Microwave synthesis of charge-compensated dodecaborates bearing exohedral boron-phosphorus connectivities

Article abstract of DOI:10.1016/j.ica.2017.12.027

Solution pyrolysis of arylphosphonium salts of B₁₂H₁₂^2?,1, utilizing microwave heating provided charge-compensated monophosphine dodecaborate derivatives of general formula B₁₂H₁₁(PPh_nR_3-n)^? (n = 1, R = Me; n = 2, R = Me, Et) in high yield. The resulting anions B₁₂H₁₁(PPh₂Me)^? (2), B₁₂H₁₁(PPhMe₂)^? (3), and B₁₂H₁₁(PPh₂Et)^? (4) were characterized by ¹¹B, ¹H, ¹³C, and ³¹P NMR spectroscopy, and by negative mode ESI-TOF mass spectrometry. A single-crystal X-ray diffraction study of [Ph₃PMe]2 confirmed an icosahedral boron cluster fragment appended by a PPh₂Me moiety through a P-B bond.

Full text of DOI:10.1016/j.ica.2017.12.027

Accepted Manuscript
Research paper
Microwave Synthesis of Charge-Compensated Dodecaborates Bearing Exohe-
dral Boron-Phosphorus Connectivities
Joel A. Dopke, Zachary S. Lincoln, Jacob Blazejewski, Richard J. Staples, Mark
E. Lee
PII:
S0020-1693(17)31435-4
https://doi.org/10.1016/j.ica.2017.12.027
ICA 18061
DOI:
Reference:
Inorganica Chimica Acta
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Revised Date:
Accepted Date:
28 September 2017
18 December 2017
19 December 2017
Please cite this article as: J.A. Dopke, Z.S. Lincoln, J. Blazejewski, R.J. Staples, M.E. Lee, Microwave Synthesis
of Charge-Compensated Dodecaborates Bearing Exohedral Boron-Phosphorus Connectivities, Inorganica Chimica
Acta (2017), doi: https://doi.org/10.1016/j.ica.2017.12.027
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Microwave Synthesis of Charge-Compensated Dodecaborates Bearing
Exohedral Boron-Phosphorus Connectivities
Joel A. Dopkea*, Zachary S. Lincolna, Jacob Blazejewskia, Richard J. Staplesb, Mark E.
Leec
a-Alma College, Department of Chemistry, Alma, MI, USA, 48801
b-Michigan State University, Department of Chemistry and Biochemistry, East
Lansing, MI, USA, 48824
c-University of Missouri-Columbia, Department of Chemistry, Columbia, MO, USA,
65211
*-corresponding author. Email: jdopke@alma.edu

Abstract
Solution pyrolysis of arylphosphonium salts of B12H122–,1, utilizing
microwave heating provided charge-compensated monophosphine dodecaborate
derivatives of general formula B12H11(PPhnR3-n)– (n=1, R=Me; n=2, R=Me, Et) in high
yield. The resulting anions B12H11(PPh2Me)– (2), B12H11(PPhMe2)– (3), and
B12H11(PPh2Et)– (4) were characterized by 11B, 1H, 13C, and 31P NMR spectroscopy,
and by negative mode ESI-TOF mass spectrometry. A single-crystal X-ray diffraction
study of [Ph3PMe]2 confirmed an icosahedral boron cluster fragment appended by a
PPh2Me moiety through a P-B bond.
Keywords: Phosphine, phosphonium, dodecaborate, pyrolysis, microwave
1. Introduction
Since its initial synthesis by Hawthorne, et al.,[1] the icosahedral
dodecaborate anion closo-B12H122–,1, has received significant attention due to its
inherent thermal stability, and low toxicity and nucleophilicity. These properties
have led to a broad range of possible applications for 1 and its derivatives, such as
cancer therapeutics and drug delivery agents,[2,3] and radionucleotide extraction
compounds.[4] However, these same properties have made the exohedral
modification of 1 challenging, as early attempts at its derivatization often utilized
harsh reagents and forcing conditions.[5] The chemistry of 1 and its potential
applications has been reviewed.[6]
While there have been recent successes in the production of exohedrally-
substituted dodecaborates bearing B-N, B-O, B-S, and boron-halogen bonds,[7] there
are remarkably few reports of the synthesis of charge-compensated derivatives of 1
bearing phosphine substituents. Muetterties reported the mono and diphosphine-
substituted dodecaborates of PMe3 as byproducts in the preparation of 1 by
pyrolysis of Me3P·BH3 in the presence of diborane.[8] Todd, et al., prepared the
neutral diphosphine derivative, B12H10(PhPMe2)2, and anionic monophosphine
complex B12H11(PhPMe2)– in low yield through the oxidation of 1 with
(PhPMe2)2PdCl2.[9,10] Similarly, the synthesis of B12H11(PPh3)– was achieved in
moderate yield by Bernard and coworkers by reaction of the halogenated cluster
B12H11I2– with Pd(PPh3)4 in the presence of a base.[11] Aside from these reports,
accounts of B12 icosahedra complexed with phosphines in the literature have been
rare. Given recent interest in the use of analogous mono and dicarborane species
(CB11H12– and C2B10H10, respectively) bearing exohedral phosphorus-containing
fragments as ligands in transition metal chemistry, new avenues toward generating
B-P connectivities in the dodecaborate system are particularly attractive.[12]
This report details the direct synthesis of charge-compensated cluster anions
of the general formula B12H11(PPhnR3-n)– (n=1, R=Me; n=2, R=Me, Et) through
solution pyrolysis of simple arylphosphonium salts of 1 by microwave heating.

