Three-coordinate, 12-electron organometallic complexes of iron(II) supported by a bulky β-diketiminate ligand: Synthesis and insertion of CO to give square-pyramidal complexes

Article abstract of DOI:10.1021/om020571v

The preparation of a series of three-coordinate, 12-electron organometallic complexes of iron(II) supported by a bulky β-diketiminate ligand is described. The thermally stable complexes LFeR (R = Et, CH₂^tBu, ⁱPr) are three-coordinate in both the solid state (single crystal X-ray diffraction) and solution. They react rapidly with CO to form the diamagnetic complexes LFe(CO)₂(COR), which have an unusual square-pyramidal geometry. Spectroscopic and crystallographic studies show that the acyl group is in the axial position. As a result, there are two orientations of the acyl group about the Fe-C bond, and the isomeric ratio is dependent on the size of R. The two isomers are in equilibrium in solution at room temperature.

Full text of DOI:10.1021/om020571v

4808
Organometallics 2002, 21, 4808-4814
Th r ee-Coor d in a te, 12-Electr on Or ga n om eta llic
Com p lexes of Ir on (II) Su p p or ted by a Bu lk y
â-Dik etim in a te Liga n d : Syn th esis a n d In ser tion of CO
To Give Squ a r e-P yr a m id a l Com p lexes
J eremy M. Smith, Rene J . Lachicotte, and Patrick L. Holland*
Department of Chemistry, University of Rochester, Rochester, New York 14627
Received J uly 16, 2002
The preparation of a series of three-coordinate, 12-electron organometallic complexes of
iron(II) supported by a bulky â-diketiminate ligand is described. The thermally stable
t
i
complexes LFeR (R ) Et, CH2 Bu, Pr) are three-coordinate in both the solid state (single
crystal X-ray diffraction) and solution. They react rapidly with CO to form the diamagnetic
complexes LFe(CO)2(COR), which have an unusual square-pyramidal geometry. Spectroscopic
and crystallographic studies show that the acyl group is in the axial position. As a result,
there are two orientations of the acyl group about the Fe-C bond, and the isomeric ratio is
dependent on the size of R. The two isomers are in equilibrium in solution at room
temperature.
In tr od u ction
We have recently shown that the bulky â-diketimi-
nate ligand 2,2,6,6-tetramethyl-3,5-bis((2,6-diisopropyl-
phenyl)imido)hept-4-yl leads to isolable three-coordinate
complexes of iron, cobalt, and nickel (Figure 1).16,17 Since
only two donor atoms exert the steric hindrance in these
complexes, it is possible to prepare heteroleptic three-
coordinate complexes in which the third coordination
site is occupied by a chloride ligand. Selective reaction
of the chloride ligand gives products in which the low
coordination number at the metal is maintained. Thus,
for example, it is possible to prepare three-coordinate
complexes of iron and cobalt in which methyl ligands
occupy the third coordination site.17,18 Other researchers
have also used â-diketiminate ligands to prepare para-
magnetic organometallic complexes,19-22 albeit with
greater coordination numbers.
This contribution shows that the synthetic method
used for LFeMe is generally applicable for creating a
series of stable three-coordinate, 12-electron organome-
tallic complexes of iron. We also report initial reactivity
studies of our three-coordinate alkyl complexes with the
prototypical organometallic ligand CO. In contrast to
the more common 18-electron organometallic complexes,
low-electron-count, coordinatively unsaturated organo-
metallic complexes have not been as extensively stud-
Low-coordinate, coordinatively unsaturated late-
transition-metal complexes are often invoked as reactive
intermediates in catalytic processes. In the case of iron,
low-coordinate metal centers have been proposed as the
active species in alkene polymerization1,2 as well as
C-H activation3-5 and functionalization6,7 processes.
Stable three-coordinate complexes are rare in com-
pounds with less than 10 d electrons.8,9 Most three-coor-
dinate complexes of iron are homoleptic, and reactions
of these complexes tend to be at the expense of the low
coordination number.8 Few reported three-coordinate
ferrous complexes contain Fe-C bonds. Among these
are the dimeric diaryl complexes [FeR2]2 (R ) 2,4,6-
Me3C6H2, 2,4,6-iPr3C6H2) and their monomeric adducts
FeR2L (L ) donor ligand)10-14 and the N-functionalized
alkyl complex Fe2(η2-CHSitBuMe2C5H4N-2)4.15
* To whom correspondence should be addressed. E-mail: holland@
chem.rochester.edu.
(1) Britovsek, G. J . P.; Gibson, V. C.; Wass, D. F. Angew. Chem.,
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1169-1203.
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2001, 1542-1543.
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10.1021/om020571v CCC: $22.00 © 2002 American Chemical Society
Publication on Web 10/04/2002

12-Electron Organometallic Complexes
Organometallics, Vol. 21, No. 22, 2002 4809
F igu r e 1. Three-coordinate chloride complexes supported
by a bulky â-diketiminate ligand.
Sch em e 1
F igu r e 2. Ball and stick diagram of the complex LFeEt.
