and
[ArNdC(CH3)-C(CH3)dNAr]FeCl2 (1) were isolated as
blue crystalline solids in >90% yield (Scheme 1). Al-
though R-diimine iron dihalides have been known for
quite some time and have been the subject of numerous
studies, 1H NMR spectroscopic data have not been
reported. Given the high-spin (S ) 2) electronic config-
uration of these molecules, the NMR peaks are expected
to be paramagnetically shifted and broadened but
Another attractive feature of R-diimine ligands is the
ease with which deuterium can be incorporated into the
ligand backbone, providing a diagnostic handle for
characterization by 2H NMR spectroscopy. For para-
1
observable.4,6,12 In chloroform-d, the H NMR spectrum
of 1 displays five of the six expected ligand resonances,
some of which can be readily assigned on the basis of
integration. Diastereotopic isopropyl methyl groups
appear at -3.83 and 3.10 ppm, whereas the methyl
backbone of the ligand is centered at 60.82 ppm.
Confirmation of the latter assignment was obtained
2
magnetic molecules, H NMR resonances are sharper
1
2
than corresponding H NMR lines, and as a result, H
NMR spectroscopy has proven useful in characterizing
a range of paramagnetic transition metal complexes,17
including iron(II) compounds.18 Synthesis of the desired
deuterated ligand, ArNdC(CD3)-C(CD3)dNAr (Ar )
2,6-diisopropylphenyl), was accomplished by initial H/D
exchange of 2,3-butadione with D2O in D2SO4 to afford
butadione-d6,19 followed by condensation with 2,6-di-
isopropylaniline to provide the deuterated R-diimine in
84% yield and 67% isotopic purity (eq 1). Improved
1
2
from both H and H NMR spectra of 1-d 6. As expected,
2
the H NMR resonance is much sharper (∆ν1/2 ) 18.4
Hz) than the corresponding H signal (∆ν1/2 ) 120 Hz).
1
Enantiopure iron(II) dihalide complexes were also of
interest. The naturally occurring alkaloid (-)-sparteine
has been shown to be an excellent ligand for a variety
of transition metals and has enjoyed success in catalytic
asymmetric transformations.20 Synthesis of (-)-
(sparteine)FeCl2 (2) was first reported by Long21 and
more recently crystallographically characterized by
Lorber.22 Although the synthesis of 2 has been described
previously, we have found that simply stirring stoichio-
(15) (a) tom Dieck, H.; Dietrich, J . Chem. Ber. 1984, 117, 694. (b)
tom Dieck, H.; Dietrich, J . Angew. Chem., Int. Ed. Engl. 1985, 24, 781.
(16) (a) Gibson, V. C.; O’Rielley, R. K.; Wass, D. F.; White, A. J . P.;
Williams, D. J . Macromolecules 2003, 36, 2591. (b) Gibson, V. C.;
O’Reilley, R. K.; Reed, W.; Wass, D. F.; White, A. J . P.; Williams, D. J .
Chem. Commun. 2002, 1850.
(17) La Mar, G. N., Horrocks, W. D., J r.; Holm, R. H., Eds. NMR of
Paramagnetic Molecules: Principles and Applications: Academic
Press: New York, 1973.
(18) Stokes, S. L.; Davis, W. M.; Odom, A. L.; Cummins, C. C.
Organometallics 1996, 15, 4521.
(19) Arduengo, A. J .; Dias, H. V. R.; Dixon, D. A.; Harlow, R. L.;
Klooster, W. T.; Koetzle, T. F. J . Am. Chem. Soc. 1994, 116, 6812.
(20) For recent examples of palladium-catalyzed aerobic oxidation
of alcohols see: (a) Mueller, J . A.; Sigman, M. S. J . Am. Chem. Soc.
2003, 125, 7005. (b) Ferreira, E. M.; Stoltz, B. M. J . Am. Chem. Soc.
2001, 123, 7725.
(21) Wrobleski, J . T.; Long, G. J . Inorg. Chim. Acta 1978, 30, 221.

Four-Coordinate Fe(II) and Mn(II) Alkyl Complexes
Organometallics, Vol. 23, No. 2, 2004 239
metric quantities of commercially available (-)-spar-
tiene with FeCl2 in THF at ambient temperature
afforded the desired iron dihalide complex in near
quantitative yield (Scheme 1). Furthermore, high-spin
2 may also be easily characterized by 1H NMR spec-
troscopy. Both the free ligand and diamagnetic transi-
tion metal complexes containing (-)-sparteine display
complicated 1H NMR spectra that cannot be readily
interpreted due to the presence of 26 inequivalent
hydrogens with a relatively narrow chemical shift
1
dispersion. In contrast, the H NMR spectrum of 2 in
chloroform-d displays 26 distinct resonances over a 500
ppm range, facilitating characterization of 2. Unfortu-
nately, attempts to obtain 13C NMR data have been
unsuccessful.
F igu r e 1. Molecular structure of (S)-(tBuBox)FeCl2 (3)
with 30% probability ellipsoids. Hydrogen atoms omitted
for clarity.
Despite its relatively low cost and commercial avail-
ability, (-)-sparteine is not an easily manipulated
ligand. Access to the other antipode, (+)-sparteine, or
derivatization of (-)-sparteine requires sophisticated
synthesis,23 potentially limiting their utility in asym-
metric transformations. However, C2-symmetric bis-
oxazoline ligands are easily synthesized, are readily
tuned, and in some cases are commercially available as
both antipodes. This class of ligand has enjoyed consid-
erable success in coordination chemistry24 and asym-
metric catalysis.25 With respect to iron, both Corey26 and
Takacs27 have implicated iron(III) halide complexes with
2,2-bis[2-[4(S)-(R′)-1,3-oxazolinyl]propane (R′ ) C6H5,
CH2Ph) as chiral Lewis acids in the Diels-Alder reac-
tion and in the carbocyclizations of trienes, respectively.
Provided these reports, we explored the preparation and
characterization of well-defined iron(II) dialkyl com-
pounds supported by bis-oxazoline ligands.
crystallographically characterized four-coordinate Fe-
(II) dihalide complexes,16b,22,28 and the CdN bond dis-
tances of 1.282(8) and 1.292(9) Å are in the range
typically observed in other structurally characterized
transition metal bis-oxazoline complexes.29
Synthesis of an enantiopure, manganese(II) dihalide
complex has also been achieved. Stirring stoichiometric
quantities of (-)-sparteine and MnBr2 in THF at ambi-
ent temperature for 16 h followed by filtration afforded
(-)-(sparteine)MnBr2 (4) in 93% yield (eq 2). Unlike the
Fe(II) dihalide complexes, 4 is NMR silent. The solution
magnetic moment of 4 was measured as 5.96 µB in
chloroform-d and is consistent with the spin-only value
expected for five unpaired electrons.
Reaction of 2,2-bis[2-[4(S)-(CMe3)-1,3-oxazolinyl]pro-
pane ((S)-(tBuBox)) with FeCl2 in THF at ambient
temperature afforded (S)-(tBuBox)FeCl2 (3) as a white
crystalline solid in 72% yield. As with 1 and 2, 3 was
readily identified by 1H NMR spectroscopy. Chloro-
form-d solutions of 3 display five resonances, consistent
with a C2-symmetric Fe(II) complex. Equivalent tert-
butyl groups are observed at 20.01 ppm, whereas the
methyl groups on the methylene bridge are observed at
17.71 ppm. The solution magnetic moment was found
to be 4.26 µB (Evans method) in chloroform-d and is
consistent with a tetrahedral, high-spin Fe(II) complex.
