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German Edition:DOI:10.1002/ange.201706002
Uranium Complexes International Edition:DOI:10.1002/anie.201706002
Crystalline Diuranium Phosphinidiide and m-PhosphidoComplexes
with Symmetric and Asymmetric UPU Cores
Thomas M. Rookes,Benedict M. Gardner,G#bor Bal#zs,Matthew Gregson, Floriana Tuna,
Ashley J. Wooles,Manfred Scheer,* and Stephen T. Liddle*
Abstract: Reaction of [U(TrenTIPS)(PH2)] (1,TrenTIPS =
N(CH2CH2NSiPri3)3)with C6H5CH2Kand [U(TrenTIPS)-
(THF)][BPh4](2)afforded arare diuranium parent phosphi-
nidiide complex [{U(TrenTIPS)}2(m-PH)] (3). Treatment of 3
with C6H5CH2Kand two equivalents of benzo-15-crown-5
ether (B15C5) gave the diuranium m-phosphido complex
[{U(TrenTIPS)}2(m-P)][K(B15C5)2](4). Alternatively,reaction
of [U(TrenTIPS)(PH)][Na(12C4)2](5,12C4 =12-crown-4
ether) with [U{N(CH2CH2NSiMe2But)2CH2CH2NSi(Me)-
(CH2)(But)}] (6)produced the diuranium m-phosphido com-
plex [{U(TrenTIPS)}(m-P){U(TrenDMBS)}][Na(12C4)2][7,
TrenDMBS =N(CH2CH2NSiMe2But)3]. Compounds 4and 7
are unprecedented examples of uranium phosphido complexes
outside of matrix isolation studies,and they rapidly decompose
in solution underscoring the paucity of uranium phosphido
complexes.Interestingly, 4and 7feature symmetric and
asymmetric UPU cores,respectively,reflecting their differing
steric profiles.
Inrecent years there has been burgeoning interest in the
synthesis and chemistry of uranium–ligand multiple bonds,[1]
which stems from adesire to better understand the chemical
bonding of uranium and to correlate this to observed
physicochemical properties.However,most progress has
been made regarding complexes where uranium engages in
aformal multiple bond to C-/N-/O-based donor ligands,and
examples of second row-centered, and beyond, donor ligands
generally continue to be rare.[2] Where uranium–phosphorus
multiple bonding is concerned,[3] only two structurally
authenticated phosphinidene complexes have been
reported,[4] and investigations into uranium phosphido com-
plexes are exceedingly rare and restricted to cryogenic matrix
isolation and/or computational studies.[5] Thus,there are no
reports of uranium phosphido complexes on macroscopic
scales under conditions that would permit further investiga-
tion;indeed, the phosphido linkage,whether terminal or m-
bridging,remains arelatively rare structural motif even in
transition-metal chemistry.[6]
As part of our work on actinide–ligand multiple bonds,[7]
we reported dithorium phosphido and arsenido complexes
that are supported by the very sterically demanding triamido-
amine ligand N(CH2CH2NSiPri3)3(TrenTIPS).[7a,d] Forthe
ThPThderivative this ligand combination produced aseem-
ingly optimal balance of steric shielding of the ThPThcore
versus inter-TrenTIPS steric repulsion. We therefore considered
whether the analogous diuranium complex might be acces-
sible;however,uranium has potentially deleterious and facile
redox chemistry compared to the more redox-robust thorium,
and is smaller than thorium by 0.05–0.18 c,[8] so uranium with
the same ligand set might well be too strained to form astable
UPU linkage and could very easily decompose.Herein,
however, we report two different methods for the bulk-scale
preparation and subsequent characterization of diuranium m-
phosphido complexes,utilizing Tr enTIPS and the related
TrenDMBS (TrenDMBS =N(CH2CH2NSiMe2But)3)ligands,that
are the first examples of uranium phosphido complexes
outside of cryogenic spectroscopic experiments.[5b,c] These
complexes can be isolated and manipulated in the solid state,
but we find that they are indeed highly sensitive and
decompose rapidly in solution, which is in-line with the
prior absence of any synthetically accessible actinide phos-
phido complexes.Interestingly,depending on the steric
profiles of the Tren ligands that support these phosphido
complexes,symmetric and asymmetric UPU cores are
observed in the solid state structures.
