Phosphine-Stabilized Diiododiborenes: Isolable Diborenes with Six Labile Bonds

Article abstract of DOI:10.1002/anie.201814230

The lability of B=B, B?P, and B–halide bonds is combined in the syntheses of the first diiododiborenes. In a series of reactivity tests, these diiododiborenes undergo cleavage of all six of their central bonds in different ways, leading to products of B=B hydrogenation and dihalogenation as well as halide exchange.

Full text of DOI:10.1002/anie.201814230

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Accepted Article
Title: Phosphine-Stabilized Diiododiborenes: Isolable Diborenes with
Six Labile Bonds
Authors: Jonas Muessig, Melanie Thaler, Rian Dewhurst, Valerie
Paprocki, Jens Seufert, James Mattock, Alfredo Vargas, and
Holger Braunschweig
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201814230
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10.1002/anie.201814230
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Phosphine-Stabilized Diiododiborenes: Isolable Diborenes with
Six Labile Bonds
Jonas H. Muessig, Melanie Thaler, Rian D. Dewhurst, Valerie Paprocki, Jens Seufert, James D.
Mattock, Alfredo Vargas,* Holger Braunschweig*
Abstract: The lability of B=B, B-P and B-halide bonds is combined
in the syntheses of the first diiododiborenes. In a series of reactivity
tests, these diiododiborenes demonstrate cleavage of all six of their
central bonds in different ways, leading to products of B=B
hydrogenation and dihalogenation as well as halide exchange.
[B2Br4(IDip)2].[4a] In 2017, Kinjo et al. prepared an interesting
unsymmetrical dibromodiborene stabilized by one N-heterocyclic
carbene and one cyclic (alkyl)(amino)carbene (CAAC),[5] which
underwent facile B=B bond cleavage and isonitrile insertion (B,
Figure 1).[4b] Most recently, we reported the cycloaddition and
cleavage of CO2 at dibromodiborene [B2Br2(IDip)2] (C, Figure
1).[4c]
One of the oldest and most compelling techniques of chemical
research is to make molecules “unhappy” (i.e. reactive) – either
electronically, photolytically or through strain – and see how they
behave. Throughout chemistry, it is an imperative of
fundamental curiosity, not to mention old-fashioned bravado, to
see just how unhappy we can make
a certain class of
compounds. In main-group chemistry, as in other fields, the
synthesis of a new class of reactive, low-valent molecule
inevitably leads to attempts to further destabilize the compounds
by the inclusion of increasingly labile bonds. Disilenes
[R2Si=SiR2] are an archetypal example of these chemical
instincts. The first isolated disilenes bore bulky, kinetically-
stabilizing mesityl groups bound through relatively non-labile Si-
C(sp2)
bonds.[1]
Later,
however,
the
synthesis
of
dibromodisilenes [RBrSi=SiBrR] provided further points of
reactivity for these already reactive molecules, leading to a
range of interesting reactions such as reduction to form disilynes
[RSiºSiR], their use as synthons for halosilylene fragments of
the form [:SiXR], and even as precursors to transition-metal
silylidyne complexes [LnMºSiR].[2]
The chemistry of B-B multiple bonds has seen much activity
recently,[3] however, the inclusion of boron-halide bonds in such
compounds is far less well-explored. The first dihalodiborene[4]
to be reported was the doubly NHC-stabilized example
[B2Br2(IDip)2] (IDip = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-
ylidene), which was prepared by two-electron reduction of the
tetrabromo derivative [B2Br4(IDip)2]. In the same communication,
the dibromodiborene was shown to undergo two-electron
reduction to form the diboryne [B2(IDip)2] (A, Figure 1), from
Figure 1. Currently known reactivity of dihalodiborenes.
The use of phosphine donors in place of carbenes is a more
well-established method to enhance the reactivity of diborenes;
phosphine-stabilized diborenes[6] have shown quite diverse
reactivity at different positions since their discovery in 2014.
Unlike carbene-stabilized diborenes, phosphine-stabilized
diborenes offer the additional opportunity (or hindrance) of labile
B-P bonds, as demonstrated by a number of rearrangement
processes both in the presence and absence of other
reagents.[6g,h,i] However, the B=B bond is also labile, and a
number of reactions involving its cleavage have been presented,
some of which are followed (or preceded) by B-P bond cleavage.