Hydrogen atoms are omitted for clarity.
ied.23,24 The three-coordinate [FeR2]2 complexes men-
tioned above undergo insertion reactions with isocyanides
and nitriles.13,14 There are a few examples of 14-electron,
4-coordinate organometallic complexes of iron.15,25-32
Among the most extensively studied are the 1,2-bis-
(diisopropylphosphino)ethane complexes Fe(dippe)R2
and Fe(dippe)(R)Cl, which display reactivity patterns
similar to those of 18-electron complexes,27,28,33 and
the hydrotris(3,5-diisopropylpyrazolyl)borate complexes
Fe(TpiPr)R, which display some unusual properties, such
as stability toward â-hydride elimination.31,32
Resu lts a n d Discu ssion
t
F igu r e 3. ORTEP diagram of the complex LFeCH2 Bu.
Hydrogen atoms are omitted for clarity; thermal ellipsoids
are given at the 50% probability level.
Syn th esis a n d Ch a r a cter iza tion of 3-Coor d in a te
Ir on (II) Alk yl Com p lexes. Preparation of the three-
coordinate organometallic complexes LFeR (R ) Me, Et,
t
i
CH2 Bu, Pr) was achieved by reaction of the three-
coordinate chloride complex LFeCl16 with the appropri-
ate Grignard or alkyllithium reagent in ether solutions
at room temperature (Scheme 1). The preparation and
characterization of LFeMe has been previously re-
ported.18 The organometallic complexes were isolated
as orange solids in high yield by crystallization from
pentane solutions at -35 °C. Bulky alkyl groups are not
required, as is evident from the isolation of LFeMe.
The low-coordinate nature of the iron atom in all
complexes was confirmed by X-ray crystallographic
(23) Theopold, K. H. Acc. Chem. Res. 1990, 23, 263-270.
(24) Poli, R. Chem. Rev. 1996, 96, 2135-2204.
(25) Akita, M.; Shirasawa, N.; Hikichi, S.; Moro-oka, Y. Chem.
Commun. 1998, 973-974.
(26) Fryzuk, M. D.; Leznoff, D. B.; Ma, E. S. F.; Rettig, S. J .; Young,
V. G., J r. Organometallics 1998, 17, 2313-2323.
(27) Hermes, A. R.; Girolami, G. S. Organometallics 1987, 6, 763-
768.
F igu r e 4. ORTEP diagram of the complex LFeiPr. Hy-
drogen atoms are omitted for clarity; thermal ellipsoids are
given at the 50% probability level.
(28) Hermes, A. R.; Girolami, G. S. Organometallics 1988, 7, 394-
401.
studies (Figures 2-4). In the case of LFeEt, suspected
twinning problems led to high residuals (R1 ) 0.15),
but we were able to confirm the connectivity of the
molecule. The experimental data and selected structural
parameters are listed in Tables 1 and 2. All complexes
are three-coordinate in the solid state. For the com-
(29) Kisko, J . L.; Hascall, T.; Parkin, G. J . Am. Chem. Soc. 1998,
120, 10561-10562.
(30) Hursthouse, M. B.; Izod, K. J .; Motevalli, M.; Thornton, P.
Polyhedron 1996, 15, 135-145.
(31) Shirasawa, N.; Akita, M.; Hikichi, S.; Moro-oka, Y. Chem.
Commun. 1999, 417-418.
(32) Shirasawa, N.; Nguyet, T. T.; Hikichi, S.; Moro-oka, Y.; Akita,
M. Organometallics 2001, 20, 3582-3598.
plexes LFeCH2 Bu and LFeiPr, confirmation of the
t
(33) Hermes, A. R.; Warren, T. H.; Girolami, G. S. J . Chem. Soc.,
Dalton Trans. 1995, 301-305.
trigonal-planar geometry is obtained from the sum of

4810 Organometallics, Vol. 21, No. 22, 2002
Smith et al.
Ta ble 1. Exp er im en ta l Da ta for X-r a y Diffr a ction
Stu d ies of LF eR (R ) CH2tBu , P r ) a n d
i
LF e(CO)2(COMe)
LFeCH2tBu
LFeiPr
LFe(CO)2(COMe)
39H55FeN2O3
655.70
formula
fw
C
40H64FeN2
C
38H60FeN2
600.73
C
628.78
cryst size (mm) 0.18 × 0.32 × 0.20 × 0.26 × 0.02 × 0.20 ×
0.44
9.7226(7)
18.187(1)
21.785(2)
90
96.771(1)
90
3825.5(5)
4
0.38
9.6306(6)
17.388(1)
21.749(1)
90
95.884(1)
90
3623.0(4)
4
0.24
a (Å)
b (Å)
c (Å)
R (deg)
â (deg)
γ (deg)
V (Å3)
Z
9.5385(8)
12.519(1)
17.043(2)
72.705(2)
82.442(2)
67.729(1)
1797.8(3)
2
space group
T (K)
P21/n
P21/n
193
0.710 73 (Mo KR)
0.442
1.101
0.068
0.155
1.073
P1h
F igu r e 5. 1H NMR spectrum of the complex LFeCH2 Bu.
