Mixed sandwich imido complexes of Uranium(V) and Uranium(IV): Synthesis, structure and redox behaviour

Article abstract of DOI:10.1016/j.jorganchem.2017.08.019

The mixed sandwich U(III) complex {U[η⁸-C₈H₆(1,4-Si(ⁱPr)₃)₂](Cp*)(THF)} reacts with the organic azides RN₃ (R = SiMe₃, 1-Ad, BMes₂) to afford the corresponding, structurally characterised U(V) imido complexes {U[η⁸-C₈H₆(1,4-Si(ⁱPr)₃)₂](Cp*)(NR)}. In the case of R = SiMe₃, the reducing power of the U(III) complex leads to reductive coupling as a parallel minor reaction pathway, forming R-R and the U(IV) azide-bridged complex{[U]}₂(μ-N₃)₂, along with the expected [U] = NR complex. All three [U] = NR complexes show a quasi-reversible one electron reduction between ?1.6 and ?1.75 V, and for R = SiMe₃, chemical reduction using K/Hg affords the anionic U(IV) complex K⁺{U[η⁸-C₈H₆(1,4-Si(ⁱPr)₃)₂](Cp*) = NSiMe₃}^-. The molecular structure of the latter shows an extended structure in the solid state in which the K counter cations are successively sandwiched between the Cp* ligand of one [U] anion and the COT^tips2 ligand of the next.

Full text of DOI:10.1016/j.jorganchem.2017.08.019

Journal of Organometallic Chemistry xxx (2017) 1e9
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Mixed sandwich imido complexes of Uranium(V) and Uranium(IV):
Synthesis, structure and redox behaviour
Nikolaos Tsoureas, F. Geoffrey N. Cloke*
Department of Chemistry, School of Life Sciences, University of Sussex, Brighton BN1 9QJ, United Kingdom
a r t i c l e i n f o
a b s t r a c t
The mixed sandwich U(III) complex {U[h
8-C8H6(1,4-Si(iPr)3)2](Cp*)(THF)} reacts with the organic
Article history:
Received 29 June 2017
Received in revised form
23 August 2017
Accepted 24 August 2017
Available online xxx
azides RN3 (R ¼ SiMe3, 1-Ad, BMes2) to afford the corresponding, structurally characterised U(V) imido
complexes {U[
h
8-C8H6(1,4-Si(iPr)3)2](Cp*)(NR)}. In the case of R ¼ SiMe3, the reducing power of the
U(III) complex leads to reductive coupling as a parallel minor reaction pathway, forming R-R and the
U(IV) azide-bridged complex{[U]}2(
m
-N3)2, along with the expected [U] ¼ NR complex. All three [U] ¼ NR
complexes show a quasi-reversible one electron reduction between ꢀ1.6 and ꢀ1.75 V, and for R ¼ SiMe3,
chemical reduction using K/Hg affords the anionic U(IV) complex Kþ{U[ 8-C8H6(1,4-
h
Dedicated to Bill on the occasion of his 70th
birthday - and wishing him many more!
Si(iPr)3)2](Cp*) ¼ NSiMe3}-. The molecular structure of the latter shows an extended structure in the
solid state in which the K counter cations are successively sandwiched between the Cp* ligand of one [U]
anion and the COTtips2 ligand of the next.
Keywords:
Uranium
Mixed-sandwich
Imido
© 2017 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license
(http://creativecommons.org/licenses/by/4.0/).
Electrochemistry
1. Introduction
by the chemistry of the uranyl moiety [11]. These investigations
have led to the isolation of novel compounds with multiple U-E
Low valent actinide (essentially uranium and thorium) chem-
istry has attracted significant attention, both from a purely aca-
demic viewpoint but also in the context of activation of important
small molecules [1]. Examples of the latter include CO cyclo-
oligomerisation [2], CO2 [3] and SO2 [4] reductive coupling, NO/
CO co-disproportionation [5], CO/H2 coupling [6], N2 activation [7],
and electro-catalytic splitting of H2O promoted by sterically
encumbered U(III) complexes [8]. The common denominator
amongst all these examples is that they employ the An(III)/An(IV)
(An ¼ U, Th) redox couple to promote reactivity (hereafter the
discussion will focus on uranium). On the other hand, uranium
complexes in oxidation state (IV) exhibit reactivity attributed
mainly to the insertion of reactive molecules (CO, CO2) into U(IV)-X
(X ¼ C, H, heteroatom) [9], although there are examples of U(III)
alkyl complexes with bulky substituents that will also undergo
such reactivity rather than promote a redox event [10]. Recently,
more attention has been shifted towards higher oxidation com-
plexes (ie (V), (VI)) of uranium that historically has been dominated
bonds (E ¼ N [12], O [13], S [14], Se [14], Te [14]) that have not only
provided useful insights into the nature of the U-E bond but have
also provided novel examples of chemical reactivity [15]. In this
context, uranium imido {[U] ¼ NR} complexes have provided an
important entry for the study of these higher oxidation [16] states.
