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Internationale Ausgabe:DOI:10.1002/anie.201711824
Metal–Metal Bonding Deutsche Ausgabe:DOI:10.1002/ange.201711824
Actinide–Pnictide (An@Pn) Bonds Spanning Non-Metal, Metalloid,
and Metal Combinations (An =U, Th;Pn=P, As,Sb, Bi)
Thomas M. Rookes+,Elizabeth P. Wildman+,G#bor Bal#zs,Benedict M. Gardner,
Ashley J. Wooles,Matthew Gregson, Floriana Tuna, Manfred Scheer,* and Stephen T. Liddle*
Dedicated to Professor William J. Evans on the occasion of his 70th birthday
Abstract: The synthesis and characterisation is presented of the
compounds [An(TrenDMBS){Pn(SiMe3)2}] and [An(TrenTIPS)-
{Pn(SiMe3)2}] [TrenDMBS =N(CH2CH2NSiMe2But)3,An=U,
Pn =P, As,Sb, Bi;An=Th, Pn =P, As;TrenTIPS =
N(CH2CH2NSiPri3)3,An=U, Pn =P, As,Sb; An =Th, Pn =
P, As,Sb] .The U@Sb and Th@Sb moieties are unprecedented
examples of any kind of An@Sb molecular bond, and the U@Bi
bond is the first two-centre-two-electron (2c–2e) one.The Th@
Bi combination was too unstable to isolate,underscoring the
fragility of these linkages.However,the U@Bi complex is the
heaviest 2c–2e pairing of two elements involving an actinide on
amacroscopic scale under ambient conditions,and this is
exceeded only by An@An pairings prepared under cryogenic
matrix isolation conditions.Thermolysis and photolysis experi-
ments suggest that the U@Pn bonds degrade by homolytic bond
cleavage,whereas the more redox-robust thorium compounds
engage in an acid–base/dehydrocoupling route.
The preparation, isolation, and study of new molecular
element–element bonds remains afundamentally important
endeavour because it informs us about chemical character-
istics and reactivity,allows us to probe and refine periodic
trends,and provides vital benchmarking for structural and
theoretical predictions and modelling. Reflecting much
progress over decades of synthetic effort, there are now few
places left in the Periodic Table where new element–element
bonds can be regularly discovered. However,the actinides,
where progress has generally lagged owing to their radio-
activity and the inherent challenges involved in stabilizing
chemical bonds to some of the largest metal ions in existence,
remains arich seam from which to mine new chemical bonds.
Forexample,only very recently have the first Am@S, Pu@C,
and Bk@Obonds been crystallographically authenticated.[1]
Theaforementioned examples all involve synthetic trans-
uranic examples with unique associated challenges;studies
have in particular been impeded by their radioactive nature,
need for specialist handling facilities,and limited availabil-
ities.H
owever,e
ven for naturally occurring neighbour
elements like uranium and thorium, which can be handled
in normal laboratories,there are chemical bond combinations
yet to be realised. Forexample,regarding An@Pn bonds
(An =U, Th;Pn=N, P, As,Sb, Bi), although covalent An@N,
An@P, and An@As bonds are known,[2–4] somewhat remark-
ably given the burgeoning nature of non-aqueous actinide
chemistry,[5] An@Sb and An@Bi derivatives are conspicuous
by their absence even though analogous examples are known
in transition-metal[6] and even in lanthanide chemistry.[7]
Indeed, there are no structurally characterized U@Sb,Th
@
Sb,orTh
@Bi bonds and there is only one report of U@Bi
bonds,[8] which involves open-shell, delocalised radical Zintl
clusters that reside at the molecular–periodic interface.
Seeking to remedy this situation, we sought to extend our
previous work on early metal@PnH2complexes.[3c,4,9] Since
discrete (PnH2)@anions are not available for Sb and Bi, and
noting that many heavy PnHxR3@xreagents are prone to facile
Pn@Cbond homolysis,weutilised the more sterically
demanding pnictides {Pn(SiMe3)2}@for P, As,Sb, and Bi,
though even these reagents are prone to easy decomposi-
tion.[10,11] We reasoned that this would present the opportunity
to prepare astructurally homologous series of An@Pn
covalent bond benchmarks,whilst enabling meaningful com-
parison of the Pn geometries (that is,development of trigonal
pyramidal from trigonal planar) as the pnictide group is
descended.
