9,12-Di­iodo-1,2-dicarba-closo-dodecaborane(12)

The title compound, C₂H₁₀B₁₀I₂, has a pseudo-icosahedral cluster geometry. The crystal structure features an intermole­cular C—H...I—B hydrogen bond with a normalized H...I distance of 3.00 Å.

S1. Comment

Carboranes with I atoms substituted at boron are precursors to a large number of B-alkyl-, B-allyl-, B-aryl- and B-ethynylcarboranes (Zakharkin et al., 1982; Li et al., 1991; Yang et al., 1994; Zheng et al., 1995; Jiang et al., 1995; Harakas et al., 1998; Lee et al., 2000; Fox & Wade, 2002). Some B-iodocarboranes have been structurally characterized (Zheng et al., 1995; Jiang et al., 1995; McGrath et al., 2000; Marshall et al., 2001). However, the most widely used precursor, 9,12-diiodo-1,2-dicarba-closo-dodecaborane(12), or 9,12-diiodo-ortho-carborane, (I), has not yet been structurally studied as an individual compound, although the structure of the 1:1 host–guest complex of (I) with the mercuracarborand (C₂B₁₀H₈Et₂Hg)₄ has been reported by Yang et al. (1994). In this complex, the mercuracarborand host acts as an `inverse crown-ether' to which (I) is coordinated via Hg···I interactions. This nucleophilic role of the iodine substituents is of particular interest since (I) is also a precursor to a mercuracarborand, (C₂B₁₀H₈I₂Hg)₄, which exhibits remarkable modular self-assembly in the solid state (Lee et al., 2001).

In this paper, we report the structure of (I), which we prepared by reacting 1,2-dicarba-closo-dodecaborane(12) and I₂ in acetic acid with an HNO₃/H₂SO₄ mixture. This method, developed in the syntheses of several B-iodocarboranes by Fox & Wade (1999, 2002) and McGrath et al. (2000), is more convenient than those reported earlier by Zakharkin & Kalinin (1966), Stanko et al. (1968), Li et al. (1991) and Zheng et al. (1995).

The molecular geometry of (I) is unremarkable (Fig. 1 and Table 1). The iodine substituents effectively prevent the rotational disorder of the highly symmetrical carborane cage which often hampers crystallographic studies of icosahedral carboranes. The location of the C atoms is not an issue, since the spectroscopic data locate them uniquely para to the substituted B atoms. An interesting feature of the crystal packing is the C—H···I hydrogen bonding. A covalently bonded I atom is not known as a good acceptor of hydrogen bonds, in contrast to the I⁻ anion (Desiraju & Steiner, 1999). This is not surprising, as in the covalent species in question were mostly either I₂ molecules, or organic molecules with C—I bonds. The electronegativities of iodine and carbon being equal (ca 2.5 on Pauling's scale; see Batsanov, 1990, and references therein), the C—I bond is non-polar, and so, of course. is the I—I bond. Thus, in either case, the I atom carries no net negative charge. In (I), however, iodine is bonded to an atom with much lower electronegativity (1.9) than itself. Thus, there should be a substantial shift of electron density toward the I atoms, which thus becomes capable of accepting hydrogen bonds. On the other hand, polarization of the B—I bonds can further enhance the acidity of the carborane CH groups, which, even in unsubstituted carborane, are sufficiently acidic to act as donors of hydrogen bonds.

In fact, the intermolecular contact C1—H1···I12(1/2 − x, 1 − y, 1/2 + z) can be regarded as a hydrogen bond; the H···I distance of 3.00 (3) Å, calculated for the corrected C—H bond distance of 1.083 Å (Allen et al., 1987), is shorter than the relevant van der Waals sum of 3.14 Å (Rowland & Taylor, 1996), and the C—H···I angle of 141 (3)° is sufficiently close to 180°.

These bonds link molecules into chains which spiral around 2₁ screw axes parallel to z. However, the other acidic H atom participates only in a much weaker C2—H2···I9(-x, 1/2 + y, 1/2 − z) interaction, with a corrected H···I distance of 3.32 (5). On the other hand, there is a relatively close B6—H6···I12(−1/2 − x, 1 − y, 1/2 + z) contact, with an H···I distance of 3.15 (4) Å (for the B—H bond length corrected to 1.216 Å; see Bohn & Bohn, 1971), which can not be described as a hydrogen bond.

A broadly similar pattern was observed in the crystal of the dibromo analogue of (I), 9,12-dibromo-1,2-dicarba-closo-dodecaborane(12) (Potenza & Lipscomb, 1966), although the latter has different symmetry (space group Pbn2₁). This structure also contains only one symmetrically independent C—H···Br hydrogen bond, with an H···Br distance of 2.53 Å (relative to the sum of the van der Waals radii for H and Br of 2.99 Å; Rowland & Taylor, 1996), the next shortest H···Br distance being 2.98 Å.

