New Benzylpotassium Reagents and Their Utility for the Synthesis of Homoleptic Uranium(IV) Benzyl Derivatives

Article abstract of DOI:10.1021/acs.organomet.5b00212

A new family of benzylpotassium reagents, KBn′(1-Bn′) (Bn′ = p-ⁱPrBn, p-^tBuBn, p-NMe₂Bn, p-SMeBn, m-OMeBn, o-OMeBn, 2-picolyl), was synthesized using a modified literature procedure and characterized by multinuclear NMR sp

Full text of DOI:10.1021/acs.organomet.5b00212

Article
pubs.acs.org/Organometallics
New Benzylpotassium Reagents and Their Utility for the Synthesis of
Homoleptic Uranium(IV) Benzyl Derivatives
Sara A. Johnson, John J. Kiernicki, Phillip E. Fanwick, and Suzanne C. Bart*
H.C. Brown Laboratory, Department of Chemistry, Purdue University, West Lafayette, Indiana 47907, United States
S
* Supporting Information
ABSTRACT: A new family of benzylpotassium reagents,
KBn′(1-Bn′) (Bn′ = p-iPrBn, p-tBuBn, p-NMe2Bn, p-SMeBn, m-
OMeBn, o-OMeBn, 2-picolyl), was synthesized using a modified
literature procedure and characterized by multinuclear NMR
spectroscopy. Combining four equivalents of 1-Bn′ with UCl4 at
low temperature in THF afforded the homoleptic uranium(IV)
derivatives 2-Bn′ (2-p-iPr, 2-p-tBu, 2-p-NMe2, 2-p-SMe, 2-o-
Picolyl, 2-m-OMe, 2-o-OMe). In addition to 1H NMR
spectroscopic characterization, structural studies of five of these
organouranium compounds (2-p-iPr, 2-p-tBu, 2-o-Picolyl, 2-m-
OMe, 2-o-OMe) were performed, showing that in many cases the
benzyl groups are coordinated in an η4-fashion, lending stability to
these otherwise low-coordinate molecules. In the cases of U(o-OMeBn)4 (2-o-OMe) and U(2-picolyl)4 (2-o-Picolyl),
heteroatom coordination to the uranium center is observed.
benzyl groups, which prevent unwanted side reactions that have
plagued previous synthetic attempts.5,6,10
INTRODUCTION
■
Metal alkyls serve as important intermediates in a variety of
industrial1 and biological2 processes; thus, new synthetic routes
are always desirable. Such species can be formed via
organometallic processes, including oxidative addition and β-
alkyl elimination, but salt metathesis using organolithium or
Grignard reagents remains a popular method for installation of
alkyl and aryl groups on metal centers.3,4 Less attention has
been devoted to other alkali metals, specifically sodium and
potassium, for their ability to install M−C bonds. Although
these reagents tend to be more reducing than their lithium
counterparts, the formation of insoluble NaX and KX salts is
advantageous as compared to LiX salts, which have increased
solubility in polar solvents typically required for organometallic
synthesis.
The successful isolation and characterization of 2-Bn
demonstrated that perhaps this synthetic route could be
expanded to produce organouranium species with alkyl-
substituted benzyls, benzyls with functional groups, and 2-
picoline. Along these lines, we targeted a general procedure to
synthesize a new family of substituted benzylpotassium
reagents, 1-Bn′ (Bn′ = p-iPrBn, p-tBuBn, p-NMe2Bn, p-
SMeBn, m-OMeBn, o-OMeBn, 2-picolyl), which would provide
diversity in steric, electronic, and coordination environments
for the resulting uranium complexes. Four members feature
benzyl groups substituted in the para position with electron-
donating groups, specifically iPr, tBu, NMe2, and SMe. Methoxy
derivatives are substituted in both the ortho and meta positions,
while the picolyl group has the nitrogen in the ortho position.
Given the variety of benzyl substituents, the synthesis for the
potassium salts, 1-Bn′, needs to be tolerant of functional groups
and easy to perform while generating appreciable quantities.
Since the original report of benzylpotassium by Gilman in the
1940s, many variations on its synthesis have been described.
One of the earliest, which avoided the use of potassium metal,
was that by Lochmann, who reported that addition of an
alkyllithium to a solution of potassium (−)-(1R)-menthoxide
was stirred at “a chosen temperature for the appropriate
time”.11 This procedure was a significant improvement over the
previously reported synthesis for benzylpotassium, which
involved treating potassium metal with organomercury
Historically, the generation and isolation of tetra(alkyl)
uranium species of the form UR4 has been attempted using
alkyllithiums, but was not successful due to instability of the
products at temperatures above 0 °C.5,6 We recently
demonstrated the utility of potassium alkyls, specifically
benzylpotassium, KCH2Ph, and its methylated derivatives,
7
KCH2-p-CH3C6H4 and KCH2-m-(CH3)2C6H3,8 in the syn-
thesis of the first thermally stable neutral homoleptic
uranium(IV) benzyl compounds.9 Treating UCl4 with four
equivalents of each of these salts at −108 °C (frozen THF)
resulted in formation and isolation of the corresponding
uranium products, U(CH2Ph)4 (2-Bn), U(CH2-p-CH3C6H4)4,
and U(CH2-m-(CH3)2C6H3)4, in moderate yields. These
reactions proceeded cleanly with elimination of KCl during
workup with diethyl ether.9 In the case of 2-Bn and its
derivatives, stability is imparted by the η4-coordination of the
Received: March 13, 2015
© XXXX American Chemical Society
A
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compounds.12 From this work stemmed the advancement
reported by Schlosser, who determined that commercially
available potassium tert-butoxide and n-butyllithium could be
combined with toluene to generate benzylpotassium cleanly.13
By eliminating elevated temperatures and potassium metal,
Schlosser’s procedure creates milder reaction conditions that
are amenable for functionalized toluenes. Thus, herein we
present a general synthesis for substituted benzylpotassium
reagents, 1-Bn′, by adaptation of this method, along with the
characterization of these salts by multinuclear NMR spectros-
copy. We also report the utility of 1-Bn′ in the generation of
homoleptic uranium(IV) benzyl compounds, 2-Bn′, which are
significant given the rarity of neutral homoleptic uranium alkyls.
