Fluorinated diarylamide complexes of uranium(iii, iv) incorporating ancillary fluorine-to-uranium dative interactions

Article abstract of DOI:10.1039/c2sc21458j

The fluorinated diarylamines HNPhPh^F, HNPh^F ₂, HNPhAr^F, Ph^F = 2,3,4,5,6-pentafluorophenyl, Ar^F = 3,5-bis(trifluoromethyl)phenyl, are used to prepare complexes of uranium(iii, iv) ions. Despite being electron-poor amines with little steric bulk, their coordinated amide ligands exhibit direct control over the coordination environment through a subtle, cooperative interplay of multiple labile F→U dative interactions and favorable arene-arene interactions. The C-F→U interactions, ～8.9 kcal mol^-1 as determined by variable temperature NMR experiments, persist in solution and allow the isolation of otherwise unstable species as well as the first pseudo-square planar uranium complex. The Royal Society of Chemistry 2013.

Full text of DOI:10.1039/c2sc21458j

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EDGE ARTICLE
Fluorinated Diarylamide Complexes of Uranium(III, IV) Incorporating
Ancillary Fluorine-to-Uranium Dative Interactions
Haolin Yin, Andrew J. Lewis, Ursula J. Williams, Patrick J. Carroll and Eric J. Schelter*
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
5
DOI: 10.1039/b000000x
F
F
2
F
F
F
The fluorinated diarylamines HNPhPh , HNPh , HNPhAr , Ph =2,3,4,5,6ꢀpentafluorophenyl, Ar = 3,5ꢀ
bis(trifluoromethyl)phenyl, are used to prepare complexes of uranium(III, IV) ions. Despite being
electronꢀpoor amines with little steric bulk, their coordinated amide ligands exhibit direct control over the
coordination environment through a subtle, cooperative interplay of multiple labile F→U dative
interactions and favorable areneꢀarene interactions. The C–F→U interactions, ~8.9 kcal/mol as
determined by variable temperature NMR experiments, persist in solution and allow the isolation of
otherwise unstable species as well as the first pseudoꢀsquare planar uranium complex.
1
0
F–
F –
2
F–
F
Introduction
NPhPh , NPh
,
and NPhAr , where Ph
= 2,3,4,5,6ꢀ
F
pentafluorophenyl and Ar = 3,5ꢀbis(trifluoromethyl)phenyl. We
demonstrate that C–F→M interactions can be exploited to
influence the structures of the complexes both in the solid state
and solution, and isolate the first example of a pseudoꢀsquare
planar uranium(IV) complex. We also show the C–F→M
interactions induce larger ligand field splitting of electronic states
at the uranium(IV) ion compared to arylꢀamide complexes
without the C–F→M interactions. The C–F→M interactions also
afford isolation of otherwise reactive uranium(III) complexes, as
compared to amides of similar basicity.
The coordination geometries at midꢀ to low valent uranium
ions are typically nonꢀdirectional and unpredictable. Applying
1
1
2
2
3
3
4
4
5
0
5
0
5
0
5
5
0
electronꢀrich ligand systems with large steric demands to confer
stability and direct reactivity has been a fruitful approach in
2
uranium chemistry. Multidentate ligands derived from TREN,
TREN
= tris(2ꢀaminoethyl)amine, have been extensively
explored in this context, as they are electronꢀrich and direct
55
3
ꢀ6
coordination to a confined pocket. These multidentate amide
ligands typically prevent oligomerization that has been observed
for monodentate uraniumꢀamide complexes without sufficient
7, 8
steric protection.
Conversely, stable monomeric uranium
Results and Discussion
complexes can be obtained in relatively open ligand frameworks
incorporating energetically weaker contributions such as
agostic interactions to the coordination sphere of uranium ions.
β
ꢀ
Recently we demonstrated that the use of interꢀligand, nonꢀ
9
ꢀ
60 covalent interactions such as π–π stacking is a viable strategy for
12
28
The combination of sterically demanding ligands and weak
manipulation of reactive uranium centers. In initial steps toward
agostic interactions has been shown to enforce geometries that
understanding the coordination chemistry of dative C–F→U
interactions, we hypothesized that uranium complexes
comprising fluorinated diarylamide ligands could potentially gain
1
3
direct reactivity.
