Low-valent α-diimine iron complexes for catalytic olefin hydrogenation

Article abstract of DOI:10.1021/om050625b

A family of low-valent α-diimine iron complexes has been synthesized and their utility in catalytic olefin hydrogenation reactions evaluated. Reduction of the ferrous dichloride complex [ArN=C(Me)C(Me)=NAr]FeCl₂ (Ar = 2,6-(CHMe₂)₂-C₆H₃) with sodium amalgam in benzene or toluene furnished the iron arene complexes, [ArN=C(Me)C(Me)=NAr]Fe(η⁶-C₆H₅R) (R = H, Me). The solid-state structure of the toluene adduct revealed a contracted carbon-carbon backbone, short iron-imine bonds, and elongated imine nitrogen-carbon distances, suggesting significant reduction of the α-diimine ligand. The analogous reduction in alkane solvents afforded the bis(α-diimine) complex [ArN=C(Me)C(Me)=NAr]₂Fe, which has also been crystallographically characterized. The arene complexes and the bis(a-diimine) complexes are inactive for catalytic olefin hydrogenation. Performing the reduction in the presence of internal alkynes such as diphenylacetylene and bis(trimethylsilyl)acetylene furnished the alkyne adducts [ArN=C(Me)C(Me)=NAr]Fe(η²-RC=CR) (R = Ph, SiMe₃ ). Analogous olefin complexes with 1,5-cyclooctadiene and cycloctene have also been isolated using similar reduction procedures. The olefin adducts provide more active precatalysts than the alkyne compounds for the hydrogenation of 1-hexene. In each case, formation of rfarene adducts serves as a major catalyst deactivation pathway.

Full text of DOI:10.1021/om050625b

5518
Organometallics 2005, 24, 5518-5527
Low-Valent r-Diimine Iron Complexes for Catalytic
Olefin Hydrogenation
Suzanne C. Bart, Eric J. Hawrelak,† Emil Lobkovsky, and Paul J. Chirik*
Department of Chemistry and Chemical Biology, Baker Laboratory, Cornell University,
Ithaca, New York 14853
Received July 25, 2005
A family of low-valent R-diimine iron complexes has been synthesized and their utility in
catalytic olefin hydrogenation reactions evaluated. Reduction of the ferrous dichloride complex
[ArNdC(Me)C(Me)dNAr]FeCl2 (Ar ) 2,6-(CHMe2)2-C6H3) with sodium amalgam in benzene
or toluene furnished the iron arene complexes, [ArNdC(Me)C(Me)dNAr]Fe(η6-C6H5R) (R )
H, Me). The solid-state structure of the toluene adduct revealed a contracted carbon-carbon
backbone, short iron-imine bonds, and elongated imine nitrogen-carbon distances, sug-
gesting significant reduction of the R-diimine ligand. The analogous reduction in alkane
solvents afforded the bis(R-diimine) complex [ArNdC(Me)C(Me)dNAr]2Fe, which has also
been crystallographically characterized. The arene complexes and the bis(R-diimine)
complexes are inactive for catalytic olefin hydrogenation. Performing the reduction in the
presence of internal alkynes such as diphenylacetylene and bis(trimethylsilyl)acetylene
furnished the alkyne adducts [ArNdC(Me)C(Me)dNAr]Fe(η2-RCtCR) (R ) Ph, SiMe3).
Analogous olefin complexes with 1,5-cyclooctadiene and cycloctene have also been isolated
using similar reduction procedures. The olefin adducts provide more active precatalysts than
the alkyne compounds for the hydrogenation of 1-hexene. In each case, formation of η6-
arene adducts serves as a major catalyst deactivation pathway.
Introduction
oligomerization of ethylene and R-olefins promoted by
pyridine(diimine) (PDI) iron dihalides activated with
methylalumoxane. Inspired by these findings, we sought
to prepare well-defined, single-component iron com-
plexes that could serve as effective catalysts for carbon-
hydrogen, carbon-carbon, and carbon-silicon bond
construction in small organic molecules.
Previous work from Wrighton and co-workers estab-
lished that photogenerated [Fe(CO)3] is an active cata-
lyst for the hydrogenation, hydrosilation, and isomer-
ization of olefins.8 Similar results were obtained from
thermal activation, although temperatures in excess of
200 °C were required for reasonable activity.9 Seeking
access to a similar low-valent iron fragment under mild
conditions, our laboratory has been exploring the reduc-
tion chemistry of aryl-substituted pyridine(diimine)
iron(II) dihalide complexes.10 Tridentate pyridine(di-
imines) were chosen as initial supporting ligands due
to their ease of synthesis, steric and electronic modular-
ity, and ability to support catalytically active iron
complexes.5-7 Furthermore, this class of ligand is known
to engage in redox chemistry with the metal center,
delocalizing charge over the conjugated π system of the
pyridine-diimine core,11 making them attractive for
Organometallic and coordination complexes of iron
have received considerable interest, due to their utility
in biomimetic chemistry1 as well as in small-molecule2
and hydrocarbon activation processes.3 Because of high
natural abundance, environmental compatibility, and
relatively low cost, iron complexes are emerging as
potential surrogates for precious metals in catalytic
bond-forming processes.4 A recent breakthrough in this
area was provided in independent reports by Brookhart,5
Gibson,6 and others7 of the catalytic polymerization and
* To whom correspondence should be addressed. E-mail: pc92@
cornell.edu.
† Present address: Department of Chemistry, 237A Hartline Science
Center, Bloomsburg University, Bloomsburg, PA 17815.
(1) (a) Evans, D. J.; Pickett, C. J. Chem. Soc. Rev. 2003, 32, 268. (b)
MacKay, B. A.; Fryzuk, M. D. Chem. Rev. 2004, 104, 385.
(2) (a) Vela, J.; Stoian, S.; Flaschenriem, C. J.; Munck, E.; Holland,
P. L. J. Am. Chem. Soc. 2004, 126, 4522. (b) Smith, J. M.; Lachicotte,
R. J.; Pittard, K. A.; Cundari, T. R.; Lukat-Rodgers, G.; Rodgers, K.
R.; Holland, P. L. J. Am. Chem. Soc. 2001, 123, 9222. (c) Betley. T. A.;
Peters, J. C. J. Am. Chem. Soc. 2004, 126, 6252. (d) Komiya, S.; Akita,
M.; Yoza, A.; Kasuga, N.; Fukuoka, A.; Kai, Y. Chem. Commun. 1993,
787. (e) Hirano, M.; Akita, M.; Morikita, T.; Kubo, A.; Fukuoka, A.;
Komiya, S. J. Chem. Soc., Dalton Trans. 1997, 3453. (f) Chatt, J.;
Hayter, R. G. J. Chem. Soc. 1961, 5507.
(3) Perthuisot, C.; Jones, W. D. New J. Chem. 1994, 18, 621.
(4) Bolm, C.; Legros, J.; Le Paih, J.; Zani, L. Chem. Rev. 2004, 104,
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(5) (a) Small, B. L.; Brookhart, M.; Bennett, A. M. A. J. Am. Chem.
Soc. 1998, 120, 4049. (b) Small, B. L.; Brookhart, M. J. Am. Chem.
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V. C.; White, A. J. P.; Williams, D. J. Polyhedron 2004, 23, 2921. (c)
Britovsek, G. J. P.; Gibson, V. C.; Kimberley, B. S.; Maddox, P. J.;
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Passaglair, E.; Gragnoli, T. Organometallics 2004, 23, 6087. (b) Sun,
W. H.; Tang, X. B.; Gao, T. L.; Wu, B.; Zhang, W. J.; Ma, H. W.
Organometallics 2004, 23, 5037. (c) Kim, I.; Han, B. H.; Ha, Y. S.; Ha,
C. S.; Park D. W. Catal. Today 2004, 93-95, 281.
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10.1021/om050625b CCC: $30.25 © 2005 American Chemical Society
Publication on Web 10/13/2005

Low-Valent R-Diimine Iron Complexes
Organometallics, Vol. 24, No. 23, 2005 5519
stabilizing low-valent iron species. Indeed, reduction
of (iPrPDI)FeBr2 (iPrPDI ) ((2,6-CHMe2)2C6H3NdCMe)2-
C5H3N) with excess 0.5% sodium amalgam under an
atmosphere of N2 furnished an unusual bis(dinitrogen)
complex, (iPrPDI)Fe(η1-N2)2.10 Both solid-state and solu-
tion magnetic susceptibility measurements revealed a
paramagnetic ground state with two unpaired electrons,
giving the appearance of a high-spin iron(0) d8 complex.
