Substitution of the laser borane: Anti -B18H22 with pyridine: A structural and photophysical study of some unusually structured macropolyhedral boron hydrides

Article abstract of DOI:10.1039/c7dt03823b

Reaction of anti-B₁₈H₂₂1 with pyridine in neutral solvents gives sparingly soluble B₁₆H₁₈-3′,8′-Py₂3a as the major product (ca. 53%) and B₁₈H₂₀-6′,9′-Py₂2 (ca. 15%) as the minor product, with small quantities of B₁₈H₂₀-8′-Py 4 (ca. 1%) also being formed. The three new compounds 2, 3a and 4 are characterized by single-crystal X-ray diffraction analyses and by multinuclear multiple-resonance NMR spectroscopy. Compound 2 is of ten-vertex nido:ten-vertex arachno two-atoms-in-common architecture, long postulated for a species with borons-only cluster constitution, but previously elusive. Compound 3a is of unprecedented ten-vertex nido:eight-vertex arachno two-atoms-in-common architecture. The single-crystal X-ray diffraction analysis for the picoline derivative B₁₆H₁₈(NC₅H₄Me)₂3b, similarly obtained, is also presented. B₁₈H₂₀Py 4 is also previously unreported but is of known ten-vertex nido:ten-vertex nido two-atoms-in-common architecture of anti configuration, but now with the pyridine ligand positioned differently to other reported examples of B₁₈H₂₀L compounds. Factors behind the remarkably low solubility of 3a and 3b are elucidated in terms of electrostatic potential (ESP) calculations, polarity, and van der Waals complementarities. In view of contemporary developing high interest in the fluorescent properties of macropolyhedral boron-containing species, a detailed assessment of the photophysical characteristics of 3a and 4 is also presented. In contrast to the thermochromic fluorescence of 2 (from 620 nm brick-red at room temperature to 585 nm yellow at 8 K, quantum yield 0.15), compound 3a is only weakly phosphorescent in the yellow region (590 nm, quantum yield 0.01), whereas compound 4 exhibits no luminescence. The far more photoactive nature of compound 2 is associated with S₁ excited-state minima structures that differ from each other only by the relative rotational positions of the pyridine substituents on its disubstituted ten-vertex {arachno-B₁₀Py₂}-subcluster. The wavelength and relative intensity of fluorescence from these structures depends on the rotational positions of the pyridine ligands, which in turn are influenced by temperature and/or rotational inhibition in the solid-state.

Full text of DOI:10.1039/c7dt03823b

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Francés-Monerris, K. Lang and W. Clegg, Dalton Trans., 2018, DOI: 10.1039/C7DT03823B.
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Substitution of the Laser Borane anti-B18H22 with Pyridine:
A Structural and Photophysical Study of some Unusually
Structured Macropolyhedral Boron Hydrides
Received 00th January 20xx,
Accepted 00th January 20xx
Michael G. S. Londesborough,a Jiří Dolanský,a,b Tomáš Jelínek,a,c John D. Kennedy,a,d Ivana
Cisařová,b Robert D. Kennedy,d Daniel Roca-Sanjuán,e Antonio Francés-Monerris,e Kamil Lang,a
and William Clegg f
DOI: 10.1039/x0xx00000x
www.rsc.org/
Reaction of anti-B18H22 1 with pyridine in neutral solvents gives sparingly soluble B16H18-3′,8′-Py2 3a as the major product
(ca. 53 %) and B18H20-6′,9′-Py2 2 (ca. 15 %) as the minor product, with small quantities of B18H20-8′-Py 4 (ca. 1 %) also being
formed. The three new compounds 2, 3a and 4 are characterised by single-crystal X-ray diffraction analyses and by
multinuclear multiple-resonance NMR spectroscopy. Compound 2 is of ten-vertex nido : ten-vertex arachno two-atoms-in-
common architecture, long postulated for a species with borons-only cluster constitution, but previously elusive.