Landmark examples include: the synthesis and isolation of the U(V)
complexes UCpMe3(¼NR) (R ¼ Ph, SiMe3) [17], the U(VI) complex
UCp*2(¼NPh)2 [18], and more recently the synthesis of imido an-
alogues of the uranyl ion [19], high valent uranium imido com-
plexes supported by non-innocent ligands [20], as well as the
isolation of ‘homoleptic’ U(VI) imido complexes [21]. These well-
defined molecular examples have allowed the study of the nature
of the U¼NR bond and its relationship to the isoelectronic U¼O
bond, and have been shown to display a diverse electrochemical
behaviour. In terms of reactivity towards small molecules, it has
been shown that U(V) imido complexes can undergo a metathesis
type reaction with CO2 to release R ¼ N¼C¼O and produce stable
terminal U(V) oxo complexes [22]. We have recently described how
steric manipulation of the ligand environment around the uranium
centre in mixed sandwich complexes can facilitate the isolation of
U(V) nitride and terminal oxo complexes [23], and were thus
interested in expanding this chemistry to include the closely
* Corresponding author.
E-mail address: f.g.cloke@sussex.ac.uk (F.G.N. Cloke).
http://dx.doi.org/10.1016/j.jorganchem.2017.08.019
0022-328X/© 2017 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).
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N. Tsoureas, F.G.N. Cloke / Journal of Organometallic Chemistry xxx (2017) 1e9
related imido derivatives. Herein we report on the synthesis and
NR ligand, and therefore increasing the U-N bond distances in the
isolation of U(V) complexes of the type [U] ¼ NR (where [U] is {U
series. Furthermore, in the case of (4) elongation of the U-N bond
could also be due to p-donation to the empty p orbital located on
[
h
8-C8H6(1,4-Si(iPr)3)2](Cp*)}, R ¼ SiMe3, 1-adamantyl, B(2,4,6-
Me3-C6H2)2) and some preliminary investigations into their redox
behaviour.
the boron atom. The R-N-U angles fall well within the range of
previously reported values found in [U¼NSiMe3] and [U¼NAd]
complexes as well as [U¼NAr] (160-179ꢁ for [U¼NSiMe3], 167-175ꢁ
for [U¼NAd] and 154-180ꢁ for [U¼NAr]) [26]. Complex (3) consti-
tutes, to the best of our knowledge, the first example of the NBMes2
(Mes ¼ 2,4,6-Me3-C6H2) imido ligand complexed to an actinide
centre and only the second structurally characterised example of
this type of imido ligand [27]. The N1-B1 distance of 1.417(6) Å is
2. Results and discussion
2.1. Synthesis of [U] ¼ NR complexes
The synthesis of the new [U] ¼ NR complexes ([U] ¼ {U[h8
-
similar within esds to that of 1.404(9) Å in {[(NPN)Ta](m- :h
h1 1-N)
C8H6(1,4-Si(iPr)3)2](Cp*)}; R ¼ SiMe3 (2), Ad (3), BMes2 (4)) can be
[Ta(¼N-BC8H12)(NPN*)]} [28]. The Ct(COT)-U-Ct(Cp*) angle in the
case of (4) is more obtuse compared to the ones in (2) and (3)
(which are the same within esds), probably due to the flexibility of
the mesityl substituents to orient themselves in such a way as to
minimise steric repulsions.
achieved in a straightforward manner by reaction of the U(III)
complex {U[h
8-C8H6(1,4-Si(iPr)3)2](Cp*)(THF)} (1) with the corre-
sponding azide in hydrocarbon solvents at room temperature
(Scheme 1).
The new complexes were fully characterised by spectroscopic
and analytical methods; their 1H NMR spectra feature para-
magnetically shifted broad peaks, in agreement with the proposed
structures, while in all cases a single broad peak in the region
between ꢀ60 and ꢀ78 ppm was observed in their 29Si{1H}-NMR
spectra. This is shifted downfield in comparison to the U(III)
starting material (ꢀ133), [2a] and is in accordance with the general
trend observed upon oxidation of the U(III) metal centre to U(V) but
out of the range of þ160 to ꢀ90 ppm quoted for the seven examples
of U(V) complexes compared in this study [24]. In the case of (4),
the 11B{1H}-NMR spectrum displayed a very broad signal (Dn1/
It is worth noting that the synthesis of both (3) and (4), proceeds
with the title compounds being the only observable uranium
containing species in solution as evidenced by NMR scale reactions.