However, a combination of spectroscopic and computa-
tional studies support involvement of the conjugated π
system of the pyridine(diimine), suggesting a higher
oxidation state. After our findings, Danopoulos and co-
workers12 synthesized an iron(0) bis(dinitrogen) complex
with an S ) 0 ground state. In this case, the supporting
bis(N-heterocyclic carbene)pyridine ligand is a weaker
π acceptor and is less likely to facilitate metal-ligand
electron transfer.
complexes is described and their utility in catalytic
hydrogenation reactions evaluated.
Results and Discussion
Reduction of [ArNdC(Me)C(Me)dNAr]FeCl2. Fol-
lowing the method used for the preparation of (iPrPDI)-
Fe(η1-N2)2,10 a toluene slurry of the four-coordinate
iron(II) dichloride complex [ArNdC(Me)C(Me)dNAr]-
20
FeCl2 was stirred with an excess of 0.5% sodium
amalgam under 1 atm of dinitrogen. Filtration and
recrystallization from pentane afforded a bright red-
orange diamagnetic powder identified as the iron arene
complex [ArNdC(Me)C(Me)dNAr]Fe(η6-C7H8) (1) (eq 1).
Significantly, (iPrPDI)Fe(η1-N2)2 is an effective catalyst
for both the hydrogenation and hydrosilation of olefins
and alkynes at 22 °C. Synthetically useful activities are
observed at part per million levels of iron and, unlike
precious metal catalysts, do not require polar organic
solvents for optimal activity.13,14 These observations
prompted investigation into other formally iron(0) plat-
forms with the goal of generating more active catalysts
with a wider substrate scope. Specifically, we were
interested in generating [L2Fe0] fragments with the
expectation that bidentate “ancillary” ligands would
engender more reactive metal centers than their tri-
dentate counterparts by virtue of their lower formal
electron count.
The analogous reduction in benzene furnished the
corresponding η6-benzene adduct [ArNdC(Me)C(Me)d
NAr]Fe(η6-C6H6) (2). These results highlight the pro-
pensity of less congested, formally iron(0) compounds
to attain a coordinatively saturated 18-electron con-
figuration. Both 1 and 2 are similar to the R-diimine
iron(0) toluene compounds [RNdC(Me)C(Me)dNR]Fe-
t
i
(η6-C7H8) (R ) Bu, Cy, Pr, 2,6-Me2-C6H3), prepared
previously by displacement of ethylene from (η6-C7H8)-
Fe(η2-C2H4)2.18b
R-Diimine ligands, [RNdC(R1)C(R1)dNR], seemed
ideal for this purpose, offering a low-lying molecular
orbital that is antibonding with respect to the imine
portion of the molecule but bonding with respect to the
carbon-carbon backbone.15,16 This synergistic interac-
tion may facilitate back-donation from an electron-rich
iron center and allow isolation of suitable catalyst
precursors. Iron R-diimine complexes have precedent in
catalytic bond-forming reactions serving as initiators in
atom transfer radical polymerization,17 catalysts for the
dimerization of dienes,18 and the polymerization of
olefins.19 In this article we describe our initial explora-
tion of reduced R-diimine iron chemistry. Synthesis of
a family of R-diimine iron arene, alkyne, and olefin
Diamagnetic 1 and 2 were readily characterized by a
1
combination of H and 13C NMR spectroscopy. For 1,
the number of resonances expected for a C2v-symmetric
ligand environment is observed, indicating rapid rota-
tion of the toluene ring on the time scale of the NMR
experiment. The aromatic hydrogens of the coordinated
toluene appear at 4.40 (meta), 4.80 (ortho), and 5.69
(para) ppm in benzene-d6, shifted well upfield from those
of free toluene, and are diagnostic of coordination to
iron.18b Significantly, these resonances are retained in
benzene-d6, demonstrating that arene exchange does not
occur rapidly at ambient temperature. The spectroscopic
features of both 1 and 2 are in good agreement with
previous observations with [ArNdC(Me)C(Me)dNAr]-
Fe(η6-C7H8) compounds, where both fast arene rotation
and reluctance to participate in arene exchange reac-
tions were observed.18b
(11) (a) Sugiyama, H.; Korobkov, I.; Gambarotta, S.; Mo¨ller, A.;
Budzelaar, P. H. M. Inorg. Chem. 2004, 43, 5771. (b) Enright, D.;
Gambarotta, S.; Yap, G. P. A.; Budzelaar, P. H. M. Angew. Chem., Int.
Ed. 2002, 41, 3873. (c) Budzelaar, P. H. M.; de Bruin, B.; Gal, A. W.;
Wieghardt, K.; van Lenthe, J. H. Inorg. Chem. 2001, 40, 4649. (d) de
Bruin, B.; Bill, E.; Bothe, E.; Weyermu¨ller, T.; Wieghardt, K. Inorg.
Chem. 2000, 39, 2936.
(12) Danopoulos, A. A.; Wright, J. A.; Motherwell, W. B. Chem.
Commun. 2005, 784.
(13) Schrock, R. R.; Osborn, J. A. J. Am. Chem. Soc. 1971, 93, 3091.
(14) For a zwitterionic Rh catalyst that displays high activity in
nonpolar media see: Betley, T. A.; Peters, J. C. Angew. Chem., Int.
Ed. 2003, 42, 2385.
(15) Uhlig, E. Pure Appl. Chem. 1988, 60, 1235.
(16) Reinhold, J.; Benedix, R.; Birner, P.; Hennig, H. Inorg. Chim.
Acta 1978, 33, 209.
(17) (a) Gibson, V. C.; O’Reilly, R. K.; Wass, D. F.; White, A. J. P.;
Williams, D. J. Macromolecules 2003, 36, 2591. (b) Gibson, V. C.;
O’Reilly, R. K.; Reed, W.; Wass, D. F.; White, A. J. P.; Williams, D. J.
Chem. Commun. 2002, 1850.
(18) (a) tom Dieck, H.; Stamp, L.; Diercks, R.; Mu¨ller, C. New J.
Chem. 1985, 9, 289. (b) Le Floch, P.; Knoch, F.; Kremer, F.; Mathey,
F.; Scholz, J.; Scholz, W.; Theile, K.-H.; Zenneck, U. Eur. J. Inorg.
Chem. 1998, 119.
(19) Lorber, C.; Choukroun, R.; Costes, J.-P.; Donnadieu, B. C. R.
Chim. 2002, 5, 251.
Crystals of 1 suitable for X-ray diffraction were
obtained from a concentrated pentane solution chilled
to -35 °C. Figure 1 depicts the solid-state structure of
the molecule, while selected bond distances and angles
are reported in Table 1. Also included in Table 1 are
the metrical parameters for the tetrahedral ferrous bis-
(alkyl) complex [ArNdC(Me)C(Me)dNAr]Fe(CH2SiMe3)2
and the formally iron(I) monochloride dimer [(ArNd
C(Me)C(Me)dNAr)Fe(µ2-Cl)]2 (3).20 In the solid-state
structure of 1, the iron atom lies essentially in the plane
of the R-diimine ligand with a deviation of only 0.0071-
(4) Å. As is typically observed with this class of ligand,
(20) Bart, S. C.; Hawrelak, E. J.; Schmisseur, A. K.; Lobkovsky, E.;
Chirik, P. J. Organometallics 2004, 23, 237.

