Paramagnetic bis(amidinate) iron(II) complexes and their diamagnetic dicarbonyl derivatives

Article abstract of DOI:10.1002/1099-0682(200103)2001:3<707::aid-ejic707>3.0.co;2-z

Reactions of two equivalents of the lithium amidinate salts Li[tBuC(NR)₂] (R = iPr, cyclohexyl) with FeCl₂ have been found to yield paramagnetic bis(amidinate) iron(II) compounds of the type [tBuC(NR)₂]₂Fe. In the case of R = cyclohexyl, the product has been characterized by single-crystal X-ray diffraction analysis as having a distorted tetrahedral geometry. The derivative with R = iPr is an oil, but its ¹H and ¹³C NMR spectra indicate a similar monomeric structure. Both species have been found to react readily with CO to give the new diamagnetic Fe^II dicarbonyls [tBuC(NR)₂]₂Fe(CO)₂. The compound with R = iPr has been structurally characterized, which showed it to have a strongly distorted octahedral structure with the carbonyls in a cis arrangement.

Full text of DOI:10.1002/1099-0682(200103)2001:3<707::aid-ejic707>3.0.co;2-z

FULL PAPER
Paramagnetic Bis(amidinate) Iron(II) Complexes and their Diamagnetic
Dicarbonyl Derivatives[‡]
Beatrice Vendemiati,[a] Giansiro Prini,[b] Auke Meetsma,[a] Bart Hessen,*[a] Jan H. Teuben,[a]
and Orazio Traverso[b]
Keywords: Iron / N ligands / Magnetic properties / X-ray diffraction / Carbonyl complexes
Reactions of two equivalents of the lithium amidinate salts
Li[tBuC(NR)2] (R = iPr, cyclohexyl) with FeCl2 have been
found to yield paramagnetic bis(amidinate) iron(II) com- to
pounds of the type [tBuC(NR)2]2Fe. In the case of R = cyclo-
hexyl, the product has been characterized by single-crystal
X-ray diffraction analysis as having a distorted tetrahedral
geometry. The derivative with R = iPr is an oil, but its 1H
and 13C NMR spectra indicate a similar monomeric structure.
Both species have been found to react readily with CO
give
the
new
diamagnetic
FeII
dicarbonyls
[tBuC(NR)2]2Fe(CO)2. The compound with R = iPr has been
structurally characterized, which showed it to have a strongly
distorted octahedral structure with the carbonyls in a cis ar-
rangement.
Introduction
We are interested in the chemistry of iron(II) amidinates,
especially with respect to their Lewis acidic behavior, their
redox chemistry, and their potential as catalyst precursors.
We describe herein the synthesis and characterization of
paramagnetic monomeric iron(II) complexes of the highly
substituted amidinate ligands [tBuC(NR)2]Ϫ (R ϭ iPr,
cyclohexyl).[9] The bis(amidinate) species [tBuC(NR)2]2Fe
has been structurally characterized for R ϭ Cy. These com-
plexes appear to be quite light-sensitive. They react with
CO to give the new distorted octahedral dicarbonyls cis-
[tBuC(NR)2]2Fe(CO)2, of which the derivative with R ϭ iPr
has also been structurally characterized.
