Imido exchange in bis(imido) uranium(VI) complexes with aryl isocyanates

Article abstract of DOI:10.1021/ja7107454

Addition of 1 and 2 equiv of ArNCO (Ar=Ph, 2,4,6-Me₃C₆H₂) to U(N_tBu)₂(I)₂(OPPh₃)₂ yields the aryl-imido complexes U(NAr)(NtBu)(I)₂(OPPh₃)₂ and U(NAr)₂(I)₂(OPPh₃)₂, respectively. Unlike analogous transition metal reactions this imido exchange reaction does not proceed through a metal oxo intermediate. Density functional theory calculations and ¹⁵N-labeling studies suggest this transformation involves the [2 + 2] cycloaddition of the aryl isocyanate C=N bond across the U=N imido ligand to form an N,N-bound ureato intermediate. Copyright

Full text of DOI:10.1021/ja7107454

Published on Web 02/14/2008
Imido Exchange in Bis(imido) Uranium(VI) Complexes with Aryl Isocyanates
Liam P. Spencer,† Ping Yang,‡ Brian L. Scott,† Enrique R. Batista,*,‡ and James M. Boncella*,†
Materials, Physics and Applications DiVision, Los Alamos National Laboratory, MS J514, Los Alamos New Mexico,
87545, and Theoretical DiVision, Los Alamos National Laboratory, MS B268, Los Alamos New Mexico, 87545
Received December 11, 2007; E-mail: boncella@lanl.gov; erb@lanl.gov
For the past 150 years, studies of uranium(VI) have been
generally directed toward understanding the chemical behavior and
unique bonding in the uranyl ion (UO22+). This ion possesses U-O
bonds which have high thermodynamic stability and extreme kinetic
2+
inertness. As a result, the majority of UO2 reaction chemistry
involves substitution of equatorially coordinated ligands while
leaving the U-O bond unaffected.1 Recently, we have reported
the syntheses of isoelectronic bis(imido) [U(NR)2]2+ and oxo-imido
[U(NR)(O)]2+ ions which possess many of the bonding features
found in UO22+.2 Our recent isolation of these ions has stimulated
Figure 1. Solid-state molecular structure of [U(NPh)(NtBu)(I)2(OPPh3)2]
(1) and [U(NMes)2(I)2(OPPh3)2] (6) (Mes ) 2,4,6-Me3C6H2). Selected bond
lengths (Å) and angles (deg) for 1: U1-N1 ) 1.832(8), U1-N2 ) 1.841-
(8), U1-O1 ) 2.320(7), U1-O2 ) 2.306(7), U1-I1 ) 3.0623(9), U1-I2
) 3.0486(9), O1-P1 ) 1.493(7), O2-P2 ) 1.497(7), N1-U1-N2 )
177.4(4), N1-U1-O1 ) 92.6(4), O1-U1-O2 ) 178.2(3). Selected bond
lengths and angles for 6: U1-N1 ) 1.867(5), U1-O1 ) 2.313(4), U1-I1
) 3.0618(5), O1-P1 ) 1.513(5), N1-U1-N1A ) 180.0(5), N1-U1-
O1 ) 89.9(2).
us to investigate their reactivity to elucidate a more complete
understanding of uranium(VI) chemistry.
In this communication, we report our investigations of the
reactivity of the [U(NtBu)2]2+ ion toward aryl isocyanates. Given
the strength and stability of the U-O multiple bond, these reactions
were anticipated to produce the oxo-imido framework. Instead, we
observe an unexpected reaction that does not involve UdO bond
formation but rather an imido exchange reaction in which aryl-
imido for alkyl-imido substitution occurs.
Scheme 1
Organic isocyanates have often been used in transition metal
chemistry to effect transformations of MdN imido functional
groups.3 For instance, the reactions of R*NCO with MdNR imido
