Terminal Uranium(V/VI) Nitride Activation of Carbon Dioxide and Carbon Disulfide: Factors Governing Diverse and Well-Defined Cleavage and Redox Reactions

Article abstract of DOI:10.1002/chem.201605620

The reactivity of terminal uranium(V/VI) nitrides with CE₂(E=O, S) is presented. Well-defined C=E cleavage followed by zero-, one-, and two-electron redox events is observed. The uranium(V) nitride [U(Tren^TIPS)(N)][K(B15C5)₂] (1, Tren^TIPS=N(CH₂CH₂NSiiPr₃)₃; B15C5=benzo-15-crown-5) reacts with CO₂to give [U(Tren^TIPS)(O)(NCO)][K(B15C5)₂] (3), whereas the uranium(VI) nitride [U(Tren^TIPS)(N)] (2) reacts with CO₂to give isolable [U(Tren^TIPS)(O)(NCO)] (4); complex 4 rapidly decomposes to known [U(Tren^TIPS)(O)] (5) with concomitant formation of N₂and CO proposed, with the latter trapped as a vanadocene adduct. In contrast, 1 reacts with CS₂to give [U(Tren^TIPS)(κ²-CS₃)][K(B15C5)₂] (6), 2, and [K(B15C5)₂][NCS] (7), whereas 2 reacts with CS₂to give [U(Tren^TIPS)(NCS)] (8) and “S”, with the latter trapped as Ph₃PS. Calculated reaction profiles reveal outer-sphere reactivity for uranium(V) but inner-sphere mechanisms for uranium(VI); despite the wide divergence of products the initial activation of CE₂follows mechanistically related pathways, providing insight into the factors of uranium oxidation state, chalcogen, and NCE groups that govern the subsequent divergent redox reactions that include common one-electron reactions and a less-common two-electron redox event. Caution, we suggest, is warranted when utilising CS₂as a reactivity surrogate for CO₂.

Full text of DOI:10.1002/chem.201605620

A Journal of
Accepted Article
Title: Terminal Uranium(V/VI)-Nitride Activation of Carbon Dioxide and
Disulfide: Factors Governing Diverse and Well-Defined Cleavage
and Redox Reactions
Authors: Stephen Liddle, Peter Cleaves, Christos Kefalidis, Benedict
Gardner, Floriana Tuna, Eric McInnes, William Lewis, and
Laurent Maron
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
published online in Early View as soon as possible and may be different
to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: Chem. Eur. J. 10.1002/chem.201605620
Link to VoR: http://dx.doi.org/10.1002/chem.201605620
Supported by

10.1002/chem.201605620
Chemistry - A European Journal
Terminal Uranium(V/VI)-Nitride Activation of Carbon Dioxide and Disulfide: Factors
Governing Diverse and Well-Defined Cleavage and Redox Reactions
Peter A. Cleaves,[a] Christos E. Kefalidis,[b] Benedict M. Gardner,[a] Floriana Tuna,[c] Eric J. L.
McInnes,[c] William Lewis,[d] Laurent Maron,*[b] and Stephen T. Liddle*[a]
[a]
School of Chemistry, The University of Manchester, Oxford Road, Manchester, M13 9PL, UK.
*Email: steve.liddle@manchester.ac.uk.
[b]
LPCNO, CNRS & INSA, Université Paul Sabatier, 135 Avenue de Rangueil,
Toulouse 31077, France. *Email: laurent.maron@irsamc.ups-tlse.fr.
[c]
School of Chemistry and Photon Science Institute, The University of Manchester, Oxford Road,
Manchester, M13 9PL, UK.
[d]
School of Chemistry, The University of Nottingham, University Park, Nottingham, NG7 2RD,
UK.
1
This article is protected by copyright. All rights reserved.

10.1002/chem.201605620
Chemistry - A European Journal
Abstract
We report the reactivity of terminal uranium(V/VI)-nitrides with CE2 (E = O, S), where we observe
well-defined C=E cleavage followed by zero-, one-, and two-electron redox events. The
uranium(V)-nitride [U(TrenTIPS)(N)][K(B15C5)2] [1, TrenTIPS = N(CH2CH2NSiPri3)3; B15C5 =
benzo-15-crown-5] reacts with CO2 to give [U(TrenTIPS)(O)(NCO)][K(B15C5)2] (3), whereas the
uranium(VI)-nitride [U(TrenTIPS)(N)] (2) reacts with CO2 to give isolable [U(TrenTIPS)(O)(NCO)]
(4); complex 4 rapidly decomposes to known [U(TrenTIPS)(O)] (5) with concomitant formation of N2
and CO proposed, with the latter trapped as a vanadocene adduct. In contrast, 1 reacts with CS2 to
give [U(TrenTIPS)(κ2-CS3)][K(B15C5)2] (6), 2, and [K(B15C5)2][NCS] (7), whereas 2 reacts with
CS2 to give [U(TrenTIPS)(NCS)] (8) and “S”, with the latter trapped as Ph3PS. Calculated reaction
profiles reveal outer-sphere reactivity for uranium(V) but inner-sphere mechanisms for
uranium(VI); together the experimental and theoretical data reproduce the experimental outcomes
and suggest that despite the wide divergence of products the initial activation of CE2 follows
mechanistically related pathways, providing insight into the factors of uranium oxidation state,
chalcogen, and NCE groups that govern the subsequent divergent redox reactions that include
common one-electron reactions and a less-common two-electron redox event. This work highlights
that caution is warranted utilising CS2 as a reactivity surrogate for CO2.
Introduction
The activation and cleavage of carbon dioxide and carbon disulfide, CE2 (E = O, S), are burgeoning
areas given current environmental and sustainability agendas.[1] The former is an abundant, low-
cost, and renewable C1-source for fine chemicals and fuels production,[2] and the latter is a versatile
building block in organic synthesis that is often used to model CO2 reactivity.[3] In recent years f-
elements,[4] and especially uranium,[5,6] have emerged as effective, appealing candidates for the
reductive activation of CE2,[7] but studies of such activations remain in the shadow of those
involving transition metals.[1-3,8]
2
This article is protected by copyright. All rights reserved.

10.1002/chem.201605620
Chemistry - A European Journal
Metal-nitride chemistry is now well-developed for transition metals with a wide range of reactivity
types.[9] However, although electron-rich terminal M≡N triple bonds are primed for bond metathesis
chemistry, and are therefore excellent candidates for nitrogen-atom transfer reactions with
heteroallenes,[10] there are few examples of transition metal-nitride reactivity with CE2
molecules.[8,9] Germane to this work, [V{N(R)(Ar)}3(N)Na]2 (R = But or Ad; Ar = 3,5-Me2C6H3)
reacts with CE2 to give [V{N(R)(Ar)}3(NCE2)][Na(THF)2], which for E = O reacts no further but
for E = S and R = But extrudes NaNCS following bond metathesis to give [V{N(But)(Ar)}3(S)].[10d]
In contrast, [Nb{N(R)(Ar)}3(N)Na]2 reacts with CO2 to give [Nb{N(R)(Ar)}3(NCO2)][Na(THF)n],
but C-O splitting only occurs with addition of external electrophiles (such as Ac2O, O{C(O)CF3}2,
-
or ButCOCl); this opens the door to elimination of the CO2-derived oxide as RCO2 and
decarbonylation of the resulting niobium-cyanate to -nitride to close a synthetic cycle.[10b]
Regarding uranium, a promising candidate in this arena given its redox chemistry,[5,6] it was not
until recently that terminal uranium-nitride complexes became available under ambient
conditions,[11] and so their inherent reactivity patterns are yet to be established. Indeed, there are no
other classes of terminal f-block nitrides available for reactivity studies,[12] and although a number
of bridged diuranium-nitrides are known,[6a,b,13] their reactivity is also almost unknown so
determining and comparing the reactivity of terminal to bridging uranium-nitrides would present
opportunities to better understand their intrinsic reactivities. Furthermore, uranium sometimes
exhibits reactivity that parallels transition metal chemistry, but also often exhibits divergent
reactivity, and so it is of interest to compare terminal uranium- and transition metal-nitride
reactivity. Recently, we showed that the terminal uranium-nitrides [U(TrenTIPS)(N)][K(B15C5)2] [1,
TrenTIPS = N(CH2CH2NSiPri3)3; B15C5 = benzo-15-crown-5][14] and [U(TrenTIPS)(N)] (2)[11a] react
with CO by reductive carbonylation,[14,15] to give cyanate derivatives followed by complete
denitrification and N-atom transfer.[16] Whilst terminal M≡N triple bond reactivity with CE2 has
potential for C-N bond formation chemistry, since so little reactivity has been reported the factors
3
This article is protected by copyright. All rights reserved.

