Coordination of isocyanide and reduction of cyclooctatetraene by a homoleptic uranium(III) aryloxide, and characterisation of the heteroleptic uranium(III) dimer [{U(N″)2(thf)(μ-I)}2]

Article abstract of DOI:10.1016/j.poly.2016.03.048

[U(ODtbp)₃] (ODtbp?=?O-2,6-^tBu₂C₆H₃) reacts in a 1:1 ratio with the isocyanide CN-Xyl (Xyl?=?2,6-Me₂C₆H₃) to form the pseudo-tetrahedral 4-coordinate adduct [U(CNXyl)(ODtbp)₃] with ν_CN24?cm^?1higher compared to the free isocyanide. Uranium(III) complexes with bulky ligands UX₃(X: ODtbp, N″?=?N(SiMe₃)₂) react with cyclooctatetraene (COT) in a 2:1 U:COT ratio to generate the half-sandwich U^IV[U(COT)X₂] and [UX₄] (which for X?=?N″ spontaneously converts into the more stable metallacycle [U(N″)₂{κ²-N(SiMe₃)SiMe₂CH₂}] and HN″), as opposed to the other potential product, the inverse COT-sandwich [(UX₂)₂(μ-COT)]. The heteroleptic U^IIIamido-iodide [{U(N″)₂(thf)(μ-I)}₂] can be isolated in a low yield (14%) from the 2:1 reaction of KN″ and [UI₃(thf)₄] in thf, and its molecular structure was shown to be dimeric with iodine atoms bridging the U centres.

Full text of DOI:10.1016/j.poly.2016.03.048

Polyhedron xxx (2016) xxx–xxx
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Polyhedron
journal homepage: www.elsevier.com/locate/poly
Coordination of isocyanide and reduction of cyclooctatetraene
by a homoleptic uranium(III) aryloxide, and characterisation
of the heteroleptic uranium(III) dimer [{U(N00)2(thf)(
l-I)}2]
Stephen M. Mansell a,b, Polly L. Arnold a,
⇑
a EaStCHEM School of Chemistry, University of Edinburgh, The King’s Buildings, Edinburgh EH9 3FJ, UK
b Institute of Chemical Sciences, Heriot-Watt University, Edinburgh EH14 4AS, UK
a r t i c l e i n f o
a b s t r a c t
[U(ODtbp)3] (ODtbp = O-2,6-tBu2C6H3) reacts in a 1:1 ratio with the isocyanide CN-Xyl (Xyl = 2,6-
Me2C6H3) to form the pseudo-tetrahedral 4-coordinate adduct [U(CNXyl)(ODtbp)3] with mCN 24 cmꢀ1
higher compared to the free isocyanide. Uranium(III) complexes with bulky ligands UX3 (X: ODtbp,
N00 = N(SiMe3)2) react with cyclooctatetraene (COT) in a 2:1 U:COT ratio to generate the half-sandwich
UIV [U(COT)X2] and [UX4] (which for X = N00 spontaneously converts into the more stable metallacycle
[U(N00)2{j2-N(SiMe3)SiMe2CH2}] and HN00), as opposed to the other potential product, the inverse COT-
Article history:
Received 25 January 2016
Accepted 22 March 2016
Available online xxxx
Keywords:
Actinide complexes
f-Block organometallics
Amide
Aryloxide
Reduction
sandwich [(UX2)2(l l-I)}2] can be isolated in a
-COT)]. The heteroleptic UIII amido-iodide [{U(N00)2(thf)(
low yield (14%) from the 2:1 reaction of KN00 and [UI3(thf)4] in thf, and its molecular structure was shown
to be dimeric with iodine atoms bridging the U centres.
Ó 2016 Published by Elsevier Ltd.
1. Introduction
was proposed [20]. The first crystallographically characterised f-
block CO compound, [U(CO)(C5Me4H)3], contained a similar ligand
Low oxidation state uranium chemistry has attracted a lot of
interest over the past decade [1–4], highlighted by the recent char-
acterisation of the U2+ oxidation state in molecular compounds
[5,6]. In particular, the ability of molecular U3+ compounds to react
directly with small molecules such as arenes [7,8], dinitrogen [9–
15] and CO2 [16] has raised the potential of f-block catalysts which
can react with cheap and readily available starting materials using
distinctly different reactivity compared to the transition metals
[17].
