Deboronation of ortho-carborane by an iminophosphorane: Crystal structures of the novel carborane adduct nido ortho carborane tris(dimethylamino)iminophosphorane complex and the borenium salt [(Me2N)3PNHBNP(NMe2)3],O2+(C2B9H12-)2

Article abstract of DOI:10.1039/a903030a

The structures of the adduct nido-C₂B₁₀H₁₂·HNP(NMe₂)₃ and the borenium salt [(Me₂N)₃PNHBNP(NMe₂)₃]₂O²⁺(C₂B₉H₁₂^-)₂, both obtained from 1,2-C₂B₁₀H₁₂ and HNP(NMe₂)₃, were determined by X-ray crystallography; the adduct represents the first structurally determined carborane of its type, a possible intermediate in the well known conversion of Closo-1,2-C₂B₁₀H₁₂ into the nido-7,8-C₂B₉H₁₂^- anion by bases.

Full text of DOI:10.1039/a903030a

Deboronation of ortho-carborane by an iminophosphorane: crystal structures
of the novel carborane adduct nido-C2B10H12·HNP(NMe2)3 and the borenium
2
salt [(Me2N)3PNHBNP(NMe2)3]2O2+(C2B9H12
)
2
Matthew G. Davidson, Mark A. Fox,* Thomas G. Hibbert, Judith A. K. Howard, Angus Mackinnon, Ivan S.
Neretin and Kenneth Wade
Chemistry Department, Durham University Science Laboratories, South Road, Durham, UK DH1 3LE.
E-mail: m.a.fox@durham.ac.uk
Received (in Cambridge, UK) 16th April 1999, Accepted 20th July 1999
The structures of the adduct nido-C2B10H12·HNP(NMe2)3
and
the
borenium
salt
[(Me2N)3PNHBNP(N-
Me2)3]2O2+(C2B9H122)2, both obtained from 1,2-C2B10H12
and HNP(NMe2)3, were determined by X-ray crystallog-
raphy; the adduct represents the first structurally deter-
mined carborane of its type, a possible intermediate in the
well known conversion of closo-1,2-C2B10H12 into the nido-
7,8-C2B9H122 anion by bases.
The closo-icosahedral carboranes C2B10H12 have an extensive
(3-dimensional aromatic) chemistry with implications for
neutron scavenging, for materials (thermally stable or conduct-
ing or otherwise electroactive oligomers and polymers), for
metal extraction, supramolecular chemistry and catalysis.1
Remarkably resilient to heat and oxidising agents, they are
known to have one very important degradation reaction, being
susceptible to nucleophilic attack by a select few powerful
Lewis bases (e.g. alkoxides, fluoride, amines)2–5 which can
remove one of their BH units, formally as BH2+, leaving nido-
shaped C2B9H11 (or C2B9H122) anionic residues that can
22
strongly bind metal ions to their open C2B3 faces,6 useful for
metal-extraction and catalytic applications. However, although
such base deboronations have been known for 35 years, details
of their mechanisms have remained elusive. Here, we describe
work that sheds light on the mechanism, reporting the X-ray
structural characterization of an adduct C2B10H12·HNP(NMe2)3
2, formed in the nucleophilic attack by the new deboronating
base HNP(NMe2)3 on ortho carborane, 1,2-C2B10H12 1. We
also show that the hitherto unknown monoboron cation
Scheme 1
toluene (15 ml) to a solution of 1 (1.44 g; 10 mmol) in toluene
(15 ml) resulted in the formation of a crystalline product (ca.
0.03 g) overnight. Although this product was identified by
2
boron NMR spectroscopy as a mixture of 1 and 7,8-C2B9H12
in solution, an X-ray structural determination on a crystal
selected from the material revealed a carborane adduct
C2B10H12·HNP(NMe2)3 2 (Fig. 1).‡ The volume of the
decanted solution was halved by solvent removal in vacuo and,
after standing for a week at room temperature, the crystals
+
(Me2N)3PNHB[NP(NMe2)3]2 is a later product of the same
reaction, and that the dication [(Me2N)3PNHBNP(NMe2)3]2O2+
is an unexpected product if traces of water are present.