As an alternative to
β
ꢀagostic interactions, we hypothesized
that incorporation of labile, intramolecular, dative C–F→M 65 stability from areneꢀarene interactions. Thus, we set out to
interactions could similarly direct geometries. These C–F→M
prepare homoleptic complexes akin to the uranium(IV) complex
interactions are wellꢀknown in the chemistry of electropositive,
U(NPh2)4 (1–Ph ), reported previously by Edelstein and coꢀ
2
1
4ꢀ17
18ꢀ22
29
earlyꢀtransition metals,
and the lanthanides,
but there are
workers.
few examples of complexes featuring actinideꢀfluorine dative
In the reported synthesis, complex 1–Ph was prepared from
2
23, 24
interactions despite the fluorophilicity of actinides.
example, short contacts between fluorine atoms and
uranium(IV) ion were observed in the crystal structure of
For 70 salt metathesis with UCl in low yield or through transamination
4
29
a
from U(NEt ) in 80% yield. However, the precursor U(NEt )
2 4 2 4
7
was only prepared in 30% yield. We reasoned that an improved
Ar
24
[Cp* Co][U(OB(C F ) ) ( acnac)(OEt )].
In contrast, such
interactions are not evident in the solidꢀstate structures of
reported uranium pentafluoroꢀphenoxide (–OC F ) complexes, 75 weaker U–I bond and low tendency of alkali metal iodides to
yield of 1–Ph could be achieved using UI (Et O) as a starting
2
6
5 3 2
2
2
4
2
2
material through a single step salt metathesis reaction due to the
6
5
2
5, 26
e.g. Li [U(OC F ) ],
or in
a
series of uranium(IV)
form “ꢀate” complexes. Furthermore, we expected that the KI
2
6 5 6
2
7
fluoroketimides, Cp* U[–N=C(CH )(Ar)] .
2
3
2
In this work, we investigate the role of U–F interactions in
uranium complexes of the fluorinated diarylamide ligands
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F
H
PhF
Ph
N
H
N
Ph
Ph
N
N
F
F
N
U
N
U
PhF
Ph
Ph
Ph
F
Ph
4 KNPh2
4 KNPh Ph
- 4 KI
F
N
N
N
- 4 KI
PhF
N
Ph
F
HNPh2
Ph
F
Ph Ph
Ph Ph
F
HNPhPh
F
(
1-Ph2)
(1-PhPh )
UI (OEt )
4
2 2
H
ArF
PhF
PhF
Ph
N
N
CF3
F
F
F
4 KNPhF
- 4 KI
PhF
N
PhF
ArF
4 KNAr Ph
- 4 KI
2
F
H
N
F
Ph
U
N
N U N
N
N
PhF
PhF
ArF
N
Ph
F
F
CF3
F
Ph
Ar
F
PhF
PhF
F F
F
HNPhArF
F
1-PhF
2
( )
HNPhF
2
(
1-PhAr )
Scheme 1. Synthesis of uranium(IV) diarylamide complexes from UI
4 2 2
(Et O) .
side product could be easily removed from the reaction mixture.
30
A convenient synthesis of UI (Et O) was recently reported, and
4
2
2
5
0
5
0
5
the compound has been shown to be an effective starting material
1
1, 31
for the synthesis of uranium amide complexes.