5520 Organometallics, Vol. 24, No. 23, 2005
Bart et al.
Chart 1
to the LUMO of the R-diimine, suggesting an important
contribution from the “ene-diamide” canonical form
(Chart 1). In this limit, the arene complexes can be
viewed as low spin ferrous compounds similar to (η5-
C5Me5)Fe(acac) (acac ) acetylacetonate).21 Interestingly,
the degree of R-diimine ligand reduction observed in 1
is comparable to that reported in the formally d2 group
4 metallocene complexes (η5-C5H5)2M(PhNC(Ph)C(Ph)-
NPh) (M ) Ti, Zr, Hf),22 highlighting the reducing
nature of the iron(0) fragment.
Comparison of the R-diimine bond distances in 1 and
[(C6H11)NdC(H)C(H)dN(C6H11)]Fe(η6-C7H8) to that
in [ArNdC(Me)C(Me)dNAr]Fe(CH2SiMe3)2 revealed
longer iron-nitrogen and carbon-carbon backbone
bond lengths in the dialkyl complex, suggesting a
diminished back-bonding interaction from the more
oxidized ferrous center. The monochloride dimer 3 has
metrical parameters between these two extremes with
Fe-N bond distances of 1.979(1) Å and a backbone
carbon-carbon bond length of 1.420(3) Å. This con-
tinuum of bond lengths demonstrates the ability of the
R-diimine to adjust for the electronic requirements of
the metal.
Figure 1. Molecular structure of 1 with 30% probability
ellipsoids. Hydrogen atoms are omitted for clarity.
Table 1. Selected Bond Distances (Å) and Angles
(deg) for 1, (DI)Fe(CH2SiMe3)2, and 3
a
1
(DI)Fe(CH2SiMe3)2
3a
Fe(1)-N(1)
Fe(1)-N(2)
N(1)-C(9)
N(2)-C(10)
C(9)-C(10)
C(1)-C(2)
C(1)-C(6)
C(2)-C(3)
C(3)-C(4)
C(4)-C(5)
C(5)-C(6)
C(1)-C(2)
1.886(2)
1.886(2)
1.356(3)
1.361(3)
1.358(4)
1.405(5)
1.417(4)
1.409(4)
1.421(4)
1.389(4)
1.429(4)
1.405(5)
2.025(2)
2.013(2)
1.329(2)
1.320(3)
1.507(3)
1.979(1)
1.979(1)
1.339(2)
1.339(2)
1.420(3)
The iron arene complexes 1 and 2 were evaluated for
the catalytic hydrogenation of 1-hexene. No hydroge-
nated product was observed up to temperatures of 80
°C in benzene-d6 or cyclohexane-d12 with 4 atm of
dihydrogen. Monitoring the addition of H2 to a benzene-
N(1)-Fe(1)-N(2) 81.23(8)
79.59(9)
80.87(8)
C(9)-N(1)-Fe(1) 114.96(16) 111.81(17)
N(1)-C(9)-C(10) 115.0(2) 115.4(2)
112.87(10)
115.36(9)
1
d6 solution of 1 by H NMR spectroscopy indicated no
reaction, even after several days. Displacement of the
arene can be achieved by addition of 1 atm of carbon
monoxide to afford the previously reported iron tricar-
bonyl complex [ArNdC(Me)C(Me)dNAr]Fe(CO)3 (eq
2).19,23
a Values taken from ref 20 and are for the corresponding bond
lengths and angles. DI ) [ArNdC(Me)C(Me)dNAr].
the ortho-substituted aryl groups are oriented perpen-
dicular to the iron-R-diimine plane, while the methyl
group on the toluene ring is nestled between the
isopropyl substituents to avoid unfavorable steric in-
teractions.
The structural data contained in Table 1 establish the
redox flexibility of the R-diimine ligand and its ability
to smoothly adjust to the electronic requirements of the
metal. In the electron-rich iron toluene complex 1,
relatively short Fe(1)-N(1) and C(9)-C(10) bond dis-
tances of 1.886(2) and 1.358(4) Å are observed, respec-
tively. Concomitantly, the imine bonds, N(1)-C(9) and
N(2)-C(10), elongate to 1.356(3) and 1.361(3) Å, respec-
tively. In contrast, little perturbation of the carbon-
carbon bonds in the toluene ring is observed. These
findings are in good agreement with the structural data
reported18b for the R-diimine ligand in [(C6H11)Nd
C(H)C(H)dN(C6H11)]Fe(η6-C7H8), where the C-C back-
bone distance was contracted by 0.078 Å and the
average CdN distance was elongated by 0.043 Å com-
pared to the free R-diimine. Observation of such pro-
nounced bond alternation in both cases is consistent
with π-back-donation from the electron-rich iron center
To circumvent the formation of robust arene com-
plexes, the reduction of [ArNdC(Me)C(Me)dNAr]FeCl2
with sodium amalgam was performed in alkane solvent.
1
Aliquots of the reaction mixture were analyzed by H
NMR spectroscopy and revealed the R-diimine iron
chloride dimer 3 as the initial reduction product, fol-
lowed by formation of small quantities of the bis-
(21) Bunel, E. E.; Valle, L.; Manriquez, J. M. Organometallics 1985,
4, 1680.
(22) (a) Scholz, J.; Dlikan, M.; Stro¨hl, D.; Dietrich, A.; Schumann,
H.; Theile, K.-H. Chem. Ber. 1990, 123, 2279. (b) Scholz, J.; Hadi, G.
A.; Theile, K.-H.; Go¨rls, H.; Weimann, R.; Schumann, H.; Sieler, J. J.
Organomet. Chem. 2001, 626, 243.
(23) tom Dieck, H.; Bruder, H. Chem. Commun. 1977, 24.

Low-Valent R-Diimine Iron Complexes
Organometallics, Vol. 24, No. 23, 2005 5521
Figure 2. (a) Molecular structure of 4 with 30% probability ellipsoids. Hydrogen atoms are omitted for clarity. (b) View
of the core of the molecule.
Table 2. Selected Bond Distances (Å) and Angles
(deg) for 4
(R-diimine) iron complex [ArNdC(Me)C(Me)dNAr]2Fe
(4) with deposition of iron metal (eq 3). An improved
Fe(1)-N(1)
Fe(1)-N(2)
2.013(9)
1.988(9)
Fe(1)-N(3)
Fe(1)-N(4)
2.061(8)
2.077(8)
N(1)-C(2)
N(2)-C(3)
1.354(14)
1.431(12)
N(3)-C(30)
N(4)-C(31)
1.275(12)
1.349(13)
C(2)-C(3)
1.351(14)
C(30)-C(31)
1.472(13)
N(1)-Fe(1)-N(2)
82.7(4)
N(1)-Fe(1)-N(4) 126.25(11)
N(2)-Fe(1)-N(3) 144.33(12) N(3)-Fe(1)-N(4)
N(1)-Fe(1)-N(3) 116.4(3)
79.6(3)
magnetic measurements produced a magnetic moment
of 2.8 µB, consistent with the spin-only value for two
unpaired electrons.
Chilling a concentrated pentane solution of 4 to -35
°C produced crystals suitable for X-ray diffraction. The
molecular structure of the compound is depicted in
Figure 2, and selected metrical parameters are reported
in Table 2. In agreement with the magnetic data, a
tetrahedral geometry is observed, where the iron atom
is displaced by 0.524(16) Å from the core of the R-diimine
ligand. Significant variation in the bond distances and
angles in the R-diimine core are also observed. For one
ligand, a short carbon-carbon backbone distance of
1.351(14) Å is observed for C(2)-C(3), suggesting sig-
nificant back-donation from the iron center. In agree-
ment with this assertion is the elongated N(2)-C(3)
distance of 1.431(12) Å, much longer than the corre-
sponding N(1)-C(2) distance of 1.354(14) Å. Curiously,
the Fe(1)-N(2) distance of 1.998(9) Å is longer than
those found in 1, where there is significant electron
donation from the iron to the R-diimine ligand.