+
formed were identified by NMR as the salt H2NP(NMe2)3
Iminotris(dimethylamino)phosphorane HNP(NMe2)3 is very
effective in the conversion of closo-C2B10H12 1 into nido-
7,8-C2B9H122. Several NMR-scale reactions of carborane 1
with dry HNP(NMe2)3 in anhydrous deuterated toluene at
varying temperatures (220 to 20 °C), ratios (carborane+base
1+10 to 10+1) and concentrations (0.1 to 0.01 M) were
monitored by 11B NMR spectroscopy. We found that boron
peaks corresponding to a carborane intermediate were best
observed using a 1+1 ratio mixture of low concentration (0.01
M) at room temperature after only two minutes reaction time.
The peaks observed arising from the intermediate at 34.8 (br),
218.2 (d) and 227.8 (d) ppm accounted for only 2% of the total
boron peak intensities (the major carborane is 1 with peaks at
22.0, 28.7, 213.2 and 214.4 ppm at this stage) and
disappeared rapidly to peaks corresponding to the carborane
anion, C2B9H122.4 This anion was observed by multinuclear
NMR spectroscopy accompanied by a 1+1 mixture of the cation
(7,8-C2B9H12)2 3 (0.32 g) and confirmed by an X-ray structural
determination.‡ The mother liquor was cooled to 220 °C to
yield a white solid identified by NMR spectroscopy as the
protonated tris(imino)borane salt 4 (0.43 g).
Calculated (GIAO HF/6-31G*) boron NMR shifts§ gen-
erated from the X-ray geometry of the carborane adduct 2 show
good agreement with the intermediate observed in the NMR-
scale reactions once masking of certain peaks by the starting
carborane 1 is taken into account. This suggests that adduct 2 is
the first structurally characterized intermediate in the well
known conversion of closo-1,2-C2B10H12
1 into nido-
7,8-C2B9H122. As revealed by the X-ray structure of 2, the first
step of the closo–nido conversion is the attachment of the base
to the most positively charged boron atom near the two
neighbouring carbon atoms which pivots about B(10), cleaving
the two B–C bonds and stretching the two B–B bonds to B(9)
and B(11). The structure of adduct 2 may be contrasted with the
hydrogen-bonded structures of C2B10H12·OP(NMe2)3 and
C2B10Cl10H2·OSMe2 adducts.7
+
H2NP(NMe2)3 3 and the novel protonated tris(imino)-
+
borane (Me2N)3PNHB[NP(NMe2)3]2 4 respectively (Scheme
1).†
As expected from simple skeletal-electron counting rules for
a 12-vertex 28 skeletal electron nido geometry, the cluster
framework of carborane 2 is viewed as a 13-vertex closo-
deltahedron with a 5-coordinate vertex removed.8 Positions of
A preparative scale reaction was carried out to allow
structural characterization of the products. Under nitrogen, slow
addition of dry HNP(NMe2)3 (1.78 g, 10 mmol) in anhydrous
Chem. Commun., 1999, 1649–1650
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BOB(NHR)(NR)]2+ there is strong BN p bonding within each
RN(RNH)B unit to the imino (NR) residue, much less to the
imine (RNH) residue, though evidently sufficient to ensure
planarity of the (PN)2BO unit.
Our work suggests that the formation of an adduct
C2B10H12·L, with an expected nido-structure, is the first step in
the deboronation of ortho-carborane by HNP(NMe2)3 and
presumably other Lewis bases L. Further studies, both experi-
mental and theoretical, will be needed to clarify the later
steps.
