Fourteen-vertex homo- and heterobimetallic metallacarboranes

Article abstract of DOI:10.1039/b500236b

Reduction of 4-(p-cymene)-4,1,12-closo-RuC₂B₁₀H ₁₂ followed by metallation with {M′} fragments (M′ = {CpCo²⁺}, {(arene)Ru²⁺} or {(dppe)Ni²⁺}) affords 14-vertex bimetallic 1,14,2,10-RuM′C

Full text of DOI:10.1039/b500236b

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Fourteen-vertex homo- and heterobimetallic metallacarboranes
David Ellis, Maria Elena Lopez, Ruaraidh McIntosh, Georgina M. Rosair and Alan J. Welch*
Received (in Cambridge, UK) 7th January 2005, Accepted 25th January 2005
First published as an Advance Article on the web 15th February 2005
DOI: 10.1039/b500236b
Reduction of 4-(p-cymene)-4,1,12-closo-RuC2B10H12 with Na
in THF followed by treatment with CoCl2–NaCp afforded the
14-vertex ruthenacobaltacarborane {(p-cymene)Ru}{CpCo}-
C2B10H12 (1) in modest yield.{Compound 1 was characterised
by microanalysis, mass spectrometry and 1H and 11B NMR
spectroscopies but did not produce crystals suitable for X-ray
diffraction study. Treatment of the reduced metallacarborane with
[(g-C6H6)RuCl2]2 or [(p-cymene)RuCl2]2 similarly yielded the
diruthenacarboranes {(p-cymene)Ru}{(g-C6H6)Ru}C2B10H12 (2)
and {(p-cymene)Ru}2C2B10H12 (3) respectively, but in much better
yields.{ Compound 3 is revealed by NMR spectroscopy to be
symmetric, with only one set of resonances assigned to the
Reduction of 4-(p-cymene)-4,1,12-closo-RuC2B10H12 followed
by metallation with {M9} fragments (M9
{CpCo2+},
{(arene)Ru2+} or {(dppe)Ni2+}) affords 14-vertex bimetallic
1,14,2,10-RuM9C2B10 species having bicapped hexagonal anti-
prismatic structures.
5
Supraicosahedral heteroborane chemistry, established nearly
35 years ago with the synthesis of the 13-vertex metallacarborane
4-Cp-4,1,6-closo-CoC2B10H12,1 continues to be an area of
significant research activity: the possible structures of supraicosa-
hedral boranes and heteroboranes attract major interest from
computational chemists2 and, at least in principle, several of the
current applications of boron cluster compounds would be
enhanced if larger clusters could be prepared.3 Within carborane
chemistry the initial goals are large, supraicosahedral, carboranes,
e.g. C2BxH2+x with x . 10. Although we were recently successful
in preparing the first such compound (x 5 11)4 we have always
appreciated that valuable lessons could be learned by working first
with analogous metallacarboranes in which one or more {BH}
fragment was replaced by an isolobal transition metal fragment.5
Although there are in excess of a hundred 13-vertex metalla-
carboranes known there are fewer than ten 14-vertex bimetalla-
carboranes. From Me4C4B8H8 and subsequent thermolysis
Grimes has produced a series of 14-vertex compounds with
Fe2C4B8 cores.6 In these a closo Wadian skeletal electron count7
results from there being four {CMe} fragments (3e sources) and
two {CpFe} fragments (1e sources) per cluster: however, only the
final two isomers in the series are actually closed. A 14-vertex
bimetallacarborane analogous to Wadian 12-vertex MC2B9 and
13-vertex MC2B10 species would have only two {CH/CR}
fragments and two 2e {M} fragments, truly isolobal with {BH}.
In 1974 Evans and Hawthorne prepared such a compound,
(CpCo)2C2B10H12, in two isomeric forms (one from reduction and
metallation of a 4,1,8-CoC2B10 precursor, the other from similar
treatment of the analogous 4,1,12-CoC2B10 species). However,
these 14-vertex clusters were only principally characterised by mass
spectrometry and low-resolution 11B NMR spectroscopy.8
1
p-cymene ligand and one CcageH resonance in the H spectrum,
and only four resonances (relative integrals 1 : 2 : 1 : 1) in the
11B{1H} spectrum. Compound 2 has only five, equally intense,
resonances in its 11B{1H} spectrum, although eventual structural
assignment (vide infra) suggests that these are all 1 + 1 coincidences.
One particularly notable feature of the spectra of 2 and 3 is the low
frequency of the weighted average 11B chemical shifts, 221.5 and
221.2 ppm, respectively.