These include the insertion and complete CºC bond cleavage of
an alkyne,[6c] one-electron oxidation with a tropylium cation[6a]
and two-electron oxidation with selenium,[6d] coordination to
Cu(I),[6e] Zn(II)[6f] and Cd(II)[6f] (the former being followed by
hydroarylation of the B=B bond), inverse-electron-demand Diels-
Alder reactions with dienes,[6h] and 1,1- and 1,2-diboration of the
B=B bond.[6i]
which the dibromodiborene could be regenerated by
a
comproportionation reaction with the tetrabromo derivative
[*]
J. H. Muessig, M. Thaler, Dr. R. D. Dewhurst, V. Paprocki, Dr. J.
Seufert, Prof. Dr. H. Braunschweig
Institute for Inorganic Chemistry
Julius-Maximilians-Universität Würzburg
Am Hubland, 97074 Würzburg (Germany)
E-Mail: h.braunschweig@uni-wuerzburg.de
and
Institute for Sustainable Chemistry & Catalysis with Boron
Julius-Maximilians-Universität Würzburg
Am Hubland, 97074 Würzburg (Germany)
Dr. J. D. Mattock, Dr. A. Vargas
Department of Chemistry, School of Life Sciences
University of Sussex
Brighton BN1 9QJ, Sussex, UK
E-Mail: alfredo.vargas@sussex.ac.uk
Given the fascinating reactivity of both phosphine-stabilized
diborenes and dihalodiborenes, we have for some time sought
to combine the potential lability of B-P and B-X (X = halide)
Supporting information for this article is given via a link at the end of
the document.
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10.1002/anie.201814230
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bonds into a single diborene, i.e. [B2X2(PR3)2]. Several years
ago we were able to obtain a single-crystal X-ray structure of a
phosphine-stabilized dibromodiborene, however, the compound
was highly reactive, could not be reliably isolated, and the
diffraction data were poor. We have recently reported simple
solution-phase syntheses of the tetrahalodiboranes B2X4 (X = F,
Cl, I),[7] and our recent work with B2I4 has shown in a number of
cases that it reacts differently to the other tetrahalodiboranes.[8]
As our extensive attempts to prepare stable phosphine-
stabilized dichloro- and dibromodiborenes were unsuccessful,
we set our sights on the diiodo variants, which we considered
may show different behavior and perhaps allow isolation.
In this work, we report the successful synthesis of three
phosphine-stabilized diiododiborenes, the first examples of iodo-
containing species with B-B multiple bonding. In a number of
reactions, a diborene is shown to undergo 1,2-addition, B-B
cleavage, and halide exchange with dihydrogen and halogens,
demonstrating the lability of all six of the central bonds of the
molecule (B=B, B-I and B-P).
tetraiododiborane, [B2I4(SMe2)2] (δB −20) is in agreement with
the formation of the tetraiododiborane adducts 1a-c. The
crystallographically-derived structures of 1a-c show the
expected doubly base-stabilized diborane structures, with
phosphine ligands bound in anti conformation, and only slight
differences in their B−B bond distances (1a: 1.727(7); 1b:
1.765(5); 1c: 1.759(4) Å).
The reduction of the phosphine-stabilized diiododiboranes
1a-c with excess potassium graphite (KC8) led to yellow reaction
mixtures and new 11B NMR spectroscopic signals (δB 5−6).
While these signals are significantly downfield of those of the
precursors 1a-c, they are ca. 10−14 ppm upfield of those of both
phosphine-stabilized diaryldiborenes ([B2Mes2(PMe3)2]: δB 16.7;
[B2Mes2(PEt3)2]: δB 16.3; [B2An2(PMe3)2]: δB 22; Mes = 2,4,6-
trimethylphenyl; An
=
9-anthryl) and NHC-stabilized
dibromodiborenes (δB ca. 20). The relatively upfield nature of the
11B NMR signals of 2a-c is presumably a result of the relatively
low electronegativities of the atoms bound to the B=B core (P
and I) and suggest a high electron density at this unit thanks to a
combination of good s-donor and poor p-acceptor substituents.
Interestingly, the 31P NMR signal of 2a (PMe3) is found to higher
field (δP −18) than that of precursor 1a (δP –11), while the
signals of the diborenes with sterically more demanding
phosphines are found to lower field (2b: δP 8; 2c: δP −4) than
their corresponding starting material (1b: δP –9; 1c: δP –5).
