A new polymorph of tetra­phenyl­diboroxane

A new polymorph of tetra­phenyl­diboroxane [or oxybis(diphenyl­borane)], C₂₄H₂₀B₂O, (Ia), has been found. It is monoclinic, like the already known form, (Ib), and can be refined in the same space group, namely P2₁/c, or in the equivalent setting P2₁/n. The mol­ecular conformations of the two polymorphs differ in the rotations of two of the phenyl rings about the B-C bonds, leading to markedly different packing patterns and cell dimensions.

S1. Comment

Recently, we have synthesized the anionic [1,1]diborataferrocenophane, (II), which represents a highly efficient Li⁺ scavenger (see scheme) (Scheibitz et al., 2003), and anionic ferrocene derivatives such as the ferrocenyl borate, (IV) (Kaufmann et al., 2007). In an attempt to synthesize the borane, (III), from FcHgCl (Fc = C₅H₄FeC₅H₅) and Ph₂BBr, we have obtained the title compound, tetraphenyldiboroxane, Ph₂B—O—BPh₂, (Ia), as a side-product. The ferrocene derivative, (III), represents a versatile starting material for the synthesis of a broad range of ferrocenyl-substituted polyborates.

Both B atoms in (Ia) are in trigonal planar environments (Fig. 1). The sums of the bond angles are 360.0 (2) and 359.8 (2)° for atoms B1 and B2, respectively. Another monoclinic polymorph of the title compound, (Ib), has been described at 190 K in space group P2₁/c, with cell parameters a = 12.378 (5), b = 8.019 (2) and c = 19.180 (7) Å, β = 100.89 (3)°, and V = 1869.51 ų (Lange et al., 2002). The only similarity between the cell parameters of the two polymorphs is that the c axis of (Ia) and the b axis of (Ib) differ by just 0.1 Å. Since the cell parameters are otherwise completely different, the setting in P2₁/n has been selected for (Ia). A least-squares fit of the two polymorphs shows that the orientations of the phenyl rings attached to atom B2 are almost identical in (Ia) and (Ib), while the orientations of the phenyl rings attached to atom B1 differ significantly (Fig. 2). This difference can also be illustrated by a comparison of the corresponding torsion angles (Table 1); only the torsion angles about the B1—C1 and B1—C7 bonds differ significantly.

An analysis of short intramolecular distances for both polymorphs reveals two somewhat shorter X···H distances in (Ib), which might be the reason for the different orientations of the phenyl rings [B2···H10 = 2.63 Å in (Ib) versus 2.74 Å in (Ia), and C2···H6 = 2.71 Å in (Ib) versus 2.79 Å in (Ia); all C—H distances are 0.93 Å in both polymorphs]. A molecular mechanics calculation with the molecular modelling program MOMO (Beck et al., 1991) shows that the conformation of (Ia) is more stable than that of (Ib), by 4.2 kcal mol⁻¹ [1 kcal mol⁻¹ = 4.184 kJ mol⁻¹].

The packing motifs of the two polymorphs are quite different, presumably due to the different orientations of two of the four aromatic rings. The orientations of the packing diagrams (Figs. 3 and 4) have been chosen so that one molecule (shown with dashed bonds) has approximately the same orientation in both figures. Comparison of the diagrams shows that the orientation of the remaining molecules differs markedly.

An analysis of short intermolecular distances in the crystal packing of both polymorphs shows only two notable short distances in (Ib) [C14···H8ⁱ = 2.96 Å and C21···H2ⁱⁱ = 2.93 Å; symmetry codes: (i) x, y − 1, z; (ii) x + 1, y, z], whereas there are five intermolecular C···H distances of less than 3 Å in (Ia): C6···H15ⁱ = 2.99 Å, C3···H9ⁱⁱ = 2.91 Å, C10···H17ⁱⁱⁱ = 2.95 Å, C17···H5^iv = 2.90 Å and C20···H2^v = 2.99 Å [symmetry codes: (i) 2 − x, 1 − y, 1 − z; (ii) −1/2 + x, 1/2 − y, 1/2 + z; (iii) 3/2 − x, −1/2 + y, 1/2 − z; (iv) x, −1 + y, −1 + z; (v) −1/2 + x, 1/2 − y, −1/2 + z]. In addition, there is one very short intermolecular H···H distance in (Ia) [H4···H22^vi = 2.40 Å; symmetry code: (vi) 1 + x, y, 1 + z], for which there is no equivalent in (Ib). The difference in all these intermolecular contacts shows that a single molecule is in a different environment in both structures.