The solid state structure of 3 was determined by X-ray
diffraction and confirms the chirality and distorted
tetrahedral geometry of the iron center (Figure 1). An
acute N(1)-Fe(1)-N(2) angle of 86.6(2)° is observed due
to the chelating bis-oxazoline, which is compensated by
a more open Cl(1)-Fe(1)-Cl(2) angle of 117.12(9)°. The
core of the ligand is slightly puckered, with a dihedral
angle of 46.7° arising from the planes formed by N(1)-
Fe(1)-N(2) and C(3)-C(4)-C(5). The Fe-Cl distances
of 2.2416(9) and 2.266(2) Å are in accord with other
P r ep a r a tion of F e(II) a n d Mn (II) Dia lk yl Der iva -
tives. Addition of 1 equiv of LiCH2SiMe3 to a diethyl
ether slurry of 1 at -78 °C followed by filtration and
recrystallization from pentane afforded [ArNdC(CH3)-
C(CH3)dNAr]Fe(CH2SiMe3)Cl (5) as green needles in
84% yield (eq 3). The solution magnetic moment was
determined in benzene-d6, and the value of 4.88 µB is
consistent with the spin-only value for an S ) 2 iron
complex. In benzene-d6, 5 displays five broad resonances
in the H NMR spectrum, whereas the H NMR spec-
trum of 5-d 6 contains a single peak centered at -77.71
ppm, assigned to the methyl groups on the backbone of
the R-diimine ligand.
1
2
(22) Lorber, C.; Choukroun, R.; Costes, J .-P.; Donnadieu, B. C. R.
Chim. 2002, 5, 251.
(23) Smith, B. T.; Wendt, J . A.; Aube, J . Org. Lett. 2002, 4, 2577.
(24) Go´mez, M.; Muller, G.; Rocamora, M. Coord. Chem. Rev. 1999,
193-195, 769.
(25) Ghosh, A. K.; Mathivanan, P.; Cappiello, J . Tetrahedron:
Asymmetry 1998, 9, 1.
(26) Corey, E. J .; Imai, N.; Zhang, H.-Y. J . Am. Chem. Soc. 1991,
113, 728.
(27) Takacs, J . M.; Weidner, J . J .; Takacs, B. E. Tetrahedron Lett.
1993, 34, 6219.
(28) Hermes, A. R.; Girolami, G. S. Inorg. Chem. 1988, 27, 1775.
(29) J ohnson, J .; Evans, D. A. Acc. Chem. Res. 2000, 33, 325.

240 Organometallics, Vol. 23, No. 2, 2004
Bart et al.
1
The chloride dimer, 7, has been characterized by H
NMR spectroscopy, elemental analysis, and X-ray dif-
fraction. The 1H NMR spectrum displays two diaste-
reotopic isopropyl methyl groups at -21.19 and -18.06
ppm as well as other resonances arising from the
methine and aryl protons at -9.50, -0.01, and 5.42
ppm. As with the dialkyl complex, 6, no resonance for
the methyl backbone of the ligand was observed. Simi-
F igu r e 2. Molecular structure of [ArNdC(CH3)-
C(CH3)dNAr]Fe(CH2SiMe3)2 (6) with 30% probability el-
lipsoids. Hydrogen atoms omitted for clarity.
2
Ta ble 1. Selected Bon d Dista n ces (Å) a n d An gles
(d eg) for 6
larly, no peak was observed in the H NMR spectrum
of 7-d 6. A solution magnetic moment of 3.86 µB was
measured at ambient temperature in benzene-d6 and
is indicative of antiferromagnetic coupling through the
bridging chloride ligands.31
bond distances
bond angles
Fe(1)-C(1)
2.042(3)
2.072(3)
2.025(2)
2.013(2)
1.329(3)
1.320(3)
1.507(3)
N(2)-Fe(1)-N(1)
79.59(9)
114.88(15)
113.70(12)
107.41(11)
106.49(13)
125.31(16)
20.3
Fe(1)-C(5)
Fe(1)-N(1)
Fe(1)-N(2)
N(1)-C(10)
N(2)-C(11)
C(10)-C(11)
N(2)-Fe(1)-C(1)
N(1)-Fe(1)-C(1)
N(2)-Fe(1)-C(5)
N(1)-Fe(1)-C(5)
C(1)-Fe(1)-C(5)
fold anglea
The solid state structure of 7 is shown in Figure 3
and displays idealized D2h symmetry with two tetrahe-
dral iron centers. The Fe-Fe distance of 2.9764(6) Å is
considerably longer than that of iron metal (2.48 Å) and
is longer than the sum of the covalent radii (2.50 Å),
making it outside of the range typically ascribed to
iron-iron bonds.32 The carbon-carbon bond length of
the ligand backbone, C(1)-C(1A), is 1.420(3) Å and is
slightly shorter than the corresponding C(10)-C(11)
bond length of 1.507(3) Å in 6, suggesting that the ene-
diamide form of the ligand is an important contributor
to the resonance hybrid. In such a limit, one can
consider the iron centers in the (+3) ferric form rather
than the uncommon monovalent oxidation state.
Enantiopure dialkyl derivatives, 8-12, were also
prepared by reaction of the corresponding iron(II)
dichloride complex with 2 equiv of the appropriate
alkylating reagent in diethyl ether (Scheme 2). (-)-
Sparteine iron(II) bis-trimethylsilylmethyl, neopentyl,
and benzyl complexes were prepared in approximately
65% yield, whereas bis-oxazoline iron(II) trimethylsi-
lylmethyl and benzyl derivatives were prepared in 88%
and 57% yields, respectively. Solution magnetic data
obtained in benzene-d6 are consistent with an S ) 2
ground state for each dialkyl complex. As with 5 and 6,
the enantiopure iron(II) dialkyl complexes are readily
a
Angle between N(1)-C(10)-C(11)-N(2) and N(1)-Fe(1)-N(2)
planes.
Synthesis of the dialkyl complex [ArNdC(Me)-(Me)Cd
NAr]Fe(CH2SiMe3)2 (6) was accomplished by addition
of 1 equiv of LiCH2SiMe3 to 5 or by direct alkylation of
1 with 2 equiv of LiCH2SiMe3 (eq 3). Purple, benzene-
d6 solutions of 6 have a magnetic moment of 4.71 µB
1
and display six of the eight expected H NMR signals.
2
Curiously, no H NMR signal could be detected for the
methyl backbone of 6-d 6. However, the identity of the
molecule was confirmed by single-crystal X-ray diffrac-
tion. The solid state structure of 6 is shown in Figure 2
and reveals the expected pseudotetrahedral geometry
about iron. The aryl rings are rotated such that the
isopropyl substituents flank the plane that defines the
ligand core. Selected bond distances and angles are
reported in Table 1. The iron carbon distances of 2.042-
(3) and 2.072(2) Å are similar to those reported by
Holland4b for three-coordinate iron(II) alkyls and are
slightly shorter than the value of 2.120(6) reported by
Girolami12 for (dippe)Fe(CH2C6H5)2.