The diborenes 2a-c were isolated as yellow solids in varying
yields (2a: 76%; 2b: 84%; 2c: 27%), and while 2b and 2c were
indefinitely stable at room temperature under inert conditions,
the sterically unencumbered trimethylphosphine derivative 2a
was found to be very temperature sensitive, decomposing within
30 mins at RT and within a few hours at –30 ºC. The instability of
2a precluded its structural authentication, however, the solid-
state structures of 2b/2c confirmed their identity as diborenes,
with B=B distances (2b: 1.539(8) Å; 2c: 1.546(5) Å) only
marginally shorter than those observed in other phosphine-
stabilized diborenes (1.57-1.59 Å).[3] Interestingly, the average
B-P distances of the diborenes 2b,c are significantly shorter
than their corresponding tetraiodo precursors (by 4.4% and
3.5%, respectively). The average B-I distances of the diborenes
are also shorter than those of their precursors, but to a lesser
extent (1.1% for 2b and 1.9% for 2c).
Heating benzene or toluene solutions of 2b for 2 h at 60 °C
or 1 h at 80 °C provided no evidence for decomposition by NMR
spectroscopy. Longer heating (12
h at 60 °C) induced
decomposition to phosphonium salts of the type [Cy3PH][I] or
[Cy3PH][I3] as well as free phosphine (among other unidentified
products), as detected by 31P NMR spectroscopy. The 11B NMR
spectrum of this mixture showed new signals corresponding to
unknown decomposition products (δB = –26 (s), –37 (br s), –43
(br s), –73 (s)). Full decomposition of the diborene can be
observed after 48 h at 80 °C. Diborene 2c is even more
thermally stable than 2b, showing no signs of decomposition
after 24 h at 80 °C. Diborenes 2b,c were also found to remain
unchanged after one week in solution in direct sunlight, and 2b
requires longer than 2 h under a UV lamp to show discernible
decomposition. Overall, aside from the highly unstable (but
sterically unprotected) diborene 2a, the diborenes 2b,c show
surprising stability given the observed instability of the dibromo
analogues, and their presumably reactive B=B, B-P and B-I
bonds.
Figure 2. Synthesis and reactivity of diiododiborenes 2a-c.
Reactions of tetrahalodiborane B2I4 with 2 equiv of PR3 (R = Me,
Cy or CH2Cy) led to isolation of colorless solids 1a-c (Figure 2)
in good yield (73−97%), showing 11B NMR and 31P NMR signals
in very narrow ranges (δB −26 to −28; δP –5 to –11). The 11B
NMR signal of the only known double Lewis base adduct of
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Figure 3. Crystallographically-derived structures of diborane adducts 1a-c, diborenes 2b,c, and diborene reaction products 3-5. Ellipsoids shown at the 50%
probability level. Some ellipsoids, all carbon-bound hydrogen atoms and solvent molecules (if present) have been removed for clarity. Selected bond lengths (Å)
and angles (º) for 1a: B1-B1’ 1.727(7), B1-I1 2.290(5), B1-I2 2.298(5), B1-P1 1.958(5). For 1b: B1-B2 1.766(6), B1-I1 2.293(4), B1-I2 2.304(4), B2-I3 2.294(4), B2-
I4 2.313(4), B1-P1 2.013(4), B2-P2 2.027(4). For 1c: B1-B2 1.759(5), B1-I1 2.302(3), B1-I2 2.281(3), B2-I3 2.306(3), B2-I4 2.289(3), B1-P1 1.980(4), B2-P2
1.981(4). For 2b: B1-B2 1.539(7), B1-I1 2.276(5), B2-I2 2.277(5), B1-P1 1.935(6), B2-P2 1.934(6). For 2c: B1-B1’ 1.545(4), B1-I1 2.251(3), B1-P1 1.913(3). For 3:
B1-B2 1.746(7), B1-Br1 2.051(4), B1-Br2 2.073(5), B2-Br3 2.063(4), B2-Br4 2.063(4). For 4: B1-B1’ 1.733(4), B1-I1 2.338(4), B1-H2 1.04(3). For 5: B-I 2.273(3),
B1-H2 1.09(3), B1-H3 1.10(3).