The crystal structure of 3-iodo-1,2-dicarba-closo-dodecaborane(12) has recently been reported (Barberà et al., 2002) and reveals self-assembly of carborane molecules via C—H···I hydrogen bonds. The C—H···I distance is 3.10 Å, with a C—H···I angle of 131° for a normalized C—H bond distance of 1.083 Å. Unlike in (I), the carbon/boron ordering of the cage in 3-iodo-1,2-dicarba-closo-dodecaborane(12) is directed by these hydrogen bonds.

S2. Experimental

A stirred solution of 1,2-dicarba-closo-dodecaborane(12) (1.44 g, 10 mmol) and iodine (2.54 g, 10 mmol) in glacial acetic acid (40 ml) was heated to 353 K and treated slowly with a mixture of HNO₃ and H₂SO₄ (1:1 v/v, 25 ml). A brown vapour was observed above the solution and within 1 h the solution became colourless. It was poured into distilled water (300 ml) and the precipitate was washed with water and then dissolved in ether. The ether solution was dried and the ether evaporated. The residue was recrystallized from hexane to give 3.25 g (82%) of crude (I), which contained a ca 13% admixture of the 8,9-diiodo isomer, (Ia) [m.p. 463–464 K, cf. 466–467 K for pure (I) (Stanko et al., 1968)]. This product displayed the NMR signals of (I) as reported by Li et al. (1991) and He˘rmánek (1999) [viz. ¹H {¹¹B} (δ in CDCl₃): 4.00 (C1,2H), 2.89 (B8,10H), 2.74 (B4,5,7,11H), 2.49 (B3,6H)], as well as those of (Ia) [¹¹B −0.8 (B12), −9.4 (B10), −13.2 (B4), −18.8 (B6), −20.9 (B8)]. Further recrystallization from a hexane–acetone (1:1) mixture gave X-ray quality crystals of pure (I).

S3. Refinement

All H atoms were located in difference Fourier maps. Their geometry was then idealized and they were refined using a riding model, with B—H = 1.12 Å and C—H = 1.00 Å, and with U_iso(H) values fixed to 1.2U_eq(B,C).

Computing details

Data collection: SMART (Bruker, 1997); cell refinement: SMART; data reduction: SAINT (Bruker, 1997); program(s) used to solve structure: SHELXS97 (Sheldrick, 1990); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: SHELXTL (Bruker, 1998); software used to prepare material for publication: SHELXTL.

Figures

Fig. 1. View of the molecular structure of (I) and the intermolecular hydrogen bonds [symmetry codes: (i) 1/2 − x, 1 − y, 1/2 + z; (ii) 1/2 − x, 1 − y, z − 1/2]. Displacement ellipsoids are drawn at the 50% probability level.

9,12-diiodo-1,2-dicarba-closo-dodecaborane(12)

Crystal data

C2H10B10I2	Dx = 2.155 Mg m^−3
Mr = 396.00	Melting point: 466–467 K K
Orthorhombic, P212121	Mo Kα radiation, λ = 0.71073 Å
a = 7.126 (3) Å	Cell parameters from 959 reflections
b = 12.320 (4) Å	θ = 11.9–25.9°
c = 13.905 (5) Å	µ = 5.10 mm^−1
V = 1220.8 (8) Å^3	T = 120 K
Z = 4	Block, colourless
F(000) = 712	0.22 × 0.18 × 0.16 mm

Data collection

Bruker SMART 1K CCD area-detector diffractometer	3233 independent reflections
Radiation source: normal-focus sealed tube	3133 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.034
Detector resolution: 8 pixels mm^-1	θmax = 29.1°, θmin = 2.2°
ω scans	h = −9→9
Absorption correction: integration (XPREP; Bruker, 1998)	k = −11→16
Tmin = 0.391, Tmax = 0.513	l = −18→17
9618 measured reflections

Refinement

Refinement on F^2	Secondary atom site location: difference Fourier map
Least-squares matrix: full	Hydrogen site location: difference Fourier map
R[F^2 > 2σ(F^2)] = 0.019	H-atom parameters constrained
wR(F^2) = 0.047	w = 1/[σ^2(Fo^2) + (0.018P)^2 + 1.295P] where P = (Fo^2 + 2Fc^2)/3
S = 1.05	(Δ/σ)max = 0.001
3233 reflections	Δρmax = 0.57 e Å^−3
127 parameters	Δρmin = −0.86 e Å^−3
0 restraints	Absolute structure: Flack (1983)
Primary atom site location: structure-invariant direct methods	Absolute structure parameter: −0.01 (3)

Special details

Experimental. Four sets of ω scans (each scan 0.3° in ω, exposure time 15 s), each set at different ϕ and/or 2θ angles, nominally covered over a hemisphere of reciprocal space. The first 50 scans were repeated at the end of data collection, revealing no significant crystal decay. Crystal to detector distance 4.56 cm. 1833 unique reflections after merging Friedel equivalents.

