Exploring the coordination modes of pyrrolyl ligands in bis(imido) uranium(VI) complexes

Article abstract of DOI:10.1039/c002440f

The preparation of a family of bis(imido) uranium(vi) complexes stabilized by mono- and bidentate pyrrolyl ancillary ligands is described. X-ray crystallographic studies of dipyrrolylmethane (dpm) derivatives show that the pyrrolyl coordination mode in these uranium(vi) ions is unexpected in comparison to analogous transition metal and lanthanide chemistry. The ability of the coordinated pyrrolyl moieties to undergo pyrrolyl isomerization has also been explored and demonstrates reactivity that is unique from structurally similar uranium(vi)-bis(cyclopentadienyl) derivatives. The Royal Society of Chemistry 2010.

Full text of DOI:10.1039/c002440f

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Exploring the coordination modes of pyrrolyl ligands in bis(imido)
uranium(VI) complexes†
Douglas L. Swartz II,a Liam P. Spencer,b Brian L. Scott,b Aaron L. Odom*c and James M. Boncella*b
Received 9th February 2010, Accepted 22nd April 2010
First published as an Advance Article on the web 18th May 2010
DOI: 10.1039/c002440f
The preparation of a family of bis(imido) uranium(VI) complexes stabilized by mono- and bidentate
pyrrolyl ancillary ligands is described. X-ray crystallographic studies of dipyrrolylmethane (dpm)
derivatives show that the pyrrolyl coordination mode in these uranium(VI) ions is unexpected in
comparison to analogous transition metal and lanthanide chemistry. The ability of the coordinated
pyrrolyl moieties to undergo pyrrolyl isomerization has also been explored and demonstrates reactivity
that is unique from structurally similar uranium(VI)-bis(cyclopentadienyl) derivatives.
moieties have also been effectively integrated into multidentate
ancillary ligand architectures and utilized to stabilize the ura-
Introduction
20
nium(III) oxidation state,19 cleave the N N bond found in N2,
Pyrrolyl complexes of transition metals have attracted consider-
≡
and serve as actinide cation receptors.21 We sought to extend our
general knowledge of the chemistry of these donors by examining
their reactivity with the [U(NR)2]2+ ion. In this paper, we report
the synthesis of pyrrolyl complexes of the [U(NR)2]2+ ion utilizing
mono- and bidentate ligand sets. In coordination studies with a
sterically encumbered dipyrrolylmethane ancillary ligand, an un-
expected coordination mode is observed which can be induced to
undergo pyrrolyl isomerization via the addition of dmpe (dmpe =
1,2-bis(dimethylphosphino)ethane).
able attention as a result of their rich and diverse coordination
chemistry and their continuing role as essential ancillary ligands in
catalysis.1-14 One defining feature that has driven their use in these
areas is their isoelectronic relationship to cyclopentadienyl frag-
ments. Like their cyclopentadienyl analogues, pyrrolyl-based lig-
ands possess the capacity to assume either h - or h -coordination
modes; this ability appears to be dependent upon features like
the nature of the metal centre or the organic substituents on
the N-heterocyclic molecule.15,16 In contrast to cyclopentadienyl
derivatives however, pyrrolyl ligands show a greater propensity
5
1
1
to bind in an h -fashion which has provided promise to yield
new avenues of reactivity in areas of chemistry that have been
dominated by metallocenes. While most of the interest in these
ancillary ligands has focused on high oxidation state transition and
lanthanide metal centers, actinide elements like uranium possess
tremendous potential to further our understanding of pyrrolyl
chemistry. Given that these metal centers can attain large formal
coordination numbers and unusual coordination geometries, it is
plausible that pyrrolyl moieties may assume coordination modes
that are very different from early transition metal and lanthanide
systems.
Results and discussion
We have recently reported that the iodide ligands of a fam-
ily of uranium(VI) bis(imido) diiodide complexes can be ef-
ficiently substituted to yield a diverse library of uranium(VI)
derivatives.22-25 Guided by this reactivity, we investigated the
reaction of U(NtBu)2(I)2(THF)2 (1) with 2 equiv of the 2,5-
dimethylsubstituted pyrrolide anion shown in Scheme 1. Surpris-
ingly, no tractable uranium materials were isolated as evidenced
by NMR spectroscopy; the only product identified was 2,5-
dimethylpyrrole. In contrast, an analogous reaction between
the OPPh3-adduct U(NtBu)2(I)2(OPPh3)2 (2) and 2 equiv of the
pyrrolide anion produces the bis(pyrrolyl) complex U(NtBu)2(2,5-
Me2C4H2N)2(OPPh3)2 (3) in reasonable yield. This complex was
identified by NMR spectroscopy, elemental analysis, and a single
crystal X-ray diffraction study.