The interaction of f-block organometallic compounds with CO
has a long history with pioneering reactivity demonstrated over
30 years ago for Cp-supported thorium dialkyls and dihydrides
[18]. The first f-block complex of CO stable above cryogenic tem-
peratures was reported in 1986 [19,20]. Exposure of green solu-
tions of [U(C5H4SiMe3)3] to CO caused a change of colour to
burgundy as one equivalent of CO binds, and IR spectroscopy
showed a mCO band at 1976 cmꢀ1. This band has lowered in fre-
quency significantly upon coordination (free CO: 2143 cmꢀ1),
which is reminiscent of transition metal carbonyl binding, and
population of the CO p⁄ orbital with electron density from uranium
set, and an even lower mCO band at 1900 cmꢀ1 was observed in
solution demonstrating a remarkable ligand effect [21,22]. Subse-
quently the crystal structure of [U(CO)(Cp⁄)3] was also reported
with m
CO at 1922 cmꢀ1 (Cp⁄ = C5Me5) [23]. A theoretical study char-
acterised the back-bonding interaction in [U(CO)(Cp0)3] (Cp0 = Cp,
C5H4SiMe3, C5Me4H, Cp⁄) as resulting from the transfer of electron
density from the Cp0 ligands to the CO ligand [24]. A bridging CO
has also been characterised in a mixed-valent UIV/III diuranium sys-
tem [25]. It is the subsequent reactivity of U carbonyls that has
been a particularly important development and one which differ-
entiates 5f reactivity with d-block carbonyl chemistry. A recent
report of the conversion of CO and H2 into a uranium-bound
methoxide is a remarkable example of small molecule activation
by an f-block complex [26], and U3+ compounds have also demon-
strated reductive coupling of two [14,27–29], three [30] and four
[31] molecules of CO.
Due to the relative scarcity of f-block CO complexes, other
ligands have also been used to probe the potential p-donating nat-
ure of uranium compounds. Dinitrogen and isocyanides (CN-R) are
isoelectronic to CO, and isocyanides are known to function as
stronger
r-donors but weaker p-acceptors compared to CO. The
uranium isocyanide complex [U(CNEt)(C5H4SiMe3)3] was originally
studied as a proxy for the CO compound which was not crys-
⇑
Corresponding author.
E-mail addresses: S.Mansell@hw.ac.uk (S.M. Mansell), Polly.Arnold@ed.ac.uk
tallised. The energy of the CN stretch,
mCN, of the complex
(P.L. Arnold).
http://dx.doi.org/10.1016/j.poly.2016.03.048
0277-5387/Ó 2016 Published by Elsevier Ltd.
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2
S.M. Mansell, P.L. Arnold / Polyhedron xxx (2016) xxx–xxx
(2160 cmꢀ1) was higher than in free CNEt (2151 cmꢀ1) [20,32],
similar to [U(CNCy)(Cp)3] (Cy = cyclohexyl) which showed mCN to
be 25 cmꢀ1 higher than in the free isocyanide [33]. Both indicated
be a better
good target for this work. We have also explored for UX3 whether
the readily reducible, - and d-acceptor cyclo-octatetraene (COT)
reactivity mimics that of 6- and 10- arenes which underwent
p
-acceptor ligand than CNtBu, and was considered a
p
a strengthening of the CN bond and no evidence of
p
back-dona-
p
tion from the U centre. In a subsequent in-depth study of many
direduction to yield inverse UIII arene complexes and UIVX4 by-
products. The reaction of COT with [U(N00)3] has briefly been
reported [42], but herein we clarify the reaction products and
report the crystal structure of [(COT)U(N00)2].
U(Cp0)3 derivatives with both alkyl and aryl isocyanides [21], the
authors found that mCN increased slightly for complexes of the alkyl
isocyanides but decreased slightly for the aryl isocyanides giving
evidence of U back donation to the isocyanide ligand, particularly
when the aryl isocyanide CNXyl (Xyl = 2,6-Me2C6H3) was used.
The difference in binding between alkyl- and aryl-isocyanides is
mirrored in their reactivity as UIII complexes of alkyl isocyanides
were often found to be unstable with respect to the formation of
UIV cyanide compounds [21]. This was also observed in the bond
cleavage reaction of CNtBu with UCp⁄3 which generated the trimeric
During this work, a by-product from the synthesis of [U(N00)3], a
common UIII starting material, was discovered which has great
potential as a starting material in its own right.
2. Results and discussion
cyanide complex [{U(Cp⁄)2(CNtBu)(
l-CN)}3] instead of simple coor-
2.1. Isocyanide coordination
dination [34]. [U(N00)3] has not been found to coordinate or react
with CNtBu [19].