Absorption correction used XPREP (SHELXTL); before correction R(int)=0.042; afterwards it was 0.034.

Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes.

Refinement. Refinement of F² against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F², conventional R-factors R are based on F, with F set to zero for negative F². The threshold expression of F² > σ(F²) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq
I9	0.26319 (3)	0.303961 (16)	0.068963 (14)	0.02331 (5)
I12	−0.14555 (3)	0.53312 (2)	−0.006576 (16)	0.03045 (6)
C1	0.3072 (5)	0.5824 (3)	0.3006 (3)	0.0252 (7)
H1	0.3886	0.5932	0.3584	0.030*
C2	0.1499 (5)	0.6720 (3)	0.2712 (2)	0.0246 (6)
H2	0.1317	0.7391	0.3106	0.030*
B3	0.3527 (6)	0.6641 (3)	0.2028 (3)	0.0250 (7)
H3	0.4648	0.7277	0.1988	0.030*
B4	0.4045 (5)	0.5227 (3)	0.2029 (3)	0.0225 (7)
H4	0.5525	0.4925	0.1983	0.027*
B5	0.2339 (6)	0.4544 (3)	0.2755 (2)	0.0223 (6)
H5	0.2705	0.3800	0.3178	0.027*
B6	0.0755 (6)	0.5530 (3)	0.3205 (3)	0.0248 (7)
H6	0.0086	0.5447	0.3931	0.030*
B7	0.1270 (5)	0.6823 (3)	0.1492 (3)	0.0227 (7)
H7	0.0907	0.7581	0.1088	0.027*
B8	0.2876 (5)	0.5841 (3)	0.1030 (3)	0.0215 (7)
H8	0.3572	0.5942	0.0313	0.026*
B9	0.2135 (5)	0.4544 (3)	0.1479 (2)	0.0188 (6)
B10	0.0094 (5)	0.4729 (3)	0.2217 (3)	0.0201 (6)
H10	−0.1035	0.4099	0.2275	0.024*
B11	−0.0428 (6)	0.6141 (3)	0.2223 (3)	0.0216 (7)
H11	−0.1895	0.6458	0.2294	0.026*
B12	0.0429 (5)	0.5524 (3)	0.1152 (2)	0.0196 (7)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
I9	0.02952 (10)	0.01995 (9)	0.02046 (9)	0.00366 (9)	0.00342 (8)	−0.00220 (7)
I12	0.02560 (10)	0.04025 (12)	0.02551 (11)	0.00658 (9)	−0.00666 (9)	−0.00562 (10)
C1	0.0269 (17)	0.0242 (16)	0.0243 (16)	0.0025 (13)	−0.0039 (12)	−0.0002 (13)
C2	0.0253 (15)	0.0220 (16)	0.0267 (16)	0.0031 (13)	−0.0017 (14)	−0.0027 (12)
B3	0.0218 (16)	0.0224 (18)	0.0309 (19)	−0.0033 (14)	−0.0026 (16)	−0.0007 (14)
B4	0.0172 (15)	0.0257 (18)	0.0248 (17)	0.0011 (14)	−0.0029 (13)	0.0000 (15)
B5	0.0263 (17)	0.0200 (15)	0.0207 (15)	0.0031 (15)	0.0001 (14)	−0.0003 (13)
B6	0.0329 (19)	0.0220 (19)	0.0196 (16)	0.0010 (15)	−0.0007 (14)	−0.0016 (14)
B7	0.0259 (17)	0.0167 (16)	0.0253 (17)	0.0002 (14)	−0.0004 (14)	0.0011 (13)
B8	0.0179 (16)	0.0204 (16)	0.0263 (17)	−0.0020 (13)	0.0013 (13)	0.0024 (13)
B9	0.0186 (15)	0.0179 (15)	0.0199 (15)	0.0006 (12)	0.0012 (12)	−0.0002 (13)
B10	0.0220 (16)	0.0184 (16)	0.0198 (16)	−0.0020 (14)	0.0032 (13)	−0.0017 (14)
B11	0.0207 (16)	0.0184 (16)	0.0258 (17)	0.0016 (12)	0.0025 (13)	−0.0029 (13)
B12	0.0207 (15)	0.0207 (17)	0.0174 (15)	0.0008 (13)	−0.0009 (12)	−0.0006 (13)