With respect to actinide elements, less is known about the
coordination chemistry of these N-heterocyclic fragments. Early
1
5
findings suggest that both h - and h -pyrrolyl configurations
are possible as evidenced in the homoleptic uranium(IV) com-
plex U(N-2,5-Me2C4H2)4.17 Furthermore, these pyrrolyl rings
1
5
are susceptible to rapid h - and h -interconversions which are
analogous to those reported for transition metal cyclopentadienyl
complexes.18 Aside from their coordination chemistry, pyrrolyl
aKutztown University, Department of Chemistry, Kutztown, PA, 19530,
USA. E-mail: swartz@kutztown.edu
bMaterials, Physics and Applications Division, Los Alamos National Labo-
ratory, Los Alamos, NM, 87545, USA. E-mail: boncella@lanl.gov
cMichigan State University, Department of Chemistry, East Lansing, MI,
48824, USA. E-mail: odom@chemistry.msu.edu
† Electronic supplementary information (ESI) available: Solid state molec-
ular structure information for 3–6. CCDC reference numbers 765726–
765729. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c002440f
Scheme 1
This journal is
The Royal Society of Chemistry 2010
Dalton Trans., 2010, 39, 6841–6846 | 6841
©

The solid state molecular structure of 3 is shown in Fig. 1 and
features bis(imido), triphenylphosphine oxide and pyrrolyl ligands
in an all trans-orientation. As a result of these interactions, the ura-
nium centre assumes a pseudo-octahedral geometry with pyrrolyl
state.13 In addition, this trans configuration is also prevalent in
other actinide dihalide complexes.28-32
Given the successful coordination of a monodentate pyrrolyl
ligand to the [U(NR2)2+]2+ ion, we investigated reactions with
the bidentate dipyrrolylmethane species H2dpm (H2dpm = 5,5-
dimethyldipyrrolylmethane). The addition of K2dpm to a THF so-
lution of U(NtBu)2(I)2(THF)2 affords the dipyrrolylmethane ura-
nium(VI) complex U(NtBu)2(dpm)(THF)2 (4) as a red solid in rea-
sonable yield (Scheme 2). Crystallization of 4 from THF/hexanes
results in the formation of crystals suitable for an X-ray diffraction
experiment; the solid state molecular structure of 4 is shown in
Fig. 2. Both pyrrolyl moieties in the dpm ancillary ligand adopt an
1
ligands that are bound in an h -geometry. The U–N(pyrrolyl) bond
˚
distances in 3 (ave = 2.468(4) A) are similar to the U–N(amide)
bond lengths found in the bis(imido) uranium(VI) disulfonamide
1
3
complex U(NtBu)2(h -N(Me)SO2Ar)(h -N(Me)(SO2Ar)(Me2bpy)
(Ar = 4-Me-C6H4; Me2bpy = 4,4¢-dimethyl-2,2¢-bipyridyl, ave =
25
2+
˚
2.481(8) A) and significantly longer than those found in UO2
-
26
˚
amide derivative [Na(THF)2][UO2(N(SiMe3)2)3] (ave 2.310(4) A).
˚
Of further note, the U–N(imido) (1.842(3) A) and U–O(OPPh3)
1
1
˚
(2.383(2) A) bond lengths are similar to the those distances found
h ,h -bonding mode that possesses a U–N(pyrrolyl) bond distance
in the diiodide analogue U(NtBu)2(I)2(OPPh3)2 (2).22
(2.356(6) A) which is significantly shorter than analogous lengths
˚
found in complex 3. As a result of this pyrrolyl geometry, the
uranium centre assumes a distorted octahedral geometry with
trans-oriented imido ligands occupying the axial coordination
sites (N1–U1–N2 = 172.8(3)◦). Furthermore, the U–N(imido) (ave
˚
˚
= 1.856(7) A) and U–O(THF) (2.461(4) A) bond distances are
similar to previously reported bond lengths in U(NR)2(I)2(THF)x
(R = Bu, x = 2; R = Ph, x = 3) complexes.22 In the case of solution
t
studies, the 1H NMR spectrum of 4 is consistent with one species
as evidenced by pyrrolyl resonances at 6.59, 7.22, and 7.37 ppm.
