Electronic Delocalization in Two and Three Dimensions: Differential Aggregation in Indium “Metalloid” Clusters

Article abstract of DOI:10.1002/anie.201708496

Reduction of indium boryl precursors to give two- and three-dimensional M?M bonded networks is influenced by the choice of supporting ligand. While the unprecedented nanoscale cluster [In₆₈(boryl)₁₂]^? (with an In₁₂@In₄₄@In₁₂(boryl)₁₂ concentric structure), can be isolated from the potassium reduction of a bis(boryl)indium(III) chloride precursor, analogous reduction of the corresponding (benzamidinate)In^IIIBr(boryl) system gives a near-planar (and weakly aromatic) tetranuclear [In₄(boryl)₄]^2? system.

Full text of DOI:10.1002/anie.201708496

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Accepted Article
Title: Electronic delocalization in two and three dimensions: differential
aggregation in indium 'metalloid' clusters
Authors: Simon Aldridge, Andrey Protchenko, Juan Urbano, Joseph
Abdalla, Jesus Campos, Dragoslav Vidovic, Andrew Schwarz,
Matthew Blake, Philip Mountford, and Cameron Jones
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201708496
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10.1002/anie.201708496
Angewandte Chemie International Edition
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Electronic delocalization in two and three dimensions: differential
aggregation in indium ‘metalloid’ clusters
Andrey V. Protchenko,a Juan Urbano,a,b Joseph A.B. Abdalla,a Jesús Campos,a,c Dragoslav Vidovic,a,d
Andrew D. Schwarz,a Matthew P. Blake,a Philip Mountford,a Cameron Jones,e and Simon Aldridgea,*
R4In4.[11] Planar systems offering the potential for Hückel-type
Abstract: Reduction of indium boryl precursors to give two- and
three-dimensional M-M bonded networks is influenced by the choice
of supporting ligand. While the unprecedented nanoscale cluster
[In68(boryl)12]- (with an In12@In44@In12(boryl)12 concentric structure),
aromaticity, and concentric polyhedral clusters are unknown: the
largest polynuclear indium system yet reported to our knowledge
is a compound of composition In19X6,[11q] which features a cubic-
close-packed In13 core closely related to the tetragonal structure
of the metal itself.
Recently we have been exploiting the extremely sterically
demanding boryl ligand [B(NDippCH)2] as an ancillary donor for
the stabilization of novel main group metal systems,[11p,11q,12,13]
and in the current contribution report on its use for the
encapsulation of highly reduced indium clusters with widely
differing degrees of nuclearity.
can be isolated from the potassium reduction of
a
bis(boryl)indium(III) chloride precursor, analogous reduction of the
corresponding (benzamidinate)InIIIBr(boryl) system gives a near-
planar (and weakly aromatic) tetranuclear [In4(boryl)4]2- system.
M
etal-metal bonded compounds of aluminium and gallium
have been the subject of a number of landmark studies in the
last 20 years,[1] ranging from dimetallic systems such as
[2]
Reduction of the InIII bisboryl complex In{B(NDippCH)2}2Cl
with Cp*2Sm(thf) has previously been shown to offer clean
access to the InII radical In{B(NDippCH)2}2.[11p] The corres-
ponding chemistry using a more aggressive reductant such as
potassium metal, also leads to the formation of the InII complex
(albeit in lower yields), but also to the formation of more highly
reduced species, including indium metal itself, and a metalloid
cluster of the composition [In68(boryl)12]-, which can be isolated in
very low yield and characterized by X-ray crystallography
(Figure 1).[14] Aspects of the geometric structure of this cluster
are reminiscent of the landmark [Al77{N(SiMe3)2}20]2- system
{(Me3Si)2CH}2AlAl{CH(SiMe3)2}2 and the ‘digallyne’ Na2[ArTrip
2
Ga2] (ArTrip
=
2,6-(2,4,6-iPr3C6H2)2C6H3)),[3] through planar
trinuclear systems of the type Na2[ArMes3M3] (M = Al, Ga; ArMes
=
2,6-(2,4,6-Me3C6H2)2C6H3),[4] all the way to large ‘metalloid’
clusters such as [Al77{N(SiMe3)2}20]2- and [Ga84{N(SiMe3)2}20]n- (n