10.1002/chem.201605620
Chemistry - A European Journal
that govern their reactivity are yet to be elucidated. With the reactivity of 1 and 2 towards CO
suggesting a rich seam of N-atom transfer reactivity we turned our attention to the reactivity of 1
and 2 towards CO2 and CS2.
Here, we report the first benchmarking study of the reactivity of terminal f-block-nitrides towards
CO2 and CS2 with uranium(V) and (VI). In all cases we find facile CE2 cleavage chemistry and that
the split E and NCE products react further in zero-, one- and two-electron redox processes. In
contrast to the d-block, one-electron redox chemistry generally dominates in the f-block, and
although becoming more common two-electron redox process are unusual so understanding the
factors that promote such reactivity is important given the fundamental role electron transfer plays
in chemical reactivity.[11,13a,b,c,e,g,17] Calculated reaction profiles reveal outer-sphere reactivity for
uranium(V) but inner-sphere mechanisms for uranium(VI) and provide insight into the factors that
govern the observed reactivity. Thus this work furthers our understanding of the nature and
reactivity of terminal metal-nitride reactivity, permitting mechanistic similarities and differences
between terminal d- and f-block and bridging and terminal f-block metal-nitride reactivity to be
determined. Lastly, the divergent reactivities observed show that care should be taken extrapolating
surrogate reactivity between CO2 and CS2.
4
This article is protected by copyright. All rights reserved.

10.1002/chem.201605620
Chemistry - A European Journal
Results and Discussion
Experimental terminal uranium(V)-nitride reactivity with CO2
Scheme 1. a) Reactions of the terminal uranium(V)-nitride complex 1 with CO2 and CS2 to give the
ate uranium(V)-oxo-cyanate 3 and the uranium(IV)-trithiocarbonate 6, uranium(VI)-nitride 2, and
potassium bis(crown) thiocyanate 7, respectively. b) Reactions of the terminal uranium(VI)-nitride
complex 2 with CO2 and CS2 to give the uranium(VI)-oxo-cyanate 4, which decomposes to the
uranium(V)-oxo 5, N2, and CO2, and the uranium(IV)-thiocyanate 8 with expulsion of elemental
“S” that can be trapped by Ph3P as Ph3PS. A control reaction between 8 and Ph3P confirms that the
S does not originate from the NCS group of 8.
Exposure of a degassed toluene solution of 1 to an atmosphere of CO2 at −78 °C resulted in a colour
change from yellow-brown to red-brown. After work-up and recrystallisation from benzene the
orange complex formulated as the oxo-cyanate derivative [U(TrenTIPS)(O)(NCO)][K(B15C5)2] (3)
was isolated in 36% crystalline yield, Scheme 1a, which reflects the high solubility of this complex;
analysis of the mother liquor by NMR spectroscopy suggests the formation of 3 is essentially
quantitative (Figure S1).
1
The H NMR spectrum of 3 spans 16 to −3 ppm, which suggests the presence of uranium(V) and
the resonances are broad so inferences about the symmetry of the TrenTIPS ligand (Cs- or C3-
5
This article is protected by copyright. All rights reserved.

10.1002/chem.201605620
Chemistry - A European Journal
coordination) could not be discerned; however, the 29Si NMR spectrum exhibits two resonances
(−27.1 and −27.8 ppm), suggesting that the anion component of 3 possesses approximate Cs rather
than C3v symmetry.
Figure 1. Variable temperature SQUID magnetometry of the effective magnetic moment (µB) of 3,
6, and 8 in the temperature range 2-298 K. Lines are to guide the eye only and have no further
significance.
The formation of cyanate and oxo functional groups is supported by inspection of the FTIR
spectrum of 3, which exhibits absorptions at 2194, 800, and 742 cm-1, with the first attributed to
cyanate and the latter two to oxo stretches, and these compare very well to calculated frequencies of
2195 and 799 and 747 cm-1, respectively. The two stretches for the oxo result from coupling to the
uranium-amine stretch to give asymmetric and symmetric combinations. The cyanate stretch
compares well to a value of 2187 cm-1 for [U(TrenTIPS)(NCO)] (D),[14] prepared by reductive
carbonylation of 2, and those found in related examples of uranium cyanate complexes (ca 2122-
2201 cm-1).[5a,17i]
The UV/Vis/NIR spectrum of 3 (Figure S8) exhibits a characteristic absorption at 6693 cm-1 (ε = 43
M-1 cm-1), consistent with uranium(V), as well as three other absorptions in that region resulting
2
from splitting of the 7/2 excited state of F by the Oh crystal field into four levels (Γ7', two Γ8' and
6
This article is protected by copyright. All rights reserved.

10.1002/chem.201605620
Chemistry - A European Journal
Γ6 in double-group notation).[18] The uranium(V) assignment is confirmed by variable temperature
SQUID magnetometry of 3 (Figure 1), which reveals a magnetic moment of 1.86 µB at 298 K, that
compares well to a solution magnetic moment of 1.69 µB at 298 K; this changes little until 50 K
where it falls more rapidly to 1.20 µB at 2 K consistent with the presence of uranium(V) that is a
magnetic doublet.[19] Final confirmation of the presence of uranium(V) in 3 is provided by EPR
spectroscopy (see Figure S15) which reveals a strong absorption at g = 2.85 at 5 K, which is
consistent with a Kramers uranium(V) ion; uranium(IV) would be expected to be EPR-silent under
these measurement conditions.[20]
Figure 2. Molecular structure of 3 at 120 K with displacement ellipsoids at 50% and hydrogen
atoms omitted for clarity. Only one of the two independent ate pairs in the crystallographic
asymmetric unit is shown. The other pair is essentially identical with statistically indistinguishable
bond lengths and angles. Selected bond lengths (Å) for the ate pair shown: U1-N1, 2.316(7); U1-
N2, 2.302(7); U1-N3, 2.342(7); U1-N4, 2.517(7); U1-N5, 2.404(8); U1-O1, 1.848(7); N5-C34,
1.199(13); C34-O2, 1.163(13).
In order to confirm the structure of 3 we determined the molecular structure by single crystal X-ray
diffraction (Figure 2). Complex 3 retains the separated ion pair motif from 1, and the structure
reveals a pseudo-octahedral uranium centre where the oxo is trans to the Tren-amine and the
cyanate group is trans to a Tren-amide and mutually cis to the oxo. Competitive refinement of the
7
This article is protected by copyright. All rights reserved.

10.1002/chem.201605620
Chemistry - A European Journal
diffraction data suggest that the cyanate is N- not O-bound, and this is supported by DFT
calculations that show the former isomer to be more stable than the latter by 9 kcal/mol. The U-O,
U-Ncyanate, and U-Namine bond lengths of 1.848(7), 2.404(8), and 2.517(7) Å, respectively, are typical
of such linkages,[14,17d,21] though the latter is towards the lower end of the range of Tren-Namine-U
distances, which suggests there might be a weak inverse-trans-influence in the N→U=O unit.[22]
The U-Namide bond lengths span the range 2.302(7)-2.342(7) Å and can be considered long for
uranium(V) when compared to Tren uranium complexes, which most likely reflects the electron
rich nature of this anionic fragment of the compound. The O-U-Namine and Namide-U-Ncyanate angles
deviate from the octahedral ideal at 168.0(3) and 168.3(3)°, respectively, reflecting the steric
demands of the TrenTIPS ligand.
Experimental terminal uranium(VI)-nitride reactivity with CO2
When a degassed toluene solution of [U(TrenTIPS)(N)] (2) is exposed to an atmosphere of CO2 at
−78 °C the brown solution turns wine-red (Scheme 1b). After quick work-up a red solid is obtained,
and on the basis of the data presented below we formulate this red solid as [U(TrenTIPS)(O)(NCO)]
1
(4), obtained in essentially quantitative yield. The H NMR spectrum of the red solid retains the
appearance of a diamagnetic complex, spanning 0-10 ppm, suggesting the retention of uranium(VI),
but it is not particularly informative. We were unable to obtain the potentially more informative 29Si
NMR spectrum due to rapid decomposition of 4 in solution.
The FTIR spectrum of 4 exhibits strong absorptions at 2176 and 827 cm-1, which are assigned as
cyanate and oxo stretches, respectively. The former absorption compares well to the cyanate stretch
for 3 and is supported by a calculated stretching frequency of 2198 cm-1. Interestingly, the oxo
stretch for 4, unlike 3, is now decoupled from the U-Namine vibration resulting in only one
absorption being observed; this is also reproduced by the frequency calculation, which yields a
calculated oxo stretching frequency of 812 cm-1. Although the formation of 4 seems certain on the
8
This article is protected by copyright. All rights reserved.