These organouranium species have been fully characterized
using 1H NMR and electronic absorption spectroscopic
methods, as well as by X-ray crystallography.
synthesized these according to the same conditions as for the
rest of the 1-Bn′ family, as the previous procedure for 1-p-tBu
requires potassium 3-methyl-3-pentanolate, which is not
commercially available. The KOtBu/nBuLi method is also an
improvement over that used to generate 1-o-OMe,15 which was
previously performed by ether cleavage of the benzylmethyl
ether on a K mirror at −70 °C. Additionally, the synthesis for 1-
m-OMe was previously reported,15 but not accompanied by a
repeatable detailed synthetic procedure.
With the new family of substituted benzylpotassium reagents
in hand, attempts were made at the synthesis of the
corresponding neutral homoleptic uranium(IV) benzyl deriva-
tives, 2-Bn′. By analogy to 2-Bn, their synthesis using the 1-Bn′
family of salts was performed in frozen THF, and samples were
not handled in solution at room temperature for prolonged
periods. Mixing thawing THF solutions of UCl4 and four
equivalents of 1-Bn′ rapidly produced dark red or orange
reaction mixtures. Volatiles were removed immediately,
followed by workup in diethyl ether and filtration of KCl.
The new uranium tetrabenzyls were isolated as dark red-brown
solids in good yields and were stable in the solid state (eq 1).
RESULTS AND DISCUSSION
■
The 1-Bn′ family was generated by freezing a slurry of KOtBu
and an excess of the substituted toluene reagent (p-iPr, p-tBu, p-
NMe2, p-SMe, o-OMe, m-OMe) or 2-picoline. Upon thawing,
this solution was treated with a cooled solution of n-BuLi in
hexanes, which instantly produced a color change to bright
yellow or orange. Solutions were stirred at ambient temperature
for the time given in Table 1, at which point conversion to deep
Table 1. Reaction Times and Abbreviations for 1-Bn′
Exceptions to this are 2-p-NMe2 and 2-p-SMe, which were
found to decompose during workup. Successful synthesis of the
2-Bn′ family is significant, as substituted benzyl ligands on
uranium are rare.
1
The organouranium products were analyzed by H NMR
spectroscopy (23 °C, benzene-d6), and these data are presented
in Table 2 along with a comparison of 2-Bn. For the majority of
substituted toluene
reaction time (h)
abbreviation
CH3-p-iPr-C6H4
CH3-p-tBu-C6H4
CH3-C6H4N
3
3
1-p-iPr
1-p-tBu
1
Table 2. H NMR Spectroscopic Shifts for 2-Bn′ (C6D6, 23
4
1-o-Picolyl
1-p-NMe2
1-p-SMe
1-m-OMe
1-o-OMe
a
°C)
CH3-p-NMe2-C6H4
CH3-p-SMe-C6H4
CH3-m-OMe-C6H4
CH3-o-OMe-C6H4
2
7
benzyl-uranium
−CH2
x-CH
12
4
2-Bn
−30.41
−31.09
−32.07
−32.21
−13.99 (m), 9.48 (o), 0.39 (p)
−13.20 (m), 9.40 (o)
−13.78 (m), 9.86 (o)
16.15 (m-NCH or o), 9.19 (m)
4.84 (m-NCH or o), 4.94 (p)
−11.56 (m), 8.46 (o)
−13.78 (m), 9.84 (o)
−19.23 (o), −14.77 (o), 0.09 (p), 9.40 (m)
−4.79 (o), 1.79, 5.79
2-p-iPr
2-p-tBu
2-o-Picolyl
orange or yellow was noted. The substituted benzylpotassium
reagents were collected as solids by filtration, washed
thoroughly with pentane to remove LiOtBu, and dried under
vacuum.
2-p-NMe2
2-p-SMe
2-m-OMe
2-o-OMe
−32.44
−32.50
−29.17
4.16
Characterization of 1-Bn′ was possible by H and 13C NMR
1
spectroscopic analysis of THF-d8 solutions of each (25 °C). For
1-p-iPr, 1-p-tBu, 1-p-NMe2, 1-p-SMe, 1-m-OMe, and 1-o-
OMe, the resonances for the methylenes were visible in the
range 2−3 ppm (1H) and 48−59 ppm (13C). The chemical
shifts for additional resonances are presented in the
a
o = ortho; m = meta, p = para.
the U(IV) complexes, the methylene protons have the most
extreme chemical shifts due to their proximity to the
paramagnetic uranium center, appearing between −29 and
−33 ppm. The small range of chemical shifts for the methylene
protons is not surprising given the similarity in electron-
donating nature of the benzyl substituents. In the case of 2-o-
OMe, the methylene protons appear at 4.16, indicating a
potentially different coordination mode than the rest of the
family. For the complexes with para substitution, 2-p-iPr, 2-
p-tBu, 2-p-NMe2, and 2-p-SMe, equivalent resonances are
noted for both the ortho- and meta-protons of the aryl rings.
