New UIII and UIV silylamides and an improved synthesis of NaN(SiMe2R)2 (R = Me, Ph)

Article abstract of DOI:10.1016/j.jorganchem.2010.08.019

It is shown that the deprotonation of bulky amides such as HN(SiMe ₂Ph)₂ may be accelerated by the use of catalytic quantities of an alkali metal tert-butoxide salt, affording, for example, overnight syntheses of NaN(SiMe₂Ph)₂. The new uranium(IV) and uranium(III) complexes [U{N(SiMe₂H)₂}₄] and [U{N(SiMe₂Ph)₂}₃] are both accessible from the Group 1 salts of the amides and UI₃(thf)₄ in thf. The choice of sodium or potassium salt made no difference to the reaction outcome. Both exhibit Weak interactions between uranium and with silyl-H or silyl-Ph groups in the solid-state.

Full text of DOI:10.1016/j.jorganchem.2010.08.019

Journal of Organometallic Chemistry 695 (2010) 2814e2821
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Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
New UIII and UIV silylamides and an improved synthesis of NaN(SiMe2R)2
(R ¼ Me, Ph)
*
Stephen M. Mansell, Bernabé Fernandez Perandones, Polly L. Arnold
School of Chemistry, Joseph Black Building, University of Edinburgh, The King’s Buildings, Edinburgh, EH9 3JJ, UK
a r t i c l e i n f o
a b s t r a c t
Article history:
It is shown that the deprotonation of bulky amides such as HN(SiMe2Ph)2 may be accelerated by the use
of catalytic quantities of an alkali metal tert-butoxide salt, affording, for example, overnight syntheses
of NaN(SiMe2Ph)2. The new uranium(IV) and uranium(III) complexes [U{N(SiMe2H)2}4] and [U{N
(SiMe2Ph)2}3] are both accessible from the Group 1 salts of the amides and UI3(thf)4 in thf. The choice of
sodium or potassium salt made no difference to the reaction outcome. Both exhibit Weak interactions
between uranium and with silyl-H or silyl-Ph groups in the solid-state.
Received 14 May 2010
Received in revised form
6 August 2010
Accepted 12 August 2010
Available online 20 August 2010
Ó 2010 Elsevier B.V. All rights reserved.
Keywords:
Uranium
Amide ligands
Actinide complexes
Reduction
Agostic
Weak interactions
1. Introduction
U-metal bonding. [17,18] It is increasingly important to investigate
the fundamental chemistry of these species in order to expand on
Low-valent uranium chemistry has caused much recent excite-
ment due to the unusual reactions that can be accomplished [1e3].
In this respect, convenient syntheses of low-valent starting mate-
rials have been crucial to the development of this field such as
uranium triiodide [4,5] for use in salt metathesis reactions with
sodium and potassium ligand precursors. Uranium amides have
also become very important precursors to many low-valent
uranium complexes [6], and the synthesis of [U{N(SiMe3)2}3] in
particular was a landmark in low-valent uranium chemistry [7,8]
allowing the facile exploration of uranium(III) and its comparison
with lanthanide(III) chemistry [9]. Examples where this method-
ology has been used include in the synthesis of tripodal trisalkox-
ideuranium(III) systems [10,11] and in the synthesis of an alkoxy-
tethered uranium(III) carbene complex [12]. Compared to the
cyclopentadienyl and pentamethylcyclopentadienyl ligands, which
have a very rich f-element chemistry [13e15], simple amido species
are comparatively underdeveloped, but glimpses of fascinating
chemistry have been observed including reversible coordination of
dinitrogen between two uranium centres supported by trisami-
doamine ligands [16] and the ability of the same ligand to stabilise
known chemistry and find new reactivity unachievable with the
cyclopentadienyl ligand system [19,20].
Homoleptic uranium(III) amides are relatively rare [21], and
apart from [U{N(SiMe3)2}3], the structure of which was published in
1998 [22], other examples include the “ate” complex [U{N
(SiMe3)2}4][K(thf)6] [23], and along with the La, Ce and Pr analogues,
these represent the only crystallographically characterised
complexes with four N(SiMe3)2 ligands around one metal centre
[23]. Another “ate” complex that has been identified is [K(THF)2]2[U
(N(H)Dipp)5] (Dipp ¼ 2,6-iPr2C6H3) [24]. From a U(IV) compound, [U
{N(3,5-Me2C6H3)tBu}3(thf)] was synthesised by potassium reduc-
tion and was crystallographically characterised [25].
Homoleptic uranium(IV) amides were investigated initially in
the search for volatile uranium compounds for separations tech-
nologies; the highly volatile tetravalent complex [U(NEt2)4] was
reported in 1956 [26]. This compound is dimeric with a five-coor-
dinate uranium centre in the solid-state [27], but the methyl
analogue [U(NMe2)4] has a trimeric structure with six-coordinate
uranium centres, again characterised as containing ligands bridging
through the N atoms [28]. Using the proligand MeN(H)CH2CH2N(H)
Me, a tetrameric cluster was characterised [28] whereas [U(NPh2)4]
is monomeric with a four-coordinate uranium centre [29].