Geometric parameters (Å, º)

I9—B9	2.183 (3)	B5—B6	1.773 (6)
I12—B12	2.173 (4)	B5—B10	1.781 (5)
C1—C2	1.626 (5)	B5—B9	1.780 (5)
C1—B4	1.693 (5)	B5—H5	1.1200
C1—B5	1.697 (5)	B6—B10	1.757 (5)
C1—B6	1.712 (6)	B6—B11	1.773 (6)
C1—B3	1.723 (5)	B6—H6	1.1199
C1—H1	1.0000	B7—B12	1.774 (5)
C2—B11	1.690 (5)	B7—B8	1.785 (5)
C2—B6	1.704 (5)	B7—B11	1.790 (6)
C2—B7	1.708 (5)	B7—H7	1.1201
C2—B3	1.733 (6)	B8—B12	1.795 (5)
C2—H2	1.0000	B8—B9	1.795 (5)
B3—B8	1.764 (5)	B8—H8	1.1199
B3—B4	1.781 (6)	B9—B12	1.773 (5)
B3—B7	1.786 (6)	B9—B10	1.794 (5)
B3—H3	1.1200	B10—B11	1.780 (5)
B4—B9	1.774 (5)	B10—B12	1.792 (5)
B4—B8	1.789 (5)	B10—H10	1.1200
B4—B5	1.790 (5)	B11—B12	1.781 (5)
B4—H4	1.1200	B11—H11	1.1200
C2—C1—B4	112.1 (3)	C2—B7—B12	103.3 (3)
C2—C1—B5	111.6 (3)	C2—B7—B8	104.3 (3)
B4—C1—B5	63.7 (2)	B12—B7—B8	60.6 (2)
C2—C1—B6	61.3 (2)	C2—B7—B3	59.4 (2)
B4—C1—B6	115.7 (3)	B12—B7—B3	107.6 (3)
B5—C1—B6	62.6 (2)	B8—B7—B3	59.2 (2)
C2—C1—B3	62.2 (2)	C2—B7—B11	57.7 (2)
B4—C1—B3	62.8 (2)	B12—B7—B11	60.0 (2)
B5—C1—B3	116.0 (3)	B8—B7—B11	108.7 (3)
B6—C1—B3	115.7 (3)	B3—B7—B11	108.3 (3)
C2—C1—H1	120.8	C2—B7—H7	125.5
B4—C1—H1	117.7	B12—B7—H7	122.7
B5—C1—H1	117.9	B8—B7—H7	122.2
B6—C1—H1	117.2	B3—B7—H7	121.4
B3—C1—H1	116.6	B11—B7—H7	121.3
C1—C2—B11	112.0 (3)	B3—B8—B7	60.4 (2)
C1—C2—B6	61.8 (2)	B3—B8—B4	60.2 (2)
B11—C2—B6	63.0 (2)	B7—B8—B4	107.8 (3)
C1—C2—B7	111.5 (3)	B3—B8—B12	107.6 (3)
B11—C2—B7	63.6 (2)	B7—B8—B12	59.4 (2)
B6—C2—B7	115.7 (3)	B4—B8—B12	106.7 (3)
C1—C2—B3	61.6 (2)	B3—B8—B9	107.5 (3)
B11—C2—B3	115.7 (3)	B7—B8—B9	106.8 (2)
B6—C2—B3	115.6 (3)	B4—B8—B9	59.3 (2)
B7—C2—B3	62.5 (2)	B12—B8—B9	59.18 (19)
C1—C2—H2	120.8	B3—B8—H8	121.4
B11—C2—H2	117.6	B7—B8—H8	122.0
B6—C2—H2	116.8	B4—B8—H8	122.1
B7—C2—H2	118.1	B12—B8—H8	122.6
B3—C2—H2	117.1	B9—B8—H8	122.6
C1—B3—C2	56.1 (2)	B12—B9—B4	108.3 (3)
C1—B3—B8	104.2 (3)	B12—B9—B5	108.2 (2)
C2—B3—B8	104.1 (3)	B4—B9—B5	60.5 (2)
C1—B3—B4	57.8 (2)	B12—B9—B10	60.3 (2)
C2—B3—B4	103.1 (3)	B4—B9—B10	108.4 (2)
B8—B3—B4	60.6 (2)	B5—B9—B10	59.8 (2)
C1—B3—B7	103.5 (3)	B12—B9—B8	60.4 (2)
C2—B3—B7	58.0 (2)	B4—B9—B8	60.2 (2)
B8—B3—B7	60.3 (2)	B5—B9—B8	108.8 (2)