Experimental Section. The H and 13C NMR spectroscopic
1
data for 1-o-Picolyl compare well to previously acquired data,
which showed an η3-azaallyl coordination mode with
potassium.14 The purity of these salts was also assessed by
elemental analysis. Attempts were made at the synthesis of the
p-OMe variant; however, its instability precluded isolation and
full characterization.15
A variation of Schlosser’s procedure has already been
reported for 1-o-Picolyl16 and 1-p-tBu.17 However, we have
B
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Figure 1. Molecular structures of 2-p-iPr (left), 2-p-tBu (center), and 2-m-OMe (right) shown at 30% probability ellipsoids. H atoms, cocrystallized
solvent molecules, and additional U−C bonds depicting η4-coordination have been omitted for clarity. Inset depicts geometry for sigma bonds to
uranium.
Table 3. Structural Parameters for 2-p-iPr, 2-p-tBu, 2-m-OMe, 2-o-OMe, and 2-o-Picolyl, with 2-Bn for Comparison
2-Bn
2-p-iPr
2-p-tBu
2-m-OMe
2-o-OMe
2-o-Picolyl
U−C bonds (Å)
U−Cipso bonds (Å)
U−C−Cipso angles
2.446(7), 2.454(8),
2.477(7), 2.462(7)
2.839(7), 2.762(7),
2.777(6), 2.700(8)
89.4(4)°, 86.0(4)°,
85.7(4)°, 82.7(4)°
2.438(19)
2.424(10)
2.450(5),
2.462(5)
2.806(6),
2.772(6)
87.4(3)°,
86.4(4)°
2.446(8),
2.466(9)
3.365,
2.555(5), 2.598(4),
2.611(4), 2.656(4)
2.818(4), 2.832(4),
2.834(4), 2.842(4)
86.4(3)°, 84.3(2)°,
82.0(2)°, 84.1(2)°
2.791(15)
2.792(9)
3.356
87.2(10)°
87.0(3)°
114.7(5)°,
113.7(6)°
The ortho-protons typically appear between 8 and 10 ppm,
while those in the meta position show more variability,
appearing between −11 and −14 ppm; these chemical shifts
are as expected based on those for 2-Bn. The rest of the family,
2-o-Picolyl, 2-m-OMe, and 2-o-OMe, have m-CH resonances
appearing within the same range, 8−10 ppm. The resonances
for the ortho-protons, however, are quite shifted, appearing at
16.15 ppm for 2-o-Picolyl, at −19.23 and −14.77 for 2-m-
OMe, and at −4.79 for 2-o-OMe. While this resonance for 2-o-
Picolyl could not be definitively established based on
integration value alone from the meta position, the assignment
is proposed based on the chemical shift in the analogous
species.
toluene and ∼15% bibenzyl.9 Both 2-p-iPr and 2-p-tBu were
stable for over 24 h in solution, a significant improvement over
2-Bn. 2-p-iPr showed the same percentages as 2-Bn for
production of p-cymene and 4,4′-di-isopropyldibenzyl, whereas
the 2-p-tBu was slightly different at 84% 4-tert-butyltoluene and
16% 4,4′-di-tert-butyldibenzyl. The OMe-substituted com-
pounds were both stable for less than 24 h, with the 2-m-
OMe producing >99% 3-methylanisole and trace 3,3′-
ethylenebis(anisole), where 2-o-OMe showed only 2-methyl-
anisole and no 2,2′-ethylenebis(anisole) over the course of the
experiment. In the case of 2-o-Picolyl, only formation of 2-
picoline was noted over the course of 1 h. The coupled product
2,2′-(1,2-ethanediyl)bis(pyridine) was not detected, likely due
to the fact that dissociation of this chelate from the uranium
center is unfavorable.
The isolation of 2-p-iPr, 2-p-tBu, 2-m-OMe, 2-o-OMe, and
2-o-Picolyl expands the number of neutral homoleptic
uranium(IV) alkyl compounds that have been characterized
to eight (including 2-Bn, U(CH2-p-CH3C6H4)4, and U(CH2-m-
(CH3)2C6H3)4). These species persist at ambient temperature,
avoiding decomposition pathways. Typical breakdown of
uranium alkyls includes U−C homolytic cleavage, which
produces the corresponding substituted toluene or 2-picoline,
and reductive elimination/radical coupling, both of which
produce the substituted bibenzyl and are thus indistinguish-
able.5,10,18,19 To evaluate the relative stabilities of 2-p-iPr, 2-
p-tBu, 2-m-OMe, 2-o-OMe, and 2-o-Picolyl, the same method
as for 2-Bn was used.9 Benzene-d6 solutions of each (with a
Following NMR spectroscopic analysis, electronic absorption
spectroscopy was used to help elucidate the electronic
structures of 2-Bn′. Data for 2-Bn′ were collected from 380
to 2100 nm at ambient temperature in toluene (Figure S20). As
expected, the spectra for the family are very similar. The UV−
visible region has very broad transitions that are difficult to
resolve. Broad, low-intensity bands assigned as Laporte-
forbidden f−f transitions are found throughout the near-
infrared region, as would be expected for uranium(IV) f 2
centers. These spectra are reminiscent of those noted for the
(NNR)U(CH2Ph)2 (NNR = fc(NR)2, fc = 1,1′-ferrocenediyl; R
= SitBuMe2, SiMe3, SiMe2Ph) family recently reported by
Diaconescu.20 Interestingly, lower molar absorptivities are
noted in the spectra for 2-o-OMe and 2-o-Picolyl as compared
to the rest of the family, perhaps indicating a difference in
coordination of these ligands.
1
ferrocene internal standard) at 23 °C were monitored by H
NMR spectroscopy over 3 days or until no starting material
remained (Table S1). Following this, samples were filtered over
a silica gel plug and washed with diethyl ether to remove
uranium, without being subjected to vacuum. Solutions were
analyzed by GC/MS to analyze the relative percentages of
substituted toluenes and bibenzyls formed.