F
2
Addition of 4 equiv of KNPh (Et O) to UI (Et O) in Et O
2
4
2
2
2
resulted in an immediate color change from cherryꢀred to orange
Scheme 1). Removal of KI by filtration and removal of the
(
1
1
2
2
volatiles under reduced pressure followed by extraction with
F
hexanes and storage at –21 °C yielded red crystals (1–Ph ) in
2
F
3
9% yield. The low yield is due to poor solubility of 1–Ph 2 in
hexanes which hampers its extraction. Partial decomposition of
the complex is observed upon treatment with hot hexanes. The
1
9
F
2
F NMR spectrum of 1–Ph in tolueneꢀd at 300 K exhibits
8
three resonances, indicating an equivalent environment of the
four ligands on the NMR timescale as well as free rotation around
F
F
2
N–Ph bonds. Resonances observed for 1–Ph at –156.0 and –
1
64.3 ppm in a 1:2 ratio are attributed to the para- and metaꢀ
fluorines, respectively. The orthoꢀfluorine resonance is broadened
and shifted significantly upfield to –276.8 ppm. The chemical
shift and broadness of the orthoꢀfluorine resonance is suggestive
of a direct interaction with the paramagnetic uranium(IV) ion in
solution at room temperature. No resonances were observed in
1
the H NMR spectrum of the complex.
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IV
F
2
F
2
Fig. 1. Thermal ellipsoid plot of U (NPh
)
4
(1–Ph
) at the 30%
probability level as viewed from the top and side. Selected bond length
Å) and angles (deg): U(1)–N(1) 2.387(2), U(1)–N(2) 2.370(2), U(1)–
(
N(3) 2.2781(17), U(1)–F(1) 2.6480(11), U(1)–F(2) 2.5989(11); N(1)–
5
U(1)–N(3) 95.76(5), N(2)–U(1)–N(3) 84.24(5), N(1)–U(1)–F(1) 63.43(3),
N(2)–U(1)–F(2) 64.21(3).
F
2
Xꢀray crystallographic analysis of 1–Ph revealed a surprising
planar UN4 geometry (Figure 1); the complex resides on a
crystallographic 2ꢀfold axis. The overall geometry is slightly
3
2
1
1
2
2
3
3
4
0
5
0
5
0
5
0
distorted from D symmetry. Represented in τ notation, the
2 4
F
local geometry of 1–Ph exhibits τ = 0.082, a nearly square
planar geometry (τ = 0). The solidꢀstate structure of 1–Ph is
unique compared to the three other reported fourꢀcoordinate
homoleptic uranium(III) and uranium (IV) amide complexes:
{K(thf) }{U[N(SiMe ) ] } (τ = 0.931), U(NPh ) (τ = 0.808),
The unique coordination
environment in 1–Ph is rationalized by the presence of close
contacts between the uranium(IV) ion and four of the orthoꢀ
fluorine atoms of two NPh 2 ligands, at U–F distances of
2.6480(11) and 2.5989(11) Å. In order to accommodate the short
U–F contacts, the bonds U(1)–N(1) and U(1)–N(2), which
include N–C–C–F chelate rings, are ~0.10 Å longer than the
U(1)–N(3) and U(1)–N(3') bonds, which do not include chelating
interactions. The single reported structurally characterized
uranium complex containing dative F→U interactions is
Cp*Co][U(OB(C F ) ) ( acnac)(OEt )], with U–F distances at
.762(6) and 2.789(5) Å. The shorter U–F distances in 1–Ph 2
indicate tighter binding at a more electron deficient metal center
2
4
F
4
2
3
3
29
6
3 2 4
4
2 4
4
3
4
U[N(SiHMe ) ] (τ4
=
0.765).
2
2 4
F
2
F –
Ar
[
2
6
5 3 2
2
2
4
F
Ar
than in [Cp* Co][U(OB(C F ) ) ( acnac)(OEt )]. Finally, the
eight –Ph groups are oriented into four parallel displaced areneꢀ
2
6
5 3 2
2
F
arene interactions at 3.4–3.5 Å (Figure 1).
F
2
Complex 1–Ph can also be formed in low yield by addition
of KNPh (Et O) to UI suspended in toluene at –21 °C,
presumably through disproportionation (4 U → 3 U + U ).
On the other hand, UCl was generally found to be a poor starting
F
2
2
3
III
IV
0 13
4
Fig. 2. Calculated HOMO–5 to HOMO and LUMO to LUMO+4 of 1–
F –
F
2
Ph
, with percentage uranium 5fꢀAO contribution shown in italics for the
material for the synthesis of complexes of the NPh 2 ligand,
respective orbitals. Larger renderings of the orbitals are available as
supplementary information.
generating inseparable mixtures of “ꢀate” complexes retaining
one or two chloride ligands in a variety of solvents including
toluene, ethyl ether or THF.