On the other hand, when a solution of SiMe3N3 in C7D8 was vacuum
transferred to a solution of (1) in the same solvent at ꢀ78 ꢁC fol-
lowed by slow warming to RT, 1H and 29Si{1H}-NMR spectroscopy
showed the existence of two species in solution in an approximate
ratio of 4:1. The major constituent was indeed complex (2) (as
evidenced by comparison with an authentic sample) while the
minor constituent displayed paramagnetically shifted sharp peaks
consistent with a symmetric U(IV) complex. Furthermore, a singlet
located at 0.07 ppm was also observed and was attributed to
Si2Me6. Based on these observations, and literature precedents
[26a,g,m] documenting the reductive coupling of the SiMe3 group
upon reaction of [U(III)] complexes with SiMe3N3 and concurrent
formation of [U(IV)-N3] complexes, we tentatively assigned the
afore-mentioned minor species to a U(IV) azide complex (Scheme
2). X-ray diffraction studies confirmed this to be the case and the
¼ 1688 Hz) centred at 135.86 ppm. Mass spectrometry (EI)
2
showed the expected molecular ions with the correct isotopic
distributions and microanalysis was consistent with the molecular
formulations, with the exception of (4), where it has been docu-
mented that higher deviations from theoretical values are common
for boron azides [25]. The structures of all three complexes have
been confirmed by single crystal X-ray diffraction studies. They are
extremely soluble in common hydrocarbons and crystalline solids
could only be obtained from SiMe4 or mixtures of SiMe4 and n-
pentane (see experimental section). Figure 1 shows the ORTEP di-
agrams of (2), (3) and (4), while a comparison of selected bond
lengths and angles is given in Table 1.
molecular structure of the bridging azide complex {U[
h
8-C8H6(1,4-
Si(iPr)3)2](Cp*)( h1 1-N3)}2 (5) is shown in Fig. 2, together with
m
- :h
selected bond lengths and angles. The latter are unremarkable,
falling well within the range of the corresponding values of pre-
viously reported [U]-N3 complexes [29], and therefore do not
warrant further discussion.
The U-N bond distances are characteristic of a U-N multiple
bond and are all in the range of 1.90e2.12 Å, as previously observed
for [U(n)-N(imido)] (n ¼ IV-VI) complexes [20e22,26]. In the case
of (2), the U-N bond length is at the shorter end of this range, and
also slightly shorter than those found in both (3) and (4). This trend
correlates well with the decreasing linearity of the R-N-U moiety in
the series ((2) > (3) > (4)) that probably accounts for decreased
Complex (5) can be independently synthesised by reaction of {U
[h
8-C8H6(1,4-Si(iPr)3)2](Cp*)Cl} (6) [9j] with an excess of NaN3 in
THF (Scheme 2). In this case, (5) is easily isolated as a dark brown
crystalline solid after extraction of the reaction mixture in hot
toluene and displays identical spectroscopic (1H and 29Si{1H}-NMR)
features to that obtained from the reaction of (1) with SiMe3N3.
effective overlap between
p orbitals of the uranium centre and the
2.2. Electrochemical studies
All three imido complexes display a quasi-reversible (over a
range of potential scan rates) reduction process at ꢀ1.76 V for
(2), ꢀ1.74 V for (3) and ꢀ1.62 V for (4) vs Fc0/þ (Fig. 3), which is
attributed to the U(V)/U(IV) redox couple in each case.
The observed E1/2 values for (2) and (3) are almost identical with
that previously reported for the terminal oxo complex {U[h8
-
C8H6(1,4-Si(iPr)3)2](Cp*) ¼ O} (7) (ꢀ1.78 V vs Fc0/þ) [23], consistent
with the isoelectronic relationship between the imido and oxo li-
gands. These values are more cathodically shifted compared to the
corresponding U(V)/U(IV) redox potentials for the U(V) imido
complexes of the formula [UN00 ¼ NR] (N00 ¼ N(SiMe3)2, R ¼ CPh3,
3
C(2-napth)Ph2, C(2-napth)3, 2-napth) (ꢀ1.37 to ꢀ1.25 V) [20c], and
rather more cathodically shifted with respect to the observed
reduction potentials (ꢀ0.34 Ve0.36 V) for the series UCp*2(¼N-
Dipp)X (Dipp ¼ 2,6-iPr2-C6H3; X ¼ halide, OTf, SPh, CCPh, NPh2,
Scheme 1. General synthetic route to [U] ¼ NR ([U] ¼ {U[
h
8-C8H6(1,4-Si(iPr)3)2](Cp*)})
complexes.
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N. Tsoureas, F.G.N. Cloke / Journal of Organometallic Chemistry xxx (2017) 1e9
3
Fig. 1. ORTEP diagrams of the molecular structures, from left to right, of (2), (3) and (4) respectively displaying 50% thermal ellipsoids (hydrogen atoms and iPr groups as well as a
molecule of SiMe4 in the asymmetric unit of (4), have been removed for clarity).
Table 1
reduction proceeds very cleanly in the presence of a small excess of
K/Hg (0.4e0.5% w/w) within a few minutes in THF as solvent, as
Comparison of key metric parameters of complexes (2), (3) and (4).