Herein, we report the synthesis and characterisation of
molecular compounds containing new An@Pn bonds that
include the first structurally authenticated U@Sb and Th@Sb
bonds of any kind and the first two-centre-two-electron (2c–
2e) U@Bi bond. Thecorresponding Th@Bi bond was too
unstable to isolate,highlighting the major challenges of
preparing these ill-suited hard–soft linkages generally.These
complexes present chemical bond benchmarks,and the U@Bi
bond is the heaviest 2c–2e pairing of two elements involving
[*] T. M. Rookes,[+] Dr.E.P.Wildman,[+] Dr.B.M.Gardner,
Dr.A.J.Wooles, Dr.M.Gregson, Dr.F.Tuna, Prof. Dr.S.T.Liddle
School of Chemistry,The University of Manchester
Oxford Road, Manchester,M13 9PL (UK)
E-mail:steve.liddle@manchester.ac.uk
Dr.G.Bal#zs, Prof. Dr.M.Scheer
Institute of Inorganic Chemistry,University of Regensburg
Universit-tsstr.31, 93053 Regensburg (Germany)
E-mail:manfred.scheer@ur.de
[++]These authors contributed equally to this work.
All data are available from the authors on request. Supporting
informationand the ORCID identification number(s) for the
author(s) of this article can be found under:
https://doi.org/10.1002/anie.201711824.
T2018 The Authors. Published by Wiley-VCH Verlag GmbH &Co.
KGaA. This is an open access article under the terms of the Creative
Commons AttributionLicense, which permits use, distribution and
reproduction in any medium, provided the original work is properly
cited.
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an actinide under macroscopic,ambient conditions,exceeded
experimentally only by An@An pairings in matrix isolation
experiments.[12] Preparing homologues spanning non-metal,
metalloid, and metal within asingle element group has
permitted elucidation of aformal periodic break-point;DFT
calculations suggest that Pand As adopt formal @3oxidation
states whereas Sb and Bi are more appropriately assigned as
+1.
To assemble the desired An@Pn linkages we utilised asalt
elimination strategy with separated ion pair An precursors to
avoid complications with installation of soft Pn centres at
hard An ions since the outer-sphere borate is an excellent
leaving group.[3c,4, 9] Thus,treatment of [An(TrenDMBS)(L)]-
[BPh4](An =U, L =THF, 1U;An=Th,L=DME, 1Th)or
[An(TrenTIPS)(L)][BPh4](An =U, L =THF, 2U;An=Th,
L=DME, 2Th)[3c,4,9] with KPn(SiMe3)2(Pn =P, As,Sb, Bi)[10]
afforded [An(TrenDMBS){Pn(SiMe3)2}] (3AnPn)and [An-
(TrenTIPS){Pn(SiMe3)2}] (4AnPn).[11] Most combinations
proved accessible,and 3UP,3UAs,3USb,3UBi,3ThP,
3ThAs,4UP,4UAs,4USb,4ThP,4ThAs,and 4ThSb were
isolable (Scheme 1). Forcompleteness,weexamined installa-
tion of the analogous amide {N(SiMe3)2}@,but found only the
formation of Tren-cyclometallates,which seems to be steri-
cally driven, since for example the dicyclohexylamide com-
plex [U(TrenDMBS){N(C6H11)2}] is isolable.[13] Thecharacter-
isation data for the isolable complexes are consistent with
their formulations,and for uranium the variable temperature
magnetisation data (Supporting Information, Figures S1–
S7)[11] corroborate the uranium(IV) assignments,but other-
wise are not particularly informative so we determined their
molecular structures to gain further insight.
Thesolid-state molecular structures of 3UP,3UAs,3USb,
3UBi,3ThP,3ThAs,4UP,4UAs,4USb,4ThP,4ThAs,and
4ThSb were all determined by single-crystal X-ray diffraction.