The good yield and ease of handling of 2b led us to use this
compound as representative for the class of compounds in
reactivity studies. As an initial test, 2b was treated with a slightly
sub-stoichiometric amount of iodine, which, after being left for 24
h in an ultrasound bath, showed complete conversion back to
the tetraiododiborane adduct 1b (Figure 2; 73% isolated yield),
indicating the 1,2-addition of I2 to the B=B double bond. Addition
of a large excess of bromine to 2b provided a new compound
with a high-field 11B NMR signal (δB –3) and a 31P NMR signal
(δP –8) matching those of 1a-c. NMR spectroscopy, elemental
analysis and single-crystal X-ray diffraction allowed identification
of the new compound as the tetrabromodiborane adduct
[B2Br4(PCy3)2] (3, Figures 2 and 3), presumably a product of
both iodide/bromide exchange and the bromination of the B=B
double bond of 2b. Notably, to our knowledge, the reactions of
2b with Br2/I2 are the first reported halogenations of B-B double
bonds.
Heating a benzene solution of diborene 2b (δB 6; δP 8) to
80 °C under 1 atm of H2 for 1 h led to full conversion to a new
product with a 11B NMR signal at δB −26. The 31P NMR spectrum
showed multiple signals indicative of phosphonium salts. The 1H
NMR of the mixture spectra showed a broad signal at δH 3.53,
which sharpened upon 11B decoupling, indicating the presence
of boron-bound hydrogen atoms. The upfield shift of the 11B
NMR signal into the typical region of Lewis-base-stabilized
tetraiododiboranes ([B2I4(PR3)2] (R = Me, Cy, CH2Cy): δB −25 to
−27), together with the aforementioned signal in the 1H NMR
spectrum led us to assume a 1,2-addition of H2 across the B=B
double bond of 2b. Removal of the solvent and extraction of the
residue with hexane provided a solution from which single
crystals of 1,2-dihydro-1,2-diiododiborane adduct [B2H2I2(PCy3)2]
(4, Figures 2 and 3) grew at −30 °C.
Despite the formation of a small batch of crystals of 4, we
were unable to reliably isolate the compound due to its instability,
precluding its full characterization. All attempts to obtain
substantial amounts of the compound by workup under an Ar
atmosphere led instead to isolation of its decomposition product,
the known monoborane adduct [BH2I(PCy3)] (5, Figures 2 and 3;
δB −37; δP 5; isolated yield 54%). The 11B NMR signal at δB −37
matches that observed after heating 2b in benzene for 12 h at
60 °C (as described above), thus, the hydrogenation experiment
was repeated with D2 to confirm that the diborene is indeed
reacting with dihydrogen and to exclude reaction with the solvent.
Heating 2b for 1 h at 80 °C under one atmosphere of D2 resulted
in a broad signal in the 2H{11B} NMR at δD = 3.50 (δH = 3.53),
confirming reaction with H2/D2. It should be noted here that
diborynes [LBºBL],[9] diboracumulenes [L=B=B=L][9] and even
diboranes(4)[10] have shown the ability to undergo hydrogenation
with H2, leaving diborenes as the missing link in the
hydrogenation chemistry of B-B bonds.
DFT calculations were performed on 2b and its hypothetical
dibromo variant [B2Br2(PCy3)2] (2b’) at the OLYP/ZORA/TZ2P
level, in an attempt to rationalize the apparent, and surprising,
stability of diiododiborenes relative to dibromodiborenes. Despite
the differences in the nature of the halides in 2b and 2b’, almost
all key metrics of the two molecules are very similar, including
the B-B and B-P distances, the interaction energies concerning
fragmentation at the B=B bond, the Hirshfeld charges at B and P,
and the HOMO-LUMO gaps. The Hirshfeld charges at I (2b; –
0.079) and Br (2b’; –0.107) are significantly different, however.