evidenced by the colour change from brown to a deep wine-red
(see Scheme 3). The 1H NMR of the reaction mixture (in C4D8O)
shows the clean conversion to a new species (9) with much sharper
resonances. Again, an important diagnostic value was the 29Si{1H}
NMR which displayed a signal centred at ꢀ159.6 ppm attributed to
the SiiPr3 groups (the NSiMe3 signal could not be located). The 29Si
{1H}-NMR shift follows the trend (ie shifted upfield compared to (2)
(2) [U] ¼ NSiMe3 (3) [U] ¼ NAd (4) [U] ¼ NBMes2
U1-N1 (Å)
1.935(7)
172.6(4)
1.979(7)
2.500(1)
1.964(5)
171.7(4)
1.999(1)
2.515(3)
133.76(9)
1.980(4)
168.4(4)
1.983(6)
2.474(9)
136.16(3)
R-N1-U1 (ꢁ)
Ct(COT)-U (Å)
Ct(Cp*)-U (Å)
Ct(COT)-U-Ct(Cp*) (ꢁ) 133.71(3)
(ꢀ70.6 ppm)) observed for the reduction of {U[
h
h
8-C8H6(1,4-
8-C8H6(1,4-
Si(iPr)3)2](Cp*)O} (7) with K/Hg to yield {U[
OPh, Me, N¼CPh2) [30]. Unlike the afore-mentioned compounds, in
our case no anodic waves corresponding to an oxidation process
were observed over the scanned solvent window.
In the case of (4) the measured E1/2 value is approximately 0.16 V
higher compared to (2), (3) and (7) meaning that (4) is reduced
more easily. This is most likely due to the Lewis acidic boron atom
accepting electron density from the nitrogen, thus making the
metal centre less electron rich. This observation is also consistent
with the explanation for the elongated U-N bond length in the
molecular structure of (4).
Si(iPr)3)2](Cp*)(
m
-O)K(18-c-6)} (18-c-6 ¼ 18-crown-6) (8), and is
in fair agreement with the corresponding 29Si{1H}-NMR spectrum
of the latter (ꢀ172.2 ppm) [23].
Red crystals of (9) suitable for a single crystal X-ray diffraction
study were obtained from slow evaporation of a 2:1 C6H6/Et2O
solution at room temperature, and the molecular structure is
shown in Fig. 4 (left), together with selected bond lengths and
angles.
The U-N bond (1.972(5) Å) in the anionic complex (9) is slightly
elongated compared to that in the parent complex (2) (1.935(7) Å);
a similar phenomenon has been observed with the lengthening of
the U-O bond upon the reduction of (7) (see above). Nevertheless,
this bond is still in the range of U-N multiple bonds observed for
[U(n) ¼ N(imido)] (n ¼ IV-VI) complexes. Similarly the U-Ct(Cp*)
and U-Ct(COT) in (9) are elongated compared to (2), while the
2.3. Chemical reduction of (2)
Based on the voltammograms in Fig. 3 (and the precedence of
the reduction of (7) [23]), the anionic [U(IV) ¼ NR]- complexes
should be chemically accessible. Indeed, in the case of (2) this
Scheme 2. Formation of (5) as the minor by-product of the synthesis of (2) and its independent synthesis from (6) and NaN3.
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N. Tsoureas, F.G.N. Cloke / Journal of Organometallic Chemistry xxx (2017) 1e9
Fig. 2. ORTEP diagram of the molecular structure of (5) displaying 50% probability ellipsoids (Hydrogen atoms and iPr groups have been omitted for clarity). Selected bond lengths
(Å) and angles (ꢁ) (the dimer is generated via inversion over a centre of symmetry and therefore distances and angles of symmetry equivalent atoms are equal and so omitted): U1-
N1 2.490(3), N1-N2 1.165(4), N2-N3 1.176(4), Ct(COT)-U 1.968(8), Ct(Cp*)-U 2.495(1); N2-N1-U1 142.9(3), N1-N2-N3 177.7(4) Ct(COT)-U-Ct(Cp*) 137.36(3).
Fig. 3. From left to right, overlaid CV scans (3 cycles) in the cathodic direction of (2), (3) and (4) in 0.05 M [N(n-Bu)4][B(C6F5)]4 in THF (1 mL) (150 mV sꢀ1 scan rate in all three cases).
For (2) ipa/ipc 0.85, j
DEppj ¼ 256 mV; for (3) ipa/ipc 0.94, j
D
Eppj ¼ 425 mV and for (4) ipa/ipc 0.96, j
D
Eppj ¼ 293 mV ( Epp ¼ Epc-Epa).
D
sphere of the anion, in the absence of a coordinating terminal
heteroatom as is the case for (8) [23]. Indeed, this results in the
formation of an extended polymeric structure in the solid state
(Fig. 4, right), in which the Kþ cations are bridging the mixed ura-
nium sandwich units through the aromatic ring systems. The U-
Ct(Cp*/COT)-K angles are almost linear while the Ct(COT)-K-Ct(Cp*)
angle is 156.5(1)ꢁ. Remarkably, when the reaction is repeated in the
presence of 18-crown-6 in THF, (9) is still the only compound iso-
lated after work-up. Furthermore, the 1H NMR signals attributed to
(9) generated in-situ remain unchanged both in the presence or the
absence of 18-crown-6 in C4D8O, further confirming that the
crown-ether is not able to disrupt the Kþ cation environment in
solution. Nevertheless, since (9) is only soluble in (the presence of)
THF, the formation of species where the intercalation of the po-
tassium ion between the two carbocycles is disrupted by coordi-
nating THF, cannot be excluded; nevertheless, the isolation of
crystals of (9) in the presence of Et2O suggests that if such an
interaction exists in solution then it is very weak. Unfortunately (9)
Scheme 3. Synthesis of (9).