Additionally, there are singlets at 1.41 and 1.38 ppm that can
be attributed to the two methyl groups of the dipyrrolylmethane
backbone and tert-butyl imido ligand, respectively.
1
Fig.
1
Solid-state molecule structure of U(NtBu)2(h -2,5-Me2-
C4H2N)2(OPPh3)2-2CH2Cl2 (3) with thermal ellipsoids drawn at the 50%
˚
probability level. Selected bond lengths (A) and angles (deg): U1–N1 =
1.842(3), U1–N2 = 2.459(4), U1–N3 = 2.476(4), U1–O1 = 2.383(2),
N1–U1–N1A = 171.42(17), N1–U1–O1 = 93.64(11), N1–U1–N2 =
85.71(9).
In solution, two distinct species are observed as evidenced by
1
1H and 31P{ H} NMR spectroscopy. We believe this observation
can be attributed to cis- and trans-dipyrrolyl isomers of complex
3; this phenomenon has been observed in dihalide bis(imido)
uranium(VI) complexes U(NtBu)2(X)2(OPPh3)2 (X = Cl, Br)25
and in uranyl phosphine oxide and phosphinimine complexes
UO2Cl2(EPCy3)2 (E = O, NH).27 To investigate this phenomenon,
Fig. 2 Solid-state molecule structure of U(NtBu)2(dpm)(THF)2 (4) with
thermal ellipsoids drawn at the 50% probability level. Selected bond lengths
˚
(A) and angles (deg): U1–N1 = 1.856(6), U1–N2 = 1.856(7), U1–N3 =
2.356(6), U1–O1 = 2.461(4), N1–U1–N2 = 172.8(3), N2–U1–N3 =
we examined the H and 31P{ H} NMR spectra of 3 in solvents
with differing polarities. In solvents which possess a high dielectric
constant, we expected the cis-isomer to be stabilized whereas the
trans-isomer is expected to predominate in solvents which possess
a lower dielectric constant.27 Examining the NMR spectra in
CD2Cl2 (e = 8.93 for CH2Cl2) and CDCl3 (e = 4.81 for CHCl3),
we found that the concentration of the cis-isomer increased with
an increase in the concentration of CD2Cl2. For example, in
CD2Cl2, the ratio of major/minor isomers of 3 as indicated
by the 31P NMR spectrum and integration of tert-butyl imido
1
1
91.52(19).
Encouraged by the successful coordination of this simple
dipyrrolylmethane ligand set, we next investigated reactions
with the more sterically encumbered ligand derivative dpmmes
(dpmmes = 2,2¢-bis(mesityl)-5,5-dimethyldipyrrolylmethane). We
believed that increasing the steric profile of the dipyrrolylmethane
architecture may alter the pyrrolyl coordination mode and allow
us to evaluate the influence of steric effects on the pyrrolyl
coordination geometry. The addition of K2dpmmes to a stirring
orange solution of U(NtBu)2(I)2(THF)2 (1) in THF provides a
black solution from which U(NtBu)2(dpmmes) (5) can be isolated
as a black solid in reasonable yield (Scheme 2). The solid
state molecular structure is shown in Fig. 3 and features two
1
resonance in the H NMR spectrum was 0.64/0.36 whereas in
CDCl3 the ratio was 0.89/0.11. These solution studies along with
the solid state molecular structure of 3 suggest that the dominant
species in solution is the trans-isomer. Such a hypothesis is also
consistent with the trans-diiodide complex U(NtBu)2(I)2(OPPh3)2
(2) in which no cis-isomer is observed in solution or the solid
5
5
pyrrolyl moieties in an h ,h -coordination mode. As a result, the
uranium centre possesses a geometry reminiscent of transition
6842 | Dalton Trans., 2010, 39, 6841–6846
This journal is
The Royal Society of Chemistry 2010
©

2
1
5
of bimetallic derivatives.7,16 For instance, a dimeric bis(m :h ,h -
dpm) zirconium derivative, [Zr(dpm)(NMe2)2]2, has been prepared
5
which possesses two h -pyrrolyl ligands bound to one of the
zirconium centers. The second zirconium atom is coordinated to
1
5
two h -pyrrolyl ligands and the two nitrogen atoms of the h -
pyrrolyl ligands that are facially bound to the first zirconium
centre.7 In contrast to zirconium and lanthanide chemistry, the
coordination of dpm-based ancillary ligands to the [U(NtBu)2]2+
ion provides well-defined mononuclear uranium(VI) derivatives
with structural pyrrolyl motifs that appear to be dependent upon
the steric properties of the ancillary ligand. Furthermore, these
coordination modes are in contrast to detailed studies of zirconium
complexes stabilized by monodentate 2-arylpyrrolyl ligands in
which decreased sterics and decreased electron density at the metal
5
centre favour the h -coordination mode.15 In the study presented
herein, it appears the opposite effect is true, that is, uranium
complexes with a decreased dipyrrolyl steric profile like 4 possess
1
an h -coordination mode while those with an increased steric
5
component (ie. complex 5) maintain an h -coordination profile.