A search of the Cambridge Structural Database (Version 5.28, November 2006, updated May 2007; Allen, 2002) for the fragment (C_ar)₂B—O—B(C_ar)₂ shows just four entries, namely BUBXEB (Cardin et al., 1983), BZBPER (Cynkier & Furmanova, 1980), DTBOPE (Aurivillius, 1974) and UCITEF, which is (Ib) (Lange et al., 2002). It is worth noting that in all of these structures the B—O—B angle is wider than in (Ia) and the B—O bond lengths are shorter (Table 2), although the measurement temperatures of 173 K for (Ia) and 190 K for (Ib) are almost equal.

S2. Experimental

In an attempt to synthesize the borane, (III), from FcHgCl (0.88 g, 2.09 mmol) with Ph₂BBr (0.51 g, 2.09 mmol) in hexane (45 ml) at 281 K, we obtained Ph₂B—O—BPh₂, (Ia), as a side-product. X-ray quality crystals of the title compound were grown from hexane at 281 K.

S3. Refinement

H atoms were located in a difference map, but were positioned geometrically and refined using a riding model, with fixed bond lengths and individual displacement parameters [C—H = 0.95 Å and U_iso(H) = 1.2U_eq(C)].

Computing details

Data collection: X-AREA (Stoe & Cie, 2001); cell refinement: X-AREA (Stoe & Cie, 2001); data reduction: X-AREA (Stoe & Cie, 2001); program(s) used to solve structure: SHELXS97 (Sheldrick, 1990); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: PLATON (Spek, 2003) and XP in SHELXTL-Plus (Sheldrick, 1991); software used to prepare material for publication: PLATON (Spek, 2003) and SHELXL97 (Sheldrick, 1997).

Figures

	Fig. 1. Perspective view of the title compound, with the atom-numbering scheme (identical to that used for Ib). Displacement ellipsoids are drawn at the 50% probability level and H atoms are shown as small spheres of arbitrary radii.
	Fig. 2. A least-squares fit (fitting the B—O—B fragment and the two phenyl rings C13–C18 and C19–C24) of (Ia) (full bonds) with (Ib) (open bonds); r.m.s. deviation for the fitted atoms = 0.103 Å.
	Fig. 3. A packing diagram for (Ia), viewed approximately onto the (011) plane. H atoms have been omitted for clarity. The molecule drawn with dashed bonds has the same orientation as the molecule drawn with dashed bonds in Fig. 4.
	Fig. 4. A packing diagram for (Ib), viewed onto the (101) plane. H atoms have been omitted for clarity. The molecule drawn with dashed bonds has the same orientation as the molecule drawn with dashed bonds in Fig. 3.

oxybis(diphenylborane)

Crystal data

C24H20B2O	F(000) = 728
Mr = 346.02	Dx = 1.174 Mg m^−3
Monoclinic, P21/n	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -P 2yn	Cell parameters from 7643 reflections
a = 10.1489 (11) Å	θ = 3.8–25.5°
b = 19.2851 (16) Å	µ = 0.07 mm^−1
c = 10.1658 (11) Å	T = 173 K
β = 100.286 (8)°	Block, orange-brown
V = 1957.7 (3) Å^3	0.19 × 0.17 × 0.17 mm
Z = 4

Data collection

Stoe IPDS II two-circle diffractometer	2205 reflections with I > 2σ(I)
Radiation source: fine-focus sealed tube	Rint = 0.062
Graphite monochromator	θmax = 25.7°, θmin = 3.8°
ω scans	h = −12→12
19779 measured reflections	k = −23→23
3682 independent reflections	l = −12→12