compounds have been reported to produce either (1) carbodiimides
(R*NCNR) and metal oxo complexes or (2) a new isocyanate
(RNCO) and MdNR* metal imido compound. The latter transfor-
mation is unusual, although there are several reports in which high
oxidation state titanium, vanadium, and molybdenum complexes
can accomplish this conversion.4-6 The isolation of N,O- or N,N-
bound ureate complexes during these reactions suggests that the
reaction proceeds by the formal [2 + 2] cycloaddition of either the
CdO or CdN bond of the isocyanate across the MdN imido
functional group.7 Given the strength of UdO bonds, it was
anticipated that the reaction of the bis(imido) complex U(NtBu)2-
(I)2(OPPh3)2 (1) with aryl isocyanates would yield the oxo-imido
complex 2 (Scheme 1) or possibly an N,O-bound ureate.
(3) Å) and U(NPh)2(I)2(THF)3 (U-N avg ) 1.863(2) Å) complexes
as is the N1-U1-N2 bond angle (177.4(4)°).2b The U-I and
U-O(phosphine oxide) bond lengths are also similar to those of
other previously described uranium trans-bis(imido) complexes.
The reactions between 1 and 2 equiv of ArNCO (Ar ) Ph, 2,4,6-
Me3C6H2) yield symmetric bis(imido) complexes 5 and 6 (Scheme
1). 31P NMR spectroscopy confirms the formation of 6 as observed
Stirring orange-red solutions of 1 with ArNCO (ArdPh, 2,4,6-
Me3C6H2) in CH2Cl2 generates dark red-brown solutions. In the
reaction with PhNCO, the mixed bis(imido) complex, U(NPh)(Nt-
Bu)(I)2(OPPh3)2 (3), can be isolated from this reaction as a dark
1
red powder in 74% yield. The H NMR spectrum of 3 displays
1
by a singlet at 48.5 ppm. The H NMR spectrum indicates only
phenyl resonances at 5.35, 5.82, 6.92 ppm and a tert-butyl imido
resonance at 0.19 ppm (Scheme 1).
one set of aryl-imido resonances, with two methyl groups located
1
at 2.41 and 2.45 ppm. In the case of 5, the H NMR spectrum of
Single crystals of 3 were grown from a mixture of CH2Cl2/
hexanes and analyzed by X-ray crystallography. The solid-state
molecular structure of 3 is shown in Figure 1. Complex 3 possesses
a pseudo-octahedral geometry at the uranium center with trans-
oriented bis(imido) ligands. The U-N(imido) bond lengths (U-
N(alkyl) ) 1.832 Å, U-N(Ph) ) 1.841(8) Å) are comparable to
bond lengths found in U(NtBu)2(I)2(OPPh3)2 (U-N avg ) 1.840-
the product matches the compound U(NPh)2(I)2(OPPh3)2 which has
been previously described. Interestingly, there are no reverse
reactions between tert-butyl isocyanate and aryl imido complexes
3-6 as indicated by 31P NMR spectroscopy.
Single crystals of 6 suitable for an X-ray diffraction study were
grown from CH2Cl2/hexanes, and the solid-state molecular structure
is shown in Figure 1. Complex 6 was found to crystallize in the
triclinic space group P1h, with an inversion center at the uranium
metal atom. The molecule possesses short U-N bonds (U1-N1
† Materials, Physics and Applications Division.
‡ Theoretical Division.
9
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10.1021/ja7107454 CCC: $40.75 © 2008 American Chemical Society