Our initial approach was to target aUP(H)U core via
deprotonation/salt elimination and then effect deprotonation
to give aphosphido complex. Accordingly,sequential treat-
ment of the uranium(IV) phosphanide complex [U(TrenTIPS)-
(PH2)] (1)[4a] with benzyl potassium and then the separated
ion pair [U(TrenTIPS)(THF)][BPh4](2)[4a] afforded, after
work-up and recrystallization, dark red-brown crystals of
the diuranium(IV) parent phosphinidiide complex [{U-
(TrenTIPS)}2(m-PH)] (3)in67% isolated yield, Scheme 1.[9]
Thesynthesis of 3requires 2as elimination of KBPh4is
favorable owing to the outer sphere nature of the BPh4@anion
in 2whereas any uranium-coordinated halide is not displaced
by the relatively soft Pcenter.[4a] The 1HNMR spectrum of 3
spans the range @27 to +8ppm and the 29Si NMR spectrum
exhibits asingle resonance at +11.6 ppm, which are both
[*] T. M. Rookes, Dr.B.M.Gardner,Dr. M. Gregson, Dr.F.Tuna,
Dr.A.J.Wooles, Prof. 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-tsstrasse 31, 93053 Regensburg (Germany)
E-mail:manfred.scheer@ur.de
Supportinginformation and the ORCID identification number(s) for
the author(s) of this article can be found under:
https://doi.org/10.1002/anie.201706002.
T2017 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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consistent with the uranium(IV) formulation of 3.[10] No
31PNMR resonance could be detected for 3,most likely due
to the phosphinidiide being bonded to two uranium(IV) ions.
TheATR-IR spectrum of 3exhibits aweak, broad feature at
approximately 2169 cm@1,consistent with the presence of the
m-PH unit.[11] SQUID magnetometry on powdered 3gives
magnetic moments of 4.3 and 0.8 mBat 298 and 2K,
respectively,with asteady fall in-between these two extremes.
These data are entirely consistent with the presence of two
uranium(IV) ions in 3and alow temperature magnetic
moment that is tending to zero and dominated by temper-
ature independent paramagnetism from the spin-orbit cou-
pled ground-state multiplet of 3H4uranium. Ashoulder in the
cvs. Tdata is apparent at about 25 Kwhich is most likely due
to single-ion crystal field effects rather than any magnetic
exchange.[12] Confirmation of the formulation of 3was
provided by the solid state crystal structure,Figure 1, which
reveals U–P distances of 2.8187(12) and 2.8110(12) cthat,
considering steric profiles,compares well to aU–P distance of
2.743(1) cin [{U(C5Me5)2(OMe)}2(m-PH)][11] and the sum of
the single-bond covalent radii of uranium and phosphorus
(2.81 c).[8]
With complex 3secured, we attempted deprotonation of
the phosphinidiide group.Treatment of 3with one equivalent
of benzyl potassium in the presence of two equivalents of
benzo-15-crown-5 ether (B15C5, to completely sequester the
Kion) produced, after work-up and recrystallization, asmall
crop (<5% yield) of black crystals of the diuranium(IV) m-
phosphido complex [{U(TrenTIPS)}2(m-P)][K(B15C5)2](4),
Scheme 1.[9] Thesolid-state crystal structure of 4,Figure 1,
confirms the separated ion pair formulation and reveals U–P
Scheme 1. Synthesis of complex 3from 1and 2,the conversion into 4,
and the formation of 7from 5and 6.B15C5 =benzo-15-crown-5 ether,
12C4=12-crown-4 ether,Bn=benzyl.
