Stable Analogs of the Uranyl Ion Containing the -N=U=N- Group

Article abstract of DOI:10.1021/ic951386v

A series of compounds containing the -N=U=N- group has been prepared. Their physical and chemical properties are reported. They adopt a trans configuration, and are thermally robust. The chemistry of some recently-prepared O=U=N- compounds is also described. These are the first examples of stable analogs of the uranyl ion UO₂²⁺. The factors that determine their stability are discussed.

Full text of DOI:10.1021/ic951386v

6158
Inorg. Chem. 1996, 35, 6158-6163
Stable Analogs of the Uranyl Ion Containing the -NdUdN- Group
Dean R. Brown and Robert G. Denning*
Inorganic Chemistry Laboratory, University of Oxford, South Parks Road,
Oxford OX1 3QR, United Kingdom
ReceiVed October 27, 1995X
A series of compounds containing the -NdUdN- group has been prepared. Their physical and chemical
properties are reported. They adopt a trans configuration, and are thermally robust. The chemistry of some
recently-prepared OdUdN- compounds is also described. These are the first examples of stable analogs of the
uranyl ion UO22+. The factors that determine their stability are discussed.
Introduction
Results and Discussion
The uranyl ion UO22+ is well-known for its chemical stability,
being resistant to reactions which modify the dioxo unit. This
property is of particular importance in the nuclear fuel cycle
and has stimulated detailed interest in its electronic structure.1
However, only a few compounds of uranium containing the
analogous imido and iminato ligands are known, and these bear
little resemblance to the uranyl ion.2 Syntheses of compounds
containing these ligands have relied on the presence of bulky
organometallic groups in compounds such as (Me5Cp)2U(O)-
NC6H3-2,6-i-Pr23 or (Me5Cp)2U(NPh)2,4 both of which approxi-
mate to tetrahedral geometry, and on trimethylsilyl-imido and
amido compounds, such as Me3SiNdU[N(SiMe3)2]3.5 By
contrast, imido and iminato analogs of oxo compounds of the
transition metals are common, and methods for their intercon-
version have been established.6,7
Preparation and Properties. Deep red [Ph4P][OUCl4-
(NPR3)] salts 1 are formed by the reaction of the oxopentachlo-
rouranate(VI) salt [Ph4P][UOCl5] with trimethylsilylphospho-
ranimines R3PNSiMe3 (eq 1). The phosphorane substituents
can either be aryl groups, or a mixture of aryl and alkyl groups.
When R ) m-Tolyl, the crystal structure2 shows the
OdUdN- unit to be nearly linear (179o) with a short U-N
distance (1.90 Å), indicative of a multiple bond between U and
N. Bis(iminato) compounds 2 (eq 2) can be isolated from the
same reaction mixture by controlling the conditions of workup.
Until our recent report2 of the preparation and crystal structure
of the phosphorane-iminato compound [Ph4P][OUCl4(NPTol3)]
containing a linear OdUdN- group, no stable structural
analogs of the uranyl ion had been isolated. However, unstable
analogs, such as UN2 in an N2 matrix, have been identified,8
and the OUN+ ion has been characterized in a mass spectrom-
eter.9 Recent theoretical studies of OUO2+, NUO+, and NUN,
as well as other isoelectronic species, suggest that many of these
triatomic units could have significant stability.10
We now describe the synthesis of a series of compounds
containing the linear -NdUdN- group, and report their
physical and chemical properties together with those of related
compounds containing the OdUdN- group by way of com-
parison. Both classes of compound resemble the dioxo species
in being thermally robust; the solids remain unchanged in air,
and their solutions are stable at high temperatures in the absence
of moisture.
The oxopentachlorouranate(VI) is a useful starting material,
because it is both reactive and convenient to handle. As such,
it represents a compromise between the very robust uranyl ion
in UO2Cl42-, and the highly unstable and oxidizing hexachloride,
UCl6. It is prepared as a red solid by the partial deoxygenation
of [Ph4P]2[UO2Cl4] in refluxing thionyl chloride.11 The chloride
X Abstract published in AdVance ACS Abstracts, September 1, 1996.
(1) Denning, R. G. Struct. Bonding 1992, 79, 215-276.
(2) Brown, D. R.; Denning, R. G.; Jones, R. H. J. Chem. Soc., Chem.
Commun. 1994, 2601-2602.
-
in the UOCl5 anion provides a driving force for reactions 1
and 2, through the elimination of volatile trimethylsilyl chloride.
Trimethylsilylphosphoranimine precursors are prepared by the
reaction of tertiary phosphines R3P with trimethylsilyl azide.12,13
In a typical preparation, 1-2 g of [Ph4P][UOCl5] was stirred
for several hours with a slight molar excess of triphenylphos-
(3) (a) Arney, D. S. J.; Burns, C. J. J. Am. Chem. Soc. 1993, 115, 9840-
9841. (b) Arney, D. S. J.; Burns, C. J. J. Am. Chem. Soc. 1995, 117,
9448-9460.
(4) Arney, D. S. J.; Burns, C. J.; Smith, D. C. J. Am. Chem. Soc. 1992,
114, 10068-10069.
(5) Zalkin, A; Brennan, J. G.; Andersen, R. A. Acta Crystallogr. 1988,
C44, 1553-1554.
(6) Dehnicke, K.; Stra¨hle, J. Polyhedron 1989, 8, 707-726.
(7) Green, M. L. H.; Hogarth, G.; Konidaris, P. C.; Mountford, P. J. Chem.
Soc., Dalton Trans. 1990, 3781-3787.
(10) Pyykko¨, P.; Li, J.; Runeberg, N. J. Phys. Chem. 1994, 98, 4809-
4813.