Attempts to prepare other dialkyl complexes from 1
have met with limited success. Reaction of 1 with MeLi,
MeMgBr, EtMgBr, and KCH2Ph resulted in deposition
of Fe(0) and dissociation of the R-diimine ligand. Inter-
estingly, addition of 1 equiv of LiCH2CHMe2 to 1
resulted in reduction, rather than alkylation, forming
the formally iron(I)-iron(I) dimer, [ArNdC(Me)-(Me)Cd
NAr)Fe]2(µ-Cl)2 (7), as green crystals in low yield (eq
4). Monitoring the reaction by 1H NMR spectroscopy also
revealed formation of isobutene and isobutane. Similar
results are obtained from the attempted arylation of 1
with PhLi.30
1
characterized by H NMR spectroscopy, the details of
which can be found in the Experimental Section.
Alkylation of the manganese(II) dibromide complex,
4, with 2 equiv of LiCH2SiMe3 resulted in clean conver-
sion to (-)-(sparteine)Mn(CH2SiMe3)2 (13) in 79% yield.
Clear, colorless 13 has been characterized by elemental
analysis, magnetic susceptibility, and X-ray diffraction.
The solution magnetic moment in benzene-d6 was found
to be 5.77 µB, consistent with the spin-only value for
five unpaired electrons.
(31) (a) Zang, Y.; J ang, H. G.; Chiou, Y.-M.; Hendrich, M. P.; Que,
L. Inorg. Chim. Acta 1993, 213, 41. (b) Hay, P. J .; Thibeault, J . C.;
Hoffmann, R. J . Am. Chem. Soc. 1975, 97, 4884.
(30) Reaction of 1 with PhLi also produces a significant quantity of
the iron(0) arene complex, ArNdC(Me)-(Me)CdNAr(η6-biphenyl).
Hawrelak, E. J .; Chirik, P. J . Manuscript in preparation.
(32) (a) Fehlhammer, W. P.; Stolzenberg, H. In Comprehensive
Organometallic Chemistry; Wilkinson, G., Stone, F. G. A., Abel, F. W.,
Eds.; Pergamon Press: Oxford, 1982; Vol. 4, pp 515-524.

Four-Coordinate Fe(II) and Mn(II) Alkyl Complexes
Organometallics, Vol. 23, No. 2, 2004 241
F igu r e 4. Molecular structure of (-)-(spartiene)Fe(CH2-
SiMe3)2 (8) with 30% probability ellipsoids. Hydrogen atoms
omitted for clarity.
F igu r e 3. Molecular structure of [ArNdC(CH3)-C(CH3)d
NArFe]2(µ2-Cl)2 (7) with 30% probability ellipsoids. Hydro-
gen atoms omitted for clarity.
Sch em e 2
The dialkyl complexes, 8, 13, and 11 were character-
ized by X-ray diffraction, and their solid state structures
are shown in Figures 4-6. Selected bond distances and
angles are provided in Table 2. The (-)-(sparteine) iron-
(II) and manganese(II) dialkyls, 8 and 13, are nearly
isostructural, with 13 having slightly longer metal-
carbon and metal-nitrogen bonds arising from the
larger Mn(II) center. The iron complex 8 is a distorted
tetrahedron with an acute N(1)-Fe(1)-N(2) angle of
80.20(4)° arising from the bite angle of the diamine
ligand. To compensate, the C(16)-Fe(1)-C(20) angle
opens to 129.46(6)°. The trimethylsilyl groups are
oriented in an “up-down” fashion as to avoid unfavor-
able steric interactions. The iron-carbon bond lengths
of 2.0856(16) and 2.0963(13) Å are similar to those
observed in the R-diimine iron(II) dialkyl complex 6.
The bis-oxazoline iron(II) dialkyl complex 11 displays
similar structural features. Similar to the dichloride
complex 3, a puckered ligand core is observed with a
dihedral angle of 25.0° between the planes formed by
N(1)-Fe(1)-N(2) and C(17)-C(16)-C(15). The tri-
methysilyl groups of the alkyl ligands are canted away
from the tert-butyl groups of the bis-oxazoline ligand to
avoid unfavorable steric interactions. The acute N(1)-
Fe(1)-N(2) bond angle of 85.17(15)° and iron-carbon
bond distances of 2.074(5) and 2.087(4) Å are in accord
with those observed in 6 and 8.
F igu r e 5. Molecular structure of (-)-(spartiene)Mn(CH2-
SiMe3)2 (13) with 30% probability ellipsoids. Hydrogen
atoms omitted for clarity.
Rea ctivity of Ir on (II) Dia lk yl Com p lexes. The
competency of the iron complexes to promote catalytic
C-C bond formation was initially assayed by polymer-
ization of ethylene. Although the goal of our work is not
to develop new catalysts for polyolefin synthesis, eth-
ylene polymerization was chosen as a test reaction due
to its simplicity and the rate with which precatalysts
can be screened for activity. The four-coordinate iron-
(II) dichloride complexes 1-3 when activated with
methylalumoxane (MAO) are active for the solution
phase polymerization of ethylene.22 Although each
catalyst did produce several hundred milligrams of
polyethylene, the activity of the four-coordinate iron
precatalysts was significantly diminished in comparison
to the Brookhart-Gibson, five-coordinate Fe(II) dichlo-
ride complexes, which produced multigram quantities
of polymer under identical conditions. Stephan has
recently reported diminished ethylene polymerization
activity with four-coordinate pyridine- and imidazole-
phosphinimine-based iron(II) dihalide catalyst precur-
sors.33 The origin of this effect is most likely electronic
rather than steric in origin. Although the structure of

242 Organometallics, Vol. 23, No. 2, 2004
Bart et al.
F igu r e 6. Molecular structure of (S)-(tBuBox)Fe(CH2-
SiMe3)2 (11) with 30% probability ellipsoids. Hydrogen
atoms omitted for clarity.
F igu r e 7. Molecular structure of ArNdC(CH3)-C(CH3)d
NArFe(CH2SiMe3)(C6F5) (15) with 30% probability el-
lipsoids. Hydrogen atoms omitted for clarity.
Ta ble 2. Selected Bon d Dista n ces (Å) a n d An gles
(d eg) for 8, 11, a n d 13
8
11
13
in the phosphine system no competing reaction produc-
ing free ligand and Fe(CO)5 was reported. Red, diamag-
netic 14 is related to R-diimine iron tricarbonyl com-
plexes reported by tom Dieck prepared by reaction of
the free ligand with either Fe(CO)5 under photochemical
conditions or by thermal reaction with Fe2(CO)9.36 The
corresponding reaction with the enantiopure dialkyl
complex 8 resulted in clean formation of ketone along
with free (-)-sparteine and Fe(CO)5, suggesting (-)-
sparteine is not a good ligand for Fe(0).
Ma-C
2.0856(14)
2.0963(13)
2.2662(11)
2.2475(12)
80.20(4)
106.77(5)
106.27(6)
103.96(5)
117.85(4)
129.46(6)
2.074(5)
2.087(4)
2.140(4)
2.173(4)
85.17(15)
120.14(18)
109.36(18)
98.10(17)
113.80(17)
123.74(18)
2.1582(18)
2.165(17)
2.2967(14)
2.3288(13)
78.12(5)
102.92(6)
117.68(6)
106.89(6)
103.76(7)
132.84(7)
M-N(1)
M-N(2)
N(2)-M-N(1)
N(2)-M-C(1)b
N(1)-M-C(1)
N(2)-M-C(2)
N(1)-M-C(2)
C(1)-M-C(2)
a
b
M ) Fe (8, 11) or Mn (13). For 8 and 11, C(1) refers to C(16),
and C(2) refers to C(20) in Figures 4 and 5.
the active polymerization catalyst is open to speculation,
the S ) 2 electronic configuration of the precatalyst has
no low lying empty molecular orbitals available for olefin
binding.
