Polyhedral azadicarbaborane chemistry. The 'converse' synthesis of a small family of 6,9-(NHR)-bridged-arachno-5,10-dicarbadecaboranes

Article abstract of DOI:10.1016/S0020-1693(00)00099-2

Addition of PhC≡CH to hypho-type species [(R'H₂N)B₈H₁₁NHR] proceeds readily with the displacement of R'NH₂ to give corresponding N-bridged arachno ten-vertex species [6,9-μ-(NHR)-5-Ph-arachno-5,10-C₂B₈H₁₀] (15-34%), for which DFT structural and GIAO ¹¹B nuclear shielding calculations suggest the syn configurations that have the R and Ph groupings on the same side of the molecule. (C) 2000 Elsevier Science S.A.

Full text of DOI:10.1016/S0020-1693(00)00099-2

www.elsevier.nl/locate/ica
Inorganica Chimica Acta 304 (2000) 268–273
Polyhedral azadicarbaborane chemistry. The ‘converse’ synthesis of
a small family of
6,9-(NHR)-bridged-arachno-5,10-dicarbadecaboranes
Udo Do¨rfler, Daniel L. Ormsby, Robert Greatrex, John D. Kennedy *
The School of Chemistry of the Uni6ersity of Leeds, Leeds LS2 9JT, UK
Received 30 November 1999; accepted 31 January 2000
Abstract
Addition of PhCꢀCH to hypho-type species [(R%H2N)B8H11NHR] proceeds readily with the displacement of R%NH2 to give
corresponding N-bridged arachno ten-vertex species [6,9-m-(NHR)-5-Ph-arachno-5,10-C2B8H10] (15–34%), for which DFT struc-
tural and GIAO 11B nuclear shielding calculations suggest the syn configurations that have the R and Ph groupings on the same
side of the molecule. © 2000 Elsevier Science S.A. All rights reserved.
Keywords: Boron cluster calculations; Azadicarborane chemistry
1. Introduction
eroatom insertion reagent. The converse of this, the
addition of dicarbon residues such as acetylenes into
preformed heteroboranes is, interestingly, little investi-
gated, in spite of the difficulties of some dicarbaborane
syntheses and the ease of some heteroborane syntheses.
Here, there is a recent initiative on the addition of
acetylenes to metallaboranes in a ‘converse’ synthesis of
metalladicarbaboranes [18–20], whereas, classically,
metalladicarbaboranes are formed by the addition of
metal centres to dicarbaboranes [21]. These consider-
ations, and the observation of double metal-atom inser-
tion noted above, encouraged us to examine for
azadicarbaborane synthesis by the similarly ‘converse’
route of double carbon-atom addition to the hypho-
type azaboranes 1, 2 and 3. Here therefore we report
results from the reaction of phenylacetylene PhCꢀCH
with [(R%H2N)B8H11NHR], where both R and R% are
We have become interested in the unusual hypho-
type eight-boron species [(EtH2N)B8H11NHEt] (com-
pound 1), and its N-isopropyl and N-tertiary-butyl
cognates (compounds 2 and 3 respectively) (schematic
skeletal structure I, see also IV in Scheme 1 below)
[1,2]. Although the N-ethyl derivative has been known
for many years [3,4], the further chemistry of these
species has hitherto been sparsely investigated. This is
surprising, because (a) these compounds are readily
prepared in good yield in easy steps from nido-B10H14
[1–4], and (b) in principle they constitute useful syn-
thons for eight-boron, and, via boron-vertex loss,
seven-boron cluster chemistry. In this latter regard, for
example, we have recently noted double metal-atom
addition to give {M2NB8} and {M2NB7} azadimetal-
laboranes [5,6]. There is contemporary interest in ex-
tending classical dicarbaborane chemistry to include a
third carbon [7–9] or other heteroatom [10–17]. Syn-
theses in these last two areas are usually approached by
the reaction of a preformed dicarbaborane with a het-
i
t
Et, Pr or Bu (compounds 1, 2 and 3, respectively), and
also with the mixed species [(iPrH2N)B8H11NHtBu], i.e.
t
where {R,R%}={iPr, Bu} (compound 4).
* Corresponding author. Tel.: +44-113-233 6414; fax: +44-113-
233 6565.
