Simple N≡UF3 and P≡UF3 molecules with triple bonds to uranium

Article abstract of DOI:10.1002/anie.200801120

UN-beatable? Laser-ablated uranium atoms activate NF₃ and PF₃ to form the N≡UF₃ and P≡UF₃ molecules containing novel terminal nitride and phosphide functional groups. These molecules are identified from matrix infrared spectra and theoretical methods. The N≡UF₃ molecule contains the strongest triple bond to uranium in a ternary compound.

Full text of DOI:10.1002/anie.200801120

Communications
DOI: 10.1002/anie.200801120
Multiple Bonds
ꢀ
ꢀ
Simple N UF3 and P UF3 Molecules with Triple Bonds to Uranium**
Lester Andrews,* Xuefeng Wang, Roland Lindh, Björn O. Roos, and Colin J. Marsden
=
=
=
ꢀ
ꢀ
The chemical behavior of actinide elements must be under-
stood to manage effectively the many uses of actinide
materials in todayꢀs world. Considerable interest has devel-
oped in recent years in actinide (An) complexes with metal–
ligand multiple bonds. Most of these investigations have
centered on organometallic systems,[1] and molecular com-
plexes containing metal nitride units have been prepared.[2–6]
CH2 UH2, CH2 UHF, CH2 UF2, HC UF3, and FC UF3
molecules.[21–24] Analogous methylidene complexes are impor-
tant reagents in organometallic chemistry, particularly for the
early transition metals.[25] Although alkylidyne complexes are
not as prevalent, a number of simple Group 6 methylidyne
complexes have been prepared through metal-atom reactions
with methyl halides.[25,26]
=
These compounds with U N linkages and organoimido
Fluorine is a very important element in uranium chemistry
because of its role in uranium-isotope separation using
gaseous diffusion of the volatile uranium hexafluoride.
Accordingly UF6 has been the subject of considerable
experimental and theoretical investigations.[27,28] The smaller
UFn molecules have been studied less often,[28,29] but fluorine
has demonstrated its ability to facilitate the participation of
uranium in important chemical and physical processes. The
strong inductive effect of fluorine was central to the
stabilization of the first uranium methylidyne complex
(An NR) and phosphinidene (An PR) groups[7,8] are repre-
sentative. Matrix-isolation infrared spectroscopy has contri-
buted to the short list of uranium compounds containing
triple bonds with discovery of the NUN and CUO molecules
and preparation the [NUO]+ cation first detected by
mass spectrometry.[9–13]
The common first-row elements carbon, nitrogen, and
oxygen form multiple bonds with their 2p valence orbitals,
and these bonds are responsible for the chemical properties of
many simple compounds, such as HC CH, N N, and O O.
However, heavier main group elements from the second,
third, and fourth complete rows of the periodic table are much
less inclined to form multiple bonds, and their chemistry
markedly reflects this difference.[14] Although the heavier p-
block elements are involved in double bonds, such triple
bonds are seldom found. On the other hand f elements
(lanthanides and actinides) with multiple bonds are not
common, and the quest for such compounds with multiple
bonds between two actinide metals and between actinide and
main-group elements has evolved vigorously with computa-
tions leading the former[15–20] and synthesis the latter.[1–8]
We recently reported the first examples of uranium
methylidene and methylidyne molecules using laser-ablated
uranium atoms as the reagent. These species include the
=
=
[24]
ꢀ
ꢀ
=
ꢀ
FC UF3,
and fluorine should be equally as useful in
assisting other even heavier main-group elements to stabilize
UVI and to form novel triple bonds to uranium.
We report herein a combined experimental and theoret-
ical investigation of uranium-atom reactions with NF3 and
PF3, designed to prepare terminal uranium nitride and
phosphide functional groups. Our knowledge of terminal
ꢀ
U N bonds in neutral molecules is limited to matrix-isolation
[9,10]
ꢀ
ꢀ ꢀ
ꢀ
studies of U N and N U N,
species have been prepared.
and to date no terminal U P
Uranium atoms, laser-ablated from a solid-metal target
(Oak Ridge National Laboratory, high purity, depleted of
235U) were reacted with NF3 (Matheson) and PF3 (PCR
Research Chemicals) diluted in argon during condensation on
a 4 K cesium iodide window using methods described
previously.[26,30–32] After reaction, ultraviolet irradiation, and
annealing, infrared spectra were recorded at a resolution of
0.5 cmꢁ1.