To establish the hapticity of the benzyl groups in 2-p-iPr, 2-
p-tBu, 2-m-OMe, 2-o-OMe, and 2-o-Picolyl, X-ray crystallog-
raphy was used to evaluate bonding in these representative
members of the family. On the basis of the analysis, the
Previously, it was noted that decomposition of 2-Bn occurred
in approximately 6 h under these conditions, giving ∼85%
C
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Table 4. Comparison of the Structural Parameters for 2-p-iPr, 2-p-tBu, 2-m-OMe, and 2-Bn
a
b
MCipso−MCH2 (Å)
MCo−MCH2 (Å)
MCo′−MCH2 (Å)
Δ
Δ′
2-Bn9
0.39, 0.31 0.30, 0.24
0.353
0.95, 0.84 0.75, 0.71
0.822
0.98, 0.86 0.91, 0.83
0.950
0.56, 0.53 0.45, 0.47
0.469
0.59, 0.55 0.61, 0.59
0.597
2-p-iPr
2-p-tBu
2-m-OMe
0.368
0.836
1.054
0.468
0.686
0.344, 0.322
0.694, 0.642
1.034, 1.031
0.350, 0.320
0.690, 0.709
a
b
Δ = [MCo−MCH2] − [MCipso−MCH2]. Δ′ = [MCo′−MCH2] − [MCipso−MCH2].
Figure 2. Molecular structures of 2-o-Picolyl (left) and 2-o-OMe (right) shown at 30% probability ellipsoids. Hydrogen atoms and cocrystallized
solvent molecules have been omitted for clarity. Inset depicts geometry for sigma bonds to uranium.
homoleptic uranium(IV) compounds presented here can be
divided into two groups: (1) those with para or meta
substitution that have similar coordination geometries to 2-
Bn and (2) those with divergent geometries due to heteroatom
donation.
p-tBu, and 2-m-OMe, respectively, and compare well to that for
2-Bn (U−C−Cav = 86.0°).
The members of the second category, 2-o-Picolyl and 2-o-
OMe, display coordination by an additional heteroatom in the
ortho position and thus will be discussed individually. Crystals
of 2-o-Picolyl were obtained by cooling a concentrated diethyl
ether solution of this material layered with pentane to −35 °C.
Analysis showed an eight-coordinate uranium center in 2-o-
Picolyl, where all distances are crystallographically distinct
(Figure 2, left, structural parameters in Table 3). The U−C σ-
bond distances range from 2.555(5) to 2.656(4) Å, while those
to the ipso-carbon of the phenyl group are ∼2.8 Å. While the
latter is on par with the compounds previously discussed, the σ-
interaction is actually about 0.1 Å longer. The nitrogen atoms
of the pyridine rings are coordinated, with U−N interactions
averaging 2.452 Å, as expected for dative interactions. The U−
N distances are on the order of those displayed in
Cp*2UMe(Spy) (2.448 Å)22 (py = pyridine) and Cp*2U(η2-
picoline)CH3 (2.394(3) Å).23 The coordination mode noted
for 2-o-Picolyl is similar to that previously observed by
Rothwell for the thorium(IV) species Th(OAr′)2(CH2-py-6-
Me)2 (Ar′ = 2,6-di-tert-butylphenyl).24 In this molecule, the
respective Th−C σ-bonds and Th−N distances of 2.55(1) and
2.61(1) Å are on the order of those for 2-o-Picolyl. Similarly,
Kiplinger notes corresponding Th−C and Th−N distances of
2.642(10) and 2.574(7) Å in Cp*2Th(CH3)[η2-(N,C)-2-CH2-
NC5H3].23 The M−C−C angles in 2-o-Picolyl range from
82.0(2)° to 86.4(3)°, as compared to 91.6(8)° for Th-
(OAr′)2(CH2-py-6-Me)2 and 87.9(5)° for Cp*2Th(CH3)[η2-
(N,C)-2-CH2-NC5H3]. As in both of these cases, the actinide
lies out of the C−C−N plane of the pyridine ring, most likely in
order to relieve steric pressure in the eight-coordinate species.
While this decreases the nitrogen lone pair−metal overlap, this
The members of the first category include 2-p-iPr, 2-p-tBu,
and 2-m-OMe and were crystallized by cooling concentrated
diethyl ether solutions to −35 °C; for 2-p-iPr, the solution was
further layered with pentane to precipitate X-ray quality
crystals. Analysis of each sample showed that both 2-p-iPr
and 2-p-tBu are crystallographically symmetric with pseudo-
tetrahedral geometries; thus their structural parameters are
equivalent (Figure 1, structural parameters in Table 3). 2-p-iPr,
2-p-tBu, and 2-m-OMe have respective U−C σ-bond averages
of 2.438(19), 2.424(10), and 2.456 Å, similar to 2-Bn
(2.460(8) Å). Additionally, the distances to the ipso-carbon
average 2.791(15), 2.792(9), and 2.789 Å, respectively, and are
on the order of those observed for the parent species.
The hapticity of the benzyl substitutents was established
using parameters defined by Andersen for benzyluranium
species, Δ and Δ′, which are based on the U−C distances for
the methylene (MCH2), ipso- (MCipso), and ortho-carbons
(MCo is the shorter contact length, MCo′ is the longer contact
length) of the benzyl groups.21 Thus, Δ = [MCo−MCH2] −
[MCipso−MCH2] and Δ′ = [MCo′−MCH2] − [MCipso
−
MCH2]. These data are tabulated for the 2-Bn′ series presented
here along with 2-Bn for comparison (Table 4). On the basis of
the U−C distances for 2-p-iPr, 2-p-tBu, and 2-m-OMe, the
calculated Δ and Δ′ values, ranging from 0.320 to 0.469 and
0.597−0.709, respectively, support η4-benzyl coordination, as in
the case of 2-Bn. This is also evident from the U−C−C angles
that average 87.2(10)°, 87.0(3)°, and 86.9(4)° for 2-p-iPr, 2-
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decrease is insignificant compared to the relief from steric
crowding that is gained.24 Thus, the bonding in 2-o-Picolyl is
analogous to that noted for Cp*2Th(CH3)[η2-(N,C)-2-CH2-
NC5H3], in which the metal−pyridine interaction can best be
described as a distorted η2-(C,C′,N)-azaallylic interaction.