The amidinates [RC(NRЈ)2]Ϫ (R ϭ H, alkyl, aryl; RЈ ϭ
alkyl, aryl, SiMe3) constitute
a
class of versatile
monoanionic ancillary ligands for transition metals.[1] They
can bind in a chelating N,NЈ-η2 fashion to a single metal
center, or form N,NЈ-µ bridges between two metals. In the
latter case, ‘‘lantern’’ complexes of the type {[µ-
RC(NRЈ)2]2M}2 can be formed that may contain very short
MϪM bonding contacts.[2,3] Amidinates have also found
extensive use as ancillary ligands in catalytically active
metal complexes, e.g. in Group 4 metal mono- and bis(ami-
dinate) complexes, which can be activated with methylalu-
moxane (MAO) to give active olefin polymerization cata-
lysts,[4] in cationic aluminum amidinate alkyls,[5] and in
neutral (alkyl)bis(amidinate)vanadium ethene oligomeriz-
ation catalysts.[6] To date, the chemistry of amidinate com-
plexes of iron has yielded only a few well-defined com-
plexes. For FeII, one type of dimeric bis(amidinate) complex
has been reported, {[RC(NPh)2]2Fe}2 (R ϭ H, Ph), which
has a ‘‘twisted’’ A-frame structure with two bridging and
two dihapto amidinate ligands.[7] In addition, one example
of a monomeric bis(amidinate) FeII complex is known,
which contains amidinate ligands bearing ferrocenyl sub-
stituents on their backbone carbons, i.e. {[FcC(NCy)2]2Fe}
(Fc ϭ ferrocenyl).[8] As yet, no details of the reactivities of
these complexes have been reported.
Results and Discussion
The lithium amidinates Li[tBuC(NR)2] (R ϭ iPr, Cy)
used in this study were readily available from the reaction
of the corresponding carbodiimides with tBuLi.[9] Reaction
of two equivalents of Li[tBuC(NCy)2] with FeCl2 in THF
solution produced a brown-yellow colored solution, which
gradually turned red-brown on exposure to ambient light.
When the reaction mixture was worked-up under these con-
ditions, a significant amount of a red-brown oil was reco-
vered, which hampered isolation of the desired product.
However, when the reaction and subsequent workup were
performed with the exclusion of light, extraction with and
crystallization from pentane yielded the bis(amidinate) FeII
complex [tBuC(NCy)2]2Fe (1a) as yellow crystals in 54%
isolated yield (Scheme 1). An analogous procedure with
Li[tBuC(NiPr)2] resulted in a relatively low yield of a deep-
brown/yellow oil that resisted all attempts to crystallize it.
NMR spectroscopy (vide infra) indicated that this oil con-
sisted mainly of [tBuC(NiPr)2]2Fe (1b), although some im-
purities were present.
[‡]
Netherlands Institute for Catalysis Research (NIOK)
publication RUG-00-4-3.
[a]
Center for Catalytic Olefin Polymerization, Stratingh Institute
for Chemistry and Chemical Engineering, University of
Groningen,
Nijenborgh 4, 9747 AG Groningen, The Netherlands
E-mail: hessen@chem.rug.nl
[b]
´
Dipartimento di Chimica, Centro di Studio su Fotoreattivita e
´
Catalisi, Universita di Ferrara,
Via Borsari 46, 44100 Ferrara, Italy
Eur. J. Inorg. Chem. 2001, 707Ϫ711
WILEY-VCH Verlag GmbH, 69451 Weinheim, 2001
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707

B. Vendemiati, G. Prini, A. Meetsma, B. Hessen, J. H. Teuben, O. Traverso
FULL PAPER
with µeff ϭ 4.82 and θ ϭ Ϫ1.36 K, consistent with a mag-
1
netically dilute solid with tetrahedral d6 ions (S ϭ 2). H
NMR spectra of the compounds 1 in C6D6 (25 °C) con-
sequently show broad resonances, but nevertheless allow as-
signments to be made. For 1b, three resonances are ob-
served at δ ϭ 190.4, 8.8, and Ϫ2.2 (in a 2:9:12 ratio) which
can be attributed to the iPr CH, tBu CH3, and iPr CH3
groups, respectively. For 1a, the first two resonances are
also present (at δ ϭ 188.6 and 9.0), but these are accompan-
ied by additional resonances (at δ ϭ 11.9 and 10.7, as well
as a group of less-well-resolved resonances in the δ ϭ 4Ϫ0
Scheme 1. Formation of the bis(amidinate) iron(II) complexes
A crystal structure determination of 1a was performed
(Figure 1; selected interatomic distances and angles are
listed in Table 1). It showed the compound to be mono-
meric, with two dihapto amidinate ligands and the iron in
a distorted tetrahedral environment. The geometry of the
two FeϪNϪCϪN four-membered rings is essentially planar
[largest deviation seen in the FeϪN(3)ϪC(30)ϪN(4) dihed-
ral angle of 10.3(2)°, FeϪN(1)ϪC(13)ϪN(2) ϭ Ϫ4.9(2)°].
