Synthesis of a phosphorano-stabilized U(IV)-carbene via one-electron oxidation of a U(III)-Ylide adduct

Article abstract of DOI:10.1021/ja2001133

Addition of the Wittig reagent Ph₃P=CH₂ to the U(III) tris(amide) U(NR₂)₃ (R = SiMe₃) generates a mixture of products from which the U(IV) complex U=CHPPh₃(NR ₂)₃ (2) can be obtained. Complex 2 features a short UdC bond and represents a rare example of a uranium carbene. In solution, 2 exists in equilibrium with the U(IV) metallacycle U(CH₂SiMe ₂NR)(NR₂)₂ and free Ph₃P=CH ₂. Measurement of this equilibrium as a function of temperature provides ΔH_rxn = 11 kcal/mol and ΔS_rxn = 31 eu. Additionally, the electronic structure of the U=C bond was investigated using DFT analysis.

Full text of DOI:10.1021/ja2001133

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pubs.acs.org/JACS
Synthesis of a Phosphorano-Stabilized U(IV)-Carbene via
One-Electron Oxidation of a U(III)-Ylide Adduct
Skye Fortier,† Justin R. Walensky,*,‡ Guang Wu,† and Trevor W. Hayton*,†
†Department of Chemistry and Biochemistry, University of California, Santa Barbara, California 93106, United States
‡Department of Chemistry, University of Missouri, Columbia, Missouri 65211, United States
S Supporting Information
b
Scheme 1
ABSTRACT: Addition of the Wittig reagent Ph3PdCH2 to
the U(III) tris(amide) U(NR2)3 (R = SiMe3) generates a
mixture of products from which the U(IV) complex
UdCHPPh3(NR2)3 (2) can be obtained. Complex 2 fea-
tures a short UdC bond and represents a rare example of a
uranium carbene. In solution, 2 exists in equilibrium with
the U(IV) metallacycle U(CH2SiMe2NR)(NR2)2 and free
Ph3PdCH2. Measurement of this equilibrium as a function
of temperature provides ΔHrxn = 11 kcal/mol and ΔSrxn
=
31 eu. Additionally, the electronic structure of the UdC
bond was investigated using DFT analysis.
hile transition metal alkylidenes and carbenes are well
atom transfer reagents, in an attempt to form uranium complexes
possessing metalꢀligand multiple bonds.12 We have turned our
attention toward the reactivity of U(NR2)3 with Wittig reagents
(R3PdCHR0) with the goal of forming complexes possessing
UdC bonds. While a rare transformation, Wittig reagents are
capable of effecting alkylidene transfer in some transition metal
systems, with concomitant generation of PR3.13
W
established,1,2 actinide carbene complexes (excluding
those possessing N-heterocyclic carbene ligands) remain exceed-
ingly rare.3 Uranium carbenes have been observed in inert gas
matrices and are postulated as intermediates in McMurry-type
reactions,4,5 but well-defined, isolable examples are limited. The
first uranium carbenes to be synthesized, Cp3UdCHPMe2R (Cp =
η5-C5H5; R = Ph6 and Me7), were generated by treatment of
Cp3UCl with the lithiated phosphoylide Li[(CH2)2PMeR].
These complexes feature a short uraniumꢀcarbon bond dis-
tance, suggestive of a formal UdC interaction. Nearly 30 years
passed before other uranium carbenes were reported, specifically
[U{C(PPh2S)2}(BH4)2(THF)2]8,9 and [U{C(PPh2NMes)2}2]
(Mes = 2,4,6-Me3C6H2).10 These U(IV) complexes utilize a
chelating methandiide ligand which serves to stabilize the UdC
interaction. Interestingly, every uranium carbene isolated thus far
features at least one phosphorus substituent attached to the R-
carbon.
Treatment of a diethyl ether solution of U(NR2)3 with 1 equiv
of Ph3PdCH2 rapidly generates the U(III)-ylide adduct U-
(CH2PPh3)(NR2)3 (1) in good yield (Scheme 1), which can
be isolated as a deep purple crystalline solid. Inspection of the
solid-state molecular structure of 1 (see the Supporting In-
formation) reveals a UꢀC bond distance of 2.686(6) Å. This
is comparable to the UꢀC distance (2.672(5) Å) found in the
U(III) N-heterocyclic carbene adduct U(ImMe4)[N(SiMe3)2]3
(ImMe4 = tetramethylimidazol-2-ylidene),14 signifying a simple
Lewis acidꢀbase interaction between the U(III) metal center
and the phosphoylide ligand in 1. The 1H NMR spectrum of 1 in
C6D6 indicates the U(III)-Wittig interaction is retained in
solution, as the methylene protons of the Wittig reagent appear
at ꢀ110.55 ppm. Similarly, the 31P{1H} NMR spectrum exhibits
a singlet at ꢀ46.9 ppm, which is shifted significantly from that
observed for the uncomplexed ylide.15 Complex 1 exhibits an
effective magnetic moment of 3.01 μB at 300 K which decreases
to 2.12 μB at 4 K, as determined by SQUID magnetometry. This
is consistent with the proposed U(III) oxidation state.16
The paucity of actinide complexes possessing AndC inter-
actions likely results from an energetic disparity between the
carbon and metal valence orbitals, as has been reasoned for the
absence of lanthanide alkylidene and carbene complexes.11
However, the ability of uranium to form multiple bonds with
heteroatoms, such as nitrogen and oxygen,3 suggests that
complexes possessing UdC double bonds should be attainable.