The reaction between [U(ODtbp)3] and one equivalent of CNXyl
in toluene immediately produces a dark blue solution, and the iso-
cyanide adduct [U(ODtbp)3(CNXyl)] (1) was crystallised from hot
n-hexane in 54% yield (Scheme 1). 1H NMR spectroscopy reveals
sharp resonances assignable to the ODtbp and isocyanide ligands
coordinated to a paramagnetic centre. IR spectroscopy as a Nujol
mull shows the bound isocyanide mCN at 2138.8 cmꢀ1 which is
24 cmꢀ1 higher when compared to the free isocyanide
(2114.8 cmꢀ1). In previous work, the coordination of one equiva-
lent of CNtBu to [U(ODtbp)3] showed an increase in mCN of
Uranium compounds feature large ionic radii and the stability
of these compounds depends on the ligand framework. Multi-den-
tate ligands have often been utilised (such as triamidoamine [35]
and the tris-aryloxide substituted triazacyclononane [36]), but
the traditional set of non-chelating and kinetically stabilising
ligands such as N(SiMe3)2 (N00) and other bulky amides [37], alkox-
ide and aryloxide ligands [38] and Cp derivatives still predominate.
The dianion of cyclo-octatetraene, [COT]2ꢀ, is a particularly useful
dianionic ligand for the f-block ions due to their large ionic radii,
and has a long track record of use as a supporting ligand for ura-
nium [39]. [U(COT)2] was an instrumental molecule in the history
of actinide chemistry and [COT]2ꢀ is still widely used as a support-
ing ligand for small molecule activation [40]. With UIII, the ancil-
lary ligand is predominantly Cp⁄ or another derivative [41], but
with UIV the ancillary ligands are much more varied, and examples
of piano-stool complexes with halide [42,43], acetylacetonate [42],
BH4 [44], alkoxide [45] and amide [46,47] co-ligands are known.
COT has also demonstrated binding modes beyond standard half-
sandwich complex formation as seen in reactions using a 2:3 ratio
43 cmꢀ1 which is ascribed to
nating over other interactions [54]. The increase in wavenumber
r-donation of the isocyanide domi-
is less for CNXyl, possibly reflecting weaker
r-donation, but this
is distinctly different behaviour compared to [U(Cp0)3] complexes
of CNXyl. With Cp0 = C5H4Me, mCN was 2060 cmꢀ1 (a decrease of
54 cmꢀ1), and with Cp0 = C5Me4H, mCN was 2052 cmꢀ1 (a decrease
of 62 cmꢀ1). In comparison to the Ce analogue [Ce(CNXyl)
(C5H4Me)3], where
r-donation is expected to dominate, mCN was
found to be 2150 cmꢀ1, 36 cmꢀ1 higher energy compared to free
CNXyl and 90 cmꢀ1 higher than that seen for the analogous U com-
of [U(Cp⁄)3]:COT; [{(Cp⁄)(C8H8)U}2(
unusual
3-bridging binding mode [48]. The inverse sandwich-
complex [{U{NC(tBu)(Mes)}3}2(
-COT)], with stabilising ketimide
co-ligands, has been synthesised from the reaction of COT with
[U{NC(tBu)(Mes)}{NC(tBu)(Mes)( -0.5 K)}2]2( -naphthalene), or
from the reaction of [U(COT){NC(tBu)(Mes)}{NC(tBu)(Mes)(
l- :g
g3 3-C8H8)] formed with an
plex. These results suggest that [U(ODtbp)3] acts as a weaker
p-
donor than [U(Cp0)3] derivatives which does not shed any further
light on the reasons behind the reactions of [U(ODtbp)3] to reduce
N2 and CO [14]. The complex [U(ODtbp)3(CNXyl)] is thermally
stable in C6D6 solution at 80 °C overnight and showed no signs of
dissociation of the isocyanide ligand; we note that even the poor
donor thf cannot be displaced readily from [U(ODtbp)3(thf)] [53].
The crystal structure of [U(ODtbp)3(CNXyl)] (Fig. 1) showed two
molecules in the asymmetric unit with four coordinate, distorted
tetrahedral U centres. There are some minor differences in metrics
between the two molecules, particularly for the U-CNXyl unit, and
g
l
l
l
l
-
0.5 K)}2] with [U(I){NC(tBu)(Mes)}3(dme)], demonstrating d-bond-
ing in the inverse sandwich interaction [49]. Analysis of the bond-
ing situation in 5f complexes of unsaturated carbocycles continues
to be an active field of study in order to probe covalency and the
degree of different orbital contributions [50]. f-Block complexes
of benzene and COT have been found to contain significant d-inter-
actions [49–51], but unlike complexes with CO (and isoelectronic
species) that can be probed using IR and Raman spectroscopy,
direct information about the degree of metal back-bonding in
COT complexes is much harder to establish [52].