t
λ (Å)
µ (mm-1
)
0.421
1.092
0.063
0.182
1.011
0.457
Approximate T2 values and assignments are indicated (s,
pcalcd (g cm-3
R1a
)
1.211
0.068
0.146
1.109
residual C6D5H; x, solvent impurities).
wR2a
GOFb
R1 ) (∑||Fo| - |Fc||)/|Fo|; wR2 ) [∑[w(Fo2 - Fc2)2]/∑[w(Fo2)2]]1/2
,
a
where w ) 1/[σ2(Fo2) + (aP)2 + bP and P ) [(Max{0,Fo2}) + 2Fc2]/
b
2
3. GOF ) [∑[w(Fo - Fc2)2/(n - p)]1/2, where n and p denote the
numbers of data and parameters, respectively.
Ta ble 2. Selected Bon d Len gth s (Å) a n d An gles
(d eg) for th e Com p lexes LF eR
LFeMe
LFeCH2tBu
LFeiPr
Fe-C
Fe-N
2.009(3)
1.973(1)
2.027(4)
1.996 (2)
1.994(2)
120.4(2)
142.9(2)
94.62(7)
126.3(2)
127.9(2)
2.933(3)
2.048(3)
1.994(2)
1.995(2)
130.8(1)
134.5(1)
94.10(8)
125.5(2)
127.6(2)
2.920(3)
1.50 (0.38)
N-Fe-C
132.57(4)
N-Fe-N (bite angle) 96.4(1)
C-N-C
128.56(1)
N‚‚‚N
2.906(3)
0.49 (0.06) 0.71 (0.36)
fold anglea
a
Angle between the least-squares NCCCN and NMN planes of
the six-membered diketiminate-metal ring.
bond angles around iron (∼360°). There are no agostic
interactions in either molecule, as the closest intramo-
lecular Fe‚‚‚H-C contact is 2.70 Å. From the data in
Table 1, it is evident that the larger alkyl groups are
accommodated solely by an increase in the iron-carbon
distance. The bulkier alkyl groups do not cause a
decrease in the bite angle, force the aryl groups to bend
away from the alkyl ligand (C-N-C angle), or change
the geometry of the rigid diketiminate ligand (N‚‚‚N
distance).
A number of three-coordinate iron â-diketiminate
complexes have been crystallographically characterized.
The Fe-N bond lengths in all these complexes are
similar to each other (∼2.0 Å)16-18,34,35 and similar to
those of the alkyl complexes reported here. The low
coordination number of the metal center results in re-
markably short Fe-C bond lengths (2.009(3)-2.048(3)
Å).17,36 A three-coordinate iron benzyl complex sup-
ported by a less bulky â-diketiminate ligand16 has an
Fe-C bond length of 2.042(2) Å.35
F igu r e 6. (a) ORTEP diagram of the complex LFe(CO)2-
(COMe). Hydrogen atoms are omitted for clarity; thermal
ellipsoids are given at the 50% probability level. (b)
Expanded view of the â-diketiminate ring, showing the
square-pyramidal geometry around the metal. Aromatic
rings on the nitrogen atoms and tert-butyl groups on the
ligand backbone are omitted for clarity.
All complexes are paramagnetic, with solution mag-
netic moments around 5.5 µB, suggestive of high-spin
iron(II) (S ) 2).18 Consistent with the high-spin nature
1
of the compounds, the H NMR spectra show paramag-
netically shifted resonances. The solution 1H NMR
(34) Panda, A.; Stender, M.; Wright, R. J .; Olmstead, M.; Klavins,
P.; Power, P. P. Inorg. Chem. 2002, 41, 3909-3916.
(35) Sciarone, T. J . J .; Meetsma, A.; Hessen, B.; Teuben, J . H. Chem.
Commun. 2002, 1580-1581.
(36) Balch, A. J .; Olmstead, M.; Safari, N.; St. Clair, T. N. Inorg.
Chem. 1994, 33, 2815-2822.
t
spectrum of the complex LFeCH2 Bu in benzene-d6
(Figure 5) is representative. The spectrum is consistent
with the solid-state structure: signals from all protons
in the complex are observed, with the exception of those

12-Electron Organometallic Complexes
Organometallics, Vol. 21, No. 22, 2002 4811
Sch em e 2
on the R-carbon of the alkyl group, most likely due to
their close proximity to the paramagnetic iron center.
Most of the resonances can be assigned on the basis of
their relative integration,17 although by this criterion
alone we cannot distinguish the two sets of chemically
inequivalent isopropyl methyl protons. However, we
have previously shown that the distance of the protons
from the paramagnetic center can be correlated with
both the chemical shift and relaxation time of the peak
(estimated from peak broadness).17,37 Thus, the reso-
nance at δ -140 can be assigned to the two methyl
groups adjacent to the iron atom, and the peak at δ -30
can be assigned to the methyl groups closer to the
backbone of the ligand. Similarly, we assign the peak
at δ -111 to the four methine protons and the peak at
δ -4.5 to the four aryl protons meta to the nitrogen
atoms. Similarly for the other alkyl complexes, signals
for all the protons of the â-diketiminate ligand were
observed, while the protons of the alkyl ligand in close
proximity (R or â to iron) to the metal center were not
observed.
