A weak interaction between iron and uranium in uranium alkyl complexes supported by ferrocene diamide ligands

Article abstract of DOI:10.1021/om700541u

Alkyl complexes of uranium supported by ferrocene diamide ligands are described along with the corresponding cationic species. Synthesis of uranium dialkyl compounds fc(NSi^tBuMe₂)₂UR₂ [R = Np (neo-pentyl), Bz (henzyl)] was accomplished by salt metathesis between fc(NSi^tBuMe₂)₂UI₂(THF) and neo-pentyl lithium or benzyl potassium. Protonation of one alkyl ligand led to the isolation of a cationic uranium alkyl complex. DFT calculations and X-ray crystallography data support the existence of a donor-acceptor, weak interaction between iron and uranium.

Full text of DOI:10.1021/om700541u

1702
Organometallics 2008, 27, 1702–1706
A Weak Interaction between Iron and Uranium in Uranium Alkyl
Complexes Supported by Ferrocene Diamide Ligands
Marisa J. Monreal and Paula L. Diaconescu*
Department of Chemistry & Biochemistry, UniVersity of California, Los Angeles, California 90095
ReceiVed June 1, 2007
Alkyl complexes of uranium supported by ferrocene diamide ligands are described along with the
corresponding cationic species. Synthesis of uranium dialkyl compounds fc(NSitBuMe2)2UR2 [R ) Np
(neo-pentyl), Bz (benzyl)] was accomplished by salt metathesis between fc(NSitBuMe2)2UI2(THF) and
neo-pentyl lithium or benzyl potassium. Protonation of one alkyl ligand led to the isolation of a cationic
uranium alkyl complex. DFT calculations and X-ray crystallography data support the existence of a
donor–acceptor, weak interaction between iron and uranium.
The organometallic chemistry of uranium has been dominated
around iron;19 and (iii) a weak interaction of donor–acceptor
type19–23 could occur between iron and uranium that might
influence the reactivity of the complex. Incorporation of
electron-rich ferrocene ligands has been shown19 to stabilize a
highly reactive, cationic titanium species, an example of a d0
metal compound. To this end, we synthesized mono(1,1′-
diamidoferrocene) complexes of tetravalent uranium and report
our studies on the interaction between uranium and iron in a
cationic benzyl complex.
by cyclopentadienyl complexes.1,2 Recently, new reactivity
patterns, beyond those of d elements, have been reported for
noncyclopentadienyl complexes.3–6 Similarly, although alkyl
complexes of uranium(IV) have been investigated for cyclo-
pentadienyl systems,7–13 systems supported by other ancillary
ligands are increasingly studied.14–18 We are interested in
exploiting ferrocene 1,1′-disubstituted ligands as versatile
frameworks to support uranium(IV) alkyl moieties. Such ligands
have the following characteristics: (i) the 1,1′-disubstituted
ferrocene enforces cis-coordination of the two donors, thus only
one side of the metal center is blocked; (ii) the ferrocene
backbone has the ability to accommodate changes in the
electronic density at a metal center by varying the geometry
Results and Discussion
24
The reaction between UI3(THF)4
and [K2(OEt2)2]-
25
fc(NSitBuMe2)2 in tetrahydrofuran led to a mixture of
compounds, U(fc(NSitBuMe2)2)2 and fc(NSitBuMe2)2UI2-
(THF) (1-I2(THF)), from which the latter was separated as a
consequence of its insolubility in hexanes. Both products
obtained are uranium(IV) compounds, a common occurrence
when starting from UI3(THF)4;26,27 presumably, uranium(III)
disproportionates to uranium(IV) and some form of uranium(0),
which is removed by filtration. Several reports show that
employing a stoichiometry consistent with this disproportion-
ation reaction increases the yield of the uranium(IV) product.26,27
Optimization of the reaction conditions allowed the isolation
of 1-I2(THF) in 60% yield reproducibly. Reactions between
compound 1-I2(THF) and alkyl sources (benzyl potassium, neo-
pentyl lithium) led to uranium dialkyl complexes (Scheme 1,
only the uranium dibenzyl complex is shown; for the synthesis
of the analogous di-neo-pentyl complex, see the Supporting
Information). The yields of these reactions vary with the
* Corresponding author. E-mail: pld@chem.ucla.edu.