Xyl
N
C
C
N Xyl
U
The synthesis of [U(O-2,6-tBu2C6H3)3], [U(ODtbp)3], was origi-
nally reported by Sattelberger and co-workers in 1988 [53], and
we have recently studied its reactivity with small molecules,
together with the 2,4,6-tBu3C6H2 derivative, and demonstrated
reductive coupling of two molecules of CO, binding of N2, its reac-
tion with CO2 [14] and the reduction followed by subsequent bory-
lation of arenes (benzene, toluene, biphenyl and naphthalene) [7].
Here, we have sought to find the limits to the capability of simple
DtbpO
2 UX3
ODtbp
ODtbp
toluene
U
ODtbp
ODtbp
DtbpO
+ UX4
1
U
toluene
X
X
H2C SiMe2
2, X = ODtbp
3, X = N''
X = N''
-HN''
N''
N''
U
N
UX3 systems to reductively couple a p-acceptor such as CO, and to
SiMe3
reductively activate d-acceptors such as benzene. We noted previ-
ously that simple coordination of CO was not observed to UX3, but
complexes of [U(ODtbp)3] have been shown to coordinate one or
Scheme 1. Reactions of homoleptic UIII compounds with the neutral molecules,
xylyl isocyanide and cyclo-octatetraene. Xyl = 2,6-Me2C6H3, N00 = N(SiMe3)2,
ODtbp = O-2,6-tBu2C6H3.
two p
-acceptor CNtBu as ligands [54]. The molecule CNXyl should
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S.M. Mansell, P.L. Arnold / Polyhedron xxx (2016) xxx–xxx
3
by-product which provided the two electrons to direduce the arene
and maintained the +3 oxidation state of the two bound U centres
[7]. Reactions with [U(ODtbp)3] and COT were carried out in
toluene to see if an inverse COT sandwich complex could be
formed directly, Scheme 1. 1H NMR spectroscopy of the reaction
mixture shows resonances for the UIV compound [U(ODtbp)4]
together with a singlet resonance for COT bound to a paramagnetic
U centre at ꢀ35.3 ppm. Integration of the resonances associated
with the other ligands in this complex revealed a ratio of two
ODtbp ligands to one COT ligand ruling out the formation of the
inverse sandwich [{U(ODtbp)2}2(l-COT)]. Extraction of the product
into n-hexane and filtration removed the [U(ODtbp)4] by-product
and crystallisation of the filtrate yielded pure [U(COT)(ODtbp)2]
(2) as orange crystalline material. Although crystals suitable for
X-ray crystallography have not been grown, the NMR data,
together with elemental analysis data, support the formulation of
the complex as [U(COT)(ODtbp)2]. In seeking structural data for
[U(COT)X2] complexes, the analogous reaction with [U(N00)3] was
carried out to see if [U(COT)(N00)2] could be crystallised instead.
This compound is known and was synthesised from [U(COT)Cl2(-
thf)2] and NaN00 in 76% yield [46], but we were also interested in
re-visiting a report detailing the reaction of [U(N00)3] with COT
which stated that a COT-bound UIV compound was observed by
1H NMR, without any additional details [42].
Fig. 1. Thermal ellipsoid plot (50% ellipsoid probability) of one of the two molecules
of [U(CNXyl)(ODtbp)3] (1) present in the asymmetric unit. All hydrogen atoms have
been omitted for clarity. Selected bond lengths: U1-O1 2.172(6), U1-O2 2.213(6),
U1-O3 2.170(6), U1-C1 2.761(12), N1-C1 1.090(11), U2-O4 2.189(6), U2-O5 2.169
(5), U2-C5 2 2.649(10), N2-C52 1.197(12).
The reaction of [U(N00)3] with COT can be monitored by param-
agnetic 1H NMR spectroscopy which revealed resonances that
match those of [U(COT)(N00)2] together with resonances for the
UIV metallacycle [U(N00)2{j2-N(SiMe3)SiMe2CH2}] and HN00 arising
from the well-known decomposition of [U(N00)4] [7]. The reaction
therefore proceeds as for [U(ODtbp)3], i.e. generating one equiva-
lent of UIV by-product for every equivalent of [U(COT)X2] formed.