E-mail address: john@chem.leeds.ac.uk (J.D. Kennedy)
0020-1693/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved.
PII: S0020-1693(00)00099-2

U. Do¨rfler et al. / Inorganica Chimica Acta 304 (2000) 268–273
269
2. Results and discussion
Reaction of PhCꢀCH with species [(RH2N)-
i
t
B8H11NHR], where R=Et, Pr and Bu (compounds 1,
2 and 3, respectively, reaction scale ca. 500 mmol), in
boiling toluene for 3 h, followed by chromatography,
resulted in the isolation of [6,9-m-(NHEt)-5-Ph-arachno-
5,10-C2B8H10] (compound 5, 34%), [6,9-m-(NHiPr)-5-
Ph-arachno-5,10-C2B8H10] (compound 6, 20%) and
[6,9 - m - (NHtBu) - 5 - Ph - arachno - 5,10 - C2B8H10] (com-
pound 7, 18%), respectively, as colourless oils (sche-
matic cluster structure II, see also VII in Scheme 1
below). The mixed species [(iPrH2N)B8H11NHtBu]
(compound 4) reacted with PhCꢀCH under the same
conditions to form [6,9-m-(NHtBu)-5-Ph-arachno-5,10-
C2B8H10] (compound 7, 14%). The monitoring by
NMR spectroscopy of the reaction products showed no
evidence of the formation of the isopropyl compound 6
from this mixed species 4 showing that it is the exo-ter-
minal R%NH2 group that is lost, rather than the bridg-
ing {NHR} residue. The stoichiometry of the process is
therefore as in Eq. (1).
Scheme 1.
fragmentation (to generate the base peak envelope)
corresponded to the loss of methyl as classically ex-
pected for RNH-group fragmentation where R=Et,
iPr or tBu. Clean 11B and 1H NMR spectra were
additional criteria of purity.
The products had similar NMR spectra (Table 1),
with single-resonance and double-resonance correlation
spectroscopy establishing eight BH(exo) units, one
BHB bridging hydrogen atom, one CH(exo) unit, one
phenyl group and the appropriate alkyl group in each
case. The NMR experiments also established the con-
nectivities of the {B8H9} system. No NH proton reso-
nance was definitively observable in the CDCl3 solution
spectra at 400 MHz, but diamagnetism and electron
count [22,23] suggested this must be present. This is
corroborated by the results of calculations discussed
below. Additionally, comparison with the previously
reported C-unsubstituted species [tBuNHC2B8H11]
(compound 8) [18] demonstrated ‘isospectrality’ [24]
PhCꢀCH+(R%H2N)B8H11NHR
“PhC2B8H10NHR+R%NH2+H2
(1)
The constitution and purity of each of the compounds
5, 6 and 7 was established by NMR spectroscopy and
mass spectrometry. In their 70 eV EI mass spectra, each
exhibited a molecular ion envelope at ca. 30% of base-
envelope intensity, with a high-mass cut-off M at a
value corresponding to the {11B813C1} isotopomer of the
molecular ion, with an M/M−1 ratio of ca. 10–15% as
expected for 10, 11 and 12 carbon atoms, respectively.
For each, the molecular ion profile suggested some
contribution from dihydrogen loss, and the principal
Table 1
Observed 11B and 1H NMR data for [6,9-m-(NHEt)-5-Ph-arachno-5,10-C2B8H10] (compound 5), [6,9-m-(NHiPr)-5-Ph-arachno-5,10,C2B8H10
]
(compound 6) and [6,9-m-(NHtBu)-5-Ph-arachno-5,10-C2B8H10] (compound 7) in CDCl3 solution at 294–297 K
Assignment
Compound 5 a
l(11B)
Compound 6 b
l(11B)
Compound 7 c
l(11B)
l(1H)
l(1H)
l(1H)
BH(1)
BH(2)
BH(3)
BH(4)
BH(6)
BH(7)
BH(8)
BH(9)
CH(10)
m-H(7,8)
−7.4
+0.7
−42.2
−2.9
−12.7
+4.3
+4.3
+2.74
+2.74
+0.35
+2.22
+2.22
+3.49
+3.49
+2.13
+3.77
−3.85
−7.7
+0.5
−42.3
−3.1
−13.9
+4.0
+4.0
+2.66
+2.74
+0.36
+2.17
+2.30
+3.44
+3.44
+2.22
+3.77
−3.78
−5.3
+0.2
−42.6
−4.3
−13.9
+5.8
+5.8
+2.75
+2.58
+0.35
+2.43
+2.21
+3.57
+3.57
+2.14
+3.79
−3.71
−14.6
−15.6
−15.5
a l(1H) +3.48 (CH2, 2H, quartet), +1,15 (CH3, 3H, triplet), +7.15 to +7.58 (Ph, 5H, multiplet).