To address this issue, experiments with the more
well-defined four-coordinate iron(II) dialkyl complexes
were examined. In general, the dialkyl complexes
display good thermal stability, eventually undergoing
slow decomposition after heating to 85 °C for several
days. Addition of 1 atm of dihydrogen to 6, 8, or 11
resulted in alkane formation along with decomposition
to free ligand and metallic iron. Each of the dialkyl
complexes was also unreactive toward excess ethylene
and not active for the hydrogenation of olefins such as
1-hexene and styrene.
Addition of strong field ligands to the dialkyl com-
plexes 6 and 8 has also been examined. Carbonylation
of the R-diimine iron(II) dialkyl, 6, with 1 atm of CO at
ambient temperature produced an immediate reaction,
affording the ketone Me3SiCH2C(O)CH2SiMe3 and [ArNd
C(Me)-(Me)CdNAr]Fe(CO)3 (14) (eq 5). In addition,
small (∼10%) amounts of free R-diimine ligand and Fe-
(CO)5 are also observed. The identity of the ketone was
established by NMR spectroscopy, mass spectrometry,
and infrared spectroscopy.34 Similar observations have
been reported by Girolami for the carbonylation of
(dippe)FeR2 with 3 atm of carbon monoxide.35 However,
To generate more reactive iron(II) organometallic
complexes, alkyl abstraction with B(C6F5)3 was at-
tempted. Reaction of 6 with 1 equiv of B(C6F5)3 produced
green crystals upon recrystallization from pentane at
-35 °C. A combination of NMR spectroscopy, elemental
analysis, and X-ray diffraction revealed the product of
the reaction to be [(ArNdC(Me)-(Me)CdNAr)Fe(CH2-
SiMe3)]C6F5 (15) arising from aryl group transfer rather
than the expected contact ion-pair, [(ArNdC(Me)-
(Me)CdNAr)] Fe(CH2SiMe3)][Me3SiCH2B(C6F5)3] (eq 6).
The borane byproduct Me3SiCH2B(C6F5)3 was also
detected by both 1H and 19F NMR spectroscopy.37
Attempts to generate the desired contact ion-pair by
conducting the alkyl abstraction reaction at low tem-
perature consistently yielded 15. No 1H NMR reso-
(35) Hermes, A. R.; Girolami, G. S. Organometallics 1988, 7, 394.
(36) tom Dieck, H.; Orlopp, A. Angew. Chem., Int. Ed. Engl. 1975,
14, 251.
(37) Spence, R. E.; Piers, W. E.; Sun, Y.; Parvez, M.; MacGillivray,
L. R.; Zaworotko, M. J . Organometallics 1998, 17, 2459.
(33) Spencer, L. P.; Altwer, R.; Wei, P.; Gelmini, L.; Gauld, J .;
Stephan, D. W. Organometallics 2003, 22, 3841.
(34) Schleis, T.; Spaniol, T. P.; Okuda, J .; Heinemann, J .; Mu¨lhaupt,
R. J . Organomet. Chem. 1998, 569, 159.

Four-Coordinate Fe(II) and Mn(II) Alkyl Complexes
Organometallics, Vol. 23, No. 2, 2004 243
Sch em e 3
Although the dialkyl complexes are inert toward excess
ethylene, the contact ion-pair 16 is modestly active for
polymerization. Stirring a toluene solution of 16 with
90 psi of ethylene yielded 0.104 g of monodisperse
polyethylene (PDI ) 2.74, Mn ) 333 000 versus poly-
ethylene) with a melting temperature of 132 °C.
Con clu d in g Rem a r k s
Synthesis of a series of tetrahedral, high-spin iron-
(II) dialkyl complexes of the general form L2FeR2 has
been described. This straightforward methodology has
been expanded to include the preparation of enantiopure
dialkyl derivatives from readily available and inexpen-
sive ligands and metal sources. Despite their relatively
low formal electron count, the 14-electron iron(II) dialkyl
derivatives are unreactive due to electronic factors
arising from their S ) 2 ground states rather than steric
considerations. However, addition of strong field ligands
such as carbon monoxide is sufficient to induce spin
crossover and, in the case of the R-diimine-ligated iron-
(II) dialkyl, produce ketone and the iron(0) tricarbonyl
complex. Reaction of the dialkyl complexes with the now
ubiquitous Lewis acid B(C6F5)3 produced results that
were dependent on the ancillary ligation. In the case of
the R-diimine ligand, aryl group transfer was observed,
whereas in the case of the (-)-sparteine and (S)-
(tBuBox) ligands, contact ion-pair complexes were ob-
tained. In the latter case, the contact ion-pairs were
found to be active for catalytic carbon-carbon bond
formation as judged by the polymerization of ethylene.
Ta ble 3. 19F a n d 11B NMR Ch em ica l Sh ifts for
F e(II) Ion -P a ir s
δ
19F NMR
(ppm)
δ
11B NMR
(ppm)
compound
[(-)-(sparteine)Fe(CH2SiMe3)]-
[B(C6F5)3CH2SiMe3] (16)
-161.49
-14.52
-14.75
-14.48
-13.12
-158.59
-124.45
-164.72
(-)-(sparteine)Fe(CH2C6H5)-
(B(C6F5)3CH2C6H5) (17)
-163.00
-126.41
-162.61
[(S)-(tBuBox)Fe(CH2SiMe3)]-
[B(C6F5)3CH2SiMe3] (18)
-160.53
-128.18
-162.93
[(S)-(tBuBox)Fe(CH2C6H5)]-
[B(C6F5)3CH2C6H5] (19)
-161.82
-129.24
Exp er im en ta l Section
Gen er a l Con sid er a tion s. All air- and moisture-sensitive
manipulations were carried out using standard vacuum line,
Schlenk, and cannula techniques or in an M. Braun inert
atmosphere drybox containing an atmosphere of purified
nitrogen. Solvents for air- and moisture-sensitive manipula-
tions were initially dried and deoxygenated using literature
procedures.39 The M. Braun drybox was equipped with a cold
well designed for freezing samples in liquid nitrogen. Argon
and hydrogen gas were purchased from Airgas Incorporated
and passed through a column containing manganese oxide
supported on vermiculite and 4 Å molecular sieves before
admission to the high-vacuum line. Benzene-d6 was purchased
from Cambridge Isotope Laboratories and distilled from
sodium metal under an atmosphere of argon and stored over
4 Å molecular sieves or sodium metal. Bromobenzene-d5 and
chloroform-d were purchased from Cambridge Isotope Labs
and distilled from CaH2. Iron(II) chloride, (-)-sparteine, carbon
monoxide, and LiCH2SiMe3 were purchased from Aldrich. The
FeCl2 was used as received, whereas (-)-sparteine was dis-
tilled from CaH2 and the LiCH2SiMe3 was recrystallized from
pentane and used as a solid. Carbon monoxide was passed
through a liquid nitrogen-cooled trap immediately before use.
B(C6F5)3 was purchased from Strem Chemicals and was used
as received. LiCH2CMe3,40 (S)-(tBuBox),41 and ArNdC(Me)-
(Me)CdNAr42 were prepared according to literature proce-
dures.
nances were observed for 15, although a single peak
centered at 28.20 ppm (∆ν1/2 ) 12.02 Hz) was detected
in the corresponding 2H NMR spectrum of 15-d 6. In
addition, three broad 19F resonances were observed at
-139.06, -154.04, and -162.32 ppm. The magnetic
susceptibility of 15 was determined by SQuID magne-
tometry and yielded a value of 5.38 µB, indicative of a
high-spin, S ) 2 molecule. Variable-temperature mag-
netic data collected between -80 and 40 °C yielded a
Curie constant of 3.60 mol‚K/emu, consistent with the
expected value (Θ > 0) for a paramagnetic molecule.