Crystals of [U(COT)(N00)2] (3) suitable for X-ray crystallography
were grown from n-hexane, and as the structure has not been
described before, it is included here, Fig. 2. The U centre is bound
symmetrically to the COT ligand with two N00 ligands making up
the rest of the U coordination sphere. The U-C distances (2.668
(3)–2.700(2) Å) are similar to previous UIV examples, such as [U
{C8H6(SiiPr3)2}(dimethylpyrazolyl)(CNMe)] [56], and the U-N dis-
tances (2.285(2) and 2.270(2) Å) are similar to those seen in [U{N
(SiHMe2)2}4] (2.280(4) and 2.281(4) Å) [60]. It is isostructural to
the Th analogue, although described in P21/a in the original paper,
with the Th-N (2.32(1) and 2.35(1) Å) and Th-C distances (2.71(2)–
2.79(2) Å) found to be slightly longer [46]. Potential agostic inter-
actions between Th and two of the carbon atoms in the amide
groups were identified in the original publication with distances
combined with some minor twinning, the slightly elongated ther-
mal ellipsoids of the C and N atoms in the isocyanide ligands and
the possibility of trace Cl, I or CN impurities co-crystallised in the
structure, discussion of the structure will be limited to listing the
parameters, and those of the other eight uranium isocyanide com-
plexes listed in the CSD, without drawing any conclusions from the
minor differences in bond lengths between U complexes (Table 1).
Interestingly, all but one contains Cp0 co-ligands, and as mentioned
above, the spectral data of these complexes points to their better
p-donor properties than for the [U(ODtbp)3].
2.2. Reactions with cyclooctatetraene (COT)
Previously, we showed how reactions between homoleptic UIII
amides and aryloxides with arenes have demonstrated the
formation of one equivalent of inverse arene sandwich complex
[(l-arene)(UX2)2] together with two equivalents of a
UIV
Table 1
Selected bond lengths (Å) and angles (°) for structurally characterised uranium isocyanide complexes.
U-CCN
C = N
U-C-N
C-N-C
Ref.
UIV
[UCp⁄2(NMe2)(CNtBu)2][BPh4]
2.60(1)
2.58(1)
2.675(3)
2.59(2)
1.14(1)
1.16(1)
1.140(3)
1.13(2)
169.7(6)
171.3(6)
170.0(2)
175(1)
179(1)
178(1)
177.3a
174a
[55]
[U{C8H6(SiiPr3)2} (dimethylpyrazolyl)(CNMe)]
[UCp3(OSO2CF3)(CNtBu)]
[56]
[57]
UIII
[U(C5H4SiMe3)3(CNEt)]b
2.57(2)
2.464(4)
2.61(5)
2.662(8)
2.697(7)
2.654(9)
2.681(9)
2.76(1)
2.65(1)
1.16a
1.166(6)
1.164(3)
1.128
1.126
1.164
1.144
1.09(1)
1.20(1)
173.6(2.0)
173.7(9)
176.5(5)
177.1
178.4
168.1
171.1
173(1)
175(1)
170.2(2.6)
173.5a
177.9a
173.1
176.1
171.1
177.5
176(1)
175(1)
[20]
[21]
[34]
[58]
[U(C5Me4H)3(CNC6H4OMe)]
[{UCp⁄2(CNtBu)( -CN)}3]c
l
[U{C5H3(SiMe3)2}2Br(CNtBu)2]a
[U{C5H3(SiMe3)2}2Cl(CN-2,6-Me2C6H3)2]a
[U(ODtbp)3(CN-2,6,-Me2C6H3)]d
[59]
This work
a
b
c
Only some E.S.D.s were available.
Four molecules in the asymmetric unit.
Av. of three CNtBu ligands.
d
Two molecules in the asymmetric unit.
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4
S.M. Mansell, P.L. Arnold / Polyhedron xxx (2016) xxx–xxx
Fig. 2. Thermal ellipsoid plot (50% ellipsoid probability) of [U(COT)(N00)2] (3); All
hydrogen atoms have been omitted for clarity.
Fig. 3. Thermal ellipsoid plot of [{U(N00)2(thf)(
l-I)}2] (4) (symmetry operator for
symmetry generated atoms: ꢀx + 1, ꢀy + 1, ꢀz + 1). All hydrogen atoms have been
omitted for clarity.