B4—B3—B7	108.1 (3)	B10—B9—B8	108.9 (2)
C1—B3—H3	125.6	B12—B9—I9	124.1 (2)
C2—B3—H3	125.6	B4—B9—I9	119.7 (2)
B8—B3—H3	122.6	B5—B9—I9	119.2 (2)
B4—B3—H3	122.4	B10—B9—I9	121.8 (2)
B7—B3—H3	122.3	B8—B9—I9	122.2 (2)
C1—B4—B9	103.7 (2)	B6—B10—B11	60.2 (2)
C1—B4—B3	59.4 (2)	B6—B10—B5	60.1 (2)
B9—B4—B3	107.7 (3)	B11—B10—B5	108.1 (3)
C1—B4—B8	104.4 (3)	B6—B10—B12	107.7 (3)
B9—B4—B8	60.5 (2)	B11—B10—B12	59.8 (2)
B3—B4—B8	59.2 (2)	B5—B10—B12	107.3 (2)
C1—B4—B5	58.2 (2)	B6—B10—B9	107.5 (3)
B9—B4—B5	59.9 (2)	B11—B10—B9	107.2 (3)
B3—B4—B5	108.7 (3)	B5—B10—B9	59.7 (2)
B8—B4—B5	108.7 (3)	B12—B10—B9	59.25 (19)
C1—B4—H4	125.2	B6—B10—H10	121.7
B9—B4—H4	122.7	B11—B10—H10	121.7
B3—B4—H4	121.3	B5—B10—H10	121.8
B8—B4—H4	122.3	B12—B10—H10	122.2
B5—B4—H4	121.0	B9—B10—H10	122.4
C1—B5—B6	59.1 (2)	C2—B11—B6	58.9 (2)
C1—B5—B10	104.1 (3)	C2—B11—B10	104.2 (3)
B6—B5—B10	59.3 (2)	B6—B11—B10	59.3 (2)
C1—B5—B9	103.3 (2)	C2—B11—B12	103.8 (3)
B6—B5—B9	107.5 (3)	B6—B11—B12	107.5 (3)
B10—B5—B9	60.5 (2)	B10—B11—B12	60.4 (2)
C1—B5—B4	58.0 (2)	C2—B11—B7	58.7 (2)
B6—B5—B4	108.0 (3)	B6—B11—B7	108.4 (3)
B10—B5—B4	108.2 (2)	B10—B11—B7	108.3 (3)
B9—B5—B4	59.6 (2)	B12—B11—B7	59.6 (2)
C1—B5—H5	125.5	C2—B11—H11	125.2
B6—B5—H5	121.6	B6—B11—H11	121.6
B10—B5—H5	122.3	B10—B11—H11	122.4
B9—B5—H5	122.9	B12—B11—H11	122.8
B4—B5—H5	121.5	B7—B11—H11	121.2
C2—B6—C1	56.8 (2)	B9—B12—B7	108.3 (3)
C2—B6—B10	104.6 (3)	B9—B12—B11	108.1 (2)
C1—B6—B10	104.5 (3)	B7—B12—B11	60.5 (2)
C2—B6—B5	104.5 (3)	B9—B12—B10	60.4 (2)
C1—B6—B5	58.3 (2)	B7—B12—B10	108.5 (2)
B10—B6—B5	60.6 (2)	B11—B12—B10	59.8 (2)
C2—B6—B11	58.1 (2)	B9—B12—B8	60.4 (2)
C1—B6—B11	104.1 (3)	B7—B12—B8	60.0 (2)
B10—B6—B11	60.6 (2)	B11—B12—B8	108.6 (3)
B5—B6—B11	108.8 (3)	B10—B12—B8	109.1 (2)
C2—B6—H6	125.0	B9—B12—I12	123.3 (2)
C1—B6—H6	125.1	B7—B12—I12	121.0 (2)
B10—B6—H6	122.6	B11—B12—I12	119.1 (2)
B5—B6—H6	121.8	B10—B12—I12	120.1 (2)
B11—B6—H6	122.0	B8—B12—I12	123.4 (2)

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
C1—H1···I9^i	1.00	3.37	4.017 (4)	124
C1—H1···I12^i	1.00	3.05	3.877 (4)	141
C2—H2···I9^ii	1.00	3.37	4.031 (4)	125

Symmetry codes: (i) −x+1/2, −y+1, z+1/2; (ii) −x, y+1/2, −z+1/2.