The angle between these two least-squares planes is
86.4(1)°. The structure is similar to that of the only other
region of the spectrum) that can be attributed to the cyclo-
hexyl methylene protons. NMR spectroscopy thus suggests
that complexes 1a and 1b have a similar structure in solu-
tion. The 13C NMR spectrum of 1b (C6D6, 25 °C) consists
of two broad resonances at δ ϭ 336 and 310, most probably
attributable to the tBu and iPr methyl groups, respectively,
and a third broad feature at δ ϭ 661. The assignment of
the latter resonance is ambiguous, as it could feasibly be
attributed to the iPr CH group or the tBu quaternary car-
bon. The 13C NMR spectrum of 1a shows similar reson-
ances (at δ ϭ 628, 335, and 304), with two additional nar-
rower resonances at δ ϭ 27.9 and 20.1, associated with the
δ- and γ-CH2 groups, respectively, of the cyclohexyl moiety.
Both 14-electron bis(amidinate) FeII complexes 1 react
with CO in hexane solution in the absence of light to give
the diamagnetic carbonyl derivatives [tBuC(NR)2]2Fe(CO)2
(R ϭ Cy, 2a; iPr, 2b), which were obtained in the form of
orange crystals (Scheme 2). The IR spectra of these com-
pounds show two carbonyl vibrations [2a: ν(CO) ϭ 1999
and 1929 cmϪ1], indicative of a cis-dicarbonyl structure.
known
monomeric
bis(amidinate)
FeII
complex,
{[FcC(NCy)2]2Fe},[8] although in the latter the average
˚
˚
FeϪN distance is longer (2.037 A vs. 2.020 A in 1a) and
the dihapto amidinates are less symmetrically bound (the
largest difference between the FeϪN distances for one
˚
˚
amidinate ligand is 0.025 A vs. 0.011 A in 1a).
1
The H and 13C NMR spectra of the derivative with R ϭ
iPr (2b) show the resonances of one tBu group and two
nonequivalent iPr groups, each with two diastereotopic
methyl groups. This, together with the IR data, suggests a
C2-symmetric octahedral cis-bis(η2-amidinate)Fe(CO)2
structure for 2b that is nonfluxional on the NMR time scale
1
at ambient temperature. The H NMR spectrum of 2a is
less-well-resolved, but is consistent with the same structure
type. For both complexes, the carbonyl 13C NMR reson-
ance is found at δ ϭ 219.
Figure 1. Molecular structure of 1a; hydrogen atoms are omitted
for clarity
˚
Table 1. Selected interatomic distances (A) and bond angles (°)
in 1a
FeϪN(1)
2.014(2)
2.025(2)
2.026(2)
2.015(2)
65.38(8)
65.15(8)
137.38(8)
109.2(2)
135.8(2)
133.8(2)
N(1)ϪC(13)
1.336(3)
1.340(3)
1.328(3)
1.347(3)
143.41(8)
124.71(8)
132.90(8)
108.9(2)
133.8(2)
137.3(2)
FeϪN(2)
N(2)ϪC(13)
FeϪN(3)
FeϪN(4)
N(3)ϪC(30)
N(4)ϪC(30)
N(1)ϪFeϪN(2)
N(3)ϪFeϪN(4)
N(1)ϪFeϪN(3)
N(1)ϪC(13)ϪN(2)
FeϪN(1)ϪC(1)
FeϪN(3)ϪC(18)
N(1)ϪFeϪN(4)
N(2)ϪFeϪN(3)
N(2)ϪFeϪN(4)
N(3)ϪC(30)ϪN(4)
FeϪN(2)ϪC(7)
FeϪN(4)ϪC(24)
Scheme 2. Formation of the bis(amidinate) iron(II) dicarbonyl
complexes
The bis(amidinate) FeII compounds 1 are paramagnetic.
