Polyhedral nonaborane chemistry: An unsuspected new isomeric type in the nine-vertex arachno-B9H13L system

Article abstract of DOI:10.1016/S0022-328X(97)00539-1

NMR spectroscopy reveals that the preparation of conventionally-structured 4-(ligand)-arachno-B₉H₁₃ species 1 by ligand exchange on 4-(SMe₂)-arachno-B₉H₁₃ 1a is often accompanied by the formation of their hitherto unsuspected and unreported 5-(ligand)-arachno-B₉H₁₃ isomers 2, which can be isolated by chromatography. 5-(4′-PhC₅H₄N)-arachno-B₉H₁₃ 2c is characterised by a single-crystal X-ray diffraction analysis that also reveals an interesting stacking of the organic aromatic residues in the solid state.

Full text of DOI:10.1016/S0022-328X(97)00539-1

Ž
.
Journal of Organometallic Chemistry 550 1998 441–444
Short communication
Polyhedral nonaborane chemistry: an unsuspected new isomeric type in
the nine-vertex arachno-B9H13L system
Karen L.F. Callaghan, Udo Dorfler, Thomas D. McGrath, Mark Thornton-Pett,
¨
)
John D. Kennedy
The School of Chemistry of the UniÕersity of Leeds, Leeds LS2 9JT, England
Received 6 February 1997
Abstract
NMR spectroscopy reveals that the preparation of conventionally-structured 4- ligand -arachno-B9H13 species 1 by ligand exchange
Ž
.
Ž
.
Ž
.
on 4- SMe2 -arachno-B9H13 1a is often accompanied by the formation of their hitherto unsuspected and unreported 5- ligand -arachno-
X
Ž
.
B9H13 isomers 2, which can be isolated by chromatography. 5- 4 -PhC5H4N -arachno-B9H13 2c is characterised by a single-crystal
X-ray diffraction analysis that also reveals an interesting stacking of the organic aromatic residues in the solid state. q 1998 Elsevier
Science S.A.
Keywords: NMR spectroscopy; Arachno-B9 H13L; X-ray diffraction; Borane cluster
It is a pleasure for us to dedicate this paper to Ken
Wade. Ken Wade’s Rules have great predictive and
rationalisation power in polyhedral borane, carborane
and heteroborane chemistry, extending into
organometallic assemblies and metal cluster species
tural characterisations using techniques that may not
have revealed certain species, or that may have involved
preparative conditions that may have destroyed them.
The example that is relevant to the work presented here
consists of the series of well-known arachno nine-vertex
compounds LB9 H13, where L is a two-electron donor
such as triorganophosphine, diorganosulphide, amine,
1
w
x
1,2 . They are based initially on the recognition of
w x
empirical structural patterns 3 . Within series of com-
pounds that obey Wade’s Rules, a variety of isomeric
structural configurations may exist, and it is important
to recognize the associated patterns of cluster-sub-
stituent and cluster-constituent behaviour so that other
useful empirical rules may ultimately emerge. Here, for
example, empirical ‘rules’ governing the stabilities of
the absolute and relative positioning of carbon atoms in
mono-, di- and polycarborane clusters are well-estab-
w x
etc. 8 . These are readily prepared in high yield from
ten-vertex arachno-6,9-L2 B10 H12 by alcoholysis Eq.
Ž
Ž .. w x
1
9 , or by ligand exchange on a preformed LB9 H13
Ž
Ž ..
w
x
species Eq. 2 10–12 . They are generally held to
have the configuration I in which the ligand is in the
4-position in the open face.
w
x
lished 4–7 . In this context it may be noted that many
of the perceived configurational stereotypes in poly-
hedral boron-containing cluster chemistry derive from
initial pioneering preparations, separations, and struc-
)
Corresponding author.
1 We have pleasure in expressing our best wishes to Ken Wade on
the occasion of his 65th birthday, and are pleased to dedicate this
paper to him in recognition of his outstanding contributions and
astute insights into organometallic, inorganic and boron cluster chem-
istry.
L2 B10 H12 q3ROH™
LB H qH qLqB OR
1
Ž
.
Ž .
3
9
13
2
X
X
LB9 H13 qL ™L B9 H13 qL
2
Ž .
0022-328Xr98r$19.00 q 1998 Elsevier Science S.A. All rights reserved.

