Low-Coordinate Iron(II) Amido Complexes of β-Diketiminates: Synthesis, Structure, and Reactivity

Article abstract of DOI:10.1021/ic035483x

The synthesis, structure, and reactivity of a series of low-coordinate Fe(II) diketiminate amido complexes are presented. Complexes L ^RFeNHAr (R = methyl, tert-butyl; Ar = para-tolyl, 2,6-xylyl, and 2,6-diisopropylphenyl) bind Lewis bases to give trigonal pyramidal and trigonal bipyramidal adducts. In the adducts, crystallographic and ¹H NMR evidence supports the existence of agostic interactions in solid and solution states. Complexes L^RFeNHAr may be oxidized using AgOTf, and the products L^RFe(NHAr)(OTf) are characterized with ¹⁹F NMR spectroscopy, UV/vis spectrophotometry, solution magnetic measurements, elemental analysis, and, in one case, X-ray crystallography. In the structures of the iron(III) complexes L^RFe(NHAr)(OTf) and L ^RFe(OtBu)(OTf), the angles at nitrogen and oxygen result from steric effects and not π-bonding. The reactions of the amido group of L ^RFeNHAr with weak acids (HCCPh and HOtBu) are consistent with a basic nitrogen atom, because the amido group is protonated by terminal alkynes and alcohols to give free H₂NAr and three-coordinate acetylide and alkoxide complexes. The trends in complex stability give insight into the relative strength of bonds from three-coordinate iron to anionic C-, N-, and O-donor ligands.

Full text of DOI:10.1021/ic035483x

Inorg. Chem. 2004, 43, 3306−3321
Low-Coordinate Iron(II) Amido Complexes of â-Diketiminates:
Synthesis, Structure, and Reactivity
Nathan A. Eckert, Jeremy M. Smith, Rene J. Lachicotte, and Patrick L. Holland*
Department of Chemistry, UniVersity of Rochester, Rochester, New York 14627
Received December 24, 2003
The synthesis, structure, and reactivity of a series of low-coordinate Fe(II) diketiminate amido complexes are presented.
Complexes LRFeNHAr (R ) methyl, tert-butyl; Ar ) para-tolyl, 2,6-xylyl, and 2,6-diisopropylphenyl) bind Lewis
bases to give trigonal pyramidal and trigonal bipyramidal adducts. In the adducts, crystallographic and 1H NMR
evidence supports the existence of agostic interactions in solid and solution states. Complexes LRFeNHAr may be
oxidized using AgOTf, and the products LRFe(NHAr)(OTf) are characterized with 19F NMR spectroscopy, UV/vis
spectrophotometry, solution magnetic measurements, elemental analysis, and, in one case, X-ray crystallography.
In the structures of the iron(III) complexes LRFe(NHAr)(OTf) and LRFe(OtBu)(OTf), the angles at nitrogen and
oxygen result from steric effects and not π-bonding. The reactions of the amido group of LRFeNHAr with weak
acids (HCCPh and HOtBu) are consistent with a basic nitrogen atom, because the amido group is protonated by
terminal alkynes and alcohols to give free H NAr and three-coordinate acetylide and alkoxide complexes. The
2
trends in complex stability give insight into the relative strength of bonds from three-coordinate iron to anionic C-,
N-, and O-donor ligands.
Introduction
Low-coordinate amido complexes have been known for
several decades, and most of them are homoleptic.6,7 In
general, sterically hindering ligands are used to create
coordinatively and electronically unsaturated compounds. To
isolate the reactivity of a single M-N bond, we are interested
in designing three-coordinate amido complexes that are
heteroleptic. Bulky â-diketiminates (Figure 1) are ideal for
this purpose and have been shown to sterically encourage
low coordination in transition metal complexes.8 Specifically,
Amido compounds of the late transition metals are of
interest because they are intermediates in catalysis, especially
catalytic N-C bond formation.1,2 For example, the hy-
droamination of olefins and alkynes3 and catalytic aryl group
amination2 involve transition metal amido intermediates.
Transition metal amido compounds have also been synthe-
sized to study the fundamentals of metal-ligand π-bonding.4
Finally, amido complexes are precursors to late transition
metal imido compounds.5
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* Author to whom correspondence should be addressed. E-mail:
holland@chem.rochester.edu.
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3306 Inorganic Chemistry, Vol. 43, No. 10, 2004
10.1021/ic035483x CCC: $27.50 © 2004 American Chemical Society
Published on Web 04/23/2004

Iron(II) Amido Complexes of â-Diketiminates
Figure 1. â-Diketiminate ligands used in this study: R ) methyl, LMe; R
) tert-butyl, LtBu
.
three-coordinate compounds supported by â-diketiminates
have been synthesized and characterized for several late
transition metals.9 Previously, we reported Mo¨ssbauer pa-
rameters for a pair of three-coordinate Fe(II) â-diketiminate
amido compounds.10 Here we report the synthesis, structure,
and reactivity of a larger series of Fe(II) amido compounds
with â-diketiminates. Because the three-coordinate iron(II)
compounds are very coordinatively unsaturated (formal
valence electron count of 12 at iron), they display an
interesting combination of ligand exchange, ligand binding,
and oxidation reactivity.
Figure 2. ORTEP drawing of the molecular structure of [LMeFeCl]2.
Thermal ellipsoids are shown at the 50% probability level. Hydrogen atoms
have been omitted for clarity. There is a crystallographic inversion center
in the middle of the dimer. Important bond distances (Å) and angles (deg):
Fe-N1 2.006(1), Fe-N2 2.002(1), Fe-Cl 2.3582(5), Fe-Cl′ 2.4046(5);
N1-Fe-N2 94.50(5), Cl-Fe-Cl′ 88.32(2).
Scheme 1
Results
Synthesis and Structure of [LMeFeCl]2. We were inter-
11
ested in a solvent-free analogue of LMeFeCl2Li(THF)2 to
eliminate potential ligands from reaction solutions. The
preparation of [LMeFeCl]2 was completed by stirring equimo-
13
lar amounts of LMeLi12 and FeCl2THF1.5 in hot toluene
overnight. Due to the low solubility of [LMeFeCl]2 in
hydrocarbon solvents, it could conveniently be freed from
LMeFeCl2Li(THF)2 by washing repeatedly with pentane.
Although the resultant material is contaminated with LiCl,
it is useful for further transformations. Analytically pure
[LMeFeCl]2 was isolated by filtering a hot toluene solution
and crystallizing at -35 °C. A single crystal of [LMeFeCl]2
was grown from a hot toluene solution and subjected to X-ray
diffaction analysis. The molecular structure is shown in
Figure 2. The X-ray crystal structure shows that each Fe atom
(8) Bourget-Merle, L.; Lappert, M. F.; Severn, J. R. Chem. ReV. 2002,
102, 3031.
has a distorted tetrahedral geometry. The average Fe-Cl
distance in [LMeFeCl]2 (2.3814(5) Å) is slightly longer than
that found in LMeFeCl2Li(THF)2 (2.331(1) Å),11 possibly due
to steric interference between the two diketiminates. The bite
angles of the two compounds are similar, 94.50(5)° for
[LMeFeCl]2 and 93.21(14)° for LMeFeCl2Li(THF)2.11
[LMeFeCl]2 can also be synthesized by dissolving LMeFe-
Cl2Li(THF)2 in hot toluene, stirring overnight, and filtering,
a method identical to that used for the transformation of LMe-
NiCl2Li(Et2O)(THF) into [LMeNiCl]2.9k
(9) (a) Budzelaar, P. H. M.; de Gelder, R.; Gal, A. W. Organometallics
1998, 17, 4121. (b) Holland, P. L.; Tolman, W. B. J. Am. Chem. Soc.
1999, 121, 7270. (c) Holland, P. L.; Tolman, W. B. J. Am. Chem.
Soc. 2000, 122, 6331. (d) Fekl, U.; Kaminsky, W.; Goldberg, K. I. J.
Am. Chem. Soc. 2001, 123, 64234. (e) Smith, J. M.; Lachicotte, R. J.;
Holland, P. L. Chem. Commun. 2001, 1542. (f) Dai, X.; Warren, T.
H. Chem. Commun. 2001, 1998. (g) Yokota, S.; Tachi, Y.; Itoh, S.
Inorg. Chem. 2002, 41, 1342. (h) Spencer, D. J. E.; Aboelella, N. W.;
Reynolds, A. M.; Holland, P. L.; Tolman, W. B. J. Am. Chem. Soc.
2002, 124, 2108. (i) Panda, A.; Stender, M.; Wright, R. J.; Olmstead,
M. M.; Klavins, P.; Power, P. P. Inorg. Chem. 2002, 41, 3909. (j)
Holland, P. L.; Cundari, T. R.; Perez, L. L.; Eckert, N. A.; Lachicotte,
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1542.
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Calabrese, J. C.; Arthur, S. D. Organometallics 1997, 16, 1514. (b)
Stender, M.; Wright, R. J.; Eichler, B. E.; Prust, J.; Olmstead, M. M.;
Roesky, H. W.; Power, P. P. J. Chem. Soc., Dalton Trans. 2001, 23,
3465.
Synthesis and Solid-State Structures of Three-Coor-
dinate Fe(II) Amido Compounds of LtBu. Because the
products are simpler, the amido complexes of LtBu will be
discussed first. By a simple salt metathesis between LtBu
-
FeCl11 and LiNHR [R ) tert-butyl (tBu), p-tolyl (tol), 2,6-
xylyl (xyl), and 2,6-diisopropylphenyl (dipp)] in Et2O, three-
coordinate LtBuFeNHR could be obtained in 57-73% yield
(Scheme 1). All arylamido compounds are isolated as red to
dark red crystals, while LtBuFeNHtBu crystallizes as dark
brown blocks. All the compounds are extremely air and
(13) Kern, R. J. J. Inorg. Nucl. Chem. 1962, 24, 1105.
Inorganic Chemistry, Vol. 43, No. 10, 2004 3307

Eckert et al.
1.938(2) Å).7,9i,18 There are no close contacts with the
calculated positions of hydrogen atoms (shortest Fe-H >
2.8 Å), arguing against the presence of agostic interactions
in the compounds.
The three-coordinate amidoiron(II) compounds are all
high-spin (S ) 2), as indicated by their solution magnetic
moments (5.1 ( 0.3 µΒ) and their paramagnetically shifted
1H NMR resonances. Signals for the amido NH protons could
1
not be found in the H NMR spectra of any of the Fe(II)
amido compounds, presumably due to the close proximity
1
of these protons to the paramagnet. In some cases other H
NMR signals, in addition to those of the amido NH protons,
could not be found, presumably due to extreme broadening.
Weak bands assigned to N-H stretching vibrations were
observed between 3300 and 3450 cm-1 in FTIR spectra of
the (amido)iron(II) complexes.
Figure 3. ORTEP drawing of the molecular structure of LtBuFeNHxyl.
Thermal ellipsoids are shown at the 50% probability level. Hydrogen atoms
have been omitted for clarity.
Synthesis and Solid-State Structures of Low-Coordi-
nate Fe(II) Amido Compounds of LMe. A similar meta-
thetical procedure may be used to synthesize Fe(II) amido
complexes of the smaller â-diketiminate, LMe (Figure 1, R
11
) Me). When LMeFeCl2Li(THF)2 and LiNHAr were
stirred in Et2O to give red-brown or brown solutions,
LMeFe(µ-NHtol)(µ-Cl)Li(THF)(Et2O), LMeFe(NHxyl)(THF),
and LMeFe(NHdipp)(THF) were isolated by crystallization
from diethyl ether or pentane in yields of 49-77% (Scheme
1). LMeFe(µ-NHtol)(µ-Cl)Li(THF)(Et2O) crystallizes as dark
brown blocks, and one was subjected to X-ray diffraction
analysis. Unfortunately, the data were only sufficient to
determine the connectivity of the molecule. The iron is in a
tetrahedral coordination environment, bound to the diketimi-
nate, a bridging tolylamido group, and a bridging chloride.
The chloride and the amido group each bridge to a tetrahedral
lithium ion.
LMeFe(NHxyl)(THF) and LMeFe(NHdipp)(THF) each crys-
tallize as golden-brown needles. A single crystal of
LMeFe(NHdipp)(THF) was studied by X-ray diffraction, and
an ORTEP diagram is shown in Figure 5. The molecular
structure of LMeFe(NHxyl)(THF) (Figure S-3) is analogous
to that of LMeFe(NHdipp)(THF). Data collection parameters
are given in Table 1, and pertinent molecular data are listed
in Table 2. In stark contrast to the LtBu analogues,
these compounds are not planar at the metal center.
LMeFe(µ-NHtol)(µ-Cl)Li(THF)(Et2O) is tetrahedral about
iron, while LMeFe(NHxyl)(THF) and LMeFe(NHdipp)(THF)
have distorted square pyramidal geometries. A THF molecule
occupies the apical position, while the equatorial coordination
site is occupied by an agostic C-H bond. The agostic
interactions are between a C-H bond of an amido isopropyl
methine group or a C-H bond of an amido methyl group
and the Fe(II) center. As judged by this distance, each agostic
interaction (Fe-C ) 3.401(5) Å, Fe-H ) 2.569(5) Å for
Ar ) dipp; Fe-C ) 3.152(5) Å, Fe-H ) 2.577(5) Å for
Figure 4. ORTEP drawing of the molecular structure of LtBuFeNHtBu.
Thermal ellipsoids are shown at the 50% probability level. Hydrogen atoms
have been omitted for clarity.
moisture sensitive and must be handled under nitrogen. The
Fe(II) arylamido compounds are stable in the solid state at
room temperature for months under an inert atmosphere,
while LtBuFeNHtBu must be stored at -35 °C, where it is
stable as a solid for months.
Molecular structures of all Fe(II) amido compounds were
determined using X-ray diffraction, and ORTEP drawings
of LtBuFeNHxyl and LtBuFeNHtBu are shown in Figures 3
and 4, respectively. The structures of LtBuFeNHtol and
LtBuFeNHdipp (Figures S-1 and S-2) are analogous to
LtBuFeNHxyl. Pertinent collection data are given in Table 1,
and metrical data are included in Table 2. All of the
compounds are planar at iron, with the sum of the bond
angles greater than 359°. The bite angle of the diketiminate
(N-Fe-N; 94.36(9)-94.85(6)°) and the bond lengths of the
Fe-N(diketiminate) (1.961(3)-2.0176(15) Å) are typical of
three-coordinate Fe-diketiminate compounds.9i,11,14-17 The
Fe-N(amido) bond distances (1.787(11)-1.9066(17) Å) are
short but in the range of other crystallographically character-
ized, low-coordinate Fe-N(amido) bond distances (1.84(2)-
(14) Smith, J. M.; Lachicotte, R. J.; Holland, P. L. Organometallics 2002,
21, 4808.
(15) Sciarone, T. J. J.; Meetsma, A.; Hessen, B.; Teuben, J. H. Chem.
Commun. 2002, 1580.
(16) Vela, J.; Smith, J. M.; Lachicotte, R. J.; Holland, P. L. Chem. Commun.
2002, 2886.
(17) Gibson, V. C.; Marshall, E. L.; Navarro-Llobet, D.; White, A. J. P.;
Williams, D. J. J. Chem. Soc., Dalton Trans. 2002, 23, 4321.
(18) (a) Andersen, R. A.; Faegri, K., Jr.; Green, J. C.; Haaland, A.; Lappert,
M. F.; Leung, W.; Rypdal, K. Inorg. Chem. 1988, 27, 1782. (b) Stokes,
S. L.; Davis, W. M.; Odom, A. L.; Cummins, C. C. Organometallics
1996, 15, 4521. (c) Siemeling, U.; Vorfeld, U.; Neumann, B.;
Stammler, H. Organometallics 1998, 17, 483. (d) Siemeling, U.;
Vorfeld, U.; Neumann, B.; Stammler, H.-G. Inorg. Chem. 2000, 39,
5159.
3308 Inorganic Chemistry, Vol. 43, No. 10, 2004