Magnetic susceptibility measurements on solid 1a showed
An X-ray crystal structure determination of 2b corrobor-
CurieϪWeiss behavior over the temperature range 5Ϫ300 K ated the conclusions drawn from the spectroscopic studies.
708
Eur. J. Inorg. Chem. 2001, 707Ϫ711

Paramagnetic Bis(amidinate) Iron(II) Complexes
FULL PAPER
helpful in establishing their structural relationship, which
was especially useful as 1b could not be crystallized. We are
presently studying the Lewis acidities, electrochemistry, and
photochemistry of the new complexes. Preliminary results
suggest that the binding of the two CO molecules to the
(amidinate)2Fe moiety can be readily reversed upon irradi-
ation.
Experimental Section
General: All experiments were performed under a nitrogen atmo-
sphere using standard Schlenk, glove-box, and vacuum line tech-
niques. All manipulations involving compounds 1 were performed
in the absence of light, the samples being protected by enveloping
glassware in a black plastic bag whenever possible. Solvents (pent-
ane, hexane, THF) were distilled from Na/K alloy prior to use.
Deuterated benzene was dried over Na/K alloy and vacuum trans-
Figure 2. Molecular structure of 2b; hydrogen atoms are omitted
for clarity
˚
Table 2. Selected interatomic distances (A) and bond angles (°)
in 2b
ferred before use. The Li salts Li[tBuC(NR)2] (R ϭ Cy, iPr)[9] and
[12]
anhydrous FeCl2
were prepared according to literature proced-
FeϪN(1)
1.986(1)
2.031(1)
1.771(2)
62.53(5)
90.48(6)
99.85(6)
165.01(5)
93.09(7)
N(1)ϪC(1)
1.342(2)
1.324(2)
1.143(2)
104.21(5)
90.65(6)
163.99(6)
90.01(5)
179.1(1)
FeϪN(2)
N(2)ϪC(1)
ures. Ϫ NMR spectra were recorded on Varian VXR-300 or Unity
500 spectrometers. The 1H NMR spectra were referenced to the
resonances of residual protons in the deuterated solvent. Chemical
shifts (δ) are given relative to tetramethylsilane (downfield shifts
are positive). Ϫ IR spectra were recorded on a Mattson 4020 Gal-
axy FT-IR spectrophotometer. Ϫ Elemental analyses were per-
formed at the Microanalytical Department of the University of
Groningen; all data are the average of at least two independent
determinations. Ϫ Magnetic susceptibility measurements on solid
1a were performed on an MPMS-7 Quantum Design instrument
under zero-field cooled conditions (1000 T field, 5Ϫ300 K temper-
ature range). The EMU and temperature data are the average of
three independent determinations; µeff was calculated from the total
spin quantum number S ϭ 1.96 obtained from the CurieϪWeiss
law (C ϭ 2.90, θ ϭ Ϫ1.35 K).