In contrast, the other R-diimine ligand in 4 contains
an elongated carbon-carbon backbone with a C(30)-
C(31) distance of 1.472(13) Å. Accordingly, the nitrogen-
carbon bond lengths are relatively short with N(3)-
C(30) and N(4)-C(31) distances of 1.275(12) and
1.349(13) Å, respectively. The iron-nitrogen bond lengths
of 2.061(8) and 2.077(8) Å are comparable to those found
in [ArNdC(Me)C(Me)dNAr]Fe(CH2SiMe3)2,20 where
there is little back-donation from the iron to the R-di-
imine. The metrical data, in conjunction with the
magnetic susceptibility measurements, suggest that 4
can be viewed as a tetrahedral iron(0) d8 complex or as
procedure for the preparation of 4 has been devised by
reducing the R-diimine ferrous dichloride complex with
sodium amalgam in the presence of a stoichiometric
quantity of ArNdC(Me)C(Me)dNAr. These results are
analogous to previous observations by tom Dieck and
Bruder,23 who reported the synthesis of several [RNd
i
t
CHCHdNR]2Fe (R ) Pr, Bu, C6H11, CHiPr2) deriva-
tives by reduction of a mixture of FeCl2 and the free
ligand. These complexes, when activated with orga-
noaluminum compounds, are active for the selective
dimerization of 1,3-dienes.24
Red-brown 4 displays paramagnetically broadened 1H
NMR resonances over a relatively narrow chemical shift
window. Diastereotopic isopropyl methyl resonances
appear at -2.2 and 4.0 ppm, relatively close to their
diamagnetic reference values, consistent with the aryl
groups being orthogonal to the R-diimine ligand plane.
In contrast, the methyl groups on the R-diimine back-
bone appear as a broad (∆ν1/2 ) 41 Hz) singlet centered
at -22.2 ppm, indicative of spin delocalization over the
core of the ligand. Although 4 was sufficiently soluble
in benzene-d6 to collect NMR data, reliable solution
magnetic data could not be obtained, due to partial
solubility of the compound. Solid-state (Gouy balance)
(24) tom Dieck, H.; Dietrich, J. Angew. Chem., Int. Ed. Engl. 1985,
24, 781.

5522 Organometallics, Vol. 24, No. 23, 2005
Bart et al.
a intermediate-spin iron(II) d6 complex, where the
R-diimine acts as a strong field ligand, engaged in π
back-bonding with the iron center.
As was observed with 1, 4 is not active for the
catalytic hydrogenation of 1-hexene and does not react
with excess dihydrogen. Replacement of one or both of
the R-diimine ligands can be accomplished by exposure
of the compound to 1 atm of carbon monoxide. A mixture
of [ArNdC(Me)C(Me)dNAr]Fe(CO)3, Fe(CO)5, Fe2(CO)9,
and free ligand were observed upon treatment of 4 with
CO (eq 4). As with previously reported bis(R-diimine)
whether the formally 18-electron, η6-arene isomer was
accessible and perhaps thermodynamically favored, the
thermal stability of 5 was studied. Gently warming a
benzene-d6 solution of 5 to 60 °C for approximately 48
h furnished a new, diamagnetic iron product, identified
as the η6-diphenylacetylene complex [ArNdC(Me)C-
(Me)dNAr]Fe(η6-C6H5CtCPh) (6), arising from migra-
tion of the alkyne ligand (eq 6). While detailed kinetic
studies have not been performed, competitive formation
of 2-d6 was not observed, suggesting an intramolecular
rearrangement process.
Due to complications from competitive η6 coordination
in aryl-substituted alkynes, the synthesis of alkyl- or
silyl-substituted acetylene complexes was explored (eq
7). Stirring a pentane slurry of [ArNdC(Me)C(Me)d
iron complexes,23 4 is catalytically active for the dimer-
ization of 1,3-butadiene to 1,5-cyclooctadiene at 23 °C.
Significantly, no trialkylaluminum activator was re-
quired for catalytic activity.
Isolation of the bis(R-diimine) ligand complex 4
prompted reduction of [ArNdC(Me)C(Me)dNAr]FeCl2
in the presence of other neutral donors. An ideal ligand
is one that would allow isolation of an [ArNdC(Me)C-
(Me)dNAr]FeLn species but is sufficiently labile to
dissociate (or be hydrogenated) to engender catalytically
active iron centers. The utility of (iPrPDI)Fe(η2-PhCt
CPh) in catalytic hydrogenation and hydrosilation reac-
tions10 suggested that alkynes may be effective in
stabilizing R-diimine iron(0) complexes while maintain-
ing sufficient lability to generate catalytically active
metal centers. Reduction of a pentane slurry of [ArNd
C(Me)C(Me)dNAr]FeCl2 with an excess of 0.5% sodium
amalgam in the presence of a stoichiometric quantity
of PhCtCPh furnished the desired R-diimine iron(0) η2-
alkyne complex [ArNdC(Me)C(Me)dNAr]Fe(η2-PhCt
CPh) (5), as a red-brown solid (eq 5). The solid-state
NAr]FeCl2 with an excess of 0.5% sodium amalgam in
the presence of a slight excess (∼3 equiv) of bis-
(trimethylsilyl)acetylene furnished [ArNdC(Me)C(Me)d
NAr]Fe(η2-(Me3Si)CtC(SiMe3)) (7). As with 5, 7 is
paramagnetic with a benzene-d6 solution magnetic
moment of 2.9 µB (Evans method, 23 °C). Accordingly,
the 1H NMR spectrum displays five broad and feature-
less signals. Additional characterization has been ob-
tained from combustion analysis and degradation ex-
periments. Treatment of 7 with a benzene-water
mixture produced free R-diimine ligand and bis((trim-
ethylsilyl)acetylene) along with insoluble iron products.
Performing an analogous reduction of the R-diimine iron
dichloride in the presence of 2-butyne furnished an oily
compound, tentatively assigned as [ArNdC(Me)C(Me)d
NAr]Fe(η2-MeCtCMe) (8). However, the inability to
obtain the compound in the solid state and purify it by
recrystallization prevented combustion analysis and
determination of a reliable magnetic moment.
The solid-state structure of 7 was determined by
X-ray diffraction and is presented in Figure 3, while
selected metrical parameters are reported in Table 3.
The data were modeled such that one isopropyl methyl
group on the R-diimine ligand is positionally disordered,
and one representation is presented in Figure 3. As is
typically observed with hindered R-diimine ligands, the
sterically demanding aryl groups are oriented perpen-
dicular to the iron-ligand plane. The alkyne ligand is
canted with respect to the iron-diimine plane, forming
a dihedral angle of 25.5°. The iron-nitrogen bond
lengths of 1.9843(8) and 1.9821(7) Å and the carbon-
carbon backbone distance of 1.4139(12) Å are between
the limiting “ene-diamide” form of the ligand principally
magnetic moment of 3.2 µB (Guoy method) is consistent
with two unpaired electrons and is in accord with a
trigonal iron(0) or tetrahedral iron(II) complex. The 1H
NMR spectrum of 5 in benzene-d6 exhibits nine para-
magnetically broadened and shifted resonances over a
300 ppm chemical shift range, in agreement with a C2v-
symmetric η2-alkyne complex.
The observed paramagnetism of 5 is consistent with
coordination of one of the π faces of the carbon-carbon
triple bond rather than formation of the corresponding
arene complex. If the alkyne ligand was η6 coordinated
through one of the phenyl substituents, a closed-shell,
diamagnetic compound would be expected. To determine

Low-Valent R-Diimine Iron Complexes
Organometallics, Vol. 24, No. 23, 2005 5523
Figure 3. Molecular structure of 7 with 30% probability
ellipsoids. Hydrogen atoms are omitted for clarity.
Figure 4. Molecular structure of 9 with 30% probability
ellipsoids. Hydrogen atoms are omitted for clarity.