Amides which are similar to the ubiquitous and highly useful N
(SiMe3)2 ligand include N(SiMe2H)2 and N(SiMe2Ph)2 which have
* Corresponding author. Tel.: þ44 131 650 5429; fax: þ44 131 650 6453.
E-mail address: polly.arnold@ed.ac.uk (P.L. Arnold).
0022-328X/$ e see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2010.08.019

S.M. Mansell et al. / Journal of Organometallic Chemistry 695 (2010) 2814e2821
2815
both received attention in the field of f-element chemistry [30e33].
Considering steric factors, replacement of a methyl group for
a hydrogen atom would be expected to decrease the kinetic stabi-
lisation of the metal centre whereas the effect of replacement of
a methyl group by phenyl is less clear. Since agostic and other weak
interactions can be hard to predict a priori but can play significant
roles in the chemistry of low-coordinate metal complexes, it is
notable that both ligands offer the potential for weak interactions
with the metal centre through SieH agostic-type interactions or
associated electron density [32]. Interestingly, NaN(SiMe2H)2 forms
an extended ladder structure in the solid-state with SieH/Na
interactions and NaN(SiMe2Ph)2 shows coordination of thf (retained
from the synthesis) whereas KN(SiMe2Ph)2 forms a dimeric struc-
ture with bridging amide groups and incorporated toluene mole-
cules indicating incomplete encapsulation of the potassium atom by
the ligand despite the larger steric bulk and presence of SiePh
groups. The structure of [{(Me2HSi)N(SiMe3)K}2thf] is dimeric with
one bridging thf molecule and one K atom has SieH and SieMe
contacts as well as bonds to two nitrogen atoms [46].
h
n-Ph interactions. Examples of agostic interactions in low-coordinate
d-block chemistry are well-documented [34,35], including a notable
recent example of CeH agostic interactions in titanium amido
b
2. Results and discussion
compounds [36]. The most notable weak CeH interactions in
uranium(III) coordination chemistry are the silylmethyl agostic
interactions found in the pyramidal [U{N(SiMe3)2}3] [22] and [U{CH
(SiMe3)2}3] [37], and the interaction with trapped hydrocarbon
solvent in [{(ArO)3tacn}U(cy-C6H11CH3)]$(cy-C6H11CH3) [38e40].
Despite in general being less well-documented, uranium has an
extensive CeH activation and cyclometallation chemistry [41,42].
Homoleptic lanthanide complexes of the N(SiMe2H)2 ligand
(Ln ¼ Lu, Y, Er, Nd, La) have been shown to form agostic-type inter-
actions as observed by I.R. spectroscopy with a stretch at lower
wavenumber (1931e1970 cmꢀ1) as well as one at higher wave-
number (2051e2072 cmꢀ1) [31]. Their solid-statestructuresshowed
two molecules of coordinated thf and limited evidence for Ln/Si
interactions (Lu/Si distances 3.271(1) to 3.476(1) Å, La/Si
distances 3.337(3) to 3.575(3) Å) [31]. Recently, thf-free structures
have been realised by the reaction of HN(SiMe2H)2 with either
[(YMe3)n] or [La{N(SiMe3)2}3] and dimeric, four-coordinate lantha-
nide complexes were formed with bridging N(SiMe2H)2 ligands
[43,44]. Agostic interactions were clearly evident both by
X-ray crystallography (Y/Si: 3.0521(7) and Y/H: 2.41(3) Å,
La/Si: 3.191(2) and La/H: 2.56(6) Å) and by I.R. spectroscopy via
identification of the SieH stretch (Ln ¼ Y; 2095 e non-agostic,
1931 cmꢀ1 e agostic, Ln ¼ La; 2092 and 2060 e non-agostic, 2023,
1920 cmꢀ1 e agostic). However, lanthanide complexes containing
only one N(SiMe2H)2 ligand with Cp* (Cp* ¼ C5Me5) coligands gave
much lower stretching frequencies for SieH agostic interactions
(as low as 1827 cmꢀ1) [45]. The amide N(SiMe2Ph)2 has been char-
acterised coordinating to a bis(Cp*) lanthanum fragment and
showed agostic interactions with the methyl groups (La/C
distances of 3.121(2) and 3.388(2) Å, La/H distances of 2.86(2) to
3.46(2) Å) despite the presence of phenyl groups with their
2.1. An improved synthesis of MN(SiMe2R)2 and the solid-state
structure of KN(SiMe2H)2
We started our survey of new uranium amides by noting an
efficient and general route to improving the synthesis of the Group
1 metal salts, and investigating the solid-state structure of one of
the ligand precursors, KN(SiMe2H)2. This ligand contains a
b SieH
bond which could lead to interesting interactions or reactivity with
low-valent uranium centres.
We note that for bulkier silyl substituents R, the synthesis of MN
(SiMe2R)2 from reaction of the amine with NaH can take over 72 h
even in refluxing thf or toluene to react completely with the amine.