These syntheses benefit from nearly quantitative yields and clean product mixtures,
leading to minimal waste and easy purification.
2. Experimental
2.1. Materials and Methods
All manipulations were carried out under nitrogen utilizing standard inert
atmosphere techniques unless otherwise stated. Phosphonium salts of 1 were
prepared by cation exchange utilizing K21 (Boron Specialties, Inc.) and simple
phosphonium halides in water. Phosphonium halide salts were purchased from
Sigma-Aldrich ([Ph3PMe]Br) or Acros Organics ([Ph3PEt]I), or were synthesized
according to literature methods.[13] Acetonitrile (HPLC grade) was purchased from
Fisher and dried under argon using a Waters Seca solvent purification system. All
other solvents were purchased from Fisher and used without further purification.
Microwave syntheses were carried out using a Biotage Initiator Microwave
Synthesizer. NMR spectra were collected using a Bruker AVANCE HD spectrometer
1
at the following frequencies: H (400 MHz), 31P (162 MHz), 13C (100 MHz), 11B (128
MHz). NMR spectra were referenced internally to residual solvent peaks (1H, 13C),
or externally to standard compounds (31P: 85% H3PO4, δ = 0 ppm; 11B: Na2B12H12 in
(CD3)2CO), δ = –15.6 ppm[14]). Cation resonances have been excluded from the
reported NMR data. Electrospray ionization mass spectra (negative mode) were
recorded on a Perseptive Biosystems Mariner API-TOF mass spectrometer in
CH3CN.
2.2. Synthesis
2.2.1. Ph3PMe[B12H11(PPh2Me)], [Ph3PMe]2.
A 0.204 g (0.293 mmol) sample of [Ph3PMe]21 was loaded into a 5 mL
microwave vial and sealed with a septum cap. The vial was purged with N2 for 10
minutes prior to the addition of 3 mL of CH3CN through the cap by syringe. The vial
was then subjected to microwave heating for 8 h at 180˚C resulting in a clear,
colorless solution. The vial was vented with a needle and opened to air, and the
reaction mixture was filtered. The volatile components were removed from the
filtrate under reduced pressure to yield a colorless solid that was recrystallized
from ethanol. The resulting colorless solid was demonstrated to be pure by 11B and
1H NMR analysis. Yield: 0.172 g, 0.278 mmol (94.9%). 11B NMR, ppm ((CD3)2CO): -
9.2 (B12, JB-H=130 Hz); -12.5 (B7-B11, JB-H=141 Hz); -14.1 (B2-B6, JB-H=143 Hz); -
1
18.4 (B1, JB-P=142 Hz). H NMR, ppm ((CD3)2CO): 7.79-7.98 (m, 8H); 7.51-7.58 (m,
2H); 2.18 (d, 3H, JP-H= 12.5 Hz). 13C NMR ((CD3)2CO): 132.8 (2C, JP-C= 9 Hz); 131.2
(1C, JP-C= 3 Hz); 128.5 (1C, 74 Hz); 128.3 (2C, JP-C= 10 Hz); 10.7 (1C, JP-C= 49 Hz). 31P
NMR ((CD3)2CO): 23.0 (Ph3PMe+), -2.6 (JP-B= 142 Hz). ESI-MS, m/z (M-): 341.280
(calc.); 341.259 (obs.).