(
)
442
K.L.F. Callaghan et al.rJournal of Organometallic Chemistry 550 1998 441–444
Ž
We now report preliminary results that show that the
Thin-layer chromatographic separation silica gel G,
CH2Cl2 of the sec BuNH2 products 1b and 2b resulted
.
formation of these 4-substituted species 1 by the ex-
Ž .
change route of Eq. 2 above is in fact often accompa-
in the isolation of each of these compounds in yields of
ca. 45%, with ca. 10% of the SMe2 starting compound
1a recovered. In the 4-Ph-C5H4 N system, the filtered
CH2Cl2 extract of an evaporated reaction mixture con-
taining 2c and 1c in proportion 2:1 was fractionally
nied by the hitherto unsuspected formation of their
5-substituted isomers 2 of configuration II in propor-
tions of up to ca. 90%. The five open-face bridging and
terminal hydrogen atoms of these new 5-isomers 2
mutually rapidly exchange sites at ambient tempera-
tures. Moderate heating converts the 5-isomers 2 to the
conventional 4-isomers 1.
Ž
.
crystallized twice CH2Cl2rhexane to afford essen-
Ž
.
Ž
tially pure 2c 41%, unoptimised ; and thence 1c -ca.
.
5% was recovered by repeated thin-layer chromatogra-
phy silica gel G, CH2Cl2-hexane mixtures of the
combined supernatants. Moderate heating e.g.,
perdeuterotoluene solutions, 758C, 15 h for 2b, 1008C,
Ž
.
Thus, in the product mixture from the treatment of
B9 H13 SMe2 1a, of structure type I with sec BuNH2
Ž
. Ž
.
Ž
Ž
Ž .. Ž
Eq. 2 608C, benzene solution, 2 h, reaction scale
900 mmol , integrated NMR spectroscopy showed
that the 4-substituted species 4- NH2 Bu -arachno-
B9 H13 1b was accompanied by an approximately
2
.
sec
Ž
.
equimolar proportion of the 5-substituted species 5-
sec
Ž
.
NH2 Bu -arachno-B9 H13 2b. A reaction at lower tem-
Ž
perature between 1a and 4-Ph-C5H4 N room tempera-
ture, toluene solution, 16 h, 600 mmol scale showed an
excess of 5- 4 -Ph-C5H4 N -arachno-B9 H13 2c over the
4-substituted species 4- 4 -Ph-C5H4 N -arachno-B9 H13
1c in ca. 9:1 proportion. In the former case ca. 10% of
.
X
Ž
.
X
Ž
.
Ž
.
the starting material B9 H13 SMe2 1a was unreacted.
2 NMR parameters, measured at B0 ca. 5.875 T, CDCl3 solution,
Ž11
.
298 K, ordered for BH groups as tentative assignment d B rppm
w Ž1 .Ž
.
x
relative to BF3OEt2
d
H
exo rppm relative to TMS as follows:
Ž .
y8.3 q2.85 , BH 7 and BH 9 y8.3 and y7.3 q2.35 and
Ž .
Ž .
w
x
for 2b BH 1 and BH 2 q2.4 and q2.8 q3.06 and q2.98 , BH 8
w
x
Ž . Ž .
w
1
x
Ž .
w
Ž
.x
Ž .
q2.25 , BH 5 y24.6 ligand position, no H exo , BH 4 and
Ž .
w
x
Ž .
w
x
BH 6 both ca. y21.7 both ca. q1.55 , BH 3 y50.5 y0.41 ,
Ž1
.
Ž
.
additionally, d H y0.91 broad, 5H, endo and bridging , q4.24,
Ž
.
Ž
q4.02 broad, 2=1H, NH2 , q1.91, q1.70 multiplets, 2=1H,
.
Ž
.
Ž
.
CH2 , q3.35 multiplet, 1H, CH , q1.45 doublet, 3H, CH3 , and
q1.03 triplet, 3H, CH3 note the chiral nature of the sec Bu group
Ž
. Ž
Ä
4
.
Ž
.
w
x
gives asymmetry to the B9 cluster ; for 2c BH 1,2 q2.3 q3.12 ,
Ž . Ž . Ž . Ž .
w
x
w
x
BH 8 ca. y7.3 q2.96 , BH 7 and BH 9 ca. y7.3 q2.56 , B 5
1
w
Ž
.x Ž .
Ž .
w
w
x
y18.8 ligand position, no H exo , B 4 and BH 6 y22.0 q1.81 ,
Ž .
w
x
Ž1
.
B 3 y50.0 y0.21 , additionally d H y0.52 broader, 5H, mutu-
.x
Ž
ally exchanging bridging and endo , q7.37 to q7.87 multiplets,
3
1
.
Ž
Ž1
7H, C6 H5 and C5H4 N , q9.06 d, 2H, J H– H ca. 6Hz, C5H4 N ;
.
.
Ž .
w
x
Ž .
w
x
Ž
.
for 1a BH 7 q18.1 q4.07 , BH 1 q4.4 q3.04 , BH 5,9 y17.7
w
x
Ž
.
w
Ž
.
Ž
w
.x Ž .
q1.81 , BH2 6,8 y21.5 q1.96 exo , y0.01 endo , BH 4
w
Ž
.x
Ž
.
x
y 23.3 q 0.39 endo
,
BH 2,3 y 39.2 q 0.44 and
Ž1 .Ä
Ž
.4
Ž1 .Ž
.
Ž .
d
H
mH 5,9;6,8 y3.53, d H SMe2 q2.54; for 1b BH 7
w
x
Ž . x
w
x
Ž
Ž
.
w
q15.6 q4.00 , BH 1 q4.2 q2.88 , BH 5,9 y17.7 q1.78 ,
Ž
.
Ž
w
Ž
.
.x
Ž .
w
x
BH 6,8 y20.8 q1.93 exo and y0.18 endo , BH 4 y20.9
Fig. 1. ORTEP 15 type drawing of the crystallographically deter-
X
w
.x
Ž
.
w
x
Ž1 .Ä
Ž
.4
Ž
.
q0.60 endo , BH 2,3 y39.8 q0.39 and
d
H
mH 5,9;6,8
mined molecular structure of 5- 4 -PhC5H4 N -arachno-B9 H13 2c.
Ž1
.
.
Ž
.
˚
Ž .
Ž .
Ž
.
Ž . Ž .
y3.53; additionally, d H q4.04 broad, 2H, NH2 , q1.83, q1.71
Selected interatomic distances A are: B 5 –N 51 1.558 2 , B 4 –
Ž
Ž
Ž
.
Ž . Ž . Ž . Ž .
Ž . Ž . Ž .
Ž . Ž . Ž .
multiplets, 2=1H, CH2 , q3.25 multiplet, 1H, CH , q1.42
B 5 1.759 3 , B 5 –B 6 1.884 3 , B 6 –B 7 1.973 3 , B 7 –B 8
.
Ž
. Ž
Ž . Ž . Ž .
Ž .
Ž . Ž .
Ž .
doublet, 3H, CH3 , and q0.99 triplet, 3H, CH3 note that, in
1.789 3 , B 8 –B 9 1.787 3 and B 9 –B 4 1.970 3 , with other
˚
Ä
4
Ž .
Ž .
contrast to 2b above, any asymmetry in the B9 cluster arising from
interboron distances in the range 1.713 3 –1.801 3 A. Note that
the chiral sec Bu group is not apparent at the polarizing field strength
there is crystallographic disorder with half-occupancies of the two
.
Ž .
w
x
Ž
.
Ž
.
used in these NMR experiments ; for 1c BH 7 q16.7 q4.14 ,
m 4,5 and m 5,6 bridging hydrogen atom positions. These are
Ž . Ž .
w
x
Ž
.
w
x
Ž
.
BH 1 q4.8 q3.24 , BH 5,9 ca. y15.5 q1.95 , BH 4 ca. y15.5
shown with hatched connectivities. This disorder between the m 4,5
and m 5,6 bridging hydrogen atoms may obscure differential solid-
state bridging versus endo character in the endo H 4 and endo H 6
w
Ž
.x
Ž
.
w
Ž
.
Ž
.x
Ž
.
q1.54 endo , BH 6,8 y20.3 q2.10 exo and q0.00 endo ,
Ž
.
w
x
Ž1 .Ä
Ž
.4
Ž .
Ž .
BH 2,3 y38.9 q0.57 and d
H
mH 5,9;6,8 y3.18; addition-
Ž1
Ž
.
Ž
.
ally, d H q7.60 to q7.95 multiplets, 7H, C6 H5 and C5H4 N and
positions, and thence differential endo versus bridging character in
3
Ž1
q8.88 doublet, 2H, J H– H ca. 6 Hz, C5H4 N .
.
.
Ž
.
Ž
.
1
the mH 7,8 and mH 8,9 positions.

