A simple entry into nido-C2B10 clusters: HCl promoted cleavage of the C-C bond in ortho-carboranyl diphosphines

Article abstract of DOI:10.1039/b719404h

Treatment of the diphosphines ortho-B₁₀H₁₀C(P ^tBu₂)C(PR₂) (R = Et, Cy, Ph) with HCl gives the zwitterionic, nido-12-vertex species B₁₀H₁₀C(PH ^tBu₂)C(PClR₂); these reactions are reversed by the addition of NEt₃.

Full text of DOI:10.1039/b719404h

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A simple entry into nido-C2B10 clusters: HCl promoted cleavage of the C–C
bond in ortho-carboranyl diphosphines†‡§
Jonathan P. H. Charmant, Mairi F. Haddow, Rakesh Mistry, Nicholas C. Norman,* A. Guy Orpen and
Paul G. Pringle*
Received 17th December 2007, Accepted 17th January 2008
First published as an Advance Article on the web 7th February 2008
DOI: 10.1039/b719404h
Treatment of the diphosphines ortho-B10H10C(PtBu2)C(PR2)
(R = Et, Cy, Ph) with HCl gives the zwitterionic, nido-12-
vertex species B10H10C(PHtBu2)C(PClR2); these reactions are
reversed by the addition of NEt3.
The high thermodynamic stability of closo-12-vertex clusters
such as closo-1,2-dicarbadodecaborane(12) (commonly known as
ortho-carborane) is an impediment to the synthesis of supra-
icosahedral clusters1 although alkali metals do reduce ortho-
carborane to the nido-anion [C2B10H12]2− which provides a gateway
to 13-vertex clusters and beyond.2 Here we report a very mild and
reversible cage opening reaction of ortho-carboranyl diphosphines.
The bulky monophosphine 1 was prepared from ortho-
˚
Fig. 1 The molecular structure of 3. Important molecular dimensions (A)
include: P1–C1 1.767(4), C3–P1 1.859(5), C7–P1 1.854(4), P2–C2 1.701(5),
C13–P2 1.767(6), C11–P2 1.802(5), Cl1–P2 2.041(2), C1 · · · C2 2.851(9).
carborane by the route shown in Scheme 1; no ortho-
B10H10C2(PtBu2)2 was observed presumably because of the steric
t
inhibition to disubstitution by the bulky Bu2P substituent in 1.
n
Deprotonation of 1 with BuLi followed by addition of ClPEt2
gave the unsymmetrical diphosphine 2. The 31P NMR spectrum of
2 in CDCl3 shows a characteristic AX pattern with a large 3J(PP)
of 99 Hz.¶
(1)
gave 3 rapidly and quantitatively. This reaction is readily and
quantitatively reversed by treatment of 3 with NEt3 (eqn (1)). It is
notable that a single regioisomer of 3 is formed in this reaction,
i.e. the PHEt2/PCltBu2 isomer was not observed.
A reaction outline which accounts for the formation of 3 is
shown in Scheme 2. The ylide structure 3y is a resonance form
of the zwitterionic structure 3z which has the charges on the two
phosphonium centres balanced by the dianionic charge on the
carborane cage.
Scheme 1
It was noted that the 31P NMR spectrum of a sample of 2 that
had been standing in CDCl3 for 28 d was very different from that of
a freshly prepared solution. The AX pattern for 2 was absent and
had been replaced by two singlets, one of which had an unusual
chemical shift of 122 ppm and the other at 28 ppm showed a large
J(PH) of 435 Hz, consistent with the presence of a P–H bond.
Crystals of the new product 3 grew from its CDCl3 solution and
its structure was determined by X-ray crystallography (Fig. 1).
The reaction between 2 and CDCl3 is formally the addition
of DCl to the two phosphine groups with concomitant cluster
opening via C–C cleavage. A more rational synthesis of 3 is shown
in eqn (1);the addition of one equivalent of ethereal HCl to 2
Scheme 2
From the zwitterion formulation 3z, a 12-vertex, nido structure
is predicted on the basis of the 14 cluster valence electron pairs.3
However the 13-vertex polyhedron on which 3 is based is not
a C2v-docosahedron,4 a henicosahedron5 nor an edge-bridged
nido-icosahedral fragment.6 Intriguingly, the structure is formally
derived from an unprecedented C2-symmetric docosahedron
(Scheme 3). The additional vertex in the parent deltahedron is
connected to 6 other vertices; two of these are 6-connected BH
School of Chemistry, University of Bristol, Cantock’s Close, Bristol, UK
BS8 1TS. E-mail: paul.pringle@bristol.ac.uk, n.c.norman@bristol.ac.uk;
Fax: +44 117 929 0509; Tel: +44 117 928 8114
† This Communication is dedicated to Professor Ken Wade on the occasion
of his 75th birthday.