Figure 1. Molecular structures of 3(top) and the anion components of
4(middle),and 7(bottom) at 150 K. Displacementellipsoids set at
40%probability and non-phosphorus-bound hydrogen atoms, minor
disorder,and cation components are omitted for clarity.Ugreen,
Ppurple, Nblue, Si yellow.Selected bond lengths [b]: 3,U1–P1 2.8187-
(12), U2–P1 2.8110(12), U1–N1 2.270(4), U1–N2 2.258(4), U1–N3
2.264(4), U1–N4 2.685(5), U2–N5 2.254(5), U2–N6 2.253(4), U2–N7
2.265(4), U2–N8 2.682(6); 4,U1–P1 2.653(4), U2–P1 2.665(4), U1–N1
2.330(8), U1–N2 2.277(10), U1–N3 2.305(9), U1–N4 2.766(9), U2–N5
2.308(8), U2–N6 2.307(12), U2–N7 2.296(10), U2–N8 2.745(9); 7,U1–
P1 2.657(2), U2–P1 2.713(2), U1–N1 2.309(4), U1–N2 2.309(5), U1–
N3 2.324(4), U1–N4 2.765(4), U2–N5 2.276(5), U2–N6 2.284(5), U2–
N7 2.263(5), U2–N8 2.840(5).[20]
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distances of 2.653(4) and 2.665(4) c,which represents acon-
traction of approximately 0.15 cfrom 3.The U–P distances in
4can be considered to be short when considering the bridging
nature of the phosphido;for example,although the sum of the
covalent uranium and phosphorus double bond radii is
2.36 c,[8] the U=Pdistances in the terminal uranium(IV)
phosphinidene complexes [U(TrenTIPS)(PH)][K(B15C5)2][4a]
and [U(C5Me5)2(P-2,4,6-But3C6H2)(OPMe3)][4b] are 2.613(2)
and 2.562(3) c,respectively.Furthermore,the Th–P distances
in [{Th(TrenTIPS)}2(m-P)][Na(12C4)2](12C4 =12-crown-4
ether)[7d] are significantly longer [2.735(2)/2.740(2) c]than
the U–P distances in 4,even when factoring in the covalent
radii differences between thorium and uranium.[8] TheU–
Namide distances are around 0.1 clonger than is typical for
uranium(IV) Tren complexes,[13] reflecting the anionic for-
mulation of the phosphido moiety.Wenote that the U–Namine
distances are long,which infers a trans-influence from the
phosphido ligand,[7d] but this cannot be stated with confidence
due to U@Nbond lengthening from the anionic formulation.
Complex 4decomposes in solution, which, together with
the low yield, precluded further characterization beyond the
X-ray crystal structure and elemental analyses.The reaction
that produces 4is highly capricious,and despite exhaustive
attempts the reaction conditions could not be improved;
sometimes deprotonation of 3fails,orcomplete decomposi-
tion occurs to unidentified products.Use of different organo-
alkali-metal reagents,the presence or absence of different
crown ethers,orincreasing the molar quantity of benzyl
potassium results in intractable reaction mixtures and/or
production of the known uranium(IV) cyclometallate com-
plex [U{N(CH2CH2NSiPri3)2(CH2CH2NSiPri2C(H)(Me)-
(CH2)}],[14] where the fate of the phosphorus-containing
products could not be determined.
Theabove mentioned observations likely reflect the
inherently polarized, weak, and labile nature of these U–P
linkages,asreflected by the paucity of any other macroscopic
molecular uranium phosphido complexes,and also likely
steric overloading from close proximity of two TrenTIPS
ligands.Inorder to reduce this steric strain and perhaps
obtain amore tractable phosphido complex, we adopted
adifferent strategy to introduce asterically less demanding
Tren ligand.
Reaction of the new terminal uranium(IV)-phosphini-
dene complex [U(TrenTIPS)(PH)][Na(12C4)2](5),[9] which is
only the third example of auranium phosphinidene,with the
uranium(IV) cyclometallate complex [U-
{N(CH2CH2NSiMe2But)2CH2CH2NSi(Me)(CH2)(But)}] (6)[15]
proceeds by protonolysis to give the diuranium m-phosphido
complex [{U(TrenTIPS)}(m-P){U(TrenDMBS)}][Na(12C4)2](7),
isolated as dark brown crystals in 29%yield, Scheme 1.[9]
Thecrystalline yield is low due to the oily nature of 7,and the
decomposition that occurs once it is formed (see below). The
solid-state crystal structure of 7,Figure 1, is in gross terms
very similar to that of 4,noting the change of Tr en ligand and
cation component. However,the U–P distances of 2.657(2)
and 2.713(2) care notable in that the shorter is consistent
with the U–P distances in 4,but the longer is significantly
longer and mid-way to the U–P distances in 3.Interestingly,
the shorter U–P distance is found for the TrenTIPS-bound
uranium with the longer U–P distance associated with the
sterically less demanding TrenDMBS portion, and the U@N
bonds are longer in the TrenTIPSUportion of the molecule
compared to those in the TrenDMBSUfragment, perhaps
reflecting the asymmetry of the phosphido bonding.