The UV-vis spectra in pentane solution all show a
peak at around 520 nm, with a molar extinction coef-
ficient of 0.51-0.58 mM-1 cm-1. This peak is charac-
teristic of the alkyl complexes and has not been observed
in the UV-vis spectra of other three-coordinate â-di-
ketiminate iron complexes.16,17,38
Although highly sensitive to oxygen and water, the
complexes are all thermally stable. The 1H NMR
spectrum of the methyl complex LFeMe showed no
changes after heating for 3 days at 120 °C. The complex
also survives hydrogenation pressures of up to 2000 psi
unscathed. Even more remarkably, the complex LFeEt
showed no propensity toward â-hydride elimination. No
F igu r e 7. (a) 13C{1H} NMR spectrum of the complex LFe-
(13CO)2(13COMe). The inset shows an expansion of the
signals for the acyl methyl groups. (b) 13C NMR spectrum
of the complex LFe(13CO)2(13COMe). The inset shows an
expansion of the signals for the acyl methyl groups.
1
changes in the H NMR spectrum were observed either
in the IR spectrum. The major product showed a single
νCdO absorption (1687 cm-1) and two νCO absorptions
(1998, 1934 cm-1). In the 1H NMR spectrum of this
product the methine protons of the isopropyl groups give
two sets of signals at δ 3.52 and 2.58, suggesting loss of
symmetry about the iron-diketiminate ligand plane. In
the 13C{1H} NMR spectrum of the complex prepared
from 13CO (Figure 7), the acyl ligand carbon resonates
as a triplet at δ 248.1, which is further split by coupling
to the methyl protons in the proton-coupled 13C spec-
trum. The two terminal carbonyl ligands resonate
together as a doublet at δ 208.0, suggesting the presence
of a mirror plane in the molecule. The spectroscopic data
are consistent with a square-pyramidal geometry in
which the carbonyl ligands occupy the basal positions.
This is confirmed by estimating the angle between the
two carbonyl groups in the isomers using the relation
tan2 θ ) Ias/Is, where 2θ is the angle between the two
carbonyl bands, Is is the height of the symmetric νCO
band, and Ias is the height of the asymmetric νCO band.39
For the major isomer, θ ) 86°, and for the minor isomer,
θ ) 90°. The minor product shows similar features in
its IR and NMR spectra and, thus is also proposed to
have a square-pyramidal structure with the acyl group
in the apical position.
on heating at 120 °C for 3 days or after irradiation with
visible light.
One possible explanation for the compounds’ resis-
tance to thermal degradation is the highly crowded
nature of the metal center. However, we have been able
to prepare complexes with coordination number 5 hav-
ing ligands residing between the two aryl rings of the
â-diketiminate ligand (see below), suggesting that the
diketiminate iron moiety can accommodate further
ligands. A second possible explanation is that in these
high-spin complexes18 there are no empty orbitals on
the metal and, therefore, â-hydride elimination reac-
tions are not possible.24 This explanation has been
used to rationalize the observation that tetrahedral
14-electron iron alkyl complexes supported by hydrotris-
(3,5-diisopropylpyrazolyl)borato and hydrotris(3,4,5-
trimethylpyrazolyl)borato ligands are resistant to
â-hydride elimination.29,31,32
Ca r bon yla tion of 3-Coor d in a te Alk yl Com p lexes.
Exposure of an orange solution of LFeMe to an atmo-
sphere of CO led to the formation of a red diamagnetic
product (Scheme 2), which was characterized as the
5-coordinate complex LFe(CO)2(COMe) and was ob-
served in a 2:1 isomeric mixture. The acyl-dicarbonyl
functionality shows a characteristic set of absorbances
The 2D NOESY/EXSY spectrum evidences chemical
exchange between the isomers. For example, a positive
(37) Ming, L.-J . In Physical Methods in Bioinorganic Chemistry;
Que, L., J r., Ed.; University Science Books: Sausalito, CA, 2000; pp
375-464.
(39) Beck, W.; Melnikoff, A.; Stahl, R. Angew. Chem., Int. Ed. Engl.
1965, 4, 692-693.
(38) Smith, J . M.; Eckert, N. A.; Holland, P. L. Unpublished results.

4812 Organometallics, Vol. 21, No. 22, 2002
Smith et al.
Ta ble 3. Selected Bon d Len gth s (Å) a n d An gles
(d eg) for th e Com p lex LF e(CO)2(COMe)
Carbonylation of the tetrahedral 14-electron com-
plexes Fe(dippe)(R)X similarly led to the formation of
the octahedral complexes Fe(dippe)(COR)(CO)2X. It was
possible to isolate the intermediate η2-acyl complexes
Fe(dippe)(η2-COR)(CO)2X by using a sufficiently bulky
R group and controlling the CO stoichiometry.28 How-
ever, we were not able to isolate any intermediates;
addition less than 3 equiv of CO to LFeR at -78 °C
followed by warming to room temperature resulted only
in incomplete formation of the final product.