Thestructures of 3UP,3UAs,3USb,and 3UBi are illustrated
in Figure 1and key metrical parameters are compiled in
Table 1. Theother structurally determined compounds in this
study are in the Supporting Information, Figures S10–S20.[11]
Forthe sake of brevity our discussion will largely focus on the
3UPn series since this constitutes acomplete actinide–heavy-
pnictide family.
TheU
@Pn distances of 2.8646(14), 2.9423(9), 3.2437(8),
and 3.3208(4) cfor 3UP,3UAs,3USb,3UBi,respectively,can
be compared to the respective sums of single-bond covalent
radii of 2.81, 2.91, 3.10, and 3.21 c,[14] and for 3UP to the U@P
distance of 2.789(4) cin sterically less encumbered [U(h5-
Scheme 1. Synthesis of the An@Pn complexes reported in this study:
a) utilising the triamidoamine TrenDMBS ancillary ligand ;b)utilising the
triamidoamine TrenTIPS ancillary ligand. Reagents and conditionsfor
(a) and (b): i) THF, KPn(SiMe3)2,@7888Ctoroom temperature.
Figure 1. a)–d) Solid-state molecular crystal structures of 3UP,3UAs,3USb,and 3UBi,respectively,measured at 120 K. Ellipsoids set at 40%
probability;hydrogen atoms, minor disorder components, and any lattice solvent removed for clarity.[23] e)–h) Ball-and-stick representations of the
core U-Pn(SiMe3)2units from the same side-on perspectiveineach case, where one Si perfectly obscures the other,toshow the increasing
deviation from trigonal planar to trigonal pyramidal geometry as the pnictide series is descended;all other atoms in these depictions are omitted
for clarity.Ugreen, Pn magenta, Nblue, Si orange, Cgray.
Table 1: An@Pn bond lengths [b]and sum of Pn angles [88]for the
structurally authenticatedmolecules in this study.
Combination 3UPn 4UPn 3ThPn 4ThPn
Pn=P2.8646(14)/
354.06(8)
2.8391(9)/
359.94(5)
2.9406(11)/
351.28(7)
2.9020(13)/
356.75(8)
Pn=As 2.9423(9)/
349.71(9)
2.9062(7)/
355.56(8)
3.0456(9)/
343.47(9)
2.9569(6)/
359.35(6)
Pn=Sb 3.2437(8)/
325.96(11)
3.2089(6)/
351.53(12)
–3.2849(3)/
348.13(4)
Pn=Bi 3.3208(4)/
315.98(9)
–– –
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C5Me5)2(Cl){P(SiMe3)2}];[3h] these data essentially divide into
two groups where the experimental U@Pand U@As pairs are
within 0.05 cof those predicted, but the discrepancies for the
U@Sb and U@Bi distances are >0.1 c.This suggests aperiodic
break between As and Sb,but also perhaps reflects the
changing sum of angles at each Pn (P,354.06(8) ;As, 349.71-
(9);Sb, 325.96(11);Bi, 315.98(9)88); although respective s–p
energy gaps decrease as Group 15 is descended, rehybridisa-
tion promotion energies conversely increase owing to pro-
gressively inefficient s–p orbital overlap from increasingly
diffuse orbitals.
Interestingly,for 3ThP and 3ThAs the Th@Pn distances
are 0.08–0.1 clonger than the uranium analogues even
though the single bond covalent radius of Th is only 0.05 c
larger than that of U.[14] Conversely,however, the U@Pn
distances for 4UP,4UAs,and 4USb,are all shorter than the
respective corresponding U@Pn distances in the 3UPn series.
Also,the sum of angles at Pn varies far less for the 4UPn than
3UPn series,which can be related to the sterically more
demanding nature of TrenTIPS compared to TrenDMBS restrict-
ing the tendencytowards orthogonal bonding for the heavier
pnictides.This suggests that the trigonal planar bonding mode
of these pnictides is stronger than amore trigonal pyramidal
mode.This trend is overall repeated when comparing the
3ThPn and 4ThPn series together,though we note that the
differences for 4ThPn vs. 4UPn are smaller than those of
3ThPn vs. 3UPn,reflecting the greater constraints imposed by
the more sterically demanding TrenTIPS compared to TrenDMBS.