Compound 3a is of unprecedented ten-vertex nido : eight-vertex arachno two-atoms-in-common architecture. The single-
crystal X-ray diffraction analysis for the picoline derivative B16H18(NC5H4Me)2 3b, similarly obtained, is also presented.
B18H20Py 4 is also previously unreported but is of known ten-vertex nido : ten-vertex nido two-atoms-in-common
architecture of anti configuration, but now with the pyridine ligand positioned differently to other reported examples of
B18H20L compounds. Factors behind the remarkably low solubility of 3a and 3b are elucidated in terms of electrostatic
potential (ESP) calculations, polarity, and Van-der-Waals complementarities. In view of contemporary developing high
interest in the fluorescent properties of macropolyhedral boron-containing species, a detailed assessment of the
photophysical characteristics of 3a and 4 is also presented. In contrast to the thermochromic fluorescence of 2 (from 620
nm brick-red at room-temperature to 585 nm yellow at 8 K, quantum yield 0.15), compound 3a is only weakly
phosphorescent in the yellow region (590 nm, quantum yield 0.01), whereas compound 4 exhibits no luminescence. The
far more photoactive nature of compound 2 is associated with S1 excited-state minima structures that differ from each
other only by the relative rotational positions of the pyridine substituents on its disubstituted ten-vertex {arachno-B10Py2}-
subcluster. The wavelength and relative intensity of fluorescence from these structures depends on the rotational
positions of the pyridine ligands, which in turn are influenced by temperature and/or rotational inhibition in the solid-
state.
rather than chains and rings, and the study and usage of these
as single-cage species dominates the area. A comprehensive
monograph on polyhedral boron-containing cluster chemistry
Introduction
Boron, like carbon, forms an extensive series of hydrides and
heteroatom derivatives of hydrides. Organic chemistry builds
upon the ability of smaller hydrocarbon-based units to join
together to form chains and rings. Otherwise equivalent
smaller boron hydride units are of polyhedral cage constitution
is lacking, but a recently compiled survey1 of the subdiscipline
of carborane chemistry well illustrates the single-cluster
dominance. For the extent of boron-hydride-based chemistry
to compete with that of organic chemistry, at least in
structural terms, individual cage units need to join together to
give larger molecular assemblies. Here the term
‘macropolyhedral’ has been coined2 to categorize compounds
that (a) contain more than one boron-hydride-based cage and
in which (b) individual cages fuse to each other with two or
more cage atoms held in common so that the cluster
multicentre bonding characteristics extend through, and are
part of, the inter-cage link. One way of developing this area is
to examine the chemistry of pre-formed macropolyhedral
compounds. In a seminal report, over 40 years ago now, Siedle
and Todd examined some reaction chemistry of the two most
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readily available macropolyhedral binary boron hydrides, the is of rare arachno ten-vertex : nido ten-vertex two-atoms-in-
anti and syn isomers of B18H22 (also known as n-B18H22 and iso- common macropolyhedral constitution. We also determine the
B18H22 respectively).3 Their work tentatively identified
macropolyhedral carbaborane and
a
identity of the yellow crystalline material as B16H18Py2 3a, also
a
macropolyhedral of unprecedented constitution, now ten-vertex nido : eight-
metallacarbaborane, as well as several macropolyhedral vertex arachno, with an unexpectedly very low solubility in
metallaboranes, but no definitive molecular structural work common solvents. We also identify an additional minor
was presented. Aspects of these chemistries have product as B18H20-8′-Py
4, previously unreported, but of
subsequently been developed by others, often by us in our established ten-vertex nido : ten-vertex nido eighteen-boron
own laboratories.4 Particularly interesting is the utilisation of architecture that has precedent in anti-B18H22 itself, and that is
anti-B18H22 itself as the optical gain medium in a non-organic closely
related
to
the
ligand
derivative
borane-based laser device.5 This seems to be a rare incidence B18H20{(MeNH)C3N2HMe2} that has an unusual N-heterocyclic