We find that the addition of a catalytic amount of NaOBut can act as
a transfer agent, reducing the synthetic time required for even NaN
(SiMe2Ph)2 to only 16 h in thf at reflux, eq. (1). After 7 h the NaOBut-
catalysed synthesis of NaN(SiMe2Ph)2 is almost complete, whereas
the uncatalysed reaction has formed only ca. 45% product.
þ1:0MH; þcat:MOBut
Solv
ꢀMI
HNðSiMe2RÞ2
M ¼ Na, K
MNðSiMe RÞ
(1)
ƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒ!
2
2
R ¼ H(Solv. ¼ hexanes)
Me(Solv. ¼ PhMe)
Ph(Solv. ¼ thf)
Colourless crystals of KN(SiMe2H)2 were obtained from
a toluene solution of the reaction of the amine with KH. Its struc-
ture, Fig. 1, is isomorphous to the sodium analogue in the space
Fig. 1. Thermal ellipsoid plot of KN(SiMe2H)2 (50% ellipsoid probability). Selected distances (Å) and angles (ꢁ): K(1B)eN(1B) 2.833(7), K(1)eN(1)#1 2.871(7), K(1)eN(1)#2 3.072(7),,
K(1)eSi(1)#3 3.437(2), K(1)eSi(1)#1 3.589(2).

2816
S.M. Mansell et al. / Journal of Organometallic Chemistry 695 (2010) 2814e2821
group Pnma showing an extended ladder structure parallel to the
a axis with three nitrogen atoms surrounding each potassium atom.
The NeK bond lengths that form the rungs of the ladder are the
shortest (2.833(7) Å) and another bond is of a similar length (2.871
(7) Å) with the other slightly longer (3.072(7) Å). Additional close
contacts with all of the SieH bonds are observed as well as several
K/Me contacts. Unlike in the structure of [{(PhMe2Si)2NK
(toluene)}2], no molecules of toluene coordinate to the potassium
atom presumably as a consequence of the extended nature of the
structure.
C6H6, increased to 2025 when thf is coordinated to the Li ion), E ¼ Na
(1961 and 1926 in nujol), 2004 in C6H6 and E ¼ K (1980 in nujol,1979
in C6H6). SieH stretches at lowwavenumberhavealso been reported
in homoleptic lanthanide complexes of the N(SiMe2H)2 ligand with
stretches observed in the range 1931e1970 cmꢀ1 for agostic-type
interactions [31]. It can be seen that the stretches at higher wave-
numbers are from SieH bonds without any interaction with the
metal centre, whereas stretches at lower wavenumber indicate
some form of interaction with f-element or alkali metal. Indeed, in
[(SALEN)Ln{N(SiHMe2)2}(thf)n] (Ln ¼ Y, La) compounds, bands at
only high wavenumber (ca. 2040 cmꢀ1) were observed for the SiH
moieties indicative of no agostic interaction [53].
An attempt to synthesise the mixed ligand halide complex [U{N
(SiMe2H)2}3I], which we deemed a good candidate for reduction to
the sterically unencumbered target [U{N(SiMe2H)2}3], with
2.25 equivalents of the amide anion yielded only [U{N(SiMe2H)2}4]
in sub 30% yields in our hands, Scheme 2. Given the reported
stability of the UIII complex K[U(N{SiMe3}2)4] [23], we anticipated
that reduction of the tetraamide would be straightforward: Some
reactivity with potassium or potassium graphite was observed, but
no product could be isolated, while surprisingly, no reaction with
tert-butyl lithium, which could also act as a reductant, was
observed, Scheme 2.
2.2. Syntheses of homoleptic [U{N(SiMe2R)2}n], n ¼ 3, 4, R ¼ H, Ph
Syntheses of two new homoleptic uranium(III) amides were
attempted by treatment of three equivalents of either the sodium
or potassium salts of [N(SiMe2H)2]ꢀ [47] or [N(SiMe2Ph)2]ꢀ [32]
with UI3(thf)4 in thf (Scheme 1). The choice of sodium or potas-
sium salt made no difference to the reaction outcome. However, the
two different amide ligands gave very different products.
2.2.1. Synthesis of [U{N(SiMe2H)2}4]
The reaction of MN(SiMe2H)2 and UI3(thf)4 in thf overnight
affords a brown solution. Removal of the solvent and extraction of
the product into n-hexane followed by filtration and crystallisation
gave pale-blue crystals characterised as [U{N(SiMe2H)2}4] in good
yield. The oxidation to uranium(IV) is not uncommon, and is
accompanied by the formation of grey powdered uranium metal
[25,54e56]; the smaller size of the ligand has clearly allowed the
straightforward isolation of this complex which has never been
observed in the [U{N(SiMe3)2}3] syntheses. The tetrakis complex
can also be made independently from UI4(Et2O)2 in good (46%)
yield.