b l(1H) +2.94 (CH, 1H, multiplet), +1.12 (CH3, 6H, doublet), +7.15 to +7.58 (Ph, 5H, multiplet).
c l(1H) +1.18 (CH3, 9H, singlet), +7.26 to +7.62 (Ph, 5H, multiplet).

270
U. Do¨rfler et al. / Inorganica Chimica Acta 304 (2000) 268–273
Table 2
Observed and calculated 11B NMR chemical shift values for [6,9-m-(NHEt)-5-Ph-arachno-5,10,C2B8H11] (compound 5) (DZ/IGLO//RHF/6-31G*),
[6,9-m-(NHtBu)-5-Ph-arachno-5,10-C2B8H10] (compound 7) (DFT-GIAO//B3LYP/6-31G*) and [6,9-m-(NHtBu)-arachno-5,10-C2B8H11] (compound
8, observed data from Ref. [14]) (DZ/IGLO//RHF/6-31G*)
Assignment
5obs
5scal
5acal
7obs
7scal
7acal
8sobs
8aobs
8scal
8acal
1
2
3
4
6
7
8
9
−7.4
−6.3
+0.7
−44.8
−3.7
−15.5
+6.2
+5.0
−10.6
+0.4
−45.0
−0.8
−14.5
+3.3
+2.9
−5.3
−5.5
−1.1
−41.6
−5.97
−15.1
+5.7
−13.1
−2.4
−41.5
−2.9
−15.0
+0.8
+2.6
−17.8
−2.3
−43.7
−2.3
−18.5
−1.7
−1.7
−11.0
−4.6
−46.2
−4.6
−17.5
+5.5
+5.5
−13.5
+1.6
−43.3
+1.6
−21.8
+5.3
+5.4
−8.4
−1.1
−45.5
−1.1
−20.8
+6.8
+6.9
−20.7
+0.7
−42.2
−2.9
−12.7
+4.3
+0.2
−42.6
−4.3
−13.9
+5.8
+4.3
−14.6
+5.8
−15.5
+5.4
−16.4
−15.9
−18.1
−17.8
−18.5
−17.5
−21.8
and suggested the same constitution. This last species
had previously been established, as a mixture of syn
and anti isomers (see next paragraph), by alkylamine
treatment of [1,2-C2B8H10] (Eq. (2)), in an interesting
stoichiometric closo-to-arachno cluster expansion [18].
C2B8H10+tBuNH2“tBuNHC2B8H11
(2)
In these products the N-alkyl group can in principle be
positioned either on the same (syn) or the opposite
(anti ) side of the molecule relative to the {PhCC} string
(schematics IIIs and IIIa, respectively). Whether the
present products 5, 6 and 7 have the syn or anti
configurations cannot readily be established directly
The results of these DFT-GIAO//B3LYP/6-31G* calcu-
lations gave much the better match with the experimen-
tal results for the syn configuration 7s, with mean and
maximum values for ꢀDl(11B)ꢀ of 0.8 and 1.7 ppm,
respectively when observed and calculated l(11B) values
for each site were compared. By contrast, the Dl values
for the anti isomer 7a were much larger, with a mean
for ꢀDl(11B)ꢀ of 3.0 ppm, and with the B(1) and B(7)
positions in particular showing big differences, with
ꢀDl(11B)ꢀ at 7.8 and 5.0 ppm, respectively. Here it may
be noted that the biggest shielding difference experi-
mentally observed between the previously reported syn
and anti pair of isomers of C-unsubstituted [m-6,9-
(NHtBu)-arachno-5,10-C2B8H11] (compounds 8s and 8a,
respectively) [14], was also for the B(1) position, with
ꢀDl(11B)ꢀ 6.8 ppm. We examined for this difference
calculationally at the lower DZ/IGLO//RHF/6-31G*
level for the 8s and 8a configurations, and found that
these B(1) positions were readily differentiated, even at
this lower level (Table 2, final columns)1. The clear
correspondence for the syn configuration 7s of the
C-phenylated species at the higher DFT-GIAO//
B3LYP/6-31G* level therefore tends to confirm this
configuration for compound 7 (Table 2). In these calcu-
lated structures of both 7s and 7a, the principal struc-
tural features of note are the very long interboron
connectivities B(6)ꢁB(7) and B(8)ꢁB(9) (hatched lines in
structures IIIs and IIIa), both in excess of 200 pm.