10.1002/chem.201605620
Chemistry - A European Journal
basis of 3 and computationally supported spectroscopy, despite exhaustive attempts we have been
unable to obtain a crystal structure of 4 due to decomposition during recrystallisation attempts;
however, this is consistent with the computed reaction profile (see below). Attempts to recrystallise
4 resulted in isolation of the known mono-oxo uranium(V) complex [U(TrenTIPS)(O)] (5),[17d] as
evidenced by single crystal X-ray diffraction and FTIR data which are devoid of any absorptions in
the cyanate region and that show the characteristic oxo stretch of 5 at 910 cm-1. Monitoring the
conversion of 4 to 5 by 1H NMR spectroscopy (for a representative spectrum see Figure S2) reveals
clean, quantitative conversion over 24 hours, with 4:5 ratios of 1:0 (<30 minutes), 2:1 (2 hours), 1:9
(5 hours), 0:1 (20 hours).
The isolation of 5 suggests that a cyanate radical has been ejected from the coordination sphere of
uranium; this could react with solvent to abstract a proton to give HNCO, or dimerise to the
putative molecule diisooxocyan followed by facile decomposition to N2 and CO.[23] Although the
bond dissociation enthalpy of HNCO is high (~110 kcal/mol),[24] suggesting that it could be formed
- with toluene implicated as the H-source - we have not observed at any stage of the reaction any IR
absorbances consistent with its formation;[25] further, production of HNCO (pKa = 3.7)[26] would be
anticipated to result in extensive decomposition of 5, which is not the case experimentally. To probe
the possibility that diisooxocyan is formed, we reacted 50% 15Nnitride-labelled 2 with CO2, but could
not detect a 15N NMR resonance for dissolved N2. Due to diamond absorbance from the probe-tip in
the region 2000-2250 cm-1 we could not observe CO formation in solution by ReactIR, and
sampling of the gas headspace resulted in IR spectra dominated by solvent vapour. Unfortunately,
sampling and MS-analysis of the headspace from the decomposition of 4 was inconclusive, likely
due to concentration effects and the presence of N2 even in high purity Ar gas. However, vacuum
transfer of the headspace atmosphere from reactions that generate 4 onto a thawing toluene solution
of vanadocene gave a weak absorption at 1895 cm-1 in the resulting IR spectrum, which is
consistent with the formation of a vanadocene carbonyl.[5k] Attempts to trap the putative cyanate
9
This article is protected by copyright. All rights reserved.

10.1002/chem.201605620
Chemistry - A European Journal
radical were inconclusive (see SI). Attempts to oxidise 3 to 4 with numerous oxidants gave
intractable product mixtures, but oxidation of D[14] with trimethylamine-N-oxide or 4-
methylmorpholine-N-oxide gave mixtures of D:4:5 in 6:1:1 and 3:2:2 ratios, respectively, which is
not inconsistent with oxidation of D to 4 followed by decomposition to 5 (Figures S6 and S7).
Experimental terminal uranium(V)-nitride reactivity with CS2
The reactions of CS2 with 1 and 2 proceed very differently, in terms of the final products, to the
CO2 reactivity (Scheme 1a). Addition of one equivalent of CS2 to a solution of pentavalent 1 in
toluene at −78 °C results in a colour change from yellow-brown to brown. After removal of solvent
a brown oil is formed, which after washing with hexanes is isolated as a brown solid. Work-up of
the hexane washings results in isolation of hexavalent 2, suggesting that overall disproportionation
has occurred. Extraction of the brown solid with aromatic solvent and recrystallisation resulted in
the isolation of an orange complex formulated as the uranium(IV)-trithiocarbonate complex
[U(TrenTIPS)(κ2-CS3)][K(B15C5)2] (6) in crystalline yields of ca 4%; this yield reflects the oily
nature of this complex and although it is low it is reproducible. The formulation of 6 suggests the
presence of a uranium(IV) ion, which would account for the remainder of the disproportion balance
1
together with 2. Inspection of crude reaction mixtures by H NMR spectroscopy (Figure S4)
suggests that 2 and 6 are formed in a 1:1 ratio, as would be expected, and together they constitute
~90% of the reaction mixture (ignoring the thiocyanate component, see below) and thus 2 and 6
constitute the major reaction products.
The 1H NMR spectrum of 6 (Figure S3) spans the range 9 to −26 ppm in-line with the uranium(IV)
assignment. Further confirmation that 6 is a uranium(IV) complex comes from solid-state SQUID
magnetometry (Figure 1), which reveals a magnetic moment of 2.4 µB at 298 K, whose value
decreases with decreasing the temperature to 0.6 µB at 2 K and tending to zero.[19]
10
This article is protected by copyright. All rights reserved.

10.1002/chem.201605620
Chemistry - A European Journal
The solid that remains after the toluene or benzene extraction was examined by FTIR spectroscopy
revealing strong absorbances at 2060 and 2052 cm-1 which are assigned as thiocyanate stretchs.[6c,f,j]
For mass balance this is proposed to be [K(B15C5)2][NCS] (7), confirmed by comparison to an
authentic sample prepared independently from KNCS and B15C5 that also exhibits two thiocyanate
stretches in its FTIR spectrum.
Figure 3. Molecular structure of 6 at 120 K with displacement ellipsoids at 50% and hydrogen
atoms omitted for clarity. Selected bond lengths (Å): U1-N1, 2.277(3); U1-N2, 2.290(3); U1-N3,
2.296(3); U1-N4, 2.670(3); U1-S1, 2.8520(10); U1-S2, 2.8415(8); S1-C34, 1.729(4); S2-C34,
1.731(4); S3-C34, 1.665(4).
The molecular structure of 6 was determined by single crystal X-ray diffraction to confirm its
identity (Figure 3). The U-Namine (2.670(3) Å) and U-Namide (range: 2.277(3)-2.296(3) Å) distances
are typical of U-N distances in Tren-uranium(IV) complexes.[21] Complex 6 the first example where
the (CS3)2- dianion is terminally bound to one f-element centre, and is only the third example of a f-
block trithiocarbonate complex.[6c,f,j] Befitting the unique coordination mode of the (CS3)2- dianion
in f-block chemistry, the U-S bond lengths of 2.8415(8) and 2.8520(10) Å are shorter than in
complexes where the (CS3)2- dianion bridges (range: 2.9488(19)-3.130(2) Å). The κ2-coordination
mode of the (CS3)2- dianion appears to localise negative charge on the two coordinated sulfur
atoms, resulting in some localisation of a C=S double bond on the remaining uncoordinated sulfur-
11
This article is protected by copyright. All rights reserved.

10.1002/chem.201605620
Chemistry - A European Journal
carbon linkage; this is evidenced by a terminal C=S bond distance of 1.665(4) Å compared to the
other two C-S bond lengths of 1.731(4) and 1.729(4) Å.
Experimental terminal uranium(VI)-nitride reactivity with CS2
Since the uranium(V)-nitride 1 undergoes, overall, disproportion reactivity with CS2 we were
interested to determine how the analogous uranium(VI)-nitride 2 would react since it lacks the
requisite valence electron to engage in disproportionation chemistry. When a toluene solution of 2
is treated with CS2 over six days a new uranium-containing product is formed, Scheme 1b. Work-up
and recrystallisation from pentane affords the green uranium(IV)-thiocyanate complex
[U(TrenTIPS)(NCS)] (8) in 51% crystalline yield.
1
The H NMR spectrum of 8 (Figure S5) spans the range 9 to −37 ppm, consistent with the
uranium(IV) formulation and the 29Si NMR spectrum exhibits only one resonance at 2.4 ppm that is
consistent with a C3v symmetric complex. The FTIR spectrum exhibits a strong absorbance at 2013
cm-1 and this is typical of uranium(IV)-thiocyanate stretches.[27] In support of the uranium(IV)
assignment, variable temperature SQUID magnetometry data of 8 reveal a magnetic moment of
2.52 µB at 298 K (2.91 µB by Evans method in solution) that decreases monotonously down to 0.24
µB at 2 K and tending to zero in-line with the magnetic singlet ground state of uranium(IV) at low
temperature (Figure 1).[19]
12
This article is protected by copyright. All rights reserved.