Fe(1)-C(1)
Fe(1)-C(2)
Fe(1)-C(3)
1.774(6)
1.773(6)
1.946(6)
Fe(1)-N(11)
Fe(1)-N(21)
1.983(4)
1.989(4)
C(1)-Fe(1)-C(2)
C(1)-Fe(1)-C(3)
C(2)-Fe(1)-C(3)
81.5(2)
92.1(2)
94.4(3)
N(11)-Fe(1)-N(21)a
C(21)-N(11)-C(12)
C(41)-N(21)-C(13)
94.8(2)
121.9(4)
121.9(4)
a
Bite angle.
cross-peak is observed between the diketiminate back-
bone protons of each isomer at δ 6.67 (major) and δ 6.39
(minor). Similar positive cross-peaks are observed be-
tween the other equivalent protons of each isomer. Since
both complexes are square pyramidal, it is likely that
they differ by the orientation of the acyl group around
the Fe-C bond. Unfortunately, no cross-peaks were
observed in the 2D NOESY/EXSY spectrum between the
acyl methyl group and the backbone tert-butyl groups;
therefore, it was not possible to assign the isomers
conclusively using 2D NMR.
Con clu sion
Through the use of a suitably bulky â-diketiminate
ligand, it is possible to isolate and characterize a series
of thermally stable 12-electron iron(II) alkyl complexes.
The presence of exposed â-hydrogen atoms does not
affect the stability of the complexes. Reactions with CO
result in the formation of diamagnetic square-pyramidal
complexes, again highlighting the ability of diketiminate
ligands to stabilize unusual geometries.40
t
Both LFeCH2 Bu and LFeiPr show similar CO inser-
Exp er im en ta l Section
tion chemistry (Scheme 2). Interestingly, the proportion
of the major isomer present in the equilibrium mixture
increases as the R groups increase in size from neopen-
tyl to isopropyl. This is more consistent with the major
isomer having the alkyl group pointed away from the
diketiminate ligand backbone, and these assignments
are used in Scheme 2.
Gen er a l P r oced u r es. All manipulations were performed
under a nitrogen atmosphere by standard Schlenk techniques
or in an M. Braun glovebox maintained at or below 1 ppm of
O2 and H2O. Glassware was dried at 150 °C overnight. NMR
data were recorded on a Bruker Avance 400 spectrometer (400
MHz) at 22 °C. All peaks in the NMR spectra are reported in
ppm, referenced to residual C6D5H at δ 7.16 ppm. In the
paramagnetic complexes, all peaks are singlets. In parentheses
The solution assignment of LFe(CO)2(COMe) was
confirmed by X-ray crystallography (Figure 6). The
structure is consistent with the major solution isomer,
with the acyl methyl pointed away from the diketimi-
nate backbone. The CO-Fe-CO bond angle θ, at
81.5(2)°, compares favorably with the value calculated
from the IR spectrum (86°, see above). In compari-
son to the alkyl complexes, there is not a significant
change in the bond lengths and angles of the diketimi-
nate ligand to the iron center (Table 3), despite the
increased coordination number and diamagnetism of
the complex. Both the Fe-N bond lengths and the
bite angle are similar to those of the starting ma-
terial. However, the C-N-C bond angles become smaller
as the aryl groups are pushed back toward the tert-
butyl groups on the ligand backbone, allowing more
space for other ligands to coordinate. The â-diketimi-
nate ligand further adjusts to accommodate the extra
ligands by bending the aryl groups away from the apical
acyl ligand. This pulls the methine carbons of the
isopropyl groups adjacent to the acyl ligand away from
each other to a distance of 6.950(7) Å and pushes the
other two methine carbons toward each other so that
they are separated by 4.012(7) Å. In the precursor
complex LFeMe the methine carbons are separated by
5.204(3) Å. It is likely that this deformation sterically
prohibits a sixth ligand from coordinating to the iron
center.
are listed T2 values in ms (calculated as (π∆ν1/2)
-1),17,37 integra-
tions, and assignments. In some cases, overlapping peaks
prevented T2 determinations. Coupling constants in the spec-
tra of the diamagnetic complexes are reported in Hertz. IR
spectra were recorded on a Mattson Instruments 6020 Galaxy
Series FTIR using solution cells with CsF windows. UV-vis
spectra were measured on a Cary 50 spectrophotometer, using
screw-cap cuvettes. Solution magnetic susceptibilities were
determined by the Evans method.41 Elemental analyses were
determined by Desert Analytics, Tucson, AZ.
Pentane, diethyl ether, tetrahydrofuran (THF), and toluene
were purified by passage through activated alumina and
“deoxygenizer” columns from Glass Contour Co. (Laguna
Beach, CA). Deuterated benzene was first dried over CaH2 and
then over Na/benzophenone and then vacuum-transferred into
a storage container. Before use, an aliquot of each solvent was
tested with a drop of sodium benzophenone ketyl in THF
solution. Celite was dried overnight at 200 °C under vacuum.