1
from straightforward H and 11B NMR spectra, partic-
ularly if in each case only one of the two isomers is
available, as in this present synthesis. Because of low
melting points, crystalline samples of compounds 5, 6
and 7 suitable for X-ray diffraction analysis for struc-
tural confirmation have not yet been feasible. As an
alterative structural probe, however, it is apparent that
structural calculations using ab inito methods, followed
by IGLO or GIAO boron nuclear shielding calculations
based on these calculated structures, and thence com-
parison with experimentally observed 11B shieldings
from NMR experiments, are currently proving to be
exceptionally useful in establishing structures for
boron-containing cluster molecules of about this size
[25,26]. Preliminary DZ/IGLO//RHF/6-31G* calcula-
tions of this nature on the syn and anti versions of the
simplest of our products, the N-ethyl compound 5, gave
calculated results for the two isomers that were very
similar (Table 2). Although the syn configuration 5s
seemed to give a somewhat better match between calcu-
lated and observed 11B nuclear shieldings, we did not
regard this result as unequivocal. We therefore con-
ducted higher level DFT-GIAO//B3LYP/6-31G* calcu-
lations, now using the tertiary-butyl compound 7, since
this would be likely to be subject to a larger intramolec-
ular steric effect and therefore be most likely to show
the largest differences between the two isomers.
1 These and other related species have been calculated at higher
&
levels by B. St´ıbr and coworkers, and are to be published in due
course in the full paper associated with the preliminary communica-
tion of reference 18 [27].

U. Do¨rfler et al. / Inorganica Chimica Acta 304 (2000) 268–273
271
(numbering as in Fig. 1) in the final products 5, 6 and
7. This is accompanied by the displacement of the distal
NH2R% ligand. This exo attack off the open face is at
first sight counter-intuitive. However, it has long been
recognised that there is steric crowding on the open
face of these species [4], which takes the form of a
wedge-shaped {B6} pocket filled with five closely juxta-
positioned bridging and endo hydrogen atoms [2]. At-
tack there would therefore presumably be inhibited,
and so occurs preferentially from the other side, exo to
the cluster2. The movement of the more flexible open-
face hydrogen atoms is less clear-cut; it is tempting to
postulate a swing of the endo hydrogen atom on the
NH2R%-substituted boron position to exo as the NH2R%
ligand leaves, while the adjacent endo hydrogen atom
connects across to generate the H(8,9) bridge of the
products (Scheme 1, structure IIIs), but this does not
address the fate of the two bridging hydrogen atoms
(structure I in Scheme 1) or the overall elimination of
dihydrogen (Eq. (1)).
The clean isomeric products and their syn configura-
tions contrast to the products of the amine process of
Eq. (2) which are syn/anti mixtures [14]. This stereospe-
cificity indicates quite a specific mechanistic process.