TheUV/Vis/NIR spectroscopic data for 3UPn (Support-
ing Information, Figure S8)[11] show acharacteristic absorp-
tion maxima that bathochromically shifts (3UP,19,420; UAs,
18,280; USb,15,820; UBi 14,245 cm@1); this can be related to
the increasing pyramidalisation of the Pn centres as the Pn
group is descended and adecreasing pnictide–uranium charge
transfer energy.The same trend is observed for 4UPn
(Supporting Information, Figure S9).[11]
To further probe the An@Pn linkages,weexamined them
using DFT,NBO,and QTAIM methods (Table 2; Supporting
Information, Figures S21–S32).[11] Computed structures com-
pare well with the solid-state structures,soweconclude that
these models represent qualitative pictures of the electronic
structures of these complexes.The An@Pn Mayer bond
orders,considering these linkages are expected to be polar,
are surprisingly high, suggest Pn p-donation in addition to the
anticipated s-bonds.[15]
Thecomputed An charges,and spin densities for uranium,
are overall consistent with their +4oxidation states,[2a] but the
computed charges of the Pn centres fall into two clear groups;
for P/As computed charges are about @1to@1.3 whereas for
Sb/Bi they are lower at about @0.3 to @0.4 and this does not
appear to be related in any way to the geometry of the Pn
centre as an explanation. This suggests aperiodic break where
the former pair are best described formally as being in the @3
oxidation state whereas the latter two are better formulated
as being +1.
When the Pn centre remains essentially trigonal planar, as
is the case for the 4UPn and 4ThPn series,the An charge
follows the trend An@As >An@P>An@Sb,which suggests
that As is the weakest donor ion. Interestingly,for the 3UPn
series the An charges increases from 3UP to 3UAs,but then
falls away for 3USb and 3UBi,sothat the same,but extended,
series of An charges of An@As >An@P@An@Sb &An@Bi
emerges.This is counterintuitive,because the expected trend
would be for the An charges to be ordered An@Bi >An@Sb >
An@As >An@Passuggested by the Mayer bond order data.
However,inspection of the DFT Kohn Sham and NBO
descriptions of 3UP,3UAs,3USb,3UBi,3ThP,3ThAs,4UP,
4UAs,4USb,4ThP,4ThAs,and 4ThSb reveals that whilst the
An@Pn s-bonds are largely ionic, surprisingly,since the Pn
np-orbitals (n=3–6) become increasingly diffuse,there are
significant p-bonding combinations in these complexes,and,
using series 3UPn,asthe pnictide becomes more pyramidal-
ised although one lobe of the p-orbital moves increasingly
away from the metal the other lobe approaches much more
closely and so may actually engage more effectively overall
with one orbital lobe than two.Thus,linkages that would be
Table 2: Selected computed DFT,NBO, and QTAIM data.