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10.1021/om700541u CCC: $40.75
2008 American Chemical Society
Publication on Web 03/25/2008

Weak Interaction between Iron and Uranium
Organometallics, Vol. 27, No. 8, 2008 1703
Scheme 1. Syntheses of fc(NSitBuMe2)2UBz2, 1-Bz2, and [fc(NSitBuMe2)2UBz(OEt2)][BPh4], [1-Bz(OEt2)][BPh4]
solubility of the products in nonpolar solvents, but
fc(NSitBuMe2)2UBz2, 1-Bz2 (Bz ) CH2Ph) could be obtained
in 86% yield after recrystallization. The X-ray crystal structure
of 1-Bz2 was not of very good quality and therefore discussion
of the structural parameters is not attempted here. It was of
reasonable quality though to indicate atom connectivity (see the
Supporting Information). The only other reported actinide(IV)
dialkyl complexes supported by dianionic, noncyclopentadienyl
ligands are [(DIPPNCOCN)U(CH2SiMe3)2] and [(tBuNON)-
MR2] (M ) Th or U; R ) C3H5 or CH2SiMe3; DIPPNCOCN )
O(CH2CH2NAr)2; tBuNON ) O(SiMe2NtBu)2; Ar ) 2,6-diiso-
propylphenyl),14,15 and [LTh(CH2SiMe3)2] (L ) 2,6-bis(2,6-
diisopropylanilidomethyl)pyridine, 4,5-bis(2,6-diisopropylanilino)-
2,7-ditert-butyl-9,9-dimethylxantheno).28
With the dibenzyl uranium complex in hand, we set out to
investigate the formation of a cationic complex. Alkyl cationic
uranium complexes are not numerous,29 presumably because
of their high reactivity. The reaction between 1-Bz2 and
[Et3NH][BPh4] led to [fc(NSitBuMe2)2U(CH2Ph)(OEt2)][BPh4]
([1-Bz(OEt2)][BPh4]), which was recrystallized from Et2O/
toluene. The X-ray crystal structure of [1-Bz(OEt2)][BPh4]
(Figure 1) shows an η2-coordination of the benzyl group and a
pseudotetrahedral coordination around uranium (if contacts with
iron and the ipso-carbon atoms, ca. 2.8 Å, are ignored). The
U-Fe distance of 3.08 Å is similar to the Ti-Fe distance of
3.07 Å in [fc(NSiMe3)2TiMe][MeBPh3];19 the uranium ionic
radius being larger than that of titanium (1.00 Å vs 0.74 Å),30
this distance indicates a stronger interaction of iron with uranium
than with titanium. Since both metal complexes are alkyl cations,
the stronger interaction with uranium is in accord with its higher
Lewis acidity than that of titanium. The zirconium-iron distance
in a similar alkyl cation complex, [fc(NSiMe3)2ZrCH2Ph]-
[PhCH2B(C6F5)3], is 3.20 Å, more than 0.1 Å longer than the
distance found in [1-Bz(OEt2)][BPh4].31 Examples of iron-late
transition metal center interactions in complexes of 1,1′-
substituted ferrocene ligands have been previously reported and
also probed by X-ray crystallography.20–23 Other metrical
parameters in [1-Bz(OEt2)][BPh4] agree with reported ex-
amples; the average uranium-nitrogen distance of 2.20 Å is
similar to uranium-nitrogenamide distances in other cationic
complexes: 2.17 Å in [(η5-C5Me5)2U(NEt2)(THF)2][BPh4],32
2.18 Å in [(η8-C8H8)U(NEt2)(THF)2][BPh4],33 and 2.22 Å in
[(η5-C5Me5)2U(NMe2)(CNtBu)2][BPh4].34 The U-Cbenzyl dis-
tance of 2.48 Å is slightly longer than the U-CMe distance of
2.39 Å in [(η5-C5Me5)2UMe(THF)][MeBPh3];29 the shortening
of distances involving methyl groups has been reported before.35
The Fe-C distances average 2.07 Å, closer in value to those in
ferrocenium36 complexes rather than in ferrocene; in the latter
averaged distances are closer to 2.04 Å.37,38 The distances
observed in [1-Bz(OEt2)][BPh4] likely reflect the cationic nature
of the complex. One interesting feature is the fact that the
uranium atom sits slightly out of the plane formed by the two
nitrogens and the two carbons connected to them (dihedral
angles: C10-U1-N1-C5, 17.7° and C5-U1-N2-C10, 6.5°);
such a geometrical characteristic has been correlated to π-bond-
ing with the nitrogen atoms in other chelating diamide
complexes.39,40
To gain more insight into the nature of the iron-uranium
interaction we turned to DFT calculations. Geometry optimiza-
tions were performed on model methyl compounds. The
calculated iron-uranium distance is smaller for the alkyl cation
model [fc(NSiH3)2U(CH3)(OMe2)]+ (3.04 Å) than for the dialkyl
(28) Cruz, C. A.; Emslie, D. J. H.; Harrington, L. E.; Britten, J. F.;
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Figure 1. ORTEP representation of [fc(NSitBuMe2)2U(CH2Ph)-
(OEt2)][BPh4] ([1-Bz(OEt2)][BPh4]) with thermal ellipsoids at 35%
probability. Hydrogen atoms and disordered counterparts of some
atoms were omitted for clarity. The U-Fe distance is 3.08 Å.