2.2.2. Structure of [U{N(SiMe2H)2}4]
The structure of this compound was ascertained as monomeric
by X-ray diffraction (Fig. 2) with the uranium atom situated on
a two-fold rotation axis. A uranium centre strongly distorted away
from tetrahedral (109.5ꢁ) is evident from the different NeUeN
angles; N(1A)eU(1)eN(1) 126.2(2), N(1A)eU(1)eN(2) 99.61(14), N
(1)eU(1)eN(2) 104.15(14)ꢁ. The UeN bond lengths (U(1)eN(1)
2.280(4) and U(1)eN(2) 2.281(4) Å) are typical of U(IV) amides
[6,48], Agostic-type interactions between the uranium atom and
four SieH bonds are suggested by the bent nature of the ligand as
evidenced by the UeNeSi angles (Si(1)eN(1)eU(1) 103.45(18), Si
(2)eN(1)eU(1) 129.3(2), Si(4)eN(2)eU(1) 103.83(18), Si(3)eN(2)e
U(1) 127.5(2)ꢁ).This in turn leads to four short U/H contacts (2.705
and 2.773 Å) and short U/Si contacts (3.1462(14), 3.1566(14) Å). A
distorted tetrahedral structure was also reported for [U(NPh2)4]
(NeUeN angles from 96.3(7) to 139.2(7)ꢁ) with very similar UeN
bond lengths (2.21(2) to 2.35(2) Å) [29]. This structure can be also
be compared with a transition metal analogue as the crystal
structure of [Hf{N(SiMe2H)2}4] revealed a similar, if slightly less
distorted, tetrahedral geometry with NeHfeN angles varying from
102 to 115ꢁ, and with shorter HfeN bonds lengths (2.062(2) to
1H NMR spectroscopy revealed a very low frequency resonance
at ꢀ19.85 ppm caused by the close proximity of the SieH to the
paramagnetic uranium centre which demonstrates averaging of the
two environments seen in the solid-state. Broad 29Si satellites are
observed (162 Hz) although no coupling is resolved for this reso-
nance or the SiMe groups (observed as a singlet at 1.85 ppm) unlike
in the alkali metal salts. 13C NMR spectroscopy shows one reso-
nance for the methyl groups at 7.46 ppm. The solution magnetic
moment meff (using Evans’ NMR method) is 2.94 BM, which is larger
than that measured for similar UIV amides [U{N(SiMe3)2}3H]
(2.6 BM) [50] and[{N(SiMe3)2}2U(CH2Si)N(SiMe3)] (2.7 BM) [51]
although it is close to the value reported for [U(NPh2)4] (2.84 BM)
[52].
An I.R. spectroscopic study of[U{N(SiMe2H)2}4] revealed SieH
stretches in two different regions; two closely spaced bands at
2099.1 and 2075.5 cmꢀ1 and another at 1975.2 cmꢀ1 (in hexane:
2103.2, 2075.0 and 1982.2 cmꢀ1). This can be compared to EN
(SiMe2H)2, E ¼ H (2118 in nujol), E ¼ Li (1990 cmꢀ1 in nujol, 1996 in
SiMe2H
9
+
/
4 M
N
H
SiMe2H
SiMe2H
SiMe2H
Me2Si
UIV
SiMe2H
3
N
N
UI3(thf)4
/
4
N
X
- 3 MI, -1/4
U
HMe2Si
SiMe2H
SiMe2H
SiMe2H
HMe2Si
H
N
+3 M
N
SiMe2H
SiMe2H
SiMe2H
SiMe2H
thf
3
Me 2Si
/
UIV
UI3(thf)4
4
N
N
N
- 3 MI, -1/4
U
H
UIII
HMe2Si SiMe2H
N
HMe2Si
SiMe2H
Me2Si
SiMe2H
SiMe2H
SiMe2H
+ Red
Solv.
3
/
4
N
N
SiMe2Ph
X
H
SiMe2H
SiMe2H
SiMe2H
N
Me2Si
UIV
N
+3 M
N
SiMe2Ph
HMe2Si
N
Me2Si
SiMe2H
Red =
K, (Solv. = thf)
KC8, (Solv. = thf)
UIII
N
N
N
SiMe2Ph
thf
- 3 MI
SiMe2Ph
SiMe2Ph
HMe2Si
N
SiMe2H
t
M = Na, K
PhMe2Si
SiMe2Ph
LiBu (Solv. = PhMe)
Scheme 1. Synthesis of UIV and UIII amides.
Scheme 2. Reactions of [U{N(SiMe2H)2}4] to make related amide complexes.

S.M. Mansell et al. / Journal of Organometallic Chemistry 695 (2010) 2814e2821
2817
(SiMe3)2}3] (the solid-state meff is 3.07 mB at 300 K) [7,37,57], and
only slightly higher than that measured above for the 5f2 [U{N
(SiMe2H)2}4], but the range of room temperature magnetic
moments of UIII (4I9/2 ground state) and UIV (3H4 ground state)
coordination complexes are know to have considerable overlap,
and the values reported for both of these new uranium amides are
within this range [58e60].
thf
SiMe2Ph
SiMe2Ph
SiMe2Ph
+ thf
SiMe2Ph
Me2Si
III
UIII N SiMe2Ph
PhMe2Si
PhMe2Si
U
N
N
N
N
N
SiMe2Ph
PhMe2Si
SiMe2Ph
SiMe2Ph
(2)
Attempts to coordinate additional small molecules such as thf
have shown no coordination by NMR spectroscopy, eq (2), in our
hands to date, testifying to the protection of the UIII centre in this
molecule.