Ct(Cp*)-U-Ct(COT) angle in (9) (172.1(5)ꢁ) is significantly more
obtuse compared to that in (2) (133.71(3)ꢁ). On the other hand, the
U-N-Si angle remains unchanged, signifying the role of the imido
moiety as a spectator ligand. These pronounced differences in the
metric parameters of the mixed sandwich unit could be due to the
need to accommodate the Kþ counter-ion in the coordination
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N. Tsoureas, F.G.N. Cloke / Journal of Organometallic Chemistry xxx (2017) 1e9
5
Fig. 4. ORTEP diagram of the molecular structure of (9) (left) and its extended structure in the solid state (right) (50% probability ellipsoids). Hydrogen atoms, half a molecule of
C6H6 and iPr groups have been removed for clarity. Selected bond lengths (Å) and angles (ꢁ): U1-N1 1.972(5), N1-Si3 1.712(5), U1-Ct(Cp*) 2.599(4), U1- Ct(COT) 2.066(4), K1-Ct(Cp*)
2.788(4), K1-Ct(COT) 2.466(5); Ct(COT)-U1-Ct(Cp*) 172.1(5), U1-N1-Si3 172.7(4), Ct(Cp*)-K1-Ct(COT) 156.5(1), U2-Ct(Cp*)-K1 176.5(8), K1-Ct(COT)-U1 176.4(7).
could not be isolated as a microanalytically pure compound as it is
thermally unstable; pink-red crystalline material has the tendency
to convert into an as yet unidentifiable brown non-crystalline solid,
with concomitant ligand decomposition.
When (3) was reduced in the same manner (2), a colour
change to brown red was observed; in the case of (4) when the
reduction was carried out in a non-coordinating solvent (e.g.
toluene) the almost instant formation of a red precipitate was
observed. Unfortunately, in both cases we have not managed to
isolate well-defined complexes but initial spectroscopic evidence
(sharper signals in the 1H NMR spectra consistent with
reduction to U(IV)) suggests that similar reactivity to that of (2) is
occurring.
Acknowledgements
We thank EPSRC (Grant Number EP/M023885/1) for financial
support, and Mr. Christopher Inman for assistance with the elec-
trochemical studies.
Experimental
General procedures
All manipulations were carried out in a MBraun glovebox under
N2 or Ar (O2 and H2O < 1 ppm) or by using standard Schlenk
techniques under Ar (BOC pureshield) passed through a column
containing BASF R3-11(G) catalyst and activated molecular sieves
(4 Å). All glassware was dried at 160 ꢁC overnight prior to use. Filter
cannulas were prepared using Whatman 25 mm glass microfiber
filters and were predried at 160 ꢁC overnight. THF and toluene were
dried over molten K and distilled under a N2 atmosphere and were
kept in Young ampules over activated molecular sieves (4 Å) or a
potassium mirror, respectively, under Ar. Hydrocarbons were dried
over NaK, distilled under a N2 atmosphere, and kept in Young
ampules over a potassium mirror under Ar. SiMe4 (ꢂ99%) was
purchased from Aldrich, degassed by three freezeꢀthaw cycles and
dried by stirring over NaK for three days before being vacuum
distilled and kept over molecular sieves (4 Å) in an Ar glovebox
at ꢀ35 ꢁC. Deuterated toluene, benzene, and THF were degassed by
three freezeꢀthaw cycles, dried by refluxing over K for 3 days,
vacuum distilled, and kept in Young ampoules in the glovebox
3. Conclusions
In summary, we have reported the synthesis of three new U(V)
mixed sandwich imido complexes via the straightforward reaction
of the [U(III)] precursor {U[h
8-C8H6(1,4-Si(iPr)3)2](Cp*)(THF)} (1)
with organic azides. In one case (SiMe3N3), reductive coupling of
the azide occurs as a parallel minor reaction pathway to form
Si2Me6 and the U(IV) bridging azide complex {[U](
with the expected U(V) [U] ¼ NSiMe3 complex. All three imido
complexes show
quasi-reversible reduction between ꢀ1.6
and ꢀ1.75 V, and in one case clean chemical reduction has been
achieved using K/Hg to readily afford Kþ{U[ 8-C8H6(1,4-
m-N3)}2 along
a
h
Si(iPr)3)2](Cp*) ¼ NSiMe3}-. The latter displays a novel, extended
structure in the solid state in which Kþ counter cations are sand-
wiched between between the Cp* ligand and the COTtips2 ligand of
neighbouring anionic mixed sandwich uranium units, a structural
feature that persists even when the synthesis is carried out in the
presence of 18-crown-6.
under N2. {U[h -
8-C8H6(1,4-Si(iPr)3)2](Cp*) (THF)} [2a] and {U[h8
C8H6(1,4-Si(iPr)3)2](Cp*)Cl} [9] were prepared according to litera-
ture. SiMe3N3 was purchased from Aldrich, was degassed by three
freezeꢀthaw cycles and kept in a Young's ampule at 5 ꢁC under Ar.