Refinement

Refinement on F^2	Primary atom site location: structure-invariant direct methods
Least-squares matrix: full	Secondary atom site location: difference Fourier map
R[F^2 > 2σ(F^2)] = 0.052	Hydrogen site location: inferred from neighbouring sites
wR(F^2) = 0.111	H-atom parameters constrained
S = 0.95	w = 1/[σ^2(Fo^2) + (0.0425P)^2] where P = (Fo^2 + 2Fc^2)/3
3682 reflections	(Δ/σ)max < 0.001
244 parameters	Δρmax = 0.29 e Å^−3
0 restraints	Δρmin = −0.13 e Å^−3

Special details

Experimental.

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. R-factors based on F² are statistically about twice as large as those based on F, and R- factors based on ALL data will be even larger.

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

	x	y	z	Uiso*/Ueq
O1	0.67212 (15)	0.38327 (8)	0.51141 (13)	0.0455 (4)
B1	0.7733 (2)	0.34170 (14)	0.5742 (2)	0.0366 (6)
B2	0.6135 (3)	0.41569 (13)	0.3955 (2)	0.0375 (6)
C1	0.80987 (19)	0.35039 (11)	0.73024 (19)	0.0352 (5)
C2	0.8760 (2)	0.29905 (12)	0.81529 (19)	0.0382 (5)
H2	0.8999	0.2566	0.7784	0.046*
C3	0.9073 (2)	0.30951 (14)	0.9529 (2)	0.0488 (6)
H3	0.9527	0.2744	1.0090	0.059*
C4	0.8725 (3)	0.37060 (16)	1.0078 (2)	0.0603 (8)
H4	0.8935	0.3774	1.1017	0.072*
C5	0.8068 (3)	0.42218 (15)	0.9264 (2)	0.0614 (7)
H5	0.7833	0.4644	0.9645	0.074*
C6	0.7755 (2)	0.41204 (13)	0.7891 (2)	0.0455 (6)
H6	0.7300	0.4475	0.7341	0.055*
C7	0.8415 (2)	0.29119 (11)	0.48572 (19)	0.0349 (5)
C8	0.9738 (2)	0.26868 (13)	0.5240 (2)	0.0441 (6)
H8	1.0217	0.2806	0.6100	0.053*
C9	1.0367 (2)	0.22931 (14)	0.4390 (2)	0.0524 (6)
H9	1.1269	0.2151	0.4672	0.063*
C10	0.9689 (3)	0.21056 (13)	0.3137 (2)	0.0525 (6)
H10	1.0120	0.1834	0.2560	0.063*
C11	0.8378 (3)	0.23180 (13)	0.2732 (2)	0.0492 (6)
H11	0.7905	0.2192	0.1873	0.059*
C12	0.7758 (2)	0.27113 (12)	0.3574 (2)	0.0418 (5)
H12	0.6856	0.2852	0.3279	0.050*
C13	0.7002 (2)	0.46453 (11)	0.32223 (19)	0.0369 (5)
C14	0.8409 (2)	0.46569 (12)	0.3556 (2)	0.0457 (6)
H14	0.8846	0.4342	0.4209	0.055*
C15	0.9172 (3)	0.51121 (13)	0.2963 (2)	0.0527 (6)
H15	1.0121	0.5104	0.3206	0.063*
C16	0.8562 (3)	0.55795 (13)	0.2015 (2)	0.0511 (6)
H16	0.9088	0.5894	0.1609	0.061*
C17	0.7179 (3)	0.55867 (13)	0.1661 (2)	0.0474 (6)
H17	0.6755	0.5908	0.1013	0.057*
C18	0.6414 (2)	0.51262 (12)	0.2251 (2)	0.0408 (5)
H18	0.5467	0.5135	0.1992	0.049*
C19	0.4588 (2)	0.40369 (11)	0.35428 (19)	0.0349 (5)
C20	0.3929 (2)	0.39576 (12)	0.2208 (2)	0.0429 (5)
H20	0.4421	0.4002	0.1500	0.051*