(
)
K.L.F. Callaghan et al.rJournal of Organometallic Chemistry 550 1998 441–444
443
Fig. 2. Mutually perpendicular views of the molecular p-stacking that occurs among the PhC5H4 N ligands along the crystallographic a direction
X
Ž
.
in the crystal of 5- 4 -PhC5H4 N -arachno-B9 H13 2c.
.
18 h for 2c resulted in quantitative conversions of the
the five endo terminal and bridging hydrogen atoms
about the open face being different from those in the
5-isomers 2b and 2c to the conventional 4-isomers 1b
and 1c respectively. The new 5-isomers 2 are thus
reasonably robust, requiring several hours heating for
complete conversion to isomers 1. It is therefore per-
haps remarkable that they have not previously been
detected in the 36 years since the initial reports of the
Ž
conventionally-structured 4-LB9 H13 species 1 sche-
matic I 16–18 . The 1H NMR results for each of
compounds 2b and 2c show that the five endo terminal
and bridging hydrogen atoms are rapidly interchanging
positions with each other, in contrast to neutral 4-
LB9 H13 species which are generally static, but in a
.
w
x
Ž .
syntheses of the 4-isomers 1 via the reaction of Eq. 2 .
X
w
xy
Ž
Ž
.
Recrystallization of the 5- 4 -Ph-C5H4 N derivative
similar manner to anionic species 4-XB9 H
when
w
xy xy
w
Ž
.
.
w
x
13w
xy
Xy NCS , NC BH3
19 and to the B9 H14
anion 20 and its conjugate acid, neutral ‘iso’-B9 H15
21 . An interesting feature in the solid state structure of
2c is the extended stacking along the a axis of the
phenylpyridine ligands Fig. 2 . The stacking is ‘step-
like’ in nature such that the phenyl group of one
molecule stacks directly over the pyridyl group of the
molecule beneath and so on. A similar phenomenon is
2c from dichloromethanerhexane at y308C yielded
w
x
pale yellow air-stable crystals suitable for single-crystal
3
Ž
w
x
X-ray diffraction work. The molecular structure Fig.
has the basic arachno nine-vertex shape, but is
clearly shown to have the previously unreported 5-ligand
.
1
Ž
.
Ž
.
Ž
.
configuration schematic II , with a the distribution of
Ž
.
3
recently noted in mutual polymorphs of 6,9- C5H5N -
2
Ž .
Crystal data: C11H22 B9 N, monoclinic, as5.5868 6 , bs
3
w x
arachno-B10 H12 22 . This interesting supramolecular
assembly behaviour has possible implications for crystal
engineering in precursive work to nano-layered boron,
boron–carbon, boron–nitrogen, etc. ceramic materials.
We currently extend studies to other ligands L, to the
quantitative measurement of the fluxionalities and of the
2™1 interconversions and thence to the factors that
might stabilize the structure type 2, to conditions that
might further favour the formation of the new isomers 2
rather than isomers of type 1, and to the further investi-
gation of ligand stacking phenomena in these and other
related boron systems.
˚
˚
Ž .
Ž .
Ž .
P26n, Zs4. 2486 independent reflections collected "h, qk, ql
Ž .
Ž
.
19.804 3 , cs14.428 2 A, b s97.036 13 8, Us1584.3 4 A ,
Ž
.
w
for 3.81Fu F64.398 Stoe STADI4 diffractometer, 200 2 K, Cu
˚
x
Ž
.
x
Ka , l 1.54184 A . Empirical absorption correction c scans ap-
w
x w
plied. Structure solved by direct methods SHELXS-86 13 and
w
x w x
refined by full-matrix least-squares SHELXL-93 14 . Non-hydro-
gen atoms refined anisotropically; ligand hydrogen atoms constrained
to ideal positions; borane-cluster hydrogen atoms located on a Fourier
difference map and freely refined isotropically. The bridgingrendo
Ž .
hydrogen atom associated with B 5 appeared to be disordered with
Ž . Ž . Ž . Ž .
half-occupancy over the two positions B 4 B 5 and B 5 B 6 . 2477
data refined by full-matrix least-squares on F2 12 restraints, 242
Ž
.
Ž
.
parameters to final residuals all data R1s0.0576, wR2 s0.1381,
G.o.Fs1.097.