Bond length and index[b,c] Charges[d] Spin density[e] NBO
s-component[g]
NBO
p-component[g]
QTAIM[h]
Entry[a] An@Pn BI qAn qPn mAn An[%] Pn[%] An[%] Pn[%] An 7s/7p/6d/5f 1(r) 521(r) H(r) e(r)
3UP 2.898 0.92 2.24 @1.02 2.33 0100 14 87 1:1:16:82 0.03 0.04 @0.01 0.27
3UAs 2.988 0.94 2.33 @1.32 2.37 0100 17 83 1:1:17:81 0.02 0.03 @0.01 0.29
3USb 3.265 0.87 1.85 @0.39 2.38 0100 17 83 5:1:19:75 0.02 0.02 @0.01 0.29
3UBi 3.385 0.78 1.86 @0.40 2.38 0100 18 82 9:2:22:67 0.02 0.01 @0.01 0.15
4UP 2.854 1.00 2.43 @0.97 2.37 0100 14 86 0:1:29:70 0.03 0.04 @0.01 0.22
4UAs 2.944 0.98 2.69 @1.31 2.39 0100 17 83 0:1:29:70 0.02 0.03 @0.01 0.26
4USb 3.222 1.03 2.13 @0.33 2.48 0100 89236:1:14:49 0.02 0.03 @0.01 0.13
3ThP 2.981 0.81 2.17 @1.05 –0100 6942:1:23:74 0.04 0.06 @0.01 0.22
3ThAs 3.072 0.86 2.29 @1.32 –0100 6946:1:32:61 0.04 0.05 @0.01 0.26
4ThP 2.955 0.87 2.44 @0.99 –0100 59517:1:33:49 0.04 0.07 @0.01 0.31
4ThAs 2.996 0.94 2.58 @1.33 –0100 6940:2:36:62 0.04 0.07 @0.01 0.37
4ThSb 3.315 0.92 2.14 @0.36 –0100 79319:2:36:43 0.03 0.05 @0.01 0.25
[a] All molecules geometry optimised without symmetry constraints using the BP86 GGA functional and abasis set derived from TZP/ZORA all-
electron ADF database ;calculations were unrestricted for uranium and restricted for thorium. [b] Computational An@Pn distances [b]. [c] Mayer bond
indices. [d] MDC-q charges on An. [e] MDC-m a-spin densities on An. [f ]MDC-q charges on Pn. [g] Natural bond orbital (NBO) analyses;the electron
occupancies of these orbitals are +97 %. [h] QTAIM topologicalelectron density [1(r)],Laplacian [521(r)],electronic energy density [H(r)],and
ellipticity [e(r)]bond critical point data.
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expected to be weaker may actually be stronger,with respect
to donor strength, which in this context is not synonymous
with thermodynamic enthalpic bond strength.
Theasymmetry of the An@Pn bonding,assuggested by
DFT and NBO methods,isfurther supported by analysis of
the QTAIM data;although highly polar bonds are certainly
found, the bond critical point ellipticity values are consis-
tently greater than zero and of the magnitude found for the
C@Cbonds in benzene tending to ethene.[16]
Although 4UBi,3ThSb,3ThBi,and 4ThBi have eluded
isolation, attempts to prepare them along with studies on the
subsequent reactivity of the isolable An@Pn complexes has
proven informative with respect to unravelling the under-
pinning chemistry of these An@Pn linkages.Attempts to
prepare 4UBi from 2U and KBi(SiMe3)2resulted in batch-
variable quantities of green crystals,that could not be cleanly
isolated, of (Me3Si)2Bi@Bi(SiMe3)2(Bi2)[11,17] as verified by
single-crystal X-ray diffraction. This implies that 4UBi is
transiently formed, but decomposes by homolytic U@Pn bond
cleavage to give [U(TrenTIPS)],[18] which was indeed identified
in reaction mixtures by 1HNMR spectroscopy.Interestingly,
attempts to prepare 3ThSb,3ThBi,and 4ThBi resulted in the
isolation, respectively,ofvariable levels of red (Me3Si)2Sb@
Sb(SiMe3)2(Sb2),[11,19] and green Bi2,verified by single-crystal
X-ray diffraction. Since the Th4+/Th3+reduction potential is
very negative,[20] homolytic Th@Pn bond cleavage seems
unlikely,but on one occasion, from otherwise intractable,
complex reaction mixtures,crystals of the cyclometallate
complex [Th{N(CH2CH2NSiMe2But)2(CH2CH2NSiMeBut-
CH2)}(DME)] (5)were isolated from an attempted prepara-
tion of 3ThSb.[11] This suggests that aconcerted or step-wise
deprotonation/cyclometallation–dehydrocoupling of HPn-
(SiMe3)2reaction occurs for thorium.