45
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F
2
Fig. 3. Optimized model of 1–Ph with Mayer bond orders for U–N and
U–F bonds shown in italics.
F
Electronic structure calculations were performed on 1–Ph 2
(see supplementary information) to further characterize the
5
0
5
0
5
0
5
0
5
complex. The optimized geometry was in close agreement with
the crystallographically observed unsymmetric U–N bond lengths
(see Table S2), and the corresponding four U–F contacts were
also effectively reproduced. These results provide evidence that
the crystallographically observed U–F contacts are not solely the
result of packing effects or a favourable interꢀligand collective
stacking arrangement since such weak interactions are poorly
1
1
2
2
3
3
4
4
3
5
treated in the DFT method that was applied.
The calculated filled MOs from the HOMO to HOMOꢀ3
comprise primarily amideꢀcentered ligand orbitals (Figure S30).
Because of the delocalized nature of the filled ligand orbitals it
was not possible to identify primary MOs involved in the F→U
interactions. However, the arrangement of the four interacting
fluorine atoms and two amide nitrogen atoms is symmetrically
poised to allow for collective σꢀdonation from these six atoms
5
0
IV
F
F
4
Fig. 4. Thermal ellipsoid plot of U (NPhPh ) (1–PhPh ) at the 30%
into the unfilled 5fyz
reveals the corresponding σ* MO is the LUMO+4 virtual orbital
Figure 2 and Figure S31), providing indirect evident for the
interactions.
2
orbital. Inspection of the calculated orbitals
probability level. Hydrogen atoms are omitted for clarity. Selected bond
length (Å) and angles (deg): U(1)–N(1) 2.313(2), U(1)–N(2) 2.278(2),
U(1)–N(3) 2.286(2), U(1)–N(4) 2.308(3), U(1)–F(1) 2.7035(16), U(1)–
F(2) 2.7502(17), U(1)–F(3) 2.7221(17), U(1)–F(20) 2.7347(18); N(1)–
U(1)–N(2) 106.31(8), N(1)–U(1)–N(3) 103.25(8), N(1)–U(1)–N(4)
123.15(9), N(2)–U(1)–N(3) 123.46(9), N(2)–U(1)–N(4) 98.62(9), N(3)–
U(1)–N(4) 103.85(9).
(
55
F
Mayer Bond Orders (MBO) were calculated for 1–Ph 2 to
provide the most straightforward assessment of the F→U and U–
N interactions using theory. The calculated MBO for the shorter
F
Addition of KNPhPh to a solution of UI
(Et
O) in Et
O led to
4
2
2
U–N bonds was 0.64 and the longer U–N bonds was 0.53, 60 a red solution with precipitation of KI (Scheme 1). After filtration
whereas the F→U interactions were 0.25 (Figure 3). These results
suggest that the U–N bonds for those amide ligands that include
F→U interactions are longer and weaker than the monodentate
of KI and removal of the volatiles, the product was subsequently
recrystallized from hexanes to yield dark red crystals of
(1–PhPh ) in 59% yield. Xꢀray crystallographic
analysis of 1–PhPh revealed a UN
F
F
U(NPhPh )
4
F
amide ligands, However, accounting for the sum of the MBOs
4
core with a value of τ
4
=
=
F –
between uranium and the NPh 2 ligand with the longer U–N 65 0.744, showing deviation from an ideal tetrahedral geometry (τ
4
F
bonds was larger, suggests that those ligands are more tightly
1). The deviation from tetrahedral geometry in 1–PhPh is
similarly due to four short U–F contacts (2.70–2.75 Å) between
uranium and ortho-fluorine atoms from each ligand (Figure 3).
bound.