The isolation and characterization of 2-o-Picolyl is
significant, as actinocenes with η2-picolyl ligands have been
computationally determined as products in sp3 C−H bond
activation of 2-picoline and 2,6-lutidine.25,26 Such species have
been isolated and characterized for thorium,27 but not for
uranium. The bond distances displayed by 2-o-Picolyl are
consistent with the calculations for Cp*2U(CH3)η2-(N,C)-2-
CH2-NC5H4, which has a U−C σ-bond distance of 2.598 Å and
a U−N distance of 2.570 Å.26
The second member of this family, 2-o-OMe, was crystallized
by cooling a concentrated THF/pentane solution to −35 °C
over 3 days. The solid-state structure as determined by X-ray
diffraction shows a square antiprismatic uranium center, with
each square composed of two carbon and two oxygen donors
(Figure 2, right, structural parameters in Table 3). 2-o-OMe is
the first structurally characterized actinide compound to feature
an o-OMe benzyl group; thus no direct comparisons to
previous examples could be made for the U−O and U−C
distances. 2-o-OMe possesses a C2 rotation axis, equivalencing
the metrical parameters. The U−C distances of 2.446(8) and
2.466(9) Å are on the order of those of 2-p-iPr, 2-p-tBu, and 2-
m-OMe, but slightly shorter than those in 2-o-Picolyl. The U−
O distances of 2.671(5) and 2.672(5) Å are as expected for
dative bonds to uranium.28
Interesting structural features in 2-o-OMe include the U−
C−C angles of 114.7(5)° and 113.7(6)°. These larger angles as
compared to the rest of the compounds analyzed create a near-
tetrahedral environment around the methylene carbons. This is
not unexpected, given that there are no additional π
interactions to uranium necessary, as 2-o-OMe is coordinatively
saturated. This is in contrast to 2-p-iPr, 2-p-tBu, and 2-m-OMe,
which maintain η4-bonding to prevent a sterically unsaturated
four-coordinate species.
The generation of the neutral homoleptic uranium
tetrabenzyl family presented here is an advance in under-
standing the fundamental organometallic chemistry of uranium.
Previous synthetic attempts of four-coordinate uranium
tetrabenzyl compounds were unsuccessful due to thermal
instability. However, increasing the coordination number of the
uranium center with additional π or heteroatom interactions
imparts stability, allowing these species to be handled at
ambient temperatures. Additionally, uranium complexes with
these particular substituted benzyl groups have not previously
been reported. Thus, these compounds mark new territory in
the field of σ-bonded uranium alkyls.
scission and β-hydrogen elimination. Thus, this family of
uranium alkyls is significant, given the notorious instability of
those tetraalkyls previously synthesized. Additionally, these
benzyluranium compounds provide insight into the bonding
and coordination modes of substituted benzyl ligands, as these
have not been previously structurally characterized for actinides
(except for 2-picoline). Future studies will be aimed at
understanding fundamental organometallic reactions with
these unique alkylated species.
EXPERIMENTAL SECTION
■
General Considerations. All air- and moisture-sensitive manip-
ulations were performed using standard Schlenk techniques or in an
MBraun inert atmosphere drybox with an atmosphere of purified
nitrogen. The MBraun drybox was equipped with a coldwell designed
for freezing samples in liquid nitrogen as well as two −35 °C freezers
for cooling samples and crystallizations. Solvents for sensitive
manipulations were dried and deoxygenated using literature
procedures with a Seca solvent purification system.29 Benzene-d6 was
purchased from Cambridge Isotope Laboratories, dried with molecular
sieves and sodium, and degassed by three freeze−pump−thaw cycles.
THF-d8 was purchased from Cambridge Isotope Laboratories and
used as received. Potassium tert-butoxide and n-butyllithium (2.5 M in
hexanes) were purchased from Sigma-Aldrich and used as received.
Methyl p-tolyl sulfide, 4-tert-butyltoluene, p-cymene, 2-methylpyridine,
and 4-methoxytoluene were purchased from Sigma-Aldrich and
distilled from CaH2 prior to use. Uranium tetrachloride was prepared
according to literature procedures.30
1H NMR and 13C NMR spectra were recorded on a Varian Inova
300 spectrometer operating at 299.992 and 75.424 MHz, respectively.
All chemical shifts are reported relative to the peak for SiMe4, using 1H
(residual) chemical shifts of the solvent as a secondary standard. The
spectra for paramagnetic molecules were obtained by using an
acquisition time of 0.5 s; thus the peak widths reported have an error
of 2 Hz. For paramagnetic molecules, the 1H NMR data are reported
with the chemical shift, followed by the peak width at half-height, the
integration value, and, where possible, the peak assignment.
The capillary gas chromatography/mass spectrometry analyses were
carried out with an Agilent 5975C (Agilent Laboratories, Santa Clara,
CA, USA) mass spectrometer system. Typical electron energy was 70
eV with the ion source temperature maintained at 250 °C. The
individual components were separated by using a 30 m HP-5 capillary
column (250 μm i.d. × 0.25 μm film thickness). The initial column
temperature was set at 35 °C (for 3 min) and programmed to reach
280 °C with a ramp of 10.0 deg/min. The flow rate was set at 1 mL/
min, and the injector was set at 250 °C. Samples were run through a
silica gel plug to remove uranium from organics in preparation for gas
chromatography/mass spectrometry analysis.
