Exchange of an imido ligand in bis(imido) complexes of uranium

Article abstract of DOI:10.1021/ja064400j

Addition of B(C₆H₅)₃·H₂O to U(N^tBu)₂I₂(THF)₂ provides U(N^tBu)(O)I₂(THF)₂, a complex with a trans arrangement of the oxo and imido ligands. A DFT study on the Ph₃PO adduct, U(N^tBu)(O)I₂(Ph₃PO)₂, reveals that there are six bonding orbitals in the O=U=N interaction, much like the bis(imido) N=U=N interaction. However, the calculations suggest that the multiple bonding in the oxo imido complexes is less covalent than that in the bis(imido) analogues. Copyright

Full text of DOI:10.1021/ja064400j

Published on Web 09/13/2006
Exchange of an Imido Ligand in Bis(imido) Complexes of Uranium
Trevor W. Hayton,† James M. Boncella,*,† Brian L. Scott,† and Enrique R. Batista‡
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 June 21, 2006; E-mail: boncella@lanl.gov
The study of the uranyl ion, UO22+, has been central to
understanding metal-ligand bonding in the f elements and in
determining the extent to which the f electrons participate in such
interactions.1 Our recent isolation of the imido analogues of uranyl,
namely, U(NR)22+, provides another model that can be used to
refine our understanding of these phenomena.2,3 We now report
the synthesis of a uranium oxo imido complex, trans-[U(NR)(O)]2+
,
which provides a link between the uranyl and bis(imido) extremes.
We have been studying the reactivity of the [U(NR)2]2+ fragment
with various Lewis bases, including monodentate amines. For
instance, addition of excess aniline to a toluene solution of
U(NtBu)2I2(THF)2 quickly results in the deposition of the expected
U(NtBu)2I2(NH2Ph)2 (1) as pale-orange plates, in moderate yield
(Scheme 1).4
Figure 1. Solid-state molecular structures of U(NtBu)(O)I2(THF) (NH2-
Ph)2 (2) and U(NtBu)(O)I2(Ph3PO)2 (4). Selected bond lengths (Å) and
angles (deg) for 2: U1-O1 ) 1.781(4), U1-N1 ) 1.823(4), U1-O2 )
2.452(4), U1-N2 ) 2.632(4), U1-N3 ) 2.597(5), U1-I1 ) 3.1455(5),
U1-I2 ) 3.1749(5), O1-U1-N1 ) 178.6(2), C1-N1-U1 ) 170.4(4).
Selected bond lengths (Å) and angles (deg) for 4: U1-O1 ) 1.764(5),
U1-N1 ) 1.821(7), U1-O2 ) 2.335(5), U1-O3 ) 2.302(5), U1-I1 )
3.0660(7), U1-I2 ) 3.0443(8), O1-U1-N1 ) 178.4(3), C1-N1-U1 )
172.3(7).
However, during one reaction performed in THF, a few small
orange blocks were isolated upon recrystallization from toluene.
These crystals were quite different in morphology than those of 1.
The material was analyzed by X-ray crystallography and found to
be U(NtBu)(O)I2(THF)(NH2Ph)2 (2), and its solid-state molecular
structure is shown in Figure 1. Complex 2 arises from the partial
hydrolysis of the starting bis(imido) complex by trace quantities
of water in the solvent or on the glassware (Scheme 1). It possesses
a pentagonal bipyramidal geometry similar to those found in many
uranyl and uranium bis(imido) complexes. However, its most salient
structural features are the oxo and imido ligands, which adopt a
trans orientation (O1-U1-N1 ) 178.6(2)°). The U-O bond length
is 1.781(4) Å, comparable to the U-O bond lengths found in the
uranyl ion, while the U-N(imido) bond length (1.823(4) Å) is
comparable to the U-N bond lengths in the previously characterized
uranium trans-bis(imido) complexes.
Scheme 1
Scheme 2
Only one other oxo imido complex has been structurally
characterized for uranium, Cp*2U(O)(N-2,6-(iPr)2C6H3). In this
instance, the oxo and imido ligands are in a cis configuration, and
the U-N and U-O bond lengths are 1.988(4) and 1.844(4) Å,
respectively.5,6 The structures of trans-[U(O)(NE)Cl4]- (E ) SR2,
PPh3) are also known. In these complexes, the U-O bond lengths
are comparable to those in 2, but the U-N bond lengths are
considerably longer (E ) SR2, U-N ) 1.920(3) Å; E ) PPh3,
U-N ) 1.912(3) Å).7,8
ing an imido ligand for an oxo group in tellurium imido complexes,
without unwanted reactivity.10
Monitoring an equimolar solution of B(C6F5)3‚H2O and
U(NtBu)2I2(THF)2 in THF-d8 by 1H NMR spectroscopy reveals the
t
growth of two new Bu peaks at 0.69 and 1.23 ppm with equal
intensity, consistent with the formation of a new uranium imido
complex and B(C6F5)3‚H2NtBu, respectively. The 11B NMR spec-
trum of the reaction mixture reveals that a new resonance at -11.1
ppm, assignable to B(C6F5)3‚H2NtBu, also appears over the course
of the reaction.10
On a preparative scale, the reaction of B(C6F5)3‚H2O and
U(NtBu)2I2(THF)2 in benzene results in the deposition of a red-
orange powder within several hours. Crystallization of this powder
from THF/hexanes provides U(NtBu)(O)I2(THF)2 (3) in moderate
yields (Scheme 2).
We have endeavored to develop a rational synthesis of 2 by
direct addition of water; however, addition of 1 equiv of H2O to
U(NtBu)2I2(THF)2 in mixtures of aniline/THF, or neat THF, only
t
provide the products of complete hydrolysis (e.g., BuNH3I) or
sometimes UO2I2(THF)3.9 Clearly, H2O is too indiscriminate a
reagent to be useful in this instance, and it is likely that only
extremely low concentrations of water will lead to hydrolysis of
only one imido ligand. In contrast, Chivers and co-workers found
that B(C6F5)3‚H2O was an effective reagent for selectively exchang-
Complex 3 dissolves in benzene and CH2Cl2 but quickly
precipitates in either solvent to provide a fine orange powder. We
attribute this behavior to the loss of a THF ligand and the deposition
of insoluble [U(NtBu)(O)I2(THF)]x.11 Not surprisingly, complex 3
† Materials Physics and Applications Division.
‡ Theoretical Division.
9
12622
J. AM. CHEM. SOC. 2006, 128, 12622-12623
10.1021/ja064400j CCC: $33.50 © 2006 American Chemical Society