Though surprising, this observation may be rationalized by the
difference in the ionic radii of zirconium and uranium centers and
their different Lewis acidities.
Pyrrolyl isomerisation in dipyrrolyl methane transition metal
complexes has been recently studied to provide insight into the
nature of the active species during hydroamination catalysis.42
To address the potential for pyrrolyl isomerisation in complex
5, we examined its reactivity with dmpe in an attempt to
determine if the pyrrolyl coordination mode could be altered.
Interestingly, the addition of 1 equiv of dmpe to a toluene
solution of U(NtBu)2(dpmmes) (5) gives U(NtBu)2(dpmmes)(dmpe)
Scheme 2
1
(6) in moderate yield (Scheme 2). The H NMR spectrum of 6
is consistent with a species having C2v symmetry in solution as
evidenced by equivalent dpmmes resonances at 2.26, 2.44, 3.36,
5.65, 6.37, and 7.01 ppm. There are also broad singlets at 0.99 and
1.21 ppm for the dmpe ligand which suggests rapid motion of the
uranium–dmpe metallacycle on the NMR timescale. Furthermore,
there is a singlet at 78.9 ppm in the 31P{ H} NMR spectrum.
1
The solid state structure of 6 is shown in Fig. 4 and pos-
sesses a pseudo-octahedral geometry at the uranium centre with
trans-oriented bis(imido) ligands. The N1–U1–N2 bond angle
(165.9(6)◦) is smaller than the less sterically encumbered uranium-
pyrrolyl complex 4 (172.8(3)◦) and the trans-bis(imido) complex
U(NtBu)2(I)2(THF)2 (1).22 Presumably, this difference in N–U–
N angles is a result of an increase in steric interactions at the
metal centre provided by the mesityl substituents of the pyrrolyl
Fig. 3 Solid-state molecule structure of U(NBut)2(dpmmes) (5) with
thermal ellipsoids drawn at the 50% probability level. Selected bond lengths
˚
(A) and angles (deg): U1–N1 = 1.930(4), U1–N2 = 1.946(4), U1–N3 =
2.575(4), U1–N4 = 2.592(4), N1–U1–N2 = 116.06(15).
metal metallocenes. This atomic disposition establishes a N1–U1–
N2 bond angle of 116.06(15)◦ that is significantly larger than other
cis-bis(imido) uranium(VI) complexes like (C5H5)2U(NtBu)2 (N1–
U1–N2 = 103.4(3)◦).23,33-35 Consistent with other cis-bis(imido)
uranium(VI) complexes, the average U–N(imido) bond length
˚
moiety. The average U–N(imido) bond length in 6 (1.861(14 A)
is shorter that the U–N(imido) bond lengths found in 5 and
comparable to the average U–N(imido) bond length found in
˚
(1.938(4) A) is significantly longer than U–N imido bond lengths
22
observed in trans-bis(imido) complexes like U(NtBu)2(I)2(THF)2
1 (1.844(4) A). In the case of U–P bond lengths, the average
˚
13
˚
˚
distance in 6 (3.080(6) A) is reminiscent of the average U–P bond
(1.844(4) A).