Unlike the R-diimine complex, reaction of the enan-
tiopure iron(II) dialkyl complexes with B(C6F5)3 did
produce the desired ion-pair complexes. Thus, addition
of 1 equiv of B(C6F5)3 to a pentane solution of 8 resulted
in immediate formation of a thick yellow clatharate
identified as [(-)-(sparteine)Fe(CH2SiMe3)][(C6F5)3B(CH2-
1
SiMe3)] (16) (Scheme 3). Although a H NMR spectrum
was not observed, three distinct resonances were ob-
served in the 19F NMR spectrum. Similar results were
obtained from reaction of 10 and 11 (Scheme 3). The
19F and 11B NMR data for each ion-pair complex are
compiled in Table 3 and are consistent with values
typically observed for four-coordinate borate anions.38
Although the structures of 16-18 have not been defini-
tively established by X-ray diffraction, NMR spectro-
scopic data, elemental analysis, and their solubility
properties are suggestive of alkyl group abstraction.
(39) Pangborn, A. B.; Giardello, M. A.; Grubbs, R. H.; Rosen, R. K.;
Timmers, F. J . Organometallics 1996, 15, 1518.
(40) Schrock, R. R.; Fellman, J . D. J . Am. Chem. Soc. 1978, 100,
3359.
(41) Evans, D. A.; Burgey, C. S.; Paras, N. A.; Vojkovsky, T.; Tregay,
S. W. J . Am. Chem. Soc. 1998, 120, 5824.
(42) Tempel, D. J .; J ohnson, L. K.; Huff, R. L.; White, P. S.;
Brookhart, M. J . Am. Chem. Soc. 2000, 122, 6686.
(38) Sadow, A. D.; Tilley, T. D. J . Am. Chem. Soc. 2003, 125, 9462.

244 Organometallics, Vol. 23, No. 2, 2004
Bart et al.
1H NMR spectra were recorded on Varian Mercury 300 and
Inova 400 and 500 spectrometers operating at 299.763,
399.780, and 500.62 MHz, respectively. All chemical shifts are
4H, CHMe2), 7.19 (m, 2H, Ar), 7.17 (s, 4H, Ar). 2H NMR
(benzene): δ 2.07 ppm.
P r ep a r a tion of [Ar NdC(Me)-(Me)CdNAr ]F eCl2 (1). A
1.0 L round-bottomed flask was charged with 20.7 g (51 mmol)
of ArNdC(CH3)-(CH3)CdNAr and approximately 600 mL of
THF. With stirring, 5.0 g (39 mmol) of FeCl2 was added at 25
°C. The deep blue suspension was stirred for 18 h and filtered
through Celite, and the resulting solid was extracted with an
additional 1.0 L of THF. The solvent was removed in vacuo,
and the resulting residue was washed with toluene to remove
any soluble impurities. Removal of the toluene afforded 25.3
g (47.6 mmol, 93%) of blue solid identified as 1 based on
1
reported relative to SiMe4 using H (residual) chemical shifts
of the solvent as a secondary standard. 2H NMR spectra were
recorded on a Varian Inova 500 spectrometer operating at
76.851 MHz, and the spectra were referenced using an external
benzene-d6, toluene-d8, or chloroform-d standard. 19F NMR
spectra were recorded on Varian Inova 400 and 500 spectrom-
eters operating at 376.127 and 470.997 MHz, respectively, and
were referenced to C6F6 in benzene-d6. 11B NMR spectra were
recorded on a Bruker ARX-300 spectrometer operating at
96.2936 MHz, and spectra are referenced to BF3‚Et2O in
comparison to literature data. 1H NMR (chloroform-d):
δ
1
benzene-d6. For paramagnetic compounds, H NMR data are
-18.31 (26.19, 2H, p-C6H3), -3.83 (131.62, 12H, CHMe2), 3.10
(52.54, 12H, CHMe2), 6.05 (42.95, 4H), 60.82 (120.41, 6H,
C(Me)), one not located. 2H NMR (CH2Cl2) for 1-d 6: δ 60.82
ppm (18.37, CD3).
reported with the chemical shift followed by the peak width
at half-height in hertz followed by integration value and, where
possible, peak assignment.
Unless stated otherwise, magnetic moments were measured
at 22 °C by the method originally described by Evans43 with
stock and experimental solutions containing a known amount
of a ferrocene standard. Solid state magnetic moments were
recorded using a Quantum Design SQuID magnetometer
running Magnetic Property Measurement System Revision 2
software. Data were recorded at 300 oersteads (Oe), and the
sample was prepared by sealing an NMR tube that was sewn
into a plastic straw with a needle and thread to prevent sample
migration. The ends of the straw were covered in Kapton tape.
The sample used was recrystallized several times and purity
determined by elemental analysis. The loaded sample was
centered within the magnetometer using the DC centering
scan at 20 K and 100 Oe. Data were acquired at 190-310 K
with one data point every 10 K.
Single crystals suitable for X-ray diffraction were coated
with polyisobutylene oil in a drybox and were quickly trans-
ferred to the goniometer head of a Siemens SMART CCD area
detector system equipped with a molybdenum X-ray tube (λ
) 0.71073 Å). Preliminary data revealed the crystal system.
A hemisphere routine was used for data collection and deter-
mination of lattice constants. The space group was identified
and the data were processed using the Bruker SAINT program
and corrected for absorption using SADABS. The structures
were solved using direct methods (SHELXS) completed by
subsequent Fourier synthesis and refined by full-matrix least-
squares procedures.
The molecular weight (Mn) and molecular weight distribu-
tion (Mw/Mn) of polyethylene were measured by a Waters
Alliance GPCV 2000 size exclusion chromatograph (SEC). The
SEC column set (four Waters HT 6E and one Waters HT 2)
was equilibrated at 140 °C and eluted with 1,2,4-trichloroben-
zene (1.0 mL/min) containing 0.01 wt % di-tert-butylhydroxy-
toluene (BHT). The molecular weight (Mn) and molecular
weight distribution (Mw/Mn) were measured relative to a
polyethylene calibration curve. The melting point of the
polyethylene sample was measured by a TA Instruments DSC
Q100. The sample run was performed from 0 to 220 °C with a
heating rate of 10 °C/min.
P r ep a r a tion of (-)-(Sp a r tein e)F eCl2 (2). A 20 mL scin-
tillation vial was charged with 1.60 g (12.6 mmol) of FeCl2 and
slurried in approximately 5 mL of THF. A THF solution
containing 2.95 g (12.6 mmol) of (-)-sparteine was added to
the reaction mixture and stirred. Over time, a white solid
formed. After 18 h, THF was removed in vacuo and the
resulting solid was washed with diethyl ether and collected
by filtration. The white solid was washed with several portions
of diethyl ether to yield 4.37 g (96% yield) of 2. Magnetic
1
susceptibility (chloroform-d): µeff ) 4.86 µB. H NMR (chloro-
form-d): δ -153.02 (893.84, 1H), -91.57 (751.88, 1H), -51.14
(182.43 Hz, 1H), -41.44 (178.22 Hz, 2H), -35.12 (673.38 Hz,
1H), -27.66 (418.29 Hz, 1H), -24.69 (103.04, 1H), -17.32
(79.59 Hz, 1H), -15.46 (104.58, 1H), -8.59 (139.52, 2H), 9.40
(47.82, 1H), 14.47 (94.46, 1H), 28.56 (158.56 Hz, 1H), 30.62
(169.75, 1H), 39.17 (116.59, 1H), 49.75 (184.57, 1H), 51.25
(191.47 Hz, 1H), 123.05 (929.15, 1H), 145.91 (737.87, 1H),
148.92 (820.82, 1H), 207.93 (551.37, 1H), 310.52 (747.33, 1H),
349.94 (153.88, 1H), 388.03 (124.13, 1H).