10.1002/chem.201605620
Chemistry - A European Journal
Figure 4. Molecular structure of 8 at 120 K with displacement ellipsoids at 50% and hydrogen
atoms omitted for clarity. Only one of the two parts of the disordered NCS unit are shown. Selected
bond lengths (Å): U1-N1, 2.238(3); U1-N2, 2.240(3); U1-N3, 2.237(3); U1-N4, 2.626(4); U1-N5,
2.377(4); N5-C34, 1.181(11); C34-S1, 1.589(9).
In order to confirm the identity of 8 we prepared it independently from [U(TrenTIPS)(Cl)] and KSCN
(isolated in 53% yield) and the characterisation data are in agreement. The formation of 8 suggests
that elemental sulfur is the by-product of this reaction, presumably extruded from the putative
complex [U(TrenTIPS)(S)(NCS)] analogously to 4. Therefore, either post-reaction or all in one-pot,
we treated the reactions with Ph3P and observed the formation of Ph3PS by 31P NMR spectroscopy
(31P NMR: Ph3P = −5.2; Ph3PS = 42.8 ppm). In a control experiment to rule out the abstraction of
sulfur from 8 to give the plausible cyanide product [U(TrenTIPS)(CN)], we treated 8 with Ph3P and
found that no reaction occurs, even on extended reflux in toluene.
Final confirmation of the identity of 8 was obtained by examination of its single crystal X-ray
diffraction structure (Figure 4), which unambiguously reveals a complex of approximate C3v
symmetry with an N-bound thiocyanate ligand at uranium. The metrical data for 8 are
unexceptional.[27]
13
This article is protected by copyright. All rights reserved.

10.1002/chem.201605620
Chemistry - A European Journal
Given this diverse range of reaction outcomes, we modelled the reaction profiles by using DFT
calculations (at B3PW91 level of theory) to provide useful insights for the mechanisms and in order
to understand why such a diverse range of reactivity occurs and what the controlling factors are. In
all cases the computed energy pathways were found to be kinetically accessible and
thermodynamically favourable and completely consistent with experimental outcomes.
Computed reaction profiles of terminal uranium-nitride reactivity with CO2
Figure 5. Energy profile (at 298 K) for the gas phase reaction of CO2 with the uranium(V)-nitride 1
computed at the B3PW91 level of theory. [U] = [U(TrenTIPS)].
The computed energy pathway for the reaction of complex 1 with CO2 (Figure 5) is reminiscent of
the outer-sphere CO2 addition reported by Cummins.[10a,d] In other words, the highly nucleophilic
nitride attacks the incoming CO2 molecule without any coordination to the poorly electrophilic UV
centre. Indeed, a very low-lying outer-sphere addition transition state (TS), TSA-B, was located
(enthalpy barrier of 1.4 kcal/mol-1), where the CO2 molecule bends to allow the π* of CO2, that is
mainly located at the 2p-orbital at the carbon atom, to overlap with the σ-lone pair of the nitride.
This outer-sphere TS is lower in energy than the classical [2 + 2]-cycloaddition route (ΔΗ≠ = 5.1
kcal/mol-1) (Figure S20). TSA-B leads to the formation of a η1-carbamate intermediate, that readily
isomerises (ΔΗ≠ = 0.2 kcal/mol-1) to a stable metal-based-four-membered ring (−23.1 kcal/mol-1).
14
This article is protected by copyright. All rights reserved.

10.1002/chem.201605620
Chemistry - A European Journal
Finally, following a low-lying transition state, TSI-3-, involving bond metathesis (−20.8 kcal/mol-1),
the cleavage of the C-O bond occurs to give the oxo-cyanate that is 57.1 kcal/mol-1 more stable than
the initial complex 1.
Figure 6. Energy profile (at 298 K) for the gas phase reaction of CO2 with the uranium(VI)-nitride
2 computed at the B3PW91 level of theory. [U] = [U(TrenTIPS)].
In the same way as for complex 1, the energy profile was computed for the reaction of complex 2
with CO2 (Figure 6) at the same level of theory. Unlike 1, the outer-sphere mechanism is not found
to be operative. Indeed, the UVI centre is more electrophilic and at the same time the nitride is less
nucleophilic compared to the UV case, so that the CO2 molecule needs to be activated through
coordination to the Lewis acid centre to react. Hence, the reaction profile evolves firstly by a [2 +
2]-cycloaddition step, surmounting an activation barrier of 12.1 kcal/mol-1 and leading to the
formation of the intermediate II. The geometry of the latter is close to the intermediate I in
uranium(V)-nitride reactivity (Fig. 5), but energetically more unstable by almost 22 kcal/mol-1. The
following step corresponds to the C-O bond-breaking which proceeds via a bond metathesis
transition state TSI-3- (ΔH≠ = 3.5 kcal/mol-1) reminiscent of activated alkene metathesis[28] to give
the oxo-cyanate (−38.1 kcal/mol-1). Although 4 is an enthalpic minimum with respect to the oxo 5
(−25.1 kcal/mol-1), when the Gibbs free energy term is considered the formation of 5 from 4
becomes more favourable by 5.7 kcal/mol-1 at −43.8 kcal/mol-1 due to the entropic gain from N2 and
CO elimination. This is in-line with the strong electrophilic character of UVI that prevents it from
15
This article is protected by copyright. All rights reserved.

10.1002/chem.201605620
Chemistry - A European Journal
tightly binding to the cyanate radical (U-N distance of 2.36 Å in 4 vs. 2.27 Å for 3-) as it prefers a
strongly anionic ligand. This finding is in-line with the initial experimental isolation of 4 but
conversion to 5 on attempted recrystallisation of the former.
The initial reactivity of 2 with CO2 to give 4 clearly follows the same pattern as 1 reacting to give 3.
However, although 3 is stable in the absence of O2 or H2O in solution or the solid state, 4 is stable
for only ca 30 minutes in solution and the cyanate group is rapidly lost. The formation of 5 suggests
that the cyanate group is ejected as a radical in a homolytic U-Ncyanate bond cleavage step which
would provide the requisite electron for the one-electron reduction of the uranium(VI) centre.
Comparing the computed barriers for the first step is informative regarding the nucleophilic nature
of the nitride in 1 and 2. The computed barrier for 2 (12.1 kcal/mol) although relatively low is > 30
kcal/mol higher relative to the equivalent point computed for 1. This indicates that the nitride is of
lower nucleophilic character in 2 than in 1,[29,30] which can be attributed to the shorter U-N distance
in the former than in the latter, inducing a stronger U-N interaction. On the other hand, the barrier
for the C-O bond-breaking in the carbamate intermediates are similar for both complexes,
indicating that the oxophilicity of uranium is affected very little by the oxidation state in these steps.
However, the final oxo-cyanate 4 further evolves by ejecting the coordinated cyanate group
yielding 5.
The question then arises as to the fate of the cyanate radical; H-abstraction to give HNCO can
seemingly be ruled out, but the coupling of cyanate radicals to give the putative molecule
diisooxocyan followed by facile decomposition to N2 and CO has been computationally
described,[23] and seems a credible, if not conclusively proven, fate for the cyanate radical given the
indirect supporting evidence by FTIR of the formation of vanadocene carbonyl as a CO-trap. This
step is computed to be endothermic in terms of enthalpy, but is favoured by entropic effects (Figure
6) and indeed becomes exergonic overall when entropy is factored in (although it is known that the
16
This article is protected by copyright. All rights reserved.

10.1002/chem.201605620
Chemistry - A European Journal
computation of entropy is not very accurate).[31] The difference between the reactivity of 3 and 4 is
attributed to steric hindrance around the metal centre[32] as well as electronic effects in the latter.
Indeed, the shorter distances in the UVI complexes makes the geometry more compact around the
metal centre and also induces stronger interactions with the oxo ligand with respect to the TrenTIPS
ligand. Therefore, in 4 the uranium centre is less available for interacting with the radical cyanate
that undergoes the entropically-driven radical coupling reaction to yield N2 + 2CO.[33] This is
reflected in the calculations where 4 is more stable from its start point by 38.1 kcal/mol, the
corresponding uranium(V) component of 3 is more stable than its start point by a much larger 57.1
kcal/mol, which must to a significant extent reflect the size differences between uranium(V) and
uranium(VI) where the former is larger so an additional ligand in the coordination sphere of
uranium brings greater stability through the formation of another U-N bond with only a minimal
thermodynamic penalty to pay from steric aspects. We further suggest that this reactivity can be
rationalised on the basis that uranium(VI) is more oxidising than uranium(V) and so although the
uranium(V) component in 3 is stable to such homolytic extrusion of cyanate this is not the case for
uranium(VI) in 4. A parallel can be drawn here to the chemistry of terminal uranium-nitrides under
photolytic conditions,[11a,13c] where the uranium(VI)- nitride activates ancillary ligand C-H bonds
but uranium(V) analogues do not; this can be rationalised on the basis of the oxidising power of
uranium(VI) compared to uranium(V). The fact that the cyanate linkage is lost in preference to the
oxo group is testament to the highly favourable nature of the U=O bond, but also that one-electron
redox events are far more common in f-element chemistry than two-electron redox events; if the
uranium were to be reduced by two units to generate [U(TrenTIPS)(NCO)] instead this would require
the elimination of oxidised ‘O’ which would be expected to be thermodynamically far uphill given
the hard nature of electronegative oxide and its ability to stabilise high oxidation states at metals,
i.e. resist oxidation. We note there is precedent for elimination of the cyanate radical from
uranium,[34] where in that report the reaction takes a different course because the cyanate radical can
17
This article is protected by copyright. All rights reserved.