t
22
˚
Dipyrrolyl-based ancillary ligands have shown a remarkable
lengths in U(N Bu)2(I)2(PMe3)2(THF) (3.059(3) A) but longer
than analogous U–P bond distances found in the cyclopentadienyl
bis(imido) uranium(VI) complex (C5H5)2U(NtBu)2(dmpe) (U–Pave
1
5
ability to assume both h - and h -coordination modes with
early transition metal and lanthanide centers.8,36,37 In lanthanide
chemistry, this has led to elaborate aggregation of metal centers
23
˚
= 2.999(10) A). Additionally, the average U–N(pyrrolyl) bond
1
5
˚
that possess both h - and h -pyrrolyl metal interactions that bridge
distance (2.412(15) A) is slightly longer than analogous distances
1
5
between metal centers.37-41 Similar h ,h -coordination modes are
observed in titanium-dipyrrolyl chemistry, however these inter-
actions exist on a single titanium centre.16 Substitution of the
titanium atom for a larger zirconium centre results in the formation
found in 4, possibly a result of increased steric interactions at the
metal centre.
The reactivity exhibited by
6 with dmpe is markedly
different than analogous reactions with the organouranium
This journal is
The Royal Society of Chemistry 2010
Dalton Trans., 2010, 39, 6841–6846 | 6843
©

or an atmosphere of argon. Hexanes, THF, and toluene were
dried by passage over activated alumina. CH2Cl2 was purchased
˚
anhydrous and stored over activated 4 A molecular sieves for
˚
24 h before use. C6D6 and CDCl3 were dried over activated 4 A
molecular sieves for 24 h before use. U(NtBu)2(I)2(THF)2 (1)
and U(NtBu)2(I)2(OPPh3)2 (2)22 were synthesized by published
procedures as were the pyrrolyl ancillary ligands dpm43 and
dpmmes.8 The potassium amide reagents of the dpm ligands were
prepared by adding 1.1 equiv of pyrrole to a THF suspension
of KH. After stirring overnight, the potassium reagents were
recovered by filtering the solution through Celite and precipitation
with hexanes. All other reagents were purchased from commercial
suppliers and used as received. NMR spectra were recorded
on a Bruker AVA300 spectrometer at room temperature unless
otherwise stated. 1H, 13C{ H}, and 31P{ H} NMR spectra are ref-
erenced to external SiMe4 using the residual protio solvent peaks
as internal standards (1H NMR experiments), the characteristic
resonances of the solvent nuclei (13C NMR experiments), or using
H3PO4 as an external standard (31P NMR experiments). Elemental
analyses were performed at the UC Berkeley Microanalytical
Facility, on a Perkin-Elmer Series II 2400 CHNS analyzer.
1
1
Fig. 4 Solid-state molecule structure of U(NBut)2(dpmmes)(dmpe) (6) with
thermal ellipsoids drawn at the 50% probability level. Selected bond lengths
˚
(A) and angles (deg): U1–N1 = 1.865(15), U1–N2 = 1.857(14), U1–N3 =
2.393(15), U1–N4 = 2.431(15), U1–P1 = 3.043(6), U1–P2 = 3.116(6),
N1–U1–N2 = 165.9(6), N1–U1–N3 = 97.10(6), N3–U1–N4 = 93.1(5),
P1–U1–P2 = 67.39(15).
(VI)-cyclopentadienide complex (C5H5)2U(NtBu)2. In reactions
with the Cp-derivative, the dmpe adduct (C5H5)2U(NtBu)2(dmpe)
5
X-ray crystallographic details
is isolated which possess h -coordinated Cp-rings as identified
by NMR spectroscopy and an X-ray diffraction experiment.23 In
The crystal structures of compounds 3–6 were determined as
follows: The crystal was mounted in a nylon cryoloop from
Paratone-N oil under an argon gas flow. The data were collected on
a Bruker SMART APEX II charge-coupled-device diffractometer,
with a KRYO-FLEX liquid nitrogen vapor cooling device. The
instrument was equipped with a graphite monochromatized
contrast, the addition of dmpe to 5 induces the isomerization of
1
pyrrolyl moieties to an h -binding mode presumably to alleviate
increased steric interactions in the uranium centre. Such results
highlight a significant difference in the coordination chemistry of
Cp and pyrrolyl ligands with uranium and also demonstrates the
ease with which pyrrolyl ligands can isomerize on uranium centers.