(11) Bagnall, K. W.; du Preez, J. G. H.; Gellatly, B. J.; Holloway, J. H. J.
Chem. Soc., Dalton Trans. 1975, 1963-1968.
(12) Birkofer, L.; Kim, S. Chem. Ber. 1964, 97, 2100-2101.
(13) Schmidbaur, H.; Wolfsberger, W. Chem. Ber. 1967, 100, 1000-1015.
(8) Hunt, R. D.; Yustein, J. T.; Andrews, L. J. Chem. Phys. 1993, 98,
6070-6074.
(9) Heinemann, C.; Schwarz, H. Chem.sEur. J. 1995, 1, 7-11.
S0020-1669(95)01386-3 CCC: $12.00 © 1996 American Chemical Society

Stable Analogs of the Uranyl Ion
Inorganic Chemistry, Vol. 35, No. 21, 1996 6159
Table 1. Vibrational Frequencies (cm-1) of Uranium Iminato Complexesa
compound
no.
νas(U-N-P)
νs(U-N-P)
ν(U-O)
[Ph4P][OUCl4(NPPh3)]
(Ph3PN)2UCl4
[Ph4P][OUCl4(NPTol3)]
3
4
5
1090
1042
1080
561
552
556
862
850
1079Raman
1046
849Raman
(Tol3PN)2UCl4
6
554
1068Raman
1059
1053
[Me2PhPNH2][OUCl4(NPMe2Ph)]a
(Me2PhPN)2UCl4
7
8
856
a Infrared data, unless otherwise indicated. b N-H stretching frequencies were observed at 3341 and 3260 cm-1
.
Table 2. Infrared Vibrational Frequencies (cm-1) of Transition
Metal Bis(iminato) Complexes
phoranimine, Ph3PNSiMe3, in dichloromethane at room tem-
perature. During this time the sparingly soluble [Ph4P][UOCl5]
dissolves, forming a clear red solution that contains a mixture
of [Ph4P][OUCl4(NPPh3)] (3), (Ph3PN)2UCl4 (4), and the uranyl
salt [Ph4P]2[UO2Cl4]. Compound 3 is an ionic solid, soluble
in polar organic solvents such as dichloromethane, and is
precipitated by addition of nonpolar solvents, e.g. toluene, in
which it is very insoluble. 4 is neutral and is usually precipitated
first when a solution is concentrated by removing some of the
solvent under vacuum. Crystalline samples of 3 may be isolated
by carefully adding a layer of toluene to the top of the reaction
mixture, which is left undisturbed to interdiffuse over a period
of weeks. 4 is formed as a microcrystalline solid when the
solvent is evaporated slowly.
During the preparation [Ph4P]2[UO2Cl4] is usually deposited
as yellow crystals, but these are easy to remove by hand; its
presence in powdered samples can be identified by an intense
infrared absorption14 at 912 cm-1. Reactions performed in
acetonitrile also yield a brown reduction product. On the basis
of IR, analytical, and crystallographic data,15 we tentatively
identify this as [Ph4P]2[UCl6]‚4MeCN, containing a small
amount of UO2Cl42- as a solid solution. Solids containing only
the UCl62- ion are pale green, and the brown color may be due
to uranium intervalence charge-transfer. This product forms
large crystalline masses and can also be separated by hand.
Both the bis(iminato) and the oxoiminato compounds are
stable indefinitely in air and in dry solutions. Even if solutions
are heated above 200 °C the compounds can be recovered
unchanged after removal of the solvent. However, they are
immediately hydrolyzed in solution by traces of water. The
oxo-iminato compounds, [Ph4P][OUCl4(NPR3)], 1, are dark red
crystalline solids and are very soluble in polar organic solvents
such as dichloromethane or acetonitrile. The bis(iminato)
complexes, (R3PN)2UCl4, 2, are formed as red crystalline solids,
and are less soluble in polar organic solvents.
compound
νas(M-N-P)
ref
WCl4(NPPh3)2
WF4(NPMe3)2
VCl3(NPPh3)2
VOCl(NPPh3)2
TiCl2(NPPh3)2
1110, 1220
1065-1110(br)
1080, 1110
1095, 1120
1090, 1110
6
45
46
46
46
atom, for example to the asymmetrical PhMe2P, failed to
produce an improvement in crystal quality. In an attempt to
obtain some crystallographic data, a very small single crystal
of (Ph3PN)2UCl4, 4, was mounted on a diffractometer. De-
composition occurred in the X-ray beam over a period of hours,
and only a few hundred weak reflections were recorded. It
proved impossible to obtain an acceptable refined structure from
this data set. However, the heavy atoms U, Cl, and P, could
be located, and their positions indicate that the molecule contains
a square planar UCl4 unit (U-Cl ∼ 2.6 Å) with phosphorus
atoms in trans axial positions at a distance of ∼3.5 Å from
uranium. These distances match those that have been accurately
determined in the oxo-iminato compound [Ph4P][OUCl4-
(NPTol3)].2
Table 1 shows the bis(iminato) and oxo-iminato complexes
that have been isolated as pure crystalline samples, together with
selected infrared and Raman data. The assignment of the
vibrational frequencies is made by analogy with the phospho-
rane-iminato complexes of the transition metals.16-19 These
display characteristic strong absorptions in the range 1050 to
1150 cm-1 that are attributed6 to the antisymmetric stretching
vibrations of the M-N-P group. The corresponding symmetric
M-N-P stretches occur6 at about 550 cm-1, although they are
generally weak.