The exclusive formation of the tertiary butyl derivative
7
from the mixed tertiary-butyl/isopropyl species
[(iPrH2N)B8H11NHtBu] (compound 4) shows that it is
the bridging {NHR} grouping which is retained and the
exo-terminal R%NH2 grouping that is lost (Eq. (1)
above). The N-alkyl group of the bridging {NHR}
moiety is exo to the cluster open face in the starting
species 1–4 (structure I). An attachment of the
{PhCCH} unit to the open face would result in anti
products, contrary to observation. This therefore sug-
gests a gross heavier-atom mechanism in which the
PhCꢀCH molecule attacks exo to the cluster I (Scheme
1), with scission of the N-bridged interboron linkage to
generate what become the B(6) and B(9) positions
This ‘converse’ reaction process of equation 1 (ver-
sus, for example, the more conventional direction of
Eq. (2)) and the isomeric purity of the products 5, 6 and
7 suggest, in principle, a useful general synthetic route
to C-substituted derivatives of [m-6,9-(NHR)-arachno-
5,10-C2B8H11]. In practice, however, it was disappoint-
ing to find no reaction, under the same conditions
under which PhCꢀCH reacted readily, with the sym-
metrical acetylene PhCꢀCPh. The often more reactive
electronegatively-substituted MeOꢁCOꢁCꢀCꢁCOꢁOMe
and BrCH2ꢁCꢀCH also in our hands have so far
showed no reaction under these conditions. Neverthe-
less, with both alkyl [1,2] and dialkyl [28] derivatives
[(RH2N)B8H11NHR] and [R2HNB8H11NR2] available,
[1,2,28] a family of isomerically pure N-alkyl and N,N-
dialkyl C-phenyl derivatives syn-[m-6,9-(NHR)-5-Ar-
5,10-C2B8H11] and syn-[m-6,9-(NR2)-5-Ar-5,10-C2B8H11]
should now be available in a simple three-step process
from commercially available nido-B10H14.
2 A referee has entertained the possibility that the addition could in
fact occur initially onto the open face to give the anti configuration,
which would then subsequently rearrange quantitatively to give the
syn product, which would be presumed thereby to be thermodynami-
cally much more favoured than the anti. In this context it may be
noted that the DFT//B3LYP/6-31G* calculated energy difference
between 7a and 7s is very low at 5.05 kJ mol−1, with the syn
configuration being the more stable. If there is a reasonably low
barrier to rearrangement, a real energy difference of ca. 5 kJ mol−1
would imply observable quantities of both syn and anti isomers in the
reaction product, contrary to observation. This suggests that the
rearrangement barrier is too high to permit syn/anti interconversion
under the reaction conditions, and supports the idea that the anti
configuration is indeed the kinetically more favoured as well as
(possibly) being the thermodynamically more favoured also.
Fig. 1. Drawing of the minimum-energy molecular structures of
(upper diagram) the syn and (lower diagram) the anti isomers of
[6,9-m-(NHtBu)-5-Ph-arachno-5,10-C2B8H10] (compounds 7s and 7a,
respectively), as calculated from DFT B3LYP/6-31G* structure mini-
mizations. Comparison of experimental NMR results with the GIAO-
calculated 11B nuclear shieldings for the two isomers suggest that the
syn isomer 7s is the one formed in the reaction described in this
paper. Selected dimensions (in pm) for the calculated structure of this
latter syn compound 7s are as follows C(5)ꢁB(6) 165.2, B(6)ꢁB(7)
203.1, B(7)ꢁB(8) 183.4, B(8)ꢁB(9) 203.0, B(9)ꢁC(10) 166.8, C(10)ꢁC(5)
1544.4, N(6,9)ꢁB(6) 159.7 and N(6,9)ꢁB(9) 159.5.

272
U. Do¨rfler et al. / Inorganica Chimica Acta 304 (2000) 268–273
3. Experimental
3.4. Preparation of syn-[6,9-v-(NHR)-5-Ph-
arachno-5,10-C2B8H10], where R=Et, Pr or Bu
(compounds 5–7)
i
t
3.1. General
A sample of [(EtH2N)B8H11NHEt] (compound 1)
(100 mg, 536 mmol) was dissolved in toluene (30 ml),
and then PhCꢀCH (60 ml, 55 mg, 540 mmol) was added
at room temperature. The solution was heated at reflux
for 3 h. After cooling, the more volatile components
were removed (rotary evaporator, water-pump pres-
sure). The resulting solid residue was redissolved in
CH2Cl2 (ca. 30 ml) and the products separated and
purified by repeated preparative TLC, development
with 1:1 hexane–CH2Cl2 giving [6,9-m-(NHEt)-5-Ph-
arachno-5,10-C2B8H10] (compound 5) as a colourless oil
(RF: 0.58, 43 mg, 180 mmol, 34%). Similar experimenta-
tion using [(iPrH2N)B8H11NHiPr] (compound 2) (100
mg, 466 mmol) or [(tBuH2N)B8H11NHtBu] (compound
3) (100 mg, 413 mmol) resulted in [6,9-m-(NHiPr)-5-Ph-
arachno-5,10-C2B8H10] (compound 6) (RF: 0.56, 24 mg,
94 mmol, 20.2%) and [6,9-m-(NHtBu)-5-Ph-arachno-
5,10-C2B8H10] (compound 7) (RF: 0.55, 20 mg, 74 mmol,
18.0%), both as colourless oils. Under the same condi-
tions, the mixed compound [(iPrH2N)B8H11NHtBu]
(compound 4) gave compound 7 exclusively as a
colourless oil (RF: 0.55, 16 mg, 64 mmol, 14.0%), with
no trace of compound 6.