2.2.4. Structure of [U{N(SiMe2Ph)2}3]
Crystals of [U{N(SiMe2Ph)2}3] suitable for X-ray diffraction were
grown from a concentrated 1:1 n-hexane/toluene solution at
ꢀ20 ꢁC and the high air- and moisture-sensitivity of the crystals
meant the data collected is only of moderate quality. The complex
[U{N(SiMe2Ph)2}3] crystallised in the space group R3 with the
uranium atom sitting on a three-fold rotation axis. Interestingly,
this leads to voids throughout the structure which are not filled
with solvent (no residual electron density could be identified in
these voids) and do not appear to be interconnected, and their
presence is reflected in a lower density (1.275 g cmꢀ3) compared
with [U{N(SiMe2H)2}4] (1.407 g cmꢀ3). The molecular structure
(Fig. 3) reveals a pyramidal uranium atom bonded to three nitrogen
atoms (UeN distance 2.337(15) Å) with one phenyl group of every
amide ligand pointing above the plane of the nitrogen atoms and
directed towards the uranium atom, whilst the other three phenyl
groups are below the plane and point away from the uranium atom.
This pyramidal geometry is always seen in f-element tris(amide)
complexes, such as [U{N(SiMe3)2}3] (UeN: 2.320(4) Å) [22], and can
be explained for these compounds by the polarised-ion model [22].
Three Ph groups form close contacts with the uranium atom via
the ipso-carbon and Si atom (U/Si: 3.319(5) Å) as has been seen
in [U{N(tBu)3,5-Me2C6H3}3(thf)] which showed slightly shorter
distances (UeN: 2.295(10) to 2.361(9) Å, U/ipso C: 2.886(12) to
2.980 (12) Å), and aryl interactions are also seen in uranium(IV)
benzyl compounds [54,55]. This is seen most clearly in the top view
of the complex, Fig. 3(b).
Fig. 2. Thermal ellipsoid plot of [U{N(SiMe2H)2}4] (50% ellipsoid probability). All
hydrogen atoms except for SieH have been omitted for clarity. Symmetry operator for
symmetry generated atoms: ex, y, ez þ 3/2. Selected bond lengths (Å) and angles (ꢁ):
U(1)eN(1) 2.280(4), U(1)eN(2) 2.281(4), U(1)eSi(1) 3.1462(14) U(1)eSi(4) 3.1566(14),
N(1)eSi(1) 1.701(4), N(1)eSi(2) 1.717(4), N(2)eSi(4) 1.704(4), N(2)eSi(3) 1.711(4),
Si(1)eH(1) 1.32(6), Si(2)eH(2) 1.38(2), Si(3)eH(3) 1.30(6), Si(4)eH(4) 1.47(5), N(1A)e
U(1)eN(1) 126.2(2), N(1A)eU(1)eN(2) 99.61(14), N(1)eU(1)eN(2) 104.15(14), Si(1)e
N(1)eU(1) 103.45(18), Si(2)eN(1)eU(1) 129.3(2), Si(4)eN(2)eU(1) 103.83(18),
Si(3)eN(2)eU(1) 127.5(2); S (angles at N1) 359.95, S (angles at N2) 359.83.
2.079(2) Å) [49]. Agostic interactions were not mentioned in the
paper, but the same asymmetry in the N(SiMe2H)2 ligands was seen
in the tilting of the group (HfeNeSi angles of approximately 110ꢁ
and 125ꢁ) with one set of shorter Si/Hf distances (3.090e3.169 Å)
and one longer set (3.361e3.410 Å) [49].
2.2.3. Synthesis of [U{N(SiMe2Ph)2}3]
The reaction of three equivalents of MN(SiMe2Ph)2 with
UI3(thf)4 in thf proceeded to give a dark brown solution, Scheme 1.
The solvent was removed under reduced pressure and the residue
was extracted with n-hexane which was filtered and a small
amount of impure solid precipitates after storage at ꢀ30 ꢁC. After
the transfer of the supernatant to a new vessel, brown material
crystallises upon storage at ꢀ30 ꢁC, characterised as [U{N
(SiMe2Ph)2}3]. The additional separation stage to remove impure
material, along with the low volatility of the free amine makes this
complex more difficult to isolate pure and in large quantities than
the other silylamide analogues. 1H NMR spectroscopy (C6D6) shows
a single set of broad, paramagnetically shifted resonances at 5.34,
3.89 and 3.18 ppm for the phenyl groups and ꢀ6.50 ppm for the
methyl groups. This is also observed by 13C NMR spectroscopy with
the phenyl resonances resonating at lower frequency (122.6, 120.5
and 112.0 ppm) than in the free amine (141.0, 133.4, 129.0 and
127.7) and in the K salt (149.2, 132.8 and 128.0 ppm) and a very
broad resonance at ꢀ57.1 ppm for the methyl groups. The ipso-
carbon was not observed.