Bis(iminato) compounds of d-block transition metals are
generally found with a cis geometry,6 and both symmetric and
antisymmetric combinations of the νas(M-N-P) modes are
infrared active. Table 2 gives the available infrared data for
compounds of this class. The uranium bis(iminato) compounds
have only a single infrared-active band at ∼1050 cm-1 (Table
1), while the Raman spectrum of compound 6 contains just a
single feature, 22 cm-1 above the IR frequency. The absence
of any bands common to the IR and Raman spectra supports a
centrosymmetric configuration, with the nitrogen atoms located
in trans positions. Taken together with the heavy atom positions
implied by the X-ray data on 4, we conclude that the gross
geometry of the bis(iminato) compounds is trans.
In an attempt to produce (Ph3PN)2UCl4 more directly, we
have examined the reaction of Ph3PNSiMe3 with uranium
hexachloride. Stoichiometric amounts of UCl6 and 2Ph3-
PNSiMe3 were loaded into a glass vessel and cooled to -78
°C. Slow addition of dichloromethane resulted in the precipita-
tion of a heavy olive-green solid, identical in appearance to
uranium tetrachloride, that contains no strong features in the
infrared above 400 cm-1; we infer that the hexachloride has
been reduced. No red coloration was observed in the final
mixture and no iminato complexes could be isolated.
Coordination Geometry. The full determination of a crystal
structure of a uranium bis(iminato) compound has proved elusive
because it has not been possible to grow crystals of sufficient
size and quality for X-ray work. The compounds do crystallize,
but usually as very thin platelets clumped together around a
central axis. Varying the organic groups on the phosphorus
The uranium oxo-iminato complexes have IR absorptions
at ∼850 cm-1, in addition to those of the U-N-P and organic
groups, and these are assigned to the uranium-oxygen stretch.
Their frequencies are close to that of UdO stretching mode in
(16) Griffith, W. P. J. Chem. Soc. 1962, 3248-3250.
(17) Griffith, W. P. J. Chem. Soc. 1964, 245-249.
(18) Griffith, W. P. J. Chem. Soc. A 1969, 211-218.
(19) Tatsumi, K.; Hofmann, R. Inorg. Chem. 1980, 19, 2656-2658.
(14) Day, J. P.; Venanzi, L. M. J. Chem. Soc. A 1966, 1, 1363-1367.
(15) Brown, D. R.; Denning, R. G. manuscript in preparation.

6160 Inorganic Chemistry, Vol. 35, No. 21, 1996
Brown and Denning
[Ph4P][UOCl5] (837 cm-1, Raman)11 and clearly the binding
of the oxo group is similar to that in the uranyl ion.
such a solution, crystallization of a red solid occurred over a
period of several weeks. The product was obviously different
from the starting material, but had similar elemental ratios.
Further investigation identified it as [Me2PhPNH2][OUCl4-
(NPPhMe2)], 7. The infrared spectrum displayed a medium
intensity UdO band at 856 cm-1, and symmetric and antisym-
metric N-H stretches of the cation were observed at 3341 and
The infrared-active antisymmetric U-N-P stretching fre-
quencies, listed in Table 1, fall in two regions; those in the oxo-
iminato compounds occur near 1080 cm-1, and those in the
bis(iminato) complexes occur at ∼1050 cm-1. Because of its
large mass, the uranium atom is effectively motionless in these
vibrations, and the frequencies are indicative of the force
constants within the U-N-P unit. Apparently the presence of
an oxo group trans to the iminato group in 3, 5, and 7 leads to
an increase in the U-N-P stretching frequencies relative to
those in 4, 6 and 8, where a second iminato group occupies
this position.
3260 cm-1
.
The crystal structure15 is consistent with this
formulation.
A variety of pathways can be postulated for the formation of
the bis(iminato) complexes. To investigate one of them, a
solution of pure [Ph4P][OUCl4(NPPh3)] 3 in dichloromethane
o
was heated (CAUTION! high pressure) to 170 C for 2 days,
In d-block chemistry an oxo group creates a strong trans
influence, polarizing the metal in such a way as to weaken the
bond to the group opposite, and to reduce its vibrational
frequency.20 Phosphorane iminato groups might be expected
to possess a weaker trans influence than the oxo group, because
the M-N bond is considerably longer than the M-O bond.
However, the available structural data is sparse,21-23 and all
that can be said is that the magnitudes of the trans influences
of iminato and oxo groups are similar and substantial.6
Recently it has been argued that an inverse trans influence
in an attempt to force its disproportionation into a uranyl
complex and the corresponding bis(iminato) species (eq 4). After
cooling and rapid removal of the solvent, the solid was found
to be identical to the starting material; no other products could
be detected.
Similarly, when equimolar quantities of [Ph4P]2[UO2Cl4] and
4 were heated together in dichloromethane (195 °C, 5 h), the
solid obtained after removal of the solvent exhibited an IR
spectrum identical to the starting mixture. No trace of 3 could
be detected, so the interconversion in eq 4 does not occur in
either direction.
Formation of the oxo-iminato complexes 1, from [Ph4P]-
[UOCl5] and phosphoranimines, can be postulated to occur by
one of two mechanisms. Because the equatorial coordination
number of the uranyl ion can be as large as six, an associative
mechanism, in which the imine nitrogen initially coordinates
to the uranium to give an equatorial coordination number of 5,
seems plausible (eq 5a). A coordination sphere with this form
is known in UOCl2‚H2O, which contains a linear uranyl ion
with planar equatorial coordination by four chloride ions and a
water molecule.27 Furthermore, compounds with the formula
MCl2(Me3SiNPMe3)2 (M ) Co, Zn) have been isolated28 in
which the phosphoraniminato ligands are bound in the same
manner as in the postulated intermediate. Such an intermediate
could eliminate Me3SiCl in a second step.