Table 3. Selected Bond Distances (Å) and Angles
(deg) for 7
Fe(1)-N(1)
Fe(1)-N(2)
1.9843(8)
1.9821(7)
Fe(1)-C(29)
Fe(1)-C(30)
1.9377(9)
1.9486(10)
Table 4. Selected Bond Distances (Å) and Angles
(deg) for 9
N(1)-C(2)
N(2)-C(3)
1.3423(12)
1.3393(11)
C(2)-C(3)
1.4139(12)
1.2956(14)
Fe(1)-N(1)
Fe(1)-N(2)
Fe(1)-C(29)
1.9964(18)
1.9944(18)
2.096(3)
Fe(1)-C(30)
Fe(1)-C(33)
Fe(1)-C(34)
2.115(3)
2.105(2)
2.082(2)
C(29)-C(30)
N(1)-Fe(1)-N(2)
80.83(3) C(29)-Fe(1)-C(30)
38.94(4)
147.07(6)
144.67(6)
25.5
N(1)-C(2)
N(2)-C(3)
C(2)-C(3)
1.344(3)
1.349(3)
1.413(3)
C(29)-C(30)
C(33)-C(34)
1.375(6)
1.397(4)
N(1)-Fe(1)-C(29) 119.39(4) Fe(1)-C(29)-Si(1)
N(2)-Fe(1)-C(30) 123.43(4) Fe(1)-C(30)-Si(2)
N(2)-Fe(1)-C(29) 158.00(4) dihedral anglea
N(1)-Fe(1)-C(30) 154.22(4)
N(1)-Fe(1)-N(2)
C(29)-Fe(1)-C(30) 38.12(15) C(29)-Fe(1)-C(33) 92.71(11)
C(33)-Fe(1)-C(34) 38.96(11)
79.59(7)
C(29)-Fe(1)-C(34) 85.46(12)
a The angle formed between the planes defined by N(1)-Fe(1)-
N(2) and C(29)-Fe(1)-C(30).
Chart 2
equiv of 1,5-cyclooctadiene furnished dark brown crys-
tals identified as [ArNdC(Me)C(Me)dNAr]Fe(η2:η2-1,5-
C8H12) (9) (eq 8).
observed in 1 and the neutral diimine that is the major
canonical form in [ArNdC(Me)C(Me)dNAr]Fe(CH2-
SiMe3)2. Both the carbon-carbon bond length of 1.2956-
(14) Å and the Fe(1)-C(29)-Si(1) and Fe(1)-C(30)-
Si(2) bond angles of 147.07(6) and 144.67(6)° are
consistent with considerable metallacyclopropene char-
acter. Thus, the X-ray data suggest that 7 is best
described as an iron(II) complex intermediate between
an “ene-diamide” acetylene complex (A) and an R-di-
imine compound with a metallacyclopropene ligand (B;
Chart 2).
The successful synthesis and isolation of the R-diimine
iron alkyne complexes prompted exploration of related
olefin complexes. Iron complexes containing 1,5-cyclooc-
tadiene were targeted, given the well-documented abil-
ity of this diolefin to stabilize catalytically active
precursors.25,26 Stirring [ArNdC(Me)C(Me)dNAr]FeCl2
in the presence of excess 0.5% sodium amalgam and 5
The benzene-d6 1H NMR spectrum of 9 recorded at
23 °C exhibits six paramagnetically shifted and broad-
ened peaks over a 200 ppm chemical shift range.
Accordingly, a solution magnetic moment of 2.8 µB was
observed at 23 °C in benzene-d6, consistent with a high-
spin, tetrahedral iron(0) center with two unpaired
electrons. Significantly, 9 is stable in benzene-d6 and
does not undergo substitution with the arene solvent.
Brown crystals of 9 suitable for X-ray diffraction were
obtained from a concentrated pentane-ether solution
chilled to -35 °C. The molecular structure of the
compound is presented in Figure 4, while selected
metrical data are reported in Table 4. During the final
stages of structure refinement, the R1 value could not
be lowered below 7%. A difference Fourier map showed
a strong (3.7 e/Å3) residual peak about 2 Å from Fe(1)
along with other weaker peaks. The overall residual
peak distribution resembles the data for 9, suggesting
a whole-molecule disorder. The core atoms, Fe, N, and
C, were included into the model with a 0.14 occupancy.
The resulting R factor was reduced in the subsequent
(25) (a) Schrock, R. R.; Osborn, J. A. J. Am. Chem. Soc. 1971, 93,
2397. (b) Schrock, R. R.; Osborn, J. A. J. Am. Chem. Soc. 1976, 98,
2143.
(26) (a) Green, M.; Howard, J. A. K.; Spencer, J. L.; Stone, F. G. A.
Chem. Commun. 1975, 449. (b) Krause, J.; Cestaric, G.; Haack, K.-J.;
Seevogel, K.; Storm, W.; Po¨rschke, K.-R. J. Am. Chem. Soc. 1999, 121,
9807.

5524 Organometallics, Vol. 24, No. 23, 2005
Bart et al.
Figure 5. Reactivity of 9 with dihydrogen and synthetic routes to 10.
Table 5. Assay of the Catalytic Hydrogenation
Activity of r-Diimine Iron Alkyne Complexes
refinement; however, a full second molecule could not
be completed. Interestingly, this disorder appears to be
a intrinsic property of the material, consistently ap-
pearing in multiple crystals grown under different
conditions.
compd
time (min)a
tof (mol/h)b
The molecular geometry about iron in 9 is best
described as pseudo-tetrahedral, with the centroids
formed from the midpoints of C(29)-C(30) and C(33)-
C(34) occupying two coordination sites above and below
the plane of the R-diimine ligand. The C(2)-C(3) bond
distance of 1.4139(12) Å is similar to that found in the
solid-state structure of 4 and, along with the CdN bond
lengths of the imine, suggests partial ligand reduction
and some contribution from the “ene-diamide” resonance
structure of the nitrogen chelate. Likewise, the C(29)-
C(30) and C(33)-C(34) bond distances of 1.375(6) and
1.397(4) Å are only slightly elongated from the typical
CdC distance of 1.34 Å.
Exposure of a benzene-d6 solution of 9 to 1 atm of
dihydrogen resulted in rapid conversion to 2-d6 with
concomitant loss of cyclooctene and cyclooctane (Figure
5). Performing the hydrogenation in cyclohexane-d12
rather than aromatic solvent also produced some cy-
clooctene and cyclooctane along with a new iron com-
pound, assigned as the cyclooctene complex [ArNd
C(Me)C(Me)dNAr]Fe(η2-C8H14) (10). To confirm this
assertion, an independent synthesis of 10 was explored
by reduction of [ArNdC(Me)C(Me)dNAr]FeCl2 with
excess 0.5% sodium amalgam in the presence of 5 equiv
of olefin. Excess olefin is employed to suppress formation
of 4. This synthetic procedure furnished a green crystal-
line material identified as the desired cyclooctene ad-
duct, 10, on the basis of magnetic data, combustion
analysis, NMR spectroscopy, and degradation experi-
ments. As with 9, the cyclohexane-d12 NMR spectrum
of 10 displays eight broad and featureless signals over
a 100 ppm chemical shift range. Dissolving 10 in
benzene-d6 resulted in olefin loss and formation of 2-d6.
(
iPrPDI)Fe(N2)2
12
1440d
240
1800
4
7c
9
90
10
240
90
a Time required to reach >98% conversion as judged by GC.
b Determination based on time for completion. c Hydrogenation
carried out at 60 °C. d 25% conversion to hexane, 12% internal
hexenes observed.
frequencies. The time to reach >98% conversion and the
turnover frequencies for each catalyst are reported in
Table 5.