Single crystals of 2-o-Picolyl suitable for X-ray diffraction were
coated with poly(isobutylene) oil in a glovebox and quickly transferred
to the goniometer head of a Nonius KappaCCD image plate
diffractometer equipped with a graphite crystal, incident beam
monochromator. Preliminary examination and data collection were
performed with Mo Kα radiation (λ = 0.710 73 Å). Single crystals of 2-
p-iPr, 2-p-tBu, 2-m-OMe, and 2-o-OMe suitable for X-ray diffraction
were coated with poly(isobutylene) oil in a glovebox and quickly
transferred to the goniometer head of a Rigaku Rapid II image plate
diffractometer equipped with a MicroMax002+ high-intensity copper
X-ray source with confocal optics. Preliminary examination and data
collection were performed with Cu Kα radiation (λ = 1.541 84 Å). Cell
constants for data collection were obtained from least-squares
refinement. The space group was identified using the program
XPREP.31 The structures were solved using the structure solution
program PATTY in DIRDIF99.32 Refinement was performed on a
LINUX PC using SHELX-97.31
CONCLUSIONS
■
In summary, a new family of benzylpotassium reagents has been
synthesized using a modification of Schlosser’s procedure and
fully characterized. This has been accomplished in the absence
of toxic precursors or deleterious side reactions. These reagents
have been used for the synthesis of homoleptic uranium(IV)
tetrabenzyl derivatives, as the 1-Bn′ reagents smoothly
eliminate KCl in diethyl ether from UCl4. Characterization of
these species by X-ray crystallography revealed that their
inherent stability is due to the increased hapticity of the benzyl
groups, which creates a saturated uranium center and slows
unwanted decomposition reactions, including homolytic U−C
General Synthesis of Organopotassium Reagents. A modified
procedure for organopotassium reagents, reported by Schlosser,13 was
used to generate the series of organopotassium reagents. A 250 mL
E
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round-bottom flask was charged with 2.00 g (17.8 mmol) of KOtBu
and excess substituted toluene (100 mL). The slurry was cooled in the
coldwell for ∼20 min or until frozen. While stirring, cold (−35 °C) n-
butyllithium (2.5 M in hexanes) (7.12 mL, 17.8 mmol) was added to
the thawing slurry, producing an immediate color change to yellow-
orange. The solution was allowed to warm to room temperature for 1
to 20 h (see Table 1 for specific reaction times), at which point the
mixture darkened to deep orange-red. The orange or yellow solid was
collected using a fritted funnel and washed with a continuous stream of
pentane totaling approximately ∼200 mL to remove any remaining
substituted toluene and LiOtBu. The solid was dried on the vacuum
line. Quantitative yields were obtained for all organopotassium
reagents.
2-p-tBu. A 0.099 g amount of 1-p-tBu was added (83% yield).
Single crystals suitable for X-ray crystallography were grown in
concentrated diethyl ether at −35 °C for 2 days. Anal. Calcd for
C44H60U: C, 63.90; H, 7.31. Found: C, 63.72; H, 7.40. 1H NMR
(C6D6, 25 °C): δ = −32.09 (33, 8H, CH2), −13.78 (22, 8H, m-CH),
t
1.82 (6, s, 36H, Bu−CH3), 9.86 (13, 8H, o-CH).
2-o-Picolyl. A 0.070 g amount of 1-o-Picolyl was added (62%
yield). Single crystals suitable for X-ray crystallography were grown in
concentrated diethyl ether layered with pentane at −35 °C for 5 days.
Due to the limited stability of the compound, elemental analysis was
1
unable to be obtained. H NMR (C6D6, 25 °C): δ = −32.21 (34, s,
8H, CH2), 4.84 (d, 4H, o-CH or N-CH, J = 8.1 Hz), 4.94 (t, 4H, p-CH,
J = 5.9 Hz), 9.19 (t, 4H, m-CH, J = 7.2 Hz), 16.15 (13, 4H, o-CH or
N-CH).
Reaction Times and Characterization for Organopotassium
Reagents. 1-p-iPr. After stirring for 2−3 h a bright orange solid was
produced. Anal. Calcd for C10H13K: C, 69.70; H, 7.60. Found: C,
69.53; H, 7.44. 1H NMR (THF-d8, 25 °C): δ = 1.20 (d, 6H, iPr-CH3, J
2-p-NMe2. A 0.093 g amount of 1-p-NMe2 was added. Compound
decomposes quickly upon workup. Therefore, crystals could not be
grown for X-ray crystallography, and elemental analysis and yields
could not be determined. 1H NMR (C6D6, 25 °C): δ = −32.44 (3, 8H,
CH2), −11.56 (28, 8H, m-CH), 4.06 (4, 24H, NMe2-CH3), 8.46 (93,
8H, o-CH).
i
= 6.9 Hz), 2.26 (s, 2H, CH2), 2.83 (septet, 1H, Pr-CH, J = 7.0 Hz),
7.0−7.1 (m, 4H, CH). 13C NMR (THF-d8, 25 °C): δ = 33.78 (iPr-
CH3), 34.70 (iPr-CH), 48.21 (CH2), 110.40 (CH), 115.92 (Cpara),
128.29 (CH), 152.64 (Cipso).
2-p-SMe. A 0.093 g amount of 1-p-SMe was added. Compound
decomposes quickly upon workup. Therefore, crystals could not be
grown for X-ray crystallography, and elemental analysis and yields
1-p-tBu. After stirring for 2−3 h a bright orange solid was produced.
Anal. Calcd for C11H15K: C, 70.91; H, 8.11. Found: C, 70.83; H, 7.96.
t
1
1H NMR (THF-d8, 25 °C): δ = 1.06 (s, 3H, Bu-CH3), 2.04 (s, 2H,
could not be determined. H NMR (C6D6, 25 °C): δ = −32.05 (42,
CH2), 5.54 (d, 2H, CH, J = 8.6 Hz), 6.15 (d, 2H, CH, J = 8.5 Hz). 13
C
8H, CH2), −13.78 (21, 8H, m-CH), 1.25 (6, 12H, SMe-CH3), 9.84
(12, 8H, o-CH).