= 3, 4).[5,6] Such systems have proved pivotal to the evolution of
theories of chemical bonding for the heavier p-block elements, in
particular for the formulation of delocalized bonding models in
one, two and three dimensions. Thus, intense debate
concerning the bond order in Na2[ArTrip2Ga2] and related trans-
bent alkyne analogues ultimately led to a fuller understanding of
their electronic structure and the unusual (‘transition metal like’)
reactivity displayed by some such systems.[7] In addition, planar
2π electron metallacycles have pushed the boundaries of what is
understood by ‘aromatic’ character,[8,9] and larger concentric
polyhedral clusters have been identified as nanoscale models
for bulk metallic phases.[10]
reported
by
Schnöckel
in
1997.[5a]
Thus,
the
In12@In44@In12(boryl)12 formulation revealed crystallographically
features an icosahedral In12 unit and surrounding In44 shell
similar to those found in the Al@Al12@Al44@Al20(amido)20
system. Presumably the lower nuclearity of the outermost shell
(containing 12 indium atoms, as opposed to 20 aluminiums)
reflects the greater steric bulk of the attached B(NDippCH)2
ligands, compared to N(SiMe3)2. The other substantive differ-
ence is the absence of a central metal atom in [In68(boryl)12]-,
with no significant q-peak being located in the X-ray structure
Related compounds featuring the heavier metals of group 13
(i.e. indium and thallium) are much less well known, with In-In
bonded systems being dominated by dinuclear InII systems of
the type R2InInR2 and tetrahedral InI systems of the type
[a]
Dr A. V. Protchenko, Dr J Urbano, Dr J.A.B. Abdalla, Dr J. Campos,
Dr D. Vidovic, Dr A.D. Schwarz, Dr M.P. Blake, Prof P. Mountford,
Prof S. Aldridge
Inorganic Chemistry Laboratory, Department of Chemistry,
University of Oxford, South Parks Road, Oxford, OX1 3QR, UK
E-mail: simon.aldridge@chem.ox.ac.uk
Dr J. Urbano
Departamento de Química y Ciencia de los Materiales, CIQSO,
Edificio Robert H. Grubbs, Campus de el Carmen, Universidad de
Huelva, Spain
[b]
[c]
Dr J. Campos
Instituto de Investigaciones Químicas (IIQ), Consejo Superior de
Investigaciones Científicas (CSIC) and Universidad de Sevilla,
Avda. Américo Vespucio, 49, 41092 Sevilla, Spain
Dr D. Vidovic
SPMS-CBC, Nanyang Technological University, 21 Nanyang Link,
Singapore 637371
[d]
[e]
+
Figure 1. One of the anionic components of [Li9InII6{B(NDippCH)2}6(Cl)(OH)13
]
Prof C. Jones
[In68{B(NDippCH)2}12]- in the solid state as determined by X-ray crystallography
(see SI for full details). Other anion/cation pair and hydrogen atoms omitted
and boryl ligands shown in wireframe format for clarity. The colour scheme
reflects the In12@In44@In12(boryl)12 structure, with the central In12 icosahedron
shown in red, the surrounding shell of 44 non-ligated In atoms shown in grey
School of Chemistry, PO Box 23, Monash University, Melbourne,
VIC 3800, Australia
Supporting information for this article is given via a link at the end of
the document. Structural data has been deposited with the CCDC
(ref: 1563288-1563299)
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10.1002/anie.201708496
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and the outer 12 boryl-bound indium centres shown in yellow. The cluster is
shown orientated along its crystallographic six-fold rotation axis.
refinement close to the centre of the In12 icosahedron (R factor =
0.07). While a number of metalloid clusters of this type –
featuring concentric shells of metal atoms – have been reported
for aluminium and gallium,[5,10] no significant precedent exists for
indium, with the closest analogue being an In19(boryl)6 system
which features
a
cubic-close-packed In13 core.[11q] The
icosahedral In12 core of [In68(boryl)12]- features a relatively
narrow range of In-In bond lengths [3.100(1)-3.145(2) Å], and
can be put in context by the corresponding distances measured
for the M12 cores of related aluminium and gallium systems (e.g.