Iron(II) Amido Complexes of â-Diketiminates
Table 1. X-ray Diffraction Data Collection Parameters for the Crystal Structures Presented in This Work
LtBuFeNHxyl
LtBuFeNHtBu
LtBuFeNHdipp
LtBuFeNHtol
LMeFe(NHdipp)(THF)
empirical formula
fw
C43H63FeN3
677.81
C39H63FeN3
629.77
C47H71FeN3
733.92
C42H62FeN3
664.80
C45H67FeN3O
721.87
cryst system
space group
a (Å)
b (Å)
c (Å)
monoclinic
P21/c
monoclinic
P21/n
orthorhombic
P212121
13.5816(8)
15.0241(9)
21.633(1)
90
4414.3(5)
4
1.104
monoclinic
P21/c
monoclinic
P21/c
17.0752(11)
10.3304(6)
22.5209(14)
95.816(1)
3952.1(4)
4
1.139
0.413
0.0744, 0.1436
9.7000(11)
17.906(2)
21.821(3)
96.464(2)
3766.0(7)
4
1.111
0.429
0.1708, 0.4248
9.5603(6)
20.1214(12)
22.2744(14)
99.945(1)
4220.5(5)
4
1.100
0.389
0.0610, 0.1287
10.681(2)
21.222(5)
18.542(4)
99.469(4)
4145.6(16)
4
1.157
0.400
0.1138
0.1698
â (deg)
V (Å3)
Z
F (g/cm3)
µ (mm-1
)
0.375
0.0440
0.1049
R1, wR2 (I > 2σ(I))
GOF
1.063
1.483
1.063
1.045
1.295
LMeFe(NHxyl)(THF)
LMeFe(NHtol)(Cl)(THF)2
LMeFeNHdippLMeFe(NHdipp)(NH2dipp)
LtBuFe(NHdipp)(tBuPyr)
empirical formula
fw
C41H59FeN3O
665.76
C44H67ClFeLi N3O2
768.25
C94H137Fe2N7
1476.8
C59H91FeN4
912.21
cryst system
space group
a (Å)
b (Å)
c (Å)
triclinic
P1h
monoclinic
P21/n
monoclinic
C2/c
monoclinic
P21/c
9.1937(7)
12.4377(10)
17.7907(14)
86.147(1)
2011.3(3)
2
13.5640(10)
16.0450(12)
20.4062(15)
95.597(1)
4419.9(6)
4
1.155
0.438
0.1678, 0.2979
1.783
44.079(3)
14.9748(9)
27.2296(16)
106.984(1)
17189.6(18)
8
1.141
0.386
0.0362, 0.0863
1.045
12.7218(9)
19.6354(13)
22.6539(16)
100.406(1)
5565.8(7)
4
1.089
0.309
0.0560, 0.1472
1.094
â (deg)
V (Å3)
Z
F (g/cm3)
1.099
0.407
0.0474, 0.1168
1.054
µ (mm-1
)
R1, wR2 (I > 2σ(I))
GOF
LtBuFe(NHdipp)(MeCN)
LMeFe(NHdipp)(OTf)
C49H59F3FeN3 O3S
LtBuFe(OtBu)(OTf)
LMeFe(OtBu)‚LiCl(Et2O)
empirical formula
fw
C49H74FeN4
774.97
C45H74F3FeN4O 4S
851.97
C37H60ClFeLiN 2O2
663.11
798.83
cryst system
space group
a (Å)
b (Å)
c (Å)
monoclinic
P21/n
triclinic
P1h
monoclinic
P21/n
triclinic
P1h
12.3408(6)
13.3377(7)
29.7324(15)
101.544(1)
4794.9(4)
4
1.074
0.349
0.0657, 0.1269
1.086
9.9115(6)
12.0208(8)
18.6827(12)
95.853(1)
2099.9(2)
2
10.4900(7)
21.2786(14)
43.032(3)
94.588(1)
9574.5(11)
8
1.182
0.410
0.0866, 0.2043
1.046
11.5511(10)
11.8987(10)
16.4896(14)
106.354(1)
1957.1(3)
2
â (deg)
V (Å3)
Z
F (g/cm3)
1.263
0.462
0.0403, 0.0911
1.011
1.125
0.484
0.0492, 0.1339
1.019
µ (mm-1
)
R1, wR2 (I > 2σ(I))
GOF
LMeFe(OtBu)(Cl)
LMeFeOtBu
LMeFe(OtBu)(OTf)
LtBuFeCCPh
[LMeFeCl]2
empirical formula
fw
C33H50ClFeN2 O
582.05
C33H50FeN2O
546.60
C34H50F3FeN2O 4S
695.67
C43H58FeN2
658.76
C58H82Fe2Cl2N4
1017.88
cryst system
space group
a (Å)
b (Å)
c (Å)
monoclinic
P2
orthorhombic
Pna21
18.0746(14)
8.7561(7)
21.0325(16)
90
3328.7(4)
4
1.091
monoclinic
P21/n
triclinic
P1h
monoclinic
C2/c
9.0678(6)
20.1462(12)
10.1664(16)
114.205(1)
1693.94(18)
2
1.141
0.549
0.0523, 0.1392
1.022
10.7526(7)
31.146(2)
12.1488(8)
115.481(1)
3672.9(4)
4
1.258
0.519
0.0682, 0.1487
1.104
12.3758(9)
16.7222(13)
19.5633(15)
77.668(1)
3938.5(5)
4
22.8730(15)
14.9341(10)
16.3363(11)
90.402(1)
5580.1(6)
4
1.212
0.655
0.0414, 0.0937
1.214
â (deg)
V (Å3)
Z
F (g/cm3)
1.111
0.412
0.0467, 0.1352
1.095
µ (mm-1
)
0.477
0.0352, 0.0840
1.005
R1, wR2 (I > 2σ(I))
GOF
Ar ) xyl) falls in the class of “remote” agostic interactions.19
These compounds also contain high-spin Fe(II), supported
by their solution magnetic moments (5.5 ( 0.3 µB) and their
If [LMeFeCl]2 is treated with LiNHAr in pentane, then
three-coordinate amido complexes of the smaller LMe ligand
are isolated as dark red to orange-red crystalline solids
(Scheme 1). In noncoordinating solvents such as benzene
or pentane, these compounds show spectroscopic features
identical with those of the corresponding “ate” or solvent-
1
paramagnetically shifted H NMR spectra.
(19) (a) Brookhart, M. J. Organomet. Chem. 1983, 250, 395. (b) Brookhart,
M.; Green, M. L. H.; Wong, L. Prog. Inorg. Chem. 1988, 36, 1.
Inorganic Chemistry, Vol. 43, No. 10, 2004 3309

Eckert et al.
Table 2. Relevant Distances (Å) and Angles (deg) for Amido Complexes of LtBu and LMe
Fe-N(amido)
Fe-N1
Fe-N2
N1-Fe-N2
N1-Fe-N(amido)
N2-Fe-N(amido)
LtBuFeNHxyl
1.893(3)
1.9066(17)
1.902(3)
1.787(11)
1.9494(18)
1.940(2)
1.978(2)
1.9732(15)
1.983(3)
2.016(2)
2.0176(15)
1.961(3)
94.36(9)
94.85(6)
94.51(13)
94.4(3)
95.63(7)
96.00(7)
94.59(18)
94.32(6)
93.65(6)
96.03(6)
151.54(11)
150.11(7)
144.75(14)
123.6(4)
110.25(7)
108.90(8)
143.9(2)
145.39(7)
155.78(7)
123.10(6)
114.10(11)
114.85(7)
120.70(14)
142.0(4)
136.03(7)
136.33(8)
105.6(2)
106.54(7)
110.44(7)
103.13(7)
LtBuFeNHdipp
LtBuFeNHtol
LtBuFeNHtBu
1.979(8)
1.989(8)
LtBuFe(NHdipp)(tBuPyr)
LtBuFe(NHdipp)(MeCN)
LMeFe(NHdipp)(THF)
LMeFe(NHxyl)(THF)
LMeFeNHdipp
2.0461(18)
2.0174(18)
2.008(5)
2.0240(15)
1.9736(15)
1.9986(14)
2.0312(17)
2.0154(19)
2.050(5)
2.0360(15)
2.0186(15)
1.9427(14)
1.931(5)
1.9347(17)
1.8971(17)
1.8801(15)
LMeFe(NHdipp)(OTf)
bound compounds suggesting that the bound ether ligand is
lost from the above THF adducts in solution. LMeFeNHdipp
and LMeFeNHxyl are stable as solids at room temperature
for several weeks, while LMeFeNHtol shows far less thermal
stability. After 1 day, a pure sample of LMeFeNHtol stored
The molecular structure of the LMeFeNHdipp molecule is
similar to that of the LtBu analogues. The Fe-N(diketiminate)
distances of 1.9736(15) and 2.0186(15)
Å in the
LMeFeNHdipp molecule are in the same range as other three-
coordinate iron diketiminate complexes, as is the bite angle
of the diketiminate (93.65(6)°).9,11,14-17 The Fe-N(amido)
distance of 1.8971(17) Å is again on the short end of the
range of structurally characterized low-coordinate iron amido
complexes.9i,18 The molecule is planar at iron, as is evident
from the sum of the ligand bond angles (359.87(7)°). Finally,
there is a somewhat close contact between the iron and a
methine hydrogen from the amido group (∼2.63 Å).
1
at -35 °C overnight shows modest decomposition. The H
NMR spectra of LMeFe(NHdipp)(THF) and LMeFeNHdipp
in C6D6 at ambient temperature are identical, and their UV/
vis spectra in pentane are very similar. These spectral
signatures resemble their LtBu analogues in hydrocarbon
solution. If dark red crystals of three-coordinate LMeFeNHAr
(Ar ) dipp or xyl) are dissolved in pentane and reacted with
2 molar equiv of THF, golden-brown needles precipitate after
several minutes. Finally, elemental analysis of LMeFeNHdipp
is consistent with a solvent-free formulation.
Coordination of N-Donor Lewis Bases to LRFeNHAr.
The three-coordinate Fe(II) amido compounds react with
Lewis bases to form four- and five-coordinate adducts. In a
typical reaction, a small excess of base was added to an
ethereal solution of Fe(II) amido complex. The structures
of the acetonitrile and 4-tert-butylpyridine adducts of
LtBuFeNHdipp were elucidated from X-ray diffraction
data and are shown in Figures 7 and 8, respectively.
LtBuFe(NHdipp)(tBuPyr) is a four-coordinate molecule with
trigonal pyramidal geometry about iron. The tBuPyr ligand
occupies the apical position of the pyramid: this has been
seen in several other complexes LFe(X)(L) with neutral
σ-donor ligands, for example LMeFe(NHdipp)(THF) and
LtBuFeH(tBuPyr).20 There are no significant changes in bond
lengths or angles from the binding of a fourth ligand except
the exceptional case of MeCN (see below). Similar reactions
occur with LMeFeNHAr as indicated by the new signals
Dark red, single crystals of LMeFeNHdipp were grown
from a hot pentane solution. The molecular structure was
determined by X-ray diffraction of one of these crystals.
Interestingly, there were two distinct molecules in the unit
cell. One was the desired three-coordinate LMeFeNHdipp
(Figure 6), and the second molecule in the unit cell was an
H2Ndipp adduct of LMeFeNHdipp (LMeFe(NHdipp)-
(H2Ndipp), Figure S-4). Considering that the bulk material
was spectroscopically pure, the single crystal was likely a
minor product from a small amount of H2Ndipp in the lithium
reagent LiNHAr. Unfortunately, LMeFe(NHdipp)(H2Ndipp)
could not be isolated in bulk, although the binding of
dippNH2 to LMeFeNHdipp was independently established
through 1H NMR experiments (Keq ) 5(4) × 102 M-1 at 25
°C). Interestingly, the structure of LMeFe(NHdipp)(H2Ndipp)
is trigonal pyramidal rather than tetrahedral (see Discussion).
1
observed in H NMR spectra.
Figure 6. ORTEP drawing of the molecular structure of LMeFeNHdipp,
from the crystal structure of LMeFeNHdipp‚LMeFe(NHdipp)(H2Ndipp).
Thermal ellipsoids are shown at the 50% probability level. Hydrogen atoms
have been omitted for clarity.
Figure 5. ORTEP drawing of the molecular structure of LMeFe(NHdipp)-
(THF). Thermal ellipsoids are shown at the 50% probability level. Hydrogen
atoms have been omitted for clarity.
3310 Inorganic Chemistry, Vol. 43, No. 10, 2004