-
operates in actinide compounds.1 For example, in the UOCl5
anion11,24 the U-Cl bond trans to the oxygen is shorter than
the cis U-Cl bond, and their stretching frequencies are 345
and 302 cm-1 respectively; similar observations are reported
for the PaOCl52- anion.25 The effect has been attributed1,26 to
the superposition of the 5f shell and the pseudo-core 6p shell
of the actinide ion, and is analogous to the d-s hybridization
that underlies the preponderance of linearly bonded compounds
of d10 ions. One result of this is the linearity and strength of
the uranyl ion, which contrasts with transition-metal dioxo
species that are found predominantly in the cis configuration.40
If the same effect operates in uranium iminato complexes, the
large inverse trans-influence of the oxo ligand should strengthen
the U-N bond to the trans iminato group in compounds 3, 5,
and 7 relative to that in compounds 4, 6, and 8, where the more
weakly bound trans phosphiniminato group would exert a
smaller influence. The vibrational data provide clear support
for this argument.
Reactivity and Mechanisms of Formation. Hydrolysis
converts the bis(iminato) species first into oxo-iminato com-
pounds and subsequently into uranyl compounds if suffi-
cient water is present. The first step in this conversion was
observed accidentally during an attempted recrystallization of
(Me2PhPN)2UCl4, 8 (eq 3) from inadequately dried dichlo-
Alternatively an initial nucleophilic displacement of the
phosphoraniminate anion at the silicon atom, by a coordinated
chloride ion, can be postulated. This would create a reactive
five-coordinate UOCl4 intermediate,29 to which the iminate anion
can subsequently add, eq 5b.
(Me PhPN) UCl + H O
f
2
2
8
4
2
[Me2PhPNH2][OUCl4(NPPhMe2)]
(3)
7
romethane. The solubility of 8 in dichloromethane is low, ca.
0.05 g in 30 cm3, and when a layer of toluene was added to
(20) Appleton, T. G.; Clark, H. C.; Manzer, L. E. Coord. Chem. ReV. 1973,
10, 335-422.
(21) Ho¨sler, K.; Dehnicke, K. Z. Naturforsch. 1987, 42B, 1563-1566.
(22) Mu¨ller, U.; Du¨bgen, R.; Dehnicke, K. Z. Anorg. Allg. Chem. 1981,
471, 89-101.
(23) Schmidt, I.; Kynast, U.; Hanich, J.; Dehnicke, K. Z. Naturforsch. 1984,
39B, 1248-1251.
(24) De Wet, J. F.; du Preez, J. G. H. J. Chem. Soc., Dalton Trans. 1978,
592-597.
(25) Brown, D.; Reynolds, C. T.; Moseley, P. T. J. Chem. Soc., Dalton
Trans. 1972, 857-859.
(27) Taylor, J. C.; Wilson, P. W. Acta Crystallogr., Sect. B 1974, 30, 169-
175.
(28) Ko¨cker, R. M.; Frenzen, G.; Neumu¨ller, B.; Dehnicke, K. Z. Anorg.
Allg. Chem. 1994, 620, 431-437.
(26) Bartlett, J. M.; Denning, R. G.; Morrison, I. D. Mol. Phys. 1992, 75,
601-612.

Stable Analogs of the Uranyl Ion
Inorganic Chemistry, Vol. 35, No. 21, 1996 6161
2+
The key question, however, is how the tightly bound oxo
group is lost in the formation of the bis(iminato) compounds.
Because 3 and 4 are not interconvertible even under forcing
conditions, we assume that 4 must be formed with the aid of
the other components in the initial reaction mixture, i.e.
[UOCl5]- as well as free phosphoranimine. If eq 5b is a good
model for the initial substitution, the reactive intermediate
UOCl4 could also remove an oxide ion from 3 to yield
UO2Cl42-. This is an attractive hypothesis because the latter
species is the other major product isolated from the reaction,
and it is not formed by the disproportionation of 3. This
mechanism is summarized in eq 6.
the dioxo cation is a good reference point. In comparing UO2
with the [UONPR3]3+ unit in the oxo-iminato compounds, it
is helpful to consider the results of some quasi-relativistic density
functional theory calculations on UO2 and the isoelectronic
2+
ion UON+ due to Heinemann and Schwarz.9 The replacement
of an oxide ion in the former by a nitride ion in the latter has
the effect of narrowing the HOMO-LUMO gap from 2.4 to
1.1 eV. In UON+ the LUMO is a nonbonding uranium f orbital,
and the HOMO has s symmetry with the composition 35% N,
3
2pz; 1% O, 2pz; and 41% U, 5fz . The reduction of the gap and
the strong bias of the HOMO toward the nitrogen atom can be
understood in terms of a notional removal of a positive charge
2+
from one of the oxygen nuclei in UO2 to create a nitrogen
atom. The reduced nuclear charge lowers the binding energy
of the valence electrons on the nitrogen atom relative to those
on oxygen, so that the bond to the nitrogen atom is weakened
and the system becomes more susceptible to an internal charge
transfer in which the uranium is reduced at the expense of the
nitrogen ligand. Such a reduction occurs, for example, when
phosphines are added to transition metal nitrides to give
phosphorane iminato complexes.6
The critical step in the formation of 2 is the abstraction of
the oxo atom from 1 and we believe that this succeeds because
of the intrinsic stability of the uranyl ion in the form of
UO2Cl42-. Addition of the second phosphoraniminato group
follows the deoxygenation. If this model is correct eq 5b
appears to be a better representation of the initial substitution
than eq 5a.