Selected uranium distances (Å): U(1)-Fe(1), 3.0714(16); U(1)-N(1),
2.169(10);U(1)-N(2),2.236(11);U(1)-C(23),2.482(12);U(1)-O(1),
2.4112(63). Iron-carbon distances (Å): 2.031(11); 2.052(10);
2.054(11); 2.056(12); 2.062(11); 2.0638(97); 2.069(12); 2.073(14);
2.092(10); 2.112(10). Selected angles (deg): N(1)-U(1)-N(2),
140.5(3); C(23)-U(1)-N(1), 100.7(4); C(23)-U(1)-N(2), 96.9(5);
C(23)-U(1)-Fe(1),93.0(3);N(1)-U(1)-O(1),105.6(3);N(2)-U(1)-O(1),
90.4(3); C(23)-U(1)-O(1), 128.0(3).
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(31) Shafir, A.; Arnold, J. Organometallics 2003, 22, 567–575.
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Organometallics 2001, 20, 4993–4995.

1704 Organometallics, Vol. 27, No. 8, 2008
Monreal and Diaconescu
Figure 2. SOMO-6 of fc(NSiH3)2U(CH3)2 (left) and SOMO-5 of
[fc(NSiH3)2U(CH3)(OMe2)]+ (right) from geometry optimizations
performed with ADF2007.01.
Figure 3. Plot of µeff vs T for 1-Bz2 and [1-Bz(OEt2)][BPh4].
Figure 4. Chemical shift (δ) vs 1/T plot of the 1H NMR resonances
for 1-Bz2 (top, from –80 to 75 °C) and [1-Bz(OEt2)][BPh4] (bottom,
from –85 to 115 °C) in toluened-d8.
model fc(NSiH3)2U(CH3)2 (3.10 Å), similar to the trend
observed experimentally (3.08 vs 3.18 Å). Both model com-
plexes present one molecular orbital with an iron-uranium
interaction (Figure 2). The nature41 of these interactions is very
similartothatofzirconium-orthorium-platinuminteractions,42–44
beingofdonor–acceptortype.Inthezirconium-orthorium-platinum
interactions, a molecular orbital with contributions only from
the two metal centers was found; in the present case contribu-
tions from ligand atomic orbitals are also observed (Figure 2),
making comparison with the reported examples not possible.
We also investigated the magnetic properties of [1-Bz-
(OEt2)][BPh4]. Analyzing the magnetic moments for [1-Bz-
(OEt2)][BPh4] and 1-Bz2 in the temperature range 50–300 K
(Figure 3), we noticed that the value corresponding to the
uranium benzyl cation is considerably less than that correspond-
ing to 1-Bz2 (ca. 2.4 vs 3.2 µB). For uranium compounds, a
smaller value of the room temperature magnetic moment than
the one calculated for the free ion is a consequence of partly
quenching the orbital-angular momentum either because of a
lower symmetry or higher covalency than for the free ion (3H4
for U(IV)).45 Since [1-Bz(OEt2)][BPh4] and 1-Bz2 feature
similar coordination environments, we propose that the lower
magnetic moment for [1-Bz(OEt2)][BPh4] than for 1-Bz2 might
be consistent with the fact that the iron-uranium distance is
smaller in the uranium benzyl cation than in the uranium
dibenzyl complex, allowing for a better orbital overlap in the
former than in the latter case.