C O M M U N I C A T I O N S
A vibrational mode analysis of complex 4 identified a U-O
mode at 903 cm-1, whereas the U-N mode couples with the N-C
mode, leading to three bands at 1142, 1144, and 1159 cm-1. The
simulation has helped us identify the U-O and U-N vibrations in
the IR spectrum of complex 4. As a KBr pellet, the IR spectrum of
4 exhibits a band at 858 cm-1, which we attribute to the U-O
vibration, and bands at 1128 and 1134 cm-1, which we attribute to
the U-N vibrations. For complex 3, we have unambiguously
identified the U-O vibration at 883 cm-1 (KBr pellet), as this peak
is shifted to 827 cm-1 in the IR spectrum of 3-18O.13 For
comparison, the U-O vibration in Cp*U(O)(N-2,4,6-Me3C6H2) was
observed at 775 cm-1 6
.
In conclusion, the synthesis of 4 provides a bridge between the
isostructural uranyl analogue, UO2I2(Ph3PO)2,14 and the bis(imido)
congener, U(NtBu)2I2(Ph3PO)2. The use of the borane adduct of
water to exchange oxo for imido suggests that similar synthetic
strategies using borane adducts of amines may be a viable route to
mixed bis(imido) complexes. The structural data and theoretical
results for 2 and 4 demonstrate that, like the bis(imido) complexes,
the mixed oxo imido species still maintain many of the qualities
that uranyl possesses, including significant uranium f and d electron
participation in the multiple bonds.
Figure 2. Molecular orbitals of 4 involved in the UdN and UdO bonds.
The figure shows four π bonds and two σ bonds. The π bonds are given
mostly by interaction between the U dπ and fπ and the 2p orbitals of the N
and O. In the σ bonds, the U fσ and dσ take part.
is readily soluble in THF or mixtures of CH2Cl2 and THF. Complex
3 can be conveniently converted to U(NtBu)(O)I2(Ph3PO)2 (4) by
addition of 2 equiv of Ph3PO in toluene (Scheme 2). Like 3,
complex 4 is an orange crystalline solid, which is readily soluble
in aromatic solvents and THF. Consistent with the proposed
formulation, 4 exhibits a single peak in its 31P NMR spectrum at
47.3 ppm.
Crystals of 3 proved to be badly disordered, but 4 was found to
crystallize in the triclinic space group P1h with two independent
molecules in the unit cell. An ORTEP diagram of one of those
molecules is shown in Figure 1. The metrical parameters of the
oxo imido core (U1-O1 ) 1.764(5) Å, U1-N1 ) 1.821(7) Å,
O1-U1-N1 ) 178.4(3)°) are similar to those of 2, while its U-I
and U-O(phosphine oxide) bond lengths are similar to those of
the bis(imido) analogue, U(NtBu)2I2(Ph3PO)2.3
Complexes 2 and 4 were studied using hybrid density functional
theory,12 and the optimized structures agree well with the crystal-
lographic data. For example, the calculated structure of 4 exhibits
U-N and U-O(uranyl) bond lengths of 1.818 and 1.781 Å,
respectively, and a N-U-O(uranyl) angle of 179.5°. The six
molecular orbitals involved in the UdN and UdO bonds of 4 are
shown in Figure 2.
Like uranyl and the bis(imido) complexes, a total of four π bonds
and two σ bonds were identified, giving each ligand a bond order
of 3. The participation of the uranium f orbitals in the MOs (in
order of descending energy) is 15, 19, 32, 7, 3, and 0%, while the
participation of the d orbitals (in the same order) is 8, 10, 7, 13, 9,