Iron(II) Amido Complexes of â-Diketiminates
solution of LMeFeNHdipp in toluene-d8 were collected every
10 °C over the range of 20 to -80 °C (Figure S-5; all
solutions were allowed to equilibrate for at least 10 min at
each temperature). Although broadened at low temperatures
(from increased magnetic susceptibility and attendant short
T2),21 the signals from LMeFeNHdipp do not decoalesce;
rather, an independent set of peaks is first observed at 0 °C.
These new signals grow in intensity with lower temperature
until -80 °C, where they are approximately the same
intensity as the ambient temperature signals. There are more
signals in the new set, suggesting a loss of symmetry, and
they disappear on warming to room temperature, indicating
a reversible equilibrium. The lack of decoalescence is
inconsistent with slowed motion (twisting and/or flapping
motions of the amido group). We will present evidence below
supporting the idea that the low-temperature signals cor-
respond to an agostic (four-coordinate) complex that is in
equilibrium (slow on the NMR time scale) with the three-
coordinate form of the amido complex that was characterized
by crystallography.
Figure 7. ORTEP drawing of the molecular structure of LtBuFe(NHdipp)-
(MeCN). Thermal ellipsoids are shown at the 50% probability level.
Hydrogen atoms have been omitted for clarity. Important bond distances
(Å) and angles (deg): Fe-NCMe 2.097(2), Fe-H 2.522(2); N1-Fe-H
167.45(9).
To evaluate this hypothesis, 1H NMR spectra of
LtBuFeNHdipp were recorded over a similar range of tem-
peratures. In this complex of the larger LtBu ligand, increasing
the coordination number would be more difficult, and there
is no evidence for agostic interactions in the solid-state
structure. Upon cooling of a toluene-d8 solution of
LtBuFeNHdipp from 20 to -80 °C, no new signals appeared,
only the expected temperature-dependent signal broadening
1
and chemical shift changes (Figure S-6). H NMR spectra
of LMeFeNHtol were also recorded between 20 and -80 °C
(Figure S-7), and again no new species is evident. Because
the low-temperature behavior of LMeFeNHdipp is not ob-
served in LMeFeNHtol, it is likely that the specific site of
agostic C-H binding in solution is an amido aryl isopropyl
group. Therefore, these 1H NMR experiments offer evidence
that (a) agostic interactions are present in solution and (b)
the agostic interaction is between a C-H bond of the alkyl
substituent on the amido group and iron, as in the crystal
structures.
Figure 8. ORTEP drawing of the molecular structure of LtBuFe(NHdipp)-
(tBuPyr). Thermal ellipsoids are shown at the 50% probability level.
Hydrogen atoms have been omitted for clarity. Fe-N(Pyr) ) 2.128(2) Å.
The X-ray crystal structure of the acetonitrile adduct,
LtBuFe(NHdipp)(MeCN), shows that it is five-coordinate. The
geometry about iron is trigonal bipyramidal with coordination
sites occupied by two diketiminate nitrogens, the amido
nitrogen, the axial MeCN, and an equatorial agostic C-H
bond from an amido methine. Again, this interaction can be
classified as a remote agostic interaction and appears to be
weak (Fe-C ) 3.407(5) Å, Fe-H ≈ 2.521(5) Å).19 The
Fe-N(amido) bond distance is slightly longer in the aceto-
nitrile adduct than in the three-coordinate complex
(1.940(2) Å vs 1.906(2) Å) as expected for the greater
coordination number.
1
We also inspected low-temperature H NMR spectra of
LMeFe(NHdipp)(THF) in toluene-d8 and THF-d8. At ambient
1
temperature in THF-d8, only eight H NMR signals for
LMeFe(NHdipp)(THF) are observed. The chemical shifts and
integrations are consistent with signals for the diketiminate
ligand and para-aryl proton of the amido group (see
Experimental Section for assignments). We propose that the
absence of signals for the amido isopropyl protons is due to
Low-Temperature 1H NMR Studies. As described
above, several of the Fe(II) amido compounds show agostic
interactions in the solid state. These interactions are not
1
fast exchange between agostic complexes and that the H
NMR signals from the amido isopropyl groups are broadened
due to binding to the paramagnetic iron(II). Upon cooling,
1H NMR spectra of LMeFe(NHdipp)(THF) in THF-d8 showed
a loss of symmetry between -40 and -50 °C as indicated
by decoalescence of the signals at -13 and 17 ppm (ambient
temperature chemical shifts) from the isopropyl methyl
groups of the diketiminate aryl moieties (Figure S-8). Note
that the decoalescence in this case indicates that there is fast
1
evident in solution at ambient temperature: H NMR spectra
are consistent with C2V symmetry in the diketiminate ligand
(all four isopropyl groups are equivalent). However, the
molecules have lower symmetry at low temperature, as
1
1
shown by H NMR spectroscopy. H NMR spectra of a
(20) (a) Smith, J. M.; Lachicotte, R. J.; Holland, P. L. J. Am. Chem. Soc.
2003, 125, 15752. (b) Vela, J.; Stoian, S.; Flaschenriem, C.; Mu¨nck,
E.; Holland, P. L. J. Am. Chem. Soc. 2004, 126, 4522.
(21) Ming, L.-J. In Physical Methods in Bioinorganic Chemistry; Que, L.,
Ed.; University Science Books: Sausalito, CA, 2000.
Inorganic Chemistry, Vol. 43, No. 10, 2004 3311

Eckert et al.
exchange in the presence of THF. Importantly, splitting of
the isochronous resonances of the four isopropyl groups
shows that the symmetry is reduced, and it is likely that the
low-temperature structure is reduced to idealized Cs sym-
metry, as in the solid-state structure of LMeFe(NHdipp)(THF).
Similar behavior of LMeFe(NHdipp)(THF) was evident in
toluene-d8 (Figure S-9).
The 1H NMR spectra of LtBuFe(NHdipp)(MeCN) were also
recorded between 20 and -80 °C in toluene-d8 (Figure S-10).
1
Note that the ambient-temperature H NMR spectrum of
LtBuFe(NHdipp)(MeCN) in C6D6 is identical with that of
LtBuFeNHdipp, indicating a rapid equilibrium between bound
and free acetonitrile. Upon cooling of LtBuFe(NHdipp)-
(MeCN) to -50 °C, a new set of signals and broadness is
evident, but the details are not clear due to the broadness of
the spectra. However, the contrast in behavior of
LtBuFe(NHdipp)(MeCN) and LtBuFeNHdipp in low-temper-
ature solution by 1H NMR spectroscopy, combined with the
analogy to the other complexes, supports the idea that
LtBuFe(NHdipp)(MeCN) also has an agostic interaction in
solution.
Figure 9. ORTEP drawing of the molecular structure of LMeFe(NHdipp)-
(OTf). Thermal ellipsoids are shown at the 50% probability level. Hydrogen
atoms have been omitted for clarity. Important bond distances (Å) and angles
(deg): Fe-OTf 1.988(1); N(amido)-Fe-OTf 118.78(6), N1-Fe-OTf
101.05(6), N2-Fe-OTf 112.67(6).
of AgOTf effected spectroscopic changes in LMeFe(NHdipp)-
(THF) that were similar to those seen in LtBuFeNHAr.
LMeFe(NHdipp)(OTf) can be isolated as a dark blue micro-
Synthesis of Iron(III) Complexes. The oxidation of LtBu
-
1
crystalline solid from pentane. The broad, unresolved H
FeNHAr (Ar ) dipp, tol) was accomplished by mixing an
ethereal solution of the Fe(II) amido compounds with a
stoichiometric amount of AgOTf. Dark blue microcrystalline
solids were isolated by crystallization from a concentrated
pentane solution. Evidence for oxidation to iron(III) comes
from the UV/vis and NMR spectra of the resultant com-
pounds. For Ar ) tol, an intense absorption at 615 nm (3100
M-1 cm-1), presumably amido to Fe(III) LMCT, is respon-
sible for the deep blue color of the compound in solution.
Evidence for triflate coordination comes from the high
solubility in aliphatic solvents and a 19F NMR signal for the
triflate counterion that is broadened and shifted substantially
NMR spectrum, a magnetic moment of 5.9 ( 0.3 µB, new
electronic absorptions at 415 nm (2700 M-1 cm-1) and 750
(4300 M-1 cm-1) nm, and a broad, shifted 19F NMR signal
are again consistent with the oxidation of LMeFe(NHdipp)-
(THF) to LMeFe(NHdipp)(OTf). The low-energy LMCT is
apparent at low concentrations as a shoulder to the band at
750 nm. In this case, the band at 750 nm is of much lower
intensity than the band at 820 nm in the spectrum of
LtBuFe(NHdipp)(OTf).
Single crystals of LMeFe(NHdipp)(OTf) were grown from
a pentane solution, and one was subjected to X-ray diffraction
analysis. The molecular structure is shown in Figure 9. There
is a decrease in the Fe-N(diketiminate) distances from an
average of 2.029(5) Å in the Fe(II) amido compound to
1.971(2) Å in LMeFe(NHdipp)(OTf), as expected for an
increase in oxidation state. We observe the same trend in
the Fe-N(amido) distance, 1.931(5) Å in the Fe(II) amido
and 1.880(2) in LMeFe(NHdipp)(OTf). Interestingly, the
geometry about iron is pseudotetrahedral rather than the
trigonal pyramidal geometry of the four-coordinate Fe(II)
solvent adducts. The bite angle of the diketiminate is
96.03(6)°, slightly larger than that in the Fe(II) amido
compounds. Enlargement of the bite angle can be attributed
to the higher oxidation state in LMeFe(NHdipp)(OTf), because
the shortening of the Fe-N(diketiminate) bond lengths forces
the iron deeper into the NN-binding pocket.9j
1
downfield from that of AgOTf. The H NMR spectrum
contained signals that were much broader than those for the
analogous high-spin Fe(II) amido compounds. These peaks
were too broad to integrate or assign. The solution magnetic
moment in C6D6 at room temperature is 3.6 ( 0.3 µB,
consistent with an intermediate spin Fe(III) center (S ) 3/2).
Spin states less than maximal are unusual for low-coordinate
â-diketiminate iron complexes because of the weak ligand
field;10 the electronic structure of this molecule is currently
under further investigation.
In LtBuFe(NHdipp)(OTf), the 1H and 19F NMR spectra are
similar to those of LtBuFe(NHtol)(OTf). However, there is
an extremely intense electronic absorption centered at 820
nm (6400 M-1 cm-1) that is not seen in LtBuFe(NHtol)(OTf).
The amido to Fe(III) LMCT can be seen as a high-energy
1
shoulder at ∼630 nm. The H NMR spectrum of LtBuFe-
Interested in the structural changes caused by the oxidation
of Fe(II) to Fe(III), we sought to prepare another compound
that could be characterized structurally. The previously
reported three-coordinate Fe(II) compound, LtBuFeOtBu,17
could be oxidized to give LtBuFe(OtBu)(OTf). This trans-
formation followed a similar protocol as the Fe(II) amido
compounds. Stoichiometric amounts of AgOTf caused a
color change from orange to dark green, and dark green
single crystals were isolated from pentane in 82% yield.
(NHdipp)(OTf) gives signals that are extremely broad and
difficult to resolve and the solution magnetic moment in C6D6
at room temperature is 5.9 ( 0.3 µB, both consistent with
high-spin Fe(III) (S ) 5/2).9i,22
The oxidation of LMeFe(NHdipp)(THF) proceeds in a
manner similar to its LtBu analogue. Stoichiometric amounts
(22) Bertini, I.; Turano, P.; Vila, A. J. Chem. ReV. 1993, 93, 2833.
3312 Inorganic Chemistry, Vol. 43, No. 10, 2004