C21	0.2567 (2)	0.38154 (13)	0.1919 (2)	0.0520 (6)
H21	0.2139	0.3752	0.1017	0.062*
C22	0.1830 (2)	0.37657 (14)	0.2930 (3)	0.0558 (7)
H22	0.0897	0.3671	0.2721	0.067*
C23	0.2444 (2)	0.38524 (13)	0.4248 (2)	0.0511 (6)
H23	0.1934	0.3827	0.4945	0.061*
C24	0.3807 (2)	0.39763 (12)	0.4542 (2)	0.0421 (5)
H24	0.4227	0.4022	0.5451	0.050*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
O1	0.0381 (9)	0.0575 (10)	0.0387 (8)	0.0098 (7)	0.0004 (6)	0.0082 (7)
B1	0.0250 (12)	0.0440 (15)	0.0394 (12)	−0.0067 (10)	0.0018 (10)	0.0094 (11)
B2	0.0416 (14)	0.0399 (15)	0.0296 (11)	0.0100 (11)	0.0025 (10)	−0.0023 (10)
C1	0.0237 (10)	0.0444 (13)	0.0366 (10)	−0.0039 (9)	0.0025 (8)	0.0029 (9)
C2	0.0299 (11)	0.0460 (13)	0.0374 (11)	−0.0040 (9)	0.0025 (9)	0.0063 (10)
C3	0.0382 (13)	0.0711 (18)	0.0353 (11)	−0.0059 (12)	0.0017 (10)	0.0097 (12)
C4	0.0499 (15)	0.095 (2)	0.0340 (12)	−0.0106 (15)	0.0029 (11)	−0.0131 (13)
C5	0.0521 (16)	0.0728 (19)	0.0574 (15)	−0.0035 (14)	0.0047 (13)	−0.0233 (14)
C6	0.0361 (12)	0.0481 (14)	0.0510 (13)	−0.0017 (10)	0.0040 (10)	−0.0049 (11)
C7	0.0284 (11)	0.0397 (12)	0.0357 (10)	−0.0054 (9)	0.0037 (8)	0.0082 (9)
C8	0.0310 (12)	0.0593 (15)	0.0413 (11)	−0.0005 (10)	0.0044 (10)	0.0048 (10)
C9	0.0307 (12)	0.0641 (17)	0.0644 (15)	0.0047 (11)	0.0141 (11)	0.0055 (13)
C10	0.0522 (15)	0.0527 (15)	0.0562 (14)	−0.0023 (12)	0.0196 (12)	−0.0059 (12)
C11	0.0511 (15)	0.0507 (14)	0.0446 (12)	−0.0047 (12)	0.0054 (11)	−0.0057 (11)
C12	0.0376 (12)	0.0424 (13)	0.0431 (12)	0.0001 (10)	0.0007 (10)	0.0009 (10)
C13	0.0352 (12)	0.0398 (12)	0.0349 (10)	0.0013 (9)	0.0039 (9)	−0.0045 (9)
C14	0.0384 (13)	0.0448 (13)	0.0517 (13)	−0.0008 (11)	0.0015 (10)	0.0020 (11)
C15	0.0404 (14)	0.0507 (15)	0.0656 (15)	−0.0064 (12)	0.0056 (11)	−0.0017 (12)
C16	0.0537 (16)	0.0478 (14)	0.0526 (13)	−0.0120 (12)	0.0116 (12)	−0.0027 (12)
C17	0.0568 (15)	0.0458 (14)	0.0394 (12)	0.0020 (11)	0.0082 (11)	0.0001 (10)
C18	0.0400 (13)	0.0454 (13)	0.0368 (11)	0.0011 (10)	0.0063 (9)	−0.0032 (10)
C19	0.0332 (11)	0.0369 (12)	0.0330 (10)	0.0067 (9)	0.0013 (8)	0.0042 (9)
C20	0.0427 (13)	0.0488 (14)	0.0356 (11)	0.0011 (11)	0.0031 (9)	0.0074 (10)
C21	0.0435 (14)	0.0618 (17)	0.0444 (12)	−0.0044 (12)	−0.0091 (11)	0.0132 (11)
C22	0.0307 (12)	0.0648 (17)	0.0689 (16)	−0.0013 (11)	0.0004 (12)	0.0189 (13)
C23	0.0404 (14)	0.0626 (17)	0.0529 (13)	0.0046 (12)	0.0158 (11)	0.0094 (12)
C24	0.0395 (13)	0.0500 (14)	0.0373 (11)	0.0075 (10)	0.0086 (9)	0.0054 (10)