of 3.147(15) and 3.041(13) Å [46], and the structure of 3 showed
similar contacts at 3.053 and 3.141 Å. A recent experimental and
theoretical study by us showed the effects of pressure on U. . .C dis-
In the crystal structure, a molecule of [{U(N00)2(thf)(
l-I)}2]
tances to the N00 groups in [{U(N00)2}2(
l-arene)] which demon-
(Fig. 3) is situated over an inversion centre and features approxi-
mately equal U-I distances of 3.2121(3) and 3.2562(3) Å and U-N
distances of 2.319(3) and 2.309(3) Å, which are similar to those
seen in the homoleptic UIII amides [U(N00)3] (2.320(4) Å) [64] and
[U{N(SiPhMe2)2}3] (2.337(15) Å) [60]. The U. . .U separation is long
at 5.006 Å and the U-thf distance (2.542(3) Å) is very close to the
mean U-thf distance as determined by a search of the CSD
strated no agostic interaction at ambient pressure (U. . .C: 3.025
(3) and 3.022(3) Å). Evidence for an agostic interaction was found
and characterised at increased pressure (3.2 GPa, U. . .C: 3.00(5)
and 2.95(2) Å) using QTAIM (quantum theory of atoms-in-mole-
cules) computational analysis [61]. Without structural information
on both 2 and 3, we were unable to investigate the influence of
these two monodentate ligands on the U-COT bonding situation,
although it was anticipated that structural data alone was unlikely
to offer substantial evidence. Decisively, however, neither support-
ing ligand enabled the formation of a bridged species from simple
UX3 starting materials.
(2.497 Å). The related heteroleptic UIII iodide [U{
(Me)}2C}(N00)(I)] has a monomeric structure presumably due to
the extra stabilisation of the sterically bulky
2-diketiminate
j
2-{N(Dipp)C
j
ligand, as are [U(L)2I] where L are dihydrobis(pyrazolyl)borate
derivatives [65]. Related LaIII compounds are known, although
somewhat surprisingly, given that UIII and LaIII are almost the same
size, [{La(N00)2(thf)(
{N(SiMe3)(SitBuMe2)}2(
l
-I)}2] is not isostructural [66]. The related [La
2.3. Formation of [{U(N00)2(thf)(
l-I)}2] (4)
l
-I)]2 which did not feature thf coordina-
tion was only characterized by X-ray crystallography [67].
thf
N''
thf
I
thf
- 4 KI
N''
2 UI3(thf)4 + 4 KN''
U
U
3. Conclusions
N''
N''
ð1Þ
I
4
In the coordination of the isocyanide CNXyl to a homoleptic UIII
aryloxide
r-donation effects dominate over p-acceptance accord-
During a synthesis of [U(N00)3] using [UI3(thf)4] and 2.9 equiva-
lents of KN00 in an attempt to suppress all formation of the metal-
ing to FTIR spectroscopy. Reactions of both homoleptic UIII arylox-
ides and amides tested with COT result in direduction of COT to the
[COT]2ꢀ dianion and redistribution of the monoanionic ligands,
forming the half sandwich complexes [U(COT)X2] (a known mole-
cule for the amide) together with an equivalent of UIV by-product;
[U(ODtbp)4] or [U(N00)4], the latter of which is not observed directly
lacycle [U(N00)2{ 2-N(SiMe3)SiMe2CH2}], a small quantity of black,
j
block-shaped crystals were observed, in addition to the typical for-
mation of purple needles of [U(N00)3], grown from a concentrated n-
hexane solution placed at ꢀ25 °C for 16 h. X-ray crystallographic
analysis revealed the compound to be [{U(N00)2(thf)(
l-I)}2] (4)
as it decomposed to the metallacycle [U(N00)2{ 2-N(SiMe3)SiMe2-
j
which contained two UIII centres bridged by two iodine atoms
and further ligated by two N00 ligands and a molecule of thf
(Fig. 3). Rational syntheses following Eq. (1), using two equivalents
of KN00 show the reaction to be repeatable, but the desired product
is only made pure in poor isolated yields (14%) as a consequence of
the crystallisation required to purify the compound from other by-
products.
CH2}] and HN(SiMe3)2 under the reaction conditions. This reactiv-
ity contrasts that observed previously with aromatic arenes, such
as benzene and toluene, where inverse sandwich-complexes were
formed in a 4:1 U:arene stoichiometry in arene solvent. The isola-
tion of the dimeric UIII complex [{U(N00)2(thf)(
l-I)}2] in low yield
from 2:1 KN00:[UI3(thf)4] is hampered by the isolation, but could
potentially be a useful starting material in its own right.
1H NMR spectroscopy in C6D6 revealed a moderately broad sig-
nal at ꢀ8.88 ppm due to the paramagnetic nature of the UIII cen-
tres, along with two broad resonances for the thf molecules at
ꢀ3.71 and ꢀ4.25 ppm. Using the Evans NMR method, the magnetic
4. Experimental
moment was calculated to be 4.8
l
B, which makes the value of 2.4
4.1. General details
l
B per U centre approximately in the middle of values recorded for
bimetallic UIII complexes [62], and lower than the value recorded
for monomeric [U{j2-{N(Dipp)C(Me)}2C}(N00)(I)] (3.1
B) [63].