We are grateful to the EPSRC (M. A. F.), BNFL (T. G. H.)
and Durham University (Sir Derman Christopherson Fellow-
ship to J. A. K. H. and studentship to A. M.) for financial
support.
Notes and references
† NMR data for cation of 3: dP (d8-toluene; standard 85% H3PO4) 43.2, dH
2.33 [J(PH) 10 Hz, CH3] dC 36.7 [J(PC) 18 Hz, CH3]; for cation of 4: dP
40.2 (br, 1P), 23.6 (br, 2P), dB (standard BF3·Et2O), 22.3 (s), dH 2.46
[3J(PH) 10 Hz, 18H, CH3], 2.43 [J(PH) 10 Hz, 36H, CH3], dC 37.1 [J(PC)
18 Hz], 36.8 [J(PC) 18 Hz].
‡ Crystal data for 2: C8H31B10N4P, M = 322.44, orthorhombic, space
group Pbca (no. 61), a = 10.799(2), b = 17.252(4), c = 20.326(4) Å, U =
3786.9(13) Å3, Z = 8, Dc = 1.131 g cm23, m = 0.141 mm21, F(000) =
1376, T = 153(2) K, 25525 reflections (4337 unique), 2q @55º, R1 = 0.077
(2221 data I > 2s(I)), wR(F2) = 0.187, GOF (obs) = 1.053. For 3:
C8H32B9N4P, M = 312.64, monoclinic, space group P21/n (no. 14), a =
11.580(2), b = 13.182(3) , c = 12.385(2) Å, b = 91.88(5)°, U = 1889.5(6)
Å3, Z = 4, Dc = 1.099 g cm23, m = 0.140 mm21, F(000) = 672, T =
150(2) K, 14804 reflections (4992 unique), 2q @ 58°, R1 = 0.0435 (4283
data I > 2s(I)), wR(F2) = 0.118, GOF(obs) = 1.085. All NMe2 groups of
Fig. 1 Molecular structure of adduct 2 (50% ellipsoids; methyl hydrogens
omitted for clarity). Important interatomic distances (Å) are: P(1)–N(1)
1.651(3), N(1)–H(13) 0.75(4), N(1)–B(12) 1.506(6), B(12)–H(12) 1.18(4),
C(7)–C(8) 1.530(6), C(7)–B(11) 1.654(6), C(8)–B(9) 1.651(6), B(12)–
B(10) 1.761(6), B(12)–B(9) 2.099(6), B(12)–B(11) 2.091(6), B(10)–B(9)
1.779(6), B(10)–B(11) 1.802(6). Selected angle (°): P–N(1)–B(12)
127.3(3).
the cation are disordered. For 5: C28H98B20N16P4O, M
= 1015.30,
orthorhombic, space group P212121 (no. 19), a = 14.105(3), b = 16.529(3),
c = 25.563(5) Å, U = 5960(2) Å3, Z = 4, Dc = 1.132 g cm23, m = 0.168
mm21, F(000) = 2184, T = 150(2) K, 44070 reflections (16579 unique), 2q
@ 61°, R1 = 0.062 (5792 data I > 2s(I)), wR(F2) = 0.138, GOF (obs) =
0.893. The absolute configuration was determined; Flack parameter
20.04(9). CCDC 182/1339. See http://www.rsc.org/suppdata/cc/
1999/1649/ for crystallographic files in .cif format.
§ Calculated (GIAO HF/6-31G*) 11B NMR data for 2: d 35.7 (B12), 26.3
(B5,6), 26.6 (B2,4), 213.9 (B3), 215.4 (B9,11), 218.3 (B10), 224.0
(B1).
¶ Calculated (AM1) bond orders for 5 (p bond order in parentheses): B(1)–
O(1) 1.029 (0.207), B(1)–N(1) 1.311 (0.441), B(1)–N(3) 0.829 (0.086),
B(2)–O(1) 0.932 (0.152), B(2)–N(4) 0.920 (0.129), B(2)–N(2) 1.317
(0.444).