˚
Mo-Ka X-ray source (l = 0.71073 A), with MonoCap X-ray
source optics. A hemisphere of data was collected using w scans,
with 5 s frame exposures and 0.3◦ frame widths. Data collection
and initial indexing and cell refinement were handled using APEX
II software.44 Frame integration, including Lorentz-polarization
corrections, and final cell parameter calculations were carried
out using the SAINT+ software.45 The data were corrected for
absorption using the SADABS program.46 Decay of reflection
intensity was monitored via analysis of redundant frames. The
structure was solved using direct methods and difference Fourier
techniques. All hydrogen atom positions were idealized and
rode on the atom they were attached to. The final refinement
included anisotropic temperature factors on all non-hydrogen
atoms. Structure solution, refinement, graphics, and creation of
publication materials were performed using SHELXTL.47
Conclusions
Both mono- and bidentate pyrrolyl ligands have been effectively
utilized to yield structurally intriguing bis(imido) uranium(VI)
complexes. In some cases, these pyrrolyl-derivatives exhibit coor-
dination chemistry that is different from transition metal and lan-
thanide counterparts. Of particular note, it appears that increasing
the steric profile of bidentate dipyrrolylmethane ligands results in
the formation of uranium(VI) complexes that have both pyrrolyl
5
moieties in an h -geometry; this feature is in contrast to well-
defined zirconium chemistry in which an h -coordination mode
1
would be expected with monodentate pyrrolyl ligands. Whether
this coordination mode is induced by larger uranium centers or by
the Lewis acidity of the metal centre remains a question that will be
further investigated. Bis(imido) uranium(VI) complexes stabilized
by a sterically encumbered bidentate pyrrolyl ligand design are
also susceptible to isomerization upon the addition of a Lewis
base like dmpe. Such reactivity is not observed in uranium(VI) Cp-
derivatives and highlights an important difference in reactivity
between isoelectronic Cp and pyrrolyl fragments.
Synthesis of U(NtBu)2(g1-2,5-Me2C4H2N)2(OPPh3)2 (3)
To a stirring THF suspension (2 mL) of U(NtBu)2(I)2(OPPh3)2
(500 mg, 0.42 mmol) in a 20 mL scintillation vial was slowly added
a THF solution (2 mL) of KpyrrMe (112 mg, 0.84 mmol). The
orange suspension was stirred overnight and then filtered through
Celite to yield a bright orange-red solution. After removing
the volatiles in vacuo, the red residue was dissolved in CH2Cl2
(2 mL) and layered with an equal volume of hexanes. Large red
needles were recovered after 48 h which were collected by filtration
(359 mg, Yield = 76%). were present in the solid state lattice which
are lost upon exposure to vacuum. Under vacuum, the crystalline
Experimental
General considerations
Allreactions andsubsequentmanipulations were performedunder
anaerobic and anhydrous conditions either under high vacuum
6844 | Dalton Trans., 2010, 39, 6841–6846
This journal is
The Royal Society of Chemistry 2010
©

material obtained readily loses two molecules of co-crystallized
CH2Cl2.