In general, the alkyne complexes are poor precursors
for 1-hexene hydrogenation, reaching only partial con-
version after 24 h at 60 °C. Significant (∼12%) olefin
isomerization to a mixture of internal hexenes ac-
companies the hydrogenation reaction. In contrast, the
olefin complexes 9 and 10 are efficient catalysts at 22
°C with turnover frequencies of 90 mol/h. Observation
of nearly identical hydrogenation rates with the two
olefin complexes suggests that the first hydrogenation
in the cyclooctadiene compound 9 is fast and generates
10, which in turn undergoes subsequent hydrogenation
or displacement by substrate to generate the active
species. Such an observation has been noted in stoichio-
metric reactions, where addition of H2 to 9 furnishes
10 over the course of minutes at ambient temperature.
It should be noted that variable induction periods,
ranging between minutes and 1 h, have been observed,
suggesting that generation of the catalytically active
species is not straightforward. Further investigations
into the nature of the active species and the scope and
mechanism of the catalytic hydrogenation reaction are
currently underway in our laboratory. Significantly,
both of the olefin precursors offer inferior rates of
hydrogenation compared to the bis(dinitrogen) iron
complex with the tridentate pyridine(diimine) ligand.
The poor performance of the diphenylacetylene com-
pound 5 suggested complications from competing η6
coordination during catalytic turnover. To investigate
this possibility, a pentane solution of 5 was stirred
under 4 atm of H2 for 16 h at 60 °C in a thick-walled
glass vessel. This procedure furnished an orange-red
diamagnetic solid identified as the R-diimine iron cis-
stilbene complex [ArNdC(Me)C(Me)dNAr]Fe(η6-(C6H5)-
The iron alkyne complexes 6 and 7 and the corre-
sponding olefin compounds 9 and 10 were evaluated for
the catalytic hydrogenation of 1-hexene. Each catalytic
run was conducted with a 1.25 M solution of 1-hexene
in pentane containing 0.3 mol % of the desired catalyst
with 4 atm of dihydrogen. While the olefin complexes
are efficient hydrogenation catalysts at lower H2 pres-
sures, 4 atm was used to avoid potential complications
from rate-determining gas diffusion at higher conver-
sions and to obtain reliable comparisons of turnover

Low-Valent R-Diimine Iron Complexes
Organometallics, Vol. 24, No. 23, 2005 5525
HCdCH(C6H5)) (11), arising from partial hydrogenation
of the diphenylacetylene ligand in 6 (eq 9). Similar to
Concluding Remarks
The reduction of the R-diimine ferrous dichloride
complex [ArNdC(Me)C(Me)dNAr]FeCl2 under a variety
of conditions has been described. In the presence of
aromatic organic solvents such as benzene or toluene,
robust 18-electron R-diimine iron arene complexes are
formed that display metrical parameters consistent with
a significant contribution from the “ene-diamide” form
of the bidentate nitrogen ligand. Replacing the aromatic
reduction medium with an alkane solvent such as
pentane furnished the bis(R-diimine) iron complex,
which also exhibits significant reduction of the ligand
in the solid state. Both the arene complexes and the bis-
(ligand) adducts are inactive for catalytic olefin hydro-
genation. Performing the reduction of the R-diimine
ferrous dichloride in the presence of a trapping ligand
such as an olefin or alkyne afforded the corresponding
adduct of the unsaturate, where both the R-diimine and
the organic ligand engage in π back-bonding with the
iron center. The olefin compounds are more active than
the acetylene precursors for 1-hexene hydrogenation,
and both classes of compounds are subject to forming
catalytically inactive, coordinatively saturated arene
compounds upon exposure to aromatic solvents or
substrates.
the arene complexes 1 and 2, a C2v-symmetric ligand
1
environment is observed by H and 13C NMR spectros-
copy for 11, consistent with rapid rotation of the arene
ligand. In addition, upfield-shifted resonances are ob-
served at 4.48 (meta), 5.07 (ortho), and 5.79 ppm (para),
corresponding to the aromatic protons on the coordi-
nated arene. Importantly, 11 does not react with ad-
ditional H2 or isomerize to the olefin complex, account-
ing for the lower activity of 5 in catalytic 1-hexene
hydrogenation.
To further investigate potential complications from
arene-containing substrates, the bis(trimethylacetylene)
complex 7 was treated with a functionalized aromatic
olefin. Addition of 1 equiv of methyl trans-cinnamate
to a pentane solution of 7 resulted in an immediate color
change to red, and a diamagnetic powder identified as
the methyl trans-cinnamate complex [ArNdC(Me)C-
(Me)dNAr]Fe(η6-(C6H5)CHdCHCOOCH3) (12) was iso-
lated in excellent yield (eq 10). Interestingly, the
Experimental Section
General Considerations. 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.27 The M. Braun dryboxes were equipped with cold
wells designed for freezing samples in liquid nitrogen. Argon
and hydrogen gas were purchased from Airgas Inc. 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. Chloroform-d was purchased from Cambridge
Isotope Laboratories and distilled from CaH2. Diphenylacety-
lene was purchased from Acros, dried under high vacuum, and
recrystallized from pentane before use. Bis(trimethylsilyl)-
acetylene and 2-butyne were purchased from Acros, dried over
4 Å molecular sieves, and vacuum-transferred immediately
before use. Likewise, 1-hexene, 1,5-cyclooctadiene, and cy-
clooctene were purchased from Acros and dried over LiAlH4.
[ArNdC(Me)C(Me)dNAr]FeCl2 and 3 were prepared according
to the previously reported procedure.20
substitution reaction can also be performed in benzene-
d6 without complications from formation of 2. The H
1
NMR spectrum of 12 in benzene-d6 exhibits the number
of peaks expected for a C2v R-diimine ligand environ-
ment, in addition to three upfield peaks centered at 4.46,
5.08, and 5.83 for the meta, ortho, and para hydrogen
resonances for the coordinated arene. This result dem-
onstrates the strong preference for the R-diimine iron
fragment to form coordinatively saturated arene com-
plexes, even in the presence of carbonyl functional
groups and aromatic solvent.
The preference for thermodynamic arene coordination
is reinforced by the thermal behavior of 5. While the
η2-alkyne complex is kinetically preferred from the
alkali-metal reduction of the ferrous dihalide in the
presence of diphenylacetylene, gently warming the
compound produces the thermodynamically favored,
coordinatively saturated arene complex. This thermo-
dynamic preference has ramifications on the “func-
tional” group tolerance of R-diimine-based iron hydro-
genation catalysts, as substrates with arene substituents
inhibit turnover through formation of inactive, 18-
electron arene complexes.
1H spectra were recorded on Varian Mercury 300 and Inova
400 and 500 spectrometers operating at 299.763, 399.780, and
500.618 MHz, respectively. All chemical shifts are reported
relative to SiMe4 using 1H (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. For para-
magnetic compounds, 1H NMR data are 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
(27) Pangborn, A. B.; Giardello, M. A.; Grubbs, R. H.; Rosen, R. K.;
Timmers, F. J. Organometallics 1996, 15, 1518.

5526 Organometallics, Vol. 24, No. 23, 2005
Bart et al.
Evans28 with stock and experimental solutions containing a
known amount of a ferrocene standard.
Preparation of [ArNdC(Me)C(Me)dNAr]Fe(η2-PhCt
CPh) (5). This molecule was prepared in a manner similar to
that for 4 with 1.00 g (1.90 mmol) of [ArNdC(Me)C(Me)dNAr]-
FeCl2 and 5 equiv of 0.5% Na(Hg) in the presence of 0.335 g
(1.90 mmol) of PhCtCPh. Recrystallization from pentane at
-35 °C afforded 0.759 g (63%) of brown crystals identified as
5. Anal. Calcd for C42H50FeN2: C, 78.98; H, 7.89; N, 4.39.
Magnetic susceptibility (Guoy method): µeff ) 3.2 µB. Found:
C, 79.01; H, 7.76; N, 4.40. 1H NMR (C6D6): δ -281.5 (58.2,
4H), -50.8 (219, 4H), -25.4 (38.4, 4 H), -21.2 (28.4, 6 H, Nd
C(CH3)), -18.1 (17.9, 2H), -9.5 (16.8, 4H), -6.5 (17.4, 2H),
-0.90 (55.7, 12 H, CH(CH3)2), 5.4 (12.9, 12 H, CH(CH3)2).