LiCH2CMe3 was prepared from neopentyl chloride and lithium
in pentane and purified by sublimation. Grignard reagents
(1-2 M in Et2O or THF) were obtained from Aldrich and used
without further purification.
Im p r oved Syn th esis of LF eCl. A 200 mL Schlenk flask
was charged with FeCl2(THF)1.542 (8.0 g, 34 mmol), LiL(THF)43
(20 g, 34 mmol), and toluene (150 mL). The reaction mixture
became red. The reaction mixture was then heated at 100 °C
for 24 h to form a dark red solution. The solvent was removed
in vacuo, and the red solid residue was transferred to a glass
thimble, which was placed in a Soxhlet extractor. Continuous
extraction of the residue with hot diethyl ether until the
extracting solvent was clear (1-2 days) led to the formation
In a related reaction, Akita has found that the
4-coordinate hydrotris(3,5-diisopropylpyrazolyl)borato-
and hydrotris(3,4,5-trimethylpyrazolyl)borato-supported
complexes react with CO to give hexacoordinate acyl
dicarbonyl complexes, TpFe(CO)2(COR).31,32 On the
other hand, reaction of a complex supported by the more
bulky phenyltris(3-tert-butylpyrazolyl)borato ligand re-
sults in reduction to the iron(I) complex TpFe(CO).29
(40) Fekl, U.; Kaminsky, W.; Goldberg, K. I. J . Am. Chem. Soc. 2001,
123, 6423-6424.
(41) Baker, M. V.; Field, L. D.; Hambley, T. W. Inorg. Chem. 1988,
27, 7.
(42) Kern, R. J . J . Inorg. Nucl. Chem. 1962, 24, 1105.
(43) Budzelaar, P. H. M.; van Oort, A. B.; Orpen, A. G. Eur. J . Inorg.
Chem. 1998, 1485.

12-Electron Organometallic Complexes
Organometallics, Vol. 21, No. 22, 2002 4813
of a red slurry and left a gray solid behind. The solvent was
then reduced to ca. 50 mL, and the red solid was isolated by
filtration (17.0 g). Further product was obtained by crystal-
lization from the mother liquor at -35 °C. The total yield is
Calcd for C39H56N2O3Fe: C, 71.33; H, 8.59; N, 4.27. Found:
C, 71.57; H, 8.23; N, 4.13.
i
Rea ction of LF eR (R ) Me, n P n , P r ) w ith 13CO. In a
resealable NMR tube, LFeR (R ) Me, nPn, iPr) (5-10 mg) was
dissolved in C6D6 (0.5 mL) to give an orange solution. The
solution was frozen and the headspace evacuated and back-
filled with 13CO (ca. 1 atm). The solution was thawed and
mixed, resulting in a color change to red. The complexes were
1
19.0 g (94%). H NMR (C6D6): δ 109 (0.32, 1H, CH); 43 (0.64,
9H, C(CH3)3), 0 (0.13, 4H, m-H), -29 (2.1, 12H, CH(CH3)2),
-111 (1.3, 2H, p-H), -115 (0.27, 12H, CH(CH3)2), -116 (4H,
CH(CH3)2).
1
characterized by H and 13C NMR and IR spectroscopy.
Gen er a l Syn th esis of LF eR (R ) Et, iP r ). To a red slurry
of LFeCl in diethyl ether (10 mL) was added via syringe 1
molar equiv of the appropriate Grignard reagent solution in
THF. The red color of the reaction mixture faded to orange,
with the formation of a white precipitate. The reaction mixture
was stirred overnight and the solvent removed in vacuo. The
residue was extracted with pentane and filtered through a plug
of Celite to give an orange solution. The solution was then
concentrated (ca. 2 mL) and warmed to dissolve the product.
Orange crystals were isolated after cooling to -35 °C.
LF e(13CO)2(13COMe). Major isomer: 13C{1H} NMR (C6D6)
δ 248.1 (t, J CC ) 9.0, COMe), 208.0 (d, J CC ) 9.0, CO); 13C
NMR (C6D6) δ 248.1 (tq, J CC ) 9.0, J CH ) 5.1, COMe), 208.0
(d, J CC ) 9.0, CO); IR (pentane) 1949, 1884, 1665 cm-1. Minor
isomer: 13C{1H} NMR (C6D6) δ 251.8 (tq, J CC ) 9.4, J CH ) 4.5,
COMe), 206.0 (d, J CC ) 9.4, CO); IR (pentane) 1968, 1908, 1665
cm-1
.