F
Complex 1–Ph is indefinitely stable in the solid state or in a
toluene solution at
2
F
F
2
–
21 °C. However, a toluene solution of 1–Ph 2
Similar to 1–Ph
, three resonances in 2:1:2 ratio are observed in
19
F
stored at room temperature resulted in the formation of a black 70 F NMR spectrum of 1–PhPh including a broad orthoꢀfluorine
1
9
1
precipitate over 5 days. The F NMR spectrum of the mixture
indicated two new broad peaks centred at 154.8 and 161.2 ppm
resonance at –303.3 ppm. The H NMR resonance attributed to
–
–
the ortho hydrogen atom appears at 7.45 ppm together with
resonances of meta and para hydrogen atoms at 7.25 and 5.07
ppm, respectively. The sharp contrast in both broadness and shift
F
as decomposed product. We attribute the instability of 1–Ph 2 to
weak σꢀdonating ability of the electron deficient amide ligand.
F
2
The low solubility of 1–Ph in common nonꢀcoordinating 75 of ortho hydrogen and fluorine atoms is consistent with direct U–
F
solvents at low temperature prevented investigation of F→U
F contacts but not U–H contacts in solution. Complex 1–PhPh is
; a red toluene solution of 1–PhPh can
2
be stored at room temperature for two weeks without observable
1
9
F
F
interactions using variableꢀtemperature F NMR spectroscopy.
more stable than 1–Ph
Since only four U–F contacts are observed in the structure of 1–
F
2
1
19
Ph , we sought to prepare a similar complex using a comparably
changes in either the H NMR or F NMR spectra of the
more electronꢀrich ligand that nevertheless possessed metalꢀ 80 complex.
fluorine interactions for the study of the thermodynamics of the
F→U interactions.
4
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1
19
Fig. 5. Variableꢀtemperature H NMR (top) and F NMR (bottom) data
F
F
4 8
for U(NPhPh ) (1–PhPh ) in tolueneꢀd . The orthoꢀfluorine resonance is
5
unobserved over the range of 210–270 K due to paramagnetic relaxation
and is omitted for clarity. Lines are provided as a guide for the eye.
In order to further support the presence of F→U interactions in
F
F
2
Fig. 6. NearꢀIR spectra of 1–Ph (green), 1–PhAr (purple), 1–PhPh
F
1
F
solution for the complex 1–PhPh , variable temperature H NMR 25 (red), 1–Ph
2
(blue) in a fluorobenzene solution at room temperature.
1
9
19
ꢀ1
Spectra are offset for clarity; intensity ≈ 0 at 11000 cm for all spectra.
The asterisk denotes a small contribution from solvent (fluorobenzene)
vibrational band. Solid vertical lines are provided as a guide of eye.
and F NMR experiments were performed (Figure 4). The
F
1
1
2
0
5
0
resonance corresponding to the orthoꢀfluorines broadened and
shifted upfield with decreasing temperature, and was unobserved
Example of spectra deconvolution of nearꢀIR region (bottom) of
F
between 210–270 K. However, a new resonance appeared at –152 30 electronic spectrum for 1-PhPh . Labels on component fit bands
correspond to those listed in Table S1.
ppm and ~200 K, which is assigned to the orthoꢀfluorine directed
away from uranium ion, indicating restricted rotation about the
To further understand the influence of the electronꢀ
withdrawing nature of the amide ligands and the role of U–F
F
N–Cipso bond of the –Ph group. The second expected orthoꢀ
fluorine resonance was not observed, likely due to its direct
interactions in the U(NAr ) complexes, a ligand that is
2 4
F–
interaction with the paramagnetic uranium ion. Decoalescence of 35 electronically similar to NPhPh and does not have the ability to
F
the metaꢀfluorine, orthoꢀhydrogen and metaꢀhydrogen was
observed at ~215 K. The decoalescence of the metaꢀhydrogen
resonances yields a barrier to collective rotation of the aryl
engage in U–F interactions was synthesized. The amine HNPhAr
was prepared in 83% yield on a 10 g scale by a single step
BuchwaldꢀHartwig
crossꢀcoupling
reaction
of
3,5ꢀ
3
6
37
groups about the N–Cipso bonds of ~8.9 kcal/mol.