To probe the mechanistic aspects further we investigated
thermal and photolytic U@Pn reactivity profiles ;analogous
thorium studies gave no different outcomes to the ones
described above.Complex 4USb,and putative 4UBi prepared
in situ, completely decompose at 8088Ctogive [U(TrenTIPS)]
and Sb2or Bi2,respectively,consistent with homolytic bond
cleavage.Under these conditions, 4UP and 4UAs and
surprisingly 3USb and 3UBi show little decomposition
(<5%). Photolysis (125 WUVlamp,2h) of 4USb and
putative 4UBi prepared in situ results in conversion into the
cyclometallate complex [U{N(CH2CH2NSiPri3)2(CH2CH2N-
SiPri2CHMeCH2)}][21] and elemental Sb or Bi. Interestingly,
photolysis of 3USb and 3UBi results in initial formation of Sb2
or Bi2,H
2,and the cyclometallate
[U{N(CH2CH2NSiMe2But)2(CH2CH2NSiMeButCH2)}],[22]
but 3UP and 3UAs show little decomposition under photo-
lytic conditions.Extended photolysis of 3USb and 3UBi
resulted in Sb2/Bi2decomposition to elemental Sb/Bi, which
was verified by independent decomposition of Sb2/Bi2under
the same conditions.Under photolytic conditions,[U(TrenR)]
species slowly cyclometallate with elimination of H2.So, the
above data suggest that for U@Pn bonds homolysis is the
preferred decomposition route,which may proceed to pro-
duction of uranium–cyclometallate and elemental pnictide
deposition under photolytic but not thermal conditions,but
the Th@Pn linkages undergo acid–base/dehydrocoupling
reactions owing to the redox robustness of thorium. The
more facile decomposition of 4USb and “4UBi” compared to
3USb and 3UBi suggest that although sterically demanding
ligands are necessary to stabilise these polar U@Pn linkages
that TrenTIPS may be too bulky and actually destabilise the U@
Sb/U@Bi linkages;the isolation of 3UBi is thus remarkable
because this complex has the facile uranium redox bond
homolysis route open to it yet it is still isolable.This is clearly
adelicate balance of sterics,since for the larger thorium
neither TrenDMBS nor TrenTIPS can stabilise Th@Bi linkages.
Such afine balance of metal/ligand size ratio has been found
previously with respect to m-phosphido linkages where
thorium–TrenTIPS gives stable ThPThlinkages but uranium–
TrenTIPS results in UPU linkages that readily decompose.[9a,d]
To conclude,wehave reported the synthesis and charac-
terisation of new An@Pn bonds that include the first
structurally authenticated U@Sb and Th@Sb bonds of any
kind and the first 2c–2e U@Bi bond. Thecorresponding Th@Bi
bond was too unstable to isolate,highlighting the major
challenges of preparing these mismatched hard–soft linkages
generally.These complexes present chemical bond bench-
marks,and the U@Bi bond is the heaviest 2c–2e pairing of two
elements involving an actinide under macroscopic,ambient
conditions,exceeded only by An@An pairings in matrix
isolation experiments.Preparing homologues spanning non-
metal, metalloid, and metal within asingle element group has
permitted elucidation of aformal periodic break-point
between As and Sb.Reactivity studies suggest that the
heavier U@Pn bonds decompose by homolytic U@Pn bond
cleavage,and the resulting uranium(III) and di-pnictane
compounds react further to give uranium(IV)–cyclometallate,
hydrogen, and elemental pnictide,respectively,whereas the
more redox robust thorium complexes engage in an acid–
base/dehydrocoupling route to give thorium–cyclometallate
and di-pnictane.Thus,although the same product classes
emerge from these decomposition reactions overall they
proceed via different mechanistic routes,highlighting the
different redox chemistries of uranium and thorium actinide
elements.
Acknowledgements
We thank the Royal Society (grant UF110005), EPSRC (grant
EP/M027015/1), ERC (grant CoG612724), Universities of
Manchester and Regensburg,the Deutsche Forschungsge-
meinschaft, and COST Action CM1006 for generously
supporting this work. We thank the reviewers for their
constructive comments.
Conflict of interest
Theauthors declare no conflict of interest.
Keywords: metalloids ·metal–metal bonds ·pnictides ·
thorium ·uranium
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[23] CCDC 1585332 –1585346 contain the supplementarycrystallo-
graphic data for this paper.These data are provided free of
charge by TheCambridgeCrystallographic Data Centre.
Manuscript received:November17, 2017
Acceptedmanuscript online: December 12, 2017
Version of record online: December 29, 2017
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