10.1002/chem.201605620
Chemistry - A European Journal
be intercepted by low valent and electron rich uranium(III), which is not an available pathway in the
reaction with hexavalent 2.
Computed reaction profiles of terminal uranium-nitride reactivity with CS2
Figure 7. Energy profile (at 298 K) for the gas phase reaction of CS2 with the uranium(V)-nitride 1
computed at the B3PW91 level of theory. [U] = [U(TrenTIPS)]. Extrusion of NCS⋅ from VI and
subsequently the oxidation of 1 by NCS⋅ to give 2 and 7 is highly favoured (−97.6 kcal/mol) (see
Figures S17 and S19).
As for the reaction of 1 with CO2, the reaction of 1 with CS2 (Figure 7) begins by a low energy
outer-sphere addition of CS2, that after isomerisation yields the η2-N,S carbamate intermediate
(−33.8 kcal/mol-1). From this intermediate, rather than the C-S bond breaking, which was found to
be higher in energy (Figure S21), a kinetically facile outer-sphere electrophilic attack of a CS2
molecule to the pendant sulfur atom of the carbamate intermediates occurs (enthalpy barrier of only
2.0 kcal/mol-1). Such process does not work for the reaction with CO2 as every attempt to locate this
kind of TS leads to the departure of the incoming CO2 molecule. This yields, after an isomerisation
process (ΔH≠ = 11.6 kcal/mol-1), to a metal-based-six-membered ring intermediate (−36.5 kcal/mol-
1). In this intermediate, a C-S bond is strongly activated and thus easily broken (ΔH≠ = 1.4 kcal/mol-
1) to form an isothiocyanate adduct to the thiocarbonate complex. In the last step, the driving force
18
This article is protected by copyright. All rights reserved.

10.1002/chem.201605620
Chemistry - A European Journal
⋅
for the extrusion of NCS and the subsequent oxidation of 1 by NCS⋅ to give 2 and 7 is mainly due
to the resulted high exothermicity of such sequence of events (−97.6 kcal/mol-1) as would be
expected for two open shell radicals comproportionating to two closed shell species.
Figure 8. Energy profile (at 298 K) for the gas phase reaction of CS2 with the uranium(VI)-nitride 2
computed at the B3PW91 level of theory. [U] = [U(TrenTIPS)].
Finally, the reactivity of complex 2 with CS2 was investigated (Figure 8). Once again, like the
analogous CO2 reaction the outer-sphere mechanism does not operate since the formed product of
this step is endothermic (Figure S22). Therefore, the two first steps of the reaction are similar to that
found for the CO2 reactivity previously described, that is a [2 + 2]-cycloaddition via TS2-VI (ΔH≠ =
23.4 kcal/mol-1) yielding a thiocarbamate complex (−3.7 kcal/mol-1) followed by a low-lying bond
metathesis transition state TSVI-VII (ΔH≠ = 5.1 kcal/mol-1) with C-S bond disruption to form a
terminal sulfido-thiocyanate complex VII (−19.6 kcal/mol-1). Extrusion of sulfur by the PPh3,
computed as Ph3P=S, as indeed can be done experimentally, is enthapically highly favoured (−38.3
kcal/mol-1) yielding eventually complex 8 (overall exothermicity of reaction −57.9 kcal/mol-1).
Interestingly, for the reaction of 1 with CS2 the thiocarbamate intermediate can easily be
electrophilically attacked by another CS2 molecule, accounting for the formation of the
isothiocyanate adduct of terminal uranium(IV)-trithiocarbonate. However, the NCS ligand cannot
19
This article is protected by copyright. All rights reserved.

10.1002/chem.201605620
Chemistry - A European Journal
react as the cyanate ligand in a radical way. Indeed, a radical coupling of two NCS⋅ would
ultimately lead to the formation of two CS molecules that are kinetically highly unstable.[35] Instead,
the NCS⋅ extrusion occurs in a different way, by employing a redox process. The NCS⋅ radical can
react with complex 1, which acts as a single electron donor, yielding complex 2 by an esoteric but
new way to prepare a uranium(VI)-nitride, to give NCS- that coordinates the counter-cation of 1 to
form 7. This reaction is thermodynamically highly favourable (by 97.6 kcal/mol) and represents an
alternative to the unfavourable radical coupling reaction for NCS⋅. Including a solvation model, this
reaction is of the same order of magnitude, being −87.2 kcal/mol (Figure S18).
The uranium(VI)-nitride 2 reacts with CS2 to ultimately give the uranium(IV)-thiocyanate 8 and
elemental sulfur. However, the calculated reaction profile supports the idea that the nitride
undergoes a nucleophilic attack of the CS2 and by a subsequent bond-metathesis generates a
[U(TrenTIPS)(S)(NCS)] analogue of the CO2 reaction with 2 to give 4. However, whereas for the
analogous CO2 reaction 4 extrudes cyanate, for the sulfur analogue the uranium(VI)-sulfide-
thiocyanate intermediate instead extrudes sulfur. Here, the former reaction is a more common one-
electron redox event, whereas the latter is a less frequently observed example of a two-electron
reduction. We suggest that the different reaction outcomes can be attributed directly to the hard
nature of oxide compared to softer sulfide; even highly oxidising uranium(VI) cannot oxidise oxide
so the cyanate is sacrificed and ejected for 4 but the softer sulfide can be oxidised to elemental
sulfur by uranium(VI) and thus 8 is formed. The barrier for the [2 + 2]-cycloaddition is the highest
found (23.4 kcal/mol), that is 14.7 kcal/mol higher than for the outer-sphere reaction of CS2 with 1,
possibly due to three effects: (i) the smaller size of uranium(VI) compared to uranium(V); (ii) the
difference of nucleophilicity of the nitride; and (iii) the bond polarisation in CS2. Also, this
relatively high computed energy barrier nicely explains the fact that the corresponding reaction
takes considerable time to occur experimentally with respect to the others. The barrier for the C-S
bond disruption from the thiocarbamate complex is again low (5.1 kcal/mol) and similar to that
20
This article is protected by copyright. All rights reserved.

10.1002/chem.201605620
Chemistry - A European Journal
found for 1. In the terminal sulfide-thiocyanate complex, the thiocyanate ligand is strongly bonded
(calculated BDE = 25.6 kcal/mol). This is due to the relatively long U-S bond, which renders the
uranium centre more available for coordination with an extra ligand. Indeed, we note that the
uranium(VI)-sulfide-thiocyanate is the least stable (−19.6 kcal/mol); this compares to −57.1
kcal/mol for the uranium(V)-oxo-cyanate which is more stable than the uranium(VI)-oxo-cyanate
(−38.1 kcal/mol).
Finally, the most striking feature of the CS2 reactivity is the completely different final outcomes of
the reactions compared to the CO2 reactions, but as suggested by the calculated reaction profiles the
reactions actually proceed initially through structurally related intermediates. The barriers for either
the outer-sphere addition (16 kcal/mol) or the [2 + 2]-cycloaddition (23.4 kcal/mol) are
systematically higher than for the analogous reactions with CO2 (11-13 kcal/mol). Therefore, this
cannot be attributed to a difference in nucleophilicity of the nitride, but to the difference of bond
polarisation in CS2 versus CO2. Indeed, in the latter, the carbon atom carries a formal δ+ charge
−
whereas it is δ in the former, making the formation of an empty 2p-orbital at the carbon atom more
complicated and thus increasing the barrier.
Summary and Conclusions
In summary, we report the first terminal f-block nitride reactivity study with CE2 (E = O, S), where
we observe cleavage at terminal uranium-nitrides followed by zero-, one-, and two-electron redox
events to give four different, yet related reactions that are well-defined; this highlights that care
should be taken when using either as a reactivity surrogate of the other. For CO2 the uranium(V)-
nitride reacts to give a uranium(V)-oxo-cyanate by bond metathesis and cleavage of a C=O bond
and the uranium(VI)-nitride reacts to give an analogous uranium(VI)-oxo-cyanate complex that
subsequently decomposes to a uranium(V)-oxo with elimination of a cyanate radical; the latter is
proposed to dimerise to diisooxocyan that is unstable with respect to decomposition to N2 and CO
21
This article is protected by copyright. All rights reserved.