[*] Prof. Dr. L. Andrews, Dr. X. Wang, Prof. Dr. B. O. Roos
Department of Chemistry
Ten experiments were carried out with NF3 using concen-
trations from 1.0 to 0.05% in argon and a range of laser
energies in order to minimize aggregation. The major product
absorptions common to experiments with other metals and
NF3 is the NF2 radical absorbing at 932 and 1069 cmꢁ1, which
arises from precursor photodissociation, and new bands at 999
University of Virginia
Charlottesville, VA 22904-4319 (USA)
Fax: (+1)434-924-3710
E-mail: lsa@virginia.edu
Prof. Dr. R. Lindh
Department of Theoretical Chemistry
Chemical Center, University of Lund
P.O.B. 124, 2-221 00 Lund (Sweden)
and 873 cmꢁ1 for NF2
.
Infrared spectra of low energy,
ꢁ
[33]
laser-ablated U and NF3 (0.3%) reaction products are shown
in Figure 1. The spectrum of the initial sample deposit
(Figure 1a) shows four weak new bands at 938, 613, 540,
and 533 cmꢁ1 which are marked by arrows in Figure 1b. The
four bands increased five-fold on ultraviolet irradiation and
acquired matching shoulders on the low energy side (Fig-
ure 1b). A second ultraviolet irradiation increased the four
bands 20% in concert (Figure 1c). Annealing to 20 K
increased the shoulders at the expense of the original bands
and resolved them into matching three-band sets (Figure 1d
Dr. C. J. Marsden
Laboratoire de Chimie et Physique Quantiques
UMR5626, IRSAMC, Universite Paul Sabatier
118 Route de Narbonne, 31062 Toulouse Cedex 9 (France)
[**] Supported by NSF Grant CHE 03-52487 to L.A. and by the Swedish
Research Council (V.R.) through the Linnaeus Center of Excellence
on Organizing Molecular Matter (OMM) to R.L. and B.O.R.
Supporting information for this article (experimental and theoretical
Methods with references) is available on the WWW under http://
www.angewandte.org or from the author.
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Chemie
Figure 2. Infrared spectra for U-atom reaction products with PF3 in the
630–520 cmꢁ1 regions; arrows indicate key absorptions, see text for
details; for clarity the y axis of spectra (b)–(e) has been vertically
shifted. a) spectrum after co-deposition of laser-ablated U and PF3 at
0.4% in argon at 4 K for 60 min, b) after annealing to 20 K, c) after
>290 nm irradiation for 20 min, d) after >220 nm irradiation for
20 min, and e) after annealing to 30 K.
Figure 1. Infrared spectra for U-atom reaction products with NF3 in the
940–860 and 630–480 cmꢁ1 regions, arrows indicate key absorptions,
see text for details; for clarity the y axis of spectra (b)–(f) has been
vertically shifted. a) spectrum after co-deposition of laser-ablated U
and NF3 at 0.3% in argon at 4 K for 60 min, b) after 240–380 nm
irradiation for 20 min, c) after 240–380 nm irradiation for 20 min more,
d) after annealing to 20 K, e) after annealing to 30 K, and f) after
annealing to 35 K.
have weaker fluorine bonds, as summarized in Reactions (1)
and (2).
and marked by arrows in Figure 1 f), and produced a weak
band due to UF6 (619 cmꢁ1; marked in Figure 1 f). Further
annealing to 30 and 35 K continued these trends and
markedly increased the UF6 absorption (Figure 1e and f).
The 938 cmꢁ1 band clearly tracks with the 613, 540, and
533 cmꢁ1 bands, and the observation of similar red-shifted
satellite structures for these four bands associates them with a
common product species. The UF6 band was observed in
earlier U and F2 reactions,[29] but not in our subsequent
investigations with the less reactive PF3 precursor. The UF6
peak increased more relative to the new product absorptions
on using higher reagent concentration. It is clear that uranium
fluorides are also products of the U and NF3 reaction.
Five investigations with U at different laser energies and
PF3 in different concentrations produced the representative
spectra shown in Figure 2. Absorptions common to ablation
experiments with other heavy metals and PF3 include the PF2
radical at 830 cmꢁ1 (not shown) which decreased on anneal-
ing, and PF5 at 1017 and 573 cmꢁ1, which increased on
annealing.[33] Clearly some F atoms are produced on photo-
dissociation of PF3, but not enough to form uranium fluorides.