Although compound 2 could not be persuaded to crystallise
sufficiently well for a diffraction study, good-looking crystals of 3
were obtained by slow evaporation of a CH2Cl2 solution. The
results of a crystallographic determination of 3 are shown in
Fig. 1.{The molecule is located on a crystallographic C2 axis
passing through the mid points of the B2–B2A and B5–B5A
connectivities. The dimetallacarborane cluster has a bicapped
hexagonal antiprismatic structure with the Ru atoms in the
6-connected (with respect to the polyhedron) capping vertices.
Unfortunately it proved impossible to assign the cluster C atoms
with certainty and refinement was completed with B atoms
occupying all six crystallographically unique 5-connected sites.
There is additional minor disorder in the iPr group of the p-cymene
ligand.
The mixed ruthenium–nickel species {(p-cymene)Ru}-
{(dppe)Ni}C2B10H12 (4) was similarly afforded by addition of
(dppe)NiCl2 to the reduced ruthenacarborane, and initially
1
characterised by H, 11B and 31P NMR spectroscopies.{ For the
We recently reported the first synthesis of 13-vertex 4,1,10-
MC2B10 compounds and their facile isomerisation into corres-
ponding 4,1,12-isomers,9 work which has afforded us gram
quantities of a wide range of new 4,1,12-MC2B10 species. We
now describe the reduction and subsequent metallation of one of
these, 4-(p-cymene)-4,1,12-closo-RuC2B10H12, leading to a series of
both homo- and heterobimetallic 14-vertex MM9C2B10 metalla-
carboranes (the latter reported for the first time) in sufficient yields
for complete characterisation and ultimate further polyhedral
expansion reactions.
ruthenanickelacarborane 4 the weighted average 11B chemical shift
is 216.9 ppm, essentially the same as that for the ruthenacobalta-
carborane 2 (217.0 ppm), still relatively low frequency but not so
low as for the diruthenacarboranes 2 and 3. Although 4 is afforded
in poor yield relative to 2 and 3 it formed single crystals which
proved to be free of cage atom disorder allowing unequivocal
structural determination.{
A perspective view of a single molecule is given in Fig. 2. The
bicapped hexagonal antiprismatic structure observed for 3 is again
evident, but this time the cage C atoms were readily distinguished
on the twin bases of refined (as B) equivalent isotropic thermal
*a.j.welch@hw.ac.uk
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Fig. 1 Perspective view of the diruthenacarborane compound 3. The
molecule bestrides a crystallographic C2 axis passing through the mid
points of the B2–B2A and B5–B5A connectivities. The cage C atoms were
not located, all non-Ru cluster vertices being refined as B. Selected
Fig. 2 Perspective view of the ruthenanickelacarborane compound 4.
˚
Selected interatomic distances (A) include: Ru1–C2 2.257(4), Ru1–B3
2.281(4), Ru1–B4 2.253(4), Ru1–B5 2.261(4), Ru1–B6 2.283(4), Ru1–B7
2.254(4), Ni14–B8 2.198(4), Ni14–B9 2.167(4), Ni14–C10 2.306(4), Ni14–
B11 2.244(4), Ni14–B12 2.147(4), Ni14–B13 2.215(4), C–B 1.657(5)–
˚
interatomic distances (A) include : Ru1–B2 2.255(6), Ru1–B3 2.251(7),
Ru1–B4 2.232(7), Ru1–B5 2.265(6), Ru1–B6 2.251(7), Ru1–B7 2.241(6),
…
…
1.733(6), B–B 1.713(6)–1.795(6), Ru1 Ni14 4.3225(6). The upper
Ru1 Ru1A 4.3985(9). The dihedral angle between the least-squares
planes through B2B3B4B5B6B7 and B2AB3AB4AB5AB6AB7A is
(C2B3B4B5B6B7) and lower (B8B9C10B11B12B13) hexagonal least-
i
square planes are inclined at a dihedral angle of 0.16(1)u. The Pr group
of the p-cymene ligand shows partial disorder.
1.94(16)u.
parameters and interatomic separations. The species is thus
identified as 1-(p-cymene)-14-(dppe)-1,14,2,10,-closo-RuNiC2-
B10H12. By analogy, compounds 1–3 presumably have similar
heteroatom patterns, i.e. 1,14,2,10-RuCoC2B10, 1,14,2,10-
Ru2C2B10 and 1,14,2,10-Ru2C2B10, respectively. These results
confirm Hawthorne’s proposed structure of 1,14-Cp2-1,14,2,10,-
closo-Co2C2B10H12 for the isomer (‘isomer I’) derived from 4-Cp-
4,1,12-closo-CoC2B10H12.8 With the structure of 4 thus established
we looked again at the crystallographic structure of 3 focussing on
the 3/3A and 6/6A positions as potential C atom sites (both
possibilities would have afforded 1,14,2,10-Ru2C2B10 architectures
consistent with the crystallographic C2 symmetry) but neither was
convincing and this structure remains disordered.