Isolating new uranium carbenes would both serve to provide
insight into the role of the 5f and 6d orbitals in actinide
metalꢀligand bonding and potentially reveal unprecedented
modes of reactivity.
Solutions of 1 can be stored for several weeks at ꢀ25 °C;
however, upon standing at room temperature, the deep purple
Our laboratory has been investigating the reactivity of the
highly reducing U(III) tris(amide) U(NR2)3 (R = SiMe3) with
Received: January 5, 2011
Published: April 13, 2011
r
2011 American Chemical Society
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The formation of 2 differs from the salt elimination reactions
employed in the syntheses of previous uranium carbenes (vide
supra).6ꢀ10 Instead, 2 arises from the one-electron oxidation of 1
concomitant with the formal loss of H from the coordinated
3
Wittig ligand. To determine the fate of this hydrogen atom, we
1
monitored solutions of 1 by H and 31P{1H} NMR spectro-
scopies. This revealed the formation of 2 along with two new
products, namely PPh3 and the U(IV) methyl complex U(CH3)-
(NR2)3 (3), in a 1:1:1 ratio (Scheme 1). The presence of 3 was
confirmed by comparison of its spectral properties with inde-
pendently prepared samples and by elemental analysis of isolated
material.23 Monitoring the reaction of U(CD2PPh3)(NR2)3
(1-d2) by 2H and 1H NMR spectroscopies reveals the formation
of U(CDPPh3)[N(SiMe3)2]3 (2-d1) and U(CD3)[N(SiMe3)2]3
(3-d3), as anticipated. However, this process is complicated by
Figure 1. Solid-state molecular structure of UdCHPPh3(N(SiMe3)2)3
(2) with 50% probability ellipsoids. Selected bond lengths (Å) and angles
(deg): U1ꢀC1 = 2.278(8), C1ꢀP1 = 1.679(8), U1ꢀC1ꢀP1 = 151.7(4),
U1ꢀN1 = 2.287(6), U1ꢀN2 = 2.304(6), U1ꢀN3 = 2.290(6).
2
the scrambling of the H label between the ylide methylene
group and the N(SiMe3)2 ligands in 1-d2 (see the Supporting
Information). This scrambling occurs prior to its conversion into 2-
d1 and 3-d3. As a result, we observe small amounts of 1H incorpora-
tion into the methine position of 2-d1 and the methyl position of 3-
d3. We also observe 2H incorporation into the SiMe3 groups of 2-d1,
3-d3, and 4. The mechanism by which the scrambling occurs in 1-d2
is currently under investigation; however, a related uranium system
has been observed to undergo similar scrambling.21
color of these solutions bleaches to a light red-orange within
several hours. From these solutions, the U(IV) carbene
UdCHPPh3(NR2)3 (2) can be isolated (Scheme 1) in moderate
yield. Complex 2 can also be obtained, in better yield (70% yield,
35% based on U(NR2)3), by direct reaction of U(NR2)3 with
excess Ph3PdCH2 in Et2O.
Given these results, the generation of 2 appears to arise from
an intermolecular hydrogen atom transfer between ylide ligands.