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N. Tsoureas, F.G.N. Cloke / Journal of Organometallic Chemistry xxx (2017) 1e9
AdN3 was purchased from Aldrich and was kept in an Ar glovebox
in a ꢀ35 ꢁC freezer and used as received. (2,4,6-Me3-C6H2)2BN3 was
prepared by modification of a published literature procedure (see
below). 1H, 13C{1H} and 29Si{1H} NMR data were recorded on a
Varian VNMR S400 spectrometer operating at 400 MHz (1H). The
spectra were referenced internally to the residual protic solvent
(1H) or the signals of the solvent (13C). 29Si{1H} NMR spectra were
referenced externally relative to SiMe4. All spectra were recorded at
30 ꢁC unless otherwise stated. EI-MS mass spectra were recorded
on a VG-Autospec Fisons instrument at the University of Sussex
unless otherwise stated. Elemental analyses were performed at the
Microanalysis Service of the School of Chemistry at University of
Bristol. Cyclic voltammetry studies were performed in an Ar filled
EI-MS: 877 (M), 741 (M-Cp*), 460 (M-COTtips2), 444 (M-COTtips2
-
Me), 115 (H(SiiPr3)2); Elem. Anal.: Calcd for C39H72NSi3U: C 53.39; H
8.27; N 1.60. Found: C 53.67; H 8.32; N 1.78.
In situ reaction of {U[h
8-C8H6(1,4-Si(iPr)3)2](Cp*)(THF)} with
SiMe3N3: A solution of 25 mg (0.029 mmol) {U[h
8-C8H6(1,4-
Si(iPr)3)2](Cp*)(THF)} in C7D8 (0.4 mL) in a Youngs NMR tube was
degassed and cooled at ꢀ78 ꢁC and to that was vac-transferred a
degassed solution of 4 mL (1.04 mol eq) TMSN3 in ca 0.2 mL C7D8.
Upon warming to RT effervescence was observed and the 1H NMR
spectra were recorded to show formation of (2) and (5) in
approximately 80:20 ratio respectively.
{U[
h
8-C8H6(1,4-Si(iPr)3)2](Cp*){ ¼ NAd)} (3): This was done as for
(2) starting from 550 mg (0.638 mmol) of {U[h
8-C8H6(1,4-
glovebox using
a
BaSi-Epsilon potentiostat under computer
Si(iPr)3)2](Cp*)(THF)} and 113 mg (1 mol eq) AdN3 added in small
portions over ca 5 min. After effervescence had ceased, the dark
brown solution was stripped of volatiles and the residue was dis-
solved in SiMe4 (ca 5 mL) and filtered through a glass-microfibre
filter pipette. Insoluble in SiMe4 solids were taken in a ca 1:1
mixture of nC5/SiMe4 and filtered as above (total volume 5 mL).
Both solutions upon slow evaporation started depositing crystals
after ca 20 min and the crystallisation was completed by overnight
control. CV experiments were performed using the three electrode
method with glassy carbon (7 mm2) as the working electrode, Pt
wire as the counter electrode and Ag wire as the pseudo-reference
one. Sample solutions were prepared by dissolving the electrolyte
([N(n-Bu)4][B(C6F5)4]) in 1 mL of THF (0.05 M) followed by the
analyte to give a concentration of the latter of ca 5 mM. The re-
ported half potentials are referenced to the Fc0/þ redox couple,
which was measured by adding ferrocene (ca 1 mg) to the sample
solution.
refrigeration at ꢀ35 ꢁC. Yield: 390 mg (65%).1H NMR (
C7D8): ꢀ5.91
d
and ꢀ5.44 (broad d, 42H, CH(CH3)2), 1.24 (broad s, 15H, C5(CH3)5),
10.91 (s, 3H, adamantyl), 13.67 (s, 3H, adamantyl), 17.74 (s, 3H,
adamantyl), 29.16 (broad s, 6H, adamantyl) the COT CH protons
(2,4,6-Me3-C6H2)2BN3:
300
mg
(2,4,6-Me3-C6H2)2BF
(1.12 mmol) (purchased from TCI and stored under Ar in a ꢀ35 ꢁ
C
freezer) were placed in a Young's ampoule containing a glass-
coated magnetic follower in an Ar glovebox. To this, 2 mL of
could not be observed; 29Si{1H}-NMR (
d
C7D8): ꢀ76.08 (s, SiiPr3);
EI-MS: 938 (M), 803 (M-Cp*), 522 (M-COTtips2), 388 (UNAd), 157
SiMe3N3 (d ¼ 0.868 mg/mL; 15.06 mmol) were added to produce
((SiiPr3)3), 135 (Cp*), 115 (H(SiiPr3)2); Elem. Anal.: Calcd for
a heterogeneous reaction mixture that after overnight stirring
became homogeneous. The reaction was followed by 1H NMR by
dissolving small aliquots in C6D6 (the 11B{1H}-NMR signals of the
product and the starting material are broad and are both centred
at ca 54 ppm). Full conversion to the title compound was
observed after 7 days of stirring at RT, upon which time volatiles
were removed in vacuum to give (2,4,6-Me3-C6H2)2BN3 as a
white flocculent crystalline solid which was kept in a glovebox
C
46H78NSi2U: C 58.82; H 8.37; N 1.49. Found: C 58.99; H 8.44; N
1.87.