NMR (THF-d8, 25 °C): δ = 32.71 (Me); 33.63(CtBu); 48.19 (CH2);
110.50(Cortho); 118.21 (Cpara); 127.22 (Cmeta); 152.21 (Cipso).
1-o-Picolyl. After stirring for 3−4 h a bright yellow solid was
produced. NMR data matched previously published data.14
1-p-NMe2. After 1−2 h a bright orange-red solid was produced.
Anal. Calcd for C91H12NK: C, 62.38; H, 6.98; N, 8.08. Found: C, 62.14;
H, 6.87; N, 8.01. H NMR (THF-d8, 25 °C): δ = 2.17 (s, 2H, CH2),
2.83 (s, 6H, CH3), 6.60 (d, 2H, CH, J = 8.5 Hz), 6.92, (d, 2H, CH, J =
8.5 Hz). 13C NMR (THF-d8, 25 °C): δ = 46.28 (CH2), 46.60 (Me),
110.68 (CH), 123.28 (CH), 128.53 (Cipso), 152.51 (Cpara).
2-m-OMe. A 0.085 g amount of 1-m-OMe was added (67% yield).
Single crystals suitable for X-ray crystallography were grown in
concentrated diethyl ether at −35 °C for 3 days. Anal. Calcd for
1
C32H36O4U: C, 53.19; H, 5.02. Found: C, 52.96; H, 4.94. H NMR
(C6D6, 25 °C): δ = −29.17 (29, 8H, CH2), −19.23 (67, 4H, o-CH),
−14.77 (241, 4H, o-CH), 0.09 (5, d, 4H, p-CH, J = 8.1 Hz), 1.17 (4,
12H, OMe-CH3), 9.40 (77, 4H, m-CH).
2-o-OMe. A 0.085 g amount of 1-o-OMe was added (71% yield).
Single crystals suitable for X-ray crystallography were grown in
concentrated THF and pentane at −35 °C for 3 days. Anal. Calcd for
1-p-SMe. After 6−7 h a light orange solid was produced. Anal.
1
1
C32H36O4U: C, 53.19; H, 5.02. Found: C, 52.89; H, 4.87. H NMR
Calcd for C8H9SK: C, 54.50; H, 5.14. Found: C, 54.32; H, 4.93. H
(C6D6, 25 °C): δ = −4.79 (6, 4H, CH), −4.76 (7, 12H, OMe-CH3),
−1.79 (t, 4H, CH, J = 7.4 Hz), 4.16 (45, 8H, CH2), 5.79 (t, 4H, CH, J
= 7.3 Hz), 6.92 (d, 4H, CH, J = 7.8 Hz).
NMR (THF-d8, 25 °C): δ = 1.93 (s, 3H, SMe-CH3), 2.44 (s, 2H,
CH2), 5.44 (d, 2H, CH2, J = 8.7 Hz,), 6.14 (d, 2H, CH2, J = 8.6 Hz).
13C NMR (THF-d8, 25 °C): δ = 24.25 (Me), 58.91 (CH2), 112.24
(CH), 130.28 (Cipso), 137.39 (CH), 151.23 (Cpara).
1-m-OMe. After 10−12 h a golden yellow solid was produced. Anal.
ASSOCIATED CONTENT
■
1
Calcd for C8H9OK: C, 59.96; H, 5.66. Found: C, 59.78; H, 5.74. H
S
* Supporting Information
NMR (THF-d8, 25 °C): δ = 2.28 (s, 2H, CH2) 3.72 (s, 3H, OMe-
CH3), 6.65−6.70 (m, 3H, CH), 7.09 (t, 1H, CH, J = 7.9 Hz). 13C
NMR (THF-d8, 25 °C): δ = 53.49 (Me), 54.16 (CH2), 83.73 (Cipso),
92.99 (Cmeta), 105.98 (CH), 130.57 (CH), 153.81 (CH), 163.14 (CH).
1-o-OMe. After 3−4 h a bright yellow solid was produced. Anal.
Multinuclear NMR spectra, X-ray crystallographic experimen-
tals, electronic absorption data, and table of decomposition
data. The Supporting Information is available free of charge on
the ACS Publications website at DOI: 10.1021/acs.organo-
met.5b00212.
1
Calcd for C8H9OK: C, 59.96; H, 5.66. Found: C, 59.30; H, 5.78. H
NMR (THF-d8, 25 °C): δ = 0.99 (s, 2H, CH2), 3.57 (s, 3H, OMe-
CH3), 4.63 (t, 1H, CH, J = 6.8 Hz), 5.85 (d, 1H, CH, J = 7.8 Hz),
5.94−5.99 (m, 2H, CH, J = 7.4 Hz). 13C NMR (THF-d8, 25 °C): δ =
37.40 (Cortho), 44.33 (CH2), 55.74 (Me), 67.86 (CH), 95.24 (Cipso),
112.27 (CH), 112.93 (CH), 124.86 (CH), 145.71 (CH).
AUTHOR INFORMATION
Corresponding Author
*E-mail: sbart@purdue.edu.
■
General Synthesis for U(IV) Tetra(alkyls). A 20 mL scintillation
vial was charged with 0.050 g (0.132 mmol) of UCl4 and ∼2 mL of
THF and frozen in a coldwell. A second 20 mL scintillation vial was
charged with 4 equiv (0.528 mmol) of organopotassium reagent and 2
mL of THF and frozen in a coldwell. The two solutions were
combined and stirred while thawing, creating a dark red-brown
solution. Volatiles were promptly removed in vacuo. The residue was
taken up in diethyl ether and filtered to remove KCl. Immediate
removal of the diethyl ether in vacuo left a red-brown solid.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We acknowledge the National Science Foundation (CAREER
grant to S.C.B., CHE-1149875) for funding.