Reactions were carried out in dry solvent (toluene,
hexane and CH2Cl2) under dry dinitrogen but subse-
quent manipulatory and separatory procedures were
carried out in air. [(EtH2N)B8H11NHEt] (compound 1),
[(iPrH2N)B8H11NHiPr] (compound 2), [(tBuH2N)B8H11
NHtBu] (compound 3) and [(iPrH2N)B8H11NHtBu]
(compound 4) were prepared as previously described
[2,3]. PhCꢀCH was obtained commercially. Preparative
thin-layer chromatography (TLC) was carried out using
1 mm layers of silica gel G (Fluka, type GF254),
prepared from water slurries on glass plates of dimen-
sions 20×20 cm2, followed by drying in air at 80°C;
components were located visually under ultra-violet
irradiation. Mass spectrometry was carried out on a
VG Autospec instrument using electron-impact ionisa-
tion at 70 eV.
3.2. Nuclear magnetic resonance spectroscopy
NMR spectroscopy was performed at ca. 5.9 and 9.4
1
T (fields corresponding to 250 and 400 MHz H fre-
quencies, respectively) using commercially available in-
strumentation and using techniques and procedures as
adequately described and enunciated elsewhere [29–32].
Chemical shifts l are given in ppm relative to ]=100
MHz for l(1H) (90.05 ppm) (nominally TMS), ]=
32.083972 MHz for l(11B) (90.5 ppm) (nominally
[F3BOEt2] in CDCl3) [32], and ]=25.145004 MHz for
l(13C) (90.5 ppm) (nominally TMS). ] is as defined in
reference [33].
4. Supplementary material
DFT B3LYP/6-31G*-calculated atomic coordinates
for the syn and anti isomers of [6,9-m-(NHtBu)-5-Ph-
arachno-5,10-C2B8H10] (compound 7) are available, on
request, from the appropriate authors (danielo@chem.
leeds.ac.uk or r.greatrex@chem.leeds.ac.uk) or from the
Editor.
3.3. Computational methods
For the IGLO calculations, both structures were
initially optimised with the 3-21G basis set and the final
optimization was performed using the 6-31G* basis set
and the GAUSSIAN-92 package [34]. The resultant ge-
ometry was used to predict the NMR chemical shifts
using the IGLO package [35–39] and the DZ basis set
incorporated therein. For the DFT calculations, both
structures were initially optimised with the STO-3G and
6-31G* basis sets using standard ab initio methods. The
final optimizations, frequency analyses (to confirm true
minima) and GIAO NMR predictions were performed
using the 6-31G* basis set and B3LYP methodology as
incorporated in the GAUSSIAN-98 package [40]. Gas-
phase NMR predictions were calibrated via a predic-
tion at the same level of theory on diborane, which is
taken to have an NMR shift l(11B) of −16.6 ppm with
respect to BF3OEt2.
Acknowledgements
We thank the Deutsche Forschungsgemeinschaft (for
support to UD), the EPSRC (UK) for equipment
grants (grants nos. J/56929 and GR/M/83360) and a
&
studentship (to DLO), and Dr B. St´ıbr of the Institute
of Inorganic Chemistry of the Academy of Sciences of
the Czech Republic for friendly cooperative discussions.
We also thank Dr C. van Wu¨llen for providing a copy
of the DIGLO software, and Dr J.B.C. Whitaker for
assistance with software configuration.
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