C O M M U N I C A T I O N S
(15N-5), respectively. 15N-3 and 15N-5 display single 15N resonances
at 390.5 and 393.3 ppm in the 15N{1H} NMR spectra, respectively.
While the 15N{1H} NMR chemical shifts of U(VI) imido com-
pounds have yet to be reported, the resonances exhibited for 15N-3
and 15N-5 are similar to those of previously characterized 15N-
labeled tungsten(VI) imido complexes.11
Scheme 2 Relative Free Energies of the Products and
Intermediates in the Potential Pathways for the Formation of 2
and 3a
Given the evidence from DFT calculations and the results from
15N-labeling studies, it appears the mechanism for the formation
of 3 involves the [2 + 2] cycloaddition of the aryl isocyanate Cd
N bond across the UdN imido moiety. These results are quite
surprising given the thermodynamic and kinetic stability of UdO
bonds. We are currently investigating the relative energies of the
transition states in Paths 1 and 2 to determine if the selectivity for
the formation of the mixed imido species 3 over the oxo-imido
complex 2 is kinetic in origin. Preliminary experiments show that,
at higher temperatures (50 °C), the initially formed mixed imido
a Energies reported at the hybrid DFT level of theory relative to the
t
complex, 3, reacts further with the BuNCO that is produced to
energy of 1 are provided in parentheses in kcal mol-1
.
give the mixed oxo-imido complex 2 which is consistent with the
relative energies determined by DFT calculations. While alkyl
substituted carbodiimides do not react with 1 at room temperature
or at 50 °C for 15 h, we are actively investigating the reactions of
these and other unsaturated electrophiles to determine the generality
of this unexpected imido reactivity.
) 1.867(5) Å) and a linear N-U-N bond angle. The U-O(OPPh3)
bond lengths in 6 (U1-O1 ) 2.313(4) Å) are comparable with
those in 3 and the uranyl analogue UO2I2(OPPh3)2.2b,8
Density functional theory (DFT) calculations were performed
to elucidate the relative energies of the intermediates and products
of this reaction.9 There are two reaction pathways which could
generate the bis(imido) product 3 and the oxo-imido species 2 from
U(NtBu)2(I)2(OPPh3)2 (Paths 1 and 2, Scheme 2). The first pathway
involves the [2 + 2] cycloaddition of the CdN bond of the aryl
isocyanate to form an N,N-bound ureato intermediate (7), which
can isomerize to form species 8 with the -NPh group trans to the
tert-butyl imido moiety. Compound 8 can then eliminate tBuNCO
to generate the unsymmetrical bis(imido) complex 3 (Path 1).
Alternatively, N,O-bound carbamate intermediates 9 and 10 could
form which result from the [2 + 2] cycloaddition of the CdO bond
of the aryl isocyanate across the UdN imido bond. Elimination of
a substituted carbodiimide would generate the oxo-imido complex
2 (Path 2).
Acknowledgment. P.Y. and L.S. thank the Seaborg Institute
for their postdoctoral fellowships. E.R.B. and J.M.B. were partially
supported by the Division of Chemical Sciences, Office of Basic
Energy Sciences, U.S. DOE under the Heavy Element Chemistry
program at LANL.
Supporting Information Available: Complete details of the
preparation and characterization of 3-6, including X-ray crystal-
lographic details (as CIF files) of 3 and 6. Geometries of the calculated
structures of 2, 3, and 7-10. This material is available free of charge
via the Internet at http://pubs.acs.org.
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The computational results suggest the lowest energy pathway
involves the [2 + 2] cycloaddition of the CdN bond of phenyl
isocyanate to form the N,N-bound ureato intermediate 7 (Path 1,
Scheme 2). In the calculations performed, it is assumed that OPPh3
dissociation occurs in order to generate intermediates 7 and 8.
Experimentally, the reactions between 1 and PhNCO proceed much
more slowly in the presence of excess OPPh3 as is consistent with
this assumption. Overall the transformation of U(NtBu)2(I)2(OPPh3)2
(1) to U(NPh)(NtBu)(I)2(OPPh3)2 (3) is exergonic, with the free
energy of the bis(tert-butyl)imido uranium complex (1) + PhNCO
higher than the mixed imido species 3 + tBuNCO by 6.2 kcal mol-1
.
In contrast to this mechanism, the calculated [2 + 2] CdO
cycloaddition bond pathway (Path 2) involves the formation of the
higher energy N,O-bound ureato intermediates 9 (19.8 kcal mol-1
)
and 10 (28.7 kcal mol-1). Complex 9 can then eliminate the mixed
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t
carbodiimide BuNdCdNPh and form the oxo-imido species 2.
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As was anticipated, the relative energy of 2 is substantially lower
in energy than the unsymmetrical imido species 3 (15.2 kcal mol-1).
This energy difference between UdO and UdN bond formation
has also been observed in cyclopentadienyl-substituted uranium-
(IV) complexes.10
The reaction between 1 and 2 equiv of Ph15NCO and 1 produces
U(NtBu)(15NPh)(I)2(OPPh3)2 (15N-3) and U(15NPh)2(I)2(OPPh3)2
(9) Complete details of the theoretical calculations can be found in the
Supporting Information.
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