P r ep a r a tion of (S)-(tBu Box)F eCl2 (3). A 20 mL scintil-
lation vial was charged with 0.471 g (3.74 mmol) of FeCl2 and
approximately 5 mL of THF. A THF solution containing 1.10
g (3.74 mmol) of 2,2-bis[2-[4(S)-(CMe3)-1,3-oxazolinyl]propane
was added and the resulting clear, off-white reaction mixture
stirred for 18 h. The THF was removed in vacuo, the remaining
off-white solid was dissolved in CH2Cl2, and the remaining
FeCl2 was removed by filtration. The dichloromethane was
removed in vacuo to yield 1.13 g (72% yield) of 3. Anal. Calcd
for C17H38Cl2N2O2Fe: C, 48.48; H, 7.18; N, 6.65. Found: C,
48.20; H, 6.71; N, 6.42. Magnetic susceptibility (chloroform-
d) µeff ) 4.26 µB. 1H NMR (chloroform-d): δ -20.01 (288.60
Hz, 18H, CMe3), -15.47 (484.61, 2H), 12.16 (98.20, 2H), 12.63
(73.44, 2H), 17.71 (109.08, 6H, Me).
P r ep a r a tion of (-)-(Sp a r tein e)Mn Br 2 (4). A procedure
similar to that for 2 was used with 0.916 g (4.27 mmol) of
MnBr2 and 1.09 g (4.27 mmol) of (-)-sparteine, yielding 1.78
g (93%) of 4. Anal. Calcd for C15H26Br2N2Mn: C, 40.11; H, 5.84;
N, 6.24. Found: C, 39.88; H, 5.69; N, 6.19.
P r ep a r a t ion of [Ar NdC(Me)-(Me)CdNAr ]F e(CH2Si-
Me3)Cl (5). A 100 mL round-bottomed flask was charged with
2.00 g (3.80 mmol) of 1 and 0.354 g (3.80 mmol) of LiCH2SiMe3
and attached to a 180° needle valve. On the vacuum line,
approximately 60 mL of diethyl ether was added by vacuum
transfer at -78 °C. The reaction mixture was maintained at
this temperature for 1 h, and the diethyl ether was removed
in vacuo. The blue-green residue was transferred into the
drybox, extracted with pentane, and filtered through Celite
to afford 1.85 g (84%) of a blue-green solid identified as 5. Anal.
Calcd for C32H51ClN2FeSi2: C, 65.91; H, 8.82; N, 4.80. Found:
C, 65.62; H, 8.51; N, 5.24. Magnetic susceptibility (benzene-
P r ep a r a tion of Ar NdC(CD3)-(CD3)CdNAr . A 100 mL
round-bottomed flask was charged with 0.49 g (5.7 mmol) of
2,3-butanedione-d6, 2.11 g (11.9 mmol) of diisopropylaniline,
and 5 mL of CH3OD and stirred. To the reaction mixture was
added 0.50 mL (13.2 mmol) of formic acid-d1 and resulted in
the formation of a yellow precipitate within 10 min. The
reaction mixture was stirred for 19 h, and the resulting yellow
solid was collected by filtration. The solid was washed with
20 mL of methanol and dried under vacuum to afford 1.96 g
1
(4.8 mmol, 84%) of a yellow solid. Integration of the H NMR
spectrum revealed approximately 67% D incorporation. 1H
NMR (chloroform-d): δ 1.05 (d, 4.7 Hz, 12H, CHMe2), 1.08 (d,
4.9 Hz, 12H, CHMe2) 2.07 (s, <2H, CD2H), 2.72 (sept, 6.9 Hz,
1
d6): µeff ) 4.88 µB. H NMR (benzene-d6): δ -74.36 (283.29,
6H, C(Me)), -22.29 (51.52, 2H), 2.86 (224.18, 24H, CHMe2),
15.57 (85.41, 2H), 21.10 (428.30, 9H, CH2SiMe3), one not
2
(43) Sur, S. K. J . Magn. Res. 1989, 82, 169.
located. H NMR (toluene) for 5-d 6: δ -77.71 (45.76, CD3).

Four-Coordinate Fe(II) and Mn(II) Alkyl Complexes
Organometallics, Vol. 23, No. 2, 2004 245
P r ep a r a t ion of [Ar NdC(Me)-(Me)CdNAr ]F e(CH2Si-
Me3)2 (6). A 100 mL round-bottomed flask was charged with
4.10 g (7.7 mmol) of 1 and 1.50 g (15.5 mmol) of LiCH2SiMe3.
A 180° needle valve was attached, and approximately 60 mL
of diethyl ether was added by vacuum transfer at -78 °C. The
reaction mixture was maintained at this temperature for 1 h,
and the diethyl ether was removed in vacuo. The purple
residue was transferred into the drybox and extracted with
pentane to afford 4.46 g (91%) of a purple solid identified as
6. Anal. Calcd for C36H62N2FeSi2: C, 68.10; H, 9.84; N, 4.41.
Found: C, 67.86; H, 9.57; N, 4.41. Magnetic susceptibility
as 10. Anal. Calcd for C29H40N2Fe: C, 73.72; H, 8.53; N, 5.93.
Found: C, 73.62; H, 8.07; N, 5.85. Magnetic susceptibility
(benzene-d6): µeff ) 4.59 µB. 1H NMR (benzene-d6): δ -147.96
(1019.38, 1H), -85.51 (559.09, 1H), -75.96 (583.40, 1H),
-73.66 (27.52, 1H), -57.68 (25.60, 1H), -49.61 (389.25, 1H),
-41.28 (212.29, 1H), -33.23 (95.66, 1H), -22.25 (73.69, 1H),
-21.27 (238.03, 1H), -14.81 (40.22, 1H), -12.20 (46.75, 1H),
-4.62 (36.11, 1H), 5.37 (19.53, 1H), 6.02 (25.46, 1H), 14.99
(14.23, 1H), 23.95 (19.25, 2H), 25.36 (92.17, 1H), 26.22 (77.78,
1H), 30.60 (22.06, 2H), 31.41 (29.11, 1H), 41.12 (56.27, 2H),
116.226 (129.843, 1H), 131.79 (841.68, 1H), 137.43 (701.40,
1H), 149.27 (625.18, 1H), 218.68 (130.49, 1H).
1
(benzene-d6): µeff ) 4.71 µB. H NMR (benzene-d6): δ -27.10
P r ep a r a tion of (S)-(tBu Box)F e(CH2SiMe3)2 (11). A pro-
cedure similar to that for 8 was used employing 0.200 g (0.476
mmol) of 3 and 0.090 g (0.957 mmol) of LiCH2SiMe3, yielding
0.221 g (88%) of yellow crystals identified as 11. Anal. Calcd
for C25H52N2O2Si2Fe: C, 57.23; H, 9.99; N, 5.34. Found: C,
56.95; H, 9.28; N, 5.45. Magnetic susceptibility (benzene-d6):
(111.41, 2H, p-C6H3), -0.69 (526.91, 12H, CHMe2), 1.17 (15.77,
4H), 2.72 (185.23, 12H, CHMe2), 14.17 (527.7, 18 H, CH2SiMe3),
two peaks not located.