All manipulations were carried out under a dry, oxygen-free
dinitrogen atmosphere using standard Schlenk techniques or in
l
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S.M. Mansell, P.L. Arnold / Polyhedron xxx (2016) xxx–xxx
5
MBraun Unilab or Vacuum Atmospheres OMNI-lab gloveboxes
unless otherwise stated. Tetrahydrofuran (thf) and n-hexane were
degassed and purified by passage through activated alumina tow-
ers prior to use. All deuterated solvents were boiled over potas-
sium, vacuum transferred, and freeze–pump-thaw degassed three
times prior to use. The compounds KN00 (from KH and HN00 in
toluene), CN(2,6-Me2C6H3) [68], [UI3(thf)4] (from stirring [UI3]
[69] in thf), [U(N00)3] [70] and [U(ODtbp)3] [14] were made as pre-
viously 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 spec-
trometers at 298 K. Chemical shifts are reported in parts per mil-
lion, and referenced to residual proton resonances calibrated
against external TMS. Infrared spectra were recorded on Jasco
410 spectrophotometers. Solutions for UV–Vis spectrophotometry
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
4.1.2. Synthesis of [U(COT)(ODtbp)2] (2)
To a solution of [U(ODtbp)3] (335 mg, 0.392 mmol, 1.63 equiv.)
in toluene (4 cm3) was added cyclooctatetraene (25 mg,
0.24 mmol, 1 equiv.) in toluene (1 cm3) forming an orange solution.
After 1 h, this was left to settle and the orange supernatant solution
was transferred by filter cannula to a new Schlenk vessel and all
the volatiles were removed under reduced pressure. The product
was extracted into n-hexane (20 cm3) with vigorous stirring, then
reduced in volume to ca. 2 cm3 and the mixture was left to settle
as the U(ODtbp)4 by-product is not soluble under these conditions.
The orange supernatant solution was transferred by cannula filter
into a new Schlenk vessel where upon it crystallised at room tem-
perature (66 mg, 0.088 mmol, 45%).
1H NMR (400 MHz, 298 K, C6D6) d (ppm) 15.10 (d JHH = 8.3 Hz,
3
4 H, m-Dtbp-H), 11.29 (t 3JHH = 8.3 Hz, 2 H, p-Dtbp-H), ꢀ9.77 (s, 36
H, tBu), ꢀ35.33 (s, 8 H, COT). Anal. Calc. for C36H50O2U: C, 57.44; H,
6.69. Found: C, 57.47; H, 6.75%. leff (Evans’ NMR method) 2.83 lB
4.1.3. Synthesis of [U(COT)(N00)2] (3)
.
sealed by
a
tight fitting Subaseal on
a
Unicam UV1
spectrophotometer.
A solution of COT (27 mg, 0.26 mmol, 1 equiv.) in toluene
(3 cm3) was added to [U(N00)3] (304 mg, 0.42 mmol, 1.6 equiv.)
and the reaction was stirred overnight. All of the volatiles were
removed under reduced pressure and the resulting solid was trans-
ferred into a sublimation apparatus and the metallacyclic by-pro-
duct was sublimed at 100 °C, 1 ꢁ 10ꢀ6 mbar leaving [U(COT)
(N00)2] (53 mg, 0.080 mmol, 38%). The product was recrystallised
from n–hexane yielding orange crystals suitable for X-ray
diffraction.
4.1.1. Synthesis of [U(ODtbp)3(CN-2,6-Me2C6H3)] (1)
To a solution of [U(ODtbp)3] (345 mg, 0.404 mmol) in toluene
(5 cm3) was added
toluene solution of CN(2,6-Me2C6H3)
a
(53 mg, 0.404 mmol in 1 cm3) forming a dark blue solution instan-
taneously. The solution was stirred for 16 h then all the volatiles
were removed under reduced pressure and the product was dis-
solved in hot n-hexane (30 cm3) which crystallised as dark blue
crystals upon cooling to room temperature (166 mg). The super-
natant solution was placed at ꢀ30 °C to obtain a second crop of
crystals (combined yield: 216 mg, 0.219 mmol, 54%).
1H NMR data (ꢀ33.7 ppm, COT, and ꢀ11.1 ppm, N00) match liter-
ature values [46].