10.1002/chem.201605620
Chemistry - A European Journal
formation. In contrast, for CS2 the uranium(V)-nitride reacts to give a uranium(IV)-trithiocarbonate
along with uranium(VI)-nitride and potassium thiocyanate, with formal overall disproportionation,
but the uranium(VI)-nitride reacts to give a uranium(IV)-thiocyanate complex and extrusion of
elemental sulfur that can be trapped as Ph3PS by a Ph3P scavenger.
Calculated reaction profiles consistently reproduce the experimental outcomes, and together the
experimental and theoretical data suggest that the initial activation of CE2 follows outer sphere
attack of CE2 by the nitrides for uranium(V) or direct [2 + 2]-cycloaddition for uranium(VI).
Irrespective, carbamate-type species are ultimately formed which then engage in multiple bond
metathesis steps, which may or may not involve redox chemistry. By dissecting each individual
calculated step with reference to the experimental reaction outcome it has proven possible to
delineate the effects that uranium oxidation state and nature of the coordinated E and NCE groups
have on the resulting divergent reaction pathways. In one instance there is no subsequent redox
chemistry, but the more common outcome is a subsequent one-electron redox step, which is a
common feature of f-block chemistry. However, in one instance an unusual two-electron reduction
event is found.
In a wider context, the outer-sphere reactivity of 1 parallels that of related transition metal(V)
complexes, but the inner-sphere reactivity of 2 stands in contrast, and can be related to the stronger,
less reactive U≡N triple bond in 2 compared to 1. Cleavage of CE2 by 1 and 2 here is similar to that
reported for diuranium-nitride complexes,[6a,b] but 1 and 2 react further than the bimetallics, most
likely because the bimetallics generally trap the products whereas monometallic 1 and 2 do not. A
fuller comparison of terminal and bridging uranium nitrides will require mechanistic studies of the
latter to complement the former reported here.
22
This article is protected by copyright. All rights reserved.

10.1002/chem.201605620
Chemistry - A European Journal
Acknowledgements
We thank the Royal Society, the Engineering and Physical Sciences Research Council, the
European Research Council, The University of Manchester, The University of Nottingham, the
EPSRC UK National EPR Facility, COST Action CM1006, and the National Nuclear Laboratory
for supporting this work. LM is a member of the Institut Universitaire de France. The Humboldt
foundation is also acknowledged for a grant of experienced researcher. CalMip is thanked for a
generous grant of computing time.
References
[1]. a) W. B. Tolman, Activation of Small Molecules, Wiley-VCH, Weinheim, Germany, 2006; b)
H. Arakawa, M. Aresta, J. N. Armor, M. A. Barteau, E. J. Beckman, A. T. Bell, J. E. Bercaw,
C. Creutz, E. Dinjus, D. A. Dixon, K. Domen, D. L. DuBois, J. Eckert, E. Fujita, D. H.
Gibson, W. A. Goddard, D. W. Goodman, J. Keller, G. J. Kubas, H. H. King, J. E. Lyons, L.
E. Manzer, T. J. Marks, K. Morokuma, K. M. Nicholas, R. Periana, L. Que, J. Rostrup-
Nielson, W. M. H. Sachtler, L. D. Schmidt, A. Sen, G. A. Somorjai, P. C. Stair, B. R. Stultls
and W. Tumas, Chem. Rev. 2001, 101, 953.
[2]. a) C. Costentin, M. Robert and J. -M. Savéant, Chem. Soc. Rev. 2013, 42, 2423; b) E. E.
Benson, C. P. Kubiak, A. J. Sathrum and M. Smieja, Chem. Soc. Rev. 2009, 38, 89; c) D. J.
Darensbourg, Chem. Rev. 2007, 107, 2388; d) T. Sakakura, J. -C. Choi and H. Yasuda, Chem.
Rev. 2007, 107, 2365.
[3]. a) B. Ochiai and T. Endo, Prog. Polym. Sci. 2005, 30, 183; b) K. K. Pandey, Coord. Chem.
Rev. 1995, 140, 37.
[4]. Representative examples: a) G. -C. Wang, Y. -M. So, K. -L. Wong, K. -C. Au-Yeung, H. H. -
Y. Sung, I. D. Williams and W. -H. Leung, Chem. Eur. J. 2015, 21, 16126; b) J. Andrez, J.
Pécaut, P. -A. Bayle and M. Mazzanti, Angew. Chem. Int. Ed. 2014, 53, 10488; c) Y. -M. So,
G. -C. Wang, Y. Li, H. H. -Y. Sung, I. D. Williams, Z. Lin and W. -H. Leung, Angew. Chem.
23
This article is protected by copyright. All rights reserved.

10.1002/chem.201605620
Chemistry - A European Journal
Int. Ed. 2014, 53, 1626; d) K. Wang, G. Luo, J. Hong, X. Zhou, L. Weng, Y. Luo and L.
Zhang, Angew. Chem. Int. Ed. 2014, 53, 1053; e) W. S. Ren, H. B. Song, G. F. Zi and M. D.
Walter, Dalton Trans. 2012, 41, 5965; f) W. S. Ren, G. F. Zi, D. C. Fang and M. D. Walter, J.
Am. Chem. Soc. 2011, 133, 13183; g) D. Heitmann, C. Jones, D. P. Mills and A. Stasch,
Dalton Trans. 2010, 39, 1877; h) W. J. Evans, T. J. Mueller and J. W. Ziller, J. Am. Chem.
Soc. 2009, 131, 2678; i) W. J. Evans, D. B. Rego, J. W. Ziller, A. G. DiPasquale and A. L.
Rheingold, Organometallics 2007, 26, 4737; j) N. W. Davies, A. S. P. Frey, M. G. Gardiner
and J. Wang, Chem. Commun. 2006, 4853; k) W. J. Evans, J. M. Perotti and J. W. Ziller, J.
Am. Chem. Soc. 2005, 127, 3894; l) W. J. Evans, J. M. Perotti, J. C. Brady and J. W. Ziller, J.
Am. Chem. Soc. 2003, 125, 5204; m) W. J. Evans, C. A. Seibel and J. W. Ziller, Inorg. Chem.
1998, 37, 770.
[5]. Uranium-mediated CO2 reactivity: a) C. Camp, L. Chatelain, C. E. Kefalidis, J. Pécault, L.
Maron and M. Mazzanti, Chem. Commun. 2015, 51, 15454; b) J. A. Higgins, A. S. P. Frey, F.
G. N. Cloke and S. M. Roe, Organometallics 2015, 34, 2102; c) O. Cooper, C. Camp, J.
Pécault, C. E. Kefalidis, L. Maron, S. Gambarelli and M. Mazzanti, J. Am. Chem. Soc. 2014,
136, 6716; d) A. -C. Schmidt, F. W. Heinemann, L. Maron and K. Meyer, Inorg. Chem. 2014,
53, 13142; e) C. L. Webster, J. W. Ziller and W. J. Evans, Organometallics 2014, 33, 433; f)
A. -C. Schmidt, F. W. Heinemann, C. E. Kefalidis, L. Maron, P. W. Roesky and K. Meyer,
Chem. Eur. J. 2014, 20, 13501; g) P. L. Arnold, Z. R. Turner, A. I. Germeroth, I. J. Casely, G.
N. Nichol, R. Bellabarba and R. P. Tooze, Dalton Trans, 2013, 42, 1333; h) C. L. Webster, J.
W. Zilller and W. J. Evans, Organometallics 2013, 32, 4820; i) J. A. Higgins, F. G. N. Cloke
and S. M. Roe, Organometallics 2013, 32, 5244; j) O. P. Lam, S. M. Franke, F. W.
Heinemann and K. Meyer, J. Am. Chem. Soc. 2012, 134, 16877; k) V. Mougel, C. Camp, J.
Pécault, C. Copéret, L. Maron, C. E. Kefalidis and M. Mazzanti, Angew. Chem. Int. Ed. 2012,
51, 12280; l) C. L. Webster, J. W. Ziller and W. J. Evans, Organometallics 2012, 31, 7191;
m) A. -C. Schmidt, A. V. Nizovtsev, A. Scheuer, F. W. Heinemann and K. Meyer, Chem.
24
This article is protected by copyright. All rights reserved.