of photoactive behaviour from a binary borane system. carbene (NHC) ligand.16 Some preliminary summary general
Indeed, with regards to the boron hydrides – or boranes – as aspects have been previously mentioned at a conference.17
luminescent materials,6 the general focus of work has been
instead on the isomers of closo-dicarbadodecaborane
Results and discussion
(conveniently referred to here as ‘carborane’), C2B10H12, as
enhancers of the fluorescent properties of various luminescent
organic polymers or fluorophores.7 Additionally, the principle
Synthesis and Structural Characterisation. The reaction of
of aggregation-induced-emission (AIE) claims enhanced
anti-B18H22
1 with an excess of pyridine (Py) in refluxing
luminescence of π-conjugated organic systems that are
connected to ortho-carborane, 1,2-C2B10H12.8,9 Here the
advantage of the carboranyl moieties being their suppression
of intermolecular interaction between the luminescent π-
conjugated organic ligands owing to the three-dimensional
structure of the icosahedral cluster. Carborane-based dyads
and triads that participate in photon-induced electron
transfer10 have also been made and studied,11 and, more
recently, multiple emissions from single-molecules have led to
efficient white-light production from o-carborane-based
luminophors.12 By itself, however, ortho-carborane shows no
benzene or chloroform results in the formation of B16H18Py2 3a
as a pale yellow solid precipitate (ca. 53 % yield), together with
an amber solution that contains brick-red B18H20Py2
%) and a trace quantity of yellow B18H20-8′-Py
(≤ ca. 1 %).‡
Known BH3·Py and the [anti-B18H21]¯ anion are also present in
the reaction product mixture. Compounds and can be
separated and purified by column and thin layer
chromatography on silica-gel. Pale yellow 3a is remarkably
insoluble in common solvents such as chloroform, benzene
and dichloromethane, but dissolves in dimethylformamide
from which pure crystals may be obtained by vapour diffusion
2 (ca. 15
4
2
4
of diethyl ether or dichloromethane. Pure crystals of
were obtained from concentrated solutions
2
and
4
photoluminescence in solution13 and just
a blue-green
in
phosphorescence in the solid-state at temperatures of 77 K.13
Indeed, of all the known binary boranes (compounds
consisting only of boron and hydrogen), anti-B18H22 – the
centrosymmetric isomer of octadecaborane(22) – is the only
one known to have the property of fluorescence. This
attribute was first noticed by Hawthorne et al. some years
ago,14 and subsequently described comprehensively, including
a full consideration of all transitions between ground and
excited electronic states of the molecule.15 In this latter study,
anti-B18H22 was revealed to exhibit an intense blue
fluorescence at 406 nm with a quantum yield approaching
unity (ΦF = 0.97).15 With respect to the successful combination
of carborane cluster and π-delocalised organic systems in one
molecule, as mentioned above, we became interested in such
a combination with the macropolyhedral borane species anti-
B18H22.
dichloromethane by overlayering with hexane. All three
compounds were characterised by all-atom molecular
structures as determined by single-crystal X-ray diffraction
analysis, and by 11B and 1H single- and multiple-resonance
NMR spectroscopy which in each case confirmed the identity
of the selected single crystals with the bulk samples.
From the reaction of anti-B18H22
1 with pyridine (abbreviated
as Py in formulae in this present report) Siedle and Todd
identified an orange-red species, of proposed formulation
B18H20Py2 and of asymmetric constitution, as well as noting a
pale-yellow crystalline material
3 that dissolved only in polar
solvents.3 No definitive structural work, and thence
conclusions, about the molecular structures of these species
was presented. Here we report a re-examination of this
reaction, from which we confirm the empirical formulation of
Figure 1 - The crystallographically determined molecular structure of B18H20Py2 2.
The majority of the B-B interatomic distances fall within a usual borane cluster
range of 1.74 Å – 1.88 Å with the following main exceptions: B5-B9′ = 1.94 Å; B6-
B6′ = 1.90 Å within the arachno subcluster, and with B5-B10 = 2.01 Å and B7-B8=
1.92 Å in the nido subcluster, typical of longer nido-decaboranyl ‘gunwale’
connectivities. No anomalies for the C-C, C-N and B-N distances were apparent.