LF e(13CO)2(13COCH2tBu ). Major isomer: 1H NMR (C6D6)
δ 7.00-7.13 (m, 6H, Ar H), 6.79 (s, 1H, backbone CH), 3.35
(dt, J HH ) 6, J HH ) 6, 2H, CH(CH3)2), 3.24 (s, 2H, CH2), 2.31
(dt, J HH ) 6, J HH ) 6, 2H, CH(CH3)2), 1.40 (d, 6H, CH(CH3)2),
1.40 (d, J HH ) 6, 6H, CH(CH3)2), 1.30 (d, J HH ) 6, 6H,
CH(CH3)2), 1.29 (s, 18H, C(CH3)3), 1.25 (d, J HH ) 6, 6H,
CH(CH3)2), 1.10 (d, 9H, CH2C(CH3)3); 13C{1H} NMR (C6D6) δ
1
LF eEt. Yield: 80%. H NMR (C6D6): δ 129 (0.28, 1H, CH),
42 (0.80, 18H, C(CH3)3), -5 (1.6, 4H, m-H), -29 (0.13, 12H,
CH(CH3)2), -112 (1.1, 2H, p-H), -116 (0.24, 4H, CH(CH3)2),
-136 (0.32, 12H, CH(CH3)2). µeff (Evans, C6D6): 4.9(3) µB. UV-
vis (pentane): 517 nm (ꢀ ) 0.59(2) mM-1 cm-1). Anal. Calcd
for C37H58N2Fe (586.71): C, 75.74; H, 9.96; N, 4.77. Found:
C, 74.34; H, 9.68; N, 4.68. Despite repeated attempts, we were
not able to obtain an accurate microanalysis on spectroscopi-
cally pure material.
t
247.6 (t, J CC ) 8.5, COCH2 Bu), 209.3 (d, J CC ) 8.5, CO); IR
(pentane) 1973, 1915, 1654 cm-1. Minor isomer: 1H NMR
(C6D6, not all peaks could be observed) δ 7.00-7.13 (m, 6H,
Ar H), 5.62 (s, 1H, backbone CH), 3.72 (dt, J HH ) 6, J HH ) 6,
2H, CH(CH3)2), 3.60 (dt, J HH ) 6, J HH ) 6, 2H, CH(CH3)2),
1.63 (d, 6H, CH(CH3)2), 1.55 (d, J HH ) 6, 6H, CH(CH3)2), 1.22
(s, 3H, CH2(CH3)3); 13C{1H} NMR (C6D6) δ 249.3 (t, J CC ) 8.3,
LF eiP r . Yield: 92%. 1H NMR (C6D6): δ 128 (0.29, 1H, CH),
45 (0.80, 18H, C(CH3)3), -7 (1.1, 4H, m-H), -27 (1.6, 12H,
CH(CH3)2), -104 (1.6, 2H, p-H), -110 (4H, CH(CH3)2), -139
(0.29, 12H, CH(CH3)2). µeff (Evans, C6D6): 5.4(3) µB. UV-vis
(pentane): 515 nm (ꢀ ) 0.53(2) mM-1 cm-1). Anal. Calcd for
t
COCH2 Bu), 206.4 (d, J CC ) 8.3, 2C, CO).
LF e(13CO)2(13COiP r ).1H NMR (C6D6): δ 7.01-7.14 (m, 6H,
Ar H), 6.75 (s, 1H, backbone CH), 3.43 (m, J HH ) 6, 2H,
C
38H60N2Fe (600.74): C, 75.97; H, 10.07; N, 4.66. Found: C,
75.81; H, 9.49; N, 4.63.
CH(CH3)2), 3.29 (dt, J HH ) 6, 1H, CH(CH3)2), 2.36 (dt, J HH
)
t
LF eCH2tBu . A clear solution of LiCH2 Bu (37 mg, 472 µmol)
in Et2O (5 mL) was added to a red slurry of LFeCl (280 mg,
472 µmol) in Et2O (10 mL). The reaction mixture immediately
became orange with the formation of a white precipitate. After
it was stirred for 2 h at room temperature, the mixture was
filtered through a plug of Celite to give an orange solution.
The solvent was removed under vacuum, and the residue was
dissolved in hot pentane (4 mL). The product was then
crystallized in two crops at -35 °C to give an orange solid (240
6, 2H, CH(CH3)2), 1.37 (d, J HH ) 6, 3H, CH(CH3)2), 1.35 (d,
J HH ) 6, 3H, CH(CH3)2), 1.33 (d, J HH ) 6, 3H, CH(CH3)2), 1.27
(d, J HH ) 6, 3H, CH(CH3)2), 1.18 (s, 18H, C(CH3)3), 1.16 (d,
J HH ) 6, 3H, CH(CH3)2), 1.11 (d, 6H, CH(CH3)2); 13C{1H} NMR
(C6D6): δ 252.8 (t, J CC ) 8.5, COiPr), 210.0 (d, J CC ) 8.5, CO).
IR (pentane): 1950, 1886, 1619 cm-1
.
X-r ay Str u ctu r al Deter m in ation of LFeCH2 Bu , LFeiP r ,
a n d LF e(CO)2(COMe). Crystalline samples of the three
complexes were grown from pentane solutions at -35 °C. All
samples were rapidly mounted under Paratone-8277 onto glass
fibers and immediately placed in a cold nitrogen stream at
-80 °C on the X-ray diffractometer. The X-ray intensity data
were collected on a standard Bruker SMART CCD area
detector system equipped with a normal-focus Mo-target X-ray
tube operated at 2.0 kW (50 kV, 40 mA). A total of 1321 frames
of data (1.3 hemispheres) were collected using a narrow-frame
method with scan widths of 0.3° in ω and exposure times of
30 s/frame for LFeiPr and LFe(CO)2(COMe), and 60 s/frame
t
1
mg, 81%). H NMR (C6D6): δ 129 (0.29, 9H, CH2C(CH3)3), 45
(0.91, 18H, C(CH3)3), -7 (1.1, 4H, m-H), -27 (2.1, 12H,
CH(CH3)2), -104 (1.3, 2H, p-H), -110 (0.19, 4H, CH(CH3)2),
-139 (0.32, 12H, CH(CH3)2). µeff (Evans, C6D6): 5.5(3) µB.