Finally, a comparison of the NIR-UV-vis spectra of hexanes
solutions of [U{N(SiMe2Ph)2}3] and [U{N(SiMe2H)2}4] (Fig. 4) show
maxima with molar absorptivities in the 100s for the UIII amide and
absorptions with molar absorptivities in the 10s the UIV amide,
confirming the assigned oxidation states.
3. Conclusions
A fast, efficient synthesis of Group 1 bis(silylamide) salts NaN
(SiMe2R)2, R ¼ H, Me, Ph, using NaOBut as a catalyst, has been
described, which reduced the time required to make the steri-
cally most hindered compound, NaN(SiMe2Ph)2, from three days
to 16 h. The recent renaissance in multi-electron-chemistry
The 5f3 UIII centre gives rise to a magnetic moment for [U{N
(SiMe2Ph)2}3], determined in solution at room temperature, of
3.11 mB. This is very close to the values reported for the parent [U{N

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S.M. Mansell et al. / Journal of Organometallic Chemistry 695 (2010) 2814e2821
Fig. 3. Thermal ellipsoid plot of [U{N(SiMe2Ph)2}3]. (a) side view (50% ellipsoid probability), (b) top view (space-fill). All hydrogen atoms have been omitted for clarity. Symmetry
operators for symmetry generated atoms: ex þ y, ex þ 1, z and ey þ 1, x e y þ 1, z. Selected bond lengths (Å) and angles (ꢁ): U(1)eN(1) 2.337(15), U(1)eSi(2) 3.319(5), N(1)eSi(2)
1.720(16), N(1)eSi(1) 1.739(16).
reported for f-block metals using ’sterically induced reduction’
suggests that this protocol for the acceleration of kinetically
difficult deprotonations of bulky ligands might have more
widespread use. These amide anions allow the synthesis of UIV
and UIII complexes [U{N(SiMe2H)2}4] and[U{N(SiMe2Ph)2}3], both
of which display weak interactions between the uranium centre
and silane SieH atoms (the former) and SiePh ipso C atoms (the
latter). We have been unable to isolate the sterically unencum-
bered trivalent [U{N(SiMe2H)2}3] but we anticipate that both
amide complexes will prove useful starting materials for further
redox and ligand exchange reactivity.
4. Experimental
4.1. General details
All manipulations were carried out under a dry, oxygen-free
dinitrogen atmosphere using standard Schlenk techniques or in
400
40
[U{N(SiMe2Ph)2}3]
1000
[U{N(SiMe2H)2}4]
350
35
900
800
700
600
500
400
300
200
100
0
300
250
200
150
100
50
30
25
20
15
10
5
0
600
0
1400
700
800
900
1000
1100
1200
1300
Wavelength /nm
[U{N(SiMe2H)2}4]
[U{N(SiMe2Ph)2}3]
250
500
750
1000
Wavelength /nm
1250
1500
1750
Fig. 4. NIR-Uv-vis spectra of hexane solutions of [U{N(SiMe2H)2}4] and [U{N(SiMe2Ph)2}3] (1.9 ꢂ 10ꢀ5 M and 1.5 ꢂ 10ꢀ5 M respectively).

S.M. Mansell et al. / Journal of Organometallic Chemistry 695 (2010) 2814e2821
2819
c) Four equivalents of NaN(SiMe2H)2 (61 mg, 0.39 mmol) were
added to UI4(OEt2)2 (88 mg, 0.099 mmol) in thf (2 cm3) and the
pale yellow solution was stirred for 24 h. The solvent was
removed under reduced pressure and the solid was extracted
with n-hexane (10 cm3), filtered and the solvent removed
under reduced pressure yielding a yellow solid (35 mg,
0.046 mmol, 46%) which was identified as [U{N(SiMe2H)2}4] by
NMR spectroscopy.
MBraun Unilab or Vacuum Atmospheres OMNI-lab gloveboxes
unless otherwise stated. THF and hexane were degassed and
purified by passage through activated alumina towers prior to use.
All deuterated solvents were boiled over potassium, vacuum
transferred, and freeze-pump-thaw degassed three times prior to
use. The compounds NaN(SiMe2H)2 [47], KN(SiMe2H)2 [47], KN
(SiMe2Ph)2 [32], and UI3(thf)4 (from stirring UI3 [61] in thf), were
made as previously described in the literature, whilst all other
reagents were used as purchased without further purification. 1H
and 13C NMR spectra were recorded on Bruker AVA 400 or 600 MHz
NMR spectrometers at 298 K. Chemical shifts are reported in parts
per million, and referenced to residual proton resonances cali-
brated against external TMS. Infrared spectra were recorded on
Jasco 410 spectrophotometers. Solutions for UVevis spectropho-
tometry were made in a nitrogen filled glovebox and spectra were
recorded in either a Teflon-tapped 10 mm quartz cell or a 1 mm
quartz cell sealed by a tight fitting Subaseal on a Unicam UV1
spectrophotometer.