‡ The HTML version of this article has been enhanced with colour images.
§ CCDC reference numbers 671421–671423. For crystallographic data in
CIF or other electronic format see DOI: 10.1039/b719404h
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Scheme 3
vertices leading to a highly unusual situation in which there are 3
adjacent 6-connected vertices. There are also two 4-connected CR
vertices and 8 conventional 5-connected BH vertices completing
the cage. It may be noted that the C2 symmetry of both the parent
and nido structures renders them chiral even in a situation in which
the CR groups are equivalent.
The conversion of 2 to 3 is the first example of a closo-
to nido-carborane transformation effected by manipulation of
exopolyhedral substituents. In effect, the addition of HCl has
simultaneously oxidised the P centres and reduced the carborane
cage. To explore the generality of this reaction, the unsymmetrical
diphosphines 4 and 5 (made from 1 by a modification of the route
shown in Scheme 1) and symmetrical diphosphines 6 and 7 were
treated with HCl in Et2O and the products examined by 31P NMR
spectroscopy.
˚
Fig. 2 The molecular structure of 4. Important molecular dimensions (A)
include: P1–C1 1.8891(11), P1–C3 1.8704(12), P1–C9 1.8580(11), P2–C2
1.9162(11), P2–C15 1.9075(12), P2–C19 1.9195(12), C1–C2 1.7642(16).
bond length of C–C bonds found in closo-12-vertex C2B10 cages is
˚
˚
1.686 A (with sample standard deviation 0.050 A).
During the synthesis of 2 by the route shown in Scheme 1,
a minor product crystallised from the reaction mixture and its
crystal structure (Fig. 3) revealed it to be compound 8 with a nido-
carborane cage analogous to 3 and therefore can be represented
by the zwitterionic formulation 8z. The elements of BuH have
formally added to the two P groups of 2 in the formation of
8 (though the origin of 8 is possibly a halobutane impurity in
the commercial BuLi solution). Regardless of the mechanism of
formation of 8, its crystal structure establishes that the unusual
nido-12-vertex polyhedron of 3 is not unique.
No reaction was observed between HCl and 7 but with 4–6,
it was clear that products analogous to 3 were regioselectively
formed since a single product was observed in each case. Thus
reaction between 4 and ethereal HCl (1 equiv.) gave a species
with 31P parameters (singlet at 125 ppm and doublet at 39 ppm,
J(PH) 447 Hz) similar to those for 3. The reaction of 5 required
a 2-fold excess of HCl to give a cage-opened product (singlet at
121 ppm and doublet at 21 ppm, J(PH) 476 Hz). The symmetrical
diphosphine 6 required a 10-fold excess of HCl to give the cage-
opened product (singlet at 102 ppm and doublet at 34 ppm,
J(PH) 446 Hz) but only 75% conversion of 6 was observed. From
these experiments, HCl promoted cage-opening occurs readily
with electron-rich diphosphinocarboranes to give the regioisomer
expected from a mechanism involving protonation of the more
electron-rich phosphine group (Scheme 2).
The reactions between 2 or 4–7 and HCl can be described by the
equilibria shown in eqn (2).With 2 and 4, the equilibria lie fully to
Fig. 3 The molecular structure of 8. Important molecular dimensions
˚
(A) include: C1–P1 1.763(3), C3–P1 1.868(3), C7–P1 1.874(3), C11–P1
(2)
1.822(3), C2–P2A 1.740(3), C15–P2A 1.805(3), C17–P2A 1.783(3),
C1 · · · C2 2.892(5).
the side of the nido-cage, with 7, fully to the side of the closo-cage
and with 5 and 6, between these extremes.
The propensity of diphosphines 2 and 4–6 to undergo cage-
opening by C–C rupture may be a consequence of two effects: (i)
the relief of steric congestion caused by the bulky P-substituents
and (ii) the C–C weakening which has been calculated to be
promoted by electron-rich C-substituents in o-C2R2B10H10.7 Con-
sistent with this conjecture, in the structure of 4 (Fig. 2), the C–C
distance of 1.7642(16) is at the high end of the range of C–C
bonds in ortho-carboranes. A search of the Cambridge Structural
Database (Version 5.29, November 2007) reveals that the mean
In conclusion, a surprisingly easy route to neutral, nido-12-
vertex carboranes has been discovered which might prove to be
useful en route to higher nuclearity carboranes.
1410 | Dalton Trans., 2008, 1409–1411
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1.478 mm−1, Rint = 0.0584 (for 10218 measured reflections), R1 = 0.0678
[for 3645 unique reflections with I>2r(I)], wR2 = 0.1892 (for all 4529
unique reflections).