Figure 2. Molecular structures of 2-InBr (left) and 3 (right) in the solid state as
determined by X-ray crystallography. Hydrogen atoms omitted and Dipp iPr
groups shown in wireframe format for clarity; thermal ellipsoids shown at the
35% probability level. Key bond lengths (Å) and angles (o): (for 2-InBr): In(1)-
B(1) 2.223(1), In(1)-Br(1) 2.545(1), In(1)-N(3) 2.187(1), In(1)-N(4) 2.179(1),
N(3)-C(27)-N(4) 114.3(1), N(3)-In(1)-N(4) 61.3(1); (for 3): B(1)-C(27) 1.614(2),
B(2)-Br(1) 1.926(2), B(2)-N(3) 1.410(2), B(2)-N(4) 1.412(2), N(3)-B(2)-N(4)
95.7(1), N(3)-C(27)-N(4) 87.4(1).
2.650-2.762
Å for [Al22Br20(thf)12] and 2.593-2.614 Å for
[Ga22Br2{N(SiMe3)2}10Br10]2-) with due consideration of the
metallic radii of the respective elements (Al: 1.43 Å; Ga: 1.40 Å;
In: 1.58 Å).[15]
ESI) features the expected N,N'-chelating benzamidinate ligand
and a four-coordinate metal geometry. The E-B distances
[2.114(2)/2.111(2), 2.062(2)/2.064(2) and 2.223(1) Å, respect-
ively] are consistent with the presence of conventional 2-centre
2-electron covalent bonds, being similar to those previously
In order probe the potential role of the ancillary ligands in
mediating the reduction process(es), we targeted the use of
chelating ancillary ligands, hypothesizing that such systems
might reduce the extent and/or rate of metal-metal aggregation
processes. Strong N,N'-chelating mono-anionic donors such as
amidinato, guanidinato and β-diketiminato ligands have
previously been applied to the stabilization of low-valent main
group compounds.[16] We therefore targeted the synthesis and
reduction chemistry of group 13 systems of the type
(amidinato)E(boryl)X (X = Cl, Br).[17,18] In the event, such
systems prove to be readily accessible for E = Al, Ga and In, via
metathesis chemistry utilizing the corresponding amidinato
element dihalide, {PhC(NiPr)2}EX2 (1-EX) and one equivalent of
Yamashita’s boryllithium reagent (thf)2Li{B(NDippCH)2} (Scheme
1).[12] Only in the case of the boron-containing precursor 1-BBr is
any divergence observed from this pattern of reactivity, with an
alternative product being formed resulting from C- rather than B-
centred attack by the boryl nucleophile (Scheme 1 and Figure
2).[19] For aluminium, gallium and indium, however, the
respective mono-boryl products 2-AlCl, 2-GaCl, 2-GaBr and 2-
InBr can be accessed via E-B bond formation, and each has
been characterized by standard spectroscopic, analytical and
(with the exception of 2-GaBr) crystallographic methods.
Structurally, each of 2-AlCl, 2-GaCl and 2-InBr (see Figure 2 and
reported for boryl complexes of the respective group 13 metals
[16a]
in the +3 oxidation state [e.g. Al: 2.150(2), 2.119(2) Å;
2.067(3), 2.098(2) Å;[16b] In: 2.245(7), 2.256(6) Å].[11p]
Ga:
With each of these (amidinato)boryl precursors in hand we
examined their reduction chemistry towards potassium metal
(Scheme 2). In the case of the chloro-aluminium and –gallium
systems 2-AlCl and 2-GaCl, no molecular group 13 metal-
containing products could be isolated from the reaction mixture,
while in the of 2-GaBr we could isolate and structurally
characterize trace amounts of a gallium containing species of
stoichiometry Ga2(boryl)2(amidinate),
4 (along with mainly
gallium metal; see ESI). The reduction of 2-InBr under similar
conditions, however, leads to the formation of a more highly
reduced metal-containing system, in accordance with the
established redox properties of gallium and indium.[20] As such,
NMR measurements are consistent with complete loss of the
Dipp
K
[B]
i
N
B
Pr
In
N
N
In
In
N
a)
[B]
[B]
Ph
In
In
Dipp
[B]
Dipp
Br
i
i
N
K
Pr
Pr
i
Pr
5
E = Al, Ga, In
X = Cl, Br
B
X
X
N
N
N
N
N
Scheme 2. Reduction of bromo-indium complex 2-InBr with potassium to
generate the cyclic tetraindium cluster K2[In4{B(NDippCH)2}4] (5). [B] =
B(NDippCH)2. Key reagents and conditions: a) K metal (4.4 equiv.), C6D6,
sonication for 1 h, 21%.