{U[
ampoule was charged with 100 mg (0.116 mmol) of {U[h8
h
8-C8H6(1,4-Si(iPr)3)2](Cp*){
¼
NBMes2)} (4): A Young's
-
C8H6(1,4-Si(iPr)3)2](Cp*)(THF)} and 33.7 mg (1 mol eq) of
Mes2BN3 in an Ar glovebox and C6H6 (ca 5 mL) was added to
produce a dark brown red solution. After effervescence had
ceased, volatiles were removed in vacuum and the residue
extracted in SiMe4 (ca 4 mL) and let to slowly evaporate to
approximately half before being refrigerated at ꢀ35 ꢁC over-
night to yield the title compound the SiMe4 solvate. Yield:
under N2. Yield: 300 mg (92%). 1H NMR (
d C6D6, 400 MHz): 2.09
(s, 6H, p-CH3), 2.27 (s, 12H, o-CH3), 6.71 (s, 2H, aromatic). EI-MS:
291 [M], 263 [M-N2], 248 [M-N3], 172 [M-C9H11], 144 [MesBN],
119 [C9H11], 116 [BMes-Me].
50 mg (ca 38%). 1H NMR
(d
C7D8): ꢀ4.18 (s, 18H,
NOTE: 1H NMR for Mes2BF in C6D6 is as follows: 1H NMR (
d
C6D6,
CH(CH3)2), ꢀ2.44 (broad s, 24H, CH(CH3)2), 2.78 (broad s,
15H, C5(CH3)5), 4.50 (s, 6H, Mes-CH3), 8.56 (broad s, 6H, Mes-
CH3), 11.20 (s, 6H, Mes-CH3) the COT CH proton could not be
400 MHz): 2.10 (s, 6H, p-CH3), 2.28 (s, 6H, o-CH3), 2.29 (s, 6H, o-
CH3), 6.70 (s, 2H, aromatic).
observed; 29Si{1H}-NMR
(d
C7D8): ꢀ63.05; 11B{1H}-NMR
(d
C7D8): 135.86 (broad s, Dn1/2 ¼ 1688 Hz); EI-MS: 1053 (M), 1011
(M-iPr), 637 (M-COTtips2), 517 ((M-COTtips2-Mes), 416 (COTtips2),
373 (UCp*), 265 (UNBMes2), 157 ((SiiPr3)3), 115 (H(SiiPr3)2); Elem.
Anal.: Calcd for C54H85BNSi2U.SiMe4: C 61.03; H 8.57; N 1.23.
Found: C 58.87; H 8.38; N 2.14 repeated attempts from re-
crystallised batches of different syntheses gave irreproducible
nitrogen analyses.
Scaling up this preparation was not undertaken as boron azides are
potentially highly energetic materials
{U[
(0.58 mmol) of {U[
solved in ca 10 mL of n-pentane in an Ar glovebox with vigorous
stirring and were treated with 80 L (1.04 mol eq) of TMSN3 in 2
h
8-C8H6(1,4-Si(iPr)3)2](Cp*){
8-C8H6(1,4-Si(iPr)3)2](Cp*)(THF)} were dis-
¼
NSiMe3)} (2): 500 mg
h
m
NOTE: The low yield is probably due to the high solubility of the
product in apolar solvents. When the reaction was repeated on a
NMR scale the product was found to be present in solution in >85%
yield.
portions at RT. During addition effervescence was observed and a
colour change from black brown to dark brown-orange. After
10 min of stirring volatiles were removed and the residue was
extracted in SiMe4 (ca 10 mL) and filtered through a glass-
microfibre filter pipette in an Ar glovebox. The SiMe4 extract was
let to evaporate slowly at RT to ca 2 mL before being placed
at ꢀ35 ꢁC overnight to produce the title compound as a dark brown
Independent synthesis of {U[
(5): A young's ampule was charged with 100 mg (0.121 mmol) of {U
8-C8H6(1,4-Si(iPr)3)2](Cp*)Cl} (6) and 15 mg (1.9 mol eq) NaN3
h m-N3)}2
8-C8H6(1,4-Si(iPr)3)2](Cp*)(
[h
crystalline solid in two crops. Yield: 360 mg (ca 71%). 1H NMR (
d
and THF (ca 10 mL) was added to it to give a dark red solution. The
reaction mixture was stirred overnight at RT to adopt a dark brown
colouration. Volatiles were removed in vacuum and the residue was
extracted with hot toluene (3 ꢃ 5 mL) and filtered while warm via a
filter cannula. Upon cooling brown crystals started forming which
C7D8): ꢀ5.97 (s, 18H, CH(CH3)2), ꢀ4.84 (s, 6H, CH(CH3)2), ꢀ4.30 (s,
18H, CH(CH3)2), ꢀ1.80 (s, 15H, C5(CH3)5), 20.36 (s, 9H, Si(CH3)3), the
COT CH protons could not be observed; 29Si{1H}-NMR
(d
C7D8): ꢀ70.61 (s, broad) the NSiMe3 signal could not be observed;
Please cite this article in press as: N. Tsoureas, F.G.N. Cloke, Journal of Organometallic Chemistry (2017), http://dx.doi.org/10.1016/
j.jorganchem.2017.08.019