Geometric parameters (Å, º)

O1—B1	1.367 (3)	C11—C12	1.377 (3)
O1—B2	1.372 (3)	C11—H11	0.9500
B1—C7	1.568 (3)	C12—H12	0.9500
B1—C1	1.573 (3)	C13—C18	1.408 (3)
B2—C13	1.565 (3)	C13—C14	1.408 (3)
B2—C19	1.568 (3)	C14—C15	1.379 (3)
C1—C6	1.403 (3)	C14—H14	0.9500
C1—C2	1.404 (3)	C15—C16	1.382 (3)
C2—C3	1.393 (3)	C15—H15	0.9500
C2—H2	0.9500	C16—C17	1.385 (4)
C3—C4	1.376 (4)	C16—H16	0.9500
C3—H3	0.9500	C17—C18	1.385 (3)
C4—C5	1.386 (4)	C17—H17	0.9500
C4—H4	0.9500	C18—H18	0.9500
C5—C6	1.389 (3)	C19—C24	1.401 (3)
C5—H5	0.9500	C19—C20	1.410 (3)
C6—H6	0.9500	C20—C21	1.389 (3)
C7—C8	1.399 (3)	C20—H20	0.9500
C7—C12	1.409 (3)	C21—C22	1.377 (3)
C8—C9	1.388 (3)	C21—H21	0.9500
C8—H8	0.9500	C22—C23	1.384 (3)
C9—C10	1.384 (3)	C22—H22	0.9500
C9—H9	0.9500	C23—C24	1.382 (3)
C10—C11	1.382 (4)	C23—H23	0.9500
C10—H10	0.9500	C24—H24	0.9500
B1—O1—B2	147.34 (19)	C11—C12—C7	122.2 (2)
O1—B1—C7	117.92 (18)	C11—C12—H12	118.9
O1—B1—C1	115.5 (2)	C7—C12—H12	118.9
C7—B1—C1	126.57 (19)	C18—C13—C14	116.3 (2)
O1—B2—C13	119.3 (2)	C18—C13—B2	121.79 (19)
O1—B2—C19	114.9 (2)	C14—C13—B2	121.81 (19)
C13—B2—C19	125.64 (18)	C15—C14—C13	122.0 (2)
C6—C1—C2	117.57 (18)	C15—C14—H14	119.0
C6—C1—B1	119.27 (19)	C13—C14—H14	119.0
C2—C1—B1	123.2 (2)	C14—C15—C16	120.2 (2)
C3—C2—C1	120.9 (2)	C14—C15—H15	119.9
C3—C2—H2	119.5	C16—C15—H15	119.9
C1—C2—H2	119.5	C15—C16—C17	119.7 (2)
C4—C3—C2	120.2 (2)	C15—C16—H16	120.2
C4—C3—H3	119.9	C17—C16—H16	120.2
C2—C3—H3	119.9	C18—C17—C16	120.1 (2)
C3—C4—C5	120.2 (2)	C18—C17—H17	119.9
C3—C4—H4	119.9	C16—C17—H17	119.9
C5—C4—H4	119.9	C17—C18—C13	121.7 (2)
C4—C5—C6	119.9 (3)	C17—C18—H18	119.1
C4—C5—H5	120.1	C13—C18—H18	119.1
C6—C5—H5	120.1	C24—C19—C20	117.2 (2)
C5—C6—C1	121.2 (2)	C24—C19—B2	119.23 (17)
C5—C6—H6	119.4	C20—C19—B2	123.6 (2)
C1—C6—H6	119.4	C21—C20—C19	120.5 (2)