Ph
E
Ph
E
a)
Dipp
X
i
i
Pr
Pr
1-EX (E = B-In; X = Cl, Br)
2-EX ( E = Al, Ga, In; X = Cl, Br)
Dipp
i
N
Pr
B
N
N
N
1-BBr
B
Br
a)
Dipp
Ph
i
Pr
3
Figure 3. Molecular structure of 5 in the solid state as determined by X-ray
crystallography. Hydrogen atoms omitted and iPr/Dipp groups shown in
wireframe format for clarity; thermal ellipsoids shown at the 35% probability
level. Key bond lengths (Å) and angles (o): In(1)-In(1') 2.805(1), In(1)-In(1'')
Scheme 1. Reactions of benzamidinato-stabilized Group 13 halides with the
boryllithium reagent (thf)2Li{B(NDippCH)2}: syntheses of boryl-
aluminium, -gallium and –indium complexes of type 2, and backbone-borylated
diamido-bromoborane 3. Key reagents and conditions: a)
(thf)2Li{B(NDippCH)2} (1 equiv.), benzene, 2 h, 20-55%.
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2.838(1), In(1)-B(1) 2.316(2), In(1')-In(1)-In(1'') 89.6(1), In4 centroid-In(1)-B(1)
148.0.
pπ overlap for indium, exacerbated by the bending of the boryl
substituents away from the In4 least-squares plane (even in the
‘naked’ dianion), and hence the imperfectly co-linear arrange-
ment of indium 5p orbitals. Interestingly, a similar, moderately
puckered, structure is observed for the related (6π-electron)
system [Si4(SiMetBu2)4]2-, for which the NICS(1) value (+6.1)
implies a a non-aromatic description.[26]
amidinato ancillary ligand. The presence of a single boryl ligand
environment is also signalled by 1H NMR measurements, but
definitive characterization of the product as the cyclic tetraindium
tetraboryl system K2[In4{B(NDippCH)2}4] (5) is dependent on the
results of X-ray crystallographic studies (Figure 3). The solid-
state structure features an approximately square array of four
(symmetry-related) indium atoms, with each metal centre being
additionally bound to a single terminal boryl ligand. The In4 unit
is near-planar, with each indium atom lying 0.12 Å out of the
least-squares plane, and the internal In-In-In angles being
essentially 90o [89.6(1)o in each case]. The structure is
completed by two K+ counter-ions situated above and below the
In4 unit (K…In distances: 3.73 and 3.93 Å), which are
sandwiched between the flanking Dipp aryl rings of diagonally
opposite boryl ligands.
A consequence of these K-arene
contacts (3.17-3.37 Å) is a slight ‘puckering’ of the ligand
envelope of the [In4(boryl)4]2- unit, such that the In4 centroid-In-B
angles are non-linear [148.0o], and alternate boron atoms are
positioned above/below the approximate In4 plane.
HOMO (+1.85 eV)
HOMO (-2.85 eV)
The In-In contacts [2.805(1) and 2.838(1) Å] are similar to
those found in related systems containing In-In single bonds [e.g.