In agreement with their calculations, Heinemann and Schwarz
report experimental bond dissociation enthalpies of 429 ( 122
and 739 ( 164 kJ mol-1 for the U-N and U-O bonds of
UON+ respectively.9 In [UONPR3]3+ the energy of the HOMO
should be lower in energy than that in UON+ because a nitrogen-
centered σ-lone pair now forms part of the valence shell of
the phosphorus atom; nevertheless the basis of the weakness
of the U-N bond relative to the U-O bond is clear. The
reduction in the HOMO-LUMO gap associated with the
iminato group, compared to the dioxo cation is consistent with
the red color of the phosphoraniminato compounds. The nature
of the first electronic excited states of the uranyl ion are well-
understood and correspond to parity forbidden charge-transfer
transitions from an oxygen-centered HOMO to a uranium-
centered LUMO.1 Thus the reduced gap in the UON+ unit
should lead to a red shift in these transitions. The electronic
absorption of the iminato complexes is broad and unstructured.
The low energy threshold occurs near 750 nm for 6 and 560
nm for 5, compared to less than 500 nm in yellow uranyl
compounds,1 as would be predicted by this model.
Although the U-O bond distance2 in 5 [175.9(13) pm] is
typical of uranyl compounds, the U-N distance [190.1(14) pm]
is significantly shorter than those of most other uranium iminato
and imido compounds,2 implying that there is significant mutual
cooperation in bonding between the trans components. More-
over the chemical stability of both series of compounds 1 and
2 in solution in polar solvents at 200 °C resembles that of uranyl
compounds more closely than do the strongly-oxidizing and
reactive properties associated with other uranium(VI) com-
pounds, such as UF6, UCl6, and, to a lesser extent, [Ph4P]-
[UOCl5].
Electronic Structure and Bonding. The stability of these
new compounds should be seen in the context of the properties
of the uranyl ion. The UdO bond strength in this species is
significantly larger than that of most comparable transition
metal-oxo bonds.30 For example, the mean bond enthalpies
(kJ mol-1) for dissociation to neutral oxygen atoms, in some
gaseous dioxo species are as follows: UO2, 712 ( 15;30 MoO2,
578; WO2, 635; RuO2, 509; OsO2, 770.31 The equivalent
quantity for UO2+(g) has been estimated32 to be 785 kJ mol-1
,
while a calculation of the lattice enthalpy of UO2F2,33 together
with a theoretical value for the second ionization energy of
uranium,34 gives a UO22+(g) value of 701 ( 50 kJ mol-1 30
.
Clearly the mean U-O bond strength approaches that of C-O
in carbon dioxide31 (802 kJ mol-1), and indeed gaseous UN+
reacts exothermically with CO2 to give the UON+ ion.9 The
uranyl ion is also so inert that the rate of exchange between the
oxo atoms and water occurs with a half-life approaching 40 000
h at room temperature.35
The electronic structure and bonding of actinyl ions has been
reviewed recently,1 and it is evident that the presence on ura-
nium of both f and d valence orbitals leads to unusually
favourable interactions with oxygen p orbitals, and an effective
bond order of three for the U-O bond in the uranyl ion. An
oxo group is isoelectronic with imido (RsNd) or iminato
(R3PsNd) groups with respect to their donor ability, and so
Redox Stability. We have been able to prepare compounds
of general formula 1 or 2 that include the following residues:
Ph3P, 3-Tol3P, Me2PhP, MePh2P, i-PrPh2P. It seems that stable
products are only obtained if there is at least one aromatic
substituent on the phosphorus atom. Although our study cannot
claim to be comprehensive, reactions of trialkylphosphoran-
imines with [Ph4P][UOCl5] led only to reduction products. For
example, addition of n-Bu3PNSiMe3 to a stirred suspension of
[Ph4P][UOCl5] in MeCN or CH2Cl2 resulted in the immediate
precipitation of the pale green uranium(IV) compound [Ph4P]2-
[UCl6], which was identified by its elemental analysis and
infrared spectrum. The analogous reaction with Cy3PNSiMe3
(Cy ) cyclohexyl) was very slow at room temperature, but on
(29) UOCl4 is postulated as an intermediate in the chlorination of uranium
oxides with CCl4. See, for example: Brown, D. In Gmelin Handbuch
der Anorganischen Chemie; Keller, C., Ed.; Springer Verlag: Heidel-
berg, Germany, 1979; Vol. C9, p 72.
(30) Denning, R. G. Gmelin Handbook of Inorganic Chemistry; Keller,
C., Ed.; Springer Verlag: Heidelberg, Germany, 1983; Vol. A6, pp
31-79.
(31) Glidewell, C. Inorg. Chim. Acta 1977, 24, 149.
+
(32) Thermochemical data derived for UO2 are referenced in: Pyykko¨,
P.; Li, J.; Runeberg, N. J. Phys. Chem. 1994, 98, 4809-4813.
(33) (a) Marcus, Y. J. Inorg. Nucl. Chem. 1975, 37, 493-501. (b)
Cordfunke, E. H. P.; Ouwelfjes, W. J. Chem. Thermodyn. 1977, 9,
71-74.
(34) Pyper, N. C.; Grant, I. P. J. Chem. Soc., Faraday Trans. 2 1978, 1885-
1900.
(35) Gordon, G.; Taube, H. J. Inorg. Nucl. Chem. 1961, 19, 189-191.

6162 Inorganic Chemistry, Vol. 35, No. 21, 1996
Brown and Denning
configuration and about 2° away from the axial direction. Location of
lighter atoms and further refinement were unsuccessful. Direct methods
yielded no additional structural information.
heating brown solutions were formed from which no iminato
compounds could be isolated.