1 J. Plesˆek, Chem. Rev., 1992, 92, 269 and references therein; M. F.
Hawthorne, in Advances in Boron Chemistry, ed. W. Siebert, The Royal
Society of Chemistry, Cambridge, 1997, p. 261 and references therein.
2 R. A. Wiesboeck and M. F. Hawthorne, J. Am. Chem. Soc., 1964, 86,
1642; L. I. Zakharkin and V. N. Kalinin, Tetrahedron Lett., 1965, 7,
407.
3 M. F. Hawthorne, D. C. Young, P. M. Garrett, D. A. Owen, S. G.
Schwerin, F. N. Tebbe and P. A. Wegner, J. Am. Chem. Soc., 1968, 90,
862; L. I. Zakharkin and V. S. Kirillova, Bull. Acad. Sci. USSR Div.
Chem. Sci. (Engl. Transl.), 1975, 2484.
Fig. 2 Molecular structure of cation in 5 (50% ellipsoids; methyl hydrogens
omitted for clarity). Important interatomic distances (Å) are: B(1)–N(1)
1.402(6), B(1)–N(3) 1.463(5), B(1)–O(1) 1.373(5), B(2)–N(2) 1.389(5),
B(2)–N(4) 1.464(6), B(2)–O(1) 1.416(5), N(1)–P(1) 1.555(3), N(2)–P(2)
1.554(3), N(3)–P(3) 1.630(4), N(4)–P(4) 1.614(3).
4 H. Tomita, H. Luu and T. Onak, Inorg. Chem., 1991, 30, 812.
5 M. A. Fox, J. A. H. MacBride and K. Wade, Polyhedron, 1997, 16, 2499;
M. A. Fox and K. Wade, Polyhedron, 1997, 16, 2517.
6 N. N. Greenwood and A. Earnshaw, Chemistry of the Elements, 1st edn.,
Pergamon, Oxford, 1984, p. 209 and references therein; J. Plesˆek and S.
Heˆrmánek, Inorg. Synth., 1984, 22, 231 and references therein.
7 M. G. Davidson, T. G. Hibbert, J. A. K. Howard, A. Mackinnon and K.
Wade, Chem. Commun., 1996, 2285; A. I. Yanovskii, Yu. T. Struchkov,
L. E. Vinogradova and L. A. Leites, Bull. Acad. Sci. USSR, Div. Chem.
Sci. 1983, 1988.
8 F. Meyer, J. Muller, P. Paetzold and R. Boese, Angew. Chem., Int. Ed.
Engl., 1992, 31, 1227 and references therein.
9 L. I. Zakharkin, G. G. Zhigareva, A. V. Polyakov, A. I. Yanovskii and Y.
T. Struchkov, Bull. Acad. Sci. USSR, Div. Chem. Sci. (Engl. Transl.),
1987, 798.
the cage carbons in the carborane adduct 2 were conclusively
determined by an ab initio optimization of a model of
C2B10H12·HNP(NH2)3 at the HF/6-31G* level of theory where
both theoretical and experimental geometries are nearly identi-
cal. These carbon placements are supported by a similar cage
geometry observed in the nido-C2B10 moiety of the two-cage
anion [PhCB10H10CB10H10C2HPh]2 structurally characterized
by X-ray crystallography.9
During attempts to obtain suitable crystals of the salt 4
containing the cation (Me2N)3PNHB[NP(NMe2)3]2+ from tolu-
ene, a single crystal was characterized by an X-ray study‡ as a
salt 5 containing the dication [(Me2N)3PNHBNP(NMe2)3]2O2+
(Fig. 2). The oxygen atom apparently arose from inadequately
dried toluene. According to bond order calculations¶ (AM1)
carried out on the structure of the dication [RN(RNH)-
Communication 9/03030A
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Chem. Commun., 1999, 1649–1650