2.777(1) Å for In2{B(NDippCH)2}3][11p] and significantly shorter
than the intermetallic distances associated with the boryl-ligated
indium centres in clusters such as In19{B(NDippCH)2}6 and
[In68{B(NDippCH)2}12]- (means: 2.967 and 2.966 Å, respect-
ively).[11q] The near planar In4 core of 5, and the associated 2-
charge, offers the potential for Hückel aromatic character.[9]
Although 2π electron metallo-aromatic systems have been
reported previously for the lighter congeners aluminium and
HOMO-1 (+1.64 eV)
[In4{B(NDippCH)2}4]2-
HOMO-1 (-3.24 eV)
gallium (for example, M2[Ga3ArMes
]
(M
=
Na, K),[4a,b]
3
K2[In4{B(NDippCH)2}4]
Na2[Al3ArMes3],[4c] and K2[Ga4ArTrip
]
[21]), related systems are
2
Figure 4. Electron density surfaces and energies of key molecular orbitals for
[In4{B(NDippCH)2}4]2- (left) and K2[In4{B(NDippCH)2}4] (right) calculated by
Density Functional Theory (see ESI).
unprecedented for indium. Superficially, 5 is most similar to the
silyl-ligated system [(thf)Na]2[Ga4(SitBu3)4] reported by Wiberg
and co-workers, which has been described either in terms of a
2π aromatic [Ga4(silyl)4]2- dianion, or as a Wade-Mingos five
skeletal electron pair Na2Ga4 bicapped tetrahedron.[22,23] In the
case of 5, the latter description (while correct in electron
counting terms) is geometrically less appropriate, since the
central In4 unit is clearly a long way from being tetrahedral. Thus,
the In-In-In-In torsion angles in 5 are 9.6 and 9.7o, while the
corresponding parameters in the more puckered Ga4 unit of
[(thf)Na]2[Ga4(SitBu3)4] are 39.4o.
In conclusion, we have shown that the highly sterically
demanding boryl ligand B(NDippCH)2 is capable of supporting
two and three-dimensional aggregates featuring indium in very
low oxidation states, including an unprecedented [In68(boryl)12]-
concentric cluster and a near planar 2π-electron cyclic tetramer.
Acknowledgements
DFT calculations (carried out at the BP-TZP level) reveal that
the HOMO of the [In4{B(NDippCH)2}4]2- system is indeed a
delocalized π-type orbital extending in-phase over all four indium
centres, with the HOMO-1 possessing In-In σ-bonding character
(Figure 4). The inclusion of the K+ counter-cations perturbs the
orbital picture slightly: thus for K2[In4{B(NDippCH)2}4], greater
stabilization of the In-In π bonding MO due to the proximity of the
K+ cations means that this orbital is the HOMO-1, with the less
effectively stabilized σ-orbital now being the HOMO. Nucleus
Independent Chemical Shift (NICS) values have been calculated
for the potassium-free system [In4{B(NDippCH)2}4]2- [NICS(0) = -
4.92, NICS(1) = -6.53] and can be compared with the values
calculated using the same method for benzene [NICS(0) = -7.40,
NICS(1) = -9.61].[24] The π aromatic character is therefore shown
to be relatively weak,[25] presumably reflecting relatively poor pπ-
We thank the EPSRC (EP/L025000/1) and the Leverhulme Trust
(F/08 699/E) for funding aspects of this work.
Keywords: Group 13 • indium • gallium • boryl • metal cluster
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]
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[25] NICS(0) values of -23.4, -18.5 and -17.6 have been reported for the
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This article is protected by copyright. All rights reserved.

10.1002/anie.201708496
Angewandte Chemie International Edition
COMMUNICATION
COMMUNICATION
Andrey V. Protchenko, Juan Urbano,
Joseph A. B. Abdalla, Jesús Campos,
Dragoslav Vidovic, Andrew D. Schwarz,
Matthew P. Blake, Philip Mountford,
Cameron Jones, and Simon Aldridge*
Reduction of indium boryl complexes
to give 2- and 3-D aggregates can be
controlled by the choice of supporting
ligand. The unprecedented nanoscale
cluster [In68(boryl)12]-
Page No. – Page No.
(In12@In44@In12(boryl)12) is isolated
from the potassium reduction of a
bis(boryl) precursor, while reduction of
(amidinate)InIIIBr(boryl) gives a
(weakly aromatic) planar tetranuclear
[In4(boryl)4]2- system.
Electronic delocalization in two and
three
dimensions:
differential
aggregation in indium ‘metalloid’
clusters
This article is protected by copyright. All rights reserved.