Analysis of 2 by X-ray crystallography reveals a complex
possessing a short uraniumꢀcarbon bond distance (U1ꢀC1 =
2.278(8) Å) and a large U1ꢀC1ꢀP1 bond angle (151.7(4)°)
(Figure 1). For comparison, this UꢀC distance is significantly
shorter than a typical UꢀCalkyl distance (2.4ꢀ2.5 Å), such as that
found for Cp*2UMe2 (2.41(1) Å) (Cp* = η5-C5Me5) or [Na(18-
crown-6)THF][U(NR2)(CH2SiMe2NR)2] (2.457(6) Å) (R =
SiMe3).17ꢀ19 Moreover, the UꢀC bond length is comparable to
the UdC distance found in Cp3UdCHPMe2Ph (2.29(3) Å)6
and in [Cp2U{C(PPh2S)2}] (2.336(4) Å),9 but shorter than
those observed in [U{C(PPh2NMes)2}2] (2.427(8) Å, 2.448(9)
Å).10 Finally, the PꢀC bond length (P1ꢀC1 = 1.679(8) Å) in 2
is identical to that of the uncoordinated ylide, Ph3PdCH2
(1.661(8) Å)20 suggesting that carbene formation does not
greatly affect the PꢀC interaction. Finally, complex 2 exhibits
an effective magnetic moment of 2.84 μB at 300 K which
decreases to 0.97 μB at 4 K, as determined by SQUID mag-
netometry. This temperature response is consistent with the
proposed U(IV) oxidation state.16
This is similar to the H transfer reactivity observed between
3
U(NR2)3 and the U(V) imido complex U(dNAr)(NR2)3 (Ar =
p-tolyl), which yields the U(IV) metallacycle, 4, and the U(IV)
amide U(NHAr)(NR2)3.24 A related hydrogen atom transfer
between ylide ligands in 1 would result in the formation of the
phosphoranyl radical [Ph3PCH3] , a known source of CH3 via
3
3
PꢀCalkyl bond homolysis.25,26
To test for hydrogen atom transfer, the reaction between
complex 1 and a substoichiometric amount of TEMPO
(TEMPO = 2,2,6,6-tetramethylpiperidine-1-oxyl) in C6D6 was
explored.27 Addition of 10 mol % TEMPO to 1 results in the
rapid formation of complexes 2, 3, and Ph3P. The reaction is
complete after 2.5 h, a nearly 4-fold decrease from the normal
reaction time in this solvent. The final product distribution also
reveals a deficiency of 3 and Ph3P, relative to 2, equivalent to the
amount of TEMPO added to the reaction. The only TEMPO-
derived product observed in the final reaction mixture is tetra-
methylpiperidine. Its formation may be due to an O-atom
transfer side reaction28 or decomposition of the hydroxylamine,
TEMPO-H. Overall, these observations support the premise that
The 1H NMR spectrum of 2 in C6D6 exhibits a broad singlet
at ꢀ147.1 ppm, assignable to the methine proton of the carbene
ligand, while its 31P{1H} NMR spectrum reveals a singlet at
172.1 ppm, which is shifted dramatically downfield from the
the reaction proceeds via an initial H atom transfer event.
3
The solution phase interconversion of 2 and 4 was monitored
by variable-temperature NMR spectroscopy from 243 to 295 K
in toluene-d8, and the thermodynamic parameters were extracted
from the equilibrium concentrations of the reactants. The van’t
Hoff plot is linear (see the Supporting Information) and reveals
that the forward reaction is endothermic (ΔH = 11 kcal/mol)
and entropically driven (ΔS = 31 eu). Assuming that ΔHrxn is
solely due to the two bond breaking events and the two bond
making events (Scheme 1), and the contribution of solvation
effects is negligible, then ΔHrxn can be represented by eq 1.
1
methylene signal observed for 1. Interestingly, the H and 31P-
{1H} NMR spectra of 2 show that, in solution, the complex exists
in equilibrium with uncoordinated Ph3PdCH2 and the U(IV)
metallacycle U(CH2SiMe2NR)(NR2)2 (4)21,22 (Scheme 1).
This equilibrium is highly dependent upon solvent and tempera-
ture. For instance, the metallacycle is favored in a 5:1 ratio
over 2 in 25 mM toluene solutions at room temperature, whereas
the ratio changes to 1:5, respectively, in 30 mM diethyl ether
solutions of 2 at ꢀ30 °C. Accordingly, 2 can be isolated in 59%
yield by addition of 1.2 equiv of Ph3PdCH2 to a concentrated
diethyl ether solution of 4 at ꢀ25 °C, where the lower temperature
favors the formation of 2. In contrast, in polar solvents such as THF
or pyridine, 2 converts completely to 4 and Ph3PdCH2, irrespective
of temperature.
ꢀΔHrxn ¼ DðCꢀHÞWittig þ DðUꢀCÞ ꢀ DðCꢀHÞSiMe3
ꢀ DðUdCÞ
ð1Þ
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results are nearly identical with 12% and 7% U character for the
UꢀC σ-bond (HOMO) and π-bond (HOMO-9), respectively.
The corresponding C2p contributions are 88% and 93% for
the σ- and π-bonds, respectively. Overall, the calculations
reveal that the UꢀC interaction is highly polarized with
modest π character, consistent with conclusions drawn from
the thermochemistry data.