4.3.2. Attempted synthesis of [U{N(SiMe2H)2}3I]
In an attempt to synthesise [U{N(SiMe2H)2}3I], NaN(SiMe2H)2
(290 mg, 1.87 mmol) reacted with [UI3(thf)4] (753 mg, 0.83 mmol)
yielding instead [U{N(SiMe2H)2}4] (140 mg, 0.18 mmol, 29%).
4.3.3. Reactions of [U{N(SiMe2H)2}4]
4.3.3.1. [U{N(SiMe2H)2}4] with KC8. A solution of [U{N(SiMe2H)2}4]
(150 mg, 0.20 mmol) in thf (10 cm3) was added to a bronze
suspension of KC8 (26 mg, 0.20 mmol) in thf (10 cm3) and this
mixture was stirred for 72 h. Black graphite was observed, and
the brown supernatant was isolated by filtration and the solvent
removed under reduced pressure. Extraction into C6D6 with a few
drops of thf allowed the identification of resonances for the
starting material as well a numerous paramagnetically shifted
4.2. Improved synthesis of NaN(SiMe3)2 and NaN(SiMe2Ph)2
NaH (102 mg, 4.2 mmol) and NaOtBu (7 mg, 0.02 mol) was
dissolved in thf (10 cm3) and HN(SiMe2Ph)2 (1 cm3, 3.5 mmol) was
added and the mixture was heated under reflux for 16 h. All of the
solvent was removed under reduced pressure and the colourless
solid was extracted into hot n-hexane and filtered. Removal of the
solvent under reduced pressure and extended heating with a hot
water bath gave NaN(SiMe2Ph)2 with only very small amounts of
residual coordinated thf (882 mg, 2.9 mmol, 83%).
resonances including
ꢀ3.6 (s), ꢀ4.9 (s), ꢀ10.9 (s), ꢀ22.9 (s).
d
(ppm); 21.6 (s), 12.7 (s), 9.1 (s), ꢀ0.8 (s),
4.3.3.2. [U{N(SiMe2H)2}4] with K. Potassium (20 mg, in excess) was
added to a solution of [U{N(SiMe2H)2}4] (150 mg, 0.20 mmol) in thf
(15 cm3) and the brown solution was stirred for 16 h, after which
time, potassium metal was still visible in the dark brown solution.
The supernatant was isolated by filtration and the solvent removed
under reduced pressure.1H NMR spectroscopy in C6D6 showed no U
starting material and no resonances outside the diamagnetic region:
NaH (1.21 g, 50.4 mmol) and NaOtBu (97 mg, 1.0 mmol) was
suspended in toluene (20 cm3) and HN(SiMe3)2 (20 cm3,
48.0 mmol) was added and the mixture was heated under reflux for
48 h. The solution was filtered and all of the solvent was removed
under reduced pressure and the colourless solid was washed with
n-hexane (ca. 3 cm3) and dried under reduced pressure (7.530 g,
41.1 mmol 86%).
d
(ppm) 5.13 (bs, 2H), 3.37 (bs, thf), 1.29 (bs, thf), 0.36 (bs, 12H).
4.3.3.3. [U{N(SiMe2H)2}4] with tBuLi. tBuLi (0.15 cm3, 1.7 M in
pentane, 0.25 mmol) was added to a solution of [U{N(SiMe2H)2}4]
(190 mg, 0.25 mmol) in toluene and was stirred for 72 h. The
supernatant was isolated by filtration and the solvent removed
under reduced pressure. 1H NMR spectroscopy revealed only
resonances for the starting material, [U{N(SiMe2H)2}4].
4.3. Synthesis of uranium(III) and uranium(IV) amides
4.3.1. [U{N(SiMe2H)2}4]
(a) A solution of NaN(SiMe2H)2 (490 mg, 3.15 mmol) in thf
(10 cm3) was added to a blue solution of UI3(thf)4 (954 mg,
1.05 mmol) in thf (10 cm3) giving a brown solution which was
stirred at room temperature for 16 h. The volatiles were
removed under reduced pressure and the brown solid was
extracted with n-hexane (20 cm3). This was filtered via cannula
and the supernatant was reduced in volume under reduced
pressure and pale-blue crystals were obtained after storage of
a saturated solution overnight at ꢀ30 ꢁC (182 mg, 0.24 mmol,
30%).
4.3.4. [U{N(SiMe2Ph)2}3]
a) A solution of NaN(SiMe2Ph)2 (406 mg, 1.32 mmol) in thf
(10 cm3) was added to a blue solution of UI3(thf)4 (399 mg,
0.44 mmol) in thf (10 cm3) giving a brown solution which was
stirred at room temperature for 16 h. The volatiles were
removed under reduced pressure and the brown solid was
extracted with n-hexane (40 cm3). This was cannula filtered
and reduced in volume under reduced pressure; upon storage
at ꢀ30 ꢁC an impure solid precipitates. The supernatant solu-
tion was transferred into a new Schlenk vessel and dark brown
crystals were obtained after storage of this saturated solution
overnight at ꢀ30 ꢁC (234 mg, 0.21 mmol, 49%).