Thepresence of uranium(IV) ions in 7was confirmed by
SQUID magnetometry on apowdered sample of 7;the
magnetic moments of 4.3 and 1.1 mBat 298 and 2K,
respectively,are consistent with the presence of uranium(IV)
ions.However the magnetic moment of 7at 2Kis higher than
the corresponding data for 3,which may represent the relative
crystal-field effects on uranium(IV) from HP2@versus P3@;the
P3@would be expected to present agreater point charge and
splitting of the paramagnetic excited states manifold, so alow-
lying group are still populated to some extent at low
temperature with ahigher-lying group at high temperature
that are more difficult to populate.This notion is consistent
with aslightly flatter magnetic trace at high temperature for 7
compared to 3and has been noted in other uranium(IV)
complexes with strong point-charge ligands.[2h,3b,7f,h,16] Inter-
estingly,counter to expectations the shoulder at about 25 K
for the magnetic data of 3is much less pronounced for 7which
is consistent with our suggestion that this feature is due to
single ion crystal field effects and not magnetic exchange,[12]
though magnetic exchange cannot be completely ruled out.
Complex 7is moderately more stable than 4,but although,
once isolated, solid state characterization methods were
feasible we find that redissolving 7results in rapid decom-
position so NMR and optical spectroscopic data were
unobtainable.Interestingly,wefind that the majority decom-
position products of 7are the uranium(IV)-cyclometallate
complex [U{N(CH2CH2NSiPri3)2(CH2CH2NSiPri2C(H)(Me)-
(CH2)}],[14] and what we deduce to be [U(TrenDMBS)(PH)]-
[Na(12C4)2], though the latter is not sufficiently sterically
protected so decomposes to unidentified products.Never-
theless,the more clear-cut nature of the decomposition of 7
compared to 4is instructive because it suggests that even with
reduced ligand steric demands the UPU unit is inherently
unstable.Interestingly,the decomposition reaction of 7
produces acyclometallate with aless-strained 5-membered
metallocyclic ring compared to the more-strained 4-mem-
bered metallocycle in 6.This aspect is also consistent with the
observation that mixing the five-membered-ring cyclometal-
late [U{N(CH2CH2NSiPri3)2(CH2CH2NSiPri2C(H)(Me)-
(CH2)}][14] and known [U(TrenTIPS)(PH)][K(B15C5)2][4a]
gives no reaction. Thus,the importance of metallocyclic
ring-strain as akey factor in driving the protonolysis reaction
to generate 7emerges.This point is underscored when
considering that on the basis of the solid-state structure the
phosphido appears to be more associated with the TrenTIPSU
fragment rather than the TrenDMBSUgroup,but it is the
TrenTIPSUfragment that is,inessence,the leaving group
during decomposition.
To gain agreater understanding of the bonding in the
UPU units of 4and 7,wecarried out DFT calculations on the
full anion components of these compounds, 4@and 7@,
respectively.Considerable difficulty was encountered obtain-
ing SCF-converged structures,which suggests that 4@and 7@
have multi-reference ground states.However, satisfactorily
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converged models that provide aqualitative description of the
electronic structure of these compounds could be obtained.
Both 4@and 7@exhibit four unpaired electrons of
essentially exclusive 5f character in their a-spin manifolds
as HOMO to HOMO@3, which is consistent with the
presence of two 5f2uranium(IV) ions.HOMO@4to
HOMO@6ineach case represent the principal bonding
components in the UPU units,see Figure 2and the Support-
ing Information,[9] confirming the presence of polarized
uranium–phosphido triple bonding interactions.The uranium
spin densities of @2.31/@2.33 and @2.17/@2.22, for 4@and 7@
respectively,show donation of electron density from the
ligands to uranium and support the uranium(IV) formula-
tions.The uranium charges are high for Tr en uranium(IV)
complexes,[17] at +3.79/ +3.86 for 4@and +3.52/ +3.87 for 7@
and the phosphido charges are @2.19 and @2.35, respectively.