P r ep a r a tion of [[Ar NdC(Me)-(Me)CdNAr ]F e(µ-Cl)]2
(7). A 50 mL round-bottomed flask was charged with 0.500 g
(0.940 mmol) of 1 and approximately 35 mL of diethyl ether.
The resulting slurry was chilled to -35 °C in the glovebox
freezer for approximately 15 min. Likewise, a scintillation vial
was charged with 0.060 g (0.94 mmol) of LiCH2CH(CH3)2 and
approximately 2 mL of diethyl ether and chilled to -35 °C.
With stirring, the LiCH2Si(CH3)2 solution was added to the
ethereal suspension of 1. The reaction was allowed to warm
to 25 °C and stirred for 45 min. The solution was filtered
through Celite and the solvent removed in vacuo to yield 0.180
g (19%) of a green solid identified as 7. Anal. Calcd for
1
µeff ) 4.83 µB. H NMR (benzene-d6): δ -20.64 (475.30, 18H),
-12.17 (193.93), -6.92 (41.15), -4.81 (75.82), -1.17 (40.06),
1.24 (17.88), 1.73 (22.48), 12.22 (70.57), 13.71 (115.37, 4H),
15.35 (150.24, 18H), 23.51 (194.26, 2H).
P r ep a r a tion of (S)-(tBu Box)F e(CH2P h )2 (12). This mol-
ecule was prepared in a manner identical to that for 8 with
0.208 g (0.495 mmol) of 3 and 0.128 g (0.990 mmol) of
KCH2C6H5, yielding 0.095 g (57%) of a yellow solid identified
as 12 following recrystallization from diethyl ether. Anal. Calcd
for C31H44N2O2Fe: C, 69.92; H, 8.33; N, 5.26. Found: C, 70.02;
C
56H80N4Cl2Fe2: C, 67.81; H, 8.13; N, 5.65. Found: C, 67.58;
H, 8.13; N, 5.18. Magnetic susceptibility (benzene-d6): µeff
)
H, 8.54; N, 5.63. Magnetic susceptibility (benzene-d6): µeff
)
3.86 µB. 1H NMR (benzene-d6): δ -21.19 (41.46, 12H, CHMe2),
-18.06 (34.14, 2H p-C6H3), -9.50 (25.59, 4H), -0.01 (227.57,
4H), 5.42 (22.71, 12H, CHMe2), one not located (CH3).
4.60 µB. 1H NMR (benzene-d6): δ -65.40 (29.12), -44.09
(222.68), -24.28 (293.29), -17.14 (230.16), -6.00 (64.02), 5.68
(105.38), 10.76 (56.56), 17.41 (55.88), 21.39 (73.35), 29.88
(27.2).
P r ep a r a tion of (-)-(Sp a r tein e)F e(CH2SiMe3)2 (8). A 20
mL scintillation vial was charged with 0.200 g (0.554 mmol)
of 2 and approximately 5 mL of diethyl ether. A 10 mL diethyl
ether solution containing 0.104 g (1.11 mmol) of LiCH2SiMe3
was added, and the reaction mixture was stirred for 6 h,
forming a brown solution and white precipitate. The mixture
was filtered through a pad of Celite and the diethyl ether
removed in vacuo, yielding a brown oil. Recrystallization from
pentane afforded 0.170 g (66%) of white crystals identified as
8. Anal. Calcd for C23H48N2FeSi2: C, 59.45; H, 10.41; N, 6.03.
Found: C, 59.36; H, 10.03; N, 6.00. Magnetic susceptibility
P r ep a r a tion of (-)-(Sp a r tein e)Mn (CH2SiMe3)2 (13). A
procedure similar to that for 8 was used with 0.120 g (0.267
mmol) of 4 and 0.050 g (0.531 mmol) of LiCH2SiMe3, yielding
0.097 g (79%) of 13 as white crystals. Anal. Calcd for C23H48N2-
MnSi2: C, 59.57; H, 10.43; H, N, 6.04. Found: C, 59.23; H,
10.20; N, 6.14. Magnetic susceptibility (benzene-d6): µeff ) 5.77
µB.
Ch a r a cter iza tion of Ar NdC(Me)-(Me)CdNAr F e(CO)3
(14). Anal. Calcd for C31H40N2FeO3: C, 68.38; H, 7.40; N, 5.14.
Found: C, 68.28; H, 7.06; N, 5.01. 1H NMR (benzene-d6): δ
1.04 (d, 6.87 Hz, CHMe2), 1.41 (d, 6.60 Hz, CHMe2), 1.51 (s,
6H, Me), 2.99 (sept, 6.87 Hz, 4H), 7.09-7.22 (m, 6H, Ar). 13C
NMR (benzene-d6): δ 17.59 (CHMe2), 24.72 (CHMe2), 24.72
(CHMe2), 28.34 (CH3), 124.34 (Ar), 127.53 (Ar), 140.77 (Ar),
151.37 (Ar), 152.27(CdN), 212.94 (CO). IR (pentane): ν 2030,
1
(benzene-d6): µeff ) 4.83 µB. H NMR (benzene-d6): δ -33.03
(310.62, 1H), -19.23 (129.88, 1H), -18.32 (103.50, 1H), -10.30
(79.62, 1H), -5.83 (71.18, 1H), -1.82 (1.81, 1H), 8.80 (82.76,
1H), 10.14 (156.05, 9H, CH2SiMe3), 12.67 (59.52, 1H), 13.62
(80.16, 1H), 15.71 (151.02, 9H, CH2SiMe3), 24.95 (53.17, 1H),
25.66 (64.50, 1H), 27.42 (63.30, 1H), 78.10 (606.23, 1H), 121.60
(245.85, 1H), 130.33 (296.31, 1H), 214.38 (137.82, 1H), 276.06
(235.70, 1H), 279.57 (330.70, 1H).
1959 cm-1
.
P r epar ation of Ar NdC(Me)-(Me)CdNAr Fe(CH2SiMe3)-
(C6F 5) (15). A 20 mL scintillation vial was charged with 0.100
g (0.157 mmol) of 5 and approximately 5 mL of pentane. A 10
mL pentane solution containing 0.080 g (0.157 mmol) of
B(C6F5)3 was added to the vial, resulting in a dark green
solution. After 30 min, the pentane was removed in vacuo,
yielding a green solid. Successive recrystallizations from
pentane were required to remove Me3SiCH2B(C6F5)3 and
yielding 0.060 g (55%) of pure 15. Anal. Calcd for C38H51BF5N2-
SiFe: C, 63.86; H, 7.19; N, 3.92. Found: C, 63.78; H, 6.84; N,
P r ep a r a tion of (-)-(Sp a r tein e)F e(CH2CMe3)2 (9). This
molecule was prepared in a manner identical to that for 8 with
0.105 g (0.290 mmol) of 2 and 0.045 g (0.582 mmol) of LiCH2-
CMe3 to yield 0.084 g (67%) of white crystals identified as 8.
Anal. Calcd for C25H48N2Fe: C, 69.42; H, 11.19; N, 6.48.