1H NMR (400 MHz, 298 K, C6D6)
d (ppm) 1.85 (s, 48H, Me),
ꢀ19.85 (s with 29Si satellites 1J(1He29Si) ¼ 162 Hz, 8 H, SieH). 13
C
NMR (400 MHz, 298 K, C6D6)
d (ppm) 7.46 (Me). meff (Evans’ NMR
method) 2.94. I.R. (nujol mull)
1975.2m, 1260.3s, 1093.4s, 1018.7s, 896.7s and 797.4s. I.R. (hexane
solution)
(cmꢀ1) 2103.2s, 2075.0s, 1982.2m, 1255.2m, 1099.0m,
y
(cmꢀ1) 2099.1m, 2075.5m,
1H NMR (400 MHz, 298 K, C6D6)
C6H5), 3.89 (bs, 16H, ortho-C6H5), 3.18 (bs, 16H, meta-C6H5), ꢀ6.50
(bs, 48H, Me). 13C NMR (600 MHz, 298 K, C6D6)
(ppm) 122.6
(meta-C), 120.5 (para-C), 112.0 (ortho-C), ꢀ57.1 (Me). meff (Evans’
NMR method) 3.11 mB. I.R. (nujol mull)
(cmꢀ1) 1259.1 (m), 1102.8
(m), 933.1 (m), 832.1 (w), 799.8 (w), 722.2 (w). I.R. (hexane solu-
tion)
(cmꢀ1) 1255.7 (w), 1181.2 (w), 1111.0 (m), 942.1 (m), 833.8
(w), 699.1 (w).
d (ppm) 5.34 (bs, 8H, para-
y
d
943.5m, 843.7m, 684.6m. Anal. Calcd. for C16H56N4Si8U: C, 25.04; H,
7.36; N, 7.30. Found: C, 24.31; H, 7.31; N, 7.27.
y
b) Instead of NaN(SiMe2H)2, KN(SiMe2H)2 (573 mg, 3.34 mmol)
was added to UI3(thf)4 (1.011 g, 1.11 mmol) in thf (20 cm3),
yield: 313 mg, 0.41 mmol, 37%.
y

2820
S.M. Mansell et al. / Journal of Organometallic Chemistry 695 (2010) 2814e2821
Table 1
Selected experimental crystallographic details for Compounds KN(SiMe2H)2, [U{N(SiMe2H)2}4] and [U{N(SiMe2Ph)2}3].
Compound
KN(SiMe2H)2
[U{N(SiMe2H)2}4]
[U{N(SiMe2Ph)2}3]
Colour, habit
Size/mm
Empirical Formula
M
Crystal system
Space group
a/Å
Colourless, block
0.50 ꢂ 0.50 ꢂ 0.20
C8H28K2N2Si4
342.88
Orthorhombic
Pnma
5.8432(6)
15.1901(19)
11.2466(17)
90
90
90
998.2(2)
2
0.699
Pale-blue, shard
0.12 ꢂ 0.07 ꢂ 0.02
C16H56N4Si8U
767.40
Monoclinic
C2/c
17.3799(7)
11.6699(5)
18.4494(8)
90
104.478(4)
90
Brown, block
0.15 ꢂ 0.15 ꢂ 0.03
C48H66N3Si6U
1091.61
Trigonal
R-3
18.6138(10)
18.6138(10)
28.426(3)
90
90
120
8529.5(11)
6
3.012
b/Å
c/Å
ꢁ
ꢁ
a
/
b
/
ꢁ
g
/
V/Å3
Z
3623.1(3)
4
4.756
m
/mmꢀ1
T/K
173(2)
173(2)
171(2)
qmin,max
3.62, 27.49
95.1
2486, 1131
0.0842
3.33, 27.48
99.8
16093, 4156
0.0468
2.91, 24.10
99.7
7691, 3013
0.1036
Completeness to qmax
Reflections: total/independent
Rint
Final R1 (I > 2
s
) and wR2 (all data)
0.0923, 0.2637
0.876, ꢀ0.755
1.141
0.0360, 0.0830
3.441, ꢀ1.024
1.407
0.0929, 0.2591
3.880, ꢀ1.459
1.275
Largest peak, hole/e Åꢀ3
rcalc/g cmꢀ3
b) Instead of NaN(SiMe2Ph)2, KN(SiMe2Ph)2 (539 mg, 1.67 mmol)
was added to UI3(thf)4 (504 mg, 0.56 mmol) in thf (40 cm3),
yield: 320 mg, 0.29 mmol, 52%.
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Appendix A. Supplementary data
CCDC 776998, 776999, 777000 contain the supplementary
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