bis(trifluoromethyl)bromobenzene and aniline. Addition of
F
4
4
5
0
5
0
KNPhAr to a solution of UI (Et O) in Et O similarly led to a
4
2
2
2
red solution with precipitation of KI. Storage of a concentrated
hexanes solution of the product at –21 °C precipitated a dark red
F
F
1
amorphous solid assigned as U(NPhAr ) (1–PhAr ). The
H
4
F
NMR spectrum of 1–PhAr in C D shows 5 peaks in a 2:1:2:1:2
6
6
1
9
ratio and only a single sharp resonance is observed in the
NMR spectrum, clearly indicating symmetric ligand
environments of the monomeric complex in solution.
F
F
2
F
With the four complexes 1–Ph , 1–Ph , 1–PhPh , and 1–
2
F
PhAr in hand, their electronic structures were examined in order
to detect differences based on the presence of F→U interactions.
The UVꢀVisꢀNIR electronic absorption spectra of 1–Ph , 1–
2
F
2
F
F
Ph , 1–PhPh , and 1–PhAr were collected in a fluorobenzene
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F
solution at room temperature to compare the electronic structures
of the complexes. The NIRꢀregions of the four complexes are
qualitatively similar, corresponding to transitions between 5f
electronic states split primarily by interꢀelectron repulsion and
spinꢀorbit coupling. Transitions are assigned for 1–Ph2 as
reactive metal ion. Addition of 3 equiv KNPhAr to a THF,
pyridine, or DME solution of UI at –21 °C led to the initial
45 formation of a dark purple solution with apparent precipitation of
KI in each case. Monitoring these reactions by H and F NMR
showed initial complex formation. However, removal of solvent
or addition of nonꢀcoordinating solvent such as hexanes or nꢀ
pentane induced further reactivity, typically generating red or
3
2
1
19
5
0
5
0
5
0
5
3
3
originating from the H ground state and tentatively include H
4
5
ꢀ
1
3
ꢀ1
3
ꢀ1
(
5880 cm ), F3 (8580 cm ), F4 (8619 cm ) electronic
38, 39
transitions.
The spectral data for all the complexes were fit 50 brown solutions indicative of oxidation products (Scheme 2). In
F
with a sum of 5 Gaussian bands as illustrated for 1–PhPh
one instance a few crystals of a decomposition product were
isolated and characterized by Xꢀray crystallography as
1
1
2
2
3
3
(Figure 6, bottom). A blueshift of the peak positions is clearly
F
2
evident in these deconvoluted components (Table S1).
[K(py)][U(F)(NPhAr ) (py)] (Figure S4).
4
F
2
The comparable transitions of complexes 1–Ph and 1–
F
PhPh , which contain the solution F→U interactions, are shifted
to higher energy with respect to 1–Ph by approximately 200–
2
–1
F
F
400 cm (Figure 6). The results for 1–Ph and 1–PhPh are
2
surprising considering the expected weaker basicity of the
nitrogen donor ligands in those complexes as compared to 1–Ph .
2
F
In contrast, the nearꢀIR spectrum of 1–PhAr is virtually
unshifted relative to 1–Ph , which demonstrates that the energy
2
F
2
F
shifts for 1–Ph and 1–PhPh are larger than can be explained
solely by differences in the electronꢀdonating ability of the
ligands.
4
0
The interꢀelectron repulsion
and spinꢀorbit coupling
parameters are undoubtedly very similar among these complexes.