Interestingly,the uranium ion in 7@which has the closest
association with the phosphido,that is,TrenTIPSU, has the
highest charge and lowest spin density,and recall that the U–
Ndistances are longer for that unit than the TrenDMBSUunit;
this suggests that the Natoms are better as aunit at charge
donation to uranium than the phosphido.[18]
TheUPMayer bond orders reflect multiple,but polarized
bond interactions.Specifically,in4@they are 1.41/1.43
whereas for 7@they are 1.44/1.66 reflecting the asymmetric
UP distances and bonding in the UPU core in 7@;notably,
these UP bond orders are in-line with the situation in the
Lewis bonding scheme for these units,that is,U
=P=U. These
Mayer bond orders should be viewed in the context that the
UNamide and UNamine bond orders are 0.71 and 0.18, respec-
tively,and they are surprisingly invariant across 4@and 7@.
Thebond topological data are remarkably invariant,
showing polar, quite ionic UP bonds with 1values of 0.06
(typically 1>0.1 for covalent bonds) and bond ellipticities
that are zero or close to zero[9] reflecting the formal triple
bond interactions that constitute cylindrical distributions of
electron density with respect to the inter-nuclear axes.[19] Polar
UP bonding is also suggested by NBO analyses,which finds
UP s-bonds with 16%Uand 84%Pcharacter (U:
1:1:69:29 %7s:7p:5f:6d;P:100%3p) and UP p-bonds with
26%Uand 74%Pcharacter (U:0:1:54:45 7s:7p :5f:6d;P:
100%3p).
Thedata above unequivocally suggest that the UPU
interactions in 4@and 7@are polarized and weak, which is
consistent with the observed instability of 4and 7.Interest-
ingly,the UP bonds for 4@and 7@have higher Mayer bond
orders,exhibit more metal component, and utilize more 5f
character (relative to 6d) than the ThPbonds in [{Th-
(TrenTIPS)}2(m-P)][Na(12C4)2],[7d] consistent with the general
view that uranium engages in more covalent bonding,and
with greater 5f character,than thorium, but we note that the
bond topological data are essentially invariant for uranium
and thorium. This suggests that the instability of 4and 7is
most likely of kinetic origin.
To conclude,wehave reported two structurally authenti-
cated examples of uranium phosphido complexes.These
linkages are unprecedented outside of cryogenic matrix
isolation conditions,remain rare even in the d-block, and
indeed uranium–phosphorus multiple bonding remains
exceedingly rare overall. These complexes have been pre-
pared on macroscopic scales by two different methodologies
that could greatly expand uranium–phosphido chemistry:
1) construction of aUP(H)U unit by salt elimination and
subsequent deprotonation;or2)protonation of acyclometal-
late by aparent phosphinidene.Although both complexes can
be prepared and isolated they exhibit intrinsic instability that
is consistently reflected in quantum chemical calculations.
Low-temperature magnetism studies also suggest differences
in the relative crystal-field effects on uranium(IV) from HP2@
versus P3@.Most intriguingly,the UP bond lengths can be
perturbed by co-ligand steric demands,which suggests that
with suitably chosen co-ligands perhaps aUPU linkage,or
perhaps aUPM unit that might be prepared by method (2),
could be polarized to the point of rupture in order to produce
aterminal uranium phosphido complex under ambient
conditions.Efforts in that regard are on-going.
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.
Figure 2. Kohn–Shamfrontier molecular orbitals of that represent the principal bonding components of the UPU unit in the anion component of
7,7@:Left, HOMO@6(403a, @1.332 eV);Middle, HOMO@5(404a, @0.997 eV);Right, HOMO@4(405a, @0.983 eV). Hydrogen atoms are
omitted for clarity.
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Conflict of interest
Theauthors declare no conflict of interest.
Keywords: density functional theory ·metal–
ligand multiple bonding ·phosphido ·phosphinidiide ·uranium
Howtocite: Angew.Chem. Int. Ed. 2017,56,10495 –10500
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[20] CCDC 1554770, 1554771, 1554772 and 1554773 contain the
supplementary crystallographic data for this paper.These data
can be obtained free of charge from TheCambridge
Crystallographic Data Centre.All other data are available
from the corresponding authors upon request.
Manuscript received:June 13, 2017
Acceptedmanuscript online: July 5, 2017
Version of record online: July 24, 2017
A
ngewandte
Chemie
Communications
10500 www.angewandte.org T2017 The Authors. Published by Wiley-VCH Verlag GmbH &Co. KGaA, Weinheim Angew.Chem.Int. Ed. 2017,56,10495 –10500