(
)
444
K.L.F. Callaghan et al.rJournal of Organometallic Chemistry 550 1998 441–444
w x
9
B.M. Graybill, J.K. Ruff, M.F. Hawthorne, J. Am. Chem. Soc.
Acknowledgements
Ž
.
83 1961 2669.
w
x
10 B.M. Graybill, A.R. Pitochelli, M.F. Hawthorne, J. Org. Chem.
We thank the Deutsche Forschungsgemeinschaft and
the UK EPSRC for support.
Ž
.
1 1962 626.
w
w
x
Ž
.
11 M. Roth, P. Paetzold, Chem. Ber. 128 1995 1221.
x
12 U. Doerfler, J.D. Kennedy, M. Thornton-Pett, J. Chem. Soc.,
Dalton Trans. in press.
w
w
x
13 G.M. Sheldrick, SHELXS-86, Program for Crystal Structure
Solution, University of Gottingen, Germany, 1986.
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¨
x
14 G.M. Sheldrick, SHELXL-93, Program for Crystal Structure
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Ž .
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w x
2
¨
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x
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Ž
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.
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´
Ž
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x
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w x
6
Ž
.
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Ž
.
w
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w
x
Ž
.
1963 3378.
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Ž .
x
Ž
.
7
F.J. Gerhart, R.E. Williams, J. Am. Chem. Soc. 87 1965 3513.
w x
Ž
.
x
8
S.G. Shore in: E.L. Muetterties Ed. , Boron Hydride Chem-
istry, Academic Press, New York, 1975, pp. 144–146, and
references therein.
Chemistry, Heidelberg, Germany, July 14–18 1996, Abstract
No. P117, p. 210.