Thus, although the steric bulk associated with cyclopentadi-
enyl and trimethylsilylamido ligands found in previous examples
of uranium imides is not a prerequisite for stability, the
resistance of the ligands to oxidation seems to be vital. This
can be emphasized by comparison with transition metal
analogues. The greater redox stability of W(VI) compared to
U(VI), can be seen from the fact that WCl6 boils without
decomposition at 346 °C,36 while UCl6 partially decomposes
to lower halides during sublimation near 100 °C.37 Indeed at
300 K the free energy of the reaction UCl4(s) + Cl2(g) f UCl6-
Reagents. Dichloromethane was refluxed over phosphorus pentox-
ide for several hours, distilled, and stored over molecular sieves (type
4A). Acetonitrile was refluxed over phosphorus pentoxide for several
hours and distilled onto fresh P2O5, the last quarter fraction being
discarded, and the process was repeated three times prior to storage
over molecular sieves. Toluene was refluxed over sodium metal, and
distilled onto molecular sieves. Phosphines were obtained from Aldrich
Chemical Co.; the solids were melted under vacuum prior to use.
Trimethylsilyl azide was obtained from Lancaster Synthesis and used
as supplied.
[Ph4P][UOCl5] was prepared by the method of Bagnall et al.,11 but
the solid produced in this way contained occluded thionyl chloride.
This interfered with subsequent reactions, but could be removed by
heating the solid to 120 °C at 10-3 torr until evolution of the gas had
ceased. The solid may be recrystallized from dichloromethane, in which
it is very soluble at elevated temperatures (∼120 °C). Caution! High
pressure. Uranium hexachloride was prepared according to the
published procedure.37
(s) is reported to be only -2.3 kJ mol-1 38
The phosphoran-
.
iminato group can be written as R3P+-N2-, and so can be
notionally derived from the isoelectronic imido group R3C-
N
2- by introducing an extra nuclear charge at the R-carbon atom.
It is this increase in positive charge that provides the superior
stability of the iminato ligands to oxidation in relation to imido
ligands.
Even so it appears from our results that electron-withdrawal
by at least one aromatic group in the phosphorane is required
to prevent the reduction of the uranium by the ligand. Therefore,
despite the stability of nitrido and imido ligands in tungsten
(VI) species such as [Cl5WN-i-Pr]-,39 attempts to prepare their
counterparts using the more oxidizing uranium (VI) are likely
to lead to the reduction of the uranium. Clearly other strongly
covalent ligands, such as the cyclopentadienyl groups in
(CpMe5)2U(NPh)2,4 should be bound to the uranium atom if its
charge is to be reduced sufficiently to overcome this source of
instability.
Phosphorane imines were generally prepared by the method of
Staudinger.41 Triaryl-, trialkyl-, or mixed aryl/alkylphosphines were
heated for several hours with a small mole ratio excess of trimethylsilyl
azide in a sealed vessel, after which all volatiles were removed and
the product purified by distillation or sublimation. In each case we
list the reaction temperature (°C), its duration (hours), the physical state
at room temperature, and the temperature (°C) and pressure (Torr) of
sublimation or distillation: Ph3PNSiMe3, 150, 5, s, 170, 0.04;42,43 3-Tol3-
PNSiMe3, 150, 5, s, 120, 0.02; Me2PhPNSiMe3, 110, 3, l, 52-54, 0.01;44
MePh2PNSiMe3, 140, 5, l, 105-106, 0.02; n-Bu3PNSiMe3, 110, 3, l,
85-86, 0.01;12 i-PrPh2PNSiMe3, 160, 8, s, 125, 0.01; Cy3PNSiMe3,
150, 8, s, 120, 0.01. 2-Tol3P did not react with trimethylsilyl azide
below 200 °C. All the imines were characterized by elemental analysis
and infrared spectroscopy. Analyses for new imines: Calcd for i-PrPh2-
PNSiMe3, C18H28NPSi: C, 68.14; H, 8.33; N, 4.42. Found: C, 68.36;
H, 8.35; N, 4.42. Calcd for Cy3PNSiMe3, C21H42NPSi: C, 68.66; H,
11.44; N, 3.81. Found: C, 68.67; H, 12.31; N, 4.70. Calcd for 3-Tol3-
PNSiMe3, C24H30NPSi: C, 73.66; H, 7.67; N, 3.58. Found: C, 74.03;
H, 7.52; N, 4.71. Calcd for MePh2PNSiMe3, C16H22NPSi: C, 66.90;
H, 7.67; N, 4.88. Found: C, 66.48; H, 6.74; N, 4.57.
Syntheses. (Ph3PN)2UCl4, 4. [Ph4P][UOCl5] (1.5 g, 1.95 mmol)
and Ph3PNSiMe3 (0.68 g, 1.95 mmol) were loaded into a flamed-out
Schlenk tube inside a nitrogen-purged glovebox. A 30 cm3 aliquot of
dry acetonitrile was transferred onto the mixture, which was heated
gradually to 90 °C while stirring. The red solid dissolved giving a
dark red solution which was cooled to room temperature and allowed
to stand for 1 day, when a fine deposit of red microcrystalline product
was formed. This was washed with acetonitrile and dried under
vacuum. Yields were about 20%. Anal. Calcd for C36H30N2P2Cl4U:
C, 46.35; H, 3.22; N, 3.00, P, 6.65; Cl, 15.21; U, 25.21. Found: C,
47.04; H, 3.34; N, 3.27; P, 6.50; Cl, 15.18; U, 25.36. At room
temperature the red starting material reacts over several hours, during
which time the product begins to deposit. The precipitation is complete
in ∼12 h. Dichloromethane can also be used as the solvent.
Crystals of 4 were grown by scaling up the above preparation 4-fold.