and 7%. The uranium 6p orbital was found to participate in one σ
bond (HOMO-20) with a contribution of 6.5%. Interestingly, the
two π bonding orbitals involved in the U-O bond have a larger
component of d character, while in the U-N bond the uranium f
orbitals plays a larger role. Contrary to the bis(imido) complexes,
the different nature of the two ligands (O vs NtBu) breaks the
symmetry of the orbitals. The bonding orbitals are, therefore, more
clearly defined as belonging to the U-N (HOMO-6, -7, -29) or
the U-O (HOMO-20, -27, -28) bonds. The total Mulliken and NBO
calculated positive charges on the metal center are 1.60 and 1.45,
respectively, compared to 1.50 and 1.27, respectively, for the bis-
(imido) system.3 The greater charge at the metal center suggests
that the M-L multiple bonding in 2 and 4 is less covalent than in
the bis(imido) analogues.
Acknowledgment. T.W.H. thanks NSERC (Canada) and the
Seaborg Institute for postdoctoral fellowships. E.R.B. was supported
by the Division of Chemical Sciences, Office of Basic Energy
Sciences, U.S. Department of Energy under the Heavy Element
Chemistry program at LANL. We also thank Dr. Andrew J. Gaunt
for help with the UV/vis measurements.
Supporting Information Available: Complete details of the
preparation and characterization of 1-4, including X-ray crystal-
lographic details (as CIF files) of 1, 2, and 4. Geometries of the
calculated structures of complexes 2 and 4. This material is available
free of charge via the Internet at http://pubs.acs.org.
References
(1) (a) Denning, R. G. Struct. Bonding 1992, 79, 215-276. (b) King, R. B.
Inorg. Chem. 1992, 31, 1978-1980.
(2) Hayton, T. W.; Boncella, J. M.; Palmer, P. D.; Scott, B. L.; Batista, E.
R.; Hay, P. J. Science 2005, 310, 1941-1943.
(3) Hayton, T. W.; Boncella, J. M.; Scott, B. L.; Batista, E. R.; Hay, P. J. J.
Am. Chem. Soc. 2006, 128, 10549-10559.
(4) Full details of the characterization of complex 1 can be found in the
Supporting Information.
(5) Arney, D. S. J.; Burns, C. J. J. Am. Chem. Soc. 1993, 115, 9840-9841.
(6) Arney, D. S. J.; Burns, C. J. J. Am. Chem. Soc. 1995, 117, 9448-9460.
(7) Williams, V. C.; Mu¨ller, M.; Leech, M. A.; Denning, R. G.; Green, M.
L. H. Inorg. Chem. 2000, 39, 2538-2541.
(8) Only one transition metal trans oxo imido complex appears to have been
structurally characterized, namely, Os(O)(NtBu)(TTP). Li, Z.-Y.; Huang,
J.-S.; Chan, M. C.-W.; Cheung, K.-K.; Che, C.-M. Inorg. Chem. 1997,
36, 3064-3071.
(9) Berthet, J.-C.; Nierlich, M.; Ephritikhine, M. Chem. Commun. 2004, 870-
871.
(10) Schatte, G.; Chivers, T.; Tuononen, H. M.; Suontamo, R.; Laitinen, R.;
Valkonen, J. Inorg. Chem. 2005, 44, 443-451.
(11) Examination of the 1H NMR spectrum of the orange powder in pyridine-
d5 reveals that only 1 equiv of THF remains in the compound after it has
precipitated from benzene or CH2Cl2. Dissolution of the precipitate in
THF regenerates 3.
(12) Complete details of the theoretical calculations can be found in the
Supporting Information.
(13) 3-18O was synthesized using B(C6F5)3‚H218O.
(14) Crawford, M.-J.; Ellern, A.; Karaghiosoff, K.; Mayer, P.; No¨th, H.; Suter,
M. Inorg. Chem. 2004, 43, 7120-7126.
JA064400J
9
J. AM. CHEM. SOC. VOL. 128, NO. 39, 2006 12623