Acknowledgements
We thank Sasol for a studentship (to RM), and Professor Alan
Welch (Heriot-Watt University) and Dr John Jeffery (University
of Bristol) for useful discussions.
1 (a) A. S. F. Boyd, A. Burke, D. Ellis, D. Ferrer, B. T. Giles, M. A. Laguna,
R. McIntosh, S. A. Macgregor, D. L. Ormsby, G. M. Rosair, F. Schmidt,
N. M. Wilson and A. J. Welch, Pure Appl. Chem., 2003, 75, 1325 and ref.
therein; (b) L. D. Brown and W. N. Lipscomb, Inorg. Chem., 1977, 16,
2989; (c) P. v. R. Schleyer, K. Najafian and A. M. Mebel, Inorg. Chem.,
1998, 37, 6765.
2 (a) W. N. Lipscomb and L. Massa, Inorg. Chem., 1992, 31, 2297; (b) L.
Deng and Z. Xie, Organometallics, 2007, 26, 1832; (c) L. Deng and Z.
Xie, Coord. Chem. Rev., 2007, 251, 2452 and ref. therein.
Notes and references
¶ Unless otherwise stated, all compounds reported here have been
characterised by a combination of 1H, 13C and 31P NMR spectroscopy.31P
NMR data (CDCl3): d 1, 94.8; 2, 71.6 (d, J(PP) 99 Hz, PtBu2), 1.4 (d,
J(PP) 99 Hz, PEt2); 4, 70.3 (d, J(PP) 87 Hz, PtBu2), 23.4 (d, J(PP) 87 Hz,
PCy2); 5, 71.0 (d, J(PP) 97 Hz, PtBu2), 3.0 (d, J(PP) 97 Hz, PPh2); 6, 27.0.
Crystal data: 3: C14H39B10ClP2, M = 412.94, monoclinic, a = 17.625(4),
3 (a) K. Wade, Adv. Inorg. Chem. Radiochem., 1976, 18, 1; (b) R. E.
Williams, Adv. Inorg. Chem. Radiochem., 1976, 18, 67.
4 (a) M. F. Hawthorne, G. B. Dunks and M. M. McKown, J. Am. Chem.
Soc., 1971, 93, 2541; (b) L. I. Zakharkin, G. G. Zhigareva, A. V. Polyakov,
A. I. Yanovskii and Yu. T. Struchkov, Bull. Acad. Sci. USSR Div. Chem.
Sci. (Engl. Transl.), 1987, 36, 798; (c) A. S. Batsanov, R. C. B. Copley,
M. G. Davidson, M. A. Fox, T. G. Hibbert, J. A. K. Howard and K.
Wade, J. Cluster Sci., 2006, 17, 119.
5 A. Burke, D. Ellis, B. T. Giles, B. E. Hodson, S. A. Macgregor, G. M.
Rosair and A. J. Welch, Angew. Chem., Int. Ed., 2003, 42, 225.
6 (a) E. I. Tolpin and W. N. Lipscomb, Inorg. Chem., 1973, 12, 2257;
(b) M. R. Churchill and B. G. DeBoer, Inorg. Chem., 1973, 12, 2674.
7 (a) J. M. Oliva, N. L. Allan, P. v. R. Schleyer, C. Vinas and F. Teixidor,
J. Am. Chem. Soc., 2005, 127, 13538; (b) M. A. Fox, C. Nervi, A. Crivello
and P. J. Low, Chem. Commun., 2007, 2372.
◦
3
˚
˚
b = 8.5424(17), c = 17.324(4) A, b = 113.60(3) , V = 2390.0(11) A , T =
100 K, space group P21/c, Z = 4, l = 0.293 mm−1, Rint = 0.1403 (for
19691 measured reflections), R1 = 0.0717 [for 2388 unique reflections with
>2r(I)], wR2 = 0.1794 (for all 3746 unique reflections); 4: C22H50B10P2,
˚
M = 484.66, ◦monoclinic, a = 9.936(2), b = 17.900(4), c = 16.742(3) A,
3
˚
b = 106.23(3) , V = 2859.0(10) A , T = 100 K, space group P21/c, Z =
4, l = 0.164 mm−1, Rint = 0.0384 (for 31046 measured reflections), R1 =
0.0349 [for 5875 unique reflections with I >2r(I)], wR2 = 0.1085 (for all
6552 unique reflections); 8: C18H48B10O0.12P2, M = 436.52, triclinic, a =
˚
8.964(3)◦, b = 10.496(3), c = 15.234(4) A, a = 70.82(2), b = 80.68(2), c =
3
¯
˚
89.78(3) , V = 1334.1(7) A , T = 100 K, space group P1, Z = 2, l =
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The Royal Society of Chemistry 2008
Dalton Trans., 2008, 1409–1411 | 1411
©