FeϪC(12)
OϪC(12)
N(1)ϪFeϪN(2)s
N(1)ϪFeϪC(12)
N(1)ϪFeϪC(12a)
N(1)ϪFeϪN(1a)
C(12)ϪFeϪC(12a)
N(1)ϪFeϪN(2a)
N(2)ϪFeϪC(12)
N(2)ϪFeϪC(12a)
N(2)ϪFeϪN(2a)
FeϪC(12)ϪO
The structure is shown in Figure 2, while pertinent in-
teratomic distances and angles are listed in Table 2. The
compound crystallizes in the space group C2/c, with a C2
symmetry axis passing through the Fe atom. The amidinate
ligands are again bound in a dihapto fashion with the
FeNCN ring having a planar geometry [the dihedral angle
FeϪN(1)ϪC(1)ϪN(2) is Ϫ4.6(1)°]. The two Fe-amidinate
planes, related by C2 symmetry, are essentially orthogonal
Preparation of [tBuC(NCy)2]2Fe (1a): All manipulations were per-
formed under the exclusion of light (vide supra). To a stirred sus-
˚
[87.49(8)°]. The FeϪN(2) distance is 0.045 A longer than
the FeϪN(1) distance, as a consequence of the trans-posi- pension of FeCl2 (0.653 g, 5.51 mmol) in THF (40 mL), solid Li[t-
BuC(NCy)2] (2.843 g, 10.5 mmol) was added at ambient temper-
ature. After stirring for 2 h, the solvent was removed in vacuo, and
then any residual THF was removed by stirring the mixture with
pentane (20 mL) and subsequently pumping off the volatiles. Ex-
traction of the residue with pentane (30 mL), concentration of the
extract, and cooling it to Ϫ25 °C afforded 1.640 g (2.81 mmol,
54%) of 1a as analytically pure yellow crystals. Ϫ 1H NMR
([D6]benzene, 25 °C): δ ϭ 188 (∆ν1/2 ϭ 280 Hz, 4 H, iPr CH), 11.9
(∆ν1/2 ϭ 40 Hz, 8 H, Cy CH2), 10.7 (∆ν1/2 ϭ 60 Hz, 8 H, Cy CH2),
9.0 (∆ν1/2 ϭ 60 Hz, 18 H, tBu Me), 2.7 (∆ν1/2 ϭ 470 Hz, 8 H, Cy
tion of N(2) in relation to the CO ligand. Compared to
the two known FeII cis-dicarbonyl complexes with bidentate
monoanionic ligands — the phosphanyl-enolate and phos-
phanyl-carboxylate derivatives [Ph2PCHC(Ph)O]2Fe(CO)2
[10]
[11]
and [Ph2PCH2C(O)O]2Fe(CO)2
— complex 2b has a
much more strongly distorted octahedral geometry. This is
due to the very small ‘‘bite angle’’ of the dihapto amidinate
ligand, i.e. N(1)ϪFeϪN(2) is just 64.53(5)°, as compared to
OϪFeϪP angles of 82Ϫ86° in the other two complexes. In
all three complexes, the CϪFeϪC angle is about 93°, and CH2), 1.5 (∆ν1/2 ϭ 73 Hz, 4 H, Cy CH2), 1.1 (∆ν1/2 ϭ 33 Hz, 4 H,
Cy CH2), 0.5 (∆ν1/2 ϭ 785 Hz, 8 H, Cy CH2). Ϫ 13C{1H} NMR
([D6]benzene, 25 °C): δ ϭ 628 (assignment uncertain), 335 (tBu
hence this feature would appear to be quite insensitive to
changes in the ligand. In the IR spectra, the carbonyl vibra-
Me), 304 (Cy β-CH2), 27.9 (Cy δ-CH2), 20.1 (Cy γ-CH2). Ϫ
tions for 2 are found at noticeably lower wavenumbers than
C34H62N4Fe (582.7): calcd. C 70.08, H 10.72, N 9.61, Fe 9.58;
for the phosphanyl-carboxylate and -enolate complexes
found C 69.74, H 10.79, N 9.39, Fe 9.75.
(2048, 1998 cmϪ1 and 2023, 1969 cmϪ1, respectively), indic-
ating that the amidinate ligands are better donors.