New absorption peaks (marked by arrows in Figure 2e) were
observed at 581, 579, 542, 538, 536 and 535 cmꢁ1. These bands
sharpened on annealing. Near-UV irradiation increased these
absorptions and the very weak 1051 cmꢁ1 NUN band,[10] which
shows that U atoms undergo further reactions even with trace
N2 impurity.
U þ NF3 ! F2NꢁUF ! FN¼UF2 ! NꢀUF3
U þ PF3 ! F2PꢁUF ! FP¼UF2 ! PꢀUF3
ð1Þ
ð2Þ
Calculations were performed with density functional
(DFT, PW91) and wave function (CASSCF/CASPT2) meth-
ods[36,37] to support the above reaction mechanism, to identify
the new molecules, and to explore their bonding and
structure. The details of these theoretical methods are
summarized in the Supporting Information. The first quintet
insertion product, F2N-UF, failed to converge, but the NF-UF2
imine is + 345 kJmolꢁ1 higher in energy than the final nitride
product (quoted energies from PW91 density functional
ꢁ
calculations). Furthermore, the strongest U F stretching
ꢁ1
=
mode calculated (PW91) for NF UF2 at 528 cm is clearly
lower than the strongest observed absorptions. However, the
four new product bands observed at 938, 613, 540, and
533 cmꢁ1 correlate nicely with the U N and U F stretching
ꢀ
ꢁ
ꢀ
vibrations computed for the final nitride product N UF3, as
ꢁ1
ꢀ
shown in Table 1. First, the N U stretching mode at 938 cm
is bracketed by the DFT (976 cmꢁ1) and CASPT2 (921 cmꢁ1
)
theoretical values. The isolated UN molecule absorption was
observed at 1001 cmꢁ1 in solid argon.[8] Second, the symmetric
ꢁ1
ꢁ
U F stretching mode observed at 613 cm is calculated at
634 cmꢁ1 for the C3v molecule at the CASPT2 level and at
603 cmꢁ1 with DFT. Third, the degenerate U F stretching
ꢁ
Our previous investigations of metal-atom reactions with
mode calculated at 548 cmꢁ1 by CASPT2 is split at 541 and
533 cmꢁ1 using DFT, and the observed solid-argon-matrix
band is split at 540 and 533 cmꢁ1. The calculated frequencies
are harmonic and are expected to be slightly higher than
observed values, particularly in solid argon. The general
agreement between calculated and observed frequencies
ꢁ
fluorocarbons have shown that C F bond insertion occurs
followed by one, two, or three F-atom transfers to give the
more stable products.[23,24,34] Such was the case with CF4 to
ꢀ
give the final FC UF3 methylidyne. The driving force was the
ꢁ
formation of exceedingly strong U F bonds at the expense of
C F bonds that though strong, are less so.[28,35] This situation
is even more dramatic for the NF3 and PF3 precursors which
substantiates the present observation of the N UF3 molecule.
The molecule may be distorted slightly by the argon matrix to
ꢁ
ꢀ
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Table 1: Observed and calculated frequencies of the N UF3 and P UF3 molecules in the singlet ground electronic states with C3v or Cs structures.[a]
ꢀ
ꢀ
ꢀ
ꢀ
Approximate Description
N UF3
int
P UF3
calcd(CA)
obs
calcd(CA)
calcd(PW)
int
obs
int
calcd(PW)
int
ꢀ
N U str, a1,a’
938
613
540
533
921
634
548
71
171
454
976
603
541
533
110
98
155
141
U-F str, a1,a’
U-F str, e, a’’
U-F str, a’
581
542
536
619
560
259
430
600
557
548
374
128
124
103
62
166
140
140
11
10
2
9
13
14
P U str, a’
n.o.[b]
404
148
111
26
19
12
ꢀ
F-U-F bend, a1, a’
F-U-F bend, e, a“
F-U-F bend, a’
143
135
20
30
148
135
119
82
8
5
17
39
34
P-U-F bend, e, a’’
P-U-F bend, a’
22i
118
25
54
51
34
[a] Frequencies and intensities (int) are in cmꢁ1 and kmmolꢁ1, respectively. Observed in an argon matrix. Frequencies and intensities computed with
CASPT2 (CA) or PW91(PW) methods in the harmonic approximation. Symmetry notations are first for C3v from CASSCF and second for Cs from PW91.