Iron(II) Amido Complexes of â-Diketiminates
Figure 11. ORTEP drawing of the molecular structure of LMeFeOtBu‚
LiCl(Et2O). Thermal ellipsoids are shown at the 50% probability level.
Hydrogen atoms have been omitted for clarity. Important bond distances
(Å) and angles (deg): Fe-N1 2.024(2), Fe-N2 2.0347(17), Fe-OtBu
1.9233(15), Fe-Cl 2.4691(6); N1-Fe-N2 92.132(7), OtBu-Fe-Cl
88.36(5).
Figure 10. ORTEP drawing of the molecular structure of one of the two
independent molecules of LtBuFe(OtBu)(OTf) in the asymmetric unit.
Thermal ellipsoids are shown at the 50% probability level. Hydrogen atoms
have been omitted for clarity. Important bond distances (Å) and angles
(deg): Fe-N1 1.964(3), Fe-N2 1.969(3), Fe-OtBu 1.750(3), Fe-OTf
1.972(3); N1-Fe-N2 97.8(1), OtBu-Fe-OTf 110.8(1).
LtBuFe(OtBu)(OTf) is spectroscopically similar to its amido
analogues, with extremely broad, paramagnetically shifted
1H and 19F NMR spectra, a charge-transfer band at 650 nm
(1500 M-1 cm-1), and a solution magnetic moment of 5.5
( 0.3 µB. All of these data are consistent with a high-
spin Fe(III) metal center. The molecular structure of
LtBuFe(OtBu)(OTf) was determined via X-ray diffraction
analysis, and an ORTEP diagram is shown in Figure 10.
There was disorder over two positions in both the OTf and
the tert-butyl substituent of the alkoxide, and each was clearly
coordinated through one oxygen atom. They are shown in
one of the half-occupancy positions. As predicted, the
molecule contains a formally Fe(III) center in a distorted
tetrahedral coordination environment. The inner-sphere tri-
flate is consistent with the paramagnetic shift of the 19F NMR
signal. The bond distances are shorter in the Fe(III) tert-
butoxide (Fe-OtBu: 1.75(3) Å) than those in the Fe(II) tert-
butoxide (Fe-OtBu: 1.786(3) Å) as expected for an increase
in oxidation state. Surprisingly, the Fe-O-C angle is nearly
linear for Fe(III) (170.5(3)°, compared to 150.3(3)° for
LtBuFeOTf). We initially guessed that strong π-donation to
tetrahedral Fe(III) (versus trigonal planar Fe(II)) was the
reason for the substantial increase in bond angle. However,
to evaluate the importance of steric effects, we attempted to
prepare the LMe analogues, LMeFeOtBu and LMeFe(OtBu)-
(OTf).
Initially, we sought to prepare LMeFeOtBu via salt me-
tathesis between LMeFeCl2Li(THF)211 and LiOtBu. However,
the isolated product was LMeFe(OtBu)‚LiCl(Et2O). Orange
single crystals of this compound could be isolated from Et2O,
and one was subjected to X-ray diffraction analysis. An
ORTEP diagram of the molecular structure is shown is Figure
11. LMeFe(OtBu)‚LiCl(Et2O) contains distorted tetrahedral
iron(II) ligated by the diketiminate, a bridging chloride, and
a bridging tert-butoxide group. The Fe-N and Fe-O
distances of LMeFe(OtBu)‚LiCl(Et2O) are slightly longer than
in LtBuFeOtBu as expected for an increase in coordination
number as well as the bridging nature of the tert-butoxide
Figure 12. ORTEP drawing of the molecular structure of LMeFe(OtBu)-
(Cl). Thermal ellipsoids are shown at the 50% probability level. Hydrogen
atoms have been omitted for clarity. Important bond distances (Å) and angles
(deg): Fe-O 1.780(2), Fe-N1 1.987(6), Fe-N2 1.967(6), Fe-Cl
2.221(1); N1-Fe-N2 95.09(9), O-Fe-Cl 119.9(1), Fe-O-C 139.8(4).
group. The Fe-Cl distance is approximately 0.1 Å longer
than in LMeFeCl2Li(THF)2.11
Addition of AgOTf to an ethereal solution of
LMeFe(OtBu)‚LiCl(Et2O) gave a dark green solution from
which dark green crystals could be isolated. This compound
had 1H NMR and UV/vis spectra similar to those for
LtBuFe(OtBu)(OTf), but single-crystal X-ray diffraction stud-
ies showed that the fourth ligand was chloride instead of
triflate. The molecular structure of LMeFe(OtBu)(Cl) is shown
in Figure 12. The Fe-O distance is much shorter in
LMeFe(OtBu)(Cl) than in LMeFe(OtBu)‚LiCl(Et2O)
(1.780(2) and 1.9236(16) Å, respectively), due to the increase
in oxidation state as well as loss of the tert-butoxide bridge.
The isolation of LMeFe(OtBu)(Cl) rather than the desired
LMeFe(OtBu)(OTf) suggested that the starting material
needed to be free of LiCl to create the desired compounds.
Isolation of LMeFeOtBu was finally accomplished through
extraction of LMeFe(OtBu)‚LiCl(Et2O) with toluene and
crystallization from pentane. Yellow-green crystals of
LMeFeOtBu were isolated, and one was subjected to an X-ray
Inorganic Chemistry, Vol. 43, No. 10, 2004 3313

Eckert et al.
Figure 13. ORTEP drawing of the molecular structure of LMeFeOtBu
(major conformer). Thermal ellipsoids are shown at the 50% probability
level. Hydrogen atoms have been omitted for clarity. Important bond
distances (Å) and angles (deg): Fe-O 1.761(10), Fe-N1 1.970(4), Fe-
N2 1.981(4); N1-Fe-N2 94.04(5), O-Fe-N1 141.1(4), O-Fe-N2 124.7-
(4), Fe-O-C 147.5(12).
Figure 14. ORTEP drawing of the molecular structure of LMeFe(OtBu)-
(OTf) (major conformer). Thermal ellipsoids are shown at the 50%
probability level. Hydrogen atoms have been omitted for clarity. Important
bond distances (Å) and angles (deg): Fe-OtBu 1.758(2), Fe-OTf 1.968-
(2), Fe-N1 1.962(2), Fe-N2 1.967(2); N1-Fe-N2 96.77(9), O-Fe-O
115.48(10), Fe-O-C 153.0(2).
diffraction study. There was disorder of the tert-butyl
substituent of the alkoxide, with two conformations in a
61:39 ratio. The major component is shown in Figure 13.
The molecule is planar at iron, as seen from the sum of the
bond angles (359.8(5)°). The Fe-O distance (1.78(1) Å) and
Fe-O-C angle (147.5(13)°) are similar to those of
LtBuFeOtBu (1.786(3) Å and 150.3(3)°).17 Alternatively,
LMeFeOtBu could be prepared by addition of 1 molar equiv
of HOtBu to LMeFeNHAr (see below).
Fe-OtBu distance (1.758(2) Å) is similar to that of
LtBuFe(OtBu)(OTf) (1.75(3) Å). However, the Fe-O-C
angle in LMeFe(OtBu)(OTf) is much smaller (153.0(2)°) than
that observed in LtBuFe(OtBu)(OTf) (170.5(3)°) and is only
slightly larger than that observed in LMeFeOtBu (147.5(13)°).
Therefore, the overall structural evidence suggests that steric
effects are more important than π-bonding in determining
the Fe-O-C angles in these pseudotetrahedral iron(III)
complexes.
Acid-Base Chemistry of the Fe-N(amido) Bond.
Deprotonated anilines are strong bases (pKa ∼ 31),23 and
therefore one expects the basic amido group of the Fe(II)
compounds to react with weak acids. Similar acid-base
reactions have been performed with heavier group VIII metal
amido compounds.24 The following reactions were performed
on a 5-10 mg scale in an NMR tube and, unless otherwise
mentioned, were similar for both LMe- and LtBu-supported
Fe(II) amido compounds. In some cases, the product was
characterized structurally and spectroscopically.
We observed solvent-dependent characteristics in the
LMeFeOtBu compounds that were similar to those observed
in the [LMeNiCl]2 series.9k The 1H NMR spectra of
LMeFe(OtBu)‚LiCl(Et2O) and LMeFeOtBu are identical in
C6D6. There is a light-colored precipitate when LMeFe(OtBu)‚
LiCl(Et2O) is dissolved in C6D6, suggesting that LiCl
precipitates from nonpolar solvents, leaving the LMeFeOtBu
fragment in solution. Interestingly, if dissolved in THF-d8,
1
LMeFe(OtBu)‚LiCl(Et2O) has an H NMR spectrum that
contains signals for two compounds, neither of which is
similar to that of LMeFeOtBu in C6D6. If the solution is treated
with excess LiCl and sonicated, a yellow solution is obtained
for which the 1H NMR spectrum contains one set of signals,
presumably corresponding to pure LMeFe(OtBu)‚LiCl(Et2O).
This set of signals was identifiable in the spectrum of an
orange THF-d8 solution of LMeFe(OtBu)‚LiCl(Et2O). The
second set of signals is identical with those obtained from
an orange THF-d8 solution of LMeFeOtBu. Therefore, we
explain this behavior by a reversible equilibrium between
the butoxide and its LiCl adduct, which is dependent on
solvent and LiCl concentration.
The preparation of LMeFe(OtBu)(OTf) finally proceeded
by stoichiometric addition of AgOTf to an ethereal solution
of LMeFeOtBu. Dark blue-green LMeFe(OtBu)(OTf) displays
solution spectroscopic properties much like LtBuFe(OtBu)-
(OTf), with an intense charge-transfer band at 625 nm (1500
M-1 cm-1) as well as broad 1H and 19F NMR spectra. Single
crystals were obtained, and the structure of LMeFe(OtBu)-
(OTf) was elucidated with X-ray crystallography (Figure 14).
The bite angle and Fe-N(diketiminate) distances are similar
to other pseudotetrahedral Fe(III) diketiminate complexes
(e.g. LMeFe(NHdipp)(OTf) and LtBuFe(OtBu)(OTf)). The
The low-coordinate amidoiron complexes did not insert
alkynes into the Fe-N bond.25 Internal alkynes did not react
with the amido complexes even at elevated temperatures.
However, when a stoichiometric amount of phenylacetylene
(PhCCH) was added to a sample of LtBuFeNHdipp in C6D6,
(23) Smith, M. B.; March, J. AdVanced Organic Chemistry: Reactions,
Mechanisms and Structure, 5th ed.; Wiley & Sons: New York, 2001;
p 331.
(24) (a) Bryndza, H. E.; Fong, L. K.; Paciello, R. A.; Tam, W.; Bercaw, J.
E. J. Am. Chem. Soc. 1987, 109, 1444. (b) Hartwig, J. F.; Andersen,
R. A.; Bergman, R. G. Organometallics 1991, 10, 1875. (c) Fulton, J.
R.; Bouwkamp, M. W.; Bergman, R. G. J. Am. Chem. Soc. 2000,
122, 8799. (d) Jayaprakash, K. N.; Gunnoe, T. B.; Boyle, P. B. Inorg.
Chem. 2001, 41, 6481. (e) Jayaprakash, K. N.; Connor, D.; Gunnoe,
T. B. Organometallics 2001, 20, 5254. (f) Fulton, J. R.; Sklenak, S.;
Bouwkamp, M. W.; Bergman, R. G. J. Am. Chem. Soc. 2002, 124,
4722.
(25) (a) Kemmitt, R. D. W.; Mason, S.; Moore, M. R.; Fawcett, J.; Russell,
D. R. J. Chem. Soc., Chem. Commun. 1990, 1535. (b) Villanueva, L.
A.; Abboud, K. A.; Boncella, J. M. Organometallics 1992, 11, 2963.
(c) VanderLende, D. D.; Abboud, K. A.; Boncella, J. M. Inorg. Chem.
1995, 34, 5319. (d) Boncella, J. M.; Eve, T. M.; Rickman, B.; Abboud,
K. A. Polyhedron 1998, 17, 725. (e) Katayev, E.; Li, Y.; Odom, A.
L. Chem. Commun. 2002, 838.
3314 Inorganic Chemistry, Vol. 43, No. 10, 2004