N. Tsoureas, F.G.N. Cloke / Journal of Organometallic Chemistry xxx (2017) 1e9
7
were isolated by filtration, washed with nC5 (5 mL) and dried in
vacuum. Reducing the supernatant and washing and cooling
at ꢀ80 ꢁC produced a second crop. Yield: 75 mg (74.5%). 1H NMR (
d
C7D8): ꢀ82.43 (s, 4H, CH), ꢀ76.69 (s, 4H, CH), ꢀ6.09 (s, 36H,
CH(CH3)2), ꢀ3.79 (s, 12H, CH(CH3)2), ꢀ0.95 (s, 36H, CH(CH3)2), 8.10
(s, 30H, C5(CH3)5), 60.78 (s, 4H, CH); 29Si{1H}-NMR (
d
C7D8): ꢀ74.57
(s, SiiPr3). Elem. Anal.: Calcd for C72H126N6Si4U2: C 51.96; H 7.63 N
5.05. Found: C 52.12; H 7.84; N 5.14.
Kþ{U[
h
8-C8H6(1,4-Si(iPr)3)2][Cp*](
¼
NSiMe3)}- (9): 100 mg
(0.114 mmol) of (2) were dissolved in 0.5e0.7 mL of C4D8O in an Ar
glovebox and with stirring K/Hg amalgam (0.4% w/w) (2e3 drops
ca 5 fold excess) was added. After a few minutes the brown solution
adopted a deep red colouration and the solution was carefully
decanted from the amalgam using an elongated Pasteur pipette.
After NMR were collected, the solvent was quickly removed in
vacuum and the residue taken in a 2:1 mixture of C6H6/Et2O (ca
3 mL) in an Ar glovebox, and the solvent removed in vacuum slowly
until red crystals suitable for single crystal XRD started forming. 1H
NMR
(
d
-C4D8O): ꢀ19.28 (s, 6H, CH(CH3)2), ꢀ16.78 (s, 18H,
CH(CH3)2), ꢀ11.73 (s, 18H, CH(CH3)2), ꢀ9.89 (s, 15H, C(CH3)5), 41.78
(br s, 9H, Si(CH3)3), the COT CH protons could not be observed; 29Si
{1H}-NMR (
d
-C4D8O): ꢀ159.6 (s, Si(iPr)3) the NSiMe3 signal could
not be observed; No molecular ion could be observed in ESIꢀ mode.
No elemental analyses could be obtained from the sample as the
red crystalline material readily decomposes to a yet unidentified
brown compound.
X-ray crystallography
The dataset for (5) was collected on a Bruker-Nonius KappaCCD
area detector diffractometer with a sealed-tube source (Mo K
an Oxford Cryosystems low-temperature device (173 K), operating
in scanning mode with and scans to fill the Ewald sphere. The
a) and
u
j
u
programs used for control and integration were Collect [31], Sca-
lepack, and Denzo [32]. Absorption corrections were based on
equivalent reflections using SADABS [33]. In the case of (4) data
were collected using an Agilent Gemini Ultra diffractometer with
an Enhance source (Mo Ka) equipped with an Eos CCD area detector
and the same temperature device as above. The strategy used for
data collection is the same as previously. Control, integration and
absorption correction were handled by the CrysAlis Pro software. In
the case of (2) and (3), data were collected using the same Agilent
single crystal diffractometer as above with an Enhance Ultra source
(Cu K
handled by the CrysAlis Pro software. In the case of (9) data were
collected on a Rigaku MicroMax-007 HF rotating anode (Cu K
a). Control, integration and absorption correction were
a)
equipped with VariMax HF confocal mirrors at 100 K. Control of the
instrument was handled by the d-Trek software and the data set
was processed after its collection using the CrysAlis Pro software
suite. In the case of (3) a numerical absorption correction based on
gaussian integration over a multifaceted crystal model was applied.
The crystals were mounted on a glass fiber with silicon grease or
MiTiGen loops, from dried vacuum oil kept over 4 Å in a MBraun
glovebox under Ar. All solutions and refinements were performed
using the WinGX [34] package and all software packages within. All
non-hydrogen atoms were refined using anisotropic thermal pa-
rameters, and hydrogens were added using a riding model. CCDC
deposition numbers 1570061e1570065 for (2)e(9), respectively.
Crystal structure and refinement details are given in the following
Table 2:
Please cite this article in press as: N. Tsoureas, F.G.N. Cloke, Journal of Organometallic Chemistry (2017), http://dx.doi.org/10.1016/
j.jorganchem.2017.08.019

8
N. Tsoureas, F.G.N. Cloke / Journal of Organometallic Chemistry xxx (2017) 1e9
[13] (a) S. Fortier, J.L. Brown, N. Kaltsoyannis, G. Wu, T.W. Hayton, Inorg. Chem. 51
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