[b] Not observed because it falls below the instrument limit.
facilitate a bonding interaction of the first argon atom with
the uranium metal center.[38]
the global minimum, but the energy gain compared to the C3v
structure is minute.
ꢀ
We have found the N UF3 molecule to be extremely
The active molecular orbitals that contribute to this triple
bond are shown in Figure 4. The natural orbital occupation
difficult to preserve in these experiments and dilute samples
were required to trap the isolated product in solid argon. In
fact, the red-shifted satellites that increase in intensity on
ꢀ
ꢁ
annealing are most likely to be due to (N UF3) (NF3)
complexes in two different orientations. We note a dramatic
increase in UF6 absorption intensity on the final annealing at
ꢀ
the expense of both N UF3 and NF3 and propose the highly
exothermic (995 kJmolꢁ1) mechanism given in Reaction (3)
ꢀ
to explain the favorable reaction of N UF3 under these
conditions.
F3N þ NꢀUF3 ! NꢀN þ UF6
ð3Þ
ꢀ
The N UF3 molecule contains a strong uranium–nitrogen
triple bond with a calculated length of 1.76 Š (CASPT2) or
1.75 Š (PW91). The C3v symmetry structure converged by
CASSCF/CASPT2 is given in Figure 3. The degenerate
ꢁ3
ꢀ
Figure 4. Active molecular orbitals in N UF3. Contour line 0.05 eau
Natural orbital occupation numbers given below the orbital.
.
numbers for the s- and p-bonding and antibonding orbitals
ꢀ
add to give a strong triple bond in N UF3 with an effective
ꢀ
bond order of 2.78. Recent characterization of the HC UF3
molecule with a 2.56 bond order using the same level of
theory[24] indicated that N UF3 has the strongest triple bond
ꢀ
ꢀ
ꢀ
Figure 3. Structures of N UF3 and P UF3 in C3v symmetry calculated
at the CASSCF/CASPT2 level of theory. Bond lengths [Š] and angles [8].
to uranium yet prepared in a ternary molecule. The bonding-
orbital occupancy is higher and the antibonding orbital
ꢀ
ꢀ
occupancy lower for N UF3 than for HC UF3. Thus, it
appears that nitrogen valence orbitals are particularly favor-
able for forming strong multiple bonds to uranium.
N-U-F bending frequency is imaginary (i22 cmꢁ1), and a
distorted structure is found 19 cmꢁ1 lower in energy. PW91
also finds an imaginary degenerate bending frequency
(i13 cmꢁ1): geometry optimization in Cs symmetry leads to a
true minimum that is distorted from C3v geometry, but which
lies only 17 cmꢁ1 lower in energy. Vibrational frequencies of
For the reaction with PF3 [Reaction (2)] the first and
second intermediate species are + 184 and + 50 kJmolꢁ1
higher than the final phosphide product. Although we find
ꢀ
two sets of bands for assignment to P UF3, which may be due
to the isolated molecule and a complex with PF3, there
appears to be no reaction of product with the more stable PF3
molecule as no UF6 is detected in this system. In Table 1 the
ꢀ
the N UF3 molecule are given in Table 1: it is important to
note that the stretching frequencies are almost identical for
the C3v and Cs structures. Hence, both DFTand the CASSCF/
CASPT2 methods predict a slightly distorted molecule to be
ꢀ
observed and calculated frequencies for P UF3 are com-
pared; the correlation again substantiates our assignments.
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ꢀ
The P UF3 molecule has a weak uranium–phosphorus
measured for the larger complexes prepared by Evans, et al.
triple bond with a calculated bond length of 2.38 Š (PW91) or
2.40 Š (CASPT2; Figure 3). The natural orbital occupation
numbers (Figure 5) give an effective bond order of 2.4
in the 2.01–2.07 Š double-bond range, the first UVI organo-
metallic complex reported by the Burns group, 1.95 Š, and
[1,4,6]
ꢁ ꢁ
the shorter 1.85 Š N U N distances of Hayton, et al.