Iron(II) Amido Complexes of â-Diketiminates
recent activity.11,14-17 Herein we add [LMeFeCl]2 to the list
of convenient Fe(II) starting materials for the preparation of
molecules with three-coordinate iron. Three-coordinate
iron(II) amido complexes are rare,18 and we were motivated
by the spectacular reactivity of many three-coordinate
transition metal amido complexes. For example, a three-
coordinate amido complex of Mo cleaves the triple bond of
dinitrogen to give a Mo(VI) terminal nitride.26 Recently
Hillhouse and co-workers used a cationic three-coordinate
Ni(II) amido complex as the precursor to an isolable, reactive
three-coordinate Ni(II) imido complex.5e â-Diketiminate
ligands have been used to stabilize three-coordinate late
transition metal amido compounds.9i,k One of these reports,
by Power and co-workers, included several structurally
characterized â-diketiminate iron(II) amido compounds:
LMeFeN(SiMe3)2; LMe,1FeN(SiMe3)2; LMe,2FeN(SiMe3)2;
LMe,3FeN(SiMe3)2; LMe,4FeN(SiMe3)2 [LMe,1, Ar ) C6F6; LMe,2
,
Ar ) 2,4,6-trimethylphenyl; LMe,3, Ar ) 2,6-dimethylphenyl;
LMe,4, Ar ) 2,6-dichlorophenyl].9i No reactivity was reported.
All four of these Fe(II) amido compounds have Fe-
N(amido) distances between 1.908(1) and 1.928(1) Å. These
distances are similar to those in the Fe(II) amido complexes
discussed in this work (1.84(5)-1.935(2) Å).
The three-coordinate Fe(II) amido complexes described
here are heteroleptic with a single amido group and show
remarkable stability for such electronically and coordinatively
unsaturated complexes. Some of the amidoiron(II) complexes
are stable to thermolysis in C6D6 solution up to temperatures
of 100 °C and can be stored at room temperature for several
months without substantial decomposition. However, strongly
basic or unhindered complexes are less stable: LtBuFeNHtBu
and LMeFeNHtol show moderate decomposition after several
days at room temperature. The amido complexes do not react
with strong bases such as LiN(SiMe3)2 and NaOtBu and
decompose in the presence of n-butyllithium.
The Fe(II) amido compounds form adducts with σ-donor
Lewis bases such as THF, acetonitrile, and 4-tert-butyl-
pyridine. Interestingly, increasing the coordination number
in this way causes the formation of additional bonds in the
form of weak agostic interactions. The formation of agostic
interactions is not caused by a high-spin to low-spin
transition, because the five-coordinate products remain in
an S ) 2 state. Coordinatively induced agostic interactions
have precedent in reactions where phosphines bind to four-
coordinate alkyltitanium complexes.27
Geometries: Electronic Considerations. The four-
coordinate Fe(II) diketiminate complexes LFe(X)(L) gener-
ally adopt geometries that are distorted from tetrahedral
toward trigonal pyramidal, with the exception of the aceto-
nitrile adduct, which is discussed below. To explain these
observations qualitatively, we use a crystal-field model that
borrows from one used to explain the preference for trigonal
Figure 15. ORTEP drawing of the molecular structure of LtBuFeCCPh.
Thermal ellipsoids are shown at the 50% probability level. Hydrogen atoms
have been omitted for clarity. Important bond distances (Å) and angles
(deg): Fe-N1 1.966(2), Fe-N2 1.974(2), Fe-C 2.000(2); N1-Fe-N2
96.70(6), N1-Fe-C 137.44(7), N2-Fe-C 125.80(7).
the color immediately changed from dark red to red-orange
1
and all H NMR signals from the amido starting material
were replaced by signals for LtBuFeCCPh. The acetylide
compound was independently prepared from LtBuFeCl and
LiCCPh. A single crystal was subjected to an X-ray
diffraction analysis, and a diagram of one of the two
crystallographically independent but metrically similar mol-
ecules from the asymmetric unit is shown in Figure 15.
LtBuFeCCPh is a three-coordinate, trigonal planar molecule
as indicated by the sum of the bond angles about iron
(360.0(7)°). The acetylide C-C triple bonds are intact
(1.19(1) Å), and the C-C-Cipso angles are linear
(177.6(3)°).
LtBuFeNHdipp also reacted with 1 equiv of tert-butyl
1
alcohol (tBuOH) in C6D6 to give a product with H NMR
signals identical with LtBuFeOtBu as reported by Gibson et
al.17 In a comparison of M-X basicity, 2 equiv of HCCPh
was added to a C6D6 solution of LtBuFeOtBu. Both
1
LtBuFeOtBu and LtBuFeCCPh were evident in the H NMR
spectrum, but eventually LtBuFeCCPh precipitated leaving
only LtBuFeOtBu in solution. LtBuFeOtBu did not react with
a 2-fold excess of H2Ndipp. However, with a 3-fold excess
or more of H2Ndipp, an equilibrium between LtBuFeOtBu
and a new compound was established. By analogy to the
formation of LMeFe(NHdipp)(H2Ndipp), we assign this
compound as LtBuFe(NHdipp)(H2Ndipp); supporting this idea,
an 1H NMR spectrum with identical peaks was observed in
the reaction of LtBuFeNHdipp and a 50-fold excess of H2-
Ndipp. The same adduct is observed when 2 equiv or more
of H2Ndipp are added to a C6D6 solution of LtBuFeCCPh.
(26) (a) Laplaza, C. E.; Cummins, C. C. Science 1995, 268, 861. (b)
Laplaza, C. E.; Johnson, M. J. A.; Peters, J. C.; Odom, A. L.; Kim,
E.; Cummins, C. C.; George, G. N.; Pickering, I. J. J. Am. Chem.
Soc. 1996, 118, 8623.
(27) Dawoodi, Z.; Green, M. L. H.; Mtetwa, V. S. B.; Prout, K.; Schultz,
A. J.; Williams, J. M.; Koetzle, T. F. J. Chem. Soc., Dalton Trans.
1986, 1629.
Discussion
Synthesis and Stability. The synthetic utility of LtBuFeCl
and LMeFeCl2Li(THF)211 for the preparation of a wide variety
of three-coordinate compounds is evident from a flurry of
Inorganic Chemistry, Vol. 43, No. 10, 2004 3315

Eckert et al.
Table 3. Torsion Angles Fe-N(amido)-C(R)-C(â) (deg)
LtBuFeNHtol
25.9(5)
4.3(5)
LtBuFeNHxyl
LtBuFeNHdipp
71.1(5)
38.3(5)
39.2(5)
28.8(5)
47.7(5)
LMeFeNHdipp
LtBuFe(NHdipp)(tBuPyr)
LMeFeNHxyl(THF)
LMeFeNHdipp(THF)
the metal-binding pocket to a higher degree and in most cases
prevents the complexation of LiCl or THF solvent molecules.
However, with a small donor such as MeCN, complexes of
LtBu may accommodate the additional ligand with geometrical
perturbations.
Figure 16. Crystal splitting of the d-orbitals of a trigonal planar, C2V-
A closer look at the structures of the Fe(II) amido
complexes reveals that the metal geometry must distort to
fit large amido groups into the third coordination site of the
complex. The amido complexes have an almost T-shaped
geometry, which probably arises from the need to accom-
modate the aryl or alkyl substituent. Consistent with this idea,
the N-Fe-C angles in the acetylide complex LtBuFeCCPh
are much closer to each other (Figure 15). In addition, the
steric hindrance causes the amido group aryl substituents to
be twisted. The amido twist from the N3Fe plane is quantified
by a measure of the Fe-N(amido)-C(R)-C(â) angles
(Table 3). All of the amidoiron(II) compounds except
LtBuFeNHxyl show a significant amount of amido twisting.
In the THF adducts of the Fe(II) arylamido complexes of
LMe, agostic interactions exist between the amido alkyl
substituents and iron. These interactions are not present in
the three-coordinate LtBu analogues. The agostic interactions
arise from the electronically unsaturated nature of the Fe(II)
amido complexes and the freedom of the aryl group of the
amido moiety to twist in complexes of the smaller diketimi-
nate. Typically, Fe-H distances for agostic alkyl complexes
are on the order of 1.5-2.0 Å.19,30 However, weak interac-
tions have been observed in some remote agostic Fe
compounds on the order of 2.0-2.5 Å.19 This suggests that
our compounds have interactions that are weak due to the
steric bulk of the ligand set. These interactions are present
in solution as well, as concluded from the low-temperature
1H NMR studies described above. Although the agostic C-H
bond itself is thought to be sterically unimpeding,19 it has
been argued that bulky groups near the agostic C-H bond
may inhibit its interaction with a transition metal.31 It has
been shown by Caulton and co-workers that bulky groups
force the smaller C-H group to interact with the electroni-
cally and coordinatively unsaturated metal in (Ir(H)2(PR2-
symmetric compound upon addition of an axial ligand.
bipyramidal or square pyramidal geometry in five-coordinate
transition metal complexes.28 The relative d-orbital energies
for three-coordinate, trigonal planar iron diketiminate com-
plexes have been determined using EPR and Mo¨ssbauer
spectroscopies in combination with density functional theory
(Figure 16).10,29 The z axis is taken to be perpendicular to
the Fe-ligand plane, and the Fe-X bond is along the x axis.
Addition of an axial ligand along the z axis (giving trigonal
pyramidal geometry) should raise the energy of the z2 orbital
as shown, making the yz orbital lowest in energy and
therefore doubly occupied.
Consider the interactions of a π-donor with these d orbitals
in a basal or axial position. For a basal ligand, π-overlap is
possible with the half-filled xy or xz orbitals, and for an axial
ligand π-overlap is possible with the xz or yz orbitals.
Because the yz orbital is doubly occupied, the axial position
benefits from fewer stabilizing π-interactions, and therefore
a π-donor might be expected to gravitate toward the basal
sites of the trigonal pyramid. This agrees with the observation
of basal amido and alkoxo ligands in the four-coordinate
amido and alkoxo complexes. The crystal-field model also
predicts that a weaker σ-donor should gravitate toward the
lowest-energy d orbital of the trigonal planar complexes, the
z2 orbital. This is consistent with the preference of Lewis
bases and solvents for the axial position. Finally, only axial
π-acceptors can interact with the completely filled metal yz
orbital, explaining the axial position of tBuPyr. This also
rationalizes the trigonal pyramidal geometry of the recently
reported terminal hydride, LtBuFeH(tBuPyr),20 in which the
π-acceptor pyridine again inhabits the axial site in order to
maximize back-bonding. Therefore, a simple crystal-field
model can explain many of the geometric preferences of the
four-coordinate compounds. In high-spin iron(III) complexes,
the d orbitals are all singly occupied, and so there are no
electronic effects on geometry.
+
Ph)2 (R ) tert-butyl, isopropyl, cyclohexyl).32 A similar
phenomenon, where the added bulk of the MeCN and the
(30) (a) Tachikawa, M.; Muetterties, E. L. J. Am. Chem. Soc. 1980, 102,
4541. (b) Crabtree, R. H.; Holt, E. M.; Lavin, M.; Morehouse, S. M.
Inorg. Chem. 1985, 24, 1986. (c) Barreto, R. D.; Fehlner, T. P. J. Am.
Chem. Soc. 1988, 110, 4471. (d) Simons, R. S.; Tessier, C. A.
Organometallics 1996, 15, 2604. (e) Evans, D. R.; Drovetskaya, T.;
Bau, R.; Reed, C. A.; Boyd, P. D. W. J. Am. Chem. Soc. 1997, 119,
3633. (f) Chen, W. C.; Hung, C. H. Inorg. Chem. 2001, 40, 5070. (g)
Siemer, C. J.; Meece, F. A.; Armstrong, W. H.; Eichorn, D. M.
Polyhedron 2001, 20, 2637. (h) Rachlewicz, K.; Wang, S. L.; Peng,
C. H.; Hung, C. H.; Latos-Grazynski, L. Inorg. Chem. 2003, 42, 7348.
(31) Crabtree, R. H.; Hamilton, D. G. AdV. Organomet. Chem. 1988, 28,
299.
Geometries: Steric Considerations and Agostic Inter-
actions. The difference in steric hindrance supplied by the
two diketiminates is apparent from the geometries of
LtBuFeNHAr and LMeFeNHAr. While amido compounds can
be three-coordinate utilizing either diketiminate, LtBu protects
(28) Rossi, A. R.; Hoffmann, R. Inorg. Chem. 1975, 2, 365.
(29) Zhang, Y.; Oldfield, E. J. Phys. Chem. B 2003, 107, 7180.
3316 Inorganic Chemistry, Vol. 43, No. 10, 2004