The dark black-brown solution was stirred overnight and then
filtered through Celite to yield a dark black solution. After
removing the volatiles in vacuo, the dark brown residue was
dissolved in a minimum of toluene (0.5 mL) and layered with
hexanes overnight to yield the title compound as a black solid,
(0.047 g. 47%)
1H NMR (CDCl3): trans-isomer d -0.018 (s, 18H, -C(CH3)3),
1.96 (s, 12H, -CH3pyrr), 5.92 (s, 4H, -pyrrH), 7.64 (m, 18H, -ArH),
8.42 (m, 18H, -ArH). cis-isomer d -0.024 (s, 18H, -C(CH3)3), 1.94
(s, 12H, -CH3pyrr), 5.85 (s, 4H, -pyrrH), 7.62 (m, 18H, -ArH),
8.40 (m, 18H, -ArH) and aryl resonances overlapping with trans-
1H NMR (C6D6): d 0.45 (s, 18 H, -C(CH3)3), 0.99 (br s, 12H, -
PCH3), 1.21 (br s, 4H, -PCH2), 2.26 (s, 6H, -C(CH3)2), 2.44 (s, 6H,
-ArCH3), 3.36 (s, 12H, -ArCH3), 5.65 (app s, 2H, -pyrrH), 6.37
1
isomer. 13C{ H} NMR (CDCl3): trans isomer d 20.7 (-CH3pyrr),
30.1 (-C(CH3)3), 73.6 (-C(CH3)3), 107.9 (-pyrrC), 125.4 (-pyrrC),
124.9 (-ArC), 128.1 (-ArC), 132.2 (-ArC), 136.9 (-pyrrC), 139.5
1
(app s, 2H, -pyrrH), 7.01 (s, 4H, -ArH). 13C{ H} NMR (C6D6):
1
13
13
(d, J{ C-31P}=14Hz, -ArC). 31P{1H} NMR (CDCl3): d 45.1 (s,
major), 44.8 (s, minor). Anal. Calcd for C56H64N4O2P2U: %C,
59.78; %H, 5.73; %N, 4.98. Found: %C, 59.69; %H, 5.68; %N,
4.93.
d 16.9 (t, 1J{ C-31P} = 7 Hz, -PCH3), 20.2 (-C(CH3)2), 25.1
1
13
(-ArCH3), 27.4 (s, -NC(CH3)3), 29.6 (t, J{ C-31P} = 14 Hz, -
PCH2), 30.6 (-C(CH3)3), 31.2 (-ArCH3), 39.6 (-C(CH3)2), 74.0
(-C(CH3)3), 107.2 (-pyrrC), 113.1 (-pyrrC), 128.3 (-ArC), 133.0 (-
pyrrC), 137.2 (-pyrrC), 138.5 (-ArC), 138.7 (-ArC), 159.8 (-ArC).
1
31P{ H} NMR (C6D6): d 78.9. Anal. Calcd for C43H66N4P2U: %C,
Synthesis of U(NtBu)2(dpm)(THF)2 (4)
55.00; %H, 7.09; %N, 5.97. Found: %C, 55.03; %H, 7.12; %N, 6.03.
To a stirring THF solution (4 mL) of U(NtBu)2(I)2(THF)2 (0.050 g,
0.064 mmol) in a 20 mL scintillation vial was slowly added a THF
solution (2 mL) of K2dpm (0.016 g, 0.064 mmol). The deep red
suspension was stirred overnight and then filtered through Celite
to yield a dark red solution. After removing the volatiles in vacuo,
the red residue was triturated with hexanes (5 mL) overnight and
isolated by filtration. X-ray quality crystals of 5 were obtained
from a layered THF/hexanes solution at -40 ◦C (0.018 g 42%).
1H NMR (C6D6): d 1.38 (s, 18 H, -C(CH3)3), 1.41 (br s, 8H,
-CH2), 1.76 (s, 6H, -C(CH3)2), 3.61 (br s, 8H, -OCH2), 7.22 (app s,
Acknowledgements
DLS and ALO thank the National Science Foundation of the
United States and the Petroleum Research Fund administered
by the American Chemical Society for funding. DLS thanks
the Seaborg Institute (Los Alamos National Laboratory) for a
fellowship to support this work. LPS thanks the LANL LDRD
office for partial support of this work.
1
2H, -pyrrH), 7.37 (app s, 2H, -pyrrH). 13C{ H} NMR (C6D6): d
References
27.2 (br s, OCH2CH2), 27.8 (-C(CH3)2), 32.3 (-C(CH3)3), 40.6
(-C(CH3)2), 70.9 (-C(CH3)3), 72.1 (br s, OCH2CH2), 108.3
(-pyrrC), 111.6 (-pyrrC), 126.8 (-pyrrC), 142.0 (-pyrrC). Anal.
Calcd for C27H46N4O2U: %C, 46.54; %H, 6.66;%N, 8.04. Found:
%C, 46.59; %H, 6.58; %N, 8.10.
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1
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The Royal Society of Chemistry 2010
Dalton Trans., 2010, 39, 6841–6846 | 6845
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6846 | Dalton Trans., 2010, 39, 6841–6846
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The Royal Society of Chemistry 2010
©