In summary, the U(III) ylide adduct U(CH2PPh3)(NR2)3
undergoes an intermolecular H atom transfer between ylide
3
ligands, to form the U(IV) carbene complex UdCHPPh3(NR2)3,
the U(IV) methyl complex U(CH3)(NR2)3, and Ph3P, revealing
a new mode of ylide reactivity with a metal complex. This result also
highlights the propensity of U(NR2)3, and U(III) reagents more
generally, to favor one-electron redox chemistry.12,34ꢀ36
Given that the formation of a metal carbene by group transfer
typically requires a formal two-electron oxidation at the metal
center, new carbene transfer reagents exploiting the one-electron
redox chemistry of U(III) appear necessary for the synthesis of
carbenes from U(III). Utilizing this knowledge, we intend to
investigate this strategy of carbene synthesis with the goal of
generating new uranium carbene complexes in which the carbene
ligand is not stabilized by a phosphorano substituent.
Figure 2. KohnꢀSham orbital representations of the π (left) and σ
(right) bonds of the UdC interaction in 2, plotted at the 0.02 isolevel.
The strengths of UꢀCalkyl bonds are highly sensitive to the
coordination environment provided by the ancillary ligands,29,30
making it difficult to calculate an absolute D(UdC) value.
Nonetheless, using eq 1 it is possible to determine the relative
difference between D(UdC) and D(UꢀC). A reliable bond
enthalpy is known for D(CꢀH)SiMe3; however, to our knowl-
edge, the D(CꢀH) for an R3PdCHꢀH ylide has not been
measured. Thus we were required to use the calculated D(CꢀH)
for the parent phosphonium salt, [Me3PCH2ꢀH]þ, the closest
analogue for which data are available. Substituting these litera-
ture values into eq 1 (D(CꢀH)SiMe3 = 99 kcal/mol31 and
D(CꢀH)Wittig = 103 kcal/mol32) provides an estimate of the
relative difference between D(UdC) and D(UꢀC) of 15 kcal/mol.
Previously reported D(UꢀC) values for uranium alkyls vary
from 29 kcal/mol for Cp03U(nBu) (Cp0 = Me3SiC5H4)30 to
75 kcal/mol for Cp*2U(Me)Cl,29 allowing us to assess an
upper limit of 90 kcal/mol for D(UdC). Given this, it is
apparent that D(UdC) of 2 is only slightly greater than a
typical uranium carbon single bond, such as that found for
Cp*2U(Me)Cl.29 It is also clear that D(UdC) of 2 is sig-
nificantly weaker than the D(TadC) = 126 kcal/mol for
TadCHR(CH2R)3 (R = SiMe3),33 highlighting the difference
between MdC bond strengths of an actinide carbene and a
traditional transition metal alkylidene.
’ ASSOCIATED CONTENT
S
Supporting Information. Experimental procedures, crys-
b
tallographic details (as CIF files), and spectral data for 1ꢀ3. This
material is available free of charge via the Internet at http://
pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
hayton@chem.ucsb.edu; walenskyj@missouri.edu
’ ACKNOWLEDGMENT
We thank the University of California, Santa Barbara and the
Department of Energy (BES Heavy Element Program) for
financial support of this work, and Theodore A. Matson for his
preliminary investigations in this area. J.R.W. thanks Prof.
Michael B. Hall for helpful discussions and NFEAP for start-
up funds.
To gain further insight into the electronic structure of the
UdC bond, a DFT analysis was performed on 2 at the B3LYP
level of theory. The bond distances compare well with those
obtained experimentally: the calculated UꢀC bond distance is
2.284 Å, while the calculated UꢀN bond distances are 2.313,
2.324, and 2.331 Å. The Mulliken spin density of 2.133 is
consistent with a U(IV) complex having two unpaired 5f
electrons. These reside in HOMOꢀ1 and HOMOꢀ2, which
are both predominantly metal based and nonbonding (see
Supporting Information). The main orbitals involved in the
uraniumꢀcarbon interaction are the HOMO, which consti-
tutes the π interaction, and HOMO-6, which constitutes the
main σ interaction (Figure 2). According to Mulliken popula-
tion analysis, the HOMO is mostly carbon 2p character with
22% uranium character (51% C2p, 16% U5f and 6% U6d).
This is slightly more U character than that calculated for
previous uranium carbenes.8,10 HOMOꢀ6 is also mostly
carbon centered (24% C2p, 4% C2s, 9% U6d, 4% U5f).
A natural bond orbital (NBO) analysis was also conducted on
complex 2, and on Cp3UdCHPMe2Ph,6 for comparison. The
UꢀC σ-bond (HOMO-6) in complex 2 includes 12% U charac-
ter (10% 7s, 15% 7p, 35% 6d, 40% 5f) while the UꢀC π-bond is
composed of 8% U character (18% 7s, 11% 7p, 54% 6d, 17% 5f).
The corresponding C2p contributions are 88% and 92% for the
σ- and π-bonds, respectively. For Gilje’s carbene complex,6 the
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