REFERENCES
Characterization for U(IV) Tetra(alkyls). 2-p-iPr. A 0.091 g
amount of 1-p-iPr was added (51% yield). Single crystals suitable for
X-ray crystallography were grown in concentrated diethyl ether layered
with pentane at −35 °C overnight. Anal. Calcd for C40H52U: C, 62.32;
■
(1) Hoff, R.; Mathers, R. Handbook of Transition Metal Polymerization
Catalysts; Wiley: Hoboken, 2010.
(2) Sigel, H.; Sigel, A. Biological Properties of Metal Alkyl Derivatives;
Marcel Dekker: New York, 1993.
(3) Crabtree, R. H. The Organometallic Chemistry of the Transition
Metals, 4th ed.; Wiley-Interscience: Hoboken, 2005.
1
H, 6.80. Found: C, 62.19; H, 6.82. H NMR (C6D6, 25 °C): δ =
i
−31.09 (43, 8H, CH2), −13.20 (21, 8H, m-CH), 1.77 (7, 24H, Pr-
i
CH3), 5.88 (14, m, 4H, Pr-CH), 9.40 (12, 8H, o-CH).
F
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Article
(4) Hartwig, J. F. Organotransition Metal Chemistry: From Bonding to
Catalysis; University Science Books, 2010.
(5) Marks, T. J.; Seyam, A. M. J. Organomet. Chem. 1974, 67, 61−66.
(6) Seyam, A. M. Inorg. Chim. Acta 1983, 77, L123−L125.
(7) Bambirra, S.; Meetsma, A.; Hessen, B. Organometallics 2006, 25,
3454−3462.
(8) Bailey, P. J.; Coxall, R. A.; Dick, C. M.; Fabre, S.; Henderson, L.
C.; Herber, C.; Liddle, S. T.; Lorono-Gonzalez, D.; Parkin, A.; Parsons,
S. Chem.Eur. J. 2003, 9, 4820−4828.
(9) Kraft, S. J.; Fanwick, P. E.; Bart, S. C. J. Am. Chem. Soc. 2012, 134,
6160−6168.
(10) Marks, T. J.; Seyam, A. M. J. Am. Chem. Soc. 1972, 94, 6545−
6546.
(11) Lochmann, L.; Lim, D. J. Organomet. Chem. 1971, 28, 153−158.
(12) Schlosser, M. Angew. Chem., Int. Ed. Engl. 1964, 3, 287−306.
(13) Schlosser, M.; Hartmann, J. Angew. Chem., Int. Ed. Engl. 1973,
12, 508−509.
(14) Kennedy, A. R.; Mulvey, R. E.; Urquhart, R. I.; Robertson, S. D.
Dalton Trans. 2014, 43, 14265−14274.
(15) Vanermen, G.; Van Beylen, M.; Geerlings, P. J. Phys. Chem.
1986, 90, 603−607.
(16) Kim, Y. H.; Kim, T. H.; Kim, N. Y.; Cho, E. S.; Lee, B. Y.; Shin,
D. M.; Chung, Y. K. Organometallics 2003, 22, 1503−1511.
(17) Harder, S.; Mueller, S.; Huebner, E. Organometallics 2004, 23,
178−183.
(18) Marks, T. J.; Seyam, A. M.; Kolb, J. R. J. Am. Chem. Soc. 1973,
95, 5529−5539.
(19) Davidson, P. J.; Lappert, M. F.; Pearce, R. Chem. Rev. 1976, 76,
219−242.
(20) Duhovic, S.; Oria, J. V.; Odoh, S. O.; Schreckenbach, G.; Batista,
E. R.; Diaconescu, P. L. Organometallics 2013, 32, 6012−6021.
(21) Edwards, P. G.; Andersen, R. A.; Zalkin, A. Organometallics
1984, 3, 293−298.
(22) Evans, W. J.; Walensky, J. R.; Ziller, J. W. Inorg. Chem. 2010, 49,
1743−1749.
(23) Kiplinger, J. L.; Scott, B. L.; Schelter, E. J.; Pool Davis Tournear,
J. A. J. Alloys Compd. 2007, 444−445, 477−482.
(24) Beshouri, S. M.; Fanwick, P. E.; Rothwell, I. P.; Huffman, J. C.
Organometallics 1987, 6, 2498−2502.
(25) Castro, L.; Yahia, A.; Maron, L. Dalton Trans. 2010, 39, 6682−
6692.
(26) Yang, P.; Warnke, I.; Martin, R. L.; Hay, P. J. Organometallics
2008, 27, 1384−1392.
(27) Pool, J. A.; Scott, B. L.; Kiplinger, J. L. J. Alloys Compd. 2006,
418, 178−183.
(28) Avens, L. R.; Bott, S. G.; Clark, D. L.; Sattelberger, A. P.;
Watkin, J. G.; Zwick, B. D. Inorg. Chem. 1994, 33, 2248−2256.
(29) Pangborn, A. B.; Giardello, M. A.; Grubbs, R. H.; Rosen, R. K.;
Timmers, F. J. Organometallics 1996, 15, 1518−1520.
(30) Kiplinger, J. L.; Morris, D. E.; Scott, B. L.; Burns, C. J.
Organometallics 2002, 21, 5978−5982.
(31) Sheldrick, G. M. Acta Crystallogr. 2008, A64, 112−122.
(32) Beurskens, P. T.; Beurskens, G.; De Gelder, R.; Garcia-Granda,
S.; Gould, R. O.; Smits, J. M. M. Dirdif2008 Program System;
Crystallography Laboratory, University of Nijmegen: The Nether-
lands, 2008.
G
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