Characterization of [ArNdC(Me)C(Me)dNAr]Fe(η6-
C6H5CtCPh) (6). 1H NMR (C6D6): δ 0.99 (d, 6.1 Hz, 18H,
CH(CH3)2), 1.10 (s, 6H, NdC(CH3)), 1.60 (d, 6.1 Hz, 12H, CH-
(CH3)2), 3.53 (sept, 6.1 Hz, 4H, CH(CH2)3), 4.41 (t, 6.1 Hz, 2H,
m-C6H5CCPh (bound)), 5.17 (d, 6.1 Hz, 2H, o-C6H5CCPh
(bound)), 5.86 (t, 6.1 Hz, 1 H, p-C6H5CCPh (bound)), 6.97-
7.35 (m, 5 H, PhCCC6H5 (free)).
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
) 0.710 73 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.
Preparation of [ArNdC(Me)C(Me)dNAr]Fe(η6-C7H8)
(1). A 500 mL round-bottomed flask was charged with 129 g
of Hg (643 mmol) and approximately 200 mL of toluene. With
stirring, 0.660 g (28.2 mmol) of Na metal was added, and the
resulting amalgam was stirred for 30 min. After this time, 3.00
g (5.60 mmol) of [ArNdC(Me)C(Me)dNAr]FeCl2 was added,
forming a bright orange-red solution over time. After it was
stirred at ambient temperature for 2 days, the reaction mixture
was decanted and filtered through Celite and the filtrate
collected. The solvent was removed in vacuo and the resulting
solid recrystallized from pentane to afford 2.30 g (74%) of deep
red crystals identified as 1. Anal. Calcd for C35H46FeN2: C,
Preparation of [ArNdC(Me)C(Me)dNAr]Fe(η2-Me3SiCt
CSiMe3) (7). A 100 mL round-bottomed flask was charged
with 64.8 g (324 mmol) of mercury and 50 mL of pentane.
Small pieces of sodium (0.324 g, 14.1 mmol) were added with
stirring and the resulting amalgam mixed for 20 min. After
this time, 1.505 g (2.11 mmol) of [ArNdC(Me)C(Me)dNAr]-
FeCl2 was added immediately, followed by 1.438 g (8.46 mmol)
of bis(trimethylsilyl)acetylene. The resulting reaction mixture
was stirred for 48 h and then filtered through Celite and the
filtrate collected, and the volatiles were removed in vacuo. The
resulting residue was recrystallized from a pentane-diethyl
ether mixture to yield 0.957 g (54%) of a red-brown solid
identified as 7. Anal. Calcd for C36H58N2Si2Fe: C, 62.41; H,
8.44; N, 4.04. Found: C, 62.28; H, 8.51; N, 4.40. Magnetic
1
76.06; H, 8.75; N, 5.07. Found: C, 75.93; H, 8.28; N, 4.95. H
NMR (C6D6): δ 1.01 (m, 6.1 Hz, 18H, CH(CH3)2), 1.04 (s, 6H,
NdC(CH3)), 1.53 (d, 7.6 Hz, 12H, CH(CH3)2), 2.30 (s, 3H,
C6H5CH3), 3.52 (sept, 6.1 Hz, 4H, CH(CH2)3), 4.40 (t, 6.1 Hz,
2H, m-C6H5CH3), 4.80 (d, 6.10 Hz, 2H, o-C6H5CH3), 5.69 (t,
6.0 Hz, 1 H, p-C6H5CH3), 7.27-7.39 (m, 7.63, 6 H, C6H3). 13C
NMR (C6D6): δ 18.51 (CH(CH3)2, 20.26 (C6H5CH3), 25.06 (CH-
(CH3)2), 25.41 (CH(CH3)2), 28.23 (NdC(CH3)), 80.46 (o- or
m-C6H5CH3), 80.81 (o- or m-C6H5CH3), 82.10 (p-C6H5CH3),
97.27 (i-C6H5CH3), 123.70 (aryl), 126.17 (aryl), 142.23 (aryl),
143.73 (aryl), 154.36 (NdC(CH3)).
1
susceptibility (benzene-d6): µeff ) 2.9 µB. H NMR (benzene-
d6): δ -25.3 (47.0), -17.7 (26.1), -13.3 (21.1), -2.7 (208), 4.1
(14.7).
Preparation of [ArNdC(Me)C(Me)dNAr]Fe(η2-MeCt
CMe) (8). A thick-walled glass vessel was charged with 13.0
g (65.0 mmol) of mercury and approximately 50 mL of pentane.
With stirring, 0.065 g (2.83 mmol) of sodium, cut into small
pieces, was added to the flask. After approximately 10 min,
0.300 g (0.564 mmol) of [ArNdC(Me)C(Me)dNAr]FeCl2 was
added to the flask. The vessel was transferred to the high-
vacuum line, where it was submerged in liquid nitrogen and
evacuated. Using a 31.6 mL calibrated gas volume, 331 Torr
(0.564 mmol) of 2-butyne was added. The resulting reaction
mixture was thawed and stirred for 28 h at ambient temper-
ature, forming a dark brown solution. After this time, the
toluene was removed in vacuo and the desired product was
extracted into pentane. Recrystallization from pentane at -35
°C yielded 0.255 g (88%) of a dark brown solid identified as 8.
1H NMR (benzene-d6): δ -21.6 (49.0), 0.0 (376).
Preparation of [ArNdC(Me)C(Me)dNAr]Fe(η6-C6H6)
(2). This molecule was prepared in a manner identical with
that for 1 with 0.500 g (0.941 mmol) of [ArNdC(Me)C(Me)d
NAr]FeCl2 and 5 equiv of 0.5% Na(Hg) to yield 0.296 g (58%)
of 2 as bright red crystals. Anal. Calcd for C34H46FeN2: C,
1
75.82; H, 8.61; N, 5.20. Found: C, 75.47; H, 8.69; N, 5.22. H
NMR (C6D6): δ 1.00 (d, 6.8 Hz, 12H, CH(CH3)2), 1.05 (s, 6H,
NdC(CH3)), 1.47 (d, 6.8 Hz, 12H, CH(CH3)2), 3.52 (sept, 6.8
Hz, 4H, CH(CH3)2), 5.03 (s, 6H, C6H6), 7.25 (d, 7.3 Hz, 4H,
m-C6H3), 7.32-7.35 (m, 2H, p-C6H3).13C NMR (C6D6): δ 17.93
(CH(CH3)2), 24.38 (CH(CH3)2), 25.05 (CH(CH3)2), 27.99 (Nd
C(CH3)), 81.02 (C6H6), 123.33 (aryl), 125.886 (aryl), 141.68
(aryl), 143.79 (aryl), 153.99 (NdC(CH3)).
Preparation of [ArNdC(Me)C(Me)dNAr]2Fe (4). A 100
mL round-bottomed flask was charged with 43.8 g (219 mmol)
of mercury and approximately 50 mL of pentane. With stirring,
0.219 g (9.52 mmol) of sodium was added. After 10 min, 1.01
g (1.90 mmol) of [ArNdC(Me)C(Me)dNAr]FeCl2 was added to
the flask and the resulting red-green reaction mixture stirred
for approximately 24 h. After this time an additional 0.716 g
(1.90 mmol) of ArNdC(Me)C(Me)dNAr was added and the
reaction mixture stirred for an additional 24 h. The liquid was
decanted from the flask and filtered through Celite. The
volatiles were removed in vacuo, and the resulting red-brown
solid was recrystallized from pentane to afford 1.45 g (94%) of
4. Anal. Calcd for C56H80FeN4: C, 77.75; H, 9.32; N, 6.48.
Found: C, 77.43; H, 9.54; N, 5.99. Magnetic susceptibility
(Guoy balance): µeff ) 2.8 µB.1H NMR (C6D6) δ -22.2 (41.5, 6
H, NdC(CH3)), -15.9 (24.72, 4H), -6.0 (15.0, 4H), -3.6 (24.5,
2H, p-C6H3), -2.2 (28.3, 12 H, CH(CH3)2), 3.96 (88.0, 6H, CH-
(CH3)2).