10.1002/chem.201605620
Chemistry - A European Journal
Commun. 2012, 48, 8634; n) E, M. Matson, W. P. Forrest, P. E. Fanwick, S. C. Bart, J. Am.
Chem. Soc. 2011, 133, 4948; o) S. M. Mansell, N. Kaltsoyannis and P. L. Arnold, J. Am.
Chem. Soc. 2011, 133, 9036; p) E. M. Matson, P. E. Fenwick and S. C. Bart, Organometallics
2011, 30, 5753; q) O. Bénaud, J. -C. Berthet, P. Thuéry and M. Ephritikhine, Inorg. Chem.
2010, 49, 8117; r) W. J. Evans, J. R. Walensky and J. W. Ziller, Organometallics 2010, 29,
945; s) O. P. Lam, S. C. Bart, H. Kameo, F. W. Heinemann and K. Meyer, Chem. Commun.
2010, 46, 3137; t) O. T. Summerscales, A. S. P. Frey, F. G. N. Cloke and P. B. Hitchcock,
Chem. Commun. 2009, 198; u) S. C. Bart, C. F. Anthon, W. Heinemann, E. Bill, N. M.
Edelstein and K. Meyer, J. Am. Chem. Soc. 2008, 130, 12536; v) I. Castro-Rodriguez and K.
Meyer, J. Am. Chem. Soc. 2005, 127, 11242; w) I. Castro-Rodriguez, H. Nakai, L. N.
Zhakharov, A. L. Rheingold and K. Meyer, Science 2004, 305, 1757; x) K. G. Moloy and T.
J. Marks, Inorg. Chim. Acta, 1985, 110, 127; y) A. L. Arduini, J. D. Jamerson and J. Takats,
Inorg. Chem. 1981, 20, 2474.
[6]. Uranium-mediated CS2 reactivity: a) L. Chatelain, R. Scopelliti and M. Mazzanti, J. Am.
Chem. Soc. 2016, 138, 1784; b) M. Falcone, L. Chatelain and M. Mazzanti, Angew. Chem.
Int. Ed. 2016, 55, 4074; c) C. Camp, O. Cooper, J. Andrez, J. Pécault and M. Mazzanti,
Dalton Trans. 2015, 44, 2650; d) E. M. Matson, A. T. Breshears, J. J. Kiernicki, B. S. Newell,
P. E. Fanwick, M. P. Shores, J. R. Walensky and S. C. Bart, Inorg. Chem. 2014, 53, 12977; e)
O. P. Lam, S. M. Franke, F. W. Heinemann and K. Meyer, J. Am. Chem. Soc. 2012, 134,
16877; f) O. P. Lam, L. Castro, B. Kosog, F. W. Heinemann, L. Maron and K. Meyer, Inorg.
Chem. 2012, 51, 781; g) N. A. Siladke, J. LeDuc, J. W. Ziller and W. J. Evans, Chem. Eur. J.
2012, 18, 14820; h) O. P. Lam, F. W. Heinemann and K. Meyer, Angew. Chem. Int. Ed.
2011, 50, 5965; i) E. M. Matson, W. P. Forrest, P. E. Fanwick and S. C. Bart, J. Am. Chem.
Soc. 2011, 133, 4948; j) C. Lescop, T. Arliguie, M. Lance, M. Nierlich and M. Ephritikhine,
J. Organomet. Chem. 1999, 580, 137; k) J. G. Brennan, R. A. Andersen and A. Zalkin, Inorg.
25
This article is protected by copyright. All rights reserved.

10.1002/chem.201605620
Chemistry - A European Journal
Chem. 1986, 25, 1756; l) A. L. Arduini, J. D. Jamerson and J. Takats, J. Inorg. Chem. 1981,
20, 2474.
[7]. a) S. T. Liddle, Angew. Chem. Int. Ed. 2015, 54, 8604; b) H. S. La Pierre and K. Meyer, Prog.
Inorg. Chem. 2014, 58, 303; c) B. M. Gardner and S. T. Liddle, Eur. J. Inorg. Chem. 2013,
3753; d) A. R. Fox, S. C. Bart, K. Meyer, and C. C. Cummins, Nature 2008, 455, 341.
[8]. a) J. P. Krogman, B. M. Foxman and C. M. Thomas, J. Am. Chem. Soc. 2011, 133, 14582; b)
A. R. Sadique, W. W. Brennessel and P. L. Holland, Inorg. Chem. 2008, 47, 784; c) C. H.
Lee, D. S. Laitar, P. Müller and J. P. Sadighi, J. Am. Chem. Soc. 2007, 129, 13802; d) C. C.
Lu, C. T. Saouma, M. W. Day and J. C. Peters, J. Am. Chem. Soc. 2007, 129, 4; e) D. S.
Laitar, P. Müller and J. P. Sadighi, J. Am. Chem. Soc. 2005, 127, 17196; f) G. Fachinetti, C.
Floriani, A. Chiesi-Villa and C. Guastini, J. Am. Chem. Soc. 1979, 101, 1767.
[9]. a) J. M. Smith, Prog. Inorg. Chem. 2014, 58, 417 b) C. T. Saouma and J. C. Peters, Coord.
Chem. Rev. 2011, 255, 920; c) R. A. Eikey and M. M. Abu-Omar, Coord. Chem. Rev. 2003,
243, 83; d) K. Dehnicke and J. Strähle, Angew. Chem. Int. Ed. Engl. 1992, 31, 955; e) K.
Dehnicke and J. Strähle, Angew. Chem. Int. Ed. Engl. 1981, 20, 413.
[10]. a) S. P. Semproni and P. J. Chirik, J. Am. Chem. Soc. 2013, 135, 11373; b) J. S. Silva and C.
C. Cummins, J. Am. Chem. Soc. 2010, 132, 2169; c) U. J. Kilgore, F. Basuli, J. C. Huffman
and D. J. Mindiola, Inorg. Chm., 2006, 45, 487; d) J. K. Brask, V. Durà-Villà, P. L.
Diaconescu and C. C. Cummins, Chem. Commun. 2002, 902.
[11]. a) D. M. King, F. Tuna, E. J. L. McInnes, J. McMaster, W. Lewis, A. J. Blake and S. T.
Liddle, Nat. Chem. 2013, 5, 482; b) D. M. King, F. Tuna, E. J. L. McInnes, J. McMaster, W.
Lewis, A. J. Blake and S. T. Liddle, Science 2012, 337, 717.
[12]. D. M. King and S. T. Liddle, Coord. Chem. Rev. 2014, 266-267, 2.
[13]. a) N. Tsoureas, A. F. R. Kilpatrick, C. J. Inman, and F. G. N. Cloke, Chem. Sci. 2016, 7,
4624; b) C. Camp, J. Pécaut and M. Mazzanti, J. Am. Chem. Soc. 2013, 135, 12101; c) R. K.
Thomson, T. Cantat, B. L. Scott, D. E. Morris, E. R. Batista and J. L. Kiplinger, Nat. Chem.
26
This article is protected by copyright. All rights reserved.