2.2.2. Ph2PMe2[B12H11(PPhMe2)], [Ph2PMe2]3.
This product was obtained by similar method to that of [Ph3PMe]2 above
save for the heating time was extended to 18 h. Pyrolysis of 0.200 g (0.349 mmol)
[Ph2PMe2]21 provided 0.161 g (0.325 mmol, 93.1%) [Ph2PMe2]3. 11B NMR, ppm
((CD3)2CO): -10.9 (B12, JB-H= 131 Hz), -13.5 (B7-B11, JB-H= 133 Hz), -15.4 (B2-B6, JB-
1
H= 132 Hz), -19.4 (B1, JP-B=145 Hz). H NMR, ppm ((CD3)2CO): 7.81-7.92 (m, 3H);
7.47-7.57 (m, 2H); 1.79 (d, 6H, JP-H= 12.2 Hz). 13C NMR, ppm ((CD3)2CO): 131.4 (2C,
JP-C= 9 Hz); 131.0 (1C, JP-C=3 Hz); 128.3 (2C, JP-C= 10 Hz); 122.6 (1C, JP-C= 73 Hz); 10.7
+
(2C, JP-C=47 Hz). 31P NMR ((CD3)2CO): 20.8 (Ph2PMe2 ), -8.8 (JP-B=145Hz). ESI-MS,
m/z (M-): 279.265 (calc.); 279.216 (obs.).
2.2.3. Ph3PEt[B12H11(PPh2Et)], [Ph3PEt]4.
This product was obtained by similar method to that of [Ph3PMe]2 above
save for the heating time was extended to 18 h. Pyrolysis of 0.200 g (0.276 mmol)
[Ph3PEt]21 provided 0.175 g (0.270 mmol, 98.2%) [Ph3PEt]4. 11B NMR, ppm
((CD3)2CO): -9.9 (B12, JB-H= 130 Hz), -13.5 (B7-B11, JB-H= 144 Hz), -14.9 (B2-B6, JB-
1
H= 145 Hz), -19.7 (B1, JP-B=143 Hz). H NMR, ppm ((CD3)2CO): 7.89-7.97 (m, 4H);
7.76-8.39 (m, 4H); 7.53-7.59 (m, 2H); 2.58 (dq, 2H, JP-H= 11.1 Hz, JH-H= 7.5 Hz); 1.02
(dt, 2H, JP-H= 17.3 Hz, JH-H= 7.5 Hz). 13C NMR, ppm ((CD3)2CO): 133.6 (2C, JP-C= 9Hz);
130.9 (1C, JP-C=3 Hz); 127.8 (2C, JP-C=11Hz); 126.6 (1C, JP-C=65 Hz); 18.4 (1C, JP-C= 44
Hz); 6.6 (1C, JP-C=4 Hz). 31P NMR, ppm ((CD3)2CO): 25.9 (Ph3PEt+), 5.3 (JB-P=143 Hz).
ESI-MS, m/z (M-): 355.296 (calc.); 355.262 (obs.).
2.3. Crystallographic Determination of [Ph3PMe]2.
Single, colorless, block-shaped crystals were grown from a CH3CN solution of
[Ph3PMe]2 by vapor diffusion of methyl-tert-butylether (MTBE). Data were
collected on a crystal with dimensions 0.47×0.46×0.29 mm3 mounted with paratone
oil on a glass loop using a Bruker APEX-II CCD diffractometer (MoKα radiation,
173(2) K). Using Olex2 [15], the data was reduced and refined using SAINT. The
structure was solved with the ShelXS structure solution program by direct methods
on F2. The model was refined by least squares minimization using version 2014/6
of XL.[16] All non-hydrogen atoms were refined anisotropically. Hydrogen atom
positions were calculated geometrically and refined using the riding model.
Crystallographic data in CIF format have been included in the Supplementary
Information.
3. Results and Discussion
The solution pyrolysis of arylphosphonium salts of 1 utilizing microwave
heating provided charge-compensated mono-phosphine dodecaborate salts 2-4
cleanly and in high yields. While the anion 3 has been synthesized by Todd, et