Iron(II) Amido Complexes of â-Diketiminates
amido ligands force the isopropyl methine C-H group of
the amido ligand toward the iron, may occur in
LtBuFe(NHdipp)(MeCN). A similar argument may be made
for LMeFe(NHdipp)(THF) and LMeFe(NHxyl)(THF), although
presented data that showed a very good 1:1 correlation
between H-X and M-X bond energies with a Ru-X bond
strength trend: Ru-C(sp) > Ru-OR > Ru-H > Ru-
C(sp3) > Ru-N.24a However, other deviations from the 1:1
correlation are present in the literature. For example, a study
by Hartwig, Andersen, and Bergman on a different ruthenium
system, L4RuH(X) (L ) CO or PR3; X ) H, CH2Ph, OAr,
NHPh), gives the following relative bond energy scale:
Ru-H > Ru-OAr > Ru-NHPh > Ru-CH2Ph.24b The
relative bond energy trend shown here (Fe-OtBu ∼ Fe-
CCPh > Fe-NHdipp) similarly argues against a 1:1 MX:
HX bond energy correlation since the displacement from a
1:1 correlation of the Fe-OtBu bond is greater than that of
the Fe-CCPh. Interestingly, the iron trend correlates with
neither π-donor ability nor steric effects. Our results on three-
coordinate alkyliron(II) complexes35 strongly suggest that the
polarity of the iron-ligand bonds explains the deviations,
because the most polar bond (Fe-O) is most stable.
However, our inability to perform detailed thermodynamic
measurements on the amido complex prevents detailed
thermodynamic conclusions on this system.
The proton exchange products are interesting in their own
right. To our knowledge, LtBuFeCCPh is the first example
of a three-coordinate transition metal acetylide. Terminal
acetylide complexes of the late transition metals are catalysts
or intermediates in alkyne dimerization and the living
polymerization of alkynes.36 Acetylide complexes are im-
portant in many other fields, including luminescence, non-
linear optics, and rigid organometallic polymers.37 Three-
coordinate Fe alkoxides are also interesting because they are
catalysts for the polymerization of lactides.17
The structures of LRFe(OtBu)(OTf) show that iron(III)
complexes prefer a tetrahedral geometry rather than the
trigonal pyramidal geometry described above for iron(II)
complexes. The molecular structure of LMeFe(NHdipp)(OTf)
similarly reveals a pseudotetrahedral geometry with an η1-
triflate ligand in addition to the diketiminate and the amido
group, as suggested by spectroscopy and solubility. The
spectroscopic similarity between all these iron(III) complexes
suggests structural similarity. All Fe(III) compounds contain
broad 1H NMR spectra as well as intense low-energy bands
in their UV/vis spectra, which were assigned as LMCT
transitions. Interestingly, LtBuFe(NHtol)(OTf) has a solution
magnetic moment that is consistent with an intermediate-
spin (S ) 3/2) Fe(III) rather than a high-spin (S ) 5/2) Fe-
(III) as is observed in LtBuFe(NHdipp)(OTf), LMeFe(NHdipp)-
(OTf), LtBuFe(OtBu)(OTf), and LMeFe(OtBu)(Cl). Also,
LtBuFe(NHdipp)(OTf) has a drastically different absorption
spectrum from the other Fe(III) complexes. These differences
in the absorption spectra suggest that LtBuFe(NHtol)(OTf)
and LtBuFe(NHdipp)(OTf) have divergent electronic struc-
tures, which are currently under investigation.
1
low-temperature H NMR experiments indicate the acces-
sibility of an agostic complex of LMeFeNHdipp in solution.
Reactivity. The amido groups of these Fe(II) compounds
are strong bases, and their relative basicity was explored
through the addition of weak acids (HX). PhCCH and tBuOH
protonated the amido group from the Fe(II) complex, giving
aniline and the resultant Fe-X compound. When excess H2-
Ndipp was added to a C6D6 solution of LtBuFeOtBu or
LtBuFeCCPh, signals from LtBuFe(NHdipp)(H2Ndipp) were
observed, and the observation of signals from free diisopropyl-
aniline in the mixture suggested a slow-exchange process.
The identity of LtBuFe(NHdipp)(H2Ndipp) was confirmed by
adding a 50-fold excess of H2Ndipp to a C6D6 solution of
LtBuFeNHdipp. These additional equilibria made quantitative
analysis of relative bond dissociation energies (BDEs)
impractical, but the results of stoichiometric proton exchange
reactions make possible some rough statements about relative
Fe-X bond energies, using eqs 1-3, using the values for
the BDEs of HX as 132 (H-CCPh),24a 106 (H-OtBu),33
and 90 (H-NHdipp)34 kcal/mol, using the consideration that
LtBuFe(NHdipp)(H2Ndipp) must have a lower energy than
LtBuFe(NHdipp), and assuming that entropic effects are
minimal.
LtBuFe-NHdipp + HOtBu f LtBuFe-OtBu + HNHdipp
(1)
L
tBuFe-NHdipp + HCCPh f LtBuFe-CCPh + HNHdipp
(2)
LtBuFe-CCPh + HOtBu f LtBuFe-OtBu + HCCPh (3)
According to eq 2 the BDE for the Fe-CCPh bond must
be more than 42 kcal/mol higher than that of the Fe-NHdipp
bond. From eq 1, the BDE for the Fe-OtBu bond must be
more than 16 kcal/mol higher than the BDE for the Fe-
NHdipp bond. Even though eq 3 proceeds to the right as
shown, BDEFe-OtBu is not necessarily higher than BDEFe-CCPh
because the H-CCPh bond is much stronger than the
H-OtBu bond. Nevertheless, it shows that the difference
BDEFe-OtBu - BDEH-OtBu is substantially larger than
BDEFe-CCPh - BDEH-CCPh
.
Note that, for these three-coordinate iron(II) compounds,
the M-X bond strengths do not appear to follow a 1:1
correlation with H-X bond strengths, in contrast with
observations by Bercaw and co-workers with 18-electron
Cp*(PMe3)2RuX compounds.24a Bercaw and co-workers
(32) (a) Cooper, A. C.; Streib, W. E.; Eisenstein, O. E.; Caulton, K. G. J.
Am. Chem. Soc. 1997, 119, 9069. (b) Ujaque, G.; Cooper, A. C.;
Maseras, F.; Eisenstein, O.; Caulton, K. G. J. Am. Chem. Soc. 1998,
120, 361. (c) Cooper, A. C.; Clot, E.; Huffman, J. C.; Streib, W. E.;
Maseras, F.; Eisenstein, O.; Caulton, K. G. J. Am. Chem. Soc. 1999,
121, 97.
(33) Blanksby, S. J.; Ellison, G. B. Acc. Chem. Res. 2003, 36, 255.
(34) Here we assume that the BDE of dippNH-H is the same as the BDE
of PhNH-H: McMillen, D. F.; Golden, D. M. Annu. ReV. Phys. Chem.
1982, 33, 493.
(35) Vela, J.; Vaddadi, S.; Cundari, T. R.; Smith, J. M.; Gregory, E. A.;
Lachicotte, R. J.; Holland, P. L. Submitted for publication.
(36) (a) Kishimoto, Y.; Eckerle, P.; Miyatake, T.; Ikariya, T.; Noyori, R.
J. Am. Chem. Soc. 1994, 116, 12131. (b) Bassetti, M.; Marini, S.;
D´ıaz, J.; Gamasa, M. P.; Gimeno, J.; Rodr´ıguez-AÄ lvarez, Y.; Garc´ıa-
Granda, S. Organometallics 2002, 21, 4815 and references therein.
(37) Long, N. J.; Williams, C. K. Angew. Chem., Int. Ed. 2003, 42, 2586.
Inorganic Chemistry, Vol. 43, No. 10, 2004 3317

Eckert et al.
The Fe(III) compounds are tetrahedral rather than trigonal
pyramidal, because for a high-spin d5 electronic configuration
there can be no electronic preference for different geometries.
Accordingly, four-coordinate Fe(III) molecules are typically
tetrahedral.38 The only complexes with terminal Fe(III)
imido39 and Fe(IV) imido groups40 have roughly tetrahedral
geometries at iron, suggesting good π-acceptor ability by
the iron center. A number of other late transition metal
compounds with strong π-donating imido ligands also have
tetrahedral geometries.41 Considering these examples and the
π-donor ability of alkoxo and amido groups,42 we were
tempted to believe that significant π-bonding explained the
nearly linear Fe-O-C angle observed in LtBuFe(OtBu)(OTf).
However, the Fe-O-C angles are smaller in the less
hindered LMeFe(OtBu)Cl and LMeFe(OtBu)(OTf) complexes,
indicating that sterics are more important than π-bonding in
determining this angle in our complexes.
Although LRFeNHAr was easily oxidized with AgOTf to
give a stable product, attempts to synthesize a three-
coordinate iron(III) amido compound have thus far failed.
Oxidants such as AgBArF (BArF ) tetrakis(3,5-bis(trifluoro-
methyl)phenyl)borate) and AgBPh4 give extremely insoluble
and/or intractable materials. Therefore, the current evidence
suggests that a fourth ligand is necessary to stabilize Fe(III)
amido compounds of â-diketiminates.
bond is at least 42 kcal/mol weaker than the Fe-CCPh bond
and at least 16 kcal/mol weaker than the Fe-OtBu bond.
Experimental Section
General Methods. 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.
1
Glassware was dried at 150 °C overnight. H NMR spectra were
recorded on a Bruker Avance 400 spectrometer (400 MHz) at 22
°C and referenced internally to residual protiated solvent (C6D5H
at 7.16 ppm, C4D7HO at 1.73 ppm, and C7D7H at 2.08 ppm). 19F-
{1H} NMR spectra were recorded on the same instrument (376.5
MHz) at 22 °C and referenced to external C6H5CF3 at 0 ppm.
Resonances are broad singlets unless otherwise specified. IR spectra
were recorded on a Mattson Instruments 6020 Galaxy Series FTIR
using solution cells with CsF windows or KBr pellets. UV/vis
spectra were measured on a Cary 50 spectrophotometer, using
screw-cap or Schlenk-adapted cuvettes. Solution magnetic suscep-
tibilities were determined by the Evans method.43 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 obtained from Glass Contour Co. (Laguna Beach, CA).
Deuterated benzene, toluene, and THF were 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. The lithiated anilines,
n
Conclusions
LiNHAr, were prepared by adding 1.1 molar equiv of BuLi to a
pentane solution of the appropriate aniline. This mixture was dried
in vacuo after 1 h of stirring, rinsed with pentane, and used without
further purification. The PhCCH was distilled under vacuum and
passed through a short alumina column prior to use. All other
chemicals were used as received.
A series of Fe(II) amido complexes has been synthesized
using â-diketiminates to enforce low coordinate numbers.
Complexes of LtBu are three-coordinate trigonal planar
molecules, while complexes of LMe may be three-, four- or
five-coordinate depending on the identity of the aryl moiety
of the amido group, the solvent, and the iron starting material
employed. The three-coordinate amidoiron(II) complexes are
stable 12-electron coordination compounds. In a few ex-
amples, structural data reveal remote agostic interactions
between a C-H bond of an ortho-alkyl substituent on the
aryl group of the amido ligand and the iron(II) center in the
X-ray Crystallography. Single crystals of each compound were
mounted on a fiber under Paratone-8277 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 Siemens
SMART CCD area detector system equipped with a normal focus
molybdenum-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 varying from 10 to 60 s/frame using a detector-to-
crystal distance of 5.09 cm (maximum 2θ angle of 56.5°). The total
data collection time was varied but was typically between 12 and
24 h. Frames were integrated to a maximum 2θ angle of 56.5°
with the Siemens SAINT program to yield a total amount of
reflections. Laue symmetry revealed the respective crystal systems,
and the final unit cell parameters (at -80 °C) were determined
from the least-squares refinement of three-dimensional centroids
of the reflections. Data were corrected for absorption with the
SADABS44 program.
1
solid state. VT H NMR experiments support the presence
of the same agostic interactions in solution. It is likely that
these remote agostic interactions are adopted for steric
reasons. The amido complexes may be oxidized with AgOTf,
and they undergo acid-base chemistry with weak acids. The
results of the acid-base reactions reveal that the Fe-NHdipp
(38) Gibson, V. C. J. Chem. Soc., Dalton Trans. 1994, 11, 1607.
(39) Brown, S. D.; Betley, T. A.; Peters, J. C. J. Am. Chem. Soc. 2003,
125, 322.
(40) Verma, A. K.; Nazif, T. N.; Achim, C.; Lee, S. C. J. Am. Chem. Soc.
2000, 122, 11013.
The space groups were assigned using XPREP, and the structures
were solved with direct methods by using Sir9245 (WinGX v1.63.02)
and refined by employing full-matrix least squares on F2 (Siemens,
SHELXTL,46 version 5.04). All of the atoms were refined with
(41) (a) Glueck, D. S.; Wu, J.; Hollander, F. J.; Bergman, R. G. J. Am.
Chem. Soc. 1991, 113, 2041. (b) Schofield, M. H.; Kee, T. P.; Anhaud,
J. T.; Schrock, R. R.; Johnson, K. H.; Davis, W. H. Inorg. Chem.
1991, 30, 3595. (c) Michelman, R. I.; Bergman, R. G.; Andersen, R.
A. Organometallics 1993, 12, 2741. (d) Hay-Motherwell, R. S.;
Wilkinson, G.; Hussain-Bates, B.; Hursthouse, M. B. Polyhedron 1993,
12, 2009. (e) Burrell, A. K.; Steedman, A. J. J. Chem. Soc., Chem.
Commun. 1995, 2109. (f) Jenkins, D. M.; Betley, T. A.; Peters, J. C.
J. Am. Chem. Soc. 2002, 124, 11238.
(43) (a) Baker, M. V.; Field, L. D.; Hambley, T. W. Inorg. Chem. 1988,
27, 2872. (b) Schubert, E. M. J. Chem. Educ. 1992, 69, 62.
(44) The SADABS program is based on the method of Blessing: Blessing,
R. H. Acta Crystallogr., Sect. A 1995, 51, 33.
(42) Nugent, W. A.; Mayer, J. A. Metal-Ligand Multiple Bonds; Wiley
(45) Altomare, A.; Cascarano, G.; Giacovazzo, C.; Gualardi, A. J. Appl.
Crystallogr. 1993, 26, 343.
& Sons: New York, 1988; and references therein.
3318 Inorganic Chemistry, Vol. 43, No. 10, 2004