To characterize further the dibenzyl and benzyl cation
uranium complexes, we collected variable-temperature (VT) 1H
NMR spectroscopic data (Figure 4). For both complexes, 1-Bz2
and [1-Bz(OEt2)][BPh4], nonlinear dependence of the chemical
shifts (δ) versus 1/T was observed. The nonlinearity is more
pronounced for protons belonging to the cyclopentadienyl rings
and the benzyl fragment. For some protons, which would be
affected the most by the presence of unpaired electron density,
the CH2 benzyl group and some cyclopentadienyl protons (for
the cationic complex), we could not trace the chemical shift
over the entire range of temperature. Nonlinear dependence of
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Weak Interaction between Iron and Uranium
Organometallics, Vol. 27, No. 8, 2008 1705
received. 1H NMR spectra were recorded on Bruker300 or
Bruker500 spectrometers at room temperature in C6D6 unless
otherwise specified (the UCLA NMR spectrometers are supported
by NSF grant CHE-9974928). Chemical shifts are reported with
respect to internal solvent, 7.16 ppm (C6D6). CHN analyses were
performed by Desert Analytics (3860 S. Palo Verde Rd., Suite 303,
Tucson, AZ 85714) and UC Berkeley Micro-Mass facility (8 Lewis
Hall, College of Chemistry, University of California, Berkeley, CA
94720).
Synthesis of 1-Bz2. A 20 mL scintillation vial was charged with
1-I2(THF) (0.4508 g, 0. 448 mmol) and diethyl ether (10 mL) and
the suspension was cooled in a -35 °C freezer for 30 min. Benzyl
potassium (KBz, 0.1223 g, 0.941 mmol) was added to the stirring
suspension. The reaction mixture was stirred for 3 h, warming it
to room temperature. The reaction mixture was filtered through
Celite and washed with hexanes. The volatiles were removed under
reduced pressure, hexanes was added to the solid, and the solution
was filtered through Celite. Solvent was removed and the extraction/
filtration procedure was repeated. The dried product was dissolved
in hexanes and filtered through Celite one final time, concentrated,
and set to crystallize at -35 °C overnight. Total yield (over three
crops): 86%. 1H NMR (C6D6, 300 MHz, 25 °C): δ, 44.79 (s, 12H,
SiCH3), 29.53 (s, 18H, SiCCH3), -8.30 (s, 4H, Cp-CH), -12.91
(t, J ) 6.42 Hz, 2H, Ar-CH), -16.79 (s, 4H, Ar-CH), -17.84
(s, 4H, Ar-CH), -34.10 (s, 4H, Cp-CH), -154.46 (s, 2H,
Ar-CH2).
Figure 5. NIR spectra (toluene) for 1-Bz2 and [1-Bz(OEt2)][BPh4].
the chemical shift vs 1/T46,47 has been used as an indicator of
changes in the spin density at specific protons in a complex,
but the interpretation of such data is not straightforward.
The NIR spectra recorded at room temperature for both 1-Bz2
and [1-Bz(OEt2)][BPh4] showed absorption bands with ꢀ ≈ 102
M-1 cm-1 (Figure 5). These bands are consistent with f-f
transitions;48,49 similar spectra have been reported for (η5-
C5Me5)2UMe2.50 From Figure 5 it is apparent that the NIR
spectra for the two complexes are not analogous; some of the
lines for the two complexes have similar shapes, but the peaks
are present at rather different absorption energies. Considering
that the ligands are not the same in the two complexes, it is
difficult to interpret this difference in energies as a consequence
of other causes.
Anal. Calcd for C36H52FeN2Si2U: C, 50.11; H, 6.14. Found: C,
50.11; H, 5.91.
Synthesis of [1-Bz(OEt2)][BPh4]. A concentrated toluene solu-
tion of 1-Bz2 (0.1000 g, 0.116 mmol) was added to solid
[Et3NH][BPh4] (0.0488 g, 1 equiv). The reaction was stirred at room
temperature for 1 h and filtered through Celite. The solvent was
removed under reduced pressure and the dried product was
dissolved in toluene, filtered through Celite, layered with diethyl
ether, and placed in a -35 °C freezer to crystallize. Yield: 69%.
1H NMR (C6D6, 300 MHz, 25 °C): δ, 57.23 (s, 6H, Si-CH3), 37.12
(s, 18H, SiC(CH3)3), 36.67 (s, 6H, Si-CH3), 24.65 (s, CH2-Ar),
1.73 (s, 4H, OCH2CH3), -10.30 (s, 6H, OCH2CH3), -18.45 (s,
4H, Cp-H), -22.14 (s, 4H, Cp-H), -40.71 (s, 2H, Ar-H),
-41.76 (s, 2H, Ar-H), -44.96 (s, 1H, Ar-H).
In conclusion, a stable uranium benzyl cation complex
supported by a ferrocene diamide ligand was isolated and
characterized. DFT calculations and X-ray data support the
existence of a weak interaction between iron and uranium.