Other dimensions may be found in the supplementary information
(SI 1).
Displacement ellipsoids are shown at the 50% probability level in these Figures.
the bright orange-red B18H20Py2 species
2 and now show that it
2 | J. Name., 2012, 00, 1-3
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10) at intermediate field, and 11B(1and 3), together with 11B( 6
and 9), tending towards lower field, is traceable. This pattern
A representation of the molecular structure of brick-red
follows through to the nido subcluster of B18H20Py2
2 but the
B18H20Py2
2 is in Figure 1. It is seen to consist of a nido ten-
overall differences from nido-B10H14 itself are now more
marked. Compared to nido-B10H14, therefore, the nido
vertex subcluster and an arachno ten-vertex subcluster,
respectively modelled by the single-cluster species nido-B10H14
subcluster of compound
By contrast, comparison of the shieldings of the single-cluster
arachno model with the arachno subcluster of shows much
2 shows quite a considerable change.
and 6,9-Py2-arachno-B10H12
5, fused with two boron atoms
held in common (schematic connectivity diagrams
I
and II).
5
2
The fusion of these two classic nido and arachno ten-vertex all-
boron cluster types, although often speculated upon, has
hitherto been elusive, although an analogous macropolyhedral
thiaborane, the [SB17H19]− anion, fusing nido ten-vertex borane
and arachno ten-vertex thiaborane units (schematic III), has
been isolated and characterised.18
less perturbation.27 Since magnetic nuclear shieldings depend
intimately and directly upon molecular-orbital structure,
similarities in shielding patterns imply similarities in molecular-
orbital structure. On this criterion, the arachno subcluster of
is electronically much more similar to that of 6,9-Py2-arachno-
B10H12 than the nido subcluster is to that of nido-B10H14. For
2
5
an expansion on this discussion and a rationalisation of the
electronic characteristics of the two constituent subclusters of
compound
structures
2
, refer to Supporting Information, SI 2 (schematic
V
One overall inference is that the physicochemical
to IX).
characteristics of the arachno single-cluster model 6,9-Py2-
arachno-B10H12
than those of the nido-B10H14 single-cluster model
the arachno subcluster is concerned, the eight-boron non-
common {B8H11} element of the nido subcluster of is
effectively behaving as a substituent that bridges the B(5,10)
‘gunwale’ positions on the arachno unit , and does not
5
should be much better mimicked in B18H20Py2
2
1. As far as
I
II
2
5
significantly affect the overall electronic structure of that unit.
III
IV
The dimetallaborane [(PMe2Ph)3Pt2B16H18(PMe2Ph)], fusing
nido ten-vertex {PtB9} and arachno ten-vertex {Pt2B8} units
(schematic IV), is also related, but somewhat more distantly.19
Here it may be noted that macropolyhedral thiaboranes and
transition-element metallaboranes have proved to be more
flexible than corresponding binary boranes in their reaction
chemistry and structural variants,
a versatility probably
facilitated by the flexible valency states of sulphur and of
transition-element centres.20,21 It also may be noted in this
context that attempts to make the purely binary boron hydride
−
analogue of
2
, viz. the [B18H22]2 dianion, by reduction of anti-
B18H22
1
, surprisingly led instead in the isolation of the
22
macropolyhedral [OB18H21]− oxaborane monoanion .
Measured H and 11B NMR data for B18H20Py2
2 are in Table 1.
1
Assignments were made by the established procedure 23,24 and
by combining results from 11B and 1H single- and multiple-
resonance spectroscopies.25,26 Stick representations of the
chemical shifts and relative intensities are shown in Figure 2,
together with those of nido-B10H14, anti-B18H22
1, and 6,9-Py2-
arachno-B10H12 5 for comparison. There is a significant change
when B10H14 and anti-B18H22
1
are compared,26 although the
general nido pattern, with 11B(2,4) to high field, 11B(5,7,8 and
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