Iron(II) Amido Complexes of â-Diketiminates
anisotropic thermal parameters unless otherwise noted. Hydrogen
atoms were included in idealized positions unless otherwise
specified.
(2, p-Ar), 58 (6, Me), 37 (18, tBu), 20 (4, ligand m-Ar), -25 (12,
i
i
iPr-Me), -105 (4, Pr-CH), -110 (12, Pr-Me).
LtBuFeNHxyl. To a suspension of LtBuFeCl9e (2.5 mmol, 1.5 g)
in Et2O (10 mL) was added solid LiNHxyl (2.7 mmol, 340 mg).
The mixture immediately became dark red, with precipitation of a
light-colored solid. The dark red solution was stirred for 2 h, filtered,
and pumped to dryness. The dark red solid was extracted with
pentane (20 mL) and filtered again. LtBuFeNHxyl was isolated as
dark red crystals (970 mg, 57%) from a concentrated pentane
For the structures of LtBuFeNHtBu and LMeFe(µ-NHtol)(µ-Cl)-
Li(THF)(Et2O) the diffraction data that were collected were weak
and, therefore, the subsequent solutions were of low quality. The
structure of LtBuFe(OtBu)(OTf) contained disordered OTf and OtBu
moieties which were both refined over two positions at 50%
occupancy. The strucuture of LMeFeOtBu contained a disordered
butoxide group. The butoxide was refined over two positions in a
61:39 ratio. Each inversion possibility gave a Flack parameter of
0.4-0.5, and therefore the structure was refined as a racemic twin.
[LMeFeCl]2. A Schlenk flask was charged with LiLMe 12 (5.0 g,
21 mmol), FeCl2(THF)1.513 (9.1 g, 21 mmol), and toluene (150 mL).
The resulting orange mixture was heated at 100 °C for 12 h. The
solvent was removed in vacuo to give an orange solid. Impurities
were removed by stirring the solid with pentane (100 mL) and
filtering to yield an orange solid that was dried under vacuum. The
resulting solid was continuously washed with pentane (5 × 25 mL)
to remove any trace amounts of LMeFe(µ-Cl)2Li(THF)2.11 Separation
from LiCl is not readily achieved. For example, if the yellow solid
1
solution (8 mL) at -35 °C. H NMR (C6D6): δH 142 (6, amido
Me), 108 (2, amido m-Ar or ligand p-Ar), 42 (18, tBu), -6 (4,
m-Ar), -17 (1, amido p-Ar or C-H), -28 (12, iPr-Me), -43 (1,
amido p-Ar or C-H), -92 (2, amido m-Ar or ligand p-Ar), -106
(4, iPr-CH), -114 (12, iPr-Me) ppm. UV/vis (pentane): 255 (16
000 M-1 cm-1), 335 (16 000 M-1 cm-1), 415 (8000 M-1 cm-1),
500 (sh) nm. µeff (C6D6, 298 K): 5.4 ( 0.3 µB. FTIR (pentane):
3368 (νN-H) cm-1. Anal. Calcd: C, 76.22; H, 9.31; N, 6.20.
Found: C, 76.26; H, 9.62; N, 6.06.
LtBuFeNHtBu. To a solution of LtBuFeCl9e (0.840 mmol, 501
mg) in Et2O (10 mL) was added solid LiNHtBu (0.85 mmol, 71
mg). The mixture immediately became dark yellow-brown, with
formation of a precipitate. The yellow-brown solution was stirred
for 2 h, filtered, and pumped to dryness. The dark brown solid
11
is stirred in Et2O for several hours, then LMeFe(µ-Cl)2Li(THF)2
is isolated. However, [LMeFeCl]2 is pure enough for synthetic use.
The complex is insoluble in pentane, benzene, and toluene at room
temperature and reacts immediately with CH2Cl2. We therefore have
been unable to obtain a satisfactory 1H NMR spectrum in a
was extracted with pentane (10 mL) and filtered again. LtBu
-
FeNHtBu was isolated as dark brown crystals (350 mg, 66%) from
a concentrated pentane solution (4 mL) at -35 °C. 1H NMR
(C6D6): δH 136 (9, amido tBu), 95 (1, C-H), 38 (18, tBu), -2 (4,
m-Ar), -28 (12, iPr-Me), -95 (4, iPr-CH), -104 (2, p-Ar), -122
(12, iPr-Me) ppm. UV/vis (Et2O): 510 (530 M-1 cm-1), 560 (270
M-1 cm-1) nm. µeff (C6D6, 298 K): 4.8 ( 0.3 µB. FTIR (Nujol):
3282 (νN-H) cm-1. Due to the thermal instability of LtBuFeNHtBu,
a suitable elemental analysis has not been obtained.
noncoordinating solvent. H NMR (THF-d8; LMeFeCl(THF)): δH
1
15 (4, m-Ar), 2.5 (12, iPr-Me), -17 (12, iPr-Me), -34 (4, iPr-
CH), -44 (2, p-Ar), -47 (1, backbone), -65 (6, Me) ppm. An
LiCl-free sample of [LMeFeCl]2 was isolated by crystallizing from
hot toluene. Anal. Calcd: C, 68.17; H, 8.48; N, 5.48. Found: C,
68.63; H, 8.16; N, 5.29.
11
LMeFe(µ-NHtol)(µ-Cl)Li(THF)(Et2O). LMeFe(µ-Cl)2Li(THF)2
LtBuFeNHtol. To a solution of LtBuFeCl9e (420 µmol, 250 mg)
in Et2O (5 mL) was added solid LiNHtol (420 µmol, 48 mg). The
mixture immediately became dark red, with formation of a gray
precipitate. The mixture was stirred for 2 h, filtered, and pumped
to dryness. The dark red product was extracted with pentane (15
mL) and filtered again. LtBuFeNHtol was isolated as dark red
crystals (200 mg, 73%) from a concentrated pentane solution (5
mL) at -35 °C. 1H NMR (C6D6): δH 122 (3, amido p-Me), 90 (1,
CH), 87 (2, amido o- or ligand m-Ar), 38 (18, tBu), 6 (4, m-Ar),
-26 (12, iPr-Me), -98 (4, iPr-CH), -113 (2, p-Ar), -114.5 (12,
iPr-Me) ppm. UV/vis (pentane): 265 (15 000 M-1 cm-1), 330 (11
000 M-1 cm-1), 480 (sh) nm. µeff (C6D6, 298 K): 5.3 ( 0.3 µB.
FTIR (KBr): 3402 (νN-H) cm-1. Anal. Calcd: C, 76.02; H, 9.20;
N, 6.33. Found: C, 75.58; H, 9.62; N, 5.82.
(0.3 mmol, 200 mg) was dissolved in Et2O (5 mL), and to this
yellow solution was added a slight excess of LiNHtol (0.3 mmol,
40 mg). The mixture rapidly became red-brown with precipitation
of light-colored solid. The mixture was filtered, and the resultant
brown solution was concentrated (2 mL) and placed in a -35 °C
freezer. Brown crystals were isolated in two crops (120 mg, 52%).
1H NMR (C6D6): δH 115 (3, amido Me), 108 (1, C-H), 80 (2,
amido o-Ar), 18 (6, Me), -10 (4, m-Ar), -20 (12, iPr-Me), -36
(2, amido m-Ar), -80 (2, p-Ar), -105 (4, iPr-CH), -110 (12, iPr-
Me) ppm. UV/vis (pentane): 235 (26 000 M-1 cm-1), 330 (22 000
M-1 cm-1), 464 (sh) nm. µeff (C6D6, 298 K): 5.3 ( 0.3 µB. FTIR
(KBr): 3389 (νN-H) cm-1. Anal. Calcd: C, 68.97; H, 8.55; N, 5.48.
Found: C, 62.23; H, 8.14; N, 6.96. It is possible that thermal
instability (as in LMeFeNHtol) is the reason for the poor
LtBuFeNHdipp. To a suspension of LtBuFeCl9e (6.70 mmol, 4.01
g) in Et2O (20 mL) was added solid LiNHdipp (6.8 mmol, 1.2 g).
The dark red mixture was stirred for 2 h, filtered, and pumped to
dryness. The dark red solid was extracted with pentane (50 mL)
and filtered again. LtBuFeNHdipp was isolated as dark red crystals
(3.4 g, 68%) from a concentrated pentane solution (18 mL) at -35
1
microanalysis; however, H NMR spectra indicated high purity.
LMeFeNHtol. [LMeFeCl]2 (320 µmol, 330 mg) was suspended
in 5 mL of Et2O. To this yellow slurry was added LiNHtol (640
µmol, 75 mg). The mixture immediately became red-brown with
formation of a light-colored precipitate. After 2 h, the mixture was
filtered and pumped dry. The orange-red solid was extracted with
pentane (10 mL), and the extract was filtered, concentrated (4 mL),
and placed in a -35 °C freezer. Orange-red solid LMeFeNHtol was
1
°C. H NMR (C6D6): δH 103 (1, C-H), 101 (2, amido iPr-CH),
41 (18, tBu), 32 (12, amido iPr-Me), -4 (4, m-Ar), -12 (2, amido
m-Ar), -23 (12, iPr-Me), -33 (1, amido p-Ar), -89 (2, p-Ar),
-111 (4, iPr-CH), -112 (12, iPr-Me) ppm. UV/vis (pentane): 335
(14 000 M-1 cm-1), 415 (sh), 560 (sh) nm. µeff (C6D6, 298 K): 5.1
( 0.3 µB. FTIR (pentane): 3417 (νN-H) cm-1. Anal. Calcd: C,
1
isolated in two crops (101 mg, 54%). H NMR (C6D6): same as
LMeFe(µ-NHtol)(µ-Cl)Li(THF)(Et2O). UV/vis (pentane): 330 (14
000 M-1 cm-1), 418 (sh), 455 (sh) nm. µeff (C6D6, 298 K): 5.0 (
0.3 µB. FTIR (pentane): 3431 (νN-H) cm-1. Due to the extreme
thermal instability of LMeFeNHtol, a suitable elemental analysis
could not be obtained.
1
76.94; H, 9.69; N, 5.73. Found: C, 76.40; H, 9.11; N, 5.60. H
NMR (LtBuFe(NHdipp)(H2Ndipp), C6D6): δH 68 (1, backbone), 65
LMeFe(NHdipp)(THF). LMeFe(µ-Cl)2Li(THF)211 (0.6 mmol, 400
mg) was dissolved in Et2O (8 mL), and to this yellow slurry was
(46) SHELXTL: Structure Analysis Program, version 5.04; Siemens
Industrial Automation Inc.: Madison, WI, 1995.
Inorganic Chemistry, Vol. 43, No. 10, 2004 3319