ꢀ
1
F
The 200–400 cm blueshift of the 5fꢀ5f bands for 1–Ph 2 and 1–
F
PhPh are rationalized on the basis of a larger ligand field
splitting of those complexes than the parent 1–Ph . It is evident
2
that the ancillary F→U interactions play a measurable role in the
F
2
F
electronic structures of complexes 1–Ph and 1–PhPh . Because
the overall ligand field splittings are blueshifted by more than kT
III
F
5
6
5
0
3 3 2
Fig. 7. Thermal ellipsoid plot of U (NPhAr ) (OPPh ) (2) at 30%
at room temperature, the data imply a collective structural
probability. Hydrogen atoms omitted for clarity. Selected bond length (Å)
and angles (deg): U(1)–N(1) 2.390(5), U(1)–N(2) 2.426(5), U(1)–N(3)
.390(5), U(1)–O(1) 2.388(5), U(1)–O(2) 2.387(4); U(1)–O(1)–P(1)
69.7(3), U(1)–O(2)–P(2) 172.3(3), O(1)–U(1)–O(2) 175.99(14), N(1)–
U(1)–N(2) 118.53(17), N(2)–U(1)–N(3) 122.26(17), N(1)–U(1)–N(3)
19.20(17).
F
2
F
stabilization of 1–Ph and 1–PhPh in solution as compared to
2
1
1
–Ph . In light of the collective F→U stabilization that is larger
2
than kT at room temperature, we next considered the application
of ligands that confer ancillary F→U interactions to stabilize and
isolate highly reactive complexes.
1
This unexpected product indicates the instability of the U(III)
complex formed initially. The reaction of UI with KNPhAr
forms a highly reactive complex that is ultimately unstable
towards fluoride abstraction, as a result of the absence of
stabilizing F→U interactions. In an effort to confirm initial
uranium(III) complex formation with NPhAr , the same reaction
was performed in the presence of triphenylphosphine oxide
(TPPO) to trap the reactive uranium(III) product.
Addition of 2 equiv TPPO to UI , followed by subsequent
addition of 3 equiv KNPhAr led to the formation of greenꢀblack
F
3
F
3
KNPhAr , THF
Decomposition
products
-
3 KI
6
5
PPh3
Fꢀ
Ph
O
ArF
ArF
N
N
F
U
O
N
3
KNPhAr , THF, 2 Ph PO
3
( )
2
UI3
Ph
Ph
70
3
-
3 KI
F
ArF
PPh3
solution and precipitation of KI. Recrystallization from a toluene
solution layered with nꢀpentane afforded dark green
U(NPhAr ) (OPPh ) (2) in 76% yield. The H NMR spectrum of
3 3 2
PhF
F
1
O
3
KNPhF2, THF
3 KI
PhF
PhF
75 2 reveals a single ligand environment with freely rotating N–Ph
N
(3)
F
U
O
N
and N–Ar groups and resonances between +18 and –7 ppm. The
-
PhF
PhF
N
19
F NMR spectrum similarly shows a single peak at room
PhF
temperature. The P NMR spectrum shows a single broad peak
31
centered at 266.2 ppm, strongly shifted due to the proximity of
the phosphorous atoms to the metal center. The Xꢀray crystal
structure of 2 reveals an approximately trigonal biꢀpyramidal
3
Scheme 2. Syntheses of uranium(III) diarylamide complexes from UI .
80
Complexes of uranium(III) are typically less stable than their
uranium(IV) counterparts due to the strong reducing potential of
40
geometry, in which the phenyl groups of the Ph PO ligands
3
4
1
the uranium(III) ion. The coordination chemistry of U(III) was
investigated to evaluate the impact of F→U interactions at a more
Fꢀ
cooperatively direct the aryl groups of the NPhAr ligands into
the equatorial plane (Figure 7). As expected from the structure of
6
| Chemical Science, 2012, [vol], 00–00
This journal is © The Royal Society of Chemistry [year]

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evident by the large paramagnetic shifts of orthoꢀfluorine atoms.
F
1
–PhAr , F→U interactions are not present in the structure of 2.
The stability of 2 was thus rationalized based on the ability of the
The F→U interactions also exert larger crystal field splittings
strong donor Ph PO ligands to temper the U(III) ion as well as the 45 than related complexes without F→U interactions, as evident in
3
steric protection provided by the Ph PO ligands that prevents
interꢀmolecular fluorine abstraction. Finally, the NPh 2 ligand
was interrogated for the synthesis of a simple uranium(III)
complex.
the electronic absorption spectra. The F→U interactions were
also shown to stabilize and saturate the coordination sphere of a
U(III) ion, thus allowing the isolation of the uranium(III)
complex, 3.