the C atom its preferred low connectivity.11 Thus 4,1,8-, 4,1,12-
and 4,1,11-MC2B10 species would seem to be attractive 13-vertex
metallacarboranes for further reduction–expansion reactions (see I
for numbering) whilst expansion starting from the 4,1,2-, 4,1,6-
and 4,1,10-isomers could be more difficult (the remaining 4,1,X-
MC2B10 isomer, 4,1,5-, is unlikely to yield to synthesis since it
contains a high-connected C atom). So far both we and
Hawthorne have been successful in reduction–expansion of 4,1,8-
and 4,1,12-MC2B10 compounds. The 4,1,11-MC2B10 isomer has
not yet been reported but represents an attractive future target.
These studies establish the first complete characterisation of 14-
vertex M2C2B10 metallacarboranes and report the first examples of
heterobimetallic analogues. They provide a firm basis on which to
develop 15-vertex (and possibly beyond) heteroborane chemistry,
which is currently without precedent.
We thank the EPSRC (DE, MEL) and the Carnegie Trust
(RMcI) for support, Dr R. Ferguson and Mr G. P. Smith for mass
spectrometry, Dr A. F. S. Boyd for NMR spectroscopy and Ms C.
Graham for microanalysis.
David Ellis, Maria Elena Lopez, Ruaraidh McIntosh,
Georgina M. Rosair and Alan J. Welch*
School of Engineering & Physical Sciences, Heriot-Watt University,
Edinburgh, UK EH14 4AS. E-mail: a.j.welch@hw.ac.uk;
Fax: +44 131 4513180; Tel: +44 131 4513217
Attempted syntheses of 14-vertex bimetallacarboranes starting
from 4-(p-cymene)-4,1,10-closo-RuC2B10H12 were unsuccessful:
this isomer of the 13-vertex precursor does not appear to undergo
reduction by Na–naphthalene in THF.10 In cases where reduction
is successful we believe that the reduced (nido) species is stabilised
by the presence of a cage C atom in its open face, since this affords
Notes and references
{ Experimental procedure. For 1: 4-(p-cymene)-4,1,12-closo-RuC2B10H12
(0.10 g, 0.26 mmol) was dissolved in dry, oxygen-free THF (30 ml). Sodium
1918 | Chem. Commun., 2005, 1917–1919
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pieces (0.06 g, 2.50 mmol) and naphthalene (ca. 0.010 g) were added and
the solution was stirred at room temperature for 72 hours. The deep red
solution was cooled to 0 uC. NaCp (0.39 ml of a 2 M solution in THF,
0.78 mmol) was added followed by solid CoCl2 (0.13 g, 0.95 mmol) and the
reaction mix stirred at room temperature for 18 hours. The purple product
isolated by TLC (80% : 20% CH2Cl2 : 40/60 petrol, Rf 0.62). Yield 0.010 g
(8%). IR (CH2Cl2): nmax at 2503 cm21 (B–H). 1H NMR (CDCl3, 298 K): d
5.00–5.20 (m, 4 H, C6H4), 4.79 (s, 5 H, C5H5), 2.75 (sept, 1 H, CH(CH3)2),
1.95 (s, 3 H, CH3), 1.10 (d, 3 H, CH(CH3)), 1.06 (d, 3 H, CH(CH3)), neither
cage CH resolved. 11B{1H} NMR (CDCl3, 298 K): d 211.69 (3B), 215.31
(1B), 217.67 (3B), 220.17 (1B), 223.18 (2B). Mass spectrometry: m/z 503
reflections, R1 5 0.0670, wR2 5 0.1175, S 5 1.054 for refinement with all
data. CCDC 260453 and 260454 for 3 and 4, respectively. See http://
www.rsc.org/suppdata/cc/b5/b500236b/ for crystallographic data in .cif or
other electronic format.
1 G. B. Dunks, M. M. McKown and M. F. Hawthorne, J. Am. Chem
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2 L. D. Brown and W. N. Lipscomb, Inorg. Chem., 1977, 16, 2989;
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17, 2041; J. Bicerano, D. S. Marynick and W. N. Lipscomb, Inorg.
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We restrict this list to calculations on true supraicosahedral species and
do not include the elegant work of Jemmis et al. on large fused
icosahedral and subicosahedral species, e.g. E. D. Jemmis,
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102, 93.
3 A. H. Soloway, W. Tjarks, B. A. Barnum, F. G. Rong, I. M. Codogni
and J. G. Wilson, Chem. Rev., 1998, 98, 1515; I. Krossing and I. Raabe,
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G. M. Rosair and A. J. Welch, Angew. Chem., Int. Ed., 2003, 42, 225.