Found: C, 69.08; H, 10.51; N, 6.45. Magnetic susceptibility
1
(benzene-d6): µeff ) 4.68 µB. H NMR (benzene-d6): δ -50.03
(3406.62, 1H), -46.55 (3762.96, 1H), -38.14 (252.55, 1H),
-22.02 (130.08, 1H), -19.77 (112.80, 1H), -11.35 (60.2, 1H),
-10.60 (57.7, 1H), -1.47 (102.50, 1H), 0.091 (41.26, 1H), 3.27
(18.11, 1H), 4.15 (78.50, 1H), 10.26 (67.65, 1H), 17.51 (86.58,
1H), 19.41 (96.41, 1H), 22.25 (242.54, 9H, CMe3), 27.77 (72.95,
1H), 28.44 (72.82, 1H), 30.15 (70.81, 1H), 33.33 (290.80, 9H,
CMe3), 66.50 (334.70, 1H), 117.64 (3024.48, 1H), 129.80
(1124.15, 1H), 205.30 (271.45, 1H), 260.82 (381.55, 1H), 275.94
(377.38, 1H).
2
3.28. H NMR (benzene): δ 28.20 ppm (12.02, 6D). 19F NMR
(benzene-d6): δ -162.32 (m, 2F, m-C6F5), -154.04 (s, 1F,
C6F5), -139.06 (m, 2F, C6F5).
P r ep a r a t ion of [(-)-(Sp a r t ein e)F e(CH2SiMe3)][B(C6-
F 5)3CH2SiMe3] (16). A 20 mL scintillation vial was charged
with 0.020 g (0.0431 mmol) of 8 and 0.022 g (0.0431 mmol) of
B(C6F5)3. Approximately 5 mL of pentane was added. The
solution turned bright yellow and a viscous oil settled out. The
pentane was decanted and the oil dried in vacuo, yielding 0.024
g (57%) of 16. Anal. Calcd for C41H48N2Si2F15BFe: C, 50.42;
H, 4.94; N, 2.87. Found: C, 50.01; H, 4.43; N, 3.03. 19F NMR
(bromobenzene-d5): δ -161.49 (62.05, 3F), -158.69 (96.74, 6F),
P r ep a r a tion of (-)-(Sp a r tein e)F e(CH2P h )2 (10). This
molecule was prepared in a manner identical to that for 8 with
0.203 g (0.562 mmol) of 2 and 0.146 g (1.12 mmol) of
KCH2C6H5, yielding 0.242 g (93%) of a yellow solid identified

246 Organometallics, Vol. 23, No. 2, 2004
Bart et al.
-124.45 (88.82, 6F). 11B NMR (bromobenzene-d5): δ -14.52
ppm (∆ν1/2 ) 92 Hz).
temperature for approximately 12 min, at which time the
reaction mixture was very viscous. The reaction was quenched
with a 25% hydrochloric acid/methanol solution. Wa r n in g:
High ly exoth er m ic; vigor ou s bu bblin g a n d h ea t evolu -
tion obser ved . The solution was transferred to a beaker with
300 mL of the acidic methanol solution. This was allowed to
stir for several hours, resulting in the precipitation of poly-
ethylene. The polymer was then collected by filtration and
dried in vacuo. The resulting polymer was characterized by
its melting temperature as determined by DSC.
Gen er a l Hyd r ogen a tion P r oced u r e. A thick walled glass
vessel was charged with 0.028 mmol of catalyst, approximately
1 mL of toluene, and a stirbar. Approximately 0.5 mL of
1-hexene was vacuum transferred into a calibrated tube. This
was then vacuum transferred into the thick walled glass vessel
followed by the addition of 4 atm of dihydrogen. The reaction
was stirred for 22 h at ambient temperature and the reaction
periodically analyzed by GC.
P r ep a r a tion of (-)-(Sp a r tein e)F e(CH2C6H5)(B(C6F 5)3-
CH2C6H5) (17). The same procedure was used as that for 16
using 0.020 g (0.042 mmol) of 10 and 0.022 g (0.042 mmol) of
B(C6F5)3, yielding 0.025 g (60%) of a thick yellow oil identified
as 17. Anal. Calcd for C47H40N2F15BFe: C, 57.34; H, 4.10; N,
2.85. Found: C, 56.97; H, 3.84; N, 2.96. 19F NMR (bromoben-
zene-d5): δ -164.72 (113.11, 6F), -163.00 (112.17, 3F),
-126.41 (114.19, 6F). 11B NMR (bromobenzene-d5): δ -14.75
ppm (∆ν1/2 ) 113 Hz).
P r ep a r a tion of [(S)-(tBu Box)F e(CH2SiMe3)][B(C6F 5)3-
CH2SiMe3] (18). The same procedure was used as that for 16
using 0.020 g (0.0382 mmols) of 11 and 0.020 g (0.0382 mmols)
of B(C6F5)3 and yielding 0.029 g (72%). Anal. Calcd for
C
43H52N2O2F15BSi2Fe: C, 49.82; H, 5.06; N, 2.70. Found: C,
49.54; H, 5.16; N, 2.67. 19F NMR (bromobenzene-d5):
δ
-162.61 (65.29, 3F), -160.53 (126.89, 6F), -128.18 (144.03,
6F). 11B NMR (bromobenzene-d5): δ -14.48 ppm (∆ν1/2 ) 181
Hz).
Ack n ow led gm en t. For financial support we would
like to thank Cornell University and the National
Science Foundation for a CAREER award to P.J .C.
S.C.B. also thanks the National Institutes of Health for
support through the Chemistry and Biology Interface
Training Grant at Cornell. A.K.S. acknowledges support
from the Cornell Center for Materials Research REU
program. We would also like to thank Sara Barron and
Louis Whaley for assistance with SQuID magnetometry
and Anthony Condo for assistance with DSC and GPC
measurements.
P r ep a r a t ion of [(S)-(t Bu Box)F e(CH2C6H 5)][B(C6F 5)3-
CH2C6H5] (19). The same procedure was used as that for 16
using 0.020 g (0.038 mmols) of 12 and 0.19 g (0.038 mmols) of
B(C6F5)3 and yielding 0.026 g (65%) of 19. Anal. Calcd for
C
49H44N2O2F15BSi2Fe: C, 56.35; H, 4.25; N, 2.86. Found: C,
56.01; H, 4.44; N, 2.81. 19F NMR (bromobenzene-d5):
δ
-162.93 (90.90, 3F), -161.82 (76.11, 3F), -129.24 (75.55, 6F).
11B NMR (bromobenzene-d5): δ -13.12 ppm (∆ν1/2 ) 108 Hz).
Eth ylen e P olym er iza tion P r oced u r e. A 100 mL Fis-
cher-Porter bottle was charged with approximately 40 mL of
toluene, 400 equiv of solid MAO, and a stirbar and was sealed.
A syringe was charged with 0.025 mmol of the appropriate
iron catalyst slurried in approximately 3 mL of toluene. The
needle of the syringe was capped with a septum to avoid
exposure to air and leakage. The Fischer-Porter bottle was
Su p p or tin g In for m a tion Ava ila ble: SQuID magnetic
data for 15 as well as crystallographic data for 3, 6, 8, 11, and
15 including full atom-labeling schemes, bond distances, and
bond angles. This material is available free of charge via the
Internet at http://pubs.acs.org.
then purged through several times with ethylene. At
a
pressure of 15 psi, the catalyst solution in the syringe was
added. The bottle was then sealed and the pressure was
increased to 90 psi. The polymerization was run at ambient
OM034188H