10.1002/chem.201605620
Chemistry - A European Journal
2010, 2, 723; d) T. K. Todorova, L. Gagliardi, J. R. Walensky, K. A. Miller and W. J. Evans,
J. Am. Chem. Soc. 2010, 132, 12397; e) S. Fortier, G. Wu and T. W. Hayton, J. Am. Chem.
Soc. 2010, 132, 6888; f) A. R. Fox, P. L. Arnold and C. C. Cummins, J. Am. Chem. Soc.
2010, 132, 3250; g) A. R. Fox and C. C. Cummins, J. Am. Chem. Soc. 2009, 131, 5716; h) G.
Nocton, J. Pécaut and M. Mazzanti, Angew. Chem. Int. Ed. 2008, 47, 3040; i) W. J. Evans, K.
A. Miller, J. W. Ziller and J. Greaves, Inorg. Chem. 2007, 46, 8008; j) W. J. Evans, S. A.
Kozimor and J. W. Ziller, Science 2005, 309, 1835; k) I. Korobkov, S. Gambarotta and G. P.
A. Yap, Angew. Chem. Int. Ed. 2002, 41, 3433.
[14]. P. A. Cleaves, D. M. King, C. E.; Kefalidis, L. Maron, F. Tuna, E. J. L. McInnes, J.
McMaster, W. Lewis, A. J. Blake and S. T. Liddle, Angew. Chem. Int. Ed. 2014, 53, 10412.
[15]. a) A. F. Cozzolino, J. S. Silvia, N. Lopez and C. C. Cummins, Dalton Trans. 2014, 43, 4639;
b) B. L. Tran, M. Pink, X. Gao, H. Park and D. J. Mindiola, J. Am. Chem. Sos. 2010, 132,
1458; c) J. S. Silvia and C. C. Cummins, J. Am. Chem. Soc. 2009, 131, 446.
[16]. a) F. Dielmann, O. Back, M. Henry-Ellinger, P. Jerabek, G. Frenking and G. Bertrand,
Science 2012, 337, 1526; b) J. J. Scepaniak, R. P. Bontchev, D. L. Johnson and J. M. Smith,
Angew. Chem. Int. Ed. 2011, 50, 6630.
[17]. a) D. P. Halter, F. W. Heinemann, J. Bachmann and K. Meyer, Nature 2016, 530, 317; b) C.
Camp, M. A. Antunes, G. Garcia, I. Ciofini, I. C. Santos, J. Pecault, M. Almeida, J. Marcalo
and M. Mazzanti, Chem. Sci. 2014, 5, 841; c) E. Lu, O. J. Cooper, J. McMaster, F. Tuna, E. J.
L. McInnes, W. Lewis, A. J. Blake and S. T. Liddle, Angew. Chem. Int. Ed. 2014, 53, 6696;
d) D. M. King, F. Tuna, J. McMaster, W. Lewis, A. J. Blake, E. J. L. McInnes and S. T.
Liddle, Angew. Chem. Int. Ed. 2013, 52, 4921; e) J. L. Brown, S. Fortier, G. Wu, N.
Kaltsoyannis and T. W. Hayton, J. Am. Chem. Soc. 2013, 135, 5352; f) D. P. Mills, O. J.
Cooper, F. Tuna, E. J. L. McInnes, E. S. Davies, J. McMaster, F. Moro, W. Lewis, A. J. Blake
and S. T. Liddle, J. Am. Chem. Soc. 2012, 134, 10047; g) S. Fortier, J. L. Brown, N.
Kaltsoyannis, G. Wu and T. W. Hayton, Inorg. Chem. 2012, 51, 1625; h) C. R. Graves and J.
27
This article is protected by copyright. All rights reserved.

10.1002/chem.201605620
Chemistry - A European Journal
L. Kiplinger, Chem. Commun. 2009, 3831; i) I. Castro-Rodríguez, H. Nakai and K. Meyer,
Angew. Chem. Int. Ed. 2006, 45, 2389; j) D. S. J. Arney and C. J. Burns, J. Am. Chem. Soc.
1995, 117, 9448; k) D. S. J. Arney and C. J. Burns, J. Am. Chem. Soc. 1993, 115, 9840; l) D.
S. J. Arney, C. J. Burns and D. C. Smith, J. Am. Chem. Soc. 1992, 114, 10068; m) J. G.
Brennan and R. A. Andersen, J. Am. Chem. Soc. 1985, 107, 514.
[18]. J. Selbin and J. D. Ortego, Chem. Rev. 1969, 69, 657.
[19]. D. R. Kindra and W. J. Evans, Chem. Rev. 2014, 114, 8865.
[20]. D. Patel, F. Tuna, E. J. L. McInnes, J. McMaster, W. Lewis, A. J. Blake and S. T. Liddle,
Dalton Trans. 2013, 42, 5224.
[21]. B. M. Gardner and S. T. Liddle, Chem. Commun. 2015, 51, 10589.
[22]. a) H. S. La Pierre, M. Rosenzweig, B. Kosog, C. Hauser, F. W. Heinemann, S. T. Liddle and
K. Meyer, Chem. Commun. 2015, 51, 16671; b) H. S. La Pierre and K. Meyer, Inorg. Chem.
2013, 52, 529; c) A. J. Lewis, P. J. Carroll and E. J. Schelter, J. Am. Chem. Soc. 2013, 135,
13185; d) B. Kosog, H. S. La Pierre, F. W. Heinemann, S. T. Liddle and K. Meyer, J. Am.
Chem. Soc. 2012, 134, 5284; e) E. O’Grady and N. Kaltosyannis, J. Chem. Soc. Dalton Trans.
2002, 1233; f) R. G. Denning, J. Phys. Chem A 2007, 111, 4125; (g) R. G. Denning, Struct.
Bonding, Springer, Berlin 1992, 79, 215.
[23]. A. Schulz and T. M. Klapötke, Inorg. Chem. 1996, 35, 4791.
[24]. B. Ruscic and J. Berkowitz, Chem. Sci. 1994, 100, 4498.
[25]. Recorded at 145 K (b.p. 296 K), the N-H IR frequency of HNCO is 2254 cm‒1 (ν2). See, M. S.
Lowenthal, R. K. Khanna and M. H. Moore, Spectrochim. Acta A 2002, 58, 73.
[26]. A. R. Amell, J. Am. Chem. Soc. 1956, 78, 6234.
[27]. a) C. Camp, N. Settineri, J. Lefèvre, A. R. Jupp, J. M. Goicoechea, L. Maron and J. Arnold,
Chem. Sci. 2015, 6, 6379; b) E. Hashem, J. A. Platts, F. Hartl, G. Lorusso, M. Evangelisti, C.
Schulzke and R. J. Baker, Inorg. Chem. 2014, 53, 8624; c) A. J. Lewis, E. Nakamaru-Ogiso,
J. M. Kikkawa, P. J. Carroll and E. J. Schelter, Chem. Commun. 2012, 48, 4977.
28
This article is protected by copyright. All rights reserved.

10.1002/chem.201605620
Chemistry - A European Journal
[28]. See for instance P. Liu, B. L. H. Taylor, J. Garcia-Lopez, and K. N. Houk, Computational
Studies of Ruthenium-Catalyzed Olefin Metathesis, in Handbook of Metathesis, Set, ed R. H.
Grubbs, A. G. Wenzel, D. J. O'Leary and E. Khosravi, Wiley-VCH Verlag GmbH & Co.
KGaA, Weinheim, Germany, 2015, doi: 10.1002/9783527674107.ch7.
[29]. C. E. Kefalidis, A. Stasch, C. Jones and L. Maron, Chem. Commun. 2014, 50, 12318.
[30]. C. E. Kefalidis and L. Maron, Chem. Eur. J. 2016, 22, 4743.
[31]. L. Watson and O. Eisenstein, J. Chem. Ed. 2002, 79, 1269.
[32]. a) B. M. Gardner, G. Balázs, M. Scheer, A. J. Wooles, F. Tuna, E. J. L. McInnes, J.
McMaster, W. Lewis, A. J. Blake and S. T. Liddle, Angew. Chem. Int. Ed. 2015, 54, 15250;
b) B. M. Gardner, G. Balázs, M. Scheer, F. Tuna, E. J. L. McInnes, J. McMaster, W. Lewis,
A. J. Blake and S. T. Liddle, Nat. Chem. 2015, 7, 582; c) B. M. Gardner, F. Tuna, E. J. L.
McInnes, J. McMaster, W. Lewis, A. J. Blake and S. T. Liddle, Angew. Chem. Int. Ed. 2015,
54, 7068; d) B. M. Gardner, P. A. Cleaves, C. E. Kefalidis, J. Fang, L. Maron, W. Lewis, A. J.
Blake and S. T. Liddle, Chem. Sci. 2014, 5, 2489; e) B. M. Gardner, G. Balázs, M. Scheer, F.
Tuna, E. J. L. McInnes, J. McMaster, W. Lewis, A. J. Blake and S. T. Liddle, Angew. Chem.
Int. Ed. 2014, 53, 4484; f) D. M. King, J. McMaster, F. Tuna, E. J. L. McInnes, W. Lewis, A.
J. Blake and S. T. Liddle, J. Am. Chem. Soc. 2014, 136, 5619.
[33]. For examples of sterically compressed complexes see: a) N. A. Siladke, K. R. Meihaus, J. W.
Ziller, M. Fang, F. Furche, J. R. Long and W. J. Evans, J. Am. Chem. Soc. 2012, 134, 1243; b)
W. J. Evans, T. J. Mueller and J. W. Ziller, J. Am. Chem. Soc. 2009, 131, 2678; c) W. J.
Evans, S. A. Kozimor and J. W. Ziller, J. Am. Chem. Soc. 2003, 125, 14264; d) W. J. Evans,
S. A. Kozimor, G. W. Nyce and J. W. Ziller, J. Am. Chem. Soc. 2003, 125, 13831; e) W. J.
Evans, and B. L. Davis, Chem. Rev. 2002, 102, 2119; f) W. J. Evans, G. W. Nyce, M. A.
Johnston and J. W. Ziller, J. Am. Chem. Soc. 2000, 122, 12019.
[34]. C. E. Kefalidis, A. S. P. Frey, S. M. Roe, F. G. N. Cloke and L. Maron, Dalton Trans. 2014,
43, 11202.
29
This article is protected by copyright. All rights reserved.