Experimental Section
Anal. Calcd for C57H75BFeN2OSi2U: C, 59.58; H, 6.58; N, 2.44.
Found: C, 60.25; H, 6.69; N, 2.17.
All experiments were performed under a dry nitrogen atmosphere
with standard Schlenk techniques or in an MBraun inert-gas
glovebox. Solvents were purified with use of a two-column solid-
state purification system by the method of Grubbs51 and transferred
to the glovebox without exposure to air. NMR solvents were
obtained from Cambridge Isotope Laboratories, degassed, and stored
over activated molecular sieves prior to use. Uranium turnings were
purchased from Argonne National Laboratories. UI3(THF)4,24
X-ray Crystal Structure of [1-Bz(OEt2)][BPh4]. X-ray quality
crystals were obtained from an Et2O/toluene solution placed in a
-35 °C freezer in the glovebox. Inside the glovebox, the crystals
were coated with oil (STP Oil Treatment) on a microscope slide,
which was brought outside the glovebox. The X-ray data collections
were carried out on a Bruker AXS single-crystal X-ray diffracto-
meter with use of Mo KR radiation and a SMART APEX CCD
detector. The data were reduced by SAINTPLUS and an empirical
absorption correction was applied, using the package SADABS.
The structure was solved and refined with SHELXTL (Brucker
1998, SMART, SAINT, XPREP, and SHELXTL, Brucker AXS
Inc., Madison, WI). Some atoms were disordered and therefore
modeled with different occupancies over two sites. Hydrogen atoms
were placed in calculated positions and refined isotropically. Crystal
25
KBz,52 [Et3NH][BPh4],53 and fc(NHSitBuMe2)2 were prepared
following published procedures. Other chemicals were used as
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and refinement data for [1-Bz(OEt2)][BPh4]: formula C284H360
-
B4N8Si8Fe4U4O4, space group Pna21, a ) 16.1229 Å, b ) 21.8507
Å, c ) 18.5574 Å, R ) ꢀ ) γ ) 90°, V ) 6537.71 Å3, Z ) 4, µ
) 2.78 mm-1, F(000) ) 2756, R1(based on F) ) 0.0689 for 9372
data (I > 2σ(I)), and wR2(based on all data) ) 0.1440.
Susceptibility Measurements. Measurements for each com-
pound were carried out on batches obtained independently until at
least two different experiments gave superimposable results. The
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tibility measurements were recorded with a SQUID magnetometer
at 5000 G. The samples were prepared in the glovebox (ca. 50
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1706 Organometallics, Vol. 27, No. 8, 2008
Monreal and Diaconescu
mg), loaded in a gelatin capsule that was positioned inside a plastic
straw, and carried to the magnetometer in a tube under N2. The
sample was quickly inserted into the instrument and centered and
data were obtained from 5 to 300 K. The contribution from the
sample holders was not accounted for. Effective magnetic moments
were calculated either by linear regression from plots of 1/ꢁmol
exchange and correlation corrections that are used by default by
the ADF2007.01 program suite. Calculations for both model
compounds were carried out with the spin-unrestricted, noncollinear,
ZORA spin–orbit relativistic formalism.
Acknowledgment. The authors thank Dr. Saeed Khan for
help with crystallography and Prof. Jeffrey Zink for allowing
the use of the UV–vis-NIR spectrometer. This work was
supported by the UCLA and the UC Energy Institute (EST
grant).
versus T (K) for Curie–Weiss behavior or by using the formula
1/2
2.828(Tꢁmol
)
for non-Curie–Weiss behavior.
DFT Calculations. The Amsterdam Density Functional (ADF)
package (version ADF2007.01) was used to do geometry optimiza-
tions on Cartesian coordinates of the model compounds specified
in the text. For the uranium and iron atoms, standard triple-ꢂ STA
basis sets from the ADF database ZORA TZP were employed with
1s-3p (Fe) and 1s-5d (U) electrons treated as frozen cores. For all
the other elements, standard double-ꢂ STA basis sets from the ADF
database ZORA DZP or DZ were used, with the 1s electrons treated
as frozen core for non-hydrogen atoms. The local density ap-
proximation (LDA) by Becke-Perdew was used together with the
Supporting Information Available: Experimental details for
synthesis of 1-I2(THF) and 1-Np2, X-ray crystal structures of
1-I2(THF) and 1-Bz2, NIR spectra, and DFT calculation details.
This material is available free of charge via the Internet at
http://pubs.acs.org.
OM700541U