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+
(M ). Satisfactory microanalytical data were obtained for all compounds
reported. For 2: 4-(p-cymene)-4,1,12-closo-RuC2B10H12 (0.26 mmol)
reduced and treated with [(g-C6H6)RuCl2]2 (0.13 mmol). Yield 35%. IR
(CH2Cl2): nmax at 2492 cm21 (B–H). 1H NMR (CDCl3, 298 K): d 5.35 (s,
6 H, C6H6), 5.05–5.25 (m, 4 H, C6H4), 2.60 (sept, 1 H, CH(CH3)2), 1.95 (s,
3 H, CH3), 1.75 (br s, 1 H, CcageH), 1.15 (d, 3 H, CH(CH3)), 1.07 (d, 3 H,
CH(CH3)), 0.80 (br s, 1 H, CcageH). 11B{1H} NMR (CDCl3, 298 K): d
216.49 (2B), 220.68 (2B), 221.27 (2B), 222.90 (2B), 225.94 (2B). Mass
spectrometry: m/z 560 (M +). For 3: 4-(p-cymene)-4,1,12-closo-RuC2B10H12
(0.75 mmol) reduced and treated with [(p-cymene)RuCl2]2 (0.38 mmol).
Yield 40%. IR (KBr): nmax at 2520 cm21 (B–H). 1H NMR (CDCl3, 298 K):
d 5.15–5.25 (m, 8 H, C6H4), 2.65 (sept, 2 H, CH(CH3)2), 2.00 (s, 6 H, CH3),
1.70 (br s, 2 H, CcageH), 1.05 (overlapping d, 12 H, CH(CH3)2). 11B{1H}
NMR (CDCl3, 298 K): d 216.14 (2B), 220.69 (4B), 223.01 (2B), 225.60
(2B). Mass spectrometry: m/z 615 (M +), 470 (M–C2B10H12). For 4: 4-
(p-cymene)-4,1,12-closo-RuC2B10H12 (0.26 mmol) reduced and treated with
(dppe)NiCl2 (0.26 mmol). Yield 7%. IR (CH2Cl2): nmax at 2502 cm21
(B–H). 1H NMR (CDCl3, 298 K): d 7.2–7.7 (m, 20H, C6H5), 5.02–5.19 (m,
4 H, C6H4), 2.62 (sept, 1 H, CH(CH3)2), 2.19 (br s, 2 H, CH2), 1.90 (s, 3 H,
CH3), 1.77 (br s, 2 H, CH2), 1.05 (d, 6 H, CH(CH3)2), neither cage CH
resolved. 11B{1H} NMR (CDCl3, 298 K): d 29.34 (2B), 213.40 (2B),
216.72 (3B), 220.34 (1B), 224.80 (1B), 227.78 (1B). 31P{1H} NMR
(CDCl3, 298 K): d 61.9.
6 W. M. Maxwell, R. F. Bryan and R. N. Grimes, J. Am. Chem. Soc.,
1977, 99, 4008; W. M. Maxwell, R. Weiss, E. Sinn and R. N. Grimes,
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{ Crystal data. For 3: C22H40B10Ru2, Mr 5 614.78, monoclinic, C2/c,
˚
a 5 16.319(2), b 5 16.9323(18), c 5 10.3987(12) A, b 5 115.413(5)u,
V 5 2595.3(5) A , Z 5 4 (C2 symmetry imposed), m 5 1.175 mm21
,
3
˚
8 W. J. Evans and M. F. Hawthorne, J. Chem. Soc., Chem. Commun.,
1974, 38.
F(000) 5 1240. Data to hmax 5 28.7u collected at 100(2) K on a Bruker
AXS X8 diffractometer using Mo-Ka radiation. 14050 reflections, 3319
independent reflections, R1 5 0.1231, wR2 5 0.1082, S 5 0.961, for all
data. For 4: C38H50B10NiP2Ru, Mr 5 836.60, monoclinic, P21/n,
9 D. Ellis, M. E. Lopez, R. McIntosh, G. M. Rosair, A. J. Welch and
R. Quenardelle, Chem. Commun., 2005, DOI: 10.1039/b416646a.
10 A. Vicente, M. E. Lopez, D. Ellis and A. J. Welch, unpublished results.
11 R. E. Williams, Inorg. Chem., 1971, 10, 210; R. E. Williams, Chem. Rev.,
1992, 92, 177.
˚
a 5 12.7337(9), b 5 17.0657(12), c 5 17.4612(12) A, b 5 92.338(4)u,
3
V 5 3791.3(5) A , Z 5 4, m 5 1.009 mm21, F(000) 5 1720. Data collection
˚
as for 3 except hmax 5 24.6u. 85924 reflections collected, 6348 independent
This journal is ß The Royal Society of Chemistry 2005
Chem. Commun., 2005, 1917–1919 | 1919