al.,[10] anions 2 and 4 have not previously appeared in the literature. The product
salts were isolated as colorless powders upon recrystallization from ethanol, and
were found to be stable indefinitely to air and moisture both in solution and in the
solid state.
Scheme 1. General reaction scheme for the formation of [Ph3PMe]2 from
[Ph3PMe]21 utilizing microwave heating. Terminal cluster hydrides have been
omitted for clarity.
In each case, the pyrolysis of arylphosponium salts of 1 yielded a product in
which an aryl group was preferentially lost from a phosphonium cation over an
alkyl moiety with accompanying removal of a hydride fragment from the cluster
(Scheme 1). Analysis of the pot mixtures of these reactions by 13C NMR
spectroscopy showed the appropriate resonances for the products and a single peak
consistent with the presence of benzene, suggesting the transfer of a cluster hydride
to a C6H5+ substituent of the phosphonium cation. The liberation of benzene from
PPh3 by reaction with a B-H moiety from a dicarbollide derivative has been
reported, although this reaction requires a transition metal (Pd) to occur.[17,18] No
investigation of the mechanism of this transfer was undertaken. However, the
extended reaction time (18 h) required to convert [Ph3PEt]21 to [Ph3PEt]4 in
comparison to (Ph3PMe)21 suggests that the additional steric bulk afforded by the
ethyl group could impact the rate of conversion. Accordingly, similar salts of cations
containing bulkier alkyl groups, such as Ph3PBn+ (Bn = benzyl), were unreactive by
this method under the pyrolysis conditions utilized.
Phosphonium cations containing mixed aryl and alkyl moieties were
+
+
necessary for this reaction to proceed. Simple PR4+ (PMe4 , PBu4 ) and PPh4+ salts of
1, as well as Ph3P(CH2OCH3)+, failed to yield the analogous phosphine-ligated
pyrolysis products. Similar reactions carried out in other common organic solvents
(ethanol, THF, acetone, DMF, DMSO) were either unsuccessful or failed to produce
ligated dodecaborate salts in significant yields. Salts of 1 and simple ammonium
+
+
+
cations (NEt4 , NBu4 , NBnMe3 ) were ineffective in yielding amine-substituted
clusters by this route.
The ratio of aryl to alkyl groups present in the phosphonium cation was also
important in the ease of the reaction. While the pyrolysis of PPh4+ salts of 1 yielded
no B12H11(PPh3)–, the replacement of a single aryl group with a methyl moiety
(Ph3PMe+) provided the most facile reaction, reaching completion in 8 h at 180˚C.
As the number of methyl groups present on the cation was increased, however, the
reaction required more forcing conditions to reach completion. The pyrolysis of
[Ph2PMe2]21 required 18 h at 180˚C to reach completion, and, in the pyrolysis of
[PhPMe3]21, only small quantities of starting material were consumed using

extended heating times (up to 36 h) and higher temperatures (up to 220˚C). No
B12H11(PMe3)– was isolated by this method.
Figure 1. 31P{1H} NMR and MHz 11B{1H} NMR (inset) spectra of [Ph3PMe]2
measured in CH3CN.
The identity of 3 was confirmed by comparison of its 11B NMR spectrum to
literature values.[10] 11B{1H} NMR spectra of anions 2-4 are nearly identical and
feature peaks with a 1:5:5:1 ratio of intensities, consistent with a monosubstituted
icosahedron of idealized C5v symmetry (Figure 1). The upfield signal is attributed
to the phosphine-substituted vertex by virtue of its splitting into a doublet by the
adjacent phosphorus atom (Table 1). In the 11B NMR spectrum, the remaining
resonances were observed to split into doublets due to coupling with terminal
hydrides while the phosphine-substituted peak remained unchanged. Analysis of
the substituted products by 11B-11B COSY NMR spectroscopy allowed for the
unambiguous assignment of cluster resonances. The locations of the individual
resonances are only slightly impacted by changes in the phosphine fragment.
Resonance
B1 (JP-B)
B2-B6 (JB-H)
B7-B11 (JB-H)
B12 (JB-H)
P (JP-B)
2
3
4
-18.4 (142)
-14.1 (143)
-12.5 (141)
-9.2 (130)
-2.6 (142)
-19.4 (145)
-15.4 (132)
-13.5 (133)
-10.9 (131)
-8.8 (145)
-19.7 (143)
-14.9 (145)
-13.5 (144)
-9.9 (130)
5.3 (143)
Table 1. Summary of 11B and 31P NMR spectroscopic data (in ppm) for anions 2-4 in
(CD3)2CO. Relevant coupling constants, in Hz, are given in parentheses.
The 31P NMR spectra of the product salts of 2-4 featured two signals: a sharp
downfield singlet corresponding to the phosphonium cation, and a broad upfield
quartet of approximately equal integration due to the phosphine-ligated product.
The JB-P coupling observed in the product was consistent with other phosphine-
dodecaborates.[9-11] All expected 1H and 13C NMR signals due to the phosphine
were observed. Molecular ions (M–) of the appropriate masses for compounds 2-4
were also observed by negative-mode ESI-MS (CH3CN) analysis of each of the
products, and matched well with calculated values.
Empirical Formula
C32H42B12P2
618.31
Fw
T, K
173(2)
Crystal System
Monoclinic