C8—C7—C12	116.4 (2)	C21—C20—H20	119.7
C8—C7—B1	122.24 (18)	C19—C20—H20	119.7
C12—C7—B1	121.13 (18)	C22—C21—C20	120.6 (2)
C9—C8—C7	121.5 (2)	C22—C21—H21	119.7
C9—C8—H8	119.3	C20—C21—H21	119.7
C7—C8—H8	119.3	C21—C22—C23	120.2 (2)
C10—C9—C8	120.5 (2)	C21—C22—H22	119.9
C10—C9—H9	119.7	C23—C22—H22	119.9
C8—C9—H9	119.7	C24—C23—C22	119.3 (2)
C11—C10—C9	119.3 (2)	C24—C23—H23	120.3
C11—C10—H10	120.3	C22—C23—H23	120.3
C9—C10—H10	120.3	C23—C24—C19	122.14 (19)
C12—C11—C10	120.1 (2)	C23—C24—H24	118.9
C12—C11—H11	120.0	C19—C24—H24	118.9
C10—C11—H11	120.0
B2—O1—B1—C7	−24.0 (5)	C8—C7—C12—C11	−0.3 (3)
B2—O1—B1—C1	155.6 (3)	B1—C7—C12—C11	174.2 (2)
B1—O1—B2—C13	−54.2 (4)	O1—B2—C13—C18	−163.16 (19)
B1—O1—B2—C19	130.2 (3)	C19—B2—C13—C18	12.0 (3)
O1—B1—C1—C6	−21.7 (3)	O1—B2—C13—C14	13.0 (3)
C7—B1—C1—C6	157.8 (2)	C19—B2—C13—C14	−171.9 (2)
O1—B1—C1—C2	158.3 (2)	C18—C13—C14—C15	−0.2 (3)
C7—B1—C1—C2	−22.2 (3)	B2—C13—C14—C15	−176.5 (2)
C6—C1—C2—C3	−0.6 (3)	C13—C14—C15—C16	0.4 (4)
B1—C1—C2—C3	179.4 (2)	C14—C15—C16—C17	−0.2 (4)
C1—C2—C3—C4	0.5 (3)	C15—C16—C17—C18	−0.2 (4)
C2—C3—C4—C5	−0.3 (4)	C16—C17—C18—C13	0.5 (3)
C3—C4—C5—C6	0.3 (4)	C14—C13—C18—C17	−0.3 (3)
C4—C5—C6—C1	−0.4 (4)	B2—C13—C18—C17	176.1 (2)
C2—C1—C6—C5	0.6 (3)	O1—B2—C19—C24	34.5 (3)
B1—C1—C6—C5	−179.5 (2)	C13—B2—C19—C24	−140.8 (2)
O1—B1—C7—C8	152.9 (2)	O1—B2—C19—C20	−142.9 (2)
C1—B1—C7—C8	−26.6 (3)	C13—B2—C19—C20	41.8 (3)
O1—B1—C7—C12	−21.3 (3)	C24—C19—C20—C21	−0.9 (3)
C1—B1—C7—C12	159.2 (2)	B2—C19—C20—C21	176.5 (2)
C12—C7—C8—C9	0.6 (3)	C19—C20—C21—C22	1.5 (4)
B1—C7—C8—C9	−173.8 (2)	C20—C21—C22—C23	−0.4 (4)
C7—C8—C9—C10	−0.7 (4)	C21—C22—C23—C24	−1.2 (4)
C8—C9—C10—C11	0.4 (4)	C22—C23—C24—C19	1.8 (4)
C9—C10—C11—C12	−0.1 (4)	C20—C19—C24—C23	−0.8 (3)
C10—C11—C12—C7	0.1 (4)	B2—C19—C24—C23	−178.3 (2)