Preparation of [ArNdC(Me)C(Me)dNAr]Fe(η2:η2-1,5-
COD) (9). This compound was prepared in a manner similar
to that for 7 with 1.124 g (2.11 mmol) of [ArNdC(Me)C(Me)d
NAr]FeCl2, 5 equiv of 0.5% Na(Hg), and 0.685 g (6.34 mmol)
of 1,5-cyclooctadiene in approximately 20 mL of pentane.
Following stirring for 48 h and standard workup, 0.596 g (50%)
of 9 was obtained as a brown solid. Anal. Calcd for C36H52N2-
Fe: C, 76.04; H, 9.22; N, 4.93. Found C, 76.19; H, 9.23; N,
4.96. Magnetic susceptibility (benzene-d6): µeff ) 2.8 µB.1H
NMR (benzene-d6): δ -61.5 (305.0), 5.9 (25.3), 0.0 (57.6), 1.70
(24.9), 93.0 (525.4), 143.5 (889).
Preparation of [ArNdC(Me)C(Me)dNAr]Fe(η2-COE)
(10). This molecule was prepared in a manner similar to 9
with 8.65 g (43.3 mmol) of mercury, 0.043 g (1.88 mmol) of
sodium, 0.200 g (0.376 mmol) of [ArNdC(Me)C(Me)dNAr]-
FeCl2, and 0.206 g (1.88 mmol) of cyclooctene, yielding 0.127
g (59%) of a brown solid identified as 10. Anal. Calcd for
C36H54N2Fe: C, 75.77; H, 9.54; N, 4.91. Found: C, 75.52; H,
(28) Sur, S. K. J. Magn. Reson. 1989, 82, 169.
9.78; N, 4.87. Magnetic susceptibility (Guoy method): µeff )

Low-Valent R-Diimine Iron Complexes
Organometallics, Vol. 24, No. 23, 2005 5527
3.1 µB. 1H NMR (cyclohexane-d12): δ -75.2 (530.4), -46.7
(657.5), -32.9 (170.0), -17.4 (115.1), -13.2 (30.2), 0.39 (36.1),
11.8 (93.3), 19.5 (74.0).
NdC(CH3)), 1.52 (d, 6.8 Hz, 12H, CH(CH3)2), 3.50 (q, 6.8 Hz,
4H, CH(CH3)2), 3.54 (s, 3H, C6H5CHCHCOOCH3), 4.46 (t, 5.9
Hz, 2H, m-C6H5CHCHCOOCH3), 5.08 (d, 6.4 Hz, 2H, o-C6H5-
CHCHCOOCH3), 5.83 (t, 5.9 Hz, 1H, p-C6H5CHCHCOOCH3),
6.42 (d, 15.6 Hz, 1H, C6H5CHCHCOOCH3), 7.21 (d, 7.8 Hz,
4H, m-aryl), 7.30 (t, 7.8 Hz, 2H, p-aryl), 8.18 (d, 15.6 Hz, 1H,
C6H5CHCHCOOCH3). 13C NMR (C6D6): δ 18.95 (CH(CH3)2),
25.12 (CH(CH3)2), 25.42 (CH(CH3)2), 28.27 (NdC(CH3)), 51.49,
80.40 (o-, m-, or p-C6H5CHCHCOOCH3), 82.45 (o-, m-, or p-
C6H5CHCHCOOCH3), 82.63 (o-, m-, or p-C6H5CHCHCOOCH3),
92.55 (ipso-C6H5CHCHCOOCH3), 116.57, 123.93 (aryl), 126.60
(aryl), 141.97 (aryl), 147.03 (aryl), 153.55 (NdC(CH3)), 197.16,
236.17.
General Hydrogenation Procedure. A thick-walled glass
vessel was charged with 0.02 mmol of the desired iron complex,
along with approximately 4 mL of pentane and a stir bar.
Approximately 4.90 mmol of substrate was added to the vessel.
The vessel was submerged in liquid nitrogen and evacuated.
One atmosphere of hydrogen was admitted, and the reaction
mixture was warmed to ambient temperature and stirred for
the appropriate time. The progress of the reaction was assayed
periodically by gas chromatography.
Preparation of [ArNdC(Me)C(Me)dNAr]Fe(η6-(C6H5)-
HCdCH(C6H5)) (11). A thick-walled glass vessel was charged
with 0.094 g (0.147 mmol) of 5 and approximately 20 mL of
pentane. On the high-vacuum line, the vessel was frozen in
liquid nitrogen and evacuated and 1 atm of dihydrogen was
admitted. The vessel was thawed and the mixture stirred for
16 h at 60 °C. After this time, the brown solution turned bright
orange-red. The volatiles were removed in vacuo to yield 0.090
g (96%) of an orange red solid identified as 11. Purification
was accomplished by recrystallization from pentane. 1H NMR
(C6D6): δ 1.00 (m, 7.63 Hz, 18H, CH(CH3)2), 1.01 (s, 6H, Nd
C(CH3)), 1.47 (d, 6.10 Hz, 12H, CH(CH3)2), 3.46 (sept, 6.10 Hz,
4H, CH(CH2)3), 4.48 (t, 6.10 Hz, 2H, bound m-C6H5CH2CH2-
C6H5), 5.07 (d, 6.10 Hz, 2H, bound o-C6H5CH2CH2C6H5), 5.73
(t, 4.58 Hz, 1H, bound p-C6H5CH2CH2C6H5), 6.64 (d, 10.68 Hz,
1H, free p-C6H5CH2CH2C6H5), 6.87 (m, 3.06 Hz, 4H, m-C6H3),
7.11-7.27 (m, 6.10 Hz, 8 H, p-C6H3, free C6H5CH2CH2C6H5,
free o- ,m-, and p-C6H5CH2CH2C6H5). 13C NMR (C6D6): δ 18.33
(CH(CH3)2, 24.82 (CH(CH3)2), 25.10 (CH(CH3)2), 27.98 (Nd
C(CH3)), 80.65 (o- or m-C6H5CH2CH2C6H5), 81.08 (o- or m-C6H5-
CH2CH2C6H5), 81.93 (p-C6H5CH2CH2C6H5), 96.06 (i-C6H5CH2-
CH2C6H5), 123.40 (C6H3), 126.00 (C6H3), 127.20, 129.56, 130.17,
130.91, 137.21 (C6H3), 141.71 (C6H3), 144.63 (C6H3), 153.70
(NdC(CH3)).
Acknowledgment. We thank Cornell University,
Research Corp. (Cottrell Scholarship to P.J.C.), and the
Packard Foundation for financial support. S.C.B. ac-
knowledges support from the National Institutes of
Health as part of the Chemistry and Biology Interface
Training Grant at Cornell.
Preparation of [ArNdC(Me)C(Me)dNAr]Fe(η6-(C6H5)-
CHdCHCOOCH3) (12). A 20 mL scintillation vial was
charged with 0.100 g (0.157 mmol) of 7 and approximately 10
mL of diethyl ether. Solid methyl trans-cinnamate (0.026 g
(0.157 mmol)) was added to the vial, immediately forming a
red solution. The volatiles were removed in vacuo, and the
residue was recrystallized to yield 0.080 g (82%) of a red solid
identified as 12. Anal. Calcd for C38H50N2O2Fe: C, 73.30; H,
8.09; N, 4.50. Found: C, 72.94; H, 8.27; N, 4.25. 1H NMR
(benzene-d6): δ 0.92 (d, 6.8 Hz, 12H, CH(CH3)2), 0.96 (s, 6H,
Supporting Information Available: Crystallographic
data for 1, 4, 7, and 9, including full atom labeling schemes
and bond distances and angles. This material is available free
of charge via the Internet at http://pubs.acs.org.
OM050625B