Eckert et al.
added LiNHdipp (0.60 mmol, 110 mg). The mixture immediately
became red-brown, and solid precipitated from solution. The
reaction was stirred for 2 h and was then filtered and pumped dry.
The brown residue was extracted with pentane (12 mL), and the
resultant red-brown pentane solution was filtered, concentrated to
6 mL, and cooled to -35 °C. Golden brown needlelike crystals
volatile materials were removed under vacuum. The dark blue solid
was extracted with pentane (15 mL) and filtered. The dark blue-
green pentane solution was concentrated to 5 mL and cooled to
-35 °C, and microcrystalline LtBuFe(NHtol)(OTf) was isolated in
two crops (90 mg, 70%). 19F NMR (C6D6): δF 107 ppm. UV/vis
(Et2O): 435 (2800 M-1 cm-1), 615 (3100 M-1 cm-1) nm. µeff
1
were isolated in two crops (230 mg, 49%). H NMR (C6D6): δH
(C6D6, 298 K): 3.5 ( 0.3 µB. FTIR (KBr): 3295 (νN-H) cm-1
.
110 (1, C-H), 109 (2, amido m-Ar), 42 (6, Me), 32 (12, amido
iPr-Me), -8 (4, m-Ar), -15 (12, iPr-Me), -40 (1, amido p-Ar),
-60 (2, p-Ar), -100 (12, iPr-Me), -120 (4, iPr-CH) ppm. 1H NMR
(THF-d8): δH 56 (4, iPr-CH), 17 ppm (12, iPr-Me), 15 (4, m-Ar),
-13 (12, iPr-Me), -29 (2, p-Ar), -60 (1, C-H), -62 (6, Me).
UV/vis (pentane): 240 (17 000 M-1 cm-1), 295 (14 000 M-1 cm-1),
325 (15 000 M-1 cm-1), 415 (sh), 470 (sh) nm. µeff (C6D6, 298 K):
5.8 ( 0.3 µB. FTIR (pentane): 3366 (νN-H) cm-1. Anal. Calcd:
C, 74.81; H, 9.36; N, 5.82. Found: C, 74.17; H, 8.79; N, 5.92.
Anal. Calcd: C, 63.54; H, 7.56; N, 5.17. Found: C, 64.00; H, 7.31;
N, 5.03.
LtBuFe(NHdipp)(OTf). LtBuFeNHdipp (140 µmol, 100 mg) was
dissolved in Et2O (5 mL), and to this dark red solution was added
AgOTf (140 µmol, 36 mg). The mixture immediately became dark
blue, and a mirror of Ag formed on the bottom of the reaction
vessel. The mixture was stirred for 10 min, and then the volatile
materials were removed under vacuum. The dark blue residue was
extracted with pentane (15 mL), filtered, concentrated to 5 mL,
and cooled to -35 °C. Microcrystalline LtBuFe(NHdipp)(OTf) was
isolated in two crops (40 mg, 30%). 1H NMR (C6D6) (peaks listed
as δH (∼fwhm)): 60 (3200 Hz), 30 (2800 Hz), 22 (1200 Hz), 10
(1200 Hz), 3 (800 Hz), -58 (800 Hz) ppm. 19F NMR (C6D6): δF
102 ppm. UV/vis (Et2O): 435 (3400 M-1 cm-1), 630 (sh), 820
(6400 M-1 cm-1) nm. µeff (C6D6, 298 K): 5.9 ( 0.3 µB. FTIR
(KBr): 3277 (νN-H) cm-1. Anal. Calcd: C, 65.29; H, 8.11; N, 4.76.
Found: C, 65.67; H, 8.18; N, 4.66.
LMeFeNHdipp. [LMeFeCl]2 (200 µmol, 200 mg) was suspended
in 5 mL of pentane. To this yellow slurry was added LiNHdipp
(400 µmol, 80 mg). The workup was identical with that of LMe
-
Fe(NHdipp)(THF). Dark red crystals were isolated from a pentane
solution at -35 °C in two crops (170 mg, 67%). 1H NMR (C6D6):
same as LMeFe(NHdipp)(THF). UV/vis (pentane): 285 (11 000 M-1
cm-1), 330 (15 000 M-1 cm-1), 470 (sh) nm. µeff (C6D6, 298 K):
4.5 ( 0.3 µB. FTIR (KBr): 3436 (νN-H) cm-1. Anal. Calcd: C,
75.86; H: 9.15; N, 6.47. Found: C, 75.23; H, 9.47; N, 5.97. For
the fast equilibrium between LMeFeNHdipp and H2Ndipp, Keq was
obtained from changes in chemical shifts of signals in the 1H NMR
spectra of a C6D6 solution containing different concentrations of
H2Ndipp.47
LMeFe(NHxyl)(THF). LMeFe(µ-Cl)2Li(THF)211 (0.21 mmol, 140
mg) was dissolved in Et2O (6 mL), and to this yellow slurry was
added LiNHxyl (0.23 mmol, 41 mg). The mixture immediately
became red-brown, and solid precipitated from the solution. The
reaction was stirred for 2 h, filtered, and pumped dry under vacuum.
The brown residue was extracted with pentane (8 mL), filtered,
concentrated to 3 mL, and cooled to -35 °C. Golden brown
needlelike crystals were isolated in a single crop (106 mg, 76%).
1H NMR (C6D6): δH 161 (6, amido Me), 104 (2, amido m-Ar), 99
(1, C-H), 40 (6, Me), -10 (4, m-Ar), -17 (12, iPr-Me), -42 (1,
amido p-Ar), -62 (2, p-Ar), -100 (12, iPr-Me), -114 (4, iPr-CH)
ppm. UV/vis (pentane): 285 (24 000 M-1 cm-1), 330 (16 000 M-1
cm-1), 394 (sh), 466 (sh) nm. µeff (C6D6, 298 K): 5.3 ( 0.3 µB.
FTIR (KBr): 3342 (νN-H) cm-1. Anal. Calcd: C, 73.97; H, 8.93;
N, 6.31. Found: C, 73.94; H, 8.95; N, 6.83.
LMeFeNHxyl. [LMeFeCl]2 (1.0 mmol, 1.0 g) was suspended in
10 mL of pentane. To this yellow slurry was added LiNHxyl (2.2
mmol, 330 mg). The mixture became dark red as it was stirred for
a period of 12 h. The workup was identical with that of
LMeFe(NHxyl)(THF). Dark red crystals were isolated from a pentane
solution at -35 °C in one crop (1.18 g, 76.6%). 1H NMR (C6D6):
same as LMeFe(NHxyl)(THF). UV/vis (pentane): 330 (21 000 M-1
cm-1), 465 (sh) nm. µeff (C6D6, 298 K): 5.0 ( 0.3 µB. FTIR
(KBr): 3319 (νN-H) cm-1. Anal. Calcd: C, 74.86; H, 9.41; N, 7.08.
Found: C, 73.85; H, 8.96; N, 6.61.
LtBuFe(NHtol)(OTf). LtBuFeNHtol (150 µmol, 100 mg) was
dissolved in 4 mL of Et2O. To this dark red solution was added
AgOTf (150 µmol, 39 mg). The mixture immediately became dark
blue-green, and a mirror of Ag developed on the bottom of the
reaction vessel. The mixture was stirred for 10 min, and then the
LMeFe(NHdipp)(OTf). LMeFe(NHdipp)(THF) (0.7 mmol, 500
mg) was dissolved in Et2O (5 mL), and to this brown solution was
added AgOTf (0.72 mmol, 190 mg). The mixture immediately
became dark blue, and a mirror of Ag formed on the bottom of the
reaction vessel. The mixture was stirred for 2 h. The volatile
materials were then removed under vacuum. The dark blue residue
was extracted with pentane (15 mL) and filtered. The dark blue
pentane solution was concentrated to 4 mL and cooled to -35 °C.
Microcrystalline LMeFe(NHdipp)(OTf) was isolated in three crops
1
(205 mg, 37%). H NMR (C6D6) (peaks listed as δH (∼fwhm)):
162 (1500 Hz), 63 (500 Hz), 52 (500 Hz), 30 (1500 Hz), 11 (750
Hz), -45 (500 Hz), -52 (1500 Hz) ppm. 19F NMR (C6D6): δF
108 ppm. UV/vis (Et2O): 415 (2700 M-1 cm-1), 750 (4300 M-1
cm-1) nm. µeff (C6D6, 298 K): 5.9 ( 0.3 µB. FTIR (KBr): 3272
(νN-H) cm-1. Anal. Calcd: C, 63.15; H, 7.44; N, 5.26. Found: C,
61.90; H, 7.58; N, 5.11.
11
LMeFeOtBu‚LiCl(Et2O). LMeFeCl2Li(THF)2 (360 µmol, 250
mg) was dissolved in Et2O (6 mL). To this yellow-green solution
was added LiOtBu (360 µmol, 29 mg). The mixture became orange-
red, and a light-colored precipitate formed. The mixture was stirred
for 6 h, filtered, concentrated to 1 mL, and cooled to -35 °C.
LMeFeOtBu‚LiCl(Et2O) was isolated as a yellow-orange solid in
1
one crop (150 mg, 66%). H NMR (C6D6): δH 135 (9, OtBu), 62
(6, Me), -17 (4, m-Ar), -21 (12, iPr-Me), -70 (2, p-Ar), -109
(12, iPr-Me), -122 (4, iPr-CH) ppm. UV/vis (Et2O): 240 (14 000
M-1 cm-1), 330 (18 000 M-1 cm-1) nm. Anal. Calcd: C, 67.02;
H, 9.12; N, 4.22. Found: C, 64.83; H, 8.54; N, 4.38. Repeated
elemental analyses of spectroscopically pure material failed to give
the expected results.
LMeFeOtBu. A vial was charged with [LMeFeCl]2 (221 µmol,
225 mg) and 2.1 molar equiv of LiOtBu (460 µmol, 37 mg).
Toluene (4 mL) was added, and the mixture was stirred for 12 h.
The cloudy orange mixture was dried under vacuum. The yellow-
orange residue was extracted with pentane (6 mL) and filtered twice.
The yellow-green solution was concentrated (0.5 mL), and hex-
amethyldisiloxane was added (1 mL). The solution was cooled to
-35 °C, and LMeFeOtBu was isolated as yellow-green crystals in
a single crop (110 mg, 49%). 1H NMR (C6D6): identical with that
(47) Connors, K. A. Binding Constants: The Measurement of Molecular
Complex Stability, 1st ed.; Wiley & Sons: New York, 1987; pp 189-
205.
1
of LMeFeOtBu‚LiCl(Et2O) above. H NMR (THF-d8): δH 49 (9,
3320 Inorganic Chemistry, Vol. 43, No. 10, 2004

Iron(II) Amido Complexes of â-Diketiminates
OtBu), 7 (4, m-Ar), -2 (12, iPr-Me), -8 (1, C-H), -32 (12,
iPr-Me), -40 (6, Me), -41 (2, p-Ar), -48 (4, iPr-CH) ppm. UV/
vis (pentane): 280 (11 000 M-1 cm-1), 330 (14 000 M-1 cm-1),
375 (8000 M-1 cm-1), 490 (6000 M-1 cm-1) nm. µeff (C6D6, 298
K): 5.1 ( 0.3 µB. Anal. Calcd: C, 72.65; H, 9.05; N, 5.13. Found:
C, 71.52; H, 9.43; N, 5.50.
LtBuFe(OtBu)(OTf). LtBuFeOtBu17 (140 µmol, 88 mg) was
dissolved in Et2O (5 mL), and to this orange solution was added
AgOTf (150 µmol, 39 mg). The mixture immediately became dark
green and was stirred for 2 h. The reaction mixture was filtered,
and LtBuFe(OtBu)(OTf) was crystallized from the mother liquor at
-35 °C. LtBuFe(OtBu)(OTf) was isolated as dark green single
crystals in two crops (91 mg, 82%). 1H NMR (C6D6) (peaks listed
as δH (∼fwhm)): 23 (1600 Hz), 9 (800 Hz), 4 (600 Hz), 3 (600
Hz) ppm. 19F NMR (C6D6): δF 109 ppm. UV/vis (Et2O): 422 (3100
M-1 cm-1), 650 (1500 M-1 cm-1) nm. µeff (C6D6, 298 K): 5.5 (
0.3 µB. Anal. Calcd: C, 61.38; H, 7.98; N, 3.58. Found: C, 61.50;
H, 7.98; N, 3.55.
with pentane (12 mL). The dark blue-green pentane solution was
concentrated (4 mL) and cooled to -35 °C. Dark blue-green
crystalline LMeFe(OtBu)(OTf) was isolated in two crops (91 mg,
1
69%). H NMR (C6D6) (peaks listed as δH (∼fwhm)): 64 (2000
Hz), 52 (1000 Hz), 40 (2500 Hz), 9 (1200 Hz), 3 (2000 Hz), -46
(1500 Hz), -63 (4000 Hz) ppm. 19F NMR (C6D6): δF 118 ppm.
UV/vis (Et2O): 295 (9500 M-1 cm-1), 325 (sh), 400 (3000 M-1
cm-1), 625 (1500 M-1 cm-1) nm. µeff (C6D6, 298 K): 5.2 ( 0.3
µB. Anal. Calcd: C, 58.79; H, 7.11; N, 4.03. Found: C, 58.72; H,
7.54; N, 4.16.
LtBuFeCCPh. LtBuFeCl (340 µmol, 200 mg) was suspended in
Et2O (4 mL). To this red solution was added LiCCPh (340 µmol,
40 mg). The mixture immediately became bright red-orange and
was stirred for 2 h. The red-orange solution was filtered, concen-
trated to 2 mL, and cooled to -35 °C. LtBuFeCCPh was isolated in
a single crop (120 mg, 56%). LtBuFeCCPh may also be synthesized
by adding 1 molar equiv of PhCCH to a solution of LtBuFeNHR.
This reaction is quantitative on the 1H NMR scale, and on a
preparative scale LtBuFeCCPh may be isolated in approximately
the same yield as the metathetical procedure above. 1H NMR
(C6D6): δH 65 (2, o/m-Ph), 42 (18, tBu), 25 (2, o/m-Ph), 3 (4, m-Ar),
-16 (1, p-Ph), -27 (12, iPr-Me), -113(12, iPr-Me), -115 (2,
p-Ar), -117 (4, iPr-CH) ppm. UV/vis (Et2O): 335 (17 000 M-1
cm-1), 385 (sh), 510 (2300 M-1 cm-1), 540 (2100 M-1 cm-1) nm.
LMeFe(OtBu)Cl. LMeFeOtBu‚LiCl(Et2O) (230 µmol, 150 mg)
was suspended in Et2O (5 mL). To this yellow-orange slurry was
added AgOTf (230 µmol, 59 mg). The mixture immediately became
extremely dark, and a mirror of Ag metal formed on the bottom of
the reaction vessel. The dark mixture was stirred for 10 min, filtered,
and pumped to dryness. The residue was extracted with pentane
(20 mL) and filtered again. The dark green pentane solution was
concentrated (3 mL) and cooled to -35 °C. LMeFe(OtBu)Cl was
µ
eff (C6D6, 298 K): 5.3 ( 0.3 µB. FTIR (KBr): 2085 (νC≡C) cm-1
.
Anal. Calcd: C, 78.40; H, 8.87; N, 4.25. Found: C, 77.90; H, 8.94;
1
isolated as dark green crystals (70 mg, 53%). H NMR (C6D6)
N, 4.23.
(peaks listed as δH (∼fwhm)): 60 (1600 Hz), 45 (800 Hz), 30 (4000
Hz), 5 (800 Hz), -30 (2000 Hz), -45 (800 Hz) ppm. UV/vis
(Et2O): 410 (3700 M-1 cm-1), 630 (1400 M-1 cm-1) nm. µeff
(C6D6, 298 K): 5.8 ( 0.3 µB. Anal. Calcd: C, 68.09; H, 8.66; N,
4.81. Found: C, 67.90; H, 8.42; N, 4.72.
Acknowledgment. The authors thank the University of
Rochester and the National Science Foundation (Grant CHE-
0134658) for funding. P.L.H. gratefully acknowledges an
Alfred P. Sloan Research Fellowship.
LMeFe(OtBu)(OTf). LMeFeOtBu (0.190 mmol, 101 mg) was
dissolved in Et2O (3 mL). To this yellow solution was added AgOTf
(0.19 mmol, 49 mg). The mixture immediately became dark in
color, and a mirror of Ag developed on the bottom of the reaction
vessel. After 2 h, the mixture was dried under vacuum and extracted
Supporting Information Available: Additional figures and
crystallographic data in CIF format. This material is available free
of charge via the Internet at http://pubs.acs.org.
IC035483X
Inorganic Chemistry, Vol. 43, No. 10, 2004 3321