3
F ꢀ
5
5
5
6
0
5
0
We expect that the incorporation of multiple, weak metalꢀ
fluorine interactions will be a useful strategy to saturate the
coordination sphere of uranium, obviating the need for sterically
9ꢀ12
bulky ligands or agostic interactions.
Moreover, the 8.9
kcal/mol upper limit of the F→U donation suggests that these
interactions are sufficiently labile to allow for facile ligand
substitution at the uranium center. However, in the fourꢀ
coordinate uranium diarylamide case, the displacement of F→U
interactions is complicated by simultaneous displacement of
favourable πꢀπ interactions. Efforts are currently underway to
develop relevant systems aimed at exploring functionalization of
the metal centre with displacement of F→U interactions.
Acknowledgements
The University of Pennsylvania is acknowledged for financial
support. We thank the U.S. National Science Foundation for
Fig. 8. Thermal ellipsoid of U (NPh ) (THF)
2 3 2
(3) at 30% probability. 65 support of the Xꢀray diffractometer used in this work (CHEꢀ
III
F
1
1
0
5
Hydrogen atoms are omitted for clarity. Selected bond length (Å) and
angles (deg): U(1)–N(1) 2.4252(17), U(1)–N(2) 2.4401(16), U(1)–N(3)
0
840438). We also thank the Penn University Research
Foundation for support of the UVꢀVisꢀNIR spectrometer and
JohnsonꢀMatthey for a donation of palladium reagents. This work
used the Extreme Science and Engineering Discovery
Environment (XSEDE), which is supported by U.S. National
Science Foundation grant number OCIꢀ1053575.
2
2
.4367(16), U(1)–O(1) 2.5101(14), U(1)–O(2) 2.4700(14), U(1)–F(1)
.7843(12), U(1)ꢀF(2) 2.9279(12), U(1)ꢀF(3) 2.8131(12); N(1)–U(1)–
N(2) 118.98(6), N(1)–U(1)–N(3) 132.45(6), N(2)–U(1)–N(3) 107.84(6),
O(1)–U(1)–O(2) 154.12(5).
70
F
2
Addition of KNPh (Et O) to UI in THF, resulted in a purple
2
3
solution with precipitation of KI (Scheme 2). Recrystallization
F
from hexanes yielded red crystals of U(NPh ) (THF) (3). The
Notes and references
2
3
2
19
F NMR spectrum of 3 in THF shows three peaks indicating that
a
Department of Chemistry, University of Pennsylvania, 231 S. 34th
F
2
2
3
0
5
0
the Ph groups are equivalent on the NMR time scale. The broad
ortho-fluorine resonance was centered at –302.8 ppm, supporting
the presence of F→U interactions in this complex. Dissolution of
Street, Philadelphia,PA, USA. Fax:(215) 573 6743; Tel: (215) 898 8633;
E-mail: schelter@sas.upenn.edu
† Electronic Supplementary Information (ESI) available: full
experimental details, NMR spectra, . See DOI: 10.1039/b000000x/
75
3
in a nonꢀcoordinating solvent such as benzene or intensive
drying under vacuum led to desolvation and gave multiple
1
9
1. S. Bart and K. Meyer, ed. T. AlbrechtꢀSchmitt, Springer Berlin /
Heidelberg, 2008, vol. 127, pp. 119ꢀ176.
unidentified products by F NMR spectroscopy. Complex 3 was
characterized by Xꢀray crystallography, revealing a pseudoꢀ
trigonal bipyramidal geometry in which three amide ligands
occupy the equatorial plane such that the sum of the N–U–N
angles is 359.3°. There are three observed short F→U contacts
80
2
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1
evident in the structure, 2.8131(20) Å, 2.9279(12) Å, 2.7843(12)
F
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4
2
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potassium salts affords homoleptic uranium(IV) complexes with
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This journal is © The Royal Society of Chemistry [year]
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| Chemical Science, 2012, [vol], 00–00
This journal is © The Royal Society of Chemistry [year]