=
Our computed double-bond length for FN UF2 is 0.20 Š
ꢀ
longer than our triple-bond length for N UF3. Likewise the
phosphinidene complexes of Burns et al.[8] with U P bonds
ꢁ
=
2.74 Š and 2.79 Š and U P bond lengths of 2.56 Š are longer
than the 2.38 Š (PW91) and 2.40 Š (CASPT2) values
ꢀ
ꢀ
ꢀ
calculated by us for P UF3. Clearly, the N UF3 and P UF3
molecules reported herein represent uranium–Group 15
triple bonds as shown by the molecular orbital analysis,
ꢀ
although the U P bond is weak. A final comparison with
ꢀ
ꢀ
terminal W N and W P bond lengths of 1.67 Š and 2.12 Š in
analogous organometallic complexes[41,42] shows that the
triple-bond lengths computed herein for uranium are 5 and
13% longer than their tungsten analogues. We see that the
ꢀ
ꢀ
U P bond is clearly weaker in comparison than W P, which is
ꢀ
ꢀ
in marked contrast to U N in comparison with W N. This
ꢁ3
ꢀ
Figure 5. Active molecular orbitals in P UF3. Contour line 0.05 eau
Natural orbital occupation numbers given below the orbital.
.
situation is further supported by compiled triple-bond radii,[43]
ꢀ
which predict a U N length (1.72 Š) that is 2% shorter than
ꢀ
our computed value but a U P length (2.12 Š) that is 12%
corresponding to a considerably weaker triple bond than in
shorter than our CASPT2 value.
ꢀ
ꢀ
N UF3. The terminal U P bond is expected to be weaker
As for the uranium methylidyne systems,[24] macroscopic
ꢀ
ꢀ
ꢀ
than the terminal U N bond as the more diffuse phosphorus
synthetic sources of terminal U N and U P triple bonds have
not yet been found. This matrix-isolation work shows that
with the assistance of strongly electronegative ligands such as
F, the UVI oxidation state can be stabilized and terminal triple
bonds can be formed with N and reasonably well with the
heavier analogue P. Comparison of the nitride and phosphide
triple bonds to uranium demonstrates that U 5f orbitals bond
more effectively with the smaller nitrogen ligand.
orbitals are less effective in forming multiple bonds than
nitrogen atomic orbitals.[14] The U N and U P bond energies
can be estimated as the energy of the atom reaction with UF3,
ꢀ
ꢀ
ꢁ
where UF3 has a Cs structure with U F bond lengths of 2.055
and 2.074 Š. These CASSCF/CASPT2 calculations are
approximate without ZPE or spin-orbit correction. Never-
theless, the difference between the U N and U P bond
energies is striking. While the U N bond is strong
ꢀ
ꢀ
ꢀ
ꢁ1
ꢁ1
ꢀ
Received: March 7, 2008
(460 kJmol ), the U P bond is much weaker (176 kJmol ).
ꢀ
ꢀ
Why is the U N bond so much stronger than the U P
bond? The explanation is found, in part, in the character of
the U3+ moiety. It is the 5f orbitals on this ion that are mainly
used to form the bond, and they are quite contracted and
hard. Therefore a more balanced covalent bond can be
formed with a hard ligand such as nitrogen. The softer 3p
orbitals of P do not have the same ability to form a strong
bond with U3+. It is interesting to look at the charges of the
Keywords: density functional calculations · IRspectroscopy ·
matrix isolation · multiple bonds · uranium
.
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ꢀ
atoms in the two cases. The U P bond is slightly more ionic
ꢀ
than the U N bond (Mulliken charges ꢁ0.29(P) and
ꢁ0.25(N)) while the charges on F are the same in both
cases. The total occupation of the U 5f orbitals is 2.21 for
ꢀ
ꢀ
P UF3 and 2.14 electrons for N UF3. Also the U 6d orbitals
participate in the bonding with a total population of 1.34
ꢀ
ꢀ
electrons (in P UF3) and 1.43 electrons (in N UF3), thus,
slightly larger in the nitrogen compound, which is another
indication of a less-ionic bond in this case. The very strong
uranium–nitrogen bond is also found in the molecule
NUN[9,10] in which the stabilization energy relative to U +
N2 has been estimated to be about 250 kJmolꢁ1.[39] Adding the
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molꢁ1 per bond in NUN.
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ꢁ
The U N bond lengths in our present system, 1.75 Š
(PW91) and 1.76 Š (CASPT2), and those determined earlier
for N U N, 1.73 Š,
[40]
ꢀ ꢀ
can also be compared to those
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