Samples of the resulting solution were then removed periodically (every
24 h), filtered, and transferred into Schlenk tubes that had been
previously silylated with Me2SiCl2. The solution remains supersaturated
for several days and crystallization is very slow; early samples deposit
large amounts of powdered solid. Those that are removed later deposit
smaller amounts more slowly, and so on, until the crystal quality of
the product is good. At this stage there is very little product left in
solution, and the crystals formed are very small. A similar method
Experimental Section
General Data. All reactions were performed under an inert
atmosphere. Involatile solids were generally handled in a nitrogen-
purged glovebox, while all other operations were performed using
Schlenk techniques. IR spectra were recorded between 400 and 4000
cm-1 on a Mattson Galaxy 6020 Fourier transform infrared spectrom-
eter. Samples were dispersed in a dry potassium bromide matrix
prepared under nitrogen and mounted in a cell equipped with cesium
iodide windows. Raman spectra were measured on a Perkin-Elmer
2000 Fourier transform Raman/infrared instrument, equipped with an
InGeAs detector. The source was a 100 mW, 1064 nm Nd:YAG laser
and the spectral resolution was 4 cm-1
. 5 did not fluoresce when
excited at 1064 nm, but 6 was found to fluoresce weakly. Fluorescence
prevented any measurements under 647 nm laser excitation.
X-ray Crystallography. Crystals of 4 (with dimensions ∼0.1 ×
0.1 × 0.005 mm) were mounted on an Enraf Nonius CAD4 diffrac-
tometer. Data were collected with the following parameters: λ )
0.701 69 Å, µ(Mo KR) ) 29.41 cm-1, room temperature, and θ range
1-16° yielding a total of 350 unique reflections, although intensity
checks decreased by 75% over 12 h. The data were of insufficient
quality to present a completed structure. No improvements were
obtained on cooling the samples or by the use of plate-detection systems.
Crystal data: C36H30Cl4N2P2U, M ) 931.8, space group P21/a
(nonstandard setting), a ) 12.187 Å, b ) 15.143 Å, c ) 10.176 Å, â
) 97.32°, V ) 1862.7 Å3, Z ) 2, Dc ) 1.64 g cm-1. The heavy atoms
(U, Cl, P) were located with Patterson methods; their arrangement
showed clearly a square-planar array of four chlorine atoms at a distance
of ca. 2.6 Å, and two phosphorus atoms at ca. 3.5 Å in a trans
(36)
(37) O’Donnell, T. A.; Wilson, P. W.; Hurst, H. J. Inorg. Synth. 1976, 16,
143-147.
(41) Staudinger, H.; Hauser, E. HelV. Chim. Acta 1921, 4, 861-886.
(42) Stuckwisch, C. G. Synthesis 1973, 469-483.
(38) Brown, D. ComprehensiVe Inorganic Chemistry, The Actinides;
Trotman-Dickenson, A. F., Ed.; Pergamon: Oxford, England, 1973;
pp 141.
(43) Birkofer, L.; Ritter, A.; Richter, P. Chem. Ber. 1963, 96, 2750-2757.
(44) Buchner, W.; Wolfsberger, W. Z. Naturforsch. 1974, 29B, 328-334.
(45) Roesky, H. W.; Katti, K. V.; Seseke, U.; Scholz, U.; Herbst, R.; Egart,
E.; Sheldrick, G. M. Z. Naturforsch. 1986, 41B, 1509-1512.
(46) Choukroun, R.; Gervais, D.; Dilworth, J. R. Transition Met. Chem.
1977, 4, 242-252.
(39) Ashcroft, B. R.; Clark, G. R.; Nielson, A. J.; Pickard, C. E. F.
Polyhedron 1986, 5, 2081-2091.
(40) Nugent, W. A.; Mayer, J. M. In Metal Ligand Multiple Bonds;
Wiley: New York, 1985; Chapter 5.

Stable Analogs of the Uranyl Ion
Inorganic Chemistry, Vol. 35, No. 21, 1996 6163
can be used with (Tol3PN)2UCl4. Crystals produced in this way had
dimensions of about 0.1 × 0.1 × 0.005 mm.
at room temperature. When the mixture was heated, the [Ph4P][UOCl5]
dissolved but the resultant solution was brown and failed to yield any
isolable products.
Preparation of [Ph4P][OUCl4(NPPh3)], 3. [Ph4P][UOCl5] (0.5 g,
0.65 mmol), Ph3PNSiMe3 (0.23 g, 0.66 mmol) and CH2Cl2 (30 cm3)
were loaded into a glass vessel guarded by a Young’s greaseless valve,
and heated to 100 °C. The solids dissolved, forming a clear dark red
solution. After cooling, the solution was filtered, and an equal volume
of dry toluene added carefully so as to form a layer above the CH2Cl2.
After several weeks small red crystals had formed and were washed
with a dry dichloromethane/toluene mixture. [Ph4P]2[UO2Cl4] is also
produced during the reaction; if crystallization occurs it may normally
be removed by hand. Yield: ∼20%. Anal. Calcd for C42H35NOP2-
Cl4U: C, 49.55; H, 3.46; N, 1.38, P, 6.13; U, 23.54. Found: C, 49.75;
H, 3.38; N, 1.21; P, 6.06; U, 23.58.
Reaction of [Ph4P][UOCl5] with Ph2MePNSiMe3 or
i-PrPh2PNSiMe3. [Ph4P][UOCl5] (0.5 g, 0.65 mmol) and Ph2-
MePNSiMe3 (0.20 g, 0.70 mmol) were mixed in MeCN (12 cm3) or
CH2Cl2 (12 cm3). In both solvents dark red solutions were produced
suggesting the presence of uranium phosphoraniminato complexes; the
products were, however, exceedingly soluble and could not be isolated
pure. In another reaction, [Ph4P][UOCl5] (0.5 g, 0.65 mmol) and
i-PrPh2PNSiMe3 (0.21 g, 0.67 mmol) were heated in MeCN (12 cm3)
yielding a dark red solution, but for similar reasons no iminato
compounds could be isolated.
Reaction of UCl6 with Ph3PNSiMe3. Uranium hexachloride (0.20
g, 0.44 mmol) and Ph3PNSiMe3 (0.33 g, 0.95 mmol) were loaded into
a glass vessel and cooled to -78 °C. CH2Cl2 (4 cm3) was slowly
transferred onto the stirred mixture, which was held at this temperature.