Space Group
Cc
4
Z
a, Å
b, Å
c, Å
α, ˚
23.7603(14)
9.6076(6)
18.4055(1)
90
123.3200(10)
90
3510.9(4)
1.170
β, ˚
γ,˚
V, Å3
dcalc, g⋅cm-3
µ, mm-1
0.147
F (000)
1296
2Θmax, ˚
25.369
Measured Reflections
Independent Reflections (Rint)
Obs. Refl./restraints/parameters
R, % [I>2σ(l)]
14201
6312
5937/2/417
3.35
Rw, %
8.17
GooF
1.029
0.200/–0.180 (–1.11)
Residual Electron Density, e⋅Å-3
(dmin/dmax
)
Table 2. Crystallographic parameters for [Ph3PMe]2.
Crystals of [Ph3PMe]2 suitable for crystallographic study were grown and
analyzed (Table 2). The anion structure showed the icosahedral cluster fragment
with a single PPh2Me unit attached to the cluster through a B-P bond (Figure 2,
Table 3). The B-P bond distance, 1.920 Å, was consistent with those distances
reported in B12H11(PPh3)– (1.928 Å)[11] and B12H10(PPhMe2)2 (1.906, 1.911 Å).[10]
In the solid state, the phosphine moiety was slightly tilted away from a single phenyl
substituent, resulting in slightly restricted bond angles between P1-B1 and B4, B5,
and B6 (avg. 116.7˚), and larger angles between P1-B1 and B2 and B3 (avg. 126.3˚).
This also resulted in a slight opening of the B1-P1-C8 angle (117.6˚) of one
phosphine phenyl substituent relative to the B1-P1-C1 and B1-P1-C2 angles of the
remaining phenyl and methyl groups (109.8˚ and 109.2˚, respectively). The boron-
boron distances (1.765-1.797 Å ) and angles within the cluster fell within the
expected ranges for dodecaborates and were otherwise unremarkable.
Figure 2. The crystallographically-determined structure of the anion, 2, with
ellipsoids drawn at the 50% level. The Ph3PMe+ cation has been omitted.

In light of the findings of Bernard, et al.,[11] the pyrolysis of the Ph3PMe+ salt
of B12H11I2– was also investigated to determine the suitability of the halogenated
cluster for the pyrolysis reaction. The starting material was consumed upon heating
for 18 h at 180˚C resulting in a complex mixture. The primary product identified
was [Ph3PMe]2 as evidenced by its 11B NMR and mass spectra. The anion was
collected in 24.7% yield after column chromatography (5% CH3CN in CH2Cl2; silica
gel). It is presumed that this pyrolysis yields iodobenzene as its byproduct. Also
observed, but not isolated, from the reaction mixture was the product anion
B12H10I(PPh2Me) –. The anion was identified by its mass spectrum (M–: 467.136
m/z (obs.), 467.177 m/z (calc.)) and was expected to be generated by the loss of
benzene in analogous fashion to the production of 2.
The anion 2 proved to be inaccessible by pyrolitic reactions featuring
different cation combinations. As an example: the pyrolysis of a solution containing
equimolar quantities of [NBu4]21 and [Ph3PMe]21 failed to produce the anion 2.
When a solution of K21 was heated in the presence of [Ph3PMe]Br, the only
substituted cluster product observed was tentatively identified as B12H11Br2- by its
11B NMR spectrum.[19] Neither the anion B12H11(OH)2– nor B12H11(NH3) – provided
phosphine-substituted products when their Ph3PMe+ salts were subjected to
reaction conditions similar to those studied here.
4. Conclusions.
The microwave pyrolysis of simple arylphosphonium salts of 1 provided charge-
compensated, phosphine-derivatized products, representing a direct method for the
production of new borane anions bearing phosphine substituents. It is anticipated
that the reaction chemistry detailed here could potentially be applied to a range of
analogous arylphosphonium salts of 1. With the greater applicability of boron
hydrides, the ability to produce clusters in high yield with a broader variety of
substituent groups can only enhance the viability of cluster anions in both emerging
and established applications.
Acknowledgements.
We thank Drs. Nancy Dopke and Robert Burns for fruitful discussions, and a
reviewer for helpful reference information. We gratefully acknowledge the Herbert
H. and Grace Dow Foundation for support of ZSL. We also recognize support from
Alma College and the Alma College Department of Chemistry, and the National
Science Foundation (USE-8951401) for research and instrumentation support.
Conflict of Interest: none.
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Graphical abstract

Highlights
Microwave methodologies were used to synthesize charge-compensated
dodecaborates with ancillary phosphine moieties from arylphosphonium salts of
dodecaborates.
Benzene was liberated by loss of a phenyl fragment from a phosphonium cation and
a hydride from the cluster.
The rate of conversion to products depends strongly on the nature of the
phosphonium cation.