1H NMR (400 MHz, 298 K, C6D6) d (ppm) 15.75 (d, 2JHH = 8.0 Hz,
6 H, m-Dtbp-H), 12.77 (t, 2JHH = 8.0 Hz, 3 H, p-Dtbp-H), 7.55 (br. s, 1
H, p-Xyl-H), ꢀ1.83 (s, 2 H, m-Xyl-H), ꢀ2.45 (s, 54 H, tBu), ꢀ14.55 (s,
6 H, o-Xyl-CH3). IR (Nujol mull) cmꢀ1: 2138.8 (C@N). UV–Vis-NIR
4.1.4. Synthesis of [{U(N00)2(thf)(
l-I)}2] (4)
To a solution of [UI3(thf)4] (2.648 g, 2.919 mmol, 1 equiv.) in thf
(20 cm3) was added KN00 (1.1645 g, 5.84 mmol, 2 equiv.) in thf
(20 cm3) and the dark purple/black solution was stirred for 16 h.
The solution was left to settle then filtered and the volatiles were
removed under reduced pressure. The dark residue was extracted
into n-hexane (50 cm3) and transferred by filter cannula into a
(1.14 ꢁ 10ꢀ3 M n-hexane solution) k, nm (
e
, Mꢀ1 cmꢀ1): 655
(867), 773 (494), 1044 (353), 1197 (164), 1219 (172), 1397 (144),
1415 (138). Anal. Calc. for C51H72NO3U2: C, 62.18; H, 7.37; N,
1.42. Found: C, 61.89; H, 7.20; N, 1.38%.
Table 2
Crystal data and structure refinement details.
Identification code
1
3
4
Empirical formula
Formula weight
T (K)
C
54H79NO3U
C20H44N2Si4U
662.96
170.15
C32H88I2N4O2Si8U2
1515.64
170.15
1028.21
170.15
Crystal system
Space group
a (Å)
monoclinic
P21
11.1182(2)
monoclinic
P21/c
17.8873(2)
orthorhombic
Pbca
18.5264(3)
b (Å)
20.3955(5)
13.0193(1)
16.3899(2)
c (Å)
22.5361(5)
11.9118(1)
19.1118(3)
a
(°)
90
90
90
b (°)
90.4202(17)
90.964(1)
90
c
(°)
90
90
90
V (Å3)
5110.17(19)
2773.63(4)
5803.22(15)
Z
4
4
4
qcalc (g cmꢀ3
)
1.336
3.216
1.588
6.033
1.735
6.831
l
(mmꢀ1
)
F(000)
Crystal size (mm)
range for data collection (°)
2104.0
1304.0
2904.0
0.45 ꢁ 0.08 ꢁ 0.05
0.49 ꢁ 0.26 ꢁ 0.20
0.15 ꢁ 0.12 ꢁ 0.09
2H
5.706–54.994
6.258–54.97
5.838–54.968
Index ranges
ꢀ14 6 h 6 13, ꢀ26 6 k 6 26,
ꢀ29 6 l 6 29
ꢀ23 6 h 6 23, ꢀ16 6 k 6 16,
ꢀ15 6 l 6 15
ꢀ24 6 h 6 24, ꢀ21 6 k 6 21,
ꢀ24 6 l 6 24
Reflections collected
48945
78463
57615
Independent reflections
Data/restraints/parameters
Goodness-of-fit (GOF) on F2
22113 [Rint = 0.0600, Rsigma = 0.1017]
22113/1/1106
1.038
6347 [Rint = 0.0318, Rsigma = 0.0138]
6347/0/256
1.129
6650 [Rint = 0.0690, Rsigma = 0.0407]
6650/0/238
1.042
Final R indexes [I P 2
Largest difference in peak/hole
(e Åꢀ3
r
(I)]
R1 = 0.0524, wR2 = 0.0696
1.87/ꢀ1.41
R1 = 0.0162, wR2 = 0.0371
0.57/ꢀ0.71
R1 = 0.0317, wR2 = 0.0495
0.75/ꢀ0.52
)
Please cite this article in press as: S.M. Mansell, P.L. Arnold, Polyhedron (2016), http://dx.doi.org/10.1016/j.poly.2016.03.048

6
S.M. Mansell, P.L. Arnold / Polyhedron xxx (2016) xxx–xxx
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We thank EaStCHEM, the University of Edinburgh and the Engi-
neering and Physical Sciences Research Council EPSRC, grants EP/
H004823/1 and EP/M010554/1. PLA also thanks the Technische
Universität München – Institute for Advanced Study, funded by
the German Excellence Initiative.
Appendix A. Supplementary data
CCDC 1446017, 1432001 and 1432002 contains the supplemen-
tary crystallographic data for 1, 3 and 4. These data can be obtained
free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.
html, or from the Cambridge Crystallographic Data Centre, 12
Union Road, Cambridge CB2 1EZ, UK; fax: (+44). Supplementary
data associated with this article can be found, in the online version,
at http://dx.doi.org/10.1016/j.poly.2016.03.048.
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