When the mixture was allowed to stand, a volume of olive green solid
UCl4 was precipitated below a brown solution. No (Ph3PN)2UCl4 could
be detected.
Reaction between [Ph4P]2[UO2Cl4] and (Ph3PN)2UCl4. [Ph4P]2[UO2-
Cl4] (0.16 g, 0.15 mmol) and (Ph3PN)2UCl4 (0.14 g, 0.15 mmol) were
loaded into a glass vessel, and 20 cm3 of CH2Cl2 was added. The
mixture was heated to 195 °C for 5 h, during which time the solids
dissolved. When the solution was cooled to room temperature, a red
solid was precipitated and its infrared spectrum was identical with that
of (Ph3PN)2UCl4.
Attempted Disproportionation of 3. A sample of [Ph4P][OUCl4-
(NPPh3)] was dissolved in CH2Cl2 and heated to 170 °C for 2 days,
during which time no visible change was observed; after removal of
the solvent the IR spectrum of the solid was found to be unchanged.
Preparation of [Me2PhPNH2][OUCl4(NPMe2Ph)], 7. 50 cm3 of
toluene was carefully added to form a layer above a saturated solu-
tion of (Me2PhPN)2UCl4 8 (∼0.05 g) in CH2Cl2 (50 cm3), and
allowed to stand for 2 months. The product was formed as thin
flat red platelets and identified by its elemental analysis, infrared
spectrum (NH2 group, 3341 and 3260 cm-1; NsP, 1058 cm-1; UdO,
856 cm-1) and X-ray structural data.15 Anal. Calcd for C16H24N2OP2-
Cl4U: C, 27.37; H, 3.45; N, 3.99. Found: C, 28.01.01; H, 3.36; N,
4.17.
Preparation of [Ph4P][OUCl4NPTol3], 5. [Ph4P][UOCl5] (1.39 g,
1.8 mmol) and 3-Tol3PNSiMe3 (0.78 g, 2.00 mmol) were allowed to
react in dry CH2Cl2 (30 cm3) at room temperature for 5 h, forming a
clear red solution. Onto this was added an equal volume of dry toluene.
After interdiffusion for several weeks, well-formed needles of 5 were
isolated in ca. 30% yield. Large clusters of [Ph4P]2[UO2Cl4] crystals
were codeposited, but were easily removed by hand. Anal. Calcd for
C42H35NOP2Cl4U: C, 51.28; H, 3.89; N, 1.33. Found: C, 50.49; H,
4.39; N, 1.45.
Preparation of (Me2PhPN)2UCl4, 8. [Ph4P][UOCl5] (1.5 g, 2
mmol) and Me2PhPNSiMe3 (0.6 g, 2.7 mmol) were stirred together in
CH2Cl2 (30 cm3) at room temperature for 1 h, after which all of the
solid had dissolved leaving an orange solution. After filtering, the
solution was left undisturbed, and was observed to darken considerably
in color. Over a period of 1 day, the red microcrystalline product was
deposited and was washed carefully with a small volume of dry CH2-
Cl2. Yield: ∼15%. Typically, yellow [Ph4P]2[UO2Cl4] also crystal-
lized, and was removed by hand. Anal. Calcd for C16H22N2P2Cl4U:
C, 28.09; H, 3.24; N, 4.09, P, 9.05; Cl, 20.73; U, 34.79. Found: C,
29.01; H, 3.24; N, 3.81; P, 8.31; Cl, 20.12; U, 33.80.
Preparation of (Tol3PN)2UCl4, 6. [Ph4P][UOCl5] (0.53 g, 0.69
mmol) and 3-Tol3PNSiMe3 (0.32 g, 0.82 mmol) were reacted together
in MeCN (12 cm3) at 90 °C for 1 h, yielding a dark red solution. This
was cooled, filtered, and left to stand. The product was deposited as
a dark orange powder, and was washed with MeCN and dried under
vacuum. Yields were typically 20%. As in the case of 4 it was
necessary to separate the product by hand selection. Anal. Calcd for
C42H42N2P2Cl4U: C, 49.61; H, 4.13; N, 2,76; P, 6.10; U, 23.43.
Found: C, 49.31; H, 3.39; N, 3.57; P, 5.03; U, 23.54.
Acknowledgment. We thank the EPSRC (formerly SERC)
and British Nuclear Fuels Limited for the support of D.R.B.
and Andrew Jeapes of BNFL for valuable discussion and advice.
We are most grateful to Heather Wilson of Southampton
University for the FT-Raman measurements. For much valuable
advice on the crystallography of UCl4(NPPh3)2 we would like
to thank David Watkin, Keith Prout, Ann Chippindale, and
James Bartlett of Chemical Crystallography, Oxford, U.K., and
Dai Hibbs of Cardiff University (EPSRC Crystallography
Service).
Reaction of [Ph4P][UOCl5] with n-Bu3PNSiMe3. MeCN (15 cm3)
was transferred to a glass vessel containing [Ph4P][UOCl5] (1.00 g,
1.30 mmol) and n-Bu3PNSiMe3 (0.47 g, 1.63 mmol). Reduction
occurred immediately, and large amounts of pale green solid were
produced, shown by analysis and infrared spectroscopy to be [Ph4P]2-
[UCl6]. No red coloration was observed, and no iminato compounds
could be detected.
Reaction of [Ph4P][UOCl5] with Cy3PNSiMe3. Addition of MeCN
(12 cm3) to a mixture of [Ph4P][UOCl5] (0.75 g, 0.97 mmol) and Cy3-
PNSiMe3 (0.38 g, 1.04 mmol, Cy ) cyclohexyl) resulted in no reaction
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