UV-vis (pentane): 520 nm (ꢀ ) 0.58(2) mM-1 cm-1). Anal.
Calcd for C40H64N2Fe (628.79): C, 76.40; H, 10.26; N, 4.46.
Found: C, 76.86; H, 10.33; N, 4.44.
LF e(CO)2(COMe). A resealable flask was charged with
LFeMe (200 mg, 349 µmol) and diethyl ether (15 mL) to give
an orange solution, which was frozen at -196 °C. The
headspace was evacuated and refilled with CO to approxi-
mately 1 atm. The solvent was then thawed to give a red
solution that was stirred at room temperature for 4 h. The
volatiles were removed in vacuo and the red residue dissolved
in a diethyl ether/pentane mixture. Red crystals (205 mg, 90%)
were grown at -35 °C. 1H NMR (C6D6): major isomer, δ 7.08-
t
for LFeCH2 Bu, with a detector-to-crystal distance of 5.09 cm.
Frames were integrated to a maximum 2θ angle of 56.5° with
the Bruker SAINT program. Laue symmetry revealed mono-
t
clinic crystal systems for LFeCH2 Bu and LFeiPr and a triclinic
system for LFe(CO)2(COMe). The final unit cell parameters
were determined from the least-squares refinement of three-
dimensional centroids of >3400 reflections for each crystal.44
Data were corrected for absorption with SADABS.45
7.16 (m, 6H, Ar H), 6.67 (s, 1H, backbone CH), 3.47 (dt, J HH
)
The space groups were assigned as P21/n (No. 14) for
6, J HH ) 6, 2H, CH(CH3)2), 2.65 (s, 3H, COCH3), 2.59 (dt, J HH
) 6, J HH ) 6, 2H, CH(CH3)2), 1.46 (d, J HH ) 6, 6H, CH(CH3)2),
1.45 (d, J HH ) 6, 6H, CH(CH3)2), 1.34 (d, J HH ) 6, 6H,
CH(CH3)2), 1.30 (s, 18H, C(CH3)3), 1.23 (d, J HH ) 6, 6H,
CH(CH3)2); minor isomer, δ 7.08-7.16 (m, 6H, Ar H), 6.39 (s,
1H, backbone CH), 3.64 (dt, J HH ) 6, J HH ) 6, 2H, CH(CH3)2),
3.47 (s, 3H, COCH3), 2.51 (dt, J HH ) 6, J HH ) 6, 2H, CH(CH3)2),
1.55 (d, J HH ) 6, 6H, CH(CH3)2), 1.48 (d, J HH ) 6, 6H,
CH(CH3)2), 1.33 (d, J HH ) 6, 6H, CH(CH3)2), 1.25 (s, 18H,
C(CH3)3), 1.23 (d, J HH ) 6, 6H, CH(CH3)2). IR (pentane): 1998,
1934 (major isomer), 2012, 1948 (minor), 1687 cm-1. Anal.
LFeCH2 Bu and LFeiPr, and P1h (No. 2) for LFe(CO)2(COMe),
t
and the structures were solved by direct methods using
SIR9246 and refined employing full-matrix least squares on
F2 (SHELXTL-NT,47 version 5.10). The disordered methyl
(44) It has been noted that the integration program SAINT produces
cell constant errors that are unreasonably small, since systematic error
is not included. More reasonable errors might be estimated at 10×
the reported value.
(45) The SADABS program is based on the method of Blessing;
see: Blessing, R. H. Acta Crystallogr., Sect. A 1995, 51, 33.

4814 Organometallics, Vol. 21, No. 22, 2002
Smith et al.
carbon atoms C(82) and C(82A) in LFe(CO)2(COMe) were
refined anisotropically using the PART instruction. All other
non-H atoms in all three complexes were refined with aniso-
tropic thermal parameters. Hydrogen atoms were included in
idealized positions. The structures were refined to goodness
of fit (GOF) values and final residuals found in Table 1.
Ack n ow led gm en t. We thank the National Science
Foundation (Grant No. CHE-0134658) and the Univer-
sity of Rochester for financial support.
Su p p or tin g In for m a tion Ava ila ble: Tables giving X-ray
crystallographic information; these data are also available in
CIF format. This material is available free of charge via the
Internet at http://pubs.acs.org.
(46) Altomare, A.; Cascarano, G.; Giacovazzo, C.; Gualardi, A. J .
Appl. Crystallogr. 1993, 26, 343-350.
(47) SHELXTL NT: Structure Analysis Program, version 5.10;
BRUKER-AXS, Madison, WI, 1995.
OM020571V