In conclusion, we have prepared two paramagnetic 14-
Preparation of [tBuC(NiPr)2]2Fe (1b): Following a similar proced-
ure as described above for 1a, but using Li[tBuC(NiPr)2] (0.934 g,
4.91 mmol) and FeCl2 (0.308 g, 2.43 mmol), 0.26 g (0.61 mmol,
25%) of crude 1b was obtained as a brown-yellow oil upon evapora-
electron bis(amidinate) iron(II) complexes and their dia-
magnetic 18-electron dicarbonyl derivatives. 1H and 13C
NMR spectroscopy of the paramagnetic complexes proved tion of the solvent from the pentane extract. Ϫ 1H NMR ([D6]ben-
Eur. J. Inorg. Chem. 2001, 707Ϫ711
709

B. Vendemiati, G. Prini, A. Meetsma, B. Hessen, J. H. Teuben, O. Traverso
FULL PAPER
Table 1. Data relating to the crystal structure determinations of 1a and 2b
1a
2b
Empirical formula
Formula mass
Crystal size (mm)
Temperature [K]
Radiation
C34H62FeN4
582.74
C24H46FeN4O2
478.50
0.25 ϫ 0.30 ϫ 0.38
130
0.20 ϫ 0.25 ϫ 0.50
130
Mo-Kα
Mo-Kα
˚
Wavelength [A]
0.71073
0.71073
Crystal system
Space group
monoclinic
P21/n
monoclinic
C2/c
˚
a [A]
10.872(2)
16.962(3)
18.644(1)
93.69(1)
3431.0(9)
4, 1.128
0.466
17.195(1)
8.282(1)
˚
b [A]
˚
c [A]
18.779(1)
103.099(7)
2604.7(4)
4, 1.220
β [°]
3
˚
Volume [A ]
Z, calcd. density [MgmϪ3
]
µ [mmϪ1
Scan
]
0.605
ω/2θ
1280
ω/2θ
F(000)
1040
θ range [°]
1.10 to 26.0
Ϫ13 Յ h Յ 13
0 Յ k Յ 20
Ϫ22 Յ l Յ 0
7232/6702
4892
1.11 to 27.0
Ϫ21 Յ h Յ 21
Ϫ10 Յ k Յ 0
Ϫ23 Յ l Յ 23
6057/2841
2647
Index ranges
Refl. collected/unique
Refl. observed [I Ͼ 4.0σ(I)]
Parameters refined
wR(F2)[a]
600
0.1004
222
0.0913
(for F2o Ͼ 0)
Weighting scheme: a, b
0.0441, 1.234
0.0443
Ϫ0.30, 0.26(6)
0.0595, 2.3807
0.0347
Ϫ0.40, 1.90(7)
R(F)[b] [for Fo Ͼ 4.0 σ(Fo)]
3
˚
Diff. peak and hole (e/A )
[a]
2 2
2 2
2
2
2
[b]
wR(F2) ϭ {Σ[w(Fo2 Ϫ Fc ) ]/Σ[w(Fo ) ]}1/2, where w ϭ 1/[σ2(Fo ) ϩ (aP)2 ϩ bP] and P ϭ [max(Fo ,0) ϩ 2Fc ]/3. Ϫ R(F) ϭ Σ(||Fo| Ϫ
|Fc||)/Σ|Fo|.
zene, 25 °C): δ ϭ 190 (∆ν1/2 ϭ 240 Hz, 4 H, iPr CH), 8.8 (∆ν1/
cooled to Ϫ25 °C. This led to the deposition of 0.158 g (0.33 mmol,
28% overall) of orange crystalline 2b; m.p. 112Ϫ113 °C. Ϫ 1H
ϭ 55 Hz, 18 H, tBu Me), Ϫ2.2 (∆ν1/2 ϭ 305 Hz, 24 H, iPr Me).
2
3
Resonances attributable to a diamagnetic impurity (probably de-
rived from the ligand) are visible in the NMR spectrum at around
NMR ([D6]benzene, 25 °C): δ ϭ 4.58 (quint., JHH ϭ 5.8 Hz, 2 H,
3
iPr CH), 3.95 (quint., JHH ϭ 6.3 Hz, 2 H, iPr CH), 1.50 and 1.19
δ ϭ 3Ϫ4 and 0.6Ϫ1.6. Ϫ 13C{1H} NMR ([D6]benzene, 25 °C): δ ϭ (d, JHH ϭ 5.8 Hz, 6 H each, iPr Me), 1.27 and 1.15 (d, JHH
ϭ
3
3
661 (assignment uncertain), 336 (tBu Me), 310 (iPr Me).