Such a difference is somewhat to be expected with different
elements, however, the lower negative charge at the iodo
groups could indicate increased electron donation from I to B in
2b. While the Mayer bond orders (MBOs) of the B-X interactions
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in 2b (0.89) and 2b’ (0.75) are by no means indicative of strong
multiple bonding, the B-X MBO of the iodo variant is significantly
(19%) higher than that of the bromo variant. That the B-I bonds
of 2b are significantly stronger than the B-Br bonds of 2b’ –
while metrics related to the rest of the molecules remain
effectively constant – suggests a rationale for the higher stability
of diiododiborenes than dibromodiborenes.
represents out-of-phase p orbitals at the iodide groups, and the
HOMO–4 is a combination of the in-phase iodide p orbitals with
smaller contributions from the boron atoms, making up a
delocalized, fully in-phase p system across the I-B-B-I unit
(Figure 4). While this p bonding interaction could in principle
rationalize the higher stability of 2b relative to diborenes with the
lighter halides, an entirely analogous interaction is found in the
HOMO–4 of hypothetical dibromodiborene 2b’. Interestingly,
preliminary calculations suggest that similar fully in-phase X-C-
C-X p interactions also exist in the organic analogues E-
dibromo- and E-diiodoethene.
Closer inspection of 2b and 2b' in terms of conceptual DFT
(the branch of the discipline that formalizes the extraction of
chemically meaningful properties from electron density[11]) and
calculations based on the atoms-in-molecules (AIM)[12] approach
suggests that differences between the two species lie in
diiododiborene 2b being 'softer' in the Pearson sense. Indeed,
the value of the atomic Fukui function for nucleophilic attack f+ of
the boron centres is higher in 2b (0.1402) than 2b’ (0.111) while
for electrophilic attack both have the same reactivity (2b: 0.2919;
2b’: 0.2889). Overall, the two diborenes present nucleophilic
character as the values of their dual descriptors are both
negative, but more so for 2b' (–0.1777) than 2b (–0.1519). This
suggests that 2b is more able to accommodate additional
negative charge, and conversely there is a higher incentive for
2b' to give away electrons. These points are supported by the
Shannon entropy values at the boron atomic basins, which
indicate that 2b’ (4.641) has a more structured and localized
density than 2b (5.364), that is, given the augmented electron
flow towards the B-B core in 2b, higher disorder and entropy are
induced. Further descriptors such as Jenkins metallicity indices
and the so-called localization l(A) and delocalization d(A, B)
indices (LI/DI) are all in line with the points discussed above
(see Supporting Information). Overall, the results suggest that a
BB core like those of 2b and 2b' can impart hardness (or
softness) all over a large part of a molecule, which in turn
confers additional stability. Given that diborene 2b was
experimentally isolated, one can deduce that such stabilization
is higher in this diborene given the higher polarizability of the
halide centres compared to those of 2b', which is less stable.
Our
successful
syntheses
of
phosphine-stabilized
diiododiborenes – including even a relatively unprotected PMe3
derivative – stands in stark contrast to our work with the dibromo
analogues, which have thus far resisted isolation even with
strong steric shielding. Computational results point to stronger
B-X bonding with iodide groups, due likely to their higher
polarizability, and the ensuing lower reactivity leading to the
observed stabilization of the diiododiborenes. The presented
reactivity tests demonstrate the value of maximizing the number
of labile bonds in small, low-valent molecules, and showcase a
handful of unique compounds able to be deconstructed in a
range of different ways, including both their B-B s and p bonds,
as well as the sets of B-P and B-I bonds.
Acknowledgements
Figure 4. Calculated frontier orbitals of hypothetical dibromodiborene 2b’ and
isolated diiododiborene 2b.
Financial support from the European Research Council (ERC)
under the European Union Horizon 2020 Research and
Innovation Program (grant agreement no. 669054) is gratefully
Inspection of the frontier orbitals of 2b shows that the HOMO
consists of the conventional B-B p interaction, the HOMO–3
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10.1002/anie.201814230
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support.
Keywords: boron; low-valent main-group species; iodine;
multiple bonding; 1,2-additions
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10.1002/anie.201814230
Angewandte Chemie International Edition
COMMUNICATION
Entry for the Table of Contents
FULL PAPER
Jonas H. Muessig, Melanie Thaler, Rian
D. Dewhurst, Valerie Paprocki, Jens
Seufert, James Mattock, Alfredo Vargas,
Holger Braunschweig
Page No. – Page No.
Phosphine-Stabilized
Diiododiborenes: Isolable Diborenes
with Six Labile Bonds
The lability of B=B, B-P and B-halide bonds is combined in the syntheses of the first
diiododiborenes. In a series of reactivity tests, these diiododiborenes demonstrate
cleavage of all six of their central bonds in different ways.
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