6.3 Hz, 6 H each, iPr Me), 1.17 (s, 18 H, tBu). Ϫ 13C (APT) NMR
([D6]benzene, 25 °C): δ ϭ 219.4 (CO), 177.1 (NCN), 49.6 and 48.5
(NCH), 41.1 [C(CH3)], 30.7 [C(CH3)3], 27.5, 26.3, 24.7, and 22.8
[CH(CH3)2]. Ϫ IR (Nujol, KBr): ν˜ ϭ 2009, 1942 cmϪ1 [ν(CO)].
Preparation of [tBuC(NCy)2]2Fe(CO)2 (2a): A 50 mL flask was
charged with 1a (0.323 g, 0.55 mmol) and hexane (10 mL). The
flask was attached to a vacuum line, the solution was degassed by
several freeze-pump-thaw cycles, and then CO (140 Torr, 1.2 mmol)
was admitted. The mixture was allowed to stand (protected from
light) for two days at ambient temperature. It was then concen-
trated and cooled to Ϫ25 °C, whereupon orange bar-shaped crys-
tals were deposited. Yield: 0.244 g (0.38 mmol, 69%) of analytically
X-ray Crystal Structure Determinations: Diffraction data were col-
lected on an EnrafϪNonius CAD4-F diffractometer. Pertinent
crystallographic data and information concerning the data collec-
tion and residuals can be found in Table 3. For 1a, the cell para-
meters were derived from the setting angles of 22 reflections in the
range 16.37° Յ θ Յ 21.54°; for 2b from 22 reflections in the range
17.95° Յ θ Յ 20.39°. Intensity data were corrected for Lorentz and
polarization effects, but not for absorption. The structures were
solved by Patterson methods and the models were extended by dir-
ect methods applied to difference structure factors using the pro-
gram DIRDIF.[13] In both structures, all hydrogen atoms were loc-
ated and refined with isotropic displacement parameters. Final re-
finement on F2 was carried out by full-matrix least-squares tech-
niques. Calculations were performed with the program packages
SHELXL[14] (least-squares refinement) and PLATON[15] (geomet-
ric data). Crystallographic data (excluding structure factors) for the
1
pure 2a. Ϫ H NMR ([D6]benzene, 25 °C): δ ϭ 4.19 (m, 2 H, Cy
CH), 3.53 (m, 2 H, Cy CH), 2.2Ϫ1.5 (m, 32 H, Cy CH2), 1.26
(s, 18 H, tBu Me), 1.20 (m, 8 H, Cy CH2). Ϫ 13C (APT) NMR
([D6]benzene, 25 °C): δ ϭ 219.4 (CO), 176.6 (NCN), 57.4 and 58.0
(NCH), 40.9 [C(CH3)], 38.6, 36.3, 35.1, and 32.3 (CH2), 30.5
[C(CH3)3], 26.5 (2ϫ), 26.2, 26.1, 25.9, and 25.8 (CH2). Ϫ IR (Nu-
jol, KBr): ν˜ ϭ 1999, 1929 cmϪ1 [ν(CO)]. Ϫ C36H62FeN4O2 (638.8):
calcd. C 67.69, H 9.78, N 8.77, Fe 8.74; found C 67.56, H 9.67, N
8.58, Fe 8.60.
Preparation of [tBuC(NiPr)2]2Fe(CO)2 (2b): For this preparation,
compound 1b was first generated by reaction of FeCl2 (0.150 g,
1.18 mmol) with Li[tBuC(NiPr)2] (0.450 g, 2.36 mmol) in THF structures reported in this paper have been deposited with the Cam-
(15 mL) as described above, and the product was extracted with
hexane (20 mL). A flask charged with this hexane extract was at-
tached to a vacuum line and the solution was degassed; thereafter
CO (1 bar) was admitted. After allowing the solution to stand for
two days (protected from light), it was filtered, concentrated, and
bridge Crystallographic Data Centre as supplementary publication
nos. CCDC-147087 for 1a and -147086 for 2b. Copies of the data
can be obtained free of charge on application to the CCDC,
12 Union Road, Cambridge CB2 1EZ, U.K. [Fax: (internat.) ϩ 44-
1223/336-033; E-mail: deposit@ccdc.cam.ac.uk].
710
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